Molecular Nano Dynamics: Vol. I: Spectroscopic Methods and Nanostructures Vol. II: Active Surfaces, Single Crystals and Single Biocells

Molecular Nano Dynamics Edited by Hiroshi Fukumura, Masahiro Irie, Yasuhiro Iwasawa, Hiroshi Masuhara, and Kohei Uosak...

Author: Hiroshi Fukumura | Masahiro Irie | Yasuhiro Iwasawa | Hiroshi Masuhara | Kohei Uosaki

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Molecular Nano Dynamics

Edited by Hiroshi Fukumura, Masahiro Irie, Yasuhiro Iwasawa, Hiroshi Masuhara, and Kohei Uosaki

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Molecular Nano Dynamics Volume I: Spectroscopic Methods and Nanostructures


The Editors Prof. Dr. Hiroshi Fukumura Tohoku University Graduate School of Science 6-3 Aoba Aramaki, Aoba-ku Sendai 980-8578 Japan

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for

Prof. Dr. Masahiro Irie Rikkyo University Department of Chemistry Nishi-Ikebukuro 3-34-1 Toshima-ku Tokyo 171-8501 Japan Prof. Dr. Yasuhiro Iwasawa University of Electro-Communications Department of Applied Physics and Chemistry 1-5-1 Chofu Tokyo 182-8585 and Emeritus Professor University of Tokyo 7-3-1 Hongo, Bunkyo-ku Tokyo 113-0033 Japan Dr. Hiroshi Masuhara Nara Institute of Science and Technology Graduate School of Material Science 8916-5 Takayama, Ikoma Nara, 630-0192 Japan and National Chiao Tung University Department of Applied Chemistry and Institute of Molecular Science 1001 Ta Hsueh Road Hsinchu 30010 Taiwan Prof. Dr. Kohei Uosaki Hokkaido University Graduate School of Science N 10, W 8 , Kita-ku Sapporo 060-0810 Japan

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de. # 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Cover Design Adam-Design, Weinheim Typesetting Thomson Digital, Noida, India Printing and Binding betz-druck GmbH, Darmstadt Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32017-2

V

Contents to Volume 1 Contents to Volume 2 XIII Preface XVII About the Editors XIX List of Contributors for Both Volumes

XXIII

Part One Spectroscopic Methods for Nano Interfaces 1

1.1 1.2 1.2.1 1.2.2 1.3 1.4 1.4.1 1.4.2 1.5

2 2.1 2.2 2.3 2.4 2.4.1 2.4.2 2.4.3

1

Raman and Fluorescence Spectroscopy Coupled with Scanning Tunneling Microscopy 3 Noriko Nishizawa Horimoto and Hiroshi Fukumura Introduction 3 Outline of STM Combined with Optical Spectroscopy 4 Raman Spectroscopy 4 Fluorescence Spectroscopy 6 Theoretical Approaches 6 Experimental Approaches 8 STM Combined with Raman Spectroscopy 8 STM Combined With Fluorescence Spectroscopy 12 Future Prospects 13 References 16 Vibrational Nanospectroscopy for Biomolecules and Nanomaterials 19 Yasushi Inouye, Atsushi Taguchi, and Taro Ichimura Introduction 19 Surface Plasmon Polaritons 20 Near-Field Optical Microscopy Using a Metallic Nano-Tip 22 Tip-Enhanced Near-Field Raman Spectroscopy and Imaging 24 Raman Spectroscopy 25 Near-Field Nano-Raman Microscopy 25 Tip-Enhanced Near-Field Raman Spectroscopy and Imaging 26

VI

Contents to Volume 1

2.5 2.6

Tip Effect on Near-Field Raman Scattering 30 Conclusion 36 References 36

3

Near-Field Optical Imaging of Localized Plasmon Resonances in Metal Nanoparticles 39 Hiromi Okamoto and Kohei Imura Introduction 39 Near-Field Spectroscopic Method 40 Fundamental Spectroscopic Characteristics of Gold Nanoparticles 42 Wavefunction Images of Plasmon Modes of Gold Nanorod –— Near-Field Transmission Method 42 Ultrafast Time-Resolved Near-Field Imaging of Gold Nanorods 45 Near-Field Two-Photon Excitation Images of Gold Nanorods 47 Enhanced Optical Fields in Spherical Nanoparticle Assemblies and Surface Enhanced Raman Scattering 48 Concluding Remarks 51 References 52

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

4

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.4

5

5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4

Structure and Dynamics of a Confined Polymer Chain Studied by Spatially and Temporally Resolved Fluorescence Techniques 55 Hiroyuki Aoki Introduction 55 Conformation of a Confined Polymer Chain 56 Polymer Ultra-Thin Film 56 Near-Field Optical Microscopy 56 Structure of a Single Polymer Chain 58 Dynamics of a Confined Polymer Chain 61 Polymer Brush 61 Fluorescence Depolarization Method 61 Dynamics of a Polymer Brush 63 Summary 67 References 68 Real Time Monitoring of Molecular Structure at Solid/Liquid Interfaces by Non-Linear Spectroscopy 71 Hidenori Noguchi, Katsuyoshi Ikeda, and Kohei Uosaki Introduction 71 Sum Frequency Generation Spectroscopy 72 Brief Description of SFG 72 Origin of SFG Process 73 SFG Spectroscopy 74 Experimental Arrangement for SFG Measurements 77


5.2.4.1 5.2.4.2 5.3 5.3.1 5.3.2 5.3.3 5.4 5.4.1 5.4.2 5.4.3 5.5 5.5.1 5.5.2 5.5.3 5.6 5.6.1 5.6.2 5.6.3 5.7

6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

7 7.1 7.1.1 7.1.2 7.2 7.2.1

Laser and Detection Systems 77 Spectroscopic Cells 78 SFG Study of the Potential-Dependent Structure of Water at a Pt Electrode/Electrolyte Solution Interface 80 Introduction 80 Results and Discussion 80 Conclusions 83 Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode 84 Introduction 84 Results and Discussion 85 Conclusions 88 Interfacial Water Structure at Polyvinyl Alcohol (PVA) Gel/Quartz Interfaces Investigated by SFG Spectroscopy 89 Introduction 89 Results and Discussions 90 Conclusions 92 Hyper-Raman Spectroscopy 94 Selection Rules for Hyper-Raman Scattering 94 Enhancement of Hyper-Raman Scattering Intensity 94 Conclusion 96 General Conclusion 96 References 97 Fourth-Order Coherent Raman Scattering at Buried Interfaces Hiroshi Onishi Why Buried Interfaces? 103 Optical Transitions 104 Experimental Scheme 106 Application to a Liquid Surface 107 Application to a Liquid/Liquid Interface 108 Applications to Solid Surfaces 109 Frequency Domain Detection 112 Concluding Remarks 113 References 113

103

Dynamic Analysis Using Photon Force Measurement 117 Hideki Fujiwara and Keiji Sasaki Introduction 117 Weak Force Measurements 117 Potential Analysis Method Using Photon Force Measurement 118 Measurement of the Hydrodynamic Interaction Force Acting between Two Trapped Particles Using the Potential Analysis Method 121 Two-Beam Photon Force Measurement System 121

VII

VIII


7.2.2 7.2.3 7.3 7.4

8

8.1 8.2 8.2.1 8.2.2 8.2.3 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4 8.4.1 8.4.2 8.5

9

9.1 9.2 9.3 9.4 9.5

Potential Analysis Method for Hydrodynamic Force Measurement 122 Trapping Potential Analysis 124 Kinetic Potential Analysis 125 Summary 129 References 130 Construction of Micro-Spectroscopic Systems and their Application to the Detection of Molecular Dynamics in a Small Domain 133 Syoji Ito, Hirohisa Matsuda, Takashi Sugiyama, Naoki Toitani, Yutaka Nagasawa, and Hiroshi Miyasaka Introduction 133 Development of a Near-Infrared 35 fs Laser Microscope and its Application to Higher Order Multiphoton Excitation 133 Confocal Microscope with a Chromium: Forsterite Ultrafast Laser as an Excitation Source 134 Detection of Higher Order Multiphoton Fluorescence from Organic Crystals 135 Multiphoton Fluorescence Imaging with the Near-Infrared 35 fs Laser Microscope 137 Application of Fluorescence Correlation Spectroscopy to the Measurement of Local Temperature at a Small Area in Solution 139 Experimental System of FCS 139 The Principle of the Method of Measurement of Local Temperature Using FCS 140 Measurement of Local Temperature for Several Organic Solvents 141 Summary 146 Relaxation Dynamics of Non-Emissive State for Water-Soluble CdTe Quantum Dots Measured by Using FCS 147 Samples and Analysis of Experimental Data Obtained with FCS 147 Non-Emissive Relaxation Dynamics in CdTe Quantum dots 148 Summary 150 References 152 Nonlinear Optical Properties and Single Particle Spectroscopy of CdTe Quantum Dots 155 Lingyun Pan, Yoichi Kobayashi, and Naoto Tamai Introduction 155 Nonlinear Optical Properties of CdTe QDs 156 Optical Trapping of CdTe QDs Probed by Nonlinear Optical Properties 158 Single Particle Spectroscopy of CdTe QDs 162 Summary 166 References 167


Part Two Nanostructure Characteristics and Dynamics 10

10.1 10.2 10.2.1 10.2.2 10.2.3 10.2.4

10.2.5

10.3

11

11.1 11.2 11.3 11.4 11.5

12

12.1 12.2

12.3

12.3.1

171

Morphosynthesis in Polymeric Systems Using Photochemical Reactions 173 Hideyuki Nakanishi, Tomohisa Norisuye, and Qui Tran-Cong-Miyata Introduction 173 Morphosynthesis of Polymeric Systems by Using Light 174 Significance of Photochemical Reactions 174 Polymer Mixtures Used in this Study 175 Polymers with Spatially Graded Morphologies Designed from Photo-Induced Interpenetrating Polymer Networks (IPNs) 175 Designing Polymers with an Arbitrary Distribution of Characteristic Length Scales by the Computer-Assisted Irradiation (CAI) Method 177 Reversible Phase Separation Driven by Photodimerization of Anthracene: A Novel Method for Processing and Recycling Polymer Blends 181 Concluding Remarks 184 References 185 Self-Organization of Materials Into Microscale Patterns by Using Dissipative Structures 187 Olaf Karthaus Self-Organization and Self-Assembly 187 Dissipative Structures 189 Dynamics and Pattern Formation in Evaporating Polymer Solutions 191 Applications of Dewetted Structures in Organic Photonics and Electronics 196 Summary 198 References 198 Formation of Nanosize Morphology of Dye-Doped Copolymer Films and Evaluation of Organic Dye Nanocrystals Using a Laser 203 Akira Itaya, Shinjiro Machida, and Sadahiro Masuo Introduction 203 Position-Selective Arrangement of Nanosize Polymer Microspheres Onto a PS-b-P4VP Diblock Copolymer Film with Nanoscale Sea–island Microphase Structure 205 Nanoscale Morphological Change of PS-b-P4VP Block Copolymer Films Induced by Site-Selective Doping of a Photoactive Chromophore 208 Nanoscale Surface Morphology of PS-b-P4VP Block Copolymer Films 208

IX

X


12.3.2

12.4

12.5

13

13.1 13.1.1 13.1.2 13.1.3 13.1.4 13.1.5 13.2

14

14.1 14.2 14.3 14.3.1 14.3.2 14.3.3 14.3.4 14.4 14.5

15

15.1 15.2 15.2.1

Nanoscale Surface Morphological Change of PS-b-P4VP Block Copolymer Films Induced by Site-Selective Doping of a Photoactive Chromophore 208 Site-Selective Modification of the Nanoscale Surface Morphology of Dye-Doped Copolymer Films Using Dopant-Induced Laser Ablation 211 Photon Antibunching Behavior of Organic Dye Nanocrystals on a Transparent Polymer Film 217 References 221 Molecular Segregation at Periodic Metal Nano-Architectures on a Solid Surface 225 Hideki Nabika and Kei Murakoshi Molecular Manipulation in Nano-Space 225 Lipid Bilayer and its Fluidic Nature 225 Controlling Molecular Diffusion in the Fluidic Lipid Bilayer 227 Self-Spreading of a Lipid Bilayer or Monolayer 229 Controlling the Self-Spreading Dynamics 230 Molecular Manipulation on the Self-Spreading Lipid Bilayer 233 Summary 235 References 236 Microspectroscopic Study of Self-Organization in Oscillatory Electrodeposition 239 Shuji Nakanishi Introduction 239 Dynamic Self-Organization in Electrochemical Reaction Systems 240 Oscillatory Electrodeposition 241 Formation of a Layered Nanostructure of Cu–Sn Alloy 242 Layered Nanostructures of Iron-Group Alloys 246 Layered Nanostructure of Cu/Cu2O 247 Nanostructured Metal Filaments 250 Raman Microspectroscopy Study of Oscillatory Electrodeposition of Au at an Air/Liquid Interface 252 Summary 255 References 256 Construction of Nanostructures by use of Magnetic Fields and Spin Chemistry in Solid/Liquid Interfaces 259 Hiroaki Yonemura Introduction 259 Construction of Nanostructures by the use of Magnetic Fields 260 Magnetic Orientation and Organization of SWNTs or their Composite Materials Using Polymer Wrapping 260


15.2.2

15.2.3 15.3 15.3.1 15.3.2

15.4

16

16.1 16.1.1 16.1.2 16.1.3 16.1.4 16.2 16.2.1 16.2.2 16.3

17

17.1 17.2 17.2.1 17.2.1.1 17.2.1.2 17.2.2 17.3 17.4 17.4.1 17.4.2

Effects of Magnetic Processing on the Morphological, Electrochemical, and Photoelectrochemical Properties of Electrodes Modified with C60-Phenothiazine Nanoclusters 264 Effects of Magnetic Processing on the Luminescence Properties of Monolayer Films with Mn2þ-Doped ZnS Nanoparticles 268 Spin Chemistry at Solid/Liquid Interfaces 270 Magnetic Field Effects on the Dynamics of the Radical Pair in a C60 Clusters–Phenothiazine System 270 Magnetic Field Effects on Photoelectrochemical Reactions of Electrodes Modified with the C60 Nanocluster-Phenothiazine System 272 Summary 274 References 274 Controlling Surface Wetting by Electrochemical Reactions of Monolayers and Applications for Droplet Manipulation 279 Ryo Yamada Introduction 279 Self-Assembled Monolayers 279 Preparation of Gradient Surfaces 280 Spontaneous Motion of a Droplet on Wetting Gradients 281 Surface Switching 282 Ratchet Motion of a Droplet 284 Ratchet Motion of a Droplet on Asymmetric Electrodes 284 Ratchet Motion of a Droplet Caused by Dynamic Motions of the Wetting Boundary 285 Conclusion 289 References 291 Photoluminescence of CdSe Quantum Dots: Shifting, Enhancement and Blinking 293 Vasudevanpillai Biju and Mitsuru Ishikawa Introduction 293 Synthesis of CdSe Quantum Dots 295 Synthesis of CdSe Quantum Dots in Organic Phases 295 Synthesis of CdSe Quantum Dots from Dimethyl Cadmium 295 Synthesis of CdSe Quantum Dots from Cadmium Sources Other Than Dimethyl Cadmium 296 Synthesis of Water-Soluble Quantum Dots 296 Bandgap Structure and Photoluminescence of CdSe Quantum Dots 298 Photoluminescence Spectral Shifts 299 Physical Effects on Spectral Shifts 300 Chemical Effects on Spectral Shifts 301

XI

XII


17.5 17.6 17.6.1 17.6.2 17.7

Enhancement of Photoluminescence in CdSe Quantum Dots 303 On and Off Luminescence Blinking in Single Quantum Dots 306 Power-Law Statistics of On and Off Time Distributions 308 Modified Blinking 308 Conclusions 312 References 312

XIII

Contents to Volume 2 Part Three Active Surfaces

315

18

The Genesis and Principle of Catalysis at Oxide Surfaces: Surface-Mediated Dynamic Aspects of Catalytic Dehydration and Dehydrogenation on TiO2(110) by STM and DFT 317 Yohei Uemura, Toshiaki Taniike, Takehiko Sasaki, Mizuki Tada, and Yasuhiro Iwasawa

19

Nuclear Wavepacket Dynamics at Surfaces Kazuya Watanabe

20

Theoretical Aspects of Charge Transfer/Transport at Interfaces and Reaction Dynamics 357 Hisao Nakamura and Koichi Yamashita

21

Dynamic Behavior of Active Ag Species in NOx Reduction on Ag/Al2O3 401 Atsushi Satsuma and Ken-ichi Shimizu

22

Dynamic Structural Change of Pd Induced by Interaction with Zeolites Studied by Means of Dispersive and Quick XAFS 427 Kazu Okumura

Part Four Single Crystals

337

441

23

Morphology Changes of Photochromic Single Crystals 443 Seiya Kobatake and Masahiro Irie

24

Direct Observation of Change in Crystal Structures During Solid-State Reactions of 1,3-Diene Compounds 459 Akikazu Matsumoto

Molecular Nano Dynamics, Volume I: Spectroscopic Methods and Nanostructures Edited by H. Fukumura, M. Irie, Y. Iwasawa, H. Masuhara, and K. Uosaki Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32017-2

XIV


25

Reaction Dynamics Studies on Crystalline-State Photochromism of Rhodium Dithionite Complexes 487 Hidetaka Nakai and Kiyoshi Isobe

26

Dynamics in Organic Inclusion Crystals of Steroids and Primary Ammonium Salts 505 Mikiji Miyata, Norimitsu Tohnai, and Ichiro Hisaki

27

Morphology Changes of Organic Crystals by Single-Crystal-to-SingleCrystal Photocyclization 527 Hideko Koshima

Part Five

Single Biocells

28

Femtosecond Laser Tsunami Processing and Light Scattering Spectroscopic Imaging of Single Animal Cells 547 Hiroshi Masuhara, Yoichiroh Hosokawa, Takayuki Uwada, Guillaume Louit, and Tsuyoshi Asahi

29

Super-Resolution Infrared Microspectroscopy for Single Cells Makoto Sakai, Keiichi Inoue, and Masaaki Fujii 571

30

Three-Dimensional High-Resolution Microspectroscopic Study of Environment-Sensitive Photosynthetic Membranes 589 Shigeichi Kumazaki, Makotoh Hasegawa, Mohammad Ghoneim, Takahiko Yoshida, Masahide Terazima, Takashi Shiina, and Isamu Ikegami

31

Fluorescence Lifetime Imaging Study on Living Cells with Particular Regard to Electric Field Effects and pH Dependence 607 Nobuhiro Ohta and Takakazu Nakabayashi

32

Multidimensional Fluorescence Imaging for Non-Invasive Tracking of Cell Responses 623 Ryosuke Nakamura and Yasuo Kanematsu

33

Fluorescence Correlation Spectroscopy on Molecular Diffusion Inside and Outside a Single Living Cell 645 Kiminori Ushida and Masataka Kinjo

34

Spectroscopy and Photoreactions of Gold Nanorods in Living Cells and Organisms 669 Yasuro Niidome and Takuro Niidome

545

571


35

Dynamic Motion of Single Cells and its Relation to Cellular Properties 689 Hideki Matsune, Daisuke Sakurai, Akitomo Hirukawa, Sakae Takenaka, and Masahiro Kishida Index

703

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Preface Over the past two decades, studies of chemical reaction dynamics have shifted from ideal systems of isolated molecules in the gas phase, of molecular clusters in jet beams, on ultra-clean surfaces, in homogeneous and in dilute molecular solutions, and in bulk crystals, towards nanosystems of supramolecules, colloids, and ultrasmall materials, following the contemporary trends in nanoscience and nanotechnology. The preparation, characterization, and functionalization of supramolecules, molecular assemblies, nanoparticles, nanodots, nanocrystals, nanotubes, nanowires, and so on, have been conducted extensively, and their chemical reactions and dynamic processes are now being elucidated. The systematic investigation of molecular nanosystems gives us a platform from which we can understand the nature of the dynamic behavior and chemical reactions occurring in complex systems such as molecular devices, catalysts, living cells, and so on. Thus we have conducted the KAKENHI (The Grant-in-Aid for Scientific Research) Project on Priority Area ‘‘Molecular Nanodynamics’’ (Project Leader: Hiroshi Masuhara) for the period from 2004 April to 2007 March, involving 86 laboratories in Japan. For the investigation of such complex systems new methodologies which enable us to analyze dynamics and mechanisms in terms of space and time are indispensable. Methods for simultaneous direct dynamic measurements in both time and real space domains needed to be devised and applied. Spectroscopy with novel spaceresolution and ultrafast spectroscopy with high sensitivity have been developed, the manipulation and fabrication of single molecules, nanoparticles, and single living cells have been realized, molecules and nanoparticles for probing chemical reactions spectroscopically and by imaging have been synthesized, new catalyses for cleaning air and new reactions have been found, and the way in which a reaction in a single molecular crystal leads to its morphological change has been elucidated under the umbrella of this research program. The recent development of these new methods and the advances in understanding chemical reaction dynamics in nanosystems are summarized in the present two volumes. The presented results are based on our activities over three years, including 1146 published papers and 1112 presentations at international conferences. We hope readers will understand the present status and new movement in Molecular

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Preface

Nano Dynamics and its relevant research fields. The editors thank the contributors and the Ministry of Education, Culture, Sport, Science, and Technology (MEXT), Japan for their support of the project. We would also like to thank our publishers for their constant support. Sendai, Tokyo, Nara and Sapporo August 2009

Hiroshi Fukumura Masahiro Irie Yasuhiro Iwasawa Hiroshi Masuhara Kohei Uosaki

XIX

About the Editors Hiroshi Fukumura received his M.Sc and Ph.D. degrees from Tohoku University, Japan. He studied biocompatibility of polymers in the Government Industrial Research Institute of Osaka from 1983 to 1988. He became an assistant professor at Kyoto Institute of Technology in 1988, and then moved to the Department of Applied Physics, Osaka University in 1991, where he worked on the mechanism of laser ablation and laser molecular implantation. Since 1998, he is a professor in the Department of Chemistry at Tohoku University. He received the Award of the Japanese Photochemistry Association in 2000, and the Award for Creative Work from The Chemical Society Japan in 2005. His main research interest is the physical chemistry of organic molecules including polymeric materials studied with various kinds of time-resolved techniques and scanning probe microscopes.

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980s he invented a new class of photochromic molecules – diarylethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives.

XX

About the Editors

Yasuhiro Iwasawa received his B.S., M.S. and Ph.D. degrees in chemistry from The University of Tokyo. His main research interests come under the general term ‘‘Catalytic Chemistry’’ and ‘‘Surface Chemistry’’, but more specifically, catalyst surface design, new catalytic materials, reaction mechanism, in situ characterization, oxide surfaces by SPM, time-resolved XAFS, etc. His honors include the Progress Award for Young Chemists in The Chemical Society of Japan (1979), The Japan IBM Science Award (1990), Inoue Prize for Science (1996), Catalysis Society of Japan Award (1999), The Surface Science Society of Japan Award (2000), Medal with Purple Ribbon (2003), and The Chemical Society of Japan Award (2004). The research reported by Yasuhiro Iwasawa represents a pioneering integration of modern surface science and organometallic chemistry into surface chemistry and catalysis in an atomic/ molecular scale. Iwasawa is a leader in the creation of the new filed of catalysis and surface chemistry at oxide surfaces by XAFS and SPM techniques. Hiroshi Masuhara received his B.S. and M.S. degrees from Tohoku University and Ph.D. from Osaka University. He started his research in photochemistry and was the first to use nanosecond laser spectroscopy in Japan. He studied electronic states, electron transfer, ionic photodissociation of molecular complexes, polymers, films, and powders by developing various time-resolved absorption, fluorescence, reflection, and grating spectroscopies until the mid 1990s. The Masuhara Group combined microscope with laser and created a new field on Microchemistry, which has now developed to Laser Nano Chemistry. After retiring from Osaka University he shifted to Hamano Foundation and is now extending his exploratory research on femtosecond laser crystallization and laser trapping crystallization in National Chiao Tung University in Taiwan and Nara Institute of Science and Technology. He is a foreign member of Royal Flemish Academy of Belgium for Science and the Arts and his honors include The Purple Ribbon Medal, Doctor Honoris Causa de Ecole Normale Superier de Cachan, Porter Medal, the Chemical Society of Japan Award, Osaka Science Prize, and Moet Hennessy Louis Vuitton International Prize ‘‘Science for Art’’ Excellence de Da Vinci.

About the Editors

Kohei Uosaki received his B.Eng. and M.Eng. degrees from Osaka University and his Ph.D. in Physical Chemistry from Flinders University of South Australia. He was a Research Chemist at Mitsubishi Petrochemical Co. Ltd. From 1971 to 1978 and a Research Officer at Inorganic Chemistry Laboratory, Oxford University, U.K. between 1978 and 1980 before joining Hokkaido University in 1980 as Assistant Professor in the Department of Chemistry. He was promoted to Associate Professor in 1981 and Professor in 1990. He is also a Principal Investigator of International Center for Materials Nanoarchitectonics (MANA) Satellite, National Institute for Materials Science (NIMS) since 2008. His scientific interests include photoelectrochemistry of semiconductor electrodes, surface electrochemistry of single crystalline metal electrodes, electrocatalysis, modification of solid surfaces by molecular layers, and non-linear optical spectroscopy at interfaces.

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List of Contributors for Both Volumes Hiroyuki Aoki Kyoto University Department of Polymer Chemistry Katsura, Nishikyo Kyoto 615-8510 Japan Tsuyoshi Asahi Osaka University Department of Applied Physics Suita 565-0871 Japan Vasudevanpillai Biju National Institute of Advanced Industrial Science and Technology (AIST) Health Technology Research Center Nano-bioanalysis Team 2217-14 Hayashi-cho, Takamatsu Kagawa 761-0395 Japan Masaaki Fujii Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Hideki Fujiwara Hokkaido University Research Institute for Electronic Science Kita-12, Nishi-6, Sapporo Hokkaido 060-0812 Japan Hiroshi Fukumura Tohoku University Graduate School of Science 6-3 Aramaki Aoba Sendai 980-8578 Japan Mohammad Ghoneim Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Makotoh Hasegawa Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

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List of Contributors for Both Volumes

Akitomo Hirukawa Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan

Isamu Ikegami Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Ichiro Hisaki Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan

Kohei Imura The Graduate University for Advanced Studies Institute for Molecular Science Myodaiji, Okazaki Aichi 444-8585 Japan

Noriko Nishizawa Horimoto Tohoku University Graduate School of Science 6-3 Aramaki Aoba Sendai 980-8578 Japan

Keiichi Inoue Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Yoichiroh Hosokawa Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan

Yasushi Inouye Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan

Taro Ichimura Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan Katsuyoshi Ikeda Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan

Masahiro Irie Rikkyo University Department of Chemistry Nishi-Ikebukuro 3-34-1, Toshima-ku Tokyo 171-8501 Japan Mitsuru Ishikawa National Institute of Advanced Industrial Science and Technology (AIST) Health Technology Research Center Nano-bioanalysis Team 2217-14 Hayashi-cho, Takamatsu Kagawa 761-0395 Japan


Kiyoshi Isobe Kanazawa University Graduate School of Natural Science and Technology Department of Chemistry Kakuma-machi Kanazawa 920-1192 Japan Akira Itaya Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan Syoji Ito Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Yasuhiro Iwasawa The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan Yasuo Kanematsu Osaka University Center for Advanced Science and Innovation Venture Business Laboratory JST-CREST Suita Osaka 565-0871 Japan

Olaf Karthaus Chitose Institute of Science and Technology 758-65 Bibi, Chitose Hokkaido 066-8655 Japan Masataka Kinjo Riken Hirosawa 2-1, Wako Saitama 351-0198 Japan Masahiro Kishida Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Yoichi Kobayashi Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan Seiya Kobatake Osaka City University Graduate School of Engineering Department of Applied Chemistry Sugimoto 3-3-138, Sumiyoshi-ku Osaka 558-8585 Japan Hideko Koshima Ehime University Graduate School of Science and Engineering Department of Materials Science and Biotechnology Matsuyama 790-8577 Japan

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Shigeichi Kumazaki Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Guillaume Louit Osaka University Department of Applied Physics Suita 565-0871 Japan Shinjiro Machida Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan Hiroshi Masuhara Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan and Osaka University Department of Applied Physics Suita 565-0871 Japan Sadahiro Masuo Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan

Hirohisa Matsuda Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Akikazu Matsumoto Osaka City University Graduate School of Engineering Department of Applied Chemistry 3-3-138 Sugimoto, Sumiyoshi-ku Osaka 558-8585 Japan Hideki Matsune Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Hiroshi Miyasaka Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Mikiji Miyata Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan


Kei Murakoshi Hokkaido University Graduate School of Science Department of Chemistry Sapporo Hokkaido 060-0810 Japan

Hisao Nakamura The University of Tokyo Graduate School of Engineering Department of Chemical System Engineering Tokyo 113-8656 Japan

Takakazu Nakabayashi Hokkaido University Research Institute for Electronic Science (RIES) Sapporo 001-0020 Japan

Ryosuke Nakamura Osaka University Center for Advanced Science and Innovation Venture Business Laboratory JST-CREST Suita Osaka 565-0871 Japan

Hideki Nabika Hokkaido University Graduate School of Science Department of Chemistry Sapporo Hokkaido 060-0810 Japan Yutaka Nagasawa Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Hidetaka Nakai Kanazawa University Graduate School of Natural Science and Technology Department of Chemistry Kakuma-machi Kanazawa 920-1192 Japan

Hideyuki Nakanishi Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan Shuji Nakanishi Osaka University Graduate School of Engineering Science Division of Chemistry Toyonaka Osaka 560-8531 Japan Takuro Niidome Kyushu University Department of Applied Chemistry 744 Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan

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Yasuro Niidome Kyushu University Department of Applied Chemistry 744 Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Hidenori Noguchi Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan Tomohisa Norisuye Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan

Kazu Okumura Tottori University Faculty of Engineering Department of Materials Science Koyama-cho, Minami Tottori 680-8552 Japan Hiroshi Onishi Kobe University Faculty of Science Department of Chemistry Rokko-dai, Nada, Kobe Hyogo 657-8501 Japan Lingyun Pan Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan

Nobuhiro Ohta Hokkaido University Research Institute for Electronic Science (RIES) Sapporo 001-0020 Japan

Makoto Sakai Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Hiromi Okamoto The Graduate University for Advanced Studies Institute for Molecular Science Myodaiji, Okazaki Aichi 444-8585 Japan

Daisuke Sakurai Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan


Keiji Sasaki Hokkaido University Research Institute for Electronic Science Kita-12, Nishi-6, Sapporo Hokkaido 060-0812 Japan Takehiko Sasaki The University of Tokyo Graduate School of Frontier Science Department of Chemistry Kashiwanoha, Kashiwa Chiba 277-8561 Japan Atsushi Satsuma Nagoya University Graduate School of Engineering Department of Molecular Design and Engineering Chikusa Nagoya 464-8603 Japan Takashi Shiina Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Ken-ichi Shimizu Nagoya University Graduate School of Engineering Department of Molecular Design and Engineering Chikusa Nagoya 464-8603 Japan

Takashi Sugiyama Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Mizuki Tada The University of Tokyo Graduate School of Frontier Science Department of Chemistry Kashiwanoha, Kashiwa Chiba 277-8561 Japan Atsushi Taguchi Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan Sakae Takenaka Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Naoto Tamai Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan

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Toshiaki Taniike The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan

Yohei Uemura The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan

Masahide Terazima Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Kohei Uosaki Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan

Norimitsu Tohnai Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan

Kiminori Ushida Riken Hirosawa 2-1, Wako Saitama 351-0198 Japan

Naoki Toitani Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Qui Tran-Cong-Miyata Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan

Takayuki Uwada Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan and National Chiao Tung University Institute of Molecular Science Department of Applied Chemistry 1001 Ta Hsueh Road Hsinchu 30010 Taiwan


Kazuya Watanabe Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Hiroaki Yonemura Kyushu University Department of Applied Chemistry 6-10-1 Hakozaki, Higashi-ku Fukuoka 812-8581 Japan

and

Takahiko Yoshida Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

PRESTO, JST 4-1-8 Honcho Kawaguchi Saitama Japan Ryo Yamada Osaka University Graduate School of Engineering Science Division of Materials Physics Department of Materials Engineering Science Toyonaka, Osaka Japan

XXXI

Part One Spectroscopic Methods for Nano Interfaces

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1 Raman and Fluorescence Spectroscopy Coupled with Scanning Tunneling Microscopy Noriko Nishizawa Horimoto and Hiroshi Fukumura

1.1 Introduction

Optical spectroscopy is a very powerful tool in many fields, especially in chemistry. Light irradiates molecules (input) and, after interaction with the molecules, light is collected (output). In Raman spectroscopy, light is inelastically scattered by molecules and, owing to molecular vibration excitation or de-excitation, light with higher or lower energy is observed. Raman scattering is emitted together with a vast amount of elastically scattered light known as Rayleigh scattering, which generally makes the measurement of Raman spectra difficult. Raman spectra are, however, very valuable because we can obtain directly the vibration frequencies of bonds contained in matter by using visible light. In fluorescence spectroscopy, light is absorbed by molecules and the molecules are excited to the electronically excited state, resulting in the emission of fluorescence when the molecules relax to the ground state. Fluorescence spectra and decay processes are generally sensitive to intermolecular interactions in excited states. By the use of such optical spectroscopy, chemical species can be identified and the average environment surrounding target molecules can be deduced. Conventional optical spectroscopy has the drawback that the spatial resolution is limited because of the diffraction limit of light. The resolution of a conventional microscope is about half of the observation wavelength, which is several hundred nm in the case of visible light. A variety of attempts to overcome the diffraction limit realizing better spatial resolution have been performed to date. Scanning near-field optical microscopy (SNOM) is one such methods, which has a spatial resolution of about 50 nm [1]. In aperture SNOM, light is irradiated from a tapered optical fiber tip so that the spatial resolution is limited by the aperture. Aperture-less SNOM uses a sharp tip which scatters the evanescent field containing the information in the sub-wavelength range. In both types of SNOM, the tip is placed close to or in contact with a sample, and light signals are collected while the tip scans the sample surface.

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The combination of atomic force microscopy (AFM) and Raman spectroscopy is another approach to attain high spatial resolution. AFM also employs a sharp tip close to a sample surface. When the tip is made of metal and light is irradiated onto the tip and surface, Raman scattering is largely enhanced. In this way, a spatial resolution of 15 nm is achieved [2]. However, there is demand for better spatial resolution – ultimately atomic scale resolution. If this is realized we will be able to distinguish individually molecules densely packed on a surface, or even identify a part of a single molecule. For example, it could be clarified which part of a large molecule is modified with which kind of functional group. This kind of instrument may become essential for the preparation and analysis of single molecular devices. Combining scanning tunneling microscopy (STM) with optical spectroscopy is considered to be a feasible approach. This is because, at present, STM has the highest resolution among the various types of scanning probe microscopy. STM was developed by Binnig et al. in 1982 [3], and now enables us to observe a sample surface with atomic-scale resolution. The principle of the method is based on the tunneling current between the sample and a sharp metal tip as a tunneling probe [4, 5]. Very recently, attempts to combine STM with Raman or fluorescence spectroscopy have emerged and these will be described in this chapter. Here we will focus on studies performed under ambient conditions, that is, at room temperature and in air. Therefore, inelastic electron tunneling STM spectroscopy (IETS) is omitted from this chapter. STM induced luminescence, (alternatively called photon scanning tunneling microscopy, PSTM) which observes the luminescence induced by the STM tunneling current, is also beyond the scope of this chapter.

1.2 Outline of STM Combined with Optical Spectroscopy 1.2.1 Raman Spectroscopy

STM-Raman spectroscopy utilizes the effect that Raman scattering is enhanced for a molecule in the vicinity of a metal nanostructure. This enhancement effect is generally called surface-enhanced Raman scattering (SERS). When a sharp scanning probe, such as a tunneling tip for STM, is used as a metal nanostructure to enhance Raman intensity, it is called tip-enhanced Raman scattering (TERS). The concept of STM combined with Raman spectroscopy is presented in Figure 1.1. The mechanisms of surface enhanced Raman scattering have been well discussed in the early review papers [6, 7]. Briefly, Raman enhancement is considered to occur by two mechanisms: chemical enhancement and electromagnetic enhancement. The chemical enhancement is due to resonance Raman scattering based on charge transfer between a metal nanostructure and a molecule, and takes place only when the metal nanostructure is in contact with the molecule. In STM, the tip is close to the sample molecule but not close enough to induce charge transfer, so the enhancement by the chemical effect may be negligibly small. The electromagnetic enhancement

1.2 Outline of STM Combined with Optical Spectroscopy

Figure 1.1 Concept of STM combined with Raman spectroscopy.

arises from the field enhancement in the vicinity of a metal nanostructure when it is illuminated by probe light. This is due to the excitation of localized surface plasmons in the metal nanostructure, which is a collective oscillation of the electron gas at the subsurface of the metal. The enhancement factor of Raman intensity is roughly proportional to M4, where M is the factor indicating by how much the electric field is enhanced by the nanostructure, since both the incident light intensity and the scattered light intensity are enhanced [8, 9]. The enhancement factor is especially large when the nanostructure is made of silver, gold, or other metals containing free electrons. By changing the shape, size, or material of the metal nanostructure, and the surrounding medium, the wavelength dependence of the enhancement efficiency changes due to changes in plasmon resonance. The optical response of metal nanospheres can be described precisely by using the Mie theory, which is an analytical solution of Maxwells equation. Its details and numerical calculation programs are fully and comprehensively presented by Bohren and Huffman [10]. For non-spherical particles, numerical approximation methods are generally required. The finite different time domain (FDTD) method [11], and the discrete dipole approximation (DDA) [12, 13] are commonly utilized. For a classical SERS substrate, which is a roughened silver electrode, the average enhancement factor is known to be about 106 [14]. For a selected single silver nanoparticle with a single molecule, the enhancement factor is reported to be 1014 [15]. A maximum enhancement factor of about 5 109 is reported for TERS [16]. It should be noted that the net enhancement factor by TERS is large but the observed signal increase is sometimes small because the spot having the intense electric field under a tunneling tip is very limited compared with the area illuminated by the probe light beam.

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1.2.2 Fluorescence Spectroscopy

The effect of a tunneling tip on fluorescence is rather complicated compared with that on Raman scattering. The enhancement of the electric field is induced by approaching the metal tip to a fluorescent molecule; however, the quenching of fluorescence is also caused in the vicinity of or in contact with the surface of the metal. Generally, four major processes in fluorescent molecules are known to compete with the emissive decay: energy transfer, electron transfer, intersystem crossing, and internal conversion. Energy transfer can take place when an energy acceptor comes as close as about 5 nm to a fluorescent molecule, depending on the interaction between the excited state of the molecule and the ground state of the acceptor. Here the energy acceptor can be the nanostructured metal tip which shows an absorption spectrum in the visible region. The interaction is roughly estimated from the overlap of the molecular fluorescence and the acceptor absorption spectra. When the metal tip approaches to less than 1 nm from the fluorescent molecule, strong interactions like electron transfer and electron exchange can occur, which results in the fast decay of excited states. Intersystem crossing to triplet states is also known to be accelerated by approaching heavy atoms to fluorescent singlet states. This is caused by the enhancement of spin–orbit coupling, which requires the overlap of electron orbitals. Although internal conversion is rather intrinsic to molecular electronic states, it may be affected by approaching a metal tip if the vibronic coupling in a fluorescent molecule is modified through the molecular structure distortion. Thus, there are plural processes reducing fluorescence intensity, in addition to the electromagnetic field enhancement effect. Roughly speaking, the enhancement effect extends to 10 nm or more around the tunneling tip although the quenching effects become dominant only at a few nm from the tip, which results in a donut-like enhancement pattern for fluorescence.

1.3 Theoretical Approaches

Demming et al. calculated the field enhancement factor (FEF) for a silver tip and a flat gold surface by the boundary element method [17]. They demonstrated that the peak position and the magnitude of the FEF maximum depend on the geometry of the tip. For smaller tip radii, the FEF maximum increases and shifts to longer wavelength. The wavelength dependence of the FEF for a silver ellipsoid also behaves in this manner when its aspect ratio is changed. The spatial distribution of the FEF also becomes narrower and the peak intensity becomes stronger when the tip shape becomes sharper. The same research group calculated the FEF for a small spheroid shape as a model of tips made of gold, platinum, silver, p-doped silicon, and tungsten, by solving the Laplace equation analytically [18]. They calculated the dependence of FEF on the aspect ratio of the ellipsoid and wavelength, as shown in Figure 1.2. They also calculated the FEF space distribution for a tip with a flat shape by a boundary

1.3 Theoretical Approaches

Figure 1.2 Modulus of the field enhancement factor versus the aspect ratio a ¼ b and wavelengths l for SPM tips of different materials: (a) gold, (b) platinum, (c) silver, (d) p-doped silicon, (e) tungsten. Reprinted with permission from J. Jersch, Applied Physics A, 66, 29 (1998). Copyright 1998, Springer-Verlag.

element method. The intensity showed a double peak. These FEF calculation results were consistent with interesting experimental results: a flat Au surface was topographically changed to form a double structure by the illumination of the surface and a flat-shaped tip with laser light. Mills calculated the enhancement of dynamic dipole moments for a dipole moment on a smooth silver or copper surface with a silver tip above it, which

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leads to fluorescence enhancement and TERS [19]. They used a sphere as a model of the tip and calculated the enhancement factors for various wavelengths. It was found that the enhancement factors are larger for a dipole moment oriented perpendicular to the surface than for one oriented parallel to the surface and are also larger when the tip–sample distance is small. Using the same condition and a similar method, Wu and Mills performed calculations of the enhancement factors for various wavelengths for several tip–sample distances [20]. They also calculated the enhancement factor for dipoles at different lateral distances from directly beneath the tip. They confirmed that the lateral resolution scales as (RD)1/2, with R the radius of curvature of the tip, and D its distance from the substrate, as was first suggested by Rendell et al. [21]. Klein et al. used the same model as Demming et al. [17], and calculated the FEF for an Ag tip near an Au surface [22], which is similar to the experimental conditions of Pettinger et al. [23]. They showed that Raman signals of molecules below or in the near field of the tip can be enhanced to practically measurable levels, and the FEF is larger for smaller tip–sample gaps, being localized below the tip, as shown in Figure 1.3. Downes et al. calculated the enhancement of the scattered light intensity for a tip over a substrate using finite element electromagnetic simulations [24]. A gold tip or silicon tip over a gold, mica, or silicon substrate in air or water was considered. They calculated the enhancement dependence on incident light polarization, wavelength, tip–substrate separation, Raman shift, distance beneath the tip apex, angle between the tip axis and incident radiation, tip radius, and lateral displacement from the tip apex. The enhancement exceeded 1012 or 108 for a 20 nm radius Au tip over an Au or mica substrate, respectively, for a tip–substrate separation of 1 nm. In both cases, the enhancement gradually decreased as the tip–substrate separation increased. They showed that with this enhancement, high-speed (10 ms integration time) mapping is possible considering the signal contrast against the far-field signal. The lateral resolution dependence on tip–substrate separation was also calculated, and the lateral resolution was better than 1 nm for a 10 or 20 nm radius Au tip over an Au substrate. The lateral resolution became worse as the tip–substrate separation increased.

1.4 Experimental Approaches 1.4.1 STM Combined with Raman Spectroscopy

Pettinger et al. observed a TERS spectrum of monolayer-thick brilliant cresyl blue (BCB) adsorbed on a smooth Au film surface by using a Ag tip, while no STM image of the adsorbed surface was shown [23]. The Raman intensity increased when the tip was in the tunneling position, meaning that the tip–surface distance was around 1 nm. They calculated the field enhancement factor by the method described by

1.4 Experimental Approaches 800 τ = 0.5 nm

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Figure 1.3 Field distributions along the Ag-tip surface and corresponding Ag-tip geometry. z ¼ 0 corresponds to the Au-substrate. r/R is the normalized radius from the point directly beneath the tip (R is the Rayleigh length R ¼ l/2p). Reprinted with permission from S. Klein, Electrochemistry, 71, 114 (2003). Copyright 2003, The Electrochemical Society of Japan.

Demming et al. [17], and compared it with their experimental results. The observed increase in Raman intensity by a factor of 15 within the laser focus of about 2 mm, corresponds to a maximum local enhancement at the center of the tip apex by a factor of about 12 000 under the assumption that the tip radius is 100 nm. More precisely, about 20% of the Raman intensity is considered to originate from an area with a radius of 14 nm under the tip, which is roughly equivalent to 400 molecules out of 2 106 in the total focused area.

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Pettinger et al. further reported the TERS spectra of CN and BCB on a smooth and rough Au substrate with a silver tip [25]. They observed a signal enhancement by a factor of 4 for CN, and estimated the real enhancement to be at least three orders of magnitude, assuming the laser spot size to be 2 mm, and the tip radius at the apex to be 100 nm. They also observed TERS spectra of CN ions on SERS active Au surfaces [26], and compared the experimental result of the illumination from the bottom through a thin metal film with that of direct excitation from the top at an incident angle of 60 . It was found that subtraction of the SERS spectra from the TERS spectra shows narrow bandwidth spectra, suggesting that the TERS signal arises from a rather small amount of molecules. Ren et al. reported a method to prepare a gold tip with a tip apex radius of 30 nm reproducibly [27]. They observed the TERS of a Malachite Green isothiocyanate (MGITC) monolayer on an Au(111) surface and obtained an enhancement factor of about 1.6 105, by using the relation, q ¼ ITERS/IRRS ¼ g4a2/Rfocus2, where q is the net increase in the signal, ITERS and IRRS are the signal intensities for TERS and RRS (resonance Raman scattering), respectively; g4 is the TERS enhancement (g is the field enhancement), a denotes the radius of the enhanced field, and Rfocus the radius of the laser focus. Pettinger et al. observed TERS with a sharp Au tip for MGITC dye on Au(111) with a side illumination [28]. They studied the bleaching of the dye and fitted the data by taking into account the radial varying intensity distribution of the field as a Gaussian profile instead of a Heaviside profile. For the former profile, the TERS radius is smaller by a factor of 1/2 than for the latter profile. They obtained a TERS enhancement factor of 6.25 106. The radius of the enhanced field Rfield is about 50 nm, which results in a TERS radius of 25 nm, which is smaller than the radius of the tip apex (about 30 nm). Domke et al. also studied the TERS of the MGITC dye on Au(111) in combination with the corresponding STM images of the probed surface region [29]. They estimated an enhancement factor of 106 107, where five molecules should be present in the enhanced-field region, assuming an enhanced field with a radius of 20 nm, thus they claim that single-molecule Raman spectroscopy is possible. In this work, the origin of the background signal is considered to be mainly due to the adsorbate (or an adsorbate–metal complex) which most likely emits enhanced fluorescence, whereas, as a first approximation, the contribution from the substrate and contamination are negligibly small compared with resonant Raman scattering. Wang et al. developed a method to prepare a sharp Au tip, as shown in Figure 1.4, and built an apparatus for Raman spectroscopy [30]. By using the original apparatus, they measured an STM image of a monolayer of 4, 40 -bipyridine (4BPY) on Au(111) as well as a TERS spectrum at the tip-approached condition. They found strong enhancement of the Raman spectrum compared to the tip-retracted condition. From the STM image, it can be seen that 4BPY is adsorbed flat on the surface, but the TERS signal seems to originate from 4BPY perpendicular to the surface. Picardi et al. introduced a method to fabricate a sharp Au tip for STM by electrochemical etching [31]. The efficiency of TERS for a thin BCB dye layer using the etched sharp tip was then compared with that using an Au-coated AFM tip.

1.4 Experimental Approaches

Figure 1.4 (A) Current–time curves for Au wires etched in a mixture of HCl:ethanol (1 : 1v/v) at different voltages. (B) SEM images of the etched Au tips. A: 2.1 V, B: 2.2 V, C: 2.4 V. Reprinted with permission from Xi Wang, Applied Physics Letters, 91, 101105 (2007). Copyright 2007, American Institute of Physics.

A higher enhancement factor and better reproducibility were obtained when the etched sharp tip was used. This is probably due to the intrinsically lower optical quality of the coated tips with respect to the massive metal ones as well as to the differences in tip shapes. They calculated the Raman signal enhancement factor (EF) using the equation C ¼ Itip engaged/Itip withdrawn ¼ 1 þ EF(prtip2/A), where C is the ratio of the Raman scattered intensity with the tip engaged (Itip engaged) and the tip withdrawn (Itip withdrawn), rtip is the tip radius, A is the laser focal area. They estimated the EF to be about 5 102 for coated AFM tips and 4 104 to 2 105 for etched STM tips. The same research group further studied TERS by changing the polarization of the incident laser light [32]. They measured TERS of Si(111) and BCB dye on Au(111). The experimental result with the former sample is in accord with a model reported by Ossikovski et al. [33]. For both samples, the TERS enhancement was larger when the incident light had an electric field component along the tip axis, while a nonnegligible enhancement was also observed for the field component perpendicular to the tip axis. We have observed the TERS of a single wall carbon nanotube (SWCNT) on highly oriented pyrolytic graphite (HOPG) [34]. The STM image and TERS image were obtained simultaneously, as shown in Figure 1.5. The TERS image was obtained by mapping the Raman signal intensity at the 1340 cm1 peak (D-band) of SWCNT. An aggregate of SWCNT was observed in the STM topographical image, and strong Raman signals were observed in the TERS image. Thus, we have succeeded in observing the simultaneous spectral mapping of TERS, although the position of the SWCNT in the STM image was shifted from the TERS image by several hundred nm.

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Figure 1.5 Simultaneously obtained STM (a) and TERS (b) image of SWCNT on HOPG. The TERS image was obtained for the 1340 cm1 peak (D-band) of SWCNT.

This probably arises from the different position of tunneling (STM) and of field enhancement (TERS) owing to the tip shape and the height of the observation object. 1.4.2 STM Combined With Fluorescence Spectroscopy

Anger et al. measured the fluorescence rate of a single Nile Blue molecule as a function of its distance from a gold nanoparticle using a scanning probe technology [35]. The fluorescence rate shows a maximum around the molecule–nanoparticle distance (z) of 5 nm and decreases for smaller or larger z. This is due to the competition between the increased rate of excitation due to local field enhancement and the decrease in the quantum yield due to nonradiative energy transfer to the nanoparticle. They also calculated the quantum yield, excitation rate, and fluorescence rate by the multiple multipole method (MMA) and the dipole approximation. The curve of the quantum yield and fluorescence rate was reproduced only with the MMA calculation. Kuhn et al. observed the fluorescence enhancement and fluorescence decay rate of a single terrylene molecule when a spherical gold nanoparticle was approached to the

1.5 Future Prospects

molecule, using scanning probe technology [36]. The dependence of the fluorescence intensity and fluorescence decay rate on the lateral and vertical displacement of the gold nanoparticle from the molecule was measured. The fluorescence intensity was enhanced 19 times with a simultaneous 22-fold shortening of the excited state lifetime when the gold nanoparticle was in the vicinity of the molecule. The decay rates cr (radiative decay rate), cnr (nonradiative decay rate), and c (total decay rate), were calculated neglecting the effect from the substrate following a formalism presented by Puri et al. [37], and were compared with the experimentally obtained c. At the closest molecule–nanoparticle separation achieved experimentally, the values cr ¼ 11c0 and cnr ¼ 11c0 were deduced, where c0 is the unperturbed decay rate. The enhancement was larger when using an excitation wavelength near the gold nanoparticle plasmon resonance maximum wavelength than for a wavelength at the tail of the plasmon resonance, revealing the importance of the plasmon resonance in the excitation enhancement. Nishitani et al. observed the tip-enhanced fluorescence of 8 nm thick meso-tetrakis (3,5-di-tertiarybutyl-phenyl)porphyrin (H2TBPP) films on ITO with an Ag tip [38]. They reported that the fluorescence was enhanced by the locally confined electromagnetic field in the vicinity of the tip. The enhancement factor is evaluated to be larger than 2000.


We have reviewed the present situation of research on STM tip-enhanced Raman and fluorescence spectroscopy. There have been several papers showing the enhancement of Raman spectra when tips are in the approached condition. However, STMTERS intensity mapping – scanning the tip to acquire a TERS intensity image and a STM topographical image simultaneously – seems to remain a difficult task. Very recently, Steidtner et al. have reported the TERS spectra and the TERS intensity mapping of a single BCB molecule on an Au(111) surface using a gold tip in ultrahigh vacuum [39]. Observation under ambient conditions is also expected for various systems. We propose here a method to achieve single (or even sub-) molecule spectroscopy based on an additional principle to modify conventional TERS. The method utilizes the vibrational excitation of molecules by inelastic scattering of tunneling electrons, leading to further localization of the excited area. This can be applied to both fluorescence and Raman spectroscopy. For fluorescence spectroscopy, an area including the tip is illuminated with light having photon energies smaller than the absorption edge of the target molecules. When the molecule is observed by STM and electrons tunnel through the tip–molecule gap, some of the electrons are inelastically scattered and the molecule is vibrationally excited if the energy of the electron is higher than the energy of the molecular vibration. The vibrational excitation by tunneling electrons was first demonstrated by Stipe et al. for adsorbed single molecules under a high vacuum and at ultra-low temperature [40]. The

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Figure 1.6 Concept of single molecule fluorescence observation using STM.

vibrationally excited molecule can be electronically excited by the absorption of light, resulting in the emission of fluorescence. The concept of this method is presented in Figure 1.6. Because STM has an atomic scale spatial resolution, only one molecule (or part of a molecule) among other surrounding molecules can be selectively excited. Therefore, the above mentioned method would enable us to perform single (sub-) molecule fluorescence spectroscopy, even if the molecules are densely packed. The STM tunneling current and the light intensity necessary for this method can be estimated as follows. If both the tunneling current and the light intensity are continuous with time, the probability for simultaneous injection of electron and photon will be very low. Therefore, the tunneling bias of STM and the excitation light signal should be applied as pulses in order to induce vibrational excitation and absorption of a photon simultaneously. This is shown schematically in Figure 1.7. First we consider the electronic excitation probability Pelec for a single molecule during a single laser pulse. When a molecule has the absorption coefficient eabs(dm3 mol–1 cm1), its absorption cross section sabs is given by 3.81 1021eabs cm2 molecule–1. Since the probability Pelec is proportional to the light intensity (photons s1 cm2) under the objective lens, it is given by pelec ¼

1:91 105 eabs Iph lph Tph tph S

ð1:1Þ

where Iph (W) is the average laser power, Tph (Hz) the laser repetition rate, lph (nm) the laser wavelength, tph (s) the laser pulse width, and S (cm2) the illuminated area under the objective lens. For an allowed transition in typical organic molecules, eabs is larger than 104 dm3 mol–1 cm1, and a conventional table-top ps laser can safely emit 1 mW green light (lph ¼ 500 nm) with a repetition rate of 10 MHz. Assuming that the illuminated area under the objective lens is 100 mm in diameter, we obtain Pelec ¼ 1.2 105. This means that the molecule under the microscope can be definitely excited during a short ps pulse. The next step is to estimate how long a molecule can stay in its vibrational excited state during a single electric pulse. A conventional electronic circuit cannot generate a short pulse like 10 ps, so that the use of electric pulses longer than 100 ps is more


Figure 1.7 Schematics of simultaneous incoming probability of electrons and photons for (a) continuous mode and (b) pulsed mode.

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realistic. The lifetime of vibrational excited states in organic molecules is generally in the range of 10 ps, which means that the molecule can relax immediately and be excited again during the electric pulses. The average dwelling time for the molecule in its vibrational excited states during a single electric pulse is given by Tdwell ¼

6:25 109 hel Iel tvib Tel

ð1:2Þ

where hel denotes the probability for vibrational excitation by a single electron, Iel (nA) average tunneling current, tvib (s) the lifetime of vibrational excited states, Tel (Hz) the tunneling bias repetition rate. Stipe et al. reported that the total conductance increase induced by inelastic electron tunneling for acetylene molecules is of the order of several percent [40]. Therefore, several percent of the electrons injected into the molecule are considered to induce vibrational excitation, although this is dependent on the molecule, its vibrational mode, and other surrounding conditions. By assuming hel ¼ 1%, Iel ¼ 1 nA, tvib ¼ 10 ps, and Tel ¼ 10 MHz, we obtain Tdwell ¼ 62.5 ps. If we can utilize electric pulses shorter than this time duration as well as synchronized with the light pulses, we would be able to further excite the molecule in its vibrationally excited state to the electronically excited states. Of course, the values of the above variables depend on each experimental system, and optimization of the values will be necessary. For Raman spectroscopy, the experimental set-up is similar to that for fluorescence spectroscopy, but the observation wavelength should be in the range of anti-Stokes Raman spectra. When a molecule is excited vibrationally with pulsed tunneling currents, the molecule is expected to emit anti-Stokes scattered light when the probe light pulses are synchronized. In general, Raman scattering efficiency is lower than that of fluorescence, meaning that the anti-Stokes Raman measurement may be difficult. In any case, the key is how to increase the dwelling time for a molecule in its vibrationally excited states. In summary, recent progress and future prospects in the research field of fluorescence and Raman spectroscopy combined with STM in order to achieve high spatial resolution spectroscopy have been reviewed. In the near future, single (sub-) molecule STM spectroscopy is expected to be applied to the nano-world of science and engineering.

References 1 Imura, K. and Okamoto, H. (2008) Development of novel near-field microspectroscopy and imaging of local excitations and wave functions of nanomaterials. Bull. Chem. Soc. Jpn., 81, 659–675. 2 Anderson, N., Hartschuh, A., Cronin, S. and Novotny, L. (2005) Nanoscale vibrational analysis of single-walled carbon

nanotubes. J. Am. Chem. Soc., 127, 2533–2537. 3 Binnig, G., Rohrer, H., Gerber, Ch. and Weibel, W. (1982) Surface studies by scanning tunneling microscopy. Phys. Rev. Lett., 49, 57–61. 4 Bai, C. (2000) Scanning Tunneling Microscopy and Its Applications, (Springer

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Raman spectroscopy with single-molecule sensitivity. Phys. Rev. B, 73, 1934061–193406-4. Demming, F., Jersch, J., Dickmann, K. and Geshev, P. I. (1998) Calculation of the field enhancement on laser-illuminated scanning probe tips by the boundary element method. Appl. Phys. B, 66, 593–598. Jersch, J., Demming, F., Hildenhagen, L. J. and Dickmann, K. (1998) Field enhancement of optical radiation in the nearfield of scanning probe microscope tips. Appl. Phys. A, 66, 29–34. Mills, D. L. (2002) Theory of STM-induced enhancement of dynamic dipole moments on crystal surfaces. Phys. Rev. B, 65, 125419-1–125419-11. Wu, S. and Mills, D. L. (2002) STMinduced enhancement of dynamic dipole moments on crystal surfaces: theory of the lateral resolution. Phys. Rev. B, 65, 2054201–205420-7. Rendell, R., Scalapino, D. and M€ uhlschlegel, B. (1978) Role of local plasmon modes in light emission from small-particle tunnel junctions. Phys. Rev. Lett., 41, 1746–1750. Klein, S., Geshev, P., Witting, T., Dickmann, K. and Hietschold, M. (2003) Enhanced Raman scattering in the near field of a scanning tunneling tip – an approach to single molecule Raman spectroscopy. Electrochemistry, 71, 114–116. Pettinger, B., Picardi, G., Schuster, R. and Ertl, G. (2000) Surface enhanced Raman spectroscopy: towards single molecule spectroscopy. Electrochemistry, 68, 942–949. Downes, A., Salter, D. and Elfick, A. (2006) Finite element simulations of tipenhanced Raman and fluorescence spectroscopy. J. Phys. Chem. B, 110, 6692–6698. Pettinger, B., Picardi, G., Schuster, R. and Ertl, G. (2002) Surface-enhanced and STMTip-enhanced Raman spectroscopy at metal surfaces. Single Mol., 5–6, 285–294.

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26 Pettinger, B., Picardi, G., Schuster, R. and Ertl, G. (2003) Surface-enhanced and STM tip-enhanced Raman spectroscopy of CN ions at gold surfaces. J. Electroanal. Chem., 554, 293–299. 27 Ren, B., Picardi, G. and Pettinger, B. (2004) Preparation of gold tips suitable for tipenhanced Raman spectroscopy and light emission by electrochemical etching. Rev. Sci. Instrum., 75, 837–841. 28 Pettinger, B., Ren, B., Picardi, G., Schuster, R. and Ertl, G. (2005) Tip-Enhanced Raman spectroscopy (TERS) of malachite green isothiocyanate at Au(111): bleaching behavior under the influence of high electromagnetic fields. J. Raman Spectrosc., 36, 541–550. 29 Domke, K. F., Zhang, D. and Pettinger, B. (2006) Toward Raman fingerprints of single dye molecules at atomically smooth Au(111). J. Am. Chem. Soc., 128, 14721–14727. 30 Wang, X., Liu, Z., Zhuang, M.-D., Zhang, H.-M., Wang, X., Xie, Z.-X., Wu, D.-Y., Ren, B. and Tian, Z.-Q. (2007) Tipenhanced Raman spectroscopy for investigating adsorbed species on a singlecrystal surface using electrochemically prepared Au tips. Appl. Phys. Lett., 91, 101105–101105-3. 31 Picardi, G., Nguyen, Q., Schreiber, J. and Ossikovski, R. (2007) Comparative study of atomic force mode and tunneling mode tip-enhanced Raman spectroscopy. Eur. Phys. J. Appl. Phys., 40, 197–201. 32 Picardi, G., Nguyen, Q., Ossikovski, R. and Schreiber, J. (2007) Polarization properties of oblique incidence scanning tunneling microscopy – tip-enhanced Raman

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2 Vibrational Nanospectroscopy for Biomolecules and Nanomaterials Yasushi Inouye, Atsushi Taguchi, and Taro Ichimura

2.1 Introduction

When metallic nanostructures are illuminated with white light, they give us a variety of colors by scattering the light. For example, old stained glass windows in a church show a distinguished and glorious scene because nanoparticles of noble metals, for example, gold, are present in the glass. Stained glass does not bleach, but keeps its beauty forever, as long as metallic particles exist in the glass. Why do the metallic nanoparticles provide various colors which are different from the color of the bulk metal? In fact, their coloration is strongly related to the size of the nanoparticles. Plasmon polaritons, which are coupled quanta of the collective oscillation of free electrons in a metal and photons, are generated on/around the surface of the nanoparticles [1]. As the plasmon polaritons are resonant phenomena, the frequency of the resonance is very sensitive to the dielectric constant and the size. Hence, metallic nanoparticles produce a variety of colors according to their size and composition. Features of the plasmon polaritons correlated with resonant phenomenon are enhancement and confinement of light field/photons in the nano-dimensions. In particular, photons accompanied by plasmons are localized in the vicinity of the metallic nanostructures although the wavelength of the photon is of the order of several hundred nanometers. Hence we create a nano-light-source around a metallic nanostructure by virtue of the localization of plasmon polaritons. Furthermore, the field enhancement effect provides the nano-light-source capability for achieving highly sensitive optical measurement. In 1994, we proposed that a metallic needle having a nano-tip at its apex be employed as a nano-light-source for microscopy attaining nanometric spatial resolution [2]. Later, we expanded the technique to Raman spectroscopy for molecular nano-identification, nano-analysis and nano-imaging. In this chapter, we give a brief introduction to local plasmons and microscopy using a metallic nano-needle to produce the local plasmons. Then, we describe the microscope that we built and

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show some experimental results of nano-Raman analysis and nano-imaging with the microscope.

2.2 Surface Plasmon Polaritons

Electrons in a metal move around freely as electron gas while the atomic nuclei are located at each lattice position. The free electrons are collectively oscillating due to Coulomb interaction between the electric charges. As the density of the electric charges moves back and forth in an oscillating electric field, such a phenomenon is called plasma oscillation and the quantum of the plasma oscillation is called a plasmon. In such a phenomenon, an electric field is induced owing to the collective oscillation of electric charges, thereby a magnetic field arises due to generation of the electric field. This means that an electromagnetic wave is accompanied by the plasmon. Reciprocally, plasma oscillation is induced because an electric field or light exerts a Coulomb force on free electrons. Hence, the plasma oscillation and the electromagnetic field are coupled with each other. The coupled quanta are called plasmon polaritons. Plasmons generated on a metallic surface are a longitudinal wave propagating on the surface as a transverse magnetic wave [3]. Such plasmons are called surface plasmons and can arise on a metallic thin film deposited on a glass substrate [4]. The metallic film works as a waveguide on which the plasmons propagate. Here, the wavenumber of the plasmon has to be matched with that of the photons in order for coupling to occur. According to the dispersion relation of the plasmon, as shown in Figure 2.1, the wavenumber of a light field propagating in a lower refractive index medium is not coincident with that of the plasmon except at the origin of the coordinates, which means that it is impossible to generate a plasmon by using a light field propagating in a lower refractive index medium. On the other hand, the dispersion curve of the plasmon intersects that of a light field propagating into the substrate (higher refractive index material). The wavenumber of the plasmon is

k1 =

angular frequency

ω

ωi

ω n c 1

kx = ω n2 sinθ i c ε m(ω) n 12

Ksp = ω c

wavenumber Figure 2.1 Dispersion relation of surface plasmon.

ε m(ω) + n 12

kx

2.2 Surface Plasmon Polaritons

matched with that of the light field through the substrate at the crossing point, hence the plasmon can be generated by being coupled resonantly with the light field. Considering the wavenumber of the light field through the lower refractive index medium, the component perpendicular to the surface is imaginary while the component parallel to the surface is the same as that of the plasmon. The light field accompanied by the plasmon does not propagate into the lower refractive index medium but the field is localized on the boundary. Such a light field is called an evanescent field. By tuning the incident angle of the light field propagating into the substrate properly for coincidence with both the wavenumbers of the plasmon and the light field, plasmons can be generated in the three layer system (called the Kretchmann configuration) [4]. As the resonant condition, that is, the coincidence of the wavenumbers of both the plasmons and the light field, is very sensitive to the refractive index of the medium located on the metallic thin film, this configuration is widely used for refractive index measurement [5]. The sensor, which is called a surface plasmon resonance (SPR) sensor, is applied as an immunosensor and so on. A plasmon can be generated in a metallic nanoparticle [1]. When the nanoparticle is irradiated with a light field, free electrons in the metallic nanoparticle are forced to oscillate synchronously with the electric polarity of the light field. Polarization is then induced in the nanoparticle. The polarization P is given by the product of the incident light field E and the polarizability of the metallic nanoparticle a (P ¼ aE). Assuming that the nanoparticle is a sphere and that its diameter is much smaller than the wavelength of the light field, the polarizability of the nanoparticle is given by the following equation, a¼

3 4prm fem ðwÞe1 g fem ðwÞ þ 2e1 g

ð2:1Þ

Here, rm, em(w), e1, and w are the radius, the dielectric constant of the nanoparticle, the dielectric constant of the medium surrounding the nanoparticle, and the angular frequency, respectively. Accordingly, the polarization is diverged if the denominator of Eq. (2.1) approaches zero (Re{em(w)} þ 2e1 ¼ 0). The divergence is equivalent to the resonant phenomenon which corresponds to surface plasmon resonance generated in the metallic nanoparticle. Compared with the surface plasmons generated in a thin metallic film, the plasmons are localized around the nanoparticle because they do not propagate into the space. The light field or evanescent field is accompanied by the plasmons in the same manner as the surface plasmons on a metallic film. This type of plasmon is called a localized surface plasmon (or local plasmon). As can be deduced from Eq. (2.1), the resonant frequency of the local plasmons is determined by the dielectric constants of the nanoparticle material and the medium surrounding the nanoparticle provided that the diameter of the nanoparticle is negligible compared to the wavelength. If the diameter is not negligible then Mie scattering theory gives the precise resonant frequency for the local plasmons of a metallic sphere. In this case, the size of the metallic particle is an additional parameter in the determination of the resonant frequency.

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2.3 Near-Field Optical Microscopy Using a Metallic Nano-Tip

Optical microscopy is used for observation, analysis, fabrication, and manipulation in a wide number of fields ranging from basic science to industrial applications. The light field that is focused with an objective lens on the sample plane interacts with the sample (e.g., absorption, scattering, fluorescence) at a micro or sub-micro scale. As the focusing or imaging phenomena of the light field are a result of the nature of the wave, the spatial resolution of optical microscopy is limited to about half the wavelength [6]. This is the so-called diffraction limit of a light wave. How do we overcome the limitation of spatial resolution? One answer is the use of a metallic nanoparticle or nanostructure which confines the light field around the nanostructure by generating local plasmons. The nanostructure works as a nano-light-source. In 1985, Wessel proposed such a new type of microscope which used a submicrometer-sized metal particle attached to a glass substrate, as shown in Figure 2.2 [7]. As the metal particle confines and enhances the light field by coupling with local plasmons, the light field can be interacted with the sample in a much smaller region than the wavelength and the light field can be detected with high sensitivity. Optical mapping and imaging of a sample can be achieved with superresolved power by scanning the metal particle on the sample surface. Wessel called this technique surface-enhanced optical microscopy. In 1989, Pohl and Fisher reported experimental results of such optical microscopy for the first time [8]. They formed a metallic nano-protrusion by evaporating gold film onto a polystyrene particle adsorbed on a glass substrate. When the protrusion approached the sample surface, surface plasmons generated on the gold film were coupled with local plasmons. Here the local plasmon is generated on the gold protrusion due to matching of the resonant condition with the relative refractive index of the environment configured with the metallic film, protrusion and sample. Since plasmon resonance is highly sensitive to changes in the refractive index of the sample, the optical properties of the sample surface were measured by detecting the light field

incident field

scattered field

glass substrate PZT transducer a metallic nanoparticle Figure 2.2 Optical probe of surface-enhanced microscopy proposed by J. Wessel. A metallic nanoparticle attached to a glass substrate confines and enhances the light field.

2.3 Near-Field Optical Microscopy Using a Metallic Nano-Tip

Figure 2.3 Numerical analysis of a nano-light-source generated by a metallic nano-tip. (a) Model for numerical analysis. (b) Intensity distribution of light scattered by the metallic nano-tip.

scattered from the nano-protrusion. In 1994, we proposed the use of a metallic needle with a nano-sized apex which enables us to scan the probe on the sample surface easily by combining a scanning tunneling microscope and an atomic force microscope to regulate the gap between the tip apex and the sample surface [2]. Next, we explain how to confine the light field at a metallic nano-tip by using electromagnetism. Figure 2.3a shows a model for the calculation of light field scattering at the nano-tip. The metallic nano-tip is located on the glass substrate while plane light propagates in the glass substrate towards the boundary at an incident angle of 45 , as shown in the figure. Here, we assume that the material of the tip is silver and the diameter of the tip apex is 20 nm. The finite-differential timedomain (FDTD) method was employed in the calculation. Polarization of the incident field is parallel to the incident plane, which corresponds to p-polarization. Figure 2.3b shows the intensity distribution of a scattered light field just under the metallic nanotip. As shown in the figure, the light field is confined under the nano-tip due to generation of the local plasmon polaritons. As the dimension of field confinement coincides with the size of the nano-tip, the localized field works as the nano-lightsource. Furthermore, the light field is enhanced by a factor of 100 or more, depending on the structure and size of the tip apex, due to the resonant effect of the plasmon polaritons. Assuming that the incident field is perpendicular to the incident plane, that is s-polarization, confinement and enhancement of the light field do not occur because the light field oscillating in this direction does not couple with the collective oscillation of electrons at the nano-tip. Even if the incident light field possesses p-polarization, a dielectric or sharpened glass fiber nano-tip exhibits much less confinement and enhancement than a metallic nano-tip. To summarize, a metallic nano-tip and p-polarized incident light field are requisite to nano-imaging with high sensitivity, which indicates that local plasmons play an important role in the enhancement of the electric field.

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Figure 2.4 A scanning electron microscopy image of an AFM cantilever tip covered with a thin silver film.

As a near-field probe, many types of nano-tips are proposed and employed, for example, an STM needle tip [2], AFM tips covered with metallic thin film [9], tetrahedral tips [10], laser-trapped metallic nanoparticles [11], bow-tie antennas [12], and so on. In general, metal-coated AFM cantilevers are used for near-field observation and analysis due to their simple preparation and ease of handling. Figure 2.4 shows a scanning electron microscopy image of an AFM tip covered with a thin silver film. As the film was coated by thermal evaporation in vacuum with a thickness of 20 nm, a metallic nanostructure of this size is formed at the tip apex. The evaporation rate should be set carefully to avoid bending of the lever due to additional tension. Chemical reduction is another method for the creation of a metallic nanostructure or thin film on the apex. In order to obtain extreme enhancement of the light field, the design or optimization of nano-tips should be taken into account. For example, a triangular structure having a certain shape and size shows, from electromagnetic theory, an enhancement factor of the light field intensity up to 106 under a welltuned resonant condition of local plasmon polaritons [12].

2.4 Tip-Enhanced Near-Field Raman Spectroscopy and Imaging

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution


by introducing nonlinear Raman spectroscopy into the tip-enhanced near-field technique. 2.4.1 Raman Spectroscopy

Molecules vibrate due to displacement of the relative position of the atoms since covalent bonds among the atoms work as springs. Each vibration mode has a specific frequency. When laser light hits molecules, the energy of the laser light is partly transferred to the molecules due to light scattering and the molecules begin to vibrate by acquiring the energy. The frequency of the laser light decreases (accordingly, the wavelength becomes longer) because the laser light loses the energy corresponding to the molecular vibration energy due to inelastic scattering. Such a phenomenon is called Raman scattering (more precisely, Stokes Raman scattering). Since each vibrational mode has an intrinsic frequency shift of the Raman scattering, the scattering spectrum indicates which kind of vibration modes a molecule possesses or what kind of molecules are present in a sample. Raman spectroscopy is a very powerful tool for the analysis of molecules and their dynamics, like infrared absorption spectroscopy and fluorescence spectroscopy. While transition among electronic states plays a part in fluorescence spectroscopy, vibrational transition occurs in Raman scattering and mid-infrared absorption. Accordingly, molecular vibration is directly observed for the latter. Not all molecules emit fluorescence, on the other hand Raman scattering is induced for all molecules due to the vibration of chemical bonding. Furthermore, the quenching and photobleaching phenomena that are sometimes seen in fluorescence spectroscopy do not occur in Raman and IR absorption spectroscopy. Visible lasers are available in Raman spectroscopy while a broadband light source is required for IR absorption spectroscopy. However, as Raman scattering is a two-photon process, the probability of the Raman scattering process is lower than that of fluorescence and IR absorption processes. The cross section of Raman scattering is 1030 cm2, which is much smaller than that of fluorescence (1016 cm2) and IR absorption (1020 cm2). When we detect Raman scattering at the nanoscale, the number of photons obtained is less than with the usual micro-Raman spectroscopy due to reduction in the detection area or the number of molecules. To overcome this problem, we need to devise a method for amplification of Raman scattering. 2.4.2 Near-Field Nano-Raman Microscopy

With regard to the confinement and enhancement ability of a metallic nano-tip, we have proposed near-field Raman microscopy using a metallic nano-tip [9]. The metallic nano-tip is able to enhance not only the illuminating light but also the Raman scattered light [9, 15, 16]. Figure 2.5 illustrates our nano-Raman microscope that mainly comprises an inverted microscope for illumination and collection of Raman

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Figure 2.5 A tip-enhanced near-field Raman microscope which we have developed. The microscope is based on AFM for control of the metallic nano-tip, an inverted optical microscope for illumination/collection of the light field and a polychromator for measurement of the Raman signal.

scattering from the nano-tip, an atomic force microscope (AFM) for control of the nano-tip and a dispersive spectrophotometer for detection of the Raman spectrum [17]. Laser light (wavelength: 532 nm) is focused onto the sample plane through an objective lens (NA: 1.4, magnification: 100). A focused spot is formed by using an annular mask in the pupil plane to reject any light component for which NA is less than 1.0 [18]. When put into the focused spot, the metallic nano-tip produces a nanolight-source at an apex of the tip and induces Raman scattering from molecules located under the tip. Raman scattered light is collected via the same objective and guided to the spectrophotometer after passing a notch filter to eliminate Rayleigh scattering. An AFM cantilever, the tip of which is coated with a thin silver film of thickness 40 nm, is operated in contact mode. 2.4.3 Tip-Enhanced Near-Field Raman Spectroscopy and Imaging

In this section, we will describe some experiments which we have performed using the above-mentioned nano-Raman microscope. Figure 2.6a shows the Raman spectrum of an adenine nanocrystal of height 7 nm and width 30 nm [19]. Several Raman bands are observed as the probe tip is near enough to the sample (AFM operation is made in contact mode). These bands, except the one appearing at 924 cm1, are assigned as the vibrational modes, inherent to the adenine molecule, according to the molecular orbital calculation. For examples, two major bands, one at

Intensity [a.u.]


(a) (b)

glass 600

800

1000

1200

1400

1600

1800

-1

Raman shift [cm ] Figure 2.6 Raman spectrum of an adenine nanocrystal obtained (a) with and (b) without the metallic tip. Spectrum (a) corresponds to the tip-enhanced near-field Raman spectrum while spectrum (b) shows the conventional micro-Raman spectrum.

739 cm1 and the other at 1328 cm1 are the ring breathing mode, and a combination of the CN stretching mode and CC stretching mode, respectively. The spectral peak at 924 cm1 is assigned as a Raman band of the glass substrate. This was proved from the experimental result shown in Figure 2.6b, which is the same as Figure 2.6a except that measurement was made when the tip was far from the sample. This indicates not only that the bands, except that at 924 cm1, are all due to Raman scattering by adenine molecules located near the probe tip, but also that the Raman spectrum is detected only when the probe is in the near-field of the molecules of interest, otherwise the photon field is not enhanced enough to scatter Raman-shifted photons. The laser power for illumination was 2.5 mW and the exposure time for obtaining the Raman spectra was 1 min. Comparing the intensity per unit area of the Raman band at 739 cm1 in Figure 2.6a and b, the enhancement factor afforded by the metallic nano-tip is estimated at 2700. We carried out spectral mapping of the nanocrystals in order to evaluate the spatial response of the measurement. Figure 2.7a shows the spectral mapping attained by scanning the sample at intervals of 30 nm. Near-field Raman spectra were detected for 10 s at each position using a nitrogen-cooled CCD camera. The optical response of the two major Raman bands at 739 cm1 and 1328 cm1, taken from Figure 2.7a, is shown in Figure 2.7b. The intensity of the two Raman bands exhibits a similar optical response and the minimum spatial response is 30 nm. This value corresponds to the size of the metallic nano-tip. This experimental result agrees well with the numerical analysis shown in Figure 2.3. Nonlinear optical phenomena, as well as near-field optics, provide us with super resolving capability [20]. The probability of nonlinear optical phenomena is proportional to the number of photons which participate in the phenomenon. For example, the intensity distribution of two-photon excited fluorescence corresponds to the square of the excitation light. Thus, we proposed a combination of the field

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(b)

30nm

Intensity [counts/10s]

600 739 cm-1 400

200 1328 cm-1 0 0

100

200

300

400

X axis [nm] Figure 2.7 (a) Tip-enhanced near-field Raman spectral mapping of the adenine nanocrystal at 30 nm intervals. (b) Raman intensity distribution of two major bands at 739 cm1 and 1328 cm1.

enhancement effect of a metallic nano-tip and coherent anti-Stokes Raman scattering (CARS) spectroscopy, a third-order nonlinear Raman spectroscopy [21]. With the tip enhancement of CARS, the excitation of CARS polarization can be further confined and highly enhanced at the very end of the probe tip owing to its third-order nonlinearity, providing higher spatial resolution capability. In addition, because of the nonlinear responses, even a small enhancement of the excitation field can lead to a huge enhancement of the emitted signal, allowing a reduction of the far-field background.


CARS spectroscopy utilizes three incident fields including a pump field (w1), a Stokes field (w2; w2 < w1), and a probe field ðw0 1 ¼ w1 Þ to induce a nonlinear polarization at wCARS ¼ 2w1 w2. When w1 w2 coincides with one of the molecular-vibration frequencies of a given sample, the anti-Stokes Raman signal is resonantly generated [22, 23]. We induce the CARS polarization by the tip-enhanced field at the metallic tip end of the nanometric scale. In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration: 5 ps, spectral width: 4 cm1) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)poly(dA-dT) [24]. The frequency difference of the two excitation lasers (w1 w2) was set at 1337 cm1, corresponding to the ring stretching mode of diazole. After the onresonant imaging, w2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample (off-resonant). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. A spontaneous Raman spectra is shown in Figure 2.8d in which the on- and offresonant frequencies are indicated. The DNA bundles are observed at the resonant frequency, as shown in Figure 2.8a, while they cannot be seen at the off-resonant frequency in Figure 2.8b. This indicates that the observed contrast is dominated by the vibrationally resonant CARS signals. Figure 2.8c shows a cross-section of Figure 2.8a denoted by two solid arrows, which were acquired with a 5 nm step. The FWHM of

Figure 2.8 CARS images of the DNA network structure. (a) A tip-enhanced CARS image in the on-resonant condition (w1 w2 ¼ 1337 cm1). (b) A tip-enhanced CARS image in the offresonant condition (w1 w2 ¼ 1278 cm1). (c) Line profile of the row indicated by the solid arrows in (a). (d) A spontaneous Raman spectrum of the DNA sample, in which the arrows indicate the frequencies adopted for the on- and off- resonant conditions.

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the narrowest peak is found to be 15 nm, as shown in Figure 2.8c. This spatial response is better than that of the spontaneous Raman scattering appearing in Figure 2.7b. The improvement of spatial resolution is attributed to the nonlinearity of the CARS process. The size of the effective excitation volume of the DNA structure and the enhancement factor of the CARS signal was estimated to be 1 zeptoliter 106, respectively. This huge enhancement factor is also attributed to the nonlinear effect.

2.5 Tip Effect on Near-Field Raman Scattering

Tip-enhanced near-field Raman scattering is the same phenomenon as surfaceenhanced Raman scattering (SERS) except that only one metallic nanostructure participates in tip-enhanced Raman scattering while many metallic nanostructures are associated with SERS. Two enhancement mechanisms, that is, electromagnetic enhancement and chemical enhancement, exist in TERS as in SERS. The former is related to local plasmon polaritons, as described earlier, the latter is caused by formation of metal-molecule complexes by chemical adsorption and the change in the electronic state of the molecule [25]. Specific changes in Raman spectra are often seen in SERS spectroscopy due to the first layer effect of chemical adsorption; for example, peak shift and/or a huge enhancement of specific Raman bands and the appearance of new Raman bands. We observed such phenomena in tip-enhanced near-field Raman spectroscopy. Furthermore, we found a new phenomenon in which particular Raman bands in TERS spectra of adenine nanocrystals shift to higher wavenumber than those observed in SERS [19]. In this section, we will demonstrate such phenomena appearing in TERS. Figure 2.9a shows a TERS spectrum of a single adenine nanocrystal. Several Raman bands, including two intense bands at 739 and 1328 cm1 due to the ring breathing mode of the whole molecule and the ring-stretching mode of diazole, were detected. These peaks were not observed in the far-field Raman spectra of the same sample obtained by retracting the tip from the sample. The TERS spectrum is compared with an ordinary SERS spectrum (Figure 2.9b) and an ordinary near-infrared (NIR) Raman spectrum (Figure 2.9c) both of which we measured. The peak frequency of the RBM (nRBM) is 739, 733, and 723 cm1 in the TERS, SERS and normal Raman spectra, respectively. The peak frequency of the TERS spectrum shifted to higher frequency than those of SERS and the normal Raman spectra. It is also seen that the peak frequency in the SERS spectrum shifted with respect to the normal Raman spectrum. These frequency shifts are strongly related to the chemical interaction between the silver and the molecules. However, the Raman band shift appearing in TERS is definitely different from that of SERS. Why were both spectra not coincident with each other? In TERS, a metallic nano-tip is controlled with an AFM which applies a constant force onto the sample to get the topography of the sample surface while observing the tip-enhanced near-field Raman scattering from the sample. As the nano-tip causes unidirectional pressure, the anisotropy in our techniques allows us to change the molecular bond lengths in one direction in a controlled manner, and the simultaneous


Intensity [arb. unit]

(a) Tip-enhanced Raman

(b) Ordinary SERS

(c) Ordinary NIR Raman

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800 1200 Raman shift [ cm-1]

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Figure 2.9 Spectra of a single adenine nanocrystal. (a) TERS spectrum, (b) ordinary SERS spectrum, and (c) ordinary nearinfrared (NIR) Raman spectra. For the SERS measurement, a silver island film was used. For the NIR Raman measurement, a thick sample of adenine was used with a 1 h exposure.

near-field Raman scattering measurement allows us to perform an in situ nanoanalysis. The uniaxial pressure effect shows up in interesting spectral changes, such as peak shifts, peak broadening and new peak appearance, as the AFM-controlled tip force changes. Accordingly, the difference between TERS and SERS is interpreted as a result of a mechanical pressure effect due to the dynamic contact of the metallic nano-tip and the molecules in addition to the chemical effect. The chemical and mechanical effects were analyzed by using quantum chemical calculations of the simplest adenine-silver complex model where a single silver atom represents the silver nano-tip employed in our experiment. In order to improve the accuracy of the calculation for the vibrational frequencies, we use a complex model consisting of an adenine molecule and a silver cluster (quadrimer). In the model, a silver cluster is adjacent to the nitrogen atom at N3 (Ad-N3) (Figure 2.10a and b), which was found to be the best model to give good agreement with the experimental results. The vibrational properties were calculated using the UB3LYP/6-311 þ G (for adenine)/SDD (for Ag) [26]. For analysis of the mechanical effect, the bond distance between the N3 atom and the adjacent silver atom was changed, and the vibrational frequencies were calculated for the different bond distances. The calculated frequency shifts of nRBM as well as the calculated potential curves (binding energy) are plotted as a function of the bond distance between the silver atom and the N3 nitrogen of adenine in Figure 2.10c. The calculated frequency nRBM demonstrated a significant shift towards

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(a)

νRBM in TERS νRBM in SERS

νRBM in NR

Binding energy [kcal/mol]

Raman shift [cm-1]

(c)

∞ Bond distance [Å] Figure 2.10 (a) Molecular structure and atomic numbering of adenine. (b) The calculated model of the adenine-silver quadrimer complex. (c) The calculated frequency shifts nRBM of the Ad-N3 Ag quadrimer and the calculated binding energy as a function of the bond distance for the Ag–N linkage.


higher frequency with the contraction of the bond distance. The calculated frequency agrees with the corresponding band of the experimental TERS spectra of adenine when the bond distance of the Ag–N linkage is 5% contracted from the equilibrium. In our TERS experiment, the adenine nanocrystals are pressurized by the nano-tip with a constant atomic force (ca. 1–5 pN molecule–1). When the bond distance of the Ag–N linkage is reduced by 5%, the repulsive force of 6 pN molecule–1 is derived from a harmonic oscillation of the binding energy difference. In this case, the repulsive force obtained from the calculated potential surface of the adenine silver quadrimer coincides quantitatively with the atomic force. These results support the idea that the frequency shifts occur due to the deformation of adenine molecules by the silver tip. Moreover, we have found temporal fluctuation in TERS spectra of an adenine nanocrystal when we left the silver-coated cantilever (operated in contact mode) on the surface of the nanocrystal for 600 s [27]. Figure 2.11a shows a waterfall plot of a

Figure 2.11 (a) Waterfall plot of the time evolution of Raman spectra of adenine polycrystal. (b) Five spectra taken from (a). The exposure time for each spectrum is 10 s.

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time series of tip-enhanced near-field Raman spectra of the nanocrystal. Five characteristic spectra, indicated by arrows in Figure 2.11a, are shown in Figure 2.11b for facile comparison of their spectral shapes. Most of the Raman peaks observed in Figure 2.11a and b are assigned to adenine except for one peak at 511 cm1 which is assigned to silicon, the material of the cantilever. It is obvious from Figure 2.11 that the intensities and frequencies of many peaks of adenine fluctuate temporally. This is quite a contrast to the stable peak of the silicon at 511 cm1. Moreover, several Raman peaks suddenly appeared at 250 s and disappeared at 550 s. This phenomenon can be referred to as blinking. Furthermore, the relative intensities of several Raman peaks, for example, at 800 cm1, 855 cm1, and 945 cm1 are exceptionally strong in the time range 250–550 s, although these peaks are relatively weak in the normal Raman spectrum of adenine. The observed phenomena in the tip-enhanced near-field Raman spectra, including fluctuation, blinking, and extraordinary enhancement of several peaks, are analogous to those observed in previous studies on surface-enhanced Raman spectroscopy of a single molecule. We suppose that this phenomenon was caused by temporal fluctuation of molecular adsorption on the silver tip. Then, we analyzed the vibrational modes of an adenine molecule for different polarization of the incident field by using density functional theory, as shown in Figure 2.12. The calculated result indicates that the Raman spectrum of the adenine molecule is strongly dependent on the polarization

Figure 2.12 Calculated Raman spectra of an adenine molecule with three orthogonal polarization directions (a), (b): in-plane polarization, c: out-of-plane polarization).


of the incident field denoted by E in the figure. As the tip-enhanced field has polarization parallel to the axis of the nano-tip, orientation of the molecules is supposed to change relative to the polarization. The experimental and calculated results support the fact that temporally fluctuating tip-enhanced near-field Raman spectra are affected by the molecular orientation. Hence, TERS spectroscopy enables us to determine the orientation of a molecule with high sensitivity as well as to analyze, identify and image molecules at the nanometric scale. The force effect is applicable to investigation of the mechanical properties of nanomaterials [28, 29]. We measured TERS spectra of a single wall carbon nanotube (SWCNT) bundle with a metallic tip pressing a SWCNT bundle [28]. Figure 2.13a–e show the Raman spectra of the bundle measured in situ while gradually applying a force up to 2.4 nN by the silver-coated AFM tip. Raman peaks of the radial breathing

RBM

G-band

(a) 0 nN (far field) ×4.4

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(b) 0.6 nN

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(d) 1.8 nN

(e) 2.4 nN ×1

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Raman shift [cm-1] Figure 2.13 TERS spectra of an SWCNT bundle measured with an applied tip-force up to 2.4 nN.

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mode bands and the lower-frequency Raman peak (stretching mode along the circumferential direction) of the G-band down-shifted by as much as 18 cm1 as the tip force increased while no change was observed in the higher-frequency Raman peak of the G-band (the mode along the axial direction). The peak shift was caused by radial deformation of SWCNTs in the bundle. More interestingly, the Raman intensity of the two peaks in the G-band increased with increasing force. The intensity increase is attributed to the resonant Raman effect caused by modification of the electronic band gap energies of the SWCNTs.

2.6 Conclusion

Local plasmon polaritons that are generated in the vicinity of a metallic nano-tip work as a nano-light-source for revealing the nano-world with light. As the light field is strongly enhanced as well as confined at nano-volume due to the resonance effect of local plasmon, the metallic nano-tip is of benefit to Raman spectroscopy and imaging with spatial resolution at the nanoscale. Metallic atoms of the nano-tip are able to form complexes with molecules in the near-field measurement thanks to chemical adsorption. This phenomenon changes the Raman spectrum drastically, depending on the orientation of the molecules, thus the method is suitable for determination of the molecular orientation. The nano-tip also provides perturbative force on the sample at the nano/atomic level owing to the use ofAFM for position control of the nano-tip. Hence, high-pressure Raman spectroscopy is feasible without any special equipment for high pressure. As the chemical and force phenomena are generated at the molecular or atomic scale, spatial resolution of the tip-enhanced Raman spectroscopy may reach the molecular or atomic levels. Nano-spectroscopy with the metallic nano-tip opens the way to real nano-imaging and nano-analysis of biosamples with a gentle and safe light.

References 1 Kawata, S. (2001) Near-Field Optics and Surface Plasmon Polaritons, Springer, Heidelberg. 2 Inouye, Y. and Kawata, S. (1994) Near-field scanning optical microscope using a metallic probe tip. Opt. Lett., 19, 159–161. 3 Raether, H. (1988) Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Springer-Verlag, Heidelberg. 4 Kretschmann, E. (1971) Die Bestimmung Optischer Konstanten von Metallen durch Anregung von Oberfl€achenplasmaschwingungen. Z. Phys., 241, 313–324.

5 Matsubara, K., Kawata, S. and Minami, S. (1988) Optical chemical sensor based on surface plasmon measurement. Appl. Opt., 27, 1160–1163. 6 Born, M. and Wolf, E. (1980) Principle of Optics, 6th edn, Pergamon Press, Oxford. 7 Wessel, J. (1985) Surface-enhanced optical microscopy. J. Opt. Soc. Am. B, 2, 1538–1540. 8 Fischer, U. Ch. and Pohl, D. W. (1989) Observation of single-particle plasmons by near-field optical microscopy. Phys. Rev. Lett., 62, 458–461.

References 9 Inouye, Y., Hayazawa, N., Hayashi, K., Sekkat, Z. and Kawata, S. (1999) Near-field scanning optical microscope using a metallized cantilever tip for nanospectroscopy. Proc. SPIE, 3791, 40–48. 10 Koglin, J., Fischer, U. C. and Fuchs, H. (1997) Material contrast in scanning nearfield optical microscopy at 1–10 nm resolution. Phys. Rev. B, 55, 7977–7984. 11 Sugiura, T., Okada, T., Inouye, Y., Nakamura, O. and Kawata, S. (1997) Gold-bead scanning near-field optical microscope with laser-force position control. Opt. Lett., 22, 1663–1665. 12 Kottmann, J. P., Martin, O. J. F., Smith, D. R. and Schultz, S. (2001) Non-regularly shaped plasmon resonant nanoparticle as localized light source for near-field microscopy. J. Microsc., 202, 60–65. 13 Sanchez, E. J., Novotny, L. and Xie, X. S. (1999) Near-field fluorescence microscopy based on two-photon excitation with metal tips. Phys. Rev. Lett., 82, 4014–4017. 14 Knoll, B. and Keilmann, F. (1999) Nearfield probing of vibrational absorption for chemical microscopy. Nature, 399, 134–137. 15 Stockle, R. M., Suh, Y. D., Deckert, V. and Zenobi, R. (2000) Nanoscale chemical analysis by tip-enhanced Raman spectroscopy. Chem. Phys. Lett., 318, 131–136. 16 Anderson, M. S. (2000) Locally enhanced Raman spectroscopy with an atomic force microscope. Appl. Phys. Lett., 76, 3130–3132. 17 Hayazawa, N., Inouye, Y., Sekkat, Z. and Kawata, S. (2000) Metallized tip amplification of near-field Raman scattering. Opt. Commun., 183, 333–336. 18 Hayazawa, N., Inouye, Y. and Kawata, S. (1999) Evanescent field excitation and measurement of dye fluorescence using a high N.A. objective lens in a metallic probe near-field scanning optical microscopy. J. Microsc., 194, 472–476. 19 Watanabe, H., Ishida, Y., Hayazawa, N., Inouye, Y. and Kawata, S. (2004) Tip-enhanced near-field Raman analysis

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of tip-pressurized adenine molecule. Phys. Rev. B, 69, 155418. Denk, W., Strickler, J. H. and Webb, W. W. (1990) Two-photon laser scanning fluorescence microscopy. Science, 248, 73–76. Ichimura, T., Hayazawa, N., Hashimoto, M., Inouye, Y. and Kawata, S. (2004) Tip-enhanced coherent anti-Stokes Raman scattering for vibrational nano-imaging. Phys. Rev. Lett., 92, 220801. Duncan, M. D., Reintjes, J. and Manuccia, T. J. (1982) Scanning coherent anti-Stokes Raman microscope. Opt. Lett., 7, 350–352. Zumbusch, A., Holtom G. R. and Xie, X. S. (1999) Three-dimensional vibrational imaging by coherent antiStokes Raman scattering. Phys. Rev. Lett., 82, 4142–4145. Tanaka, S., Cai, L. T., Tabata, H. and Kawai, T. (2001) Formation of twodimensional network structure of DNA molecules on Si substrate. Jpn. J. Appl. Phys., 40, L407–L409. Chang, R. K. and Furtak, T. E. (1982) Surface Enhanced Raman Scattering, Plenum Press, New York and London. Becke, A. D. (1993) Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys., 98, 5648–5652. Ichimura, T., Watanabe, H., Morita, Y., Verma, P., Kawata, S. and Inouye, Y. (2007) Temporal fluctuation of tipenhanced Raman spectra of adenine molecules. J. Phys. Chem. C, 111, 9460–9464. Verma, P., Yamada, K., Watanabe, H., Inouye, Y. and Kawata, S. (2006) Near-field Raman scattering investigation of tip effects on C60 molecules. Phys. Rev. B, 73, 045416. Yano, T., Inouye, Y. and Kawata, S. (2006) Nanoscale uniaxial pressure effect of a carbon nanotube bundle on tip-enhanced near-field Raman spectra. Nano Lett., 6, 1269–1273.

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3 Near-Field Optical Imaging of Localized Plasmon Resonances in Metal Nanoparticles Hiromi Okamoto and Kohei Imura

3.1 Introduction

Metal nanostructures show peculiar optical characteristics totally different from bulk metals. For example, metal nanoparticles show characteristic resonance peaks in extinction spectra, which are greatly affected by the nanoscale geometry differences [1, 2]. It is also known that strongly enhanced optical fields localized in a nanometric spatial scale are generated under certain conditions in the vicinity of metal nanostructures. Novel functions of metal nanostructures to enhance interaction between light and molecules can be developed based on this property. It is of fundamental importance to study the optical and spectroscopic characteristics of single metal nanostructures (nanoparticles in particular) and their origins, as a basis for nano-optics and physics and for chemical and biological applications. The optical characteristics of metal nanostructures originate primarily from the collective oscillation of conduction electrons coupled with electromagnetic fields, known as localized plasmon polariton resonance (hereafter simply called plasmon) [1, 2]. The peculiar optical characteristics of metal nanostructures arising from plasmon resonances are found in a spatial scale of the order of 100 nm or less. It is therefore essential to reveal the optical characteristics in a nanometric spatial scale in order to understand and control the physical and chemical natures of metal nanostructures. In conventional optical microscopy in the visible wavelength region, the spatial resolution is restricted by the diffraction limit of light, which is a sub-micrometer regime in so far as the optical process concerned is linear. Consequently, it is difficult or impossible to apply such conventional microscopy to characterize the optical properties of materials in nanometric spatial resolution. For the purpose of topographic measurements, the electron microscope, scanning tunneling microscope, atomic force microscope, and so forth are very powerful and can achieve atomic level spatial resolution. However, these methods are not suitable for the study of spectroscopic characteristics. Scanning near-field optical microscopy (SNOM) has been developed together with the progress of the scanning probe microscopy technique,

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as a method to meet the demand for optical measurements with high spatial resolution [3–6]. The highest spatial resolution achieved by this method is around 10 nm. This is not as high as those of other probe microscopes, but is still much higher than that of the conventional optical microscope determined by the diffraction limit of light. No vacuum environment is needed for SNOM measurements and even sample surfaces covered by liquids can be measured under appropriate conditions. Since near-field microscopy is based on optical measurements, we can combine SNOM with various advanced techniques developed in the field of laser spectroscopy, such as nonlinear and time-resolved methods, to develop new microscopic methods. The near-field optical method is thus expected to provide various unique nanoscale imaging techniques. In this chapter, we review the studies of fundamental optical and plasmon characteristics of metal nanoparticles by near-field optical imaging and spectroscopy. We will show, on gold nanorods, as typical non-isotropic metal nanoparticles, that wavefunctions of plasmon modes can be imaged by near-field microscopy. We will also show ultrafast time-resolved imaging of a gold nanorod to detect dynamic changes of plasmon waves induced by photoexcitation. For assembled spherical gold nanoparticles, we will demonstrate that strongly enhanced optical fields confined in the interstitial gaps between nanoparticles can be visualized using near-field imaging. The contribution of the enhanced fields to surface enhanced Raman scattering (SERS) will also be discussed.

3.2 Near-Field Spectroscopic Method

Two different types of near-field microscopic methods are currently used. In one, a tiny aperture in an opaque metal film, with diameter less than the wavelength of light, is used [3, 5]. When the aperture is irradiated with light from one side of the film, an optical near-field is generated in the vicinity of the aperture on the back side of the film, and this is used to locally excite the sample. For the other type, we use a localized optical field generated in the neighborhood of a metal tip (or a nanoparticle) by irradiation of light [4]. This method is called the scattering type or apertureless type, whereas the former one is called the aperture type. The spatial resolution is given approximately by the aperture diameter or the radius of curvature of the metallic tip, respectively, for the aperture type and the scattering type. Both methods have advantages and drawbacks which we do not discuss in detail in this chapter. In the authors laboratory, we use home-built aperture-type SNOM systems [7–12]. The apparatus is shown schematically in Figure 3.1. We use an apertured near-field probe made of a single-mode optical fiber. The core of the optical fiber is sharpened by chemical etching and coated with metal, and an aperture with a diameter of typically 50–100 nm is opened at the apex of the sharpened tip. High-efficiency probes with doubly tapered structure are available from JASCO Corp. The incident light on the sample is introduced from the other end of the optical fiber. We use a Xe discharge arc lamp as the light source for near-field transmission spectral measurements, as well as

3.2 Near-Field Spectroscopic Method

Figure 3.1 Schematic diagram of the near-field optical microscope system. The structure of the near-field probe tip is illustrated in the circle. (Reproduced with permission from Royal Society of Chemistry [10]).

various lasers for other purposes. The sample is set just beneath the probe tip, and the position is controlled by a piezo-driven high-resolution translation stage. The height of the probe tip from the sample surface is maintained at a few nanometers to about 10 nm by the shear-force feedback method, and scanned in the xy-plane. The samples for measurements are usually prepared on transparent substrates. The light from the probe excites the sample, and the radiation transmitted through, scattered by, or emitted from the sample is collected by a microscope objective lens beneath the substrate and conveyed to the detection system. For detection, a single-channel photodetector (such as a photodiode, avalanche photodiode, or photomultiplier tube) or a multi-channel spectral detector (such as a charge coupled device (CCD)) is used, depending on the purpose. To select the detected polarization, a polarizer is placed in front of the detector. If a CCD is adopted as a detector, it is possible to take two polarization components of spectra at the same time, by using a Wollaston prism to separate the two components into different directions. To select the polarization of the incident light on the sample, we adjust the ellipticity and polarization axis of the incident light by a quarter-wave plate and a half-wave plate prior to coupling to the optical fiber, so as to get the desired polarization at the tip of the fiber probe. For near-field imaging based on nonlinear or ultrafast spectroscopy, light pulses from a femtosecond Ti:sapphire laser (pulse width ca. 100 fs, repetition rate ca.

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80 MHz) is used as a light source. Since femtosecond laser pulses are spectrally broad, the dispersion effect arising from the fiber optics seriously broadens the pulse duration. This causes lower signal efficiency in nonlinear imaging and lower time resolution in ultrafast measurements. To avoid this effect, we place a grating pair device [13], to compensate for dispersion in the fiber, before coupling the beam into the fiber [7, 8, 10–12, 14]. The grating separation is adjusted to obtain the shortest pulse width at the tip of the probe.

3.3 Fundamental Spectroscopic Characteristics of Gold Nanoparticles

In this section, the fundamental spectroscopic characteristics of gold nanoparticles are described. The samples studied are crystalline gold nanoparticles synthesized in water solution. For spherical gold nanoparticles, commercially available colloidal solutions were used in most cases. Non-isotropic nanoparticles, such as rod-shaped particles (nanorods), were prepared by the seed-mediated growth method where a colloidal solution of small spherical nanoparticles was mixed with growth solutions containing a high concentration of surfactant molecules [15, 16]. As is well known, a colloidal water solution of spherical gold nanoparticle shows a strong extinction peak at about 530 nm [1, 2, 4]. This extinction band is attributed to the collective oscillation of conduction electrons in the nanoparticle, known as localized plasmon resonance. The extinction peak shifts toward the longer wavelength side and becomes broad when the diameter of the particle exceeds 100 nm. In non-spherical or aggregated gold nanoparticles, strong extinction peaks are, in general, found in longer wavelength region. Gold nanorods, for example, show strong extinction peaks in the longer wavelength region [1, 2, 17], in addition to the peak at about 530 nm that is also found for the spherical nanoparticles. The peak shifts towards the longer wavelength side with increasing aspect ratio (ratio of the length of the rod to its diameter) (Figure 3.2). The peak at about 530 nm and that at a longer wavelength are attributed, respectively, to plasmon modes polarized along the short axis (hereafter called transverse mode) and the long axis (longitudinal mode) [14]. In a nanorod with large aspect ratio, the longitudinal plasmon shifts to the near-infrared region, and an additional extinction peak appears near the transverse-mode plasmon peak. This new peak is attributed to a higher longitudinal mode [18–21], where the direction of the electronic oscillation is dependent on the position on the rod, as will be explained later.

3.4 Wavefunction Images of Plasmon Modes of Gold Nanorod — Near-Field Transmission Method

We usually use white light from a Xe discharge arc lamp for the measurement of near-field transmission images and spectra [9]. The spectrum of transmitted light

3.4 Wavefunction Images of Plasmon Modes of Gold Nanorod — Near-Field Transmission Method

Figure 3.2 Extinction spectra of colloidal water solutions of gold nanospheres and nanorods. Dotted curve: nanospheres (diameter 15–25 nm). Solid curve: nanorods, low aspect ratio. Dashed curve: nanorods, high aspect ratio. Extinction is normalized at about 520 nm. (Reproduced with permission from Royal Society of Chemistry [10]).

from the sample is recorded as a function of lateral position by means of a polychromator equipped with a CCD. Figure 3.3 shows near-field transmission images of a gold nanorod (diameter 20 nm, length 510 nm), and transmission spectra taken at positions 1 and 2 on the rod [10, 22]. The near-field image consists of several dark spots along the long axis, instead of the uniform shadow of the rod. The number of spots decreases one by one on increasing the wavelength of observation. In the near-field transmission spectra, several resonant extinction peaks are found [20, 21]. The number of dark spots in the image is characteristic of each resonant peak. This result suggests that the images obtained correspond to visualization of wavefunctions of longitudinal plasmon modes. Near-field observation of wavefunctions was also reported for quantum well structures of semiconductors [23] Plasmon modes of gold nanorods are shown schematically in Figure 3.4 [10, 12]. For both longitudinal and transverse modes, the direction of the collective electronic oscillation is uniform on the rod in the case of the fundamental dipolar mode. There are higher modes, in addition to the dipolar modes, where the direction of the electronic oscillation is dependent on the position on the rod. The oscillation amplitude as a function of the position gives a wavefunction of the plasmon mode [12]. The sign of the wavefunction does not alter for the dipolar mode, whereas the wavefunctions have nodes for higher plasmon modes. The oscillation frequency is approximately constant, regardless of the modes, for transverse modes. In contrast,

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Figure 3.3 Near-field transmission spectra and images of a single gold nanorod (length 510 nm, diameter 20 nm). The two transmission spectra were obtained at positions 1 and 2 indicated in the inset. Each image was obtained at the resonance peak wavelength. (Reproduced with permission from Royal Society of Chemistry [10]).

the dipolar longitudinal mode gives the lowest frequency among the longitudinal modes, and the frequency becomes higher in higher modes, finally approaching the transverse-mode frequency as a limit. The magnitude of optical extinction is given approximately by the square of the electronic oscillation amplitude, or the square modulus of the wavefunction. Consequently, the magnitude of the optical extinction is expected to oscillate along the long axis of the rod in the near-field images of higher longitudinal modes. The experimentally observed near-field transmission image in Figure 3.3 is in good accordance with this expectation. The number of dark spots increases in higher optical frequencies in Figure 3.3, which is again consistent with the scheme given in Figure 3.4. The longitudinal plasmon resonances interact with the radiation field polarized along the long axis of the nanorod, and interaction with radiation polarized along the short axis is negligible. In fact, the images shown in Figure 3.3 are not observable when the polarization is perpendicular to the long axis [14, 20]. These results strongly support that the transmission images observed can be interpreted as visualization of square moduli of wavefunctions of longitudinal plasmon modes. Theoretical simulation of plasmon-wavefunction amplitudes (or more precisely

3.5 Ultrafast Time-Resolved Near-Field Imaging of Gold Nanorods

Figure 3.4 Schematic view of plasmon modes of a metal nanorod. (Reproduced with permission from The Japan Society of Applied Physics [12]).

photon local density of states) based on Green dyadic formalism [24–26] reproduces well the observed near-field images [14, 20]. In summary, it has been demonstrated that plasmon-mode wavefunctions of gold nanoparticles resonant with the incident light can be visualized by near-field transmission imaging.

3.5 Ultrafast Time-Resolved Near-Field Imaging of Gold Nanorods

Imaging of ultrafast processes occurring in the sample is realized by a pump–probe transient absorption scheme at the near-field probe tip, using femtosecond pulses as the incident light [7, 8]. In the present study, we adopted an equal-pulse transmission correlation method to get time-resolved images, with femtosecond pulses from a mode-locked Ti:sapphire laser at a wavelength of 780 nm [14]. The pulse from the laser was split into pump and probe pulses of approximately equal intensities by a beam splitter. The beams passed through optical delay lines to adjust the time separation between the pump and probe pulses, and then were combined again collinearly. The combined beam was incident on the optical fiber probe after a dispersion compensation device described in the previous section. The total intensity of the light transmitted through the sample was detected by a single-channel

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Figure 3.5 Near-field static ((a), (b)) and transient ((c)–(e)) transmission images of a single gold nanorod (length 300 nm, diameter 30 nm). Observed wavelengths are 750 nm (a), 900 nm (b), and 780 nm ((c)–(e)). The pump–probe delay times in ((c)–(e)) are 0.60,

1.03, and 8.2 ps, respectively. The dark and bright parts in (c)–(e) indicate the bleached and induced absorption regions, respectively. The approximate position of the nanorod is indicated by the broken line in each panel.

photodetector, while the sample position was scanned laterally. The transmission intensity difference between pump-on and -off was detected using a mechanical chopper and a lock-in amplifier, to get an image of transient transmission change induced by the pump pulse. By changing the delay time between the pump and probe pulses, variation of the image was recorded which gives information on the dynamics after photoexcitation. Figure 3.5 shows time-resolved near-field transient transmission images of a gold nanorod (diameter 30 nm, length 300 nm) and a static near-field image of the same rod observed at 780 nm [14]. In the static image, the wavefunction of the plasmon resonant with the incident light is visualized (with a node in the center), as mentioned in the previous section. In the transient transmission immediately after the excitation (0.6 ps), a characteristic image with bleached and induced absorptions at the center and at both ends, respectively, is observed, for the single nanorod. The induced absorption at both ends rises up to about 1 ps, and then gradually decays to thermal equilibrium. Based on the previous ensemble measurements of ultrafast spectroscopy on gold nanoparticles, the rapid rise may be attributed to an electron–electron scattering process in the nanoparticle, whereas the slower decay may arise from electron–photon scattering. To get insight into the origin of the characteristic image observed shortly after photoexcitation, we tried a numerical simulation of the transient near-field image based on electromagnetic theory [27]. We do not describe the details of the theoretical formulation here. We assumed that the energy of the pump pulse dissipates quickly in the particle after photoexcitation, and as a result the electronic temperature of the rod rises uniformly. The temperature rise causes temporal change of dielectric function of the material (gold), and induces changes in plasmon modes as a consequence. By taking this effect into account, a simulated transient image, as shown in Figure 3.6, was obtained. The simulated image corresponds well with the observed images of transient transmission changes. We sometimes observed inverted transient transmission images with respect to that of Figure 3.5 (i.e., induced absorption in the center and bleached absorptions at both ends), depending on the

3.6 Near-Field Two-Photon Excitation Images of Gold Nanorods

Figure 3.6 Near-field transient transmission image of a single gold nanorod observed at 0.6 ps (a) and corresponding simulated image (b). (Reproduced with permission from The American Physical Society [27]).

dimension of the rod [27]. In the simulation, this result can be reproduced as arising from the difference in the resonance condition that is determined by the dimension. This result supports the reasonableness of the simulation. From these facts, it has been clarified that the characteristic transient transmission image observed reflects changes in plasmon wavefunctions accompanied by electronic temperature rise induced by the photoexcitation.

3.6 Near-Field Two-Photon Excitation Images of Gold Nanorods

Some kinds of gold nanoparticles (or assemblies) show quite strong two-photon induced photoluminescence (TPI-PL) in the region between 500 and 700 nm, when excited by femtosecond pulses at around 800 nm (i.e., Ti:sapphire laser wavelength) [28–32]. By detecting this TPI-PL, we can obtain near-field two-photon excitation probability images of the samples [28–30, 33, 34]. Figure 3.7 shows typical examples of two-photon excitation images of gold nanorods [28, 29]. In Figure 3.7a, an image corresponding to the steady-state wavefunction of the plasmon resonant with the incident light (780 nm) is observed, in a similar way to the near-field transmission images in Figure 3.3. For another rod, shown in Figure 3.7b, in contrast, no wavefunction is visible, and two-photon excitation probability is found only in the edge region of the rod. The electric fields are expected to be localized in the edges of metal rods due to the so-called lightning rod effect [35, 36]. The observed image in Figure 3.7b corresponds well with the optical-field distribution expected for this effect. The results suggest that images corresponding to the spatial distribution of optical fields are obtainable by near-field two-photon excitation imaging. As shown in Figure 3.7, some rods give images of steady-state plasmon wavefunctions, while others show localized excitation probability at the edges. Since the resonance frequency is strongly dependent on the rod dimension, the resonance condition with the incident light is different for each nanorod. The difference in the observed images illustrated in Figure 3.7 probably originates from such an effect. In the next section, we will apply this near-field two-photon excitation imaging method to investigate enhanced-field distribution in nanoparticle assemblies.

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Figure 3.7 Near-field two-photon excitation images of single gold nanorods detected by two-photon induced photoluminescence. Nanorod dimensions (length, diameter) are 540 nm, 20 nm for (a) and 565 nm, 21 nm in (b). Scale bars: 100 nm. (Reproduced with permission from The Chemical Society of Japan [11]).

3.7 Enhanced Optical Fields in Spherical Nanoparticle Assemblies and Surface Enhanced Raman Scattering

In late 1990s, Raman scattering from adsorbed species on noble metal (principally silver) nanoparticle aggregates was reported to be so enormously enhanced that even single-molecular detection might be possible [37, 38]. This discovery is important not only for analytical applications, but the mechanism of the enhancement has also attracted fundamental interest from many researchers. Many reports have been published since then on this topic. A number of studies have also been devoted to simulation of model systems to explain the enhancement based on electromagnetic theory. The general conclusion of these simulation studies is as follows. When an assembly of noble metal nanoparticles is irradiated by light, a strongly enhanced electric field is induced in the gap between the nanoparticles [39–41]. This makes the optical field applied to the molecule adsorbed in the gap very strong, and very strong Raman scattering is induced. The scattered Raman radiation is further influenced by the field enhancement effect in the gap. As a result, the Raman-scattered field is enhanced with a factor given by approximately the fourth power of the local electric field enhancement. Enhanced electric-field distribution is illustrated schematically in Figure 3.8, based on reported electromagnetic simulations, for a dimer of a noble metal spherical nanoparticle. The optical field enhancement at the gap site occurs only when the incident polarization is parallel to the interparticle axis of the dimer. According to the electromagnetic simulation, the enhancement factor of the electric field can be as high as a few thousand under some conditions, and

3.7 Enhanced Optical Fields in Spherical Nanoparticle Assemblies

Figure 3.8 Schematic view of enhanced field distribution in the vicinity of a dimer of noble metal nanospheres. (Reproduced with permission from The Japan Society of Applied Physics [12]).

consequently Raman enhancement factor can reach as high as 1011 [41]. Such a field enhancement in the gap site of a noble metal nanoparticle aggregate is called a hot spot, and is believed to be a major origin of the extremely high Raman enhancement. However, it is practically impossible to prove this mechanism by observation of optical field distribution with a conventional optical microscope, since the spatial scale of the whole aggregate is in the subwavelength region. To observe the optical field distribution in the aggregates, optical measurements on a subwavelength scale are essential. We tried to visualize the hot spot using the near-field two-photon excitation imaging, on a dimeric assembly of spherical gold nanoparticles as a model of nanoparticle aggregates [33, 34]. The dimer sample was prepared on a glass substrate by self-assembly of spherical gold nanoparticles (diameter 100 nm). Near-field twophoton excitation images of the sample were obtained with a Ti:sapphire laser (at 785 nm) as an incident light source. Figure 3.9 shows the near-field two-photon excitation images of the sample obtained with two mutually perpendicular incident polarization directions, together with the topograph image. In the topograph image, a few dimers and isolated nanoparticles are found. In the two-photon excitation images, high two-photon excitation probability is found in the vicinity of the gap sites of the dimers when the incident polarization is parallel to the interparticle axis. This spatial distribution of two-photon excitation probability is consistent with the theoretically predicted structure of the electric field for the hot spot of the spherical metal nanoparticle dimer (Figure 3.8). In contrast, enhanced two-photon excitation is scarcely observed in the neighborhood of isolated nanoparticles. From these results, it becomes evident that we can visualize nanoscale optical field distributions in nanoparticle assemblies by the near-field method, and that the hot spot predicted theoretically can be observed as a real image by this method. We also tried measurements to demonstrate that hot spots make significant contributions to surface enhanced Raman scattering [34]. For this purpose, the sample of nanoparticle assembly was doped with Raman active molecules by a spincoating method, and near-field excited Raman scattering from the sample was recorded. We adopted Rhodamine 6G dye as a Raman active material, which is

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Figure 3.9 Near-field two-photon excitation images of gold nanosphere dimers. (a) Topography. Scale bar: 500 nm. (b) and (c) Two-photon excitation images. The excitation wavelength is 780 nm. Incident polarization directions are indicated by arrows. The approximate positions of the particles are indicated by circles. (Reproduced with permission from The Japan Society of Applied Physics [12]).

sometimes used for examination of surface enhanced Raman scattering. In the nearfield Raman experiment, the Raman excitation light (785 nm, cw) was illuminated from the near-field probe, and the spectrum of the scattered photons in the far field was recorded. The result is shown in Figure 3.10. When the near-field probe for excitation is positioned at the dimer of the nanoparticle, strong Raman scattering was observed when the incident polarization was parallel to the interparticle axis. The observed Raman bands are attributed to Rhodamine molecules by comparing with previous reports. We could not observe any Raman scattering signal when the probe was positioned at an isolated nanoparticle or at the substrate, regardless of the incident polarization. This indicates that Raman scattering is enhanced only at the dimeric sites. We can take a near-field Raman excitation probability image by scanning the sample laterally while monitoring the Raman-band intensity at around 1600 cm1, as shown in Figure 3.10. These images clearly demonstrate that Raman enhancement is observed only when the incident polarization is parallel to the dimer axis, and is localized at the interparticle gap site. The result is again excellently consistent with the prediction of the hot spot model, and strongly supports the idea that electric field enhancement at hot spots makes a major contribution to the mechanism of surface enhanced Raman scattering. To summarize, we have shown here that enhanced electric-field distribution in metal nanoparticle assemblies can be visualized on the nanoscale by a near-field two-photon excitation imaging method. By combining this method and near-field Raman imaging, we have clearly demonstrated that hot spots in noble metal nanoparticle assemblies make a major contribution to surface enhanced Raman scattering.

3.8 Concluding Remarks

Figure 3.10 (a) Topography of the sample. (b), (c) Near-field excited Raman spectra at dimers 1 and 2, respectively, taken at two different incident polarizations. The peaks marked with # are unassigned. (d) Near-field two-photon excitation images of dimers 1 and 2. (e) Near-field Raman excitation images of dimers 1 and 2 obtained for

bands near 1600 cm1. Incident polarizations are indicated by arrows. White lines in (d) and (e) indicate the approximate shapes of the dimers. The average diameter of the spheres is about 100 nm. (Reproduced with permission from The American Chemical Society [34]).


In this chapter, we have provided an overview of near-field imaging and spectroscopy of noble metal nanoparticles and assemblies. We have shown that plasmon-mode wavefunctions and enhanced optical fields of nanoparticle systems can be visualized. The basic knowledge about localized electric fields induced by the plasmons may lead to new innovative research areas beyond the conventional scope of materials.

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The contribution of near-field microscopy is not limited to studies on plasmon-based nanomaterials, but may also provide valuable fundamental information on novel functions of various nanomaterials. The theoretical framework for near-field imaging, on the other hand, is not as straightforward as that for the far-field optical measurements. This is primarily because the effects of perturbation from the near-field probe on the optical characteristics of the samples are not well known. Further developments in theoretical treatments and practical and precise simulation methods for realistic near-field measurement systems are desired.

Acknowledgments

The authors are grateful to Drs T. Nagahara, J. K. Lim, N. Horimoto, M. K. Hossain, T. Shimada, and Professor M. Kitajima for their contributions to this work and fruitful discussion. The authors also thank the Equipment Development Center of IMS for their collaboration in the construction of the near-field apparatus. This work was supported by Grants-in-Aid for Scientific Research (Nos. 17655011, 18205004, 18685003) from the Japan Society for the Promotion of Science and for Scientific Research on Priority Areas (Area No. 432, No. 17034062) from the Ministry of Education, Culture, Sports, Science and Technology.

References 1 Bohren, C. F. and Huffman, D. R. (1983) Absorption and Scattering of Light by Small Particles, Wiley, New York. 2 Kreibig, U. and Vollmer, M. (1995) Optical Properties of Metal Clusters, Springer, Berlin. 3 Ohtsu, M. (ed.) (1998) Near-Field Nano/ Atom Optics and Technology, Springer, Tokyo. 4 Kawata, S. (ed.) (2001) Near-Field Optics and Surface Plasmon Polariton, Topics in Applied Physics, vol. 81, Springer, Berlin. 5 Courjon, D. (2003) Near-Field Microscopy and Near-Field Optics, Imperial College Press, London. 6 Novotny, L. and Hecht, B. (2006) Principle of Nano-Optics, Cambridge University Press, Cambridge. 7 Nagahara, T., Imura, K. and Okamoto, H. (2003) Spectral inhomogeneities and spatially resolved dynamics in porphyrin

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J-aggregate studied in the near-field. Chem. Phys. Lett., 381, 368–375. Nagahara, T., Imura, K. and Okamoto, H. (2004) Time-resolved scanning near-field optical microscopy with supercontinuum light pulses generated in microstructure fiber. Rev. Sci. Instrum., 75, 4528–4533. Imura, K., Nagahara, T. and Okamoto, H. (2004) Characteristic near-field spectra of single gold nanoparticles. Chem. Phys. Lett., 400, 500–505. Okamoto, H. and Imura, K. (2006) Nearfield imaging of optical field and plasmon wavefunctions in metal nanoparticles. J. Mater. Chem., 16, 3920–3928. Imura, K. and Okamoto, H. (2008) Development of novel near-field microspectroscopy and imaging of local excitations and wave functions of nanomaterials. Bull. Chem. Soc. Jpn., 81, 659–675.

References 12 Okamoto, H. and Imura, K. (2008) Nearfield optical imaging of nanoscale optical fields and plasmon waves. Jpn. J. Appl. Phys., 47, 6055–6062. 13 Nechay, B. A., Siegner, U., Achermann, M., Bielefeldt, H. and Keller, U. (1999) Femtosecond pump-probe near-field optical microscopy. Rev. Sci. Instrum., 70, 2758–2764. 14 Imura, K., Nagahara, T. and Okamoto, H. (2004) Imaging of surface plasmon and ultrafast dynamics in gold nanorods by near-field microscopy. J. Phys. Chem. B, 108, 16344–16347. 15 Busbee, B. D., Obare, S. O. and Murphy, C. J. (2003) An improved synthesis of highaspect-ratio gold nanorods. Adv. Mater., 15, 414–416. 16 Murphy, C. J., Sau, T. K., Gole, A. M., Orendorff, C. J., Gao, J., Gou, L., Hunyadi, S. E. and Li, T. (2005) Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications. J. Phys. Chem. B, 109, 13857–13870. 17 Link, S. and El-Sayed, M. A. (1999) Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods. J. Phys. Chem. B, 103, 8410–8426. 18 Schider, G., Krenn, J. R., Hohenau, A., Ditlbacher, H., Leitner, A., Aussenegg, F. R., Schaich, W. L., Puscasu, I., Monacelli, B. and Boreman, G. (2003) Plasmon dispersion relation of Au and Ag nanowires. Phys. Rev. B, 68, 155427 (4 pages). 19 Hohenau, A., Krenn, J. R., Schider, G., Ditlbacher, H., Leitner, A., Aussenegg, F. R. and Schaich, W. L. (2005) Optical nearfield of multipolar plasmons of rodshaped gold nanoparticles. Europhys. Lett., 69, 538–543. 20 Imura, K., Nagahara, T. and Okamoto, H. (2005) Near-field optical imaging of plasmon modes in gold nanorods. J. Chem. Phys., 122, 154701 (5 pages). 21 Lim, J. K., Imura, K., Nagahara, T., Kim, S. K. and Okamoto, H. (2005) Imaging and dispersion relations of surface plasmon

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modes in silver nanorods by near-field spectroscopy. Chem. Phys. Lett., 412, 41–45. Imura, K. and Okamoto, H. (2006) Reciprocity in scanning near-field optical microscopy: illumination and collection modes of transmission measurements. Opt. Lett., 31, 1474–1476. Matsuda, K., Saiki, T., Nomura, S., Mihara, M., Aoyagi, Y., Nair, S. and Takagahara, T. (2003) Near-field optical mapping of exciton wave functions in a GaAs quantum dot. Phys. Rev. Lett., 91, 177401 (4 pages). Girard, C. and Dereux, A. (1996) Near-field optics theories. Rep. Prog. Phys., 59, 657–699. Girard, C., Weeber, J.-C., Dereux, A., Martin, O. J. F. and Goudonnet, J.-P. (1997) Optical magnetic near-field intensities around nanometer-scale surface structures. Phys. Rev. B, 55, 16487–16497. Girard, C. (2005) Near fields in nanostructures. Rep. Prog. Phys., 68, 1883–1933. Imura, K. and Okamoto, H. (2008) Ultrafast photoinduced changes of eigenfunctions of localized plasmon modes in gold nanorods. Phys. Rev. B, 77, 041401(R) (4 pages). Imura, K., Nagahara, T. and Okamoto, H. (2004) Plasmon mode imaging of single gold nanorods. J. Am. Chem. Soc., 126, 12730–12731. Imura, K., Nagahara, T. and Okamoto, H. (2005) Near-field two-photon-induced photoluminescence from single gold nanorods and imaging of plasmon modes. J. Phys. Chem. B, 109, 13214–13220. Imura, K., Nagahara, T. and Okamoto, H. (2006) Photoluminescence from gold nanoplates induced by near-field two-photon absorption. Appl. Phys. Lett., 88, 023104 (3 pages). M€ uhlschlegel, P., Eisler, H.-J., Martin, O. J. F., Hecht, B. and Pohl, D. W. (2005) Resonant optical antennas. Science, 308, 1607–1609. Ueno, K., Juodkazis, S., Mizeikis, V., Sasaki, K. and Misawa, H. (2008) Clusters

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37 Nie, S. and Emory, S. R. (1997) Probing single molecules and single nanoparticles by surface-enhanced Raman scattering. Science, 275, 1102–1106. 38 Kneipp, K., Wang, Y., Kneipp, H., Perelman, L. T., Itzkan, I., Dasari, R. R. and Feld, M. S. (1997) Single molecule detection using surface-enhanced Raman scattering (SERS). Phys. Rev. Lett., 78, 1667–1670. 39 Kelly, K. L., Coronado, E., Zhao, L. L. and Schatz, G. C. (2003) The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment. J. Phys. Chem. B, 107, 668–677. 40 Futamata, M., Maruyama, Y. and Ishikawa, M. (2003) Local electric field and scattering cross section of Ag nanoparticles under surface plasmon resonance by finite difference time domain method. J. Phys. Chem. B, 107, 7607–7617. 41 Xu, H., Aizpurua, J., K€all, M. and Apell, P. (2000) Electromagnetic contributions to single-molecule sensitivity in surfaceenhanced Raman scattering. Phys. Rev. E, 62, 4318–4324.

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4 Structure and Dynamics of a Confined Polymer Chain Studied by Spatially and Temporally Resolved Fluorescence Techniques Hiroyuki Aoki

4.1 Introduction

A polymer molecule is a chain-like molecule consisting of thousands of monomer units. The polymer chain has numerous degrees of freedom and can take various conformations in a three-dimensional space. The variety of the chain conformation is the origin of the characteristic properties of polymeric materials such as rubber elasticity and viscoelasticity. The fabrication of polymer materials with an ordered structure on a nanometric scale has been studied intensively. In a nanostructured polymeric material, the polymer chain is confined in a local space smaller than its unperturbed dimension and the degree of freedom of the polymer chain is greatly suppressed. Consequently, the physical properties of polymer nanomaterials will be different from the bulk systems, and it is important to understand the fundamental aspects of polymer systems confined in a nanometric space [1–5]. One of the simplest systems of polymer confined in a small space is a polymer thin film. In an ultra-thin film with a thickness less than the unperturbed size of a polymer chain, the chain dimension is regulated between the interfaces with air and a substrate, and loses the degree of freedom in the height direction. Another example of a restricted polymer is a graft polymer chain, one end of which is tethered on a solid substrate. When the polymer chain is grafted at a high density on the substrate, the graft chains interact with each other. The high-density graft chain is restricted not only by the fixation of a chain end on a substrate but also by the strong inter-chain repulsion. For the investigation of polymer systems under spatial confinement, fluorescence microscopy is a powerful method providing valuable information with high sensitivity. A fluorescence microscopy technique with nanometric spatial resolution and nanosecond temporal resolution has been developed, and was used to study the structure and dynamics of polymer chains under spatial confinement: a polymer chain in an ultra-thin film and a chain grafted on a solid substrate. Studies on the conformation of the single polymer chain in a thin film and the local segmental motion of the graft polymer chain are described herein.

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4.2 Conformation of a Confined Polymer Chain 4.2.1 Polymer Ultra-Thin Film

The conformation of polymer chains in an ultra-thin film has been an attractive subject in the field of polymer physics. The chain conformation has been extensively discussed theoretically and experimentally [6–11]; however, the experimental technique to study an ultra-thin film is limited because it is difficult to obtain a signal from a specimen due to the low sample volume. The conformation of polymer chains in an ultra-thin film has been examined by small angle neutron scattering (SANS), and contradictory results have been reported. With decreasing film thickness, the radius of gyration, Rg, parallel to the film plane increases when the thickness is less than the unperturbed chain dimension in the bulk state [12–14]. On the other hand, Jones et al. reported that a polystyrene chain in an ultra-thin film takes a Gaussian conformation with a similar in-plane Rg to that in the bulk state [15, 16]. The real-space imaging of a single chain provides direct information on the chain conformation in an ultra-thin film. Fluorescence microscopy would be a powerful tool to examine the conformation of a single polymer chain because it can distinguish the contour of a single chain embedded in a bulk medium. However, the spatial resolution of the conventional fluorescence microscopy is limited to a half of the wavelength due to the diffraction limit of light. Fluorescence microscopy has been used to observe a huge bio-macromolecule such as DNA, while conventional optical microscopy is not applicable to the imaging of a synthetic polymer chain with chain dimensions of the order of 10–100 nm. In 1990s, scanning near-field optical microscopy (SNOM) was developed as an optical microscopy to overcome the diffraction limit [17, 18]. SNOM allows fluorescence detection with nanometric spatial resolution; therefore, it is a powerful tool for the real-space imaging of a single polymer chain. In the next section, the conformation of a single polymer chain in a thin film less than the unperturbed dimension, is studied by fluorescence imaging with near-field optical microscopy [19, 20]. 4.2.2 Near-Field Optical Microscopy

Scanning near-field optical microscopy is a novel imaging technique to achieve high spatial resolution free from the diffraction limit. When the light is incident on an object smaller than its wavelength, there arises not only scattering but also a nonpropagating electric field. The non-propagating light is called the optical near-field, and it is restricted to the vicinity of the object. SNOM illuminates the sample with the optical near-field localized in a nanometric area. Figure 4.1a illustrates an experimental set-up of SNOM, which uses a probe tip with an aperture much smaller than the wavelength of light. The SEM image of the sub-wavelength aperture is shown on the in Figure 4.1c. The typical size of the aperture is less than 100 nm. When light is

4.2 Conformation of a Confined Polymer Chain

Figure 4.1 Block diagram of a SNOM apparatus (a), SEM images of a cantilever-type SNOM probe (b) and the aperture at the tip end (c).

delivered to the backside of the aperture at the probe end, the optical near-field generates around the aperture. The near-field is confined to an area as large as the aperture size; therefore, SNOM allows us to illuminate an area much smaller than the wavelength by approaching the specimen with the tip end. The scattering or fluorescence from the sample is collected by an objective lens and detected by a photodetector. The optical signal is recorded as a function of the position of the scanning probe to construct a microscope image. SNOM provides not only the signal intensity, but also spectroscopic and time-resolved information from a nanometric area. Figure 4.2 shows the fluorescence SNOM image of a single Rhodamine 6G (Rh6G) molecule. The molecular size of Rh6G is much smaller than the SNOM aperture; therefore, the fluorescence image of a single Rh6G molecule corresponds to a point spread function of the apparatus. The single Rh6G molecule, which had a diameter of 300 nm when observed by conventional confocal microscopy, was observed as a circular bright spot with a diameter of 75 nm. This indicates the high spatial resolution of SNOM, beyond the diffraction limit of light. The unperturbed dimension of a single polymer chain with a molecular weight of 106 is of the order

Figure 4.2 Fluorescence SNOM image of a single Rhodamine 6G molecule. Panel (b) indicates the cross-section profile for the dashed line in panel (a).

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of 100 nm; therefore, SNOM is applicable to the observation of the conformation of individual polymer chains. 4.2.3 Structure of a Single Polymer Chain

The conformation of poly(methyl methacrylate) (PMMA) was investigated. For the SNOM measurement, PMMA was labeled with a fluorescent moiety of perylene. The perylene-labeled PMMA (PMMA-Pe) was prepared by the copolymerization of methyl methacrylate and 3-perylenylmethyl methacrylate [21]. The number-average molecular weight was 4.16 106 with a relatively narrow molecular weight distribution, and the fraction of perylene was less than 0.8% to avoid the influence of the introduction of the dye moiety on the properties of PMMA. Ultra-thin films with a thickness of 1–100 nm were prepared on glass plates by spin-coating and Langmuir– Blodgett methods from a mixed benzene solution of PMMA and PMMA-Pe, where the amount of PMMA-Pe was less than 0.01% of the unlabeled PMMA. Figure 4.3 shows the fluorescence SNOM images of single PMMA-Pe chains embedded in the unlabeled PMMA matrix. In the SNOM image, each PMMA-Pe chain was observed as an isolated fluorescent spot. The molecular weight of each chain can be estimated from the integrated fluorescence intensity [19], and Figure 4.3

Figure 4.3 SNOM images of the single PMMA-Pe chains in an ultra-thin film. The scanned area is 1.4 1.4 mm2 for each image. The molecular weight (M) and the value of Rxy for each chain were evaluated as follows: (a) M ¼ 4.2 106 and Rxy ¼ 122 nm; (b) M ¼ 4.2 106 and Rxy ¼ 160 nm; (c) M ¼ 4.0 106 and Rxy ¼ 180 nm; (d) M ¼ 3.6 106 and Rxy ¼ 256 nm. Reproduced with permission from The Society of Polymer Science, Japan.

4.2 Conformation of a Confined Polymer Chain

shows four PMMA chains with a molecular weight of about 4 106. The four chains have a similar chain length, but different chain conformations. The PMMA chain shown in Figure 4.3a has a shrunken shape and that in Figure 4.3d adopts a stretched conformation. The broad distribution of the chain conformation indicates the flexibility of the PMMA chain. For the statistical analysis, the chain dimension, Rxy, was evaluated for each chain according to the following equations. X Ii ðri rcm Þ2 i X R2xy ¼ ð4:1Þ Ii i

rcm

X I i ri i ¼ X Ii

ð4:2Þ

i

where ri and Ii are the position vector of the ith pixel in the SNOM image and the fluorescence intensity therein, respectively. Rxy corresponds to the radius of gyration for the projection of the polymer chain in the film plane. Figure 4.4 shows the histogram of Rxy, which corresponds to the probability distribution function of the chain dimension. Information on the distribution was not available from the previous experiments in inverse space. The average radius of gyration, hRxyi, was 138, 145, and 143 nm for the PMMA chains in thin films with thickness 15, 50, and 80 nm, respectively. The thickness of 15–80 nm is relatively 15 nm

60 40

Number of Chains

20 0 50 nm

60 40 20 0

80 nm

60 40 20 00

100

200

300

Rxy / nm Figure 4.4 Histogram of the lateral chain dimension for the PMMA-Pe chains in ultra-thin films with thickness 15, 50, and 80 nm. The PMMA chains with a molecular weight of 4 106 were selected in the SNOM images and analyzed to construct the histogram. Reproduced with permission from The Society of Polymer Science, Japan.

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small compared to the unperturbed chain size of 100 nm in the bulk state [22]. This thickness range corresponds to an intermediate region between the two- and three-dimensional systems. Then we evaluated also the conformation in the model systems of two- and three-dimensional limits, which are a Langmuir–Blodgett monolayer and a 1-mm-thick film, respectively. The values of hRxyi were 139 nm for the monolayer and 138 nm for the thick film. This indicates that the chain dimension in the lateral direction is not dependent on the film thickness in the range from a monolayer to the bulk. Although a polymer chain is spatially restricted in the height direction, the radius of gyration normal to the restriction remains a similar dimension to that in the unperturbed state. This result revealed by the direct observation with SNOM is consistent with the SANS result reported by Jones et al. [15, 16]. In a three-dimensional bulk state, the polymer chain adopts a random coil conformation. Considering the molecular size of the single chain (larger than 100 nm for a chain with a molecular weight of 4 106), there is much free space to allow the intrusion of the neighboring chains, as shown schematically by the solid curve in Figure 4.5a. In the bulk system, such free space is filled with the surrounding chains (the gray curves in Figure 4.5a), resulting in entanglement of the polymer chains. In an ultra-thin film, the SNOM revealed that the radius of gyration in the film plane is similar to that in the bulk state, whereas the chain dimension in the height direction is regulated by the film thickness. This indicates the reduction of the pervaded volume of a single chain, as shown in Figure 4.5b. Consequently, the surrounding chains are squeezed out and the entanglement among the chains is reduced in the thin film. The characteristic behavior of a two-dimensional polymer chain was predicted by the scaling theory [6]. The characteristic properties of the polymer ultra-thin film, different from those of the bulk state, have been attributed to lowered entanglement in a thin film state [23, 24]. SNOM provides direct information on the relationship between the structure at the single chain level and the macroscopic properties of polymer materials. (a)

Rxy (b)

Rxy Figure 4.5 Schematic drawing of the chain conformations in the bulk (a) and ultra-thin film (b). To clarify the contour of the single chain, one chain is indicated as the solid curve. Reproduced with permission from The Society of Polymer Science, Japan.

4.3 Dynamics of a Confined Polymer Chain

4.3 Dynamics of a Confined Polymer Chain 4.3.1 Polymer Brush

Grafting of polymer chains onto a solid substrate is a common method to control the surface properties of materials [25–27]. At a low density of the graft chain on the substrate, the polymer chain adopts a mushroom conformation resembling the coiled conformation of a linear chain in solution. When the graft density increases and the average distance between the fixed ends of the graft chains is smaller than the unperturbed chain dimension, the polymer chains overlap. Due to the repulsive interaction among the graft polymer chains, the chain conformation is altered from a mushroom conformation. The graft chain with a high density is stretched away from the substrate due to the repulsion among the chains, which is called the polymer brush.Recent development oftheliving polymerizationtechniqueallows ustoincrease the graft density up to 0.7 chains nm2 [28, 29]. In such a high-density polymer brush, the structure and dynamics of the polymer chain are greatly constrained due to strong interaction among the brush chains. The high-density polymer brush shows unique properties, which are different from the equivalent polymer film; for example, a high glass transition temperature and a low plate compressibility [30, 31]. The dynamics of the polymer brush has been extensively studied theoretically and numerically to elucidate the characteristic properties of polymer brushes [32–37]. The collective motion of polymer brushes in the time range of micro- to milliseconds has been extensively studied by the dynamic light scattering technique [38–40]; however, there are few studies especially on the chain dynamics of the polymer brush in the time range of nanoseconds because of the experimental difficulty. The time-resolved fluorescence depolarization method provides us with direct information on the dynamics of molecules with high sensitivity and high sub-nanosecond temporal resolution. In the following sections, the dynamics of a high-density polymer brush, studied by the fluorescence depolarization technique, is discussed [41]. 4.3.2 Fluorescence Depolarization Method

The fluorescence depolarization technique excites a fluorescent dye by linearly polarized light and measures the polarization anisotropy of the fluorescence emission. The fluorescence anisotropy, r, is defined as r¼

Ijj I? Ijj 2I?

ð4:3Þ

where I|| and I? are the polarization components of the fluorescence in the directions parallel and perpendicular to the excitation polarization, respectively. The fluorescence anisotropy is dependent on the difference of the angle between the electronic transition moments for excitation and emission. When the transition moments for

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excitation and emission are the same, the fluorescence anisotropy corresponds to the angular variation within the fluorescence lifetime, that is, the rotational motion of the molecule in the time range of nanoseconds. The following equation expresses the relationship between the fluorescence anisotropy, r(t), and the transition moment, M(t), at time t after the instantaneous excitation of a fluorescent dye. rðtÞ ¼ r0

3hfMð0Þ MðtÞg2 i1 2

ð4:4Þ

where r0 is the anisotropy at t ¼ 0. This indicates that the fluorescence anisotropy is equivalent to the second-order orientational auto-correlation function of the transition dipole; therefore, the time-resolved fluorescence depolarization method is able to directly probe the rotational relaxation of the molecule. Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect I|| and I? by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of I|| and I? simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function, t t þ ð1xÞexp ð4:5Þ rmodel ðtÞ ¼ r0 x exp t1 t2 where 0 x < 1. The mobility of the chain segment was discussed in terms of the correlation time for the anisotropy relaxation, tc, which is defined as Ð¥ tc ¼ r01 0 rðtÞdt ð4:6Þ ¼ xt1 þ ð1xÞt2

Figure 4.6 Block diagram of the apparatus for the fluorescence depolarization measurement. The dashed and solid arrows indicate the light paths of the excitation pulse and the fluorescence from the sample. OBJ: microscope objective, M: mirror, L: lens, DM: dichroic mirror, LP: long-pass filter, PH: pin-hole, PBS: polarizing beam splitter, P: polarizer, PMT: photomultiplier.


Figure 4.7 Fluorescence anisotropy decay curves for the PMMA brush swollen in benzene (filled circles) and the free PMMA chain in benzene solution at concentrations of 0.33 (triangles) and 2.9 103 g L–1 (open circles). The graft density of the brush is 0.46 chains nm2. The solid curve indicates the instrument response function. Reproduced with permission from the American Chemical Society.

4.3.3 Dynamics of a Polymer Brush

The dynamics of a perylene-labeled PMMA brush which was prepared by surfaceinitiated atom transfer radical polymerization is discussed here [42]. The molecular weight of the sample polymer was 1.0 105. Figure 4.7 shows the fluorescence anisotropy decay for the linear PMMA chain in benzene solution and the PMMA brush with a graft density of 0.46 chains nm2 swollen in benzene. The linear chain solution showed the concentration dependence of the chain dynamics. The solution at a concentration of 0.33 g L–1 showed the decay of fluorescence anisotropy with a correlation time of 9.9 ns slower than that in the dilute solution (tc ¼ 2.3 ns). The motion of the polymer chains is suppressed by the inter-chain interaction at the concentration of 0.33 g L–1. This concentration corresponds to the average polymer concentration in the PMMA brush layer with a graft density of 0.46 chains nm2. Even at the same polymer concentration, the brush chain showed a large correlation time of 31 ns compared to the corresponding solution of the linear PMMA. Such low mobility of the brush chain can be attributed to the effect of fixation of the chain end onto the solid substrate. For the linear polymer chain in solution, both chain ends show high mobility compared with the center segment in the main chain. One end of the polymer brush is chemically bound to the substrate. Therefore, the segmental motion is greatly suppressed at the fixed end, resulting in a large correlation time compared to the free chain in a solution. Figure 4.8 shows the fluorescence anisotropy decay curves for PMMA brushes with various graft densities swollen in benzene and acetonitrile. Benzene and acetonitrile are good and Q solvents for PMMA. As clearly shown in this figure,

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Benzene

Acetonitrile

Figure 4.8 Fluorescence anisotropy decay curves for the PMMA brush swollen in benzene (a) and acetonitrile (b). The filled circles, triangles, and open circles indicate the data for the brush sample with graft densities of 0.46, 0.37, and 0.11 chains nm2, respectively. The solid curves indicate the instrument response function. Reproduced with permission from the American Chemical Society.

the chain mobility of the PMMA brush chain decreases with increasing graft density. A linear PMMA chain with a molecular weight of 105 shows an unperturbed dimension of 7 nm in a Q solvent acetonitrile [43]. Therefore, at a density higher than 0.02 chains nm2, the distance between the brush chain is smaller than the chain dimension in solution. The steric interaction among the brush chains suppresses the chain motion. The higher the graft density, the stronger the interchain interaction, resulting in low segmental mobility of the high-density brush. Next, the solvent dependence of the dynamics of the PMMA brush is discussed. Figure 4.9 summarizes the correlation time of the fluorescence anisotropy decay for the PMMA brushes of 0.11–0.46 chains nm2 swollen in poor and good solvents. The solvent dependence of the PMMA brush at the graft density of 0.11 chains nm2 was different from that at 0.46 chains nm2. Although the mobility of the low-density brush increases increasing solvent quality, the dynamics of the high-density brush is not dependent on the solvent. The solvent dependence of the low-density PMMA brush is similar to that of a linear chain in dilute solutions. The local chain motion of a linear PMMA in solutions depends on the solvent quality [44]. A polymer chain adoptss a relatively shrunken conformation in a poor solvent; therefore, the chain motion of the polymer chain is suppressed by an intra-chain interaction among the chain segments. In a good solvent, the polymer chain adopts an expanded conformation, resulting in high chain mobility due to little intra-chain interaction. Thus the chain dynamics of the linear chain in solution depends on the solvent quality. The similar solvent dependence of the linear chain solution and the low-density brush indicates that the nanosecond dynamics of the PMMA brush is affected by the chain


τc / ns

102

101

100 0

0.1

0.2

0.3

0.4

0.5

σ / chains nm–2 Figure 4.9 Correlation time of the fluorescence anisotropy decay for the PMMA brush. The open and closed circles indicate the correlation times for the brush in acetonitrile and benzene, respectively.

conformation dependent on the solvent quality. On the other hand, the high-density PMMA brush did not show significant solvent dependence. This implies that the conformation of the brush chain with a high graft density is not dependent on the solvent quality. The chain dimension in the height direction was evaluated as the thickness of the brush layer, L, relative to the chain contour length, L0, by atomic force microscopy (AFM). Figure 4.10 shows the solvent dependence of the conformation of the PMMA brush. Whereas the brush chain changes its conformation in response to the solvent quality at the low graft density, the high-density PMMA brush does not show

L / L0

100

10–1

10–2 0

0.1

0.2

0.3

0.4

0.5

0.6

σ / chains nm–2 Figure 4.10 Chain dimension of the PMMA brush in the normal direction to the substrate. L and L0 are the thickness of the brush layer and the contour length of the brush chain. The open and filled circles indicate the chain dimension in acetonitrile and benzene.

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(a) Low density

(b) High density

Figure 4.11 Schematic drawing of the PMMA brush chain swollen in solvents. (a) brush with low graft density, (b) brush with high graft density. The left- and right-hand sides of each image illustrate the brush chains in poor and good solvents, respectively. Reproduced with permission from the American Chemical Society.

significant dependence of the chain dimension on the solvent. For the PMMA brush with a low graft density of 0.11 chain nm2, the chain conformation in the dimension normal to the substrate is greatly altered by the swelling solvent quality: the chain dimension in benzene is more than five times larger than that in acetonitrile. Figure 4.11a shows a schematic drawing of the chain conformation of the PMMA brush with a low graft density. In the case of the low-density brush, the interaction among the brush chains is relatively weak and the brush chain can easily change its conformation, resulting in a sensitive response of the chain conformation to the solvent quality. On the other hand, it should be noted that the difference between the chain dimensions in both solvents is only 36% for the high-density PMMA brush. The chain dimension normal to the substrate is 161 nm in acetonitrile whereas the unperturbed dimension is estimated to be about 7 nm. Because of the strong repulsive inter-chain interaction, the brush chain is extremely stretched, even in the poor solvent. The conformation of the high-density brush chain is affected mainly by the strong inter-chain interaction rather than by the solvent quality. Thus, the chain conformation of the high-density PMMA brush does not show significant solvent dependence. The brush chain conformation in poor and good solvents is illustrated schematically in Figure 4.11b. As mentioned above, the segmental mobility of the polymer chain is dependent on the chain expansion in a solvent. Due to the similar chain conformations in the poor and good solvents, the micro-Brownian motion of the high-density PMMA brush chain is not affected by the solvent quality, whereas the low-density brush chain changes its mobility depending on the solvent. Next we discuss the effect of the structure of the brush layer on the chain dynamics. The spatially resolved fluorescence depolarization measurement was performed using a high NA objective. Figure 4.12a shows the fluorescence image of the perylene-labeled PMMA brush. The bright area on the left-hand side indicates the PMMA brush layer, and the dark region on the right-hand side corresponds to the quartz substrate exposed by scratching the brush layer. The fluorescence anisotropy decay was observed in the polymer brush layer and at the scratched edge indicated by the filled and open circles in Figure 4.12a, respectively. The correlation time for the anisotropy decay inside the brush layer was 68 ns. On the other hand, the fluorescence anisotropy at the scratched edge showed the decay with a correlation time of 23 ns, indicating that the chain mobility of the PMMA brush increases at the

4.4 Summary

Figure 4.12 Fluorescence image of PMMA brush layer (a) and schematic drawing of the brush chain (b). The dark region (a) corresponds to the substrate surface exposed by scratching off the brush layer. The filled and open circles indicate the points where the fluorescence anisotropy decay was acquired.

scratched edge. This spatial heterogeneity of the chain dynamics results from the conformation of the brush chain dependent on the position in the brush layer. Figure 4.12b illustrates the conformation of brush chains near a scratched region. As mentioned above, a brush chain with a high graft density is surrounded by the neighboring chains and shows slow dynamics due to the strong steric inter-chain interaction. As shown in Figure 4.12b, a brush chain at the scratched edge is under less constraint from the neighboring chains because there is no brush chain on the right-hand side of the figure. Consequently, the degree of freedom of the brush chain at the edge increases, resulting in a relatively expanded conformation. The high segmental mobility at the scratched edge region is attributed to the relaxation of the constraint of the brush chain by the structural inhomogeneity of the brush layer.

4.4 Summary

The structure and dynamics of polymer systems confined in a small space was studied by the spatially and temporally resolved fluorescence methods, scanning near-field optical microscopy and the fluorescence depolarization technique. The conformation of PMMA was studied by optical imaging with nanometric resolution by near-field microscopy. The direct observation of the single chain of PMMA revealed the chain conformation characteristics of a thin film with a thickness less than the unperturbed size of the polymer chain. In an ultra-thin film, the chain dimension in the normal direction to the film plane is similar to that in a threedimensional bulk state. The dynamics of the polymer brush was investigated by scanning optical microscopy combined with the time-resolved fluorescence depolarization technique. The PMMA brush with a high graft density adopts an extremely stretched conformation in the normal direction to the substrate and shows low segmental mobility due to the strong interaction among the brush chains. The fluorescence technique with nanosecond temporal resolution and nanometric spatial resolution would provide indispensable information from a specimen and the further development of novel fluorescence techniques would reveal more insight into polymer systems on the molecular scale.

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Acknowledgments

The author appreciates support by a Grant-in-Aid for Scientific Research in Priority Area Molecular Nano Dynamics from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS), Japan. The Innovative Techno-Hub for Integrated Medical Bio-imaging Project of the Special Coordination Funds for Promoting Science and Technology from MEXT is also acknowledged.

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9 Muller, M. (2002) Chain conformations and correlations in thin polymer films: a Monte Carlo study. J. Chem. Phys., 116, 9930–9938. 10 Reiter, J., Zifferer, G. and Olaj, O. F. (1989) Monte-Carlo studies of polymer-chain dimensions in the melt in 2 dimensions. Macromolecules, 22, 3120–3124. 11 Yethiraj, A. (2003) Computer simulation study of two-dimensional polymer solutions. Macromolecules, 36, 5854–5862. 12 Kraus, J., Muller-Buschbaum, P., Kuhlmann, T., Schubert, D. W. and Stamm, M. (2000) Confinement effects on the chain conformation in thin polymer films. Europhys. Lett., 49, 210–216. 13 Brulet, A., Boue, F., Menelle, A. and Cotton, J. P. (2000) Conformation of polystyrene chain in ultrathin films obtained by spin coating. Macromolecules, 33, 997–1001. 14 Shuto, K., Oishi, Y., Kajiyama, T. and Han, C. C. (1993) Aggregation structure of a twodimensional ultrathin polystyrene film prepared by the water casting method. Macromolecules, 26, 6589–6594. 15 Jones, R. L., Kumar, S. K., Ho, D. L., Briber, R. M. and Russell, T. P. (1999) Chain conformation in ultrathin polymer films. Nature, 400, 146–149. 16 Jones, R. L., Kumar, S. K., Ho, D. L., Briber, R. M. and Russell, T. P. (2001) Chain conformation in ultrathin polymer films using small-angle neutron scattering. Macromolecules, 34, 559–567.

References 17 Ohtsu, M. (ed.) (1998) Near-Field Nano/ Atom Optics and Technology, Springer, Tokyo. 18 Paesler, M. A. and Moyer, P. J. (1996) NearField Optics: Theory, Instrumentation, and Applications, John Wiley & Sons, New York. 19 Aoki, H., Anryu, M. and Ito, S. (2005) Twodimensional polymers investigated by scanning near-field optical microscopy: conformation of single polymer chain in monolayer. Polymer, 46, 5896–5902. 20 Aoki, H., Morita, S., Sekine, R. and Ito, S. (2008) Conformation of single poly(methyl methacrylate) chains in ultra-thin film studied by scanning near-field optical microscopy. Polym. J., 40, 274–280. 21 Aoki, H., Tanaka, S., Ito, S. and Yamamoto, M. (2000) Nanometric inhomogeneity of polymer network investigated by scanning near-field optical microscopy. Macromolecules, 33, 9650–9656. 22 OReilly, J. M., Teegarden, D. M. and Wignall, G. D. (1985) Small-angle and intermediate-angle neutron-scattering from stereoregular poly(methyl methacrylate). Macromolecules, 18, 2747–2752. 23 Sato, N., Ito, S. and Yamamoto, M. (1998) Molecular weight dependence of shear viscosity of a polymer monolayer: evidence for the lack of chain entanglement in the two-dimensional plane. Macromolecules, 31, 2673–2675. 24 Si, L., Massa, M. V., Dalnoki-Veress, K., Brown, H. R. and Jones, R. A. L. (2005) Chain entanglement in thin freestanding polymer films. Phys. Rev. Lett., 94, 127801. 25 Kato, K., Uchida, E., Kang, E. T., Uyama, Y. and Ikada, Y. (2003) Polymer surface with graft chains. Prog. Polym. Sci., 28, 209–259. 26 Senaratne, W., Andruzzi, L. and Ober, C. K. (2005) Self-assembled monolayers and polymer brushes in biotechnology: Current applications and future perspectives. Biomacromolecules, 6, 2427–2448. 27 Uyama, Y., Kato, K. and Ikada, Y. (1998) Surface modification of polymers by grafting. Adv. Polym. Sci., 137, 1–39.

28 Pyun, J., Kowalewski, T. and Matyjaszewski, K. (2003) Synthesis of polymer brushes using atom transfer radical polymerization. Macromol. Rapid Commun., 24, 1043–1059. 29 Tsujii, Y., Ohno, K., Yamamoto, S., Goto, A. and Fukuda, T. (2006) Structure and properties of high-density polymer brushes prepared by surface-initiated living radical polymerization. Adv. Polym. Sci., 197, 1–45. 30 Yamamoto, S., Tsujii, Y. and Fukuda, T. (2002) Glass transition temperatures of high-density poly(methyl methacrylate) brushes. Macromolecules, 35, 6077–6079. 31 Urayama, K., Yamamoto, S., Tsujii, Y., Fukuda, T. and Neher, D. (2002) Elastic properties of well-defined, high-density poly(methyl methacrylate) brushes studied by electromechanical interferometry. Macromolecules, 35, 9459–9465. 32 Lai, P. Y. and Binder, K. (1992) Structure and dynamics of polymer brushes near the theta point – a Monte-Carlo simulation. J. Chem. Phys., 97, 586–595. 33 Wittmer, J., Johner, A., Joanny, J. F. and Binder, K. (1994) Chain desorption from a semidilute polymer brush - a Monte-Carlo simulation. J. Chem. Phys., 101, 4379–4390. 34 Semenov, A. N. (1995) Rheology of polymer brushes - Rouse model. Langmuir, 11, 3560–3564. 35 Semenov, A. N. and Anastasiadis, S. H. (2000) Collective dynamics of polymer brushes. Macromolecules, 33, 613–623. 36 He, G. L., Merlitz, H., Sommer, J. U. and Wu, C. X. (2007) Static and dynamic properties of polymer brushes at moderate and high grafting densities: A molecular dynamics study. Macromolecules, 40, 6721–6730. 37 Dimitrov, D. I., Milchev, A. and Binder, K. (2007) Polymer brushes in solvents of variable quality: Molecular dynamics simulations using explicit solvent. J. Chem. Phys., 127, 084905. 38 Fytas, G., Anastasiadis, S. H., Seghrouchni, R., Vlassopoulos, D., Li, J. B.,

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Factor, B. J., Theobald, W. and Toprakcioglu, C. (1996) Probing collective motions of terminally anchored polymers. Science, 274, 2041–2044. 39 Michailidou, V. N., Loppinet, B., Vo, D. C., Prucker, O., Ruhe, J. and Fytas, G. (2006) Dynamics of end-grafted polystyrene brushes in theta solvents. J. Polym. Sci. B Polym. Phys., 44, 3590–3597. 40 Yakubov, G. E., Loppinet, B., Zhang, H., Ruhe, J., Sigel, R. and Fytas, G. (2004) Collective dynamics of an end-grafted polymer brush in solvents of varying quality. Phys. Rev. Lett., 92, 115501. 41 Aoki, H., Kitamura, M. and Ito, S. (2008) Nanosecond dynamics of poly(methyl methacrylate) brushes in solvents studied by fluorescence depolarization method. Macromolecules, 41, 285–287.

42 Ejaz, M., Yamamoto, S., Ohno, K., Tsujii, Y. and Fukuda, T. (1998) Controlled graft polymerization of methyl methacrylate on silicon substrate by the combined use of the Langmuir-Blodgett and atom transfer radical polymerization techniques. Macromolecules, 31, 5934–5936. 43 Arai, T., Sawatari, N., Yoshizaki, T., Einaga, Y. and Yamakawa, H. (1996) Excludedvolume effects on the hydrodynamic radius of atactic and isotactic oligo- and poly(methylmethacrylate)s in dilute solution. Macromolecules, 29, 2309–2314. 44 Horinaka, J., Ono, K. and Yamamoto, M. (1995) Local chain dynamics of syndiotactic poly(methyl methacrylate) studied by the fluorescence depolarization method. Polym. J., 27, 429–435.

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5 Real Time Monitoring of Molecular Structure at Solid/Liquid Interfaces by Non-Linear Spectroscopy Hidenori Noguchi, Katsuyoshi Ikeda, and Kohei Uosaki

5.1 Introduction

Many important processes such as electrochemical reactions, biological processes and corrosion take place at solid/liquid interfaces. To understand precisely the mechanism of these processes at solid/liquid interfaces, information on the structures of molecules at the electrode/electrolyte interface, including short-lived intermediates and solvent, is essential. Determination of the interfacial structures of the intermediate and solvent is, however, difficult by conventional surface vibrational techniques because the number of molecules at the interfaces is far less than the number of bulk molecules. Recently, sum frequency generation (SFG) spectroscopy has been shown to be a very powerful technique to probe molecular structure at an interface [1–6]. SFG is a second-order nonlinear optical (NLO) process, in which two photons of frequencies w1 and w2 generate one photon of the sum frequency (w3 ¼ w1 þ w2) [2]. Secondorder NLO processes, including SFG, are prohibited in media with inversion symmetry under the electric dipole approximation and are allowed only at the interface between these media where the inversion symmetry is necessarily broken. By using visible light of fixed wavelength and tunable IR light as the two input light sources, SFG spectroscopy can be a surface sensitive vibrational spectroscopy as the SFG signal is resonantly enhanced when the energy of the IR beam becomes equal to that of a vibrational state of the surface species [2]. SFG spectroscopy is particularly useful for studying the structure of water molecules, the most common solvent, at various interfaces where the presence of a much larger amount of bulk water than interfacial water makes the measurement of interfacial water by other vibrational techniques very difficult. Furthermore, SFG is free from the ambiguity associated with the choice of reference spectrum as required for linear spectroscopy applied to interfaces such as surface-enhanced IR spectroscopy (SEIRAS), [7, 8] and surfaceenhanced Raman scattering (SERS) [9]. Furthermore, time-resolved vibrational spectroscopy based on SFG is possible because a short pulse laser is used. Thus,

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SFG spectroscopy is an ideal technique to investigate the mechanism of interfacial processes at solid/liquid interfaces [5, 6, 10–16]. Hyper-Raman scattering (HRS), which is inelastic scattering in the second-order nonlinear optical process, is also expected to be a useful spectroscopic technique to obtain vibrational information [17–24]. According to the molecular symmetry selection rules, IR-active vibrational modes, which are usually not evident in normal Raman spectra, are all active in HRS. Moreover, so-called silent mode vibration, which is not observed in either Raman or IR absorption spectra, can be revealed in HRS [17, 18]. However, since the HRS effect is extremely weak, enhancement of the HRS signal is essential for HRS to be a useful spectroscopic technique. Similar to the enhancement of Raman scattering, SERS, surface-enhanced hyper-Raman scattering, SEHRS, has been reported, utilizing metal colloids or a roughened electrode surface [21, 23, 24]. Here, we describe the basic principles and detailed experimental arrangement of SFG spectroscopy and present several examples of SFG study at solid/liquid interfaces. HRS is also described briefly.

5.2 Sum Frequency Generation Spectroscopy 5.2.1 Brief Description of SFG

Non-linear optical phenomena that can be observed with static electric and magnetic fields such as the P€ockels and Faraday effects have been known since the nineteenth century. Frequency conversion such as second harmonic generation (SHG) and SFG requires intense optical fields and its realization had to wait until the birth of the pulsed laser at the end of the 1950s. SHG was first observed in 1961 by Franken et al. in a bulk quartz crystal [25]. The foundation of the theory of NLO was laid by Bloembergen in the early 1960s [26]. Surface SHG was detected for the first time in 1974 from the surface of silver[27].ThefirstobservationofsurfaceSFGwasin1986byShenetal.forthecoumarin dye on glass [28]. SFG spectroscopy has become extremely attractive for analysis of interface science, thanks to the recent development of a tunable laser source. SFG is one of the second-order NLO processes, in which two photons of frequencies w1 and w2 generate one photon of a sum frequency (w3 ¼ w1 þ w2) as shown in Figure 5.1. Second-order NLO processes, including SFG, are strictly forbidden in media with inversion symmetry under the electric dipole approximation and are allowed only at the interface between these media where the inversion symmetry is necessarily broken. In the IR–Visible SFG measurement, a visible laser beam (wVis) and a tunable infrared laser beam (wIR) are overlapped at an interface and the SFG signal is measured by scanning wIR while keeping wVis constant. The SFG intensity (ISFG) is enhanced when wIR becomes equal to the vibration levels of the molecules at the interface. Thus, one can obtain surface-specific vibrational spectra at an interface

5.2 Sum Frequency Generation Spectroscopy

ωVis

ωSFG

ωIR Figure 5.1 Energetic scheme for the SFG process.

between two phases with inversion symmetry, which cannot be obtained using traditional vibrational spectroscopy such as IR and Raman scattering. In addition to the surface/interface selectivity, IR–Visible SFG spectroscopy provides a number of attractive features since it is a coherent process: (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region. (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29–32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33–37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 5.2.2 Origin of SFG Process

The induced dipole ð~ m Þ of a molecule placed in an electric field ð~ E Þ is given by $ $ $ 0 ~ E þ b~ E~ Eþ g~ E~ E~ E þ ð5:1Þ m ¼~ m þ a~ $$ $ Where a ; b and g are the linear polarizability, the first- and second-hyper polarizabilities, respectively, and are represented by second, third and fourth rank 0 tensors, respectively, and ~ m is a static polarizability. In condensed phases, it is more convenient to consider the dipole moment per unit volume or polarization ð~ PÞ ð0Þ ð1Þ ð2Þ ð3Þ ~ P þ~ P þ~ P þ P ¼~ P þ~ 0 $ ð2Þ ~ $ ð3Þ~~~ $ ð1Þ Eþ c ~ EE þ c EEE þ Þ ¼~ P þ e0 ð c ~

ð5:2Þ

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$ ð1Þ $ ð2Þ $ ð3Þ where, c ; c and c are the first-, second, and third-order susceptibilities, respectively, and e0 is the dielectric constant in vacuum. For a simple molecular material, the susceptibility depends on the number of molecules per unit volume, N, multiplied by the molecular polarizability averaged over all orientations of molecules in the material. $ $ ð1Þ c ¼ Nh a i=e0 ;

$ ð2Þ $ c ¼ Nh b i=e0 ;

$ ð3Þ $ c ¼ Nh g i=e0

ð5:3Þ

where the brackets < > denote the ensemble average of the molecular orientation. ð0Þ Since few materials have a static polarization, the first term in Eq. (5.2), ~ P can be dropped. $ ð2Þ ~ $ ð3Þ~~~ $ ð1Þ ~ Eþ c ~ EE þ c EEE þ ð5:4Þ P ¼ e0 c ~ When the electric field ð~ E Þ is weak, as in a non-laser light source, the second and third terms can be neglected and polarization ð~ PÞ can be expressed as below, corresponding to linear optics. $ ð1Þ ~ E P ¼ e0 c ~

ð5:5Þ

When the electric field ð~ E Þ is strong, as in a laser light source, the second and third terms cannot be neglected and NLO processes take place. A remarkable consequence of the higher-order terms in Eq. (5.4) is that the frequency of the light can change. If we consider two electric fields with frequencies w1 and w2, ~ r ; tÞ ¼ ~ E 1 ð~ r Þcosðw1 tÞ; E 1 ð~

~ E 2 ð~ r ; tÞ ¼ ~ E 2 ð~ r Þcosðw2 tÞ

ð5:6Þ

where ~ r and t represent a position vector and time, respectively. The second-order ð2Þ $ ð2Þ polarization, ~ P , in a material in which c is non-zero is given by: ð2Þ $ ~ E 1~ E2 P ¼ e0 c ~ $ ð2Þ~ ¼ e0 c E 1 ð~ r Þcosðw1 tÞ~ E 2 ð~ r Þcosðw2 tÞ ð2Þ $ ~ ¼ 1=2e0 c E 1 ð~ r Þ~ E 2 ð~ r Þfcosððw1 þ w2 ÞtÞ þ cosððw1 w2 ÞtÞg ð2Þ

ð5:7Þ

This shows that there are now oscillating dipoles at frequencies (w1 þ w2) and (w1 w2), which give rise to SFG and difference frequency generation (DFG), respectively. When w1 ¼ w2, Eq. (5.7) becomes: ð2Þ $ ð2Þ ~ E 1 ð~ r Þ~ E 2 ð~ r Þf1 þ cosð2w1 tÞg P ¼ 1=2e0 c ~

ð5:8Þ

The first term in the brackets represents a static electric field in the material and the second term represents a dipole oscillating at 2w, twice the frequency of the incident light. This is a process known as SHG. 5.2.3 SFG Spectroscopy

In SFG vibrational spectroscopy, w1 is usually fixed in the visible region and w2 is scanned in the infrared region. In the most widely used geometry, the two laser


Figure 5.2 Schematic drawing of the optical arrangement of an SFG measurement.

beams are incident in either counter- or collinear-propagating geometry and the reflected SF light (wsum) is detected. Let us consider the case where visible and infrared beams are incident from medium 1 to the interface with medium 2, as depicted in Figure 5.2. The direction of emission is determined by conservation of momentum parallel to the surface. wSFG ¼ wVis þ wIR wSFG sinqSFG ¼ wVis sinqVis þ wIR sinqIR

ð5:9Þ ð5:10Þ

where qSFG is the emission angle of SFG light and qVis and qIR are the incident angles of the visible and infrared light. Since the angles qVis, qIR and the frequency wVis are constant in a given SFG measurement, qSFG can be expressed as a function of wIR. wvis sinqvis þ wIR sinqIR qSFG ðwIR Þ ¼ sin1 ð5:11Þ wvis þ wIR As shown in this equation, the angle of emission changes as wIR is scanned. The intensity of emitted SF light (ISFG) is expressed by the following equation [38] ISFG ¼

$ $ ð2Þ 2 8p3 wSFG 2 ðsecqSFG Þ2 F : c IVis IIR AT hc 3 n1 ðwSFG Þ n1 ðwVis Þ n1 ðwIR Þ

ð5:12Þ

where Ivis and IIR are the intensities of incident visible and infrared light, respectively, c is the speed of light in vacuum, A is an overlapping beam cross section at the sample, T is a pulse duration, and n1(wi) is the refractive index of medium 1 at frequency wi. $ and F is a Fresnel factor. $ ð2Þ For a polar monolayer of molecules at an interface, the resonant c is typically ð2Þ $ 1014 1016 esu. If we take c 1015 esu, IVis 1 mJ pulse1 and IIR 100 mJ pulse1, T 25 ps, wVis ¼ 5.64 1014 s1 (532 nm), wIR ¼ 9.0 1013 s1 (3333 nm), wSFG ¼ 6.55 1014 s1 (458.77 nm), A 103 cm2, qSFG 67 , Eq. (5.12) predicts a signal of 1.5 104 photon pulse1. Such a signal can be readily detected by a PMT or CCD detector. If neither wVis nor wSFG is in resonance with an electric dipole transition in the material and only electric dipole transitions are considered, the hyperpolarizability,

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blmn, near a vibrational transition in a molecule is given by [38] 1 X hgjml jsihsjmm jvi hgjmm jsihsjml jvi blmn ¼ 2 wSFG wsg wVis þ wsg 2h s hvjmn jgi þ const: wIR wvg þ iG

ð5:13Þ

where |gi is the ground state, |ni is the excited vibrational state, |si is any other state, G is the relaxation time of the excited vibrational state, const. is an off-resonant term, which can be considered as a constant and m ¼ er is the electric dipole operator. The first bracket is identified as a Raman transition dipole moment Mlm, and the term vg hn|mn|gi as an IR transition dipole moment mn . If we substitute wvg by w0, Eq. (5.13) becomes: bijk ¼

aijk 1 2 w w iG þ const: 2h IR 0

ð5:14Þ

where vg

aijk ¼ Mij mk

From the above equation, it is clear that bijk takes a maximum value when the IR frequency is in resonance with the molecular vibration, that is, wIR ¼ w0, and only the molecular vibration, which is both IR- and Raman-active is SFG-active. $ $ If we introduce A ¼ N=en h a i, the description of macroscopic resonant second$ ð2Þ order susceptibility, c R , is given by $ 1 A $ ð2Þ cR ¼ 2 w w 2h IR 0 iG

ð5:15Þ

The emitted SFG light is a sum of resonant contributions with nth vibrational states, $ ð2Þ $ ð2Þ c R;n and nonresonant contributions, c NR . $ An $ ð2Þ X 1 $ ð2Þ $ ð2Þ X $ ð2Þ c R;n ¼ c NR þ c ¼ c NR þ 2 w w iG 2 h IR n n n n

ð5:16Þ

$ $ ð2Þ Here, c NR is the sum of nonresonant contributions from molecules ð c NR;mol Þ, $ $ substrate ð c NR;sub Þ and the interaction between molecules and substrate ð c NR;m-s Þ. $ $ c NR ¼ c NR;mol þ cNR;sub þ cNR;ms Combining Eqs. (5.12) and (5.16), the SFG intensity can be written as: 2 $ ð2Þ X A eff ;n $ ISFG / c eff ;NR þ wIR wn iG n n

ð5:17Þ

ð5:18Þ

$ $ ð2Þ $ $ $ $ ð2Þ where c eff ;NR ¼ F : c NR and A eff ;n ¼ F : A n . This is the equation generally used to analyze an observed SFG spectrum.


Figure 5.3 Schematic drawing of a picosecond SFG system.

5.2.4 Experimental Arrangement for SFG Measurements 5.2.4.1 Laser and Detection Systems Figure 5.3 shows the schematic diagram of a picosecond SFG spectrometer used in this work. A passive mode-locked Nd:YAG laser system (EKSPLA, PL2143B) generates 25 ps pulses of 1064 nm (fundamental), 532 nm (second harmonic: SHG) and 355 nm (third harmonic: THG) at a repetition rate of 10 Hz. The total output power was about 3335 mJ pulse1 and typical pulse energies of 1064 nm, 532 nm and 355 nm were 8, 8 and 6 mJ pulse1, respectively. The ratio of the harmonic generations can be adjusted to some extent by tuning the phase match angles of the mixing crystals. The 50% of SHG and 15% of THG were split and transferred to a pump line. The residual SHG was directed to a sample through a time-delay and used as the incident visible light for an SFG measurement. The power of the incident visible light can be adjusted by a combination of a half wave plate and a Gran–Taylor prism and the polarization can be changed by an additional half wave plate. The 85% of THG output (about 5 mJ pulse1) was used to pump an optical parametric generation and amplification (EKSPLA, PG401VIR/DFG) system, containing a LBO crystal and double gratings. The output from the OPG/OPA was mixed with the 1064 nm laser output in a nonlinear crystal, Ag2GaS2, to generate a tunable infrared output from 2.3 to 8.5 mm by the difference frequency generation (DFG) process. The power of the incident infrared light can be roughly adjusted by tuning a total power of the laser and the ratio of the harmonic generator. The polarization of the IR beam can be changed by exchanging the mirror sets, which contains three and four mirrors. The line-width of the tunable infrared output was 6 cm1, determined by the double grating set-up in the OPA path.

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The visible output from the laser and tunable infrared output from the DFG were incident and overlapped at the sample surface with angles of 70 and 50 , respectively. Either a 532 nm or a 1064 nm laser output was used as the visible light in the SFG measurement. A BK7 lens ( f ¼ 600) and a ZnSe lens ( f ¼ 50) were used to focus the visible and IR beams, respectively. The position of the ZnSe lens was controlled by an XY stage to perform the spatial overlapping of the two incident beams. The beam spot of the visible and infrared lights had an elliptical shape with dimensions of about 2.5 0.9 mm2 and 0.3 0.2 mm2, respectively. The SFG signal was separated from the reflected visible and IR lights by passing through irises and a monochromator (Oriel Instruments, MS257) and was detected by a photomultiplier tube (PMT: Hamamatsu, R630-10 or R3896) with a gated electronic system (Stanford Research System). When the 532 nm visible light was used, two holographic Super Notch filters (Kaiser Optical System, INC, HSPF-5321.0) were placed in front of the monochromator to further reduce scattered visible light. All SFG spectra were obtained by averaging 50400 pulses per data point and were normalized against the intensity of the visible and infrared inputs, which were simultaneously monitored by a power meter (Oriel instruments, Model 70833 and 70811) and Si photodiode, respectively. 5.2.4.2

Spectroscopic Cells

Spectroelectrochemical Cell Figure 5.4 shows spectroelectrochemical cells used in electrochemical SFG measurements. An Ag/AgCl (saturated NaCl) and a Pt wire were used as a reference electrode and a counter electrode, respectively. The electrolyte solution was deaerated by bubbling high-purity Ar gas (99.999%) for at least 30 min prior to the electrochemical measurements. The electrode potential was controlled with a potentiostat. The electrode potential, current, and SFG signal were recorded by using a personal computer through an AD converter. After introduction of the working electrode to the spectroelectrochemical cell, continuous potential cycling was performed to obtain a clean surface before each

Figure 5.4 Spectroelectrochemical cells for electrochemical SFG measurements.


electrochemical SFG experiment until the cyclic voltammogram (CV) became that of a clean electrode. When single- or poly-crystalline disk electrodes are used as a working electrode, the electrode must be gently pushed against the CaF2 window of the spectroelectrochemical cell to achieve a thin layer (about 5 mm) configuration so that strong absorption of the IR beam by the electrolyte solution was avoided. A thin metal electrode deposited on a hemispherical prism can also be used as a working electrode. Strong absorption of the IR beam by the electrolyte solution can be more easily avoided in this configuration. Flow Cell In situ SFG measurements were carried out in the internal reflection geometry to avoid the strong absorption of IR incident beam by water. Since the SFG signal from the quartz disk/water interface is very weak if a flat quartz disk is used as a window, due to the very large reflection loss of the incident beams at the air/quartz interface and the small Fresnel factors at the quartz/water interface, a hemi-cylindrical prism, where higher enhancement of the sensitivity is expected, was used for the in situ SFG measurements. The flat face of the quartz prism was in contact with electrolyte solution and the infrared and visible beams were incident from the quartz prism side and were overlapped at the quartz/electrolyte solution interface. The incident angle of the visible light was about 70 , which is near the critical angle of total reflection (qc) of quartz/water (72 ), so that a strong surface field can be obtained. The incident angle of the infrared light was set at 50 , which is far from qc, so that a large change in the Fresnel factor can be avoided when the IR frequency is scanned over the OH-stretching vibration region. A flow cell made of polychlorofluoroethylene, shown schematically in Figure 5.5, was used to change between gas or liquid during SFG measurements. Electrolyte solution was introduced into the flow cell by the pressure of the pure argon gas so that the solution could be exchanged without being exposed to the atmosphere and the optical alignment and the sample position were not affected when the solution was exchanged.

Figure 5.5 A flow cell for SFG measurements.

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5.3 SFG Study of the Potential-Dependent Structure of Water at a Pt Electrode/Electrolyte Solution Interface 5.3.1 Introduction

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39–41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47], surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49], the exact nature of the structure of water at an electrode/solution interface is still not fully understood. Here, we demonstrate the usefulness of SFG spectroscopy in the study of water structure at electrode/electrolyte solution interfaces by showing the potential dependent SFG spectra in the OH-stretching vibration region at a Pt/thin film electrode/0.1 M HClO4 solution interface in internal reflection mode. 5.3.2 Results and Discussion

Figure 5.6 shows a typical CV of a thin Pt film electrode in a 0.1 M HClO4 solution. The hydrogen waves in the potential range between 200 and about 50 mV were observed and surface oxidation and reduction peaks were observed in the positive potential region (>600 mV). This result confirmed that the conductivity of the Pt thin film was good enough to be used as an electrode. Figure 5.7 shows SFG spectra in the OH-stretching region (2800–3800 cm1) obtained at the Pt electrode in a 0.1 M HClO4 solution at various potentials. Two broad peaks were observed at about 3200 cm1 and 3400 cm1. These peaks have been assigned to the vibration of OH oscillators of three-coordinated hydrogen-bonded water, that is, less ordered liquid-like water, molecules and that of the fourcoordinated hydrogen-bonded water, that is, highly ordered ice-like water molecules, respectively, based on an IR study of water clusters [50]. Thus, the intensity ratio between these two peaks can be considered as an index of the order of the interfacial water [51]. Our previous work showed that the SFG spectra of Au thin film/0.05 M H2SO4 solution interface were dominated by the peak corresponding to the liquidlike water at all potentials we investigated [13]. Thus, water seems to be more highly oriented at the Pt electrode than at the Au electrode.

5.3 SFG Study of the Potential-Dependent Structure of Water

Figure 5.6 CV obtained with a sweep rate of 50 mV s1 (solid line) and potential dependence of integrated SFG intensity in the OH-stretching region (.) of a Pt thin film electrode in 0.1 M HClO4 solution.

While the shape of the SFG spectra did not change significantly with potential, the intensity depended on potential. To clarify the potential dependences of the SFG intensity, the integral intensities of SFG spectra of the Pt electrode between 2800 cm1 and 3800 cm1, taken from Figure 5.7 were plotted against electrode potential,l as shown also in Figure 5.6. Parabolic behavior of the SFG intensity was observed between 200 and 600 mV with a minimum around 200 mV, which is close to the potential of zero charge, pzc, of Pt electrode in a HClO4 solution [52, 53], although the value of the pzc of a Pt electrode is still debated [54]. Previously, we have proposed that SFG intensity due to interfacial water at quartz/ water interfaces reflects the number of oriented water molecules within the electric double layer and, in turn, the double layer thickness based on the pH dependence of the SFG intensity [10] and a linear relation between the SFG intensity and (ionic strength)1/2 [12]. In the case of the Pt/electrolyte solution interface the drop in the potential profilein the vicinity of electrode become precipitous as theelectrode becomes more highly charged. Thus, the ordered water layer in the vicinity of the electrode surface becomes thinner as the electrode is more highly charged. Since the number of ordered water molecules becomes smaller, the SFG intensity should become weaker at potentials away from the pzc. This is contrary to the experimental result. When the electrolyte concentration is relatively high, the potential dependence of the double layer thickness is low and the potential dependence of the fraction of oriented water predominantly determines that of the SFG intensity. Since the polarization of IR in the present experiment is p, water oriented normal to the surface is effectively detected by SFG. Water molecules are expected to lie parallel to the surface around the pzc and reorient from oxygen up to oxygen down as the surface charge of electrode surface changes from negative to positive so long as no specific adsorption of ions take place. IR study [7, 8] as well as computer simulation [55, 56] also suggest that

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Figure 5.7 SFG spectra in the OH-stretching region at a Pt electrode at each potential in 0.1 M HClO4 solution.

5.3 SFG Study of the Potential-Dependent Structure of Water

Figure 5.8 Potential dependence of relative phase difference between cNR and cR.

water molecules at the metal electrode surface have an oxygen up and oxygen down orientation on negatively and positively charged surfaces, respectively, on average. ð2Þ For fixed non-resonant cNR sign, when the dipole moment of a molecule rotates by 180 , the relative phase between the resonant and non-resonant part of the signal also ð2Þ changes by 180 [57]. Thus, by analyzing the relative phase difference between cNR ð2Þ and cR , the potential dependent orientation of water molecules on a Pt electrode surface can be determined. Figure 5.8 shows the potential dependence of the relative phase difference between ð2Þ ð2Þ cNR and cR . The relative phase was changed by about 180 at 200 mV, which is close to the pzc for a Pt electrode in HClO4 electrolyte solution [52, 53]. This orientation change is most probably associated with a change in sign of the charge at the Pt surface. This clearly demonstrates that the orientation of water dipoles flips by 180 at the pzc. SFG intensity in the OH-stretching region decreased as the potential became more positive where Pt oxide was formed, as shown in Figure 5.6. There are several possibilities for this decrease. One is the disruption of the well-ordered hydrogen bonded network structure of water molecules at a roughened Pt electrode surface compared to that at an atomically flat surface, since it is well known that the atomically flat surfaces of Pt were roughened by surface oxide formation [58]. The other possibility is the electric effect. Since Pt oxide is an insulating thin film [59], an additional potential drop takes place within the Pt oxide layer, resulting in a smaller electric field within the double layer. Furthermore, surface charge should be also affected by the oxide formation. 5.3.3 Conclusions

In conclusion, electrochemical SFG measurements showed that the SFG spectra in the OH-stretching region (2800–3800 cm1) at the Pt electrode in a 0.1 M HClO4

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solution showed two broad peaks at about 3200 cm1 and 3400 cm1, which are assigned to the vibration of OH oscillators of three-coordinated hydrogen-bonded water, that is, less ordered liquid-like water, molecules and that of the fourcoordinated hydrogen-bonded water, that is, highly ordered ice-like water molecules, respectively, in contrast to the Au thin film/0.05 M H2SO4 solution interface where the spectra were dominated by the peak corresponding to the liquid-like water at all potentials, showing that water molecules are more highly oriented at the Pt electrode than at the Au electrode. The SFG intensity of the OH-stretching region of interfacial water at a Pt/electrolyte solution interface exhibits a strong dependence on electrode potential. Parabolic behavior of SFG intensity was observed between 200 and 600 mV with a minimum around 200 mV, which is close to the pzc for a Pt electrode in HClO4 electrolyte solution. In the Pt oxide formation region (more positive than 600 mV), SFG intensity due to interfacial water decreased.

5.4 Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode 5.4.1 Introduction

The dynamics of interactions between molecules and a surface such as vibrational excitations, energy exchange, and relaxation are of fundamental importance in surface science [60, 61]. The time scale of these processes is in the ps to fs regime. Recent development of short pulse laser techniques has enabled direct observation of ultrafast surface dynamics, not only to identify the surface species but also to probe the transient species generated by the pump pulse in real time [62–64]. Although most of the SFG studies so far have been concerned with the static structure of molecules at interfaces, a more important contribution of SFG spectroscopy should be its high time resolution and time-resolved SFG (TR-SFG) is expected to be one of the most powerful methods for observing and identifying transient states of surface adsorbates [65–68]. Adsorbed CO on a metal surface is one of the simplest adsorbates and has attracted significant interest within the areas of fundamental surface science, catalysis, and electrochemistry. An understanding of the oxidation mechanism of adsorbed CO is important to design and develop electrocatalysts for fuel cells [69–73] and the surface dynamics of adsorbed CO on electrode surfaces in electrolyte solutions is, therefore, very important. TR-SFG seems to be an ideal tool to study the surface dynamics of adsorbed CO at solid/liquid interfaces. Although there are several reports of TR-SFG studies on an electrode, they are only of investigations of vibrational relaxation lifetime by IR excitation [34, 65, 67]. In the present study, we investigated the SFG response of CO adsorbed on a Pt electrode surface in an electrolyte solution upon irradiation of intense visible pulses with time resolution of about 20 ps.

5.4 Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode

Figure 5.9 CVs of a Pt-poly electrode in 0.1 M HClO4 with and without (inset) 0.1 M HCHO with sweep rate of 20 mV s1.

5.4.2 Results and Discussion

Figure 5.9 shows CVs of the Pt electrode in 0.1 M HClO4 solution with and without (inset) 0.1 M HCHO. They are in good agreement with the results reported previously [74]. The hydrogen waves in the potential range between 250 and about 50 mV (inset) were suppressed in the solution containing HCHO, indicating the existence of a CO adlayer on the Pt electrode surface [72, 73]. SFG measurements were carried out in the potential range 0–300 mV, where the presence of adsorbed CO was expected, after a CO adlayer was formed at 0 V. Figure 5.10 shows SFG spectra of the Pt electrode in 0.1 M HClO4 solution containing 0.1 M HCHO at various potentials. A peak centered at about 2055 cm1 was observed, in agreement with results of previous studies by IR spectroscopy [75], and was assigned to the stretching vibration of CO (nCO) adsorbed on a one-fold coordinated (atop) site of the Pt surface. As the potential became more positive, the peak position of this band shifted to a higher wavenumber by approximately 33 cm1 V1 up to 200 mV (Figure 5.10 inset), which also agrees with the previous IR results [75]. This shift has often been referred to as electrochemical Stark tuning. A decrease in intensity and a red shift of the SFG peak were observed at 300 mV, indicating the loss of adsorbed CO as a result of CO oxidation. When the potential was made more positive, the SFG peak was no longer observed (data not shown), indicating complete loss of the adsorbed CO from the Pt surface. The CO oxidation threshold potential observed by SFG in the present experiments was about 200 mV more negative than that expected from the CV (Figure 5.9). A similar discrepancy between the CO oxidation potential in CV and the potential of the disappearance of the SFG signal was reported previously [76, 77]. The difference in the data acquisition

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Figure 5.10 SFG spectra of the Pt surface in a solution of 0.1 M HCHO in 0.1 M HClO4 at (a) 0, (b) 100, (c) 200, and (d) 300 mV.

time (SFG: 20–60 min for a spectrum at a given potential, CV: typically less than 1 min) is considered to be the origin of this discrepancy. Figure 5.11 shows the temporal profile of the intensity change in the SFG signal at the peak of the nCO mode (2055 cm1) at 0 mV induced by visible pump pulse irradiation. The solid line is the least-squares fit using a convolution of a Gaussian function for the laser profile (FWHM ¼ 20 ps) and a single exponential function for the recovery profile. The SFG signal fell to a minimum within about 100 ps and recovered

5.4 Photoinduced Surface Dynamics of CO Adsorbed on a Platinum Electrode

Figure 5.11 Temporal profile of SFG signal intensity at 2055 cm1 at a potential of 0 mV. The solid line is the result of a least-squares fitting.

to the initial value. There was a linear correlation between the pump fluence and the intensity decrease induced by the pump pulse and no intensity change was observed when the pump fluence was less than that of the SFG probe (532 nm) pulse. Figure 5.12 shows TR-SFG spectra at delay times of 80, 0, and 70 ps. The spectrum observed at 80 ps, that is, 80 ps before pumping, is the same as those observed without pumping (Figure 5.10), indicating that the irradiation-induced changes in the spectra were restored during the 0.1 s interval of pump pulse repetition (10 Hz). When the delay time was 0 ps, a maximum change in spectral features was observed. The height of the SFG peak at 2055 cm1 was decreased, the peak position was slightly shifted to a lower wavenumber, and the peak was broadened. The values of FWHM derived from the fits are about 20 cm1 and about 25 cm1 at 80 ps and 0 ps delay, respectively. At 0 ps, in addition to the changes in the peak at 2055 cm1, a new peak appeared at around 1980 cm1. One possible origin of the new peak is n ¼ 1 ! 2 transition (hot band) of the stretching vibration of CO adsorbed on the Pt surface. Due to a vibrational anharmonicity, a hot band will give rise to a new peak in a lower wavenumber region than the fundamental peak (n ¼ 0 ! 1). The frequency shift from the initial CO stretching band to the new broad peak observed in the present study was, however, about 88 cm1, which is much larger than the previously reported value for CO on a Pt surface (about 30 cm1) [78] and in the gas phase (about 27 cm1) [79]. Thus, it is unlikely that the new peak is due to excitation of the hot band generated by pump pulses. Another possible reason for the appearance of the new peak is the transient site migration of CO on the Pt surface. It has been reported that CO adsorbed on a multi-bonded [80] or asymmetric bridge site [81] gives a peak at about 1980 cm1, which is in agreement with the position of the transiently observed peak in the present study. Thus, it is reasonable to assume that the decrease

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Figure 5.12 TR-SFG spectra in the CO-stretching region at delay times of 80, 0, and 70 ps.

in the peak at 2055 cm1 and the appearance of the new peak at 1980 cm1 were caused by reversible site migration of CO on the Pt surface from the on-top site to a multi-bonded or asymmetric bridge site induced by intense pump pulse irradiation. The low-frequency shift and the broadening of the CO spectra at 0 ps suggest that the low-frequency modes of adsorbed CO, that is, stretching, frustrated rotation, and frustrated translation modes of Pt–CO, were thermally excited by pump pulses, as reported by Bonn et al. [82] Thus, it is concluded that the transient site migration of adsorbed CO on the Pt electrode surface was caused by a transient rise in the surface temperature of Pt induced by pump pulses. 5.4.3 Conclusions

TR-SFG measurements at a Pt electrode/electrolyte interface covered with a CO monolayer excited by the irradiation of picosecond visible pulses showed that the

5.5 Interfacial Water Structure at Polyvinyl Alcohol (PVA) Gel/Quartz Interfaces

Figure 5.13 Transient CO migration induced by intense pump pulse irradiation.

population of on-top CO instantly decreased, accompanied by an increase in multibonded CO due to the transient temperature jump at the surface and the initial state was recovered within 100 ps, showing the transient reversible migration of CO molecules on the Pt surface under electrochemical conditions, as shown schematically in Figure 5.13.

5.5 Interfacial Water Structure at Polyvinyl Alcohol (PVA) Gel/Quartz Interfaces Investigated by SFG Spectroscopy 5.5.1 Introduction

Hydropolymer gel has been considered as a possible candidate for an artificial articular cartilage in artificial joints because it exhibits very low friction when it is in contact with a solid. The origin of such low friction is considered to be associated with the water absorbed in the gel [83–86], some of which is squeezed out from the gel under the load and serves as a lubricant layer between the gel and solid surface, resulting in hydrodynamic lubrication [87, 88]. Although the structural information about the interfacial water is important to understand the role of water for the low frictional properties of hydrogel in contact with a solid and the molecular structure of lubricants other than water at solid/solid interfaces have been investigated theoretically [89–91] and experimentally [92–98], no experimental investigations on water structure at gel/solid interfaces have been carried out due to the lack of an effective experimental technique. Attractive or repulsive interaction between two solid surfaces should play an important role in the interfacial frictional behavior [87, 92–95]. From previous theoretical [89] and experimental investigations [87, 95], it was known that the attractive interaction result in a high friction and repulsive interaction results in low friction force. To characterize the interfacial molecular structure between two solids under electrostatic interaction is also important to elucidate the frictional properties of two solids.

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Figure 5.14 SFG spectra in the OH-stretching region (2800–3800 cm1) at a quartz surface in water before (a) and after contact of the PVA gel with applied pressure of (b) 0 (just in contact), (c) 0.2, (d) 0.4, (e) 0.6, and (f) 0.8 MPa.

Here, the structures of interfacial water at a fused quartz surface with and without contact of polyvinyl alcohol (PVA) were investigated by in situ SFG spectroscopy and their role in low friction between PVA and a fused quartz surface is discussed. 5.5.2 Results and Discussions

Figure 5.14 shows SFG spectra in the OH-stretching region (2800–3800 cm1) obtained at a quartz surface in water before (a) and after (b–f) the contact of PVA


with various applied pressures. Solid lines indicate least-square fits with ice-like and liquid-like water components using Eq. (5.1). At the quartz surface before the PVA gel was contacted (Figure 5.14a), two broad peaks at about 3200 cm1 and 3400 cm1 were observed. It is known that the higher the wavenumber, the lower the degree of hydrogen bonding. Based on IR studies on water clusters [50], the band at 3200 cm1 is attributed to the vibration of OH oscillators of three-coordinated hydrogen-bonded water molecules at the surface and that at 3400 cm1 to the four-coordinated hydrogen-bonded water molecules [6]. The intensity ratio between the two peaks can be considered as an index of the order of the interfacial water [51]. The SFG intensity decreased when PVA gel was in contact with the quartz surface. Weakening of SFG intensity is mostly due to the change in the Fresnel coefficients upon contact with the PVA gel [99]. The shape of the spectra also changed as the PVA gel was pressed toward the quartz surface and the pressure on the PVA gel was increased (Figure 5.14b–f ). The SFG signal from the liquid-like water component became dominant as the pressure on the PVA gel surface was increased. This trend is shown more clearly in Figure 5.15 where the intensity ratio between the two peaks, which can be considered as an indicator of the order of the interfacial water, is plotted against the pressure applied to the PVA gel. The friction behavior of PVA gel and solid is strongly dependent on the nature of the solid surface. While the friction is low when the PVA gel is in contact with a quartz surface, which is hydrophilic, it is very high when the PVA gel is in contact with a

Figure 5.15 Effect of applied pressure on the intensity ratio between the SFG signals due to ice-like and liquid-like water components.

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hydrophobic surface [100]. To further clarify the importance of water structure in interfacial friction behavior, SFG measurement was also carried out at an interface between PVA gel and a hydrophobic surface, which was prepared by modifying the quartz surface with a octadecyltrichlorosilane (OTS) monolayer [101]. Figure 5.16 shows SFG spectra in the region of 2800–3800 cm1 obtained at the OTS modified quartz in water before (a, b) and after (c–f) the contact of PVA with various applied pressures. Solid lines indicate least-square fits with ice-like and liquid-like water components using Eq. (5.1), except for Figure 5.16a where fitting for CH-stretching peaks is also included. Before the PVA gel was contacted to the quartz surface (Figure 5.16a and b), a sharp peak was observed at 3680 cm1 corresponding to non-hydrogen bonded OH, that is, free OH, in addition to a broad peak at about 3200 cm1 with a shoulder at around 3400 cm1. A peak at 3400 cm1 corresponding to the liquid-like water was significantly suppressed compared to the SFG spectrum of Figure 5.2a and the spectrum was similar to that of an ice/air interface, showing that the water molecules at the OTS/quartz interface are extremely well ordered. In addition to the peaks corresponding to OH-stretching, two large peaks were observed at 2879 and 2940 cm1 (Figure 5.16a), which are attributed to the CH-stretching of the CH3 group, indicating that the OTS monolayer maintained its highly ordered structure in water. These results are in agreement with those reported previously by our group [10]. No intensity change in the CH-stretching region was observed when pressure was applied to the PVA gel, although detailed discussion about these peaks is not possible because the SFG spectra were obtained in 10 cm1 steps. Although the total intensity of the SFG spectra decreased as the pressure on the PVA gel was increased, the intensity ratio between the peaks corresponding to icelike water and liquid-like water was almost constant. Since the OTS-modified quartz surface was hydrophobic, the water squeezed from the bulk gel was ice-like at the PVA gel/OTS-modified quartz interface. These results at the interfaces between the PVA gel and quartz surfaces, with and without modification by OTS, suggest that the weakly hydrogen bonded, that is, liquid-like, water plays an important role for the low friction at the PVA gel/quartz interface. 5.5.3 Conclusions

The structure of water at the PVA/quartz interface was investigated by SFG spectroscopy. Two broad peaks were observed in the OH-stretching region at 3200 and 3400 cm1, due to ice-like and liquid-like water, respectively, in both cases. The relative intensity of the SFG signal due to liquid-like water increased when the PVA gel was pressed against the quartz surface. No such increase of the liquid-like water was observed when the PVA gel was contacted to the hydrophobic OTS-modified quartz surface where friction was high. These results suggest the important role of water structure for low friction at the polymer gel/solid interfaces.


Figure 5.16 SFG spectra in the OH-stretching region (2800–3800 cm1) at the OTS-modified quartz surface in water before (a), (b) and after contact of the PVA gel with applied pressure of (c) 0 (just in contact), (d) 0.2, (e) 0.6, and (f) 0.8 MPa.

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5.6 Hyper-Raman Spectroscopy 5.6.1 Selection Rules for Hyper-Raman Scattering

The dipole moment ~ m induced in a molecule placed in an electromagnetic field is $ ~ given by Eq. (5.1). As mentioned previously in Section 5.2, the third term b ~ E E acts as the source of SFG. This SF radiation process may be accompanied by inelastic nonlinear scattering, which can be understood by analogy with Raman scattering. Such an inelastic scattering effect in the second-order NLO effects is called hyperRaman scattering [102]. (Since practical hyper-Raman spectroscopy is usually carried out under the condition of input frequency-degeneracy, that is, SHG.) This is described by a first-order expansion of b in the normal coordinate of vibration Q: b ¼ b0 þ

X

ðqb=qQl Þ0 Ql þ

ð5:19Þ

l

Similarly, the first-order expansion of the m0 and a of Eq. (5.1) is, respectively, responsible for IR absorption and Raman scattering. According to the parity, one can easily understand that selection rules for hyper-Raman scattering are rather similar to those for IR [17, 18]. Moreover, some of the silent modes, which are IR- and Ramaninactive vibrational modes, can be allowed in hyper-Raman scattering because of the nonlinearity. Incidentally, hyper-Raman-active modes and Raman-active modes are mutually exclusive in centrosymmetric molecules. Similar to Raman spectroscopy, hyper-Raman spectroscopy is feasible by visible excitation. Therefore, hyper-Raman spectroscopy can, in principle, be used as an alternative for IR spectroscopy, especially in IR-opaque media such as an aqueous solution [103]. Moreover, its spatial resolution, caused by the diffraction limit, is expected to be much better than IR microscopy. Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 5.6.2 Enhancement of Hyper-Raman Scattering Intensity

The hyper-Raman scattering cross section is extremely small, typically of the order of 1065 cm4 per molecule [24]. Therefore, an enhancement of signal intensity is essential in order to utilize this phenomenon as a practical spectroscopic tool in the field of molecular science. In a similar manner to the enhancement of Raman scattering

5.6 Hyper-Raman Spectroscopy

Figure 5.17 Energy diagrams of various vibrational spectroscopies.

intensities, there are two methods for the signal enhancement in hyper-Raman scattering: resonance hyper-Raman scattering [104, 105] and surface-enhanced hyper-Raman scattering [24]. The former gains intensities from resonances to electronic excited states of molecules. Therefore, this enhancement is practically useful only for centrosymmetric molecules [106]. (In the case of non-centrosymmetric molecules, Raman-active modes are dominantly enhanced via a Frank–Condon mechanism; both two-photon upward and one-photon downward transitions are allowed.) On the other hand, the latter gains intensities from plasmonic resonances at metal surfaces. Therefore, surface-enhanced hyper-Raman spectroscopy is surfacespecific, although the enhancement is restricted by surface selection rules [102]. Figure 5.18 shows resonance hyper-Raman spectrum of fullerene C60 microcrystals [22] using pulsed excitation for which 2w was near-resonant with the dipole-allowed 1T1u–1Ag transition band (the input wavelength of 790 nm and the spectral-width of 14 cm1 FWFM). According to group theory, C60 has 174

Figure 5.18 Resonance hyper-Raman spectrum of C60 microcrystals with theoretically calculated hyper-Raman-active modes (black bars) and Raman-active modes (gray bars).

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internal degrees of freedom with 46 distinct vibrational coordinates having the representation. G vib ¼ 2ag þ 1au þ 3t1g þ 4t1u þ 4t2g þ 5t2u þ 6gg þ 6gu þ 8hg þ 7hu

ð5:20Þ

The numbers of IR- and Raman-active modes are 4 (4t1u) and 10 (2ag þ 8hg), respectively. On the other hand, hyper-Raman-active modes are all of the modes with u symmetry, including the silent modes. Compared with the theoretically calculated result, the expected modes are clearly seen in the spectrum. (The appearance of Raman-active modes is due to magnetic dipole contributions.) In a similar manner to surface-enhanced Raman scattering, surface-enhancement of hyper-Raman scattering is a promising method to study adsorbed molecules on metal surfaces [24]. Based on recent developments in plasmonics, design and fabrication of metal substrates with high enhancement activities is now becoming possible [21]. Combination of the surface enhancement with the electronic resonances would also be helpful for the practical use of hyper-Raman spectroscopy. Development of enhanced hyper-Raman spectroscopy is awaited for the study of solid/liquid interfaces. 5.6.3 Conclusion

The background of hyper-Raman scattering has been described and several enhancement mechanisms presented. Unique features of hyper-Raman scattering have been demonstrated for C60.

5.7 General Conclusion

Here we have described two second-order non-linear spectroscopies, SFG in detail and hyper-Raman scattering briefly. SFG spectroscopy was applied to various solid/liquid interfaces and the following results were obtained. (i) SFG spectra in the OH-stretching region at a Pt/electrolyte solution interface showed two broad peaks at about 3200 and 3400 cm1, corresponding to liquid-like water and ice-like water molecules, respectively, in contrast to the Au thin film/0.05 M H2SO4 solution interface where the spectra were dominated by the peak corresponding to liquid-like water at all potentials, showing that water molecules are more highly oriented at the Pt electrode than at the Au electrode. (ii) SFG intensity in the OH-stretching region at a Pt/electrolyte solution interface exhibited parabolic behavior between 200 and 600 mV with a minimum around 200 mV, which is close to the pzc of a Pt electrode in HClO4 electrolyte solution, and decreased again in the more positive potential region where Pt oxide was formed (more positive than 600 mV). (iii) The population of on-top CO at a Pt electrode/electrolyte interface instantly decreased, accompanied by an increase in multi-bonded CO, due to the transient temperature

References

jump caused by the irradiation of picosecond visible pulses; the initial state was recovered within 100 ps. (iv) The relative intensity of the SFG signal at the PVA/ quartz interface, where friction was low, due to the presence of liquid-like water rather than ice-like water, became higher when the PVA gel was pressed against the quartz surface while no such increase was observed when the PVA gel was contacted to the hydrophobic OTS-modified quartz surface where friction was high, suggesting the important role of water structure for low friction at polymer gel/ solid interfaces. The background of hyper-Raman scattering was described, several enhancement mechanisms were presented, and the unique features of hyper-Raman scattering were demonstrated for C60.

Acknowledgments

This work was partially supported by Grants-in-Aid for Scientific Research (KAKENHI) in the Priority Area of Molecular Nano Dynamics (No. 16072202) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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6 Fourth-Order Coherent Raman Scattering at Buried Interfaces Hiroshi Onishi

6.1 Why Buried Interfaces?

Interfaces between two different media provide a place for conversion of energy and materials. Heterogeneous catalysts and photocatalysts act in vapor or liquid environments. Selective conversion and transport of materials occurs at membranes of biological tissues in water. Electron transport across solid/solid interfaces determines the efficiency of dye-sensitized solar cells or organic electroluminescence devices. There is hence an increasing need to apply molecular science to buried interfaces. However, analyses of the interface surrounded by some medium are not easy. When an interface of interest is exposed to a vacuum, electron-based or ion-based methods are available to determine the chemical composition and molecular structure of the top layers. The charged particles with limited penetration range result in a good vertical resolution. Buried interfaces are beyond the range of penetration. Photons, an alternative class of probe particles, have better ability for penetration. When the linear response to the incident electric field is analyzed, the vertical resolution is limited to the order of the wavelength, which is greater than the thickness of the top layers. Sum-frequency (SF) spectroscopy [1] has been used to achive vertical resolutions much better than the wavelength. Sum-frequency light is generated at an interface irradiated with infrared (IR) and visible light. The probability of sum-frequency generation is governed by a second-order susceptibility c(2) to be zero in any medium with inversion symmetry. The second-order transition is allowed at interfaces where the symmetry breaks. The generation probability, when allowed, is enhanced by the vibrational resonance of the IR light, as shown in Figure 6.1a. The SF light intensity as a function of the IR wavenumber provides a vibrational spectrum at the interface. Vibrational SF spectroscopy is successful in probing interfaces exposed to vapor [2–5]. Infrared light is still sensitive to absorption by condensed media. Access to interfaces buried in liquids is achieved by irradiating the probe light from the side of a weak IR-absorptive material [6–9].

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Figure 6.1 Nonlinear optical responses. (a) Second-order SF generation, the transition probability is enhanced when the IR light is resonant to the transition from the ground state g to a vibrational excited state n. w is the angular frequency of the vibration. (b) Third-order coherent Raman scheme, the vibrational coherence is generated via impulsive stimulated

Raman excitation. WL and WS are the highfrequency and low-frequency components of the pump light pulse. A probe pulse of frequency W interacts with the coherence to present the optical response of the fundamental frequency W þ w W. (c) Fourth-order coherent Raman scattering, the optical response of the second harmonic frequency 2 W þ w 2 W is modulated by the vibrational coherence.

The author has proposed application of fourth-order coherent Raman scatterring to provide IR-free observation of vibrations at buried interfaces. This method was originally developed in 1997 by Chang, Xu and Tom to observe coherent vibrations of the GaAs lattice [10]. The vibrations are coherently pumped with near-infrared light via a stimulated Raman transition instead of the IR resonant transition used in the SF scheme. Another ability of this Raman-based method is to access lowfrequency vibrations. It is difficult to prepare low-frequency IR light compatible with SFgeneration. A specifically modified tabletop laser light source [11] or a free-electron laser facility [12] is required to observe an SF spectrum of wavenumbers below 1000 cm1. Low-frequency vibrations are important in dissipating the excess energy released in endothermic reactions. Lateral and vertical transport of molecules at an interface is initiated by multiple excitation of low-frequency modes, including frustrated rotations and frustrated translations. In the current chapter, the principles of Raman excitation and interface-selective detection of vibrational coherence are described, including applications to air/liquid, liquid/liquid, air/solid interfaces, and an organic submonolayer.

6.2 Optical Transitions

A light pulse of a center frequency W impinges on an interface. Raman-active modes of nuclear motion are coherently excited via impulsive stimulated Raman scattering, when the time width of the pulse is shorter than the period of the vibration. The ultrashort light pulse has a finite frequency width related to the Fourier transformation of the time width, according to the energy–time uncertainty relation.

6.2 Optical Transitions

When the full width at half maximum (fwhm) of a Gaussian pulse is 20 fs, its frequency width is 740 cm1 as the fwhm. Frequency components WL and WS are present in the pulse and are used to generate the vibrational coherence, where WL WS is equal to the vibration frequency w. Another light pulse of frequency W comes at a time delay td and interacts with the vibrationally excited molecules. The intensity of the probe light transmitted through the interface is modulated as a function of the delay. The modulation is Fourier-transformed to provide the frequency and phase of the vibrational coherence. The decay of the coherence is traced with the modulation amplitude. This series of optical transitions shown in Figure 6.1b contains three incident electric fields (WL, WS, and W) and offers bulk-sensitive, time-domain detection of vibrational coherence. The whole portion of the material having interacted with the pump pulse contributes to the generation of the response. This method is known as third-order coherent Raman spectroscopy in the time domain and has been successfully applied to bulk liquids [13], bulk solids [14, 15], and molecular submonolayers [16, 17]. To ensure interface-selective detection of the Raman-pumped vibrational coherence, one more incident electric field is required. A fourth-order optical response is thereby generated. The requirement is fulfilled by observing the second harmonic (SH) light generated at the interface, instead of the transmitted fundamental light. The SH probe scheme contains four incident electric fields (WL, WS, W, and W) as shown in Figure 6.1c. The two-photon transition probing the coherence is equivalent to a hyper-Raman scattering from v to g [18]. The cross section of the pump-and-probe transitions is therefore proportional to the product of the Raman tensor for the pump from g to v, and the hyper-Raman tensor for the probe from v to g. When two or more vibrational modes are excited, the fourth-order response field Efourth is presented as the sum of exponentially decayed modulations, X Efourth ðtd ; 2WÞ / Av cosðwv td þ jv Þexpðtd =Tv Þ ð6:1Þ v

where Av, wv, jv, and Tv are the amplitude, frequency, phase, and dephasing time of each mode. The time-domain response is Fourier-transformed to a frequency spectrum of the fourth-order susceptibility, c(4)(wv). The efficiency of the impulsive stimulated Raman excitation is enhanced when an electronic transition from g to an electronic state e is resonant with the photon energy. In the on-resonance extreme a cosine-like coherence is generated with jv ¼ 0 or p. In the out-of-resonance extreme a sine-like coherence is expected with jv ¼ p/2 or 3 p/2. This presentation is analogous to what has been established in third-order Raman spectroscopy [19]. In addition to the fourth-order response field Efourth, the probe light generates two SH fields of the same frequency 2 W, the pump-free SH field E0(2 W), and the pumpinduced non-modulated SH field Enon(td, 2 W). The ground-state population is reduced by the pump irradiation and the SH field is thereby weakened. The latter term Enon(td, 2 W) is a virtual electric field to represent the weakened SH field. Timeresolved second harmonic generation (TRSHG) has been applied to observe Enon (td, 2 W) with a picosecond time resolution [20–25]. The fourth-order field interferes with the two SH fields to be detected in a heterodyned form.

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The pump-affected intensity of SH light I(td, 2 W) is given by, 2 Iðtd ; 2WÞ E0 ð2WÞ þ Enon ðtd ; 2WÞeiF þ Efourth ðtd ; 2WÞeif ¼ E0 ð2WÞj2 I0 ð2WÞ

ð6:2Þ

with respect to the pump-free intensity I0(2 W). F is the phase shift of Enon(td, 2 W) relative to E0(2 W) and assumed to be zero, since Enon and E0 are generated in a common, second-order transition. f represents the phase shift of Efourth relative to E0(2 W). With Efourth being much smaller than the other terms, Eq. (6.2) is simplified to, Efourth ðtd ; 2WÞEsecond ðtd ; 2WÞ / Iðtd ; 2WÞIsecond ðtd ; 2WÞ

ð6:3Þ

where Esecond(td, 2 W) ¼ E0(2 W) þ Enon(td, 2 W). Isecond(td, 2 W) is the SH light intensity relevant to Esecond and determined with a non-oscillatory numerical function fitted to the observed I(td, 2 W). The right-hand side of Eq. (6.3) represents the intensity modulation of the SH light, Ifourth(td, 2 W), which is the experimetally determined quantity. On the left-hand side of the equation the fourth-order field Efourth is multiplied by Esecond. The intensity modulation is proportional to the amplitude of Efourth and hence to the number of coherently vibrating molecules or atoms. Hirose et al. [26] proposed a homodyne scheme to achieve the background-free detection of the fourth-order field. With pump irradiation in a transient grating configuration, the fourth-order field propagates in a direction different from that of the second-order field because of different phase match conditions. The fourth-order field is homodyned to make Ifourth(td, 2 W) and spatially filtered from the second-order response Isecond(td, 2 W).

6.3 Experimental Scheme

Figure 6.2 illustrates the spectrometer used in our laboratory. A noncollinear optical parametric amplifier (TOPAS-white, Quantronix) is pumped by a Ti:sapphire regenerative amplifier (Hurricane, Spectra Physics, 800 nm, 90 fs, 1 kHz). The time width of the amplified light pulse is less than 20 fs, and the wavelength is tunable at 500–750 nm. A p-polarized pump and p-polarized probe pulses are focused at an interface with an incident angle q of 50 . The spot diameter of the focused beams is 0.1 mm. The p-polarized SH light beam emitted to the reflected direction is conducted to a photomultiplier tube. The multiplier output is gated with a boxcar integrator and sent to a PC on a pulse-to-pulse basis. The pump pulse is chopped at 500 Hz. The pump-on signal and pump-off signal are separately accumulated and the former is divided by the latter. The time origin is determined by monitoring the SH light intensity. A more detailed description is available [27]. The time resolution of the instrument determines the wavenumber-dependent sensitivity of the Fourier-transformed, frequency-domain spectrum. A typical response of our spectrometer is 23 fs, and a Gaussian function having a half width

6.4 Application to a Liquid Surface

Figure 6.2 A fourth-order coherent Raman spectrometer constructed with a Ti:sapphire regenerative amplifier (Ti:sapphire) and noncollinear optical parametric amplifier (NOPA).

at half maximum (hwhm) of 640 cm1 centered at 0 cm1 represents the wavenumberdependent sensitivity. Frequency spectra are presented in the following sections without correcting the sensitivity.

6.4 Application to a Liquid Surface

The fourth-order coherent Raman spectrum of a liquid surface was observed by Fujiyoshi et al. [28]. The same authors later reported a spectrum with an improved signal-to-noise ratio and different angle of incidence [27]. A water solution of oxazine 170 dye was placed in air and irradiated with light pulses. The SH generation at the oxazine solution was extensively studied by Steinhurst and Owrutsky [24]. The pump and probe wavelength was tuned at 630 nm to be resonant with the one-photon electronic transition of the dye. The probability of the Raman transition to generate the vibrational coherence is enhanced by the resonance. The efficiency of SH generation is also enhanced. With the resonance to the electronic transition, the ground-state population is partially depleted by the pump irradiation and restored with the time delay. The raw intensity of SH light was accordingly damped at td ¼ 0 and recovered in picoseconds, as seen in Figure 6.3a. Intensity modulation due to the vibrational coherence was superimposed on the non-modulated evolution as expected from Eq. (6.3). The coherence continued for picoseconds on this solution surface. The non-modulated component Isecond(td, 2 W) was fitted with a multiexponential

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Figure 6.3 Vibrational coherence at a solution surface. (a) The raw SH intensity, (b) the modulated component, and (c) the Fourier-transformed spectrum. The surface was irradiated with p-polarized pump (5 mJ cm2) and p-polarized probe (2.5 mJ cm2) pulses.

function. The modulated component Ifourth(td, 2 W) was determined by subtracting the fitted Isecond(td, 2 W) from the raw intensity. Ifourth(td, 2 W) was multiplied with a window function and then converted to a frequency-domain spectrum via Fourier transformation. The window function determined the wavenumber resolution of the transformed spectrum. Figure 6.3c presents the spectrum transformed with a resolution of 6 cm1 as the fwhm. Negative, symmetrically shaped bands are present at 534, 558, 594, 620, and 683 cm1 in the real part, together with dispersive shaped bands in the imaginary part at the corresponding wavenumbers. The band shapes indicate the phase of the fourth-order field f to be p. Cosine-like coherence was generated in the five vibrational modes by an impulsive stimulated Raman transition resonant to an electronic excitation. The wavenumbers of the observed bands are identical with those of the spontaneous Raman spectrum of the solution and oxazine solid [27]. The impulsive stimulated Raman transition may initiate coherent vibrations in the electronic excited state. However, there was no sign of the excited-state vibrations superimposed on the ground-state bands in the spectrum of Figure 6.3.

6.5 Application to a Liquid/Liquid Interface

A 0.2-mm thick hexadecane layer was placed on the oxazine solution. The vibrational coherence at the hexadecane/solution interface was pump–probed in a similar manner [27]. The light pulses traveled in the hexadecane layer and experienced group velocity dispersion before arriving at the interface. This undesired dispersion

6.6 Applications to Solid Surfaces

Figure 6.4 Vibrational coherence at a liquid/liquid interface. (a) The raw SH intensity, (b) the modulated component, and (c) the Fourier-transformed spectrum. A 0.2-mm thick hexadecane layer was placed on the oxazine solution of Figure 6.3. The interface was irradiated with p-polarized pump (5 mJ cm2) and p-polarized probe (2.5 mJ cm2) pulses of a 630-nm wavelength.

was compensated by adding an extra negative chirp in the noncollinear amplifier. As shown in Figure 6.4, the raw SH intensity and the Fourier-transformed spectrum were identical to those observed at the air/solution interface, being insensitive to the hexadecane overlayer. Vibrations in a chromophore are thought to be sensitive to the solvent structure around the chromophore [6, 7, 23]. This was not the case for this particular chromophore. The spontaneous Raman spectrum of oxazine was observed in the bulk solution and bulk solid. Dye monomers are equilibrated with dimerized species in the solution [24]. The solvated monomer, solvated dimer, and solid dye are in different dielectric environments. The observed Raman bands are, nevertheless, identical with the two spectra shown in Figures 6.3 and 6.4. It is, hence, not surprising that the oxazine vibrations are insensitive to the interface composition.


A number of solid compounds have been examined with this time-domain method since the first report of coherent phonons in GaAs [10]. Coherent phonons were created at the metal/semiconductor interface of a GaP photodiode [29] and stacked GaInP/GaAs/GaInP layers [30]. Cesium-deposited [31–33] and potassiumdeposited [34] Pt surfaces were extensively studied. Manipulation of vibrational coherence was further demonstrated on Cs/Pt using pump pulse trains [35–37]. Magnetic properties were studied on Gd films [38, 39].

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Vibrational coherence at a TiO2 surface covered with different organic molecules is described in this section. Titanium oxide is a wide-bandgap semiconductor. When buried in some medium a wide range of applications are known including photocatalysts, solar cells, hydrophilic coatings. The (1 1 0) plane of rutile polymorph is the most extensively studied single-crystalline surface of metal oxide [40]. Atomically flat surfaces of (1 1 0) orientation are routinely prepared and modified with chemisorbed formate [41], acetate [42], benzoate [43], fluorescein [44], N3 dye [45, 46] and so on. The (1 1 0) surface with and without adsorbates is characterized with various techniques including probe microscopes [47]. The vacuum-prepared surface may be contaminated when taken out of the vacuum chamber and exposed to laboratory air. White et al. [48] found that a trimethylacetate (TMA) monolayer with hydrophobic tert-butyl groups exposed to the environment passivates the surface in air. The vibrational coherence at the TMA-covered surface was observed in air with pump and probe pulses of a 630-nm wavelength [49]. This red light is out of resonance with the bandgap excitation of TiO2 (3 eV). The coherent response presented in panels Figure 6.5a and b decayed much more rapidly than the modulation at the solution interfaces described in the preceding sections. Four major bands are recognized at 180, 357, 444, and 826 cm1 in the Fourier-transformed spectrum of Figure 6.5c. The wavenumbers of the first, third, and fourth bands agreed with those of Raman-active bulk phonon bands. In the bulk rutile having D4h symmetry a Raman-active mode is inactive for hyper-Raman transitions [18]. Hence, the fourth-order transition of the three bands is forbidden in the

Figure 6.5 Vibrational coherence at a TiO2(110) surface covered with TMA monolayer. (a) The raw SH intensity, (b) the modulated component, and (c) the Fourier-transformed spectrum. The TMA-covered surface was irradiated in air with p-polarized pump (14 mJ cm2) and p-polarized probe (6 mJ cm2) pulses.


bulk and should occur at the surface. The shape of the three bands is negative and symmetric in the imaginary part, while dispersive in the real part. The phase of the fourth-order field f relative to Esecond is 3p/2. The most intense 826-cm1 band is broader than the other bands. The broadened band suggests a frequency distribution in the observed portion of the surface. Indeed, the symmetric peak in the imaginary part of the spectrum is fitted with a Gaussian function rather than with a Lorenz function. The bandwidth was estimated to be 56 cm1 by considering the instrumental resolution, 15 cm1 in this particular spectrum. This number is larger than the intrinsic bandwidth of the bulk modes [50]. Lanz and Corn [51] proposed a 20-nm thick space charge layer on the TiO2 surface. When the fourth-order response with our TMA-covered surface is generated in the space charge layer, the broad width of the 826-cm1 band is understood as a depthdependent wavenumber of the lattice vibration. Bulk phonon modes are absent in wave numbers near 357 cm1, the center-frequency of the second band. According to electron energy loss studies done in a vacuum [52, 53], TMA-free TiO2(110) surfaces exhibit surface optical phonons at 370–353 cm1. The 357-cm1 band is related to the surface optical phonons. The four bands observed on the TMA-covered TiO2 surface are lattice vibrations of the substrate. Molecular vibration of TMA was not observed, probably because TMA has no resonance with the fundamental wavelength (630 nm) and SH wavelength (315 nm) of the pump and probe pulses. By putting p-nitrobenzoate (NO2C6H4COO, pNB) on the TiO2 surface, vibrations of molecular adsorbates were first observed on a solid [54]. pNB has more Raman-active vibrational freedoms than TMA. A TMA-covered TiO2(1 1 0) surface was immersed in dichloromethane containing p-nitrobenzoic acid. The acid in the solution is expected to be dissociatively adsorbed as pNB by exchanging the preadsorbed TMA. Similar exchange reactions are known in formate–acetate exchange [55], TMA–retinoate exchange [56], TMA–fluorescein exchange [44], and TMA–N3 exchange [45]. The immersed surface was characterized with XPS and a number density of 0.35 nitrogen atom nm2 was determined. As shown in Figure 6.6, the raw SH intensity and modulated component were almost identical to what was observed on the TMA-covered surface. A slight bump appeared at 570 cm1 in addition to the four major bands of lattice vibrations. The Forurier-transformed spectrum was simulated using the sum of Lorentzian functions to properly identify the weak band. As a result, five bands are definitely identified at 825, 572, 439, 363, 180 cm1. Corrugations at 100 cm1 or below are artifacts caused by fitting residue. There is no corresponding bulk and surface phonon mode of TiO2 in this wavenumber region. The 572 cm1 band should have originated from adsorption of pNB on TiO2. Assignment of the band is not easy, unfortunately. According to a normal mode analysis [57] skeletal vibrations are predicted at 550–500 cm1 and related to the vibrations observed in surface-enhanced Raman scattering of p-nitrobenzoic acid adsorbed on metal surfaces [58]. On the other hand, photoinduced coupling of p-nitrobenzoic acid to 4,40 -azodibenzoate

j111


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Figure 6.6 Vibrational coherence on a pNB-adsorbed TiO2(110) surface. (a) The raw SH intensity, (b) the modulated component, (c) the Fourier-transformed spectrum, the gray lines show the transformed spectrum. The spectrum simulated with Lorentzian functions is overlaid with broken lines. The pNB-adsorbed surface was irradiated in air with p-polarized pump (8 mJ cm2) and p-polarized probe (8 mJ cm2) pulses of a 550-nm wavelength.

(OOCC6H4N¼NC6H4COO) was found on metal surfaces. The coupled product also presents vibrations at around 550 cm1 [58].

6.7 Frequency Domain Detection

One latest technical development is the direct observation of c(4) in the frequency domain. Yamaguchi and Tahara [59, 60] irradiated a Rhodamine solution surface with a white light continuum pulse and a narrow-bandwidth picosecond pulse. Vibrational coherence having different frequencies is generated in a stimulated Raman transition caused by the continuum and the narrow-bandwidth light. The energy distribution of the vibrational excited state v is projected on the narrowbandwidth SH light by the hyper-Raman transition from v to g. The frequencydomain spectrum of the coherence is observed by using a polychrometer with a multichannel CCD. On the solution surface intra-molecular bands at 1200, 1350, 1500, 1650, and 2220 cm1 were observed. In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of TiO2 at 826 cm1. Direct observation of a frequency-domain spectrum is free from the high-wavenumber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses.

References


Successful applications of fourth-order coherent Raman scattering are presented. Interface-selective detection of Raman-active vibrations is now definitely possible at buried interfaces. It can be recognized as a Raman spectroscopy with interface selectivity. Vibrational sum-frequency spectroscopy provides an interface-selective IR spectroscopy in which the vibrational coherence is created in the IR resonant transition. The two interface-selective methods are complementary, as has been experienced with Raman and IR spectroscopy in the bulk. On the other hand, we cannot ignore drawbacks in observing fourth-order responses. The desired response is always weak due to the high optical order. The damage threshold of the interface to be analyzed is severe with intense irradiation. The difficulty has been overridden by one-photon resonant enhancement of Ramanpump efficiency. The observable range of materials is somewhat limited as a result. There is still much room for technical improvements and the author is optimistic for the future.

Acknowledgments

The work reviewed in this chapter has been carried out in collaboration with Satoru Fujiyoshi, Tomonori Nomoto, and Taka-aki Ishibashi. We started this series of studies in Kanagawa Academy of Science and Technology and continued in Kobe University with support from Core Research for Evolutional Science and Technology of the Japan Science and Technology Agency, and from a Grant-in-Aid for Scientific Research in Priority Area Molecular Nano Dynamics.

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7 Dynamic Analysis Using Photon Force Measurement Hideki Fujiwara and Keiji Sasaki

7.1 Introduction 7.1.1 Weak Force Measurements

Micro- and nano-particles dispersed in a liquid experience various forces [1–5]. Although gravity, viscous drag, Van der Waals interactions, surface double layer forces, and Brownian motion exerted on a single particle or between particles and those between a particle and a solid surface are only of the order of femto-newtons in strength, these forces play an important role in the adhesion mechanism. Shortrange forces due to hydrophobic and hydrophilic interactions, hydration and solvation energy, and hydrogen bonding networks sometimes govern the behavior of microparticles in solution. In addition, depending on local environmental conditions (temperature, concentration gradient of gases, solvents, and molecules) around the single particles, the chemical and physical characteristics of particles and cells are also changed due to the changes in molecular conformation and reaction yields. Thus, various phenomena (aggregation, adhesion, sedimentation, chemical reactions, and transition processes) of organic, metallic, semiconductor colloidal particles, surfactant micelles, macromolecules, and so on, should be strongly affected by the strength of the balances of those forces and environmental conditions around the nanoparticles [1]. To measure the strength of the forces exerted on particles, various analytical techniques have been developed [6, 7]. Unfortunately, since most of these techniques are based on hydrodynamics, assumption of the potential profiles is required and the viscosities of the fluid and the particle sizes must be precisely determined in separate experiments, for example, using the viscous flow technique [8, 9] and power spectrum analysis of position fluctuation [10]. Furthermore, these methods provide information on ensemble averages for a mass of many particles. The sizes, shapes, and physical and chemical properties of individual particles may be different from each other, which will result in a variety of force strengths. Thus, single-particle

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measurement is indispensable for the analysis of van der Waals interactions and electric double layer forces and for observation of ionic dissociation and adsorption processes. Recently, a technique for measuring the adhesion force between single particles and a solid surface has been developed by bringing a single particle attached to the cantilever of an atomic force microscope (AFM) close to the solid surface and by monitoring the displacement of the cantilever [11]. However, it is technically difficult to attach the single particle with a sub-micrometer diameter to the cantilever and to eliminate the influence of the cantilever on the physical and chemical characteristics of a sample. Furthermore, since the detectable force using the AFM cantilever is of the order of pico-newtons, the sensitivity of the AFM is insufficient for the measurements of very weak forces of the order of femto-newtons. 7.1.2 Potential Analysis Method Using Photon Force Measurement

For this purpose, we have proposed a method that makes it possible to precisely and instantaneously observe arbitrary profiles of three-dimensional potentials exerted on a single microparticle [12]. This method utilizes the photon force as an optical spring to characterize the physical and chemical forces exerted on single particles [13]. The force measurement technique we used is based on thermodynamic analyses of the Brownian motion, evaluated with nanometer position sensing [14]. Neither the assumption of potential profiles nor knowledge of media viscosities or particle diameters is necessary. The only physical parameter that needs to be known is the temperature of the sample [15]. The principle is shown in Figure 7.1. When a microparticle is irradiated with a focused laser beam, the particle can be threedimensionally trapped at the focal spot [16, 17] and fluctuate within the trapping potential well due to the Brownian motion [14]. In the method, this fluctuation of particle position is sequentially measured for a sufficiently long time by a position sensing system and a histogram is obtained as a function of the three-dimensional position. This histogram gives a probability density function of the particle position. Since the particle motion is caused by thermal energy, the three-dimensional potential profile can be determined from the position histogram by a simple logarithmic transformation of the Boltzmann distribution. By measuring the

Figure 7.1 The principle of thermodynamic analysis for the measurement of three-dimensional potentials exerted on a single microparticle.

7.1 Introduction

Figure 7.2 A schematic diagram of nanometer position sensing. Light from the evanescent field scattered by the microparticle is measured with a quadrant photodiode detector, whose differential outputs correspond to the x and y displacements and the total intensity depends exponentially on the distance z between the particle and the glass plate.

potentials with and without the additional weak force acting on the particle, the subtraction of these potentials gives the potential of the weak force, resulting in the determination of the strength of the additional weak force. The fluctuation of the particle position is measured by the nanometer position sensing system based on total internal reflection microscopy [12, 14, 18] and quadrant photodiode detection. A schematic diagram of the system is shown in Figure 7.2. A microparticle dispersed in a liquid is optically trapped by a focused near-infrared trapping laser beam under an optical microscope. The particle is positioned in the close vicinity of the surface of a microscope glass slide. A weak S-polarized illumination laser beam is introduced into a prism, which is optically coupled to the glass slide through matching oil, with the incident angle on the glass slide surface larger than the critical angle of the total internal reflection condition. Thus the laser beam induces an evanescent field, which illuminates the trapped particle. The scattered light from the microparticle is collected by the objective lens, passed through an interference filter, and then the particle position is detected by a quadrant photodiode (QPD). Using this photon force measurement technique, radiation pressure induced by a focused laser beam and an evanescent field [12, 14, 19, 20] was investigated for polymer latexes and metallic particles. Electrostatic forces of charged particles in

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solutions have also been precisely observed using this method [21]. This photon force measurement can be extended to the analysis of radiation pressure exerted on a single absorbing particle and to the estimation of the imaginary part of the refractive index [22]. Furthermore, using this technique, we have clarified that the trapped particle position in the vicinity of an interface is discretely shifted by the interference effect of the incident trapping laser beam and reflected beams from a substrate and the focal point of the trapping laser beam [23–25]. Thus, by using the photon force measurement technique, we have succeeded in analyzing the potential profiles and precisely evaluating femto-newton order forces acting on single particles. Since this technique measures the thermal fluctuation of single particles and analyzes the potentials acting on the particle on the basis of the thermodynamic analysis, the only required physical parameter for the highly precise weak force measurement is the temperature of the sample and a priori information, such as medium viscosity, particulate mass and size. However, although various weak force measurements using the potential analysis method have been reported, these measurements have been mainly of the forces acting on single particles, not those between two particles. It is well known that various forces, such as the Van der Waals force, the electric double layer effect, the electrostatic force, the hydrodynamic interaction force, and so on, are also exerted between two adjacent particles dispersed in liquid [1]. Therefore, the weak force acting between particles should also be of importance for a wide range of fields, such as colloidal science, biology, medical science, and so on. For the measurement of the interaction forces acting between two particles, the interaction force has been mainly investigated by use of a cross-correlation analysis of the temporal position fluctuations of two particles [26, 27]. This method typically requires a priori knowledge such as particle size, mass, and media viscosity, and so on, in order to evaluate the interaction. Since the photon force measurement technique we used is based on the thermodynamic analysis of the Brownian motion evaluated with the nanometer position sensing system, the only physical parameter needed is the temperature of the sample; neither a priori knowledge of media viscosity and particle size nor the assumption of potential profiles is necessary [14]. However, in order to investigate the interaction force by the use of potential analysis, it is necessary to know the effect of the interaction forces on the potentials and how the trapping or kinetic potential profiles are changed by the interaction. This is still unclear. In this chapter, we introduce a two-beam photon-force measurement system using a coaxial illumination for sensing the particle position as a new technique for dynamically analyzing the interaction forces exerted between two adjacent objects. By utilizing this potential analysis method for the interaction forces, especially, we focused on the hydrodynamic interaction force between adjacent trapped particles and the potentials at different distances between two particles were measured. Then, we discussed the validity of the proposed method, comparing with the typically used correlation method and/or the theoretical calculation. From the results, we found that the profiles of the two-dimensional plots of the trapping or kinetic potentials of two particles, were collapsed on decreasing the distance between two particles, indicating the increase in the interaction force between two particles.

7.2 Measurement of the Hydrodynamic Interaction Force Acting between Two Trapped Particles

From the changes in the obtained potential profiles, we evaluated the interaction coefficient b and, compared it with the theoretical calculation of b, we also confirmed that the proposed method could evaluate the interaction force without any fitting parameters, which have usually been required in the typical correlation method.

7.2 Measurement of the Hydrodynamic Interaction Force Acting between Two Trapped Particles Using the Potential Analysis Method 7.2.1 Two-Beam Photon Force Measurement System

Figure 7.3 shows the two-beam photon-force measurement system using a coaxial illumination photon force measurement system. Two microparticles dispersed in a liquid are optically trapped by two focused near-infrared beams (1 mm spot size) of a CW Nd:YAG laser under an optical microscope (1064 nm, 1.2 MW cm2, 100X oilimmersion objective, NA ¼ 1.4). The particles are positioned sufficiently far from the surface of a glass slide in order to neglect the interaction between the particles and the substrate. Green and red beams from a green LD laser (532 nm, 21 kW cm2) and a He–Ne laser (632.8 nm, 21 kW cm2) are introduced coaxially into the microscope and slightly focused onto each microparticle as an illumination light (the irradiated area was about 3 mm in diameter). The sizes of the illumination areas for the green and red beams are almost the same as the diameter of the microparticles (see Figure 7.4). The back scattered light from the surface of each microparticle is

Figure 7.3 A schematic of a two-beam photonforce measurement system. Obj: objective lens (100 oil immersion, N.A. 1.4), PBS: polarization beam splitter, F1: color filter for eliminating red illumination laser beam, F2: color filter for eliminating green illumination laser

beam DM: dichroic mirror, QPD: quadrant photo diode. Two polystyrene latex particles dispersed in water were trapped by a focused CW Nd:YAG laser beam. Two particles were illuminated by red and green lasers, respectively.

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Figure 7.4 A microscope image of two trapped microparticles in water, illuminated by a green and red laser beam, respectively. The diameter of the particles is 3 mm.

collected by the same objective lens. Then, the green and red scattered lights are divided by a dichroic mirror and imaged on QPDs set in the individual paths of the green and red scattered lights. The sum of the four output signals (z-signal) and two differential outputs normalized by the sum (x- and y-signals) from each QPD are transferred, with analog-to-digital conversion, to a computer for data acquisition and analysis of each position of the two trapped particles. Calibrations of the x- and y- position were performed by analyzing the variation of long-time-averaged signals while changing the focal position of the trapping laser beam. The sampling rate for the output signals of the QPDs was set to be 10 kHz, that is, shorter than the mechanical response time of the particle. The fluctuations of the particle positions are recorded sequentially for a sufficiently long time (60 s) by position sensing systems in each path. In the experiments, polystyrene particles (diameter 3 mm) dispersed in water were used as a sample. On changing the distance between the surfaces of two trapped particles by manipulating the focused positions of the trapping laser beams (from 1 to 20 mm), we measured the position fluctuations of each particle in the x- and y-directions. 7.2.2 Potential Analysis Method for Hydrodynamic Force Measurement

For the analysis, we developed a new method that makes it possible to observe correlated potentials between two trapped particles. The principle is shown in Figure 7.5. From the recorded position fluctuations of individual particles (indicated by the subscripts 1 and 2), histograms are obtained as a function of the threedimensional position. Since the particle motion is caused by thermal energy, the three-dimensional potential profile can be determined from the position histogram by a simple logarithmic transformation of the Boltzmann distribution. Similarly, the

7.2 Measurement of the Hydrodynamic Interaction Force Acting between Two Trapped Particles

Figure 7.5 The principle of thermodynamic analysis for measuring trapping or kinetic potentials exerted between two trapped particles.

velocity of each particle motion was also derived from the differentiation of the recorded position fluctuations, resulting in the kinetic potentials. In order to discuss the correlation between the position fluctuations, the potentials are two-dimensionally plotted against the positions or velocities of individual particles (indicated as x1 and x2 or v1 and v2). From the Langevin equations, assuming that the mass and diameter of the particles and the irradiation power of the trapping laser beams were the same, the motions of each trapped particle can be given by [28] ð dv1j m þ gv1j þ kj v1j dt bj v2j ¼ n1j ð7:1Þ dt and m

ð dv2j þ gv2j þ kj v2j dt bj v1j ¼ n2j dt

ð7:2Þ

ð dðv1j v2j Þ þ ðg þ bj Þðv1j v2j Þ þ kj ðv1j v2j Þdt ¼ n1j n2j dt

ð7:4Þ

Ð where j ¼ x, y, and z. In the equations, m, gvij ; kj vij dt; bj vij , and nij (i ¼ 1, 2) indicate the particle mass, the viscosity, the restoring force of the optical trap, the interaction force, and the random force in the j-direction ( j ¼ x, y, z), respectively. Note that bj is the interaction coefficient and the function of the distance between two particles. From the sum and difference of these equations, we can derive the following equations, ð dðv1j þ v2j Þ ð7:3Þ m þ ðg bj Þðv1j þ v2j Þ þ kj ðv1j þ v2j Þdt ¼ n1j þ n2j dt and m

Since the histogram gives a probability density function of the particle position, the correlation in the velocities v1j and v2j in the j-direction causes the change in the shape of the histogram plotted against v1j and v2j, due to the different coefficient g bj in

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the (v1j þ v2j) direction and g þ bj in the (v1j v2j) direction in the second terms of the Eqs. (7.4) and (7.5). If the interaction becomes large on decreasing the distance between the particles, the shape of the histogram is squeezed in the (v1j v2j) direction and extended in the (v1j þ v2j) direction, depending on the strength of the interaction. However, if the interaction can be neglected (b j ¼ 0), as the coefficients in the second terms in Eqs. (7.4) and (7.5) become the same, the histogram shape does not change in the (v1j þ v2j) and (v1j v2j) directions. For example, let consider the case in the direction between the central axes of the two trapped particles, which is indexed as v1x and v2x. The correlation of the kinetic potential in the vx-directions changes depending on the coefficients of g bx in the (v1x þ v2x) direction and g þ bx in (v1x v2x) direction. Thus, as the interaction coefficient bx changes with the distance between the two particles, the shape of the kinetic potential plotted against v1x and v2x should change in the (v1x þ v2x) and (v1x v2x) directions. 7.2.3 Trapping Potential Analysis

Figure 7.6 shows the two-dimensional plots of trapping potentials in the x1- and x2-directions, when the distances between adjacent particles were (a) 1, (b) 5, and (c) 20 mm. In the figure, the vertical and horizontal axes indicate the x positions of the particles 1 and 2, respectively, and the depth of the trapping potential is presented by the gray-scale image. From the results, although the collapse of the potential shape should appear with decreasing distance (increasing interaction), the observed potentials were round-shaped and it was difficult to identify the change in the shape. We considered that this is due to the fluctuation of the system, such as fluctuation in the trapping laser and noise of electronic circuits, and the change in the trapping potential would be hidden in the noise. We also calculated the cross-correlations of the position fluctuations in the x1- and x2-directions at each distance, which method has been utilized for analyzing the

Figure 7.6 Two-dimensional plots of x-directional trapping potentials. The particles were trapped in water by two focused laser beams with a power of 10 mW. The distances between the surfaces of the two trapped particles were (a) 1, (b) 5, and (c) 20 mm.

7.3 Kinetic Potential Analysis

Figure 7.7 Cross-correlations of position fluctuations in the x1- and x2-directions. The distances between the surfaces of the two trapped particles were (a) 1, (b) 3, (c) 5, and (d) 20 mm.

interaction. Figure 7.7 shows the results when the distances were (a) 1, (b) 3, (c) 5, and (d) 20 mm. From the results, we can surely confirm the typical cross-correlation curves and found the correlation dip around 20 ms. On decreasing the distance, the correlation dip becomes deeper and this behavior coincided well with the experimental data on the hydrodynamic force in ref. [26, 27]. From the correlation data, the observed position fluctuation includes the influence of the interaction, however, it is difficult to confirm the influence of the interaction from the potential analysis. We considered that since the position fluctuation induced by the interaction was smaller than the amplitude of the system noise and was buried in the noise, the collapse of the trapping potential could not be clearly observed.


On the other hand, in order to observe the influence of the interaction, we also measured the kinetic potentials, which are plotted against the velocities of the particles 1 and 2, when changing the distance from 1 to 20 mm. Since the velocities of the x- and y-directions were obtained by the derivation of the position fluctuations of each particle, the low-frequency components were suppressed and the interaction of two particles would be clearly obtained rather than the position fluctuation. Surely, from the cross-correlation of the velocities v1x and v2x (Figure 7.8), we can observe much clearer correlation peaks with the change in the distance from 1 to 20 mm, in which the cross-correlation of the derivation of the position should be identical to the second-order derivation of the position cross-correlation. From the results, different from the position cross-correlations, the system noise components were eliminated from the results and the correlation peak was increased with decreasing distance. Thus, we were able to observe the interaction clearly.

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Figure 7.8 Cross-correlations of velocity fluctuations in the x1- and x2-directions. The distances between the surfaces of the two trapped particles were (a) 1, (b) 3, (c) 5, and (d) 20 mm.

Figure 7.9 shows the two-dimensional plots of kinetic potentials in the v1x- and v2xdirections, when the distances between adjacent particles were (a) 1, (b) 5, and (c) 20 mm. In the figure, the vertical and horizontal axes indicate the velocity of the particles 1 and 2 in the x-direction, and the depth of the kinetic potential is shown by the gray-scale image. From the results, when the distance was sufficiently large (see Figure 7.9c), the profile was almost round-shaped and the profiles in the

Figure 7.9 Two-dimensional plots of x-directional kinetic potentials exerted on two trapped particles. The particle was trapped in water by two focused laser beams with a power of 10 mW. The distances between the surfaces of the two trapped particles were (a) 1, (b) 5, and (c) 20 mm.


Figure 7.10 Cross-section of the kinetic potentials at the dashed lines in Figure 7.9. The distances between the surfaces of two trapped particles were (a) 1, (b) 5, and (c) 20 mm. Solid and dashed lines indicate the cross-section of the (v1x v2x)- and (v1x þ v2x)-directions, respectively.

(v1x þ v2x)- and (v1x v2x)-directions (dashed lines in the figure) almost coincided with each other, although the potential profile was slightly distorted. However, when the distance became shorter (see Figure 7.9a and b), we found that the profile was squeezed in the (v1x v2x)-direction and extended in the (v1x þ v2x)-direction. In order to confirm the collapse of the potential profiles, Figure 7.10 shows the cross-section of the kinetic potentials in the (v1x v2x)- and (v1x þ v2x)-directions, when the distances were (a) 1, (b) 5, and (c) 20 mm. In the figure, black and dashed lines indicate the cross-sections in the (v1x v2x)- and (v1x þ v2x)-directions, respectively. The profiles of the kinetic potentials collapsed in the (v1x v2x)-direction on decreasing the distance. Since the collapse was caused by the increase in the influence of the interaction via the coefficient b in the x-direction, which should be the function of the distance, we estimated the value of bx/g at each distance by fitting the curves with hyperbolic functions (av2) and taking the ratio of a(v1xv2x) and a(v1x þ v2x). Since the ratio of a(v1xv2x) and a(v1x þ v2x) should be proportional to the ratio of the coefficients in the second term in Eqs. (7.3) and (7.4), bx/g can be estimated from the results as 0.304, 0.177, and 0.038 at the distances of (a) 1, (b) 5, and (c) 20 mm, respectively. Thus, we confirmed that the interaction coefficient was changed with the change in the distance. Similarly, we measured the kinetic potentials in the vy-direction, which was perpendicular to the direction between the central axes of the two trapped particles. Since the hydrodynamic force exerted similarly on the y-direction as on the x-direction, we expected similar results to Figures 7.9 and 7.10. Figure 7.11 shows the kinetic potentials in the v1y- and v2y-directions, when the distances were (a) 1, (b) 5, and (c) 20 mm. Comparing Figure 7.11b and c at the distances of 5 and 20 mm, the clear difference was not found in the (v1y v2y)- and (v1y þ v2y)-directions. However, at a distance of 1 mm (Figure 7.11a), the potential profile was modified and the changes in the (v1y v2y)- and (v1y þ v2y)-directions were observed. From the plots of the cross-sections in Figure 7.12, the interaction coefficients by/g in the vy-direction were estimated to be 0.207, 0.037, and 0.004, when the distances were (a) 1, (b) 5, and (c) 20 mm, respectively. From the results, the tendency was similar to the case of the vx-direction, but the strength of the interaction was smaller.

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Figure 7.11 Two-dimensional plots of y-directional kinetic potentials exerted on two trapped particles. The particle was trapped in water by two focused laser beams with a power of 10 mW. The distances between the surfaces of the two trapped particles were (a) 1, (b) 5, and (c) 20 mm.

From the theoretical calculations of the hydrodynamic force, we can estimate the distance dependences of the interaction coefficients in the x- and y-directions. According to ref. [29], the interaction coefficients bx/g and by/g are calculated from the following approximated equations, assuming that the two particles are spherical in shape with the same diameter. 3 k þ 19 8 k 1 þ 94 k2

ð7:5Þ

3 3 k þ 59 64 k by =g ¼ 4 9 2 1 þ 16 k

ð7:6Þ

3

bx =g ¼ 2 and

where

k¼

a d þ 2a

Figure 7.12 Cross-section of the kinetic potentials at the dashed lines in Figure 7.11. The distances between the surfaces of the two trapped particles were (a) 1, (b) 5, and (c) 20 mm. Solid and dashed lines indicate the cross-section of the (v1x v2x)- and (v1x þ v2x)-directions, respectively.

ð7:7Þ

7.4 Summary

Figure 7.13 Distance dependence of b/g. Solid and dashed lines indicate the theoretical calculation of b/g in the x- and y-directions. Triangles and circles indicate the experimentally obtained data for b/g in the x- and y-directions.

a and d indicate the radius of the trapped particle and the distance between the surfaces of the two trapped particles. Figure 7.13 shows the plots of the calculated results from Eqs. (7.5) and (7.6) against the distance d, in which the solid and dashed lines indicate the calculation of bx/g and by/g, respectively. In addition, the experimentally obtained data of bx/g and by/g were also plotted in the same figure as triangles and circles. From the results, the absolute values were about half the values obtained from the calculation. We consider one possible origin of the difference to be that the electronic noise from the electronic circuits with high-frequency components, which could not be eliminated by the derivation procedure, distorted the potential profiles, resulting in the poor estimation of b/g. However, we would like to emphasize here that the tendencies of the distance dependence of both experimentally obtained bx/g and by/g coincided well with the calculated results and the data were obtained only by measuring the potentials with no fitting parameters.

7.4 Summary

We introduced the technique for measuring the weak interaction forces acting between two particles using the photon force measurement method. Compared with the previous typically used methods, such as cross-correlation analysis, this technique makes it possible to evaluate the interaction forces without a priori information, such as media viscosity, particle mass and size. In this chapter, we focused especially on the hydrodynamic force as the interaction between particles and measured the interaction force by the potential analysis method when changing the distance between particles. As a result, when the particles were close to each other, the two-dimensional plots of the kinetic potentials for each particle were distorted in the diagonal direction due to the increase in the interaction force. From the results, we evaluated the interaction coefficients and confirmed that the dependence of the

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obtained values on the distance coincided well with the theoretical calculations and the experimental results from the cross-correlation analysis method. Thus, we have developed the two-beam photon-force measurement system and the potential analysis method for evaluating the weak interaction force acting between two trapped particles using the photon force measurement technique. Although in the experiments introduced here we used micrometer-sized particles, if the scattered light from particles is sufficiently intense to observe with a quadrant photodiode, it can be applicable to particles of any size and shape (metal nanoparticles, semiconductor powders, swelled micelles, organic nanocrystals, etc.).

Acknowledgments

This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Area from MEXT.

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the ray optics regime. Biophys. J., 61, 569–582. Ashkin, A., Dziedzic, J. M., Bjorkholm, J. E. and Chu, S. (1986) Observation of a single-beam gradient force optical trap for dielectric particles. Opt. Lett., 11, 288–290. Prieve, D. C. (1986) Hydrodynamic measurement of double-layer repulsion between colloidal particle and flat plate. Science, 231, 1269–1270. Wada, K., Sasaki, K. and Masuhara, H. (2000) Optical measurement of interaction potentials between a single microparticle and an evanescent field. Appl. Phys. Lett., 76, 2815–2817. Sasaki, K., Hotta, J., Wada, K. and Masuhara, H. (2000) Analysis of radiation pressure exerted on a metallic particle within an evanescent field. Opt. Lett., 25, 1385–1387. Wada, K., Sasaki, K. and Masuhara, H. (2002) Electric charge measurement on a single microparticle using thermodynamic analysis of electrostatic forces. Appl. Phys. Lett., 81, 1768–1770. Matsuo, Y., Takasaki, H., Hotta, J. and Sasaki, K. (2001) Absorption analysis of a single microparticle by optical force measurement.J. Appl. Phys., 89,5438–5441. Inami, W. and Kawata, Y. (2001) Analysis of the scattered light distribution of a

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tightly focused laser beam by a particle near a substrate. J. Appl. Phys., 89, 5876–5880. Fujiwara, H., Takasaki, H., Hotta, J. and Sasaki, K. (2004) Observation of the discrete transition of optically trapped particle position in the vicinity of an interface. Appl. Phys. Lett., 84, 13–15. Hotta, J., Takasaki, H., Fujiwara, H. and Sasaki, K. (2002) Precise analysis of optically trapped particle position and interaction forces in the vicinity of an interface. Int. J. Nanosci., 1, 645–649. Meiners, J.-C. and Quake, S. R. (1999) Direct measurement of hydrodynamic cross correlations between two particles in an external potential. Phys. Rev. Lett., 82, 2211–2214. Bartlett, P., Henderson, S. I. and Mitchell, S. J. (2001) Measurement of the hydrodynamic forces between two polymer-coated spheres. Philos. Trans. R. Soc. London, Ser.A, 359, 883–895. Doi, M. and Edwards, S. F. (1986) The Theory of Polymer Dynamics, Clarendon Press, Oxford, Chapter 3. Happel, J. and Brennner, H. (1983) Low Reynolds Number Hydrodynamics with Special Applications to Particulate Media, Martinus Nijhoff Publishers, The Hague, Chapter 6.

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8 Construction of Micro-Spectroscopic Systems and their Application to the Detection of Molecular Dynamics in a Small Domain Syoji Ito, Hirohisa Matsuda, Takashi Sugiyama, Naoki Toitani, Yutaka Nagasawa, and Hiroshi Miyasaka

8.1 Introduction

The elucidation of molecular dynamics in a small domain, especially intermolecular interaction via rotational/translational diffusion, is of significant importance for a rational understanding of cooperative/hierarchical phenomena of molecular systems, such as crystallization from the solution phase, crosslinking of polymers, and segregation processes. In order to comprehensively elucidate the dynamics in these complex phenomena taking place in a small domain, an apparatus with spatial and temporal resolution is indispensable. In this chapter, we will introduce several microscopic systems that have been developed by the combination of a confocal optical set-up and appropriate laser light sources. First, we will show a laser microscope with 35 fs excitation pulse under objectives and its application to higher order multiphoton excitation and fluorescence imaging. Then we will introduce the measurement of temperature in micro-space under a laser trapping condition. In this measurement, we have utilized a fluorescence correlation technique. In the third section, we will discuss the single-particle-level relaxation dynamics of quantum dots diffusing freely in water, as revealed by fluorescence correlation detection under the microscope.

8.2 Development of a Near-Infrared 35 fs Laser Microscope and its Application to Higher Order Multiphoton Excitation

Spatially resolved measurements, based on the confocal laser microscope and related techniques, have recently enabled direct detection of individual molecules, single nanoparticles, and molecular assemblies, leading to elucidation of the heterogeneous nature of these systems and its dependence on the individual environments.

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In addition, combining the microscope with the use of a pulsed laser light source provides temporal information on these systems in a small domain. The dispersion of refractive index, however, strongly affects the temporal resolution in the measurements of dynamics under the microscope and typical resolution stays around 100 fs when a Ti:Sapphire laser is used as an excitation source. The dispersion of the refractive indices is small in the near-infrared (NIR) region >1 mm, compared to that in the visible region. Hence, we can expect ultrashort pulse duration attainable under the microscope in the NIR region. The ultrashort NIR pulse can also provide several advantages: (i) the low probability of light scattering leading to deeper penetration depth than that of visible light can improve imaging capability for opaque materials, such as biological specimens; (ii) the NIR pulse causes less damage to samples; and (iii) the quite high photon intensity in the short pulse duration can easily induce higher order multiphoton processes, leading to higher spatial resolution in three dimensions. From these viewpoints, we have developed a femtosecond NIR laser microscope with a home-built cavity dumped chromium: forsterite (Cr:F) laser as an excitation light source whose output wavelength is centered at 1260 nm. In the following the set-up of the NIR laser microscope and its application to multiphoton imaging are presented. 8.2.1 Confocal Microscope with a Chromium: Forsterite Ultrafast Laser as an Excitation Source

A schematic diagram of the confocal microscope system with the Cr:F laser is shown in Figure 8.1a. The spectrum of the laser pulse covers the wavelength region from 1.2 to 1.35 mm with an FWHM of 77 nm, as shown in Figure 8.1b. The output of the Cr:F laser was guided into an optical microscope (IX 71, Olympus) after passing through a prism pair for the optimal compensation of the pulse duration at the sample plane of the microscope. In the optical set-up, a Michelson interferometer was introduced for the time-resolved measurements; the NIR pulse was divided into two pulses with the same intensity. The optical delay unit (nanomover, Melles Griot) with a minimum step of 10 nm enabled us to measure ultrafast excitation dynamics at 66.7 as (attosecond) intervals. A dichroic mirror (800DCSX, Chroma Technology) attached to the optical microscope realized the selective reflection of the NIR laser pulse toward an objective (MPlan 100 IR, Olympus, NA ¼ 0.95) and selective transmission of visible light to the detection system of the microscope. The emission in the visible region from samples was guided into an avalanche photodiode (C5460-01, Hamamatsu) or an optical fiber. For the effective detection of the fluorescence signal, a lock-in amplifier (Model 5210, EG&G Instruments) was introduced into the detection system. A fiber-coupled spectrometer (SD2000, Ocean Optics) was employed for the spectral measurements. Figure 8.1c shows the interferometric autocorrelation signal of second harmonic generation (SHG) from a BBO crystal positioned at the sample plane of the microscope. The shape of the SHG trace was symmetrical with respect to the time origin; the ratio of the maxima to the background was 8 : 1, indicating that nearly ideal

8.2 Development of a Near-Infrared 35 fs Laser Microscope

Figure 8.1 (a) Block diagram of the femtosecond near-infrared laser microscope system. (b) Spectrum of the light pulse from the Cr:F laser. (c) Interferometric autocorrelation trace of SHG signal with envelope curve calculated assuming a chirp-free Gaussian pulse with 35 fs fwhm.

irradiation conditions were preserved even under the objective with high refractive index dispersion. The envelope of the SHG signal in Figure 8.1c is the result calculated on the assumption of a Gaussian pulse with 35 fs duration under the objective. From the reproduction of the SHG autocorrelation signal by the envelope thus calculated, we can safely conclude that the dispersion of the ultrashort laser pulse in the NIR region could be well compensated to almost its transform limit. 8.2.2 Detection of Higher Order Multiphoton Fluorescence from Organic Crystals

Figure 8.2a–c shows optical transmission images of organic microcrystals of perylene, anthracene, and pyrene, excited at a laser power of 1.7 nJ pulse1 under similar excitation conditions as in Figure 8.1c. The bright spots l/20, Lorenz–Mie theory based on plane wave scattering is used to describe the optical trapping of an object placed in an arbitrary field distribution. For the particle size l/20, such a small particle behaves as an induced elementary dipole and the gradient and scattering forces act on it. In this approximation, the scattering and gradient force components are readily separated. Chaumet and NietoVesperinas obtained the expression for the total time average force on a sphere, j F i ¼ ð1=2ÞRe½aE0j qi ðE0 Þ , which includes Fgrad / a3 rI0 and Fscatt / a6 I0 , where a is the polarizability of the dielectric particles, E0 is the incident optical field [32, 33]. High incident beam intensity is needed to give a balance between the gradient force Fgrad towards the origin direction and the scattering force Fscatt and drag force Fdrag from a medium pushing the particle away from the focusing point. For stable trapping in all three dimensions, the axial gradient components of the force pulling the particle towards the focal region must exceed the scattering component of the force pushing it away from that region [32, 33]. For particle size l/20, for example, 10 nm, the scattering force can be ignored as compared with the gradient force since it is proportional to a6 and is two orders weaker than the gradient force. As described by Ashkin and Chu [34], a sufficient trapping condition is that the time to pull particles into the trap potential should be much less than the time for the particles to diffuse out of the trap by Brownian motion. Based on Stokes law for a small sphere, the viscosityinduced drag force is expressed by, Fdrag ¼ 6phva, where h is the viscosity of D2O, v is the velocity of the QDs driven by Brownian motion. When ~ Fgrad ¼ ~ Fscatt þ ~ Fdrag is 2 satisfied, QDs can be trapped in the focusing volume with pw0 /l length. With decreasing size, a much higher laser intensity is required to overcome the drag force, due to the rapid decrease in the gradient force from the a3 term. In the Lorenz–Mie regime, it was believed that there is no difference between cw laser and pulse laser trapping, and average power rather than peak power is the key parameter for optical trapping [35]. This comparison may not be adopted for small particles, such as semiconductor QDs with size l/20. When the high-repetition pulse laser is used to induce the gradient force, the highest dipole interaction between particles and the electromagnetic field of incident light appears at the peak position of the pulse between successive pulses. As driven by inertia, particles at the edge of the

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potential well will experience motion toward deep trapping because of the small diffusion distance (a few nm) of nm-sized QDs within the 80 MHz repetition rate. As a result, Rayleigh particles can be optically trapped with a high repetition rate ultrashort pulse laser beam. Figure 9.3 illustrates the photon pressure potential

Figure 9.3 (a): Calculation of photon pressure potential against the radius and trapping power of CdTe QDs based on the peak power of YLF laser with 3.5 ps FWHM and 80 MHz repetition rate. (b) Parts of cross sections of (a), the corresponding average power is given in the figure.

9.3 Optical Trapping of CdTe QDs Probed by Nonlinear Optical Properties

Figure 9.4 Two-photon absorption-induced luminescence of CdTe QDs in D2O and H2O obtained from the luminescence spectrum as a function of time. The diameter and solvent are (a) 4.5 nm in D2O, (b) 3.7 nm in D2O, (c) 4.5 nm in H2O, (d) 3.7 nm in H2O. Incident average laser power is 140 mW.

versus the radius of CdTe QDs based on the calculation of peak power transient trapping by a picosecond YLF laser (1047 nm, 3.5 ps FWHM, 80 MHz). If the photon pressure potential is above the thermal potential of the solution, the particles are supposed to be optically trapped. As clearly shown in the figure, 2-nm radius CdTe QDs can be optically trapped with an incident average power larger than 20 mW, suggesting that a high-repetition pulse laser can trap nanosized particles much more easily than a cw-mode laser with the same average power. Figure 9.4 shows the time-dependent luminescence intensity of CdTe QDs in D2O and in H2O induced by the focused irradiation of a ps YLF laser at 1047 nm [31]. Since CdTe QDs have no absorption in the near-IR wavelength region and the luminescence intensity is proportional to the second power of the laser intensity, the luminescence is induced by a two-photon absorption process of CdTe QDs. As mentioned in our Z-scan experiments, the two-photon absorption cross section, s, is of the order of 103–104 GM, which is much larger than a typical organic nonlinear material. In D2O, two-photon luminescence of CdTe QDs increases with time, while the luminescence intensity of CdTe QDs in H2O decreases with time. This result suggests that in D2O the number of CdTe QDs increases with time within a focusing volume, indicating that CdTe QDs are optically trapped by a picosecond YLF laser. In H2O, the number of CdTe QDs decreases with time within a focusing volume, due to the local heating of H2O by the excitation of the vibrational overtone at 1047 nm resulting in an increase in thermal Brownian motion that exceeds the optical trapping force. As expected, large sized CdTe QDs are more easily trapped than small sized CdTe QDs, which is confirmed by the initial slope of the two-photon luminescence intensity against time in Figure 9.4. In addition, the intensity decrease in 4.5 nm CdTe QDs in H2O is almost negligible

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Figure 9.5 AFM images of CdTe QDs (3.3 nm) on a hydrophilic glass substrate after drying the D2O solution without (a) and with (b) a trapping laser beam. Incident average laser power is 120 mW.

although faster decay is observed for 3.7 nm QDs, suggesting that the gradient force on 4.5 nm CdTe QDs is comparable to the thermal Brownian motion. The optically trapped CdTe QDs can be fixed on a substrate. The sample was naturally dried with and without trapping laser beam. The volume of trapped particles may be similar to or less than that of a focusing beam. Figure 9.5 illustrates the surface topography of a naturally dried sample with and without a picosecond YLF laser. As clearly shown, an assembled structure of CdTe QDs less than 1 mm diameter is detected only when the trapping beam is applied. Furthermore, the height of the assembled structure is 150 nm, much larger than CdTe QDs dried naturally without a trapping laser beam. By using a high repetition-rate near-IR pulse laser, as just mentioned above, nm-sized CdTe QDs can be optically trapped and fixed on a substrate, in which the two-photon absorption-induced luminescence of CdTe QDs as a nonlinear optical property is used to monitor the optical trapping process. These results will give a basic technique for quantum information science and technology to control and arrange semiconductor QDs freely [36–38].

9.4 Single Particle Spectroscopy of CdTe QDs

Single chromophore detection, that is, the study of single nano-objects such as molecules, QDs, metal colloids, and so on is a modern tool for investigating the physicochemical properties of a single object in interaction with surrounding environments. Several prominent features of single particle spectroscopy are the observation of luminescence intermittency or blinking [39–41], that is a random switching between an emitting and a non-emitting state, and spectral diffusion [42], that is a random shift of luminescence spectra. These observations cannot be detected in ensemble measurements and are characteristic for a single particle. Fluorescence correlation spectroscopy has also been used for analysis of single particle dynamics in


solution [43]. Luminescence blinking of single QDs was first observed and analyzed by Nirmal et al. using single molecule spectroscopy and microscopy [44, 45]. In their investigations the luminescence intensity time-trajectories of single QDs were observed for an extended period of time, in contrast to single molecules which often suffer fast photobleaching. This high photostability of QDs offers better statistical accuracy with respect to the underlying kinetic phenomena, even on a single particle. In contrast to exponential processes like intersystem crossing-related photon bunching in single molecules, a single CdSe QD follows an inverse power law behavior over many decades in probability density and in time. The control of the blinking behavior of single QDs is not so easy, which may become obstructive for imaging and single-photon source applications. Several studies have been carried out to understand the blinking mechanism and to suppress the blinking by changing environmental conditions or ligands [46, 47]. A time trace of luminescence intermittency (blinking) is shown in Figure 9.6 for water-soluble CdTe QDs embedded in polyvinyl alcohol (PVA) and trehalose [48]. From these two trajectories one can preliminarily predict that in trehalose media a longer on time is observed as compared to that in a PVA matrix. The plausible reason behind this is that we prepared the QDs in aqueous solution and hence the QDs preserve their luminescence properties better in an aqueous environment. Trehalose is a common non-reducing disaccharide of glucose found in a variety of organisms such as fungi, plants and animals, where it has a protective role in the case of water stress or dehydration and freezing. It is well known that trehalose helps to reduce the evaporation of water. These results suggest that trehalose is a better environmental matrix as compared to PVA, although both compounds have a hydrophilic OH group. The blinking phenomenon of QDs and its unusual power-law distribution in the histogram of on–off emission events are still not clearly understood though the origin of luminescence intermittency of QDs has been studied in detail previously. It has been suggested that an electron tunneling from the excited QD to a trap state or Auger ionization may be responsible for the blinking of QDs [49, 50]. After electron transfer to trap states or ionization, the charged QD still absorbs, but is dark because of charge-induced nonradiative relaxation of the exciton energy. QDs become bright again when the trapped electron hops back. Thus the on state depends on the number of surface states or tunneling efficiency, and the off state depends on the stability of charged QDs and trapped electrons. Various parameters such as temperature, QD size, coated-shell thickness, excitation intensity, and so on are considered to affect the on state of QDs [39, 51]. We also found that the excitation intensity has a clear effect on the blinking behavior. Blinking dynamics and its dependence on excitation intensity have been examined by recording luminescence time trajectories and determining statistically the duration of the bright and dark periods for a large number of QDs at different excitation intensities from 0.28 to 7.1 kW cm2. As clearly shown in Figure 9.7, with increase in excitation intensity the duration of the bright interval values of CdTe QDs in PVA reduced dramatically. This is qualitatively expected for a competing photophysical branching process, the Auger effect. At higher excitation intensity multiple carriers are formed in a single CdTe QD, in which the possible ionization of CdTe QDs will increase due to

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Figure 9.6 Typical luminescence intensity versus time trajectories of a single CdTe QD (4.6 nm) embedded in a PVA (a) and a trehalose (b) matrix dispersed on a cover glass surface at room temperature. The excitation intensity was 1.7 kW cm2 and the integration time was 200 ms bin1.

carrier–carrier interactions. This nonlinear Auger process turns the particle luminescence off. In contrast, the statistics for off times is independent of the excitation intensity [48], so that the transition from a dark state to a bright state is not a light driven process and is probably a charge recombination process. From a theoretical point of view, the blinking kinetics of these CdTe QDs can be quantified by analysis of the on- and off- time probability densities, P(ton) and P(toff), respectively. Figure 9.8 displays the luminescence intermittency statistics for CdTe QDs in trehalose environment. The distribution of off times involved in the blinking is almost linear on this scale, indicating that the lengths of off times events are distributed according to an inverse power-law of the type, P(toff ) ¼ P0ta, where P0 is


Figure 9.7 On-time histogram of single CdTe QDs in PVA matrix at 0.28 kW cm2 and 7.1 kW cm2. All histograms were built from time traces recorded under identical experimental conditions except the excitation intensity.

the scaling coefficient and a is the power law exponent characterized by the statistics of each type of event. On the other hand, the on times were fitted with a power law with an exponentially decaying tail with the relaxation time ton, thereby limiting very long on times, P(ton) ¼ P0taexp (t/ton) [39, 52]. Fitting the data of P(toff ) to a power law distribution yields aoff ¼ 1.55 in PVA and aoff ¼ 1.55 in trehalose, and the data of P(ton) to the power law distribution with exponential tail yields aon ¼ 1.40 in PVA and aon ¼ 1.24 in trehalose for all QDs. This result suggests that trehalose helps to reduce the formation of charged CdTe QDs. Trehalose, as compared with PVA, binds to a large number of water molecules that can be adsorbed on the surface of the QDs and passivate surface traps, which results in an increase in luminescence intensity. For the off time, on the other hand, Issac et al. have observed that an increase in the dielectric constant of the matrix surrounding the QDs leads to a decrease in the value of aoff [40]. The matrix in this case was thought to solvate the ejected charge carriers and stabilize the charge separated state. The value of the power law exponent of the off time (aoff ) statistics scaled linearly with the reaction field factor [ f (e)], which

Figure 9.8 Log vs. log plots of off (a) and on (b) probability distribution compiled from a set of CdTe QDs in a trehalose matrix at room temperature.

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correlates the stabilization energy with the dielectric properties of the matrix, f (e) ¼ 1 1/e, where e is the static dielectric constant of the matrix. The dielectric constant of trehalose and PVA (e ¼ 14) are high enough and hence the aoff values are considered to be similar to each other [48].

9.5 Summary

Third-order nonlinear optical properties of CdTe QDs were examined by Z-scan and FWM experiments in the nonresonant wavelength region. We found that the twophoton absorption cross section, s, is as high as 104 GM, although this value decreases with decreasing size. In addition, the nonlinear response is comparable ð3Þ ð3Þ to the pulse width of a fs laser and the figures of merit ðFOM ¼ Re cQD =Im cQD Þ is 40 times enhanced for small-sized CdTe QDs as compared with the bulk, suggesting that CdTe QDs are suitable for all optical switching applications. The optical trapping of nanometer-sized CdTe QDs in D2O and fixation of QDs on a substrate were also demonstrated by high repetition-rate ps near-IR laser, in which the nonlinear optical properties of QDs were used to monitor the optical trapping process. By using singleparticle spectroscopy, we have shown that the emission intermittency is dependent on the aqueous environment surrounding the CdTe QDs, and the lifetimes of the bright on states are prolonged in a trehalose matrix compared to a PVA one. Since the power law statistics for off times are excitation intensity independent, the process that couples a dark state to a bright state is probably a charge recombination process and not the light-driven process. The influence of the environment on the blinking behavior confirms that the charge rearrangements of QDs are relevant for the blinking statistics. Recent developments for the synthesis of zero-, one-, and two-dimensional semiconductor nanomaterials such as nanowires, nanotubes, and nanosheets, and various microspectroscopy techniques provide systematic understanding of the photophysics and photochemistry of these nanomaterials. Linear and nonlinear optical properties, and Auger and inverse Auger effects and so on, will be analyzed as a function of size and shape of these semiconductor nanomaterials. Blinking dynamics of single nanomaterials are believed to be related to electron or charge transfer/shift processes, so that the real-time observation of electron or charge movements in semiconductor nanomaterials may be analyzed by time-resolved microspectroscopy techniques. Careful tuning of the environments also makes the semiconductor nanomaterials even more suitable for various applications or as model systems for different research.

Acknowledgments

The authors gratefully thank Dr K. Kamada, Dr V. Biju, and Dr M. Ishikawa. This work was partly supported by Grant-in-Aid for Scientific Research on Priority Areas of Molecular Nano Dynamics, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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Abstreiter, G., Finley, J. J. and Zrenner, A. (2005) Recent advances in exciton-based quantum information processing in quantum dot nanostructures. New J. Phys., 7, 184-1–184-27. Shimizu, K. T., Neuhauser, R. G., Leatherdale, C. A., Empedocles, S. A., Woo, W. K. and Bawendi, M. G. (2001) Blinking statistics in single semiconductor nanocrystal quantum dots. Phys. Rev. B, 63, 205316-1–205316-5. Issac, A., Borczyskowski, C. V. and Cichos, F. (2005) Correlation between photoluminescence intermittency of CdSe quantum dots and self-trapped states in dielectric media. Phys. Rev. B, 71, 161302 (R)-1–161302(R)-4. Biju, V., Makita, Y., Nagase, T., Yamaoka, Y., Yokoyama, H., Baba, Y. and Ishikawa, M. (2005) Subsecond luminescence intensity fluctuations of single CdSe quantum dots. J. Phys. Chem. B, 109, 14350–14355. Empedocles, S. A. and Bawendi, M. G. (1999) Influence of spectral diffusion on the line shapes of single CdSe nanocrystallite quantum dots. J. Phys. Chem. B, 103, 1826–1830. Ito, S., Toitani, N., Pan, L., Tamai, N. and Miyasaka, H. (2007) Fluorescence correlation spectroscopic study on watersoluble cadmium telluride nanocrystals: fast blinking dynamics in the ms–ms region. J. Phys.: Condens. Matter, 19, 486208-1–486208-10. Nirmal, M., Dabbousi, B., Bawendi, M. G., Macklin, J. J., Trautman, J. K., Harris, T. D. and Brus, L. E. (1996) Fluorescence intermittency in single cadmium selenide nanocrystals. Nature, 383, 802–803. Nirmal, M. and Brus, L. E. (1999) Luminescence photophysics in semiconductor nanocrystals. Acc. Chem. Res., 32, 407–414.

46 Odoi, M. Y., Hammer, N. I., Sill, K., Emrick, T. and Barnes, M. D. (2006) Observation of enhanced energy transfer in individual quantum dot–oligophenylene vinylene nanostructures. J. Am. Chem. Soc., 128, 3506–3507. 47 Hohng, S. and Ha, T. (2004) Near-complete suppression of quantum dot blinking in ambient conditions. J. Am. Chem. Soc., 126, 1324–1325. 48 Mandal, A., Nakayama, J., Tamai, N., Biju, V. and Isikawa, M. (2007) Optical and dynamic properties of water-soluble highly luminescent CdTe quantum dots. J. Phys. Chem. B, 111, 12765–12771; Mandal, A. and Tamai, N. (2008) Influence of acid on luminescence properties of thioglycolic acid-capped CdTe quantum dots. J. Phys. Chem. C, 112, 8244–8250. 49 Shimizu, K. T., Woo, W. K., Fisher, B. R., Eisler, H. J. and Bawendi, M. G. (2002) Surface-enhanced emission from single semiconductor nanocrystals. Phys. Rev. Lett., 89, 117401-1–117401-4. 50 Verberk, R., Oijen, A. V. and Orrit, M. (2002) Simple model for the power-law blinking of single semiconductor nanocrystals. Phys. Rev. B, 66, 2332021–233202-4. 51 Banin, U., Bruchez, M., Alivisatos, A. P., Ha, T., Weiss, S. and Chemla, D. S. (1999) Evidence for a thermal contribution to emission intermittency in single CdSe/ CdS core/shell nanocrystals. J. Chem. Phys., 110, 1195–1201. 52 Tang, J. and Marcus, R. A. (2005) Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles. Phys. Rev. Lett, 95, 1074011–107401-4; Tang, J. and Marcus, R. A. (2005) Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots. J. Chem. Phys., 123, 0547041–054704-12.

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Part Two Nanostructure Characteristics and Dynamics

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10 Morphosynthesis in Polymeric Systems Using Photochemical Reactions Hideyuki Nakanishi, Tomohisa Norisuye, and Qui Tran-Cong-Miyata

10.1 Introduction

In general, chemical reactions give rise to changes in chemical structures. For most cases, structures with spatial symmetry, such as lamellae or hexagons, cannot be solely produced by chemical reactions due to their short-range nature. In order to generate and, particularly, control structures with correlations over long ranges, it is necessary to couple chemical reactions, a microscopic phenomenon, to macroscopic phenomena such as flow induced by osmotic pressure or to the thermodynamics of the system. This coupling would result in long-range cooperative phenomena such as phase transition or phase separation. In this chapter, to express the unique roles of chemical reactions in the generation and control of polymer morphology via phase separation, we adopt the term morphosynthesis [1]. The scope of this chapter will be focused on inducing and controlling the morphology of multi-component polymers by taking advantage of photochemical reactions. First, the significant roles of photochemical reactions in coupling with the thermodynamics of polymer mixtures are described. Subsequently, several examples are provided to demonstrate the advantages of this coupling over thermally activated reactions. We also show that light or photochemical reactions can be utilized as an efficient tool to drive the system from one location to another on the phase diagram of the mixture, that is, walking the mixture on its phase diagram. As a consequence, a number of unique morphologies were generated and subsequently controlled by using the intensity and wavelengths of the excited light. Finally, the mechanism for the formation of these structures is discussed in conjunction with the universality of systems with competing interactions [2].

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10.2 Morphosynthesis of Polymeric Systems by Using Light 10.2.1 Significance of Photochemical Reactions

Among various photochemical reactions, photodimerization of trans-cinnamic acid and anthracene [3, 4] was chosen in this work for the morphosynthesis of multicomponent polymers. The scheme for these two specific reactions is illustrated in Figure 10.1. By labeling at a specific site on a polymer chain, anthracene derivatives were used as a cross-linker for UV-sensitive polymer systems, whereas the transcinnamic acid derivative can be photosensitized and used to crosslink polymers under visible light. The latter would be useful for computer-assisted irradiation (CAI) experiments, as described later. Furthermore, the photodimers of anthracene generated by irradiation with 365 nm UV light can undergo photodissociation to recover anthracene upon irradiation with shorter UV wavelengths (below 300 nm). Since the viscosity of the mixture containing anthracene-labeled polymers continues to increase during the photodimerization process, the photodissociation of anthracene photodimers can be used to regulate the viscosity of the reacting medium by

Figure 10.1 Photodimerization of anthracene (a) and trans-cinnamic acid (b).

10.2 Morphosynthesis of Polymeric Systems by Using Light

de-crosslinking the forming networks. As described below, coupling this reversibility of anthracene to phase separation could provide a polymer blend exhibiting reversible phase separation behavior drivable by two different wavelengths. 10.2.2 Polymer Mixtures Used in this Study

Samples used in this work are the binary polymer mixtures with the characteristics illustrated in Table 10.1. Here, PSA and PSAF stand, respectively, for polystyrene labeled with anthracene and polystyrene doubly labeled with anthracene and fluorescein used as a fluorescent marker. On the other hand, PSC and PVME stands respectively for polystyrene labeled with trans-cinnamic acid and poly(vinyl methyl ether). The factor a in Table 10.1 indicates the label content of anthracene in the polystyrene chain in the unit of number of labels per one chain. For PSC, the label content is 1 cinnamic acid per 28 styrene monomers. 10.2.3 Polymers with Spatially Graded Morphologies Designed from Photo-Induced Interpenetrating Polymer Networks (IPNs)

Spatially graded structures observed in Nature, such as bamboo, have been of great interest to materials scientists for many years because of their strength against hostile environments [5]. This idea was first realized in several research laboratories for metallic materials, particularly ceramics and was later applied to polymer materials by a number of different experimental techniques. So far, the method of producing polymers with graded structures is essentially based on the gradient of the guest monomer generated in the host polymer matrix followed by their polymerization [6]. Alternatively, the method of generating spatial gradients of a guest polymer in a host polymer matrix by sorption was utilized for the synthesis of polymers with spatially gradient structures [7]. These methods have a common drawback caused by the difference in chemical affinity between the guest and the host at the experimental temperature that could induce phase separation and consequently alter the concentration gradient initially built in the host polymer. To resolve these problems, we have used light, instead of heat, to induce polymerization. The concentration gradient of the reactants was therefore generated by

Table 10.1 Characteristics of polymers used in this study. a indicates the label content.

Polymer PSA PSAF PSC PVME

Mw

Mw/Mn

a (anthracenes/chain)

2.3 · 105 3.4 · 105 3.0 · 105 1.0 · 105

1.5 2.6 2.8 2.5

43 44 — —

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taking advantage of the Lambert–Beer law applied to the UV absorption of the sample. From the viewpoint of phase separation, the characteristic length scale of the morphology emerging at the initial stage of phase separation is inversely proportional to the quench depth [8]. This quench depth is, in turn, governed by the change in the phase diagram upon reaction. Under irradiation with strong light, the reaction yield increases quickly with irradiation time, pushing the reacting mixture deeply into the two-phase region. As a result, the quench depth DT often increases with increasing light intensity, thus changing the characteristic length scales of the morphology. Figure 10.2 shows the co-continuous structures with a gradient of characteristic length scales developing along the direction of incident light [9]. By changing the light intensity, the gradient of the characteristic length scales can also be modified. The dependence of the structural gradient on irradiation intensity, illustrated in Figure 10.3, indicates that the spatial gradient of the characteristic length scales in the mixture can be modified by changing the light intensity, except for the region in the vicinity of the boundary between the glass

Figure 10.2 Graded co-continuous morphology obtained at different depths along the propagation direction of light in a PSAF/MMA (10/90) blend irradiated with 365 nm UV light at room temperature. The number on the upper left in each figure indicates the Z-coordinates of the sample.


Figure 10.3 Dependence of the gradient of the characteristic length scales on the irradiation intensity observed for PSAF/MMA mixtures with different compositions at room temperature. The 2D power spectra corresponding to the morphologies are indicated in the inset.

substrate and the polymer mixtures where the effects of wetting are pronounced due to the selective adsorption of the PMMA component. It was found that the gradient of the characteristic length scales increases with increasing irradiation intensity. An example of a spatially graded structure is illustrated in Figure 10.4 which was obtained by stacking morphologies observed by a laser-scanning confocal microscope (LSCM) at different depths along the propagating direction of the UV light. These results suggest that by taking advantage of the Lambert–Beer law, polymers with spatially graded morphology can be generated and controlled by irradiation with UV light. 10.2.4 Designing Polymers with an Arbitrary Distribution of Characteristic Length Scales by the Computer-Assisted Irradiation (CAI) Method

In order to establish a general method of producing morphology with an arbitrary distribution of characteristic length scales, instead of utilizing the intensity gradient of light inside the sample, we developed a method of using a computer to design a light pattern with arbitrary characteristic length scales and time scales, the so-called computer-assisted irradiation (CAI) method [10]. Irradiation using a light pattern generated by a personal computer was initially performed to control chemical

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Figure 10.4 Three-dimensional morphology of the spatially graded morphology obtained for a PS/MMA (10/90) mixture: Dark region: PSAF-rich phase; transparent region: PMMA-rich phase [9].

reactions far-from-equilibrium such as the Belousov–Zhabotinsky (BZ) reaction in solution [11, 12]. Here, we have developed a method for controlling the phase separation of polymer mixtures in the bulk state. The block diagram of the instrument is given in Figure 10.5. A light pattern with a characteristic length and a characteristic time was generated on a laptop computer and then transferred to a digital projector. Subsequently, these patterns were projected onto a photosensitive polymer placed under a microscope through a series of lenses, as sketched in Figure 10.5. The time-evolution of the images of morphology under irradiation was observed in situ under a bright field optical microscope and stored in a second computer for further analysis. Since the digital projector only transmits visible light, polymer mixtures with the capability of being cross-linked upon irradiation with visible light were particularly designed for this CAI method. Since it is well known that photodimerization of cinnamic acid can be induced by irradiation with visible light via photosensitization, we took advantage of this mechanism and synthesized a polystyrene derivative labeled with trans-cinnamic acid (PSC). The chemical structure of the polymer is illustrated in Figure 10.6. The resulting polymer was mixed with poly (vinyl methyl ether)PVME) to form a blend which possesses a lower critical solution temperature (LCST), that is, undergoes phase separation with increasing temperature. The phase diagram obtained by light scattering detected at a fixed angle for this PSC/PVME blend is illustrated in Figure 10.7. The details of the experiments are given elsewhere [10]. Upon irradiation with 405 nm light, the photosensitizer 5-nitroacenaphthene is excited and transfers its excited energy to the triplet state of trans-cinnamic acid, triggering the dimerization of the cinnamic acid group. This


Figure 10.5 The block diagram of the apparatus for the computer-assisted- irradiation method.

photosensitized reaction of cinnamic acid leads to the formation of PSC networks in the blend. As the fraction of PSC networks exceeds a certain threshold, the PSC/ PVME blend enters the two-phase region and undergoes phase separation. Under this circumstance, there exists a competition between phase separation and the

Figure 10.6 Chemical structure of the polystyrene component labeled with trans-cinnamic acid (PSC).

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Figure 10.7 The phase diagram (a) and the glass transition temperatures (b) of a PSC/PVME mixture obtained, respectively, by light scattering and differential scanning calorimetry (DSC). Irradiation experiments were performed in the miscible region at 127 C indicated by (X) in the figure of trans-cinnamic acid-labeled polystyrene/poly(vinyl methyl ether) blends.

network formation process of the PSC component. The characteristic length scales of the morphology are determined by this competition. Illustrated in Figure 10.8 are the morphologies of a PSC/PVME (20/80) blend irradiated by a 405 nm light stripe with a width of 150 mm. The intensity of the stripe at the center is 1.0 mW cm2 and its background is 0.01 mW cm2. It was found that the blend underwent phase separation after 40 min of irradiation, exhibiting spinodal structures in the region irradiated with the light stripe, whereas no structure was observed at either side of the stripe with much lower intensity. This result clearly indicates that phase separation of the mixture can be locally induced at an arbitrary position in the blend by using this CAI method. Further investigation is currently underway to examine


Figure 10.8 (a) The light intensity profile used in this experiment and the morphology obtained at low magnification obtained for a PSC/PVME (20/80) blend irradiated at 127 C for 240 min. (b) Morphologies of the same sample observed under high magnification at different positions corresponding to the light intensity profile. L ¼ 150 mm.

phase separation phenomena under spatio-temporally non-uniform conditions imposed by this experimental technique. 10.2.5 Reversible Phase Separation Driven by Photodimerization of Anthracene: A Novel Method for Processing and Recycling Polymer Blends

Among a number of reversible photochemical reactions, photodimerization of anthracene would be one of the most convenient reactions to drive the phase separation process of polymer blends because of its easy accessibility in the range of absorption wavelengths and, particularly, without catalyst. Figure 10.9 shows the reversibility of the photodimerization of anthracene moieties labeled on polystyrene in a blend with poly(vinyl methyl ether) monitored under irradiation with two wavelengths 365 and 297 nm at room temperature [13]. Obviously, the reaction rate increases with increasing the light intensity upon irradiation with 365 nm. On the other hand, the recovery process of anthracene via photodissociation of its photodimers induced by 297 nm UV light becomes faster with increasing light intensity. These results clearly indicate that the cross-linking process of the PSAF component

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Figure 10.9 Reversible photodimerization of anthracene induced by light of two wavelengths 365 and 295 nm in a PSAF/PVME (20/80) mixture observed at 25 C.

in the blend can be reversibly induced by using two wavelengths 365 and 297 nm. Consequently, the phase separation of PSA/PVME blends can also be reversibly driven by these two wavelengths. As expected, phase separation of the blend was promoted by irradiation with 365 nm light, as shown in Figure 10.10a where the scattering profile with the characteristic peaks appears as a result of the spinodal decomposition process induced by photodimerization of anthracenes. The peak moves toward the side of large wavenumbers, indicating that the phase-separated structures have shrunk as the phase separation proceeds. This unusual behavior of the scattering profiles was reported for the phase separation of epoxy systems under curing, but different explanations were provided for this peculiar behavior [14]. To elucidate the mechanism of this unusual scattering behavior, the local deformation of the same sample was monitored under the same irradiation conditions by using Mach–Zehnder interferometry (MZI) [15]. By comparison of the scattering results with the deformation data observed from the MZI for the same blend under the same irradiation conditions, it was found that there exists a strong correlation between the two sets of data. Furthermore, as seen in Figure 10.10b, the scattering intensity decreases gradually with time upon dissociation of the cross-link network by irradiation with 297 nm light, suggesting that the homogenization process of the phase-separated blends was induced by the photodissociation of anthracene photodimers. The phase separation behavior depicted in Figure 10.10a and b is quite different from the scattering data obtained previously in the reverse quenched experiments [16], revealing the effects of crosslink-induced shrinkage on the critical behavior of polymer blends. The experimental results described above suggest not only a potential method for photo-recycling of polymer blends, but could also provide a novel method of processing multiphase polymer materials using two UV wavelengths.


Figure 10.10 Scattering profiles of a PSA/PVME (20/80) blend driven by irradiation with light of wavelength (a) 365 nm and (b) 297 nm.

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We have shown that by coupling chemical reactions to cooperative phenomena such as phase separation, chemical reactions become a long-range effect via which a wide variety of morphologies can be generated. Particularly, the long-range effects of this coupling are further enhanced by the viscoelasticity of polymeric networks. Since photochemical reactions can be started and stopped independently from the thermodynamics of the polymer systems, light can be used as a useful tool to control the morphology of polymeric systems. On the other hand, light-induced phase separation is also an interesting physical phenomenon because of its relation to the mode-selection processes in systems far from equilibrium. By systematically examining this reaction-induced phase separation, the mode selection process in polymeric systems can be understood and the information obtained would provide a novel way of designing morphology of polymer materials. Finally, the morphologies with a variety of ordered structures, obtained by the methods described here, could be utilized as templates for dispersion of nanofillers such as dendrimers, fullerenes or carbon nanotubes for practical applications in materials science.

Abbreviations

CAI EGDMA IPNs LCST PSA PSAF PSC PVME

computer-assisted irradiation method ethyleneglycol dimethacrylate interpenetrating polymer networks lower critical solution temperature anthracene-labeled polystyrene polystyrene doubly labeled with anthracene and fluorescein polystyrene labeled with trans-cinnamic acid poly(vinyl methyl ether)

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research on Priority Area Molecular Nanodynamics from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank Professor Hiroshi Masuhara (Nara Institute of Science and Technology, Nara, Japan) and Professor Hiroshi Fukumura (Tohoku University, Sendai, Japan) for their encouragement throughout the course of this study. The efforts of former graduate students Y. Adachi, J. Fukuda, S. Ishino, N. Namikawa and X.-A. Trinh in our research group are greatly appreciated.

References

References 1 Antonietti, M. and Ozin, G. A. (2004) Promises and problems of mesoscale materials chemistry or why meso. Chem-Eur. J, 10, 28–41. 2 For example, see, Vedmedenko, E. Y. (2007) Competing Interactions and Pattern Formation in Nanoworld, Wiley-VCH, Weinheim. 3 See, for example, Cowan, D. C. and Drisko, R. L. (1976) Elements of Photochemistry, Plenum Press, New York. 4 See, for example, McCullough, J. J. (1987) Photoadditions of aromatic compounds. Chem. Rev., 87, 811–860. 5 Rabin, B. H. and Shiota, I. (1995) Functionally gradient materials. MRS Bull., 20, 14–18. 6 Akovali, G., Biliyar, K. and Shen, M. (1976) Gradient polymers by diffusion polymerization. J. Appl. Polym. Sci., 20, 2419–2427. 7 Agari, Y., Shimada, M., Ueda, A. and Nagai, S. (1996) Preparation, characterization and properties of gradient polymer blends: discussion of poly(vinyl chloride)/poly(methyl methacrylate) blend films containing a wide compositional gradient phase. Macromol. Chem. Phys., 197, 2017–2033. 8 For example, see: Cahn, J. W. (1961) On spinodal decomposition. Acta Metal., 9, 795–801. 9 Nakanishi, H., Namikawa, N., Norisuye, T. and Tran-Cong-Miyata, Q. (2006) Autocatalytic phase separation and graded co-continuous morphology generated by photocuring. Soft Matter, 2, 149–156.

10 Ishino, S., Nakanishi, H., Norisuye, T., Awatsuji, Y. and Tran-Cong-Miyata, Q. (2006) Designing a polymer blend with phase separation tunable by visible light for computer-assisted irradiation experiments. Macromol. Rapid. Commun., 27, 758–762. 11 Petrov, V., Ouyang, Q. and Swinney, H. L. (1997) Resonant pattern formation in a chemical system. Nature, 388, 655–657. 12 Kadar, S., Wang, J. and Showalter, K. (1998) Noise-supported travelling waves in sub-excitable media. Nature, 391, 770–772. 13 Trinh, X. A., Fukuda, J., Adachi, Y., Nakanishi, H., Norisuye, T. and TranCong-Miyata, Q. (2007) Effects of elastic deformation on phase separation of a polymer blend driven by a reversible photo-cross-linking reaction. Macromolecules, 40, 5566–5574. 14 Kyu, T. and Lee, J.-H. (1996) Nucleation initiated spinodal decomposition in a polymerizing system. Phys. Rev. Lett., 76, 3746–3749. 15 Inoue, K., Komatsu, S., Trinh, X. A., Norisuye, T. and Tran-Cong-Miyata, Q. (2005) Local deformation in photocrosslinked polymer blends monitored by Mach-Zehnder interferometry. J. Polym. Sci. Part B: Polym. Phys., 43, 2898–2913. 16 Akcasu, Z. A., Bahar, I., Erman, B., Feng, Y. and Han, C. C. (1992) Theoretical and experimental study of dissolution of inhomogeneities formed during spinodal decomposition in polymer mixtures. J. Chem. Phys., 97, 5782–5793.

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11 Self-Organization of Materials Into Microscale Patterns by Using Dissipative Structures Olaf Karthaus

11.1 Self-Organization and Self-Assembly

First, it has to be noted that there is no clear-cut definition of self-organization, nor is there a sharp boundary between self-organization and self-assembly [1]. Both, selfcitation for organization and self-assembly, give rise to ordered one-, two-, or threedimensional structures on various length scales from a disordered precursor state without outside guidance. Some authors want to restrict the phrase self-assembly to the formation of monolayers of reactive organic compounds, often functionalized alkanes, on solid substrates. The most famous examples of these types of layers are the self-assembled monolayers (SAMs) of alkylthiols on gold or silver [2], and monolayers of mixed alkoxy-alkyl silanes on glass or SiO2 [3]. By restricting the phrase self-assembly to these few examples, all other types of structures formed by spontaneous aggregation of atoms and molecules have to be categorized as selforganized structures. Another school of thought makes a distinction between self-assembly and selforganization on the basis of their building blocks, their dynamics and energetics. Here, self-assembled structures are made from preformed building blocks and the structures are static and near the thermodynamic equilibrium. Self-organized structures on the other hand are independent of the component shape, they are dynamic and far away from equilibrium. In that sense, self-assembly is a directed process in which the obtained structure is stable and predictable. Interactions between the building blocks, atoms or molecules, are short range and can be between the same kind of building block (homologous) or between different kinds of building blocks (heterologous). Self-organization, on the other hand, is only present in states that are far from equilibrium, and relies on the collective interaction of many parts in a dynamic fashion. It should be noted that in this definition a system that ends up having a selfassembled structure may also start far from equilibrium and may be dynamic until the final structure has been reached.

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The following structures are examples that are grouped according to this latter definition of self-assembly and self-organization: Self-assembly: .

Snow crystals [4]: Their macroscopic structure is different from a bulk threedimensional ice crystal, but they are formed by homologous pair–pair interaction between water molecules and are static and in thermodynamic equilibrium. It should be noted, however, that dendritic crystal growth is a common phenomenon for metals [5–7] and polymers. The crystals grow under non-equilibrium conditions, but the final crystal is static.

.

Self-assembled monolayers (SAMs) [8]: The layers are formed by heterologous interaction between reactive groups, such as thiols, and noble metals, such as gold or silver. Since the molecules are selectively adsorbed on these metals, film growth stops after the first monolayer is completed. The molecular aggregation is enthalpy driven, and the final structure is in thermodynamic equilibrium.

.

Blockcopolymer microphase separation [9]: Depending on the length of chemically different blocks of monomers in a block copolymer, ordered nanostructures can be obtained in bulk samples and thin films. The film morphology can differ significantly from the bulk morphology, but because the structure is determined by the pair–pair interaction of monomers and/or an interface, and it is a thermodynamically stable structure, it is classified as self-assembly.

.

Micelles: the mostly spherical nanoscale aggregates formed by amphiphilic compounds above their critical micelle concentration in aqueous solution have a narrow size distribution and are dynamic, because there is a fast exchange of amphiphiles in solution and those incorporated in micelles. However, micelles are defined as self-assembled structures, since the structure is in thermodynamical equilibrium. Self organization:

.

Vesicles [10, 11]: these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase.

.

Belouzov–Zhabotinsky reaction [12, 13]: This chemical reaction is a classical example of non-equilibrium thermodynamics, forming a nonlinear chemical oscillator [14]. Redox-active metal ions with more than one stable oxidation state (e.g., cerium, ruthenium) are reduced by an organic acid (e.g., malonic acid) and re-oxidized by bromate forming temporal or spatial patterns of metal ion concentration in either oxidation state. This is a self-organized structure, because the reaction is not dominated by equilibrium thermodynamic behavior. The reaction is far from equilibrium and remains so for a significant length of time. Finally,

11.2 Dissipative Structures

when the reducing organic acid and the oxidizing bromate concentrations are too low for further redox cycles, the reaction stops. .

Biological cell: This is an often cited prime example of a self-organized structure. A cell is made up of many different building blocks that interact with each other in various ways and in which compartments form ordered structures. A living cell requires energy (light or chemical energy) to maintain its function and structure. However, a cell is actually more than just a complex self-organized structure, in order to be functional it needs preexisting information, passed down from the previous generation through self-replication of hereditary material (DNA or RNA).

It should be noted that the glassy state of amorphous polymers, metals, or other inorganics is neither self-assembled, nor self-organized. Even though it is not an equilibrium structure, it lacks the characteristic order and regular structure that is inherent in self-assembly or self-organization systems. Even though the concept of self-organization can be followed back to Descartes, who theorized that the ordinary laws of nature tend to produce organization [15], the phrase self-organization did not appear until 1947 when it was used by the psychologist and engineer W. Ross Ashby [16]. Even after that, the concept lay dormant with little progress until the 1980s. Only more recently has a significant increase in scientific dissertations been noted, reaching nearly 600 for the years 1991–2000. Self-organization seems to be counterintuitive, since the order that is generated challenges the paradigm of increasing disorder based on the second law of thermodynamics. In statistical thermodynamics, entropy is the number of possible microstates for a macroscopic state. Since, in an ordered state, the number of possible microstates is smaller than for a more disordered state, it follows that a self-organized system has a lower entropy. However, the two need not contradict each other: it is possible to reduce the entropy in a part of a system while it increases in another. A few of the systems macroscopic degrees of freedom can become more ordered at the expense of microscopic disorder. This is valid even for isolated, closed systems. Furthermore, in an open system, the entropy production can be transferred to the environment, so that here even the overall entropy in the entire system can be reduced.

11.2 Dissipative Structures

The flow of matter and energy through an open system allows the system to selforganize, and to transfer entropy to the environment. This is the basis of the theory of dissipative structures, developed by Ilya Prigogine. He noted that self-organization can only occur far away from thermodynamic equilibrium [17]. Dissipative, open systems that allow for the flux of energy and matter may exhibit non-linear and complex behavior. Following the above argumentation, complex systems are usually far from thermodynamic equilibrium but, despite the flux, there may be a stable pattern, which may arise from small perturbations that cause a larger, non-proportional effect. These patterns can be stabilized by positive (amplifying)

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Open System Flow of matter and energy

Fluctuations Positive feedback

Enhancement Collectivity

Temporal and spatial patterns Figure 11.1 Concept of dissipative structures for the emergence of spatio-temporal patterns.

feedback as illustrated in Figure 11.1. Another quality of complex systems is that they often exhibit hysteresis or periodic behavior and may be nested (hierarchical), meaning that the parts of a complex system are complex themselves. Dissipative structures can occur from the nanometer to the megameter scale. At the smaller end of the scale, it is argued that dissipative structures in liquid water are responsible for homeopathic effects [18]. Large-scale dissipative structures are found, for example, in the hydrodynamic instabilities of the ozone layer [19] and the eye in the atmosphere of the planet Jupiter. The complexity of life requires complex order principles, and it is only logical that dissipative structures have been proposed, both for the origin of life [20], as well as for biological rhythms [21] and noise [22]. It goes without saying that pattern formation occurs not only during physical processes such as drying [23] and phase separation [24], but also during chemical reactions [25], a specific example being the passivation of metals [26]. In the following, two types of dissipative structures are explained in more detail, because they can be used to produce nano- and micropatterns of organic materials. .

Benard convection cells [27, 28]: a liquid with an inverse temperature gradient (hot below and cool on top) may exhibit thermal convection. Less dense parts of the liquid well upward whereas denser parts show down-welling. The convection cells may arrange in hexagonal order in which the center of each cell wells downwards and the rim wells upwards. The cells stem from the concerted movement of many molecules and cease when the temperature gradient is below a threshold at which the thermal equilibrium can be reachedsolely by thermal conduction and not convection.

.

Tears-of-Wine: The surface motion of mixed liquids was described in ancient times [29], and scientifically in a paper by James Thompson, the brother of Lord Kelvin [30] in 1855. Recently Neogi [31], and Cazabat [32] reported on the formation of ordered structures in evaporating solutions of two liquids. The meniscus of wine in a glass is drawn upward on the glass surface and forms a thin film. Due to its

11.3 Dynamics and Pattern Formation in Evaporating Polymer Solutions

lower boiling point, evaporation of alcohol leads to a water-rich liquid surface that has a higher surface tension. This leads to a surface tension gradient and a flow by which accumulated liquid forms tears, very often at regular intervals along the edge of the liquid film. The tears-of-wine phenomenon can be generalized to describe the flow near the surface and the edge of liquid films driven by surface tension gradients that lead to ordered structures. These two examples show that regular patterns can evolve but, by definition, dissipative structures disappear once the thermodynamic equilibrium has been reached. When one wants to use dissipative structures for patterning of materials, the dissipative structure has to be fixed. Then, even though the thermodynamic instability that led to and supported the pattern has ceased, the structure would remain. Here, polymers play an important role. Since many polymers are amorphous, there is the possibility to freeze temporal patterns. Furthermore, polymer solutions are nonlinear with respect to viscosity and thus strong effects are expected to be seen in evaporating polymer solutions. Since a macromolecule is a nanoscale object, conformational entropy will also play a role in nanoscale ordered structures of polymers.


The two dissipative structures in the examples given above are observable with the naked eye. Tears of wine have a periodicity of a few mm. The size of Benard convection cells depends on the thickness of the liquid layer, because the convection cells have the same height and diameter. Since a macroscopic layer of liquid is often used to demonstrate this effect, the cells have a diameter of at least a few mm. By reducing the thickness of the layer, convection cells are expected to become smaller. On the other hand, an effect of a thinner layer on the tears of wine would not be that obvious. Instead of heating a solution from below to induce convection, the cooling of the surface should be as effective in creating a temperature gradient. When a polymer is dissolved in a volatile solvent, such surface cooling occurs spontaneously by allowing the solvent to evaporate. The heat of evaporation is enough to drive the system out of equilibrium. For example, by placing a few ml of a dilute chloroform solution of a fluorescent-labeled polymer on a substrate we could observe the formation of Benard-like convection cells on a sub-millimeter scale [33] as can be seen in Figure 11.2. The polymer is a polyion complex [34] of poly(styrene sulfonate) having bisoctadecyldimethylammonium as counterion, containing 1 mol% of Rhodamine B, an ionic red-fluorescent dye. By exciting with green light in an epifluorescence microscope, areas of the solution that have higher polymer concentration are brighter. Because the polymer solution droplet has a spherical-cap shape, the solution height is not constant along its radius. Thus it is impossible to develop a two-dimensional regular convection-cell pattern. Instead, there is just one ring of laterally connected Benard cells. At thinner areas, closer to the edge of the polymer solution droplet, the fluorescence is weaker, because the amount of polymer in the light-pass is less.

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Figure 11.2 (a) Microscope image of Benard convection cells (indicated by the circle in the upper left corner) and tears of wine (indicated by the white arrows) in an evaporating fluorescence-labeled polymer solution (adapted

from Ref. 33). (b) Two microscope snapshots of an evaporating polystyrene solution on silicon wafer. The time between the two frames is approximately 100 ms. The polymer droplets have a diameter of approximately 3 mm.

In addition, a periodic intensity profile along the edge of the droplet can be seen. Areas with strong fluorescence are separated by areas that do not fluoresce. This indicates a periodic concentration profile, very similar to the tears-of-wine effect in which surface tension gradients lead to periodic build-up of solution droplets. Similar to the tears of wine, this concentration profile is dynamic. Monitoring the time course revealed that the higher concentrated areas merge, and new areas of higher concentration evolve between existing concentrated areas [35]. These dynamic processes, convection cells and polymer-rich finger structures, are due to solvent evaporation. Consequently, the volume of the solution droplet becomes constantly smaller, and thus the diameter of the solution droplets begins to decrease shortly after the dynamic structures are formed. Hence, the dynamic structures, because they are linked to the edge of the solution droplet, move across the substrate. In this process, polymer may be deposited on the substrate, and droplet or line patterns are commonly observed. Line patterns are formed when the polymer is continuously deposited at the highly concentrated finger positions [36]. Droplets are formed when the emerging line exhibits another instability, the so-called Plateau–Rayleigh instability, in which the line decomposes into droplets, very much like a dripping faucet [37]. These line and droplet patterns are similar to those obtained by heating a thin polymer film that has been spun cast on a substrate. Here, too, a fingering


instability is observed while circular holes develop in the film [38–41]. Such a process occurs because the surface and interfacial energies of polymer and substrate favor droplet formation, that is, dewetting occurs when the polymer is above its glass transition temperature and thus mobile. Droplet formation is triggered by randomly generated holes in the continuous polymer films, thus no regular array of droplets can be formed, even thought the droplets may have a narrow size distribution and have an average nearest neighbor distance [42]. By contrast, the dewetted polymer solution may produce two-dimensionally ordered droplet arrays, because the droplet formation occurs only at the edge of the evaporating solution. Typically, the droplets have a diameter of 200 nm to 10 mm, and they have been used to immobilize organic dyes [43] and inorganic nanoparticles [44] in an orderly fashion on a substrate. [45] Dye aggregates may also accumulate along the rim of the microscopic polymer droplet, leading to hierarchically ordered structures (dye molecule–nanoscale dye aggregate–circular arrangement within droplet–two-dimensional array of droplets), as can be seen in Figure 11.3. In the case of organic cyanine dyes a dependence of the fluorescence spectra on the droplet size has been found [46], which indicates that the nanoscale aggregation of dye molecules can be regulated by micrometer size confinement. Recently it has been reported that even colloidal particle suspensions themselves, without added polymers, can form dissipative structures. Periodic stripes of colloidal particles (monodisperse particles of diameter 30 nm and 100 nm, respectively) and polystyrene particles (monodisperse; diameters from 0.5 to 3 mm) can be formed from dilute aqueous suspensions. The stripes are parallel to the receding direction of the edge of the suspension droplet and thus indicate that a fingering instability

Figure 11.3 Hierarchical pattern of cyanine-dye J-aggregates. Dye molecules form strongly fluorescent nanoscale J-aggregates; J-aggregates arrange at the rim of the micrometer-sized polymer droplet; droplets arrange in a regular mm-sized two-dimensional array (adapted from Ref. 39).

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is the driving force for patterning [47, 48]. The thickness of the stripes is one monolayer of particles and their width is from three to ten particles, depending on the preparation conditions. As explained above, the formation of regular droplet arrays stems from the regular fingering instability at the liquid edge of the solution droplet and the smooth receding of the solution. Thus, the surface roughness of the substrate plays an important role. Regular two-dimensional arrays can easily be produced on atomically flat mica and semiconductor-grade polished silicon wafers with a surface roughness of less than 1 nm. Rougher surfaces, like float glass or indium tin oxide (ITO) covered substrates rarely yield regular patterns since surface inhomogenities (either topological or chemical) disturb the regular fingering patterns. Stick–slip motion of the edge of the solution causes the three-phase-line to recede with non-uniform speed. One possibility to prepare regular dewetted patterns on rough substrates is by pattern transfer. A regular pattern is formed on mica, on which then another substrate is placed. The pattern is released from the mica and fixed on the other substrate at the original droplet positions [49]. In order to allow for more control during the evaporation, the solution can be placed in a motor-controlled sliding [50] or rolling apparatus [51] which uses capillary force to confine the solution between two glass surfaces. The motor determines the speed at which the solution edge is drawn over the substrate and is one of the main parameters to control patterning. Droplet, stripe and ladder patterns have been observed. Recently, we explored the effect of molecular weight on the pattern and employed post-dewetting processes to alter the shape of the dewetted polymer droplets. Since the viscosity of a polymer solution is nonlinear with respect to concentration and also strongly dependent on polymer weight, we expected a drastic effect. Figure 11.4

Figure 11.4 Optical and scanning electron micrographs of patterns obtained by using polystyrene with different molecular weight. The black arrows indicate the rolling direction. The black scale bar is 10 mm.


Random dots

Molecular weight (g/mol)

2,000,000

2-D dots

983,400 565,000 96,900

Dotted lines

44,000

lines

13,700 4,100 2,460 0

10

20

30

40

50

Roller Speed (mm/min) Figure 11.5 Dependence of the polymer dewetting pattern on molecular weight and dewetting speed.

shows examples of the different patterns that were obtained by using polystyrene with different molecular weights. The regular fingering instability that leads to regular 2-D patterns is only achieved with medium molecular weights. Below those, irregular patterns prevail. At higher molecular weights, the fingering instability is suppressed and gelation occurs at the solution edge. If the molecular weight is below 1 000 000, a Plateau–Rayleigh instability leads to the formation of lines of droplets, while at molecular weights above 1 000 000, the lines perpendicular to the receding direction remain. The periodic gelation of polymers at the edge of an evaporating solution has been modeled [52] and is in good agreement with the experimental data. Figure 11.5 shows the approximate regions where the patterns emerge. Dewetting of polymers leads to droplets that have a shallow contact angle of 5–10 [39, 53]. When considering applications, it would be beneficial to be able to control the contact angle, thus allowing for various droplet shapes. By applying a mixed solution containing a good solvent as well as a miscible non-solvent for a polymer, it is possible to swell the polymer without dissolving it. For polystyrene a good solvent is tetrahydrofuran (THF) and a non-solvent is water. By adjusting the volume ratio of both liquids, we succeeded in swelling the dewetted polymer microdroplets. Electron microscope imaging shows high aspect-ratio spherical droplets after the THF–water treatment. The swelling does not overcome the polymer–substrate interaction, so that the position of the droplets is not changed. These microdroplets have been demonstrated to act as a microlens arrays [54] as shown in Figure 11.6. The shape change is reversible by raising the temperature above the glass transition temperature of the polymer [55]. Wetting and dewetting depend strongly on the surface energy of the substrate and the interfacial energy of the polymer and the substrate. Thus dewetting on a patterned substrate with areas of different surface energies gives rise to selective deposition

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Figure 11.6 Scanning electron micrographs of a dewetted polymer droplet before and after solvent treatment. Viewing angle is 45 and the scale bar is 1 mm.

of the polymer on one surface functionalization only. Selective deposition of conducting polymers has been demonstrated for dewetted structures on Au patterns on Si wafer [56]. Similarly, dewetting can be used to form micron-sized gaps that can be used for the self-organized assembly of electrodes for organic field effect transistors [57]. Besides polymers, other amorphous materials such as dendrimers or even small molecules with a molar mass as low as 300 g mol1 can also be dewetted. The reason for this is that dissipative structures in solution are independent of the molecular structure of the solute, as long as the solute does not crystallize during pattern formation. Hence, dendrimers have been reported to form submicron-sized droplets on mica [58], and submicron-sized droplets of arylamines can be used as hole conductors in organic light emitting diodes [59]. Since the amorphous state of small molecules is thermodynamically unstable, dewetted droplets of organic molecules that are initially amorphous tend to crystallize. Depending on the compound and the annealing conditions, several crystal morphologies can be obtained. Single crystalline, polycrystalline or fibrous crystals have been reported [60].

11.4 Applications of Dewetted Structures in Organic Photonics and Electronics

Controlled micro-structuring of surfaces is important for a wide variety of applications. Using organics in electronics devices has multiple advantages. Organics are lightweight and easy to apply via solution processes or vacuum evaporation. Many organics are flexible and transparent. Since their electrical, mechanical and magnetic properties can be tailored, the interaction with magnetic, electric and electromagnetic fields offers the possibility to control electric current, light emission or absorption. The first recombination of electric charges that led to light emission in anthracene crystals was reported in the 1960s [61–63]. Because trapping sites in crystals require high quality single crystals for efficient luminescence, research quickly moved to

11.4 Applications of Dewetted Structures in Organic Photonics and Electronics

amorphous materials as charge carriers and in the 1980s vacuum-deposited thin films of low molar mass compounds were shown to exhibit strong electroluminescence [64]. Patterning is crucial for display applications, and shadow-mask evaporation, ink-jet printing, or screen printing is widely used. Besides these top-down methods, bottom-up self-assembly has alsobeen reported as an effective patterning tool. A phase separated mixture of two polymers, a matrix polymer and an electroluminescent guest polymer, shows electroluminescence from the micron-sized phase-separated guest polymer [65]. However, since the phase separation occurs randomly in the thin film, only some guest domains are in contact with the ITO electrode and emit light. Furthermore, the spinodal decomposition of the host and guest leads to a broad size distribution of the luminescent domains. By dewetting a solution of a hole-transport material, alq3, we were able to produce a device with submicron-sized emitting dots, a size that cannot be reached by the conventional top-down patterning methods described above. Low molar mass organics tend to crystallize and crystals have a series of advantages over amorphous materials in electronic applications, once the difficulties with trap sites due to crystal heterogeneities can be overcome. Thus we attempted to create an array of microcrystals of the hole conductor by annealing the dewetted samples. Bright luminescence was observed, even though the aggregation state of the molecules is still unclear [66]. Another, related organic device that is receiving increasing attention recently is the organic field effect transistor (OFET). Dewetting can be used for fabricating shortchannel polymer field effect transistors with conducting polymer employed as electrodes. Source and drain contacts can be patterned by splitting a conducting polymer water solution over a hydrophobic self-assembled monolayer [67], or by selfaligning ink-jet printed gold contacts [68]. Besides forming the source-drain gap by dewetting, patterning of the organic semiconductor itself can also be achieved by self-organization. Pentacene is a promising material, and OFETs have been prepared by a conventional vacuum evaporation process [69]. Grain boundaries inevitably present in the resulting polycrystalline film prepared by this method restrict the carrier mobility. Spin coating from hot trichlorobenzene solution led to a preferred radial crystal orientation, but grain boundaries were still present [70]. By using the roller apparatus to cast a hot trichlorobenzene solution, we were able to produce unidirectionally oriented crystal fibers with a length of several hundred micrometers [71] as shown in Figure 11.7. As in the case of polymers, a concentration profile develops along the solution edge during solvent evaporation and at areas with highest concentration, crystal seeds are formed. Since the edge of the solution is dragged over the substrate, the crystal seed can only grow in one direction – perpendicular to the solution edge. Thus ordered arrays of unidirectionally aligned crystals are formed, when the receding speed of the edge matches the crystal growth rate. Even though the source-drain gap was only partially covered by pentacene, charge mobilities of 0.1 cm2 V1 s1 have been obtained. It can be expected that, in the future, other organic electronic devices and circuits, such as sensors [72], radio-frequency identification tags (RFIDs) [73], and ring oscillators [74] may be fabricated using dissipative structures.

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Figure 11.7 Scanning electron micrograph of pentacene fibers prepared by dewetting of a hot trichlorobenzene solution using the roller apparatus. The fibers are aligned along the rolling direction (reprinted with permission from Ref. 71). The scale bar is 5 mm.

11.5 Summary

Dissipative structures may form when a system is in a non-equilibrium state that allows the flow of energy and/or matter. The self-organized structures formed cover a wide rangeoflengthscalesandeven though they aretemporalbydefinition, itispossibletofix them on solid substrates to produce nano- to micrometer-sized regular patterns. By using functional organic materials, functional patterns can be prepared. This opens up the possibility of using bottom-up self-organization in connection with top-down patterning processes to produce functional devices for use in photonics and electronics.

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45 Suematsu, N. J., Ogawa, Y., Yamamoto, Y. and Yamaguchi, T. (2007) Dewetting selfassembly of nanoparticles into hexagonal array of nanorings. J. Colloid. Interface Sci., 310, 684–652. 46 Karthaus, O., Okamoto, K., Chiba, R. and Kaga, K. (2002) Size effect of cyanine dye J-aggregates in micrometer-sized polymer Domes. Int. J. Nanosci., 1, 461–464. 47 Sawadaishi, T. and Shimomura, M. (2005) Two-dimensional patterns of ultra-fine particles prepared by self-organization. Colloid. Surf. A, 257–258, 71–74. 48 Sawadaishi, T. and Shimomura, M. (2006) Control of structures of two-dimensional patterns of nanoparticles by dissipative process. Mol. Cryst., Liq. Cryst., 464, 227–231. 49 Suematsu, N. J., Nishimura, S. and Yamaguchi, T. (2008) Release and transfer of polystyrene dewetting pattern by hydration force. Langmuir, 24, 2960–2962. 50 Yabu, H. and Shimomura, M. (2005) Preparation of self-organized mesoscale polymer patterns on a solid substrate: continuous pattern formation from a receding meniscus. Adv. Func. Mater., 15, 575–581. 51 Karthaus, O., Mikami, S. and Hashimoto, Y. (2006) Control of droplet size and spacing in micronsize polymeric dewetting patterns. J. Colloid. Interface Sci., 301, 703–705. 52 Nonomura, M., Kobayashi, R., Nishiura, Y. and Shimomura, M. (2003) Periodic precipitation during droplet evaporation on a substrate. J. Phys. Soc. Jpn., 72, 2468–2471. 53 Karthaus, O., Ijiro, K. and Shimomura, M. (1996) Ordered self-assembly of nanosize polystyrene aggregates on mica. Chem. Lett., 821–822. 54 Kiyono, Y. and Karthaus, O. (2006) Reversible shape change of polymer microdomes. Jpn. J. Appl. Phys., 45, 588–590. 55 Nussbaum, P., V€olkel, R., Herzig, H. P., Eisner, M. and Haselbeck, S. (1997) Design, fabrication and testing of

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12 Formation of Nanosize Morphology of Dye-Doped Copolymer Films and Evaluation of Organic Dye Nanocrystals Using a Laser Akira Itaya, Shinjiro Machida, and Sadahiro Masuo

12.1 Introduction

The formation of nanopatterned functional surfaces is a recent topic in nanotechnology. As is widely known, diblock copolymers, which consist of two different types of polymer chains connected by a chemical bond, have a wide variety of microphase separation structures, such as spheres, cylinders, and lamellae, on the nanoscale, and are expected to be new functional materials with nanostructures. Further modification of the nanostructures is also useful for obtaining new functional materials. In addition, utilization of nanoparticles of an organic dye is also a topic of interest in nanotechnology. In most studies using block copolymers, the block copolymer films are used as a scaffold for patterning inorganic nanoparticles such as metals and are removed from the substrate at the end of a process. On the other hand, there are fewer studies on the functionalization of a block copolymer film by the introduction of a functional chromophore into a part of the nanoscale microphase separation structure. However, the introduction of a functional chromophore, that exhibits charge transport [1–8], a photovoltaic effect [5–8], nonlinear optical effects [4, 9, 10], emission [11], photochromism [12, 13], and so on, into a block copolymer film is very attractive in view of the recent development of organic electronic and photonic devices. Several methods are adopted for the site-selective introduction of a functional chromophore into a block copolymer film: (i) chemical introduction of a functional chromophore to one component or the junction of a block copolymer [1–11], (ii) immersion of a block copolymer film into the solution of a functional chromophore using a solvent that can dissolve only one component [14], and (iii) vapor transportation of a functional chromophore whose compatibility with the two components of a diblock copolymer is different [12, 13]. Although chemical introduction is the best method with regard to position selectivity, it lacks versatility, requires intensive processing, and is expensive. Vapor transportation is also costly since it needs vacuum and high temperature but the immersion method is simple and convenient.

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Laser ablation of polymer films has been extensively investigated, both for application to their surface modification and thin-film deposition and for elucidation of the mechanism [15]. Dopant-induced laser ablation of polymer films has also been investigated [16]. In this technique ablation is induced by excitation not of the target polymer film itself but of a small amount of the photosensitizer doped in the polymer film. When dye molecules are doped site-selectively into the nanoscale microdomain structures of diblock copolymer films, dopant-induced laser ablation is expected to create a change in the morphology of nanoscale structures on the polymer surface. First, we describe the position-selective arrangement of polystyrene microspheres with a diameter of 20 or 50 nm onto a diblock copolymer film of polystyrene-blockpoly(4-vinylpyridine) (PS-b-P4VP) with a nanosize sea–island microphase structure. The microspheres contain a fluorescent chromophore, and their surface is modified with carboxylic acid groups. Hydrogen bonding between the pyridyl groups of P4VP and the carboxylic acid groups of the microspheres is expected to be the driving force for position-selective adsorption [17]. COOH

HOOC N H N

N H2C CH

n

CH2

CH

m

H N COOH

HOOC

N

Polystyrene-block-poly(4-vinylpyridine)

Tetrakis-5,10,15,20-(4-carboxyphenyl)porphyrin

(PS-b-P4VP)

(TCPP)

O COOH

HOOC

O

O

N

N

O

O

COOH

HO

OH

Aurintricarboxylic (ATA)

N,N’-bis(2,6-dimethylphenyl)-3,4,9,10-perylenedicarboxyimide (DMPBI)

Secondly, we describe the site-selective introduction of a functional molecule, tetrakis-5,10,15,20-(4-carboxyphenyl)porphyrin (TCPP), into the microphase separation structure of a diblock copolymer film of PS-b-P4VP. Since porphyrin derivatives show various functionalities such as sensitization, redox activity, and nonlinear optical effect, a polymer nanodot array containing a porphyrin at a high concentration would be applicable to a light-harvesing and charge transporting nanochannel.

12.2 Position-Selective Arrangement of Nanosize Polymer Microspheres

Site-selective doping was performed by immersing the copolymer film into the dopant solutions with different solubilities for the two components. For the siteselective doping, we also utilized multipoint hydrogen bonding between the four carboxylic acid groups of TCPP and pyridyl groups of P4VP [18]. Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea–island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostructures. Organic nanoparticles, including nanosized crystals and aggregates, have attracted growing attention in recent years because of their potential application to optoelectronics, pharmaceuticals, cosmetics, and so on [22, 23]. Finally, we describe how the emission from a single nanocrystal consisting of organic dye molecules, namely, N,N0 -bis(2,6-dimethylphenyl)-3,4,9,10-perylenedicarboxyimide (DMPBI) shows photon antibunching [24]. That is, even molecular assemblies can be made to behave as single-photon sources by controlling the size on the nanometer scale. The photon antibunching means that the probability of detecting two simultaneous photons drops to zero; therefore, materials that exhibit photon antibunching are called single-photon emitters or single-photon sources. A single molecule is a typical single-photon source because the molecule cannot emit two photons simultaneously [25, 26]. Generally, single-photon emission can be observed from so-called single quantum systems. However, it is considered that the multiquantum systems, namely, multichromophoric systems, can also be single-photon sources if one exciton remains as a result of exciton–exciton annihilation among the generated excitons. This phenomenon is demonstrated.

12.2 Position-Selective Arrangement of Nanosize Polymer Microspheres Onto a PS-b-P4VP Diblock Copolymer Film with Nanoscale Sea–island Microphase Structure

PS-b-P4VP (Mn; PS:P4VP ¼ 301 000 : 19 600) films were prepared by spin-casting of the toluene solution onto glass substrates. The adsorption of microspheres on PS-bP4VP films was performed by immersing the film in a microsphere/methanol suspension and subsequently rinsing the film in methanol to remove the microspheres that were not adsorbed. On the basis of the brightness of the fluorescence microscope images of P4VP homopolymer films after immersing in a microsphere/water suspension and rinsing in water, we determined the most appropriate immersion time of the block copolymer films for adsorption and rinsing.

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Figure 12.1 AFM images of a PS-b-P4VP (301 000 : 19 600) film (a) before and (b) after immersion in methanol for 75 min and the height profiles. S. Machida, H. Nakata, K. Yamada, A. Itaya: Position-selective arrangement of nanosized polymer microsphere on diblock copolymer film with sea–island microphase structure. Jpn. J. Appl. Phys. 2006, 45, 4270–4273. Copyright Wiley InterScience. Reproduced with permission.

First, we observed the AFM images of a PS-b-P4VP film before and after immersion in methanol for 75 min. For the AFM image before immersion (Figure 12.1a), a clear sea–island structure is observed. On the basis of the molecular weight ratio of the PS-b-P4VP, we can safely judge that the island part corresponds to the P4VP domain and the sea part to PS. The AFM image observed after immersion in methanol (Figure 12.1b) shows islands with a smaller height and a larger width than those before immersion. Moreover, a hole is created at the center of each island. This morphological change is attributed to the difference in solubility between P4VP and PS chains in methanol. A similar morphological change (formation of a hole in the P4VP domain) of a PS-b-P4VP film was reported to be also induced by exposure of the film to methanol vapor [27]. Figure 12.2showstheAFM images andheight profilesof PS-b-P4VP films immersed in the microsphere/methanol suspension (diameter: 20 nm for (a) and 50 nm for (b)) for 75 min and subsequently rinsed in pure methanol for 90 min. The height profile in Figure 12.2a shows a larger island-top curvature radius and a larger island height (15–30 nm) than those before immersion (Figure 12.1a). This morphological change is

12.2 Position-Selective Arrangement of Nanosize Polymer Microspheres

Figure 12.2 AFM images of a PS-b-P4VP (301 000 : 19 600) film after immersion in a microsphere/methanol ((a): 20 nm, (b): 50 nm) suspension for 75 min and rinsing in methanol for 90 min and the height profiles. S. Machida, H. Nakata, K. Yamada, A. Itaya: Position-selective

arrangement of nanosized polymer microsphere on diblock copolymer film with sea–island microphase structure. Jpn. J. Appl. Phys. 2006, 45, 4270–4273. Copyright Wiley InterScience. Reproduced with permission.

attributed to the adsorption of a single microsphere onto each island of the block copolymer film. For most islands in the height profile, the increase in height is smaller than the diameter of the microsphere (20 nm). This is reasonable because P4VP chains swell in methanol so that the microspheres would sink into the islands. The birds-eye view in Figure 12.2a, compared with those in Figure 12.1a and b, indicates that most of theislandparts in the film adsorb 20 nm microspheres.On theother hand, in the case of the 50 nm microspheres, only a small number of the islands show an increase in height (Figure 12.2b). In the height profile, the height and width of the two islands are approximately equal to the microsphere diameter (50 nm). Hence, we conclude that, although some 50 nm microspheres are adsorbed onto the block copolymer film, most islands do not adsorb 50 nm microspheres. Here, we have demonstrated that it is possible to arrange successfully polystyrene microspheres with a diameter of 20 nm on each island (P4VP domain) of a PS-b-P4VP block copolymer film using hydrogen bonds. A 50 nm-large microsphere was rarely adsorbed to the PS-b-P4VP film. Since the present technique does not require an

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expensive apparatus and the chemical modification of block copolymers, it can be applied to various types of organic functional nanoparticle.

12.3 Nanoscale Morphological Change of PS-b-P4VP Block Copolymer Films Induced by Site-Selective Doping of a Photoactive Chromophore 12.3.1 Nanoscale Surface Morphology of PS-b-P4VP Block Copolymer Films

It is widely known that the microphase separation structures of block copolymers depend on the volume fraction of the components [28] and film preparation conditions [29–33]. As aforementioned, when PS-b-P4VP (Mn; PS:P4VP ¼ 301 000 : 19 600) films were prepared by the spin-coating of the toluene solution, a clear sea–island microphase separation structure was formed (Figure 12.1a). On the other hand, when PS-b-P4VP (162 400 : 87 400) and chloroform solvent were used with a polymer concentration of 2.5 wt%, regular nanoscale networklike phase separation structures with P4VP-cylinder parts as holes in the PS-matrix parts are obtained (Figure 12.3a). The film thickness was about 250 nm, and the P4VP-cylinder parts were about 2 nm lower in height than the PS-matrix parts (Figure 12.3g). Since the solvent is one of the key factors that affects the resultant microphase separation structures [29], 3-pentanone, which has a high boiling point, was used instead of chloroform, which has a lower boiling point. The film spin-cast from a PS-b-P4VP/3-pentanone solution of 0.52 wt%, with a thickness of about 25 nm, showed a surface morphology with network-like phase separation structures (Figure 12.4a and d), which is similar to that of the film prepared using chloroform solvent (Figure 12.3a). However, the structure is nonuniform, and both the average diameter and depth of the holes (P4VP-cylinder parts) are larger than those of the above-mentioned film. On the other hand, symmetric PS-b-P4VP (20 000 : 19 000) diblock copolymer films spin-cast from a 3-pentanone solution (0.2 wt%) show nanoscale worm-like phase separation structures with an average roughness height of about 3 nm, as shown in Figure 12.5a and d. In the AFM image, the bright (high) and dark (low) parts correspond to P4VPand PS components, respectively. 12.3.2 Nanoscale Surface Morphological Change of PS-b-P4VP Block Copolymer Films Induced by Site-Selective Doping of a Photoactive Chromophore

The selective doping of TCPP chromophore into the nanoscale domains of P4VP was carried out by immersing the thin films in a methanol solution of TCPP and subsequently rinsing them in pure methanol. As aforementioned, the driving force of the selective doping is the hydrogen bonds between the nitrogen atoms of the pyridyl groups and the carboxylic groups of TCPP. Figure 12.6a and b shows AFM images of PS-b-P4VP (301 000 : 19 600) films before and after doping of TCPP, respectively. The regular sea–island structure with

12.3 Nanoscale Surface Morphological Change

Figure 12.3 AFM images of thin PS-b-P4VP (162 400 : 87 400) films (chloroform solvent) with network-like structures of P4VP cylinders in PS matrices on glass substrates, height profiles of horizontal lines in these images. (a) Before and (b) after immersion in methanol, (c) after doping with TCPP, (d) TCPP-doped films irradiated using one shot with fluence of about 170 mJ cm2 in air, (e) TCPP-doped films irradiated using one shot with fluence of about 150 mJ cm2 in methanol and (f ) 100-shot

irradiation with the same fluence. (g)–(l) Show the height profiles of the horizontal lines shown in the AFM images of (a)–(f ), respectively. Z. Wang, S. Masuo, S. Machida, A. Itaya: Siteselective doping of dyes into polystyrene-blockpoly(4-vinyl pyridine) diblock copolymer films and selective laser ablation of the dye-doped films. Jpn. J. Appl. Phys. 2007, 46, 7569–7576. Copyright the Japan Society of Applied Physics. Reproduced with permission.

nanoscale size is observed clearly for both of the films before and after doping. The average heights of the island parts are estimated to be about 8 and 12 nm before and after doping, respectively. On the basis of these AFM images, an AFM image of PS-bP4VP films after immersing in methanol (Figure 12.1b), and the results of the doping of TCPP into PS and P4VP homopolymer films, we can judge that the selective doping of TCPP into P4VP-island parts is successful [18]. TCPP was doped selectively into the P4VP-cylinder domains of the PS-b-P4VP (162 400 : 87 400) films prepared from the chloroform solution. Figure 12.3c and i show an AFM image of a film doped with TCPP and the height profile of the line indicated in the image, respectively. On the surface of the TCPP-doped films, there

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Figure 12.4 AFM images of thin PS-b-P4VP (162 400 : 87 400) films (3-pentanone solvent) with phase separation structures of P4VP cylinders in PS matrices on glass substrates, and height profiles of horizontal lines in these images. (a), (d) Before and (b), (e) after immersion in methanol; (c), (f) after being doped with TCPP; (d)–(f) are the height profiles of the horizontal lines shown in the AFM images

of (a)–(c), respectively. Z. Wang, S. Masuo, S. Machida, A. Itaya: Site-selective doping of dyes into polystyrene-block-poly(4-vinyl pyridine) diblock copolymer films and selective laser ablation of the dye-doped films. Jpn. J. Appl. Phys. 2007, 46, 7569–7576. Copyright the Japan Society of Applied Physics. Reproduced with permission.

are some conglomeration parts (P4VP þ TCPP), which are attributed to the TCPP doped in the P4VP cylinders. When the undoped films are immersed in TCPP/ methanol solution, the hole volume in the matrices does not change, because methanol is a poor solvent for PS matrices, that is, there is no redundant space to accept the swollen P4VP domains doped with TCPP. Thus, the conglomeration parts remain on the film surface, even after being dried under vacuum. The coloration of the doped film was observed clearly, suggesting that a large number of TCPP molecules were doped into the thick films. In the case of the PS-b-P4VP (162 400 : 87 400) films prepared from the 3-pentanone solution (Figure 12.4c and f ), the protruding parts consisting of many small spheres are formed around the holes; thus, the size of the holes is decreased. The small spheres are likely to be composed of P4VP chains crosslinked by many TCPP molecules. As aforementioned, symmetric PS-b-P4VP (20 000 : 19 000) diblock copolymer films show the wormlike phase separation structures with an average roughness height of about 3 nm (Figure 12.5a and d). The doping of TCPP into the P4VP domain of the high part induces an increase in the average roughness height of about 8 nm (Figure 12.5b and e).

12.4 Site-Selective Modification of the Nanoscale Surface Morphology

Figure 12.5 AFM images of thin PS-b-P4VP (20 000 : 19 000) films (3-pentanone solvent) with worm-like structures on mica substrates and height profiles of horizontal lines in these images. (a), (d) Before and (b), (e) after being doped with TCPP. (c), (f) TCPP-doped films irradiated with fluence of about 150 mJ cm2 in air. (d)–(f) show the height profiles of the

horizontal lines shown in the AFM images of (a)–(c), respectively. S. Machida, H. Nakata, K. Yamada, A. Itaya: Position-selective arrangement of nanosized polymer microsphere on diblock copolymer film with sea–island microphase structure. Jpn. J. Appl. Phys. 2006, 45, 4270–4273. Copyright Wiley InterScience. Reproduced with permission.

When PS-b-P4VP diblock copolymers were spin-cast using different solvents, the surface morphology of the films depended on the solvent used and the numberaverage molecular weight. Utilizing the multiple hydrogen bonds between the nitrogen atoms of pyridyl groups and the carboxylic groups, a selective doping into the films with different morphologies of nanoscale phase separation structures was carried out by immersing the films in a methanol solution of TCPP chromophore, resulting in a further nanoscale surface morphological change of the films. One can see schematic illustrations of the surface modification process of PS-b-P4VP films during doping in the TCPP/methanol solution in Refs. [18] and [20].

12.4 Site-Selective Modification of the Nanoscale Surface Morphology of Dye-Doped Copolymer Films Using Dopant-Induced Laser Ablation

As aforementioned, laser ablation of polymer films themselves and dopant-induced laser ablation of polymer films have been extensively investigated. The photochemical or photothermal mechanism has been discussed. The feature of the dopant-

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Figure 12.6 AFM images of thin PS-b-P4VP (301 000 : 19 600) films with sea–island structures (sea parts of PS components and island parts of P4VP components) on glass substrates, and height profiles of horizontal lines in these images. (a) Before and (b) after being doped with TCPP. Doped films irradiated using one shot with fluences of about (c) 110 and (d) 150 mJ cm2 in methanol. Height profiles of

(e)–(g) correspond to the horizontal lines shown in the AFM images of (a), (b), and (d), respectively. Z. Wang, S. Masuo, S. Machida, A. Itaya: Application of dopant-induced laser ablation to site-selective modification of sea– island structures of polystyrene-block-poly(4vinylpyridine) films. Jpn. J. Appl. Phys. 2005, 44, L402–L404. Copyright the Japan Society of Applied Physics. Reproduced with permission.

induced laser ablation is that ablation is induced by excitation not of the target polymer film itself but of a small amount of the photosensitizer doped in the polymer film. By choosing a molecule with a large p-electronic conjugated system as a dopant, the ablation of polymer films is induced easily with longer wavelength lasers and lower fluences than those employed in laser ablation with excitation of the polymer film itself. For photostable dopants, the cyclic-multiphoton absorption mechanism


has been proposed for the dopant-induced laser ablation [34]. This mechanism is applied to molecules whose transient states have substantial absorption coefficients at the excitation wavelength. In the mechanism, relaxation from the excited states of the transient states of a dopant to the transient states involves internal conversion in the dopant and subsequent intermolecular vibrational energy transfer from the dopant to the surrounding polymer matrix. Thus, the polymer matrix is heated up repeatedly during the multiphoton absorption. The irradiation of a laser with high fluences results in rapid thermal decomposition of the polymer matrix, that is, ablation. As aforementioned, diblock copolymer films have a wide variety of nanosized microphase separation structures such as spheres, cylinders, and lamellae. As described in the above subsection, photofunctional chromophores were able to be doped site-selectively into the nanoscale microdomain structures of the diblock copolymer films, resulting in nanoscale surface morphological change of the doped films. The further modification of the nanostructures is useful for obtaining new functional materials. Hence, in order to create further surface morphological change of the nanoscale microdomain structures, dopant-induced laser ablation is applied to the site-selectively doped diblock polymer films. First, the laser ablation behavior of PS and P4VP homopolymer films and TCPPdoped P4VP homopolymer films was investigated. PS and P4VP homopolymer films show no absorption at a laser wavelength of 532 nm (second harmonic output of a Nd3 þ :YAG laser with full width at half maximum of about 8 ns); however, the ablation phenomena of these films were observed for laser irradiation with this wavelength. Hence, the ablation is likely to be attributed to multiphoton absorption of these films. The ablation thresholds were determined to be about 190 and 320 mJ cm–2 for PS and P4VP films, respectively. The ablation thresholds of the TCPP-doped P4VP films were about 220 and 150 mJ cm2 for dopant concentrations of 1.0 and 3.5 wt%, respectively, which corresponds to the previous reports that indicate that the ablation threshold strongly depends on the absorbance, that is, the concentration of the dopant [16, 35–38]. Since these values are smaller than that of the neat P4VP films, the dopant-induced laser ablation of TCPP-doped P4VP films is successfully induced upon excitation at this wavelength. According to the above ablation thresholds, the concentration of TCPP is considered to be one of the important parameters for ablating the P4VP parts effectively. However, it is difficult to measure the concentration of TCPP doped in the P4VP parts of PS-b-P4VP films. Thus, it is difficult to estimate the ablation thresholds of TCPP-doped P4VP parts. Hence, we irradiated TCPP-doped PS-b-P4VP films with laser fluences lower than the threshold fluence of PS parts to prevent damage to the PS parts but to induce the selective ablation of the TCPP-doped P4VP parts. Next, laser ablation of the TCPP-doped PS-b-P4VP (301 000 : 19 600) films with a sea–island structure was carried out in air. However, ablation of the P4VP-island parts did not occur after one-shot laser irradiation with a fluence of 170 mJ cm2, but both the P4VP-island and PS-sea parts were ablated by laser irradiation with fluences higher than 190 mJ cm2. Hence, the ablation threshold of the P4VP-island parts doped with TCPP may be even higher than 170 mJ cm2 in air.

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As mentioned, methanol is a good solvent for P4VP and a poor solvent for PS. When the TCPP-doped thin films are immersed in methanol, the TCPP-doped P4VPisland parts swell. However, TCPP molecules doped in the P4VP-island parts do not dissolve in methanol, because the TCPP molecules are connected with P4VP chains via multiple hydrogen bonds. This is supported by the fact that the surface morphology of the TCPP-doped thin films was unaffected by immersing the film in methanol. Thus, the laser ablation experiment of the films was carried out in methanol. Figure 12.6c and d show AFM images of the films irradiated using one laser shot with fluences of 110 and 150 mJ cm2, respectively. For the irradiation in methanol, a selective ablation of the TCPP-doped P4VP-island parts is obtained. However, for ablation with a fluence of 110 mJ cm2, a small number of fragments are redeposited. The irradiation with a fluence of 150 mJ cm2 induces the ablation of the topside of the island parts, and the shape of the island parts becomes that of a flower. The average heights of the edge and center in the flowerlike-shaped island parts were about 5 and 3 nm from the surface of the surrounding sea parts, respectively (Figure 12.6g). These small etch depths suggest that the distribution of TCPP in the P4VP-island parts is not homogeneous; the concentration of TCPP in the topside is larger than that in the underside. For ArF excimer laser (193 nm) ablation of hydrated collagen gel films in the swollen state, the boiling of the water is reported to be responsible for the ablation [39]. In the present case, the photon energy is low (532 nm), and laser fluences are not particularly high. In addition, the swollen island parts are covered with methanol. Hence, laser irradiation is unlikely to induce the boiling of methanol in the swollen parts. That is, the ablation of the swollen P4VP-island parts is not attributable to the boiling of methanol. A portion of the irradiated energy is considered to be absorbed by TCPP, and dopant-induced laser ablation occurs in the swollen state, but not in the dry state. The dopant-induced laser ablation can likely be attributed to the photothermal effect due to the cyclic multiphotonic absorption mechanism. The reason why the ablation threshold for the swollen state is lower than that for the dry state is still unclear. In this experiment, however, it is suggested that the swollen state causes the selective ablation of TCPP-doped P4VP-island parts to become easier. When the doped films were irradiated with a fluence of 150 mJ cm2 in a methanol vapor environment, where the TCPP-doped P4VP-island parts were in a swollen state as they were in methanol, selective ablation was not induced as it was in the dry condition. This can likely be attributed to the low degree of the swollen state of the TCPP-doped P4VP parts in solvent vapor compared with the degree in the solvent. In the present experiment, the concentration of TCPP is one of the most important parameters for obtaining a sufficient laser energy for ablating the island parts. In order to remove further P4VP-island parts, we tried to dope TCPP into the sample films again after the first laser irradiation and to irradiate the sample film. However, AFM images showed that the average height of the P4VP-island parts did not increase after the second doping and that the remaining parts of the island could no longer be ablated. These results suggest that the first laser irradiation induces not only the ablation of P4VP-island parts but also a cross-linking reaction among P4VP chains. The presence of the cross-linking reaction was suggested by the fact that the solubility


rate of the surface of the TCPP-doped P4VP homopolymer films in methanol was smaller for the part irradiated in acetone solvent than for the nonirradiated one. As for PS-b-P4VP (162 400 : 87 400) films with a different surface morphology (Figure 12.3), the conglomeration parts on the film surface are ablated away selectively to a certain extent by one-shot laser irradiation with a fluence of about 170 mJ cm–2 in air (Figure 12.3d and j). On the other hand, one-shot laser irradiation with a fluence of about 150 mJ cm2 in methanol results in a further selective ablation of P4VP-cylinder parts, although the degree of ablation is not homogeneous for each cylinder (Figure 12.3e and k). The latter result suggests that the concentration of TCPP doped into each cylinder is not homogeneous and/or that the phase separation structures in the interior of the present thick films are not uniform throughout the films [29]. The same film was also irradiated using 100 shots of the laser pulse with the same fluence in methanol. Figure 12.3f and l show an AFM image of this irradiated film and the height profile of the line indicated in the image, respectively. The depths of the ablated cylinder parts indicate that the TCPP-doped P4VP-cylinder parts are selectively ablated away to a larger extent by 100-shot laser irradiation. However, the laser irradiation of 100 shots results in a morphological change of the PS-matrix parts. That is, the PS parts were also ablated in part. As aforementioned, site-selective laser ablation was also successful using the diblock copolymer films with a nanosize regular network-like phase separation structure with P4VP cylinders in PS matrices. In the present case, because of the large film thickness, a large amount of TCPP was doped into the P4VP-cylinder parts compared with the amount doped into the above-mentioned films with the sea–island structure. This larger concentration of TCPP also induced selective ablation in air, although the degree of ablation was low. A further selective ablation of the doped matrices was also achieved by irradiation in methanol. For symmetric PS-b-P4VP (20 000 : 19 000) diblock copolymer films with the wormlike phase separation structures, the TCPP-doped films were irradiated using one laser shot with a fluence of 150 mJ cm2 in air. The ablation phenomenon is observed for this irradiation fluence (Figure 12.5c and f), but it is difficult to conclude that this is a selective ablation of the doped-P4VP parts. We cannot deny the possibility that the decomposition of the P4VP parts affects the PS parts because of the existence of large interfaces between the two symmetric blocks in wormlike structures. Thus, for the site-selective ablation of diblock copolymer films, the surface morphology of the phase separation structures is one of the most important parameters. Next, in order to investigate the effect of the dye on selective ablation, another dye, namely, aurintricarboxylic (ATA), was doped selectively into the PS-b-P4VP (162 400 : 87 400) diblock copolymer films spin-coated from a 3-pentanone solution using the same immersion method. As shown in Figure 12.7, the surface morphology of the films was changed markedly by the doping with ATA (Figure 12.7a), and the morphology of the ATA-doped films is very different from that of the TCPPdoped films (Figure 12.4c). The ATA-doped copolymer films were irradiated using one laser shot with a fluence of 150 mJ cm–2 in air. An AFM image of the irradiated films and the height profile of the line indicated in the image are shown in

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Figure 12.7 AFM images of thin PS-b-P4VP (162 400 : 87 400) films (3-pentanone solvent) on glass substrates, height profiles of horizontal lines in these images. (a), (c) After being doped with ATA, and (b), (d) ATA-doped films irradiated using one shot with fluence of about 150 mJ cm2 in air. (c) and (d) Show the height profiles of the horizontal lines shown in the AFM

images of (a) and (b), respectively. Z. Wang, S. Masuo, S. Machida, A. Itaya: Site-selective doping of dyes into polystyrene-block-poly(4-vinyl pyridine) diblock copolymer films and selective laser ablation of the dye-doped films. Jpn. J. Appl. Phys. 2007, 46, 7569–7576. Copyright the Japan Society of Applied Physics. Reproduced with permission.

Figure 12.7b and d, respectively. Selective laser ablation of the ATA-doped P4VP parts is observed, even though some parts are not ablated completely. According to Figure 12.7d, some matrices are ablated markedly, particularly where the substrate surface comes out. In the present case, we also achieved selective ablation under dry conditions, which is attributed to a sufficient number of ATA molecules being present in the P4VP components. In conclusion, when the diblock copolymer films, whose P4VP parts were selectively doped with dyes, were irradiated using appropriate laser fluences lower than the threshold fluences of the PS and P4VP parts, selective ablation of the P4VP parts in the PS-b-P4VP films was obtained. The concentration of dyes in the P4VP parts was an important parameter for ablating P4VP parts effectively. For the effect of irradiation environments, further selective ablation of the dye-doped parts was obtained in methanol with respect to the ablation both in solvent vapor and under dry conditions, suggesting that the dye-doped parts have a lower ablation threshold in the swollen state and that the ablation thresholds depend on the degree of the swollen state. For the effect of the dye, it was suggested that the difference in the cross-linked structures between the TCPP-doped and the ATA-doped P4VP parts is an important factor both for the morphology of the nanoscale phase-separation structures of the dye-doped films and for the photothermal-energy release relating to ablation. For the effect of the number-average molecular weight of the diblock copolymers, we considered the following two factors: (i) The amount of dye per diblock copolymer; diblock copolymers with a large number-average molecular weight of P4VP blocks can contain a large number of dye molecules. (ii) The morphology of the phase separation structures of the neat films. In sea–island and network-like structures,

12.5 Photon Antibunching Behavior of Organic Dye Nanocrystals on a Transparent Polymer Film

selective ablation was achieved easily when the doped parts were the island parts or cylinder parts, respectively. However, in worm-like structures, both PS and P4VP parts were easily ablated away together, because of the larger interfaces between the two blocks. To our knowledge, there has been no report concerning the dopantinduced selective ablation of block copolymer films. No completely selective ablation was obtained in the present study, but we were able to confirm the possibility of obtaining a completely selective ablation of block copolymer films. We consider that a completely selective ablation of block copolymer films can be carried out when the selective doping of dopant is improved such that an increase in the dopant concentration in the target domains occurs.


As mentioned, photon antibunching is when the probability of detecting two simultaneous photons drops to zero. Hence, the materials that exhibit photon antibunching are called single-photon emitters or single-photon sources. Since molecules cannot emit two photons simultaneously, a single molecule is the typical single-photon source [25, 26]. However, it is considered that multichromophoric systems can also be single-photon sources if one exciton remains as a result of exciton migration and subsequent exciton–exciton annihilation. It was demonstrated, on the basis of nanosecond laser flash photolysis of poly(N-vinylcarbazole) in solution, that only one excited-state remains in the single polymer chain, even when more than one excited-state is generated by an intense excitation pulse [40]. Densely generated excited-states undergo mutual interaction such as singlet–singlet annihilation during their migration along the polymer chain, and this sequential annihilation results in one excited-state in the single chain. This idea was extended to multichromophore-substituted dendrimer molecules which have definitively designed interchromophore distances and the number of chromophores, and singlephoton emission from single dendrimers was observed by single molecule spectroscopy [41, 42]. The results suggest that multichromophoric systems also behave as single-photon sources so long as efficient exciton migration and exciton–exciton annihilation are possible. In this section, we report that the emission from a single nanocrystal consisting of N,N0 -bis(2,6-dimethylphenyl)-3,4,9,10-perylenedicarboxyimide (DMPBI) molecules shows photon antibunching. The nanoparticles of DMPBI prepared by the reprecipitation method [43, 44] are square or rectangular in shape (Figure 12.8a), suggesting strongly that they are in a crystalline state. This was also confirmed by the X-ray diffraction measurement. The size distribution of the nanocrystals (Figure 12.8b) indicates that the size of the longaxis ranges from 35 to 85 nm, and that the average size is 50 nm. Figure 12.8c shows the absorption and fluorescence spectra of the DMPBI nanocrystal-dispersed aqueous solution and the DMPBI chloroform solution. The spectra of the nanocrystals are quite different from those of the DMPBI molecules in solution. These differences

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Figure 12.8 (a) FESEM image of prepared nanocrystals. (b) Size distribution of the nanocrystals evaluated from the FESEM image (a). This distribution was built up by scaling the long-axis of 100 individual nanocrystals. (c) Absorption and fluorescence spectra of the DMPBI nanocrystal-dispersed aqueous solution (solid line), and of the DMPBI chloroform

solution (dotted line). S. Masuo, A. Masuhara, T. Akashi, M. Muranushi, S. Machida, H. Kasai, H. Nakanishi, H. Oikawa, A. Itaya: Photon antibunching in the emission from a single organic dye nanocrystal. Jpn. J. Appl. Phys. 2007, 46, L268–L270. Copyright the Japan Society of Applied Physics. Reproduced with permission.

indicate that a strong intermolecular interaction exists in the nanocrystals, and the fluorescence spectrum of the nanocrystals is attributed to a self-trapped exciton state (excimer-like emission) [45]. The photon antibunching measurement of single nanocrystals was carried out using a classical Hanbury-Brown and Twiss type photon correlation set-up [46] combined with pulsed laser excitation (488 nm, 8.03 MHz, 100 fs fwhm). Single nanocrystals were selected from a scanned image and positioned in the laser focus of a confocal microscope (objective: 100 , numerical aperture ¼ 1.3) in order to collect the emission from the single nanocrystal. The emitted photons over all wavelengths were collected by the same objective, divided by a 50/50 nonpolarizing beam splitter, and detected by two avalanche photodiodes. An aqueous solution with an appropriate concentration of the nanocrystals was spin-coated onto PMMA-coated clean coverglasses. The samples were measured under nitrogen atmosphere at room temperature. First, the excitation-power dependence of the emission count rate of single DMPBI nanocrystals was examined (Figure 12.9). The emission count rate of the single


Emission count rate (kHz)

300

200

100

0 0

1 2 3 4 Excitation power (kW/cm2)

Figure 12.9 The excitation-power dependence of the emission count rate of single DMPBI nanocrystals (dots), and a saturation curve calculated from a two-level model (solid line). One count rate value to one laser power was calculated as an average of 30 nanocrystals. S. Masuo, A. Masuhara, T. Akashi, M. Muranushi,

5

S. Machida, H. Kasai, H. Nakanishi, H. Oikawa, A. Itaya: Photon antibunching in the emission from a single organic dye nanocrystal. Jpn. J. Appl. Phys. 2007, 46, L268–L270. Copyright the Japan Society of Applied Physics. Reproduced with permission.

nanocrystals saturates for laser powers over 4 kW cm2, and is not well fitted by the saturation law of a two-level system, which is the same as the case of the multichromophoric dendrimers [41]. If the single nanocrystals emit one photon every excitation pulse, the maximum count rate should reach 400 kHz by taking into account the excitation laser repetition rate of 8.03 MHz and the detection efficiency of about 5%. However, the measured count rate reaches a maximum value of 250 kHz; therefore, the emission efficiency of the nanocrystals is evaluated to be 0.63. For the photon correlation measurements, excitation laser power of 2–4 kW cm2 was used, and thereby some excitons were generated in a single nanocrystal by a single excitation pulse. Typical results of simultaneous measurements for a single nanocrystal (time traces of emission intensity and lifetime, and photon correlation) are shown in Figure 12.10. The time traces of both the emission intensity and lifetime show a constant value (Figure 12.10a and b), and the lifetime is about 3.7 ns, indicating that the nanocrystal behaves like a single molecule. The central peak at 0 ns in the photon correlation histogram (Figure 12.10c) corresponds to the photon pairs induced by the same laser pulse. In all other cases, the interphoton times are distributed around a multiple of the repetition rate of the laser pulses (8.03 MHz), that is, one peak every 125 ns. In Figure 12.10c, there is apparently no central peak at 0 ns, only the background is observed. This means that the emission from the nanocrystal shows photon antibunching, that is, this nanocrystal behaves as a single-photon source. It was reported that the ratio (NC/NL) of the number of photon pairs contributing to the central peak, NC, to the average number of counts in the lateral peaks, NL, can be used to estimate the number of photons induced by the same laser pulse. NC/NL ratios of 0.0, 0.5, 0.67, and 0.75 are expected for 1, 2, 3, and 4 photons per one laser pulse, respectively [47].

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Figure 12.10 Typical time traces of (a) emission intensity and (b) lifetime, measured from a single DMPBI nanocrystal. (c) Photon correlation histogram obtained from the time trace of the emission intensity (a). The lifetimes were obtained by fitting a single exponential function to the decay curves constructed for every 2000

photons. S. Masuo, A. Masuhara, T. Akashi, M. Muranushi, S. Machida, H. Kasai, H. Nakanishi, H. Oikawa, A. Itaya: Photon antibunching in the emission from a single organic dye nanocrystal. Jpn. J. Appl. Phys. 2007, 46, L268–L270. Copyright the Japan Society of Applied Physics. Reproduced with permission.

However, a nonzero background in the experiment always produces a small contribution from the detection events that fall in the central NC peak. For example, a signalto-background ratio of 7 leads to an NC/NL ratio of about 0.2. In the present case, an NC/NL ratio of 0.06 was determined. Hence, the NC/NL value also indicates that the present nanocrystal behaves as a single-photon source. To investigate the antibunching behavior of other nanocrystals, the photon correlations of 213 different nanocrystals were measured, and the histogram of their NC/NL ratios is shown in Figure 12.11. Most of the ratios are distributed around a fairly low value of about 0.1. This result indicates that most of the single nanocrystals of size 35–85 nm behave as single-photon sources. An intense single excitation pulse of the present excitation condition generates simultaneously several excitons in the single nanocrystals. In order to behave as a single-photon source even when more than one exciton is generated, efficient exciton–exciton annihilation has to occur in the nanocrystal, while inter-exciton distances for the efficient annihilation have to be below a few nanometers. Thus, efficient exciton migration also has to occur in the nanocrystals in order to show photon antibunching. As shown in Figure 12.8c, spectral overlap between the absorption and fluorescence spectra of DMPBI nanocrystals is observed; consequently, excitons can migrate in the present nanocrystal. Hence, it can be regarded that the photogenerated excitons can access each other by migration and efficient exciton–exciton annihilation can occur, resulting in the photon antibunching. However, the maximum size of the nanocrystal which shows the photon antibunching has not yet been revealed. In general, the migration length of an exciton in a molecular assembly depends strongly on both the spectroscopic properties of the chromophore and the molecular alignment in the assembly. In addition, defects in the assembly also play an important role in the exciton dynamics. In another experiment, we confirmed that several-micrometer-sized DMPBI crystals

References

Figure 12.11 Histogram of NC/NL ratios obtained from the photon correlation measurement of 213 nanocrystals. S. Masuo, A. Masuhara, T. Akashi, M. Muranushi, S. Machida, H. Kasai, H. Nakanishi, H. Oikawa, A. Itaya: Photon antibunching in the emission from a single organic dye nanocrystal. Jpn. J. Appl. Phys. 2007, 46, L268–L270. Copyright the Japan Society of Applied Physics. Reproduced with permission.

show no antibunching; therefore, the antibunching behavior of the DMPBI crystals is limited to nanometer-sized crystals. In this section, we have demonstrated that a single organic dye nanocrystal comprised of many chromophores shows photon antibunching when the size is sufficiently small. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources.

Acknowledgments

The present work was partly supported by a Grant-in –Aid for Scientific Research in the Priority Area Molecular Nano Dynamics from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors wish to express their sincere thanks to Professor H. Nakanishi, Professor H. Oikawa, Dr H. Kasai, Dr A. Masuhara, Dr Z. Wang, Dr K. Yamada, H. Nakata, T. Akashi, and M. Muranushi.

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29 Gong, Y., Huang, H., Hu, Z., Chen, Y., Chen, D., Wang, Z. and He, T. (2006) Inverted to normal phase transition in solution-cast polystyrene-poly(methyl methacrylate) block copolymer thin films. Macromolecules, 39, 3369–3376. 30 Funaki, Y., Kumano, K., Nakao, T., Jinnai, H., Yoshida, H., Kimishima, K., Tsutsumi, K., Hirokawa, Y. and Hashimoto, T. (1999) Influence of casting solvents on microphase-separated structures of poly(2vinylpyridine)-block-polyisoprene. Polymer, 40, 7147–7156. 31 Zhang, Q., Tsui, O. K. C., Du, B., Zhang, F., Tang, T. and He, T. (2000) Inverted to normal phase transition in solution-cast polystyrene-poly(methyl methacrylate) block copolymer thin films. Macromolecules, 33, 9561–9567. 32 Huang, H., Zhang, F., Hu, Z., Du, B., He, T., Lee, F. K., Wang, Y. and Tsui, O. K. C. (2003) Study on the origin of inverted phase in drying solution-cast block copolymer films. Macromolecules, 36, 4084–4092. 33 Gong, Y., Hu, Z., Chen, Y., Huang, H. and He, T. (2005) Ring-shaped morphology in solution-cast polystyrene-poly(methyl methacrylate) block copolymer thin films. Langmuir, 21, 11870–11877. 34 Fukumura, H. and Masuhara, H. (1994) The mechanism of dopant-induced laser ablation. Possibility of cyclic multiphotonic absorption in excited states. Chem. Phys. Lett., 221, 373–378. 35 Bolle, M., Luther, K., Troe, J., Lhlemann, J. and Gerhardt, H. (1990) Photochemically assisted laser ablation of doped polymethyl-methacrylate. Appl. Surf. Sci., 46, 279–286. 36 Fukumura, H., Mibuka, N., Eura, S. and Masuhara, H. (1991) Porphyrin-sensitized laser swelling and ablation of polymer films. Appl. Phys. A, 53, 255–259. 37 Fukumura, H., Mibuka, N., Eura, S., Masuhara, H. and Nishi, N. (1993) Mass spectrometric studies on laser ablation of polystyrene sensitized with anthracene. J. Phys. Chem., 97, 13761–13766.

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13 Molecular Segregation at Periodic Metal Nano-Architectures on a Solid Surface Hideki Nabika and Kei Murakoshi

13.1 Molecular Manipulation in Nano-Space

Exploring new molecular diffusion modes in confined spaces has long been an attractive challenge for both science and technology. When free space is reduced to sizes below a few hundred nanometers, interaction between molecules and their confining walls cannot be ignored. Especially for a macromolecule such as DNA or proteins, geometrical constraint significantly alters the molecular diffusion dynamics. One trend in this field is to apply these unique diffusion phenomena to a molecular separation or sieving system in microfluidic devices [1–3]. These findings help to promote the development of future integrated bio-analytical devices. In order to manipulate biomaterials in these microfluidic systems, it is useful to use a lipid bilayer as a molecular manipulation and separation medium in order to avoid alternating the structure and properties of the biomaterials. Below we will first describe how a molecule diffuses in a two-dimensional lipid bilayer. Then, several attempts to apply the phenomenon for molecular manipulation and separation techniques in both solid-supported and self-spreading lipid bilayers are introduced. 13.1.1 Lipid Bilayer and its Fluidic Nature

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects of biological cell membranes [5–7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, twodimensional fluidity, bio-compatibility, elasticity, and rich chemical composition.

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Figure 13.1 Schematic illustrations of vesicle fusion process. (a) An adsorbed vesicle ruptures and forms a bilayer. (b) Two adjacent vesicles fuse and eventually rupture. (c) A ruptured bilayer patch promote the rupture of the adjacent vesicle. (d) The cooperative action of the vesicle rupture. Adapted from Ref. [9] with permission.

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir–Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11–16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17]. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. Due to the high compatibility of artificial lipid bilayers with biomolecules, they are often used as a bio-assay or bio-detection system [22]. For development of combinatorial analysis based on the lipid bilayer, a substrate on which has been deposited a well-defined micro- or nano-patterned lipid bilayer has been fabricated in various ways [22–30]. Well-defined patterning of the lipid bilayer can be used to fabricate substrates to be used as bio- or chemical-sensors and in drug screening. Normally, patterning based on the vesicle fusion process yields a single component pattern of the lipid bilayer [23, 26, 30]. Alternatively, multi-component bilayer patterns can be obtained via multistep vesicle fusion [24] or dip-pen nanolithography (DPN) (Figure 13.2) [29]. For an artificial lipid bilayer of any size scale, it is a general feature that the bilayer acts as a two-dimensional fluid due to the presence of the water cushion layer between the bilayer and the substrate. Due to this fluidic nature, molecules incorporated in the lipid bilayer show two-dimensional free diffusion. By applying any bias for controlling the diffusion dynamics, we can manipulate only the desired molecule within the artificial lipid bilayer, which leads to the development of a molecular separation system.


Figure 13.2 Fluorescence micrographs of DOPC multi-layer patterns fabricated by dip-pen nanolithography. (a) An array of 25 contiguous line features. Red color is from doped rhodaminelabeled lipid. (b) A higher magnification of the region highlighted by the white square in (a). (c) Two-component patterns containing two different dyes. Green color is from doped NBD-labeled lipid. Scale bars: 5 mm. Adapted from Ref. [29] with permission.

13.1.2 Controlling Molecular Diffusion in the Fluidic Lipid Bilayer

Direct observation of molecular diffusion is the most powerful approach to evaluate the bilayer fluidity and molecular diffusivity. Recent advances in optics and CCD devices enable us to detect and track the diffusive motion of a single molecule with an optical microscope. Usually, a fluorescent dye, gold nanoparticle, or fluorescent microsphere is used to label the target molecule in order to visualize it in the microscope [31–33]. By tracking the diffusive motion of the labeled-molecule in an artificial lipid bilayer, random Brownian motion was clearly observed (Figure 13.3) [31]. As already mentioned, the artificial lipid bilayer can be treated as a two-dimensional fluid. Thus, an analysis for a two-dimensional random walk can be applied. Each trajectory observed on the microscope is then numerically analyzed by a simple relationship between the displacement, r, and time interval, t, hr 2 i ¼ 4Dt

ð13:1Þ

where hr2i and D are the mean-square displacement (MSD) and the two-dimensional (lateral) diffusion constant, respectively. MSD plots shown in Figure 13.3 demonstrate

Figure 13.3 (a–c) Trajectories of the diffusion motion of a gold nanoparticle probe on a planar lipid membrane. (d) Mean-square displacement plots for the diffusion shown in (a–c). Adapted from Ref. [31] with permission.

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a linear increase in a short time interval region, which is characteristic of random diffusion. It should be noted here that molecular diffusion is strongly suppressed when the lipid matrix is in the gel phase [34]. In this chapter, only the lipid bilayer in the liquid phase will be treated as a molecular manipulating medium. In addition to the direct observation of the diffusing molecules, fluorescence correlation spectroscopy (FCS) [35–37] and fluorescence recovery after photobleaching (FRAP) [38, 39] are also useful alternative techniques to characterize the molecular diffusivity in the lipid bilayer. However, we have to note that the data acquired from FCS and FRAP are essentially based on the ensemble characteristics derived from a number of molecules. In any experimental approach, the artificial lipid bilayer on the solid substrate demonstrated sufficient fluidity for molecular diffusion if the temperature was above a phase transition temperature. Two-dimensional fluidity enables the manipulation of molecules in the bilayer by applying an appropriate bias. A typical example of this is electrophoresis, a method in which an electric field is applied to the bilayer [40–42]. Charged molecules present in the bilayer drift along the electric field lines. The drift velocity is dependent on the electric field strength, the size and charge of the molecule, bilayer fluidity, and so on. Molecular dependence of the drift velocity is a basic principle of an electrophoretic molecular separation (Figure 13.4) [43]. Recent investigations have revealed that electrophoresis on the artificial lipid bilayer can separate Texas Red DHPE isomers [44]. A POPC bilayer doped with 20 mol% cholesterol was used. The addition of cholesterol reduces band broadening during the electrophoresis. After applying an electric field of 100 V for 30 min, two Texas Red DHPE isomers were separated into two distinct bands in the POPC bilayer. Other than the lipid molecules, DNA-tethered vesicles and GPI-tethered proteins can also be separated via electrophoresis [45, 46]. As mentioned above, the charged molecules drift along the electric field direction in the electrophoretic manipulation. In principle, the drift direction is determined by the electric field direction. Differences in the drift velocity are what act to separate

Figure 13.4 Schematic illustration of the electrophoretic molecular separation. (a) The charged molecules drift according to the electric field direction. (b) Separation of each fraction by applying several separate laminar flows. (c) Alternatively, each fraction can be separated by scanning the stripping laminar flow across the sample channel. Adapted from Ref. [43] with permission.


molecules into isolated bands. However, constructing an asymmetric obstacle on the substrate can modify and control the drift direction. This method of drift manipulation is known as the process based on the Brownian ratchet mechanism [47]. The deviation angle is determined by the original drift velocity, the diffusion coefficient, and the structure of the obstacle. This system is capable of separating molecules even if they have the same drift velocity but different diffusion constants. All of these electrophoretic manipulations in the lipid bilayer can be used to purify or separate biomaterials in a micro-fluid. However, electrophoresis can manipulate only charged molecules, which is a severe limitation of the electrophoretic system. To develop a versatile system that enables the manipulation of any molecule irrespective of its charge, a new concept must be introduced. One possibility is to exploit the selfspreading nature of a lipid bilayer, a macroscopic fluidic phenomenon, that will be introduced in the next section. 13.1.3 Self-Spreading of a Lipid Bilayer or Monolayer

The lipid bilayer grows spontaneously from a lipid aggregate on a hydrophilic substrate on immersing the substrate in an aqueous solution (Figure 13.5) [48, 49]. This phenomenon is called the self-spreading of a lipid bilayer. Any molecules in the lipid bilayer can be collectively transported by the molecular flow of the selfspreading. Since the self-spreading is a thermodynamically driven phenomenon, no input energy such as an electric field is needed for the molecular transportation. By taking advantage of this aspect, we can transport and manipulate any collection of molecules in the lipid bilayer, irrespective of their charge. By constructing a microchannel on the substrate, we can control the direction of the self-spreading. Furukawa et al. fabricated photo-lithographic micro-channels with a width of 1–20 mm [50].

Figure 13.5 (a) Fluorescence micrograph of the self-spreading lipid bilayer doped with a dye molecule. The lipid bilayer spread on an oxidized silicon wafer from a deposited lipid aggregate illustrated on the left. (b) A schematic drawing of the selfspreading lipid bilayer from the lipid aggregate. Adapted from Ref. [48] with permission.

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Figure 13.6 (a) Confocal micrograph of a circularly self-spreading lipid monolayer. A rhodamine-labeled lipid is doped to visualize the spreading behavior. (b) A schematic illustration of the front edge of the self-spreading lipid monolayer [51].

By placing the lipid aggregate near the micro-channel, the bilayer spreads into the micro-channel without changing the spreading dynamics. This system has several advantages compared with a conventional three-dimensional microfluidic system. First, input energy for the molecular transport is no longer needed, whereas the conventional system relies on an external bias such as a syringe pump or an electric field. This enables the miniaturization and simplification of the device. The second advantage is the low dimensionality of the lipid bilayer. The conventional system manipulates molecules in three-dimensional free space. However, the lipid bilayer confines the molecules into a quasi-two-dimensional plane. A small amount of molecule can be concentrated in the bilayer, leading to improvement in the manipulation and detection limits. Additionally, the lipid bilayer offers a field to manipulate a biomolecule in its native environment. In addition to the self-spreading lipid bilayer, it was also found that a lipid monolayer showed similar spreading behavior on a hydrophobic surface (Figure 13.6) [51]. By fabricating an appropriate hydrophobic surface pattern, the spreading area and direction can be easily controlled. For both the self-spreading bilayer and monolayer, non-biased molecular transportation is an important key concept for the next generation of microfluidic devices. 13.1.4 Controlling the Self-Spreading Dynamics

To utilize the self-spreading bilayer in microfluidic devices, an understanding of their dynamics and other characteristics is necessary. The driving energy for the selfspreading has been explained as a gain in free energy via bilayer–substrate interaction [49]. Similar to the supported bilayer prepared by vesicle fusion, a hydration water layer is inserted under the spreading bilayer. Through this hydration layer, several bilayer–substrate interactions are imposed, such as the van der Waals interaction. Interactions involving lipid molecules and lipid membranes have long been investigated and discussed from both experimental and theoretical


Figure 13.7 (a) The self-spreading distance and (b) velocity of egg-PC lipid bilayer in NaCl aqueous solutions with different concentrations. () 100 mM, () 10 mM, and (^) 1 mM. Adapted from Ref. [53] with permission.

viewpoints [52]. Owing to this vast knowledge, the bilayer–substrate interaction energy can be controlled via several experimental conditions. For example, a dependence on the electrolyte concentration has been investigated by experimental and theoretical approaches [53]. Figure 13.7 shows the experimental results for the dependence of the spreading distance and the velocity on the NaCl concentration. It was clearly demonstrated that the spreading velocity increased on increasing the NaCl concentration. The self-spreading velocity as a function of time is given by 1 1 log vðtÞ ¼ log b log t 2 2

ð13:2Þ

where v, b, t are the spreading velocity, spreading coefficient, and time, respectively. Fitting the experimental date to Eq. (13.2) gives the value of b as 48, 33, and 21 mm2 s1 in 100 mM, 10 mM, and 1 mM NaCl solutions, respectively. The spreading coefficient b is given by b¼

Ed 2h

ð13:3Þ

where E, d, h are the bilayer–substrate interaction energy, thickness of the water layer, and water viscosity, respectively. As is clear from Eq. (13.3), the spreading dynamics are closely correlated with the bilayer–substrate interaction energy. Assuming d ¼ 2 nm and h ¼ 103 N s m2, we can expect the value of E to be 48, 33, and 21 mJ m2 in 100 mM, 10 mM, and 1 mM NaCl solutions, respectively. For a theoretical estimation of E, three interaction energies were considered as the dominant components, that is, the van der Waals, the electrostatic double layer, and the hydration energies. Figure 13.8 shows the calculated interaction energy curves considering these three interaction energies on an egg-PC bilayer and hydrophilic glass substrate system. The minimum at around 2 nm corresponds to E in Eq. (13.3). The theoretically estimated values for E were calculated as 35, 33, 22 mJ m2 in 100 mM, 10 mM, and 1 mM NaCl solutions, respectively, which is in good agreement with the experimental values.

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Figure 13.8 Calculated interaction energy curves for egg-PC on a surface-oxidized silicon substrate system. The NaCl concentration is 100 mM (solid line), 10 mM (broken line), and 1 mM (dotted line). Adapted from Ref. [53] with permission.

This result demonstrates that the self-spreading dynamics are controllable by tuning the bilayer–substrate interactions. The above-mentioned electrolyte dependence is an example of this fact. Considering that there are many parameters that alter the bilayer–substrate interaction, a diverse approach can be proposed. For example, Nissen et al. investigated the spreading dynamics on the substrate coated with polymetic materials [48]. They found that insertion of a hydrophilic and inert polymer layer under the self-spreading lipid bilayer strongly attenuated the bilayer–substrate interaction. Of course, other physical and chemical conditions also affect the self-spreading dynamics. Figure 13.9 shows the dependences of b on the temperature and lipid

Figure 13.9 (a) Temperature dependence of b on the DMPC bilayer on glass (white) and silicon (black). (b) Dependence of b on the molar fraction of a cationic DMTAP additive in the DMPC bilayer. Adapted from Ref. [48] with permission.


composition. A gradual increase in b with temperature has been explained as being due to a change in the viscosity of water. However, the observed increase was more rapid than the change in the water viscosity. This discrepancy was attributed to the formation of water clusters in the hydration layer. It should also be taken into consideration that the viscosity of the lipid assembly itself is dependent on the temperature. This will change the frictional force present at the transfer of lipid molecules from the aggregate to the spreading bilayer, which Nabika et al. assumed to be negligible. Figure 13.9a demonstrates the bilayer composition dependence of the DMPC/DMTAP binary system. The bilayer composition is a critical parameter that determines the bilayer–substrate interaction energy, the bending energy of the bilayer, the viscosity, domain formation, and so on. DMTAP is a cationic lipid, with which the bilayer can interact more strongly with the negatively charged glass substrate. Thus, the spreading velocity was expected to be increased with DMTAP content. However, the experimental results showed the completely opposite behavior. This fact strongly suggests that it is not sufficient to consider only the bilayer–substrate interaction for a comprehensive understanding of the self-spreading dynamics. By comparing the structures of DMPC and DMTAP, it is clear that the structure of the head group is different and that DMTAP has a smaller head group. Thus, addition of DMTAP disturbs the formation of a thermodynamically stable bilayer structure. This energy cost reduces the self-spreading driving energy, which could be one of the reasons why the addition of DMTAP led to a decrease in b. In addition to the spreading dynamics, the stacking structure of the self-spreading lipid bilayer is also controllable via the NaCl concentration [54, 55]. Further experimental and theoretical investigations regarding the control of self-spreading are required before we will be able to easily control the self-spreading behavior in microfluidic devices. 13.1.5 Molecular Manipulation on the Self-Spreading Lipid Bilayer

The most intriguing aspect of the self-spreading lipid bilayer is that any molecule in the bilayer can be transported without any external bias. The unique characteristic of the spreading layer offers the chance to manipulate molecules without applying any external biases. This concept leads to a completely non-biased molecular manipulation system in a microfluidic device. For this purpose, the use of nano-space, which occasionally offers the possibility of controlling molecular diffusion dynamics, would be a promising approach. The first successful example was the use of a metallic nano-gate, in which a periodic array of nano-gates was constructed on the self-spreading substrate [56]. Due to its simplicity, the nano-gate substrate was prepared via nano-sphere lithography (NSL) (Figure 13.10b). However, to carry out a systematic and quantitative experiment on the structural dependence, the fabrication process was shifted to electronbeam lithography (EBL) (Figure 13.10b) [57, 58]. Figure 13.11a shows a fluorescence microscope image of the self-spreading egg-PC bilayer doped with TR-DHPE. The fluorescence intensity directly reflects the molecular concentration of TR-DHPE in

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Figure 13.10 AFM image of (a) NSL and (b) EBL substrates. Lower panels show a enlarged three-dimensional images of the regions highlighted with the white squares with permission.

the self-spreading bilayer, under the condition that the dye content is below its selfquenching concentration. It is clear that the fluorescence intensity is not constant within the bilayer. The intensity gradually decreases from the spreading edge inwards. This is a characteristic aspect of the self-spreading lipid bilayer. During the selfspreading on a hydrophilic substrate, an elastic tension is imposed on the bilayer, causing area dilation [48]. This effect becomes more significant in the spreading edge

Figure 13.11 (a–c) Fluorescence microscope images of the selfspreading lipid bilayer on (a) flat glass, (b) EBL nano-gate with width 500 nm, and (c) EBL nano-gate with width100 nm. (d–f) Averaged and area calibrated line profiles of the self-spreading bilayer on (d) flat glass, (e) EBL nano-gate with width 500 nm, and (f) EBL nano-gate with width100 nm. Adapted from Ref. [57] with permission.

13.2 Summary

region. For a bulky molecule such as TR-DHPE, the less dense region is thermodynamically more favorable. Thus, TR-DHPE accumulates at the less dense spreading edge. This distribution gradient differs from molecule to molecule, depending on the molecular size and configuration. When the bilayer spreads on the nano-gate substrate, the fluorescence intensity appears to be reduced, indicating a molecular filtering effect. Periodic dark spots observed in the fluorescence micrographs on the nano-gate channel correspond to the non-wetting metallic architectures. The observed molecular filtering effect can be explained along the same lines with a partitioning phenomenon on the lipid bilayer, in which doped molecules such as dye-labeled lipids exhibit inhomogeneous distribution when the bilayer has more than two coexisting phases [59]. The inhomogeneous distribution is the result of a difference in chemical potential among different phases. In the present case, the lipid density is thought to increase in the nano-gate region, judging from the attenuation in spreading dynamics [58]. This creates a similar situation to the above-mentioned phase coexisting system. In the case of TR-DHPE with a bulky head group, the solubility at the dense phase is known to be reduced compared to that of a noncompressed phase [60]. Therefore, the penetration ability of the TR-DHPE molecule into the densely packed nano-gate region is reduced. This is the proposed mechanism for the observed molecular filtering effect. The chemical potential is highly sensitive to any structural parameter of both the doped molecules and the medium lipid molecules. Based on the suggested mechanism, the nano-gate system can filter any molecule by recognizing any structural parameter, such as molecular size, charge, polarity, hydrophilicity, chirality, and configuration.

13.2 Summary

In this chapter, we have introduced a novel molecular filtering system using a self-spreading lipid bilayer and a periodic array of metal nano-gates. The filtering effect could be the result of the formation of a local chemical potential barrier in the nano-gate region during spreading. Since the self-spreading is a thermodynamically driven collective molecular flow, any molecule including non-charged molecules can be manipulated in this system, which is completely different from other ordinary separation systems such as those based on a conventional electrophoretic approach. The present system could be applied in micro- and nano-scopic device technologies, as it provides a versatile and completely non-biased filtering methodology.

Acknowledgment

This work was supported in part by Grants-in-Aid for Scientific Research on Priority Area Molecular Nano Dynamics (Area No. 432, No. 16205026) and for Grant-in-Aid for Young Scientist (B) (No. 18750001) from MEXT, Japan. We would like to thank Prof. H. Misawa and Prof. K. Ueno for the fabrication of the substrate with electronbeam litography.

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14 Microspectroscopic Study of Self-Organization in Oscillatory Electrodeposition Shuji Nakanishi

14.1 Introduction

Solid surfaces with ordered nanostructures composed of periodic layers, dots, holes, and grooves (or ridges), provide unique optical, electronic, magnetic, and mechanical properties [1]. Photo- and electron beam lithography and ion beam etching (top-down method) have been widely used for creating desired nanostructures at the surface. Recently, atomic-scale fabrication using surface probe microscopy (SPM), such as scanning tunneling microscopy (STM) and scanning near-field optical microscopys (SNOM), has been developed as a powerful technique (bottom-up method) for designed, well-controlled nanostructure fabrication. However, these techniques now face serious problems, including the challenge of mass production and cost increases due to the expensive specialized apparatus required. With the goal of overcoming problems associated with conventional techniques, self-organization (bottom-up method) has recently attracted much attention. In general, self-organization can be categorized into two different types, that is, static and dynamic. The former (static) is self-organization under conditions of thermodynamic equilibrium, in which ordered structures are formed on the basis of specific properties of the intermolecular forces. These structures, which can have regularity with almost the same size as the system components, are simple, rigid and stable. Self-assembled structures, such as lipid bilayers, close-packed crystals of nanospheres, and monolayers of thiol molecules on gold surfaces, are representative examples of this type. Numerous studies have been carried out on this type of selforganization, as summarized in a number of reviews [2–9]. The critical issues to be tackled next for these methods are to improve the regularity and to place the nanostructures with the desired sizes at specific desired locations. In the other type of self-organization (dynamic self-organization), spontaneous ordering of the systems occurs under thermodynamically non-equilibrium conditions, in which various ordered structures with wavelengths tens to hundreds of thousands times larger than the size of the system components are formed by spatiotemporal synchronization of various factors [10–12]. The spatiotemporal order

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appearing in dynamic self-organization phenomena has unique and attractive properties for producing materials with ordered structures: (i) complex patterns appear spontaneously without any external control, (ii) the observed patterns have long-range order, and (iii) various ordered patterns are obtained simply by changing the experimental parameters. Since dynamic self-organized ordering in chemical systems vanishes when reactions stop and the systems are back in their equilibrium state, some sort of strategy to solidify the dynamic patterns is required for the fabrication of structures [2, 10, 11]. One of the strategies for achieving this is to use electrodeposition, in which the histories of ever-changing spatiotemporal orders caused by reactions can be recorded in a form of ordered architecture of the electrodeposited material. In this chapter, self-organization studies in oscillatory electrodeposition are reviewed, the focus being on nanostructure formation.

14.2 Dynamic Self-Organization in Electrochemical Reaction Systems

Chemical oscillation is a typical example of dynamic self-organization under nonequilibrium conditions [13–16]. Chemical reactions in oscillating systems proceed spatio-synchronically, and, in certain cases, produce a range of spatiotemporal patterns including dots, target patterns, and spirals. However, due to the paucity of examples of such pattern development, chemical oscillations have, in general, been regarded as specific or discrete phenomena. In contrast, a large number of oscillatory reactions have been observed in an electrochemical reaction at an electrode (solid) surface [17–20], indicating that electrochemical oscillations are not specific but general phenomena. Electrochemical systems more frequently display oscillatory behavior due to the effect of an autocatalytic (positive feedback) mechanism derived from the coupling of the electrical factor with chemical dynamics. Such a mechanism, which is rarely active in pure chemical systems, contributes considerably to the appearance of oscillation. Electrochemical oscillations have been reported for a variety of systems, including anodic metal dissolution, cathodic metal deposition, oxidation of hydrogen molecules and small organic compounds, and reduction of hydrogen peroxide and persulfate ions. In comparison with other systems, electrochemical systems have strong advantages for the study of dynamic self-organization phenomena. For example, (i) the Gibbs energies for reactions can be regulated continuously and reversibly by tuning the electrode potential, and (ii) the oscillations can be observed via electric signals such as current or potential. (iii) The diffusion process can also be controlled by changing the sizes and geometrical arrangements of the electrodes in electrochemical cells, and (iv) the mode, period, and amplitude of the spatiotemporal patterns can be tuned easily by changing the geometrical arrangements of the electrodes and the applied potential or current. Based on the above, in the 1990s, detailed mathematical models for oscillations and spatial patterns in electrochemical systems were successfully constructed, which have enabled electrochemical oscillations and patterns to be controlled and designed.

14.3 Oscillatory Electrodeposition

According to the literature [21], all reported electrochemical oscillations can be classified into four classes depending on the roles of the true electrode potential (or Helmholtz-layer potential, E). Electrochemical oscillations in which E plays no essential role and remains essentially constant are known as strictly potentiostatic (Class I) oscillations, which can be regarded as chemical oscillations containing electrochemical reactions. Electrochemical oscillations in which E is involved as an essential variable but not as the autocatalytic variable are known as S-NDR (Class II) oscillations, which arise from an S-shaped negative differential resistance (S-NDR) in the current density (j) versus E curve. Oscillations in which E is the autocatalytic variable are known as N-NDR (Class III) oscillations, which have an N-shaped NDR. Oscillations in which the N-NDR is obscured by a current increase from another process are known as hidden N-NDR (HN-NDR; Class IV) oscillations. It is known that N-NDR oscillations are purely current oscillations, whereas HN-NDR oscillations occur in both current and potential. The HN-NDR oscillations can be further divided into three or four subcategories, depending on how the NDR is hidden.


As mentioned in the introduction, oscillatory electrodeposition is an interesting target from the point of view of the production of micro- and nanostructured materials because it has the possibility to produce ordered electrodeposits by recording ever-changing self-organized spatiotemporal patterns during the oscillation. Schlitte et al. were the first to report the formation of ordered architecture via an oscillatory electrodeposition [22]. They showed that the electrodeposition of Cu with a potential oscillation gave layered deposits. Krastev et al. reported that the oscillatory electrodeposition of Ag–Sb alloy gave similar layered deposits [23]. Interestingly, in this system, spiral and stripe patterns appeared at the surface of the deposits during the oscillation. The thickness of the layers in these examples was rather large, of the order of 100 mm or more. On the other hand, Switzer et al. reported that the oscillatory electrodeposition of Cu in alkaline solutions produced alternate Cu and Cu2O multilayers with thickness of about 90 nm [24]. We have also reported that oscillatory electrodeposition of Cu–Sn alloy [25] and iron-group alloys [26] produced nanoperiod layered deposits. Thus, for all the examples shown above, layered deposits are formed in synchronization with electrochemical oscillations. Another example is dendritic crystal growth under diffusion-limited conditions accompanied by potential or current oscillations. Wang et al. reported that electrodeposition of Cu and Zn in ultra-thin electrolyte showed electrochemical oscillation, giving beautiful nanostructured filaments of the deposits [27, 28]. Saliba et al. found a potential oscillation in the electrodeposition of Au at a liquid/air interface, in which the Au electrodeposition proceeds specifically along the liquid/air interface, producing thin films with concentric-circle patterns at the interface [29, 30]. Although only two-dimensional ordered structures are formed in these examples because of the quasi-two-dimensional field for electrodeposition, very recently, we found that

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three-dimensional metal latticeworks lying vertical to the electrode surface were spontaneously produced in synchronization with a potential oscillation in a normal electrochemical condition [31, 32]. Thus, these types of deposits are very attractive from the point of view of two- or three-dimensional structurization. 14.3.1 Formation of a Layered Nanostructure of Cu–Sn Alloy

Electrochemical oscillation during the Cu–Sn alloy electrodeposition reaction was first reported by Survila et al. [33]. They found the oscillation in the course of studies of the electrochemical formation of Cu–Sn alloy from an acidic solution containing a hydrosoluble polymer (Laprol 2402C) as a brightening agent, though the mechanism of the oscillatory instability was not studied. We also studied the oscillation system and revealed that a layered nanostructure is formed in synchronization with the oscillation in a self-organizational manner [25, 26]. Figure 14.1a shows a j vs. U curve in Cu2 þ þ Sn2 þ þ H2SO4 with (solid curve) and without (dashed curve) cationic surfactant. The addition of the surfactant causes a drastic change in the j vs. U curve. Namely, an NDR appears in a narrow potential region of about 5 mV near 0.42 V, where the Cu–Sn alloy is electrodeposited. Another notable point in the surfactant-added solution is that a current oscillation appears when the U is kept constant in (and near) the potential region of this NDR, as shown in Figure 14.1b. It was also revealed that both the NDR and current oscillation appeared only in the presence of cationic surfactant and not in the presence of anionic surfactant. The structure of alloy films deposited during the current oscillation was investigated by scanning electron microscopy (SEM) and scanning Auger electron microscopy (AEM). Figure 14.1c illustrates schematically the procedure of sample preparation. The deposited film was etched with an Ar þ -ion beam, with the film being rotated. This procedure gave a bowl-shaped hollow of about 1 mm in diameter at the bottom, together with a slanting cross section of the deposited film. Figure 14.1d shows an SEM image (top view) of a sample thus prepared. Uniform concentric rings of gray and black colors in the region of the slanting cross-section clearly indicate the formation of a quite uniform layered structure spreading over a macroscopically wide range of 1 1 mm. It was confirmed that the number of sets of the gray and black layers (one period of the multilayer) agreed with the number of cycles of the current oscillation during which the deposit was formed, indicating that one oscillation cycle produced one layer of the deposit. Figure 14.1e compares the expanded SEM image in the region of the slanting cross-section with the profile (white curve) of the atomic ratio [Cu/(Cu þ Sn)] in this region, in which we can see that Cu is rich in the black layer, whereas Sn is rich in the gray layer. The average thickness of one period of the multilayer was estimated from the sputter time to be about 38 nm. The thickness can be controlled via tuning of the oscillation period. The NDR and the oscillation appear only in the presence of cationic surfactant and not in the presence of anionic surfactant, suggesting that the NDR arises from electrostatic adsorption of a cationic surfactant on a (negatively polarized) Cu–Sn


Figure 14.1 (a) Current density vs. potential curves obtained for the electrodeposition of Cu–Sn alloy in the presence (solid curve) and absence (dashed curve) of cationic surfactant. (b) Typical example of the current oscillation observed for Cu–Sn alloy electrodeposition. (c) Schematic illustration of sample preparation for SEM and AEM

analyses. (d) SEM (top view) of a bowl-shaped hollow with a slanting cross-section, prepared in the deposited alloy film by Ar þ ion etching. (e) Expanded SEM image, compared with the distribution of the atomic ratio [Cu/(Cu þ Sn)] obtained with scanning AEM. (Reprinted from Ref. [25] with permission from the American Chemical Society.)

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alloy (deposit) surface because the adsorption will retard the diffusion of electroactive metal ions to the alloy surface and thus decrease j. The negative polarization at the surface with negative potential shift will increase the amount of cationic surfactant, resulting in more decrease in j, that is, the appearance of the NDR (Figure 14.2). On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the true electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given by E ¼ U jAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, and j is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, E is much more

Figure 14.2 Schematic representation of surface reactions in the (a) low- and (b) high-current states, respectively.


Figure 14.3 Schematic illustrations of expected current density vs. time, true electrode potential vs. time, and coverage of the surfactant vs. time curves.

positive than U, and hence the coverage (q) of the adsorbed surfactant in this stage is small. Thus, effective diffusion of electroactive metal ions to the electrode surface occurs without retardation, which leads to a high j value due to active electrodeposition of Cu–Sn alloy (stage a in Figure 14.3). The active alloy deposition, however, causes a decrease in the surface concentration of electroactive metal ions (hereafter denoted as C ) owing to their slow diffusion from the solution bulk. This leads to a gradual decrease in j (in the absolute value) and thus to a decrease in the ohmic drop and a negative shift in E. The negative shift in E, in turn, leads to an increase in q (Figure 14.2a). Thus, j decreases (stage b) owing to a decrease in the diffusion of metal ions to the electrode surface, and the system goes to a low-current stage, accompanied by a negative shift in E. In the low-current stage (stage c), only slow deposition (or slow reduction of metal ions) occurs at vacant sites (atomic pinholes); thus, C gradually increases by diffusion from the solution bulk. The increase in C induces an increase in j and thus causes a positive shift in E (and a decrease in q). When E is shifted to the positive, j increases (stage d) owing to an increase in the diffusion of metal ions, and the high-current stage is restored again. The alternate-multilayer formation in the alloy deposit can also be explained on the basis of the above mechanism. First, we have to note that the j value in the low-current

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stage is very low; hence, this stage hardly contributes to the alloy formation. In the high-current stage, the E shifts gradually to the negative with time, as seen in Figure 14.3, and the Sn content in the alloy deposit increases with this negative shift in E. Accordingly, one period of the multilayer is formed by one cycle of the current oscillation. 14.3.2 Layered Nanostructures of Iron-Group Alloys

The electrodeposition of the iron-group alloys has an interesting aspect in that it leads to self-organized formation of layered structures in which the iron-group metals and the incorporated elements change their contents periodically. The formation of the layered structures was commonly observed in various electrodeposition systems of iron-group alloys [34–36]. It was also reported that an electrochemical oscillation was observed when the layered Ni–P alloy was deposited [37], implying that the layered structures of iron-group alloys are also formed by electrochemical oscillation. Figure 14.4a shows the j vs. U curve in an electrolyte of NiSO4 þ H3BO3 þ NaCl with (solid-curve) and without (dashed-curve) NaH2PO2, which is the P-source of the Ni–P alloy. In the presence of NaH2PO2 (solid curve), the current started to flow at about 0.5 Vand an NDR appeared in the potential region from 0.87 to 1.12 V. On the other hand, in the absence of NaH2PO2 (dashed curve), the current started to flow at a more negative potential than in the presence of NaH2PO2 and showed no NDR. It is to be noted that the j values in the presence and the absence of NaH2PO2 became nearly the same in a U range more negative than 1.0 V. This implies that the added NaH2PO2 has almost no effect on the deposition reaction in this U range. Figure 14.4b shows a time course of a potential oscillation obtained in electrolyte with slightly different composition, which appeared spontaneously when j was kept at a constant value in the range 55 < j < 75 mA cm2, that is, in a range of j where U was in the NDR region of Figure 14.4a. It may be noted that the highest and lowest values of the oscillating potential in Figure 14.4b nearly coincide with the highest and lowest potentials of the NDR region, respectively. Figure 14.4c shows an Auger depth profile for a deposit film formed during the potential oscillation, in which the formation of the layered structure of Ni–P with different P-ratio in the alloy can be clearly seen. The thickness of one layer was estimated to be a few hundred nanometers, by dividing the thickness of the deposit, measured with an optical microscope, by the number of oscillation cycles. Note that essentially the same behavior as for the Ni–P alloy deposition was observed in electrodeposition of other iron-group alloys, such as Co–W and Ni–W alloys. Namely, the deposition current in the presence of Na2WO4 (the W-source of the Co–W and Ni–W alloys) started to flow at a more positive potential than in the absence of Na2WO4, indicating that the electrodeposition of the Co–W and Ni–W alloys occurs by essentially the same mechanism as that of the Ni–P alloy, suggesting the presence of a general mechanism for the induced co-deposition of these alloys. As mentioned above, the Ni–P (Co–W, Ni–W) alloy deposition current in the presence of the P-source (W-source) starts to flow at a more positive potential than in


Figure 14.4 (a) Current density vs. potential curves obtained for electrodeposition of Ni–P alloy in the presence (solid curve) and absence (dashed curve) of NaH2PO2 (P-source of the alloy). (b) Typical example of the potential oscillation observed for Ni–P alloy electrodeposition. (c) An Auger depth profile for a deposit produced under the potential oscillation. (Reprinted from Ref. [26] with permission from American Chemical Society.)

its absence (Figure 14.4a). This fact indicates that P-source (W-source) or a species related to it acts as a promoter for the alloy deposition reaction. Sakai et al. attributed the origin of the NDR to desorption of the adsorbed promoter. The desorption of the adsorbed promoter (anionic species) may be caused by an increase in negative charges at the electrode surface by the negative potential shift (Figure 14.5) [26]. It was also revealed that the oscillation was caused by a positive feedback mechanism originating from the NDR, in a similar way to that in the Cu–Sn alloy electrodeposition system (Section 3.1.1). 14.3.3 Layered Nanostructure of Cu/Cu2O

Switzer et al. found that Cu/Cu2O layered nanostructures are electrodeposited with spontaneous potential oscillations from alkaline Cu(II)-lactate solution in a self-

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Figure 14.5 Schematic illustrations to explain the promotion effect of adsorbed H2PO2.

organized manner [24, 38]. Since the resultant deposits with layered nanostructures show interesting electronic [24, 39, 40] and optical [41] properties, this system has attracted a lot of attention from the point of view of self-organization of nanofunctional materials. Figure 14.6a shows an example of the potential oscillation that is observed when the electrodeposition is performed in a solution with pH 8.3. Direct evidence for layering was obtained by Auger depth profile with Ar þ ion sputtering [24], SEM [38], and STM [24]. Figure 14.6b shows an SEM cross-section of a film, from which the thickness of one layer is estimated to be about 62 nm. The phase composition, layer thicknesses, and resistivity of the films can be tuned by varying the applied current density of the solution pH. Essentially the same behavior can be observed in tartrate and citrate solutions [42–44]. Very interestingly, it was revealed that the resultant deposits can work as resonant tunneling devices, which show sharp NDR signatures at room temperature in perpendicular transport measurement [40]. The bias for the NDR maximum can be controlled simply by tuning the oscillation period (Figure 14.6c). Leopold et al. and Nyholm et al. have investigated this oscillatory system by in situ confocal Raman spectroscopy [43], and in situ electrochemical quartz crystal microbalance [44], and in situ pH measurement [45] with the focus being on clarification of the oscillation mechanism. Based on the experimental results, a mechanism for the oscillations was proposed, in which variations in local pH close to the electrode surface play an essential role. Cu is deposited at the lower potentials of the oscillation followed by a simultaneous increase in pH close to the surface due to the protonation

Figure 14.6 (a) Typical example of the potential oscillation observed for Cu/Cu2O electrodeposition. (b) SEM image of a cross-section of the Cu/Cu2O film grown under the oscillatory condition. (c) NDR curves for layered Cu/Cu2O

nanostructures as a function of the Cu2O layer thickness. The NDR maximum shifts to higher applied bias for samples with thinner Cu2O layers. (Reprinted from Refs. [38, 40] with permission from American Chemical Society.)


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of the lactate (or tartrate) liberated from the Cu(II) complex. As the pH is increased, the deposition of Cu2O becomes more favorable and a positive shift in potential is observed. When Cu2O starts to electrodeposit, the production of OH stops and the pH decreases. The rate of the pH decrease is significantly increased by the presence of comproportionation between Cu(II) and copper as this reaction consumes OH. As a result of the decreasing local pH, the rate of electrodeposition of Cu2O decreases, which has to be compensated for by a negative shift in the potential. On the other hand, Switzer et al. proposed a different model for the oscillation. They attributed the oscillation to repetitive build-up and breakdown of a thin Cu2O layer, which is a p-type semiconductor and acts as a thin rectifying (passivating) layer [24]. Disappearance of the oscillation under irradiated condition supports this model. Light will generate electron–hole pairs in the Cu2O and lower the rectifying barrier at the semiconductor/solution interface. 14.3.4 Nanostructured Metal Filaments

More than 20 years ago, Matsushita et al. observed macroscopic patterns of electrodeposit at a liquid/air interface [46, 47]. Since the morphology of the deposit was quite similar to those generated by a computer model known as diffusion-limited aggregation (DLA) [48], this finding has attracted a lot of attention from the point of view of morphogenesis in Laplacian fields. Normally, thin cells with quasi 2D geometries are used in experiments, instead of the use of liquid/air or liquid/liquid interfaces, in order to reduce the effect of convection. Recently, Wang et al. found electrodeposition in an ultra-thin layer of CuSO4 showed spontaneous electrochemical oscillations and formation of straight Cu filaments with periodic corrugated nanostructures [28, 49–52]. Figure 14.7a shows schematically the experimental set-up with an ultra-thin layer of an electrolyte. To generate the ultra-thin electrolyte, the CuSO4 solution was solidified by decreasing the temperature (The thickness of the electrolyte layer is about 200 nm). A typical example of the oscillation is shown in Figure 14.7b. Figure 14.7c shows the macroscopic morphology of the deposit obtained under the oscillation, in which finger-like branches developing outwards are seen. It can be seen by optical microscopy that the finger-like branches grown on the glass plate consist of long, narrow filaments, as shown in Figure 14.7d. AFM inspection of the microstructure of the deposit revealed that periodic corrugated structures exist on the filament (Figure 14.7e). The periodicity varies from several tens of nanometers to a few microns, depending on temperature, voltage or current applied across the electrodes, and the pH of the electrolyte. Analysis of the deposits by TEM diffraction [52] and scanning near-field optical microscopy (SNOM) [51] revealed that the periodic nanostructures on the electrodeposits correspond to the alternating growth of Cu and Cu2O. Similar electrochemical oscillation and formation of nano-filaments were also observed in Zn electrodeposition (Figure 14.8) [27]. A simple model was proposed on the basis of the experimental findings, in which the solution pH plays a key role for the oscillatory instability [51]. The theoretical


Figure 14.7 (a) A schematic diagram of the experimental set-up for the generation of an ultrathin electrolyte film and for electrodeposition. The cell for electrodeposition shown here has two parallel electrodes. (b) Voltage oscillation during the electrodeposition of Cu in the ultrathin

electrolyte. Inset: the Fourier transform of the voltage oscillation. (c) The macroscopic view of the electrodeposits of copper grown from a circular electrodeposition cell. (d) The AFM view of the copper filaments. (Reprinted from Refs. [28] with permission from the American Physical Society.)

approach suggested that the oscillating local concentration of Cu2 þ and H þ triggers an alternating deposition of Cu and Cu2O. Monte Carlo simulation of the spontaneous oscillation was also performed by the same authors [49]. The simulated layered structure of the deposited Cu/Cu2O, as well as the correspondence between the pH oscillation and phase composition agreed qualitatively with the experimental results.

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Figure 14.8 (a) A voltage oscillation obtained for Zn electrodeposition from an ultra-thin electrolyte. (b) SEM images of the Zn deposit obtained under the voltage oscillation. (Reprinted from Ref. [27] with permission from John Wiley & Sons Ltd.)

14.4 Raman Microspectroscopy Study of Oscillatory Electrodeposition of Au at an Air/Liquid Interface

As has been shown above, oscillatory electrodeposition is interesting from the point of view of the production of micro- and nanostructured materials. However, in situ observation of the dynamic change of the deposits had been limited to the micrometer scale by use of an optical microscope. Inspections on the nanometer scale were achieved only by ex situ experiments. Thus, information with regard to dynamic nanostructural changes of deposits in the course of the oscillatory growth was insufficient, although it is very important to understand how the macroscopic ordered structures are formed with their molecular- or nano-components in a self-organized manner. Surface-enhanced Raman scattering (SERS) is a candidates for resolving this issue. Since the SERS effect is observed only at metal surfaces with nanosized curvature, this technique can also be used to investigate nanoscale morphological structures of metal surfaces. It is thus worth investigating SERS under oscillatory electrodeposition conditions. The author of this chapter and coworkers recently reported that

14.4 Raman Microspectroscopy Study of Oscillatory Electrodeposition of Au at an Air/Liquid Interface

in situ SERS from a gold film formed at a liquid/air interface by oscillatory electrodeposition can probe the dynamic nanostructural change of the deposits during the oscillatory growth [53]. Figure 14.9a is a schematic drawing of the experimental set-up for electrochemical formation of Au film. The working electrode (WE) was positioned at the center of the Pt-ring counter-electrode, and its tip was located just at the liquid/air interface. In the present electrodeposition system, the potential oscillates spontaneously (Figure 14.9b) and the Au deposition proceeds specifically along the liquid/air interface, resulting in the formation of an Au film with a concentric-circle pattern. Figure 14.10 shows SEM images of the Au film, taken by pulling it out from the electrolyte at stage 2 in Figure 14.9b (positive-end) of the potential oscillation. From this procedure, the growing front, namely the area denoted

Figure 14.9 (a) Schematic illustration of the experimental set-up used for Au electrodeposition at a liquid/air interface. (b) Typical example of the potential oscillation during Au electrodeposition. (Reprinted from Ref. [53] with permission from the American Chemical Society.)

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Figure 14.10 (a) SEM image of the growing front of the Au film at a stage where the potential is at the positive end of the potential oscillation (e.g., at stage-2 of Figure 14.9b). (b–d) Expansions of a part of b–d with increased magnification, respectively. The areas denoted

A and B represent deposited parts in the preceding stages of the oscillation and newly deposited part, respectively (see text for details). The dotted lines in panels a and b are the border between areas A and B. (Reprinted from Ref. [53] with permission from the American Chemical Society.)

by B, can be regarded as presenting a newly deposited part during the transition from stage 1 to 2, that is, from the negative end to the positive end of the oscillation, whereas the area denoted by A represents a deposited part in the preceding stages of the oscillation. Figure 14.10b–d are the expanded images, from which we can see clearly that the deposit in area B is composed of numerous numbers of nanoneedles with width 20–80 nm, whereas the continuous film is formed in area A. The SERS activity of the deposited Au film was investigated by measuring the Raman scattering intensity from bipyridine (bpy) adsorbed on the surface. Figure 14.11a is a schematic illustration to explain how the Raman signals were obtained under in situ conditions. The laser beam was focused at a position close to the growing front of the Au film, so that it could move and pass the laser spot at the liquid/air interface during one cycle of the oscillation. The Raman spectrum observed is shown as a function of t in Figure 14.11b, in which the intensity is expressed by the gray scale. All peaks at 1020, 1076, 1227, and 1293 cm1 are assignable to vibrational modes of adsorbed bpy. The change in the Raman spectrum with t was measured concurrently with the measurement of the potential oscillation shown in Figure 14.11c The SERS intensity becomes stronger while U moves from the negative end to the positive end of the potential oscillation (e.g., from stage 1 to stage 2), whereas the intensity is weakened when U shifts from the positive end to the negative

14.5 Summary

Figure 14.11 (a) Schematic illustrations to explain how the SERS signal was measured under in situ conditions. (b) Raman spectrum as a function of time. (c) The potential oscillation measured concurrently with the measurement of panel (b). (Reprinted from Ref. [53] with permission from the American Chemical Society.)

end (e.g., from stage 2 to stage 3). As mentioned earlier, when U moves from the negative end to the positive end of the oscillation, a 2D film composed of numerous numbers of nanoneedles is formed (Figure 14.10) and the SERS intensity increases, indicating that the nanoneedles are SERS-active. On the other hand, when U moves from the positive end to the negative end, the SERS intensity is weakened because the nanoneedles change the morphology into a continuous film via thickening and coalescing. These results clearly show that the SERS under in situ conditions probes the dynamic nanostructural change in the deposits proceeding under the oscillatory electrodeposition.

14.5 Summary

We have reviewed studies of the self-organized formation of ordered nanostructures by oscillatory electrodeposition. Although the mechanism is totally different in different cases and the structures of the resultant deposits vary greatly, they agree in that a unit structure is formed with one cycle of the oscillation. Periodic ordered

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structures are formed by periodic oscillation, and modulated oscillations give rise to the formation of modulated periodic structures. The important point with regard to the self-organized formation of ordered structures by oscillatory electrodeposition is that all the processes are spatiotemporally synchronized under non-equilibrium, nonlinear electrochemical dynamics. This principle is quite unique and is never realized by other methods. In order to design and tune the ordered structures, it is very important to understand the mechanism of the self-organization on a molecular or nano-level. From this point of view, it is very interesting to perform in situ microspectroscopic studies. In the present review, we described how the dynamic nanostructural change of a gold film formed by oscillatory electrodeposition was probed by in situ SERS. We can thus expect that further study along this line will greatly contribute to the preparation of designed and controlled ordered nanostructures at solid surfaces.

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36 Ogburn, F. and Johnson, C. E. (1975) Mechanical properties of electrodeposited brass. Plating, 62, 142–147. 37 Lee, W. G. (1971) Improvement of solder connections by gold alloy plating. Plating, 58, 997–1001. 38 Bohannan, E. W., Huang, L. Y., Miller, F. S., Shumsky, M. G. and Switzer, J. A. (1999) In situ electrochemical quartz crystal microbalance study of potential oscillations during the electrodeposition of Cu/Cu2O layered nanostructures. Langmuir, 15, 813–818. 39 Switzer, J. A., Hung, C. J., Bohannan, E. W., Shumsky, M. G., Golden, T. D. and VanAken, D. C. (1997) Electrodeposition of quantum-confined metal semiconductor nanocomposites. Adv. Mater., 9, 334–338. 40 Switzer, J. A., Maune, B. M., Raub, E. R. and Bohannan, E. W. (1999) Negative differential resistance in electrochemically self-assembled layered nanostructures. J. Phys. Chem. B, 103, 395–398. 41 Mishina, E. D., Nagai, K. and Nakabayashi, S. (2001) Self-assembled Cu/Cu2O multilayers: Deposition, structure and optical properties. Nano Lett., 1, 401–404. 42 Eskhult, J., Herranen, M. and Nyholm, L. (2006) On the origin of the spontaneous potential oscillations observed during galvanostatic deposition of layers of Cu and Cu2O in alkaline citrate solutions. J. Electroanal. Chem., 594, 35–39. 43 Leopold, S., Arrayet, J. C., Bruneel, J. L., Herranen, M., Carlsson, J. O., Argoul, F. and Serant, L. (2003) In situ CRM study of the self-oscillating Cu-(II)-lactate and Cu(II)-tartrate systems. J. Electrochem. Soc., 150, C472–C477. 44 Leopold, S., Herranen, M. and Carlsson, J. O. (2001) Spontaneous potential oscillations in the Cu(II)/tartrate and lactate systems, aspects of mechanisms and film deposition. J. Electrochem. Soc., 148, C513–C517. 45 Leopold, S., Herranen, M., Carlsson, J. O. and Nyholm, L. (2003) In situ pH

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measurement of the self-oscillating Cu(II)lactate system using an electropolymerised polyaniline film as a micro pH sensor. J. Electroanal. Chem., 547, 45–52. Matsushita, M., Hayakawa, Y. and Sawada, Y. (1985) Fractal structure and cluster statistics of zinc-metal trees deposited on a line electrode. Phys. Rev. A, 32, 3814–3816. Matsushita, M., Sano, M., Hayakawa, Y., Honjo, H. and Sawada, Y. (1984) Fractal structures of zinc metal leaves grown by electrodeposition. Phys. Rev. Lett., 53, 286–289. Witten, T. A. and Sander, L. M. (1981) Diffusion-limited aggregation, A kinetic critical phenomenon. Phys. Rev. Lett., 47, 1400–1403. Ha, M. J., Fang, F., Liu, J. M. and Wang, M. (2005) Monte carlo simulation of the spontaneous oscillation in electrochemical deposition. Eur. Phys. J. D, 34, 195–198. Wang, M., Feng, Y., Yu, G. W., Gao, W. T., Zhong, S., Peng, R. W. and Ming, N. B. (2004) Self-organization of nanostructured copper filament array by electrochemical deposition. Surf. Interface Anal., 36, 197–198. Wang, Y., Cao, Y., Wang, M., Zhong, S., Zhang, M. Z., Feng, Y., Peng, R. W., Hao, X. P. and Ming, N. B. (2004) Spontaneous formation of periodic nanostructured film by electrodeposition: Experimental observations and modeling. Phys. Rev. E, 69, 021607. Zhang, M. Z., Wang, M., Zhang, Z., Zhu, J. M., Peng, R. W. and Ming, N. B. (2004) Periodic structures of randomly distributed Cu/Cu2O nanograins and periodic variations of cell voltage in copper electrodeposition. Electrochim. Acta, 49, 2379–2383. Fukami, K., Nakanishi, S., Sawai, Y., Sonoda, K., Murakoshi, K. and Nakato, Y. (2007) In situ probing of dynamic nanostructural change of electrodeposits in the course of oscillatory growth using SERS. J. Phys. Chem. C, 111, 3216–3219.

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15 Construction of Nanostructures by use of Magnetic Fields and Spin Chemistry in Solid/Liquid Interfaces Hiroaki Yonemura

15.1 Introduction

Applying the strong magnetic fields (>6 T) of a superconducting magnet to materials induces huge magnetic field effects (MFEs), in comparison with applying the magnetic fields (1 T) of an electromagnet. It is expected that highly functional nanomaterials with new properties will be created, since new interfaces or nanostructures are constructed by strong magnetic fields [1]. The magnetic orientation of crystals [2–4], polymers [5, 6], fibrin fiber [7, 8], and carbon nanotubes [9, 10] by the strong magnetic fields of a superconducting magnet has been widely investigated. Two- or three-dimensional patterns of silver dendrites in a strong magnetic field were reported by Mogi et al. [11, 12] and Katsuki et al. [13, 14]. Significant morphological changes induced by magnetic fields were interpreted by the magnetohydrodynamic (MHD) mechanism in which the motions of ions in a magnetic field are influenced by the Lorentz force, and/or by the magnetic force of a gradient magnetic field. Magnetic field effects on the growth morphology in electropolymerization, photoelectrochemical reactions, and the redox behavior of polypyrrole as a conducting polymer, have been reported by Mogi et al. [15–17]. They were explained by the diamagnetic orientation of the polymers. Recently, Tanimoto et al. reported attractive 3D morphological chirality in membrane tubes prepared by a silicate garden reaction using a strong magnetic field [18, 19]. Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20–23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor–acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1 T [24]. It has been reported that C60 and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. The clustering behavior of fullerene (C60) is mainly associated with the strong three-dimensional hydrophobic interactions between the C60 units. Photoinduced

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electron-transfer and photoelectrochemical reactions using C60 clusters have been extensively reported because of the interesting properties of C60 clusters [25, 26]. However, MFEs on the photoinduced electron-transfer reactions using the C60 cluster in mixed solvents had not until now been studied. Magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes are very important for practical applications of the MFEs in controlling the photoelectronic functions of molecular devices. Previously, we have examined MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with zinc-tetraphenylporphyrin-viologen linked compounds [27, 28] and semiconductor nanoparticles [29, 30]. However, MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with nanoclusters have not yet been reported. First, we attempted to create highly functional nanomaterials containing new physical or chemical properties using single-walled carbon nanotubes (SWNTs) (Section 15.2.1), a C60 derivative (Section 15.2.2), and diluted magnetic semiconductor (Zn1xMnxS) nanoparticles (Section 15.2.3), since new interfaces or nanostructures are constructed by strong (>6 T) or normal magnetic fields due to an electromagnet (1 T). Secondly, we attempted to obtain new information to examine spin chemistry at the solid/liquid interface or the nanostructures using electrodes modified with C60 nanoclusters (Section 15.3.2).

15.2 Construction of Nanostructures by the use of Magnetic Fields 15.2.1 Magnetic Orientation and Organization of SWNTs or their Composite Materials Using Polymer Wrapping

Since the discovery of SWNTs, they have been expected to become the building blocks of the next generation of functional nanomaterials. However, their strong cohesive property and poor solubility have restricted the use of SWNTs for fundamental and applied research fields. One method to overcome these problems is to make the SWNTs more soluble by wrapping them with polymers [31]. At the same time, the fabrication of high-performance carbon nanotube (CNT)-based composites is driven by the ability to create anisotropy at the molecular level to obtain appropriate functions. Many groups have reported the orientation of CNTs by various methods, such as spinning CNTs [32], condensing viscous flow-aligned polymer-CNT composites [33], electric field [34, 35], and the use of the cooperative reorientation of a liquid crystalCNT suspension in an electric field [36]. Several groups have reported magnetic orientation of CNTs by using strong magnetic fields [9, 10, 37, 38]. However, the magnetic orientation of composites between CNT and a conducting polymer have been scarcely reported. In addition, SWNTs have been expected to act as acceptors or molecular wires in molecular photoelectric conversion since they have attractive electron-accepting

15.2 Construction of Nanostructures by the use of Magnetic Fields

properties and a one-dimensional nanowire structure. Recently, the photoelectrochemical reactions of electrodes modified with SWNT composites with a photosensitizer such as a semiconductor or donor molecules have been reported [39–41]. For example, novel SWNT composites due to electrostatic interaction using poly(sodium 4-styrenesulfonate) (PSS) or pyrene derivatives have been reported [39, 40, 42–44]. Recently, we examined the photoelectrochemical reactions of a modified electrode of composite materials consisting of poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) as a conjugated polymer or ruthenium tris(2,20 -bipyridine) (Ru(bpy)32 þ )-PSS complex and SWNT [45]. The magnetic orientation of these composites on the electrodes is expected to improve the photoelectrochemical properties of the electrodes. We examined the magnetic orientation or organization of the SWNTs or the polymer-wrapped SWNTs using MEHPPV by measurements of AFM images and polarized absorption spectra [46–48]. SWNTs (HiPco, Carbon Nanotechnologies Incorporated) were shortened by ultrasonication with a probe-type sonicator in mixed acids (H2SO4 and HNO3) under ice-cooling. After diluting the mixture with water (MiliQ), the shortened SWNTs were purified by filtration through a PTFE membrane filter (pore size: 1 mm or 0.2 mm) or by chromatography (Sepadex G-50). The SWNT/MEHPPV composites were prepared by the following procedure. The shortened SWNTs were added to a DMF solution of MEHPPV. The suspension was then sonicated with a bath-type sonicator. Centrifugation (6000 rpm) of the suspension for 15 min gave a DMF solution of the SWNT/MEHPPV composite. The shortened SWNTs were added to the aqueous solution in the absence and the presence of NaHCO3. These suspensions were then sonicated with a bath-type sonicator. Centrifugation (6000 rpm) of the suspensions gave aqueous solutions of the shortened SWNTs with and without NaHCO3. The formation of SWNT/MEHPPV composites was confirmed by absorption and fluorescence spectra. The DMF solution of SWNT/MEHPPV composites or the aqueous solution of the shortened SWNTs was then dropped onto a mica or glass plate. The magnetic processing of the composites or the SWNTs was carried out by using a superconducting magnet (8 T) in the horizontal direction, as described below. The magnetic field was applied by using two superconducting magnets (horizontal and vertical direction of magnetic field). In the horizontal direction, a superconducting magnet (Oxford Instruments Spectromag-1000) was used, as reported in the previous papers [9, 10]. In the superconducting magnet a bore tube (50 mm diameter) was installed horizontally. Distribution of the magnetic field was approximated by a Gaussian distribution. The maximum strength of the magnetic field was 8.0 T at the center position. In the case of SWNT/MEHPPV composites, after drying at 283 K under the magnetic field of 8 or 0 T (control), AFM images of the SWNT/MEHPPV composites on the mica were measured (Figure 15.1). The heights of the top of the SWNT/ MEHPPV composites were 6–15 nm, indicating that they consist of bundles of 4–21 SWNTs, since the diameter of the SWNTs is 0.7–1.5 nm.

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Figure 15.1 AFM images of SWNT/MEHPPV composites on mica placed in (a) 0 and (b) 8 T.

The AFM images strongly indicate that the SWNT/MEHPPV composites are oriented randomly in the absence of a magnetic field (0 T) (Figure 15.1a), while they are oriented with the tube axis of the composites parallel to the magnetic field in the presence of a magnetic field (8 T) (Figure 15.1b). We examined the effect of the length of the shortened SWNT on the magnetic orientation of the SWNT/MEHPPV composites. In the long SWNTs (average length; 2.2 mm), the magnitude of the magnetic orientation (Figure 15.1b) was greater in comparison with that in the short SWNTs (average length; 1.3 mm) in the AFM measurement. In the case of the shortened SWNTs in the absence and the presence of NaHCO3, after drying at 283 K under a magnetic field of 8 or 0 T (control), the AFM images of the shortened SWNTs on the mica were also measured. The images strongly indicate that the SWNTs are oriented randomly in the absence of a magnetic field, while the SWNTs oriented with the tube axis of the composites parallel to the magnetic field in the presence of a magnetic field (8 T). On the basis of comparison of the AFM images between the SWNT/MEHPPV composites and the SWNT in the absence and presence of NaHCO3, the magnetic orientation of SWNT/MEHPPV composites can most likely be ascribed to the anisotropy in susceptibilities of the SWNTs. Polarized absorption spectra of the SWNT/MEHPPV composites on glass plates were measured in the near-IR region (900–1600 nm) with a UV–VIS–NIR spectrometer. In the absence of a magnetic field, the absorbance of the band (at about 1150 nm) due to the semiconducting SWNT was almost the same in both polarization directions (horizontal and vertical) (Figure 15.2a). On the other hand, in the presence of a magnetic field (8 T), the absorption band (at about 1150 nm) of the SWNT/MEHPPV composites on the glass plates in the horizontal polarization direction (B(//); 0 ) was larger than that in the vertical polarization direction (B(?); 90 ) (Figure 15.2b). Next, the polarized absorption spectra of the shortened SWNTs in the absence and presence of NaHCO3 on the glass plates were measured in the near-IR region (10001600 nm). In the absence of a magnetic field, the absorption band (at about 1450 nm) due to the semiconducting SWNT was observed in both polarization directions (horizontal and vertical). The absorption bands on the glass plate are similar to that in aqueous solution. In contrast, in the presence of a magnetic field (8 T), the absorbance (at about 1450 nm) due to the semiconducting SWNT in the


Figure 15.2 Polarized absorption spectra of SWNT/MEHPPV composites on glass plates in (a) the absence (0 T) and (b) the presence of magnetic processing (8 T). In the absence of magnetic processing (0 T), the polarization direction of the light against the longitudinal direction of the glass plates is horizontal

(0 ; solid line) or vertical (90 ; broken line). In the presence of magnetic processing (8 T), the polarization direction of the light against the direction of the magnetic field is horizontal B(//) (0 ; solid line) or vertical B(?) (90 ; broken line). AFM images of SWNT/MEHPPV composites on mica placed in (a) 0 and (b) 8 T.

horizontal polarization direction (B(//); 0 ) was much larger than that in the vertical polarization direction (B(?); 90 ). The above results of polarized absorption spectra on glass plates support the magnetic orientation of the SWNT/MEHPPV composites and the shortened SWNTs in the absence and presence of NaHCO3, where the composites or the SWNTs were oriented with the tube axis of the composites or the SWNTs parallel to the magnetic field (8 T), suggested by the results of the AFM images (Figure 15.1). On the basis of the polarized absorption spectra (Figure 15.2), the magnetic orientation of SWNT/MEHPPV composites can also most likely be ascribed to the anisotropy in susceptibilities of SWNTs, similar to the comparison of the AFM images (Figure 15.1), as described above. In the vertical direction, a superconducting magnet (Japan Superconductor Technology, JMTD-LH15T40) was used in the present study, as reported in the previous papers [18, 19]. It has a room temperature bore tube width of 40 mm. The distribution of the magnetic field is reported in the previous papers [18, 19]. The maximum field (Bmax(vertical)) and field (B) gradient field (dB/dz) were 15 T and 1500 T2 m1, respectively, where z is the distance from the center of the bore tube along the tube. Three samples were placed at positions in the bore tube, for which B and BdB/dz were 5.6 T and 940 T2 m1 for the top position, 15 T and 0 T2 m1 for the middle position, and 9.8 T and þ 1070 T2 m1 for the bottom position and one was placed outside the tube as a control. In the case of the vertical direction of magnetic field, after drying at ambient temperature under the magnetic field at the three positions (top, middle, and bottom) and in the absence of a magnetic field (outside the bore tube) as the control, the AFM images of the SWNTs on the mica were measured (Figure 15.3). An organized network of bundles consisting of a certain amount of nanofibers, several nanometers in width, was observed at the top position, as shown in Figure 15.3a and b. Interesting nanostructures were not observed at the other positions. The heights of the top of the nanofibers were 2–3 nm. The results indicate

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Figure 15.3 AFM images of SWNTs on mica placed at the top ((a) and (b)), the middle (c), and the bottom (d) positions using magnet apparatus (vertical direction of magnetic field) and outside position (e) of the bore tube (0 T).

that the nanofibers consist of individual SWNTor bundles of some SWNTs, since the diameter of the SWNTs is 0.7–1.5 nm. On the basis of these observations, an interesting formation of nanostructures consisting of SWNTs was probably achieved by magnetic force, magnetic orientation, interaction of induced magnetic moment of SWNTs due to strong magnetic fields, and self-assembly of SWNTs due to hydrophobic interaction in aqueous solution and so on [46, 48]. 15.2.2 Effects of Magnetic Processing on the Morphological, Electrochemical, and Photoelectrochemical Properties of Electrodes Modified with C60-Phenothiazine Nanoclusters

We examined the effects of magnetic processing on the morphological, electrochemical, and photoelectrochemical properties of electrodes modified with nanoclusters of C60N þ and MePH (Figure 15.4) using a strong magnetic field [49]. Clusters of C60N þ and MePH were prepared by dissolving C60N þ and MePH in THF-H2O (2 : 1) mixed solvent using first injection methods [50]. C60N þ and MePH form optically transparent clusters. The formation of nanoclusters of C60N þ –MePH (diameter about 100 nm) was verified from absorption measurements and AFM. The magnetic processing of the samples was carried out using a superconducting magnet (Oxford Instruments Spectromag-1000) as described in Section 15.2.1. The THF–H2O mixed solution of C60N þ and MePH was dropped onto a mica or indium tin oxide (ITO) electrode. The samples were placed at three positions in the strong magnetic field and one outside the magnetic field (control). The magnetic field was applied horizontally to the surface of the mica or ITO electrode at 283 K. After drying


Figure 15.4 Chemical structures of a C60 derivative (C60N þ ) and methylphenothiazine (MePH).

the solvent at 283 K, AFM of the sample on the mica and electrochemical and photoelectrochemical measurements of the sample on the ITO electrode, that is, the electrodes modifed with C60N þ –MePH clusters, were carried out. The formations of clusters of C60N þ –MePH were also examined by AFM. From the AFM image on a mica surface in the absence of magnetic field at 283 K (Figure 15.5a), many roughly spherical nanoclusters were clearly observed. The diameters of the clusters of C60N þ were estimated to be about 100 nm. The results were in fair agreement with those of the C60N þ –MePH clusters prepared using a different method in previous studies [51, 52]. The morphological effects of applying magnetic fields to C60N þ –MePH clusters on mica were also examined by AFM. The images indicate that the C60N þ –MePH clusters in the presence of magnetic processing (Figure 15.5b) were smaller than those in the absence of magnetic processing (Figure 15.5a). In the AFM images obtained for clusters in the presence of magnetic processing (Figure 15.5b), nonspherical clusters were observed. The morphological effects of applying magnetic fields can probably be ascribed to the MHD mechanism and/or the convective flow of the suspension produced by the

Figure 15.5 AFM images of C60N þ –MePH clusters obtained from C60N þ and MePH in THF–H2O (2 : 1) mixed solvent on a mica surface in the absence (a) and presence of magnetic processing at 8 T (b) at 283 K.

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magnetic force. In other words, these phenomena can probably be ascribed to the effect of the Lorentz force since the C60N þ –MePH clusters had positive charge. These morphological effects of applying magnetic fields can also probably be ascribed to the magnetic force due to the gradient magnetic fields. The redox potentials of the ITO electrodes modified with C60N þ –MePH clusters were measured by cyclic voltammetry and differential pulse voltammetry in the absence and presence of magnetic processing. In the cyclic voltammetry, the oxidation peaks of PH were clearly observed in positive scans for all the modified electrodes. In contrast, reduction peaks of C60 were clearly observed in the absence of magnetic processing but not in the presence of magnetic processing. Next, differential pulse voltammetry of the ITO electrodes modified with C60N þ –MePH clusters was carried out to observe clearly the peak corresponding to the reduction of C60 in the presence of magnetic processing. The first reduction peaks corresponding to the C60N þ nanocluster in the presence of magnetic processing were negative-shifted compared with those in the absence of magnetic processing (Figure 15.6a, (1200 0 mV vs Ag/AgCl)). In contrast, with MePH, the first oxidation peaks were observed at about 600 mV vs Ag/AgCl in the absence and presence of magnetic processing. Nakashima and coworkers reported that the DPVs due to the reduction of C60 were influenced by the type of cation electrolyte in the cast films of C60 lipids on electrodes [53]. The results showed the importance of the cationic charge of the lipids and the microenvironments in the films for the electrochemistry of C60. Therefore, the Coulombic interaction between C60. and the cation is most likely responsible for the marked negative shifts in the present study, since the interaction between the anion C60. and the cation C60N þ was changed by the morphology of the C60N þ nanoclusters. These considerations are in fair agreement with the small shifts in the redox potential for MePH. The MFEs on the electrochemical properties of C60N þ clusters are in good agreement with the morphological effects of applying magnetic fields seen in the AFM images (Figure 15.5). The photoelectrochemical properties of ITO electrodes modified with C60N þ –MePH clusters were also measured in the presence and absence of magnetic

Figure 15.6 Effects of magnetic processing on (a) DPV curves (b) potential dependences of photocurrents of C60N þ –MePH clusters on ITO electrodes.


processing. In both cases, photoirradiation of the modified electrodes afforded anodic photocurrents. The photocurrent action spectrum in the absence of magnetic processing was in fair agreement with the absorption spectrum of the mixture of C60N þ and MePH in the THF:H2O (2 : 1) mixed solvent. A similar photocurrent action spectrum was also observed in the presence of magnetic processing. These results indicate that the photocurrents are caused by the photoexcitation of C60N þ clusters in the presence and absence of magnetic processing. The potential dependences of the photocurrents of the electrodes modified with C60N þ –MePH nanoclusters in the presence of magnetic processing were also different from those in the absence of magnetic processing (Figure 15.6b). The magnetic field effects during AFM (Figure 15.4), and differences in electrochemical and photoelectrochemical measurements (Figure 15.5) can most likely be ascribed to the difference in the reduction potentials between the absence and the presence of magnetic processing due to the morphological change of the C60N þ nanoclusters. The potential dependences of the photocurrents of the modified electrodes were examined in all the ITO electrodes modified with C60N þ –MePH clusters in the absence and presence of magnetic processing. There were appreciable differences (Figure 15.6b). The potentials at zero photocurrent in the presence of magnetic processing were slightly negatively shifted compared with those in the absence of magnetic processing. The reaction scheme of the photoelectrochemical reactions of the ITO electrodes modified in the absence and presence of magnetic processing is shown in Figure 15.7. On the basis of the reaction scheme (Figure 15.7), the results can most likely be ascribed to the difference in the first reduction potentials of the C60N þ clusters in the presence and absence of magnetic processing and are in good agreement with those of the electrochemical properties.

Figure 15.7 Schematic presentation of reaction scheme of photoelectrochemical reactions of ITO electrodes modified with C60N þ –MePH clusters in the absence and presence of magnetic processing.

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The MFEs on electrochemical and photoelectrochemical measurements can most likely be ascribed to the difference in the reduction potentials between the clusters in the absence and presence of magnetic processing using the morphological change of C60N þ nanoclusters. We demonstrated that the morphology of nanostructures, electrochemical, and photoelectrochemical properties in the electrodes modified with nanoclusters of C60 can be controlled by applying a strong magnetic field. The present study provides useful information for designing novel nanodevices whose photofunctions can be controlled by a magnetic field. 15.2.3 Effects of Magnetic Processing on the Luminescence Properties of Monolayer Films with Mn2 þ -Doped ZnS Nanoparticles

Semiconductor nanoparticles have been intensively studied because of their properties of quantum size effects [54]. A number of synthetic techniques have been reported and their characteristics have been studied by various spectroscopic methods [55, 56]. However, magnetic field effects (MFEs) on the photoelectrochemical properties of semiconductor nanocrystals had not until now been reported. We reported, for the first time, MFEs on photocurrents of modified electrodes with cadmium sulfide nanoparticles (Q-CdS) ascribing them to their quantum size effects [29]. However, the MFEs were smaller than those on the modified electrodes with the porphyrin-viologen linked compounds as reported in previous papers [27, 28]. In a diluted magnetic semiconductor and its nanoparticles, a variety of unusual magnetic and magneto-optical properties due to exchange interaction between the band electrons and the magnetic ions have been reported [57, 58]. We found that the MFEs on the photocurrent responses from modified electrodes with diluted magnetic semiconductor nanoparticles (Q-Cd1xMnxS) were substantially enhanced by the presence of Mn2 þ ions [30]. The MFEs on photocurrents observed in the modified electrodes with semiconductor nanoparticles (Q-CdS and Q-Cd1xMnxS) are most likely explained by an electron–hole pair mechanism. We prepared monolayered films with Q-Zn1xMnxS and examined the effects of magnetic processing during their preparation on their luminescence properties [59]. Q-Cd1xMnxS was prepared using the bis(2-ethylhexyl) sulfosuccinate (AOT) reversed micelle method. The size of the nanoparticles was controlled by changing the W(¼[H2O]/[AOT]) values (2.5, 3.0, 5.0, 7.5, 10). Luminescence peaks were observed at 583589 nm in the Q-Zn1xMnxS or the alkanethiol-capped Q-Zn1xMnxS, except for the Q-Zn1xMnxS (W ¼ 2.5) samples. Luminescence at 583 589 nm was observed in the Q-Cd1xMnxS samples and is ascribed to Mn2 þ ion in the nanoparticles due to energy transfer from ZnS. The luminescence was enhanced by capping with alkanethiol. Monolayer films with the alkanethiol-capped ZnS:Mn nanoparticles were fabricated on quartz substrates by the layer-by-layer method using a self-assembled monolayer of 1,6-hexanedithiol.


Figure 15.8 Experimental set-up for magnetic processing in the preparation of monolayer films consisting of Q-Zn1xMnxS.

Magnetic processing in the preparation of monolayer films was carried out as shown in Figure 15.8. Polarized luminescence spectra for monolayer films prepared in the absence and the presence of magnetic processing were measured without a magnetic field. The polarization degrees (p-values) of luminescence for the monolayer films consisting of Q-Zn1xMnxS (W ¼ 5.0, 10 and x ¼ 0, 0.05, 0.10) were compared in the absence and the presence of magnetic processing (0.2, 0.6, 0.8 T). The p-value of luminescence for a cyclohexane solution of alkanethiol-capped Q-Zn1xMnxS (W ¼ 5.0 and x ¼ 0.10) was estimated to be 0.007. The polarized luminescence spectra for the monolayer film consisting of Q-Zn1xMnxS (W ¼ 5.0 and x ¼ 0.10) in the absence of magnetic processing was recorded and the p-value was estimated to be 0.024. The increment in the p-value can probably be ascribed to the immobilization of the Q-Zn1xMnxS on the quartz plates. The p-values for the monolayer films consisting of Q-Zn1xMnxS (W ¼ 5.0, 10 and x ¼ 0.10) increased with increasing magnetic field in the magnetic processing, as shown in Table 15.1. At the same magnetic field (0.8 T), the effect of x-values on the Table 15.1 Effect of magnetic processing on polarization degrees (p-values) of luminescence for monolayer films consisting of Q-Zn1xMnxS (x ¼ 0.10).

Magnetic Field (B)

W¼5 W ¼ 10

0.2 T

0.6 T

0.8 T

0.028 0.029

0.050 0.060

0.075 0.076

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Table 15.2 Effect of magnetic processing on polarization degrees (p-values) of luminescence for monolayer films consisting of Q-Zn1xMnxS (x ¼ 0.10).

x-value

W¼5

0.02

0.05

0.10

0.028

0.050

0.079

p-values for the monolayer films consisting of Q-Zn1xMnxS (W ¼ 5.0) was examined (Table 15.2), the p-values increased with increasing x-value. As a reference, magnetic processing in the preparation of monolayer films consisting of Q-ZnS without Mn2 þ ion was also carried out. The luminescence at 400 nm was observed in the monolayer films, however, no magnetic field effects on the p-values for the monolayer films were observed. On the basis of the observations, the enhancements of the p-values for the Q-Zn1xMnxS monolayer films can most likely be ascribed to the interaction between Mn2 þ ion as a magnetic ion and the external magnetic field. The enhancements are probably caused by magnetic orientation of the Q-Zn1xMnxS on the quartz substrates.

15.3 Spin Chemistry at Solid/Liquid Interfaces 15.3.1 Magnetic Field Effects on the Dynamics of the Radical Pair in a C60 Clusters–Phenothiazine System

Recently, photochemical and photoelectrochemical properties of fullerene (C60) have been widely studied [60]. Photoinduced electron-transfer reactions of donor–C60 linked molecules have also been reported [61–63]. In a series of donor–C60 linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin–C60 linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. It has been reported that C60 and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C60 clusters have been extensively reported because of the interesting properties of C60 clusters [25, 26]. The MFEs on the decay of the radical pair between a C60 cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the C60 cluster in mixed solvents have not yet been studied.

15.3 Spin Chemistry at Solid/Liquid Interfaces

A fullerene derivative containing a positive charge (C60N þ ) (Figure 15.4) forms optically transparent clusters (diameter about 100 nm) in THF/H2O mixed solvent as seen from the results of the absorption spectra and AFM images, and dynamic light scattering as described above (Section 15.2.2). We examined photoinduced electrontransfer reaction and MFEs on the dynamics of the radical pair generated by the intermolecular electron transfer reaction between the C60N þ cluster (C60N þ )n and MePH (Figure 15.4). Transient absorption spectra of the C60N þ cluster (C60N þ )n–MePH system following laser excitation at 355 nm indicate that the photoinduced intermolecular electron-transfer from the triplet excited state of PH to the C60N þ cluster (C60N þ )n occurs as shown in Figure 15.9a. MFEs on the dynamics of the radical pair in C60N þ clusters (C60N þ )n–MePH system were examined in THF–H2O (2 : 1) mixed solvent. MFEs on the decay profiles of the transient absorption at 520 nm due to the phenothiazine cation radical (PH þ ) are shown in Figure 15.9b. The decay was retarded in the presence of the magnetic field. In addition, the absorbance at 10 ms after laser excitation increased with increasing magnetic field. The result indicated that the yield of the escaped PH. þ increased with the increase in magnetic field. Therefore, the MFEs on the decay profile were clearly observed. The magnitudes of the MFEs were evaluated by the following equation; D ¼ Abs(B T)/Abs(0 T), where Abs(B T) and Abs(0 T) represent the absorbance at 10 ms at 520 nm in the presence (B T) and the absence of magnetic field. The D-values increased with increasing magnetic field. The D-value became 2.7 at 1.2 Tas shown in Figure 15.9b. The MFEs on the C60N þ cluster–MePH are explained by a radical pair mechanism between the C60N þ cluster, (C60N þ )n and MePH as shown in Figure 15.9a. MePH is mainly photoexcited by the 355 nm laser light, because of the excess amount of MePH in comparison with C60N þ . The singlet excited state of PH (1PH ) is generated by the

Figure 15.9 (a) Reaction scheme of photoinduced intermolecular electron-transfer in C60N þ cluster- MePH system. (b) MFEs on D ¼ Abs(B T)/Abs(0 T) in C60N þ –MePH in THF–H2O (2 : 1) mixed solvent.

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laser excitation. The intersystem-crossing process (kisc) occurs and 3PH is generated. The intermolecular electron-transfer process (kCS(T)) from 3PH to the C60N þ cluster (C60N þ )n, occurs and generates the triplet radical pair, 3((C60N þ )n. þ PH. þ ). The triplet radical pair disappears partly via a spin–orbit coupling (SOC)-induced intersystem-crossing process (kSOC) to the ground state. The process (kSOC) is independent of the magnetic field. The triplet radical pair decays to the ground state via the singlet radical pair, 1((C60N þ )n. þ PH. þ ). The intersystem-crossing process (kisc2) for the radical pair, but not the reverse electron-transfer process (kCR) from the singlet radical pair to the ground state, becomes a rate-determining step for the radical pair to decay via a reverse electron-transfer reaction. The intersystemcrossing process (kisc2) is influenced by the magnetic field. In the absence of magnetic field, the three sublevels of triplet radical pair are degenerate. In the presence of a magnetic field, Zeeman splitting of the triplet sublevels occurs. As a consequence of Zeeman splitting, the intersystem crossing process (kisc2) is retarded in the presence of a magnetic field. As a result, the escape process (kesc) from the triplet radical pair increases with increasing magnetic field and the yield of the escaped PH. þ increases with increasing magnetic field. In the presence of a magnetic field, the intersystemcrossing process (kisc2) is controlled by the spin–lattice relaxation from triplet sublevels. Thus the MFEs are explained in terms of a spin–lattice relaxation mechanism [20–23]. 15.3.2 Magnetic Field Effects on Photoelectrochemical Reactions of Electrodes Modified with the C60 Nanocluster-Phenothiazine System

As a study of spin chemistry at solid/liquid interfaces, we have examined MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with nanoclusters containing C60N þ and MePH (Figure 15.4), intended for utilization of C60 as photofunctional nanodevices. The nanoclusters of the mixture of C60N þ and MePH (Figure 15.4) were prepared by dissolving C60N þ and MePH in the THF–H2O mixed solvent prepared using a different method to that described above (Section 15.2.2) [49]. Similar spherical nanoclusters (diameter about 100 nm) were also observed, similar to Figure 15.5a. Self-assembled monolayers of HS(CH2)2SO3Na þ (MPS) were prepared by immersing a gold electrode in an ethanol solution of MPS. Modified electrodes with clusters of C60N þ alone and of the mixture of C60N þ and MePH were fabricated by immersing the MPS-modified electrode in the THF–H2O mixed solution containing the respective nanoclusters. The AFM image (Figure 15.10a) indicated that the modified electrodes with nanoclusters of C60N þ and MePH were fabricated by immersing the MPS-modified electrode in the THF–H2O mixed solution containing the nanoclusters. Photoelectrochemical measurements were carried out by using a three-electrode cell containing the modified electrode as a working electrode, a platinum electrode as a counter electrode, and an Ag/AgCl electrode as a reference electrode. Na2SO4 was used as the supporting electrolyte. Photocurrents from the modified electrode were

15.3 Spin Chemistry at Solid/Liquid Interfaces

Figure 15.10 (a) AFM image and schematic illustration of electrode modified with nanocluster of C60N þ and MePH. (b) Magnetic field dependence on Q-values.

measured under a controlled potential at 0 V vs Ag/AgCl with a potentiostat in the presence of triethanolamine as a sacrificial electron donor under nitrogen atmosphere. The MFEs on the photoelectrochemical measurements were carried out according to the previous paper [29, 30]. The electrode cell was placed at the pole-gap of an electromagnet. Photoirradiation of the modified electrode with nanoclusters of C60N þ alone or the mixture of C60N þ and MePH afforded anodic photocurrents. The photocurrent action spectrum was in fair agreement with the absorption spectrum of the THF–H2O (2 : 1) mixed solution containing nanoclusters of the mixture of C60N þ and MePH or C60N þ alone. These results strongly indicate that the photocurrents can be ascribed to photoexcitation of the nanoclusters of C60N þ . MFEs on the photocurrents of the modified electrode with nanoclusters of the mixture of C60N þ and MePH were examined to verify the photocurrent generation mechanism. In the presence of a magnetic field, the photocurrents clearly increased. The magnitude of the MFEs on the photocurrent is expressed as follow: Q ¼ ðIðBÞIð0ÞÞ=Ið0Þ 100

ð15:1Þ

where I(B) and I(0) are the photocurrent in the presence and absence of magnetic fields, respectively. The Q-value (%) increased gradually with increasing magnetic field up to 0.5 T (about 3% at 0.5 T), as shown in Figure 15.10b. Though the photoexcited species of the C60N þ cluster, (C60N þ )n in the present study is different from that of MePH, as described in Section 15.3.1, the formation of a triplet radical pair occurred in both cases. Therefore, the present MFEs observed in the photocurrents (Figure 15.10b) are expected to be explained by a similar mechanism to that shown in Figure 15.9a In fact, the magnetic field dependence on the yield of the escaped PH. þ (D-value) (Figure 15.9b) was in good agreement with that on the Q-value in Figure 15.10b. Accordingly, the MFEs on the photocurrents can be explained by a spin–lattice relaxation mechanism in a radical pair mechanism [20–23].

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15.4 Summary

We first demonstrated that the morphology of nanostructures, electrochemical, photoelectrochemical, or luminescence properties in the functional materials consisting of SWNTs, their composites, diluted magnetic semiconductor (Zn1xMnxS) nanoparticles, and nanoclusters of C60 can be controlled by magnetic processing, including the use of a strong magnetic field. The magnetic processing in the present studies provides useful information for designing novel nanodevices whose photofunctions can be controlled by a magnetic field. The MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with nanoclusters containing C60N þ and MePH were examined as a study of spin chemistry at solid/liquid interfaces. The results can be expected to lead to an epochmaking means of reaction control involving photoelectrochemical processes. The results also provide useful information for designing novel nanodevices whose photofunctions can be controlled by a magnetic field.

Acknowledgments

The author is grateful to Mr. H. Horiuchi for the preparation of photoelectrochemical cells, Professor S. Yamada for discussion, and Professor Y. Tanimoto and Associate Professor Y. Fujiwara for using the superconducting magnets. The author also thanks The Center of Advanced Instrumental Analysis, Kyushu University, for 1H-NMR measurements. This study was financially supported by the Grant-in-Aids for Scientific Research: Priority Areas (Area 767, No. 15085203) and (Area 432, No. 17034051), Scientific Research (C) (No. 17550131), and twenty first century COE Program Function Innovation of Molecular Informatics from MEXT of the Japan.

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16 Controlling Surface Wetting by Electrochemical Reactions of Monolayers and Applications for Droplet Manipulation Ryo Yamada

16.1 Introduction

Reversible control of surface physicochemical properties has received much attention [1, 2] because it enables researchers to construct novel fluidic devices such as pumps [3], micro-shutters [4] and variable focus lenses [5]. Organic monolayers are of particular interest for controlling surface characteristics because surface structural changes and chemical reactions are designed with an accuracy of a single molecular thickness. In this chapter, I would like to give a brief review of self-assembled monolayers and describe our studies on the droplet manipulation realized by controlling the wetting distribution of the surface. 16.1.1 Self-Assembled Monolayers

When appropriate molecular–molecular and molecular–surface interactions are present, an ordered monolayer is formed spontaneously on surfaces (Figure 16.1). This process is called self-assembly (SA) and monolayers formed in this manner are called self-assembled monolayers (SAMs). SAMs of alkanethiols on an Au(1 1 1) surface are widely used to control surface properties, electron transfer processes and to stabilize nano-clusters [6, 7]. SAMs are formed by chemical bond formation between S and Au when an Au(1 1 1) substrate is immersed in a solution containing several mM of alkanethiols for hours to days. Various functions have been realized by using SAMs of alkanethiols on Au substrates as listed in Table 16.1. Figure 16.2a and b show schematic drawings of top and side views of the SAMs of alkanethiols on the Au(111) surface, respectively [28]. The basic molecular arrangep p ment is ( 3 3)R30 with respect to the Au(111) surface. Closer inspection of the p p structure revealed the existence of a c(4 2) superlattice of ( 3 3)R30 . The alkyl chain is tilted from the surface normal by about 30 with all-trans conformation. This

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Figure 16.1 Spontaneous formation of a monolayer on a clean surface.

tilt angle comes from the conditions for close packing of alkyl chains. The plane defined by an all-trans carbon molecular skeleton alternatively changes its direction. 16.1.2 Preparation of Gradient Surfaces

A surface whose physicochemical properties gradually change as a function of the position is called a gradient surface. While the gradient surface has potential applications as a substrate for combinatorial studies [29–31], the gradient of surface wetting is of particular interest because spontaneous motions of a droplet are induced on the surface [32]. The gradient surface is easily prepared by using SA technique. For example, vapor phase deposition of the molecules is useful to change the spatial distribution of the coverage, as shown in Figure 16.3. In this method, a droplet containing molecules is put beside the substrate [32]. The vaporized molecules diffuse and are adsorbed on the substrate. The closer the position on the substrate to the droplet is, the faster the adsorption of the molecules is. As a result, a gradient of coverage is formed on the surface. SA in dilute solutions is also used to make a gradient [33]. When the substrate is slowly pulled up from the dilute solution of molecules, the bottom part of the substrate possesses higher coverage of molecules. Other

Table 16.1 Functions realized by SAMs and terminal functional groups.

Functions

Functional Groups

Redox Photochemistry Catalysis

ferrocene [8], quinone [9], Ru(NH3)62þ [10] porphyrin [11], Ru(bpy)32þ [12] ferrocene [13], Ni-azamacrocyclic complex [14], metal-porphyrin [15] ferrocenyl-nitrophenyl ethylene [16]

Optical second harmonic generation Sensor Structural transition Mediator Wetting control Bonding

quinone [17], cyclodextrin [18], enzyme [19] azobenzene [20], spiropyran [21] ferrocene [22], pyridine [23] carboxyl group [6], hydroxyl group [6], sulfonic acid [6], methyl group [6] carboxyl group [24], amine [25], phosphonate [26], thiol [27]

16.1 Introduction

Figure 16.2 (a) Model of molecular arrangement (shaded circle) with respect to the Au(111) surface (open small circle). The diagonal slash indicates the azimuthal orientation of the plane defined by the C–C–C backbone of the all trans-hydrocarbon chain. (b) Side view of the molecules. Circles represent sulfur atoms.

methods using microfluidics [34] and partial decomposition of monolayers [35, 36] have been developed. 16.1.3 Spontaneous Motion of a Droplet on Wetting Gradients

Manipulation of a droplet on a solid surface is of growing interest because it is a key technology to construct lab-on-a-chip systems. The imbalance of surface tensions is known to cause spontaneous motion of a droplet on the surface, as mentioned above. The wetting gradient causing liquid motion has been prepared by chemical [32], thermal [37], electrochemical [3] and photochemical [38–40] methods. Chaudhury and Whitesides prepared the gradient surface by vapor deposition of hexamethyldisilazane (HMDS) molecules on silicon oxide surface [32]. Because of the gradual change in the coverage of the molecules, gradients in the wetting by water were generated. When a water droplet was put on the gradient surface, the droplet moved spontaneously. The droplet moved on the gradient surface because of the difference in advancing and receding contact angles at the front and rear ends of the droplet [32] (Figure 16.4). Interestingly, the droplet can climb the slope without any external power source. Abott et al. demonstrated that the flow in a microfluidic channel can be controlled by using the wetting gradient generated by electrochemical reactions of the surfactant

Figure 16.3 Vapor phase deposition of molecules and gradients of coverage of molecules formed on a surface.

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Figure 16.4 Spontaneous motion of a droplet on wetting gradients.

dissolved in the solution [3]. Two electrodes were placed in the micro-fluidic channel. The surfactant was oxidized at one electrode and reduced at the other electrode. As a result, the concentration of oxidized molecules gradually changed as a function of the distance between the two electrodes and a flow of liquid was generated. By using this method, they succeeded in moving the solid particles in micro-fluidic channels. 16.1.4 Surface Switching

Physicochemical properties of surfaces can be reversibly changed by using reversible reactions of monolayers and polymers [2]. This phenomenon is called surface switching and the surfaces possessing this characteristic are sometimes called smart surfaces. Surface switching using monolayers was proposed by Lahann et al. [41]. They prepared a loosely packed monolayer of 16-mercapto-hexadecanoic acid. The orientation of the molecule was altered by the surface charge controlled by an external voltage source. When the surface was negatively charged, the molecules were straightened due to the electrostatic repulsion. On the contrary, the molecules made a bow shape when the surface was positively charged. These conformational changes were confirmed by surface analysis techniques such as sum frequency generation spectroscopy. As a result of surface switching, the contact angle of the water droplet was reversibly changed as a function of the voltage applied to the surface. Ichimura et al. used photoisomerization of the azobenzene molecule to create wetting gradients [38]. The silicon oxide surface was modified with a monolayer of calyx[4]resorcinarene derivative possessing azobenzene units. The azobenzene was in the trans-state under ambient conditions. When the surface was exposed to UV light (365 nm), photoisomerization took place and the cis-isomer, which has a high dipole moment, was generated on the surface. This isomerization resulted in a change in the surface wetting. The cis-isomer is converted to the trans-isomer by illumination with blue light (436 nm). The gradient of surface wetting was generated by asymmetrical illumination of light. The molecular shuttle was also used to generate a wetting gradient by photo-irradiation [40]. Electrochemical reactions of Fc-alkanethiol monolayers have been used to control the wetting of the gold surface [41–45]. We observed the change in surface wetting accompanying the electrochemical reactions by putting an oil (nitrobenzene) droplet on the surface, as shown in Figure 16.5. The surface covered with Fcalkanethiol is hydrophobic and, thus, the oil was spread on the surface as shown in Figure 16.5a. When Fc was oxidized, the anions in the solution were trapped on the surface to compensate the positive charge of Fc þ and determined the surface wetting. The surface became hydrophilic when the anions were hydrophilic, which

16.1 Introduction

Figure 16.5 Change of contact angle of nitrobenezene droplet. The droplet was put on a gold substrate covered with Fcalkanethiols in a 1 M aqueous solution of HClO4. Fc was neutral (a) and positively charged (b).

is usual for aqueous solutions, and, thus, the oil droplet was repelled as shown in Figure 16.5b. Figure 16.6a shows an experimental set-up to monitor the interfacial tension of a nitrobeneze/1 mM HClO4 aqueous solution interface as a function of the potential of the electrodes by a hanging meniscus method. A gold plate modified with Fcalkanethiol was first hung in the aqueous phase (the upper open area in Figure 16.6a). Then, the gold plate was lowered. When the gold plate reached the aqueous solution/ nitrobenzene interface, the mass measured by the microbalance changed due to the interfacial force working on the plate. The change in measured mass, DW, is given by DW=p ¼ g cosq where p is the perimeter of the plate (4 cm), g is the interfacial tension of the liquid/ liquid interface, and q is the contact angle [46]. Since g was almost constant during the potential change, DW was attributed to the change in the contact angle q. Figure 16.6b shows the experimental results. Open and closed circles represent the values measured after the plate was moved downward and upward in Figure 16.6a, respectively, at each potential. In both processes, the contact angles increased as the potential was changed towards the positive direction. The observed behavior of the contact angles is attributed to the change in surface wetting of the gold-substrate due to the oxidation and reduction of Fc. The hysteresis of the contact angle during the upward and downward process was apparent from the measurement. Note that inserting and pulling steps are related to the advancing and receding processes of droplet motion on the surface, respectively. The hysteresis in contact angles caused the hysteresis of the deformation of the droplet put on the substrate during the potential cycle. When the potential was

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Figure 16.6 (a) Schematic drawing of experimental set-up for the evaluation of the interfacial tension under potential control. (b) Relative change in contact angle as a function of the potential after the substrate was inserted into (open circles) and pulled from the nitrobenzene phase. Insets are schematic drawings of the side views of the contact lines. The potential was described with respect to the Au/AuOx reference electrode.

scanned in a positive direction, the nitrobenzene droplet shrank on the substrate since the surface became repulsive to nitrobenzene. The contact line of the droplet receded in this process and, therefore, the change in the contact angle followed the curve A–B in Figure 16.6b. At the point B, when the potential was scanned in the negative direction, the droplet kept its shape until the potential reached 0.66 V (point C). At point D, the droplet spread. When the direction of the potential scan was changed from negative to positive, the droplet did not deform until the potential reached about 0.68 V (point E).

16.2 Ratchet Motion of a Droplet 16.2.1 Ratchet Motion of a Droplet on Asymmetric Electrodes

When a droplet is deformed asymmetrically, the ratchet motions of the droplet can be induced as demonstrated on the vibrated gradient surface and on a saw-shaped electrode on which the wetting was changed by electrowetting [48]. We showed that the ratchet motions of a droplet were induced by very simple asymmetric guides [45]. Figure 16.7 shows a schematic drawing of the electrode causing the ratchet motions of a droplet.

16.2 Ratchet Motion of a Droplet

Figure 16.7 Schematic drawing of the asymmetric electrode pattern. The gold electrode was covered with a Fc-alkanethiol monolayer. The wetting of the gold electrode was switched from wetting to repulsive and vice versa by changing the electrochemical potential of the electrode.

The surface of the V-shaped electrode was covered with Fc-alkanethiol monolayers. The droplet of nitrobenzene having a diameter larger that the width of the electrode was put on the electrode. The droplet of nitrobenzene was deformed following the electrode structure because it did not spill from the area of the electrode into the surrounding glass surface, which was hydrophilic. The contact line at the left side of the droplet, denoted A in Figure 16.8a, was longer than that at the right side, denoted B, on the V-shaped electrode. The difference in the length of the contact lines generated the imbalance in net forces acting on the droplet in the deformation process. When the surface wetting was changed from repulsive to wetting, only the contact line facing the wider side (A in Figure 16.8b) proceeded because of the stronger net-force generated for the longer contact line of the droplet. On the contrary, only the narrow contact line (denoted B in Figure 16.8c) retracted, as shown in Figure 16.8c, when the surface was changed from wetting to repulsive. The sequential spreading and shrinking of the droplet resulted in its net transport. The presented results show that the simple asymmetric pattern caused directional deformations and transport of a droplet. This technique is applicable to generation of a flow in microfluidic devices. 16.2.2 Ratchet Motion of a Droplet Caused by Dynamic Motions of the Wetting Boundary

The ratchet motion of a droplet was induced by electrochemical control of the wetting distributions on the surface [44]. Figure 16.9 shows the principles for the generation

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Figure 16.8 Directional deformations of the nitrobenzene droplet. The length of the scale bar in (a) is 3 mm. When the surface was changed from repulsive to wetting (a–b), the left side of the droplet (A) spread. When the surface was back to repulsive (b–c), the right side of the droplet (B) shrank. These directional deformations resulted in a net transport of the droplet.

and control of the wetting gradient by the electrochemical method. The gold thin-film covered with Fc-alkanethiol monolayer was immersed in the electrolyte solution and the potential of the gold substrate, Eoffset, was controlled with respect to the reference electrode in the solution. A lateral bias voltage, Vbias, was applied to the gold substrate to generate in-plane gradients in the electrochemical potential [49]. The electrochemical potential between A and B denoted in Figure 16.9a is expected to change linearly when only a negligible current flows into the counter electrode in the electrochemical cell. Figure 16.9b schematically shows the relationships between these electrochemical parameters and the concentration of Fc þ in the monolayer as a function of the position on the gold substrate, x. When the redox potential of Fc þ /Fc in the monolayer is in the electrochemical potential window generated in the gold substrate, the concentration of Fc þ gradually changes around the point P shown in

16.2 Ratchet Motion of a Droplet

Figure 16.9 (a) Schematic drawing of the experimental configuration. (b) Potential profile and wetting distribution on the substrate under the biased condition.

Figure 16.9b. Since Fc- and Fc þ -covered surfaces are known to be hydrophobic and hydrophilic, respectively [42, 43], the wetting gradient is expected to be formed around P. Therefore, we call P the wetting boundary. The position of the wetting boundary and the magnitude of the wetting gradient can be reversibly controlled as functions of Eoffset and Vbias/l, respectively, where l is the length of the substrate in the biased direction. Figure 16.10 show pictures of the nitrobenzene droplets on the gold electrode covered with Fc-monolayer in aqueous solution when Vbias (0.35 V) was applied. The potential of the substrate was measured by two wires in contact with the substrate. It was clearly shown that the position of the wetting boundary moved when Eoffset was changed, as shown in Figure 16.10b–d. Figure 16.11 shows the motions of the droplet caused by the shift in the wetting boundary. Initially, Eoffset and Ebias were set to 300 mV and 500 mV, respectively. The left side of the substrate in Figure 16.11 was more wetting to nitrobenzene than the right side of the substrate under this bias condition. The initial appearance of the droplet, shown in Figure 16.11a, indicated that the droplet was on the repulsive side of the wetting boundary at this stage. When Eoffset was changed to more negative values to move the wetting boundary to the droplet, the droplet gradually spread into the wetting areas. Figure 16.11b shows the droplet when Eoffset was 340 mV. The droplet was almost completely wet, indicating that the wetting boundary was at the right side of the droplet. When Eoffset was changed to the initial value to move the wetting boundary, the droplet gradually shrunk from the right edge and returned to its original appearance, as shown in Figure 16.11c. The unidirectional spreading-andshrinking cycle resulted in the net transport of the droplet by an inchworm motion. The rate of the droplet motion was limited by the rate of the viscous flow of the liquid because electrochemical reaction took place much faster (in less than 1 s) than the deformation of the droplet (10 s). Pinning of the droplet sometimes occurred at a defect on the surface. In most cases, the pinned droplet could be moved again by

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Figure 16.10 Photographs of nitrobenzene droplets. Vbias was fixed at 0.35 V and Eoffset was varied (a–d). The line in the photograph represents the position of the wetting boundary estimated from the shape of the droplets. Note that the current peak for the Fc þ /Fc reaction was observed at about 0.5 V in the cyclic voltammogram.

making the wetting gradient stronger or changing the limits of the potential cycles. The inchworm motion could be repeated many times, as shown in Figure 16.11d, and the reverse motion was possible by changing the bias direction. The positions of the contact lines moved as a result of the imbalance between the forces acting on the droplet edges during the deformation of the droplet. Since the surface had wetting gradient, the forces acting on the contact lines at the more wetting side were always larger than those acting on the other side during the deformation of the droplet. As a result, the rear and front ends were pinned when the droplet spread and shrank, respectively. It should be noted that the directional spreading of the droplet was caused by the imbalance of advancing contact angles acting on the opposite sides of the droplet while the directional shrinking was due to the imbalance of receding contact angles. These mechanisms differentiate the motion of the droplet presented here from the motion driven by the static wetting gradient originating from the difference in the contact angles between advancing and receding sides of the droplet. The motion of droplets in solution was used to manipulate micro-particles on surfaces. The dichloromethane droplet pushed aside hydrophilic glass beads of about

16.3 Conclusion

Figure 16.11 Photographs of inchworm motion of the droplet in solution. See text for details. EBIAS ¼ 0.5 V. Eoffset ¼ 300 mV (a), 340 mV (b) and 300 mV (c). (d) Trace of the multi-step inchworm motions of the droplet. Six photographs were superimposed.

40 mm diameter on the substrate in an aqueous solution, as shown in Figure 16.12a, because the hydrophilic glass beads could not get into the oil droplet. In contrast, the hydrophobic beads, which initially dispersed randomly in the dichloromethane droplet, were gathered at the receding end of the droplet during its motion, as shown in Figure 16.12b and c, because the hydrophobic beads could not get out of the droplet. In both cases, the liquid/liquid and/or solid/liquid/liquid interfaces acted as barriers to move the particles directly.

16.3 Conclusion

Surface switching coupled with geometric and potential asymmetry was used to cause directional motion of a droplet. Sophisticated design and active control of surface properties are important technology for motion control on the micro/nanoscales.

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Figure 16.12 Transportation of glass-beads by the droplet. (a) Hydrophilic glass beads were pushed by the oil droplet. The droplet moved from the upper right of the figure. A magnified image of the region within the square is shown in the inset. Hydrophobic beads in the droplet (b) were carried with the motion of the oil droplet (c). The droplet shown in (c) was moved for several mm in the direction shown by the arrow in (b).

References

Acknowledgments

This work was supported by Grants-in-Aid for Scientific Research on Priority Areas Molecular Nano Dynamics from Ministry of Education, Culture, Sports, Science and Technology.

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17 Photoluminescence of CdSe Quantum Dots: Shifting, Enhancement and Blinking Vasudevanpillai Biju and Mitsuru Ishikawa

17.1 Introduction

Quantum dots are nanocrystals of semiconductors, metals and organic materials in which excitons (electrons and holes) are three-dimensionally confined. The dimension of quantum dots is typically on the 1–25 nm scale (nanoscale). At this scale, the surface-to-volume ratios of materials become large and their electronic energy states become discrete. The large surface-to-volume ratios and the discrete energy states give unique electronic, optical, magnetic, and mechanical properties to nanomaterials. While nanoscience advances with the size-dependent properties of nanomaterials, large surface tension and friction limit their applications in the advancement of nanotechnology. In general, as the sizes of semiconductor, metal and organic materials are decreased towards the nanoscale, their optical and electronic properties become size- and shape-dependent and vary largely from those in the bulk and at the atomic/molecular levels. The size- and shape-dependent properties on the nanoscale are attributed to the quantum confinement effect, strong confinement of electrons and holes when the radii of nanoparticles are less than the exciton Bohr radius of the material. Remarkable advances in the field of semiconductor quantum dots emerged recently when the fundamental principles underlying light–matter interactions and the quantum confinement effect became clearly understood. A rationale between size and energy states in semiconductor quantum dots was developed by Luis Brus [1, 2] by applying the particle-in-a-sphere-model approximation to the bulk Wannier Hamiltonian. According to the approximation, the energy of the lowest excited state in a quantum confined system is given by Eq. 17.1. h 2 p2 1 1 1:8e2 E¼ þ ð17:1Þ 2R2 me mh cR where, R is the radius of a nanocrystal, me is the effective mass of an electron, mh is the effective mass of a hole, and e is the electronic charge. In simple words, as the size decreases the bandgap increases. This equation was proposed even before strong quantum confinement was experimentally observed in colloidal quantum dots.

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Figure 17.1 (A) Size-dependent photoluminescence color of ZnSshelled CdSe quantum dots. (B) Schematic presentation of size in Å, color, and photoluminescence spectral maxima of CdSe quantum dots. (C) Size-dependent absorption (solid lines) and photoluminescence (broken lines) spectra of CdSe quantum dots. Reprinted with permission from references [4] (A) and [5] (C); copyright [1997, 2001], American Chemical Society.

Among different quantum dots, cadmium selenide (CdSe) attracted much attention in almost all branches of science and technology, especially in nanoscience, nanotechnology and nanobiotechnology (the Nano World), rooted in the size-tunable bandgap and photoluminescence color (Figure 17.1) extending throughout the visible region of the electromagnetic spectrum [3, 4]. Probably, the Nano World would not be as exciting and colorful as it is today without CdSe quantum dots. Although extensive investigations of the optical and electronic properties of CdSe quantum dots lifted their status from a fundamental scientific level to direct applications in lasers [6], light emitting diodes [7], solar cells [8], and bioanalyses and bioimaging [9–11], surface defects (defects in the bandgap) and intrinsic on and off (blinking) photoluminescence remain unresolved. The surface defects and blinking of quantum dots limit the advancement of their applications towards singlequantum dot logic devices and single-molecule imaging. Efforts to improve the photoluminescence properties of quantum dots by decreasing the surface defects and suppression of blinking are underway. Modified syntheses, post-synthesis surface modifications, and photo- and thermal-activations are promising approaches to control the surface defects and blinking of quantum dots. This chapter presents an overview of the synthesis, ensemble photoluminescence properties and blinking of single CdSe quantum dots. The stress is on (i) widely accepted methods of synthesis, (ii) the origin of photoluminescence and variations of photoluminescence as functions of surface-coating, surface-passivating molecules, chemical environment, and thermal- and photo-activations, and (iii) photolumines-

17.2 Synthesis of CdSe Quantum Dots

cence blinking and variations of blinking as functions of modified synthesis and postsynthesis modifications.

17.2 Synthesis of CdSe Quantum Dots

In the 1980s, CdSe quantum dots were prepared by top-down techniques such as lithography; however, size variations, crystal defects, poor reproducibility, and poor optical properties of quantum dots made them inadequate for advanced applications. Introduction of bottom-up colloidal synthesis of CdSe quantum dots by Murray et al. [3] and its further advancements brought radical changes in the properties of quantum dots and their applications in devices and biology. The colloidal syntheses of CdSe quantum dots are broadly classified into organic-phase synthesis and aqueousphase synthesis. 17.2.1 Synthesis of CdSe Quantum Dots in Organic Phases 17.2.1.1 Synthesis of CdSe Quantum Dots from Dimethyl Cadmium In 1993, Murray et al. synthesized hydrophobically-capped colloidal CdSe quantum dots by the pyrolysis of organometallic precursors of cadmium and selenium [3]. In this synthesis, dimethyl cadmium [CdMe2, 13.35 mmol in 25 mL trioctylphosphine (TOP)] was reacted at high-temperature with the selenide of TOP (TOPSe, 10 mmol in 15 mL TOP) in the presence of trioctylphosphine oxide (TOPO, Tech. grade). TOPSe was prepared by dissolving Se powder or Se shots in TOP at 150 C. In a typical synthesis of CdSe quantum dots, TOPO was heated to 300 C under vacuum for 20 min followed by injection of a mixture of CdMe2 and the TOPSe in an atmosphere of Ar. The growth of CdSe nanocrystals was carried out at 230–260 C. The as-synthesized sample contained a size distribution (1.2–11.5 nm) of CdSe quantum dots which were separated by size-selective centrifugation from a mixture of 1-butanol and methanol. One of the main limitations to producing narrow sizedistributed or size-focused quantum dots by this method was Ostwald ripening, a gradual growth of larger quantum dots at the cost of a gradual dissolution of smaller ones. In 1994, Katari et al. lifted the limitation of size-distribution by replacing TOP with tributylphosphine (TBP) and avoiding Ostwald ripening [12]. In the modified synthesis, a precursor solution was prepared by dissolving Se powder (800 mg) in TBP (8 g) followed by the addition of 2 g CdMe2. This mixture was diluted to 1/4 using TBP and injected into a hot (350 C) solution of TOPO (12 g). The injection of CdSe precursors into the hot solution of TOPO resulted in spontaneous nucleation of CdSe nanocrystals and a decrease in temperature. Once the temperature was stabilized, an additional amount (0.4 mL) of the precursor solution was added for the growth of the nucleated nanocrystals. Here, Ostwald ripening was avoided by separating the nucleation and growth processes. All the reagents and the reaction were kept under an Ar atmosphere to avoid fire hazard and surface oxidation of the nanocrystals.

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17.2.1.2 Synthesis of CdSe Quantum Dots from Cadmium Sources Other Than Dimethyl Cadmium With the introduction of the colloidal syntheses of CdSe quantum dots, researchers were interested in cadmium precursors other than the volatile and toxic CdMe2. In 2001, Qu et al. replaced CdMe2 with cadmium oxide (CdO), cadmium acetate [Cd(AcO)2], and cadmium carbonate (CdCO3) and synthesized high-quality CdSe quantum dots [13]. A typical example is the synthesis of CdSe quantum dots from Cd (AcO)2 and trioctylphosphine selenide (POTSe). In this reaction, a suspension of Cd (AcO)2 in TOPO or TOPO–phosphonic acid mixture was heated to 250–360 C under an Ar atmosphere followed by injection of TOPSe. After nucleation of CdSe nanocrystals, the reaction temperature was decreased to 200–320 C and nanocrystals with different sizes were grown. CdSe nanocrystals with strong quantum confinement (size 100 K, which is a more general observation [21]. The red shift of the photoluminescence or decreasing bandgap with increasing temperature is due to the dilation of the crystal lattice and lattice–electron interactions, which are bulk properties of CdSe. Thus, the red shift is less related to the quantum confinement effect. We observed reversible red shifts and blue shifts in the photoluminescence spectra of closely-packed aggregates of CdSe quantum dots [21]. Figure 17.4 shows the photoluminescence spectral shifts during heating and cooling of a solution of CdSe quantum dot aggregates; the photoluminescence spectrum red shifted when the temperature was increased from 298 to 353 K and returned to the original position when re-cooled. In addition to the intrinsic thermal effects on the bulk bandgap energy of CdSe, the red shift is contributed by the relaxation of photoactivated excitons through surface states and interparticle interface states, both of which are thermally populated. Also, carrier tunneling and inter-dot dipole–dipole interactions are involved in the red shift. Figure 17.5 shows various relaxation processes in a photoactivated CdSe quantum dot aggregate. Similar red shifts are observed in the photoluminescence spectra of CdSe quantum dot superstructures in which quantum mechanical tunneling through inter-dot barriers and thermal activation followed by hopping above energy barriers due to exciton-coupling are proposed to account for the red shift. In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kV cm–1. Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field; the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots.

17.4 Photoluminescence Spectral Shifts

Figure 17.4 (A) Photoluminescence spectral shifts (Dl) of a solution of CdSe quantum dot aggregates during heating–cooling cycles: photoluminescence spectral maxima were recorded at 298 K during cooling and 353 K during heating. Reversibility of the photoluminescence spectral shift was attained after four heating–cooling cycles. (B) Photoluminescence spectra of a solution of CdSe

quantum dot aggregates at (C) 298 K before heating–cooling cycles and (b–e) 353 K after one to four heating cycles. Spectra (b–e) are normalized for intensity with respect to the spectrum before heating (a) [21]. The photoluminescence measurements were carried out by exciting the samples at 450 nm. Reprinted with permission from reference [21]; copyright [2005], American Chemical Society.

17.4.2 Chemical Effects on Spectral Shifts

In addition to the bulk bandgap (1.74 eV), the total energy of an exciton in a CdSe quantum dot is contributed by the kinetic energy of confinement, electron–hole coulomb interactions, self-charging energy of charge carriers and polarization energy. These factors increase the quantum confined bandgap of CdSe by 1 eV. The contribution of the polarization energy to the total energy is relatively small. For example, 1Se–1S3/2h transitions in a size-series of CdSe quantum dots were only slightly (30%) and considerably stabilized under photoactivation in polar solvents and in the presence of dissolved polymer molecules (Figure 17.8A) [25]. We also found a decreasing photoluminescence quantum efficiency of CdSe quantum dots under continuous photoactivation. The decrease in the photoluminescence quantum efficiency indicates photobleaching of quantum dots or a gradual formation of surface defects, that is, defects are continuously formed on and removed from the surface. Therefore, the stability of the photoactivated photoluminescence depends on the stability of the equilibrium between the photoactivated formation and removal of surface defects (Figure 17.8B). We attributed the enhanced photoluminescence quantum efficiency to a static passivation of the surface defects by polymers and polar molecules. In general, there are multiple ways to improve the photoluminescence quantum efficiency of as-synthesized CdSe quantum dots: (i) Preparation shells from ZnS and other higher bandgap materials, (ii) surface passivation by oxygen, ligands and polar molecules and (iii) photoactivation in the presence of various surface passivating molecules. It has been possible in recent years to synthesize highly luminescent CdSe quantum dots in a one-pot approach and, therefore, the interest in the surface modifications of quantum dots is focused on providing physical stability, chemical and bioconjugations, reduced cytotoxicity and suppressed blinking.

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Figure 17.8 (A) Photoluminescence spectra of CdSe quantum dots in CHCl3 in the presence of polybutadiene at different times under photoactivation at 400 nm. The blue shift of the photoluminescence spectra is due to a gradual decrease in quantum dot size. (B) Schematic

presentation of the surface passivation and the formation of surface defects in CdSe quantum dots under photoactivation [25]. Reprinted with permission from reference [25]; copyright [2007], American Chemical Society.

17.6 On and Off Luminescence Blinking in Single Quantum Dots

Intermittent on and off photoluminescence (blinking) property of single CdSe and CdSe–ZnS quantum dots was first observed by Nirmal et al. in 1996 [36]. Figure 17.9A shows the typical blinking behavior of a single quantum dot. To date, the origin of the blinking has been correlated with Auger ionization and trapping of photoactivated charge-carriers in defect states. Applications of quantum dots both in technology and biology are limited to a certain extent by this intrinsic blinking. For example, blinking


Figure 17.9 (A) Photoluminescence intensity trajectory (gray) of a CdSe–ZnS quantum dot. The high intensity level is the on state and the low intensity level is the off state. The trace in black is the background intensity. Reprinted with permission from reference [14]; copyright [2005], American Chemical Society. (B) Schematic

presentation of Auger ionization in a CdSe quantum dot: (a) photoactivation of a single exciton, (b) photoactivation of a second exciton, (c) electron–electron annihilation and Auger ionization and (d) neutralization of an ionized quantum dot.

of quantum dots is not promising for single-photon logic devices, on demand light emitters, quantum dot switches and continuous imaging and tracking of single molecules. Thus, synthesis of non-blinking quantum dots or complete suppression of blinking by post-synthesis modifications would bring far-reaching changes in the applications of single quantum dots. Auger ionization and transient trapping of electrons (or holes) in surface states and environment are the origins of blinking [37]. Unlike single molecules, the blinking of a single quantum dot is photoinduced and not spontaneous, which was characterized by an inverse-relation between excitation light-intensity and average on times, and excitation light intensity independent off times of single CdSe quantum dots [36]. Auger ionization is one of the best models to explain the blinking of quantum dots, which occurs by exciton–exciton annihilation when more than one exciton pairs are photoactivated (Figure 17.9B). Exciton–exciton annihilation creates a high-energy exciton that escapes from the quantum dot and ionizes it. Photoexcitation of an ionized quantum dot always results in non-radiative exciton recombination due to

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strong coulombic repulsive potentials. Thus, an ionized quantum dot continues to be in the dark state until neutralized. Simply, photoinduced ionization turns off a quantum dot and neutralization turns it back on. On the basis of the Auger ionization model, an increase in the excitation light intensity increases the possibility to turn off a quantum dot. On the other hand, neutralization of a photoionized quantum dot is an uncontrolled process. This is the reason why the average on time of a quantum dot is excitation light intensity dependent. Thus, blinking can be suppressed if (i) excitation light intensity is kept at a low level, which has practical limitations in single-molecule microscopy and (ii) resonant tunneling of a charge carrier to an outside trap is prevented by removing defects. 17.6.1 Power-Law Statistics of On and Off Time Distributions

Although the Auger ionization model is widely accepted to account for the blinking of quantum dots, exponential decays of on and off probability densities involving a single trap state are inconsistent with experimental observations of the distributed kinetics; that is, on and off times are distributed over different (3–5) decades of time (power-law behavior) [38]. A distribution of off times over three decades of time for a single CdSe quantum dot is shown in Figure 17.10. The distribution of off times indicates the presence of distributed trap states. Thus, trap states with different energies are present on the surface and in the surrounding matrix of a quantum dot. Although an Arrhenius model and a hopping mechanism are assumed to account for the power-law distribution of off times, the hopping mechanism is significant based on the lower sensitivity of off time distributions to temperature changes. The concepts of the single-exponential behavior and the distributed kinetics have recently been revisited by Tang and Marcus [39] and Frantsuzov and Marcus [40] to account for a cut-off of the power-law behavior at longer time scales. In the latest models, blinking is attributed to hole-trapping and phonon-assisted diffusion-controlled electron transfer processes [39, 40]. 17.6.2 Modified Blinking

There are advances in both experimental and theoretical studies to gain an understanding of the origin and distribution of trap states in quantum dots and to prepare non-blinking quantum dots. Electron transfer from a photoexcited quantum dot to an outside trap or an acceptor is a barrier-less process. Thus, preparation of a physical barrier such as an insulating layer on the surface of a quantum dot is insufficient to control the blinking. However, post-synthesis surface modifications suppress the blinking of quantum dots to a certain extent by removing electron traps on the surface. For example, on and off times in the intensity trajectories of single CdSe quantum dots are considerably increased when overlaid with ZnS shells of different thickness [36]. The increases in the on and off times show a reduced density of surface defects and a suppression of Auger ionization. Similarly, blinking of single


Figure 17.10 Three successive enlargements of the off time probability density for a single CdSe quantum dot. Reprinted with permission from reference [38]; copyright [2000], American Institute of Physics.

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Figure 17.11 (a) A single dot (emission peak at 585 nm) intensity trace as tris-HCl buffer with 140 mM BME was injected at 40 s (dotted lines) into the sample, displacing the buffer from the sample. (b) A time trace of a reverse case where BME buffer was washed away using the buffer at 40 s. Reprinted with permission from reference [28]; copyright [2004], American Chemical Society.

quantum dots is suppressed in the presence of ligands such as b-mercaptoethanol (Figure 17.11), oligo(phenylene vinylene) and aliphatic amines. In the presence of these ligands, the surface traps are efficiently quenched by electron transfer. Blinking is also considerably affected when the dielectric environment of a quantum dot is changed. Typical examples are suppression of blinking when quantum dots are placed on Ag [41], Au [42] and ITO (indium-tin-oxide) glass [43] surfaces. Recently, we observed a considerable increase in the on time of single CdSe–ZnS quantum dots placed on an Ag nanoparticle film [41]. Figure 17.12B shows the intensity trajectories of single CdSe–ZnS quantum dots on an Ag nanoparticle surface and a glass surface. Interestingly, complete off levels are absent in the intensity trajectory of a quantum dot placed on the Ag nanoparticle film; instead, stochastic fluctuations of intensity with non-zero intensity levels (pseudo off states) are observed. This modifiedblinking is associated with reduced average photoluminescence intensity (Figure 17.12C) and lifetime values at ensemble and single dot levels. The reduced photoluminescence intensity and lifetime values are due to non-radiative energy transfer from quantum dots to Ag nanoparticles. However, quantum dots with enhanced and intact photoluminescence intensities were also observed on the Ag nanoparticle film (Figure 17.12D). Figure 17.12A shows the energy transfer and various relaxation processes in a photoactivated quantum dot. The competence of the energy transfer process to compete with carrier-trapping and Auger ionization is the origin of the modified blinking. In other words, the relaxation of excitons in a photoactivated quantum dot that is present on an Ag nanoparticle film is dominated by ultra-fast non-radiative energy transfer to a proximal Ag nanoparticle; the rate of energy transfer is comparable to that of the trapping of charge-carriers. The suppression of blinking by electron donor molecules, ZnS shells, and noble metal nanoparticles validates the electron trapping mechanism and the distributed kinetics model of blinking. The suppression of blinking and increase in the on times are due to (i) the suppression of Auger ionization by removing traps (by electron donor molecules and ZnS shells), and (ii) deactivation of charge carriers via energy transfer to Ag nanoparticles before carrier trapping and Auger ionization.


Figure 17.12 (A) Schematic presentation of deactivation and energy transfer processes in a single quantum dot placed on an Ag nanoparticle film. (B) Photoluminescence intensity trajectories of single quantum dots on a glass substrate (a) and on an Ag nanoparticle film (b). The traces in green represent background intensities. (C) Photoluminescence spectra of quantum dot solutions in the presence of

different concentrations of Ag nanoparticles. (D) Schematic presentation of (i) a quantum dot without proximal Ag nanoparticles, (ii) a quantum dot involved in energy transfer to a proximal Ag nanoparticle, and (iii) a quantum dot in a strong plasmon field of Ag nanoparticles. Reprinted with permission from reference [41]; copyright [2008], American Chemical Society.

Also, we identified that the on and off times in the intensity trajectories of single CdSe quantum dots in a sample synthesized at a slow rate at a low temperature (75 C) are considerably decreased and uniformly distributed [14]. The short-lived on states indicate the presence of a high density of surface traps and the short-lived off states indicate a narrow distribution of the surface traps. In other words, the rate of ionization was increased by the high density of surface traps, and the narrow distribution of surface traps increased the probability of neutralization. This is an example of deviation from the distributed-kinetics model of blinking. In short, the blinking of quantum dots is due to transient trapping of charge carriers in trap states distributed on the surface and environment and the discrepancies in the blinking in

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different environments and in the presence of different ligands are due to a difference in the distribution of trap states.

17.7 Conclusions

Strong three-dimensional confinements of excitons provide size-tunable photoluminescence color to CdSe quantum dots. Additionally, brightness and stability of photoluminescence make the CdSe quantum dot a promising material for optoelectronic and electro-optic devices, bioanalyses, and bioimaging. Systematic investigations of various chemical and physical parameters involved in the colloidal synthesis of CdSe quantum dots made their preparation straightforward and improved their optical and electronic properties. The large surface-to-volume ratios and the presence of surface states give temperature-, pressure- and environment-sensitive photoluminescence properties to CdSe quantum dots. However, the intermittent blinking due to Auger ionization makes quantum dots less attractive for various single quantum dot applications. Photoluminescence properties and the stability of quantum dots are considerably improved through advances in the colloidal synthesis and post-synthesis surface modifications. Also, the blinking of single quantum dots is suppressed by preparing shells, tethering electron donor molecules, modified synthesis, and introducing dielectric environments. In addition to the improvement of the optical and electronic properties of CdSe quantum dots, all the above efforts considerably contributed to the general understanding of bandgap structure and exciton relaxation processes in quantum confined systems.

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26 Nazzal, A. Y., Wang, X. Y., Qu, L. H., Yu, W., Wang, Y. J., Peng, X. G. and Xiao, M. (2004) Environmental effects on photoluminescence of highly luminescent CdSe and CdSe/ZnS core/shell nanocrystals in polymer thin films. J. Phys. Chem. B, 108, 5507–5515. 27 Kanemoto, R., Anas, A., Matsumoto, Y., Ueji, R., Itoh, T., Baba, Y., Nakanishi, S., Ishikawa, M. and Biju, V. (2008) Relations between dewetting of polymer thin films and phase-separation of encompassed quantum dots. J. Phys. Chem. C, 112, 8184–8191. 28 Hohng, S. and Ha, T. (2004) Near-complete suppression of quantum dot blinking in ambient conditions. J. Am. Chem. Soc., 126, 1324–1325. 29 Myung, N., Bae, Y. and Bard, A. J. (2003) Enhancement of the photoluminescence of CdSe nanocrystals dispersed in CHCl3 by oxygen passivation of surface states. Nano Lett., 3, 747–749. 30 Wuister, S. F., van Houselt, A., Donega, C. D. M., Vanmaekelbergh, D. and Meijerink, A. (2004) Temperature antiquenching of the luminescence from capped CdSe quantum dots. Angew. Chem. Int. Ed., 43, 3029–3033. 31 Jones, M., Nedeljkovic, J., Ellingson, R. J., Nozik, A. J. and Rumbles, G. (2003) Photoenhancement of luminescence in colloidal CdSe quantum dot solutions. J. Phys. Chem. B, 107, 11346–11352. 32 Cordero, S. R., Carson, P. J., Estabrook, R. A., Strouse, G. F. and Buratto, S. K. (2000) Photo-activated luminescence of CdSe quantum dot monolayers. J. Phys. Chem. B, 104, 12137–12142. 33 Maenosono, S., Dushkin, C. D., Saita, S. and Yamaguchi, Y. (2000) Optical memory media based on excitation-time dependent luminescence from a thin film of semiconductor nanocrystals. Jpn. J. Appl. Phys., 39, 4006–4012. 34 Uematsu, T., Maenosono, S. and Yamaguchi, Y. (2006) Photoinduced fluorescence enhancement in CdSe/ZnS quantum dot monolayers: Influence of substrate. Appl. Phys. Lett., 89, 031910.

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Molecular Nano Dynamics Volume II: Active Surfaces, Single Crystals and Single Biocells


Molecular Nano Dynamics


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Molecular Modeling Basic Principles and Applications Third, Revised and Expanded Edition 2008 ISBN: 978-3-527-31568-0

Matta, C. F., Boyd, R. J. (eds.)

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Rode, B. M., Hofer, T., Kugler, M.

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Molecular Nano Dynamics Volume II: Active Surfaces, Single Crystals and Single Biocells


The Editors Prof. Dr. Hiroshi Fukumura Tohoku University Graduate School of Science 6-3 Aoba Aramaki, Aoba-ku Sendai 980-8578 Japan

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for

Prof. Dr. Masahiro Irie Rikkyo University Department of Chemistry Nishi-Ikebukuro 3-34-1 Toshima-ku Tokyo 171-8501 Japan Prof. Dr. Yasuhiro Iwasawa University of Electro-Communications Department of Applied Physics and Chemistry 1-5-1 Chofu Tokyo 182-8585 and Emeritus Professor University of Tokyo 7-3-1 Hongo, Bunkyo-ku Tokyo 113-0033 Japan Dr. Hiroshi Masuhara Nara Institute of Science and Technology Graduate School of Material Science 8916-5 Takayama, Ikoma Nara, 630-0192 Japan and National Chiao Tung University Department of Applied Chemistry and Institute of Molecular Science 1001 Ta Hsueh Road Hsinchu 30010 Taiwan Prof. Dr. Kohei Uosaki Hokkaido University Graduate School of Science N 10, W 8 , Kita-ku Sapporo 060-0810 Japan

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de. # 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Cover Design Adam-Design, Weinheim Typesetting Thomson Digital, Noida, India Printing and Binding betz-druck GmbH, Darmstadt Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32017-2

V

Contents to Volume 2 Contents to Volume 1 XV Preface XVII About the Editors XIX List of Contributors for Both Volumes Part Three Active Surfaces 18

18.1 18.2 18.2.1 18.2.2 18.3 18.3.1

18.3.2 18.3.2.1 18.4

19 19.1 19.2

XXIII

315

The Genesis and Principle of Catalysis at Oxide Surfaces: Surface-Mediated Dynamic Aspects of Catalytic Dehydration and Dehydrogenation on TiO2(110) by STM and DFT 317 Yohei Uemura, Toshiaki Taniike, Takehiko Sasaki, Mizuki Tada, and Yasuhiro Iwasawa Introduction 317 Experimental 318 STM Measurements of TiO2(110) 318 Computational Methods 318 Results and Discussion 319 Dynamic Mechanism for Catalytic Dehydration of Formic Acid on a TiO2(110) Surface, Much Different from the Traditional Static Acid Catalysis 319 Dynamic Catalytic Dehydrogenation of Formic Acid on a TiO2(110) Surface 327 Mechanism of the Switchover of Reaction Paths 331 Conclusion and Perspective 332 References 333 Nuclear Wavepacket Dynamics at Surfaces Kazuya Watanabe Introduction 337 Experimental Techniques 338

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19.2.1 19.2.1.1 19.2.1.2 19.2.2 19.2.2.1 19.2.2.2 19.3 19.3.1 19.3.2 19.3.3 19.4 19.4.1 19.4.2 19.4.3 19.4.4 19.5

20

20.1 20.1.1 20.1.2 20.1.3 20.1.4 20.1.5 20.2 20.2.1 20.2.2 20.2.3 20.2.4 20.3 20.3.1 20.3.2 20.4

Time-Resolved Two-Photon Photoemission with Femtosecond Laser Pulses 338 Principles 338 Experimental Set-Up 339 Time-Resolved Second Harmonic Generation 340 Principles and Brief History 340 Experimental Set-Up 341 Nuclear Wavepacket Motions of Adsorbate Probed by Time-Resolved 2PPE 343 Alkali Atom Desorption from a Metal Surface 343 Solvation Dynamics at Metal Surfaces 344 Ultrafast Proton-Coupled Electron Transfer at Interfaces 345 Nuclear Wavepacket Motion at Surfaces Probed by Time-Resolved SHG 345 Vibrational Coherence and Coherent Phonons at Alkali-Covered Metal Surfaces 345 Dephasing of the Vibrational Coherence: Excitation Fluence Dependence 347 Excitation Mechanisms 349 Mode Selective Excitation of Coherent Surface Phonons 351 Concluding Remarks 352 References 353 Theoretical Aspects of Charge Transfer/Transport at Interfaces and Reaction Dynamics 357 Hisao Nakamura and Koichi Yamashita Introduction and Theoretical Concepts 357 Introduction 357 Molecular Orbital Theory and Band Theory 358 Charge Transfer vs. Charge Transport 359 Electronic Excitation 361 Reaction Dynamics 363 Electrode–Molecule –Electrode Junctions 365 Nonequilibrium Greens Function Formalism 365 Efficient MO Approach 367 Ab Initio Calculations: Single Molecular Conductance and Waveguide Effects 370 Inelastic Transport and Inelastic Electron Tunneling Spectroscopy 375 Photochemistry on Surfaces 381 Theoretical Model of Hot Electron Transport and Reaction Probability 381 Photodesorption Mechanism of Nitric Oxide on an Ag(111) Surface 384 Summary and Outlook 392 References 394


21

21.1 21.1.1 21.1.2 21.2 21.2.1 21.2.2 21.3 21.3.1 21.3.2 21.4 21.4.1 21.4.2

22

22.1 22.2 22.3

22.4 22.5 22.6

Dynamic Behavior of Active Ag Species in NOx Reduction on Ag/Al2O3 401 Atsushi Satsuma and Ken-ichi Shimizu Introduction 401 NOx Reduction Technologies for Diesel and Lean-Burn Gasoline Engines 401 Selective Catalytic Reduction of NOx by Hydrocarbons Over Ag/Al2O3 402 Hydrogen Effect of HC-SCR Over Ag/Al2O3 403 Boosting of HC-SCR Activity of Ag/Al2O3 by Addition of H2 403 Surface Dynamics of Ag Species Analyzed by in situ UV–Vis 405 The Role of Surface Adsorbed Species Analyzed by in situ FTIR 410 Reaction Scheme of HC-SCR Over Ag/Al2O3 410 Effect of H2 Addition on Reaction Pathways of HC-SCR Over Ag/Al2O3 414 Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons 416 Debates on Role of Ag Clusters 416 Reductive Activation of O2 and Promoted HC-SCR on Ag Cluster 420 References 422 Dynamic Structural Change of Pd Induced by Interaction with Zeolites Studied by Means of Dispersive and Quick XAFS 427 Kazu Okumura Introduction 427 Formation and Structure of Highly Dispersed PdO Interacted with Brønsted Acid Sites 428 Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and Reversible Formation of Stable Pd Clusters in H-ZSM-5 and H-Mordenite 430 In Situ QXAFS Studies on the Dynamic Coalescence and Dispersion Processes of Pd in USY Zeolite 432 Time-Resolved EXAFS Measurement of the Stepwise Clustering Process of Pd Clusters at Room Temperature 435 Summary 438 References 439

Part Four Single Crystals 23 23.1 23.2 23.3 23.4

441

Morphology Changes of Photochromic Single Crystals 443 Seiya Kobatake and Masahiro Irie Introduction 443 Photochromic Diarylethene Crystals 444 X-Ray Crystallographic Analysis 444 Reactivity in the Crystal 447

VII

VIII


23.5 23.6 23.7

Photomechanical Effect 448 Crystal Surface Changes 449 Photoreversible Crystal Shape Changes References 454

24

Direct Observation of Change in Crystal Structures During Solid-State Reactions of 1,3-Diene Compounds 459 Akikazu Matsumoto Introduction 459 Crystal Engineering Renaissance 459 EZ-Photoisomerization 460 Model of Photoisomerization 460 Photoisomerization of Benzyl Muconate 462 Change in Crystal Structures During Photoisomerization 463 [2 þ 2] Photodimerization 465 [2 þ 2] Photodimerization of 1,3-Dienes 465 [2 þ 2] Photodimerization of Benzyl Muconates 465 Topochemical Polymerization 469 Features of Topochemical Polymerization 469 Monomer Stacking Structure and Polymerization Reactivity 470 Shrinking and Expanding Crystals 473 Accumulation and Release of Strain During Polymerization 474 Homogeneous and Heterogeneous Polymerizations 476 Conclusion 480 References 481

24.1 24.1.1 24.2 24.2.1 24.2.2 24.2.3 24.3 24.3.1 24.3.2 24.4 24.4.1 24.4.2 24.4.3 24.4.4 24.4.5 24.5

25

25.1 25.2 25.3 25.3.1 25.3.2 25.3.3 25.4

26

26.1 26.2 26.2.1

450

Reaction Dynamics Studies on Crystalline-State Photochromism of Rhodium Dithionite Complexes 487 Hidetaka Nakai and Kiyoshi Isobe Introduction 487 Photochromism of Rhodium Dithionite Complexes 488 Reaction Dynamics of Crystalline-State Photochromism 490 Dynamics of Molecular Structural Changes in Single Crystals 490 Dynamics of Reaction Cavities in a Crystalline-State Reaction 495 Dynamics of Surface Morphology Changes of Photochromic Single Crystals 498 Summary 499 References 500 Dynamics in Organic Inclusion Crystals of Steroids and Primary Ammonium Salts 505 Mikiji Miyata, Norimitsu Tohnai, and Ichiro Hisaki Introduction 505 Dynamics of Steroidal Inclusion Crystals 506 Guest-Responsive Molecular Assemblies 506


26.2.2 26.2.3 26.3 26.3.1 26.3.2 26.4 26.4.1 26.4.2 26.4.2.1 26.4.2.2 26.4.2.3 26.4.3 26.4.4 26.5

27

27.1 27.2

27.2.1 27.2.2 27.2.3 27.2.4 27.2.5 27.3 27.3.1 27.3.2 27.4

Intercalation in Steroidal Bilayer Crystals 508 Guest Fit Through Weak Non-Covalent Bonds 510 Dynamics of Organic Crystals of Primary Ammonium Salts 512 Solid-State Fluorescence Emission 512 Hydrogen Bond Clusters 514 Dynamical Expression of Molecular Information in Organic Crystals 516 Hierarchical Structures with Supramolecular Chirality 516 Expression of Supramolecular Chirality in Hierarchical Assemblies 517 Three-Axial Chirality 517 Tilt Chirality 518 Helical and Bundle Chirality in a 21 Assembly 519 Supramolecular Chirality of Hydrogen Bonding Networks 520 Expression of Molecular Information 522 Conclusion and Perspectives 523 References 523 Morphology Changes of Organic Crystals by Single-Crystal-to-SingleCrystal Photocyclization 527 Hideko Koshima Introduction 527 Surface Morphology Changes in the Salt Crystals of a Diisopropylbenzophenone Derivative with Amines via Single-Crystal-to-Single-Crystal Photocyclization 528 Solid-State Photocylization 528 Crystal Structures and the Reaction Mechanism 529 Morphology Changes in Bulk Crystals 531 Morphology Changes in Microcrystals 532 Correlation between the Morphology Changes and the Crystal Strucural Changes 535 Morphology Changes in Triisobenzophenone Crystals via Diastereospecific Single-Crystal-to-Single-Crystal Photocyclization 537 Solid-State Photocyclization and the Crystal Structures 537 Morphology Changes 539 Concluding Remarks 541 References 541 545

Part Five

Single Biocells

28

Femtosecond Laser Tsunami Processing and Light Scattering Spectroscopic Imaging of Single Animal Cells 547 Hiroshi Masuhara, Yoichiroh Hosokawa, Takayuki Uwada, Guillaume Louit, and Tsuyoshi Asahi Introduction 547

28.1

IX

X


28.2 28.2.1 28.2.2 28.2.3 28.2.4 28.3 28.4

29 29.1 29.1.1 29.1.2 29.1.3 29.1.4 29.2 29.2.1 29.2.2 29.2.2.1 29.2.2.2 29.2.3 29.3 29.3.1 29.3.2 29.3.3 29.3.4 29.3.4.1 29.3.4.2 29.4

30

30.1 30.1.1

Femtosecond Laser Ablation and Generated Impulsive Force in Water: Laser Tsunami 548 Manipulation of a Single Polymer Bead by Laser Tsunami 551 Manipulation of Single Animal Cells by Laser Tsunami 554 Modification and Regeneration Process in Single Animal Cells by Laser Tsunami 556 Injection of Nanoparticles into Single Animal Cells by the Laser Tsunami 558 Development of Rayleigh Light Scattering Spectroscopy/Imaging System and its Application to Single Animal Cells 561 Summary 565 References 566 Super-Resolution Infrared Microspectroscopy for Single Cells 571 Makoto Sakai, Keiichi Inoue, and Masaaki Fujii Introduction 571 Infrared Microscopy 571 Super-Resolution Microscopy by Two-Color Double Resonance Spectroscopy 571 Transient Fluorescence Detected IR Spectroscopy 572 Application to Super-Resolution Infrared Microscopy 573 Experimental Set-Up for Super-Resolution Infrared Microscopy 574 Picosecond Laser System 574 Fluorescence Detection System 574 Optical Layout for the Solution and Fluorescent Beads 574 Optical Layout for Biological Samples 575 Sample 576 Results and Discussion 576 Transient Fluorescence Image with IR Super-Resolution in Solution 576 Picosecond Time-Resolved Measurement 578 Application to Fluorescent Beads 579 Application to Whole Cells 581 Super-Resolution IR Imaging of Arabidopsis thaliana Roots 581 Vibrational Relaxation Dynamics in the Cells 582 Summary 584 References 585 Three-Dimensional High-Resolution Microspectroscopic Study of Environment-Sensitive Photosynthetic Membranes 589 Shigeichi Kumazaki, Makotoh Hasegawa, Mohammad Ghoneim, Takahiko Yoshida, Masahide Terazima, Takashi Shiina, and Isamu Ikegami Introduction 589 Thylakoid Membranes of Oxygenic Photosynthesis 589


30.1.2 30.1.3 30.1.4 30.1.5 30.2 30.2.1 30.2.2 30.2.2.1 30.2.2.2 30.2.2.3 30.2.2.4 30.2.3 30.2.3.1 30.2.3.2 30.3 30.4

31

31.1 31.2 31.2.1 31.2.2 31.2.3 31.3 31.3.1 31.3.2 31.3.3 31.3.4 31.4

32

32.1 32.2 32.2.1

Thylakoid Membranes in Chloroplasts 590 Thylakoid Membrane of Cyanobacteria 590 Applications of Fluorescence Microscopy to a Thylakoid Membrane 590 Simultaneous Spectral Imaging and its Merits 591 Spectral Fluorescence Imaging of Thylakoid Membrane 592 Realization of Fast Broadband Spectral Acquisition in Two-Photon Excitation Fluorescence Imaging 592 Spectral Imaging of a Filamentous Cyanobacterium, Anabaena 594 Thylakoid Membrane of Cyanobacterium 594 Stability of the Anabaena Fluorescence Spectra Under Photoautotrophic Conditions 594 Change of the Anabaena Fluorescence Spectra by Dark Some Conditions 595 Intracellular Spectral Gradient in Anabaena Cells 596 Spectral Imaging of Chloroplasts 598 Chloroplasts from a Plant, Zea mays 598 Chloroplast from the Green Alga, Chlorella 600 Technical Verification and Perspective 601 Summary 602 References 604 Fluorescence Lifetime Imaging Study on Living Cells with Particular Regard to Electric Field Effects and pH Dependence 607 Nobuhiro Ohta and Takakazu Nakabayashi Introduction 607 Experimental 608 FLIM Measurement System 608 Preparation of Hb. salinarum Loaded with BCECF 610 Measurements of External Electric Field Effects 610 Results and Discussion 611 FLIM of Hb. salinarum 611 pH Dependence of the Fluorescence Lifetime in Solution and in Living Cells 614 External Electric Field Effect on Fluorescence of BCECF 616 Electric-Field-Induced Aggregate Formation in Hb. salinarum 617 Summary 619 References 619 Multidimensional Fluorescence Imaging for Non-Invasive Tracking of Cell Responses 623 Ryosuke Nakamura and Yasuo Kanematsu Introduction 623 Materials and Methods 625 Time-Gated Excitation–Emission Matrix Spectroscopy 625

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32.2.2 32.2.3 32.2.4 32.3 32.3.1 32.3.2 32.3.3 32.4 32.4.1 32.4.2 32.4.3 32.4.4 32.5

33

33.1 33.1.1 33.1.2 33.1.2.1 33.1.2.2 33.1.2.3 33.1.3 33.1.4 33.2 33.2.1 33.2.2 33.2.3 33.3 33.3.1 33.3.2 33.4

Time- and Spectrally-Resolved Fluorescence Imaging 626 PARAFAC Model 628 Sample Preparation 630 Time-Gated Excitation–Emission Matrix Spectroscopy 630 The 3D Fluorescence Properties of Dye Solutions 630 The 3D Fluorescence Property of a Mixed Solution 631 PARAFAC Decomposition Without any Prior Knowledge of Constituents 633 Time- and Spectrally-Resolved Fluorescence Imaging 635 Characterization of y–Em Maps 635 Spatial Localization of Fluorescent Components 637 PARAFAC Decomposition 637 Possible Assignments of Fluorescent Components 639 Concluding Remarks 640 References 642 Fluorescence Correlation Spectroscopy on Molecular Diffusion Inside and Outside a Single Living Cell 645 Kiminori Ushida and Masataka Kinjo Introduction 645 Investigation on Biological System Based on Molecular Identification and Visualization 645 Technical Restrictions and Regulations in Real-Time Visualization of Material Transport in Biological System 647 Spatial Resolution 647 Time Resolution 648 Sensitivity 648 Time and Space Resolution Required to Observe Anomalous Diffusion of a Single Molecule in Biological Tissues 648 General Importance of Anomalous Diffusion in a Signaling Reaction 652 Use of Fluorescence Correlation Spectroscopy (FCS) for Investigation of Biological Systems 655 Use of FCS for Biological Systems 655 Experimental Example of Anomalous Diffusion Observed in a Model System for Extracellular Matrices 656 Quantitative Estimation of Reaction Volume in Signaling Reaction 661 A Short Review of Recent Literature Concerning FCS Inside and Outside a Single Cell 662 FCS Measurement Inside Single Cells 662 FCS Measurement Outside Cells 664 Summary 664 References 665


34

34.1 34.1.1 34.1.2 34.2 34.2.1 34.2.2 34.3 34.4

35

35.1 35.1.1 35.1.2 35.2 35.2.1 35.2.2 35.2.3 35.3 35.3.1 35.3.2 35.4 35.4.1 35.5

Spectroscopy and Photoreactions of Gold Nanorods in Living Cells and Organisms 669 Yasuro Niidome and Takuro Niidome Introduction 669 Spectroscopic Properties of Gold Nanorods 669 Biocompatible Gold Nanorods 670 Spectroscopy of Gold Nanorods in Living Cells 674 Gold Nanorods Targeting Tumor Cells 674 Spectroscopy of Gold Nanorods In Vivo 675 Photoreactions of Gold Nanorods for Biochemical Applications Conclusions and Future Outlook 682 References 683

680

Dynamic Motion of Single Cells and its Relation to Cellular Properties 689 Hideki Matsune, Daisuke Sakurai, Akitomo Hirukawa, Sakae Takenaka, and Masahiro Kishida Introduction 689 Single Cell Analysis 689 Dynamic Motion of Murine Embryonic Stem Cell 690 Laser Trapping of Biological Cells 691 Optical Tweezers 691 Set-up for Optical Trapping of a Living Cell 692 Murine Embryonic Stem Cell Trapped with Optical Tweezers 693 Relationship Between Cellular Motion and Proliferation 694 Dynamic Motion of a Murine Embryonic Stem Cell 694 Experimental Procedure 695 Cell Separation by Specific Gravity 699 Cell Separation 699 Summary 700 References 701 Index

703

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Contents to Volume 1 Part One Spectroscopic Methods for Nano Interfaces

1

1

Raman and Fluorescence Spectroscopy Coupled with Scanning Tunneling Microscopy 3 Noriko Nishizawa Horimoto and Hiroshi Fukumura

2

Vibrational Nanospectroscopy for Biomolecules and Nanomaterials 19 Yasushi Inouye, Atsushi Taguchi, and Taro Ichimura

3

Near-Field Optical Imaging of Localized Plasmon Resonances in Metal Nanoparticles 39 Hiromi Okamoto and Kohei Imura

4

Structure and Dynamics of a Confined Polymer Chain Studied by Spatially and Temporally Resolved Fluorescence Techniques 55 Hiroyuki Aoki

5

Real Time Monitoring of Molecular Structure at Solid/Liquid Interfaces by Non-Linear Spectroscopy 71 Hidenori Noguchi, Katsuyoshi Ikeda, and Kohei Uosaki

6

Fourth-Order Coherent Raman Scattering at Buried Interfaces Hiroshi Onishi

7

Dynamic Analysis Using Photon Force Measurement Hideki Fujiwara and Keiji Sasaki

8

Construction of Micro-Spectroscopic Systems and their Application to the Detection of Molecular Dynamics in a Small Domain 133 Syoji Ito, Hirohisa Matsuda, Takashi Sugiyama, Naoki Toitani, Yutaka Nagasawa, and Hiroshi Miyasaka

Molecular Nano Dynamics, Volume II: Active Surfaces, Single Crystals and Single Biocells Edited by H. Fukumura, M. Irie, Y. Iwasawa, H. Masuhara, and K. Uosaki Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32017-2

103

117

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9

Nonlinear Optical Properties and Single Particle Spectroscopy of CdTe Quantum Dots 155 Lingyun Pan, Yoichi Kobayashi, and Naoto Tamai

Part Two Nanostructure Characteristics and Dynamics

171

10

Morphosynthesis in Polymeric Systems Using Photochemical Reactions 173 Hideyuki Nakanishi, Tomohisa Norisuye, and Qui Tran-Cong-Miyata

11

Self-Organization of Materials Into Microscale Patterns by Using Dissipative Structures 187 Olaf Karthaus

12

Formation of Nanosize Morphology of Dye-Doped Copolymer Films and Evaluation of Organic Dye Nanocrystals Using a Laser 203 Akira Itaya, Shinjiro Machida, and Sadahiro Masuo

13

Molecular Segregation at Periodic Metal Nano-Architectures on a Solid Surface 225 Hideki Nabika and Kei Murakoshi

14

Microspectroscopic Study of Self-Organization in Oscillatory Electrodeposition 239 Shuji Nakanishi

15

Construction of Nanostructures by use of Magnetic Fields and Spin Chemistry in Solid/Liquid Interfaces 259 Hiroaki Yonemura

16

Controlling Surface Wetting by Electrochemical Reactions of Monolayers and Applications for Droplet Manipulation 279 Ryo Yamada

17

Photoluminescence of CdSe Quantum Dots: Shifting, Enhancement and Blinking 293 Vasudevanpillai Biju and Mitsuru Ishikawa

XVII

Preface Over the past two decades, studies of chemical reaction dynamics have shifted from ideal systems of isolated molecules in the gas phase, of molecular clusters in jet beams, on ultra-clean surfaces, in homogeneous and in dilute molecular solutions, and in bulk crystals, towards nanosystems of supramolecules, colloids, and ultrasmall materials, following the contemporary trends in nanoscience and nanotechnology. The preparation, characterization, and functionalization of supramolecules, molecular assemblies, nanoparticles, nanodots, nanocrystals, nanotubes, nanowires, and so on, have been conducted extensively, and their chemical reactions and dynamic processes are now being elucidated. The systematic investigation of molecular nanosystems gives us a platform from which we can understand the nature of the dynamic behavior and chemical reactions occurring in complex systems such as molecular devices, catalysts, living cells, and so on. Thus we have conducted the KAKENHI (The Grant-in-Aid for Scientific Research) Project on Priority Area ‘‘Molecular Nanodynamics’’ (Project Leader: Hiroshi Masuhara) for the period from 2004 April to 2007 March, involving 86 laboratories in Japan. For the investigation of such complex systems new methodologies which enable us to analyze dynamics and mechanisms in terms of space and time are indispensable. Methods for simultaneous direct dynamic measurements in both time and real space domains needed to be devised and applied. Spectroscopy with novel spaceresolution and ultrafast spectroscopy with high sensitivity have been developed, the manipulation and fabrication of single molecules, nanoparticles, and single living cells have been realized, molecules and nanoparticles for probing chemical reactions spectroscopically and by imaging have been synthesized, new catalyses for cleaning air and new reactions have been found, and the way in which a reaction in a single molecular crystal leads to its morphological change has been elucidated under the umbrella of this research program. The recent development of these new methods and the advances in understanding chemical reaction dynamics in nanosystems are summarized in the present two volumes. The presented results are based on our activities over three years, including 1146 published papers and 1112 presentations at international conferences. We hope readers will understand the present status and new movement in Molecular

XVIII

Preface

Nano Dynamics and its relevant research fields. The editors thank the contributors and the Ministry of Education, Culture, Sport, Science, and Technology (MEXT), Japan for their support of the project. We would also like to thank our publishers for their constant support. Sendai, Tokyo, Nara and Sapporo August 2009

Hiroshi Fukumura Masahiro Irie Yasuhiro Iwasawa Hiroshi Masuhara Kohei Uosaki

XIX

About the Editors Hiroshi Fukumura received his M.Sc and Ph.D. degrees from Tohoku University, Japan. He studied biocompatibility of polymers in the Government Industrial Research Institute of Osaka from 1983 to 1988. He became an assistant professor at Kyoto Institute of Technology in 1988, and then moved to the Department of Applied Physics, Osaka University in 1991, where he worked on the mechanism of laser ablation and laser molecular implantation. Since 1998, he is a professor in the Department of Chemistry at Tohoku University. He received the Award of the Japanese Photochemistry Association in 2000, and the Award for Creative Work from The Chemical Society Japan in 2005. His main research interest is the physical chemistry of organic molecules including polymeric materials studied with various kinds of time-resolved techniques and scanning probe microscopes.

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980s he invented a new class of photochromic molecules – diarylethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives.

XX

About the Editors

Yasuhiro Iwasawa received his B.S., M.S. and Ph.D. degrees in chemistry from The University of Tokyo. His main research interests come under the general term ‘‘Catalytic Chemistry’’ and ‘‘Surface Chemistry’’, but more specifically, catalyst surface design, new catalytic materials, reaction mechanism, in situ characterization, oxide surfaces by SPM, time-resolved XAFS, etc. His honors include the Progress Award for Young Chemists in The Chemical Society of Japan (1979), The Japan IBM Science Award (1990), Inoue Prize for Science (1996), Catalysis Society of Japan Award (1999), The Surface Science Society of Japan Award (2000), Medal with Purple Ribbon (2003), and The Chemical Society of Japan Award (2004). The research reported by Yasuhiro Iwasawa represents a pioneering integration of modern surface science and organometallic chemistry into surface chemistry and catalysis in an atomic/ molecular scale. Iwasawa is a leader in the creation of the new filed of catalysis and surface chemistry at oxide surfaces by XAFS and SPM techniques. Hiroshi Masuhara received his B.S. and M.S. degrees from Tohoku University and Ph.D. from Osaka University. He started his research in photochemistry and was the first to use nanosecond laser spectroscopy in Japan. He studied electronic states, electron transfer, ionic photodissociation of molecular complexes, polymers, films, and powders by developing various time-resolved absorption, fluorescence, reflection, and grating spectroscopies until the mid 1990s. The Masuhara Group combined microscope with laser and created a new field on Microchemistry, which has now developed to Laser Nano Chemistry. After retiring from Osaka University he shifted to Hamano Foundation and is now extending his exploratory research on femtosecond laser crystallization and laser trapping crystallization in National Chiao Tung University in Taiwan and Nara Institute of Science and Technology. He is a foreign member of Royal Flemish Academy of Belgium for Science and the Arts and his honors include The Purple Ribbon Medal, Doctor Honoris Causa de Ecole Normale Superier de Cachan, Porter Medal, the Chemical Society of Japan Award, Osaka Science Prize, and Moet Hennessy Louis Vuitton International Prize ‘‘Science for Art’’ Excellence de Da Vinci.

About the Editors

Kohei Uosaki received his B.Eng. and M.Eng. degrees from Osaka University and his Ph.D. in Physical Chemistry from Flinders University of South Australia. He was a Research Chemist at Mitsubishi Petrochemical Co. Ltd. From 1971 to 1978 and a Research Officer at Inorganic Chemistry Laboratory, Oxford University, U.K. between 1978 and 1980 before joining Hokkaido University in 1980 as Assistant Professor in the Department of Chemistry. He was promoted to Associate Professor in 1981 and Professor in 1990. He is also a Principal Investigator of International Center for Materials Nanoarchitectonics (MANA) Satellite, National Institute for Materials Science (NIMS) since 2008. His scientific interests include photoelectrochemistry of semiconductor electrodes, surface electrochemistry of single crystalline metal electrodes, electrocatalysis, modification of solid surfaces by molecular layers, and non-linear optical spectroscopy at interfaces.

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List of Contributors for Both Volumes Hiroyuki Aoki Kyoto University Department of Polymer Chemistry Katsura, Nishikyo Kyoto 615-8510 Japan Tsuyoshi Asahi Osaka University Department of Applied Physics Suita 565-0871 Japan Vasudevanpillai Biju National Institute of Advanced Industrial Science and Technology (AIST) Health Technology Research Center Nano-bioanalysis Team 2217-14 Hayashi-cho, Takamatsu Kagawa 761-0395 Japan Masaaki Fujii Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Hideki Fujiwara Hokkaido University Research Institute for Electronic Science Kita-12, Nishi-6, Sapporo Hokkaido 060-0812 Japan Hiroshi Fukumura Tohoku University Graduate School of Science 6-3 Aramaki Aoba Sendai 980-8578 Japan Mohammad Ghoneim Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Makotoh Hasegawa Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

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Akitomo Hirukawa Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan

Isamu Ikegami Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Ichiro Hisaki Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan

Kohei Imura The Graduate University for Advanced Studies Institute for Molecular Science Myodaiji, Okazaki Aichi 444-8585 Japan

Noriko Nishizawa Horimoto Tohoku University Graduate School of Science 6-3 Aramaki Aoba Sendai 980-8578 Japan

Keiichi Inoue Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Yoichiroh Hosokawa Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan

Yasushi Inouye Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan

Taro Ichimura Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan Katsuyoshi Ikeda Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan

Masahiro Irie Rikkyo University Department of Chemistry Nishi-Ikebukuro 3-34-1, Toshima-ku Tokyo 171-8501 Japan Mitsuru Ishikawa National Institute of Advanced Industrial Science and Technology (AIST) Health Technology Research Center Nano-bioanalysis Team 2217-14 Hayashi-cho, Takamatsu Kagawa 761-0395 Japan


Kiyoshi Isobe Kanazawa University Graduate School of Natural Science and Technology Department of Chemistry Kakuma-machi Kanazawa 920-1192 Japan Akira Itaya Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan Syoji Ito Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Yasuhiro Iwasawa The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan Yasuo Kanematsu Osaka University Center for Advanced Science and Innovation Venture Business Laboratory JST-CREST Suita Osaka 565-0871 Japan

Olaf Karthaus Chitose Institute of Science and Technology 758-65 Bibi, Chitose Hokkaido 066-8655 Japan Masataka Kinjo Riken Hirosawa 2-1, Wako Saitama 351-0198 Japan Masahiro Kishida Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Yoichi Kobayashi Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan Seiya Kobatake Osaka City University Graduate School of Engineering Department of Applied Chemistry Sugimoto 3-3-138, Sumiyoshi-ku Osaka 558-8585 Japan Hideko Koshima Ehime University Graduate School of Science and Engineering Department of Materials Science and Biotechnology Matsuyama 790-8577 Japan

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Shigeichi Kumazaki Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Guillaume Louit Osaka University Department of Applied Physics Suita 565-0871 Japan Shinjiro Machida Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan Hiroshi Masuhara Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan and Osaka University Department of Applied Physics Suita 565-0871 Japan Sadahiro Masuo Kyoto Institute of Technology Department of Polymer Science and Engineering Matsugasaki, Sakyo-ku Kyoto 606-8585 Japan

Hirohisa Matsuda Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Akikazu Matsumoto Osaka City University Graduate School of Engineering Department of Applied Chemistry 3-3-138 Sugimoto, Sumiyoshi-ku Osaka 558-8585 Japan Hideki Matsune Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Hiroshi Miyasaka Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Mikiji Miyata Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan


Kei Murakoshi Hokkaido University Graduate School of Science Department of Chemistry Sapporo Hokkaido 060-0810 Japan

Hisao Nakamura The University of Tokyo Graduate School of Engineering Department of Chemical System Engineering Tokyo 113-8656 Japan

Takakazu Nakabayashi Hokkaido University Research Institute for Electronic Science (RIES) Sapporo 001-0020 Japan

Ryosuke Nakamura Osaka University Center for Advanced Science and Innovation Venture Business Laboratory JST-CREST Suita Osaka 565-0871 Japan

Hideki Nabika Hokkaido University Graduate School of Science Department of Chemistry Sapporo Hokkaido 060-0810 Japan Yutaka Nagasawa Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Hidetaka Nakai Kanazawa University Graduate School of Natural Science and Technology Department of Chemistry Kakuma-machi Kanazawa 920-1192 Japan

Hideyuki Nakanishi Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan Shuji Nakanishi Osaka University Graduate School of Engineering Science Division of Chemistry Toyonaka Osaka 560-8531 Japan Takuro Niidome Kyushu University Department of Applied Chemistry 744 Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan

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Yasuro Niidome Kyushu University Department of Applied Chemistry 744 Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Hidenori Noguchi Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan Tomohisa Norisuye Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan

Kazu Okumura Tottori University Faculty of Engineering Department of Materials Science Koyama-cho, Minami Tottori 680-8552 Japan Hiroshi Onishi Kobe University Faculty of Science Department of Chemistry Rokko-dai, Nada, Kobe Hyogo 657-8501 Japan Lingyun Pan Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan

Nobuhiro Ohta Hokkaido University Research Institute for Electronic Science (RIES) Sapporo 001-0020 Japan

Makoto Sakai Tokyo Institute of Technology Chemical Resources Laboratory 4259 Nagatsuta-cho, Midori-ku Yokohama 226-8503 Japan

Hiromi Okamoto The Graduate University for Advanced Studies Institute for Molecular Science Myodaiji, Okazaki Aichi 444-8585 Japan

Daisuke Sakurai Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan


Keiji Sasaki Hokkaido University Research Institute for Electronic Science Kita-12, Nishi-6, Sapporo Hokkaido 060-0812 Japan Takehiko Sasaki The University of Tokyo Graduate School of Frontier Science Department of Chemistry Kashiwanoha, Kashiwa Chiba 277-8561 Japan Atsushi Satsuma Nagoya University Graduate School of Engineering Department of Molecular Design and Engineering Chikusa Nagoya 464-8603 Japan Takashi Shiina Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan Ken-ichi Shimizu Nagoya University Graduate School of Engineering Department of Molecular Design and Engineering Chikusa Nagoya 464-8603 Japan

Takashi Sugiyama Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Mizuki Tada The University of Tokyo Graduate School of Frontier Science Department of Chemistry Kashiwanoha, Kashiwa Chiba 277-8561 Japan Atsushi Taguchi Osaka University Graduate School of Frontier Biosciences & Graduate School of Engineering Suita, Osaka Japan Sakae Takenaka Kyushu University Faculty of Engineering Department of Chemical Engineering Moto-Oka, Nishi Ku Fukuoka 819-0395 Japan Naoto Tamai Kwansei Gakuin University School of Science and Technology Department of Chemistry 2-1 Gakuen Sanda 669-1337 Japan

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Toshiaki Taniike The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan

Yohei Uemura The University of Tokyo Graduate School of Science Department of Chemistry Hongo, Bunkyo-ku Tokyo 113-0033 Japan

Masahide Terazima Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Kohei Uosaki Hokkaido University Graduate School of Science Division of Chemistry Sapporo 060-0810 Japan

Norimitsu Tohnai Osaka University Graduate School of Engineering 2-1 Yamadaoka, Suita Osaka 565-0871 Japan

Kiminori Ushida Riken Hirosawa 2-1, Wako Saitama 351-0198 Japan

Naoki Toitani Osaka University Graduate School of Engineering Science Center for Quantum Science and Technology under Extreme Conditions Division of Frontier Materials Science Toyonaka Osaka 560-8531 Japan Qui Tran-Cong-Miyata Graduate School of Science and Technology Kyoto Institute of Technology Department of Macromolecular Science and Engineering Matsugasaki Kyoto 606-8585 Japan

Takayuki Uwada Nara Institute of Science and Technology Graduate School of Materials Science Takayama 8916-5 Ikoma 630-0192 Japan and National Chiao Tung University Institute of Molecular Science Department of Applied Chemistry 1001 Ta Hsueh Road Hsinchu 30010 Taiwan


Kazuya Watanabe Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

Hiroaki Yonemura Kyushu University Department of Applied Chemistry 6-10-1 Hakozaki, Higashi-ku Fukuoka 812-8581 Japan

and

Takahiko Yoshida Kyoto University Graduate School of Science Department of Chemistry Kyoto 606-8502 Japan

PRESTO, JST 4-1-8 Honcho Kawaguchi Saitama Japan Ryo Yamada Osaka University Graduate School of Engineering Science Division of Materials Physics Department of Materials Engineering Science Toyonaka, Osaka Japan

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Part Three Active Surfaces

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18 The Genesis and Principle of Catalysis at Oxide Surfaces: Surface-Mediated Dynamic Aspects of Catalytic Dehydration and Dehydrogenation on TiO2(110) by STM and DFT Yohei Uemura, Toshiaki Taniike, Takehiko Sasaki, Mizuki Tada, and Yasuhiro Iwasawa

18.1 Introduction

An important catalytic property of metal oxides is the surface acid–base property, which regulates the performance of catalysts for the dehydrogenation and dehydration of alcohols, formic acid, ethanolamine, and so on [1]. The acid–base property of heterogeneous oxide catalysts has been characterized by the decomposition reaction of formic acid as a probe reaction, because the products are regulated by the acid–base property of the oxide surface [2–8]. Typically, the dehydration reaction (HCOOH ! CO þ H2O) occurs on acidic oxides such as Al2O3 and TiO2, while the dehydrogenation reaction (HCOOH ! CO2 þ H2) occurs on basic oxides such as MgO and ZnO. Both the reactions proceed via the formate anion intermediate (HCOO). We found that the catalytic reaction of HCOOH on a TiO2(110) surface violated the uniqueness rule of the acidity–basicity of the catalysts [9–11]. The HCOOH dehydration dominantly occurred under a low-pressure atmosphere of HCOOH and at high temperatures, above 500 K, while the dehydrogenation of HCOOH occurred under a relatively high-pressure atmosphere and at low temperatures, below 450 K [12–15]. In the latter case an additional acidic HCOOH molecule promotes the dehydrogenation categorized as basic catalysis. Note that TiO2 powder predominantly catalyzes the dehydration. We proposed the mechanism for the dehydrogenation of HCOOH on a TiO2(110) surface by means of density functional theory (DFT) calculations and scanning tunneling microscopy (STM) [14]. A key issue of developing catalytic technologies is to understand site specific surface dynamic processes from the atomic-scale view point. The inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their functions extremely difficult. STM has particularly great potential to overcome the difficulty of heterogeneity of oxide surfaces, discriminating specific sites from the other sites on an atomic scale. It is well known that the images obtained by STM do not naturally reflect the physical geometry of surfaces because the tunneling current used for regulating the tip–sample separation for STM reflects the local electronic density of states of the

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sample and tip in addition to physical geometry. Thus additional experimental evidence may be demanded for identification of the STM image. Chemical identification of Ti atoms at a TiO2(110)–(1 1) surface has been performed by the use of formic acid as a probe molecule that adsorbs selectively on Ti4 þ cations of the surface, which can be imaged by STM [16, 17]. Thus, formic acid is regarded not only as a probe reactant molecule for catalysis research but also a useful molecule for visualization of chemical events on oxide single crystal surfaces. The present in situ STM and DFTstudies on the dehydration and dehydrogenation of formic acid on a TiO2(110) surface as typical probe catalytic reactions document new dynamic aspects of acid–base catalysis. Active sites for the dehydration are oxygen vacancies that are produced in situ under the catalytic reactions and the formate intermediates migrate on the surface to find the oxygen vacancies where they decompose to CO and OH. The pre-adsorbed formates modify the acidic surface to a new surface with basic character on which acidic HCOOH adsorbs with Coulombic attractive interaction to promote a bimolecular dehydrogenation. These findings provide a new concept of the genesis of acid–base catalysis on oxide surfaces and also a new implication to oxide catalyst design on a molecular scale.

18.2 Experimental 18.2.1 STM Measurements of TiO2(110)

All experiments were performed in an ultrahigh vacuum (UHV) STM (JEOL JSTM 4500VT) with ion guns and low-energy electron diffraction (LEED) optics. The base pressure of the chamber was less than 1 108 Pa. Electrically etched tungsten tips were used in the STM observation, and all STM images were taken in constant current mode. A polished TiO2(110) wafer of 6.5 1 0.25 mm3 (Earth Chemical) was used after deposition of Ni film on the rear side of the sample to resistively heat the sample on a sample holder. The surface was cleaned by cycles of Ar þ ion sputtering (3 keV for 3 min) and annealing under UHV at 900 K for 30 s until a clear 1 1 LEED pattern was obtained. Deuterated formic acid (DCOOD, Wako, 98% purity, most of the contaminant is water) was purified by repeated freeze–pump–thaw cycles and introduced into the chamber by backfilling. The surface temperature of the crystal was monitored by an infrared radiation thermometer. 18.2.2 Computational Methods

Density functional calculations with the GGA-PBE [46] functional were performed with Materials Studio Dmol3 from Accelrys. The basis sets were DND and effective core potentials (DND is as accurate as 6-31G ) [18]. A transition state (TS) between

18.3 Results and Discussion

two immediate stable structures was first identified by linear synchronous transit (LST) [19], and then cyclically refined by quadratic synchronous transit and conjugate gradient methods. Each TS was converged within 0.1 Ha nm1. The slab method was employed to model a TiO2(110) surface, where the thickness of a vacuum layer was 1 nm. The slab thickness was decided to be three layers (one layer involves three atomic layers, i.e., OTiO) for the reason given later. The atomic arrangement of a bottom layer was fixed at the bulk arrangement. In the calculations of the adsorption energies of HCOOH on a defective surface, an oxygen defect (O-defect) was formed by removing a bridging oxygen from a stoichiometric p(1 2) surface. For the transition state search, p(1 4) or p(2 2)-HCOOH surfaces were used, where the k-point meshes were 3 2 1 and 2 3 1, respectively. In the calculations for the transition state search on the defective surface, an O-defect was formed by removing 1/4 bridging oxygen from a stoichiometric p(2 2) surface, and thermal smearing of moderate strength [20] was imposed to improve the SCF convergence. The chosen adsorption states were the most stable bridging formate and a quasistable monodentate formate produced by the dissociative adsorption of HCOOH, and a less stable molecular form. We employed the most efficient three-layer slab throughout the calculations involving TS search. Independently, very recently, Perron et al. have also reported that the surface energy on TiO2(110) was almost convergent at four-layers slab [21]. At the cost of the thin slab in deciding plausible reaction paths for HCOOH dehydrogenation, the difference of 0.5 eV in the activation barrier was regarded as a margin, and all the reaction paths with activation energies less than 0.5 eV larger than the smallest activation energy were considered as candidates.

18.3 Results and Discussion 18.3.1 Dynamic Mechanism for Catalytic Dehydration of Formic Acid on a TiO2(110) Surface, Much Different from the Traditional Static Acid Catalysis

The rutile-TiO2(110)–(1 1) surface has been studied extensively and well characterized experimentally and theoretically to be close to that of the bulk-truncated structure [22, 23]. It consists of alternating rows of fivefold coordinated Ti4 þ rows at the troughs and bridging O (denoted OB hereafter) rows at the ridges that locate 0.11 nm above the underlying TiO plane (Figure 18.1(b)). The O-ridges and the Ti rows are aligned with a 0.649-nm separation. Figure 18.1(a) shows a typical STM image of the TiO2(110)–(1 1) surface at positive sample bias voltages [23, 24]. Bright rows consisting of bright spots along the [001] direction are imaged with a constant separation of 0.649 nm. The periodic bright spots form a rectangle of 0.649 0.296 nm2 which coincides with the (1 1) unit cell. At a positive sample bias voltage, the major path for tunneling electrons is from the Fermi energy of the tip to the unoccupied states of the sample surface [25]. Rutile TiO2 bulk has a filled valence band of predominantly O 2p character and an empty conduction band of

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Figure 18.1 Surface structure of TiO2(110) characterized by STM and non-contact atomic force microscope (NC-AFM). (a) An empty state STM image (7.3 7.3 nm2, Vs ¼ 1.2 V, It ¼ 0.15 nA) of a TiO2(110) surface at RT. (b) A structure model of a clean TiO2(110) surface. The fivefold coordinated Ti atoms are visualized. (c) A perspective view of a TiO2(110) surface with oxygen defect.

predominantly Ti 3d character, separated by a band gap of 3.1 eV [26]. However, it becomes an n-type semiconductor due to a slight deficiency of oxygen atoms from the stoichiometry caused by Ar þ ion sputtering and annealing at 900–1000 K under UHV, which are typical procedures used for cleaning the surface. The Fermi level is close to the conduction-band minimum and electrons can tunnel to the states at positive sample bias voltages. Thus, the Ti atom is imaged as the bright spot by STM through its 3d empty state although the topmost atoms are the oxygen atoms.


Figure 18.2 (a) An STM image (7.1 7.1 nm2, Vs ¼ 1.2 V, It ¼ 0.1 nA) of TiO2(110) exposed to 1 L formic acid at room temperature followed by annealing at 350 K. Three types of formate species are observed. The lengths of each formate along the [001] and [1 10] directions are indicated in nanometers. (b) Models of the three formate configurations.

Figure 18.2(a) shows a typical STM image of a TiO2(110) surface exposed to 1 L (1 L ¼ 1.33 104 Pa s) of formic acid at room temperature followed by annealing at 350 K. In this STM image, titanium rows are observed as bright lines in spite of the poor atomic resolution, and traced with white lines on which formate species are observed as white protrusions with different shapes in three different configurations, labeled A, B, and C, as shown in our recent report [12]. Species A in the A configuration is located on a Ti row, species B in the B configuration is located

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between Ti and O rows, and species C in the C configuration is located on an O row. STM can discriminate the three different kinds of formates by the locations at the surface. The directions of OCO planes of species A and B can be determined using the shape of the STM image in Figure 18.2(a) because the shapes of the observed formates reflect the shape of their lowest unoccupied molecular orbitals (LUMOs), which expand to the direction perpendicular to the OCO plane. The observed species A and B have ellipsoidal shapes, and the major axes of them are along [1 10] and [001], respectively. Thus, the OCO planes of species A and B are along the [001] and [110] directions, respectively. This result was also confirmed through our theoretical calculation, and the obtained configurations are shown in Figure 18.2(b). The detail of the C configuration was also determined through the same calculation. Formate species A in the A configuration is on two fivefold coordinated Ti4 þ ions (denoted by Ti(5), hereafter) in a bridge configuration, and is the same configuration as that previously reported by STM observations at room temperature [16, 27–29]. Species B is located on an oxygen vacancy site and a Ti(5) ion, respectively, at an oxygen atom and another oxygen atom of formate species in the B configuration. When the Ti(5)O bond is broken to transform to a monodentate type, the STM image is observed as a bright contrast of a round shape on an oxygen vacancy site. This formate species is assigned as species C in the C configuration, as shown in Figure 18.2(b). The B and C configurations were not observed at room temperature though the B type was observed on a highly defective surface or by much greater exposure of formic acid than 1 L. Among the three formate species the monodentate formate species in the C configuration is imaged as a circular protrusion in Figure 18.2(a) although the OCO plane is illustrated along the [001] direction in Figure 18.2(b). This is because the monodentate species is expected to rotate thermally around the CO bonding at 350 K. The thermal effect yields a circular shape. It is believed that formate ions are reaction intermediates in catalytic formic acid decomposition. However, the three formates A, B, and C were found at a catalytic reaction temperature of 350 K [13]. The dynamic behavior of these formates was found through successive STM observation at 350 K. Figure 18.3 shows successive STM images of the same area on a TiO2(110) surface. Although many successive STM images were taken at intervals of 80 s, only five selected ones are shown in Figure 18.3. One finds two formate species in the yellow rectangular area in each image. In the image (a), both of them are seen as ellipses along the [1 10] direction on Ti rows. Thus, they are in the A configuration at t ¼ 0 s. However, in the image (b), one of them, which is indicated by a white arrow in the lower part, changes into an ellipse along the [001] direction. It also shifts slightly to the right side of the Ti row with a white center line. Since these features are characteristic of the B configuration, we can safely say that the initial A configuration, which is the major configuration at RT, has changed into the B configuration at t ¼ 80 s. Since this kind of dynamics was not observed at RT, it must be due to the elevated temperature effect, and hence can be considered as a process of the catalytic reaction. We superimposed white solid and broken lines along the [1 10] direction in each STM image. The


Figure 18.3 Successive STM images of formates on TiO2(110) at 350 K (7.5 7.5 nm2, Vs ¼ 1.2 V, It ¼ 0.1 nA). (a) t ¼ 0, (b) t ¼ 80 s, (c) t ¼ 320 s (d) t ¼ 640 s, (e) t ¼ 880 s. The structure model inside the yellow rectangle is shown in the right panel of each STM image. The interval

of solid and dotted lines in each figure is 0.296 nm, which corresponds to half of the distance between two adjacent Ti atoms on the Ti row. The solid lines are drawn so that one of them passes over the center of the upper formate species.

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interval between a solid line and the adjacent broken line is 0.148 nm, which corresponds to half of the TiTi distance along the [001] axis. Since one of the solid lines is drawn so as to pass through the center of the upper formate species, each solid line is on the second layer O atoms, and the broken line is on the Ti atoms in the same atomic layer. The center of the lower formate species is crossed also by a solid line in the image (a), but it shifts to the lower side along the Ti row and is crossed by a broken line in the image (b). This situation is illustrated schematically illustrated at the right side of the STM images. The change from A to B yields a shift of the species by 0.148 nm along the Ti row. We found another dynamics of the species through further observation. As mentioned above, the species is in the B configuration on the right side of a Ti row in (b). The species remains at the same site until (c), recorded at t ¼ 240 s. However, the species shifts to the left side of the same Ti row in the image (b), which corresponds to the mirror site of the B configuration in (b) against the Ti row. Since a formate species is stable in the B configuration only at the vacancy of OB, two O defects are needed on both sides of the corresponding Ti row for this kind of hopping behavior. It is known that hydroxy groups on TiO2(110) generate H2O and make O vacancy defects at 350 K. When this kind of H2O generation occurs on the neighboring O site to a formate in the A configuration, the formate is thermally activated into the B configuration. Furthermore, when another O vacancy is generated on the other side of the formate on the Ti row, the species hops between the two B configurations. The hopping is repeated unless the species changes into a configuration other than B. The present formate species hops again to the initial B configuration in the image (d) at t ¼ 640 s, then to the mirror B configuration again in the image (e) taken at 880 s. The hopping event occurred three times in the present observation of 800 s from (b) to (e). Assuming a typical value of the vibration frequency of the formate adsorbate as 1013 s1, the activation energy E (kJ mol1) for a formate to hop between the two B configurations is simply estimated by the following relation: n0 expðE=RTÞ ¼ 3=800 This relation leads to E 100 kJ mol1, which is of the order of magnitude for a chemical binding energy. The value is also similar to the value for the binding energy estimated by temperature programmed desorption (TPD) [10]. Although we could not observe other dynamics than hopping for the formates in the B configuration, the observed hopping event indicates that the formate species B can change its configuration easily, just like the species A. Our successive STM observation in another sample area revealed the final step of the formate dehydration process. Figure 18.4a and b are STM images of the same sample area, but image (b) was taken at 80 s after image (a). The formate species in the top part of (a) is located on the OB row, and hence it is species C in the C configuration. However, after 80 s in (b), the species changed into a rather faint protrusion, and shifted a little upward. This indicates some kind of reaction for the species. The line-profiles along 1–2 in (a) and 3–4 in (b) are shown in Figure 18.4(c) and (d). The formate in the C configuration is known as a protrusion of 0.20 nm


Figure 18.4 Successive STM images (7.5 7.5 nm2) taken under the same conditions as Figure 18.3. A decomposition process of species C and a formation process of an OH group are visualized. The OH species diffused along the oxygen row by a

distance of three oxygen atoms. Image (b) was taken 80 s after image (a). Line profiles of species C in (a) and an OH group in (b) are shown in (c) and (d), respectively. The location models of the two species are shown in (e) and (f), respectively.

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Scheme 18.1 A proposed reaction mechanism for the HCOOH dehydration at oxygen vacancy on a TiO2(110) surface.

height measured from the top of a Ti row in (a). However, it changes to a 0.05 nm height protrusion in (b). Considering the previous study [17], the protrusion is assigned to a surface OH group adsorbed on a bridge oxygen row. Thus, it is suggested that an OH group and a CO molecule were produced from the formate species C in the monodentate C configuration. A proposed reaction mechanism based on the STM images and DFT calculations is shown in Scheme 18.1. Now we succeeded in observing the change in formate configuration from A to B, and the dehydration process from C. As mentioned above, these formates A, B, and C can be considered as reaction intermediates because they are only observed at elevated temperature. Thus we elucidated that the formate on the TiO2(110) surface changed its configuration from A to C through B in the dehydration process. This scenario has actually been confirmed by our previous theoretical calculations [12]. A bridging formate species A first changes into a bridging formate species B, then into a monodentate formate species C, and the monodentate formate C finally decomposes. The first and the last processes were shown directly in the present STM observations. Although the B to C transition was not observed in the present study, it is inevitable for the decomposition process. However, the observed dynamics of the species B strongly indicates such a transition. In this mechanism, formate species A migrates to find an oxygen vacancy on which one of the two oxygen atoms in formate A is trapped with the molecular axis rotated by 90 (formate B). It is to be noted that the active site for the dehydration is the oxygen vacancy but it is not necessary for the oxygen vacancies to be present at the surface. Even if there are no active oxygen vacancies at the catalyst surface, the active site can be produced in situ under the catalytic reaction conditions. This indicates the importance of in situ characterization of the catalyst surface under the working conditions. The formate species has long been regarded to be the reaction intermediate for the formic acid dehydration, but the dehydration of formic acid involves four dynamic processes including dynamic migration of formic acids and the production of different formate species in three configurations.


18.3.2 Dynamic Catalytic Dehydrogenation of Formic Acid on a TiO2(110) Surface

Various adsorption configurations of HCOOH on a TiO2(110) surface (Figure 18.5) were first investigated to seek possible reactants and intermediates for the HCOOH dehydrogenation by DFT calculations, considering previous experimental and theoretical studies on adsorption structures on TiO2 (110) surfaces [12–14]. Table 18.1 summarizes their adsorption energies and the Mulliken charges of the hydrogen atoms of associative and dissociative adsorbates [14]. R1 and R2 are molecularly adsorbed species, where R1 adsorbs on a fivefold coordinated Ti4 þ with a carbonyl group of formic acid, while R2 adsorbs with a hydroxy group. The adsorption energies of the molecular adsorption states were similar and small, and they may be regarded as precursor states for the dissociative adsorption of formic acid. For the dissociative adsorption on a stoichiometric surface, there were two configurations of R3 and R4. R3 is monodentate formate which binds to a fivefold coordinated Ti4 þ with an O atom of the carbonyl group. R4 is a bridging formate, where two O atoms of a formate bind to the neighboring surface Ti4 þ ions, and the bridging formate (R4) was most

Figure 18.5 Energy diagrams for a proposed dehydrogenation pathway on a stoichiometric TiO2(110) surface. The zero-level energy (E0) as reference is defined as the sum of the energies of the clean surface and of two HCOOH(g).

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Table 18.1 Adsorption energies of HCOOH on a TiO2 (110)

surface and Mulliken charges of the hydrogen atoms of CH and OH. R1 and R2: associative adsorption, R3-R8: dissociative adsorption.

Adsorbate conformations R1 R2 R3 R4 R5(d)c R6(d)c R7(d)c R8(d)c Gas phase

Adsorption energy/eV 0.559 0.562 1.059 1.872 1.584 1.992 0.292 0.181 —

Charge of H1a

Charge of H2b

0.330 0.241 0.215 0.219 0.181 0.221 0.047 0.006 0.140

0.437 0.552 0.552 0.472 0.490 0.480 0.476 0.468 0.421

a

H1 is the hydrogen atom of CH. H2 is the hydrogen atom of OH groups. Note that there are two types of OH groups for molecularly adsorbed HCOOH and bridging OH at the surface. c Adsorption on oxgen defect sites. b

stable. The adsorption energies of R1–R4 were very similar to the values found by previous DFT calculations, which confirms the quality of the present calculation method. On the other hand, four dissociative configurations (R5(d)–R8(d)) were found to be specific to a defective surface. R5(d) offers an O atom to a defect site and another O atom directs to the surface normal, while R6(d) binds to a defect site with an O atom and another O atom binds to a fivefold coordinated Ti4 þ in front of the defect site. R7(d) adsorbs on a fivefold coordinated Ti4 þ with an O atom and on a defect site with a CH hydrogen. R8(d) is a formate species, where the CH bond of R7(d) is nearly broken (not shown). The most stable species on the defective surface was R6(d), while unstable R7(d) and R8(d) may be regarded as transient configurations. This is due to an electron transfer from the neighboring Ti3 þ ions at the defect site, which weakens the Lewis acidity of the five-coordinated Ti4 þ site in front of the defect site. Experimentally, only R4 and R6(d) were observed at RT on a TiO2(110)


surface by means of Fourier transform reflection-absorption infrared (FT-RAIR) spectroscopy [15] and X-ray photoelectron diffraction (XPD) [30, 31]. The majority was R4 species, as imaged by STM [16] at elevated temperatures. STM revealed an increasing population of R6(d) and the existence of R5(d) as intermediate species for the unimolecular dehydration, which was suggested to proceed by the steps R4 ! R6 (d) ! R5(d) [12]. The unimolecular decomposition of R5(d) to CO þ OH is rate determining, as suggested by STM [16] and DFT calculation [12]. The electron density at the transition state in the most plausible dehydrogenation pathway (Path 2) is also shown. Note that both the reactant molecules at the transition state interact with the surface. In all the configurations, the CH hydrogen atoms denoted as H1 are electron richer than the OH hydrogen atoms denoted as H2 (Table 18.1) and, particularly, H1 of R7(d) and R8(d) is richest due to direct electron donation from the two Ti3 þ ions. Except for these species, the adsorption on the Ti4 þ Lewis acid site decreased the electron negativity of H1 compared to that of the gas-phase molecule. On the other hand, the most positively charged H2 was observed with R2 and R3, which hydrogen bonded with the neighboring surface oxygen atoms. The other H2 had a similar charge to that of a gas-phase molecule. The modification of the H charge through adsorption was much larger for electron-negative H1 than for H2. These charges on H1 and H2 were strongly related to the genesis of a bimolecular reaction path, as described below. The dehydrogenation of HCOOH on a TiO2 (110) surface is given by the secondorder rate equation which is proportional to the formate coverage and the gas-phase HCOOH pressure [10]. We performed transition state searches for the bimolecular dehydrogenation reaction between a HCOOH(g) or molecularly weakly adsorbed HCOOH and dissociatively adsorbed species on the basis of the adsorption states mentioned above. The energy diagrams of the obtained pathways on the stoichiometric and defective surfaces have been reported previously by us [14]. Two kinds of activation energies for each path are listed in Table 18.2; DEel is the activation energy of the rate-limiting elementary step, which corresponds to the height of the transition state from the neighboring stable or quasi-stable state, and DEap is the apparent activation energy, which is the height of the transition state from the zero-level reference energy, that is, the energy of a clean surface and two HCOOH(g). Note that the experimentally observed activation barrier 0.16 eV corresponds to DEap. As a result of the series of transition state searches, we found two important factors to decide the height of the transition state. One factor is the charges of the reacting H atoms to produce H2(g). Since H1 and H2 are electro-negative and positive, respectively, the dehydrogenation reaction tends to be a heterolytic reaction. This fact suggests that the polarization between the two H atoms was important. The relation between the activation energies and the charges of the reacting H atoms in Table 18.2 indicates that the H1 charge is critical for DEel. Concretely, the defect-trapped H1 atom gave very small DEel in Paths 5 and 7 (Path 6 was an exceptional case, where the large distance between the reacting H atoms raises DEel).

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The other factor is the amount of the adsorption energy, which affects the energy level of the transition states. DEap of Path 2 was comparable to or even smaller than those of Paths 5 and 7, in spite of the much larger value of DEel. This was because the stabilization of the transition state by the adsorption energies was most effective in Path 2, as shown in Figure 18.5. At the transition state in Path 2 the two reactant molecules have a monodentate-like configuration, whose adsorption energies should be about 1 eV, as listed in Table 18.1 and Figure 18.5. As seen in the electron density distribution of the transition state in Path 2 (Figure 18.5), both the leaving and adsorbing reactants interact with surface fivefold coordinated Ti4 þ ions. On the other hand, at the rate-limiting transition states of Paths 5 and 7, the adsorption energy of only one monodentate-like reactant is available because of the instability of R7(d) and R8(d). Furthermore, the transition state in Path 2 seems not to have any notable steric repulsion or distortion because of the moderate distance between the two neighboring Ti4 þ ions (Figure 18.5). On the contrary, the elongated distance between the two reacting H atoms in Paths 5 and 7 made the use of the adsorption energy at the transition states ineffective. The results of Table 18.2 lead to the conclusion that Paths 2, 5 and 7 are plausible pathways with sufficiently low activation energies compared to the experimental value (0.16 eV). Path 2 on the stoichiometric surface is advantageous for the available adsorption energies at the transition state, whereas Paths 5 and 7 on the defective surface are stable due to the electronegativity of the defect-trapped H atom [14]. Which pathway is the most plausible under the actual catalytic reaction conditions? While Path 2 proceeds on the three neighboring fivefold coordinated Ti4 þ ions at a perfect surface, Paths 5 and 7 require a priori an oxygen-defect site. It is known that the concentration of oxygen defects on a TiO2(110) surface is typically only less than 5% under the dehydrogenation reaction conditions [32]. In addition, the defect formation by H2O removal from two OH groups observed at elevated temperature, >500 K, is not probable at a dehydrogenation temperature below 450 K [10, 12, 32]. Considering that the R6(d) species was the most stable one at the oxygen defects, most of the defects must be occupied by R6(d), particularly at the higher pressures of HCOOH under the dehydrogenation reaction conditions. Namely, Paths 5 and 7 cannot be reaction paths for the HCOOH. The R6(d) species is a reaction intermediate for the HCOOH dehydration, as reported previously [33]. Thus, Paths 5 and 7 are excluded, based on the lack of the reactants R7 and R8 under the reaction conditions [14]. On the other hand, the concentrations of the reactants, bridging formate R4 and weakly adsorbed formic acid, in Path 2 were sufficient at the surface because no special sites are required for this path and one of the reactants, R4, is the most stable species. Furthermore, high HCOOH coverage by increasing HCOOH pressure increases the coadsorption state in such a manner that a weakly adsorbed HCOOH is located adjacent to a R4 species. Thus Path 2 via the transition state activated in a concerted manner by three Ti4 þ ions (Figure 18.5) should be the most plausible dehydrogenation pathway under the reaction conditions [14].

18.3 Results and Discussion Table 18.2 Relationship between the activation energies and the Mulliken charges of the H atoms of two reactants at the transition state.

Path 1 2 3 3 5 6 7

H1

H2

DEel/eVa

DEap/eVb

0.14 0.219 0.221 0.221 0.047 0.046 0.046

0.552 0.437 0.437 0.552 0.437 0.552 0.552

2.708 2.662 2.424 2.173 0.952 1.782 0.907

0.585 0.353 1.508 1.257 0.212 1.007 0.041

DEel is the intrinsic activation energy between the transition state (TS) and the adsorption state, as shown below. b DEap is the apparent activation energy which is the height of the TS from the initial state before adsorption, as shown below. a

18.3.2.1 Mechanism of the Switchover of Reaction Paths We suggested the bimolecular dehydrogenation mechanism, where the reaction proceeds on the three neighboring Ti4 þ sites, with the aid of the large adsorption energy of the reactants. As this mechanism requires neither special sites nor unstable reactants, the concentration of the reactants as well as the apparent activation barrier are quite reasonable. The dehydrogenation pathways on the oxygen defect sites were discarded because the concentration of the active site was regarded to be too small at the dehydrogenation temperature below 450 K. Henderson and Bowker et al. stated that there was no evidence for the dehydrogenation of HCOOH on TiO2(110) [34, 35]. This discrepancy with our results should be attributed to the difference in the employed reaction atmospheres. We performed the decomposition of HCOOH under

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HCOOH atmospheres, whereas they examined it under ultra-high vacuum. According to our results, the bimolecular dehydrogenation never proceeds in the absence of the gas-phase HCOOH. Kecskes et al. [36] reported that HCOOH was converted to HCHO on a defective TiO2 (110) surface at 300 K, and that the produced HCHO reacted with HCOOH(g), leading to CO and H2 at 473 K. We could not find any evidence for the previous STM work [7–10]. Wang et al. studied the decomposition of DCOOD at 500 K on TiO2(110) surfaces with different initial concentrations of the oxygen defects [34]. They observed the production of CO2 þ D2 on a perfect TiO2 (110) surface, similar to our result [9–11], while the product changed to CO þ D2 as the concentration of the defect sites increased. These results on the defective TiO2(110) surface reproduce the dehydration reactivity of TiO2 power catalysts [37]. We found a denuded zone of 1.4 nm on the terrace from the edge of an atomicheight step on a TiO2(110) surface. This denuded zone may be explained by the assumption that formate species near a step are more reactive for decomposition or more easily desorbed compared to other formate species. Alternatively, formate species are much less stable near a step, which leads to almost no population of formate species near steps because anions like chloride ions do not adsorb near a step. Anyhow, it means that no dehydrogenation reaction proceeds unless there are flat areas larger than 2.8 nm dimension without defects and steps on the surface of TiO2 powders [11]. Regarding the formic acid dehydration, oxygen vacancies which are formed in situ under the catalytic reaction conditions are indispensable for catalysis at higher temperatures than the dehydrogenation reaction. Thus the reaction pathways of HCOOH decomposition depend on the defect concentration as well as the geometric arrangement of the TiO2 surface.

18.4 Conclusion and Perspective

We investigated the dehydration of formic acid on a TiO2(110) surface. Contrary to the conventional knowledge of acid–base catalysis, which is regulated by the intrinsic acid–base property of a catalyst surface, we showed experimental and theoretical evidence for a surface-mediated catalytic reaction mechanism on TiO2(001), where the existence of oxygen vacancies as active sites at the beginning of the catalysis is not necessary and the active oxygen vacancies are produced in situ under the catalytic reaction. We propose a dynamic acid–base concept for catalysis at oxide surfaces. The mechanism of the bimolecular dehydrogenation reaction of HCOOH on a TiO2(110) surface was also investigated by DFT calculations. The most plausible reaction pathway was that between a bridging formate adsorbed on two fivefold coordinated Ti4 þ ions and HCOOH molecule weakly adsorbed at the adjacent Ti4 þ ion. The dehydrogenation occurs by Coulombic interaction between the two adsorbates. The intrinsic acidic TiO2 surface is modified by adsorbed formate anions, and the new basic surface reacts with the second formic acid. The difference in the reactivity between the TiO2(110) single crystal surface and TiO2 powder catalysts should be due to the concentration of oxygen defects and the dimensions

References

of the flat terrace area without steps. The classical acid–base concept for oxide catalysis is not appropriate for explanation of the oxide catalysis and should be modified. The present finding shows the critical importance of the atomic-scale design of the surface of a material. The arrangements of Ti4 þ ions and O2 ions create new catalytic functions for desired chemical processes. The active sites can be produced in situ under the catalytic reaction conditions, even if there are no active sites at the surface before the catalysis. The surface is also modified by adsorption of a reactant to form a new surface with a different acid–base character from the intrinsic property. The dynamic acid–base aspect at a catalyst surface is the key issue to regulate the acid–base catalysis, which may provide a new strategy for creation of acid–base catalysts.

References 1 Tanabe, K., Hattori, H., Yamaguchi, T. and Tanaka, T. (1989) Acid-Base Catalysis, Kodansha. 2 Mars, P., Scholten, J.F. and Zwietering, P. (1963) The catalytic decomposition of formic acid. Adv. Catal., 14, 35–113. 3 Aramedia, M.A., Borau, V., Garcia, I.M., Jimenez, C., Marinas, A., Marinas, J.M., Porras, A. and Urbano, F.J. (1999) Comparison of different organic test reactions over acid–base catalysts. Appl. Catal. A-General, 184, 115–125. 4 Patermarakis, G. (2003) The parallel dehydrative and dehydrogenative catalytic action of g-Al2O3 pure and doped by MgO: kinetics, selectivity, time dependence of catalytic behaviour, mechanisms and interpretations. Appl. Catal. A-General, 252, 231–241. 5 Poulston, S., Rowbotham, E., Stone, P., Parlett, P. and Bowker, M. (1998) Temperature-programmed desorption studies of methanol and formic acid decomposition on copper oxide surfaces. Catal. Lett., 52, 63–67. 6 Halawy, S.A., Al-Shihry, S.S. and Mohamed, M.A. (1997) Gas-phase decomposition of formic acid over Fe2O3 catalysts. Catal. Lett., 48, 247–251. 7 Haffad, D., Chambellan, A. and Lavalley, J.C. (2001) Propan-2-ol transformation on

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Oxygen vacancy promoting catalytic dehydration of formic acid on TiO2(110) by in situ scanning tunneling microscopic observation. J. Phys. Chem. B, 109, 18831–18838. Uemura, Y., Taniike, T., Tada, M., Morikawa, Y. and Iwasawa, Y. (2007) Switchover of reaction mechanism for the catalytic decomposition of HCOOH on a TiO2(110) surface. J. Phys. Chem. C, 111, 16379–16386. Hayden, B.E., King, A. and Newton, M.A. (1999) Fourier transform reflection absorption IR spectroscopy study of formate adsorption on TiO2(110). J. Phys. Chem. B, 103, 203–208. Onishi, H. and Iwasawa, Y. (1994) STMimaging of formate intermediates adsorbed on a TiO2(110) surface. Chem. Phys. Lett., 226, 111–114. Onishi, H., Fukui, K. and Iwasawa, Y. (1995) Atomic-scale surface structures of TiO2(110) determined by scanning tunneling microscopy: a new surfacelimited phase of titanium oxide. Bull. Chem. Soc. Jpn., 68, 2447–2458. Delley, B. (1990) An all-electron numerical method for solving the local density functional for polyatomic molecules. J. Chem. Phys., 92, 508–517. Halgern, T.A. and Lispscomb, W.N. (1977) The synchronous-transit method for determining reaction pathways and locating molecular transition states. Chem. Phys. Lett., 49, 225–232. Delley, B. (1995) Modern Density Functional Theory: A Tool for Chemistry; Theoretical and Computational Chemistry, Vol. 2 (eds J.M. Seminario and P. Politzer), Elsevier, Amsterdam. Perron, H., Domain, C., Rogers, J., Drot, R., Simon, E. and Xatalette, H. (2007) Optimisation of accurate rutile TiO2 (110), (100), (101) and (001) surface models from periodic DFT calculations. Theor. Chem. Acc., 117, 565–574. Lai, X., St Clair, T.P., Valden, M. and Goodman, D.W. (1998) Scanning tunneling microscopy studies of metal

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clusters supported on TiO2 (110): morphology and electronic structure. Prog. Surf. Sci., 59, 25–52. Diebold, U. (2003) The surface science of titanium dioxide. Surf. Sci. Rep., 48, 53–229. Onishi, H. and Iwasawa, Y. (1994) Reconstruction of TiO2(110) surface: STM study with atomic-scale resolution. Surf. Sci., 313, L783–L789. Wiesendanger, R. (1994) Scanning Probe Microscopy and Spectroscopy: Methods and Applications, Cambridge University Press, New York. Henrich, V.E. and Cox, P.A. (1994) The Surface Science of Metal Oxides, Cambridge University Press, Cambridge. Onishi, H. and Iwasawa, Y. (1994) Observation of anisotropic migration of adsorbed organic species using nanoscale patchworks fabricated with a scanning tunneling microscope. Langmuir, 10, 4414–4416. Onishi, H., Fukui, K. and Iwasawa, Y. (1996) Molecularly resolved observation of anisotropic intermolecular force in a formate-ion monolayer on a TiO2 (110) surface by scanning tunneling microscopy. Colloids Surf., A: Physicochem. Eng., Aspects, 109, 335–343. Iwasawa, Y. (1997) Recent progress in surface scientific approaches to oxide catalysis. Catal. Surv. Jpn., 1, 3–16. Thevuthasan, S., Herman, G.S., Kim, Y.J., Chambers, S.A., Peden, C.H.F., Wang, Z., Ynzunza, R.X., Tober, E.D., Morais, J. and Fadley, C.S. (1998) The structure of formate on TiO2(110) by scannedenergy and scanned-angle photoelectron diffraction. Surf. Sci., 401, 261–268. Chambers, S.A., Thevuthasan, S., Kim, Y.J., Herman, G.S., Wang, Z., Tober, E., Ynzunza, R., Morais, J., Peden, C.H.F., Ferris, K. and Fadley, C.S. (1997) Chemisorption geometry of formate on TiO2(110) by photoelectron diffraction. Chem. Phys. Lett., 267, 51–57.

References 32 Wang, Q., Biener, J., Guo, X.C., FarfranArribas, E. and Madix, R.J. (2003) Reactivity of stoichiometric and defective TiO2(110) surfaces toward DCOOD decomposition. J. Phys. Chem. B, 107, 11709–11720. 33 Bennett, R.A., Stone, P., Smith, R.D. and Bowker, M. (2000) Formic acid adsorption and decomposition on non-stoichiometric TiO2(110). Surf. Sci., 454, 390–395. 34 Henderson, M.A. (1997) Complexity in the decomposition of formic acid on the TiO2(110) surface. J. Phys. Chem. B, 101, 221–229.

35 Bowker, M., Stone, P., Bennett, R. and Perkins, N. (2002) Formic acid adsorption and decomposition on TiO2(110) and on Pd/TiO2(110) model catalysts. Surf. Sci., 511, 435–448. 36 Keceskes, T., Nemeth, R., Rask, ó J. and Kiss, J. (2005) New reaction route of HCOOH catalytic decomposition. Vacuum, 80, 64–68. 37 Anpo, M. (1997) Photocatalysis on titanium oxide catalysts: Approaches in achieving highly efficient reactions and realizing the use of visible light. Catal. Surv. Jpn., 1, 169–179.

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19 Nuclear Wavepacket Dynamics at Surfaces Kazuya Watanabe

19.1 Introduction

Since the development of ultrashort lasers, nuclear wavepacket dynamics of various matters have attracted continuing attention [1, 2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump–probe spectroscopy. The direct timedomain observation of the nuclear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. Although there is an accumulated body of literature on nuclear wavepacket dynamics, those of surface adsorbates are less explored. This is mainly attributed to the fact that the experiments are demanding: one needs to combine surface science techniques and ultrafast spectroscopy and, generally, signals from monolayer adsorbates are far smaller than those from the bulk. Nevertheless, detailed information on nuclear wavepacket dynamics of surface adsorbates is important both from fundamental and from practical points of view. As is evidenced from the huge success of catalysis, solid surfaces sustain various kinds of reactions [8]. In order to understand the elementary steps of these reactions, the electronic and vibrational dynamics of surface adsorbates should be investigated in depth. Photochemistry at surfaces involves the photoinduced nuclear dynamics of adsorbates, which needs to be elucidated by ultrafast spectroscopy. Furthermore, combining with recently developed pulse shaping technologies [9], elucidation of the wavepacket dynamics will open up a novel laser control scheme of surface photochemical reactions. This chapter will first describe the principles of experimental techniques which enable us to study the nuclear wavepacket dynamics at surfaces. We focus

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on time-resolved two-photon photoemission (2PPE) and time-resolved second harmonic generation (SHG). After discussing these techniques, selected studies conducted by our group and others by using these methods are reviewed.

19.2 Experimental Techniques 19.2.1 Time-Resolved Two-Photon Photoemission with Femtosecond Laser Pulses 19.2.1.1 Principles Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum level. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 2PPE signals from metal surfaces can be observed with nanosecond laser pulses. Early studies with tunable nanosecond dye lasers focused on image potential states formed at clean metal surfaces [11]. When the light is replaced by two ultrashort pulses and the 2PPE spectra are obtained as a function of the relative delay time of the pulses, the signal, which depends on the time delay, contains important physical information, that is, excited state lifetime, dephasing time, and so on. Time-resolved 2PPE has been proven to be a powerful technique to investigate ultrafast electron dynamics in solid states, bulk carrier dynamics in semiconductors and metals, electron dynamics at well-defined surfaces, and so on [12–14]. 2PPE is sensitive to the electron dynamics in the adsorbate induced states, when molecules or atoms cover the surfaces. Upon electron injection into the unoccupied states with an ultrashort pulse, adsorbate molecules or atoms respond to the sudden change in their electronic states, and a nuclear wavepacket motion towards a new equilibrium starts. The adsorbate induced states change their energy and/or width as the wavepacket evolution, and it emerges as time-dependent changes of the 2PPE spectra as a function of the delay time. 2PPE signals give direct information on the density of states of the unoccupied states which is obtained only indirectly with other optical methods. One drawback is that since the excited electrons are detected, the observation time window is limited to the lifetime of the excited electrons. The excited state lifetimes at metal surfaces are typically less than a few hundreds of femtoseconds and much shorter than vibrational relaxation times. Hence the information is limited to that in the very beginning of the nuclear wavepacket motion, right after the photoexcitation.

19.2 Experimental Techniques

19.2.1.2 Experimental Set-Up For time-resolved 2PPE spectroscopy, a combined set-up of an ultrafast laser system and an ultrahigh-vacuum photoemission spectroscopic system is indispensable. Typical electron energy analyzers have been used as the spectrometer, such as a cylindrical mirror analyzer, a hemispherical analyzer and a time-of-flight (TOF) analyzer. The TOF analyzer is mainly used for low repetition rate ( NEGF theory, the lesser and greater Greens functions, G< mn ðEÞ and Gmn ðEÞ are defined as follows: ð 0 G< ðEÞ ¼ i dðtt0 Þ < dn ðt0 Þdm ðtÞ > eiEðtt Þ mn ð20:6Þ ð 0 0 iEðtt0 Þ > Gmn ðEÞ ¼ i dðtt Þ < dm ðtÞdn ðt Þ > e

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Hence, the integral of the lesser/greater Greens function is equal to the density matrix of the electron/hole. The density matrix of an electron is represented by ð i dEG< Dmn ¼ ð20:7Þ mn ðEÞ: p Note that we incorporate the factor 2 for spin degeneracy, and we always include it without reference in this chapter. The Keldysh formalism leads to the following Keldysh–Ladanoff–Baym (KKB) equation: GÞ ðEÞ ¼ GðEÞSÞ ðEÞG ðEÞ

ð20:8Þ

where G(E) is the retarded Greens function, and is given as the formal solution of the Dyson equation, GðEÞ ¼ ½EHSðEÞ1 :

The terms S ðEÞ; S ðEÞ and SðEÞ are lesser, greater, and retarded self-energy terms, respectively. The word retarded for Greens function and self-energy is often omitted, and we adopt this omitted notation. In the present Hamiltonian system, the self-energy terms consist of the renormalized electrodes, which are denoted as L and R, and electron correlation denoted as ee S ¼ SL þ SR þ See Þ

S ¼ SL

Þ

þ SR

ð20:10Þ

Þ þ See

If electron correlation is incorporated within the DFT level, SÞ is equal to 0, and ee P ee is the sum of the Hartree potential VH and the exchange-correlation (XC) P potential VXC. Thus, ee can be implicitly included if the KS Hamiltonian is adopted for HC, where the KS Hamiltonian is determined self-consistently by the density matrix of the nonequilibrium state. P The lead self-energy terms L/R are the renormalized parts of the left and right electrodes. To represent Greenss function on the C region in the matrix form, the C region is divided into three blocks, L, R and central contact, c regions. The L and R regions should have bulk properties, and the c region is the contact. The electronic state in c can be changed by applied bias, due to induced polarization and net charge. The Greens function matrix (GFM) is expressed in AO basis as 0 11 ESLL HLL SL ðEÞ ESLc HLc 0 A EScc Hcc EScR HcR GCC ðEÞ ¼ @ ESLc HLc 0

EScR HcR

ESRR HRR SR ðEÞ

ð20:11Þ where S is an overlap matrix, and H is now the KS-Hamiltonian. From now on, we adopt the terminology AO basis for both standard AOs or two-dimensional Bloch basis in terms of a linear combination of AOs. The left lead self-energy is written by using the clean surface GFM Gsur LB LB , which is constructed by the complete bulk Hamiltonian with a Direchlet boundary condition, SL ðEÞ ¼ ðESLB L HLB L Þ Gsur LB LB ðEÞðESLB L HLB L Þ:

ð20:12Þ

20.2 Electrode–Molecule –Electrode Junctions

The self-energy of the right electrode is similarly defined. How to define the lesser Þ (greater) self-energy, SL=R , which represents the scatter-in (out) function of electrons provided by electrodes, is the central issue. Practical applications of the NEGF will be possible if the (generalized) Kadanoff–Baym ansatz is applicable [67–69, 77]. In the E–M–E system under constant bias Vb, electrodes are the electron reservoirs by means of the Landauer picture: thus they can be approximated as a non-interacting quasi-equilibrium system, Vb Vb Vb S< ðEÞ ¼ f ðEÞ S E E ðEÞG E S ¼ if L=R L=R L=R L=R L=R L=R 2 2 2 ð20:13Þ Vb Vb S ðEÞ ¼ ð1f ðEÞÞ S E E S> L=R L=R L=R L=R 2 2 Vb ¼ iðfL=R ðEÞ1ÞGL=R E 2

ð20:14Þ

where fL/R are the Fermi functions for the left and right electrodes. Their Fermi levels are shifted to Vb/2. The value E in the self-energy terms is also shifted by Vb/2 because the bulk Hamiltonian should be shift up (down) due to the bias. The current I is formally expressed as a function of Vb: ð 1 > > < IðVb Þ ¼ dETr½S< ð20:15Þ L ðEÞGCC ðEÞSL GCC ðEÞ; p and the Landauer–Buttliker formula, which corresponds to Eq. (20.2), can be derived from Eqs. (20.8),(20.13) and (20.14), ð 1 dETr½GL ðE þVb =2ÞGCC ðEÞGR ðEVb =2ÞGCC ðEÞðfL ðEÞfR ðEÞÞ p ð 1 ¼ dET0 ðE;Vb ÞðfL ðEÞfR ðEÞÞ p G0 T0 ðEF ;0ÞVb

IðVb Þ ¼

ð20:16Þ where the last equation is derived by the wide-band-limit (WBL) approximation. The eigenchannel ti is obtained by diagonalizing the transmission matrix tt, where t is 1=2 1=2 the transmission amplitude matrix and written as GR GCC GL . As stated above, the KS-Hamiltonian should be determined self-consistently through updating the change of VH and VXC in the NEGF-DFT. Recall that the updated part is only the c region, and the L and R blocks are fixed for the whole NEGF-SCF. 20.2.2 Efficient MO Approach

The NEGF-DFT methods to calculate charge transport in the E–M–E systems have recently been incorporated into several ab initio program codes with various practical

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models of systems [70–72, 73–78, 79]. There are two routes to construct the Hamiltonian in the C region that is, starting from the cluster model or the slab model. In addition, correct estimation of semi-infiniteness for electrodes is important for quantitative (ab initio) NEGF-DFTcalculations, and it depends on the method of calculation of surface GFMs. The cluster model is flexible for incorporation into various well-established quantum chemistry programs, hence it is convenient to combine other methods in MO theory. However, there are two disadvantages of the cluster model. The first is that modeling of the surface is somewhat artificial, and it is difficult to reproduce a suitable external potential that eliminates artificial surface effects. This could also lead to unphysical waveguide effects [40, 80], which is particularly important to predict energy-dependent terminal current or sensitive quantities against bias, for example, inelastic electron tunneling spectroscopy (IETS) signal, as shown in the section 20.2.4. The second is the difficulty in reproducing bulk properties by applying the same level cluster approximation to the electrodes. The alternative route is to adopt the slab model. So long as the slab is set sufficiently thick, it is free from the above two problems. However, the thick slab requires an estimation of large GFMs and, hence, the computational cost is very high. Furthermore, the numerical singularity of self-energy matrices and the instability for integration of the lesser Greens function lead to an increase in the computational cost due to the difficulty of convergence in the NEGF-SCF. More recently, we have proposed the efficient MO approach [38], which is intermediate between the cluster and the slab. We give a brief review of our scheme and show an application to the benzene-dithiol (BDT) molecule attached to Au(111) electrodes and an Au atomic wire on Au(001) electrodes. Our efficient MO approach consists of new features, an embedding potential scheme, an O(N) method to calculate lead self-energy matrices, and the use of perturbation Greens functions (PT-GFs) expanded by the restricted MO space in the NEGF-SCF step. When the SCF cycle of NEGF-SCF is performed, the PT-GFMs in the MO basis, which are the eigenstates of the matrix HCC, are adopted. The PT-GFMs are diagonal and represented as follows: " 1 # i PT 0 ð20:17Þ GCC ðEÞ ¼ diag EeI ðGL;I ðE þ Vb =2Þ þ GR;I ðEVb =2ÞÞ 2 where the label I is the index of the MO, and e0I is the MO energy. In each NEGF-SCF step, the density matrix is evaluated as an electron occupation number of MOs, ð i ð20:18Þ dI ¼ dEG< CC ðEÞ p then it is transformed to the AO (Bloch) basis to update the Hartree and XC potentials. The use of PT-GFs in MO basis allows much easier and faster evaluation of the density matrix than calculating Eq. (20.7) in the AO basis directly. In addition, one can introduce the WBL approximation by replacing E with the equilibrium Fermi level EF or the golden-rule type approximation by replacing with e0I in the term GL/R,I. These simplifications lead to further computational efficiency in the NEGF-SCF procedure.


To make the NEGF-SCF step even more efficient, the restricted MO space idea is proposed. The idea is similar to the scheme of the complete active space (CAS)-SCF method in quantum chemistry [81, 82]. The MOs, whose occupation number should be determined by NEGF-SCF, are the only active MOs, and their energies cover the region close to EF Vb/2. The inactive MOs, which are core orbitals, are always fully occupied. The MOs of much higher energy than EF are virtual MOs, and their electron occupations are always equal to zero. In typical cases, the applied bias is within a few volts, and the active MOs in the restricted MO space are only about 10% of the MOs in the whole MO space. Note that orbital relaxation is allowed for all MOs because the Hamiltonian is updated. The fixed values in the inactive and virtual MOs are only occupation numbers. Although the sub-block parts, HLL/RR, are incorporated in the bulk Hamiltonian, which can be obtained by separate band calculations, the long-range potential created by the deep bulk parts is not accounted for when constructing the c region. Our starting point is the assumption that the Hamiltonian matrix HCC obtained by clipping the KS-Hamiltonian and the density matrix resulting from a standard KS~ region is a good approximation of the exact subpart C of the real semiDFT for the W ~ region is set sufficiently large. Usually infinite system labeled as W, so long as the W ~ can be defined by adding two or three outer layers to the focused C region. Then the W the embedding potential Vemb CC can be obtained as the potential, which enforces the ~ and the NEGF results agreement in the C region between the KS-DFTresults of the W of the zero-bias, which is sometimes referred to as EGF. Once the embedding potential is determined, it is fixed in the whole NEGF-SCF for any bias: thus the calculation for the large W system is required only once. The embedding potential includes (at least a part coming from the outer layers) a long-range potential. We proposed a concrete procedure to construct Vemb CC [38], and details can be found in the literature. In Figure 20.2, a brief schematic picture and flow chart of the procedure to construct the embedding potential are given. The evaluation of self-energy matrices is also a computationally expensive task. To incorporate both correct bulk properties and semi-infiniteness (Direchlet boundary condition), we adopted the tight-binding-layer (TBL) method proposed by Sanvito et al. [72, 83] The TBL scheme requires a converged Hamiltonian matrix for the bulk, which can be obtained easily by ab initio band calculations, and solutions of generalized (complex) eigenvalues for each E value for the TBL block diagonal matrix derived from the bulk Hamiltonian. Therefore, the simple use of the TBL scheme is O(N3) for each E. The basic strategy is the reverse procedure of the k-sampling in band theory, as follows. The electrodes usually have smaller unit cells (highly two-dimensional periodicity) than the whole contact. We describe the method when the electrodes have Ncell Ncell structure with G point approximation as an example. Instead of calculating the self-energies of the Ncell Ncell structure directly, one can use a Fourier expansion of the self-energies of the 1 1 structure, S0 ð~ k== Þ, as follows: X X0 i~ k ð~ R ~ R Þ Sm0 n0 ¼ ð~ k Þe == m0 n0 ð20:19Þ mn == ~ k==

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Figure 20.2 (a) Schematic picture of generic system to model the device part. (b) Flow chart of the preliminaries in the efficient MO approach. Recall that we need only the matrices in the C region after the preliminary procedure, and the elements on the buffer part are never used.

where m0 is the set of AOs of atoms in the Ncell Ncell cell, but m is in the unit 1 1 cell. The ~ k== vector is a wave vector parallel to the surface, and sufficient numbers of P ~ k== should be sampled if the cell for consists of many numbers of cells for S0. Because Eq. (20.19) allows one to calculate two-dimensionally wide electrodes by using only the self-energy matrices of the smaller unit cell, it provides the O(N) algorithm. 20.2.3 Ab Initio Calculations: Single Molecular Conductance and Waveguide Effects

As examples, we apply the presented efficient MO approach to the Au(111)/BDT/Au (111) system and atomic Au wire with Au(001) electrodes. The former is the first observed molecular conduction system [17] and one of the most popular benchmark systems for theoretical studies. First, we perform ab initio calculations for it. From now on, we set EF to 0. We adopted PBE XC functionals [84] for the DFT. The details of the parameters to carry out the ab initio (DFT) calculations, for example, basis set and pseudopotentials,


Figure 20.3 Modeled system for BDT on Au(111) electrodes. ~ used to define the embedding (a) The C region and (b) the W potential. The largest spheres denote the Au atoms, and the BDT molecule is at the center. The y and z axes are inserted in (a). The dashed lines represent the boundaries of the c and L/R regions.

are given in Ref. [38]. The C region was modeled using three Au layers for each side and a single BDT molecule, as shown in Figure 20.3a. The c region consists of ~ region is set by adding the top two layers for left and right and the BDT, and the W outer three layers on each side, as given in Figure 20.3b. The BDT is on the hollow sites, and the distance between the S atom and the surface is 1.97 Å. The (111) surface plane was constructed as a 4 4 (i.e., Ncell is set to 4), and the z axis is set as the direction of transport. The y axis is the ½110 direction. The contour plot of the mean-field potential in the C region for the EGF is shown in Figure 20.4, and the calculated induced potential by the applied bias, where Vb is 1.6 V, is given in Figure 20.5. We found that the embedding potential (and the resulting mean-field KS potential under the zero bias) converged rapidly for more than three additional outer layers of the W system. The I–V curve is given in Figure 20.6 and shows a rapid increase (decrease) at Vb ¼ 0.75 1.00 V. Our I–V characteristic is in good agreement with other theoretical data, and gives

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Figure 20.4 A contour plot of the mean-field potential in the C region obtained by the standard KS-DFT for the W region. The contours are separated by 20 eV. The x coordinate is fixed at the center of the system, and the (y,z) positions of the S atoms are set to (0.833,1.97) and (0.833, 8.098).

Figure 20.5 The induced potential with an applied bias (i.e., voltage drop) of 1.6 V.


Figure 20.6 The I–V characteristic calculated by the present NEGF-DFT method.

qualitative agreement with experiments. In the present range of the applied bias, polarization or net charge in the contact is not important. However, the absolute value of the conductance is overestimated by more than one or two orders compared with the experiments of Reed et al. [16, 17] The large quantitative disagreement of the absolute value of conductance in organic molecular conductance between theory and experiment remains an open question and is debated [85]. In Figure 20.7, the induced Mulliken charges are shown as functions of Vb, for the average of the first Au layers, the average of the nearest three Au atoms of the hollow site, and the S atoms for the left and right sides, that are denoted as Au(L1), Au(R1), Au(LH), Au(RH), and S(L), S(R), respectively. Here we set the neutral charge to zero for each case. In the case of zero bias, BDT was negatively charged due to electron donation (i.e., static charge transfer) from the Au atoms nearest to the hollow site. Excess charge from the Au atoms in the left and right layers shows a clear dependence on Vb and, hence, the induced potential is dominated by the net charge on these Au atoms. Table 20.1 shows the orbital energies for the renormalized MOs (RMOs), which are defined as the eigenstates of the sub-block part of the Hamiltonian projected onto the BDT molecule, for each Vb. The labels HOMO and LUMO are those defined by a free BDT molecule. The LUMO level of the RMO is lower than the Fermi level, just as shown in Table 20.1. The occupation numbers of electrons in the HOMO, LUMO, and LUMO þ 1 for the RMOs are 1.87, 1.53, and 0.31, respectively. Thus, LUNO þ 1 provides a small contribution to the conduction by coupling with the broad self-energies, although the RMO energy is much higher than the bias. Compared to the free BDT, the LUMO is almost occupied, which relates to the rise in the I–V curve described above. It can be seen that the fluctuation in the RMO energies is not so strongly a function of bias in the present range.

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Figure 20.7 The Mulliken excess charge (in units of electron charge) on each atom (or atom average) as a function of Vb. The filled and open triangles denote S(L) and S(R), respectively. The filled and open circles denote the average of the Au atoms nearest to BDT for the left and right

leads, that is, Au(L1) and Au(R1), respectively. The data for the Au(LH) and Au(RH) denoting the Au atoms of the left and right electrodes on the first layer are plotted as filled and open squares, respectively.

We give a brief analysis of the computational efficiency in our approach. To expand ~ region, the required number of AOs is 2026 but the C region can be expanded the W with 1546 AOs. Since we focused on the applied bias range of Vb 2 ½1:6; 1:6 V, the active MOs should cover the energy 0.8 V. We found that the sufficient size of restricted MO space to get converged results was [2.35,2.65] V. The total number of active MOs was 185. Therefore, the time-consuming step becomes more than 50 times faster because each matrix inversion requires O(N3).

Table 20.1 Energy of the restricted MO as a function of the bias.

Vb

HOMO 1

HOMO

LUMO

LUMO þ 1

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5

2.82 2.81 2.84 2.82 2.75 2.76 2.80 2.77

2.81 2.74 2.73 2.73 2.64 2.57 2.61 2.54

1.24 1.22 1.26 1.25 1.19 1.18 1.23 1.21

1.79 1.80 1.77 1.76 1.83 1.18 1.79 1.82

The units are volts for the bias and eV for the energies. The equilibrium Fermi level is set to 0.


Next we calculated the Au atomic wire system, which consists of six atoms in the wire and 3 3(001) electrodes [40]. Only the top apex layer on each side is taken as 2 2. To analyze the contributions of the periodic boundary condition for the direction normal to the transport direction, we introduced two models, model (I) and (II) for the Au(001)/Au6/Au(001) system. Model (I) is entirely the 2D periodic system, that is, both the C region and the self-energy matrices are calculated by a Blochtype AO basis maintaining the 2D periodic boundary condition. To incorporate the periodic boundary condition, we adopted G point sampling. In model (II), the lead self-energies are the same as those for model (I), but the C region is 1D nonperiodic. Since direct interactions between the wires in the neighboring cells are negligible, model (I) corresponds to a single wire and (semi) infinite bulk as the physical situation. The relating physical situation is that the wire attached on the finite 3 3 cross-section is then connected to the (semi-) infinite bulk. The schematic diagrams of the above physical situations for models (I) and (II) are given in Figure 20.8. The calculated T0(E) in the model (I) is given in Figure 20.9 a. It is almost constant in the energy range [0.2,0.2] eV and has the value 1.0 due to conduction of the 6s electron of Au. Our result of single open eigenchannel transmission agrees with the experimental results as well as the previous theoretical studies [66, 86]. T0(E) for model (II) is presented in Figure 20.9b. One can find the oscillation structure of T0(E) in the given energy window; the minimum value is about 0.7. The position of the Fermi level gives a value close to the bottom of T0(E). Recall that the differences between models (I) and (II) are periodicity for only the C region, and the self-energies are common. Since the unit cell size 3 3 is sufficient to avoid interaction between chains (and apex parts) in the neighboring cells, the local electronic structure in the contact is similar for each model. The large fluctuation of T0(E) is caused by the waveguide effect, which is a result of quantum confinement, that is, interference of transverse modes of one-electron wavefunctions. Another example of waveguide effects has been studied by Ke et al. with detailed analyses [80]. They concluded that the properties obtained by the integral of E, such as I–V curves are insensitive to the waveguide effects due to washing out of fluctuations during integration. However, their analysis was restricted to tunneling current (i.e., small T0(E )) cases. When the system is conductive (e.g., a metallic wire), or the focus is on more sensitive quantities for the bias (e.g., IETS signals), unphysical fluctuation can lead to more serious errors. The waveguide effect is a clear examples of the importance of the correct models for both the C region and the electrodes. In particular, one needs careful modeling and checking of the results when the cluster model is adopted. 20.2.4 Inelastic Transport and Inelastic Electron Tunneling Spectroscopy

The interaction between electrons and the bridge molecular vibrations plays an important role in E–M–E junctions, and its effect on the change of current caused by energy dissipation from eN interactions is relevant experimentally [8, 65, 66]. Inelastic transport affects device characteristics and Joule heating, which ultimately

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Figure 20.8 The schematic figures relating to periodic and nonperiodic boundary conditions for the Au wire with Au(001) electrodes. (a) and (b) relate to model (I) and (II), respectively, see the text.

controls the mechanical stability of created devices. Another interesting function of inelastic transport is an electron-stimulated reaction, which is the fundamental process of STM-chemistry, and it is of promising use in nanotechnology [8, 27–30, 87]. The inelastic currents also provide an experimental technique of vibrational spectroscopy, IETS [88, 89]. The IETS signal is defined as the second derivative of the current (first derivative of the conductance) to the bias, and its usefulness has been successfully demonstrated. When the molecule is rigid for the applied bias, approximations of harmonic motions for nuclei and linear couplings with electrons will be sufficient. Then we can adopt the (nonlocal) Holstein model [2], and the total Hamiltonian is expressed by


Figure 20.9 The transmission coefficient T0(E) for (a) model (I) and (b) model (II).

adding the two terms Heph and Hph to Eq. (20.15) as follows: H ¼ HC þ HLB þ HRB þ HT þ Heph þ Hph

ð20:20Þ

where Heph and Hph are the eph interaction term and the phonon Hamiltonian, respectively, and are written as X a Heph ¼ Mmm ð20:21Þ 0 dm dm0 ðba þ ba Þ a

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Hph ¼ Wa ba ba

ð20:22Þ

a Mmm 0 is

The term the eph coupling constant, and ba is the annihilation operator of the mode a, whose frequency and normal mode coordinate are represented by Wa and Qa, respectively. The sites for electrons m(m0 ) coupled with phonons are restricted to the C region or a subpart of C. The focused modes should be sufficiently localized on the molecule in term of their definition. Practically, these internal modes can be calculated by means of a frozen-phonon approximation, where displaced atoms are atoms in the c region (or its subpart) denoted as a vibrational box though a check for convergence to the size of the vibrational box is necessary [90]. In general, the time to pass through the molecule is fast in the E–M–E system (in particular, when the electrodes are metal) so long as the bridge molecule is not too large; thus eph couplings are sufficiently weak (thus inelastic transport, not transfer!). In other words, the transient anion is short-lived. In this case, perturbation expansion in the term of the HF diagram can be adopted to incorporate eph interactions into the NEGF formalism [91, 92]. If the coupling is strong and the transient anion is long-lived, the perturbation theory breaks down. In this case the present HF diagram expansion is also not suitable though the NEGF formalism is formally applicable. The eph couplings with the mode a can be renormalized by eph self-energies from an HF diagram as follows: ð i a < a S< ðEÞ ¼ ð20:23Þ dwD< a:eph a ðwÞM GCC ðEwÞM 2p Sa:eph ðEÞ ¼

ð a >

i a a a dwfD< a ðwÞ½M GCC ðEwÞM þ Da ðwÞ M GCC ðEwÞM 2p a Da ð0ÞMa Tr½Ma G< CC ðwÞM g

ð20:24Þ

where D< a and Da are lesser and retarded Greens functions of the phonon, determined by the KKB and Dyson equations, respectively. Since the equations of the electron and phonon Greens functions are coupled, one can solve them selfconsistently combined with the Born approximation (self-consistent Born approximation: SCBA). The details of the SCBA and expressions for lesser/retarded Greens functions and self-energies for phonons are given in the literature [91, 93, 94]. The SCBA includes high order perturbation terms; thus multiple electron–phonon scatterings are partially incorporated. However a single electron– phonon scattering is most important when the SCBA is a good approximation. In this case, the lowest order expansion (LOE) by Ma (i.e., second order in Ma) is often valid, and it is much more convenient and practical than the SCBA for ab initio calculations [86, 94, 95]. We only show the resulting LOE expressions for a system constructed from symmetric electrodes for simplicity but it is extendable to a non-symmetric system [40]. In the LOE, the current can be divided into the three terms, I0,dIel, and Iinel, which are ballistic, elastic correction and inelastic terms, respectively. The last two terms are effects of inelastic transport. The elastic correction term relates to virtual phonon


excitation and background scatterings caused by eph interactions. The corresponding terminal currents are given as follows: i0 ðEÞ ¼

1 T0 ðEÞðfL ðEÞfR ðEÞÞ p diel ðEÞ ¼

X a

diel a ðEÞ ¼

ð20:25Þ X1 a

p

Taec ð2Na þ 1ÞðfL fR Þ

þ TaecLR ðfL fR ÞðfL þ fL þ fR þ fR Þg iinel ðEÞ ¼

X a

iinel a ðEÞ ¼

X1 a

p

ð20:26Þ

Tain f2Na ðfL fR Þ þ fR þ ð1fL Þ þ fL ð1fR Þg ð20:27Þ

where f represents f(E Wa) for the Fermi functions. Na is the nonequilibrium phonon distribution, which is a function of Vb, temperature T, and broadening parameter h. Na is also determined by the LOE framework. The parameter h represents the dissipation to bath phonon in the electrodes. When dissipation to bath modes is zero (i.e., the electron energy is completely transferred to molecular motions), h should be set to 0 and called the externally undamped limit. In contrast, if the phonon modes in the molecule are in completely thermal equilibrium by coupling with the bath, h is infinite, and the resulting Na should be set to the Bose–Einstein distribution, NBE. This is called the externally damped limit. The functions in Eqs. (20.26) and (20.27) are then written as follows: Taec ðEÞ ¼ 2ReTr½Ma G0CC ðEÞMa G0CC ðEÞGR ðEVb =2ÞGCC ðEÞ 0

GL ðE þ Vb =2ÞG0CC ðEÞ

ð20:28Þ

TaecLR ¼ 2Im Tr½Ma ImG0CC ðEÞMa G0CC ðEÞGR ðEVb =2ÞGCC ðEÞ 0

GL ðE þ Vb =2ÞG0CC ðEÞ

ð20:29Þ

Tain ðEÞ ¼ Tr½Ma G0CC ðEÞGR ðEVb =2ÞGCC ðEÞMa GCC ðEÞGL ðE þ Vb =2ÞG0CC ðEÞ 0

0

ð20:30Þ where G0CC is the retarded GFM without eph interactions. The ab initio calculation of inelastic transport in the LOE was also applied to models (I) and (II) of Au(001)/Au6/Au(001). In Figure 20.10a and b, the change in conductance, dG, caused by inelastic transport is shown for model (I) and (II), respectively. Here we only included the highest alternative bond length (ABL) mode in the wire, and the undamped limit is assumed. The calculated value of Wa for the ABL mode was 128.3 cm1. The term dG is the sum of the elastic correction dGel, which is the derivative of the elastic correction current to Vb, and the inelastic term dGinel, which is a derivative of the inelastic current term. The term dGinel is always positive mathematically from the definition, but the sign of dGel depends on the system. In both models, dGel is negative and dominates dG, that is, eph interactions provide the conductance-drop for the Au(001)/Au6/Au(001) system.

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Figure 20.10 The change in conductance as a function of the voltage for (a) model (I) and (b) model (II), in (a) and (b). The total change dVdIb is labeled as dG and shown by the solid line. The elastic Þ dðI Þ el term dðdI dV b and the inelastic term dV b are denoted as (dG ) and inel (dG ) inset and given by the dashed and dotted lines, respectively. el

inel

20.3 Photochemistry on Surfaces

The decrease in the conductance is close to linear when the bias is larger than Wa. Comparing with model (I), the magnitude of dGel becomes small (about 1/3 of the value in model (I)) due to enhancement of dGinel. The eph couplings in the wire are dominated by the local electronic structure. As a result, the difference in Ma between models (I) and (II) is small: thus the main reason for the difference is waveguide effects, that is, properties relating to inelastic transport are sensitive not only to the local geometry (e.g., adsorbed site) but also to the shape of the contact. To create molecular devices, increases or decreases in conductance by eph interactions are quite important. The LOE form tells us that the sign of Taec;ecLR and their magnitude relative to Tain are key factors. Although the simple rule for a rough estimation of increase/decrease in conductance is proposed, it is not certain that the rule is general for any kind of phonon modes and applicable to any systems. Hence ab initio calculation in the LOE is a powerful tool to understand inelastic transport processes. Although we introduced the Holstein model, it is not always suitable. When one focuses on vibrational excitation dynamics, the use of the Holstein model and perturbation expansion is not applicable to DIET type dynamics in the two-state mode because the linear eph coupling in the Holstein model cannot treat multiphonon excitation by single electron excitation. Readers should distinguish two perturbation concepts, where the first is a high order perturbation scheme to represent electron transitions between two (symbolic) states in the two-state model, as stated in Section 20.1.5, and the other is, as given in the present section, perturbation expansion for the self-energies in the Greens function theory. If DIET is dominant to the dynamics in the focused inelastic transport process, one needs to modify the model by including higher order coupling terms than linear couplings. To control chemical reactions on surfaces by injected electrons, a DIET type mechanism should be considered.

20.3 Photochemistry on Surfaces 20.3.1 Theoretical Model of Hot Electron Transport and Reaction Probability

Photo-induced reaction on a metal surface usually consists of several elementary reactions and it is difficult to model the whole reaction process. However, any reactions need to be triggered by electronic excitation. As stated in Section 20.1.4, the major mechanism is indirect excitation; thus we focus on modeling the indirect excitation reaction. Since desorption from the surface is one of the simplest processes and can be a prototype for other complex surface reactions, DIETor DIME are clearly the best to study [10, 48, 53, 57, 96]. In photochemistry, continuous wave or nanosecond lasers lead to DIET, where desorption increase linearly with fluence. In contrast, the DIMET process is caused by intense and short laser pulses on the picosecond or femtosecond time scale, with nonlinear dependence on fluence. Since the fluence is proportional to the number of created hot electrons in the bulk, linear

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and nonlinear dependences reflect whether desorption is triggered by tunneling of a single electron or sequential tunneling of multiple electrons. From the discussion in Section 20.1.5, this is consistent with the definitions of DIETand DIMET. There have been many studies of DIET/DIMET dynamics within the two-state model, where electronic excitations between the two PESs are often reduced to a two-temperature model, that is, the concept of thermal electron bath and electronic temperature is introduced phenomenologically [48]. While the electron excitation is a quite important step for the surface chemistry, the two-state model usually omits a detailed excitation (and de-excitation) mechanism in the reaction process. One of the most important issues in reaction dynamics is to estimate reaction probability, and the main features of indirect excitation are that the resulting reaction probability is a continuous function of photon energy. It is insensitive to incident direction and polarization of light [97]. In the reaction caused by the indirect excitation, the electronic transition in the two-state model should be described by electron–hole pair creation and charge transport, that is, hot electron dynamics. To account for these features, a more rigorous theoretical treatment of hot electron dynamics, in particular, quantitative estimation of heterogeneous charge transport, is necessary. Hence, we focus on the DIET under continuous wave to develop the theory [37]. To treat charge transport, the Landauer approach should be a good start, and we have seen the usefulness of the NEGF framework to perform practical (ab initio) calculations in the previous section. First, we set the Hamiltonian to represent an indirect DIET system. Referring to Eq. (20.5), we introduce the Hamiltonian: H ¼ HC þ HLB þ HT þ HeN X X ðH Þ c L c L X LB nn0 n n0 þ Vmn dm cnL þ ðH:CÞ þ HeN ¼ ðHC Þmm0 dm dm0 þ mm0

nn0

mnn0

ð20:31Þ The terms HC ; HLB ; and HT are the same as those in Eq. (20.20), but there is no right bulk part because the system is a molecule adsorbed on a surface. The C region is also divided into the two regions, L and c. The term HeN is an electron–nuclear interaction, where we use eN, instead of eph, because nuclear motion is now an anharmonic large amplitude motion. The first task is to model HeN. In the direct excitation mechanism, the energy transfer between an electron and nuclear DoFs is caused by the attachment of a hot electron and formation of a transient anion: thus HeN can be defined as follows: ( ) X res res 0 HeN ¼ heN hmjf ihf jm iðdm dm hnif Þ ð20:32Þ mm0

The orbital fres is a resonant orbital, in which a hot electron is attached to form a temporary anion. We assume that desorption is triggered by single orbital resonance to fres through an extension to multi-resonant levels is straightforward. The term hnif is the occupation of the electron in the ground state, and is required to keep the Pauli principle. The coefficient heN is the reactive eN coupling factor and assumed to be


constant. By using the KKB and Dyson equations, we can write down lesser, greater, and retarded Greens functions formally. For simplicity, we omit the contribution of the retarded self-energy of the eN term in the retarded Greens function. To combine the DFT, we adopt the KS-Hamiltonian for HC. In the present case, it is practically quite difficult to include electron-correlation in the nonequilibrium case; thus we P always adopt the Hartree and XC potentials of the ground state. The self-energy L is the same as that in Section 20.2.1. From Eq. (20.32), the eN self-energy is easily obtained as follows: ( ) X res res 0 res 0 res hmjf ihf jm iGC ðEÞhn jf ihf jni qh ðEEF Þ SeN ðEÞ ¼ iIm heN m0 n0

i ¼ GeN ðEÞqh ðEEF Þ 2 ð20:33Þ where qh is the Heaviside function, and we set the real part to 0, that is, the level shift of a resonant orbital by attachment of a hot electron is omitted. To determine lesser (greater) eN self-energy, another condition is required. We introduce an assumption that is called electronic transition state theory (eTST) by analogy with transition state theory. The eTSTassumes that a reaction is promoted by attachment of a transient electron to freswithout any energetic threshold, and there is no recrossing of the transient electron. This leads to the following lesser and greater self-energy terms: S< eN ðEÞ ¼ 0

ð20:34Þ

i S> eN ðEÞ ¼ GeN ðEÞ 2

ð20:35Þ

Then we can obtain the cumulative reaction probability, Pcum(E), as an in-flux current to the resonant orbital, which is represented by the NEGF framework as follows: Pcum ðEÞ ¼

1 < > < Tr SeN GCC S> eN GCC 2p

ð20:36Þ

Eq. (20.36) is a relation of (cumulative) reaction probability, that is, reaction dynamics, and hot electron dynamics. Because the rate of energy transfer is much higher in DIET than DIMET per single hot electron attachment, the eTST is suitable for a DIET process as a first approximation. Although we have written the lesser (greater) Greens functions as functions of E, they should be also functions of the photon energy w. The photon-energy dependence is caused by generation of photo-excited hot electrons in the bulk, and transport to the surface region. From the viewpoint of theoretical formulation, this gives a way Þ to determine the lesser (greater) self-energy of the substrate SL ðE; wÞ, which now depends on the photon energy and should be defined explicitly because we want to Þ apply the NEGF-DFT calculation. To define SL ðE; wÞ we introduce an ad hoc extension of the Kadanoff–Baym ansatz,

S< ð20:37Þ L ðE; wÞ ¼ f ðE; wÞ SL ðEÞSL ðEÞ

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The function f(E,w) is the pseudoequilibrium distribution of photo-excited hot electrons, and we assume its existence. Introducing Eq. (20.37) means fictitious partitioning of the scenario, that is, (i) electron excitation in bulk, these electrons are called primary electrons, (ii) transport of excited electrons to the surface, including generation of secondary electrons by electron–electron scattering during the transport; and (iii) tunneling of hot electrons to the resonant orbital, that is already formulated. The pseudoequlibrium distribution is far from the Fermi function (this is trivial because we treat photo-excitation!), and written as f ðE; wÞ ¼ ðconstÞ

AðwÞ½N1 ðEÞ þ N2 ðEÞ Tr½ImGsur LB LB ðEÞ

ð20:38Þ

where A(w) is the absorbance of the substrate, which can be calculated by the Fresnel formula using a complex refractive index. The terms N1 and N2 are the numbers of primary and secondary electrons reaching the surface region. To calculate N1 and N2, we employed semi-classical transport theory developed by Berglund and Spicer [98]. The details can be found in Ref. [37]. Now we arrived at the final formula for the reaction probability P(w), which forms a connection between two processes, i.e., between the charge transport and reaction dynamics described by the two-state-model, EF ðþ w

dEPcum ðE; wÞ

PðwÞ ¼

EF EF ðþ w

¼

dE EF

i 1 h Tr GeN ðEÞGC ðEÞImSL ðEÞGC ðEÞ f ðE; wÞqh ðEEF Þ p ð20:39Þ

Recall that Eq. (20.39) can be estimated by ab initio NEGF-DFT calculations. 20.3.2 Photodesorption Mechanism of Nitric Oxide on an Ag(111) Surface

Adsorption of NO on a metal surface is important in heterogeneous catalysis [99], and the photodesorption of NO on an Ag(111) surface is an intensely studied DIETprocess. The action spectra (reaction probability) have been measured by several groups, and the data analyzed [100–102]. Recently, Carlisle and King have observed four different ordered dimer phases by STM, named as the a, b, g and d phases [103]. Furthermore theoretical calculations also support a stable dimer as an adsorbate, the photoactive species can be considered as a dimer. Recently Kidd et al. carried out a comparative study of the action spectra of NO and OCS photodesorption on Ag(111), and analyzed it using a phenomenological model [100]. Their analysis can be summarized as follows: (1) Two resonance levels are located at 1.2 and 3.9 eV above EF. The lower level is needed to reproduce a long tail in the action spectra at the long wavelength ( 600 nm). The presence of the higher level results in the rapid rise in the action spectra at 350 nm ( 3.6 eV).


(2) Because of the existence of the lower resonance level, the contribution of secondary electrons must be considered. (3) The resonance widths of the lower and higher levels are 1.0 and 0.5 eV, respectively. However, a resonance level, whose position is close to 3.9 eV, has not been observed experimentally, and the predicted parameters such as resonance width are results of fitting. Therefore, a more detailed analysis based on first principles is desired, and we applied our ab initio scheme to NO photodesorption [39]. Since, experimentally, the most stable phase is the d phase, we performed DFT calculations to find the details of the adsorbed structure of dimer in the d phase, which was modeled by a 2 4 supercell with four Ag layers. For these calculations, we adopted the PBE XC function and the polarized double zeta (DZP) level basis set. According to the STM image, the adsorption sites of the dimers can be selected as the bridge and the threefold hollow sites, that are shown in Figure 6 of Ref. [102]. We adopted two possible models for the ordered phase. One consists of bridge and fcc sites, which we called the d 1 model, and the other has bridge and hcp sites and is called the d2 model. The geometry optimization was carried out for the above two models. The resulting dimer structures and related adsorption energies are given in Table 20.2. The bond axis of NN is parallel to the surface in both cases. The height from the surface to the N atoms, which is denoted as dh is 2.27 Å for the bridge and 2.11 Å for the dimers on the threefold site. The differences between the two NO distances, and the angles made by the NN and NO axes of each dimer are negligible: thus only one set of NO distances and NNO angles is given for each dimer in Table 20.2. From now on, we focus only on the d1 model when refering to the d phase, because the difference between the two models is equivalent to the difference between a dimer on the fcc or hcp site and is not important in the analysis. In addition, we estimated the contribution of the dimmer–dimer interaction on the stability of the d phase. The stabilization energy due to the interaction is only 0.03 eV, that is, the interaction is so weak that formation of bonding between the dimers is negligible. Hence, the dimers on the fcc and bridge site in the d phase can be treated separately in the hot electron transport process. To apply our NEGF-DFT approach, we set the C

Table 20.2 The calculated results of geometric structures and adsorption energies Ead of dimers for the d1 and d2 models.

Site

NN

NO

NNO

dh

Ead

fcc (d1) bridge (d1) hcp (d2) bridge (d1)

1.62 1.64 1.63 1.64

1.22 1.21 1.21 1.21

116.5 115.9 116.4 116.4

2.11 2.27 2.11 2.27

1.47 1.48 1.44 1.48

The heights of the dimers from the surface are also given as dh. All distances, angles, and energies are in Å, degrees, and eV, respectively.

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Figure 20.11 The atomic structures for (a) the fcc model and (b) the bridge model. The largest balls are the Ag atoms. The Ag atoms, which give the threefold or bridge sites, are denoted as bright balls. The N and O atoms are small bright and light gray balls, respectively.

region for the dimer on the fcc and bridge, which are denoted as the fcc model and the bridge model, respectively. The structure of the C region is outlined in Figure 20.11. In Figure 20.12, the numbers of electrons reaching the surface, N(E,w), for w ¼ 3.0 (a) and 3.9 eV (b) are shown. Since the inter-band transition occurs in the case of w ¼ 3.9 eV, the ratio of electrons close to EF (recall we set it to 0) becomes large; thus N(E,w ¼ 3.9) decreases much faster than N(E,w ¼ 3.0). In Figure 20.13(a) and (b), we plot the absorption coefficient a(w) and the absorbance A(w) as functions of w, respectively. Since N(E,w) is roughly proportional to A(w), there is a possibility that N(E,w) will also have a sharp peak, which causes an unphysical peak in the action


Figure 20.12 The number of electrons calculated by our theoretical model, N(E,w) and by the empirical form, Neff(E,w), with w ¼ (a) 3.0 eV and (b) 3.9 eV. In each panel, the solid line is N, the dashed line is Neff with b ¼ 1.6. For comparison, the value of Neff with b ¼ 1.0 is plotted as the dotted line.

spectra. To avoid such an unphysical peak, previous theoretical works introduced a modified absorbance, Aeff(w), which is termed an effective absorbance [100, 102]. In our approach, the term N(E,w) is determined by microscopic view without any phenomenological modifications. The E-dependence in N(E,w) includes both

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Figure 20.13 Absorption coefficient (a) and absorbance (b) plotted as a function of photon energy.

inter/intra-band transitions, and we found that it eliminated a possible unphysical peak in the action spectra. The values of N(E,w) used for the analysis by Kidd et al. [100] are also shown for w 3.0 and 3.9 eV in Figure 20.11a and b, respectively. To distinguish N(E,w) determined by Eq. (20.38), we denote it as Neff, which is a phenomenological empirical form [104]


N ef f ðE; wÞ /

Aef f ðwÞ w b w E

ð20:40Þ

where a concrete form of Neff is given in Ref. [102]. The value of b, which is a fitting parameter of the action spectra, can be interpreted as the contribution of secondary electrons. With increasing b, the contribution of secondary electrons becomes important. When b is set to 1, no secondary electrons contribute. In the NO/Ag (111), b was set to 1.6 by Kidd et al. [100]. For a comparison we give Neff obtained by setting b to 1 and 1.6. When E is larger than 1.0 eV, our N(E,w) and Neff obtained using b ¼ 1.6 agree well. In other words, our formulation by Eq. (20.38) can give two empirical parameters Neff(w) and b by the microscopic viewpoint without any arbitrariness. When E is close to 0, the difference between N and Neff increases rapidly. If the resonant level is close to 0, this difference leads to serious disagreement for the action spectra obtained by the phenomenological model and our model. However, as shown later, the resonance level is sufficiently higher than 1.0 eV above the EF; thus the disagreement of N and Neff in the low energy region does not play any important role in the action spectra. Now we need to introduce a resonant orbital. By calculating the free NO dimers relating to the fcc and bridge models, we found that only the LUMO þ 1 level was a suitable resonant orbital for both cases. We denote these resonant orbitals as fres fcc and fres brd , respectively. The selected two orbitals form clear p-type bonding for NN and NO axes, and one can consider that the resonance for NO photodesorption first triggers decomposition of a dimer to two monomers, rather than directly initiating cleavage of a bond between NO and Ag atoms. In our calculation, each absorbed dimer has only one resonant orbital as a candidate. The projected density res of states (PDOS) for fres fcc and fbrd are given in Figure 20.14a and b), respectively. In Figure 20.14a, there is one large peak at E ¼ 1.98 eV. Another small peak close to E ¼ 2.5 eV can be regarded as part of the tail of the large peak, that is, not another resonant level. This structure of the PDOS is similar to Figure 20.14b. Therefore we conclude that the resonant level is single for both fcc and bridge models, as res well as the resonant orbital. The resonant energy Ebrd and width g res brd are 1.97 and res res res 0.85 eV, respectively. The difference between Efcc and Ebrd or g res fcc and g brd is negligible, and they can be treated as almost the same narrow resonance positions for the photon-energy dependence, even if the photoactive species is assumed to be a dimer. The action spectra (reaction probability), Pfcc(w) and Pbrd(w), were calculated for 12 photon energies, in the photon energy range of 2.1–4.1 eV, for the fcc and bridge models, respectively. We estimated the rate Pbrd(w)/Pfcc(w) and found that it is almost 50% for all the given photon energies. Therefore the dimer on the fcc site is more reactive than that on the bridge site for photodesorption in the present model. The calculated and experimental action spectra are plotted in Figure 20.15. In addition we plotted Pd(w), which is the sum of the fcc and bridge models, as the reaction probability of the d phase. Note that in Figure 20.15, they are scaled by setting the values to unity at w ¼ 3.6 eV for comparison. In the complete range of w, the theoretical results are in reasonably good agreement with experiment as a function

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Figure 20.14 PDOS of the resonant orbital for the fcc model (a) and the bridge model (b).

of w. In particular, the experimental features: (i) a long tail in the region w 2.0 eV and (ii) a rapid increase at w 3.6 eV, are perfectly reproduced. We applied our approach under the assumption that the monomer is the photoactive species, and compared the results for the case of the dimer to check whether or not another photoactive species exists. The procedure for the calculation is the same with the case of dimer, although we considered only the fcc site, which is more stable than the bridge in monomer adsorption. The resonant orbital is p for the free NO molecule, and the resulting action spectra is shown in Figure 20.16 with the experimental and theoretical dimer action spectra. At low photon energy, they agree well, but the increase at w 3.6 eV seems to be too rapid in the monomer model.


Figure 20.15 Calculated reaction probabilities of NO desorption for the fcc model (open triangles), the bridge model (open squares), and the total d phase (crosses). The experimental action spectrum is also plotted as filled circles. All the data are scaled by setting the reaction probabilities to 1 at w ¼ 3.6 eV.

From the ab initio calculations and analysis based on our NEGF theory, we can suggest a mechanism of the interfacial charge transport for NO photodesorption (1) The photoactive species is the NO dimer (2) Although the dimer phase consists of two kinds of structures, the resonant level and width are similar, hence, the dimer phase can be treated as a single resonant level system. (3) The long tail of the action spectrum is caused by low resonant energy, which is about 2 eV above the Fermi level. (4) The effect of secondary electrons gradually increases, reaching 40% at high photon energy. In addition, the interference for tunneling of hot electrons contributes to the increase in the action spectrum. Furthermore, the results support that NO photodesorption and decay of a dimer itself should be competing channels because the introduced resonant orbital is the only one for a dimer which has anti-bonding character for both NO and NONO. This is consistent with the experimental results that N2O desorption is also observed by substrate-mediated excitation, and its action spectrum exactly mirrors that for NO photodesorption. Again we emphasize our results and analyses are obtained by ab iniio calculations within the model of NEGF-DFT focusing on hot electron transport and a charge injection process. This is an example to show the importance and

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Figure 20.16 Calculated reaction probabilities for an assumed photoactive monomer (open diamonds). The reaction probability of NO for the total d phase (crosses) and the experimental action spectrum (filled circles), which are as in Figure 20.15, are plotted for comparison. All the data are scaled by normalizing to a value of 1 at w ¼ 3.6 eV.

necessity of a theoretical model which connects reaction dynamics and excited electron dynamics followed by heterogeneous transport.

20.4

Summary and Outlook

Throughout this chapter, we have tried to set the problems in a unified theoretical viewpoint. To do so, we rearranged the terminologies used in each research field, particularly transfer and transport, which are often treated in terms of Marcus and Landauer theories. We showed which approach is the better description as a starting point for several systems by focusing on the classification of electronic excitation mechanisms. In addition, we gave a conceptual insight of the connection between traditional models of reaction dynamics, which is close to molecular science, and CTs in heterogeneous (surface) systems. We have focused on the charge transport at interfaces and nonadiabatic interactions between injected electrons and nuclear motions. Our purpose is to establish practical models, which enable us to perform ab initio calculations. We adopted the NEGF formalism, and developed theoretical models combined with a practical ab initio scheme by means of DFT. We chose two systems as examples, the E–M–E junction and photoreaction on metal surfaces.

20.4 Summary and Outlook

One of the advantages in adopting the NEGF for charge transport is that semiinfiniteness, which is a most important feature of nanoscale heterogeneous systems, can be renormalized by self-energies, and the non-equilibrium one-particle state of continuous electronic excitations/de-excitations can be expressed formally. In addition, within DFT (or, more widely, the mean-field approximation) level, the surface Greens functions are obtained exactly, that is, the retarded Greens function can be estimated by ab initio calculations as a function of energy. However, although we have shown successful models by NEGF formalism, an extension to general interfacial CTs is not straightforward. To apply the NEGF, lesser (greater) self-energies are required, and determination of them usually requires the (generalized) Kadanoff– Baym ansatz; thus the possibility of application of the NEGF depends on the detail of electronic excitation processes, and how to introduce the ansatz as suitable physical models. Even if the NEGF is applicable, the combination with DFT, that is, NEGFDFT (or other mean-field theory such as Hartree–Fock) is not often sufficient for the requirements of highly accurate quantitative estimations. This is because the strict applicability of DFT is limited to the ground state; thus the extension to excited states gives a crude approximation although it is often useful and qualitatively sufficient. As stated in Sections 20.1 and 20.2, the separation of transfer and transport is the classification relating to coherent and decoherent limiting cases, respectively. Each case can be described by a model expressed in terms of Landauer (coherent limit) and Marcus (decoherent limit) approaches. However, many heterogeneous CTs processes are intermediate between transfer and transport, or consist of both transfer and transport steps in the whole process while the process is ultra-fast. In the present study we tried to start from the Landauer approach, then extended it to incorporate decoherence for the complex heterogeneous CTs. One of the applications was a calculation of the IETS, where the LOE formulation was adopted, and the other was photodesorption on metal surfaces. The former is formally rigorous, but application is limited to the weak eph coupling, that is, weak decoherence only. Furthermore, it is difficult to develop the scheme for the case where nuclear motion is (anharmonic) large amplitude motion during transport. To model the latter case, we proposed a fictitious separation of the scenario for the entire photodesoprtion process. Though the fictitious partitioning combined with NEGF-DFT is a promising model, the procedure required some premises or ad hoc (crude) assumptions, for example, indirect excitation is assumed, eTST, omitting details of reaction dynamics, extension of the Kadanoff–Baym ansatz. Generally, it is not always possible to introduce such a tricky fictitious separation, and the required theory should predict the details of a process without any pre-knowledge of the mechanism. One of the other frameworks, time-dependent-DFT may be listed because the formulation is very simple and independent of the strength of nonadiabatic couplings. Several applications to heterogeneous systems have been carried out, for instance, charge injection of a dyesensitized TiO2 solar cell [105–107] or nanoscale conductance [61, 108]. However, there are a few serious difficulties in performing practical (ab initio) calculations, for example, incorporation of semi-infiniteness of the substrate into the time propagation of electronic wavefunctions and representing the decoherence of nuclear motions by their stochastic property [109, 110]. As another promising formalism,

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time-dependent NEGF is also extensively studied, but practical applications are limited to a few cases [111]. Comparing homogeneous charge transfer and reactions, the theoretical understanding of heterogeneous CTs is very insufficient although it is a common problem underlying many important chemical and physical processes. The difficulty arises from the coexistence of locality and nonlocality in the electronic state to be considered and competition between coherence and decoherence in the particle dynamics as well as the resulting nonadiabatic reaction dynamics. In this sense, the heterogeneous CTs just border on molecular science (chemistry) and condensed physics (physics), and it is challenging to develop a theoretical model that is comparable with recent advances in experiments, and the model should be unified with traditional theoretical models in the fields of both chemistry and physics.

Acknowledgments

This research was supported by a Grant-in-aid for Scientific Research in Priority Area, Molecular Nano Dynamics (Grant No. 16072206) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). One of the authors (HN) would like to give thanks for financial support by a Grant-in-aid for Scientific Research in Priority Area, Electron transport through a linked molecule in nanoscale (Grant No. 20027002) from MEXT, and a Grant-in-aid for Scientific Research (C) (Grant No. 20613002) from the Japan Society for the Promotion of Science.

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105 Craig, C.F., Duncan, W.R. and Prezhdo, O.V. (2005) Trajectory surface hopping in the time-dependent Kohn-Sham approach for electron-nuclear dynamics. Phys. Rev. Lett., 95, 163001-1–163001-4. 106 Duncan, W.R. and Prezhdo, O.V. (2005) Electronic structure and spectra of catechol and alizarin in the gas phase and attached to titanium. J. Phys. Chem. B, 109, 365–373. 107 Duncan, W.R., Stier, W.M. and Prezhdo, O.V. (2005) Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the alizarin-TiO2 interface. J. Am. Chem. Soc, 127, 7941–7951. 108 Gebauer, R. and Car, R. (2004) Current in open quantum systems. Phys. Rev. Lett., 93, 160404-1–160404-4. 109 Miyamoto, Y. and Sugino, O. (2000) Firstprinciples electron-ion dynamics of excited systems: H-terminated Si(111) surfaces. Phys. Rev. B, 62, 2039–2044. 110 Sugino, O. and Miyamoto, Y. (1999) Density-functional approach to electron dynamics: Stable simulation under a selfconsistent field. Phys. Rev. B, 59, 2579–2586. 111 Kurth, S., Stefanucci, G., Almbladh, C.O., Rubio, A. and Gross, E.K.U. (2005) Timedependent quantum transport: A practical scheme using density functional theory. Phys. Rev. B, 72, 035308-1–035308-13.

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21 Dynamic Behavior of Active Ag Species in NOx Reduction on Ag/Al2O3 Atsushi Satsuma and Ken-ichi Shimizu

21.1 Introduction 21.1.1 NOx Reduction Technologies for Diesel and Lean-Burn Gasoline Engines

In view of the reduction of total CO2 emissions from automobiles, the production of diesel and lean-burn gasoline engine cars is expected to increase because of their greater fuel economy than conventional gasoline engine cars. Especially, the total CO2 emission (well-to-tank and tank-to-wheel) from diesel hybrid cars is almost equivalent to fuel-cell engine cars using H2 produced from fossil fuels. Further reduction of CO2 emission can be expected by the use of biodiesel fuels. However, serious drawbacks of diesel engines are emissions of particulate materials (PM) and nitrogen oxides (NOx: NO and NO2). Removal of PM is well established by the use of ceramic filters (DPF: diesel particulate filter) [1]. For NOx removal, the following three technologies are being developed and coming onto the market: NSR (NOx storage-reduction) system, urea-SCR (selective catalytic reduction by urea) and HC-SCR (selective catalytic reduction by hydrocarbons). The NSR system developed by Toyota [2–4] is characterized as an evolved three-way catalyst combined with NOx storage. The catalyst contains NOx storage materials, such as BaO, which store NOx as nitrates during lean conditions. When turning the engine to rich conditions for a short period, the stored NOx is released from the storage materials and reduced over noble metals by hydrocarbons and CO. The improvement in SOx tolerance of NSR catalysts is a further problem to be solved. Urea-SCR is thought to be effective for large-scale diesel engine cars such as tracks and buses [5]. The injection of urea into a catalytic converter results in the generation of NH3 via hydrolysis, and then NOx in exhausts is effectively reduced to N2 by generated NH3 over zeolitebased catalysts. The effective reduction of NO at low temperatures is the merit of

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the urea-SCR technology, however, this technology still has some problems to be solved, such as refilling with urea solution, appropriate injection of urea solution, and so on. 21.1.2 Selective Catalytic Reduction of NOx by Hydrocarbons Over Ag/Al2O3

Selective catalytic reduction of NOx by hydrocarbons (HC-SCR) is also thought to be a potential method for removing NOx from lean-burn and diesel exhausts. In 1990, Iwamoto et al. [6] and Held et al. [7] discovered that NOx reduction under excess oxygen could be accomplished using hydrocarbons as reducing agents over Cu-MFI. Unburned hydrocarbons in exhausts or fuels can be used as reductants, which is the main advantage of the HC-SCR system. Since the air/fuel ratio in exhausts of lean burn and diesel engines is very high, the catalysts for the HC-SCR system should work in the presence of excess oxygen, around 10%, and the following functions are required: (i) promotion of the reduction of NO (less than 1000 ppm) by HC and (ii) the suppression of the undesired combustion of HC with excess O2 (around 10%). Through numerous screening runs after the discovery of the HC-SCR technology, various types of catalysts, which can be classified into three categories: ion-exchanged zeolites, supported precious metals, and metal oxide-based catalysts, have been found [1, 8, 9]. Ion-exchanged zeolites, especially Cu-MFI, show very high performance in HC-SCR. However, deactivation due to instability of the zeolite framework under hydrothermal conditions is a serious problem. Supported precious metal catalysts, such as Pt/Al2O3, show high stability and high tolerance to sulfur oxides (SOx) and water vapor. They show very high activity at lower temperatures, however, the formation of N2O and a narrow temperature range for NOx reduction are the problems to be solved. Metal oxide-based catalysts show high stability and moderate tolerance to SOx and water vapor. Among a number of catalysts reported, Ag/Al2O3 is thought to be the most promising candidate for practical applications. Several reports on engine bench tests have demonstrated a high possibility of Ag/Al2O3 for practical applications [10–14], but low HC-SCR activity at lower temperatures should be improved. Miyadera first reported the high efficiency of Ag/Al2O3 catalysts in NO reduction to N2 by alcohols [15, 16]. By the use of ethanol as a reductant, Ag/Al2O3 catalysts showed 80–100% conversion of NO in the range 300–400 C, as shown in Figure 21.1. He et al. demonstrated a diesel engine bench test by using ethanol-SCR over Ag/Al2O3 [17]. Ag/Al2O3 showed average NOx conversion of greater than 80% in the temperature range 300–400 C. For a space-velocity of 30 000 h1, the light-off temperature was 270 C and the highest NOx conversion was 92.3% at 400 C. Thomas et al. tested SCR by various alcohols for the exhaust from a 5.9 L diesel engine using a 7.0 L Ag/Al2O3 catalyst system with a reductant injector [14]. The low price of Ag is also attractive for practical use. The Ag/Al2O3 catalyst shows very high SCR activity when alcohols are used as reductants, however, it is not effective for SCR by light hydrocarbons such as methane and propane.

21.2 Hydrogen Effect of HC-SCR Over Ag/Al2O3

Figure 21.1 Conversion of NO by oxygen-containing organic compounds over 2 wt.%Ag/Al2O3 catalyst. Reaction conditions: 500 ppm NO, 1000 ppm (as CHx) reductant (333 ppm), 10% O2, 10% CO2, N2 balance, 10% H2O, SV ¼ 6400 h1. Symbols: (*) 1-propanol, (&) 2-propanol, (~) ethanol, ( ) acetone, and (~) methane [16].

.

21.2 Hydrogen Effect of HC-SCR Over Ag/Al2O3 21.2.1 Boosting of HC-SCR Activity of Ag/Al2O3 by Addition of H2

The most significant and surprising promotion effect of Ag/Al2O3 can be obtained by the addition of a small amount of H2 into the reaction atmosphere. Satokawa first reported the promotion effect of H2 on the HC-SCR activity of Ag/Al2O3 at lower temperatures [18, 19]. As shown in Figure 21.2, the addition of H2 into propane-SCR atmosphere boosted the NO conversion to N2. The effect is greater at lower temperatures, below 773 K. For example, the conversion of NO at 673 K was nearly zero, but it increased to about 50% by co-feeding 909 ppm of H2. The promotion effect of H2 on NO and propane conversions was reversible, depending on the presence of H2 in the gas phase (Figure 21.3). Interestingly, NO conversion is nearly zero when only hydrogen is used as the reductant. Therefore, hydrogen does not act as a reducing agent, but plays the role of promoter for the NO reduction by hydrocarbons [19]. Actually, the steep increase in propane conversion with the introduction of H2 suggests that the activation of hydrocarbon reductants is a key step.

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NOx conversion/%

60

40

20

0 500

600

700 Temperature/K

800

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Figure 21.2 NOx conversion over Ag/Al2O3 at various H2 concentrations. Feed: 91 ppm NO, 91 ppm C3H8, 9.1% O2, 9.1% H2O, and 0 ppm ( ), 227 ppm (*), 451 ppm (~), 909 ppm (&), or 1818 ppm (^) H2 in He. GHSV ¼ 44 000 h1 [19].

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This remarkable role of hydrogen is called the hydrogen effect [1] and has been re-examined by various research groups, such as Burch et al. [1, 20], Sazama et al. [21], Klingstedt et al. [22], Richter et al. [23] and Zhang et al. [24]. Burch et al. reported superior H2-HC-SCR performance of Ag/Al2O3 by using octane as a reductant [1, 20]. As shown in Figure 21.4, the conversion of NO increased from 450 C in the absence

Figure 21.3 Effect of H2 on the NOx and C3H8 conversions at 673 K as a function of time over Ag/Al2O3. Feed: 91 ppm NO, 91 ppm C3H8, 9.1% O2, 0 or 455 ppm H2, and He as balance. GHSV: 44 000 h1 [19].


Figure 21.4 The effect of H2 on NOx reduction by octane on Ag/ Al2O3 catalyst. (a) ( ) NO conversion octane alone; (^) NOx conversion octane alone; (&) NO conversion with 0.72% H2; (&) NOx conversion with 0.72% H2. (b) (&) H2 conversion; (^) octane conversion with 0.72% H2; (^) octane conversion in the absence of H2 [20].

.

of H2, while sufficient NO conversion was observed from 200 C. Surprisingly, the NO conversion started from a temperature 250 C lower than in the absence of H2, and nearly 100% conversion of NO was achieved over the wide temperature range of 200–500 C. They also confirmed that the conversion of octane increased with the addition of H2. The hydrogen effect is also achieved by using a wide range of reductants, C2–C4 hydrocarbons [18], ethanol [24], and NH3 [25, 26]. The hydrogen effect is of practical interest because (i) the temperature of diesel exhaust ranges from 200 to 400 C, (ii) the light olefins, the main components of unburned hydrocarbons, or higher paraffins, such as cetane, the main component of diesel fuel, can be used as reductants. The hydrogen effect was also observed over Ag-zeolite catalysts, such as Ag/MFI and Ag/BEA [27]. Furthermore, the co-feeding of hydrogen is also preferable for SOx tolerance [28, 29]. 21.2.2 Surface Dynamics of Ag Species Analyzed by in situ UV–Vis

For a better understanding of the hydrogen effect, in situ analysis is applied to the Ag/Al2O3 under the working state of HC-SCR. In the UV–Vis region, LMCT bands and d–d transfer bands of various elements can be observed, and thus UV–Vis spectra may give directly the states of active elements on solid surfaces. Figure 21.5 shows the

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Figure 21.5 The exterior and cross-section of the in situ UV–Vis cell [30].

exterior and the cross-section of an in situ UV–Vis cell (JASCO VHR-630) developed by our group and the JASCO corporation [30]. The apparatus can be used outside the main optical system of the UV–Vis spectrophotometer (JASCO V-570). Inside the lower unit is a diffuse reflectance sample cell with a heating system, which is connected to a gas flow system. In the upper unit, the light source is led to the center of an integrating sphere by an optical fiber. The transient reaction experiment was carried out under the same conditions as the flow reaction of the HC-SCR. Figure 21.6 shows the in situ UV–Vis spectra thus measured [31]. It is accepted that the bands above 40 000 cm1 correspond to the 4d10 A

B

0.2

b c

Kubelka-Munk

Kubelka-Munk

0.3

a

0.1

e

0.2

d 0.1

f g

0

0 20000 30000 Wavenumber/cm-1

40000

Figure 21.6 In situ UV–Vis spectra of (a) 2 wt.% Ag/Al2O3 in O2 after oxidation at 823 K, (b) during the steady state H2 þ O2 reaction, and (c–g) difference spectra subtracted by (a) in different conditions at 573 K: (c) 2 wt.% Ag/ Al2O3 during the steady state H2 þ O2 reaction,

20000 30000 -1 Wavenumber/cm

40000

(d) 2 wt.% Ag/Al2O3 in H2 reduction of 3 min, (e) 2 wt.% Ag/Al2O3 in H2 reduction of 30 min, (f) 2 wt.% Ag/Al2O3 in reoxidation with O2 for 3 min after d, (g) 0.5 wt.% Ag/Al2O3 after H2 reduction of 30 min [31].


to 4d95s1 transition of Ag þ ions, and the bands between 25 000 and 40 000 cm1 are due to silver clusters with different size and oxidation states [21, 27, 31]. In a flow of 10% O2 (spectrum a), a band centered above 40 000 cm1 due to Ag þ ions was observed. During the steady-state of H2 þ O2 reaction, a broad band centered around 30 000 cm1 assignable to Agnd þ (n 8) clusters was observed in a difference spectrum (spectrum c). When 2 wt.% Ag/Al2O3 was exposed to a flow of H2 (0.5%) for 3 min (spectrum d), a broad band in a similar position was monitored. We have reported that bands at 32 800 and 38 000 cm1 were observed in the UV–Vis spectrum of Ag-MFI zeolite reduced by 0.5%H2 at 573 K and assigned them to the Ag42 þ cluster. In the EXAFS spectra during H2 þ O2 reaction and during H2 reduction for 3 min described later, the coexistence of the Ag42 þ cluster and highly dispersed Ag þ ions is suggested by the low Ag–Ag coordination number (0.6) and Ag–Ag distance, indicative of the Ag42 þ cluster as well as a large Ag–O contribution. With an increase in H2 reduction time (t ¼ 30 min, spectrum e), a broad band centered around 25 000 cm1 assignable to large Agn (n > 4) clusters and a shoulder around 28 600 cm1 due to the Ag42 þ cluster were observed. When the flowing gas was switched from H2 to O2 (10%), the intensity of the band due to the Ag clusters, including Agnd þ and Agn (n > 4), decreased (spectrum e to f). This result is consistent with the EXAFS, which also indicated the re-dispersion of Ag clusters to Ag þ ion under O2. The states of surface Ag were dependent on the Ag content. The UV–Vis spectrum of 0.5 wt.% Ag/Al2O3 (spectrum g) was almost unchanged after 30 min of H2 reduction. This indicates that reductive agglomeration of Ag þ to Ag cluster does not occur on this sample. In contrast, Ag/Al2O3 containing higher Ag (5 and 10 wt.%) showed that the bands are observed at lower wavenumbers at 25 000 cm1 assignable to the large Ag clusters and at 17 500 cm1 assignable to Ag particles (figures are not shown). At higher Ag loadings, reduced Ag species of larger size are dominant. The structure of surface Ag species was also identified by in situ Ag K-edge EXAFS measurements. Figure 21.7 shows Fourier transforms of k3-weighted EXAFS of Ag/Al2O3 at 573 K [31]. The EXAFS of 2 wt.% Ag/Al2O3 after oxidation at 773 K (spectrum b) shows a large Ag–O contribution. From curve-fitting analysis, the Ag–Ag coordination number was negligibly small (0.1), indicating that Ag þ ions dispersed on the alumina surface are dominant. Under the steady-state H2(0.5%) O2(10%) reaction (spectrum a), the Ag–Ag shell with coordination number 0.6 and bond distance 2.74 A was observed. In a flow of 0.5% H2 for 3 min (spectrum c), the bond distance of the AgAg contribution was the same (2.74 A). According to the Ag K-edge EXAFS measurements of Ag-MFI zeolite, the Ag–Ag shell with a bond distance of 2.74 A is tentatively assigned to a small Agnd þ cluster such as Ag42 þ . This assignment is based on the following results of Ag-MFI zeolite reduced by 0.5% H2 at 573 K [32]: (i) the coordination number of the AgAg shell with bond distance 2.73 A was around 3 (3.3), (ii) the average valence of Ag is þ 0.5 from H2-TPR. The average valence of Ag cluster on alumina was also around þ 0.6 from a microcalorimetric study of H2 adsorption on Ag/Al2O3: The H2/Ag ratio was around 0.19 for the H2 adsorption with heats above 30 kJ mol1 [31]. As shown in Figure 21.7, the intensity of the AgAg peak increased while that of the Ag–O shell decreased with increase in H2 reduction time. After the steady-state (30 min, spectrum d), the AgAg contribution

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f

FT [k3 χ(k)]

e

d

c

b

a 0

1

2

3 R/Å

4

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Figure 21.7 Fourier transforms of Ag K-edge in situ EXAFS measured at 573 K: (a) 2 wt.% Ag/Al2O3 during the steady state H2 þ O2 reaction, (b) 2 wt.% Ag/Al2O3 in He after oxidation at 773 K, (c) 2 wt.% Ag/Al2O3 in H2 reduction of 3 min, (d) 2 wt.% Ag/Al2O3 in H2 reduction of 30 min, (e) 2 wt.%Ag/Al2O3 in re-oxidation with O2 for 3 min after d, and (f) 0.5 wt.% Ag/Al2O3 after H2 reduction of 30 min [31].

with coordination number of 3.2 and bond distance of 2.83 A was observed. The AgAg shell with a bond distance of 2.83 A is assigned to large Ag cluster because the distance is shorter than that of bulk Ag and is the same as that reported for silver clusters in Ag-X zeolite formed by H2 reduction at 573 K [33]. When the flowing gas was switched from 0.5% H2 to 10% O2, the features of spectrum d changed quickly to spectrum e. The Ag–Ag coordination number decreased from 3.2 to 0.7 and the AgO coordination number increased from 1.6 to 4.4. This indicates that oxidation with O2 results in the re-dispersion of Ag cluster to Ag þ ion. In the case of 0.5 wt.%


H2= 0.5%

H2= 0%

0.01

60

0.006 40 0.004

NO conversion/%

Kubelka-Munk

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20 0.002

0

0

10

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30

0 40

Time / min Figure 21.8 Effect of hydrogen switching on/off on NO conversion and the UV–Vis band height at 28 500 cm1 during propane–SCR over at 573 K. Conditions: 0.1% NO, 0.1% C3H8, 0% or 0.5% H2, 10% O2, catalyst weight ¼ 50 mg [31].

Ag/Al2O3, on the other hand, the Ag–Ag contribution was negligibly small, even after 30 min of H2 reduction treatment (spectrum f), indicating that reductive agglomeration of Ag þ to silver cluster does not occur. These results are consistent with the in situ UV–Vis spectra shown in Figure 21.6: The small Agnd þ clusters are formed by slight reduction of 2 wt.% Ag/Al2O3, further reduction or higher Ag loading results in the formation of large Ag clusters, Ag clusters are reversibly re-dispersed in the oxidative atmospheres, and there is no change of Ag þ ion 0.5 wt.% Ag/Al2O3 by reduction with H2. Figure 21.8 shows the time dependence of NO conversion for the C3H8-SCR at 573 K over 2 wt.% Ag/Al2O3. The NO conversion was only 1% in the C3H8-SCR condition (t ¼ 0 min), but significantly increased to 68% upon addition of H2. Upon removal of H2 from the reaction mixture at t ¼ 16.5 min, the NO conversion decreased and at t ¼ 43 min reached nearly the same conversion level as before the addition of H2. In the same figure, the time course of the band height at 28 500 cm1 is also plotted as an indicator of the relative amount of Agnd þ clusters. The reversible change in the NO conversion is in harmony with the time-dependence of the band assignable to the Agnd þ cluster. The band of the Agnd þ cluster was not observed in the propane-SCR condition, but increased after the addition of H2, steeply decreased on removal of H2 in the reaction atmosphere, and finally reached nearly the original level. The results suggest that the formation of Agnd þ cluster is essential for the hydrogen effect on HC-SCR.

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21.3 The Role of Surface Adsorbed Species Analyzed by in situ FTIR 21.3.1 Reaction Scheme of HC-SCR Over Ag/Al2O3

Dynamic analysis of Ag species by in situ UV–Vis and in situ EXAFS indicated the contribution of Ag cluster to the hydrogen effect. But we still have the question as to why the addition of hydrogen promotes the activity of HC-SCR. In this section, studies on the dynamic analysis of surface adsorbed species by in situ FTIR are summarized. The measurement of in situ FTIR spectra during H2-HC-SCR over Ag/Al2O3 was carried out using a quartz in situ IR cell connected to a gas flow apparatus, as shown in Figure 21.9 [24]. The self-supporting catalyst disk (20 mm in diameter, about 0.08 g) is mounted at the center of the cell, and the thermocouple can be positioned very near to the catalyst disk. CaF2 or KBr windows are fixed on both sides of the cell, which is surrounded by water jackets to cool the windows. A reference spectrum of the catalyst wafer, which was taken in flowing He, was subtracted from each spectrum. The compositions of the feeds are the same as in the flow reaction apparatus. The kinetics of the surface adsorbed species can be determined through measuring transient phenomena of spectra by a stopped-flow technique. Figure 21.10 shows the IR spectrum of adsorbed species on Ag/Al2O3 after exposing the catalyst to a flow of NO þ O2, followed by a flow of n-hexane þ O2 on Ag/Al2O3 at 623 K [35]. Just after the pretreatment in NO þ O2 (0 min), the bands assignable to unidentate nitrate (1554, 1292 cm1) and bidentate nitrate (1580, 1246 cm1) and a small band due to weakly adsorbed NO2 (1624 cm1) were

Figure 21.9 Quartz in situ IR cell [34].

21.3 The Role of Surface Adsorbed Species Analyzed by in situ FTIR

Figure 21.10 Dynamic changes in the IR spectra as a function of time in flowing n-hexane þ O2 on Ag/Al2O3 at 623 K. Before the measurements, the catalyst was pre-exposed to a flow of NO þ O2 for 120 min at 623 K [35].

observed. The formation of nitrates proceeds by NO oxidation to NO2 followed by adsorption of NO2 on basic oxygen sites. The surface concentration of these species can be estimated from the band intensities and the extinction coefficients of these bands determined by separate experiments [34]. After switching the flow gas to n-hexane þ O2, the nitrate bands gradually decreased and new bands appeared at around 1570–1580 and 1460 cm1, assigned to nas(COO) and ns(COO) of adsorbed acetate. Shoulder bands assignable to formate (1378 and 1390 cm1) and carbonate (1630, 1410, 1300–1336 cm1) were also observed. From the time courses of the surface concentration of these species, the surface reaction rates of the adsorbed species in the stopped-flow experiments were estimated and compared with those of gas phase products in n-hexane-SCR over Ag/Al2O3. Figure 21.11 shows Arrhenius plots of the reaction rate of nitrate consumption measured by in situ FTIR, and the steady state reaction rates of NO reduction and n-hexane conversion. In the temperature range 573–648 K, the initial rates of nitrates consumption were close to the steady-state rates of NO reduction. The apparent activation energy of nitrates consumption was 61 kJ mol1, which is comparable to that for NO reduction in the gas phase (67 kJ mol1). The good agreement of these rates clearly indicates that

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Figure 21.11 Arrhenius plots of ( ) the initial rate of NO3 consumption in n-hexane þ O2, and the steady-state rates of (*) NO reduction to N2 and (~) n-hexane conversion to CO, for NO þ n-hexane þ O2 reaction over Ag/Al2O3. Condition: NO ¼ 1000 ppm, HC ¼ 6000 ppm C, O2 ¼ 10% [35].

N2 is formed via surface nitrates as intermediates. It should be noted that the apparent activation energy for NO reduction increased to 156 kJ mol1 below 573 K. The apparent activation energy for n-hexane conversion to COx also increased below 573 K. The higher activation energy at lower temperature suggests self-poisoning of nitrates on Ag/Al2O3, which can be correlated to the low HC-SCR activity of Ag/Al2O3 below 573 K. As well as the surface NOx species, the reaction rates of hydrocarbon-derived species were also estimated by in situ FTIR. Figure 21.12 shows the transient behavior of the IR spectra of surface adsorbed species after NO þ n-hexane þ O2 reaction followed by treatment in a flow of NO þ O2 [35]. During the NO þ n-hexane þ O2 reaction, at 0 min in the figure, bands due to acetate (1460 cm1), formate (1378 and 1390 cm1) and unidentate and bidentate nitrates (1292 and 1246 cm1, respectively) were observed. Minor bands are assigned to carbonyl (1720 cm1), carbonate (1630, 1410, 1300–1336 cm1), -NCO species bound to Ag þ ions (2232 cm1), and -CN species (2162 and 2130 cm1). After the flowing gas was switched to NO þ O2, the bands due to acetate (1460 cm1) decreased, while nitrates bands progressively appeared. The result indicate that acetate is a reactive species toward nitrates. The surface oxygenated hydrocarbons, including acetate [35–41], acrylate [42], and enolate species [43], are generally accepted as possible intermediates for HC-SCR. The presence of acetate species is also supported by DFT calculation reported by Gao et al. [41]. From a comparison of the theoretical IR spectra of 10 possible model oxygenated compounds with the experimental spectra in a flow of ethanol þ O2, they


Figure 21.12 Dynamic changes in the IR spectra as a function of time in a flow of NO þ O2 on Ag/Al2O3 at 623 K. Before the measurement, the catalyst was pre-exposed to a flow of NO þ n-hexane þ O2 for 120 min at 623 K [35].

concluded that the acetate species is the most likely structure on the Ag/Al2O3 surface. The reaction of nitrates with oxygenated hydrocarbon species is proposed to be a crucial step in the initial stage of the HC-SCR reaction. The bands due to Ag þ NCO and CN decreased in a flow of both NO þ O2 and O2, indicating the reaction between these species. From the kinetic analysis of these species, the main reaction pathways on Ag/Al2O3 catalyst can be depicted as shown in Scheme 21.1. These reaction pathways are common to the other alumina-based HC-SCR catalysts [36–40]. The selective reduction of NO to N2 initially proceeds by the parallel reactions: (i) the oxidation of NO to surface nitrates, and (ii) the oxidation of hydrocarbons to surface oxygenates, followed by selective reaction between these species. The surface reaction between NO + O2

step 1

NO3step 3

CnHm +

O2

step 2' step 2

CH3COO-

-NCO -CN

N2 CO2 H 2O

Scheme 21.1 Proposed reaction mechanism of HC-SCR over alumina-based catalysts.

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Figure 21.13 Dynamic changes in the IR spectra as a function of time in flowing HC þ O2 on Ag/Al2O3 at 623 K. Conditions: HC ¼ 6000 ppm C, O2 ¼ 10% [40].

nitrates and oxygenates leads to the formation of NCO and CN species, and finally leads to the formation of N2 through oxidation or hydrolysis of these nitrogencontaining species. A sufficient reaction rate of NO to N2 can be achieved in n-hexane-SCR, however, the reaction rates are too low at lower temperatures when light hydrocarbons are used as reductants. Figure 21.13 shows the dynamic changes in the acetate formation in flowing hydrocarbons þ O2 on Ag/Al2O3 at 623 K after adsorption of nitrates in a flow of NO þ O2 [40]. The rate of acetate formation is very low when propane and n-butane are used as reductants. It is clear that the low activity for partial oxidation of light hydrocarbons is the main problem with the use of Ag/Al2O3 as an HC-SCR catalyst. 21.3.2 Effect of H2 Addition on Reaction Pathways of HC-SCR Over Ag/Al2O3

Shibata et al. clearly demonstrated that the boosting of the HC-SCR activity in the presence of H2 is mainly attributable to the promotion of partial oxidation of hydrocarbons to surface oxygenates [39], and this conclusion is now generally accepted [44]. Figure 21.14 shows the dynamic changes of surface adsorbed species on Ag/Al2O3. Even in a flow of propane þ O2, the adsorbed nitrates (1251 and 1296 cm1) cannot be removed in the absence of H2. In the presence of H2, the nitrates gradually decreased and then the bands assignable to acetate (1457 cm1) appeared, indicating formation of oxygenated species significantly boosted in the


(b)

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Absorbance

180 min

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NO+O2 3h

NO+O2 3h 1800

1600

1400

1200

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1800

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Wavenumber / cm-1

Figure 21.14 Dynamic changes in the IR spectra as a function of time in a flow of propane þ O2 in the (a) absence and (b) presence of H2 on Ag/Al2O3 at 473 K. Before the measurement, the catalyst was pre-exposed to a flow of NO þ O2 for 3 h at 473 K. [40].

presence of H2. Figure 21.15 shows the time course of the acetate and nitrates concentrations in the presence and absence of H2 [40]. Clearly, the formation of acetate by the oxidation of propane and the consumption of nitrates were much higher in the presence of H2 than in the absence of H2. From the slope of the transient responses, the initial rate of nitrate consumption was below 0.1 nmol g1 s1 in the absence of H2, but increased to 75 nmol g1 s1 in the presence of H2 at a surface concentration of nitrates of 280 mmol g1. This rate corresponds well to the consumption rate of gaseous NO (68 nmol g1 s1) in the steady state. The comparison of the surface reaction rates should reflect the hydrogen effects observed in the gas phase NO conversion. The formation of acetate by propane oxidation (step 2 and 20 in Scheme 21.1) and nitrates by NO oxidation (step 1) were remarkably promoted by the addition of H2. The acceleration of step 3 by the addition of H2 is caused by the increases in concentration terms of adsorbed species, which is due to the promotion of steps 1 and 20 . Comparing the promotion effect on NO oxidation (step 1) and propane oxidation (step 2 and 20 ), the former rate is already sufficient (66 nmol g1 s1), even in the absence of H2, but the latter became two orders of magnitude faster on addition of H2 (from less than 0.1 to 19 nmol g1 s1 for step 2, and to 40 nmol g1 s1 for step 20 ). From the kinetic analysis of the adsorbed species, the promotion effect of the addition of H2 on the propane-SCR is attributed to the remarkable promotion of partial oxidation of propane to mainly surface acetate.

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C3H8 + O2 or C3H8 + O2 + H2

He purge after NO+O2 3h

50

adsorbed NO3- / μmol g-1

40 300 30 200 20 100

adsorbed CH3COO- / μmol g-1

400

10

0

0

20

40

60

0

Time / min

.

Figure 21.15 Time dependence of (*, ) nitrate and (&, &) acetate concentration in (*, &) propane þ O2 and ( ,&) propane þ O2 þ H2 on Ag/Al2O3 at 473 K. Before the measurement, the catalyst was pre-exposed to a flow of NO þ O2 for 3 h at 473 K [40].

.

21.4 Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons 21.4.1 Debates on Role of Ag Clusters

Although in situ analysis by UV–Vis and EXAFS indicated that the Ag cluster is the active species for H2-HC-SCR, there are negative results reported about the role of Ag cluster as active species [45]. Sazama et al. investigated the dynamic analysis of the formation and re-dispersion of Ag clusters on alumina under realistic reaction conditions using a homemade in situ UV–Vis spectrometer [21]. They also confirmed the presence of an additional band at around 31 000 cm1, assignable to Agnd þ clusters having cubic symmetry, [46] under the decane-SCR in the presence of H2, while only bands at 41 600 and 46 600 cm1, attributable to isolated Ag þ ions, were observed without H2. Figure 21.16 shows the response of NO conversion and the intensity of the UV–Vis band assignable to the Ag cluster (31 000 cm1) during hydrogen switching on/off. The NOx conversion and the band of the Agnd þ cluster increased sharply after H2 addition. After the hydrogen was switched off, NOx

21.4 Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons

Figure 21.16 Effect of hydrogen switching on/off in decane-SCRNO over Ag/Al2O3 at 523 K, 0.1% NO, 0.06% decane, 6% O2 and 0 or 0.2% H2. GHSV ¼ 60 000 h1. NOx consumption and intensity of the UV–Vis band at 31 000 cm1 (323 nm) as a function of time and hydrogen switch (a) on and (b) off [21].

conversion decreased with longer response within 4 min. Comparing the responses of the Agnd þ cluster and NO conversion, the dispersion of Agnd þ clusters to Ag þ ions was significantly slower than the decrease in NO conversion. From this result, they claimed that the formation of Ag cluster is not essential for the enhancement of HC-SCR activity in the presence of H2. By using in situ EXAFS, Been at al. analyzed Ag species of Ag/Al2O3 under the H2-octane-SCR conditions [47]. In their EXAFS spectra, shown in Figure 21.17, the changeofAgstructure inthepresence ofH2 was not significant. TheAgnd þ (n ca. 3from the coordination number) cluster was already present on the alumina support under octane-SCR conditions without H2 (shoulder in spectrum b). Furthermore, the Agnd þ cluster was also observed in the co-presence of CO. Although the promoting effect of octane-SCR was only observed in the presence of H2, the Agnd þ cluster is present under the octane-SCR without H2 and also under the octane-SCR with CO. They concluded that the hydrogen effect could not be attributed to changes in Ag particle size. Table 21.1 summarizes the relationships between reaction conditions, presence of Agnd þ cluster, and hydrogen effect on HC-SCR. Sazama et al. and Burch et al. have pointed out that the hydrogen effect cannot be attributed only to the formation of Agnd þ cluster because of (i) the delay in the disappearance of Ag cluster after H2 cutoff [21], and (ii) the formation of Agnd þ cluster by other non-effective reductants such as octane and CO [47]. However, most importantly, the hydrogen effect was not observed in the absence of Agnd þ clusters. It is clearly shown that neither Ag cluster nor hydrogen effect were observed in H2-propane-SCR over 0.5 wt.%Ag/Al2O3. Therefore, the formation of Agnd þ cluster is not a sufficient condition, but a necessary condition. There must be another factor for the hydrogen effect than the formation of Agnd þ cluster, which cannot be achieved by the co-presence of CO or hydrocarbons.

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Figure 21.17 Comparison of the experimental (solid line) and fitted (dashed line) pseudo-radial distribution (lower) functions from 2%Ag/alumina (a) as received, under SCR reaction conditions at 225 C with (b) no co-reductant (c) 0.72% hydrogen and (d) 0.72% carbon monoxide and (e) at 224 C in 8% hydrogen/helium [47].

What is another factor for the hydrogen effect? In relation to this question, Sazama et al. published a very interesting communication [48]. Table 21.2 shows the effect of hydrogen and hydrogen peroxide on the decane-SCR over Ag/Al2O3 at 473 K. The conversion of NO is negligible without co-feeding of hydrogen or hydrogen

Table 21.1 Relationships between reaction conditions, presence of Ag cluster, and hydrogen effect on HC-SCR.

Reaction conditions

Presence of Agndþ cluster

Hydrogen effect on HC-SCR

Reference

H2-HC-SCR in steady state 0–20 min after H2 cut-off CO-HC-SCR Octane-SCR H2-HC-SCR over 0.5 wt.%Ag/Al2O3

Yes Yes Yes Yes No

Yes No No No No

[31] [21, 31] [47] [47] [31]

21.4 Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons Table 21.2 Comparison of the effect of hydrogen and hydrogen peroxide on the decane-SCR reaction over Ag/Al2O3 at 473 K [48].

Reducing agent

X NOa Yield of N2 Yield of NO2

Decane

Decane þ H2O2

Decane þ H2

2.5 0 2.5

60.0 11.8 48.2

49.5 21.0 28.5

a

Conversion of NO to nitrogen and nitrogen dioxide.

peroxide, but the NO conversion increased significantly to 60% in the presence of 0.2% H2. The promotion effect was also observed in decane-SCR with 0.2% of hydrogen peroxide with an increase in decane conversion from nearly zero to 12%. The results suggest a role for highly reactive hydroxy and hydroperoxy radicals formed from hydrogen peroxide on HC-SCR. In the co-presence of hydrogen peroxide, the NO2 yield was higher than that with H2, which may be due to the reaction between hydroxy radicals as follows: HO2 þ NO ! NO2 þ OH. The similarity of the effect of H2 and hydrogen peroxide on the decane-SCR performance strongly suggests the role of hydroperoxy-like species during HC-SCR in the presence of H2. Since the addition of hydrogen promotes partial oxidation of hydrocarbons, the hydrogen effect can be rationalized by the formation of reactive oxygen species. It is well known that the addition of hydrogen promotes the partial oxidation of hydrocarbons such as CH4 [49], ethane [50], and so on. For example, the formation of peroxide (O22) on iron phosphate catalyst is the cause of the promotion of the selective oxidation of methane or ethane by oxygen with hydrogen [49, 50]. A similar mechanism is proposed for H2-assisted allyl alcohol epoxidation over titanosilicate catalyst; the formation of O2 intermediates via H2 þ O2 reaction is proposed as a key step [51]. In these cases, hydrogen reductively activates molecular oxygen as the electron donor as well as the proton donor. In general, the term reductive oxidation stands for the following process: O2 þ 2H þ þ 2e ! [O] þ H2O, where [O] represents reactive oxygen species. This idea can be extended to the H2-HC-SCR over Ag/Al2O3. Reductive activation of molecular oxygen on an Ag cluster was confirmed by EPR spectroscopy. Figure 21.18 shows EPR spectra recorded at 77 K for Ag/Al2O3 after various pre-treatments [31]. After the propane-SCR reaction at 573 K (spectrum a), the sample showed no EPR signal. After the H2 þ O2 reaction at 573 K (spectrum b), 2 wt.% Ag/Al2O3 showed an EPR spectrum with anisotropic g values gxx ¼ 2.001, gyy ¼ 2.009, and gzz ¼ 2.030. This signal can be identified as O2 (super oxide) ion on a silver site [52]. The EPR signals due to O2 ion were observed even after the H2-propane-SCR reaction at 573 K (spectrum c). These results clearly indicate the reductive activation of molecular oxygen to the reactive oxygen species, O2. It is important to note that no signal was observed after CO (0.5%) þ O2(10%) reaction on 2 wt.% Ag/Al2O3 at 573 K (spectrum d). In the case of 0.5 wt.% Ag/Al2O3, no EPR

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gyy=2.009 gzz=2.030

gxx=2.001 e d

c

b

a

2.04

2.02

2

1.98

g Figure 21.18 EPR spectra recorded at 77 K for Ag/Al2O3 after various pre-treatments at 573 K: (a) 2 wt.% Ag/Al2O3 after propane-SCR (b) 2 wt.% Ag/Al2O3 after H2 þ O2 reaction, (c) 2 wt.% Ag/Al2O3 after H2-propane-SCR, (d) 2 wt.% Ag/Al2O3 after CO þ O2 reaction, (e) 0.5 wt.% Ag/Al2O3 after H2-C3H8-SCR [31].

signals due to O2 ion were observed after the H2-propane-SCR reaction (spectrum e) nor after H2 þ O2 reaction (not shown). The EPR results provide direct evidence for the reductive activation of molecular oxygen into reactive oxygen species, superoxide anion (O2), which is known to be reactive toward CH bonds of hydrocarbons. 21.4.2 Reductive Activation of O2 and Promoted HC-SCR on Ag Cluster

In this final section, the role of H2 in the reductive activation of O2 on Ag cluster is discussed. Baba et al. reported that silver cations in Ag-exchanged zeolites [53] and silver salts of heteropoly acids [54] are reduced by hydrogen to generate protons

21.4 Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons

and silver clusters according to the following scheme. Ag þ þ 1=2H2 ! Ag0 þ H þ ðn1ÞAg0 þ Ag þ ! Agnþ Agnþ þ 1=2H2 ! Ag0n þ H þ On the Ag/Ag2O3 catalyst, the production of OH groups as a result of generation of protons on an alumina surface was also observed in the IR spectrum [31]. After dehydration at 823 K followed by exposure to a flow of H2 (0.5%) at 423 K, a broad band characteristic of a new OH stretching vibration was observed in the 3700–3000 cm1 region, while the intensity of the band due to the deformation vibration of water at 1617 cm1 was relatively small. This indicates the formation of OH groups as well as water as minor species. Due to the low signal/noise ratio in the OH stretching vibration region, the formation of OH species was also confirmed by reduction with D2. The resulting IR spectrum shows broad bands due to a new O–D stretching vibration at 2642, 2672, 2704, 2744, and 2772 cm1, which correspond to the OH bands with positions at 3494, 3552, 3588, 3672, and 3720 cm1. The bands at lower stretching frequency assignable to the OH (OD) group having higher Brønsted acidity are more intense than those at higher frequencies. Furthermore, the intensity of the band due to the DOD deformation vibration of water is absent, indicating the formation of H þ (D þ ). These IR results indicate that Ag þ species are reduced by hydrogen to generate acidic protons together with the formation of Agnd þ clusters. As described in Section 21.3.2, the H2 addition promotes the oxidation of hydrocarbon to surface oxygenates and the oxidation of NO to nitrates. It is clear, that the co-presence of protons and reduced Ag species is indispensable for reductive oxidation of O2 to yield reactive oxygen species (O2). This clearly explains why CO is not effective as a co-reductant and why the formation of Ag clusters is not sufficient to improve the catalytic activity. Over 0.5 wt.% Ag/Al2O3, showing no activity for NO reduction, monomeric Ag þ species are not reduced to clusters, and O2 is not produced because of the absence of reduced Ag species. The co-presence of CO leads to the formation of Ag clusters, though the condition is not sufficient because of the absence of protons. The co-presence of both the dissociated hydrogen as acidic proton and Ag cluster are indispensable for the O2 activation.

H2

Ag+

Agnδ+ +

H

C3H8 O2(+H2O) O2

NO

CH3COO-

O2-

NO3NO3-

CH3NO2

N2 (+H2O) NH3 (+COx)

Scheme 21.2 Proposed reaction scheme of H2-HC-SCR over Ag/Al2O3 catalyst.

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As discussed above, the reaction mechanism of H2-HC-SCR is proposed as shown in Scheme 21.2. The initial steps of the reaction are: (i) H2 dissociation on a Ag site, (ii) spillover of H atom to form proton, (iii) aggregation of isolated Ag to form the reduced Agnd þ cluster, (iv) O2 reduction with Agnd þ and H þ to yield O2 and H2O. Then (v) O2 activates the CH bond of hydrocarbons to yield hydrocarbon radicals, which will be finally converted to acetate ions, and (vi) NO oxidation to NO2. These intermediates react to produce N2: (vii) acetate reaction with NO2 to yield nitromethane, which is then converted to NH3 via NCO species, followed by (viii) reduction of NO2 with adsorbed NH3 (NH4 þ ) to produce N2. The reaction scheme thus obtained can rationalize the role of Agnd þ cluster as a necessary condition and the role of protons as another condition for the hydrogen effect.

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8 Iwamoto, M. and Hamada, H. (1991) Removal of nitrogen monoxide from exhaust gases through novel catalytic processes. Catal. Today, 10, 57–71. 9 Iwamoto, M. (2000) Air pollution abatement through heterogeneous catalysis. Stud. Surf. Sci. Catal., 130, 23–47. 10 Kikuchi, T. and Kumagai, M. (1998) Selective reduction of NOx in diesel engine exhaust over supported Co, Cu catalysts. Sekiyu Gakkaishi, 41, 173–174. 11 Nakatsuji, T., Yasukawa, R., Tabata, K., Ueda, K. and Niwa, M. (1998) Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection. Appl. Catal. B: Environ., 17, 333–345. 12 Er€anen, K., Lindfors, L.-E., Niemi, A., Elfving, P. and Cider, L. (2000) Influence of hydrocarbons on the selective catalytic reduction of NOx over Ag/Al2O3 laboratory and engine tests. SAE paper, 012813. 13 Lindfors, L.-E., Eranen, K., Klingstedt, F. and Murzin, D.Yu. (2004) Silver/alumina catalyst for selective catalytic reduction of NOx to N2 by hydrocarbons in diesel powered vehicles. Top. Catal., 28, 185–189. 14 Thomas, J.F., Lewis, S.A., Bunting, G.B., Storey, J.M., Graves, R.L. and Park, P.W. (2005) Hydrocarbon selective catalytic reduction using a silver-alumina catalyst

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23 Richter, M., Bentrup, U., Eckelt, R., Schneider, M., Pohl, M.-M. and Fricke, R. (2004) The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al2O3. Appl. Catal. B: Environ., 51, 261–274. 24 Zhang, X., He, H. and Ma, Z. (2007) Hydrogen promotes the selective catalytic reduction of NOx by ethanol over Ag/Al2O3. Catal. Commun., 8, 187–192. 25 Richter, M., Fricke, R. and Ecklt, R. (2004) Unusual activity enhancement of NO conversion over Ag/Al2O3 by using a mixed NH3/H2 reductant under lean conditions. Catal. Lett., 94, 115–118. 26 Shimizu, K. and Satsuma, A. (2007) Reaction mechanism of H2-promoted selective catalytic reduction of NO with NH3 over Ag/Al2O3. J. Phys. Chem. C, 111, 2259–2264. 27 Satsuma, A., Shibata, J., Shimizu, K. and Hattori, T. (2005) Ag cluster as active species for HC-SCR over Ag-zeolites. Catal. Surv. Asia, 9, 75–85. 28 Shimizu, K., Higashimata, T., Tsuzuki, M. and Satsuma, A. (2006) Effect of hydrogen addition on SO2-tolerance of silveralumina for SCR of NO with propane. J. Catal., 239, 117–124. 29 Breen, J.P., Burch, R., Hardacre, C., Hill, C.J., Krutzsch, B., Bandl-Konrad, B., Jobson, E., Cider, L., Blakeman, P.G., Peace, L.J., Twigg, M.V., Preis, M. and Gottschling, M. (2007) An investigation of the thermal stability and sulphur tolerance of Ag/g-Al2O3 catalysts for the SCR of NOx with hydrocarbons and hydrogen. Appl. Catal. B: Environ., 70, 36–44. 30 Satsuma, A., Shibata, J., Wada, A., Shinozaki, Y. and Hattori, T. (2003) In-situ UV-visible spectroscopic study for dynamic analysis of silver catalyst. Stud. Surf. Sci. Catal., 145, 235–238. 31 Shimizu, K., Tsuzuki, M., Kato, K., Yokota, S., Okumura, K. and Satsuma, A. (2007) Reactive activation of O2 with H2 reduced silver clusters a key step in the H2promoted selective catalytic reduction of

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NO with C3H8 over Ag/Al2O3. J. Phys. Chem. C, 111, 950–959. Shibata, J., Shimizu, K., Takada, Y., Shichi, A., Yoshida, H., Satokawa, S., Satsuma, A. and Hattori, T. (2004) Structure of active Ag clusters in Ag-zeolites for SCR of NO by propane in the presence of hydrogen. J. Catal., 227, 367–374. Suzuki, Y., Matsumoto, N., Aitani, T., Miyanaga, T. and Hoshino, H. (2005) In situ infrared and EXAFS studies of an Ag cluster in zeolite X. Polyhedron, 24, 685–691. Satsuma, A. and Shimizu, K. (2003) In-situ FT/IR study of selective catalytic reduction of NO over alumina-based catalysts. Prog. Energy Combust. Sci., 29, 71–84. Shimizu, K., Shibata, J., Yoshida, H., Satsuma, A. and Hattori, T. (2001) Silveralumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism. Appl. Catal. B: Environ., 30, 151–162. Shimizu, K., Kawabata, H., Satsuma, A. and Hattori, T. (1998) Formation and reaction of surface acetate on Al2O3 during NO reduction by C3H6. Appl. Catal. B: Environ., 19, L87–L92. Shimizu, K., Kawabata, H., Satsuma, A. and Hattori, T. (1999) Role of acetate and nitrates in the selective catalytic reduction of NO by propene over alumina catalyst as investigated by FTIR. J. Phys. Chem. B, 103, 5240–5245. Shimizu, K., Kawabata, H., Maeshima, H., Satsuma, A. and Hattori, T. (2000) Intermediates in the selective reduction of NO by propene over Cu-Al2O3 catalysts: transient in situ FTIR study. J. Phys. Chem. B, 104, 2885–2893. Shimizu, K., Shibata, J., Satsuma, A. and Hattori, T. (2001) Mechanistic causes of hydrocarbon effect on the activity of Ag-Al2O3 catalyst for selective reduction NO. Phys. Chem. Chem. Phys., 3, 880–884. Shibata, J., Shimizu, K., Satokawa, S., Satsuma, A. and Hattori, T. (2003) Promotion effect of hydrogen on surface steps in SCR of NO by propane over

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53 Baba, T., Komatsu, N., Sawada, H., Yamaguchi, Y., Takahashi, T., Sugisawa, H. and Ono, Y. (1999) 1 H magic angle spinning NMR evidence for dissociative adsorption of hydrogen on Ag þ exchanged A- and Y-zeolites. Langmuir, 15, 7894–7896. 54 Baba, T., Nimura, M., Ono, Y. and Ohno, Y. (1993) Solid-state proton MAS NMR study on the highly active protons in partially reduced trisilver dodecatungstophosphate (Ag3PW12O40). J. Phys. Chem., 97, 12888–12893.

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22 Dynamic Structural Change of Pd Induced by Interaction with Zeolites Studied by Means of Dispersive and Quick XAFS Kazu Okumura

22.1 Introduction

X-ray absorption fine structure (XAFS) is a useful technique for the analysis of the local structure of heterogeneous catalysts, whose structural information is otherwise difficult to obtain. In the conventional XAFS technique, the data collection is carried out under static conditions using double monochromators that are moved stepwise. Along with the recent development of various techniques for high-speed measurement, that is, QXAFS (quick XAFS) and DXAFS (dispersive XAFS), in situ measurements were realized. In QXAFS, the double monochromator is moved continuously to obtain a monochromatic X-ray beam. On the other hand, in DXAFS, the whole energy region is collected at the same time using a bent polychromator, as shown in Figure 22.1. Bragg and Laue configurations are chosen, depending on the X-ray energy region for XAFS measurements. Using these techniques combined with an appropriate cell and a gas-flow line, in situ measurements of the various chemical processes, such as formation of the active sites in heterogeneous catalysts and clustering of metal atoms, have been realized [1, 2]. In recent years, much effort has been devoted to the design of various in situ cells and studies combined with other techniques, that is, XRD and a mass spectrometer. Hannemann et al. proposed a versatile in situ cell for fluorescence/transmission EXAFS and XRD of heterogeneous catalysts in the gas and liquid phases [3]. Clausen et al. and Sankar et al. used a capillary cell for combined XRD-EXAFS studies [4, 5]. A capillary-type high pressure cell was also designed by Bare et al. [6]. Grunwaldt et al. designed various types of in situ cell and applied them to kinetic studies of the reduction of CuO/ZnO and PdO/ZrO2 catalysts [7]. They reported the two-dimensional distribution of Rh with different valence states in the course of the partial oxidation of methane, which was realized through the combination of XAFS, mass spectroscopy and a CCD camera [8]. The local structure of Al in zeolites was measured by in situ Al K-edge XAFS investigation [9–13]. In this chapter, we will focus on our studies concerning the structural change of Pd induced by the metal–support interaction with zeolite supports. Small metal clusters occluded in a zeolite pore

j 22 Dynamic Structural Change of Pd Induced by Interaction with Zeolites

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Figure 22.1 An energy-dispersive XAFS instrument installed in SPring-8 BL28B2. Bragg (Ex-ray < 12 keV) (a), and Laue configurations (Ex-ray > 12 keV) (b).

have been studied primarily from the viewpoint of the formation of uniform active sites for catalytic reactions. The introduction of metals into zeolites has been achieved through various techniques including chemical vapor deposition (CVD), the ship-inbottle method and decarbonylation of carbonyl clusters in zeolite pores [14–18]. Palladium clusters in zeolites have been paid special attention since Pd exhibits high catalytic activity in many valuable reactions, such as the selective reduction of NO, the total combustion of hydrocarbons and organic reactions. Recently, small metal Pd clusters were obtained through the introduction of Pd into the pores of Na-Y zeolite, followed by a successive reduction at low temperatures. Furthermore, it was found that the location of the metal Pd cluster in X and Y-zeolites was significantly affected by the calcination temperature [19, 20]. We have focused on the influence of the Brønsted acid site as well as the structure of zeolite on the generation of metal Pd clusters and highly dispersed PdO. This is because it has been revealed that the structure and the acid sites of zeolites remarkably affect the generation of active sites for catalytic reactions. The catalytic performance of Pd was found to be highlyly dependent on the structure and composition of the zeolite support [21, 22]. This suggested that the strong metal– support interaction between PdO and Brønsted acid sites is an important factor not only in the generation of the active Pd center but also in its catalytic performance. The interaction was directly evidenced from the structural change in Pd induced by the Brønsted acid sites of zeolites in an oxidative or reductive atmosphere. However, the formation and structure of the active Pd species or its precursor and the role of the Brønsted acid sites associated with Pd are rather ambiguous. In this study, DXAFS and QXAFS techniques were utilized to follow the dynamic structural change of Pd induced by interaction with the Brønsted acid sites of zeolites.

22.2 Formation and Structure of Highly Dispersed PdO Interacted with Brønsted Acid Sites [23–25]

First, H-ZSM-5 zeolites with different Al content were employed as supports for palladium and the structure of Pd was measured by Pd K-edge EXAFS in the static

22.2 Formation and Structure of Highly Dispersed PdO Interacted with Brønsted Acid Sites

Figure 22.2 k3-weighted Pd K-edge EXAFS Fourier transforms of 0.4 wt% Pd loaded on H-ZSM-5 with different Al concentrations (a) and H-ZSM-5 (Si/Al2 ¼ 24) treated with H2, and O2 at 773 K (b).

mode. The catalyst exhibited high activity for the selective reduction of NO with methane in an atmosphere of excess O2 [26]. Figure 22.2a shows the Fourier transforms of the k3c(k) EXAFS for Pd/H-ZSM-5 with different Al content. All samples were oxidized under an oxygen flow at 773 K for 3 h before the measurement. With increasing Al content of the H-ZSM-5, the intensity of the PdPd peaks appearing at 0.26 and 0.31 nm gradually decreased, and finally disappeared for the Pd/H-ZSM-5 with the highest Al content (Si/Al2 ¼ 24), where high activity toward the NOCH4O2 reaction was observed. The results indicated that the size of PdO is a function of the amount of Brønsted acid in H-ZSM-5 and decreases with an increase in the acid amount, since the intensity of the PdPd shell seems to reflect the size of PdO. On the other hand, for the PdO bond observed at 0.16 nm (phase shift uncorrected) in Figure 22.2a, the spectra for bulk PdO and highly dispersed PdO on H-ZSM-5 are quite similar. In addition, the coordination number and bond distance of PdO determined by curve fitting analysis on highly dispersed PdO agreed well with those for bulk PdO, implying that the local structure of highly dispersed PdO is closely similar to that of bulk PdO. Therefore, it can be noted that the role of the Brønsted acid sites of H-ZSM-5 is not to provide ionexchange sites for Pd2 þ but to stabilize the dispersed state of PdO. Based on the analysis, the local structure of Pd in the oxidized Pd/H-ZSM-5 was proposed by Liu et al. as Pd surrounded by four oxygen atoms in a square planar arrangement, a part of which came from the zeolite structure [27]. In order to confirm the ability of zeolite Brønsted acid sites to anchor PdO, the regeneration of dispersed PdO with repeated reduction and oxidation treatments was followed by EXAFS. The experiment was conducted on the Pd/H-ZSM-5 (Si/Al2 ¼ 24) where highly dispersed PdO was observed by the oxidation treatment, as described above. Figure 22.2b shows the EXAFS FT spectrum measured after the reduction of previously oxidized Pd/ H-ZSM-5. The formation of metal Pd was confirmed by the appearance of an intense

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peak at 0.24 nm (phase shift uncorrected). The particle size of the metal Pd calculated from the PdPd coordination number (CN ¼ 10.6) was estimated to be >3 nm, which was far larger than the zeolite pore diameter. Thus, with treatment under H2, the highly dispersed PdO was reduced and migrated to form aggregated Pd metal particles on the external surface of H-ZSM-5. The reduced sample was subsequently oxidized again under oxygen flow at 773 K for 3 h. The spectrum measured after oxidation was identical with that measured after initial oxidation treatment, as shown in Figure 22.2b. Therefore, the aggregated Pd de-aggregated and returned to the H-ZSM-5 pores as the highly dispersed PdO. This behavior of Pd demonstrates the high mobility of PdO and the presence of strong interaction between the Brønsted acid sites of H-ZSM-5 and PdO. Probably, the acid–base interaction between highly dispersed PdO and Brønsted acid sites of zeolite promoted the de-aggregation and fixation of highly dispersed PdO.

22.3 Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and Reversible Formation of Stable Pd Clusters in H-ZSM-5 and H-Mordenite [28, 29]

As described in the previous section, we found that the agglomerated metal Pd spontaneously migrated into zeolite pores to form the molecular-like dispersed PdO on acid sites of H-ZSM-5 under an O2 atmosphere at elevated temperatures. From these findings, it was postulated that the Brønsted acid sites played a key role in the generation of metal Pd clusters. In order to further reveal the dynamic behavior of Pd with zeolite supports, we have tried to follow the local structure of Pd in the course of the clustering process of metal Pd supported on H-form zeolites in an atmosphere of H2. For these purposes, an in situ DXAFS experiment was applied to the determination of the Pd structure during temperature programmed reduction. Figure 22.3a and c show the coordination number (CN) of the nearest neighboring PdPd (metal Pd) peak determined based on the curve-fitting analysis of the EXAFS spectra for HZSM-5 and H-Mordenite, respectively. A slight increase in the PdPd was observed from the beginning of the reduction. At the same time, the CN of the PdO bond decreased, suggesting that reduction of PdO to metal Pd took place up to 440 K. After completion of the reduction, the CN of the metal PdPd remained constant at 4 between 440 K and 620 K on both H-ZSM-5 and H-Mordenite. The appearance of this plateau indicated the generation of a stable Pd cluster in this temperature region. From the CN value, the Pd cluster was estimated to consist of six atoms. On further heating of the samples under flowing 8% H2, the CN increased steeply from 620 to 770 K. Probably, the Pd6 cluster migrated into the external surface of the zeolites to form agglomerated metal Pd particles. The change in CN on Pd/H-Mordenite was similar to that on Pd/H-ZSM-5, except that the growth of metal Pd observed above 620 K was steeper on the H-Mordenite. After the measurements given in Figure 22.3a and c, the samples were oxidized at 773 K for 4 h in an O2 flow. Then the in situ cell was cooled to room temperature and temperature programmed reduction was carried out in an 8% H2

22.3 Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO

Figure 22.3 Dependence of coordination number of Pd–Pd (.) and Pd–O (~) on the temperature measured in an 8% H2 flow; Pd/H-ZSM-5(a,b); Pd/H-Mordenite (c,d).

flow again. Figure 22.3b and d shows the CN of H-ZSM-5 and H-Mordenite measured during the second run, respectively. A similar pattern for the change of CN (PdPd) to that in the first runs was observed on both H-ZSM-5 and H-Mordenite. That is to say, a plateau of CN (PdPd) was observed from 440 to 620 K, similar to the first run of the experiment. Therefore, it was confirmed that the generation of a stable metal Pd cluster was reversible upon oxidation with O2 at 773 K and successive reduction with H2, as illustrated in Figure 22.4. The phenomenon

Figure 22.4 Reversible structural change of Pd induced by the interaction with acid sites of H-ZSM-5 and H-Mordenite.

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could be explained by the re-dispersion of the agglomerated metal Pd onto the acid sites of zeolites through oxidation at 773 K in an O2 flow, as evidenced in the previous section.

22.4 In Situ QXAFS Studies on the Dynamic Coalescence and Dispersion Processes of Pd in USY Zeolite [30]

As described in the previous section, we found that Pd6 clusters were obtained through the introduction of Pd into the pores of H-ZSM-5 and H-Mordenite and a successive reduction in a H2 stream. The formation of the Pd6 clusters was observed to be reversible upon repeated treatments with O2 and H2. Such regeneration of clusters was promoted through the spontaneous migration of molecular PdO onto the acid sites of H-type zeolites under an O2 atmosphere, which was evident from the EXAFS analysis. Resasco et al. also reported that the morphology of the oxidized Pd species strongly depended on the acidity of the support [17]. From these findings, it was postulated that the acid sites played a key role in the generation of metal Pd clusters inside zeolites. In fact, in contrast to the H-type zeolites, the severe aggregation of Pd progressed over the Na-type zeolite without the formation of stable metal clusters. Here, the dynamic behavior of Pd in the pore of a USY zeolite was measured in atmospheres of H2 (TPR) and O2 (TPO). The Pd clusters generated in the supercage of an FAU-type zeolite were active and reusable in Heck reactions [31]. The Pd or bimetallic PdPt supported on the USYzeolite has been found to exhibit high sulfur tolerance in the hydrogenation of aromatics and hydrodesulfurization activity [32]. For this purpose, the QXAFS technique was first applied to detect the detailed structural change of Pd in the USY zeolite. TPR and TPO measurements were repeated one after another to follow the structural change of Pd induced by reduction and oxidation; first, the changes in the structure of Pd loaded on the USY zeolite were measured by using the QXAFS technique in an atmosphere of hydrogen (first TPR). The Fourier transforms of the k3c(k) EXAFS collected after every 10 K are given in Figure 22.5. Initially, a single PdO peak could be seen at 0.16 nm, which was directly assigned to the PdO bond characteristic of PdO. However, the PdPd bonds that are characteristic of bulk PdO were not seen in the spectrum; this implies the formation of highly dispersed PdO in the initial stage. The intensity of the PdO peak gradually decreased. This was accompanied by an increase in the temperature, and a new peak attributable to the PdPd bond of Pd metal appeared at 0.24 nm as a result of the reduction of the dispersed PdO. On further increasing the temperature above 673 K, a new peak appeared at 0.18 nm (phase shift uncorrected), which corresponded to a longer bond in comparison to the covalent PdO bond of PdO. The peak could probably be assigned to oxygen in the framework of the USY zeolite (denoted as PdOsurface1) by considering that Pd was already reduced to Pd0. The CNs of these bonds were determined by curve-fitting analysis and the data are summarized in Figure 22.6.

22.4 In Situ QXAFS Studies on the Dynamic Coalescence

Figure 22.5 Pd K-edge EXAFS Fourier transforms for Pd/USY measured in the TPR (r.t.–773 K). Fourier transforms range, 30–120 nm1.

In the initial step of the first TPR, the CN of the PdO bond (D) decreased up to 523 K, which alternated with an increase in the CN of the Pd–Pd bond (.) up to 7.5 at 673 K. The CN of the Pd–Pd bond decreased with further increase in the temperature, indicating the dispersion of previously agglomerated Pd metal at elevated temperatures. At the same time, the CN of the PdOsurface1 bond (&) increased, accompanied by an increase in the temperature. This fact suggested that a strong interaction between the framework of the USY zeolite and the Pd led to the dispersion of the Pd metal. This phenomenon appears to be interesting, taking into account that heating at a high temperature usually results in severe sintering of the metal. The sample was cooled to room temperature and the first TPO experiment was subsequently carried out after switching the flowing gas to an 8% O2 flow. The CNs determined from the spectra are given in Figure 22.7 (1st).

Figure 22.6 Coordination numbers determined by curve fitting analysis plotted as a function of temperature measured in TPRs. (.) PdPd (metal), (~) PdO, (&) PdOsurface1.

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Figure 22.7 Coordination numbers determined by curve fitting analysis plotted as a function of temperature measured in TPOs. (.) PdPd (metal), (~) PdO, (&) PdOsurface1.

The CN of PdOsurface1 decreased up to 500 K. On the other hand, the CN of the Pd metal reached 7.1 at 513 K. This change implied that removal of the Pd metal clusters from the framework of the zeolite and agglomeration took place in the initial step. On further increasing the temperature, the CNs of PdPd decreased and, in turn, the CN of covalent PdO increased. Despite the disappearance of the PdPd bond attributable to the Pd metal at 773 K, the PdPd bond of PdO did not appear, suggesting the formation of highly dispersed PdO. After the first TPO experiment, the sample was cooled to room temperature and the second TPR experiment was carried out after switching the flowing gas to an 8% H2 flow. The CNs determined by the curve-fitting analysis are shown in Figure 22.6 (2nd). As can be seen in the Fourier transforms, the PdPd bond of Pd metal was already observed at 313 K. With increasing temperature, the covalent PdO decreased and finally disappeared at 500 K to yield Pd0. The CN of Pd metal obtained at 500–573 K was as low as 2.7, indicating the formation of extremely small metal Pd clusters having about 4 atoms. Subsequently, the CN(PdPd) of the Pd clusters started to increase to a temperature above 600 K. At the same time, the PdOsurface1 bond appeared. The appearance of the PdOsurface1 bond suggested a strong interaction between the Pd clusters and the zeolite wall, as observed in the first TPR. This change implied that the Pd4 clusters agglomerated and migrated onto the wall of the USY zeolite. The changes in the CNs in the second TPO experiment are summarized in Figure 22.7 (2nd). The profile of the second TPO was different from that of the first TPO in that the oxidation of Pd clusters progressed without an increase in the CN (PdPd). As a result, the removal of the Pd clusters from the zeolite framework and their oxidation took place simultaneously in the second TPO. The curve-fitting analysis of the EXAFS data obtained in the third TPR run is included in Figure 22.6 (3rd). The changes in the CNs were very similar to those of the second run. In other words, the formation of Pd4 clusters was observed after the disappearance of the

22.5 Time-Resolved EXAFS Measurement of the Stepwise

Figure 22.8 A proposed structural change of Pd in the course of temperature programmed reduction and oxidation.

PdO bond; this was followed by an increase in the CN(PdPd) and the appearance of the PdOsurface1 bond at an elevated temperature. This implied that the formation of the Pd4 clusters was reversible upon repeated oxidation and reduction treatments. The proposed structural change of Pd in the USY zeolite is summarized in Figure 22.8. In the first TPR process, dispersed PdO was reduced to Pd0, followed by agglomeration to give Pd clusters inside the supercage of USY. In the subsequent first TPR, the Pd0 clusters migrated and were dispersed on the acid sites of the sodalite cage as the PdO form. In the second TPR process, Pd was reduced to give Pd4 clusters inside the sodalite cage, followed by agglomeration to larger clusters. The Pd4 clusters were generated repeatedly in further TPO and TPR processes.

22.5 Time-Resolved EXAFS Measurement of the Stepwise Clustering Process of Pd Clusters at Room Temperature [33]

Finally, the time-resolved QXAFS technique was applied to follow the clustering process of Pd in the cages of H-USY zeolite. The measurements carried out in atmospheres of H2 and O2 were repeated successively at room temperature; first, the changes in the structure of the Pd loaded on the H-USY zeolite were measured using the QXAFS technique in an H2 atmosphere. The Fourier transforms of the k3c(k) data of 0.4 wt%-PdðNH3 Þ2þ 4 /H-USY collected after every 0.6 min are given in Figure 22.9. Although it was difficult to distinguish between PdO and PdN due to the similarity of the backscatters, the peak that appeared at 0.16 nm in the initial stage may be assigned either to the PdN bond of Pd(NH3)42 þ or the PdO bond of H2O

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Figure 22.9 Pd K-edge EXAFS Fourier transforms for 0.4 wt%-Pd/ H-USY measured in the atmosphere of 8% H2. Fourier transforms range, 30–130 nm1.

coordinated to Pd2 þ . The intensity of the PdO(N) peak gradually decreased with time. This was accompanied by an increase in a new peak attributable to the PdPd bond of Pd metal at 0.24 nm as a result of the reduction of Pd2 þ . In this process, the color of the sample gradually changed from white to the gray that is characteristic of Pd0. Then the flowing gas was switched to 8% O2 for 20 min, followed again by 8% H2; at this point, the second QXAFS measurement was carried out. In the second step, a small PdO bond could be seen in the initial stage, indicating that the metal Pd clusters generated in the first step were partially oxidized by exposure to 8% O2. The intensity of the PdO bond decreased while that of the PdPd bond increased quickly within 3 min after switching to 8% H2. It was clear that the intensity of the metal PdPd bond became larger than in the first step. A similar change was observed on further switching the flowing gas to O2 and then to H2 (third and fourth steps). A comparison of the spectra at 20 min. revealed that the intensity of the PdPd bond increased in a stepwise fashion, suggesting that the sizes of the Pd clusters increased with the repetition of the O2 and H2 exposures. The CNs of the PdO(N) and PdPd bonds were determined by curve-fitting analysis; the data are summarized in Figure 22.10. In the first H2 exposure, the CN of the PdO(N) bond decreased up to about 10 min, accompanied by an increase in the CN of the PdPd bond. The growth of the PdPd bond stopped when the CN of PdPd reached 5.1. A small PdO bond appeared on exposure to O2, as can be seen at the beginning of the second exposure to H2. In the second exposure, the partially oxidized Pd clusters were quickly reduced on exposure to H2, in less than 2 min; the CN reached 6.7, which was larger than after the first process. In the third and fourth runs, a similar change in the CN was observed, where the ultimate CN of PdPd reached 7.8 and 8.7 after the third and fourth runs, respectively. The progressive increase in the CN with the number of H2 exposure cycles suggests that stepwise growth of Pd clusters occurred in the HUSY support. Information on the valence state of Pd can be obtained from the analysis of the Xray absorption near-edge structure (XANES) region. The calculated ratio of Pd0 and the total amount of Pd in the 0.4 wt%-Pd/H-USY are summarized in Figure 22.11.

22.5 Time-Resolved EXAFS Measurement of the Stepwise

Figure 22.10 CNs of PdO(N) and nearest-neighboring PdPd bonds determined by curve-fitting analysis of 0.4 wt%-Pd/H-USY plotted as a function of time measured in an 8% H2 flow: (.) PdPd (metal); (*) PdO or PdN.

It can be seen that the reduction of Pd2 þ proceeded slowly and was complete in 20 min. from the start of the introduction of H2 in the first exposure. Unlike in the first H2 exposure, the reduction of Pd in the second, third, and fourth exposures was quickly completed, in less than 3 min. In the latter cases, 10%–20% of the Pd content was oxidized after the introduction of O2 and before the admission of H2. These facts are consistent with the change in the PdO and PdPd bonds, as observed in the Fourier transforms of Figure 22.9. The oxidation state of Pd in the first H2 exposure was kinetically analyzed using the data of Figure 22.11: the first-order rate constant k was determined to be 0.28 min1. In addition, the first-order rate constant of the CN of the PdPd bond was determined independently, based on the data of Figure 22.10. The obtained k value was 0.35 min1, which is close to that of the kinetic constant k for the reduction of Pd2 þ , suggesting the reduction of Pd2 þ and the coalescence of Pd clusters progressed simultaneously in H-USY. As demonstrated here, in situ QXAFS was effectively applied to follow precisely the clustering process of Pd in zeolites to show that bare Pd clusters were

Figure 22.11 Relative concentrations of Pd0 in 0.4 wt%-Pd/ H-USY plotted as a function of time, measured in an 8% H2 flow.

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Figure 22.12 A proposed stepwise growth of Pd clusters in H-USY at room temperature.

easily formed at room temperature on exposure to H2. The maximum CN was close to that of Pd13 clusters (CN ¼ 5.5), corresponding to the one-shell structure of a cuboctahedron. The formed clusters were stable up to 443 K, as evidenced by temperature-programmed measurements (not shown). Another finding was that the Pd clusters grew in a stepwise manner on repeated exposure to O2 and H2 to form larger clusters after the second and third stages of H2 flow, as illustrated in Figure 22.12.

22.6 Summary

We successfully applied the DXAFS and QXAFS techniques to follow the versatile structural change of Pd induced by interaction with zeolite supports. That is to say, from the measurements using different kinds of zeolites, the dynamic behavior as well as the formation of various Pd clusters was revealed. In an oxygen atmosphere, the spontaneous dispersion of PdO was observed on H-ZSM-5 and H-Mordenite. Furthermore, it was revealed that the crystal structure and acid sites of zeolites had a profound effect on the generation of stable metal Pd clusters in an atmosphere of H2. In the case of H-ZSM-5 and H-Mordenite, the formation of Pd6 cluster was observed. The formation was repeatedly observed upon oxidation and successive reduction with H2. In the case of Pd loaded on USY zeolite, complex behavior of Pd was observed through interaction with the framework structure in the temperature programmed measurement under an atmosphere of H2 and O2. Furthermore, time-resolved measurement revealed the clustering and stepwise growth process of Pd over Pd2 þ /USY at room temperature. I believe that the finding obtained here sheds light on the importance of the metal–support interaction in Pd/zeolite catalysts.

Acknowledgments

The author is very grateful to SPring-8 staff: Dr T. Uruga, Dr H. Tanida, Dr T. Honma, Mr K. Kato, and Ms S. Hirayama for technical support. The present work is supported by the Grant-in-Aid for Scientific Research (KAKENHI) in Priority Area Molecular Nano Dynamics from the Ministry of Education, Culture, Sports, Science and Technology.

References

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Part Four Single Crystals

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23 Morphology Changes of Photochromic Single Crystals Seiya Kobatake and Masahiro Irie

23.1 Introduction

Molecular materials that change shape and/or size reversibly in response to external stimuli such as light have attracted much attention as photomechanical actuators because the materials can allow remote operation without any direct contact. The photomechanical phenomena are potentially induced by the photoisomerization of constituent molecules. Reversible photo-transformation reactions of a chemical species between two isomers having different absorption spectra are called photochromism [1, 2]. The two isomers differ from one another not only in the absorption spectra but also in various physical and chemical properties, such as refractive indices, dielectric constants, and oxidation–reduction potentials. The instant property changes upon photoirradiation without processing lead to their use in various optoelectronic devices, such as optical memory [3–6], photoswitching [7, 8], display materials [9, 10], and nonlinear optics [11, 12]. In addition to the above electronic property changes, the photochromic compounds change their geometrical structures during photoisomerization. In this chapter, we focus on the geometrical structure changes and describe the photochromic reactions of diarylethene derivatives in the single-crystalline phase and their photomechanical phenomena. Although many photochromic compounds have already been reported, compounds that show photochromic reactions in the crystalline phase are rare [13]. Typical photochromic compounds such as spiropyran and azobenzene do not show any photochromism in the crystalline phase because large geometrical structure changes are prohibited in the crystals. Typical examples of crystalline photochromic compounds are paracyclophanes [14], triarylimidazole dimer [15, 16], diphenylmaleronitrile [17], aziridines [18], 2-(2,4-dinitrobenzyl)pyridine [19–22], N-salicylideneanilines [23–25], and triazenes [26]. Figure 23.1 shows some examples of crystalline photochromic compounds. In many cases, their photogenerated isomers are thermally unstable. Thermally irreversibleandfatigue-resistantphotochromic diarylethene crystals have been developed in the past decade. The colored isomers are stable in the crystals, even at 100 C and hardly return to the initial colorless isomers in the dark. The thermally

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Figure 23.1 Typical examples of photochromic crystals.

irreversible crystalline photochromic materials are potentially applicable not only to optical memory, photoswitching, and display, but also to photomechanical actuators.

23.2 Photochromic Diarylethene Crystals

Some diarylethene derivatives were found to undergo reversible photochromic reactions in the single-crystalline phase as shown in Figure 23.2. Figure 23.3 shows the typical color changes of several diarylethene single crystals [27]. Upon irradiation with ultraviolet light, the colorless crystals change to yellow, red, blue, or green, depending on the molecular structure of the diarylethenes. The colors of the crystals are due to the formation of the closed-ring isomers. The colors remain stable in the dark, but they disappear on irradiation with visible light. The photoinduced coloration/ decoloration cycles of the crystals can be repeated more than 104 times without any destruction of the crystals. Upon irradiation with ultraviolet light, the light penetrates the crystals in the bulk, and the photochromic reaction takes place not only on the crystal surface but also inside the crystal.

23.3 X-Ray Crystallographic Analysis

The molecular and geometrical structure changes in the diarylethene crystals can be directly observed by X-ray crystallographic analysis. When diarylethene crystal 3 was

23.3 X-Ray Crystallographic Analysis

Figure 23.2 Diarylethene derivatives that show photochromism in the crystalline phase.

Figure 23.3 Photochromism of diarylethene derivatives in the single-crystalline phase.

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irradiated with polarized 360 nm light, the unit cell dimension changed according to the photochromic reaction [28]. All unit cell lengths and unit cell volumes tended to decrease during the photocyclization. This corresponds to the decrease in the molecular volume by the transformation from the open-ring isomers to the closed-ring ones. The crystal irradiated for 24 h was analyzed by single-crystal X-ray diffraction [28]. The difference Fourier electron density map of the crystal indicates the existence of two quite high electron density peaks corresponding to the sulfur atoms of the photogenerated closed-ring isomer. The locations of these peaks are close to positions expected for the closed-ring isomer photogenerated in a conrotatory mode. Electron density peaks corresponding to two carbons at the reacting points also appeared. Figure 23.4 shows ORTEP drawings of the molecular structures that are a mixture of open- and photogenerated closed-ring isomers. The occupancy factor for the photogenerated closed-ring isomer was 0.084(2), which indicates that about 8% of the molecules in the crystal underwent the photocyclization reaction upon 360-nm light irradiation. The molecular structure of the photogenerated closed-ring isomer was compared with that of the isolated closed-ring

Figure 23.4 ORTEP drawings of diarylethene 3 upon irradiation with ultraviolet light. The open-ring isomer and the photogenerated closed-ring isomer are shown in black and gray, respectively.

23.4 Reactivity in the Crystal

isomer. The structural difference between the photogenerated closed-ring isomer in crystal 3 and the isolated closed-ring isomer in the crystal appeared as the difference of the distance between two sulfur atoms [29]. The structure of the closed-ring isomer produced in the open-ring form crystal is distorted. The structure difference was reflected in the absorption maximum of the closed-ring isomer. The closed-ring isomer in the closed-ring form crystal had an absorption maximum at 485 nm and an edge at 610 nm [29]. However, the photogenerated closed-ring isomer in the openring form crystal shifted to longer wavelength. The maximum was observed at 535 nm with an edge at 650 nm. The red shift of the absorption maximum of the closed-ring isomer is ascribed to the strained structure [29].

23.4 Reactivity in the Crystal

It is of interest to know the reactivity in the crystalline phase. In most cases, the cyclization quantum yields of diarylethenes in crystal were twice as large as those in solution. The low quantum yield in solution is due to the presence of molecules in photoinactive parallel conformation. On the other hand, the cyclization quantum yields in the crystal were around unity (100%). This means that photon energy absorbed in the crystal is used quantitatively for the cyclization reaction. The single crystal utilizes all absorbed photons for the coloration chemical reaction. X-ray crystallographic analysis of the crystals indicated that diarylethene molecules in the crystals were fixed in the antiparallel conformation. The distances between the reactive carbon atoms were estimated to be 3.48–3.96 A, which are close enough for the conrotatory cyclization reactions. Figure 23.5 shows a correlation between the cyclization quantum yields of diarylethenes in crystals and the distances between the reactive carbon atoms of the diarylethenes [30]. When the distance is larger than 4.2 A, the photocyclization reaction in the crystal is suppressed. The reaction process was analyzed based on ab initio and DFT calculation of the initial geometries, the

Figure 23.5 Relationship between photocyclization quantum yield and distance between the reacting carbon atoms.

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relaxation from the Franck–Condon states, the shapes of the potential energy surface of the ground states, and the geometry change by the large amplitude motions [31]. The large cyclization quantum yield in the crystalline phase can be ascribed to three factors. One is a high population of the photoreactive antiparallel conformation in the crystalline phase, in which the distance between the reacting carbon atoms is less than 4 A. All photoexcited molecules in the antiparallel conformation fixed in the crystal lattice readily undergo the photocyclization reactions. Other factors are the very low activation energy of the conrotatory cyclization reaction [32, 33] and the rapid cyclization rate, less than 10 ps [34]. The rapid reaction rate prevails over other relaxation processes, such as radiative and nonradiative transitions from the excited states to the ground state.

23.5 Photomechanical Effect

The photoinduced deformation phenomenon of materials is called a photomechanical effect, and it has been so far reported for photoresponsive polymer films and gels [35–43]. When azobenzene is isomerized from the trans form to the cis form, the length of the molecule is shortened from 0.90 to 0.55 nm. The size change of the molecule on photoirradiation is expected to alter the shape of the polymers which contain the azobenzene molecules. However, it is not the case in polymer systems. The transformation in polymer films does not change the polymer shape because of the large free volumes of the polymer bulk. Suitable organization or assembly of the molecules is required for the photoinduced deformation of materials. The reversible shape change in molecular materials was found for the first time in 2001 by using azobenzene-containing liquid crystal elastomers [39]. Figure 23.6

Figure 23.6 Contraction fraction of azobenzene-containing liquid crystal elastomers, (L0 Lt)/L0, at 298 K against the time upon irradiation with ultraviolet light and in the dark. L0 and Lt represent lengths of the elastomers in the initial state and after the time, respectively.

23.6 Crystal Surface Changes

shows the fractional contraction of the elastomers, (L0 Lt)/L0, at 298 K against the time of irradiation with ultraviolet light and in the dark for recovery [39]. L0 and Lt represent the lengths of the elastomers in the initial state and after the time, respectively. The elastomers contract by almost 20% upon photoisomerization of azobenzene chromophores. After shutting off the illumination, the elastomers recovered to their initial length. The effect can give rise to directed bending of elastomer films when the chromophores are selectively excited with linearly polarized light [40] or unidirectionally aligned in the film by a rubbing procedure [41, 42]. In these cases, the light-induced trans–cis photoisomerization of the azobenzene chromophores reduces the ordering of the liquid-crystal material, which can give rise to macroscopic contraction or bending. However, the phenomenon occurs only around the phase transition temperature. The response time of these systems is rather slow, and the deformed states are unstable because the cis-azobenzene isomers relax back to trans-azobenzenes.

23.6 Crystal Surface Changes

The colorless single crystals of diarylethenes change color by the formation of closedring isomers upon irradiation with ultraviolet light. As described in Section 23.3, the component diarylethene molecules shrink during the photoisomerization. The molecular-scale shape change induces nano-scale morphological changes of the crystal surfaces. The morphology changes were detected by an atomic force microscope (AFM) [44]. Two crystal surfaces, (100) and (010), of diarylethene crystal 7 were used for observation of the surface morphological changes. The crystal surface before photoirradiation was flat (Figure 23.7a). Upon irradiation for more than 10 s with

Figure 23.7 AFM images of the (100) surface (a) and the (010) surface (b) of crystal 7 upon alternate irradiation with ultraviolet and visible light.

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ultraviolet light, steps appeared on the (100) surface. No step formation was discerned during the irradiation for the initial 10 s but appeared after the induction period. The step height was 1.0 nm. The step disappeared by bleaching upon irradiation with visible light (l > 500 nm). When the irradiation time was prolonged, the number of steps increased and steps with heights of 2.0 and 3.0 nm appeared. The height was always a multiple of the minimum step height in 1.0 nm, and any steps with a height lower than the unit height of 1.0 nm were not observed. Each step of 1.0 nm requires reactions to depths of about 600 molecular layers. The morphological change was reversible and correlated with the color change of the crystal. The AFM images of the (010) surface before and after ultraviolet light irradiation are shown in Figure 23.7b. Upon irradiation for 15 s with ultraviolet light, the crystal turned blue and valleys appeared on the crystal surface. The depth of the valleys was estimated to be 10 to 50 nm. The valley almost disappeared by bleaching upon irradiation with visible light (l > 500 nm). The morphological change was again reversible and correlated with the color change.

23.7 Photoreversible Crystal Shape Changes

The crystal surface morphology changes encouraged us to study the shape change of bulk crystals upon photoirradiation. Diarylethene molecular crystals with sizes ranging from 10 to 100 mm exhibit rapid and reversible macroscopic changes in shape and size induced by ultraviolet and visible light [45]. The changes occur about five orders of magnitude faster than the response time of the azobenzene-containing liquid crystal elastomers [39–43]. Single crystals of diarylethenes 4 and 8 with sizes on the 10–100 mm scale were prepared by sublimation of the compounds on glass plates. Figure 23.8 shows a

Figure 23.8 Microcrystals of diarylethene 8 prepared by sublimation.


microscopic picture of the microcrystals of diarylethene 8. Upon irradiation with ultraviolet light, the molecules in the crystals underwent a cyclization reaction that transformed open-ring isomers into closed-ring isomers. The colors of crystals 4 and 8 turned to violet and blue, respectively. The colors were stable in the dark, but disappeared on irradiation with visible light [46, 47]. The crystal shape changes during the photochromic reactions were observed directly with a microscope. Figure 23.9 illustrates the deformations of diarylethene single crystals 4 and 8 on alternate irradiation with ultraviolet (l ¼ 365 nm) and visible (l > 500 nm) light. As shown in Figure 23.9a, a rectangular single crystal of 4

Figure 23.9 Photoresponsive deformation of diarylethene crystals 4 (a) and 8 (b).

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Figure 23.10 Relationship between the corner angle of the single crystal of 8 and the absorption intensity of the crystal measured at 600 nm upon alternate irradiation with ultraviolet light (&) and visible light (.).

induced contraction and expansion by as much as 7% on irradiation with ultraviolet and visible light, respectively. Ultraviolet irradiation of a single crystal of 8 changed its corner angles from 88 and 92 to 82 and 98 , respectively, changing its shape from squares to lozenges, as shown in Figure 23.9b. The angle changes can be repeated for more than 20 cycles of alternate irradiation with ultraviolet and visible light without any evidence of a change in the performance of the crystal. Figure 23.10 shows the time dependence of the color and shape changes on alternate irradiation with ultraviolet and visible light by showing the relation between the absorption intensity of the crystal at 600 nm wavelength and its corner angle. The absorption intensity increases with the amount of photogenerated closed-ring isomers in the crystal, which reaches 70% of all molecules in the photostationary state. The angle initially remains unchanged and then decreases by as much as 5 to 6 . No hysteresis between the forward and reverse processes was observed. An interesting correlation between the absorption maximum and the shape change was observed. The absorption maximum initially remained constant at 625 nm, but then shifted to 585 nm as the crystal shape changed. The spectral shift is attributable to the interaction of adjacent closed-ring isomers [29]. An induction period for the changes in the crystal shape is necessary for the photomechanical phenomenon to take place. The crystallinity of the small crystal was evaluated from its melting point and the order parameter of the visible absorption in the photostationary state. The melting point of crystal 8 before photoirradiation was 164 C, and decreased to 45–55 C upon irradiation with ultraviolet light. The decrease in the melting point upon ultraviolet irradiation is due to the coexistence of two isomers in the same crystal. The crystal became colorless again on irradiation with visible light. The recovery of the melting


Figure 23.11 Reversible bending of a rod-like crystal of 4 upon alternate irradiation with ultraviolet and visible light.

point (164 C) upon visible-light irradiation indicates that crystal 8 remains highly crystalline after a cycle of irradiation with ultraviolet and visible light. The order parameter ((A// A?)/(A// þ 2A?)) at 600 nm in the photostationary state was 0.53, which is identical to the value measured with a large crystal [47]. The constant order parameter during photochromic reactions also indicates that the crystallinity is maintained even in the photostationary state. Rod-like crystals of 4 were also prepared by sublimation. X-ray crystallographic analysis revealed that the thin plate-like crystal and the rod-like crystal have the same crystal structure. The rod-like crystal mounted at one end on a glass surface bent upon irradiation with ultraviolet light, as shown in Figure 23.11, with the bending moving towards the direction of the incident light [45, 48]. This effect is due to a gradient in the extent of photoisomerization caused by the high absorbance of the crystal, so that the shrinkage of the irradiated part of the crystal causes bending, just as in a bimetal. The bent rod-like crystal became straight again upon irradiation with visible light. Such reversible bending could be repeated over 80 cycles. The power produced during bending can move a gold

Figure 23.12 Photoresponsive anthracenecarboxylates.

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microparticle with a weight 90 times greater than the single crystal over a distance of 30 mm. The bending of the crystal was found to be almost complete within 25 ms, whereas the photoreaction of diarylethenes in crystals takes place in less than 10 ps [34]. This very fast response time of the bending is about 105 times faster than those of the azobenzene-containing elastomer systems [39–43], and is comparable to the response time of piezoelectric devices. The shape change of molecules upon photoirradiation is directly linked to the macroscale shape change of the crystals. The suitable arrangement of molecules in crystals induces a cooperative motion of the crystal lattice and results in the mechanical motion of the crystals. The specific molecular packing in the crystals is therefore considered to play an important role in macroscale motion. Bardeen et al. have reported analogous effects with 200-nm-diameter nanorods of 9-tert-butylanthracenecarboxylate 11 [49] and 9-anthracenecarboxylic acid 13 (Figure 23.12) [50]. Both crystals were prepared by solvent annealing in Al2O3 templates [51]. Crystals of 11 were shown to expand irreversibly by as much as 15% along the long axis in a photochemical reaction to give dimer 12. Exposure of 13 to ultraviolet light resulted in formation of the thermally unstable syn 4 þ 4 photodimer 14, which spontaneously returns to the starting anthracenecarboxylic acid [50]. These photomechanical materials can be useful for applications to photomechanical actuators in many fields of electronic, photonic, mechanical, medical, and functional materials [52].

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24 Direct Observation of Change in Crystal Structures During Solid-State Reactions of 1,3-Diene Compounds Akikazu Matsumoto

24.1 Introduction 24.1.1 Crystal Engineering Renaissance

Solid-state organic reactions often provide a high regio- or stereoselectivity because the structure of a product is strictly determined by the crystal structure of the reactant, that is, the reaction proceeds under crystalline-lattice control [1–10]. Especially, topochemical reactions promise the formation of products with a highly controlled structure, which can be predicted on the basis of the structure of a reactant. The crystallographic investigation of topologically controlled reactions was first carried out by Schmidt and coworkers during the early 1960s, and the principles of the photodimerization, that is, the Schmidt rule, as well as the concept of crystal engineering, were established [11, 12]. They examined a relationship between the crystal structure of the reactant and the chemical structure of the product to discuss topochemical principles for the reactions. Since the finding of the topochemical polymerization of diacetylenes [13] and diolefins [14] via a chain or stepwise reaction mechanism, many studies on the solid-state polymerization of various kinds of monomers have been carried out [15, 16]. For example, Tieke [17] reported the radiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures, but the details of topochemical polymerization of 1,3-diene monomers have still been unknown until recent years. Thirty years later, we discovered the topochemical polymerization of various 1,3-diene monomers giving a highly stereoregular polymer in the form of polymer crystals. When ethyl (Z,Z)-muconate was photoirradiated in the crystalline state, a tritactic polymer was produced [18, 19], in contrast to the formation of an atactic polymer by conventional radical polymerization in an isotropic state. Thereafter, comprehensive investigation was carried out, for example, the design of monomers, the crystal structure analysis of monomers and polymers, and polymerization reactivity control, in order to reveal the features of the polymerization of 1,3-diene monomers [20–23]. Eventually, it was revealed that the solid-state photoreaction

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pathway depended importantly on the chemical structure of the reacting 1,3-diene compound. As a result of the solid-state photoreaction, one of the corresponding EE-isomers, [2 þ 2] cyclodimer, and 1,4-polymer were obtained by unimolecular, bimolecular, and polymerization reactions, respectively. The pathway is selectively determined by the structure of the substituent, that is, the molecular packing fashion in the crystals. A crystal engineering approach has changed since the 1990s, and been renewed as a strategy for the rational design of organic solid architecture utilizing supramolecular chemistry [24–26]. Furthermore, studies on organic solid-state reactions and crystal structure analysis have been accelerated by the development of methods and apparatus for crystal structure analyses [27–29]. In this chapter, we describe the isomerization, dimerization, and topochemical polymerization of benzyl muconates with various kinds of substituents on the benzyl ester group (Scheme 24.1), in order to reveal the solid-state reaction mechanism by the direct observation of crystal structures which change during the reactions.

24.2 EZ-Photoisomerization 24.2.1 Model of Photoisomerization

Only a limited number of isomerizations of olefins between the E and Z forms in the crystalline state have been reported because of the difficulty in the inevitable CO2R

hν crystal-to-crystal CO2R

(Z,Z)-isomer R=

CH2 X

RO2C

CO2R CO2R

(E,E)-isomer CO2R n RO2C-HC=HC

RO2C

CO2R

1,4-polymer

CH=CH-CO2R

cyclodimer Scheme 24.1 Possible pathways for the solid-state photoreactions of benzyl (Z,Z)-muconates.

24.2 EZ-Photoisomerization

change in the size and shape of the space occupied by the substituents of a double bond [30–32].The isomerization of polyenes is an important photochemical process in biological systems, and a bicycle-pedal model as the volume-conserving reaction mechanism was first pointed out by Warshel in 1976 [33]. Later, Liu et al. [34] proposed the hula-twist process to explain the results of the picosecond time-resolved kinetics of the reaction in Rhodopsin. The hula-twist model has been applied to various reactions in confined media such as a viscous fluid, a rigid matrix, an organic glass, and organic crystals [35, 36]. Reactions proceeding according to the bicyclepedal [37] and hula-twist models, as well as reactions including crankshaft motion [38–40], require only a small change in the molecular shape, differing from the conventional one-bond-flip motion, which is usually observed during many reactions in solution (Figure 24.1). Several years ago, we found that the isomerization of n-butylammonium (Z,Z)-muconate produces the corresponding EE-isomer in a high yield in the crystalline state under photoirradiation [41]. This solid-state photoisomerization was revealed to be a one-way reaction and no EZ-isomer was formed during the reaction, while unsaturated compounds such as olefins, polyenes, and azo compounds generally undergo reversible one-bonded photoisomerization to form a mixture of isomers. Previously, we pointed out the possibility that the isomerization of the muconic derivatives in the solid state follows the bicycle-pedal reaction mechanism, but the details of the molecular dynamics of the reaction had not been clarified. Saltiel et al. [42–44] and Liu et al. [45, 46] have independently discussed volume-conserving reaction mechanisms for the isomerization of 1,4-diphenyl-1,3-butadienes, and

Figure 24.1 Photoisomerization mechanism of (Z,Z)-muconate to the (E,E)-muconate via one-bond-flip (OBF), bicycle-pedal (BP), and hula-twist (HT) models. The OBT and HT models include two-step reactions, while the BP reaction provides the product by a one-step reaction.

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O O O O (Z,Z)-Bn

O O

UV irradiation crystal-to-crystal

O O (E,E)-Bn

Scheme 24.2 One-way photoisomerization of benzyl muconate from the (Z,Z)- to (E,E)-isomer in the crystalline state.

1,6-disubstitued hexatrienes [47] in the crystalline state and other confined media. They also referred to the isomerization mechanism of the muconate derivatives. More recently, we succeeded in clarification of the reaction mechanism for the solid-state photoisomerization by the direct observation of a change in the single crystal structure during the photoisomerization of benzyl (Z,Z)-muconate [(Z,Z)-Bn] to the (E,E)-muconate [(E,E)-Bn] in the solid state (Scheme 24.2) [48]. 24.2.2 Photoisomerization of Benzyl Muconate

The (Z,Z)-Bn provided three polymorphic forms of crystals [(Z,Z)-Bn-a, b, and g] when it was recrystallized from n-hexane or chloroform the same at room temperature. Each polymorph is preferentially obtained, depending on the recrystallization conditions, such as the temperature and evaporation rate. For example, the (Z,Z)-Bn-a crystals most frequently appeared when the solution of (Z,Z)-Bn was slowly evaporated, while a fast evaporation sometimes results in the growth of the crystals as (Z,Z)-Bn-b. The structures of these polymorphs were determined by single crystal X-ray analyses. The packing structures of (Z,Z)-Bn in the crystals are shown in Figure 24.2. The crystal systems of all the polymorphs are monoclinic and the space group is P21/c or P21/n. The lattice volumes (833.8–841.7 Å) and packing densities (1.272–1.284 g cm–3) are very similar to each other. The photoisomerization reactivity of (Z,Z)-Bn depended significantly on the polymorphic structures. After photoirradiation of the powdered (Z,Z)-Bn-a crystals for 5 h at room temperature, the conversion to (E,E)-Bn was 71%, but the crystals with the other forms had a lower reactivity (13 and 6% conversions for (Z,Z)-Bn-b and g, respectively) under the same conditions. In general, isomerization requires a reaction space to change its molecular conformation. However, the crystallographic data for the three polymorphs indicated a similar average density for the molecular

24.2 EZ-Photoisomerization

Figure 24.2 Top and side views of single crystal structures of polymorphic (Z,Z)-Bn crystals. (a) (Z,Z)-Bn-a, (b) (Z,Z)-Bn-b, and (c) (Z,Z)-Bn-g.

packing in the crystals. The total volumes of the void spaces in each unit cell were 51.8, 61.2, and 83.6 Å3 for (Z,Z)-Bn-a, b, and g, respectively. These results disagree with the order of the isomerization reactivity. 24.2.3 Change in Crystal Structures During Photoisomerization

To reveal the reaction mechanism, we directly monitored the change in the single crystal structure of (Z,Z)-Bn during the photoisomerization using a band path filter to irradiate with light longer than 300 nm, corresponding to the absorption edge of the (Z,Z)-Bn crystals. As a result, the crystal structure of (Z,Z)-Bn after UV light irradiation was successfully solved. The space group of the product crystals was the same as the crystal before photoirradiation, and the b- and c-axis lengths as well as the cell volume increased slightly during the initial stage of the reaction. The crystal of (Z,Z)-Bn after photoirradiation has cell volume and density values (V ¼ 858.8 Å3, r ¼ 1.247 g cm3) identical to those of the crystal of (E,E)-Bn prepared by recrystallization (V ¼ 859.5 Å3, r ¼ 1.246 g cm3), but the packing mode of the molecules in the crystals is different. Namely, the crystal of (Z,Z)-Bn has the space group P21/c while the crystal of (E,E)-Bn has P21 [48]. For the crystal structure after photoirradiation, a disordered structure was observed around the diene moiety. This disordered structure contains both (Z,Z)Bn and (E,E)-Bn as the molecular structures, as shown by the ORTEP drawing in Figure 24.3. Both the ZZ- and EE-isomers included in the crystal after photoirradiation have similar molecular shapes and planar conformations. The direct observation of a crystal structure during the reaction has revealed that the diene moiety changes its geometric structure from the ZZ-isomer into the EE-isomer consistent with the

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Figure 24.3 ORTEP drawings of (a) (Z,Z)-Bn crystal, (b) disordered structure observed after 27 h photoirradiation, and (c) separated into (Z,Z)-Bn and (E,E)-Bn with a site occupancy factor of 35% for (E,E)-Bn.

bicycle-pedal model. The change in the volume and shape of the reacting molecule [49–51] is small, as expected for a reaction proceeding via the bicycle-pedal mechanism. The topochemical isomerization of (Z,Z)-Bn is accompanied by the expansion of the lattice lengths and the cell volume, but the space group of the crystals does not change. As a result, a structural strain may be produced by a mismatch in the structures of the ZZ- and EE-derivatives in the crystal lattice. When the (E,E)-Bn molecules are produced in the crystal lattice of the (Z,Z)-Bn molecules during the solid-state reaction, a structural strain is accumulated in the (E,E)-Bn molecules as a strained molecular conformation. By comparison of the structures of the recrystallized or calculated molecules, it was revealed a small change in the molecular structure of (Z,Z)-Bn in the product crystal, and the structural strain is found at the bond angles in the produced EE-molecules. Thus, the analysis of an intermediate single crystal structure during the reaction has revealed the evolution of a significant strain in the crystal lattice during the topochemical EZ-isomerization. Furthermore, a structural change in the (Z,Z)-Bn-a crystals was observed at a higher conversion. The X-ray diffraction lines after photoirradiation for 120 h agreed well with those for the recrystallized (E,E)-Bn. The molecular conformation of (E,E)Bn includes an asymmetric structure with a bent benzyl moiety, being different from the symmetric conformation of the (Z,Z)-Bn molecules, and also the (E,E)-Bn molecules as the cocrystals during the photoisomerization. These results indicate that a phase transition occurs in the final stage of the isomerization process. The (E,E)-Bn molecules can change their conformation in the solid state using the void

24.3 [2 þ 2] Photodimerization

space around the benzyl moiety during the phase transition. Namely, the primary EEisomer produced by photoirradiation has a molecular length similar to that of the original (Z,Z)-Bn isomer in the initial stage of the reaction, but the cell length increases along the directions of the b- and c-axes during the reaction, leading to an increase in the total volume of the crystals. Drastic changes in molecular conformation and packing structure occur during the crystal-to-crystal transition to the stable crystal structure of (E,E)-Bn, accompanying a phase separation. Thus, we clarified the isomerization process of the muconates according to a bicycle-pedal model. The isomerization occurs via a topochemical reaction process which does not require significant movement of the atoms. The void space included in the crystals plays an important role in the phase transition rather than in the isomerization. In future, the photochemical process of polyene systems performed in confined media will be further clarified using a simple model such as the isomerization of the muconates.

24.3 [2 þ 2] Photodimerization 24.3.1 [2 þ 2] Photodimerization of 1,3-Dienes

Since the works of Schmidt et al. [52, 53], the [2 þ 2] photodimerization of various olefin and diene compounds has been investigated to control reactivity in the solidstate by advanced molecular design using crystal engineering and host–guest chemistry. For example, halogen–halogen, donor–acceptor, and phenyl–perfluorophenyl interactions, as well as hydrogen bonding, are used to control the organization of reacting olefins. The molecular design includes the introduction of appropriate functional groups or atoms, which interact with each other to make the desired molecular assembly in the crystals [54–57]. Another approach using inclusion crystals or templates has also been reported [58, 59]. MacGillivray et al. [58] succeeded in the supramolecular construction of molecular ladders in the solid state using a linear template approach. They designed cocrystals of resorcinol with butadiene and hexatriene derivatives, in which two resorcinol molecules preorganize two polyene molecules through two hydrogen bond interactions, appropriate for [2 þ 2] cycloaddition. In this design, two polyenes are positioned at a distance less than 4.2 Å by the templates, leading to the production of the targeted ladderane with the fused cyclobutane framework. 24.3.2 [2 þ 2] Photodimerization of Benzyl Muconates

We have examined the [2 þ 2] photodimerization of the fluorine-substituted benzyl esters of (Z,Z)- and (E,E)-muconic acids in the solid state (Scheme 24.3) [60]. The muconates undergo [2 þ 2] cyclodimerization, EZ-isomerization, and

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CO2R UV irradiation

CO2R

crystal-to-crystal

CO2R

RO2C

RO2C-HC=HC

CO2R

RO2C

CH=CH-CO2R

CH=CH-CO2R RO2C

CH=CH-CO2R

or RO2C

R=

F

CH2

F

F CH2

CH2 F

F

F F

CH2

35F2 F

CH2

2345F4 F

CH2

F

2356F4

CH2

F

2346F4 F

CH2 F

F

F

F

345F3

F

F F

F

F

F

26F2

4F

F F

F

23456F5

Scheme 24.3 [2 þ 2] photodimerization of fluorine-substituted benzyl (Z,Z)- and (E,E)-muconates in the crystalline state.

polymerization, depending on the position and number of the fluorine substituents and the EZ-structure of the diene moiety, as well as the kind of irradiation. We clarified the crystal structure of these compounds in order to discuss the relationship between the molecular packing of the muconates and the photoreaction behavior. The (Z,Z)-isomers provided either the corresponding dimers or (E,E)-isomers during UV irradiation. During the photoreaction of (E,E)-isomers, some crystals provided the dimer and others showed no reaction. Neither (Z,Z)- nor (E,Z)-isomers were formed because the photoisomerization of muconates occurs via a one-way reaction mechanism. On the basis of the results of single crystal structure analysis, various molecular packing structures were observed, depending on the number and

24.3 [2 þ 2] Photodimerization

position of the fluorine substituents. In the molecular packing structure of (E,E)3454F3 and (E,E)-2356F4, we can see one-dimensional zigzag hydrogen bonding with a ladder structure through mutual CHF interactions as well as cyclic CHO intermolecular interaction. In most crystals, the characteristic patterns consisting of cyclic CHF and CHO interactions between adjacent molecules are observed. In these crystals, close FF contact (2.82–3.09 Å) was also observed, but such a close contact may be interpreted as the result of simple close packing, being different from clear interactions for ClCl and BrBr contacts. There are two possible structures for the products obtained during the photodimerization of the muconates. One is a syn-product with mirror-symmetry, and another is an anti-product with centro-symmetry (Figure 24.4). The pathway for the reaction is determined by the molecular packing in the crystal. Two reacting double bonds are required to take an appropriate position and direction for the process of dimerization. The reaction needs a specific geometrical structure for the translational packing of reacting double bonds on exactly parallel locations at a smaller stacking distance [61, 62]. The results of the photodimerization and the distance between double bonds (dCC-dim) for the muconates are summarized in Table 24.1. The dimerization of the muconates occurred when the crystals included a dCC-dim value less than 4.2 Å, as expected. In the crystals of (Z,Z)-4F, the dCC-dim for syn-dimer formation is 4.00 Å, which is allowed to undergo [2 þ 2] dimerization, but another distance (4.58 Å) is too long to form an anti-dimer. The values for (E,E)-2356F4 and (E,E)-23456F5 (4.20 and 4.28 Å) are at the boundary between reactive and non-reactive crystal structures, and the lack of dimer formation is due to the disadvantageous stacking angles instead of an ideal 90 . Some crystals have an acceptable dCC-dim value for both reactions providing syn- and anti-dimers, for example, dCC-dim ¼ 3.8–4.1 Å to produce both dimers of (Z,Z)-26F2, (Z,Z)-35F2, and (Z,Z)-345F3. However, the NMR data of the isolated products clarified that (Z,Z)-26F2 and (Z,Z)-345F3 give only anti-dimers and (Z,Z)-4F and (Z,Z)-35F2 give only syn-dimers. For all the cases shown in this study, only one type of dimer was produced, but not a mixture of both dimers. A high selectivity during syn- and anti-dimer formation is quite consistent with the dCC-dim values. Namely, the reaction proceeds favorably according to the path accompanying a smaller dCC-dim value when both the dCC-dim values are less than 4.2 Å. Furthermore, we revealed the photoreaction mechanism of the [2 þ 2] photodimerization of (E,E)-345F3 on the basis of the direct observation of single-crystal-tosingle-crystal transformation during the reaction [63]. The photodimerization occurred randomly in the columnar assembly of monomer molecules in the crystals of (E,E)-345F3, followed by formation of a trimer. When we carefully carried out the photoreaction of (E,E)-345F3, a disordered structure was observed. The structure was separated into the monomer and a product with an occupancy factor of 28% for the latter. The product appeared to have a ladder structure but, in an actual case, the dimer including a cyclobutane ring was randomly formed in the lattice. The molecule (E,E)345F3 has double bonds at four crystallographically equivalent positions for cyclobutane ring formation. A cyclobutane ring can be randomly formed in the

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(a)

RO 2 C

RO2C H

RO2C

RO 2 C

H

H

CO 2R CO 2R dCC-dim(anti) = 3.83 Å

anti-product

RO2C

RO 2 C

H H

RO2C

RO 2 C

H H

CO 2R

CO2R

CO 2R

CO2R

dCC-dim(syn) = 4.08 Å

syn-product

RO2C

(b)

H

H

RO2C

H

H

Ha

H

Hb

d H e

CHCl3

CO2R

H CO2R

CO2R

F

CO2R

H

f g

c R=

H

CH2 F

H

H 2O c

f

4.70 4.60 3.45 3.40

g

8

d

e

7

a

b

6

5

4

3

2

1

0 ppm

Figure 24.4 (a) Structure of syn- and anti-type [2 þ 2] cyclodimers of (Z,Z)-26F2, (b) 1 H NMR spectrum of the obtained anti-dimer. The dimer was isolated by column chromatography after UV irradiation.

columnar structure consisting of the translational arrangement of the (E,E)-345F3 molecules, which have a centro-symmetrical structure. When we investigated the photoreactions up to high conversion of (E,E)-345F3, a trimer was also confirmed, in addition to the formation of a dimer, in the NMR spectrum of the photoproducts

24.4 Topochemical Polymerization Table 24.1 Structure of obtained dimers and stacking parameters for [2 þ 2] photodimerization of fluorine-substituted muconates in the crystalline state.

Monomer

(Z,Z)-4F (Z,Z)-26F2 (Z,Z)-35F2 (Z,Z)-345F3 (Z,Z)-2345F4 (Z,Z)-2346F4 (E,E)-4F (E,E)-26F2 (E,E)-35F2 (E,E)-345F3 (E,E)-2356F4 (E,E)-23456F5

Ester

4-fluorobenzyl 2,6-difluorobenzyl 3,5-difluorobenzyl 3,4,5-trifluorobenzyl 2,3,4,5-tetrafluorobenzyl 2,3,4,6-tetrafluorobenzyl 4-fluorobenzyl 2,6-difluorobenzyl 3,5-difluorobenzyl 3,4,5-trifluorobenzyl 2,3,5,6-tetrafluorobenzyl 2,3,4,5,6-pentafluorobenzyl

Dimer structure

syn-type anti-type syn-type anti-type no dimer no dimer syn-type no dimer syn-type syn-type no dimer no dimer

dCC-dim (Å) syn

anti

4.00 4.08 3.83 3.93 7.01 5.20 3.88 5.43 3.97 3.90 5.13 5.20

4.58 3.83 3.91 3.90 5.27 5.74 4.31 4.76 4.34 4.54 4.20 4.28

(Figure 24.5). The trimer was isolated as a white powder. A trace of a higher oligomer was also detected in the NMR spectra at higher conversion.

24.4 Topochemical Polymerization 24.4.1 Features of Topochemical Polymerization

The topochemical polymerization of 1,3-diene monomers proceeds under photo-, X-, and g-ray irradiation or upon heating [64–70], similar to the solid-state polymerization of diacetylene compounds [71–73]. The polymerization proceeds via a radical chain-reaction mechanism and the propagating radicals are readily detected by EPR spectroscopy during polymerization in the crystalline state, because termination between the propagating radicals occurs less frequently in the solid state. The stacking structure of the monomers used for the topochemical polymerization is evaluated using the following parameters: the intermolecular distance between carbons that react to form a new bond during the topochemical polymerization (dCC), the stacking distance along the column (ds), and the angles between the stacking direction and the molecular plane in orthogonally different directions (q). The polymerization principle (the 5 Å rule) that the ds values are in an exclusively limited region of 4.74–5.21 Å for the 1,3-diene monomers [74, 75] has features similar to the empirical rules for the polymerization of diacetylene compounds [72].

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(a) F

=

a b

e f CO2CH2

M c d

F F

D 1H T 1H

T D

D 1H

T 2H

T 1H Ar + d + f

after UV irradiation

3.9 3.8 3.7

c+e

3.1 3.0

3.5 3.4

Ar

monomer b

8

7

a

CH2

6

5

4

3

2

1

0 ppm

(b) 100 D

Fraction (%)

80

60

40 T 20 M 0

0

2

4 6 8 10 Irradiation Time (h)

12

14

Figure 24.5 (a) 1H NMR spectra of (E,E)-345F3 and photoproducts at 92% conversion after UV irradiation, (b) timedependence of the fractions of monomer, dimer, and trimer during the photoreaction.

24.4.2 Monomer Stacking Structure and Polymerization Reactivity

We determined the single crystal structures for the monomer and polymer crystals of several kinds of benzyl muconates (Scheme 24.4) [76–79]. The polymer crystals were

24.4 Topochemical Polymerization

CO2R CO2R

RO2C CO2R CO2R

UV, γ-ray, X-ray crystal-to-crystal

CO2R n

O

X O

O X

O CH2

Cl

CH2

(Z,Z)-4Cl

Br

(Z,Z)-4Br F

CH2

NO2

(Z,Z)-4NO2

CH2 F (Z,Z)-26F2

O H2C O

O O

O O

O CH2 O

(E,E)-MDO O C2H5O

OC2H5 O

(Z,Z)-Et Scheme 24.4 Topochemical polymerization of (Z,Z)- and (E,E)-muconates in the solid state.

prepared by the g-radiation polymerization of the monomer single crystals. The changes in the selected unit cell lengths and volume before and after the polymerization are summarized in Table 24.2. The ds and fiber period (FP) values are lattice lengths parallel to a fiber axis for the monomer and polymer crystals, respectively. A model for the solid-state polymerization of 1,3-diene monomers is illustrated in Figure 24.6. When the polymerization proceeds in a domino-type reaction mechanism, all the monomer molecules include a conrotatory molecular motion for the formation of the polymer chain. Each molecule rotates and changes its conformation to make a new covalent bond between molecules with the least translational movement of the center of the molecular mass.

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Table 24.2 Change in the crystal structures and polymerization rates for solid-state polymerization of muconates

Monomer

Lattice length (Å)

Cell volume (Å3)

Monomer(ds)

Polymer(FP)

Monomer

Polymer

Polymerization rate constant k · 103 (s1)

4.931 5.122 4.432 5.21 5.239 4.177

4.839 (1.9%) 4.863 (5.1%) 4.702 (þ6.1%) 4.856 (6.8%) 4.855 (7.3%) 4.716 (þ12.9%)

554.3 923.0 941.9 953.0 947.6 871.3

525.3 (5.2%) 886.5 (4.0%) 920.0 (2.3%) 927.8 (2.6%) 927.8 (2.1%) 905.5 (þ3.9%)

5.16 1.15 0.75 0.25 0.10 0

(Z,Z)-Et (Z,Z)-4Cl (E,E)-MDO (Z,Z)-4Br (Z,Z)-4NO2 (Z,Z)-26F2

When monomer molecules stack translationally in a column with a ds value larger than the FP of the corresponding polymer, shrinking is observed along the column direction. In fact, the change in the lattice lengths along the fiber axis was 1.9 to 7.3% for the crystals of (Z,Z)-Et, (Z,Z)-4Cl, (Z,Z)-4Br, and (Z,Z)-4NO2. The other two axis directions involved expansion, except for shrinking in the perpendicular axis for (Z,Z)-Et. Conversely, the fiber axis length increases by þ 6.1 and þ 12.9% during the polymerization of (E,E)-MDO and (Z,Z)-26F2, respectively. Simultaneously, a large shrinking occurs in the orthogonal direction. Overall, the unit cell volume of these crystals is reduced during the polymerization by 2–5%, except for the volumeexpanding polymerization of (Z,Z)-26F2. Volume shrinking is usually observed for the addition polymerization of unsaturated monomers because it is a covalent bondforming reaction. In Table 24.2, we notice the exactly identical FP values for the polymer crystals, irrespective of the ds values for the monomers; that is, FP 4.84–4.86 Å and 4.70–4.72 Å for the polymers obtained by the shrinking and expanding polymerizations, respectively. This indicates that the polymer chains have a specific conformation in the crystals, depending on the polymerization mode.

ds FP d s > FP shrinking

d s < FP expanding

ds

Figure 24.6 Schematic model for the shrinking and expanding of crystals during topochemical polymerization in the solid state.


(a)

(b) 0

8

lnC1

-1

(Z,Z)-4Br

-2

(E,E )-MDO

-3

(Z,Z )-4Cl

k x 103 (s-1)

(Z,Z )-4NO2

6

4

2

(Z,Z )-Et

-4

0

10 20 30 40 50 60 70 Time (min)

0

0

2 4 6 8 10 Shrinkage or Expansion (%)

Figure 24.7 (a) Semilogarithmic plots of monomer fraction versus UV irradiation time for the topochemical polymerization of muconates, (b) relationship between relative polymerization rate and degree of shrinking (open circles) or expansion (closed circle) during topochemical polymerization.

Previously, a zigzag-type reaction mechanism was proposed for the diacetylene polymerization [55] and the step-wise [2 þ 2] photopolymerization of diolefin compounds [80], which have a large stacking angle (i.e., a small tilt angle) in the columnar structure of the monomers. However, we can conclude that the polymerization of the muconates proceeds via a domino-type polymerization mechanism, irrespective of the shrinking and expanding polymerizations. Next, we investigated the relationship between the stacking structure and the polymerization reactivity. The polymerization was carried out using powdered crystals embedded in a KBr pellet under UV irradiation. The conversion was determined from a change in the absorption intensity of the C¼C stretching around 1600 cm1 in the IR spectrum [66, 76]. The first-order plot of the conversion is shown in Figure 24.7, as a function of the UV irradiation time. Acceleration of polymerization was observed for the shrinking polymerization of (Z,Z)-4Cl, while the polymerization rate decreased during the expanding polymerization of (E,E)-MDO. This is due to a different mode for quenching the evolved strain through a crystal lattice change and a polymer conformational stress. The polymerization rate depended significantly on the magnitude of change in the lattice length during polymerization. The polymerization rate constant, k, becomes greater as the change in the lattice is smaller. Interestingly, this unique relationship is true for both shrinking and expanding polymerizations. 24.4.3 Shrinking and Expanding Crystals

We further examined a continuous change in the crystal structure and the strain accumulated in the crystals during the shrinking and expanding polymerizations [79].

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Figure 24.8 Single crystal structure of the monomers and a change in the lattice lengths during the continuous X-ray radiation of (a) (Z,Z)-4Cl and (b) (E,E)-MDO. Polymer chains are formed along the b-axis. Circle: a-axis, triangle: b-axis, square: c-axis.

The change in the X-ray diffraction profiles of the muconates was investigated by continuous X-ray radiation. Figure 24.8 shows the single crystal structure of (Z,Z)-4Cl and (E,E)-MDO and a change in the lattice lengths during the X-ray radiation polymerization. A temporary increase was observed up to the about 2% increment in the lattice length along the a- and c-axes during the initial stage of the polymerization of (Z,Z)-4Cl. The lattice lengths then gradually decreased and approached the values for the single crystals of poly(4Cl); that is, þ 0.9 and 0.1% increase. In the diffraction profile of (E,E)-MDO, the shift of each line was more clearly observed and all the axes change their length monotonically without exhibiting a temporary peak. An increase in the b-axis length as the fiber axis was observed due to the expanding polymerization, and a decrease in the other orthogonal axes. An induction period was detected in the structural change. 24.4.4 Accumulation and Release of Strain During Polymerization

The thermally-induced polymerization of the 1,3-diene monomers occurs at a much lower rate than the rates of the UV- and X-ray-induced polymerizations. As a result, we can readily observe an induction period during the initial stage of the polymerization and the subsequent acceleration of the rate by IR microscope


(a) 1.0 Fraction of Monomer

80 ºC

90 ºC

0.8 0.6

100 ºC

0.4

110 ºC UV

0.2

120 ºC 0 0

50

100

150

200

Time (min) (b)

Fraction of Monomer

1.0 60 μm 80 μm

0.8

0.6 thickness 20 μm

0.4 0

30 μm

100

200

300

Time (min) Figure 24.9 (a) Time-conversion relationship for the thermallyinduced polymerization of (Z,Z)-4Cl in the dark at 80–120 C, (b) photopolymerization under UV irradiation, and the effect of crystal size.

spectroscopy under temperature control [81]. The platelet crystals of (Z,Z)-4Cl were set on a thin KBr plate for the IR measurement, and the light was induced from the 100 face of the crystals. In the time dependence of the absorbance of some selected bands in the IR spectrum observed during the polymerization, we detected an induction period. The length of the induction period depended on the temperature, the kind of monomer, and also the size of the crystals. The results are represented as a change in the monomer fraction as a function of the polymerization time (Figure 24.9). The length of the induction period was dependent on the temperature; for example, about 10 min at 120 C and 1 h at 80 C. The polymerization of (Z,Z)-4Br occurred similarly with a longer induction period for the polymerizations at 110 and 100 C. In contrast to the observation of the induction period during the polymerization of (Z,Z)-4Cl and (Z,Z)-4Br, no induction period was detected during the polymerization of (E,E)-MDO at each temperature in the range 110–130 C. The polymerization rate decreased with the increase in the polymerization time.

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In contrast to the results for the polymerization using the larger crystals (about 40–100 mm in length), the induction period was drastically reduced at each temperature when microcrystals prepared by a precipitation method were used. In order to further examine the effect of the crystal size on the length of the induction period, we prepared various platelet or block-like crystals with different thicknesses and similar sizes. We found that an increase in the thickness of the crystals lengthened the induction period. The size effect on the induction period is related to the difference in the occurrence of the phase transition of the crystals. Eventually, it was concluded that the formation of polymer chains in the monomer crystals leads to the evolution of strain in the crystals during the initial stage of the polymerization. The accumulated strain induces the phase transition from the monomer crystal phase to the polymer crystal phase just after the induction period. In other words, the strain accumulated during the induction period is a trigger for the phase transition and the acceleration of the polymerization. 24.4.5 Homogeneous and Heterogeneous Polymerizations

Previously, we found that the di(4-alkoxybenzyl) esters of (E,E)- and (Z,Z)-muconic acids have different geometric structures, but the obtained polymers have completely identical chemical structures with the same tacticity (Scheme 24.5) [82–84]. We investigated the molecular dynamics during the polymerization of (E,E)-4MeO, (Z,Z)-4MeO, monitored by in situ single and powder X-ray diffraction experiments in order to discuss a reaction mechanism for the solid-state polymerization CO2R UV irradiation

CO2R

crystal-to-crystal

CO2R

RO2C

CO2R

CO2R

CO2R

CO2R

n

R= CH2

OCH3

4MeO

CH2

OCH2CH3

4EtO

Scheme 24.5 Formation of syndiotactic polymers during the solid-state polymerization of alkoxybenzyl (Z,Z)- and (E,E)muconates.


that proceeds via a chain reaction mechanism with or without crystal phase separation [85]. The change in the lattice parameters before and after the polymerization was 0.2–3.0% during the polymerization of (E,E)-4MeO, much smaller than that of (Z,Z)-4MeO (2.7–6.8%). Interestingly, both polymer crystals have completely identical structures regarding not only the stereochemical structure (tacticity) but also the crystal structure (molecular conformation). In other words, both polymer crystals have the same crystallographic parameters. This indicates that a greater movement of atoms is required in the crystals of (Z,Z)-4MeO than in (E,E)-4MeO during the polymerization. We examined the change in the crystal lattice for (E,E)4MeO and (Z,Z)-4MeO by continuous X-ray radiation at room temperature [85]. In the powder X-ray diffraction profiles of the (E,E)-4MeO crystals observed during the polymerization, all diffraction lines continuously shifted from a position for the monomer to that for the polymer without broadening of the line. On the other hand, the diffraction lines of (Z,Z)-4MeO showed a discontinuous change under the same radiation conditions. The profiles of the diffraction from the (Z,Z)-4MeO crystals consisted of the lines due to the monomer and the polymer accompanied by a crystal phase separation, despite highly penetrating X-ray radiation conditions. From the diffraction profile of the (E,E)-4MeO crystals, the c-axis length along the fiber axis of the resulting polymer crystals decreased by about 6%. The a-axis length first increased by about 1% and then decreased, finally leading to shrinking along that axis direction. The b-axis length gradually increased. The expanding and shrinking of the lattice length agree well with the results of single crystal structure analysis of the monomer and polymer. Such a continuous change in the crystal lattice had already been observed for the polymerization of other muconate monomers, for example (Z,Z)-4Cl and (E,E)-MDO [79]. On the other hand, the polymerization of (Z,Z)-4MeO induced a crystal phase separation from the initial stage of the reaction up to at least about 20% conversion. At higher conversion, no diffraction line due to the monomer phase was observed. The polymer phase included a small change in all the axis lengths, while the lattice parameters of the monomer phase changed drastically. We also tried to determine the structural change in the crystals during the polymerization more precisely by in situ X-ray single crystal structure analysis and eventually observed a disordered structure around the diene moiety of (E,E)-4MeO, as shown in the ORTEP drawings in Figure 24.10. The disordered structure was divided into the monomer and polymer structures. Furthermore, we noticed a maximum value for the lattice parameters of (E,E)-4MeO (a- and c-axis lengths, and cell volume) after starting the polymerization within 1 h of radiation, that is, below 30% conversion. The lattice lengths initially include an expansion and consequently the cell volume also temporarily increases. Such a temporary increase in the lattice constants is due to the coexistence of the monomer and polymer molecules in the common crystal lattice. The coexistence of monomer and polymer structures in a solid solution should cause a strain in the molecular conformation of the monomer or polymer as the intermediate structure of the cocrystals. The initial change in the lattice parameters is ascribed to the polymer chain produced in the monomer crystal.

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Figure 24.10 ORTEP drawing of the (E,E)-4MeO crystal as the intermediate structure during the polymerization with a site occupancy factor of 53% for poly[(E,E)-4MeO].

Topochemical polymerization includes two kinds of polymerization mechanisms as the extreme cases, as shown in Figure 24.11. One is the homogeneous reaction mechanism, as was seen in the polymerization of (E,E)-4MeO. In this model, polymerization occurs at random positions in the crystals, and both the monomer and polymer molecules include structural strain during the reaction. This forms a solid solution and no phase separation is observed at the intermediate stage of the reaction. Therefore, we can directly observe a change in the single crystal structure from 0 to 100% conversion throughout the reaction. Another is the heterogeneous reaction mechanism, as observed for (Z,Z)-4MeO. The reaction starts preferentially

Figure 24.11 Schematic model for homogeneous and heterogeneous polymerization mechanism.


near specific defect sites and accompanies the nucleation of a polymer phase. The produced polymer forms a new domain in the monomer crystals in the process of the polymerization, and consequently, phase separation is observed at the intermediate stage. In the latter model, the strain is possibly concentrated around the interface of the monomer and polymer phases, and the polymerization is accelerated near the phase boundary. In this model, a conformational change in the unreacted monomer domains is emphasized for the homogeneous polymerization model. Such a change often results in the drastic phase transition of the whole crystals from the starting structure into the structure of the product. Furthermore, the proposed model is specific to polymerization that proceeds via a chain reaction mechanism in the solid state. This model includes the strain accumulated in all the crystals during the reaction because of the presence of long-chain polymer molecules as the products. For the reactions of low-molecular-weight compounds yielding the corresponding low-molecular-weight products, such as the isomer or dimer products, the reactivity in the solid state has exclusively been explained by the mobility of the atom in the solid state. A free volume surrounding a reacting center is also important because the intermolecular space can act as a buffer to reduce the strain evolved during the reactions. Thus, we have clarified the polymerization mechanism of the muconates during solid-state polymerization via a crystal-to-crystal transformation by the direct observation of a change in the crystal structures during the polymerization. We revealed a change in the structure of the muconate crystals accompanying the shrinking and expanding of the lattice lengths, on the basis of the X-ray single crystal structure analysis of the monomers and polymers as well as a change in the transient structure during the continuous X-ray radiation. During the polymerization, an initially evolved strain induces the temporary expansion of the crystal. A strain is accumulated in both the formed polymer chain and the remaining monomer crystal during the initial stage of the reaction and is finally released after the polymerization. Furthermore, we observed a definite induction period during the initial stage of the thermally-induced polymerization. Such polymerization behavior was observed for the shrinking-type polymerization, but not for the expanding polymerization. Furthermore, we have concluded that the solid-state polymerization of 1,3-diene monomers is divided into a homogeneous polymerization which proceeds in the solid solution of the remaining monomer and the resulting polymer and a heterogeneous polymerization in which a crystal phase separation occurs between the monomer and polymer crystal domains during the reaction. According to the former model, the monomer crystal phase changes continuously into the polymer crystal phase without any phase separation when the least movement of atoms induces less strain in the crystals. We succeeded in the direct observation of the expanding crystal lattices and cell volume in the initial stage of polymerization, followed by the subsequent shrinking during the progress of the homogeneous reaction. Whereas, a phase separation was observed during the reaction according to the heterogeneous polymerization model, in which a considerable movement of atoms is required. The monomer stacking structure determines the reaction path during the solid-state polymerization.

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24.5 Conclusion

For a long time, it has been believed that molecules can react only in fluid media such as solution, liquid and gaseous states. However, recent progress in crystal engineering and crystallography has revealed that various reactions can be performed successfully in the solid state. During investigation of solid-state organic chemistry and materials science, including polymer chemistry, the direct observation of a continuous change in the conformational and chemical structures of reactant molecules during reactions in the crystalline state can give us important information. In the project of Molecular Nano Dynamics, we have investigated a change in the crystal structures and the accumulation of a strain in the crystals during the reaction to reveal the mechanism of the topochemical polymerization, EZ-photoisomerization, and [2 þ 2] photodimerization of muconic esters as the 1,3-diene monomers via a crystal-to-crystal transformation. We proposed a solid-state reaction mechanism for the crystal-lattice controlled reactions of various unsaturated organic compounds based on the results of in situ X-ray crystal structure analysis during the polymerization. In addition, we revealed the structures and the intercalation behavior of several stereoregular polymers [86, 87] obtained by the topochemical polymerization during the course of the project. Furthermore, we developed a double intercalation method, using an alkylamine and pyrene as the guests, to control fluorescence properties [88]. Some collaboration with the other research groups in the project was also successfully carried out, regarding the fabrication of the nanocomposite consisting of a crystalline organic polymer and fine metal particles [89–91], and crystal engineering using naphthylmethylammonium supramolecular synthons [92–95]. Studies on the two-dimensional polymerization of the muconates to fabricate polymer thin films are also continuing. We also investigated diacetylene polymerization [96–99], which is useful for the fabrication of conjugating polymers, on the basis of the results obtained from the studies on the diene polymerization. The direct observation of molecular motion in the crystals will bring new insight to the mechanism of solid-state reactions and also the design of new functional solid materials.

Acknowledgments

The author acknowledges Daisuke Furukawa, Dr Yutaka Mori, Takako Ueno, Katsuya Onodera, Natsuko Nishizawa, and the other members of the research group of Osaka City University. The author also acknowledges Prof. Seiya Kobatake, Osaka City University, Prof. Mikiji Miyata, Prof. Norimitsu Tohnai, and Dr Ichiro Hisaki, Osaka University, Dr Toru Odani, Prof. Hidetoshi Oikawa, and Prof. Hachiro Nakanishi, Tohoku University, Prof. Shuji Okada, Yamagata University, Prof. Masato Suzuki, Nagoya Institute of Technology, and Dr Ken Nakajima, Tokyo Institute of Technology for their fruitful collaborations in the project Molecular Nano Dynamics. The author gratefully thanks Prof. Kunio

References

Oka, Osaka Prefecture University, for his kind assistance with the g-radiation experiment. This work was supported by Grants-in-Aid for Scientific Research on Priority Areas (Area No. 432, No. 16072215) and for Scientific Research (No. 16350067) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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solution by the self-organization of polymer chains containing no polar group. Macromolecules, 41, 2467–2473. 98 Dei, S., Shimogaki, S. and Matsumoto, A. (2008) Thermochromism of polydiacetylenes containing robust 2D hydrogen bond network of naphthylmethylammonium carboxylates. Macromolecules, 41, 6055–6065. 99 Dei, S. and Matsumoto, A. (2009) Synthesis, structure, chromatic property, and induced circular dichromism of polydiacetylenes including an extended conjugating system in the side chain. Macromol. Chem. Phys., 210, 11–20

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25 Reaction Dynamics Studies on Crystalline-State Photochromism of Rhodium Dithionite Complexes Hidetaka Nakai and Kiyoshi Isobe

25.1 Introduction

A photochromic reaction is a reversible isomerization reaction induced by photoirradiation [1]. Crystalline-state photochromic reactions, especially, are becoming an important topic because of their potential advantages for construction of novel switching devices and materials [2]. Although many photochromic compounds have been reported so far, compounds that undergo photochromic reactions in the crystalline state are rare and their reaction dynamics are not well characterized as a result of the low degree of interconversion ratios and/or instability of the photogenerated isomers in the solid phase [3, 4]. We have recently found that a new class of photochromic compounds, rhodium dinuclear complex [(RhCp )2(m-CH2)2 (m-O2SSO2)] (1) (Cp ¼ h5-C5Me5) with a photo-responsive dithionite group (m-O2SSO2), shows the reversible crystalline-state photochromic reaction between 1 and [(RhCp )2(m-CH2)2(m-O2SOSO)] (2) with essentially 100% interconversion ratio (Figure 25.1) [5]. Taking advantage of the full reversibility of 1, we started the unexplored reaction dynamics study of crystalline-state photochromism [6]. This chapter presents the following three reaction dynamics related to single crystals of the dithionite complexes: (i) dynamics of molecular structural changes in single crystals, (ii) dynamics of reaction cavities in the crystalline-state reaction, and (iii) dynamics of surface morphology changes of photochromic single crystals.

Figure 25.1 Crystalline-state photochromism between 1 and 2.

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25.2 Photochromism of Rhodium Dithionite Complexes

Recently some metal complexes showing a photochromic reaction have been prepared [7, 8]. However, almost all of these compounds possess an organic photochromic moiety as a ligand. For instance, the diarylethene derivatives have been widely used as powerful ligands to synthesize the photo-functional metal complexes [7]. On the other hand, the preparation of a metal complex having photochromic properties by itself, transition-metal based photochromic compound (Figure 25.2) [4–6, 8], is still difficult because of a lack of systematic synthetic strategies. Further, finding new transition-metal based photochromic compounds poses a challenge because of the lack of understanding of the photochromic mechanism in these complexes. The rhodium dithionite complex 1 is one of the rare examples of transition-metal based photochromic compounds, as described in the following discussion. The rhodium dithionite complex 1 is composed of the rhodium dinuclear moiety (RhCp )2(m-CH2)2 and the dithionite ion S2O42. The dithionite ion is very attractive as the external stimuli-responsive ligand because the ion has a weak covalent SS bond (2.389 10 Å), which is easily cleaved to form radical species by stimuli [9].

Figure 25.2 Some examples of transition-metal based photochromic compounds.

25.2 Photochromism of Rhodium Dithionite Complexes

However, it has not been well established whether S2O42 has an ability to coordinate [10]. The organorhodium dinuclear moiety is a soft acidic center and can accept a soft donor atom like sulfur. Indeed, the dinuclear moiety forms the complexes [(RhCp )2(m-CH2)2(m-S2)] and [(RhCp )2(m-CH2)2(m-SSO2)] where the m-S2 and m-SSO2 ligands are coordinated parallel to the RhRh bond through two S atoms (side-on fashion) [11]. Treatment of trans-[(RhCp )2(m-CH2)2Cl2] [12] with S2O42 affords the first novel side-on type dithionite complex 1. The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without O2 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2(m-CH2)2(m-SO3)] and [(RhCp )2 (m-CH2)2(m-SO4)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. Crystalline-state photochromism usually proceeds with considerably lower interconversion ratios of less than 15% because the light penetration into the bulk crystal is prohibited by the absorption of the photo-generated isomer (inner-filter effects) [3, 4]. The fully reversible crystalline-state photochromism of 1 can be partly attributed to its photochromic property. The rhodium dithionite complex 1 belongs to a unique class of photochromic compounds, which exhibits a unimolecular type T inverse photochromism [13]. The type T inverse photochromism means that the back reaction occurs thermally and the lmax of the absorption spectrum of 1 is longer than that of 2. If the back reaction occurs photochemically and the lmax of the initial absorption spectrum is shorter than that of the photo-generated isomer, it is called type P positive photochromism and is known as a common photochromic system. Figure 25.3 shows the UV–vis spectral change from 1 to 2 in a microcrystalline powder film. The hypsochromic (blue) shift of lmax from 510 to 475 nm was

Figure 25.3 Irradiation time-resolved UV–vis spectra of 1 to 2 in a micro-crystalline powder film.

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observed as a characteristic of inverse photochromism. The B3LYP hybrid DFT and the time-dependent DFT calculations support the assignment of the absorption band at 510 nm for 1 as the charge transfer band from s(SS) to s (SS) and s (RhRh) orbitals and the assignment of the absorption band at 475 nm for 2 as the charge transfer band from s(RhSO) to s (RhRh) orbital. More importantly, the absorption coefficient of lmax in 2 is smaller than that in 1 by about one third. Thus, the light is able to pass more easily through the crystals when the photoreaction of 1 proceeds. This is one of the reasons why the photoreaction of crystals of 1 proceeds with almost 100% interconversion ratio. The back reaction from 2 to 1 is an exothermic reaction with liberation of about 4.5 kcal mol1 of heat. On leaving crystals of 2 in the dark for three weeks at room temperature, the back reaction takes place leading to complete regeneration of crystals of 1. The unique full reversibility of the crystalline-state photochromic system between 1 and 2 provides us with an opportunity to clarify the reaction dynamics by X-ray diffraction analysis and spectroscopic methods, as described in the next section.

25.3 Reaction Dynamics of Crystalline-State Photochromism 25.3.1 Dynamics of Molecular Structural Changes in Single Crystals

When single-crystal integrity is preserved during the reaction, the reaction is called a crystalline-state reaction and the process of the crystalline-state reaction can be observed by stepwise crystal structure analysis [4d, 14]. Although some crystallinestate reactions, including the photochromic reaction, have been investigated based on crystal structures before and after reactions, little is known about the dynamics of the molecular structural changes in the crystalline-state reaction [3d, 4a, 15]. Intriguingly, the crystalline-state photochromic process between 1 and 2, including the stereoselective oxygen-atom transfer and thermodynamically controlled photoisomerization, can be directly followed by conventional single-crystal X-ray diffraction analyses. The solid-state molecular structures of 1 and 2 are shown in Figure 25.4. The m-O2SSO2 ligand in 1 is coordinated parallel to the RhRh bond and has a weak SS bond of 2.330(2) Å. Complex 2 contains a new type of oxysulfur species, O2SOSO, having one oxo-bridge between the two S atoms: S1 has two terminal O atoms, while S2 has only one terminal O atom and is asymmetric. Although 2 has an asymmetric sulfur atom, S2, there are pairs of enantiomers in one single crystal of 2 because of the centrosymmetric space group P21/n. The O2SOSO type of oxysulfur compounds has been considered as an important unstable intermediate for the oxidation of disulfide compounds to SO32 and SO42 and has only been characterized theoretically [16]. The bridging S1O5 and S2O5 bonds in 2 differ significantly in length (S1O5: 1.709(5), S2O5: 1.636(6) Å), and the latter has double-bond character.

25.3 Reaction Dynamics of Crystalline-State Photochromism

Figure 25.4 ORTEP drawings of 1 and 2 with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles ( ) for 1: Rh1Rh2 2.6224(5), Rh1S1 2.279(1), Rh2S2 2.277(1), S1S2 2.330(2), S1O1 1.462(5), S1O2 1.459(4), S2O3 1.467 (5), S2O4 1.464(4), Rh2Rh1S1 86.63(4), Rh1Rh2S2 86.00(3), Rh1S1S2 93.29(6)

Rh2S2S1 94.06(5). Selected bond lengths (Å) and angles ( ) for 2: Rh1Rh2 2.6257(6), Rh1S1 2.270(2), Rh2S2 2.285(2), S1O1 1.445(7), S1O2 1.440(6), S1O5 1.709 (5), S2O5 1.636(6), S2O4 1.486(6), S1 S2 2.964(3), Rh2Rh1S1 94.15(5), Rh1Rh2S2 94.35(5), Rh1S1O5 107.9(2), Rh2S2O5 107.8(2), S1O5S2 124.7(3).

Since the crystals of 1 and 2 possess the same space group and molecular arrangement, by comparison of a certain molecule in the unit cell of 1 with that of 2, it can be readily seen which terminal oxygen in the m-O2SSO2 ligand of 1 is transferred to the bridging oxygen in 2 (Figure 25.5). It is the O3 atom. In the m-O2SSO2 ligand, the four oxygen atoms are stereochemically nonequivalent in the cavity formed by six Cp ligands. As shown in Figure 25.6, the O3 atom is the most congested by methyl groups of the Cp ligands. Based on this situation in the crystal of 1, what kind of selectivity and specificity is observed through the photoreaction? During the photoreaction from 1 to 2, positional disorder and occupancy changes of the oxygen atoms with irradiation time are observed in X-ray diffraction analyses (Figure 25.7). A careful analysis of the time dependence of the positional disorder and occupancy changes of the oxygen atoms during photoreaction at 20 C indicates that species 2a–d are formed in the initial and middle stages of the reaction (Figure 25.8a). The species 2a–d that cause the positional disorder are the stereoisomers of 2 in the crystal. Although the crystal has mirror image species of 2a–d as a set, in the present treatment only one asymmetric unit in the crystal is considered. Isomers 2a and 2c are a pair of enantiomers, 2a and 2b are identical species but differ orientationally in the cavity, and 2c and 2d are also identical but differ in orientation. Surprisingly, in the final stage of the reaction, only 2a is formed and the positional disorder almost disappears. This disappearance means that the thermodynamically unstable m-O2SOSO species 2b–d generated in the crystal convert under irradiation to the most stable species 2a. Thus, the specificity of oxygen transfer is observed during the photoreaction at 20 C (Figure 25.7).

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Figure 25.5 Superimposition of (a) unit cells and (b) crystal structures of 1 (black) and 2 (gray).

The solid-state dynamics from 1 to 2a is disclosed by using low temperature experiments. The photoreaction at 163 C shows that, in the initial stage of the reaction, the highest population product is not the most stable product 2a but 2b (Figure 25.8b). As described above, the O3 atom migration, which forms 2a, has much difficulty compared with other O atoms. The initial population of the products at 163 C reflects the selectivity based on the shape of the cavity in the crystal of 1 (topochemical principle [17]). It must be emphasized that no obvious conversions between the isomers are observed through the photoreaction at 163 C. These results strongly indicate that the kinetically controlled reaction takes place predominantly at 163 C. On the other hand, in the dark, the time dependence of the population of the isomers generated at 163 C after 190 h of irradiation shows that isomer 2c is directly transformed into 2a without irradiation (Figure 25.9). At the same time, the unchanged 2b and 2d populations confirm that the direct thermal


Figure 25.6 Reaction cavity around the m-O2SSO2 unit in the crystal of 1.

Figure 25.7 Positional disorder of oxygen atoms during the photoreaction.

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Figure 25.8 Change in population of the photochemically generated isomers with irradiation time at (a) 20 C and (b) 163 C.

conversion process of 2b (or 2d) to 2a is not present at 20 C. Based on the facts that the photoreactions from 2 to 1 and between the isomers of 2 do not proceed within photoirradiation, we conclude that the conversion of 2b (or 2d) into 2a contains a photochromic process of 2b (or 2d) ! 1 ! 2a. Thus, the specific photoreaction from 1 to 2a at 20 C was observed because thermodynamically favorable 2a accumulates through the direct thermal conversion from 2c to 2a and the repetitive photochromic process (2b (or 2d) ! 1 ! 2a) (Figure 25.10). In contrast, the thermal back reaction from 2a to 1 does not show any disorder phenomena except for that due to the formation of 1. The bonding character in 2a supports that specific cleavage of the weak S1–O5 bond is the lowest-energy path for isomerization to 1 and may be the only path for isomerization to 1.


Figure 25.9 Time dependence of the population of the lightinduced isomers generated at 163 C for 190 h irradiation (during the experiment, the backward reaction scarcely takes place).

Figure 25.10 Selective oxygen transfer through thermodynamic control.

25.3.2 Dynamics of Reaction Cavities in a Crystalline-State Reaction

It is well known that flat disk-like ligands such as Cp (h5-C5H5) and Cp can undergo 2p/5 jumping motions around the ligand-metal coordination C5 axis in the crystal [18]. The crystalline-state photochromic reaction between 1 and 2 proceeds in the reaction cavities formed by the Cp ligands. Figure 25.11 shows the packing diagram of 1 and thermal ellipsoids (50% probability) of C-atoms of two crystallographically independent Cp rings in 1 at 163 C. In Figure 25.11, the following three features can be pointed out. 1. The crystallographically independent Cp ligands are in the parallel and perpendicular Cp ring plane arrangements. Thus, the chemical environment of the

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Figure 25.11 Packing diagram of 1 and thermal ellipsoids (50% probability) of C-atoms of two crystallographically independent Cp rings in 1.

two Cp ligands in one molecule in the crystal is different. Indeed, the solid-state 13 C CPMAS NMR spectrum of the Cp ligands in the crystals of 1 at 21 C shows two methyl carbon signals at 10.5 and 10.8 ppm and two ring carbon signals at 104.0 and 106.0 ppm, reflecting the chemical environment in the crystal. Of course, in solution, only one methyl and one ring carbon signal appear at 9.6 and 104.1 ppm, respectively. 2. The shapes of the thermal ellipsoids of the C-atoms, which contain information on the dynamic process in the solid state [18–20], are quite different between the parallel and perpendicular Cp rings. The shape difference indicates that an activation energy for the jumping motion of parallel Cp rings is higher than that for perpendicular Cp rings. 3. The parallel Cp ligands are the closest to the terminal O3 atoms (indicated pale gray in Figure 25.11) while the parallel Cp rings in two adjacent molecules are staggered (two parallel Cp rings form an intermolecular staggered arrangement). These crystallographic situations indicate that the motion of the parallel Cp ligands in 1 is restricted by the steric hindrance of the O3 atom, in addition to the intermolecular staggered form itself. In other words, complex 1 has a molecular stress in the crystal. Figure 25.12 shows the packing diagram and thermal ellipsoids of complex 2 similar to Figure 25.11 of 1. Two features can be noted by comparison between Figures 25.11 and 25.12. First, the increase in packing distance for the parallel Cp rings is observed after irradiation: 3.5793(20) and 3.6323(25) Å in crystal 1 and 2, respectively. Secondly, the shapes of the thermal ellipsoids of the C-atoms in 2 are different from those


Figure 25.12 Packing diagram of 2 and thermal ellipsoids (50% probability) of C-atoms of two crystallographically independent Cp rings in 2.

in 1. The shape differences indicate that the activation energies for the jumping motion of the parallel and perpendicular Cp rings in 2 are lower than those in 1. These crystallographic features strongly suggest that the molecular motions of the Cp ligands, which form the reaction cavities of the m-O2SSO2 and m-O2SOSO groups in the crystals of 1 and 2, are closely connected to the photochromic reaction. In order to elucidate the relationship between the molecular motion of the Cp ligands and the photochromic reaction, the dynamic behavior of the Cp in the crystals of 1 and 2 was examined by the variable temperature (VT) solid-state NMR analyses [6d]. The activation energies for the Cp ligand motions in 1 and 2 were unequivocally determined by the VT solid-state 2 H NMR analyses of deuterated analogues 1-d30 and 2-d30, in which the Cp d15 ligands (h5-C5(CD3)5) are used instead of Cp ligands in 1 and 2. The resulting activation energies (kJ mol1) are as follows: 33 3 and 7.8 1 for the parallel and perpendicular Cp ligand in 1, respectively; 21 2 and 4.7 0.5 for the parallel and perpendicular Cp ligand in 2, respectively. The results show that the molecular motion of the Cp ligands couples to the photochromic reaction (atom rearrangement of the dithionite ligand). This strongly indicates that the dynamic behavior of the Cp ligands assists the crystalline-state reaction to proceed, maintaining the single-crystal integrity and forming only one enantiomeric pair of the m-O2SOSO complex in the final stage of the photoreaction at 20 C. The activation energy change of 12 kcal mol1 (subtract 21 from 33) for the parallel Cp ligand motion with the photoreaction from 1 to 2 is much greater than that of 3.1 kcal mol1 (subtract 4.7 from 7.8) for the perpendicular Cp ligand motion. The results indicate that the relaxation of the above-mentioned molecular stress by the O3 atom and between the parallel Cp rings in the crystal of 1 mainly contribute to a reduction in the activation energy for the parallel Cp ligand motion.

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In order to find the role of the dynamic reaction cavity in 1, we are currently preparing various derivatives of the dithionite complex by chemical modification of the Cp ligands of 1. Very recently, we have successfully synthesized a new dithionite complex [(RhCpEt)2(m-CH2)2(m-O2SSO2)] (1Et) with CpEt (h5-C5Me4Et) ligands instead of Cp . The crystal structure of the CpEt derivative 1Et was determined by X-ray diffraction analysis. The three-dimensional crystal packing of 1Et is similar to that of the Cp complex 1 (The CpEt ligands also form the reaction cavities in the crystal). However, the flexibility of the cavities related to the rotational motion of the CpEt ligands, which is disclosed by variable temperature CP/MAS 13 C NMR measurements, is substantially less than that of 1. Intriguingly, in the case of 1Et, the single crystal integrity is sometimes not preserved during the photoreaction. Thus, the dynamic behavior of the Cp ligands in 1 plays an important role in keeping the single-crystal integrity during the crystalline-state reaction. 25.3.3 Dynamics of Surface Morphology Changes of Photochromic Single Crystals

Surface nano-morphology changes of photoreactive molecular crystals are an attractive area of research, because the phenomena could potentially be applied to photodriven nanometer-scale devices and provide important information on crystalline-state reaction mechanisms and dynamics [2a, 21]. As described in Section 25.3.2, the single crystal of 1Et, in which the CpEt rings have no reorientation freedom in the crystal, tends to collapse and degrade as the reaction proceeds. This observation for the crystal of 1Et can be explained by the local stress induced by the photoreaction that is not suitably released by the crystal lattice. In such a crystal, does the surface morphology of the crystal change? To observe the changes on the nanometer-scale, we have examined the surface morphology changes by using atomic force microscopy (AFM). When a crystal of 1Et is mildly irradiated such that macroscopically it does not collapse, the light-induced surface nano-morphology changes on the crystal of 1Et could be reproducibly observed on the nanometer-scale. Careful AFM experiments show two kinds of stepwise surface morphology changes triggered by the photoreaction, as described in the following. First, a crystal of 1Et was irradiated for 2 min with visible light (>500 nm, intensity 5.0 mW cm2). The conversion ratio from 1Et to 2Et in the crystal was about 5%. This roughly corresponds to the photoisomerization reactions of the molecules in 3000 layers. Continuous AFM measurements of the photo-irradiated crystal were then performed at certain intervals under dark conditions and the AFM image changes were observed, as shown in Figure 25.13 (A prominent surface was used for the AFM experiments). Figure 25.13a is the schematic representation of the AFM image at 5 min after stopping the photoirradiation. No morphological change of the surface was discerned during this initial 5 min interval. After this induction period, steps gradually appeared on the surface and formed a ripplelike pattern (Figure 25.13b). The height of the steps is 6.6 2.4 nm, which corresponds to 13 molecular layers. The steps are nearly parallel to a line X in Figure 25.13b. When the observation time was prolonged,

25.4 Summary

Figure 25.13 Schematic-representation for AFM image changes of the (11-1) crystal surface of 1Et at the following intervals: (a) 5 min. (b) 30 min (c) 120 min after irradiation.

a new cross-stripe pattern with regularity appeared and the initial ripplelike pattern gradually disappeared (Figure 25.13c). The heights of the steps along lines Y and Z in Figure 25.13c were determined to be 8.1 5.8 and 7.9 8.4 nm, respectively. When a long photoirradiation time (>5 min) was employed, the height of the crossstripe steps became higher though this pattern remains. Of course, without photoirradiation, no morphological changes were observed after more than 3 h (blank experiments). The key to the observation of the pure ripplelike pattern lies in the choice of a suitable light intensity and irradiation time for the photoisomerization. If the light intensity is high and/or the irradiation time is prolonged, only the crossstripe pattern is observed. Thus, the present experiment using the devised AFM equipment adduces clear evidence to show surface morphology dynamics along with the formation of the ripplelike pattern. The results indicate that the development of surface structures is a consequence of the relaxation of strain. Further work including the elucidation of this dynamic behavior is currently in progress.

25.4 Summary

In this chapter, crystalline-state photochromic dynamics of rhodium dithionite complexes are reviewed. The chemistries described here have been achieved not only by recent developments of the analytical technique but also by discovery of a new class of transition-metal based photochromic compounds. One of the advantages of transition-metal complexes is structural diversity. In order to find the rule of an exquisite combination of metal ions and ligands, we are currently synthesizing various dithionite derivatives with other metal ions and/or modified Cp ligands. As shown in this chapter, dithionite complexes are a very useful photochromic system to investigate crystalline-state reaction dynamics. We believe that dynamics studies of newly synthesized dithionite derivatives provide useful insight into the construction of sophisticated molecular switches. A dithionite complex may appear in a practical application field in the near future.

Acknowledgments

This work was financially supported by the Grant-in-Aids for Scientific Research (KAKENHI) in Priority Area Molecular Nano Dynamics, No. 17034018,

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Chemistry of Coordination Space, No. 180330, Synergy of Elements, No. 190270, and Photochromism, No. 20044010 and by the Grant-in-Aids No. 1635002 and Nanotechnology Support Project from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was also supported in part by funds from the Tokuyama Science Foundation.

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References Ozawa, Y., Yonezawa, T., Toriumi, K. and Isobe, K. (2006) Direct observation of photochromic dynamics in the crystalline state of an organorhodium dithionite complex. Angew. Chem. Int. Ed., 45, 6473–6476. 6 (a) Miyano, Y., Nakai, H., Hayashi, Y. and Isobe, K. (2007) Synthesis and structural characterization of a photoresponsive organodirhodium complex with active SS bonds: [(CpPhRh)2(m-CH2)2 (m-O2SSO2)] (CpPh ¼ h5-C5Me4Ph). J. Organomet. Chem., 692, 122–128; (b) Nakai, H., Uemura, S., Irie, M., Miyano, Y., Hayashi, Y. and Isobe, K. (2007) Exploration of dynamics of photoreaction and morphology change of dithionite complex in crystalline-state. 1st Asian Conferece on Coordination Chemistry, Okazaki, Japan, July 29-August 2, Abstr. OA-14; (c) Miyano, Y., Nakai, H., Mizuno, M. and Isobe, K. (2008) Substitution effects of Cp ring benzyl groups on photoisomerization of a rhodium dithionite complex in the crystalline state. Chem. Lett., 826–827; (d) Nakai, H., Nonaka, T., Miyano, Y., Mizuno, M., Ozawa, Y., Toriumi, K., Koga, N., Nishioka, T., Jrie, M. and Jsobe, K., (2008) Photochromism of an organorhodium dithionite complex in the crystalline-state: molecular motion of pentamethylcyclopentadienyl ligands coupled to atom rearrangement in a dithionite ligand. J. Am. Chem. Soc.,130, 17836–17845. 7 (a) Han, J., Maekawa, M., Suenaga, Y., Ebisu, H., Nabei, A., Kuroda-Sowa, T. and Munakata, M. (2007) Photochromism of novel metal coordination polymers with 1,2-bis(20 -methyl-50 -(carboxylic acid)-30 thienyl)perfluorocyclopentene in the crystalline phase. Inorg. Chem., 46, 3313–3321; (b) Lee, P.H.-M., Ko, C.-C., Zhu, N. and Yam, V.W.-W. (2007) Metal coordination-assisted near-infrared photochromic behavior: a large perturbation on absorption wavelength properties of N,N-donor ligands containing diarylethene derivatives by

coordination to the rhenium(I) metal center. J. Am. Chem. Soc., 129, 6058–6059; (c) Uchida, K., Inagaki, A. and Akita, M. (2007) Preparation and photochemical behavior of organoruthenium derivatives of photochromic dithienylethene (DTE): DTE–(RRuLm)n (RRuLm ¼ (h6-C6H5)Ru (h5-C5Me5), (h6-C6H5)RuCl2(PPh3), (h5-C5Me4)Ru(CO)2; n ¼ 1, 2). Organometallics, 26, 5030–5041. 8 (a) Kawano, Y., Tobita, H. and Ogino, H. (1992) Reversible thermal and photochemical interconversion between cis and trans isomers of the silylenebridged diiron complex Cp0 2Fe2(CO)3 (m-SiHTol) (Cp0 ¼ h-C5Me5, Tol ¼ p-CH3C6H4). Organometallics, 11, 499–500; (b) Adams, R.D., Cortopassi, J.E., Aust, J. and Myrick, M. (1993) Energy capture by a tetranuclear metal cluster complex. Synthesis and characterization of PtOs3(CO)10(m-h2-dppm)[Si(OMe)3](m-H) and its metastable photoisomer. J. Am. Chem. Soc., 115, 8877–8878; (c) Boese, R., Cammack, J.K., Matzger, A.J., Pflug, K., Tolman, W.B., Vollhardt, K.P.C. and Weidman, T.W. (1997) Photochemistry of (fulvalene)tetracarbonyldiruthenium and its derivatives: efficient light energy storage devices. J. Am. Chem. Soc., 119, 6757–6773; (d) Yuki, M., Okazaki, M., Inomata, S. and Ogino, H. (1998) Formation of isomeric tetrathiotungstate clusters [{Cp Ru(CO)}2(WS4){W(CO)4}] by the reaction of [Cp 2Ru2S4] with [W(CO)3(MeCN)3]. Angew. Chem. Int. Ed., 37, 2126–2128; (e) Burger, P. (2001) [Me2C (h5-C5H4)2Ru2(CO)4]–An organometallic thermo-optical switch. Angew. Chem. Int. Ed., 40, 1917–1919; (f) Shibahara, T., Tsuboi, M., Nakaoka, S. and Ide, Y. (2003) Photochromic isomerization of a dinuclear molybdenum complex with ethylene-1,2-dithiolate and disulfur ligands: X-ray structures of the two isomers. Inorg. Chem., 42, 935–937. 9 (a) Pearson, W.B. (ed.) (1956) Structure Reports, vol. 20, International Union of

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Crystallography, p. 331; (b) Cotton, F.A. and Wilkinson, G. (1988) Advanced Inorganic Chemistry, 5th edn, Wiley-Interscience, p. 522. (a) Poffenberger, C.A., Tennent, N.H. and Wojcicki, A. (1980) Synthesis, characterization and some reactions of organometallic complexes containing bridging dithionite ligand. J. Organomet. Chem., 191, 107–121; (b) Kubas, G.J., Wasserman, H.J. and Ryan, R.R. (1985) Reduction of SO2 by (C5R5)M(CO)3H (M¼Mo, W; R¼H, Me). Chemistry and structures of (C5H5)Mo(CO)3(SO2H), the first example of insertion of SO2 into a MH bond, and [(C5Me5)Mo(CO)3]2 (m-S2O4), an S-bonded dithionite complex. Organometallics, 4, 2012–2021; (c) Bitterwolf, T.E. (1987) Reaction of sulfur dioxide with new dinuclear rhodium compounds. J. Organomet. Chem., 320, 121–127; (d) Matsumoto, K., Koyama, T. and Koide, Y. (1999) Oxidation of the sulfide ligands to SO2– 4 in the dinuclear complex {RuCl[P(OMe)3]2}2(m-S2)(m-Cl)(mN2H4): synthesis and characterization of {RuCl[P(OMe)3]2}2(m-S2)(m-Cl)(mN2H4) þ HSO4, {RuCl2[P(OMe)3]2}2(m-S) (m-N2H4), and {RuCl[P(OMe)3]2}2(m-S2O5) (m-N2H4). J. Am. Chem. Soc., 121, 10913–10923. (a) Nishioka, T., Kitayama, H., Breedlove, B.K., Shiomi, K., Kinoshita, I. and Isobe, K. (2004) Novel nucleophilic reactivity of disulfido ligands coordinated parallel to MM (M¼Rh, Ir) bonds. Inorg. Chem., 43, 5688–5697. (a) Isobe, K., Okeya, S., Meanwell, N.J., Smith, A.J., Adams, H. and Maitlis, P.M. (1984) Synthesis and characterisation of disubstituted Di-m-methylene-bis (m-pentamethylcyclopentadienyl) dirhodium(IV) complexes; X-ray structure of [{(C5Me5)Rh}2(mCH2)2(CO)2]2 þ . J. Chem. Soc., Dalton Trans., 1215–1221. Bouas-Laurent, H. and D€ urr, H. (2001) Organic photochromism. Pure Appl. Chem., 73, 639–665.

14 Ohashi, Y. (1998) Real-time in situ observation of chemical reactions. Acta. Crystallogr., A54, 842–849. 15 (a) Ramamurthy, V. and Venkatesan, K. (1987) Photochemical reactions of organic crystals. Chem. Rev., 87, 433–481; (b) Desiraju, G.R. (1989) Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam; (c) Braga, D. and Grepioni, F. (2004) Reactions between or within molecular crystals. Angew. Chem. Int. Ed., 43, 4002–4011; (d) Kawano, M. and Fujita, M. (2007) Direct observation of crystalline-state guest exchange in coordination networks. Coord. Chem. Rev., 251, 2592–2605. 16 Lacombe, S., Loudet, M., Dargelos, A. and Robert-Banchereau, E. (1998) Oxysulfur compounds derived from dimethyl disulfide: an ab initio study. J. Org. Chem., 63, 2281–2291. 17 (a) Cohen, M.D. and Schmidt, G.M.J. (1964) Topochemistry. Part I. A survey. J. Chem. Soc., 1996–2000; (b) Schmidt, G.M.J. (1971) Photodimerization in the solid state. Pure Appl. Chem., 27, 647–678. 18 Braga, D. (1992) Dynamical processes in crystalline organometallic complexes. Chem. Rev., 92, 633–665. 19 (a) Garcia-Garibay, M.A. (2005) Crystalline molecular machines: encoding supramolecular dynamics into molecular structure. Proc. Natl. Acad. Sci. U. S. A., 102, 10771–10776; (b) Khuong, T.-A.V., Nuñez, J.E., Godinez, C.E. and GarciaGaribay, M.A. (2006) Crystalline molecular machines: a quest toward solid-state dynamics and function. Acc. Chem. Res., 39, 413–422; (c) Horike, S., Matsuda, R., Tanaka, D., Matsubara, S., Mizuno, M., Endo, K. and Kitagawa, S. (2006) Dynamic motion of building blocks in porous coordination polymers. Angew. Chem. Int. Ed., 45, 7226–7230. 20 Bryan, R.F., Greene, P.T., Newlands, M.J. and Field, D.S. (1970) Metal-metal bonding in coordination complexes. Part X. Preparation, spectroscopic properties,

References and crystal structure of the cis-isomer of di-m-carbonyl-dicarbonyldi-ncyclopentadienyldi-iron. J. Chem. Soc. A, 3068–3074. 21 (a) Kaupp, G. (1992) Photodimerization of cinnamic acid in the solid state: new insights on application of atomic force microscopy. Angew. Chem. Int. Ed., 31, 592–595; (b) Kaupp, G. (1992) Photodimerization of anthracenes in

the solid state: new results from atomic force microscopy. Angew. Chem. Int. Ed., 31, 595–598; (c) Kaupp, G. (1995) Advances in Photochemistry, vol. 19 (eds Neckers, D.C., Volman, D.H. and von B€ unau, G.), Wiley-Interscience, New York, pp. 119–178; (d) Kaupp, G. (2002) Solidstate reactions, dynamics in molecular crystals. Curr. Opin. Solid State Mater. Sci., 6, 131–138.

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26 Dynamics in Organic Inclusion Crystals of Steroids and Primary Ammonium Salts Mikiji Miyata, Norimitsu Tohnai, and Ichiro Hisaki

26.1 Introduction

Organic inclusion crystals are attractive substances from the viewpoint of the dynamics of molecular assemblies. The inclusion crystals comprise nanocomposites between host and guest molecules and exhibit supramolecular properties through non-covalent bonds [1]. The host molecules provide molecular-level cavities where the guest molecules undergo intercalations and reactions. We have elucidated such dynamics by using the inclusion crystals of steroids and primary ammonium salts (Figure 26.1). This chapter consists of three sections. The first is an overview of comprehensive steroidal crystals on the basis of our lengthy research [2]. The steroidal hosts yield diverse host–guest assemblies which display a variety of dynamical behaviors. For example, classical studies of inclusion polymerization established a dynamical process of monomeric guests in channel-type cavities, leading to recognition of molecular-level space effects therein [3]. Such polymerization research gave us the chance to observe intercalation of the guest molecules with retention of the host assemblies in the crystalline state, compared to working with inorganic compounds such as clays and graphite [4]. Intercalation with sandwich-type crystals and layerreversion crystals is highlighted, followed by consideration of the fit of the guest molecules in the cavities. The second section deals with the dynamics of organic primary ammonium salts of carboxylic acids, sulfonic acids, and phosphonic acids. Inspired by the steroidal crystals described above, our research has been extended to the supramolecular properties of organic salts. We focus on the latest research on solid-state fluorescence emission and pseudo-cubic hydrogen bonding clusters. It is noteworthy that the salts have practical advantages, including the relatively simple preparation of their crystals,

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Figure 26.1 Dynamical properties related to organic inclusion crystals of steroids and primary ammonium salts.

the enormous number of combinations of acids and bases, and diverse crystal structures with different hydrogen bonding networks. Such organic salts could serve as useful materials for exploring supramolecular functions toward crystal engineering. The third section describes the hierarchy and supramolecular chirality of molecular assemblies in the crystalline state. The steroidal molecules construct hierarchical assemblies on the basis of sequential information, as in the case of proteins. The notable feature is that each hierarchical assembly exhibits supramolecular chirality, such as three-axial, tilt, helical, and bundle chirality. On the other hand, the primary ammonium salts construct hierarchical hydrogen bonding networks which, in some cases, create supramolecular chirality from achiral components. The creation of chirality can be interpreted from a topological viewpoint, leading us to define the handedness of the supramolecular chirality. At the end of this section we present the general concept that molecular-level information on organic substances can be expressed by their assemblies through non-covalent interactions. Finally, perspectives are briefly provided from the viewpoint of information and expression of organic molecules through their supramolecular architectures.

26.2 Dynamics of Steroidal Inclusion Crystals 26.2.1 Guest-Responsive Molecular Assemblies

Naturally-occurring steroidal bile acids involve cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid, as shown in Figure 26.2. These acids consist of skeletons and side-chains, and can be modified to more than one hundred derivatives. For example, their side-chains may have different chain length and hydrogen bonding groups in the form of carboxylic acids, amides, esters, alcohols and so on. Their highly asymmetric skeletons have one or more hydroxy groups with different

26.2 Dynamics of Steroidal Inclusion Crystals

Figure 26.2 Naturally-occurring steroidal bile acids involving cholic acid and its related derivatives.

locations and directions. Such unique molecular structures cause extensive formation of their guest-responsive assemblies through cooperative non-covalent bonds. These steroidal derivatives yield inclusion crystals with a variety of organic substances. The enormous range of inclusion experiments indicated that each host exhibits unique inclusion ability. In other words, comparative studies with a pair or set of steroidal hosts show slight or drastic differences in their inclusion behavior. For example, both cholic acid and deoxycholic acid include a wide range of organic substances, while chenodeoxycholic acid includes only a little and lithocholic acid not at all. Their amide derivatives can include many aliphatic alcohols, except for the amide of chenodeoxycholic acid. The alcohol derivatives of cholic acid and deoxycholic acid include aromatic guests, while those of chenodeoxycholic acid and lithocholic acid do not. Bishomocholic acid with two additional methylene units has larger inclusion spaces than cholic acid, while bisnorcholic acid with two decreased methylene units has no inclusion spaces. Epimers of cholic acid and deoxycholic acid include various aliphatic alcohols, in contrast to their original acids. Such diverse inclusion behavior is interpreted on the basis of X-ray crystallographic data and molecular graphics. Nowadays, more than three hundred sets of crystal data give us valuable information about non-covalent bonds between the host and guest molecules. The dominant interactions are attributed to conventional hydrogen bonds among oxygen and nitrogen atoms. Moreover, we can confirm a variety of weak hydrogen bonds, such as CH O, CH p, NH p, p p and so on [5]. These hosts have multiple hydrogen bonding groups, which combine in many ways among four host molecules to make linear, cyclic, branched or arched networks. These networks are often accompanied by additional guest groups and are modified by slightly different hydrogen bonding groups. For example, cholic acid forms a cyclic network with four host molecules, and includes over a hundred aromatic molecules through CH p interactions. The additional hydrogen of its amide induces a cyclic network with an extra hydrogen bonding donor which catches over fifty kinds of aliphatic alcohols. On the other hand, an epimer of cholic acid produces a branched network instead of the cyclic one, allowing aliphatic alcohol guests to insert between two hydrogen bonding groups of the host.

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Figure 26.3 Representative packing diagrams for inclusion crystals of cholic acid and its derivatives. The crystals exhibit guestresponsive structures.

The steroidal acids and their derivatives form various crystal structures. Figure 26.3 illustrates four kinds of crystal structures, such as a bilayer, a herringbone, a triangular prism, and a honeycomb. The representative bilayer structures have diverse guest-dependent assembly modes. This diversity comes from facial molecular structures with three-axial chirality, resulting in four kinds of combinations of hydrophilic and lipophilic sides in a parallel- or anti-parallel manner. In addition, the cumulated bilayers can slide on the lipophilic sides, followed by conformational changes of the side-chains. Such flexibilities of the bilayers can explain the versatile inclusion behavior mentioned above. Another feature is that the bilayers are constructed by connection of 21 helical assemblies of the host molecules, indicating that their side-chain length decisively influences the assembly modes of the helices [6]. 26.2.2 Intercalation in Steroidal Bilayer Crystals

The amphiphilic structures of steroidal molecules are closely related to the dynamical properties of their crystals. The hydrophilic sides are used to make the host framework through strong hydrogen bonds, while the lipophilic sides are used to make the cavities through weak hydrogen bonds and van der Waals forces. The latter weak interactions cause intercalation phenomena which correspond to reversible


Figure 26.4 Schematic representation of intercalation phenomenon in a layer material.

insertion, release, and exchange of guest molecules accompanying any fluctuation of the host framework (Figure 26.4). An important problem lies in whether the host frameworks are preserved during the intercalation or not. This is the reason why the intercalation of steroidal crystals was quite rare among organic crystalline materials about twenty years ago [4a]. Recently, the intercalation phenomena have become ubiquitous for organic and inorganic layered materials as well as nano-porous crystalline materials based on three-dimensional organic or organometallic frameworks [7]. Hereafter, we introduce the latest two examples of the attractive intercalation of steroidal bilayer crystals. One is the intercalation and deintercalation by using sandwich-type inclusion crystals of cholic acid, as shown in Figure 26.5 [8]. The sandwich-type structure has the same host bilayers as the bilayer-type, but involves additional sandwiched guest molecules between the bilayers. The intercalation and deintercalation of the guest molecules take place smoothly with retention of the crystalline state, resulting in reversible changes of the interlayer distances. The sandwich-type crystals in a 1 : 2 host-to-guest ratio are formed with five disubstituted benzenes, such as o-toluidine, m-fluoroaniline, o-chlorotoluene, o-bromotoluene and indene. Their thermogravimetric analyses revealed that the guest molecules, except o-toluidine, are released in two steps, indicating formation of intermediate crystals in a 1 : 1 or 2 : 1 host-to-guest ratio. X-ray powder diffraction patterns of the intermediate crystals revealed that the crystals have the same bilayer structures as those of the common inclusion crystals. Furthermore, reverse structural changes were achieved with absorption of the guest

Figure 26.5 Intercalation in sandwich-type inclusion crystals of steroidal cholic acid. Benzene derivatives undergo reversible desorption and absorption, accompanied by changes in bilayer distances.

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molecules to regenerate the starting sandwich-type inclusion crystals. The host bilayers expand and shrink by 1.8 to 4.5 A perpendicular to the layer direction. Such notable changes serve as a novel example of organic intercalation materials. Moreover, the intercalation method was applied to the enantioresolution of racemic alcohols. The amide derivative of cholic acid, called cholamide, serves as an excellent host for enantioresolution of 2,2-dimethyl-3-hexanol among secondary aliphatic alcohols. The intercalation method gave highly effective enantioresolution of the alcohol, as in the case of the recrystallization method. For example, dioxane molecules were exchanged with the alcohol molecules in channels of the bilayers with a high enantioselectivity of 96% ee, which was close to the value (98% ee) obtained by the recrystallization method. X-ray diffraction studies proved that the host bilayer framework has an anti-parallel arrangement on the lipophilic side in the case of dioxane, while the framework has a parallel one after intercalation of the alcohol. This indicates that the intercalation accompanies a layer-reversion. Such dynamical enantioresolution confirms the efficiency of the intercalation method. 26.2.3 Guest Fit Through Weak Non-Covalent Bonds

The guest molecules are accommodated in molecular cavities with different strengths, relating to the dynamical behavior of organic inclusion crystals. In this section we introduce three related examples about guest fit in the host cavities through weak non-covalent bonds. The first example is concerned with the packing coefficient of the host cavity, termed the PCcavity, which corresponds to the ratio of the volume of the guest molecule to the volume of the host cavity [9]. The PCcavity values of the inclusion crystals of cholic acid and cholamide were evaluated for a wide range of aromatic compounds and aliphatic alcohols. The values are approximately in the range 45–75%, meaning inhibition of too close or loose packing. In the case of the same host frameworks the values tend to increase with increasing guest volumes, indicating that the guest molecules exceeding this range force the host to adopt different assembly modes or molar ratios. Otherwise, they may form guest-free crystals or not form any crystals. The values in the inclusion state (45–75%) are intermediate between those in the liquid state (44–56%) and in the crystalline state (66–77%). The second example deals with weak hydrogen bonds such as NH p, CH p, CH O and so on [5, 10]. We compared these bonds between the inclusion crystals of cholamide and cholic acid, where weak hydrogen bonds play a key role in linking the host and guest molecules. The steroidal side-chains involving methyl, methylene, and amide groups serve as the hydrogen bond donors, and aromatic guest molecules serve as the acceptors. The inclusion crystals are categorized into two host frameworks, b-trans- and DCA-type. The former involves four weak hydrogen bonds; one NH p, two CH p, and one CH O interactions, whereas the latter involves only CH p interactions. Figure 26.6 displays these interactions and the Hirshfeld surface of the guest molecule. It can be seen that


Figure 26.6 Weak intermolecular interactions among two cholamide host and one aromatic guest molecules. (a) Four kinds of the interactions around the guest molecule. (b) Hirshfeld surface of the guest molecule, which shows four spots corresponding to the interactions shown in (a).

the NH p weak hydrogen bond forces cholamide to choose the b-trans-type crystal rather than the a-gauche-type. This exemplifies that the weak hydrogen bond enables the host to select the framework having low steric matching. In this way our systematic investigation provides effective examples for displaying weak hydrogen bonds among the host and guest molecules. The third example deals with efficient enantioresolution due to the highest guest fit in the steroidal crystals. When the host assemblies have guest-dependent crystal structures, it is not so easy to obtain resolution greater than 90% ee. This is because the assembly modes can change to different modes before the highest PCcavity values. So far we have obtained two successful enantioresolutions of secondary alcohols. One is the remarkable recognition of the (2R,3S)-isomer among four isomers of 3-methyl-2-pentanol by epicholic acid [11]. The other is the enantioresolution of (S)-2,2-dimethyl-3-hexanol by cholamide [12]. As for the recognition mechanism, we proposed that a four-location model [13] is more suitable than the conventional three-point attachment model. As shown in Figure 26.7, the former is based on a deformed hole, while the latter on a planar surface. As the fourth location of the former model, the smallest hydrogen atom together with a chiral carbon or methyl group have to be recognized in the case of secondary alcohols. In principle, at least, two kinds of disordered structures of guest components may be observed in the concave cavity. One is that in which a stereogenic carbon of the guest is disordered together with the fourth site (D, hydrogen). Such a disordered structure was confirmed in the case of the inclusion crystals of cholamide with 2-methyl-3hexanol. An additional methyl group brought about highly efficient enantioresolution of (S)-2,2-dimethyl-3-hexanol. The other is that in which the stereogenic carbon is almost fixed while two neighboring substituents (C, D), such as a methyl group and hydrogen, are disordered.

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Figure 26.7 Three- (left) and four- (right) location models for chiral recognition. The former is based on a planar surface, while the latter on a deformed hole where the fourth substituent (D) together with stereogenic carbon has to be recognized.

26.3 Dynamics of Organic Crystals of Primary Ammonium Salts 26.3.1 Solid-State Fluorescence Emission

Solid-state luminescent properties of organic materials have attracted much attention because of their direct and widespread applications in organic light-emitting diodes, organic dye laser photonic and photoelectronic devices. Currently, research on the luminescent properties is faced with the serious problem that identical organic fluorophores emit strongly in solution but weakly in the solid state. This is probably because the luminescent properties depend not only on the molecular structures but also on the molecular arrangements of the fluorophores. In order to develop sophisticated crystalline materials, it is essential to screen the arrangements necessary for solid-state fluorescence emission as well as to regulate slight movements of the fluorophores through intermolecular interactions. We employed organic salts of anthracene-2,6-disulfonic acid (ADS) with various aliphatic primary amines (Scheme 26.1), and investigated their solid-state luminescent properties as well as their arrangements of anthracene moieties. The ADS salts exhibited amine-dependent properties, such as emitting color and intensity (Figure 26.8). Single X-ray crystallographic studies revealed that the salts have amine-dependent arrangements, which are classified on the basis of hydrogen bonding networks between sulfonate anions and ammonium cations. These results indicate that the luminescent properties depend closely on the molecular arrangements. We have found that the ADS salt with n-heptylamine, which has discrete anthracene moieties in the crystal structure, exhibits the highest quantum yield in

26.3 Dynamics of Organic Crystals of Primary Ammonium Salts

Scheme 26.1 Preparation of organic salts of anthracene-2,6disulfonic acid (ADS) and various primary amines.

a series of ADS salts and luminesces more strongly than intact anthracene [14]. Moreover, we have demonstrated that a powerful strategy for luminescent enhancement lies in the rigidity of the luminant packing in the solid-state [15]. X-ray crystallographic studies confirmed the formation of a rigid one-dimensional arrangement by inserting benzylamine molecules into tubulate spaces of an anthracene host framework. This result shows that the strong fluorescence intensity observed is due to the increase in rigidity around the fluorophores, and not to the decrease in intermolecular interactions, as previously thought. A ternary system was found to afford different molecular arrangements and excimer emission from a binary system. The former system involves guest molecules as the third modulators which control the solid-state emission modes by chemical or physical stimuli. In our sophisticated system the modulators act as molecular information sources which are transcribed to the arrangements, and then are translated into the emission mode.

Figure 26.8 Solid-state fluorescence spectra of the ADS salts with benzyl amine (solid line), n-amyl amine (dashed line), and n-butyl amine (dotted line). Their spectra in solution (inset).

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Figure 26.9 Solid-state fluorescence spectra in the ADS salts with rac-sec-butyl amine involving dioxane or benzene as the third modulators.

For instance, 1,4-dioxane as the modulator yields a repetitive cycle consisting of desorption, absorption, and cooling processes among three different crystal forms. We observed switching of the corresponding solid-state emission modes between monomer and excimer states (Figure 26.9) [16]. In contrast, benzene as the modulator did not show such switching, although benzene was absorbed into the crystals and desorbed from them. The former crystals showed a change from a monomer emitting arrangement to an excimer emitting one of anthracene moieties under the cooling process, while the latter crystals kept the monomer emitting arrangement. Moreover, the switching of the solid-state emission modes can be repeated with good reproducibility. This programmed switching system is based on a dynamical change of arrangements of the anthracene moieties. These observations led to the idea that these crystal dynamics depend on molecular information of the guest molecules. Such programmed dynamics may lead to a novel paradigm of organic crystalline materials. 26.3.2 Hydrogen Bond Clusters

Recently we have found that triphenylmethylamine and various sulfonic acids form supramolecular cubic-type clusters in the crystalline and solution states (Scheme 26.2) [17]. This finding originates from a combinatorial study for screening new host compounds other than steroidal compounds. The research gave us the idea that the bulkiness of the components can regulate the dimensionalities of hydrogen bonding networks. Such an idea has been applied to the preparation of novel materials such as artificial clusters, clays, zeolites and so on. The hydrogen bonded [4 þ 4] clusters have a cubic hydrogen bonding network in a core-shell, which is covered by bulky triphenylmethyl groups. Figure 26.10 shows three kinds of [4 þ 4] ion-pair clusters of organic salts composed of four triphenylmethylamines and four organic carboxylic acids, sulfonic acids, and phosphonic acids in the crystalline state. Such clusters involving various carboxylates are obtained

Scheme 26.2 Formation of [4 þ 4] clusters from salts of organic sulfonic acids and primary amines.

26.3 Dynamics of Organic Crystals of Primary Ammonium Salts

Figure 26.10 Hydrogen bonding [4 þ 4] clusters composed of primary ammonium salts of triphenylmethylamine with organic acids, such as carboxylate, sulfonate and phosphonate.

only in a limited range of combinations, whereas those involving sulfonates are obtained in a wide range. The retention of the sulfonate clusters in solution was confirmed by 1 HNMR and mass spectroscopy. It is attractive that the cubic clusters exhibit supramolecular chirality like dice in our daily lives, since the clusters have two vortex-like patterns of the hydrogen bonding network; in a clockwise or anticlockwise direction. A wide range of organic sulfonic acids yield clusters with diverse sizes and shapes (Figure 26.11). The clusters have potency which derives from: the topology of the hydrogen bond networks, steric effects of the substituents and the acidities of the sulfonic acids. A sulfonate is a tridentate hydrogen bond acceptor and a primary ammonium is a tridentate hydrogen bond donor. Since each direction of the hydrogen bond is at the same tetrahedral angle, complementary pairs among primary ammoniums and sulfonates assemble into a cubic hydrogen bonding cluster without strain. The closed hydrogen bond network can be considered as zero dimensional. Bulky triphenylmethyl groups cover a core-shell which consists of a cubic hydrogen bonding network. High acidity overcomes other hydrogen bond acceptors and solvent molecules. These factors enable the cubic hydrogen bond networks to be tightly held together. The twelve hydrophobic phenyls of four triphenylmethyl groups

Figure 26.11 Different sizes and shapes of the [4 þ 4] clusters composed of various sulfonates: methane sulfonate (a), benzene sulfonate (b), and 2-anthraquinone sulfonate (c).

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shield the hydrophilic core, and segregate it from the outside. This unique feature, like a reverse micelle, gives high solubilities of the clusters in nonpolar solvents. Moreover, organic salts of triphenylmethylamine with some phosphonic acids yield more complicated [4 þ 4] clusters than those with sulfonates. The second hydrogen atoms in the ammonium phosphonates remain as OH groups without deprotonation. Hence, the additional hydrogen atoms serve as hydrogen-bond donors to yield four OH O hydrogen bonds in addition to the cubic-type hydrogen bonds. As a result, a total of sixteen hydrogen bonds produces a novel pseudodecahedral hydrogen-bond network with different topology. Since these complicated clusters form topological isomers, dynamical control of the topology may contribute to molecular informatics.

26.4 Dynamical Expression of Molecular Information in Organic Crystals 26.4.1 Hierarchical Structures with Supramolecular Chirality

At present there are no general principles for explaining crystal growth processes of organic molecules. In addition, it still remains very difficult to observe the process with the latest microscopes. These situations prompted us to overview the abovementioned studies which clarified various relations between the molecular structures and the crystal structures. Such an overview gave us an analogy between the crystals and proteins, resulting in an interpretation of hierarchy and three-dimensional chirality of the crystalline assemblies. This interpretation comes from the brief hypothesis that various small assemblies can be formed before total crystal structures. Figure 26.12 shows schematically that the steroidal molecules construct a hierarchical structure, as in the case of proteins. Namely, the molecules serve as primary structures (Figure 26.12a) and associate with their hydrogen-bonding groups of the skeletons and side-chains. The resulting secondary structures are naturally bimolecular assemblies (Figure 26.12b), followed by helical assemblies (Figure 26.12c). The helices are tied up in a bundle (Figure 26.12d), which corresponds to the tertiary structures and leaves cavities for accommodating guest components (Figure 26.12e). Since the steroidal molecules are highly asymmetric, the resulting hierarchical assemblies may have the corresponding three-dimensional structures with supramolecular chirality. Starting from molecular chirality, each assembly must be chiral. In this context, we encountered a new problem, how we describe such molecular and supramolecular chirality. The first idea is that the steroidal molecules are analogous to a vertebrate animal which has three-axial chirality based on three directions such as head–leg, right–left, and belly–back. The three-axial chirality enables us to determine the three-axes of the hierarchical assemblies, as in the case of the helices of proteins and DNA.

26.4 Dynamical Expression of Molecular Information in Organic Crystals

Figure 26.12 Hierarchical structures with supramolecular chirality for a folded polypeptide chain (top) and a cholic acid crystal (bottom). (a) A single molecule, (b) a bimolecular assembly, (c) a helical assembly, (d) a bundle of the helices, and (e) a host–guest system.

Kitaigorodskii pointed out the fact that molecules without symmetry elements tend to form 21 helical assemblies predominantly and induce close packing modes with chiral space groups, such as P21, P212121, and so on [18]. It was strange for us that there are no general rules to determine handedness of the 21 helical assemblies, and therefore we cannot determine their handedness [19]. In our daily lives, however, we have experiences of going up and down right- or left-handed stairs with 21 helical arrangements, indicating that the key structures, 21 helical assemblies, may have their handedness. We have introduced the term, three-axial and tilt chirality to define the handedness. This definition has proved to be powerful in the elucidation of many structural problems of hierarchical assemblies, prompting further research not only for steroidal molecules but also the surrounding organic molecules. The next section deals with our research directed toward this problem. 26.4.2 Expression of Supramolecular Chirality in Hierarchical Assemblies 26.4.2.1 Three-Axial Chirality In order to distinguish enantiomorphic structures, molecular chirality has conventionally been expressed in terms of center-, axis-, and plane-chirality. In the case of compounds involving several stereogenic centers, however, these terms seem to be insufficient to express their whole molecular chirality or anisotropy, although their local chirality is well confirmed in the conventional manner. Indeed, the steroidal molecules (Figure 26.13a), which have asymmetric, amphiphilic, facial structures with multiple stereogenic carbon atoms, are saddled with such a structural complexity. In order to solve this problem, we introduced a simple but unique concept, three-axial chirality, as shown in Figure 26.13b. Such three-axial chirality is based on the orthorhombic three axes applied in a molecular structure and is expressed by

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Figure 26.13 Expression for three-axial chirality of cholic acid molecule. (a) A molecular structure of cholic acid, (b) a space-filling model of cholic acid with the three-axial chirality expression, as in the case of (c) a vertebrate amimal.

the following terms: head and tail (leg), right and left, as well as belly and back, just like the direction of vertebrate animals in our daily lives (Figure 26.13c). 26.4.2.2 Tilt Chirality Generally, a symmetrical object is naturally achiral; its mirror image is identical with the original. On the other hand, an assembly composed of two objects can exhibit chirality and have distinguishable enantioisostructures, as shown in an illustration of polyhedrons (Figure 26.14a). This example is classical and must be well understood. The important thing here is that the appearance of supramolecular chirality requires a tilt between the two objects [20]. Thus, we have emphasized the use of the term, tilt chirality, to express such chirality [21], although the conventional term axis-chirality essentially comes from the same origin as the tilt chirality. Right- or left-handedness of the 21 helical molecular assembly cannot be determined from a mathematical and crytallographical viewpoint, because the assembly is produced by a twofold screw axis operation which includes 180 rotation and translation. However, since the assembly obviously gives a distinguishable enantiomorphic pair, it is reasonable to aspire to define the handedness of the 21 helical assembly. Indeed, we have proposed how to determine the handedness on the basis of the tilt chirality. As shown schematically in Figure 26.14b, given that the molecules are inclined to the right or left in front of a 21 screw axis, the assembly is defined to be right- or left-handed, respectively.

Figure 26.14 Supramolecular chirality provided by the tilt of the elements. (a) An enantiomorphic pair of polyhedrons, (b) definition of tilt chirality of a 21 helical assembly.


26.4.2.3 Helical and Bundle Chirality in a 21 Assembly In the case of facial and asymmetric molecules such as steroids, combination of the three-axial and tilt chirality enables us to express supramolecular chirality of the 21 helical assembly. The first step directed toward the molecular assembly lies in the formation of a bimolecular assembly. We employ a simple example to describe the helical chirality, as shown in Figure 26.15. When the molecules with three-axial chirality (Figure 26.15a) align in the same head-to-tail direction, they have the following three association modes; belly-to-back, belly-to-belly as well as back-toback (Figure 26.15b). In the case of the latter two modes, the bimolecular assemblies with a right- or left-tilt are expanded to the corresponding asymmetric 21 helical assemblies by twofold screw axis operation. In addition, an asymmetric helix is generally designated by the following three axes; right and left, up and down, and in and out (Figure 26.15c). Combination of these three-axial and tilt chirality enables us to define the helical chirality of the 21 helical assemblies, as shown in Figure 26.15d. In this figure, the up- and in-side of the helix may correspond to the head- and bellyside of the asymmetric molecule, respectively. The molecules are stacked with combinations of the belly-to-belly or back-to-back mode on the in-side, accompanied by the tilt, to give four kinds of bimolecular assemblies (a1)–(d1). They are extended to the corresponding 21 helical assemblies (a2)–(d2) [22].

Figure 26.15 Schematic representation of supramolecular chirality in the 21 helices and their bundles. (a) Three-axial chirality in an asymmetric molecule, (b) assembly modes of the two molecules, (c) terms expressing the chirality of the helix: right- or left-handed, up or

down, and in or out, (d) schematic representation of chirality in four kinds of bimolecular and 21 helical assemblies, and (e) chirality of the bundle of the helices, where the space groups of crystals are represented by stacking modes of the helices.

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The 21 helical assemblies are then tied up to yield bundles as tertiary structures. Considering the way to assembly the helices with helical chirality, the most popular and fundamental aggregation pattern in crystals is a parallel or anti-parallel alignment. In Figure 26.15e, the typical seven kinds of bundles of helices are shown. A uniform alignment of either left- or right-handed helices gives a chiral structure which is observed in the crystal with space group P21. Another bundle of either left- or right-handed helices with the reverse up-down directions also gives the corresponding chiral crystal with space group P212121. On the other hand, bundles composed of both left- and right-handed helices in a parallel or antiparallel fashion give achiral crystals with space groups such as Pbn21, Pnma, and P21/n. 26.4.3 Supramolecular Chirality of Hydrogen Bonding Networks

It is natural that steroidal crystals belong to chiral space groups, such as P21, P212121, and so on. But it was surprising to us that primary ammonium salts sometimes gave chiral crystals, even when achiral amines and carboxylic acids were employed [23]. Such chirality creation was ascertained in various crystalline assemblies with zero-, one-, and two-dimensionalities. This suggests that the assemblies involve supramolecular chirality through non-covalent bonds in contrast to molecular chirality through covalent bonds. This idea prompted us to thoroughly analyze the hydrogen bonding networks of the ammonium salts. Mathematical interpretation led us to the conclusion that we should recognize a topological aspect of the hydrogen bond networks in the crystals of primary ammonium carboxylates. This comes from the following two facts. First, that each cation and anion acts as a tridentate hydrogen bond donor and acceptor, respectively. Secondly, that one of two oxygen atoms of the carboxylate anion (O(a)) acts as a two hydrogen bond acceptor, while another (O(b)) acts as a one hydrogen bond acceptor (Scheme 26.3). This discrimination of the two oxygen atoms enables us to recognize absolute configurations of the primary ammonium cations, leading to the interpretation of supramolecular chirality of the zero-, one-, and two-dimensional networks [24]. Such a consideration enabled us to clarify the topological diversity of pseudocubic hydrogen bond networks composed of primary ammonium triphenylacetates. The carboxylates involve three kinds of hydrogen bond which display different combinations of four carboxylates on the vertices of the cube to generate nine different topologies. Figure 26.16a exemplifies an enantiomeric pseudocubic network with C2 symmetry. Such a topological issue may appear in onedimensional hydrogen bond networks of the carboxylates. Absolute configurations of the ammonium cations play a decisive role for discriminating supramolecular chirality of one-dimensional ladder-type hydrogen bond networks. As shown in Figure 26.16b, the ladder network involving the 21-axis is not superimposable on its mirror image, leading to the first definition of right- or left-handedness of its 21 helicity on the basis of supramolecular tilt chirality. Moreover, the 21 helical assemblies with three-axial chirality can be bundled in various ways to yield chiral crystals.


Scheme 26.3 Schematic representation of ladder hydrogen bond networks composed of primary ammonium cations and carboxylate anions with three possible alignments. The nitrogen atoms are classified into four achiral and four chiral isostructures.

Figure 26.16 Enantiomorphic pairs of chiral clusters (a) and ladders (b) which are composed of primary ammonium carboxylates.

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On the other hand, two-dimensional networks of the carboxylates involve a more complicated problem to discriminate the configurations of the nitrogen cations and the supramolecular chirality. 26.4.4 Expression of Molecular Information

For the past century, various relations between organic molecules and their assemblies have been made clear. It is well-known that spherical, axial, and facial molecules form hexagonal close packing, layered, and stacked assemblies, respectively. These relations have currently been extended to supramolecular synthons towards crystal engineering [25]. In this context, steroidal molecules with chiral, facial, curved, and amphiphilic features correspond to chiral supramolecular synthons for self-organization. In addition, our long-term study on the steroidal assemblies made clear a relation between organic molecules and their assemblies, bridging the gap between small molecules and biopolymers. We noticed that the steroidal assemblies bear a resemblance to proteins and gave us a novel concept for elucidating the relation between organic molecules and their assemblies. As mentioned above, steroidal assemblies consist of hierarchical structures with supramolecular chirality, leading us to find an analogy on the basis of the concept; molecular information and their expression of biopolymers. The information originates from sequential arrangements of a-amino acids. Since it is considered that the steroidal molecules consist of chains of methylene units with various substituents, the concept may be applied to steroidal molecules. Such sequential chains may be considered to hold for other related organic molecules, leading to the idea that chiral methylene chains with various substituents function as universal molecular storage. The methylene chains can be chemically modified to various sequential chains, such as polypetides, polynucleotides, polysaccharides, and ster-

Figure 26.17 Dynamical processes for molecular information and expression in the cases of protein (upper) and steroidal molecules (lower). (a) A molecule as an information carrier, (b) a molecular architecture based on folding or assembly, (c) a host-guest complexation, and (d) a guest exchange reaction.

References

oids. Their information of the chains can be expressed through three-dimensional architectures by the best use of non-covalent bonds. The architectures are folding structures of proteins as well as assemblies of the steroidal molecules, as shown schematically in Figure 26.17.

26.5 Conclusion and Perspectives

We have demonstrated dynamics in organic inclusion crystals of steroids and primary ammonium salts from various viewpoints. We started such a dynamical study with inclusion polymerization in channel-type cavities. Subsequently, we directed our attention toward host–guest assemblies of the steroids. Although we suffered from diverse crystal structures of steroids at the beginning, the dynamics of the host and guest molecules in the assemblies always encouraged us. During the long-term studies we acquired valuable knowledge about diversity, hierarchy and supramolecular chirality. Analogy with proteins played a decisive role in reaching the unique concept that chiral carbon chains of the steroidal molecules are similar to sequential peptide chains. The chains involve enormous isomers. For example, when twenty different amino acids make a sequential chain having ten amino acids, the chain has over a trillion isomers. This reminds us of a form of Japanese literature, thirty one- or seventeen-syllable short poems, called Tanka (Waka) or Haiku, respectively. The short poems have infinite expression, much as the steroidal molecules do. We hope such analogy plays a significant role in performing various research studies towards supramolecular crystal engineering as well as the prediction of crystal structures.

References 1 (a) MacNicol, D.D., McKendrick, J.J. and Wilson, D.R. (1978) Clathrates and molecular inclusion phenomena. Chem. Soc. Rev., 7, 66–87; (b) Davies, J.E.D., Kemula, W., Powell, H.M. and Smith, N.O. (1983) Inclusion compounds–past, present, and future. J. Inclusion Phenom., 1, 3–44; (c) Weber, E.(ed.) (1987) Top. Curr. Chem., 140; (1988) Top. Curr. Chem., 149; (d) Atwood, J.L., Davies, J.E.D. and MacNicol, D.D. (eds) (1984) Inclusion Compounds, Academic Press, London, vols 1–3;(1991) Oxford Press, Oxford, vols 4, 5; (e) MacNicol, D.D., Toda, F. and Bishop, R. (eds) (1996) Comprehensive Supramolecular Chemistry, Solid-State Supramolecular

Chemistry: Crystal Engineering, Vol. 6, Pergamon, Oxford; (f) Bishop, R. (1996) Designing new lattice inclusion hosts. Chem. Soc. Rev., 25, 311–319; (g) Herbstein, F.F. (2005) Crystalline Molecular Complexes and Compounds, vols. 1, 2, Oxford University Press, New York. 2 (a) Miyata, M., Tohnai, N. and Hisaki, I. (2007) Crystalline host–guest assemblies of steroidal and related molecules: diversity, hierarchy, and supramolecular chirality. Acc. Chem. Res., 40, 694–702; (b) Miyata, M., Tohnai, N. and Hisaki, I. (2007) Supramolecular chirality in crystalline assemblies of bile acids and

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their derivatives; three-axial, tilt, helical, and bundle chirality. Molecules, 12, 1973–2000. (a) Miyata, M. (1996) in Comprehensive Supramolecular Chemistry, Supramolecular Technology, vol. 10 (ed. D.N. Reinhoudt,), Pergamon, Oxford, pp. 557–582; (b) Miyata, M. (2004) in Encyclopedia of Supramolecular Chemistry, vol. 1 (eds J.L. Atwood and J.W. Steed), Marcel Dekker, New York, pp. 705–711. (a) Miyata, M., Shibakami, M., Chirachanchai, S., Takemoto, K., Kasai, N. and Miki, K. (1990) Guest-responsive structural changes in cholic acid intercalation crystals. Nature, 343, 446–447; (b) Matsumoto, A., Odani, T., Sada, K., Miyata, M. and Tashiro, K. (2000) Intercalation of alkylamines into an organic polymer crystal. Nature, 405, 328–330. (a) Yoswathananont, N., Sada, K., Nakano, K., Aburaya, K., Shigesato, M., Hishikawa, Y., Tani, K., Tohnai, N. and Miyata, M. (2005) The effect of a host-guest hydrogen bond on the inclusion of alcoholic guests in the host cavities of cholamide. Eur. J. Org. Chem., 5330–5338; (b) Aburaya, K., Nakano, K., Sada, K., Yoswathananont, N., Shigesato, M., Hisaki, I., Tohnai, N. and Miyata, M. (2008) Importance of weak hydrogen bonds in the formation of cholamide inclusion crystals with aromatic guests. Cryst. Growth Des., 8, 1013–1022. Kato, K., Sugahara, M., Tohnai, N., Sada, K. and Miyata, M. (2004) Drastic increase of flexibility of open host frameworks of a steroidal host compound by the shortened spacer. Eur. J. Org. Chem., 981–994. M€ uller, A., Reuter, H. and Dillinger, S. (1995) Supramolecular inorganic chemistry: small guests in small and large hosts. Angew. Chem., 107, 2505–2539; (1995); Angew. Chem., Int. Ed. Engl., 34, 2328–2361. Nakano, K., Sada, K., Nakagawa, K., Aburaya, K., Yoswathananont, N., Tohnai, N. and Miyata, M. (2005) Organic intercalation material: reversible change in

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interlayer distances by guest release and insertion in sandwich-type inclusion crystals of cholic acid. Chem. Eur. J., 11, 1725–1733. Nakano, K., Sada, K., Kurozumi, Y. and Miyata, M. (2001) Importance of packing coefficients of host cavities in the isomerization of open host frameworks: guest-size-dependent isomerization in cholic acid inclusion crystals with monosubstituted benzenes. Chem. Eur. J., 7, 209–220. (a) McKinnon, J.J., Mitchell, A.S. and Spackman, M.A. (1998) Hirshfeld surfaces: a new tool for visualising and exploring molecular crystals. Chem. Eur. J., 4, 2136–2141; (b) McKinnon, J.J., Spackman, M.A. and Mitchell, A.S. (2004) Novel tools for visualizing and exploring intermolecular interactions in molecular crystals. Acta Crystallogr. B, 60, 627–668. Kato, K., Aburaya, K., Miyake, Y., Sada, K., Tohnai, N. and Miyata, M. (2003) Excellent enantio-selective enclathration of (2R, 3S)3-methyl-2-pentanol in channel-like cavity of 3-epideoxycholic acid, interpreted by the four-location model for chiral recognition. Chem. Commun., 2872–2873. Aburaya, K., Hisaki, I., Tohnai, N. and Miyata, M. (2007) Dependence of the enantioselectivity on reversion of layer directions in cholamide inclusion compounds. Chem. Commun., 4257–4259. Mesecar, A.D. and Koshland, D.E. Jr, (2000) Structural biology: a new model for protein stereospecificity. Nature, 403, 614–615. Mizobe, Y., Tohnai, N., Miyata, M. and Hasegawa, Y. (2005) Tunable solid-state fluorescence system consisted of organic salts of anthracene-2, 6-disulfonic acid with primary amines. Chem. Commun., 1839–1841. Mizobe, Y., Ito, H., Hisaki, I., Miyata, M., Hasegawa, Y. and Tohnai, N. (2006) A novel strategy for fluorescence enhancement in the solid-state: affording rigidity to fluorophores packing. Chem. Commun., 2126–2128.

References 16 Mizobe, Y., Miyata, M., Hisaki, I., Hasegawa, Y. and Tohnai, N. (2006) Anomalous anthracene arrangement and the corresponding rare excimer emission in the solid-state by transcription and translation of molecular information. Org. Lett., 8, 4295–4298. 17 Tohnai, N., Mizobe, Y., Doi, M., Sukata, S., Hinoue, T., Yuge, T., Hisaki, I., Matsukawa, Y. and Miyata, M. (2007) Well-designed Supramolecular Clusters Comprising Triphenylmethylamine and Various Sulfonic Acids. Angew. Chem., 119, 2270–2273; (2007) Angew. Chem. Int. Ed., 46, 2220–2223. 18 Kitaigorodskii, A.I. (1973) Molecular Crystals and Molecules, Academic Press, London. 19 Hahn, T. (ed.) (1983) International Tables for Crystallography, Vol. A, Space-Group Symmetry, Kluwer Academic Publishers, London, pp. 6048–6055. 20 Tanaka, A., Hisaki, I., Tohnai, N. and Miyata, M. (2007) Supramolecular tiltchirality derived from symmetric benzene molecules: handedness of the 21 helical assembly. Chem. Asian J., 2, 230–238. 21 Hisaki, I., Tohnai, N. and Miyata, M. (2008) Supramolecular tilt chirality in crystals of steroids and alkaloids. Chirality, 20, 330–336.

22 Watabe, T., Hisaki, I., Tohnai, N. and Miyata, M. (2007) Four kinds of 21 helical assemblies with the molecular tilt as well as three-directional and facial chirality. Chem. Lett., 36, 234–235. 23 Tanaka, A., Inoue, K., Hisaki, I., Tohnai, N., Miyata, M. and Matsumoto, A. (2006) Supramolecular chirality in layered crystals of achiral ammonium salts and fatty acids: a hierarchical interpretation. Angew. Chem., 118, 4248–4251; (2006) Angew. Chem. Int. Ed., 45, 4142–4145. 24 (a) Yuge, T., Tohnai, N., Fukuda, T., Hisaki, I. and Miyata, M. (2007) Topological study of pseudo-cubic hydrogen bond networks in a binary system composed of primary ammonium carboxylates: an analogue of ice cube. Chem. Eur. J., 13, 4163–4168; (b) Yuge, T., Sakai, T., Kai, N., Hisaki, I., Miyata, M. and Tohnai, N. (2008) Topological classification and supramolecular chirality of 21-helical ladder-type hydrogen-bond networks composed of primaryammonium carboxylates: bundle controls in 21-helical assemblies. Chem. Eur. J., 14, 2984–2993. 25 Desiraju, G.R. (1995) Supramolecular synthons in crystal engineering—a new organic synthesis. Angew. Chem., Int. Ed. Engl., 34, 2311–2327.

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27 Morphology Changes of Organic Crystals by Single-Crystal-to-Single-Crystal Photocyclization Hideko Koshima

27.1 Introduction

Since the study of [2 þ 2] photocyclization of trans-cinnamic acids in 1964 [1], a large number of crystalline state reactions have been reported [2–6]. The relationship between the crystal structures and the reactions has been elucidated. Crystalline state photoreactions should give rise to molecular motion in the crystal lattice and thereby changes in surface morphology. However, the morphological changes in crystals during the reactions have never been intensively studied. Recently, actuators based on molecular crystals that change morphology in response to light have attracted interest. For the development of crystal actuators, it is necessary to elucidate the correlation between the structural changes and the morphological changes. It was first reported that the topochemical photopolymerization of diolefin crystals gave rise to cracks and deformation [7]. An atomic force microscopic (AFM) study made possible the observation that the photodimerizations of trans-cinnamic acids and anthracenes in the crystalline state induced surface morphological changes at the tens and hundreds of nanometers level by the transportation and rebuilding of the surface molecules [8]. The appearance of a surface relief grating on the single crystal of 4-(dimethylamino)azobenzene was demonstrated by repeated irradiation with two coherent laser beams [9]. A recent study of the mechanical motion of photochromic diarylethene crystals is outstanding [10–14]. New steps appeared on the crystal surface on irradiation with UV light and disappeared with visible light [10]. The square single crystals changed reversibly to the lozenge shape upon irradiation and the rectangular crystals reversibly contracted and expanded [11]. The rapid and reversible bending of the crystalline rod on irradiation gave us the visual imagination for the development of crystal actuators [11]. The rolling thin crystals and the formation of microfibrils were reported [12, 13]. Furthermore, the diarylethene microcrystals made directional jumps upon UV irradiation [14]. Other jumping crystals induced by thermal phase

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transition are also known, myo-inositol [15, 16], hexadecahydropyrene [17, 18], and inorganic spinels [19]. Isopropylbenzophenone derivatives are well known compounds to undergo Norrish type II photocyclization in the crystalline state on UV irradiation [20]. Asymmetric induction is also possible by using the salt crystals of carboxylic acids with the chiral amines [21–24]. Furthermore, the absolute asymmetric photocyclization was achieved using the chiral salt crystals self-assembled from diisopropylbenzophenone derivative and achiral amine [25]. Some of the crystals reacted without any cracks or break due to small changes in the crystal structures, which involved single-crystal-to-single-crystal transformation [22–27]. For correlation between morphological changes and structural changes, single-crystal-to-single-crystal reactions are indispensable because the changes in morphology can be continuously determined throughout the reactions and the reaction processes can be traced by X-ray crystallographic analysis of the single crystals. We would like to introduce our recent study of the morphological changes in isopropylbenzophenone crystals via single-crystal-to-single-crystal photocyclization. First, the salt crystals gave rise upon photoirradiation to unevennesses on the surfaces and returned to the initial smooth surfaces after completion of the reaction [28, 29]. Second, the bulk crystals of the triisoproylbenzophenone derivative broke on irradiation despite the single-crystal-to-single-crystal transformation, but the microcrystals maintained the single-crystalline morphology during the course of the photocyclization [26].

27.2 Surface Morphology Changes in the Salt Crystals of a Diisopropylbenzophenone Derivative with Amines via Single-Crystal-to-Single-Crystal Photocyclization 27.2.1 Solid-State Photocylization

The salt crystals of 1(S)-2 of 4-(2,5-diisopropylbenzoyl)benzoic acid 1 with (S)-phenylethylamine (S)-2 were prepared by recrystallization from a methanol solution of both components (Scheme 27.1a). The crystals underwent enantiospecific photocyclization on irradiation at >290 nm with a high-pressure mercury lamp through Pyrex glass under argon at 288 K to give a cyclobutenol (S)-4 in almost quantitative optical yield and 100% chemical yield [23]. The chiral crystals were spontaneously obtained from the solutions of 1 and 2,4-dichlorobenzylamine 3 (Scheme 27.1b). This kind of chiral crystallization of achiral molecules leads neccessarily to left- and right-handed crystals. The enantiomeric crystals of M- and P-13 were selectively prepared by seeding. Irradiation of the crystals of M-13 at around 350 nm through a UV filter gave a cyclopentenol (R,R)-5 as almost the sole product, achieving absolute asymmetric photocyclization [25]. In contrast, irradiation at >290 nm afforded (R,R)-5, (R)-4, and (R)-hydrol in a 6:3:1 molar ratio; the enantiomeric excesses of the three products were higher than

27.2 Surface Morphology Changes in the Salt Crystals

Scheme 27.1 (a) Enantiospecific and (b) absolute asymmetric photocyclization via single-crystal-to-single-crystal transformation.

80% ee at about 90% conversion, that is, showing lower product selectivity than the irradiation at around 350 nm. 27.2.2 Crystal Structures and the Reaction Mechanism

Photoirradiation of a single crystal of 1(S)-2 maintained the initial transparency, confirming the single-crystal-to-single-crystal transformation. Finally, a piece of single crystal (1.28 0.21 0.09 mm3) of 1(S)-2 was submitted to X-ray crystallographic analysis at 293 K before and after successive irradiation at >290 nm at 293 K [23]. The absolute structure was determined on the basis of the S configuration of the phenylethylamine molecule 2. The reaction proceeded smoothly and was completed after irradiation for 45 min. The crystal data are summarized in Table 27.1. Table 27.1 Unit cell constants before and after photoirradiation.

M-13

1(S)-2 Before irrad.

Irrad. for 45 min (S)-4(S)-2

Before irrad.

Irrad. for 712 ha

(change, %) Meas. temp./K Space group a/Å b/Å c/Å V/Å3 Dc/g cm1

293 P212121 6.1890(5) 14.411(1) 28.732(2) 2562.6(4) 1.119

293 P212121 6.3312(3) 13.776(1) 28.510(2) 2486.6(3) 1.153

Product: reactant ¼ (R,R)-5: 1 ¼ 53: 47.

a

(þ2.29) (4.41) (0.77) (2.97) (þ3.04)

(change, %) 293 P212121 6.3028(5) 12.7985(9) 31.838(4) 2568.2(4) 1.258

293 P212121 6.235 13.146 31.913 2616.0 1.235

(1.08) (þ2.71) (þ0.24) (þ1.84) (1.83)

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The sizes of the unit cells changed slightly, increasing ( þ 2.29%) along the a-axis, decreasing (4.41%) along the b-axis, and decreasing (0.77%) along the c-axis, resulting overall in a decrease (2.97%) in total cell volume. The ORTEP drawing of a salt bond pair arranged in the reactant 1(S)-2 is shown in Figure 27.1a. Irradiation of the reactant 1(S)-2 causes np excitation of the C1¼O1 carbonyl group of the benzophenone unit. The O1 atom abstracts the methine H1 hydrogen atom of the o-isoproyl group in the highest priority due to the shortest O1 H1 distance (3.15 Å) to give the ketyl radical .C1 and the methine radical .C2. Another hydrogen abstraction from the m-isoproyl group does not occur due to the

Figure 27.1 ORTEP drawings of (a) the reactant 1(S)-2 and (b) the product (S)-4(S)-2 at 25% probability level, (c) stereoview of 1(S)-2, and the molecular arrangements on the (100) (d and e) and (010) faces (f and g) in 1(S)-2 and (S)-4

(S)-2, respectively. Note that the methine H1 hydrogen atom in (a) is projected towards the carbonyl O1 oxygen atom for abstraction to yield the cyclobutenol (S)-4. (From Ref. [28] with permission. 2008 Am. Chem. Soc.)


long O1 H2 distance (5.19 Å). Subsequently, the .C1 and .C2 biradicals are enantiospecifically coupled to produce the cyclobutenol (S)-4 (Figure 27.1b). Such a photocyclization, however, did not greatly change the molecular conformation. The ionic bridge between the carboxylate anion of 1 and the ammonium cation of (S)-2 forms a twofold helical chain in the reactant crystal along the a-axis. The stereoview and the molecular arrangements in the (100) and (010) planes are shown in Figure 27.1c, d, and f, respectively. The helical salt bridges remain after the reaction and are arranged in a similar manner in the product crystal (Figure 27.1e and g). The strong salt bonding is believed to fix the molecules and promote complete reaction without destroying the crystal, leading to single-crystal-tosingle-crystal transformation. The single crystals of 13 were also transparent after irradiation, confirming the single crystalline phase reaction. The single crystal of M-13 was successively irradiated at around 350 nm with a superhigh-pressure mercury lamp through a UV transparent filter. The reaction proceeded very slowly and was not complete even after irradiation for 712 h (Table 27.1). The structure was solved as a disordered structure of the product (R,R)-5 and the remaining reactant 1 in 53 : 47 occupancy. The sizes of the unit cells changed slightly, decreasing (1.08%) along the a-axis, increasing ( þ 2.71%) along the b-axis, and increasing ( þ 0.24%) along the c-axis, resulting overall in an increase ( þ 1.84%) in total cell volume. The molecular arrangement was similar to that of 1(S)-2; a twofold helical chain through the salt bridge between the carboxylate anion of 1 and the ammonium cation of 3. The reaction path to give (R,R)-5 could be explained based on the crystal structure [26]. 27.2.3 Morphology Changes in Bulk Crystals

The morphological changes in the salt crystals of 1(S)-2 on photoirradiation were determined by AFM [28]. The single crystals prepared by evaporating the methanol solutions of both components at room temperature gave needle-shaped crystals grown along the a-axis (Figure 27.2A). The (001) face was the most developed, followed by the (010) face. The (100) face was cut perpendicular to the (001) and (010) faces with a razor blade to afford a piece of single crystal (210 209 68 mm3). The crystal was then irradiated at >290 nm with a high-pressure mercury lamp through Pyrex glass under argon at 288 K. Figure 27.2B shows the AFM images of the changes in surface morphology during the reaction. Before irradiation, the (001) face was flat (a). Upon UV irradiation, numbers of hemispheric unevennesses appeared on the (001) surface and these gradually grew to heights of tens of nanometers after irradiation for 20 min (b). The relative unevenness was less than 0.01% of the crystal thickness (68 mm), indicating that it was limited to the crystal surface. Prolonged irradiation for 100 min resulted in merger of the hemispheres and a return to the initial smooth surface (c). Similar morphological changes appeared on the (010) face on photoirradiation (d–f). In contrast, the (100) face was very rough with heights of several nanometers due

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Figure 27.2 (A) Crystal habit of 1(S)-2 and (B) the AFM images of the morphological changes on UV irradiation for 0, 20, and 100 min: (a–c) (001), (d–f) (010), and (g–i) (100) faces. (From Ref. [28] with permission. 2008 Am. Chem. Soc.)`

to the cut with the razor blade (g). UV irradiation decreased the roughness (h) and finally gave a very smooth surface (i). The completion of reaction after irradiation for 100 min was confirmed by HPLC analysis. The smooth progress of the reaction is consistent with the result obtained in the X-ray crystallographic analysis of a single crystal of 1(S)-2 (1.28 0.21 0.09 mm) in which irradiation for 45 min gave the product crystal (S)-4(S)-2 (Table 27.1). 27.2.4 Morphology Changes in Microcrystals

Next, the microcrystals of 1(S)-2 were submitted to UV irradiation [29]. The microcrystals of 1(S)-2 were prepared by dropping equimolar solutions of both components in methanol onto quartz plates and evaporating the solvents to give the long needle-shaped crystals (Figure 27.3A). The top surface was determined to be


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Figure 27.4 (a) IR spectral changes and (b) the conversion of the photocyclization of 1 in the microcrystals of 1(S)-2 on UV irradiation.

the (001) face from comparison with the needle-shaped bulk crystals grown along the a-axis (Figure 27.2A). The photocyclization process was monitored by FTIR spectroscopy. Figure 27.4a shows the change in the IR spectrum under UV irradiation at >290 nm. The intensities of the reactant 1(S)-2 band at 1667 cm1, the stretching vibration due to the benzophenone carbonyl group, decreased with increasing UV irradiation time. Irradiation for longer than 30 min did not change the IR spectrum, showing that the photocyclization was complete within 30 min. The conversion was calculated based on the intensity change in the absorption at 1667 cm1 to give an almost linear relationship with the irradiation time (Figure 27.4b). The morphological changes in the microcrystal of 1(S)-2 during the photocyclization were observed by AFM (Figure 27.3C and D). Before irradiation, the (001) surface was flat (a). Upon UV irradiation at >290 nm, unevennesses appeared on the surface (b), and these gradually grew to a height of 50–70 nm at 75% conversion after 20 min (c); the relative unevenness was about 7–10% for the crystal thickness (680 nm). Irradiation for 30 min completed the reaction and decreased the height of unevennesses to about 20 nm (3% relative unevenness) (d). Prolonged irradiation for 90 min further decreased the unevenneses to several nanometers (1% relative unevenness), returning to a nearly smooth surface (e). The whole crystal shape did not change much before and after irradiation. In the case of the bulk crystal (68 mm thickness), similar unevennesses with heights of tens of nanometers appeared on the (001) surface (290 nm due to the decrease in light intensity. Similar unevennesses appeared on the (001) surface and these gradually grew to a height of 15 nm at 67% conversion after 70 min; about 2% relative unevenness for the crystal thickness (830 nm). Irradiation for 110 min completed the reaction, increasing the height of the unevennesses to about 25 nm (3% relative unevenness). On irradiation for 180 min the unevennesses grew large,r to about 40 nm (5% relative unevenness). Prolonged irradiation for 300 min at last decreased the unevenneses to about 30 nm (4% relative unevenness). Thus, irradiation at around 350 nm delayed the appearance and disappearance of unevennesses. The morphological changes in the microcrystals of 13 during the photocyclization were also determined by AFM. The microcrystals of 13 prepared by dropping equimolar solutions of both components in ethanol onto quartz plates and evaporating the solvents gave long needle-shaped crystals, similar to those of 1(S)-2 in Figure 27.3A. The top surface was determined to be the (001) face from comparison with the needle-shaped bulk crystals grown along the a-axis. Figure 27.5 shows the morphological changes in the microcrystals of 13 during photocyclization. Before irradiation, the (001) face was nearly flat (a). Upon UV irradiation at >290 nm, unevennesses appeared on the (001) surface and these gradually grew to a height of about 15 nm at 66% conversion after 12 h (b); the relative unevenness was about 2% of the crystal thickness (700 nm). Irradiation for 25 h completed the reaction and slightly decreased the height of unevennesses to about 10 nm (1.5% relative unevenness) (c). Prolonged irradiation for a further 40 h decreased the unnevennesses to 2 nm (0.7% relative unevenness) (d). The whole crystal shape did not change much before and after irradiation. 27.2.5 Correlation between the Morphology Changes and the Crystal Strucural Changes

The crystalline state reaction proceeds heterogeneously from the surface to the inside on UV irradiation. The X-ray crystal data (Table 27.1) of 1(S)-2 before and after irradiation reveal that the crystal expands slightly along the a-axis and contracts along the b- and c-axes, without destroying the crystal. Therefore, some stress should be induced within the crystal lattice. The helical salt bond chains are strong, like molecular springs. In contrast, the intermolecular interaction among the neighboring salt bond chains is weak due to van der Waals forces alone, as shown in Figure 27.1d and f, suggesting that the (001) plane can be cleaved in a thin layer. Each molecular chain near the (001) surface can move along the a-axis, which leads to the appearance of hemispheric unevennesses on the (001) surface shown in Figures 27.2b and 27.3b–d. When the reaction is complete, the inner stress disappears, and an essentially smooth surface is restored (Figures 27.2c and 27.3e).

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Figure 27.5 (A) AFM images of the changes in surface morphology of the microcrystal of 13 and (B) the section on photoirradiation for (a) 0 h, (b) 12 h (66% conversion), (c) 25 h (100% conversion), and (d) 40 h (100% conversion).

A similar explanation can be applied for the morphological changes of the (010) surface (Figure 27.2d–f ). In contrast, the roughness of the (100) face of 1(S)-2 decreased on irradiation and the surface ultimately became flat (Figure 27.2g–i). The experimental results provide the visual evidence that the molecules on the (100) surface moved inside or outside the crystal leading to a decrease in the surface energy to a minimum,

27.3 Morphology Changes in Triisobenzophenone Crystals

resulting in the formation of a very smooth surface. This is like photochemical annealing of a crystal. To evaluate the molecular mobility during the morphological change, the melting point change was measured. The melting point of the microcrystals of 1(S)-2 before photoirradiation was 163–164 C, decreasing to a minimum of 92–106 C on irradiation due to the coexistence of the reactant 1(S)-2 and the product (S)-4(S)2 in the crystals. The melting point then reached an almost constant value of 102–107 C due to the formation of (S)-4(S)-2 alone at the completion of photocyclization. The considerable decrease in the melting point should facilitate the movement of molecules in the photochemical process. The same explanation for the morphological changes in the salt crystal of 13 is applicable because the molecular arrangement is similar to that of 1(S)-2.

27.3 Morphology Changes in Triisobenzophenone Crystals via Diastereospecific Single-Crystal-to-Single-Crystal Photocyclization 27.3.1 Solid-State Photocyclization and the Crystal Structures

Crystals of 2-(2,4,6-triisopropylbenzoyl)((S)-1-phenylethyl)benzamide (S)-6 were irradiated at >290 nm with a high-pressure mercury lamp through Pyrex glass under argon at 293 K for 8 h to afford a cyclobutenol (R, S)-7 in 100% chemical yield and 100% de, revealing the diastereospecific photocyclization in the crystalline state (Scheme 27.2) [26]. In contrast, the solution photolysis of (S)-6 in acetonitrile gave (R, S)- and (S, S)-7 in 24% and 21% chemical yield, respectively, revealing the low diastereoselectivity of only 6% de. X-Ray crystallographic analyses of the reactant and the product were carried out to elucidate the reaction mechanism for the diastereospecific photocyclization. Irradiation of the single crystal of (S)-6 at >290 nm with a high-pressure mercury lamp led to breakage of the crystal. Therefore, the reaction was not thought to be single-crystal-to-single-crystal transformation. However, careful irradiation at around 350 nm through a UV filter for 180 h completed the reaction without any cracking so that the X-ray structure analysis was successful [27]. Table 27.2 summarizes the crystal data. The size of the unit cells changed slightly, increasing

Scheme 27.2 Diastereospecific single-crystal-to-single-crystal photocyclization.

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Table 27.2 Selected crystal data before and after photoirradiation.

(S)-6 Before irrad.

Irrad. for 180 h (R,S)-7 (change, %)

Meas. temp./K Space group a/Å b/Å c/Å V/Å3 Dc/g cm3

173 P212121 9.5739(16) 13.154(3) 21.267(5) 2678.3(10) 1.130

173 P212121 9.742(2) 13.735(4) 20.009(6) 2677.4(13) 1.130

(þ1.76) (þ4.42) (–5.92) (–0.03) (0.00)

( þ 1.76%) along the a-axis, increasing ( þ 4.42%) along the b-axis, and decreasing (5.92%) along the c-axis, resulting overall in no change (0.0%) in the total cell volume. ORTEP drawings of the reactant (S)-6 and the product (R, S)-7 are shown in Figure 27.6a and b, respectively. The molecular conformations are very similar. In the

Figure 27.6 ORTEP drawings of (a) (S)-6 and (b) (R,S)-7 at 15% probability level, and the molecular arrangements in (c) (S)-6 and (d) (R,S)-7.

27.3 Morphology Changes in Triisobenzophenone Crystals

crystal of (S)-6, a twofold helical chain is formed among the amide groups through the NH O¼C hydrogen bond (2.12 Å) along the b axis (Figure 27.6c). A similar twofold helical chain is also formed in the product crystal of (R, S)-7 among the amide groups through the NH O¼C hydrogen bond (2.32 Å) along the b axis, and a further intramolecular OH O¼C hydrogen bond (1.86 Å) is formed between the hydroxy group and the amide group. Irradiation of the crystals of (S)-6 at >290 nm with a high-pressure mercury lamp through Pyrex glass causes np excitation of the carbonyl group of the (S)-6 molecule (Figure 27.6a). The O1 atom has the possibility of abstraction of both the methine H1 and H2 hydrogen atoms of two o-isopropyl groups to produce the ketyl radical and corresponding methine radicals, because both distances of O1 H1 (2.84 Å) and O1 H2 (2.97 Å) are short enough and similar, and the angles of C1O1H1 (57.1 ) and C1O1H2 (53.2 ) are also similar. Then, the radicals should approach each other and finally couple to afford (R)- and (S)-cyclobutenol, that is, both the diastereomeric products (R,S)-7 (Figure 27.6b) and (S,S)-7. However, only (R,S)-7 was obtained by the crystalline state photocyclization. This means that the occurrence of the diastereospecific O1 H1 hydrogen abstraction could not be explained from the distance and angle parameters between the carbonyl oxygen atom and the two g-hydrogen atoms. Recent intensive studies have revealed that crystalline state reactions proceed generally with the minimum molecular motion in the crystal lattice because the molecules are arranged at close positions in three-dimensional regularity and the motion is very restricted [2–6]. As shown in Figure 27.6, the similarity of the molecular shapes as well as the twofold helical arrangements in the crystals suggests that the transformation from (S)-6 to (R, S)-7 can proceed smoothly with minimum motion within the helical chains, leading to the singlecrystal-to-single-crystal reaction. 27.3.2 Morphology Changes

Despite the single-crystal-to-single-crystal transformation of (S)-6, photoirradiation of the bulk crystals led to cracking after several minutes (Figure 27.7). The following

Figure 27.7 Morphology change in the bulk crystal of (S)-6 (a) before and (b) after UV irradiation. The top surface is the (100) plane.

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explanation is possible. The photocyclization proceeds from the surface to the inside of the bulk crystal. Therefore, upon UV irrradiation, the (100) surface expands slightly along the b-axis and contracts along the c-axis, as understood from the crystal data in Table 27.2, and the strain is generated within the crystal lattice leading to cracking and breaking into polycrystals. A recent report encouraged us that in the topochemical polymerization by successive [2 þ 2] photocyclization of diolefin derivatives, the bulk crystals were broken into fragments during polymerization but the nanocrystals maintained the single-crystalline phase in the course of polymerization [30]. We tried UV irradiation of the microcrystals of (S)-6. The microcrystals were prepared by vaporizing the powder samples on a heater at slightly lower temperature than the melting point, collecting the vapor on a quartz plate, and crystallizing at room temperature (Figure 27.8a). The photocyclization process was monitored by FTIR spectroscopy. The intensities of the reactant (S)-6 band at 1678 and 1640 cm1 for the stretching vibration due to the benzophenone carbonyl group and the amide carbonyl group, respectively, decreased with increasing irradiation time. Conversely, the intensity of the product (R,S)-7 band at 1609 cm1 for the stretching vibration due to the amide carbonyl group increased. Irradiation for longer than 10 min did not change the IR spectrum, revealing that the photocyclization was complete within 10 min. The conversion was calculated based on the intensity change in the absorption at 1678 cm1 to give an almost linear relationship with irradiation time. Figure 27.8b shows the AFM image of a piece of microcrystal before irradiation. The top surface is the (100) plane. In contrast to the bulk crystals, the microcrystal maintained the single-crystalline phase during photocyclization (Figure 27.8c and d).

Figure 27.8 Microcrystals of (S)-6 and AFM images of the morphological change on UV irradiation: (a and b) before irradiation, (c) irradiation for 4 min (50% conversion), and (d) 10 min (100% conversion).

References

No cracks were found during or after the completion of reaction. The surface morphology and the shape of the microcrystal were the same as the reactant crystal. The crystal should expand slightly along the a- and b-axis and contract along the c-axis, but not change the volume (Table 27.2). Therefore, the inner strain accumulated in the microcrystals is so small that the reaction can be completed while keeping the initial single-crystalline shape.


During single-crystal-to-single-crystal phototransformation, the salt crystals of the diisopropylbenzophenone derivative with amines changed their surface morphology on photoirradiation but returned to the initial smooth surfaces after completion of the reaction. We also found that the bulk crystals of the triisoproylbenzophenone derivative broke on UV irradiation, but the microcrystals maintained the singlecrystalline morphology during photocyclization. The morphological changes could be explained based on the crystal structure changes. However, many subjects remain for the elucidation of the correlation between the motion of molecules and the morphological changes in bulk crystals. More intensive and basic research is necessary for the development of mechanical crystal devices.

Acknowledgments

This study was supported by the Grant-in-Aid for Scientific Research in Priority Area (Area No. 432, No. 17034047) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Asahi Glass Foundation.

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6 Inoue, Y. and Ramamurthy, V. (eds) (2004) Chiral Photochemistry, Marcel Dekker Inc., New York. 7 (a) Nakanishi, H., Hasegawa, M. and Sasada, Y. (1972) Four-center type photopolymerization in the crystalline state. V. X-Ray crystallographic study of the polymerization of 2,5-distyrylpyrazine. J. Polym. Sci., A-2, 10, 1537–1553; (b) Nakanishi, H., Hasegawa, M., Kirihara, H. and Yurugi, T. (1977) Various morphological changes in the solid-state photopolymerization of diolefinic compounds. Nippon. Kagaku. Zasshi., 1046–1050.

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8 (a) Kaupp, G. (1992) Photodimerization of cinnamic acid in the solid state: new insights on application of atomic force microscopy. Angew. Chem. Int. Ed. Engl., 31, 592–595; (b) Kaupp, G. (1992) Photodimerization of anthracenes in the solid state: new results from atomic force microscopy. Angew. Chem. Int. Ed. Engl., 31, 595–598; (c) Kaupp, G. and Plagmann, M. (1994) Atomic force microscopy and solid state photolyses: phase rebuilding. J. Photochem. Photobiol. A: Chem., 80, 399–407. 9 Nakano, H., Tanino, T. and Shirota, Y. (2005) Surface relief grating formation on a single crystal of 4-(dimethylamino) azobenzene. Appl, Phys. Lett., 87, 061910. 10 Irie, M. Kobatake, S. and Horichi, M. (2001) Reversible surface morphology changes of a photochromic diarylethene single crystal by photoirradiation. Science, 291, 188–191. 11 Kobatake, S., Takami, S., Muto, H., Ishikawa, T. and Irie, M. (2007) Rapid and reversible shape changes of molecular crystals on photoirradiation. Nature, 446, 778–781. 12 Uchida, K., Sukata, S., Matsuzawa, U., Akazawa, M., de Jong, J.J.D., Katsonis, N., Kojima, Y., Nakamura, S., Areephong, J., Meetsma, A. and Feringa, B.L. (2008) Photoresponsive rolling and bending of thin crystals of chiral diarylethenes. Chem. Comm., 326–328. 13 Uchida, K., Izumi, N., Sukata, S., Kojima, Y., Nakamura, S. and Irie, M. (2006) Photoinduced reversible formation of microfibrils on a photochromic diarylethene microcrystalline surface. Angew. Chem. Int. Ed., 45, 6470–6473. 14 (a) Colombier, I., Spagnoli, S., Corval, A., Baldeck, P.L., Giraud, M., Leaustic, A. and Yu, P. (2005) Strong photomechanical effects in photochromic organic microcrystals. Mol. Cryst. Liq. Cryst., 431, 195–199; (b) Colombier, I., Spagnoli, S., Corval, A., Baldeck, P.L., Giraud, M., Leaustic, A., Yu, P. and Irie, M. (2007) Diarylethene microcrystals make

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References 22 Hirotsu, K., Okada, K., Mizutani, H., Koshima, H. and Matsuura, T. (1996) X-ray study of asymmetric photoreaction of 2,4,6-triisopropyl-40 -benzophenone. Mol. Cryst. Liq. Cryst., 277, 99–106. 23 Koshima, H., Matsushige, D. and Miyauchi, M. (2001) Enantiospecific single crystal-to-single crystal photocyclization of 2,5-diisopropyl-40 -carboxybenzophenone in the salt crystals with (S)- and (R)phenylethylamine. Cryst.Eng.Comm., 33, 1–3. 24 Koshima, H., Ide, Y., Fukano, M., Fujii, K. and Uekusa, H. (2008) Single-crystal-tosingle-crystal photocyclization of 4-(2,4,6triisopropylbenzoyl)benzoic acid in the salt crystal with (S)-phenylethylamine. Tetrahedron Lett., 49, 4346–4348. 25 Koshima, H., Kawanishi, H., Nagano, M., Yu, H., Shiro, M., Hosoya, T., Uekusa, H. and Ohashi, Y. (2005) Absolute asymmetric photocyclization of isopropylbenzophenone derivatives using cocrystal approach involving single-crystalto-single-crystal transformation. J. Org. Chem., 70, 4490–4497.

26 Koshima, H., Fukano, M. and Uekusa, H. (2007) Diastereospecific photocyclization of isopropylbenzophenone derivative in crystals and the morphological changes. J. Org. Chem., 72, 6786–6791. 27 Fujii, K., Uekusa, H., Fukano, M. and Koshima, H., unpublished. 28 Koshima, H. Yuya, I. and Ojima, N. (2008) Surface morphology changes of a salt crystal of 4-(2,5-diisopropylbenzoyl) benzoic acid with (S)-phenylethylamine via single-crystal-to-single-crystal photocyclization. Cryst. Growth Des., 8, 2058–2060. 29 Koshima, H., Ide, Y., Yamazaki, S. and Ojima, N. (2009) Changes in the surface morphology of salt crystals of 4-(2,5diisopropylbenzoyl)benzoic acid with amines via single-crystal-to-single-crystal photocyclization. J. Phys. Chem. C., 113, 111683–111688. 30 Takahashi, S., Miura, S., Okada, S., Oikawa, H. and Nakanishi, H. (2002) Single-crystalto-single-crystal transformation of diolefin derivatives in nanocrystals. J. Am. Chem. Soc., 124, 10944–10945.

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Part Five Single Biocells

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28 Femtosecond Laser Tsunami Processing and Light Scattering Spectroscopic Imaging of Single Animal Cells Hiroshi Masuhara, Yoichiroh Hosokawa, Takayuki Uwada, Guillaume Louit, and Tsuyoshi Asahi

28.1 Introduction

Spectroscopy and imaging have received much attention, not only for fundamental research in physics and chemistry but also for analyzing structures and functions of materials, devices, biological systems, and so on. Nowadays such studies utilizing spectroscopy and imaging are oriented toward bio and medical applications, and many researchers in life science, biotechnology, and medicine have introduced advances in laser and microscope technologies to their specific fields and proposed new measurement, processing, molecular and nanoparticle probes, and so on [1–5]. In general in situ analysis of dynamic structures, properties, and functionalities of living cells and tissues is performed by fluorescence and Raman spectroscopies, coherent anti-Stokes Raman spectroscopy(CARS), second harmonic generation, fluorescence lifetime measurement, and their imaging [6–11]. Fluorescent, Raman-active, and/or centro-symmetric molecules are necessary, but they are not always incorporated in living systems. Therefore new spectroscopic and imaging methods with higher spatial and temporal resolutions are needed for unraveling novel functionality. Spectroscopy and imaging need suitable molecules and nanoparticles which can probe the properties and functions of components and organs in cells and tissues. When we do not add such probes, we consequently detect signals from the component molecules constituting living systems, for example, autofluorescence. The analysis is usually not easy as the photophysical and photochemical natures of the molecules in the systems are not clarified, and even molecular structures are mostly beyond our knowledge. Note that so many molecules, maybe more than 1000 kinds of molecules, are included in a single cell, meaning that identification of molecular structure should be the first step for understanding biological function in terms of molecules and molecular interactions. Thus for probing and understanding biological activity in living cells and tissues, many fluorescent molecules have been designed and synthesized [12] and, recently, metallic nanoparticles, gold nanorods,

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and semiconductor dots have been developed and applied [13]. Inorganic nanoparticles have the advantages of high emission and scattering efficiencies and of no bleaching upon long illumination and measurement. It is assumed that the physical and chemical properties in homogeneous media of all these probe molecules and particles are elucidated well and their studies in living systems give information on the properties and functions at the molecular or single nanoparticle level. Small molecules can be easily pinocytosed by living cells, but nanoparticles larger than 100 nm are not easily transferred to their interiors. It sometimes takes a few hours for such nanoparticles to enter inside the cells. Therefore manipulation of cells and injection of probe molecules and nanoparticles are extremely important and their development is strongly needed to fulfill the promise of potential applications of spectroscopy and imaging in life science, biotechnology, medicine, and so on. Based on our systematic studies on nano-spectroscopy and nano-photochemistry, nano-manipulation and chemistry of photon pressure, and nano-ablation and its biological application [14–16], we have planned to develop new methodologies for answering some of the above described problems. One area is to develop novel spectroscopy and imaging which makes it possible to interrogate non-fluorescent nanomaterials incorporated in living cells. If this is realized, we will have more potential probes and elucidate a wider range of functionalities. Rayleigh light scattering spectroscopy has given one answer, namely, electronic transitions of single polymer nanocrystals were sensed by the scattering spectroscopy by us, although their size was too small for absorption spectroscopy [14–18]. Now the high potential of scattering spectroscopic measurement has been widely demonstrated for surface plasmon resonance of gold and silver nanoparticles [13, 19–23]. Indeed scattering spectroscopy and imaging are complementary to fluorescence spectroscopy and its imaging. The second area is to develop new manipulation and modification of living cells and injection of nanoparticles into living cells. Our approach is based on femtosecond laser processing, but the laser pulse is not introduced to target cells and tissues. That makes it possible to introduce artificial nanoparticles, whose physical and chemical nature is well known, into the targets. By combining new spectroscopy/ imaging and injection methods, we will be able to reach one of the milestones in the understanding of molecular level dynamics and the functionality of single living cells. Some of our novel results obtained between April 2004 and March 2007 are summarized and considered here.

28.2 Femtosecond Laser Ablation and Generated Impulsive Force in Water: Laser Tsunami

Laser ablation is the base on which many laser processing technologies for living systems have been developed, where the targets which should be fabricated are, in general, directly exposed to intense laser pulse irradiation. Thus laser processing is always accompanied by some damage to their surface and internal structures, which is indeed critical in application, especially to living systems. By tuning laser fluence


and pulse width, better solutions to achieve the manipulation and injection with less damage have been sought. Among the trials, femtosecond laser processing has been receiving much attention, as three-dimensional processing is made possible in aqueous solution, culture medium, and even inside living cells and tissues. The conventional Ti:Sapphire femtosecond laser oscillates around 800 nm, at which wavelength most biological materials are transparent. By focusing it, multi-photon absorption is induced selectively at the central part of the focal point, where the ablation takes place. Namely, ablation can be caused freely in the three-dimensional space of transparent media, which is indeed one advantage of the femtosecond laser compared to the nanosecond one. It had been believed that intense femtosecond laser excitation results in plasma formation, causing ablation phenomena [24]. This explanation can be applied to metals and semiconductors but we doubted whether this plasma is responsible for the ablation of molecular materials. We applied femtosecond absorption spectroscopy and femtosecond surface light scattering imaging to phthalocyanine films and liquid benzenes and revealed the primary electronic and morphological processes [25–30]. We could not detect ionic species of the corresponding molecules as the main transient species just at and moderately above the ablation threshold, although some ionic radicals should be observed, at least in the initial stage of ablation involving plasma formation. Thus we pointed out the negative possibility of the plasma mechanism, at least for molecular solids and liquids in the fluence range not far above the ablation threshold. Here, we briefly describe the laser ablation dynamics of phthalocyanine film where the primary species was confirmed, by femtosecond transient absorption spectroscopy [25], to be its electronically excited state. The exciton absorption band was replaced in 20 ps by a hot band of the ground electronic state. This rapid decay was ascribed to mutual interactions between densely formed excitons leading to sudden temperature elevation. The elevated temperature was estimated by comparing transient absorption spectra with the temperature difference ones. It is worth noting that quite normal dynamics of the excited states are detected, even under ablation conditions, which is the reason why we do not accept the plasma mechanism. Then we considered how to correlate these electronic processes of phthalocyanine films with their fragmentation. The evolution of the surface roughening just before the ejection of fragments was evaluated by time-resolved surface light scattering imaging. The root-mean-squared roughness of the phthalocyanine film increased up to a few tens of nm around a few ns after excitation, although the film was excited by a 170 fs laser pulse. The temperature elevation was completed in a few tens of ps after excitation, while appreciable surface roughening was not clearly detected before a few ns. These dynamic processes are depicted schematically in Figure 28.1. Vigorous molecular and lattice vibrations are enhanced in the irradiated area for 20 ps – a few ns delay time, which may be represented as a transient pressure [31]. This idea of laserinduced transient high pressure was already proposed by Dlott by using a picosecond CARS experiment on an anthracene single crystal [32]. The irradiated part is surrounded by an unexcited area and a substrate glass, while thermal conduction

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Figure 28.1 Molecular electronic and vibrational relaxation processes and following morphological changes under laser ablation condition.

does not take place rapidly in the organic film. Therefore the transient pressure can be released only by fragmentation of the film to open air. This explanation of femtosecond laser ablation is consistent with the experimental result of etching behavior (discrete etching) which is quite different from that in nanosecond laser ablation (gradual etching). In the latter case, electronic excitation, conversion to molecular motions, and surface roughening take place simultaneously in hundreds of ns, and the melting phenomenon is surely involved. On the basis of the spectroscopic consideration, we proposed that a photomechanical mechanism operates for femtosecond laser ablation of molecular materials, which is contradictory to the conventional plasma mechanism of femtosecond laser ablation [24]. During these laser ablation studies to elucidate the relevant mechanism, we have come to the idea that the photomechanical force generated by laser ablation can be utilized as a useful perturbation to manipulate and fabricate living systems in aqueous solution. Namely, multi-photon excitation of water which is induced by a focused near-IR femtosecond pulse under a microscope evolves to ablation of water at the focal point, which is followed by shockwave propagation, cavitation bubbling, and convection flow. The phenomenon is already well known [33], and we have utilized this as an impulsive force and demonstrated how the force is useful to manipulate, modify, and pattern various nanomaterials and single living cells in solution [34]. Here we name this phenomenon laser-induced micro/nano tsunami (laser tsunami) in view of the impulsive water flow and describe some recent advances in our studies. Ablation is induced near the target, and the generated laser tsunami attacks the target. Of course, the laser pulse is not irradiated onto the target so that photochemical decomposition leading to damage is fully avoided. A schematic illustration of the laser tsunami and laser power dependence of shockwave generation, plasma emission, and cavitation bubbling is given in Figure 28.2. It is noticeable that the threshold of the shockwave is lowest and that of the bubbling is highest [35]. Of course this tendency is rather qualitative, as direct


Figure 28.2 A schematic representation of the laser tsunami and laser energy necessary to induced shockwave, emission, and cavitation.

observation of shockwave, emission, and bubbling depends on the sensitivity/ resolution of the time-resolved detection instruments and experimental conditions such as laser wavelength, fluence, pulse width, temperature, solvent and so on. The threshold of the pulse energy to induce the laser tsunami is relatively low for a femtosecond laser compared with nanosecond and picosecond lasers. The laser tsunami expands to a volume of (sub mm)3 around the focal point, when an intense laser pulse is focused into an aqueous solution by a high numerical aperture objective lens. When a culture medium containing living animal cells is irradiated, they could be manipulated by laser tsunami. Mouse NIH3T3 cells cultured on a substrate can be detached and patterned arbitrarily on substrates [36]. We have also demonstrated that the laser tsunami is strong enough to transfer objects with size of a few 100 mm [37], which is impossible by conventional optical tweezers because the force due to the optical pressure is too weak. In addition we demonstrated for the first time the crystallization of organic molecules and proteins in their super-saturated solution by laser tsunami [38–40]. 28.2.1 Manipulation of a Single Polymer Bead by Laser Tsunami

The laser tsunami can push a small object in solution, which was clearly visualized by conducting a model experiment. As shown in Figure 28.3, a single polymer bead moved when a femtosecond laser pulse was focused near to it. Upon one shot of irradiation, the bead was pushed step-wise away from the focal point. More detailed

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Figure 28.3 An experimental set-up for direct observation of the laser tsunami pushing a single polymer bead in solution.

observation was carried out by employing a high-speed camera to record the whole process of the motion of a larger bead after the femtosecond laser irradiation as shown in Figure 28.4 [41]. The phenomenon is due to the laser tsunami and can be divided into three stages. After femtosecond laser irradiation, a cavitation bubble is soon generated and the nearby bead is pushed away by bubble expansion, which is considered stage A. The bubble expands up to about 200 mm diameter and then collapses within 30 ms, and consequently the bead is pulled back to a certain extent, which is stage B. Following this, a jet water flow is induced, which carries small remaining gas bubbles to the surface of the solution. The jet flow is regarded as convection of water, which is stage C. During stages A and B, the maximum velocity was only 2.8 m s1, but the maximum acceleration was about 3.6 105 m s2. As the mass density of the polystyrene spheres is 1.05 g ml1, the force exerted on the bead can be estimated, fA ¼ 1.2 mN. This force is much larger, by a few orders of magnitude, than the trapping force of the conventional optical tweezers. However, this force itself makes little contribution to the total displacement of the bead. The final position of the bead is determined mainly by the jet flow, that is, stage C. The maximum acceleration of the bead induced by the jet flow was about 9.0 104 m s2, so the force exerted on the bead is estimated to be fC ¼ ma ¼ 0.29 mN. This force is smaller than that in stages A and B, but still much larger than the conventional optical trapping force. Now we describe a novel method to trap and manipulate a micro-object with the laser tsunami. As a demonstration, we chose polystyrene beads of 90 mm diameter and dispersed them in de-ionized water. As the bead is relatively large and heavy, it


Figure 28.4 Time-resolved observation of the laser tsunami pushing a single polymer bead in solution and its model explanation.

remained at the bottom of the specimen cell without visible Brownian motion. When the laser was introduced, the water near the laser focus clearly caused a laser tsunami. When the laser focal point was moved toward the target bead by adjusting the mortardriven microscope stage, and the distance between them reached about 300 mm or so, the bead started to move. The force was so strong that even the beads adhering to the substrate could be moved easily. In order to manipulate the sample with the laser tsunami, the laser beam was scanned around the target bead by using a Galvano mirror to form a trapping circle [37], as shown in Figure 28.5. The trapping circle

Figure 28.5 Manipulation of a single polymer bead by spatially controlling the laser tsunami.

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radius was set to 150 mm, the scanning frequency was tuned to 10 cycles s1, the laser pulse energy was adjusted to 5 mJ, and its repetition rate was set to 100 Hz, so that one cycle consisted of ten laser shots. The femtosecond laser was irradiated and the trapping circle was brought close to the target bead from its upper position. When the circle plane was shifted to about 50 mm above the bead, it began to move to the center of the trapping circle. As shown in Figure 28.5, the dust on the substrate which looked like black dots were moved from left to right by driving the stage, so that we can say that the target bead was transferred from right to left, while still remaining in the center area of the trapping circle. Thus, it was obvious that the bead was successfully trapped inside the circle and manipulated by the laser tsunami. It is worth noting that even the bead attached to the substrate could be trapped and moved easily by the laser tsunami, which is usually impossible by conventional optical tweezers. Optical trapping has been used as a powerful tool and it is being developed for exploring new molecular phenomena and clarifying novel molecule–light interactions. Nevertheless, optical trapping has some limitations preventing practical application to living systems. The first is that the trapping force by the CW laser is not strong, as pointed out above. The second is due to the damage caused by intense CW laser irradiation. Manipulation by the laser tsunami overcomes these limitations, as its mechanical force is strong despite its transient nature and the laser tsunami is caused by irradiation of the surrounding medium, not the target. The advantages of manipulation by laser tsunami will receive much attention as an alternative to optical trapping in the near future. 28.2.2 Manipulation of Single Animal Cells by Laser Tsunami

Usually the living animal cells are cultured on a substrate and its removal is realized by chemical treatments, while laser tweezers cannot be applied as the cells are strongly adhered to the substrate. As described above, the impulsive force of the laser tsunami reaches mN order, so that its one-by-one removal is now possible. The first demonstration of non-destructive isolation was given for an animal cell, mouse NIH3T3, having lamellipodia and filopodia, with which the cells were firmly attached in the collagen matrix [36]. By just applying the laser tsunami, we found that nothing happened, which is of course because they were connected to the substrate using filopodia. Therefore we tried to cut each filopodium by direct femtosecond irradiation. The filopodia contracted soon upon one shot irradiation and the cell changed its shape to spherical. As Brownian motion of the cell was not observed, the cell should still be, at least partly, adhered to the matrix. Then a laser with pulse energy 0.51 mJ pulse was focused at 20 mm, far from the ventral side of the cell, to cause laser tsunami. As a result the cell was detached from the matrix and pushed away. The observed behavior is summarized in Figure 28.6, where a schematic illustration is also given. The isolated cell showed Brownian motion, then it stopped on the substrate again and the filopodia were confirmed to be regenerated, namely, the cell adhered again to the matrix. This means that the cells were not killed. The viability of the cells was also confirmed by examining the conventional dye exclusion test


Figure 28.6 Non-destructive removal of single living cells from a substrate.

before and after laser irradiation. These results indicate that the one-by-one removal of cells utilizing the laser tsunami is a method with high potential. A successful example is summarized above, while the result of the non-destructive removal should be statistical as the cell has distributions in shape, cell cycle, viability, and so on. To estimate the physical damage to the cells, unspecified 80 NIH3T3 cells were selected and examined. First, their filopodia were cut directly, as described above, and then the laser irradiation was started at a position where the distance between the focal point and the end of the cell was 60 mm. The distance was then reduced until the cell was detached by laser tsunami, which was examined as a function of laser fluence. Of course, as the distance becomes shorter, the necessary energy to remove the cells is smaller. When the distance is typically less than 35 mm, however, the laser probably irradiates and kills a cell. Note that the height of the present mouse cell is usually about 10 mm and a lens with high NA giving large incident angle was used. The probability of direct laser irradiation should increase with the decrease in distance, although the laser energy is decreased. One result is given here as a concrete example. 80% of detached cells kept their viability when a 0.72 mJ pulse1 was applied at a distance greater than 15 mm, while 80% of cells were killed even at 0.41 mJ pulse1 upon decreasing the distance to less than that. We consider that the higher probability at a greater distance promises a practical extension of the laser tsumani method in the near future. For example, once a cell is detached from the collagen matrix by laser tsunami, it can be successfully manipulated by employing conventional laser trapping with a CW Nd3 þ :YAG laser [35], which should realize fine manipulation and arrangement of the cell. The combination of laser tsunami and conventional laser tweezers will open new horizons in the studies on single cell analysis of differentiation and issue formation. The removal by laser tsunami has important advantages which are not available by conventional removal and patterning methods for living cells: ink jet printing,

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micro-contact printing, laser-induced forward transfer. In printing methods, liquid droplets with a volume less than 1 nl containing cells, proteins, and so on, are deposited on a substrate. These small droplets are very volatile due to the relatively large surface-to-volume. The laser transfer technique also involves vaporization. Drying of cells and proteins may lead to death and de-naturarization, respectively, which is critical for cell manipulation. On the other hand our laser tsunami method never exposes the cells to air during the manipulation. Moreover, the cells are never irradiated by the laser, so that photochemical and photothermal damage are not, in principle, involved. These features are very important and promise high potential for our laser tsunami method in bio-related research involving cell manipulation. The present removal technique by laser tsunami was developed as a non-destructive micro-patterning method for living animal cells [41]. To conduct the whole process in solution, we set two substrates at a distance of 100 mm; a source substrate where NIH3T3 cells were cultured and a target substrate onto which the cells are transferred. Immediately after the treatment the cells kept their spherical shape and adhered rather weakly to the source substrate. The femtosecond laser pulse was focused at 5 mm below the source substrate. The resultant tsunami attacked the cells and they fell down to the target substrate. By motor-driving the microscope stage laterally, we could demonstrate in situ micro-patterning of living cells. Similarly, we applied the laser tsunami to micro-patterning of protein cubes a few mm in size, and demonstrated how easily two kinds of cubes with and without green fluorescence protein are alternately arranged [42, 43]. The whole process was performed in water, so that the cells were never exposed to air and de-naturalization of proteins would not be expected. The spatial resolution of the micro-pattern of the protein crystal was determined to be 80 mm, which is sufficient for practical applications. 28.2.3 Modification and Regeneration Process in Single Animal Cells by Laser Tsunami

By decreasing laser fluence, the force caused by the laser tsunami decreases, so that the cell cannot be detached. However, we found a fluence region where the cell position was shifted by the effect of the laser tsunami [34]. High-speed imaging of the culture medium containing NIH3T3 cells revealed how the cell is shifted by the laser tsunami. Focusing the femtosecond pulse at 20 mm from one side of the cell, bubbling was clearly observed and the cell was shifted. It is interesting to see that the cell still adhered to the substrate. The distance of the shift and its velocity were estimated to be 5 mm and 10 mm s1, respectively, under some conditions. We consider that the cell movement is induced by mechanical shock due to the laser tsunami, while another possible explanation is that the shift is due to cellular biological processes such as de-polymerization, synthesis, and polymerization of cytoskeletons which are stimulated by the laser tsunami. The latter seems important, but such biological and chemical processes will not take place in the present time scale of milliseconds. The shift of the living cell is accompanied by modification of the cell shape. The elucidation of this phenomenon should be important for the further development


of the laser tsunami as a general tool for manipulation of living cells. As one of such approaches we studied the dynamic change of actin stress fibers induced by the laser tsunami. It is well known that the fibers are the inside framework giving the cell shape, while actin is a cytoskeleton protein and forms actin filament. The three-dimensional network of actin stress fiber, which is an association of actin filaments, provides mechanical support for the cell, determines the cell shape, and enables cell movement. Thus the shape change in the cell due to the laser tsunami can be examined by observing the laser-induced dynamics of fibers. The actin stress fiber was visualized by binding it with enhanced green fluorescence protein (EGFP), and monitored by total internal reflection fluorescence (TIRF) imaging [34]. Epi-fluorescence and TIRF images of a single NIH3T3 cell before and after laser irradiation are shown in Figure 28.7. Although the EGFP-actin is expressed in the whole cell, only the actin fibers at the interface between the cell and substrate were selectively detected as a TIRF image. This is one of the suitable conditions to interrogate the initial process of how the laser tsunami detaches the cell. The bright lines in the TIRF image can be attributed to actin stress fibers which are located close to or adhered to the substrate. The time evolution of the cell when the laser with 0.3 mJ pulse1 was focused at the ventral side of the cell with the 60 objective lens is

Figure 28.7 Epi-fluorescence (a) and total internal reflection fluorescence (b–e) images of a single NIH3T3 living cell before (a, b) and after (c–e) inducing a laser tsunami.

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summarized in Figure 28.7. Immediately after the laser shot, fluorescence of actin filaments near the laser focal point was lost, while the intensity of the filaments increased at the opposite side to the laser focal point. The loss of the actin filaments near the laser focal point will be due to the detachment of the cytoskeleton from the culture substrate. The detachment will induce relaxation of tension of the cytoskeleton, which may be the one of the origins of cell adhesion and migration processes. On the other hand, in the area far from the laser focal point, there is a possibility that the generation of cytoskeleton in the cell is enhanced, because the increase in the fluorescence intensity means that the actin molecules accumulate at the adhesion area. A few tens of minutes after laser irradiation, the actin filaments were regenerated near the substrate. The direct cutting of actin fibers by focussing a femtosecond laser pulse has already been reported by Mazur et al. [44], however, the regeneration process of the cell had not been described in the literature. Our result deals with the regeneration of the fiber cut by the laser tsunami, which is a unique result on regeneration dynamics. 28.2.4 Injection of Nanoparticles into Single Animal Cells by the Laser Tsunami

The laser tsunami changes the shape and morphology of the membrane of living NIH3T3 cells without their death, while the change does not take place uniformly. The membrane structure should be affected mechanically and recover spontaneously due to the self-assembling nature of lipid molecules. We considered that this deformation may facilitate the penetration of external nanoparticles into the cells. If the nanoparticles with several kinds of functionalities can be freely injected into the living cells, probing and sensing the cell from its inside will be much advanced. Therefore, there is a strong need to develop a new injection technique for nanoparticles with size ranging from 10 to a few hundreds of nm into a specific cell. Recently, we have demonstrated for the first time that a fluorescent polymer latex bead located on the surface of an animal cell can be injected by applying the laser tsunami [45]. It is well known that those nanoparticles cannot enter quickly into living cells, although small molecules can be pinocytosed spontaneously. A schematic illustration and our experimental set-up for the laser tsunami injection are shown in Figure 28.8. Mouse NIH3T3 fibroblasts were cultured on a glass bottom dish, and polystyrene nanoparticles of 200 nm diameter containing a fluorescent dye were dispersed in the culture medium. Without the laser tsunami the particles can penetrate into the cell by themselves, but this endocytosis takes 2 h after the nanoparticles were dispersed in the culture medium. Since the laser was irradiated within 10 min after the dispersion of the nanoparticles, the penetration through spontaneous endocytosis was negligible in the experiment. In order to observe simultaneously the fluorescent nanoparticles and the cell membranes and to know their relative geometry, the membrane was stained with a styryl dye, N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino) styryl) pyridinium dibromide for its imaging. The three-dimensional fluorescence image was obtained by employ-


Figure 28.8 A confocal microscopy system for injecting nanoparticles into a single living cell.

ing a confocal fluorescence microscope system, where the fluorescence through a confocal aperture was split into two wavelength regions, giving two fluorescence distributions together. After dispersion of the nanoparticles in the sample solution, Brownian motion of the nanoparticle was observed in the medium. Once the nanoparticles were attached to the cell, the Brownian motion stopped. Here, we present the transmission and confocal fluorescence images at 10 min after adding the polystyrene nanoparticles to the culture medium. Initially, fluorescent nanoparticles and stained cell membranes were observed as white spots and a red wide silhouette, respectively, and the particles were observed only on and near the cell membrane. However, after 20 pulses of the focused femtosecond laser with energy 10 nJ pulse1 were shot into the culture medium at 20 mm from the edge of the cell, the nanoparticles were observed inside the cell. This is clearly shown in Figure 28.9, where the fluorescence images at different heights are included. White spots were observed at heights of 2 and 4 mm in the upper part inside the cell, confirming the successful injection of the 200 nm particles. The injection behavior strongly depended on the shape and size of the cells, penetration was confirmed for 10% of the targeted cells. We consider that the injection of the nanoparticles is achieved not only by pushing the nanoparticle onto the cell membrane but also by enhancing its fluidity with the laser tsunami.

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Figure 28.9 Fluorescence images of the polymer particles at different heights on/in a single NIH3T3 cell in culture medium. The laser was focused at 20 mm from the right edge of the cell, and some nanoparticles were injected at the upper right part inside the cell, which is shown at 2 and 4 mm.

How the single cell is affected by the laser tsunami was interrogated by time-resolved imaging, and indeed the cell facing the laser focal point was pushed and the nucleus shape was bent to some extent, corresponding to the morphological change in the cell. It is worth noting that the size, shape, and distribution of cellular organelles, giving white micro-objects inside the cell, were modified. Such observation suggests that the cell morphology is much affected by the impact caused by the laser tsunami and penetration through the cell membrane is greatly enhanced. Direct irradiation of a single cell by a femtosecond laser pulse is an alternative and more conventional method to inject nanoparticles, which has already been reported by some groups [46–48]. A small ablated hole in the cell membrane may easily allow the nanoparticles to penetrate into the cell. However, some photochemical damage should occur, so that viability of the injected cells may be lower than after processing with the laser tsunami. As a more conventional injection method, high pressure has been employed as an impactor, using a particle gun [49], but selective injection into a specific single cell is not possible. Mechanical injection using a micropipette is another popular tool [50], but it suffers from clogging problems. The present laser tsunami injection is, in principle, a non-invasive, non-clogging, and

28.3 Development of Rayleigh Light Scattering Spectroscopy/Imaging System

three-dimensionally controllable method, and directional injection giving specific selective penetration of nanoparticles is realized, which is indeed one of the advantages of this method.

28.3 Development of Rayleigh Light Scattering Spectroscopy/Imaging System and its Application to Single Animal Cells

Most spectroscopy and imaging methods for single-cell analysis are based on fluorescence measurement of molecules and nanoparticles. The non-luminescent molecules and materials have not received attention in the relevant research fields, while electronic absorption spectroscopy is another important tool for the general identification and understanding of the optical properties and the molecular electronic structure. However, one can hardly measure the visible absorption spectrum under a microscope, because much of the illumination light cannot interact with the sample due to the short pass length and the large illumination spot compared to molecules, aggregates, and particles in small domains, resulting from the diffraction limit of light. This results in low sensitivity, therefore, conventional absorption spectroscopy is not a useful technique for investigating single cells. Instead we have developed a light scattering microspectroscopic system using darkfield illumination and investigated single gold nanoparticles [22, 23, 51–54]. The dark background realized high contrast imagings like fluorescence detection leading to spectroscopic measurements at the single particle level. In particular, the light scattering spectra are closely related to electronic absorption ones, so that this spectralmeasurementisanalternative approachtounderstandingmolecularelectronic structures. On the other hand, the spatial resolution of light scattering microspectroscopy remains lower than conventional confocal laser scanning microscopy because a halogen lamp is applied as a light source. Light scattering microspectroscopy with high three-dimensional resolution is strongly required for spectroscopy and imaging of living cells. Recently, we have developed a confocal light scattering microspectroscopy and imaging system for three-dimensional spectroscopic investigation and imaging with submicron resolution [55, 56]. Femtosecond laser-induced supercontinuum light obtained by a photonic crystal fiber [57] has been applied to microspectroscopy, and indeed the spectral coverage of supercontinuum light makes it possible to achieve cellular imaging of multicolor two-photon fluorescence [58–60] and mutiplex CARS [10, 11, 61]. Here the supercontinuum allows us to obtain light scattering spectra covering the whole visible region. This means that we are able to investigate electronic spectral properties without measuring auto-fluorescence and without adding probe molecules. Some groups have reported light scattering microscopic imaging by means of supercontinuum light [62–64], however, no one has achieved simultaneous three-dimensional spectroscopy and imaging yet, and, as far as we know no one has examined living cells. By combining the supercontinuum light with the confocal microscope technique, we have developed a light

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Figure 28.10 A confocal microscopy set-up for light scattering spectroscopy and imaging using femtosecond white light continuum as a probe light.

scattering spectroscopy/imaging system, whose spatial and spectral resolutions were evaluated on the basis of single gold nanoparticle measurements. Here, we describe its performance and application to the three-dimensional spectroscopic imaging of single unstained cells with and without gold nanoparticles and consider its promising future. Figure 28.10 shows a schematic diagram of the experimental set-up of our confocal light scattering microspectroscopy/imaging system. A femtosecnd laser pulse train of 780 nm, 200 fs, 800 mW, and 80 MHz is introduced into a photonic crystal fiber by a microscope objective. The out-out pulse was spectrally broadened in the wavelength region from 500 nm to 800 nm, and its output power was several tens of megawatts. The supercontinuum light was coupled into an inverted microscope, focused to a sample with an objective lens (100, N.A. ¼ 1.30). The sample was set on a piezoelectric controlled-stage which was mounted on an XY motorized microscope stage. The back scattered light from the sample was collected with the same objective lens and introduced to an imaging pinhole, its spectrum was then measured with a polychromator coupled with an image-intensified charge-coupled device (ICCD). The sample was raster scanned by the piezo stage, which was synchronized with ICCD detection. Thus the light scattering spectra were measured at each focused

28.3 Development of Rayleigh Light Scattering Spectroscopy/Imaging System

Figure 28.11 Light scattering spectra and images of single gold nanoparticles dispersed on/in a single living NIH3T3 cell.

point. By sampling an arbitrary wavelength region of the light scattering spectrum at each point of the target, a spectroscopic image was obtained. Confocal light scattering spectroscopic imaging of mouse NIH-3T3 living cells at different depths are shown in Figure 28.11, which was obtained after incubation with dispersed gold nanoparticles. Images are roughly similar to that before incubation with nanoparticles, while some bright points are interestingly overlapped. These features suggest that the points seem to be gold nanoparticles. The light scattering spectra of the bright points A-D in Figure 28.11 are also given. As the particles A and C were located at the end of the cell, we can conclude that they are adsorbed on the surface of the cellular membrane, while the particle B is clearly included in the cell. The latter is surrounded in the scattering medium with relatively high refractive index. On the other hand the particle D at 8 mm should be near the top of the cell. Each bright point shows a spectral peak in the wavelength range 600–700 nm, which is clearly ascribed to the surface plasmon resonance band of single gold nanoparticles [13]. Without adding gold nanoparticles we could not find any bright point showing such a spectral character. The spectral peak and intensity differ from particle to particle, which could indicate not only variation in particle size or shape but also the change in intercellular environmental condition, depending on the particle position [22]. The spectrum of the particle labeled C strongly indicates the aggregation of particles, as the peak is much shifted to the red compared with others [13]. The present results clearly demonstrate that the wide spectroscopic wavelength and the submicron spatial resolution, which are characteristics of our system, allow us to distinguish precisely single gold nanoparticles on or in the single cell. It is important to note that the incident supercontinuum light into the cell is bright enough to penetrate into the cell and to give reliable spectra of gold nanoparticles. Since light scattering spectra of gold nanoparticles reflect the environmental condition, simultaneous monitoring of the position and spectrum give us a new way to understand dynamic aspects of single living cells. When they are dissolved in

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homogeneous solution or fixed in a matrix no spectral change is expected. If nanoparticles move in and on a single cell, the scattering spectrum changing from position to position can be followed in the time scale determined by the employed instruments. In this case spatial and spectral tracking of a single nanoparticle can be elucidated and real time observation of how nanoparticles behave will receive much attention. One example is shown in Figure 28.12, where the nanoparticle near the top of the single mouse NIH3T3 cell started its migration at the position of 12.5 mm from the bottom of the dish. During the measurement the particle migrated laterally keeping constant height for a while, giving constant peak wavelength of the scattering spectra. At about 28 min after the start the particle sank down by 3 mm and the peak showed a quick change. After that the position and the peak recovered to their original values in a few minutes. Later the particle rose to a higher position in the cell, accompanied by a small spectral shift.

Figure 28.12 Single gold nanoparticle tracking on/in a single living NIH-3T3 cell by light scattering spectroscopy and imaging. (a) Scattering spectrum maximum wavelength evolution with time. (b) Particle position evolution with time (x, y and z positions in light

gray circles, gray triangles and black squares, respectively); x and y positions are measured starting from the initial tracking position, while z values are the distance from the upper surface of the dish. (c) Typical spectrum (at 28 min).

28.4 Summary

A possible explanation of this interesting behavior is as follows. After an initial contact of the gold nanoparticle with the cell, it may migrate laterallly and then may be included in the cell either through pinocytosis or by entering through a small channel [65]. At this point the particle would be tightly surrounded by a membrane layer, and this process would be followed by rapid efflux out of the cell in the next few minute because of its exogenous character. Short endocytosis and exocytosis cycles have indeed been reported by Xu et al. [66]. The shift of 12 nm in the scattering spectral maximum may represent an increase in the local dielectric constant upon penetration of the particle into the cell. The aparent return to the initial maximum could be explained by the opposite process, possibly escape of the particle from the cell before the endocytosis is completed.

28.4 Summary

In this chapter we have described two methodologies of femtosecond laser tsunami processing and light scattering spectroscopic imaging, and have shown their application to single animal cells. The first is a new processing method based on femtosecond laser ablation in solution. The femtosecond pulse from a Ti:Sapphire laser (800 nm, 150 fs) is focused into water, culture medium, and so on, under a microscope, leading to multiphoton excitation at the focal point. Above a certain threshold shockwave propagation, plasma emission, and cavitation bubbling are induced, which is a well known phenomenon. We have named the phenomenon laser tsunami and have paid attention to the resultant high pressure and convection flow. When the femtosecond laser is focused near a target animal cell, the high pressure and convection flow affect the cell transiently and locally, which is regarded as an impulsive force and should be a soft mechanical perturbation. As the targets are not irradiated directly, photochemical damage can be avoided. Three-dimensional processing is possible even inside a single living cell, which is ascribed to multiphoton excitation by femtosecond pulse. The force is of the order of mN, although it is transient, and extremely stronger than that produced by conventional optical trapping with CW laser beam irradiation which is of the order of pN. By taking advantage of the laser tsunami, we have developed new methods which enable us to manipulate and to process single living animal cells in solution without damaging them photochemically and exposing them to air. Manipulation of single large polymer beads and living cells, modification of actin stress fibers in a single cell, and injection of nanoparticles into a single cell are presented here to show the high potential of laser tsunami processing. The second methodology is a novel spectroscopy and imaging method which makes it possible to interrogate non-fluorescent nanomaterials incorporated in living cells. This is very promising, since we will be able, in principle, to probe more materials and elucidate a wider range of functionalities. Rayleigh light scattering spectroscopy using a supercontinuum which is generated by focusing a femtosecond laser pulse to a photonic crystal fiber is combined with a confocal microscope.

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Electronic transitions of single molecular aggregates, nanocrystals, polymer beads, metal nanoparticles, and so on can be followed three-dimensionally. As an example, gold nanoparticles were dispersed in a culture medium containing living cells and their tracking behavior was monitored. Spectral measurement of the surface plasmon resonance of gold nanoparticles gives information on the surrounding environment, so we can consider where the nanoparticle is located: in water, on the membrane surface, or in the membrane. These two methodologies are novel and complementary to conventional laser manipulation, fabrication, spectroscopy, and imaging methods. Processing by laser tsunami is clearly different from laser processing which is based on direct laser irradiation. Manipulation by laser tsunami is due to an impulsive force, which is in contrast to the laser tweezer by continuous laser irradiation. The present light scattering spectroscopy and imaging are complementary to fluorescence spectroscopy and its imaging. Our approaches are new and thus expected to contribute to systematic exploration of new phenomena in living cells, particularly at molecular and nanoparticle levels.

Acknowledgments

The preset work is supported by KAKENHI (the Grant-in-Aids for Scientific Research) on Priority Area Molecular Nano Dynamics (April 2004-March 2007) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). HM also thanks MOE-ATU (the Ministry of Education-Aiming for Top University) Project (National Chiao Tung University) by the Ministry of Education, Taiwan and the National Science Council, Taiwan (0970027441) for their support.

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29 Super-Resolution Infrared Microspectroscopy for Single Cells Makoto Sakai, Keiichi Inoue, and Masaaki Fujii

29.1 Introduction 29.1.1 Infrared Microscopy

Molecular vibration is often referred to as a fingerprint of molecules, because it sensitively reflects their geometry and environment. The technique of infrared microscopy; combining optical microscopes with IR spectroscopy, enables one to visualize the molecular fingerprint in various materials including biological samples. The infrared microscope has been used widely for the following reasons: (i) The IR absorption spectrum of small samples and domains can be easily measured, (ii) without causing any damage, and (iii) spatial mapping of specific materials by IR is possible [1–8]. However, the spatial resolution of infrared microscopes is very poor relative to other optical microscopes, meaning that other analytical methods are used, such as Raman microscopy, in order to achieve subcellular resolution [9–13]. This is because the spatial resolution of a conventional infrared microscope is restricted by the diffraction limit, which is almost the same as the wavelength of IR light. This diffraction limit prevents conventional infrared microscopes from achieving a better spatial resolution, because the IR wavelength is very long, ranging from 3 to 25 mm in the mid-infrared region. If a specific IR absorption band can be mapped with sub-micron spatial resolution, visualization of the molecular structure and reaction dynamics in a non-uniform environment such as a cell becomes a possibility. 29.1.2 Super-Resolution Microscopy by Two-Color Double Resonance Spectroscopy

In recent years, we have developed a two-color super-resolution laser scanning fluorescence microscope based on the up-conversion fluorescence depletion

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Figure 29.1 (a) Scheme of two-color fluorescence dip spectroscopy. The pump beam excites a molecule from the ground state (S0) to the S1 state. Then, the erase beam further excites the S1 molecule to a higher excited state, Sn. Due to various relaxation processes from Sn states, such as internal conversion to the ground state,

the molecule can decay from Sn without fluorescence. (b) Excitation scheme of transient fluorescence detected IR (TFD-IR) spectroscopy. The IR light excites a molecule to a vibrationally excited level. The visible light further excites it to the electronically excited S1 state, where the transient fluorescence is emitted.

technique, that is a form of two-color double resonance spectroscopy [14–17]. The excitation scheme is shown in Figure 29.1a. In this technique, when a molecule is excited to the S1 state by the first beam of laser light (pump light), the fluorescence intensity from S1 is monitored. Here, the fluorescence intensity is proportional to the population in S1. The S1 molecule is further excited to a higher excited state, Sn by irradiating with a second beam of laser light (erase light). Because non-radiative relaxation processes, such as internal conversion or inter-system crossing, are generally accelerated in the Sn state, the fluorescence intensity from S1 is depleted (fluorescence depletion effect). Therefore, excitation to the Sn state can be detected by fluorescence depletion. By applying this fluorescence depletion process to fluorescence microscopy, the size of the fluorescent region becomes smaller than the diffraction limit of the focused pump beam because the up-conversion fluorescence depletion process occurs only in the overlapping area of the two laser beams. This approach, which we have developed for over 20 years [18–20], proves the principle that super-resolution optical microscopes can be achieved by combining optical microscopes with two-color double resonance spectroscopies. In fact, super-resolution optical microscopes employing two-color double resonance spectroscopy are a hot topic, chosen by Science as one of the top 10 breakthroughs of 2006, and many scientists and biologists have noted the significance of this technique [21–24]. 29.1.3 Transient Fluorescence Detected IR Spectroscopy

Figure 29.1b shows the principle of transient fluorescence detected IR (TFD-IR) spectroscopy, which was developed about 30 years ago by Kaiser and Laubereau [25, 26]. This technique is also a form of two-color double resonance spectroscopy, and allows operation in the IR region. Briefly, tunable IR light is introduced together with visible light of which the wavelength is fixed to be slightly longer than the visible

29.1 Introduction

absorption band. If the frequency of the IR light is not resonant with a vibrational level, no fluorescence will appear because the wavelength of visible light does not match the absorption band. When the IR frequency is resonant with an excited vibrational level, the vibrationally excited molecule generated by the IR absorption can absorb the visible light, and generate fluorescence (transient fluorescence). Thus the vibrational transition can be detected as this transient fluorescence. TFD-IR spectroscopy has high sensitivity because it is zero-background, and the IR absorption can be detected by the electronic transition, which has a large absorption crosssection. Moreover, it is also possible to observe the population dynamics of vibrational cooling by adjusting the time delay between the IR and visible light. 29.1.4 Application to Super-Resolution Infrared Microscopy

Super-resolution infrared microscopy is achieved by applying TFD-IR spectroscopy to a fluorescence microscope. A sample is irradiated with IR and visible light. At the focal point, IR and visible light are focused to diameters restricted by their respective diffraction limits. The transient fluorescence due to IR excitation appears only in the spatial region where both IR and visible beams overlap. This overlap region can be smaller than the diffraction limit of IR or visible light (see Figure 29.2a). Thus it is possible to obtain the IR absorption spectrum of a region smaller than the diffraction limit. More realistically, samples are typically irradiated with co-linear IR and visible light beams. In this case, super-resolution is also achieved in the IR light. When the same objective lens is used, the diffraction limit is proportional to the wavelength. The IR

Figure 29.2 Concept of super-resolution infrared microscopy based on TFD-IR spectroscopy. (a) Principle of super-resolution infrared microscopy. The transient fluorescence due to IR excitation takes place in the area where the IR and visible lights overlap. This overlapped region can

be smaller than the diffraction limit of IR or visible light. (b) Co-linear visible and IR light beams are introduced to the sample. The size of the overlapping region is identical to the diffraction limit of the visible light employed.

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wavelength is much longer than the visible wavelength; visible light can be focused much more tightly than IR light. Transient fluorescence appears only in the overlapping region of IR and visible light, and the size of the overlapping region is precisely the same as the diffraction limit of visible light (see Figure 29.2b). This means that IR information is obtained with the spatial resolution of visible light, that is, the IR is super-resolved. Furthermore, picosecond time-resolved TFD-IR spectroscopy gives access to the time-evolution of the vibrationally excited molecule. This means that the measurement of picosecond dynamics is possible with IR superresolution in this form of microscopy.

29.2 Experimental Set-Up for Super-Resolution Infrared Microscopy 29.2.1 Picosecond Laser System

In super-resolution infrared microscopy, we monitor the transient fluorescence pumped from a vibrationally excited level. In a vibrationally excited level, vibrational relaxation occurs on a picosecond timescale, which means that the use a picosecond laser system is required. The laser set-up for infrared microscopy is essentially the same as that for the transient fluorescence detected IR spectroscopy [27, 28]. Seed pulses from a cw mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) were introduced into a regenerative Ti:sapphire amplifier (Quanta Ray, TSA-10). The amplified output was composed of 2–3 ps pulses with energies of 6 mJ at 800 nm. 20% of the output pulse was used to pump a traveling-wave optical parametric amplifier system (Light conversion, TOPAS 400) after frequency doubling in BBO, to provide tunable visible light. The remaining 80% of the output pulse was split into two beams, and one was introduced into another OPA system (Light conversion, TOPAS 800). The second harmonic of the idler wave from the OPA and the remaining output pulse were differentially mixed in a KTA crystal to generate tunable IR light in the 3 mm region. For the generation of mid-IR light around the 6 mm region, the signal and idler waves from the OPA were differentially mixed in a AgGaS2 crystal. Both visible and IR light have a spectral bandwidth of 15 cm1 and a pulse width of 3 ps. 29.2.2 Fluorescence Detection System 29.2.2.1 Optical Layout for the Solution and Fluorescent Beads The fluorescence detection system employed is shown in Figure 29.3. Both the IR and visible light (3 ps, 15 cm1) generated by the picosecond laser system were introduced into a home-made laser fluorescence microscope [29, 30]. For the measurement of both solutions and fluorescent beads, both beams were adjusted onto a co-linear path by a beam-combiner and focused into the sample by an objective

29.2 Experimental Set-Up for Super-Resolution Infrared Microscopy

Figure 29.3 (a) Optical layout for experiments on solutions and fluorescent beads. Picosecond IR and visible light beams were coaxially combined by a beam combiner, and were focused on the sample through an objective lens. The transient fluorescence at the focal point was

collected by the same objective lens and was focused onto a photodetector. (b) Optical layout for the cells. Both IR and visible light beams are used as the illumination light, and the transient fluorescence is collected from the opposite side by an objective lens.

reflection lens (Sigma, OBLR-20, NA ¼ 0.38). Here, the sample used was Rhodamine6G dissolved in chloroform-d1 to give a concentration of 5 104 mol dm3 or fluorescent beads of 15 mm diameter. The transient fluorescence from the sample at the focal point was collected by the same objective lens and was projected onto a photodetector. For the measurement of the transient fluorescence image, the signal was recorded by a CCD camera with an image-intensifier (Princeton Instruments Inc., PI-MAX-512) and stored on a personal computer as a fluorescence image. For the measurement of IR spectra, the transient fluorescence signal was detected by a photomultiplier (Hamamatsu, 1P28) and integrated by a digital boxcar (EG&G PARC, 4420/4422) before being recorded by a personal computer as a function of the IR laser frequency. The delay time between the IR and visible light was varied by an optical delay system (Sigma, LTS400X). The optical layout is shown in Figure 29.3a. 29.2.2.2 Optical Layout for Biological Samples The optical layout for the measurement of biological samples (cells) is shown in Figure 29.3b. The sample was irradiated with co-linear IR and visible light beams. The transient fluorescence from the sample was collected from the opposite side by an objective lens. In this optical layout, the spatial resolution was determined by the objective numerical aperture (NA) and the visible fluorescence wavelength; IR superresolution smaller than the diffraction limit of IR light was achieved. Here, Arabidopsis thaliana roots stained with Rhodamine-6G were used as a sample. We applied this super-resolution infrared microscope to the Arabidopsis thaliana root cells, and also report the results of time-resolved measurements. IR and visible light beams were superposed onto a co-linear path by a beamcombiner, and focused into the sample by a CaF2 lens (f ¼ 100 mm). The focal spot

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sizes of the IR and visible light beams were adjusted to about 200 mm at the sample position. The transient fluorescence from the sample was collected from the opposite side by a NA ¼ 0.25 objective lens (Newport, M-10X), and was projected onto a CCD camera with an image-intensifier (Princeton Instruments Inc., PI-MAX-1K-HB) and recorded by a personal computer as a fluorescence image. For measuring the timeresolved transient fluorescence detected IR image, the delay time between the IR and visible light beams was varied by an optical delay system (Sigma, LTS-400X). In this optical layout, IR and visible light beams were used as the illumination light for the microscope, and the spatial resolution at 570 nm, which is the fluorescence wavelength of Rhodamine-6G, was 1.4 mm from the Rayleigh diffraction limit. This is much smaller than the diffraction limit (8.1 mm) of IR light. 29.2.3 Sample

Rhodamine-6G, a typical dye for fluorescence probes, shows S1–S0 absorption around the 500–550 nm region, with the maximum at 530 nm [31]. The visible light was fixed to around 610 nm, which is longer than the S1–S0 absorption. The IR light was scanned in the region of the CH and NH stretching vibrations of Rhodamine-6G (2700–3700 nm). Under these conditions, the total energy of visible plus IR reaches somewhere around the absorption maximum at 530 nm. On the other hand, the fluorescence region of Rhodamine-6G is from 550 to 590 nm, with the maximum at 565 nm [32]. Therefore, the transient fluorescence was monitored through a 555–575 nm band-pass filter. The visible light around 610 nm was cut off by a notch filter. Dye laser grade Rhodamine-6G was purchased from Exciton (R590) and was used without any further purification. Fluorescent beads of 15 mm diameter were purchased from Molecular Probes (Orange, excitation 540 nm/fluorescence 560 nm) and were used after spreading on a glass plate. Arabidopsis thaliana seedlings were grown using a 1 104 M Rhodamine-6G aqueous solution on a filter paper of glass fiber. For the preparation of microscopic specimens, Arabidopsis thaliana roots were crushed between two glass slides after soaking in 1 M HCl in a hot-water bath for 10 min, and washed with 45% acetic acid and 3 : 1 methanol/acetic acid. The preparation was sealed in nujol mull with a cover slip.

29.3 Results and Discussion 29.3.1 Transient Fluorescence Image with IR Super-Resolution in Solution

Figure 29.4 shows the fluorescence images of Rhodamine-6G in a chloroform-d1 solution (concentration, 5 104 mol dm3) observed by introducing (a) only IR


Figure 29.4 The fluorescence images of Rhodamine-6G in a chloroform-d1 solution (concentration, 5 104 mol dm3) observed by introducing (a) only IR (IR wavelength, 3400 nm; diameter, 2.5 mmf), (b) only visible (visible wavelength, 621 nm; diameter, 7.0 mmf), (c) both IR (IR wavelength, 3400 nm)

and visible light, and (d) both IR (IR wavelength, 2750 nm) and visible light into a quartz cell. (e) TFD-IR spectrum of Rhodamine-6G. The IR wavelength was scanned from 2.7 mm (3700 cm1) to 3.8 mm (2650 cm1) while monitoring the transient fluorescence signal of (c).

light (IR wavelength, 3400 nm; diameter, 2.5 mmf), (b) only visible light (visible wavelength, 621 nm; diameter, 7 mmf), (c) both IR (IR wavelength ¼ 3400 nm) and visible light, and (d) both IR (IR wavelength ¼ 2750 nm) and visible light into a quartz cell. No fluorescence appears with either (a) (only IR) or (b) (only visible light). In contrast, the transient fluorescence image is clearly observed when the sample is irradiated with both infrared and visible light (c). Moreover, the transient fluorescence image disappears when the IR wavelength is changed from 3400 nm to 2750 nm (d), which is out of the range of IR absorption of Rhodamine-6G. This clearly demonstrates that the transient fluorescence images originate from an IR–visible double resonance signal that appears via the vibrationally excited level. To confirm that the transient fluorescence is indeed an action signal due to resonant infrared absorption, the fluorescence intensity in the image was measured as a function of IR wavelength (TFD-IR spectroscopy, [27]). Figure 29.4e shows the TFD-IR spectrum of Rhodamine-6G. When the IR wavelength was scanned over the

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CH/NH/OH stretching vibration region from 2650 to 3650 cm1, the transient fluorescence signal clearly varied as a function of the IR wavelength. This spectrum matches well the IR spectrum of Rhodamine-6G obtained using a conventional IR absorption spectrometer [33]. Therefore, it can be concluded that the observed fluorescence image shown in Figure 29.4c does indeed correspond to the infrared image at 3400 nm. The observed diameter of the transient fluorescence image, that is the TFD-IR image, is 6.8 mm FWHM (see Figure 29.4c). The theoretical diffraction limit for the IR light (3400 nm, 2.5 mmf, f ¼ 10 mm; effective NA ¼ 0.125) is about 16 mm FWHM. Thus it is clear that the TFD-IR image obtained by this technique gives an infrared image smaller than the diffraction limit. The expected spatial resolution of the infrared microscopy is at least two times higher than the observed diffraction limit. Furthermore, the resolution can be improved because the TFD-IR image resolution of 6.8 mm is still larger than the diffraction limit of the visible light, which is 1 mm under these particular experimental conditions (621 nm, 7.0 mmf, f ¼ 10 mm; effective NA ¼ 0.35). In principle, it should be possible to reach a resolution of about 1 mm. The loss of resolution observed here may be due to low axial resolution and we intend to improve it significantly by means of a confocal optical system. This is significant for the application of this microscopy and microspectroscopy to realistic systems such as biological tissues and so on. One of the important functions of this infrared microscope is the measurement of the IR spectrum from a spatial region smaller than the diffraction limit. This possibility is already illustrated in Figure 29.4e. The TFD-IR spectrum, that corresponds to the IR absorption spectrum, was measured from a fluorescence region smaller than the IR diffraction limit. Infrared spectroscopy in a sub-micron region will be possible by using a high NA objective lens with the confocal optical system. 29.3.2 Picosecond Time-Resolved Measurement

Another important aspect of this method is picosecond time-resolved IR imaging. Originally TFD-IR spectroscopy was used to investigate vibrational relaxation processes [25–27]. In this form of spectroscopy, the population dynamics can be observed by the time-evolution of transient fluorescence images as the delay time between IR and visible light is adjusted. Figure 29.5 shows picosecond time-resolved transient fluorescence images when the IR and visible lights are fixed to 3400 nm and 621 nm, respectively. At a delay time of 3 ps, when the visible is applied before the IR, no signal is observed. At 0 ps delay time when the IR and visible beams are applied at the same time, a transient fluorescence image clearly appears. The transient fluorescence increases in intensity up to 3 ps and subsequently decays with time. The observed time-evolution of the fluorescence image represents the population decay from the vibrational level prepared by the IR together with the spatial mapping of the hot molecules at a specific time. This time-resolved IR imaging shows excellent promise for space and time-resolved spectroscopy, which is necessary


Figure 29.5 Picosecond time-resolved transient fluorescence images at several delay times, when the IR (diameter, 2.5 mmf) and visible (diameter, 7 mmf) lights are fixed to 3400 nm and 621 nm, respectively.

for the study of non-uniform systems such as catalysts, or biological systems like whole cells. 29.3.3 Application to Fluorescent Beads

In this section, we report the application of super-resolution infrared microscopy to a Rhodamine-6G doped fluorescence bead; a minute sample. Figure 29.6a shows a fluorescence image of a 15 mm diameter fluorescent bead observed by using 539 nm pumping light. This corresponds to the fluorescence image taken by a conventional fluorescent microscope. An image of about 15 mm diameter is observed. The images of the fluorescent bead measured by our super-resolution infrared microscope are shown in Figure 29.6b–d. No fluorescence is observed when only visible light at 621 nm (Figure 29.6b) or only IR light at 3300 nm (Figure 29.6c) are applied to the bead. However, when both IR and visible lights are introduced, the transient fluorescence is clearly observed, as shown in Figure 29.6d. The fluorescence intensity decreased remarkably when the IR wavelength was changed from 3300 nm to 2810 nm. This shows that transient fluorescence also depends on IR absorption in the fluorescent bead sample. The spatial resolution of the observed TFD-IR image can be evaluated from Figure 29.7. Figure 29.7a is a normal fluorescence image obtained by 539 nm pumping, and Figure 29.7b is a TFD-IR image measured using both IR and visible pumping lasers. Cross-section profiles along the white lines drawn in Figure 29.7a

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Figure 29.6 (a) A fluorescence image of a 15 mm diameter fluorescent bead observed by using 539 nm pumping light. The image of a fluorescent bead measured by applying (b) only IR (IR wavelength, 3300 nm; diameter, 2.5 mmf), (c) only visible (visible wavelength, 621 nm; diameter, 7 mmf), (d) both IR and visible light.

and b are shown in c and d. The simulated fluorescence intensity for the 15 mm diameter bead with the diffraction limit (16 mm FWHM) of IR light (IR wavelength, 3300 nm; effective NA ¼ 0.125) is shown in Figure 29.7d as a solid curve. It is clear that the TFD-IR image is smaller than the IR diffraction limit, and super-resolution is achieved for IR microscopy. At a glance, both the normal fluorescence image and the TFD-IR image are similar. This suggests that the spatial resolution of the TFD-IR image is close to the visible fluorescence resolution. The dashed curve shows the simulated fluorescence intensity for the 15 mm diameter bead with the assumption that the effective spatial resolution of this microscope is 6.8 mm FWHM, which was measured by the TFD-IR image in the Rhodamine-6G solution (see Figure 29.4c). Here, the size of the bead is large enough relative to the calculated focal spot size (1 mm) and depth of focus (2.5 mm) that we used the size of the image in the dye solution as the effective spatial resolution. The simulated profile matches well both the normal fluorescence and the TFD-IR images. This means that the spatial resolution is determined only by visible light, not IR light, and provides evidence of super-resolution in the TFD-IR image.


Figure 29.7 (a) A normal fluorescence image by 539 nm pumping. (b) TFD-IR image measured by IR and visible pumping lasers. (c), (d) Crosssection profiles along the white lines of (a) and (b). The simulated fluorescence intensity for the

15 mm diameter bead with 6.8 mm FWHM is shown by a dashed white curve, and with 16 mm FWHM that is the diffraction limit of IR light (IR wavelength, 3300 nm; diameter, 2.5 mmf; effective NA ¼ 0.125) is shown by a gray curve.

29.3.4 Application to Whole Cells 29.3.4.1 Super-Resolution IR Imaging of Arabidopsis thaliana Roots Figure 29.8a shows a fluorescence image of Arabidopsis thaliana roots labeled with Rhodamine-6G, observed by introducing 539 nm light. This corresponds to a fluorescence image taken by a conventional fluorescent microscope. As can be seen from the figure, Rhodamine-6G stains the inside of the cell uniformly. Figure 29.8b–d show transient fluorescence detected IR (TFD-IR) images of Arabidopsis thaliana roots labeled by Rhodamine-6G, observed by introducing: (b) only visible light (wavelength, 607 nm), (c) only IR light (wavelength, 3300 nm), and (d) both IR and visible light. No fluorescence appears in (b) only visible and in (c) only IR light. On the other hand, transient fluorescence clearly appears by introducing in (d) both IR and visible light, and a TFD-IR image that is almost the same as (a) is observed with a spatial resolution higher than the diffraction limit of IR light. TFD-IR images disappear when the visible light is applied before the IR light. We also measured picosecond time-resolved TFD-IR images from 10 to 50 ps. Figure 29.9 shows the picosecond time-resolved TFD-IR images obtained around a 0 ps delay time. For these TFD-IR images, the population decay of the vibrationally excited Rhodamine-6G molecule in a cell is demonstrated as the delay-time dependent fluorescence. At a 5 ps delay time, when visible light is applied before IR light,

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Figure 29.8 (a) Fluorescence image of Arabidopsis thaliana roots labeled by Rhodamine-6G, observed by introducing 539 nm light. (b)–(d) TFD-IR images of Arabidopsis thaliana roots by introducing (b) only visible (wavelength, 607 nm), (c) only IR (wavelength, 3300 nm), and (d) both IR and visible light. The spatial resolution observed from the cross-section of the image in (d) is almost the same as the diffraction limit of 1.4 mm.

the TFD-IR image is not observed at all. However, at 0 ps when visible and IR light are applied simultaneously, a TFD-IR image clearly appears, and subsequently decays with time. 29.3.4.2 Vibrational Relaxation Dynamics in the Cells Figure 29.10 shows time-profiles of the TFD-IR signal intensity at positions A–C on the TFD-IR image in Figure 29.9. As can be seen, all of the time-profiles show the same behavior. An interesting feature is that the vibrational energy of Rhodamine-6G is not completely lost, even at a 50 ps delay time, which is sufficiently longer than the fast vibrational energy flow with an exponential decay constant of 1.5 ps. Many previous reports [34–44] on the relaxation dynamics of vibrational energy in molecules of a similar size have shown that vibrational energy flows from a solute to solvent molecules on a picosecond time scale, and is completely lost after typically 20 ps in most solutions. This suggests that the vibrational relaxation dynamics in


Figure 29.9 Picosecond time-resolved TFD-IR images of Arabidopsis thaliana roots labeled with Rhodamine-6G at delay times around 0 ps. The IR and visible light are fixed to 3300 nm and 607 nm, respectively.

the cell is quite different from that of the solute–solution system. We assume that the vibrational relaxation of Rhodamine-6G in the cell is greatly influenced by the neighboring environment. Inside the cell is a non-uniform environment, consisting of many parts, including a nucleus, cytoplasm, and a cell membrane, which interact

Figure 29.10 Time-profiles of the TFD-IR signal intensity at positions A–C on the TFD-IR image of Figure 29.9. All of the time-profiles show that the vibrational energy of Rhodamine-6G is not completely lost, even at a 50 ps delay time. The IR and visible lights are fixed to 3300 nm and 607 nm, respectively.

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with each other or water molecules in the cell. This may cause the site dependence of vibrational relaxation in a cell. In order to understand the more detailed characteristic dynamics occurring within cells, vibrational relaxation of probe molecules in specific parts of cells, such as the nucleus, cytoplasm, and cell membrane, will be measured by using other probe fluorescent dyes in the next stage of our investigations.

29.4 Summary

We have performed super-resolution infrared microscopy by combining a laser fluorescence microscope with picosecond time-resolved TFD-IR spectroscopy. In this chapter, we have demonstrated that the spatial resolution of the infrared microscope improved to more than twice the diffraction limit of IR light. It should be relatively straightforward to improve the spatial resolution to less than 1 mm by building a confocal optical system. Thus, in the near future, the spatial resolution of our infrared microscope will be improved to a sub-micron scale. Furthermore, this new super-resolution infrared microscopy is capable of capitalizing on TFD-IR spectroscopys ability to study vibrational relaxation processes.

Figure 29.11 Picosecond time-resolved TFD-IR spectra of Rhodamine-6G at several delay times. The visible light is fixed to 585 nm and the IR wavelength was scanned from 2.7 mm (3700 cm1) to 8 mm (1250 cm1).

References

Therefore, by using this super-resolution infrared microscope, we will be able to carry out space- and time-resolved vibrational microspectroscopy in the IR super-resolved region. Given that IR absorption is regarded as the fingerprint of a molecule, the new super-resolution infrared microspectroscopy will become an extremely important tool, not only in microscopy but also in spectroscopy. Recent tunable picosecond IR lasers can generate IR light up to 10 mm. This means we can easily extend this imaging and microspectroscopic method into the mid-IR region, where the strong IR absorption of water around the 3 mm region can be avoided. Figure 29.11 shows a demonstration of picosecond time-resolved TFD-IR spectra of Rhodamine-6G/chloroform-d1 solution in the 1250–3800 cm1 (8–2.7 mm) mid-IR region. The visible light is fixed to 585 nm. As can be seen, we are able to perform infrared microspectroscopy at least up to the 8 mm (1250 cm1) region. The resolution of this infrared microscopy is determined by the diffraction limit of visible light, not by IR. Consequently, sub-micron resolution should be retained, even in the mid-IR region for which the diffraction limit can be as high as a few tens of micrometers. We have also demonstrated picosecond time-resolved TFD-IR imaging of the vibration relaxation of Rhodamine-6G in Arabidopsis thaliana roots, and found an abnormally long-lived component of vibrational relaxation in a cell. This may result from a site dependence of vibrational relaxation within whole cells. These results indicate the possible utility of the two-color super-resolution infrared microscope in mapping specific IR absorptions with high spatial resolution, and the observation of dynamics in a non-uniform environment, such as a cell. By using this infrared super-resolution microscope, we will be able to visualize the structure and reaction dynamics of molecules in a wide range of non-uniform environments.

Acknowledgments

The present work was financially supported in part by a Grants-in-Aid for Scientific Research (KAKENHI) on Priority Areas (Area No. [432] and Area No. [477]) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The authors gratefully thank Professor M. Kinjo and Professor N. Ohta in Hokkaido University for providing the Arabidopsis thaliana sample, and Dr T. Ohmori for stimulating discussion. The authors also thank Dr T. Watanabe and Dr Y. Iketaki of the Olympus Company for providing technical support for the laser fluorescence microscope, and Professor J. R. Woodward for valuable comments on the manuscript.

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30 Three-Dimensional High-Resolution Microspectroscopic Study of Environment-Sensitive Photosynthetic Membranes Shigeichi Kumazaki, Makotoh Hasegawa, Mohammad Ghoneim, Takahiko Yoshida, Masahide Terazima, Takashi Shiina, and Isamu Ikegami

30.1 Introduction 30.1.1 Thylakoid Membranes of Oxygenic Photosynthesis

In nearly all biological systems, synthesis of organic molecules directly or indirectly requires energy from the sun. The light-to-chemical energy conversion is called photosynthesis, which occurs in various forms around the globe. Oxygenic photosynthesis in plants takes place in chloroplasts, which are believed to be descendants of intracellular symbionts, probably cyanobacteria. The thylakoid membrane in both chloroplasts and cyanobacteria is the most essential part of the light-to-chemical energy conversion for oxygenic photosynthesis. The primary photochemical reactions of oxygenic photosynthesis are essentially achieved by two types of pigment– protein complexes photosystem I and photosystem II (PSI and PSII), which form a serial photoinduced electron-transfer chain [1]. The ideal condition for the serial electron-transfer system is that the two photosystems are equally excited within their maximum reaction rates. The two photosystems possess slightly different spectral features, and the spectral features and the numbers of light-harvesting systems (antenna) interacting with each of the two photosystems varies. If only PSII is highly and continuously excited, the acceptor side of PSII is fully reduced and some of the extra chlorophylls in the singlet excited state are converted into the lowest triplet state. The triplet chlorophyll can generate highly reactive and damaging singlet oxygen. In natural environments, both the intensity and spectral composition of solar light fluctuate with time, which necessitates an active balancing of excitation energy distribution between the two photosystems according to the light conditions. Such adaptation is achieved through changes in the distance relation and number of pigment–protein complexes, and through the morphology of the thylakoid membrane on a time scale from seconds to months [2]. The adaptation mechanism is a

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subject of active research, but this active balancing has not yet been well visualized for either cyanobacteria or chloroplasts. 30.1.2 Thylakoid Membranes in Chloroplasts

In higher plants, thylakoid membranes in chloroplasts form a flattened sac and are impermeable to most molecules and ions. There are two major light-harvesting chlorophyll–protein complexes LHCI and LHCII, which work as the main antenna for PSI and PSII, respectively. The light-harvesting complexes, LHCI and LHCII, absorb light and transfer the excitation energy to the two photosystems. The fine structure of the thylakoid membrane has been studied mainly using electron microscopy. A well accepted model of the structure suggests that it consists of two types of membranes [3]. One type is a stacked form of membranes known as grana membranes. The other form is unstacked or stroma-exposed membranes. It has been established that PSI and PSII are segregated. About 85% of PSII and 90% of its light-harvesting complexes, LHCII, are localized in the grana thylakoid. About 90% of PSI is localized in the unstacked part of the membrane, the stroma-exposed thylakoid [4]. The dimensions of a single grana region of the thylakoid membrane are of the order of submicrometers. Since PSI and PSII show different colors in fluorescence, red and far-red, respectively [5, 6], it could be expected that color-selected fluorescence imaging may resolve the fine structures of the thylakoid. Optical microscopy, including fluorescence imaging, has the potential to trace physiological changes of the thylakoid membrane during various adaptation processes. Whilst the electron microscopy is necessary to reveal the fine structure on a nanometer scale in solidified samples, such techniques may not provide details of all physiological aspects of the thylakoid membrane. 30.1.3 Thylakoid Membrane of Cyanobacteria

One unique feature of the thylakoid membrane in cyanobacteria is that the main light-harvesting antenna for PSII is phycobilisome (PBS), which is a pigment– protein complex containing phycobilins with absorption peaks at about 530–620 nm, between carotenoids and chlorophylls [1]. PBS is not a transmembrane complex, but is attached to the stromal side of the thylakoid membrane. Another unique feature of the cyanobacterial thylakoid is that PSI and PSII are not segregated, unlike in higher plants [7]. This raises a question: why are grana necessary in higher plants and some algae? To answer this question is not simple, but it may be addressed if we understand the detailed responses of the thylakoid membrane of various organisms to oxygenic photosynthesis: higher plants, alga and cyanobacteria. 30.1.4 Applications of Fluorescence Microscopy to a Thylakoid Membrane

The last two decades have seen tremendous progress in optical microscopy, especially, confocal fluorescence microscopy. Confocal laser-scanning fluorescence

30.1 Introduction

microscopy (CLSM) has been applied to chloroplasts. Chloroplasts in green algae were found to show different levels of complexity, when they were studied through PSII fluorescence [9]. Grana regions in the thylakoid membrane of chloroplasts have been successfully observed as intense fluorescence spots of chlorophyll autofluorescence inside chloroplasts [8–12]. The quantum yield of the PSII fluorescence is known to be far higher than that of PSI at physiological temperatures, such that chlorophyll autofluorescence usually infers fluorescence from PSII and its closely associated antenna, LHCII. For example, time-lapse imaging of chloroplast division was achieved using CLSM based on two-photon-absorption induced chlorophyll autofluorescence [13]. Two-photon excitation of pigments based on a near-infrared short pulsed laser is an attractive technique since it can suppress out-of-focus excitation and has high penetration ability through thick samples, such as a leaf [14, 15]. In studies of thylakoid membranes using fluorescence microscopy, the excitation light necessary for generating fluorescence inevitably causes some adaptive changes of the thylakoid membrane, depending on the wavelength and power. Suppression of the out-of-focus excitation, as realized with nonlinear excitation, is thus favorable for minimizing the disturbance to the physiological state of the thylakoid membrane. Images with spectral information have been frequently obtained from leaves of plants [16]. However, fluorescence microscopic studies on chloroplasts to resolve the spectrum have been very limited. To resolve the spectrum a decrease in the number of photons per single detector channel is required, which is problematic in highly spatially resolved microscopic studies requiring photons. The weak fluorescent signal may be compensated by using a relatively long accumulation time for the imaging, but slow image acquisition may not enable us to study relatively fast physiological processes. In spite of such difficulties in spectral microscopy, several works have recently been published, focusing on chloroplasts [12, 17, 18] or cyanobacteria [19–21]. Although there are several types of commercial microscopic systems that can measure fluorescence spectra with a reasonably high wavelength resolution (2 nm), most record spectra using stepby-step tuning of detectable wavelength regions and/or sacrifice the wavelength resolution and/or number of channels. The most serious drawback in the choice of a limited number of spectral windows and detectors is a significant rejection of fluorescence in the other spectral regions that could be physically detectable and scientifically informative. 30.1.5 Simultaneous Spectral Imaging and its Merits

A line-scan of the illumination laser light has been used to enhance the scan rate, at least compared to the point-by-point scan [18, 21–25]. It remains compatible with a spectral acquisition of high resolution, since total fluorescence from the linear region can be projected onto the slit of an imaging polychromator. A CCD camera at the exit port records the fluorescence intensity as a function of the spatial coordinate and the wavelength in the two-dimensional sensitive area of the camera. This method

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sustains the advantage of nearly parallel illumination and simultaneous detection of fluorescence, as far as the linear region is concerned. A microscopic fluorescence spectrum is potentially very informative, since it reflects stoichiometric ratios, efficiencies of electronic excitation transfers among the pigment–protein complexes, and quenching mechanisms inherent in the photosynthetic reactions. Since the multiple fluorescence bands are overlapping, spectral detection based on a polychromator and multichannel detector is more informative than detection using a few channels based on dichroic mirrors and band-pass filters. Cyanobacterial thylakoid membranes show fluorescence not only from chlorophylls but also from phycobilins. Even in the case of plant and algal thylakoid membranes, in which chlorophyll fluorescence is predominant in the red to far-red region, it is possible that multiple fluorescence components arise. The lightharvesting complexes, LHCI and LHCII, are characterized by slightly different chlorophyll fluorescence peaks, the distinction of which is usually feasible only at cryogenic temperatures [1, 6]. However, the ratios of their contributions certainly change during some physiological changes, and these may potentially be detectable at room temperature if the fluorescence spectrum is recorded at a high resolution (2 nm).

30.2 Spectral Fluorescence Imaging of Thylakoid Membrane 30.2.1 Realization of Fast Broadband Spectral Acquisition in Two-Photon Excitation Fluorescence Imaging

Full details of our-home-made spectromicroscope system have been described previously [21]. The set-up is shown in Figure 30.1a. A near-infrared femtosecond pulse train at 800 nm is illuminated on a line (one lateral axis, denoted as the X axis) on a specimen using a resonant scanning mirror oscillating at 7.9 kHz. Total multiphoton-induced fluorescence from the linear region was focused on the slit of an imaging polychromator. An electron-multiplying CCD (EMCCD) camera is used to resolve fluorescence of different colors at different horizontal pixels and fluorescence of different spatial positions in the specimen at different vertical pixels. Scanning on the other two axes (Y and Z) is achieved by a closed-loop controlled sample scanning stage and a piezodriven objective actuator. The full widths at half maximum of the point-spread function of the system have been estimated to be 0.39–0.40, 0.33 and 0.56–0.59 mm for the X (lateral axis along the line-scan), Y (the other lateral axis) and Z axes (the axial direction), respectively, at fluorescence wavelengths between 644 and 690 nm. The efficiency of the new line-scanning spectromicroscope has been estimated in comparison with our own point-by-point scanning spectromicroscope [21]. Under typical conditions for observations of cyanobacterial cells (cf. Section 30.2.2), the total

30.2 Spectral Fluorescence Imaging of Thylakoid Membrane

Figure 30.1 (a) Schematic of the optical set-up of the line-scanning spectromicroscope. (b) Typical wide-field microscopic image of the cyanobacterium Anabaena PCC7120 under the illumination of a halogen lamp. The scale bar is 10 mm. (c) Reflection image of the femtosecond

pulsed laser from the surface of an objective scale with a spacing of 10 mm. (d) Typical image of fluorescence intensity distribution detected by the EMCCD camera. The horizontal and vertical axes correspond to the fluorescence wavelength and a lateral axis in the specimen, respectively.

exposure time was shortened by about 50 times for a constant average excitation density. The improvement factor was proportional to the length of the line-scanned region, as expected. The idea of line-scanning or line-focus is not new. There is only one drawback which is that a confocal effect does not occur along the slit direction. Nevertheless, it is still adopted in a number of commercial fluorescence and Raman spectromicroscopes (LSM5-LIVE from Carl Zeiss, Raman 11 from Nanophoton, etc.) when spectral information is desirable. Our set-up seems to be unique in that multiphoton excitation with the rapidly moving resonant scan mirror is used for broadband spectral imaging. Linear illumination has been achieved either by expanding a laser beam along one axis with a cylindrical lens [22–24] or by scanning a focal point with a scan mirror [21, 25]. In the former case, the laser intensity (W cm2) is inversely proportional to the out-of-focus distance in the optic axial coordinate, as far as the region outside the beam waist is concerned. In the latter case, the laser intensity is inversely proportional to the square of the out-of-focus distance in the optic axial coordinate. Thus, in cases employing multiphoton excitations to suppress outof-focus excitation and improve penetration ability, better depth sectioning is expected in the case of linear illumination by a rapid scan mirror than in the case of linear illumination by a cylindrical lens. We have actually confirmed that the depth resolution is equal to the diffraction limit [21]. However, the lateral resolution is not equal to the theoretical limit, which may be limited by the imaging polychromator.

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30.2.2 Spectral Imaging of a Filamentous Cyanobacterium, Anabaena 30.2.2.1 Thylakoid Membrane of Cyanobacterium Anabaena is a filamentous cyanobacterium (Figure 30.1b), which is well known for its ability to fix nitrogen. Under nitrogen-starved conditions, the nitrogen fixation is undertaken only in terminally differentiated heterocyst cells that are separated from each other at a constant spacing in the same filament [26, 27]. A great deal of attention has been paid to Anabaena by a broad range of biologists, since it is regarded as one of the simplest organisms exhibiting cell differentiation. In our study, intracellular spectral features of vegetative cells, which are normal cells without nitrogen-fixation ability, were studied using fluorescence spectromicroscopy. 30.2.2.2 Stability of the Anabaena Fluorescence Spectra Under Photoautotrophic Conditions Before any active research to induce and observe responses of the thylakoid membrane to environmental conditions can take place, it is important for us to understand the stability in fluorescence properties of cells, especially under probing light. Total fluorescence intensity is equal to the wavelength-integrated fluorescence signal in the case of spectrally resolved data, which is shown in Figure 30.2a for photoautotrophically grown Anabaena cells. All cells typically show a dark region at the center of

Figure 30.2 (a) A typical image of the total fluorescence intensity of filamentous Anabaena cells grown photoautotrophically. The scale bar on the right represents 2 mm. (b) Fluorescence spectra generated from the subregions numbered 1 to 4 in (a). The averaging for individual spectra was performed on all of the 15 Z-positions sharing the same XY coordinates. (c) Fluorescence spectra of the region 1 in (a) in consecutive three 3D scans.


cells, which correspond to regions free of thylakoid membranes, as shown by electron microscopic images [28]. In contrast to the thylakoid membrane in plants and some algae, grana-like regions do not occur in cyanobacterial cells [7, 28]. It is thus believed that PSI and PSII are not segregated. Fluorescence spectra of individual cells in the same filament are compared in Figure 30.2b. These were generated from the subregions numbered from 1 to 4, as indicated in Figure 30.2a. The spectra are very similar. It should be noted that the spectra here are all generated based on the average of about 1000–2000 fluorescent points. In many articles studying fluorescence spectra of plants and cyanobacteria, cryogenic temperatures are used to decompose spectra. This technique is powerful and informative, but the cooling procedure is not compatible with real-time recording of physiological phenomena. It is noteworthy that the spectra, even at room temperature, show some shoulders corresponding to peaks of constituent fluorescent components (at around 680, 710 nm), which can be visualized only with a high spectral resolution, as used in the current set-up. Stability or sensitivity of the fluorescence spectra under the current observation conditions has been investigated by repeating fluorescence spectral imaging in the same region. The spectra in Figure 30.2c show little change in terms of intensity and shape between three sets of consecutive 3D scans, one of which includes 15 z-sections with spacing of 0.90 mm. This suggests that the fluorescence properties of the cells are preserved even with the laser scanning. 30.2.2.3 Change of the Anabaena Fluorescence Spectra by Dark Conditions Some Anabaena can grow heterotrophically under dark conditions if fructose is supplied as a respiratory substrate [29], although the growth rate is not so fast. It is interesting that some thylakoid membrane is preserved even in the dark. However, the morphology and response to light exposure or laser scanning is thought to be different from those grown photoautotrophically, as has been shown in part previously [29]. Under photoautotrophic growth conditions long filaments of cells (containing 10 cells or more) are frequent. In the case of Anabaena PCC 7120 that is unable to grow under dark conditions, we were unable to find long filaments of cells, reflecting stop of growth. A typical image of total fluorescence intensity for a couple of Anabaena cells with dark treatment for about 30 days is shown in Figure 30.3a. The central dark regions in the cells were smaller than those in photoautotrophically grown cells. This is consistent with early electron microscopic observations [29]. Fluorescence spectra of individual cells are shown in Figure 30.3b and c for the left and right cells, respectively. Two sets of 9 z-sections of spectral images with a spacing of 1.00 mm were acquired consecutively from these cells. In comparison with photoautotrophically grown cells, several differences in the spectral properties were noted. First, the two cells clearly exhibit different spectral shapes, which are largely characterized by the ratio between the 660 and 740 nm peaks. Second, the spectra obtained in the second 3D scan were significantly weaker for the whole wavelength region than in the first scan. Third, the spectral shapes in the second 3D scans were different from those in the first scans. The overall decrease in fluorescence intensity in the

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Figure 30.3 (a) A typicalimage ofthe total fluorescenceintensity ofa couple of Anabaena cells with dark treatment. The scale bar on the right represents 2 mm. (b) Fluorescence spectra averaged over the 3D space of the left cell in (a). (c) Fluorescence spectra averaged over the 3D space of the right cell in (a). The spectra in (b) and (c) were generated from two consecutive 3D scans.

consecutive scans is most likely due to photobleaching and/or photoinhibition of all types of photosynthetic pigment–protein complexes. This was not observed for photoautotrophically grown cells (Figure 30.2b), probably due to activation of photochemical and non-photochemical quenching (quenching for photosynthetic charge separation and protective quenching other than photosynthetic charge separation) only under photoautotrophic growth conditions. 30.2.2.4 Intracellular Spectral Gradient in Anabaena Cells Our previous publication showed an intracellular gradient in the ratio of fluorescence intensities between two wavelength regions of 644–664 nm and 680–690 nm (F685/F654 ratio), for photoautotrophically grown cells [21]. The two wavelength regions centered at 654 and 685 nm are selected to focus probing on the fluorescence from phycobilisome (PBS) and PSII, respectively [19, 20]. An example is shown for the F685/F654 ratio map in Figure 30.4b, together with the total fluorescence intensity map in Figure 30.4a. These indicate that the ratio value is lower at the core than at the periphery of each cell. The locally averaged spectra in Figure 30.4c show that the difference at 680 nm fluorescence, after normalization using amplitudes at 645 nm, amounts to as much as 30%. We have obtained such F685/F654 ratio maps for about 70 cells from 25 Anabaena filaments. Among them, about 60% of the cells showed substantially lower F685/ F654 ratios (by 10–30%) in the central regions than in the peripheral regions. The


Figure 30.4 A typical case for the fluorescence ratio, F685/F654, being lower in the central parts of the cells than the periphery. (a) Gray-scale image of total fluorescence intensity. The scale bar represents 2 mm. (b) F685/F654 Ratio map. The corresponding ratio value is shown by the horizontal bar. The region outside the cell is also shown in black, where fluorescence intensity is too weak for the ratio value to be defined. (c)

Local fluorescence spectra. They were calculated from the region demarcated by black circles with a diameter of 1.0 mm in (b). The spectral imaging data in this figure were obtained using a commercial point-scanning spectromicroscope (NanoFinder, Tokyo Instruments, Tokyo, Japan) modified to use two-photon excitation using a 820 nm femtosecond pulse train [21].

opposite trend has not been observed. The other 40% of cells did not show a clear gradient in the ratio maps. To the best of our knowledge, there has been no report of such a systematic modulation of the F685/F654 ratio on the same strain. Even if the intrinsic fluorescence spectrum of the thylakoid membrane is constant, there are several possible causes for such a spectral gradient. Wavelength dependence of reabsorption and/or point-spread functions can, in principle, give rise to such a dependence of the spectra on the position of the probing fluorescence (e.g., whether the position is at the central part or near the edge). These two possible artifacts were ruled out in our previous work, based on measurements of wavelength dependence of transmission and point-spread function [21]. The intracellular gradient in the F685/F654 ratio map indicates that fluorescence of PSII is relatively lower in the central regions than in the peripheral regions. There are at least three interpretations of this phenomenon. The first and most straightforward interpretation is that the stoichiometric ratio of PBS is higher in the central regions than in the peripheral regions. It is easy to hypothesize a physiological meaning for this, since the thylakoid membrane at the deepest position in the cell receives less external light than that nearest to the outer membrane. The loss of light during transmission from the surface to the deepest part of the thylakoid membrane is estimated to be about 24% (assuming one half of the measured apparent absorbance of 0.25) [21]. This may be a small change in light intensity, but the increase in the effective absorption cross section would certainly benefit PSII in the deepest part of the thylakoid membrane. The second interpretation is that the

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efficiency of energy transfer from PBS to PSII is somehow lower in the central regions than in the peripheral regions. This interpretation may be relevant to the socalled state transition, in which the yield of the energy transfer from PBS to PSI and/ or PSII is regulated by the intensity and spectrum of the incident light [2]. The third interpretation is a natural hybrid of the first and second interpretations. When there is more PBS per PSII in the central regions than in the peripheral regions, some of the extra PBS associated with the central thylakoid membrane cannot be closely connected with PSII. This, on average, results in higher weights of PBS fluorescence at the central thylakoid membrane than at the peripheral thylakoid membrane. It would be necessary to apply time-resolved fluorescence decay measurements and/or excitation wavelength dependence of the fluorescence ratio image to this system to determine which of the above-mentioned mechanisms is most appropriate. Two recent works using spectral microscopy have reported dependence of fluorescence spectrum on radial positions in cyanobacterial cells [20, 30]. One is about variation between subunits of phycobilisomes in Nostoc punctiforme [20], the other is reporting that phycobilin-to-chlorophyll fluorescence ratio (phycobilin/chlorophyll) is higher along the periphery than inside the cells in Synechocystis sp. PCC6803 [30]. The latter phenomenon seems to be opposite to what we have found. How such different trends arise is interesting future subject to be addressed with the help of spectral microscopy. 30.2.3 Spectral Imaging of Chloroplasts 30.2.3.1 Chloroplasts from a Plant, Zea mays In an effort to generate images of the morphology of thylakoid membranes inside plant chloroplasts, isolated chloroplasts resuspended in a buffer solution containing a herbicide, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), have often been used [10, 11]. The DCMU inhibits photosynthesis by blocking electron transport at the electron acceptor side of PSII and enhances chlorophyll fluorescence from PSII. In our study, we tried to resolve PSI and PSII without introducing DCMU or any other chemical to enhance fluorescence. Chloroplasts were placed in situ, in a leaf, which was detached but not sliced. One prerequisite to acquire a fluorescence map of PSIand PSIIis to determine the fluorescence spectra of the two photosystems. From careful examination of the sensitivity of the microscopic fluorescence spectra, the following rule was suggested. A volume-averaged fluorescence spectrum of a single chloroplast exhibits intensity changes at wavelengths between 655 and 715 nm, but the intensity between 715 and 740 nm remains more or less constant with repeated 3D scans for spectral imaging and illumination with a halogen lamp under the microscopic observation conditions. The volume-averaged fluorescence spectrum was obtained using 3D spectral imaging of a whole chloroplast, which gave chloroplast volume and spectra for every scanned point. The sum of the fluorescence spectra was divided by the number of scanned points with substantial chlorophyll fluorescence (relative volume). The two photosystems are known to show different dependence on redox-conditions and varying stress-sensitivity in fluorescence quantum yields [31].


Under our measurement conditions, unavoidable lamp illumination, laser scanning, and the enclosure of the leaf in a thin chamber seem to have induced changes in the fluorescence quantum yield of mainly PSII. This procedure was used to approximately decompose the observed fluorescence spectra into at least two components, which helped us to obtain fluorescence maps of PSI and PSII. The wavelength for PSII imaging was thus selected to be between 665 and 681 nm (hereafter designated red image), and that for PSI was between 715 and 740 nm (farred image), as shown in Figure 30.5. It should be noted that the fluorescence bands of PSII antenna are believed to be located at 683–685 nm at both a physiological temperature and 77 K [1, 6]. Our wavelength selection for the PSII fluorescence in Figure 30.5 was between 665 and 681 nm in order to minimize overlap with the fluorescence of PSI. The fluorescence images in Figure 30.5a and b are shown using a normal gray scale. In order to visualize small amplitude modulation clearly, pixels with intensities of at least 50% of the maximum intensity in each XY image are shown in Figure 30.5c and d. There are distinctly bright spots at a sub-micrometer scale in the red fluorescence images in Figure 30.5c. The bright spots in Figure 30.5c are attributable to grana that are rich with PSII. Compared with the nearly circular grana regions obtained in previous works [8–12], the putative grana region in our red-fluorescence images is anisotropically elongated in the direction of the laser scan (X axis). This may simply reflect an artifact due to the lack of a confocal effect along the direction of the slit. According to the concept of a string of grana, grana are not scattered at random, but tend to be arranged in strings of varying length, like beads on a necklace. This has been supported by previous CLSM images and electron microscopic images [3, 10]. With our resolution limitations, elongated spots may actually contain separate multiple grana in the same string. Closer inspection of Figure 30.5c also reveals that the putative grana regions are connected not only in the single XY planes, but also along the different Z positions. Our data seem to directly support the model of a string of grana rather than randomly scattered grana, based on the 3D red fluorescence distribution. The PSI indicated by the far-red images in Figure 30.5d seems to be distributed almost homogeneously down to a sub-micrometer scale, which is in contrast to the case of the red fluorescence images. It seems that PSI is distributed not only in the non-appressed part (non-grana part) of the thylakoid membrane but also in the grana part of thylakoid membrane. This is possible if we consider the model in which PSI reside not only between neighboring grana regions but also in the stromaexposed thylakoid closely surrounding the grana. Given the typical size of grana (0.3–0.6 mm) and the relatively weak far- red fluorescence than the red one [3, 10], it is probably hard for our optical microscopy to resolve local minima of PSI fluorescence intensity corresponding to the center of grana. The fluorescence images attributed to PSI in a chloroplast of the plant simplex var. metallica in Ref. [10] show a clear intensity modulation with local maxima coincident with local minima of PSII fluorescence, in contrast to our far-red images that show rather homogeneous distribution of PSI. This difference may be attributable to the abnormally large size of the grana (up to 1.9 mm) in this special plant which is found in shady, dense and humid tropical forest.

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Figure 30.5 Fluorescence images of two neighboring chloroplasts in a mesophyll cell of Zea mays at different Z positions. (a) Integrated intensities between 665 and 681 nm (red image). The scale bar is 5.0 mm. The direction of the line scan is along the X axis. (b) Integrated intensities between 715 and 740 nm (far-red image). In (a) and (b), the maximum intensity in each image is indicated in white and a threshold intensity

reasonably above noise level in black. (c) and (d) Generated from the same data set as in (a) and (b), respectively, but the threshold intensity was set at 50% of the maximum intensity in each image. Fluorescence points below the threshold are thus black. This figure is adapted from Figure 1 in Ref. [18] with kind permission of Springer Science and Business Media.

30.2.3.2 Chloroplast from the Green Alga, Chlorella Figure 30.6 shows fluorescence images of a chloroplast from a cell of Chlorella kessleri using the same wavelength selection as for Zea mays (cf. Figure 30.5). The red and far-red fluorescence images are rather similar to each other. Although the red fluorescence is relatively localized near the edge of the cells compared to the

30.3 Technical Verification and Perspective

Figure 30.6 Fluorescence images of a Chlorella cell at different Z positions. (a) Integrated intensities between 665 and 681 nm (red image) The scale bar is 2.0 mm. The direction of the line scan is along the X axis. (b) Integrated intensities between 715 and 740 nm (far-red image). The maximum intensity in each XY image is indicated

in white and the intensity at 50% of the maximum intensity in black. Fluorescent points below the threshold (50% of the maximum intensity) are thus black. This figure is adapted from Figure 2 in Ref. [18] with kind permission of Springer Science and Business Media.

far-red one, the overall difference between the red and far-red images is far less evident for Chlorella than for Zea mays. The fluorescence spectrum of the Chlorella chloroplast is relatively flat between 715 and 740 nm compared with that of Zea mays, which often shows an increase in fluorescence intensity from 715 to 740 nm. The situation suggests that the far-red component of the Zea mays is substantially influenced by PSI whilst the far-red component of the Chlorella is dominated by a vibronic progression of the chlorophyll fluorescence of PSII. It is then reasonable that the red and far-red fluorescence images are relatively similar to one another. These results seem to be consistent with the previous findings that segregation between PSI and PSII, or grana formation, is observed in only a limited number of taxa of green algae [9]. Electron microscopic images of Chlorella do not usually show typical grana stacking [32, 33], except when excess carbon dioxide is supplied [34].

30.3 Technical Verification and Perspective

One drawback of spectral microscopy is ambiguity in interpreting the observed spectral shape and intensity which can be affected by scattering and/or re-absorption

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(see Section 30.2.3.4). Lifetime is a more robust parameter which is not easily affected by reabsorption or scattering. Several types of fluorescence lifetime imaging microscopy (FLIM) are already available as commercial systems. Fluorescence decay rate can be used to record the extent of non-photochemical quenching and the effect of photosynthetic inhibitors. This has been studied for the green alga Chlamydomonas reinhardtii using FLIM [35]. However, time-resolved spectra are still difficult to generate, mainly due to the high cost of preparing multiple detector channels with time-resolving ability. Spectral imaging of time-integrated fluorescence will continue to be a basic microscopic tool, particularly for the research of the thylakoid membrane, because of the complexity of the spectra. Fast confocal fluorescence imaging at a video rate or far better (1000–2000 Hz) is now available for several commercial systems. Imaging using a single focus requires higher peak power for faster imaging, which may lead to faster photobleaching and photodamage. Some of the systems thus employ a parallel illumination using multiple foci and parallel imaging by CCD cameras [36]. The responses of chloroplasts are not as fast, but the faster imaging capability is a benefit for observing slow phenomena. It enables fluorescence images to be obtained with a lower average and/ or peak power by distributing the excitation photons over a number of scans. The number of photons necessary for fluorescence (spectral) imaging may be reduced as far as possible, ideally to the level of natural solar illumination. There are some negative opinions on the use of multiphoton microscopy in botanical specimens, including chloroplasts [15]. In Ref. [15], it is reported that short-pulsed near-IR excitation (780 nm) leads to faster damage to the specimens than single photon excitation. However, our results show that at least light-adapted chloroplast and cyanobacteria grown photoautotrophically can preserve their fluorescence spectrum even after a few 3D scans. As mentioned previously, time-lapse imaging of chloroplast division was also demonstrated based on two-photon fluorescence [13]. In addition, we have estimated the relative densities of excited chlorophylls under the three experimental conditions detailed in Table 30.1. The rapid photodamage reported in Ref. [15] seems to be due to high peak power and high excitation density. It should be also noted that so far we have only encountered fluorescence with a substantial intensity at wavelengths longer than 600 nm, in both Anabaena cells and chloroplasts. In contrast, significant autofluorescence was observed not only for chlorophylls but also for other shorter-wavelength emitting species in the multiphotoninduced fluorescence spectra of a chloroplast in Ref. [15]. The significantly different spectra are most plausibly explained by differences in the peak power of the laser and/or possible (near-) resonance effects. Excitation wavelength as short as 780 nm (in Ref. [15]) at the peak power level may lead to higher order nonlinear absorption by so many types of molecules in cells that it hampers non-invasive fluorescence imaging.

30.4 Summary

We have shown an application of fluorescence spectral imaging for the detailed study of thylakoid membranes in three types of organisms with oxygenic photosynthesis.

30.4 Summary Table 30.1 Comparison of excitation density between the line scan system and two point-scan systems in application to plant leaves.

Excitation wavelength: lex (mm) Average power of the laser at the sample position: PA (mW) Pulse repetition rate: R (Hz) Single pulse energy: EP ¼ PA/R (pJ) Numerical aperture: NA Spot Size: Se(0.61lex/NA)2 (cm2) Pulse duration: tp (ps)c peak power density: IeEP/tp/S (W cm–2) Dwell time or exposure time per pixel (s): td Number of pulses per pixel: NP ¼ Rtd/6 · (0.25/20)dor NP ¼ Rtd Relative two-photon absorption cross section: s, Ref. [14]e pixel size in data acquisition (mm2) Relative two-photon excitation probability per single pulse: PSesI2tp · 105 Relative excitation density DEe PSNP · 105

Our linescan System

CLSM, pointscan [15]

CLSM, pointscan [13]

0.805 0.03a

0.780 6.4

0.800 1b

76 · 106 0.39a 1.4 1.23 · 109 0.2 1.6 · 109 1 1.58 · 105

82 · 106 78 1.2 1.57 · 109 0.1 5.0 · 1011 8.4 · 106 6.89 · 102

80 · 106 4.5 1.3 1.41 · 109 0.18 1.8 · 1010 30 · 106 2.4 · 103

0.9

8

1

0.25 · 0.25 4.50

0.234 · 0.234 1.97 · 106

0.24 · 0.24 5.7 · 102

7.13

1.36 · 104

14

a

Single pulse energy previously reported in Ref. [21] was overestimated, since it was based on a power measured outside the microscope. In Ref. [13], the power used for the time-lapse imaging of a chloroplast division was not exactly described, but power as high as 2 mW was used without causing damage. c In our study, pulse width at the sample position was not measured. In Ref. [15], it is not clear where the pulse width was obtained. d In the case of the line scan, the number of pulses illuminated on the linear region to be imaged (20 mm) was about one-sixth of the total laser pulses, because of an angle-range selection of the scan mirror in which its angular speed is approximately constant. e The pulse number per pixel was given by the number of pulses in the line divided by the number of pixels therein (80 ¼ 20/0.25). b

In the case of Anabaena, we found that the morphology and sensitivity of the thylakoid membrane, which are dependent on growth conditions, can be readily studied. Although the fluorescence spectra at room temperature are not so easily, or so well, resolved, they certainly show shoulders and dips, which reflect constituent pigment–protein complexes. It should be noted that the ratio of concentrations of the pigment–protein complexes and/or energy transfer efficiencies between them are position dependent in some cases. PSI fluorescence in Zea mays was distributed rather homogeneously down to the submicrometer level. This was in contrast to the distribution of PSII which formed distinctly bright fluorescent spots. This supports the model in which PSI resides not only in the non-appressed regions of thylakoid membrane connecting neighboring grana, but also in the stroma-exposed part closely surrounding the grana.

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Although high resolution of wavelengths is not necessarily regarded as important in all research areas, it is certainly advantageous in deriving the detailed spectral shape of fluorescence from microscopic regions. It is also necessary in order to select the best set of dichroic mirrors and filters for a limited number of detection channels. Nearly diffraction-limited and truly simultaneous fluorescence spectral imaging of chloroplasts in situ using the line-scanning spectromicroscope seems to be achieved with an excitation density comparable to that for commercial CLSM with only a few detector channels. The functionality of fluorescence spectral imaging needs to be further improved to study a wider range of physiological changes and developmental stages of chloroplasts and cyanobacteria.

Acknowledgments

The authors gratefully thank Professors H. Masuhara, K. Yoshihara and, S. Itoh for their encouragement of this study, and Drs M. Nishiyama, H. Oh-Oka and K. Okamoto for advice. This work was supported in part by Grants-in-Aid for Scientific Research (No.17750069 to SK) and for Scientific Research on Priority Areas (Area No. 432, No.16072209 to SK, and Area No. 477, No. 19056012 to SK) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also supported in part by funds from the Kurata Memorial Hitachi Science and Technology Foundation (to SK), and the Asahi Glass Foundation (to SK).

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6 Itoh, S. and Sugiura, K. (2004) in Chlorophyll a Fluorescence a Signature of Photosynthesis (eds C. Papageorgiou and Govindjee), Springer, Netherlands, pp. 231–250. 7 Gantt, E. (1994) in The Molecular Biology of Cyanobacteria, (ed D.A. Bryant), Kluwer Academic Publisher, Netherlands, pp. 119–138. 8 van Spronsen, E.A., Sarafis, V., Brakenhoff, G.J., van der Voort, H.T.M. and Nanningga, N. (1989) Threedimensional structure of living chloroplasts as visualized by confocal scanning laser microscopy. Protoplasma, 148, 8–14. 9 Gunning, B.E.S. and Schwartz, O.M. (1999) Confocal microscopy of thylakoid autofluorescence in relation to origin of

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26 Thiel, T. and Pratte, B. (2001) Effect on heterocyst differentiation of nitrogen fixation in vegetative cells of the cyanobacterium Anabaena variabilis ATCC 29413. J. Bacteriol., 183, 280–286. 27 Meeks, J.C. and Elhai, J. (2002) Regulation of cellular differentiation in filamentous cyanobacteria in free-living and plantassociated symbiotic growth states. Microbiol. Mol. Biol. Rev., 66, 94–121. 28 Black, K., Buikema, W.J. and Haselkorn, R. (1995) The hglK gene is required for localization of heterocyst-specific glycolipids in the cyanobacterium Anabaena sp. Strain PCC7120. J. Bacteriol., 177, 6440–6448. 29 Pescheck, G.A. and Sleytr, U.B. (1983) Thylakoid morphology of the cyanobacteria Anabaena variabilis and Nostoc MAC grown under light and dark conditions. J. Ultrastruct. Res., 82, 233–239. 30 Vermaas, W.F.J., Timlin, J.A., Jones, H.D.T., Sinclair, M.B., Nieman, L.T., Hamad, S.W., Melgaard, D.K. and Haaland, D.M. (2008) In vivo hyperspectral confocal fluorescence imaging to determine pigment localization and distribution in cyanobacterial cells. Proc. Natl. Acad. Sci. USA, 105, 4050–4055.

31 Lichtenthaler, H.K. and Babani, F. (2004) in Chlorophyll a Fluorescence a Signature of Photosynthesis (eds C. Papageorgiou and Govindjee), Springer, Netherlands, pp. 713–736. 32 Reger, B.J. and Krauss, R.W. (1970) Photosynthetic response to a shift in chlorophyll-a to chlorophyll-b ratio of chlorella. Plant Physiol., 46, 568–575. 33 Hatano, S., Kabata, K., Yoshimoto, M. and Sadakane, H. (1982) Studies on frost hardiness in Chlorella-ellipsoidea. 8. Accumulation of free fatty-acids during hardnening of Chlorella-ellipsoidea. Plant Physiol., 70, 1173–1177. 34 Gergis, M.S. (1972) Influence of carbon dioxide supply on chloroplast structure of Chlorella pyrenoidosa. Arch. Mikrobiol., 83, 321–327. 35 Holub, O., Seufferheld, M.J., Gohlke, C., Govindjee, Heiss, G.J. and Clegg, R.M. (2007) Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient. J. Microsc., 226, 90–120. 36 Bewersdorf, J., Egner, A. and Hell, S.W. (2006) in Handbook of Biological Confocal Microscopy (ed. J. Pawley), Springer, New York, pp. 550–560.

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31 Fluorescence Lifetime Imaging Study on Living Cells with Particular Regard to Electric Field Effects and pH Dependence Nobuhiro Ohta and Takakazu Nakabayashi

31.1 Introduction

Fluorescence microscopy techniques have become invaluable tools for the study of biological systems because they can be applied to living cells under native, physiological conditions [1, 2]. The conventional fluorescence microscopy techniques generally use fluorescence intensity measurements to reveal chromophore concentration and location in cells. Numerous fluorescent chromophores are now available that enable selective imaging at the microscopic level. However, fluorescence intensity depends on excitation intensity fluctuations, absorption by the sample, and photobleaching of fluorescent chromophores, and thus is difficult to analyze quantitatively. To overcome these problems, either excitation ratio or emission ratio methods have been employed for quantitative imaging [3, 4]. However, the ratio methods are difficult to combine with a confocal microscope because of wavelengthdependent focal depth and absorption. Measurements of fluorescence lifetime of a chromophore can enhance the potential of fluorescence microscopy [1, 2, 5–8]. Fluorescence lifetime is an inherent property of a chromophore, and thus is independent of chromophore concentration, photobleaching and, excitation intensity, but highly dependent on pH, ion concentration, and local environment that affects the non-radiative rate of a chromophore. This makes fluorescence lifetime imaging (FLIM) a powerful tool for quantitative imaging of cellular conditions as well as the circumstances around the fluorescent dyes. In the present study, a FLIM measurement system was constructed and applied to Halobacterium salinarum (Hb. salinarum) loaded with 20 ,70 -bis-(carboxyethyl)-5(6)carboxyfluorescein (BCECF) to obtain information on the intracellular environment as well as the intracellular pHin each of the cells [9–12]. Hb. salinarum belongs to the family of extreme halophilic archaebacteria, and considerable attention has been paid to this bacterium in relation to proton transport, phototaxis or the adaptation of an organism to extreme environments [13–15]. Intracellular pH is an essential parameter for Hb. salinarum in the regulation of intracellular processes [14, 16, 17], and fluorescence intensity ratio methods have been used to measure the intracellular pH [18, 19]. The

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Figure 31.1 Chemical structure of BCECF.

fluorescence probe, BCECF, whose chemical structure is shown in Figure 31.1, is one of the most widely used fluorescent dyes for evaluation of pH because of its highly pHdependent fluorescence intensity [4, 6, 18–22]. It is shown that the intracellular pH of Hb. salinarum can be evaluated from the fluorescence lifetime of BCECFretained in the cells, so that the intracellular pH as well as other intracellular properties is expected to be obtained from FLIM measurements on the halobacteria. Application of electric fields to cells has been widely used in chemical biology for gene transfection, drug delivery, and modulation of intracellular ion concentrations [23–25]. However, the detailed mechanisms of such electroperturbation processes remain unclear. Therefore, the effect of an externally applied electric field on living cells was also examined by taking halobacteria as an example. We measured the time course of the space-resolved images of both the fluorescence intensity and the fluorescence lifetime of BCECF loaded in Hb. salinarum in the presence of external electric fields, which allow modulations of membrane permeabilization and intracellular environments [12]. Then, the field-induced changes in the shape and in the intracellular environment of the halobacteria were measured using the fluorescence of BCECF inside the cell. Charged and polar groups within protein structures have been reported to produce an electric field of 1–80 MVcm1 for embedded molecules [26–28]. The fluorescence chromophore, which may be embedded in a protein cavity or attached to a membrane, is surrounded by both apolar and polar functional groups [29–31]. These groups produce a strong electric field inside the protein cavity or toward the chromophore. As a result, the excitation dynamics of the fluorescence chromophore may be affected by such a field [32, 33]. It is conceivable that the local electric field is one of the vital factors that control the rate of the non-radiative process of the fluorescent chromophore in cells. To analyze the fluorescence lifetime, this kind of effect may have to be considered in living cells. An external electric field effect on the fluorescence spectrum has also been observed for the fluorescent chromophore of BCECF in a polymer film.

31.2 Experimental 31.2.1 FLIM Measurement System

FLIM measurements were carried out using a four-channel time-gated detection system [9, 11, 34]. The experimental system is shown in Figure 31.2. A mode-locked

31.2 Experimental

Figure 31.2 (a) Schematic diagram of a FLIM system. (b) Schematic illustration of the time-gating method. Fluorescence decay profile (solid line) and excitation pulse (dotted line). LIMO captures the fluorescence in the four time-windows.

Ti:sapphire laser (Spectra-Physics, Tsunami) pumped by a diode laser (SpectraPhysics, Millennia Xs) was used as the excitation light source. The pulse duration and the repetition rate of the laser pulse were 80 fs and 81 MHz, respectively. The second harmonic of an ultrafast harmonic system (Inrad) was used for excitation. The excitation beam was coupled to a single-mode optical fiber by a fiber coupler (Five Lab) and was introduced into the scanner head (Nikon, C1) of a confocal microscope (Nikon, TE2000-E). The intensity of the excitation pulse was attenuated to be

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