Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 47 PHOTOCHEMISTRY ON SOLID SURFACES

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates

Vol. 47

PHOTOCHEMISTRY ON SOLID SURFACES Editors M. Anpo Department of Applied Chemistry, University of Osaka Prefecture,Sakai, Osaka 59 I , Japan

T. Matsuura Department of Synthetic Chemistry, Kyoto University,Sakyo-ku, Kyoto 606, Japan

ELSEVIER

Amsterdam - Oxford

- New York -Tokyo

1989

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distributors for the hired States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U.S.A.

ISBN 0-444-874 13-5 (VOl. 47) ISBN 0-444-41 801-6 (Series) 0 Elsevier Science PublishersB.V., 1989 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences & EngineeringDivision, P.O. Box 330, 1000 AH Amsterdam, The Netherlands.

Special regulationsfor readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein Printed in The Netherlands

V

CONTENTS

XI

PREFACE

LIST OF CONTRIBUTORS

Chapter 1

1.1

1.2

2.2

2.3

2.4

INTRODUCTION Introduction (I. Tanaka)

1

General Aspects of Photochemistry. Search for New Basic Types of Photochemical Reactions (K. Tokumaru, T. Arai, and T. Karatsu)

5

Chapter 2

2.1

XI11

PHOTOCHEMICAL TECHNIQUES TO UNDERSTAND PHOTOCHEMICAL AND PHOTOPHYSICAL FEATURES ON SOLID SURFACES

Fluorescence and Transient Absorption Spectra of Solid Surface: Development of Time-Resolved Total Internal Reflection Spectroscopy (H. Masuhara)

15

Laser Flash Photolysis on Solid Surfaces (F. Wilkinson and G. P. Kelly)

30

Excimer Formation with Pyrenes on Silica Surfaces ( K . A. Zachariasse)

48

Photophysics of Acridone, N-Methylacridone, Acridine, and Pyrene Adsorbed on Silica Gel ( S . Suzuki and T. Fujii)

7')

VI

2.5

2.6

Heterogeneous Molecular Environments Probed by Fluorophores Bonded to Chemically Modified Silica Gel: Fluorescence Decay Measurements under a Microscope ( S . Hirayama, T. Kubo, and H. Yamasaki) Photoacoustic and Fluorescence Measurements of Energy Transfer in Adsorption Layers (H. D. Breuer)

Chapter 3

3.1

3.2

93

106

SPECIFIC FEATURES OF PHOTOCHEMICAL REACTIONS ON SOLID SURFACES

Photochemistry of Alkyl Ketones in the Adsorbed State: Effects of Solid Surfaces upon the Photolysis (M. Anpo)

119

Decomposition of Azocurnene on Silica Surfaces ( J . E. Leffler and J. J. Zupancic)

138

3.3

Photolytic and Redox Mechanisms €or the Photodecomposition of Ethanoic Acid Adsorbed over Pure and Mixed Oxides (M. Schiavello, V. Augugliaro, S. Coluccia, L. Palmisano, 149 and A. Sclafani)

3.4

ESR Studies of Alkyl Radicals Adsorbed on Porous Vycor Glass (H. D. Gesser)

168

Chemiluminescence Properties of Adsorbed Biacridylidenes (K. Maeda and S. Yamada)

184

3.5

Chapter 4

NEW DEVELOPMENTS OF ORGANIC PHOTOCHEMISTRY ON SOLID

SURFACES

4.1

Photochemistry of Dibenzyl Ketone Adsorbed on SizelShape Selective Faujasite Zeolites: Steric Effects on Product Distributions ( N . J. Turro and 2 . Zhang)

197

VII

4.2

4.3

Photochemistry of Organic Cations at Charged Interfaces (C. A. Backer and D. G. Whitten)

216

Electron Transfer between Adsorbed Dye Molecules and Organic Crystals: Model Character of the Adsorption System for Certain Aspects in Photosynthesis (K. Kemnitz, N. Nakashima, and K. Yoshihara)

236

Chapter 5

NEW DEVELO&S

OF INORGANIC P H O ~ I S T R YON SOLID

SURFACES

5.1

5.2

5.3

5.4

Inorganic Photochemical Reactions in Low Temperature Matrices and in the Surface-s of Solids (T. Tominaga)

255

Photochemistry of Metal Carbonyls Physisorbed on Porous Vycor Glass (H. D. Gafney)

272

Photochemistry of Silica-Adsorbed Fe(C0)5 (R. L. Jackson)

288

Photopreparation of Supported Metal Oxide and Metal Carbonyl Catalysts (A. Morikawa and Y. Wada)

303

LASER INDUCED PHOTOREACTIONS AND P H O T O - 0 ON SOLID

Chapter 6

SURFACES 6.1

6.2

6.3

UV Laser Photodissociation of Small Molecules on Solid Surfaces (H. Sat0 and M. Kawasaki)

317

C02 Laser Induced Surface Reaction

(M. Kawai)

329

Photochemical Aspects of Amorphous-Si Nucleation by Photo-CVD (H. Hada and M. Kawasaki)

339

VIII

Chapter 7

Photoprocesses on Fractal Surfaces (A. Seri-Levy, J. Samuel, D. Farin, and D. Avnir)

353

New Aspects in Area-Selective Electrode Reactions on I1luminated Semiconductors (M. Okano, R. Baba, K. Itoh, and A. Fujishima)

375

Photoluminescent Properties of Cadmium Sulfide Contacted with Gaseous Lewis Acids and Bases ( G . J. Meyer, E. R. M. Luebker, G . C. Lisensky, and A. B. Ellis)

388

Fluorescence of Dye Molecules Adsorbed on Semiconductor Surfaces (A. M. Ponte Goncalves)

403

7.1

7.2

7.3

7.4

Chapter 8 8.1

TOPICS OF PHOTOCHEMISTRY ON SEMICONDUCTING MATERIALS

APPLICATIONS OF PHOTOCHEMISTRY TO OPTICAL MEDIA

Photostability of Near-Infrared Absorbing Organic Dyes in New Optical Media (H. Nakazumi and T. Kitao)

8.2. Photoinduced Phase Transition in Liquid Crystals (S. Tazuke and S. Kurihara) 8.3

Photochemical Surface Reactions of Polymeric Systems: Lithographic Applications ( H . Hiraoka)

Chapter 9

41 9

435

448

RECENT DEVELOPMENTS OF PHOTOCHEMISTRY IN LIQUID CRYSTALS AND PROTEINS

9.1 Photoreactivity of Carbonyl Compounds in the Solid State (Y. Ito)

469

IX

9.2 Ketone Photochemistry as a Probe of Conformational Mobility in Nematic and Smectic Liquid Crystals (W. J. Leigh) 9.3

9.4

9.5

48 1

Absolute Asymmetric Synthesis via Photochemical Reactions of Chiral Crystals (J. R. Scheffer and M. Garcia-Garibay)

501

Fluorescence Quenching of Pyrene as a Monitor of Intermolecular Diffusion and Intramolecular Chain Bending in Cholesteric Liquid Crystalline Phases(1) (M. F. Sonnenschein and R. G. Weiss)

526

Dynamics of Excited State Relaxations in Some Proteins (F. Tanaka and N. Mataga)

551

SUBJECT INDEX

567


XI PREFACE

Research into light-initiated chemical reactions and processes on solid surfaces is a growing new field which promises to yield a number of useful applications: molecular photo-devices for super memory, photo-chemical vapor deposition to produce thin-layered electronic semiconducting materials, sensitive optical media, and the control of photochemical reaction paths, etc. In fact, photochemistry on solid surfaces is now a major field in a national research project on "Frontiers of Highly Efficient Photochemical Processes" sponsored by the Ministry of Education, Science and Culture of Japan. Photochemical techniques in homogeneous systems, i. e., gas-phase and liquid-phase, have been developed over the past several decades by many scientists and engineers, especially in the fields of physical chemistry, organic chemistry, inorganic chemistry, polymer chemistry, and laser chemistry. There are many useful books about photochemistry in the homogeneous systems, but no book has yet appeared regarding photochemistry on solid surfaces. The object of this book, "Photochemistry on Solid Surfaces", is to present an overview of the latest developments in photochemistry on solid surfaces, i. e., in heterogeneous systems for the further development of photochemistry, and in searching for useful applications of photochemistry in industry. Thus, this book covers those areas in which fundamental aspects are incorporated in order to elucidate the features of photochemistry on solid surfaces as well as those in developmental and research stages and suggests a promise for the future by many distinguished photochemists in those fields. The first chapter is an introduction of the general and basic aspects of photochemistry on solid surfaces. The second chapter covers the application of photochemical techniques in order to understand the photochemical and photophysical nature on solid surfaces. Chapter 3 covers the special features of adsorbed-state photochemistry as compared with those of gas-phase and solution-phase photochemistry. Chapter 4

XI1 covers the new developments of organic photochemistry on s o l i d s u r f a c e s , and i n c h a p t e r 5 t h e i n o r g a n i c p h o t o c h e m i s t r y on s o l i d s u r f a c e s i s described.

Chapter 6 c o v e r s r e c e n t developments i n l a s e r photo-

c h e m i s t r y and i n p h o t o - d e p o s i t i o n on s o l i d s u r f a c e s . Some t o p i c s i n t h e photochemistry on semiconducting m a t e r i a l s a r e included i n chapter

7. The a p p l i c a t i o n s of photochemistry t o o p t i c a l media such a s compact l a s e r d i s k s i s o u t l i n e d i n chapter 8. The recent developments of photochemistry i n l i q u i d c r y s t a l s t o c o n t r o l p h o t o c h e m i c a l r e a c t i o n s , and t h e photochemistry i n p r o t e i n s a r e a l s o included i n t h i s l a s t chapter. Thus, " P h o t o c h e m i s t r y on S o l i d Surfaces" should n o t only a c t a s an overview a s intended, but should a l s o p l a y a s i g n i f i c a n t r o l e i n t h e progress of t h i s most important f i e l d of high technology. The Editors October 1988

LIST OF CONTRIBUMRS

ANPO, Masakazu Department of Applied Chemistry, University of Osaka Prefecture Mom-Umemachi, Sakai, Osaka 591, Japan

ARAI, Tatsuo Department of Chemistry, The University of Tsukuba Tsukuba, I b a r a k i 305, Japan AUGUGLIARO, Vincenzo Dipartimento d i Ingegneria Chimica d e i Processi e d e i M a t e r i a l i , U n i v e r s i t a ' Degli Studi d i Palermo Viale Delle Scienze, 90128 Palermo, I t a l y AVNIR, David

Department of Organic Chemistry and t h e F r i t z Haber Research Center f o r Molecular Dynamics, The Hebrew University of Jerusalem Jerusalem 91904, I s r a e l BABA, Ryo

Department of Synthetic Chemistry, The University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan BACKER, Carol A. Department of Chemistry, The University of Rochester River S t a t i o n , Rochester, New York 14627, U. S. A. BRJWER, H. D.

I n s t i t u t f u r Physikalische Chemie, U n i v e r s i t a t des Saarlandes 6600 Saarbrucken, Federal Republic of Germany COLUCCIA, Salvatore Dipartimento d i Chimica Inorganica, Chimica F i s i c a e Chimica d e i M a t e r i a l i , Universita' d i Torino Corso Massimo d'Azeglio 48, 10125 Torino, I t a l y

XIV

ELLIS, Arthur B. Department of Chemistry, University of Wisconsin-Madison Madison, Wisconsin 53706, U. S. A . FARIN, Dina Department of Organic Chemistry and the Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem Jerusalem 91904, Israel FUJII, Tsuneo Department of Industrial Chemistry, Shinshu University Wakasato, Nagano 380, Japan FUJISHIMA, Akira Department of Synthetic Chemistry, The University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan GAFNEY, Harry D. Department of Chemistry, City University of New York Queen's College, Flushing, New York 11367-0904, U. S. A . GARCIA-GARIBAY, Miguel Department of Chemistry, The University of British Columbia 2036 Main Mall, Vancouver, British Columbia V6T 1Y6, Canada GESSER, Hyman D. Department of Chemistry, The University of Manitoba Winnipeg, Manitoba R3T 2N2, Canada HADA, Hiroshi Department of Industrial Chemistry, Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan HIRAOKA, Hiroyuki IBM Research Center 650 Harry Roag,; K92/802, San Jose, California 95120-6099, U. S. A. HIRAYAMA, Satoshi Faculty of Textile Science, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, Japan

XV

ITO, Yoshikatsu Department of Synthetic Chemistry, Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan ITOH, Kiminori Institute of Environmental Science and Technology, Yokohama National University, 156 Tokiwadai, Hodogaya, Yokohama 2 4 0 , Japan JACKSON, Robert L. IBM Research Center 650 Harry Road, K91/802, San Jose, California 95120-6099, U. S. A.

KARATSU, Takashi Department of Chemistry, The University of Tsukuba Tsukuba, Ibaraki 305, Japan KAWAI, Maki The Institute of Physical and Chemical Research Wako, Saitama 351-01, Japan KAWASAKI, Masahiro Research Institute of Applied Electricity, Hokkaido University Sapporo 060, Japan KAWASAKI, Hitsuo Department of Industrial Chemistry, Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan KELLY, G. P.

Department of Chemistry, Loughborough University of Technology Loughborough, Leics. LEll 3TU, U. K. KEMNITZ, Klaus Institute for Molecular Science, Myodaiji, Okazaki 444, Japan KITAO, Teijiro Department of Applied Chemistry, University of Osaka Prefecture Mom-Umemachi, Sakai, Osaka 591, Japan

XVI

KUBO, Takashi Faculty of Textile Science, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, Japan

KURIHARA, Seiji Research Laboratory of Resources Utilization, Tokyo Institute of Technology 4 2 5 9 Nagatsuta, Midori-ku, Yokohama 227, Japan LEFFLER, JohnE.

Department of Chemistry, The Florida State University Tallahassee, Florida 32306-3006, U. S. A .

LEIGH, William J. Department of Chemistry, McMaster University Hamil ton, Ontario L8S 4M1, Canada LISENSKY, George C. Department of Chemistry, Beloit College Beloit, Wisconsin 53511, U. S. A . LUEBKER, Elizabeth R. M.

Department of Chemistry, University of Wisconsin-Madison Madison, Wisconsin 53706, U. S. A .

MAEDA, Koko Department of Chemistry, Ochanomizu University Otsuka, Bunkyo-ku, Tokyo 112, Japan MASUHARA, Hiroshi Department of Polymer Science and Engineering, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, Japan MATAGA, Noboru

Department of Chemistry, Osaka University Toyonaka, Osaka 560, Japan

MATSUURA, Teruo Department of Synthetic Chemistry, Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan

XVII E Y E R , Gerald J.

Department of Chemistry, University of Wisconsin-Madison Madison, Wisconsin 53706, U. S. A. MORIKAWA, Akira Department of Chemical Engineering, Tokyo Institute of Technology Ohokayama, Meguro-ku, Tokyo 152, Japan NAKASHIMA, Nobuaki Osaka Institute of Laser Engineering, Osaka University Yamada-Oka, Suita, Osaka 565, Japan NAKAZUMI , Hiroyuki Department of Applied Chemistry, University of Osaka Prefecture Mozu-Umemachi, Sakai, Osaka 591, Japan Mitsutoshi Department of Synthetic Chemistry, The University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan

OKANO,

PAlMISANO, Leonard0 Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita' Degli Studi di Palermo Viale Delle Scienze, 90128 Palermo, Italy PONTE GONCALVES, A. M. Department of Chemistry, Temple University

Philadelphia, Pennsylvania 19122, U. S. A.

SAMUEL, Joshua Department of Organic Chemistry and the Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem Jerusalem 91904, Israel SATO, Hiroyasu Chemistry Department of Resources, Mi'e University Tsu, Mie 514, Japan SCHEFFER, John R.

Department of Chemistry, The University of British Columbia 2036 Main Mall, Vancouver, British Columbia V6T 1Y6, Canada

XVIII SCHIAVEUO, Mario

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita' Degli Studi di Palermo Viale Delle Scienze, 90128 Palermo, Italy SCLAFANI, Antonino Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita' Degli Studi di Palermo Viale Delle Scienze, 90128 Palermo, Italy

SERI-LEVY, Alon Department of Organic Chemistry and the Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem Jerusalem 91904, Israel SONNENSCHEIN, Mark F. United States Naval Research Laboratory Code 6546, Washington, D. C. 20375, U. S. A .

SUZUKI, Satoshi Department of Industrial Chemistry, Shinshu University Wakasato, Nagano 380, Japan

TANAKA, Fumio Mie Nursing College 100 Torii-cho, Tsu, Mie 514, Japan

TANAKA, Ikuzo The President, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152, Japan TAZUKE, Shigeo

Research Laboratory of Resources Utilization, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan TOKUMARU, Katsumi Department of Chemistry, The University of Tsukuba Tsukuba, Ibaraki 305, Japan

XIX

TOMINAGA, Takeshi Department of Chemistry, The University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan TURRO, Nicholas J.

Department of Chemistry, Columbia University New York, New York 10027, U. S. A. WADA, Y u j i Department of Chemical Engineering, Tokyo Institute of Technology Ohokayama, Meguro-ku, Tokyo 152, Japan WEISS, Richard G. Department of Chemistry, Georgetown University Washington, D. C. 20057, U. S. A.

WHITTEN, David G. Department of Chemistry, The University of Rochester River Station, Rochester, New York 14627, U. S. A . WILKINSON, Francis Department of Chemistry, Loughborough University of Technology Loughborough, Leics. LEll 3TU, U. K. YAMADA, Sachiko Department of Chemistry, Ochanomizu University Otsuka, Bunkyo-ku, Tokyo 112, Japan YAMASAKI, Hirohisa Faculty of Textile Science, Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, Japan

YOSHIHARA, Keitaro Institute for Molecular Science, Myodaiji, Okazaki 444, Japan ZACHARIASSE, Klaas A. Max-Planck-Institut ffir Biophysikalische Chemie Postfach 2841, D-3400 Gzttingen, Federal Republic of Germany

xx ZHANG. Zhenyu

Department of Chemistry, Columbia University New York, New York 10027, U. S. A.

ZUPANCIC, J. J. Department of Chemistry, The Florida State University Tallahassee, Florida 32306-3006, U. S. A .

Chapter 1

INTRODUCTION

Contents

1.1

Introduction 1

(Ikuzo Tanaka)

1.2

General Aspects of Photochemistry.

Search for New Basic

Types of Photochemical Reactions (Katsumi Tokumaru, Tatsuo Arai, and Takashi Karatsu)

5


1

INTRODUCTION

I. TANAKA Photochemistry on solid surfaces has unveiled the important role of sufaces as reactant media.

Solid surfaces work as acids o r bases;

sensitizers or quenchers: reaction space for size-specific reactions:

seed

crystals for epitaxial growth etc. Thus, the molecule-surface interaction enhances or reduces photoabsorption, reaction rates, and selectivities. Since there are a lot of parameters f o r surface reactions, such as adsorption, desorption, diffusion, nucleation etc., i t has been difficult to control the photochemistry on solid surfaces. Recently, as it becomes possible to characterize the surface conditions with techniques of ESCA,

SIMS, and STM and also to use new light sources, new research field appears as

Surface Photochemisty

".

The simplest treatment of surface reactions involving one molecule of a reactant is in terms o f the Langmuir adsorption isotherm, according to which the rate of reaction is given by, VOCK*P/(

l + K *P),

where K is an equilibrium constant for adsorption and desorption and P is the pressure of the reactant. This simple and important equation is often used to verify whether the reaction occurs on the surface or not. Reactions on the surface are not random processes but ordered ones by layer-by-layer techniques.

What is important to notice about surface

photochemistry is the major role of Langmuir type adsorption/ desorption processes.

Reaction systems that involve these adsorption/ desorption

processes have been characterized as being either Eley-Rideal or LangiauirHinshelwood. Reaction between gas-phase and surface-adsorbed molecules are Eley-Rideal, and those reactions involving two adsorbed molecules on neighboring surface sites are Langmuir-Hinshelwood. These fundamental processes are often discussed for understanding surface photochemistry. From kinetic theory of gases, the number of particles hitting unit area

2

of surface per unit time is,

r = 3 . 5 1 3 x 1 O Z 2 P/(MT)

[cm

1.

sec

I ]

,

where P is pressure in Torr, M is a molecular weight, and T temperature in K. Assuming the sticking coefficient is unity, then one monolayer is formed in 3 sec at P =10

Torr and 0 . 8 hr at P =10

Torr. Thus, both the ultra-

high vacuum system and surface cleaning technique are essential to the surface photochemistry. The mean residence time is given by,

Tr = ( 1 / v

)exp( E n /kT).

Thus, for E d >> kT, 'pr i s long and thermal equilibrium occurs rapidly, e. g.,

L',

2

/1

s for E d = 1 . 5 eV. On prepared surfaces where E d is

controllable, 27 can be adjusted to suitable times for desired reactions. Physically adsorbed

layer characterized as weakly bound to a

substrate forms a thin liquid-like layers of 8 - 1 0

folds.

Absorption

spectra of this thick layer are close to what observed in liquid phase. Spectral changes brought about by adsorption onto solid surfaces can be characterized by, ( 1 ) spectral shifts, ( 2 ) changes in the extinction coefficients, ( 3 ) broadening and

( 4 ) appearance of new absorption bands.

Thus, on surfaces photoabsorption wavelengths are changeable to desired regions even if the molecules have no absorption in the gas phase.

These

characters appear also in chemisorption of molecules; for example, new absorption bands appear in the visible region for organometallic compounds on the substrate, which have absorption limits typically at uv region. Not only UV light but also visible light are useful in photochemical reactions on surfaces. Enhancement in the extinction coefficient is expected for molecules on metals. When the frequency of the incident light is near a plasma of the metal structure , i t is predicted that there should be two effects leading to enhanced photochemistry of molecules ; (a) The photoabsorption of molecules will increase as the distance decreases from the surface of a silver sphere due to the enhanced local field ;(b) There will be a new photodissociation process by energy transfer

to

molecules,

then, strong enhancement of photochemistry is expected. The energy transfer from the surface to adsorbates is thus expected as one of enhancement processes. The photodissociation of Clz is enhanced on

3

an Si wafer due to strong photoabsorption of the wafer.

UV or visible

photons can generate electron-hole pairs in a semiconductor. This nonthermal photon effect was clearly demonstrated by excitation of

the

silicon band gap. For example, the Si etch rate could be enhanced by the laser irradiation at visible region. The surface works not o n l y as a quencher but also as a spacial-selected sensitizer. Rapid relaxation on a metal surface occurs usually to quench longlived excited states. This is clearly shown when time resolved luminescence decay of a molecule has been measured as a function of distance between the molecular layer and a silver surface. The decay decreases with decreasing the distance due to relaxation of electronic energy even through the solid argon layer. In order to reduce this quenching effect, the reaction chosen must involve a short-lived dissociative state; that is, the chemical reaction should occur within

-10

I 4

s

.

On the contrary, increase of

emission lifetimes is observed, when a cyanine dye is dissolved in a micellar solution of the detergent. The dye attaches to the surface of the micell is protected from the self-quenching process of the electronically excited states of the dye molecules. Lifetimes of photo-excited molecules may be controlled by proper use of surfaces. Direct photon effect on a substrate is photoablation of a solid surface. After the substrate' absorbs strong laser light, the material at the irradiation site is spontaneously etched away to a depth of -1

&m.

This

technique is applicable to cell surgery, gene transfection, and repair of mask patterns for integrated circuits. To understand ablation mechanisms we must know the difference between photolytic and thermal processes. Reaction rates for both processes are characterized as, for thermal processes,

R(I) = A.exp( - Ea/kT

for photolytic processes,

R(I) = A In ,

),

where I i s the intensity of incident photons. The Arrhenius equation is quite familiar, while the unfamiliar power index equation i s to be used in analysis of surface photochemistry.

4

When IR laser excitation supplies energy as thermal energy, the ablation is far less efficient than UV laser excitation. Obviously, UV laser induces some electronic excitation on the surfaces, which may generate local heating or may cause Coulomb repulsion. The thermal effect on light irradiation is used for laser-assisted chemical vapor deposition of Si for low-temperature growth o f epitaxial and polycrystalline layers.

It

improves quality of Si thin layers due to annealing of amorphous silicon on the surface. In this case photo-irradiation works as a very local thermal energy source on a specific region of the surface. This shows the spacial selectivity of reactions by photo-irradiation

. Strong laser irradiation

may result in ablation of ions and electrons with translational energy of more than 100 eV due to Coulomb

repulsion. These ions can react with

molecules. In this case, surface works as a source of a reactant. Concerning applications of photochemistry on solid surfaces, it has been recently applied to microelectronics industry. About thirty years ago, metal atoms were deposited by pyrolysis of organowetallic compounds. By this method, heat destroys the layer already deposited on the substrates. Since metals can be deposited by UV photodissociation of organometallic compounds, very fine lines of metals can directly be drawn on a substrate with microscopic features and without thermal damage. This technique is applicable to semiconductor industries. Moreover, atomic layer epitaxy of compound semiconductors was performed without any damage caused by ions and electrons those are formed in plasma chemical vapor deposition, Insulators are also deposited on the epitaxial layers of

semiconductors. Thus,

photochemistry on solid surfaces will be widely used as one of most advanced chemical vapor deposition techniques in microelectronics industry. Photochemical etching has been also used in microchemistry. Irradiation of light enhances etching rate in solution. Gratings in semiconductors are processed through laser irradiation of GaAs in solution. This technique, for example, allows etching of integral lenses on LEDs by shinning light through a photomask having patterned transmission. Thus, photochemistry on solid surfaces is a new and fruitful research field in which basic and applied photochemistry have to be studied.

5

General Aspects of Photochemistry. Search for New Basic Types of Photochemical Reactions

K. TOKUMARU, T. ARAI, and T. KARATSU 1.

INTRODUCTION Recently, many photochemical reactions of organic molecules have extensively been investigated particularly in solution, and their essential mechanisms seemed to be well established (1). Accordingly, many workers might suppose that new types o f reactions are observed in solid state or in organized media (2). It may be true. However, in this chapter on general aspects of photochemistry, we would like to point out, based on our recent findings, that still there are possibilities to find out in solution new basic types of reactions, which might serve for understanding essential features of photochemical reactions and be extended to other media. NOVEL TYPE OF ISOMERIZATION OF OLEFINS As one of the key types of photochemical reactions, cis-trans isomerization of olefins has well been investigated from mechanistic as well as synthetic aspects of the reaction (3-16). Many olefins seemed to isomerize mutually between their cis- and trans-isomers. A following well accepted mechanism was proposed. In the singlet and triplet excited states, cis (c‘:;) and trans (t“’) forms are less stable than the perpendicular form (p”), and c” and t“ twist around the C=C bond to p”, which deactivates to the ground state giving a mixture of the cis- and trans-isomers (Fig. 1 ) . This mechanism had been believed to cover all olefins until we found that introduction of a 2-anthryl group on the ethylenic bond dramatically changed the course of isomerization (17-22). 2.

_I.

c i s - 1 . a : R=’Bu,b:

R=Me.

trans - 1

6

A Q g k of Twist /Degree

Fig. 1. Calculated potential energy surfaces of ethylene.

Angle of Twist /Degree

Fig. 2. Proposed potential energy surfaces for anthrylethylenes.

The anthryl olefins ( l g - l d ) undergo solely cis-to-trans isomerization, and no reverse isomerization takes place at all. A l s o , surprisingly, the quantum yields of the cis t o trans isomerization far exceed unity and increase with the cis-isomer concentration attaining 10-20 depending on the olefins and their concentration. Once a photon is absorbed, many cis-molecules isomerize into trans by a quantum chain process. Therefore, this reaction also can serve for amplification of the effect of photons. We named this type of reaction as one-way isomerization and proposed its mechanism. The actual isomerization proceeds on the triplet energy surface, and we believe that there is no energy minimum at the 3-k perpendicular conformation. The initially resulting c twists around the C=C bond passing through a perpendicular conformation At the perpendicular conformation, the triplet and the to 3t". ground state energy surfaces are close to each other; however, the absence of energy minimum at this conformation do not enable a triplet molecule to undergo intersystem crossing to the ground state in preference to twisting along the triplet energy surface. The resulting 't" undergoes unimolecular deactivation to the ground-state trans (t 0 ) or bimolecular energy transfer to the 0 3 6 ground state-cis (c ) to regenerate c , therefore accomplishing a quantum chain process (Fig. 2). The existence of 't" as the most stable triplet states was supported by their transient absorption profiles and their lifetimes (Fig. 3)(18). Their transient absorption does n o t

7

A /nm

1. / n m

Fig. 3. Observed Tn-T1 absorption spectra of anthrylethylenes. correspond to that of an anthrylmethyl radical moiety, the most and shifts to longer wavelengths important chromophore of 3p'', depending on the extent of the conjugation brought about by the substituents of the other ethylenic carbon. Their lifetimes are extraordinarily long for olefin triplet states, in the order of 100 microseconds, which is in remarkable contrast with much shorter lifetimes of 3pf', e.g., 60 nanoseconds for stilbene 3p" (6). These long lifetimes are due to larger energy gaps between the triplet and ground-state in the trans-conformation as compared t o those in the perpendicular conformation. The mode of isomerization, one-way or conventional two-way, has been found to be governed, first, by triplet energies of the aromatic group on the ethylenic carbon, and, second, by the type of substituents on the other carbon. The lower the triplet energy 3 f' of the aromatic group, the more stable 3tf' compared to p . However, the presence of a conjugative group like a phenyl group, 3 f' tends to stabilize p , unless the main aromatic group is not so sufficiently low in triplet energy as anthracene (17-25). For example, in fluoranthenylethylenes, substitution on the other carbon by a simple alkyl group like a t-butyl group causes one-way isomerization; however, substitution by a phenyl group, brings about two-way isomerization (Table 1 ) ( 2 3 - 2 5 ) . A s regards the profile of the potential energy surface, the behavior of deuterated vinyl anthracene (&> provided valuable informations (26). On benzil sensitized irradiation, its E-isomer

8

53

-

t

3 3

-

0

=1

f

2 w

.

0 a

ya

1

ids

,

0


90

iaa


Fig. 5. Modified potential energy surfaces of anthrylethlenes.

Fig. 4 . Potential energy surfaces of vinylanthracene.

did not isomerize at all below 10°C in benzene; however, rise of temperature at more than 1 5 ° C led to the isomerization with a quantum yield increasing with temperature. These results give an activation barrier of 11 kcal mol-’ and a preexponential factor of 5x1Ol1 s - ’ for the isomerization process in the triplet state (Fig. 4 ) .

cis-le

trans-le

The above results together with determination of the quantum yield for singlet oxygen produced by energy transfer from the olefin triplet to oxygen have enabled us to propose the potential energy surface as depicted in Fig. 4 for vinylanthracene and Fig. 5 generally for the one-way isomerizing olefins, where, for the 3 isomerization to take place, c has to overcome a barrier of more or less energies depending on the compounds ( 2 7 ) . A s described above, introduction of aromatic groups with low triplet energies drastically changes behavior of the olefins, and the novel one-way and the traditional two-way can be understood by a new wide concept; the mode o f isomerization gradually changes from two-way to one-way by reduction of the triplet energies of the aromatic groups. :#;

9

Table 1. Ar -

Triplet energies of aromatic substituents and isomerization modes of aromatic olefins.

_-----

_---

ET (ArH) /kcal mol-I Ar-CH=CH-tB~ Ar-CH=CH-Ph 84.3

two-way

two-way

60.9

two-way

two-way

56.6

two-way

two-way

54.2

one-way

two-way

Moreover, the quantum ampli€ication effect o f the one-way isomerizing olefins could be utilized for practical purposes particularly in the solid state (28). It is w o r t h m e n t i o n i n g t h a t t h i s a p p r o a c h p r o v i d e s experimental data to depict the potential energy surface of the excited state along the reaction coordinate. SEMICONDUCTOR PHOTOCATALYZED ISOMERIZATION OF OLEFINS Irradiation of particles of semiconductors like Ti02 o r CdS suspended in a solution of stilbene led to isomerization of its cis- to trans-isomer (29-30). The reaction seems to be initiated b y electron transfer from cis-stilbene to the positive hole generated in the semiconductor by irradiation. The resulting cis radical cation (c") is several kcal mol-I higher'in energy than a trans radical cation (t") over the ground state trans isomer is (31). Accordingly, the isomerization from c" to t" exothermic. Likewise, irradiation of semiconductors, TiO2, WOg, CdS, and CdSe in the presence of 1-(4-methoxyphenyl)propene (anethole, A) in acetonitrile resulted in isomerization of its cis- to transi s o m e r (32). T h e v a l e n c e b a n d s o f T i 0 2 , W O 3 , and C d S (electrochemical potentials: 2 . 7 , 2.6 and 1.7 V v s . S C E , respectively) are more anodic than trans- and cis-A (oxidation potentials: 1.39 and 1.60 V vs. SCE respectively). Therefore, the electron transfer from c i s - A to the valence band of these

3.

10

hY

hole+

Fig. 6 .

Electrochemical p o t e n t i a l s of semiconductors and c i s - and trans-anethole.

semiconductors must be exothermic. On the o t h e r hand, w i t h CdSe, i t s v a l e n c e b a n d ( 1 . 2 V ) i s m o r e c a t h o d i c t h a n c i s - A , arid t h e r e f o r e , e l e c t r o n t r a n s f e r i t s seeni endothermic, which s u g g e s t s t h a t a mechanism o t h e r t h a n e l e c t r o n t r a n s f e r works f o r t h e isoiiierization. An a c t i v e s i t e i n t h e CdSe s u r f a c e , f o r example, g e n e r a t i n g Se atoms might work t o i n i t i a t e t h e i s o i i i e r i z a t i o n . Lt

.

ISOIlEP~IZATI0L.I IN M D I C A L CATIONS OF OLEFIIIS

Lehavior of o l e f i n r a d i c a l c a t i o n s has n o t been well investigated until recently, Lewis r e p o r t e d t h a t 9 , l O dicyanoanthracene (DCA) s e n s i t i z e d i r r a d i a t i o n of s t i l b e n e l e d t o i t s c i s t o t r a n s isomerization (33). ' t h e r e s u l t i n g c + - from e l e c t r o n t r a n s f e r f r o m c i s - i s o m e r t o ~ D C A " i s o m e r i z e s t o t-'*, which s u b s e q u e n t l y a c c e p t s an e l e c t r o n from c i s t o g i v e t r a n s and r e g e n e r a t e c+* l e a d i n g t o a c h a i n p r o c e s s . The quantum y i e l d f o r c i s t o trans isomerization increases with increasing cis-isomer concentration. 2 , 4 , 6 - T r i p h e n y l p y r i l i u m t e t r a f l u o r o b o r a t e (TPP+BFt,-) c a n s e n s i t i z e t h e i s o n i e r i z a t i o n of c i s - t o t r a n s - s t i l b e n e ( 3 4 , 3 5 ) .

CN

11

However, the sensitized isomerization takes place when not the singlet state but the triplet state of TPP' is quenched by cisstilbene, which actually gives transient absorptions due to c+' as well as to t+* One of the problems in the isomerization of the olefin radical cations is whether the isomerization proceeds really through a unimolecular process or through other processes. If one assumes that the pi-bond energy of an unsaturated bond of the radical cation is nearly half the value of the corresponding olefin, the barrier for twisting of the double bond in the radical If cis-stilbene is cation must be nearly 20 kcal mol-l for t+' nearly 5 kcal mol-I higher in energy than trans-stilbene, c+' must overcome a barrier of nearly 13 kcal mol-l, since cis-stilbene is nearly 2 kcal mol-' higher in the oxidation potential than for trans-stilbene (36). The preexponential factor for the twisting can be assumed as 1011-1012 s - ' , and then the unimolecular rate constant for isomerization c" to t+- would be 102-103 s-' a t room temperature. Therefore, c+' could isomerize only when it can exist without participating in any reaction. It is known that radical cations generated by pulse radiolysis of aromatic olefins can add to olefins to give olefin dimer cations (37). In view of the above facts, the isomerization would be accelerated by the participation of olefin molecules.

5.

PHOTOISOMERIZATION IN SOLID Recently 1,2-diarylethylenes are reported to undergo isomerization only from cis to trans in small monocrystals ( 3 8 ) . The isomerization occurs within a limit of cis-isomer crystalline lattice. The reaction seems to proceed through excitation walk or by addition-elimination mechanism resulting in the change of the crystal form from the cis- to trans-isomer.

6.

SUMMARY In this chapter, we attempted to show an approach to find out new types of basic photochemical reactions, particularly, the isomerization of olefins. REFERENCES

1

2

N.J. Turro, Mordern Molecular Photochemistry, Benjamin/ Cummings, Menlo Park, 1978; L. Salem, Electrons in Chemical Reactions: First Principles, John Wiley, New York, 1982. M.A. Fox, Semiconductor-Catalyzed Photoreaction of Organic

3 4

5 6

7

8

9 10

11 12 13 14 15

16 17

18 19 20 21 22 23 24 25

Compounds, ACS Symposium Series No.278, American Chemical Society, Washington D.C., 1985; I.C. Paul and D.Y. Curtin, Acc. Chem. Res. 6 (1973) 217. G.S. Hammond, J. Saltiel, A.A. Lamola, N.J. Turro,. J.S. Bradshaw, D.O. Cowan, R.C. Counsell, V. Vogt and C. Dalton, J. Am. Chem. SOC. 86 (1964) 3197. A.J. Merer and R.S. Mulliken, Chem. Rev. 63 (1969) 639; R.S. Mulliken, J. Chem. Phys. 66 (1977) 2448. D. Gegiou, K.A. Muszkat and E. Fischer, J. Am. Chem. SOC. 90 (1968) 3907; V.Krongauz, N. Caste1 and E. Fischer, J. Photochem. 39 (1987) 285; K. Sandros and H.-D. Becker, J. Photochem. 39 (1987) 301. H. G6rner and Schulte-Frohlinde, J. Phy. Chem. 85 (1981) 1835; D. Schulte-Frohlinde and H. GGrner, Pure Appl. Chem. 51 (1979) 2 7 9 ; H . Gorner and Schulte-Frohlinde, J . Photochem. 8 (1978) 91. J. Saltiel, S. Ganapathy and C. Werking, J . Phys. Chem. 91 (1987) 2755; J. Saltiel and J.L. Charlton, in: Rearrangement in Ground and Excited States, ed. P. de Mayo, vo1.3 Academic Press, New York, (1980) p . 2 5 ; J . Saltiel, G.R. Marchand, E. Kirkor-Kaminska, W. K. Smothers, W.B. Mueller and J.L. Charlton, J. Am. Chem. S O C . 106 (1984) 3144; J . Saltiel, A.D. Rousseau and B. Thomas, J. Am. Chem. SOC. 105 (1983) 7631. R. Bonneau, J. Photochem. 10 (1979) 439; S . Lazare, R. Lapouyade and M.-P. Robert, Nouv. J. Chim. 8 (1984) 407; S. Lazare, R. Bonneau and R. Lapouyade, J. Phy. Chem. 88 (1984) 18. R.A. Caldwell, G.W. Sovocool 'and R.J. Peresie, J. Am. Chem. SOC. 95 (1973) 1496; R.A. Caldwell and C.V. Cao, J. Am. Chem. SOC. 104 (1982) 6174. T. Arai, H. Sakuragi and K. Tokumaru, Bull. Chem. SOC. Jpn. 55 (1982) 2204. V. Malatesta, C. Willis and P.A. Hacckett, J. Am. Chem. SOC. 103 (1981) 6781; H.J. Hansen and K. Pfoertner, Eur. Pat. Appl., EP 130509 (1985). R.M. Hochstrasser, Pure Appl. Chem. 52 (1979) 2683. M. Sumitani and K. Yoshihara, J. Chem. Phys. 76 (1982) 738. H. Garner, D. W. Eaker, and J. Saltiel, J. Am. Chem. SOC. 103 (1981) 7164. T. Arai, H. Sakuragi and K. Tokumaru, Chem. Lett. (1980) 1335; T. Arai, T. Karatsu, H. Sakuragi and K. Tokumaru, Chem. Lett. (1981) 1377; T. Arai, H. Sakuragi, K. Tokumaru, Y. Sakaguchi, J. Nakamura and H. Hayashi, Chem. Phys. Lett. 98 (1983) 4 0 . K. Tokumaru and T. Arai, Kagaku 37 (1982) 63. T . Arai, T . Karatsu, H . Sakuragi and K. Tukumaru, Tetrahedron Lett. 24 (1983) 2873. T. Karatsu, T . Arai, H. Sakuragi and K. Tokumaru, Chem. Phys. Lett. 115 (1985) 9. T. Karatsu, H. Sakuragi and K. Tokumaru, Kagaku 39 (1984) 201. T. Arai and K. Tokumaru, Yuki Gosei Kagaku Kyokaishi (J. Synth. Org. Chem. Jpn) 44 (1986) 999. H. Hamaguchi, M. Tasumi, T . Karatsu, T. Arai and K. Tokumaru, J. Am. Chem. SOC. 108 (1986) 1698. T. Karatsu, Thesis ( University of Tsukuba, 1985). T. Arai, Y. Kuriyama, T. Karatsu, H. Sakuragi, K. Tokumaru and S. Oishi, J. Photochem. 36 (1987) 125. T . Arai, Kagaku to Kogyo, 41 (1988) 238. T. Arai, T. Karatsu, H. Misawa, Y. Kuriyama, H. Okamoto, T.

13

26 27

28 29

30

31 32 33 34 35 36 37 38

Hiresaki, H. Furuuchi , H.-L. Zeng, H. Sakuragi and K. Tokumaru, Pure Appl. Chem. 60 (1988) 898. H . Misawa, T. Karatsu, T. Arai, H. Sakuragi and K. Tokumaru, Chem. Phys. Lett. 146 (1988) 405. T. A r a i , T . Karatsu, M. Tsuchiya, H. Sakuragi and K. Tokumaru, Chem. Phys. Lett. 149 (1988) 161. T. W. Ebbesen and K. Tokumaru, Appl. Opt. 25 (1986) 4618. H. Al-Ekabi and P. de Mayo, J . Chem. SOC., Chem. Commun. (1984) 1231; T. Hasegawa and P. de Mayo, J. Chem. S O C . , Chem. Commun., (1985) 1534; H. Al-Ekabi and P. de Mayo, J. Phys. Chem. 89 (1985) 581. K . Tokumaru, H. Sakuragi, T. Kanno, T. Oguchi, H. Misawa, Y. Shimamura and Y. Kuriyama, in:Semiconductor-Catalyzed Photoreactions of Organic Compounds, M.A. Fox ed., ACS Symposium Series No. 278, American Chemical Society, Washington, D.C. (1985), p. 43. T.W. Taylor and A.R. Murray, J. Chem. SOC. (1938) 2078. Y. Kuriyama, T. Arai, H. sakuragi and K. Tokumaru, Hyomen Gijutsu, 40 (1989) 80. F.D. Lewis, J.R. Petisce, J.D. Oxman and M.J. Nepras, J. Am. Chem. SOC. 107 (1985) 203. R . Searle, J.L.R. Williams, D.E. DeMeyer and J.C. Doty, J . Chem. Soc., Chem. Commun. (1967) 1165. Y. Kuriyama, T . Arai, H. Sakuragi and K. Tokumaru, Chem. Lett. (1988) 1193. H. Suzuki, K. Koyano, T. Shida and A. Kira, Bull. Chem. S O C . Jpn. 55 (1982) 3690; H. Suzuki, K. Ogawa, T. Shida and A . Kira, Bull. Chem. SOC. Jpn. 56 (1983) 66. Y . Chikai, Y . Yamamoto and K. Hayashi, Bull. Chem. SOC. Jpn. 61 (1988) 2281. S . Aldoshin. M. Alfimov. L. Atovmvan, A. Ivanchenko, A. Rachinskii and V . Rasumov, XIIIth International Conference on Photochemistry, Budapest, Abstracts (1988) p.406.


Chapter 2

PHOTOCHEMICAL TECHNIQUES TO UNDERSTAND PHOTOCHEMICAL AND PHOTOPHYSICAL FEATURES ON SOLID SURFACES

Contents

2.1

Fluorescence and Transient Absorption Spectra of Solid Surface: Development of Time-Resolved Total Internal Reflection Spectroscopy (Hiroshi Masuhara)

15

2.2 Laser Flash Photolysis on Solid Surfaces

(Francis Wilkinson and G . P. Kelly)

2.3

Excimer Formation with Pyrenes on Silica Surfaces (Klaas A. Zachariasse)

2.4

30 %

48

Photophysics of Acridone, N-Methylacridone, Acridine, and Pyrene Adsorbed on Silica Gel (Satoshi Suzuki and Tsuneo Fujii)

2.5

79

Heterogeneous Molecular Environments Probed by F l u o r o phores Bonded to Chemically Modified Silica Gel: Fluorescence Decay Measurements under a Microscope (Satoshi Hirayama, Takashi Kubo, and Hirohisa Yamasaki)

2.6

93

Photoacoustic and Fluorescence Measurements of Energy Transfer in Adsorption Layers (H. D. Breuer)

106


15

FLUORESCENCE AND TRANSIENT ABSORPTION SPECTRA OF SOLID SURFACE : DEVELOPMENT OF TIME-RESOLVED TOTAL INTERNAL REFLECTION SPECTROSCOPY H . MASUHARA 1.

INTRODUCTION

Under the total internal reflection (TIR) condition, light penetrates into materials with lower refractive index from a glass plate with higher value. This is called an evanescent wave and can be used as excitation and probe beams for surface photochemical studies. In the case of s-polarization, an intensity of this wave is given by the following equations. E = Eoexp(-rz)

t11 r = ( 2 m l/ X)(sin2e-(n2/nl )2)0.5 [21 Here, z is the depth from the interface between both materials,

Eo and E are intensities of the evanescent wave at the interface and at the depth z , respectively, 8 is an incident angle of the beam, h is its wavelength, and n1 and n2 are the refractive indices of the denser and rarer materials, respectively. A surface area whose depth is determined by these experimental parameters can be monitored and its photochemical phenomena elucidated. we summarize here two time-resolved spectroscopic methods, giving direct information o n picosecond and nanosecond photodynamics of solid surface. One is a fluorescence spectroscopy which analyzes fluorescence behavior of the surface area excited by the evanescent laser pulse. The other is to get transient UV-visible absorption spectra by using the evanescant light as a probe beam. 2. TIME-RESOLVED TOTAL INTERNAL REFLECTION FLUORESCENCE SPECTROSCOPY 2.1 Experimental Sapphire was selected as an internal reflection element with a high value of refractive index (nl = 1.81 at 313 nm). It is transparent down to 200 nm, mechanically hard, chemically stable, and has a small birefringence. The dimension of the used plate

16

was 30 x 10 x 1 mm3, and the longest edge was along the c-axis. The contact surface (30 x 10 mm2) with the sample was the (1010) plane. As an example, an optical set for a model system of layered polymer films described later ( 1 - 2 ) is shown in Fig. 1. The sapphire-sample combination was fixed on a goniometer by which an incident angle was adjusted with the precision of less than 0.lo. A synchronously pumped, cavity-dumped dye laser (Spectra Physics 375 and 3 4 4 s ) combined with a mode-locked Art laser (Spectra Physics 1 7 1 - 1 8 ) was used as a picosecond excitation light source. The laser was operated with a repetition rate of 800 kHz. The pulse duration was 6 ps (fwhm). The excitation wavelength was 315 nm which was obtained by using a KDP crystal. Its beam divergence was 1.7 5 0.4O. A Nikon monochromator was used, and the fluorescence w a s detected by a HTV R 1 2 9 4 U microchannel plate photomultiplier. Time-resolved fluorescence spectra and rise as well as decay curves were measured with timecorrelated single photon counting method. This system gave an instrumental response function with 60 ps (fwhm), details of which were published elsewhere ( 3 ) . Polarizers and filters were set in front of the monochromator, and a quartz lens was used to collect the fluorescence efficiently. Excitation polarization was usually adjusted to be perpendicular to the plane of incidence which include s t h e c-axis o f t h e sapphire and to be normal to the contact surface, using Babinet-Soleit plate. In this case of s polarization, the polarization of the evanescent wave is independent of the incident angle.

Sapphire I I

'

I 1

Laser

I

b Fluorescence

Fig. 1. Optical set for total internal reflection fluorescence spectroscopy. 8 : incident angle of an excitation laser beam. S film: thin surface film. B-film: thick bulk film.

17

2.2 Demonstration experiment & a model bilayer system A model system of bilayered polystyrene films was adopted for demonstrating a high potential of this spectroscopy and for proposing an analysis method (2). Thin surface ( S ) films with 0.01, 0.09, and 0.4 um thickness were doped with p-bis(2-(5phenyleneoxazoly1)benzene) (POPOP). Thick bulk (B) film with

about 30 v m were doped with N-ethylcarbazole. Absorbance of both films at the excitation wavelength was smaller than 0.1. The S film was coated on the sapphire, while the B-film was pressed firmly to the S-film in order to obtain an intimate contact. The refractive index of polystyrene (n2) is 1.68 at 313 nm, s o that the critical angle (8,) given by sin BC = n2/n1 was calculated to be 68.15O. Conventional fluorescence spectra measured under the TIR condition and by the normal illumination a r e s i m i l a r to fluorescence spectrum of N-ethylcarbazole in the B-film. This is because the latter was far thicker than the S-film. The timeresolved fluorescence spectra of 0.01 U r n S-film measured with an incident angle of €Ic + 1-45' is shown in Fig. 2 (left). The A

r

A

C

400

500

Wavelength/nm Fig. 2. Time-resolved fluorescence s ectra of a bilayer system of the 0.01 urn S-film doped Tith 1x10-zp mol dm-3 POPOP and the thick B-film doped with 1.3~10- mol dm-3 N-ethylcarbazole. 8 is 69.6O (left) and 71.9O (right). Gated times are ( A ) 0.4-1.4 ns, (B) 10.4-12.4 ns, and (C) 24.4-44.4 ns.

18

structured bands of N-ethylcarbazole below 3 8 0 nm were observed as a main band independent of the gated time. Only at the early gated interval, POPOP fluorescence above 3 8 0 nm was slightly observed. On the other hand, when 0 = Bc+ 3 . 7 5 O (Fig. 2(right)), POPOP fluorescence intensity is larger compared to N ethylcarbazole of the B-film at the early gated time. As the fluorescence lifetime of POPOP ( 1 . 2 n s ) is shorter than that of N-ethylcarbazole ( 1 3 ns), the spectrum at the late gated time was independent of 8. This result demonstrated a possibility that a surface information is emphasized to a great extent by applying time-resolved measurement. An analysis of fluorescence decay curves gives m o r e quantitative information on the TIR phenomena compared to the time-resolved spectra. The results on the present bilayer system with the 0.09 um S-film at 3 8 5 nm are shown in Fig. 3. At 0 = BC+ 0.27O, a contribution of the short component POPOP is slightly detected, while it increased sharply upon a little

4

3 Er,

tn 0

4

2

1

........ .. . ..--. ................. 1

9.76nsldiv

05

a bilayer system of the 0.09 Fig. 3. Fluorescence decay curves um S-film doped wi h 0.1 mol POPOP and the thick B-film doped with 1 . 4 ~ 1 0 - ’ m o l dm-gmi-ethylcarbazol Observation wavelength was 3 8 5 nm. Incident angles are (a) 0 +0.27O, (b) BC+0.68O, (c) Bc+0.980, (d) eC+1.49O, and (e) BC+l -89%.

.

19

increase of 9 by about lo.

All the curves were reproduced by a

s u m of two exponentials; F(t) = FSexp(-t/rS) + FBexp(-t/TB) where T~ and T~ are fluorescence lifetimes of POPOP and N ethylcarbazole, respectively. Although pre-exponential factors F, and FB change upon absorbance, fluorescence spectrum, yield, and lifetime of dopants, the values of FB/Fs are related to a ratio of the number of fluorescent molecules in each layer. Therefore, a relation between FB/Fs and 9 can be used as a measure for surface analysis. One of the examples is shown in Fig. 4 . W e proposed how to analyze these 9-dependence of fluorescence decay curves. The FB/FS values are plotted against 6 by normalizing them to the value obtained by excitation with 8250 ps [34]. Another interesting observation [33] was the fact that the ALPCS triplet on the fabric was only quenched by oxygen when the opaque sample was water-saturated. Another sample studied on a dyed fabric was 1,5-diphenyl-3-styryl-2-pyrazolineI in which a weak triplet-triplet transient absorption was observed. In figure 7 ( a ) and (b) the time resolved spectra for the above mentioned pyrazoline is shown for the dyed fabric and for a microcrystalline sample, respectively [15]. The photosensitising dye rose bengal has been studied in a variety of environments including chemically bound within cross-linked polymers (i.e. as the commercially available heterogeneous photosensitisers known as Sensitox I and II), adsorbed on polyacrylamide and polystyrene and dyed on woven For each sample the kinetics of the fabric and nylon [35]. transient absorption and emission decays, after 354 nm excitation, are virtually identical and thus the transient is assigned to triplet rose bengal. Two other studies on polymer systems have been published [36,37]. Imagi et a1 [36] have used the new technique to obtain a transient signal from poly(ethy1ene terephthalate) powder, where two broad bands were observed in the spectrum. The first band at 520 nm decayed with a half life of 20 ps and is assigned to the triplet state of the rr-electronic unit, while the higher energy band (430 nm) with a longer half-life is ascribed to a radical formed by an intermolecular interaction between carbonyl and OH groups. Wilkinson et a1 [ 3 7 ] studied polymeric benzophenone (benzoylated polystyrene beads) which yield a characteristic transient absorption typical of the triplet benzophenone moiety.

43

E.0440

I

I

460

a

I.o---

0

480

--I

I

500

520

WAVELENGTH / nm

I

I

560

540

I

580

FIGURE 7(a) - Time resolved transient absorption spectra obtained f r o m a sample of 1,S-diphenyl3-styryl-2-pyrazoline dyed o n cotton fabric

"1

TIME / p S . n

z 0

F

a Ce

2m a

01

400

I

450

I

500

WAVELENGTH / nm

I

550

I

600

FIGURE 7(b) - Time resolved transient absorption spectra for microcrystalline 1,5-diphenyl-3-styryl-Z-pyrazoline

6.3

Organic microcrystalline samples

Microcrystalline benzophenone [38] and benzil [16] were two of the first systems studied by nanosecond diffuse reflectance flash photolysis. Both samples gave transient absorptions which were positively identified as triplet-triplet absorptions. In the case of benzophenone an absorption, centred at 540 nm, was observed which has, within experimental error, identical kinetics to the phosphorescence decay, which is predominantly second order. In the case of benzil a transient absorption of 60% at 510 nm was observed after 354 nm excitation. The assignment as triplettriplet absorption was made on the basis of the absorption and phosphorescence kinetics being virtually identical, namely a mixture of first and second order kinetics. Ikeda et a1 [39] have also studied microcrystalline benzophenone on the picosecond time scale. Another microcrystalline sample studied is 1,5-diphenyl-3styryl-2-pyrazolineI in which the triplet-triplet transient absorption was identified within the microsecond time domain [15] (see figure 7(b)). However, as mentioned above (see section 4 and figure 5 ) , the transient absorption due to the excited singlet state has been observed on a picosecond time domain [17]. 6.4

Inorganic systems

The technique has been applied to yield information about the transient events occurring in zinc sulphide phosphors [40] and zinc oxide powders [41], both doped and undoped. In the latter case transient absorption spectra for zinc oxide undoped and doped with copper, nickel, cobalt, iron and manganese have been observed, following sub-bandgap excitation at 532 nm. An absorption, with a first half life ranging between 10-25 ps, was observed in all samples at 400 nm (approximately) and is assigned to the host zinc oxide lattice. A second absorption is observed at longer wavelengths which is characteristic of the dopant used. These transients have first half-lifes typically of 80-100 ps depending on the system and involve redox reactions of the dopants. In addition to these two systems an earlier study of polycrystalline LaA103 doped with 1% CrlI1 yielded a transient absorption spectrum following millisecond flash lamp excitation [42]., assigned to the 2E excited state of Cr(II1) in this environment.

45

15

I 1ASERSCATKR

I

I

I

REGION

I

I

10

I

I

I

A

TIME/~S.

I

I

z

I I I

0 I-

I I

n

lx

s:m

1

5

6

R

0

-5

I

I

400

I

450

I

500

I

550

I

600

I

650

WAVELENGTH/n m

I

700

1

750

I

800

FIGURE 8 - Time resolved transient absorption spectra of cobalt doped zinc oxide after 532 nm laser excitation 7.

CONCLUSIONS

The above provides a broad overview of some of the areas of study to which the technique of laser flash photolysis, in diffuse reflectance mode, has been applied with respect to opaque samples. It is hoped that this chapter in conjunction with other published review material [13,14,43,44] provide a complete picture of "the state of the artt1of laser flash photolysis of solid surfaces, and reiterates the great potential of this new mode for flash photolysis studies at interfaces.

46

8.

ACKNOWtEDGEMENTS

It is a pleasure to thank our colleagues in Loughborough, Philip Leicester and Charles Willsher for many discussions and for allowing us to use their results in this Chapter. We also wish to thank many scientists who have collaborated with us to exploit this technique. These include J.R.M. Barr and M.J.C. Smith (Rutherford-Appleton Laboratory) , D. Oelkrug , W. Honnen and S. Uhl (Institut fur Physikalische Chemie, University of Tubingen), and J. Kossanyi and J. Pouliquen (CNRS, Thiais, Paris). Financial support from the E.E. C. , S.E.R. C. , and Minnesota 3M Research Ltd U.K. is gratefully acknowledged. REFERENCES 1. 2.

G. Porter, Proc. R. SOC. London Ser. A, 1950, 200, 284. G. Porter and M.A. West, Techniques of Organic Chemistry, Ed. A. Weissberger (Wiley-Interscience, New York) VI, Chapter X

3.

M.A. West, Creation and Detection of the Excited State, (Dekker, New York), 4, (19701, 217. J.F. Rabek, Experimental Methods in Photochemistry and Photophysics, Part 2 (Wiley, New York), Chapter 23 (1982). R.V. Bensasson, E.J. Land and T.G. Truscott, Flash Photolysis and Pulse Radiolysis; Contributions to the Chemistry of Biology and Medicine, (Pergamon Press, Oxford), (1983). G. Porter, Techniques of Organic Chemistry, 8 (11), 1055, (New York : Interscience) 1963. Picosecond Phenomena 111, ed. by K.B. Eisenthal, R.M. Hochstrasser, W. Kaiser and A. Laubereau (Springer-Verlag, Berlin) 1982. N.J. Harrick, Ann. N.Y. Acad. Sci., 101, (1963) 928. W. Wendlandt and H.G. Hecht, Reflectance Sepctroscopy., (Wiley, New York), 1966. P. Kubelka, J. Opt. SOC. Am. 38, (1948) 448. G. Kortum, W. Braun and G. Herzog, Angew. Chem. Int. Ed. Engl. 2, (1963) 333. R.W. Kessler and F. Wilkinson, J. Chem. SOC., Faraday Trans

4. 5. 6. 7. 8. 9. 10. 11. 12.

(1974).

I,77, (1981) 309.

13. 14. 15. 16.

C.J. Willsher, J. Photochem., 33, (1986) 273. F. Wilkinson, J. Chem. SOC. Faraday Trans. 2, 82, (1986) 1. G.P. Kelly, Ph.D. thesis, University of Loughborough (1987). F. Wilkinson and C.J. Willsher, Applied Spectroscopy, 38,

17.

F. Wilkinson, G.P. Kelly and P.A. Leicester, Laser Chemistry in press. R.E. Imhof, D.J.S. Birch, F.R. Thornley, J.R. Gilchrist and A. Stevens, J. Phys. E., 17, (1984) 521. F. Wilkinson, C.J. Willsher, S. Uhl, W. Honnen and D. Oelkrug, J. Photochem. 33, (1986) 273. T. Lin and H.K.A. Kan. J. Opt. SOC. Am., 60, (1970) 1252. R.W. Kessler, G. Krabichler, S.Uh1, D. Oelkrug, W.P. Hagan, J. Hyslop and F. Wilkinson, Optica Acta., 30, (1983) 1099. D. Oelkrug, W. Honnen, F. Wilkinson and C.J. Willsher, J. Chem. SOC., Faraday Trans 2, 83, (1987) 2081.

18. 19. 20. 21. 22.

(1984) 897.

47

23. 24. 25. 26. 27. 28. 29.

W. Honnen, Ph.D. thesis, University of Tubingen (1986). J.K. Thomas, J. Phys. Chem. 91, (1987) 267. N.J. Turro, I.R. Gould, M.B. Zimmit and C.C. Cheng, Chem. Phys. Lett. 119, (1985) 484. N.J. TUrrO, M.B. Zimmit, I.R. Gould and W. Makler, J. Am. Chem. SOC. 107, (1985) 5826. M.L. CaSal and J.C. Scaiano, Can. J. Chem. 62, (1984) 628. F. Wilkinson, C.J. Willsher, M.L. Casal, Linda J. Johnston and J.C. Scaiano, Can. J. Chem. 64, (1986) 539. E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchener and J.V. Smith, Nature (London) 271, (1978) 512.

30.

A.Garner and F. Wilkinson, J. Chem.

31.

F. Wilkinson, C. J. Willsher, P.A. Leicester, J.R.M. Barr and M.J.C. smith, J. Chem. SOC., Chem. Commun. 1216 (1986). F. Wilkinson and C.J. Willsher, J. Chem. SOC., Chem. Commun.

32. 33.

34. 35. 36. 37. 38.

(1976) 1010.

SOC.

Faraday Trans 2 72,

142 (1985).

F. Wilkinson and C.J. Willsher, Photochemistry and Photophysics of Coordination Compounds Ed. by H. Yersin/A. Vogler (Springer-Verlag, Berlin), 327 (1987). J.R. Darwent, I. McCubbin and D. Phillips, J. Chem. SOC. Faraday Trans 2, 78, (1982) 347. F. Wilkinson, C.J. Willsher and R.B. Pritchard, Eur. Polym. J, 21, (1985) 333. K. Imagi, N. Ikeda, M. Masuhara, M. Nishigaki and M. Isogawa, Polymer J. 19, (1987) 999. F. Wilkinson, C.J. Willsher, J.L. Bourdelande, J. Font and J.Greuges, J. Photochem. 38, (1987) 381. F. Wilkinson and C.J. Willsher, Chem. Phys. Lett. 104, (1984) 272.

40.

N. Ikeda, R. Imagi, H. Masuhara, N. Nakashima and K. Yoshihara, Chem. Phys. Lett. 140, (1987) 281. F. Wilkinson and C.J. Willsher, J. Luminescence, 33, (1985)

41.

J. Poliquien, D. Fichou, P. Valat, J. Kossanyi, F. Wilkinson

39.

42. 43. 44.

187.

and C.J. Willsher, J. Photochem. 35, (1986) 381. R.W. Kessler, D. Oelkrug and F. Wilkinson, Applied Spectroscopy 36, (1982) 673. F. Wilkinson and C.J. Willsher, Tetrahedron 43, (1987) 1197. F. Wilkinson and G.P. Kelly, Handbook of the Photochemistry of Organic Compounds in Condensed Media; Chapter on diffuse reflectance laser flash photolysis., Ed. J. Scaiano (CRC Press Florida) (1988) in press.

-

48

EXCIMER FORMATION W I T E PYRENES ON SILICA SURFACES

K . A. ZACHARIASSE 1.

INTRODUCTION

I n t h i s chapter i n v e s t i g a t i o n s u s i n g excimer f o r m a t i o n w i t h p y r e n e (Py) and a number o f i t s d e r i v a t i v e s a d s o r b e d on d r y and m o d i f i e d s i l i c a g e l s u r f a c e s a r e d i s c u s s e d . These s t u d i e s were c a r r i e d o u t t o o b t a i n i n f o r m a t i o n on t h e p r o p e r t i e s o f t h e s i l i c a s from t h e s u r f a c e b e h a v i o u r of t h e probe m o l e c u l e s . The p r e f e r e n t -

i a l u s e o f p y r e n e s for i n v e s t i g a t i o n s o f s i l i c a g e l (1-3) and

o t h e r o x i d e m a t e r i a l s (3-9) , f i n d s i t s o r i g i n i n t h e f a c t t h a t Py p o s s e s s e s a number o f d i s t i n c t a d v a n t a g e s as a f l u o r e s c e n t p r o b e (1G-15):

c a p a b i l i t y t o forr.1 e x c i m e r s , f a v o u r a b l e r a t i o o f t h e excimer-to-monomer

r a d i a t i v e rates

kf ' / k f I l o n g f l u o r e s c e n c e l i f e t i m e ( e . g . 457 n s i n n-decane a t 2G°C), e n a b l i n g e f f i c i e n t excimer f o r m a t i o n a t r e l a t i v e l y low conc e n t r a t i o n s , and s e n s i t i v i t y t o t h e p o l a r i t y of t h e e n v i r o n m e n t of t h e v i b r a t i o n a l s t r u c t u r e i n t h e f l u o r e s c e n c e s p e c t r u m ( t h e Ham e f f e c t ,

see S e c t i o n 3 . 5 ) . o t h e r aromatic hydrocarbons c a p a b l e o f i n t e r m o l e c u l a r e x c i n e r f o r m a t i o n (10-14), i.e. b e n z e n e , n a p h t h a l e n e , 9 , f O - a l k y l a t e d anthracenesI1,2-benzanthracene and a l s o p e r y l e n e , have c o n s i d e r a b l y smaller v a l u e s f o r t h e r a t i o k f ' / k f , s h o r t e r f l u o r e s c e n c e l i f e t i m e s and moreover much smaller excimer s t a b i l i z a t i o n e n e r g i e s . These f a c t o r s ( 1 5 ) have a n e g a t i v e i n f l u e n c e on t h e excimer-to-monomer f l u o r e s c e n c e i n t e n s i t y r a t i o I ' / I , u s e d as a major e x p e r i m e n t a l t o o l i n excimer f l u o r e s c e n c e s t u d i e s . 2.

PHOTOPHYSICS O F EXCIMER FORMATION 2.1 P h o t o s t a t i o n a r y Aspects As c o i n e d by S t e v e n s ( 1 6 ) , a n excimer i s t a k e n t o be a d i m e r i c

species, only s t a b l e i n t h e e l e c t r o n i c a l l y excited s t a t e , i.e. diss o c i a t i v e i n t h e ground s t a t e . I n t e r m o l e c u l a r excimer f o r m a t i o n

49

between a m o l e c u l e i n t h e s i n g l e t e x c i t e d s t a t e 'M*

and i t s

c o u n t e r p a r t i n t h e ground s t a t e M, l e a d i n g t o t h e excimer I D * ,

can

be t r e a t e d w i t h i n t h e s c o p e o f Scheme ( I ) .

'M*+

k

M

EM1

'O* Scheme ( I )

Here, k and kd a r e t h e r a t e c o n s t a n t s o f e x c i m e r f o r m a t i o n and a d i s s o c i a t i o n , kf and k f ' d e s c r i b e t h e monomer and excimer r a d i a t i -

v e r a t e s , and x0 and T ~ are ' t h e monomer and excimer l i f e t i m e s . 2.1.1 Excimer-to-Monomer

F l u o r e s c e n c e I n t e n s i t y R a t i o (I'/I).

I n most p r o b e s t u d i e s u s i n g e x c i m e r f o r m a t i o n , a s mentioned above, t h e excimer-to-monomer

f l u o r e s c e n c e i n t e n s i t y r a t i o I'/I i s

used a s a n e x p e r i m e n t a l c r i t e r i o n (11,17-19).

I n [ I ] , Q' and Q are t h e f l u o r e s c e n c e quantum y i e l d s o f t h e e x c i m e r and t h e monomer, r e s p e c t i v e l y . The r a t e c o n s t a n t s ka and kd depend on s o l v e n t v i s c o s i t y and hence o n t e m p e r a t u r e ( 1 0 - 1 5 ) . I n c o n t r a s t , t h e r a t i o of t h e r a d i a t i v e r a t e c o n s t a n t s k f ' / k f e x c i m e r l i f e t i m e -cot

i s shown i n F i g .

a s w e l l as t h e

d o n o t s t r o n g l y depend on t e m p e r a t u r e . T h i s

1 , where ka, k

t i m e 1 / ~ f~o r' Py i n n-decane

d

and t h e r e c i p r o c a l excimer l i f e -

( 1 5 ) a r e p r e s e n t e d i n an A r r h e n i u s -

p l o t , g i v i n g v a l u e s f o r t h e a c t i v a t i o n e n e r g i e s Ea and Ed.

It is

s e e n t h a t t h e excimer l i f e t i m e T ~ 'i s l a r g e l y i n d e p e n d e n t o f temper a t u r e below 2 O o C ( 6 3 n s ) , i t s v a l u e d e c r e a s i n g somewhat a t h i g h e r t e m p e r a t u r e s , down t o 4 9 n s a t 7OOC. From t h e a c t i v a t i o n e n e r g i e s Ea = 13.4 kJ/mol and Ed = 55.1 kJ/mol, a v a l u e f o r t h e excimer s t a b i l i z a t i o n e n e r g y -AH

-

=

(Ed E a ) = 41.7 kJ/mol i s o b t a i n e d . The a c t i v a t i o n e n e r g y Ea i s p r a c t i c a l l y i d e n t i c a l t o t h e a c t i v a t i o n e n e r g y of v i s c o u s flow:

E ( T / r l ) = 13.8 kJ/mol, where

i s t h e macroscopic v i s c o s i t y ( 1 5 ) .

T h i s p r o v e s t h a t excimer f o r m a t i o n w i t h Py i n decane s o l u t i o n i s indeed a d i f f u s i o n - c o n t r o l l e d p r o c e s s ( 1 3 , 1 5 ) . 2.1.2

Low-TemperaturejHigh-Viscosity C o n d i t i o n s . When t h e

t e m p e r a t u r e i s s u f f i c i e n t l y low, o r t h e s o l v e n t v i s c o s i t y s u f f i c :11 t r a n s f o r m s i e n t l y h i g h , t o make kd much smaller t h a n

50

-TI'Cl

log ki

I

.t 7;

2.5

-

10001TlK)

F i g . 1 . A r r h e n i u s p l o t f o r p y r e n e i n n-decane of t h e r a t e c o n s t a n t s o f e x c i m e r f o r m a t i o n (k,), excimer d i s s o c i a t i o n ( k d ) , and t h e recip r o c a l excirner l i f e t i m e ( l / ~ ~ ' S) e.e Scheme ( I ) . i n t o [ 2 ]:

I' I

kf'

N - .

kf

ka To'

Under t h e s e c o n d i t i o n s , a n A r r h e n i u s p l o t o f t h e r a t i o I'/I w i l l r e s u l t i n a s t r a i g h t l i n e , w i t h t h e s l o p e Ea/R,

when t h e t e m p e r a t -

u r e dependence o f k f ' / k f and T ~ 'c a n be n e g l e c t e d ( 1 5 ) . 2 . 2 Time-Resolved Measurements 2.2.1

Monomer and Excimer F l u o r e s c e n c e Decays. When o n e ex-

c i t e d s t a t e monomer 'M* i n t e r a c t s w i t h one excimer 'D*

(Scheme ( I ) ) ,

t h e monomer and e x c i m e r f l u o r e s c e n c e d e c a y s are d o u b l e - e x p o n e n t i a l :

1 1M " ( t ) l

= A l l e -'It

+ A12e-'2t

i,(t) = k f 8 [ ' D * ( t ) ] = AZle-'lt

+ A22e-'2t

i,(t) = kf

1 * 1 * where [ M ( t )] and [ D ( t ) ] d e s c r i b e t h e t i m e e v o l u t i o n of t h e mono-

m e r and e x c i m e r c o n c e n t r a t i o n s . The a n a l y t i c a l e x p r e s s i o n ( 1 1 ) f o r t h e r e c i p r o c a l d e c a y t i m e s ,

X1 and h 2 , i s g i v e n by 151:

51

[51

where X = ka[M] + l/-r and Y = kd + 1 / - r O 1 . 0 C l e a r l y , X 1 c a n a t most be e q u a l t o X2:

X1

=

XI < A2

(the case

X 2 h a s been d i s c u s s e d e l s e w h e r e ( 1 5 ) ) . When t h e excimer, a s i m p l i e d by i t s d e f i n i t i o n (see a b o v e ) ,

i s n o t formed by d i r e c t e x c i t a t i o n o f a g r o u n d - s t a t e d i m e r , t h e n

*

[D I = 0 a t t = 0 , and A21 + A22 = 0, l e a d i n g t o A22/A21 = -1. However, t h e c o n d i t i o n s set b y Scheme (I) are n o t a l w a y s e x a c t l y fulfilled. The R a t i o o f t h e Excimer A m p l i t u d e s A22/A21. Under ex-

2.2.2

p e r i m e n t a l c o n d i t i o n s t h e v a l u e s f o r A22/A21 c a n d e v i a t e from - 1 . Two p o s s i b l e r e a s o n s f o r t h i s d e v i a t i o n a r e : ( a ) The p r e s e n c e o f g r o u n d - s t a t e d i m e r s . These d i m e r s t h e n l e a d " i n s t a n t a n e o u s l y " , i . e . w i t h i n t h e t i m e - r e s o l u t i o n of t h e equipment u s e d , t o e x c i m e r s upon e x c i t a t i o n . T h i s makes [ 4 ] A21 + A22 = * * ID ( 0 1 1 , where [D ( 0 1 1 i s t h e c o n c e n t r a t i o n o f e x c i m e r - l i k e g r o u n d - s t a t e d i m e r s a t t = 0 , r e s u l t i n g i n A22/A21 > -1.

(b) The p r e s e n c e o f monomer e m i s s i o n a t t h e w a v e l e n g t h where t h e e x c i m e r e m i s s i o n i s m o n i t o r e d . T h i s i s e s p e c i a l l y i m p o r t a n t when t h e r a t i o I'/I i s s m a l l , and when t h e e x c i m e r e m i s s i o n o f t h e p y r e n e s i s measured a t w a v e l e n g t h s s h o r t e r t h a n 500 nm ( 1 9 ) . Hence, p r o v i d e d t h a t t h e c o n t r i b u t i o n o f monomer f l u o r e s c e n c e c a n be n e g l e c t e d a t t h e e x c i m e r w a v e l e n g t h , d e v i a t i o n s from u n i t y

o i t h e r a t i o (-A22/A21) i n d i c a t e t h a t t h e e x c i m e r i s i n f a c t p r e formed i n t h e ground s t a t e . T h i s h a s r e c e n t l y b e e n shown t o be t h e case f o r t h e meso d i a s t e r e o i s o m e r o f 2,4-di(2-pyrenyl)pentaneI b a s e d on a c o m b i n a t i o n o f t i m e - c o r r e l a t e d s i n g l e - p h o t o n c o u n t i n g and NMR measurements (20). 2.2.3

The P h y s i c a l Meaning o f t h e Excimer-Growing-In.

cimer growing-in,

w i t h t h e decay t i m e

T~

The ex-

[41 , h a s o f t e n been d i s -

c u s s e d i n i n v e s t i g a t i o n s w i t h p y r e n e s on s i l i c a s u r f a c e s , see S e c t i o n 4.5.

F o r an i n t e r p r e t a t i o n of t h e p h y s i c a l meaning o f t h i s

growing-in, decay t i m e

it i s i m p o r t a n t t o r e c a l l t h a t t h e s h o r t e s t e x c i m e r T~

a l w a y s h a s t h e n e g a t i v e a m p l i t u d e . T h i s is t h e c a s e ,

i r r e s p e c t i v e of t h e p h y s i c a l p r o c e s s e s t h a t determine i t s v a l u e [ 5 1 ,

*

a s t h e e x c i m e r c o n c e n t r a t i o n [D I c a n n o t become n e g a t i v e . A s -r2,

j u s t a s -rl,

depends on a l l r a t e c o n s t a n t s and r e c i p r o -

c a l f l u o r e s c e n c e l i f e t i m e s i n Scheme ( I ), ka[Ml

, kd,

1/-r0 and l / - r o l ,

i t w i l l be c l e a r t h a t t h e e x c i m e r r i s e t i m e ( T ~ )c a n n o t a u t o m a t i c -

52

a l l y be r e l a t e d t o t h e p r o c e s s of e x c i m e r f o r m a t i o n ka[M] a l o n e . Moreover, i t s h o u l d b e n o t e d t h a t t h e same p a i r of d e c a y t i m e s app e a r s i n t h e monomer a s w e l l a s i n t h e e x c i m e r d e c a y , see [ 3 ] and [41.

The p h y s i c a l meaning o f t h e excimer growing-in

( 1 5 ) depends

on t h e r e l a t i v e magnitude o f X a n d Y, see [ 5 1 . Two s e p a r a t e c a s e s w i l l be considered ( l e a v i n g o u t X = Y ) : (a)

x >

Y. Whether t h e c o n d i t i o n X > Y, i . e .

ka[M1 + l

/ >~

l / . r o i , w i l l h o l d f o r a p a r t i c u l a r molecule with t h e l i f e t i m e s

~ kd + 0

and T ~ ' ,d e p e n d s on t h e v a l u e s o f ka[M] and kd and h e n c e on t e m p e r a t u r e , s o l v e n t f l u i d i t y a n d c o n c e n t r a t i o n [ M I . When X > Y 1

and kd = 0 , t h e n X 2 = z [ ( X + Y ) + (X-Y)] = X = ka[M1 + l/-c0. Th

S

means t h a t o n l y u n d e r t h e s e c o n d i t i o n s i n f o r m a t i o n o n e x c i m e r forma t i o n a n d h e n c e on d i f f u s i o n o f p y r e n e m o l e c u l e s c a n d i r e c t l y be o b t a i n e d from t h e e x c i m e r rise t i m e T ~ .I t s h o u l d b e n o t e d , t h a t when X > Y and kd = 0 , T~ w i l l become s h o r t e r w i t h i n c r e a s i n g Py c o n c e n t r a t i o n , -cl (b)

x
was introduced. (A2T2 ) 2+~2T2 (AITl) = T = 191 ( A , T , + A ~ T ~ )‘1 + ( A , T , + A ~ T ~ )2 (A,-r1+A2i2) 2~2 the ) fraction of the toHere, as an example, ( A 1 ~ l ) / ( A 1 ~ 1 + Ais tal fluorescence associated with the decay time T ~ . The usefulness of C T > was shown by fluorescence quenching studies of Py with 2-halonaphthalenes on dry silica, where < T > yielded linear quenching plots (30), from which the rate constants for this quenching process could be calculated, see Section 4.2.2. 4.2.2. Dynamic Excimer Formation of Pyrene on Decanol-Covered Silica. The temperature dependence of the monomer and excimer fluorescence decays of Py adsorbed on silica gel with a monolayer coverage of 1-decanol has been studied by de Mayo et al. (39). The literature data for the two times T~ and T~ of the double-exponential excimer decays are depicted in Fig. 9A. It is seen that the shorter decay time T~ approaches a value of about 60 ns upon lowering the temperature, a value similar to the excimer lifetime of Py in methylcyclohexane (60), Fig. 9B. From the observation that the

64 600

600

500

500

LOO

Ti1 ns)

1

300 \

200

200

100

100

0

100 TIK)

-

200

300

0

100

-

200 TIK)

300

Fig. 9. ( A ) Literature data (from ref. (39)) for the decay times T~ and T~ of the double-exponential excimer decay of pyrene ( 2 . 5 mg/g of silica) on silica gel covered with 1 x mol 1decanol/g of silica. ( B ) Decay times ? 1 and -r2 of the excimer fluorescence decay of pyrene ( 1 x M ) in methylcyclohexane. The values for the monomer lifetime T~ ( 5 x 10 M ) and for the also indicated. calculated excimer lifetime T ~ ' , are longer excimer decay time ( T ~ )for Py on silica/decanol reaches a plateau at temperatures below 2 2 5 K, it is concluded that dynamic excimer formation no longer takes place under these conditions. In this temperature range, the monomer decay time ( T ~ )has become single-exponential (39). The double-exponential excimer decay of Py on silica/decanol at 2 0 ° C , where excimer formation is completely dynamic ( A 2 2 / A 2 1 = -1, see Section 3 . 2 and Table l ) , will now be analyzed. This is done within the context of Scheme (I), on the basis of the observed liquid-like nature of silica/decanol (36). It first should be recalled which information is contained in a double-exponential excimer decay. From the sum of the reciprocal decay times (1, + A 2 ) , the sum of the rate constants and reciprocal lifetimes can be obtained. From [ 5 ] , with l / X 1 = 1 0 4 ns and l / h Z = 26 ns (Table 1 ) : X, + X2 = X + Y = ka[Pyl + 1 / + ~kd ~+

65

.

6 -1 It is now assumed that the value for T o is 1/TO1 = 48.1 x 10 s equal to 430 ns (15), whereas T I is taken to be 60 ns, the low0 temperature value measured by de Mayo (Fig. 9 A ) . With these data, it is found that ka[Pyl + kd = 29.1 x 106s-1 From this result and X 2 = 38.5 x 106s-l using [5] the values ka[Pyl = 16.4 x 106s-l and kd = 10.7 x 106s -1 can then be obtained, see Table 2. The validity of these values obviously depends on the correctness of the data for T~ and T ~ that ' were adopted here.

.

TABLE 2 Data for excimer formation with pyrene on silica/l-decanol at 2OoC

104

26

-1.02

(60)

(430)

18.4

10.7

2 With the coverage of 2.5 mg Py on 560 m silica (39), one obtains, within the picture of a two-dimensional liquid as adopted by de Mayo, a two-dimensional Py concentration of 2.21 x 10-l' mol/dm2. Taking ka[Py] = 18.4 x 106s-l, this leads 16 2 to ka = 8.3 x 1 0 dm mo1-ls-l for the rate constant of excimer formation on a two-dimensional silica/decanol surface. It is of interest to note that this value is considerably larger than that obtained by de Mayo for the fluorescence quenching of Py by e.g. 2-iodonaphthalene on dry silica, having a value of 2.3 x 1 0 l 5 dm mol -'s-l, which process was considered to be diffusion controlled (30) T h r e e - D i m e n s i o n a l L i q u i d . When the monolayer Of 1-decanol mol) is treated as a three-dimensional hydrocarbon layer (1 x on silica, the follQwing results are obtained. For a Py concenmol/l (2.5 mg of Py in 1 x loq3 mol 1-decanol) tration of 6.5 x 8 and ka[Py] = 18.4 x 106s-', see above, a value ka = 2.8 x 10 Imol- 's-l results. For Py in n-decane solution at 20°C (151, by 9 comparison, a value of ka = 4.9 x 1 0 lmol-'s-' is obtained, see Fig. 1. 4 . 2 . 3 Pyrene in Reversed-Phase Silica Si-C18 with Methanol/ Water. Excimer formation with Py was also used to study (21) laterTwo-Dimensional

Liquid.

.

66

a 1 d i f f u s i o n i n r e v e r s e d - p h a s e s i l i c a , Si-C18, i n t h e p r e s e n c e o f a w e t t i n g m o b i l e phase o f methanol ( 7 5 % ) and water ( 2 5 % ) . The ex-

cimer decay was f i t t e d w i t h two e x p o n e n t i a l s ( F i g . l o ) , h a v i n g an

000

-

750

-

V

z

$

500-

ul W

3 _1 -

250-

oy

I

I

50

0

I

150

I00

I

200

TIME (nsrc)

’

F i g . 10. F l u o r e s c e n c e c a y c u r v e s o f t h e monomer ( M ) and e x c i m e r ( E ) o f p y r e n e ( 5 . 3 x 10 M) i n r e v e r s e d - p h a s e Si-C i n t h e pres e n c e of a w e t t i n g mobile p h a s e of methanol ( 7 5 % ) l a a n d w a t e r ( 2 5 % ) ( R e p r i n t e d w i t h p e r m i s s i o n from A n a l y t i c a l C h e m i s t r y , 56 (1984) 1080, o u r r e f . ( 2 1 1 , C o p y r i g h t (1984) American Chemical Society).

.

[ 8 ] approximately e q u a l t o - 1 . T h e r e f o r e excimer f o r m a t i o n o f Py i n t h e hydrocarbon l a y e r i s dynamic, n o t o r i g i n a t i n g from ground s t a t e dimers.

a m p l i t u d e r a t i o A22/A2, (Section 2.2.3), on S i - C 1 8

T h i s c o n c l u s i o n i s s u p p o r t e d by t h e o b s e r v a t i o n t h a t t h e e x c i t a t i o n s p e c t r a a t t h e monomer and e x c i m e r wavelength a r e i d e n t i c a l

(see S e c t i o n 3.2.2)

and t h a t t h e e x c i m e r e m i s s i o n i s c o m p l e t e l y

f r o z e n o u t a t I 7 K ( 2 1 ) . The c o r r e s p o n d i n g monomer d e c a y , h a v i n g a v a l u e s i m i l a r t o t h e excimer d e c a y t i m e

T

~

w, a s r e p r e s e n t e d a s a

single-exponential. These o b s e r v a t i o n s l e a d t o t h e f o l l o w i n g scheme, see [ 2 1 , s e c t i o n 2.1.2

;’”

py

kaPY1-

’( py pyf

\

I * where -r (Q) and T ~ (Q) ’ a r e t h e monomer ( Py ) and excimer 1 o* ( (PyPy) ) l i f e t i m e s i n t h e p r e s e n c e o f oxygen. E x c i m e r G r o w i n g - I n N o t R e f l e c t i n g E x c i m e r F o r m a t i o n . The monomer

and e x c i m e r d e c a y s were measured f o r v a r i o u s c o n c e n t r a t i o n s o f Py.

67 A s it was n o t e d t h a t t h e excimer growing-in

(decay t i m e

i n d e p e n d e n t o f c o n c e n t r a t i o n (see S e c t i o n 2.2.3) cay t i m e

,

T ) was 2 t h e excimer de-

was t a k e n t o d e t e r m i n e t h e r a t e c o n s t a n t of excimer

T~

f o r m a t i o n ka

(see [ 5 ] , w i t h kd = 0 ) .

From a p l o t o f A ,

a g a i n s t Py c o n c e n t r a t i o n , t h e v a l u e s f o r ka and

~ / T ~ ( Q were ) o b t a i n e d from t h e s l o p e and t h e i n t e r c e p t , r e s p e c t i -

v e l y (see T a b l e 3 ) . Table 3 Excimer f o r m a t i o n w i t h p y r e n e i n r e v e r s e d - p h a s e s i l i c a Si-C18 i n c o n t a c t w i t h methanol/water. Data from r e f . ( 2 1 ) .

[FYI

ka

(moll

(Irr0l-ls-l)

l/T0(Q)

kd

a

l/To'(Q)

1/11

l/T2

(106s-l 1

5 . 7 ~ 1 0 - ~3 . 4 ~ 1 0 ~ 7.9

0

71.0

10.6

71.0

Yd

tmax

xc

(ns)

(1 06s-l )

33.5

10.6 71.0

a bThe t h e r m a l excimer d i s s o c i a t i o n was assumed t o be a b s e n t . The t i m e f o r t h e maximum i n t h e excimer r i s e and decay c u r v e : tmax = l n ( A 2 / A , )/ ( A 2 - A l 1 ( 1 1 ) . C X = 1 h 1 . From (ka[Pyl + ~ / T ~ ( Q ) ) s,e e [ 5 1 , a v a l u e f o r X of 9.84 x 106s-' i s obtained. d Y = I / T ~= I / T ~ ' ( Q ) , s e e [ 5 ] , S e c t i o n 2 . 2 . 1 . To r e l a t e t h e r a t e c o n s t a n t ka t o a d i f f u s i o n c o e f f i c i e n t D , t h e

well-known

e x p r e s s i o n from t h e Einstein-Smoluchowski d i f f u s i o n

t h e o r y (ka = 16nNDa /1000, where a i s t h e i n t e r a c t i o n r a d i u s and N

i s t h e Avogadro number ( 1 1 , 1 3 ) ) was used. The m i c r o v i s c o s i t y o f t h e Py environment was t h e n deduced from D, employing t h e Stokes-

E i n s t e i n e q u a t i o n ( r l = k T / 6 n D b , where b i s t h e S t o k e s ' r a d i u s of -7 2 -1 PY ( 7 1 , 1 3 ) ) . Using t h i s p r o c e d u r e , v a l u e s f o r D ( 2 . 5 x 1 0 c m s ) and f o r t h e m i c r o v i s c o s i t y of t h e hydrocarbon l a y e r of t h e S i - C I 8 around Py ( 1 9 cP) were o b t a i n e d . It i s of i n t e r e s t t o n o t e t h a t f o r t h e s u r f a c e r e g i o n o f aqueous micelles of sodium d o d e c y l s u l f a t e (SDS) a similar v a l u e ( 0 = 1 9 CP a t 20D C ( 1 7 ) ) was found. From t h e d a t a i n Table 3 , i t i s s e e n t h a t t h e i n e q u a l i t y X < Y holds. This explains, a s discussed i n Section 2.2.3,

the experi-

m e n t a l l y observed c o n c e n t r a t i o n independence o f t h e excimer rise time ( ~ ~ 1 . I t should be p o i n t e d o u t , however, t h a t t h e v a l u e s 1 2 7 n s and ro'(Q)

= 1 4 . 1 ns a r e u n u s u a l l y s h o r t , c . f .

T ~ ( Q )=

Table 2.

68

T h i s w 11 be due t o oxygen q u e n c h i n g , a s t h e samples were n o t degassed

Taking a v a l u e f o r T~

,

u n d e r oxygen-free c o n d i t i o n s , o f

436 n s (n-hexadecane a t 25OC ( 6 0 ) ) t h e o b s e r v e d l i f e t i m e . r o ( Q )

of

1 2 7 ns g i v e s v i a t h e r e l a t i o n

a v a l u e of k

Q

= 2.66

x l o 9 lmol-'s-'

f o r t h e r a t e c o n s t a n t of

oxygen q u e n c h i n g , when an oxygen c o n c e n t r a t i o n of 2.1 x

mol/l

i n t h e a l k a n e l a y e r of t h e r e v e r s e d - p h a s e Si-C18 i s assumed, t h e v a l u e of c y c l o h e x a n e a t 25OC ( 1 1 ) . T h i s t h e n l e a d s t o a v i s c o s i t y o f around 1 1 CP f o r t h e a l k a n e l a y e r , u s i n g k ( f o r cyclohexane;

r(

Q

= 3

101olmol-'s-l

= 0.898 CP a t 25OC ( 5 3 ) ) , i n f a i r a g r e e m e n t , i n

view o f t h e a s s u m p t i o n s made, w i t h t h e v a l u e of 19 cP d e r i v e d by Bogar e t a l . from t h e monomer f l u o r e s c e n c e d e c a y . 4.3 T r i p l e - E x p o n e n t i a l Decays T r i p l e - e x p o n e n t i a l monomer and e x c i m e r f l u o r e s c e n c e d e c a y s

were r e p o r t e d f o r t h e f o l l o w i n g s y s t e m s : Py on Si-C18 ( 3 8 ) , 1Py (3)1Py a d s o r b e d on s i l i c a / o c t a n o l and on r e v e r s e d - p h a s e o c t a d e c y l s i l i c a Si-CI8 ( 3 8 ) , a s w e l l as PPS and PDS c h e m i c a l l y bound t o silica i n contact with solvents (42,43). 4.3.1 I n t e r m o l e c u l a r Excimer F o r m a t i o n w i t h P y r e n e on si-C,+,,. The monomer as w e l l as t h e e x c i m e r f l u o r e s c e n c e d e c a y s o f Py on rev e r s e d - p h a s e Si-C1

, were

s t u d i e d a t low p y r e n e c o v e r a g e (38),

w h e r e , n e v e r t h e l e s s , a s u p e r p o s i t i o n o f s t a t i c and dynamic e x c i m e r f o r m a t i o n was found ( S e c t i o n 3 . 2 . 2 ) .

The excimer d e c a y c o u l d o n l y

be f i t t e d w i t h t h r e e e x p o n e n t i a l s ( 3 8 ) , see F i g . 1lB. Excimer f o r mation i s p r e d o m i n a n t l y dynamic, as shown by t h e e x c i m e r growingi n ( 4 5 n s ) . That g r o u n d - s t a t e d i m e r s o n l y p l a y a minor r o l e c a n be s e e n from t h e r a t i o o f t h e n e g a t i v e - t o - p o s i t i v e h a v i n g a v a l u e of - 0 . 9 0 ,

excimer amplitudes,

see S e c t i o n 2 . 2 .

I n T a b l e 4 , d a t a from a s i m u l t a n e o u s ( " g l o b a l " ( 6 1 , 6 2 1 ) anal y s i s of t h e e x c i m e r and monomer f l u o r e s c e n c e d e c a y s f o r Py on Si-C18 a r e c o l l e c t e d , u s i n g t h e e x c i m e r d e c a y c u r v e d e p i c t e d i n F i g . 11B. The d e c a y t i m e s found i n t h i s p r o c e d u r e are n e a r l y i d e n t -

i c a l t o t h o s e o b t a i n e d from t h e a n a l y s i s o f t h e excimer d e c a y curve alone. I n t e r e s t i n g l y , t h e monomer decay i s p r a c t i c a l l y double-expon e n t i a l (Table 4 1 , t h e t r i p l e - e x p o n e n t i a l

f i t having only a r e l a t -

i v e l y s m a l l a m p l i t u d e f o r t h e d e c a y t i m e T~ c o r r e s p o n d i n g t o t h e growing-in o f t h e e x c i m e r . I t i s s e e n t h a t t h e two l o n g e r monomer

t i m e s -r2 and r 1 r e p r e s e n t i m p o r t a n t f r a c t i o n s o f t h e t o t a l d e c a y .

3

520nm

a

A

,n+.

31

0.45

i l l * '

-

B

530nm

c----l

2LONS

NSEC

AMPL(IO-*) - I7

"$

69

45

186

AMPL(IO-') -16

2.9

389 1.1

K

HlO.OEVIRTION

~ o . o E ' v L t R ; I G o ' +Jhl

'

x2 : 349

X2 : 9.29

d3'

'

*'

a-

F i g . 1 1 . R i s e and d e c a y c u r v e o f t h e e x c i m e r f l u o r e s c e n c e o f ( A ) I P y ( 3 ) l P y a d s o r b e d on Si-CI8 ( 3 x 10-6 m o l / g ) and (B) Py a d s o r b e d on Si-C18 ( 6 . 5 x 10-6 mol/g) , f i t t e d t o t h r e e e x p o n e n t i a l s . The e x c i t a t i o n p u l s e (337 nm) i s also d e p i c t e d i n e a c h case. The v a l u e s f o r t h e decay parameters X ( i n n s ) and t h e i r a m p l i t u d e s are g i v e n (see t e x t ) . The w e i g h t e d d e v i a t i o n s i n u n i t s o f o ( e x p e c t e d dev i a t i o n ) , t h e a u t o c o r r e l a t i o n f u n c t i o n (A-C) , and t h e v a l u e f o r x2 a r e a l s o i n d i c a t e d . ( R e p r i n t e d w i t h p e r m i s s i o n from t h e J o u r n a l of P h y s i c a l C h e m i s t r y , 86 (1985) 3521, o u r r e f . ( 3 8 ) , C o p y r i g h t (1985) American Chemical S o c i e t y )

.

TABLE 4 Decay p a r a m e t e r s , from g l o b a l a n a l y s i s ( 6 1 , 6 2 1 , f o r i n t e r m o l e c u l a r e x c i m e r f o r m a t i o n w i t h p y r e n e on r e v e r s e d - p h a s e o c t a d e c y l m e t h y l s i l i c a Si-C 18'

A23a

T3 T2 T1 (ns) (ns) (ns)

A22

%I

A1 3

A12

A1 1

aThe q u a n t i t y i n p a r e n t h e s e s is t h e f r a c t i o n o f t h e t o t a l d e c a y : A . . T . / I ~ ( A . . T . ) , t a k i n g t h e a b s o l u t e v a l u e f o r A23. 71 1

31

1

70

The o b s e r v e d monomer and e x c i m e r f l u o r e s c e n c e d e c a y s o f PY on Si-C18 can be u n d e r s t o o d w i t h i n t h e c o n t e x t o f a s i m p l i f i e d k i n e t -

i c scheme c o m p r i s i n g two groups o f k i n e t i c a l l y d i s t i n c t p y r e n e mol e c u l e s , P y ( 1 ) and P y ( 2 1 , see Scheme ( I V )

. In

the octadecyl layer,

t h e s e two s e t s o f m o l e c u l e s are assumed t o r e s i d e i n r e g i o n s o f

I

/

1/t;

l/t,

d i f f e r e n t f l u i d i t y , forming excimers w i t h d i f f e r e n t r a t e c o n s t a n t s , k a ( l ) and k a ( 2 ) . The o b s e r v a t i o n t h a t t h e s h o r t e s t decay t i m e

T

~

,

s t r o n g l y p r e s e n t i n t h e excimer decay, is p r a c t i c a l l y a b s e n t i n t h e monomer d e c a y , i n d i c a t e s t h a t t h e r m a l d i s s o c i a t i o n o f t h e ex-

cimer back t o t h e e x c i t e d s t a t e monomer c a n be n e g l e c t e d as a f i r s t ' , Section 2.1. a p p r o x i m a t i o n : k d ( l ) , k d ( 2 ) < < l / ~ ~ see F o r Scheme ( I V ) , one o b t a i n s a d o u b l e - e x p o n e n t i a l monomer de-

ca

(i,(t)) and a t r i p l e - e x p o n e n t i a l excimer d e c a y (i,(t)) :

i t ) = A t 2 e- X 2 t M

+ A

~

~

~

-

~

I

~

[ I 21

D t ) = A23e-X3t + A22e-X2t + AZ1e-'lt

i

~131

The r e c i p r o c a l decay times f o r t h e monomer are: A 1 = l / T o k a ( l ) [ P y ( l ) l and X 2 = l/.ro + k a ( 2 ) [ P y ( 2 ) 1 , where ( [ P y ( l ) l

+ +

[ P y ( 2 ) ] ) i s t h e t o t a l Py c o n c e n t r a t i o n . F o r t h e e x c i m e r , two d e c a y

times

( T ~and T

~

a) r e e q u a l t o t h o s e of t h e monomer and t h e f l u o -

r e s c e n c e grows i n (T3 ) w i t h t h e excimer l i f e t i m e i o n 2.2.3.

T ~ ' , see

Sect-

I n f o r m a t i o n on t h e excimer f o r m a t i o n , k a ( l ) and k a ( 2 ) i n Scheme ( I V ) , c a n now be deduced from t h e e x c i m e r d e c a y t i m e s (390 n s ) and -c2

T 1 (187 n s ) ( T a b l e 4 ) . Adopting f o r t h e monomer l i f e -

t i m e -co = 430 n s ( 1 1 , 1 5 ) , t h e f o l l o w i n g r e s u l t s are o b t a i n e d : from 6 -1 l / r l = 2.6 x 10 s = k (1) [ P y ( l ) l + l / , ~one ~c a l c u l a t e s k a ( l ) [ p y ( l ) ] = 0.2 x 1 t 6 s m 1 and from l / = ~5 . 4~ x 106 s-1 --

, a v a l u e f o r k,(2) [Py(2)1 of 3 . 0 x 106s-l i s ka(2)[Py(2)] + found. Using a n e x p r e s s i o n f o r t h e excimer-to-monomer quantumyield ratio

@ $ / @

v a l i d f o r Scheme ( I V ) , t o be p u b l i s h e d e l s e w h e r e ,

t h e o b s e r v e d v a l u e f o r t h e r a t i o 11/1o f 0 . 0 7

(see F i g . 6 ,

(3811,

71

c a n be u s e d t o d e t e r m i n e k a ( l ) , k a ( 2 ) , [ P y ( l ) ] and [ P y ( 2 ) 1 s e p a rately. 4.3.2

I n t r a m o l e c u l a r Excimer Formation w i t h l P y ( 3 ) l P y .

S i l i c a / l - o c t a n o l . The monomer and excimer f l u o r e s c e n c e d e c a y s o f

l P y ( 3 ) I P y i n t h e system s i l i c a / o c t a n o l were f i t t e d w i t h t h r e e exp o n e n t i a l s ( 3 8 ) , d o u b l e - e x p o n e n t i a l f i t s g i v i n g u n a c c e p t a b l e res u l t s . The d e c a y times a t 25OC ( 3 8 ) f o r t h e monomer ( 2 0 , 43 and 1 4 6 n s ) , have v a l u e s i n t h e same r a n g e as t h o s e o f t h e e x c i m e r ( 2 7 ,

51 and 1 0 6 n s ) . As w a s n o t e d i n s t u d i e s w i t h l P y ( 3 ) l P y and r e l a t e d

compounds i n homogeneous s o l u t i o n ( 2 0 , 6 2 ) , t h e monomer decay o f t e n c o n t a i n s a c o n t r i b u t i o n from an i m p u r i t y w i t h a l i f e t i m e s i m i l a r

to t h a t of e.g.

I-methylpyrene

(-c0),

becoming more i m p o r t a n t w i t h

i n c r e a s i n g f l u o r e s c e n c e quenching. T h i s t h e n l e a d s t o t h e d i f f e r e n c e o b s e r v e d i n t h e l o n g e s t decay times o f excimer and monomer. These monomer and e x c i m e r d e c a y s were r e a n a l y z e d u s i n g t h e method of g l o b a l a n a l y s i s ( 6 1 - 6 2 ) , employing t h e same set o f t h r e e d e c a y t i m e s f o r excimer and monomer. From s u c h a n a n a l y s i s ( 6 3 ) , good t r i p l e - e x p o n e n t i a l

f i t s r e s u l t , w i t h decay t i m e s s i m i l a r t o

t h o s e p r e v i o u s l y o b t a i n e d f o r t h e excimer decay a l o n e ( F i g . 1 2 ) .

EXC

MON

CHANNEL

NSEC

25.5 -34.1 14.5

52 14.1 4.6

107 (227) 22.0 EXC 0.63 ( 0 . 2 ) MON

F i g . 1 2 . Monomer and e x c i m e r f l u o r e s c e n c e d e c a y c u r v e s o f 1 , 3 - d i ( 1 - p y r e n y l ) p r o p a n e , I P y ( 3 )l P y , i n s i l i c a / l - o c t a n o l a t 25OC, a n a l y z e d by t h e method o f " g l o b a l a n a l y s i s " , see r e f . ( 6 3 ) and c a p t i o n t o F i g . 11. T h i s shows t h a t f o r i n t r a m o l e c u l a r e x c i m e r f o r m a t i o n w i t h 1 P y ( 3 ) 1 P y I t h r e e (and o n l y t h r e e ( 2 5 ) ) e x c i t e d s t a t e s p e c i e s govern t h e k i n e t -

12

ics. It can, therefore, be concluded that a double-layer equivalent of 1-octanol on silica behaves as a homogeneous solution. R e v e r s e d - P h a s e S i - C 1 8 - The excimer decay of lPy(3)lPy on Si-C18 likewise were found to be triple-exponential (38), with a substantial contribution of excimer growing-in, see Fig. 11A. These mol/g measurements were carried out with a coverage of 3 x of Si-C18, i.e. at a concentration where both static and dynamic excimer formation occurs, see Fig. 13 and Section 3.2.2. The presence of static excimer formation, can also be seen from the stepwise growing-in of the excimer fluorescence in Fi'g. 11A.

3%

X nm

400 440 480 520 300 340 380

X

nrn

Fig. 13. Emission and excitation spectra of lPy(3)lPy adsorbed on Si-Cl8. Probe concentration: ( A ) 2.6 x mol/g; 2.6 x lo-' mol/g. (Reprinted with permission from the Journal (B) of Physical Chemistry, 86 (1985) 3521 , our ref. (38), Copyright (1985) American Chemical Society). 4.3.3 Alkylpyrenylsilanes Chemically Bound to Silica. Lochmiiller et al. reported on time-dependent fluorescence studies with the alkylpyrenylsilanes PPS and PDS chemically bound to microparticulate silica (41-43). These compounds undergo intermolecular excimer formation in contact with a number of solvents such as tetrahydrofuran and methanol. For all solvents, at different surface concentrations of PPS and PDS, the monomer fluorescence decays i,(t) could be fitted with three exponentials. The excimer decays were also found to be triple-exponential, although only the excimer rise times, having values comparable with the shortest monomer decay times T ~ were , reported.

73

i,(t)

=

A 1 1 e-'lt

+ A12e-A2t + A13e

-X3t

It was concluded that the observation of these three exponentials indicated the presence of three kinetically non-interacting, each in itself apparently completely isotropic, monomer populations of PPS or PDS. Lochmuller et al. based.their discussions on the values of the (normalized) amplitudes Ali in the triple-exponential monomer decays, and stated that these amplitudes were' a direct measure of the three fractions of monomers. Of these fractions, one (associated with A13 of the shortest decay time -r3) formed excimers at all surface coverages studied, whereas a second (A12) only underwent excimer formation at intermediate and higher coverages. The third fraction (Al1), finally, was supposed not to form excimers at all. The conclusion was then drawn that the chemically reactive silanols -Si-OH are inhomogeneously distributed at the sur€ace, and, consequently, that PPS and PDS are clustered predominantly into regions of high density. Whereas Avnir (64) critized these conclusions on the basis of deficiencies in the concept of flat surfaces, invoking fractals (64,65), the three amplitudes Ali:14] do in fact not reflect the presence of three populations of different monomers, making the interpretation intrinsically unfounded. Just as in the case of the double-exponential decays of Scheme (I), the normalized amplitudes in the triple-exponential decays of a system undergoing excimer formation do not have a straightforward meaning, see Section 2.2.4. In the derivation (42) of the physical meaning of the monomer amplitudes in [14], the concept of molecular fluorescence quantum yield is used, as if the decay times in the triple-exponential monomer decay were in fact the lifetimes .ro(i) of three molecules that do not interact in any way, i.c. do not form excimers. Therefore, valid information from these normalized amplitudes can only be obtained for such a mixture of completely noninteracting molecules. The ratio Ai~o(i)/(F.iAi~o(i)), see [9], then is the fraction of each molecular species, provided any difference in excitation cross-section has been taken into account. Three further points regarding the interpretation given by Lochmuller should be made. First, in a system for which tripleexponential monomer (and excimer) fluorescence decays are observed, at most two kinetically different groups of monomers can play a role in the kinetics (Section 2.2.4). Secondly, when one of the

74

three monomer distributions of PPS or PDS cannot form excimers (42,43), its fluorescence decay time should be identical to the monomer lifetime of, for example, I-methylpyrene (188 ns in tetrahydrofuran at 25°C (60)). Finally, it cannot be concluded from the absence of a growing-in (i.e. a negative amplitude Ali) for a decay time in i,(t) [14], that thermal backtransfer (kd) from the excimer to that particular singlet excited state monomer does not occur. In fact, the amplitudes of a monomer fluorescence decay are always positive, when the excitation light is exclusively absorbed by the pyrenylsilane monomers (42). 4.4 Distributions of Fluorescence Decay Times In a treatment of fluorescence decays of mDlecules adsorbed on solid surfaces such as silica gel (59,661, it was concluded that it is practically impossible, under normal experimental conditions, to differentiate a distribution of decay times from two (or three) separate decay times. It was then argued that it would be difficult to acquire significant information from such decays. Although it is not surprising that unresolvable distributions of decay times are encountered for systems such as Py on dry silica, this conclusion should not be carried over to fluid-like media such as silica with adsorbed alcohols and reversed-phase Si-CI8. For lPy(3)lPy in fluid solution, the concept of three discrete molecular states (one monomer and two excimers, Scheme (11)) has .been shown to be valid (22-25). Similarly, the use of such a discrete state model can very well lead to interpretable results for the photophysical behaviour of Py and IPy(3)lPy in media such as silica/octanol and Si-C,8, see Section 4.3. 4.5 Excimer Growing-In on Silica Surfaces A growing-in of the excimer fluorescence of Py on silica s u r faces was first observed by de Mayo and Ware (321, for silica gel covered with a monolayer of 1-decanol (Section 4.2.2). It was shown that this growing-in, i.e. the negative sign of the amplitude AZ2 of the shorter decay time -r2 [ 4 1 , disappeared upon cooling below -23OC, see Fig. 9A. From the observation that the excimer amplitude ratio A22/A21 was equal to -1.0, at relatively hi h Py surface coverage (Table l), it was concluded that excimer formation was completely dynamic (Section 2.2.31. Results were not as clear for Py adsorbed on silica gel in the presence of a series of other coadsorbates (water methanol, 1hexanol, glycerol and 1-adamantanol), for which f uorescence decay

75

.

t i m e measurements were a l s o c a r r i e d o u t by d e lrlayo e t a 1 ( 3 2 , 3 6 ) negative v a l u e s f o r t h e excimer

.

I n contrast to silica/decanol,

a m p l i t u d e r a t i o were n o t o b s e r v e d f o r Py on s i l i c a / m e t h a n o l o r s i l i c a / w a t e r , and t h e excimer e x c i t a t i o n s p e c t r a w e r e s t r o n g l y r e d s h i f t e d a s compared t o t h o s e of t h e monomer. F o r s i l i c a / l - h e x a n o l (and a l s o f o r s i l i c a / g l y c e r o l ) a s u p e r p o s i t i o n of dynamic and

s t a t i c e x c i m e r f o r m a t i o n was o b s e r v e d , w i t h A22/A21 = - 0 . 6 1

(1-

h e x a n o l ) and a r e l a t i v e l y s m a l l r e d - s h i f t between t h e excimer and monomer e x c i t a t i o n s p e c t r a ( 3 6 ) . The f o l l o w i n g p i c t u r e t h e n emerges: Py on s i l i c a / d e c a n o l moves t h r o u g h a l a y e r o f a l k a n e c h a i n s t h a t a r e l o n g enough t o c r e a t e a f l u i d medium, s i m i l a r t o b u l k d e c a n o l ( S e c t i o n 3 . 5 ) . Howe v e r , w i t h l - h e x a n o l , and a f o r t i o r i w i t h m e t h a n o l , t h e a l k a n e c h a i n s a r e a p p a r e n t l y t o o s h o r t t o match t h e dimensions of Py. F o r Py on d r y s i l i c a w i t h r e l a t i v e l y s m a l l p o r e s i z e s ( 6 0 A ) , a growing-in o f t h e excimer f l u o r e s c e n c e ( i . e . a n e g a t i v e a m p l i t u d e A22,

[ 4 ] ) w a s not detected. This i s i n accord with t h e i n t e r p r e t a t -

i o n t h a t g r o u n d - s t a t e d i m e r s govern t h e excimer f o r m a t i o n . I t

s h o u l d be n o t e d , however, t h a t t h e p r e s e n c e of a s m a l l p e r c e n t a q e o f dynamic e x c i m e r f o r m a t i o n c a n n o t be e x c l u d e d on t h e b a s i s of t h e s e r e s u l t s , a s t h i s p r o c e s s c a n be masked by a much s t r o n g e r s t a t i c e x c i m e r f o r m a t i o n . Whether t h e s e d i m e r s have t h e same r e l a t i v e c o n f i g u r a t i o n f o r t h e two p y r e n e s a s p r e s e n t i n t h e e x c i m e r (52),

c a n n o t be a s c e r t a i n e d ( S e c t i o n 3 . 2 ) . The o n l y case of a n excimer growing-in on unmodified s i l i c a

was r e c e n t l y d e t e c t e d by W e l l n e r e t a1 ( 4 0 ) , f o r s u r f a c e s w i t h v a r y i n g p o r e s i z e d i s t r i b u t i o n s , F i g . 1 4 . The excirner growing-in c o n s i s t s o f a f a s t subnanosecond component common t o a l l s i l i c a s u r f a c e s s t u d i e d , n e x t t o a s l o w e r excimer r i s e t i m e i n c r e a s i n g from 1 . 9 n s t o 5 n s f o r p o r e s i z e s between 6 0 and 1000 A . These excimer r i s e t i m e s were i n t e r p r e t e d as g i v i n g i n f o r m a t i o n on t h e r a t e o f excimer f o r m a t i o n from t h e p r i m a r i l y e x c i t e d Py m o l e c u l e s

( s e e S e c t i o n 2.2.31,

viewed as m o l e c u l a r r e a r r a n g e m e n t s between

n e i g h b o u r i n g Py m o l e c u l e s on t h e s u r f a c e ( n o t n e c e s s a r i l y d i m e r s ) . I t w a s concluded t h a t a s u b s t a n t i a l f r a c t i o n o f t h e e x c i m e r s are

formed on a nanosecond t i m e scale, a l t h o u g h t h e p r e s e n c e o f g r o w d

s t a t e d i m e r s o f p y r e n e w a s c o n f i r m e d i n t h i s s t u d y ( a t l e a s t up t o average pore d i a m e t e r s of 1000 A ) .

76

T nsec

F i g . 14. Growing-in p r o f i l e s of p y r e n e e x c i m e r f l u o r e s c e n c e o n v a r i o u s s i l i c a s u r f a c e s . Laser p u l s e i s a l s o p r e s e n t e d . ( R e p r i n t e d w i t h p e r m i s s i o n from L a n g m u i r , 2 ( 1 9 8 6 ) 6 1 6 , o u r ref. ( 4 0 1 , Copyr i g h t ( 1 986) American C h e m i c a l S o c i e t y )

.

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64 65 66

448-452.

Compounds, E l s e v i e r , Amsterdam, 1965. N a k a j i m a , B u l l . C h e m . SOC. J a p a n , 46 (1973) 2602-2604. Kalyanasundaram and J . K . Thomas, J . Am. Chem. S O C . , 99 ( 1 9 7 7 ) 2039-2044. D . C . Dong and M.A. Winnik, Photochem. P h o t o b i o l . , 35 ( 1 9 8 2 ) 17-21. D.C. Dong and M.A. Winnik, Can. J . Chem., 6 2 ( 1 9 8 4 ) 2560-2565. K.A. Zachariasse, B. K o z a n k i e w i c z , and W. Kuhnle, i n : K.L. M i t t a l ( E d . ) , S u r f a c t a n t s i n S o l u t i o n , V o l . 1 , Plenum, N e w York, 1984, pp. 565-584. D . R . James, Y . - S . L i u , P. de Mayo, and W.R. Ware, Chem. P h y s . L e t t . , 120 (1985) 460-465. K.A. Z a c h a r i a s s e , u n p u b l i s h e d r e s u l t s . G . S t r i k e r , i n : M. Bouchy ( E d . ) , D e c o n v o l u t i o n and Reconvol u t i o n o f A n a l y t i c a l S i g n a l s , U n i v e r s i t y P r e s s , Nancy, 1982, pp. 329-354. K.A. Z a c h a r i a s s e , G . Duveneck, W . K u h n l e , P . R e y n d e r s , and G . S t r i k e r , Chem. P h y s . L e t t . , 1 3 3 ( 1 9 8 7 ) 390-398. The g l o b a l a n a l y s i s , see r e f s . ( 6 1 , 6 2 ) , was c a r r i e d o u t u s i n g a program w r i t t e n by P. R e y n d e r s . I n t h i s program, t w o t i m e s h a v e b e e n f i x e d t o i n c o r p o r a t e t h e c o n t r i b u t i o n s from s c a t t e r ed l i g h t and from a n i m p u r i t y h a v i n g o n l y o n e p y r e n y l g r o u p w i t h T~ = 227 n s , see F i g . 1 2 and r e f . ( 6 2 ) . A. K.

D. A v n i r , J. Am. Chem. SOC., 109 ( 1 9 8 7 ) 2931-2938. V . R . Kaufman and D. A v n i r , Langmuir, 2 (1986) 717-722. D.R. James and W.R. Ware, Chem. P h y s . L e t t . , 120 ( 1 9 8 5 )

455-459.

79

Photophysics of Acridone. N-Methylacridone, Acridine, and Pyrene Adsorbed on Silica Gel S. Suzuki and T. Fujii

1.

INTRODUCTION I n recent years, there has been a sharp increase in the study of the photophysics of organic molecules adsorbed on metal oxides including silica gel (1). Many of these investigations are aimed at studying the nature of the surface itself and contributing to elucidate the mechanism of catalysis on which the adsorption on solid surfaces may play an important role. T.his chapter outlines the electronic spectroscopy o f acridone, N-methylacridone, acridine, and pyrene adsorbed on silica gel and discusses the assignment of emitting species on a silica gel surface. The authors are interested in studying factors perturbing the electronic states of organic molecules through the adsorption on solid surfaces. The former three compounds have functional groups which are expected to contribute to hydrogenbond formation but pyrene has no such a group. Emphasis is also placed on the procedures to realize the observation of electronic spectra intrinsic to adsorbed species on silica gel.

2.

SAMPLE PREPARATION 2.1 Pretreatment of silica gel An enormous number of papers have been accumulated on the surface structure of silica gel ( 2 ) . The nature of a silica gel surface is highly complicated and depends on various factors including its preparation. A simplified picture of the chemical structure of a silica gel surface may be summarized as follows. The active surface of silica gel is covered with OH groups bound to an S i 0 2 skeleton (SiOH groups). The silanol groups are the sites on which water molecules and many organic molecules with polar groups are adsorbed through hydrogen bonding. When silica gel is exposed to the atmosphere, the water molecules cover a l l SiOH groups on the surface and come off at ordinary temperature in a vacuum or at 150°C in the atmosphere, Increase in temperature

80

above 200°C results in dehydration to form siloxane groups which are hardly active to adsorption. In the present chapter silica gel was activated in the f o l lowing two ways. One method was to activate it in the atmosphere; Silica gel (Wako Pure Chemical Industries, Ltd., Wakogel 4 - 2 3 for chromatography) was heated at 200°C for 24 hr in the atmosphere and then cooled to room temperature in a dry atmosphere. Hereafter, the silica gel thus pretreated is referred to as SG200A and the silica gel pretreated at 800°C in the same way as SG800A. The other method was t o activate it in a vacuum; Silica gel was heated at 200°C for 5 hr at a pressure of less than 2 x Torr and all the succeeding procedures for adsorption was carried out in a vacuum (SG200V). 2.2 Procedures for sample preparation Organic molecules can be adsorbed on adsorbents such as silica gel either in solutions or' from gas phases. In the earliest stage of spectroscopic studies of adsorbed organic species ( 3 ) , electronic spectra of adsorbed samples were observed without elimination of solution phases (slurry samples) ; The spectra o f the species in solution phases overlapped with those of adsorbed species, so we were unable to observe the spectra of adsorbed species separately. In the next stage of the studies ( 4 ) , s o l vents were eliminated by forced vaporization and the spectra were observed on "dried" samples. By adopting such adsorption procedures, we can avoid observing the spectrum of species in a solution phase which disturbs the "pure" one of adsorbed species, but still have the difficulties that the non-intrinsic spectra will be observed in addition to the intrinsic spectra of adsorbed species to adsorption sites on adsorbents. Forced vaporization of solvents leaves solute molecules on the surface of adsorbents. Some of the molecules may be deposited on the surface of adsorbents to form aggregates which prevent observing the "pure" spectrum of adsorbed species. In a system containing a solution and an adsorbent, solute and adsorbate molecules are considered to be in dynamic equilibrium. It appears that a solute molecule is adsorbed at an adsorption site from the solution but an adsorbed molecule is dissolved into the solution by the solvation power of the solvent. This idea is supported by the fact that fairly satisfactory empirical isotherms have been presented by H. Freundlich and others.

81

The existence of adsorption equilibrium allows us to determine the amount of chemical species adsorbed on silica gel. The authors adopt the following procedures for sample preparation of adsorbed species. The pretreated silica gel is admitted into a solution of a known concentration which contains a compound to be adsorbed and is deaerated for the adsorption in deaerated atmosphere. The solution is allowed to stand for an appropriate time to establish equilibrium and then is separated from the silica gel by decantation. The residual solvent impregnated into silica gel is evaporated under reduced pressure. In case of SGZOOV, all the adsorption procedures were carried out in a vacuum. In case of SG200A and SGEIOOA, samples are prepared in a dry atmosphere. The amount o f adsorbed species was calculated from the observed absorbance data of solutions separated from an adsorbent by decantation. The amount o f adsorbed species C is expressed in mg/g as the mass of adsorbate on 1 g silica gel. 3.

RESULTS 3.1 Acridone Acridone in solutions (5) shows three types of emission spectra (Fig. 1). Acridone in benzene emits fluorescence with two vibrational peaks at 2 5 2 0 0 and 2 4 1 0 0 cm-l. In protic solvents such as ethanol and water, the fluorescence spectra of acridone

Fig. 1. Fluorescence spectra of acridone in various solvents.

....... :

in benzene,

18N sulfuric acid.

-:

in ethanol,---

:

in water, ---.-:

in

82

resemble that in benzene in shape but are displaced to the red compared with that in benzene. In 18N sulfuric acid, acridone emits fluorescence with three vibrational peaks, the maximum of which is located at 2 1 8 0 0 cm -1 , The fluorescence excitation spectrum o f acridone observed at 21800 cm-l in the sulfuric acid solution has a band characteristic of protonated acridone, which is essentially different from that in ethanol (see Fig. 3). The emitting species for three types of fluorescence i n solution phases are reasonably assigned to the free, the hydrogen-bonded, and the protonated species of acridone. The fluorescence spectra of acridone adsorbed on SG200V are given in Fig. 2 , together with those in ethanol and in 18N sulfuric acid. The fluorescence band of adsorbed acridone has a maximum of 22800 cm-l, which is located at a lower wavenumber than the corresponding peak in ethanol but at a slightly higher wavenumber than that in water. The 0 - 0 vibrational band has the highest intensity in ethanol and water, while the 0-1 band has the highest on silica gel. The difference between the spectral shape in ethanol and that on SG200V appears to result from the reabsorption of fluorescence. The idea is supported by the fact that the intensity of the 0-0 band decreases with increase in the amount of adsorbed acridone.

20 U/I

0 3 d

25

Fig. 2 . Fluorescence spectra of acridone adsorbed on SG2OOV. : 0.014 mg/g,----: 0.13 mg/g,....... : in ethanol,--.--.: in 18N sulfuric acid.

-

83

Figure 3 shows the fluorescence excitation spectrum of acridone adsorbed on SG200V as well as those in ethanol and in 1 8 N sulfuric acid. The fluorescence excitation spectrum of acridone in 1 8 N sulfuric acid has the band characteristic of protonated acridone which has an intrinsic peak near 30000 cm-l. The excitation spectrum of adsorbed acridone on SG200V resembles that in ethanol. These findings indicate that the dominant emitting species on SG200V is safely assigned to the hydrogen-bonded acridone to silanol groups on the silica gel surface.

20

25

30

u/ 1 o3c m-’

35

-

Fig. 3 . The fluorescence excitation spectrum of acridone. : on SG200V,----: on SG200A, ....... : in Coverage: 0.01 mg/g. ethanol.-.- , : in 18N sulfuric acid. Figure 4 shows the fluorescence spectra of acridone adsorbed on SGZOOA, the sample preparations of which were carried out in a dry atmosphere. The fluorescence band has an identical spectral shape with that on SG200V but is slightly displaced to the red compared with that on SG200V. The band maximum is quite similar to that of the corresponding band in water, but is slightly lower than that in ethanol. The fluorescence excitation spectrum of acridone on SG200A is substantially the same as that on SG200V. The fluorescence spectra of N-methylacridone adsorbed on SG200A are given in Fig. 5. The fluorescence band of adsorbed Nmethylacridone is located at the wavenumber between the bands in ethanol and in 0 . 1 N sulfuric acid and the situation is identical to the case of acridone. This suggests that the hydrogen bond is

84

formed between the carbonyl group of acridone or N-methylacridone and the silanol group on silica gel and that the imino group of acridone is inactive to adsorption sites on the silica gel surface. The fluorescence spectrum of acridone which is prepared by sublimation in a vacuum ( S G 2 0 0 S ) is given in Fig. 6 as well as those on SG200V and SG200A for the purpose of comparison. In the

I

20 a/~ o

~

25

~

-

~

-

~

Fig. 4 . Fluorescence spectra of acridone adsorbed on SG200A. : 0.001 mg/g,----: 0,103 mg/g, . : in ethanol, -.-. 18N sulfuric acid.

......

20

u/

1

o3cm-’

25

:

L

Fluorescence spectra of N-methylacridone on 0.001 mg/g,----: 0.103 mg/g, ....... in ethanol, -.-.-: in 0.1N sulfuric acid.

Fig. 5.

:

:

SG200A.

in

85

case that the samples are prepared by sublimation, it is difficult to control the amount of adsorbate at low coverage. A s a result of the high coverage, the spectral shape is modified by the reabsorption of the 0 - 0 fluorescence band. The band maximum is in agreement with that on SG200V. The emitting species on SG200V is actually identical with that on SG200S and is safely assigned to the ones hydrogen-bonded directly to the silanol group on neat silica gel. On the other hand, acridone adsorbed on SG200A shows a n emission band of a slightly lower wavenumber than that o n SG200V and the emission is tentatively assigned to the hydrogenbonded species to hydrated spheres covering silanol groups on the silica gel surface.

20

o

~

25

~

~

-

~

Fig. 6 . Comparison of acridone fluorescence due to different preparations. : on SG200V,----: on SGZOOS, : on SG200A.

-

a m . . . . .

The emission of acridone on SG800A has a similar spectrum to that on SG200A but its intensity decreases due to the decrease i n the relative amount of silanol groups on silica gel. The increase of the amount of siloxane groups has n o substantial effect on the emission spectrum; The siloxane group is inactive to the acridone adsorption onto the silica gel surface. There is n o spectral evidence for the hydrogen-bond formation of acridone with the siloxane group of silica gel through the imino group. The fluorescence lifetimes of acridone adsorbed on silica gel at various coverage are given in Table 1 as a function of emission

86

wavelength. The observed fluorescence decay curves can be satisfactorily fitted as the two-component decay except for the samples which have a low amount of adsorbate. The third fluorescence component with a decay time of about 1 ns seems to originate from acridone molecules left on silica gel without specific interaction with a surface. The scattering of exciting light may make some contribution to this component. The fluorescence lifetimes of acridone in ethanol and in 18N sulfuric acid are observed to be 10.5 ns and 20.2 ns, respectively, with the same instrument used for the measurements of the fluorescence lifetimes of adsorbed samples. TABLE 1 Fluorescence lifetimes of acridone adsorbed on SG200V and SG200A. Acridone on SG200V C mglg

A nm

- Tl

ns

2 ns

Acridone on SG200A

K

C

mgJg

418 0.014 436 465

15.2 25.6 15.5 25.0 15.4 25.6

0.20 0.14 0.40

0.001

418 0.070 436 465

14.2 24.2 15.6 23.5 12.5 24.3

0.31 0.37 0.50

418 436 465

16.2 30.5 16.0 30.1 16.2 30.3

0.54 0.64 0.76

0.130

A Tl T2 nm ns ns

420 440 470

10.2 9.9 9.6

K

22 25 30

0.21 0.36 0.54

420 0.010 440 470

13.4 28 13.7 28 13.6 28

0.03 0.06 0.16

420 0.103 440 470

10.1 10.2 10.2

1.4 1.7

18 18 19

1.8

Based on the values of fluorescence lifetimes in solutions, the main decay component -tl is assigned to the hydrogen-bonded species of acridone adsorbed on silica gel. The r2 component seems to correspond to the protonated species of acridone. The value of K increases with increase in the observation wavelength o f emission. This implies that a weak band underlies the main emission band of hydrogen-bonded species at a longer wavelength side. The dominant emission component of acridone adsorbed on the silica gel surface can be assigned to the hydrogen-bonded species with the silanol group on silica gel through the carbonyl group of acridone. Evidence for the existence of protonated acridone was

found from the fluorescence decay data, but there is still some doubt whether the formation of protonated species is intrinsic nature to the silica gel surface or not; Silica gel used in this study is for chromatography so that the purity is not sufficiently guaranteed. 3.2 Acridine Acridine in benzene emits the fluorescence with three main vibrational bands. The most intense vibronic band is at 24500 cm-1 The fluorescence band of acridine in ethanol is shifted to the red by 500 cm-l compared with that in benzene. In an acidic solution, acridine shows a broad emission band which was assigned to the fluorescence of acridinium cation ( 6 ) . The peak wavenumber band is 20700 cm-'.

.

20

-

25

Fig. 7. Fluorescence spectra of acridine. : on SG200V,. in ethanol, -.---: acid.

in 0.1N sulfuric

Figure 7 shows the fluorescence spectrum of acridine adsorbed on SG200V. The figure includes the fluorescence spectra in ethanol and in 0.1N sulfuric acid. The emission band of acridine o n SG200V is largely displaced to the red compared with that in ethanol and its location and shape are quite similar to that in the acidic solution. Acridine adsorbed on SG200A shows substantially the same emission as that on SG200V. The fluorescence spectra both on SG200V and on SG200A are little affected by surface coverage.

88

The fluorescence excitation spectrum of acridine in 0.1N sulfuric acid has a band characteristic of acridinium cation at about 25000 cm-', but on the other hand, acridine in ethanol has no band below 25000 cm-l (see Fig. 8). The fluorescence excitation spectra of acridine both on SG200V and SG200A have a band near 25000 cm-'. These findings indicate that a proton is transferred to acridone from silanol groups in its ground state as a result of adsorption on the silica gel surface. The emission of acridine adsorbed on silica gel are reasonably assigned to the protonated species of acridine.

Fig. 8. Fluorescence excitation spectra of acridine. -: On SG200V, ..... .. : in ethanol, -.-. : in 0.1N sulfuric acid. Acridine adsorbed on SG800A shows a similar fluorescence spectrum with that on SG200A. The apparent fluorescence intensity is reduced but the spectral shape is quite identical with that of acridine adsorbed o n SG200A. These observations are consistent with the decrease in silanol groups in SG800A than in SG200A. The fluorescence lifetimes of acridine adsorbed on SG200A are given in Table 2. The fluorescence decay data of adsorbed acridine can be reasonably fitted as the two-component decay, except for the samples of the lowest coverage which have a shorter lifetime. The situation is quite similar to the case of adsorbed acridone and the short-lifetime component seems to originate from acridine molecules left on the silica gel surface without any

89

specific interaction with adsorption sites. The scattering o f exciting light may also make some contribution to this component. The fluorescence lifetimes of acridine in ethanol and in 0.1N sulfuric acid are observed to be 0.7 ns and 26 ns by the same instruments, respectively. The r2 component is the dominant fluorescence of acridine on SG200A because of the large value o f K and is assigned to the fluorescence lifetime of protonated acridine on SG200A. Acridine has a very short lifetime in ethanol, however, the r 1 component may be tentatively assigned to the fluorescence lifetime of the hydrogen-bonded acridine on SG200A. The value of K increases with increase in the observation wavelengths and this is consistent with underlying the emission band of hydrogen-bonded species at a longer wavelength side. TABLE 2 Fluorescence lifetimes of acridine on SG200A

450 0.010 470 500

11.0 30 11.5 30 11.2 30

1.26 1.97 3.05

0.101

450 470 500

11.2 11.5 11.5

35

35 35

3.85 4.11 5.41

1.01

450 470 500

9.9 10.8 11.2

30 32 33

0.96 1.78 2.11

The dominant fluorescence component of adsorbed acridine is assigned to the protonated species as the results of adsorption to silanol sites on the silica gel surface. The fluorescence decay data suggest that the emission band o f hydrogen-bonded species underlies that of protonated acridone but no positive evidence was found on the fluorescence spectra. 3.3 Pyrene Acridone and acridine molecules have functional groups which are expected to interact with the silanol groups or other adsorption sites on a silica gel surface, and as was discussed in the

90

preceding sections, electronic spectroscopic data can reasonably be explained by the interaction of the silanol group on a silica gel surface with functional groups of adsorbates. Although aromatic hydrocarbon molecules such as pyrene have no such functional groups, the evidence for the interaction with a silica gel surface has been found by several authors (7). Figure 9 gives the fluorescence spectra of pyrene adsorbed on SG200A at various coverage. Adsorbed pyrene emits both monomerand excimer-like fluorescence (hereafter abbreviated as FM and FE, respectively). The FE component becomes dominant at higher pyrene coverage and its intensity ratio to the FM component decreases with decreasing in pyrene coverage. The spectral shape of the FM band in cyclohexane is considerably different from those of adsorbed pyrene on the silica gel surface; The vibronic band o f 26100 cm-’ has the highest intensity in cyclohexane, whereas those of 2 5 5 0 0 and 26900 cm-’ have the highest intensities on adsorbed

Fig. 9. The fluorescence spectra of pyrene excited at 3 3 5 nm. 1 mg/g on SG200A, : 2 0 mg/g -: 0.24 mg/g on SG200A,----: on SG200A, .... : in cyclohexane. ---a

pyrene. It is known that the vibrational structure o f pyrene The obserfluorescence is very sensitive to the environment ( 8 ) . vations suggest that pyrene adsorbed on the silica gel surface has some different interaction from that dissolved in solution. The fluorescence excitation spectra of adsorbed pyrene observed at the wavelengths of the FM and FE bands are shown in Fig.

91

10. The spectrum observed a t the FE band is different in shape from that observed at the FM band in contrast to the case in solutions; This finding indicates that pyrene molecules emitting the FE fluorescence have the specific conformation in which they considerably interact with each other in the ground state and are favorable for excimer formation. The fluorescence polarization has been observed for the FM and FE bands of pyrene adsorbed on silica gel (9). The excimer fluorescence of pyrene is completely depolarized even in viscous solvents. Rotational diffusion is involved in the process of excimer formation in solutions. The degree of polarization P for the F E fluorescence of adsorbed pyrene has non-zero positive values and the value of P is larger on the 'La excitation than on the 'I, excitation. On the contrary to this, the value of P for the FM emission is larger on the 'Lb excitation than on the 'La excitation. These findings show that the FE transition of adsorbed pyrene has the transition polarization along the a-axis of a pyrene molecule.

25

35

30

4 1 0 cm-l ~

-

Fig. 10. The fluorescence excitation spectra of pyrene on SG200A. Coverage: 20 mg/g. : observed at 392 nm,----: observed at 470 nm. The observation o f non-zero P values for the FE emission implies that it is not essential to involve rotational diffusion on the excimer formation process f o r adsorbed species on the silica gel surface. A s is seen previously, pyrene molecules

92

resulting in excimer emission are supposed to have specific conformations in their ground state which are favorable for excimer formation. It is concluded that a pyrene pair interacting in the ground state forms an excimer as a result of slight translational motion on the silica gel surface and emits the FE fluorescence (9). Even in molecules with no functional groups which are expected to interact with silanol sites on silica gel through hydrogen bonding, adsorption on solid surfaces modifies their electronic states and their photophysical processes but the mechanism of adsorption interaction is not elucidated in detail. S. Suzuki thanks the Ministry of Education, Science and Culture of Japan (Grant-in-Aid for Special Project Research, No. 61123004). REFERENCES P. de Mayo and L. J. Johnston, in: Preparative Chemistry Using Supported Reagents. Academic Press, Inc., 1987. R. K. Iler. The Chemistry of Silica, John Wiley fi S o n s , New York, 1979; M. Robin and K. N. Trueblood, J. Am. Chem. SOC. 79 (1957) 5138-5142; P. A . Leermakers and H. T. Thomas, J. Am. Chem. SOC. 87 (1965) 1620-1622. D. Oelkrug, M. Radjaipour, and H. Erbser, Z. Phys. Chem. Neue Folge, 88 (1974) 23-36; D. Oelkrug, G. Schrem and I. Andra, ibid., 106 (1977) S.197-210. H. Kokubun, Z. Elektrochem., 62 (1958) 599-607; H. kokubun, Z. Phys. Chem. Neue Folge, 101 (1976) 137-142; K. Fushimi, K. Kikuchi and H. Kokubun, J. Photochem. 5 (1976) 457-468. F. Doerr and M. Held, Angew. Chem. 72 (1960) 287-294. K. Hara, P. de Mayo, W. R. Ware, A. C. Weedon, G. S. Wong, and K. C. Wu, Chem. Phys. Lett., 69 (1980) 105-108; V. R. Kaufman and D. Avnir, Langmuir 2 (1986) 717-722. A. Nakajima, Bull. Chem. SOC. Jpn. 44 (1971) 3272-3277. T. Fujii and E. Shimizu, Chem. Phys. Lett. 137 (1987) 488452; T. Fujii, E. Shimizu, and S. Suzuki, J. Chem. S O C . , Faraday Trans. 1. in press.

93

HETEROGENEOUS MOLECULAR ENVIRONMENTS PROBED BY FLUOROPHORES BONDED TO CHEMICALLY M O D I F I E D S I L I C A GEL

FLUORESCENCE DECAY

MEASUREMENTS UNDER A MICROSCOPE

S.HIRAYAMA, T.KUB0 and H.YAMASAK1

INTRODUCTION In homogeneous systems such as solutions molecular environments and their role in determining the fate of photoexcited states are not complicated at least conceptually. On the other hand, in the heterogeneous systems such as interfaces, biological polymers, cells etc., even the definition of molecular environments is ambiguous and we are still far from the true understanding of them. Chemically modified silica gel is well known to provide very attractive molecular environments in which a wide variety of physico-chemical processes can be examined. One of the advantages of using the chemically modified silica gel is the easiness of the control of the molecular environments, for instance, by varying the extent of chemical modification or by exposing it to different kinds of solvents or gases. By combining these features of the chemically modified silica gel with the technique of the lifetime measurements under a microscope developed recently in our laboratory ( 1 ) , we can expect to get a much closer view of the heterogeneous molecular environments provided by the interfaces. There are numerous spectroscopic studies of the chromophores bound to the chemically modified silica gel(2-4) but dynamic studies such as fluorescence lifetime measurements are rather > limited. In their recent work, Lochmuller and Hunnicutt (5) have employed the time-correlated single photon counting technique and analyzed the non-exponential decays in detail to disclose the complex features of the interfaces through ( 1 0 - ( 3 pyreny1)decyl)dimethylmonochlorosilane chemically bonded to silica as a probe. Unfortunately, however, their method, though sophisticated enough to monitor heterogeneous fluorescence decays, cannot distinguish one microparticle from the other and hence unavoidably follows overall decays. The advantage of introducing a microscope in order to pick up 1.

94

a single microparticle for observation is obvious since not only the fluorescence decay features can be followed on a specific particle but a l s o external control of its environments is feasible. With this in mind, erythrosin was bonded as a probe to the amino group of aminopropyl silica gel and its fluorescence decays were examined in detail under various conditions. EXPERIMENTAL Materials The aminopropyl silica gel ( A P S ; surface area determined by N 2 adsorption, 300 m2/g; average pore diameter, 100 particle size, 3 - 1 0 urn) was synthesized from spherical porous silica gel obtained from Nomura Chemical Co.,Ltd. The silica gel was treated with 6 N hydrochloric acid first and then 1 2 N hydrochloric acid and dried at 2 O O 0 C i n vacuo before use. T h e n u m b e r of aminopropyl groups per unit area ( 1 0 0 i2) was estimated to be two from the surface area and elemental analysis. Erythrosin ( E R ) was bonded to the amino group of A P S by the coupling reaction promoted by N , N ' - d i c y c l o h e x y l c a r b o d i i m i d e (DCC)(6). An appropriate volume of 4.6 x 1 O-5M tetrahydrofuran (THF) solution of ER and 30 mg of A P S were mixed to make a 1 0 0 mll THF solution and were reacted in the presence of DCC for 4 hrs at 5OoC with the solution stirred gently. Then the solvent w a s removed by filtration. The residue was rinsed with 0.1 N acetic acid once and then with 1 0 0 mR of ethanol and finally dried in a desiccator The a m o u n t of unreacted ER w a s under a reduced pressure. determined by the absorption photometric measurements. The percentage of the amino propyl groups linked to ER, which was dependent on the volume of the THF solution of ER used, was evaluated from both elemental analysis and absorption photometric measurements. Both methods yielded the values in a reasonable agreement. When the surface concentration of ER was low, the absorption measurements gave more reliable results since they were not sensitive to remaining adsorbed solvent. Spectro-grade ethanol, THF, acetonitrile, ethylene glycol (E.G.), benzene and pxylene were used as received without further purification. Silicone oil supplied by Kyouei Oil Co.,Ltd. was of 300 cSt. 2.2 Fluorescence Lifetime Measurements on a Sinqle Microparticle The fluorescence lifetimes were measured by the timecorrelated single photon counting method with a mode-locked Ar' 2.

2.1

i;

95

laser as an excitation source. The experimental details were as have already been reported previously ( 1 ). The obtained fluorescence decay curves were analyzed by the iterative nonlinear least squares fit with the automatic shift. The computer program which can cope with the saw-tooth like decays was developed in our laboratory (7). The experimental procedure to pick up a single silica gel particle for observation is as follows. For the aerated samples, APS particles with ER bonded (APS-ER) were suspended in a solvent and sucked into a glass capillary. To make the deaerated samples, small amounts of APS-ER and solvent were degassed separately in an H-shaped cell with a side capillary tube. After cutting the cell off from a vacuum line, APS-ER and the solvent were mixed and then the mixture was introduced into the glass capillary tube which was finally chopped off from the cell. To freeze APS-ER at 77 K for degassing was thus avoided. The samples under a gas were prepared in a similar way. The glass capillaries thus prepared were mounted on the stage of an epi-illumination fluorescence microscope. By employing glass capillary tubes we were able to pick up a single particle in the solvent under the microscope without any difficulty. The fluorescence spectra on a single particle under a microscope were taken by the photon counting method with a 1 5 0 W xenon lamp as a light source (8). '

RESULTS AND DISCUSSION A relatively small percentage of the amino groups could be bonded to ER even when the concentration of ER used far exceeded the chemical equivalent. Table 1 presents the results obtained. Throughout t h e present study APS-ER ( V ) w a s used unless otherwise stated and it is denoted just by APS-ER for convenience. A pictorial representation of APS-ER is given in Fig.1. Electron micrographs (SEM and TEM) of APS are shown in Fig.2 to give an idea how the surface and inside of the particle look. TEM was 0 taken o n a ultra thin section (800 A i n thickness) of the particle. We can easily imagine that the microscopic environments around the ER moiety may be varied quite readily by immersing it in different kinds of solvents or exposing it to various gases. It is also possible to introduce hydrocarbon chains of a different length to modify silica gel itself. It is seen that even in the most concentrated case the average number of the amino groups coupled is not more than one out of 3.

96

TABLE 7 Surface concentration of bonded ER for the APS-ER's prepared. ER:APS in reaction mixture (by weight)

Elementa 1 ana 1y s i s

Absorption photometry

mo.i/m2

( X Io 9

110 150 41 13 9.3

1:2 (I) 1 : 2 0 (11) 1.100 111) 1I300 1 . 1 1 :400 V)

%a

mol/m2

3.8 5.1 1.4 0.4 0.3

160 -

( X Io 9

%a 5.5 1.9 -

57 -

a Percentage of the amino groups bonded to ER.

SOLVENT I

I

-C-

v,

0 r

4

rn

0 r L

z

I

4

y 2

m

3

y 2

HO-Si-OH OH

Fig. 1.

NH

I 0

OH

Pictorial representation of APS-ER in a solvent.

(a) Fig. 2. Electron micrographs of APS.

(b) SEM ( a ) and TEM (b).

97

ten. Therefore, the interaction between two ER moieties can be neglected. However, the fluorescence decay features definitely depend on the concentration of the bonded ER as are exemplified in Fig.3. The fluorescence decay curves are non-exponential and can be resolved into two components but not more than two. When the concentration is high ( A P S - E R ( I 1 ) ) the decay is dominated by the shorter component as is shown by the curve (b) in F i g . 3 . On the other hand, the fluorescence spectra of APS-ER ( 1 1 ) and ( V ) in E.G. ( Fig.4) are not appreciably different from the spectrum of ER in solution with the fluorescence maxima at around

0.0

I

5.0 T I E / “OsEcONDs

I

10.0

Fig. 3 . Fluorescence deca curves of a single APS-ER particle in aerated E.G. observed unJer, a microscope. APS-ER(V) (a) and APS-ERlTT)lb). The best fitted curves are shown by the-solid .-_ _ ,. - , lines through the dotts. The reduced residuals are shown only for the curve (a) f o r simplicity.

---.

98

500

550

600

650

WAVELENGTH L nmI Fi 4. Fluorescence s ectra of a single APSTER particle in E.G. APZIER ( V ) (a) and APS-E)R(II) (b) For comparison the spectrum of ER in E.G. is shown by the curve'(c).

5 5 8 nm. The emissions at the wavelengths longer than 530 nm were monitored for the decay measurements. The decay curve obtained for APS-ER particles suspended in E.G. in a rectangular cell (10x10 mm) was significantly different from that obtained for a single particle of the same kind. In the cell, the long-lived component is almost missing, though the decay still remains non-exponential. Therefore, it must be emphasized that the particles suspended in a cell or packed intoa column may smear out the true picture of the heterogeneous molecular environments. We have a feeling that the exciting light penetrates deeper into a particle under a microscope, thus uncovering a detailed picture of the interface of porous silica (see Fig.2).

Environments Influenced by Solvent The fluorescence lifetimes of ER measured in a variety of solvents are summarized in Table 2. The lifetime varies widely from 85 ps in water to 2.05 ns in acetonitrile. In any case the decay follows satisfactorily a single exponentia1,indicating the absence of any kind of complication in solution. In fact, the 3.1

99

shortening of the lifetime roughly parallels the blue shift of the absorption spectrum of ER (9). On the other hand, for APS-ER the fluorescence decay curve F(t) is always expressed by a double exponential. r(t) = A 1 exp(-t/rl ) + A2 exp(-t/r2). The obtained results a r e s u m m a r i z e d in Table 2 and the fluorescence decay curves of a single APS-ER particle immersed in THF and ethanol are compared in Fig.5. The magnitudes of the lifetimes for the short- and long-lived components are not much different between these two solvents, but a remarkable thing is a large difference in the fraction of each component. They are so

L n Z I

1

Fig. 5. Fluorescence decay curves of a single APS-ER. particle in ethanol (a) and THF (b). Both curves can be best fitted to a sum of two ex onentials but not more than two. The reduced residuals are s\own only for the curve (a) for simplicity.

100

TABLE 2 Fluorescence lifetimes and pre-ex onential factors of the double exponential decays of a single !PS-ER particle obtained under various conditions. Solvent

ERa T Ins

THF CH3 CN EtOH E.G. Hz0 S.0.b Benzene Benzene' n-Hexane p-Xylene Air Vacuum

1.91 f 0.03 2.05 f 0.03 0 . 6 4 2 0.01 0.39 f 0.01 0.085 t 0.004

Insoluble Insoluble Insoluble Insoluble Insoluble

APS-ER Ins

T~

~2 /ns 0.04 0.05

0.452 0.912 0.800 0.881

0.548 0.400 0.088 0.200 0.119

0.06 0.09 0.07 0.06 0.06

0.747 0.877 0.581 0.789 0.783

0.253 0.123 0.419 0.211 0.217

1.24 f 0.07 2.33 f 0.08

0.830 0.702

0.170 0.298

1.43 1.32 1.10 2.90 0.79

f

0.03 0.02

0.95 1.22 2.65 1.00 1.13

i f f f

f f f f f

0.06 0.03 0.03

0.60 t 0 . 0 3 0.56 f 0.04

A2

f 0.10 t 0.10 f 0.09

0.75 2 0.07 0.75 2 0 . 0 5 0.62 t 0.02 0.42 t 0 . 0 3 0.22 f 0.02 0.51 0.57 0.71 0.51 0.53

A1

f

k

0.600

a The fluorescence decays in solution follow single exponentials

satisfactorily. b Silicone oil. Under the deaerated conditions.

greatly different that the decay curves shown in Fig.5 are clearly distinguishable from each other. Except for E.G., the long-lived components of APS-ER in polar solvents appear to parallel the lifetimes of ER in homogeneous solutions. When it is taken into account that in protic solvents a rather short lifetime is realized for ER, the short-lived component ( " 5 0 0 ps) of APS-ER may reflect the presence of the bonded ER which experiences more polar (and/ or protic) environments governed presumably by the remaining silanol groups. It is interesting to find that in non-polar solvents such as benzene and p-xylene, the decay features (lifetime and fraction) are not much different from those observed in the air (i.e., in the absence of any solvent). In poor solvents like benzene it is very likely that the solvent molecules can neither penetrate into small cavities of silica gel nor get close to the ER moiety, thus leaving a gas-like atmosphere around the chromophore. Under the deaerated conditions, however, benzene can penetrate into the cavities. In fact, a rather large change in the fluorescence decay features was found upon degassing as is seen from Table 2. In water the photo-decomposition of APS-ER occurred very rapidly. For this it was not easy to obtain a very accurate fluorescence decay curve but the short-lived component is

101

TABLE 3 Fluorescence lifetimes and pre-exponential factors of APS-ER particle exposed to various gases. Gas

TI /ns

0.56 (400 torr) Moisture ( H z O ) 0.56 ~ 0.64 Xe (600 torr) Vacuum 0.56 0 2

a A I

/Tf'g,

arent

quenchin

0.04

f

f 0.06

f 0.04

0.04

f

- 1 /r(vacuumT.

rate

/ns

T~

1.21 f 0.04 2. 77 f 0 . 0 8 2.22 f 0.08 0.08 2.33

*

A1 0.654

0.589 0.702 0.702

constant ( X ~ O - S~ - l )

a

single

A2 0.346

0.411 0.298 0.298

4.0

f

0.3

-

~

~

calculated

from

At 20'C.

0 104

I

0.0

im

400

I

5.0

TIME /

I

10.0

"oSECOH]S

Fi 6. Fluorescence decay curves of a single APS-ER article The eBfect of unlgr oxygen (a) and deaerated ( b ) conditions. oxvaen is remarkable. The reduced residuals are shown onlv for th6 <decay curve (b) for simplicity.

102

as long as 200 ps, much longer than that in water. Being consistent with the results obtained for other solvents, this appears to reflect the fact that, even in water, the ER moiety of APS-ER is not under the full influence of water. 3.2 Effect of Gas Gaseous molecules, when they are adsorbed on silica surface or collide with the ER moiety, may well affect the fluorescence decay of APS-ER. The results obtained are summarized in Table 3. The two decay curves measured under oxygen and vacuum are compared in Fi9.6. Although ER is not quenched in solution measurably by dissolved oxygen, the effect of gaseous oxygen on the fluorescence decay of APS-ER is remarkable. The introduction of Xe (600 torr) did not lead to the quenching of the fluorescence of either component. This is not surprising since the four iodine atoms in ER are already sufficient enough to enhance the intersystem The apparent quenching rate constant by crossing process in ER. oxygen is also given in Table 3 . Dividing this pseudo first order rate constant by the concentration of oxygen (400 torr) gives the second order rate constant of 2 x l o l o M-lS-l , which is similar to the rate constant of the diffusion-controlled oxygen quenching well-documented in solution. The lifetime of the shortlived component seems to be too short to be quenched. Water moisture, which is always contained in the air, was proved not to have any effect in shortening the decay time. Thus, the adsorbed water molecules do not appear to assemble to form clusters large enough to provide a very polar atmosphere. This is supported by the fact that the addition of small amounts of water (up to 5 % ) to the THF solution of ER has practically no effect on the fluorescence lifetime of ER. Since water is a good coordinating molecule and is expected to be more "sticky" than oxygen, the effect of oxygen should be regarded to be collisional but is not due to complex formation. Since the gas phase data of ER, which does not have sufficient vapour pressure at accessible temperatures, is not available, it is difficult to say definitely, but from the pictorial representation of APS-ER given in Fig.1 the ER moiety of APS-ER may well be expected to be partially in the gas phase. Consequently, a high collision frequency of oxygen may be responsible for the shortened fluorescence lifetime of APS-ER in the air. Based on the refractive index correction for the radiative

lifetime (lo), the radiative lifetime in the gas phase T~ (hence the longest lifetime achievable in the gas phase) can be estimated by the following equations,

?,(gas)

2

n2Tr(solution), n: refractive index.

From the value of gf ( 7 1 and the lifetime in ethanol, the radiative lifetime in the gas phase is estimated as 12 ns. Even in the gas phase, some radiationless process may still be open to ER, the actual lifetime could be much shorter than 12 ns. However, the lifetime of the long-lived component in vacuo is longer than those measured in solution except for E.G. and it must be indicative of the existence of a sparse space around the ER moiety, which may provide the environments similar to the gas phase. Owing to a rapid development of the supersonic free jet technique, a wide variety of clusters, i.e., "a solvated molecule in the gas phase'' are now under intense investigation (11) and may give a clue to answer such an important question of microscopic environments in the near future. Gaseous oxygen seems to play another important role which might often be overlooked i n the study of interfaces. As mentioned before, benzene gives rise to a large effect on the lifetime only when t h e a i r is removed. The air s e e m s t o block the approach of solvent molecules toward the bonded chromophore in When the degassed sample was exposed to the air again, pores. the fluorescence decay resumed the previous features. Thus, the blocking process must be reversible. 4. CONCLUSIONS AND PROSPECTS

It is now seen that the fluorescence lifetime measurements on a single particle are much more informative than the similar measurements on a packed or suspended sample in a conventional cell. In fact, when the distribution of the particle size is rather broad, our technique will allow us to differentiate particle-size dependent phenomena from the others. The double exponential decays, the fraction of which sensitively depends on the nature of the solvent around the ER moiety, definitely tell us the existence of the two different kinds of molecular environments, one of which is governed mainly by the silica body itself and the other by solvent. However, the fact

104

that the lifetime of the long-lived component of APS-ER always takes a value appreciably different from the lifetime in solution indicates that the ER moiety surrounded by solvent molecules is still affected by silica to some extent. We have already shown that our lifetime-measurement apparatus and computer program can cope with multi-exponentials consisting of more than three components and can recover very reliable decay parameters ( 1 , 7 ) . Therefore, the fact that the double exponential decays were always recovered for APS-ER is neither fortuitous nor due to the limitation of the decay curve analysis, but must have a physical significance. The role of oxygen revealed in the present study is still puzzling since E R in solution is not quenched by oxygen significantly. Nevertheless, oxygen quenching of this type must always be taken into account when the photophysics of molecules in the interfaces is investigated. Wellner et al. examined the effect of pore size on oxygen quenching of pyrene adsorbed on silica surface and discussed several relevant mechanisms ( 1 2 ) . It is interesting to note that the apparent quenching rate constant which they obtained is of the same magnitude as ours, though the chromophores are different in the two cases. Extension of our technique described here is of course limited by various factors. The laser intensity cannot be increased greatly because of the damage of lenses and mirrors of the microscope. Therefore, the concentration of a fluorophore to be bonded as a probe must be rather high, especially when its fluorescence quantum yield is low. Thus, it is difficult to say definitely the smallest size of a particle which is amenable to the decay measurement, but in a fortunate case, a particle as small as 1 um can be examined (8). When a particle is larger than a few tens urn, it is also possible to examine spatially heterogeneous decays within the particle (see Fig.2 ). Our goal is to put molecules in a well specified and externally controllable environment and then to know how the fluorescence decay features are influenced. We believe that further elegant controls of the molecular environments, for instance, by changing temperature or extent of the modification of silica gel, will make it possible for us to understand more deeply what the microscopic molecular environments are and how they affect the fluorescence lifetime.

105

Acknowledgements T h e a u t h o r s a r e e x t r e m e l y g r a t e f u l t o P r o f e s s o r N . T a n a k a of

K y o t o I n s t i t u t e of T e c h n o l o g y f o r h i s g e n e r o u s g i f t o f APS a n d valuable discussions.

T h a n k s a r e a l s o d u e t o H - T s u c h i y a of N i t t o

T e c h n i c a l I n f o r m a t i o n C e n t e r Co.,Ltd.

for his

m e a s u r e m e n t s of SEM

a n d TEM of APS. REFERENCES 1

T.Minami, J.Lumin.,

M-Kawahigashi Y Saksi, 3 5 ( 1 9 8 6 ) 247-1253'.

2

R G Bo a r 1b 8 b - 1 3 8 4 :

3

J.K.Thomas,

4

C.H.Lochmiller (1987)1244-1245.

5

C.H.LochmGller 4322.

6

J.C.Sheehan

7

Y.Sakai

J.C.Thomas

and J.B.Callis,

J.Phys.Chem., and

S.S.Saavedra,

a n d G.P.Hess,

M.Kawahigashi

a n d S.Hirayama,

9

G.R.Fleming, G.W.Robinson,

A.W.E.Knight, J.Am.Chem.Soc.

S.Hirayama

56

J.Am.Chem.Soc.

J.Phys.Chem.,

(1984)

and D.Phillips,

90

I

109

(1986)4318-

7 7 ( 1 955) 1067.

J.Am.Chem.Soc., J.Lumin.,

8

10

Anal-Chem.,

91 ( 1 9 8 7 ) 2 6 7 - 2 7 6 .

a n d M.L.Hunnicutt,

and S . H i r a y a m a ,

KShimamoto and S . H i r a y a m a ,

3 9 ( 1 9 8 8 ) 1 4 5 - 1 51.

unpublished results.

J.M.Morris I

R.J.S M o r r i s o n 9 9 (1977)'4306-4311.

J.Photochem.,

1 2 (1980)

and

1 3 9 - 1 45.

!

11

a ) S.Hirayama K . S o b a t a k e and K . T a b a y a s h i Chem.Ph s . L e t t . , ( b ) S . H i r a y a m a , F.Tanak& a n d K S X o b a t a k e , 21 ( 1 9 8 5 ) 228-'232. J.Chem.Soc., Faraday Trans 2 in r e s s ( c ) E.A.Mangle a n d ~ . ~ . ~ o pJ .pP h, y s . C h e m . , 9 0 c i 9'86) a t ; 2 - a o j .

12

E.Wellner, D.Ro'anski, M.Ottolenghi, J.Am.Chem.Soc., 1 0 9 ( 19 8 7 ) 575-56.

D.Huppert a n d D.Avnir,

106

PHOTOACOUSTIC AND FLUORESCENCE MEASUREMENTS OF ENERGY TRANSFER IN ADSORPTION LAYERS

H. D. BREUER

1. INTRODUCTION

Transfer of electronic excitation energy has received much attention as a tool for understanding complex molecular aggregates. Time-resolved fluorescence studies of energy transfer can provide information concernig distances and densities of molecules interacting with each other. Some observations must be made concerning the nomenclature: Intermolecular energy transfer is a term used to describe the transfer of electronic excitation energy from one molecule to another molecule of different species. Transfer between identical molecules is called energy migration. Transfer of energy between different parts of one molecule is an intramolecular process. It will not be discussed here. Transfer of electronic excitation energy from a donor molecule, D, to an acceptor molecule, A, may be represented by D*+A-

D+A*

where the asterisk denotes an electronically excited state. The result is that the emission of D is quenched and the emission of A is sensitized. The first observations of energy transfer were made in gas phase atomic systems. There have been many publication dealing with similar studies on polyatomic systems in the gas phase, liquid phase and in molecular crystals. Only very little, however, is known about transfer processes and mechanisms when donor and acceptor molecules are adsorbed at the surface of a solid (13). In this paper we present some photoacoustic measurements and compare the results with fluorescence decay time measurements, As model substances we have chosen four different donor dyes and two acceptor dyes. The dyes were adsorbed on silica. There are several processes which may be responsable for the transfer of electronic excitation energy:

107

1.1 Radiative transfer

This is the simplest type of transfer. The efficiency depends only on the extent of overlap of the donor emission spectrum with the acceptor absorption spectrum. Ultraviolet or visible light is emitted by the excited donor molecule and absorbed by the acceptor molecule in a two-step process. Increasing the acceptor concentration results in a decrease of fluorescence yield of the donor but the decay time of the donor fluorescence remains unaffected. The range of energy transfer by this mechanism depends on the undisturbed path of the emitted light and falls off with distance as R-2. 1.2 Radiationless transfer

In contrast to radiative transfer nonradiative resonance transfer is a single step process in which the donor is de-excited and the acceptor is excited. Thus both donor fluorescence yield and fluorescence lifetime are affected by interaction with the acceptor. Resonance transfer can only occur if donor emission and acceptor absorption at least partially overlap in wavelength. Therefore the transfer probability increases with spectral overlap. s in Since vibrational relaxation steps require about contrast to 10-9 - 10-8 s for emission, energy transfer will start from the vibrational ground state of the excited donor. The reverse transfer process from an excited acceptor to a de-excited donor need not be considered here because the acceptor and donor are, in general, not in resonance after vibrational relaxation of the acceptor.

The formulation of the electronic interaction between donor and acceptor can be separated into a long-range Coulomb term and a short-range exchange term. The Coulomb term may be represented by a multipole expansion. In this expansion the first term describes the dipole-dipole interaction, the second term the dipole-quadrupole interaction and so on. If the optical transitions involved are permitted by the selection rules, dipole-dipole interaction contributes most to the energy transfer process. In this case the higher order terms and the exchange term may be neglected. Dipole-dipole interaction varies with distance as R-3 , the probability for transfer by this mechanism is proportional to the square of this quantity, i.e. R-6.

108

The theory of electronic energy transfer in terms of a dipoledipole interaction has been developed by Forster (4). In this theory the rate constant for energy transfer, kD-A, is given by

- 9000 5 In 410

kD.”

- 128

R

K2

n NA

r D R6

J fD(u:~A(u)

dv

where n is the refractive index, K is an orientation factor, NA is the Avogadro number and T O is the natural fluorescence lifetime of the donor in absence of an acceptor. R is the distance between donor and acceptor. fD(u) is the normalized spectral distribution of the donor fluorescence and EA(v) is the extinction coefficient of the acceptor. Equation [ll can be written as

with

Q, is the fluorescence quantum yield of the donor in the absence of an acceptor. It follows from equations [21 and [31 that Ro can be identified with a critical distance for which the probability for energy transfer is equal to that of deactivation of the donor by all other processes. Following the literature values for Ro will be given in A . Ro increases with the overlap of the spectra. Since absorption spectra of adsorbed molecules are, in general, broader than the corresponding spectra in the gas phase or in solution there will be a different degree of overlap and thus different Ro values.

The critical distance can be related to another experimentally important quantity, the critical concentration, Co,

109

-

3

co - 2 r 3’2

L41

NA R i

The influence of radiationless energy transfer on the fluorescence lifetime T O in three dimensions is well known and described by the following expression

D(t)

=

with y

exp

[ -$-

1/2

= C/Co.

analytical decay curve for two dimensions can be derived by integrating the randomly distributed acceptor sites (5) An

Here, y is the ratio n/no, where n is the number of acceptor molecules m-2 and no is a critical number corresponding to the critical concentration. Since the surface of silica used as support in this study has a complex structure we also adopted the model suggested by Klafter and Blumen (6). In this model the survival probability p(t) of an excited donor in a fractal environment is given by

is a measure of the surface concentration of the acceptor molecules, D can be regarded as the fractal dimension of the distribution of acceptor molecules on the surface around a donor molecule, and s = 6 for dipole-dipole interactions.

110 2.

EXPERIMENTAL 2.1 Sample preparation

The choice of donor and acceptor dyes used in this study was determined by several criteria: Surface concentrations had to be varied over as large a range a s possible. For this reason only cationic dyes could be employed. The spectral overlap between donor emission and acceptor absorption also had to be as large as possible. The donor dyes should have a sufficiently high fluorescence quantum yield while the deactivation of the acceptor dyes should be predominantly radiationless. The dyes employed in this study were rhodamine B (RHB), pyronine G (PYG) , lissamine-rhodamine (LIS) and acridine red (ACR) as donors and thionine (THI) and alfazurine (ALF) as acceptors. The dyes were adsorbed from aqueous solution onto silica having a specific surface area of 300 m2 9-l and a mean pore diameter of 35 d Surface concentrations were determined by measuring the absorptions of the solutions before and after the adsorption.

.

2.2 ApDaratus In most experiments until now energy transfer has been observed by fluorescence measurements. Exciting the donor causes fluorescence of the acceptor. This is true for the gas phase, the liquid phase and for crystals, but only if the acceptor exhibits fluorescence. In adsorption systems fluorescence experiments are possible but more difficult mainly because of reflections and stray light effects. On the other hand, Photoacoustic Spectroscopy ( P A S ) is a very powerful tool for the study of photophysical and photochemical phenomena in adsorption layers. In this technique, the incident monochromatic radiation is timemodulated. The radiationless de-excited part of the absorbed energy is converted into heat which in turn in a closed volume, the photoacoustic cell, gives rise to pressure fluctuations which are detected by a sensitive microphone. There are no stray light problems since only absorbed energy contributes to the photoacoustic signal. The spectrum obtained by this technique resembles the normal optical absorption spectrum in that, in general, it exhibits the same wavelength dependence. The main difference is that the rela-

111

tive intensity is reduced wherever a channel for radiative de-excitation exists. This implies that strongly fluorescing molecules will show only poor photoacoustic spectra. On the other hand, if an additional channel for radiationless de-excitation is opened, e.g. by energy transfer to co-adsorbed nonfluorescing molecules the photoacoustic signal will be enhanced at the wavelength of excitation. So, in contrast to fluorescence measurements the acceptor should mainly deactivate by radiationless transitions. The photoacoustic spectrometer we used has been described elsewhere (7). In Figure 1 the influence of energy transfer on the PA-signal is demonstrated with rhodamine B as donor and thionine as acceptor. Curve 1 is the spectrum of the pure donor dye at a surface coverage of 1.34*10-7 mole m-2. Keeping the donor concentration constant and coadsorbing acceptor molecules results in an increase of the PA signal at the wavelength of the donor absorption. Curve 2 is the spectrum obtained by co-adsorption of 5.36*10-8 mole m-2 thionine. The shoulder at 600 nm corresponds to the thionine absorption. The highest possible enhancement of the donor signal is shown in Curve 3. Here the acceptor concentration is 8.71*10-* mole m-2. A nitrogen pumped dye laser and boxcar averaging were employed for the fluorescence decay time measurements.

)

600 Wavelength [nml

500

709

Fig. 1. Photoacoustic spectrum of RHB and THI on silica. For explanations see text.

112

RESULTS AND DISCUSSION

3.

3 . 1 Photoacoustic spectra at constant surface coverage A compilation of critical distances and critical concentrations calculated from the spectral data of the adsorbed dyes is given in Tab. 1.

TABLE 1

Calculated critical distances and critical concentrations.

Alfazurine

Thionine

RHB PYG LIS ACR

62.1 57.5 61.2 62.5

4.095 3.953 4.781 4.609

58.8 54.4 62.1 59.9

4.122 4.064 4.529 4.902

The enhancement of the photoacoustic signal by energy transfer is illustrated in Fig. 2 with RHB as donor and THI as acceptor. Here the total surface concentration was kept constant at mole m-2 (except for Curve 1). The donor concentration was varied between l o m 6 and mole m-'; the acceptor concentration was varied accordingly. In this Figure Curve 1 is the intensity of the pure donor dye as a function of surface coverage. In this curve the donor concentration corresponds to the donor mole fractions in the mixtures. Curve 2 is the observed intensity when acceptor molecules are added. This curve can be described by the empirical formula

I(C)

=

.I q

D' 'max 'A

181

where 10 is the intensity of the donor in the absence of an acceptor, XA and XD are the mole fractions of acceptor and donor respectively, Xmax is the donor mole fraction at the maximum of enhancement and q is a measure of spectral overlap between donor emission and acceptor absorption. Since the spectra of adsorbed molecules are, in general, different in shape than they

113

I PAS

0

x [RHBI

1

Fig. 2. Enhancement of the photoacoustic signal by energy transfer from RHB to "HI. are in solution, the spectral overlap has to be determined from the photoacoustic and fluorescence spectra of the adsorbed molecules. Curve 3 corresponds to the calculated intensities.

As long as XD<XA there is good agreement between experimental and theoretical curves. However, there are differences at higher donor concentrations. In the case XWXA there will always be an acceptor within the critical distance of an excited donor molecule. At higher donor and lower acceptor concentrations the probability for finding an acceptor and thus the probability for deactivation by energy transfer decreases. 3.2 Photoacoustic spectra at constant donor concentrations

In order to demonstrate the influence of donor-acceptor distance more clearly Fig. 3 illustrates the enhancement of the photoacoustic signal that is observed when the donor concentration is held constant at 2.78*10'8 mole m-2 and the acceptor concentration is varied as indicated. The donor- donor distance at the surface concentration given above is about 4 4 so that energy migration can be neglected. It is evident that in all cases the addition of an acceptor leads to an increase in the photoacoustic signal. The threshold is at about lo'* mole m-2 corresponding to an intermolecular dis-

114

.

tance of 35 The final values of enhancement are 2.28 for RHB, 2.20 for ACR, 1.98 for LIS and 1.69 for PYG. If the concentration at which these quantities have reached half their final values are taken as a measure of the critical distance, Ro, the following values are obtained: RHB : 31 , ACR : 30 i, L I S : 30 and PYG : 28 These values are considerably lower than those obtained in solution.

.

Fig. 3. Enhancement of the photoacoustic signal by energy transfer to THI.1: PYC, 2: RHB, 3: ACR, 4: LIS.

The influence of acceptor concentration on energy transfer can be demonstrated if fluorescence quantum yield is plotted against the reduced acceptor concentration. Quantum yields of adsorbed molecules can be derived from photoacoustic measurements (8). Figures 4 and 5 illustrate the concentration dependence of donor fluorescence as a result of energy transfer. These curves will be compared to the concentrational dependence of the donor lifetime in Section 3.3 3.3. Fluorescence lifetime measurements

The time-resolved fluorescence decay of donors without and in the presence of acceptor molecules in various concentrations were simulated by using Eq. I51 , [6 1 , and [ 7 1 The goodness of the fit was determined by the statistical parameters chi squared( acetone.

10

0

20

Amount adsorbed, F i g . 2.

2-pentanone>2-butanone

Although t h e t r u e n a t u r e o f s u c h a d i f f e r e n c e is u n c l e a r ,

30 lom6 mol

R e l a t i o n s h i p bwteen absorbance and amounts adsorbed o f t h e

*

a l k y l k e t o n e s a t w a v e l e n g t h o f maximum a b s o r p t i o n o f n , x t r a n s i t i o n ( a : 2-pentanone,

b: 2-butanone,

c: a c e t o n e ) .

122

i t seems t o b e c l o s e l y a s s o c i a t e d w i t h t h e a d s o r p t i o n s t r e n g t h of t h e s e A marked

molecules.

reduction

i n t h e e x t i n c t i o n c o e f f i c i e n t on

a d s o r p t i o n h a s been r e p o r t e d by a number o f w o r k e r s (16).

3.

EXCITED STATES OF ALKYL KETONES ADSORBED ON VYCOR GLASS F i g u r e 3 shows t h e e m i s s i o n s p e c t r u m o f a c e t o n e a d s o r b e d on Vycor

g l a s s and t h e e f f e c t o f t h e a d d i t i o n of NO m o l e c u l e s upon t h e e m i s s i o n . I t i s s e e n t h a t t h e a d d i t i o n o f NO m o l e c u l e s l e a d s t o t h e q u e n c h i n g o f t h e e m i s s i o n , t o a n e x t e n t d e p e n d i n g on t h e amount o f added NO.

By t h e

a d d i t i o n o f 02, t h e e m i s s i o n was a l s o q u e n c h e d b u t w i t h a d i f f e r e n t e f f i c i e n c y from t h a t w i t h NO m o l e c u l e s .

From t h e s e r e s u l t s ,

together

w i t h a good a c c o r d a n c e o f t h e e m i s s i o n w i t h t h e p h o s p h o r e s c e n c e o f a c e t o n e m e a s u r e d a t 77 K i n EPA m a t r i x ( 2 3 ) , t h e o b s e r v e d e m i s s i o n i s e a s i l y a s s i g n e d t o t h e p h o s p h o r e s c e n c e o f a c e t o n e m o l e c u l e s a d s o r b e d on V y c o r g l a s s (5,

7).

A s i m i l a r p h o s p h o r e s c e n c e s p e c t r u m was a l s o

o b s e r v e d w i t h a d s o r b e d 2-butanone b e i n g 2-pentanone>2-butanone

w

and 2-pentanone,

acetone.

t h e order of y i e l d s

Phosphorescence s p e c t r a of

a d s o r b e d k e t o n e s e x h i b i t e d b l u e s h i f t s a s compared w i t h t h e c o r r e s p o n d i n g s p e c t r a i n EPA m a t r i x , b e i n g a t t r i b u t a b l e t o hydrogen b o n d i n g , a s i n

a manner similar t o t h o s e o b s e r v e d w i t h t h e UV a b s o r p t i o n s p e c t r a .

Wavelength, nm

F i g . 3.

Phosphorescence spectra o f a c e t o n e a d s o r b e d on Vycor g l a s s a t

77 K i n t h e

a b s e n c e ( a ) and p r e s e n c e o f NO m o l e c u l e s (b-e) (amount of

a d s o r b e d a c e t o n e , 2.8 umol, 0.49,

c: 0.91,

d : 2.02,

a m o u n t s o f added NO m o l e c u l e s ( i n T o r r ) , b:

e : 4.04).

123

Recently,

t h e a u t h o r h a s i n v e s t i g a t e d t h e d e a c t i v a t i o n pathways o f

t h e a l k y l k e t o n e s a d s o r b e d on Vycor, and i n d i c a t e d t h a t t h e d e a c t i v a t i o n p a t h w a y s i n v o l v i n g s u r f a c e OH g r o u p s - a s s i s t e d

photoenol i z a t i o n

play a

d e c i s i v e role i n d e t e r m i n i n g t h e t r i p l e t l i f e t i m e o f a c e t o n e m o l e c u l e s a d s o r b e d on Vycor g l a s s .

The f o l l o w i n g r e a c t i o n sc h e m e h a s b e e n

p r o p o s e d t o s h o w t h e f a t e o f t h e e x c i t e d t r i p l e t s t a t e of a c e t o n e m o l e c u l e s a d s o r b e d on Vycor g l a s s by a hydrogen bonding.

Photolysis

OD

I

T

Si R e a c t i o n scheme 1.

-..-.-/Deactivation pathways of t h e e x c i t e d t r i p l e t s of

a c e t o n e i n v o l v i n g s u r f a c e hydroxyl groups-assisted r e a c t i o n s (KO: g r o u n d s t a t e o f k e t o n e s , K*3: ketones,

deactivation

e x c i t e d t r i p l e t s t a t e of

s u r f a c e h y d r o x y l g r o u p s are d e u t e r a t e d t o show t h e H-D photo-

exchange r e a c t i o n ) . The d e c a y c u r v e o f t h e p h o s p h o r e s c e n c e o f a c e t o n e a d s o r b e d on Vycor g l a s s w a s f o u n d t o b e b i - e x p o n e n t i a l w i t h lifetimes of

= 1.1

,us.

A s o f t e n suggested, (4)

TI =

6.7

and

xz

t h e i d e n t i f i c a t i o n of s i n g l e or

mu1 t i - e x p o n e n t i a l d e c a y o f a d s o r b e d m o l e c u l e s is a q u i t e c o m p l i c a t e d problem.

.

I n t h e p r e s e n t case, h o w e v e r , t h e s e c o m p o n e n t s seem t o b e

well a t t r i b u t a b l e t o t h e e x c i t e d t r i p l e t a c e t o n e s a d s o r b e d on s u r f a c e

--,Si-OH a n d

>B-OH

groups,

respectively.

Indeed, ketone molecules

a d s o r b e d o n t h e s u r f a c e S S i - O H a n d Z B - O H g r o u p s by h y d r o g e n b o n d i n g h a v e d i f f e r e n t l i f e t i m e s due t o t h e d i f f e r e n t a c i d i t y o f t h e s e h y d r o x y l g r o u p s (24).

The l i f e t i m e s o f t h e e x c i t e d t r i p l e t o f a c e t o n e m o l e c u l e s

a d s o r b e d on V y c o r g l a s s a r e f o u n d t o b e

much s h o r t e r t h a n t h o s e i n

water ( 2 3 AE) and i n a c e t o n i t r i l e (44 4 s ) (25).

Much h i g h e r r e a c t i v i t y

o f s u r f a c e OH g r o u p s o n V y c o r g l a s s t h a n t h a t o f water r e s u l t s i n m o r e e f f i c i e n t deactivation of t h e excited t r i p l e t state of acetone molecules a d s o r b e d on Vycor g l a s s .

124 4.

PHOTOLYSES OF ALKYL KETONES ADSORBED ON VYCOR GLASS 4.1. Features of t h e Photolyses R e s u l t s o f t h e p h o t o l y s e s o f a c e t o n e , 2-butanone,

a d s o r b e d o n V y c o r g l a s s a r e s h o w n i n T a b l e 2.

and 2-pentanone

I t i s w e l l known t h a t

a l k y l k e t o n e s w i t h d - h y d r o g e n atoms, s u c h a s 2 - p e n t a n o n e , u n d e r g o t h e N o r r i s h Type I 1 p r o c e s s e s ( i n t r a m o l e c u l a r e l i m i n a t i o n ) a s well a s t h e N o r r i s h Type I p r o c e s s e s ( d - c l e a v a g e i n t o r a d i c a l p a i r s ) , a s s h o w n i n t h e f o l l o w i n g r e a c t i o n mechanisms.

I n t h e g a s p h a s e p h o t o l y s i s o f 2-

p e n t a n o n e a t room t e m p e r a t u r e , t h e amount o f p r o d u c t s d e r i v e d from t h e T y p e I p r o c e s s e s is l e s s t h a n 5-15% of t h a t d e r i v e d f r o m t h e Type I 1 process (26).

A s s e e n i n T a b l e 2 , t h e r a t e o f C3H8 f o r m a t i o n is more

t h a n 75% t h a t o f C2H4 f o r m a t i o n . TABLE 2 R e s u l t s o f t h e p h o t o t o l y s i s o f t h e a l k y l k e t o n e s a d s o r b e d o n Vycor g l a s s

a t 298 K -

Compounds

Y i e l d s o f major p r o d u c t s

Amounts a d s o r b e d mol/g)

(10-5 m l / h ) -_

Acetone

4.73

CH4 ( 1 2 . 0 ) ,

C2H6 ( 8 . 0 )

2-Butanone

3.97

CH4 ( 3 0 . 0 ) ,

C2Hg ( 9 3 0 )

2-Pentanone

3.30

CH4 ( 5 0 . 0 ) , C2H4 ( 3 2 0 0 ) , C3Hg ( 2 4 0 0 )

A s shown i n T a b l e 2 , k e t o n e s c a n b e compared: p e n t a n o n e (202).

t h e efficiencies of the photolyses of these a c e t o n e (l.O),

A marked d i f f e r e n c e

2-butanone

(34.3),

a n d 2-

is s e e n i n t h e i r e f f i c i e n c i e s ,

i n c o n t r a s t w i t h e s s e n t i a l l y t h e same e f f i c i e n c i e s o b s e r v e d f o r t h e g a s phase p h o t o l y s e s o f t h e s e ketones (6).

From t h e s e ,

together with t h e

r e s u l t s m e n t i o n e d i n s e c t i o n 3, t h e f o l l o w i n g c o n c l u s i o n emerges.

The

p h o t o c h e m i c a l r e a c t i o n e f f i c i e n c i e s of a l k y l k e t o n e s a d s o r b e d o n Vycor

g l a s s are d e c r e a s e d on a d s o r p t i o n owing t o t h e i n c r e a s e i n t h e i r e f f i c i e n c i e s of r a d i a t i o n l e s s d e a c t i v a t i o n and also t o t h e d e c r e a s e i n their extinction coefficients.

T h u s , t h e more b l u e s h i f s , i. e., t h e

more s t r o n g l y hydrogen bonded a k e t o n e molecule is, t h e more e f f i c i e n t r a d i a t i o n l e s s d e a c t i v a t i o n becomes. F i g u r e 4 s h o w s t h e r a t e s o f C3H8

and C2H4 f o r m a t i o n v e r s u s t h e

amount o f 2 - p e n t a n o n e a d s o r b e d on Vycor g l a s s .

I t is s e e n t h a t t h e

r a t e s o f f o r m a t i o n i n c r e a s e by i n c r e a s i n g t h e amount a d s o r b e d .

In t h e

r a n g e of small amounts adsorbed, t h e e x t e n t of an i n c r e a s e is l a r g e ,

125

Amount adsorbed, Fig.

4.

mol/g

E f f e c t o f t h e a m o u n t a d s o r b e d u p o n t h e p h o t o l y s i s o f 2-

p e n t a n o n e o n Vycor glass a t 298 K (propane:

Type I r e a c t i o n ,

ethylene:

Type I 1 r e a c t i o n ) . d e c r e a s i n g w i t h i n c r e a s i n g t h e amount a d s o r b e d .

Such a t r e n d is more

s i g n i f i c a n t f o r t h e C3H8 t h a n f o r t h e C2H4 f o r m a t i o n .

r a t i o o f C3H8

Accordingly, t h e

( T y p e I ) t o C2H4 ( T y p e 1 1 ) d e c r e a s e s by i n c r e a s i n g t h e

amount a d s o r b e d , f i n a l l y a p p r o c h i n g 0.4. A c c o r d i n g t o Wagner e t a l . , ( 2 7 ) t h e r a t e o f t h e Type I 1 r e a c t i o n i n c r e a s e s by i n c r e a s i n g t h e p o l a r i t y o f t h e s o l v e n t owing t o t h e s t a b i l i z a t i o n of the 1,4-biradical r e t u r n o f t h e {-hydrogen

intermediates.

Probability for

i n t h e 1 , 4 - b i r a d i c a l s d e c r e a s e s due t o hydrogen

b o n d i n g b e t w e e n t h e p o l a r s o l v e n t a n d t h e h y d r o x y l g r o u p o f t h e 1,4biradicals.

A s w i l l b e d i s c u s s e d i n s e c t i o n 4.3, t h e e f f e c t s o f t h e

s u r f a c e p r e t r e a t m e n t s u p o n t h e y i e l d s o f t h e Type I 1 r e a c t i o n o f a d s o r b e d 2 - p e n t a n o n e i s i n t e r p r e t e d on a s i m i l a r b a s i s .

I t h a s also

b e e n r e p o r t e d t h a t t h e r a t e o f t h e Type I r e a c t i o n i n c r e a s e s by i n c r e a s i n g t h e p o l a r i t y of t h e s o l v e n t due t o t h e i n c r e a s e i n s t a b i l i z a t i o n o f t h e r a d i c a l s , which r e s u l t s i n t h e s u p p r e s s i o n o f t h e r e c o m b i n a t i o n o f t h e ( a l k y l - a c e t y l ) r a d i c a l p a i r s , a s well a s t h e i n c r e a s e d probability ofq-cleavage

i n t h e e x c i t e d states.

I n t h e r a n g e of t h e a m o u n t a d s o r b e d a b o u t 9.0 x

moljg, t h e

m a g n i t u d e o f t h e b l u e s h i f t o f t h e n , d t r a n s i t i o n , a s well a s t h e r a t i o o f t h e y i e l d f o r t h e Type I t o t h a t f o r t h e Type I 1 r e a c t i o n was e s s e n t i a l l y t h e same a s t h o s e o b s e r v e d i n w a t e r s o l u t i o n ( 1 1 ) .

These

r e s u l t s i n d i c a t e t h a t , a t least i n t h e r a n g e o f a small amount a d s o r b e d , an enhancement o f t h e Type I r e a c t i o n i n t h e a d s o r b e d l a y e r arises f r o m

126 t h e h i g h e r p o l a r i t y o f t h e surfaces which is r e f l e c t e d by t h e m a g n i t u d e of t h e blue s h i f t . The p h o t o l y s e s o f a d s o r b e d a l k y l k e t o n e s i n t h e p r e s e n c e o f NO m o l e c u l e s h a v e been i n v e s t i g a t e d .

R e s u l t s of p h o t o l y s i s o f 2-pentanone

i n t h e p r e s e n c e o f NO m o l e c u l e s are shown i n Fig. 5.

In t h e photolysis

t h e y i e l d s o f C2H4 f o r m a t i o n ( T y p e I 1 p r o c e s s ) a r e

of 2 - p e n t a n o n e ,

d e c r e a s e d by i n c r e a s i n g t h e NO p r e s s u r e , l e v e l i n g o f f t o a c o n s t a n t C o n s i d e r i n g t h a t NO m o l e c u l e s are a n e f f i c i e n t t r i p l e t q u e n c h e r

value.

and t h e r e is a marked d i f f e r e n c e i n t h e i r r e a c t i v i t y t o w a r d t h e e x c i t e d s i n g l e t and t r i p l e t s t a t e s , t h e amount o f nonquenchable reaction ( a b o u t 45%) is a t t r i b u t e d t o t h e r e a c t i o n from t h e e x c i t e d s i n g l e t state.

The

r e m a i n d e r o f t h e r e a c t i o n , (55%), o c c u r s f r o m t h e e x c i t e d t r i p l e t s t a t e , though t h e o b s e r v e d e m i s s i o n s p e c t r u m o f adsorbed 2-pentanone c o m p l e t e l y a t t r i b u t e d t o t h e phosphorescence.

is

Ausloos, e t al. (28) have

s t u d i e d t h e p h o t o l y s i s of 2-pentanone i n t h e p r e s e n c e o f O2 and o b s e r v e d t h a t t h e nonquenchable f r a c t i o n of C2H4 f o r m a t i o n is 43%, a t t r i b u t i n g it t o t h e r e a c t i o n from t h e e x c i t e d s i n g l e t s t a t e .

As s e e n i n

t h e y i e l d o f C3H8 f o r m a t i o n i n t h e p h o t o l y s i s of 2-pentanone, butanone,

and CH4 and C2H6

F i g . 5,

C2H6 i n 2-

i n a c e t o n e d e c r e a s e d m a r k e d l y by i n c r e a s i n g

a d d e d NO p r e s s u r e , f i n a l l y a p p r o a c h i n g z e r o a r o u n d 0.3 T o r r f o r C3H8,

1.0 T o r r f o r C2H6,

0

0.2

0.4

a n d 0.5 Torr f o r C2H6 a n d 8.3 T o r r f o r C H 4 ,

0.6

08

Initial NO Pressure, Fig. 5.

40

as

50

Ton

E f f e c t s of t h e a d d i t i o n o f NO m o l e c u l e s upon t h e p h o t o l y s i s o f

2-pentanone

a d s o r b e d on Vycor g l a s s a t 2 9 8 K ( e t h y l e n e a n d p r o p a n e

formations)

and 2-butanone ( e t h a n e f o r m a t i o n ) ( a m o u n t o f 2 - p e n t a n o n e

a d s o r b e d : 3.29 x

mol/g,

amount o f 2-butanone:

3.99 x

loq5

mol/g),

expected from t h e a c t i o n of NO molecules as a r a d i c a l scavenger as well a s an e x c i t e d t r i p l e t quencher. I t i s worth mentioning t h a t a n e g l i g i b l e formation of CO o c c u r s i n t h e p h o t o l y s e s of adsorbed ketones.

This would be a s s o c i a t e d w i t h t h e

f a c t t h a t a c e t y l r a d i c a l s escaped from t h e r e a c t i o n w i t h p a i r e d a l k y l r a d i c a l s are bound t o t h e s u r f a c e s ,

rearranging t o stable surface

complexes. E f f e c t of S u r f a c e OH Groups upon t h e P h o t o l y s i s

4.2.

F i g u r e 6 shows t h e y i e l d s of CH4 and CZHs i n t h e p h o t o l y s i s o f a c e t o n e v e r s u s t h e amount adsorbed on Vycor g l a s s .

I t is seen t h a t CHq

f o r m a t i o n i n c r e a s e s w i t h i n c r e a s i n g t h e amount o f a c e t o n e a d s o r b e d , w h i l e C2H6 f o r m a t i o n a p p e a r s t o l e v e l o f f t o a c o n s t a n t v a l u e .

The

r a t i o o f t h e y i e l d o f CH4 t o t h a t o f C2H6 r a n g e s from 0.6 t o 4.3, b e i n g l a r g e r t h a n t h e corresponding v a l u e i n t h e g a s phase p h o t o l y s i s (8). The CDQH c o n t e n t o f m e t h a n e f o r m e d i n t h e p h o t o l y s i s o f a c e t o n e - d 6 adsorbed on Vycor g l a s s has been

investigated.

A s shown i n Fig. 6, it

was f o u n d t h a t a l a r g e f r a c t i o n o f t h e m e t h a n e c o n s i s t s o f CD3H, There is

d e c r e a s i n g by i n c r e a s i n g t h e amount of acetone-d6 adsorbed.

no d o u b t t h a t l i g h t hydrogen i n t h e m e t h a n e comes from t h e s u r f a c e OH groups.

The hydrogen atom a b s t r a c t i o n r e a c t i o n f r o m t h e s u r f a c e OH

groups by methyl r a d i c a l s , i. e., CD,

+

+

(+~~-) (+si-b), NCD~H

o c c u r s on t h e s u r f a c e o f Vycor g l a s s .

5

Amount of adsorbed, I0 ml/g

Fig. 6.

E f f e c t of t h e amount adsorbed upon t h e p h o t o l y s i s of acetone-d6

on Vycor g l a s s a t 298 K and t h e c o n t e n t of CD3H i n t h e produced methane.

128

The a c t i v a t i o n e n e r g y f o r t h i s h y d r o g e n a b s t r a c t i o n r e a c t i o n was d e t e r m i n e d t o b e l e s s t h a n 9 k c a l / m o l , b e i n g smaller t h a n t h e corres p o n d i n g v a l u e f o r t h e hydrogen atom a b s t r a c t i o n from a c e t o n e m o l e c u l e s

(8).

T h e r e f o r e , t h e hydrogen atom a b s t r a c t i o n f r o m s u r f a c e OH g r o u p s on

V y c o r g l a s s is e x p e c t e d t o o c c u r more e a s i l y .

T h e r e is some p o s s i -

l i g h t h y d r o g e n atoms a r e t r a n s f e r r e d i n t o a c e t o n e - d 6

bility that

m o l e c u l e s by t h e H-D exchange r e a c t i o n b e t w e e n t h e s u r f a c e OH g r o u p s and acetone-d6

molecules ( 5 ) .

H o w e v e r , t h i s i s e x c l u d e d s i n c e t h e H-D

exchange r e a c t i o n is f o u n d t o b e much slower e v e n u n d e r UV i r r a d i a t i o n . The o c c u r r e n c e o f hydrogen atom a b s t r a c t i o n from t h e s u r f a c e OH g r o u p s on Vycor g l a s s h a s i n d i c a t e d t h a t t h e s u r f a c e s p l a y a s i g n i f i c a n t role i n t h e p h o t o l y s e s o f a d s o r b e d a l k y l k e t o n e s n o t o n l y as a n a d s o r b e n t b u t a l s o a s a reactant. 4.3.

E f f e c t s o f S u r f a c e P r e t r e a t m e n t upon t h e P h o t o l y s i s

As d e s c r i b e d above,

blue shifts.

a l k y l k e t o n e s a d s o r b e d on Vycor g l a s s e x h i b i t

F i g u r e 7 s h o w s t h e m a g n i t u d e of b l u e s h i f t s o f t h e n , $

t r a n s i t i o n o f a d s o r b e d 2-pentanone t e m p e r a t u r e o f Vycor g l a s s .

and

2-butanone v e r s u s t h e d e g a s s i n g

I t is s e e n t h a t t h e magnitude o f b l u e

s h i f t s d e c r e a s e by r a i s i n g t h e d e g a s s i n g t e m p e r a t u r e .

Since alkyl

k e t o n e s a r e a d s o r b e d on Vycor g l a s s by h y d r o g e n b o n d i n g b e t w e e n t h e s u r f a c e OH g r o u p s a n d

C=O g r o u p s of t h e k e t o n e s , s u c h a d e c r e a s e i n

b l u e s h i f t s w i t h i n c r e a s i n g t h e d e g a s s i n g t e m p e r a t u r e o f Vycor g l a s s is

273 473 673 Degassing F i g . 7.

873

1073

Temperature, K

E f f e c t of d e g a s s i n g t e m p e r a t u r e upon t h e n ,

a*t r a n s i t i o n

of

t h e k e t o n e s a d s o r b e d on Vycor glass (amount o f a d s o r b e d , a: 2-pentanone, 2.37 x

mol/g,

b: 2-butanone,

2.10 x

loq5

mol/g).

129

a t t r i b u t a b l e t o a d e c r e a s e i n t h e p o l a r i t y o f t h e s u r f a c e OH g r o u p s . The change o f I R a b s o r p t i o n b a n d s a t a p p r o x i m a t e l y 3750 cm'l

due t o t h e

s u r f a c e OH g r o u p s s u g g e s t e d t h a t a d e c r e a s e o f t h e s u r f a c e p o l a r i t y is c l o s e l y a s s o c i a t e d w i t h a d e c r e a s e of t h e c o n c e n t r a t i o n of t h e s u r f a c e OH g r o u p s .

T a b l e 3 s h o w s t h e e f f e c t o f t h e d e g a s s i n g t e m p e r a t u r e upon t h e phosphorescence spectrum o f ad s o r b ed 2 - p en ta n o n e

i n t h e i r wavelengths

R e l a t i v e i n t e n s i t y of t h e p h o s p h o r e s c e n c e

and r e l a t i v e i n t e n s i t y .

i n c r e a s e s by r a i s i n g t h e d e g a s s i n g t e m p e r a t u r e o f Vycor g l a s s .

Taking

i n t o a c c o u n t t h e f a c t t h a t t h e r e was no c h a n g e i n t h e r e l a t i v e i n t e n s i t y of t h e a b s o r p t i o n s p e c t r a w i t h a rise i n t h e d e g a s s i n g t e m p e r a t u r e , s u c h

a n i n c r e a s e i n t h e p h o s p h o r e s c e n c e i n t e n s i t y i s a t t r i b u t a b l e t o an i n c r e a s e i n t h e population of t h e e x c i t e d t r i p l e t state of ketones. d e s c r i b e d i n s e c t i o n 4.1.,

As

i t was f o u n d t h a t t h e m o r e b l u e s h i f t e d , i.

e., t h e m o r e s t r o n g l y h y d r o g e n b o n d e d a k e t o n e m o l e c u l e i s , t h e m o r e e f f i c i e n t r a d i a t i o n l e s s d e a c t i v a t i o n becomes.

Therefore, t h e increase

i n t h e p o p u l a t i o n of t h e e x c i t e d t r i p l e t s t a t e of 2 - p e n t a n o n e w i t h a

r i s e i n t h e d e g a s s i n g t e m p e r a t u r e o f V y c o r g l a s s is a t t r i b u t a b l e t o a s u p p r e s s i o n i n t h e r a d i a t i o n l e s s d e a c t i v a t i o n i n t h e e x c i t e d s t a t e due t o a decrease i n t h e surface polarity. TABLE 3 E f f e c t o f d e g a s s i n g t e m p e r a t u r e upon t h e p h o s p h o r e s c e n c e o f 2-pentanone a d s o r b e d on Vycor g l a s s i n t h e w a v e l e n g t h and t h e i n t e n s i t y ( a t 77 K ) Degassing

Amounts of

Phosphorescence

R e 1a t i v e

temperature

adsorbed

wavelength a t

intensity

mol/g)

(K)

IMax

(nm)

--

._-__

573

7.01

445

5

1.00

773

7.01

450 2 5

1.23

973

7.01

455

5

1.70

573

20.1

445 f 5

1.00

973

20.1

445 2 5

1.68

The e f f e c t o f t h e d e g a s s i n g t e m p e r a t u r e o f V y c o r g l a s s u p o n t h e y i e l d s o f t h e Type I and Type I 1 r e a c t i o n s h a s been i n v e s t i g a t e d .

By

i n c r e a s i n g t h e d e g a s s i n g t e m p e r a t u r e t h e y i e l d of t h e Type I1 r e a c t i o n d e c r e a s e d , w h i l e t h e y i e l d o f t h e Type I r e a c t i o n p a s s e d t h r o u g h a maximum a t 573 K

and t h e n d e c r e a s e d a t h i g h e r d e g a s s i n g t e m p e r a t u r e s .

130

T a b l e 4 s h o w s t h e y i e l d s o f C2H4 a n d C3H8 p e n t a n o n e a d s o r b e d on Vycor g l a s s .

i n t h e p h o t o l y s i s of 2-

A l s o , t h e r e s u l t s o b t a i n e d on Vycor

g l a s s , w h i c h h a d b e e n p r e t r e a t e d w i t h NH4F, a r e s h o w n .

Taking i n t o

a c c o u n t a d e c r e a s e i n t h e s u r f a c e a r e a of V y c o r g l a s s a f t e r t r e a t m e n t w i t h NH4F, it is c o n c l u d e d t h a t b o t h t h e T y p e I 1 a n d T y p e I r e a c t i o n s

are enhanced a f t e r t h e t r e a t m e n t w i t h NH4F, t h e e x t e n t b e i n g l a r g e r f o r t h e Type I t h a n f o r t h e Type 11.

Hair, ( 2 9 )

A c c o r d i n g t o t h e work o f Chapman and

a f t e r p r e t r e a t m e n t w i t h NH4F, t h e s u r f a c e of V y c o r g l a s s

c o n s i s t s o f t h e s i l a n o l g r o u p s ($Si-OH)

and

t h o s e s u b s t i t u e d by F atom

(GSi-0-F).

Owing t o t h e e l e c t r o n e g a t i v i t y o f F atoms, t h e weakening

of t h e

b o n d t a k e s p l a c e a t t h e n e i g h b o r i n g OH g r o u p s o f + S i - O F ,

0-H

r e s u l t i n g i n a n i n c r e a s e i n s u r f a c e a c i d i t y or s u r f a c e p o l a r i t y . Thus,

essentially,

t h e same t r e n d i s o b s e r v e d f o r t h e s e

e x p e r i m e n t s , i. e., p r e t r e a t m e n t w i t h NH4F a n d c h a n g i n g a d e g a s s i n g t e m p e r a t u r e o f Vycor g l a s s , i n d i c a t i n g t h a t t h e s i g n i f i c a n t role o f t h e p o l a r i t y of t h e s u r f a c e OH g r o u p s i n d e t e r m i n i n g t h e rates of b o t h t h e Type I a n d 11 r e a c t i o n s .

A s d e s c r i b e d a b o v e , t h e Type I 1 r e a c t i o n

p r o c e e d s v i a t h e 1 , 4 - b i r a d i c a l s which are H-atom a b s t r a c t i o n .

f o r m e d by t h e i n t r a m o l e c u l a r

The Type I 1 r e a c t i o n i s known t o b e e n h a n c e d i n

p o l a r s o l v e n t s i n comparison w i t h nonpolar s o l v e n t s .

T h i s is w e l l

a t t r i b u t e d t o a d e c r e a s e d p r o b a b i l i t y f o r r e t u r n o f t h e 1-hydrogen i n t h e 1 , 4 - b i r a d i c a l s due t o hydrogen b o n d i n g b e t w e e n t h e p o l a r s o l v e n t and t h e hydroxyl group o f t h e 1 , 4 - b i r a d i c a l s

(27). T h e r e f o r e , t h e d e c r e a s e

i n C2H4 f o r m a t i o n by r a i s i n g t h e d e g a s s i n g t e m p e r a t u r e o f Vycor g l a s s is a s s o c i a t e d w i t h t h e d e c r e a s e i n t h e s u r f a c e p o l a r i t y which r e s u l t s i n t h e d e s t a b i l i z a t i o n of t h e 1,4-biradicals. TABLE 4 E f f e c t of s u r f a c e p r e t r e a t m e n t s u p o n t h e p h o t o l y s i s o f 2 - p e n t a n o n e a d s o r b e d o n Vycor g l a s s a t 298 K ~-

Condition of

Y i e l d s o f C3H8

pretreatment(a)

(Type I ) ml/min)

Only d e g a s s e d

1.47

NH4F s o l u t i o n ~~~~

~~

Y i e l d o f C2H4 ( T y p e 11) (10-4 m l /min)

3.48 ~~~~

____

~-

Ratio (C3H8)/(C2H4)

3.33

0.44

3.35

1.04

~

( a ) : The s p e c i m e n s were d e g a s s e d a t 1 0 7 3 K. The amounts o f 2-pentanone a d s o r b e d were (6.4

-

7.8) x

mol/g.

131

On t h e o t h e r h a n d , t h e y i e l d o f C3H8 f o r m a t i o n is d e t e r m i n e d by a b s t r a c t i o n e f f i c i e n c y a s well

hydrogen

c o n c e n t r a t i o n o f C3H7 r a d i c a l s (10).

as t h e s t e a d y s t a t e

A s described i n section 3 . 4 ,

it

is e x p e c t e d t h a t e a s i n e s s o f t h e hydrogen a b s t r a c t i o n f r o m t h e surface

OH g r o u p s c o n t r i b u t e c o n s i d e r a b l y t o t h e o v e r a l l y i e l d s o f C3H8 f o r m a t i o n a t l e a s t i n t h e r a n g e o f t h e small amount adsorbed. hydrogen a b s t r a c t i o n e f f i c i e n c y would d e c r e a s e by r a i s i n g

The

t h e degassing

t e m p e r a t u r e o f Vycor g l a s s owing t o t h e d e c r e a s e i n t h e c o n c e n t r a t i o n of On t h e o t h e r h a n d , t h e s t e a d y s t a t e c o n c e n -

t h e s u r f a c e OH g r o u p s .

t r a t i o n o f k3H7 r a d i c a l s is d e t e r m i n e d by t h e lifetimes o f k3H7 and t h e e x c i t e d s t a t e o f 2-pentanone as well as t h e r a t e o f a n o c c u r r e n c e o f C-C bond f i s s i o n ( d - c l e a v a g e ) . The c h a n g e i n r a d i c a l l i f e t i m e s o n V y c o r g l a s s w h i c h h a d b e e n d e g a s s e d a t v a r i o u s t e m p e r a t u r e s was d e t e r m i n e d by t h e e f f e c t of O2 upon t h e C3H8 f o r m a t i o n by means o f t h e Stern-Volmer r e l a t i o n s h i p : QO/Q

=1

where

+

Tr*kS*( 0 2 )

lr is

t h e r a d i c a l l i f e t i m e on Vycor g l a s s and ks is t h e s c a v e n g i n g

r a t e c o n s t a n t w i t h 02, r e s p e c t i v e l y .

Q a n d Qo a r e t h e r a t e s o f C3H8

f o r m a t i o n i n t h e p r e s e n c e a n d a b s e n c e of 02, r e s p e c t i v e l y .

Figure 8

s h o w s a S t e r n - V o l m e r p l o t f o r s c a v e n g i n g o f C3H8 f o r m a t i o n w i t h t h e a d d e d O2 o n V y c o r g l a s s w h i c h h a d b e e n d e g a s s e d a t 573 K , 773 K, a n d

973K, r e s p e c t i v e l y .

a

lot

0

b

0.1 Initial

F i g . 8.

A s s u m i n g t h a t t h e q u e n c h i n g r a t e c o n s t a n t (k,)

0.2

Pressure,

Stern-Volmer

Torr

p l o t s f o r scavenging o f t h e propane f o r m a t i o n i n

t h e p h o t o l y s i s of 2 - p e n t a n t m e a d s o r b e d o n V y c o r g l a s s w h i c h h a d b e e n d e g a s s e d a t v a r i o u s t e m p e r a t u r e s ( a : d e g a s s e d a t 573 K,

773 K,

c: d e g a s s e d a t 973 K ) .

b: d e g a s s e d a t

132 s c a r c e l y c h a n g e s on Vycor g l a s s , w h i c h had b e e n d e g a s s e d a t v a r i o u s temperatures, t h e changes i n t h e r a d i c a l l i f e t i m e c a n b e d e t e r m i n e d f r o m t h e Stern-Volmer p l o t . l i f e t i m e o f C3H7

(T)o n V y c o r

Thus,

glass

f r o m F i g . 8, t h e

r a d i c a l s on t h e s u r f a c e s was found t o d e c r e a s e by i n -

c r e a s i n g t h e d e g a s s i n g t e m p e r a t u r e of Vycor g l a s s . The l i f e t i m e o f C3H7 r a d i c a l s is m a i n l y c o n t r o l l e d by t h e r a t e s o f r e a c t i o n s b e t w e e n (e3H7

-

COCH3) r a d i c a l p a i r s s u c h as t h e r e c o m b i n a t i o n

and d i s p r o p o r t i o n a t i o n r e a c t i o n s (6, 1 3 ) .

In t h e adsorbed l a y e r ,

such

r e a c t i o n r a t e s depend on t h e s u r f a c e m o b i l i t y o f t h e r a d i c a l s , which would i n c r e a s e w i t h d e c r e a s i n g s t r e n g t h of t h e i n t e a c t i o n b e t w e e n t h e r a d i c a l s a n d t h e s u r f a c e OH g r o u p s .

The d e t a i l s o f t h e weak i n t e r -

a c t i o n of t h e a l k y l r a d i c a l s w i t h s u r f a c e OH g r o u p s is d e s c r i b e d i n t h e p r e v i o u s c h a p t e r by Gesser by means o f t h e ESR t e c h n i q u e .

A decrease

i n t h e s t r e n g t h o f t h e i n t e r a c t i o n b e t w e e n t h e r a d i c a l s and t h e s u r f a c e by r a i s i n g t h e d e g a s s i n g t e m p e r a t u r e s u g g e s t s t h e d e c r e a s e i n s t a b i l i z a t i o n o f t h e r a d i c a l s f o r m e d i n t h e p r i m a r y p r o c e s s , which is e x p e c t e d t o r e s u l t i n a decreased p r o b a b i l i t y of 4-cleavage. T h u s , it is c o n c l u d e d t h a t t h e d e c r e a s e i n C3H8 f o r m a t i o n by r a i s i n g t h e d e g a s s i n g t e m p e r a t u r e o f Vycor g l a s s i s a t t r i b u t a b l e t o a d e c r e a s e d p r o b a b i l i t y o f d - c l e a v a g e as well as t h e d e c r e a s e i n t h e r a d i c a l l i f e t i m e s and t h e hydrogen a b s t r a c t i o n e f f i c i e n c y .

On t h e o t h e r

as d e s c r i b e d i n s e c t i o n 4.1, i t is a l s o e x p e c t e d t h a t C3H8 f o r m a t i o n increases w i t h i n c r e a s i n g d e g a s s i n g t e m p e r a t u r e owing t o t h e

hand,

i n c r e a s e i n t h e l i f e t i m e of t h e e x c i t e d t r i p l e t s t a t e .

I t appears t h a t

s u c h a s i t u a t i o n is r e a l i z e d i n t h e d e g a s s i n g t e m p e r a t u r e r a n g e b e l o w 5 7 3 K.

T h u s , a maximum i n t h e C3H8

f o r m a t i o n a t a r o u n d 573 K is

explicable. 4. 4 . S t e r i c H i n d r a n c e E f f e c t upon t h e Type I 1 R e a c t i o n The r e s u l t s o f t h e p h o t o l y s i s of 3-methyl-2-pentanone 2-pentanone

and

a d s o r b e d on Vycor g l a s s are g i v e n i n T a b l e 5, t o g e t h e r w i t h

t h o s e of t h e p h o t o l y s i s i n t h e g a s phase. o f 3-M-2-P,

(3-M-2-P)

In t h e g a s phase p h o t o l y s i s

t h e rate o f C2H4 f o r m a t i o n (Type 1 1 ) is t h r e e times h i g h e r

t h a n t h a t o f r a d i c a l p r o d u c t f o r m a t i o n (Type I ) .

I t is l i k e l y t h a t t h e

r a t e o f r a d i c a l p r o d u c t f o r m a t i o n i n t h e a d s o r b e d l a y e r is much h i g h e r t h a n t h a t o f C2H4 f o r m a t i o n .

I t is a l s o s e e n t h a t t h e Type I

s e l e c t i v i t y i n t h e p h o t o l y s i s o f 2-pentanone l a y e r t h a n i n t h e g a s phase.

is h i g h e r i n t h e a d s o r b e d

I t is s e e n i n T a b l e 5 t h a t t h e s e l e c t i v i t y

of t h e Type I is much l a r g e r f o r 3-M-2-P

t h a n f o r 2-pentanone.

F u r t h e r m o r e , t h e marked i n c r e a s e i n t h d Type I r e a c t i o n o b s e r v e d i n g o i n g from 2-pentanone t o 3-M-2-P

r a t e o f C2H4

formation.

a r i s e s m a i n l y from t h e d e c r e a s e i n t h e

A s described above,

s u c h a h i g h Type

I

133 TABLE 5

R e s u l t s o f t h e p h o t o l y s i s o f 3-methyl-2-pentanone

and 2-pentanone

a d s o r b e d o n Vycor g l a s s a t 298 K

Compounds

Amount a d s o r b e d

Type I1 y i l e d

mol/g) 3-Methyl-2p e n t anone 2-Pentanone

Type I y i e l d

ml/min)

ml/min)

Ratio (Type 1 / 1 1 )

5.36

0.42

2.14

5.1

21.40

0.96

4.42

4.6

42.80

1.30

5.18

4.0

4.28

1.44

1.30

0.89

3.84

2.48

0.65

22.0

-

s e l e c t i v i t y i n t h e a d s o r b e d l a y e r would b e a t t r i b u t a b l e t o a s u p p r e s s i o n of

t h e recombination of t h e radical p a i r s ,

p r o b a b i l i t y o f o(-cleavage, product formation.

a s well as i n c r e a s e d

l e a d i n g t o an i n c r e a s e i n t h e rate of r a d i c a l

i t is d i f f i c u l t t o e x p l a i n t h e marked as

However,

h i g h e r T y p e I s e l e c t i v i t y i n t h e p h o t o l y s i s o f a d s o r b e d 3-M-2-P compared w i t h t h a t o f 2-pentanone. A s d e s c r i b e d i n s e c t i o n 4.1,

1,4-biradicals.

t h e Type I 1 r e a c t i o n p r o c e e d s v i a t h e

A favorable conformation is required for ghydrogen

a b s t r a c t i o n s t o take place.

A six-membered t r a n s i t i o n s t a t e h a s been

p r o p o s e d f o r t h i s r e a c t i o n (30). motion of molecules,

On a s o l i d s u r f a c e t h e r a p i d t u m b l i n g

e s p e c i a l l y f o r t h e m o l e c u l e s which make hydrogen

b o n d i n g w i t h t h e s u r f a c e OH g r o u p s , i s h i n d e r e d .

As a result, the

f o r m a t i o n of s u c h a s i x - m e m b e r e d t r a n s i t i o n s t a t e is c o n s i d e r a b l y disturbed.

S u c h s t e r i c h i n d r a n c e is e x p e c t e d t o b e more s i g n i f i c a n t

f o r 3-M-2-P

t h a n for 2-pentanone,

m e t h y l g r o u p i n s t e a d o f hydrogen. f o r m a t i o n o b s e r v e d w i t h 3-M-2-P, explicable.

owing t o t h e presence of a bulky Thus, t h e marked lower r a t e of C2H4

as compared w i t h 2-pentanone,

is

T u r r o and Wan have shown t h a t s u b s t a n t i a l c h a n g e s i n t h e

r e l a t i v e y i e l d s o f t h e Type I / T y p e I 1 r e a c t i o n s i n t h e p h o t o l y s i s o f p h e n y l a l k y l k e t o n e s a d s o r b e d o n z e o l i t e s (31).

S t e r i c hindrance

e f f e c t s f o r t h e Type I 1 r e a c t i o n a p p e a r t o o p e r a t e i n t h o s e s y s t e m s .

5.

EFFECT OF SURFACE MODIFICATION BY SUPPORTING N i 2 + IONS The p h o t o l y s e s o f a l k y l k e t o n e s h a v e b e e n i n v e s t i g a t e d o n V y c o r

g l a s s which had been f u n c t i o n a l i z e d by s u p p o r t i n g a small amount of N i 2 + ions,

i n o r d e r t o seek t h e f a c t o r t o enhance t h e e f f i c i e n c y of t h e

photoreaction i n t h e adsorbed l a y e r (32).

S u p p o r t i n g o f N i 2 + i o n s on

V y c o r g l a s s ( N i 2 + / V y c o r g l a s s ) was c a r r i e d o u t by a c o n v e n t i o n a l

0

02 0.4 0.6 0.8 1.0

2+ -4 Amount of supported N1 ,I0 m o l h

Fig. 9.

E f f e c t s o f t h e amount o f s u p p o r t e d N i 2 + i o n s upon t h e y i e l d s o f

t h e p h o t o l y s i s o f 2-butanone

a d s o r b e d o n V y c o r g l a s s m o d i f i e d by

s u p p o r t i n g N i 2 + i o n s and t h e i n t e n s i t y o f t h e a b s o r p t i o n band due t o t e t r a h e d r a l l y coordinated Ni2+

ions.

i m p r e g n a t i o n m e t h o d u s i n g a d i l u t e d a q u e o u s s o l u t i o n o f N2+ i o n s a n d t h e n d r i e d a t 227 K (14-21).

Figure 9 shows t h e y i e l d of t h e photo-

l y s i s o f 2 - b u t a n o n e a d s o r b e d on Ni2+/Vycor g l a s s v e r s u s t h e amount of

I t was found t h a t , w i t h i n c r e a s i n g t h e amount of

Ni2+

i o n s supported.

Ni2+

ions supported, t h e y i e l d of photolysis increases remarkedly,

p a s s i n g t h r o u g h a maximum a t t h e amount of 2 x

mol/g o f Ni2+ i o n s ,

and t h e n d e c r e a s i n g w i t h f u r t h e r i n c r e a s e o f N i 2 + i o n s . The p h o t o l y s i s o f a c e t o n e a d s o r b e d on Ni2+/Vycor glass was f o u n d t o l e a d t o t h e f o r m a t i o n o f CH4 and C2Hs

w i t h almost e q u a l y i e l d s .

y i e l d s i n c r e a s e d w i t h t h e amount o f N i 2 + i o n s

These

loaded, passing through a

maximum a t t h e same a m o u n t o f N i 2 + i o n s t o t h o s e i n F i g . 9 , a n d t h e n The y i e l d s of t h e p h o s p h o r e s c e n c e o f a c e t o n e and 2-butanone were a l s o f o u n d t o i n c r e a s e b y a d s o r p t i o n o n N i 2 + / V y c o r g l a s s , a s decreased.

compared w i t h t h o s e on un-loaded p u r e Vycor glass. F i g u r e 1 0 s h o w s t h e UV a b s o r p t i o n s p e c t r a o f N i 2 + / V y c o r g l a s s b e f o r e and a f t e r t h e a d s o r p t i o n o f 2-butanone.

UV a b s o r p t i o n bands a t

a p p r o x i m a t e l y 280, 4 0 0 , a n d 530 nm c a n b e a t t r i b u t a b l e t o t h e c h a r g e t r a n s f e r bands o f t h e t e t r a h e d r a l co o r d i na te d Ni2+ io n s, i n d i c a t i n g t h a t t h e Ni2+

i o n s s u p p o r t e d on Vycor glass are i n t e t r a h e d r a l c o o r d i n a t i o n .

As shown i n Fig.

10, a d s o r p t i o n o f k e t o n e m o l e c u l e s on Ni2+/Vycor l e a d s

t o a marked c h a n g e i n t h e i r a b s o r p t i o n s p e c t r a .

The e x t e n t o f change

135

I

Wavelength,

new

nm

Fig. 10. UV absorption spectra of 2-butanone adsorbed on Vycor glass involving Ni2+ ions (0: back ground from Vycor glass, 1: Vycor glass involving Ni2+ ions of 5.1 x mol/g, 2: after adsorption of 2butanone of 2.2 x loq5 mol/g, 3: after adsorption of 2-butanone of 5.0 x 10-5 mol/g). depended on the amount of ketone molecules. UV absorption band appeared at approximately 445 nm after the adsorption of 2-butanone onto the Ni2+/Vycor glass shows a good agreement with the characteristic absorption band of octahedral Ni2+ complexes (33). IR absorption spectrum at approximately 3750 cm-l due to surface OH groups scarcely changed by the adsorption of ketones of about mol/g onto Ni2+/Vycor glass. These results clearly indicate that on Ni2+/Vycor glass ketone molecules adsorb on Ni2+ ions, but not Vycor glass, i. e, surface hydroxyl groups. It is well known that tetrahedrally coordinated Ni2+ ions located on the support surfaces such as Si02 act as adsorption centers for polar molecules, which enter the coordination sphere of Ni2+ ions and change their coordination from tetrahedral to octahedral. Figure 9 also shows the relative intensity of the absorption of tetrahedrally coordinated Ni2+ ions versus the amount of Ni2+ ions supported on Vycor glass. A complete parallel between the intensity (i. e., concentration) of tetrahedrally coordinated Ni2+ ions and the yields of the photolysis of 2-butanone (or acetone) adsorbed on Ni2+/Vycor glass is found. This indicates that tetrahedrally coordinated Ni2+ ions play a significant role in the enhancement o f the

136

p h o t o l y s e s o f a l k y l k e t o n e s a d s o r b e d on Ni2+/Vycor g l a s s .

Being i n

a g r e e m e n t w i t h t h e s e r e s u l t s , t h e d e c a y c u r v e of t h e p h o s p h o r e s c e n c e of acetone

adsorbed

exponential,

on Ni2+/Vycor

and t h e

g l a s s was

l i f e t i m e was much

found t o

be

a single

longer than t h a t

on pure

u n s u p p o r t e d Vycor glass by a b o u t one o r d e r o f magnitude. Thus,

t h e enhancement o f t h e p h o t o l y s e s o f a l k y l k e t o n e s is

a c h i e v e d on Vycor g l a s s w h i c h h a d b e e n f u n c t i o n a l i z e d by s u p p o r t i n g

on which k e t o n e m o l e c u l e s a d s o r b a s some l i g a n d s . Although t h e d e t a i l e d mechanism is n o t clear a t p r e s e n t , t h e r e is n o d o u b t t h a t d e a c t i v a t i o n o f t h e e x c i t e d s t a t e of t e t r a h e d r a l l y coordinated

Ni2+

i' o n s ,

ketone molecules adsorbed on t h e t e t r a h e d r a l l y coordinated Ni2+ i o n s m i g h t t a k e p l a c e much l e s s e f f i c i e n t l y t h a n t h o s e o n V y c o r g l a s s , r e s u l t i n g i n t h e enhancement o f t h e p h o t o l y s i s . 6.

CONCLUSION S t u d i e s o f t h e p h o t o l y s e s o f a l k y l k e t o n e s a d s o r b e d on Vycor g l a s s

by hydrogen b o n d i n g h a v e d e m o n s t r a t e d t h a t t h e p h o t o c h e m i c a l r e a c t i o n s i n t h e a d s o r b e d l a y e r i n d i c a t e some d e v i a t i o n s i n t h e r e a c t i o n y i e l d s and s e l e c t i v i t y as compared w i t h t h o s e i n t h e g a s phase.

These a r e

g e n e r a l l y c a u s e d by h i g h r e a c t i v i t y and h i g h p o l a r i t y o f t h e s u r f a c e OH g r o u p s on Vycor g l a s s , wh i ch p l a y s a s i g n i f i c a n t r o l e n o t o n l y i n t h e r e a c t i v i t y and s t a b i l i z a t i o n o f i n t e r m e d i a t e s p e c i e s i n t h e c h e m i c a l r e a c t i o n s , b u t also i n t h e photophysical pathways.

R e s t r i c t i o n s by

s o l i d s u r f a c e s on t h e p a r t i c u l a r m o l e c u l a r c o n f o r m a t i o n h a v e b e e n observed.

I t h a s also been i n d i c a t e d t h a t t h e r e are p o t e n t i a l l y u s e f u l

s u r f a c e m o d i f i c a t i o n s t o enhance t h e photochemical

r e a c t i v i t y of

a d s o r b e d molecules. REFERENCES P. A. L e e r m a k e r s , H. T. T h o m a s , L. D. Weis, a n d F. C. James, J. Am. Chem. SOC., 88 (1966) 5075-5083, and r e f e r e n c e s t h e r e i n .

Y. Kubokawa a n d M. Anpo, Hyomen ( S u r f a c e S c i e n c e ) , 1 6 ( 1 9 7 8 ) 463480, and r e f e r e n c e s t h e r e i n . N. J. T u r r o , T e t r a h e d r o n , 4 3 ( 1 9 8 7 ) 1589-1616.

P. d e Mayo a n d L. J. J o h n s t o n , i n P r e p a r a t i v e C h e m i s t r y U s i n g Supp o r t e d R e a g e n t s , Academic P r e s s , Inc.,

M. Anpo, Chem. L e t t . ,

N e w York,

1987, p. 61-75.

(1987) 1221-1224.

Y. Kubokawa and M. Anpo, J . Phys. Chem.,

78 ( 1 9 7 4 ) 2442-2446.

M. Anpo and Y. Kubokawa, J. Phys. Chem.,

7 8 ( 1 9 7 4 ) 2446-2449.

M. Anpo,

S. H i r o h a s h i , a n d Y. Kubokawa, B u l l . Chem. SOC. J p n . ,

(1975) 985-987.

48

137 9

M. Anpo, T. Wada, a n d Y. Kubokawa, B u l l . Chem. SOC. J p n . , 48 ( 1 9 7 5 ) 2663-2666 and M . Anpo, Chem. E x p r e s s , 3 (1988) 000-000.

10

M. Anpo a n d Y. Kubokawa, B u l l . Chem. SOC. J p n . ,

11

M. Anpo and Y. Kubokawa, J. Phys. Chem.,

12

M. Anpo and Y. Kubokawa, B u l l . Chem. SOC. J p n . , 49 (1976) 2623-2624.

13

M . Anpo, T. Wada, a n d Y. Kubokawa, B u l l . Chem. SOC. J p n . , 50 ( 1 9 7 7 )

48 (1975) 3085-3087.

79 (1975) 2225-2228.

31-35.

14

M. Anpo, I. T a n a h a s h i , a n d Y. Kubokawa, J. Phys. Chem.,

84 (1980)

3440-3444. 15

M. Anpo, I . T a n a h a s h i , a n d Y. Kubokawa, J. P h y s . Chem., 86 ( 1 9 8 2 ) 13.

16

M. Anpo, I. T a n a h a s h i , a n d Y. Kubokawa, J. Chem. S O C . , F a r a d a y T r a n s . 1, 78 (1982) 2121-2128.

17

M. Anpo and Y. Kubokawa, J. C a t a l . , 75 (1982) 204-206.

18

M. Anpo, T. S u z u k i , Y. Kubokawa, F. T a n a k a , a n d S. Y a m a s h i t a , J. Phys. Chem., 88 (1984) 5778-5779.

19

M. Anpo,

N. A i k a w a , Y. Kubokawa, M. Che, C. L o u i s , a n d E. G i a m e l l o ,

J. Phys. Chem.,

89 (1985) 5017-5021 and 5689-5694.

20

M. Anpo and Y. Kubokawa, J . C a t a l . , 97 (1986) 272-276.

21

M. Anpo and Y. Kubokawa, Rev. Chem. I n t e r m e d i . , 8 (1987) 105-124.

22

M. Anpo, T. S u z u k i , Y. Yamada, a n d M. Che, Proc. 9 t h I n t e r n . C o n g r . C a t a l . , ( C a l g a r y ) 4 (1988) 1513-1520.

23

C. T. L i n , W. L. HSU,

Chem.,

C. L. Yang, a n d M. A. E l - S a y e d ,

J.

Phys.

9 1 (1987) 4556-4559.

24

M. 0 S u l l i v a n and A. C. Testa, J. Phys. Chem., 92 (1970) 258-261.

25

M, J. D. Low a n d N, R a m a s u b r a m a n i a n , J. P h y s . Chem., 7 1 (1967) 30773081, a n d t h e i r e a r l i e r series.

26

G. P o r t e r , S. K. D o g r a , R. 0. L o u t f y , S. E. S u g a m o r i , a n d R. W. Y i p , J. Chem. S O C . , F a r a d a y T r a n s .

27

1, 69 (1973) 1462-1474.

P. J. Wagner, T e t r a h e d r o n L e t t . , and J. Am. Chem. SOC.,

(1968) 5385-5387,

(1967)

1753-1755,

89 ( 1 9 6 7 ) 5898-5892.

28

R. P. Borkman and P. A u s l o o s , J. Phys. Chem.,

29

I . D. Chapman a n d M. L. Hair, J. Catal., 2 ( 1 9 6 3 ) 145-149,

65 (1961) 2257-2261,

Trans.

F a r a d a y S O C . , 61 (1965) 1507-1561. 30

N . J. Yang, S. P. E l l i o t t , a n d B. K i m , J. Am. Chem. S O C . , 87 ( 1 9 6 9 ) 7551-7552.

31

N . J. T u r r o and P. Wan, T e t r a h e d r o n L e t t . , 25 (1984) 3655-3658.

32

M. Anpo, t o b e s u b m i t t e d f o r p u b l i c a t i o n .

138

OF AZOCUMENE ON SILICA SURFACES

DECOMPOSITION

LEFFLER and J.J.

J. E. 1.

ZUPANCIC

INTRODUCTION Azocumene

(1)

is a two-bond

initiator

whose

decomposition

into cumyl radical pairs has been extensively studied solvents

and

includes

rates,

and

the

extent

in

(1-3).

glasses radical

The

available

in liquid informatioil ratios,

disproportionation/combination

of geminate

recombination.

The

of

purpose

tiic.

our laboratory was t o compare the 1 iqiiid

research carried out in

phase results with those f o r the reaction in the quite diffei-cnt medium

afforded

by

during

the

present

silica gel reaction

surfaces

stage of

(4).

our

No

solvents were

experiments

and

the

temperature was such that vapor phase reaction could be ignored.

1 A z oc u me ne

The

variables

monolayer

in

covered,

azocumene,

our

the

thermolysis

experiments presence

at

of

55' versus

were

the

fraction

adsorbates photolysis

other at

25',

of

it

than the

presence of a stable radical scavenger, the purity of the s i 1 ica, and the hydroxylated versus dehydroxylated nature of the surface.

2.

CHARACTERISTICS OF THE Four

SURFACES

types of chromatographic grade silica were used.

F'or

convenience they are designated by Pillj, i,j = 0 or 1. P indicates the purity of the gel

and H the degree of hydroxylation.

For

139

example,

the less pure gel, containing 0 . 0 3 wt. % Fe,O,

PoHl is

and 0.09 wt. % TiO,, and having more hydroxyl groups than the H a gels.

The HI gels were merely dried at 85' whereas the Ha gels (5) by outgassing at 750 to 850'.

were extensively dehydroxylated

The hydroxyl groups of Ha gels are not only fewer, but also tend to be

isolated

2.1

or

rather than geminal

diameters of P,H,

vicinal.

The mean

pot-c

and P,Hl were 22 and 25-27 Angstroms.

EPR OF ADSORBED RADICALS

At -153' cumyl radicals on the surface (POHI) are rotationally mobile

as shown

by

the

degree

structure in Fig. 1. At -129'

of

resolution

of

the

hyperfine

the radicals become translationally

mobile as well, and the signal decays with a half-life of about, one hour.

i

-80

Fig.

1.

photolysis

I

I

-60

A

I

I

-40

,

I

-20

I

l

0

.

I

20

.

H (GAUSS)

9-GHz EPR

spectrum

I

I

40

of

I

60

I

I

80

cumyl

radicals

from

o f azocumene adsorbed on P,H, silica, photolyzed

190 'C and observed at - 153 'C.

The

the at -

g value is 2.0026 and the p -

H hyperfine splitting constant i s 1 6 . 6 G . In

contrast

to

cumyl,

bisdiphenylene-2-phenylallyl

larger

radicals

such

(BDPA) are nearly

the temperature of o u r thermolysis experiments. resolution of this radical greater than

in ethanol

at -113'

as

1,1,3,:5-

immobile even a t The degree 01' suggests a

sec. On a silica gel surface the

same

T

~

degree of

resolution is not reached until the temperature is raised t o +lS5" (Fig. 2.2

2). Summary

of the Surface Characteristics

The surfaces were also characterized by their proton exchangr or

prototropic

catalysis,

by

their

shifts of

the

diphenyl

~

~

,

140 nitroxide g value ( 6 ) , by their indicator acidities, and their Rf values. The

both the EPR and the other experiments

results of

Higher surface purity ( P , )

lead t o the following generalizations: is associated

slightly

with lower acidity, lower catalytic activity, and

higher

adsorbate (HI)

hydroxyl

groups

adsorbed

molecules

acidity

.

Fig. 2.

is

or

A

mobility. associated

radicals

higher with

and

with

level

greater a

of

surface

mobility

somewhat

BDPA hyperfine structure on silica compared to that

ethanol. The degree of resolution that o n silica at plus 181'.

at about minus 125'

of

higher

i l l

resembles

Although BDPA begins to decay at h i g h

temperatures, this is not the cause of the low resolution. T h e resolution was unchanged on running the temperature down to 105O, obtaining a spectrum, raising the temperature again t o 180°, and again running a spectrum. 3.

KINETICS Reactions

on

surfaces

will

almost

always

differ

in

their

kinetics from otherwise similar reactions in 1 iquid solvents. I n

141 broader distribution of rates would be expected. 3.2 Site Preemption Other compounds that preferentially adsorb on the sil c a gel

S sites can interfere with their occupation by azocumene causing

the

monolayer

rate

constants

of

fraction

about

to

be

higher.

0.019

on

about an equal amount of azobenzene by about 7%.

For

P,H,

silica,

to P,H,

It is important to note that

silica after the

lo%,

about

applying

increased the rate constaiit a similar experinieiit

in which the azobenzene was applied before the azocumene the rate constant by about 15%.

aga i n

azocuinenc

an

increased

Similarly, applying BDPA radical

azocumene

increased

put on before, about 1 5 % .

the

rate constant

Apparently the competitors

for the slow sites are more successful in preempting those sites if they get there first and need compete only with

inert solveiit

molecules.

D

P

3.5

,

0

1

1

2

1

5

1

4

5

MOF(0LAYER FRACTION OF AZOCUMENE

Fig. 3. Effect of monolayer fraction and type of silica on the 55' f i rst-order rate constants for the thermolysis. Independent

variables are purity (Po, low, and P I , surface OH groups (H,, low, and H,, high). We

would

expect

high)

similar effects to show

and

up

in

density

of

bimolecular

reactions. A transition state in which reagent A of a reactitlg A13 pair be

is solvated by a strongly o r different

from

one

in

rapidly adsorbing site

will

which B is solvated by such a s i t e .

This means that the rates, and perhaps even the products, of a bimolecular reaction on a surface could depend o n is applied

depend

on

first. whether

In

the

photochemical photochemical

which

reactions, the reagent

or

a

reagent

result might quencher

appl ied first.

3.3 The Nature of the Slow Site Rates for the decomposition of azocumene on silica gels at

is

142

liquids, although variously treated

the nominal

solvated

as a single

equilibration ordinary

A

reactions.

of

(7).

chemical species

is in fact a mixture

reagent nevertheless The

reason

of the solvation subspecies simi lar

complexity of a n adsorbed mobi 1 ity

reagent

subspecies, the

adsorbed

is that

thc

the

t.0

funtlarncnt.nl

reagent is usually not posssible. ' 1 ' 1 1 ~ .

reagents

among

1 imited.

microenvironments

is

azocumene,

essentially

though

be-

is fast compared of

neglect

01'

can

the

Thus

a

the

simple

various

siii.I'ac(~

decoioposition

01'

~>i'occss,

unimolecular

gives first order plots that are somewhat concave down. Motions on the surface are too slow to repopulate the faster-decomposi ng subspecies while they are being depleted by the main in the

However,

present

example, the

non-equivalent

reactiori. acIsoi.pt.i

~ i i

sites are enough alike so that the rate constants from tlre I ' i i . - x two half-lives o r so provide a useful basis f o r disciission.

Site Discrimination

3.1

A s can be seen from Fig. 3, the approximate first-oi-dei. I-at(' constants depend on the initial fraction of the surface occiil>ied rapidly with monolayer f ract- ion

by the azocumene. They increase

at first, but then level off. This can be most easily esplainccl

if the azocumene has preferentially adsorbed on a minorit>. of silica gel sites that happen to be

slow

( S ) microen\~ii-onniciitRC,II.

the decomposition reaction. At higher monolayer fractions. sites

are

filled

and

any

additional

~ l i c .

azocurnene

tlrc.

molecules

5

t~i'e

forced to occupy different, faster, sites. Apparently m o s t ol' t . l i e sites

belong to the faster subspecies, and the nearly level rate

constant

later in the reaction suggests that these sites d o

not

differ greatly among themselves o n a given silica. The division of the reaction sites into just two c a t e g o i . i ( , > . slow and fast, is undoubtedly

an

over-simp1 i ficatioii.

to the two-site model, plots of rate constant, fraction

Accc,ixl i

11s

vei~sus iiioiio1;iy~~i

should have abrupt changes in slope, and

t.lic

p03i1

i'jii

of the break should be a measure of the fraction of slow s i t c , s . The

curves in Fig. 3 show a fairly sharp break for the H, si 1 icas

but not f o r the H, silicas. Apparently the H, silicas more ncarlj' fit the

two-site model. On H, silicas most of the hydroxyl

are believed the

t o be

isolated rather than geminal

H, silicas there

involving

isolated

are

probably

hydroxyl s

and

a

variety

others

or of

sites.

i nvol v i ng

constellations of geminal and vicinal groups. A

groiips

vicinal. O I I soiue

d i f fc i ' c i i1

correslx)lldii i g l y

143

.02 monolayer, and an extrapolated rate (1)

55 ,

rate i n toluene,

ai'c

the rates on the hydroxyl-rich H , silicas

and

are lower than the rates on the H, silicas. that the slow sites

fox- toluene

All the rates on silica are lower thaii t h e

compared in Table 1.

The implication is

involve hydroxyl groups (8).

The lower rate on the more acidic, better hydrogen-bonding Polll silica t h a n o n the less acidic P,H, silica is also consistent wit11 a n important role of hydrogen bonding.

lu TABLE 1

lo5

at 55O and 0.02 monolayer o n silicas PiHj and

sec-'

x k

it1

to1 uene .a

3.9

a.

4.7

Extrapolated to 55' from data in ref. 1.

4.

PRODUCTS The

products

from the decomposition

of azocumene

on

silica

include cumene 3, a-methylstyrene 2, and dicumyl 4 . These are a l ~ ; a in liquid o r glassy media, but with major differences i n

formed

relative

yields.

ratio of the

The

rate of

disproportionat i o ~ ~

(kd, reaction 2) to coupling (kc, reaction 3 )

is estimated

-t't.otn

the cumene to dicumyl yield ratio.

4.1

Disprowortionation versus Couwlinq

In liquid benzene solution kd/kc

i s small, 0.055 f 0.005 arid

does not appear to vary with the thermolysis temperature or with

I n contrast,

thermolysis versus photolysis of the azocumene ( 1 ) . changes

in

the

versus glassy nature of the medium are large. for

the

photolysis

Experiments

or

structure of the azo compound

with

of

a

standard deviations)

azocumene

radical

ranged

scavenger

in

the

fluid

For example, l < < , / k C

from showed

1.5 to a

3

modest

(1).

(two

increase in kd/kc f o r cage versus non-cage

reaction in liquid solution (2). On

silica

deviation

surfaces, kd/kc

represents

the

is 0.28f.07. where

dispersion

caused

the by

staiiclai-cl

different

combinations of four indepnedent variables at two levels. 'These

144 variables were monolayer fraction

(H),

(C),

grade of the silica ( P ) ,

(T). The levels W C I - ~ 0.02 and 0.45 f o r monolayer fraction, and Po, P I , H , , 1-11, clcfinecl

degree of hydration before.

as

For

T

thermolysis at 5 5 ' .

the

and temperature

levels

were

photolysis

at

25'

versus

An analysis of the variance showed that the

only variables whose levels had significant effects on kd/kc w e r e the monolayer fraction and the grade of the silica.

It may be recalled that low coverage C, and low puri1.y Po both

associated

with

that

these were

ascribed

stabilization

lower to

azocumene

decomposit ion

a stronger

adsorption

~ICI'C

rates, and

and

greatel-

of the azocumene. Putting all the observations u f '

kd/kc together leads to the following generalization: constrai n t a o n the motion of the azocumene, and inferentially o n

of

the

cumyl

radicals

as

well,

almost

relative amounts of disproportionat ion. the

constraint

is a glassy

always

the motion increase

This is the case

solvent, adsorption

on

a

the

wl~etliei.

surfilcc,

adsorption on a more strongly adsorbing surface, or adsorption

01)

a more strongly adsorbing site (S site) o n a given surface.

As was pointed out by Nelsen and Bartlett (l),

there are more

orientations of the radical pair suitable for disproportionation than for coupling. A less constrained radical pair has a better chance of achieving one of the rarer orientations suitable for coupling before reaction takes place.

We would s a y that the r a t e

145

of the coupling reaction is controlled not only by trans1atioiixI diffusion but also by rotational diffusion.

4.2 Head-to-Tail Gumvl Dimers Photolysis of azocumene reported to give isolated

(1).

solution

by

low temperatures is

dimers 5 and 6 although they could not be

the On

in solution at

standing, the

dissociation, made

dimers

disappeared

irreversible

by

the

from

the

subsequent

head-to-head coup1 ing and disproportionat ion reactions. No traces

of

the

arornatbed

dimers

7 and

8 were

found

in

the

soliltion

experiments.

6

7

The dimers

8

results on silica were quite different.

7 and

8 were

formed

all

on

of the

The aromatizcd silicas

both

in

thermolysis and photolysis experiments. The ratio of the orthocoupled

to

para-coupled

product was about 3 : 1 , obtained f row

the 13C nmr signals of the methyl groups. Experiments

with

isotopically

labeled

azocuniene established

that 7 and 8 are exclusively products of the geminate reaction. probable is

explanation is that the S sites, on which the azocuniene

preferentially

aromatization tail

adsorbed,

are

of the head-to-tail

dimers, .formed at

undergo

A

aromatization.

majority

also

dimers.

catalysts

for

Non-geminate

sites, dissociate

the

head-to-

rather than

A s expected on the basis of this model,

the yield of 7 and 8 is greater when the S sites accomodate

a

146 larger fraction of the adsorbed azocumene. Photolysis also gavr more

the

of

aromatized

dimers

thermolysis experiments. We did

than

did

otherwise

sinliln~.

not determine whether

this

was

due to photoarornatization o r merely to a longer residence tirnc o f the geminate pair on the active site at the lower temperature.

4.3 The Radical Efficiencv In toluene at 55' that proved

to be

(4 as

the radical efficiency

means of scavengers i s about 75%.

determined

by

On silica the only scaveilgei.

suitable, of several tested, was B D P A .

radical efficiency o n silica, as determined from

l'hc

the fadillg of

the BDPA epr signal, is only 10-20%. This was supported by t . l ~ e ratios of do, d,, and d,, coupling products found

in experiments

with unsymmetrically deuterated azocumene l - d 6 . A

molecule

should diffuse

adsorbed

at an

isolated

hydrogen

bond donor site

less readily than it might on a surface with many

such sites close together, where potential energy minima begill to Thus in all instances but one, the radical efficiency

overlap.

was lower on the H,

dehydroxylated silica than on li, silica. ' J ' l ~ e

apparent

was

exception

efficiency

is already

a

photolysis,

in

which

low f o r other reasons.

t,he

raclical

Covering pait o f

the surface with azobenzene also appeared to decrease f, possibl~. because

azocumene

blocks

of

some

the

hydroxyl-rich

d if1'usioii

paths.

CD3

I

+

CgH5-C-H

1

+

a-methylstyrenes

CD3

3-d6 4.4

The Distribution of Hvdrocen and Deuterium

Experiments

in which

azocumene was thermolyzed

deuterated silica, and experiments in which or

photolyzed

cumene

on

non-deuterated

silica,

1-d,

on

part i a l I,\'

was thermolyzed

showed

that

neither

nor cumyl radicals exchange deuterium with the surface.

With this fact established, the combined primary and secoi~dnrg.

147 isotope effects for the disproportionation of the adsorbed ciiiii>,l radicals

5)

(reaction

can

be

obtained

from

from

the

3-d,/3-dl

product ratio.

It is about 2 . 2 , not significantly different from the ratio toluene, and

the

which

a radical-like transition state

compatible with

bond

is

distance

too

great

for

tunneling

to

ilr

in lbr

important.

The

case

of

a-methylstyrene,

disproportionation

other

disproportionation

reactions,

each

with

a

different

iaotupc

I n addition, each a-methylstyrene i s subject to isotopir

effect. exchange

with

reactions The

the

That product i s formed by four diffcrcnt

product, is different.

.

the

aromatized

decomposition

silica,

and

head-to-tail

of 1-d,

on

Po

on

dimers

silica,

7 and

to

8

clinici-izal i C J l l by

formed

the

the silicas all have precisely thc s i x

deuterium atoms required by an exclusively geminate reaction. The product

from

partially dimers

in

of non-deuterated azocunicnr ~ j i i P,H, s i 1 ica gives the arornat izccl

t h e decomposition

deuterated which

POHI o r

only

the

methine

hydrogen

(Fig. 4). The lower yield

deuterium

position

is consistent

with

contain-

a

iate-

determining transfer of a deuteron from the silica to the metliine pos i ti o n .

0

I

-Si-

Fig.

I

4. Aromatization of the head-to-tai 1 dimers.

REFERENCES 1

S . F . Nelsen and P.D.

2

S . F . Nelsen and P . D .

3

W.A.

Pryor and I < .

Bartlett, Bartlett,

Smith,

J . A m . Chem. S o c . 1966,88,137. J . Am. Chem. SOC. 196G,RS,143.

J . A m . Chem. SOC. 1970,92, 5103.

148

7

J .E. Leff ler and J . J . Zupancic, J . A m . Chem. SOC. 1980,101,259. G. J . Young, J. Colloid Sci. 1 9 5 8 , 1 3 , 6 7 . T. Kawamura, A . Matsunami, T. Yonezawa, and I < . Fuliui, Bull. Chem. SOC. Jpn. 1 9 6 5 , 3 8 , 1 9 3 5 . J. E. Leffler and E. Grunwald, Rates and Equilibria of

8

The loss of OH and replacement by Si-0-Si in the Ho silicas

4

5 6

Organic Reactions, Wiley, may have an indirect some of the pores.

New York, 1963.

effect on the rate blocking access to Other things being equal, constraints on

molecular motions and hence on reaction rates should be more severe within the pores.

149

PHOTOLYTIC AND REDOX MECHANISIYIS FOR THE PHOTODECOMPOSITION OF ETHANOIC ACID ADSORBED OVER PURE AND MIXED OXIDES M. SCHIAVELLO, V. AUGUGLIARO, S. COLUCCIA, L. PALMISANO,

A. SCLAFANI INTRODUCTION

1.

The

chemical

adsorption of

involves a chemical transfer of less

strong,

molecule. the on

net

The

effect

of

molecule

on a solid surface

interaction which in many case determines

electrons within

species. The

a

is

the

the

adsorbent

and

d

the adsorbing

energetic perturbation, more or

an

electronic

structure of

the

adsorbed

spectral behaviour of many compounds adsorbed onto

surface of solids, and problems as spectral shifts, variations the

absorption

coefficients, behaviour

of

vibrational

frequencies, appearance of new bands and so on have been addressed and

reviewed

For

(1,2).

1,3,5-trinitrobenzene is

instance, Kortiim et al. report that the not

coloured

when

adsorbed on silica

gel (acidic oxide) while it becomes red when adsorbed on magnesia oxide) (3). The

(basic

shift of

the

absorption

chromophore groups may

of

bathochromic

or

molecules

having

ipsochromic

according to the direction and the intensity of charge

transfer in the adsorbed layer ( 4 ) . of when

adsorbed

molecules

known ( 1 , 2 , 5 ) .

Therefore, the photochemistry

molecules may be different with respect to that shown

they are free.

of

be

spectrum

The

In literature several photochemical reactions adsorbed

over

acidic

nature

the

interaction

of

compounds

are

also affects the

selectivity. When adsorbed

photons

species are

species can an

impinge over

the

surface of a solid on which

present, chemical transformations of these

occur via two different mechanisms.

If the solid is

insulator compound, one o r more adsorbed species may be solely

excited

by

light of suitable energy and then may undergo chemical

150

transformations.

type

This

of

adsorbed

referred

is

process

of

semiconductor

compound, t h e l i g h t may a l s o e x c i t e t h e s o l i d , which electron-hole

electrons

and

irradiated

semiconductor

adsorbed noting are

species that

of

species.

after

and

the

p a i r s ( e--hf). migration

to

solid

The

the

is

a

separated

surface

of

the

are c a p t u r e d by r e d u c i b l e and o x i d i z a b l e

a

"redox

mechanism"

occurs.

I t i s worth

t h e p r o c e s s e s which o c c u r by t h e p h o t o l y t i c mechanism

clearly

role

holes

If

as

"photolysis photogenerates

species" ( 6 ) .

to

o n l y when t h e s o l i d i s an i n s u l a t o r and t h e

possible

the

light

When

a

is

mainly

that

photochemical

of

exciting

reaction

is

the

adsorbed

occurring

over

a

s e m i c o n d u c t o r b o t h mechanisms c a n o c c u r , i n p r i n c i p l e . present

The

devoted

over

semironductor 2nd

various

series

a

solids.

series

A

of

p u r e i n s u l a t o r and

o x i d e s c o v e r i n g a wide range of acid-base p r o ; ) e r t i e s

semiconductor

of

mixed

oxides

insulator-insulator

of

various

composition

and

insulator-

were

used

for

r e a c t i v i t y s t u d i e s i n a continuous p h o t o r e a c t o r working

performing in

reviews t h e r e s u l t s of an i n v e s t i g a t i o n

t h e study of t h e photodecarboxylation of e t h a n o i c a c i d

to

adsorbed

chapter

gas7solid

regime.

The n a t u r e o f t h e s p e c i e s a d s o r b e d o v e r t h e

v a r i o u s s o l i d s was m o n i t o r e d by I R s p e c t r a . The

main one

of

aim

this

was t h a t of d e t e r m i n i n g t h e

investigation

f a c t o r s which a f f e c t t h e o c c u r r e n c e o f t h e r e a c t i o n a c c o r d i n g mechanism

semiconductor

or

the

oxides

other the

and

how

for

t h e mixed i n s u l a t o r -

l e v e l o f p h o t o r e a c t i v i t y i s a f f e c t e d by

e l i k e l y o c c u r r e n c e of b o t h mechanisms. EXPERIMENTAL 2.1

A p p a r a t u s and P r o c e d u r e The

r e a c t i v i t y e x p e r i m e n t s were p e r f o r m e d i n a f l o w a p p a r a t u s

a

f i x e d bed f l a t reactor, whose i n t e r n a l d i m e n s i o n s ( w i d t h ,

using

thickness

and

photoreactor (Hanovia bubbled

L

height) was

5173).

were 3 . 7 , 0 . 2 and 9 cm, r e s p e c t i v e l y .

irradiated An

He

on

one

(99.5 %

The

s i d e by a 1000 W Hg-Xe lamp purity)

i n a b o t t l e c o n t a i n i n g e t h a n o i c acid.

or

an

a i r f l o w was

The g a s e o u s m i x t u r e

151

from

the

bubbling

photoreactor. to

a

Finally,

the

was

gas

then

fed

to

the

from the reactor outlet was sfant

gaschromatograph (Varian, Vista 65001, equipped with FID and

TCI)

detectors

an6

the

for continuously monitoring the mixture composition

products.

photoreactor all

bottle

The

the values of 313 and 323 K, respectively.

had

the catalysts t k

usins

temperatures of the saturator and of the For

the photoreactivity experiments were performed

He/ethanoic acid mixture.

In addition, the pure oxides

were also tested usinr; l-he air/ethanoic acid mixture. The with

run procedure was as follows: the photoreactor was filled

the

powder

(1.5 g) and

thermal equilibrium gaseous same

and

Analyses

of

after

the

minutes

hours. The

all the system. Then, the feeding of the

and the irradiation of the reactor started at the

mixture instant

time.

in

time was allowed in order to reach

this

gas

time

leaving

was

considered

the

run

zero

the reactor were performed a few

start and thereafter approximately every three

runs lasted

several hours and they were stopped when

no changes in catalytic activity were observed. 2,2

Catalyts Preparation The

used

catalysts were

insulator powders prepared with

various

and

mixed semiconductor and

sources. Two

SiO catalysts were 2

contacting for one week silica gel powders with HF o r

by

NaOH

of

pure

aqueous

solutions, whose

pH

values

were

3 and 10,

respectively. The mixed

insulator-insulator catalysts were prepared in the

following ways. prepared

by

Mg(N0, 1

3 2

The

SiO -MgO 2

impregnating solution

decomposition.

and

2 3

or A1 0

SiO

2

and

A1 0 -MgO binary samples were 2 5’

then

by

respectively, with a performing

a

thermal

SiO -A1 0 binary samples were prepared by a 2 7 3 coprecipitation method at pH=5.5. In order to correctly compare the

The

reactivity

components,

the

of

these

binary

same methods

catalysts with

were

that

of

pure

also used for preparing pure

SiO

A1 0 and IIgO. All the samples were fired at 673 K in air 2 3’ fo:. 24 h. 2’

The mixed

insulator-semiconductor catalysts were prepared in

152

the

following ways.

The catalysts with a variable ratio of MgO in

Ti02 were prepared by impregnating for 24 hours Ti0 aqueous acidic

solution containing Mg(NC3I2.

evaporated and

the

thermal

resulting

solids were

treatment. Afterwards

they were

(BDH) with an 2 The slurries were

slowly decomposed by heated in air at 673 K

for 24 h. The

catalysts with

prepared

by

solution

a

dissolving

of

NaOH

variable

SiO

2

ratio

(BDH) in

in which

Ti0

2

of SiO

in Ti0 were 2 2 strong alkaline hot

a

( B D H ) was

suspended. The

suspension was

reacted with an aqueous solution of HC1, which was

added

pH

a

value

of 3 was reached. The resulting solids

washed and dried in air at 393 K for 24 h and subsequently at

were 673 K of

until

for

these

24 h. binary

same methods MgC.

In order to compare the photoreactivity results mixtures

were

In Table 1

specimen for standard

also

and

preparing pure Ti0 SiO and 2’ 2 Table 2 MgO home prepared is the comparison

used

the MgO-Ti0

specimen whose

elsewere ( 7 ) .

with those of the pure components, the for

mixture and Ti0 home prepared is a 2 2 preparation method has been reported

reactivity of Ti0 reported in Fig. 2 refers to 2 the comparison specimens for the series MgO-Tic and SiO -Ti0 . 2 2 2 The surface areas of all the powders were measured by a

dynamic

The

BET

method

using

dinitrogen

as

adsorbate

and

a

Micromeritics Flowsorb 2300 apparatus. 2.3 Infrared Measurements

Self were

obtained

the

range

introducing which

was

allowed in

supporting pellets by

pressing 6

142.10 KPa. The the

pellets

permanently

thermal

in

(

20-40 mg.cm-2

of the catalysts

the powders at a suitable pressure in infrared a

spectra were

obtained

by

cell, equipped with CsI windows,

connected

to

a vacuum line (10-jKPa) and

treatments and adsorption-desorption experiments

situ. The spectrometer was a Perkin-Elmer 5808 equipped with a

3600 data station.

The spectra were obtained in the 4000-700 cm- 1

range, limited

the

the oxides.

on

low frequency side by bulk absorptions of

153

EXPERIMENTAL RESULTS

3.

3.1 Catalytic Results

The

reactivity results obtained with pure oxides using HAc/He

or HAc/Air

mixture

Figure 1

Table 2.

as

reactant

reports

the

are

reported in Table 1 and in

results

for

insulator-insulator

mixtures and Fig. 2 those for insulator-semiconductor mixtures. The and

main

reaction

sometimes,

traces

products of

ethane.

the

rate of production in mo1.h

the

activity

geometry

of

the bed

comparison

of

rate

product the

m

The reported figures indicate

-2

.

These units allow to compare

were

the height

always

production

of the reaction. In

of

and

maintained

the thickness of the

constant.

However, the

of the various compounds is based on CH

4’

this

compound

The values of CO

fact, carbon

2

being a direct

production rate may be

dioxide may originate not only from

photoreaction, but also from the likely presence of substances

such

as

carbonaceous on

CH

The

etc.

conditions, compounds

deposits, carbonate

and

bicarbonate

the surface, from the possible further oxidation of

impurities 4’

reactor,

photoactivity

of

misleading.

-1

of the various compounds, since the photon flow, the

catalyst the

were methane and carbon dioxide

reported

which

were

while

semiconductors.

No

results

are

reached

larger

in

periods

photodecomposition

indicative of steady state 1-2 h

were

for

the

insulator

necessary

for

was detected in absence of

the catalysts and/or of the light. From

the results obtained with pure oxides in the presence of

HAc/He mixture the following considerations were reached:

- both for semiconductor and insulator oxides it is clearly visible

that

the

basic

compounds

are more photoactive than the

acidic ones;

- among the semiconductor oxides, the basic ZnO is sensibly more active are

all

than more

the acidic Ti0

F o r the other semiconductors, which

2’

acidic than Ti0

2

(

8,91, the activity is very scarce

or almost disappears;

- worth noting is the behaviour exhibited by SiO2 samples whose activity was

strongly affected by the pH value of the solution in

154

Table 1 P h o t o r e a c t i v i t y r e s u l t s o b t a i n e d w i t h p y r e o x i d e s u s i n g HAc-He m i x t u r e a s r e a c t a n t ( A : s u r f a c e a r e a , m / g ; t : r u n time, h ; C H 4 , C02: s p e c i f i c p r o d u c t i o n r a t e , 10” mo1es.h-I m - 2 \ I Catalyst ZnO

(Hoechst (home p r e p a r e d 1 ( BDH ) (Carlo E r b a ) wo3 ( Carlo E r b a ) Mn02 (Ventron ) (Hoechst ) ( Carlo E r b a Fe203 (home p r e p a r e d MgO (JMC Specpure 1 MgO ( B D H , pH = 1 0 ) Si02 (BDH, not t r e a t e d ) Si02 ( B D H , pH = 3 ) Si02 ( Akzo Chemie ) Y-A1203 ( Carlo E r b a ) CaO MgAc2*4H20 ( S i g m a ) Ti02 Ti02,

A

t

4 11 0 10 15 21 1 2.5 11 2.5 10 260 240 256 160 6

68 38 73 29 27 15 23 20 25 25 26 33 38 56 19 46

---

CH4 84 35 18 1*7

traces 0 0 0 179.4 717 14 7 2.3 5.4 0 0

co 2 244 59 370 17 traces 0 0 0

922.6 176 29.7 14 11 21 0 0

Table 2 P h o t o r e a c t i v i t y results obtained with pure oxides using HAc-Air m i x t u r e as r e a c t a n t ( A : s u r f a c e a r e a , m2/g; t : r u n time, h ; CH 4’ C02: s p e c i f i c p r o d u c t i o n r a t e , 1 0 ” mo1es.h-I m - 2 ) A

Catalyst ZnO Ti02

(Hoechst ) ( home p r e p a r e d )

MgO Mg 0 Si02 y-A120 3

(Ventron ) ( Carlo E r b a ) (home p r e p a r e d ) (JMC Specpure ) ( B D H , pH = 1 0 ) ( Akzo Chemie 1

wo3 Mn02

(Carlo Erba) ( Carlo E r b a )

4 110 15 21 1 11 2.5 10 260 160

c02 72 31 40 21 23 23 225 5 13 52

484 26.4 33.8 traces 0 29.4 179.4 117 14.2 5.6

0

922.6 176

155

180

170

? /

/ /

8

I

A

I

I

I

//-

I

I

* / -

'A

I

I

i

i

i

I I

I

I

I I

I

I

I

;

i

;

IF-*: Al(0) ,

00

1

I

I

I

I

o----...................................... I

I I

I

0 .................................. ..'"'..'.."~-'' I

50

M g CA,O)

I

I

(%

I

at)

F i g . 1 . E x p e r i m e n t a l r e s u l t s of methane p r o d u c t i o n r a t e f o r t h e Ai 0 - S i 0 2 MgO-Si02 and Mg0-A1203 ( 0 )s y s t e m s . 2 3

(o),

(A,,

100

0

CH4 0 0

-.

production rate 0

0

13

\

(mol. h-l. m-2).1010

\

\

\

\

0

0

w

/

\

/

\

0 /

\

0 0

Q1

157

which

the

samples were

equilibrated

for 'a long

time

before

performing the reactivity tests; - finally

series

the

of

inactive

insulator MgO is the most active of the two

oxides. CaO, which

.

When

basic

the

is

still more basic than MgO, is

reactivity runs were performed in the presence of

HAc/Air mixture, the results suggest the following considerations: - the

presence

of

semiconductors in

oxygen

enhances

the

activity of

the

different amounts, while that of the insulators

is not affected;

- ZnO becomes more active than MgO. The as

photoactivity

expected, i.e.

more

on

In

photoactive.

increases

the

behaviour of insulator-insulator oxides is the line that the more basic compounds are fact, adding MgO

activity, while

decreases the

activity.

specimen MgO-SiO

1:lOO

2

A

the

to

addition of

singular point

which

SiO

resulted

in

is

and

to A1 0

A1 0

to SiO

2

2 3

2 3

2

observed f o r the

being

unexpectedly

active.

For

the

previously

insulator-semiconductor

reported

photoreactivity features ( 8 , 9 ) . photocatalysts and MgO-Ti0 An

1:l

X-ray

solids and they

2

were

on

is not

does In

specimens the general trend

strictly observed, i.e. the observed

not

follow

particular

for

the

reported

the MgO-Ti0

2

acid-base

system,

the

exhibit an activity higher than the pure components showed an exceptionally high activity. analysis carried the

out

on

the insulator-insulator


solids revealed that

amorphous o r badly crystallized. Since the preparation

temperature was rather low, the presence of bulk compounds such as silicates, spinels and

titanates

incipient formation of

disordered

excluded. Thus in which in the

a

the

the

that

very unlikely although the

surface compounds cannot be

solids should be considered intimate mixtures

chemical features of each component are not changed

significant way. idea

is

Indeed the I.R. measurements also support

interaction between

oxides is not strong.

the components of the binary

158

3.2 Infrared Results

The

samples for

for

catalytic

the infrared experiments were the same used

measurements

and

underwent

identical

pretreatment.

Substantial amounts of surface hydroxyl groups were

left

outgassing

after

due

pretreatment, as monitored bands

present

in

all

pure

the

reactions of heavy

examined

low

temperature of

such

by the intensity of the OH stretching

3000-3800 cm-’ region of the background of

ethanoic

acid

with surface sites described as

intense and generally broad OH bands, together with

scattering of

radiation observed with all the samples

in this work, rendered the optical conditions in the high

frequency

region

conditions during acid

the

and mixed oxides. More hydroxyls were formed by some of

follows. The the

the

to

vapour

extremely

was

severe. Moreover,

to

simulate the working

the catalytic runs, a high pressure of ethanoic allowed

onto

the

pellets

and

intense absorptions associated with

adsorbed

species. Full

surface

species

experiments.

Under

this

produced

the newly formed

coverage must be obtained to observe all present

such

during

the

circumstances

photocatalytic

spectra are

better

illustrated in a transmission scale than in an absorbance one. The of

I.R.

spectra of ethanoic acid adsorbed on various types

oxides, described in detail elsewhere

large

showed that a

(7,10-12),

number of different adsorbates are always produced.

present

on

any

A l l are

system though in varying proportions depending on

chemical

properties of the catalyst. The spectra obtained in the

case

a

of

overall

silica support was

picture

first described

(10).

Here, an

of the surface species will be given referring to

the spectra obtained with a tipically basic oxide. Fig. 3

In after for

the

a

curve

standard

few minutes

is

the

spectrum of a MgO pellet which,

pretreatment, was contacted with water vapour and then outgassed at room temperature.

produces

a

catalyst

under working

obtained

by

the

largely

a

hydroxylated

This

surface, similar to that of the

conditions. Curves

b-i

are the spectra

allowing, in succession, doses of ethanoic acid onto

sample at

progressively

increasing coverages.

The residual

I59

vapour at

pressure was lower than 10-3KPa for curves b-e.

1580 and

1440 cm-’

dominate

the

spectra in

Two bands the

first and

sta

I

Fig. 3. Infrared spectra of ethanoic acid adsorbed on MgO. a: hydroxylated MgO ( see text ) ; b-e: after the admission of successive doses of acid vapour (residual pressure lower than 10-3 KPa); f-i: in presence of 0.06,0.26, 1.3 and I . 8 kPa of acid vapour respectively; 1 (dashed): after subsequent outgassing at room temperature for 1 0 minutes.

more

complex

as

the

coverage increases. The absorptions in the

1400-1500 and 1500-1650 cm-’ regions were named A bands ( 7 ) .

high

coverage new

1400-1200 cm-I

curve

ranges with

of

outgassing at

B bands

surface

decreases

species are

in

the

At a

1650-1800

and

growing intensities from curve g to

named B bands ( 7 ) .

i. These were

effect the

absorptions appear

Curve 1 illustrates the

room temperature: only the intensity of

significantly, showing that

largely reversible. The

the related

intensity

of the

A bands is not affected. The are were

bands

observed described

due in in

to

the

the stretching modes

high

Of

surface OH g r o u p s

frequency region ( 3 0 0 0 - 3 8 0 0 cm-I

and

details elsewhere (13,141. They are extremely

160

intense in the background and become more intense and broader upon adsorption relevant

of

and

ethanoic

acid. Apart

detailed

species can

be

from

information on

derived

from

the

this

observation no

the structure of adsorbed

spectra in

that region and,

consequently, they are not shown. A

and

B bands were constantly observed in the spectra of all

pure

oxides ( 7 1 , with minor differences in position and shapes but

with

major differences in relative intensities. The importance of

the

B bands

grows

catalyst. The

in

proportion to the acidic properties of the

spectra of

resemble

those

confirming

that

ethanoic acid adsorbed on mixed oxides

obtained with no

new

phases

the are

parent

produced

pure

components,

in the conditions

adopted in these studies for the preparation of the catalysts.

4.

DISCUSSION The

vast

amount

of

reported

the

nature

of

the interaction adsorbate-adsorbent and

following for

understanding

several

photodecarboxylation discuss

first

detected

the

of

data

presents

aspects

of

adsorbed ethanoic acid.

various

adsorbed

the

a

basis

reaction

for of

Let us therefore

species and their nature as

by I.R. spectra. Then a discussion on the photocatalytic

behaviour of the various types of catalysts will follow. The acid of

infrared

spectra obtained

after adsorption of ethanoic

on oxides are rich with bands and this indicates the presence

a

large variety

of

surface species. Moreover, each band is

complex, showing that families of slightly different structures of the various types exist due to surface heterogeneity. The

results of

desorbed

at

room

Fig. 3

temperature

general observation valid the

product on

for

and

others

are

not.

This

is a

all oxides, and it was shown that

former reversible species (easily desorbed species) retain the

molecular the

show that some adsorbates are easily

structure of of

case

acid-base

ethanoic acid and the latter ones are the

reactions with surface sites (7,lO-12).

In

of MgO, the surface sites are cations and anions exposed 2+

the surface with coordination lower than in the bulk, (Mg and LC

161

and

groups which

OH-

are

not

fully desorbed during the

thermal pretreatment of the oxides. 4.1 Irreversible Species Cation-anion couples may react with an acid molecule to give:

Acetates

are

produced which may be stabilized essentially in two

ways as shown in the following schemes:

In

the bidentate structure, the oxygen atoms of the carboxyl group

equally only

interact with the metal cation, whereas in the monodentate

one

oxygen

carbonylic

nature

carbonyl

is

of

coordinated to a cation preserving the

the

surface species. This

structural

be relevant in photocatalysis as the excitation of

difference may the

atom

group

is

believed to play a role in the photolytic

mechanism to be discussed later. Both

acetate

spectrum. The in

the

species are related to

1400-1600 cm-'

species

(curves b - d )

bands in the infrared

bidentates are monitored by a couple of absorptions

and

region

associated with

the

asymmetric

of the carboxyl group. Figure 3 shows that

stretching vibrations these

A

appear that

the

in

the

first

stages of

separation between

adsorption

the two bands is

compatible with the bidentate structure ( 7,11 ,12,15-171. The monodentate carbonyl band shoulder of

acetates

are

associated with a stretching

contributing to the intensity in the high frequency the

broad

A band

(

1650 crn-' 1.

such ester species are at lower frequencies It

has

acetates and

been this

(

Other absorptions of

WCO

group shifts

or in non polar

Thus the donor effect

larger than that of the hydroxyl group, is

able to induce a red shift of 6 6 . 5 nm on the

)CO

group.

Therefore, it seems likely that the photoactive center is the chromophore

\

/

group of particular species adsorbed and probably

CO

These groups can be free or engaged with

the monodentate acetate. hydrogen n-@

bonds

and-

over acid Brijnsted sites. For the first, both the transitions are red shifted, for the second the n-;rt*

is blue shirted while then-* The

broad

red

transition is red shifted.

shift can

be

caused

essentially by

the

following factors: a ) electron donor

effect

of

the

solid surface on the ,\CO

group

adsorbed : b ) solid surface potential:

c) interaction of

\

excited state ,CO

group with bipolar couples of

the solid surface. The allows with

synergic effect

absorption the

of

all the previous factors eventually

radiation at wavelengths higher than 300 nm

of

final effect of

dissociating the

CH COOH 3

adsorbed over solid oxides. The experimental data by

decreasing

the

molecule

show, f o r the studied catalysts, that

electron-donor

properties

of

the

surface a

parallel decrease in the photoreactivity of the adsorbed molecules occurs.

Mainly

photoreactive sites are

acid likely

higher

surfaces, also because

than

the

pK

if

in a lesser extent, are

pertaining to some acid a that of the,adsorbed ethanoic acid. When

165

the

surface acidity

with

or

NaOH

HF

is alterated, as in the case of SiO

treated

aqueous solutions, the photoreactivity changes,

namely it decreases when the basic properties decrease. As

for

the

photocatalytic

behaviour of the semiconductors,

the situation is more complex.

For absence

the of

pure

oxides, when

oxygen,

determined

by

the

the

order

the and

acid-base

reaction is carried out in

the

level of

properties.

This

activity is

a

is

clear

indication that under those conditions the photolytic mechanism is prevailing.

As

photoactivity

oxygen of

present

is

pure

the reactant mixture, the

semiconductors is much enhanced at various

degrees, Evidently, this much

in

occurs

because

the redox mechanism is

favoured by the presence of oxygen, most likely adsorbed onto

the

surface, acting

as

a

trap

affinity.

strictly

followed due to the mixing of both mechanisms.

under

resulted

these

this

electrons due to its high

electron also

Thus, in

for

conditions a

case the general trend is not However,

basic semiconductor, namely ZnO,

in being the most active, indicating that the interaction

between ethanoic acid and a basic surface is still important. For

the


experiments were activity

does

carried

not

follow

out

specimens, for which

the

in absence of oxygen, the order of

the

acid-base

behaviour, as already

mentioned.

Evidently, the occurrence of both mechanisms at levels

which

probably

are

does

not

affected by the ratio insulator/semiconductor €or

allow

explaining

the

trend.

more

A

complete

discussion can be found elsewhere ( 12 ). Worthy

of

system, For the high

pure

attention are

any

the MgO-Ti0 2 compositions the mixtures are more active than Evidently, a

considerations can experimentally heteroexchange the

2

results

for

components, the specimen MgO-Ti0 2 1 : l displaying a very

activity.

MgO-Ti0

the

1:l

be

is more

used Ti02.

advanced

found, ( OIE ) ,

synergic

by that

for

effect is operative. Few

this

effect.

experiments the

oxygen

of

It

oxygen

for

the

has been isotope specimen

labile than for the other specimens and for

This finding indicates a higher suitability of the

166

oxygen

species present

reactions ( 2 1 ) . photoreactive

It

on

can

adsorbed

this be

specimen

envisaged

species, which

to

also react

perform oxidation that by

the

excited

the photolytic

mechanism, may further be oxidized by the photoproduced holes. ACKNOWLEDGEMENT The

financial support of CNR (Roma) and MPI (Roma) is gratefully

acknowledged.

REFERENCES 2

H. Terenin, Adv. Catal. , 1 5 ( 1 9 6 4 ) 227-283. C.H. Nicholls, P.A. Leermakers, Adv. Photochem., 8 ( 1 9 7 1 )

3

G. Kortum, J. Vogel, W. Braun, Angew. Chem., 7 0 ( 1 9 5 8 )

4 5 6

M. Anpo, Y. Kubokawa, J. Phys. Chem., 7 8 ( 1 9 7 4 ) 2446-2449. Y. Kubokawa, M. Anpo, J. Phys. Chem., 7 8 ( 1 9 7 4 ) 2442-2446. M. Schiavello, in: M. Schiavello (Ed), Photocatalysis and Environment, D. Reidel Publishing Company, Dordrecht, 1 9 8 8 , in press. A, Sclafani, L. Palmisano, M. Schiavello, V. Augugliaro, S. Coluccia, L. Marchese, New J. Chem., 1 2 ( 1 9 8 8 ) 129-135. K. Tanabe, in: Solid Acids and Bases, Academic Press, New York, 1 9 7 0 . K. Tanabe, in: J.R. Anderson and M. Boudart (Eds), Catalysis, Springer Verlag, Berlin, 1 9 8 1 , vol. 2 pp. 231-273. V. Augugliaro, L. Palmisano, M. Schiavello, A. Sclafani, J. Cat., 9 9 ( 1 9 8 6 ) 62-71. L. Palmisano, A. Sclafani, M. Schiavello, V. Augugliaro, s. Coluccia, L. Marchese, New J. Chem., 1 9 8 8 , 1 2 ( 1 9 8 8 )

1

31 5-336.

7 8 9 10 11

651 -655.

137-1 41

.

I,. Palmisano, M. Schiavello, A. Sclafani, S. Coluccia, L. Marchese. New J. Chem., in press. 1 3 S. Coluccia, L. Marchese, S. Lavagnino, M. Anpo, Spectrochim. Acta, 43A ( 1987 1573-1 576. 14 S . Coluccia, S . Lavagnino, L. Marchese, Mat. Chem. Phys. 12

1 8 ( 1 9 8 8 ) 445-464.

15

D.M. Griffiths, C.H. Rochester, J. Chem. SOC., Faraday (

16

1977

1988-1 997.

C.H. Rochester, S.A. Topham, J. Chem. SOC., Faraday

1,

1 , 73

75

( 1 9 7 9 ) 1259-1 267.

17

V. Lorenzelli, G. Busca, N. Sheppard, J. Catal. 66 ( 1 9 8 0 )

18

V. Augugliaro, L. Palmisano, M. Schiavello, A. Sclafani, in: E. Pelizzetti and N. Serpone (Eds), Homogeneous and Heterogeneous Photocatalysis, D. Reidel Publishing Company, Dordrecht, 1 9 8 6 , pp. 567-580.

28-35.

167

19 M. Simonetta, S. C a m & , in: S. Patai (Ed), The Chemistry of Carboxylic Acids and Esters, Wiley, New York, 1969. 20 C.N.R. Rao, in: Ultaviolet and Visible Spectroscopy, Butterworths, London, 1967. 21 P. Pichat, in: M. Schiavello (Ed), Photoelectrochemistry, Photocatalysis and Photoreactors Fundamental and Developments, D. Reidel Publishing Company, Dordrecht, 1985, pp. 425-455.

168

ESR STUDIES OF ALKYL RADICALS ADSORBED ON POROUS VYCOR GLASS H. D. GESSER INTRODUCTION

1.

Nwrerous properties have been used to character%ze the surfaces of solids but

fPw

have been as useful as Electron Spin Resonance Spectroscopy (ESR).

C4R has been effective in elucidating the active surface sites as well as the

surface reactions. Hydrocarbons are involved in a variety of reactions and the alkyl radical is a primary intermediate in many such reactions. The alkyl radical is usually the first product in the pyrolysis and combustion of hydrocarbons.

Such radicals have been detected and stabilized in glasses at

low temperatures and on surfaces at low and ambient temperatures. The properties of such radicals, their formation, detection and reactions is the subject o f this chapter. ?he earliest ESR study of the CH and C H radicals was reported by Gordy 3 2 5 and McCormick (1) who investigated the X-ray decomposition of CH and C 11 3 2 5 compounds of zinc, mercury and tin at 77 K. Irradiated Zn(CH ) gave a 3 2 quartet having a total spread of 70 to 80 G (gauss) which decayed over a period of ten days. This quartet was assigned to the CH3 radical trapped in a symmetrical cage and rapidly inverting (non-planar) at 77 K.

Irradiated

Hg(C H ) produced a 6-line spectrum (1:5:10:10:5:1) which was assigned to 2 5 2 C H where the 5 protons were equally coupled to the spin vector of the odd 7 5 PI ec tron. Smaller and lfatheson (2) y-irradiated methane at liquid helium temperatures. The CH radical was not observable until the temperature was 3 raised to 20 K (1iq.H ) . The spectrum had a total spread of 80 G and the 2 relative intensities were 1.01:2.84:3.00:1.16. The linewidth was 3.5 G. In order to confirm the above assignment (CH radical) they theny-irradiated 3 mercury dimethyl at 77 K. The observed four line spectrum had a total spread of 76

G,

relative intensities of 1.30:3.00:2.74:.73,

and linewidth of 10.8 G.

Jen, et al. (3) discharged methane at 4.2 K and subsequently observed the radical spectrum. The hyperfine splitting (hfs) was 6 4 . 3 9 MHz and the 3 g-value was 2.00242. The relative intensities at 4 . 2 K were 1:2.2:2.2:1. At

CH

4.2 K the spertrum was corrsidered to be partially saturated since on warming the relative intensities approached 1:3:3:1.

A similar spectrum was observed

for the IJV irradiation of 1% CH I in an argon matrix. 3

Jen, et al, (3) a l s o

169

TABLE 1 Effect of Different Matrices on Some Methyl Radical E.S.R. Parameters ( 3 ) MATRIX L . W . 1/2Max.Ht. G ) a(MHz) g.J H: Ar CH4

2.00266

65.07

1.4

2.00203

64.64

3.7

2.00242

64.39

4.5

discharged partially deuterated methane at liquid heljum temperatures. The resulting spectrum was analyzed and the following species were reported: CD3, Jen, et al, studied the CH radical in different CD2H, CHD2 and CH3. matrices.

3

(Table 1 ) .

The binding energy of the matrices increased from H2 through CH4.

The g-value was unaffected while the hfs decreased slightly on increasing binding energies.

Jen, et al, concluded that the linewidths are more dependent on the

nature of the matrices than on the properties of the radicals.

T. Cole, et

al, ( 4 ) looked at C 1 3 enriched CH3 radicals at 77 K produced by X-irradiation of methyl iodide. The C 1 3 hfs was measured to be 4123 G which was taken as strong evidence for a planar structure for the methyl radical.

Other early

ESR studies on alkyl radicals were reviewed by Garbutt. (5). 2.

SLTZFACE STABILIZED ALKYL RADICALS. Some of the earliest ESR studies of surface stabilized radicals were

performed by Russian workers.

Pariiskii, et al, (6) have studied CH3 radicals

adsorbed on silica gel at low temperatures. 2

The silica gel, having a specific

surface area of 700 m /g, was outgassed in a vacuum for 6 h at 573 K before methyl iodide adsorption at room temperature.

The silica gel samples were

then irradiated at 77 K for 4-6 h with a UV source. The ESR spectra, recorded at 77 K, consisted of four lines of 1-2 G width, spaced 24.2 and the g-factor was 2.001 were 1:8.5:13:2.5

?:

f

0.5 G apart,

0.001. The relative amplitudes of the four lines

instead of the binomial values 1:3:3:1.

described above was assigned to the CH

3

The spectrum

radical. The unusual relative

amplitudes were ascribed to incomplete neutralization of the anisotropic hyperfine spin-orbital interactions. Pariiskii, et al, thought of the CH radical as being bound to the surface by a one-electron bond due to the

3

attraction of the unpaired electron to the adsorbent. In this situation only the rotation about the three-fold symmetry axis would remain. The small linewidths (1-2 gauss), however, indicates that other motions of the stabilized CH radical are still important. The CH radical was found 3 3 to be very stable on the silica gel surface. Kazanskii and Pariiskii ( 7 )

170

ohserved the ESR spectra of C2H5 radicals after Y-irradiating ethane adsorbed H cn silica gel at 77 K. The reported proton hfs were: la I = 20.5 G H and IU I = 27 G . The C2H5 radical was pictured as being bound to the surface at one end only.

The C H radicals were found to be quite stable on 2 5 n H were increasing the temperature but no measurements of l a 1 and 16 I

reported at increased temperatures. The adsorbed radical resembled an entirely free one very closely; its unpaired electron taking no appreciable part in the bond to the surface.

Pariiskii and Kazanskii (8) have also used

the ESR technique in studying the recombination of hydrogen atoms, formed on a silica gel surface by the radiolysis of its hydroxyl groups, and their reactions at low temperatures (103 to 173 K) with oxygen and ethylene.

Much

nf the work done by Kazanskii and Pariiskii on the ESR of free radfcals

adsorbed on catalysts has been presented in summary ( 9 ) .

They concluded, on

comparing the hfs in the spectra of radicals adsorbed on silica gel with the hfs for the corresponding free radicals, that the disturbing action of the surface on the unpaired electron cloud of the adsorbed radicals is very small. They estimated the strength of the one-electron bond to the surface to be 3-5 hca?/mole. Analysis of the amplitude ratios of components in the ESR spectra of adsorbed alkyl radicals gives some information about the geometry of these

radicals and their motion in the adsorbed state. The conclusion reached, for the CH3 radical, was that there is a loss of two rotational degrees of freedom and there remains only one axis of rotation which is the three-fold symmetry axis. Interpretation of the adsorbed C2H5 radical spectrum showed that the radfcal "lies on the adsorbent surface on its side", and there is rotation of its CH and CH groups relative to each other. The C H radical was 3 2 2 5 postulated to "roll" along the surface as well. Adsorbed alkyl polymer radicals also "lie on their sides". Kinell, et al, (10) studied the ESR absorption of hexane/silical gel systems y-irradiated at low temperatures. Several lines due to defects in the silica gel were observed.

One of these lines (at g = 2.0008)

by two satellite lines (separation of 10 G ) assigned to Si29.

was accompanied Ethyl radicals

have been definitely identified as one of the irradiation products. The yields of paramagnetic species in the two phases imply an interaction between the components and the magnitudes of the yields strongly suggest an energy transfer from the silica gel in the formation of free radicals. Noble

et al,

showed that CH radicals may be stabilized for long periods in zeolites at 3 These radicals were generated by Y-irradiation temperatures below 90 K (11). of methane sorbed on synthetic zeolite (Na12[(A102),2(Si0 Linde Type A ) .

) ].27H20 2 12 Several unidentified species were observed besides the more

stable CH3 radical. The CH radical had the following parameters: 3

171

g

=

H 2.0026; la I = 21.9 G; relative intensities 1.0:3.1:2.8:0.9. Electron spin resonance studies have been made of the defects introduced

in porous Vycor glass (PVG) by Y-irradiation. ( 1 2 ) .

On analysis of the

defects, boron (which occurs as an impurity in PVG) is postulated to be trigonally coordinated in PVG in contrast to the tetrahedral coordination

ESR techniques have been used to study the effects of added gases on the paramagnetic defects introduced in PVG by y-irradiation. observed in other glasses. (13).

Three paramagnetic defects have been identified on Y-irradiation of PVG

(12):

(a)

an electron trapped on a silicon atom with at least one oxygen

atom missing (Si site), (b) a hole trapped predominantly on an oxygen atom bonded

to

a trigonally coordinated boron atom (B-0 site). and (c) a hole

trapped in the Si-0 network (Si-0 site).

Defects (b) and (c) are thought On

to

be formed by the removal of a hydrogen atom from a hydroxyl group.

adsorption of hydrogen and deuterium, defects (b) and (c) are removed but defect (a) remains (13).

This effect is irreversible which implies that

strong chemisorption is occurring, possibly with the formation of SiOH and BOH groups.

Adsorbing oxygen, nitric oxide, nitrogen and ammonia did not affect Since oxygen and nitric oxide have no

the ESR spectrum of the defects.

effect, it may be concluded that the defects are located in the bulk of the solid and are not accessible to oxygen and nitric oxide. Turkevitch and Fujita ( 1 4 ) were the first to report CH3 radicals The CH radicals were produced by 3 UV photolysis of adsorbed methyl iodide. Deuteromethyl and carbon-I3 enriched stabilized on PVG (Corning Glass No. 7930).

2

radicals were also studied. The PVG, which had a surface area of 144 m /g 3 (B.E.T. method), was pretreated in oxygen at 873 923 K for flow system

CH

-

experiments and was further treated at 773 K in a vacuum for static experiments. The surface coverage of methyl iodide was estimated to be about

2% of a monolayer for the static experiments. However since the sample was loaded with liquid substrate and excess subsequently removed by evacuation it is most probable that the actual value was much larger than the 2% estimated. The sample irradiation was carried out with a 500 W low-pressure mercury lamp. A sample of frozen methyl iodide was also irradiated at 77 K. and the ESR

parameters of the resulting CH

a/

From Table 2 the ratio la

the theoretical ratio of 6 . 5 1 4 .

I

la

I

22.7/3.54 = 6 . 4 1 which is lower than

The linewidths indicate motion of the CH3

radicals on the surface even at 77 K. 38.5 G.

-

radicals were measured (Table 2).

The measured value of Iac131 was

It was found that the CH radicals were stable for days at room 3

temperature, decaying with a half-life of about 100 h ( 1 5 ) .

It was also found

t h a t in the static system, prolonged irradiation resulted in a constant

concentration of the radical.

In the flow system the yield increased linearly

172 TABLE 2 Characteristics of Methyl and Deuteromethyl Radicals (12)* Condition Total Spread (G) HFS(G) L.K.(G)

Relative Intensities

METHYL RADICAL CH31 Matrix at 77 K

68.8

Adsorbed on PVG at Rm Temp 67.8 Adsorbed on PVG at 77 K

68.8

Adsorhed on PVG at Rm Temp 21.2

23.2

3.6

1.0:2.8:2.3:1.0

22.7

1.0

1.0:3.0:3.3:1.1

22.9 0.8 1.0:3.7:4.1:1.3 DEUTERO METHYL RADICAL

3.54

1.8

1.0:3.3:6.8:7.8: 6.5:3.2:1.4

*Copyright 1966 by the AAAS

with time of irradiation and eventually leveled off.

The interaction of the

CH radicals was studied with several gases, Fig. 1. 3 for the CH radical with C2H6, n-C4H10 or toluene. 3

No reaction was observed

3.

FORMATION OF RADICALS

Methyl radicals have been formed and stabilized on surfaces by (a) Y-radiation of adsorbed CH4 (11, 16, 17), CH3C1 (18, 19) or CH31 (20) at 7 7 K (b) the UV photolysis of adsorbed CH31 at 77 K (3, 5 , 14, 21) (c) the

photosensitized decomposition of CH on PVG coated with V205 or Moo3 (22, 23) 4 or on ZnO withX>400 nm (24, 2 5 ) , (d) the triphenylamine photosensitized decomposition of adsorbed CH3Br at 77 K and A> 300 nm (26) (e) the photolysis of adsorbed trimethylamine, dimethylamine and tetramethyl urea (27) and azomethane (28) at 77 K. Ethyl radicals were surface stabilized by the Y-radiolysis of ethane (7, 18) ethylene (8, 18) and ethyl chloride (18) adsorbed at 77 K. Ally1 and methyl allyl radicals were formed and stabilized on silica gel at 77 K by the y-radiolysis of allyl iodide and butadiene

respectively (20).

Methyl radicals were also detected at room temperature

when Al(CH ) on silica was exposed to air or oxygen.(29) 3 3 No stabilized alkyl radicals were detected at 77 K when the following adsorbed substances were U.V. photolyzed:

methylethyl ketone (28) acetone,

dimethyl mercury, C2H51, i-C H I, C6H5CH3, C H CH C1 (14). Attempts to 3 7 6 5 2 produce and stabilize the methylene radical, CH2 by photolyzing ketene on PVG proved to be unsuccessful (28). It must be stressed that the co-adsorption of mercury (from the vacuum line) could lead to mercury photosensitization of the adsorbed gas and great care must be taken to distinguish sensitizatlon by the surface or other substances on the surface with mercury sensitized decomposition. The use of l o w pressure mercury resonance lamps which produce the 253.7 nm mercury line

4

173

D

3a

s H \

H

t

TIME

IN MINUTES

Fig. 1. Tnteraction of gases with methyl radicals at room temperature. ( 1 4 ) Copyright 1966 by the AAAS

Fig. 2 . Build-up of methyl radicals stabilized on PVG surface at 7 7 K. The amplitudes (peak-to-peak) of the f o u r lines are standardized to the same gain setting. ( 5 )

is most likely to cause sensitjzation, though a matrix shift or broadening of the mercury resonance transition, which can overlap with the reversal of this line in a medium o r high pressure mercury lamp, can also occur to cause sensitized decomposition. This aspect warrants further study. The time dependence of the CH radical concentration on a surface during 3 jts formation by the photolysis of adsorbed CH31 at 7 7 K results in the ccncentration approaching a constant value ( 1 4 ) .

This was explained by the

possible back reaction of CH3 and iodine.

This was confirmed by Fujimoto et a1 (30) who indicated that the saturation value (about 1 0 l 6 radicalslm2 ) seemed to be independent of surface coverage if the coverage ( 0 ) was less than 10 layers.

However the approach to saturation depended on the coverage and, The relative amplitudes of the 4 lines of the

of course, the absorbed light.

CH radical as a function of W radiation time is shown in Fig. 2 (5). The 3 CH3 was produced by the photolysis at 77 K of CH31 adsorbed on PVG (e = 0.05). Similar plateaus ( 1 9 ) were also reported by Oduwole and Wiseall ( 3 1 ) arid Barnes et a1 (32) for the formation of the abnormal CH3 radicals H

(la

1

= 19

-

71 G) formed on silica gel and A 1 2 0 3 respectively and in the

y-radiolysis of CH4 encapsulated in a 3A molecular sieve ( 1 7 ) . During the photolysis of CH I on PVG at 77 K the PVG is colored yellow due to the 3 formation of I on the surface. At room temperature the PVG becomes colored 2

174 violet.

Hence it would appear that the plateau is probably not due to the

back reaction CH3

-I

I -)CH31

nor due t o the reaction

+

CH3

12-J

CH31

+ I

because of the energetics involved.

An appropriate explanation must await the

results of further work. 4.

THE METHYL RADICAL

There is more than one type of CH radical (30) which is capable of being 3 a normal CH3, (Me), for which h H 19.3 20.2 G la I= 23.4 GI g = 2.0024, and an abnormal CH3, (Me'), where la

formed on a surface and stabilized: and g

=

2.0016.

I=

-

The formation and characteristics of Me' will be treated in

the next section. Garbutt (5) made a detailed study of the ESR of CH3 radicals stabilized on PVG.

The temperature dependences of the hyperfine interaction, linewidths

and line asymmetries of CH3 radicals on PVG were reported (33).

Comparisons

were made with Schrader's calculated values (34) based on a model of

A'2 vibrational mode (out-of-plane bending The temperature range studied was

incomplete orbital following in the mode) for a planar CH3 radical.

K for CH3, CD3 and 13CH3. The agreement was excellent except for the C-13 hfs which dropped off at temperatures below 120 K. This was attributed by Symons (35) to restricted tumbling and a perpendicular coupling 7 7 K to 400

of CR to the surface and not due to a flattening of the radical or some major 3 delocalization. Further evidence in support of this interpretation has been

presented (36). Fujita et a1 (37) have reexamined the 13C hfs of both normal CH3. (Me), and the abnormal radical (Me') and report no fall-off for the C-13 hfs at low temperatures for Me.

The temperature study for Me' could not be made due

its reactivity at temperatures above 77 K.

to

Other theoretical studies and

models of the CH radical have been described (38-40) but surface interactions 3 havc yet to be resolved. tiarbutt and Gesser (41) were able to partially resolve the second-order splitting of the center two lines of the CH3 radical stabilized on PVG at 7 7

P.

This second-order splitting was predicted by Fessenden (42) and confirms

the presence of only weak interaction between the radical and the surface. Such second-order splitting was also reported for CH3 on silica gel, PVG, and Cabosil (43).

Simulation showed the 1:2 relative intensities predicted when

linewidth and separation were considered. It was also shown that the linewidth varied considerably with surface coverage by the CH I. 3

(See Table 3).

175 TAHLF 3

Li.newidtiia dependence on s u r f a c e coverage observed a t 77 E from s i l i c a g e l samples p r e t r e a t e d a t 1073 K ( 4 3 ) -.

x

Coverage

Line 1

Linewidths/G (20.02) Line 3 Line 3'

2nd o r d e r splitting/ G(+O. 02)

L i n e 1'

0.24

0.65

0.32

0.20

0.41

0.23

2.1

0.63

0.32

0.22

0.39

0.23

8.6

1.1

0.70

0.40

0.57

c a . 0.16

1.8

1.25

0.52

0.70

30

--

aLinewidths r e f e r t o t h e peak-to-peak w i d t h of t h e 1st d e r i v a t i v e c u r v e . G a r h u t t and G e s s e r ( 4 1 ) a l s o showed t h a t t h e l i n e w i d t h measured a t 7 7 K depended on t h e t h e r m a l h i s t o r y of t h e sample even under c o n d i t i o n s where r a d i c a l decay d i d n o t occur.

Thus a sample which was r e p e a t e d l y warmed t o

193 K showed a narrower l i n e w i d t h when measured a t 77 K and which broadened when s t o r e d o v e r n i g h t a t 77 K.

T h i s was e x p l a i n e d by p o s t u l a t i n g t h e

e x i s t e n c e of two t y p e s of p o t e n t i a l w e l l s o r s i t e s on t h e s u r f a c e .

Radicals

i n Tvpe I w e l l s , which have v a r y i n g d e p t h s , can decay o n l y by a c q u i r i n g s u f f i c i e n t energy t o l e a v e t h e s i t e . (30) and w i l l b e d i s c u s s e d l a t e r .

T h i s e x p l a i n s t h e cascade decay observed

S i n c e t h e r a d i c a l s do n o t decay a t 77 K ,

any change i n l i n e w i d t h could b e due t o m i g r a t i o n of t h e r a d i c a l s t o d i f f e r e n t s i t e s (Type 11) w i t h i n t h e Type I p o t e n t i a l w e l l .

Thus Type I1 s i t e s a r e

s h a l l o w w e l l s which a l l o w f o r m i g r a t i o n o r r e o r i e t a t i o n b u t n o t decay. The absence of s u r f a c e d i f f u s i o n by CH (28).

on PVG a t 77 K h a s been v e r i f i e d 3 A 9.5 cm l o n g r o d of PVG was c l e a n e d and evacuated a t 873 K and t h e n

loaded w i t h CH I t o a 5% s u r f a c e coverage and s e a l e d o f f . The sample w a s 3 photolyzed a t one end and CH r a d i c a l s were d e t e c t e d . The sample was t h e n 3 i n v e r t e d i n l i q u i d n i t r o g e n and t h e o t h e r end showed no ESR s i g n a l w h i l e s t o r e d a t 77 K f o r s e v e r a l days. f o r CH- was o b t a i n e d .

When r e - i n v e r t e d ,

The s u r f a c e d i f f u s i o n of CH

c r u l d b e followed by t h i s approach.

3

t h e o r i g i n a l ESR s i g n a l

a t higher temperatures

Oduwole and W i s e a l l ( 4 4 ) were a b l e t o s t a b i l i z e t h e normal CH

r a d i c a l on 3 The r a d i c a l s were produced Ly

b a s i c and n e u t r a l A 1 0 a t room t e m p e r a t u r e s . 2 3 t h e y - r a d j o l y s i s of CH I ( €i= 2X) a t 300 K. The CH3 r a d i c a l could n o t b e 3 s t a b i l i z e d a t 300 K when produced on a c i d i c A1203, s i l i c a g e l o r PVG. The

s t a b i l i z e d r a d i c a l , on A1203, d i d n o t r e a c t w i t h 0 2 , H and showed almost 2 complete r e c o v e r y of t h e s i g n a l w i t h H,O, CH I , a c e t o n e and methanol upon L 3 e v a c u a t i o n . Linewidths of CH on A 1 0 were 2 t o 4 t i m e s wider than on PVG 3 3 3

176

and was interpreted to mean that the CH3 radical is fixed and immobilized. Though this explapation is consistent with the low reactivity of the radical, it is inconsistent with CH3 motion in fused silica (45).

5.

THE ABNORMAL METHYL RADICAL It was initially (30) believed that the abnormal CH3 radical (Me') which

decayed at 77 K (half-life of about 12 h) formed another radical X.

In a

second paper ( 4 6 ) the authors assigned X to the interaction of CH3 with boron on the surface. This interpretation was soon corrected by Garbutt and Gesser

(LO) who showed the Me' does Cot decay into X and is only formed when the PVG had been outgassed at 973 to 1123 K under high vacuum, and with less than n monolayer of CH I (preferably 8 = 0.01). The values of the proton hfs was 3 corrected to 19.3 f 0.05 G. The radical X was primarily formed on PVG which had been outgassed at about 723 K. each of the four proton lines.

Three satellite lines were identified for

Based on the pretreatment of the PVG with H20

(which lead to CH with large satellite line intensities) and D20 (which 3 yielded CH with almost no satellite lines) it was concluded that X is due to 3 CH on various sites, e.g. H3C..***HO-Si< for the nearest satellites (the AA' 3 lines). The BB' satellites were assigned to concurrent spin-flip transitions of the electron and a nearby proton whereas the CC' satellites were assigned

to spin-flip transitions due to two neighbouring protons concurrently changing state. / The radical Me' was assigned to H C""B9 interaction in view of the 3 reported absence of Me' on a silica aerogel. This has since been shown to be incorrect ( 4 7 ) insofar as abnormal CH3 radicals can be produced on silica gel

(21) and A1203 (32).

H Shimamoto et a1 (49) produced Me' ( I a I = 19.3 f 0.3 G, g = 2.0023 f 0.0003) by the photolysis of CH4 adsorbed on PVG using a low pressure mercury lamp. The radical was stable at 77 K but decayed rapidly when warmed. In the presence of I2 only the normal CH3 radical was produced. This could explain why Me' is only formed during the early stages fn the photolysis of CH3 I when I2 has not had a chance to form. When PVG was loaded with V205, C r 0 or Moo3 ( 8 = 0.01 to 0.001 X) the resulting radical formed was Me' when 2 3 the adsorbed CH ( 8 = 0.1) was photolyzed at 77 K with a high pressure mercury 4 lamp (23). The PVG(V 205) showed a photosensitized formation of CH3 even at A > 420 nm. In the presence of trace amounts of O2 the peroxyradicnl was observed instead of CH3. Fujita et a1 (37) In their C-13 study on PVG concluded that Me' was probably trapped at a siloxane oxygen bridge. Me' was also reported to be formed when ZnO doped silica gel with adsorbed CH31 was photolyzed at 77 K

177 (48).

I? The smaller hfs ( la

I

= 18.8 2 0.6 G)

was attributed to the interaction

of CH with the ZnO and delocalization of the radical's unpaired electron. 3 The Me' was not formed when the ZnO/Si02 gel was dried in the presence of 0 Could a surface oxygen deficiency be

2' requirement for the formation of Me'?

a

Using the photolysis of CH I or the photosensitized decomposition of CH 3

4

to generate the CH radicals Kubota et a1 (49) showed that two types of 3 abnormal radicals Me' and Me" were formed in the early stages when the gel was heated at 873 K in vacuum for 5 h.

The intensity of these two radicals

increased with irradiation time up to about 30

s

and thereafter decreased

while the normal radical, Me, appeared and eventually dominated. The H

following characteristics were reported: Me' la I= 20.7 G, g = 2.0024; Me" l aH I= 21.2 G ; g = 2.0023 and Me laH I = 23.0 G , g = 2.0027. Based on linewidth properties and p values it was concluded that Me' and Me" are non-planar with strong surface interaction probably due to the siloxane site. When CH is produced by the photosensitized decomposition of CH4, the 3 abnormal CH did not change implying that the iodine produced during the 3 photolysis of CH I destabilizes the non-planar abnormal radical. Kubota et a1 3

were also able to obtain the normal CH radical and its satellites lines on silica gel pretreated at below 30OoC.

3

They associated the outer lines with a

spJn flip-type transition and the inner lines with two different adsorption and H C-.-*HO-Sie. This is partly sites of CH such a s H C-*-.~O-Si< 3' 3 3 consistent with the H-D surface exchanged work previously described ( 4 1 ) . The abnormal CH radical has been stabilized on various silica surfaces 3 Low surface coverage by CH I (6* 0.2%) was required with high (31). 3 temperature (TdelOOO K) pretreatment under vacuum. Second-order splitting was discernable and two species were identified (Td" 870 K) confirming Kubota's ( 4 9 ) observation, Me" was unstable and associated with siloxane bridges.

Intense W irradiation was shown to increase the decay rate of the

abnormal CH radical. In a continuing study (5C) a comparison was made 3 hetween unmodified silica gel ( S ), pre y-irradiated silica gel ( S ) and S 2 1 2 samples flame annealed ( S ) as substrates for the adsorption of over a 3

monolayer of CH I followed by the removal of excess weakly bound CH I by 3 3 evacuation. S samples led to normal CH radicals with prolonged photolysis 1 3 at 77 K reaching a saturation value which increased with increase in Td. For Td

=

1073 K, both Me and Me' were obtained where Me' disappeared and the Me

signal was reduced as the sample was warmed from 77 K to 300 K. CH were formed bhen the sample was rephotolyzed at 77 K.

Both types of

On UV photolysis at

3 77 K S2 samples (Td = 1073 K) showed o n l y Me which disappeared on warming to

room temperature. UV photolysis of S3 samples at 77 K produced only Me radicals which persisted at room temperature.

In constrast to S2 samples, Me

178

could be obtained at room temperature by photolysis of S3 sample. Two explanations were offered to explain the results.

(1)

The residual Si-OH

groups (either isolated or geminal) are the adsorption sites of CH31. strong adsorption distorts the CH3 group which on photolysis leads

The

Me'. The absence of Me' on non-porous Cabosil (31) further suggested that a pore structure was responsible for the strongly adsorbed CH31. ( 2 ) The high to

temperature treated silica gel enhances the formation of siloxane bridges which results in electron deficient point defects.

Such electron deficient

sites are neutralized by radiolysis and accounts for the absence of Me' on S2 samples. The authors conclude that a combination of both explanations is responsible for the abnormal CH,, radical. A comparable study (52) on A 1 0 was consistant with the results obtained with silica gel (50). CD

radical and C2H5 radicals were obtained (51) by the sensitized photoC H on PVG(V205) at 77 K respectively. For the

3

decomposition of CD4 and

abnormal C2H5, g = 2.002, la

2 3

The abnormal

H

I=

H

21.5 G and 18

I=

'Ik

I=

15.1 G,

I$ I =

27.3 G compared to g

=

2.003,

26.7 G for the normal C2H5 radical.

It is believed that the correlation of IR spectra of the OH region (functional group variation ( 5 1 ) ) with the pretreatment of the silica, alumina or PVG and the formation of the Me and/or Me' would help elucidate the difference. 6.

THE TUMBLING FREQUENCY (TF) Several attempts have been made to relate the surface stability of the CH radical with its tumbling frequency as determined by the dependence of the 3 linewidth on the nuclear spin quantum number ( 5 2 ) . Thus Gardner and Casey (53) measured the linewidth of one line and calculated the widths of the other

lines from the relation: (derivative height) x (derivative width)'

=

constant.

Tumbling frequencies were thus calculated to be 20 MHz for CH3 and 1 3 MHz for CD3 at 77 K on silica gel. This method was criticized by Garbutt (5) who showed that the actual measured linewidths are essential and lead to about ten times higher values. The TF of CH3 on PVG at 77 K was determined by Garbutt ( 5 ) to be 0.26 GHz. b%en the surface was chlorinated by heating PVG in the presence of CC14 the TF dropped to 0.23 GHz. When the surface of the PVG was treated with water the

TF fell to 160 2 23 MHz, It was not possible to distinguish between effects due to H20 and D20 on the surface.

179

The effect of temperature on the TF was also investigated (5) and the activation energies were determined to be 0 . 2 9 kcal/mol for CH3 on PVG pretreated at high temperatures (973 K) in vacuum and 0 . 2 8 kcal/mol for CE PVG(C1).

3 These values are of the correct order of magnitude for transitions

o r exchange energies between different conformations of the radical at

different sites on the surface. It should be noted that the temperature dependence of the linewidths of the four lines were not identical. Similar results were reported by Oduwole and Wiseall (44) for CH3 on A1203, silica gel and PVG. Kubota et a1 (47) determined the TF for CH3 (H3C-.HO-S&)

to be 0.2 GHz.

The TF for the normal and abnormal CN3 radicals on silica gel at 77 K were determined (50) to be 11 MHz and 3.3 MHz respectively. These values are significantly lower than those of Garbutt (5) and imply an extra stability associated with the pretreatment of the silica gel and selective adsorption sites for the CH31. The TF for CH trapped in synthetic fused silica was determined to be 11 3 MHz ( 4 5 ) . Based on the expected relative motion of CH3 in different environments, we would expect the TF of the CH radical to decrease in the 3 following order: 10 8 7 Free (10 Hz)(53)> Surface Stabilized (10 Hz)(5)> Bulk Stabilized (10 Hz)(45). Thus the TF values of Gardner and Casey (53) for surface stabilized CH3 which are comparable to bulk trapped CH must be suspect. Similarly the low values 3 reported by Barnes et a1 (50) could depend on the choice of parameters used in the Kivelson equation ( 5 2 ) . The TF of CH3 radicals on silica gel at 77 K was determined by Oduwole et a1 (43) to be 2

x 10l1 Hz where second-order splitting was considered. This

is equivalent to free rotation and cannot be reconciled with the absence of

diffusion and decay at 77 K.

It is obvious that a more consistent set of data for tumbling frequency of the CH radical on the surface would be highly desirable. 3 7.

THE DECAY KINETICS Fujimoto et a1 (30) studied the decay kinetics of CH3 radicals on PVG.

Gas chromatographic analysis of the products of the sttrface photolysis of CR3T 77 K showed the presence of both CH4 and C2H6 with the ratio C2H6 /CH4 being larger at lower surface coverage. The CH was believed to be formed by

at

4

H-abstraction by hot CH3 radicals. They reported a cascade type of decay as the temperature was raised from 77 K to 132 K.

The decay followed first order

kinetics where the rate constant was dependent on the surface loading of the CH I, decreasing a s the loadjng increased (see Fig. 3 and 4). This was 3 interpreted by assuming that the rate controlling step is the release of the

180 S u r f a c e Coverage a -0- I l a y e r

1.0

b

-t-

I00 SO

71ayers

-t

%

.-

H

v)

-c .w

A .

0.1

5 layers

Y

-

z.

4

Y

a

I layer 0

60

iao

160 mln.

IS

45

30

60

m i n. Time o f Radical Decay Time o f Radlcal Decay F i z . 3 ( l e f t ) , Decay of CH r a d i c a l s on porous g l a s s s u r f a c e a t v a r i o u s t e m p e r a t u r e s . Fig. 4 ( r i g a t ) . Decay of CH r a d i c a l s a t 103 K as t h e 3 l o g a r i t h m of t h e change i n ESR spectrum i s p l o t t e d a g a i n s t time i n d i c a t i n g (30) first-order k i n e t i c s , with t h e rate constant k = (1-0.4) x 10-kin-l. Copyright 1966 by t h e AAAS.

r a d i c a l s from a s u r f a c e s i t e and t h a t s u r f a c e d i f f u s i o n i s f a s t r e s u l t i n g 3 i n a low s t a t i o n a r y c o n c e n t r a t i o n of mobile r a d i c a l s . High s u r f a c e coverage

CH

of CH I would t h u s slow down t h e m i g r a t i o n of t h e r a d i c a l t o t h e s u r f a c e . The 3 cascade decay observed was a t t r i b u t e d t o a continuum of t r a p p i n g p o t e n t i a l s on the surface. The cascade decay of CH on s i l i c a g e l h a s a l s o been r e p o r t e d by Joppien 3 and W i l l a r d (19) over t h e t e m p e r a t u r e range of 7 7 K t o room t e m p e r a t u r e . The decay r a t e was r e s o l v e d i n t o m u l t i p l e f i r s t and second o r d e r p r o c e s s e s .

The

a c t i v a t i o n e n e r g i e s f o r t h e 3 f i r s t o r d e r decay p r o c e s s e s were 3, 5 and 6 kcal/mol-l

respectively.

The decay of H-atoms on PVG (51) a l s o showed a similar cascade decay b u t followed second o r d e r k i n e t i c s .

S i m i l a r cascade decay of r a d i c a l s have been

r e p c r t e d t o occur i n polymers which have been Y - i r r a d i a t e d

(54) and is

a t t r i b u t e d t o t h e t r a p p i n g of r a d i c a l s i n p r e f e r e n t i a l s i t e s w i t h i n t h e s o l i d G a r b u t t (5) showed t h a t t h e decay of s u r f a c e s t a b i l i z e d CH r a d i c a l s was 3 complex and appeared t o f o l l o w second o r d e r k i n e t i c s more c l o s e l y t h a n f i r s t order.

A t y p i c a l d e t a i l e d decay curve f o r CH3 on PVG a t 152 K

CH I ) i s shown i n F i g . 5.

3 c o n c e n t r a t i o n of CH

A p l o t of

-

(e=

0.05 f o r

l o g Rate v s log C (where C i s t h e

as determined by peak-to-peak 3 about 10 ( 7 t o 12) f o r t h e o r d e r of t h e r e a c t i o n .

amplitude) gave a s l o p e of

Dole (55) h a s r e c e n t l y shown t h a t t h e decay of f r e e r a d i c a l s i n polymers can b e made t o g i v e a good f i t t o t h e second o r d e r e q u a t i o n when i t is rearranged t o t/(Co

-

C) = t/Co

+ 1/Coik2

181

1

0

20

''

20

~~rnin.)

60

Fig. 5. Decay of 3a line of CH3 on PVG at 152 K. (5)

40 T i m e lmin)

60

Fig. 6. Second order Dole plot of decay of CH from data in Fig. 5. 3

When the data i n Fig. 5 is plotted according to this relation (i.e. t/(Co C) vs t ) a straight line is obtained (see Fig. 6 ) with o n l y

-

slight curature at low values of t where a first order plot (log C vs t) shows reasonable straight line. Similar early curvature (56) in analogous plots was attributed to the time required for temperature equilibration. This j s a

opposite to what is expected here since the concentration

-

time curve

indicates a rapid drop in concentration in the early stages rather than a slower decay rate. A cascade decay of CH radicals on basic A1203 was followed over the 3 temperature range of 298 K to 474 K (57). The radicals were stable at room temperature for months and did not decay in the presence of 02. A study of the decay kinetics of such stable CH3 radicals would be most interesting.

a.

CONCLUSION It is obvious that consistency is lacking in the reported studies of

CH radicals stabilized on PVG or other surfaces. Because of the inherent 3 effect of the iodine formed during the photolysis of CH3I some alternate sources for CH are highly desirable. One possible candidate is azomethane. 3 One of the greatest difficulty in comparing surfaces from one study to another has been the degree of dehydration. It would be most helpful if the 1 K spectrum i n the 3700 cm-l region of the 0-H stretching mode could be

associated with the sample (51,58).

This is possible if OH free quartz is

used for the ESR sample tube and if PVG or transparent monolithic aerogels of silica or alumina is used. Densities and surface area of the solid and path-length would also be required if meaningful comparisons are to'.bemade. The role of mercury as a photosensitizer when co-adsorbed on the surface

182

requires clarification. This can be readily tested by using mercury-free vacuum lines in comparison with doped mercury on the surface. The tumbling frequency determination must be resolved with respect to the second-order splittjng. The values of the parameters used in the Kivelson equation or its equivalent must be stated if direct comparison between studies is to be possible. A

reasonable interpretation of the decay characteristics of CH3 on the

surface is in terms of trapping sjtes which have a continuum of energies. Thus when the sample is changed from one temperature, T1, (at which equilihrium has been established and little or no decay occurs) to a higher temperature, T 2 , there is a fast decay due to the release of radicals with sufficient energies between T1 and T2 followed by a normal decay pattern at T2 which appears to obey second order kinetics. A more uniform surface could help clarify the problem. ACKNOWLEDGMENT Grateful acknowledgment is made to the Natural Sciences and and Enpineering Research Council of Canada for the support of this work. REFERENCES 1 2 3

4 5 6 7 8

9 JO

11 12

13 14

15 16 17 18 19 20 21 22

W. Gordy and C. G. McCormick, J. h e r . Chem. SOC., 78 (1956) 3243. B. Smaller and M. S . Matheson, J . Chem. Phys., 2 (1968) 1169. C. K. Jen, S. N, Foner, E. L. Cochran and V. A. Bowers, Phys. Rev., (1958) 1169.

112

T. Cole, H. 0. Pritchard, N. R. Davidson and H. M. McConnell, Mol. Phys.,

-I

(1958) 406.

G. B. Garbutt, Ph.D. Thesis, University of Manitoba. (1968). G. ti. Pariiskii, G. M. Zhidomirov and V. R. Kazanskii, Zhurnal Strukturnoi Khimmi, (1963) 364. English Translation. V. B. Kazanskii and G. B. Pariiskii, Kinet. Katal., 2 (1961) 507. English Translation. G. B. Pariiskii and V. B. Kazanskii, Kinet. Katal., 2 (1964) 96. English Translation. V. B. Kazanskii and G. B. Pariiskii. Proc. 3rd Int. Cong. Catalysis, (1964) 367.

P. Kinell, A. Lund and T. Vanngard, Acta Chemica Scand., 2 (1965) 2113. G. A. Noble, R. A. Serway, A . O'Donnell and E. S. Freeman, J. Phys. Chem., 2 (1967) 4326. G. M. Muha, J. Phys. Chem., 2 (1966) 1390. G. M. Muha and D. J. C. Yates, J. Phys. Chem., 2 (1966) 1399. J. Turkevich and Y. Fujita, Science, 152 (1966) 1619. J. Turkevich and Y. Fujita, Discuss. Far. SOC., 4s (1966) 181, M. Shiotani, F. Yuasa and J. Sohma, J. Phys. Chem., 2 (1975) 2669. A. E. Lemire and H . D. Gesser, J. Chem. SOC., Chem. Comm., (1983) 1175. G. R. Joppien and J. E. Willard, J . Phys. Chem., 76 (1972) 3158. G. R. Joppien and J. E. Willard, J. Phys. Chem,, 78 (1974) 1391. T. Shiga and A. Lund, J. Phys. Chem., (1973) 453. D. A . Oduwole and B. Wiseall, Bull. Chem. SOC. Japan, 53 (1980) 3037. T. Katsu, M. Yanagita and Y. Fujita, J. Phys. Chem., c ( 1 9 7 1 ) 4064.

183 23

Y . F u j i t a , K. Hatano, M. Yanagita, T. Katsu, M. Sato and T. Kwan, B u l l .

24

A . M. Volodin and A. E. Cherkashin, React. K i n e t . Catal. L e t t .

25 26 27 28 29 30 31 32

23 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Chem. SOC. Japan,

44

(1971) 2884.

243.

18 (1981)

2

E. Cherkashin, K i n e t . Katal., (1981) 598, 979. P. K. Wong, Photochem. P h o t o b i o l . 19 (1974) 391. P. W. J o n e s and H. D . Gesser, ChemF& Ind. (1970) 566. H. D. G e s s e r , Unpublished r e s u l t s . R. J. P e g l a r , J . Murray, F. H. Hambleton, M. J . Sharp, A. J. P a r k e r and J . A . Hockey, J. Chem. S O C . A, (1970) 2170. M. Fujimoto, H. D. Gesser, B. G a r b u t t and A. Cohen, S c i e n c e , (1966) 381. D. A . Oduwole and B. W i s e a l l , Appl. S u r f . S c i . , 2 (1980) 429. J . D. Barnes, M. A. T r i v e d i , D. A. Oduwole and B. W i s e a l l , Bull. Chem. SOC. Japan, (1985) 1865. G. B. G a r b u t t , H. D. Gesser and M. Fujimoto, J. Chem. Phys. 48 (1968) 4605. D. H. Schrader, J. Chem. Phys., 46 (1967) 3895. M. C . R. Symons, Ann. Rev. Phys. Chem., 2 (1969) 219. S. P. Mishra and M. C. R. Symons, J. Chem. SOC. P e r k i n I1 (1971) 391. Y . F u j i t a , T. Katsu, M. S a t o and K. Takahashi, J. Chem. Phys., 61 (1974) 4307. , L. Beveridge and I;. Miller, Molec. Phys. (1968) 401. (1971) 1033. M. Schrader and K. Morokuma, Molec. Phys., Botschwina, J. F l e s c h and W. Meyer, Chem. Phys., 74 (1983) 321. B. G a r b u t t and H. D. Gesser, Can. J. Chem., 48 (1970) 2685. W. Fessenden, J. Chem. Phys., 37 (1962) 747. Oduwole, J. D. Barnes and B. Wiseall, J. Chem. SOC., Chem. Comm., (1978) 164. S u r f . S c i . , 8 (1981) 260. D. A . Oduwole and B. W i s e a l l , Appl. E. J . F r i e b e l e . G. L. Griscom and K. Rau. J. Non-Crvstall. S o l i d s , 57 (1983) 167. (1967) M. Fujimoto, H. D. Gesser, R. G a r b u t t and M. Shimizu, S c i e n c e 1105. S. Kubota, M. Iwaizumi and T. I s o b e , B u l l . Chem. SOC. Japan 44 (1971) 2684. N. Shimamoto, Y. F u j i t a and T. Kwan, B u l l . Chem. SOC. J a p a n , 43 (1970) 580. E. J. Casey, C. W. M. Grant and C. L. Gardner. Can. J. Chem., (1969) 3367. J. D . Barnes, D. A . Oduwole, M. A . T r i v e d i and B. Wiseall, Appl. S u r f . Sci., (1985) 249. L . W. Bader and H. D. Gesser, Can. J. Chem., 2 (1972) 2305. D. Kivelson, J . Chem. Phys., 2 (1960) 1094. C. L. Gardner and E. J . Casey, Can. J . Chem., (1968) 207. B. Ranby and J. F. Rabek, ESR Spectoscopy i n Polymer Research Springer-Verlag, N.Y. (1977). M. Dole, J . Phys. Chem., 91 (1987) 3117. N. Gvozdic, R. Basheer, M. Mehta and M. Dole, J . Phys. Chem., (1981) 1563. (1981) 2551. D. A . Oduwole and B. Wescall, B u l l . Chem. SOC. Japan. H. D. Gesser, L. Bader and M. Shimizu, Chem. & Ind. (1972) 297. A . K. Volodin and A.

154

14

. . . . .

2

156

7-

2

0

5

54

184

CHEMILUMINESCENCE PROPERTIES OF ADSORBED BlACRlDYLlDENES

K. hlk\EDA and S. YiW4DA

1.

INTROWCTION

10,10'-Dimethyl-9,9'-biacridylidene ( l a ) is a two-electron reduction compound produced from lO,IO'-dimethyl-9,9' biacridinium dication ( l a 2 ' ) via cation radical ( l a t ) as shown in Scheme 1. The dinitrate of la2', lucigenin, exhibits powerful blue chemiluminescence in the reaction with hydrogen peroxide i n basic solutions ( I ) . Numerous studies concerning the mechanism of the chemiluminescence have been carried out (2-9). I t w a s concluded that N-methylacridone (3a) is the light emitter ( 6 , 9 ) . It has been shown that the chemiluminescence from lucigenin is either green o r blue depending on the conditions under which the reaction w a s carried out ( 1 1 , and that the reaction is of a (8). The electrochemical generation of the redox type chemiluminescence of lucigenin has also been investigated ( 1 0 , l l ) . A long wavelength component, green emission, was observed in the electrochemical chemiluminescence spectrum and is assigned to la*, which was formed by reduction o f lucigenin and then excited by energy transfer from excited 38, the primary emitter (11). O n e of the authors reported that lucigenin i s a charge-transfer complex between la 2 + and two nitrate anions, that la2' has reduction potentials -0.035 and -0.45 V versus an s.c.e. in acetonitrile, and that the chemiluminescence of lucigenin generates o n the reaction with nucleophiles which are able to reduce la2+ to the biradical la2' o r l a by electron transfer ( 1 2 ) . The fact that a reduction step is responsible for the formation o f an excited state makes lucigenin unique among powerful chemiluminescent compounds. The mechanism of the chemiluminescence o f la in reac i o n s with singlet oxygen o r ozone in homogeneous media has also been investigated in connection with that of lucigenin. The ight emitter o f the chemiluminescence of l a was found to be the

185

1

R

1+*

1*+

Scheme I

R

(g R

R

a -Me b -CH,CH=CH2 c

-CH2Ph

0 \

12*

Scheme 2 .

N

&

0

186 b

excited sicjlet state of N-methylacridone (3a 1 , produced by the decomposition of the important intermediate, lO,lO'-dimethylb i a c r i d y l - 9 , 9 ' - d i o x e t a n e ( 2 a ) , as shown in Scheme 2 ( 1 3 - 1 5 ) . I t is interesting that the same emitter i s responsible for both 2+ chemiluminescence reactions of l a and l a . During the course of the investigation o f photoreactions of 10,10'-dialIyl-9,9'-biacridylidene ( I b ) , w e found a n e w type of chemiluminescence reaction. When alumina was added in portions to a benzene solution of l b , o r the benzene solution of Ib w a s added to alum na, chemiluminescence appeared instantly from the surface of a urnina. Appropriate control experiments indicated that biacridy idene as a substrate, nonpolar solvent, alumina as an adsorbent, and molecular oxygen were necessary for generation of the chemiluminescence. Removal of one o f the components from the reaction system stopped the light emission imnediately. Readmittance of the component to the system gave rise to regeneration of chemiluminescence. Generation o f similar chemiluminescence w a s also observed on 10,10'-dimethyl- ( l a ) and lO,IO'-dibenzyl-9,9'-biacridylidene ( 1 ~ ) . The features of emission depend markedly on the activity of alumina. T h e properties of the chemiluminescence of adsorbed biacridylidenes, l a - I c , o n alumina o r activated alumina in a benzene slurry are described in the following.

2.

Q44TERIALS AND INSTRUMENTS

10,10'-Disubstituted-9,9'-biacridylidenes, la-lc, and N-substituted-acridones, 3a-312, were prepared according to the method of Aniet ( 1 6 ) . Alumina (Wako, f o r column chromatography, 300 m e s h ) w a s used as the adsorbent. Alumina without activation (activity grade ca. 3 ) is referred to as "alumina", and that activated by heating at ca. 700°C for 4 h i n air (activity grade ca. 1) as "activated alumina". Spectroscopic grade solvents were

used as received. The chemiluminescence (intensity of light was rather weak and decreased slowly) and fluorescence spectra were recorded on a Shimadzu R F - 5 1 0 spectrofluorometer. The chemiluminescence spectra (intensity o f light was very strong but decreased instantly) were recorded on an Otsuka Electronics IMCPD-110 spectro multi channel photodetector. The absorption spectra were obtained with a Shimadzu UV-240 spectrophotometer.

187

Concentration o f the benzene solutions of l b and lc used for the measurements of the chemiluminescence spectra was I X mol d ~ n - ~and , the concentration of the benzene solution of la was 1 X mol d ~ n - ~because , of the small solubility o f l a into benzene.

RESULTS AND DISCUSSION 3.1 Chemiluminescence of Biacridylidenes Adsorbed on Alumina When a small volume of a benzene solution of lb is added to alumina placed in a transmitted cuvette and the mixture is then stirred to make a slurry, chemiluminescence takes place. The chemiluminescence spectrum recorded in the shortest poss ble time on the spectrofluorometer is shown in Fig. 1 . The chemiluminescence spectrum with a maximum at 507 nm (curve 1 ) coincides with the fluorescence spectrum of I b adsorbed on alumina in a benzene slurry (curve 2 1 , though the I gh t intensity of the former i s much weaker. These luminescence spectra shift to longer wavelength regions than the fluorescence spectrum of I b obtained in a benzene solution (curve 3 ) . The obtained under the same chemiluminescence spectrum of Ic 3.

Wavelength ( m1 Fig. 1 . Emission spectra o f lb. ( 1 ) Chemiluminescence, adsorbed o n alumina in a benzene slurry. ( 2 ) Fluorescence, adsorbed on alumina i n a benzene slurry. (3) Fluorescence in a benzene so l u t i on.

188

conditions as for that o f Ib also shows a maximum at 5 0 7 n m with similar spec ral dist ibution to Ib. I n the chemiluminescence spectrum of either b o r Ic, emission maximum and spectral distribution remained unchanged during the course of emission, the intensity decreasing with time. These observations indicate that only one emitter is responsible f o r the generation of each chemiluminescence, which i s the excited singlet state of Ib o r Ic. Therefore, the formation and decomposition of 1,2-dioxetane, which should give N-substituted-acridone (3b or 3 c ) as an emitter, and also contribution of alumina as a catalyst for decomposition of the dioxetane, as reported on silica gel ( 1 7 ) , are ruled out from the mechanism of this chemiluminescence react ion. T h e absorption spectrum of lb obtained i n benzene shows a maximum at 420 nm ( E 12000). However, Ib does not dissolve into ethanol, methanol o r acetonitrile, unless accompanying oxidation o f Ib to lb2' takes place. Thus, in order to reveal the type of excitation, the absorption spectrum w a s recorded in N,Ndimethylformamide, which showed a maximum at 4 2 4 n m ( E 8 8 0 0 ) . This shows that the absorption of Ib is probably due to T I , T I * * excitation. I n most cases, T I , TI excited states are m o r e polar than the ground states. I f the surface of alumina contains highly polar sites, I b in the excited state should be m o r e strongly adsorbed than the molecule i n the ground state. The * T I , TI excited state might be involved in the fluorescence and chemiluminescence, produced by excitation and a chemical reaction, respectively, giving rise to a longer wavelength of the emission maxima in a slurry than that in a benzene solution, a s shown in Fig. 1 . Although l a adsored on alumina also gave rise to chemiluminesc,ence, the light intensity was too weak to record the emission spectrum accurately, because of the low concentration in a benzene slurry. Variation of the light intensity at the emission maximum ( 5 0 7 nm) of lb with time recorded on the spectrofluorometer is shown in Fig. 2. The curve is a typical one for the chemiluminescence of l a - l c adsorbed on alumina. I n a definite concentration of each sample, both the initial rate to attain the maximum o f light intensity and the rate o f disappearance from the maximum to become nearly zero decrease with the decrease in the activity of alumina by addition of a trace of

189

>r

r .-

VI

C

@J

4

C

I

U

I

0

5

I

10 Time ( m i n )

I

15

Fig. 2. Variation of maximum chemiluminescence intensity adsorbed Ib on alumina in a benzene s l u r r y with time.

35 0

400

Wavelength ( nm)

of

45 0

Fig. 3. The absorption spectra of a mixture of recovered a n d desorbed substances from the alumina s l u r r y of lb after v a r i n g contact time interval. (1) 0 min, ( 2 ) 1.5 min, ( 3 ) 3 min, ( 4 ) 6 min, ( 5 ) 9 min, ( 6 ) 12 min.

190

water. The maximum light intensity attained also decreases with the decrease in the activity of alumina. I n order to detect the reaction products, the chemiluminescence reaction of l b was monitored by t.I.c., and recovered l b and two-electron oxidation product, 10,10'-diallyl-9,9'biacridinium ( 1 b 2 ' ) , were identified. Formation of l b 2 + was also confirmed by spectroscopic methods in the following way. A known volume of the benzene solution of I b was added to a set o f alumina samples. After varying contact time intervals, the material was desorbed from the surface by shaking the sample with ethanol. Each extractant was made up to a standard volume, and the absorption spectra of the samples were obtained as shown in Fig. 3 . The absorption intensity of the maximum at 4 2 0 nm due to l b decreases with the increase of contact time, accompanied by an appearance and the increase of a new band with a maximum 2+ at 3 6 7 nm corresponding to the absorption maximum of l b . These observations indicate that electronic oxidation o f I b is involved in the formation of the excited state on the alumina surface. The contribution of electron-transfer from I b to a Lewis type acid center ( 1 9 ) or to adsorbed oxygen molecule to give superoxide (02;) ( 2 0 ) is suggested for the oxidation. 3 . 2 Chemiluminescence of Biacridylidenes Adsorbed on Activated

A1 umi n a When activated alumina was added to the benzene solution of I b in a cuvette, very strong chemiluminescence (emission I ) appeared instantly, followed by a weak light emission (emission I1 ) which continued for ca. 10 min. After the emission disappeared, ethanol was added to the slurry in order to desorb the reaction products from the surface o f activated alumina. Instantaneous blue light emission (emission 111) was observed. The relative intensities at each maximum wavelength of emissions I, 11, and 111 from l b versus time recorded on the spectrofluorometer, are shown in Fig. 4 . These observations are quite different from those obtained in the chemiluminescence reaction which caused with use of alumina. with very Chemilurninescence spectra of emissions I and 111 short duration were successfully recorded on the photodetector. Fig. 5 shows variation of the spectra of emission 1 with time, whose maxima appeared at around 520 nm. Since emission I] was too weak to record on the photodetector, the emission spectrum

191

0.5

0

1 .o 1.5 Time (rnin)

-.

0.5

0

Fig. 4. Relative intensity o f emissions I ,

I I

4 00

~~

45 0

560

Wavelength ( n m )

II and III with time.

550

Fig. 5. Variation of chemiluminescence spectra of emission I of adsorbed 1b on activated alumina in a benzene slurry with time. C u r v e I ; imnediately after addition of activated alumina to the benzene solution of Ib. C u r v e 2 and following curves measured after every 0.8 sec.

192

with a broad maximum at around 5 0 7 n m was obtained with the spectrofluorometer. The spectrum of emission I1 coincided with that of the chemiluminescence from adsorbed l b on alumina. This suggests that the excited singlet state of I b is also responsible f o r emission 11. T h e relationship between the surface hydration and catalytic properties of alumina has been investigated by Peri ( 2 1 ) . T h e oxide surface, unless highly dried, is usually covered with hydroxyl groups formed by chemisorption of water. O n dehydration, adjacent hydroxyl ions combine to form water molecules, which are then removed. F o r each molecule of water formed, one oxide ion is left in the top layer (electron-donor defect site), and one aluminium ion is left in an incomplete octahedral site in the next layer (electron-acceptor defect site). I t is shown in Scheme 3 ( 2 2 ) .

Scheme 3.

T h e dehydration pretreatment at ca. 700 "C to activate alumina probably increases both amounts of the electron-acceptor defect sites (acid centers ( 1 9 ) ) and electron-donor defect sites on the surface. Excited molecules of l b formed chemically on the surface of activated alumina should be adsorbed more strongly than on alumina surface, giving rise to a longer wavelength of the maximum of emission I than that o f emission 11. The excited singlet state of I b is a l s o suggested a s the light emitter of emission I. T h e spectrum of emission 111 , whose maximum intensity is stronger than that of emission 11, shows two maxima at 4 2 3 and 4 4 5 nm. After emission 111 disappeared, N-allylacridone (3b) was and also by spectrophotometry. 3b is detected on t.1.c. suggested to be the light emitter. The fluorescence spectrum of 3b in a mixed solution of benzene and ethanol which is similar to the solution over the slurry in the emission 111, showed two maxima at 4 2 1 and 4 3 3 nm. T h e observed shifts of maxima

193

in the chemiluminescence to longer wavelength regions than those of the fluorescence in the mixed solution are probably due to the weak interaction between the excited state of 3b and the surface of activated alumina. T h e fact that the maxima o f the fluorescence spectrum of adsorbed 3b on activated alumina in a benzene-ethanol slurry show further red shifts to 4 4 3 and 4 6 5 n m supports the assumption. Compounds l a and 1c adsorbed on activated alumina also generated emissions I , I1 and 111. 3 . 3 Mechanistic Considerations I n the chemiluminescence reaction of 1 adsorbed on activated alumina, emissions I and I11 were generated as well as emission 1 1 . I n the case of alumina, emission 11 was generated. I n the latter case, a small amount of the corresponding oxidation product j2' w a s detected by desorption. I t seems that the surface of alumina has a small amount of electron-acceptor defect sites which are capable of oxidizing 1 to corresponding j2' via 1: as an intermediate i.n the presence of oxygen. However, the resulting oxidation product 1 2 + is adsorbed loosely on the surface and is easily desorbed by addition o f ethanol to the slurry. Thus, no emission 111 takes place. On the other hand, when alumina is heated to a high temperature, electon-acceptor defect sites are produced exceedingly, and o n adsorption of 1 o n the surface a large amount of 12' is produced in a strongly adsorbed state. For the generation of emissions I and 11, the excited singlet state of'1 has been determined as the light emitter. Although i t has not yet been ascertained h o w 1 accepts the energy for excitation to give I * , the occurrence o f emission I may be rationalized by assuming an electron-transfer mechan sm. I n this case, a reaction of the cation radical I t , which is the intermediate in the oxidation-reduction reaction between 1 apd 12', with some suitable electron donors, which are capable of reducing by electron-transfer to lead to the excited s ate I * , 1: + e- += 1 * , is suggested. I t has been reported that, in the chemiluminescence of polynuclear aromatic hydrocarbons and heterocyclic compounds, the electron-transfer reaction, from a radical anion to a radical cation, is responsible for the formation o f an excited state, A; + CT + A + C* ( 2 3 ) . O n e possible candidate for the electron donor is oxide ions which

194

constitute the electron-donor defect sites on the surface of activated alumina. For the generation of emission 111, a reaction of the strongly adsorbed dication 12' o n the surface o f activated alumina with superoxide 02; to give the excited singlet state of 3 is suggested. Although the nature of the reaction occurring in the desorption process of the 12+ by the addition of ethanol cannot be clarified, the fact that, in electrochemical chemiluminescence of lucigenin, superoxide reacts with lucigenin to cause light emission ( 1 1 ) supports the suggestion. As mentioned above, oxygen is needed f o r the generation of emissions I and 11. I f an oxygen molecule acts as an electron acceptor in the oxidation process of 1 to give 1 2+ by electron transfer with accompanying a change from O2 to 02;, as suggested by Flockhart and coworkers in the oxidation of aromatic hydrocarbons on activated alum na (20), the resu ting 02;might be involved in emission 111. I t was found that in the generat on of the chemi umi nescence of 1 , silica gel, zeolite, N O , Ti02, and ZnO also act as adsorbents.

REFERENCES I 2

K. Gleu and W. Petsch, Angew. Chem., 45 (1935) 57-59. 13. Kautsky and K. H. Kaiser, Z. Naturforsch., 56 (1950

3

8.

4 5 6 7

A. V. Kariakin, Opt. Spectrosk., 7 (1959) 75-76. J . R. Totter, V. J. Medina and J. L. Scoseria, J. Biol. Chem., 235 (1960) 238-241. J. R. Totter, Photochem. Photobiol., 3 (1964) 231-234. F. McCapra and P. G . Richardson, Tetrahedron Lett., (1964)

8

J. R. Totter and G. E. Philbrook, Photochem. Photobiol.,

9 10 11

12 13

353-361.

D. Ryzhikov, Izvest. Akad. Nauk S . S. S. R., 20

533-536.

3167-3172.

( 956 1

5 (1966) 177-188. E. G. Janzen, J. 8 . Pickett, J . W. Happ and W. DeAngelis, J . Org. Chern., 35 (1970) 88-95. B. Tamamushi and H. Akiyama, Trans. Faraday SOC., 35 (1939) 491-494. K. D. Legg and D. M. Hercules, J . Am. Chem. SOC., 91 (1969) 1902-1907. K. Maeda, T. Kashiwabara and M. Tokuyama, Bul I . Chem. SOC. Jpn., 50 (1977) 473-481. F. McCapra and K. A . Hann, J . Chem. Soc., Chem. C o m u n . , (1969) 442-443.

195 14 15

16 17 18 19 20 21 22 23

E. G. Janzen, I . G. Lopp and J . W. Happ, J . Chern. S o c . , Chern. Comnun., ( 1 9 7 0 ) 1 1 4 0 - 1 1 4 2 . Kyu-Wang L e e and L. A. Singer, J. Org. Chem., 41 ( 1 9 7 6 )

2685- 2688. R . G. Amiet, J . Chem. Educ., 59 ( 1 9 8 2 ) 1 6 3 - 1 6 4 . K. A . Zaklika, P. A. Burns and A. P. Schaap, J . Am. Chem. SOC., 100 ( 1 9 7 8 ) 3 1 8 - 3 2 0 . P. A. Leermakers and H. T. Thomas, J. Am. Chern. SOC., 8 7 ( 1 9 6 5 ) 1620-1622. M. Okuda and T. Tachibana, Bul 1 . Chem. SOC. Jpn., 3 3 (1960) 863-864. B. D. Flockhart, J. A. N. Scott, and R. C. Pink, Trans. Faraday SOC., 62 ( 1 9 6 6 ) 7 3 0 - 7 4 0 . J . B . Peri, J . Phys. Chern., 6 9 ( 1 9 6 5 ) 2 2 0 - 2 3 0 . S . G. Hindin and S . W. Weller, J . Phys. Chern., 6 0 ( 1 9 5 6 ) 1501-1506. E. H. White, J . D. Miano, C. J . Watkins and E. J. Breaux, Angew. Chem. Int. Ed. Engl., 1 3 , ( 1 9 7 5 ) 2 2 9 - 2 4 3 .


NEW DEVELOPMENTS OF ORGANIC PHOTOCHEMISTRY ON SOLID

Chapter 4

SURFACES

Con t e n t s

4.1

P h o t o c h emi s t r y of Dibenzyl Ketone Adsorbed on S i z e I S h a p e S e l e c t i v e F a u j a s i t e Zeolites: S t e r i c E f f e c t s on P r o d u c t Distributions ( N i c h o l a s J. T u r r o and Zhenyu Zhangf

4.2

P h o t o c h emi s t r y o f Or g an i c C a t i o n s a t Charged I n t e r f a c e s ( C a r o l A. Backer and David G . W h i t t e n )

4.3

197

216

E l e c t r o n T r a n s f e r between Adsorbed Dye Molecules and O r g a n i c C r y s t a l s : Model C h a r a c t e r of the A d s o r p t i o n System f o r C e r t a i n As p ect s i n P h o t o s y n t h e s i s ( K l a u s Kemnitz, Nobuaki Nakashima, and K e i t a r o Y oshiha ra )

236


197

PHOTOCHEMISTRY OF DIBENZYL KETONE ADSORBED ON SIZE/SHAPE SELECTIVE FAUJASITE ZEOLITES. STERIC EFFECTS ON PRODUCT DISTRIBUTIONS N. J. T W O and 2. ZHANG

1. ABSTRACT Zeolites are robust, crystalline, porous aluminosilicates possessing an enormous internal surface area that is capable of adsorbing large quantities of guest molecules, the size and shape of whose structures allow them to pass from the external to the internal zeolitic surface and to diffuse on the internal surface. The framework composition, the presence of cations associated with the framework, and the topology of the void space internal to the zeolite all contrive to imbue these materials with special properties that contribute to their widespread use as catalysts, ion exchange materials and molecular sieves. Photochemical probes have been developed to explore the structure of zeolites near the sites of adsorption and to examine the dynamics of reactions of molecules adsorbed on

the internal zeolitic surface. In this chapter review the structure of zeolites in general and then survey the structure of an important class of zeolites, the faujasites. We then show how a photochemical probe, the photochemistry of dibenzyl ketone, can yield information on how intracrystalline dynamics can be influenced by cation type, cation number density and coadsorbed guests and, in turn, how intracrystalline dynamics can determine the products of photoreactions.

2. CHEMICAL REACTIONS IN RESTRICTED REACTION SPACES The development of theories and methods which provide an understanding of the dynamics of reactions of species embedded in restricted spaces should help to unravel the important factors that determine the catalytic properties of porous solids. For example, a common theme of current investigations of porous solids is the examination of adsorbed molecular probes which possess pore dimensions that are small enough to influence molecular dynamics. Typically, the molecular probes occupy one or more specific sites in the restricted space of the porous solid and possess the possibility of executing rotational and diffusional motion prior to being irreversibly consumed by deactivation (excited states) or by reaction (excited states and reactive intermediates). Among the most fascinating attributes of porous solids which can affect the dynamic properties of probe molecules are

198

the steric factors resulting from "spectator" co-adsorbents and the tortuosity of void space. Experimentally, in order to obtain meaningful information related to the catalytic functions of the porous solids one must characterize the locations of the probe molecules and how geometric and chemical issues control the dynamics which determine the observed products under a given set of conditions. Among the porous solids available, zeolites possess a number of features that make them particularly attractive as restricted spaces which can be investigated with photochemical probes. Because of the incredible complexity of the many important porous solid systems, there is little hope in the near future to treat the restricted spaces quantitatively or to model them in terms of small perturbations of idealized systems; as a result, broadly applicable qualitative models are the order of the day and these are precisely the models we seek to develop and to test in this report.

3. THE ZEOLITE PARADIGM Zeolites are fascinating materials whose unusual chemical and physical properties derive from their porous yet crystalline structure (1-6). Since the reader is probably unfamiliar with these materials, it will be useful to express the attributes of zeolites that are pertinent to the photochemistry of organic molecules adsorbed on their surface in terms of qualitative concepts that will allow an appreciation of the strategies available to the experimentalist. Such concepts will allow both a clear understanding of experimental results in terms of zeolite attributes and a means of extending the.approaches to other areas of chemistry. It is informative to discuss zeolite properties in terms of their geometric, physical, steric and chemical properties, even though a merging of these properties is inevitable as one becomes more demanding for detail. The important geometric properties include the pore structure and topology of the internal surface and the overall three dimensional network of void space that is available for molecular diffusion of adsorbed guest molecules. The important physical properties include the weak dispersion forces and other non-covalent electronic interactions responsible for adsorption of molecules on the zeolitic surface, in addition to the steric properties which result from the space filling features of species such as exchangeable cations and adsorbed guest molecules, within the internal surface. The important chemical properties include the framework composition, especially the Si to Al ratio, which will be a critical factor in determining the qualitative and quantitative aspects of the physical and steric properties involved in diffusion within the internal surface. We shall now consider each of these attributes in turn and indicate how they can contribute to modify the course of photochemical reactions of organic molecules adsorbed on zeolitic surfaces by controlling the rotational and diffusional characteristics of reactive intermediates produced by light absorption.

I99

4. ZEOLITE COMPOSITION The typical composition (for dehydrated material) of the "classical" aluminosilicate zeolites may be represented by the empirical formula: M+(AlO2)-(SiO2)nAm where M+ denotes an exchangeable singly charged cation (which can also be replaced by 112 the number of Mf2 or 113 the number of M+3 cations), and A is a physiadsorbed guest molecule (such as water or an organic molecule). The value of n may be almost any whole number greater than one. The number density of cations is determined by the number of aluminum atoms and by the charge of the cation (each aluminum atom contributes a single negative charge that must be compensated for by a cation to maintain electroneutrality). The constitutional or "framework" structure of zeolites is based on an infinitely extending three-dimensional network of A104 and SiO4 tetrahedra that are linked to each other by shared oxygen atoms and may be represented as: hydrophilic

M+

hydrophobic

hydrophilic

M+

5. GEOMETRIC AND TOPOLOGICAL ATTRIBUTES The important qualitative features of the various geometric descriptions of the void space are that the topologies of the internal surface of the faujasite represent an interlinked three dimensional network for diffusion, i.e., if a molecule is of appropriate size to diffuse throughout the framework, it can move from any initial site to any other site on the internal surface. The important quantitative features of the void space are related to size and shape features of diffusing species relative to the size/shape features of the void space. The interactions of sizehhape features of the zeolite with sizehhape features of the adsorbed molecules are critical in determining the rotational and diffusional characteristics of species within the internal surface. The geometric features of the void space available for diffusion within a zeolite may be viewed in terms of an idealized geometry and topology that would represent the effective void space available for diffusional and rotational motion of adsorbed molecules. For a more realistic picture we must add any itnportant, but complicating features such as the occurrence of exchangeable ions and guest molecules. Figure 1 shows the general situation with respect to the nature of the surface framework "walls". Essentially, an observer would see only oxygen atoms, since the silicon and aluminum atoms are buried at the center of the tetrahedral arrays.

200 F I G U R E 1.

Si (Al) tetrahedra

Faujasite pore openings

6. GEOMETRIC DESCRIPllONS OF THE VOID SPACE OF THE FAUJASITE (X AND Y) ZEOLITES The void space topologies of zeolites results from the surface generated by the zeolite framework structure. With modern computers the internal void space is easy to visualize (7). For example, computer graphics representations of the void space of a faujasite type zeolite are shown in Figure 2. In Figure 2a, a representation of the faujasite supercage with the exchangeable cations absent is shown; in Figure 2b representions of the X zeolite (Si/Al ratio = 1.2) and of the Y zeoIites (Si/Al = 2.4) are shown with exchangeable cations included. In Figure 3 the building blocks and a simple topological representation of the framework structure of the faujasite zeolites are shown. As depicted in Figure 4, the X and Y (faujasite) zeolites possess independent, but interconnected three dimensional networks of cavities. One network consists of relatively large and roughly spherical cavities, termed sunercages, that possess a diameter of about 13A (1). The supercages are linked by four tetrahedrally disposed, roughly cylindrical pores which serve as windows to the supercage. The free diameter of these windows is about 8A. The faujasite zeolite's internal topology is one of the most open of all known zealite structures and, therefore, presents many opportunities to investigate diffusional and rotational processes within the internal surface. The framework is rigid and stable, even though about 50% of the crystal volume is void space (1). The chemical compositions of X and Y differ in that the X zeolite contains roughly 1 A1 atom for each Si atom, whereas the Y zeolite contains roughly 2 Si atoms for each A1 atom. The other void space network in the zeolite framework can be considered in terms of polyhedral units formed by linked A104 and SiO4 tetrahedra (Figure 4). These are cage-like units designated by Greek letters a,

i3, etc. The smaller, truncated octahedra is called the 13 cage. The pore size dimensions of this cage are too small (ca. 3 A) to allow access to most organic molecules, but are large enough to allow access to certain cations. The faujasite framework consists of linked truncated octahedra (I3 cages) characteristic of the structures of sodalite (1). The pore size of the sodalite p cages is too small (ca. 2 A) to allow e n q to organic molecules.

201

F I G U R E 2a. Computer Graphic Representation of

the Void Space with the Exchangeable Cations Absent.

X-Zeolite

F I G U R E 2b.

y-zeolite

Computer Graphic Representations of the Void Space of X and Y Zeolites with Exchangeable Cations. Filled circles represent Site 111 cations; open circles represent Site 11 cations.

6.1 Phvsical and Steric Atmbutes The physical and steric attributes of the internal surface must be considered together, since the effects of non-covalent adsorption and space occupation are both important factors in determining the rotational and diffusional characteristics of guest molecules. In general, the major driving forces for adsorption on the internal surface are of two types: (a) an entropic driving force resulting from the enormous ratio of internal surface to internal surface ( 8 ) typically >lO,OOO for the system discussed in this paper; and (b) an enthalpk driving force resulting from various electronic interactionsbetween the surface and the adsorbed species, typically resulting from weak forces between exchangeablecations and guest molecules. 7. CHEMICAL ATTRIBUTES It is useful to think of the chemical attributes of the internal surface as only indirectly influencing the rotational and diffusional characteristic of species adsorbed on the internal surface. Thus, the composition of the zeolite internal surface, especially the Si to A1 ratio, will determine the hydrophilicity and hydrophobicity of the internal surface and the number density of the exchangeable cations. The hydrophilicity (hydrophobicity) of the

202

INTERNAL CAGES

SURFACE

OF INTERNAL CAGE

FIGURE 3.

Building blocks and simple topological representation of the framework structure of faujasite zeolites.

surface and the cation density, in turn, will influence the strength of adsorption and mobility of adsorbed species.

8. DIFFUSION IN ZEOLITES; THE BASIS OF SIEVING AND CATALYTIC AnION The enormous internal free volume of zeolites may be conveniently classified in terms of a local porous structure consisting of channels (cylindrical shapes) and cages (spherical shapes) which are connected to one another through intersections containing "windows" or "pores" that determine which molecules can diffuse through the internal surface (Figure 4 shows a representation of the NaX and NaY structures showing the pores, windows and cages along with the exchangeable cations). Similar windows presumably occur at the external surface of the zeolite crystal and determine which molecules can access the internal surface. Molecular motions such as diffusion and rotation which occur on the internal surface are at the heart of the sieving and catalytic action of zeolites. In catalytic action the ability of a reactant to diffuse to an active site is a critical step in the reaction sequence. In zeolitic structures, the geometry associated with the size and shape of the porous structure, in addition to the chemical and steric effects associated with the framework cations and adsorbed molecules, can control the diffusional and rotational motions of reactants within the zeolite. The intimate interactions between the size and shape of the reactant species and the dimension, geometry and the chemical species occupying the channels and cages will play a dominant role in determining the catalytic effectiveness of a zeolite. The very same features will determine the molecular sieving characteristics. There are three distinct diffusional situations that can be distinguished: (a) the diffusion of a guest molecule from the external surface of the crystal through a window that leads to the internal surface; (b) the diffusion of a guest molecule within the internal

203

Figure 4a.

Cation sites in zeolite X

@

Typemcations

Figure 4b. zeolite Y

surface; (c) the diffusion of a guest molecule on the external surface of the crystal. Let us consider each in turn. 8.1 Diffusion from the External to the Internal Surface. Entry into the internal pores of a zeolite will depend strongly on the size and shape of the guest molecule and the size and shape of the windows controlling access to the internal channels and cages of the zeolite. Although it is expected that the size and shape of the windows at the externalhnternal crystal interface will be similar to the windows in the internal surface, they cannot be identical. Thus, the rate at which molecules pass through the "external" surface windows will only be qualitatively similar to the rate at which molecules pass through analogous internal windows. 8.2 Diffusion within the Internal Surface. Once a guest molecule has entered the internal surface, its rate of diffusion will be strongly dependent on the size and shape of the channels and cages of the internal pores compared to its own size and shape. In addition, the number density, the location and the size of the exchangeable cations associated with the internal framework will influence diffusion by both chemical (electronic) effects e.g., electrostatic and dispersion factors, and by steric (space filling) effects.

8.3 Diffusional on the External Surface Guest molecules adsorbed on the external surface will diffuse on the external

204

surface until they react or enter the internal surface. The rate of diffusion on the external surface is expected to be much faster than that on the internal surface, because size and shape factors are absent. In the studies to be reported diffusion on the external surface is inconsequential, since all of the ketones (due to the low coverages investigated) are adsorbed completely on the internal surface. 9. PHOTOCHEMICAL PROBES OF THE MOBILITY OF MOLECULES ADSORBED ON ZEOLITES Photochemistry provides a powerful and versatile means of probing the mobility of species adsorbed on surfaces (9,lO). The basic reason for this power is that the absorption of light can produce, instantaneously on the time scale of diffusion, reactive intermediates whose chemistry is totally determined by their mobility on the surface of the porous solid. With proper selection of the reactant species, information concerning the mobility of the precursor reactive intermediates can be. locked into the structure of the stable, isolable products. In such cases, (11-15) product analysis provides a simple, yet elegant method to obtain information on the dynamics of motion of molecules adsorbed on the zeolites. In one method, the "cage" effect, or the percentage of an initial number of geminately produced radical pairs which react with each other within the "cage" in which they were born together, has been employed to examine the translational diffusion of radicals adsorbed on the external and internal surfaces of zeolites. In a second method, the formation and the structures of isomers from a geminate radical pair have been employed to examine the rotational and diffusional motion of radical pairs generated on a zeolite surface. 10. FURTHER DETAILS ON THE STRUCTURE OF THE X AND Y ZEOLITES As discussed above, the "openness" of the faujasite void space and the diffusional and rotational processes that occur within the internal surface are expected to depend on variables such as the number of cations in a supercage, their size, charge and location within the supercage and also on the presence of "spectator" guest additives (such as water and organic molecules). We shall assume that only the exchangeable cations are important for reactions that occur in the supercages, because the exchangeable cations in the I3 cages are not accessible to the photochemical probes employed in our studies. The extent of diffusional and rotational motion that occurs for probe molecules can be examined for the X and Y zeolites as a function of the following parameters: 10.1. The nature of the exchaneeable cations (2,1617). The charge of the cation may be kept constant, and the atomic number of the cation may be varied. For example, the X or the Y zeolites containing alkali ions Li, Na, K, Rb and Cs may be compared. Similarly, the X and the Y zeolites containing the alkali earth ions Mg, Ca, Ba and Sr may be compared. In changing the cations in a column of the periodic table, the charge is kept constant, but the size and the electrostatic features of the ion are changed. For example, the ionic diameters in the alkali ions (Li = 1.4A, Na +

205

1,9A, K = 2.7A, Rb = 3,OA and Cs = 3.4A) increase by over a factor of 2, implying an increase in volume of a factor of about an order of magnitude in going from Li to Cs. The biggest change, however, is expected in going from Li to K, with a smaller change upon going from K to Cs (Figure 5). 10.2. The number densitv of exchangeable cations in a cage. The number of cations in a cage may be varied in one of two ways: either variation of the charge of the ion, or variation of the SVA1 ratio. A di-cation such as Mg+2 can neutralize the negative charge of two A1 atoms, whereas a mono-cation such as Na+ can neutralize only one negative charge. Thus, a cage contains one half of the number of dications as mono-cations. Similarly, if the number of A1 atoms is cut in half (as is roughly the case in going from the X zeolite to the Y zeolite) the number of cations required to neutralize the framework negative charge is decreased by a factor of two. 10.3. SDectator-guestmolecules in a cage, Molecules whose kinetic diameter is about 8A or smaller are able to pass through the windows of the X and Y zeolites and be adsorbed within the internal surface. Water (1 8) and benzene (19-21), for example, can be added as "Spectator" guest molecules which do not participate in a reaction sequence in a direct chemical manner, but may strongly influence the course of reaction of another adsorbed reactant by controlling factors such as the site of substrate adsorption or by influencing the diffusional or rotational motion of the reactants. The number and position of these guest molecules within a supercage may also be varied. 10.4. The location of exchangeable cations and stxctator-guest molecules in a

a

There is no guarantee that an exchanged cation will position itself in the same location as its predecessor. Indeed, in some cases it it highly unlikely that this will be the case. An ion or a spectator guest that positions itself at or near a window may exert a significant influence on diffusional processes in and out of the supercage. An ion or a spectator guest that positions itself inside the supercage may exert a significant influence on diffusional and rotational motions within the supercage.

Figure 5 .

Schema$ic representation of the arrangement of the cations ,'iL K" and Cs at Site I1 (hatched circles) and Site I11 (filled circles) in zeolite X.

11. ION LOCATIONS IN THE MX AND MY FAUJASITE ZEOLITES

The sites available for occupancy exchangeable ions in the MX and MY families

206

may be classified in terms of their locations within the supercage framework. According to convention, (1) three typical sites (Figure 4) have been recognized: (a) Site I consists of the "inside" hexagonal prisms of the wall of the super cage. These pores, which make up the sodalite building blocks of the faujasite structures, have ca. 2.4 A openings so that the cations at these sites will not be accessable to a probe whose size compels it to be located in the supercage. (b) Site I1 consists of the "external" six membered rings of the sodalite which form the walls of the supercage containing DBK molecules. From information in the literature (19-21), approximately four benzene molecules may be absorbed into a supercage and the adsorbed benzenes are associated with the Site I1 cations (19-21). (c) Site I11 consists of four or five cations which cannot interact significantly with specific rings of oxygen atoms or negative aluminum atoms and, as a result, the Site 111cations are not localized and possess a strong binding affinity. Given the occurrence of three sites, we now recognize three general and important features of variation of the SilAl ratio of the faujasite composition: (a) As the ratio increases, the number density of the cations decreases, because there are fewer (negatively charged) A1 atoms in the framework. (b) As the ratio increases, the steric inhibitions to rotational and diffusional motion within the supercage will decrease, because the number density of cations will decrease. (c) As the ratio increases, the polarity of the supercage will increase. Furthermore, we shall assume that the Site I11 cations, being unable to find specific site binding, will be mobile and serve as "marbles" that take up space within the supercage. Starting from this hypothesis we shall design experiments to test it and to determine when and if factors other than steric effects are significant in determining the products of photochemical reactions of molecules adsorbed in the supercage. 12. PHOTOCHEMISTRYOF DIBENZYL KETONE ADSORBED ON MX AND MY ZEOLITES We have developed a well-established paradigm which expresses the "static structure'' of faujasite zeolites. Below we shall develop one which expresses the "dynamic structure" of a photochemically generated radical pair. We shall merge the two paradigms to generate a working hybrid paradigm which can drive experiments to obtain information on the dynamics of photochemically generated radical pairs adsorbed on the internal surface of faujasite zeolites. This working paradigm will pre-suppose no novel feature of either the photochemical system or the zeolite system, unless compelled to be experimental information. The simplicity of the working paradigm, of course, prevents quantitative experimental comparison with theory, but the simplicity is blessed with qualitative precision of prediction which must precede any quantitative modelling. 12.1 The Photochemical Paradigm. The probe structure, dibenzyl ketone, was selected from consideration of the quantitative aspects of the size/shape constraints of the faujasites zeolites, from the global

207

and local void space topologies and from the ability to manipulate framework ,cation, and guest properties, experimentally (Figure 6). The mechanism of photolysis of dibenzyl ketone (hereafter referred to as DBK) has been firmly established (22-24) and proceeds R two important stages as shown in Figure 7.

Figure 6

Computer Graphic Representa tion of DBK fitting into the void spaces of the faujasite zeolite.

First we consider the global overall mechanism and then the details of the structures of the products which will lead to reporters of zeolite structure and of rotational and diffusional motion of small molecules on the zeolite internal surface. The top of Figure 7

C&&H,I!

a

CH~C~HS .

primary geminate triplet radical pair

1

0

II

C~H~CHZCCHZC~H~

primary geminate combination

Figure 7. Schematic representation of the mechanism of photolysis of DBK.

208

shows the mechanism for photolysis of DBK. In order to consider the general case, Figure 8 depicts an asymmetric ketone, ACOB, as an example of the important mechanistic features of the system. The A group conforms to an isotopically labeled benzyl group of DBK, it was found (25) that even the modest substitution of a para methyl group in one of the rings of DBK changed the observed photochemistry, presumably because of differing locations of DBK and the methyl substituted derivative. The absorption of a photon cleaves the ketone into two fragments (ACOD), a carbonyl-containing acyl and a benzyl radical, termed a primary geminate radical pair. The acyl radical is known to persist for about 100 ns and then decarbonylates to produce a second benzyl radical (26). decarbonvlation reaction serves as a clock which can monitor reaction rates and molecular motion. This primary geminate pair can undergo geminate coupling processes in competition with decarbonylation. The geminate combination processes will regenerate the starting structure (ACOB) or an isomer (ACOB') of the starting structure. If decarbonylation occurs, a secondary geminate pair of benzyl radicals (AD) is produced. This pair may undergo a competition between geminate coupling and diffusional separation to form free radicals (A + B). In the systems under investigation the structure of the products and their absolute and relative yields are the data needed to infer the manner in which adsorption on the zeolite surface controls the chemistry of the various radicals. Let us now see how this statement is justified mechanistically.

A-C

Jf!

-B

Primary geminale coupling

Figure 8.

AA+AB+BB

A-B Secondary geminate coupling

The mechanism of the photolysis of an

Free radical coupling

asymmetric DBK system.

Consider the primary geminate pair (ACO/B) for the specific case of dibenzyl ketone (Figure 9). If the primary geminate radical pair can rotate within the 100 ns time window allowed by the rate of decarbonylation, the carbonyl fragment can attach itself to the ortho or the para position of the benzyl radical, to yield oDBK and pDBK, respectively. If the primary radical pair can diffuse apart and remain apart for the 100 ns, decarbonylation occurs and a secondary geminate radical pair (A/B) is produced. If the mechanistic ideas are correct, the ratio of oDBK to pDBK provides ;Isimple probe of the rotation degrees of freedom available to the primary radical pair. Let us now consider the possible fates of this secondary geminate radical pair. Since the product of reaction, DPE, does not contain the carbonyl group, we can easily

209

a ,

@-CH2C

CH2-@

n

Q'CH2C

--

pDBK oDBK

-@-CH,

no rotation

- 30' rotation - 180'rotation

pDBK degree of

freedom

Figure 9.

The ratio of ODBK to ~ D B Kas a measurement of the degree of rotational freedom.

exploit the structures of the products of the photolysis of DBK to provide information concerning the diffusional motion of benzene-like molecules adsorbed on the zeolite surface and on the ability of the zeolitic cavities and channels to serve as "cages" which promote geminate combination. Figure 10 depicts the competition between the diffusional trajectories of the secondary radical pair and the rotational motion of the primary radical pair. If the mechanistic ideas proposed are correct, the ratio of DPE to the (sum of) isomeric DBK products provides a simple probe of the diffusional and rotational motion of the primary radical pair. The zeolite systems selected for detailed investigation by the DBK probe were the X and Y faujasites. These systems possess a common internal surface topology but differ in the Si/Al composition of the framework, with the X zeolite possessing a lower ratio than the Y zeolite (Si/AI = 1.2 and 2.4, respectively). As a result of the differing compositions, the X zeolite possesses about 4 to 5 Site In cations per supercage, whereas the Y zeolite possesses less than one cation per supercage (Figure 4)(27). Thus, it is expected that the steric factors experienced by DBK adsorbed in the supercage of a X zeolite will be much more severe than those experienced by a DBK molecule adsorbed in a Y zeolite. Experimentally, this expectation can be tested by simply examining the ratio of products from the photolysis of DBK adsorbed in a X or Y zeolite (Figures 9 and lo), both of which have been fully exchanged with the same cation. Based on the steric effect hypothesis, the products which require more rotational and/or diffusional degrees of freedom will be strongly favored when DBK is photolyzed in the Y zeolite. The influence of electrostatic and related electronic (dispersion and quadrapolar) effects on the course of reaction can be tested by examining the products of photolysis of DBK adsorbed on a MX (or MY) zeolite as a function of the exchangeable cations and coadsorbed "inert" gases. In this case it is expected that if electronic factors are important, steric factors alone will not be able to explain the results. For example, the steric factors are qualitatively clear as one proceeds from Li to Na to K exchanged zeolites, whereas the electronic factors cannot be qualitatively predicted.

210

DPE

DPE

Isomers

-Diffusion Rotation

Isomers ‘Figure 1 0 .

Competition between diffusional motions of secondary radical pairs and rotational motions of primary radical pairs.

12.2 Photolvsis of Ketones Adsorbed in Fauiasites Only a brief outline of the salient results will be given here and the reader is referred to the original literature for details (9,11,13,14). The photochemistry of DBK adsorbed on faujasite zeolites was examined under various conditions. Under all conditions the amount of DBK adsorbed on the zeolites was sufficiently low that only one out of every several supercages would contain a DBK molecule. Controls of various sorts including the results described below provide convincing evidence that essentially all of the observed photochemistry occurred from species adsorbed on the internal surface of the zeolite. For an overview of the results, consider Table 1 which shows how the products of photolysis of DBK may be varied in a “catalytic” manner by varying zeolite characteristics. First consider the samples in the absence of any added guest molecules. The major product of photolysis of DBK adsorbed on MY is DPE (>go%) and is completely independent of M for the samples in the absence of added guests. In contrast, the major product of photolysis of DBK adsorbed on MX zeolites is strongly dependent on M for the samples in the absence of added guests (Table l), with DPE the major product for LiX,p-DBK the major product for NaX, and o-DBK the major product for KX. The influence of an added guest molecule, benzene, to the samples results in an equally remarkable variation in the reaction products. For example, addition of sufficient benzene to saturate the void space in the supercages causes the following changes in the major product: (a) for LiX, from DPE to p-DBK; (b) for NaX, from p-DBK to o-DBK; (c) for Kx, from o-DBK to p-DBK. In the case of the photolysis of DBK on the MY zeolites, addition of benzene causes a dramatic increase in the isomeric products (Table 2). Let us consider the photolysis of DBK on NaX as a standard system (Table 3). Under a vacuum or in an argon atmosphere, the major product is 1,2-diphenyl ethane (DPE) which results from diffusional separation of the primary radical pair, followed by decarbonylation and random coupling of the benzyl radicals produced in the secondary radical pair. These results, along with isotopic labelling experiments, show that diffusional motion of primary and secondary radical pairs is fast compared to coupling or decarbonylation reactions of the primary or the secondary radical pairs. The results for

21 1

Table 1.

Product distributions from the photolysis of dibenzyl ketone adsorbed on ion exchanged X and Y zeolites under vacuum.

DBK

CY

DPE

pDBK

oD8K

Zeoiite

aEE

DDnK

onnK

LiX

80%

16%

3%

NaX

55%

26%

17%

KX

40%

16%

40%

LiY

100%

0%

0%

NaY

95%

5%

0%

KY

94%

4%

2%

photolysis of DBK adsorbed on NaY leads to completely different results for the case of vacuum and added guests (Table 3 and 4). The effect of added guest molecules or of variation of the exchangeable cations on the product distributions is remarkable. Addition of benzene vapor causes the photochemistry to change from decarbonylation to formation of coupling products of the primary radical pair, i.e., coupling now occurs faster than decarbonylation. Thus, we can conclude that the diffusional and rotational motion of the primary radical pairs has been tremendously inhibited by the benzene molecules that fill the supercage. A similar result occurs upon changing the cation from Na to K (Table 1). In this case the yield of coupling products generated from the primary pair increased substantially relative to that found for NaX. With LiX the yield of primary pair coupling decreases substantially relative to NaX. These results reflect the fact that the steric constraints of the cations influence the rotational and diffusional motions of the radicals in the supercage. Li being smaller than Na allows more freedom of motion, and K being larger than Na provides more constraints on motion of the radicals. The effect of adding benzene on MX zeolites is more substantial than on MY zeolites (Table 2). These results reflect the fact that the Y zeolite

has fewer exchangeable cations per supercage and, therefore, the changes in the space occupied by the larger or smaller cations does not influence the rotational or the diffusional motions of the radicals in a significant way. Furthermore, the lack of any significant difference in the product distribution (Table 1) in the MY samples (M = Li, Na or K) provides support for the proposal that steric effects

212

dominate the rotational and diffusional dynamics of the primary and secondary radical pairs and thereby determine the product distributions. The effect of added "inert" gases on the product distributions provides the example of the influence of electrostatic and electronic (dispersion and quadrapole) effects on the course of reaction. It is believed (28-30) that interaction between the electrostatic field of the cations and the quadrapole moment of the adsorbed gas molecules contributes significantly to the adsorption energy. Nitrogen and carbon dioxide have substantial quadrapole moments, while argon has zero quadrapole moment. Coadsorption of nitrogen and carbon dioxide causes the photochemistry to change to formation of the coupling products of the primary radical pair. The strong association of nitrogen and carbon dioxide molecules with the cations exerts restrictions on the diffusional and rotational motion of the primary radical pairs due to the filling of the supercage. The yields of the coupling products generated from the primary radical pairs increased from that observed under a vacuum (or in an argon atmosphere) to that observed in a nitrogen or in a carbon dioxide atmosphere (Table 3). Y zeolite has fewer exchangeable cations per supercage, and therefore, the change of the reaction space due to adsorption of nitrogen or carbon dioxide does not influence the rotational or diffusional motions in a significant way (Table

4).

Table 2.

0

II D C H l - C -CHl

Product distributions from the photolysis of dibenzyl ketone adsorbed on ion exchanged X and Y zeolites with coadsorption of benzene.

a

hv

-

0

~

c

~

0

+~ Q w z-' b e CcH ) + ~

~Q W 1 '~A - Q CH3

pDBK

DPE

Df(K

oDBK

Zeolite

LEE

aDBK

oJ-)BK

LiX

17%

65%

6%

NaX

6%

30%

47 %

KX

20%

33%

29%

LiY

74%

26%

0%

Nay

25 %

28%

11%

KY

23%

30%

30%

213

Table 3.

DBK

Product distributions from the photolysis of dibenzyl ketone adsorbed on NaX.

pDBK

DYE

aDBK

m

LEE

DDBK

oDBX

Vacuum

55%

26%

17%

Ar (600 torr)

56%

26%

15%

CO2 (600 torr)

32%

33%

26%

N2(600torr)

24%

35%

32%

Benzene

6%

30%

50%

Table 4.

Product distributions from the photolysis of dibenzyl ketone adsorbed on Nay.

DBK

DYE

CHI

pOBK

uDBK

w

PEF,

nDBK

oDBK

Vacuum

95%

5%

0%

Ar (600 torr)

95%

5%

0%

CO2 (600 torr)

100%

0%

0%

N2 (600 torr)

96%

4%

0%

Benzene

25 %

28%

11%

13. CONCLUSION The products produced by the photoexcitation of dibenzyl ketone adsorbed on faujasite (X and Y) zeolites has been shown to be sensitive to variations in zeolite smcture such as the Si/Al ratio of the framework composition, the number density of the exchangeable cations and the nature (size) of the exchangeable cation. I n addition,

214

additives such as permanent gases and organic molecules may exert a profound influence on the product distributions. All of the results are consistent with the hypothesis that steric factors play a dominant role in determining the product distribution. In its simplest form, the paradigm which rationalizes the results is as follows. The photochemistry of DBK adsorbed on the internal surface of faujasite zeolites is the same as that of DBK in homogeneous solvents (Figure 7). The competition between rotation, diffusion and decarbonylation determines the observed products (Figures 9 and 10). The dynamics of these three processes are modulated by the zeolite topology, the zeolite composition, the cation type and cation number density, in addition to the number density and type of coadsorbed guest molecules. In particular, the ability of the zeolite attributes to allow or to inhibit the three competing processes to occur are directly related to steric factors. It is unlikely that steric factors will always dominate product ratios. However, in the case of photolysis DBK adsorbed on faujasite zeolites, such clearly appears to be the case. Acknowledgments. The authors thank the Air Force Office of Scientific Research, the National Science Foundation, the Department of Energy and the IBM Corporation for their generous support of this research. REFERENCES 1 2 3 4 5 6 7

8 9 10 11 12

13

14 15 16 17 18 19 20

D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. N.N. Avgul, A.G.Bezus, and O.M. Dzhigit, in E.M. Flanigen and L.B. Sand (Eds), Molecular Sieve Zeolites 11. Adv. Chem. Series 102, A. C. S., Washington, D.C. 1971, p. 185 J.M. Thoma, in R. Vanselow and R. Howe '(Eds), Chemistry and Physics of Solid Surfaces , Springer-Verlag, 1986, p. 107 R.M. Barrer, Pure & Appl. Chem., 58,1986, 1317. E.G. Derouane, in M.S. Whittingham and A.J. Jacobson (Eds.) Intercalation Chemistry, Academic Press, New York, NY, 1982, p. 101 E.M. Flanigen, in L.V.C.Rees (Ed.), Proceedings of the Fifth International Conference on Zeolites,. Heyden, London, Philadelphia, Rheine, 1980, p. 760. S Ramdas, J.M. Thomas, P.W. Betteridge, A.K. Cheetham, E.K. Davies, Angew. Chem. Int. Ed. Engl., 23,1984, p. 671. I. Suzuki, S. Oki, S. Namba, J. Catal., 100, 1986, p. 219. N.J. Turro, Pure & Appl. Chem., 58, 1986, p. 1219. J. K. Thomas, J.Phys. Chem., 91, 1987, p. 267. N.J. Turro, C.-C. Cheng, L.Abrams and D.R.Corbin, J . Am. Chem. Soc., 109, 1987, p. 2429. B. Frederick. L.J. Johnston, P.de Mayo and S.-K. Wong, Can. J. Chem., 62, 1984, p. 403. N.J. Turro, C.-C.Cheng, X.-G.Lei, and E.M. Flanigen, J . Am. Chern. SOC., 107, 1985, p. 3739. N.J. Turro and Z. Zhang, Tetrahedron Lett., 1987, p. 5637. D.R. Corbin, D.F. Eaton and V. Ramamurthy, submitted for publication. H.S. Sherry, J . Phys. Chem., 70, 1966, p. 1158. M. Onaka, K. Ishikawa and Y. Izumi, J. Inclusion Phenomena., 2, 1984, p. 359. M.M. Dubinin, A.A.Isirikyan, G.U. Rakhmatkariev, V.V. Serpinskii, Bull. Acad. USSR. Div. Chem., 1973, p. 900. H.L. Lechert, K.-P. Wittern, Ber. Bunsenges. Phys. Chem., 82, 1978, p. 1054. H. Jobic. A. Renouurez, A.N.Fitch. and H.J. Lauter, J . Chern. SOC.Faraday Trans.'I, 83, 1987, p: 3199.

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21 22 23 24 25 26 27 28 29 30

M.L. Unland and J.J. Freeman, J . Phys. Chem., 82, 1978, p. 1036. (a) P.S. Engel, J. Am. Chem. Soc., 92, 1970, p. 6074; (b) W.K. Robbins and R.H. Eastman, ibid., 92,1970, pp. 6076, 6077. N.J. Turro and G.C.Weed,J. Am. Chem. SOC., 105,1983, p. 1861. N.J. Turro and B. Kraeutler, Acc. Chem. Res., 13, 1980, p. 369. N.J. Turro and Z. Zhang, unpublished results. I.R. Gould, B.H. Baretz and N.J. Turro, J . Phys. Chem., 91, 1987, p. 925. H.A. Resing, C.G. Wade, in Magnetic Resonance in Colloid and Interface Science ACS Symp. Series 34, Washington, D.C., 1976, p. 36. R.M. Barrer and W.I. Stuart, Proc. Roy. SOC.Ser. A , 249, 1959,~.464. R.M. Barrer, and W.I. Stuart, Proc. Roy. SOC.Ser. A , 249, 1959, p. 484. H.W. Habgood, Can. J . Chem., 42, 1964, p. 2340.

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PHOTOCHEMISTRYOF ORGANIC CATIONS AT CHARGED INTERFACES C. A. BACKER and D. G. WHITTEN

INTRODUCTION Intermolecular cycloaddition reactions are of considerable importance in synthetic organic chemistry and have been studied extensively. The Diels Alder reaction which involves the thermal ground state addition of a diene to an olefin is the best known example of such a reaction. Also, it is well established that cycloaddition reactions of two olefins to give cyclobutane occur under both thermal and photochemical conditions. Although these photocycloaddition and cycloaddition reactions are widely used, the [2+2]photoaddition of an olefin in homogeneous solution is limited in synthetic value due to the lack of stereospecificity. In contrast, [2+2]photodimerizations in the solid state can be very selective, often with only one product being formed (16). The stereochemical or regiochemical selectivity or control of these solid state photochemical reactions can be related to a "topochemical" control in which the monomer packing within the crystal lattice dictates the formation of product with minimal atomic or molecular rearrangement (1-4). However, the utility of solid state photochemistry is limited because it is often difficult to predict or influence the way in which the monomer orients within the crystal (5-8). In an effort to retain both the topochemical control afforded by the solid crystal and the flexibility provided by homogeneous solution, we have focused our efforts on the intermediate environments which can be provided by microheterogeneous media. We have been particularly interested in photodimerization and addition reactions involving reagents which localize at interfaces. The concentrating and orienting effects of the interface may be expected to favor such topological control. In this paper we discuss the photodimerization and excimer fluorescence of stilbazolium cations at charged and structured interfaces, intermediate in organization between homogeneous solution and solid state.

BACKGROUND( Work by Cohen, Schmidt, and Sonntag (1) sought to correlate the molecular structure of the photoproduct in dimerization reactions with

217

the packing orientation of the reactant species in the crystal lattice. This work focused on trans-cinnamic acid and some of its ring substituted derivatives. In homogeneous solution, the distribution of the photoproducts is dependent on the steric and electronic effects of the reagents. Irradiation of a melt or solution of trans-cinnamic acid does not cause dimerization, only isomerization (9,lO). Dimers of transcinnamic acid can form in the solid state with retention of crystal symmetry because the monomers are held rigidly within the lattice in a uniform and repeating manner forcing an association between monomers. There are three known crystal modifications of trans-cinnamic acid: a, p and y. The different crystal structures are comprised of identical molecules arranged differently in space. a is the most stable form and its corresponding photoproduct is the centrosymmetric dimer, a-truxillic acid. p is metastable and yields pure photodimer p-truxinic acid (a dimer with mirror symmetry), only under low temperatures (below 50°C) where the B+a phase transformation does not occur. At higher temperatures, the photoproduct a-truxillic acid can be formed from the conversion of metastable p+a. Four dimers are possible, but only the a and p dimers are formed from the irradiation of crystals of transcinnamic acid. Schmidt (1) found that the conformation of the transcinnamic dimers was related to the packing arrangement of the nearest neighbors in the monomer lattice. Dimerization occurs only when the crystal lattice places the two olefins parallel to one another and at separations no greater than 3.6-4.2A. The third crystalline form of cinnamic acid, y, is light stable and photochemically inactive due to a greater separation between the monomer units (-4.7A) within the crystal. Schmidt's (1) rules for photodimerization can be summarized as follows: Reactions in the solid state occur with a minimal amount of atomic or molecular rearrangement. 2. Bimolecular reactions are expected to take place between nearest neighbors. 3. "...the stereochemistry of the dimer is determined by the contact geometry of nearest neighbor double bonds, provided that the center-to-center distance d of these double bonds is of the order of 4A (experimentally observed limits 4.2>d>3.5A. At 4.7A and above reaction does not take place.)" 4. Potentially reactive double bonds must be in exact parallel alignment within a regular crystal lattice.

1.

Over the last six years there have been several reports of unusual topological behavior which deviate from these accepted topochemical

218

rules. Rarnarnurthy et a/. (11) reported that 7-rnethoxycournarin dimerized in the solid state even though the double bonds were in a nonparallel alignment (the two double bonds were rotated 65O with respect to one another). In order for dimerization to occur, the molecules would have to be brought together in the proper orientation. Ramamurthy suggested that the favorable orientation could occur by a translationcum-rotation involving a screw dislocation.

0

0 BBCP

DBCP

0 [DB(+)3 MeC P]

0 DDBCP

Fig. 1. Family of compounds studied by Theocharis eta/. (12) based on 2benzyl-5- benzyl idenecyclopen tanone. A more in depth study of the interrelationships between photoreactivity and crystal packing was undertaken by Theocharis et a/. (12) They studied a family of compounds based on 2-benzyl-5benzylidenecyclopentanone (BBCP) (Fig.1) and found that 2,5dibenzylidenecyclopentanone (DBCP) dimerized even though the geometry of the reactive centers was not conducive to a topochemical reaction as defined by Schmidt and Cohen (1). The potentially reactive double bonds were well within the limits for dirnerization to occur (3.714 and 3.725A), however, as with 7-rnethoxycournarin, the double bonds were not in parallel alignment. Although the reactive centers were not parallel, the overall planes of the R bonds to which these reactive centers belong were parallel. Therefore upon irradiation the pz orbitals were able to interact and dimerization occured. Another molecule studied by Theocharis and coworkers (12) was (+)-2,5-dibenzylidene-3-rnethylcyclopentanone [DB(+)3MeCP]. This molecule was nearly planar but had a methyl group which extended 1.65A above the mean plane. Although the distance beween potentially reactive

219

centers was 3.871A and they were related by a two-fold screw axis, the crystal was found to be photostable. Theocharis attributed the lack of dimerization to the fact that the benzylidene groups to which the two reactive centers belonged were nonparallel and didn't allow for overlap of the pz orbitals. Theocharis (12) found many interesting contrasts between the dimerization of BBCP and DBCP, even though the only difference between these two molecules is an additional double bond in DBCP. BBCP photodimerization proceeds in a single crystal+ single crystal manner whereas DBCP yields an amorphous product with mostly DDBCP being formed. The photodimer of BBCP has a planar cyclobutane ring, whereas the cyclobutane ring of DDBCP was found to be puckered. This is probably due to dipole-dipole repulsion between the carbonyl groups. The additional double bond makes DBCP a more rigid molecule than BBCP. It was proposed that while BBCP can change conformationally during dimerization within the geometric constraints of the crystal lattice, the more rigid structure of DBCP results in formation of a dimer which is unable to fit within the dimensions of the monomer crystal lattice. Strain develops within the crystal during dimerization and a loss in integrity of the crystal results. Most crystals contain some impurities and structural defects which can affect the outcome of the photodimeritation. Cohen (13) and Ludmer studied the effects of impurities and defects on the dimerization of 9cyanoanthracene. With the monomer units stacked parallel (p type), the expected photoproduct would be cis-dianthracene; however it is the trans-isomer which is formed. Further investigation of the monomer crystal revealed the presence of structural defects where the monomer units were aligned in an antiparallel manner. These investigators showed that the dimerization occurs at specific sites within the crystal. Cohen noted that there was strong circumstantial evidence indicating that the reactions occurred at defect sites and hypothesized that absorbed energy was rapidly transmitted through the crystal to the defect sites. In support of this theory, they incorporated several impurities within 9cyanoanthracene and found that the impurities trapped the absorbed excitation energy and inhibited the dimerization reaction. Others have reported the cyclodimerization of olefin monomers in parallel alignment but at distances up to 4.8A (14). Considerable progress has been made in understanding the conditions required for dimerization within the crystal lattice (with the ultimate goal of crystal engineering). However, even if all the technical problems of forming uniform crystals with the appropriate geometry from dissimilar

220

monomers were overcome, not all chemical reactions can take place in the solid state. An alternate approach to exerting a controlling influence on photodimerization would be through the use of microheterogenous media. Organized media such as micelles, vesicles, films, or microemutsions can provide an environment of variable properties and order. If a molecule has a hydrophilic end and hydrophobic end, it might be expected to align at an interface in a predictable manner. Quina and Whitten (5,6) studied the photodimerization, photoisomerization and excimer fluorescence of stilbazolium derivatives in homogeneous solution, CTAB micelles, monolayer assemblies, and the solid state. The photoproducts of N-(1-Octadecyl)-4-stilbazolium salts (ClaStzX, where X- is the counterion) were found to be extremely dependent upon the molecular environment. In acetonitrile, C1&3tzX exhibits a weak structureless fluorescence with a maximum peak at 430nm. Irradiation of a nearly saturated solution results in fransj cis isomerization without evidence of bimolecular interactions occurring. Prolonged irradiation of the ClaStzX solution results in cyclization to the corresponding azaphenathrene salt from cis-C1&tzX. The photochemical behavior of ClaStzX in CTAB micelles was found to be quite similar to that observed in homogeneous solution. Trans+ cis isomerization with eventual cyclization occurs upon irradiation. No dimerization was observed. The photochemistry of CIaStzX in the solid state was found to be dependent upon the counterion used. A weak blue fluorescence with a maximum of 430nm was evident for solid samples of ClaStzBr and Cl&tzBF4. Furthermore, these samples remained unchanged after irradiation. Changing the counterion to pchloro- or p-bromobenzene sulfonate resulted in an intense yellow-green fluorescence with a maximum at 470nm. Irradiation of these solids caused only dimerization of the stilbazole and no detectable trans+ cis isomerization. The structure of the photodimer was determined by NMR to be the p dimer. This result was surprising because of the four possible dimers, one would expect a head-to-tail alignment in order to minimize steric and coulombic repulsion. Yet the p dimer, which is aligned head-to-head and expected to have the most coulombic repulsion between the positively charged pyridinium rings, is formed. Monolayer assemblies containing mixtures of arachidic acid and CIeStzX exhibited an intense yellow-green fluorescence (maximum=49Onm) which was independent of the counterion. This red shifted emission was also evident in the solid state and was attributed to stilbazole excimer fluorescence. Irradiation led to bleaching of the

221

long-wavelength absorption band, a concomitant increase in a shorterwavelength band, and a decrease in the emission at 490nm. The photobleaching was similar to that seen in the solid state and was again attributed to a [2+2] photoaddition. Profound differences were evident between the photochemical behavior of stilbazole in micelles and in monolayers. As in homogeneous solution, the stilbazole isomerized without producing excimer emission or photodimeritation within the micelle. Assuming an aggregation number of 75 for CTAB, each micelle should contain approximately six to seven stilbazolium molecules for a 1 : l O or 1:12 mole ratio. The same concentration in the monolayer assemblies produced significant excimer fluorescence and photodimerization. The stilbazolium should concentrate at the interface of the micelle. However, the lack of dimerization within the micelle suggests that concentrating the stilbazole at the hydrophilichydrophobic interface is not sufficient to bring about the geometric conditions needed for bimolecular reactions to occur; a highly organized interface must also be present. Although bimolecular reactions do take place within the monolayer, these assemblies are not ideal reaction media. Monolayering is a difficult and time consuming technique in which only small amounts of reagents can be used. A more ideal reaction medium would be one which incorporated the ease of preparation of the micelle with the more ordered environment of the monolayer assembly. Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosucinnate) reversed micelles are easily prepared and have a highly organized hydrophilic-hydrophobic interface. The structure of the reversed micelle is such that the polar head groups of the surfactant occupy the structure's core and the branched hydrocarbon chains extend into the hydrocarbon continuous phase (15-20). The properties of the reversed micelle are generally described in terms of o where w=[HpO]/[AOT]. At low values of cu (04-10) all of the water is involved in hydrating the surfactant head groups (20,21). Increasing w results in a larger water pool and the presence of two types of water; "interfacial" water which is chiefly associated with the polar and ionic portions of the surfactant and associated counterion, and the presence of "normal" or "bulk" water in the interior of the reversed micelle (16,20,21). In the present work, two types of polymeric supports (inorganic and organic) have also been studied as potential reaction surfaces: montrnorillonite clay and Nafions (22) 117 perfluorinated membrane. Clays are hydrous silicates or aluminosilicates, many of which occur in nature and make up the colloidal fraction of soils, sediments, rocks and water. Most clays belong to the class of layer silicates or phyllosilicates

222

because they are composed of sheets of silicon-oxygen tetrahedra and alumina-oxygen-hydroxyl octahedra joined together in various proportions (23,24). The tetrahedral layer consists of two planes of oxygen-hydroxyl ions in which the oxygen atoms are located on the four corners of the tetrahedron with the silicon atom in the center. Three of the four oxygen atoms are shared by neighboring tetrahedra. The octahedral layer is comprised of an aluminum ion octahedrally coordinated to six oxygen or hydroxyl groups located in planes above and below the aluminum ion. Recently there has been much interest in adsorbing organic compounds and carrying out organic reactions on the surface of clay (16,20,23,25). The most extensively used clay for these purposes has been Montmorillonite, or "swelling clay", which is part of the dioctahedral smectite group. The structure of montmorillonite clays (23,24) consists of a unit layer comprised of on octahedral layer sandwiched between two tetrahedral layers. The unit layers are stacked parallel to one another with water channels between the units. In clays, isomorphous substitution which is the replacement of one atom for another often occurs. Silicon(1V) sometimes replaces aluminum(lll) in montmorillonite's tetrahedral layer. Within the octahedral layer, aluminum atoms may be replaced by Mg, Fe, Cr, Zn, Li and other atoms. The replacement of an atom with a higher valence by that of a lower valence results in the lattice having an excess negative charge. The excess negative charge on the clay lattice can be reduced by the addition of exchangeable cations. If the cations are too large to be absorbed into the interior of the lattice, they will exchange on the surface of the clay. Nafiona, a perfluorinated sulfonic acid ion-membrane, has both hydrophobic regions (-CF2CF2-) and hydrophilic regions (-SOd-i). The fluorocarbon backbone provides the membrane with exceptional chemical, thermal, and mechanical stability, while the hydrophilic region is responsible for the ion exchange capabilities and the ability to absorb relatively large amounts of water and other solvents (26) Komoroski and Mauritz (27) found the behavior of Nafion membranes similar to that of the reversed micelle. Gierke and Hsu (28) proposed an ion cluster model in which the sulfonate head groups are located in spherical clusters surrounded by the fluorocarbon background. The ionic clusters are connected by short channels containing sulfonate exchange sites which are not included in the clusters (29).

223

Results and Discussion In the present paper, we report a study of the photodimerization and "excimer" fluorescence of tfans-2- and 4-stilbazolium cations in AOT reversed micelles, NafionB perfluorinated membranes, and montmorillonite clay. Both cations were converted into their corresponding salts with the anionic surfactant AOT and were incorporated into reversed micelles comprised of hexane-AOT-water. For both polymeric supports, solutions of the two cations were prepared and the film or clay was added and allowed to stir overnight. The substrates were irradiated using a 200W Hg lamp with a Corning 0-52, 7-37 and water filter. The product distribution was quantitatively analyzed using a GE 300MHz NMR and evaluating the chemical shifts and coupling constants of the methine protons of the cyclobutane rings and the Ha protons which are ortho to the pyridine nitrogen.

1 . trans-4-Stilbazolium

2 . t f a n s - 2 - S t i Ib a z o I i u m

Aqueous acidic solutions of either stilbazolium cation exhibit only "monomer" absorption and fluorescence at concentrations of 0.005M or lower. Increasing the stilbazolium concentration in aqueous HCI (0.11.OM) to 0.01M or higher results in a red-shifted, broadened and structureless "excimer" fluorescence and a slightly broadened absorption spectrum (Table 1). Irradiation of these concentrated stilbazoliurn solutions result in a decrease in the trans+ cis isomerization and the formation of the a dimer as the principle photoproduct for both cations (Tables 2 and 3). Although both the cis isomer and the a dimer are formed from each cation in the concentrated homogeneous solution, only the 6 dimer forms from cation 1 while the p dimer and small amounts of the E dimer form from cation 2. The most probable cause of the predominant a dimer formation in concentrated homogeneous solutions of cations 1 and 2 is that this dimer has the least repulsion between the two positively charged pyridinium rings; the p, or head-to-head dimer has the most electrostatic repulsion. The difference in the distribution of photoproducts for cations 1 and 2 in concentrated homogeneous solution indicates that the position of the nitrogen in the pyridinium ring has a profound effect on the photochemistry of stilbazole. Results in concentrated aqueous acid

224

solutions suggest that dimerization of trans-4-stilbazolium is a more selective process than that of trans-2-stilbazolium. This is demonstrated by the total lack of any f3 dimer formation for 4stilbazolium. TABLE 1

-

Absorption and Emission Maxima of trans-4- and 2-Stilbazole in Homoaeneous Solution and Reversed Micelles Absorption

protonated freebase o=lO W-40

338nm 310 335 330

336nm

334 334

Emission

O.OOlM,Aq. HCI O.OlM,Aq. HCI w-20 0~40

u 446nm 442nm 469 469 490

471 50 1 51 0

TABLE 2 Photoproducts from Irradiation of trans-4-Stilbazolium in Reversed Micelles and Homogeneous Solution (30) 0.028 0.0024 0.0013 0.0010 0.0024 0.0010

lAOTl 0 0.0065 0.0065 0.0020 0.0048 0.0020

w 78 64 20 10 5

Solvent %Conv. CIS 13 1NHCI 78 84 77 n-hexane 49 n-hexane 86 88 27 n-hexane 27 92 n-hexane 27 94 n-hexane

-

€3

0 6 29 60 51 56

6

a

5

60 17 15 6

--

7 8 21 12

-5

Dla

0 0.4 2.0 10. >20. 12.3

TABLE 3

Photoproducts from Irradiation of 2-Stiibazolium in Reversed Micelles and Homogeneous Solution

0.028M 0.0010 0.0010 0.0010 0.0010 0.0010 0.0010

0 0.0080 0.0080 0.0020 0.0020 0.0020 0.0020

SolventO/Conv.

60 15 40 20 15 10

1N HCI n-hexane n-hexane n- hexane n-hexane n-hexane n-hexane

>90 95 95 93 89 92 80

cis

16 80 83 67 59 53 48

0 20 11 11 26 36 40 42

13/a 0.3 1.2 1.8 3.7 7.2 5.7 4.2

225

As the counterion for AOT, stilbazolium is located and concentrated at the anionic interface. This results in changes in the photophysics of the cations relative to that in homogeneous solution. Although the absorption spectrum in the AOT reversed micelle remains unchanged for cation 1 versus homogeneous solution, hmax,abs for cation 2 is blueshifted and broadens slightly with increasing w (Table 1). "Excimer" fluorescence occurs at much lower stilbazolium concentrations for both 1 and 2 within the reversed micelle and is accompanied by the onset of photodimerization. A decrease in w results in a significant blue shift in the fluorescence emission maximum and an increase in the fluorescence intensity for both cations (Table 1).

6

&

Fig. 2. Cyclobutane Dimers from Irradiation of trans-1.2-Disubstituted Ethylenes. In addition to the spectroscopic changes observed for cations 1 and 2 in the AOT reversed micelle, significant differences in the distribution of photoproducts are observed relative to that in homogeneous solution (Tables 2 and 3). At large w values, the photoproduct distribution is similar to concentrated aqueous acid solution with ths cis isomer and the a dimer as the principle photoproducts. However as w is decreased, the p dimer becomes the predominant photoproduct and the ratio of p/a reaches a maximum value: at 0-10 for 4-stiibazolium and at 0 ~ 1 5 - 2 0for 2stilbazolium. Formation of the 6 dimer from cation 1 foilows the same trend as the p dimer; the amount of 6 dimer formed increases as the water pool size is reduced. Decreasing the ratio of [HStz]/[AOT] appears to have an effect similar to increasing w on the photoproduct distribution. These results clearly demonstrate that the size of the water pool and/or

226

the ratio of [HStz]/[AOT] has a profound effect on the distribution of the cyclobutane dimers. These results can be explained by considering: the electrostatic interactions between the cations, the structure of the AOT reversed micelle interface at varying o, and the transition states leading to the formation of the cyclobutane dimers. As mentioned above, p is the only photoproduct formed in studies with N-octadecyl-trans-4-stilbazolium p-chlorobenzenesulfonate. This can be attributed to the packing constraints of the monomer within the crystal lattice. The fact that the ratio of pla increases with decreasing o indicates that the way in which the stilbazolium monomer units pack at the interface is affected by a. The smaller the value of o, the larger the amount of curvature at the interface and the greater the polarity (17,18,21). The greater curvature should result in a larger surface pressure at the interface which could force the stilbazolium molecules to align more uniformly at the interface in a manner similar to monolayers. In AOT reversed micelles, pla reaches a maximum value at a-10 for 4-stilbazolium, but occurs somewhere between o=l5-20 for 2stilbazolium. Stilbazolium with the nitrogen in the 4 position can probably align more uniformly at an anionic interface than it can with the nitrogen in the 2-position. 2-Stilbazolium may even be disruptive to the AOT reversed micelle at low o,such as o=10. In a system lacking stilbazolium, an a, value of 10 represents the appearance of "free water" (19). The maximum value of p/a for 4-stilbazolium is approximately 2.5 times greater than that for 2-stilbazolium. This difference may be attributable to both the difference in selectivity between the two molecules and the manner in which they align at the interface. The transition states leading to the formation of the cyclobutane dimers must also be considered in any explanation of the distribution of photoproducts (31). Although there is little evidence for strong ground state interaction, our results are most consistent with a reaction in which the ground state geometry of the reactant pair can strongly influence if not dictate which product is formed. Therefore, we refer to a pro-a or -p reactant pair as one in which two monomers are aligned at the interface with a proper geometry to allow maximum overlap of the pz orbitals. The transition state leading to the formation of the a or p dimer should have the greatest stabilization energy. The stabilization energy for the pro4 reactant pair should be considerably lower than the transition state leading to the Q or p dimer. If the cationic groups of the pro-6 reactant pair are bound to the interface, the double bonds would be in an antiparallel arrangement and, according to the cinnamic acid crystal

227

study, dimerization would not occur (Scheme 1-a.). Therefore, the molecules of the 6 reactant pair must undergo a rotation so that a favorable overlap between the orbitals can occur (scheme l-b) and lead to the formation of the 6 dimer. It is difficult to rationalize why the 6 dimer is formed only from cation 1 and the E dimer is formed only in homogeneous solution from cation 2. There is precedent for the formation of the 6 dimer in the solid state. Previous work in our laboratory indicates that the dioctadecyl ester of 6-truxinic acid is formed by irradiation of C crystals of the octadecyl ester of trans cinnamic acid (32).

a.

b.

c. 6 Transition State

Scheme 1. Possible Geometries of Stilbazolium Cations at Interface Leading to Formation of 6 Dimer from Cations 1 (31). An apparent anomaly is the enhanced excimer versus monomer fluorescence and simultaneous increase of trans+ cis isomerization in the reversed micelles compared to concentrated aqueous acid solutions. We suggest that one of three possible explanations may account for this. One possibility could be that, even though the stilbazolium AOT salt is formed by the addition of protonated stilbazole to protonated AOT and acidic water is added to form the water pool, the stilbazolium is being deprotonated within the AOT reversed micelle. In support of the deprotonation theory, previous investigations in our laboratory with the methyl iodide salt of 4-stilbazole suggest that, at low to moderate o , dimerization of the olefin is the only photoreaction within the AOT reversed micelle (30,31). The transfer of a proton from HStz+ to AOTcould reduce some of the electrostatic repulsion between the negatively charged surfactant head groups at the interface. Whether associated with the interface as a moderately polar molecule or located in the continuous phase, the stilbazolium molecule would be more free to isomerize than if it was bound to the interface. Trans+cis isomerization also increases with larger o values. If deprotonation of the stilbazolium in the reversed micelle is the cause for the higher concentration of cis in the AOT solution, perhaps the addition of water forms H3O+ rather than HStz+.

228

Bardez and coworkers (33) showed that the ability of water to accept a proton is related to its H-bonded structure. At low w values, the protolysis of sodium 2-naphthol-6-sulfonate (NSOH) which resides near the interface was hindered. However, at ~ 4 0 the , rate of deprotonation of NSOH was equivalent to bulk water even though the rate of recombination was much faster. At low w values in the AOT reversed micelle, significantly more cis isomer is formed from cation 2 than from cation 1. This difference could be attributed to a difference in the alignment at the interface, reactivity, or ~ K values A between the two cations. The absorption spectrum of 2stilbazole is significantly blue shifted relative to the protonated form. The slight blue shift of the 2-stilbazolium absorption spectrum with increasing w could suggest that deprotonation occurs with increasing water pool size. A second explanation for the greater amount of cis isomer formed is that the photoreaction of stilbazolium in the AOT reversed micelle is a more "static" process than in homogeneous solution. Activation of the reactant-pair leads to dimeritation. However, activated stilbazolium monomers bound to the interface can only isomerize, whereas, in solution, the activated monomers are free to diffuse and dimerize as well as isomerize. Another possible explanation for this anomaly could be the influence of the interface on the isomerization reaction. A maximum value of pla at low w values suggests a more ordered interface. Increasing the water pool size reduces the curvature at the interface and could decrease the overlap between monomers needed for bimolecular reactions to occur. The appearance of at least two fluorescence lifetimes for cations 1 and 2 indicates that more than one excimer is present in the reversed micelle solution (Table 4). The percent contribution of the lifetime components change with w. This finding is consistent with the notion that the size of the water pool affects the photochemistry of the stilbazolium cation. At this point we do not have enough information to determine whether or not the different excimer lifetimes represent different states which lead to the production of different photodimers. It is clear from these results that the AOT reversed micelles have more than a concentrating effect upon the stilbazolium cations. Changes in the size of the water pool have a profound effect upon the outcome of the dirnerization reaction. The interface not only concentrates the monomers but alsp orients the cations in such a way as to force a nonpreferential association between the stilbazolium molecules. These results indicate that it is possible to exert topological control over the

229

[2+2] cycloaddition reaction in the AOT reversed micelle and suggest the possibility that these effects may be general for reactions of cations and anions at charged and highly organized interfaces.

TABLE 4 Fluorescence Lifetimes for trans-4- and 2-Stilbazolium AOT Salts in Reversed Micellesa

w

Xmax

T1

4-HStzAOT 4- HStz AOT

10 50

51 1 51 8

2.8( 53%) 2.0(44%)

7.5(47%) 6.5(56%)

2-HStz AOT 2-HStzAOT

10 40

480 485

0.9(42%) 1.3(46%)

3.4(37%) 4.0(30%)

Probe

T2

T3

11.3(21%) 1 1.2(24%)

aFluorescence Lifetime measurements were made on a PRA Single Photon Counter. Lifetime values reported are in nanoseconds. Lifetimes from cation 1 are from ref. 31. We have recently been investigating the behavior of stilbazolium cations at other anionic interfaces such as Nafiona 117 and montmorillonite clay. Nafiona 117 readily adsorbs the stilbazolium cations with a maximum coverage level (ratio of stilbazolium to exchangeable cations) of 100% and 90% for cations 1 and 2, respectively. When irradiated by a UV hand lamp, Nafiona films with 11% coverage levels or greater exhibited a yellow-green fluorescence, which is characteristic of the stilbazolium "excimer". Films with 5% coverage exhibited a blue fluorescence, characteristic of the stilbazolium "monomer". The Nafiona control (no probe) had a very faint blue fluorescence. A similar weak fluorescence was reported by Lee and Meisel (34) for wet Nafion-120 and by Childs and Mika-Gibala(35) for dry Nafion-125 and was also attributed to an impurity in the membrane. As the concentration of cation was increased above 5 percent, the fluorescence was significantly red-shifted and broadened for cation 1 and slightly red-shifted for cation 2. The excimer fluorescence of cation 1 at a 50% coverage level was red-shifted -30nm relative to cation 2. The presence of "excimer" fluorescence at coverage levels as low as 11Yo indicates the concentrating effect of Nafion's* interface. However, in order to investigate whether the interface could exert topological control over the reactivity, the films were irradiated and the photoproducts were extracted (three times with a 5M pyridinium

230

solution) and analyzed by NMR. Consistent with the findings in AOT reversed micelles and homogeneous solution, excimer fluorescence was accompanied by the onset of dimerization. The photoproducts of trans-2stilbazolium in Nafionm are very similar to those found in aqueous HCI, with the principal photoproduct being the Q dimer (Table 6 ) . As the coverage level decreases, the amount of trans cis isomerization increases significantly. For all coverage levels investigated, the ratio of pla remained constant and at a level comparable to that in homogeneous solution. More E dimer is formed in the Nafion than in homogeneous solution and reaches a maximum value at moderate coverage levels (2550%). Results with trans-4-stilbazolium were more unusual (Table 5). At high coverage levels, the Q dimer was the predominant photoproduct and almost no cis isomer was formed. (In the reversed micelle solution 6 dimer formation followed the same trend as p dimer formation.) However, in the NafionB, significantly more 6 dimer is formed even though no p dimer was detected. During irradiation, the color of the films changed from the yellowgreen "excimer" fluorescence to the blue "monomer" fluorescence. Two 90% coverage films with cation 2 were carried to different percent conversions in order to determine if trans+ cis isomerization occurred only after the "dimer" sites were exhausted. Similar product distributions were found regardless of percent conversion. Significant amounts of cis-2-stilbazolium formed at high coverage levels with both low and high percent conversions, suggesting the presence of different environments within the film for "monomer" and "excimer" emission. Formation of the "water pool" or "ionic cluster" within the film is a dynamic process, but perhaps the cis isomer is formed within the channel rather than within the water pool. Another possibility could be deprotonation of the stilbazolium. If the cis isomer is formed by deprotonation of the stilbazolium in the organized media, one would predict significantly less deprotonation in the Nafion than in the reversed micelle. Nafion is often used as a superacid catalyst (29,36). The electron withdrawing ability of the perfluorinated backbone is responsible for the superacidity of the sulfonate head groups. Significantly less cis isomer is formed in the Nafion film. This result in addition to the low level of p formation in the film relative to that in the reversed micelle suggests that the high percentage of cis formed in reversed micelle is due to deprotonation. As expected, the amount of trans+ cis isomerization increases with decreasing coverage level.

+

231

TABLE 5 Photoproducts From the Irradiation of trans-4-Stilbazolium (HStz) in Nafion Films. o/ ov O/ CIS 0 6 a 0f 0.028M HStz inlN HCI 7 8 13 0 5 60 0 5 40 100 11 72 50 - - 20 30 - 50 70 trace 13.5 85.5 - 100 83 trace 10 90 --

--

TABLE 6 Photoproducts from Irradiation of trans-2-Stilbazolium (HStz) in Nafion Films o/

C

0.028M HStz in 1N HCI 11 25 50 90 90

>90 60 47 36 24 93

D

e 16 46 29 29 11 8

20 7 6 7 12 6

E

U

Dfa

4

60 43 53 50 73 78

0.3 0.2 0.1 0.1 0.2 0.2

4 12 14 4 8

A third and significantly longer lifetime component for cation 1 was present in the Nafion film but not detected in the AOT reversed micelle (Table 7). The lifetimes for cation 2 in the film were comparable YI those in the reversed micelle. For both cations, the percent contribution of the components changed with coverage levels of the film. These results suggest that the photochernisty of stilbazolium is affected by the coverage level in the Nafionaeven though the actual photoproduct distribution does not change appreciably with different levels of coverage. The absence of or low levels of p dimer formed from cations 1 and 2 in the film was disappointing. Attempts to increase or reduce the hydration of the film had little effect upon the photoproduct distribution. Although more than 90 percent of cation 1 is extracted by the pyridinium solution, it is possible that some of the dimers formed within the film are not being removed. A control study was done in which a known distribution of dimers was added to the support and then re-extracted. Greater than 75 percent of the dimers were extracted and the distribution of dimers remained unchanged. The presence of stilbazolium "excimer"

232

fluorescence at coverage levels as low as 11% in the Nafionm film indicates the concentration of cation at the interface. Although the structure of Nafionm has been compared to that of the reversed micelle, these studies suggest that the interface is less ordered than that of the AOT reversed micelle. TABLE 7 Fluorescence Lifetimes of trans-4- and 2-Stilbazolium in Nafiona T1 Coverage by 4-Stilbazolium 11% 1.5 (25%) 50% 1.6 (29%) 85% 1.7 (35%) Coverage by 2-Stilbazolium 11% 0.5 (58%) 50% 0.8 (47%) 75% 0.7 (42%)

T2

T3

5.4 (42%) 5.6 (41%) 6.0 (42%)

15.5 (33%) 16.6 (30%) 17.2 (22%)

2.9 (29%) 3.6 (38%) 3.2 (41%)

9.9 (13%) 11.4 (15%) 11.3 (17%)

aFluorescence lifetime measurements were made on a PRA Single Photon Counter. Lifetime values reported are in nanoseconds. Montmorillonite clay readily adsorbed cations 1 and 2 at coverage levels of 130 and 90% respectively. At coverage levels of 50% or lower, the colloidal solution, or sol was off white in color and the particles remained dispersed for a reasonable amount of time (>5 hours). Clay suspensions at 90% coverage or greater appeared yellow-green in color and flocculated (the dispersed particles were large and settled in less than 2 minutes). The addition of salt to a sol has no effect upon the attractive van der Waals forces between particles, but does reduce the repulsion between particles due to the electric charge associated with the clay. As the stilbazolium salt concentration is increased, the clay particles which collide due to their Brownian motion begin to stick to one another and agglomerates form in solution (23,24). The spectral properties of the the two cations in the montmorillonite clay were affected by the percent coverage. Although the absorption maximum remained constant for cation 1, the peak broadened with decreasing coverage levels. This is presumably due to increased scattering caused by the higher concentration of clay relative to the stilbazolium concentration. In contrast, Xmax for cation 2 was blue shifted 22nm when the coverage was increased from 25% to 90%. A broadened absorption spectrum was also found at lower coverage levels.

233

"Excimer" fluorescence was seen for all coverage levels reported, however the intensity of the fluorescence was decreased by approximately 40 percent in a 100% coverage sample relative to homogeneous solution. lsomorphous substitution of Fe3+ for aluminum causes the stilbazolium fluorescence to be quenched. The fluorescence of cation 1 was again broadened relative to cation 2 at coverage levels of 50% or less. However, the fluorescence maxima at 100% coverage levels or greater were significantly blue shifted for both cations (40nm at 130% coverage level for cation 1 and 22nm at the 100% coverage level for cation 2). Xmax for 130% coverage by cation 1 was shifted -25nm to the blue of the maximum for 100% coverage suspension. However, addition of 30% more cation (NaCI) to the 100% 4-stilbazolium-clay suspension resulted in a blue shift of the fluorescence spectrum resulting in equivalent Xmax for both suspensions. The blue shift of the stilbazolium fluorescence at high cation concentrations was an interesting finding. Deprotonation of the stilbazolium is again probably responsible for the blue shift in the fluorescence at high cation concentrations. By definition, the clay is neutral at a 100% coverage level. The sodium cation, due to its smaller size be might be more efficient at neutralizing the negative charge on the clay surface than the stilbazolium cation. The exchange of sodium for stilbazolium at the anionic interface could facilitate deprotonation of the stilbazolium. The photoproducts formed within the clay were very similar to those in the Nafionm (Tables 8 and 9). However, only the a dimer was formed from cation 1 in the montmorillonite clay. The a dimer was the principle photoproduct from cation 2 at all coverage levels reported. However, small amounts of p and E dimers were also recovered. The clay was irradiated as a suspension or a film, but the form did not appear to have any effect on the distribution of photoproducts. The low level of cis isomer formed in the clay is probably a result of the enhanced acidity of the lamellar water which would minimize the deprotonation of the stilbazolium (37). Although greater than 90% of the frans-4-stilbazolium is extracted from the montmorillonite clay, there appears to be selective retention of dimers within the clay. Most of the cis isomer was extracted from the clay, however only a small percentage of p dimer put into the clay was re-extracted (-25%). Therefore, the percentages of photoproducts recovered from the clay probably do not reflect the actual distribution formed. However, based on the findings in the AOT reversed micelle and the Nafion, we would not expect the surface of the clay to exert

234

sufficient pressure on the monomer units to cause high quantities of the P dimer to be formed. TABLE 8 Photoproducts from the Irradiation of trans-4-Stilbazolium in Montmorillonite Clay % Coverage

50 100 1o o * 100' 130

% Conversion

>95 >95 28 87 >95

cis trace trace 2

a >95 95 95 >95 >95

--

trace

~

Montmorillonite clay was irradiated as a film.

~~

-

TABLE 9 Photoproducts from the Irradiation of trans-2-Stilbazolium in Mont morillonite Clay % Coverage 50 90

% Conversion 75 95

cis

---

P

3 10

E

8 trace

a

pla

89 90

0.0 0.1

The authors are grateful to the National Science Foundation (Grant CHE-8616361) for funding, and Dr. J. C. Scaiano for his helpful suggestions regarding sample preparation and extraction in NafionB. We would also like to acknowledge Dr. W. Hagan, Dr. F. Quina, Dr. B. Suddaby, Ms. S. Vadas, and Dr. K. Takagi all of whom have contributed during the course of this project. REFERENCES M. D. Cohen, G.M. J. Schmidt and F. I. Sonntag, Chem. SOC.,(1964) 1996. For a review see G. M. J. Schmidt, Pure Appl. Chem., 27 (197 647. H. I. Bernstein and W. C. Quimby, J. Am. Chem. SOC.,65 (1943) 1845. M. Hasegawa, Chem. Rev., 83 (1983) 507. M. Hasegawa, Pure Appl. Chem., 58 (1986) 1179. F. H. Quina and D. G. Whitten, J. Am. Chem. SOC.,97 (1975) 1602. F. H. Quina and D. G. Whitten, J. Am. Chem. SOC.,99 (1977) 877. L. Addadi and M. J. Lahav, J. Am. Chem. SOC., 100 (1978) 2838. L. Addadi, J. van Mil and M. J. Lahav, J. Am. Chem. Sac., 104 (1982) 3422.

235

9 10 11 12 13 14 15

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

G. Ciamician and P. Silber, Ber., 35 (1902) 4128. G. Ciamician and P. Silber, Ber., 36 (1903) 4266. N. Ramasubbu, T. N. Guru Row, K. Venkatesan, V. Ramamurthy and C. N. Ramachandra Rao, J. Chem. SOC., Chem. Comm., (1982) 178. C. R. Theocharis, W. Jones, J. M. Thomas, M. Montevalli and M. B. Hursthouse, J. Chem. SOC. Perkin Trans. II, (1984) 71. M. D. Cohen, Angew. Chem. Int. Ed., 14 (1975) 386. H. Nakanishi, M. Hasegawa and T. Mori, Acta. Cryst., C41 (1985) 70. M. Wong, J. K. Thomas and M. Gratzel, J. Am. Chem. SOC., 98(9) (1976) 2391, J. K. Thomas, The Chemistry of Excitation at Interfaces, ACS Monograph 181, Washington, 1984. J. H. Fendler, ACC.Chem. Res., 9 (1976) 153. M. Kotlarchyk, J. S. Huang and S-H. Chen, J. Phys. Chem., 89 (1985) 4382. C. A. Martin and L. J. Magid, J. Phys. Chem., 85 (1981) 3938. K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, New York, NY, 1987. P. L. Luisi, Angew. Chem. Int. Ed., 24 (1985) 439. "Nafion" is a registered trademark of the E. I. du Pont de Nemours and Company for its Nafion perfluorinated membranes. B. K. G. Theng, Formation and Properties of Clay-Polymer Complexes, Developments in Soil Science, Vol. 1, Elsevier, New York, 1979. H. van Olphen, An Introduction to Clay Colloid Chemistry, Interscience, New York, 1963. T. Nakamura and J. K. Thomas, Langmuir, 1 (1985) 568. R. S. Yeo, Polymer, 21 (1980) 432. R. A. Komoroski and K. A. Mauritz, J. Am. Chem. SOC., 100 (1978) 7487. T. D. Geirke and W. Y. Hsu, in: A. Eisenberg and H. L. Yeager (Eds), Perfluorinated lonomer Membranes , ACS Symposium Series 180; American Chemical Society, Washington, 1982. F. J. Waller and R. W. Van Scoyoc, Chemtech., (1987) 438. K. Takagi, B. R. Suddaby, S. L. Vadas, C. A. Backer and D. G. Whitten, J. Am. Chem. SOC., 108 (1986) 7865. B. R. Suddaby, Ph.D Dissertation, University of Rochester, 1986. J. Bolt, F. H. Quina and D. G. Whitten, Tetra. Lett., 30 (1976) 2595. E. Bardez, E. Monnier and 8. Valeur, J. Phys. Chem., 89 (1985) 5031. P. C. Lee and D. Meisel, J. Am. Chem. SOC.,102 (1980) 5477. R. F. Childs and A. Mika-Gibala, J. Org. ;hem., 47 (1982) 4204. D. Weir and J. C. Scaiano, unpublished manuscript. We thank Dr. Scaiano for a preprint of this paper. M. M. Mortland, J. J. Fripiat, J. Chaussidon and J. B. Uytterhoeven, J. Phys. Chem., 67 (1963) 248.

236

ELECTRON TRANSFER BETWEEN ADSORBED DYE MOLECULES AND ORGANIC CRYSTALS: Model Character of the Adsorption System for Certain Aspects in Photosynthesis

K. KEMNITZ, N. NAKASHIMA. and K. YOSHIHARA 1.

INTRODUCTION The adsorption system, comprised of electronically excited organic dye molecule adsorbed on organic single crystals, has been extensively studied over the past twenty years (1) using standard electrochemical methods. The main result of these studies consists in the fact that electronic relaxation in these systems occurs by electron transfer from the crystal substrate electrode (e.g. anthracene) to the singlet state of the excited dye (e.g. ‘(Rhodamine B)*). By variation of the substrate crystal material, the dependence of tha electron transfer rate constant on the free energy gap AGO can be studied (1). It was found that the rate constant increases with increased energy-gap according to the Marcus theory (2), and that the reorientation energy ( A ) in these wet adsorption systems is about 0.3 eV (3). Recently, the first direct determination of the rate constant of electron transfer using the quenching of the dye’s fluorescence was performed in a wet system at a dye coverage of about 1/2 monolayer (4). More recently, to avoid in-plane energy transfer among the dye population (5), the coverage has been reduced to 1/100 monolayer and the single photon counting technique was The employed to monitor the weak fluorescence signal (6,7). rinverted regiont of the Marcus theory could be observed in these diffusionless, dry systems and a very small reorientation energy of 0.18 eV has been found. The dry adsorption systems invite to study the temperature dependence and to examine solvent effects (7) on the rate constant of electron transfer. It turned out that the rate constant is almost independent of temperature from 300 to 77 K, but that it is drastically slowed down by the addition of a thin layer of water (7). The dry adsorption system with its small reorientation energy, at -AGO = A , has certain features in common with biological electron transfer systems; to emphasize these parallels is the main intention of this paper.

237

Fig. 1. Structures of model systems and the photosynthetic unit. (a) Fthodamine B adsorbed on the ab-plane of the anthracene single crystal (l), (b) steroid skeleton with biphenyl as donor and acceptor A (8), (c) donor and acceptor molecules bonded by methylene chain (28), (d) molecule with completely rigid skeleton (13) , (e) methylviologen-capped porphyrin (9) (f) reaction center of RDS viri.dis (29). D:chlorophyll diner, M: Chlorophyll monomer, P:Pheophytin, Q:Quinone.

.

I

288

2.

COMPARISON OF THE ADSORPTION SYSTEM WITH BRIDGED SYSTEMS As a system for the study of electron transfer, the adsorbed molecule/substrate system is distinguished mainly by the direct mutual contact of .electron donor and acceptor (Fig.la) in absence of any solvent. Complications, as originating from diffusion in homogeneous solution, are avoided in the adsorption system, and so we were able to observe the inverted region of the Marcus theory (6,7). The inverted region, where the rate constant decreases with increasing exothermicity, has so far been observed for a few systems only, i.e., exclusively in systems of bridged donor and acceptor moieties (8-11)(Fig.1). These bridges, which create a diffusionless system, have a certain disadvantage, inasmuch they themselves act as conducting paths for the electron, namely, by through-bond electron transfer. This has recently been observed even €or 'isolating' saturated hydrocarbon Numerous types of molecular links have been chains (8). developed and investigated in experiment and theory (8-27). Among them are steroid (8)(Fig.lb) , -(CH2)n-chain (28)(Fig.1~), aromatic (12), spiro (15), bicyclo-[2,2,2,]-octyl(26a, 26b) or protein (25,27) bridges. These bridges vary in the chemical nature of their constituents and in the architecture of their design. Bridges of high rigidity and conformational constraint of their donor and acceptor end groups have' been conceived (13,14 ,22 ,26a) (Fig.ld) Capped porphyrins (Fig.le) with two and four bridges (9,11,18,24) seem to be systems in which throughspace electron transfer might dominate through-bond contributions. In all of the above systems, however, electron transfer measurements comprise through-space and through-bond contributions. A precise knowledge of the individual throughspace and through-bond contributions is highly desirable, for example in view of the continuing discussion of the detailed nature of electron transfer in photosynthesis. The major point of controversy centers around the question whether the chlorophyll monomer is an intermediate in the electron transfer from the excited chlorophyll dimer to the pheophytin acceptor. This transfer over a distance of 17 A (29) takes place in 2.8 ps (30)(Fig.lf). There are three main conceptual approaches to model the first step of charge separation in the reaction center of the photosynthetic bacterium ~ D S . viridis, i.e., the two-step mechanism (31,32), the superexchange-mechanism (33-35) and the

.

239

exciton-electron transfer mechanism (36). The discrimination of these mechanisms would greatly be facilitated by the precise knowledge of the individual through-space and through-bond electron-exchange matrix elements between chlorophyll dimer and monomer, between chlorophyll monomer and pheophytin, and between chlorophyll dimer and pheophytin. So far, the elucidation of the electron-exchange matrix elements of through-space electron transfer has only been possible by calculation (25), since no experimental system was available. Above adsorption system of non-bridged donor and acceptor allows the experimental determination of the pure through-space matrix element for the first time. Another important characteristic of the solvent-free adsorption system has t o be seen in the very small reorientational energy, which mainly consists of inner, intramolecular reorientation, X i , but not much of outer, solvent As X i > As, one might even hope to observe reorientation, As. the quantum-mechanical oscillations in the rate constant vs. energ-gap relation. These fluctuations are discussed in context with biological control (turning on and off of electron transfer (37)), since small changes in the free energy AGO would create a large change in the rate constant. There might be a fair chance to observe such fluctuations in the adsorption system, since only a few vibrational modes are influenced by the electron transfer. Therefore, a complete washing-out due to interference effects might not take place (38). Electron transfer in biological systems is strongly influenced by charge separated ion-pair states. Electric fields generated by polar groups of the protein environment have a decisive effect on the rate of electron transfer by modifying the These chargefree energy of the electron transfer (35). separated states can be seen in analogy to the ion-pairing of the charged products in the adsorption system, which also has a decisive influence on AGO in this system and actually makes electron transfer possible (AGO c 0). Another parallel to biological systems, more fortuitous in character, can be seen in the small value of AGO in Some of the crystal-dye systems. In analogy to the photosynthetic unit -AGO .u X w 0.15 eV, and the electron transfer is an activationless process and independent of temperature in both systems. The adsorption system does not contain a solvent in the

240

classical sense. The interaction of donor and acceptor with the environment consists in a coupling of the electron transfer to vibrational modes of the crystal substrate lattice and to the vibration of the adsorbed dye molecule with respect to the surface (7). In biological systems, as for example in the reaction center of photosynthetic bacteria, the environment consists of a semi-rigid matrix of protein molecules with their low frequency skeletal modes. The common frequency range for crystal lattice vibrations (39) and for protein skeletal modes (40,41) is 10-150 cm”. Summarizing, we state that the following features are common in adsorption and biological systems: solvent-free, diffusionless, non-polar, fixed two-sitesystem with semi-rigid environment, environment with modes of identical frequency range, activationless electron transfer due to -AGO LI A , presence of ion-pair states, potential presence of fluctuative rate constant/energy-gap relationship due to X i > As (fiw > 2(XskT) ’12(42)), and similar sets of parameters like frequencies, coupling strengths, reorganization energies and tunneling matrix element. 3.

RESULTS AND ANALYSES

In the present system adsorbed dye molecules are excited by picosecond laser light and electron transfer takes place from the valence band of the substrate molecular crystals to the excited dye and concomitantly quenches the fluorescence (6,7). We are able to determine the electron transfer rate by measuring the fluorescence decay dynamics of the adsorbed dye. Figure 2 shows the energy-gap (a) and temperature dependence (b) of the rate constant of electron transfer in the adsorption systems calculated according to eq.[l], which is Sarai8s three-modevariant (38) of Jortner‘s original equation (43):

n’

N N-N’ 03 W = ( ~ x / ~ J ~ w ,exp ) I V[-ixA:(2iji+l)] ~~ 2 2i i IJ~=-W [ ( i j i + 1)/6;]”i’2 ln.JA:(fii(ijj+l ) ) l n ) x ffi

24 1

11.0 c

l

0 \ v)

w 8

=z

10.0

v)

e

I-

c

e

L

=a 0

s

9.0

1

t

100

200

300

Tomp.raturo/K

Fig. 2. Energy gap (a) and temperature (b) dependence of the rate constant of electron transfer in the adsorption system. Best fitting model with Rwl = Rwz = 1400 ern", hws = 20 cm”, S1 = S2 = 0.375, S, = 20, V = 0.0023 eV for the dry system according to [l] Dashed lines are calculated according to the Marcus theory (2) with k = (2r/h)V2 (4rXkgT)-1/2[-(X-AE)/(4XkgT) 1, with X(tota1) being 0.18 eV, -AGO = AE = 0.0, 0.1, 0.53 eV, for anthracene, pyrene, and perylene crystals, respectively.

.

242

'with thermal populations Ti= (exp(fiwi/kT)- 1 ) -II the modified Bessel functions I(ni) and Ip, and the reduced displacement A1 = (miwi/R) (ARi)2, with mi being the reduced mass and ARi the displacement. P(m) is given by p(m) = (AE mfiwl - mfrw2)/hws. The parameters occuring in the above equation are fiwsr frequency frequency of soft mode (solvent, environment) , Rwi , of quantum mode (intramolecular), Ss, S i r the corresponding coupling strengths, and V(R), distance dependent electronexchange matrix element. The reorientation energies of soft and quantum mode are obtained as X, = SsRws and X i = SRw, respectively. Figure 2a shows the inverted region type behaviour of dry and wet adsorption systems. The vertical shift of the wet system is due to a fourfold decrease of the electron-exchange matrix element. Figure 2b displays the temperature dependence of the rate constant of electron transfer at three different values of the energy gap, AE = 0.0, 0.1, 0.53 eV. The dashed lines in Fig. 2a and b show the corresponding behaviour according to the Marcus-theory and clearly demonstrate that electron transfer in the adsorption system is coupled t o intramolecular and environmental modes.

-

DISCUSSION OF PARAMETERS In this chapter parameters which enter the quantummechanical calculation of the rate constant of electron transfer are discussed in more detail. 4.

4 . 1 Fle ctron-Exchanse Matrix Element VfRl. The electronexchange matrix element depends on the center-to-center distance

R of electron donor and acceptor and is usually given by (8,43):

with Ro being the center-to-center distance in van-der-Waals contact and a the attenuation factor. In the following, we are discriminating through-vacuum, through-solvent, through-bond, and through-space electron transfer, where the latter comprises solvent and vacuum contributions. The pure through-vacuum tunneling matrix element in the adsorption system is found to be V(6A) = 0.0023 eV ( 7 ) , at an angle of donor and acceptor of about 64" (Fig.la). Recently, a homogeneous system which comprises the corresponding crystal melt and dissolved dye has been

243

investigated, and V(4A) was determined to be 0.004 eV (44), that is, the value of the heterogeneous adsorption system. The maximal rate constant in the melt. system was about 1/(2 ps). This value should be taken as a lower limit, since the employed method of determination of quantum yield provides only an average value of the rate constant. We fancy that there is a wide distribution of reaction complexes in the above melt systems of varying geometry, tunneling matrix element, and rate constant. The electron-exchange matrix element determined for the capped porphyrins is V(5A) = 0.0041 eV (9,11), very close to adsorption and melt systems. Harrison et al. (11) use a(throughspace) = 0.55 A-l to calculate v(4A) = 0.0054 ev. using &(through-vacuum) = 2A-’, instead, yields V(4A) = 0.011 eV. In contrast to the magnitude of above values of V is the tunneling element in the steroid-bridged compound which is considerably larger, V(6A) = 0.23 eV (8). This value, however, was obtained by extrapolation over distances as far as 10 A and might be questionable, as we will see in the next paragraph. 4.2 Attenuation Factor a. The attenuation factor a is identical with the Gamov factor in quantum-mechanical tunneling of an electron with mass m through a barrier of width b and height B, a = (2mB)lj2fi. The probability of penetrating the barrier is proportional to exp(-ab) (37). The knowledge of a is of great importance in the estimation of the tunneling elements in biological systems, and its determination in through-space and through-bond electron transfer is an area of current research activity. The values for a found in literature scatter considerably, a = 0.5 - 3.4 (2,8,9,27). Theory predicts a(vacuum) > a(bond) (45), but some experiments (9,ll) do not seem to be consistent with this prediction. Experimental and theoretical evidence is mounting that a is a function of distance, free energy, and orientation: a = a(R,AGo,)) (11,4648), where .$ symbolizes mutual orientation and stands for the angle between the r-orbitals of donor and acceptor. To give an example: a experimentally determined for short distances of 5 to 15 A is 0.55 A (9,ll), whereas a determined for a larger donoracceptor-separation of 10-17 A is found to be 1.03 A ( 8 ) . The dependence of a on the free energy of the reaction has been calculated for a polymeric bridge, e.g. a(AGo = -2 eV)/a(AGo = 0) = 1.24 (48). The value for a can differ for forward and backward

244

electron transfer, e.g. a(forward)/a(backward) = 1.8, as calculated for the bicyclo[2.2.2]-octyl-bridge (26b). A factor of 20 for the ratio V2 (forward)/V2(backward) has been postulated in electron transfer from the chlorophyll dimer to pheophytin and was explained with differing donor energy levels (49), and a ratio as high as 600 has actually been obtained in a recent experiment (34) Facing the above theoretical and experimental evidence of the dependence of a on free energy and distance, the commonly performed extrapolation of V(R) to the distance of closest approach Ro by using a constant a seems very doubtful. It seems also very important to choose the proper a, i.e., to distinguish between through-vacuum and through-medium electron transfer. Long-range electron transfer through solvent (8,50) or along bonds (8,15) is usually modelled by using the superexchange mechanism, which has recently been applied also to biological systems (33-35). The electron-exchange matrix element in the superexchange mechanism is calculated according to V = p (p/B) “, with B the energy difference from the highest occupied donor level to the lowest vacant level of the medium, p the overlap matrix element of neighbouring solvent molecules and n = R/d, where R is the donor-acceptor distance and d the distance between two solvent neighbours (50). For n >> 1, a is given by a = 2d’ln(B/p) and is a function of the donor energy level via the Typical values for P I E, d are taken from energy separation B. the pyrene-tetramethyl-p-phenylenediamine system in chlorobutane matrix (50): p = 0.2 eV, B+ = 3.1 eV, and d = 4.7 A (a = 1.17 A’), where B+ refers to the hole-tunneling in this system and means the energy of the positive ion state of the solvent. Values of a found in literature for long distance through-solvent tunneling center around 1.1 A” and are compiled in Table 1. The decay of the wave function in vacuum is thought to be considerably faster than the above value of 1.1 A’I. A frequently used form for the tunneling through-vacuum matrix element is V = 12exp(-1/2aR) (49) at Ro = 0, with a = 2.0 (51) or 2.6 (43) A-l, also used is 13 eV as prefactor and 2.7 A-’ as attenuation factor (34). ~/~ Hopfield derived the formula v = ~ . ~ ( N A N D ) exp(1/2 (1.44R) ) for aromatic donor and acceptor in edge-to-edge configuration (52), where NA and ND are the number of aromatic atoms in acceptor and donor, respectively, and R the edge-to-edge

.

245

separation. The prefactor of 2.7 was obtained by adjustment of V to the resonance integral (1 eV) of two carbon atoms in r-bonding configuration at the binding distance of 1.4 A. The value a = 1.44 was obtained by using a barrier height of 2 eV for the intervening medium. Beratan et al. calculate a(through-vacuum) = 3.4 for the non-covalent interaction of two a-orbitals of carbon bridge atoms by assuming a barrier height of 10 eV (25). 3 A'1 are in contrast with the Above large values of Q = 2 recent experimental values of a = 0.52 - 0.55 A - l , obtained in capped porphyrins (9,11) In these systems , a quinone , viologen or pyromellitimide moiety is held face-to-face to the porphyrin by help of two bridges containing 7-10 covalent bonds each. The result is a donor-acceptor pair with a center-to-center 8 A. Assuming 4 A as the contact van-der-Waals separation of 5 distance, the system in case of the shortest chains studied (R = 5 A), can be considered as being in direct contact of the donor and acceptor r-electrons with no solvent molecules (e.g. toluene) in between. At longer chain lengths, however, solvent might be interspersed, and observed a should then be compared to systems with through-solvent transfer. Since the bridges in the capped porphyrins are considerably longer than the direct through-space distance, it is thought that electron transfer occurs mainly through space (solvent) (11) Larsson compares r - and a-contributions of aromatic bridges and finds that both can be of the same magnitude, e.g. in pyrazin bridges between metal centers (17). In Table 1, we are summarizing experimental and theoretical values of a from literature, separated according to intervening transfer medium, i-e., vacuum, solvent, protein, and bond.

-

.

-

.

Recently, the first 4.3 Orientational DeDendence of V. quantum-mechanical calculations have been performed (46) on the dependence of V on distance and orientation for two porphyrin molecules. These calculations showed pronounced dependence of V on orientation of donor and acceptor, as had been presumed qualitatively before (8,22,24,53-55). The above calculations used the specialized orbitals of the porphyrin systems and can, therefore, only be a qualititative guide for other systems. Until specific calculations for other r-systems exist, it seems reasonable t o resort t o the simple cosine-dependence of the overlap on the angle 4 between the r-orbitals of donor and

246

acceptor (35,54).

Comparing the geometry of the adsorption

system with the possible face-to-face configuration in the melt, we obtain a geometric enhancement for the melt system of cos(0')/cos(64") = 2.3. An additional factor of 7.4 (using a = 2.0 A0-l) can be expected as an effect of the smaller distance of 4 A, which is possible in the melt compared to the 6 A of the adsorption system. Using k(6A) = 1 . 4 ~ 1 0 s-l ~ ~ yields a crude upper estimate k a x ( 4 A ) = 4.0~10'~ s-l for the rate constant of TABLE 1. Attenuation factor a of various media

through-vacuum

through-bond

through-solvent

2.0-2.7

ac/ac- , vacuum

3.4(th)

non-covalent bridge

(25)

0.74

spiroalkane

(56)

0.8 1.4

phenyl-bridged aromats bicyclo[2.2.2]-bridged

(12)

porphyrin/quinone rigid non-conjugated

(26a)

0.8

hyrocarbon bridge spiroalkane, AGO-dependent forward, bicyclo[2.2.2]-

(57)

0.0-2.O(th) 1.8 (th)

(15)

bridged quinone/porphyrin

(26b)

1.0 (th)

backward

(26b)

0.52-0.55

capped porphyrins

1.16

pyrene/TMPD in chlorobutane-glass steroid-bridged aromatics non-bridged aromatics in

1.01 1.21

rigid glass through-protein

(34,43,51)

0.7-0.9

protein complexes

1.2(th)

covalent

3.4(th) 0. 51B1I2

non-covalent B = binding energy

(9 r

11)

(501

247

electron transfer in the melt system. Maximal rate constant and maximal electron-exchange matrix element are also discussed in Chapter 6. 4.4 Influence of Electric Fields on AGOL A pronounced influence of polar groups in the protein environment on AGO of donor and acceptor can be expected and has been postulated to explain the unidirectionality of electron transfer along the Lbranch in the reaction center of ~ Q S .viridis (35). The reaction center displays a curious symmetry of two almost identical electron transfer chains(Fig. lf), only one of which, however, transfers the electron. It ha6 been suggested that minor differences of electrostatic stabilization of the chargeseparated state, together with slightly different tunneling elements, account for the predominance of the L-branch (35)(right one in Fig. If). The free energy in this system is calculated according to:

AGO = I*

-

EA

+

P

+

C

+

6e(D+)

+

6e(A-)

- &(D) - &(A),

[31

with I* being the ionization energy of the excited chlorophyll dimer, EA, the electron affinity of the pheophytin, P I the polarization energies of the non-polar protein environment by C, the Coulomb ionized donor and acceptor D+ and A-, stabilization of the separated charges at distance d, and the 6eterms, the electrostatic stabilization originating from polar protein residues. In detail, C = -e2/de and P = -(e2/2r)(1-1/~), with d being the distance of charge separation, r the effective radius, and E the effective dielectric constant. The differences of the individual contributions to AGO between the M- and L-branch are obtained as AC = -0.02 eV, AP = 0.02 eV, C6e(i) = -0.05, summing up to A(AGo) = -0.09 eV. The difference in the free energies of L- and M-branch leads to a ratio of k(L)/k(M) = 3. The above example pointedly demonstrates the importance of electrostatic forces acting on electron transfer in biological systems. The adsorption system features an ion-paired product state (dye-donor') with the Coulomb stabilization energy C(dry) = -1.33 eV and C(wet) = -0.10 eV for dry and wet system, respectively (d = 3 A and e = 3). The polarization energies in dry and wet adsorption systems are P(anthracene)dry = -1.2 eV (58) ,

-

248

= -1.6

P(anthracene),et

eV (59) for the anthracene surface and

A and Eeff(dye) The corresponding

= -1.34 ev, P(dye)wet = -2.33 ev (r = 3

P(dye)dq

= 2, Eeff(wet)

= 40) for the adsorbed dye.

values for the reaction center are C = -0.27 eV, P(D+) + P(A-) = -2.8 eV, and ZCo(i)r -0.5 eV. The Ce-terms in the melt and adsorption system, describing the electrostatic influence of the perchlorate counter-anion on D, A, ,'D and A-, are assumed to cancel mutually, so that CCe(i) = 0 at the first approximation.

e t s d eor' a ional 4.5 Enemies In this chapter, we discuss frequencies and coupling

.

strengths and compile them together with reorientation energies, free energies, and the electron-exchange matrix elements of A comparison is made various biological systems (Table 2, b-f)

.

with the corresponding values obtained for the adsorption system TABLE

2.Physical

parameters pertinent t o the multiphonon

theory of electron transfer system

R (A)

V (ev)

fiws

fiw

S

S,

(cm-l)(cm-1)

X

Xi

As

(eV) (ev) (ev)

AGO

ref.

(ev)

.................................................................. (a) 2.6X10-3

~ ( c c )20

1400 0.7

20 0.18 0.13 0.05 -0.10

(b) 9.

(100) 5 (ee)100

(4) 1400 0.7 4.5 0.18 0.12 0.06 -0.26

(7) (60)

(b) 2 . l ~ l O - 17(cc)50,300 ~

1360 0.2,0.3

0.14 0.03 0.11 -0.20

(36)

(c) ~ . Z J C ~21(cc)50,100 O-~ 3.7~10-~

1000

0.18 0.06 0.12 -0.19 -0.43

(40)

100

0.43

-0.43

(41)

100

0.14

-0.43

(c) 9.9x10-8 6.8~10-~

(c) (d) 4 . 6 ~ 1 014(cc) ~~ 100 (d) 3 . 7 ~ 1 0 - ~9(cc) 250 (e) 3 . 7 ~ 1 0 - 28(cc) ~ 100

(fl

0.10 0.04 -0.50 1550 0.6

(53) (41)

0.12

-0.50

(61)

0.52

-0.52 -0.30

(41) (62)

...................................................................

(a): adsorption system RhB/anthracene (b): chlorophyll dimer/ pheophytin, (c): cytochrome/chlorophyll dimer (a): quinone reduction/pheophytin, (e): quinone/chlorophyll dimer recombination, (f): blue copper proteins,(b),(d),(e) see Fig. If. cc stands for center-to-center and ee for edge-to-edge.

249

{Table 2,a). Table 2 shows that the reorganization energy A is smaller than 0.20 eV for all systems with distances below 10 A and in close proximity to the value of the adsorption system. The AGO values are less negative than -0.2 eV when donor and acceptor are immediate neighbours, also in analogy to the adsorption system of RhB/anthracene. Frequently in biological systems -AGO = A , and the reaction is designed t o occur at maximal speed and at minimized energy costs of AGO. The smaller the value of X the smaller the necessary driving force of AGO can be. Thus a maximal number of electron transfer steps can be performed using the energy of the absorbed photon most economically. As seen from Table 2, the frequencies for the intramolecular mode and solvent mode are about 1400 cm” and 100 cm-’, respectively, both for the adsorption system and the biological system. 4.6 Maximal Rate constant.

-

b,and

Maximal Electron-Exchanse

.

Matrix Element. v u Of Non Bridaed r-Svstems The maximal rate constant of nonadiabatic electron transfer in the medium temperature regime is given by (43)

and is a function of solvent reorientation energy As and the intramolecular mode coupling strength S. R = 4 A is taken as vander-Waals distance of aromatic molecules in the face-to-face configuration. The experimental data of the adsorption system are consistent with Gax(6A) = 1 . 4 3 ~ 1 0 s-l ~ ~ at S = 0.7, As = 0.05 eV, V = 0.0023 eV, and T = 300 K (7). Using the lower value Q = 2.0 A’l for through-vacuum attenuation (which is used throughout this chapter) and employing a geometric correction factor for the face-to-face configuration (see section 4.3), results in Qax(4A) = 4 . 0 ~ 1 0 s~ -’.~ Harrison et al. (11) investigated a system of capped porphyrins with fastest obsenred rate constant k(5A) 5 1x10l2 s”; using Q = 2.0 A-’ for through-vacuum attenuation, ~ ~ is obtained. Closs et al. (8) studied a kmax(4A) = 7 . 4 ~ 1 09-l steroid-spaced donor-acceptor system with the fastest rate constant k(10.5 A) = 4.2~10’ s-’. From this value, Gax(4A) =

250

1 . 8 ~ 1 0s-l ~ ~ is obtained by using a = 1.01 A-'. haxfor the diffusionless melt system of unbridged donor and acceptor was found to be hax(4A) > 5x1011 s-l (44). A prefactor Aet = 4. 5x1Ol2 s-l is reported in heterogeneous electrode systems (63) with molecules surface-attached by conjugated bridges with 8 A. separation from the surface shorter than 6 In the following we would like to speculate on the maximal rate constant and maximal electron-exchange matrix element possible in pure through-vacuum electron transfer of organic molecules in the face-to-face geometry at van-der-Waals contact, and on a potential adiabatic-to-nonadiabatic transition. The the Landau-Zenner parameter -fLz is used to characterize transition from nonadiabatic t o adiabatic electron transfer (64,65):

-

qLz

= 2rrV2/ [ Rw ( 2ASk~T) 'I2 1

[51

is less than unity for nonadiabatic and greater than unity for adiabatic transfer. Three systems are discussed in which one might anticipate adiabatic electron transfer at van-der-Waals contact: (1) the heterogeneous adsorption system of crystal and adsorbed dye, (2) the homogeneous analog-system comprised of the crystal melt and the dissolved dye, and (3) the capped porphyrin system 4.6.1 The AdsorDtion Svstem. From the experimental value V(6A) = 0.0023 eV one extrapolates, using a = 2 A ' l , V(4A) = 0.017 eV at an angle of 64' of donor and acceptor. For the parallel face-to-face geometry, a correction factor of cos(o')/cos(64") = 2.28 i s obtained, which leads to V(4A),,, = 0.039 eV. Such a situation might be realized for dye molecules adsorbed on the ac-plane of the anthracene single crystal, to give on example. With [5] and As = 0.05 eV, kT = 0.026 eV, Rw = 0.174 eV, and V = 0.039 eV follows -fLz = 1.08 and k(4A),,, = 4.05 x 1013 s-l; the frequency factor for adiabatic electron transfer s ' . in case of the 1400 c m ' l skeletal mode is 4.2 x d3 4.6.2 The other Svstems. With A, = 0.03 ev, kT = 0.044 ev, and V(4A) from 4.6.1 follows for the melt systems -fLz = 1.07. With A, = 0.3 ev, Rw = 0.20 eV, V(5A) = 0.0041 eV, V(4A) = 0.011 eV ( a = 2.0 A'l) follows for the capped porphyrins qLz = 0.03 and k(4A),,, = 7.4 x 1012 s ' . Even if a = 3.0 A-' is used, qLz is as small as 0.08, and k(4A),,, = 2 x d3s . ' The adiabatic qLz

.

25 1

limit does not seem to be realized in above system of the cappea porphyrins. The adsorption and melt systems, on the other side, seem to be two candidates for the experimental determination of the maximal through-vacuum electron-transfer matrix element in the face-to-face configuration at closest possible contact as seen from the above estimations. In the adsorption and melt system, the adiabatic limit should be reached at van-der-Waals contact. This short distance cannot be realized in the porphyrin systems, where 5 A seems to be the minimal distance possible. In case of the adsorption and melt systems, an experimental verification of the above computational estimations is highly desirable, in view of the uncertainty of the applied correction factors used in extrapolation to Ro = 4 A and to the cofacial alignment.

5.

SIMmRY

The adsorption system is in many respects similar t o biological systems. Especially the smallness of the solvent reorientation energy, with As < Xi and -AGO N A , of a donoracceptor system with ion-paired product state in a diffusionless, semi-rigid environment, opens many possibilities to study certain aspects of electron transfer which are of importance t o biological systems. The adsorption and melt systems are able to provide direct information concerning the maximal electron transfer rate constant of r-systems in van-der-Waals contact and thus the maximal possible through-space electron-exchange matrix element. It appears that electron transfer of non-bridged r-systems in van-der-Waals contact might be approaching the adiabatic limit with v(4A) u 0 . 0 4 eV. The maximal rate constant should be close to the frequency factor of 4x1Ol3 s”, corresponding to the 1400 cm-’ nuclear mode of the aromatic skeletons of donor and acceptor. The adsorption and melt systems seem to be systems in which kmax(4A) might be accessible to experimental determination. REFERENCES 1 H . Gerischer and F. Willig, in: F . L . Boschke (Ed.), Topics of Current Chemistry, Springer, Berlin, 1976. 2 R.A. Marcus and N. Sutin, Biochim. Biophys. Acta, 811 (1985) 3

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265.

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NEW DEVELOPMENTS OF INORGANIC PHOTOCHEMISTRY ON SOLID

Chapter 5

SURFACES

Contents

5.1

Inorganic Photochemical Reactions in Low Temperature Matrices and in the Surfaces of Solids (Takeshi Tominaga)

5.2

255

Photochemistry of Metal Carbonyls Physisorbed on Porous Vycor Glass (Harry D. Gafney)

5.3

Photochemistry of Silica-Adsorbed Fe(C0I5 (Robert L. Jackson)

5.4

272

288

Photopreparation of Supported Metal Oxide and Metal Carbonyl Catalysts (Akira Morikawa and Y u j i Wada)

303


255

INORGANIC PHOTOCHEMICAL REACTIONS IN LOW TEMPERATURE MATRICES AND IN THE SURFACES OF SOLIDS T. TOMINAGA

INTRODUCTION Studies of photochemical reactions of metal complexes and organometallic compounds in solid phase are still far from complete since various difficulties have been encountered in elucidation of their reaction mechanisms. There are a number of factors which influence to a greater or lesser degree the overall spectra of reaction products. Ordinary transmission (absorption) spectroscopic means are generally inconvenient for quantitative characterization of the products since the reactions often take place only in the thin surface layer through which incident light can be transmitted, while the bulk interior remains unreacted. With a view to overcoming such experimental difficulties, we have developed a promising technique - conversion electron MHssbauer spectroscopy(CEb1S) and depth-resolved conversion electron Mtls sbauer spectroscopy (DCENS) - for monitoring photochemical reactions in the surfaces of inorganic solids (iron and europium oxalates). lVe have also initiated MHssbauer spectroscopic studies of photochemical reactions of inorganic and organometallic compounds (iron chelate complexes, iron carbonyl and organotin compounds) isolated in low temperature matrices. Such systems provide a complimentary approach to solid-phase photochemistry since the reaction mechanisms may be simplified by isolating the reactant molecules in inert matrices (light transmission through the reactant is also facilitated by diluting the colored reactant with a transparent medium). Furthermore, we have a hope for the possible outcome of such studies - their application to syntheses of novel species which may be unstable at ordinary temperatures, unless trapped in inert matrices. 1.

2.

PIIOTOCHEMISTRY IN TIiE SURFACES OF INORGANIC SOLIDS 2.1 Photolysis of Iron(II1) and Europium(II1) Oxalato Complexes

256

Our early work has demonstrated that Mtlssbauer spectroscopy is a useful means for detecting changes in oxidation (electronic) states and structures induced by radiolysis, photolysis and pyrolysis in solid iron compounds (1-6). In a more detailed Mtlssbauer/IR study of the photolysis of potassium tris(oxa1ato)ferrate(III), we characterized several products observed in solids under a variety of conditions and proposed a mechanism with the following sequence of photolytic and subsequent reactions in s u c h systems (7):

In photolysis of solid systems, however, reactions proceed only in the surface layers through which incident light can be transmitted; ordinary transmission MtJssbauer spectroscopy is inconvenient for detection of such surface products because their signals are usually weak compared with the peak intensities of unreacted parent material and the background count rates. In contrast, the conversion electron MUssbauer spectroscopy(CEb1S) with which we look at the low energy electrons (i.e., with short ranges) scattered resonantly from the excited Mtlssbauer nuclei shows a high surface sensitivity and provides information

.

.

.

,

.

.

,

229 228

2 27

? -

88

Fig. 1. Mtlssbauer spectra at 293 K of K3["Fe(C204)3j.3Mzo

87

86

(8).

a) Scattered electron spectrum b) y-ray transmission spectrum be fore photo i rrad i a t i on c) Scattered electron spectrum d) y-ray transmission spectrum after 10-sec photoirradiation

(Y

C

.c U

: 571 n

4 572 Y

9

570 2 58 256 2SL

- 2 - 1 0 1 2 V e l o c i t y , m m /scc

3

4

257

regarding the chemical states of M s s b a u e r a t o m within the s u r (in the case of 57Fe). In the face depth of several thousand scattered electron Pltlssbauer spectrum of potassium tris (oxalato) ferrate(II1) exposed to light briefly (Fig. lc), a doublet due to an iron(I1) product becomes predominant at the cost of the parent peak, revealing that the parent complex within the surface depth 0 of a few thousand A has been almost completely reduced photochemically to the iron(I1) product ( 8 ) . The iron(I1) photolysis product was characterized as K2 [57Fe(C204)2 (H20)2] based on the Pltlssbauer parameters. Although the y-ray transmission hltlssbauer spectrum of the same photoirradiated sample (Fig. Id) also indicates the presence of absorption peaks due to the same photolysis product, it is obvious that only 20% of the parent complex as whole was converted to this product. IVe have also studied the photolysis of europium(II1) oxalate by means of the conversion electron Mtlssbauer and E S R technique (9). Figure 2 shows the conversion electron and y-ray transmission Fltlssbauer spectra of E u (C204)3-10H20 ~ after UV-irradiation with a low-pressure mercury lamp. Although the formation of Eu(1I) was not detected i n the y-ray transmission spectrum (Fig. Z b ) , a Eu(I1) peak showed i n the conversion electron spectrum (Fig. 2a), obviously indicating that Eu(II1) was reduced to E u ( 1 I ) by UV-irradiation. This was also confirmed by E S R measurements.

c

.. \

.:..

216

Fig 1. Yie ds of CO (O), CH4 (A),H2 (+), and W(CO)5 (ads) (m) during 310-nm photolysis of 8.50 x 10- mole of W(C0)dg.

d

The induction period preceding CH4 evolution (Figure I ) indicates that M(CO)g(ads) is photochemicallyconverted to species that mediates CH4 evolution. Stoichiometricmeasurements indicate that CH4 evolution initiates when the complex achieves an average stoichiometry of M(CO)4.me2(27). H2 evolution concurrent with the CH4 suggests metal oxidation and ESR spectra, recorded during 254-nm photolysis of Mo(C0)6(ads), confirm the formation of Mo020H (27). Since the ESR resonance of the oxide decays on continued photolysis, the hydrous oxide, where the metal is in the +5 oxidation state, is an intermediate oxidation state. Optical spectra show that the final product with each metal complex is the fully oxidized metal, i.e., M03 (27). CH4 evolution occurs with a concurrent oxidation of the metal, and the stoichiometry of the reaction (Figure 2) indicates two reaction pathways. Initially, CH4 evolution occurs with a stoichiometric oxidation of the metal complex, but at later times becomes independent of metal oxidation. Both reactions are photochemically driven, yet the later reaction indicates that excitation of the M03 generated in the stoichiometric reaction catalyzes CH4 evolution (27). The sudden susceptibility to oxidation when the complex achieves the molecularity M(C0)4(ads) is attributed to an inability of the tetracarbonyl to complete its coordination shell.

277

As pointed out above, 1-3 silanol groups are in the immediate vicinity of the photogenerated pentacarbonyl. Consequently, the pentacarbonyl readily completes its coordination shell by biiiding to a single silanol group, and as found, is relatively stable on PVG (18,32). The

I

2 I

10

6 I

I

I

I

' 6 moles CH4 x 10

Fig 2. Moles of CH4 evolved vs. the moles of W(CO)6 irreversibly oxidized. tetracarbonyl is unable to achieve the same stability. Although dependent on previous heatings, studies of a variety of hydroxylated silica indicate 4-7 silanol groups/100A* (33,34). Assuming an average of 5 silanol groups/100A2 and that these are present at the comers and center of a 10 x 1OA square, spanning any two of these groups requires M-0 (0being the silanol oxygen) bond lengths, 25A,considerably longer than known bond lengths (38). Of course, the assumption that the silanol groups are uniformly distributed, particularly on an amorphous solid, is suspect since DRIFT spectra of the glass show both free and associated silanol groups (19,20,25,26). However, this does not change the fundamental argument that, at some point during the photoinduced decarbnylation, the decarbonylated fragment is unable to complete its coordination shell by binding to the surface silanol groups. At this point, which our data and that obtained in the temperature programmed decomposition (TPD) of these complexes on silica gel (39,40), indicate occurs at an average stoichiometry of M(CO)4, the photoproduct becomes significantly more susceptible to oxidation, and as a result switches to a photoinduced reaction pathway leading to CH4 evolution. At this point, the photoinduced evolution of CH4 closely resembles. both in the products evolved and their order of appearance, the results of the TPD of these complexes on silica gel (39,40). However, two surprising differences exist. First, CH4 evolution continues after complete oxidation of the metal complex (Figure 2), i.e., photoexcitation of M03 on the glass catalyzes

278

further CH4 evolution (27). Second, the mass balance and photolysis of W(13C0)6(ads) unequivocally establish that none of photochemically evolved CH4 derives from the initially coordinated CO (27). CH4 evolution is an impurity effect, but suffice it to say here, not one that derives from conventional impurity sources (27). Since the total amount of CH4 evolved falls within the known C impurity level in the glass (43), (ads)+CO

>-.

W C Q 4 (ads1

H 2C=O

71

C oxide

M 0 3 +4CO

[31 [41

CH30H

L

CH4

Because of an inability to complete it coordination shell, M(C0)4(ads) becomes susceptible to oxidation, and undergoes a series of photoinduced oxidations leading to a stoichiometric conversion of the C1 oxide to formaldehyde, methanol, and ultimately CH4. The wavelength dependence of reaction 4 indicates that oxidation of M(C0)4(ads) requires 293 kcal/mol (27). The sequence is equivalent to the Fischer-Tropsch hydrogenation of CO (46.47) except that the carbon being hydrogenated is a C1 oxide in the glass matrix (27). There is no evidence that this carbonaceous impurity agglomerates on the surface. Therefore, unlike the Fischer-Tropsch process. where concatenation occurs by successive additions of CO, the carbon content of the hydrocarbon product is limited by the carbon content of the impurity. Consequently, the hydrocarbon evolved is, as experimentally found, limited to only CH4 (27). Following the stoichiometric reaction, CH4 evolution becomes independent of metal oxidation (Figure 2). During this stage of photolysis, electronic spectra, which are identical to the spectra ofMO3 powder dispersed on PVG,indicate an aggregation of the metal oxide on the glass

279

surface. Thus, excitation of the agglomerate, which now possesses an electronic spectrum equivalent to that of a semiconductor,catalyzes CHq evolution. Since photoinduced reductions on dispersed, semiconductor particles are well established (48-55). we propose that the agglomerated oxide behaves as a semiconductor, and on excitation. facilitates the reduction of the C1 oxide. The continued presence of bands assigned to formaldehyde suggests that the reduction proceeds by the sequence described for the stoichiometric reduction except that the metal no longer supplies the reducing equivalents. Rather, excitation of the metal oxide promotes the transfer of electrons to the C1 oxide. The nature of the electron donor is not known, but previous results and the dependence of CH4 evolution on the extent of dehydration of the glass suggests that chemisorbed water is the ultimate electron source (27). Agglomeration is not only essential to the formation of a semiconducting solid, but its occurance also indicates the mobility of the metal oxides on the glass surface. Since CHq continues to derive from the impurity, mobility of the metal oxide is essential for continued access to the carbon source. In other words, the metal oxide photocatalyst randomly migrates to the carbon source rather than the more conventional migration of the carbon source to the catalyst. Consideringthe parameters that control this photocatalytic evolution of CHq, it is not surprisingthat the turnover number ranges from ca. 5:l to a high of 12:l (27).

IL Fe(C0)g The primary photoprocess of Fe(CO)~(ads)is again similar to that found in fluid solution. Excitation with light of S350 nm leads CO evolution with a quantum efficiency, 0.9630.05. equivalent to that in fluid solution (6).The electronic spectrum of the photopduct is consistant with the formation of an Fe(C0)4L species (56,57). However, DRIFT spectra reveal the immediate appearance of two distinct photoproducts (19). One photoproduct exhibits bands at 2073 and 2048 cm-1, while the other exhibits a band at 2062 cm-1. Consistant with the electronic spectra, which indicate occupation of the vacated coordination site, neither species can be assigned to free Fe(C0)4, which generally exhibits bands in the 2000-1950 cm-1 region (56,57). Based on their close similarity to known oxidative addition products (58-61), the bands at 2073 and 2048 cm-1 are assigned to H-Fe(CO)4-OSi, where the primary photoproduct oxidatively adds a surface silanol group (19). The 2062 cm-1 band is assigned to H-Fe(C0)4-OH where the tetracarbonyl oxidatively adds chemisorbed water (19). In spite of their similarity, these are distinct, noninterconvertible photoproducts. Both species decay independently under CO to regenerate Fe(CO)5(ads), and the more rapid decay of H-Fe(C0)q-OH indicates that this is the more reactive product (19). Although more stable, H-Fe(CO)4-OSi is formed in limited amounts. Why this occurs in spite of an abundance of silanol groups (see above), is not clear. One possible explanation is that the topology of the surface curtails access to the silanol groups so that continued photolysis leads to preferential formation of H-Fe(C0)4-OH (19). In contrast to the photolysis of Fe(CO)5 on silica gel, where secondary photolysis yields Fe3(C0)12 regardless of initial Fe(C0)s surface coverage (17), secondary photolysis on PVG

280

exhibits a pronounced dependence on the initial loading (19). With samples containing 110-6 mol of Fe(CO)g(ads)/g, photolysis with light of 5300 nm leads to the evolution of 4.9-10.2 mol of CO/mol of Fe(CO)g(ads). Decarbnylation occurs without CHq or H2 evolution, indicative of metal oxidation, or any spectral indication of Fe2(CO)9 or Fe3(CO)12 formation (19). In fact, electronic and ESR spectra in a number of experimentsindicate the formation of atomic iron on the glass surface (19). Although stoichiometic measurements continually indicate complete decarbonylation without oxidation of the metal atom, the level of aggregation of the resultant iron varies from sample to sample. Nevertheless, the photochemistry at low loadings, i.e., progressive decarbonylation without spectral evidence of dimer or trimer formation, is reproducible and differs from that found in hydrocarbon solution (6) and on silica gel (17), where Fez(C0)9 and Feg(CO)12 are the respective reaction products. With samples containing 210-6 mol of Fe(CO)$g, photolysis with S350-nm light leads to rapid formation of Fe3(CO)12(ads) (19). The quantum yield of trimer formation is dependent on irradiation time, and reveals an induction period preceeding cluster formation (19). In contrast to H-Fe(C0)4-OSi, which is formed immediately after excitation in both CO and CO-free experiments, H-Fe(C0)4-OH formation is not detected in the CO-free experiments. Rather, under these conditions (Figure 9, reference 19), there is an immediate appearance of bands in the 1750-1800cm-1 bridging CO region. In addition, H-Fe(CO)4-OH formation and cluster formation are both favored by longer excitation times. Although these correlation points to H-Fe(C0)4-OH as the precursor to cluster formation, DRTFT spectra clearly establish that it does not per se thermally react to form a cluster (19). In fact, since the thermal decays of both oxidative addition products after photolysis occur without a concurrent increase in bands attributable to dimeric or trimeric clusters, neither surface-bound tetracarbonyl reacts thermally, at least on PVG, to form a cluster. Additional photonic energy is required to convert the surface bound tetratcarbonylto more reactive intermediate. Steady-state photolyses of samples containing S10-6 mol of Fe(CO)s(ads)/g establish a facile progressive decarbonylation, and trapping experiments with trimethylphosphine (TMP) indicate rapid formation of Fe(CO), (11.53) under conditions of cluster formation. Under 100 tom of TMP, for example, 350-nm photolysis of Fe(CO)s(ads) results in the rapid appearance of bands characteristic of Fe(CO)4(TMP) and Fe(CO)3(TMP)2 (19). Continued photolysis leads to bands that are tentatively assigned to the unsaturated intermediate Fe(CO)~(TMP)and Fe(CO)2(TMP)2. In addition to the monomeric substitutionproducts, TMP trapping reveals additional bands that are tentatively assigned to monosubstituted analogue of Fe2(CO)g, Fe2(C0)8(TMP) (19). Monosubstituted, dimeric complexes have not been isolated in solution. However, the decay of the bands atmbuted to Fe2(CO)g(TMP) occurs with a concurrent increase in the bands assigned to Fe(C0)4(TMP), which is consistant with the known disproportionation of Fe2(CO)9 into Fe(C0)4L and Fe(C0)g on silica gel (17) and PVG (19). Whether these intermediates are generated via multiple CO loss, as is the case in the vapor phase (62), or by sequential photoinduced decarbonylation steps is not established by these data (19). Nevertheless. photolysis quickly generates in addition to the surface-bound tetracarbonyl, more highly unsaturated intemidates that satisfy the criteria for surface mobility essential to cluster formation.

28 1

Unlike the tetracarbonyls that can bind to a single surface functionality, more highly unsaturated intermdiates are unable to complete their coordination shells by binding to the surface (see above discussion of M(CO)4 (M = Cr, Mo. and W) coordination to the glass). Consequently, Fe(CO), (1113) species are either coordinatively unsaturated or weakly bound to the surface. Either possibility would promote surface mobility by the molecules exchanging from site to site trying to achieve complete coordination and a stable configuration. The drive to complete its coordination shell promotes mobility, and the more highly unsaturated intermediates are the mobile precursors to cluster formation. The large increase in the bands assigned to Fe(C0)3(TMP)2 and a corresponding increase in the bands assigned to the dimeric intermediate Fez(CO)g(TMP) specifically point to Fe(CO)3 as the mobile precursor to dimer and most likely trimer formation (19).

Since H-Fe(C0)4-OSi is formed in limited amounts whereas continuous photolysis leads to preferential formation of H-Fe(C0)4-OH, the latter is the dominant tetracarbonyl on PVG. Being more reactive and in the largest amount, its conspicuous absence under all conditions that yield clusterification suggests that H-Fe(C0)4-OH rather than H-Fe(C0)4-OSi is the tetracarbonyl that reacts with the more highly unsaturated,mobile Fe(CO)3 to form the dimeric and trimeric clusters. Although laser excitation is sufficient to photolyze essentially all of the Fe(CO)s, recombination with the photoejected CO and/or diffusion rapidly replenishes the Fe(C0)s in the photolyzed region. Analogy to the photoinduced dimerization of the complex in noncoordinating solvents suggests that Fe(CO)5 may also be involved in clusterification on the glass. A summary of the intermediates and their interconversions on PVG is presented in Scheme I. Although written in terms of sequential photoinduced decarbonylation steps, the data do not preclude, particularly with laser excitation, multiple CO loss. In either case, however, the result is the same since steady state photolyses indicate that secondary photolysis occurs with a relatively Scheme 1

Fe(CO), (ads)

H-Fe(COf4 -OH

H-Fe(CO)A -0Si

1

F e ( C 0 y Y

q

-

O

H

high quantum efficiency (19). In a general sense, the reaction mechanism (Scheme I) agrees with

282

the postulated mechanism for the photochemistry of Fe(C0)s on silica gel (17). However, there are two obvious differences. First, bands in the 1960-1940 cm-1 region, which have been assigned by several investigators to Fe(CO)4(OSi) as the primary photoproduct (17), are not observed on PVG. Instead, UV photolysis of Fe(C0)s on this glass leads to the immediate formation of two species that are assigned to H-Fe(C0)4-OSi and H-Fe(C0)4-OH. The latter is more reactive and reacts with the mobile intermediate, principally Fe(CO)3, to form the cluster products. The second difference is that, unlike the photolysis of Fe(C0)s physisorbed onto silica gel where Fe3(CO) 12 formation occurs at considerably less than monolayer coverage (17), clusterificationon PVG is very dependent on initial surface covemge. UV photolysis (1350-MI) of PVG samples containing 110-6 mol of Fe(CO)g(ads)/g causes progressive decarbonylation of with no indication of either dimer or mmer formation (19), whereas photolysis of samples containing >10-6 movg results immediate evidence (Figure 9, reference 19) of dimer formation and subsequent trimer formation (19).

Recent studies of the photochemistry of Ru3(C0)12 suggest that the dominant reaction in fluid solution, fragmentation to monomeric products, proceeds via an isomeric form of the complex that possesses a coordinatively unsaturated metal center (63,64). Trapping experiments with CO and various phosphines indicate that the quantum yield of the precursor to fragmentation in fluid solution is 7.4k3.0 x 10-2 in fluid solution (20,63-65). Photosubstitution of the timer has also been observed, and is thought to originate from a higher energy excited state where CO dissociation yields an unsaturated or solvated intermediate that can be trapped to yield the monosubstituted mmer (63,641. The agreement with solution spectral data (Table I) establishes that Ru3(C0)12 physisorbes onto PVG without molecular change. The photochemistry of Ru3(C0)12(ads), however, differs from that in fluid solution. Scavenging experiments with CO and tris(t-butyl)phosphine,P(t-Bu)g, indicate that the quantum yield of formation of monomeric products on PVG is 110-5 (20). Instead, photolysis with light of 1350 nm leads to the evolution of 1.8M.2 mol of CO/mol of Ru3(C0)12(ads) reacted. H2 or CH4 evolution, indicative of metal oxidation, is not detected, and exposing the photoproduct to CO regenerates Ru3(CO) 12(ads) in 290% yield. Electronic spectra (Figure 3) are consistent with quantitative formation of an oxidative addition product (66-70), and the DRIFT spectrum of the photoproduct, a weak band at 2109 cm-1, intense bands at 2078 and 2069 cm-1, and a broad band at 2034 cm-1 with shoulders at 2017 and 1999 cm-1, confirms the formation of HRu3(CO)1o(OSi). Thus, UV photolysis Ru3(C0)12(ads) leads to the reaction:

where HRug(CO)1o(OSi)(ads)represents oxidative addition of a surface silanol group. Reaction 5 is independent of excitation wavelength and quantitative provided the photodissociated CO is

283

continuously removed. Oxidative addition to either an individual Ru atom or across a Ru-Ru bond reduces the molecular symmetry from D3h to C2v. The weak band at 2109 cni-l in the HRu3(CO)10(OSi)(ads) spectrum is assigned to symmetric A1 mode and is consistant with a product of CzVsymmetry (71). There is no indication in the HRu3(CO)10(OSi)(ads)spectra of a band assignable to a terminal Ru-H vibration. Rather, the photoproduct spectra are equivalent to a series of Ru and 0 s oxidative addition products, HM3(CO)ioL (L = SR, OH, NO, and OSi), where addition across the metal-metal bond has been established (66,68,69).Thus, UV photolysis of Ru3(C0)12(ads) leads to the surface grafted species, where a surface silanol group has

\I/

nu

I

ma77777 oxidatively added across an Ru-Ru bond (20). Oxygen, which anchors the complex to the support, acts as a three electron donor and hydrogen binds to the metal cluster with a two electron, three center bond. The stability of the grafted cluster, which exists for weeks in vacuo at room temperature, is due to the formation of the R u - 0 bonds. Yet, stability is not at the expense of subsequent thermal reactivity. Exposing (p-H)Ru3(CO)10(p-OSi)(ads) to CO leads to quantitative regeneration of Rug(CO)l2(ads). The temperature dependence of the pseudo first-order rate

Wavelength (nm)

Fig 3. Spectral changes during 350-nm photolysis of 2.0 x 10-7 mol of Ru3(C0)12 (ads) / g.

284

constants yields an activation energy of 5.1kO.4 kcal/mole (20). Comparing this value with that for Reaction 5, which can also be driven thermally and occurs with an activation energy of 6.20.4 kcal/mole (20), indicates that the grafted cluster, (p-H)Rq(CO) 1o(p-OSi)(ads), is ca. 0.5 kcal/mole higher in energy than the physisorbed trimer, Ru3(CO)12(ads). The thermal reactions are reversible, and the interconversion between these energetically similar species can be considered a dynamic equilibrium where the extent of reaction is subject to factors which control the position of equilibrium. In a closed cell, for example, the extent of reaction 5 is proportional to the volume of the photolysis cell (20). This is due to the fact that as the pressure of the photodissociated CO increases, the fraction of CO that desorbs deceases thereby increasing the probability of the back reaction. Continuous removal of the photodissociated CO leads to quantitative conversion to the grafted cluster (20). The dominant photochemical reaction of Ru3(C0)12 in fluid solution is fragmentation (63-65) whereas that on PVG is an oxidative addition to the intact mmer (20). This difference in the photoreactivity does not appear to be due to the formation of a different primary photoproduct. The electronic spectrum of the adsorbed complex is within experimental error of that in fluid solution, and as found for fragmentation in fluid solution (63), reaction 5 is independent of excitation wavelength. We propose that the primary photoproduct generated on PVG is equivalent to that generated in fluid solution, i.e., an isomeric form of the trimer possessing a coordinatively unsaturated Ru center. Subsequent reactivity of the primary photoproduct, however, is subject to constraints imposed by the support. One potential constraint is the availability of silanol groups. Yet, based on the average silanol number of various hydroxylated silicas. 4-7/100A2 (33,34), and the planar surface area covered by the adsorbed trimer, 3-5 silanol groups are in the immediate vicinity (20). Of course, the assumption that all silanol goups are equally reactive is suspect since, by the very nature of an amorphous material, not all are equivalent (25,26). Nevertheless, a reaction controlled only by the silanol number would be expected to produce a mixture of monomeric and cluster reaction products. Since UV photolysis of Ru3(C0)12 physisorbed onto partially dehydroxylated silica (Carbosil) is reported to yield monomeric Ru(C0)4-OSi complexes (72), in those regions where the density of silanol groups is sufficient, fragmentation to monomeric products would occur, whereas in regions of lower silanol number, oxidative addition of a single group may occur. The result would be a mixture of reaction products that is inconsistent with the observed specificity of reaction 5 (20). Based on 3-5 silanol groups in the immediate vicinity, the quantum yield of formation of (pL-H)Ru3(CO)1o(p-OSi),1.6kO.3 x (20), is a reasonable approximation of the limiting yield. Therefore, relative to the limiting yield of fragmentation in fluid solution, 7.4k3.0 x 10-2 (20,63-65), the smaller yield of the precursor complex on PVG is not due to a lack of available silanol groups. Rather, the smaller yield found on PVG must reflect a more efficient nonradiative relaxation of the excited complex and/or a less efficient formation of the isomeric form possessing a coordinatively unsaturated metal center. In a sense, formation of the oxidative addition product is similar to the steps leading to fragmentation in fluid solution. Yet, relative to fluid solution where fragmentation occurs with even less than a stoichiometricamount of a scavenger, the absence of photofragmentation on PVG is not due to an inability to form the precursor complex or a lack of potential two electron donors. In our

285

opinion, it is the rigidity and topology of the PVG surface, as opposed to the fluidity of a solvent cage, that limits the reaction to the oxidative addition of a single silanol group, and curtails the ability of the primary photoproduct to fragment to monomeric products. While rigidity and topology offer a rational for the different products obtained in fluid solution and on PVG, it is not immediately apparent that they account for the differences in reactivity on Carbosil and PVG (20,71). SUMMARY

The primary photoprocess of these physisorbed metal carbonyl closely resemble those in fluid solution. With the monomeric complexes, UV photolysis causes CO dissociation and formation coordinatively unsaturated species that is stablized by an interaction with a surface silanol group. Continued photolysis leads to further CO loss and the formation of intermediates that are unable to achieve the same stability by reaction with the glass surface. At this point, particulary with the group 6B metals, the chemistry deviates sharply from that found in fluid solution. The tetracarbonyls become susceptible to oxidation and switch to a photoinduced oxidation that results in the reduction of a C1 oxide impurity to CH4. Since CHq evolution does not occur during continued photolysis of Fe(CO)j(ads), but does occur during photolysis of Fe3(C0)12(ads), a prerequisite for this reaction pathway is not simply the level of decarbnylation, but also whether the individual metal atoms can supply an adequate number of reducing equivalents. The secondary photochemistry of Fe(COh(ads) depends on the initial loading. With relatively high initial loadings, continued photolysis generates the mobile intermediate Fe(C0)3 which reacts with principally H-Fe(C0)4-OH and Fe(C0)s to form cluster compounds. UV excitation of Ru3(C0)12(ads). on the other hand, yields an isomeric form of the ground state which, due to the topology of the glass surface, is restricted to oxidative addition of a single silanol group as opposed to the trimer fragmentation that occurs in fluid solution. ACKNOWLEDGMENT Support of this research by the Research Foundation of the City University of New York, the Dow Chemical Company's Technology Acquisition Program, and the National Science Foundation (CHE-8511727) is gratefully acknowledged. H.D.G. thanks Queens College for a Presidential Research Award during 1987, and the Coming Glass Works for samples of porous Vycor glass.

REFERENCES 1

2

M. S. Wrighton, in: D. W. Slocum and 0. R. Hughes (Eds) Ann. N.Y.Acud. Sci., 333 (1980) 188-207. M. S. Wrighton, D. S. Ginley, M. A. Schroeder, and D. L. Morse, Pure Appl. Chem., 41 (1975) 67 1-679.

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44 45 46 47 48 49 50 51 52 53 54 55

M. S . Wrighton and M. A. Schrocder, J. Am. Chem.SOC.,95 (1973) 5764-5765. M. S. Wrighton, Inorg. Chem.,13 (1974) 905-909. J. Nasielski, P. Kirsch and L. Wilputte-Steinert, J. Organomet. Chem., 27 (1971) C13-14. M. S. Wrighton, Chem.Rev.,74 (1974) 401-30. D. B. Chase and F. J. Weigert, J. Am. Chem.SOC.,103 (1981) 977-978. R. L.Whetten, K.4. Fu, and E. R. Grant, J. Am. Chem. SOC., 104 (1982) 4270-4272. M. A. Schroeder, and M. S. Wrighton, J. Am. Chern. SOC.,98 (1976) 551-558. J. C. Mitchner and M. S. Wrighton, J. Am. Chem. SOC., 103 (1983) 975-977. J. C. Mitchner and M. S. Wrighton. J. Am. Chem. SOC.,105 (1983) 1065-1067. Y.-M. Wuu, J. G. Bentsen, C. G. Brinkley, M. S. Wrighton, inorg. Chern.,26 (1987) 530-540. J. B. Kinney, R. H. Staley, C. L. Reichel, and M. S. Wrighton, J. Am. Chem.SOC.,103 (1981) 4273-4275. C. L. Reichel and M.S. Wrighton, J. Am. Chem. SOC., 103 (1981) 7180-7189; Inorg. Chem.,19 (1980), 3858-3863. D. K. Liu and M. S. Wright0n.J. Am. Chern.SOC.,104 (1982) 898-902. B. Klein, R. J. Kazlauskas, and M. S. Wrighton, Organornetallics,1(1982) 1338-1350. R. L.Jackson and M. R. Thrusheim, J. Am. Chem.Soc., 104 (1982) 6590-6596. R. C. Simon, H. D. Gafney, and D. L. Morse, Inorg. Chem., 22 (1983) 573-574. M. S. Darsillo, H. D. Gafney, and M. S. Paquette, J. Am. Chm. SOC.,109 (1987) 3275-3286. T. Dieter, and H. D. Gafney, Inorg. Chem.,27 (1988) OOO. D. C. Bailey and S. H. Langer, Chem.Rev., 81 (1981) 109-148. P. Pfiefer and D. Avnir, J. Chem.Phys., 79 (1983) 3558-3565. N. J. Turro, Pure Appl. Chem., 58 (1986) 1219-1228. R. K. Jler, The Chemistry of Silica, Wiley-Interscience, New York, 1979, p 551. M. L. Hair and I. D. Chapman, J. Am. Cerm. SOC.,49 (1966) 651; Trans.Faruduy SOC., 61 (1965) 1507. N. W. Cant and L. H. Little, Can.J. Chem., 42 (1964) 802-809; 43 (1965) 1252-1254. R. C. Simon, E. A. Mendoza andH. D. Gafney, Inorg. Chem.. submitted. R. N. Perutz and J. J. Turner, J. Am. Chem. SOC.,97 (1979) 4791-4800. M. A. Graham, M. Poliakoff and J. J. Turner, J . Chem.SOC.A., (1971) 2939-2948. W. Strohmeier and K. Gerlach, Chem. Ber., 94 (1961) 398-406. J. Nasielski and A. Cola, J. Organomet. Chem., 101(1975) 215-219. R. C. Simon, D. L. Morse and H.D. Gafney, Inorg. Chem., 24 (1985) 2565-2570. L. R. Synder and J. W.Ward, J. Phys.Chem.,70 (1966) 3941-3952. Reference 24, pp 622-714. U. Evan, K. Rademan, J. Jortner, M. Manor and R. Reisfeld, Phys. Rev.Lett., 52 (1984) 2164-2 167, M. Drake, personal communication, 1985. R. F. Howe and I. R. Leith, J . Chem.Soc. Faraday Trans.,69 (1973) 1967-1977. N. A. Beach and H. B. Gray, J. Am. Chem. Soc., 90 (1968) 5713-5721. A. Brenner, D. A. Hucul and S. J. Hardwick, Inorg. Chem.,18 (1979) 1478-1484. A. Brenner and D. A. Hucul, J. Am. Chem. SOC.,102 (1980), 2484-2487. D. L. Morse, Coming Glass Works, private communication, 1984. C. W. Burnham, in: H. S. Yoder (Ed.), The Evolution of the Igneous Rocks, Princeton University, Princeton, N. J., 1979, p 454. D. R. Fahey,J. Am. Chem. Soc., 103 (1981) 136-141. E. L. Muetterties and J. Stein, Chem.Rev., 79 (1979) 479-90. C. Masters, Adv. Organomer. Chem.,17 (1979) 61-100. A. J. Bard, Science, 207 (1980) 139-44. A, J. Nozik, in: J. S. Connolly (Ed.),Photochemical Conversionand Storage of Solar Energy, Academic Press, New York, 1981, pp. 271-295. A. H. Boonstra and C. A. H. A. Mutsaers, J. Phys. Chem., 79 (1975) 2025-2027. G. N. Schrauzer and T.D. Guth, J. Am. Chem. SOC.,99 (1977) 7189-7193. F. Kahn,P. Yue, L. Rizzuti, V. Augugliaro and M. Schiavello, Chem. Commun., (1981) 1049-50. Q. Li, K. Domen, S. Naito, T. Onishi and K. Tamaru, Chern.Lett., (1983) 321-324. H. Miyama, N. Juji and Y. Nagae, Chem.Phys. Lett., 74 (1980) 523-524. E. Endoh, J. K. Leland and A. J. Bad, J. Phys. Chem., 90 (1986) 6223-6226.

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PHOTOCHEMISTRY OF SILICA-ADSORBED Fe(C0)S R. L. JACKSON 1.

INTRODUCTION Surface effects in the photochemistry and photophysics of adsorbed molecules have been the subject of numerous investigations in the last several years. Fundamental studies have examined topics such as the role of surfaces in photochemical reaction dynamics (1) as well as the role of photophysics and photochemistry in affecting gas-surface interactions (2). The photochemistry of organometallics and metal complexes on surfaces has been an important subset of this work (3-11). The driving force behind many of the studies of organometallics and metal complexes on surfaces stems from potential applications in catalysis ( 3 - 7 ) and microelectronics (8-13). Here, we will examine the photochemistry of Fe(C0)s adsorbed on the surface of porous silica ( 4 , 5 ) . Using IR and W-visible spectroscopy to monitor photoproduct formation, we find that surface functional groups play a key role in determining the outcome of photochemical reactions in this system. The effects of surface coverage and surface temperature are particularly important. We will discuss these effects in detail, and we will propose a mechanism for the participation of silica surface groups in the photochemical reactions of Fe(C0)s. Finally, the results of our experiments on porous silica will be compared to the results of recent experiments on the photochemistry of Fe(C0)s adsorbed onto other surfaces. EXPERIMENTAL Experimental details are outlined in Refs. 4 and 5, so only the essential features of the experiments are reproduced here. Fe(C0)5 was obtained from Strem and was separated from non-volatile byproducts by a bulb-to-bulb distillation. The purified Fe(C0)s was stored at liquid nitrogen temperature in a Pyrex bulb sealed with a greaseless stopcock, except when in use. The bulb was degassed before each use. Fe3(C0)12 and Fez(C0)g were also obtained from Strem and were used without purification. 2.

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-KBr

Windows-

Fig. 1. Cell used in the photolysis of silica-adsorbed Fe(C0)s room temperature.

at

Silica samples were prepared according to the procedure described in reference 14. This method of preparing silica results in a high surface area (400-600 m2/g) very low-density material with reasonably high transmission in the visible to near-W. Transmissions of 25% were obtained for a typical pathlength of 2 . 5 mm at the photolysis wavelengths. The silica particle size ranged from 0.25-0.65 mm (20-60 mesh). The samples were pretreated by heating for at least two hours in air at 56OoC. This treatment removes all organic contaminants and adsorbed water, as shown by the presence of a sharp surface -OH stretch at 3750 cm-'. In experiments performed at room temperature, the cell shown in Fig. 1 was used. This cell is a 31 mm x 100 mm Pyrex cylinder. The silica particles were contained in a 1 cm3 volume next to the front window of the cell. The particles were held in place by a second window mounted in a clamp that presses against the inner diameter of the cell at three small contact points. This holds the sample firmly, yet allows free diffusion of Fe(C0)s vapor to the sample from the large gas reservoir behind the inner window. The inner window is typically germanium, to allow transmission of IR light but to prevent transmission of W-visible light. In experiments where UV-visible spectra were recorded, a KBr inner window was used. The cell was evacuated to

L

.-2

-+ c.’

150K

m

al 0

+51 C

m

n

Q I

I

I

Wavenumbers

I

1

I

Fig. 6. IR spectrum of silica-adsorbed Fe(C0)s before photolysis Silica bands have be n ( A ) and after photolysis (B) at 150K. subtracted, although a weak silica band is observed at -1880 cm-f . intensity during the course of photolysis, suggesting that they arise from a single species. The bands may be assigned to the species Fe(CO)4(SiO2), based on the similarities of their frequencies to the carbonyl stretching bands of Fe(C0)4(THF). It is also

297

possible that these bands are due to free Fe(C0)4, but this seems unlikely since Fe(C0)4 complexes readily with extremely weak electron donors in low temperature matrices (28). Fe(C0)4 is thus expected to complex with silica surface groups immediately after it is formed. Based on the lack of bridging carbonyl bands in the IR spectrum observed after photolysis, we can rule out the formation of significant quantities of dinuclear iron carbonyl species, such as Fez(C0)g or Fez(co)8(sioz). It thus appears that Fe(CO)q(SiOz) is the only major product formed at 150K, other than Fe3(C0)12. Fe(CO)q(SiOz) is thermally stable at 150K, but it is clearly unstable at higher temperatures. Raising the cell temperature to 200K after photolyzing a sample of silica-adsorbed Fe(C0)s at 150K results in complete loss of the bands at 1960 and 1940 cm-', leaving a spectrum identical to that obtained upon photolysis at 200K. This is consistent with our suggestion that Fe(C0)4(Si02) is an intermediate in the pathway leading to Fe3(C0)12 formation. By slowly raising the cell temperature between 150-200K, the disappearance of the IR bands assigned to Fe(CO)4(SiOz) and concomitant growth of bands assigned to Fe3(C0)12 can be observed. At 165K for example, the half-life of the bands at 1960 and 1940 cm-' is several hours. We find that the bands assigned to Fe3(C0)12 grow in smoothly with time and without an induction period as the bands assigned to Fe(CO)4(SiO2) decay. This suggests that there is no long-lived, dinuclear intermediate on the pathway leading to Fe3(C0)12 formation and is consistent with our earlier suggestion that Fe3(CO)12 is formed by simple trimerization of Fe(C0)4(SiOz). This result also indicates that a secondary photochemical reaction is not necessary for Fe(CO)4(SiOz) to form Fe3(C0)12. Interestingly, there is no growth of bands in the bridging carbonyl region at any point during the decay of Fe(CO)4(SiOz), indicating that Fez(C0)g is not an intermediate in the formation of Fe3(C0)12 or that it is unstable on a silica surface at temperatures above 150K. 3 . 3 Room temperature photochemistry - hiqh surface coverage All of the experiments discussed thus far were performed at Fe(C0)s pressures 13 torr, where the surface coverage is on the order of a monolayer or less. At higher surface coverages, however, very different results are observed. For example, photolysis of silica-adsorbed Fe(C0)s at a pressure of 10 torr in the room temperature cell with the nitrogen laser resdts in formation of the usual green Fe3(C0)12 product, but an opaque yellow deposit that is not observed in lower pressure experiments is also formed.

298

1.0

0.8

I

I

I

-

-

Wavenumberr

Fig. 7 . IR spectrum of silica-adsorbed Fe(C0)s prior to irradiation (top), following photolysis at an Fe(C0)5 pressure of 5 torr (middle), and following photolysis at an Fe(CO)s Pressure of 10 torr (bottom). Bands due to silica and due to unreacted Fe(C0)S were not subtracted. Some insight into the nature of this deposit is obtained by examining the IR spectrum during deposition. We cannot monitor the terminal carbonyl stretching region in the room temperature cell, as we pointed out earlier, but we can monitor the bridging carbonyl region from 1800-1900 cm-'. A s shown in Fig. 7, a strong band grows in at 1820 cm-' with increasing photolysis time. The increase in opacity is illustrated by the increasing baseline absorption in the IR spectrum. The appearance of the strong bridging carbonyl band at 1820 cm-' is consistent with formation of the yellow solid Fe~(C0)g in the high pressure experiments (19), where the Fe(C0)s surface coverage is well above one monolayer. We could not obtain the spectrum of the yellow deposit in the terminal carbonyl region by pumping away unreacted Fe(C0)5, since this deposit decomposes as the cell is evacuated. Within a few minutes after pumping out the cell, the yellow color and the opacity disappear, leaving only the IR spectrum of Fe3 (CO)12. The formation of the yellow solid product only at high surface coverage and its instability upon pumping away Fe(C0)5 is consistent with the following mechanism. The Fe(C0)s overlayers on porous silica are reasonably well-described as a liguid phase condensed

299

in the micropores. Most of the Fe(C0)5 molecules in this liquid phase will not be in intimate contact with the silica surface, as is the case with the first monolayer. Thus, the photolysis of the Fe(C0)s liquid condensed in the pores will most likely lead to formation of Fez(CO)g, which is the product formed upon photolysis of neat liquid Fe(C0)s (20). FeZ(C0)g is not soluble in Fe(C0)5, so any FeZ(C0)g that forms will precipitate as a solid. This accounts for the increase in opacity of the silica sample upon increasing irradiation time in the high pressure experim@nts. The solid Fez(C0)g formed in the micropores is unstable upon pumping out the cell, since the Fe(C0)5 condensed in the pores in rapidly lost on evacuation. FeZ(C0)g is not appreciably volatile, however, so as the Fe(C0)s is removed, the solid Fez(C0)g comes in contact with the silica surface. As we showed earlier by sublimation of pure Fez(C0)g onto silica, dissociation of Fez(C0)g to Fe3(C0)12 and Fe(C0)5 is expected to occur. As the solid Fez(C0)g decomposes, the transmission of the silica sample is restored. leaving only the spectrum of the Fe3(CO)12 product. 3 . 4 ComDarison to Fe(C0)q photochemistry on other surfaces The photochemistry of adsorbed Fe(C0)s has been investigated on a variety of surfaces. For example, Gafney and coworkers have examined the photochemistry of Fe(C0)s adsorbed onto porous Vycor glass (7). This substrate is chemically and structurally similar to the microporous silica used in our work, in that it is primarily a silicon dioxide lattice covered with surface silanol and siloxane groups and has a network of fine pores (30). One chemical difference is that Vycor contains an appreciable fraction of acid sites, due to the incorporation of about 3% boron oxide, plus smaller quantities of other metal oxides into the lattice. The high boron content is due to the method by which porous Vycor is made: An alkali-borosilicate glass is annealed to induce phase-separation into a boron-oxide-rich phase and a silica-rich phase, and the boron-oxide-rich phase is then leached out of the glass by an acid treatment, leaving a network of micropores (30). Differences are observed in the photochemistry of Fe(C0)5 adsorbed on silica and on porous Vycor. Irradiation of Fe(C0)S on Vycor at 350, 310, or 254 nm in a Rayonet reactor (-1 mW/cm2) or with a frequency-tripled Nd:YAG laser (-400 mJ/cm2- pulse) at Fe(C0)s loading levels of mol/g results in formation of a species assigned as the oxidative addition product of photochemicallyproduced Fe(C0)d with surface silanol groups and with chemisorbed

Water, i.e HFe(C0)4(0Si) and HFe(C0)4(0H). Neither Fe3(C0)12 nor Fez(C0)g are formed. The actual surface coverages are not known precisely, but mol/g corresponds roughly to 6 x lom4 monolayers, using the reported surface area of 250 m2/g (31) and assuming uniform coverage. This is actually a lower limit to the surface coverage, however, since Fe(C0)5 does not appear to disperse uniformly on porous Vycor glass. At loading levels mol/g, Fe3(C0)12 is formed along with the Fe(C0)4 oxidative addition products. Spectroscopic evidence suggests that the Fe(C0)4 oxidative addition products do not react by a simple trimerization mechanism to give Fe3 (CO)12, but that secondary photoreactions are required. This reaction scheme is different from the one we proposed for photochemical reactions of Fe(C0)5 on silica. The formation of species like HFe(C0)4(0Si) on Vycor and not on silica is unexpected, since Fe(C0)4 should be the primary photoproduct on both substrates and both surfaces are covered with a high concentration of silanol groups. One poseible source of the difference between the results observed on silica and on porous Vycor glass could be the presence of the acid sites in Vycor. It is possible that at very low surface coverages, Fe(CO)4(SiO2) is formed but migrates to these sites and reacts to give a mononuclear oxidative addition product, rather than Fe3(C0)12. This explanation is attractive, since the lower loading levels where Fe3(C0)12 is not produced correspond to extremely low surface coverages, where reaction with the acid sites may be favored over a trimerization process to form Fe3(C0)12 One other difference between our work and the work of Gafney and coworkers is our use of a pulsed laser as our only light source. At high fluences, pulsed-laser irradiation of strongly absorbing solids causes rapid heating to high temperatures (32); this could lead to desorption of Fe(C0)5, suggesting that the photochemistry observed under pulsed-laser irradiation may actually occur in the gas phase (7). Gas-phase photolysis could favor iron carbonyl cluster formation over the formation of surface reaction products such as HFe(C0)4(0Si). Gafney and coworkers in fact observed significant differences in the photochemistry of Fe(C0)5 on porous Vycor glass, depending on whether the light source was a low-intensity mercury lamp or a pulsed frequency-tripled Nd:YAG laser. We note, however, that our laser fluence in all cases is extremely low (sl mJ/cm2), at least a factor of 100 lower than the laser fluence used by Gafney and coworkers. The low laser fluence we used is not sufficient to cause significant heating of the silica sample,

301

especially since both silica and Fe(C0)5 absorb very weakly at our photolysis wavelengths of 337 and 355 nm. More recent experiments have addressed the photochemistry of Fe(C0)5 on the surface of single crystals under ultra-high vacuum conditions (10,33,34). It is difficult to compare our results with the results of these experiments, but some interesting results have been obtained. In the experiments by Friend and coworkers (10) for example, Fe(CO)5 is adsorbed onto the surface of Si(ll1) at low temperature, yielding roughly monolayer coverage. Photolysis is carried out by pulsed laser irradiation at an average fluence of 1-2 mJ/cm2 with the crystal maintained at low temperature. Surprisingly, no iron carbonyl photofragments on the silicon surface were detected by total internal reflectance IR measurements after photolysis at 248 nm (KrF* laser). Friend and coworkers suggested that this was due to complete decarbonylation of adsorbed Fe(C0)5 by a two-photon process under the conditions of their experiments. Auger electron spectra of the photolyzed surface after the sample was returned to room temperature, resulting in loss of unreacted Fe(C0)5, showed the presence of iron, while neither carbon nor oxygen was detected. 4. CONCLUSION Photolysis of silica-adsorbed Fe(C0)5 at surface coverages below one monolayer with a pulsed nitrogen laser (337 nm) or a pulsed frequency-tripled Nd:YAG laser (355 nm) at fluences i l mJ/cm2 yields adsorbed Fe3(C0)12 as the only major product, as shown by 1R and W-visible spectroscopy. The formation of Fe3(C0)12 was attributed to trimerization of a species designated as Fe(CO)4(SiOz), which is formed by reaction of the Fe(C0)4 primary photoproduct with silanol or siloxane groups on the silica surface. Fe(C0)4(SiOz) was detected by IR spectroscopy in photoreactions carried out at 150K and was observed to react by a purely thermal process at temperatures 2160K to give Fe3(C0)12. At surface coverages above one monolayer, Fe~(C0)g is formed, in addition to Fe3(C0)12, upon photolysis of silica-adsorbed Fe( C0)5 at room temperature and appears to aggregate as a solid. Formation of Fe~(C0)g occurs above monolayer coverage because Fe(C0)5 overlayers on porous silica exist as liquid Fe(CO)5 condensed in the micropores and photolysis of neat liquid Fe(C0)5 is known to give solid Fe~(C0)g.

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REFERENCES See f o r example F.L. Tabares, E.P. Marsh, G . A . Bach, and 1 J . P . Cowin, J . Chem. Phys., 86 (1987) 738. See f o r example T . J . Chuang, S u r f . S c i . , 178 (1986) 2 763-786. N.B. Nagy, M.V. Eenoo, and E.G. Derouane, J . C a t a l . , 58 3 (1977) 230-237. R . L . Jackson and M.R. Trusheim, J . Am. Chem. S O C . , 104 4 (1982) 6590-6596. M.R. Trusheim and R.L. Jackson, J . Phys. Chem., 87 (1983) 5 1910-1916. R. Simon, H.D. Gafney, and D . L . Morse, Inorg. Chem., 24, 6 (1985) 2565-70. M.S. D a r s i l l o . H.D. Gafnev. and M.S. P a s- u e t t e . J . Am. 7 Chem. SOC., 109 (1987) 3i75-3286. R . L . Jackson and G.W. Tyndall, J . Appl. Phys., i n p r e s s . 8 N.S. Gluck, G . J . Wolga, C . E . Bartosch, W. Ho, and Z . Ying, 9 J . Appl. Phys., 61 (1987) 998-1005. J.R. Swanson, C.M. Friend, and Y . Chabal, J . Chem. Phvs., 10 87 (1987) 5028-5037. 11 D . J . E h r l i c h and R . M . Osgood, J r . , Chem. Phys. L e t t . , 79 (1981) 381-388. M.R. A y l e t t , Chemtronics, 1 (1986) 146-149. 12 D . J . E h r l i c h , R.M. Osgood, J r . , and T.F. Deutsch, J . Vac. 13 S c i . Technol., 21 (1982) 23-32. E . J . Carlson and J . N . Armor, G e r . Offen. DE 3 534 970. 14 R . Aveyard and D . A . Haydon, I n t r o d u c t i o n t o t h e P r i n c i p l e s 15 of S u r f a c e Chemistry, Cambridge U n i v e r s i t y P r e s s , Camb r i d g e , 1973, pp. 151-177. R. C a t a l i o t t i , A . F o f f a n i , and L. M a r c h e t t i , Inorg. Chem., 16 10 (1971) 1594. F. A . Cotton and D. L . Hunter, Inorg. Chim. Acta, 101 17 (1974) 273. 18 J . L . G r a f f . R . D . Sanner. and M . S . Wriahton. J. Am. Chem SOC., 101 (1979) 273. . J . S . K r i a t o f f and D.F. S h r i v e r , Can J. S p e c t r o s c . , 19 19 (1974) 156-158. J . Dewar and H.O. Jones, Proc. R . SOC. London, S e r . A, 76 20 (1905) 558. E . H . Braye and W . Huebel, Inorg. Synth., 8 (1966) 178. 21 M . Wrighton, Chem. Rev., 74 (1974) 401-430. 22 W . Strohmeier, Angew. Chem. I n t . Ed. Engl., 3 (1964) 730. 23 24 F. A. Cotton, Prog. Inorg. Chem., 21 11976) 1. E.H. Schubert and R . K . S h e l i n e , Inorg. Chem., 5 (1966) 25 1071. 26 E . Koerner von G u s t o r f , M.C. Henry, and C . Z . D i P e t r o , 2 . Naturforsch B, 21 (1966) 42. G . E h r l i c h and K . S t o l t , Annu. Rev. Phys. Chem., 31 (1980) 27 603. 28 M. Poiakoff and J . J . Turner, J . Chem. SOC., Dalton T r a n s . , (1974) 2276. J . D . Black and P.S. Braterman, J . Organomet. Chem., 85 29 (1975) C7. 30 M.E. Nordberg. J. Am. Ceram. SOC., 27 (1944) 299-305. Corning Glass Works, Corning, N e w York. 31 P . C . S t a i r and E . Weitz, J . Opt. SOC. Am. B , 4 (1987) 32 255-260. 33 N. Bottka, P.J. Walsh, and R . Z . Dalbey, J. Appl. Phys., 54 (1983) 1104-1109. 34 J.S. Foord and R . B . Jackman, Chem. Phys. L e t t . , 112 (1984) 190-194.

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PHOTOPREPARATION OF SUPPORTED METAL OXIDE AND METAL CARBONYL CATALYSTS

A. MORIKAWA and Y. WADA INTRODUCTION Precursors of catalysts are usually activated by thermal treatments resulting in their reduction, oxidation, dehydration, etc. Although this activation step in the preparation of catalysts is very important to give rise to high activity and high selectivity of catalysts, the treatment at high temperatures often causes unfavorable effects which lower the performance of catalysts, such as, aggregation andfor crystallization of amorphous active components, vaporization or sublimation of essential compounds, and elimination of active components by the unfavorable chemical reactions with supports. If catalysts are activated at low temperatures, these disadvantages can be avoided. Photoreaction is one of the methods to activate catalysts at low temperatures. Irradiation of the catalyst precursors supported on a solid surface by visible or ultraviolet light can induce a photoreaction to convert them to a catalytically active species. The purpose of this article is to present a possibility in applying photoreactions to catalyst preparation. Two systems will be discussed. The first is the system of metal oxides in which photoreduction of metal oxides is applied to controlling the valency of metal ions. The second is the system of metal carbonyls which shows the photoinduction of its catalytic activity by the irradiation of metal carbonyls adsorbed on metal oxide supports. 1.

2.

ACTIVATION OF METAL OXIDE CATALYST BY PHOTOREDUCTION Some metal oxide catalysts are activated by thermal reduction with hydrogen or carbon monoxide. For example, the catalytic activity of molybdenum oxide and tungsten oxide for the metathesis reaction of olefins is very much enhanced by their slight reduction ( 1 ) . The catalytic activity for butene isomerization and ethene oligomerization appears on niobium oxide by its

304

reduction with hydrogen ( 2 , 3 , 4 ) . These experimental results lead us to an idea that thermal reduction can be replaced by photoreduction. The following advantages are expected from the catalyst activation by photoreduction. 1) Coordinatively unsaturated metal ion, which is essentially required for catalysis, is stably situated i n the surface structure of the catalyst, since reconstruction of the structure is suppressed when the activation is carried out a t low temperatures. 2 ) Irradiation by light of a selected wavelength can provide selective reduction of surface species so that the desirable active site is specifically prepared. 2.1

Photoreduction of Metal Oxides Kazansky et al. have reported that photoreduction of ions of usual valence state in oxides, such as Mo6+, W 6 + , and Cr6+, results in selective formation of their lower valence state ( 5 - 8 ) . Photoreduction of these ions in hydrogen medium is one electron process described in Eq[l] with the example of molybdenum oxide. Mo5+ is selectively formed by the photoreduction in the presence of hydrogen with the light of wavelength shorter than 320nm ( 5 , 7 ) .

On the other hand, CO induces two electron photoreduction as shown in Eq[2], resulting in the selective formation of Mo4’ ion (6,7).

o* 0’

//

Mo

0

‘0

+

2co

hu

0,

0’

4+

MO

,..a

co

‘0

+COz D l

I

More complicated structures of surface molybdenum oxide species have been reported in the photoreduction of Mo/Si02 catalyst by carbon monoxide ( 9 ) . Metal ions of low valency formed by the photoreduction have a high degree of coordinative unsaturation,

305

causing the high reactivity of the ions. Kazansky and his coworkers have demonstrated the high reducing character of the They have a l s o reported the photoreduced ions ( 1 0 , 1 1 1 . activation of Moo3 supported on silica for olefin metathesis by the photoreduction with carbon monoxide (11,12), and have shown that the photoreduced catalyst exhibits a higher turnover frequency for the reaction than the catalysts prepared by thermal reduction. Niobium oxide and titanium oxide are also reduced in the presence of alcohols o r hydrocarbons under UV irradiation (2,13,14,15). The changes in the absorption spectra and in the color of the oxides indicate the formations of Nb4+ and Ti3+ ion, respectively (2,14,15). The selective photoreduction described here can be utilized for the preparation of catalytically active sites on catalyst surfaces. 2.2

Catalytic Reactions on Photoreduced Niobium and Tantalum Oxide Catalysts --- Ethene Oligomerization Activity Induced bv Irradiation Induction of the catalytic activity of niobium oxide by its irradiation with UV light in the presence of hydrocarbons has been

6 - Light on 4-

2-

0 Ti me /min Fig.1 Photoinduction of the catalytic activity of Nb205/PVG for the formation of butenes from ethene. The reaction was carried o u t at room temperature.

306

demonstrated by one of the authors(A.M.1 (2,131. The formation of I-butene and 2-butenes from ethene is initiated on niobium oxide by irradiation, though this oxide is Fig.1 shows catalytically inactive for the reaction under dark. the induction of the catalytic activity for the formation of butenes from ethene on niobium oxide supported on porous Vycor glass(Nb205/PVG) by irradiation for 10min. The amount of 2 butenes formed in the reaction is less than 1/20 of that of 1 butene. The special property of this system is that the catalytic activity is maintained even after ceasing the irradiation, indicating that the catalytically active site is formed during irradiation in the presence of ethene. Since products of the reaction induced by irradiation are not only dimers of ethene(butenes1 but also oligomers of ethene, the reaction occurring on the photoactivated catalyst is oligomerization of ethene. The composition of the oligomers formed on the photoactivated niobium. oxide follows Schultz-Flory distribution. The similar induction of the catalytic activity by irradiation is also observed for tantalum oxide supported on porous Vycor glass(Ta205/PVG). The irradiation of light filtered by various optical glass filters have shown that the light of wavelength below 300nm

a

Wavel ength/n m Fig.2 VIS-UV absorption spectra of Nb205/PVG. (a)before the photoactivation, (b)after the photoactivation in the presence of ethene, (clafter exposure of b to oxygen.

307

corresponding to the UV absorption band of supported Nb2O5 (see Fig.2-a) is effective for the induction of the catalytic activity. Fig.2 shows the VIS-UV absorption spectra of Nb205/PVG. The spectrum (a) is a difference spectrum between Nb205/PVG and PVG. When Nb205/PVG is irradiated in the presence of ethene, the spectrum(a) is changed into (b). New, very broad absorption in to the spectrum(b) shows the occurrence of reduction of Nb205 Nb02, in other words, formation of Nb4+ ion. In this system, N b 4 + ion formed by photoreduction with ethene acts a s a catalytically active site. Exposure of the sample(b) to oxygen gas changes the spectrum(b) into (c), and the photoinduced catalytic activity is suppressed when oxygen gas is brought into contact with the catalyst. Both of these results draw an explanation that Nb4+ ion is a catalytically active species and is reoxidized to Nb5+ ion by oxygen gas. The difference of absorbance between the spectrum (b) and the spectrum (c) at a wavelength, for example, 500nm, denoted by A , must be proportional to the amount of Nb4+ ion. The shaded part shown in the figure must result from the adsorbed ethene oligomers. The change in the value of A along the irradiation time is displayed in Fig.3. The linear increase of A or the

h

E

c

0 0 Ln

v

a

0

5 10 T ime /mi n

15

Fig.3 Increa e of the absorbance at 500nm corresponding to the amount of Nb" ion formed on Nb205/PVG by the irradiation in the presence of ethene.

308

Time /min Fig.4 The formation of butenes during the irradiation.

Nb/mg- g-WG” Fig.5 D e p e n d e n c e o f t h e c a t a l y t i c a c t i v i t y photoreduced with ethene on the Nb content.

of N b 2 0 5 / P V G

amount of Nb4+ ion well explains the formation of the active species by the light absorption, and the cease of the increase a f t e r 1 0 m i n c o r r e s p o n d s t o t h e t h o r o u g h a c t i v a t i o n of a l l

309

precursors of the active species. When the rate of active site formation, B sites min-l, is constant and the rate of ethene dimerization on an active site is constant, R mol min-’ site-’, then the amount of butene formed, n mol, at a time of t min, is expressed by n= R(t-r)BdI=(l/2)RBt2. This relation well explains the change of the amount of butenes formed on Nb205/PVG during the irradiation(disp1ayed in Fig.4). Therefore, the Nb(1V) species must be closely related to the intermediate of the reaction. Fig.5 shows the dependence of the catalytic activity of Nb205/PVG photoactivated in the presence of ethene on the niobium content in the catalyst. The activity increases monotonously with increase in the Nb content. On the other hand, Anpo et al. have shown that tetrahedrally coordinated Mo or V ion with doublebonded oxygen ions(see Reactionsrl J and [2]) , which take part in photochemical process of their oxides, is predominantly formed at a rather low content in porous Vycor glass (16,171. Furthermore, it has been found that photocatalyzed metathesis ( 1 3 ) and photooxidation of carbon monoxide observed for Nb205/PVG show a maximum activity at 1 mg 9-PVG-l of the Nb content. This difference in the dependence of the activities on the Nb content

It

TABLE 1 Catalytic activity of niobium oxide for ethene oligomerization, photoinduced in the presence of various compounds. Compound

Reaction rate / I 0-6 mol min-1 1.34 1.90 0.60

ethene 1 -butene 1 - hexene 2.3-dimethyl2 -butene 0.17 0.08 H2a) H2b) 0.05 The Nb205/PVG catalyst was photoirradiated for 10 min in the presence of each compound shown above before the reaction of ethene was carried out. alphotoirradiated in the presence of hydrogen of 6.7kPa for 3h. b)The catalyst was reduced thermally with hydrogen of 4.OkPa at 823K for lh.

310

seems to indicate the contributions of different surface species of niobium oxide to the two photochemical processes, the photoactivation and the photocatalyzed metathesis. The catalytic activities for ethene oligomerization, induced by the photoreduction of niobium oxide with ethene and other various compounds at room temperature, are listed in Table 1 . The catalysts photoreduced with H2 at room temperature and reduced thermally with H2 at 823K show comparable activity, but with much lower values than the catalyst photoactivated with ethene. Not only ethene but also other olefins are effective for the photoactivation. Low activation ability of 2,3-dimethyl-2butene, however, has been observed, suggesting that vinyl hydrogen in olefins may play a role in the photoactivation. PHOTOACTIVATION OF METAL CARBONYL ON METAL OXIDE SURFACE Metal carbonyls adsorbed on solid surfaces have been used as precursors to develop well-defined, catalytically active site (18). They are usually treated at rather high temperatures in vacuo or in reducing or oxidizing atmosphere to convert them to catalytically active species. Metal carbonyl loses carbon monoxide, reacts with chemical species of the support surface and/or aggregates during the treatment. Metal carbonyl in homogeneous solutions is known to lose its ligand under its photoirradiation and to form a coordinatively unsaturated subcarbonyl which is very reactive ( 1 9 ) . Since this subcarbonyl combines immediately with CO again or with solvent molecules, or with another subcarbonyl to form polynuclear clusters, it is usually short-lived. When the photo-decarbonylation of metal carbonyl is applied to the preparation of catalytically active species on solid surface, the following advantages can be offered for the catalyst preparation. 1) Metal subcarbonyl, unstable in liquid phase, is stabilized on solid surface, and can be a catalytically active species possessing high activity and selectivity. 2) Light is a more controllable energy source than heat for the catalyst activation so that the number of detached carbon monoxide molecules can be controlled. In other words, the degree of unsaturation of the metal in the subcarbonyl formed can be manipulated. 3) Since photoreaction is carried out at low temperatures, the 3.

311

thermal reaction of metal carbonyl with surface hydroxyls is avoidable, resulting in maintaining a low oxidation state of the nuclear metal. Photoinduction of Catalytic Activity of Mo(COI6 and W(CO)6 Adsorbed on Porous Vycor Glass The authors have tried the method to apply photoreaction of metal carbonyl on solid surface for preparation of catalysts, and have found that metal carbonyls, M(C0)6(M=Mo, W), adsorbed on porous Vycor glass are activated as catalysts by their irradiation with UV light in the presence of reactants ( 2 0 ) . Fig.6 shows a time course of the propene reaction carried out on the M O ( C ~ ) ~ - , W(CO)6- or Cr(CO)6-adsorbed porous Vycor glass(denoted by No reaction occurs under dark. Mo(CO)~/PVG, etc., hereafter). Propene is converted to equal molar amounts of ethene and butene under the irradiation of MO(CO)6/PVG and W(C0)6/PVGl but no activation of Cr(CO)6/PVG has been observed. This result indicates the photoinduction of the catalytic activity of Mo(CO)~/PVG and W(C0)6/PVG for the metathesis of propene. The reaction continues for about 30 min even after the stop of the 3.1

15 10

5

0 0

1 2 Time/h

3

Fig.6 Photoinduced metathesis of propene on Mo(CO)~/PVG(O), w(CO)~/PVG(A), and Cr(C0)6/PVG(O). Ethene of the comparable amount to that of butenes is formed.

312

irradiation at lOmin, suggesting that the catalytically active species is formed by the irradiation. The light of the wavelength between 320 and 360nm is effective for the induction of the catalytic activity of Mo(CO)~/PVG. This region of the wavelength corresponds to the light absorption band of Mo(COI6. Therefore, the carbonyl absorbs light and is converted to the active species through the photoreaction on the surface. The irradiation of M o ( C O ) ~ / P V G with the light of the wavelength region, 320 360nm, causes desorption of carbon monoxide, and in parallel the formation of molybdenum subcarbonyls, M0(Co)~-~(n=1-3),has been recognized by applying VISUV absorption spectroscopy to MO(C0)6/PVG as shown in Fig.7. A very intense absorption below 400nm observed for Mo(CO)~/PVG is attributed to physisorbed Mo(C0I6. When it is irradiated at room temperature by light of the wavelength region effective for the induction of the catalytic activity, a new band with an absorption maximum at 400nm appears. This absorption corresponds to MO(CO)~ formed by the irradiation of Mo(CO)6 in the rigid matrices at low temperatures (21 1 , although the value of n in M o ( C O ) ~ - ~the , subcarbonyl formed on PVG, has not been determined yet. This

a c u

m

2 0 m

n

a 200 400 Wavelengthhm

600

Fig.7 VIS-UV absorption spectra of Mo(C0)6/PVG. (a)PVG, (~)Mo(CO)~/PVG, (c)after the irradiation with 320 360nm light.

313

type of subcarbonyl is usually unstable at room temperature, but the results mentioned above show that the subcarbonyl is stabilized on the surface of PVG. This absorption band decreases gradually under dark, suggesting a loss of the subcarbonyl by restoration of Mo(C0I6 through its recombination with carbon monoxide or its reaction with another subcarbonyl to make a polynuclear complex. Mo(CO15 or a subcarbonyl formed by further loss of carbon monoxide from that subcarbonyl is considered to act as the catalytically active species for the propene reaction. Stabilization of W(CO)5 on PVG formed by irradiation has been reported by Simon et al. ( 2 2 , 2 3 ) . They have observed that this unsaturated carbonyl is coordinated again by CO, or cis or trans2-butene when these compounds are brought into contact ( 2 3 ) . The fact that butenes form a complex with W(CO)5 on PVG especially suggests a potentiality of the unsaturated metal carbonyl on the surface to be a catalytically active species. In fact, photoinduced isomerization of butene on W(C0)6/PVG has been briefly reported by Perettie et al. ( 2 4 ) . The authors have found that beside metathesis, double-bondmigrating isomerization of olefins also proceeds on the irradiated Mo(C0)6/PVG. The result in Fig.8 shows production of ethene and

-0

-

60 Ti me/m in

120

Fig.8 Formations of ethene(0) and butenes(A) from 1-butene on the irradiated MO(CO)6/PVG.

314

2-butenes by the reaction of I-butene on Mo(CO)~/PVG irradiated with a reduced intensity of light. Ethene is a product of metathesis of 1-butene(the other is hexene) and 2-butenes are produced by double-bond-migration of 1-butene. Only a very little amount of ethene is formed at the early stage of the irradiation, while an appreciable amount of 2-butenes is detected from the beginning. This difference in the formation of the two products suggests that the two reactions, metathesis and isomerization, proceed on different active species. The amount of ethene formed increases along a parabolic function of the irradiation time, suggesting that ethene forms proportionally to the reaction time on the active site which i s produced proportionally to the time, as discussed in the previous section. It has been found that the two reactions on the irradiated Mo(CO)~/PVG are retarded by admission of carbon monoxide, and the metathesis is poisoned more strongly by carbon monoxide than the isomerization. This result supports the following mechanism for the formation of the catalytically active species from M o ( C O ) ~ / P V G by the irradiation.

Mo(C0)6

-

hu, -m C 0 IC--

+mCo

MO(CO)6-m

Active for Isomerization

-

hu, -nCO +nCO

MO(CO)6-m-" Active for Metathesis

M o ( C O ) ~loses carbon monoxide under the irradiation to form one subcarbonyl within a short duration. It acts as the catalytically active species for the isomerization or as a precursor of the active species. This subcarbonyl gradually liberates carbon monoxide further to form another subcarbonyl which is highly unsaturated in coordination to be adequate to catalyze the metathesis of olefins. In homogeneous system, Mo(C0I6 or W(COI6 dissolved in pentane shows no catalytic activity for metathesis under irradiation. Only when a cocatalyst, such as metal chlorides or CC14, is p r e s e n t in t h e s y s t e m , W ( C 0 ) 6 i s p h o t o a c t i v a t e d t o b e catalytically active for metathesis (25,26,27). Therefore, the surface of PVG not only stabilizes the subcarbonyls but also plays an equivalent role to the cocatalysts. This hypothesis is

315

plausible with the reason that Fe(CO)5 reacts with the surface of s i l i c a o r P V G t o f o r m i t s a d d u c t s w i t h the s u r f a c e b y photoirradiation (28,291. 4.

SUMMARY The two systems of heterogeneous catalyst activated by photoirradiation have been demonstrated. The first part has shown that photoreduction can not only replace thermal reduction for an activation method of catalysts but also give rise to higher activity than the thermal reduction. Reduction at low temperature in the photochemical method results in maintaining high coordinative unsaturation of a catalytically active site. The second part has described a special catalyst system, where photochemically formed metal subcarbonyls are developed on a surface. High catalytic activity and high selectivity are expected for this catalytic system. A solid surface can contribute to stabilization of a catalytically active species to maintain good catalyst performance. REFERENCES 1

2

3

4 5 6 7 8

9 10 11

12

J. C. Mod and J. A. Moulijn, in :J. R. Anderson and M. Boudart(Eds), Catalysis, Springer-Verlag, Berlin, 1987, pp. 69-129. A . Morikawa, T. Nakajima, I. Nishiyama, and K. Otsuka, in:Proceedings of 8th International Congress on Catalysis, IV, Berlin(west), 2-6 July 1984, DECHEMA, Frankfurt am Main, 1984, IV-815-825. Y. Murakami, Y. Wada, and A . Morikawa, Bull. Chem. SOC. Jpn., 61 (1988) 2747-2752. Y. Wada, M. Inaida, Y. Murakami, and A. Morikawa, Bull. Chem. SOC. Jpn., in press. A . N. Pershin, B. N. Shelimov, and V. B. Kazanskii, Kinet. Katal., 20 (1979) 1298(Russ.), 1071(Eng.). A. N. Pershin, B. N. Shelimov, and V. B. Kazanskii, Kinet. Katal., 21 (1980) 494(Russ.), 388(Eng.). B. N. Shelimov, A. N. Pershin, and V. B. Kazansky, J. Catal., 64 (1980) 426. A. N. Pershin, B. N. Shelimov, and V. B. Kazanskii, Doklady, Physical Chemistry, 267 (1982) 412(Russ.), 929(Eng.). L. R o d r i g o , K. M a r c i n k o w s k a , P. C. R o b e r g e , a n d S. Kaliaguin, J. Catal., 107 (1987) 8. A. N. Pershin, B. N. Shelimov, and V. B. Kazanskii, Kinet. Katal., 21 (1980) 753(Russ.), 561(Eng.). V. B. Kazansky, A. N. Pershin, and B. N. Shelimov, in:T. Seiyama and K. Tanabe(Eds), Proceedings of 7th International Congress on Catalysis, Kodansha, Tokyo/Elsevier, Amsterdam, 1981, pp.1210. B. N. Shelimov, I. V. Elev, and V. B. Kazansky, J. Catal., 98 (1986) 70.

316

13 14 15 16 17 18

19 20 21 22 23

24 25 26 27 28 29

A. Morikawa, T. Nakajima, I. Nishiyama, and K. Otsuka, Nippon Kagaku Kaishi(Journa1 of the Chemical Society of Japan, Chemistry and Industrial Chemistry), (1984) 239. Y. Wada and A. Morikawa, Bull. Chem. SOC. Jpn., 60, (1987) 3509. P. Pichat, J.-M. Herrmann, J. Disdier, H. Courbon, and M.N. Mozzanega, Nouv. J. Chim. , 5 (1981) 627. M. Anpo, I. Tanahashi, and Y. Kubokawa, J. Phys. Chem., 86 (1982) 1. Y . Kubokawa and M. Anpo, in:M. Che and G. C. Bond(Ed.), Adsorption and Catalysis o n Oxide Surfaces, Elsevier, Amsterdam, 1985, pp.127-138. R. F. Howe, in:Y. Iwasawa(Ed), Tailored Metal Catalysts, D. Reidel Publishing Co. , Holland, 1986, pp.141-182. M. Wrighton, Chem. Rev., 74, (1974) 401. Y. Wada and A. Morikawa, Chem. Lett., (1988), 25. M. A. Graham, M. Poliakoff , and J. J. Turner, J. Chem. Soc. , A, (1971 2939. R. Simon, H. P. Gafney, and D. L. Morse, Inorg. chem., 22 (1983) 573. R. C. Simon, H. D. Gafney, and D. L. Morse, Inorg. Chem., 22 (1985) 2565. D. J. Perettie, M. S. Paquette, R. L. Yates, and H. D. Gafney, in: Nato AS1 Ser., Laser Appl. Chem., 1984, p p . 2 5 1 257. A. A g a p i o u a n d E. M c N e l i s , J. C h e m . S O C . , C h e m . Commun. ,(1 975) 187. P. Krausz, F. Garnier, and J. E. Dubios, J. Am. Chem. SOC. , 97 (1975) 437. M. Nagasawa, K. Kikukawa, M. Takagi, and T. Matsuda, Bull. Chem. SOC. Jpn., 51 (1978) 1291. M. R. Trusheim and R. L. Jackson, J. Phys. Chem., 87 (19831 191 0. M. S. Darsillo, H. D. Gafney, and M. S. Paquette, J. Am. Chem. SOC., 109 (1987) 3275.

Chapter 6

LASER 1NDUCF.D PHOTOREACTIONS AND PHOTO-CVD ON SOLID SURFACES

Contents

5.1 UV Laser Photodissociation of Small Molecules on Solid Surf aces

(Hiroyasu Sato and Masahiro Kawasaki)

6.2

GO2 Laser

Induced Surface Reaction

(Maki Kawai)

6.3

317

329

Photochemical Aspects of Amorphous-Si Nucleation by Photo- CVD (Hiroshi Hada and Mitsuo Kawasaki)

339


317

UV LASER PHOTODISSOCIATION OF SMALL MOLECULES ON SOLID SURFACES

H. SAT0 and M. KAWASAKI INTRODUCTION Laser s u r f a c e c h e m i s t r y h a s b e e n u s e d a s a b a s i s for many new methods in surface processing, for example, photochemical deposition of metals and photochemical etching of solid substrates, which are potentially useful techniques for the microelectronics industry ( 1 - 3 ) . However, molecular dynamical studies of UV photodissociation of adsorbates on solid surfaces have been very scarce (4-7). We have studied UV laser photodissociation of small molecules on solid surfaces using photofragment spectroscopy. During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 1.

PHOTODISSOCIATION OF CHLORINE AND TRIMETHYLGALLIUM ON SOLID SURFACES 2.1 Chlorine molecules on an Si wafer Reactive ion etching is now very widely used for delineating fine patterns in the fabrication of VLSI circuits. Recently, fluorine atoms produced in CF4 plasma have been used in the etching of Si wafers, generating SiF2 (8). As an alternative dry etching approach, a photo-excited etching has been reported by Okano et al. (9) using chlorine molecules. It is reported that C1 atoms attack an Si wafer to form Sic1 ( 1 0 ) and SiC12 ( 1 1 ) . 2.

318

In order to investigate the photochemistry of C12 on a solid substrate, multilayered C12 condensed on an Si wafer (doped by P+ at a rate of 3 x 1015 was irradiated by excimer laser light

-

( 10 mJ/cm2) at 193, 2 4 8 , and 3 5 2 nm. The substrate was cooled to 100 K. Photoproducts ejected from the substrate were analyzed by a mass spectrometer (12).

a ) Effect of thickness of C12 multilayers on TOF distributions of photoproducts. When C12 deposited on an n-type Si wafer was irradiated at 193 nm, C1 atoms were detected as one of the photoproducts. The TOF distributions for thin and thick deposition of C12 on the substrate are contrasted in Fig. 1. The TOF distribution is bimodal for thin deposition. The low-energy component appeared only for thin-deposited C12. It contains information on the C1 atoms photoproduced on the substrate surface by laser irradiation. C1 atoms photogenerated on the substrate surface are decelerated by strong interaction between these atoms and the surface. A part of C1 atoms may result in the generation of etching products: C1 + Si (substrate) -SiC1,

SiC12.

Mass analysis gave signals at m/e = 6 3 (SiCl')

0

and 98 (SiCl?), but

400 800 1200 1600 2000 Time a f t e r laser pulse / p S

Fig. 1. Time-of-flight distributions of photofragments obtained in the photodissociation of C12 molecules deposited thin and thick on an Si wafer cooled to 100 K. Laser wavelength is 193 nm. Flight length is 16 cm. Drift time in the mass filter is 26 p s for C1+.

319

neither at 1 3 3 (SiC1;) nor 1 6 8 (SiCl;). The TOF distribution of Sic1 consists only of the low-energy component as shown in Fig. 2. The high-energy component of the C 1 atom is considered to be formed by photodissociation of C 1 2 molecules on the top molecular layer of C12 “ice“, that is, the C1 atoms are photoejected from the solid surface of C12 molecules. The interaction between photoejected atoms and the solid parent molecules makes the width of the TOF spectrum larger than that of the gas phase photodissociation. b) Translational energy of photoproducts. The high-energy component in Fig. 1 is considered to be generated by the direct photodissociation process on the top

g

3

576

1 t

2 .? t0 _ , d ‘

“

0

400 800 1200 1600 2000 Time a f t e r laser pulse / us

Fig. 2. TOF distribution of Sic1 from the photodissociation of C12 on an Si wafer at 1 9 3 nm. Drift time is 3 5 )IS for SiC1+. TABLE 1

Threshold appearance times and maximum translational energies of C1 photofragments from C12 solid photodissociation.

x nm 193 248 352

hw

kca1/mol 148 115 82

PS 40 54 65

kcal/mol

kcallmol

67 37

91 58 25

25

1 ) Threshold appearance time in TOF spectra. 2 ) maximum translational energy obtained from t

.

3) Do stands for the bond dissociation energy, T7.3k0.1

reported by Darwent ( 3 1 ) .

kcal/mol

320

molecular layer. In order to prove this direct process, the threshold appearance times (tTH) were measured at 193, 248, and The tTH position 352 nm. Results are tabulated in Table 1. shifts toward a longer time as the laser wavelengths increase. This result indicates that the high-energy component in the TOF spectrum is attributable to the direct dissociation of the top monolayer of C 1 2 :

Photodesorption of molecules from substrate surfaces has been studied by infrared laser irradiation ( 1 3-1 5). The translational energy distribution for such a thermal desorption is well repreFor laser sented by a Maxwell-Boltzmann (MB) distribution (1 3 ) . wavelengths in the visible region, however, there appear at least two translational energy distributions of molecules desorbing from the surface. Two-components fit of the MB distributions represents well the experimental data (14). Lin and co-workers ( 1 5 ) have developed a theoretical model for the photodesorption using the transition state theory. The number of photoproducts at the translational energy ET is given by:

1 +

w

u

10.7 kcal/mol

a

0

--.__ .- -_* 0

--i-

10 20 30 Translational Energy / kcal -rnol-’

Fig. 3. A typical translational energy distribution of C1 atomic photofragments generated in the 193 nm photodissociation of solid C12 molecules on the Siwafer. The smoothed curves are sum of two Maxwell-Boltzmann distributions with two different temperatures (ET = (3/2)kT).

321

Figure 3 depicts an example of the translational energy distribution P(ET) of C1 atoms obtained in the 193 nm photodissociation. The smooth curve through the experimental points shows a fit to a composite of two ME distributions with the values of low and high translational energies tabulated in Table 2. c) Formation of etching products By irradiation at 193 and 248 nm, Sic1 and a small amount of SiC12 were detected as photoproducts. Neither SiC13 nor SiC14 was detected in the present experiment. When SiC14 "ice" was irradiated at 193 nm, Cl was detected as a photoproduct, but Sic1 and SiC12 were not. This result suggests that Sic1 and SiC12 generated in the photodissociation of C12 on the Si wafer are produced by reaction of C1 atoms with the Si wafer and not by the secondary photodissociation of SiC14. It is reported that Sic1 and SiC12 are detected by mass spectrometry in an ion-assisted C1

TABLE 2 Averaged translational energy of photoproducts and photodesorbates. species

laser

substrate

h/nm

N1

193 248 352 193 248 308

1000 500 5000 2000 2500 36000

Sic1

193 248

3000 5000

Si Si

SiC12

248

5000

Si

c12

193 248 352

7 00

Si Si Si

c1

150 1800

E,2) I1

E2'

l2

kcal/mol Si Si Si SiOz

sio in

802

9.5k2.0 6.7

* 17 **

7.2 l.Ok0.3 1.2 (1.3)** 7.2 1.2 1.5

1 ) Number of laser shots required to obtain analyzable data. 2) Averaged kinetic energy (3/2)kT. T is given in eq. (1). 3) Ratio of coefficients in eq. ( 1 ) . 4 ) 7 % in CO matrix. * unanalyzagle because of mixing of surface and gas phase dissociations. ** Signal to noise ratio was not good.

322

atomic reaction with an Si wafer (16). F atomic reaction produces SiF2 that has been detected by the laser-induced-fluorescence (LIF) technique (8). The TOF spectra of Sic1 and Sic12 are analyzed assuming the M B distribution. The results are summarized in Table 2. The ET values are -1.2 kcal/mol, which are close to those observed in photodesorption of C12 from the Si wafer and the low-energy component of the C1 atoms. d) Comparison with a quartz plate The signals of C1 atoms observed at 193 nm for two kinds of substrates, an Si wafer and a quartz plate, are contrasted in Fig. 4. The signal intensity ’was stronger on Si than on quartz. Since C12 gas has only a weak absorption ( E 5 Rcm-’ m o l - l ) at this wavelength, some enhancement by the Si wafer must be present. This enhancement may be caused by energy transfer from the Si substrate excited electronically by the laser photons. The underlying Si wafer can absorb UV photons quite well.

-

C12/substrate + h w

--i

C12/substrate* + 2C1.

The substrate excitation has been discussed theoretically (17) and experimentally (1 8,19). XeF2, optically transparent in the visible region, dissociates on the Si substrate by excitation at 515 nm. The electronic excitation in the Si semiconductor causes this dissociation ( 1 8 ) . SF6 dissociation is also enhanced by laser

3

5000

. ._ on Si

on Si02

u

0

200

400

Time a f t e r l a s e r pulse 1 us

0

0

--

I

200 400 Time a f t e r l a s e r pulse / ~.rs

Fig. 4. Change in the yield of C1 photoproducts on a quartz plate (left) and on an Si wafer (right). The former is weaker than the latter by a factor of -.80.

323

irradiation on an Si wafer ( 1 9). Electronic-energy transfer in rare gas matrix has also been reported to occur by excitation of group IIB metal atoms to the 3 P state in the presence of hydrocarbons at 12 K (20). 2.2 Trimethylqalliumon a quartz substrate Laser stimulated growth of metal thin films is 0r.e of the important methods of surface processing. However, only a little is known about the fundamental physicochemical mechanisms pertinent to these techniques (1,19,21,22). The mechanism of the photo-MOCVD (metal organics chemical vapor deposition) process of trimethylgallium was investigated using the time-resolved LIF technique (23). After a quartz substrate was dosed by a pulsed molecular beam of trimethylgallium in Ar (1:25), it was irradiated by a pulsed light of an excimer laser (248 nm), or the 4th, 3rd and 2nd harmonics of a YAG laser (266, 355, and 532 nm). A pulsed dye laser beam traversing the space above the substrate surface parallel to the surface probed the concentration of Ga atoms by LIF as shown schematically in Fig. 5. In order to cut off the strong scattered light of the photolysis laser, the fluorescence was detected by a gated multichannel plate photomultiplier. Thus, concentration of Ga atoms can be probed in a spatially- and time-resolved way. An example of the results is shown in Fig. 6. The peak in the LIF at 0.5-2 u s corresponds to Ga atoms desorbed from the

I

Fluorescence

Fig. 5. A schematic diagram of experimental setup for LIF detection of metal atoms from photo-CVD processing of metal organics.

324

J

3.7mm

0 1 2 3 4 Delay Time / u s

Fig. 6. Temporal variation of L I F signal of Ga atoms from trimethylgallium after the 248 nm photodissociation laser pulse, probed at various vertical distance Ilfrom a quartz substrate. Substrate temperature = 2OoC. Signals at t = 0 is a scattered light of the photolysis laser. substrate surface after laser irradiation of trimethylgallium on a quartz plate. Ga atoms are generated at the laser wavelengths 248 and 355 nm but not at 532 nm for the same laser intensity. Since the photoabsorption of trimethylgallium starts at 260 nm in the gas phase, the absorption coefficient of the adsorbate would be small at near UV and much smaller at visible region as reported for dimethylcadmium (24). The wavelength effect suggests that the photoabsorption of the parent molecules adsorbed on the substrate initiates the formation of Ga atoms. The signal intensity increased with the stagnation pressure of the molecular beam and finally leveled off. This behavior is well represented by the Langmuir adsorption isotherm which is applicable to chemical adsorption. A monolayer on the substrate is photodissociated to generate Ga atoms. Ga(CH3)3 (ads.) + nfiw-Ga

+ - . a -

The effect of substrate temperature was investigated at T = 20- 40OoC. At T < 2OO0C, the LIF intensity was essentially temperature-independent, while it decreased with temperature at T >

325

20OoC. This finding indicates that Ga atoms are generated by thermally assisted photodissociation of the adsorbed molecules. If the dissociation reaction takes place purely photochemically in the adsorbed state, the dissociation rate would be temperatureindependent ( 2 ) . Then the yield of Ga atoms would be simply proportional to the amount of trimethylgallium adsorbed on the substrate. It would be larger at a lower temperature at which adsorption occurs favorably. The role of thermal energy would be considered such as, a) the thermal energy activates the Ga-CH3 bond scission in the hot trimethylgallium molecules, and/or b) the thermal energy facilitates the desorption of Ga atoms generated photochemically from the substrate, etc. REACTION OF MOLECULES IN THE GAS PHASE WITH PHOTOELECTRONS OR PHOTOABLATED MET.AL I O N S FROM THE SURFACE OF METAL SUBSTRATES The photo-stimulated ablation of neutrals and ions from various substrates has been studied experimentally and theoretically ( 1 2 , 1 5 , 2 5 , 2 6 ) . This technique can be used for study of ionThe ions photoablated may have quite molecule reactions (27,281. a wide range of kinetic energy of up to 1000 eV (29). The essential part of our experimental set-up is schematically illustrated in Fig. 7. This is a very simple apparatus. A quadrupole mass spectrometer was placed in a vacuum chamber. The substrate of various metals was placed at a distance of 4 cm from 3.

Molecular Beam

n

II

Quadrwole Mass

l,-l_l Metul : ubstrate

D------a

Region Detector .

(Ceratron)

It

li

Laser Light

Fig. 7. A schematic diagram of experimental setup for reaction of molecules in a beam with photoablated metal ions and photoelectrons from the substrate.

326

t h e e n t r a n c e r e g i o n o f t h e i o n - l e n s a s s e m b l y o f t h e mass f i l t e r . The m a g n e t i c f i e l d was a p p l i e d i n t h e p o s i t i o n b e t w e e n t h e mass s p e c t r o m e t e r and t h e s u b s t r a t e when n e c e s s a r y .

The s e c o n d harmon-

i c s f r o m a n N d : Y A G l a s e r ( 5 3 2 nm) was f o c u s e d o n t h e s u r f a c e o f a m e t a l s u b s t r a t e ( A l , C r , Cu, I n , N i , Nb, Pd o r Zn). together with photoelectrons,

Metal ions,

a r e p h o t o a b l a t e d and e n t e r i n t o t h e

e n t r a n c e r e g i o n of t h e mass f i l t e r .

The a p p l i c a t i o n of m a g n e t i c

f i e l d b e n d s t h e p a t h s of s u b s t r a t e metal i o n s and p h o t o e l e c t r o n s Owing t o t h e l a r g e d i f f e r e n c e i n t h e i r mass,

i n t h e o p p o s i t e way.

e l e c t r o n s make a s h a r p bend and d o n o t r e a c h t h e e n t r a n c e r e g i o n , w h i l e metal i o n s s t i l l r e a c h .

A p u l s e d m o l e c u l a r beam of g a s e o u s

m o l e c u l e s was i n j e c t e d i n t o t h e e n t r a n c e r e g i o n s i m u l t a n e o u s l y with t h e l a s e r pulse.

The s u b s t r a t e m e t a l i o n s a n d p r o d u c t i o n s

c a u s e d by t h e i r r e a c t i o n w i t h m o l e c u l e s i n t h e beam were m e a s u r e d

a s a f u n c t i o n o f mass numbers (mass a n a l y s i s ) and t i m e a f t e r l a s e r p u l s e (TOF a n a l y s i s ) . For

p u l s e d beam o f

metal organics

( t r i m e t h y l g a l l i u m and

t e t r a m e t h y l t i n (30)), n e i t h e r m e t a l i o n a d d u c t s n o r m i x e d i o n p r o d u c t s were f o u n d .

O n l y f r a g m e n t i o n s were d e t e c t e d , w h o s e

c r a c k i n g p a t t e r n i s c l o s e t o t h a t o b t a i n e d i n t h e e l e c t r o n bomWhen a m a g n e t i c f i e l d was a p p l i e d b e t w e e n t h e

bardment i o n i z e r .

s u b s t r a t e and t h e e n t r a n c e region, peared.

fragment ion signals disap-

T h e r e f o r e , m e t a l o r g a n i c compounds a r e i o n i z e d by t h e

Nb' I

100 Mass Number

50

150

m/z

200

F i g . 8. Mass s p e c t r u m o b t a i n e d o n l a s e r i r r a d i a t i o n ( 5 3 2 nm) f o c u s e d o n a n N b s u b s t r a t e w i t h a m o l e c u l a r beam o f CgHg i n j e c t e d CgHg+ i o n s d i s a p p e a r e d when t h e m a g n e t i c f i e l d (1.8 m T ) near-by. was a p p l i e d v e r t i c a l l y t o a v e c t o r L of F i g . 7.

327

photoelectrons from the surface irradiated by strong laser pulses. metal organics + e- (photoelectron)d fragment ions. For pulsed beam of metal carbonyls (e.g. Mn2(CO)10) mass analysis shows ions of many types containing both Mn and substrate metal atoms besides the substrate metal ions (M+) and the fragment ions (Mn2(CO),+, Mn(CO),+) resulting from the metal carbonyls. Obviously, the substrate metal ions react with Mn2(CO)10 to generate these mixed ions.

A pulsed beam of benzene was used together with an Nb substrate. Besides the substrate ion and benzene parent ion, Nb(CsH6)' ion was generated by the ion molecule reactions (Fig. 8). The application of a magnetic field revealed that, among these three kinds of ions, benzene ion is due to the reaction with electrons from the metal substrate. A Similar finding was made with other metal substrates, as well as with other organic molecules.

This is a simple and versatile method for investigating many ion-molecular reactions caused by metal ions. REFERENCES R.M. Osgood, Jr., Ann. Rev. Phys. Chem., 34 (1983) 77-101; Phys. Today (1988) S71-572. M. Hanabusa, Material Sci. Rep., 2 (1987) 51-97. Y. Matsumi, S. Toyoda, T. Hayashi, M. Miyamura, H. Yoshikawa and S. Komiyama, J. Appl. Phys., 60 (1986) 4102-4108. E.B.D. Bourdon, J.P. Cowin, I. Harrison, J.C. Polanyi, J. Segner, C.D. Stanners and P.A. Young, J. Phys. Chem., 88 (1984) 6100-6103. E.B.D. Bourdon, P. Das, I. Harrison, J.C. Polanyi, J. Segner, C.D. Stanners, R.J. Williams and P.A. Young, Faraday Discuss. Chem. SOC., 82 (1986) 343-358. F.L. Tabares, E.P. Marsh, G.A. Bach and J.P. Cowin, J. Chem. Phys., 86 (1987) 738-744. T.J. Chuang and K. Domen, J. Vac. Sci. Technol., A5 (1987) 473-475. D.L. Flamm, V.M. Donnelly and J.A. Mucha, J. Appl. Phys., 52 (1981 ) 3633-3639.

328

9 10 11 12

13

14 15 16 17 78

19 20 21 22 23 24 25 26 27 28 29 30 31

H. Okano, Y. Horiike and M. Sekine, Jpn. J. Appl. Phys., 24 (1985) 68-74. K.C. Pandey, T. Sakurai and H.D. Hagstrum, Phys. Rev., B16 (1 977) 3648-3651. R.J. Madix and J.A. Schwarz, Surf. Sci., 24 (1971) 264. N. Nishi, M. Shinohara and T. Okuyama, J. Chem. Phys., 8 0 (1984) 3898-3910. a) T. Kawai and T. Sakata, Chem. Phys. Lett., 69 (1980) 3336. b) D. Burgess, Jr., R . Viswanathan, I. Hussla, P.C. Stair and 2. Weitz, J. Chem. Phys., 79 (1983) 5200-5202. c) B. Schaeffer and P. Hess, Chem. Phys. Lett., 105 (1984) 563-566. D. Burgess, Jr., D.A. Mantell, R.R. Cavanagh and D.S. King, J. Chem. Phys., 85 (1986) 3127-3128. S.H. Lin, I.S.T. Tsong, A.R. Ziv, M. Symonski and C.M. Loxton, Phys. Scrip. , T6 (1987) 106-109. F.H.M. Sanders, A.W. Kolfschoten, J. Dielman, R.A. Haring, A. Haring and A.E. de Vries, J. Vac. Sci. Technol., A 2 (1984) 487-491. H. Metiu and J.W. Gadzuk, J. Chem. Phys., 74 (1981) 26412653. F . A . Houle, 3. Chem. Phys., 80 (1984) 4851-4858. T.J. Chuang, Surf. Sci. Rep., 3 (1983) 7-105. H.E. Cartland and G.C. Pirnentel, J. Phys. Chem., 90 (1986) 5485-5491. D.J. Ehrlich, R.M. Osgood, Jr. and T.F. Deutch, J. Vac. Sci. Technol., 21 (1982) 23-32. G.S. Higashi, L.J. Rothberg and C.G. Fleming, Appl. Phys. Lett. , 47 (1985) 1288-1290. H. Suzuki, K. Mori, M. Kawasaki and H. Sato, J. Appl. Phys. 62 (1988) in press. Y. Ritz-Froidevaux, R.P. Salathe, H.H. Gilgen and H.P. Weber, Appl. Phys., A27 (1982) 133-138. M. Kawasaki, H. Sat0 and G. Inoue, Jpn. J. Appl. Phys., 26 (1987) 1604-1605. L.E. Steenhoek and E.S. Yeung, Anal. Chem., 53 (1981) 528532. R.B. Cody, R.C. Burnier, W.D. Reents, Jr., T.J. Carlin, D.A. McCrery, R.K. Lengel and B.S. Freiser, Int. J. Mass Spectrom. Ion Phys., 33 (1980) 37-43. J.S. Uppal and R.H. Staley, J. Am. Chem. SOC., 102 (1980) 4144-4149. a ) W.I. Linlor, Appl. Phys. Lett., 3 (1963) 210-211. b) S. Namba, P.H. Kim, T. Itoh, T. Arai and H. Schwarz, Sci. Papers I.P.C.R., 60 (1966) 101-106. K. Toya, M. Kawasaki and H. Sato, Jpn. J. Appl. Phys. 27 (1988) in press. B. de B. Darwent, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 31 (1970) 1-48.

329

COq LASER INDUCED SURFACE REACTION M. KAWAI 1.

INTRODUCTION r e a c t i o n h a s been w i d e l y used t o s t i m u l a t e

Laser-induced

L a s e r s are also used t o probe molecular

gas-surface interaction.

d y n a m i c s i n h e t e r o g e n e o u s systems a s w e l l ,

I n t h e a p p l i e d area,

t h e laser photochemical techniques are successfully applied t o produce

w e l l defined

microstructures

m i c r o e l e c t r o n i c d e v i c e s (1).

and

new materials

for

Enhanced a d s o r p t i o n and c h e m i c a l

r e a c t i o n o n s u r f a c e s c a n b e a c h i e v e d by a p h o t o e x c i t a t i o n o f a d s o r b e d s p e c i e s as well as s o l i d s u b s t r a t e s .

gaseous molecules,

The modes o f t h e e x c i t a t i o n i n c l u d e v i b r a t i o n a l and e l e c t r o n i c s t a t e s of

t h e gaseous s p e c i e s and of

complexes.

the adsorbates surface

Both a s i n g l e a n d a m u l t i p l e p h o t o n a b s o r p t i o n may b e

involved i n t h e e x c i t a t i o n process. 2.

VIBRATIONALLY EXCITED PROCESSES Vibrationally

excited

processes

formation of semiconductor films,

can

One o f t h e

be

applied

in

the

e x a m p l e s t o demon-

strate t h e photoenhanced chemisorption and r e a c t i o n due t o t h e

v i b r a t i o n a l e x c i t a t i o n i s t h e i n t e r a c t i o n of s i l i c o n (2).

I n t h i s case,

SF6 m o l e c u l e s w i t h

SFg m o l e c u l e s c a n b e c h e m i c a l l y

a c t i v a t e d by m u l t i p l e p h o t o n a b s o r p t i o n o f CO2 l a s e r e i t h e r i n t h e g a s phase o r i n t h e adsorbed state.

D e p o s i t i o n of S i o n

q u a r t z o r g l a s s s u r f a c e c a n a l s o be s t i m u l a t e d by t h e decompos i t i o n o f S i H 4 e n h a n c e d b y t h e i r r a d i a t i o n o f C02 l a s e r t o t h e g a s p h a s e (3). E f f o r t s t o e l u c i d a t e t h e r e a c t i o n mechanisms o f t h e laseri n d u c e d s u r f a c e r e a c t i o n s h a v e b e e n i n c r e a s i n g d u r i n g t h e s e few years. I n o r d e r t o e l u c i d a t e t h e s e r e a c t i o n m e c h a n i s m s , f e a t u r e s of t h e r e a c t i o n on a s o l i d s u r f a c e s h o u l d be s t u d i e d i n d e t a i l , namely, e f f e c t of laser e n e r g i e s , laser f l u e n c e , c o n c e n t r a t i o n of adsorbed species,

t h e p r e s s u r e o f t h e g a s p h a s e a n d so on.

The photon-stimulated

surface i n t e r a c t i o n s are o f t e n divided

330 into three basic processes,

namely,

photons with (a) gaseous species,

the interaction of

(b) adsorbed s p e c i e s ,

A l a r g e number o f s t u d i e s

s o l i d s u b s t r a t e (4).

laser

and (c) a

concerning laser

induced d e s o r p t i o n have been d e m o n s t r a t e d s o far.

In the basic

t h e r e are f u n d a m e n t a l q u e s t i o n s c o n c e r n i n g e n e r g y t r a n s f e r

area,

from photons i n t o chemical.

The k i n e t i c s and t h e dynamic proce-

sses i n v o l v i n g energy a c q u i s i t i o n ,

s t o r a g e and decay

i n hetero-

g e n e o u s s y s t e m s n e e d t o be u n d e r s t o o d (4). A laser

desorption

beam s t r i k i n g a s u b s t r a t e c a n c a u s e r e a c t i o n o r

by d i r e c t h e a t i n g o f

the substrate,

resonant

excita-

t i o n of a n i n t e r n a l a d s o r b a t e v i b r a t i o n o r r e s o n a n t e x c i t a t i o n o f the external adsorbate-adsorbent

vibration.

E x c i t a t i o n of t h e

a d s o r b e d s p e c i e s h a s b e e n s t u d i e d i n t h e case o f p h o t o n s t i m u l a t e d d e s o r p t i o n o f p y r i d i n e f r o m Ag a n d K C 1 ( 5 ) , CH3F f r o m NaC1(6), CO f r o m N a C 1 ( 7 ) , S F g f r o m N a C 1 ( 8 ) , C 2 H 4 f r o m N a C 1 ( 9 ) , CC14 f r o m G e ( l 0 ) a n d

NH3 f r o m C u ( l 1 )

u s i n g C02 l a s e r . E x c i t a -

t i o n o f t h e v i b r a t i o n a l m o d e o f p y r i d i n e o r CH3F r e s u l t e d i n t h e desorption of

t h e s e molecules from t h e surface.

m e c h a n i s m of t h e s e p h o t o d e s o r p t i o n s

The e x c i t a t i o n

c a n b e e x p l a i n e d by a mecha-

n i s m t h a t t h e m o l e c u l e s were e x c i t e d t o t h e v = l l e v e l by t h e

l a s e r a n d b y t h e i n t e r m o l e c u l a r e n e r g y t r a n s f e r , a n d t h e y were excited t o higher l e v e l s s u f f i c i a n t f o r t h e desorption(l2-16). Apart

from desorption,

surface reaction with adsorbate can

b e s t i m u l a t e d by t h e laser i r r a d i a t i o n .

In this chapter we w i l l

d e m o n s t r a t e t h e f o r m a t i o n of n e w s u r f a c e s p e c i e s b y t h e C 0 2 l a s e r i n d u c e d r e a c t i o n o f CDF3 w i t h t h e s u r f a c e o f S i 0 2

(17,18).

In

o r d e r t o e l u c i d a t e t h e mechanism of t h e r e a c t i o n e s p e c i a l l y t o

i r s p e c t r o s c o p y was u s e d .

determine the surface species,

A

s y s t e m a t i c i n v e s t i g a t i o n was p e r f o r m e d i n c l u d i n g t h e d e t e r m i n a t i o n of r e a c t i o n y i e l d s as a function of t h e laser frequency,

the laser i n t e n s i t y and t h e gas p r e s s u r e as w e l l as t h e r e a c t i o n

products,

a n d t h e d e t e r m i n a t i o n of

the

correlation

between t h e

e x c i t e d s p e c i e s and t h e r e a c t i o n path.

3.

EXPERIMENTAL Si02(Cab-O-Sil

HS5)

s u b s t r a t e s were p r e p a r e d

s u s p e n s i o n o f SiO2 i n e t h a n o l plate,

by

spraying a

o n t o a s i n g l e c r y s t a l o f NaCl

t h e s i z e o f w h i c h was 2 cm i n d i a m e t e r a n d 3 m m t h i c k ,

f o l l o w e d by d r y i n g a t 1OO'Cin a i r . g l a s s I R r e a c t i o n cell.

T h e y were t h e n

mounted i n a

T h e s a m p l e was r o t a t a b l e i n t h e c e l l , s o

t h a t t h e a n g l e o f i n c i d e n t laser beam t o t h e s u r f a c e n o r m a l c o u l d

331

be varied from 4 5 t o 90degree.

T h e S i 0 2 s a m p l e t h u s m o u n t e d was

30 T o r r o f o x y g e n f o r 4 h r s ,

p r e t r e a t e d a t 4OO0C u n d e r

by t h e e v a c u a t i o n a t r o o m t e m p e r a t u r e .

followed

After t h i s pretreatment,

t h e S i 0 2 sample became f r e e from a c a r b o n i c r e s i d u e mainly o r i g i nated from t h e solvent used i n t h e mounting process. CDFj(Merck

Co.

D content

>

Gaseous

99%) w a s u s e d w i t h o u t f u r t h e r p u r i f i -

cation. A TEA C 0 2 l a s e r ( L u m o n i c s 1 0 3 - 2 ) w a s u s e d t o s t i m u l a t e t h e reaction.

I n o r d e r t o d e t e r m i n e t h e c o n f i g u r a t i o n and t h e

elec-

tronic state of the surface adsorbed species, I R transmission s p e c t r a (NICOLET FT-IR

5DX) a n d X-ray

HP 5950A) were o b s e r v e d .

p h o t o e l e c t r o n s p e c t r a (XPS;

The b i n d i n g e n e r g y of t h e e m i s s i o n peak

i n XPS w a s c a l i b r a t e d a s s u m i n g t h a t t h e Au 4 f 7 / 2 o f t h e e v a p o r a t e d Au f i l m on t h e s a m p l e t o b e 84.0 eV. 4.

RESULTS A N D DISCUSSION

OH 3750

0.005 a c C C

nL 0 VI

n

a

3 0

3700 2 10 wavenumber (cm-')

2700

F i g . 1. T h e I R a b s o r p t i o n s p e c t r a o f S i O 2 b e f o r e ( a ) a n d a f t e r ( b ) t h e l a s e r i r r a d i a t i o n i n t h e p r e s e n c e o f CDF3 v a p o r ( 1 2 L a s e r w a v e n u m b e r was 9 7 1 . 9 cm-'. Torr).

332

The I R s p e c t r u m o f S i 0 2 a f t e r t h e p r e t r e a t m e n t s h o w s a n a b s o r p t i o n b a n d a t 3 7 5 0 cm-l (OH(ads)).

due t o t h e surface hydroxyl group

Although S i 0 2 h a s v e r y s t r o n g Si-0

absorption i n the

l o w f r e q u e n c y r e g i o n , t h e r e g i o n b e t w e e n 1000 a n d 8 5 0 cm-'

t r a n s p a r e n t t o some e x t e n t . d u e t o C-D

is

G a s e o u s CDF3 h a s a n a b s o r p t i o n b a n d

w a g g i n g mode i n t h e r e g i o n o f t h e C 0 2 l a s e r .

When we i r r a d i a t e d S i 0 2 w i t h t h e 971.93

cm-I

,in

t h e p r e s e n c e o f t h e CDF3 v a p o r ,

laser l i n e , which c o r r e s p o n d e d t o t h e ab-

s o r p t i o n m a x i m u m o f t h e p b r a n c h o f C-D

w a g g i n g m o d e o f CDF3

m o l e c u l e , a n I R a b s o r p t i o n b a n d a t 2 7 6 2 cm-'

d u e t o t h e OD(ad)

s t r e t c h i n g mode o n t h e S i 0 2 s u r f a c e a p p e a r e d .

The i n t e n s i t y of

t h e a b s o r p t i o n band o f s u r f a c e OD g r a d u a l l y i n c r e a s e d w i t h t h e number o f i r r a d i a t e d laser p u l s e s ,

w h e r e a s t h a t o f t h e s u r f a c e OH

Fig. 1 shows t h e I R a b s o r p t i o n s p e c t r a o f S i 0 2 b e f o r e

decreased.

a n d a f t e r t h e l a s e r i r r a d i a t i o n u n d e r t h e CDF3 v a p o r ( 1 0 T o r r ) . A s i s c l e a r l y shown, OD i s f o r m e d by t h e l a s e r i n d u c e d r e a c t i o n

o f CDF3 a n d S i O 2 t o g e t h e r w i t h t h e d e h y d r a t i o n o f s u r f a c e OH. T h e X-ray

photoelectron spectra(XPS)

showed t h a t t h e s u r f a c e of

t h e S i 0 2 a f t e r t h e r e a c t i o n w i t h CDF3 b e s i d e s t h e o r i g i n a l S i a n d 0.

c o n s i s t e d of

F and C

T h e b i n d i n g e n e r g y o f F I S was

o b s e r v e d a t 6 8 8 . 4 eV, w h e r e a s t h a t o f C I S a t 285.2

eV a n d 2 8 7 . 4

eV was d u e t o t h e c o n t a m i -

eV. T h e e m i s s i o n p e a k o f C l S

a t 285.2

n a t e d c a r b o n on t h e s u r f a c e .

i t i s w e l l k n o w n t h a t a m o n g t h e CFx

compounds, t h e b i n d i n g e n e r g y of t h e C l S d i f f e r s s i g n i f i c a n t l y a c c o r d i n g t o t h e amount o f F t h a t bounds t o t h e c a r b o n a t o m (19). T h e b i n d i n g e n e r g y of C l s eV f o r CF a n d 2 8 9 . 7

r e f e r r e d f r o m t h e l i t e r a t u r e was 287.7

eV f o r CF2.

b i n d i n g e n e r g y (B.E.),

287.4

From t h e v a l u e o f t h e o b s e r v e d

eV,

i t w a s c o n c l u d e d t h a t CF was

f o r m e d o n t h e s u r f a c e , c o n c o m i t a n t w i t h t h e OD f o r m a t i o n . 4.1

D e h y d r a t i o n o f s u r f a c e OH; E x c i t a t i o n o f l a t t i c e v i b r a :

t i o n a l mode. The dependence o f t h e l a s e r f r e q u e n c y on t h e y i e l d o f t h e s u r f a c e r e a c t i o n i s examined.

The d e c r e a s e i n t h e a m o u n t o f t h e

I R a b s o r p t i o n b a n d o f s u r f a c e OH i s s h o w n i n F i g . 2 , i n w h i c h t h e d e h y d r a t i o n y i e l d o f s u r f a c e OH e x h i b i t e d t h e f r e q u e n c y depend e n c e similar t o t h a t of t h e a b s o r p t i o n p r o p e r t y o f t h e SiO2 s u b s t r a t e r a t h e r t h a n t h a t o f g a s e o u s CDF3. was

second

order

A[OH = A [ 0 H l 2 , t h i s reaction.

t o t h e amount

of

the

Dehydration yield

surface

OH,

that

is

w h i c h i n d i c a t e d t h a t t w o -OH w e r e i n v o l v e d i n Furthermore,

t h e e f f e c t of laser fluence t o t h e

333

aJ

V

C

b

11 L

0

v)

n a la00

1100

Wavenumber (cm-'1

900

F i g . 2. The dependence of t h e r e a c t i o n y i e l d of t h e d e c r e a s e i n t h e i n t e n s i t y o f s u r f a c e OH t o t h e l a s e r f r e q u e n c y . Absorpt i o n s p e c t r a o f S i 0 2 a n d g a s e o u s CDF3 a r e a l s o s h o w n i n t h e figure. (broken line).

y i e l d was e x a m i n e d .

The d e h y d r a t i o n y i e l d i n c r e a s e d exponen-

t i a l l y a g a i n s t t h e f l u e n c e of t h e i n c i d e n t laser, which s u g g e s t e d

t h e thermal process. concluded follows.

that

the

On t h e b a s i s o f a b o v e e v i d e n c e s , dehydration

of

surface

OH

i t was

proceeded

as

L a s e r l i g h t was a b s o r b e d b y t h e S i 0 2 s u b s t r a t e t o b e

t r a n s f e r r e d t o h e a t e n e r g y a n d t h e n t w o s u r f a c e OH g a t h e r e d together

t o f o r m H20 a n d w e r e d e s o r b e d r e m a i n i n g S i O o n t h e

s u r f a c e as follows. 2 Si-OH-,

4.2 bed

H20

+

Si-0-Si

R e a c t i o n o f s u r f a c e OH a n d CDF3;

E x c i t a t i o n of p h y s i s r r z .

species. T h e d e p e n d e n c e o f t h e l a s e r f r e q u e n c y o n t h e y i e l d o f OD

f o r m a t i o n showed s h a r p f r e q u e n c y dependence a t t h e wave l e n g t h w h e r e i t c o r r e s p o n d s t o t h e s t r o n g a b s o r p t i o n band o f t h e g a s e o u s CDF3 r a t h e r t h a n t h a t o f S i O q f F i g . 3). T h e O D f o r m a t i o n y i e l d

w a s f i r s t o r d e r t o t h e a m o u n t o f s u r f a c e OH.(Fig.

4)

T h e OD

f o r m a t i o n y i e l d vs laser f l u e n c e is shown i n t h e Fig.

5.

T h e r e a c t i o n y i e l d w a s ca. 3 r d o r d e r t o t h e l a s e r f l u e n c e . T h i s l e a d s us t o c o n c l u d e t h a t t h e s e p r o c e s s e s a r e n o t p u r e t h e r m a l

334

1100

1000

W av e nu m b e r ( cm-’)

9(

F i g . 3. The dependence of t h e r e a c t i o n y i e l d of t h e OD formation t o t h e laser frequency. Absorption s p e c t r a o f S i 0 2 and g a s e o u s CDF3 a r e a l s o s h o w n i n t h e f i g u r e ( b r o k e n l i n e ) .

processes but include 3 photon process. It is important t o n o t e t h a t u n l i k e t h e case of p h o t o d e s o r p t i o n of s u r f a c e OH, f o r m a t i o n was n o t s t i m u l a t e d by t h e h e a t i n g o f

t h e OD

t h e s u b s t r a t e by

irradiating the Si02 surface. T h e f o r m a t i o n o f t h e s u r f a c e OD was o b s e r v e d o n l y when t h e s y s t e m was e x c i t e d by t h e l a s e r o f t h e f r e q u e n c y t h a t c o r r e s p o n d e d t o t h e s t r o n g a b s o r p t i o n b a n d o f g a s e o u s CDF3(971.9

cm-l).

And t h e r e a c t i o n p r o c e e d e d o n l y when t h e t o t a l g a s e o u s p r e s s u r e exceeded ca> 6 t o r r .

Moreover,

when t h e g a s p h a s e a l o n e was

i r r a d i a t e d from t h e d i r e c t i o n p a r a l l e l t o t h e s u r f a c e ,

the yield

o f t h e s u r f a c e r e a c t i o n was l e s s t h a n o n e t e n t h o f t h e c a s e when t h e s u r f a c e was i r r a d i a t e d . the clean Si02 surface

,

T h e s e f a c t s seem t o s u g g e s t t h a t , o n

l a s e r was a b s o r b e d by t h e p h y s i s o r b e d

s p e c i e s o n t h e s u r f a c e o r by t h e m o l e c u l e s n e a r t h e s u r f a c e .

As

it was c l e a r l y s h o w n i n t h e case o f t h e d e h y d r a t i o n r e a c t i o n , t h e

s u r f a c e temperature under t h e i r r a d i a t i o n of increased extremely.

t h e laser l i g h t

is

I n t h e laser induced r e a c t i o n of s u r f a c e

OH a n d CDF3 o v e r S i 0 2 , t h e e x c i t a t i o n o f p h y s i s o r b e d CDF3 was indispensable. T h e r e a c t i o n b e t w e e n s u r f a c e O H a n d t h e g a s e o u s CDF3 a f t e r t h e f o r m a t i o n o f s u r f a c e OD(ad) a n d C F ( a d ) s p e c i e s s h o w e d d i f f e r -

335

- 1.,5

0 p ti c a I de nsi t ( log [OH) )

- q.:

,o

-1

t i

n

0

t a -5.0

F i g . 4. Y i e l d o f t h e O D f o r m a t i o n v e r s u s s u r f a c e OH c o n c e n t r a tion. L a s e r f r e q u e n c y was 9 7 1 . 9 3 cm-’ a n d t h e l a s e r f l u e n c e was 0. 95/cm2.

ent features.

The f r e q u e n c y dependence of t h e laser on t h e y i e l d

o f t h e r e a c t i o n b e c a m e b r o a d e r , T h i s may b e d u e t o t h e b r o a d e n i n g o f t h e v i b r a t i o n a l a b s o r p t i o n b a n d o f a d s o r b e d CDF3(ad) by t h e c h a n g e s i n t h e s u r f a c e of Si02.

Among t h e n e w l y f o r m e d s u r f a c e

s p e c i e s , OD(ad) s h o u l d b e c h e m i c a l l y e q u i v a l e n t w i t h t h e o r i g i n a l OH(ad).

Accordingly,

interacts

w i t h CDF3(ad)

gives t h e broadening

CF(ad) m u s t b e t h e s u r f a c e s p e c i e s t h a t o r p r e d o m i n a n t l y a d s o r b s CDF3 o n , of

the absorption

band

of

which

CDF3(ad).

A l t h o u g h t h e r e were g a s p r e s s u r e d e p e n d e n c e i n t h e i n i t i a l s t a g e , the

yield

pressure.

of

t h e OD f o r m a t i o n

It

is important

was

independent

on

the

CDF3

t o n o t e t h a t a b o v e f e a t u r e s were

o b s e r v e d o n l y when t h e SiOz s u r f a c e was i r r a d i a t e d .

It is

l i k e l y t o c o n s i d e r t h a t o n c e t h e s u r f a c e s p e c i e s were f o r m e d , t h e y i n f l u e n c e s t h e p r o p e r t y of t h e p h y s i s o r b e d CDF3(ad),

espe-

c i a l l y t h e v i b r a t i o n a l s t a t e of t h e a d s o r b e d s p e c i e s , which l e a d s t o the differences i n the

frequency dependence.

336

4.3

Energy t r a n s f e r As f o r t h e m e c h a n i s m o f

t h e photoinduced r e a c t i o n s ,

includ-

L i n e t . a 1 (13) h a v e p r o p o s e d a m e c h a n i s m t h a t I R laser induced photodesorption. I n t h e case o f laser induced d e s o r p t i o n , very narrow frequency dependence on t h e d e s o r p t i o n y i e l d h a s b e e n s h o w n f o r t h e d e s o r p t i o n o f CH3F f r o m N a C l b y H e i d b e r g (6). a n d f o r t h e p y r i d i n e f r o m KC1 a n d Ag s u r f a c e by C h u a n g (4). T h e b e h a v i o r o f t h e s e d e s o r p t i o n s wer e e x p l a i n e d by a mechanism t h a t t h e l a s e r e x c i t e s a n a n h a r m o n i c molecule t o t h e f i r s t e x c i t e d l e v e l from which h i g h e r v i b r a t i o n a l l e v e l s were a c h i e v e d by a n i n t e r m o l e c u l a r e n e r g y t r a n s f e r . The b e h a v i o r o b s e r v e d i n o u r s t u d y o f t h e s u r f a c e r e a c t i o n o f CDF3 o n S i O z was q u i t e s i m i l a r i n i t s l a s e r f r e q u e n c y a n d f l u e n c e d e p e n d e n c e o n t h e r e a c t i o n y i e l d . Here t h e f w h m v a l u e o f t h e f r e q u e n c y d e p e n d e n c e was 15 t o 20 cm- 1 Therefore, i t is n a t u r a l t o c o n s i d e r t h a t t h e I R laser induced s u r f a c e r e a c t i o n i n our case may h a v e t h e s i m i l a r mechanism t o t h a t of t h e p h o t o d e s o r p t i o n through t h e v i b r a t i o n a l e x c i t a t i o n of t h e adsorbed s p e c i e s o n NaCl o r on Ag. As s h o w n i n F i g . 5 , t h e y i e l d o f t h e s u r f a c e r e a c t i o n by t h e a d s o r b a t e e x c i t a t i o n e x i b i t e d grd o r d e r d e p e n ing desorption, deals with the

.

dence - t o t h e

e x c i t e d laser fluence.

This f a c t suggests that the

s u r f a c e r e a c t i o n b e t w e e n p h y s i s o r b e d CDF3 w i t h S i 0 2 p r o c e e d e d v i a the

jrd

e x c i t e d l e v e l o f t h e CDF3(ad).

On t h e o t h e r h a n d ,

Laser intensity( log J/cm2 )

aJ

c

U

cr

F i g . 5.

Y i e l d o f t h e OD f o r m a t i o n v e r s u s l a s e r f l u e n c e .

i f the

337

1050

rdoo

Wavenumber (cm-I)

950

Fig. 6 . Changes in the laser frequency dependence on the reaction yield caused by the different laser fluence. (a) 0.9 J/cm2 and (b) 0.7 J/cm2. excitation of the physisorbed species is a multiphoton process, then the frequency dependence on the reaction yield should change with the fluence of the laser. Fig. 6 shows the frequency dependence observed with different fluence of the laser. In the observed frequency dependence with defferent laser fluence, there is not a significant changes in the frequency where it gives the maximum yield. Accordingly, it can be concluded that the excitation process i s achieved via the excitation of the physisorbed species t o v=l state f r o m which higher vibrational levels a r e created by the intermolecular energy transfer. Once the desired vibrational level is reached, v=3 in the case of the reaction of CDF3 and the Si02 surface, reaction with the surface took place.

5.

CONCLUSION

338

T h e r e a c t i o n b e t w e e n CDF3 a n d t h e S i 0 2 s u r f a c e u n d e r t h e i r r a d i a t i o n o f t h e C02 l a s e r c o n s i s t s o f

two r e a c t i o n paths.

N a m e l y , d e h y d r a t i o n o f t h e s u r f a c e OH a n d t h e r e a c t i o n o f OH a n d p h y s i s o r b e d CDF3.

F o r t h e s u r f a c e r e a c t i o n o f O H a n d t h e CDF3,

i t i s i n i t i a t e d by t h e e x c i t a t i o n o f

p h y s i s o r b e d CDF3 t o t h e

f i r s t v i b r a t i o n a l s t a t e ( v = l ) , f o l l o w e d by a n e x c i t a t i o n t o v = 3

by a n i n t e r m o l e c u l a r e n e r g y t r a n s f e r .

These e x c i t e d s p e c i e s react

w i t h t h e s u r f a c e OH t o f o r m O D ( a d ) a n d C F ( a d ) .

The s u r f a c e

s p e c i e s f o r m e d by t h i s r e a c t i o n i n f l u e n c e t h e a b s o r p t i o n p r o p e r t y of t h e s u r f a c e and p l a y s

an important role i n the reaction

a f t e r the i n i t i a l stage. REFERENCES 1

2 3 4 5 6 7

8

9 10 11

12 13 14 15 16 17

18 19

A. W. J o h n s o n , D. J. E h r l i c h a n d H. R. R s c h l o s s b e r g ( e d ) ; Laser Controlled Chemical Processing of Solid Surface, Elsevier, N o r t h H o l l a n d ( 1 9 8 4 ) N. Y., A m s t e r d a m , L o n d o n . T. J. C h u a n g , J. C h e m . P h y s . 7 4 ( 1 9 8 1 ) 1 4 5 3 - 1 4 6 0 . M. H a n a b u s a , A. N a m i k i , K. Y o s h i h a r a , A p p l . P h y s . L e t t e r s , 35 (1979) 6 2 6 - 6 2 7 . T. J. C h u a n g , S u r f a c e S c i . 1 7 8 ( 1 9 8 6 ) 7 6 3 - 7 8 6 . T. J. C h u a n g , S u r f a c e S c i e n c e R e p o r t , 3 ( 1 9 8 3 ) 1. J. H e i d e b e r g , H. S t e i n , E. R i e h l , 2. S z i l a g y i a n d H. W e i s s , S u r f a c e S c i . 158 ( 1 9 8 5 ) 5 5 3 - 5 7 8 . J. H e i d b e r g , H. S t e i n a n d H. Weiss, S u r f a c e S c i . 1 8 4 ( 1 9 8 7 ) L431-438. J. H e i d b e r g , H. S t e i n , A. N e s t m a n n , E. H o e f s a n d I. H u s s l a , i n : P r o c . M a t e r . Res. S O C . S y m p . o n L a s e r - S o l i d I n t e r a c t i o n s a n d L a s e r P r o c e s s i n g , E d s . S. D. F e r r i s , H. J. L e a m y a n d J. M. D o a t s , ( A m e r i c a n I n s t i t u t e o f P h y s i c s , N.Y. 1 9 7 9 ) 49. J. H e i d b e r g a n d D. H o g e , J. O p t . SOC. A m . B4 ( 1 9 8 7 ) 2 4 2 - 2 4 7 . B. S c h a f e r a n d P. Hess, Chem. P h y s . L e t t e r s , 105 ( 1 9 8 4 ) 5 6 3 566. T. J. C h u a n g a n d I. H u s s l a , P h y s . R e v . L e t t e r s , 5 2 ( 1 9 8 4 ) 2045-2048. B. F a i n a n d S. H. L i n , S u r f a c e S c i . 1 4 7 ( 1 9 8 4 ) 4 9 7 - 5 3 6 . G. S. Wu, B. F a i n , A. R. Z i v a n d S. H. L i n , S u r f a c e S c i . 1 4 7 ( 1 9 8 4 ) 537-554. B. F a i n a n d S. H. L i n , Chem. P h y s . L e t t e r s , 1 1 4 ( 1 9 8 5 ) 4 9 7 502. T. J. C h u a n g , H. S e k i a n d I. H u s s l a , S u r f a c e S c i . 158 ( 1 9 8 5 ) 525-552. Z. W. G o r t e l , P. P i e r c y , R. T e s h i m a a n d H. J. K r e u z e r , S u r f a c e Sci. 1 7 9 ( 1 9 8 7 ) 176-186. M . K a w a i , Y. T s u b o i , K. T a n a k a , S. T e r a t a n i a n d K. T a y a , P r o c . o f t h e S y m p o s i u m o n D r y P r o c e s s E d s . J. N i s h i z a w a e t al., T h e E l e c t r o c h e m i c a l SOC., 8 8 - 7 ( 1 9 8 8 ) 3 1 0 - 3 1 6 . M. K a w a i , Y. T s u b o i , K. T a n a k a , S. T e r a t a n i a n d K. T a y a , s u b m i t t e d t o S u r f a c e Sci. P. C o d m a n , J. D. S c o t t a n d J. M. T h o m a s , C a r b o n , 1 5 ( 1 9 7 7 ) 75.

339

PHOTOCHEMICAL ASPECTS OF AMORPHOUS-Si NUCLEATION BY PHOTO-CVD H. HADA

and

M. KAWASAKI

INTRODUCTION There is little information regarding the surface chemistry involved in the nucleation of amorphous silicon by photo-induced chemical vapor deposition (photo-CVD). The reason seems to be that effective chemical and physical means of detecting a small 1.

amount of silicon are hardly available at present. In our laboratory, the initial process of amorphous silicon (a-Si) formation from silanes or disilanes on Si02 substrate by photo-CVD has been studied by a new technique of chemical amplification of Si nuclei to colloidal silver particles, by which the Si nuclei become observable and countable with a usual transmission electron microscope without necessitating a very high magnification. This method does not always provide atomistic information about the nucleation process of a-Si. However, the results obtained by this method will reflect the initial stage of a-Si formation and provide significant knowledge for the formation of a final a-Si film which is highly important in the LSI industry. The photo-CVD should be recognized as a photo-physical and photo-chemical phenomenon caused on the solid surface. In this chapter, therefore, first a general classification of photochemistry in which the solid surface is concerned - the general subject of the present publication - will be made. Then, our study on the initial process of photo-CVD a-Si formation on the solid surface will be specified. TOPOCHEMICAL CLASSIFICATION OF PHOTOCHEMISTRY ON SOLID SURFACE A topochemical classification of photochemistry at the interface between solid and gas or solid and liquid is

2.

illustrated schematically in Fig.1. In Fig.1 , the signs S*I etc. stand for the types of photochemical reaction at the interfaces. The S*I reaction implies that the electrons and/or positive holes produced in the inside of solid by light

340

V

Solid Phase

I.

A*S

I'S

S'A

S'G S'L

Fig. 1. Topochemical classif,icationof photochemical reactions :Position of light absorption. S*:Excited at interfaces. species in solid phase. G*,L*:Excited species in gaseous or liquid phase. A*: Excited species in adsorbed layer. S:Solid Phase. G,L:Gaseous or liquid phase. A:Adsorbed layer. 1:Chemical species at solid surface. n:Chemical species in adsorbed layer. Arrows show the directions of transfer of the energy states such as excited state, electron, positive hole, radical etc. Points of the arrows indicate initial reaction sites or chemical species. absorption diffuse to the solid interface and react with the surface species. The G*I reaction implies that the excited molecules or radicals etc. react with the solid surface or they are adsorbed on the surface. A*A reaction implies that the excited molecules or radicals etc. adsorbed on the surface diffuse along the surface and react with the other adsorbed species. The meanings of the other signs are to be recognized in the same manner as mentioned above, respectively. G*I, A*A, G*A, and S*A reactions would be involved in photo-CVD reactions depending upon materials used and experimental conditions employed. 3.

TECHNIQUE OF CHEMICAL AMPLIFICATION OF Si NUCLEI The rather unfamiliar method to detect small Si nuclei is based on the technique conventionally referred to as diffusion transfer physical development in photographic imaging science (1). More specifically it is modeled on the skillful experiment reported by Hamilton and Loge1 in which a similar technique was utilized with the aim of evaluating the minimum size of evaporated gold and silver clusters needed to initiate the physical development (2). The details of the technique have been

341

described elsewhere (3-51, but for reference, an overview of the procedures is restated hereinafter. The a-Si deposits on a substrate removed from the reactor are first coated with thin gelatin film. Since coalescence of the Si nuclei must be strictly avoided, the coating is made in a special manner. A thin gelatin film can be prepared in advance from aqueous gelat4n solution by a casting method on a cellulose acetate substrate from which the gelatin film is easily stripped mechanically. The gelatin film is floated on a water surface whereby the film expands about two folds or more in area. The substrate with the deposits down is then passed through the floating film from the air side, quickly withdrawn from the water, and dried. The development is done in a dish with the arrangement shown in Fig.2. An experimental silver bromide emulsion -coating, covered with a photographic developer containing a small amount of thiosulfate as silver halide solvent, is placed at the bottom of the dish. The gelatin coated substrate is separated by a spacer about 100Um from the top of the photographic coating to allow the developer access. During the development the silver bromide grains slowly dissolve to form free silver-thiosulfate complex ions, which diffuse toward the Si nuclei where they are selectively reduced to metallic silver particles. In other words, each of the Si nuclei is amplified to a colloidal particle of silver. If the gelatin film has been made thick enough, it can

gelat in Fig. 2

Arrangement for the development of Si nuclei to colloidal silver particle.

342

be stripped easily from the substrate. Furthermore, since the growth of the silver particle develops into the gelatin layer, they are all retained inside the stripped gelatin film. However, the thickness required for easy stripping from the substrate is too dense to transmit the electron beams of microscopes for general use. To solve this problem a small strip of the film is again floated on the surface of a slightly warmed aqueous solution of a proteolytic enzyme, and by making use of the film expansion and an etching action of the enzyme a thin enough specimen for TEM observation is finally obtained. The factor of the expansion is checked each time and used as a correction factor for the number density of nuclei counted from the electron micrographs. Experimental errors with respect to the number density are estimated to be 2 10~20%. Unfortunately, the minimum size of a-Si cluster that can be amplified by this method still remains unclear as we have not been able to measure effectively the too small total mass of the deposits. The number density of silver particles is, therefore, the only quantity directly accessible in this experiment, but the number density of Si nuclei and its variation with deposition parameters should be properly reflected on those of the developed silver particles. 3.

SOME EXPERIMENTAL RESULTS OBTAINED BY THE CHEMICAL AMPLIFICATION TECHNIQUE Raw gas materials used in our recent studies include both monosilane and disilane. Disilane has a practical advantage over monosilane since the absorption edge is located at a much longer wavelength (near 200 nm) than monosilane, so as to allow a wider choice of light source capable of causing dissociative excitation. Besides, the experimental results are quite similar in many respects for both parent gases. Therefore, our discussions in the remaining part of this chapter will be centered around the disilane photochemistry relevant to the a-Si nucleation. As can be inferred from the diversity of reaction types at the gas-solid interface shown in Fig.1, the choice of substrate material is also a key factor influencing the features of photochemical or photophysical processes involved in the a-Si deposition. In our experiments, the Si02 thin layer prepared by thermal oxidation of silicon wafers has been used as a substrate. The experimental results introduced hereinafter are those

343

derived in a gas flow system where 10% Si2H6/He (reactant gas) and pure He gas (to purge the reactor window) were admitted to the reactor at a constant flow rate of 50 and 100 sccm respectively, and a pulsed Xe lamp (about IlOmJ input energy per single flash) was operated at a frequency of 50Hz outside the reactor at a position about 21 cm above the substrate through the synthetic quartz window. Under almost all the deposition conditions we have studied, the number density of nuclei first increased approximately linearly to irradiation time with a negligible induction period, and then the rate of increase was gradually slowed down due to a saturation effect (3-5). We have to properly discriminate between a true induction period and only an apparent one. The latter is connected to the detection limit and means a period during which the nuclei are too small to be observed even if they actually exist. Provided that some induction period was detected, one must be very careful in judging its origin. Fortunately, we have not encountered such a situation, which in turn implies that the minimum size of Si nuclei detectable by the chemical amplification is actually very small, though precise size evaluation has not been successful, as stated before.

Fig. 3 The number density of nuclei (developed silver particles) at low ( ~ O S ) ,intermediate ( 1 6 0 ~ )and ~ high (320s) irradiation levels vs. substrate temperature. Total gas pressure is 3.5 torr.

344

The substrate temperature is the most important factor influencing these relationships between the number density and the total irradiation. Instead of reproducing a number of such response curves, the number densities at three typical irradiation levels(40, 160, and 320 s ) are plotted as a function of substrate temperature in Fig.3. The lowest irradiation level in Fig.3 is within or close to the linearly increasing initial part in the number density vs. irradiation time relationships, from which we can directly compare the initial nucleation rates at various substrate temperatures, in relation to the nucleation mechanism. The number densities at the intermediate and the highest irradiation levels in Fig.3 are more or less affected by the aforementioned saturation effect, and the very small increase of number density from that at 1 6 0 s to that at 320 s above 200 OC manifests a nearly complete saturation of number density with irradiation time under high enough substrate temperatures. Such a definite saturation of number density of nuclei has frequently been observed in a variety of deposition systems where the film growth obeyed island growth mode (6,7), and implies a high surface mobility of nucleation precursors at high temperatures. Indeed, when the nucleation experiment was done on a mechanically roughed Si02 substrate the saturation was observed no more due to the decreased surface mobility on the roughed substrate (5). On the other hand, the initial nucleation rate which can be evaluated from the number density at the lowest irradiation level in Fig.3 reveals a unique feature of the nucleation of photo-CVD a-Si. One can clearly recognize two different regions of substrate temperature characterized by the opposite temperature dependences. It seems quite rare that such a minimum shows up in the relation between reaction rate and temperature. It is strongly suggested that a different nucleation mechanism begins to work in the high temperature region assisted by thermal activation. One of the most important and fundamental questions in relation to the topochemical classification of Fig.1 is which contributes more to the a-Si nucleation, the gaseous phase photo decomposition or the decomposition of adsorbed molecules. The unambiguous answer to the question can be derived if we could eliminate illumination to the substrate surface without affecting excitation of gaseous phase molecules. Though it is hard to realize such a situation with the diffuse light, a similar

345

condition can be set by using a horizontal irradiation with respect to the substrate surface along with some auxiliary contrivances. We found that under the conditions where the illuminance on the substrate surface was reduced to less than one per cent of that under the vertical irradiation, the nucleation rate did not decrease so much as one would expect if the decomposition of adsorbed molecules play a significant role. This implies that for the a-Si nucleation from disilane on Si02 substrate active species (products of photodecomposition) are produced predominantly in the gaseous phase and transported to the substrate by diffusion through the gaseous phase. The effect of gas pressure on the nucleation rate gave further support to this conclusion. Fig.4 shows a typical example of number density vs. total gas pressure profile, where disilane partial pressure is varied in proportion to the total gas pressure. Because of the increase of reaction intermediates produced, the number density first increases with total gas pressure, but soon it decreases passing a maximum, most likely due to the increased loss of the active species by gaseous phase secondary reactions. Thus it follows that in the disilane/Si02 reaction system, G*I process in Fig.1 plays a decisive role.

Total gas pressure/torr Fig. 4 Dependence of the number density of nuclei deposited Substrate during 80s irradiation on the total gas pressure. temperature is 200 OC. Disilane partial pressure is varied in proportion to the total gas pressure.

346

Then what kinds of active species are transported to the substrate? The most useful answer to the question can be obtained by studying the effect of a radical scavenger. NO has widely been used as an effective radical scavenger for the purpose of studying the kinetics of gaseous reactions of silicon compounds (8-12). It is well-established that NO selectively reacts with silyl radicals and in the presence of silanes initiates a chain reaction in which polysiloxane is one of the main products (8,13). Our experiment demonstrates that a small amount of NO added to the reactor decreased the number density of nuclei significantly and it seemed that the a-Si nucleation could be almost completely suppressed by the radical scavenger (5). This means that silyl radicals are the predominant species that serve as nucleation precursors in the disilane/Si02 photo-CVD system. In the following section, we will first explain the reason for the peculiarity of the silyl radical based on the known photochemistry of silanes and then discuss the surface reactions in the early stages of the a-Si nucleation on Si02 substrate. 4.

PHOTOCHEMISTRY OF SILANES AND SURFACE REACTIONS 4-1. Photolysis of Disilane Photochemistry of silane and its derivatives has been investigated extensively and a great deal of knowledge has been accumulated. Perkins and Lampe studied the direct photolysis of disilane at 147 nm and proposed the following primary processes (11).

Si2Hfj + hv Si2H6 + hv Si2Hfj + hv

-

-

SiH2 + SiH3 + H

(a)

SiH3SiH + 2H

(b)

Si2H5 + H

(C)

The reported values of quantum yields for each of the above processes are 0.61 , 0.1 8, 0.21, respectively. Of course these quantum yields are not necessarily applicable if excitation energy is different. For example, according to the recent theoretical thermochemical data given by Ho and co-workers (14,15) the reaction enthalpy for the formation of a silylsilylene and two hydrogen atoms from disilane (decomposition (h)) is ahout 166 kcal/mol, which is equivalent to the photon energy at 172 nm. On the other hand, the dissociative excitation of disilane can be effected with even longer wavelength of up to

347

about 190nm or more, under which the contribution of the decomposition channel (b) should be negligible as long as the above reaction enthalpy is valid. Nevertheless, it is even more unlikely that under the irradiation of light longer than 147 nm a new reaction channel is allowed through which a species having higher coordination-deficiency than silylene is produced. Therefore, unless light shorter than 147 nm is used it would be sufficient to take only those active species appearing in (a)%(c) into consideration. It is well known that silylenes easily undergo the insertion reaction with silanes to form higher silanes. The rate constant of the insertion of SiH2 to disilane is estimated to be about 6x1 0-12cm3/s at 300K according to the Arrhenius parameters reported by John and Purnell (16). Provided that disilane partial pressure is 1.0 torr, the above rate constant results in the lifetime of SiH2 of about 5 us. The rate constant of the insertion of SiH3SiH was not available, but it would be similar to that of SiH2 and the same situation would also be anticipated for SiH3SiH. Such short lifetimes expected for silylenes mean that unless the gas pressure is decreased to an impractical value to increase the diffusion length, most of the silylenes produced in the gaseous phase are lost before they reach the substrate surface. Hydrogen atoms also react readily with disilane to form either H2+Si2H5 or SiH4+SiH3, with the reported rate constant of 3 . 7 ~ 1 0 - ~ ~ c mat ~ /32 s OC (17). It is worth emphasizing that from both reaction channels silyl radicals are created in addition to those directly produced by the photolysis. As can be expected from the above rate constant the lifetime of the hydrogen atoms is also very short. Though the average diffusion length of H atom can be much greater than that of silylenes because of its small mass and collision diameter, a high transportation efficiency is not expected yet. Silyl radicals SiH3 and Si2H5 can, in principle, also react with disilane. However, the only possible reaction is the hydrogen abstraction from disilane. This reaction seems to be a rather slow process necessitating more than a little activation energy (18). Moreover, even if it occurred, it would merely result in the reproduction of another Si2H5. Therefore, the only path through which the silyl radicals are lost in the gaseous phase is their recombination, so they can have a much longer

348

lifetime than silylenes and hydrogen atoms so as to allow a high transportation efficiency, because the concentration of silyl radicals is expected to be much smaller than that of parent disilane molecules under usual deposition conditions. In this way the peculiarity of the silyl radicals with respect to the transportation efficiency can be understood from the known gaseous chemistry of silicon compounds. A s a summary, we have shown a schematic flow diagram of the gaseous phase reactions and transportation starting from the photolysis of disilane in Fig.5.

Fig. 5 A flow diagram of gaseous phase secondary reactions initiated by the photolysis of disilane. Transportation of silyl radicals to substrate is shown by thick arrows.

Surface Reaction Now, there is almost no doubt that silyl radicals (SiH3 and Si2H5) are predominant nucleation precursors. However, they have only one dangling bond. How can they polymerize on the surface to form amorphous silicon hydride? We found that in the low temperature region the nucleation rate was increased by lowering the substrate temperature, and even at room temperature the polymerization could occur with a high efficiency (Fig.3). Obviously there must be a polymerization mechanism necessitating only a small activation energy. Of course when the substrate temperature is raised enough, a number of reaction channels will begin to be thermally activated to make the surface chemistry increasingly more diverse, which will be responsible for the reversal of the temperature dependence of the nucleation rate at 4.2

349

around 150 OC (Fig.3). On the other hand, in the low temperature region, there are not so many candidates for the possible surface reactions of adsorbed silyl radicals, so it would be worth discussing what they possibly are.

Fig. 6 A speculative surface reaction model between two adsorbed SiH3 radicals.

Fig.6 shows a speculative model for the surface reactions between two SiHj radicals. Here we have assumed that the main force for adsorption is afforded by interaction between the substrate and the dangling bonds of the surface reactants or products. R1 is the surface recombination between the silyl radicals to produce a vibrationally activated disilane, and R2 the deactivation to the ground state. It should be noted that, unlike the gaseous phase recombination, R1 will need some activation energy because in the configuration shown in Fig.6 the adsorption of SiH3 has to be loosened for the recombination to take place. It is not certain whether (SizHg)* is as highly activated as one formed by gaseous phase recombination. If so, a

350

disproportionation to form an adsorbed silylene (SiH2, or SiH3SiH) will be allowed as indicated by R3 in Fig.6. It may be questionable, however, that a highly activated species can stay on the substrate surface long enough to induce R3 because it seems that such a species will easily desorbs or the deactivation will be enhanced by the substrate. If the possibility of R3 can be disregarded the surface recombination plays only a negative role to act against the nucleation. R4 represents the hydrogen abstraction by one of the couples of radicals from the other to likewise form a silylene species. The dissociation energy of H atom from SiH3 to form SiH2 is estimated to be about 20kcal/mol less than the first H atom dissociation energy of SiH4 (14). Therefore, unless the adsorption energy of SiH3 is much greater than that of SiH2, R4 is exothermic, but since R 4 p l s o needs a direct participation of the dangling bond of SiH3, an activation energy greater than that for the similar reaction in the gaseous phase may be required. R3 and R4 represent reaction channels capable of producing a species having higher coordination-deficiency than the silyl radical, but they do not necessarily guarantee a high polymerization efficiency. If the silylene formed in this way coupled with a silyl radical, a higher silyl radical will be produced, but in the next step a similar process to convert it to a silylene-like structure is again necessitated for the polymerization to continue. Such a problem can formally be avoided by assuming a silylene-silylene coupling, but in that case it turns out that the actual nucleation precursor (the silylene) must be continuously supplied by the binary reactions of silyl radicals, so that a high polymerization efficiency is not expected. The reaction R5 in Fig.6, apart from its validity, is the most favorable one in view of polymerization efficiency. A thermochemical basis for R3 is the exothermicity of the process generally expressed as ( 1 9 ) ;

-

-SiH + HSi-Si-Si- + H2 It is worth noting that unlike R1 ?r R4, which are merely

(d) surface

versions of well known gaseous phase reactions, R5 is most probably allowed only in the adsorbed state where the dangling bond of SiH3 interacts with the substrate. In contrast, R1 and R4 are likely to be slowed down in the adsorbed state because of the direct participation of the dangling bonds in these reactions

351

as already pointed out. The existence of the coupling mode R S removes all difficulties arising from the fact that silyl radicals have only one dangling bond without reducing the polymerization efficiency, and is h e m e very attractive. We would like to point out that the hydrogen elimination by the process (a), as has been supposed elsewhere ( 1 9 ) , can easily explain the much lower hydrogen content in photo-CVD or plasma-CVD films of amorphous silicon hydride than that expected for the SinH2, composition. The temperature dependence of the nucleation rate in the low temperature region was characterized by a negative activation energy. Any process which promotes the nucleation, like surface diffusion of the precursors, invariably gives positive contribution to the apparent activation energy. For the net activation energy to become negative there must be some thermally activated process inhibiting the nucleation. A common cause of such negative activation energy, frequently observed in the nucleation process, is the loss of nucleation precursors due to thermal desorption. As for the present case, the desorption of the silyl radicals seems to be the major process which hinders the nucleation and, at the same time, strongly depends on the substrate temperature. Another process which may be partly responsible for the negative temperature dependence is the surface recombination R1. This is because when R3 is a minor reaction path R1 serves primarily as an inhibitive process against the nucleation, and due to the adsorption of the silyl radicals it is likely to require some finite activation energy, as stated before. As for the nucleation mechanism in the high temperature region, any discussion at the present stage would inevitably be even more speculative. Instead we give only two general predictions. First, the nature of surface reactions between silyl radicals will gradually resemble those in the gaseous phase, and a process like R 5 in Fig.6 will no longer be favored. Secondly, instead of the low temperature binary reactions, a unimolecular thermal decomposition of the silyl radical to species having higher coordination-deficiency should be taken into account as one of the possibilities, since some of the thermal decomposition paths of silyl radicals are likely to need smaller activation energy than that of parent silanes ( 1 4 , 1 5 ) and some catalytic effect of the surface can be expected.

352

In summary, we have introduced a new experimental technique for studying the very early stages of photo-CVD a-Si nucleation, and based on some of the results obtained by this method and the known chemistry of silane related compounds, some features of the gaseous phase and surface reactions relevant to the early stages of nucleation of photo-CVD a-Si have been surveyed. Acknow1edement:The nucleation experiments have been done under the support by the Ministry of Education, Science and Culture, Grant-in-Aid for Special Project Research No.62113004. The authors also thank Mr. A. Yamano of DAINIPPON SCREEN MFG Co. Ltd. for his cooperation. REFERENCES 1

2

3 4 5 6 7 8

9 10 11

12 13 14 15 16 17 18 19

G.1.P Levenson, in: T.H. James (Ed.), The Theory of Photographic Process, Macmillan, New York, 1977, p.466-p.480. J.F. Hamilton and P.C. Logel, Photogr. Sci. Eng., 18(1974) 507-51 2. M. Kawasaki, K. Hayashi, and H. Hada, OYO BUTURI, 55(1986) 606-61 1. M. Kawasaki, K. Hayashi, Y. Tsukiyama, and H. Hada, NIPPON KAGAKU KAISHI, 1987(1987) 1928-1933. submitted for publication J. Bloem, J. Cryst. Growth, S O ( 1980) 581 -604. J.A. Venables, G.D.T. Spiller and M. Hanbucken, Rep. Proq. Phys. 1 47(1984) 399-459. E. Kamaratos and F.W. Lampe, J. Phys. Chem., 74(1970) 2267-2274. A.G. Alexander and O.P. Strausz, J. Phys. Chem., 80(1976) 2531 -2538. G.G.A. Perkins, E.R. Austin, and F.W. Lampe, J. Amer. Chem. SOC. I 101 (1979) 1 1 09-11 15. G.G.A. Perkins and F.W. Lampe, J. Amer. Chem. SOC., 102(1980) 3764-3769. P.A. Longeway, R.D. Estes, and H.A. Weakliem, J. Phys. Chem., 88(1984) 73-77. J.P. Bare and F.W. Lampe, J. Phys. Chem., 81(1977) 1437-1441. P. Ho, M.E. Coltrin, J . S . Binkley, and C.F. Melius, J. Phys. Chem., 89(1985) 4647-4657. P. Ho, M.E. Coltrin, J.S. Binkley, and F.F. Melius, J. Phys. Chem., 90(1986) 3399-3406. P. John and J.H. Purnell, J. Chem. SOC., Faraday Trans. 1, 69 (1973) 1455-1 461 * E.R. Austin and F.W. Lampe, J. Phys. Chem., 81 1977) 1 1 34-1 138. J. T.L. Pollock, H.S. Sandhu, A. Jodhan, and O.P. Strausz Amer. Chem. SOC., 95(1973) 1017-1024. A. Matsuda and K. Tanaka, J. Appl. Phys., 60(1986) 2351 2356.

.

Chapter 7

TOPICS OF PHOTOCHEMISTRY ON SEMICONWCTING MATERIALS

Contents

7.1

Photoprocesses on Fractal Surfaces (Alon Seri-Levy, Joshua Samuel, Dina Farin, and David Avnir)

7.2

353

New Aspects in Area-Selective Electrode Reactions on Illuminated Semiconductors (Mitsutoshi Okano, Ryo Baba, Kiminori Itoh, and Akira Fujishima)

7.3

375

Photoluminescent Properties of Cadmium Sulfide Contacted with Gaseous Lewis Acids and Bases (Gerald J. Meyer, Elizabeth R. M. Luebker, George C. Lisensky, and Arthur B. Ellis)

7.4

388

Fluorescence of Dye Molecules Adsorbed on Semiconductor Surfaces (A. M. Ponte Goncalves)

403


353

PHOTOPROCESSES ON FRACTAL SURFACES A.

SERI-LEVY, J. SAMUEL, D. FARIN, and D. AVNIR

1. INTRODUCTION THE FRACTAL APPROACH TO PROBLEMS OF COMPLEX GEOMETRY.

The initial interest in Fractal Geometry stemmed mainly from the smking similarity between computer-generated objects, as formed by applying the rules of this geometry (1,2), and "real" objects as found in many natural or man-made objects. The algorithms used for creating the fractal objects are usually quite simple and involve an iterative construction procedure (Fig. 1). Because of this iterative procedure, the objects thus obtained have the property of being self-similar, i.e., that various magnifications of the object look similar. This. in turn, results in a simple relation between the magnification power and a measurable geometric feature of the object, say its length. The relation has the form of a power-law: length

0

magnificationD

Dl

where D, the fractal dimension, carries information on the degree of geomemc irregularity of the object. Let us look at two extremes for the case of a line: if it is a straight smooth line then magnifying it by a factor of two will double the number of, say pixels, necessary to present it; the D value in eq. [ 11 is 1, i.e., the familiar dimension of a line. Let us assume now that one has a line which is so convoluted and irregular that it actually fills the plane. In this case the relation between length and magnification will be through D=2 (actually D+2) in eq. [ 11. One can see therefore that the 1cDR range provides a measure for the degree of line irregularity. (Readers interested in a rigorous discussion which is beyond the intuitive picture presented here are referred to ref's 1.2). In order to test whether fractal geometry is applicable for problems of heterogeneous chemistry, one has to translate eq. [l] to actual experimental techniques in chemistry.

On a first level one can in principle use eq. [ 11 directly by microscopy image analysis techniques, for instance by measuring the fractal dimension of an irregular boundary line of an object (3). However, the accumulated experience in our laboratory has been that the real geometry as reflected, e.g., in the image of an object need not coincide with the effective geometry as "seen" by a specific chemical process. So while very useful at least for comparative purposes, we shall not discuss image analysis techniques here (these are reviewed in ref. 4) but concentrate only on effective geometries for (photo)chemical interactions. In order to do so, we first generalize eq. [ 11and write it in the form: (molecule-surface interaction parameter) = k (resolution of measuremmt)b

121

354

D = LOG 9 / LOG 5

=: 1.365

1

Fig. 1: The iterative process in forming a fractal line is shown. Each of the straight line segments of the top line (the generator) is replaced by a smaller version of the generator. Magnifying the generator by a factor of 5 reveals for each of the line segments 9 smaller new segments.

355

where p. as we shall see below, is a simple function of the effective D for the process. The generalization of eq. [2] over eq. [ 13 is through defining "magnification" as any procedure in which one observes a property or a process with a set of yardsticks varying in size, i.e.. one observes how a measurable property characteristic of molecule-surface interactions changes with the resolution of observation. Four types of yardstick-sets are discussed below: (a) The cross sectional area of a molecule interacting with a surface. (b) The size of an interacting particle (of an, e.g., dispersed metal catalyst). (c) The static intermolecular distances between adsorbed molecules. (d) The diffusional distances of molecules moving from the bulk volume (of pores) to the reactive surface. Obviously, use of the approach of q.121 can be justified only if it is indeed applicable for the analysis of chemical process on surfaces, and if p can serve for characterization of the effective geometry details and their role in affecting the process. We found that the yardstick sets (a) and (b) are quite general in heterogeneous chemistry (5.6); methods (c),(d), however, are still at an exploratory stage but some interesting results related to surface photochemistry are already at hand and are described below. In the following Sections we concentrate on results obtained in our laboratories. The applications of fractal geometry to problems of photochemistry and photophysics is spreading fast, and some examples of recent studies in other laboratories are collected in ref. 7.

2. THE USE OF MOLECULES AS YARDSTICKS. THE MOLECULAR ACCESSIBILITY OF A SUR-

FACE (89).

The first yardstick for the resolution analysis we discuss is the molecule interacting with the surface. For an irregular surface, the smaller the molecule is. the finer are the surface details it can probe and the larger will be the apparent surface area, A, as determined from monolayer coverage. In principle one can use the sensitivity of A or of n, the monolayer value. to changes in the size of the molecule (its radius, r, or its cross sectional area, a), as a measure for the degree of surface irregularity and molecular accessibility. A flat surface will be equally accessible to all sizes of adsorbed molecules, but the more irregular the surface is, the faster A or n will drop with increasing Q or r; and if the irregularity is fractal. then the sensitivity of A or n to changes in Q or r is given directly by (10):

n = b-D'

[3a1

where Da is the fractal dimension of the accessible surface for molecular interactions. The prefactor k, here and in all the equations below, has the value of the "property" (eq. [2]) for a

356

yardstick of unit length; its units change, of course, from equation to equation. For most cases 1cDaS3.Notice that for the classical two dimensional surone finds (Sa,6a,6b,6e-g,61,10-12,44) face, D,=2, A becomes independent of U. The consequences of the A-u dependency for photoprocesses of adsorbates on surfaces have been discussed in great detail in ref. 9; only the main points and conclusions are summarized here: (a) The Concept of Reaction Area. The N2-BET surface area (or any other value of A obtained with a small molecule) which serves routinely in surface photochemistry studies (13) is applicable only for smooth surfaces, for which the accessibility towards N2 and towards the larger photolabile molecule are equal. If this is not the case, then the use of the N2-BET area leads to erroneous values of intermolecular distances (too large), of the area occupied by an adsorbed molecule (too large) and of rate constants (too high). For several cases (13) it has been shown (9) that interpretations of experimental results may change, if the accessibility factor is neglected. For a bimolecular process, e.g., the collisional energy transfer: Q + B * +(QB)* + P there are three distinct available areas: The area accessible for Q. the area accessible for the excited molecule B* (which need not be equal to the area accessible to the ground state B), the area accessible for the product molecules, P, and, most important, the area accessible to the encounter complex (QB)*. Since the latter is the largest species in the reaction scheme, the area accessible to it is the smallest. In other words, regardless of the size of Q and B, the reaction is limited only to what we call the reaction area, AR:

AR = k N ~ ( Q B )

[41

This is perhaps the most pronounced effect of a reaction on a surface: Q and B cannot react along any point of their diffusional trails which pass in AQ or AB, but only when they collide in AR. Estimation of U(QB) is not a trivial task; perhaps the best approximate method for doing it is to assume (T(QB) = GQ + CSB or ~ ( Q B ) ap, and then to calculate these u valuesby one of the several methods available for that purpose, as reviewed in ref. 8. (b) The Distance between Two Adsorbed Molecules. There are three principal types of distances between two adsorbed molecules: First, the "aerial" distance through the bulk (solid and pore volume) of the material. This distance may be relevant for non-collisional energy transfers (radiative and non-radiative; see Section 4). Second, the shortest distance on the surface. For molecules of the same size, this distance, d, is given by: d = (NC)11'2

[51

where N is Avogadro's number and C is the concentration in units of moles/m2 of the available

357

surface area, A (and not the N2-BET area). In the case of two different molecules (say, a donor, B, and an acceptor, Q). then for a flat surface (13d.14): dQB = ( 2 / 3 ) d ~ ~

[61

On an irregular surface we encounter the following, perhaps unexpected phenomenon: Since AQ#AB, then

This is another unique feature of heterogeneous chemishy, compared to homogeneous situations. The third distance, perhaps the most relevant to reactions on surfaces, is the actual distance traversed by a diffusing molecule. This is a very complex issue which we only begin to understand. The diffusional distance reflects not only the geometric considerations made above, but also the facts that the surface is energetically heterogeneous, and that the diffusion is some combination of movements which follow closely the surface features, and of jumps from pore-wall to pore-wall and from one tip to the next. Obviously this diffusional distance is also a function of the temperature and of the solvent interfaced with the solid. Furthennore, since different types of connectedness can yield the same D value, this textural characteristic is an additional parameter to be considered (the fracton or spectral dimension '(15)). In view of this complex picture, what is then the practical advise? Under the current state of art, the best one can do is to get a preliminary estimate of d from eq's [4]-[6]; the direct observation of actual diffusional process in disordered systems, is still in its infancy. For some recent studies see ref. 16.17. (c) Estimation of Molecule-Surface Parameters From the Fractal Dimension and the N2BET Value. The recommended practice for the estimation of these parameters is to perform an actual measurement of the available surface area. If it has been established, however, that the surface is fractal, then these parameters can be estimated, using D, and the N2-BET value. An example given in Fig. 2; for many others see ref. (9). (d) The Application of the Accessibility Problem to Photochemical Studies. The correct estimation of surface concentrations and intermolecular distances is of course critical for the interpretation of bi-molecular photoprocesses. Applying the concepts described above, we have re-analysed a number of recent surface photochemistry studies (13). in which the N2-BET value was taken as a starting point. We showed that if the surfaces employed in these studies have different accessibilities to N2 and to the larger organic adsorbates, then conclusions (on static vs dynamic processes, for instance) change and can actually be reversed (9). Examples included the photodimerization of 9-cyanophenantrene, the chemiluminescent oxidation of adsorbed fatty acids, the triplet energy transfer from benzophenone to naphtalene and the pyrene excimerization in a pyrene-derivatized surface, all of these on porous silica surfaces for which molecular accessibility strongly depends on molecular size (18).

358

b2.0

Y)

C

t L

0

0.72-

d

\ a

Fig. 2: The ratio of apparent surface areas, A1/As, as seen by large, 01, and small, 0,. molecules. or the ratio of adsorbed moles, nl/n,. at the same surface concentrations (moles/available m2)as a function of 01/Gs.

2HzO + 2 W +

hv ___)

R

2w+2 Hz + 20H- + 2MV+2

h

8 P .$

‘3

2 0.90

~=1.93fo.08

-

3

3

3 8

3 0.60 -

0.30

2.00

I

2.20

I

2.40

2.b

2.60

Fig. 3: The dependence of the initial H2 evolution rate on the hydrodynamic radius of polyvinylalcohol supported Pt catalyst.

359 3. THE USE OF PARTICLE SIZES AS YARDSTICKS PHOTOCATALYSIS ON DISPERSED METALS.

Rather than measuring area with a set of molecules, one can pcrPorm an equivalent resolution analysis, namely, to take only one yardstick and to measure the area of the object as a function of its size (19). Both procedures amount to an observation at different magnifications. For instance, the area, A' per one spheroidal object of size 2R is given by: A' = k RDr

[81

(where the subscript r is added to emphasize the different technique by which D was measured). Theomically, for fractal objects D, = De,but for highly porous materials this need not coincide

(20). Not all of the surface sites which are available for the formation of a physisorbed blanket, necessarily participate in a (catalytic) reaction. In other words, in the case of reactions with a surface, the reaction area. AR. defined in Section 2, may be further diminished by chemical (and not only geomemcal) selectivities. The reactive sub-set of sites has a distinctive geometry of its own, and a recent finding we made is that not only is eq. [81 obeyed in many materials (6b,6g,Sh,6j,6n,60,11,12,18,2I ,23a24), but so do also their conjugated reactive areas (22-24):

n, = k RDR

[91

in which na is the number of active sites, and DR,defined as the reaction dimension, is a characteristic parameter of the heterogeneousreaction which quantifies the sensitivity of the process to the particle size. From the relation between Dr (eq. 181) and DR (eq.[9])one can elucidate useful information on the location of the reactive sub-set (23). For instance, if for a porous object DR a Dr,then one can assume a screening effect, i.e., that only the outer perifery of the particle reacts. The simplest way to count na is to use the surface equivalent of the mass-law (Wenzel law (23)), i.e., the assumption that the reaction rate, a, moletime-', is first order in n,: a = na

[I01

or: a=kRDR If the mass of the particle scales like R3 (20), then in units of mole time-'. gr-': ag = k RDR-3

[Ilal

As an example we re-analyse (25) the results of the study of Kiwi and Gratzel (26)on the

photocatalytic hydrogen production from water in the system Pt/dimethylviologen/EDTA, in

360

which the Pt was dispersed in polyvinyl alcohol (PVA). The activity was measured as a function of the hydrodynamic radius of the Pt-containing PVA. Analysis of the data according to eq. [lla] clarifies some of the debate over the correct interpretation, as appeared in ref. 27. The D~=1.93iO.08value obtained for this case, (Fig. 3). is quite close to D=2.0, the scaling exponent of an area of a sphere. This suggests that in the Pt-PVA particle, the only Pt crystallites which participate in the reaction are those which are exposed at the outer bounds of the polymer, whereas all of the Pt crystallites which are trapped inside are stericaly hindered and become inaccessible for the large organic reactants. Perhaps the most widely used dispersed photocatalyst is Ti02. We have analysed (25) a number of studies in which Ti02 particle size effect was recorded and found that the sensitivity of activity on particle size can be characterized by a typical DR value. Some examples: 1. Harada and Ueda have studied the photocatalytic decomposition activity of watedmethanol mixtures on particles of 5% Pfli02, generating H2 and C02 (29). They found sensitivity to particle size in the range 80-350A. and presented it by a straight line on logarithmic axes with a slope of -1.6 (Fig. 3 in ref. 28). A slope of that value means DR = 3-1.6 = 1.4, i.e., that only a sub-set of all surface sites is active in that reaction. 2. In a similar study, Sakata et al studied the photodecomposition of watedethanol (29). The preparation and size of the particles are quite different than the previous example. A DR value of 2.62 f 0.02 over three and a half orders of magnitude, was obtained for this case (Fig. 4). It seems unlikely that this high DR value reflects a highly irregular surface morphology, because the particles were prepared by crushing of large crystals (29); cleavage along crystal imperfections during the crushing procedure should not produce more than crystal-face imperfections (unless aggregation is also involved in the procedure). However, as discussed in detail elsewhere (23,30.31). a situation of DR > Dr is possible, although in such cases the pattern of distribution of active sites is not fractal. 3. Anpo et al have studied Ti02 sizeeffects on the photocatalytic hydrogenation of methylacetylene with water (32) and found a dependency of the quantum-yield on Ti02 anatase particle size. The relation between the Ar-BETsurface area and particle size obeys eq. [l la], (Fig. 3, but (Table 11 in ref. 32) with Dr=l.59rH).07, smaller than the possible minimal value for this case, Dr=2.0. Dr M + sol + kH20 Mxoysurf 1.

sol

+

376

Other investigations revealed the importance of this technique in the fabrication of microcircuitry ( 4 ) , which is now totally dependent on the photolithographic techniques. Electroless process (photocatalytic process) was also proved to be useful for the same purpose (5). Not only inorganic compounds such as metallic ions but also organic compounds react with the photogenerated electrons and (6). We obtained an image on Ti02 according to holes photoelectrochemical decomposition of organic compounds (7). Originally hydrophilic Ti02 surface was silylized by the reaction with trichloromethylsilane. The silyl group decomposes and the surface becomes the original hydrophilic surface when the electrode is irradiated. Thus obtained hydrophilic-hydrophobic pattern turns to a visible image if ink is applied to the surface. This procedure can be used for printing. In recent years, from a new point of view, works have been done using this characteristic of semiconductors (8-11). Those works, described in this section, adopted organic materials, especially polymers, in the method. Since organic materials have the wide variety of compounds, introduction of organic materials in the field has stimulated works to search for application of this technique in various ways. On the following pages, we describe (1)general reaction process, (2)photoelectrochemical deposition of polpyrrole, and (3)photoelectrodeposition coating. Deposition of polypyrrole by photocatalytic process has also been reported. However, we do not discuss it here because it is fully understood through the idea of photoelectrochemical process (12) though it is as important as the photoelectrochemical process. 2. CLOSE EXAMINATION OF REACTION PROCESS Irradiation of Semiconductor Electrode Area-selective irradiation of the semiconductor electrode requires irradiation through a mask or irradiation with a scanned laser beam. Considering the nature of light and the present technology level, it is impossible to obtain light beam whose diameter is less than submicrons. To the authors' knowledge, excitation of a semiconductor with any high energy electromagnetic waves such as X-ray has not yet successfully done. (Electromagnetic waves with shorter wave length can be focused on a smaller area.) Fate - -of Electron-Hole Pairs

311

Electrons and holes are generated by light with a certain depth profile, which is dependent on the light absorption of the semiconductor material. Those electrons and holes move according to a potential slope (which is perpendicular to the electrode surface). They also diffuse toward every direction because of thermal diffusion. Those electrons or holes, which survived and reached to the electrode surface, undergo electron transfer reactions. Diffusion lengths of electron and hole differ For instance, depending upon the semiconductor material (13). diffusion length of holes in single crystal silicon is 0.05 cm and that in polycrystaline Ti02 is ca. 100 i. Consequently, holes and electrons comes to the electrode/electrolyte solution interface with a certain area profile. After Electron Transfer Reactions Substrates in the electrolyte solution, after reacting with the holes or electrons on the electrode surface, deposit on the surface, or otherwise they begin to diffuse to the bulk solution. In case of polypyrrole, which is described in the following paragraphs, an oxidized pyrrole molecule diffuses and meets with another pyrrole molecule to react and finally becomes a high molecular weight polymer. Then the resultant polymer diffuses(?) and deposits on the electrode. Resolution of the deposited material is primarily determined by the contribution of above factors. 3.

DEPOSITION OF ORGANIC CONDUCTING POLYMERS It is well known that organic conducting polymers such as polypyrrole, polythiophene, and polyaniline can be deposited on electrodes by means of electrochemical polymerization, which is successfully carried out through oxidation of monomers in the solution (1 4 ) . These polymers have been investigated by many workers because (a)they have high conductivity in the doped state (15), (b)they show electrochromism (16), (c)they can be used as electrodes for secondary batteries (171, (d)they show semiconductor properties in the slightly doped state (18), and etc. Area-selective deposition of such polymers is important as (1)wiring material and device material in organic ICs (19) and (2)material for image formation using electrochromism. Here, we take polypyrrole as an example and discuss the areaselective deposition process.

378

Photoelectrochemical Deposition of Polypyrrole Before discussing the area-selective deposition of polypyrrole, photoelectrochemical (not area-selective) deposition of polypyrrole should be discussed. Polypyrrole was first deposited on a semiconductor electrode in order to protect it from photodecomposition ( 2 0 - 2 3 ) . However, the importance of the areaselective deposition was riot pointed out there. It is well known that pyrrole molecules are oxidized on a platinum electrode at potentials more positive than +0.6 V vs. SCE in aqueous solutions and the resultant oxidized molecules react to form polypyrrole on the electrode surface. The oxidation of pyrrole, however, takes place with less bias on an illuminated semiconductor electrode owing to the photosensitized electrolysis (24). Figure 1 is the current-potential relationship measured in M Na2S04 an aqueous solution containing 0.1 M pyrrole and 0.1 using a polycrystalline TiOZ thin layer electrode prepared by spray pyrolysis method ( 2 5 ) . 3.1

150

'

light' on o

N

6

U

100

I

--

50 0 0

E I V vs. SCE

i + 1.0

Fig. 1. Current-potential characteristics for oxidation of pyrrole on an n-TiOZ semiconductor electrode under irradiation and in the dark. In the dark, no anodic current was observed up to +1.5 V vs. SCE. (The current above +1.5 V is a leak current.) Under irradiation, however, an anodic photocurrent was observed at potentials more positive than -0.6 V vs. SCE. In fact, deposition of polypyrrole was observed on the semiconductor electrode when the electrode was kept at +0.5 V vs. SCE under irradiation. Therefore, the observed anodic current must include the current for oxidation of pyrrole. It was revealed in a separate experiment that most of the current is used for oxidation of pyrrole and that for the partial

379

oxidation (doping) of polypyrrole because the anodic photocurrent in the absence of pyrrole was negligibly small. The onset potential, -0.6 V vs. SCE is, of course, the characteristic of the Ti02 electrode and employment of other semiconductors gave other onset potentials. Only one requirement for the semiconductor electrode for this purpose is to have the positive upper valence band level which keeps the photogenerated holes positive enough to oxidize monomer molecules. In case of TiO2, even thiophene molecules, which have fairly positive oxidation potential, were easily oxidized. Polypyrrole grow at a constant current efficiency because the amount of polypyrrole was determined spectroscopically to be proportional to the charge passed through the circuit. This current efficiency was as high as that for the conventional electrochemical polymerization on Pt electrode. Therefore, the deposition rate is totally governed by the photocurrent. A s the deposited polypyrrole film absorbs the light, the photocurrent decreases gradually when semiconductor is irradiated from the solution side. Ti02 thin layer electrode employed above enabled irradiation from the back side because Ti02 is coated on a transparent Sn02 electrode. Cyclic voltammetry of thus prepared polypyrrole on Ti02 electrode showed oxidation and reduction current in a solution containing only supporting electrolyte accompanying change in color of the film. Thus, these currents can be assigned to the doping and undoping currents. Therefore, the conductivity of the deposited polypyrrole film can be controlled. 3.2 Area-selective Deposition Ilf. Polypyrrole N o w we are ready to discuss the area-selective deposition. Figure 1 suggested the possibility of area-selective deposition of polypyrrole when the electrode potential is set between -0.6 V and +1.5 v vs. SCE. In order to obtain a polypyrrole pattern on Ti02, the electrode was irradiated through a mask pattern with an expanded beam of He-Cd laser. The electrode potential was set at +0.5 V vs. SCE in order to avoid the ordinary electrochemical polymerization of pyrrole. The width of the smallest line in Fig. is 4 5 urn. To know the possibility of maskless pattern generation, the electrode was irradiated with a laser beam (the beam was ca. 1.0 mm in diameter), and the growth of polypyrrole on the electrode was observed. After passing 400 mC per 1 cm2 of irradiated area, the diameter of the polypyrrole deposit looked 2

380

Fig. 2. Picture of a photoelectrochemically formed pattern on an n-Ti02 electrode (black part is polypyrrole). the same as that of the light beam. However, the diameter became almost 8.0 mm after passing 5 C per 1 cm2 Probably, scattered light by the deposited polypyrrole was absorbed by Ti02 around the first deposition and that caused photoelectrochemical reactions there. 3 . 3 Functionalization of Deposited Films Since the conductivity of deposited polypyrrole film can be controlled as mentioned above, insulating, semiconducting, and Conducting patterns can be obtained. When such polypyrrole micro depositions with different conductivities are combined, more complicated and functional depositions are prepared. (However, such experiments have not yet been tried.) One of the attracting subjects is to make different polymers in layers, to make p-n junction with two slightly doped polymers. Fabrication of p-n junction with organic conducting polymers is widely investigated (18,26). However, all such p-n junctions have problems in stability. This is one of the reasons that a micro deposition of p-n junctions has not yet been tried. Another way to functionalize depositions, is to entrap functional molecules in polypyrrole. Incorporation of functional molecules during electrochemical polymerization an3 the application of so prepared films to sensors have been studied by many workers (27). By means of area-selective deposition much more complicated functional depositions can be prepared on an electrode. The authors have shown that organic dyes (an example of functional molecules) are incorporated in the process of photoelectrochemical deposition of polypyrrole ( 2 8 ) . Figure 3

.

381

shows the absorption spectrum of the polypyrrole film containing Rose Bengal which was incorporated in the film in the process of photoelectrochemical deposition. A spectrum of an aqueous Rose Bengal solution is shown together. Incorporation of Rose Bengal is indicated by the broad peak between 500 and 600 nm in the spectrum (a). The broadening of the peak is probably the result of the interaction between dye molecules and the polypyrrole matrix. Furthermore, the spectrum of the incorporated dye molecules changes depending upon the doping state of polypyrrole. Therefore, we have some chance to control the molecules in the film by changing the potential of the electrode. Experiments are under way to accomplish such control.

b

Q,

0

r-,

C

U

aJ V c

0.25

0

w

n

n

a 0

400

600 700 WaveIength I nrn

500

Fig. 3 . An absorption spectrum of the polypyrrole film cogtaining Rose Bengal (a) and that of 1 x 1 0 mol dm- Rose Bengal aqueous solution (b). 4.

PATTERN FORMATION BY THE PHOTOELECTRODEPOSITION COATING (29)

In this section a new method of polymer coating is presented which is also based on an area-selective reaction occurring on an illuminated semiconductor electrode. This method was designed to perform an area-selective electrodeposition coating (ED coating) which is widely employed in the automobile industry for the undercoat of a car-body. The deposited film by the ED coating is compact, firm and electrically insulating. Although the detailed mechanism of ED coating is not fully understood yet, it is generally accepted that the electrolysis of water is indispensable which leads to a pH change around the electrode to be coated. The electrophoresis of the resin particles (size of ca. 0.1 urn) toward

382

the anode in the case of anionic ED coating (or toward the cathode for the cationic ED coating) is also significant under the ( 150-250 V) Recalling a practically applied bias photoelectrochemical reaction on an illuminated semiconductor electrode (24) it is easily imagined that a photo-assisted ED coating (PED coating) can be performed when n-type semiconductors such a s Ti02 and ZnO are employed as a photoanode in contact with an anionic ED paint. Indeed, the PED coating was successfully demonstrated on illuminated Ti02 and ZnO thin-film electrodes at the bias of as small as +1.0 V vs. Ag/AgC1 with several kinds of anionic ED paint emulsions. Furthermore, a micro-patterned PED coating of a line was readily achieved in a preliminary width of ca. 100 pm experiment. Based on the properties of the PED films as mentioned above, the micro organic patterns thus obtained are applicable, for example, to a novel printing material, a photo-resist material for printed circuits, a material served for a color-filter.mosaic of liquid-crystal display devices when the PED patterns are incorporated with dyes, and possibly to organic IC's or sensor devices together with the conductive polymers like polypyrrole upon controlling their electric conductivity. and ZnO film electrode illuminated 4 . 1 PED patterning Utilizing the photoelectrochemical reaction occuring on an illuminated semiconductor electrode the area-selective deposition not only of a conductive polymer pattern, as described in a preceding section, but also of an insulating resin film (PED film) can be performed almost in a same manner with a common experimental setup. In principle the photogenerated electrons and holes are both available in these reactions, but the latters are much convenient to be adopted, since most of the stable semiconductors such as Ti02 and SrTi03 are often n-type materials. Thus an anionic PED coating was investigated on n-Ti02 and n-ZnO film electrodes in terms of the properties of the ED paints which affect the resolution and the thickness of the deposited PED-film patterns. The anionic ED coating is assumed to be induced by the reverse reaction of the electric dissociation of surface carboxylic groups on the resin particles of a paint, which is motivated by the accumulation of H+ ions upon the electrolysis of water (30); 2H 0 ---> 4H+ + O2 + 4e[41 2

.

x2

383

R-COO- + H+ --- > R-COOH [51 and the followings are also supposed to occur when the metal anode dissolves (31 ) M - - - > M n+ + ne161 n(R-COO-) + Mn+ - - - > (R-COOlnM 171 where R-COO- and M represent an anionic resin particle in an aqueous dispersion and the metal atom of an anode material, respectively. Thus the PED patterning on Ti02 electrode is supposed to proceed along eqs. [41 and [51, while eq. [ 7 ] is also valid in the case of ZnO electrode which undergoes an anodic photocorrosion under bandgap illumination ( 3 2 ) . In connection with the reaction mechanism of the anionic PED process it should be noted that the oxidative discharge of the paint resin can be involved in the ED and the PED processes as well, i.e. in the forms of the Kolbe reaction (eqs. [ 8 J , [ 9 ] ) or of another radical reaction (eq. [lo]) (33, 34). R-COO- - - - > 2R-CO0' - - - >

R-COO' + eR-R + 2C02

181

191 R-COOH + OH' [I01 R-COO' + H2 0 - - - > However, the reported analysis of the evolved gas at the anode have revealed that C02 gas was scarcely produced during an ED deposition onto a stainless steel plate with a current density of 2-4 mA/cm2 (35). Since the present PED pattern generation was 2 performed with not more than ca. 1 mA/cm , the Kolbe reaction was possibly not the main reaction path. Additionally, the reaction mechanism based on the H+ accumulation was supported by the fact that the dropping of 10 % aqueous HC1 solution into the paint emulsion gave a rapid coagulation of the paint resin around the acid droplets. A PED film of ca. 100 urn in line width and a few tenth of micrometer in thickness was readily obtained with each ED paint employed both on Ti02 and ZnO film electrodes which were anodically biased at +1.0 V vs. Ag/AgC1, where neither the dark current flowed nor even an ED film deposited. In the case of ZnO a better shaped line pattern was obtained compared to that of Ti02. This was probably because the ZnO film was dissolved photoelectrochemically at the borders of the deposited resin stripes and consequently a lateral growth of the PED film was prevented. On the other hand a slightly faded border of the PED pattern on Ti02 films was probably due to the throwing power which is characteristic of an ED resin used. The throwing power is the

384

1 0 0 Urn

(b)

100

urn

Fig. 4 Examples of the PED patterns on Ti02-film electrodes observed by means of a phase-contrast microscope. Triplicated lines of each deposited stripe of Fig.4a reflect the light The cracks seen on diffraction at the edge of the photomask. the right above in Fig. 4b appeared when dried. nature of ED paints to spread evenly over the substrate to be Under the controlled irradiation and the bias coated ( 3 1 ) . applied, however, quite a sharp line pattern of down to 20 Um in line width was successfully obtained even on Ti02 film electrodes (Fig. 4a). Furthermore] a PED film of as thick as ca. 10 um was deposited when a different ED paint was employed (Fig. 4b). The electric conductivity of a paint emulsion and the vitreous transition temperature of the paint resin are both supposed to affect the appearance of the resulted PED patterns. The conductivity of an ED paint can be controlled by adding a certain amount of an electrolyte till the concentration at which the salting-out starts. When a given amount of an electrolyte such as NaC104, NaI, Na2SO4] or triethylamine was added, a thicker but quite a damaged PED filrn was obtained, except in the case of NaI where oxygen evolution was partly suppressed. From this resu t a higher electric conductivity of an ED paint was found to contribute to a thicker PED film probably due to the accelerated photoelectrochemical reaction on the semiconductor electrode (the electric conductivity of ED paints employed was originally ca. 500-5000 uS/cm). On the other hand, however, the concurrent gas evolution, unless suppressed, will damage the film to be obtained. The vitreous transition temperature (Tg) of the paint resin can be one of the important properties in order to get an uniform film of a significant thickness. The resin is known to change its

385

characteristics with the ambient temperature ( T ) ; when T > Ty the resin particles are elastic and easy to stick to each other, and Therefore a when T < Tg the resin turns vitreous in nature. compact and finely resolved PED pattern is expected to be produced when the temperature of the paint emulsion is kept higher than Ty, and otherwise a porous but thick PZD layer will be obtained. As a preliniinary result, however, little differences were observed in the experiments at various temperatures (45, 25, and 5 o C ) using a series of the ED paints with different Tg‘s (36, 16, and -6 0 C ) . A possible explanation to this unexpected result is that the quick diffusion of H+ at higher temperature and the misted photomask at lower temperature have blurred the resolution of the PED gattern obtained. 4.2 Hybridized patterning with PED and polypyrrole As mentioned previously a hybridization of an insulatiny PED film arid a conductive polymer layer is quite attractive when they are applied to an electric device. In this case a hybridized multi-layered structure is expected to be fabricated, and the throwing power of the PED film can conveniently be utilized for this purpose. Additionally the photoelectrochemical areaselective clo2iilG anllor undoping of a polypyrrole filin as c!escriveC~ earlier iii this section eilcouracje us to i~ursuc “Ckis objective. The k e y probleln in the fabricatioii of a hyhriC:i--. LCC? rnulti-lLjered skrructure i s how to control the lesosi-kioaof a i’z!> uvercozt oileo E: poly,;Lri^jr~lelayei- zaC. vicc versa. FicJui-e5 t;ho::s th2 i>reli;.iiiiary r e s u l ’ t of a success5ullj CGiltrCllleC?. t€?ositiGil of the P E 3 fi1i.i onto t h e poly$yrrole line 2attei-il. As saan i i l ti?is fiC,,ure t h e overcoat wit;? a DXD fili;i $.id aot occurred on

Fig. 5 Preliminary results of ( A ) filling of spaces between polypyrrole (PPy) stripes with PED film, and ( B ) overcoating of PPy stripes with PED layer.

386

polypyrrole stripes in the case of A), while in the case of B ) it occurred. Although the experimental conditions, e.g. the applied bias voltage, the light intensity, the duration of PED, the thickness of the underlying polypyrrole film, and the variety of ED paints, are not established yet, this result suggests that an organic multi-layered structure combining a conductive polymer with an insulating resin can be achieved. 5.

FINAL REMARKS

The photo-induced area-selective reactions occuring on semiconductor electrodes can be utilized as a new method of the micro-patterning of polymer materials. We have shown here that both the conductive and the insulating polymers are deposited area-selectively on the electrodes employed, giving examples of photodeposition of polypyrrole and PED coating on Ti02. The organic materials have a wide variety of compounds and thus the photo-induced micro-patterning with organic polymers can be applied not only to an imaginy system but also to an integrated electronic device. In addition, when an organic semiconductor is employed as an electrode a total organic inteligent device can be realized combining the conductive and insulating polymers. REFERENCES 1

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T. Inoue, A . Fujishima, and K. Honda, Jpn. J. Appl. Phys.,

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T. Inoue, A . Fujishima, and K. Honda, J. Electrochem. SOC.,

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R. H. Micheels, A. D. Darrow, 11, and R. D. Rauh, Appl. Phys. Lett., 3 9 ( 1 9 8 1 ) 4 1 8 - 4 2 0 . T. L. Rose, D. H. Longendorfer, and R. D. Rauh, Appl. Phys. Lett., 4 2 ( 1 9 8 3 ) 1 9 3 - 1 9 5 . H. Masuda, M. Shimidzu, and S. Ohno, Chem. Lett.,

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33

34 35

S. M. Sze, "Physics of Semiconductor Devices," p.851 , John Wiley & Sons, Inc., New York(1981). A. F. Diaz and K. K. Kanazawa, J.C.S. Chem. Commun., (1979) 635-635. K. K. Kanazawa, A . F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwak, J. A. Logan, J. F. Rabolt, and G. B. Street, J.C.S. Chem. Commun., (1979) 854-855. E. M. Genies, G. Bidan, and A. F. Diaz, J. Electroanal. Chem., 149 (1983) 101-113. J. H. Kaufman, T.-C. Chung, A. J. Heeger, and F. Wudl, J. Electrochem. Soc., 131 (1984) 2092-2093. H. Koezuka, K. Hyodo, and A. G. MacDiarmid, J. Appl. Phys., 58 (1 985) 1279-1284. M. Hikita, 0. Niwa, A. Sugita, and T. Tamamura, Jpn. J. Appl. Phys., 24 (1985) L79-L81. R. Noufi, D. Tench, and L. F. Warren, J. Electrochem. Soc., 127 ( 1980) 231 0-2311. F.-R. Fan, B. L. Wheeler, A. J. Bard, and R. N. Noufi, J. Electrochem. Soc., 128 (1981) 2042-2043. R. Noufi, D. Tench, and L. F. Warren, J. Electrochem. Soc., 128 (1 981 ) 2596-2597. R. Noufi, A. J. Frank, and A. J . Nozik, J. Am. Chem. Soc., 103 (1981) 1849-1850. A . Fujishima and K. Honda, Nature (London), 238 (1972) 37. E. Kikuchi, K. Itoh and A . Fujishima, Nippon Kagaku Kaishi ( J . Chem. SOC. Jpn.), (1987) 1970-1973. K. Kaneto, S. Takeda, and K. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L553-L555. T. Shinidzu, and T. Iyoda, Membrane, 1 1 (1986) 71-82. M. Okano, K. Itoh, and A. Fujishima, Chem. Lett., (1987) 21 29-21 30. R. Baba, T. Manabe, N. Yokote, Y. Ishida and A. Fujishina, submitted for publication. S. Maeda, N. Hirai, H. Okada and K. Inoue, Denki Kayaku, 34 (1966) 705; F. Beck, Farbe und Lack, 72 (1966) 218; H. Schene, Methllober Flache, 22 (1968) 299; N. Yoshino and K. Ooyabu, Membrane, 2 (1977) 362. A. R. H. Tawn and J. R. Berry, J. Oil. C o l . Chem. A S S O C . , 48 (1965) 790; W. Funke, Farbe und Lack, 72(9) (1966). F. Lohrnann, Ber. Bunsenges. Phys. Chem., 70 (1966) 87; H. Gerischer, J. Electrochem. Soc., 113 (1966) 1174; J. Electroanal. Chem., 82 (1977) 133; A. J. Bard and M. S. Wrighton, Electrochem. Soc., 124 (1977) 1706. J. P. Giboz and J. Lahaya, J. Paint Tech., 42 (1970) 501; ibid., 43 (1971) 79; R. Matzuda, T. Hisano, T. Terazawa and N. Shinohara, Bull. Chem. SOC. Jpn., 35 (1962) 1233. S. R. Finn and C. C. Mell, J. Oil. Col. Chem. A S S O C . , 47 (1964) 219. Y. Nakamura, H. Kaneko, N. Higashiyama and H. Nozaki, Denki Kagaku, 36 (1968) 217; 36 (1968) 278.

J.

388

PHOTOLUMINESCENT PROPERTIES OF CADMIUM SULFIDE CONTACTED WITH GASEOUS LEWIS ACIDS AND BASES G.J.

MEYER, E.R.M. LUEBKER, G.C. LISENSKY and A . B . ELLIS

INTRODUCTION Despite the technological significance of semiconductorderived interfaces, much remains to be learned about their physicochemical properties. We have previously demonstrated that photoluminescence (PL) is a contactless, nondestructive technique for characterizing the electro-optical properties of the following interfaces: semiconductors in contact with electrolytes (11, other semiconductors (l), metals ( 2 1 , and covalently-bonded, redox-active species (3). More recently, by extending these studies to semiconductor-gas interfaces, we have found evidence for adduct formation at the semiconductor surface ( 4 , 5 ) . A unifying feature of all of these studies is the use of a deadlayer model for quantifying changes in PL intensity: electron-hole pairs formed within a distance on the order of the depletion width are separated by the electric field characterizing this region and thus do not contribute to PL (1). Using etched and cleaved single-crystal n-CdSe substrates, we found that the solid's PL intensity could be enhanced or diminished by exposure to Lewis bases and acids, respectively, relative to a N2 ambient. In the former case, the increase in PL intensity observed with a family of amines could be correlated with the basicity of the molecules. Moreover, Langmuir adsorption isotherms could be constructed from PL data to provide additional evidence for adduct formation and estimates of the formation constants governing the reactions. A key question raised by these studies involves the site(s) on the semiconductor surface participating in adduct formation; the ambient conditions employed mean that Cd, Se, 0 and H (from H2O) atoms could be participating in adduct formation. To investigate this question we sought to vary the chemical composition of the surface. A logical candidate for study is CdS, whose structural and electronic properties are similar to those of CdSe, but clearly 1.

389

reflect chalcogen substitution (6). We report herein on the PL properties of etched and cleaved single-crystal n-CdS substrates in contact with various gaseous ambients. We demonstrate that this compound behaves similarly to n-CdSe in its PL response to gaseous Lewis acids and bases. Use of Te-doped CdS (CdS:Te; 100 pprn Te) leads to gas-induced PL changes that parallel those observed with the undoped samples; the same fractional changes may be observed either in the band edge PL or in the Te-based subband gap PL, indicative of excitedstate communication in the solid. Alteration of the PL spectral distribution has been accomplished by exposing graded n-CdS,Sel-, substrates (x-1.0 at the surface, decreasing to 0.0 over a distance of -1 urn) to gaseous Lewis acids and bases; the colorcoded nature of the materials permits mapping of the effective electric field in the solids. Use of the samples comprising this study as chemical sensors is discussed. 2. EXPERIMENTAL 2.1 Materials and SamDle Preparation The following gases were used as received: ammonia (Matheson; 99.99%); methylamine (Aldrich; 98+%); dimethylamine (Aldrich; 99+%); trimethylamine (Aldrich;99+%); trifluoroamine (Air Products; 99.4%); perdeuterated ammonia (Cambridge Isotope; 98%); oxygen, nitrogen, hydrogen, helium, and argon (Badger Welding; 99.95%); and carbon monoxide (Matheson; 99.9%). Impurities in the amines were determined by mass spectroscopy using a Kratos MS80RFA mass spectrometer. Methanol, bromine (Fisher; 99.9%), and glacial acetic acid (Columbus Chemical; 99.7%) were used as received for etching. Potassium ferricyanide (Baker; 99.7%) and HC1 (Fisher; ACS Reagent grade) were used to prepare a solution filter. S i n g l e - c r y s t a 1 , v a p o r - g r o w n c-plates (10xlOx1 mm) of undoped n-CdS and 100-ppm Te-doped CdS (CdS:Te) with resistivity of -2 ohm-cm (four-point-probe method) were obtained from Cleveland Crystals, Inc., Cleveland, OH; the Te concentration is an estimate based on starting quantities. Graded n-CdSXSel-, samples (x-1.0 at the surface, decreasing to 0.0 over a distance of -1 pm> were synthesized as previously described (7). CdS and CdS:Te samples were prepared for use by either etching or cleaving. In the etching procedure, samples were scored and broken to -4~4x1-mm dimensions, alternately

390

wiped and etched (Brz/MeOH, 1:30 v/v, 5 s ) until the shiny Cd-rich, 0001 face was observed, ultrasonicated in MeOH for 15 min, and edge-suspended with DUCO cement from a glass rod. The samples were left in air until use. For the cleaving procedure, the CdS or CdS:Te crystals were simply cleaved parallel to the c-axis with a razor blade, mounted on a glass rod with the cement, and left in air until use. The 1-mm edge produced by cleaving was irradiated in PL measurements. 2.2 Gas-Handling ADparatus An all-glass setup for exposing n-CdS to various gases is shown in Fig. 1. The mounted crystal was placed in an adapter that served as a stopper for a 20-mm diameter tube that surrounded the sample. Two diametrically opposed stopcocks, placed near the

vent to hood

1,

I1

flowmeter 1'

ad!-

needle valve

gas in

Fig. 1. Apparatus used for measuring PL changes as a function of gaseous ambient. A more detailed description is given in the Experimental Section. bottom of the tube and below the semiconductor, served as the gas inlets, and a stopcock near the top of the tube and above the semiconductor served as the gas outlet and was connected to a Gilmont Model 11 glass flowmeter that was vented into a hood. One inlet was used for pure N z ; the other was used for gas mixtures and was connected to a Gilmont Model 10 flowmeter (or a Gilmont ruby-ball microflowmeter) and a cell bypass. Experiments were

391

conducted at atmospheric pressure and flow rates of 100 mL/min. Except for concentration studies, the gas of interest was mixed with N2 so as to be present as a 10% component (flowmeter values were corrected for gas density). Cells were cleaned between experiments in a saturated KOH/&-PrOH bath, rinsed with distilled water, and oven-dried at 100°C. In most experiments new gases were introduced without changing the sample geometry. 2.3 ODtical Measurements The irradiation source was the 458-nm line of a Coherent Radiation Model CR-12 Ar' laser. Laser plasma lines were eliminated by passing the beam through Oriel interference filters (fwhm of 10 nm, centered at the laser line). The laser beam was passed through a lox beam expander and masked to illuminate only the semiconductor surface. Laser intensity was measured with a Coherent Model 212 power meter and corrected for spectral sensitivity. Uncorrected, low-resolution ( - 3 nm) PL spectra were obtained with a McPherson Model 270, 0.35-111monochromator, equipped with a grating blazed at 500 nm, a R928 PMT, and an EG&G ratemeter; laser excitation was filtered by placing a Corning 3-71 or ferricyanide solution (-0.06 M ferricyanide, adjusted to a pH of - 0 . 3 with HC1) filter at the entrance slit to the emission monochromator. The PL signal w a s brought to the spectrometer using a 3-mm-diameter optical fiber. Spectra were displayed on a Houston Model 2000 x-y recorder. Typically, PL intensity was monitored by sitting at the band maximum (-510 nm), with the recorder operated in time-base mode as the gaseous ambient was varied. The detection optics were positioned between the incident and reflected excitation beams. RESULTS AND DISCUSSION In sections below we describe how the PL properties of n-CdS, n-CdS:Te, and graded n-CdS,Sel-, samples are affected by exposure to a variety of Lewis acids and bases. 3.1 CdS PL ResDonse The majority of our studies were conducted with the shiny Cd-rich, 0001 face of CdS that is produced by etching with Brz/MeOH. We also studied samples that were cleaved along the c-axis, exciting the newly-exposed surface. Ultraband gap excitation (Eg -2.4 eV(8)) produces green band edge PL 3.

392

(lmax -510 nm; fwhm -15 nm). Because the spectral distribution was unaffected by exposure to various gases, we were able to monitor PL changes by sitting at the band maximum. Exposure of n-CdS to gaseous NH3, ND3, (CH3)NH2, (CH3)2NH, and (CH3)3N causes enhancement in the PL intensity relative to the intensity in a N2 ambient. The PL signatures of the amines are shown in Fig. 2 for an etched sample; the ambient was alternated between N2 and 10% mixtures of the amines in N2. The Fig. 2 data are among the largest changes we have seen and illustrate the typical ordering of the PL response.

Fig. 2. Changes in PL intensity at -505 nm, resulting from exposure of an etched n-CdS sample to N2 (initial response) and the indicated amine. Superimposed on the plot is the original PL spectrum obtained in N2. Flow rates for all gases were 100 mL/min at 1 atm pressure. The sample was excited with 458-nm light. For both etched and cleaved samples, the magnitude of the PL increase follows the order NH3, ND3 < (CH3)NHz < (CH3)zNH > (CH3)sN; the reversal of the inequality with (CH3)3N serves to indicate that there is some variability in its position along the series but that it is always less t h a n (CH312NH. To more firmly establish the ordering of amines, we exposed individual n-CdS samples to pairs of amines, obtaining the same trend in each case; this result was independent of the order of exposure within a pair of amines.

393

With the exception of the reversal at (CH3)3N, the PL trend parallels the intrinsic basicity of the gases (9). Interestingly, such a reversal occurs in the aqueous basicities of this family of amines and is ascribed to a trend in hydrogen bonding that counters the trend in intrinsic basicities (10). In order to examine the role of hydrogen bonding, we compared the enhancement in PL intensity arising from NH3 and ND3. As shown in Fig. 3 , they are identical within experimental error. This suggests that hydrogen bonding interactions are not dominant in our system. Alternatively, the reduced enhancement with (CH3)3N may be due to steric effects. This notion is supported by the increase in PL response time with the degree of methyl substitution. We generally find that (CH3)zNH and (CH313N require the most time to produce their maximum PL enhancements and return to the N2 level.

Fig. 3. Changes in PL intensity at 505 nm, resulting from alternating exposure of an etched n-CdS sample to N2 (initial response) and the indicated amine. Superimposed on the plot is the original PL spectrum obtained in N2. Other experimental conditions are as described in Fig. 2 . While adduct formation is an appealing explanation for the PL changes we observe, we wished to find additional evidence for it. To this end we surveyed other gases, using the same flow setup used to study the amines. Using SO2 (1 atm), a Lewis acid, we found a reduction in PL intensity. Furthermore, with gases having

394

weaker or less well defined acid-base characteristics - He, A r , C O , and H2 - the PL signal remained within 5% of that observed in a N2 ambient. Our data thus suggest that good Lewis acids reduce PL intensity and good Lewis bases enhance it. 3 . 2 Mechanism The direction of these changes is consistent with data obtained using metal or metal oxide substrates: a gaseous base alighting at an acid surface site lowers the work function of the solid by donating electron density to it (11). As illustrated in Fig. 4, the reduction in work function for a semiconductor corresponds to a thinner depletion width and a more emissive sample, based on the dead-layer model. For this system, we can envision the nonbonding lone pair of electrons from an amine being donated to a vacant orbital at surface Cd or H sites, for example, to provide the bonding interaction needed for adduct formation. Conversely, a Lewis acid perched on the surface can withdraw electron density from the substrate, increase its work function, and thereby reduce its PL intensity. Besides altering the energetics of the interface, adduct formation can also affect recombination dynamics through the perturbation of the semiconductor's surface electronic structure.

b) Amine PL24,exp(.a'D2)

Interface Fig. 4. The dead-layer model for analyzing changes inPLintensity for two states, a) and b). As indicated in the figure, state a) corresponds to the PL intensity in a N2 ambient and state b) corresponds to the PL intensity in the presence of a gaseous amine The symbols CB and VB represent the solid's conduction andvalence band edges, respectively. For each state, the PL intensity is proportional to the amount of incident light (intensity 10; absorptivity a ' ) absorbed beyond the nonemissive layer whose thickness is D. The ratio of the two PL intensities leads to eq. 1.

395

We can quantitatively estimate the reduction in depletion width using eq. 1, PLi/PL2

=

exp ( - a ' A D ) .

[ll

In this expression, whose origin is indicated in Fig. 4, PL1 and PL2 are the PL intensities in N2 and amine ambients; AD is the corresponding change in dead-layer thickness (equated with the change in depletion width); and a ' = a + 0 , where a and 0 are the absorptivities for the exciting and emitted light; this treatment assumes that the surface recombination velocity is either very large or insensitive to the introduction of the amine (1). Ideally, accord with the dead-layer model is assessed by determining whether a range of interrogating excitation wavelengths leads to a constant value for AD. However, the ultrabandgap laser lines available correspond to only a modest change in a', limiting our ability to check the validity of the dead-layer model for this material. If we assume that the model is applicable, as it was to etched n-CdSe samples ( 4 , 5 ) , then the literature values of a and 6 (-9 x lo4 and 5.5 x lo3 cm-l at 458 and 510 nm, respectively ( 8 ) ) lead to representative reductions in depletion width ranging from -200 1 for NH3 ( 2 0 % enhancement) to -400 H for (CII3)2NH (40% enhancement). Caveats to our data concern thermal effects and gaseous impurities. We believe thermal effects are minimal for several reasons: 1) heating the sample causes PL intensity to decline, not to increase; 2) at the low incident powers employed (-1-4 mW/cm2) and with the convective cooling supplied by a substantial gas flow rate (100 mL/min), the sample should remain near ambient temperature (in fact, even when the incident intensities were increased to -10 mW/cm2, the PL enhancements were unchanged from their values at 1 mW/cm2); and 3) there appears to be no correlation between PL response and thermal conductivity of the ambient gas (4,12). We emphasize, too, that in most of our experiments we are only changing the gas composition by 10%. The effects of gaseous impurities were also examined. Mass spectroscopy reveals that the methyl-substituted amines contain traces of other methyl-substituted amines. While there may be an effect on PL intensity from these other alkylated amines, their PL signatures are sufficiently different (Fig. 2) as to imply domination by the principal gas present. Besides these gases, we assessed the effects of 02 and H20 vapor. Oxygen decreases PL

396

intensity relative to N2, typically by 5-15%. On the other hand, when the cell is "spiked" with a few drops of water, evaporation results in a slow, steady PL increase. At this point, exposure to NH3 gives only small PL changes. However, once the sample and cell have been dried in a stream of N2, the original PL response to NH3 returns. The ability to see reproducible PL signals upon cycling is important for chemical sensing (vide infra). In carrying out our measurements, we noted that in some instances reproducible cycling between N2 and amine ambients does not occur immediately. Instead, the PL intensity in N2 often shifts after the first several exposures of CdS to the amine, and it is at this shifted level that cycling occurs. We are uncertain as to why this "conditioning" period occurs in some cases; it may reflect some form of surface reconstruction. It is worth noting that some CdS samples exhibit red emission that dominates the PL spectrum as a broad band with Xmax -700 nm. This band was not studied in detail but its intensity was also enhanced by exposure to amines, albeit to a lesser extent than was found for the band edge PL. 3.3 Comparisons with n-CdSe Our general observation is that lattice substitution of Se with S has little effect on the PL changes observed. The direction and relative magnitude of PL changes are the same for the two solids for all of the gases examined. The lone exception occurs with NF3. For n-CdSe, 10% NF3 gave PL enhancements of -20% (5). With n-CdS, however, we saw no net increase within experimental error. Occasionally we saw a slight initial increase in PL intensity with NF3, but this response quickly returned to the N2 level. While we cannot say for certain whether n-CdS exhibits a response to NF3, the effect is plainly smaller than observed for NH3, as was the case with n-CdSe. The general insensitivity of PL changes to the identity of the chalcogen is intriguing and suggests that these atoms provide roughly interchangeable surface binding environments. 3.4 CdS:Te PL Response Samples of 100 ppm CdS:Te afford an opportunity to examine the effect of gaseous ambient on PL arising from intraband gap states. As shown in Figs. 5 and 6 for etched and cleaved samples, respectively, the PL of CdS:Te consists of two bands: edge

397

emission at 510 nm and a broad band with Xmax -600 nm. The latter has been assigned to a transition involving intraband gap states introduced by lattice substitution of Te for S; holes trapped in these states, estimated to lie -0.2 eV above the valence band edge, can recombine with electrons in or near the conduction band to yield the observed emission (13). Scheme I incorporates these assignments into a crude energy diagram, wherein filled and open circles represent electrons and holes, respectively; the symbols ECB, EVB, and ETe denote energies of the conduction and valence band edges and Te states, respectively. Variable temperature PL studies revealed that the two excited states leading to the two PL bands appear to be thermally equilibrated (13). Conduction Band

-510 -600 nm nm

-2.4 eV

Valence Band

Scheme I

Wavelength,nm Fig. 5. PL spectral distributions of an etched n-CdS :Te sample in the indicated gaseous ambients. The sample was excited at 458 nm.

398

Wavelength, nm Fig. 6 . PL spectral distributions of a cleaved n-CdS:Te sample in the indicated gaseous ambients. The sample was excited at 458 nm. Figs. 5 and 6 reveal that both PL bands are affected by exposure to amines and S 0 2 . Quenching of PL intensity with SO2 and enhancement with amines follows the pattern observed with undoped n-CdS. Moreover, the fractional changes in the two PL bands are roughly equal. This is most easily seen in F i g s . 6 and 7 , the latter displaying 25% enhancements in both peaks in passing from N 2 to 10% N H 3 ambients. Fig. 7 also demonstrates that the two PL bands display a comparable temporal response. The parallel response of the two bands is reminiscent of the behavior of CdS:Te electrodes: increasingly positive applied potential quenched both bands in parallel, an effect observed at several temperatures ( 1 3 ) . The existence of communication between the emissive excited states was inferred from this result, and we would draw the same conclusion here: the notion that the two excited states are interconverting means that whatever processes are responsible for increasing the population of one state in amine ambients will increase the population of the other state proportionately.

399

Fig. 7. Changes in PL intensity at 508 and 600 nm, resulting from alternating exposure of an etched n-CdS:Te sample to N2 (initial response) and 10% NH3. Superimposed on the plot is the original PL spectrum obtained in N2. Other experimental conditions are as described in Fig. 2 . The first three gas cycles were monitored at 508 nm and the last three at 600 nm. 3.5

Concentration DeDendence The concentration dependence of the PL response provides another means for characterizing adduct formation and can be used for constructing optically-coupled chemical sensors. Ammonia is a particularly appealing case, since, as shown in Figs. 2 and 3 , the substrate responds within seconds to changes from N2 to NH3 ambients. Fig. 8 presents a working response curve for NH3, showing that a PL enhancement is readily seen at a partial pressure of 5 x 10-4 atm and that the enhancement increases monotonically to 1 x 10-2 atm. At 1 atm pressure we often see a large initial response that decays to a substantially lower value. Remote sensing can be accomplished by use of a bifurcated optical fiber: the sample can be excited through one leg of the optical fiber and PL detected, after filtering the exciting wavelength, through the other leg.

400

'i Fig. 8. PL response monitored at 508 nm of an etched n-CdS:Te sample to NH3 relative to N2 as a function of the partial pressure of the gas in a NH3/Nz mixture. The inset shows the regime over which the PL change is linear. Flow rates of 100 mL/min were used in all experiments. The sample was excited using 458-nm light. 3.6 Graded CdSx&l-xPL - -

Response Inhomogeneous semiconducting substrates provide an opportunity to alter the PL spectral distribution upon exposure to various gases. We have demonstrated this effect using graded CdS,Sel-, substrates (x -1.0 at the surface, decreasing to 0.0 over a distance of -1 pm). The surface of these materials is CdS-like, leading us to expect similar interactions with gaseous ambients to those observed with CdS and CdS:Te. As shown in Fig. 9 , the graded solids emit from -510 to 720 nm, consistent with PL contributions from all of the CdSXSel-, alloy compositions comprising the graded region: Studies of homogeneous CdS,Sel-, samples (14) show that these solid solutions emit with band maxima given by eq. 2 , Amax (nm) = 718 - 210

X.

C2l

The color-coded nature of the PL in conjunction with Auger electron spectroscopy (AES)/depth profile analysis (which spatially resolves the alloys relative to the surface) permits mapping of the effective electric field (EEF) in the solid; the term "effective electric field" reflects the idea that the field has contributions from, for example, band-edge and effective-mass gradients in addition to contributions from band bending (7).

401

r

500

6oonm

Wavelength

700

Fig. 9 . Changes in PL intensity resulting from exposure of a graded n-CdSXSel-, sample sequentially to N2, NH3, N2, S 0 2 , and N2. Other experimental conditions are as described in Fig. 2. Fig. 9 demonstrates that exposure of the graded substrates to NH3 and SO2 causes asymmetric enhancement and quenching, respectively. The spectral changes correspond to a modest color change. Furthermore, the fact that the curves all coalesce at about 600 nm indicates that the EEF is influenced to a depth of about 0.2 pm: the composition leading to 600-nm PL,-CdSo*5Se0.5, occurs at about this distance from the surface based on AES data. Our expectation was that repetition of this experiment with the more basic amines would affect the EEF to greater depths (coalescence at longer wavelengths for spectra like those in Fig. 9 ) . However, spectral noise from these weakly emissive samples has frustrated our attempts to clearly see this effect thus far. CONCLUSION Samples with CdS-like surfaces exhibit similar responses to gaseous ambients as have been observed with CdSe. The enhancements and diminutions in PL intensity observed with Lewis bases and acids is of both fundamental and technological interest. In the former case, PL provides a simple in situ technique for correlating physicochemical interactions occurring at the interface with changes in electronic structure. In the latter case, the technique is readily adapted to opticallycoupled chemical sensing. Studies in progress have as their 4.

402

objective the use of other combinations of gaseous molecules and semiconducting surfaces to better map the steric and electronic properties of these interfaces. ACKNOWLEDGMENT We are grateful to the Office of Naval Research and the 3M Company for generous support of this research. We thank Dr. J. Brown for obtaining the mass spectroscopic data and Mr. Steven Zuhoski for helpful discussions. REFERENCES A.B. Ellis, in: R.B. Hall and A.B. Ellis (Eds), Chemistry 1 and Structure at Interfaces: New Laser and Optical Techniques, VCH Publishers, Deerfield Beach, FL, 1986, Chapter 6. M.K. Carpenter, H. Van Ryswyk, and A.B. Ellis, Langmuir, 2 1 (1985) 605-607. 3 H. Van Ryswyk and A.B. Ellis, J. Am. Chem. Soc.,108 (1986) 2454-2455. 4 G.J. Meyer, G.C. Lisensky, and A.B. Ellis, Proc. of the Electrochem. SOC.,87-9 (1987) 438-448. G.J. Meyer. G.C. Lisensky, and A.B. Ellis, J. Am.Chem. SOC., 5 in press. 6 M. Aven and J. Prener, Physics and Chemistry of 11-VI Compounds, North-Holland Publishing Ca, Amsterdam, 1967. M.K. Carpenter, H.H. Streckert, and A.B. Ellis, J. Solid 7 State Chem., 45 (1982) 51-62. 8 D. Dutton, Phys. Rev.,112 (1958) 758-785. D.H. Aue and M.T. Bowers, in: M. T. Bowers (Ed.), Gas Phase 9 Chemistry, Vol. 2, Academic Press, New York, 1979, Chapter 9 . 10 J.E. Huheey, Inorganic Chemistry, 3rd ed.; Harper & Row, New York, 1983, pp. 299-302. 11 P.C. Stair, J. Am. Chem. Soc.,104 (1982) 4045-4052. N.V. Tsederberg, Thermal Conductivities of Gases and 12 Liquids, MIT Press, Cambridge, MA, 1965, p. 89. B.R. Karas, H.H. Streckert, R. Schreiner, and A.B. Ellis, 13 J. Am. Chem. SOC., 103 (1981) 1648-1651. H.H. Streckert, M.K. Carpenter, J. Tong, and A.B. Ellis, 14 J . Electrochem. Soc.,129 (1982) 772 -780.

403

FLUORESCENCE OF DYE MOLECULES ADSORBED ON SEMICONDUCTOR SURFACES A. M. PONTE GONCALVES

INTRODUCTION Adsorbed dye molecules may be used to sensitize wide bandgap n-type semiconductors to visible light, (1,2) and thus make them potentially useful as electrodes in photoelectrochemical cells for Sensitization occurs when the solar energy conversion ( 3 , 4 ) . lowest excited singlet state of the dye, D*, is above the conduction band of the semiconductor, in which case the molecule becomes oxidized by injecting an electron into the semiconductor. Bending of the conduction band at the semiconductor-electrolyte junction promotes escape of the injected electron into the bulk of the semiconductor. Operation of the cell is sustained by a reducing agent present in the electrolyte, which reduces D+ and regenerates the dye. The injection efficiency, @i, is governed by the competition between injection of an electron into the semiconductor (rate constant ki) and other processes which shorten the lifetime of D* (rate constant ka): 1.

The escape efficiency, @,, is determined by the competition between escape of the injected electron away from the surface (rate constant ke) and any other processes, such as trapping at surface states, which favor recombination of the electron with D+ (rate constant kr):

The overall scheme is

t

ka

kr

I

r31

It is assumed here that the light intensity is weak enough for the relative concentration of D+ to remain low. The conversion

404

efficiency, defined as 9 = [(number of electrons detected)/(number of photons absorbed)], is then simply (1,2,7)

Reported conversion efficiencies cover a wide range of values, from a few percent for single crystal electrodes ( 5 , 6 ) to near unity for sputtered thin film electrodes (7,8). One widely accepted explanation (1,2) for low 9 is that injection is made with near-unity efficiency, but most of the electrons are unable to escape the surface, i.e., ki >> ka but ke lo9 s-l was estimated. The effect of energy transfer could not be assessed since B was not varied and @ was not determined. The dependence of @f on B was studied in our laboratory in an attempt to clarify the competition between energy transfer and injection for dyes adsorbed from solution (14). The fluorescence quantum yield of rhodamine B adsorbed on Sn-doped In203 and on glass was determined as a function of B (measured by the optical density of the adsorbed dye). A sharp difference was found between the behavior of qif on glass and on the semiconductor as B -* 0: ef increased by more than one order of magnitude on glass but only by a factor of two on the semiconductor. The increase in qif at low B reflects the decrease in the ability of energy transfer quenching to compete with the other D* decay channels. Assuming that neither kf nor ko depend on the surface, we estimated ki = 1.2 x lolo s-l by comparing @f on the semiconductor and on glass at the same B , i.e. , the same kq. This ki value differs by less than a factor of two from those obtained by Arden and Fromherz (7) as described above and by Nakashima et al. (11) for hole injection from rhodamine B into anthracene. Use of our ki value together with T~ = 2.7 ns, the lifetime in dilute ethanol solution, (15) gave @i = 1 at B = 0. At the highest optical These densities we estimated k = 1.9 x lolo s-' and @i = 0 . 4 . 4 results indicate that competition from energy transfer quenching is not the principal cause of low @ values, even at high B . Itoh et al. (16) conducted a detailed investigation of @f as a function of B for rhodamine B on glass, Sn02, and Ti02. From

407

the results in the limit 9 + 0, they calculated ki = 7.4 x lo8 s-' and rpi = 0.72 for Sn02. These ki values are over one order of magnitude lower than those obtained by us and by Arden and Fromherz (7) for In203. The authors also reported that very little fluorescence quenching was observed on Ti02, leading to ki < 3 x lo7 s-' and ei < 0.1. The wide range of ki values obtained from steady-state fluorescence experiments suggested that injection may vary significantly from semiconductor to semiconductor. TIME-RESOLVED FLUORESCENCE MEASUREMENTS In order to examine more closely the processes which determine @i, time-resolved fluorescence measurements on dyes adsorbed on semiconductor and glass surfaces were carried out in our laboratory. The first set of experiments used a low repetition rate, mode-locked Nd:glass laser and streak camera detection (17). For rhodamine B adsorbed from 4 x M aqueous solutions, we obtained rf = 55 ps on Sn-doped In203 and rf = 46 ps on glass. Because these experiments were carried out at high 8 , we concluded that the short rf on both surfaces was determined mostly by efficient energy transfer quenching. The low sensitivity of the experimental system did not permit experiments at low e . In order to overcome this obstacle, we used a synchronously pumped, mode-locked dye laser, cavity-dumped at 4 MHz and timecorrelated single-photon counting detection (18). Because of the higher sensitivity of this experimental system we were able to work at low 9 , using aqueous rhodamine B solutions with concentrations down to M. To examine the dependence of the fluorescence decays on 9 we chose to work with surface-solution interfaces, so as to minimize the problems associated with inhomogeneous surface coverage, which arise with dry surfaces (14). The semiconductor surfaces were those of thin films of Sndoped In203 or Sb-doped Sn02, vacuum deposited on glass substrates. Each I1wetl1sample was a sandwich of two surfaces of the same type with a thin layer (a few microns thick) of dye solution trapped between them. lrDrytl samples were prepared by separating wet samples and letting the solution film evaporate. The fluorescence intensity was analyzed by an iterative nonlinear least-squares reconvolution of the instrument response function and biexponential decays of the form 3.

408

F(t)

=

A1 exp(-t/rl) + (1

- A1)

exp(-t/r2).

[91

The functional form [9] was chosen because two fluorescent species (on the surface and in solution) were expected. No decay component approaching the instrument response function was found in any experiment. The goodness of each fit was judged by the x2, the distribution of weighted residuals, and the autocorrelation function. The fits used not only the fluorescence decay but also all of the rise, which gave higher x2 than when only the decay was included. Because all fits were quite good (i.e. , had low x2 and apparently random distributions of weighted residuals) no attempt was made to explore functional forms more complex than [9]. The results of typical individual fits are shown in Table 1. TABLE 1 Parameters of best fits of [9] to rhodamine B fluorescence decays for surfaces in contact with loe7 M aqueous solutions of the dye. Surface

r1 (ns)

72

(ns)

A1

x2

glass Sn02

0.68

2.6

0.36

1.31

0.41

0.71

1.35

InZ03

0.40

1.4 1.3

0.75

1.21

It is clear from these results that the fluorescence decays are considerably different at glass-solution and at semiconductorsolution interfaces. We note, in particular, an important point: The dominant component is the slow one on glass and the fast one on both semiconductors. (A detection polarizer set at the magic angle ruled out rotational depolarization as the source of the fast component.) No effect of solution concentration on r1 for semiconductor surfaces was found in the M range, which shows that interference from energy transfer quenching may be neglected at loe7 M. Experiments were also performed on dry samples in order to avoid interference from solution dye. A dry glass sample, prepared from the same wet sample which gave the results at the top of Table 1, had rl = 0.60 ns, r 2 = 3.1 ns, and A1 = 0.22. This is typical of results from dry glass samples prepared with M solutions: The dry sample always had r1 slightly shorter than the original wet sample, r 2 = 3.1 ns, and A1 5 0.25. The

409

shorter r 2 obtained with wet glass samples reflects the unresolved contributions from dye molecules on the surface (rf = 3.1 ns) and in solution (rf = 1.5 ns) (11). The fact that as many as 25% of the molecules adsorbed on glass have a lifetime considerably shorter than in solution is less readily understood. We attributed r1 on glass to molecules which inject into surface states or other localized states near the surface. On the other hand, Kemnitz et al. (12) observed a similar two-component fluorescence decay for rhodamine B adsorbed on silica ( r l = 0.9 ns, r 2 = 3.8 ns), and attributed the fast component to molecules loosely attached to the surface. We return to this point in the Discussion. M solutions Dry Sn02 and In203 samples prepared from were always found to have r 1 slightly shorter than the original wet sample, r 2 = 3.1 ns, and A1 z 0.97. We take particular note of the fact that the same r 2 was determined for dry glass as for both dry semiconductors. Also, the smallness of the slow component on dry semiconductor surfaces explains why r 2 obtained with wet semiconductor samples was close to the solution lifetime. The similarity of the decays on the two semiconductors does not support the earlier suggestion that the disparity in the steadystate results was due to the semiconductor. We attributed also to injection the dominant fast component of the fluorescence on the semiconductors. The minor slow component was attributed to molecules adsorbed at a few semiconductor surface sites where injection cannot take place. The rate constants ka and ki were estimated from the data obtained at M ) , so as to minimize energy the lowest concentration transfer effects. The values r f = r2(dry) = 3.1 ns and ki = 0 were used in [5] to derive ka = 3.2 x 108 s-’. This ka value and

r 1 were then used in [5] and in [7] to derive ki = 2.1 x lo9 s-’ and @i = 0.88 both for Sn02 and for In203. This ki value is rf

=

six times lower than that estimated from our steady-state measurements for In20g and three times higher than calculated by Itoh et Because the time-resolved measurements are al. (16) for Sn02. much more direct, we consider this ki value to be more reliable than those derived from steady-state measurements. As expected, r l became progressively shorter as the solution M, a consequence of energy concentration increased above transfer quenching. A dry Sn02 sample prepared from a M solution gave r 1 = 0.07 ns and no detectable slow component, in

410

good agreement with our streak camera results (obtained with 4 x M solutions). With this rl value we calculated @i = 0.13, which shows that energy transfer lowers @i to a greater degree than previously estimated. Nevertheless, the lower @i is still well above the @ values measured for single crystal electrodes. Only three other time-resolved measurements of the fluorescence of dyes adsorbed, on wide bandgap semiconductor surfaces seem to have been reported to date. In the earliest of these, Kamat and Fox (19) investigated erythrosin B solutions containing Ti02 colloidal particles. The decays followed [9], with r1 in the 0.20 0.36 ns range attributed to adsorbed dye, quite similar to our results for rhodamine B on Sn02 and In203. On the other hand, the results of Kamat and Fox are in sharp contrast to those of Itoh et al. (16), who found that sputtered TiOZ thin film surfaces had almost no effect on the fluorescence of rhodamine B. This may mean that some physical or chemical characteristic of these poorly defined surfaces is more important than the identity of the semiconductor. More recently, Crackel and Struve (20) investigated the fluorescence decay of cresyl violet adsorbed at high -9 on Ti02 single crystal surfaces. The decay could be fitted by a sum of three exponentials with lifetimes 36.8 ps (78%), 209 ps (19%), and 1555 ps (3%). Similar results were reported later by Anfinrud et al. (21) for rhodamine 3B adsorbed at low .9 on ZnO single crystal surfaces: 79 ps (55%), 337 ps (32%), and 1221 ps (13%). The extremely short lifetime of the dominant component in both decays (not observed in our experiments at low R ) was attributed by the authors to energy transfer from the dye to the semiconductor. Both decays also have an intermediate component in the same range as the r1 values obtained by Kamat and Fox (19) and by us. Finally, the smallness of the slow component is in general agreement with our results for dry semiconductor surfaces.

-

DISCUSSION We have outlined experimental results which show that adsorbing dye molecules on semiconductor surfaces (with D* above the conduction band) will lead to lower fluorescence quantum yields and shorter fluorescence lifetimes than on insulator surfaces. While it has been generally assumed that this is due to injection from D*, other processes have been suggested which are discussed in this Section. 4.

411

4.1 Dimers Energy transfer among identical molecules does not, by itself, quench the fluorescence: quenching occurs when the excitation is transferred to a trap, a species of lower excited state energy. It has been suggested that non-fluorescent dimers (or larger aggregates) serve as excitation traps for adsorbed rhodamine B (11). For two-dimensional Forster energy transfer quenching by isotropically distributed traps, the fluorescence decay is expected to be non-exponential and of the form F(t) = A exp[-t/rf

-

B(t/rf)1/3],

[lo1

in which rf is the lifetime in the absence of energy transfer and Other functional forms may be appropriate to other types of energy transfer (13,22). Even though the dye fluorescence is appreciably quenched at high 8 , Spitler and Calvin (5) have shown that 9 is independent of B (at least at high 8 ) . This led Itoh et al. (16) to suggest that dimers (which trap the excitation energy) must inject as efficiently as monomers, Nakashima et al. (11) reached a similar conclusion for hole injection from rhodamine B into anthracene. Spectral evidence of dimers offered by Kemnitz et al. (12,23) for rhodamine B adsorbed at high 8 on glass and on organic molecular crystals. These authors found strong evidence of room temperature dimer emission (exponential decay), in addition to the quenched monomer emission (non-exponential decay following [lo]). The different spectrum and temperature dependence of the two emissions helped distinguish between them. The dimer was found to have rf = 120 ps at room temperature and rf = 3.8 ns at 77K, which suggests that the room temperature lifetime is determined by dissociation (23). If we use the monomer ki value derived from our time-resolved experiments together with rf = 120 ps, we obtain @i = 0.20 for dimers, which is significantly lower than @i = 0 . 8 8 we calculated for monomers. Thus, while dimers may harvest most of the energy from monomers, as proposed by Itoh et al. (16), the injection efficiencies of the two species will be similar only if ki is much greater for dimers than for monomers. Our time-resolved experiments focused on the low 8 limit, in which case the fluorescence decays were found to be independent of wavelength. For the few experiments performed at high 0 we did not examine the wavelength dependence of the fluorescence. B is proportional to the trap concentration (11,12).

WAS

412

Therefore, it is quite possible that the value r1 = 0.07 ns we measured at high 8 on dry Sn02 results from an unresolved combination of dimer and quenched monomer contributions. An investigation similar to that of Kemnitz et al. (23) for semiconductor surfaces is needed to help clarify the role dimers play in the sensitization at high 8 . 4 . 2 . Multiele Sites Indications that molecules adsorbed at different sites on semiconductor surfaces may contribute very differently to I$ have been in the literature for a long time (6). However, recent work by Spitler (24) offers valuable new insight. The optical density of the adsorbed dye and the photocurrent were measured simultaneously in a cell using a ZnO single crystal electrode. Operation at low B eliminated problems associated with energy transfer quenching and dimer formation. A rhodamine B solution was first placed in contact with the electrode until equilibrium adsorption was reached. The dye was then desorbed by flushing the surface with electrolyte solution. With this simple but ingenious approach Spitler found that, although less than one-tenth of the adsorbed dye could be removed, cp decreased in the process by more than one order of magnitude. The results showed that 9% of the surface dye was weakly adsorbed and had cp = 0.15, while the rest was strongly adsorbed and had cp = 0. Spitler's work suggests time-resolved fluorescence experiments before and after weakly adsorbed dye is removed from single crystal surfaces, as a means to clarify the relationship between cp and I$i at each type of site. Although the non-exponentiality of a fluorescence decay may be clear, the distinction between [ 9 ] (or even more complex multiexponential forms) and [lo] can be rather difficult. This problem is alleviated when the experiments are performed at low 8 , so that components of the form [lo] arising from energy transfer within the adsorbed dye layer need not be considered. However, the fits are not unique even then: For example, a reasonably good fit with [ 9 ] will often be improved if a sum of three exponentials is used instead (25). Whatever the details of the functional form used in the fit, there is clear evidence for more than one adsorption site in the fluorescence decays of dye molecules adsorbed on a variety of surfaces (12,18,25). At low 8 the emitting species are monomers with lifetimes which vary from site to site. The existence of multiple adsorption sites on the same surface raises the concern that ka may not be surface independent.

413

Internal Conversion A simple view of the effect of placing a dye molecule on a surface is that adsorption inhibits some of the low frequency skeletal motions which contribute to the radiationless decay of D* to the ground st?te (internal conversion). Thus, it is easy to understand how adsorption of rhodamine B can lead to a lifetime which is longer than in solution, just as the solution lifetime increases significantly with solvent viscosity (15,26). We have taken r 2 to be l/ka and recall, somewhat reassuringly, that r 2 was the same on glass and on both semiconductors. The origin of the short lifetime r1 on glass is not as clear. We attributed r1 to injection and suggested that surface states, or some other localized states near the surface, serve as electron acceptors on insulators as well as on semiconductors (18,27). Kemnitz et al. (12,25) proposed instead that I1irregular1lsurface sites enhance internal conversion and lead to rl, while llidealll sites hinder internal conversion and lead to r 2 . These authors supported their view with two extremely interesting observations: (a) The introduction of imperfections into molecular crystals increases the contribution of fast fluorescence decays, and (b) dyes with greater skeletal rigidity have smaller contributions from fast decays. The view that adsorption may enhance as well as suppress internal conversion was also expressed by Nakashima and Phillips (28). The work of Kemnitz et al. (25) suggests a similar investigation for semiconductor surfaces. Dyes of various degrees of skeletal rigidity should be used in time-resolved fluorescence experiments in order to clarify the processes responsible for r 1 on semiconductors. The effects of mechanical polishing and of etching of the semiconductor surface (which introduce and remove surface traps, respectively) on the fluorescence decay might help identify the surface sites. 4 . 4 Intersvstem Crossinq Skeletal motions can also induce transitions from the lowest excited singlet state to the lowest triplet state (intersystem crossing). Thus, changes in ka may be due not only to internal conversion but also to intersystem crossing. It should be noted that injection can also take place from the triplet state, as discussed by Spitler et al. (29). Although the question of injection from the triplet state remains largely unexplored, the possibility of a heavy atom effect 4.3

414

(which enhances intersystem crossing) on Sn-doped Inj02 surfaces was considered by Arden and Fromherz (7). These authors placed a dye monolayer on a LaF3 film (expected to produce a heavy atom effect comparable to that of In and Sn) but found no evidence of enhanced intersystem crossing. Another exploration of possible heavy atom effects was conducted by Tanimura et al. ( l o ) , also with negative results. 4.5 Enercrv Transfer to the Semiconductor Quenching of D* by energy transfer to the semiconductor has This is not received much attention as a possible cause of low @. because interest in dyes adsorbed on semiconductors has focused on the sensitization of wide bandgap semiconductors. In this case the molecular excitation energy is smaller than the bandgap (although D* is above the conduction band) and cannot generate electron-hole pairs. The early work of Tanimura et al. ( 1 0 ) examined tetraphenylporphine thin films separated from Sn02 surfaces by KC1 spacers of varying thickness. These authors found that @f decreased as the distance to the semiconductor surface was reduced, and suggested the possibility of energy transfer to surface trap states below the conduction band. The strategy of using spacers of varying thickness to probe energy transfer to the semiconductor has been used since by other authors. (Unlike energy transfer, electron transfer is a very short range process.) Hayashi et al. ( 3 0 ) measured cpf and r f f o r 50 A thick tetracene films separated from narrow bandgap semiconductors by LiF spacers. Because the tetracene excitation energy was greater than the bandgap, energy transfer to the semiconductors could readily produce electron-hole pairs ( 3 1 ) . Surprisingly, although cpf for the tetracene films was found to decrease as the distance to the semiconductor decreased, rf did not become shorter. Deri ( 3 2 ) interpreted these results according to the Chance-Prock-Silbey model ( 3 3 ) for molecule-metal interactions, and suggested interference effects (radiative) rather than energy transfer (radiationless). Alivisatos et al. ( 2 2 ) measured r f for pyrene layers separated from Si single crystal surfaces by Xe spacers. In contrast to the results of Hayashi et al. ( 3 0 ) , rf was found to be considerably shorter at smaller separations: 2 3 ns at 1 9 6 A and 3 . 3 ns at 17 A. Detailed analysis of the results was hindered by energy transfer within the pyrene layer and by the formation of excimers, both due to high 8 . Nevertheless, these experiments

415

showed the importance of energy transfer to the semiconductor. Because the pyrene excitation energy is not much lower than the Si bandgap, some energy transfer leading to electron-hole pair formation was expected. Two recent papers have suggested quenching of D* by energy transfer to the semiconductor even though the excitation energy was much lower than the bandgap. In the first of these, Crackel and Struve (20) recorded the fluorescence decay of cresyl violet separated from Ti02 single crystal surfaces by arachidic acid multilayer spacers of thickness between 80 A and 509 A. The authors assigned the two-fold reduction in r f at the smallest separation to energy transfer to the semiconductor according to the Chance-Prock-Silbey model (33). Analysis was again difficult because of high 8 . For dye molecules adsorbed directly on the semiconductor the fluorescence decays were fitted, as we saw in Section 3, by tri-exponential functions for cresyl violet on TiOZ (20) and for rhodamine 3B on both ZnO and Ti02 (21). However, the fluorescence decays could also be fitted with [lo], an example of the lack of uniqueness of the fits to which we alluded earlier. Anfinrud et al. (21) chose the fit with [lo] to assign the fast decay to energy transfer to discrete trap states on the surface of the semiconductor, as suggested earlier by Tanimura et al. (10). Kavassalis and Spitler (34) have discussed a model in which injection takes place into acceptor traps on the surface, rather than directly into the conduction band; escape would then be from the traps to the conduction band. Fluorescence experiments cannot alone determine whether quenching of D* for adsorbed dye takes place through electron injection or through energy transfer. Anfinrud et al. (21) argued that the ultrashort lifetimes (under 100 ps), and an even faster ground state recovery (13 ps, measured by a pump-and-probe technique for rhodamine 640 adsorbed on a ZnO single crystal), ruled out injection. The argument was that the decay of photogenerated charge carriers is known to be several orders of magnitude slower than this. However, electron injection into surface traps may be followed by extremely fast recombination, so that the observed ground-state recovery could be due to fast injection followed by even faster recombination. If the decay of D* (by either energy or electron transfer) is trap-mediated, then polishing and etching of the surface (which change the trap It has been shown density) should have opposite effects on rf.

416

that such surface treatment can change @ significantly (35,36). Different evidence for energy transfer to the semiconductor was provided by Nakao et al. ( 8 ) , who noted that ef decreased by a factor of two when the doping level of the semiconductor was increased. This effect was attributed to quenching of D" by energy transfer to conduction electrons, which reach closer to the surface at higher doping levels (thinner space charge layer). This is of particular interest because the authors also found that @ increased by more than one order of magnitude as the doping level increased. For highly doped Sn02, sensitization with rose bengal and rhodamine B yielded @ = 0.9 and 0.3, respectively. Since the doping level affected C#I much more than @fl the effect on @ was attributed to enhanced escape arising from the thinner space charge layer. An examination of the effect of doping level on dye fluorescence decays could establish whether adsorption in the vicinity of dopant atoms affects the lifetime of D*. 4 . 6 Summarv A great deal of knowledge has been gained from work in this field, although it has served largely to better define the problem and to unveil its complexity, rather than to provide final answers. The results surveyed in this chapter raise several important questions and suggest future experiments which may help answer them. It will be important to extend the approach of Arden and Fromherz (7) and tie more closely together time-resolved fluorescence and @ measurements in the experimental design, especially where the distribution of adsorption sites can be manipulated. It would also be extremely useful if semiconductor surfaces used in time-resolved fluorescence experiments were better characterized physically as well as chemically. Thus, some future experiments should take advantage of ultra-high vacuum (22) and surface analysis techniques to prepare clean and well characterized surfaces prior to deposition of the dye. REFERENCES 1

H. Gerischer and F. Willig, Top. Current Chem., 61 (1976),

2

H. Gerischer, M. T. Spitler, and F. Willig, in: S. Bruckenstein (Ed.), Proceedings of the Third Symposium on Electrode Processes 1979, The Electrochemical Society, Princeton, NJ, 1980, p. 115. A. Heller, Acc. Chem. Res. , 14 (1981) 154. J.H. Fendler, J. Phys. Chem. 89 (1985) 2730.

3 4

31..

417

5 6

M. T. Spitler and M. Calvin, J. Chem. Phys., 67 (1977) 5193. M. Spitler, M. Lubke, and H. Gerischer, Ber. Bunsenges. Phys.

10

Chem., 83 (1979) 663. W. Arden and P. Fromherz, Ber. Bunsenges. Phys. Chem. 82 (1978) 868; J. Electrochem. SOC., 127 (1980) 370. M. Nakao, K. Itoh, T. Watanabe, and K. Honda, Ber. Bunsenges. Phys. Chem., 89 (1985) 134. T. Iwasaki, T. Sawada, H. Kamada, A. Fujishima, and K. Honda, J. Phys. Chem., 83 (1979) 2142. K. Tanimura, T. Kawai, and T. Sakata, J. Phys. Chem. 83

11

N. Nakashima, K. Yoshihara, and F. Willig, J. Chem. Phys., 73

12

K. Kemnitz, T. Murao, I. Yamazaki, N. Nakashima, and K. Yoshihara, Chem. Phys. Lett., 101 (1983) 337. F. Willig, A. Blumen, and G. Zumofen, Chem. Phys. Lett., 108

7 8 9

13

(1979) 2639.

(1980) 3553.

(1984) 222.

18

Y. Liang, P. F. Moy, J.A. Poole, and A.M. Ponte Goncalves, J. Phys. Chem., 88 (1984) 2451. M. J. Snare, F. E. Treloar, K. P. Ghiggino, and P.J. Thistlethwaite, J. Photochem., 18 (1982) 335. K. Itoh, Y. Chiyokawa, M. Nakao, and K. Honda, J. Am. Chem. SOC., 106 (1984) 1620. Y. Liang, A. M. Ponte Goncalves and D. K. Negus, J. Phys. Chem., 87 (1983) 1. Y. Liang and A. M. Ponte Goncalves, J. Phys. Chem., 89 (1985)

19 20

P. V. Kamat and M. A. FOX, Chem. Phys. Lett., 102 (1983) 379. R. L. Crackel and W. S. Struve, Chem. Phys. Lett., 120 (1985)

21

28

P. A. Anfinrud, T. P. Causgrove, and W. S . Struve, J. Phys. Chem. , 90 (1986) 5887. A. P. Alivisatos, M.F. Arndt, S. Efrima, D.H. Waldeck, and C. B. Harris, J. Chem. Phys., 86 (1987) 6540. K. Kemnitz, N. Tamai, I. Yamazaki, N. Nakashima, and K. Yoshihara, J. Phys. Chem., 90 (1986) 5094. M. T. Spitler, J. Phys. Chem., 90 (1986) 2156. K. Kemnitz, N. Tamai, I. Yamazaki, N. Nakashima, and K. Yoshihara, J. Phys. Chem., 91 (1987) 1423. T. Karstens and K. Kobs, J. Phys. Chem., 84 (1980) 1871. T. Kajiwara, K. Hasimoto, T. Kawai, and K. Yoshihara, J. Phys. Chem. 86 (1982) 4516. N. Nakashima and D. Phillips, Chem. Phys. Lett. 97 (1983)

29

M. Spitler, M. Lubke,

14 15 16 17

22 23 24 25 26 27

3290.

473.

337.

56 (1978) 577.

and H. Gerischer, Chem. Phys. Lett.,

30

T. Hayashi, T. G. Castner, and R. W. Boyd, Chem. Phys. Lett.,

31 32 33

D. L. Dexter, J. Lumin., 18/19 (1979) 779. R. J. Deri, Chem. Phys. Lett., 98 (1983) 485. R. R. Chance, A. Prock, and R. Silbey, in: S. A. Rice and I. Progogine (Eds.) , Advances in Chemical Physics, Vol 37 , Wiley-Interscience, New York, 1978, p. 1. C. Kavassalis and M.T. Spitler, J. Phys. Chem., 87 (1983)

34 35 36

94 (1983) 461.

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3166.

H. Gerischer, F. Hein, M. Lubke, E. Meyer, B. Pettinger, and H.R. Schoppel, Ber. Bunsenges. Phys. Chem., 77 (1973) 284. M. Matsumura, Y. Nomura, and K. Honda, Bull. Chem. SOC. Jpn., 52 (1979) 1559.


Chapter 8

APPLICATIONS OF PHOTOCHFMISTRY TO OPTICAL MEDIA

Contents

8.1

P h o t o s t a b i l i t y of Near-Infrared Absorbing Organic Dyes i n New O p t i c a l Media (Hiroyuki Nakazumi and T e i j i r o Kitao)

419

8.2. Photoinduced Phase T r a n s i t i o n i n Liquid C r y s t a l s (Shigeo Tazuke and S e i j i Kurihara)

8.3

435

Photochemical Surface Reactions of Polymeric Systems: Lithographic Applications (Hiroyuki Hiraoka)

448


419

PHOTOSTABILITY OF NEAR-INFRARED ABSORBING OPTICAL MEDIA

ORGANIC

DYES

IN NEW

H. NAKAZUMI and T. KITAO INTRODUCTION Nowadays colourants having absorption in the near-infrared region have become more important as the use has grown for laser diodes that have oscillation wavelengths in 780-830 nm in practical systems, both for reading and writing of the informations; Particularly, the near-infrared absorbing dyes (1-3) have been provided interesting research in optical disc memory used for the storage of large amounts of data and laser printers with high speed. Though the ROM, DRAW, and erasable discs in optical discs are known, at present, work to introduce organic colourants into DRAW disc technology is being actively pursued. Photostability of these functional dyes on polymer substrate will be described after first outlining the basic principle of the optical DRAW disc. 1.

PRINCIPLES OF THE OPTICAL DRAW DISC MEMORY The DRAW disc which offers the facility to make one recording consists of a plastic substrate such as polycarbonate (PC) or polymethylmethacrylate (PMMA) and a recording layer made in amorphous colourants. The thickness of the recording layer is approximately 30-100 nm. The principle of the optical DRAW disc memory, called heat mode system, is shown in Fig. 1. In this system, microscopic pits in the coloured recording layer of a disc are formed by thermal energy transformed from photo energy of a 2.

Coloured Recording Layer ___)

T

I

Laser Beam Fig.

1.

P i t Formation

Laser Beam

Principle of the mono-layer type of

DRAW d i s c .

420

M= VO, A l l SiOR 6

-

laser. In the reproducing process the presence of pits is detected by differences in reflectivity using a laser beam, and the signal is picked up by a photo-diode for subsequent reproduction. Inorganic colourants such as Te-based alloys or tellurium oxides and organic colourants, such as compounds 3-1, are known as colourants for the DRAW disc (1-3). The research to introduce organic colourants instead of inorganic colourants to the DRAW disc is being actively pursued for benefits, namely, lower manufacturing costs of the optical disc, high recording density, low toxicity, and easy modification to improve disc characteristics.

421

J

;1.5

Y

500

600 700 800 900 1000

Wavelength, nm

Fig. 2 .

t “5 8o 60

40

Optical constants for organic dye.

-

I

0

.

A

s

4

*-

20

40

60

80

100

120

Thickness, nm

Fig. 3. Film thickness dependence of reflectance and absorbance at 830 nm. Curves are calculated data. The complex index 6 of refraction (’i=n + ik) in the optical is calculated numerically from the measured layer thickness (4). The layer of organic colourants are expected to be highly sensitive optical media for recording used laser diodes, as the optical constants (n and k) at ca. B O O nm are very high values for organic dyes as shown in Fig. 2 (5). Utilizing these values, the dependence of the reflectivities on the layer thickness is calculated, as shown in Fig. 3 (5). Generally, reflectivity of disc

422

recording layer of organic colourants shows a maximum value of ca. 2 0 - 4 0 % in the range 40-80 nm of layer thickness Though cyanine dyes indicating high reflectivity are typical of organic colourants for the DRAW disc, there are some problems beside the inherent photostability of these colourants. There are two kinds of photofadings in the DRAW disc. The first is photofading of colourants by a laser beam in the reproducing process. The second is photofading of colourants in a recorded disc by sunlight upon storage. The lightfastness must be good, as recorded discs have to have a working life of ten years or more. Though the mechanisms of these photofadings of colourants in the optical disc are not known in detail, it has been found that an addition of a singlet oxygen quencher to inhibit photofading of cyanine dye in the DRAW disc is very effective (1-3). The application of nickel complexes, as a singlet oxygen quencher to inhibit photofading of cyanine dyes, to a optical disc will be described here.

.

PHOTOFADING IN THE DRAW DISC BY LASER BEAM 3.1. Photofading of Dyes on Substrate The photofading process of dye which leads to permanent fading are dependent on fibers (6). Thus, the oxidative process on cellulose and reductive process on wool play the most important role for permanent photofading processes, respectively. The effect of various singlet oxygen quenchers on photostability of coloured materials, derived from colour formers such as Crystal Violet As Lactone and fluoran, on silica gel plate was examined ( 7 ) . shown in Table 1, nickel complexes NBC and la (R=H) are effective inhibitors to photooxidation of Crystal Violet and fluoran dyes by singlet oxygen on silica gel. Thus, the relative conversions in photofading of Crystal Violet in the presence of NBC and & after 10 hours were found as 4 7 . 7 % (no quencher), 16% (NBC), and 13% (la), respectively, whereas 2,6-di-r-butyl-p-cresol had no influence on the photofading. These results indicate that the photooxidation of these dyes on silica gel plate takes place predominantly via the singlet oxygen mechanisms. Griffiths and Hawkins ( 8 ) also demonstrate that the photostability of an o-hydroxyazo dye on silica gel or on polypropylene film increases when complexed with Ni ions and that the photofading on these substrates occurs via hydrazones and by the singlet oxygen mechanisms. The photochemistry of all major classes of dyes on 3.

423

TABLE 1 Effect of additives on the phogostability of Crystal Violet dye and fluoran dye on silica gel1 Run no.

Dye

Crystal Violet

Fluoran

Addi tiveb None NBC la HP None DABCO NBC la HP NBDB + HP

ConversionC ( 8 ) 47.7

16.0 13.0 48.5

54.1 38.0 28.8 8.3 54.3 10.7

aIn all runs, tlc $lica gel plates bearing 20 p l of the dye M, in 20 ml acetone-chloroform(l:l,v/v)] were solutions [2.5x10 exposed to air and light (2>300 nm). Crystal Violet Dye and bfluoran dye were exposed for 10 and 20 h, respectively. As additives, DABCO, nickel dibutyldithiocarbamate (NBC), nickel complex fa, and 2,6-di-r-butyl-p-cresol (HP) were used (Additive/ dye=l.O). ‘From tlc analysis. various substrates are summarized in comprehensive reviews (6,9). From analogy of photofading mechanisms of dyes on the substrates and silica gel, it is assumed that the photofading in the DRAW disc principally is photooxidation fading of colourants by the singlet oxygen while oxidation fading of colourants plays the more important role for permanent photofading process, since no singlet oxygen quencher such as histidine present in wool is contained in PC or PUMA. 3.2.

Photofading of the DRAW Disc In photofading by the laser beam, two mechanisms are considered. One, called the photon mode, is photooxidation of cyanine dyes by the singlet oxygen mechanisms. The second, called the heat mode, is decomposition of cyanine dyes by thermal energy transformed from the photo energy of a laser beam. Though two mechanisms which occurred in the reproducing process could not be distinguished clearly, photodecomposition of cyanine dyes in the region of lower powers of laser to reproduce the signal is at the photon mode, and the rest is at the heat mode, as shown in Fig. 4 (10). Lifetimes of optical disc in the reproducing process depend on the line velocity of the head with laser beam. The curve of

424

Read Power, mW Fig. 4 . Lifetimes of DRAW disc using cyanine dye in reproducing process.

ln

aJ

E

.r

.

c, ln W

E

.r

c, W

rc

.r

-1

~

0.3 0.4 0.5 Read Power, mW Fig. 5. Effect of nickel complex on the lifetimes of the DRAW disc (L.v.=1.3 m/sec, 0:cyanine dye C1, 8:cyanine C1 stabilized by nickel complex, A:cyanine dye C 2 , A : cyanine C2 stabilized by nickel complex). lifetime is shifted to the right hand side in Fig. 4 (10) and the lifetime is higher when higher velocity is used. In the practical system, the signal is picked up by use of a laser beam of weak power (approx. one-tenth of that for recording) to inhibit

425

photofading of colourants in the reproducing process. There have been some investigations to inhibit photofading of cyanine dyes in the DRAW disc. They were originally described in patents (11). The first is the addition of typical singlet oxygen quenchers and the second is introducing a cyclic unsaturated group, carbonyl group or hetero atom to the center of the polyalkene in cyanine dyes to protect photooxidation of cyanine dyes (12). Figure 5 (10) can be viewed as an example of inhibiting for the laser-fading of the DRAW disc in the photon mode region by use of cyanine dyes containing a dithiolene nickel complex, Like compound 4 , as at lower read power lifetimes are significantly improved and at higher read power they do not change.

c

-1 Hamnett 6' constant

Fig. 6. Relationship between kmax and Hammett %?constants of substituent groups of nickel complex & with a correlation coefficient of 0.985,

4.

DITHIOLENE NICKEL COMPLEXES AS ABSORBERS IN RECORDING LAYER IN THE DRAW DISC The dithiolene nickel complexes have the first allowed IL-A transition in the near infrared region (13). A close linear relationship exists between the Amax values and Hammett modified substituent constants, 6 + , as shown in Fig. 6 (14). As most of absorption bands of known dithiolene nickel complexes (13-16) are shifted to longer wavelengths ( 8 6 5 - 1 0 5 0 nm) compared to the oscillation wavelengths of the Ga-As laser diode, they do not act as absorbers in the optical disc. Introduction of halogen atom to

426

TABLE 2

H 4-OCH3 4-Cl 4-CF 3A-?Cl) 2,4- (C1I2 2-c1 2-Br Hr 2-C1 2-CH3

855 894 861 832 850 787 783 783 820 812 870

-

3.02 3.17 3.54 2.99 3.10 2.23 2.56 2.52 2.77 2.74 1.40

a dichloromethane. Prepared from asymmetrically substituted benzoin. the 2-position of benzene ring of nickel complex 1 is very effective in producing a pronounced hypsochromic shift of their absorption bands shown in Table 2 (17,18), and 2-halogenated compounds such as lf, act as absorbers in the optical disc. Replacement of four sulfur atoms of nickel complex 2 with amino groups and replacement of nickel of nickel complex 1with platinum also lead to significantly hypsochromic shifts of their absorption bands (19-21). Though they generally display low reflectivity in a disc, the carrier-to-noise ratio (C/N) for the optical disc using If exceptionally rises to ca. 30 dB at 6-8 mW (22). 5. DITHIOLENE NICKEL COMPLEXES AS SINGLET OXYGEN QUENCHER 5.1 Singlet Oxygen Quenching Mechanisms of Dithiolene Nickel Complexes in Solution The singlet oxygen quenching abilities for various nickel complexes have been measured by using rubrene and 2,5-dimethylfuran (23-25), which react with singlet oxygen to form a peroxide. When a non-degassed solution of rubrene and a singlet oxygen quencher is irradiated, the following reactions occur mainly: Ru 1 Ru + O2 Ru3 + O2 lo2 + Ru

hv

k

s

kt

ko

Ru 1 Ru3 + lo2 Ru + ‘02 Ru02

427

0 -0-

N i complex

a+

50

-= 0

.r v)

L a, 3.

c

0

1ooo

20

40

60

Irradiation time, min

pig. 7. (Q)

Photofading of rubrene with and without nickel complexes CH3CN-C6H6 (4:l) (Airr=525 nm).

1-2 in

TABLE 3 Rate constantsa k and kr for singlet oxygen quenching of complexes 1 - 2 q Ni complex No. R H 4-OCH3 4-C1 4-CF 3 I 4 - b1 2 ,4- (C1) 3 4 I 5- (08H31 H 4-CF3

-

kq xlO-lO/M'lsec-l 1.20 1.05 1.17 1.35 1.23 1.29 1.02 1.12 1.00 0.67

nickel

kr x10-6/M-1sec-1 0.9 0.5 1.1 2.7 2.4 3.1 0.7 3.0 1.9 1.2

aThese valles-lwere determined by7equatioqf [ 8 ] and [9] using k =3.44x10 M sec-l and k -4.2~10 M sec (26) in CH3CN--4 -1 bgnzene (4:l). Initial congentration of rubrene was 1.0 x10 M

.

where Ru is rubrene, Ru1 is the rubrene singlet, Ru3 is the rubrene triplet, Ru02 is rubrene peroxide, Q is the quencher and Q02 is the oxidation product of the quencher (23). The rate for

428

consumed amounts of rubrene and singlet oxygen quencher calculated from the following equations: -d [Ru] ko [Rul -dt - 'oabc kd + ko[Ru] + (kq -d [QI -= dt

'oabc kd

+

kr [QI ko[Rul

+

(kq

+

kr) [QI

+

kr) [QI

can

be

where . I is the incident light intensity, a the fraction of light absorbed by rubrene, b the fraction of rubrene singlets which undergo intersystem crossing, c the fraction of rubrene triplet which transfers energy to oxygen to give singlet oxygen, [Ru] and [Q] are concentrations of rubrene and quencher, respectively. This experimental technique first is introduced by Smith (26). The kd and ko have been determined by Merkel and Kearns (27). Singlet oxygen total quenching rate constants (k + kr) in 9 air-saturated solution by other various techniques have been also measured for various metal complexes, amines, carotenes, phenols, and sulfides (28). The values for the quenching constants are in the range of lo6 lolo M-lsec-'. Rates of quenching of carotenes and nickel complexes, which have the highest values in known quenchers, are on the order of 1 . 0 ~ 1 0-~ 2~ . 0 ~ 1 0M~ls~ec-'. More complete tables of quenching rate constants are summarized in the book described by Foote (28). In general, the quencher abilities of metal in metal complexes decrease in the order Ni>Co>Cu,Pt. Rates for most of nickel complexes have been observed at near diffusion controlled rate limit and many measurements yield values of the rate constants for total quenching (k + kr) (23). To determine one of q them, the k or kr must be measured independently. Recently, we q separated k and kr by simulation calculation using equations 181 9 and [9] at various concentrations, as shown in Fig. 7 (29), and examined substituent effect on quenching ability of nickel complexes 1 and 2. The k of neutral nickel complex 1 is nearly q constant (ca. 1.1 1 . 3 ~ 1 0M~ls~ec-') and kr is an order of magnitude significantly lower (4O6M-'sec-') ,In spite of rough calculations, the values of kr for these complexes may be reliable values, as those are nearly to values of kr which was determined separately for phenols and sulfides (30-32). Obviously substituent effect of neutral complex 1 on the kr was observed, whereas only a

-

-

.

429

few substituent effect for reduced complex 2 was indicated, shown in Table 3. Though the mechanisms of singlet oxygen quenching by nickel complexes are not clear, some mechanisms of quenching by the other quenchers have been proposed ( 2 8 ) . One is the energy-transfer quenching mechanism which involves formations of triplet state quencher and ground state oxygen, and requires that they have the triplet energy estimated to be very near or below the energy excitation of singlet oxygen (rr22KcalM-I). This mechanism has been well documented for B-carotene.The second mechanism is the chargetransfer quenching, and involves interaction of the electron-poor singlet oxygen molecule with electron donors to give a chargetransfer complex as follows:

0

0.5

0

-0.5

-1.0

-1.5

V v s Ag/AgCl, V

Fig. 8. Typical cyclic voltammogram of nickel complex dichloromethane.

in

Other complex mechanism would also be possible for nickel complexes, since photoreduction of nickel complex 1 such as 1-2 and photooxidation of 2 (2-1) in the presence of dye-sensitizers in solution have been observed (33,34) and nickel complex 1 has shown two electrochemically reversible electron transfers, as shown in voltamperometric curves of Fig. 8. The quantum yields (birr=313 nm) of photooxidations of complex 2 to 1 and dianionic nickel complex to monoanionic complex are 0.1 and 0.25 in

430

chloroform, respectively (34). However, the kr

l * b l

102

I

I

1

I

1

l

l

l

l

I

1o4 Absorption Coefficient (crn-')

1o3

I

2 0

LL

0.0

Fig. 2 The threshold fiuence (Fo) and axFo versus the absorption coefficient ( a ) for PMMA doped with pyrene. (T. J. Chuang, H. Hiraoka, A. Modl, 1988)

452

'

I

'

b) PMGVPyrene, CMS/Pyrene

'I

I 0.85 J/crn2

2.0

5

-6 v)

2

1.5

L

a,

Q

a,

c)

2 c

1.0 -0-

z

0

c

+ 0

-_ --

0.34 J/crn2 ,A-

0.5 /

/

/

,

-A- -A-

/

A

0.0

0.5

lo2

0.5

lo3

0.5

lo4

Absorption Coefficient, cm-'

Fig. 3 Etch depth per pulse versus absorption coefficient at 308 nm for various laser fluences; (a) PMMNpyrene film, (b) PMGllpyrene film (solid lines) and CMS/pyrene film (broken lines).

453

shown in Fig. 3b (20). However, for CMS/pyrene and CMS/ABH films, the etch depth per pulse does not change very much as a function of the absorption coefficient, shown also in Fig. 3b. This etching behavior of chlorinated polfimethylstyrene) systems is somewhat similar to the blended polymers of PMMAlpolyimide (14). With a strongly absorbing film, the optical penetration depth becomes rather shallow, limiting the etch yield per pulse. It is clear that there are at least two classes of polymeric materials exhibiting distinctive photoetching characteristics. One group of polymers shows a higher photoetching yield at F 2 0.6 J/cmz and a stronger yield dependence on a than the other group. This distinction is very similar to the reactive ion etching (RIE) behaviors of polymeric materials: aliphatic polymers like PMMA and others exhibit high RIE rates, whereas aromatic polymers l i e polystyrene and others show good RIE resistance (21), mainly due to better energy dissipation in these polymers. This distinction does not indicate that one group is more likely to photochemically ablate while others are not. The fact that distinctly different dopant molecules as pyrene and ABH imbedded in a polymer produced practically the same photoetching results, and the fact that the low energy photons at 308 nm can induce effective decomposition and ablation at relatively low fluences, strongly indicate that the photoetching of polymers is not caused by photochemical bond breaking. Aliphatic polymers are more readily photoetchable in comparison with aromatic polymers, but at high a the etch yield per pulse is limited by shallow optical penetration and slow mass transport rates. Fig. 4 and Fig. 5 show the surface morphologies after photo-ablation of PMMA/pyrene, and CMS, CMS/pyrene respectively. In these figures a copper-mesh mask was used in contact with the polymer fhms for the generation of sharp etch patterns. For PMMA/pyrene f b , in the low pyrene concentration range, the etched surface is quite rough with dendritk and needle-like structures as well as solidified droplets on the bottom of the irradiated areas present although the edges of the etched features are reasonably sharp (Fig.4a). When the laser fluence is increased, the etched surface becomes cleaner and more smooth. At a high pyrene concentration, clean and

sharp etch patterns can be obtained at a relatively low fluence, e.g., at a = 1.04 x I04 cm-' and F = 0.3 J/cm2, as shown in Fig. 4b. In most cases, periodic structures are observed near the edges of the wire mask, which most likely result from the optical diffraction effect of the wires. When a neat chlorinated poly(methylstyrene) film is irradiated by 308 nm pulses at low fluences (e.g., F =. 0.25 J/cml), surface roughening due to melting is clearly evident (Fig. Sa). In the intermediate fluence (Le., 0.8 J/cma 2 F 2 0.4 J/cmq, the etched surface is quite smooth (Fig.5b). The topographical features associated with thermal melting, material flow and resolidification are not so evident from the SEM photos (Fig. Sa) as in the low fluence region (Fig. 5b). There are formations of bubbles and holes in

454

the etched area, indicating the transition from melting to rapid vaporization when the laser fluence is raised. At F = 1 J/cmf the ablated area is very smooth and very little deposited material is observed (Fig. 5c). The presence of pyrene or ABH in the films does not cause significant changes in the surface morphology in this case. For CMS films with or without dopants, the periodic structure is observed in the entire fluence range above the threshold for ablation, even when evidence of thermal melting is clearly present. Using the second harmonic of a Nd-YAG laser, polyimide films with Rhodamine 6G were photoetched at 532 nm, with a dye concentration of 0.1 mole dye per 1 equivalent mole of polyimide unit. The SEM photographs of ablated polyimide films are shown in Fig. 6 (22). Under weak pulses, the polyimide films expanded, exhibiting periodic diffraction patterns (10 pulses with 10 mJ/cm' in Fig. 6a). With median strength pulses (5 pulses with 30 mJ/cm2 in Fig. 6b), the film was partially ablated. With stronger pulses albeit a lower total dose (2 pulses with 45 mJ/cm2 in Fig. Sc), the 2.7 gum thick polyimide film was completely ablated. The pulsing frequency was 1 Hz

Fig. 4 SEM photographs of laser etched PMMA/ yrene:(a) a=1.95 x lo2 cm-', F30.6 Jlcm', 20 pulses; (b) a 31.04 x 10 cm", F=0.3 Jlcm', 1 pulse. The widths of the relief features 30 m In both cases.

B

455

and the pulse width was cu. 25 nsec. The etching is clearly photo-thermal, and the result indicates the importance of laser pulse strength rather than the total amount of

light absorbed for photo-etching.

Fig. 5 (Left) SEM photographs of etched surfaces. (a) CMS alone, 0.25 Jlcm2 (b) CMSlpyrene, 1 Jlcm2, 0.70x104cm-' (c) CMS alone, 0.85 Jlcm'

Etched surfaces of polyimide/Rhodamine 6G by 532 nm Nd-Yag laser. (a)lO pulses, 10 mJ/cm2/pulse, (b) 5 pulses, 30 mJ/cm2/pulse, (c) 2 pulses, 45 mJlcm2/pulse. (H. Hiraoka, T. J. Chuang, C. T. Rettner, 1987)

Fig. 6

456

Polyimides, chlorinated poly(methylstyrene) and novolac resins are all crosslinking polymers under ordinary UV-light irradiation, whereas PMMA and PMGI are main-chain scission-type polymers under deep UV irradiation (23). Even for such scission-type polymers, photo-thermal effects cannot be neglected. For crossliiking-type polymers, photo-thermal effects become more important and are the predominant mechanism for photo-ablation.

PHOTO-OXIDATION Let us now consider photochemical reactions at polymer surfaces which occur with UV-light intensity of 1 W/cmz or less, quite low in comparison to photo-ablation by intense excimer laser pulses of 10' w/cm2 output. When UV-light irradiates polymer surfaces in air or another oxidizing atmosphere, three photochemical events may take place: [l] dinct photolysis of oxygen by short wavelength UV-light, [2] photo-sensibtion of oxygen within the polymer surfaces, and [S] direct photolysis of polymer structures followed by subsequent reactions with oxygen. Direct oxygen photolysis initiated by light absorption in the vacuum UV region has been extensively studied and reviewed (24). The oxygen atom OCP) reacts with polymers in hydrogen abstraction, followed by oxygen molecular attachment and others, leading to oxidative degradation of polymers. However, these direct oxygen atom reactions take place only in space and in laboratory experiments. Photo-sensitization by aromatic systems to yield singlet state oxygen molecules are well-known. Additives to polymers, or the polymer systems themselves, can sensitize oxygen to give singlet oxygen, which subsequently oxidizes the polymer (25). Direct photolysis of polymers, or of additives to polymers, followed by naction with oxygen, could be more frequently encounterred when chromophores are present than in cases of polymers without chromophores. Photo-oxidation of polystyrene has been studied extensively (2,26,27). Recent reports show that the formation of a contact charge transfer complex between polystyrene and oxygen may play an important role in the Initiation step of photodegradation by extending the UV-absorption range of the polymers to longer wavelength (28). Atmospheric pollutants such as NO2 ,0,and others are reported to enhance the photo-oxidation rate of polystyrene. The formation of a radical center at the a-C atom during photo-oxidation has been well-established. Reaction of molecular oxygen with an a radical of polystyrene results in the following series of reactions. In this reaction scheme the products are themselves photolabile so the rate of photo-oxidation will increase as the reaction proceeds (26): 3.

457

-

0

II

-CHaC

+

+ OH

CtizCH-

66 00 88 06 \e;-c".". 4

-CH,CHCHICH-

---CH1~-CH2CH-

oxidation

OOH

I

-CHaCCHaCH-

/

6

CTwlco

3.1 Photo-oxidation of Substituted Polystyrenes and Novolac Resins

Several functionally substituted polystyrenes and novolac resins are important for electronic applications, particularly as resins for lithographic resists. We have studied the photo-oxidation of these materials using an ESCA technique (29). Fig. 7 shows change of the carbon core level signal of polystyrene. After UV-exposure for 2 hrs in air with a medium pressure mercury lamp, new carbon core level signals appear at 289 eV and 286 eV, which correspond respectively to a carbonyl carbon and a carbon bonded to a hydroxyl group. Formation of these types of carbon can be explained by the mechanism described above. The carbonyl carbon signal can be readily removed by low energy electron or proton beam irradiation as shown in Fig. 7c. Cresol-formaldehyde novolac resin Polystyrene c,, Core Level Signals ;.

- .

._

I

291.0

--

I 292

(C)

~

I

284 Binding Energy eV

282

Fig. 7 The change of the C1, core level signal of polystyrene: (a) original film, (b) UV-exposure in air, (c) subsequent proton beam exposure at 4 KeV.

288.5

286.0

283.5

I

281.0

8 The change of the C1, core level signal as a function of UV-exposure (lmW/cm2 at 254 nm): (a) 0 hr, (b) 0.5 hr, (c) 1 hr, (d) 1.5 hr. Fig.

- _ _..

.

458

Substituted polystyrenes undergo photo-oxidation more readily with halogen substitution. Poly(p-hydroxystyrene) is as susceptible to photo-oxidation as polystyrene. Under deep UV-irradiation, both poly(pch1orostyrene) and chlorinated poly(methylstyrene) undergo crosslinking reactions, initiated by chlorine removal. This provides negative polymer patterns after image development. The chlorine removal appears insensitive to the presence of oxygen. Electron and ion beams also remove chlorine effectively, which is the basis of the high radiation sensitivity of these materials as negative working resists (30). The primary mechanism of photo and radiation chemistry of these chlorinated polystyrenes has been extensively studied (31). Deep UV-irradiation of the polymer films in air causes photooxidation as demonstrated by new IR bands at 1720 cm-1 (indicative of carbonyl carbon) and by new carbon core level signals at 286 and 289 eV ,similar to polystyrene. Cresol-formaldehyde, phenol-formaldehyde, and chlorinated cresol-formaldehyde novolac resins all undergo photo-oxidation upon UV-irradiation in air. The change of the carbon core level signals is shown in Fig. 8. The new IR band at about 1720 cm-I, corresponding to formation of a carbonyl group, is also found after photo-oxidation of novolac resin. These oxidized layers of the novolac film are limited to a very shallow superficial surface only about 500 thick, even after long deep UV irradiation, as discussed below in reference to photostabilization of resist images. 3.2. AppcicCrtionS to PorymCr Image Fabrication Upon UV irradiation in air, polystyrene, chlorinated poly(methylstyrene), and novolac resins all aquire acidic surfaces by photo-oxidation. Photo-oxidation is not limited to these polymers. A wide range of polymers undergo such reactions, although the oxidation rate and the depth of the oxidized layer depend on the nature of polymers, and the presence of sensitizers or stabilizers (2,26,27). These acidic surfaces can be readily altered by low energy electron or ion beam irradiation, as shown in Fig. 7c for polystyrene. Not only acidic groups generated by photooxidation, but also carboxylic groups attached to polymer main chains can readily be removed by such low energy radiations, or by deep UV-irradiation in vacuum. Fig. 9 shows the change in the carbon core level signal of poly(methacrylic acid) when exposed to (b) electron beams, (c) proton beams, (d) deep UV in air, and (d’) deep UV in vacuum. The change can be summarized in the following way:

Similarly, the series of reactions of chlorinated poly(methylstyrene) can be summarized in the following:

459

The different chemical natures of the exposed and unexposed areas can be used for polymer pattern fabrication using a silylation technique, e.g., vapor phase hexamethyldisilazane (HMDS) treatment of the polymer films (32). HMDS is well-known to react with acidic hydroxyl groups to yield silylated products. Here, HMDS reacts with phenolic hydroxyl groups, carboxylic groups, and peroxide groups, as shown below: Me,SiNHSiMe, + ROH + ROSiMe,, + RCOOH RCOOSiMe,, + ROOH 4 ROOSiMe,. Areas exposed in vacuum or in an inert atmosphere by electrons, ions or deep UV-liiht cannot be silylated because of elimination of these reactive groups; thus they are etched faster during oxygen reactive ion etching. Unexposed areas are silylated by HMDS, and in oxygen RIE the surfaces are covered and protected by SiODshowing no etch loss during the dry image development. The resulted images are shown in Fig. IOa and 10b /

Poly(methaqlic acid)

-CH3 .CH2\

_-Fig. 9

The change of the C,, core level signal of poly(methacrylic acid): (a) before UV-exposure, (b) after 3 KeV proton beam, irradiation, (c) after 25 KeV electron beam exposure, (d) UV-exposure in air, (e) aRer UV-exposure in nitrogen.

I .

(b)

- . . _ -

-.-*

(4

,. ...,.-. ,.: ..,.

.-. ...

/*-.

___+--I 290

288

.

-'

,

2114

Binding Energy, eV

I 280

460

with poly(methacrylic acid) and photo-oxidized chlorinated poly(methylstyrene),

respectively. In this scheme positive working images are always obtained despite the negative-working nature of CMS (33). The HMDS silylation method has been reported for other resist sytems (32,34).

(a)

Poiy(methacrylic acid) Positive images by Surface Modifications

(4

Poly(chioromethy1styrene) Positive Images by Surface Modifications

Flg. 10 SEM photographs of polymer images made by surface modifications: (a) poly(methacryllc acid) positive images, (b) poly(chloromethylstyrene) positive images.

PHOTO-CROSSLINKAGES Poly(methy1 methacrylate) suffers main chain scission upon deep UV, electron-beam, x-ray, and ion-beam irradiation. Other poly(methacry1ates) (except poly(glycidy1 methacrylate)), and poly(o1efin sulfones) undergo main chain scission under high energy radiation. However, a majority of polymers undergo crosslinking in absence of oxygen. Photo-oxidation leads to degradation of polymer main chains. Even novolac resins used for positive working photoresists undergo cross-liking reactions, if 4.

461

they are used alone without a dissolution inhibitor, like a diazonaphthoquinone type photoactive compound. The tendency for cross-linking is primarily due to a cage effecr in the polymer matrix. In excimer laser ablation, discussed earlier, local high temperatures during the short pulse makes the cage effect negligible, and almost all polymers ablate under such exposure conditions. The degree of cross-linking depends on the nature of the polymer. Poly(pch1orostyrene) and chlorinated poly(methylstyrene) exhibit high electron beam sensitivity, 1 x 1oC Clan', as negative working resists, whereas similar patterning of cresol-formaldehyde novolac resin in negative tone requires doses of f x 10-3 C/cml. Although unsubstituted polystyrene degrades under UV-exposure m air with considerable main chain scission (4), cresol-formaldehyde novolac =in, photoresists based on novolac resin and poly(p-hydroxystyrene) undergo oxidative coupling reactions, resulting in crosslinked networks, as shown below (35):

The formation of cross-linked networks by UV-irradiation is the basis for a photo process for stabilizing resist images. 4.1. Process for Photo-stabilization of Resist Images In micro-lithographic processes, imaged resist films must undergo many Severe process treatments which are accompanied by high temperature, such as reactive ion etching, ion implantation and so on. During these processes, resist images must maintain image integrity, including preservation of resist wall profiles. Photoresists generally lack such resistance to high tempnature and reactive ion etching. In order to enhance the toughness of resist images, we proposed the use of UV-hardening (29,36). This technique has now become a standard lithographic process. The basis of this photostabilization process is the photo-oxidative coupling reactions of novolac resins and of poly(p-hydroxystyrene), as described above. UV-hardening is not effective for PMMA-type resists. Fig. 11 shows an example of UV-hardening/photostabilization of resist images. Without UV-irradiation resist images flow when heated above 150 "C, but UV-irradiation of about 1.5 J/cm2 before heating stops the flow. Not only enhancing thermal flow resistance of resist images, UV-hardening increases resistance to chlorine-based RIE plasma, useful for aluminum etching. Diazonaphthoquinone-novolactype photoresists have strong UV-absorption below 250 nm. For the purpose of UV-hardening, UV-light should penetrate as deeply as possible into the film for uniform irradiation. From considerations of optical penetration and

462

(4

No treatment, heated at 2 5 P C for 10 min; K

1,000 magnification; 3.5pm resist thick.

(a)

Pulsed e-beam exposure at 25KeV, 200 pls 2x10-3C/cm2; heated at 2 W C for 10 mini x 1,000.

(b)

Deep UV-exposure at 75 to 100°C for 7.5m 6J/cm2; heated at 250°C for 10 min; X1,OOO magnification. (C)

Fig. 11 UV-hardening of At-2400 images: (a) original, (b) 155°C heating, (c) 155°C heating for 30 min after 10 min UV-exposure. Fig. 12 (Right) Comparison of thermal flows of 3.5 pm thick resist images at 250°C: (a) no treatment, (b) electron beam hardening, (c) Deep UV-hardening.

energetics, UV-light in the range of 300 to 320 nm has been reported to be most effective for photostabilization (37). Because of limited optical penetration, UV-hardening is limited in its use to rather thin resist images, up to about 2.5 jim thick. With thicker resist images, reticulation appears after cooling down from a high temperature. This reticulation problem is a surface problem ‘caused by a different

463

degree of crosslinkage; near the surfaces the polymers are densely crosslinked, while inside the polymer films are only slightly so. The reticulation problem is also encountered in the plasma resist image stabilization method (PRIST) (38) where the crosslinking reactions are more superficial than in UV-hardening. In PRIST, resist images are exposed to a CF, plasma for a short period, providing a surface cross-linked by interaction with the reactive environment. The oxidation and fluorine incorporation are limited to a very shallow surface. Although photostabilization/UV-hardening process is ideal for thin resist images, this process is not effective for thick resist images, 3 pm or above. Recently we developed a pulsed electron beam stabilization process, which does not have this thickness limitation (39). An example is shown in Fig. 12; here, pulsed electron beams of 25 KeV in soft vacuum were used as crossliking radiation with a dose of 2x103 C/cmz. The electron penetration depth of 25 KeV far exceeds the resist thickness, providing uniform crosslinking. This eliminates the reticulation problem. 4.2. Contrast Enhancement by Surf- Photo-crosslinkage The design of the most advanced microelectronic devices now requires with sub-half micron dimensions. Photo, electron beam and x-ray lithographies are all candidates for patterning at such resolution. Resist resolution, along with relief image wall profiles and contrast must be improved and controlled in order for the fabrication of such devices to become practical. The use of contrast enhancing layers (CEL) has been proposed for projection printing in optical lithography, in which the aerial image of a mask is used to expose the photoresist. The photoresist must correct the distorted sinusoidal aerial images caused by diffraction within the projection instrument, to provide rectangular resist images. For an aerial image of low contrast, even those parts of the image that correspond to dark regions of the mask have significant light intensity. As the contrast is reduced, discrimination of the dark areas from the lighter areas becomes increasingly difficult. The CEL improves the ability of a photoresist to effect this discrimination, thus lowering its contrast threshold. The CEL process is based on the use of photobleachable materials which are initially opaque, but which, following some dose of radiation, become relatively transparent. A number of CEL photo-breachable materials have been reported since the initial publication (40); one example of CEL photochemistry is shown below (41):

,35,000(365 -

-

Oxaziridine

Nitrone

436 nm)

P -z 5000 (365

436 nm)

The resist images obtained by the CEL technique have far better contrast.

464

Disadvantages of the technique, however, are the added complexity of the process, the requirement of an excessive UV dose for photobleachmg, and the fact that application is limited to optical lithography. The surface photo absorption for contrast enhancement (SPACE)process has been recently reported for use with a negative-working electron-beam resist (42). By addition

of a controlled UV-flood exposure step, enhancements in both contrast and sensitivity of MRS RD2000N resist, a negative working resist composed of poly(p-hydroxystyrene) and 3,3'diazido-diphenylsulfone, have been obtained. However, its application is limited to negative working resists. Our contrast enhancement and wall profile control method is applicable to both positive and negative working resists in principle (43). Surface crosslinkages are used in this process to retard dissolution at the resist surface. The creation of a slower-dissolving surface layer affords greater control of wall profiles and enhanced contrast or sensitivity. One approach is to use flood exposure with low-energy electron and ion beams to induce crosslinking at the surface. Another method is to use midand deepUV flood exposure. In the latter case, 4,4'-diazido-diphenylsulfide was used as

Fig. 13 SEM photographs showing wall profiles obtained by surface crosslinkages: (a) no UV-flood exposure, (b) 10 mJ/cm2 exposure at 300 nm, (c) 15 rnJ/crn2 exposure at 300 nrn. The resist was a bisazide-added Shipley 145OJ.

465

a crosslinking agent which was added to a conventional positive working photoresist composed of a diazonaphthoquinone-type photosensitizer and cresol-formaldehyde novolac resin. Other aromatic bisazides such as 3,3’-diazido-diphenylsulfone can be used for this purpose.

N 3 + G N 3

in

Q ?-oQ& ;5==J?2N : = & N 2 a n d

qcH2k CH3

00

00

The imagewise UV-exposure was carried out using conventional G-line (436 nm) projection printing. Then, a mid-UV flood exposure was carried out prior to development, using a dose of about 10 mJ/cmz at 300nm. Although the image development takes longer in the same strength developer solution than is required without the mid-UV exposure, there is no measurable thickness loss in unexposed areas, while the imagewise exposed areas are completely developed without scum. The resist images obtained in this wapare shown in Fig. 13, which also presents resist images formed in the conventional way, i.e., with no mid-UV flood exposure (Fig. 13a). Surface crosslinking clearly has improved the contrast and has provide a means of controlling wall prof&. The shape of the profile can be controlled by changing the bisazide sensitizer concentration and the mid-UV flood exposure dose. Similar lift-off structures of resist images can be accomplished by the chlorobenzene soaking method (44), image reversal (45), and vacuum heating with mid-UV flood exposure (46). However, this surface crosslinkage method provides a general method for the enhancement of resist characteristics, regardless of the kind of lithography process and the tone of the resist. We and others have reported several approaches to the imagewise surface modification of resist layers, using low energy proton beams (43, low energy electron beams (47,48), and deep UV-lithography (49). The advantages of top-surface imaging are no reflectivity, better depth-of-focus requirements and others. In the Built-In-Mask (BIM) method, the resist surface is exposed in an imagewise manner, followed by extension of the top surface images down to the bulk resist layer by performing a heating step and a flood exposure with near UV-light (50). During heating, an additive such as a triazene compound reacts with the indene carboxylic acid photoproduct to yield an azo dye, which serves as a mask in the subsequent near UV-flood exposure. The result is a conversion in resist tone from positive to negative. Sub-micron resist images have been obtained. The BIM method is applicable only to photo-lithography. Howver, the concept of utilizing the surface chemistry to achieve high resolution lithography is important and a more general utility.

466

PHOTO AND ION BEAM-DEPOSITION OF POLYMER FILMS Many monomeric organic compounds undergo a shift in their UV-absorption bands to longer wavelength upon adsorption to a substrate. Imagewise surface photo-polymerization which takes advantage of this shift has been reported (51). Although this study did not demonstrated a high resolution capability, a more recent study of low pressure chemical vapor deposition of aluminum from trialkylaluminum vapor by laser beams uses the same principle (52). 5.Z. Applicotwn to bilayer Resist Technology Multi-layer resist technology, particularly the bilayer portable conformable mask (PCM) process (53) is now considered as a viable manufacturing There are two versions of the PCM process; in one deep-UV exposure is used to transfer the image into the underlayer (deep UV-PCM), and the other uses reactive ion etching to effect the pattern transfer (RIE-PCM). Although deep UV-PCM is more desirable from a manufacturing point of view, many studies usbg silicon-containing imaging layers have focused on the RIE-PCM process. Focused ion beam-induced silicon-containing polymer deposition 5.

Flg. 14 SEM photographs of polyimide patterns with CVD tetravinylsilane film as an etch barrier: (a) a stencil mask used, (b) (b),(c) polyimide patterns of (PMDA-ODA) vapor deposited.

467

has been reported (54). We have used flood exposure with deep UV, low energy electron or proton beams for low pressure chemical vapor deposition of tetravinylsilane (47). For negative working polymer patterns, a stencil mask was used for imagewise exposure, depositing tetravinylsiiane polymer on a polyimide substrate. This was followed by oxygen reactive ion etching to complete patterning process. The polyimide patterns obtained in this way are shown in Fig. 14. During oxygen RIE, the tetravinylsiiane f h is converted to a silicon dioxide layer, providing a protective coating for the underlying polyimide. Non-deposited areas do not have the protective coating, thereby during oxygen RIE these areas are etched away. For positive working image fabrication, a uniformly deposited tetravinylsilane film can be imagewise exposed to low energy electron or proton beams to remove silicon from the silane layer. This selective removal can be demonstrated by ESCA measurements and such patterned films can be successfully converted to relief images by a RIE step.

CONCLUSION Photochemical surface reactions of polymer systems are an important field not only from the point of view of micro-electronic materials processing, but also from a more general scientific and materials application perspective. We have reviewed our studies in this field, which include investigations of excimer laser ablation, studies of the photo-oxidation of polymer surfaces, and the use of surface cross-linking and surface polymer depositions for microlithographic applications. With the increasing miniaturization of microelectronic devices, the fundamental and the applied aspects of surface photochemistry of polymers becomes increasingly important. 6.

REFERENCES 1 H. R. Philipp, H. S. Cole, Y. S. Liu, T. A. Sitnik, Appl. Phys. Lett.. 48(2), (1986) 192. 2 G. Geuskens, C. David, in "Degradation and Stabilization of Polymers." ed. G. Geuskens, John WSey 62 Sons,New York, 1975. 3 E. Gipstein, A. Ouano, T. Tompkins, J. Electrochem. Sac.. 129, (1982)201. 4 H. Hiraoka, L. W. Welsh, Jr., in "Polymers in Elecrronics," ed. T. Davidson, ACS Symp. Ser. #242, Amer. C h m SOC.,Washington, D.C. 1984,pp.55-64. 5 J. S. Greeneich, J. Electrochem. Soc.,l22, (1975)970. 6 L. Karapiperis, I. Adesida, C. A. Lee, E. D. Wolf, J. Vuc. Sci. Techol.,Z9(4), (1981) 1259. 7 D. Kyser, K. Murata, "Monte Carlo Simulation of E-Beam Scattering and Energy Loss", Proc. 6th Zntl. Con$ on Electron and Zon Beam Science, Electrochem. SOC.,1974. 8 R. Srinivasan, W. J. Leigh, J. Am. Chem. Soc., 104, (1982)6784. 9 R. Srinivasan, in Laser Processing and Diagnostics, ed. D. Biluerle, Springer, Heidelberg, 1984,pp.343-354. 10 R. Srinivasan and B. Braren, J. Polym. Sci.. Chem Ed.,22, (1984)2601. 11 J. E. Andrew, P. E. Dyer, D. Foster, P. H. Key, Appl. Phys. Lett.. 43, (1983)717. 12 G. Koren, J. T. C. Yeh, Appl. Phys. Lerr., (1984)1112;J. Appl. Phys., 56, (1984)2120. 13 J. Brannon, J. Lankard, A. Bake, F. Burns, J. Kaufinan, J. Appl. Phys.. 58, (1985) 2036. 14 H. Cole, Y. Liu, H. Philipp, R. Guida, Muter. Res. SOC.Symp. Proc., 72, (1986) 241. 15 H. Masuhara, H. Hmoka, K. Domen, Macromolecules, 20, (1987)450. 16 T.J. Chuang, H. Hiraoka, A. M6dl, J. Appl. Phys., in print. 17 T. F. Dcutsch, M.W. Geis, J. Appl. Phys., 54, (1983)7201. 18 J. T.C. Yeh, J. Vuc. Sci. Technol..A4, (1986)653. 19 V. Srinivasan, M. A. Smrtic, S. V. Babu, J. Appl. Phys.. 59, (1986) 3861.

468

20 H. Hiraoka, T. J. Chuang, H. Masuhara, J. Vac. Sci. Technol. Bb. (1988) pp.463-465. 21 H. Hiraoka, W. L. Welsh, Jr., J. Bargon, J. Vac. Sci. Technol.. BI, (1983) 1062. 22 H. Hiraoka, T. J. Chuang, C. T. Rettner, in Research Disclosure, No.285, 1988; Kenneth Mason Publications Ltd, England. 23 H. Hiraoka, Macromolecules, 10, (1977) 719. 24 J. Pitts, J. Culbert, "Phorochemisw, ,John Wdey & Sons, New York, 1964, pp.205-239. 25 c d , N. S. Allen, J. F. McKellar, in "Photochemistryof Dyed and Pigmented Polymers,' ed. N. S. Allen and J. F. McKellar, Applied Science Publishers Ltd., London, 1980. 26 N. Grassie, G. Scott, "Polymer Degradation and Stabilization." Cambridge University Press, Cambridge, Britain, 1985. 27 B. Ranby, J. F. Rabek, "Photodegradation, Photo-oxidation and Photostabilization of Polymers." John Wdey & Sons, New York, 1971. 28 J. F. Rabek, J. Sanetra, B. Banby, Macromolecules, 19, (1986) 1674. 29 H. Hiraoka, L.W. Welsh, Jr., in "Tenth Intl. C o d Electron and Ion Beams Science dr Technol. Proceeding," Vol. 83-2,, The Electrochem. SOC., Washington, D.C., 1983, pp. 171-178. 30 E. D. Feit, L. E. Stillwagon, "Technical Papers, Reg. Tech. Con&' Mid-Hudson Sect., SOC. Plastic Eng., 1979, p.68. 31 Y. Tabata, S. Tagawa, M. Washio, in "Materialsfor Microlithography." ed. L. F. Thompson, C. G. Willson, J. M. J. Frechet, Vol.266,, Amer. Chem. SOC.,Washington, D.C., 1984, pp. 151-163. 32 S. A. MacDonald, H. Ito, H. Hiraoka, C. G. Willson, in "Technical Papers on Photopolymers," SOC.Plastic Eng., Mid-Hudson Sec., 1985, pp.177-196. 33 H. Hiraoka, in "Ion Beam Processes in Advanced Electronic Materials and Devices," ed. B. R. Appleton, F. H. Eisen and T. W. Sigmon, Materials Research SOC.Symp. vo1.45, 1985, pp.197-202. 34 R. Visser, J. P. W. Schellekens, M. E. Reuhman-Huisken, L. J. Van Ijzendoorn, in SPIE Symp. Proc. on Adv. Resist Technology and Processing IV, ed.M. J. Bowden, Vo1.771, 1987, pp. I 11-117. 35 A. Knop, W . Scheib, "Chemistry and Applications of Phenolic Resins," Springer-Verlag, New York, 1979. 36 H. Hiraoka, J. Pacansky, J. Electrochem. Sac.. 228, (1981) 2645; J. Yac. Sci. Technol.. 19, (1981) 1132. 37 E. Spiertz, F. Vollenbroek, Microcircuit Eng., 68-1 (1984). 38 W. H.-L. Ma, United States Patent No. 4,187, 331. 39 H. Hiraoka, J. Krishnaswamy, G. Collins, 172nd Meeting of The Electrochem. SOC.,Oct. 19, 1987; see also, H. Hiraoka, Microcircuit Eng., 6, (1987) pp.407-412. 40 B.F. Griffing, P. R. West, Proceedings of SPIE No. 394. March 1983, p.33. 41 D. R. Strom, Semiconductor International. May 1986. 42 0. Suga, E. Aoki, S. Okazaki, F. Murai, H. Shiraishi, S. Nonogaki, J. Vac. Sci. Technol.,B6, (1988) pp. 366-369. 43 H. Hiraoka, K. N. Chiong, W.Hinsberg, N. Clecak, to be presented at the 32nd Symp. Electron, Ion and Photon Beams, May 1988. 44 M. Hatzakis, B. Canavello, J. Shaw, Proc. Microcircuit Eng., Amsterdam, 1980, pp. 439-452. 45 H.Moritz, G. Pad, United States Patent No. 4,104,070 (1978). 46 F. A. Vollenbroek, E. J. Spiertz, H. J. J. Kroon, Pobmer Eng. Sci.. 23, (1983) 925. 47 H. Hiraoka, J. Electrochem. SOC., 131, (1984) 2938. 48 S. A. MacDonald, L. A. Pederson, A. M. Patlach, C.G. Willson, ACS Polymeric Matl. Sci. Eng., 55 (1986) 721. 49 R. D. Allen, S. A. MacDonald, C. G. W&on, ACS Polymeric Matl. Sci. Eng.. 55, (1986) 290. 50 F. A. Vollenbroek, W. P. M. Nijssen, M. J. H. J. Geomini, C. M. J. Mutsaers ,R. J. Visser, Microcircuit Eng. 6, (1987) pp. 495-501. 51 A. Christopher, A. K. Fntzsche, A. N. Wright, in "Photochemistry of Macromolecules," ed. R. F. Reinisch, Plenum Press, New York, 1970, pp.117-127; H. Hiraoka, ZBM Techkal Discl. Bull&.. 19, (1976) 2688. 52 J. mcsteiu, Seminar at IBM on 12/17/87. 53 B. J. Lm,in 'Introduction to Microlithography." ed. L. F. Thompson, C. G. Willson, M. J. Bowden, ACS Symp. Ser. 219, Amer. Chem SOC.,Washington, D. C., 1983, pp. 287-350. 54 K. G m o , S. Namba, in "Ion Beam Processes in Advanced Electronic Materials and Device Technology," ed. B. R. Appleton, F. H. Eisen, T. W. Sigmon, Material Research SOC.Symp. Proc., Vo1.45, Material Research SOC.,Pittsburgh, P a 1985, pp.223-234.

Chapter 9

RE-

DEVEWP!E"B

OF PHOTOCHEMISTRY IN LIQUID

CRYSTAIS AND PROTEINS

Contents

9.1

Photoreactivity of Carbonyl Compounds in the Solid State (Yoshikatsu Ito)

9.2

469

Ketone Photochemistry as a Probe of Conformational Mobility in Nematic and Smectic Liquid Crystals (William J. Leigh)

9.3

481

Absolute Asymmetric Synthesis via Photochemical Reactions of Chiral Crystals (John R. Scheffer and Miguel Garcia-Garibay)

9.4

501

Fluorescence Quenching of Pyrene as a Monitor of Intermolecular Diffusion and Intramolecular Chain Bending in Cholesteric Liquid Crystalline Phases(1) (Mark F. Sonnenschein and Richard G. Weiss)

9.5

526

Dynamics of Excited State Relaxations in Some Proteins (Fumio Tanaka and Noboru Mataga)

551


469

PHOTOREACTIVITY OF CARBONYL COMPOUNDS IN THE SOLID STATE Y. IT0 INTRODUCTION Solid-state photoreactions are featured by their chemo-, regio-, and stereoselectivities, which are often quite different from those in solution ( 1 ) . These features originate from the crystal structure of the parent molecule that is ordered with respect to packing, distance, mutual orientation, space symmetry, and molecular conformation. Reactions in crystals normally proceed with a minimum of atomic and molecular movement as a result of physical restraints by the crystal lattice (topochemical principle) ( 2 ) . To predict and control the crystal structure and reactivity by designing a chemical structure (crystal engineering) is one of the most attractive challenges in modern solid-state photochemistry (3). The strategy for crystal engineering has been mostly directed to photodimerizations of cinnamic acids and related compounds. In crystalline photochemistry, however, their photoreactivities are usually classified only into two categories, i.e., photoreactive or photostable. Evidently, this is not enough since the reactivity is by nature a continuous property. Quantum yield measurements for solid-state photoreactions are highly desirable from this viewpoint. Simple carbonyl compounds, whose photochemistry in solution are fully investigated(41, are also photoreactive in solids. Typical reactions in the solution phase, e.g., hydrogen abstraction, a-cleavage, oxetane formation, and elimination of CO, are all observed in the solid phase. Recently we have studied some of these solid-state reactions and estimated their quantum yields, which will be reviewed here. 1.

HYDROGEN ABSTRACTION Photochemical hydrogen abstraction by ketones in the solid As a result of state has drawn much attention ( 5 - 1 1 , 2 0 - 2 4 ) . detailed studies on the tetrahydronaphthoquinone system (5,6), the following geometrical requirements for the reaction have been 2.

470

presented. Thus, for the occurrence of solid-state intramolecular hydrogen abstraction, the upper limit of the distance d should be the sum of the van der Waals radii of the hydrogen and oxygen atoms (2.72 A ) (Fig. 1 ) . Intermolecular hydrogen abstraction in the crystalline inclusion complexes formed between a ketone and deoxycholic acid has also been studied in a thorough manner (7,8). In these cases, the maximal distances for the reaction (intermolecular hydrogen abstraction followed by radical coupling) to occur are d = 3.5 A and d' = 4.2 A. These distances are considerably longer than the sum of the van der Waals radii (2.72 and 3.40 A , respectively). This fact can probably be ascribed to loose crystal packings of inclusion complexes a s compared with those of pure materials. From the stereoelectronic requirement f o r n ,TI*induced hydrogen abstraction (4), optimum values of the angles a and B are 0 " and 90°, respectively (25). However, a and I3 can deviate considerably from their optimum values, e.g., a successful hydrogen abstraction with a = 60" or with 8 = 80" is known (6,9).

d:

the distance between the carbonyl oxygen and the hydrogen that is abstracted d': the distance between the carbonyl carbon and the carbon to which the hydrogen being abstracted is bonded a : the angle between the O * * * Hvector and its projection on the plane of the carbonyl group 8: the angle C - O * * * H Sum of van der Waals radii: 0 + H = 2.72 A C

+

C = 3.40 A

Fig. 1 . Geometrical parameters relevant to hydrogen abstraction by the carbonyl group (5). We have found that crystalline 4,4'-dimethylbenzophenone (1) undergoes photocoupling, albeit a low maximum conversion, to give solely 2 (eq 1 ) ( 1 0 ) . A pinacol-type dimer, which is a main product in the solution phase photolysis, is not formed. The distances d and d' are 3.32 A and 3.87 A , respectively, and the angles a and 8 are 80" and 92", respectively. By contrast, 4-methylbenzophenone ( 3 ) is photostable as crystals. The values

471

of d, d', a , and B for 2 are 2.77 (or 2.72) A , 4.39 (or 4 . 5 3 ) A , 4 2 " (or 29"), 1 2 1 " (or 1 3 7 " ) , respectively: either the C * * * C distance (d') or the C = O * * * Hangle (€3) is probably too large for the reaction to proceed. The relatively long d and d' for photoreactive 1 is not exceptional ( 6 1 , but the finding that the to-be-abstracted hydrogen lies almost perpendicularly to the n-orbital direction (i.e., a = 80") is remarkable.

I

-2

CH3

Upon photolysis in the solid state ( 1 1 ) or in solution (121, 2,4,6-triisopropylbenzophenones - f are transformed completely - f. (eq 2). The into the corresponding benzocyclobutanols ester derivative 3 undergoes no photoreaction in the solid state, but in solution it photocyclizes to Because of the steric hindrance by bulky ortho isopropyl groups, the molecular structure of 4 is severely twisted (Fig. 2): the torsion angle between the J and triisopropylphenyl ring and the carbonyl plane is 82" for $ 86" for g. The values for d, a , and B are listed in Table 1. A s will be mentioned later (section 4 ) , the photocyclization of 4 in

a.

the solid state appears to occur from the T , I T * excited state. By contrast, the n,n* excited state is responsible for the solution phase photocyclization (12).

4 -

-a , -b, c,

5 R = OMe R = t-Bu

R = Me

-d , R = H

g, R = C1 f-, R = CF3 2, R = C02Me

472

Fig. 2.

Stereoviews of

(11) and

3

(13).

TABLE 1 . Distances and angles relevant to solid-state hydrogen abstraction by 5 and (11).

4a 4d ideal value

2.88

55

55

2.94 2.90

52 55

52

0

90

2.97

~2.72

58

59

57

Molecular diffusion and molecular tumbling are severely restricted in solids. However, in-plane molecular rotation is often observed in the solid state for special aromatic molecules, e.g., the activation energy Ea = 4.69 kcalfmol and the rate constant k = 4.7 x l o 8 s - l at 25 OC for 1-bromo-8-methylnaphthalene (14). Rotation about single bonds can occur freely at room temperature in particular cases. For example, a rotational barrier

473

of the methyl group can be as low as 0.8 kcal/mol ( k = 2 x 1012 s-l at 25 "C) (15). In the crystalline deoxycholic acid - acetophenone inclusion complex, even the acetyl group has significant rotational mobility. The X-ray diffraction study showed that, while the n-orbital of acetophenone is perpendicular to the C5-H bond, the photochemical hydrogen abstraction occurred and furthermore the acetyl group underwent a net rotation of 180' during the photoaddition (eq 3 ) (8). This reaction is explained by the acetyl group rotation that happens before the n-orbital-initiated hydrogen abstraction. Therefore, this reaction seems to verify the validity of the stereoelectronic requirement for the hydrogen abstraction by the n,r* excited carbonyl oxygen.

hv ____)

crystal

bH

Fig. 3 describes a simplified scheme for solid-state hydrogen abstraction. When a solid-state geometry is unfavorable for the n,r* hydrogen abstraction, e.g., long d and/or large deviations of a and B from the ideal values, partial molecular motion such as single-bond rotation, conformational change, and partial diffusion is necessary prior to occurrence of the hydrogen abstraction (path a). These motions are normally expected to be much more restricted in the solid state than in the solution state, leading to the circumstances that a slow hydrogen abstraction process such as that by the v , r * excited carbonyl (path b) may favorably compete with path a ( 1 6 ) . In fact, in rigid matrices hydrogen abstraction reactions can often occur from excited states other than the Tl(n,r*) state (18). Of course, paths a and b are competing with various nonradiative and radiative excitation decay processes (path c). When the solid-state geometry is favorable for the n,IT*hydrogen abstraction, the reaction (path d) will readily occur with a maximal rate of %lo11 s-l (19). At present, little quantitative information is available on the paths a - f.

474

R Y

J,

/c.

I

partial molecular motion a

unsuitable

suitable

?"

,c, A I

f

further reaction or reketonization

Fig. 3 . Simplified scheme for solid-state hydrogen abstraction reactions. It is now clear from the above scheme (Fig. 3 ) that critical values for d, d', a, and f3 will depend on both looseness of crystal packing and inherent rates of various elementary processes in addition to crystal structure. In this context, violation of Schmidt's criteria for solid-state photodirnerization of alkenes (20) is not surprising. Other simple carbonyl compounds that undergo photochemical hydrogen abst action in the solid state are, e.g., a-cyclohexylacetophenones (21), a-adamantylacetophenones (91, N,N-dialkyl-aoxoamides (22 , and 2-tert-butylbenzophenones (23). Norrish Type I and Type I1 reactions of phenyl alkyl ketones included in various hosts have been studied to examine medium-effects on product distr butions (24).

OTHER REACTIONS Besides the hydrogen abstraction reaction, excited states of simple carbonyl compounds can undergo several other types of reactions in the solid state: rearrangement (26-29), [2 + 2 1 cycloaddition (5,30,31), a-cleavage (241, and elimination of CO (32). Herelonly examples done by us will be mentioned. Irradiation of crystals of a-santonin (6) yields a 2 , 4 cyclopentadienone I, whereas in solution lumisantonin (g) is a photoproduct (eq 4) (27). The observed selective rearrangement to 3.

415

7 in the solid state is ascribed to larger movement of atoms

associated with the formation of 8 .

-

____.c

solution

[41

solid

etate

-8

-7

6 -

Irradiation of 2,5-cyclohexadienone 9 with visible light (>400 nm) results in quantitative formation of two isomeric

lumiketones lo and

fi

(eq 5 ) (28,291. In the solid state

lo and

1 1 are formed with comparable yields, whereas in solution the

isomer lo is the major product. Failure for crystalline 9 to photorearrange into a 2,4-cyclopentadienone structure, like a-'santonin ( 5 ) does in the solid state eq 4), is attributed to insufficient compactness of the crystal packing of 9 (28).

OMe

R

hv

( > 400 nm) +

Y

R OMe

-9

-a,

0

R = t-Bu;

b,

R = Me

,

[51

0

10

11 -

Photolysis of benzophenone (13)with l-methyl-2,4,5-triphenylimidazole (9) in either acetonitrile solution or in the solid state ( 3 3 ) produces oxetane 14 (eq 6) ( 3 0 ) . Irradiation of 4,4'-dimethylbenzophenone (I-), 4-methylbenzophenone ( 1 1 , and 4in solution in the presence of 12 gives benzoylpyridine (3) likewise oxetane a d d u c t s . In the solid state ( 3 3 ) , however, no oxetane is formed from these three ketones. It appears that only 1 3 can take a relative geometric arrangement with 12 that is suitable for the oxetane formation in the solid state.

P h qNN I PPhh I

Me

12 -

+

& 13 -

hv ___*

t61

14 -

476 TABLE 2 Quantum yields for benzocyclobutanol formation

4

+

2 at 313

quantum yield reactant

R

mP! OC

4a

OMe t-BU Me H

111-112 120-121 87-87.5 97-99 92-94 89-91 142-1 43

4b

4c 4d 4e

Zi

4f

c1

CF3 CO2Me

aReferences 1 1 and 12.

solid

solutionb

0.56 0.20 0.48 0.13 0.072 0.011

0.48 0.38 0.42 0.52 0.51 0.30 0.13

~0.001

benzene.

TABLE 3 Quantum yields for lumiketone formation

9

+

lo + 11 at

> 400 nm.a

quantum yield reactant

R

9a 9b

t -Bu Me

aReference 29. 4.

mp, OC

58-59 92-94

solid 10

024 0.14

11 022 0.14

solutionb 10

036 0.49

11 0312 0.16

hexane.

SOLID-STATE QUANTUM YIELDS From the scheme described in Fig. 3, it is evident that quantum yield measurements in solids are the first step toward understanding mechanisms of solid-state photoreactions. Such measurements, however, have not been done frequently, because they require special precautions and apparatus (34). Recently we have found that quantum yields for some solid-state photoreactions can be conveniently estimated by the usual merry-go-round technique (11,29). The procedure is very simple: dissolve a sample with ether in a Pyrex tube, evaporate the solvent to leave a coated crystalline film whose surface area is adjusted to be constant as precisely as possible, and then irradiate after degassing on a merry-go-round apparatus. The obtained quantum yields may not be very precise because of the reflection of light from the crystal surface and a variable surface area, but their reproducibilities are confirmed to be quite satisfactory (usually t 5 % ) . Quantum yields for photocyclization of 2,4,6-triisopropylbenzophenones - & in the solid state are summarized in Table 2

471

(11). Those data in solution (12) are also listed for comparison. In the solid state, the intramolecular hydrogen abstraction from the n,T* excited state would be slow without rotation of the C-C single bond linking the carbonyl group and the triisopropylphenyl group, since the geometry (d, a , and D values) of the molecule 4 is far from the ideal one (Table 1 ) . Therefore, a normally slow

hydrogen abstraction may occur under this conformation. In fact, the solid-state quantum yield increases in the order C F 3 < H < alkyl < OMe (Table 2), the order where the n,n* character of the triplet excited state in turn increases (35). Quantum yields for 2,5-cyclohexadienones and % in the solid and solution phases are summarized in Table 3 (29). In general, the quantum yield should be affected by the electronic nature of substituents, as indicated from Table 2. However, a clearly different quantum yield is found in the solid state between % (0.20) and & (0.48) or (0.46) and % (0.28) (Tables 2 and 3), although the electronic nature of t-Bu ( 0 = -0.20) and Me ( 0 = -0.17) is similar. This difference may be considered to reflect the steric effect by the bulky tert-butyl group. It is interesting to note the fact that the lower-melting counterpart or has greater photoreactivity than the higher-melting one or E , respectively). It appears that the lower-melting counterpart has looser crystal packing and thus has greater solidstate photoreactivity than the higher-melting one. TIT*

(s s) (s

SENSITIZATION AND QUENCHING Despite their widespread use in solution, such techniques as sensitization and quenching are rarely employed to study solidstate photoreaction mechanisms (36). Any dopant (sensitizer, 5.

quencher, trapping reagent, etc.) will affect crystalline phase photoreactivity through many factors, e.g., energy transfer, electron transfer, and disruption of the lattice regularity of the parent crystal. The last problem is the most difficult to evaluate. However, investigations in this field will be highly rewarding, since they may contribute to better understanding of many complex photobiological events and to the invention of the next generation of various solid electronic devices. We have found that the photocyclization of triisopropylbenzophenone @ into cyclobutenol is quenchable by added naphthalene both in the solid and solution phases as displayed in Fig. 4 (curves a and b) ( 3 7 ) . In this figure the solid-state quantum

478

1 .c

h

a

>

.r

c,

m

7

aJ

0.5

L

v

8

0

1 2 naphthalene/@, molar r a t i o

=:

Fig. 4. Effect of added naphthalene on the quantum yield ( 0 ) for the reaction + curves a and c, in the solid phase; curve b, in the benzene solution (kqT = 2 9 4 M-l from a linear Stern-Volmer plot) (37). yields are estimated by a merry-go-round apparatus by using a procedure as outlined in section 4 . The quenching is almost complete at a guest/h st ratio .L 0.8 (curve a). In naphthalene(guest) benzophenone host) mixed crystals, where the triplet exciton jump frequency is estimated to be about 1 O1o s-l , quenching of the benzophenone phosphorescence is complete at a far less guestlhost ratio ( . L I o - ~ ( 3 8 ) . One of the reasons ( 3 8 ) for the observed inefficiency of the exciton hopping in the naphthalene - % system is a large unit-cell volume ( 2 = 4 ) of % as compared with that of benzophenone (13):1 9 6 2 A3 for 4d ( 1 3 ) and 1 0 0 1 A3 for 13 ( 3 9 ) . Curiously, however, the results of curve a are not reproducible on repeated runs (curve c). Curves similar to curve c were also obtained for naphthalene - 3 and naphthalene - 4d mixed crystals. Probably 4 and naphthalene crystallize out separately, depending upon subtle variations of experimental conditions for evaporation of the ether solvent. This demonstrates possible complexities in doing solid-state quenching experiments by our method (section 4 ) .

479 6.

CONCLUSION P h o t o r e a c t i v i t y o f c a r b o n y l compounds i n t h e s o l i d s t a t e i s

l i t t l e understood.

Many f a c t o r s s u c h a s r a t e s of e l e m e n t a r y

p r o c e s s e s , l i f e t i m e s o f i n t e r m e d i a t e s , c r y s t a l s t r u c t u r e , and l o o s e n e s s o f c r y s t a l p a c k i n g must b e c o n s i d e r e d t o e l u c i d a t e t h e mechanism ( c f . F i g .

3).

I n a d d i t i o n t o t h e t r a d i t i o n a l X-ray

c r y s t a l l o g r a p h y , t h e u s e f u l n e s s o f s o l i d - s t a t e quantum y i e l d measurements i s exemplified. REFERENCES 1 V. Ramamurthy a n d K . V e n k a t e s a n , Chem. R e v . , 8 7 ( 1 9 8 7 ) 433-481. G.M.J. S c h m i d t , P u r e Appl. Chem., 2 7 ( 1 9 7 1 ) 6 4 7 - 6 7 8 . 2 3 J.A.R.P. Sarma a n d G . R . D e s i r a j u , Acc. Chem. Res., 1 9 ( 1 9 8 6 ) 222-228. 4 N . J . T u r r o , Modern M o l e c u l a r P h o t o c h e m i s t r y , B e n j a m i n / Cummings, Menlo P a r k , 1 9 7 8 . 5 J.R. S c h e f f e r , ACC. Chem. R e s . , 1 3 ( 1 9 8 0 ) 2 8 3 - 2 9 0 . 6 ( a ) S. A r i e l , S. A s k a r i , S.V. E v a n s , C . Hwang, J . J a y , J . R . S c h e f f e r , J. T r o t t e r , L. Walsh, a n d Y. Wong, T e t r a h e d r o n , 43 ( 1 9 8 7 ) 1 2 5 3 - 1 2 7 2 . ( b ) S. A r i e l , S.H. A s k a r i , J . R . S c h e f f e r , a n d J. T r o t t e r , T e t r a h e d r o n L e t t . , 2 7 ( 1 9 8 6 ) 7 8 3 - 7 8 6 . 7 R . P o p o v i t z - B i r o , C.P. Tang, H.C. Chang, M . Lahav, a n d L. L e i s e r o w i t z , J. Am. Chem. S O C . , 1 0 7 ( 1 9 8 5 ) 4 0 4 3 - 4 0 5 8 . 8 ( a ) C.P. Tang, H.C. Chang, R . P o p o v i t z - B i r o , F. F r o l o w , M . L a h a v , L. L e i s e r o w i t z , a n d R . K . McMullan, J . Am. Chem. S O C . , 1 0 7 ( 1 9 8 5 ) 4058-4070. ( b ) H.C. Chang, R. P o p o v i t z - B i r o , M. Lahav, a n d L. L e i s e r o w i t z , i b i d . , 1 0 9 ( 1 9 8 7 ) 3883-3893. 9 ( a ) S. E v a n s , N . Omkaram, J . R . S c h e f f e r , a n d J. T r o t t e r , T e t r a h e d r o n L e t t . , 2 6 ( 1 9 8 5 ) 5903-5906. ( b ) S . V . E v a n s , M. G a r c i a - G a r i b a y , N . Omkaram, J . R . S c h e f f e r , J. T r o t t e r , a n d F. W i r e k o , J. Am. Chem. SOC., 1 0 8 ( 1 9 8 6 ) 5 6 4 8 - 5 6 5 0 . 10 Y . I t o , T. M a t s u u r a , K. T a b a t a , J. Meng, K . Fukuyama, M . S a s a k i , a n d S. Okada, T e t r a h e d r o n , 43 ( 1 9 8 7 ) 1 3 0 7 - 1 3 1 2 . 11 Y. I t o a n d T. M a t s u u r a , s u b m i t t e d f o r p u b l i c a t i o n . 12 Y. I t o , H . N i s h i m u r a , Y . Umehara, Y. Yamada, M. Tone, a n d T. M a t s u u r a , J. Am. Chem. S O C . , 1 0 5 ( 1 9 8 3 ) 1 5 9 0 - 1 5 9 7 . 13 Y . Takemoto, K . Fukuyama, T. T s u k i h a r a , Y. K a t s u b e , Y. I t o , a n d T. M a t s u u r a , R e p o r t s o f t h e F a c u l t y of E n g i n e e r i n g T o t t o r i U n i v e r s i t y , 1 4 ( 1 9 8 3 ) 148-1 57. F. I m a s h i r o , K. T a k e g o s h i , S. Okazawa, J. Furukawa, T. Terao, 14 a n d A. S a i k a , J. Chem. P h y s . , 7 8 ( 1 9 8 3 ) 1 1 0 4 - 1 1 1 1 . F. I m a s h i r o , K . T a k e g o s h i , K . H i r a y a m a , T . T e r a o , a n d A. 15 S a i k a , J . Org. Chem., 5 2 ( 1 9 8 7 ) 1 4 0 1 - 1 4 0 4 . I t i s known from t h e s o l u t i o n p h a s e s t u d i e s t h a t k e t o n e s 16 having T l ( i ~ , n * ) are less r e a c t i v e t h a n t h o s e having T l ( n , r * ) by a f a c t o r of about 1000 ( 1 7 ) . 17 P . J . Wagner, A c c . Chem. R e s . , 4 ( 1 9 7 1 ) 1 6 8 - 1 7 7 . ( a ) H . Murai a n d K. O b i , J. P h y s . Chem., 7 9 ( 1 9 7 5 ) 2 4 4 6 - 2 4 5 0 . 18 ( b ) C. B r a u c h l e , D.M. B u r l a n d , a n d G . C . B j o r k l u n d , i b i d . , 8 5 ( 1 9 8 1 ) 123-127. ( c ) G.W. S u t e r , U.P. Wild, andK. S c h a f f n e r , i b i d . , 90 ( 1 9 8 6 ) 2358-2361. ( d ) S. Yamauchi, M . T e r a j i m a , a n d N . Hirota, i b i d . , 8 9 ( 1 9 8 5 ) 4804-4808. ( e ) I.C. Winkler a n d D.M. Hanson, J. Am. Chem. SOC., 1 0 6 ( 1 9 8 4 ) 923-925. F.D. L e w i s , R.W. J o h n s o n , a n d D.R. Kory, J . Am. Chem. S O C . , 19 9 6 ( 1 9 7 4 ) 61 0 0 - 6 1 07. ~~

480

20 21 22 23 24

25

26 27 28 29 30 31 32

33 34

35 36 37 38 39

M . Hasegawa, P u r e Appl. Chem., 5 8 ( 1 9 8 6 ) 1 1 7 9 - 1 1 8 8 . S. A r i e l , V. Ramamurthy, J. R . S c h e f f e r , a n d J . T r o t t e r , J . Am. Chem. S O C . , 1 0 5 ( 1 9 8 3 ) 6 9 5 9 - 6 9 6 0 . H . Aoyama, T. Hasegawa, a n d Y. O m o t e , J. Am. Chem. S O C . , 1 0 1 ( 1 9 7 9 ) 5343-5347. P . J . Wagner, B.P. G i r i , J . C . S c a i a n o , D.L. Ward, E . Gabe, a n d F.L. L e e , J . Am. Chem. SOC., 1 0 7 ( 1 9 8 5 ) 5 4 8 3 - 5 4 9 0 . ( a ) H.L. C a s a l , P. d e Mayo, J . F . M i r a n d a , J.C. S c a i a n o , J . Am. Chem. S O C . , 1 0 5 ( 1 9 8 3 ) 5 1 5 5 - 5 1 5 6 . ( b ) P.C. G o s w a m i , P. d e Mayo, N . Ramnath, G. B e r n a r d , N . Omkaram, J . R . S c h e f f e r , a n d Y.-F. Wong, Can. J . Chem., 6 3 ( 1 9 8 5 ) 2 7 1 9 - 2 7 2 4 . ( c ) B.N. Rao, N . J . T u r r o , a n d V. Ramamurthy, J. Org. Chem., 51 ( 1 9 8 6 ) ( d ) G.D. Reddy, B. J a y a s r e e , a n d V. Ramamurthy, 460-464. i b i d . , 5 2 ( 1 9 8 7 ) 31 0 7 - 3 1 1 3 .

R e c e n t t h e o r e t i c a l c a l c u l a t i o n o f hydrogen a b s t r a c t i o n from m e t h a n e by t h e f o r m a l d e h y d e t r i p l e t h a s shown t h a t a. = 9.2" a n d 6 = 108.9' i n t h e t r a n s i t i o n s t a t e . The f o r m a l d e h y d e t r i p l e t i s nonplanar: t h e degree of pyramidalization is 34.8'. D. S e v e r a n c e , B. P a n d e y , a n d H . M o r r i s o n , J . Am. Chem. SOC., 109 ( 1 9 8 7 ) 3231-3233. ( a ) W.K. A p p e l , T . J . Greenhough, J . R . S c h e f f e r , a n d J. T r o t t e r , J. Am. Chem. S O C . , 1 0 1 ( 1 9 7 9 ) 2 1 3 - 2 1 5 . ( b ) M.B. R u b i n a n d W.W. S a n d e r , T e t r a h e d r o n L e t t . , 2 8 ( 1 9 8 7 ) 5 1 3 7 - 5 1 4 0 . T. M a t s u u r a , Y. S a t a , a n d K. Ogura, T e t r a h e d r o n L e t t . , ( 1 9 6 8 ) 4627-4630. T. M a t s u u r a , J. Meng, Y. I t o , M. I r i e , a n d K . Fukuyama, T e t r a h e d r o n , 43 ( 1 9 8 7 ) 2 4 5 1 - 2 4 5 6 . Y. I t o , H . I t o , M . I n o , a n d T. M a t s u u r a , s u b m i t t e d f o r p u b l i cation. Y. I t o , J. Meng, S. S u z u k i , Y. Kusunaga, a n d T. M a t s u u r a , Tetrahedron L e t t . , 26 ( 1 9 8 5 ) 2093-2096. ( a ) B.S. G r e e n a n d G . M . J . Schmidt, Tetrahedron L e t t . , ( 1 9 7 0 ) 4249-4252. ( b ) C.R. T h e o c h a r i s , G . R . D e s i r a j u , a n d W . J o n e s , J . Am. Chem. S O C . , 1 0 6 ( 1 9 8 4 ) 3 6 0 6 - 3 6 0 9 . ( a ) G. Q u i n k e r t , T. T a b a t a , E. Hickman, a n d W . Dobrat, Angew. Chem., I n t . E d . E n g l . , 1 0 ( 1 9 7 1 ) 1 9 8 . ( b ) G. Q u i n k e r t , K . O p i t z , W.W. W i e r s d o r f f , and J. W e i n l i c h , T e t r a h e d r o n L e t t . , ( 1 9 6 3 ) 1 8 6 3 - 1 868. A s o l i d s a m p l e p r e p a r e d by e i t h e r c r u s h i n g a m i x t u r e of c r y s t a l s i n a m o r t a r o r by r e s o l i d i f i c a t i o n o f a m o l t e n mixture afforded a similar r e s u l t . ( a ) S . L a z a r e , P. de Mayo, a n d W.R. Ware, Photochem. P h o t o b i o l . , 3 4 ( 1 9 8 1 ) 1 8 7 - 1 9 0 . , ( b ) J. R e n n e r t , E.M. R u g g i e r o , a n d J . Rapp, i b i d . , 6 ( 1 9 6 7 ) 2 9 - 3 4 . ( c ) S. M i k i , Y. Asako, a n d 2. Y o s h i d a , Chem. L e t t . , ( 1 9 8 7 ) 1 9 5 - 1 9 8 . P.J. Wagner, M . J . Thomas, a n d E. H a r r i s , J . Am. Chem. SOC., 9 8 ( 1 976) 7675-7679. M . G a r c i a - G a r i b a y , J . R . S c h e f f e r , J. T r o t t e r , a n d F. W i r e k o , T e t r a h e d r o n L e t t . , 2 8 ( 1 9 8 7 ) 1 7 4 1 -1 7 4 4 . Y. I t o , u n p u b l i s h e d r e s u l t s . ( a ) D . J . Graham, J . Phys. Chem., 8 9 ( 1 9 8 5 ) 5 3 3 0 - 5 3 3 2 . (b) J.B. B i r k s , P h o t o p h y s i c s o f A r o m a t i c Molecules, W i l e y , N e w York, 1 9 7 0 , C h a p t e r 11. E.B'. F l e i s c h e r , N . Sung, a n d S. Hawkinson, J. Phys. Chem., 7 2 ( 1 9 6 8 ) 4311-4312.

481

KETONE PHOTOCHEMISTRY AS A PROBE OF CONFORMATIONAL MOBILITY IN NEMATIC AND SMECTIC LIQUID CRYSTALS

W. J. LEIGH

1. Introduction As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thennonopic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyorropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. In liquid crystalline phases of rod-like mesogens, the constituent molecules are oriented with their long axes parallel to one another, on average. This is the only degree of ordering present in nematic phases, which are the most fluid type of liquid crystal. Cholesteric phases are similar in structure to nematics, except the molecular alignment experiences a macroscopic twist, with a period (pitch) of a few thousand Angstroms, as one proceeds through the bulk sample. Cholesteric phases are thus macroscopically optically active, and are formed either by chiral mesogens or by simple nematic phases doped with an optically active solute. In smecric phases, the constituent molecules are further arranged in layers with their long axes parallel to one another and at some angle (usually perpendicular) to the plane of the layers. There is usually no positional correlation from layer to layer, and the interlayer regions are generally less-ordered (i.e., more fluid) than the intralayer regions are. A variety of different smectic types are known, and these are classified according to the type of packing within the layers and the preferred angle of tilt between the long molecular axes and the layer planes (2). Figure 1 shows an idealized representation of the molecular order present in nematic and smectic liquid crystals. It is well-established that solutes incorporated in liquid crystalline phases are oriented in a manner and to an extent that depends primarily on structural similarities between the solute and the mesogen (2a-b,3-8). The rotational and diffusive mobility of both the solute and the mesogen is rendered strongly anisotropic, an effect that also correlates (for the former) with solute/mesogen

structural similarities. There has been increasing interest over the pa.! twenty years in the possibility of exploiting the orientational order present in liquid crystals to affect or even control the chemical reactivity of

482

b.

00 00000000 0000000000 0000000000 0000000000

Figure 1. Idealized representation of the molecular order in (a) nematic and (b) smectic liquid crystalline phases. dissolved solutes. Various possibilities have been explored, including thermal sigmatropic rearrangements (9-1 1) and other isomerization reactions (12-13, thermal (16-17) and photochemical (18-20) cycloadditions, fragmentations (21-23), and polymerization reactions (24). Until fairly recently however, the observed effects have generally been modest or negligible, with disappointingly few exceptions (12,14b,17-19). It is perhaps due largely to these early frustrations that recent studies in this area have focussed on more direct examinationsof the effects of liquid crystalline media on the conformational mobility of solutes, using well-understoodphotochemical or photophysical systems whose characteristics afford greater sensitivity to detecting fairly minor, medium-inducedeffects on fast conformationalmotions. One common approach for the investigation of such effects in ordered media, and which has also found use in liquid crystals, employs intramolecular fluorescence quenching in linked fluorophore/quenchersystems. The most commonly used system of this type employs the pyrenyl moiety as the fluorophore and an amine, pyrene, erc. as the quencher (25). These systems allow investigation of conformational motions that take place with rates faster than ca. 3 x lo6 s-l (the singlet lifetime of the isolated pyrenyl lumophore), and are aptly suited for studies of conformational dynamics in the more weakly ordered nematic and cholesteric phase types (25). In more highly ordered (i.e. more rigid) media such as smectic phases, the conformational motions leading to excimer/exciplex formation may be slower than this limit (25c), so that the usefulness of the system is curtailed. Ketone photochemistry also offers a convenient tool for investigations of this type, since intramolecular photoreactivity or excited state quenching processes depend very strongly on conformational factors (26). Furthermore, ketone photochemistry offers two very distinct advantages over fluorescence methods for the study of molecular mobility in the more rigid liquid crystalline types. First, the longer intrinsic lifetimes of aromatic ketone triplets compared to typical fluorophores allows investigation of conformational motions that may take place over a wider range in time than that observable with fluorescence methods. Second, the smaller, more compact

483

structures of the probes that can be employed affords systems that are potentially less disruptive of the medium. Two such approaches that have received extensive recent use in studies of conformational mobility in thermotropic liquid crystals are the subject of this review. 2. Thermotropic Liquid Crystals as Photochemical Reaction Solvents The number of commercially available liquid crystals that exhibit smectic and/or nematic phases at or close to room temperature, are optically transparent above 250 nm, that make relatively poor triplet quenchers, and whose structures have been reasonably well-characterized is relatively small. Two liquid crystals that do satisfy these requirements -- n-butyl stearate ( B S ) and rrunr,rrunr-4’-butylbicyclohexyl-4-carbonitrile(CCH-4) -- have been investigated fairly extensively using ketone photochemistry (23,27-34) and it is appropriate to provide some description of their structures before proceeding further. The smectic phases of both of these compounds have been characterized by x-ray diffraction (35-36) and other methods (37-39).

BS

CCH-4

CCH-2

BS forms an enantiotropic smectic ’B’ (Sm-B) phase between 15 and 26OC, in which the individual constituent molecules preferentially adopt fully extended conformations in hexagonally close-packed layers, with their long molecular axes perpendicular to the layer planes. NMR and IR evidence indicates that the constituent molecules rotate rapidly about their long molecular axes in this Sm-B phase, and methylene groups near the middle of the stearyl chains are somewhat more-ordered than those near the ends (35,37-38). This material forms an isotropic liquid phase above 26OC, and a progression of differently structured solid phases below 15OC. 4’-Butylbicyclohexyl-4-carbonitrile (CCH4; a.k.a. BCCN) forms enantiotropic smectic and nematic phases between 28-54OC and 54-79OC, respectively (40). On the basis of x-ray diffraction data (36), smectic CCH-4 has been assigned the bilayer crystal-B structure, similar to the high temperature (monotropic) crystal-B phases of the 4’-propyl (CCH-3) and 4’-pentyl (CCHS) homologs whose x-ray diffraction data were reported by Brownsey and Leadbetter (41), and which exhibit similar (mosaic) textures in the polarizing microscope. The order in these phases is such that the molecules are hexagonally close-packed within double layers, with the long molecular axes oriented parallel to one another and perpendicular to the layer plane. The repeating distance defining the layer thickness (31 A for CCH-4) is somewhat less than twice the molecular length (17.5A), consistent with the interdigitated bilayer smcture, and there is some degree of positional correlation between layers (i.e., 3-dimensional order). It is this latter feature that distinguishes the crystal-B phase from the conventional Sm-B phase type (2c,41). In our studies, we have used several related materials as reference solvents for comparison purposes. 4’-EthyIbicyclohexyl-4-carboninile(CCH-2) forms enantiotropic smectic and nematic phases between 28-44OC and 44-48OC, respectively, and is an isotropic liquid above 48OC (40).

484

The smectic phase of this homolog has a rhombohedra1 structure (36) and possesses a significantly higher degree of order than the crystal-B phase of CCH-4 (39). A 2:l mixture of CCH-2 and CCH-4 (‘EB’) forms a very useful room-temperature nematic phase (25-54OC), and a smectic phase which is similar to that of pure CCH-2 between 10-25OC (39). In some studies (vide infru) we have also used a 16 mol% solution of cyclohexane in CCH-2 (‘EC’) as a model isotropic solvent in experiments conducted at 3OOC.

3. Characterization of Bulk Solutehlesonen Solution Behavior. While the phase structures of the pure mesogens employed in studies of this type may be well-characterized, it is always necessary to determine the effects of solute incorporation on bulk medium structure. The effects of the medium on solute reactivity are microscopic ones, and can only be completely understood (or at least meaningfully interpreted) when the structure of the bulk solute/mesogen system -- i.e. the macroscopic solvation behavior -- is known. The effect that is easiest to monitor is that on the phase transition temperatures of the mesogen, and this can be conveniently determined by thermal microscopy. Solute incorporation almost always results in a lowering and broadening of the bulk phase transition temperatures to an extent that increases with solute concentration. For a given concentration of added solute in a given liquid crystal, the magnitude of this effect depends primarily on how different the solute and mesogen are structurally. It has been shown by thermodynamic methods that this reflects specific disruptive effects of the solute on solvent order at the molecular level (7,8). It is thus not entirely surprising that many of the effects discussed in the following sections usually vary dramatically with subtle changes in solute structure. On the macroscopic level, nematic and cholesteric phases (being fairly weakly ordered and thus most similar thermodynamically to isotropic liquids) are normally retained intact upon incorporation of even high concentrations of a solute (8,26). The effects of incorporation of small amounts of a solute on bulk smectic phase structure may be more complicated and difficult to ascertain (or even detect) by qualitative methods like thermal microscopy. A more exhaustive approach to characterization of the solutions or mixtures of interest should therefore be used for smectic phases. In our experience, the most reliable method involves determination of the complete phase diagram for the solute/mesogen system, using a combination of analytical techniques such as differential scanning calorimetry (DSC), thermal microscopy, and deuterium NMR (using deuterated analogs of the solute and/or liquid crystal) (42). An example of the interpretative problems that arise because of incomplete characterization of mixed solute/liquid crystal systems is discussed in detail in a later section. 4. Actinometry in Liquid Crystalline Solvents.

Reaction quantum yield determinations using conventional chemical actinometers in isotropic solution are problematic in studies employing liquid crystalline solvents, because liquid crystals (particularly smectic phases) tend to scatter and/or reflect light, and impurity quenching

485

rates are much slower in these media than they are in isotropic solution (20,28,33). The light scattering problem can sometimes be overcome by orienting the sample in a magnetic field or annealing it to promote surface-induced homeotropic alignment. If the efficiency of a photoreaction is being used as a probe for liquid crystalline effects on other processes (intramolecular excited state quenching, for example), then it is also necessary to know something about the effects of the medium on the probe reaction itself. One solution to these problems is to irradiate the actinometer as a mixture in the liquid crystal of interest (33). This procedure tends to eliminate artifacts due to light scattering and impurity quenching, but can introduce new ones if the actinometer is solubilized differently than the substrate is in the liquid crystal or more generally, if the nature of its reactivity differs from that of the substrate. Thus, the actinometer should be such that its sbucture (length and breadth are most important) and reactivity are as close as possible to those of the substrate of interest. 5. Intramolecular Ketone Photoreactivity as a Probe for Conformational Mobility. The Nomsh Type I1 Reaction of Aliphatic and Aromatic Ketones.

The Nonish Type I1 reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the I,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation @-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the mplet.state reaction are illustrated in Scheme 1 (30). It is generally accepted that the product dismbution obtained in the' triplet state reaction depends primarily on the behavior of the triplet 1P-biradical prior to intersystem crossing, since the collapse of the singlet species is too fast to allow significant changes in conformation to occur (43,44). Transoid 1,4-biradicals can undergo only fragmentation, while the cisoid conformers can in principle yield fragmentation or cyclization products in addition to collapsing to starting material. Recent work suggests that cyclization dominates over fragmentation in cisoid 1,4-biradicals (Ma). How might one expect the relative yields of fragmentation and cyclization products to vary in an environment that places restrictions on the conformational mobility of the molecule or on specific biradical conformations that can be accomodated? The answer to this question depends on the relative rates of biradical conformational equilibration and collapse to products (45). When conformational equilibration is slow, cyclization products frequently dominate because the conformations required for their formation are closest to that initially formed in the hydrogen abstraction step (45-47). Recent studies of the Norrish TI reaction of aliphatic

486

Scheme 1. The Nomsh II Reaction

tiam

x

R

cis

+ x%

ketones in zeolites (46), inclusion compounds (47), and in the solid state (48) (all of which provide a rigid, specifically dimensioned environment that is capable of very strong control over the conformations accessible to included solutes) demonstrate most conclusively the effects of rigid media on Nomsh 11reactivity under these conditions. When the various biradical conformers can equilibrate fairly quickly (as appears to be the case in liquid crystalline solvents), the ratio of fragmentationlcyclization(F/C)products obtained in the reaction depends on the relative stabilities of the transoid and cisoid 1,4-biradicalintermediates and their relative kinetic behavior with respect to collapse to products (29,30,45). Nomsh I1 F/C ratios i n liquid crystalline solvents, when they do differ from their values in isotropic media, usually lie in favor of fragmentationproducts. Weiss and coworkers have proposed that this is both because transoid biradical conformers should be favored over cisoid ones in liquid crystalline media, and fragmentation involves the least demanding changes in molecular shape as the reaction proceeds (27-30). Cleavage of transoid 1,Cbiradicalsneed involve only fairly minor conformational motions, in that facile cleavage requires only that there be some degree of overlap between the p-orbitals on the biradical termini and the C2-C3 o-bond that is cleaved (43). This situation can be achieved without significant displacement of the "1,4-substituents"from their equilibrium positions. Cyclobutanol formation, on the other hand, requires that the p-orbitals on the biradical termini point toward one another. Attainment of such a geometry requires conformational motions that displace the biradical substituentsconsiderably from their equilibrium positions. Furthermore, the motions required for formation of the cis-cyclobutanol isomer are considerably more severe than those involved in formation of the zruns-isomer, with the result that the latter is the favored isomer in liquid crystals (as well as other constrained media) when products of this t w are formed. The propensity for liquid crystalline solvents to alter Nomsh I1 reactivity in this way depends dramatically on the similarities between the structures of the ketone and that of the mesogen, as well as the magnitude of the change in overall solute shape that must accompany the conformational motions of the 1,4-biradicalsas they collapse to products.

487

Weiss and coworkers have conducted thorough investigations of the Nomsh II reaction of several aromatic (1,4-7) and aliphatic (23) ketones in the isotropic, liquid crystalline, and solid phases of BS (27-29) and CCH-4 (30,49). Their experiments using BS as solvent demonstrate particularly elegantly the remarkable correspondencebetween solute/mesogen structural similarities and the ability of smectic (and solid) phases to control solute conformational motions. 0

1 a: n = 4 b: n = l O c: n = 17 d n=19 e: n = 2 1

2

3

The Nomsh II F/C product ratios from photolysis of a series of "alkylphenones (1) in the isotropic, smectic, and solid phases of BS shows significant phase dependence only for ketones whose lengths are similar to that of n-butyl stearate itself (29). The shorter ketones in the series show phase-independentbehavior, This structural dependence is partially illustrated in Figure 2, which summarizes the reported results for ketones l b and Id. Comparisons of qualitative reaction efficiencies for la-d in the smectic and isotropic phases indicate that the total reaction quantum 0.0. 10 20 30 40 yields do not vary significantly in the two phases. V°C) This was interpreted as indicating that triplet state Figure 2. F/C ratios versus temperature for y-hydrogen abstraction is not impeded significantly photolysis of lb (0) and Id (H) in the solid in the smectic phase, consistent with the benzoyl (K), Sm-B (S), and isotropic (I phases ) of group residing in the relatively disordered ester n-butyl stearate. Arrows show the phase region of the smectic layers. transition temperatures (Data from ref. 29). Clearly, the ability of the smectic phase to control biradical reactivity is greatest when the length of the solute is very similar to that of the mesogen. When this is the case, solute-induceddisruption of solvent-packingwithin the smectic phase is minimized and as a result, the effects of solvent order on the relative kinetic behavior of the cisoid and transoid biradical intermediates are maximized. Analogous results have been obtained for the reactions of 2- and sym-alkanones (2 and 3, respectively) in the same liquid crystal, and Figure 3 illustrates the correspondence between ketone chain length and the Nomsh I1 F/C ratios obtained from photolysis in smectic BS (27). For reference purposes, it should be noted that the isotropic phase (300C)F/C ratios are almost invariant throughout the two series of compounds, and lie in a range of 3-4 for 2 and 1.5-2.8 for 3. These two series of compounds probe the effects of different regions of the smectic layer on

488

*

15

FIG 10

-

a.

rn

6 -

B B

:

b.

F/C

-

4 -

'

B

2-

5 -

-

B.

1

1

1

1

1

,

1

1

1

1

i

1

s

q

I

.

,

.

,

.

,

,

Figure 3. Dependence of Norrish I1 F/C ratios on ketone chain length for photolysis of 2 (a) and 3 (b) in the Sm-B phase of n-butyl stearate (data from ref. 27b). biradical behavior: the reactive centre in 2 is expected to reside in the relatively fluid, ester region of the layers, while the carbonyl group in 3 should reside in the non-polar, tightly packed hydrocarbon region. Comparatively speaking though, the F/C ratios are in fact affected more strongly by the smectic phase for 2 than for 3. The authors interpreted this as indicating that placing a carbonyl group in the middle of the layer has greater disrupting effects on solvent order in the vicinity of the reactive center than solvation in the more accomodating interlayer region. In both cases, the effect on biradical reactivity is maximized when the length of the ketone is similar to the smectic layer thickness. Similar trends are found in the tlc cyclobutanol ratios, and in the reactivity of 2 and 3 in the solid phases of BS. The authors proposed that the reactivity of 2 and 3 in all phases of BS was dominated by that of the triplet state, because of the relatively high medium viscosity in each case. The Norrish 11reactivities of the phenylalkyl ketones 4-7 have been investigated in the isotropic, solid, and 1iquid.crystalline phases of CCH-4 by Zimmermann, Liu, and Weiss (30). These compounds do not vary appreciably in length or breadth throughout the series, and all are of

4

5

6: n = 3 7: n = 4

similar length to CCH-4. What does vary is the degree of molecular contortion involved in the reactions of the corresponding 1,4-biradicals derived from y-hydrogen abstraction. The variation in F/C ratio with bulk solvent phase for these compounds varies dramatically throughout the series, and the results have been rationalized in terms of the specific conformational motions that each of the associated 1,4-biradicals must undergo in proceeding to their respective products. The variation in F/C ratio with bulk solvent phase is particularly striking for 6 and 7,as is shown in

489

10

FIC

.

-

1 4rn

6-

4-

S-N 24.. 0

*

20

N-I

. 40. . 60.

*

80

T ("C)

0

20

40

60

80

T ("0

Figure 4. Fragmentatiodcyclization PIC) ratios from photolysis of 6 (a) and 7 (b) in CCH-4 vs. temperature. The arrows show the optically-measured phase transition temperatures of the pure mesogen (data from ref. 30). Figure 4. While solvent order in the nematic phase exerts very weak or no control over biradical reactivity, the effects in the smectic and solid phases appear to be quite pronounced. The full explanation of these results is not as straightforward as those discussed above for the n-butyl stearate systems, however. Treanor and Weiss have recently described deuterium NMR results for mixtures of the a-deuterated analogs of 4-7 in CCH-4 (49), from which they have concluded that the solubilization behavior of these compounds in CCH-4 is rather more complex than was originally thought to be the case. They have proposed that 4-7 in fact reside in a solute-induced plastic or cubic phase ("p-phase") at temperatures below 20-35OC, depending on the ketone. The ketones exhibit isotropic 2H and 13CNMR behavior in this phase. While the formation of the p-phase is clearly evident by NMR, there is no indication of it in any of the plots of Nomsh II product ratios vs. temperature: i.e., the plots are all continuous above and below the temperature at which the p-phase is formed. In order to rationalize the isotropic NMR behavior with the substantial restrictions to biradical mobility that the Nomsh I1 results imply, they have proposed that the p-phase consists of microscopic pools of solute and mesogen which tumble isotropically as a unit, but in which the ketone suffers restrictions to its conformational mobility. Similar NMR behavior has been observed as well for mixtures of a wide variety of other solutes in CCH-4 (42,50,5l), and p-phase formation now appears to be a quite general phenomenon in mixtures of non-mesomorphic solutes with this liquid crystal. However, the morphology of the solute-induced phase is not yet established, and a somewhat different interpretation of these and other results (vide infra) has recently been proposed (42). We have investigated the Nomsh I1 reactivity of 4-methoxyvalerophenone at a single temperature (3OOC) in various isotropic solvents, nematic EB, and the smectic phases of CCH-4 and CCH-2 (34). F/C and rlc cyclobutanol ratios obtained from photolysis of this compound in these solvents are shown in Table 1. Both the F/C and tlc ratios are significantly larger in CCH-4 than in the nematic or isotropic phases studied. This result seems a bit surprising at first, in view of the very small difference in the shapes of the transoid and cisoid biradicals derived from this ketone. However, we have recently obtained deuterium NMR evidence that suggests that this

490

Table 1. Fragmentatiodcyclization (FIC) and translcis (rlc) cyclobutanol ratios from photolysis of 4-methoxyvalerophenonein isotropic, nematic, and smectic solvents at 3OoC (data from reference 34). Solvent -- MeCN

MCHa

EC(isot)b

EB (nem)"

CCH-4 (sm)

CCH-2 (sm)

FIC

4.4f0.3

4.2f0.4

4.4f0.2

6.1f0.2

9.2f0.3

4.7f0.3

tlc

2.2f0.1

4.4%0.2

2.2k0.2

2.6f0.2

5.2f0.3

3.0f0.3

a methylcyclohexane

1:5 (by weight) cyc1ohexane:CCH-2 2:1 CCH-2:CCH-4

ketone may be solubilized differently in CCH-4 at this temperature than most of the other compounds that have been studied. This compound does not form a p-phase with CCH-4 in mixtures of bulk concentration less than ca. 1.0 mol%, and appears to be truly soluble (in this concentration range) in the crystal-B phase at 3OOC (5 1).

6. Intramolecular Quenching of Aromatic Ketone Triplets as a Probe for Solute Conformational Mobility. A second approach that has found effective use in probing the effects of ordered media on solute conformational motions involves the investigation of intramolecular triplet quenching processes in aromatic ketones. Provided that intramolecularquenching occurs only by direct contact of the excited carbonyl and quencher moieties (quenching by through-bond mechanisms (52) must obviously be avoided), then the effects of the medium on the rates of the conformational motions required for formation of the quenching geometry is directly reflected in the triplet lifetime of the molecule. By judicious choice of the structure of the ketone and the quenching mechanism, one can tailor the system to report on different types of conformationalmotions. The triplet lifetime of the ketone can, of course, be measured directly (e.g.by nanosecond laser flash photolysis or phosphorescence decay) or estimated by indirect methods (e.g.,by measuring the quantum yield of some characteristic triplet reaction). We have used both approaches in our studies of liquid crystalline effects on solute conformational mobilities (31-34). Scheme 2 summarizesthe main features of the two systems that we have investigated: P-phenyl quenching in P-arylpropiophenoneand -butyrophenonederivatives (8 and 9, respectively) (31-33), and intramolecular phenolic quenching in remote-functionalizedpara-alkoxy aceto- and valerophenones(10 and 11, respectively) (3453). Scheme 2 also defines a simple kinetic scheme to describe the decay of the ketone triplet state in such molecules, and this leads to the expression shown in Equation [ 11 for the overall triplet decay rate (kd). The rate constant kdorefers to the "normal" decay of the triplet in the absence of

491

Scheme 2. Intramolecular quenching of aromatic ketone triplets . A.

8(R=H) a. X = OCH3 b. X=O"C3H, C. X = O"CSH,I

O* R kC

X

lkq

~

9 (R = Me) a. X = H b. X=OCH3

Fast Decay (R = H) or No photochemistry (R = Me)

Normal Decay (R = H) Photochemistry (R = Me)

B.

10. R CH3 n*

- l T

(bc

Normal Decay (R = Me) Photochemistry (R = "Bu)

I

T

o

11. R = "CqH9

R

q

R

-

10/11

OH

the intramolecular quenching process described by k,, and includes the rates of all radiative, non-radiative, and reactive processes that normally contribute to triplet decay of the isolated moiety. It is clear from Eqn. 1 that as long as the quenching process is fast in relation to the conformational motions required for formation or collapse of the quenching geometry (i.e. kq B kc), k, can be obtained directly from kd after correcting for kdo (when necessary) using data from model compounds. a. The Investigation of "Solute-bending"Motions: D-Phenyl quenching. It is well-established that P-phenyl ketones owe their inertness towards conventional ketone

492

photoreactivity (54-57) and unusually short triplet lifetimes in fluid solution (55,56) to efficient intramolecular deactivation via exciplex interactions between the carbonyl group and the P-phenyl ring (55). The effect is dramatic: the triplet lifetime of P-phenylpropiophenone in fluid solution (=1 ns at 27OC (55)) is several orders of magnitude shorter than that of propiophenone under similar conditions, and the quantum yield for Nomsh I1 fragmentation of P-phenylbutyrophenone (9a) is roughly one-tenth that of butyrophenone in benzene solution at room temperature (54b). The rate-determining step for P-phenyl quenching has been conclusively identified as comprising the bond rotations involved in formation of the gauche-conformer in which the 0-phenyl ring is placed in close spatial proximity to the carbonyl n-orbital. For example, the activation energy for triplet decay of P-phenylpropiophenone in fluid solution (2.3 kcal/mol) is approximately that for rotation about the C,-CB bond. Furthermore, when the molecule is incorporated in rigid matrices that allow only the trans-conformation to be assumed (e.g. zeolites), the triplet lifetime is extended to ca. 2 ms at room temperature and phosphorescence can be observed (57). para-Alkoxy substitution renders the R,X* state the lowest energy triplet state in phenylalkyl ketones (43), and in B-phenyl-substituted cases, this results in a somewhat longer triplet lifetime and higher activation energy for triplet decay in fluid isotropic solvents (56). Like the Nomsh Type I and Type II reactions, P-phenyl quenching requires the proximity of the n,x* state, so that in ketones of this type, the extra energy requirement is that required for thermal population of the higher energy state (%a). This additional (potential) kinetic complexity is overshadowed by practical considerations when it comes to direct triplet lifetime measurements however, since para-alkoxy substitution affords longer-lived and much stronger triplet-triplet absorption compared to n,x* triplet ketones. Since the X,X*-n,X*triplet energy gap should not be particularly sensitive to solvent rigidity or order (58), any increase in triplet lifetime or changes in the Arrhenius parameters for triplet decay that might occur in constrained compared to isotropic media can be safely assumed to reflect medium-induced effects on the conformational motions required for P-phenyl quenching to occur. The molecular motions that are probed in P-phenyl ketones can be qualitatively described as "bending" motions of a rod-like solute. Since the molecular length of the solute can be easily altered by substitution in either the benzoyl or P-phenyl rings, this system offers a potentially very useful one with which to investigate the effects of liquid crystalline order on conformational motions involving a wide range of solute shape changes. A very rough indication of the effects of liquid crystalline order on P-phenyl quenching is reflected in the quantum yields for Nomsh I1 fragmentation of P-phenylbutyrophenone (9a) and the 4-methoxy derivative (9b) dissolved in the smectic phases of CCH-4 and CCH-2 at 3OoC(33). (For CCH-4, it is highly likely that 9a and 9b are in fact solubilized in a "p-phase" at this temperature, similar to those discussed above for 4-7 (42)). These preliminary data were measured relative to para-methoxyvalerophenone fragmentation in acetonitrile as the actinometer, and are shown in Table 2. While these data hold little real quantitative meaning (as far as isolating the medium's effect on the conformational motions involved in P-phenyl quenching) owing to the inadequate actinometric procedure employed and the fact that comparable data in viscous isotropic or nematic solvents have not yet been obtained, they do provide a somewhat novel demonstration

493

-Table 2. Quantum yields for Nomsh n fragmentation of B-phenylbutyrophenone derivatives

9a and 9b in smectic CCH-4 and CCH-2 and acetonitrile solution at 3OoC(33).,

Ketone

MeCN

CCH-4 (Sm)

CCH-2 (Sm)

9a

0.008 -f 0.002

0.20 f0.03

0.26 f 0.1 1

9b

0.0013 zk 0.0004

0.13 k 0.04

0.07 f0.01

From photolysis of deoxygenated, 0.03M (MeCN) or 1 mol% solutions with 3 10 nm light (33), relative to 4-methoxyvalerophenonefragmentation in acetonitrile (h= 0.13 (53)). Errors represent the standard deviation of the mean from duplicate vpc analyses of triplicate runs in each solvent.

a

of how photoreactivity can be enhanced in liquid crystalline relative to fluid isotropic solvents as a result of suppressing other intramolecular pathways for excited state decay. A far more quantitative indication of the effects of these liquid crystals on the conformational motions involved in P-phenyl quenching is provided by direct triplet lifetime measurements of the ~-phenyl-4-alkoxypiophenonederivatives 8a-c as solutions in CCH-4 and CCH-2, measured by nanosecond laser flash photolysis (31,59). The investigation of 1 mol% mixtures of the ketones with both mesogens over the 30-90°C temperature range allows for a complete comparison of the behavior of the solutes in isotropic, nematic, and two differently structured smectic phases of similar macroscopic polarity. The UV absorption spectra of each of these ketones in all phases of CCH-4 are very similar to that in acetonitrile solution (31,32); thus, the chromophore experiences a moderately polar environment in all phases. Arrhenius plots showing the variation in the triplet lifetimes of these ketones with temperature and solvent phase in the two mesogens are shown in Figure 5. These plots reveal several striking features. For all three ketones, the activation energy for triplet decay is 1.5-2 kcaVmol higher and AS' is 3-4 e.u. more positive in the nematic phase of CCH-4 compared to the corresponding values in the isotropic phase of CCH-2. In fact, these differences appear to be roughly constant throughout a derivatives incorporating a very wide range of substituent series of P-aryl-4-alkoxypropiophenone lengths and flexibilities, from which we have concluded that the small inhibitory effect on conformational mobility in the nematic phase is primarily the result of microviscosityfactors, and not specific solvent ordering effects (32). Second, the medium's effect on P-phenyl quenching in the smectic phase of CCH-4 is strongly dependent on the length of the ketone in relation to that of the mesogen. The effect is striking in the case of 8a; in the temperature range below the N-Sm phase transition of the bulk mixture, E, and AS' arc ca. 10 kcavmol and 30 e.u. more positive, respectively, than their values in the bulk nematic phase. This was originally attributed to strong smectic phase-induced inhibition to the conformational motions involved in P-phenyl quenching, with the large positive AS' term arising from the severe disruptive effects that such motions profer on liquid-crystallineorder in the

494

solute's vicinity (31). The response of the triplet lifetime of 8a to temperature and solvent phase in CCH-4 contrasts the phase-independent behavior of the same ketone in CCH-2 and the phase-independent behavior of 8c in both mesogens. Ketone 8a is one carbon atom shorter than CCH-4 and one carbon atom longer than CCH-2. The "propoxy-derivative 8b shows intermediate behavior in CCH-4 of a very intriguing nature; the temperature at which the change from nematic to smectic behavior occurs is at a significantly lower temperature (cu. 41O) than the bulk N-Sm transition temperature of the mixture (50-52O) (59). Third, the fact that all ketones of general structure 8 undergo a continuous variation in their 6.5 triplet state behavior (whether phase-dependent or . . . . . . not) at the bulk Sm-N solvent phase transition b. indicates that in spite of the dramatic change in bulk (thermodynamic) solvent order that occurs at the phase transition, the ketone's immediate solvation shell changes only slightly or not at all with respect to its restrictions on the 6.5 conformational mobility of the solute. A nematic-like environment in the immediate vicinity of the solute in the smectic phase, with subtle differences in structure depending on the solute, couId be suggested by this behavior (32). A recent examination of the bulk phase behavior of mixtures (0.5 - 70 mol%) of 8a with 6.5 CCH-4 by thermal microscopy, DSC, and 2.8 3.0 3.2 deuterium NMR spectroscopy with a deuterated IO-~K/T analog of 8a has provided a detailed picture of the Figure 5 . Arrhenius plots for triplet decay of solubilization of this ketone in the liquid crystalline 8a (a), 8b (b), and 8c (c) in the isotropic, phases of CCH-4 (42), and provides further clues to nematic, and smectic phases of CCH-4 (0) the origins of the solute length effect discussed and CCH-2 (0). Filled arrows show the transition temperatures for CCH-4 and the above. This study indicates that the solubilization open arrows show those for CCH-2 (Data of 8a in the crystal-B phase is rather more complex from refs. 31a, 39). than was previously thought. By deuterium NMR, we have ascertained that the limit of solubility of 8a in the crystal-B phase is in fact somewhat less than 1.0 mol%. Under the conditions of our flash photolysis experiments, 8a is distributed between both a ketone-rich nematic phase and a ketone-depleted smectic phase at temperatures between 53-35OC. At 35O, the ketone-rich phase is transformed into a "p-phase" in which the ketone exhibits isotropic 2H and I3C NMR behavior, and which eventually crystallizes (though not under these conditions) to fonn a stable binary smectic or solid modification consisting of 8-10 mol%ketone and CCH-4. Thermal microscopy experiments with mixtures containing 1-3

]

1

i

495

mol% 8a do not allow definitive conclusions to be made with respect to the morphology of the p-phase (it is very difficult to even detect it by this method). Experiments with mixtures containing larger amounts of the solute (5-10 mol%) suggest that the p-phase may in fact be a viscous isotropic liquid or a glass of composition ca. 25-40 mol% 8a, not a plastic or cubic phase as previous workers have proposed. Overall triplet state behavior in the 28-53O range may thus be the composite of two coexisting environments whose compositions and distribution vary with temperature. However, clearcut evidence for this could not be obtained from the triplet decay profiles, and triplet decay rates in the bulk nematic and isotropic phases have been found to be composition-independent up to 15 mol% 8a (42). We thus believe that the behavior of 8a below 53OC represented in Fig. 5a does indeed reflect (in some way) the inhibiting effects of the crystal-B phase on the conformational mobility of the ketone. Unfortunately, we cannot yet be certain as to the precise nature of this effect. Similar NMR behavior has been observed for a deuterated analog of 8b, and it is thus very likely that phase separation occurs as well for all the P-phenylpropiophenone derivatives that we have studied in CCH-4. Preliminary binary phase diagrams for the 8b/CCH-4 and 8c/CCH-4 systems appear to be significantly different from that of 8a/CCH-4 (although similar to one another) (59). Subtle variations in the solubility of these compounds in the crystal-B phase with substituent could result in marked variations in the temperature-dependent distribution of ketone between the two phases present in the 30-53O range, and might possibly explain the variation in Arrhenius behavior throughout the series that is exemplified in Figure 5 . The above results suggest that in general, the chemical behavior of probe molecules in highly ordered liquid crystalline phases such as this one may be governed by rather complex, heterogeneous solubilization effects. The manifestation of this in individual systems will depend primarily on solubility factors. The behavior of 8 in the smectic phase of CCH-2 is likely to be related to that in CCH-4, but a more exhaustive study of these systems has yet to be camed out.

b. End-to-end Cyclization Motions: Intramolecular Triplet quenching by Remotely-situated phenols. While the full mechanistic details of aromatic ketone triplet quenching by phenols are not yet established, it is known that quenching occurs largely by phenolic hydrogen abstraction (53,60). The bimolecular process occurs at the diffusion-controlled rate for para-alkoxy substituted ketones in fluid solution, leading to the formation of the corresponding phenoxy and ketyl radicals which have been observed directly by laser photolysis methods (60). Investigation of the photochemistry of the two systems shown in Scheme 2b in fluid solution has verified that rapid intramolecular triplet quenching occurs by the same mechanism as in the bimolecular case, leading to the formation of the corresponding 1,13-ketyl-phenoxy biradical, presumably via prior adoption of a sandwich-like conformer which places the phenolic hydrogen in close proximity to the excited carbonyl group (53). The mplet lifetime of 10 (cu. 15 ns in acetonimle solution at 25OC) is subject

496

to a substantial deuterium isotope effect, indicating that phenolic H-abstraction (not the conformational motions required for formation of the sandwich-likequenching conformer) comprises the rate determining step for intramolecular quenching in fluid solution (53). Indeed, studies of intramolecular excimer formation in 1,3-diarylpropanesindicate that these conformational motions occur within a time scale of 1-2 ns under these conditions (51). Considering the dramatic effects proferred by the liquid crystalline phases of CCH-4 on the much subtler conformational motions involved in P-phenyl quenching in 8 and 9 (31-33), we expected (and found) very large alterations in the triplet lifetime andor reactivity of 10 and 11 upon incorporation in the liquid crystalline phases of CCH derivatives (34). Again, the rate of the conformational motions leading to formation of the quenching conformer can be monitored either by direct triplet lifetime measurements in the case of the methyl ketone (lo), or indirectly by measuring the quantum yield of Nomsh I1 fragmentation in the valeryl ketone (ll),relative to that of the comsponding methoxy-substitutedhomologs (12 and 13) in which intramolecular phenolic quenching is not possible.

Thermal microscopic inspection of 1 mol% mixtures of 10-13 and 4-methoxyvalerophenone with CCH-4 indicate that not all of these ketones are equally soluble in the crystal-B phase of the mesogen. For 11 and 13, these experiments provide a very clear indication of phase separation at 30°C. The mixtures containing 4-methoxyvalerophenone and 10 appear to be homogeneous at this temperature and their phase transition temperatures are reasonably sharp. Considering our deuterium NMR results for 8a however (42), it is very likely that 10 resides largely in a solute-inducedphase (of as yet unknown morphology) in CCH-4 at this temperature. The 12/CCH-4mixture exhibits behavior intermediate between these two extremes. Photolysis of 11 at 30°C as 1 mol% mixtures in CCH-4 (Sm). CCH-2 (Sm), EB (N), and a 1:6 solution of cyclohexane in CCH-2 (EC; isot.) leads to detectable yields of Norrish I1 fragmentation products in each case (34). Relative quantum yields for fragmentation of 11 and 13 are listed as the ratios l 1 k / l 3 ~in Table 3. Since 11 and 13 can be assumed to be solvated Table 3. Relative quantum yields for the Nomsh I1 fragmentation of 11 and 13 in isotropic and liquid crystalline solvents at 30°C.' Solvent 114Q I 1 3 h

MeCNb c 0.001

EC (isot)

EB (nem)

CCH-4 (sm)

CCH-2 (sm)

0.03 k 0.02

0.09 k 0.02

0.12 f 0.04

0.015 k 0.005

497

similarly in the various solvents used, consideration of these ratios rather than the individual quantum yields relative to that of 4-methoxyvalerophenone (which may be solubilized differently in CCH-4; vide supra) removes potential interpretative problems caused by the actinometer. It is clear that intramolecular phenolic quenching is slowed down enough even in viscous isotropic solvents (EC) to allow Norrish XI abstraction to proceed to at least a detectable extent, but it is still only ca. 3% as efficient as in 13 where phenolic quenching is not possible. Nematic solvent order (EB) has a greater inhibitory effect on the process, but as anticipated, the effect is little more than one would expect from microviscosity effects (32). The relative quantum yields in CCH-4 at 30° are very close to the EB values, consistent with the indication from thermal microscopy that these ketones are solubilized in a nematic-like environment. Considering the propensity of solutes of similar structure to form "p-phases" with CCH-4 at this temperature (42,49), it is highly likely that the behavior of 11 also reflects heterogeneous solubilization effects of this type. As discussed earlier, the p-phase in mixtures of CCH-4 with solutes of this type is derived from a higher temperature, solute-rich nematic phase, and the conformational mobility of the solute is similar in the two phases. As was found previously for 8 (31,32), the smectic phase of CCH-2 provides an astonishingly mobile environment for the probe. In contrast to our results for 11, the triplet lifetime of 10 is dramatically affected in smectic CCH-4, as might be expected on the basis of its similar length to 8a. Triplet lifetime data for 10 and 12 in CCH-4, EB, and acetonitrile at 3OoC, obtained by nanosecond laser flash photolysis, are listed in Table 4. Comparison of the triplet lifetimes for 10 and 12 in the smectic phase of CCH-4 indicates that the conformational motions involved in phenolic quenching in 10 are almost completely suppressed in this phase. The transient spectroscopic behavior of the solution of 10 in nematic EB is complex owing to the temporal and spectral overlap of absorptions due to the ketone triplet and the biradical. In this solvent, the lifetime of the biradical was estimated to be ca. 320 ns by analysis of the transient decay profile at a wavelength where absorption due to the biradical is significantly stronger than that due to the triplet (34). The similarity in the triplet and biradical lifetimes in EB is due presumably to the fact that the decay of both species is controlled by the rates of similar conformational motions; both triplet quenching via phenolic hydrogen abstraction and biradical decay via intersystem crossing require end-to-end cyclization to a sandwich-like Table 4. Triplet lifetimes for 10 and 12 in isotropic and liquid crystalline solvents at 30°Ca Solvent

10

12

MeCNb

0.015 k 0.001

2.3 C 0.2

0.28 k 0.03

5.6 C 0.6

3.8 k 0.4

I l . O ? 1.1

EB (nem)c CCH-4 (sm)'

Measured on deoxygenated solutions by nanosecond laser flash photolysis; lifetimes given in microseconds. From ref. 53. .c From ref. 34. EB and CCH-4 solutions were 0.2 and 1.0 mol% in ketone, respectively.

.a

>

498

conformer (34,53,62). Preliminary Arrhenius plots for triplet decay of 10 in isotropic and nematic EB (59) indicate that the effects of the nematic phase on the mobility of this ketone are due largely to microviscosity effects, as was proposed earlier to explain the triplet state behavior of P-phenyl ketones in nematic CCH-4. 7. Liquid Crystals as Reaction Solvents: Conclusions and Scope. It has been demonstrated that liquid crystals are capable of controlling the conformational mobility of dissolved solutes in dramatic ways. While the effect in nematic phases is small and probably due mainly to simple microviscosity effects, smectic phases are capable of profemng quite substantial restrictions on the conformational motions of dissolved solutes. This may lead to drastic alterations in chemical reactivity that is associated with these motions. The ability of smectic phases to alter conformational mobility and associated chemical reactivity depends on quite subtle factors relating to the relationship between the solute’s structure and that of the mesogen, and the degree of contortion that is involved in the process. The effects can be profound, and can result from either homogeneous or heterogeneous behavior of the solute/mesogen binary system. Ketone photochemistry provides both a useful tool with which to investigate these properties, and an important facet of molecular reactivity that can exploit them. There is no question that our understanding of the subtle ways in which liquid crystals can affect the chemical behavior of dissolved solutes has progressed by leaps and bounds in a relatively short period of time; there is also no question that we have a great deal more to learn in the area. Few other chemical environments can be provoked to alter their solvation properties so dramatically with such subtle changes in guest structure. As a supplement to their already enormous technological importance, liquid crystals will undoubtedly finally emerge as significant and valuable tools for the mechanistic and synthetic chemist. References 1 . This is Part 7 of our series ”Organic Reactions in Liquid Crystalline Solvents”. For Part 6, see:

W. J.LeighandD. S.Mitchel1, J.Am.Chem. S o c . m ( 1 9 8 8 ) , 1311-1313. 2. For comprehensive descriptions of the structure and properties of liquid crystals and leading references, see (a) F. D. Saeva (Ed.), Liquid Crystals. The Fourth State of Matter, Marcel Dekker, Inc., New York, 1979; (b) H. Kelker and R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980; (c) G. W. Gray and J. W. Goodby, Smectic Liquid Crystals. Textures and Structures, Leonard Hill, Glasgow, 1984. 3. E. Sackmann, P. Krebs, H. U. Rega, J. Voss, and H. Mohwald, Mol. Cryst. Liq. Cryst. 24 (1973), 283. 4. F. D. Saeva, Pure Appl. Chem., 3 (1974), 25-36. 5. Z . Luz, in: J. W. D. Emsley (Ed.), Nuclear Magnetic Resonance of Liquid Crystals. NATO AS1 Series, Ser. C, Vol. 141, Reidel Publishing Co., Dordrecht, 1985, Chapter 13. 6. E. Meirovitch and J. H. Freed, J. Phys. Chem., @ (1984), 4995-5004; and references cited therein. 7. D. E. Martire, in: G. R. Luckhurst and G. W. Gray (Eds.), The Molecular Physics of Liquid Crystals, Academic Press, New York, 1979, Chapter 11. 8. S. Ghodbane and D. E. Martire, J. Phys. Chem., 91 (1987), 6410-6414; and references cited therein. 9. W. E. Bacon and G. H. Brown, Mol. Cryst. Liq. Cryst., 12 (1971), 229-236.

499

10. M. J. S. Dewar and B. D. Nahlovsky, J. Am. Chem. Soc., (1974), 460-465. 11. F. D. Saeva, P. E. Shaqe, and G. R. Oh,J. Am. Chem. Soc., 97 (1975), 204-205. 12. (a) P. D. Maria, A. M,B. Samori, F. Rustichelli, and G.Toquati, J. Am. Chem. Soc., 106 (1984), 653-656; (b) S. Melone, V. Mosini, R. Nicoletti, B. Samori, and G.Torquati. Mol. Cryst. Liq. Cryst. 3 (1983), 399-409; (c) B. Samori, P. De Maria, P. Mariani, F. Rustichelli, and P. Zani, Tetrahedron, 43 (1987), 1409-1427. 13. W. J. Leigh, D. T. Frendo. andP. J. KlaWUM, Can. J. Chem,@ (1985), 2131-2138. 14. (a) E. G. Cassis, Jr. and R. G. Weiss, Photochem. Photobiol., 2 (1982), 439-444, (b) J. P. Otruba III and R. G.Weiss, Mol. Cryst. Liq. Cryst., @ (19821,165-178;(c) J. P. Otruba III and R. G. Weiss, J. Org. Chen, (1983), 3448-3453. 15. C. Eskenazi, J. F. Nicoud, and H. B. Kagan, J. Org. Chem., 44 (1979), 995-999. 16. W. J. Leigh, Can. J. Chem., @ (1985), 2736-2741. 17. W. J. Leigh and D. S. Mitchell, I. Am. Chem. Soc., 110 (1988), 1311-1313. 18. G. Aviv, J. Sagiv, and A. Yogev, Mol. Cryst. Liq. Cryst., 36 (1976). 349-357. 19. T. Kunieda, T. Takahashi, and M. Hirobe, Tetrahedron Lett., 24 (1983), 5107-5108. 20. J. M. Nerbonne and R. G. Weiss, J. Am. Chem. Soc.,100 (1978), 2571-2573; 101(1979), 402-407. 21. W. E. Bamett and W. H. Sohn, J. Chem. Soc., Chem. Commun. (1971). 1002-1003. 22. P. Seuron and G. Solladie, J. Org. Chem., 45 (1980), 715-719. 23. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J. Am. Chem. Soc.,106 (1984), 5291-5295. 24. (a) C. M. Paleos and M. M. Labes, Mol. Cryst. Liq. Cryst., 11 (1970), 385-393; (b) A. Blumstein. N. Kitagawa, and R. Blumstein, Mol. Cryst. Liq. Cryst., 12 (1971), 215-227. 25. (a) V. C. Anderson, B. B. Craig, and R. G. Weiss, J. Phys. Chem., & (1982), 4642-4648; (b) V. C. Anderson, B. B. Craig, and R. G. Weiss, Mol. Cryst. Liq. Cryst., 97 (1983), 351-363; (c) V. C. Anderson and R. G. Weiss, J. Am. Chem. Soc.,106 (1984), 6628-6637. 26. For a review, see: P. J. Wagner, Acc. Chem. Res., 16 (1983), 461-468. 27. (a) R. L. Treanor and R. G. Weiss, J. Am. Chem. Soc.,10s (1986), 3137-3139; (b) R. L. Treanor and R. G. Weiss, Tetrahedron,43 (1987), 1371-1391. 28. J. M. Nerbonne and R. G. Weiss, Isr. J. Chem., U (1979), 266-271. 29. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J. Am. Chem. Soc.,106 (1984), 7033-7037. 30. R. G. Zimmermann, J. H. Liu, and R. G. Weiss, J. Am. Chem. Soc., (1986), 5264-5271. 31. W. J. Leigh, J. Am. Chem. S O C . , (1985), ~ 6114-6116. 32. W. J. Leigh,Can. J. Chem.,64(1986), 1130-1139. 33. W. J. Leigh and C. D. Ryan, unpublished results. 34. W. J. Leigh and S. Jakobs, Tetrahekon, 43 (1987), 1393-1408. 35. D. Krishnamurti, K. S. Krishnamurthy,and R. Shashidar, R., Mol. Cryst. Liq. Cryst., 4 (1969), 339-366. 36. E. Rahimzadeh, T. Tsang, andL. Yin, Mol. Cryst. Liq. Cryst., 139 (1986), 291-297. 37. K. S. Krishnamurthy andD. Krishnamurti. Mol. Cryst. Liq. Cryst., 6 (1968), 407-413. 38. K. S. Krishnamurthy, Mol. Cryst. Liq. Cryst., 132 (1986), 255-261. 39. D. S. Mitchell and W. J. Leigh, Liquid Crystals, submitted. 40. "Licristal Liquid Crystals", EM Chemicals, Hawthorne, N.Y. 1986. 41. G . J. Brownsey and A. J. Leadbetter, J. Phys. (Paris), 42 (1981), L135-LI39. 42. B. J. Fahie, D. S. Mitchell, and W. J. Leigh, Can. J. Chem., in press. 43. (a) P. J. Wagner, ACC.Chem. Res., 4 (1971). 168-177. (b) P. J. Wagner, in: P. de Mayo, Ed., Rearrangements in the Ground and Excited States, Academic Press, New York, 1980, Vol. 3. 44. (a) R. A. Caldwell, S. N. Dhawan, and T. J. Majima, J. Am. Chem. Soc.,106 (1984), 6454-6455. (b) J. C. Scaiano, Tetrahedron, 38 (1982), 819-824. (c) L. J. Johnston, J. C. Scaiano, J. W. Sheppard, and J. P. Bays, Chem. Phys. Lett., 124(1986), 493-498. 45. V. Ramamurthy, Tetrahedron, 42 (1986), 5753-5839; and references cited therein. 46. N. J. Turro and P. Wan, Tetrahedron Lett., 25 (1984), 3655-3658. 47. (a) H. L. Casal, P. de Mayo, J. F. Miranda, and J. C. Scaiano, I. Am. Chem. Soc.,105 (1983), 5155-5156. (b) P. G. Goswami, P. de Mayo, N. Ramnath, G. Bernard, N. Omkaram, J. R. Scheffer, and Y. F. Wong, Can. J. Chem., g (1986),2719-2725. 48. S. Ariel, V. Ramamurthy, J. R. Scheffer, and J. Trotter, J. Am. Chem. Soc.,105 (1983), 6959-6960. 49. R. L. Treanor and R. G. Weiss, J. Phys. Chem., 91 (1987), 5552-5554. 50. (a) B. M. Fung and M. Gangoda, I. Am. Chem. Soc.,10'1(1985), 3395-3396. (b) M. Gangoda and B. M. Fung, Chem. Phys. Lett., 120 (1985), 527-530.

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51. B. J. Fahie, D. S. Mitchell, and W. J. Leigh, to bepublished. 52. H. E. Zimmerman and R. D. McKelvey, J. Am. Chem. Soc.,93 (1971), 3638-3645. 53. J. C. Scaiano, W. G. McGimpsey, W. J. Leigh, and S. Jakobs, J. Org. Chem., 52 (1987), 4540-4544. 54. (a) D. G. Whitten and W. E. Punch, Mol. Photochem., 2 (1970), 77-80. (b) P. J. Wagner, P. A. Kelso, A. E. Kemppainen, A. Haug, and D. R. Graber, Mol. Photochem., 2 (1970), 81-85. 55. J. C. Scaiano, M. J. Perkins, J. W. Sheppard, M. S. Platz, and R. L. Barcus, J. Photochem., 21 (1983), 137-147. 56.(a) J. C . Netto-Ferreira, W. J. Leigh, and J. C . Scaiano, J. Am. Chem. Soc., 107 (1985), 2617-2622. (b) W. J. Leigh, J. C. Scaiano, C. I. Paraskevopoulos, G. M. Charette, and S. E. Sugamori, Macromolecules, 18 (1983,2148-2154. 57. (a) H. L. Casal and J. C. Scaiano, Can. J. Chem., 62 (1984), 628-629. (b) J. C. Scaiano, H. L. Casal, and J. C. Netto-Ferreira,in: M. A. Fox (Ed.), Organic Phototransformationsin NonhomogeneousMedia, ACS Symposium Series No. 278, American Chemical Society, 1985, Chapter 13. %.(a) J. Saltiel, H. C. Curtis, L. Metts, J. W. Miley, J. Winterle, and M. Wrighton, J. Am. Chem. SOC.,92 (1970), 410-411. (b) P. J. Wagner, A. Haug, and M. May, Chem. Phys. Lett., 13 (1972). 545-547. (c) P. J. Wagner, A. E. Kemppainen, and H. N. Schott, J. Am. Chem. SOC.,95 (1973), 5604-5614. 59.D. S. Mitchell, B. J. Fahie, and W. J. Leigh, to be published. 60.P. K. Das, M. V. Encinas, and J. C. Scaiano, J. Am. Chem. Soc.,103 (1981), 4154-4162. 61.(a) W. Klopffer and W. Z. Liptay, Z. Naturforsch., 25A (1970), 1091-1096. (b) F. C. De Schryver and N. Boens, Adv. Photochem., (1977), 359-465. (c) F. C. De Schryver, L. Moens, M. Van der Auweraer, N. Boens, L. Monnerie, and L. Bokobza. Macromolecules,15 (1982), 64-66. 62.(a) C. Doubleday Jr., Chem Phys. Lett., Sg. (1982), 65-68. (b) G. L. Closs, R. Miller, and 0. D. Redwine, Acc. Chem. Res., @ (1985), 196-201. (c) M. B. Zimmt, C. Doubleday Jr., and N. J. TWO, J. Am. Chem. Soc., 108 (1986), 3618-3620.

501

ABSOLUTE ASYMMETRIC SYNTHESIS VIA PHOTOCHEMICAL REACTIONS OF CHIRAL CRYSTALS J. R. SCHEFFER and M. GARCIA-GARIBAY 1.

INTRODUCTION 1.1 Asmetric Svnthesis

One of the main goals of asymmetric synthesis is the generation of optically active substances from optically inactive starting materials. To accomplish this, chemists make use of the disymmetric influence of a resolved chiral agent on a prochiral or racemic reactant thereby leading to diastereomeric transition states of different energy (1). Many examples of asymmetric syntheses have been reported over the years (1) and several sources of disymmetry have been analyzed (2). Most of the published examples involve reactions taking place in isotropic liquid media where the disymmetric influence originates from the chiral nature of resolved reagents, catalysts, solvents or host molecules. In a relatively smaller number of cases, asymmetric photochemical reactions (usually with low optical yields) have been carried out in the absence of optically active materials by utilizing the chirality of circularly polarized light ( 3 ) . The method discussed in this article relates to asymmetric syntheses performed in the solid state, and specifically involves molecules that are symmetrical (achiral) in solution but which serendipitously crystallize in chiral space groups. In this circumstance it is the crystalline environment that is chiral, and provided that one or more chiral centers are generated in the solid state reaction, the crystal chirality may be converted into permanent molecular chirality by a stereospecific solid state process ( 4 ) . Such processes have been termed flabsolutefl asymmetric syntheses (5). In this context, the term absolute refers not to the formation of products in quantitative optical yields, but rather to the fact that such processes generate optically active products without the imnosition bv man of any external chiral influence. Absolute asymmetric syntheses are therefore obvious candidates for explanations of how optical activity could have arisen under pre-biotic conditions, and this aspect of the subject has been discussed (4b, 6).

502

1.2 Crvstal Chiralitv

Chirality is the non-identity of two objects that are related by a mirror symmetry relationship and can be a property common to both molecules and crystals (7). Whereas molecular chirality results from the disymmetric, three-dimensional arrangement of the constituent atoms, crystal chirality can be considered to result from a similar spatial arrangement in which the objects to consider are the molecules in the crystal (7b). It is for this reason that resolved molecular chirality is a sufficient but not necessary requirement for crystal chirality (7a). From the laws of crystal packing it is known that there is a total of 230 possible ways in which objects can be accommodated to build a regular three dimensional structure or lattice ( 8 ) . These packing modes, called space groups, are characterized by a unique set of symmetry operations that, when applied to the simplest building block or asymmetric unit in the crystal, enable the construction of the entire lattice structure. The space groups are then classified as chiral or achiral depending on the presence or absence of symmetry elements that convert one enantiomeric object into its antipode. Of the 230 possible space groups, 65 can be categorized as being chiral, and of these it has been noted that two are most common: P212121 and P21. These space groups comprise approximately 18% of all organic compounds whose crystal structures have been determined (9). The majority of these are resolved materials with tetrahedral carbon chiral centers, primarily organic compounds derived from natural sources. Crystal chirality without permanent molecular chirality, the topic of this review, is much rarer (7a) and can be found in compounds that can be classified in two general categories according to their liquid phase conformational characteristics: (a) flexible molecules that, while maintaining overall average symmetry through rapid conformational motions in the liquid phase, adopt fixed chiral conformations in the solid state, and (b) rigid molecules that, except for minor deformations, cannot form chiral molecular structures in the solid state. The first category can be considered to be formed by a process analogous to the well known llspontaneous resolutionI1 of binaphthyl studied by Pincock (10a,b). Here random nucleation and crystal growth of one of the two enantiomeric conformations in equilibrium under the crystallization conditions leads to the formation of crystalline conglomerates containing an unequal distribution of enantiomers;

503

in some runs 100% enantiomeric excesses were obtained. Because (+) and ( - ) binaphthyl are stable in solution at room temperature, the crystals could be dissolved and their optical purities determined. In other cases, such as those to be discussed in this article, the conformational (racemization) barriers are much lower, so that the crystalline samples immediately lose their chirality upon dissolution. 2.

[2+2]

PHOTOCYCMADDITION REACTIONS

Although the possibility of exploiting crystal chirality for the purposes of asymmetric synthesis was first considered over 8 0 years ago (ll), the first development of practical strategems for this purpose resulted from the systematic investigations of [ 2 + 2 ] photocycloaddition reactions of crystalline olefinic compounds initiated by G.M.J. Schmidt and his coworkers at the Weizmann Institute of Science in the early 1960s (12a). In a pioneering series of papers concerned with the crystalline phase photochemistry of cinnamic acid and its derivatives, Schmidt and coworkers showed that solid state [ 2 + 2 ] photocycloaddition requires a parallel orientation of the reacting double bonds with a center-to-center distance of less than approximately 4 . 2 A (12a,b). Olefinic compounds can have this arrangement in the solid state when the two molecules undergoing dimerization are related by a translation axis, inversion center or mirror plane. Only the first of these is allowed in a chiral space group, but even with the required packing features present in a chiral crystal, no asymmetric induction is possible in this case because the photoproducts necessarily have a plane of symmetry (Scheme la). Green, et al. saw that this problem could be overcome by introducing a suitable guest molecule that allows the formation of two enantiomeric heterodimers in addition to the expected mirror-symmetric homodimer (Scheme lb) (13). For this approach to be successful, two further requirements must be met (14): (a) there must be an appreciable solubility of one component in the other to allow for a homogeneous distribution of both compounds throughout the entire crystalline phase, and (b) there should exist a considerable face selectivity to allow for the formation of one enantiomer in preference to the other.

504

Scheme 1

The system selected by Schmidt and coworkers to test these ideas, based on some empirical guidelines and a great deal of experience with solid state [2+2] photocycloaddition reactions, consisted of the diarylbutadiene compounds 1 and 2 (Scheme 2)(14, 4a). While not shown in Scheme 2, these compounds have chiral conformations in the solid state because of twisting around the aryl-diene bonds and crystallize in the chiral space group P212121. Irradiation of single crystals of pure 1 or 2 gave the expected optically inactive cyclobutanes 3 and 5 respectively. However, when dilute (15%) solid solutions of 2 in 1 were prepared (in order to minimize 2 * * * 2 contacts) and the longer wavelength absorbing thiophene compound 2 selectively photolyzed, a 70% enantiomeric excess of the chiral heterodimer 4 was obtained. Scheme 2 Ph

Th=

& CI

CI

Ar

Th

Dh

505

When the structure of one of the two-component crystals was obtained and analyzed (15), it was concluded that the ground state orientation of the components would predict essentially no face discrimination and that approximately equimolar amounts of the As a enantiomeric dimers 4a and 4b should have been obtained. result, it was concluded that the thiophene compound must deform in the excited state to an arrangement that favors the formation of one of the products. This hypothesis was supported by theoretical calculations based on the assumption that the reaction proceeds through an intermediate exciplex (16); spectroscopic evidence for such a species was found in the case of diene 1 (17). In a second approach to asymmetric synthesis using [2+2] photocycloaddition chemistry, Green, et al. identified benzene-lr4-diacrylates (6a-e, Table 1) as possible substrates (13). The solid state photochemistry of related compounds (e.g., 2,5-distyryl pyrazine) had been studied previously by Hasegawa and shown to lead, y & cycloaddition at both double bonds, to dimeric, trimeric and oligomeric cyclobutane-containing products (Scheme 3 , X = Y, Z = H) (review, 18). An absolute asymmetric synthesis was planned and executed by Addadi and Lahav in essentially two strategic steps (5, 19). First, a chiral handle (a resolved sec-butyl group) was introduced into the R1 ester position to ensure a chiral space group. Secondly, the crystal structure of the compound so obtained [6a] was used as a model for the design and synthesis of a compound having an R1 ester group that was achiral. Since the R1 group of one molecule of 6a lies near the R2 ester substituent of its neighbor in the lattice, it was reasoned that portions of these alkyl groups could be interchanged without altering the overall packing arrangement. This is termed the principle of isomorphous replacement. Thus hypothetical transfer of a methyl group from R2 to R1 in 6a leads to the prediction that the achiral methyl/3-pentyl derivative 6b may adopt a chiral space group. Compound 6b was found to behave in the desired manner (chiral space group P21), however this approach is not always successful. Hypothetical transfer of a methyl group in 6a in the opposite direction (R1 to R2) gives the achiral i-Pr/n-Pr diester which, when synthesized, was found to crystallize in the achiral space group (Pi) (5, 19c).

506

Table 1. Benzene-lI4-diacrylates studied in the solid state.

6

Compound 6a

6b 6c 66

6e

Scheme

R2

R1

or (-) sec-butyl 3-pentyl (+) or (-) sec-butyl (k) sec-butyl (+) or (-) sec-butyl

ethyl methyl methyl ethyl n-propyl

(+)

Space Group P1 P21 P21 (pseudo P21/a) P1 P21 and P1

3

X

y

Trirners and

+ Higher Oligomers

Solid state photolysis of 6a gave the expected diastereomerically pure dimer 7 (X = C02sec-buty1, Y = COzEt, Z = CN) (and higher oligomers) with an absolute configuration that depended on the crystal handedness imposed by the configuration of the sec-butyl group. Irradiation of single crystals of diacrylate 6b gave optically active products of a similar nature in enantiomeric excesses ranging from 0 to 95% with rotations of either sign. The variation in the optical yield from one experiment to the next was attributed to factors related to crystal perfection and not to deficient topochemical control in the ideal crystal (19c). The sec-butyl group has been found to have some unique crystallization properties that make its use as a chiral handle in

507

solid state asymmetric synthesis studies particularly interesting. For example, because sec-butyl groups are frequently disordered in the solid state, molecules containing resolved sec-butyl groups form solid solutions with their enantiomers, and both molecules can sometimes occupy equivalent lattice sites in crystals. As a result, one has to be aware of the possible existence of ffracemicff crystals composed of single enantiomers and of chiral crystals having the chemical composition of racemates. An example of the first situation was found in one of the dimorphs (the other being light-stable) of diacrylate 6c (Scheme 4 ) which, despite having sec-butyl groups of a single chirality, crystallizes in a pseudo-centrosymmetric space group that is isostructural with the racemate (19a120). Upon photolysis, two optically active photodimers ( 8 and 9 ) were obtained in equal amounts, and after exchange of the sec-butyl groups for methyl groupsl a racemic mixture was formed. Scheme

"BUOOC MeOOC

4

'BuOOC MeOOC CN NC*

bv

+

__Ic

COOsBu COOMe

6c

MeOOC

Whereas the optically pure compound 6c resembles the racemic modification in the solid state, the opposite was found to be the case for racemic 6 6 which proved to be isostructural with optically active 6a (19a-b,20); this material crystallizes in the chiral space group P1 with the two enantiomers randomly distributed throughout the lattice. Crystals of unequal R/S composition could be prepared by recrystallizing various mixtures of the optical antipodes. Irradiation of these crystals gave a diastereomeric mixture of optically active dimers, trimers and oligomers, the optical purities of which varied from 0 to 100% depending on the R/S composition of the sample photolyzed. This

508

variation with composition was explained as being due to an *!immiscibility gap" in the phase diagram from 40%R/60%S tc 60%R/40%S. In this region a racemic mixture will yield equal amounts of crystals of both chiralities, each having the composition of the phase boundary. As a result, irradiation of samples in the 40/60 to 60/40 range led to a rapid diminution in 50:50 photoproduct optical activity which became zero at composition. In a final interesting example of solid state [ 2 + 2 ] photocycloaddition chemistry involving the sec-butyl group, it was found that the optically pure compound 6e can crystallize randomly in two dimorphic modifications, A and B, as shown in Scheme 5 (21). Dimorphs A and B are formed in the same solvent, but for a given crystallization, the crystals are all of one type or the other. As can be seen by careful inspection of Scheme 5, A and B pack in pseudo-enantiomeric polymerization stacks, the only difference between them arising from the different conformations of the R sec-butyl groups. The solid state photoproducts obtained from the two dimorphs were found to be diastereomeric with opposite absolute configurations at the cyclobutane chiral centers. Transesterification of the two diastereomers with methanol gave optically pure dimers with opposite specific rotations, representing a particularly novel (but scarcely general) way in which two enantiomers of the same product can be obtained starting from the same chiral substrate. Scheme 5

6e

509

3.

UNIMOLECULAR PHOTOREARRANGEMENTS

In contrast to bimolecular solid state reactions, where the primary role of the crystal lattice is to situate adjacent molecules within bonding distance of one another with an orientation favorable for reaction, unimolecular reactions occurring in organic crystals can be thought of as being governed by the three-dimensional microenvironment formed by the ground state neighbors that surround the photoexcited species. In this model, first formulated qualitatively by Cohen (22), the transition state traversed is the one for which the sum of the non-bonded repulsive interactions between the reacting molecule and its anisotropic surroundings is minimized. As a result, the pathway followed in the solid state may be quite different from that observed when the same compound is irradiated in isotropic liquid media, where the solute-solvent steric interactions are essentially identical for all possible transition states. During the past fifteen years, the UBC solid state chemistry research group has carried out studies of a large variety of unimolecular photoreactions in crystalline media (reviews, 2 3 - 2 5 ) . Our approach has been to look for systems that exhibit differential solid state/solution state photoreactivity and then investigate the reasons for these differences using X-ray crystallography, solid state 13C CPMAS N M R spectroscopy and solid state FTIR and W - V I S spectroscopy. In favorable cases, such studies can be performed on both reactants and products, leading to profound mechanistic insights and the formulation of detailed structure-reactivity relationships. One of the factors that makes research in solid state organic chemistry both exciting and frustrating is the unpredictability of crystal packing. In seeking to carry out solid state reactions that convert reactant crystal chirality into product molecular chirality, it is (as we have seen) necessary to deal with reactants that pack in chiral space groups. One way to ensure this is to use resolved starting materials, which necessarily pack in chiral space groups. More interesting, however, is the situation treated in this article where one starts with an achiral reactant which, for reasons that are not well understood, spontaneously packs in a chiral space group. We have been fortunate to discover several photochemically reactive compounds that behave in this manner and the remainder of this review will deal with our results in this area.

510

3.1 The Norrish Tvwe I1 Reaction

The Norrish type I1 reaction is one of the most thoroughly studied of all organic photorearrangements (reviews, 26-28) and consists of six-membered transition state abstraction of a hydrogen atom by the oxygen atom of a photoexcited carbonyl compound to give (for triplet ketones) the 1,4-biradical intermediate 11 (Scheme 6). This species, which'has been detected spectroscopically and trapped chemically, undergoes three main chemical reactions: (a) reverse hydrogen atom transfer to regenerate starting material, (b) closure to a mixture of stereoisomeric cyclobutanols (12) and (c) cleavage to afford an alkene (13) and an enol, the latter finally appearing as the corresponding methyl ketone 1 4 . Scheme 6

cyciization

R 10

I

1,4-biradical

Enol 14

11

* 12

cI eavag e

13

In the course of extensive studies of the Norrish type I1 reaction in the solid state aimed primarily at establishing the geometric and distance requirements for hydrogen abstraction (29,30) and delineating the relationship between lI4-biradical structure and reactivity (30,31), we discovered a particularly striking case of an absolute asymmetric synthesis (32). This concerns the adamantyl-substituted ketone 15 shown in Scheme 7. This material forms very large prisms from ethanol that have the chiral space group P 2 1 2 ~ 2 ~ .The molecules in the crystal have the chiral conformation shown, whereas N M R spectroscopy in solution indicates that the compound has an average plane of symmetry passing through Cl, C2 and C3.

511

Previous solution phase photochemical studies on a-adamantylacetophenone derivatives had shown that the intermediate lI4-biradicals undergo exclusive closure to cyclobutanol Scheme 7

15

16

17

derivatives owing to the prohibitive strain energy involved in forming adamantene v a biradical cleavage (33). This was corroborated in the case of 15, which gave four of the six possible cyclobutanols when irradiated in benzene or acetonitrile. These cyclobutanols, of which 17 is an example, possess six chiral centers and the stage was therefore set for solid state asymmetric photorearrangement studies. In the event, a single crystal of ketone 15 weighing 313 mg was photolyzed at 8 “ C to 8 % conversion. Gas chromatography of the crude reaction mixture indicated that one photoproduct (subsequently shown to be cyclobutanol 17) was present to the extent of 7 0 % . Isolation of this material by column chromatography afforded material with an [ Q ] D of -21.6’, and chiral NbfFt shift reagent studies using Eu(hfc)j indicated an enantiomeric excess of approximately 8 0 % . One of the most interesting aspects of results such as these is what they reveal about the details of the mechanism operative in the solid state. In the present instance we may conclude that the Norrish type I1 photorearrangement of ketone 15 is largely sterespecific in the solid state despite the fact that it is almost certainly non-concerted (solution phase sensitizationquenching studies indicate reaction through a triplet excited state). There are four possible 7-hydrogen atoms in 15 (HI H4, Scheme 7) which, if abstracted, could lead to cyclobutanol 17. Of these, abstraction of H1 or H2 would lead to one enantiomer and abstraction of H3 or H4 would give the other (assuming sip2 hybridization of the radical center produced by abstraction).

-

512

Although we cannot be certain which of these is actually abstracted, a good indication comes from the O " - H distances as determined by X-ray crystallography. For compound 15 these are O"'H1 = 2.71 A, O"'H2 = 3.09 A, O"'H3 = 3.74 A and 0 6 - * H 4 = 4.56 A. It thus seems quite likely that H1 is abstracted in this case and that the crystalline medium does not allow the relatively minor conformational changes required for the oxygen atom to approach H3 or H4. There are two additional reasons to conclude (a) the that HI abstraction is preferred for ketone 15: six-membered transition state in this case is an almost perfect chair arrangement, and (b) the abstraction distance, at 2.71 A, is less than the sum of the van der Waals radii for oxygen and hydrogen, 2.72 A. In a recent review article (29), we have tabulated and discussed the geometric and distance requirements for intramolecular hydrogen atom abstraction processes as determined from our solid state photochemical studies. Almost without exception we found that abstraction occurred preferentially over distances that are less than or equal to the sum of the van der Waals radii of the atoms involved. The fact that the 0.. H2, O * * * H 3and O - * * H 4 distances for 15 are greater than 2.72 A is another indication of their non-involvement under the solid state photolysis conditions. It is important to realize, however, that complete stereospecificity is not necessarily demanded by the solid state medium. In general, it may be expected that, depending on the size and shape of the reaction cavity involved, significantly differing degrees of stereocontrol will be observed in the ideal crystal, and this may be the source of the lack of complete stereospecificity in the case of ketone 15. Another explanation is that non-stereospecific reactions may be occurring in non-ideal regions of the crystal such as defect sites or liquid domains. Partial melting with concomitant loss of topochemical control is a distinct possibility in the case of ketone 1 5 , as the material has a low melting point (45-47 " C ) and became slightly sticky following irradiation. 3.2 The Di-%-Methane Photorearranaement The di-%-methane reaction is another very general and well studied solution phase photorearrangement (review, 34). The reaction derives its name from the observation that molecules containing two independent n-systems separated by an sp3-hybridized or ffmethaneff carbon atom undergo rearrangement upon absorption of a photon of light to give vinylcyclopropane

.

513

derivatives. This process, along with the mechanism suggested by simplest Zimmerman ( 3 4 ) , is depicted in Scheme 8 for the di-=-methane reactant, lI4-pentadiene. A variation of this Scheme

8 /methane

1.4-dime

18

carbon

Pl19

vingcyclopropone

20

21

mechanism, proposed by Paquette (35), involves direct formation of biradical 2 0 from the reactant excited state. Both aliphatic and aromatic a-systems are capable of participating in the reaction, and when one of them is a carbon-oxygen double bond, the reaction is termed the oxadi-n-methane photorearrangement (36). Approximately three years ago we initiated a study of the photochemistry of dibenzobarrelene-ll,l2-diester derivatives (22, Scheme 9) as part of a general program aimed at investigating the di-%-methane reaction in the solid state (review, 37). Ciganek had shown some 2 0 years earlier that the dimethyl diester 22a undergoes smooth photorearrangement in solution to afford the This reaction is dibenzosemibullvalene derivative 23a (38). consistent with the Zimmennan (or Paquette) mechanism in which the first step is carbon-carbon bond formation between one of the carbon atoms of the bridging vinyl group and one of the four equivalent aromatic carbon atoms C4ar C8ai Cga or ClOa. Our studies showed that this reaction proceeds smoothly in the solid state as well, and we prepared a number of additional derivatives, both symmetrical (identical R groups) and unsymmetrical (non-identical R groups) for further investigation. One of the first compounds we prepared, the symmetrical diisopropyl diester 22b, proved to be dimorphic. Recrystallization from ethanol affords large prisms with the achiral space group Pbca; recrystallization from cyclohexane gives the Pbca modification plus material having the chiral space group P212121. We later found that the P212121 space group is the exclusive result of growing crystals from the melt. NMR studies showed that

514

Scheme 9 C02R

22

(a) R (b) R

= =

Me iPr

23

diester 22b possesses average CzV symmetry in solution; in the solid state, however, both dimorphs are composed of molecules that lack symmetry because the ester groups are frozen in non-Cav orientations. Both enantiomers of the disymmetric conformation are present in equal amounts in the Pbca crystal resulting in a non-chiral space group. Only one of the two possible enantiomers is present in the P212121 case, and the crystal is thus chiral. The finding that 22b crystallizes in two space groups, one chiral and one achiral, afforded a unique opportunity to study the different effects that these two fundamentally different packing modes exert on the chemical reactivity of the same substance. In addition, the expected photoproduct, 23b, possesses four chiral centers, thus raising the possibility of an absolute asymmetric photorearrangement. Large (20-85 mg) crystals of each dimorph were irradiated using a nitrogen laser (337 nm); parallel photolyses were conducted in benzene solution. At 337 nm only the tail of the ene-dioate absorption ( C < 10) is excited. This increases the likelihood that the light will penetrate deeply and uniformly into the interior of the crystal. Both in solution and the solid state, the conversions were kept low (< 25%), and the optical activity produced in each photolysis was determined by dissolving the sample in chloroform and measuring its rotation at the sodium D line. The specific rotation of photoproduct 23b, which was formed as expected, was calculated from the weight of the crystal and the percent conversion as determined by capillary gas chromatography. The unreacted starting material contributes nothing to the rotation because it is achiral in solution. The results showed that only in the case of the P212121 crystals was optical activity developed. Remarkably, NMR chiral shift reagent studies using Eu(hfc)3 established that, within the limit

515

of the method, the chiral crystals give the photoproduct 23b in quantitative enantiomeric excess. It was of interest to determine whether the resolution that diester 22b undergoes upon crystallization from the melt is truly spontaneous, that is whether it gives an equal distribution of enantiomorphic crystals. Because the sign of crystal chirality is difficult to measure and because the samples immediately lose their optical activity upon dissolution, the distribution of chirality was checked as follows: nine samples of the racemic (Pbca) modification of diester 22b were sealed in Pyrex tubes and heated for one hour at a temperature 20" above the melting point of 145-146 " C . The tubes were cooled to approximately 120°, opened, and crystallization induced by pricking the contents with a rigorously cleaned needle. This provided diester 22b in its P212121 modification in what appeared to be a polycrystalline mass. The tubes and their contents were then immediately irradiated using a nitrogen laser and the resulting mixtures dissolved in chloroform and analyzed by polarimetry. Four of the samples gave dextrorotatory photoproduct and five gave levorotatory material. From this we conclude that the resolution is indeed spontaneous and not caused by some adventitious chiral impurity. Strikingly, the specific rotations produced in these experiments were very similar to those obtained by photolyzing beautifully shaped single crystals of diester 23b. This indicates that the polycrystalline mass obtained from the melt is of high optical purity. As mentioned earlier, the solution phase di-a-methane photorearrangement of dibenzobarrelenes is four-fold degenerate: Scheme 10 depicts the four possible initial benzo-vinyl bridging possibilities. As drawn, paths 1 and 2 lead to one enantiomer of dibenzobarrelene photoproduct and paths 3 and 4 give the other. The fact that the reaction of the P212121 crystals proceeds with 100% enantioselectivity indicates complete discrimination between pathways [1+2] and [3+4] in the solid state. It does not, however, tell us whether [1+2] is favored over [3+4] or vice versa, nor does it indicate the relative importance of 1 versus 2 or 3 versus 4. In principle, it is possible to differentiate between pathways [1+2] and [3+4] by determining the absolute configuration of the molecules in a reactant crystal and correlating this with the absolute configuration of the photoproduct generated by

516

irradiation of that crystal. Preliminary crystallographic studies of this type have been carried out f o r a 22b/23b reactantphotoproduct pair using the Bijvoet method for determining Scheme 10

absolute configuration (39). Although in both cases the anomalous dispersion was low as expected for molecules with oxygen as the heavy atoms, the results were internally consistent and indicated that the crystal of 22b studied contained molecules of the llM,12P absolute configuration and that its photoproduct had the S,S,S,S-(-) configuration. The designation llM,12P for 22b focuses on the site of disymmetry in the molecule (the ester groups) and uses the conformational chirality formalism for assigning absolute configuration ( 4 0 ) . In this approach one determines the smallest torsion angle between the groups of highest priority (fiducial groups) attached to each end of the single bond about which the conformation is to be specified. A (t) torsion angle is designated P (plus) and a ( - ) torsion angle

517

is termed M (minus). In the case of diester 22b the axes to be considered are the single bonds joining the C=C and C02R substituents, and the fiducial groups are the carbonyl oxygen atom at one end and the vinyl carbon at the other. Drawings of both the reactant and photoproduct absolute configurations are given in Scheme 11. The dark circles indicate the oxygen atoms of the C=O groups.

Scheme 11

24 n

re hindered

o l e s s hindered n

I1

conformational isomerization

Crystal Conformation

25

22b

Crystal Conformation

As is indicated in Scheme 11, the absolute configuration studies indicate that the di-n-methane photorearrangement of 22b proceeds either via path 1, path 2 or a combination of the two. Two questions arise at this point: (a) with the data in hand, can we decide among these possibilities, and (b) can we identify the crystal forces that determine the preferred pathway(s)? The answer to both questions is a qualified yes. The approach to deciding between pathways 1 and 2 (or a combination of the two) involves inspection of Scheme 11. This depicts two conformations of the all S photoproduct, one of which 1241 is formed through path 1 with minimum conformational chanaes of the attached ester urouDs. In a similar fashion, application of mechanism 2 to reactant 22b, while keeping the ester groups in their original orientations, affords the all S photoproduct in conformation 25.

518

The striking fact is that the photoproduct conformation determined by crystallography closely resembles that of 2 5 , and the tentative conclusion is thus that pathway 2 is preferred in this instance. We cannot, however, rule out the possibility that the photoproduct is formed in the crystal in conformation 2 4 with subsequent isomerization to conformation 25 upon workup and recrystallization. An approach to understanding the crystal forces that seem to favor path 2 can be made by a detailed inspection of the packing diagram for diester 22b (Scheme 12). The large spheres represent the van der Waals radii of the atoms from neighboring molecules that surround the two ester groups at a distance of 5 3 A. The largest spheres represent carbon atoms, intermediate sized spheres Scheme 12

are oxygen atoms and the smallest spheres indicate hydrogen atoms. As is evident, the M ester group is less tightly surrounded by neighboring atoms than is the P ester group, and since it is the ester group which is attached to the vinyl carbon atom involved in initial vinyl-benzo bonding that moves most during the rearrangement, this is consistent with the apparent preference for path 2 over path 1 in the solid state. What is not so clear from Scheme 12 is why path 2 should be favored over path 4 as is observed experimentally. To help answer this question we carried out computer simulations of the motions involved in the

519

initial stages of bonding pathways 2 and 4. At each stage of the simulated motion, the total Lennard-Jones non-bonded repulsion energy between the moving ester group and the surrounding lattice was calculated (41). This indicated that slight displacement of the M ester group along path 2 (toward the viewer in Scheme 12, initial benzo-vinyl bonding between darkened atoms) was significantly more favorable in terms of reactant-lattice steric effects than the opposite motion away from the viewer (path 4 ) . A similar approach has been used by us to explain the solid state regioselectivities observed in the di-%-methane photorearrangements of some dibenzobarrelene-ll,l2-diesters with non-identical ester substituents ( 4 2 ) . The final example we shall discuss in this review article concerns just such a mixed diester, namely the derivative 26 (Scheme 13) in which one ester substituent is isopropyl and and the other is sec-butyl (43); this material was prepared in both optically active form using (S)-(+)-sec-butanol and in racemic form using (t)-sec-butanol. X-ray crystallographic studies revealed that both forms are isostructural with the diisopropyl derivative 22b and crystallize in the chiral space group P212121 with disorder in the sec-butyl group. Compound 26 thus resembles diacrylate 6c studied by Addadi and Lahav (19a,b) in that it approaches the composition of a racemic mixture but crystallizes in a chiral space group. Di-a-methane photorearrangement of mixed diester 26 can give rise to four possible diastereomeric products that can be designated as either 27 or 28 on the basis of the location of the two non-equivalent ester groups (regiochemistry) and as a or b on the basis of the configuration of the substituted dibenzosemibullvalene ring system (Scheme 13). There are two enantiomers of each diastereomer making a total of eight stereoisomers. Table 2 reports the experimental results of nitrogen laser irradiation of single crystals of both the optically active and racemic forms of diester 26; parallel photolyses were conducted in 0.1 M benzene solutions. The 27:28 (regiochemical) ratio was determined by capillary gas chromatographic comparison with authentic samples prepared independently ( 4 3 ) . The a:b (enantiomeric) ratio was determined by complete hydrolysis of the photoproduct mixture and re-esterification with diazomethane to yield enantiomers 29a and 29b; the enantiomeric excess in this mixture was measured by NMR at 300 MHz using the chiral shift reagent Eu(hfc)g.

520

Scheme 13

COZSBU

27a

+

27b

26 28a

Table

2.

28b

29b

Regioselectivity and enantiomeric excess as a function of photolysis medium and sec-butyl configuration.

Reactant

Photolysis Medium

(S) (+) -sec-butyl 26

solution single crystal solution single crystal

-

(R,S)-sec-butyl 26

27 :28

a:ba

55:45 55:45

51:4gb 9O:lO 50:50

62:38

90:10

90:10

aDextrorotatory enantiomer predominates; absolute configuration unknown. bDetermined by polarimetry

.

The photochemical results demonstrate only a slight preference for the formation of one regioisomer over the other in solution. This is expected in view of the similarity in size and shape of the two ester groups. Also as expected, no chiral selectivity occurred in the solution photorearrangement of racemic 26. Interestingly, the chiral ( S ) -(+) -sec-butyl tthandlett did exert a slight but measurable solution phase asymmetric induction. The solid state results are quite different. In both cases the chiral crystal environment led to a dramatic increase in the formation of one substituted dibenzosemibullvalene configuration over the other (80% ee). A similar large effect was found on the regioselectivity of the solid state reaction. Remarkably, the regioselectivities were found to be different for crystals of (+)-sec-butyl-26 as compared to the (?r)-26samples. This is no

521

doubt the result of packing differences between the two forms. Although we have not yet been able to analyze these packing differences in detail by X-ray crystallography owing to the disorder present in the sec-butyl groupsI the existence of packing differences is clearly indicated by FTIR spectroscopy of KBr pellets and by melting point differences (43). We have shown in work not covered in this review (37,42) that packing-dependent regioselectivity differences are a general feature of solid state di-n-methane photorearrangements of mixed dibenzobarrelene diesters. A final point concerns the interesting possibility (not required by the experimental results) that, in the crystalline phase photorearrangement of racemic 26 the two enantiomers react at different rates. If this is so, then the recovered starting material should be enriched in the slower-reacting enantiomer. This was verified experimentally. In the crystals examined (all from the same batch), the recovered starting material contained an excess of (S)-(+)-sec-butyl-26. From the specific rotations of these samples we could conclude that (R)-(-)-sec-butyl-26 reacts approximately twice as fast as its enantiomer in the solid state. 3.3 0x0 Amide Photochemistry A s the final example in this review article we note the recent communication by Toda, et a1.(44) which reports the very interesting solid state photochemistry of N,N-diisopropylphenylglyoxylamide (30, Scheme 14). In a reaction exactly analogous to the Norrish type I1 reaction discussed earlier (Section 3.1)1 irradiation of crystals of this material was reported to afford the p-lactam 31 (75% yield) in optically active form (93% ee). By appropriate seeding of solutions of 0x0 amide 3 0 during recrystallization followed by photolysis of the crystals so obtained, both enantiomers of photoproduct 31 could be produced. These results are almost certainly the consequence of a largely stereospecific photoreaction occuring within a crystal having a chiral space group and were so interpreted by the authors (44). Final confirmation of this mechanism, however, must await crystal structure studies which were lacking in this case.

5'22

Scheme 14

30

4.

31

CONCLUDING REMARKB

Space limitations have restricted this review to a discussion of photochemical reactions of chiral crystals. The reader should be aware, however, that significant contributions to absolute asymmetric syntheses have been made in the area of sas/solid reactions of achiral molecules that crystallize in chiral space groups. In this type of process a gaseous reagent (e.g., bromine) penetrates the crystal lattice and reacts in a stereospecific manner with the constituent organic molecules. While studies of this type are limited, significant product enantiomeric excesses have been achieved ( 4 , 4 5 , 4 6 ) . It should be clear to the reader at this stage that the majority of the published work n the field of absolute asymmetric synthesis is the result of accidental discoveries of organic compounds that crystallize in chiral space groups and which undergo chemical reactions in that medium to give products that possess permanent molecular chirality. Such circumstances are far from common. Even in some instances where these conditions have been met, the important aspect of crystal has prevented detailed studies from being carried out. As all organic chemists are aware, crystals frequently grow in the form of very fine needles, thin plates or extremely small prisms, and attempts to increase crystal size are often unsuccessful. The need to work with single crystals of uniform chirality in absolute asymmetric synthesis studies means that when small, sub-milligram crystals are used, product separation and the measurement of product optical rotations and enantiomeric excesses becomes extremely difficult if not impossible. The direction our research is currently taking to overcome these problems is to use the removable remote chiral handle approach. In this approach, which is an extension of the

523

pioneering work of the Weizmann Institute solid state chemistry group, a chemically reactive but achiral molecule is derivatized with an unreactive remote chiral handle. The chiral handle guarantees crystallization in a homochiral space group so that it is not necessary to work with single crystals. This obviates the problem of small crystal size since polycrystalline samples can be studied. The sample is caused to undergo a solid state reaction which, because of the presence of the chiral handle, necessarily yields a chiral product. The chiral handle is then removed and the residual chirality characteristic of the photoproduct alone is revealed. Using spacer groups, the chiral handle can be located at varying distances from the site of reaction. Ideally, the chiral handle should be located at a site sufficiently remote that no asymmetric induction is exerted when the reaction is carried out in solution. Under these circumstances the optical activity of the product in the solid state can be attributed exclusively to the chiral crystalline environment. 5.

ACKNOWLEDGMENT

Special thanks are due Professor James Trotter and his research group for the crystallographic studies which play such an important part in the work described from UBC. Financial support from the Natural Sciences and Engineering Research Council of Canada and the United States Petroleum Research Fund is gratefully acknowledged.

524

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8 9 10

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(a) R.R. Sauers, M. Gorodetsky, J.A. Whittle and C.K. Hu, J. Lewis, R.W. Am. Chem. S O C . , 93 (1971) 5520-5526; (b) F . D . Johnson and D.R. Kory, J. Am. Chem. SOC., 96 (1974) 6100-6107; (c) R.B. Gagosian, J.C. Dalton and N.J. Turro, J. Am. Chem. SOC., 97 (1975) 5189-5192. H.E. Zimmerman in Rearrangements in Ground and Excited States, P. de Mayo (Ed.), Academic, New York, 1980, Ch. 16, pp 131-166. (a) L.A. Paquette and E. Bay, J. Am. Chem. SOC., 106 (1984) 6693-6701; (b) L.A. Paquette, A. Varadarajan and L.D. Burke, J. Am. Chem. SOC., 108 (1986) 8032-8039; (c) For a further discussion of the two mechanisms, see H.E. Zimmerman and A.P. Kamath, J. Am. Chem. SOC., 110 (1988) 900-911. D.I. Schuster in Rearrangements in Ground and Excited States, P. de Mayo (Ed.), Academic, New York, 1980, Ch. 17, pp 2 32-27 1.

J.R. Scheffer, J. Trotter, M. Garcia-Garibay and F. Wireko, Mol. Cryst. Liq. Cryst., in press. E. Ciganek, J. Am. Chem. SOC., 88 (1966) 2882-2883. J.M. Bijvoet, A . F . Peerdeman and J.A. Van Bommel, Nature, 168 (1951) 271-272.

R.S. Cahn, C.Ingold and V. Prelog, Angew. Chem., Int. Ed. Engl., 5 (1966) 385-415. Program MOVE, written by S.V. Evans, University of British Columbia, 1986. M. Garcia-Garibay, J.R. Scheffer, J. Trotter and F. Wireko, Tetrahedron Lett., in press. M. Garcia-Garibay, J.R. Scheffer, J. Trotter and F. Wireko, Tetrahedron Lett., 28 (1987) 4789-4792. F. Toda, M. Yagi and S. Soda, J. Chem. SOC., Chem. Commun., (1987) 1413-1414. Addadi, S. Ariel,

M. Lahav, L. Leiserowitz, R. Popovitz-Biro and C.P. Tang in Chemical Physics of Solids and their Surfaces, M.W. Roberts and J.M. Thomas (Eds), The Royal Society of Chemistry, London, 1980; Specialist Periodical Reports, Vol. 8, Ch. 7. M. Garcia-Garibay, J.R. Scheffer, J. Trotter and F. Wireko, Tetrahedron Lett., 29 (1988) 1485-1488. L.

526

FLUORESCENCE QUENCHING OF PYRENE AS A MONITOR OF INTERMOLECULAR DIFFUSION AND INTRAMOLECULAR CHAIN BENDING IN CHOLESTERIC LIQUID CRYSTALLINE PHASES(1)

M. F. SONNENSCHEIN and R. G. WEISS 1.

INTRODUCTION Liquid Crystals combine several of the properties of solids

(e.g., intermolecular repeating order) and liquids (e.g., flow). As

such,

they

offer

solids capable

a

useful

of relatively

model high

for somewhat

rates

of

disordered

diffusion.

The

diversity of molecular packing arrangements among thermotropic liquid-crystalline phases makes them very attractive solvents in which to probe the influence of ordered, flexible matrices o n solute reactions(2). Some liquid-crystalline molecules exhibit more than one anisotropic phase, s o that subtle changes in a solute's cybotactic region can be investigated without altering the bulk electronic nature of the environment(3). one

cholesteric

phase, measurable

changes

Even within

can be

induced by

pressure and temperature(4). Alternatively, solutes whose reactivity in isotropic phases is well documented can be used t o probe the microscopic environments which mesophases provide(5). In this chapter we summarize critically the use of cholesteric phases

to

influence

photophysical The

utility

the

processes of

rates which

cholesteric

of

inter- and

involve

phases

to

the

intramolecular

pyrenyl

lumophore.

simplify these

complex

dynamic processes and of the probes to detect minor changes in solvent order is demonstrated.

1.1 The Cholesteric Phase(6) The constituent molecules of a cholesteric phase must contain at least one center of chirality and frequently include a steroidal ring system. They are arranged with their long ases (directors) parallel, but without longitudinal order. A convenient description of a cholesteric arrangement (which is microscopically incorrect(3a))

places the molecules in nematic

like "layers" which are very slightly twisted with respect to those above

and below

(Fig. I).

The angle and direction of

527

twist along the axis perpendicular to the layers is a function of the chirality and structure of the constituent molecules. The macrohelicel-arrangement can be characterized by its pitch, p (i.e., the distance along the helical axis between layers with parallel directors). from

Bragg-type scattering

It is obtained experimentally

detected

spectroscopically

as

reduced transmittance (pseudo absorption) or enhanced reflection in

samples

oriented

normal

to

the

incident light.

The

wavelength maximum (Ap) and the average refractive index of the medium (n) allow p to be calculated easily (Eqn. 1). The sense of

the helical twist can be ascertained from the rotation i t

induces in transmitted polarized light.

Fig. 1. Idealized model of a cholesteric phase(26). with permission of the American Chemical Society.]

[Reprinted

Cholesteric phases are non-Newtonian and exhibit enormous shear viscosities (103-105 poise) at very low shear rates(7). However, activation energies for self-diffusion are only 3 - 5 kcal/mol higher in a cholesteric phase than in the corresponding isotropic phases(8). In spite of the large bulk viscosities, diffusion constants parallel and perpendicular to the directors differ by less than an order of magnitude(9). 1.2 Emission Prooerties of Pyrenyl Containing ComDounds A

great deal of information has been extracted from static

and dynamic emission characteristics of compounds

in various anisotropic media

pyrenyl

containing

(such as micelles(lO),

polymers(ll), and solid surfaces(l2)). The attractiveness o f pyrene lies in its photophysical properties. Since the lLa band of pyrene

has

high

molar

extinction

coefficients,

concentrations can be employed as probes without signal intensity. The excited singlet of exceptionally ethanol(l3))

long

lived

(530 ns

in

small

sacrificing is pyrene

very pure,

degassed

and exhibits very high quantum efficiencies in a

528

wide variety

of

organic

solvents.

The long lifetime is an

important consideration in the highly viscous cholesteric phases which

frequently

Furthermore,

the

slow

diffusion

ratio

of

controlled

pyrenyl

processes.

fluorescence

emission

intensity from the 1-111 vibronic bands can be used quantitatively to assess the local polarity of the environment in which emission occurs(l4). Excimers and exciplexes involving pyrenyl groups are well characterized Their

temporally, spectroscopically, and structurally.

emission

wavelength

maxima

and

intensities

provide

information concerning the polarity and viscosity of the solvent. The pyrene excimer, for example, is a sandwich-like structure in which the two molecules are slightly displaced from perfect overlap(l5). The important mechanistic steps involving pyrene excitation, fluorescence, and excited complex formation are found in Scheme 1 (vide infra).

It should be noted that

anisotropic media like cholesteric phases do not change Scheme 1 qualitatively, but they can (and frequently do) alter significantly the rates at which several of the steps occur (e.g.,

as

mentioned

previously,

those

that

are

diffusion

dependent). 1.3 Physical Studies of Cholesteric Phase Order Intrinsically, the environment it seeks observations

of

neat

presence of to probe.

cholesteric

local environments to be discerned. scant,

have

focussed

on

the

any solute alters the However, spectroscopic

compounds

allow

undisturbed

Such observations, although ramifications

to

mesophase

structure, order, and mobility when solvent molecular structure is altered.

They indicate, for instance, that the length of an

alkyl side chain at C17 of the steroidal backbone does affect the

important

physical

environment. Shivaprakesh structure

of

et

properties

a1.(16)

carbonyl

IR

which

found that absorptions

define

a solute

intensities and fine of several cholesteryl

esters decrease in the cholesteric phase relative to the solid phase.

Presumably, the greater motion in the cholesteric phase

destroys the inter- and intramolecular couplings responsible for fine

structure.

similar esters.

Earlier, Bulkin and Krishnan

(17) reached a

conclusion based upon Raman studies of cholesteryl They determined that alkyl chains of the esters are

529

largely liquid-like in the cholesteric phases. Unfortunately, in neither study were data at several temperatures taken. The temperature dependent, low resolution (10 MHz) NMR spin lattice relaxation times of liquid-crystalline cholesteryl butyrate and cholesteryl nonanoate were examined by Cutler(l8).

He observed activation energies for molecular motions of 11.0 and 12.5 kcal/mol, respectively. These energies are similar to those for chain melting within the same cholesteric esters(l9). Although more experimentation is required before a firm conclusion can be reached, the activation energies for chain melting probably represent upper limits to activation energies for solute motions in a cholesteric phase. 2.

BRIEF SUMMARY OF PHOTOCHEMICAL TRANSFORMATIONS CONDUCTED IN CHOLESTERIC PHASES Several studies of photochemical transformations of solutes

have sought to exploit the unusually high viscosities, anisotropic cybotactic regions, and helical macrostructures that cholesteric phases provide. of cholesteric influences

Only in selected cases have the mesophase order been clearly

manifested in the experimental results. For optical

instance, activity

mechanistically have

been

important

observed

in

inductions products

of

from

unimolecular transformations of achiral solutes when conducted in cholesteric phases.

These transformations involve selection

by solvent matrices between subtle intramolecular shape changes of

the

solutes.

Thus,

Nakazaki

et a1.(20)

found

that

irradiation of p h e n y l - 2 - ( 2 - b e n z o - [ c ] p h e n a n t h r y l ) e t h y l e n e and I 2 (w/w) cholesteryl nonanoate/

in the cholesteric phase of 3/2

cholesteryl chloride (CN/CCl) results in a ca. 1% enantiomeric excess (ee) of (+)-hexahelicene.

hexahe licene Similar

irradiation

1,l'-binaphthyl in

the isotropic phase of CN/CC1

produces racemic hexahelicene.

Studying the same system, but

varying the pitch of the

cholesteric phase with temperature,

530

Hibert and Solladie obtained 0.43% ee of (+)-hexahelicene(21). More importantly, they found a measurable ee when the solvent pitch approaches infinity. These results indicate that cholesteric order does influence the motions which lead from reactant to product (probably by affecting the equilibrium between the reactant conformations) but that even nematic-like order, when combined with molecular asymmetry, can alter the balance between enantiomeric rate processes. Additionally, a part of the ee may arise from the partial circular polarization of the excitation light which results from selective reflection of one light component at the cholesteric surface. Ganapathy and Weiss(22), demonstrated that the selective photoproduction of one atropisomer from a racemic solution of

(u) requires

1,l'-binaphthyl macroscopically measurable

the presence o f cholesteric reaction

ordered

change

in atropisomeric

content was

a chiral and medium. No observed upon

heating racemic BN in a variety of cholesteric mixtures.

Thus,

photoequilibration is more selective than thermal equilibration:

'm,

the isomerizing excited state,

interact more strongly matrix. Since the irradiation of

than

maximum

a was

ground

and

solvent

state

atropisomeric

are

able to

and the solvent excess observed upon

1.1%, once again the solvent influence

on

the solute motions is very weak. I n each of the above experiments, the

rotation

of

plane

polarized light by the products allows very small enantiomeric excesses to be detected with excellent precision. In other cases, where the methods influence

of

of

cholesteric

detection order

on

are

less

reaction

precise, no dynamics

is

discernible (although they may be present)(23). In

studies

where

cholesteric

phase

order

appears to

influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dimethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered exclusively

glasses, the cis-syn dimer is in liquid-crystalline media.

formed almost Furthermore,

selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO).

Since all of the

531

mesophases explored seem to have a similar influence on product selectivity, the exact nature of the responsible solvent-solute interactions is not clear; however, the ability of the ordered solvents to direct product formation (and, therefore, reactant collision orientations) is obvious. I n a similar study, Nerbonne and Weiss (25) photodimerization of acenaphthylene

proceeds

10

found

that

times more

efficiently in the cholesteric than in the isotropic phase of a 1/1

(w/w) mixture of 5a-cholestan-3p-yl nonanoate/5~-cholestan-

3p-yl acetate

(CHNICHA).

was found to be interpreted these

Furthermore, the rise in efficiency

strongly pitch dependent. The authors results to indicate that diffusion of the

plate-like acenaphthylene occurs

preferentially

molecules

in

paths

which

in

the

cholesteric

phase

lead to pre-product

sandwich collisions. 3.

INTERMOLECULAR QUENCHING OF PYRENYL FLUORESCENCE IN CHOLESTERIC PHASES The

efficiency

of

intermolecular

pyrenyl

fluorescence

quenching in a liquid-crystalline solvent is limited by the rate at which the quenching partners are able to diffuse through the medium and the fraction o f collisions which allow the requisite electronic interactions. In isotropic media, collisional orientations occur randomly, with regard only for the steric and electronic exigencies of the pyrenyl-containing molecule and its quencher.

In

cholesteric

phases,

the

solutes

diffuse

preferentially along paths proscribed by the solvent matrix and adopt collisional geometries which disturb least environment.

the

local

Thus, depending upon the structures of the solutes

and their complexes, efficiencies of quenching in cholesteric phases can be either greater or less than in the corresponding isotropic phases.

In fact, both

pyrene excimer Employing

time-correlated

cases

have

been observed.

p y r e n e - u exciplex single

photon counting and

532

steady state fluorescence emission techniques, Anderson, Craig, and Weiss studied the fluorescence quenching of pyrene singlets by pyrene (26) or 5 a - c h o l e s t a n - 3 f i - y l d i m e t h y l a m i n e (=)(27) in the cholesteric and isotropic phases of a 59.5/15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride

(m).

The transition states for formation of these

complexes, being dissimilar in shape, are affected differently by cholesteric order.

The pyrene excimer is like two stacked

plates, while the p y r e n e - m exciplex is topologically more similar to a rod intersecting a plate at an acute angle (allowing the nitrogen lone pair of electrons to interact with the pyrene pi system)(28). Dynamic Stern-Volmer plots(29) were The data from obtained at 20.25 & quencher concentrations. both experiments were treated according to a general scheme for bimolecular fluorescence corresponding lifetimes of

IP-

l(PP)

k2

quenching (Eqns. the pyrenyl singlet

2-8). The state in the

P or 3~

k5

_____*

2P

+

(hvE or A )

Scheme 1. A standard mechanism for pyrene fluorescence quenching by pyrene (P) or G.

5.13

absence ( r o ) and presence respectively. k3

and

(2)

of

a

are given by Eqns. 9 and 10,

Stern-Volmer relationships for determination of

kg,

the

rate

constants

for

excited state complex

formation, are presented in Eqns. lla and llb. Solutions to the differential equations (Eqns. 12 and 13) that describe the time dependence of excited state pyrene and pyrene excimer

concentrations

are given

In these solutions, K-(X1-X)/(X1-X2),

in Eqns. 14

F-K/(Xi-X2),

and 1 5 .

A-(Xl-X)/(X-

Y), X-kl+k2+k3, Y-kq+kg, and X i and A2 are given by Eqn. 1 6 . For several well known (30) limiting cases, X i and X2 are equivalent to r 1 and 72, the lifetimes of the pyrene singlet state and excited state complexes, respectively (see Eqns. 9 11). Activation parameters for pyrene excimer formation were calculated by two independent methods. Since kl+k2 is known to be virtually temperature independent and k 4 and k7 are negligible(31),

the ratios of fluorescent intensity maxima from

the pyrene excimer and monomer maxima (IE/IM)

vs

the inverse of

temperature yield the activation energy for pyrene excimer formation, E3. A similar experiment for the p y r e n e - U system was not possible

since its exciplex is not emissive.

Activation

parameters for the excimer and exciplex were also obtained from temperature and phase dependent

pyrene

fluorescent

lifetime

data.

In the liquid-crystalline and isotropic phases of

pyrene

decays were

& l all ,

single exponential and the excimer decays

could be expressed as the difference between two exponentials.

In these studies the pyrene fluorescence rate is equal to the

sum

processes.

of

the

rates

Expressing

of

its

Eqn. 10

radiative in

its

and non-radiative

Arrhenius

form

and

differentiating by 1/T yields Eqn. 17, a closed expression for the

apparent

activation

energy,

Ea, for pyrene fluorescence

534

quenching.

Plots of l/ro vs [PI (dynamic Stern-Volmer plots)

yield kl+k2 as their intercept (Eqn. 9).

Then E3 and A3 can be

derived as a best fit value from Eqn. 17. Following a similar approach to that

of pyrene

excimer

formation, activation energies for p y r e n e - u exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. kl+k2 are obtained from data taken

[u] - 0.

in cyclohexane(32),

and A 3 and E3 from the lifetime taken at

E3 can also be obtained from the slope of the phase

dependent dynamic Stern-Volmer plots. data

from

each

method

are

in

As seen in Table 1 the

good

agreement. The small

differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions.

4L

c a

C B

0.5

1.0

1.5

2.5

2.0

[a]xlO M

Fig. 2. Dynamic Stern-Volmer plots for (A) pyrene-pyrene excimer 64.5OC (b)) and formation in the isotropic (71OC (a); cholesteric (48.5OC (c); 4OoC (d); 29OC (e)) phases of a (27) and (B) p y r e n e - u exciplex formation in the cholesteric (29OC (a); 40.5OC (b); 48.5OC (c)) and isotropic (68OC (d); 71°C (e)) phases of a(26). [Reprinted with permission of the American Chemical Society.] Although dynamic Stern-Volmer plots for pyrene fluorescence quenching derived

by from

were the

curved,

limiting

activation

energies

could be

slopes which yield k67 (Eqn. llb).

Activation parameters obtained from the Stern-Volmer treatments agree well with those which assume kl+k2 for pyrene in same as in liquid paraffin(33) limiting slopes of Fig. 2a.

and

which

take

k3

is the from

the

I n both, the activation energy

for

535

TABLE 1. Activation parameters for p y r e n e - m exciplex formation in CMa Cholesteric Dhase Method b

E6 (kcal/mol) 11.820.7

c

11.420.4

d

9.920.2

Isotropic Dhase

AxlO-I4 ( M - l s-l) 49241

-

E6 Ax10 AS t (kcal/mol) (Itm1 s - l ) (e.u.) 5.421.4 13+17

ASt (e.u.)

19210 2.121.3

5~0.6

5.520.1

1.820.2

5.320.1

1.820.4 - 1 O k O . 4

a)Error limits are standard deviations of linear least-squares fits and reflect precision.b)Using kl+k2 as found in liquid paraffin(33). c)Using kl+k2 as found in cyclohexane(32). d)From Stern-Volmer plots. quenching is ca. 5 kcal/mol higher and the activation entropy is

ca.

than

in

15

like pyrene mutual

e.u.

the

more

positive

isotropic

excimer

diffusion

is of

in

phase. most

the

the

cholesteric

Presumably,

easily

type

formed

favored

in

the by a

phase

sandwichthe

planar

cholesteric

TABLE 2. Activation parameters for pyrene fluorescence quenching by DV rene in CMa Cholesteric ohase Method

AE3 (kcal/mol)

b

8.920.4

C

9.4t2

IsotroDic ohase

St A 3 ~ 1 0 - I ~ ASt AE3 A ~ x ~ O - 'A ~ (M-I s - l ) (e.u.) (kcal/mol) (M-I s - l ) (e.u.)

9.223.9

320.5

6.6L0.6

2.120.6

-4LO.5

6.9+2

a) Error limits are one standard deviation from the mean. b) E3

calculated from the slopes of Arrhenius plots with the error expressed as one standard deviation. A3 calculated from the corresponding intercept in which the error represents the deviation from the mean using minimum and maximum Stern-Volmer k3 values. c) Values obtained from steady state plots of ln(IE/Ip) vs 1/T. mesophase. The insignificant difference in activation parameters derived from the cholesteric and isotropic phases of Q! suggests two possibilities: 1) that solvent order plays no role in

excimer

formation

formation; caused

by

or

2)

that

mediation

of

the enhancement to excimer pyrene-pyrene

collision

536

geometries in cholesteric

is offset by the lower rates of

& f

self-diffusion in the mesophase(9). In contrast, the activation parameters

very

significant

for

pyrene-&

cholesteric and isotropic

difference

exciplex

between

formation in

is compatible with the geometric

requirements for exciplex formation: the bulky exciplex must disturb cholesteric order, thus increasing the entropy of activation;

displace

to

ordered

solvent

molecules

during

exciplex formation, energy additional to that required in the isotropic phase must be infused. In

a

studied

somewhat

different

approach, Sisido et a1.(34)

cholesteryl 3-(l-pyrenyl)propanoate

containing

(by weight)

40%

of

(m).

(Cpp) in a mixture

cholesteryl

3 - ( 1 naphthy1)-

propanoate At least in theory, placing the pyrenyl group cholesteric molecules should within one of the constituent decrease its disturbing influence on the phase order and increase the anisotropy Temperature were

recorded

phases.

of its diffusion.

dependent

in the

steady

state

Based upon relative

emission

mOSt

in

formation

was

Furthermore, plots activation

4m

efficient

of

energies

500 WAVELEWTH (nml

fluorescence

crystalline, cholesteric, and

IE/IM

vs

the

intensities, cholesteric

slopes

excimer phase.

1/T, from which Arrhenius

could be extracted, were

of dramatically

800

CPP

Fig. 3 . Steady state fluorescence emission spectra of Cpp in three different phases: cholesteric (-) at 40, 6 0 , 80 and 100°C from the top; crystal ( - - - ) at 40, 60 and 8OoC from the top; and isotropic (---) at 120, 140, and 16OoC from the top(34). [Reprinted with permission of the American Chemical Society.] different

spectra isotropic

in

each

of

the

three

phases.

The

interpretation of this result is obscured somewhat by the fact

537

that measurements

were

obtained with

undegassed

samples: the

degree to which 02 quenching of the excited state occurs in each phase is not known quantitatively. phase

order

on

intermolecular

Nevertheless, an effect of

dynamics

is clearly present.

In the crystalline phase, the pyrenyl lumophores of Cpp are thought to be situated incorrectly to form an excimer.

The high

barriers to diffusion should be lowest in the isotropic phase, but the fraction of collisions which are appropriate for excimer formation is expected to be highest in the cholesteric phase. Since the fraction of cholesteric phase collisions leading to excimer formation should be higher than in the isotropic phase and rates of self-diffusion along the direction of the solvent director are probably near isotropic phase values, the maximal excimer emission intensities in the cholesteric phase are not surprising.

Combined with the previously mentioned results for

pyrene excimer formation in cholesteric and isotropic Qf, they form the basis for a cohesive picture in which pyrenyl lumophores decrease the order of their local environments but are channeled along directions which lead to enhanced probabilities of pre-excimer collisions. By attaching a cholesteric moiety influence

to

the

is decreased and

pyrenyl the

group,

selectivity

the disturbing

of intermolecular

collisions increases. 4.

INTRAMOLECULAR QUENCHING OF PYRENYL FLUORESCENCE IN CHOLESTERIC PHASES The rates of inter- and intramolecular processes often rely

upon similar solvent properties.

However, some differences do

exist: for instance, chain length is a better descriptor than solute concentration in determining the frequency ofintramolecular collisions(35).

In evidence

of

the similarities, the

rates of several intermolecular reactions can be correlated with those of their intramolecular analogues when both are conducted in isotropic solvents. The rates of intramolecular processes in anisotropic media (such as cholesteric liquid-crystalline phases) are a function of the same solvent and solute properties mentioned above and, additionally, the exigencies imposed by

solvent order on the

frequency and orientations of head-to-tail collisions(36).

The

importance of the latter considerations i s demonstrated by the

538

lack of correlation between the rates of intramolecular

frequent

processes of and ordered

a,w-disubstituted alkanes conducted in isotropic media(37). Solvent order can "turn on" some

properties of intramolecular reactions and "turn off" others. Thus, by limiting the types of collisions between the reactive groups

which

occur,

solvent

order

should

simplify

mechanistic description of an intramolecular process. it

can

also

isotropic media.

"turn on"

factors

which

are

the

However,

unimportant in

An example of the latter is the dependence of

the ability of a head and tail group to find one another on the length of the polymethylene which separates them (vide infra). These considerations are exemplified by the efficiency of intramolecular fluorescence quenching of N,N-dimethyl-4-[3-(1pyreny1)propyllaniline

(m) as

cholesteric phases of m(38). exciplex

formation

studied

in the

isotropic and

The mechanism for (non-emissive)

could be

accommodated

in

both

phases

to

Scheme 2. Fluorescence decay curves for the pyrenyl

emissions were

obtained by a time-correlated single photon counting technique. The monoexponentiality of these curves in both solvent phases is consistent with k4 99.5% and

analysis (hexanes (lit.(42a)

as

mobile

phase).

The

m,

was 99.1% pure by HPLC

m: mp

185-186.5OC

mp 188-189OC); IR (KBr) 3040, 2920, 2860, 1600, 1590,

1510, 1490 and 1470 cm-l; NMR (CDCl-j), 6 1.7-1.55 (2H, quin), mp 179.51.9 (4H, quin), 3.35 (4H,t), 8.2-7.8 (18H,m).

m:

180.5OC; IR (KBr) 3060, 2950, 2870, 1600, 1495 cm-l; NMR (CDCL3), 6 2.0-0.95 (8H,m), 3.35 (4H,t), 8.2-7.9 (18H,m).

m:

mp 123.5-L26°C; IR (KBr) 3045, 2935, 2860, 1605, 1468 cm-l; NMR (CDCl3), 6 1.55-1.36 (10H,m), 1.85 (4H,quin), 3.3 (4H,t), 8.37.8 (18H,m). p11p: mp 133.5-134.5OC; IR (KBr) 3030, 2920,

548 2840, 1590, 1581, 1160 1.82

(4H,quin),

3.30

c m - l ; NMR (CDC13), 6 1.51-1.27 (14H,m),

(4H,t),

8.3-7.8 (18H,m).

u: mp 129.5-

130.5OC; IR (KBr) 3040, 2920, 2860, 1600, 1590, 1510, 1490, 1470 c m - l ; NMR

(CDC13),

6

1.55-1.22

(16H,m),

1.85

(4H,quin),

3.3

(4H,t), 8.2-7.8 (18H,m). REFERENCES

1 2 3

4 5

6 7

8 9 10 11 12 13 14 15 16 17 18 19 20 21

Part 33 of our series "Liquid Crystalline Solvents as Mechanistic Probes." For part 32, see: R. G. Weiss, R. L. Treanor and A. Nuffez, Pure Appl. Chem., submitted. For a comprehensive review, see: R. G. Weiss, Tetrahedron, submitted. (a) P. deGennes, The Physics of Liquid Crystals, Clarendon, Oxford, 1974. (b) R. Eidenschink, J. Krause, L. Pohl, and J . Eichler in: S. Chandrasekhar (Ed), Liquid Crystal Proceedings, International Conference, Heydon, London, 1980, pp. 515-523. J. E. Adams, W. Haas, and J . Wysocki in: J. Johnson and R. S. Porter (Eds), Liquid Crystals and Ordered Fluids, Plenum, New York, 1970, pp. 463-465. (a) E. G. Cassis and R. G. Weiss, Photochem. Photobiol., 35 (1982) 439-444. (b) W. J . Leigh, Can. J . Chem., 63 (1985) 2736-2741. (c) P. DeMaria, A. Lodi, B. Samori, F. Rustichelli, G. Torquati, J . Am. Chem. S O C . , 106 (1984) 653656. H. W. Gibson in: F. D. Saeva (Ed), Liquid Crystals. The Fourth State o f Matter, Marcel Dekker, New York, 1979, pp. 99-162. (a) R. S. Porter, E. M . Barrall, and J . F. Johnson. J. Chem. Phys., 4 5 (1966) 1452-1456. (b) R. S. Porter, A. C. Griffin, and J . F. Johnson, Mol. Cryst. Liq. Cryst., 25 (1974) 131144. K. Sakamoto, R. S. Porter, and J . F. Johnson, Mol. Cryst. Liq. Cryst., 8 (1969) 443-455. M. E. Moseley and A. Lowenstein, Mol. Cryst. Liq. Cryst., 90 (1982) 117-144. T. Forster and B. Selinger, Z. Naturforsch, Teil A , 19 (1964) 38-41. N. J . Turro and K. S. Arora, Polymer, 27 (1986) 783-796. J. K. Thomas, J. Phys. Chem., 91 (1987) 267-276. B. Stevens and M. Thomaz, Chem. Phys. Lett., 1 (1968) 5 3 5 536. K. Kalyansundaram and J. K. Thomas, J. Am. Chem. S O C . , 99 (1977) 2039-2044. A. Weller in: M. Gordon and W. R. Ware (Eds), The Exciplex, Academic Press, New York, 1975, pp. 23-38. N. C. Shivaprakash, R. K. Rajalakshmi, and J . S. Prasad, Mol. Cryst. Liq. Cryst., 6 0 (1980) 319-326. B. J. Bulkin and K. Krishnan, J . Am. Chem. SOC., 93 (1971)5998-6004 D. Cutler, Mol. Cryst. Liq. Cryst., 8 (1969) 85-92. R. S. Porter and J . F. Johnson , Acc. Chem. Res., 2 (1969) 53-59. M. Nakazaki, K. Yammamoto, and K. Fujiwara, Chem. Lett., 8 (1978) 863-864. M. Hibert and G. Solladie, J . Org. Chem., 4 5 (1980) 5393-

549

5394. 22 S. Ganapathy and R. G. Weiss in: M. A. Fox (Ed), Organic Phototransformations in Non-Homogeneous Media, American Chemical Society, Washington, D. C., 1985, pp. 146-170. 23 See for instance: J. P. Otruba I11 and R. G. Weiss. J . Org. Chem, 4 8 (1983) 3448-3453. 24 T. Kunieda, T. Takahashi, and M. Hirobe, Tetrahedron Lett., 24 (1983) 5107-5108. 25 J. M. Nerbonne and R. G. Weiss, J. Am. Chem. SOC., 101 (1979) 402-407. 26 V . C. Anderson, B. B. Craig, and R. G. Weiss, J . Am. Chem. SOC., 103 (1981) 7169-7176. 27 V. C. Anderson, B. B. Craig, and R. G. Weiss, J . Am. Chem. SOC., 104 (1982) 2972-2977. 28 G. N. Taylor, E. A. Chandross, and A. H. Schiebel, J . Am. Chem. S O C . , 96 (1974) 2693-2697. 29 C. Lewis and W. R. Ware, Mol. Photochem., 5 (1973) 261-285. 30 J . A. Syage, P. M. Felker, and A. H. Zewail, J . Chem. Phys., 81 (1984) 2233-2256. 31 J. B. Birks, M. D. Lumb, and I. H. Munro, Proc. R. SOC. London, Ser A, 280 (1964) 289-297. 32 J . B. Birks, D. J . Dyson, and I. H. Munro, Proc. R. S O C . London, Ser A, 275 (1963) 575-588. 33 B. Stevens, M. F . Thomaz, and J . Jones, J. Chem. Phys., 46 (1967) 405-406. 3 4 M. Sisido, K. Takeuchi, and Y. Imanishi, J . Phys. Chem., 8 8 (1984) 2893-2898. 35 M . Winnik, Chem. Rev., 81 (1981) 491-524. 36 V . Ramesh and R. G . Weiss, Mol. Cryst. Liq. Cryst., 135 (1986) 1 3 - 2 2 . 37 Melnick, R. L., H. C. Haspel, M. Goldenberg, L. M. Greenbaum, and S. Weinstein, Biophys. J . , 34 (1981) 4 9 9 - 5 1 5 . 38 V . C. Anderson, B. B. Craig, and R. G. Weiss, J . Phys. Chem., 86 (1982) 4642-4648. 39 V. C. Anderson and R. G. Weiss, J . Am. Chem. S O C . , 106 (1984) 6628-6637. 4 0 M. F. Sonnenschein and R. G. Weiss, J . Phys. Chem., submitted. 4 1 K. Shimada and M. Szwarc, J . Am. Chem. SOC., 9 7 (1975) 33133321. 4 2 (a) K. A. Zachariasse and W. Kuhnle, Z. Phys. Chem., N. F . , 10 (1976) 267-276. (b) M. A. Winnik, A. E. C. Redpath, and D. H. Richards, Macromolecules, 13 (1980) 328-335. (c) M . Yamamoto, K. Goshiki, T. Kanaya, and Y. Nishijima, Chem. Phys. Lett., 56 (1978) 333-335. 4 3 U. Khuanga, B. Selinger, and R. McDonald, Aust. J . Chem., 29 (1976) 1-12. 44 K. A. Zachariasse, W. Kuhnle, and A. Welfer, Chem. Phys. Lett., 73 (1980) 6 - 1 1 . 4 5 (a) K. A. Zachariasse, G. Duveneck, and R. Buse, J . Am. Chem. SOC., 106 (1984) 1045-1051. (b) A. Siemiarczuk and W. R. Ware, Chem. Phys. Lett., 140 (1987) 277-280. 46 F . Saeva, P. Sharpe, and G. Olin, J . Am. Chem. S O C . , 97 (1975) 204-205. 4 7 E. Sackmann and J . Voss, Chem. Phys. Lett., 14 (1972) 528532. 4 8 J . E. Adams, W. Haas, and J . Wysocki in: J . Johnson and R. S. Porter (Eds), Liquid Crystals and Ordered Fluids, Plenum, New York, 1970, pp. 463-475. 49 (a) J. E. Leffler, J . Org. Chem., 20 (1955) 1202-1231. (b) K.

550

50 51 52 53

54 55 56

A. Zachariasse and G. Duveneck, J . Am. Chem. S O C . , 109 (1987) 3790-3792. J. E. Leffler, Rates and Equilibria of Organic Reactions, Wiley, New York, 1963. A. Pross and S . Shaik, J. Am. Chem. S O C . , 104 (1982) 11291130. (a) H. A. Kramers, Physica, 7 (1940) 284-304.(b) T. Kanaya, K. Goshiki, M . Yamamoto, and Y. Nishijima, J . Am. Chem. S O C . , 1 0 4 (1982) 3580-3587. (a) P. J . Flory, Statistical Mechanics of Chain Molecules, Interscience, New York, 1969. (b) D. M. Golden, S. Furuyama, S. W. Benson, Int. J . Chem. Kinet., 1 (1969) 57-67. (c) K. S. Pitzer, Disc. Farad. S O C . , 10 (1951) 66-73. (d) T. P. Liao and H. Morowetz, Macromolecules, 1 3 (1980) 1228-1233. G . C. Overberger and M . J . Lapkin, J . Am. Chem. SOC., 77 (1955) 4656-4657. (a) J . Buckingham (Ed), Dictionary of Organic Compounds, 5 t h Ed., Chapman Hall, New York, 1982, Vol. 5, p. 5661. (b) Ibid. Vol. 4, p . 4309. L. Fieser and M. Fieser, Reagents for Organic Synthesis, J o h n Wiley, New York, 1967, Val. 1, p. 581.

-

551

DYNAMICS OF EXCITED STATE RELAXATIONS IN SOME PROTEINS F. TANAKA and N. MATAGA INTRODUCTION A decade ago the protein structure was believed to be rigid. A number of experimental evidences, however, have revealed that protein structure is inherently dynamic ( 1 , 2 ) . Theoretical studies on the protein dynamics have been given by Karplus et al. ( 3 ) , demonstrating various atomic and molecular motions in protein over the time regions from subpicosecond to nanosecond. Such an inherently dynamic nature of the protein structure will affect profoundly the photophysical and photochemical primary processes in proteins. In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors taking into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 1.

PICOSECOND DYNAMICS OF FLAVOPROTEINS Flavins are vitamine B2 and bind to proteins as coenzyme. Some photoreceptors contain flavins which receive photons. Isoalloxazine nucleus being chromophore of various flavins is yellow dye and intensely emits greenish fluorescence in organic and aqueous solutions. The fluorescence of flavins is remarkably quenched when they bind to protein moiety. Among amino acid 2.

552

residues tryptophan and tyrosine RF FMN 10 I t I I are known as efficient quenchers CHID-+ P-o+ P -0CH: (4). The quenching mechanism has (CHOW, H Lf--+--H been investigated by a picosecond CpI laser photolysis technique for the lumiflavin (Lf) and riboflavintetrabutylate (RFTB) as fluorescers, iroalloxazinc adenine and indole, N-methylindole and I phenol as quenchers in various FAD solvents of different polarity, and the electron transfer from the F* quencher to the fluorescer has been confirmed by transient absorption spectral measurements (5). The reaction scheme of the fluore- Z I l n S scence quenching is given in Fig.1. Observed values of (Ii/If - 1 ) were proportional to the concentrations of quenchers, exF + Q .-(F ... Q) cept indole-RFTB system in CC14 solvent, where the interaction Fig. 1. Reaction scheme of the fluorescence quenching of between indole and RFTB in the flavins. ground state must be taken into Values of the fluorescence quenching rate constant (kq) account. Observed k values revealed of flavins are listed in Table 1 . 9 that most of the quenching reactions in polar solvents are diffusion-controlled, which is in accordance with the fact that the free energy changes ( A G O ) associated with electron transfer in these systems are sufficiently exothermic. TABLE 1 The rate constant is The fluorescence quenching rate greater in the system constants of flavins. of indole-lumiflavin (Lf) by three times flavins Q solvent kq than in the system of ( M - l ns-I ) Dhenol-Lf. 5.4 Lf phenol ethanol Transient absor15 indole ethanol ption spectra of Lf in indole CHClj 12 N-methyl- ethanol 17 the absence of quenindole RFTB indole cc14 ca.20 cherare shownin indole acetonitrile 13 Fig.2. Negative ab-

-

553

sorbances of the transient spectra are due to intense absorption band of isoalloxazine nucleus in the ground state at the wavelengths shorter than 480 nm, and due to an amplification of monitoThe shapes ring light pulse by S1+So transition at about 520 nm. of the band did not change appreciably with thedelay time, although the absorbances decreased. The decay constant of the absorbance of Lf in chloroform was evaluated to be 6.0 +- 1.5 ns, which is close to fluorescence lifetime of Lf, 7.0 ns. These facts indicate that the transient absorption spectra are ascribed to Sn4S1 transition. The transient absorption spectra of Lf in the presence of N-methylindole are shown in-Fig.3. The spectrum obtained immediately after excitation is similar to the transient absorption spectrum of Lf. Therefore, S1 state of Lf is also formed at the instant of excitation. The decay of the negative absorbance at about 5 2 0 nm became much faster upon the addition of the quencher. As the fluorescence of Lf decayed, a new absorption band appeared with the maximun around 580 nm and decayed Similar phenomena were also observed rather slowly within 10 ns. in the systems of indole-Lf in chloroform and indole-RFTB in methanol. The new absorption band around 500-600 nm is similar to that of indole cation radical as shown in Fig.4, produced by exciting the acetonitrile solution of indole with 4th harmonic of : YAG laser. It has been confirmed that flavin a picosecond Nd'3

0.1

c

9

100 pr

-A

0I

500

f

a

I

I

600

700

700 600 Wavelength, nm

I

800

Q

500

800

Wavelength, nm

Fig. 2. Transient absorption spectra of Lf in chloroform ( A ) and RFTB in carbon tetrachloride (B). The delay times from pulsed excitation are indicated in the Fig. (5).

Wavelength, n m

Fig. 3. Transient absorption spectra of the N-methylindole - Lf system in chloroform. The delay times from the pulsed excitation are indicated in the Fig. (5).

554

anion radicals exhibit the band maximum at 480 nm in solution ( 6 and 490 nm in flavoprotein (7). From these results the species formed by photoexcitation of the N-methylindole-Lf system was assigned to a charge transfer (CT) or ion-pair state produced as a result of photoinduced electron transfer from indole to flavin. Characteristics of the CT or ion-pair state produced by photoexcitation have been amply discussed (8-11). In contrast to the above results of the N-methylindole-Lf system, the transient behavior in the quenching of S1 state of Lf by phenol was quite different as shown in Fig.5. Although the S 1 state was observed at the early stage, the species produced from the S1 state showed averybroad spectrum, anditdecayedwithin 5 ns, which was faster than those in the systems of indole and N-methylindoleflavins. Flavodoxin from Desulfovibrio vulgaris, strain Miyazaki, is considered to maintain similar three-dimensional structure to flavodoxin from Desulfovibrio vulgaris, strain Hildenborough, since amino acid composition (12) of flavodoxin, strain Miyazaki, resembles to that of flavodoxin, strain Hildenborough, except for Val133 and Lys-145, which are replaced by Ala-133 and Arg-145 in the latter flavodoxin ( 1 3 ) . According to the three-dimensional structure of flavodoxin, Hildenborough, isoalloxazine nucleus of flavin mononucleotide (FMN) is placed between aromatic rings of Fluorescence of FMN in flavodoxin, tryptophan and tyrosine (1 3). Hildenborough, is completely quenched. Fluorescence of FMN in flavodoxin, Miyazaki, is also completely quenched. Transient absorption spectra of flavodoxin, Miyazaki, are shown in Fig.6 0.1 ( 1 4). 0 0.1

ii O.lY--

0.1

Q

OL

I

500

I

600

Wavelength,

I

700

0

I

500

nm

Fig. 4. Transient absorption spectra of indole cation radical produced by exciting the acetonitrile solution with the 4th harmonic of the ps Nd+3 : YAG laser ( 5 ) .

I

600 Wavelength,

800

700

nm

Fig. 5. Transient absorption spectra of the phenol - Lf system in ethanol. The delay times from the pulsed excitation are indicated in the Fig. (5).

555

The spectrum similar to those of the CT or ion-pair state obtained in the systems of indole-flavins was observed already at 3 3 ps of the delay time. Therefore, the photoinduced electron transfer from tryptophan to the excited FMN occurs in flavodoxin, Miyazaki. It should be noted that the formation of the CT state is much faster in the protein than in organic solution. Dynamic processes of the formation of the CT state in flavodoxin from Desulfovibrio vulqaris, Miyazaki, are illustrated in Fig.7. The fast photoinduced reaction in flavodoxin is considered to be closely related to collective motion of tryptophan in the protein, since it may be hard forisoalloxazine to move so fast due to its hydrogen bonding with amino acid residues. It is interesting that the excited state of FMN interacts with tryptophan rather than tyrosine in flavodoxin. Although the interaction energy in the ground state between tryptophan and isoalloxazine, calculated using x-ray structure of flavodoxin, Hildenborough, by the method of ab initio molecular orbital was not different much from the one between phenol and isoalloxazine, induced dipole moment due to very weak CT in the direction perpendicular to both aromatic rings was greater in the system of tryptophan-isoalloxazine by five times than the one in the system of phenol-isoalloxazine ( 1 5 ) . The quenching rate constant of Lf, in fact, is greater in the system of indole-Lf by three times than in the system of phenol-Lf

I

fery weak luorescence

Inan-fluorescent

i! '[530nm)

t

'FMN

Wavelength (nm)

Fig. 6. Transient absorption spectra of FMN in flavodoxin from Desulfovibrio vulqaris, Miyazaki. Flavodoxin (0.1 1 mM) was dissolved in glycerol (75%). Delay times from pulsed excitation are indicated in the Fig. ( 1 4 ) .

Fig. 7. Dynamic processes of the formationof CT state in flavodoxin. Electron transfertakes place within 30 ps from tryptophan to the excited FMN to form CT.or ion-pair state which decays within several ns ( 1 4 ) .

556

(5).

The t r a n s i e n t a b s o r p t i o n s p e c t r a s i m i l a r t o t h a t of t h e

i o n - p a i r s t a t e o f i n d o l e c a t i o n r a d i c a l and f l a v i n a n i o n r a d i c a l

were a l s o o b s e r v e d i n D-amino acid o x i d a s e ( 5 ) ,

although the

s p e c t r a w e r e n o t so clear as t h o s e o f f l a v o d o x i n .

I n D-amino

a c i d o x i d a s e , t h e coenzyme, f l a v i n a d e n i n e d i n u c l e o t i d e (FAD), i s The f l u o r e s c e n c e l i f e t i m e w a s r e p o r t e d t o be

weakly f l u o r e s c e n t . 40 p s ( 1 6 ) ,

w h i c h became d r a s t i c a l l y s h o r t e r ( l e s s t h a n 5 p s ) when

b e n z o a t e , a c o m p e t i t i v e i n h i b i t o r , w a s combined w i t h t h e enzyme a t FAD b i n d i n g s i t e ( 1 7 ) .

The d i s s o c i a t i o n c o n s t a n t of FAD w a s a l s o

m a r k e d l y d e c r e a s e d by t h e b i n d i n g of b e n z o a t e ( 1 7 ) .

T h e s e re-

s u l t s s u g g e s t t h a t i n t e r a c t i o n b e t w e e n i s o a l l o x a z i n e and t h e quenc h e r became s t r o n g e r a s t h e i n h i b i t o r comkiined w i t h t h e enzyme. Absorbance of t h e t r a n s i e n t s p e c t r a of D-amino a c i d o x i d a s e b e n z o a t e c o m p l e x w a s r e m a r k a b l y decreased.

I n t h i s case b o t h

r a t e c o n s t a n t s of f o r m a t i o n a n d d e c a y o f t h e CT s t a t e c o u l d become much f a s t e r t h a n t h o s e i n t h e c a s e o f D - a m i n o a c i d o x i d a s e f r e e from benzoate. When r i b o f l a v i n (RF) i s mixed w i t h r i b o f l a v i n b i n d i n g p r o t e i n from egg w h i t e , t h e f l u o r e s c e n c e i n t e n s i t y d e c r e a s e d t o a b o u t t w o thousandth.

T r a n s i e n t abso-

r p t i o n s p e c t r a of r i b o f l a v i n bind i n g p r o t e i n were a l s o m e a s u r e d w i t h t h e picosecond laser photol y s i s t e c h n i q u e (141, Fig.8.

"0.05

'

O

m

a s shown i n

The t r a n s i e n t b e h a v i o r

of t h e s p e c t r a was q u i t e d i f f e -

100 pr-

r e n t from t h a t o f f l a v o d o x i n . The b r o a d a b s o r p t i o n band e x h i b i t e d t h e maximum w a v e l e n g t h a t

1 ns

c a . 6 6 0 nm u n t i l 1 0 0 p s , a n d t h e n s h i f t e d toward longer wavelength s i d e w i t h increasing t h e absorbance.

The s p e c t r a a p p e a r e d a t

e a r l y s t a g e resemble t h e t r a n -

0.10 0.05

0

s i e n t spectrum obtained i n t h e s y s t e m of p h e n o l - L f ,

where t h e

a b s o r p t i o n band e x h i b i t e d maximum

a t ca. 6 6 0 nm.

However, c o n t r a -

r y t o t h e c a s e o f t h e phenol-Lf

system, t h e absorbance d i d n o t decay but i n c r e s e d w i t h t h e d e l a y

500

600

700

80

Wavelength (nm)

Fig. 8. Transient absorption s p e c t r a of r i b o f l a v i n b i n d i n g p r o t e i n from e g g w h i t e . Del a y times from pulsed e x c i t a t i o n a r e shown i n t h e Fig. (14).

557

time, which means that some another state was formed from the intermediate state showing absorption at 660 nm. From the above experimental results by means of the picosecond laser photolysis, it can be predicted that tryptophan is absent near isoalloxazine nucleus within the interaction radius in riboflavin binding p r o tein, while it exists in flavodoxin from Desulfovibrio vulqaris, strain Miyazaki. 3.

TIME-RESOLVED FLUORESCENCE OF TRYPTOPHAN AND ITS MOTION IN

PROTEINS Fluorescence characteristics of indole, aromatic nuleus of tryptophan, has been extensively investigated by a number of workers (18-24). Weber demonstrated by measuring fluorescence polarization spectra that two electronic states are overlapping in the absorption band in near-ultraviolet region (18). Mataga et al. ( 1 9 ) examined the solvent effect on the fluorescence spectrum and found that it displayed structured spectrum around 310 nm in nhexane, while it showed a structureless broad band around 350 nrn in acetonitrile. They concluded that the fluorescence of indole is emitted from Lb state in non-polar solvents, while it is emitted from La state in polar solvents, because dipole moment of the La state was much larger than the one of the Lb state, and accordingly, the La state couldbe stabilized m u c h m o r e i n t h e polar solvent than the Lb state. Subsequently, a number of workers reported that indole exhibited dual emission (20,21). Lumry et al. (22,231, however, claimed that the remarkable Stoke's shift of the indole fluorescence was due to the formation of "exciplex" between excited indole and solvent molecule with high polarity, since it was observed even in the non-polar solvent when a small amount of polar molecule like alcohols was added. (The term "exciplex" used in this case is misleading since the typical exciplex is a CT complex formed in the excited state while the nature of this "exciplex" could be quite different from that). Fluorescence decay curves of indole were measured in various solvents by de Lauder and Wahl (24). The fluorescence decayed with a single exponential function in every single solvent, but decayed with double exponential functions in the mixed solvents. The lifetimes were shorter in polar solvents than in non-polar solvents. Tryptophan has a group, -CH2CH(NH2)COOH, bonded to 3-position.

Rayner and Szabo (25) and Fleming et al. (26) have indepe-

558

ndently found that the fluorescence of tryptophan in aqueous solution exhibits two lifetime components. Subsequently, several groups reported the results of fluorescence lifetime measurements on tryptophan derivatives (27-30), and showed that tryptophan was in an equilibrium between rotamers with respect to the side chain. The lifetimes of tryptophan strongly depended on the solution pH and was closely related to the ionic state of the tryptophan. 3Methylindole in aqueous solution always decayed with single lifetime. Engh et al. ( 3 1 ) demonstrated by molecular dynamics simulation that rotational motion around Ca-Ca bond of the side chain was much faster than the lifetime, but the one around CB-cy bond was much slower than the lifetime. They concluded that the rotamer with close proximity of -NH; group to indole ring decayed with the shorter lifetime. Fluorescence of tryptophans in proteins also decayed with non-exponential functions, even though the proteins contained single tryptophan. The concept of rotamer was applied to explain the non-exponential decay profiles observed in proteins. This explanation for the origin of the non-exponential decays of tryptophan fluorescence, however, Fig. 9. Geometrical arrangement of Trp-29 in erabutoxin cannot rationalize the results of b. Coordinate system of a time-resolved fluorescence anisospherical protein and tryptophanyl residue are shown by tropy in these proteins. Beec(x'y'z') and (xyz), respectihem and Brand review the timevely. The origin of (x'y'z' system is chosenat the CH resolved fluorescence of tryptopgroup connecting the peptiie hans in proteins (32). The chain and indole ring. Inte rnal rotation of the tryptopfollowing conclusions concerning han from the (x'y'z')system time-resolved fluorescence can be to the (xyz) system is expres sed with Euler's angles ( a 4 y ) . derived ; i) fluorescence decays The location of the quencher, with at least two-lifetime compo-NH+ of LYS-27, is a l s o illustraged in the Fig. Polar nents, when internal rotation of coordinates of the N atom of tryptophan is observed from the the quencher in the system (x'y'z') are indicated by a time-resolved fluorescence anisoq and a, (33). tropy, ii) fluorescence decays

559

with single lifetime, when the internal rotation is not observed from time-resolved fluorescence anisotropy, iii) in some cases fluorescence decays with multi-lifetime components, even if the anisotropy decays with single-rotational correlation time, iv) no system is found where fluorescence decays with single-lifetime component, while the internal motion of tryptophan is observed during its lifetime by the decay of fluorescence anisotropy. TO explain the above behaviors of the fluorescence decay function and time-resolved fluorescence anisotropy of single tryptophan in proteins, a model was proposed (331, in which an angular-dependent quenching constant was introduced into a rotational analogue of Smoluchowski equation given by Favro ( 3 4 ) for the internal rotation of tryptophan, as expressed by [ I ] . Rotational motion of tryptophan in a spherical macromolecule is illustrated in Fig.9.

a

- G ( R ’ u R ~ ’ ~ )= at

- [kl + k ( w ) ] G(Q’d2w’t)

4

- (D Ji + JU’D1’J,)

G(R’wRw’t)

where G(Q’wQw’t) represents Green‘s function describing both rotational motions of spherical protein and tryptophan as completely asymmetric rotor. The rotational motions of the spherical rotor and the asymmetric rotor are expressed by Euler’s angles of Q and w respectively. D is rotational diffusion coefficient of the spherical macromolecule and D1 is diffusion tensor with second JQ and J, are angular rank for internal rotation of tryptorhan. momentum operators for the rotational motion of spherical macromolecule and symmetric rotor. kq(w) and ~ ( w represent ) the angular-dependent quenching constant and potential energy for the

560

internal rotation of tryptophan. These may be expanded with Wigner's rotation matrices. Expressions for fluorescence decay function and time-resolved anisotropy of tryptophan were obtained by solving [ I ] ( 3 3 ) . Results of numerical calculations predict that i) both fluorescence decay function and time-resolved fluorescence anisotropy are dependent on the diffusion coefficients of internal rotation of chromophore and strength of torque due to the presence of potential energy for the internal rotation, ii) fluorescence should always decay with non-exponential decay function, whenever internal rotation of chromophore is observable during the fluorescence lifetime by time-resolved fluorescence anisotropy, iii) however, the intensity decay should be single exponential when the internal motion is very fast compared to the lifetime, iv) fluorescence intensity also decays with non-exponential decay function, even though timeresolved fluorescence anisotropy decays with single exponential function due to slow rotational motion of the chromophore compared to the rotational motion of spherical macromolecule. These predictions concerning both features of time-resolved fluorescence can qualitatively explain the observed results documented above. Observed data on azurins are especially interesting from the view point of the model presented above. Single tryptophan of azurin from Pseudonome aeruginosa, blue copper protein, is placed inside the protein and surrounded by hydrophobic amino acid residues. Firstly Grinvald et al. (35) studied fluorescence lifetime o f t h e tryptophan of azurins by means o f a nanosecond pulse fluorometry and reported that holoazurins with different oxidation states of copper exhibit two lifetime components whereas apoazurin where copper is removed from the protein showed single lifetime component. Similar result was also obtained in the case of azurin from Pseudomfluorescens (36). Munro et al. (37) demonstrated by using synchrotron radiation for excitation that fluorescence anisotropy of tryptophan of both holo- and apoazurin from Pseudomonas aeruqinosa decayed with two rotational correlation times. Namely, tryptophan in these azurins possess appreciable freedom of internal rotation in the proteins. Recently Petrich et al. (38) have investigated time-resolved fluorescence of three homologous azurins by means of the time-resolved singlephoton-counting apparatus with a picosecond pulsed laser. Fluorescence intensity of holoazurin from Pseudomonas aeruqinosa also decayed with two-lifetime components, and that of apoazurin did

56 1

with single-lifetime component, in accordance with the data by Time-resolved anisotropy of the apoazurin Grinvald et al.( 35). decayed with single-exponential function, contrary to the observation by Munro et al. as stated above. These data are summarized as follows : i) fluorescence of holoazurin decays with twolifetime components, and the anisotropy decays with two-exponential function, if the data by Munro et al. ( 3 7 ) concerning the time-resolved anisotropy of holoazurin is reliable, ii) fluorescence of apoazurin decays with single-lifet ime component , and the anisotropy decays also with single-exponential function, if the data by Petrich et al. ( 3 8 ) I I I are reliable. In apoazurin 0 100 200 300 Channel number (51.2 ps / channel) tryptophan is fixed in the protein and accordingly fluorescence of tryptophan is single-exponenFig. 10. Fluorescence decay tial. On the contrary tryptopcurves of Trp-29 of erabutoxin b. The observed decay curve han exhibits the two-exponential at 20 " C is shown by dots in decay function in holoazurin, C. The exciting pulse is also shown by curve c in C. where tryptophan possesses appreThe theoretical decay curve ciable freedom of internal rotaobtained from the dynamical model is shown by solid curve tion. These results can be Deviations between a in C. qualitatively explained by the the observed and the theoreticaldecay curves are s h o w n i n above described dynamical model A (see text). Calculated ( 3 3 ) for the time-resolved fluodecay curve at the best-fit with a double-exponential derescence of tryptophan in procay function is shown by curve teins. Time-resolved fluoresceDeviations between b in C. the observed decay curve and nce of single tryptophan in ribocalculated decay curve with nuclease T1 (39) is also worthy to the double-exponential decay function is shown in B. The discuss. The fluorescence devalue of x 2 was 3 . 3 6 . Decay cayed with single-lifetime compoparameters at the best-fit were 1.1 ns (0.96) and 4 . 3 ns nent at the values of pH lower ( 0 . 0 4 ) , where the values in than 6.0, while it decayed with parentheses represent component fractions ( 4 3 ) . two-exponential function at the values of pH higher than 6.5.

662

TABLE 2 Parameters obtained by best-fit procedure between observed and calculated decay curves of fluorescence of Trp-29 in erabutoxin b. diffusion coefficients T

ki

6 20 40

0.668 0.935 1.24

ns-'

P

1-30 1.30 1.20

XZ

0.474 0.506 0.529

1.70 1.88 2.00

0.036 0.16 0.15

2.29 2.53 2.70

1.46 1.54 2.03

k o and p denote averaged values of angular-dependent quenching c8nstant and potential energy over the rotational angles of internal motion of Trp-29. x 2 represents chi-square distribution between the observed and calculated decay curves.

The decays of anisotropy exhibited single-exponential function at the entire values of pH. These data indicate that internal motion of tryptophan is always slower than the rotational motion of the protein. Despite of it the fluorescence decay function of tryptophan shows non-exponential behavior, which can be explained again by the above dynamical model (33). Erabutoxin b is a neurotoxic protein from sea snake and contains single tryptophan (Trp-29). Its three-dimensional structure has been determined by Tsernoglou and Petsko (40) and Low et al. (41), according to which the indole ring of Trp-29 is placed in aqueous layer of a cavity. Trp-29 is believed to play an important role when erabutoxin b binds to acetylcholine receptor (42). Fluorescence of tryptophan of erabutoxin b decayed with nonexponential function (43), as shown in Fig.10. The non-exponential decay functions were analyzed with the formula obtained by the dynamical model (33). It was assumed that quencher for the fluorescence of Trp-29 was Lys-27, since quaternary amino group was a efficient quencher for fluorescence of tryptophan in water and, moreover, NH; group of Lys-27 was quite close to the indole ring of Trp-29. The quenching rate constant and potential energy f o r t h e internal rotation were alsoassumed todepend only on a solid angle between z-axis and a vector formed by origin and nitrogen atom of Lys-27 in (x'y'z') system (see Fig.9). Theoretical decay curve shown with solid curve in Fig.lOC fit well with the observed decay curves. The best fit parameters are listed in Table 2. The observed decay curves were also analyzed with a double-exponential decay function (shown with a broken curve in

563

Fig.1 OC). The fitting between calculated and observed decay curves was always better in the case of the theoretical decay functions. The following conclusions are derived from Table 2 ; i) averaged quenching rate over the rotational angles increases with temperature, ii) most of the diffusion coefficients are proportiocal to temperature, iii) diffusion coefficients related to z-axis, a rotational axis around covalent bond between indole ring and peptide chain, are greater than the others, iv) height of potential energy for the internal rotation is approximately constant as temperature was changed. These results are all physically reasonable. INTERNAL MOTION OF TRYPTOPHAN AND EXCITATION TRANSFER IN PROTEINS The excitation energy transfer has been frequently observed in proteins ( 4 4 , 4 5 ) . The energy transfer has been used as spectroscopic ruler to evaluate distance between the energy donor and acceptor in proteins ( 4 5 ) , assuming dipole-dipole interaction (46). Certain equivocality concerning the distance cannot be avoided, since critical transfer distance contains orientational factor, which should be different in the individual system from 0 to 4 . Dale and Eisinger ( 4 7 ) proposed the method to evaluate the distance more exactly in proteins by measuring both energy transfer efficiency and fluorescence polarization. On the other hand, the fluorescence decay functions and decays of fluorescence anisotropy for the relevant system were theoretically worked out by solving directly a rotational diffusion equation (48). In this model the donor possesses a motional freedom of internal rotation, whereas the acceptor is fixed in a spherical protein. The rotational diffusion equation contains an angular-dependent probability of the energy transfer in the protein. The following conclusions on the behavior of the time-resolved fluorescence are derived by numerical calculations with the expressions thus obtained, i) the fluorescence intensity decay is closely related with the decay of anisotropy of the donor, ii) when the internal rotation is faster than the averaged probability of energy transfer over the rotational angle, the decay function of fluorescence of donor should be single-exponential, iii) the decay function is non-exponential as the internal rotation becomes slower than the rate of energy transfer, iv) the expression for the decay of anisotropy of donor is essentially the same as the one obtained by 4.

564

G o t l i e b and Wahl ( 4 9 ) ,

when t h e i n t e r n a l r o t a t i o n i s f a s t enough,

v ) i t d e v i a t e s f r o m t h e e x p r e s s i o n by G o t l i e b a n d Wahl ( 4 9 ) a s t h e r o t a t i o n becomes slower. A p p l i c a t i o n of t h e a b o v e t h e o r e t i c a l e q u a t i o n ( 4 8 ) t o t h e a n a l y s i s of t h e quenching of tryptophan fluorescence due t o t h e e f f i c i e n t e n e r g y t r a n s f e r t o heme i n m y o g l o b i n ( 5 0 , 5 ? ) G i l l be a n i n t e r e s t i n g problem.

D e t a i l e d n u m e r i c a l a n a l y s i s o f s u c h quen-

c h i n g p r o c e s s e s by t h e method o f m o l e c u l a r d y n a m i c s s i m u l a t i o n ( 5 2 ) w i l l b e a l s o a n i m p o r t a n t problem.

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F. Tanaka and N. Mataga, Biophys. J., 5 1 ( 1 9 8 7 ) 4 8 7 - 4 9 5 . L. D. Favro, in: R. E. Burgess (Ed), Fluctuation Phenomena in Solids, Academic Press, New York, 1 9 6 0 , pp. 79-101. A. Grinvald, J. Schlessinger, I. Pecht and I. Z. Steinberg, Biochemistry, 1 4 ( 1 9 7 5 ) 1 9 2 1 -1 929. A. G. Szabo, T. M. Stepanik, D. M. Wayner and N. M. Young, Biophys. J., 41 ( 1 9 8 3 ) 2 3 3 - 2 4 4 . I. Munro, I. Pecht and L. Stryer, Proc. Natl. Acad. Sci. USA, 76 ( 1 9 7 6 ) 56-60.

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F. Tanaka, N. Kaneda, N. Mataga, N. Tamai, I. Yamazaki and K. Hayashi, J. Phys. Chem., 9 1 ( 1 9 8 7 ) 6 3 4 4 - 6 3 4 6 . I. 2. Steinberg, Ann. Rev. Biochem., 4 0 ( 1 9 7 1 ) 8 3 - 1 1 4 . L. Stryer, Ann. Rev. Biochem., 4 7 ( 1 9 7 8 ) 8 1 9 - 8 4 6 . Th. Forster, Fluorezenz Organischer Verbindungen, Vandenhoek Rupprecht, Gottingen, 1 9 5 1 . R. E. Dale and J. Eisinger, in: R. F. Chen and H. Edelhoch (Eds), Biochemical Fluorescence:Concepts, Vol. 1 , Marcel Dekker, New York, 1 9 7 5 , pp. 1 1 5 - 2 8 4 . F. Tanaka and N. Mataga, Biophys. J., 3 9 ( 1 9 8 2 ) 1 2 9 - 1 4 0 . Y. Y. Gottlieb and Ph. Wahl, J. Chim. Phys., 60 ( 1 9 6 3 ) 8 4 9 -

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567 Subject Index

Abnormal methyl radical, 173 Absolute asymmetric synthesis, 501 Absorbance, 17, 121, 440 Absorption acceptor, 107 coefficient, 451 IR, 120 nanosecond UV-Visible, 26 picosecod W-Visible, 27 spectra, 2, 99, 108, 119, 187, 229, 381 transient, 6, 15, 36 triplet-triplet, 7, 36 UV, 119 W-Visible, 15 Acceptor molecule, 106, 237 Acetone, 120 Acetonitrile, 552 Acetophenone, 473 Acid, 1, 41 BrGnsted, 121 compound, 149 surfaces, 458 Acridine, 79 Acridone, 79 Activation energy, 49, 274, 348, 472, 533 Additives, 423 Adamantylacetophenone derivatives, 511 substituted ketones, 511 Adsorbed alkyl radical, 127 cumyl radical, 147 layer, 119 methyl radical, 127 molecule, 79, 108, 119, 288, 356 pyrene, 79 species, 81 state, 119 Adsorbent, 120, 160 Adsorption acridone, 185 alkyl ketone, 121 alkyl radicals, 168 biacridylidene, 185 chemical, 149

co-, 111 coefficient, 149 dry, 236 dyes, 106 dye molecule, 410 equilibrium, 81 isotherm, 289 Langmuir, 388 physical, 289 pyrene, 79 site, 80, 412 solution, 273 spectroscopic means, 255, 305 system, 236 Aggregation, 303, 360 Algorithms, 353 Alfazurine, 1 1 Aliphatic ketones, 485 polymers, 458 A1kanones, 487 Alkylpyrenylsilanes, 54 Alkyltinchlorides, 270 Alkylphenones, 487 Alumina, 41, 186 activated, 186 A1203, 151, 173, 186, 362 A104 tetrahedra, 199 Aluminosilicates, 199, 221 Amines, 389, 428 Ammonia, 389 Amorphous, 157 crystalization, 29 silicon (a-Si), 339 silicon hydride, 348 Amphiphilic molecules, 481 Anatase, 360 Angular-dependent quenching constant, 559 Anion radicals, 554 Anisotropic cybotactic regions, 529 decay, 559 hyperfine, 169 media, 528 phase, 526 Anodic photocurrent, 379 Anthracene, 27, 48, 236, 411 Anthrylethylene, 6

568 Anthrylmethyl radical, 7 Antiozonant, 432 Arachidic acid multilayer spacers, 415 Area-selective deposition, 379 reaction, 375 Argon matrix, 168 Aromatic bridges, 238 Aromatic bisazides, 465 Aromatic ketones, 482 Aromatization, 145 Arrhenius equation, 3, 49 Aryl-diene bonds, 504 Asynnnetric stretching, 161 synthesis, 501 Atropisomeric content, 530 Attenuation factors, 246 Au film, 329 Auger electron spectroscopy (AES), 340 Azobenzene, 141, 438 Azocumene, 138 Azo pigments, 433 Band gap, 30 excitation, 375 semiconductors, 405 Band edge photoluminescence, 389 Base, 1, 150 Basicity, 388 Bending motion, 492 Benzene-l,4-diacrylates, 505 Benzene slurry, 187 Benzocyclobutanols, 471 Benzophenone, 27, 41, 357 2-benzyl-5-benzylidenecyclo-

pentanone, 218 Benzyl radical, 208 BET surface area, 360 Bidentate, 161 Bilayer crystal structure, 483 1,l '-binaphthyl, 529 Binding energy, 169, 329 Biological photosynthetic, 240 reaction, 551 Biphenyl, 237 Biradical, 125, 184, 469, 485, 510 Box-counting technique, 368 Bragg-type scattering, 527

l-bromo-8-methylnaphthalene, 472 Bulk solute/mesogen system, 484

Bulk viscosity, 527 volume, 355 2-butanone, 120 Butenes, 305 4'-butylbicyclohexyl-4-carbonitrile, 483 Cadmium sulfides, 388 Cadmium sulfoselenide, 388 Cage, 168, 202 effect, 275 Capacitance change, 435 Capped porphyrin, 237 Carbon monoxide, 313 Carbonyl compounds, 119, 255, 469 Carotenes, 428 Carrier-to-noise ratio, 426 Catalysis, 41, 151, 202, 272, 289 Catalyst, 188 Cation, 197 radical, 184, 554 Centrosyrnmetric dimer, 217 CDF3, 330 CD3 Radicals, 127 CdS, 9, 388 single crystal n-, 389 CdS:Te, 396 CdSe, 18, 389 CdS,Sel-x substrates, 389 GF(,d), 332 CF4 plasma, 317 CH4 evolution, 275 CH3F, 328 (CH3)NH2, 392 (CH3)2NH, 392 (CH3)3N, 392 Chance-Prock-Silbey model, 415 Channels (cylindrical shapes), 202 Charge transfer (CT), 554 complex, 184, 429 Chemical actinometers, 484 Chemical sensors, 399 optically-coupled, 399 Chemiluminescence, 184 Chemisorption, 41, 192 Chiral vesicle structure, 438

569

Chirality, 508, 527 Chlorine molecules, 317 Chlorobutane matrix, 244 Chlorophyll, 236 dimer, 237 monomer, 237 Cholesteric liquid crystals, 439, 526 phase, 526 reflection wavelength, 439 Cholesteryl chloride, 439 esters, 528 Chromophore, 7, 20, 33, 93, 438, 456 4-cleavage, 119, 469, 485 Clusterification, 278 CO evolution, 275 Co-adsorption, 111, 172, 197 Colloidal fraction, 221 silver, 339 Collisional energy transfer, 356 Computer graphic, 207 industry, 448 Conformational barriers, 6, 503 constraint, 238 Conjugation, 7 Coordination, 162, 171 deficiency, 350 C02 laser, 327 Collision diameter, 347 geometry, 531 Colourants, 419 amorphous, 419 inorganic, 420 organic, 419 Conformational characteristics, 502 Contrast enhancement, 463 Coupling strengths, 248 Cr(C0)6, 275 Cresol-formaldehyde novolac resin, 457 Cryophotoclustering, 263 Cryostat, 260 Crystal, 219 dye system, 239 engineering, 469

growth, 502 lattice, 469, 522 organic single, 236 packings, 470, 502 size, 522 surface, 476 structures, 502 Violet Lactone, 422 Crystalization, 303, 502 Cyanine dyes, 422 4-cyano-4'-n-pentylbiphenyl, 439 9-cyanophenantrene, 357 Cyclic voltammetry, 379, 429 Cyclization, 485 Cycloaddition, 216, 482 intermolecular, 216 Cyclobutanol formation, 486 2,5-~yclohexadienone, 475 Dangling bonds, 349 Deactivation radiationless, 106, 129 Dead-layer model, 388 thickness, 395 Decanol, 63 Decarbonylation, 275 Decay channels, 405 kinetics, 170 time, 52, 86, 111 Decomposition, 138 Defect site, 187 Degradation photosensitized, 432 Dehydration, 303, 330 Delay, 38 Deoxycholic acid, 470 Depletion width, 394 Deposition rate, 265 Si, 327 systems, 344 Deprotonation, 228 Diarylbutadiene compounds, 504 Diastereomers, 508 3,3'-diazido-diphenylsulfone, 465 Dibenzobarrelene-ll,12-diesters, 519 4,4'-diazo-diphenylsulfide, 464

570

Dibenzyl ketone, 197 Dichloromethane, 429 Dichroism, 540 Diels Alder reaction, 216 Diene, 216 Diffuse motion, 202 reflectance, 31 reflectance ETIR, 273 rotation, 91 Differential scanning calorimetry (DSC), 484 Diffusion controlled, 49, 102 distance, 355 lengths, 377 motion, 207 radicals, 175 Dimension fractal, 353 Dimer, 57, 145, 411 ground-state, 51 head-to-tail, 145, 223 Dimerization, 217, 503 4,4'-dimethylbenzophenone, 470 10,101-dimethyl-9,9'-biacridylidene, 185 2,5-dimethyl-furan, 426 1,2-dimethylthymine, 530 Dioxetane, 188 Dipalmitoylphosphatidylcholine, 540 Diphenyl-butyrophenone, 41 Diphenylnitroxide, 140 Dipole-dipole interaction, 107 repulsion, 219 1,3-di(l-pyrenyl)propane, 53 Disilane, 339 Disordered solids, 526 Disymmetric influence, 501 2,6-di-t-butyl-p-cresol, 422 Dithiolene nickel complexes, 425 1-dodecylpyrene, 540 Donor molecule, 106, 237 Doping naphthalene, 110 4,4'-dimethylbenzophenone, 469 Double-bond-migration, 313 Dye

adsorbed, 112 cationic, 110 excited, 236 molecule, 236, 381 Einstein-Smoluchowski diffusion theory, 67 Electric device, 385 field, 435 Electrochemical chemiluminescence, 186 potentials, 10 reaction, 375 Electrochromism, 377 Electrode, 375 crystal substrate, 236, 403 needle counter, 375 deposition coating, 381 surface, 375 ZnO thin-film, 382 Electrolysis, 382 Electrolyte solution, 375 Electron acceptor, 192, 237 affinity, 247 Auger, 258 biological transfer, 236 bombardmen ionizer, 326 donor, 192, 237 injected, 403 hole paris, 3, 150, 375, 388, 414 photoinduced, 551 Electron transfer, 10, 184, 236, 429 chains, 247 long-range, 244 reactions, 377 Electronic energy transfer, 362 excitation energy, 106, 322 perturbation, 119 relaxation, 236 Electron spin resonance (ESR), 168, 257, 274 alkyl radical, 168 cumyl radical, 139 hyperfine structure, 139, 168 parameter, 169 Electrophilic mechanism, 430

571 Electrophoresis, 381 Electrophotography, 433 Eley-Rideal mechanism, 366 type adsorption, 1 Emission spectrum, 106, 190 decay, 3, 107 lifetime, 3 Emitter, 184 Enantiomeric dimers, 505 heterodimers, 503 object, 502 Enantiotropic smectic phase, 483 Energetics, 537 Energy excitation of singlet oxygen, 429 gap, 236 in-plane transfer, 236 transfer, 8, 106, 322, 328, 414, 551 transfer mechanism, 430 transfer quenching, 405 migration, 20, 106 Enhanced reflection, 527 Epitaxial layer, 4 Erabutoxin b, 562 Erase-and-rewrite cycle, 437 Erythrosin, 94 ESCA, 1, 457 Etch depth, 452 Etching, 4, 342 action, 342 beam, 342 Ethanoic, 150 Ethyl radical (C2H5), 172 4 '-ethylbicyclohexyl-4-carbonitrile, 483 Excimer, 48, 482, 528 decay, 53 emission, 56 fluorescence, 216 formation, 48, 533 laser photoablation, 448 Exciplex, 482, 528, 557 Excitation electronic, 106 energy, 411 mechanism, 329 spectrum, 57

Excited singlet state, 403 triplet state, 122, 403 Exothermicity, 238 Fabrics, 24 dyed, 43 Factors solid-state photoreactivity, 469 Faujasites, 200 zeolites, 200 Fe(C0)5, 264, 288 Fe3(C0)12, 288 Fischer-Tropsch hydrogenation, 278 Flash photolysis, 259 Flavins, 552 Flavodoxin, 554 Flavoproteins, 551 Fluoran, 422 Fluorescence, 323, 404, 482, 528, 551 decay, 16, 48, 97, 111, 552 decay curve, 18, 97, 118, 365, 538 dye, 236 excitation spectrum, 83 impurity, 19 lifetime, 21, 48, 85, 94, 108, 230, 404, 553 lifetime measurement, 93 microscope, 93 monomer, 21 non-exponential decay, 405 pyrene, 79, 526 quantum yield, 48, 108, 404 quenching, 93, 106, 236, 482 quenching rate constant, 552 signal, 236 spectrum, 48, 81, 95, 113, 186, 536 time-resolved, 17, 30, 106, 405, 551 Fluorine atoms, 317 Fluoranthenylethylenes, 7 Fluorophores, 19, 482 Fsrster energy transfer quenching, 411 equation, 362 theory, 108 Fractals dimension, 116, 353

572

environment, 109 geometry, 353 interfaces, 369 line, 354 objects, 353 surface, 366 Framework compositon, 198 Free radical coupling, 207 Functional dyes, 419 Functionalization, 380 FT-IR, 380 Ga atoms, 323 Ga-CH3, 325 GaAs, 4 g-value, 168 Gelatin film, 341 Geminate radical pair, 208 Geometric irregularity, 353 Geometrical parameters, 470 requirements, 469 Gest/host ratio, 478 Gest-molecules, 198 H-D exchange, 123, 329 Halides, 439 Halogen, 317 Hamett C d constants, 425 P Head-to-tail collisions, 537 Heat mode, 423 laser recording, 435 Helical axis, 527 Hexacarbonyl, 273 Hexafluoroacetyl-acetone, 263 Hexahelicene, 529 Hexamethyldisilazane, 459 Hybrid system, 272 Hybridization, 385 Hydrocarbons, 305 Hydrogen abstraction, 123, 469, 485, 510

Hydrogen bonding, 7 9 , 120, 143, 393

Hydrogen peroxide, 186 Hydrogenation photocatalytic, 360 Hydrophilic, 199 Hydrophobic, 199

Hydrophobicity, 201 Hypothetical transfer, 505 Hyperfine structure, 168, 262 Hypsochromic shift, 426 Hysteresis of transmittance, 435 Image amplification, 437 recording, 435 Imaging system, 375 Imperfections, 413 Impregnation, 134,273 Indole, 552 Induced circular dichroism (ICD), 438 Information storage material, 442 Infrared measurement, 152 Inhomogeneous semiconducting substrates, 400 Insulator, 149 Insulating materials, 375 Interface, 93 solid and gas, 339 solid and liquid, 339 Interaction charge transfer, 551 dipole-dipole, 106, 563 electrostatic, 227 gas-surface, 327 hydrogen bonding, 293 hydrophilic-hydrophobic,

221

hyperfine, 174 molecule-surface, 354 non-covalent electronic, 197 photon-stimulated surface, 327 solute-solvent, 541 spin-orbital, 169 spin-spin, 262 Interface, 221 gas-solid, 342 semiconductor-gas, 388 semiconductor-solution, 408 Internal conversion, 413 rotation, 551 Intersystem crossing, 413, 428 Intracrystalline dynamics, 197 Intralayer regions, 481 Intramolecular radiationless

573

decay, 405 Inverted region, 236 Ionization energy, 247 Ion-pair state, 239, 554 IRabsorption, 528 reaction cell, 328 spectrum, 159, 273, 288 spectroscopy, 328 transmission spectra, 329 Irregular surface, 413 Island growth mode, 344 Isoalloxazine nucleus, 553 Isomerization, 5, 217, 303, 362, 482 photocatalytic, 362 Isomorphous replacement, 505 Isotropic phases, 526 Ketone alkyl, 119 chain length, 490 Kinetic analysis, 30, 62, 179 energy, 325 Kolbe reaction, 383 Kramers derived activation energy, 544 Kubelka-Munk theory, 31 Lambert-Beer Law, 448 Langmuir adsorption isotherm, 1 , 324, 388 type adsorption, 1 Langmuir-Hinshelwood proceses, 1 Laser assisted chemical vapor depositon (CVD), 4 beam, 36, 291, 422 COZ,

327

diodes, 418 dye, 323 excimer, 323, 448 flash photolysis, 30, 490 frequency, 330 induced ablation, 449 -induced-fluorescence (LIF) technique, 322 induced surface reaction, 327

IR, 4 mode-locked A r ' ,

94

mode-locked Nd:glass, 407 mode-locked picosecond, 31 Nd-YAG, 326, 454 Nd-YAG pulsed, 36, 290 photochemical techniques, 327 photolysis, 28 photons, 322 printers, 418 Q-switched, 30 stimulated growth, 323 surface chemistry, 317 UV-, 4 , 23 YAG, 27 Lattice substitution, 397 Layer multi, 320 thickness, 421 three-dimensional hydrocarbon, 65 Lewis acids, 393 sites, 273 Lewis bases, 388 Ligands, 294 Lightfastness, 422 Linewidth, 175 Liquid crystal, 435, 481, 526 cholesteric, 435, 481 nematic, 435, 481 smectic, 435, 481 Liquid crystalline polymer, 442 Lithographic applications, 448 Luminescence, 187 time revolved, 3 Lumisantonin, 474 Macroscopic solvation behavior, 484 Marcus theory, 236 Mass analysis, 318 Katrix, 255 cocondensed, 268 gas, 261 isolation, 260 low temperature, 267 nitrogen, 262 pulsed isolation (PMI), 265 Maxwell-Boltzmann Distribution, 320 Mechanism exciton-electron transfer, 239 superexchange, 238 two-step, 238

514

Membrane, 221 Memory function, 435 Mercury lamp, 257 Mesogenic compound, 481 Mesophase structure, 528 Metal atoms, 317 carbonyls, 272, 289, 303 chelate complexes, 260 complexes, 255, 276, 288 containing molecules, 317 oxidation, 276 oxide, 303, 375, 394 thin films, 323 Metathesis, 305 4-methoxyvalerophenone, 489 1-(4-methoxyphenyl)propene, 9 Methylacethylene, 3M) Methylamine, 389 4-Methylbenzophenone, 470 Methyl ethyl ketone, 120, 172 Methyl ketone, 510 Methyl radicals, 127, 168 Methylviologen-capped porphyrin, 237 MgO, 151, 194 Micelle, 221 Microelectronics, 288, 317, 327 devices, 449 Microheterogeneous media, 216 Microparticle, 93 Micropatterning, 386 viscosity factors, 493 Microscope, 104 Microscopic environments, 526 pits, 419 Microscopy image analysis techniques, 353 Mirror-symmetric homodimer, 503 *2(cO)lo, 327 Mo(CO)6, 274, 310 Modification chemical, 93 Molecular aggregate, 437 alignment, 435 axes, 481 chirality, 501

conformation, 469 environments, 93 diffusion, 472 dynamics, 327 mobility, 435 movement, 469 order, 481 organic crystals, 411 orientation, 435 rearrangement, 216 surface parameters, 357 tumbling, 472 Monoanionic complex, 429 Monolayer, 220, 236 coverage, 301, 355 fraction, 141 Monomer, 25, 411 fluorescence, 49 molecules, 379 triplet-exponential, 53 units, 217 Monosilane, 342 Mbssbauer spectroscopy, 255 conversion electron (CEMS), 255 depth-resolved-(DCEMS) , 255 /IR study, 256 transmission spectrum, 257 Multi-layered structure, 385 Multi-layer resist technology, 466 Multi-lifetime components, 561 Multiphoton process, 335 Multiple adsorption sites, 412 Multiple photon absorption, 327 Multiplex recording, 436 Mutual orientation, 469 NaCl single crystal, 328 N-allylacridone, 192 Nafion, 221 Naphthalene, 41, 357, 478 N2-BET value, 357 Nb4+, 307 Nb(CgH6)'' 327 ND3, 392 N-ethylcarbazole, 16 Near-infrared absorbing dyes, 419 Neurotoxic protein, 562 ~ 3 9392 Nickel complexes, 422

575

Ni2+ ions, . I19 Niobium oxide, 303 N-methylacridone, 186 N-methylindole, 553 N,N-diisopropylphenylglyoxylamide, 521 N,N-dimethylformamide, 188 N,N-dimethyl-4-( 3-( I-pyreny1)propyl)aniline, 538 Non-exponential decay, 93, 403, 560 Non-linear photoresponse, 435 Norrish Type processes, 124, 474, 485 * n, 7~ excited state, 471 hydrogen abstraction, 473 transition, 120, 164, 471 Nucleation, 339 amorphous silicon, 339 process, 339 n-type semiconductors, 382 Objects computer-generated, 353 man-made, 353 real, 353 Octanol, 58 330 OD(ad)s o-hydroxyazo dye, 422 Olefins, 5, 217 Oligomerization, 303 One-way isomerization, 6 Optical absorption characteristics, 272 activity, 507 anisotropy, 435 antipodes, 507 density changes, 440 diffraction effect, 458 disc, 420 DRAW disc memory, 419 fiber, 26 Optics, 39 Organometallics, 272, 288 compounds, 4, 255 Oxetane, 469 Oxide, 48 binary, 157 Oxygen, 122, 184 atom, 456

Ozone, 184 Packing, 469, 481 modes, 502 Para-alkoxy substitution, 492 Para-methoxyvalerophenone fragmentation, 492 Parent molecule, 469 Particle interface, 31 sizes, 359 Pattern formation, 381 wise imaging, 439 2-pentanone, 120 Pentacarbonyl, 274 Pentacarbonyliron, 264 Perdeuterated ammonia, 389 Phantom spheres, 366 Phases, anisotropic, 526 boundary, 508 cholesteric, 481, 526 crystalline, 503 isotropic, 526 liquid crystalline, 481 lyotropic liquid crystalline, 481 nematic, 481 thermotropic, 481 thermotropic liquidcrystalline, 526 Phase-contrast micorscope, 384 Phase transition, 437 nematic-isotropic, 440 Phenols, 428, 495, 554 Phenol-formaldehyde, 458 Phenyl alkyl ketones, 474, 488 Phenyl-Z-(Z-benzo-(C)phenanthryl)ethylene, 529 Pheophytin, 238 Phosphorescence decay, 490 spectrum, 122 Photoablation, 3, 317, 448 acoustic spectroscopy (PAS), 110 activation, 270, 272 activity, 157 adsorption, 1

576

aggregation, 263 aromatization, 146 bleaching, 221 chemical etching, 317 chemical interactions, 353 chemical probes, 197 chemical reaction, 119, 288 chemical transformations, 529 contraction, 439 -cvD, 339 cyclization, 471 cycloaddition, 216, 503, 508 decarboxylation, 150, 310 decomposition, 100, 360, 423, 439 deposition, 4 , 448 desorption, 320, 334 dimerization, 216, 469 dissociation, 2, 282, 293, 317 enhanced chemisorption, 327 enolization, 123 etching, 450 excitation, 277, 327 irradiation, 4 M O O , 323 oxidation, 448 preparation, 303 process, 279, 353 rearrangement, 510 reduction, 263 sensitiser, 43 stability, 419 stabilization, 172, 449 thermal etching, 450 Photocatalysis, 163, 279, 360, 376 Photocatalysts, 157 Ti02, 360 Photochromic process, 436 Photoconductor materials, 433 Photocurrent, 404 Photodimers, 507 Photodiode, 38 Photoelectrons, 317 Photoelectrodeposition, 376 Photoelectrochemical cell, 403 deposition, 376 reactions, 375 Photofading, 422 Photofragment spectroscopy, 317

Photographic developer, 341 imaging, 340 Photo-induced capacitance change, 441 chemical vapor deposition, 339 decarbonylation, 277 deposition, 339 electron transfer, 551 isomerization, 438 phase transition, 438 processes, 435 reaction, 268, 304 Photolabile molecule, 356 Photolithographic, 23 techniques, 376 Photoluminescence (PL) , 388 intensity, 394 Photolysis, 119, 138, 172, 289 alkyl ketones, 119 azocumene, 138 CH31, 172 dibenzyl ketone, 207 europium (111) oxalatecomplexes, 255 Fe(C0)5, 264, 288 laser flash, 34 low temperature, 261, 295 potassium tris(oxa1ato)ferrate(III), 256 steady-state, 280 W(CO)5, 274 Ru3(CO)12, 274 Photon mode, 424 image recording, 436 processes, 435 Photooxidation, 422, 448 Photophysics, 79, 104, 288 Photoreceptor, 433 Photostationary, 48 Photosynthetic reactions, 551 unit, 238 Phyllosilicates, 221 Physical amplification, 437 restraints, 469 Physisorbed species, 272, 331 Phthalocyanine, 41

571

Picosecond laser photolysis, 30, 552 laser light, 240 spectroscopy, 30 Pinacol-type dimer, 470 Plate-like micelle, 437 p-n junction, 380 Polar solvent, 481, 552 Polarity, environmental, 61 Polarization, 436 energy, 247 Polpyrrole, 376 Polyacrylate, 442 Polyalkene, 425 Polyaniline, 377 Polycarbonate, 419 Polycrystals, 265 Polyimide films, 449 Polymers, 448 Polymer, 527 coating, 381 films, 442, 448 images, 448 substrate, 419 systems, 43 Polymerization, 348, 379, 482 electrochemical, 379 Polymethylene, 539 Polymethylmethacrylate, 419, 448 Polymethylstyrene, 450 Polypeptide chain conformation, 439 Polystyrene, 448 film, 17 Polythiophene, 377 Polyvinylalcohol, 358 Pore diameter, 7 5 , 9 4 , 110, 200 interconnected, 273 size, 7 5 , 200, 289, 364 Porous silica, 9 8 , 288 silica surfaces, 357 solids, 197 Vycor glass, 119, 171, 272, 299, 306

x,2 excitation,

188

p $ excited state, 471, 492 f l , h transition, 164, 425, 471

%-bonding, 244 t-orbital, 243 a-system, 293 Porphyrin, 237 Potential energy surfaces, 6 well, 175 Preexponential factor, 11 Proteins, 239, 551 environment, 239 Pseudo-centrosynnnetric space-group, 507 Pyrene, 48, 7 9 , 357, 414, 450, 482, 527

excimer, 531 exciplex, 531 Pyrenyl lumophore, 482, 526 Pyrolysis, 256 Quadrupole mass spectrometer, 325 Quantum amplification effect, 9 Quantum chain process, 6 Quantum mechanical calculation, 242 Quantum mode, 242 Quantum yield, 8 , 274, 294, 346, 360, 429, 469, 493

fluorescence, 49, 104, 404 Quencher, 3 , 106, 552 singlet oxygen, 422 Quenching, 22, 60, 236, 401, 482, 531 mechanism, 429 naphthalene, 478 NO, 122 oxygen, 68, 102, 122, 537 partners, 531 pyrene, 482 rate constants, 428 self-, 3 Quinone, 237 Racemization, 503 Racemic modification, 507 Radiationless transfer, 106 Radical, 317 cation, 9 chemistry, 339 coupling, 470 disproportionationlcombination, 138 pair, 197

578 reaction, 383 scavenger, I38 Radiolysis, 175, 256 Raman studies, 528 Ramifications, 528 Rate constant, 49 electron transfer, 236 Reaction center, 247 Recombination, 171, 403 radicals, 125, 171 electron and hole, 403 Recording density, 420 Reduction, 303 Reduced transmittance, 527 Redox reaction, 45, 150, 184, 192 Reflection internal, 15 Reflectivity, 421 Reflectance, 421 diffuse, 31 Refractive index, 102, 527 Regiochemical selectivity, 216, 519 Resist materials, 448 Reversed-phase silica, 52 Rhodamine By 110, 236, 364, 406 Rhodamine 6G, 364, 454 Riboflavin, 556 Rose Bengal, 381 Rotational barrier, 472 Ru~(CO)~~,272 Rubrene, 427 Rubrene peroxide, 427 RuQ2, 427 Sandwich-like conformer, 495 structure, 528 Santonin, 474 Satellite, 173 Scavenging, 274 Schmidt's criteria, 474 SEX photograph, 454 Semiconductor, 9, 279, 322, 388, 403 devices, 448 electrolyte, 403 films, 327 laser, 436 n-type, 382, 403 substrate, 375

surface, 388 Sensitization, 403 Sensitizer, 1, 106 SFg, 327 Spherical micelle, 437 Si, 3, 58, 327 n-type, 318 wafers, 317 SiZ9, 170 Si/A1 ratio, 198 SiC1, 317 SiC12, 317 SiC14, 321 Si2H6, 346 Signal-to-noise (S/N) ratio, 440 SiH3 radicals, 349 SiH4, 327 Silyl radicals, 347 Silylsilylene, 346 Silanol, (Si-OH) group, 79, 173, 197, 273, 294 Silanes, 54 Silica (gel), 41, 48, 79, 93, 138, 172, 277, 288, 329, 339, 364, 409, 422 active surface, 79 aminopropyl, 94 chemically modified, 93 dehydroxylated, 146 hydroxylated, 277 Ioctanol, 54 reversed-phase, 52 Silicates, 221 Silicon, 327 single crystaline, 377 wafers, 342 Siloxane group, 80, 176, 294 Silver clusters, 340 halide, 341 metallic, 341 thiosulfate complex ions, 341 SIMS, 1 Singlet oxygen, 8, 184, 422 Single exponential function, 557 Single photon counting, 48, 93, 236, 407 time-correlated, 93, 407, 538 SiO, 331

579

Si02, 42, 120, 151, 328, 339, 459 thin layer, 342 Si04 tetrahedra, 199 Size/shape, 199 Skeletal motions, 413 Slow spray on method (SSO), 265 Sn-doped In203, 407 Sn02, 407 so2, 393 Sol-gel transition, 439 Solar energy, 403 simulator, 430 Solid-state, 5, 216 hydrogen abstraction, 469 photoreactivity, 469 quantum yield, 469 Solid phase, 255 substrates, 317 surface, 339, 527 state, 469, 481, 501 system, 256 Solid state NMR spectroscopy, 509 Space synanetry, 469 Spectator, 198 Spectroscopy fluorescence, 15 nanosecond, 15 picosecond, 15 time-resolved, 15 Speculative surface reactionmodel, 349 Spiropyran, 437 Spontaneous resolution, 502 Stilbazolium, 221 Stilbene, 9 Stereospecificity, 216 Stereo -chemical selectivity, 216 chemistry, 217 electronic requirement, 470 isomers, 519 selectivity, 469 selective photodimerization, 530 specific solid state process, 501 Stereoviews, 472 Steric constrains, 211

effect, 203, 477 factors, 203 hindrance, 131 interactions, 509 Stern-Volmer plots, 131, 478, 532 Steroidal ring system, 526 Steroid skeleton, 237 Sticking coefficient, 2 Stilbazole, 223 Stoichiometry, 273 Stokes-Einstein equation, 67 STM, 1 Streak camera detection, 407 Sub-bandgap, 45 Subcarbonyl, 310 Substrate, 344 surface, 348 temperature, 344 Sulfur dioxide, 388 Supercage, 200 Superoxide (02-), 194 Support, 303 Supported oxide, 119 Surface alumina, 192 area, 94, 110, 120, 171, 197, 289, 318, 356, 358 characteristics, 139 catalytic effect, 351 chemistry, 339 concentration, 110 confined complex, 272 coverage, 55, 112, 120, 173, 273, 288, 405 crosslinkage, 448 density, 405 diffusion, 79, 339 dynamics, 62 electronic structure, 394 energy deposition, 448 etched, 455 external, 202 hydrophilic, 376 hydroxyl groups, 119, 140, 158, 192, 289 induced homotropicalignment, 485 insulator, 410 internal, 202

580

microenvironment, 142 mobility, 132, 272 modification, 119, 460 morphology, 458 OH groups, 119, 288, 330 polarity, 129 potential reaction, 221 pretreatment, 128 processing, 317 purity, 140 pyrene-derivatized, 357 reaction, 348 recombination, 349 recombination velocity, 395 semiconductor, 388 silica gel, 186 stabilized radicals, 169 state, 403 temperature, 288 tension, 437 topology, 272 Vycor glass, 119, 272 Surfactant, 438 Te-based alloys, 420 Tellurium oxides, 420 Temperature programed decomposition (TPD), 277 Tetraphenylporphine thin film, 414 Tetracene films, 414 Tetrahedral carbon chiralcenters, 502 Tetrahydronaphthoquinone system, 469 Thermal activation, 272 conductivity, 395 dissociation, 70 energy, 325 laser effect, 38 microscopy, 484 oxidation, 343 processes, 3, 332 reduction, 305 sigmatropic rearrangements, 482 treatments, 303 Thennotropic liquid crystalline, 481 phases, 481 Thin deposition, 318

Thiophene compound, 505 Thiosulfate, 341 Three-dimensional arrangement, 502 Threshold appearance time, 319 Ti-A1 binary metal oxide, 362, Time-resolved fluorecenceanisotropy, 558 Time-of-flight distribution, 318 Ti02, 9, 40, 152, 194, 360 anatase-type, 361 colloidal particles, 410 hydrophilic, 376 polycrystaline, 377 single crystal surfaces, 415 size-efect, 360 Ti02/A1203, 363 Pt/Ti02, 360 Titanium oxide, 305 Topochemical classification, 339 control, 216, 506 principle, 469 Topology, 198 Toxicity, 420 Transmission electron microscope, 339 spectroscopy, 255 Transesterification, 508 Transition metal carbonyl, 272 Trans-cinnamic acid, 217 Transient absorption, 30 absorption spectrum, 552 Transient decay, 30 Transition metal carbonyls, 272, 288 Translational energy, 319 distribution, 320 Transportation efficiency, 348 Trapping, 403 Trichloromethylsilane, 376 Trifluoroamine, 389 Triisopropylphenyl group, 477 2,4,6-triisopropylbenzophenones, 471 Trimerization, 294 Trimethylamine, 389 Trimethylgallium, 323 Trimethylphosphine, 280 Triplet exciton jump frequency, 478 Triplet states, 30, 119

581

Triplet-triplet absorption, 36 2,4,6-triisopropylbenzophenone, 469 Tryptophan, 551 Tumbling frequency, 168 Tungsten oxide, 303 Tunneling, 242 hole, 244 quantum mechanical, 243 Turnover frequency, 305 Twisting, 11 Two-way isomerization, 7 UV absorption, 448

spectrum, 119, 307 exposure, 462 hardening, 461 lithography, 465 laser photodissociation, 317 photodissociation, 317 Visible spectroscopy, 289 Vacuum chamber, 325 Valence band, 240, 397 state, 304 Vibration 0-0 band, 82 excitation, 327 external adsorbate-adsorbent, 328 frequency, 149 internal adsorbate, 328 structure, 60 Vinylanthracene, 8 Viologen, 245 Vitreous transition temperature (Tg), 384 V205, 172 Vycor glass, 119, 172, 272 W(CO)5,

274

Xanthone, 42 Xe lamp, 343

X-ray crystallography, 509 ray photoelectron spectra (XPS), 329 Yardsticks, 359

Zeolite, 197 cation exchanged, 200 cavities, 200 silicate, 42 x-, 211 Y-, 211 Zinc oxide, 45 Zinc sulphide phosphor, 45 ZnQ, 154, 173, 194 single crystal electrode, 412


583

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universitb Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U S A . Volume 1 Preparation of Catalysts 1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417, 1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasison the Control of the ChemicalProcesses in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second InternationalSymposium, Louvain-la-Neuve, September 4-7, 1978 edited by 8. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd InternationalMeeting of the Socibte de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an InternationalSymposium, Ecully (Lyon), September 9- 1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and

H. Praliaud

Volume 6 Catalyst Deactivation. Proceedings of an InternationalSymposium, Antwerp, October 13- 15,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th InternationalCongress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov andV.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29October 3, 1980 edited by M. LdzniEka Volume 10 Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouquerol and K.S.W. Sing Volume 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-1 6, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud. P. Meriaudeau,

P. Gallezot, G.A. Martin and J.C. Vedrine

Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. JirQand G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Volume 14 Vibrations at Surfaces. Proceedings of the Third InternationalConference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

584 Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet. P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an InternationalSymposium, LyonVilleurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedingsof an International Conference, Prague, July 9- 13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirb, V.B. Kazansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an InternationalSymposium, Villeurbanne (Lyon), September 25-27, 1984 edited by 8. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an InternationalSymposium, Portorof-Portorose, September 3-8, 1984 edited by B. Drbj, S. HoEevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the InternationalSymposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth InternationalConference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Volume 27 Catalytic Hydrogenation edited by L. Centen3 Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kniizinger Volume 30 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 32 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 34 Catalyst Deactivation 1987. Proceedings of the 4th InternationalSymposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment

585 Volume 35 Keynotes in Energy-RelatedCatalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chaney, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pa61 Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 44 Successful Design of Catalysts Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an InternationalSymposium, Wurzburg, September 4-8, 1988 edited by H.G. Karge and J. Weitkamp

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