EPR IN THE 21StCENTURY: BASICS AND APPLICATIONS TO MATERIAL, LIFE AND EARTH SCIENCES
The participants of the Third Asia-Pacific EPR/ESR Symposium (APES’Ol), 29 October- 1 November 2001, Kobe, Japan.
EPR IN THE 21StCENTURY BASICS AND APPLICATIONS TO MATERIAL, LIFE AND EARTH SCIENCES Proceedings of the Third Asia Pacific EPRESR Symposium Kobe, Japan, October 29 - November 1,2001
Edited by
Asako Kawamori Faculty of Science Kwansei Gakuin University Gakuen 2-1 Sanda 669-1337, Japan Jun Yamauchi Graduate School of Human and Environmental Studies Kyoto University Kyoto 606-8501, Japan
Hitoshi Ohta Molecular Photoscience Research Center and Dept. of Physics Kobe University 1- 1 Rokkodai, Nada Kobe 657-8501, Japan
2002
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2002 Elsevier Science B.V.
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Editorial Note The Third Asia-Pacific EPR/ESR Symposium (APES'Ol), our first meeting in the 21" century, was held from October 29 to November 1, 2001 in Kobe, Japan. About 220 participants from 20 countries from Asia, Australia, Europe, North and South America presented 210 papers. The Proceedings not only record the Symposium presentations but are also aimed as a blueprint for development of EPR/ESR in the Asia-Pacific region in the 21Sfcentury. The Symposium reflected the variety of research fields developed over half of a century since Zavoisky's discovery of EPR in 1944. Especially the most recent developments, such as high-field and high-frequency EPR, are envisaged to be firther developed and applied to various fields in the 21Sfcentury. Special topical session were devoted to these techniques at APES'Ol. All sessions consisted of Plenary, Invited and Contributed presentations. Poster sessions were also organized. The Plenary presentations, given to all participants, aimed at summarizing the overall developments. Invited presentations, reviewing the most recent developments, and Contributed ones, dealing with original research recently carried out in the EPR/ESR area, were given in one of three parallel sessions. Researchers from AsiaPacific countries as well as Europe and the United States presented their unique research works, which covered various fields and reflected the existing diversity of applications of the EPR/ESR techniques. The additional new arrangement of this Symposium was the introduction of two satellite meetings, Symposium A: 2001 ESR Dosimetry & Dating and Symposium B: International Workshop on Advanced EPR Applied to Biosciences. The organizers express their gratitude to Prof. J. Pilbrow, President of the International EPR Society, for his participation. Prof John Pilbrow also presented Prof. S. Yamauchi of Tohoku University with the IES Silver Medal. The organizers regretted that some researchers cancelled their participation due to the terrorists attacks in USA on September 11. Prof. A.A. Romanyukha (USA) contributed his plenary lecture paper in absentia. This Proceedings volume includes the papers submitted within the time limit set by the publisher. All papers have been refereed by the members of the Review Panel who are experts in their own areas. The efforts of all the reviewers and Elsevier Science in achieving the high scientific standard and technical quality of the Proceedings are gratefully acknowledged. The editors express their gratitude to the Commemorative Association for the Japan World Exposition (1970) for the major financial support of this Symposium. Asako Kawamori Professor of Kwansei Gakuin University Jun Yamauchi Professor of Kyoto University Hitoshi Ohta Professor of Kobe University
The APES President's Message It is a great pleasure for me, on behalf of the Council of the Asia-Pacific EPIUESR Society [APES], to present the Proceedings of the Third Asia-Pacific EPR/ESR Symposium [APES'OI] to the scientific community. This Symposium has been organized, under the Society, by Kobe University and the Local Organizing Committee ably headed by Professor Asako Kawamori. As the Founding President of the Society and then an elected one for two more terms, it is particularly satisfying to me to see the Asia-Pacific EPIUESR Society and the AsiaPacific EPIUESR Symposia grow in strength and maturity. Continuing with the spirit of the First APES, held at the City University of Hong Kong in 1997, and the Second APES held due to the efforts of Professor Yuanzhi - at the Zhejiang University in 1999, this Symposium, although aimed primarily at the Asia-Pacific countries, has also been open to participants from all over the world. The main aims of our Symposia are to bring together as many EPIUESR spectroscopists as possible and to promote and facilitate collaboration among the EPIUESR community. For the first time two Satellite Meetings: 2001 ESR Dosimetry and Dating and International Workshop on Advanced EPR Applied to Biosciences, have also been arranged. This is a very positive development, which shows that the idea of the APE Symposia is expanding and attracting other specialized areas of applications of EMR (encompassing EPR and ESR). Thanks are due to Prof. M. Ikeya and Prof. A. J. Hoff for their effort in organizing, in coordination with Prof. A. Kawamori, the Symposium A and B, respectively. During the Symposium the 3rd Meeting of the Asia-Pacific EPR/ESR Society was held. We were privileged to have with us Prof. John R. Pilbrow, President of the International EPIUESR Society, who delivered an Address to the participants of the Meeting. It was decided at the APES Meeting that the fourth Symposium (APES'03) will be held at the Indian Institute of Science in Bangalore in November 2003, with Prof S. V. Bhat as the Chairman of the Local Organising Committee (LOC) for APES'03. An International School on EPIUESR Spectroscopy with tutorial sessions for students and researchers will be held at Bhabha Atomic Research Centre in Mumbai prior to the APES'03, with Prof. K.P. Mishra as the Chairman of the OC for the School. All materials related to the APES'O1 as well as the APES Meeting will be available on the APES Web site at http://~.ied.edu.hk/has/phvs/apepr. The tentative schedule (to be confirmed) of the future Asia-Pacific EPR/ESR Symposia is: APES'OS in Korea, APES07 in Russia (Vladivostok), and APES09 in Australia. Please visit the APES Web site for updated information on the Asia-Pacific EPIUESR Symposia and the Society. On behalf of the APES Council I would like to thank all hard working Members of Members of the APES'O1 Local Organising Committee for their dedicated effort. Under the skilful leadership of Prof. A. Kawamori, helped especially by Prof. H. Ohta (Secretary) and Prof. T. Takui (Treasurer), APES'Ol was a very successful Symposium. Support from the International Advisory Committee in nominating the invited speakers and maintaining the
vii
high standard of this Symposium is also much appreciated. I also wish to express the Society’s gratitude to all sponsors for their financial support. Thanks are due to all speakers and participants, who by attending this meeting have helped to make this Symposium a successful event. Special thanks are due to Prof. A. Kawamori for her dedicated two-terms service as the Vice-president of the Asia-Pacific EPR/ESR Society. Czeslaw Rudowicz President of The Asia-Pacific EPR/ESR Society Professor of City University of Hong Kong
CONTRIBUTORS to Proceedings 10 Plenary lectures:
Noboru Hirota (Japan) Zohn-Li Liu (Chaina) Masaki Oshikawa (Japan)
Lawrence Berliner (USA) Jack H. Freed (USA) Reiner. Grun (Australia) Larry. Kevan (USA) Shin-ichi Kuroda (Japan)
Czeslaw Rudowicz
Kong)
Tetsuhiko Yoshimura(Japan
18 Invited Lecutures:
Seiji Miy ashita(Japan)
C . A. h e r l a a n (Netherlands) Oswald Baffa (Brazil) Sergei A. Dzuba (Russia) Arnold J. Hoff (Netherlands) Sa-Oak Kang (Korea) Yang Liu (China) Wolfgang Lubitz (Germany) David J. Lurie (England) Sushi1 Misra (Canada)
John Pilbrow (Australia)
B.J. Reddy (India) Rafic G. Saifutdinov (Russia) Kev Salikhov (Russia) R.J. Singh (India) Yuri D. Tzvetkov (Russia) Hideo Utsumi (Japan) Nicolla Yordanov (Burgaria)
Reviewers: L. Berliner: Univ. of Denver, USA S.V. Bhat S. Demishef J. Freed: Cornell Univ,. USA R. Grun M. Ikeya: Osaka UniyJapan A. Kawamori: Kwansei Gakuin Univ., Japan
R. Kevan: Univ. of Houston, USA S . Kuroda: Nagoya Univ. Japan Y. Li W. Lubitz M. Mino: Okayama Univ. Japan K.P. Mishra S.K. Misra: Concord Univ., Canada
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R. Miyamoto S. Miyashita: Tokyo Univ., Japan M. Motokawa: Tohoku Univ., Japan T. Nakamura H. Ohta: Kobe Univ., Japan J. Pilbrow: Monash Univ.. Australia C. Rudowicz: City Univ., Hong Kong K. Salikhov
C. Shiomi H. S. So S. Tero-Kubota H. Utsumi: Kyushu Univ., Japan J. Yamauchi: Kyoto Univ., Japan S. Yamauchi: Tohoku Univ., Japan N. Yordanov T.Yoshimura
COMMITTEES of APES01 International Organizing Committee
Local Organizing Committee
President:
Chairperson:Prof. Asako Kawamori Treasurer: Prof. Takeji Takui Members: Prof. Hiroaki Ohya Prof. Jun Yamauchi Prof. Motoji Ikeya
Prof.
Czslaw
Z.
Rudowicz
OIongKong) Vice President: Prof. Asako. Kawamori (Japan) Members: Prof. Albert M. Ziatdnov (Russia) Prof. Hyunsoo So (South Korea), Prof. Yong. Li (China), Prof. Hitoshi Ohta (Japan), Dr. Y.Y Yeueng(Hong Kong), Dr. Tien Tai Nguyen (Vietnam), Prof. S.V. Bhat (India), Dr. Graem R. Hanson (Australia).
Local Advisory Members Prof. Keiji Kuwata Prof. Hiroshi Watari Prof. Noboru Hirota
Scientific Program Committee
International Advisory Members Prof. Lawrence J. Berliner: (USA) Prof. Muneyuki Date: (Japan) Prof. Arnold J. HOE(Netherlands) Prof. Come1is.A.J.Ammerlaan: (Netherlands)
Institutional Sponsors Commemorative Association for the Japan World Exposition (1970) Nakauchi Foundation for Convention Portonia. 8 1 Foundation
Prof. Jun Yamauchi Prof. Mitsuhiro Motokawa: Prof. Shozo Tero: Dr. Hiroshi Hori Prof. Chihiro Yamanaka Prof. Hideo Utsumi Dr. Tetsuhiko Yoshimura
Industrial Sponsors Bruker Biospin K.K. JEOL co.
Ltd
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CONTENTS Page Editorial Note President's Message Contributors Committees
V
vi vii
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Section 1. Physics and Magnetism Plenary Lectures Electron magnetic resonance (EMR) of the spin S>1 systems: an overview of major intricacies awaiting unwary spectroscopists ......................................... Czeslaw RUDOWICZ
3
Recent developments in low-temperature ESR in quantum antiferromagnetic chains .......... 15 Masaki OSHIKAWA Invited Lectures Direct numerical study on ESR line shape ............................................................................. Seiji MIYASHITA and Akira OGASAHARA
27
g Tensor of Er3+centers in axial symmetry ............................................................................ C. A. J. AMMERLAAN
33
Continuous wave and pulsed EPR spectroscopy of paramagnetic ions in some fluoride, silicate and metaphosphate glasses ................................................................ S. C. DREW and J. R. PILBROW
39
General Papers Frequency dependence of resonance in one-dimensionalantiferromagnetic Heisenberg chain .................................................................................................................... Akira OGASAHARA and Seiji MIYASHITA
48
ESR selection rules for direct transition of spin gap .............................................................. T8ru SAKAI, Nobuhisa OKAZAKI, Takashi OHNISHI and Shuhei TAJCEMURA
54
Spin solitons in the alternate charge polarization background of MMX chains .................... Makoto KUWABARA, Kenji YONEMITSU and Hitoshi OHTA
59
Full Monte Carlo and Fourier transformed Monte Carlo EPR spectral simulations of 8s state ions ....................................................................................................................... Tomoharu TAJCEYAMA, Takato NAKAMURA, Naoyulu TAKAHASHI Veltran BELTRAN-LOPEZ and Christopher C. ROWLANDS
63
X-Band ESR measurements of spin ladder system BIP-TEN0 ............................................. Keizo KIRITA, Takahiro SAKURAI, Hitoshi OHTA, Yuko HOSOKOSHI Keiichi KATOH and Katsuya INOUE ESR Studies of a spin1/ 2 antiferromagnetic tetramer chain ........................................................................................... Masayulu HAGIWARA
69
73
ESR Studies of quasi-one-dimensional halogen-bridged mixed-metal complexes ............... 79 H. TANAKA, K. MARUMOTO, S. KURODA, T. MANABE, S. FURUKAWA and M. YAMASHITA Microwave radiation from magnetostatic mode in high power FMR .................................... Michinobu MINO, Hideaki TSUKUDA, Masayuki TSUKAMOTO and Hitoshi YAMAZAKI
85
Parametrically excited magnetoelastic waves in FeB03 ........................................................ Kenji ISHIHARA, Michinobu MINO, Masayuki TSUKAMOTO and Hitoshi YAMAZAKI
89
Slow dynamics in chaotic magnon system ............................................................................ Jiang CAI, Yoshiyasu KATO, Atsusi OGAWA, Takayuki HIRATA, Meiro CHIBA and Yoshifumi HARADA
93
ESR studies of spin-polarized atomic hydrogen adsorbed on 3He-4Hemixture film ............ 97 A. FUKUDA, T. OHMI, H. TAKENAKA, Y. WAKI, A. MATSUBARA and T. MIZUSAKI
Section 2. Materiais Sciences Plenary Lectures Electron spin resonance studies of molecular photoionization in Cr-AlMCM-41 mesoporous oxide materials ................................................................................................. Sunsanee SINLAPADECHand Larry KEVAN
105
ESR and ENDOR spectroscopy of solitons and polarons in conjugated polymers ............. 113 Shin-ichi KURODA Invited Lecture ESR study of deoxygenated high-temperature superconductorsand their constituents ...... 125 R. J. SINGH, P. K. SHARMA and Shakeel KHAN General Papers Magnetic phase separation in Lal,Ca,MnO3 near half-doped composition observed by EMR ................................................................................................................. Alexander I. SHAMES, Andrey Yu. YAKUBOVSKY and Stanislav V. GUDENKO
127
xi
Temperature rises by microwave absorptions in superconducting materials and liquid nitrogen bubbling ....................................................................................................... T. LI, A. HASHIZUME, K. ITOH, H. KOHMOTO, S. IWASAKI, M. YAMASAKI, K. YAMAGUCHI, T. ENDO and M. SHAHABUDDIN Comparison of near-Tc behaviors of microwave absorption and resistance in bulk YBCO superconductors .......................................................................... H. KOHMOTO, S. IWASAKI, M. WAKUTA, H. SARATANI, T. ENDO, S. UTHAYAKUMAR and R. DHANASEKARAN Estimation of fluxon response-delay from magnetic field variations in superconductors using ESR system ................................................................................................................. R. D. KALE, M. TADA, K. ITOH, H. KOHMOTO, S. IWASAKI, Y. NAKAUE, T. ENDO and S. V. BHAT Coexistence of ferromagnetism with superconductivity in RuSr2GdCu208 from ESR measurements ...................................................................................................... Koji YOSHIDA, Masatoshi NAKAMURA, Naoto HIGASHI and Hajime SHIMIZU Ferromagnetic resonance and intragraidintergrain crystallinity in La-Ba-Mn-0 thin films ........................................................................................................ S. IWASAKI, J. YAMADA, H. KOHMOTO, M. TADA, Y. SAKURAI, T. ENDO and B. J. REDDY Temperature dependence of paramagnetic resonance in pure and doped ferrihydrite nanoparticles ..................................................................................................... A. PUNNOOSE and M. S. SEEHRA
133
139
145
151
157
162
ESR study of Fe-SiOz granular films ................................................................................... Kazuaki KANAZAWA, Kouichi MATSUDA, Seitarou MITSUDO, Toshitaka IDEHARA, Sigeo HONDA
168
Oxygen dependent evolution of C6: EPR signal in fullerene thin films ............................. Alexander I. SHAMES, Eugene A. KATZ, Svetlana SHTUTINA, Wojciech KEMPIkXI, Szymon LOS and Lidia Piekara-SADY
174
Molecular orientations in Langmuir-Blodgett and vacuum-deposited films of VO-phthalocyanine .......................................................................................................... Yuhei SHIMOYAMA Structural elucidation of vacuum deposited films of titanyl phthalocyanine by EPR Hiroyuki KAJI, Yuhei SHIMOYAMA
180
......... 186
ESR investigation of organic conductor with itinerant and local spins, (CHTM-TTP)zTCNQ ........................................................................................................... T. NAKAMURA, M. TANIGUCHI, Y. MISAKI, K. TANAKA and Y. NOGAMI
192
X-band ESR measurements of Et2Me~P[Pd(dmit)z]2........................................................... T. SAKURAI, H. OHTA, S. OKUBO, R. KATO and T. NAKAMURA
197
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The role of Li' and Na' charge compensators in Sm3+-dopedCaF2 and SrF2 ..................... M. YAMAGA, M. HONDA, N. KAWAMATA, K. SAMEJIMA and J.-P. R. WELLS
201
The HF and SHF interactions of V02+ions in KZnClS04.3H20 single crystals ................ 207 K. V. NARASIMHULU, B. DEVA PRASAD RAJU and J. LAKSHMANA RAO EPR study of several C? centers in K2MgC14 single crystal .............................................. H. TAKEUCHI, H. TANAKA, M. MORI, H. EBISU and M. ARAKAWA
2 13
...............................................
2 19
EPR study of Cr3+centers in T12MgF4 and T12ZnF4 crystals M. ARAKAWA, H. EBISU and H. TAKEUCHI
Single crystal EPR study of Cr(II1)-doped magnesium potassium Tutton's salt .................. 225 H. ANANDALAKSHMI, P. NEERAJA, R. VENKATESAN, T. M. RAJENDIRAN and P. Sambasiva RAO ESEEM study of 14Nnuclear quadrupole resonances in S=3/2 chromium(II1) complex .... 231 Shoji UEKI and Jun YAMAUCHI EPR investigation of inhomogeneous phases in improper ferroelastic MgTiF6.6H20:Mn2+ ............................................................................................................. A. M. ZIATDINOV and P. G. SKRYLNIK
236
EPR investigations on Fe3+ions in alkali borotellurite glasses ............................................ P. Giri PRAKASH, A. MURAL1 and J. Lakshmana RAO
242
EPR study of X-ray and UV irradiated GeO2 glasses prepared by the sol-gel method K. KOJIMA, F. OGURA, N. WADA, K. YAMAMOTO, T. FUJITA and M. YAMAZAKI
....... 247
Structural studies of the fresh water (Apple) snail, globosa shells .............................. K. V. NARASIMHULU, C. P. Lakshmi PRASUNA, T. V. R. K. RAO and J. Lakshmana RAO
253
ESR study of iron-sites on Fe-ZSM5 zeolite ....................................................................... Nguyen Tien TAI, Nguyen Huu PHU, Tran Thi Kim HOA, Hidenobu HORI and Makoto TAKI
259
CW/pulsed ESR studies of Euz+-dopedSrA1204 phosphor ................................................. H. MATSUOKA, K. SATO, D. SHIOMI, Y. KOJIMA, K. HIROTSU, N. FURUNO and T. TAKUI
264
Thermoluminescent mechanism of tridymite SiOz phosphors ............................................ Masatoshi OHTA, Takato NAKAMURA and Michiko TAKAMI
270
ESR and luminescence spectral properties of europium compounds with trifluoroacetic acid ............................................................................................................... I. V. KALINOVSKAYA, A. N. ZADOROZHNAYA, V. G. KURYAVYI and V. E. KARASEV
276
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ESR and luminescence studies on formation of ions through photochemical reactions in potassium halide crystals doped with sulfur and manganese ........................... SARJONO, M. BABA, K. OHTA, K. NISHIDATE, I. SOKOLSKA and W. Ryba-ROMANOWSKI
282
Hyperfine structure ofNd3+and Er3' ions in LiNbO3 crystals ............................................ 11-Woo PARK, Sung Ho CHOH, Sang Su KIM, Kuk KANG and Deok CHOI
288
The nature of coduction ESR linewidth temperature dependence in graphite ..................... A. M. ZIATDINOV and V. V. KAINARA
293
ESR measurement of heavily doped Si:Fe ........................................................................... Ryouhei KOYAMA, Junk0 YOSHIKAWA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA and Hiroshi NAJSAYAMA
298
ESR study of heavily doped GaAs:Er grown by organometallicvapor phase epitaxy ........ 302 J. YOSHIKAWA, S. OKUBO, H. OHTA, T. KOIDE, T. KAWAMOTO, Y. FUJIWARA and Y. TAKEDA Location of dangling bonds in ELA poly-Si ........................................................................ H. FURUTA, T. KAWASHIMA, H. HARIMA, T. HIRAO, M. FURUTA, Y. TSUCHIHASHI and A. YOSHIDA
306
ESR studies of BEDT-TTF organic conductors containing supramolecular assemblies ..... 3 12 Y. OSHIMA, H. OHTA, H. M. YAMAMOTO and R. KATO EPR spectral study of gadolinium (111) cryptate .................................................................. Ryo MIYAMOTO, Hiroyuki SAT0 and Susumu SUDOH
3 16
Ground-recognitionability of p- and y-cyclodextrins as studied by the high-pressure EPR .......................................................................................................... M. KASAHARA, H. TOBISAKO, Y. SUEISHI, S. YAMAMOTO and Y. KOTAKE
322
ESR Studies on a new phenyl t-butyl nitroxide biradical based on calix[4]arene ............... 326 Q. WANG, J.-S. WANG, Y. LI and G . 3 . WU
Section 3. Chemical Reactions Plenary Lecture Time-resolved EPR studies of excited states: some old and some new stories ...................333 Noboru HIROTA General Papers Time-resolved EPR studies of the excited triplet states of p-methylcinnamic acid and its deprotonated anion ................................................................................................... Ichiro YAMAMOTO, Kanekazu SEKI and Mikio YAGI
Quenching of singlet molecular oxygen ('Ad by vitamins and polyphenols studied by time-resolved ESR .............................................................................................. Akio KAWAI. Takahito FUSE and Kazuhiko SHIBUYA
344
349
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A time-resolved EPR study of weakly coupled triplet-doublet pairs of copper(I1)-free base porphyrin dimers .......................................................................................................... 355 M. ASANO-SOMEDA, A. JINMON, Y. KAIZU, P. RAGOGNA and A. van der EST Pulsed-ESR investigations of the photo-excited triplet state of naphthalene ...................... Kunio TAGUMA, Jun YAMAUCHI, Masaaki BABA Light-induced ESR studies of regioregular K. MARUMOTO, N. TAKEUCHI and S. KURODA
36 1
composites .......... 367
ESR study of photodecomposition mechanism of a long-lived radical ESR spectrum of trifluoromethyl radical formed during solid-phase photodecomposition at 77K in glassy matrix ........................... S. R. ALLAYAROV and D. A. GORDON ESR study and quantum-chemical calculations of alkyl radicals in the matrix of polycrystalline n-alkane irradiated at 77 K. Effects of intermolecular interactions and carbon chain length on the radical formation ................................................................ S. R. ALLAYAROV, S. V. KONOVALIKHIN and T. E. CHERNYSHEVA
373
378
ESR/ENDOR study for new radical dianion species of 6-oxophenalenoxyl derivative ...... 384 Y. MORITA, S. NISHIDA, J. KAWAI, K. FUKUI, S. NAKAZAWA, D. SHIOMI, K. SATO, T. TAKUI and K. NAKASUJI Spin labeling study of polymer chain motion in PEGPVP blend ....................................... Shiming CHEN, Guidong JIN, Zhenghua PING, Sizhao JIN and Xmin SHEN
389
EPR and UV-VIS studies on the influence of solute-solvent interactions on the self-redox reaction of bis (dithiophosphate) copper(I1) ............................................ N. D. YORDANOV and K. RANGUELOVA
395
Section 4. Environmental Sciences Plenary Lecture In and ex EPR spectroscopy and imaging of endogenously produced nitric oxide under physiological and pathophysiological conditions ................................... Tetsuhiko YOSHIMURA and Naoki KATO General Papers Molecular-electronic mechanism of the toxicity of Dioxin and ability of some natural structures to concurrently interact to inhibit its activity .......................................... Nguyen Van TRI, Pham The VUNG, Dinh Pham THAI, Ha Van M A 0 and Dinh Ngoc LAM
403
412
Section 5. Biology and Life Sciences Plenary Lecture Kinetic EPR study on reactions of vitamin E radicals ......................................................... Zhihua CHEN, Bo ZHOU, Huihe ZHU, Long-Min WU, Li YANG and Zhong-Li LIU
421
Invited Lectures ESR investigation on ROS initiated by visible light in PSI1 particles of high plants .......... 429 Yang. LIU, Jian SUN, Ke LIU, Qiyuan ZHANG and Tingyun KUANG EPR and theoretical investigations of PiFe] hydrogenase: Insight into the mechanism of biological hydrogen conversion .......................................... W. LUBITZ, M. BRECHT, S. FOERSTER, M. STEIN, Y. HIGUCHI, T. BUHRKE and B. FRIEDRICH
437
EPR studies on free radical generation by the reaction of methylglyoxal amino acids and protein ............................................................................................... 446 Hyung-Soon YIM, Cheolju LEE, P. Boon CHOCK, Moon B. YIM and Sa-Ouk KANG EPR monitoring on the quality of life .................................................................................. N. D. YORDANOV, G. PETKOVA and I. NAIDENOVA
456
General Papers EPR studies of manganese spin centers in the even-number oxidation states of water oxidizing complex in photosystem I1 ........................................................................ S. ARAO, S. YAMADA, A. KAWAMORI, J.-R. SHEN, N. IONNIDIS and V. PETROULEAS
466
Magnetic resonance studies on ascorbate binding to albumin ............................................. E. LOZINSKY, A. NOVOSELSKY, A. I. SHAMES, R. GLASER, G. I. LIKHTENSHTEIN and D. MEYERSTEIN
471
Electron magnetic resonance study on the effect of radioactive radiation on the photosynthesis of chlorophyll in lipid bilayers ......................................................... Y. S. KANG, D. K. LEE and S. M. PARK and K. W. SEO
477
Effects of tannin compounds on metal ion-hydrogen peroxide systems .............................. A. NAKAJIMA, Y. UEDA, N. ENDOH, K. TAJIMA and K. MAKINO
483
The [2Fe-2S] cluster in sulredoxin from the thermoacidophilicarchaeon sulfolobus tokodaii strain 7, a novel water-soluble Rieske protein ...................................... Toshio IWASAKI, Asako KOUNOSU and Sergei A. DIKANOV
488
EPR and saturation recovery investigations of spin probes in dispersions of hydrogenated castor oil .................................................................................................... Kouichi NAKAGAWA
494
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Section 6. Medical Sciences Plenary Lecture Advances in the spin labeling method ................................................................................. Lawrence J. BERLINER Invited Lectures Recent progress and future prospects of free radical imaging by PEDRI ............................ David J. LURIE, Margaret A. FOSTER, Wiwat YOUNGDEE, Valery V. KHRAMTSOV and Igor GRIGOR’EV
503
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525
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533
General Papers ESR studies on spin-clearance in GPxl -transgenic mice ........................................ K. MURAKAMI, 0. MIROCHNITCHENKO and H. UTSUMI
542
Non-invasive analysis of stress-induced gastric ulcer in rats ............................................... K. YASUKAWA and H. UTSUMI
548
Nitric oxide production and inducible nitric oxide synthase induction in iron-loaded rats ................................................................................................................ T. KAWABATA, A. I. HIDA, M. FUJISAWA, M. KAMEKAWA and S. OKADA
552
Electron paramagnetic resonance in medicine R. SAIFOUTDINOV
Development of in vivo ESWspin probe technique for oxidative injuries H. UTSUMI, J.-Y. HAN and K. TAKESHITA
Collapse of redox state by glutamate transporter inhibition in the rat’s hippocampus Y. UEDA, Y. HAYASHI, A. NAKAJIMA, H. YOKOYAMA, Y. MITSUYAMA, H. OHYA-NISHIGUCHI and H. KAMADA
........ 556
Non-invasive assessment of oxidative stress in the brain of small animal models by using electron spin resonance (ESR) imaging system ......................................... Masaichi-Chang-il LEE, Hirofumi SHOJI, Hiroyuki MIYAZAKI, Fumihiko YOSHINO, Kohki NAKAZONO, Kazunori ANZAI and Toshihiko OZAWA Possible production of hydroxyl free radical in the gastric legion of nitroso carcinogen-administrated rats .............................................................................................. T. MIKUNI, T. MORII, H. NAJIMA, M. EHARA and M. TATSUTA
562
567
Section 7. Geology Invited Lecture EPR and optical absorption spectroscopy on minerals ........................................................ B. J. REDDY, Jun YAMAUCHI, Y. P. REDDY, A. V. CHANDRASEKHAR and R. V. S. S. N. RAVIKUMAR
575
xvii
General Papers Thermoluminescenceand ESR centers of fluorapatite crystal from Brazil ......................... Henrique K. de FRANCAF, Luciana R. P. KASSAB and Sonia H. TATUMI
585
Spectral studies of divalent copper in antlerite mineral ....................................................... R. Rama Subba REDDY, S. Lakshmi REDDY, G. Siva REDDY and B. J. REDDY
589
Paramagnetic criterions of prognosis for oil and gas rocks content ..................................... K. KUDAIBAYEV, Sz. S. SZAMAROV, B. K. KUSPANOVA, A. S. KALAUOVA and R. N. NASIROV
595
Section 8. Dosimetry EPR dose reconstruction in teeth: Fundamentals, applications, problems and perspectives ................................................................................................................... 603 A. A. ROMANWKHA and D. A. SCHAUER
Plenary Lecture ESR dating applications in archaeology and earth sciences ................................................ Rainer G R m
613
Invited Lecture ESR and NMR dosimetry ..................................................................................................... 0. BAFFA, A. KINOSHITA, F. Chen ABREGO and N. A. SILVA
6 14
General Papers K-band ESR spectra of irradiated tooth enamel ................................................................... A. KINOSHITA, C. F. 0. GRAEFF and 0. BAFFA
624
Assessment of contribution of confounding factors to cumulative dose determined by EPR of enamel ............................................................................................. S. V. SHOLOM , V. V. CHUMAK AND E.V. BAKHANOVA
628
Retrospective EPR-dosimetry in Semipalatinsk nuclear test site region ............................. S. PIVOVAROV, A. RUKHIN, T. SEREDAVINA and A. ZHDANOV
634
Determination of total ionizing radiation dose on animals from west Kazakhstan by EPR method .................................................................................................................... R. N. NASIROV, B. K. KUSPANOVA, K. KUDAIBAYEV and M. B. KILIBAYEV
640
Section 9. Cross-Disciplinary and Methodology Invited Lectures Pulsed ESR double resonance (PELDOR) spectroscopy: Application to spin-labeled peptides .................................................................................... Yuri D. TSVETKOV and Alexander D. MILOV
647
xviii
The carotenoid triplet state in reaction centers. An EPR magnetophotoselectionstudy ................................................................................. Igor V. BOROVYKH, Irina B. KLENINA, Ivan I. PROSKURYAKOV, Peter GAST and Arnold J. HOFF Electron dipole-dipole interaction in ESEEM of biradicals ................................................. S. A. DZUBA and L. V. KULIK
659
669
The influence of label spins on EPR spectra of charge separated states in photosynthetic reaction center ...................................................................................................................... 678 Kev M. SALIKHOV, Stephan G. ZECH and Dietmar STEHLIK General Papers The structural analysis of photosystem I1 by PELDOR of three spin system ...................... H. HARA, A. KAWAMORI and N. KATSUTA
679
Application of pulsed ELDOR detected NMR measurements on the studies of photosystem I1 ...................................................................................................................... 684 H. MINO and T. ON0 Pulsed-ENDOR cavities modified from the CW-ENDOR TM-mode and pulsed-ESR TE-mode cavities ............................................................................................. Jun YAMAUCHI and Kanae FUJI1
688
Ferroelectric resonators for EPR spectrometers at 35, 65 and 125 GHz ............................. I. N. GEIFMAN and I. S. GOLOVINA
694
Fourier-transform ESR spectroscopy and observation of ultrafast spin-lattice relaxation by optical means .................................................................................................. T. KOHMOTO, Y. FUKUDA, M. KUNITOMO
700
Subnanosecond relaxation of optically-induced magnetization in aqueous solutions of transition-metal ions ....................................................................... S. FURUE, K. NAKAYAMA, T. KOHMOTO, Y. FUKUDA and M. KUNITOMO Detection of the internal electric field and relaxational magnetoelectric effect in chromium mesogen .......................................................................................................... N. E. DOMRACHEVA, I. V. OVCHNNIKOV, A. TLJRANOV and G. LATTERMANN
706
7 10
Section 10. High Frequency and High Field EPR Plenary Lecture Modern ESR methods in studies of the dynamic structure of proteins and membranes ........................................................................................................................... Jack H. FREED
719
Invited Lecture High-frequency single-crystal EPR application to multifrequency approach: Study of metalloproteins ...................................................................................................... Sushi1 K. MISRA General Papers EPR evidence of onset of the quantum critical point in CuGe03:Fe ................................... S. V. DEMISHEV, R. V. BUNTING, H. OHTA, S. OKUBO, Y. OSHIMA, N. E. SLUCHANKO
Millimeter and submillimeter wave ESR measurements of spin ladder system Sr(Cul.,ZnX)2O3 ................................................................................. S. OKUBO, K. HAZUKI, T. SAKURAI, H. OHTA, H. YOSHIDA, M. AZUMA and M. TAKANO High frequency ESR on quantum spin systems by using single shot and repeating pulsed fields ................................................................................................... H. NOJIRI ESR study on magnetic ordering of spin-hstrated antiferromagnet ZnCr204 single crystal ........................................................................................................................ H. KIKUCHI, H. OHTA, S. OKUBO, I. KAGOMIYA, M. TOKI, K. KOHN and K. SHIRATORI
73 1
741
747
75 1
755
ESR study of frustrated spin chain [Cu(bpy)HzO] [Cu(bpy)(mal)H2O](C104)2 .................. 759 T. KUNIMOTO, T. KAMIKAWA, S. OKUBO, H. OHTA and H. KIKUCHI Millimeter wave ESR measurement of diamond chain substance azurite ........................... Tomohisa KAMIKAWA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA and Hikomitsu KIKUCHI
763
High field ESR of(Ca~.,Sr,)~,CuO~with edge-sharing Cu02 chain .................................. K. KAWAKAMI, A. UEDA, H. OHTA, S. OKUBO, Z. HIROI, M. TAKANO
767
High field ESR measurements of(V0)2P207 ...................................................................... Yuta NAGASAKA, Takashi KUNIMOTO, Susumu OKUBO, Hitoshi OHTA, Touru YAMAUCHI and Yutaka UEDA
771
ESR measurements on triangular antiferromagnets CsCu~.,Co,Cl~.................................... Toshio ONO, Hidekazu TANAKA, Hiroyulu NOJIRI and Mitsuhiro MOTOKAWA
775
Gyrotron ESR in CsFeC13 up to 40 T ................................................................................... M. CHIBA, K. KITAI, S. MITSUDO, T. IDEHARA, S. UEDA and M. TODA
779
Magnetic properties of Fe12 ring: ESR and magnetization measurements ......................... Y. INAGAKI, T. ASANO, Y. AJIRO, Y. NARUMI, K. KINDO, H. NOJIRI, M. MOTOKAWA, A. CORNIA and D. GATTESCHI
784
High magnetic field ESR measurements of ACu2(P04)2 (A=Ba, Sr) .................................. 788 Masayuki HATA, Seitarou MITSUDO, Toshitaka IDEHARA and Mamoru MEKATA
ESR transmission experiments on P’-(ET)~SFSCF~SO~ and (ET)2SFsNN02, investigations of spin-Peierls systems .................................................................................. I. B. RUTEL, J. BROOKS, B. H. WARD, D. VANDERVEER, M. E. SITZMANN, J. SCHLUETER, R.W. WINTER and G. GARD
793
High field ESR measurements on molecular oxygen .......................................................... Shojiro KIMURA and Koichi KIND0
799
High field ESR measurements ofCuZ(CsH12N2)2C14 under high pressure .......................... M. SARUHASHI, T. SAKURAI, K. KIRITA, T. KUNIMOTO, S. OKUBO, H. OHTA, H. KIKUCHI and Y. UWATOKO
803
ESR at ultra-low temperatures and observation of new mode in Cu-Benzoate ...................807 T. SAKON, H. NOJIRI, K. KOYAMA, T. ASANO, Y. AJIRO and M. MOTOKAWA ESR spectrometer using frequency tunable gyrotrons as a radiation source ....................... Seitaro MITSUDO, Kazuaki KANAZAWA, Masayulu HATA, Isamu OGAWA, Tomohiro KANEMAKI and Toshitaka IDEHARA
813
High-frequency (W-band) EPR studies of biological samples ............................................ KBichi FUKUI, Tomohiro IT0 and Hiroaki OHYA
8 18
AUTHOR INDEX ................................................................................................................
825
Tribute Remembering Professor Arnold Jan Hoff (1939-2002) Just a few days before this volume was to be finally closed, we received the sad news that the EPR/ESR community has lost such an eminent researcher as Professor Arnold Hoff. He was one of the invited speakers at the Third Asia-Pacific EPR/ESR Symposium (APES’Ol) held in Kobe, the Proceedings of which form this volume. Although Arnold had not been a member of the Asia-Pacific EPR/ESR Society (APES) for ‘geographical’ reasons, we all in APES greatly appreciated his recent significant contribution to the APES activities. With Prof. Kawamori, Arnold co-organized Symposium B: International Workshop on Advanced EPR Applied to Biosciences, a Satellite Meeting which followed APES’O1. Arnold looked fine at both meetings and gave fruitful lectures concerning his recent work. A paper titled: “The carotenoid triplet state in Rhodobacter Sphaeroides reaction centers: An EPR magnetophotoselection stu+” appears in this book. After APES01 and Symposium B Arnold visited Kwansei Gakuin University at Sanda, Tohoku University in Sendai, the Institute of Molecular Sciences at Okazaki, and Kyoto. His family, friends, and colleagues have suffered a great loss. On behalf of the APES Council and all APES members we express our sincere sympathy to his wife, Zina, and his children. We will value his memory and his contributions to the APES activities. As a tribute to Arnold, on the following page we reproduce a note provided to us by Peter Gast, Hans van Gorkom, T h i s Aartsma, and Thomas Schmidt, who are at the Huygens Laboratory at the University of Leiden. Asako Kawamori Chairperson, Local Organising Committee, APES’O1 Czeslaw Rudowicz President, The Asia-Pacific EPRlESR Society President of APES
Dear colleagues and friends of Arnold Hoff Arnold Jan Hoff passed away on 22 April 2002 at the age of 62. Until the very last days he chose to ignore the fatal cancer he was suffering from and continued to work with all available energy. After his study in physics Arnold graduated under Johan Blok at the Free University in Amsterdam in 1969. In 1971 Arnold was introduced to the biophysics of photosynthetic reaction centers as a post-doc in George Feher’s group at the University of California in San Diego. Fully aware of the great potential of his specialization, magnetic resonance techniques, in the field photosynthesis research, in 1974 Arnold joined Lou Duysens (Biophysics Laboratory at Leiden University), which at that time used mainly optical techniques. In 1985 he was appointed full professor in Biophysics. The impact of Arnold’s efforts on the development and application of magnetic resonance techniques for the study of primary reactions in photosynthesis is hard to overestimate. With his team of graduate students and post-docs he worked on electron spin polarization phenomena studied with cw and time-resolved EPR and ESE. Around 1982 he developed the technique of ADMR, an ODMR technique especially suited for photosynthetic samples. In recent years his attention turned to isotopically labeled reaction centers which were studied by NMR and EPR. Also, with his Russian collaborators he worked on improving and re-evaluating magneto-photoselection and applied it to radical pair spin polarization. His future plans were the study of reaction centers with site directed spin labeling and developing the technique of CD-ADMR, but this was not to be. Arnold published more than 250 articles and 19 and students graduated under him. One of the things he loved most was to travel and to meet new people and to make new friends. He must have visited hundreds of symposia and congresses. Visitors to his office will remember the large collection of photographs on the wall of friends from all around the world. His joy for travel also led to a special relation with scientists in the field of EPR and photosynthesis from Russia. Since 1992 he was director of the Dutch-Russian Research Collaboration Network. In 1999 he was awarded the Voevodsky Gold Medal of the Russian Academy of Sciences. He was also the chairman of the highly successful ESF program: Biophysics of Photosynthesis, that, in his own words, forged a network of cordial links between a great number of groups in practically all European countries. At Leiden University he was the initiator of BIOSPEC, a collaboration between physics, chemistry and biology, which led to the foundation of the research school BIOMAC. This department and the biophysics community have lost a strong leader. We feel deep compassion for Zina and his children, who have lost much more.
Section 1 Physics and Magnetism
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EPR in the 2 lstCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
3
Electron magnetic resonance (EMR) of the spin systems: an overview of major intricacies awaiting unwary spectroscopists C. Rudowicz Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong SAR, China The aim of this paper is to bring about a better understanding of the intricacies of the spin Hamiltonian (SH) theory. A number of theoretical aspects underpinning the experimental EMR (encompassing EPR, ESR and related techniques) studies of paramagnetic species with the spin S21, especially transition ions, which require a thorough clarification, are critically reviewed. Examples of the intricacies awaiting unwary spectroscopists, drawn from recent EMR literature, are discussed in a nutshell in order to illustrate the potential pitfalls and their consequences.
1. INTRODUCTION Electron magnetic resonance (EMR), which nowadays encompasses electron paramagnetic resonance (EPR - the term dominating applications in physics) and electron spin resonance (ESR - the term dominating applications in chemistry) as well as related techniques, is a mature area with applications ranging from biology to materials sciences [l - 81. However, various theoretical aspects underpinning the interpretation of experimental EMR data, which are often confused in the EMR literature, can be identified. In this paper the major points concerning EMR studies of paramagnetic species with the spin S21, especially transition ions, which have recently been clarified, are reviewed. Examples of the intricacies awaiting unwary spectroscopists, drawn from recent EMR literature, are presented a nutshell in order to illustrate the potential pitfalls, e.g. usage of erroneous relations and misinterpretation of experimental EMR data. Thus we hope to bring about a better understanding of the intricacies of the spin Hamiltonian (SH) theory [9-141, especially the zero-field splitting (ZFS) Hamiltonian, &&s, and the crystal-field (CF) Hamiltonian, &CF, which are fundamental to EMR studies [1-81. Ramifications include the magnetic susceptibility, magnetic anisotropy, Mossbauer spectroscopy, which rely on SH, as well as the optical absorption spectroscopy, inelastic neutron scattering, and infrared spectroscopy, which rely on CF Hamiltonian. First we outline briefly the reference notations for &ZFS [I, 21 to provide background for the discussion in Sections 2 to 7. terms of the extended Stevens (ES) operators [9] the compact form of &FS (as defined in [lo]; to be distinguished from the expanded form using explicitly the pairs of the tensor operators with +q in &ZFS) is given as:
4
The uniform 'scaling'ofZFS parameters b l requires the factorsfk to be taken as [l, 2, 10, 141: 1/3,
1/60,
1/1260
(2)
For numerical convenience, the second form of &DS in Equation (1) has been more often used in EMR studies of transition ions. The conventional form of &ZFS most widely used in the literature, which is suitable for paramagnetic species with the spin S2 1 at sites with triclinic symmetry, is given by [ 1-7, 101:
whereas for the spin S22 the higher-order ZFS terms are required. For orthorhombic symmetry as well as for monoclinic and triclinic symmetry with &&FS expressed in the principal axis system, the conventional form of &ZFS is given by [I-3, 10, 141:
where the axes (x, y, z) may be chosen in different ways for orthorhombic symmetry [ 10, 111 (see Section 5), whereas for monoclinic and triclinic symmetry the orientation of the principal axes (x, y, z) with respect to the crystallographic axis system (X, Y, Z) must be provided [lo]. In Sections 2 to 6 various categories of the intricacies of the SH theory are briefly outlined; for details and references the reader shall refer to the respective original papers. In Section 7 possible remedies to some of the problems encountered by experimentalists in interpretation of EMR data are presented.
2. MAJOR INTRICACIES CONCERNING NOTATIONS AND NOMENCLATURE
2.1. Multitude of notations for the operators and the ZFS parameters There exist in the EMR (as well as CF) literature a multitude of notations for the operators and the associated ZFS (CF) parameters. Such an abundance hampers comparisons of data from various sources. All notations appearing in the EMR-related literature up to late 1986 were comprehensively surveyed in [lo], whereas recent literature will be summarized in [12]. A general pitfall concerning all SH notations consists in the misleading or often inconsistent nomenclature for various terms appearing in &~Fs. Historical developments have led to the existence of various SH notations classified briefly into three groups as follows [lo, 141. A. ConventionalSH notations Any notation using explicitly the 'spin' operators S,, S,, S,, or S,, S =S, & is,, like Equations (3) and (4), belongs to the conventional notations [lo]. In fact, the 'spin' may mean the physical spin S , or the effective spin, 3 , or the fictitious spin, S' (see Section 3). The conventional notations are particularly inconvenient for low-symmetry, e.g. monoclinic and triclinic, which require a large number of ZFS terms [lo]. Hence, these notations have been mostly used for higher-symmetry and low-spin cases [l-8, 101. In spite of the widespread
5
usage of the tensor-operator notations in the literature, some recent books still adopt only the conventional notation with the former notations receiving only scant remarks, see, e.g. [4-71. The major pitfall concerning the conventional SH notation is the existence of a number of conventions for operator combinations and symbols for ZFS parameters as reviewed in [lo]. One must be cautious when comparing SH parameter values from different original sources and reviews. B. Tensor-operatornotations Tensor-operator notations may be collectively denoted by the generic symbols: xlrn for operators and At, for the associated parameters [lo, 141. Apart from the most common product form: Alm~lm, alternate tensor forms of the ZFS Hamiltonian are sporadically used in the literature [lo, 141. The advantages of the tensor operators [lo, 141 have led to the development of several notations, which can be subdivided into two subgroups, each consisting of several different types of tensor operators interrelated by conversion coefficients. The general classification briefly outlined below may serve as a quick reference guide for practitioners. The most representative symbols originally introduced for the operators and their coefficients, defining the ZFS parameters, are given for each tensor-operator notation. A full review of the present status and a detailed survey of operator and parameter notations currently in usage in the EMR area, including a list of specific intricacies, will be provided in [ 121. A general pitfall concerning the tensor-operator notations is the fact that several symbols have been used for the same quantity and conversely, the same symbols have been used to denote different quantities. B l . Tesseral-tensoroperators (TTO) Historically [9, 141 Stevens [151 introduced an incomplete set of tesseral-tensor operators (TTO), 09,(L), being “operator equivalents” of real tesseral harmonics with the components q limited to k 2 q 2 0, which latter become known as the Stevens operators [l, 3, 9, 101. Some misleading statements concerning the ‘operator equivalents’ in [4] are discussed in [14]. The , . Stevens operators, 0; (S), with L replaced by S (in fact or S‘ - see Section 3) have been widely used in the EMR area. However, the lack of the negative q components hampered application of the Stevens operators for low-symmetry cases [lo]. The extended Stevens (ES) operators O:(S) including all components: -k q +k were introduced and their transformation properties were worked out in [9]. These are now the most widely used operators in the EMR area. Thus we adopt the ES operators as the reference notation in Equation (1). The usual Stevens operators [l, 151, however, suffer from a serious drawback in that they are not normalized [9, 101. This has led to the appearance of the corresponding normalized operator sets. The various tesseral-tensor operators (TTO) existing in the literature may be classified as follows [lo, 121. a) The usual Stevens operators, where the q values are limited only to the positive
s
integers and zero (k q 2 with the coefficients B: or the ‘resealed’ b: , see Equation (1). The major pitfall concerning this notation is the occurrence of other ‘scaling‘factors than those defined in Equation (2) as reviewed in [lo]. b) The extended Stevens (ES) operators, integers (-k
, where the q values include also the negative
q I +k) [9, lo]; with the coefficients B!
(or b:).
The early inconsistent
notations for the components q I 0 may pose apitfall as discussed in [lo].
6
c) The normalized Stevens (NS) operators, OF, [lo], which are, in fact, the ES operators, each multiplied by certain normalizing factor; with the coefficients B: . Similar pitfalls as for the operators (a) and (b) apply, to a certain extend, to the NS operators [ 10, 121. d) The normalized combinations of spherical tensor (NCST) operators, [lo, 121, which are simply related to the NS operators 0: and are linear combinations of the STO (see Section B.2); with the coefficients The usage in the literature of similar symbols for the NCST operators and their STO counterparts may easily lead to confusion [lo, 121. e) Five unnamed notations of ephemeral usage in the literature reviewed in [lo]. B2. Spherical-tensor operators (STO) Historically [101 the extension of the operator equivalents method to the spherical harmonics was made subsequent to introduction of TTO and has led to introduction of the sphericaltensor operators (STO): Tkq(X), X = L, J or s. The operators Tkq(L or J) have now become widely used in CF theory, where they are usually denoted as Ckq, whereas the extended Stevens operators ( S ) play the same role in EMR. The various spherical-tensor operators (STO) existing in the literature may be classified as follows [lo, 121. a) Buckmaster, and Smith & Thornley (BST) operators of);with the coefficients B:. Note that the BST operators are mathematically equivalent to the Wybourne operators c k q used in the CF theory [ 10, 121. b) Phase-modified BST operators - these are BST operators multiplied by a phase factor:
d(k)=ik o q( ~with ) ; the coefficients c) Koster & Statz and Buckmaster, Chatterjee & Shing (KS/BCS) TI,; with the coefficients B I ~ d) Ten unnamed notations of ephemeral usage in the literature reviewed in [lo]. Full discussion of the intricacies in the above category and interrelations between various notations used in the area of EMR and related spectroscopies [ 19, 141 will be provided in [12].
2.2. Incorrect relations for ZFS parameters Recently three cases of incorrect relations between the extended Stevens ZFS parameters and the conventional ones have been identified in the literature [16]. Case 1 concerns the conversion relations for the second-order rhombic ZFS parameters. Which relation is correct: 3E = b i = 3B: [ l , 101 or E =b; =3Bi [13, 17, 18]? It turns out [16] that the former relation is correct, whereas the latter one is incorrect. The substitution E -+bi in [13, 17, 181 has three implications. First, the maximum rhombicity limit is then: 0
> the lineshape becomes generally Lorentzian. The linewidth 77 was given as
19
where xu is the magnetic susceptibility, = [%’,S+] and denotes the expectation value at temperature T under the unperturbed Hamiltonian (XO Xz).While this approach seems successful for several systems, there are still fundamental problems. Firstly, because of the difficulty in solving the dynamics of a many-body system, the Lorentzian lineshape was never derived rigorously. It was necessary to make some assumptions about the dynamics, which may be valid for some cases but not always so. The formula eq. (8) is valid (in the lowest order of the anisotropy provided that the lineshape is Lorentzian. [5] However, justification of the Lorentzian lineshape remains a problem. Secondly, even if the lineshape is indeed Lorentzian, the calculation of the lineshape by eq. (8) poses a problem, being a dynamical quantity itself. In the high-temperature limit, by further introducing assumptions, eq. (8) is reduced to a static correlation function which can be calculated rather easily. However, at lower temperature, many spins become correlated and the calculation of the correlation functions becomes difficult by traditional means. ESR in an = 1 /2 Heisenberg antiferromagnetic chain
+
(9)
at low temperature offers an extreme case where quantum fluctuations and many-body correlation becomes essential. 3. Field-theory approach to S = 1/2 antiferromagnetic chains
Interestingly, the = 1/2 Heisenberg antiferromagnet at low temperature, which is difficult for traditional theoretical approaches even for the calculation of static quantities, is one of the few quantum many-body systems which we understand the dynamics rather well. A powerful field theory approach to the S = 1/ 2 Heisenberg antiferromagnet has been developed, to provide us an asymptotically exact description of static and dynamical quantities in the low-energy limit. Here we will introduce a rough picture of the field theory approach to the S = 1/ 2 Heisenberg chain, avoiding technical and mathematical details. For the details the readers are referred, for example to Ref. [3]. A system of interacting S = 1/ 2 spins can be regarded as a quantum many-particle system in various ways. For example, “up spin” and “down spin” state can be identified with the site occupied by a particle (without spin degree of freedom). Because each site can have zero or one particle only, the system should be regarded as a system of hard-core (ie. kith infinite onsite repulsion) bosons. In one dimension, because the hard-core bosons never exchange their position, it can also identified with the system of fermions. The transverse part of the exchange interaction SS ;+;l = corresponds to a hopping of the particles to the nearest neighbor, while the longitudinal part S7Ss+I represents an interaction between the particles at nearest-neighbor sites. Another possible way is to consider the physical origin of the S = 1 /2 Heisenberg antiferromagnetic chain. Because the spins are carried by electrons in reality, we may consider the underlying system of interacting electrons (with spin). When the number of electrons is equal to the number of sites, a repulsive interaction between electrons leads to a Mott-insulator state, in which the electrons are localized so that each site is occupied by an electron. At sufficiently low energy scale, the charge fluctuation (which involves change in local number of electrons) is frozen. The spin degrees of freedom still remains alive, with an effective antiferromagnetic interaction between nearest neighbor spins. This is nothing but
+
+
20
the = 1/2 Heisenberg chain (9). Therefore, one can discuss the spin chain by studying an interacting electron system in the Mott insulator regime. In either case, the spin chain problem is mapped to a quantum many-particle problem with strong interaction. This is in clear contrast to more traditional approaches based on classical spin picture. The interacting fermions in one dimension is an important problem in its own. In real physical systems, electrons of course interact with each other via Coulomb force. However, in metallic systems a free electron model is used frequently, to describe various properties rather well at least qualitatively. This is because if one injects an electron to such a system, the electron drags other electrons around itself but this “dressed” electron - called as quasiparticles - behaves much like a free electron, except for its renormalized mass which is different from the original electron mass. Therefore the system is rather similar to (but of course not identical to) a system of free fermions, and is called as a Fermi liquid. Fermi liquid theory has been successfully applied to usual metals and to liquid 3He. However, in one dimension, quantum fluctuations are so strong that the Fermi liquid theory generally breaks down in the presence of interactions. Namely, a dressed electron is no longer a well-defined elementary excitation with a long lifetime. A one-dimensional system of interacting fermions had been often regarded as just a theoretical toy model. However, it is becoming more realistic, thanks to discovery of more quasi one dimensional materials and fabrication of “quantum wires” based on semiconductor technology. Actually, quantum spin chains, which can be regarded as interacting fermion systems in one dimension, has been studied experimentally for a long time. These real systems requires a theoretical framework which does not rely on the notion of independent fermions. It has become known [3] that an interacting fermion system in one dimension is rather better described in terms of bosons. The local density of the fermion fluctuates locally. The fluctuation of the density propagates in space-time, forming a “sound wave.” In quantum mechanics, sound wave consists of bosonic particles, namely phonons. While this argument may apply to any dimensions. the peculiar feature in one dimension is that whole (low-energy) excitation spectrum can be described in terms of the “phonons.” Thus it is possible to study the fermion system entirely in terms of the bosonic field. The beauty of this approach - bosonization - is that a certain class of interacting fermion systems, which includes the = 1/2 Heisenberg antiferromagnetic chain (9), is reduced to just a harmonic density “sound wave,” namely collection of non-interacting “phonons” which describes magnetic excitations. The interaction between the fermions is translated into the “rigidity” of the boson field. When there is a strong repulsive interaction between the fermions, the density fluctuation is suppressed, and the corresponding boson field becomes rigid. Even though the rigidity is renormalized by the interaction, the phonons are still non-interacting. This enables us to calculate various static and also quantities exactly. In the case of a system of interacting electron with spin, there are two kinds of density fluctuations: one with changes in the total density of electron (with up and down spins), and the other accompanies a difference in the densities between up and down spin electrons but with a constant total density. They correspond to charge and spin fluctuations, respectively. Each one is described by a corresponding boson field. In the Mott insulator phase, the charge fluctuations are frozen at a sufficiently low energy, and only the spin fluctuations described by a single boson field remain. Therefore the final description of the = 1/2 Heisenberg antiferromagnetic chain (9) is the same whether one identifies the system as an interacting spinless fermion or a
21
Mott insulator phase of an interacting electrons with spin.
4. Field-theory approach to ESR Let us consider ESR in = 1 / 2 Heisenberg antiferromagnetic chain (9). It turns out that the ESR absorption corresponds to creation of a single bosonic particle (“phonon”). As already discussed in Sec. 2, ESR measures excitations at zero momentum. However, it should be noted that ESR is measured in the presence of the applied field Zzwhich changes the spectrum. More specifically, the applied static field shifts the dispersion relation. To see how this happens, it is convenient to use the spinful electron model. In order to reproduce the spin chain, the charge degree of freedom is eventually frozen. For the moment, however, let us consider free electrons (with spin). Up-spin electrons and down-spin electrons fills the respective bands. At = 0, the dispersion curves for upldown-spin electrons are exactly the same. However, the magnetic field shifts the dispersion curves vertically by i H / 2 . The ground state is given by the up-spin and down-spin electrons filling the Fermi Sea of momentum -kL < k < k i and have opposite signs the calculated trace becomes s18.0, equal to the value of a doublet in cubic symmetry. Numerical calculations as presented in this paper allow the variation of g values continuously to be followed as a function of axial field between the extreme limits of pure cubic and pure axial field. This allows to follow the variation of the trace and to detect possible changes of sign. Analytical treatments which are restricted to the limiting cases do not have this feasibility.
2. OUTLINE OF COMPUTATIONAL METHOD Energy levels in zero magnetic field are obtained by solving the eigenvalue equation for the crystal field potentials in the basis set of the 16 states for the J = 15/2 spin-orbit ground state. A cubic field, valid for Td symmetry, has the forth- and sixth-order contributions
vcu4 = 35(x4+ y4 + z4)
-
2ir4
and
+ y6 + z6) - 3i5(X4+ y4 -k Z4)? + 90r6.
vcu6 = 23i(X6
Representative expressions for axial trigonal and tetragonalpotentials are, respectively, Vh = xy + yz + zx and
v, = 2. Equivalent crystal-field operators Kf acting on spin the states IJ,mJ>are derived fiom the potentials. A general expression will have the form
Kf=Vc~cosp(sina.Ku4+ cosa.KU6)+ sinfl.H@,te], Parameters a and p, with -90' 5 a$ 5 +90', describe the relative strengths of various contributionsto the potential and Vcfthe total strength. To obtain the Zeeman effect, the energy due to a magnetic field is added to the crystal field Hamiltonian. This energy is given, directly in operator form, by
For the ground state of Er3+the Land6 factor has value = 615, which is experimentally well confirmed to be the applicable value in the cubic r6 and r7 states. By this feature of not adding new fieely adjustable parameters, the Zeeman effect is a valuable tool in spectroscopy.
100
> Q)
30
60Graph2W"2'5a 90
Parameter CI (degrees) Figure 1. Crystal-field energies in cubic symmetry of the spin-orbit ground state J = 1512 of i= the E?+ ion. States are labeled with their symmetry type or F7 for the doublets and (r&, 1, 2 and 3, for the quartets. Parameter in the range -90" < < +90" controls the mixing of fourth- and sixth-order contributions to the cubic crystal field. Parameter Vcf> 0. 3. ENERGIES AND g VALUES In the absence of axial fields, the calculated energy level diagram for cubic symmetry is given in Figure 1. The results are equivalent to the classical data of Lea, Leask and Wolf [4], but are presented in a form matching the parameters V,f and introduced in Equation (5). The calculations predict a ground state of r7 symmetry for -90 5 -40.4", for instance, therefore, for the case of pure 4th-order cubic field = -90"), with corresponding g value g = 6.0. In the range -40.4' 5 5 +54.5" there will be a r6 type ground state, with g value g = 6.8. This includes the case of pure 6th-order cubic crystal field = 0"). In the remaining range +54.5" 5 a 5 +90" the ground state is a Ts quartet, to be described by an effective spin J = 312 with a Hamiltonian including cubic terms and with an anisotropic spectrum. With an axial field present energy diagrams are given in Figures 2(a) to 2(d), for two selected cases of cubic potential, = -90" and = 0", and including trigonal and tetragonal cases. In the lower axial symmetry all degeneracy is lifted and the energy spectrum consists of eight doublets. Crossings of levels a fbnction of the relative strength of the axial field, specified by parameter p, ffequently occur. Figures 3(a) to 3(d) present the calculated principal g values and g l for the ground states of the considered cases, as well as tensor trace g// + 2 g ~The . sudden changes in ground state properties are related to level crossings at particular values of p. Figures 3 indicate that for small values of the trace remains constant at the value 18.0 for states around the r7state and at 20.4 near the r6state. The numerical
36 500).
,
,
,
.I,,
,
I
, , I , ,
*9/2 +9/2 W
Parameter p (degrees)
I '
-60
30
Parameter p (degrees)
Parameterp (degrees)
~Tmi"""""'" ' I
"
I
"
' " ' '
I
'
30
'
*-h*Wll*ll."., I '
'
I
60
Parameter p (degrees)
Figure 2. Energies of the eight doublet levels for fourth- or sixth-order cubic crystal field together with a second-order trigonal or tetragonal crystal field calculated from Equation ( 5 ) for (a) a = -90", trigonal, (b) a = O", trigonal, (c) a = -90", tetragonal and (d) a = 0", tetragonal and positive V,f. Parameter p in the range -90" < p < +90" controls the mixing of the cubic and the axial crystal fields.
37
Trace .......... .................. -
..................................
....
..................................
-
........................
L
-5 -90
UMrmmm
Parameter 3! (degrees)
15
J
10
3
5
"
-90 Parameter p (degrees)
I
"
I
"
1 '
" '
tGJ
'
-60 Parameter p (degrees)
Figure 3. Principal g values and and trace + 2gl for transitions in the lowest-energy doublets. Illustrated cases, (a) a = -90°, trigonal, (b) a = O", trigOna1, (c) a = -90°, tetragonal and (d) a = O", tetragonal, correspond to the energy diagrams shown in Figure 2. Parameter in the range -90" < p < +90" controls the mixing of the cubic and the axial crystal fields.
38
calculations provide a full confirmation of the early result based on perturbation theory [2], for both trigonal and tetragonal axial fields, both for states derived fkom r6 or r7 states in the cubic symmetry, and for ground or excited states. Positive values of the parameter p correspond, according to Equation (5), to equal signs of the cubic and axial potentials. In the range 0" I p I +90" energy curves of the ground state do not tend to cross the first excited state in the crystal field. If so, then at p = +90" the doublet state 115/2,*1/2> is reached. Transitions within this doublet in the axial field have the g values = 1.2 and g l = 9.6. Inspection of Figure 3(b) for a = 0" shows that positive values are to be taken for both and resulting in the trace 20.4, equal to the trace 3g of the isotropic r6 state. For = -90" the Figures 3(a) and 3(c) show that upon mixing axial field into the cubic field the principal value g/l changes sign in the interval 0 p +90". At p = +90° the better choice for is therefore g/l= -1.2. The trace + 2 g = ~ 18.0 at this point is equal again to the trace 3g of the cubic symmetry state fkom which the axial state can be considered to be derived. For negative values of p, on increasing the axial potential, the doublet 115/2,&15/2>will be reached, or, in case a level crossing occurs, the doublet 115/2,*13/2>. The g tensors for transitions within these doublets have components = 18.0, = 0 and = 15.6, = 0, respectively. It is then immediately clear that for states derived fkom r6 in cubic symmetry, with trace 20.4, the trace cannot be a constant. In contrast, for the doublet related to the state, with trace 18.0 this might well be the case. Indeed, Figure 3(c) shows, the case of tetragonal distortion for = -90", reveals a remarkably constant value of its trace also for negative values of p. For this particular case, over the whole range of p, the trace can decrease a bit below its limiting value of 18.0, but never falls below 17.95. The more substantial reductions of trace occur in the region of adjoining p = -90". It will be noted, however, that for the corresponding tensors = 0. This implies that the states are EPR silent; these resonances with the reduced trace are not observable. It adds support to the empirical fact that observed resonances for the erbium ion its threefold ionized state are characterized by traces in the range fiom 18.0 to 20.4.
REFERENCES 1. C.A.J. Ammerlaan and I. de Maat-Gersdorf, Appl. Magn. Reson. 21 (2001) 13-33. 2. H.R Lewis and E.S. Sabisky, Phys. Rev. 130 (1963) 137c1373. 3. F.V. Strnisa and J.W. Corbett, Cryst. Lattice Defects 5 (1974) 261-268. 4. K.R. Lea, M.J.M. Leask and W.P. Wolf, J. Phys. Chem. Solids 23 (1962) 1381-1405.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
39
Continuous wave and pulsed EPR spectroscopy of paramagnetic ions in some fluoride, silicate and metaphosphate glasses S.C. Drewa* and J.R. Pilbrow” ”School of Physics and Materials Engineering] P.O. Box 27, Monash University] Victoria] Australia] 3800 Low concentrations of paramagnetic transition metal or rare earth ions often found in commercial glasses are ideal spin probes to explore the local structural disorder intrinsic to these materials. Investigations of the EPR spectra of glasses have raised the question as to the whether the spin Hamiltonian model provides a valid basis for interpreting spectra from disordered solids. This is because the connection between the spin Hamiltonian and its environment is subtle and indirect, the more so for the likely C1 spin-probe site symmetry in a glass. CW simulations of random network glasses is thought to require a distribution of spin Hamiltonians for C1 point symmetry sites (stochastic model) whereas powder simulations should be appropriate for micro-crystallite glasses. Our present understanding concerns transitions in lightly doped glasses where individual spins are well separated; it is ‘bottomup’ thinking rather than ‘top-down’ thinking normally applied to EPR simulation. 1. SIMULATION OF C W SPECTRA OF GLASSES CW-EPR of glasses has been modeled with a variety of levels of sophistication [l,2]. The typical transition metal ion EPR spectrum extends over many kilogauss at 9 GHz. The goal of simulation strategies down the years has been to explain features often observed in normal first derivative CW-EPR spectra, however the recent trend has been towards a characterisation of the entire spectrum. Most EPR in glasses is interpreted using powder models, which means that in effect one is assuming a micro-crystallite view of glass structure. Oxide and silicate glasses are believed to have random network structures. The currently accepted view is that the classic oxide glasses are an extended 3D network without symmetry or periodicity] with inter-atomic forces that are essentially the same as in crystals. That they are not microcrystallite structures follows from x-ray line broadening, which sets a limit on possible crystallite size, and the absence of intense small angle scattering [3]. X-ray scattering radial distribution functions (RDF) do not clearly discriminate between random network and micro-crystallite models of glass; this is due to the nature of statistical averaging inherent in the experiments [4]. In vitreous silica (SiO2), the average Si-Si distance is 0.36nm and no order exists for Si-Si distances 2 l n m [5, p.11. In silicate glasses, Si N
N
*Supported by the Monash Graduate Scholarship.
40
atoms are coordinated to four oxygens at the corners of imperfect tetrahedra [5, pp.25 f€l; estimates of the distribution of Si-0-Si angles give /3 = 160 20’ [6, p.282,284]. Short planes or chains of alkali or alkaline earth ion modifiers in multi-component silicate glasses define intermediate range order. The situation is not so clear for fluoride glasses although it is generally accepted that there exist octahedral sites for Cr3+ and Fe3+ in transition metal fluoride glasses (TMFG) [7,8] and higher coordination numbers for Gd3+ in TMFG that vary from site to site [9]. Nevertheless, the rare-earth ions are found in a more regular environment than in oxide glasses. Ce3+ ions in fluorozirconate glass are thought to occupy 8-fold coordinated square anti-prism sites [lo]. Griscom [ll]has this to say: “glasses differ from simple crystalline powders in a fundamental way: The intrinsic randomness of the vitreous state gives rise to statistical distributions of crystal fields that are reflected to various degrees in many spectroscopic properties”. That statement ignores the fact that randomness requires C1 point symmetry at each spin probe site. Thus ingenious explanations such as that due to Peterson [12,13] for the g 4.3 line based on the lowest Kramers doublet, assuming axial symmetry and a joint distribution function for correlated variables gll and g l is immediately ruled out. Brodbeck and Iton [14] provided helpful insights regarding the features for = 712 (Gd3+ and Eu2+) EPR spectra in glasses, particularly the g 6 feature which occurs for a range of values, provided the limiting value = 113 is included in the range. Their powder like simulation included only 2“d order spin Hamiltonian parameters with a very broad distribution and an arbitrary lineshape function of 80G width. To investigate the possibility of simulating the EPR spectrum of a random network glass, we have written a simulation program using the Eigenfields method [15] which has complete generality [16]. Any distribution of spin Hamiltonians and their parameters can be generated using a Monte Carlo approach and the EPR spectrum is built up from a summation over all randomly generated sites. Although the aim is to use delta function resonance lines without invoking an arbitrary linewidth, these lines must still be ‘binned’ to obtain a meaningful spectrum, because the resonance lines obtained using a Monte Carlo approach are not evenly spaced in the magnetic field dimension. Thus, a linewidth is involved in the sense that binning resonance lines with widths (typically 1-10G) imposes an effective residual linewidth of If we assume the glass is a network then our disorder model should be based upon site-to-site uniqueness and a random number generator should be used to generate the principal values and orientations of the interactions. The question which is hard to answer is ‘what choice of distribution of spin Hamiltonians is appropriate for a given glass and how can we physically justify our choice?’ A popular choice for high spin ions is the Czjzek function [7,8] which is used to correlate the probability densities of the and parameters of spin probes. The Czjzek method has been applied to = 312, = 512 and S = 712 transition metal ions in fluoride glasses [7,8], however a residual linewdith of up to 80G was still used. The Czjzek distribution was also trialled for S = 512 in an oxide glass by Yahiaoui it et al. (1994) but was in this case found to be unsatisfactory in comparison to a two-dimensional Gaussian distribution. Again the linewidths were up to 80G. Legein al.[8] responded to this inference by noting that Fe3+ coordination in oxide glasses is not yet clearly established. Thus there may exist more than one type of coordination site and so more than one distribution may be needed. N
N
41
Nonetheless, the Czjzek distribution is not expected to be appropriate for oxide glasses since it is based upon Newman's superposition model [17] and a point charge model of this kind is only expected to be of relevance when the structure is highly ionic as for the TMFG's. Even so, it has been noted recently that while the superposition model can be used to predict the 4th and 6th order crystal field terms for S-state ions, it is not reliable for the 2nd order terms as these require further correction [lo]. Thus, the ability to choose a set of fine structure parameters that adequately reflect the nature of the environment of the spin probe in a glass is not a simple task. For a system such as Cr3+, where g is essentially isotropic and the second order fine structure tensor is the only appreciable anisotropic interaction, the Monte Carlo method in triclinic symmetry is mathematically equivalent to a powder model with orthorhombic symmetry. This is because a set of randomly oriented 'non-coincidences with the lab frame' (C1-symmetry D tensors) is equivalent to an average over all Bo and B1 orientations with respect to the canonical directions of an orthorhombic D, provided one assumes there is no preferred orientation of D. However, the underlying physics of the two models is still different. For sites with symmetry lower than orthorhombic, there is no direct relation between the immediate surroundings of the ion and the second order tensor [18]. If there exist two or more anisotropic interactions then the stochastic and powder simulations will lead to different results since the stochastic model can have distributions of non-coincidences bewteen the different sets of principal axes in addition to a the random orientation of Bo and Trial runs for Cu2+ indicate that simulation of many spectra require that g and A have coincident principal axes and near axial symmetry (Figure 1). This is in accord with what is found in inorganic complexes, where, in a sense, copper usually constrains its own environment. So one presumes this means at the local level copper binding to the usual four oxygens in silicate glasses is near planar and not tetrahedral. This is a very interesting chemical inference on the basis of a few simulations. However, this must be weighed against the fact that these simulations have not so far taken into account any correlations between g and values or the tensor orientations. For = 512 and 712, inclusion of only an isotropic Zeeman interaction and the 2nd order fine structure tensor also leads to a powder-like result, for the same reasons as above. Furthermore, it produces sharp spikes on the main features in the simulated C W spectrum which naturally become more pronounced as the bin width is reduced. The inclusion of higher order terms removes the equivalence with a powder model by introducing another anisotropic interaction that can be randomised with respect to the 2nd order fine structure. The 4th order terms can smooth out the very sharp features obtained in their absence, as shown for = 712, for example (Figure 2). Note that the choice of distribution at this point is completely ad and has no physical basis. However we wish to indicate that the residual linewidth could in principal be substantially due to these higher order interactions. The pseudo-symmetry method of Michoulier and Gaite [19] represents one way of linking these terms to low site symmetries, but it is unclear whether this method could be meaningfully applied in our Monte Carlo model. There are perhaps too many random parameters to pursue these higher order terms in C1 symmetry.
42
Figure 1. Computer simulation of a Cu2+ system. = 1/2; = l/2; 9GHz; 300K; (Gaussian) (gz)avg= (gy)avg= 2.06; (gz)avg= 2.37; (standard deviation) Age = Agy = = = 0 . 0 0 2 ~ m - ~AA, ; = AA, = 0 . 0 0 0 5 ~ m - ~ ; Agz = 0.01; (Ax)avg = 0.014cm-'; AA, = 0.005cm-'; bin width=lG. Left: coincident g and A, 250000 sites; 4 000 000 transitions. __ Right: random relative orientation of the principal axes; 285 000 sites; 4 559 776 transitions.
2. EXPERIMENTAL Commercial silicate glass samples were provided by Mr Peter Lowe, Pilkington Glass, Melbourne. The ZBLAN glass samples were made by Dr Peter Newman and Professor D.R. MacFarlane within the Department of Chemistry, Monash University. Metaphosphate glass samples were obtained from Professor G. Saunders & Dr Richard Martin, University of Bath, UK. All EPR experiments were carried out with a Bruker ESP380E FT spectrometer equipped with an Oxford Instruments helium flow cryostat, a dielectric resonator and a 1kW TWT amplifier. 3. PULSEDEPR 3.1. Field-swept pulsed EPR of glasses The ideas presented above should also be directly applicable to the simulation of the pulsed field-swept spectra of glasses. However, the task is somewhat more complicated in instances of systems characterised by large fine structure interactions or large Zeeman anisotropy. Here transformation to the rotating-frame fails to remove the time-dependence from the Hamiltonian during a microwave pulse, due to the deviation of the electronic quantisation axis from the direction of the static magnetic field. The behaviour of the magnetisation is expected to be very complicated in such cases, with the apparent consequence that resonance features associated with large fine structure splittings are reduced or missing from the echo-detected EPR (ED-EPR) spectra. This has been observed for the geff 5 feature in Cr3+-doped ZBLAN [20], as well as the geff 4 resonance in Fe3+-doped silicate glasses, for example. [all showed that if the microwave (mw) field As a way around this problem, Oliete
-
-
43
0
2000
4000
6000
8000 (G)
0
2000
4000
6000
8000 (G)
Figure Computer simulation of an S-state ion system. = 9GHz; (Gaussian) = 0.056cm-'; (standard deviation) AD = 0.019cm-1; uniform 0 5 5 bin width=lOG. without addition of qth order fine structure; 55000 sites; 1071 transitions. Right: With 4th order terms, all -4 5 q 5 4 chosen randomly from same = Ocm-'; = 10-4cm-1; 500 sites; 999 transiGaussian distribution; tions. is a small perturbation and if the excited transitions do not share a common energy level, the problem can be reduced to a time-independent two-level system whose evolution can then be solved for using standard density matrix methods. One foreseeable drawback to this approach is the difficulty in relating the calculated magnetisation in the interaction representation at the time of echo formation with that measured by a phase detector synchronous with the mw field. For = every unitary transformation (rotation in Hilbert space) can be associated with a physical rotation in real space, because of the one-to-one correspondence between the real orthogonal group in 3-space and the unitary unimodular group in two dimensions. ie. a representation in the former corresponds to the introduction of a coordinate system in the latter. An example of this connection is afforded by the simple case of diagonalisation of the axially symmetric Zeeman Hamiltonian for an = system Appendix J.l.11, where the rotation of the coordinate system of the vector operator S through an angle about the y-axis in 3-space amounts to a rotation in the Hermitian vector space through = where is a Wigner rotation matrix. For a an angle ie. Hamiltonian expressed in the principal axis system of the g matrix, $ represents the angle in real space that the quantisation axis of the spin makes with the z-axis. however, a rotation in the complex vector space need not have anything For general to do with rotations in real space Only in special circumstances is it possible to associate a unitary operator R with a rotation in 3-space, the form of this operator being given by the Wigner rotation matrices in terms of the three Euler angles /3,
The Wigner rotation matrices possess the symmetry relation
R(J)* - (-1)
44
The rotating frame transformation represents a special case where
and = w t . However, a transformation to the interaction representation with = ePixot for general and ?to has no such rotational analogue in 3-space, since does not fulfill the symmetry property (2). This means that there is no simple way of relating the magnetisation in the rotating (detection) frame at the time of echo formation with that computed in the interaction representation except when g-anisotropy is small and when the high field approximation is valid. We conclude, therefore, that the method of Oliete [all is not a promising framework for the simulation of field-swept spectra in pulsed EPR for spin systems where the quantisation axis differs appreciably from the direction of the static magnetic field. 3.2. Quantum beat FID in glasses For most solids, a short, non-selective mw pulse will result in the decay of the freeinduction signal within the dead-time of the mw detection system. A selective mw pulse, however, burns a hole into a line whose width approaches the homogeneous linewidth. The frequency selectivity of the pulse has the advantage of generating a FID that can be used to obtain high resolution field-swept pulsed-EPR spectra in both ordered [24] and disordered systems [20]. Application of a mw pulse of appropriate duration and strength can result in electron polarisation (EP), electron coherence (EC) and nuclear coherence (NC). A number of two-pulse experiments are possible which exploit each of these to measure electron-nuclear cannot be applied to glasses, however, interactions [25-271. The method of Dzuba since it involves monitoring the FID following a non-selective mw pulse. In the present case, we observe superhyperfine (shf) interactions in disordered systems using a high-turning-angle microwave pulse. The free induction signal exhibits oscillations that are distinct from the oscillatory FID [28] observable in some inhomogeneously broadened systems. Figure 3 shows a rather good example with a 2ps rectangular pulse applied to a metaphosphate glass doped with 1%Ce3+, with oscillations appearing at the Larmor frequency of the phosphorous matrix nuclei as the pulse power is increased using the level potentiometer of the mw channel. The nominal flip angle Po = indicated next to each spectrum was estimated by comparing the (measured with the transmitter monitor) required to optimise a primary echo using a 7r/2 pulse of 1611s. This phenomenon is not unique to glasses. In polycrystalline y-irradiated alanine for example, the quantum beats due to remote protons can be seen superimposed upon the oscillatory FID (Figure 3(bottom right)). Here the nominal flip angle can easily be extracted from the number of transitory oscillations present in the FID [as]. Quantum beats have been observed for transition metal and rare earth ions in a number of silicate, fluoride and phosphate glasses at the frequency of remote matrix nuclei and should, in principal, be observable in any system possessing a large inhomogeneous linewidth. Like the FT-EPR detected NMR experiment [25,29], the quantum beat phenomenon relies on inhomogeneous broadening of the EPR line. A frequency selective mw pulse excites on-resonance forbidden transitions, which indirectly alters the population difference of the allowed transitions that share common energy level with forbidden ones. The
45
ensuing FID contains precession signals at frequencies characteristic of shf interaction splittings. A zero frequency component will also be present due to the excitation of spin packets with on-resonance allowed transitions. The depth of the oscillations is generally larger in the FT-EPR detected NMR experiment because the second non-selective mw pulse re-focusses the EP pattern created by the first pulse, taking advantage of the greater population difference that can be created for the allowed transitions by fully inverting the forbidden transition sharing the common energy level. The quantum beat FID is therefore expected to be less effective a t uncovering electronnuclear interactions than the FT-EPR detected NMR experiment. On balance, however, it is still an effect that requires some consideration, since it has the ability to distort a field-swept FID-detected spectrum in a manner analogous to the way in which the nuclear modulation effect distorts ED-EPR.
13n 30 (MHz)
1.3n
10
0
~-
90K. The R-T curve measured with I=5 mA is also shown in Figure 4 by single-dotted dashed 90 K again. This indicates that the curve. By this curve, Tc(R) is evaluated superconducting current pass is 16 not connected over long distance n above 90 K. Then MA signal 12 comes from the intragrain current above 90 K. U 8 The sample was annealed at A VI after the first transport measurement, then R-T was measured again with I=5 mA. The 3 result is shown in Figure 4 by a v1 0 dashed curve denoted by R(a). 80 90 100 110 Obviously Tc(R(a)) is shifted from T (K) 90 K to 93 K by this annealing. Figure 4. Comparison of MA signal intensities, The non-superconducting grain JC and R a function of T for the bulk. S,, SO boundaries are improved. This and SSOare defined in the inset. R(a) indicates clearly indicates that there were the resistance after the annealing. Resistance non-superconducting grain was measured with 5 mA. A bold solid arrow boundaries between 90 and 93 K and a bold dashed arrow indicate the “thermal before the annealing. The decoupling” and “magnetic decoupling”, periphery screening current had respectively.
-3
U
144
been thermally decoupled by these grain boundaries in the region of 90 < T < 94 K, leading to the intragrain screening current. When the transport current is increased from 5 mA to 150 mA, R-T curve is drastically changed shown in Figure 5. The result indicates that there are grossly two grades of grains. One TC of 90 K and the other has TC of 77 K. As increasing T, the weak links are decoupled above 77 K, and the strong links remaining above 77 K up to 90 K. This is a verification that there are strong links and weak links near TCin the sample even after the annealing.
Figure 5. Resistance vs T for the annealed bulk measured with 150 mA.
4. SUMMARY
MA measurements were done on the bulk sample. The low-field signal at the lower T decreases with increasing T due to the thermal decoupling of the periphery current. Then the signal arises from the periphery current at the lower T while from the intragrain current at the higher T than 90 K. This is supported by Jc-T curve and R-T curve both of which show Tc(Jc) and Tc(R) of 90 K. The high-field signal is much weaker than the low-field signal even at the lower T. This is caused by the magnetic decoupling.
REFERENCES 1. T. Takizawa, K. Kanbara, M. Morita and M. Hashimoto, Jpn. J. Appl. Phys. 32 (1993)
L774. 2. Y. Tanimura, T. Itoh, T. Takizawa, K. Kanbata, M. Morita and M. Hashimoto, Physica C 235-240 (1994) 3019. 3. Y. Itoh and U. Mizutani, Jpn. J. Appl. Phys. 35 (1996) 2114. 4. T. Endo and H. Yan, Jpn. J. Appl. Phys. 33 (1994) 103. 5. T. Endo, H. Yan, S. Nagase and H. Shibata, J. Supercond. 8 (1995) 259. 6. T. Endo and H. Yan, Studies of High Temperature Superconductors 14 (1994) 65. 7. T. Endo, S. Nagase, S. Sugiura, N. Hirate, M. Hone and S. Ymada, Physica C 282-287 (1997) 1591. 8. K. W. Blazey, K. A. Milller, J. G. Bednorz, W. Berlinger, G. Amoretti, E. Buluggiu, A. Vera and F. C. Matacotta, Phys. Rev. B36 (1987) 7241.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
145
Estimation of fluxon response-delay from magnetic field variations in superconductors using ESR system a R. D. Kalea, M. Tadaa, KI. Itoha, H. Kohmoto , S. Iwasakia, Y. Nakauea, T. Endoa and b S. V. Bhat
%acuity of Engineering, Mie University, Tsu, Mie 514-8507, Japan bDepartment of Physics, Indian Institute of Science, Bangalore-560 012, India Fluxon delay time zr from external field-rising is estimated by a sweep-reversal method of microwave absorption in superconducting YBa2Cu30, bulk sample at 77 K. 7,. initially increases and turns to decrease with increasing field sweep rate R,. The values of zr are estimated to be 20-120 msec depending upon R,. The initial rise reveals the true nature of fluxon delay while the following decrease is apparent one. The decrease in zr is caused by rapid reduction of the external field. 1. INTRODUCTION
One of promising large-scale applications of high temperature superconductors (HTS) is a high field magnet. A magnetic levitation is induced by high density of flux trapped in HTS bulks through application of external field [l-31. The trapped field is larger for the application of pulsed-fields than for that of slow-increasing and static fields [l-31. Therefore, in order to develop the highest superconducting magnet, we have to clarify the mechanisms of increasing and decreasing manners of flux in HTS bulks corresponding to the field increase and decrease. Especially the dynamic mechanism of flux trapping is the most important subject to be solved. The problem is not a flux creep but flux dynamics during the field variations and just after the field changes [4-61. Now ramp rates of the applied field have become an important experimental parameter in magnetization measurements [7-81. Yeshurun et al. [8] found the very important magnetization relaxation. Under the fast field sweep, so called second peak can be observed in Bi2Sr2CaCu20x crystals. While, this second peak gradually disappears with time. They suggest that there is a field region of fast flux relaxation. Takizawa group [2,9] and Ikuta and Mizutani group [1,10] have reported the delay times in detail. Judging from their results, an order of the fluxon delay time is 1-40 msec. These values are much smaller than an order of 0.5 sec estimated by a signal delay of microwave absorption after a field sweep-stop [4]. There must be some essential differences between these two kinds of experiments.
146
2. EXPERIMENTAL A sintered bulk sample of YBazCusO, (YBCO) was used in this work. Its T, is 93 K. Field modulated microwave absorption (MA) technique called “cavity perturbation method” was employed to estimate fluxon delay. The sample was placed in the center of cavity, and cooled down to 77 K. A dc magnetic filed Ha and modulation field H, were applied to the sample in the same direction. H, was fixed at 0.5 G and 100 kHz. Besides the dc and modulation fields, the microwave magnetic field H, is applied perpendicularly to Ha and H,. Therefore, screening currents I, induced by Ha and Hm flow near the surface of the sample as well as microwave-induced current. If the resultant current density exceeds some critical current densities in weak superconducting regions or weak links, reinforced microwave absorption takes place in the sample. See references [11,121 for the details. We have tried sweep-reversal method. The microwave absorption signal S is recorded against the increasing Ha with a sweep rate R,. The field is quickly reversed at some reversal fields H,, and the field is decreased with the same sweep rate R,. We can evaluate the transient field interval AH, and the corresponding fluxon delay time Tr by Tr=AH/ R,.
3. RESULTS The microwave absorption signals S are recorded under the upward and downward sweeps as shown in Figure 1 as a function of Ha for various R,. The value of H, is very small as 0.5 G. A horizontal dotted-dashed line shows the signal level when the field is reversed at H,, = 55 G. The whole signal behaviors are shown in references [4,6,11]. When R, is small enough as 3.3 G/s (a), there is no signal drop just after the field reversal. However, when R, is increased to 33.3 G/s (b), there observed the signal drop from the level just after the reversal. Then, the downward signal rises again with decreasing Ha according to the normal signal behavior as observed in (a). This drop behavior obviously evolves with increasing R, as observed in Figure 1. The evolution can be estimated by a depth of the drop AS (Figure l(e)) as a function of R, as shown in Figure 2 (a). AS steeply increases with increasing R, at the beginning, then increases gradually. We can judge that this drop certainly corresponds to a transient signal induced by delayed fluxon entrance into the sample from the external field. Because, when the field sweep is “stopped” at this field of H, the signal suddenly drops and decays with time. This signal decay is clearly shown in Reference [4], and its delay time constant z, is evaluatedas 0.5 sec. Therefore, we can estimate the delay time constant zrby this transient “drop” behavior for the sweep-reversal method. The transient time is defined as the time interval corresponding to the transient field interval AH between the reversal field Hma, and the field of signal minimum as indicated in Figure 1 (e). The value of z,. can be calculated by zr = AHRs. A time is scaled after the reversal in Figure 1 (d) along Ha axis. The zr is plotted in Figure 2 (b) as a function of R,. It increases rapidly at the beginning, shows a peak, then decreases with increasing R,.
147
Time ( s e d 0.1
0
(d)b 0
40
HdG) H = ,
[GI
3
Figure 1. The traces of signal S before and after the sweep reversal at H, = 55 G for various R, indicated. The time is scaled only for (d) R, = 133 G/s. The definitions of AS and zr are shown in (e).
4. DISCUSSION
4.1. Mechanism of signal drop and definition of z,. First of all, we have to explain why the signal drops sharply when the field sweep is stopped [4] or reversed in this work. There is a certain “viscosity” for a fluxon to move in a superconductor. If a static field is applied to the superconducting sample, a flux density distribution has U-shape inside the sample due to an exclusion nature. This is called “equilibrium distribution” hereafter. However, if the field is under rising with a certain sweep rate R,, the flux distribution inside the sample is different from the equilibrium distribution as schematically shown in Figure 3. It has “sharper” U-shape because the fluxon entering into the sample is delayed due to the viscosity. When R, is smaller as 33.3 G/s, the sharpness of U-shape is smaller. Whereas, with increasing R,, the sharpness should become larger as shown in the figure because of the larger delay of fluxon entering. This enhanced U-shape is called “dynamical distribution” hereafter. There must be screening current to exclude the flux, and its intensity is larger for larger R, because the stronger current is needed to exclude more fluxons from the sample. This can be understood also by a relation that the screening current intensity I, is proportional to the slope of flux distribution dB/dx. Then the intensity of “surface screening current” is proportional to the “surface slope”. The surface slope is larger for larger R, as mentioned above, then the signal intensity S must be larger for larger R, and I,, because our microwave absorption mechanism here is based on the “screening current
I
I
~
~
I
r
‘“*L i =, ,H ,
55G
0.04 0
0
200
400
600
Figure 2. The transient characteristics of (a) AS and (b) 7,. as a function of R,. mechanism” as mentioned in section 2. As a matter of fact, the signal S strikingly increases with increasing R, as shown in Reference It should be reminded, however, that these surface current and surface slope are exactly the dynamical ones. Then if the dynamical filed rising is ceased, the state with the dynamical surface slope and current gets back to the equilibrium one with the smaller surface slope and current. This leads to the smaller signal S within a certain delay time (the order of 0.5 sec) [4]. When the field is “reversed”, once the signal drop takes place in this transientperiod, then the signal recovers to its intrinsic nature after this transient period. These behaviors are clearly revealed in Figure 1. We characterized this transient time by the definition of in Figure 1. 4.2. Why z,. decreases According to the above interpretation, zr must keep increasing with increasing R, because the fluxon delay must be continuously enhanced. The experimental results of shown in Figure 2 (b), however, obviously differ from this expectation. It certainly increases steeply at the beginning but decreases in the larger R, region. AS shown in Figure 2 (a) also shows a kind of saturation in the larger R, region. It implies that there are some additional mechanisms which are responsible for the unexpected behaviors. We have two reasons for them, technical and thermodynamic ones. Technical reason : If the field sweep is stopped at the delayed fluxons can enter deep into the sample even in short time either in long time under the static field. This means that we can obtain the “true” delay time constant and it has considerably longer time of the order of 0.5 sec [4]. However, in this experiment, the external field Ha is suddenly swept back to the lower side as shown in Figure 3 by a dashed arrow. Due to this, two phenomena take place. The already entered fluxons near the surface in the sample lose force of magnetic pressure exerting them to the inside direction. Then they cease to move into the center of the sample. Furthermore, there is less supply of fluxons by the external field because Ha is less than in a short period after the reversal. Then the fluxons cannot enter into the sample
149
Figure 3. Model flux density (B) distributions in the sample for various sweep rates R,. The solid curves show the U-shape distributions as a function of sample position x when Ha reaches H, (the reversal field). The dashed curves show the distributions just after the reversal. The delayed fluxons can still enter into the sample just after the reversals as shown by arrows for the smaller R,. Whereas, the delayed fluxons cannot enter into the sample, rather they get out from the sample as shown by an arrow for the larger R,. Thus the delayed fluxon entrance is interrupted by the reduction of field for the larger R,. The upward sweep of Ha is shown by a solid curve and the following downward sweep by a dashed curve at the right hand side. from the outside additionally which ought to be entered if Ha is fixed at Hma. For these two circumstances, the real delay time is hidden. With increasing R,, these two phenomena are more enhanced, leading to the decrease in This indicates that the decreasing manner of is apparent and not the “true” nature. Thermodynamic reason : The fluxons are always moving in the viscous medium to the inside direction in the sample during the upward field sweep. Then there must be some heat generation in the sample. This heat generation is larger for larger R, due to the larger velocity of fluxons. Then the real sample temperature is more raised with increasing R,. The temperature rise reduces the viscosity of fluxons, resulting in the more gradual surface slope of flux distribution. This indicates the smaller difference between the equilibrium and dynamical distributions, then the shorter delay time of z,. From the above two discussions, the initial steep rise of zr in Figure 4 (b) shows the true nature of delay phenomenon, however, the following decreasing manner of z,. is the apparent nature. Thus the real delay time must be much larger in the larger R, region as 0.5 sec as obtained by the sweep-stop experiment. In the larger R, region, say R, = 533 GIs, the
150
value of zr is around 20 msec. This value well corresponds to the values of 1 - 40 msec obtained by pulsed-field experiments [1,2,9,10]. This implies that the delay time of fluxon estimated by the pulsed-field method has some uncertainty caused by the same technical reason.
5. Summary The fluxon delay time was estimated on the superconducting YBCO bulk sample by the sweep-reversal method of the microwave absorption. The signal S suddenly drops just after the field reversal of the upward sweep due to the entrance of delayed fluxons into the sample. Then it recovers to the inherent signal behavior under the downward sweep. The delay time 7,. is defined by this transient signal behavior, and increases initially but turns to decrease with increasing Rs. The initial increase in zr reveals the true nature of fluxon delay but the following decrease in is the apparent nature. There are two reasons for this decrease, the technical and thermodynamic reasons.
REFERENCES 1. Y. Itoh and U. Mizutani, Jpn. J. Appl. Phys., 35 (1996) 2114. 2. T Takizawa, K. Kanbara, M. Morita, and M. Hashimoto, Jpn. J. Appl. Phys., 32 (1993) L774. 3. Y. Tanimura, I Itoh, T Takizawa, K. Kanbara, M. Morita and M. Hashimoto, Physica C, 235-240 (1994) 3019. 4. T. Endo, S. Yamada, M. Horie, N. Hitate, K.I. Itoh and Y. Tsutsumi, Adv. Supercond., X (1998) 179. 5. J. Yamada, V.V. Srinivasu, M. Tada, K.I. Itoh, A Hashizume, I. Kometani, K. Anwar and T. Endo, Adv. Supercond., XI1 (2000) 353. 6. K.I. Itoh, A Hashizume, H. Kohmoto, M. Matsuo, T. Endo and M. Mukaida, Physica C, 357-360 (2001) 477. 7. S. Anders, R. Parthasarathy, H.M. Jaeger, P. Guptasarma, D.G. Hinks and R. van Veen, Phys. Rev. B, 58 (1998) 6639. 8. Y. Yeshurun, N. Bontemps, L. Burlachkov, A. Kapitulnik, Phys. Rev. B, 49 (1994) 1548. 9. T. Itoh, Y. Tanimura, T. Takizawa and K. Kanbara, Jpn. J. Appl. Phys., 34 (1995) L810. 10. T. Terasaki, Y. Yanagi, Y. Itoh, M. Yoshikawa, T. Oka, H. Ikuta and U. Mizutani, Adv. Supercond., X (1998) 945. 11. T. Endo and H. Yan, Jpn. J. Apl. Phys., 33 (1994) 103. 12. T. Endo and H. Yan, A.V. Narlikar (ed.) Studies of High Temperature Superconductors, 14, Nova Science, New York, 1994, pp. 65-106.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
151
Coexistence of ferromagnetism with superconductivity in RuSr2GdCu208 from ESR measurements Koji Yoshida, Masatoshi Nakamura, Naoto Higashi and Hajime Shimizu Department of Electronics and Computer Science, Tokyo University of Science in Yamaguchi, Onoda, Yamaguchi 756-0884, Japan
We have investigated the magnetic properties of RuSr2GdCu2Os by means of ESR measurements. The Gd EPR spectra suggest the predominant antiferromagnetic correlation with a small ferromagnetic moment. We found that the Ru FMR spectra consist of two components, suggesting the mixed valency in the Ru sites. In addition, we will report the coexistence of superconductivity with the magnetic order on a microscopic scale.
1. INTRODUCTION A possible coexistence of a long-range ferromagnetic order with superconductivity in RuSr2RCuzOs = Gd, Eu, Y Ru1212) has provided us a fascinating opportunity in the study of the high-T, cuprates. The hybrid ruthenium-copper oxides have an analogous crystal structure to YBazCu307, where the CuO chains are replaced by the RuO2 planes. The superconductivity is realized predominantly in the CuO2 planes, while the magnetic order is present in the RuOz planes. However, there is still no consensus for the magnetic structure, which even seems to depend on experimental probes. In the first stage of the study for Rul2 12, results of the pSR and magnetization measurements indicated ferromagnetic order of the Ru moments [I]. On the other hand, neutron diffraction measurements revealed the antiferromagnetic correlation along the c axis among the Ru ions, together with a field-induced ferromagnetic component [2]. Recently, from the NMR measurements, the magnetic ordering with a ferromagnetic component exists within the RuOz planes even in the zero field [3]. In order to settle this problem, we investigate the magnetic and superconducting properties of RuSrzGdCuzOs using an ESR technique. ESR is a good probe to study the magnetism on a microscopic scale because there is no uncertainty for a position where the internal magnetic
152
field exists. 2. EXPERIMENTAL
Polycrystalline samples of RuSrzGdCu20~were prepared by the solid-state reactions. The stoichiometric mixtures of RuO2, SrCO3, Gd203 and CuO is calcined in air at 960°C. After being palletized, they were preheated in a flowing nitrogen atmosphere for 48 h to minimize impurity inclusions. And then, the palletized samples were sintered in a flowing oxygen atmosphere at 1020, 1030, 1040 and 1050°C for a total of 236 h with intermediate grindings. The X-ray diffiaction analysis indicates no impurity phase. The onset of the superconducting transition determined by resistivity @) measurements is 45 K, as shown in fig.1. At TCo=28K,the zero resistivity is obtained. We observed a small hump structure in p at the magnetic ordering temperature the X band (- 9GHz) were performed between diffused in parafin.
= 133 K. ESR measurements using and 300 K for fine powder samples
3. RESULTS AND DISCUSSION
Figure 2 shows the typical magnetic resonance spectra of RuSr2GdCuzOg for various temperatures. The spectra can be roughly classified into two groups. One is an absorption signal near 0.32 T seen at all temperatures, and the other is an absorption structure at low fields below about We will show that Ru1212 exhibits the antiferromagnetic correlation with a small ferromagnetic component and the coexistence of superconductivity with the magnetic order on a microscopic scale. 3.1. Gd signal
First, we consider the resonance signal around 0.32 T (g = 2). This absorption originates from the electron paramagnetic resonance (EPR) of Gd3+ ions. Its derivative line can be fitted well by a Lorentz resonance curve, suggesting importance of the exchange interaction, in addition to the dipole-dipole interaction.
We plot the temperature
dependence of the peak-to-peak line width AHppin Figure 3. It follows a relation of AHpp(T) = (l+B/T)AH(w)+bT, where B is the Weiss temperature and AH(w) the high-temperature limit of Hpp(T). The second term is based on the Korringa relaxation process through the exchange interaction between Gd3+ ions and conduction electrons within the Cu02 planes. This term is absent in the previous work by Fainstein el [4]. The first term describes a divergent increase of AHppat low temperatures as a slowing down of the Gd spins toward the antiferomagnetic transition = 2.5 K) of Gd3+[2].
153
0.15 I
h
E
I
7&290K
I
I
0.10 -
.
v
0.05
0
RuSr,GdCu,O,
+
28K
0
100
300
200
Temperature (K)
1 OK
Figure 1. Temperature dependence of resistivity in Ru1212.
0.2 0.4 0.6 0.8 Magnetic Field (T)
0
Figure 2. Typical magnetic resonance spectra of RuSr2GdCu208for various temperatures.
25001 1000, , , I
I
I
~
I
120
RuSr,GdC%O,
~~
0 I
0
I
I
I
I
I
50
100
150
200
250
Temperature(K)
0 300
200
100
300
Temperature(K)
Figure 3. Temperature dependence of the peak-to-peak line width AHpp.It is replotted
Figure 4. Temperature dependence of the Gd3+ internal field f i which
on an enlarged scale in the inset.
is defined as
=
ff,Gd(300 K) -
Here, ffr,Gd is the Gd3' EPR resonance field.
ffr,Gd(T).
154
It is meaningful that AHppexhibits a negative jump at as shown in the inset of Figure 3. We assume that it is caused by a decrease of where and correspond to the dipolar and the exchange field, respectively. The change of the dipolar field at the Gd sites accompanied by the magnetic ordering of the Ru moments could be mainly responsible for the discontinuity. In this case, because antiferromagnetic order in the Ru sites reduces the dipolar field at the Gd sites, our results suggest the predominant antiferromagnetic correlation in the Ru sites. A similar decrease in AHpp at has been reported in Gdz,CexCu04 [ 5 ] . Figure 4 shows the temperature dependence of the Gd3+internal field which is defined as = H,~d(300K) - Hr,Gd(T). Here, Hr,Gd is the Gd3' EPR resonance field. The appearance of HIbelow indicates the possible existence of a ferromagnetic component because each dipolar field induced by just antiferromagnetic ordered Ru moments cancels out at the Gd sites which are located in the body-center position of the Ru sublattice. is at most 60 Oe, which is about 10 times as small as that reported by Fainstein et [4]. We guess that such the small HI is due to a weak ferromagnetic correlation, for example, which is due to a canted spin structure induced by a rotation of the Ru06 octahedra. In fact, neutron measurements revealed the rotation of the octahedral around the c axis. However, we cannot rule out other possibilities, for example, of a ferrimagmetism. As for superconductivity, we cannot observe large changes in the Gd3+ ESR spectra through the superconducting transition. 3.2. signal It is of great importance to investigate the magnetism at the Ru sites directly. We assign the resonance structure at low fields developed below (Figure 2) to the Ru ferromagnetic resonance. It could correspond to the excitation of the k = 0 magnon of the weak ferromagnetic component. An important finding is that the resonance consists of the following two components, which is clearly Separated at around 110 K, as shown in Figure 5. (i) One is a sharp resonance signal near the zero field, and (ii) the other is a broad signal at higher fields. These signals are also observed in RuSr2EuCu208 (not shown), suggesting a common feature among Ru1212 compounds. Figure 6 shows the temperature dependence of the resonance fields for the two absorption spectra. In the signal (i), decreases with lowering temperature, together with broadening of the line width. Below about 80 K, the signal changes to a non-resonant zero-field absorption, indicating the microwave energy below the magnon gap. On the other hand, of the signal (ii) increases with lowering temperature. Below about 100 K, the signal (ii) is difficult to confirm with our resolution. The most fascinating scenario is a possibility of the mixed
155
135K 130K 127K
800
1
t
1
I I
(. ii .)
' 1
110K 79K 60K
2 o o ~ l $ * l
30K 28K 1OK
0
0 60
0.05 0.10 0.15 0.20 Magnetic Field (T)
80
I
1
100 120 7M140 Temperature (K)
Figure 6. Temperature dependence of the resonance fields in the Ru FMR.
Figure 5. Variations of the Ru FMR spectra with temperature.
T=50K 40K
30K 28K 16K 13K 1OK
I
I
I
I
I
,
0
0.08 0.12 Magnetic Field (T)
Figure 7. Variations of derivative curves with across
156
valence states of the Ru ions. NMR measurements show the charge segregation of the RuSt (60%) and Ru4’ (40%) sites as a result of the charge transfer of holes from the Ru02 planes to the Cu02 planes [3]. We conjecture that two kinds of the Ru resonance signals result in the difference of the anisotropic field due to the different valence states, while the mechanism still remains unclear for the anomalous temperature dependence of Hr,~”.In addition, the FMR signal appears below 140 K > This suggests a field-induced ferromagnetic component. Figure 7 shows the variations of Ru derivative curves with below 50K. While there is = 45 K, we found an anomalous structure below Tco = no obvious change across 28K. It could be relevant to the motions of self-induced vortices which originats from fluctuations of the FM component [3]. The superconductivity clearly affects magnetic properties in the Ru sites, suggesting the coexistence of the magnetic order with superconductivity on a microscopic scale. 4. SUMMARY
We have investigated the magnetic properties of RuSr2GdCuzOg by means of ESR measurements. We suppose that the discontinuous drop of AHppat and the small in the Gd EPR are caused by the antiferromagnetic correlation with a small ferromagnetic component. The two kinds of the Ru FMR near zero-magnetic field suggest the mixed valence states of the Ru ions. In order to clarify the magnetic structure of Ru1212, we are planning to perform ESR measurements using single crystals. The change of the dx’’ldH curve below TCoclearly shows the coexistence of superconductivity with the magnetic order on a microscopic scale in Ru1212. REFERENCES 1. C. Bernhard, J. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Brucher, R.K.
Kremer, D.R. Noakes, C.E. Stronach and E.J. Ansaldo, Phys. Rev., B 59 (1999) 14099. 2. J.W. Lynn, B. Keimer, C.Ulrich, C. Bernhard, J. Tallon, Phys. Rev., B 61 (2000) R14964. 3. Y. Tokunaga, H. Kotegawa, K. Ishida, Y. Kitaoka, H. Takagiwa and J. Akimitsu, Phys. Rev. Lett., 86 (2001) 5767.
A. Fainstein, E. Winkler, A. Butera and J. Tallon, Phys. Rev., B 60 (1999) R12597. 5. H. Shimizu, S. Suzuki and K. Hatada, Physica C, 282-287 (1997) 1379.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Ferromagnetic resonance La-Ba-Mn-0 thin films
and
157
intragraidintergrain
crystallinity
in
S. Iwasaki", J. Yamada", H. Kohmotoa, M. Tada", Y. Sakurai", T. Endo" and B. J. Reddyb aFaculty of Engineering, Mie University, Tsu, Mie 5 14-8507, Japan bDepartment of Physics, Sri Venkateswara University, Tirupati-5 17 502, India
La-Ba-Mn-0 thin films were deposited by ion beam sputtering at 750°C with supply of oxygen molecules or plasma at oxygen partial pressures PO of 0.5, 1.O and 1.5 mTorr. Intergrain crystallinity (mosaicity) is the best at 1.O mTorr while intragrain (plane-distance) crystallinity is worst at 1.O mTorr. The as-grown films show ferromagnetic resonance only for Po=l .O mTorr. Therefore, the ferromagnetic property can be obtained in the film with aligned crystalline planes among grains (higher mosaicity) rather than in the film with equal plane-distances.
1. INTRODUCTION
Perovskite manganite oxides are extremely unique system in materials so called "strongly electron-correlated system". In the system, various physical properties such as magnetism, conductivity and lattice distortion, are entangling with each other. One of the resulting appearances is colossal magnetoresistance (CMR) which is a magnetically-induced metal-insulator transition [ 11. It is strongly expected that tunable microwave filters composed of stacked manganite/superconducting oxides can be applied for mobile communication system. To realize these devices, the as-grown manganite thin films should be ferromagnetic, and problems existing in interface should be cleared. There have not been so many papers reporting the as-grown ferromagnetic manganite thin films. Most of the papers showed that the ferromagnetic property can be obtain by annealing [2-41. In this work, Lal.,Ba,MnO, (LBMO) system was selected from the many of its
158
family. The reason is that this material has high potential for device applications, because it has an extremely high Curie temperature of 355 5 K then it must be easy to control its temperature-dependent magnetic properties. As the first stage of LBMO/YBCO double layer fabrications, in this work, LBMO thin films were grown at a high temperature aiming to get as-grown ferromagnetic films. To elucidate the magnetic properties, ferromagnetic resonance (FMR) measurement was done on these thin films. Then, correlation between FMR and film crystallinity was investigated, especially in terms of intergrain and intragrain crystallinities. We discuss ferromagnetic anisotropy arising from in-plane and out-of-plane FMR signals.
2. EXPERIMENTAL LBMO thin films were deposited on MgO (1 00) substrate by ion beam sputtering. A target of Lao 94Bao 39Mn10, was sputtered by 4 keV Ar’ ion beam. The substrate was heated by lamp heaters, and substrate temperature Ts was monitored during the deposition using a thermocouple. In this experiment, Ts was fixed at a high temperature of 750°C. During the deposition, either oxygen molecules (ML) or oxygen plasma (PL) was supplied. The oxygen plasma was produced in a plasma source by discharge of oxygen gas at -1 kV at 60 Hz, and it was emitted fiom nozzles to the substrate. The oxygen molecules were supplied through the same plasma source without the discharge. The flow rate of oxidants was controlled then an average oxygen partial pressure PO in a chamber was adjusted to be 0.5, 1.O and 1.5 mTorr. The deposited films were characterized by X-ray diffraction (XRD) using Cu K line in two manners, 0 -2 8 scan and -scan (rocking curve). Then, two sorts of crystallinity were estimated. One is called “intragrain crystallinity” hereafter in this paper, it indicates a distribution of crystalline plane-distances in grains whose crystallographic plane is parallel to the substrate plane. This was estimated by 8 -2 8
XRD peak half-width A
The other is called “intergrain crystallinity” or “mosaicity” hereafter, it indicates a distribution of grains which have different directions of crystallographic plane from the substrate plane. This was estimated by XRD rocking curve half-width A Ferromagnetic resonance (FMR) was measured on the as-grown films to characterize their ferromagneticy and spin properties using ESR apparatus. The sample was cooled to liquid nitrogen temperature (77.3 K) in the TE102 cavity, and dc magnetic field Ha and modulation field H, ( 5 G) were applied on the sample. Ha was swept with
159
its direction parallel (Ha 11 plane) and perpendicular (H,lplane) to the film plane. Microwave frequency was 100 kHz and power was 0.1 mW.
3. RESULTS AND DISCCUSION First of all, we characterized the two sorts of film crystallinities, A and A w. The results are shown in Figure 1 as a function of Po both for the molecular (ML) and plasma (PL) supplies. However, we do not mention here about exact differences between ML and PL, rather grossly grasp behaviors of Po-dependences of A and A for ML and PL in a bundle. The films show a “V-shape” behavior of A ”, i.e., the minimum of A at Po=l.OmTorr. It indicates that the mosaicity is the best at 1.0 mTorr. Whereas, the films show a ‘‘A -shape” behavior of A B , i.e., the maximum of A at 1.0 mTorr just except for the ML-film at 1.5 mTorr. It indicates that the intragrain crystallinity is the worst at 1.O mTorr. As a result, the intragrain and mosaic crystallinities reveal “inverse correlation”. This must be arised from lattice mismatching between the substrate and film. The grains on the substrate grow independently with each other reflecting no exact substrate lattice information. Therefore, the optimum oxygen partial pressures for the intragrain and mosaic crystallinities are not necessarily the same FMR measurements were done on these films. The results are that the films deposited at 1.0 mTorr both for ML and PL show FMR signals thought the films at 0.5 and 1.5 mTorr do not at IFMR: x o x all, as indicated by a circle and crosses 0.3 at the top of Figure 1. These results surprisingly correspond well to the 9 4 3 above mentioned behaviors of 0.1
I
2 . 4
0
0
0
0.5
1
1.5
Po [mTorr]
2
Po-dependences
of
the
two
cristallinities. Then the appearance of film ferromagneticy must strongly related to the film crystallographic
Figure 1. and A, as a function of Pofor ML and PL-films. O...FMR
structure. The films which show FMR signals
yes, X . .FMR no.
have poor intragrain crystallinity but
160
excellent mosaicity. Then the important factor must be the mosaicity rather than the intragrain crystallinity. This indicates that the film with smaller deflections of the crystalline plane can obtain the ferromagneticy. If the crystalline plane is aligned, spins must also be aligned in one direction, originating the ferromagneticy. If there are grains with much deflected crystalline plane, these spins must disturb the spins in the aligned grains. Then the film loses the ferromagneticy. On the contrary, the spins in the grains with slightly different plane-distances do not affect so much on the aligned spins in the regular grains. Resonance fields H, in the two films are plotted in Figure 2 for the field directions of Ha 11 plane and H,Iplane. The apparent H, is lowered from the true resonance field for Ha 11 plane due to a flux concentration, and the apparent HI is raised from the true one for HaIplane due to a strong demagnetizing effect. Comparing ML-film and PL-film, H, for Ha 11 plane is smaller and HI for H,Iplane is larger for PL-film. This indicates that PL-film has the stronger ferromagnetic property, and this is directly known also from FMR peak intensity I p shown in Figure 3. The both of Ip for Ha 11 plane and H,Lplane are distinguishably larger for PL-film than for ML-film. A mechanism of this plasma effect is not known at present. Half-widths r p p of FMR signals are plotted in Figure 3 together with Ip for Ha I( plane and H,Iplane. The half-widths are not so much different between ML and PL, and between Ha (1 plane and H,Iplane. Then Ip directly reveals spin orientation ability. The spins are more easily oriented when the field is applied perpendicularly to the film plane. This anisotropy must arise from the crystalline structure and orientation to the substrate. By the way, it is not shown here but effective magnetization becomes larger
-
4
1.5 7 51
Y
1
II
Figure 2. H, against Ha 11 plane and H a l plane for ML and PL-films.
Figure 3.
rpp
and Ip against Ha 11 plane
and Ha plane for ML and PL-films.
161
contrarily for Ha 11 plane than for H,Iplane when the measurement temperature is raised. An exact origin is now under investigation.
4. CONCLUSION LBMO thin films were fabricated by IBS on MgO at 750°C with supply of oxygen molecules or plasma at Po=0.5, 1.0 and 1.5 mTorr, and their intragrain and mosaic crystallinities were estimated. The mosaicity is the best at 1.O mTorr while the intragrain crystallinity is the worst at 1.O mTorr. The as-grown films deposited at 1.O mTorr only showed FMR signals, indicating that the ferromagnetic nature can be preferably obtained in the film with higher mosaicity. The resonance field of FMR is lowered for
Ha 11 plane but it is raised for H,Iplane, and the indication that PL-film has larger ferromagneticy than ML-film was obtained. Actually the FMR peak intensity Ip is much larger for PL-film than for ML-film. Its origin is now under investigation. Ip, then ferromagneticy is notably larger for HaIplane than for Ha 11 plane. This indicates that the spins are more easily oriented perpendicularly by the field application.
REFERENCES 1. K. Miyano and Y. Tokura, Solid State Physics, 34 (1999) 637 (in Japanese). 2. X. Zhu, W. Si, X. Xi, Q. Li, Q, Jiang, and M. Medici, Appl. Phys. Lett., 74 (1999) 3540. 3. K. Choi and Y. Yamazaki ,Jpn. J. Appl. Phys., 38 (1999) 56. 4. R. Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Phys. Rev. Lett., 71 (1993) 2331. 5 . J. Yamada, M. Tada, A. Hashizume, H. Kohmoto, E. Takahashi, S. Shiomi, T. Endo, J. Nogues, J. S. Munoz and T. Masui, Trans. Mat. Res. SOC.Jpn., 26 (2001) 1049. 6. J. Yamada, M. Tada, H. kohmoto, A. Hashizume, Y. Inamori, D. Morimoto, T. Endo, J. M. Colino and J. Santamaria, Trans. Mat. Res. SOC.Jpn., 26 (2001) 1053.
162
EPR in the 2 1'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Temperature dependence of paramagnetic resonance in pure and doped ferrihydrite nanoparticles' A. Punnoose and M. S. Seehra Physics Department, West Virginia University, Morgantown, WV 26506-63 15, U. S. A. Temperature variation (5 - 300 K) of the magnetic susceptibility and electron paramagnetic resonance (EPR) spectra are reported for nanoparticles of ferrihydrite (FeOOH nH2O) and the ferrihydrite (FHYD) doped with 5% of Ni, Mo and Ir. The FHYD nanoparticles (nominal size = 4 nm) order antiferromagnetically at TN = 350 K and carry a magnetic moment = 300 p~ per particle due to uncompensated surface Fe3+spins. EPR data for FHYD taken at 300 K and 9.24 shows a single line at g = 2 with linewidth H = 750 Oe. Doping with Ni lowers AH whereas AH is increased substantially on doping with Mo and Ir. The temperature variations of AH and the resonance field shift SHr were measured for FHYD and Ni/FHYD. Both samples show that AH increases and line shifts to lower magnetic fields according to SH, = (AH)3, in agreement with the predictions for randomly oriented superparamagnetic particles. Below the blocking temperature Tp(= 71 K for FHYD and 48 K for Ni/FHYD), more rapid changes in SH, and AH are observed due to progressive freezing of the uncompensated spins.
1. INTRODUCTION The nature of magnetism in antiferromagnetic nanoparticles (AF-NP) is expected to differ substantially from their bulk counterparts because of the dominant role of the surface magnetic ions with lower coordination and broken exchange bonds [l]. Because of this and the potential applications of NP, the physics of systems has become a major area of focus in recent years [ 2 ] . Ferrihydrite (FHYD) nanoparticles with the generic formula FeOOH nH2O occur naturally in the 3-5 nm size range and they are easily synthesized [3]. Recent neutron diffraction and magnetic studies have shown that the FHYD-NP orders antiferromagnetically at TN 2: 350 K and carry a magnetic moment = 300 pB/particle, due to the uncompensated Fe3+surface spins [4]. Using the XAFS (x-ray absorption fine structure) spectra of FHYD, Zhao et al [5] pro osed that Fe3+ions in the core of FHYD-NP are coordinated by six groups with Fe3+-FeP+distance of 3.01.k On the other hand, Fe3+on the surface have only tetrahedral coordination, leading to
1
This research was supported in part by the U.S. Department of Energy (contract #DE-FC2699FT40540).
163
the surface uncoordinated sites. In the FHYD-NP, nearly 30% of total Fe3+ ions are on the surface. In recent papers, changes in the magnetic properties of FHYD-NP produced by doping FHYD with Si (for x = Si/(Si + Fe) = 0.05, 0.10 and 0.15) have been reported using magnetometry [6], Mossbauer studies [7] and electron paramagnetic resonance (EPR) [8]. Since Si favors the tetrahedral bonding characteristic of surface Fe3+,these changes could be explained by Si substituting for surface Fe3+ions. Here we report studies on the temperature variation of the magnetization and EPR spectra in FHYD doped with 5% of Ni, Mo and Ir.
2. EXPERIMENTAL DETAILS The procedures for preparing these samples are described elsewhere [7]. The room temperature x-ray diffraction of all our samples yielded the familiar two broad lines characteristic of 2-line FHYD [3,4], about which various earlier studies have been reported [3-81. For EPR studies, we have used a standard reflection-type X-band (9.25 GHz) spectrometer, with a Varian microwave cavity and a variable temperature cryostat system obtained from Oxford Instruments. Measurements of magnetization as a function of temperature were done on a commercial superconducting quantum interference device (SQUID) magnetometer.
3. RESULTS AND DISCUSSION The temperature variation of the magnetic susceptibilities for undoped FHYD and FHYD samples doped with 5% Ni, Mo and Ir is shown in Figure 1. In these measurements, the samples were zero-field cooled (ZFC) to 5 K and a magnetic field H = 100 Oe was then applied and data
4
8-
21
E
x
6.
4-
2-
0
50
150 200 250 TEM PERATUR E(K)
100
4
0
Figure 1. Temperature variations of the magnetic susceptibility under the zero-field-cooled conditions at H = 100 Oe for FHYD (pure) and FHYD doped with 5% Ni, Mo and Ir.
164
were then taken with increasing temperatures, after temperature becomes stable at each point. A detailed analysis of the magnetization data on these samples as a function of temperature (T) and magnetic field (H) will be reported elsewhere [9]. Here it suffices to note that and T, (the temperature at which peaks) change on doping with Ni, Mo and Ir. This is likely related to the decrease in the magnetic moment per particle, as the dopants replace Fe3+[9]. For T, < T < TN,the vs. T data can be explained by superparamagnetism[lo].
I
1
.
8
,
1
I 5800
I ,800
MAGNETICFIELD (Oe) Figure 2. Room temperature EPR spectra of the four samples at 9.24 GHz. The inset shows the measured peak-to-peak linewidths AH.
From the room temperature EPR spectra for the four samples shown in Figure 2, it is evident that AH (the peak-to-peak linewidth in the absorption derivative) is lowered for Ni/FHYD as compared to pure (undoped) FHYD, whereas AH is increased substantially for doping with Mo and Ir. Since the core of FHYD-NP orders antiferromagnetically(AF) below TN = 350K [4], no EPR signal is expected from the core Fe3+ spins for T < TN. Therefore, the observed EPR signals must be due to uncompensated surface Fe3+ spins which are also responsible for the observed vs. T variations of Figure 1 [4, 91. In magnetic materials, AH H:mx where Ha and &, are respectively the strengths of anisotropy (e.g. dipole-dipole interaction) and exchange interactions [ll]. For NiRHYD, Ha is expected to decrease because Ni2+has a lower moment ( ~ 3 . 2 compared to that for Fe3+( ~ 5 . 9 resulting in lower AH even if there is no change in &,. A similar decrease in AH has been observed in thin Fe films on
-
165
coating with Ni [12]. For Mo and Ir doped FHYD, decrease in between Fe3+-Fe3+due to intervening Mo and Ir ions is likely to be the reason for the observed increase in AH. This is because in insulators is considerably shorter in range than the dipole-dipole interaction.
"11
- 3200
3000
h
-2800 Z
2600-
g
%
8
-rn
2000-
W
-2400
1500-
6 2
i..
A
1000
-
500~
'
I
0
50
'
I
100
'
150
I
'
200
I
'
I
'
I
~
250
I
TEMPERATURE (K) Figure 3. Temperature variations of the linewidth and resonance field for the FHYD and Ni/FHYD samples at 9.24 The lines joining the points are for visual aid. Temperature variations of the EPR spectra were investigated for two samples, pure FHYD and NiEHYD, both showing similar variations. Large increases in the linewidth AH and shifts of the resonance field H, to lower fields is observed with decreasing temperatures for both samples (Figure 3). The linewidth of Ni/FHYD is lower than that of pure FHYD throughout the temperature range but the resonance field differs only at lower temperatures. On approach to the peak temperatures T, of the samples (71 K for FHYD and 48 K for Ni/FHYD), the signals shift extensively to the lower fields and only the high field part of the signal could be observed below T,, making accurate determination of AH and H, impossible. A dramatic increase in the width of the signal could be inferred from the extensive changes observed in the position of the right peak, illustrated in Figure 4. This may be due to the progressive freezing of the surface spins expected below T, if T, is the blocking temperature. Nagata and Ishihara [13] have shown that
166
0
50
100
150
200
250
300
TEMPERATURE (K)
Figure 4. Temperature variation of the right peak position (shown in the inset) for pure FHYD. For comparison, the temperature variation of magnetic susceptibility is also shown, demonstrating the rapid broadening of the EPR line below Tp. the shift in the resonance field 6Hr and the linewidth AH of a superparamagnetic system with a statistical distribution of sizes and shapes varies as
6H,
(AH)'.
A power of n = 2 is predicted for partially oriented particles and n = 3 for randomly oriented particles. Plots of log AH verses log 6H, using the data above Tp yield n = 2.97 for pure FBYD and n = 3.19 for Ni/FHYD (Figure 5), clearly suggesting randomly oriented particles. In summary, the results presented here show that doping of FHYD-NP produced substantial changes in the measured EPR spectra and magnetic susceptibility due to the substitution of the
167
.
I
2.8
-
I1
Pure
-B-- Ni'FHY[ FHYD
II
,
I
I
3.4
.
Log AH
Figure 5. Plot of log SH, vs log AH for the FHYD and Ni/FHYD samples. Slope yield n = 3 in 8H, (AH)".
-
dopant for surface Fe3+. The temperature variation of the EPR spectra fit the model of Nagata and Ishihara for randomly oriented superparamagneticparticles.
REFERENCES 1. R. H. Kodama and A. E. Berkowitz, Phys. Rev. B, 59 (1999) 6321. 2. J. L. Dormann and D. Fiorani (eds.), Magnetic Properties of Fine Particles, Elsevier Science, Amsterdam, 1992; G. C. Hadjipanayis and R. W. Siege1 (eds.), Nanophase Materials: Synthesis, Properties, Applications, Kluwer, Dordrecht, 1994. 3. See the review by J. L. Jambor and J. E. Dutrizac, Chem. Rev., 98 (1998) 2549. 4. M. S . Seehra, V. S. Babu, A. Manivannan, and J. W. Lynn, Phys. Rev. B, 61 (2000) 3513. 5. J. Zhao, F. E. Huggins, Z. Feng, F. Lu, N. Shah and G. P. Huffman, J. Catal., 143 (1993) 499. 6. P. Jena, S . N. Khanna and B. K. Rao (eds.), Cluster and Nanostructure Interfaces, World Scientific, Singapore, 2000 ( pp. 229-234). 7. J. Zhao, F. E. Huggins, Z. Feng and G. P. Huffman, Phys. Rev. B, 54 (1996) 3403. 8. M. S . Seehra, A. Punnoose, P. Roy and A. Manivannan, IEEE Transactions on Magnetics, 37 (2001) 2207. 9. A. Punnoose, M. S . Seehra, N. Shah and G. P. Huffman, Phys. Rev. B (to be published). 10. M. S . Seehra and A. Punnoose, Phys. Rev. B, 64 (2001) 132410. 11. T. G. Castner and M. S . Seehra, Phys. Rev. B, 4 (1971) 38. 12. P. Lubitz, M. Rubinstein, D. B. Chrisey, J. S . Honvitz and P. R. Broussard, J. Appl. Phys., 75 (1994) 5595. 13. K. Nagata and A. Ishihara, J. Magn. Magn. Mater., 104-107 (1992) 1571.
168
EPR in the Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR study of Fe-Si02 granular films Kazuaki Kanazawa ‘, Kouichi Matsuda ‘,Seitarou Mitsudo Sigeo Honda a
Toshitaka Idehara
Faculty of Engineering, Fukui University, Fukui 910-8507, Japan
Research Center for Development of Far-Infrared Region, Fukui University, Fukui 910-8507, Japan Interdisciplinary Faculty of Science and Engineering, Shimane University. Fe-SiOZ granular films having Fe volume fractions smaller than 45 % are known to be characterized by the tunneling giant magnetoresistance and superparamagnetic nature [ l ] . The magnetization and magnetoresistance (MR) curves can be fitted by the summation of two Langevin functions [2]. On the other hand, the ferromagnetic thin films and ferromagnetic ultrafine particles are studied with electron spin resonance (ESR) measurement technique. Because granular films exhibit properties of both thin films and ultrafine particles, X-band ESR measurements have been performed in Fe-SiO2 granular films with Fe volume fractions of 21, 25, and 29 vol.%. Based on the results of angular dependence measurements of ESR absorption line, films with Fe volume fractions of 25 and 29 vol.% have shown behavior of a typical ferromagnetic film. However, based on the results of temperature dependence measurements of ESR absorption line, the film with Fe volume fraction of 21 vol.% could be characterized by superparamagnetism.
1. INTRODUCTION There is extensive interest in studying magnetoresistance after the discovery of the giant magnetoresistance (GMR) in Fe/Cr multilayers [3], and in magnetic heterogeneous alloys with ferromagnetic granules embedded in a nonmagnetic metallic matrix [4]. In Fe-Si02 granular films, magnetic Fe particles are imbedded into the insulator film of Si02 at the nano scale. The bias voltage effect, the temperature effect, etc. have also been reported for Fe-Si02 granular films. Experimental magnetization curves of
169
Fe-SiOz granular films can be fitted by assuming that there are magnetic particles of 2 or 3 different sizes present in these films. To further investigate magnetic nature of Fe-SiOz granular films, it is important to perform ESR measurements.
2. EXPERIMENTS X-band (9.159 GHz) ESR measurements have been performed on Fe-Si02 granular films with Fe volume fractions of 21, 25 and 29 vol.%. The films of about 8000 A thickness were used in the experiments. The diameter of particles is estimated to be 9 -16 A based on the magnetization measurements. To study thin film properties of Fe-SiOz granular films, the angular dependence of ESR absorption was measured. Magnetic field applied parallel to the sample plane corresponds to the angle of 0" in Figurel, and magnetic field applied perpendicular to the sample plane corresponds to the angle of 90" in Figure 1. Angular dependents were measured at the room temperature. On the other hand, to study properties of ultrafine particle for Fe-SiOz granular films, the temperature dependence of ESR absorption was measured. The temperature was varied from 50 to 300 K, and the magnetic field was applied parallel to the sample plane.
Fe-SiO,
/A
f = 9.159 GHz
film plane
29 vol.% 25 vol.%
a
vol.%
Y
1
a
0 2 vl 1
Figure 1. Orientation of the dc magnetic field H and of the magnetization M with respect to the coordinate system used in the calculations. The film is parallel to the x-z plane.
20
40
Angle (degrees) Figure 2. The angular dependence of ESR resonance field.
170
3. RESULTS AND DISCUSSION 3.1. The angular dependence The results of the angular dependence measurements of ESR absorption for all three Fe volume fractions are shown in Figure From these results it follows that the long-range interaction works between Fe particles, and overall granular films have ferromagnetic nature. A quantitative analysis of the results was also performed. The geometrical configuration of the problem is shown in Figure 1. The energy density function corresponding to a system with hexagonal structure, up to the second-order term in the magnetic anisotropy, is then given by the expression 1 cos(p, - r p e q ) + Z ( 4 ~ 2 ) s i nrpcq 2 - (K, + 2K,)sin2 rpeq + K, sin4 p e q ,
=
(1)
where the first term represents the Zeeman energy, the second term is magnetic energy, and the last two terms represent the axial anisotropy energy with the c-axis parallel to are the first- and second-order anisotropy constants. The static the y-axis. Kl and equilibrium position of the magnetization is given by the relation, sin(q, - qcq) =
-
The resonance field and Beljers [6],
-
)sin rpeqcosqeq+
sin’ rp, cos
.
(2)
can be calculated by using the general equation derived by Smit
[s]:=L[e.e-(&)2]. ae2 aq2
If Equation (1) is substituted into Equation
2I:[
=
cos(rpH - rpeq
+ ( ~ J ~ J-M
)cos 2 p +
sin2 peq cos2 peq- sin 4 cpeq)]x
cos(rp, - pCq) - ( 4 n -~
)sin2 peq -
]
sin 4 rpcq
where This gives a general formula of the resonance field for a ferromagnetic film. For the granular film, it is thought that crystal axes were randomly oriented. Therefore, substituting into Equation (2) and Equation the following equations for the balance of magnetic field and moment,
and resonance condition,
171
Table 1. The g-value and value of 4xM Fe volume fractions 29 vol.% 25 vol.% 21 vol.%
n-value 2.05 2.06 2.05
4x M 0.616 T 0.437 T 0.272 T
are obtained. Equation (5) and (6) were used to fit experimental data of the angular dependence of the resonance field, and theoretical curves are plotted together with experimental data in Figure 3. The g-value and value of 4 x M were obtained from Equation (6) as shown in Table 1. The obtained g-value is the same value of pure cx -Fe bulk. It means that there is no enhancement of a Fe moment. As can be seen from Figure 3, the angular dependence of ESR absorption line for granular films with Fe volume fractions of 25, 29 vol.% are well fitted by equations for a ferromagnetic thin film. However, the results obtained for the Fe 21 vol.% film are not in good agreement with a thin ferromagnetic film model.
1.0
2
1.0 -
I,::::::.;::::-/--25vol.%
,
Q 1.0 2.5
,
,
,
,
,
,
,
,
u
--&-
29vol.%
0
20
40
60
80
100
cp,,,(degrees) Figure 3 . Comparison of the experiment data 0 (Experiments) and fitting results (Calculations) for the angle dependence of resonance field.
100
200
300
temperature Figure The results of the temperature dependence of resonance field 0 ( I f r ) and line width ( ,,) in ESR measurements. ~
172
3.2. The temperature dependence The results of the temperature dependence of ESR absorption line measurements for all three Fe volume fractions are shown in Figure 4. As the decrease of temperature, the resonance field H, shift to low field side and the line width H,, increases. These features can be explained qualitatively by assuming superparamagnetic character of Fe-SiOz granular films. Figure 5 shows relation between the shift of resonance field 6H, and half line widthAHpp on double logarithmic scale. As seen in figure, the relations for the samples with Fe volume fractions of 29, 25, 21 vol.% are on a straight lines with slopes =4.6, 2.5 and 1.7, respectively. According to results reported by Nagata et al. [7], for the partially oriented elliptical particles, the resonance field shift will be proportional to ( A HPp)’. However, it particles are randomly oriented, the shift will become as ( A Hpp)3.For the films with Fe of 21 vol.%, the same dependence for the partially oriented elliptical particle was observed. However, for the films with Fe of 25 vol.% and 29 vol.%, it seems that the exponent qualitatively can’t be understand by same point of view. Because, the results of angular dependence experiments suggest that the ferromagnetic interaction ranges overall a films, for these Fe volume fraction films.
I
I
A
3000 4 0 0 0 Hpp(Gauss)
Figure 5. Experimental 6 Hr vs. line width a Hppdependence and straight line fitting results. is exponential
173
REFERENCES 1. S.Honda, T.Okada, M.Nawata, M.Tokumoto, Phys. Rev., B 56 (1997) 14566 2. T.Okada, T.Umemoto, M.Nawate, S,Honda; Tec. Rep. IEICE. MR96-87. 3. . N .Baibich, J.M.Broto, A.Fert, J.Nguyen Van Dau, F.Petroff, P.Etienne, G.Creuzet, A.Friederich and J.Chazelas, Phys. Rev. Lett., 61 (1988) 2472. 4. S.Mitani, H.Takanashi, H.Fujimori, Solid. phys., 32 No4 (1997) 231. 5. C.Chappert, K.Le Dang, P.Beauvillain, H.Hurdequint, Phys. Rev., B 34 (1986) 3192. 6. J.Smit, H.C.Beljers, Philips Res. Rep lO(1955) 113 7. K.Nagata, A.Ishihara, J.Magn.Magn.Mater., 104-107 (1992) 1571
174
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Oxygen dependent evolution of C ~ OEPR + signal in fullerene thin films Alexander I. Shames ', Eugene A. Katz Svetlana Shtutina ', Wojciech Kempinski ', Szymon Lo6 ' and Lidia Piekara-Sady ' a
Department of Physics, Ben-Gurion University University of the Negev, P.O.Box 653, 84 105 Be'er-Sheva, Israel The National Solar Energy Center, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84 990 Sede Boqer, Israel
'Institute of Molecular Physics, Polish Academy of Science, ul. Smoluchowskiego 17, 60-179 Poznan, Poland
Time evolution of c60+EPR signal (g = 2.0026) under the air/t-oom-light exposure was studied on two identical c 6 0 films of 200 nm thickness on a glass substrate. One of these fullerene films was covered by thin Au layer. For both samples the time-dependent signal growth may be divided into at least two clearly distinguished regions: the region of the fast growth at 0 < t < 1 hour and slow growth region at t > 1 hour. The slow growth rate for the Au covered sample was found about 3 times slower in comparison with the non-covered one. The rate of the EPR signal growth depends on the rate of oxygen diffusion at both early stage (fast grain-boundary diffusion) and following stage (intra-grain diffusion). The long time kinetics measured on the covered sample is one of the best evidences that the evolution of C6[ EPR signal depends only on the rate of oxygen diffusion but not on any other external or internal factor. EPR signal kinetic curves were recorded during in-sifu pumping of the noncovered film sample. The removal of air from the hllerene film leads to a decrease in the amount of EPR active C6[ centers. Opening to air caused an immediate jump of the signal's peak height. No signal restore under the nitrogen pressure was found. The role of oxygen and various models of fullerene-oxygen paramagnetic centers in solids are discussed.
1. INTRODUCTION
Solid c 6 0 is a molecular crystal with Cso molecules occupying, at room temperature, the sites of a face-centered cubic (fcc) lattice [l]. While the carbon atoms within each CSo molecule are held together by strong covalent bonding, weak van der Waals interactions are the dominant intermolecular forces [2]. Therefore well structured c 6 0 crystals and thin films in as-grown state are practically EPR silent [3 - 61. On the other hand, an aidlight exposure leads to the appearance of a sharp (AHpp- 0.1 mT) EPR signal characterized by the Lorentzian lineshape and g = 2.0026. This signal was attributed to c60+centers located in the bulk of the film
175
and generated due to oxygen diffusion from air into c 6 0 film [3]. In a joint EPWSPV experiment we demonstrated recently [4] that the airhight exposure of c 6 0 films leads to the generation of the c60+paramagnetic centers (PCls) and deep acceptor states at = + 1.3 eV. These acceptors act as recombination andor scattering centers. The paramagnetic and recombinatiodscattering centers were suggested to have the same origin. Furthermore, the time development of that EPR signal under airhight exposure of the hllerene film was found to be strongly dependent on the film crystalline structure and consists of two clearly distinguished regions of "fast" and "slow" growth. Improvement of the film structure leads to a deceleration of the "fast" growth. The results were explained assuming the EPR signal growth is controlled by oxygen diffusion, along grain boundaries and into grains, during the "fast" and "slow" periods, respectively [5]. In the present paper we suggest additional experimental arguments directly proving the hypothesis that the c60+Pck appear due to oxygen diffusion into the hllerene bulk. On the one hand, we demonstrate that covering of CSOfilm by a thin Au layer, which slows down the actual amount of air reaching the film, considerably reduces the EPR signal growth. On the other hand, by an in-situ experiment we clearly show the effect of air pressure on the EPR signal while no effect of nitrogen partial pressure was found.
2. EXPERIMENTAL c 6 0 thin films with a thickness of about 200 nm were grown on optical glass substrates of 3.5 - 20.0 mm2 area using a vacuum evaporation of CSOpowder (Hoechst AG 'Super Gold Grade', 99.9%). The vacuum chamber pressure was maintained at about 8 x Torr. Two samples (S1 and S2) were grown under identical conditions: the deposition rate was 6 3 s and the substrate temperature 300 K. However, for the sample S2, on completion of the vacuum deposition process the hllerene film was covered by the thin (1 10 nm) Au layer. Immediately after the sample preparation completed, both film samples were placed into EPR silent Wilmad quartz tubes (5mm 0.d.). Then the tubes were filled with Ar gas and sealed. It should be noted, however, that during this procedure the samples were exposed to air for a short time (of about 3 minutes). All procedures of the films' growth and transferring into the EPR tubes were done under the dark conditions. Finally, the tubes were kept in the dark before and at the first stages of EPR measurements. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) characterization shown that both samples (S1 and S2) have the same highly dispersed polycrystalline structure with average grain sizes of about 20 nm. EPR spectra were recorded using a Bruker EMX-220 X-band digital spectrometer at room temperature. The amplitude of 100 modulation and the microwave power level were chosen as 0.1 mT and 200 KW, correspondingly, for preventing saturation and obtaining better signal-to-noise ratio. Kinetic EPR measurements were carried out in Time Scan mode when the external magnetic field was fixed at the peak of the corresponding EPR signal. EPR signals' processing was done using Bruker WIN-EPR Software.
3. RESULTS
In accord with our recent results [3 - 61, c 6 0 samples in as-grown state were found to be practically EPR silent. Further aidlight exposure leads to the appearance of a sharp (AHpp=
176
0.124 f 0.005 mT) Lorentzian EPR signal with g = 1 2.0026 k 0.0002. Meanwhile no other EPR signals 1200 were observed. Since both line-shape and line- v m width of this signal are found to remain the same during the exposure, the peak-to-peak intensity of this signal is an adequate measure of the total amount of PC's in the samples under study. Figure m s o 1 displays peak-to-peak intensities of the EPR 0 0 20 40 60 80 .signal as a function of the light/air exposure time v) Time (davsl for samples S1 and S2. The first experimental points (at time, t = 0, Figures la, b) were measured in the dark conditions for the samples, sealed into the Ar-filled tube and. It is clearly seen that the S1 sample demonstrates weak, but quite distinguishable, EPR signal while the S2 sample 40 Noise Level (covered by the Au layer) exhibits a very weak EPR signal with the intensity slightly surpassing 20 0.0 0.2 0.3 0.5 0.7 over the noise level. The second experimental point Time (hours) was obtained after the film samples, remaining in Ar filled tubes, were exposed to roomlight Figure 1. Long time kinetics of the irradiation for several minutes. The following c60+EPR signal intensity obtained points were obtained after the Ar gas was removed on samples S 1 (open circles) and S2 from the tubes and both samples were open to air (closed circles) : (a) whole kinetics; and kept under the same ambient room-light (b) zoom of the first hour. conditions. Starting from this point, the EPR signals from both samples smoothly increase. However, the rate of such a growth was found to be significantly slower for the Au-covered sample S2. As we previously reported [5], the removal of oxygen from fullerene film leads to the decrease in the intensity of the c60+EPR signal. For a further clarification of the role of oxygen in creation of c60+ centers, we performed the long time in-situ pumping experiment. Sample S1, exposed to airllight for about half a year, was placed into the quartz tube connected to a diffusion pump Torr) through a three-way valve. The third input of that valve was connected to a vessel containing pure NZ gas, or could be opened to air. Then the quartz tube was placed into the cavity of the EPR spectrometer and strictly fixed there. The kinetics of the peak EPR signal intensity was recorded before, during and after the in-situ pumping (Figure 2). The whole spectrum of c60+signal was recorded before and after the pumping as well as at several intermediate points between each 45 minutes time scans. Figure 2a clearly demonstrates the moments, when the pump was turned on and when the sample was opened to air. The EPR signal intensity begins declining immediately after the pump was connected to the sample tube (f = 0 hours). During the first 1.5 hours of pumping the peak of the EPR signal drops for about 30 % of its initial value. For justification that such kinetics corresponds to the real decrease in the amount of PC's, we plotted at the same graph data, obtained by the double integration of EPR spectra, recorded at intermediate points (open circles). The latter shows that the decline of the EPR signal peak is mostly governed by the decrease in amount of EPR active centers rather than by the instability of resonant conditions during a long time experiment (without using fieldlfrequency lock accessories). Then the I
t
+
h
L
177
sample was pumped for additional 20 hours. Before the new kinetic measurement, the EPR spectrum was recorded again and the new kinetics has begun. One can clearly see that the long 20-hour pumping did not lead to the firther decrease of the amount of c60+ PC's. Five minutes later the pump was disconnected from the sample tube and the sample was open to air. It resulted in an immediate jump of the signal peak intensity. The EPR signal practically 20 40 60 Time (min) reaches its initial (before pumping) value Figure 2. kinetics of the c60+EPR signal and remained constant till the pumping started again. Then the cycle intensity obtained on sample S 1: (a) long time pumpinglopening - to - air was repeated. pumping (line - normalized intensity measured The EPR spectrum, recorded after all open circles - normalized double cycles had been done, indicates the integrated intensity, obtained between kinetics initial amount of PC's is instantly recorded in-situ); (b) under pumping and restored when the sample is subjected to nitrogen pressure. the ambient air pressure. The latter happens disregarding the number of preceding pumpinglopening-to-air cycles. Another experiment was undergone for elucidation of the effect of partial pressure of nitrogen on the EPR signal kinetics. Result of this experiment is presented in Figure 2b. Before the pump was opened, the EPR signal had not changed. Just after the rotary pump (- lo-' Torr) was opened, the signal declines for about 6% for 30 minutes. Then the pump was closed and the sample exposed to the nitrogen gas at pressure of 75 Torr. The signal kinetics shows no changes in general trend of the signal decay. In a few minutes the nitrogen pressure was removed and the tube was opened to air. It is interesting to note that in this case, contrary to the previous experiment, the kinetics curve did not jump upward. Then the pump was opened again and the pumping continued for another 15 minutes to remove the rest of oxygen and air. The effect of the next two pumpinglopen-to-air cycles looks the same as the result of the previous experiment shown in Figure 2a. Surprisingly, at each cycle the kinetics curve jumps to the level that is a little bit lower than the signal level at zero moment. On the other hand, the previous double integration data show that the EPR signal hlly restores its initial value. Such a decrease in the maximum peak value, obtained after the multiple cycling during the kinetics measurement, may be explained, presumably, by minor changes in the resonance conditions, originating in changes of the sample's position because of the pressure cycles. 4. DISCUSSION
Analysis of the first points in the long time kinetics of the EPR signal justifies the model of PC's with g = 2.0025 2.0026 in hllerene solids, which has been proposed in [3 - 61. Summarizing all discussions, such a PC's may be described as a hole localized in the close vicinity of the hllerene molecule, namely c60+The . conclusion on the hole nature of the radical
178
observed is based on the g-factor value which is higher than that for a free electron g, = 2.0023 [3], as well as on the fact of the quenching of this signal in result of the intercalation of our samples c 6 0 by such strong electron donor as an alkali metal [6]. The aforementioned c60+ center appears when the fullerene solid (either film or powder) is exposed to the simultaneous impact of air and visible light. (Here we will not discuss appearance of EPR signals due to photopolymerization of fullerenes under strong irradiation). Let us consider the starting points for the kinetics shown in Figure 1. The first, dark measurements reveal weak but non-zero EPR signals. However, even non-zero initial signals have acceptable explanation within the framework of the proposed model. Indeed, both technological and EPR recording processes assume that all samples were undergone to a short contact with air and, possibly, exposed to weak light (from the computer display, for instance). Accordingly, we suggest that initial non-zero signal may be due to oxygen diffusion during that short period (between the film growth and the beginning of the EPR measurement). As we have mentioned above, the rate of the EPR signal growth depends on the rate of oxygen diffusion at both early (fast grain boundaries diffusion) and following (intra-grain diffusion) stages. In turn, the diffusion rate depends on the crystalline structure of the film sample, i.e. grain sizes [ S ] . However, both samples under study have the same crystalline structure. The only difference is a thin Au layer covering the surface of the S2 sample, which slows down oxygen diffusion into the fullerene film. The latter results in a significant difference in the initial signals as well as the long time kinetics for both samples. The long time kinetics (Figure la) of the S1 sample is very similar to that observed for a lot of c 6 0 thin films studied [3 - 61. EPR signals in all these samples had tendency to reach saturation in about 80 days. It was supposed [2, 31 the latter reflects mostly intra-grain oxygen diffusion. On the other hand, the long time kinetics for the S2 sample is one of the best evidences that the growth of C6: EPR signals depends only on the rate of oxygen diffusion but not on any other external or internal factor. Indeed, both samples of the same crystallinity (S1 and S2) were kept together and, correspondingly, undergone the same ambient conditions. Nevertheless, in the Au-covered sample S2 the actual amount of air reaching the film was considerably less. This hampers the diffusion rate of air oxygen. Let us analyze the in-situ kinetics. Figure 2a shows the decrease in air pressure causes reduction of the amount of c60+centers. It was reliably proved by consequent detection of the signal’s integral intensity. The rate of this reduction depends on the pumping efficiency and, as it was shown in Reference [ S ] , sample’s crystallinity. However, in a few hours the number of C6: centers reaches levels down and the signal remains the same even after very long time pumping. In the present experiment (the vacuum of Torr) the amount of c 6 0 + centers dropped for about 30% of its initial value. In the recent experiment, described in [S], the vacuum was of about lo-* Tom and the signal was found to be leveled after the signal dropped for 17%. It seems us that at each actual pressure value the fullerene-air system exists in an equilibrium that is manifested in a certain value of C60+ centers detected by EPR. The experimental results presented at Figure 2b evidences that only oxygen, but not nitrogen, affects the amount of c60+centers. Indeed, when the sample, undergone to a certain pumping, was open to the pressure of nitrogen, the signal reduction rate slowed down but the tendency remained. Moreover, when, after the nitrogen pressure, sample was opened to air, no drastic fast increase in signal intensity was observed, as observed, as ill previous experiments on opening to air. Only the additional pumping of excess nitrogen turned the system into its “normal” condition and then opening to air led to its usual effect (Figure 2b). We can suppose
179
this result may be explained in terms of nitrogen that blocks internal pathways for the diffusion of oxygen and prevents oxygen molecules restoring their initial locations. Another conclusion, which may be reliably drawn from the results presented by Figure 2 is that, on opening the sample to ambient air, the EPR signal restores its initial value for a few seconds - practically immediately (within the time scale of the appearance of this signal). The actual time of signal recovery depends, most likely, on the length of the pathway (tube, valves) needed for air to reach the sample. This fact sounds the most paradoxical in the present study. Indeed, several months were required for the oxygen molecules to occupied (by the diffusion at normal atmospheric pressure) those specific sites, which are suitable for the hllereneoxygen system to produces (26: centers. And only a few moments were needed to restore the initial situation, which were created during very long time! The simplest model, which may describe the EPR signal reduction on pumping, is that the pumping significantly removes oxygen from the hllerene system. Though, the fact of the fast recovery of the EPR signal strongly contradicts that model. It is hard to imagine that oxygen returns into the film bulk. Such a situation does not look a relevant one. The most probably, the main role in the appearance of c60+ centers plays a partial pressure of oxygen. The latter creates such conditions that are energetically more favorable for the localization of holes, generating by ambient light irradiation, in the vicinity of hllerene molecules. Taking into account a very long lifetime of c60+PC's (years at ambient conditions, as stated in [I]), the positively charged hllerene-oxygen complex is very stable one but only in condition when oxygen pressure is stable as well. Any change in a partial pressure of oxygen leads to the destruction of the complex and release of the localized hole. When the localization conditions are restored, the hole trapping is favorable again and the sharp increase in EPR signal intensity is observed. The last point should be mentioned here, is that in all pressure experiments we did not succeeded in reduction of all C6: PC's. It means the airAight exposure creates also more stable hllerene oxygen complexes that could not be destroyed by changes in a partial oxygen pressure.
REFERENCES 1. R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhia and S.M. Zahurak, Phys. Rev. Lett., 67 (1991) 1886. 2. J.P. Lu, X.-P. Li and R.M. Martin, Phys. Rev. Lett., 68 (1992) 1551 , 3 . A. Shames, E.A. Katz, S. Goren, D. Faiman and S. Shtutina, Mater. Sci. Eng., B 45 (1997) 134. 4. E.A. Katz, D. Faiman, B. Mishori, Yoram Shapka, A. I. Shames, S. Shtutina and S. Goren, J. Appl. Phys., 84 (1998) 3333. 5 . E.A. Katz, A.I. Shames, D. Faiman, S.Shtutina, Y. Cohen, S. Goren, W. Kempinski and L. Piekara-Sady. Physica B, 2738~274(1999) 932. 6. W. Kempifiski, L. Piekara-Sady, E. A. Katz , A. I. Shames and S. Shtutina, Solid State Commun., 114 (2000) 173.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
180
Molecular orientations in Langmuir-Blodgett and vacuum-deposited films of VO-phthalocyanine Yuhei Shimoyama
*
Department of Physics, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040-8567, Japan Langmuir-Blodgett (LB) and vacuum-deposited (VD) films of vanadyl tetra t-butyl phthalocyanine (VOTBP) were investigated by electron paramagnetic resonance (EPR) to elucidate their film structure and molecular organization. Angular variation of the EPR spectral position revealed that VOTBP in the LB and VD films orients perpendicular and parallel to the substrate, respectively. The EPR linewidth indicated a two-dimensional spin chain in the LB films and a three-dimensional in the VD films. The order parameters evaluated from the EPR magnetic parameters demonstrated a moderate degree of ordering in both the films.
1. INTRODUCTION Phthalocyanines have long been used as active materials for molecular devices such as chemical sensors [ 11. The thin film techniques for phthalocyanines include vacuum deposition (VD), the Langmuir-Blodgett (LB) method [2], and spraying of a fine suspension, some of which involves procedures with low reproducibility [3, 41. The physical properties of vanadyl phthalocyanine, however, has received less attention than other transition metal phthalocyanines, mainly because of the complex vanadyl cation and the difficulty in obtaining well-defined thin films. Electron paramagnetic resonance (EPR) has been used for the evaluation of thin film structure and physical properties of transition metal phthalocyanines The angular dependence of EPR spectra yields information on the molecular orientation of paramagnetic moieties in the thin films. The EPR linewidth also provides information about the molecular interactions, where the mode of the spin-spin interaction determines the dimensions of the spin chain in the thin films [6-81. The dimension of spin chain in organic molecules have been utilized to analyze the anisotropic physical properties. EPR spectra have been used to evaluate the order parameter of organic molecular systems, such as liquid crystals [9, 101. The order parameter makes it possible to estimate the molecular ordering in the aligned molecular system. In the present paper, we report on the variety of molecular orientations, the spin chain systems and the order parameters found in the LB and VD films of vanadyl phthalocyanine.
"E-mail:
[email protected] 181
2. EXPERIMENTAL Vanadyl tetra t-butyl phthalocyanine (VOTBP: Wako Chemical) of 99% purity was used as the film material. The organic solvents were all spectral grade (99.9%, Kishida Chemical). The substrate used for the LB and VD films was high purity quartz glass (spracil, Eikou-sya) chosen to minimize EPR signals from impurities. The LB films were prepared using a Langmuir trough (Kyowa Kaimenkagaku: LB-5). Ultra pure water (Organo: puric-Z) with a purity of 18 MQcm was used for the subphase. A benzene solution of VOTBP was used as the developing solution that was spread onto the subphase. Since the breaking pressure is 30 mN/m, we transferred the monolayer onto a glass substrate at 20 mN/m by the standard vertical dipping method at a speed of 2.5 mm/min. Y-type deposition was achieved in all cases with transfer ratios of 0.7-0.8 for both the up and down strokes. The transfer ratio was calculated from the area of transferred Langmuir (L)-monolayers divided by the moving area of the substrate on the water surface. The VD films of VOTBP were prepared in a vacuum of Pa using a conventional bell-jar system (Hitachi: HUS-5GB). The VOTBP powder was placed in a tungsten boat with a perforated cover. We controlled the heating current using a stabilized power supply. The VD films were deposited onto the glass substrates at room temperature. The thickness of the VD films was approximately 100 nm as measured by optical absorption. Electron paramagnetic resonance (EPR) measurements were carried out by an EPR spectrometer (JEOL: JESFELXG) operating at the X-band (9.3 GHz) with a homemade goniometer. The resonant magnetic field was 3202100 mT and the microwave power was 196 mW. The angle between the film normal and the applied magnetic field 8 was varied from 0 to 2x. All the EPR measurements were carried out at room temperature. Since the EPR lines were strongly overlapped, we used the line intensities instead of the linewidths. Since all our EPR spectra show Lorentzian lineshape, we employed the reciprocal relationship between the line intensity and the square of its linewidth.
3. RESULTS AND DISCUSSION 3.1. EPR spectral feature Figures l(a) and (b) show the EPR spectra of the LB films of VOTBP. The eight peaks from the hyperfine structure (hfs) of vanadium ion (I = 7/2) were exhibited in both directions of the magnetic field (0 = 0" and 90") [5]. A wider hyperfine coupling was observed at the magnetic field parallel to the film surface, whereas the narrower hfs was observed at the perpendicular position. This indicates that VOTBP is oriented in the LB films with a significant degree of order. In contrast, the EPR spectra of the VD films [Figures l(c) and 1(d)] showed the reverse relationship between the hfs coupling and the magnetic field direction. The wider hfs coupling was found at the magnetic field normal to the film surface. Those anisotropic hfs couplings suggest that the difference in the molecular orientations is ca. 90" between the LB and VD films. In the VOTBP molecule, the VO axis is located at the center of macrocycle and is normal to its plane. The hfs tensor yields the maximum splitting when the magnetic field is parallel to the VO axis. The parameters of the LB and VD films gave similar & and go values, although the magnetic parameter resulted in different values at various orientations. Since the hfs of the LB films yielded the maximum splitting when the magnetic field was applied parallel to the films, hence VOTBP is oriented perpendicular to the substrate. The molecular orientation in the VD films has parallel orientation, in contrast to the LB film.
182
fH
20mT
Figure 1. EPR spectra of the LB (a: 90" and b: 0") and the VD (c: 90" and d: 0") films of VOTBP. 0 is defined as an angle between the film normal and the applied magnetic field.
3.2. Spin chain systems The linewidth of the EPR spectra is relates with the spin-chain system [6]. In the spin diffusion model, the spin chain in the two- (2D) and the three-dimensional (3D) systems were defined by the following equations [6-81. AH1,2 is the half-width at half-height, and 8 is an angle between the molecular axis and the magnetic field.
2D:
= (3cosz0 -1)'
3D:
= (1+ cos20)
The angular dependence of the linewidth the line intensity defined in the experimental section) of the EPR spectra of the LB film is shown in Figure 2(a). The open circles are the experimental linewidths and the solid line is the best-fit curve. The fitting to the angular variation of the linewidths of LB films reveals the 2D spin chain system. This means that the LB films have a strong in-plane interaction and a weak interlayer interaction. The angular dependence of the linewidths of the VD films is shown in Figure 2(b). The curve fitting resulted in the 3D spin chain.
3.3. Order parameter The angular dependence of the g-value of the EPR spectra may reveal the molecular orientation and distribution in the thin films. Therefore, when the g-value was hard to determine, we instead utilized the ordcr parameter to define the molecular ordcring. The spin Hamiltonian is a linear combination of Zeeman term, hyperfine structure and spin-spin
183
1 0.9 -u N 0.8
2 0.7
m .4
0.6 v
0.5
Y
2 0.4
.4
5
0.3
2 0.2 .0.1
0 -3.2 -2.4 -1.6 -0.8 0 0.8 1.6 2.4 3.2
Angle [rad] Figure 2. Angular variations and the best-fit curve of the linewidth of EPR spectra recorded from (A) the LB and (b) the VD films of VOTBP.
interaction. The hyperfine structure and the Zeeman term are linearly independent of the spin-spin interaction. Thus, we can treat the order parameter, even that from the broadening signal, through the spin-spin interactions. The degree of order, the order parameter of liquid crystals as introduced by Saupe [9], for the first time, defines the following equation
s
t
=-(3 c o s * q -1). 2
(3)
Here 0i is an instantaneous angle between the i-th principal axis and the director, and < > denotes an ensemble average. The order parameter of angular order ranged from zero, the random orientation, to unity, axial orientation. We employed the same definition for the ordering in the thin films [lo]. Experimental order parameters are summarized in Table I. The LB films indicated an order parameter of 0.71, which means good ordering along the director, an ensemble-averaged axis. The VD films exhibited an order parameter of 0.72, which is the same order of magnitude as that for the LB films. It is quite possible that the cluster of VD films possess similar order as the domains of LB films. The order parameter indicated that the VOTBP molecules align along a certain axis.
3.4. Molecular orientation In the process of L-monolayer formation, VOTBP eventually forms a monolayer even if it does not have a hydrophilic moiety in the molecule. In fact, the limiting area of L-monolayer is equal to the cross section of VOTBP, indicating that the molecule in the monolayer is Table 1 Order parameters and director angles of LB and VD films as determined by EPR. Films SII 0 (deg) SI 0 (deg) VOTBP-LB 0.72 26 -0.44 78 0.71 26 -0.46 81 VOTBP-VD
184
oriented perpendicular to the water surface. Transfer and stacking of the L-monolayers by the vertical dipping method leads to the formation of the LB films. This process conserved the perpendicular orientation inherited from the L-monolayers. The EPR spectra also indicated that molecular orientation in the LB films is perpendicular, which is illustrated in Figure 3(a). The spin chain network in the LB films yielded a 2D system, which reflects an in-plane organization. The EPR measurements of the VD films revealed the parallel orientation of VOTBP. Figure 3(b) illustrates the plausible structure. Similar structures have been reported in the epitaxial films of phthalocyanines other than VOTBP deposited on the surfaces of KCI and MoSz single crystals. Although we used a glass substrate, which has an amorphous structure and no dipole moments, the resultant structure is identical to those of the epitaxial films. The molecular orientation of VOTBP in the VD films is inferior to that of epitaxial films. During the formation of the VD film, VOTBP molecules in the vapor phase attached to the substrate as a cluster. On the cold substrate, one may expect an amorphous film. At a higher temperature, however, the clusters rearrange into a crystalline film. Simultaneously, the dipole moment of the substrate may control the molecular orientation through electrostatic interactions. It has been reported that the VD films of phthalocyanine containing metal ions other than VO are in a perpendicular position on the glass substrate. However, our VD films of VOTBP possess an parallel orientation. We attribute the parallel orientation of VOTBP to the intermolecular force being weaker than the surface force from the substrate. The 3D spin chain was found in the VD films whose structure consisted of bulk micro crystals.
3.5. Cooperative ordering Although we have discussed the molecular orientations in the thin films, as a kind of condensed phase, the three of thin films is governed by a many body problem where the collective properties of constituent molecules, such as ordering, play a crucial role. There are two concepts in molecular ordering [11]. One is orientational ordering and the other is positional ordering. The order parameter St depicts the orientational order that determines the angular correlation between two individual molecules [9, 101. A spin chain implies positional
Figure 3. The pictorial model of the molecular packing and orientation in (a) the LB and (b) the VD films of VOTBP.
185
ordering which governs the translational degree of freedom [ 111. We shall therefore discuss molecular ordering in terms of orientational and positional order in both films at the level of various hierarchies, macroscopic, microscopic and mesoscopic structures [111. The spin chain systems of both films determine their spin dynamics. The spin chain yields mesoscopic information for both films. The LB films have 2D spin chains, and the VD films have a 3D spin chain system. Since VOTBP in the LB films possesses a 2 0 spin chain, the positional order in the LB films is confined to the layer. On the other hand, VOTBP in the VD films possesses a 3D spin chain, which indicates three-dimensional degrees of freedom, such as for molecules in a bulk crystal. However, since the order parameter St shows identical values, the orientational order is nearly the same in both films. Therefore, the mesoscopic structure is a rather unique state that hardly reflects the microscopic or macroscopic structure in terms of their phase or molecular organization [11].
4. CONCLUSIONS EPR measurements revealed the molecular orientations in both LB and VD films. VOTBP in the LB films is oriented perpendicular to the glass substrate, whereas that in the VD films is oriented parallel to it. The spin chain structure was determined by the EPR linewidth measurements. The LB films have a two-dimensional spin chains, whereas the VD films have three-dimensional one. The local structures among the nearest-neighbor molecules are the same in both LB and VD films. The difference between the LB films and the VD films is due to the balance of the molecular aggregation force and the surface force of the substrate, which eventually determines the films structure.
ACKNOWLEDGMENTS This research has been supported by the Grant-in-Aid Program (11875019 to Y. S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
REFERENCES 1. C. Leznoff and A. B. P. Lever, Phthalocyanines: Properties and Applications, VCH Publishers, New York, 1989. 2. A. W. Snow and N. L. Jarvis, J. Am. Chem. SOC.106 (1984) 4706. 3. A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly, Academic Press, San Diego, 1991. 4. R. H. Tredgold, Order in Thin Organic Films, Cambridge University Press, London, 1994. 5. J. H. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon, Oxford, 1990. 6. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett. 40 (1978) 246. 7. P. Turek, Mol. Cryst. Liq. Cryst. 233 (1993) 191. 8. B. Doscher, P. Priess and W. Gunsser, Organ. Magn. Res. 22 (1984) 658. 9. A. Saupe, Z. Naturforsch 199 (1964) 161. 10. Y. Shimoyama, M. Shiotani and J. Sohma, Jpn. J. Appl. Phys. 16 (1977) 1437. 11. P. M. Chaikin and T. C. Lubensky, Principles of Condensed Matter Physics, Cambridge University Press, New York, 1995.
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EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Structure elucidation of vacuum deposited films of titanyl phthalocyanine by EPR Hiroyuki Kaji and Yuhei Shimoyama* Department of Physics, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040-8567, Japan
Using x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy, we revealed structure and spin-chain interaction of vacuum deposited (VD) films of titanyl phthalocyanine (TiOPc). The XRD and EPR spectra demonstrated that the molecular orientation is dependent upon the deposition periods. Thermal annealing for two hours induced an orientation change in the films where TiOPc reorients from the amorphous to perpendicular positions with respect to the substrate.
1. INTRODUCTION Titanyl phthalocyanine (TiOPc) has light sensitivity at the infrared region, which has attracted much attention due to the applicability of photoelectric conductivity, and toner materials for printers and copying machines [l, 21. TiOPc with Y-type has been prominently used in electro photoreceptor devices. The molecule has polymorphs of a-form (phase 11) and @-form(phase I), and structures of their single crystals have been fully elucidated [ 3 ] .Thin films of TiOPc have been an intriguing target in the recent research of nonlinear optical devices because of their functional properties [4, 51. Thus far, there are many reports on the structure and physical properties of thin films of TiOPc [1-5]. However, it is very rare to find reports on EPR studies of the thin films of TiOPc and its powder, except the work done by Enokida a1 [6]. They have reported, for the first time, EPR spectra of TiOPc powder. The isotope effects were found in hyperfine structures due to 47Tias well as 49Tinuclei. EPR has revealed the molecular interactions where the mode of spin interactions determines the dimensions and anisotropy of the spin-chain system in the film [7-111. Therefore, It is worth while to define the structure and the spin-chain property of TiOPc thin films via EPR spectroscopy. In the present paper, we will report on the fabrication method as well as structure of ultrathin films of titanyl phthalocyanine. We will further demonstrate that EPR spectra may reveal organization of the molecular orientation and the spin-chain of titanyl phthalocyanine films.
*
To whom all correspondence should be directed. E-mail:
[email protected] 187
2. EXPERIMENT The film material in the present study was titanyl phthalocyanine (TiOPc, C32H16N80Ti, Aldrich). We used the material without further purification. For substrates of the films, we used quartz glass (supracil, Eiko-sya). The substrate is highly pure, and is free from the background EPR signals. The substrate was washed several times in an ultrasonic bath of films of TiOPc were prepared in a ethanol prior to use for the vacuum deposition. The Pa) with a conventional heat evaporator. The TiOPc powder was placed in a vacuum (1.3~10.~ tungsten boat. As soon as the temperature of deposition source reached the desired value, the shutter was opened and the source started to deposit on the substrate. The temperatures of deposition source and the substrate measured by thermocouples were controlled at 210°C and 45" during the deposition, respectively. The deposition was stopped every 1 h, and repeated for three times (3 h). The distance between the deposition source and the substrate was ca. 9 cm. The thickness of TiOPc thin films was fixed at ca. 150 (220) nm. X-ray diffraction of the TiOPc films was performed by an X-ray diffractometer (Rigaku, RINT-1200). EPR measurements were carried out using an EPR spectrometer (JEOL, JES-FEIXG). All the EPR spectra were recorded at the X-band (9.3 GHz). The microwave strength was varied between 1-64 mW. Magnetic field modulation was operated at 100 kHz. The angle between the film normal and the applied magnetic field 8 was varied over 0-211, with a homemade goniometer. All the measurements were made at a room temperature (ca. 20°C).
3. RESULTS AND DISCUSSION
3.1. Molecular organization in vacuum deposited films of TiOPc Figure 1 shows XRD patterns of the TiOPc films of h deposition. A weak peak was films of 2 h deposition, whose intensity was severely interfered detected at ca. 6.3" in the by the background scattering. We therefore enhanced the signal-to-noise ratio by the moving average method, and obtained the plane separation of 1 4 A . This value is indicative that TiOPc orients perpendicular to the substrate. The corresponding plane separation is 14A, 10000 which is approximately identical with lattice 9000 constants of phase I. The data eventua8000 lly reflect the perpendicular orientation of 7000 TiOPc In our TiOPc films, the crystal structure was modified by the deposition ,X .- 6000 peaks were period. No significant 5000 observable from the films of and 3 h 4000 deposition. These films may take the 3000 amorphous structure. Only the films of 2000 h deposition exhibited peaks.The 1000 peaks seen in the films of h deposition 0 disappear in the films of 3 h deposition. This 0 10 15 20 25 30 35 40 evidence suggests that the annealing effect is induced by the substrate temperature. By Angle (degree) keeping the deposition of TiOPc for 2 h, we Figure 1. pattern of the vacuum obtained the films with a better crystallideposited films of TiOPc as annealed for 2 h.
8
188
nity by the reorganization through thermal agitation.
3.2. Spectral features of EPR Figure 2 shows EPR spectra of the TiOPc powder, and the VD films at various deposition periods. The EPR spectra of the TiOPc VD films of 1 and 3 h deposition sorely indicated a sharp signal at g = 2.003 which is attributable to the free electron spin (ge = 2.0023). Peaks of the EPR spectrum of the TiOPc VD films of 2 h deposition were found at g = 2.006 and 1.970. The EPR signal indicated a wide hyperfine structure (hfs). However, the peak intensity was relatively weak so that the hfs coupling constant (hfcc: A) was undetermined. Appearance of the hfs suggests that the distance between the central titanium ion is more than 6 A which may prevent line broadening due to the spin exchange interactions [9]. Figure 3 shows EPR spectra of the TiOPc VD films of 2 h deposition at 0 = 0 and 90". Table 1 summarizes the data from the EPR spectra with respect to the applied magnetic field. The anisotropy was described by the 11- (parallel) and I-(perpendicular) components, since TiOPc molecule has a four-fold symmetrical axis along Ti=O double bond. Similar anisotropy has been also found in the VD films of VOPc that also possesses the shuttlecock shaped structure [9]. The anisotropy g-value satisfies the following conditions, 811< g, [9, 121. In the present spectra, the I-and 11-components were evaluated from those at 0 = 0 and go", respectively. These components predict TiOPc being normal to the substrate in the VD films. In principle, the angular variation of the peak position or g-value reflects the orientation distribution of molecules in the spatial confinements. Let be the statistical tilt, and (J the deviation (or distribution) angle. We may define an extended normal distribution function by g=2.006 g~1.970
a) 0 = 90"
g=2 006
I
@)For 1 hour
(c)For 2 hours
!/
g = l 966
j
j
Figure 2. EPR spectra of TiOPc (a) powder, and VD films annealed for (b) 1 h, (c) 2 h, and (d) 3 h. All the spectra were recorded at the film surface normal to thc applied magnetic field.
Figure 3. EPR spectra of VD films (for 2 h) of TiOPc. The spectra were recorded at the film surface (a) parallel and (b) perpendicular to the applied magnetic ficld.
189
Table 1 g-values and linewidths of the powder and the vacuum deposited films of TiOPc at the deposited time for 2 h powder g-value Linewidth (mT)
2.006 0.51
1.970 3.2
vacuum deposited films (for 2h)
no
9no - .,
2.006 1.1
1.966 2.4
2.006 1.1
1.972 3.0
the following equation:
Figure 4A shows an angular variation of the g-value for the high field peak of the TiOPc VD films of 2 h deposition (open squares) and the theoretical fitting curve (solid line). The 0 = 22.1 rad. The best-fit values for the tilt and deviation g-value hit the minimum at angles were 12" and 26", respectively. EPR spectra recorded at 0 = 90 and 0" yielded 11- and I-components of the g-tensor, respectively. A simple analysis of the tensor components predicts that phthalocyanine aligns perpendicular to the glass substrate. In fact, the theoretical fit of the g-value variation for the high field peak of the VD films suggested a tilt angle of 12". The phthalocyanine ring orients along the substrate normal with a tilt angle of 12". This result coincides with those of the XRD measurements. The difference in the tilt angle between XRD and EPR results may be due to the time scales of those detection principles. Deviation angle (0 = 26") yields a spread of molecular orientation around the tilt. Therefore, TiOPc in the VD films oriented within a cone of the spread. 1
1
0.9
0.9
9 0.8
$0.8
-
N ._ 0.7
0.7
2 0.6
0.6 v
-
0
0.5
0.5
I :
4 0.4 3 0.3
gj 0.4 KJ
?
.4
0.3 0.2
2 0.2
0.1
0.1
0
0
-3.2 -2.4 -1.6 -0.8 0
0.8 1.6 2.4 3.2
Angle (rad)
-3.2 -2.4 -1.6 -0.8 0
0.8 1.6 2.4 3.2
Angle (rad)
Figure 4. Angular variation of (A) the g-value, (B) the linewidth of EPR spectra of TiOPc VD films as annealed for 2 h.
190
3.3. Spin-chain systems The spin-chain interaction that relates to the positional order of a spin-nctwork affects the linewidth and lineshape of EPR spectra. One may define the spin-chain interaction in one(lD), two- (2D) and three-dimensions (3D). In the 3D system, spins interact equally over all directions. On the other hand, 2D and 1D systems may havc the plane- and line-wise interactions, respectively. According to the spin diffusion model, complete 2D and 1D systems may exist. Howevcr, these are weak interactions in practice. Even if exchange interactions are negligibly small, dipolar interactions still remain. Therefore, the ideal 2D and 1D systems never exist. Thus, the linewidth (AH) is governed by the flowing expressions [7, 131.
2D: AH
(3cos’O -1y
3D: AH
(l+cos’H)
Figure 5. Pictorial diagrams of the molecular organization of VD films of TiOPc; a) side and b) top views.
(4) Figure 4B shows the angular variation of the normalized linewidth of the high field peak of the VD films of the 2 h deposition (open circles). Both the linewidths of the low and high field peaks varied in a periodical fashion with the minimums at 0 = 21.0 rad, k2.1 rad and the maximums at 8 = 0 rad, rad, respectively. We successfully simulate the linewidth variation of the TiOPc VD films by Eq. 3. The best-fit curve (solid line in Figure 4B) implies that the magnetic interactions of the TiOPc VD films are dominated by the in plane or 2D-order. The TiOPc VD films take the phase 1 structurc in which titanyl groups faced and interdigitated each other. Such a cofacial arrangement between titanyl groups aligns oxygen atoms along a straight line, and eventually promotes strong in-plane interactions. We depicted a model in Figure 5 that shows 2D alignment of the TiOPc VD films. On the othcr hand, in the phase I1 where titanyl moiety directs along a certain axis, every neighboring molecule locates at an equal distance, and undergoes isotropic interactions in the 3D space. Thus, 3D interactions bccome dominant in the phase 11 structure. Wc conclude that the TiOPc VD films take the slipped-stack structurc with 2D order.
191
4. CONCLUSIONS We revealed that TiOPc orients normal to the substrate in the VD films by XRD and the angular variation of g-value of EPR spectra. The VD films of TiOPc molecule may take the phase I structure in which the oxygen moiety faces each other and interdigitates simultaneously. Angular variation of the EPR linewidths revealed that the TiOPc VD films possess a 2D spin-chain. A plane-like network is promoted by the spin-chain interaction.
ACKNOWLEDGMENTS This research has been supported by the Grant-in-Aid Program (11875019 to Y. S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
REFERENCES 1. B. Dunn, A. R. West and J. W. Goodby, Phthalocyanine Materials Synthesis, Structure and Function, Cambridge University Press, Cambridge, 1998. 2. A. Kakuta, Y. Mori, S. Takano, M. Sawada and I. Shibuya, J. Imag. Tech. 11 (1985) 7. 3. W. Hiller, J. Strahle, W. Kobe1 and M. Hanack, Zeitschrift fur Kristallographie 159 (1982) 173. 4. H. Yanagi, S. Chen, P. A. Lee, K. W. Nebesny, N. R. Armstrong and A. Fujishima, J. Phys. Chem. 100 (1996) 5447. 5. Z. D. Popovic, M. I. Khan, S. J. Atherton, A. M. Hor and J. L. Goodman, J. Phys. Chem. B 102 (1998) 657. 6. T. Enokida, R. Hirohashi and T. Nakamura, J. Imag. Sci. 34 (1990) 234. 7. T. Takamura, M. Moriyama, T. Komatsu, Y. Shimoyama, Jpn. J. Appl. Phys. 38 (1999) 2928. 8. J. H. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Clarendon Press, Oxford, 1990. 9. S. Palacin, A. Ruaudel-Texier and A. Barraud, Mol. Cryst. Liq. Cryst. 156 (1988) 331. 10. M. Brinkmann, C. Chaumont, H. Wachtel and J. J. Andre, Thin Solid Films 283 (1996) 97. 11. R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett. 40 (1978) 246. 12. P. Turek, Mol. Cryst. Liq. Cryst. 233 (1993) 191. 13. J. E. Wertz and J. R. Bolton, Electron Spin Resonance, Elementary Theory and Practical Applications, McGraw-Hill Book Company, New York, 1972.
192
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR investigation of organic conductor with itinerant and local spins, (CHTM-TTP)zTCNQ T. Nakamuraa, M. Taniguchi', Y. Misakib, K. Tanaka' and Y. Nogamic aInstitute for Molecular Science, Okazaki 444-8585, Japan 'Dep. Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan 'Dep. Science, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
ESR investigation was carried out for the organic conductor with itinerant and local spins, (CHTM-TTP)2TCNQ. This salt shows two drastic successive phase transitions at low temperatures. The contributions of the two spin species of the magnetic properties are separated using with the g-tensor analyses. The electronic phases of the title salt are discussed by microscopic point of view. 1. INTRODUCTION
The recent discovery of superconductivity in BEDT-TSF salts with iron counter ions under high magnetic fields has expanded the interest in possible cooperative phenomena between spins of donors and counter ions [l]. It is interesting to investigate the electronic phases of organic charge transfer salts that are candidates with possible cooperative phenomena between two different spin species, since we can expect much stronger interactions between spins on the donor and accepter molecules. CHTM-TTP is an organic donor recently synthesized by Kyoto Univ. group [2]. Its cation salt, (CHTM-TTP)2TCNQ, is a new organic conductor that is composed of segregated donor (CHTM-TTP) and acceptor (TCNQ) layers (Figure 1). CHTM-TTP molecules stack to form one-dimensional columns. On the other hand, there is little interaction between TCNQ molecules. This salt is metallic around R.T., and its resistivity shows an abrupt jump around
193
Figure 1. Crystal structure of (CHTM-TTP)2TCNQ. 225-245K. But it remains conductive with a slight maximum around 150-180K down to 30K. At 30K, the resistivity turns to increase. In order to clarify the mechanism of the anomalies mentioned above, we performed magnetic resonance measurements. The low temperature electronic phases of the two-component system, (CHTM-TTP)2TCNQ, are discussed by microscopic point of view.
2. EXPERIMENTAL The ESR measurements were carried out for a single crystal using an X-band spectrometer, Bruker ESP-300E, with a rectangular cavity: TM110. The temperature range of the ESR measurements was between 300K and 4K. The ESR signal is a single Lorentzian in the whole temperatures.
3. RESULT AND DISCUSSION
Figure 2 shows the temperature dependence of the spin susceptibility, xspln, of (CHTM-TTP)2TCNQ determined by the EPR signal intensity. Between 240K and 300K, zspln increases gradually as temperature decreases. At 240 K, xspln shows an abrupt decrease, indicating a phase transition. In this xspln jump, we do not observe any hysteresis behavior between in cooling and in heating processes. Between 195 K and 240 K, the increase of xspln with temperature became weak. Around 180 K, another abrupt decrease of xspln was observed. As for the low-temperature phase transition, xspln shows a clear hysteresis behavior: In the case of the cooling process, xspln abruptly drops around 170 K, while it jumps around 210K for the heating process. These observations indicate that the transition is of first order. Below
194
L . L
I
I " ' " "
0
50
200 250 Temperature (K)
100
150
300
Figure 2. Temperature dependence of xspin of (CHTM-TTP)2TCNQ determined by EPR.
0
50
' I "
' I
"
1 ' ' -
200 250 Tempeartaure (K)
100
150
Figure 3. Temperature dependence AHpp of (CHTM-TTP)2TCNQ.
300
of
170K, zspin of (CHTM-TTP)zTCNQ shows a gradual decrease and no obvious anomalies down to the lowest temperature.
Temperature dependence of the EPR linewidth, AHpp, is shown in Figure 3. Clear anomalies are also observed in the temperature dependence of the EPR linewidth, AHpp: The linewidth anomalies are associated with the both 240 K and 195 K phase transitions. Abrupt increases of AHpp suggest drastic changes of the relaxation mechanism of the electron spins. These observations indicate that the spins with different character exist, and that the fraction of them changes below the phase transitions. Especially the 240 K anomaly probably corresponds with the abrupt increase of the electric resistivity. As for the 195K phase transition, the EPR parameters exhibit clear hysteretic phenomenon, indicating that the transition is of first order. Most of phase transition associated with abrupt decreases of xspln, the EPR linewidth generally decreases: It is mainly due to reduction of the spin-spin interaction associated with spin-gap formation. On the other hand, in the case of (CHTM-TTP)2TCNQ, AHpp shows opposite behavior in both phase transition in which xspin suddenly decreases. Broadening of AHpp indicates increase of the averaged EPR relaxation rate, suggesting enhancement of itinerant nature. Below 170K, AHpp shows monotonically decrease down to 12 K, turns to increase slightly
195
below 12K. It should be noted that the enhancement of AHpp below 12K is moderate and weak. Moreover zspln of (CHTM-TTPhTCNQ shows no obvious anomalies down to the lowest temperature. Hence the insulating behavior below 30K is considered to be extrinsic. The enhancement of AHpp below 12 K is considered to be inhomogeneous broadening: A possible weak localization seems to be very likely. Further investigations are now going on. Figure 4 shows the temperature dependence of g-values of (CHTM-TTPhTCNQ applying the external static field along the orthogonal three axes. The anisotropy of the g-values at R.T. is very small: These features are quite different from those of typical TTF cation radical salts. The g-values for all the directions indicate stepwise increase at two phase transitions. The principal values of the g-tensor change their absolute values at them. It cannot be explained within the framework of one spin model. It is an obvious evidence of the existence of TCNQ spins besides CHTM-TTP spins. Moreover the average of the principal values also shows significant temperature dependence. Detailed of the g-tensors analyses enable us to decompose zspln of systems with two different spin species [3]. It indicates that contribution of the TCNQ spins to the total spin susceptibility also changes; the effective local moments on TCNQ decrease at 240K, and that disappear perfectly below 170K. These considerations are consistent with the results of the temperature dependence of the 'H-NMR relaxation rate. Detailed analyses of them will be discussed elsewhere.
2.012 2.010
-F
I
'
2.008 2.006
2.002
0
50
100
150 200
250
300
Figure 4. Temperature dependence of the g-values of (CHTM-TTP)*TCNQ applying the external static field along the crystal axes.
196
4. CONCLUSION In summary, we investigated the low temperature electronic state of (CHTM-TTP)2TCNQ by the ESR measurements. The principal values of the g-tensor change their absolute values at them. It is an obvious evidence of coexistence of TCNQ spins besides CHTM-TTP spins. The effective local moments on TCNQ decrease at 240K, and that disappear perfectly below 170K.
5. ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (B) of "Molecular Conductors and Magnets" (Area No. 730h1224213) from the Ministry of Education, Science, Sports and Culture of Japan.
REFERENCES
1. S. Uji, H. Shinagawa, T. Terashima, T. Yakabe, Y. Terai, M. Tokumoto, A. Kobayashi, H. Tanaka and H. Kobayashi, Nature, 410 (2001) 908. 2. M. Taniguchi, Y. Misaki and K. Tanaka, in preparation for publication. 3. T. Nakamura, T. Takahashi, M. Taniguchi, Y. Misaki and K. Tanaka, Synth. Met., 103 (1999) 1900, and references therein.
EPR in the 21" Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
197
X-band ESR measurements of Et,Me,P[Pd(dmit),], T. Sakurai"., H. Ohta..', S. 0kubob,', R. Katodand T. Nakamurae "The Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan bVenture Business Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 'Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan dTheInstitute of Physics and Chemical Research, Wako, Saitama 351-0198, Japan 'Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
X-band ESR measurements of aligned single crystals of EGMep[Pd(dmit),],, which is considered to undergo antiferromagnetic transition at T, = 18 K, were carried out with the static magnetic field up to 1 T. ESR signal was observed below room temperature. The linewidth at room temperature is almost 100 mT and it is relatively large compared to general radical organic compounds. The linewidth shows the minimum at 25 K and then increased as the temperature approached T,. The g-values for all axes start to change below 25 - 35 K. These behaviors of the linewidth and the g-value above T, are the typical behavior in lowdimensional antiferrornagnet and they suggest that the short range order starts to develop below 25 K.
1. INTRODUCTION A series of ~'-(cation)[Pd(dmit),], salts (cation = Me,Z and Et,Me&; Z = P, As and Sb) is well known as an interesting system which has the unique two band electronic structure and shows various T-P phase diagrams depending on cations [l]. These salts have almost the same crystal structure regardless of cation, and the crystal structure of these salts is based on the stack of strongly dimerized Pd(dmit), molecules. These salts are considered to be MottHubberd insulator with half-filled HOMO band under ambient pressure. Nakamura performed X-band ESR measurements on these single crystals and proposed that the ground state of these salts is antiferromagnetic state except for E&Me,Sb satls [ 2 ] .And they revealed that the T,'s of these salts depends strongly on the cation. For instance, the T, of the title
198
compound Et$le,P salt, which is known as a superconductor under pressure, is 18 K and that of Me,P salt is 35 K, while an antiferromagnetic transition is not observed for Et$le,Sb salt. This crucial cation dependence of T, has been explained by the effect of the magnetic frustration in this system Although the X-band ESR measurements of Pd(dmit), salts their measured temperature region was limited were already performed by Nakamura et below about 100 K due to the broad linewidth and the weak intensity of these salts at higher temperature. We have performed X-band ESR measurements on larger amount of Et,Me,P[Pd(dmit),], aligned single crystals than Nakamura et in order to gain more detailed information and extend measurable temperature region.
2. EXPERIMENTAL X-band ESR measurements have been performed on EGMe$"Pd(dmit),], single crystals using a conventional X-band spectrometer EMX081 (Bruker Co. Ltd.) with a continuous He-flow cryostat. 40 - 50 single crystals were aligned along each axis. The static magnetic field is up to about 1 T. Observed temperature region is from 3 K to room temperature.
3. RESULTS AND DISCUSSION
ESR signal of EGMe,P[Pd(dmit),], was observed below room temperature. Figure 1 shows the temperature dependence of the integrated intensity normalized by that obtained at 50 K for B//c. The integrated intensity decreases abruptly below 50 K for all axes as the temperature is decreased. Below TN= 18 K, the signal starts to be distorted by an asymmetric signal which comes from the quartz glass of the sample stage. As the intensity of this asymmetric component gets stronger as the temperature is decreased, small upturn of the
h
r
n
.
'a 1.2: Y
.
P
b +
'0 ' !! '
0
199
integrated intensity below 10 K is caused by the quartz glass and it is not intrinsic. An anomaly of the integrated intensity in the temperature region from 50 to 100 K is observed for does not show such anomaly clearly due to the Blla and b, while the result by Nakamura et scattering of data. In X-ray measurements, the superlattice reflections with the reciprocal wave vector (0 114 0) which equals to superstructure having a periodicity with a+4b+c were observed below 70 K [4]. Although the origin of this superlattice reflection is not clear at the moment, the anomaly of the integrated intensity may be related with this superlattice reflection. Figures 2 and 3 show the temperature dependence of the peak-to-peak linewidth AHpp and the g-value, respectively. These results below 100 K are consistent with the results of Nakamura [2]. The linewidth decreases as the temperature is decreased. It shows the minimum around 25 K and increases as the temperature approaches T,. This temperature dependence of the linewidth shows the typical feature of low dimensional antiferromagnet [5]. The accuracy of the g-value is not good at high temperature because of the broad linewidth and the weak intensity, while it gets better as the temperature is decreased because the linewidth gets narrower. As the temperature is decreased, the g-value starts to shift below 25 K where the linewidth shows the minimum. These results seem to suggest the occurrence of the short range order around 25 K. Moreover, the g-value for B//b increases, while those for Blla and c decrease as the temperature is decreased. The typical one dimensional antiferromagnet shows that the g-value for B//chain increases and that for Blchain decreases as the temperature is decreased below the short range ordering temperature As the stacking direction of Pd(dmit), molecules is a+b direction, the temperature dependence measurements for different direction may be required. As shown in Figure 2, the large linewidth of about 100 mT as compared with general organic compounds was observed at room temperature. Due to its weak intensity and the large linewidth, the signal is affected by the background. Therefore, we can not analyze precisely its lineshape and discuss what kind of interaction is dominant. However, it is worth comparing
0
2.06
* 2.04
? 2.02 MI
IF++++
+
O O
+
2.00 1.98 50 100 150 200 250 300
0
50
100 150 200 250 300
T (K) Figure 2. Temperature dependence of the peak-to-peak linewidth.
Figure 3. Temperature dependence of the g-value.
200
the experimental result with the estimated linewidth considering the dipole-dipole interaction and the exchange interaction. First, we made the order estimation of the linewidth of this salt in the case when only the dipole-dipole interaction exists. The obtained linewidth was about 15 - 30 mT on the assumption that the spin is S = 112 and an unit of Bohr magneton is located at the center of the Pd(dmit), dimer when the external magnetic field is applied in the ab plane. Second, in the case when there exist not only the dipole-dipole interaction but also the exchange interaction, the linewidth becomes sharp to the order of 0.03 mT using the exchange field of 47 T [3]. Neither obtained linewidths can not explain the experimental result. The followings can be the origin of the discrepancy between the experimental result and the estimated linewidth; 1) The assumption that the spin exist only on the center of Pd(dmit), dimer may not be adequate. 2) The magnetic frustration may affect this large linewidth. 3) Although the temperature dependence of the electric resistance shows semiconductive behavior, the conductivity at room temperature is about 10 S/cm [7] and this value is not so low. Therefore, the broad linewidth may reflect the effect of the conduction electron. In summary, X-band ESR measurements of the aligned single crystals of Et$ie,P[Pd(dmit),], have been performed and ESR signal was observed below room temperature. The temperature behaviors of the linewidth and the g-value above TN show typical behaviors of the low-dimensional antiferromagnet and they suggest that the short range order starts to develop below 25 K. The origin of the linewidth observed at room temperature has been also discussed.
ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Area (A) (No. 11 136231, 12023232 Metal-assembled Complexes) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
REFERENCES 1. R. Kato, Y. -L. Liu, Y. Hosokoshi and S. Aonuma, Mol. Cryst. Liq. Cryst. 296 (1997) 217. 2. T. Nakamura, H. Tsukuda, T. Takahashi, S. Aonuma and R. Kato, Mol. Cryst. Liq. Cryst. 343 (2000) 187. 3. M. Mori and K. Yonemitsu, Synth. Metals 120 (2001) 945. 4. S. Rouzihe, J. -I. Yamaura and R. Kato, Phys. Rev. B 60 (1999) 3113. 5. Y. Ajiro, S. Matsukawa, T. Yamada and T. Haseda, J. Phys. SOC.Jpn. 39 (1975) 259. 6. K. Nagata and Y. Tazuke, J. Phys. SOC.Jpn. 32 (1972) 337. 7. R. Kato, Y. Kashimura, S. Aonuma, N. Hanasaki and H. Tajima, Soild State Commun. 105 (1998) 561.
EPR in the 21' Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
20 1
The role of Li' and Na" charge compensators in Sm3+-dopedC a F 2 and SrF2 M. Yamaga", M. Hondab, N. Kawamatab,K. Samejimab,and J.-P. R. Wells" "Department of Electrical and Electronic Engineering, Faculty of Engineering, Gifu University, Gifu 50 1- 1193, Japan b
Faculty of Science, Naruto University of Education, Naruto 772-8502, Japan
'FELIX Free Electron Laser Facility, FOM-Institute for Plasmaphysics 'Rijnhuizen', P.O.Box 1207,3430 BE Nieuwegein, The Netherlands We have examined the role of co-doping of Sm3+-dopedCaF2 and SrF2 with Li+ and Na' using electron paramagnetic resonance (EPR). Three Sm3+centers in CaF2:Sm3+:Li' are identified as tetragonal(C4v). trigonal(C3v),and orthorhombic ((224 symmetry centers, whereas two Sm3+centers each in CaF2:Sm3+:Na+,SrF2:Sm3+:Li+ and SrF2:Sm3+:Na+are identified as C4v and CzVsymmetry centers with correspondingly different g values. The CzVcenters correspond to Li+/Na+-substitutionat Ca2'/Sr2' sites along the [1101 direction. The absence of cubic symmetry centers, observed in optical measurements of the same samples studied here, can be explained in terms of fast spin-lattice relaxation between the levels of the 6H5/2multiplet of Sm3+. split ground state (4)r~ 1. INTRODUCTION
The alkaline-earth fluoride crystals show space group 02, where the alkaline-earth ions are eightfold coordinated. Trivalent rare-earth ions (RE3') readily substitute for the divalent alkaline-earth cations and charge compensation is required. In CaF2:Sm3+and SrF2:Sm3+crystals, the well-known C4V(F3center is predominant [I]. This center is composed of a Sm3+-F-pair with the charge-compensating fluorine ion located in the nearest-neighbor position along the [OOl] direction from the Sm3+ ion [l-41. After oxidization, these samples have trigonal C3dO2-)centers, consisting of a Sm3+-02'pair with the charge-compensating oxygen ion located in the nearest-neighbor position along the [ 11 13 direction from the Sm3+ion [ 1,2]. The effect of co-doping CaF2:RE3+and SrF2:RE3+crystals with LiF, NaF or KF was reported during the 1960's by groups in the former Soviet Union, see for example [5], and
202
more recently by Jones, Reeves and co-workers [6,7]. From a combination of infrared absorption and laser selective excitation spectroscopy, the later workers illustrated profound changes to the defect distribution; namely that the population of regular C44F') centers is reducedeliminated, cubic centers with remote charge compensation are significantly enhanced, and new orthorhombic symmetry centers with monovalent alkali ions located in the Ca2+or Sr2+site in the [1101 direction from the RE3+ion are created. This paper presents electron paramagnetic resonance (EPR) spectroscopy upon Sm3+ centers in CaF2:Sm3+and SrF2:Sm3+crystals co-doped with Li+ or Na+ in order to better understand the microscopic structure, since the anisotropy of the g tensors yields indirect information on the possible ligand positions. From this, we propose models of the Sm3+-Lif ma+ centers.
2. EXPERIMENTAL PROCEDURES CaF2 and SrF2 crystals co-doped with up to 0.04% Sm3+and up to 1.2% of Li+ and Na' were grown in graphite crucibles by the Bridgmann-Stockbarger method in an RF-induction furnace [6]. As LiF and NaF melt around 400 'C lower than the melting points of CaF2 and SrF2, there is a loss of these dopants during crystal growth and Li' is more readily prone to evaporation as LiF. To counter this, crystals can be prepared in an inert atmosphere such as argon gas. The EPR measurements of Sm3+in these crystals were performed at temperatures between 5-50 K using a Bruker EMX10/12 X-band spectrometer with microwave frequencies of 9.695-9.700 GHz, a microwave power of 0.1 mW and 100 KHz field modulation. The angular variations of the EPR spectra were measured by rotating the sample in the cavity. The full range of the applied magnetic field was between 0-1.5 T. 3. RESULTS
Figure 1 shows the typical Sm3+EPR spectrum measured for CaF2:Sm3+:Li+with Bll[OOl] at 5 K. The spectrum consists of three groups of intense resonance lines accompanied by several weak hyperfne lines due to the 14'Srn and '49Smisotopes with a non-zero nuclear spin of 1=7/2 and natural abundances of 15.1% and 13.8%, respectively. Figure 2 shows the angular variations of the g values in the (010) and { 1 iO} planes for the Sm3+ions with zero nuclear spin. As the full range of the magnetic fields is 0-1.5 T, the g values below 0.46 are not observable. The curves in Fig. 2 are calculated using a spin Hamiltonian, including only the Zeeman interaction, of the form [S]:
203 1.o
1
-1.0 0.5
I
I
I
I
0.7
0.9
1.1
1.3
1 1.5
Magnetic field ( T ) Figure 1 The EPR spectrum for Sm3+:Li+:CaF2with B//[OOl] and 5 K. The letters of C4v,C3vand C2v denote tetragonal, trigonal and orthorhombic centers.
1.o
1.o
0.5
0.5
Q)
(D
0
0
60
Angle ( degree )
90
0
0
60
90
Angle (degree )
Figure 2 The angular variations of the resonance lines (I=O) observed in Sm3+:Li+:CaF2 with T=5 K in the (010) and (1 iOf-planes
204
where is the Bohr magneton, is the applied magnetic field, and (=1/2) is the effective spin. The patterns show tetragonal, trigonal and orthorhombic symmetry. The principal x, y , and axes of the GVcenters are defined as the [loo], [OlO], and [OOl] directions, whereas those of the C3., centers are defined as the [ l iO], [i i2], and [ l l l ] directions. The principal x and y axes of the orthorhombic ((22") centers are rotated by an amount from the [loo] and [OlO] crystallographic directions in the (001) plane, respectively, and the axis is parallel to the [OOl] direction. Cubic (Oh) symmetry centers, observed in laser selective excitation measurements of the same samples studied here, could not be detected using EPR. We tentatively ascribe this to fast spin-lattice relaxation between the magnetically split ground state and consequently broadened EPR linewidths. Analogous measurements were made for the CaF2:Sm3+:Na+,SrF2:Sm3+:Li+, and SrF2:Sm3+:Na+ as discussed below. Table 1 summarizes the optimized fit parameters to the Hamiltonian above. The g values Table 1 The spin-Hamiltonian parameters of Sm3+in alkaline-earth fluoride crystals
~~~
CaF2:Sm3+:Li+
CaF2:Sm3+:Na+
SrF2:Sm3+:Li+
gI1=O 05 gi=O 821
~
~
g11=0 415 g,=o 934
gx=O583 g,=O 558 g,=O 30 9 =+32'
g 11 =0.764 glz0.35
gI1 =0.05 gi4.822
g,=0.660 g,=0.477 g,=O. 30 e =&45' g,=0.637 gy=O.577 g,=O .33 e =+34'
gx=0.606 gy=0.537 g,=O. 34 e =+45'
205
of the C4V(F-) centers observed in CaF2:Sm3+:Li+,SrF2:Sm3+:Li+ and SrF2:Sm3+:Na+ are in agreement with those measured by Newman and Woodward [2]. The tetragonal C4,(F-) and trigonal C3,(o2-) centers consist of Sm3+-F-and Sm3+-02pairs, respectively, with the charge-compensating fluorine and oxygen ions located in the nearest-neighbor interstitial positions along the [ 1001 and [ 1111 directions from the Sm3+ion [ 1-31, As the magnitude of the g-values for the tetragonal CrV(Na) center observed in CaF2:Sm3+:Na+is the opposite of the regular C4V(F-)centers in CaF2, it is clear that the charge compensation configuration is also different. It is likely that this center consists of a second nearest neighbor Na' ion which has substituted for a Ca2' ion along the [OOl] direction. Such a configuration reduces axial distortion along the [OOl] direction, resulting in reduction of the anisotropy of the g values. We denote this center as C4dNa). The orthorhombic CzVsymmetry centers are strongly associated with Li+and Na' charge compensators. The principal x-axis of the Li+ orthorhombic center (hereafter denoted C2v(Li))is tilted with an amount of about 12O from the [ 1101 direction, whereas that of the orthorhombic C2,(Na) center is parallel to the [ 1101 direction. The difference is due to the differing ionic radii of Li+ (0.088 nm) and Na' (0.130 nm) [9]. As the ionic radius of Na' is close to those of Ca2+(0.126 nm) and S?' (0.139 nm) [9], Na' ions substitute for Ca" with minimal relaxation of the surrounding ions. On the other hand, Li+ ions are smaller in size and located slightly off the substitutional position, corresponding to two minima of the chemical potential. The relative EPR intensities, I(C4v(F-))>I(C2,(Li)),in CaF2:Sm3+:Li+suggest that the dominant charge compensators of Sm3+ are interstitial F- ions. The reduction in the intensity of the C4.,(F-) centers and enhancement in the intensities of the C4,(Na) and the CzV(Na) in CaF2:Sm3+:Na+is consistent with more efficient substitution of Na+ ions than Li+ ions, which are more readily prone to evaporation as LiF during crystal growth. The anisotropy of the g values for the C2V(Na)in CaF2/SrF2:Sm3+:Nafisslightly larger than that for the C2,(Li) in CaF2/SrF2:Sm3+:Li+. This fact indicates that the non-axial distortion of the C2V(Na) center is a little larger than that of the C2,(Li) center. The difference in the non-axial electrostatic potential is due to differences in effective size and charge of the Li+ and Na+ ions. The magnetic hyperfine coupling constants of the '47Sm and I4'Sm isotopes having a non-zero nuclear spin of 1=7/2, are estimated to be (Al,A.)=(183MHz, 571MHz) and (271MHz, 692MHz) respectively, for the C4,(F-) center in CaF2:Sm3':Li+. A comprehensive report of the spectroscopic properties of these materials including optical and EPR spectroscopies, and appropriate crystal- and magnetic-field analyses will be presented in a more complete account of this work.
206
ACKNOWLEDMENTS
One of the authors (M. Yamaga) is indebted to Iketani Science and Technology Foundation for a Research Grant Award. We would like to thank Dr Glynn D. Jones from the University of Canterbury, New Zealand for supplying the crystals used in this work.
REFERENCES 1. J.-P.R. Wells and R.J. Reeves, Phys. Rev. B., 61 (2000) 13593. 2. R.C. Newman and R.J. Woodward, J. Phys. C: Solid State Phys., 7 (1974) L432. 3. M.J. Weber andR.W. Bierig, Phys. Rev., 134 (1964) A1492. 4. A.A. Antipin, 1.1. Kurkin, L.D. Livanova, L.Z. Potvorova and L. Ya. Shekun, Sov. Phys.-Tech. Phys., 11 (1967) 821. 5. F.Z. Gil’fanov, L.D. Livanova, A.L.Stolov, Sov. Phys. Solid State, 8 (1966) 108. 6. G.D. Jones and R.J. Reeves, J. Lumin., 87-89 (2000) 1108. 7. S.P. Jamison, R.J. Reeves, P.P. Pavlichuk and G.D. Jones, J. Lumin., 83-84 (1999) 429. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon Press, Oxford, 1970) Chaps. 3 and 5. 9. A.A. Kaminskii, Laser Crystals (Springer-Verlag, Berlin, 1990) Table 2.1.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
207
The HF and SHF interactions of V02+ions in KZnC1S04.3H20single crystals K.V.Narasimhulu, B. Deva Prasad Raju, J. Lakshmana Rao Department of Physics, Sri Venkateswara University, Tirupati - 5 17 502, India
The hyperfine (hf) and superhyperfine (shf) structural patterns exhibited by V02+ions in KZnClSO4.3HzO single crystals were studied using EPR technique. The angular variation of EPR spectra reveals the presence of four magnetic complexes, which correspond to two distinct sites of V02+ ion. From the angular variation EPR data, the spin-Hamiltonian parameters were evaluated. From the EPR and optical data, the molecular-orbital bonding coefficients were evaluated. The five line superhyperfine (shf) structure is seen in all the planes of the crystal, when the magnetic field is away from the crystallographic axes. This has been attributed to the four protons from H20 ligands, where the metal ion is surrounded by H20 and SO4 environment.
1.
INTRODUCTION
The ’lV nucleus (99.8 % abundant) has a nuclear spin I=7/2 and a large magnetic moment. V4+ ion has the electronic configuration [Ar] 3d’ which thereby leads to paramagnetism in V02+.Vanadyl ion (VO”> is the most stable cation among a few molecular paramagnetic transition metal ions which has been used extensively as an impurity probe for EPR studies. Vanadyl complexes have been the subject of interest to a number of workers for EPR studies over recent years, as the V02+ is found to have different behaviour in different host lattices; it may freely rotate or stay rigid in the lattice depending on the host and the environment. Such studies will provide information regarding the symmetry of crystalline electric fields [ 1-71. Potassium Zinc Chloro Sulphate Trihydrate (PZCST) is an isomorphous crystal of original kainite mineral O(MgClS04.3HzO) studied earlier [81. In continuation of our work on this host [9], the present paper is aimed at the hyperfine and superhyperfine interactions of the V02+ ion in PZCST single crystals and to get the information regarding the metal ion environment. 2. CRYSTAL STRUCTURE
PZCST single crystal is considered to be the zinc analogue of the original kainite. Kainite crystallises in monoclinic symmetry with space group Cz; (Cz,,,,). A detailed crystal structure of PZCST is not available. However, the unit cell dimensions were reported for
208
original kainite single crystal in early fifties [lo]. Later Robinson et a1 [S] studied the detailed crystal structure of kainite and reported the monoclinic symmetry with the same space group with the lattice parameters as a=l.972 nm, b=l.623 nm, c =0.953 nm p = 94" 55" and Z=16. Recently, Wolfgang Voigt's [ l l ] work on kainite confirmed the crystal structure with a slight fkactional change in the number of water molecules (2.75 H20 instead of 3 H20) present.
3. EXPERIMENTAL Single crystals of KZnClS04.3HzO were grown by the controlled evaporation technique to which about 0.5 mole % VOS04.5H20 was added as a dopant. A single crystal of about 0.3 mmz was selected for EPR measurement. For optical absorption studies, the crystals with high concentrations (1 mole %) were used. The other experimental details of the spectral measurements were given elsewhere [7, 91 in detail.
4.
RESULTS AND DISCUSSION
4.1 EPR Studies - Hypertine interactions Figure 1 shows the EPR spectrum of V02+ions doped PZCST single crystals at room temperature for an arbitrary orientation in the zx plane. The EPR spectra of V02+ions in PZCST are very complex and show more than 24 lines in any orientation away from the crystallographic axes. However, when the magnetic field is along any one of the crystallographic axes, they merge into 2 octet hyperfine lines. The presence of not less than two octets along any crystallographic axes indicates the presence of four complexes corresponding to at least two vanadyl ion sites. The above results can be explained as follows. When the vanadyl ion enters the divalent zinc site, the V=O orients itself along any of the three mutually perpendicular Zn-HZO directions within a given metal water octahedron (Zn-HzOoctahedron) with different populations. The EPR spectra were recorded for the single crystal in three mutually perpendicular planes at 5" regular intervals. As the V02+ions easily enter the divalent host ion, the Zn2+ is the most probable site for the vanadyl ions. it may be possible for the site I of V02+ complexes to occupy either the substitutional site of Zn2+or the interstitial position. The comparable ionic radii of vanadyl ion (0.63 A") and Zn" (0.74 A") make it possible for the vanadyl for substitution into Zn2+sites.
I
I
DCOB
2300
MAGNETIC FIELD (GAUSS)
43(
Figure 1: EPR spectrum of V02' ions in PZCST single crystal in the zx plane at RT.
209
The observed V02' spectra can be fitted to the spin-Hamiltoniancontaining the Zeeman and hyperfine term [ 121
where the terms have their usual meaning. The and values were calculated at each 5" interval from the angular variation spectra in the three mutually perpendicular planes. The diagonalization procedure was carried out following the standard procedure given by Schonland [ 131 for obtaining the principal values of the and tensors. The g,, and A,, values (where i = x, y or z), so calculated for V02+ ions doped PZCST single crystals are given in Table.1. The spinHamiltonian parameters indicate that the symmetry of V02+ ions in this lattice is orthorhombic or still lower. The observed spectral features and the mirror symmetry of the crystal lattice indicate that the symmetry will be lower than orthorhombic and thus demonstrates the features of monoclinic symmetry of the crystal lattice. The EPR spectrum has also been studied for the polycrystallinesample. Figure 2 shows the EPR spectra of a polycrystalline sample at room temperature. The spin-Hamiltonian parameters (SHP's) obtained from polycrystalline data will be useful as a crosscheck to the original single crystal SH parameters but of lower accuracy (see Table 1). The EPR data from the polycrystalline sample indicate that the vanadyl ion has only one type of Zn2+site axial symmetry parameters correspondingto a single site. in the lattice at RT, giving a The optical absorption spectrum of V02+doped PZCST exhibits of three bmds centred at 14405, 16497 and 25765 cm-' and they have been assigned to 2B2g-+ 2Eg, 2 B ~and g 2A~g transitions respectively [ 1,141.
Table 1: Spin-Hamiltonian parameters of V02+:PZCST single crystals, A and P values are in units of 10-4cm-'. The errors in g and A values are 0.002 and 3 10-4 cm-' respectively.
+
Site-I g
Single Crystal
Site-11 A
g
&z= 1.920 Azz=173 gyy = 1.972 Ayy = 108 gm = 1.998 Am = 61
A
*
Molecular Orbital parameters
gzz= 1.920 Azz= 174 g y y = 1.983 Ayy = 114 gm = 1.995 Axx = 52
p2= 0.91 Powder Sample
gzz = 1.936 gxx = gyy = 1.988
Azz = 173 A m =A, = 64
E~ = 0.68 P =128 k = 0.78
210
MAGNETIC FIELD ( G A U S S )
43
Figure 2: EPR spectrum of polycrystalline sample of VO2+:PZCSTcrystals at RT Using the EPR and optical data, the molecular orbital coefficients PI2, E', the dipolar hyperfine coupling parameter P and the Fermi contact interaction parameter k have been calculated using the expressions given by Kivelson and Lee [ 1, 5, 14, 151 and are given in Table 1. The values of molecular orbital coefficients obtained are pl2=0.91and s2=0.68. The parameters (I-Pl2) and ( I - E ~ )are the measures ofthe covalencyrates [l, 6,7, 141, the former (0.09) gives an indication of the influence of o-bonding between vanadium atom and equatorial ligands while the latter (0.32) indicates the influence of n-bonding with the vanadyl oxygen. The calculated values of PI' and e2 indicate that the in-plane o-bonding is ionic and out-of plane n-bonding is significantly covalent. The parameter k indicates extreme sensitivity to the deformations of the electrm orbitals of the central vanadium ion. The large value of k (0.78) indicates a large contribution to the hyperfine constant by the unpaired s-electron and also probably a contribution from spin polarization. The standard value of P for a free ion is 160 10-4cm-I [ 1, 141. The calculated value of P in the present system is 128 x cm-I, which is considerably reduced by (78 %) when compared to the free ion value ( 1 6 0 ~ 1 0cm"), - ~ which indicates a significant amount of covalent bonding in the complex. The values of P and k are close to the values reported for V02+ions in other lattices 6, 71.
4.2 The Superhyperfine Structures At certain orientations of the crystal with the magnetic field, i.e. when the crystal makes 20" or more with the magnetic field, a few sharp lines have been observed within the ml lines. Figure 3 shows five line SHF lines observed in V02+PZCST single crystals. These lines have been identified and are expected due to superhyperfine (SHF) structure [2,3]. This SHF structure must be due to interaction of unpaired electron (impurity) with the neighbouring nuclei having nuclear spin I=2. These observations show that this ligand hf structure should result from SO-: and/or the H20 molecules in the present case. Waplak [ 161 has observed a three line SHF structure in case of V02+doped K H 3 (SO& where the ligands are SO:-. He attributed these SHF lines to the three protons from so4 ligands. Jain and Venkateswarlu observed [ 171 the SHF structure containing a quintet SHF lines in case of V02' ions in Tutton salts, where the V02+ions are surrounded by both water and sulphate ligands. Hence it can be safely assumed that these five line SHF lines result from four protons of the surrounding nuclei having I=1/2. On comparison of the other such V02+systems in literature [2, 171, it is found that the SHF arises from four protons of the surrounding water coordination, which is responsible to form the vanadyl octahedra.
21 1
.
3225
,250
MAGNETIC FIELD (GAUSS)
327
Figure 3. The observed superhyperfine lines of V02+:PZCST at 296 K and 103 K. The angular variation studies indicate that these superhyperfine transition lines are confined to two octets in the zx and planes, whereas only one set of mr lines exhibit these SHF patterns in the xy plane. This indicates that superhyperfine lines have mirror symmetry in the xy plane corresponding to two vanadyl complexes and demonstrates the mirror symmetry of the crystal lattice itself. From this, it is expected that the V02+ions enter the substitutional positions of the Zn2+ions. The populations of the observed SHF lines are found to be 1 : 6 : 9 : 6 : 1. Hence the five line SHF interactions correspond to four protons in the surrounding HzO nuclei. The ligand hyperfme splitting parameter A ~ M has been computed as 4x104 cm-'. This is in good agreement with the values of V02+ligand hyperfine separation existing in literature [2, 16, 171. The most probable ligand nuclei that are responsible for SHF interaction are from HzO ligands. The linewidth (2.2 G) as well as Asm of the ligand hyperfine lines is in good agreement with those of observations [ 14, 16, 171where the metal ion is surroundedby SO:and/or the HzO ligand molecules. EPR spectra of V02+ions doped PZCST have been studied at various temperatures fiom 103-373 K. No appreciable linewidth variation is observed in both hyperfine (hf) and superhyperfine (shf) lines, whereas the intensities of both hf and shf are found to vary linearly with temperature and is in accordance with the Boltzmann law [ 181 as expected in most of V02+complexes in diamagnetic host lattices. 5. CONCLUSIONS
(i) EPR spectra of V02+ions in PZCST have been studied in the temperature range 103373 K. Four vanadyl complexes have been identified among which, two sets of each are found to have distinct orientations. The detailed EPR analysis indicates that the two sets are magnetically inequivalent and occupy the Zn" substitutional site in the lattice. (ii) From the spin-Hamiltonian parameters and the spectral features, the lattice symmetry is found to be monoclinic.
212
(iii) The molecular orbital coefficients have been calculated fiom EPR and optical data. It is found that the inplane o-bonding is ionic, whereas the out of plane n-bonding is significantly covalent in nature. (iv) A five line superhyperfine structure patterns were observed for certain orientations of the V02+:PZCST single crystals with ratios 1: 6 : 9 : 6 : 1. These have been attributed to four surrounding protons of H20 ligands in presence of SO4 environment. (v) The angular variation of the EPR spectra indicate that only a single set of octet lines exhibit superhyperfine structure in the xy plane whereas two octet hyperfine lines exhibit shf interaction in the yz and zx planes. This shows the mirror symmetry of monoclinic crystal lattice. (vi) The intensities of the HF and SHF lines are found to be linear with temperatures in accordance with the usual Boltzmann law.
Acknowledgements: One of the authors KVN is highly thankful to CSIR, New Delhi for the award of Research Associateship.
REFERENCES 1. Prem Chand, V.K. Jain, G.C. Upreti, Magn. Reson. Rev., 14 (1988) 49. 2. Geetha Jayaram, V.G. Krishnan, Phys. Rev.(B), 49 (1994) 271. 3. G.J. Edwards, O.R.Gilliam, R.H. Bartram, A. Watterich, R. Voszka, J.R. Niklas, S. Greulich-weber, J.-M. Spaeth, J. Phys. Condens. Matter., 7 (1995) 3013. 4. S. Waplak, W. Bednarski, I.V. Stasyuk, J.Phys. Condens. Matter., 10 (1998) L 373. 5. H. Kalkan, F. Koksal, Solid State Commun., 105 (1998) 307. 6. R. Tapramaz, B. Karabulut, F. Koksal, J. Phys. Chem. Solids, 61 (2000) 367. 7. N.O. Gopal, K.V. Narasimhulu, J. Lakshmana Rao, Physica B, 307 (2001) 117. 8. P. D. Robinson, J.H. Fang, Y. Ohya, Amer. Miner., 57 (1972) 1325. 9. K.V. Narasimhulu, C.S. Sunandana, J. Lakshmana Rao, J. Phys. Chem. Solids, 61 (2000) 1209. 10. H. Linstedt, Naturwissenshaften, 38 (1951) 476. 11. Wolfgang Voigt, ICSD Code No. 26003, Freiberg, Germany, Private Coinmun., January 2000 (e-mail :
[email protected]). 12. A. Abragam and B.Bleaney, Clarendon Press, Oxford, 1970, p. 175. 13. D. S. Schonland, Proc. Phys. Soc., 73 (1959) 788. 14. A. Kasi Viswanath, J. Chem. Phys., 67 (1977) 3744. 15. D. Kivelson, S.K. Lee, J. Chem. Phys., 41 (1964) 1896. 16. S. Waplak, Acta Phys. Pol. A, 86 (1994) 939. 17. V. K. Jain, P. Venkateswarlu, J. Chem. Phys., 73 (1980) 30. 18. J. A. Weil, J. R. Bolton, J. E. Wertz, Electron Paramagnetic Resonance, Vol. 1, Wiley Intersci., NY, 1994, p. 176.
EPR in the 21'' Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
213
EPR study of several Cr3+centres in K2MgC14 single crystal H. Takeuchi", H. Tanakab, M. Mori', H. Ebisud and M. Arakawae aDepartment of Advanced Science and Technology, Toyota Technological Institute, Tenpaku-ku, Nagoya, 468-85 11, Japan bFaculty of Science, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-0033, Japan 'School of Science and Informatics, Nagoya University, Chikusa-ku, Nagoya, 464-8601, Japan dDepartment of Electrical & Computer Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan eDepartment of Materials Science & Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan EPR measurements have been made at room temperature on K2MgC14 crystals doped with chromium. Spectrum observed has been analysed by direct dia onalization method of spin-Hamiltonian matrix. Ligand octahedra in the uncompensated Cr centre are compressed tetragonally along the c axis although those in the pure matrix are elongated. By the use of the spin-Hamiltonian separation anal sis, other low-symmetry centres are ascribed to the Cr3+ ions associated with the nearest Mg2v' vacancy, the nearest K' vacancy and the next nearest K+ vacancy.
q+
1. INTRODUCTION KzNiF4-like crystals A2MX4 with the space group I4/mmm are interesting in their low symmetry properties due to two-dimensional structure composed of AX and MX3 layers as shown in Figure 1. In the fluoride crystal K2MgF4 the anion octahedron is tetragonally compressed about 1% along the axis with the Mg-F distance 1.9628 along the axis and 1.9848 along the axis [l]. It was confirmed from the negative values of and gll-g, parameters obtained by EPR study of the uncompensated Cr3+ centre in the crystal that the fluorine octahedron surrounding a Cr3+substituted for a Mg2+ion is compressed tetragonally along the axis [2]. Together with the uncompensated centre, several charge-compensated Cr3+ centres were found in K2MgF4, where the spin-Hamiltonian separation (SHS) analysis was proposed to identify the observed centres by separating the ligand configuration [2]. Later, this SHS analysis has been extended to the Fe" and Gd3+ centres in K2NiF4-like fluorides [3-61. Rudowicz [7,8] proposed a net charge compensation (NCC) model and applied it to the orthorhombic Cr3+centres in layered perovskite fluorides. We showed that
214
(a)As-growncrystal
(b)X-rayed crystal
0
Figure 1. Unit cell of A2MX4 crystal.
1
2 3 4 H(k0e) Figure 2. EPR spectra at 290K with
5
11 c.
the NCC parameter b2’2 expresses the deviation of the “separated tetragonal configuration from that for the uncompensated centre [4]. It was shown using the electrostatic theory for a 3d3 ion at orthorhombic sites [9] that the two “separated” axial terms in b2(1: and b2(2: obtained by the SHS analysis from the orthorhombic fine structure terms with lbt/b21
:6-
0
I”
>I
3-!
i
5
4
f
2I- b -
/. /
,/
2 2-
1
500
0
I
2
2.5
3
WAVELENGTH(nrn)
Figure 3. (a) Ultraviolet absorption edge, (b) A plot between LBTO.SFe, LBTlFe and LBT3Fe glasses.
hv
and hv for
245
shown in Figure 3. The optical band gap is obtained by extrapolating the linear region of the curve to the hv axis.
4. DISCUSSION 4.1. EPR studies Fe3’ ions belong to d5 configuration and it has a 6S ground state. When Fe3’ impurity complexes are situated in a crystal field with a large axial component, the free ion 6S state splits into three Kramers doublets I 5/2 ), I 3/2 ) and I 1/2 )with their separations usually greater than the microwave quantum. Normally, the selection rules permit EPR transitions in the 1 1/2 ) doublet with g = 2.0 and g = 6.0 [8]. In the present work, the authors observed a sharp peak at g = 4.2 and a broad signal at g = 2.0 and a weak signal at g = 6.4. The resonances at g = 4.2 and g = 6.4 have been attributed to Fe3+ions in rhombic and axial symmetry sites respectively. The signal at g = 2.0 is attributed to paramagnetic ions involved in clusters. The g values thereby the symmetry around the paramagnetic Fe3+ions are found to be independent of alkali ions present in the samples. From the concentration dependence of the intensity of the resonance signals at g = 4.2, it is observed that the number of Fe3’ ions participating in the resonance increases with concentration of Fe3’ ions upto 3 mol % and after that it decreases. The decrease in the intensity of the resonance signal at g = 4.2 for x > 3 mol % has been reported in several systems of iron :Itailling glasses [ 1,9,10]. The decrease in intensity may be beca-.,ae of the isolated ions are eventually incorporated into growing clusters as the concentration increases. From Figure 2, we can observe that as temperature is lowered, the number of spins increases and further we can observe a linear relationship between log N and 1/T, a phenomenon that can be expected from the Boltzmann law. From this graph, the activation energy can be calculated from the slope of the straight line. The activation energy, being the energy needed to alter the number of spins (Fe3’ ions) participating in resonance and is found to be 2.9787 x 10.’’ J (0.0186 eV). The Curie constant ( 1 9 . 6 1 ~lo”) and Curie temperature (1 16 K) have been calculated from the temperature dependence of the reciprocal of susceptibility. The Curie constant calculated in the present work is of the same order to that of measured value (1OOx 10” mol) reported for Fe3’ ions in Te0,-B,O,-PbO glasses by Ardelean et a1 [l]. 4.2. Optical band gap energy (EE,,J The optical ene;,? gap values are calculated for lithium borotellurite glass samp1.s with 0.5, 1.0 and 3.0 mol % of iron content LBTO.SFe, LBTl Fe and LBT3 Fe and the values are found to be 3.26, 3.14 and 2.82 eV respectively. The values obtained in the present work are of the same order reported for iron containing borate glasses [ 113. It is found that as the concentration of iron ion increases, the energy gap value decreases as reported in the literature [7,11]. This indicates that with increase in iron concentration, the nature of the glasses become more semiconducting.
5. CONCLUSIONS 1. In all the investigated samples, the EPR spectra exhibit three resonance signals at ~ 4 . 2 ,
246
~ 2 . and 0 ~ 6 . and 4 these are attributed to Fe3' ions in rhombic and axial symmetry sites respectively. The site symmetry around Fe3' ions is found to be independent of alkali ion present in the glass. 2. The spectra exhibit a marked concentration dependence on iron content. The decrease of intensity with increase of iron content for more than 3 mol % has been attributed to the formation of clusters in the glass samples. The intensity of the resonance signals decreases with increase in temperature according to the Boltzmann law. 3. The optical band gap energy was found to decrease with increase in iron content which indicates that the glasses become more semiconductingnature with increase of iron content.
ACKNOWLEDGEMENTS One of the authors (JLR) is thankful to University Grants Commission (New Delhi) for financial support. REFERENCES 1. I. Ardelean, M. Peteanu, S. Filip,V. Simon and G. Gyro@, Solid State Commun., 102 (1997) 341. 2. C. R. Kurkjian and E. A. Sigety, Phys. Chem. Glasses, 9 (1968) 73. 3. K. Hirao, N. Soga and M. Kunugi, !. As:. Ceram. SOC.,62 (1979) 109. 4. R.P.S. Chakradhar, A. Murali, and J.L. Rao, Opt. Mater., 10 (1998) 109. 5. D. Legein, J. Y. Buzare, and C. Jacoboni, J. Non-Cryst. Solids, 161 (1993) 112. 6. J.A. Weil, J.R. Boltan, and J.E. Wertz, Electron ParamagneticResonance-ElementaryTheory and Practical Applications, Wiley, New York, 1994, p. 498. 7. M.A. Hassan and C.A. Hogarth, J. Mater. Sci., 23 (1988) 2500. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon, Oxford, 1970, p. 203. 9. R.G. Gupta, R.G. Mendiratta, S.S. Sekhon, R. Kamal, S.K. Suri and N. Ahmed, J. NonCryst. Solids, 33 (1979) 121. 10. N. Iwamoto, Y. Makino and S. Kasahara, J.Non-Cryst. Solids, 55 (1983) 113 11. A. A. Kutub, J. Mater. Sci., 23 (1988) 2495.
EPR in the 2 1'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR study of X-ray and method
247
irradiated Ge02 glasses prepared by the sol-gel
K. Kojimaa,F. Ogura", N. Wadab, K. Yamamotoa, T. Fujita' and M. Yamazakid aDepartmentof Applied Chemistry, Ritsumeikan University, Kusatsu, Shiga 525-8577 Japan bDepartment of Materials Science and Engineering, Suzuka National College of Technology, Suzuka, Mie 5 10-0294 Japan 'Nikko Co. Ltd., Matto, Ishikawa 924-8686 Japan dSumitaOptical Glass Inc., Saitama, Saitama 338-8565 Japan X-ray irradiation of sol-gel derived GeO2 and 5Na20-94Ge02-1Er203 glasses generated many EPR signals: GeE' center (g = 1.993-1.994, 1.996-1.997, 2.000); four-coordinated germanium electron trapped center (GEC, or Ge(2); g = 1.987-1.988); self-trapped hole center (g = 2.009-2.015); and another weak signals (g = 1.965, 1.971, 1.977, 1.982, 1.983, 2.015, 2.019, 2.021, 2.025, 2.033). W irradiation yielded GeE' in the Ge02glass and GEC as well GeE' in a 99.5Ge02-0.5Er203 glass. A GeOz gel and a 0.5Na20-98.5GeO2-1Er203glass, both of which contained residual OH groups and water, were relatively unaffected by irradiation.
1. INTRODUCTION Rare-earth doped oxide glasses and films have received great attention due to the possibilities of using these materials for optoelectronics applications. Among the oxide glasses, GeOpbased glass is one of the promising hosts for waveguide amplifiers and lasers, and frequency up-conversion devices because of its lower phonon energy and high optical transparency in a wide wavelength range. We have already synthesized Er3'- and Nd3'-doped Ge02-based glasses by the sol-gel method and investigated their spectroscopic properties including absorption, fluorescence, up-conversion fluorescence and EPR spectra [ 1-41. Here we report EPR studies of X-ray and ultraviolet irradiation effect on GeO2-based glasses
248
prepared by the sol-gel process. There have been no such studies, although irradiation effects on GeO2 glasses produced by the melt-quenching [5-81 and vapor-phase axial deposition [9] methods have been reported. This report will contribute to fundamental properties of GeOa-SiOz waveguide amplifiers prepared by the sol-gel method [ 101. 2. EXPERIMENTAL
Ge02-based glasses were prepared by the sol-gel method as described elsewhere [l-41. X-ray irradiation was carried out with a Rigaku IKF-3064 X-ray fluorescence spectrometer (Rh target, 50 kV, 50 mA, dose rate: 5 X lo5Wmin) at room temperature. irradiation was done with an Ushio high-pressure Hg lamp (500 W) at room temperature. EPR spectra were measured using a Jeol JES-FE2XG X-band spectrometer with 100-kHz field modulation at room temperature. 3. RESULTS AND DISCUSSION
Figure 1 shows EPR spectra of an X-ray irradiated GeO2 glass. The original glass was transparent. Spectral intensities increased with the irradiation time. A well-known GeE’ center with g values of 1.994, 1.996 and 2.000 appeared [7-91. A resonance at g = 2.011 can be attributed to self-trapped hole (STH) [ll-131. A signal of g = 1.987 may be due to four-coordinated germanium electron trapped center (GEC, or Ge(2)) as observed in Ge02 glasses [7-91 and GeOz-SiOz glass preforms [12, 131. It is found that the signals of GeE’ and STH grow simultaneously with irradiation time, while the GEC signal tends to be saturated at the early stage of irradiation. The same will be observed below in Figs. 5, 6 , and 7. K. Ichii et al. [14] have pointed out similar tendency of the GEC signal in Ge02-SiO2 fiber preforms. Figure 2 gives the sample dependence of EPR spectra for X-ray irradiated Ge02 glasses. A spectrum (a) was obtained from a somewhat translucent glass, showing many EPR signals compared with a spectrum (b). In the spectrum (a), the centers of GeE’, STH, and GEC are observed. Signals with g = 1.983 to 1.965 may tentatively be ascribed to six- or five-coordinated germanium electron trapped centers, because such highly-coordinated germanium atoms are suggested to be formed in sol-gel derived Ge02-based glasses [15]. On the other hand, signals with g = 2.018 to 2.033 may be attributed to hole-trapped centers such non-bridging oxygen hole center W O H C ) and peroxy radical (OZ-).According to Tsai and co-workers [7], there is the possibility that those centers with small and large g values are complementary electron and hole centers. EPR spectra of a irradiated Ge02 glass are shown in Figure 3. A weak resonance at
249
330
335
340
345
350
Magnetic field / mT
Figure 1. EPR spectra of an X-ray irradiated GeOz glass. Irradiation time (in min) : (a) 0 ; (b) 1 ; (c) 5 ; (d) 10 ; (e) 30 ; (0 60 ;(g) 120.
330
335
340
345
Magnetic field / mT
Figure 2. Sample dependence of EPR spectra for Ge02 glasses X-ray irradiated for 30 min. A spectrum (b) is the same as that of Figure 1 (e). A spectrum (a) was obtained from another sample.
g = 2.003, which is observed in an unirradiated sample (a), is due to a carbon-related center This signal was not found in the transparent sample (Figure l(a)), but observed in the translucent one that gives the EPR spectrum of Figure 2(a) by X-ray irradiation. It is interesting that GeE' (g = 1.993) is produced by W irradiation. This GeE' signal was notreduced with 20 min passage of time after stopping irradiation (Figure 3(f)), but reduced with 60 min passage of time (not shown). GEC as well GeE' were observed in a W irradiated 99.5Ge02-0.5Er203 glass (not shown). In some unirradiated samples, GeE' is detected together with the carbon-related center, both of which were produced in the sol-gel reaction process. EPR spectra of a W irradiated GeOz gel are shown in Figure 4. Besides the somewhat increase in the intensity of the carbon-related signal, the spectra are hardly changed by irradiation, indicating that the gel sample, which contained residual OH groups and water, is unaffected by irradiation. The changes in EPR spectra of an X-ray irradiated 99Ge02-1Er203glass (Figure 5) are similar to those of the Ge02 glass (Figure l), since the glass composition is close to each other. Figure 6 shows EPR spectra of an X-ray irradiated OSNa~0-98.5GeO~-lEr~03 glass. The
250
I
2.003
320
I
I
I
I
325
330
335
340
320
Magnetic field mT
325
330
335
340
Magnetic field / mT
Figure 3. EPR spectra of a Ge02 glass before (a), under ( (b) 0.5 min ;(c) 5 min ), and after ( (d) - ) irradiation. Spectra (d), (e), and ( f ) were measured with 0.5 min, 10 min, 20 min passage of time, respectively, after stopping irradiation.
Figure 4. EPR spectra of a irradiated GeOz gel. Letters (a) have the same meanings as those in Figure 3.
~
330
335 340 345 Magnetic field 1
350
Figure 5. EPR spectra of an X-ray irradiated 99Ge0, - 1Er203glass. Irradiation time (in min) : (a) 0 ; (b) 30 ; (c) 60 ;(d) 120.
330
335
340
345
350
Magnetic field / mT
Figure 6. EPR spectra of an X-ray irradiated 0.5Na20- 98.5Ge02- 1Er203 glass. Letters (a) - (d) have the same meanings as those in Figure 5.
25 1
330
335 340 345 Magnetic field / mT
350
Figure 7. EPR spectra of an X-ray irradiated 5Naz0 - 94Ge02- lErz03glass. Letters (a) - (d) have the same meanings as those in Figure 5.
irradiation effect is found to be small, the intensity in the carbon-related center being reduced (the brownish color of the sample faded) and weak defect centers including GeE’ being produced. This is because the sample, like the GeOz gel shown in Figure 4, contained residual OH groups and water. EPR spectra of an X-ray irradiated SNa20-94Ge02-1Er203 glass are given in Figure 7. A resonance at g = 2.004 grows by irradiation, though it is rather overlapped with the signal of the carbon-related center. The g value of 2.004 is very close to that of the “k complex” which Purcell and Weeks [6] have observed in a y -ray irradiated GeOz rutile (tetragonal) powder. They proposed interstitial Ge3+ a possible model for this complex.
4. CONCLUSIONS In the sol-gel derived GeOz-based glasses, X-ray radiation induced GeE’, GEC, OHC such as STH, and weak EPR signals with small and large g values. UV radiation generated GeE’ and GEC. The sol-gel derived GeOZ-based glasses are considered to be suitable for the study of radiation-induced defect centers. This study will Contribute to the development of sol-gel derived GeOz-Si02 glass fibers.
REFERENCES 1. M.Yamazaki and K. Kojima, J. Mater. Sci. Lett., 14 (1995) 813. 2. K. Kojima, T. Fukuda and M. Yamazaki, Chem. Lett., (1997) 931. 3. K. Kojima, T. Fujita and M. Yamazaki, J. Non-Cryst. Solids, 259 (1999) 63. 4. K. kojima, K. Tsuchiya and N. Wada, J. Sol-Gel Sci. Tech., 19 (2000) 511. 5. R. A. Weeks and T. Purcell, J. Chem. Phys., 43 (1965) 483.
252
6. T. Purcell and R. A. Weeks, Phys. Chem. Glasses, 10 (1969) 198. 7. T. E. Tsai, D. L. Griscom and E. J. Friebele, Diffusion and Defect Data, 53-54 (1987) 469. 8. T. E. Tsai, D. L. Griscom, E. J. Friebele and J. W. Fleming, J. Appl. Phys., 62 (1 987) 2264. 9. H. Itoh, M. Shimizu and M. Horiguchi, J. Non-Cryst. Solids, 86 (1986) 261. 10. A. Martucci, M. Guglielmi, J, Fick, S. Pelli, M. Forastiere, G. C. Righini and C. Battaglin, in Sol-Gel Optics V, p2 (2000). 11. D. L. Griscom, Phys. Rev. B, 40 (1989) 4224. 12. J. Nishii, N. Kitamura, H. Yamanaka, H. Hosono and H. Kawazoe, Opt. Lett., 20 (1995) 11 84. 13. J. Nishii, K. Fukumi, H. Yamanaka, K. Kawamura, H. Hosono and H. Kawazoe, Phys. Rev. B, 52 (1995) 166 1. 14. K. Ichii, M. Takahashi and T. Yoko, Abstract of the 41st symposium on Glass and Photonics Materials of Japan, p94 (2000). 15. K. Kamiya, M. Tatsumi, H. Nasu and J. Matsyoka, J. Ceram. SOC.Jpn., 101 (1993) 1201.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
Structural studies of the fresh water (Apple) snail
253
shells
K.V.Narasimhulua,C.P.Lakshmi Prasunab, T.V.R.K. Raob, J. Lakshmana Raoa aDepartmentof Physics, Sri Venkateswara University, Tirupati - 5 17 502, India bDepartmentof Physics, Sri Krishnadevaraya University, Anantapur-5 15 003, India The (apple) snail shells containing three layers (periostracum, ostracum and hypostracum) were characterized by EPR, FT-infrared (FT-IR) and optical absorption techniques. The EPR spectrum of the organic (protein) layer periostracum shows a sharp resonance signal at g=2.032 due to a free radical namely, chonchiolin and two signals characteristic of Fe3+ions. The EPR spectra of the other two layers are found to contain Mn2+resonance signals. From the EPR spectra of ostracum and hypostracum layers, the SHP's and the ZFS parameter (D) have been evaluated and discussed with regard to the symmetry of the material. The FT-IR spectral results give strong support on the structural details of shells discussed by EPR technique. From the optical absorption spectrum of the ostracum, the crystalfield parameters Dq, B and C have been evaluated.
1. INTRODUCTION Carbonate minerals or carbonaceous shells in various forms such as calcite, aragonite, dolomite etc constitute the earth's largest COz reservoir. The marine and fresh water mussel shells in particular form themselves as the carbonate shells by absorbing much of the atmospheric COz [I]. The transition metal ions such as Fe3+,Mn2' and V4+ display a rich biocoordination in biominerals. In such cases, EPR and optical absorption techniques are the useful analytical tools for the identification and characterization of such paramagnetic cations in biological and related systems and is often used to investigate their behaviour [2-51. CaC03 exists as calcite, aragonite and vaterite. Aragonite materials and/or deposits undergo slow metamorphism into calcite [2,6] whereas the vaterite form is found to be unstable [4]. The present work is aimed at the spectroscopic studies of the shells of the edible apple snail, to know their structural details. An overview of the shells and their description and composition are given elsewhere in detail [5].
2. EXPERIMENTAL The fresh living animal shells of were collected from the fresh water ponds located around Tirupati in South Indian origin. The soft parts were removed by putting the specimen in warm water and washed them thoroughly. The shells were then dried at 40°C in a hot air oven. The three layers of the shells were separated out as
254
described in our earlier paper along with other experimental details [5]. For y-irradiation, 6oCosource was used at IGCAR, Kalpakkam, India. 3. RESULTS AND DISCUSSION EPR studies Figure 1 shows the EPR spectrum of periostracum at room temperature (RT) and at liquid nitrogen temperature (LNT). The EPR spectrum of periostracum layer at RT shows a sharp signal at g = 2.032. The intensity of the signal increases when the sample is dried at 368 K. This signal has been attributed to chonchiolin, associated with an organic protein matrix [7]. Besides this, a Dysonian signal centred at gx2.0 has also been appeared around this free radical signal on either sides. This Dysonian signal at gx2.0 and another clear signal with linewidth AHx68 gauss appearing at gx4.1 have been attributed to Fe3+ ions, which are expected to be involved in biological systems in transportation of oxygen to the tissues [8]. The EPR spectrum of fresh sample of the ostracum from the shell at RT gives a complex spectrum containing number of Mn2+ signals of at least 3 sets of six line hyperfine (hf) lines arising from the central sextet I -%> ++I +%> transition. Besides this, the forbidden doublets were also observed in between the two hf lines. The g values for the three sets varies as g =2.01f0.04. This indicates the strong deposition of Mn2+ions in this layer at random orientations. The innermost of the shell namely hypostracum is carefully collected from the shell and the operculum as well separately. The EPR spectrum of hypostracum of the shell recorded at RT exhibits a broad signal centred at gx3.45 and a sextet hf spectra of the Mn2+centred at g FS 2.01 1 as shown in Figure 1. No significant change is observed in the sextet hf lines of Mn2+ ions of this sample recorded at 103K, except an increase in intensities of the hyperfine lines at low temperatures. A broad peak appears at 153 K centred at g ~ ~ 3 . 7 7with 2 a linewidth AH = 750 gauss. As the temperature increases, the g value as well as the linewidth decreases steadily as shown in Figure 2. This can be attributed to the exchange interaction of the impurity Mn2+ions. The EPR spectrum of hypostracum sample collected from the operculum recorded at room temperature exhibits two broad resonances centred at g = 3.3 and g x 2.4 and sextet hf spectrum of Mn2+ions centred at g = 2.0.
Figure 1: EPR spectra of the fresh sample of (a) periostracum and (b) hypostracum layers of shells.
25 5
I
I
C
500 200
Temperature (K)
400
Temperature (K)
Figure 2. The variation of g m3.45 resonance (observed for hypostracum layer) and its linewidth with temperature. The EPR spectrum of hypostracum sample collected from the operculum recorded at room temperature exhibits two broad resonances centred at gm3.3and gm2.4 and sextet hyperfine spectrum of Mn2+ions centred at gz2.0. The g and A parameters calculated for all layers of the shells are given in Table 1. The D value has been calculated from the forbidden transitions as given by Nedelec et a1 [9]. A clear examination of the zerofield splitting parameter D, reflects the extent to which the coordination sphere of Mn2+ions deviate from perfect cubic symmetry and is consistent with most of the Mn2+:CaC03 lattices in calcite and aragonite symmetries. The y -irradiation of the shell species produced a change in the EPR spectrum in the free radical region giving resonances at g=2.0006, 1.9996 and 1.992 respectively. Based on their linewidths also, these signals can be attributed to CO3- and C02- radicals which are in good agreement with the reported results for y-irradiated CaCO3 materials [ 3 , 101. Table 1 The spin-Hamiltonian parameters (g and A) and the zerofield splitting parameter (D) for different layers of shells (The errors in g, A and D values are 0.002, 3 x 10"' cm-' and 4 x 10"' cm-' respectively)
ern-')
A
Material of the layer
g
1. Periostracum (Fe3') of the shell (free radical)
g = 2.0, g m 4.1 g m 2.032
2. Ostracum of the shell (Mn")
g1=2.006, g2=2.017 g3=2.030
A1=81, A 2 4 4 A3=85
108
3. Hypostracum of the shell (Mn")
2.01 1
87
137
4. Ostracum of the operculum (Mn2')
2.003
84
143
5. Hypostracum of the operculum (Mn2')
2.006
83
147
--
D
--
ern-')
256
3.2 Infrared spectral studies The infrared technique has been used to know the symmetry of CO?- molecule in different layers of the shells. The carbonate ion being D3h symmetry exhibits normally four vibrational modes as discussed elsewhere [2,6,11]. Among the four, one of the vibrational frequencies namely vl(A1), the symmetric vibration of CO': ion is reported to be infrared inactive [4,7,11]. The FT-IR spectra have been recorded in the range 2000-400 cm-' for all the two layers of the shell and is shown in Figure 3. The infrared band positions of co32and their tentative assignments in different layers of the shells are presented in Table 2. From the nature of the IR band positions, the planar bending v3(E) and the asymmetric stretching v ~ E at ) 1478 cm-' and 714 cm-' respectively in addition to the presence of infrared inactive mode vl(A1) observed at 1082 cm-' clearly indicate the deviation from the perfect calcite symmetry. From the above observed bands and their nature, it can be concluded that the carbonate ion in the shells layers is in aragonite whereas the material of the operculum is composed of both calcite and aragonite symmetries indicating the active metamorphic processes in the operculum.
Figure 3. FT-IR Spectra of ostracum and hypostracum layers of shells and the operculum of Table 2 Infrared band positions and their tentative assignments of shells (m = medium, sp=sharp, w=weak, b=broad) Band positions for Ostracum of Shell Operculum 1788(m) 1478(b) 1452 (w) 1082 (SP) 862 (w) 7 14 (SP) 700 (sp)
1798 1424
-876 712
shells in cm-'
Hypostracum of Shell Operculum 1788 (w) 1476 (b) 1455 (w) 1082 (m) 862 (m) 714 (SP) 700 (SP)
co32-molecular
Band Assignment
1798 1418
(v1 + v4) V3(E)
1084 864 714 710
v 1(A) V2(B) V4(E)
ion in
257
3.3 Optical absorption studies The optical absorption spectra observed at RT for the ostracum layer of both the shell and the operculum have been assigned to the d-d transitions of the Mn2' ions having ground state 6A&3). The observed and calculated band positions and their assignments have been given in Table 3. By diagonalising the energy matrices for d5 configurations inclusive of Tree's correction [12], the crystal field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated and are included in Table 3. The magnitude of these parameters indicates that the Mn2+ion is in a distorted octahedral environment [2, 131.
Table 3 Optical absorption spectral data of the ostracum layer of the shells and ostracum layer of the operculum Shell material
Operculum material
6Aig(S) to
4TI g(G) 4T2g(G) 4EdG> 4~2g(~) 4Eg(D>
observed (cm-')
calculated (crn-')
17694 21271 22618 26308 27389
17373 21355 23242 26027 27483
B = 750 cm-', C = 2850 cm-' Dq = 750 cm-'
observed (crn-')
17998 21277 23474 25963 27168
calculated (cm-')
17731 21568 23492 26142 27558
B = 725 cm-', C = 2950 cm'' Dq = 750 cm''
4. CONCLUSIONS
(i)
(ii)
(iii)
From the EPR spectra of the periostracum, the observed sharp line at g = 2.032 has been attributed to chonchiolin and the resonances at g 2.0 and 4.1 have been attributed to Fe3+ ions in a rhombic symmetry. The presence of Mn2+ions in ostracum and hypostracum layers has been established. The hyperfine splitting constant A and the zerofield splitting parameter D are well comparable with those of Mn2+:CaCO3 materials in both aragonite and calcite symmetries. It is found that y-irradiation of the snail shells induces changes in the EPR spectrum in the free radical region, producing resonance signals characteristic of both CO3- and COz- radicals.
258
(iv) (v)
From the infrared spectral studies, the symmetry of the C032- ion has been predicted. The slight variations in the band frequencies have been attributed to the positional changes in the C032-and crystallization processes in the layers. The optical absorption studies have been performed for ostracum of the shell and the operculum. The crystalfield and the Racah interelectronic parameters Dq, B and C have been evaluated. The magnitudes of Dq, B and C in these shells suggest that Mnz+ions are in a distorted octahedral site.
ACKNOWLEDGEMENTS One of the authors, KVN is highly thankful to the CSIR, New Delhi, for the award of Research Associateship. The authors are grateful to Mr. D. Ponraj, HASD, IGCAR, Kalpakkam, India for their help in the y-irradiation of these samples.
REFERENCES 1. Margaret-Gail Medina, Gilliam M.Bond, John Stringer, J. Electrochem. SOC., Interface, 10 (2001) 26. 2. Y.Nagaraja Naidu, J.Lakshmana Rao, S.V.J.Lakshman, Polyhedron, 11 (1992) 663. 3. R. Dejehet, S. Idrissi, R. Debuyst, J. Chim. Phys. (Fr.), 96 (1999) 741. 4. A.Boughriet, B.Mouche1, B. Revel, L.Gengembre, J.Laureyns, Phys. Chem. Chem. Phys., 1 (1999) 4051. 5 . K.V.Narasimhulu, J.Lakshmana Rao, Spectrochim. Acta, 56A (2000) 1345 and references therein. 6. K.Nakamoto, Infrared and Raman spectra of Inorganic and Coordination Compounds (lothEd.), John-Wiley & Sons, Inc., New York, 1986, p.87, 124. 7. E.L. Jordan, P.S.Verma in : Invertibrates Zoology, S.Chand & Co. Ltd., New Delhi, 1992, p. 813. 8. J.B. Neilands in : Advances in Experimental Medicine and Biology, Vol. 40; Metal ions in Biological Systems, Edited by Samat K. Dhar, Plenum Press, NY 1973, p.13. 9. J.M.Nedelec, M.Bouazaoui, S.Turrel1, Phys.Chem.Glasses, 40(1999) 264. 10. Toshikatsu Miki, Ayaku Kai, Jpn. J. Appl. Phys., 29 (1990) 2191. 11. S. Bhagavantham, T. Venkatrayudu, Proc. Ind. Acad. Sci., A9 (1939) 224. 12. A. Mehra, J. Chem. Phys., 49 (1968) 3516. 13. Roger Clausen, Klause Petermann, IEEE J. Quant. Electron., 24 (1988) 1114.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
259
ESR study of iron-sites on Fe-ZSMS zeolite Nguyen Tien Tai ', Nguyen Huu Phu a, Tran Thi Kim Hoa a, Hidenobu Hori b, Makoto Taki Institute of Chemistry, National Centre for Science and Technology, HoangQuocViet Road, Hanoi, VIETNAM
a
Japan Advanced Institute of Science and Technology, 1-1Asahidai, Tatsunokuchi, Nomi-gun, Ishikawa 923-12, JAPAN Fe-ZSM-S zeolites with Si/Fe ratio varying from 25 to 90 were hydrothermally synthesized. ESR technique at room temperature and 4.2K was used to obtain information mainly on iron species. Two groups of X-band ESR signal were observed from the zeolites. They are attributed to at least two kinds of Fe-center, namely framework and extra-framework iron. The ESR signal assignment is supported by data of vacuum treatment and Fe doped dependence. The framework iron is identified as Fe species at strong rhombic distortion of the tetrahedral coordination of Fe3+, while the extra-framework iron is identified as the antiferromagnetic binuclear ions with bridging oxygen, located in the channel system or at the surface of the zeolite. The catalytic activity of each Fe-sites on the zeolites is determined.
1. INTRODUCTION 3d transition metals as dopants in catalyst are of considerable interest in catalysts. Among them, iron in zeolites is very attractive to researchers in term of both catalytic activity and selectivity enhancements. Fe silicalite is able to catalyze the one-step oxidation of benzen to phenol with nitrogen monoxide as oxidant [l], while Fe-ZSMS zeolite is the promising catalyst for the phenol oxidation [2, 31. In a previous publication, based on IR, XRD, SEM and ion exchange technique data [4], we have reported on the synthesis, characterization and catalytic activity of Fe-ZSMS zeolite system, however the nature and magnetic behaviour of iron, doped in the zeolite lattice, were only sketchily suggested. The present contribution aims at giving a deeper understanding of Fe behaviour by ESR experiments. As high sensitive technique and the most effective tool for the characterization of iron species in different lattices, ESR technique was used for study on the magnetic behaviour of Fe-centers. The discussion is mainly focused on the interpretation and understanding of X-band ESR spectra, taken in Fe-ZSMS zeolites, synthesized by hydrothermal crystallization.
260
Table 1 Fe-ZSM5 synthesized zeolites
(%)
1 2 3 4
90 60 43 25
0. 782 0.2562 0.3676 0.6307
89 87 64 44
2. EXPERIMENTAL Fe-ZSM-5 zeolite samples with Si/Fe ratio varying from 25 to 142 were hydrothermally synthesized (Table 1). Iron concentration data, given in the table, are determined by a atomic absorption spectrometry (AAS 3300 Perkin Elmer). The Fe-ZSM-5 samples were crystallized in autoclave at 443K for 72h. The obtained solids were washed with distilled water until reaching pH=7, then dried at 393K, before calcined at 723 in airflow for 4h. Synthetic procedure is described elsewhere [4]. ESR ESR was used to obtain information mainly on iron species. X-band ESR spectra were recorded on a JES-RE3X spectrometer (School of Materials Sciences, Japan Advanced Institute of Science and Technology), at room temperature (RT) and 4.2K. For comparison of different ESR spectra, the acquisition parameters were kept constant and the microwave power was maintained below the saturation level.
3. RESULTS AND DISCUSSION 3.1. ESR observation The X-band ESR spectra of sample 2 (Si/Fe=90), recorded at RT, are given in Figure la. Main spectral features for all series of synthesized samples are rather similar and can be characterized as follow: The spectra consist of two major signal groups. The signals of the low-field spectral group, designated as Lsignals, is determined by the overlapping of various spectral lines at g>3, while a high-field group, designated as H-signals, consists of resonance transitions at g-2. According to a number of publications on similar subjects [5-71, the commonly accepted assignment of ESR signals have been as follow: The H-signals are attributed to octahedral coordination of Fe3+ ions in cationic exchangeable sites, having C3" symmetry. They are probably octahedrally coordinated by oxygen and located in the channel pore system of the zeolite. The L-signals are caused by Fe (111) ions in sites of strong rhombic distortion of the tetrahedral coordination of Fe3+.These ions substituted to Si (IV) in the zeolite lattice. Some further spectral structures are observed in very low temperature experiments or in the spectra of the vacuum treated samples. These spectral features will be described in more detail below. Fe-centers can be characterized in more detail by some additional experiments, which are described just below.
26 1
Figure 1. X-band ESR spectra of Fe-ZSMS zeolite, sample 1: Si/Fe=90, recorded at RT. a. After hydrothermal synthesis b. After the first vacuum treatment at 373K c. After exposition of sample to air d. After the second vacuum treatment at 300K
3.2. Vacuum treating cycle The sample is treated firstly in vacuum at 3 7 3 y and then ESR spectrum is taken (Figure 1b). After the vacuum treatment, the spectral profile changes strongly. The H-signals, attributed to Fe in pore, are obviously reduced, while the L-signals, attributed to Fe inside lattice, are remained almost unaffected. In subsequent step, the sample is exposed to air; ESR spectrum is acquired (Figure lc). The spectrum obtained is almost the same as one before the treatment. Finally, the sample is treated once more in vacuum and the ESR spectrum is recorded (Figure Id). The spectral profile observed after the second vacuum treatment is similar to that after the first one. The unchanged of L-signals may be explained by the stability of framework iron, while the reversible change of H-signal may be related to the transformation of Fe3+ and Fe3+/Fe2+complex, which is produced by partial reduction of Fe3+ to Fez+ at air condition. Although Fez+ions are not active in X-band ESR and measurements at temperatures of T>4& but if Fez+ is combined with Fe3+,a broad signal can be observed [S]. The reversibility of oxidation-reduction cycle of Fe ions, according to the H-signals, reveal the reversible redox cycle of Fe3+ Fez+, and may allow the preparation of suitable catalysts for oxidation reaction.
-
3.3. Temperature dependence When lowering the sample temperature to 4.2K, the spectral intensity of two groups of spectral lines changes in quite different way (Figure 2). The intensity of L-signals increases as normally when temperature goes down, but H-signal almost does not change (Figures 2a and 2b), even it decreases in the case of high-doped sample (Figures 2c and 2d). This effect firmly suggests antiferromagnetic coupling of the Fe3+ions in a (Fe3+-0-Fe3+)complex ions inside the pore system of the of the zeolite. This type of temperature dependence in the range of RT down to 77K has been observed and reported on calcined Fe-ZSMS [9] and sublimated Fe-ZSMS zeolite system
POI.
262
a
em
2w
am
Magnetic field (mT)
Figure 2 . x-band ESR s p e c t r a of Fe-ZSM5 Z e o l i t e . 21 a . Sample recorded a t RT;
Si/Fe=60,
c.Sample 4, recorded a t RT
Si/Fe=25 I
b. Sample
Si/Fe=60,
d.Sample 41 recorded a t 4 . 2 K
Si/Fe=25,
21
recorded a t 4 . 2 K
3.4. Catalytic activity Correlation of catalytic activity with magnetic and structural properties was determined, based on the oxidation reaction of phenol [4]. It is revealed that both the framework and extra-framework Fe play a role of active catalytic centers for the oxidation reaction of phenol in aqueous solution. However, the catalytic activity of framework Fe is higher than that of the extra-framework Fe. This conclusion on oxidation of phenol on Fe-ZSMS zeolite seems to be opposite to the case of the dehydrogenation of ethylbenzen by using impregnated Fe-silicalite, where the high catalytic action is attributed to the extra-framework iron, but not to the framework one [12]. cst.iyt1s s.1.ctlvity
to
co,
1
20.
0
I+a
7 0
50
100
150
2w
Reastion time (min)
Figure 3. The time dependence of selectivity to carbon dioxide a. FezOs-silicalite b. Fe-ZSMS zeolite, 17%intra-framework Fe c. Fe-ZSMS zeolite, treated by EDTA
250
0
263
4. CONCLUSION Iron doped in ZSM-5 zeolite exists in two different forms: framework and extra-framework iron. X-band ESR signals of Fe-ZSM5 zeolites with g > 3 exhibit Fe species, located at framework sites, while signals at g = 2 - 2.3 are related to Fe at extra-framework positions. Both the framework and extra-framework irons exhibit catalytic activity to the oxidation reaction of phenol in aqueous solution but not equally. The Fe inside zeolite lattice can catalyze to oxidize phenol completely than the extra-framework iron does. The authors gratefully acknowledge professor Roduner E., Institute of Physical Chemistry University of Stuttgart, for helpful discussions.
REFERENCES 1. A. S. Kharitonov, G. A. Sheveleva, G. I. Panov, V. I. Sobolev, Y. A. Paukhus, V. N. Romanikov, Appl. Catal., 98 (1993) 33. 2. K. Fajenverg, H. Debellefontaine, Appl. Catal. B: Environ., 10 (1996) 229. 3. J. C. Jansen, E. J. van der Gaag, H. van Bekkum, Zeolites, 4 (1984) 369. 4. Nguyen Huu Phu, Tran thi Kim Hoa, Nguyen Van Tan, Hoang Vinh Thang, Pham Ha, Appl. Catal. B: Environ., 901 (2001) 1. 5 . G. Calis, P. Frenken, E. de Boer, A. Swolfts and M. A. Hefni, Zeolites, 7(1981) 319. 6. K. G. Ione, L. A. Vostrikova, V. M. Mastikhin, J. Mol. Catal., 31(1985) 355. 7. L. M. Kustov, V. B. Kazansky, P. Ratnasamy, Zeolites, 7(1987) 79. 8. G. Scholz, R. Stosser, T. Grande, S. Asland, Ber. Bunsenges. Phys. Chem., 101 (1997) 1291. 9. A. Bruckner, R. Luck, W. Wieker, B. Fame, Zeolites, 12 (1992) 380. 10. El. M. El-Malki, R. A. van Santen, W. M. H. Sachtler, J. Phys. Chem. B, 103 (1999) 4611. 11. F. Grafe, Ph. D., Technical University, Dresden, 1989. 12. 0. Kan, Z. Wu, R. Xu, X. Liu, J. Mol. Catal., 74 (1992) 223.
264
EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
CW/pulsed ESR studies of Eu2"-doped SrA1204phosphor H. Matsuoka", K. Sato", D. Shiomib, Kojima", K. Hmtsu",
Furunoc, and T. Takui"
a p a r t m e n t of Chemistry, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-Ku, Osaka 558-8585, Japan bepartment of Materials Science, Gmduate School of Science, Osaka City University, Sugimoto 3-3-1 38, Sumiyoshi-Ku, Osaka 558-8585, Japan "Fine Clay Co., Ltd., Osho-Kita 1-3-8,
Hyogo 660-0063, Japan
In work, X and W-band ESR measurements of a long-lasting luminescent material SrAl~O4:Eu,Jlywere carried out in order to determine precise spin Hamiltonian parameters for the eledronic ground state. The spectral analysis including higher order fine-structure parameters reproduced the observed spectra better than the previous analysis neglecting the higher order terms. We also observed the X-baud ESR spectra of SrAlzO4:EupY whose particle are controlled in the micron range in order to examine a relationship the particle size and fine-structure parameters, showing significant differences between the observed spectra for the SrAlzO4:EupY with different particle sizes. 1. INTRODUCTION
Since it was found out that SrAl~O4:Eu(II),Dy(III)shows long lasting luminescence [l], much attention has been paid to the new luminescent material [1-6]. Two proposals made for the mechanism of the long lived luminescence by several workers [2, but the detailed mechanism has not been filly clarified yet. Electron Spin Resonance (ESR) spectroscopy provides us with information on a local setucture around the Eu(I1) ion, which is the luminescent center, well its electronic structure. High-fieldhigh-frequency powder-pattern CW-ESR measurements and a analysis neglecting higher-order fine-structure terms have been carried out for the luminescent material by et ul., showing that the Eu(@ ions in non-stoichiometric SrAlz04:EQy occupy four different sites and have intermediate zero-field splitting (ZFS) parameters [3,4]. We however, that the spectral simulations using the spin Hamiltonian parameters determined by et ul. do not reproduce completely the correspondingobserved X-band CW-ESR [6]. In this work, we reconsidered the spin-Hamiltonian parameters of SrA1~04:EUpyin terms of the combined use of X- and W-band CW-ESR spectroscopies and a spectral analysis including higher order fme-structure parameters. Spectral simulationswere carried out by the numerical exact diagonalization of the spin Hamiltonian based on a hybrid eigenfield
265
approach. In addition, X-band powder-pattern CW-ESR measurements of SrAlzO4EqDy whose particle sizes controlled in the micron range were performed in order to examine a relationshipbetween the particle size and the long lasting luminescentproperty.
DISCUSSION
2.
2.1 Reconsiderationof spin-Hamiltoninn parameters of Sr&O4;Eu,Dy
Figures l(a) and 2(a) show X and W-band powder-pattern CW-ESR spectra observed for SrA1204:Eu,Dy at room temperature, respectively. For Eu2* ion with *S7n ground state orthorhombic site, fme-structure ESR spectra can be analyzed in of the following spin Hamiltonian:
In the case of sites with lower symmetry such monoclinic or lriclinic symmetry, other terms, etc., should be considered. However, the number of which we can determine h m analyses of powder-pattern ESR spectra is limited due to problematic overparameterhation. Therefore, in this work the orthorhombic Hamiltonian were employed and the sixth order terms except were neglected. the order approximation, differences resonance fields of A4.s ++ A& -1 and - A& ++ A& +1 transitions give for & // z by the following equations, / 2 ++ +5 /
/ 2 ++
-
=-(12b,O+40b,O+12b,O)/gPe,
B 0 ( + 3 / 2 t , +1/2)-B0(-1/2t, -3/2)
+
-
=
/ gPe
and for & z by the following equations, B0(+7/2t,+5/2)-B0(-5/2t,-7/2) =
-
/ 2 ++ =
+ / 2 ++ +I
+ /
cos 29 + 5(b; cos 29 -
/ 4-
/ 2-
f)
/ 4-
cos 441)]/gP,,
-5 /
cos 29 -
cos 29 -
COSQ)/~]/
gPe7
f)
+9b: + 3 5 b , O / 4 - 2 b ~ ~ 0 ~ 2 ~ - 3 ( b ~ c o s 2 c p - b , " ~ 0 ~ 4 4 I ) ] / g P ~ , where b2q= b4q= b6q= and E= A of equations to ours has also been derived by Reynolds et ul., but the term was omitted in their [7$ In this work, two approaches for spectral analyses were employed in order to reconsider
266
,, 0
,
I
,
,
I
0.1
,
I
I , , . , , , , . , LLLILLLLCl 0.2 0.3 0.4 0.5 0.6 0.7 Magnetic Field I T ,
1
I
,
1
,
I
Figure (a) Observed and (b) calculated X-band ESR spectra for SrAhO4:Eu,Dy at room temperature. v = (bl) approach (bz) approach
2.8
3
3.2 3.4 3.6 3.8 Magnetic Field I T
4
Figure (a) Observed and (b) calculated W-band ESR spectra for SrAl~04:Eu,Dy at room temperature. v = (bl) approach (bz) approach
the spin Hamiltonian parameters of SrAlzO4:Eu,Dy. (i) We determined a set of the spin Hamiltonian parameters for SrA1204:Eu,Dy in terms of a spectral analysis based on the spin Hamiltonian including fourth and sixth order terms (termed approach First, a set of approximate parameters are obtained using the equations (2) and (3) from the observed W-band ESR spectrum. Next, the approximate parameters are refined by minimizing the followin groot-mean-squareparameter R with the help of a hybrid eigenfield method:
Bobsemed(n), &alculated(n), and stand for the number of observed data points, where observed fields, calculated fields, and weights. (ii) We also determined another set of the spin Hamiltonian parameters for SrAlzO4:Eu,Dy in terms of a spectral analysis based on the spin Hamiltonian neglecting the higher order fine-structure terms (termed as approach The best set of parameters, g, D, and E values, are directly determined by minimizing the root-mean-square parameter R in terms of a numerical calculation based on the eigenfield method, not on the perturbation approach. Finally, all spectral simulations were performed based on the eigenfield method. The resonance fields were calculated by solving numerically an eigenfield Hamiltonian [8The fine-structure transition intensities were calculated by diagonalyzing the spin Hamiltonian energy matrices with exact resonance fields acquired by the eigenfield method [8, This hybrid approach enables us to save a great deal of CPU time well to avoid technical difficulties in the numerical computation. Figures 2(bl) and (b2) show the respective spectral simulations using the spin Hamiltonian parameters determined by approaches 1 and indicating that the Eu(I1) ions occupy four
267
Table 1 Spin Hamiltonian parameters and R values obtained for SrAl204:Eu,Dy Approach 1 sl
s2
s3
1.9943
1.994
1.994
1.9944
1.9932
b:
1379
b;
Approach 2 sl
s2
s3
1.9941
1.9949
1.9936
1.994
1.994
1.994
1.9941
1.9941
1.9935
1.994
1.994
1104
1028
926.3
1377
1082
1005
898
1195
944
1028
926.3
1091
1042
1005
898
b:
0.47
6.6
5.8
8.7
bl
40
-33.2
-23.8
-36.5
b:
-57.8
78.3
-3 1.2
-59
b,"
0.7
2.2
2.4
1.3
R
0.9
1.3
1.1
0.4
4.8
4.5
2.6
3
~~
g//
* 1 All fine-structure terms and R values
given in units of lo4 an-' and mT, respectively. *2 The R values were acquired from the spectral analyses of the W-band ESR different sites. The calculated spectra were acquired by superimposing all the spectra fiom the four different sites with the intensity ratio of 6:6:2:1. The number of the Eu sites and h e The intensity ratio obtained in this work are consistent with the results by Nakamura et four different sites denoted by site 1,2,3, and 4 in Figure 2(a), which correspond site I and 11 of Sr4A1~025:Eu,and site I and I1 of stoichiometricSrAI204;EuDy in the l i t e m [4], respectively. The calculated spectra both approaches seemingly consistent with the observed W-band ESR spectrum, but the R values shown in Table 1 indicate that the spectral analysis by approach 1 reproduced the observed specbetter than by approach 2. The spectral simulations given in Figures l(b1) and (bz) reveal the higher order fme-structure terms crucial for the determination of the spin Hamiltonian parameters of SrAl204;EuDy. In order to acquire a agreement the observed and simulated X-band ESR spectra, Q-band ESR measurements and a spectral analysis considering a frequency dependence of linewidth underway. 2.2 X-band CW-ESR spectra of SrAlzOd;Eu,Dy whose particle sizes are controlled in the micron range Recently, diverse quantum effects have been observed for particles with semi-macroscopic sizes. Sharma et ul. observed that decreasing the particle size of Eu20fl203 powders h m pm to nm scale yields the enhancement of the emission intensity for fluorescence of Eu(5Do + 7F2 transition) [15]. In this work, long-lived luminescent materials, SrAI204;EUDy, whose particle sizes are controlled in the micron range were newly prepared. Their emission life time and color depend on the particle sizes. We carried preliminary X-band ESR
268
-
u--u--,
0
0.1
0.2 0.3 0.4 Magnetic Field / T
0.5
Figure 3. Observed X-band ESR spectra of SrAI204:Eu,Dy with the mean particle diameter of 6.8 p.(a) v = 9.79034 GHz, at room temperature. (b) v = 9.60478 GHz, at 4.0 K.
I , ,
0
.,
I , ,
0.1
..
L-LLU-
0.2 0.3 0.4 Magnetic Field / T
0.5
Figure 4. Observed X-band ESR spectra of SrAlzO4:EUpy with the mean particle diameter of 1.8 p.(a) v = 9.79642 GHz, at room temperature. (b) v = 9.60427 GHZ, at 3.0 K.
measurements of them with different particle sizes at room and liquid helium temperatures in order to examine a correlation between the particle size and luminescentproperties. Figures 3 and 4 show the observed X-band ESR spectra of SrAlz04;EuDy with the mean particle diameters of 6.8 and 1.8 p,respectively. There are signilkant particle size dependences in the observed spectra well the temperature dependence. It is suggested that the long-lived luminescent properties may depend on local structures around the Eu@) ions a d o r the site occupation ratio the Eu(II) ions well a particle size effecf though the homogeneity of the surface may also influence them. Q- and W-band ESR measurements of them underway in order to determine appropriate spin Hamiltonian parameters and to examine a relationship between the particle size and long-lived luminescent properties. ground-state polarized ESR spectroscopy is underway in order to acquire information on the luminescent sites.
CONCLUSIONS In work, we reconsidered and refined the spin Hamiltonian parameters of the long-lived luminescent material, SrAlzO4:Eu,Dy, in terms of X and W-band ESR spectroscopies and the spectral analysis including higher order fine-structure parameters. It was concluded that the higher order fme-structure terms required for the determination of the appropriate spin Hamiltonianparametem of SrAlz04;EuDy. X-band ESR measurements of SrAlp04:EupY whose particle sizes controlled in the micron range were also carried out in order to examine a relationship between the particle size and fine-structure parameters, showing the significantparticle-size and temperature dependences in the observed spectra.
269
ACKNOWLEDGMETNS We thank Professor T. Nakamura of Shizuoka University, Japan, for providing us with valuable discussions. We are also grateful to Professor T. Kato of Institute for Molecular science, Japan, for the W-band ESR measurements. This work been partially supported by Grants-in-Aid for General Scientific Research and for Scientific Research on Priority “Molecular Magnetism” No. 228/04 242 103 and 04 242 105), “Delocalized Electronic Systems” (Area No. 297), “Molecular Conducting and Magnetic Materials”, Grants-in-Aid for Encouragement of Young Scientists (Grant No. 12 740 385 (D.S.) and 12 740 324 (K.S.)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and also by the Ministry of International Trade and Industries (NED0 project ‘‘Organic Magnets”). REFERENCES 1. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, J. Electrochem. Soc., 143 (1996) 2670. 2. M. Ohta, M. Maruyama, T. Hayakawa, and T. Nishijo, J. Ceram. SOC.Japan, 108 (2000) 284 [in Japanese]. 3. K. Kaiya, N. Takahashi, T. Nakamura, T. Matswwa, G.M. Smith, and P.C. Riedi, J. Lumin., 87-89 (2000) 1073. 4. T. Nakamura, K. Kaiya, N. Takahashi, T. Matswawa, C.C. Rowlands, V. BeWn-Lbpez, G.M. Smith, and P.C. Riedi, J. Mater. Chem., 10 (2000) 2566. 5. M. Kamada, J. M& and N. Ohno, J. Lumin., 87-89 (2000) 1042. 6.’K. Sato, H. Matsuoka, J.Y. Bae, D. Shiomi, T. Takui, K. Kaiya, N. Takahashi, H. Fujiyasu, T. Matswawa, T. C.C. Rowlands, V. BeltrAn-Lbpez, and G.M. Smith, Proc. of 2“dAsia-Pacific EPRESR symposium, (1999) 76.
R.W. Reynolds, L.A. Boatner, C.B. Finch, A. Chatelain, and M.M. Abraham, J. Chem. Phys., 56 (1972) 5607. 8. K. Sato, H. Matsuoka, D. Shiomi, and T. Takui, Mol. Cryst. Liq. Cryst., 335 (1999) 333. 9. H. Matsuoka, K. Sato, D. Shiomi, and T. Takui, Synthetic Metals, 121 (2001) 1822. 10. N. Banwell, and H. Primas, Mol. Phys., 6 (1963) 225. 11. G.G. Belford, R.L. Belford, and J.F. Brukhalter, J. Mag. Reson., 11 (1973) 251. 12. K.T. Mcgregor, R.P. Scaringe, and W.E. Hatfield, Mol. Phys., 30 (1975) 1925. 13. K. Sato, Doctoral Thesis, Osaka City University, 1994. 14. Y. Teki, I. Fujita, T. Takui, T. Kinoshita, and K. Itoh, J. Am. Chem. Soc., 116 (1994) 11499. 15. P.K. Sharma, M.H. Jilavi, R. Nass, and H. Schmidt, J. Lumin., 82 (1999) 187.
270
EPR in the 2lStCentury A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Thermoluminescent mechanism of tridymite SiO, phosphors Masatoshi OHTA", Takato NAKAMURAb,Michiko TAKAMI" "Department of Material Science and Technology, Faculty of Engineering, Niigata University, 8050, Ikarashi 2-no-cho, Niigata, 950-2 181, Japan bDepartmentof Materials Science and Technology, Faculty of Engineering, Shizuoka University, Hamamatsu, 432-8561, Japan "Graduate School of Science and Technology, Niigata University, 8050, Ikarashi 2-no-cho, Niigata, 950-2 181, Japan
Stability of traps induced in SiO, by Al-, Eu- and Tb-doping has been examined by means of ESR and TL spectroscopy in order to elucidate relation between the depth of traps and emissions; the phosphors were prepared by a sol-gel method using optically high-pure SO,. For A13'- and Eu3+-dopings the dependence of the ESR and TL intensities on the storage time after X-ray irradiation suggests formation of a deep hole trap. Also, a shallow electron trap was found in the Eu3'-doped SO,. As for Tb-doping, TL measurements indicate that the emissions are due to Tb3', while the ESR spectrum suggests the formation of Tb4+.Furthermore, TL characteristics of the Eu3+-doped phosphors using ultra fine SiO, prepared by the hydrolysis of water glass were examined since the process is cheap and commercially available for the preparation of the phosphors with blue emission.
1. INTRODUCTION It has been well known that SiO, crystals are useful as a matrix for thermoluminescent (TL) phosphors, especially for the TL dating of earthern vessel and lava [l]. This is because they show a variety of emissions in the visible region depending on the doped metal ions. In the previous papers we have clarified the mechanisms of the TL of A13+and Eu3'-doped SiO,, in which a blue emission is due to A13' and other emissions between blue and red arise from Eu3+[2-41. In this paper, therefore, we report the results
27 1
of the investigations of the stability of traps formed in the A13'-, Eu3'- and Tb-doped SiO, prepared by a sol-gel method [5-61. the TL properties of Eu3'-doped ultra fine SiO, prepared using a reaction of water glass with hydrochloric acid were described in comparison of those prepared by the sol-gel method in order to develop cheap process for the industrial production of the phosphors.
2. EXPERIMENTAL SiO, gel was prepared as follows: lOOg of water glass were hydrolyzed with 0.6 mol/l hydrochloric acid. The obtained gel was washed thoroughly with dilute hydrochloric acid and then with deionized water in order to remove acid and watersoluble components. Various SiO, samples doped with A13', Eu3' or Tb3+were prepared by the following procedure: Measured amounts of A1,0,, Eu,03 or Tb,O, were mixed homogeneously using acetone, and then mixed with the SiO, prepared both by the solgel method (Mitsubishi Chemical Co., Ltd.,) [6] and by the hydrolytic method mentioned above. The mixture was fired at 1573 K for 6 hours in air. It was confirmed by the X-ray diffractometry that the resulting phosphors have tridymite structure. The detailed procedure was described elsewhere [2-41. The doping concentrations were selected to obtain the maximum ESR and TL intensities, respectively. The apparatus used for the measurements of TL spectra consists of a black box with a projection lamp heater, optical quartz fiber, a spectrometer with multi-channel analyzer and intensifier (Hamamatsu Photonics Co., Ltd., PMA-lo), and a personal computer for data analysis, as previously described [2-41. The SiO, sample, irradiated by X-ray (about 0.8kGy, Cu K,, 8keV), was flatly spread on a silver tray in the black box and was then linearly heated from 298K to about 680K by the projection lamp with a constant heating rate. The TL emission was guided to the spectrometer equipped with a multi-channel analyzer and intensifier using an optical quartz fiber, and the spectrum was recorded on a PC. A heating rate of 0.4 deg. s' (estimated error: was controlled by an automatic temperature regulator (Simaden, FP21) equipped with a CA thermocouple. The ESR spectra were measured on the SiO, sample irradiated by X-ray (about l.SkGy, Cu K,, 8keV) using an X-band ESR spectrometer with a 100 kHz magnetic field modulator and a phase-sensitive detector (JEOL Ltd., JES-FEXlXG).
272
3. RESULTS AND DISCUSSION
Figure 1 shows the storage time dependence of ESR signal intensity of A13+-and Eu3'-doped SiO, after X-ray irradiation, in which the A1 and Eu contents are 100 and 10 mmol%, respectively (1700 and 559 ppm for A1 and Eu, respectively). It is seen that the ESR signals start decreasing after X-ray irradiation and then become constant. Based on the g-values observed, the observed signals are due to trapped holes in the host. The decrease in ESR signal
I000
suggests that the electrons and holes produced Figure1. Storage time dependence by X-ray irradiation recombine slowly. Such of ESR spin concentration and ESR slow recombination arises from electron and holes thermally released from the traps, so that signal of (a) SiO, : (1OOmmol%) and (b) of SiO, : Eu3' TL should be observed. TL spectra of the A13+-and Eu3'-doped SiO, (1Ommol%) after X-ray irradiation. are shown in Figs.2 and 3, respectively, in which the A1 and Eu contents are 100 and 1 mmol%, respectively. For the A13'-doped SiO, a broad TL peak due to A13' was observed centered around 500 K, and showed little change in intensity with respect to the storage time. On the other hand, sharp TL peaks including two strong ones at 570 and 610 nm due to Eu3+appeared both in low and high temperature regions. Also, it was found that the TL decreased rapidly soon after X-ray irradiation, and then became constant. This trend is explained in terms of two different type of Figure2. (a) The typical TL spectraps: one is shallow traps which decrease the TL of SiO, : A13' (1 00 mmol%) rapidly, while the other is deep traps whose TL is and (b) storage time dependence independent of the time being. of TL intensity after X-ray irradPreviously, we have reported the ESR of the iation. 50 I00 Storage time I hr
I50
273
Figure3. (a) The typical spectrum of SiO, : Ed' ( l m o l % ) and (b) storage time dependence of TL intensity after X-ray irradiation.
SiO,, and found that the E,' center due to oxygen defect in the SiO, matrix [7-91 is formed. For the A13+-dopedSiO, the ESR signal is assigned to a A13+-centre consisting of non-bridging oxygen and A13+on which a hole is captured [lo]. On the contrary, Eu3+-dopingleads to the ESR signals of a hole and an electron. From the comparison of the ESR spectra of the samples heated at various temperatures and their TL spectra it was deduced that both electron and hole detected by ESR contribute to TL [2]. This implies that for the A13'-doping it occupies the Si4+site, which forms a stable hole. For the E d + doping, however, both an unstable electron trap and a stable hole trap are formed although it occupies the Same Si4+site.
Taking account of the facts described above, the difference between the storage dependence of the ESR and TL intensities shown in Figs. 1 , 2 and 3 is interpreted follows: For the A13'-doped SiO,, the TL intensity for the hole trap remain constant with the storage time although ESR signal intensity decreases gradually until ca. 80 hours after X-ray irradiation. For the Eu3+-dopedSiO, the ESR intensity decreases gradually until ca. 100 hours after X-ray irradiation, and little change was observed for the hole trap like that for A13+doped SiO,. However, it is worth noting that the TL assigned to electron trap decreases in intensity rapidly until 4 hours after X-ray irradiation. This discrepancy between the TL and ESR fading phenomena is explained as follows: As is generally known, ESR signals arise from the radicals existing both in the surface layer and bulk of SiO,, while the TL is due to the radicals existing in surface layer only. Therefore, it is presumed that although the radicals in the surface layer surely quench, they are supplied by migration from bulk of SiO,. The same thing happens for the electron trap formed by Ed+-doping. This brings that ESR signal continue decreasing after the TL quenching stopped. The ESR and TL spectra of the Tb (100 mmol%, 12300 ppm) -doped SiO, are
274
332
334
336
338
shown in Fig.4. For the ESR spectrum a broad signal with hyperfine lines, assigned to Tb4+, was observed. On the contrary, TL spectrum showed the TL peaks assigned to Tb3+at low and high temperature regions, ca. 380K and ca. 480K, respectively. Therefore, it is deduced that both Tb3+ and Tb4+occupythe Si4+site in the SiO, with less distorting the crystal lattice of SiO, for crystal optics.
340
Magnetic field I
Figure4. (a) The ESR signal and (b) the typical TL spectrum of SiO, :Tb (1OOmmol%).
Figure5. Typical TL spectrum of the Eu3'doped (10mmol%) sample using prepared by hydrolytic method.
TL spectrum showed blue emission for the Eu3+doped SO, (10 mmol%, 5590 ppm) using ultra fine SiO, prepared by cheap hydrolytic method of water glass with acid (Fig.5). Based on the results mentioned above, it is suggested that the Eu easily substitutes for Si site because of the increase of the surface free energy due to microcrystalization,
SiO, 4. CONCLUSION
Behaviors of the radicals formed in SiO, by A13+,Eu3+and Tb-dopining were examined by means of ESR and TL spectroscopy. It was found that they substitute for Si4+with less distorting the crystal lattice of SiO, for crystal optics, in which Tb has two oxidation states of Tb3+and Tb4'. Phosphors with blue emission were obtained using the
275
ultra fine SiO, prepared by cheap hydrolytic method of water glass.
Acknowledgments The authors are grateful to Mitsubishi Chemical Co. for providing the SiO, quartz prepared by sol-gel method and to Shin-Etsu Chemical Co. for providing the rare earth oxide. This work was supported by the Japan Society for the Promotion of Science through a grant-in-aid for the Scientific Research (A) No.12355030.
REFERENCES 1. Hujimura, R. and Yamashita, T., Tokyo Co., Youkendou 248-254(1985) 2. Ohta, M, Yasuda, M. and. Takami, M., J. of Alloys and Compoun., 303-304,320-324 (2000). 3. Ohta, M. and Takami, M., J., of Sol-Gel Sci. and Tec., 19, 737-740 (200). 4. Ohta, M. and Takami, M., J. of Lumin., be in press. 5. Hayashi, A., Ceram., 20 (4) ,274-279 (1985). 6. Shima, K. and Akira Utunomiya, Ceram.,33 (l), 39-42 (1998). 7. Griscom,D.L., Rev. Solid State Sci., 4, 565-599 (1990). 8. Halliburton, L.E., Appl. Radiat. Isot. 40,859-863 (1989). 9. Weil, J.A., Phys. Chem. Minerals, 10, 149-165 (1984). 10. Lell, E., Kriedl, N.J. and Hensler, J.R., Progr. Ceram. SOC.,4,3 (1966).
276
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR and luminescence spectral properties of europium compounds with trifluoroacetic acid I.V. Kalinovskaya, A.N. Zadorozhnaya, V.G. Kuryavyi, V.E. Karasev. Institute of Chemistry, Far East Division, Russian Academy of Science, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022, Russia Europium trifluoroacetates of the composition Eu(TFA)32DnH20, where TFA trifluoroacetic acid anion, D - 1,lO-phenanthroline (phen), n=l; 2,2 '- bipyridil (bipy), n=3 were proven to be resistant to UV - light in crystalline state and in polyethylene matrix. Increase of luminescent emission of europium at irradiation by UV - light was revealed. The analysis of X -ray, ESR, IR and luminescence spectra of photochemical process products was carried out. The ESR study showed that increase of the luminescent emission of europium complexes is symbiosisly to increase of the contents of bipyridil anion - radical. The ESR and IR spectroscopy data allow to make the conclusion about the mechanism of the increase of europium luminescent emission in trifluoroacetates with nitrogen-containing neutral ligands.
1. INTRODUCTION
The rare earth elements ( W E ) trifluoroacetate complexes with interesting magnetic and spectral - fluorescence properties, are few studied. In works [I-31 such properties of lanthanoid compounds with trifluoroacetic acid, as solubility, density, crystalline and fluorescence character were investigated. The REE trifluoroacetates stability to the luminescent emission has not been tested as yet. At the present study the analysis of spectral - fluorescent properties and photochemical decomposition of europium trifluoroacetates with the nitrogen - containing neutral ligands with composition Eu(TFA)32DnH20, where TFA - anion of trifluoroacetic acid, D - 1,lOphenanthroline @hen), n=l; 2,2 '- bipyridil (bipy), n=3 was carried out.
2. EXPERIMENTAL Europium trifluoroacetates were synthesized according to the procedure [4]. The IR spectra in the region 400-4000 cm-' were record at a Perkin - Elmer FT - IR device using KBr. The accuracy of vibrational frequencies definition was k2.0 cm". The low-temperature luminescence spectra were recorded at a SDL - 1 spectrometer. For this purpose the samples were lowered into a Dewar vessel with liquid nitrogen. A DRSh 250 mercury lamp was used for excitation of the complexes. The error of measurements of wave numbers for the transition bands 5Do-7F, (i=0-4) did not exceed k3.0 cm-' (number of
217 definitions not less than The excitation luminescence spectra were recorded on a device made on the basis of a SDL - 1 spectrometer and a MRD - 23 monochromator, the excitation source was a xenon lamp "Tungsram" with power 2,5 kW. The luminescence spectra of the obtained compounds have an individual nature and differ by value of Stark cleavage and ~,~ (Table 1). As the main part of Eu3'radiation is necessary to intensity of ' D o - ~ F o ,transitions 5Do_7F2 electric dipole transition (h=615 nm), the excitation spectra were recorded at h,,=615 nm . For analysis of photochemical properties of compounds the samples were subjected to irradiation by unfiltered light of high - pressure mercury DRT - 250 lamp during 7 hours. The distance between the lamp and the sample was 26 cm. The X - ray research of the compounds was carried with the out both powder and singlecrystal methods. The powders were studied at a DRON - 2.0 diffractometer in CuK, radiation. The ESR - spectra were recorded at a ESR - 231 spectrometer in X - band at room temperature.
3. RESULTS AND DISCUSSION The europium trifluoroacetates obtained complexes with nitrogen - containing neutral ligands fluoresce by red light at room temperature. The luminescent spectra of this compounds have discrete structure. The compounds in crystalline state and, inserted in polymer matrix, Table 1 The wave numbers v/cm'l, relative intensities I/%, and splitting values of 'Do-~F,(i=O, 1, 2, 4) levels (AF) in the luminescence spectra of the mixed-ligand TFA europium compounds before and after irradiation Eu(TFA)32bipy Eu(TFA)32bipy Eu(TFA)32phen Eu(TFAh2phen Trasi3HzO(unirradiated) 3HzO(irradiated) HzO(unirradiated) HzO(irradiated) tion v(I) AF v(1) AF v(I) AF v(1) 5Do-7Fo 17265 (12.0) 17269(12.3) 17107(6.9) 17282(4.4) 16925 (2 1.3) 16928 (37.1) 16879 (30.6) 16918 (22.1) 5Do-7Fl 16835 (13.8) 90 16887 (30.7) 71 16841 (16.7) 72 16875 (28.3) 82 16857 (26.3) 16807 (20.8) 16836 (23.0) 16345 (6.4) 16345 (55.4) 16238 (79.2) 16228 (88.5) 16196 (100.0) 16289 (100.0) 16317 (100.0) 16179 (100.0) 5Do-7F2 16208 (42.4) 290 16292 (65.7) 200 16109 (34.7) 129 16168(92.9) 150 16152 (85.8) 16137 (2.3) 16206 (46.2) 16078 (28.3) 16105 (18.7) 16145 (25.5) 16055 (10.8) 14548 (8.3) 14550 (10.6) 5 ~ o - 7 ~ 4 14284 (41.1) 14461 (12.5) 204 14401 (142) 14354 (23.8) 445 14373 (3 1.9) 14304 (1 1.5) 14344 (27.8)
278
were tested to UV - light stability. As it is shown at Figure 1, UV - light irradiation of finecrystalline compounds for 2-3 hours 2-4 times increases europium luminescent emission, in the same conditions the intensity of activated polymer films grows in 3-4 times. Thus, there is a noticeable redistribution of intensities of some lines, in luminescence spectra as well as the ~ electric dipole considerable changes in the structure splitting of magnetic dipole ' D o - ~ Fand 'Do-'Fz levels (Table 1) are observed. The evident changes after UV - light irradiation were also detected in the excitation spectra for studied europium trifluoroacetates. Figure 2 the excitation spectra of the investigated complexes are shown. At the moment of an enhancement of luminescence of the mixed ligand complexes after 3-4 hours of UV - irradiation) intensity of the peaks increases greatly in the region 330-470 nm. The comparison of the roentgenograms of trifluoroacetates of europium complexes before and after UV - irradiation showns (Table 2), that they are different. After irradiation there are new bands. In the roentgenograms, the intensity of some reflexes and their interplanar spaces changes. The enhancement of Eu3+luminescent emission in europium trifluoroacetate with bipyridil is the most evident (Figure la). This compounds was studied by the ESR method. In the ESR spectrum of UV - unirradiated Eu(TFA)32bipy3H20 complex at room temperature one can distinguish the two components (Figure 3). 1. Narrow symmetrical peak with AH = 1.6 mT and the center at g = 2.0038+0.0002. 2. Board intensive asymmetrical line of the width AH = 20 mT and the center, defined at the interception with the zero line, disposed at g = 2.28f0.02. I"/%
I" 1 %
a
2
4
6
8 hours
1
0
2
4
8
a
i
o
TIME, hours
Figure 1. Luminescence intensity of the Eu3+ion vs. UV irradiation time for the complexes Eu(TFA)32bipy3H20 (a) and Eu(TFA)32phenH20 (b). - fine crystalline, o - in polymer matrix. The narrow peak with g = 2.0038 in all probability concerns to a signal of a radical. On the assumption of the structure of the complex and IR - spectroscopy data, this radical can be defined bipy anion - radical. Indeed, according to the X - ray crystal analysis data of
279
Table 2 X-ray diffraction data from the europium trifluoroacetates with nitrogen - containing neutral ligands studied Eu(TFA)32phenH20
Eu(TFA)32bipy3H20 Unirradiated
VIO 10 100 32 15 15 27 17 16 17 23 15 40 10 17 30 12 12 25 12 10 10 25 17 8 33 12
d, '4 13.11 10.91 9.46 7.90 7.49 6.86 6.60 6.44 6.28 5.52 5.35 5.24 5.06 4.86 4.50 4.39 4.12 4.04 3.91 3.73 3.64 3.41 3.37 3.30 3.18 3.09
irradiated (3h) VIO
6 100 23 7 10 18 12 10 12 7 14 9 26 6 7 15 5 20 6 6 9 24 11 9 9 7 15 13
d, '4 13.23 10.91 9.46 7.87 7.49 6.86 6.58 6.4 1 6.28 5.66 5.53 5.37 5.24 5.09 4.90 4.84 4.67 4.50 4.37 4.21 4.10 4.05 3.91 3.74 3.64 3.46 3.40 3.37
Unirradiated
VIO 10 100 46 48 42 34 8 6 6 6 18 11 9 9 9 12 8 14 6 14 18 12 9 9
d, '4 14.25 12.04 1 1.75 11.30 10.30 9.71 9.27 8.11 7.53 7.17 6.23 5.70 4.90 4.79 4.40 4.23 4.06 3.91 3.77 3.72 3.54 3.37 3.29 3.18
irradiated (3h) 1/10
4'4
15 18 100 44 34 39 10 13 13 13 10 19 10 10 11 11 10 11 16 10 15 18 18 8 27
14.39 13.11 12.20 11.30 10.35 9.71 9.36 8.1 1 7.60 7.19 6.46 6.23 5.75 4.81 4.26 4.18 3.92 3.79 3.73 3.66 3.58 3.54 3.37 3.30 3.18
europium trifluoroacetate with bipyridil, coordination environment of the central ion Eu 3+ includes three oxygen atoms from three monodentate trifluoroacetates ligands, two nitrogen atoms form bipyridil molecules and three oxygen atoms from water molecules. The second bipyridil molecule is not directly coordinate by the central ion and is disposed between two other bipyridil molecules of the adjacent complexes, forming a peculiar "sandwich". There are bands in the IR spectrum assigned to a "free" molecule of a neutral
280
ligand (1596, 1567, 1427, 1250, 1016, 763 cm-'), as well as of bipyridil anion - radical of (1496, 1439, 1200, 1130, 794, 646 cm-') [8,9].
'
Et? 7F0. 'L,
'Fa- 'L,
I
a
il
EI?
1 2
360
350
400
450
500
300
350
400
450
500
Figure 2. Luminescence excitation spectra of Eu3+ from Eu(TFA)32bipy3H20 (a) and Eu(TFA)32phenH20 (b) (1 - irradiated 3 hours, 2 - unirradiated).
2.0mT
1 g=2.0038
-
50mT
g=2.28
Figure 3. ESR - spectrum unirradiated Eu(TFA)32bipy3Hz0. Powder sample. X - band
Attribution of the second signal is not evident and could not be determined exclusively from ESR data. We made this attribution to Eu2+ as a probable variant. Ed' is S ion and, therefore, it can produce an intensive signal at room temperature. Eu2+ could be present in Eu(TFA)32bipy3HzOas non - stoichiometric admixture. The value of g-factor of the center of ESR spectra of S ions could differ significantly from g-2.0023, if the ion is located in ligand
281
strong fields or in low-symmetry fields or in ferromagnetic matrix. A reliably attributed experimental spectrum is, for example, in [ 101 for Eu2+in the matrix SrA1204.
8
Figure 4. Intensity dependence of anione - radical ESR signal versus time of UVirradiation. It is characteristically, that the signal intensity from the radical permanently increases with UV - irradiation during more then 4 hours (Figure 4). Thus, it was stated by the ESR method that the enhancement of the luminescent emission observed in the europium complexes is symbiosisly to increase of bipyridil of anion - radical contents. That is why we suggest an unconventional model of luminescence development. According to this model, Eu2+is photoionized at UV - irradiation, then Eu2' is transformed into excited Eu3+ that makes an additional contribution to luminescence. Electron from Eu2+is transferred to neutral bipy - additional aion - radical bipy is formed. The new bonds in roentgenogram of long time UV-irradiated sample are connected with its partial destruction.
REFERENCES. 1. V.E. Kavun, T.A. Kaidalova, V.I. Kostin etc. Koord. Khim., 10 (1984) 1502. (In Russian) 2. M Singh, S.N. Misra, R.D. Verma. J. Inorg. Nucl. Chem., 40 (1978) 1939. 3. K.W. Rillings, J.E. Roberts. Thermochemica acta., 10 (1974) 285. 4.1.V. Kalinovskaya, V.E Karasev, A.N Pyatkina. Russian J. Inorg.. Chem., 44 (1999) 432. 5. Y.G. Klyava. ESR - spectroscopy of irregular solids. Zinatne, Riga, 1988. (In Russian) 6. L.E. Iton, C.M. Brodbeck, S.L. Suib, G.D. Stucky. J. Chem. Phys., 79 (1983) 1185. 7. F. A. Cotton, G. Wilkinson. Advanced inorganic chemistry. J.Wiley & Sons, NY, 1972. 8. M.N. Bochkarev, I.L. Fedushkin, V.I. Nevodchikov, V.K. Cherkasov, H. Schumann, H. Hemling, R. Weimann. J. Organomet. Chem., 524 (1996) 125. 9. W.J. Evans, D.K. Drummond. J. Am.Chem. SOC.,111 (1989) 3329. 10. Hideto Matsuoka, Kazunobu Sato, Daisuke Shiomi, Yoshitane Kojima, Ken Hirotu, Nobuo Furuno, Takeji Takui. Third Asia-Pacific EPRESR Symposium. Abstracts. (2001) 2P44.
282
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
ESR and luminescence studies on formation of S i ions through photochemical reactions in potassium halide crystals doped with sulfur and manganese Sarjonoa, M. Babaa, K. Ohtaa, K. Nishidatea, I. Sokolskab and W. Ryba-Romanowskib aDepartment of Electrical and Electronic Engineering, Iwate University, Morioka 020-8551, Japan bInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland Remarkable photochemical reactions in potassium halide crystals doubly-doped with sulfur and manganese were investigated by electron spin resonance (ESR) and photoluminescence (PL) measurements. It was found from the PL excitation spectrum that manganese ions were dispersed into lattices as divalent Mn2+ ions and it was further determined by ESR measurements that they were displaced with potassium ions in the form of an Mn2+-vacancy in the quenched sample. After irradiated with deep uv light, it was found by observation of the characteristic vibronic structure of PL emission spectrum, these Mn2+-vacancy ESR centers were disappeared and in turn S2- molecular ions were effectively formed. Then, it was concluded from the analysis of different types of ESR spectra that monovalent Mn' ions and neutral Mno atoms existed at the same time in the lattice. Furthermore, the isolated Mnz+ions were formed from these Mn+ ions and Mno atoms by irradiation of visible light. Such photochemical reactions exhibited the extremely reversible behavior between uv and visible light irradiations. It was confirmed from the angular dependence of corresponding ESR spectra that these three types of manganese centers except for Mn2+-vacancy centers were all isotropic. By the present method, Sy molecular ions were efficiently introduced into potassium halide crystals for the first time.
*Corresponding author: Email address:
(Sarjono)
283
1. INTRODUCTION
We often observe the formation of complexes consisting of impurity ions with an opposite charge in doubly-doped crystals. If one of the ions in a complex center is paramagnetic, then we can study the nature of the electron configuration of such paramagnetic ion and also the structure, formation and decomposition processes of complexes in detail through the ESR method.') In the previous paper, it was found that Mn2+-S2-complex centers in an as-grown KC1 crystals doped with MnS were disappeared by ultraviolet (uv)-light radiation, accompanied with the formation of S< molecular ions2)On the other hand, in a quenched KCl-MnS c stal, different type of photochemical reactions may occur, because it is known that the Mn -S2complex centers are dispersed and in turn Mn2+-vacancycenters appear in such a crystal. In this paper, it is aimed that we follows in detail the photo-conversion processes from Mn'+-vacancy centers to Mn' ion and Mno atom centers and their hoto-reverse processes by ESR measurements. We also investigated the effective role of Mn ions on formation of S; molecular ions through photochemical reactions by ESR and PL measurements, comparing the two types of doubly-doped crystals prepared with different methods.
R
2. EXPERIMENTAL Double doping of S and Mn ions into lattice crystals was carried out by two different methods. One is direct doping using Czochralski method from KC1 powder with a trace of MnS (hereafter denoted by KC1-MnS). The other is the separate and sequential doping of Mn and S ions in which a trace of Na2S was first introduced into lattice powder by Czochralski method and then Mn was introduced by diffusion from MnC12 deposited on a surface of crystals (hereafter denoted by KC1-, KBr- and KI-S:Mn). The samples were annealed at 600'C and then quenched to room temperature on a copper plate. ESR measurements were carried out at room temperature by using 9.43 GHz X-band spectrometer. Calibration of g-value was done by comparison with the resonance line of polycrystalline with g=2.0036 (DPPH). A mercury (argon) or a tungsten lamp was used for ultraviolet or visible light irradiation, respectively. For the PL spectra measurements, a sample was set in a cryostat and temperature-controlled between 8.7K to room temperature using a closed-cycle He refrigerator, and then excited by light coming out from Xe or D2 lamp through a monochromator or filter. The luminescence light was detected by a photo-multiplier tube R-928P through another monochromator. The emission and excitation spectra were obtained by a computer-aided photon-counting system.
3. RESULTS AND DISCUSSION 3.1 ESR spectra of KCI-MnS crystals Figure 1 shows a typical change of the ESR spectrum caused with uv-light in the quenched KC1-MnS crystal. The spectrum of Figure l(a) before uv-irradiation is the well-known spectrum originated from two different types of Mn2+-vacancycenters, namely type 1111 and 1112 denoted by Watkin.3' The spectrum of Figure l(b) after uv-irradiation seems to be very
284
complicated. By careful observation, however, we can read three types of spectra. Namely the quenched first-type spectrum is widely and irregularly spread over the magnetic field. The second-type spectrum I that we call it a narrow-spacing sextet is a set of six lines with a relatively narrow equidistant spacing. The third-type spectrum is another sextet with a wide spacing, called a wide-spacing sextet. When this uv-irradiated sample was further illuminated with a white light from a tungsten lamp, it showed a very drastic change as shown in Figure 2. The ESR spectrum is composed of only six lines, 2&0 2Sb0 3&o 3& A0 45bo which is the same to the third-type spectrum of Figure l(b). After that, the sample was uv-irradiated (b) re-irradiated with uv-light. Then the complicated spectrum shown in Figure l(b) appeared again. Here, it should be emphasized that the -,'.-,.+ photochemical reactions (Figure l(b) S Figure 2) observed between uv-light and white-light irradiation are reversible quite well. I I I I I I In the next observation of three types of ESR 2000 2500 3s00 4M)o 4500 Magnetic field (G) spectra, we found that six lines of the spectra did not show any angular dependence about both of the Figure '. ESR spectra Of a quenched KC1-MnS crystal, (a) before and (b) after shape and position of resonance lines. Furthermore, uv-irradiation. the third-type spectrum surprisingly did not show any angular dependence in spite of its complicated shape. The above observations indicate that all of these spectra are isotropic and make a sharp contrast with the remarkable angular dependence of the type 1111and I112 spectra. From analysis of three types of ESR spectra related to manganese shown in Figure l(b) and Figure 2, we assigned the first-type, the second-type and the third-type spectrum to a monovalent Mn' ion, a neutral Mno atom and an isolated Mn2' ion, respectively. As described later, this assignment is very reasonable to explain the formation of the molecular S2- ions which is another and important product through white-light irradiated the photochemical reaction in the .KCl sample doubly-doped with S and Mn. Concerning this Mno ESR center, it was reported by Ikeya et. that the Mno atom occupies a C1- ion The g-value and hfs parameter A for the isolated Mn2+ ion and the neutral Mno atom in KCI-MnS crystal are shown in Table 1. ESR parameters of the monovalent Mn' ion are under computational calculation. Although we tried to I I I I I I do preliminary ESR measurements of the crystals 2000 2500 4ooo 4500 Magnetic field prepared by diffusion method, only a Mno atom center has been observed among the three types Figure 2. ESR spectrum of a quenched KCI-MnS crystal after white-light irradiation. of manganese ESR centers described above.
% ~
2 --
285
Table 1 ESR mrameters for the isolated Mn2+ion and the neutral Mno atom in KCl-MnS crvstal
ern-')
Center
g-value
A
Isolated Mn2' ion Mno atom
2.0043 1.9991
82.9 20.8
3.2 Luminescence spectra of KCl-S:Mn, KBr-S:Mn and KI-S:Mn crystals In the previous work6,'), luminescence properties of quenched KCl-MnS crystals have been investigated, mainly on the vibronic emission of S; ions. In order to make the existence of Mn2+ ions clear optically'), A Emission luminescence measurements of the sulfur-doped potassium halide crystals where Mn2+ions were 8 introduced by diffusion method were g systematically carried out. Figure 3 shows orange emission and its excitation spectra at 8.8K in a quenched KCl-S:Mn crystal. The 500 Wavelength characteristic excitation spectrum composed of Figure 3. Emission and excitation spectra six bands between 300 - 550 nm is very similar of Mn2+ions in a KCI-S:Mn crystal. to the absorption spectrum of the KC1:Mn ~ r y s t a l .Therefore, ~) it was concluded that this orange emission is due to Mn2+ions, and A to F bands are assigned to the transitions from 6Al(S) to 4Tl(G), 4T2 (G), 4Al (G) / 4E (G), 4T2 (D), 4E (D) and 4T1 (P) levels, respectively. Similar orange emission and its excitation spectra were observed in KBr-S:Mn and KI-S:Mn crystals. When the KCl-S:Mn crystal was irradiated by uv-light at room temperature, S; molecular ions were efficiently produced, similar to the direct-doping KCl-MnS crystal. Then the sample was colored with the formation of F-centers. In addition, these SY molecular ions were also observed in the KBr-S:Mn and KI-S:Mn crystals with and sometimes without uv-irradiation. Figure 4(a) and (b) show the vibronic emission and excitation spectra of SY molecular ions in KBr-S:Mn and KI-S:Mn crystals, respectively. I
-
0
I
I
I
I
I
I
I
I
I
KI-S:Mn crystal
I
'
I
T=8.8K
h
'9 k 0 5
d 0
Wavelength
Figure Vibronic emission and its excitation spectra of S; molecular ions in (a) KBr-S:Mn and (b) KI-S:Mn crystals irradiated with uv-light, measured at 8.8K.
286
3.3 Photochemical reactions responsible for formation of S i molecular ions When alkali halide crystals singly doped with sulfur were irradiated with uv-light, any S2molecular ion couldn't be produced. This means that there is not any effective electron trap for capturing an electron released from divalent S2' ions by uv-irradiation. A negative ion vacancy is one of such electron traps, but not much effective. On the other hand, in alkali halide crystals doubly-doped with sulfur and manganese, Mn2+ and Mn' ions are very effective for electron capture. First, release of an electron from S2-ions, or ionization of S2-ions occurs by uv-irradiation,
s2-.[-I
+
S-
-+
e-
+
so +
[-I -+
2e-
+ 1-1 ,
(1)
and partly e-
+ [-I
F.
-+
(2)
These released electrons are captured by the following reactions, Mn2+ . [+I
+
e-
-+
Mn+
+ [+I,
(31
and further into Mn'
+
e'
(4)
Mn'.
-+
As a result, a reaction into formation of Sy molecular ions proceeds,
s- + so
-+
s2-
+ [-I.
(5)
Here, F, [-I and indicate an F center, a negative-ion vacancy and a positive-ion vacancy, respectively. As described above, the reversible photochemical reactions between a group of the Mn' and Mno centers and the other Mn2' ion center can be explained by a mechanism of capture of an electron by Mn2+ion under uv-light irradiation and release of an electron by the Mnt ion and the Mno atom under visible-light illumination as follows, Mn2+
f
e-
+ visible-light uv-light -+
Mn'
f
e'
+ visible-light uv-light -+
Mno .
Detailed analysis of isolated Mn2', Mn' and MnoESR centers and also detailed theoretical treatment of excitation spectra for Mn2' emission will be given elsewhere.
REFERENCES 1. M. I. Kornfel'd and Yu. N. Tolparov, Sov. Phys. Solid State, 9 (1968) 1607 and ibid. 2. M. Baba, H. Saga, K. Nishidate, L.O. Schwan and D. Schmid, J. Phys. SOC.Jpn., 67 (1998) 3275. 3. G. D. Watkins, Phys. Rev. 113 (1959) 79. 4. M. Ikeya and N. Itoh, Solid State Commun., 7 (1969) 355. 5. M. Ikeya and N. Itoh, J. Phys. Chem. Solids, 32 (1971) 3569. 6. R.Ye, H. Tazawa, M. Baba, K. Nishidate, L.O. Schwan and D. Schmid, Jpn. J. Appl. Phys.,
281
37 (1998) 1154. 7. R. Ye, M. Baba, K. Nishidate, L.O. Schwan and D. Schmid, Journal of Luminescence, 87-89 (2000) 542. 8. I. Sokolska, phys. stat. sol. (b), K33 (1992) 172. 9. A. Mehra, phys. stat. sol., 29 (1968) 847.
288
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
Hyperfine structure of Nd3' and Er3+ions in LiNb03 crystals 11-Woo Park', Sung Ho Choh", Sang Su Kimb, Kuk Kang' and Deok Choi' 'Seoul Branch, KBSI, 126-16 Anam-dong, Sungbuk-Ku, Seoul 136-701, Korea bDepartment of Physics, Changwon National University, Changwon 64 1-773, Korea 'Department of Physics, Myongji University, Yongin, Kyunggi-Do 449-728, Korea Rare earth ions such as Nd3+ and Er3+ in congruent LiNb03 and nearly stoichiometric crystals have been investigated to identify their local symmetry and structures with an X-band electron spin resonance spectrometer at liquid helium temperature. The spin Hamiltonian parameters of Nd3+ and Er3+in LiNbO3 could be precisely determined with nearly stoichiometric crystals by the reduction of intrinsic defects.
1. INTRODUCTION Lithium niobate (LN) crystals have usually been grown nonstoichiometric and congruent, of the same composition with the melt. Consequently they contain many defects such as the Nb antisite due to Li-deficiency [l]. However, we are able to obtain nearly stoichiometric samples by the vapor transport equilibrium (VTE) treatment [2, 31 or an addition of small amount of K20 powder in the starting mixture [4]. Rare earth doped lithium niobate has been known to widespread potentials for laser devices such as laser frequency converter [5] and optical parametric oscillator [6]. In this study, we have investigated electron spin resonance (ESR) of Nd3+and Er3' in congruent (CL) and VTEtreated LiNbO3 (VL) crystals, respectively, to identify their local symmetry and structures.
2. EXPERIMENT Neodymium and erbium doped CL crystals were grown by Czochralski method, doped with 1.3 x wt% and 0.19 wt%, respectively. The space group of the crystal is R3c with the hexagonal unit cell of a=5.15A and c=13.86A. The crystal structure
289
maintains after adding small content of rare earth impurities. To obtain the nearly stoichiometric samples they were heated in Li-rich powder, LiNbOs + PLi3Nb04, at 1100°C for 60 hours and slowly cooled down to room temperature in 24 hours. Then thin samples were prepared with the thickness of 0.5 mm for the easy incorporation of Li-atoms into CL. Similar sample preparation was reported previously [3, 71. ESR measurements were carried out by employing a Bruker ESP300 spectrometer and an Oxford ESR900 cryostat.
3. EXPERIMENTAL RESULTS AND DISCUSSION It has been reported that usual ESR linewidths of the paramagnetic centers in VL become sharper than those in CL [3] without the change of spin Hamiltonian (SH) parameters. Therefore, VL samples are desirable to determine precise SH parameters in LN. The results are characterized using the usual SH of the following form with axial symmetry:
where the notations are common. ESR spectra for both rare earth ions showed axial symmetry in the crystallographic aa-plane within experimental uncertainty, and the principal z-axis of g- and A-tensor turns out to be coinciding with the crystallographic caxis. Figure 1 shows stacked ESR spectra of Nd3+in CL measured at 4 K. The spectrum at the top is obtained when the magnetic field is parallel to the c-axis, and that at the bottom being when the magnetic field is parallel to the a-axis. The angular interval between the spectra is 6 degrees. As shown clearly, ESR signals overlap and are hard to resolve one another. However, ESR spectrum of Nd3+ in VL for B 11 c-axis was resolved well, as shown in Figure 2. Also we obtained only two strong signals with the g-values of 1.443 and 1.323, and with the peak-to-peak linewidths of 2.65 mT and 3.17 mT, respectively. The disappearance of additional signals may be due to the reduction of the intrinsic defects in CL crystals by VTE treatment. The signal at gll = 1.443 is about ten times stronger than that at gll= 1.323 by the double integration. Weaker signals near the two strong ones are originated from the hyperfine transitions of 1432 '45 Nd nuclei (both I=7/2). The g-values of Nd3+centers are summarized in Table 1, together with previous reports.
290
Table 1 g-values of Nd3+centers in LN at 4 K
present work
1
1.443( 1)
1.323( 1)
2.963(4)
Ref. 8
1.43*0.02
1.33*0.02
2.95*0.05
Ref. 9
1.42
3.136( 1 )
2.94
I
T=4K
Y
0 II c
~
= 9.4 GHz 11 c-axis
g,, = 1.443
(b)
\
\ 0 II a 1
,
1000
.
,
2000
.
,
.
,
.
4000
Magnetic Field
,
5000
.
,
6000
lo4T
Figure 1. Stacked ESR spectra of Nd3+ in CL at 4 K in the crystallographic ca-plane.
g, = 1.323 1000
2000
3000
4000
Magnetic Field
5000
6000
T
Figure 2.Comparison of ESR spectra for Nd3+in (a) CL, and (b) VL.
Er3+centers in CL showed similar behavior of Nd3+centers, the splitting of the signals, as shown in Figure 3. At least, two Er3+ signals for B 11 c-axis are recorded at the same position of gll= 14.44 and clearly show hyperfine structure due to 16'Er (Figure 4). But evenEr3+ signals start to split as the direction of magnetic field deviates from the c-axis, meaning Er3+centers have different g,-values. ESR parameters of two strong traceable Er3+centers in CL are determined as in Table 2. However, we failed to obtain relatively precise ESR parameters like Nd3+ in VL, since Er3+signals were superposed with those originating from Fe3+[ 11, an unintentional impurity resulting stronger signals after VTE treatment than before. Nd3+and Er3+ions have relatively large ionic radii of 0.995 and 0.88A, respectively, compared with the cations in LN. From our axial symmetric results and large ionic radii
29 1
for the both rare earth ions, we may propose that they are located along the c-axis in the crystal. This proposal is consistent with previous reports [ 10, 111 which argue that rare earth ions shift downward along the c-axis from the Li site to the center of oxygen octahedron. 4. SUMMARY
We observed Nd3’ and Er3+signals in LN crystals, and determined their SH parameters. Two rare earth ions in CL have two or more centers, respectively. But the number of Nd3+ centers reduces to only two centers and their linewidths become narrower after VTE treatment, while we failed to trace the Er3+ signals due to the unintentional impurity remaining in the sample.
B II
c 4K v = 9.4 GHz
B I I c-axis
P”enEr( I = 0 )
H
B II a
I
I
T I 167
I 4000
Magnetic Field / 10-4 T
Figure 4.ESR spectrum of Er3+in CL.
Figure 3. Stacked ESR spectra of Er3+ in CL at 4 K in the crystallographic ca-plane. Table 2 ESR parameters of Er3+in CL g 11
I6’Er: A II (mT)
g,
1.2
1
2
present work
14.44
2.11
0.57
Ref. 10
15.14
2.15
7.35 7.73
292
ACKNOWLEDGEMENT This work has been supported by the National Research Laboratory Program of MOST. REFERENCES 1. S. H. Choh, I.-W. Park and S. S. Kim, Proc. of the lstAsia Pacific EPR Symposium, Springer (1998) 335. 2. P. E. Bordui, R. G. Nonvood, D. H. Jundt and M. M. Fejer, J. Appl. Phys. 71 (1992) 875. 3. Y. N. Choi, S. H. Choh, I.-W. Park, E. K. Koh and S. S. Kim, J. Korean Phys. SOC.32 (1998) S643 and therein. 4. G. I. Malovichko, V. A. Grachev, L. P. Yurchenko, V. Y. Proshko, E. P. Kokanyan and V. T. Gabrielyan, phys. stat. sol. (a) 133 (1992) K29. 5. J. Capmany, J. A. Pereda, V. Bermudez, D. Callejo and E. DiCguez, Appl. Phys. Lett. 79 (200 1) 1751. 6. J. Capmany, D. Callejo, V. Bermudez, E. Dieguez, D. Artigas and L. Tomer, Appl. Phys. Lett. 79 (2001) 293. 7. Y. N. Choi, I.-W. Park, S. S. Kim, S. S. Park and S. H. Choh, J. Phys.:Condens. Matter 11 (1999) 4723. 8. N. F. Evlanova, L. S. Kornienko, L. N. Rashkovich and A. 0. Rybaltovskfi , Soviet Phys. JETP, 26 (1968) 1090. 9. G. Bums, D. F. O’Kane and R. S. Title, Phys. Rev. 167 (1968) 314. 10. D. M. B. P. Milori, I. J. Moraes, A. C. Harnandes, R. R. de Souza, M. Siu Li, M. C. Terrile and G. E. Bareris, Phys. Rev. B 5 1 (1995) 3206. 11. A. Lorenzo, H. Jaffezic, B. Roux, G. Boulon and J. Garcia-Sol&,App. Phys. Lett. 67 (1995) 3735.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
293
The nature of conduction ESR linewidth temperature dependence in graphite A.M. Ziatdinov and V.V. Kainara Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, 159, Prospect 100-letiya, 690022 Vladivostok, Russia For all orientations of the external constant magnetic field, Ho, relative to the graphite plate c-axis the linewidth of graphite conduction ESR (CESR) signal increases first with decreasing temperature, forms a distinct peak at -20 K and then falls off. The value of g-factor for HO along the c-axis increases with lowering temperature by a manner similar to that of the CESR linewidth, but for HOalong the basal plane its value does not depent on temperature. Up to the present, the nature of graphite conduction ESR linewidth temperature dependence and origin of its low temperature peak were not clear. In this work we show that a low temperature peak in the CESR linewidth temperature dependence is predictable, if the surface spin relaxation effects for graphite current carriers were taken into consideration. INTRODUCTION
The first systematic study of temperature dependences of graphite Conduction ESR (CESR) signal parameters was carried out as early as 1960 by Wagoner [l] using a natural single crystal specimen in the temperature range from 77 K up to 600 K. After Wagoner a number of authors [2-71 conducted similar studies on a variety of well-defined specimens of graphite, and have obtained nearly the same results. In particular, in all samples investigated and for all orientations of the external constant magnetic field, Ho, relative to the c-axis the graphite CESR signal linewidth increases first with decreasing temperature. According to the data of Matsubara et [7], the AH(7)-dependence forms a distinct peak near 20 K and then falls off. At present there is no consensus between researchers on both the origin of graphite CESR linewidth and of its temperature dependence. Kawamura et [4] showed that at Hollc the Elliot [S] expression for the CESR linewidth, due to carriers interacting with phonons and/or impurities, which for (& is Debye temperature) can be written as: constx(AgJ'/ym
(1)
(Agc=gc-go,where gc and go are the g-factor values respectively for graphite current carriers and for free electron, y is the electronic gyromagnetic ratio, * is the carriers effective mass, and is the carriers mobility), describes the graphite CESR linewidth dependence in the interval 77+300 K qualitatively at least. Matsubara et [7] considered the temperature variation of graphite CESR linewidth at Hollc as a direct consequence of motional narrowing
294
effect through an averaging process of g-values of scattered carriers over the Fermi surface in the limit of incomplete line averaging. In this limit the g-shift is averaged over all energy states of current carriers in k-space, but the linewidth contains the components which are proportional to the square of the microwave frequency. Kotosonov [3] pointed out that the small spectral linewidth in graphite may result from complete averaging of the g-factors over all energy states of current carriers during the spin-lattice relaxation. Thus, for example, in synthetic graphite samples the temperature change from 40 K to 100 K leads to the g, changing by -0.2, which agrees with the resonance field shift by - 3 ~ 1 0T,~ whereas ~ the CESR linewidth remains within the limits of several oersteds. According to the literature data [S, 91 the Debye temperature of graphite is about 400 K. Therefore, the description of the graphite CESR linewidth temperature dependence by Eq. (l), proposed by Elliot for is not obvious. Furthermore, this expression does not explain the presence of linewidth temperature dependence at Holc even at a qualitative level since in this orientation of HO the value of g-factor, ga, does not depend on temperature. The independence of the CESR linewidth on the microwave frequency shows that Matsubara et [7] interpretation of the linewidth temperature dependence as a result of the motional narrowing of incomplete averaging line is not correct also. Besides, the presence of lowtemperature peak in AH(7') curve also at Holc, where g-factor is temperature independent, shows that the origins of low-temperature peaks in and dependences are different. The Kotosonov's [3] point of view does not contradict to the experimental data, but he did not consider the nature of linewidth temperature dependence. We have studied the dependences of CESR signal linewidth in highly oriented pyrolitic graphite (HOPG) on sample dimensions and on temperature for different orientations of HO and have shown that all obtained experimental data on CESR linewidth in graphite may be explained well, if the surface spin relaxation of current carriers was taken into consideration. 2. EXPERIMENTAL
The CESR measurements were carried out using an X-band E-line spectrometer in a rectangular cavity with TE102 mode. The frequency and amplitude of HOmodulation were 2.5 and 0.1 mT, respectively. All experiments were carried out on samples in the shape of rectangular parallelepipeds with the dimensions: width (1)xheight (h)xthickness (4,where hxl is the area of basal plane. At the experiments, the basal Zxh and lateral dxh sides were parallel to the magnetic component, Hfi, of the microwave field and the c-axis was perpendicular to it. Note, that the rectangular resonator, the structure of electromagnetic field of TEIOZmode has such a form that, at a conventional setting of the resonator, HO)IEfi(the electrical component of the microwave field). The study of dependences of graphite CESR lineshape parameters on sample dimensions were carried out on HOPG plates with dimensions: lxO.355x0.072 cm3. The accuracy in the cm. determination of the sample dimensions was 5x The temperature studies of CESR spectra of the samples investigated were carried out in the temperature range from 100 K to 350 K. The temperature was varied by regulating the rate and temperature of nitrogen or helium gas flow through the quartz Dewar tube with the sample. The temperature was maintained and measured with an accuracy of -0.1 K/h and -0.5 K, respectively.
-
295
Figure 1. Temperature dependence of g-factor (a) and linewidth (b) for graphite. In (b), the theoretical curve 1 (2) was calculated with constant (determined by the Exp. (4)) value of intrinsic conduction ESR linewidth. 3. RESULTS
For HOPG plate investigated the CESR line is of typical Dyson [l 11 form and indicates a large g-factor anisotropy. The g,-value is about 2.047 at room temperature and first increases monotonously with lowering temperature so as to exceed 2.16, forms a peak at -20 K, and then steeply falls off (Figure la). The g,-value shows almost no shift from the free-electron value irrespective of temperature ( Aga=ga - go - 3 ~ 1 0 ~(Figure ) la). When HO is in the c-direction, the linewidth as narrow as - 6 ~ 1 0 - T~ near room temperature and first increases @ @ @ p E lremarkably * with lowering temperature, 4and then the rise of the is followed 0 1 2 3 also by a distinct peak at -20 K similar to that of the gc-shift (Figure lb). When I, lo-' cm HOlc the linewidth also increases with lowering temperature (Figure lb), by a Figure 3. The experimental (dots) and manner similar to that of the despite theoretical (lines) values of CESR linewidth, the fact that Aga does not depend on AH, in graphite vs. sample width 1. The line 1 temperature (Figure la). The value of was calculated using the value of G=180 (0) CESR linewidth monotonously changes cm-'. K; H ~ l c . with on the constant magnetic field
296
orientation. At all temperatures the is larger than the The value of CESR linewidth tends to the infinity, while 1 (HOPG plate size in a basal plane) tends to zero (Figure 2). At all temperatures the microwave field power and frequency, and the frequency of Ho modulation had no observable effect on the CESR linewidth. 3. DISCUSSION
The character of temperature dependence of CESR linewidth on 1 (Figure 2) unequivocally specifies the presence of the contribution of surface spin relaxation into total spin relaxation of current carriers in HOPG plates investigated. Indeed, while the experimental linewidth tends to the infinity, the corresponding theoretical values calculated using the well-known Dyson [l 11 expression for CESR line shape, which is not containing the surface spin relaxation parameter G,=3~~/4/1, ( E ~is the mean probability of spin reorientation during the collisions of current carriers with the lateral graphite surfaces and A, is a mean free path of current carriers in a basal plane) tends to the finite value, which differs from that for wide plates by -10% only. At the same time, the theoretical curves M(I)with the value of Dyson [l 13 surface spin relaxation parameter Ga=180 cm-' describes the experimental data well (Figure 2). Basing on this fact, we also considered the temperature dependence of CESR linewidth in HOPG studied in the frameworks of the model including surface spin relaxation effects of current carriers. Moreover, we suppose the presence of a small amount of the localized spins (-1% of the current carrier concentration or near one localized spin per lo6 carbon atoms) and complete averaging of g-factors of the conduction electrons and localized spins in HOPG studied. In such case, the CESR linewidth AHi (i=a, c) can be presented in the following form:
where AH,, and AH,, are the linewidths of CESR signal due to conduction electrons and localized spins, respectively; AH,,= f AH,:tr , where and AH::tr are contributions to the total conduction electron linewidth due to their interactions with sample surface and inner imperfections, respectively; xe and xs are the Curie and Pauli paramagnetic susceptibilities, respectively. At the calculations we assumed that
where is a constant depending on physical properties of the sample surface. Because the Elliot's expressions [8] for the intrinsic spin relaxation of current carriers were calculated for the simple isotropic metals, their applications to graphite is not obvious. Therefore, the calculations of AH, were carried out by us with values of both independent, and dependent on temperature according to the Elliot [S] law for and I To with each other. The same holds for the Sz = -112, I To p> and I T., states. In the extreme of strong coupling, the above six states separate into doublet and quartet states, which can form another basis set of the system. It should be noted that the middle four state, Y, Y,, Y4,Y5have both of doublet and quartet characters whereas Y, and Y6,are of pure quartet character. The spin polarization patterns are determined by how the transient spin-states are initially populated and also by spin dynamics after the states are formed. Here, there are two possible mechanisms, which are likely to be characteristic in the weakly coupled triplet-doublet pairs but cannot operate in the absence of coupling. One is selective electronic relaxation from the middle four states to the ground state due to their doublet character and the other is spin-flipflop relaxation between the same Sz quantum-number states[2]. Both mechanisms should affect not only the spin-polarization patterns but also their time development. 3 EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Cu-C3-H2 Figure 4 shows time-resolved EPR ( TREPR ) spectra of Cu-C,-H, in a toluene frozen glass at 80 K. The upper spectrum is taken with direct excitation of the free base half at 640 nm at early delay time, 0.8 ps, whereas the lower spectrum is after excitation of the copper porphyrin with 540 nm and its delay time is 10 ps. In both spectra, the main component is the weakly-coupled triplet-doublet pair state, 2,4[2S0-’Tl] (see Figure 1) in which a triplet excitation is localized on the free base half and the copper porphyrin is the ground state doublet as described in the Introduction section. The large difference in the spin-polarization patterns initially comes from the different pathways by which this final excited state is generated. However, the width of the spectra and the magnetic field positions corresponding to canonical orientations are the same, consistent with the assumption that we see the same excited state of the dimer in both cases. When the final excited state of the dimer is formed via ISC with hex = 640 nm ( top of Figure 4 ), the observed spectrum is quite similar to that of the free base monomer. This implies that the selection rules for the ISC process within the free base half is principally the same as the monomer, although the excited states of the dimer should be described as a coupled system with the copper doublet. However, the time dependence of the EPR signal is different from the monomer. After prompt formation of the triplet excitation in the free base, the EPR signals at the field positions for the X canonical orientation in the dimer decay much
n
kx=Wnm
t
quicker than those in the monomer. In contrast, the signal decays corresponding to the Z orientation are similar in the dimer and the monomer. On the other hand, the intramolecular EnT pathway gives a considerably different spin-polarization pattern as shown in the bottom of Figure 4. The spin-polarized spectra rise slowly compared with the response time of the spectrometer and energy transfer rate. 3.2 Cu-p-Bp-H, In this dimer, the two porphyrin
halves are rigidly linked via a biphenyl spacer and in a linear geometry. Due to a L slight difference of the peripheral substituents, the zero-field splitting Magnctic parameters of the corresponding monomer free base are different from those for Cu-C,-H,. However, apart Figure 4. TREPR spectra of Cu-C,-H, in from this difference and an additional toluene at 80 K. The top spectrum is obtained signal in the middle of the resonance with 640 nm excitation ( PH, selective, see field, the observed behavior in this dimer Figure 1) at 0.8 ps delay time whereas the is very similar to Cu-C,-H,, . With the bottom spectrum is taken with 540 nm ISC pathway, the spectral pattern is due excitation ( PCu selective) at 10 ps delay to spin-orbit ISC and has large intensities time. at X-canonical orientations, the decay of which is the fastest. When the intramolecular EnT leads to the final excited state, the characteristic polarization pattern is also observed with a slow rise. These observations confirm that the common features in TREPR signal behaviors, that is, i) fast decay in specific orientations in the ISC spectrum, ii) slow rise and characteristic polarization pattern via EnT pathway, are indeed of weakly coupled porphyrin triplet - copper doublet systems. I I
I
I
I I
Mechanisms With the direct excitation of the free base of the dimers, it is equivocal that the initial spin polarization of the final state 2*4[2S0-’T,] is due to SO-ISC within the free base moiety. The fast decay observed at the X-canonical orientations is ascribed to dynamics after the ISC. Two mechanisms are proposed as characteristic features of the triplet-doublet system in the previous section: selective depopulation and spin-flip-flop relaxation. Both of the mechanisms are consistent with the fast decay since these two decrease population differences between the Y, or Y6 state and the middle of four states in the case of the free base ISC, where the selection rule is Px>Py>Pz. This is because 1 To and 1 To p> states have the Xtriplet wavefunction character with B,/K and are populated dominantly with this orientation. On the other hand, the observed spin polarization via EnT pathway is generated after the
360
final excited state is formed. Under our experimental condition, the precursor copper quartet excited state is deduced to be in a thermal equilibrium[5-6] and thus energy transfer does not cause strong intensity of the initial spectra. Under the assumption that energy transfer occurs according to the spin-quantum numbers, either of spin-selective depopulation or spin-flip-flop relaxation increases spin population differences between the Yl or Y6state and the middle of four states in the six-level system. Thus spin-polarized spectra rise with this rate via the EnT pathway. While it is difficult to determine definitely which of the mechanisms is responsible at the moment, we note that the rise time for the ISC pathway and decay time for the EnT pathway are almost identical and obtained as 2ps and 1 ps for Cu-C,-H, and Cu-p-Bp-H,, respectively.
4.
This work is supported by the Natural Sciences and Engineering Research Council (NSERC) and by two Grants-In-Aid for Scientific Research from JSPS, No. 11694061( International Joint Research) and No. 13640554.
REFERENCES 1. M. Asano-Someda, A. van der Est, U. Kriiger, D. Stehlik, Y. Kaizu and H. Levanon, J. Phys. Chem. A, 103 (1999) 6704. 2. A. van der Est, M. Asano-Someda, P. Rogogna and Y. Kaizu, submitted. 3. K. Ishii, J. Fujisawa, Y. Ohba, S. Yamauchi, J. Am. Chem. SOC.,118 (1996) 13079. 4. C. Corvaja, M. Maggini, M. Prato, G. Scorrano, M. Venzin, J. Am. Chem. SOC.,117 (1995) 8857. 5 . W. A. J. A. van derPoel, A.M. Nuijs, J.H. van der Waals, J. Phys. Chem., 90 (1986) 1537. 6. N. van Dijk, M. Noort, J.H. van der Waals, Mol. Phys., 44 (1979) 891.
EPR in the 21* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
361
Pulsed-ESR investigations of the photo-excited triplet state of naphthalene Kunio Taguma" , Jun Yamauchi" and Masaaki Babab "Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan bFaculty of Integrated Human Studies, Kyoto University, Kyoto 606-8501, Japan
Electron spin echoes of photo-excited triplet states have been observed for naphthalene and naphthalene-d, molecules oriented in single crystals of durene at low temperature. In this study, three comparisons were made of the relaxation times (TI, T,) and echo decay curves at 6 K First, relaxation times were determined and compared at three magnetic field directions parallel to the molecular axes. Secondly, there were some differences between the echo decay curves at two resonance lines (low-field resonance and high-field resonance) of each molecular axis, in terms of modulation, number of decay components, and relaxation times. Thirdly, the characteristics of the echo decay for a naphthalene/durene system and those for a naphthalene-dddurene system were also compared. The origins of these differences were discussed.
1. INTRODUCTION The first ESR study of the photo-excited triplet states of aromatic molecules was reported on naphthalene in 1961 [l]. Since then, photo-excited triplet states have been studied for various aromatic molecules by ESR and optically-detected magnetic resonance (ODMR) [2, 31. Furthermore, pulsed-ESR techniques were applied to the studies of the photo-excited states of aromatic molecules. The first pulsed-ESR study was reported on quinoxaline-d, and naphthalene-d, at liquid helium temperature in 1975 ESEEM spectra of triplet excited states have been observed at room temperature, and hyperfine tensors have been determined for polyacene (anthracene, tetracene, and pentacene) in p-terphenyl single crystals [5-71. However, there is little information available about relaxation mechanisms and relaxation times. This paper reports on investigations about the spin echo decay on naphthalene and
362
naphthalene-d, in durene single crystal. By investigating relaxation times, we have obtained the key to understanding triplet spin dynamics, especially excited triplet state dynamics.
2.
EXPERIMENTAL
Naphthalene (or naphthalene-d,)-doped durene single crystal was prepared by the Bridgman method, described by McClure [8]. A mixture of guest and host molecules is sealed in a glass tube and degassed. Then, the mixture is melted and cooled very slowly. Concentrations of both guest molecules were 2% in mole ratio when melted, but, in the single crystal used in the measurements, the guest concentrations were unknown. It is assumed that the guest molecules lies in lattice sites of durene crystal where the molecular axes of the guest molecules nearly coincide with those of the host molecules (see Figure 1). The X-ray data of the durene single crystal [9] proves that there are two inequivalent sites in the unit cell. However, the Y axes of the durene molecules in both sites are almost perpendicular to the crystal cleavage plane (ab plane), and other axes are almost in the ab plane. Therefore, two resonance lines may be observed when magnetic field is applied perpendicularly to the ab plane, and four resonance lines are visible when it is applied parallel to the ab plane, in which case the two lines come from molecules of one site and two other lines come from those of another site. All measurements were done by X-band PULSE ESR spectrometer JEOL JES-PX1050 at 6K. A Hg-Xe UV lamp with filter (230-440nm) was used for continuous photo-excitation. A magnetic field was applied along the molecular axes of the guest molecules, the directions of which were determined by CW-ESR. Microwave power, frequency, and the time between the first and second pulses of the 3-pulse sequences were adjusted to maximize all spin echoes and obtain a better S/N ratio; therefore, these conditions were slightly different for every echo decay curves. The parameters used in this experiment are listed below. Microwave power : 110-180mW, Microwave frequency : 8.91-8.92GHz 2-pulse sequences : 90"(20ns)- - 180"(40ns) 3-pulse sequences : 90"(20ns) - - 90"(20ns) - T - 90"(20ns) :300 380ns)
-
Y
Figure 1.
Y
The molecular axes of the naphthalene and durene
Z axis is perpendicular to the molecular plane.
363
3.
RESULTS
3.1. Naphthalene/durene (NWD) system 3.1.1. Modulation For the 2-pulse echo decay, echo modulations were observed along all molecular axes, which were markedly stronger at low-field lines than at high-field lines (see Figure 2). From analyses of the modulation frequencies and the previous CW-ESR data [lo], these modulations are considered to be caused by protons at the a-position. From theoretical considerations, if the magnetic field is completely parallel to the guest molecular axes, only ,8 -proton modulations should appear. However, CY -proton modulations mainly appeared because there were crystal alignment errors, and, consequently, because hyperfine interactions of the a-proton influenced decay curves. This is supported by the fact that hyperfine interactions of the -proton are larger than those of the ,8-proton [lo]. 3.1.2. Echo decay curves and relaxation times The spin-lattice relaxation time (TI) and spin-spin relaxation time (TJ were determined for both low- and high-field resonance lines of all molecular axes by a single-exponential fitting of the echo decay curves. The relaxation times determined are listed in Table 1. It seemed that multiple decay components existed for several high-field decay curves (for example the highfield curves of Figure 2), so these curves were fitted by the sum of two single-exponential functions. The two numbers in parentheses indicate the two components of the decay curves in Table 1. For a comparison of the relaxation times, the underlined components used below because the relaxation times of the same order are considered to indicate the same c
-4.
0.6
0.4
I
I
I
I
0.7
fls
1
l tm.
1
1
I
0.75 0.4 fls
Figure 2.
I
0.8
1.2 fls
1.6
2
0.4
0.7 fls
X axis Y axis Z axis The 2-pulse echo decay curves of the naphthaleneldurene system. Uppers are the curves at the low-field lines, lowers at the high-field lines.
364
Table 1
The T, and T, of the two systems ; naphthalene/durene and naphthalene-dddurene Numbers in parentheses are the relaxation times when it is assumed that a decay curve consists of two decay components. ‘Low’ or ‘High’ means low-field resonance or high-field resonance. NaDhthalene/durene Navhthalene-dddurene Magnetic field TI (W T, TI T2 Direction Low High Low High High High H II 0.69 0.55 0.61 0.71* 2.8* 0.67 (0.05-0.06, (0.07,U) 0.74-0.75) * 2.3 11 1.1 55 0.36 H 1I y 0.61 (0.45,63) (Qa4.5) (0.20,2.0) 0.36 0.10 1.1 1.4 0.51 H 11 Z *not best-fitting values but approximate value
x
relaxation phenomena. Both TI and T, were significantly different when the magnetic field was applied parallel to the three molecular axes. The TI in the Z direction was the longest. On the other hand, the T, in the X direction was the longest. Another feature is the difference in both relaxation times between the low-field resonance and high-field resonance. The T, of the low field was longer than that of the high field in the X and Y directions, although it was shorter in the Z direction. The T, of the low field was longer than that of the high field in the Y and Z directions, while it was shorter in the X direction.
3.2. Naphthalene-dddurene (ND/D) system Echo decay curves were also obtained for the naphthalene-dddurene (ND/D) system except for the low-field resonance in the all magnetic field directions and for the high-field resonance in the Z direction (see Table 1).At the four resonance lines, the relaxation was too fast to record echo decay curves. The TI changed considerably, and the T, becomes shorter than in the naphthalene/durene (NH/D) system. The magnetic field direction dependence of the T, was the same as that of the NH/D system, i.e., the T, was the longest in the X direction. The relaxation times for this system are also presented in Table 1.
4. DISCUSSION Echo modulations at low-field lines were strong, whereas there were almost no modulations at high-field lines. One of the reasons for the difference is attributed to the
365
magnetic field magnitude. The influence of magnetic field fluctuation by nuclear spin precession on the electron spins becomes weaker as the magnetic field becomes stronger, the modulations become weaker. Not one but two (fast and slow) components evidently existed for the five echo decay curves, which are the 3-pulse echo decay curves at the high field in the Y direction for the NH/D system and the four 2-pulse echo decay curves at the high field in the X and Y directions for both systems (see Table 1). Although all echo decay curves are expected to contain two (or more) decay components, only the one (fast) component appeared in all the echo decay curves except the above-mentioned five. This was because their echo intensities were so weak that the echo itself could barely be seen when the slow decay began to appear. The T, of the ND/D system is quite different from that of the NH/D system, i.e., the deuteration of naphthalene molecules changes the T,. Since the major relaxation mechanism is the Raman process in this temperature region (6K) [ll], the vibrations of naphthalene molecules mainly affect the T,. By deuteration, the energy interval of C-H vibration becomes small, so the relaxation via the Raman process becomes faster for the ND/D system. Both components (the fast and the slow one) became faster, and the T, changed from 55ps to l l p s for the high-field resonance in the Y direction. It is predicted that the T, of the ND/D system includes two components, whereas that of the NH/D system only has a fast component (except for the high-field resonance in the Y direction). This accounts for the T, of the ND/D system being longer than the T, of the NH/D system in the X direction. The T, of the NH/D system is dependent on the magnetic field directions. The T, in the Z direction is the longest. Thermal fluctuation of the spin system by lattice vibrations, in this case, by the vibrations of naphthalene molecule, leads to spin-lattice relaxation and varies in the directions of the spin. Two spins are almost quantized along the applied magnetic field; thus, the T, differs when magnetic field is applied parallel to the three molecular axes. The T, of the high field in the X and Y directions and that of the low field in the Z direction are shorter than that of another resonance line. This is attributed to the wave functions between which the transitions occur. In the case of the naphthalene molecule, I O > e l + l > transition occurs at the high field in the X and Y directions and at the low field in the Z direction; then, in these transitions, the spin-lattice relaxation between 1+1>el-1>will occur as well as between I O > e l + l > . At the low field in the X and Y directions and at the high field in the Z direction, only the relaxation between 1-1>elO>occurs. Thus, the T, becomes shorter at the high field in the X and Y directions and at the low field in the Z direction. The T,, which is underlined in the both systems in Table 1, is almost the same, but by deuteration the T, component 4.5ps of the NH/D system changes to 2.0ps for the high field in the Y direction. Therefore, the underlined components are caused by spin-spin interactions, whereas the 4 . 5 ~ scomponent (and also the 2.0ps component) is caused by hyperfine interactions. The T,, which is underlined in the both systems, is the longest in the X direction.
366
This suggests that the T, is longer when the magnetic field is parallel to the long axis of the molecules (X axis) and shorter when parallel to the short axis (Y axis), and, consequently, that the length of the T, reflects the magnitude of the spin-spin interactions, i.e., the distance between the two spins. The T, in the Z direction of both systems includes the two components. The T, of the NH/D system differs between the low field and the high field. The characteristics of the T, for zero magnetic field wave functions should be included in this result but have not yet been extracted.
5. CONCLUSION
2-pulse and 3-pulse echo decay curves were obtained, and relaxation times were determined for a naphthaleneldurene system and for a naphthalene-dddurene system at 6K. Their features were compared and discussed in terms of magnetic field directions, resonance field, and deuterium effects. None of the echo intensities in this study is strong, and the relaxation is very fast. So we find better hosts in which these problems are improved and will perform more reliable analyses of echo decay, ESEEM, and temperature dependence of relaxation times in the better hosts.
REFERENCES
1. C.A.Hutchison Jr. and B.W.Mangum, J.Chem.Phys., 34 (1961) 908. 2. C.A.Hutchison Jr., et al., The Triplet State, edited by A.B.Zahlan, Cambridge Univ. Press, London, 1967, Section 2. 3. S.Geschwind, Electron Paramagnetic Resonance, edited by S.Geschwind, Plenum Press, New York, 1972, Chapter 5. B.J.Botter, et al., Mol.Phys., 30 (1975) 609. 5. Hsiang-Lin Yu, et al., J.Phys.Chem., 86 (1982) 4287. 6. Hsiang-Lin Yu, Tien-Sung Lin, and D.J.Sloop, J.Chem.Phys., 78 (1983) 2184. 7. D.J.Sloop, et al., J.Chem.Phys., 75 (1981) 3746. 8. D.S.McClure, J.Chem.Phys., 22 (1954) 1668. 9. J.M.Robertson, Proc.Roy.Soc., A141 (1933) 594. 10. N.Hirota, C.A.Hutchison Jr., and P.Palmer, J.Chem.Phys., 40 (1964) 3717. 11. J.P.Wolfe, Chem.Phys.Lett., 10 (1977) 212.
EPR in the 21%Century A Kawamori, J Yamauchi and H Ohta (Editors)
367
2002 Elsevier Science B.V. All rights reserved.
Light-induced ESR composite
studies
of
regioregular
K. Marumoto, N. Takeuchi and S. Kuroda Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Light-induced ESR (LESR) studies of regioregular poly(3-alkylthiophene) (PAT)-& composite using variable photoexcitation energy are reported. Two LESR signals with g value of = 2.002 and g2 = 1.999 are observed below 200 K, which come from positive and negative polarons on PAT and c 6 0 due to photoinduced electron transfer between PAT and c60, respectively. Microwave power saturation studies show a higher relaxation rate of the polaron spins of C6o than that of PAT. An excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.8 eV and a monotonic increase above 2.5 eV up to 4.1 eV. The enhancement of the LESR signals due to the photoinduced electron transfer is consistent with the enhancement of the photoconductivity at around 1.8 eV where the optically forbidden transition of c 6 0 occurs.
1. INTRODUCTION Conducting nondegenerate conjugated polymers are quasi-one-dimensional electron systems, which show a wide variety of interesting physical properties such as electroluminescence, nonlinear optical effect, semiconductor-metal transition, etc. and have been investigated extensively. Photoinduced electron transfer in polymeric semiconductors is important in understanding the nature of excited states in these systems, and in producing efficient photovoltaic devices. Recently, composites of the conducting polymers poly(3-alkylthiophene) (PAT) and a high electron affinity species such as fullerene (C,) have been investigated, because highly efficient charge separation due to the photoinduced electron transfer occurs in the PAT-& composites, and the remarkable enhancement of the photoconductivity has been reported [1,2]. The photoinduced electron transfer between PAT and c 6 0 forms positive and negative polarons on PAT and 0 , respectively. Light-induced electron spin resonance (LESR) is a direct microscopic method for detecting the photogenerated polaron, as demonstrated in the studies of (PPV) composites using regiorandom PAT, derivatives [3-51 and oligothiophenes [6]. For the PAT-C~O the LESR spectra and microwave power dependence of the LESR intensity have been studied [7,8]. However, the PAT-C~O composites using regioregular PAT have not been investigated, where the regioregular PAT exhibits higher conductivity compared with the regiorandom PAT. Moreover, excitation spectra of the LESR signals of the PAT-& composites have not been reported so far. In this paper, we report on first LESR measurements of the regioregular PAT-C~O composite
368
Figure 1. Structural formula of the (a) poly(3-octylthiophene) (PAT8) and (b) fullerene (c60). using variable photoexcitation energy. Two LESR signals are observed below 200 K, which come from the positive and negative polarons on PAT and C ~ due O to the photoinduced electron transfer between PAT and C ~ Orespectively. , The microwave power saturation studies show higher relaxation rate of the polaron spins of C ~ than O that of PAT. The excitation spectrum of the LESR signals shows a remarkable enhancement due to the photoinduced electron transfer at around 1.8 eV, which is consistent with the enhancement of the photoconductivity at around 1.8 eV.
2. EXPERIMENTAL PROCEDURE Regioregular poly(3-octylthiophene) (PAT8) was used to prepare the PAT-C~O composite (Figure 1). The concentration of Cm to PAT8 is 5 mol%. Ultrasonic treatment of PAT-Cm toluene solution was carried out with an ultrasonic disintegrator for better uniform mixing. Cast films of the PAT-c60 composite were fabricated inside ESR sample tubes. ESR measurements were performed with a Bruker E500 X-band spectrometer with a microwave cavity with optical windows down to liquid He temperature using an Oxford ESR900 gas-flow cryostat. The absolute magnitude of g value was calibrated using diphenylpicrylhydrazyl (DPPH) as a standard. A JASCO SM-5 light source with a 300 W xenon lamp was used to provide excitation for 300-1100 nm (1.1-4.1 eV) at power levels up to 2 mW/cm2 with a spectral width of 10 nm. The light was delivered by an optical fiber to the quartz sample tube. For excitation spectrum, the light intensity was adjusted to give the same photon flux at each wavelength. The LESR spectrum was obtained by the subtraction between the dark ESR signal and that under illumination. The dark ESR signals after switching off light and warming the sample up to room temperature were confirmed to be completely the same with those before illumination at room temperature while measuring the temperature dependence, microwave power dependence, and excitation spectrum of the LESR signals.
3. RESULTS AND DISCUSSIONS Upper curves in Figure 2 show the observed first derivative ESR spectra of the composite under dark condition (dotted line) and 700 nm illumination (solid line) at 60 K with a microwave power of 0.06 mW. The LESR spectrum was obtained by the subtraction between the dark signal and that under illumination as shown by lower curve in Figure 2. Two LESR signals due to the photoinduced electron transfer between PAT and c 6 0 are observed.
369 6
I
I
LESR illurn. dark
-
-
.
+PAT
-
illurn. dark
I\ .a, = 1.999
2.002
V
5G
Figure 2. Upper curves: ESR spectra of the pA'-c60 composite under dark condition (dotted line) and 700 nm illumination (solid line) at 60 K. Lower curve: LESR spectrum of the PAT-60 composite obtained by subtracting the dark spectrum from that under 700 nm illumination at 60 K.
0
100 200 Temperature (K)
Figure 3. Temperature dependence of the LESR intensities of the PAT-& composite. The solid circles and open squares denote the signal intensities of PAT and Cm, respectively.
The obtained value of gl = 2.002 and g2 = 1.999 correspond to the positive and negative polarons on PAT and &, respectively, which are consistent with those of the regiorandom PAT-& composites reported [7,8]. The observed polaron spins in the dark condition are attributed to the ground state electron transfer in the PAT-C~Ocomposite [8]. The spin concentration in the dark condition is obtained as 1 spin per lx105 PAT-molecular unit. The LESR intensity is as about 10 times large as the dark ESR intensity. The dark ESR and LESR spectra are almost the same with each other and have asymmetric line shapes. The peak-to-peak linewidth (AHpp)of the ESR signals of PAT and Cm are about 3.1 G and 2.4 G, respectively. Figure 3 shows the temperature dependence of the peak-to-peak LESR intensities of the PAT-C~O composite. The solid circles and open squares denote the signal intensities of PAT (gl) and c 6 0 (gz), respectively. The data were recorded with a microwave power of 0.06 mW under 300 nm illumination. The LESR signals of both PAT and c 6 0 become undetectable above approximately 200 K due to the higher recombination rate, confirming that the LESR signals are transient in nature [3,5]. The decrease of the signals at low temperature is caused by saturation of the ESR signals due to the longer spin-lattice relaxation rate. The LESR signal of PAT tends to saturate at higher temperature than that of c60, indicating that the spin-lattice relaxation rate of the polaron spins of c 6 0 is higher than that of PAT, which is consistent with the results of the microwave power dependence of the LESR intensity as discussed below. Figure shows the microwave power dependence of the peak-to-peak LESR intensities of the PAT-C~O composite at 60 K under 700 nm illumination. The solid circles and open squares denote the signal intensities of PAT (gl) and c 6 0 (gz), respectively. In the case of low microwave power, the signal intensities of PAT are larger compared with that of c 6 0 . When
370
1200
Wavelength (nm) 600
300 I
1
Figure 4. Microwave power dependence of the peak-to-peak LESR intensities of the PAT-C~Ocomposite at 60 K under 700 nm illumination. The solid circles and open squares denote the signal intensities of PAT and C ~ Orespectively. ,
]
2 4 Photon Energy (ev)
Figure 5. Excitation spectrum of the LESR signals of the PAT-C~O composite at 60 K. The solid and dotted lines shows the absorption spectrum of PAT6-C60 composite (c60 : 10 mol%) and photocurrent spectrum of PAT18-C60 composite (c60 : 5 mol%) for the comparison, respectively.
the microwave intensity increases, however, the LESR intensity of PAT saturates, showing a maximum at around 0.6 mW, and then decreases above 2 mW. Finally, the signal of PAT becomes undetectable due to the saturation above 200 mW. On the other hand, the LESR intensity of C ~ increases O monotonically and does not saturate within the experimentally available microwave power range up to 200 mW. These features are similar to those of the regiorandom PAT-C~O composites [8] and the composites of PPV derivative and c 6 0 derivative [9]. The field for saturation maximum is related to the spin-lattice relaxation time by
where max is the excitation field amplitude at the sample where the maximum in the saturation curve occurs, T2 is the transverse relaxation time, and ye is the electronic magnetogyric ratio, respectively. The LESR signal of PAT can be saturated at around 0.6 mW (60 K), whereas that of Cm does not saturate under same conditions up to powers which are 100 times higher. Therefore, of the polaron spins of c 6 0 is approximately ten times shorter than that of PAT. Finally, we present first excitation spectrum of the LESR signals of the PAT-& composite, which provides important information concerning the mechanism of the charge separation. Figure 5 shows the variation of the normalized LESR intensity with the photon energy of the incident light for the PAT-& composite. The measurements were performed with a microwave power of 0.06 mW at 60 K. The data are plotted by using the peak-to-peak LESR intensities of the signals of PAT (gl). For the comparison, the previously reported absorption
371
spectrum (solid line) [lo] and photocurrent (P. C.) spectrum (dotted line) [ l ] of the PAT-C~O composites are shown together in Figure 5. The sample of each composite is poly(3-hexylthiophene) (PAT6)-C60 composite ( C ~ :O10 mol%) for the absorption spectrum and (PAT18)-C60 composite ((260 : 5 mol%) for the photocurrent spectrum, respectively. The absorption spectrum of the PAT-C~O composite ( C ~:O5 mol%) up composites is to 4 eV has not been reported so far. The photoconductivity of the PAT-C~O remarkably enhanced upon C60 doping due to the photoinduced electron transfer at around 1.8 eV where the optically forbidden transition of C60 + t l u ) occurs. The excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.77 eV (700 nm) and a monotonic increase above 2.5 eV up to 4.1 eV. The enhancement of the LESR signals at around 1.8 eV is similar to the enhancement of the photocurrent spectrum on the whole, which is consistent with the photogeneration of the polarons due to the photoinduced electron transfer. However, the detailed shapes at around the peaks of the excitation spectra of the LESR signals and the photocurrents are somewhat different. This difference may be caused by the difference of the measurement conditions such as the effect of electric field, which may affect the charge separation processes. The effect of the electric field on the excitation spectrum of the LESR signals, as well as the C ~ O concentration dependence and PAT side-chain-length dependence of the charge separation are left open for further LESR studies. In summary, the regioregular PAT-C~Ocomposite was investigated by means of the light-induced ESR (LESR) method. Transient two LESR signals are observed, which come from the positive and negative polarons on PAT and C60 due to the photoinduced electron transfer between PAT and Cm, respectively. The microwave power saturation studies show higher relaxation rate of the polaron spins of C60 than that of PAT. The excitation spectrum of the LESR signals shows a remarkable enhancement at around 1.8 eV. The enhancement is similar to the enhancement of the photoconductivity at around 1.8 eV, which is consistent with the photogeneration of the polarons due to the photoinduced electron transfer.
ACKNOWLEDGMENTS This work is supported by NED0 International Joint Research Program, 99MB1 ‘Nonlinear Excitations in Molecular Electronic Materials: Detection, Control and Device Application’.
REFERENCES 1. K. Yoshino, X.H. Yin, S. Morita, T. Kawai and A.A. Zakhidov, Solid State Commun., 85
(1993) 85. 2. K. Yoshino, S. Morita, T. Kawai, H. Araki, X.H. Yin and A.A. Zakhidov, Synth. Met., 55-57 (1993) 2991. 3. S. Kuroda, K. Marumoto, H. Ito, N.C. Greenham, R.H. Friend, Y. Shimoi and S. Abe, Chem. Phys. Lett., 325 (2000) 183. 4. S. Kuroda, K. Marumoto, N.C. Greenham, R.H. Friend, Y. Shimoi and S. Abe, Synth. Met., 119 (2001) 655. 5. S. Kuroda, K. Marumoto, Y. Shimoi and S. Abe, Thin Solid Films, 393 (2001) 304. 6. K. Marumoto, N. Takeuchi, S. Kuroda, R. Azumi and M. Matsumoto, Synth. Met., 119 (2001) 549.
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7. L. Smilowitz, N.S. Sariciftci, R. Wu, C. Gettinger, A.J. Heeger and F. Wudl, Phys. Rev. B, 47 (1993) 13835. 8. S.B. Lee, A.A. Zakhidov, 1.1. Khairullin, V.Yu. Sokolov, P.K. Khabibullaev, K. Tada, K. Yoshimoto and K. Yoshino, Synth. Met., 77 (1996) 155. 9. V. Dyakonov, G. Zoriniants, M. Scharber, C.J. Brabec, R.A.J. Janssen, J.C. Hummelen and N.S. Sariciftci, Phys. Rev. B, 59 (1999) 8019. 10. S. Morita, A.A. Zakhidov and K. Yoshino, Solid State Commun., 82 (1992) 249.
EPR in the 2 1'Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
373
ESR study of photodecomposition mechanism of a long-lived radical perfluoroESR spectrum of trifluoromethyl radical formed during solid-phase photodecomposition at 77 K in glassy matrix S.R. Allayarov and D. A. Gordon Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation Formation under some perfluorocompounds radiolisys of long-lived radicals (LR) incapable to mutual recombination in liquid was first discovered by our group. This work is concerned the [(CF~)~CF]~C*C~FS (LRl) formed during radiolysis or fluorination of hexafluoropropylene trimer (HFPT) by addition of F atom to double bond of HFPT. During the heating or UV irradiation of LR1 its decomposition takes place to give a molecule of perfluoroolefine and *CF3 radical initiating the polymerization of monomers but subjected to fast recombination in liquid. ESR study of mechanism of photolytic decomposition of LR1 and the analysis of spectrum of *CF3 radical formed during solid-phase photodecomposition of LR1 at 77 K has been performed. All = 25.15 mT, A 1 = 9.1 mT, and gll = 1.9996, g 1 = 2.0056 parameters were found by using computer-simulated ESR spectra. The further destiny of *CF3 radicals formed under the decomposition depends on the phase state of the matrix. About 90% of them recombine in liquid at 300 K. The rest attaches to the HFPT molecule giving another long-lived [(CF3)2CF]3C *radical. On photolysis at 77 K, a part of *CF3 radicals is stabilized in the glassy HFPT matrix. The *CF3 radicals formed during y-radiolysis of HFPT at 77 K are not stabilized. Quantum-chemical calculations showed that the mechanism of LR1 photodecomposition is different from the mechanism of thermal decomposition. That is the consequence of the fact that decay of photoexcited radical is predetermined by a radical structure and results in whereas the reaction of *CF3 detachment from CF-CF3 group being as twice as less endothermic than that from CF2-CF3 group (48-56 kJ/mole instead 98-104 kJ/mole) is to be dominant under thermolisys. The LR1 photodecomposition red cut-off has experimentally shown to be 320nm. In accordance with this the simulations manifest that LR1 electronic excitation of 4eV is sufficient for its decomposition, and, contrary to thermolisys, *CF3 radical detachment proceeds from CFz-CF3 group.
1. INTRODUCTION LR1 was obtained on radiolisys of hexafluoropropylene trimer (HFPT) [l]. LR1 has an alkyl nature [2] and is formed, as well as on fluorination of HFPT by addition of F atom to a double bond of HFPT molecule. During the heating or UV irradiation of LR1 its decomposition takes place. Earlier we assumed the mechanism of photodecomposition of LR1 is similar to the mechanism of thermal decomposition at 373 K [3] and occurs by removal of *CF3 group from perfluoroisopropyl fragment of LRl to give *CF3 radical and
374
molecule of perfluoroolefine. However, our last researches have shown the mechanisms of photo- and thermodecomposition may be different. The semiempirical quantum-chemical calculations. were carried out to check this hypothesis. The .CF3 radical probably exceeds all other low-molecular perfluoroalkyl radicals by the number of works devoted to ESR studies. In the ESR spectrum of the *CF3 radical in the liquid [4] where the dipole interactions are averaged and lines are narrowed, a quadruplet with splitting AH = 14.24 mT is detected. It is resulted from the interaction of an unpaired electron with three equivalent F nuclei. The ESR studies of the *CF3 radicals in the solid phase [5-71 exhibited complex multicomponent ESR spectra. The ESR spectrum of the *CF3 radicals chaotically oriented in the matrix of zeolite 13X was also studied However, it is difficult to interpret all lines of the multicomponent spectrum of the C F 3 radicals stabilized in polycrystalline or amorphous matrices due to the superposition of signals from other paramagnetic centers (PC). In this work, we attempted to obtain the spectrum of the *CF3 radicals in the solid matrix of the hexafluoropropylene trimer (HFPT) without admixtures of other PC during the photodecomposition of LR1.
2. EXPERIMENTAL Photolysis of samples at 300 K was performed by the full light from a high-pressure DRSh1000 mercury lamp in quartz cells 4-5 mm in diameter, which do not give ESR signals. ESR spectra were detected on a small-size PS 100.Kh radiospectrometer. ESR spectra of the radicals were automatically detected and simulated on a computer using the programs of the Scientific Technical Cooperative Center of Radiospectroscopic Instrument Production"Tsentrospektr" (Minsk). Quantum-chemical calculations were carried out by PM3 approach using the complex of MOPAC-93 program. 3. RESULTS AND DISCUSSIONS
The structure of LR1 obtained as a result of quantum-chemical calculations is given in Figure 1. Conformation of the radical is close to the determined experimentally by the ESR spectroscopy method where the spatial arrangement of p-atoms F was determined from ESR spectra on the basis of angular dependence aF@) =Q.p.cos28. The aF(P) - hyperfine interaction (HFI) constant with p - atoms F, Q - a constant, p - spin density of unpaired electron at - atom C, 8 - a corner between a projection of C-F bond and an axis of an orbit of unpaired electron. The absence of symmetry in LRl structure may be considered as a peculiarity. Similar bonds C(2)-C(1) (1.523A) and C(2)-C(7) (1.505 A), C(l)-C(5) (1.612 A) and C(1)-C(6) (1.627 A), C(7)-C(8) (1.609 A) and C(7)-C(9) (1.613 A) have different lengths due to steric interactions. Three bonds of atom C(2) lay in one plane: the departures of atoms C(1), C(2), C(3) and C(7) from the plane included these atoms does not exceed 0.015 A. At such structure of the radical (see Figure 1) there should be reduced contacts between volumetric trifluoromethyl groups. To avoid this is possible by a different turn of CF3-groups around of bonds C(2) - C(l) and C(2)-C(7). Really, the torsion angle C(3) C(2) C(l) F(18) is 141.9 whereas the similar torsion angle C(3) C(2) C(l) F(28) is 136.9 '. For the existence of steric
375
difficulties in the radical high values of a barrier of rotation (-105 kJ/mol) along the bonds C(2)-C( 1) and C(2)-C(7) argue also.
Fi191
p
plFilll
cis1
- F(221
I
Bll
Figure 1. Simulated structure of long-lived [ ( C F ~ ) ~ C F ] ~ C O C ~ F ~ radical.
Figure 2. Experimental (a-c) and simulated (d-f) ESR spectra of [ ( C F ~ ) ~ C F ] ~ C O C ~ F ~ radical in HPFT matrix before (a,d) and after UV irradiation at 77 K during 280 (b,e) and 800 min (c,Q and line-image assignment of ESR spectrum of oCF3 radical (0.
As a result of slowed down rotations along bond C(2)-C(3) of radical (a barrier of rotation along this bond 188 kJ/mol) also should become nonequivalence of F10 and F11 atoms of perfluoromethylene groups. On F atom (10) the constant of HFI is 4.57 mT whereas second F atom (I 1) lays close to nodal plane (0 = 69.5’) and is characterized by small value of splitting. Comparison of experimental data and results of quantum-chemical calculations has shown that the mechanism of photodecomposition of LR1 differs from the mechanism of its thermal decomposition. It has been experimentally determined that LR1 photodecay red cut-off is to be 320nm. Quantum-chemical calculations have shown that the energy difference between ground and exited states of LRl is 4ev. So, at an irradiation light with length of a wave less than 320 nm occurs transition of the radical in the excited state with the subsequent break of bond CFz-CF3 most lengthened in the ground state. At thermal decomposition of LR1 (heating up to 373 K) is more probable the break of bonds >CF-CF3 because the reaction of oCF3 detachment from CF-CF3 groups is as twice as less endothermic than that from CF2- CF3 group (42-54 kJ/mole instead 98-104 kJ/mole). Thus, the first is the main channel of LR1 thermal decomposition. Data obtained allow to assert, that selectivity of decomposition of LR1 at photolysis is connected to structural predetermination, whereas the most weak C(l)-C C(1)-C (6), C(7)-C(8) and C(7)-C(9) bonds are to be broken under thermolysis. Significant changes occur in the ESR spectrum during photolysis at 77 K of LR1 in the glass HFPT matrix (Figure 2). The disappearance of the spectrum of LR! is accompanied by the appearance of a complex ESR signal (see Figure 2,c) with a total splitting of 75.5 mT and characteristic four lines with a distance of 25.15 mT. In addition, two asymmetric lines are
376
detected in the central part of the spectrum. These two groups of lines have previously [l] been assigned presumably to different PC. However, the results of this work show that the dependence of all these lines on the UV irradiation time and kinetics of their appearance and decay upon heating coincide with each other and are parallel to the kinetics of changing the overall concentration of PC during the low-temperature (77 K) photolysis of the sample with LR1 followed by its heating. This suggests that all lines of the spectrum belong to the signal of the same PC. As shown by the analysis of the spectrum (see Figure 2,c), these PC are the CF3 radicals with axial symmetry. The line-image reconstruction of the spectrum of the *CF3 radical (see Figure 2,c) shows that all three F atoms of the *CF3 radical are equivalent. The following main values of the components of the g factor and components of the HFC tensor for the F atom were determined: All = 25.15 mT, A 1 = 9.1 mT, and gll= 1.9996, g 1 = 2.0056. Figure 2f presents the results of computer simulation of the ESR spectra of the *CF3 radicals in the glassy HFPT matrix at 77 K based on the experimental data. As known [7], the shape of the asymmetric ESR line at the Gaussian or Lorentzian line shape depends only on one anisotropy parameter d= (IHII-H~I)/DH~, where DHi is the individual linewidth between the points of maximum slope; HIIand H i are the magnetic field intensities corresponding to the limiting positions of the individual line for changing the angle between the Ho direction and symmetry axis from 0 to p/2. The spectra of the *CF3 radical (see Figure 2,c,f) allow the determination of four values of the d parameter: two from lateral (dl , p 9 ) and two from inner (d3,4=3) components of the Hi1 and H i components of quadruplets. According to earlier published data [7], at these d values analysis of experimental spectra is simple, and at d25 the HI]and H i values can be determined with a high accuracy (to 5%). The theoretical spectrum (see Figure 2,f) simulated taking into account the abovementioned parameters agrees well with the experimental spectrum (see Figure 2,c). Thus, simulation confirms that the central lines are, in fact, the components of the spectrum of the *CF3 radical. Only prolonged ( 4 0 0 min) photolysis allows the almost complete decomposition of LR1 to be achieved during solid-phase photolysis (see Figure 2,c). The ESR spectrum of the samples irradiated for a relatively short time (see Figure 2,b) is a superposition of the spectra of the *CF3 and LRl. The fraction of the *CF3 radicals can rather exactly be determined by computer simulation of experimental spectra. The ESR spectrum of LRl in the glassy HFPT matrix at 77 K exhibits a doublet with a splitting of 4.6 mT (see Figure 2,a). The g factor for LR1 is [3] 2.00286. Computer simulation of the spectrum of LR1 (see Figure 2,a) shows that the shape of its individual components is described by the Gaussian function and their width is -4.4 mT. Analysis of the ESR spectrum of a solution of LR1 in HFPT UV-irradiated at 77 K for 280 min showed that in this sample the ratio of concentrations of the radicals is [* CF3]:[LRl] =1:3 (see Figure 2 b,e). Thus, the simulation of the experimental spectra of the irradiated samples containing LRl and *CF3 allows the determination of the fractions of these radicals in the sample. Based on the interpretation of the ESR spectra of the *CF3 radical, which is the decomposition product, we can assume that the photodecomposition mechanism of LR1 includes the formation of the *CF3 radical. The ESR spectra of the *CF3 radicals similar to
377
that detected in the matrix of glassy HFPT were also observed in the g-irradiated at 77 K polycrystalline samples of aqueous solution of CF3COOH [6] and photolyzed at 77 K CF3COCF3 samples [5] adsorbed on zeolite I3X. The parameters of these spectra almost coincide with those obtained in this work and, therefore, the simulated spectrum of the *CF3 radicals in the HFPT matrix (see Figure 2,f) can be used for the identification of these radicals in other matrices. Russian Fund for Fundamental Research has supported this work. Cod N: 01-03-97006.
REFERENCES 1. Allayarov S.R., Asamov M.K., Barkalov I.M., Shvedova M.K. Izvestiya VUZov, Ser. Khimiya I Khim., Technologiya, 30 (1987) 98. 2. Gordon D.A., Allayarov S.R., Kuzina S.I., Barkalov I.M., Mikhailov A.I., Izvestiya AN SSSR, Ser. Khim., No. 10 (1989) 2203. 3. Scherer K.V., Ono T., Yamanouohi K., Femandez R., Henderson P., Goldwhite H., J. Amer. Chem. SOC.,107 (1985) 718. 4. R. W. Fessenden and R. H. Schuller, J. Chem. Phys., 43 (1965) 2704. 5. P. Svejda, J. Phys. Chem., 76 (1972) 2690. 6. K. Mach, Collection Czechoslov. Chem. Commun., 37 (1972) 923. 7. V. V. Voevodskii, in Fizika i khimiya elementarnykh khimicheskikh protsessov [Physics and Chemistry of Elementw Chemical Processes], Nauka, Moscow, 1969 (in Russian).
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EPR in the 2IstCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
An ESR study and quantum-chemical calculations of alkyl radicals in the matrix of
polycrystalline n-alkane irradiated at 77 K. Effect of intermolecular interactions and carbon chain length on the radical formation S. R Allayarov, S.V. Konovalikhin and T.E. Chernysheva Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. The composition of alkyl radicals (AR) formed by y-radiolysis (T=77 K) of polycrystalline n-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(l I), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiated n-alkanes are superpositions of the signals from the H~CC'HCHT and -CH2C'HCHr radicals, whose HFS constants with a and p protons well the equilibrium conformation independent of the chain length of the it-alkane molecule. A dependenceof the concentration of the radicals on the chain length of n-alkane was found. The absence of the -CH2C'H2radicals that may arise upon H atom elimination from the Me fragments of the n-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the -CH2C'H2 radicals into H&C'HCHT- radicals With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the correspondingC atom To reveal the reasons for the previously found absence of end radicals upon y-radiolysis of n-heptane polycrystals, we performed quantum-chemical calculations (SCF-MO, RHF, 6-3 1 G* basis set) of the n-heptane molecule and its four radicals. The energies of the crystal lattice were calculated by the atom-atom potential method. Comparisonof the experimental and calculateddata showed that the absence of the end radicals is not related to the intermolecular interaction in the crystals. The most probable reason for the selectiveradical formation upon radiolysis can be a transfer of the excitation energy within the n-heptane molecule occurring before the radical formation. 1.INTRODUCTION
Hyperhe structure (H9S) ofthe ESR spectrum of alkyl radicals(AR) in the solid phase reflects the interaction of an unpaired electron with the nearest environment only In these ESR spectra, the values of HFC constants with y-protons do not exceed 0.05 mT [11 which is much less than the half-width of the spectral components (-1 0 mT) [2] and a rule, they cannot be detected. Therefore, of ESR spectra of AR is the result of the interactions of an unpaired electron with (a- and fi-protons only As a result, only AR with the valence at the C atoms in positions 1,2, or 3 be distinguished in the solid phase from ESR spectra. Therefore, the composition of AR formed during y-radiolysis of n-alkanes unambiguously be determined from the ESR spectra of the latter only for molecules with a chain length of at most five C atoms. In the case of n-alkanes with a longer chain, the AR compositioncan be established indirectly. For this purpose, one should know how the carbon chain length of the n-alkane molecule affects the composition of radicals formed under its radiolysis.
379
Although numerous data have been obtained by the ESR study of irradiated solid organic substances [2], the quantitative compositionof AR and dependence on the chain length of the linear alkane are virtually unstudied. work is devoted to the ESR study of radicals formed under y-radiolysis of polycrystalline alkaneswith merent chain lengths. in this work, we compared the. ESR spectra with the results of quantum-chemical calculations of radicals to interpret the selectivity of radical formation during radiolysis of n-heptane polycrystals.In addition, we estimated the energies of the crystallinelattice of nonirradiated n-heptane and model crystals with the radicals.
2.EXPERIMENTAL Linear alkanes n-CsHlz, n-C7H16, n-CloHzz, and n-ClgH38 (content of the main substance 299.95%) and their high-mo-lecular analog, a low-pressure polyethylene (PE) powder, were
Used. Radiolysis of samples (@Coy-radiation, dose of irradiation power 28 5') was canied out in tubes of SK-4B glass in vacuo at 77 K. ESR spectra were recorded on a PS I00.Kh 3-cm radiospectrometer. Nonempirical quantum-chemical calculations of the n-C7H16 molecule (see Figure 1 a) were performed by the RHF/6-3IG* method, and the RI, Rz, R3, and & radicals (see Figure 1 b-e) were calculated in the UHF approximation in the 6-31G* basis set using the GAMESS program [8] with the complete optimization of geometry. It is that the quantum-chemically calculatedgeometry of radicalsis very sensitiveto the choice of the basis set. Comparison of the results obtained with the 3-21G, 4-3 IG, and 6-3 lG* basis sets showed that the structure of the ti-heptane molecule is well reproduced in all cases, and that of the radicals is reproduced only in the 6-3 1 G* basis set. Coincidence of the experimental and calculated data on the conformationof the radical served as a criterion for the validity of the structure found. spectra of radicals were simulated using the EPRTOOLS program (version 3, developed the Scientific Technical Cooperative Center for Radiospectroscopic Instrument-making "Tsentrospektr,"Minsk). 3.RESULTS AND DISCUSSION
The shape of ESR spectra of irradiated polycrystalline samples of linear alkanes depends, to a great extent, on the chain length of the alkane molecule. When the chain gradually elongates, the seven-component spectrum transforms into the Six-component spectrum. The ESR spectra of n-alkanes y-irradiated at 77 K are presentedin Figure 1 (spectra 3-8). The theoretical spectra of H3CC H ' CHZ- and -HKC.HCHr correspondingto the seven- and six-component spectra were obtained by the simulation of the experimental spectra of irradiated n-alkanes (see Figure 1, spectrum 2, Table 1). The H~CC'HCHTradical (R2) is formed when the H atom is eliminated fiom the second C atom of the n-alkane molecule. The HFC constants obtained for this radical agree well with the parameters of these radicals stabilized in single crystals of n-alkanes irradiated at 77 K [3]. As follows from Table 1, in the -CH?C'HCHr (R,) radical, all four Hp atoms are equivalent. The HFC constants with these protons are equal to -3.56 mT. The HFC constants
380
loll mT L 1
Figure 1 . Theoretical ESR spectra of H$.X'HCHZ- ( I ) and -CH2C'HCH2- radicals (2); experimental(3-8) and theoretical ( 3 ' 3 3 ESR spectra ofn-CjH1Z (3, 33, ~ z - C ~(4, H I43, ~ n-C~oHn ($59, n-CIIHZ4 (6(Ref 3), 67, n-ClsH3~ 73, and PE (8, 83 y-irradiated at 77 K with a of 30 kGy. close to this value have been obtained by examination of the spectra of the -CH2C'HCHr radicals stabilized in PE [MI. Examination of the ESR spectra of y-irradiated n-alkanes shows that they mainly represent a superposition signals from the R2 and %radicals. On going fiom one alkane to another, the main parameters of the spectrum of these radicals (HFC constants with and &protons) remain unchanged. However, the linewidths in the spectra of the R2 and Rm radicals can differ (Table 2). For example, in the theoretical spectrum of n-undecane Figure 1, spectrum 63, the lines of the R, radical are more narrow than those in spectra of other alkanes. This results in a substantial distinction of the spectrum of n-undecane from the spectra of other n-alkanes Figure 1). the observed distinction in the spectrum of n-undecane be explained by a decrease in the linewidths. We found that a specified quantitative correlation between the concentrations of the RZ and R, radicals was hifilled for y-radiolysis of polycrystalline n-alkanes. The ratio of the concentrations of these radicals in the theoretical spectra of ri-alkanes irradiated at 77 K are presented in Table 2. It follows from these data that the ratio of concentrations of the R2 and &radicals depends on the alkane chain length. In the case of n-pentane (see Figure 1, spectra 3 and 37, the sevencomponent ESR spectrumis mainly observed,which is attributed to the H3CC.H(CH2)2Me radical, whose concentrationis fivefold higher than that of the HC[CHzMe]2 radicals. An inverse ratio of concentrations of the R2 and R,,, radicals (5 : 14) is observed for the samples of y-in-admted n-octadecane. In the case of n-octadecane Figure 1, spectra and 73, the content of the R2 radicals (H,CC'HCH~(CH~)MM~) is almost threefold lower than that of the -CHZC'HCHT radicals in the spectra of irradiated n-CloH~and n-CilH24 contain signals &om the R2 and R, radicals with a close intensity.
381
Table 1 Parameters of the ESR spectra and equilibrium conformation of alkyl radicals formed under yradiolysis of n-alkanes 77 K Radical
HFC constantdmT
Angle Ides*
a,H amH wwl) amH'2) amH(3) m2w4)el ez H3CC'HCHr,R2 2.4 2.55 3.8 3.3 26 34 43&CAHCHr, 2.2 3.56 3.56 3.56 3.56 30 30 * The angle between the projection of the Cp-H bond of the methylene fragment of the radical and the axis of the orbital of an unpaired electron. the of the %, (see Figure 1, spectrum 2) and R2 radicals (see Figure 1, spectrum I ) substantially differ, the shape of the spectrum of irradiated n-alkane, being a superposition of signals from these radicals, is determined by the relative concentration of the latter. Thus, the main portion of radicals formed upon y-radiolysis ofpolycrystalline short-chain n-alkanes comprises R2 radicals whose free valence is localized at the second C atom. The R,,, radicals (-€H2C'HCH~) are mainly observed in the radiolysis of n-alkanes with a longer chain 77 K. With a gradual elongation of the chain of the it-alkane molecule, the &tion of the R,,, radicals increasesin the ESR spectrum, and the latter transforms from the seven-component into the six-component spectrum. These changes in the ESR spectra on going from high-molecular paraffin or PE to lowermolecularn-dodecane are explained by the transition of the radical from one conformation to another. However, our analysis showed that the observed changes were related to distinctions in the quantitative ratio between concentrations of the R2 and R,,, radicals rather than to the conformationalpeculiaritiesof the R,,, radicals. Let us consider the ratio of concentrationsof the alkyl radicals as a knction of the chain length of the molecule of irradiated n-alkanes. We suggest that the distribution of the alkyl radicals RY,R2, R3, ..., %formed by y-radiolysis of a polycrystalline n-alkane due to the elimination of the H atom from the first, second, third, and iath C atom correspondsto the equiprobableabstraction of the H atom from any C atom. In this case, the concentrations of the radicals with different structures should be proportional to the number of H atoms at the corresponding C atoms in the n-alkane molecule. For example, for n-pentane, the ratio of concentrations of the radicals should Table 2 Parameters of the theoretical spectra that most well describe the experimental ESR spectra of n-alkanes irradiated at 77 K Alkane R2 : h*/mT R2 n-C& 5:1 1.2 1.6 n47H16 5:3 1 .o 1.6 f~-CloHz 5:6 1 .o 1.8 11H24 5:7 1 .o 1 .o fsI$H38 5 : 14 1.2 1.6 PE 0:1 2.0 * h is the half-width of spectral lines.
382
d
e
Figure 2. Structures of the n-heptane molecule with numeration of the atoms.
and radicals RI (b), RZ(c), R;
(4,and R4 (e)
be the following:
The radical is not detected experimentally, and the ratio of concentrations of the observed radicalsR2 and should be 2 : 1. The experimental ratio of concentrationsof these radicals is 5 : 1 Table 2). three units, by which the concentration of the R2 pentyl radicals increases, are to the fraction of the radicals in Eq. (l), we may assume that during y-radiolysis of n-pentane at 77 K the RI radicals are transformed into Rz. Most likely, this transition occurs before the formation of the and is a result of the transfer of the excitation energy of the Me group to the adjacent CHZfiagment. The possibility of this energy transfer during radiolysis of the n-alkane molecule has been observed previously [101.Analysis of the ESR spectra of other n-alkanes irradiated at also indicates increase in the concentrations of the R2 radicals at the expense of the radicals. The ratio of the radical concentrations in the theoretical spectra (see Figure 1, spectra 3'- 77, which optimally describe the experimental ESR spectra of the corresponding irradiated n-alkanes (see Figure 1, spectra 3is presented in Table 2. The concentrationsof the alkyl radicals RI, ..., R,, (taking into account the transformation of the RI radicals into R2) are proportional to the number of H atoms at the C atoms in the n-alkane molecule. As follows from the data in Table 2, the concentrations of the alkyl radicals formed by y-radiolysis of polycrystalline n-alkanes whose molecules contain more than four C atoms are described by the ratio [H&C'HCH2]/[--CHzC'HCH~]= where is the number of C atoms in the n-alkane molecule. Thus, the observed deviation from the primary distribution of radicals proportional to the of H atoms at the atoms of the n-alkane molecule is most likely associated with the transfer of the excitation energy of the CH3 group to the adjacent methylene fiagment. The Quantum-chemical calculationsfor the R,, R2, R;, and R4 radicals Figure 1) showed that the total energies of all the radicals are almost the same: the highest difference between them does not exceed 2.3 moF' in the nonempirical calculation scheme and kcal mop' in the semiempirical Such low differences in energies do not allow one to speak with confidence about a higher stability of any of these radicals.
383
The calculation the electronic structure of the n-heptane molecule in the excited state showed that when the energy is absorbed, the excitation is mainly localized on the central C(3), C(4), and C(5) atoms Our experiments indicate that the radiation yield of the radicals is independentof the irradiation dose. we may assume the intramoleculartransfer ofthe excitation fiom the end atoms to the central atom during radiolysis. On the other hand, the excitation transfer is accompanied by the radical formation, and the rates of these processes comparable. Therefore, the indicated energy transfer fiom the end to central atoms "stops" precisely on the C(2) atom. the selectivity of radical fbrmation is due to the intramolecular transfer of the excitation energy the to central atoms; second, the intermolecular interaction has no efEa on the radical formation during irradiition of the n-heptane polycrystals; third, the PM3 method well reproduces the experimental data on the structure of molecules and radicals of n-alkanes and is appropriate for calculationsof the physicochemical properties of more complex (in composition and structure) ~ o l e c uand i ~ radical systems. Russian Fund for Fundamental Research has supported this work . Cod N 01-03-97006
REFERENCES 1. R. Fessenden and R. H. Schuler, Chem. Phys., 39 (1963) 2147 2 S. Ya. Pshezhetskii, A G. Kotov, V. Milinchuk, V. A. Roginskii, and V. I Tupikov, EPR svobodnykh radikalov v radiatsionnoi khimii P S R of Free Radicals in Radiation Chemistry], Khimiya, Moscow, 1972 (in Russian). 3. T. GiIlbro, P 0. Kinnel, A. Lund, J. Phys. Chem ,73 (1969) 4167 4. Yu. D. Tsvetkov, N. N. Bubnov, M A. Makul'skii, Yu. S Lazurkin, and V. V. Voevodskii, DOH. Nauk SSSR [Reports Acad. Sci. USSR], 122 (1958) (in Russian). Ohnishi, Bull. Chem. Jpn., (1962) 254. 6. B. Ran by and H Yoshida, Polym. Sci , 12C (1966) 263. 7 L. A. Blyumenfel'd, V. V. Voevadskii, and A G Semenov, Application of Electron Paramagnetic Resomce in Chemistry, Press of the Siberian Branch of the Academy of Sciencesof the USSR [U-vo SO AN SSSR], Novosibirsk, 1962 (in Russian) 8. S. Allayarov S. V. Konovalikhin, Izv. Akad. Nauk, Ser. Khim., No. (2000) 1038 [Russ. Chem Bull., Int 49 (2000) 1032 (Engl. Transl.)].
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
84
ESRENDOR study for new radical dianion species of 6-oxophenalenoxyl derivative Y. Morita," S. Nishida," J. Kawai," K. Fukui,b S. Nakazawa,bD. Shiomi,b K. Sato,b T. Takuib and K. Nakasuji" "Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan bDepartments of Chemistry and Materials Science, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka
8-85
Japan
A novel open-shell molecule, dipotassium
was
designed on the basis of the 6-oxophenalenoxyl system and generated by the chemical reduction of 6-hydroxyphenalenone derivative. The structure was unequivocally determined by ESR and ENDOR/TRIPLE spectroscopies, and DFT calculations, illustrating an unique spin density distribution depending on its redox states.
1. INTRODUCTION Extensive studies have been carried out on organic molecule-based ferromagnets and other molecular functionalities based on organic open-shell molecular systems. Understanding of the electronic structure of open-shell molecules is crucial for the design and preparation of such molecules and molecular assemblies. Phenalenyl is a well-known system as a highly symmetric odd-alternant hydrocarbon n-radical with high amphoteric redox ability and has been attracting great attention as a possible component for molecule-based conducting and magnetic materials [ 1,2]. We have recently reported on the design and synthesis of 2,5-di-tertbutyl-6-oxophenalenoxyl derivative 1 as a novel stable neutral radical based on the
phenalenyl skeleton [3]. Characterization of this neutral radical in terms of ESR and ENDOR/TRIPLE spectroscopies and MO calculation has illustrated that this system has an extremely spin-delocalized and highly spin-polarized nature which is similar to that of phenalenyl system. The topology of the spin density distribution in the 6-oxophenalenoxyl system, however, was contrasted with that of the phenalenyl system due to the change in the topological symmetry of the n-electron network. Thus, it is important to clarify the effect of heteroatomic perturbation on the electronic structure of the system under study. In this work, we will describe the redox properties and the spin structure of the radical dianion species of the 6-oxophenalenoxyl system.
2. RESULTS AND DISCUSSION
2.1. Synthesis and redox property In Scheme 1, a possible redox behavior of the 6-oxophenalenoxyl system is depicted. A neutral radical 1 is generated by the one-electron oxidation of the anion 3, while the radical dianion
seems to be generated by the one-electron reduction of 3. To investigate these
behaviors by the cyclic voltammetry (CV) method, we have selected the anion salt 3.(Et,N') considering its stability in solution. The neutral radical precursor 2 was prepared from
in seven
steps, which was transformed into 3.(Et4N+)by the treatment of one equivalent of NaOMe followed by Et4NC1 in THF-MeOH, exhibiting particularly high stability even in air
neutral radical
anion
radical dianion
Scheme 1 Possible redox states of the 6-oxophenalenoxyl system
386
atmosphere due to the negative-charge delocalization (Scheme 2). CV measurements in CH3CN at room temperature have given one reversible oxidation wave (-0.30 V) and one reversible reduction wave (-2.39 V), indicating the high stability of the neutral radical 1 and radical dianion species
[4]. These results have encouraged us to
generate 4"-by chemical reduction and to characterize its stability and electronic structure.
Scheme 2 Reagents: (a) cat conc H2SO4, excess t-BuOH, CF,COOH, 45 "C, 99%; (b) 10 equiv DMF-POCl3, (CHZCI)~, 90 "C, 99%; (c) 4.2 equiv LDA-~-BuCH~COOCH~P~, THF, 0 "C, 86%; (d) (i) 1.5 equiv Et,SiH, 4.5 equiv CF3COOH, CH2C12, rt; (ii) PdC, H2, EtOH, 85%; (e) (i) excess (COC1)2, reflux; (ii) 3.5 equiv AICI,, CH2C12,93%; (f) 3.3 equiv LiAIH4, THF, rt, (8) (i) 20 equiv LiI, HMPA, 170 "C; (ii) 2 M HCl aq, rt; (iii) reprecipitated from hexane-CH2C12, 62%; (h) (i) NaOMe, THF, rt; (ii) Et4NC1, THF, rt. -100%.
2.2. Generation and characterization of radical dianion species Generation of radical dianion 4"-.(2 K') was carried out by the treatment of the 6hydroxyphenalenone derivative 2
potassium mirror in a degassed diglyme (1.O x 10" M)
in a sealed tube, to afford green solution that showed well-resolved ESR signals (Figure 1A). The decrease in these signal intensities was not observed for a long time at room temperature in a sealed tube, being indicative of the high stability under such conditions. In 'H-ENDOR spectra measured in order to determine the hfcc's values, two sets of signals due to the protons on the phenalenyl skeleton were observed (Figure 1C). The observed ESR signals were fully simulated by the addition of another hfcc
0.025 mT) which is attributed to the
39
K nuclei, while signals due to 39K nuclei could not be observed clearly in the 'H-ENDOR
spectra (Figure lB, C). The relative signs of the hfcc's due to two kinds of protons were unequivocally found to be equal by invoking 'H-TRIPLE resonance measurements (Figure
387
S
A
. .
-
./1 1
1
'._
FieldhT
B
FieldmT
,
r----
t increase
t r
I
MHz
I
MHz
Figure 1 Observed hyperfine ESR spectrum (A), 'H-ENDOR (C), 'H-TRIPLE (D, pump frequency: 20.65 MHz) spectra in diglyme (1 lop3 M) at 290 K and simulated one (B); The microwave frequency used for ESR measurement was 9.6455240 Observed g-value is 2.0037. 1D). Assignments of two kinds of hfcc's of radical dianion 4"-.(2 K') were made with the help of the spin density distribution calculated in terms of a local spin density functional theory by using
method (Table 1 and
94 with
Figure 2). Agreement between the experimental and theoretical values is satisfactory. These results demonstrate that radical dianion
exhibits the remarkable change in the a-spin
density distribution as compared with the neutral radical 1. Table 1 Observed and calculated hfcc's for radical dianion 42A,/mTa 334 799 observed -0.434 -0.551 calculated -0.478 -0.508
39K *0.025 -
The observed hfcc's were determined by 'H-ENDOR spectra in diglyme at 290 K and simulation successfully reproduced the ESR hyperfine spectrum. Values in the lower line were calculated by the DFT calculations McConnell equation using the following parameters: A,, = pQ (Q = -2.7 mT).
a
388
A
B
Figure 2 The spin density distribution of neutral radical (A) and radical dianion (B) calculated in terms of a local spin density functional theory by using Gaussian 94 with uBLYPJ6-3 1G*//UBLYP/6-31G*. Vacant and filled circles denote negative and positive x-spin density, respectively.
3. CONCLUSION A novel open-shell molecule with delocalized negative charges, radical dianion 4"-.(2 K'),
was designed and successfully generated by the reduction of 2, demonstrating strikingly different n-spin density distribution from that of neutral radical
This result means that the
6-oxophenalenoxyl system is a peculiar but important chemical system in which the n-spin structure changes remarkably depending on its redox states. This study contributes not only to the molecular design for novel organic open-shell molecules, but also to the establishment of a novel approach for controlling magnetic properties by external stimulations. Consideration on the correlation between spin polarization and charge fluctuation by using VB methods will be reported in due course.
REFERENCES 1. K. Goto, T. Kubo, K. Yamamoto, K. Nakasuji, K. Sato, D. Shiomi, T. Takui, M. Kubota, T. Kobayashi, K. Yakushi, J. Ouyang, J.Am. Chem. SOC.,121 (1999) 1619. 2. X. Chi, M. E. Itkis, B. 0. Patrick, T. M. Barclay, R. W. Reed, R. T. Oakley, A. W. Cordes, R. C. Haddon, J.Am. Chem. SOC.,121 (1999) 10395. 3. Y. Morita, T. Ohba, N. Haneda, S. Maki, J. Kawai, K. Hatanaka, K. Sato, D. Shiomi,
T. Takui, K. Nakasuji, J. Am. Chem. Soc., 122 (2000) 4825. 4. CV was carried out by the following conditions: 10 mM in CH,CN with 0.1 M Bu,NClO, as supporting electrolyte at room temperature against Fc/Fc'; Au working electrode and Pt counter electrode; 0.20 Vls.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
389
Spin labeling study of polymer chain motion in PEG/PVP blend Shiming Chen", Guidong Jin", Zhenghua Pingb, Sizhao Jin' and Yimin Shena
" Center of Analysis and Measurement, Fudan University, Shanghai 200433,China Macromolecular Science Department, Fudan University, Shanghai 200433,China
' Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, China The spin label ESR method was used to study the chain motion in the PEG/PVP blends under different temperatures. In the ESR spectrum, there were only one slow motion component in the low temperature or one fast motion component in high temperature for the PEG-labeled. While two components were detected in the blends, indicating phase separation in the blends. According to the ESR spectrum parameters, the correlation time of the samples was calculated. The correlation time of all samples is between 10-9-1011s, its value increases as the content of the PVP in the blend increases. 1. INTRODUCTION Polymer blending is an attractive technique of materials manufacturing as they provide a low-price way for the design and the fabrication of new materials whose properties generally combine those of the individual component polymers. The properties of the blends depend not only on those of the individual component but also on the miscibility of the blend. In many blends, a homogeneous phase was obtained because of the existence of favourable specific interactions between different polymer components. One of these favorable interactions is the hydrogen bonding, to which the miscibility of many polymer blends was attributed 11-31. The compatibility of poly(ethy1ene glycol) (PEG) and poly(vinylpyrro1idone) (PVP) in blends was shown, with FTIR spectroscopy studies, to be due to hydrogen-bonding interactions between the hydrogen atom of the PEG-terminal hydroxyl groups and the electronegative oxygen atoms in the carbonyl groups the comparatively longer PVP chains[4251.However, it is very difficult to evidence polymer miscibility on the molecular scale with the commonly- used investigation techniques. Spin label in Electron Spin Resonance (ESR) is an effective technique to investigate the structure and dynamic behavior of polymer chains in a complicated system [61. The information on polymer chain motions and intermolecular interactions can be obtained from the ESR spectra and the spectrum parameters ['-lo]. The spin-label method has been applied to the study of phase separation PEO/poly(methyl methacrylate) The spin-label method and the spin-probe techniques have
390
been combined together to study the phase structure in poly(viny1 methylether) /poly(styrene) blends"42'51. In this work, we present the application of spin-probe and spin-label methods to study molecular motions of polymer components in PEGPVP blends. 2. EXPERIMENT 2.1 Preparation of spin- labeled PEG Anionic living polymerization of ethylene oxide with sodium 4-oxy-2,2,6,6tetramethyl-piperidinyloxy (TEMPONa) as initiator was used to attach 4-oxyfree radical to the end of PEG . The synthesis of spin-labeled PEG was quantitative, with all chain ends hnctionalized by TEMPO (Scheme 1).
Scheme 1 Synthesis of spin-labeled PEG Molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) on a Waters Instrument equipped with two Waters Styragel columns (pore size:10' and lo4 A) in series. The other parameters were: THF eluent at lml/min, 38"C, polystyrene with M,= 3750 as molecular weight standard. 2.2 Preparation of polymer blends The polymer blend samples called EV1, EV5, EVlO were prepared by mixing spin-labeled PEG and PVP ( M ~ 3 6 0 0 0 0 )with a weight ratio of 1:1, 1 5 , l:lO, respectively. Sample films was prepared by casting solutions of 5% (wt./vol.) of polymers in tetrahydrofuran (THF) onto glass Petri dishes and by drying the cast films at 323 K in vacuum for 2 days. 2.3 ESR experiment ESR spectra were recorded on a Bruker ER 200 D-SRC spectrometer with ER 4 111VT temperature controlled device under the measurement condition of X-band, microwave power 20 mW, modulation amplitude 0.lmT and modulation frequency 1OOKHZ. 3. RESULTS AND DISCUSSION 3.1 ESR spectra at different temperatures ESR spectra of the spin-labeled pure PEG and its three blends were measured at
39 1
different temperature. Selected ESR spectra are shown in Figure 1. The observed temperature dependence of the spectra was due to the changes in the rotation rate of nitroxide radical, which is characterized by the correlation time ( T c). The two motional regimes usually detected in ESR experiments correspond to the slow motional spectra, with a correlation time in the range of 10-7-10-9~, and the fast motional spectra, with a correlation time in the range of IO-'-lO-" s[16'. The spectra of pure spin-labeled PEG in Figure l(a) are characteristic of a label in one type of environment. The spectra can be divided into two types. At low temperatures (slow motion region) i.e. below 280 K, only the slow component was detected in the spectra. At 120 K, the spectrum approached to the rigid spectrum, in which there is a separation between the two outer-most peaks (2Azz') of about 70 G. As the temperature increases, the spectrum narrows and the outer peak shifts inwards. Above 320 K, the value of 2Azz' reached 30.8 G, the mobility of nitroxide radicals in pure PEG reached motionally the narrowed regime. Figure l(b) shows the ESR spectra of the EV5 sample. Two significant differences are detected in the spectra of the blend compared with those of the SLPEG:
390K
--------. 4 30 K
320K
280K 240K
120K
420K 400K 370K 310K
370K
300K 290K
380K
2 76 53 K
320K
120K
300K
(C)
- - - - - - -
& (d )
Figure 1. X-band spectra as a hnction of temperature for (a) spin-label PEG, (b) EVl,(c)EV5 and (d) EV10.
392 (1) The spectra of the blend are broader than that of the pure SLPEG above 280 K. In general, the value of ~ A z zis' a measure of the mobility of the nitroxide radical. The width of 2Azz' in the blend shows that the motion of nitroxide attached to the PEG chain in the blend was hindered by the presence of PVP. (2) The two spectral components of the blends, indicating different rates of motion, were detected in the temperature range 275-370 K. As the temperature increases, the intensity of the fast motion component increases, and that of the slow motion component decreases. When the temperature reaches 380 K, the slow component in the spectrum disappears. The same results were detected in the EV1 and EVlO samples. But the temperatures where the fast component appears and the slow component disappears are different. In EV1 (Figure l(b)), the fast component appears at 270 K and the slow component disappears at 355 K. In EVIO, the fast component appears at 3 13 K, and the slow component disappears at 396 K. These two components in the ESR spectra indicate that the nitroxide radical in the blends was located in two different molecular environments, i.e. there was phase separation in the blends"731X1. The fast and slow components could be attributed to the radicals in the high-segment-mobility and rigid regions respectively. The molecular mobility of the labeled- PEG chain in the PEG-rich regions would be higher than that in the PVP-rich regions, since the glass-rubber transition temperature of PVP (Tg=446 K) is higher than that of PEG (Tg=208K). To confirm the phase separation in the blends, we annealed the EV5 sample at 450 K for 30 min and measured the ESR spectrum at the ambient temperature. In contrast to the spectrum of the sample stored at the ambient temperature, the intensity of the slow component of the annealed sample was enhanced. It seems that after molecular mixing at 450 K, part of the labeled-PEG chains was transferred into the PVP-rich region. Thus, a thermal annealing could affect the composition of the blend microphase.
Correlation time Figure 2 shows the temperature dependence of the correlation time for different samples; the correlation time is obtained from the temperature dependence of the ESR spectra, by using Kivelson's theory with the assumption of isotropic molecular motion
3.2
U61.
Here hl, ho,and h-1 are respectively the peak heights of low field, center field and high field, and A Hppis the peak width of the center field.
393
-19
-v-
$ -
-21
-
v .
. -22
-23
-
*Be
m
-24
EVIO
v-v
P o
0
m
.
.
,
.
,
2.2
2.3
'
,
2.4
'
,
2.5
.
,
.
2.6
,
2.7
'
2.8
I
.
I
2.9
3.0
'
I
3.1
.
1
32
Figure 2. Plot of In -i versus 1/T of spin labeled PEGPVP. The correlation time increases with increasing PVP content. Figure 2 could be divided into three regions. In the A region, the correlation time of the blends decreases with increasing temperature because of the faster motion of nitroxide radicals. A sharp jump of the correlation time was found in the B region, at which the correlation time increases as temperature increases. In Figure 2, we can find the process of molecular mixing of the two polymer chains. In the A region, the PEG chain movement was restricted by that of PVP because of weak hydrogen bond; the molecular motion increases as the temperature increases. In the B region, the PVF-rich phase begins to melt, the intermolecular interactions become stronger and stronger with increasing temperature, because of the chain entanglements. In the C region, the blends become compatible. This interpretation agrees with the fact that the values of 2Azz' for the blends at higher temperatures are broader than that for the spin-labeled PEG. 4. CONCLUSION In this work, end-labeled polyethylene glycol was prepared to form blend system with polyvinyl pyrrolidone in different ratios. The spin label ESR method was used to study the chain motion in the PEGRVP blends under different temperatures. In the ESR spectrum, there were only one slow component in the low temperature and one fast component in high temperature for the PEG-labeled. While two components were detected in the blends, indicating phase separation in the blends. The fast and slow components were respectively attributed to the SLPEG trapped in the PEG-rich and
394
PVP-rich region in the blends. According to ESR spectrum parameters, the correlation time of the samples was calculated. The correlation time is between 10-9-10-'1s, its value increases as the content of the PVP in the blend increases. ACKNOWLEDGMENT This investigation was supported by National Natural Science Foundation of China (NNSFC 29974006). REFERENCE 1. Watanabe T, He Y, Asakawa N, et al. Polymer International, 50 (4) (2001) 463-468. 2. Chen H L, Wang S F, Lin T L. Macromolecules, 3 1 ( 2 5 ) (1998) 8924-8930. 3. Li L, Chan C M, Weng L T. Polymer, 39 (1 1) (1998) 2355-2360. 4. Fel'dshtein M M, Lebedeva T L, Shandryuk G A, et al. Vysokomol Soedin, 41 (8) (1999) 1316-1330. 5. Lebedeva T L, Igonin V E, Feldstein M M, Plate N A. Proc Int Symp Controlled Release Bioact Mater, 24 (1997) 447-448. 6. Veksli .Z.Andreis .M.Rakvin .B.,Progress. In polymer science. 25 949-986. 7. Boyer R F, Keinath S E. Eds. Molecular Motion In Polymers by ESR; Harwood Academic,New York, 1980. 8. Brown I M. Macromolecules,1980,14 (1980) 80 1. 9. Noel C , Laupretre F, Friedrich C , Leonard C, Halary J L, Monnerie L. Macromolecules, 19 ( 1986) 20 1. 10. Perkan 0, Kaptan Y, Demir Y,Winnik M A. J Colloid Interface Sci, 11 (1986) 269. 11. Shimada S, Keiichi Kasima, Hisatsugu Kashiwabara. Macromolecules, 23 (1990) 3769. 12. Shimada S. Polymer Journal,l996,28(8) (1996) 647. 13. Shimada S, Isogai 0. Polymer Journal, 1996,28(8) (1996) 655 14. Cameron G G, Qureshi M Y, Tavern S C. Polymer International, 47(1) (1998) 15. Muller G, Stadler R, Schlick S. Macromolecules, 27 (6) (1994) 1555-1561. 16. Kivelson D. J Chem Phys ,33 (1960) 1107. 17. Schlick S, Harvey R D, Aloson-Amigo M G, Klempner D. Macromolecules, 22 (1989) 822. 18. Harvey R D, Schlick S. Polymer, 30 (1989) 104. 19. Bullock A T,Gameron G G,Miles S. Polymer, 23 (1982) 156. 20. Braun D,Tormala P,Weber G. Polymer, 19 (1978) 598.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
395
EPR and W-VIS studies on the influence of solute-solvent interactions on the self-redox reaction of Nicola D. Yordanov and Kalina Ranguelova EPR Laboratory, Institute of Catalysis, Bulgarian Academy of Sciences, 1113Sofia, Bulgaria The present communication describes the influence of non-polar, polar and coordinating solvents, temperature of solution and disulfide concentration on the self-redox reaction taking place between two molecules of complexes, {Cun[(R0)2PS2]2, where R = Me, Et, i-Pr}. The studies are performed by EPR and spectroscopy. It is found that the EPR parameters and the position of the charge-transfer band at 420 nm of Cun[(R0)2PS2]2 are not influenced by non-polar solvents. However, this is not the case with the intensity of the spectra since both the EPR intensity and the molar absorptivity are strongly dependent on the type of the used solvents, size and shape of remote ligand substituents, time after dissolution and quantity of added disulfide of dithiophosphate, (RO)2P(S)S-S(S)P(RO)2. In coordinating solvents the g-values increase whereas copper hyperfine splitting decreases and a hypsocromic shift is observed in the absorption band as compared to non-polar solvents. The results are ex lained with the formation of adducts with an axial or equatorial coordination between Cu [(R0)2PS2]2 and the coordinating solvents. A decreasing intensity of the EPR and electronic absorption spectra of CuE[(R0)2PS2]2 is observed 24 h after preparation of solutions in two of them OMSO and pyridine). A transitory increase in intensities is recorded in these solvents after the addition of the corresponding disulfide of dithiophosphate, but the initial complex is destroyed as a result of adduct formation. The observed effects are discussed in terms of specific solute-solute-solvent interactions governing the self-redox reaction.
f
1. INTRODUCTION
Metal salts or esters of diorganoderivatives of dithiophosphoric acid have found wide application as analytical reagents for extraction and spectrophotometric determination of copper(II). In addition they are recommended as antioxidants, antiwear additives to motor oils and polymeric materials, and also insecticides, fungicides, pesticides, flotation agents, etc. These are the reasons for studying them in order to get deeper insight into their structure, properties and reactivity. These problems are often discussed in the literature, but regarding their interaction with copper(II) ions there is a significant controversy up to now. 2. MECHANISM OF THE SELF-REDOX REACTION
The reduction of Cu(II) ions by a variety of thiols in water solutions to give Cu(I) and disulfides reported for the first time 100 years ago. From that time until recently the
396
following reaction Scheme 1 has been commonly accepted on the basis of the final reaction products:
Scheme 1 RSCu'
+ Cuz+ + RS'
-
-
+
Cu' + RS' Cu'(SR)
2RS' RS-RS in toto: 4RS' + 2Cu2+ 2Cu'(SR)
-
+ RS - SR
According to this scheme electron transfer from RS to Cu2+takes place and a free radical recombination yields the final disulfide. However, in a number of observations was found that such reactions proceed through a short-lived color not typical of the reactants themselves, which disappears with a characteristic time between milliseconds and hours followed by immediate precipitation of a pale-yellow solid phase found by EPR spectroscopy to be diamagnetic. This color is independent of the copper(II)/RS- ratio and the shape and size of remote ligand substituents. The precipitate is soluble in organic solvents and each recrystallization is connected with decreasing of S, C, H and increasing of Cu content in it [l].This phenomenon could be explained taking into account the high solubility of the disulfide and the low solubility of the complex of copper (I) in the used solvents. The self-redox reaction is typical for many sulfur-containing copper complexes as dithiophosphinates, xanthates, carboxylates, etc [11. Recent studies in our laboratory on the formation of Cun(R2-dtp)2 in alcohoVwater solutions using stopped-flow technique [2,3] have shown that the absorption band of the intermediate species at 420 nm fully coincides with that of the bis(chelate)copper(II) complex dissolved in organic solvents. On the other hand the absorbance reaches maximum value at the molar ratio Cu(II)/dtp = 1:2 and within few seconds decays through a second-order reaction in respect to its concentration. The data obtained for the rate constants in water-alcohol solutions prove that the more bulky the ligand substituent, the lower is the rate constant, thus confirming the idea of an association process between two absorbing molecules. The rate constant dependence on the remote ligand substituents follows the order (1): Me > Et > Pr > i-Pr > Bu > s-Bu > t-Bu
(1)
which suggests that the stereochemistry of the molecules is an important factor in the reaction course. On the basis of these and other results [4] a different mechanism was suggested involving self-association between Cu and S atoms from one molecule and S and Cu from another molecule (Scheme 2). In this case the isolated molecules are usually paramagnetic and exchange reaction between them, called self-association, is connected with decreasing covalence of the Cu-S bonds as well as expansion and rhombic distortion in the equatorial plane. When the ligand substituents are not bulky, sulfur atoms could approach enough close to each other to form an S-S bond between the ligands of two neighbouring Cun(R2-dtp)2 molecules. This yields a disulfide from the ligand and a reduction of Cu(I1) to Cu(I). Furthermore, two copper(1) dimmers form tetramers. The self-redox reaction could be expressed with the equilibrium reaction (Scheme 3). When these complexes are dissolved,
397
Scheme 2
the equilibrium should be set up. For Cun(R2dtp)2 complexes with bulky substituents, close approach of the molecules is prevented and the monomeric structure is favoured.
Scheme 3 2RS- + Cu2+4 Cun(SR)2 2Cun(SR)2 +m [Cun(SR)2.. ..... Cun(SR)2]
4
2Cur(SR) + RS-SR [C~'(dtp)4]+ 2ds
4Cun(dtp)2
All studies reported up to now confirm this mechanism of the self-redox reaction and strongly suggest that it can be affected by the following factors [5] which are discussed in the present paper. 3. EFFECT OF COMPLEXES SOLUTESOLVENT INTERACTIONS AND TEMPERATURE ON THE SOLUTION SPECTRAL PROPERTIES
80 G
P
I
Figurel. Room temperature EPR spectrum of Cun(R2- dtp)z complexes.
398
The self-redox reaction was investigated in two groups of solvents - polar and nonpolar and with different ligand substituents @PryEt and Me). DMFA, DMSO and pyridine were used as polar solvents and CHCl3, C6HsCH3, c6&,Cc4, C7H16, C6H14 as nonpolar. The solution EPR spectrum of Cun(R2-dtp)2 at 293 K has the typical features of the chromofore CuS4 (Fig. 1). It is characterized with a hyperfine splitting due to the interaction of copper(I1) unpaired electron with 63p65Cu nuclei (1=3/2). In addition a superhyperfine (shf) splitting appears with the intensity ratio 1:2:1, due to the interaction of copper(I1) unpaired electron with two equivalent "P nuclei from the ligands which is unambiguous evidence for the presence of the CuE(R2-dtp)2. This splitting could not be observed in some polar solvents due to line broadening.
Scheme 4
I
The effect is connected with the fact that Cu(II) chelate complexes react with some basic molecules. The products of this D-A interaction are 1:l or 1:2 adducts with axial or equatorial coordination. (Scheme 4). ( In our case adduct is a molecular (D-A) complex formed between two valently saturated molecules, one of which is a metal chelate.) The influence of the axial adducts on the EPR spectra is: (i) shifting the EPR spectra to lower magnetic field; (Table 1). (ii) increasing the g-values and decreasing of The reason is axial followed by equatorial coordination of the polar solvent molecules to the metal ion in the chelate complex which changes its electronic structure lowering the covalent character of Cu-S bonds and causes deformation in the plane xy thus facilitating its destruction.
Table 1.EPR parameters of the Cu(R2-dtp)z in the used solvents. Solvent I g, [+0.001] Ah', 2.067 2.057 DMFA 2.053 74.2 solvents
I
I
399
Our investigations on the effect of solvents and time after dissolution on the EPR and electronic spectra of Cun(R2-dtp)2 show that the EPR intensity of Cun(R2-dtp)2 in the used polar and non-polar solvents one hour after dissolution is different. It becomes 2-3 times higher on keeping the complexes for 24 h at room temperature due to the shifting of equilibrium (3) towards Cun(R2-dtp)2 and remains constant in non-polar solvents and DMFA for 60 days period of monitoring (Table 2). The highest EPR-intensity of Cun(R2-dtp)2 obtained in DMFA compared to the other solvents (Mn2+/Mg0was used as a standard in all measurements) could be explained with the formation of Cun(R2-dtp)2 adducts with axial DMFA coordination. From the used non-polar solvents maximum value of the intensity is obtained in CHCl3 due to the formation of D-A complex and H-bonds, characterized by ENDOR spectroscopy, between Cun(R2-dtp)2 and CHCl3 which facilitates the reverse reaction shifting equilibrium (3) towards Cun(R2-dtp)2. It is well known that CC4 forms stronger D-A complexes with Cun-complexes but obviously it is not enough and most probably H-bonds are much more important for improving the reverse reaction. The complexes exhibit lower EPR intensity in hexane and heptane which have weaker effect on the self-redox process because these solvents are not capable for specific interactions with the solute. In the polar solvents DMSO and Pyridine the EPR intensity gradually decreases and the EPR signal disappears several days after preparation of their solutions because the obtained axial adducts in these solvents are precursors to the equatorial. After that the complexes are destroyed (Scheme 4). The obtained EPR intensities are in concert with the
Table 2. Relations of the EPR - intensity (a.u.) of Cun(i-Pr2 - dtp)z in different solvents versus intensity of Mn2+/Mg0(standard) and values of the molar absorptivities [m-’.cm-’]
DMSO Pyridine
I
11,2.10’ 7,2.103
I
10,5.10’ 5,9.103
9,6.10’ 4,5.103
22,O.lO” 9,6.103
400
electronic spectral changes: (i) the dependence of the absorbance at 420 nm vs. concentration is not linear when the electronic spectra are recorded 1h after preparation of solutions and Beer's law is not obeyed. (ii) after keeping Cuu(R2-dtp)2 solutions for 24 h at room temperature the dependence A 4 2 0 /c becomes linear, Beer's law is satisfied and the molar absorptivity values are higher than those obtained 1h after dissolution. At least 24 h after preparation the EPR spectra of Cun(R2-dtp)2 solutions were recorded at different temperatures in the interval 20 - 50 OC. The obtained spectra show increased signal intensity within 30 - 40% in non-polar solvents and almost 50% in DMFA. This effect could be also explained with the shifting of equilibrium (3) towards Cun(R2-dtp)2. After heating to 50°C and then cooling the solution to room temperature the increased EPR-intensity was kept constant for 4-5 days. Heating the solutions in DMSO and Pyridine decreases the intensity of the EPR spectra with 10% in DMSO and 30% in Pyridine. The explanation is that the process of adduct formation with these solvents is accelerated at higher temperature and contributes to the destruction of the complexes. After cooling the solutions to room temperature the previous intensities of the EPR signals are not obtained and they continue to decrease. Studies were also carried out after addition of small portions of the appropriate disulfide dissolved in the same solvent to the solution of Cuu(R2-dtp)z. The results show a significant increase of the EPR-intensity of Cuu(R2-dtp)2 (Table2). The obtained EPR intensities remain constant in DMFA but they gradually decrease in DMSO and Pyridine because of destruction of the complexes. On the other hand the results show that the more the solution is diluted the larger should be the quantity of added disulfide to obtain increased EPR intensity. By comparing the results of EPR and UV-VIS investigations the maximum molar absorptivity of Cuu(R2-dtp)2 band at 420 nm was found to be 2 . 9 ~ 1 0M-' ~ cm-' (for i-Pr substituent in DMFA). 4. CONCLUSION
The reported data show that the proceeding of self-redox reaction is influenced by many factors. The EPR intensity of Cuu(R2-dtp)2 strongly depends on the nature of the solvent, time after dissolution, temperature, and quantity of the added disulfide. Polar solvents form adducts with Cuu(R2-dtp)2 which could be axially (DMFA) or equatorially (DMSO and Pyridine) coordinated towards the plane of the chromophore CuS4. After keeping the solutions for several hours, heating them to 50°C and/or adding of disulfide, the molar absorptivity was found to increase up to 2 . 9 ~ 1 0M-' ~ cm-' for Cun(i-Pr2-dtp)2 in DMFA which is the highest reported value in the literature up to now.
REFERENCES 1. N.D.Yordanov, Transition Met. Chem,22 (1997) 200. 2. N.D.Yordanov, LAntov and G.Grampp, Inorg.Chim.Acta, 272 (1998) 291. 3. N.D.Yordanov, K.Ranguelova and G.Grampp, Inorg.Chem.Commun., 3 (2000) 383. 4. N.D.Yordanov, V.Alexiev, J.Macicek, T.Glowiak and D.R.Russe1, Transition Met. Chem., 8 (1983) 257. 5. N.D.Yordanov, KRanguelova and LGadjanova, Transition Met.Chem. in press (2001).
Section 4 Environmental Sciences
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EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
403
In and ex EPR spectroscopy and imaging of endogenously produced nitric oxide under physiological and pathophysiological conditions Tetsuhiko Yoshimura* and Naoki Kato Laboratory of Applied Biomedicinal Chemistry, Institute for Life Support Technology, Yamagata Public Corporation for the Development of Industry Matsuei, Yamagata 990-2473, Japan
It is now widely known that nitric oxide (NO) is an ubiquitous messenger molecule in physiological and pathophysiological processes. Although NO is an uncharged free radical, direct electron paramagnetic resonance (EPR) detection of endogenous NO in biological samples seems almost impossible because of its low level (less than and short half life (3-5 s for NO from endothelial cells) These difficulties can be overcome by applying a spin-trapping technique. Iron complexes with dithiocarbamate derivatives (FeDTCs) are noted among the spin-trapping reagents for NO because NO has a high affinity for the iron complexes and resultant stable nitrosyl iron complexes exhibit intense signal both at room temperature and at low temperature, enabling and determination of endogenous NO. We have been studying the roles of endogenously produced NO by employing EPR spectroscopy and Fe-DTCs as NO trap. Here, we will present our researches concerning the EPR detection and imaging of endogenously produced NO under physiological and pathophysiological conditions.
1. INTRODUCTION
NO is a small uncharged free radical containing one unpaired electron. NO exists in space as an interstellar molecule [ 11, and in the atmosphere of Venus and Mars [2,3]. On our earth, NO has been recognized as an atmospheric pollutant and a potential health hazard. In Japan, the air pollution by nitrogen oxides @Ox) has become a subject of discussion since 1970s. Although relatively high concentration of NO has been detected in urban atmosphere, we do not yet have the detailed knowledge on the biological effects of atmospheric NO. However, in 1987 it was reported that NO is identical with endothelium derived relaxing factor (EDRF), which is biosynthesized in the living body [4-61. It is now widely known that NO is an ubiquitous messenger molecule in the cardiovascular, nervous, and immune systems [7-91. In addition, L-arginine-derived NO has been found in a wide variety
404
of organisms ranging from mammals to invertebrates, bacteria, and plants. To elucidate a variety of biological actions of NO, we must have information concerning the quantities and distributions of NO in cells, tissues and organs. Because NO formed has a low level (less than yM) and a short half life (3-5 s for NO from endothelial cells), it is difficult to determine the NO. As one of the method to overcome these difficulties, spin-trapping technique has been applied to and NO determinations by electron paramagnetic resonance (EPR) spectrometry. Iron complexes with dithiocarbamate derivatives (Fe-DTCs) are noted among the spin-trapping reagents for NO because NO has a high affinity for the iron complexes. Resultant nitrosyl iron complexes are highly stable and exhibit intense three-line signal at room temperature and axial signal at low temperature, enabling and determination of endogenous NO. Therefore, iron dithiocarbamates and their NO complexes can be effectively utilized as an NO trapping reagent and a spin probe or an imaging reagent, respectively. We have been developing the quantification method of NO employing an EPR spectrometry combined with spin-trapping technique in which utilizes Fe-DTCs as NO trap [lo]. Further, we have been studying the roles of endogenously produced NO under physiological and pathophysiological conditions through the detection of NO in cultured cells, resected organs and tissues, and living small animals [ll-261. In this paper, several examples on ex and EPR detection and imaging of endogenously produced NO are presented.
2. BACKGROUND OF NO DETECTION BY EPR SPECTROMETRY The elucidation of physiological and pathological roles of NO can lead to the developments of biomedical research reagents and clinical drugs and the research results can be utilized in clinical or therapeutic trials. Successful examples are a clinical therapy with sildenafil for patients with an erectile dysfunction and an inhaled NO for infants with primary pulmonary hypertension. To clarify the roles of NO, NO detection and quantification in biological systems are required. Newly developed analytical techniques have revealed numerous roles of NO in various biological specimens [27].Determination methods of endogenous NO are classified roughly into five techniques: chemiluminescence technique using ozone or luminol, spectrophotometry using Griess reagent or hemoglobin, fluorometry using diaminonaphthalene (DAN) or diaminofluorescein (DAF), amperometry using microelectrode, and EPR spectrometry combined with spin-trapping technique. Among them, three techniques of chemiluminescence technique, amperometry, and EPR spectrometry have been reported to be feasible to detect NO in vivo. So far, EPR spectrometry alone has been employed to obtain images of endogenous NO [lo]. Typical NO trapping agents reported are as follows: hemoglobin derivatives such as deoxy hemoglobin and carbonyl hemoglobin; iron dithiocarbamate complexes; nitronyl
nitroxide such as -yloxyl-3-oxide (PTIO); NO cheletropic trap (NOCT) [27]. At present, NO trapping reagent applicable to measurements are limited to iron dithiocarbamates [lo]. Three dithiocarbamates (Figure 1) are frequently used in NO researches: diethyldithiocarbamate (DETC); N-methyl-D-glucamine dithiocarbamate (MGD); (dithiocarboxy)sarcosine ( DTCS). Fe-DTC + NO
+ NO-Fe-DTC
(1)
Ferric or ferrous dithiocarbamates react with NO to form an NO complex. The reaction of ferric dithiocarbamate with NO occurs through novel reductive nitrosylation. Resultant NO complex is highly stable and exhibits characteristic intense EPR spectra, both at room temperature and at low temperature. Recently, we reported the second-order rate constant of this reaction [23,26]. The value, 5 x lo8 M%', clearly indicates that this reaction proceeds very rapid. The differences in water solubility among the Fe-DTC complexes allow selective usages as NO traps [lo]. Three ligands in Figure 1 have high water solubility. On the other hand, the solubilities of iron complexes are different each other. Fe-DETC is essentially insoluble in water, but it is lipid soluble. In contrast, Fe-DTCS and Fe-MGD are soluble in water, but they have low lipid-solubility. As a result, Fe-DETC has an advantage for the detection of intracellular NO, while water soluble complexes are effective in the detection of extracellular NO. Therefore, we have employed the Fe-DETC as an NO trap for tissues and organs and the Fe-DTCS for extracellular NO and imaging of NO. NO is biosynthesized by three distinct isoforms of NO synthase [7,8,9]. Neuronal NOS (nNOS) and endothelial NOS (eNOS) are constitutively expressed in neuronal and endothelial cells, respectively. Hence, both enzymes are called as cNOS. On the other hand, inducible NOS @NOS)is expressed in macrophages and microglias through stimulation with cytokines and endotoxins. NO levels produced from cNOS are sub-yM at cellular level, while NO levels from iNOS are 1 to 10 pM. Thus, iNOS is high output NOS. Reported values of half life of NO under physiological conditions range from 0.1 to 5 seconds.
IN& -sbrcoo-
S
S C2H5
I
CH,
DETC
S OH
MGD
OH
CH3
OH
DTCS
Figure 1. Dithiocarbamate derivatives used as NO trapping reagents
406
In NO researches, NOS inhibitor has been utilized for judging whether the observed events are NOS dependent or independent. If biological phenomenon is reduced or eliminated on the administration or addition of NOS inhibitor, this phenomenon is NOS dependent. Here, we used three kinds of NOS inhibitor; N-nitro-L-arginine methyl ester (LNAME), N-monomethyl-L-arginine (L-NMMA), and 7-nitroindazole (7-NI).
3. EX VZVO NO DETECTION IN KAINIC-ACID INDUCED SEIZURE IN MOUCE
Kainic acid was originally isolated from the seaweed Digenea simplex, which grows near Japan and Taiwan; known as Kaininso in Japan. Kainic acid is a structural analog of the excitatory amino acid glutamate that binds and activates ionotrophic glutamate receptors. Kainic acid-induced limbic seizures have been extensively used as a model for studying human temporal lobe epilepsy [28]. Recently, the close relationships between kainic acidinduced nitric oxide (NO) and seizures has been suggested [29]. In the present study, to elucidate the involvement of kainic acid-induced NO with seizure, we attempted to determine NO level in the right temporal cortex region by EPR spectrometry [30]. In the control group, no spike discharges were observed throughout the experiments in the electroencephalogram at mouse temporal lobe. After treatments of kainate 30 mgkg, epileptic discharges began to emerge and they were observed frequently 60 minutes after it. Thus, kainate administration to animals effectively induced an epileptic seizure. Kainate Dose (mg/kg) -o-
control
+ 10
I
0
I
I
60 90 Time after kainate injection (min)
I
120
Figure 2. Sequential changes of NO signal intensity in the temporal cortex of mouse brain after kainate injection (n = 5, mean k SE)
407
Kainic acid was administered to a mouse intraperitonealy. Then, intraperitoneal administration of DETC and subcutaneous administration of iron-citrate were performed. Thirty minutes after the administration, we observed X-band EPR signals in the brain tissues at room temperature. Since it has been known that this signal height is proportional to the amount of NO generation, the signal height was measured as an index of NO generation. Moreover, we can convert the signal height into NO concentration by using a standard NO complex. Figure 2 shows sequential changes of NO signal intensity in the temporal cortex of mouse brain after kainate injection (n = 5 , mean SE). After kainate injection, NO quantities gradually increased and reached a maximum at 90 minutes, and then decreased. As shown in Figure 2, NO quantities increased dose-dependently. Maximum signal intensity corresponds to about 4 nmol/g-tissue/30 min-accumulation of NO concentrations. However, preadministration of NO synthase inhibitor (L-NAME and 7-NI) reduced NO to control level at 90 minutes after kainate injection,. This suggests that the NO production induced after kainate administration is truly derived from NO synthase. On the other hand, the severity of kainate-induced seizure increased dose-dependently. Furthermore, the seizure severity was aggravated by the preadministration of NO synthase inhibitor. In summary, kainic acid-induced NO could be measured by using EPR NO trapping technique. NO level was increased dose-dependently in kainic acid doses of 10 to 40 mg/kg, accompanying the development of seizure severity and the increase in mortality. Preadministration of NOS inhibitors suppressed the increase in NO production, but promoted the severity of kainic acid-induced seizure. These results suggest that the enhanced NO production on kainic acid treatment contributes to the suppression of seizure severity.
4. IN WVO DETECTION 4.1. EPR-CT system developed in our institute Here, in vivo EPR detection and imaging methods will be briefly presented with emphasis on the EPR-CT system developed in our institute. Since at conventional X band (9 - 10 GHz) frequencies, water-rich samples have a high dielectric loss which reduces the factor of the resonant cavity [31,32], we utilized 700 MHz microwaves to lower the dielectric loss of water. A loop-gap resonator was used because common cavity resonators have poor filling factors and have an inhomogeneous microwave field inside the resonator [33]. The resonator has the dimensions of 10 mm in axial length and 41 mm in inner diameter, which can accept the head of a rat or the whole body of a mouse. We utilized an air-core magnet to rapidly scan the magnetic field; a pair of magnetic field gradient coils for the X-, Y-, and Zaxes were attached to the surface of the main magnet [34,35]. Three-dimensional EPR images are constructed on the basis of 3D zeugmatography. We collected many EPR spectra by changing the direction of magnetic field gradients under
408
computer control. The EPR spectral data were deconvoluted by a fast Fourier transform method with low pass filtering. Then, EPR images are reconstructed from the deconvoluted data by a filtered back projection. We have limitations in the attempt of current EPR detection and imaging. First, the sensitivity of low frequency spectrometer is much lower than the X-band instrument because the sensitivity is proportional to the square of the frequency. Therefore, to obtain data, we utilized the pathological models to produce high levels of NO and the NO donating reagent to produce much NO Second, the resolution of EPR image is affected by several factors such as linewidth and field gradient. Thus, broad linewidth of NO complexes results in low resolution of image. vivo NO detection in experimental meningitis in rat In various phases during bacterial meningitis, the involvement of excessive NO synthesis has been reported [36,37]. Experimental bacterial meningitis was induced in rat by administration of LPS and IFN-y, where LPS is a bacterial endotoxin and IFN-y is a cytokine. First, we used Fe-DETC complex as an NO trapping reagent and measured X-band EPR spectra of brain tissues at 77 K [21]. Sequential changes of signal intensity were measured in the rat brain tissues during experimental meningitis. The signal intensity reached a maximum at 8 hours after the injection, and then decreased. Then, EPR measurements were performed on the head region at 8 hours after the injection and a weak triplet signal was observed (Figure 3). This signal disappeared after the pretreatment with NO synthase inhibitor, L-NMMA. These results showed that in vivo EPR signal originated from NO synthase induced by experimental meningitis. Furthermore, expression of iNOS gene was also confirmed with RT-PCR technique. 4.2.
a
1 mT
Figure 3. In vivo EPR spectra in the rat brain during experimental meningitis. (a) EPR spectrum recorded at 8 hours after the injection of LPS and IFN-y. (b) EPR spectrum after the injection of LPS and IFN-y and then of L-NMMA.
Figure 4. The structure of isosorbide dinitrate (ISDN)
409
vivo EPR detection and imaging of NO produced from isosorbide dinitrate More than 90 of information from the external environment is visual. An appeal to the eye is more effective than that to the ear. Therefore, seeing is believing. We performed in vivo EPR detection and imaging of NO produced from isosorbide dinitrate (ISDN, Figure 4) [17]. In the ISDN, NO is released from two nitrate ester groups in living body. ISDN is a long-acting nitrovasodilator while glyceryl trinitrate is a quick acting drug. Both drugs have been used as therapeutic agents for the angina pectoris for many years [27]. First, we administered iron-DTCS complex to mouse, subcutaneously. 30 Minutes later, S.C. administration of I4N- or "N-ISDN was performed. Then, in vivo EPR spectra were measured with our EPR-CT system. vivo spectrum of upper abdomen of mouse had a triplet in the treatment of I4N-ISDN and a doublet in that of "N-ISDN. The latter had a higher signal-to-noise ratio than the former. EPR-CT images from the upper abdomen of I4N- and 'jN-ISDN -administered mouse was obtained, respectively. The spatial resolution of images improved from 5.7 mm in the treatment of I4N-ISDNto 3.9 mm in the treatment of IjN-ISDN. The outline of a slice image obtained with I4N-ISDN corresponds to liver alone, while that obtained with "N-ISDN corresponds to liver and kidney, suggesting that ISDN is metabolized primarily in the liver. These results clearly demonstrated that the I5N substitution of 14Nin ISDN provides a high quality EPR image of NO in a living mouse.
4.3.
In conclusion, L-arginine-derived NO is an ubiquitous messenger molecule in cardiovascular, nervous, and immune systems. Therefore, in vitro, ex vivo, and vivo detection and imaging of NO are required elucidating the biological roles of NO. EPR NO trapping technique using iron-dithiocarbamates as NO trap is considered to be most effective for detection and imaging of endogenously produced NO. At present, vivo EPR detection and imaging are not applicable to physiological levels of NO because of limitations of currently available EPR instrumentations. However, this method has the potential to contribute to diagnosis of pathophysiological conditions involving the overproduction of NO. I believe that further developments of EPR instrumentations and NO traps with novel functions will open up new applications. We would like to acknowledge the contribution of our coworkers whose names appear in referenced papers.
REFERENCES 1.
H. S. Lisa and B. E. Turner, Astrophys. J., 224 (1978) L73-L76.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
A. J. Watson, T. M. Donahue, and D. H. Stedman, Geophys. Res. Lett., 6 (1979) 743746. W. L. Chameides, J. C. G. Walker, and A. F. Nagy, Nature, 280 (1979) 280, 820-822. R. M. J. Palmer, A. G. Femge, and S. Moncada, Nature, 327 (1987) 524-526. L. J. Ignarro, G. M. Buga, K. S. Wood, R. E. Byrns, and G. Chaudhuri, Proc. Natl. Acad.Sci. USA., 84 (1987) 9265-9269. R. F. Furchgott, Angew. Chem. Int. Ed., 38 (1999) 1870-1880. S. Moncada, R. M. J. Palmer, and E. A. Higgs, Pharmacol. Rev., 43 (1991) 109-142. P. L. Feldman, 0. W. Grifith, and D. J. Stuehr, Chem. Eng. News., 71 (1993) 26-38. J. F. Kenvin, Jr., J. R. Lancaster, Jr., and P. L. Feldman, J. Med. Chem., 38 (1995) 43434362. T. Nagano and T. Yoshimura, Chem. Rev., 102 (2001) in press. T. Yoshimura, S. Fujii, H. Yokoyama, and H. Kamada, Chem. Lett., (1995) 309-310. T. Yoshimura, H. Yokoyama, S. Fujii, F. Takayama, K. Oikawa, and H. Kamada, Nature Biotechnol., 14 (1996) 992-995. S. Fujii, T. Yoshimura, and H. Kamada, Chem. Lett., (1996) 785-786. S. Fujii, G. Miyakoda, M. Chihiro, T. Yoshimura, and H. Kamada, Chem. Lett., (1996) 1055-1056. H. Yokoyama, S. Fujii, T. Yoshimura, H. Ohya-Nishiguchi, and H. Kamada, Magn. Res. Imag., 15 (1997) 249-253. Y. Suzuki, S. Fujii, T. Tominaga, T. Yoshimoto, T. Yoshimura, and H. Kamada, Biochim. Biophys. Acta, 1335 (1997) 242-245. S. Fujii, Y. Suzuki, T. Yoshimura, and H. Kamada, Am. J. Physiol., 274 (1998) G857G862. Y. Suzuki, S. Fujii, Y. Numagami, T. Tominaga, T. Yoshimoto, and T. Yoshimura, Free Radic. Res., 28 (1998) 293-299. K. Inoue, T. Akaike, Y. Miyamoto, M. Otagiri, S. Suzuki, T. Yoshimura, and H. Maeda, J. Biol. Chem, 274 (1999) 27069-27075. T. Ueno, Y. Suzuki, S. Fujii, A. F. Vanin, and T. Yoshimura, Free Radic. Res., 31 (1999) 525-534. Y. Suzuki, S. Fujii, T. Tominaga, T. Yoshimoto, S. Fujii, T. Akaike, H. Maeda, and T. Yoshimura, J. Cereb. Blood Flow Metab., 19 (1999) 1175-1178. M. Okuyama, S. Yamaguchi, M. Yamaoka, J. Nitoube, S. Fujii, T. Yoshimura, and H. Tomoike, Arterio. Throm. Vasc. Biol., 20 (2000) 1506-1511. S. Fujii, K. Kobayashi, S. Tagawa, and T. Yoshimura, J. Chem. SOC.Dalton Trans. (2000) 33 10-3315. H. Endoh, S. Fujii, Y. Suzuki, S. Sato, T. Kayama, Y. Kotake, and T. Yoshimura, Free Radic. Res., in press. T. Yoshimura, H. Yokoyama, and S. Fujii, J. Magn. Reson. Anal., 3 (1997) 125-140. S. Fujii and T. Yoshimura, Coodin. Chem. Rev., 198 (2000) 89-99.
41 1 27. M. Feelisch and J. S. Stamler (eds.), Methods in Nitric Oxide Research, John Wiley & Sons, Chichester 1996. 28. G. Sperk, Prog. Neurobiol. 42 (1994) 1-32. 29. E. Przegalinski, L. Baran, and J. Siwanowicz, 170 (1994) 74-76. 30. N. Kato and T. Yoshimura, to be published. 3 1. K. Ohno, Magn. Reson. Rev. 11 (1987) 275-3 10. 32. G. R. Eaton, S. S. Eaton, K. Ohno, EPR Imaging and in vivo EPR; CRC Press: Boca Raton, FL, 1991. 33. M. Ono, T. Ogata, K. -C. Hsieh, M. Suzuki, E. Yoshida, and H. Kamada, Chem. Lett., (1986) 491-494. 34. H. Yokoyama, Y. Lin. 0. Itoh, Y. Ueda, A. Nakajima, T. Ogata, T. Sato, H. Ohya Nishiguchi, and H. Kamada, Free Radic. Biol. Med., 27 (1999) 442-448. 35. K. Oikawa, T. Ogata, H. Togashi, H. Yokoyama, H. Ohya-Nishiguchi, and H. Kamada, Anal. Sci., 11 (1995) 885-888. 36. A. R. Tunkel and W. M. Scheld,Annu. Rev. Med., 44 (1993) 103-120. 37. K. M. K. Boje, Brain Res., 720 (1996) 75-83.
412
EPR in the 21" Century A Kawarnori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Molecular-electronic mechanism of the toxicity of Dioxin ability of some natural structures to concurrently interact to inhibit its activity Nguyen Van Tri", Pham The Vung", Dinh Pham Thaia, Ha Van Maob and Dinh Ngoc Lamb
" Institute of Engineering Physics, Hanoi University of Technology, P.CHT, C2-101, Dai hoc Bach khoa, 1 Dai Co Viet, Hanoi, Vietnam Hanoi Cancer Research Center, 1 Tran Thanh Tong, Hanoi, Vietnam Numerous particular ESR line groups from Dioxin (2,3,7,8-TCDD) in biological fat and oil, and from its interaction effects in human blood and liver have been revealed and interpreted on the basis of the exchange combinations of the x-electrons of the Dioxin molecule and their specific superexchange complexes with the structures of the nuclei of the respective proteins. On the other hand, the results achieved in long-term experimental clinical studies suggest that some special biological natural compounds from traditional sources are able to inhibit the toxicity of Dioxin. On the basis of the ESR and also the HPLC experimental results, we have revealed some new active natural complexes present in special vegetable products. These complexes are able to bind strongly to Dioxin molecules, thereby inhibiting the toxicity of Dioxin, and its carcinogenic activity. 1. INTRODUCTION AND EXPERIMENTAL
Polychlorinated dibenzo-p-dioxins have generated considerable concern because of their resistance to biological and chemical degradation, ubiquity, fat solubility, long biological half-life and extremely high toxicity. Numerous results of experiments with dioxins in animals (especially on the liver) suggested that dioxins can induce various biological effects. They can adhere strongly to the body, have high affinity binding to protein structure, and deeply perturb genetic control centers in cell nucleus. The experiments also verified of of Dioxin toxicity is selectively dependent on the number and the positions of the substituted chlorine atoms. Especially, the prototypical and most potent member, of maximum toxicity, of these compounds is (TCDD, Dioxin) [ 1-51, On the other hand, the results achieved in long-term experimental and clinical studies [6-81 suggest that some special biological natural compounds from traditional sources have of Such a compound called "Gacavit" is presented in this paper, a Vietnamese plant drug prepared from Momordica Cochinchinensis Spreng [9]. The nutritive effect of this compound is well known and has been used by the population for centuries. It is rich in p-carotene and other carotenoids, oleic, stearic, palmitic, linoleic acids and various microelements [lo]. A primitive chemicopharmaceutical analysis showed that the Gacavit samples contain 0.165% p-carotene.
413 The complicated relationships mentioned above can only be clearly interptreted on the basis of the molecular electronic dynamics of the interaction of dioxin with the biological structures. Keynote problems include the mechanism of the apperance of the respective unpaired electrons - the prerequisite of the interactions - and the nature of the particular binding complexes between dioxin and the biological structure. In this paper, ESR is used as an especially effective means to reveal and explain the nature as well as the molecular electronic mechanism of the interactions. In order to clearly observe the respective effects, the experimental samples have been treated with Dioxin at the dose of about lo3 ppt. The Dioxin-treated samples and the original samples have been studied in situ and in vivo. The ESR measurements were performed on an ERS-220 standard electron resonance spectrometer system with the sensitivity of 10'' Spin/G. The sample temperature was controled automatically at diffrent values from 77 K to room temperature. Some results will be briefly reported as follows.
2. MOLECULAR ELECTRONIC MECHANISM OF THE DIOXIN ACTIVITY Typical ESR spectra from Dioxin [ l l ] are given in Fig.1. These spectra are exactly evidenced by a spin Hamiltonian for the exchange coupled combinations of the electrons of the Dioxin molecule. These electrons may be considered as quasi-free electrons moving in an open resonant cavity constituted by the short-range order structure [12] of the Dioxin molecule. The calculation on the measured data suggests that the line pair B,,B2 originated from the triplet state (S=2/2) combined with the two quinone electron spins corresponding to the Dewar structure of each aromatic ring of the Dioxin molecule (Fig.2). The four line group Dl,DZ,D3,D4 is the signal of the quintuplet state (S=4/2) of a super-exchange combination of the four quinone electron spins on the whole Dioxin molecule (Fig.3). This combination creates very strong electron wave bundles pointing out perpendicularly to the Dioxin molecule plane. But the ground state of the combination is singlet (S = 0). Thus, in the ground state, Dioxin molecules can easily move and worm their way deeply into biological structures. Here, they may be promoted to the quintuplet state - an excited state - and become able to interact strongly with partners. The spectrum in F i g . 1 ~shows clearly a particular binding complex between the Dioxin molecule and the heme nucleus of hemoglobin (Fig.4). A similar result was also obtained from the interaction effect of Dioxin with the Mn-bearing porphyrinoprotein in human liver [I 11. These experimental revelations suggest that the nature of the specific binding complexes between Dioxin and the functional nucleus structures of partners is just a super-exchange interaction through a combination electron spins delocalized between the two molecular systems. This interaction can develop and burrow very deeply into the nuclear framework of the nucleus structure of partners and form a super-stable complex. These results also coincide with the suggestion of M.S. Denison from many biochemical studies on dioxin toxicity [l] and especially, with a well known elementary hypothesis about the molecular-electronic mechanism of the carcinogenic activity [13-141. After this hypothesis, in a carcinogenic aromatic hydrocarbon there must be a "K activity zone" of n electrons induced from the oxidation by epoxy bridges, or N atoms, etc. When such a hydrocarbon meets an adequate partner, the odd electrons from the K activity zones will be promoted to sufficiently high energy, for them to interact strongly with an important functional structure in the cells and form there a particular binding complex.
414
Figure 2. Two triplet coupled pairs of the (1-4) and (6-9) n-electron spins in the Y B (Dewar)-configuration.
, I
,--103-4NI-HFS (Hyperfine ~ t r u c t u r e )
Figure 1. ESR spectra (first derivative) of Dioxin (2,3,7,8-TCDD): a) powder, b) in biological fat, oil and human blood plasma, c) in-a particular binding complex with human Hemoglobin.
Figure 4. Model of the particular binding complex of the Dioxin molecule to the heme nucleus structure in human Hemoglobin. This model is elaborated on the basis of the recorded ESR spectra from the interaction effect of Dioxin in human Hemoglobin (Fig. lc) : The HFS including nine lines with the relative intensities of 1,4,10,16,19,16,10,4,1 shows exchange coupling interaction between the n-electron spin combination of the whole Dioxin molecule and the nuclear spins (I=l) of the four Nitrogen atoms of the heme nucleus in Hemoglobin.
Figure 3. Super-exchange combination ofthe (1-4-6-9) n-electron spins on the whole Dioxin molecule in the (Dewar)-configuration.
415
3. ABILITY OF A NATURAL STRUCTURE TO CONCURRENTLY INTERACT TO INHIBIT THE DIOXIN ACTIVITY The typical ESR spectra of Gacavit recorded at different temperatures are shown in Fig.5 [ 151. At room temperature, the B and D* signals appear very weakly, but as the temperature falls they become stronger. It is especially notable that the positions of the lines and D* coincide respectively with the positions the lines and 4 in the spectrum Dioxin (Fig.l), and correspond to a higher spin number at the lower temperature. From the measured data it can be seen that the line group Ro in Fig.5 is the hyperfine structure (HFS) signal of a stable radical H2C. at the end of a branching chain. The signal groups BlB2, D* must derive from a specific nucleus structure of the "dibenzo" type including two aromatic rings combined with each other by the epoxy bridges similar to the dibenzo structure of dioxin. These suggest that a new structure, called "X-carotene", persists in Gacavit. According to the remarkable spin number relations
1 2
n [XI = (-n[B] + n[D*]}(at 20°C) and
n[Ro]
=
= n [D*] (at -160°C)
2n[X]
it may be imagined that the ground organization of X-carotene must include a dibenzodioxygen structure - the nucleus structure - and two non-aromatic branching chains ended by the stable free radicals R'. It is possible, X-carotene appears as a result of the epoxyreconstruction into a new marcromolecule from the two halves subdivided from an initial pcarotene molecule. In this normal structure X-carotene contains no unpaired electrons, it gives no ESR signal. However, in its own tautomeric structure, every X-carotene molecule shows four unpaired electrons in the aromatic nucleus rings and two at the ends of both branching chains. Thus, the line group Bl,B2 is the signal of the triplet state of the combination of the two odd 7t electrons in the every aromatic ring, and the line D* is the strongest of the four line signal coming from the quintuplet state of the combination of the four odd x electrons delocalized strongly on the aromatic nucleus rings of the tautomeric structure of X-carotene. It is clear that, there is a very interesting coincidence between the ESR-revelation of this new structure "X-carotene" and the corresponding peak reported directly by the HLPC study on Gacavit. This result is in conformity with the well known general rule that the natural tautomeric abundance generally occupies 7.5 % or more depending on the ambient condition [16-171.
Figure ESR spectra from "Gacavit" : a) An overview spectrum measured at 20"C, b) The overview spectrum measured at - 160°C, c) A separately described spectrum of the radical Re (C0-2H) measured at 20"C, c') Second derivative corresponding to c).
416
The measured exchange integral of the D*-system values J - 250 The minus sign of J shows that the ground state of the combination of four unpaired x electrons in the nucleus structure of X-carotene is a this is Thus, concerning the energy relation to the surrounding the X-carotene structure becomes with the high wall I J I . This structure, of course, is able to attract and bind strongly a suitable interaction partner. This micro-quantitative X-carotene complex in Gacavit may be in agreement with the result of some work maintaining that natural carotene is not only a source of A-vitamin but also has its own specific activity [18]. The ESR spectrum obtained from the Gacavit sample treated with Dioxin is given in Fig.6 [15]. This spectrum provides characteristic information. The radical R* shows no HFS with its own H protons. It can be seen that the odd electron of this radical is hybridized after the exterior orientation to exchange with an another odd electron complex, i.e. with a Dioxin molecule. On the other hand, the line D3 of Dioxin becomes very strong and shows an equivalent hyperfine interaction with eight protons. This important event suggests that the wave fimction of the combination of the four unpaired electrons of the Dioxin molecule has burrowed deeply into the nucleus structure of X-carotene. In this structure combined by two aromatic rings, eight H atoms are present. The measured HFS constant with a very characteristic value a = 21.1 G shows that the average density of the unpaired electrons distributes in the ratio: 1 electron spin of Dioxin / 1 H proton of X-carotene. Thus, it can be seen that The specific binding complex [X-carotene - Dioxin] is illustrated as in Fig.7. The measured values of the exchange integrals : J [Hemoglobin - Dioxin] = + 118.4 kK,
Figure 6. ESR spectra from the interaction effect of Dioxin in Gacavit : a',b') First derivative, and a",b") Second derivative.
Figure 7. A model of the particular binding complex [X-carotene - 2DioxinI corresponding to the ESR spectra in Fig.5 and Fig.6 : a) Conformational molecular structure, b) Block symbolic scheme of the exchange interaction of molecules. : Epoxy bridge 'CV: Unpaired electron
417 and J [X-carotene - 2DioxinI = -130.1 show that in the ground state, the electron combination in the first complex is able to transfer easily and act strongly, and in the second complex, the electron combination is confined to a deep well, moves with difficulty, and therefore, it is almost inactive. The measured values of the spin numbers of the specific binding complexes show that the probability for creating the second complex is time higher than the one of the first complex. Thus, if there is a continuous and a long concurrence and competition between the two interactions, the second interaction can predominate over the first one. Eventually, almost all the molecules may be trapped by X-carotene, and the concentration of free Dioxin will no longer be enough to provide the toxic effect. In other words, the X-structure can interact concurently to eliminate the activity of Dioxin. Obviously, this ability depends on other factors, such as the concentration of X-structure present in the sample, its tautomeric probability, the life times of the Re and the D* states, etc. This research was supported by the National Basis Research Program in Natural Sciences, the "10-80" National Committee and the Vietnamese General Union of Medicine. The authors would like to thank Doctor Elizabeth Duel1 in the Laboratory of Professor John Voorhees, University of Michigan, U.S.A., for the very helpful HPLC data on Gacavit, Professor Keith S. Henley of the Medical Center, Doctor William R. Dunham, Distinguished Research Scientist of the Biophysics Research Division, University of Michigan, U.S.A., and Professor Jim Simpson of the Chemistry Department, University of Otago, New Zealand, for their very useful suggestions as well as their read and careful correction of the manuscript.
REFERENCES 1. M.S. Denison et al, DIOXIN '90, Bayreuth, Vol. 4 (1990), 95. 2. 0. Hutzinger, Dioxins and Furans in the Environment: An overview, Oesterr. Gesellschaft fuer Natur- und Umweltschutz, Wien, Heft 18 (1984) 9 1. 3. 0. Wassermann, Toxikologie von Dioxinen und verwandten Verbindungen und des Herbizids, Oesterr. Gesellschaft fuer Natur- und Umweltschutz, Wien, Heft 18 (1984) 121. 4. T. Weidenbach et al, Dioxin - die chemische Zeitbombe, Kiepenheuer-Witsh, Koeln, 1984. 5 . D. Neubert, DIOXIN '90, Bayreuth, Vol. 4 (1990), 117. 6. H.V. Mao et al, 10th World Congress of Gastroenterology, Los Angeles, USA (1 994) 1737. 7. T.V. Bao, H.V. Mao, J. Gastroenterology and Hepatology, Vol. 8, No. 5 (1993) A42. 8. T.V. Bao, H.V. Mao, 2nd International Symposium on Herbicides in war, Hanoi 1993, 434. 9. D.T. Loi, Vietnamese Medicinal Plants and Remedies, 5th Edition, Science & Technology Publishing House, Hanoi, 1986. 10. N.V. Dan, Natunvissenschaften, Heft I (1959), 18. 11. N.V. Tri et al, Proceedings of the Fourth National Conference on Physics (1 994), 542. 12. N.V. Tri, Habilitation Dissertation, TU Ilmenau, Germany 1990. 13. A. and B. Pullman, Nature, Vol. 196 (1962), 228. 14. P.L. Grover et al, Biochem. Pharmacol. Vol. 21 (1972), 2713. J. Caldwell, Xenobiotica, Vol. 9 (1979), 63. 15. N.V. Tri et al, J. Medicine, Vol. 171, No. 5 (1993), 89. 16. J.E. Spice, Chemische Bindung und Struktur, Geest & Portig K.-G., Leipzig, 1971. 17. A.T. Pilipenko et al, Spravocnik PO Elementarnoj Chimii, Kiev Naukova Dumka, 1985. 18. O.E. Privalo et al, Vitaminy v Kormlenii Sel'skochozjajstvennych zivotnych, Kiev, 1983.
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Section 5 Biology and Life Sciences
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EPR in the 21" Century A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
42 1
Kinetic EPR study on reactions of vitamin E radicals Zhihua Chen, Bo Zhou, Huihe Zhu, Long-Min Wu, Li Yang and Zhong-Li Liu* National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China The role of vitamin E (a-tocopherol) in lipid peroxidation is discussed on the basis of kinetic studies on the reactions of vitamin E and vitamin E radical in micelles, erythocytes and low-density lipoprotein by using stopped-flow electron paramagnetic resonance (EPR) and other methods.
1. INTRODUCTION Free radical mediated lipid peroxidation and DNA damage has been suggested to be associated with a wide variety of chronic health problems, such as cancer, atherosclerosis and aging [11. Inhibition of lipid peroxidation by supplementation of antioxidants has become an attractive therapeutic strategy to prevent and possibly to treat these diseases [2]. In the antioxidant family vitamin E plays a central role. It is well-known that a-tocopherol (TOH), the principal component and the most active form of vitamin E, is the major endogenous lipid-soluble chain-breaking antioxidant in human plasma [3] and low-density lipoprotein (LDL) [4]. It could effectively trap the lipid peroxyl radical (LOO') to inhibit the free radical initiated lipid peroxidation (eqn. 1). The antioxidant efficiency of vitamin E can be enhanced by another co-existed antioxidant (AH, such as vitamin C, ubiquinol-10 and green tea polyphenols) if the
latter could reduce a-tocopheroxyl radical (TO', vitamin E radical) to regenerate vitamin E (eqn. 2). On the other hand, Bowry and co-workers [5] have reported that a-tocopherol might become a prooxidant the so-called tocopherol-mediated peroxidation (TMP) to oxidize polyunsaturated lipids (LH) in LDL particles (eqn. 3). The overall picture of the role of vitamin E in lipid peroxidation is depicted in Scheme 1. An issue of ongoing debate is whether, and under what conditions, vitamin E inhibits or ~~
~~~~~~~
* Author for correspondence, Fax: +86-931-8625657;E-mail:
[email protected] 422
promotes the lipid peroxidation. This article summarizes our recent kinetic and mechanistic studies on this issue. TOH
+
LOO'
-
TO'
TO'
+
AH
-
TOH
+
A'
TO'
+
LH
TOH
+
L'
-
--
-TO.
,.
+
(1)
LOOH
TOH
T0H7
(3)
r
AH+LJoo.l .-
A-
Scheme 1. The role of vitamin E in lipid peroxidation
2. INHIBITION OF LIPID PEROXIDATION BY VITAMIN E The inhibition of lipid peroxidation by a-tocopherol has been extensively studied in the past two decades by Ingold [3], Barclay [6], Niki [7] and others. The rate constant of reaction (l), kid, has been determined to be ranging from 3.2 x lo6 M-' s-l in chlorobenzene [3] to 5.8 x lo3 M-' s-l in dilinoleoyl phosphatidylcholine (DLPC) membrane [6], depending remarkably on the microenvironment of the reaction medium. We have pointed out that the predominant factors which control the antioxidant activity in membrane mimetic systems are the inter- and/or intra-membrane diffusion rate of the antioxidant and the electrostatic interaction between the antioxidant and the surface charge of the membrane [8,9]. Generally, the antioxidation reaction of a-tocopherol conducted in micelles and bilayer membranes follows the same classical rate law of free radical chain reactions as that conducted in homogeneous solutions (6,101. That is, a-tochopherol should decay linearly with time. However, we found recently that the linear decay of a-tocopherol no longer exist when the rate of initiation was very slow and/or the concentration of a-tocopherol was very high. Representative results are shown in Figures 1 and 2. It reveals clearly that appreciable deviation from the linear decay of a-tocopherol appeared when the concentration of the initiator,
423
dihydrochloride (AAPH), was lower than 2.5 mM and when the initial concentration of a-tocopherol was higher than 7.5 pM when the reaction was conducted in sodium dodecyl sulphate (SDS) micelles at 37 'C. The reason for this deviation will be discussed in the next section.
-.-.-.-.-.-.-.-. 40
16
f
-
9-
2 v
P
P t
8
4
0
I
. 400
t / min Figure 1. The consumption of a-tocopherol (TOW
t 1 min Figure 2. The consumption of a-tocopherol (TOH)
during the peroxidation of Iinoleic acid in SDS during the peroxidation of Iinoleic acid in SDS micelles. The peroxidation was initiated by AAPH and inhibited by TOH. The initial Concentration of
micelles. The peroxidation was initiated by 0.63 mM of AAPH and inhibited by TOH. The initial
TOH was 15.0 pM and the initial concentrations of
concentrations of TOH were: (a) 2.5 pM, (b) 5.0
AAPH were: (a) 6.3 mM, (b) 5.0 mM, (c) 3.8 mM,
p M (c) 7.5 pM, (d) 10.0 p M (e) 12.5 pM, (f) 15.0 pM.
(d) 2.5 m M (e) 1.25 pM, (0 0.63 mM.
3. a-TOCOPHENOL-MEDIATED PEROXIDATION Although a-tocopherol behaves as an antioxidant under normal experimental conditions in low-density lipoprotein (LDL) [ll-131, Bowry and Stocker reported that a-tocopherol might become a prooxidant to accelerate the peroxidation of LDL the so-called tocopherol-mediated peroxidation (TMP, eqn. 3) when the initiating radical flux was low and the concentration of a-tocopherol was high [14]. As shown in Figures 1 and 2 that the decay of a-tocopheroxyl radical in SDS micelles is decreased when the concentration of AAPH is low and the concentration of TOH is high. Therefore, it is reasonable to assume the co-existence of the antioxidative reaction (1) and prooxidative reaction (3) in SDS micelles. Computer simulation of the experimental data from Figures 1 and 2 by taking into account of the both reactions gave rate constants kih = 3.6 lo4 M-' s-l and kTMp = 0.46 M-l s-l for reactions (1) and (3) respectively. We found that although a-tocopheroxyl radical decays rapidly in homogeneous
424
solutions due to the fast self-recombination reaction ( 2 kt = 2 x lo3 M-l s-' at [15]), it decays very slowly in micelles (e.g., Figure 3). Therefore, the reaction kinetics of TMP (reaction 3 ) can also be directly determined via stopped-flow EPR by rapidly mixing the micellar solution of a-tocopheroxyl radical with the micellar solution of polyunsaturated lipids situ and stopping the flow (e.g., Figure 4). As shown in Figure 4A a-tocopheroxyl radical decayed very slow in the absence of linoleic acid, while it gave a fast and exponential decay in the presence of large excess of linoleic acid. Plot of the pseudo-first-order decay rate with the initial concentration of linoleic acid gave a straight line (Figure 4B) from which the second-order rate constant, kTMp for reaction 3 was obtained as 0.44 M-l s-', in good agreement with the result obtained indirectly by computer simulation mentioned above.
Figure 3. EPR spectra of a-tocopheroxyl radicals Figure 4. (A) Decay of a-tocopheroxyl radical in recorded in 0.2 M SDS micelles at pH 7.4 and 37 0.2 M SDS micelles at pH 7.4 and room "C. The radicals were generated by temperature under a, Intrinsic decay; b, in the oxidizing a-tocopherol with Pb02. presence of 18.75 m M linoleic acid (B) Plot of the pseudo first-order decay rate with the concentration of linoleic acid.
4. a-TOCOPHEROL REGENERATION REACTIONS The antioxidant synergism of vitamin E and vitamin C has been extensively
425
studied and proved to be due to the reduction of a-tocopheroxyl radical by vitamin c that regenerates vitamin E [16,171. We found recently that polyphenolic constituents extracted from green tea (Figure 5) are very good antioxidants against lipid peroxidation in homogeneous solutions [18], in micelles [10,19], in low density lipoproteins [13], in erythrocytes [20] and in red blood cell membranes [21]. Stopped-flow EPR showed that the decay of a-tocopheroxyl radical was greatly increased by reaction with green tea polyphenols as exemplified in Figure 6. Plot of the pseudo-first-order rate constants with the initial concentration of the green tea polyphenols gave straight lines (Figure 7) from which the rate constants for reaction 2, kRm, were obtained. It is found that the rate constants correlated linearly with the oxidation potential of these green tea polyphenols, implying that the rate-determining step is the electron transfer between a-tocopheroxyl radical (TO
and the green tea polyphenol (GOH) as shown in eqn.
Since the radical cation of GOH must be very acidic and very easy to deprotonate in the neutral medium and transfer the proton to the anion of a-tocopherol, the overall reaction is the same as eqn. (2). Rate constants and the equilibrium constants for reaction (4) are listed in Table 1.
- kREG
TO’
+
GOH
k - ~ G
TO-
+
GOH”
Figure 5. Molecular structures of green tea polyphenols When vitamin C was added together with green tea polyphenols and a-tocopherol the antioxidant efficiency of the latter two antioxidants was much more enhanced. The inhibition time of the mixed antioxidants was 200 % longer than the sum of the inhibition times when the three antioxidants were used individually (e.g., Figure 8).
426
Examination of the decay kinetics of the antioxidants revealed that a-tocopherol was regenerated by green tea polypenols and the latter was regenerated by vitamin C as depicted in Scheme 1.
1
50
100
150
200
40
250
[GOH] / 10' M
t (set)
Figure 6. Decay of a-tocopheroxyl radical in 0.2 M Figure 7. Plot of the pseudo-first-order rate SDS micelles at pH 7.4 and room temperature constants versus the initial concentration of GOH. under air. a. Self decay; b. in the presence of 0.1 A. EGCG; b. ECG; c. EGC; d. EC. mM of ECG.
GOH
I
EC EGC ECG EGCG
I
kmG
k-mG
(10' M'' s-') 0.45
( M-' s-') 4.6 1.8 1.7 1.3
1.11 1.31 1.91
I
K
I
10.2 66.8 71.3 143.9
1
E,b (V vs.-SCE) 0.33 0.29 0.27 0.23
1
427
50
-50
0
50
100 I50 200 250
t
3M
Figure 8. Formation of lipid hydroperoxides (LOOH) during the AAPH-initiated peroxidation of linoleic acid in SDS micelles and its inhibition by mixed antioxidants. a. Uninhibited reaction; Inhibited by: b. VC; c.TOH; d. TOH+VC+GA, e. TOH+VC+EC; f. TOH+VC+ECG g. TOH+VC+EGC; h. TOH+ VC+EGCG.
min
Aqueous phase
GO'
vc -*
Scheme 2. Regeneration of vitamin E by green tea polyphenol and vitamin C in membranes. 5. CONCLUSION
The antioxidation reaction of a-tocopherol is always accompanied by tocopherol-mediated peroxidation (TMP) in its reaction against lipid peroxidation. The rate constant for the antioxidation reaction, ki*, is over four orders magnitude than kTm. Therefore, the TMP is only observable when the initiating radical flux is very low andlor the concentration of a-tocopherol is very high. On the other hand, green tea polyphenols and vitamin C can regenerate vitamin E and greatly enhance the antioxidant efficiency of the latter.
Acknowledgements We thank the National Natural Science Foundation of China for financial support (Grant No. 29832040). We are also grateful to the Organizing Committee of 3rd Asia-Pacific EPRESR Symposium for financial support for attending the symposium.
428
REFERENCES 1. L. J. Marnett, Carcinogenesis, 21 (2000) 361. 2. C. A. Rice-Evans and A. T. Diplock, Biol. Free Radical Med., 15 (1993) 77. 3. G. W. Burton and K. U. Ingold, ACC.Chem. Res., 19 (1986) 194. 4. H. Esterbauer and P. Ramos, Rev. Physiol. Biochem. Pharmacol., 127 (1995) 30. 5. V. W. Bowry and K. U. Ingold, ACC.Chem. Res., 32 (1999) 27. 6. L. R. C. Barclay, Can. J. Chem., 71 (1993) 1. 7. E. Niki, T. Saito, A. Kawakami, Y. Kamiya, J. Biol. Chem., 259 (1984) 4177. 8. Z. L. Liu, in Bioradicals Detected by ESR Spectroscopy, H. Ohya-Nishiguchi and L. Parker (eds.), Birkhauser Verlag, Basel, 1995, pp. 259. 9. Z. L. Liu, Z. H. Han, K. Z. Yu, Y. L. Zhang and Y. C. Liu,. Org J. Phys. Chem., 5 (1992) 33. 10. Z. H. Chen, B. Zhou, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC.Perkin Trans. 2, (2001) 1835. 11. N. Noguchi, N. Gotoh and E. Niki, Biochim. Biophys. Acta, 1168 (1993) 348. 12. Z. Q. Liu, W. Yu and Z. L. Liu, Chem. Phys. Lipids, 103 (1999) 125. 13. Z. Q. Liu, L. P. Ma, B. Zhou, L. Yang and Z. L. Liu, Chem. Phys. Lipids, 106 (2000) 53. 14. V. W. Bowry and R. Stocker, J. Am. Chem. SOC.,115 (1993) 6029. 15. T. Doba, G. Burton, K. U. Ingold and M. Matsuo, J. Chem. SOC. Chem. Commun, (1984) 461. 16. J. E. Packer, T. F. Slater and R. L. Willson, Nature, 278 (1979) 737. 17. Y . C. Liu, Z . L. Liu and Z. X. Han, Rev. Chem. Intermed., 10 (1988) 269 and references cited therein. 18. Z. S. Jia, B. Zhou, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC.Perkin Trans. 2, (1998) 911. 19. B. Zhou, Z. S. Jia, Z. H. Chen, L. Yang, L. M. Wu and Z. L. Liu, J. Chem. SOC. Perkin Trans. 2, (2000) 785. 20. L. Ma, Z. Liu, B. Zhou, L. Yang and Z. L. Liu, Chin. Sci. Bull., 45 (2000) 2052. 21. L. Ma, Y. Cai, L. Yang and Z. L. Liu, Chin. J. Org. Chem., 21 (2001) 518.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
429
ESR investigation on ROS initiated by visible light in PSII particles of high plants Yang Liu”” Jian SunaxbKe Liu” Qiyuan Zhang” and Tingyun Kuangb ahstitUte of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China bInstitute of Botany, The Chinese Academy of Sciences, Beijing 100093, China Experimental evidence of the reactive oxygen species (ROS) generation in photosystem 2 (PS particles separated from spinach upon an irradiation of visible light have been directly obtained by Electron Spin Resonance technique in combination with several new phosphorylated spin traps, such as (DEPDMPO), (DEPMPO) and 5-diethoxy Further ESR observations indicate that not only the plastoquinones located at QA and QB of PS I1 is the primary site responsible for superoxide formation, but also the generation of superoxide anion radicals is positive correlated to the components of oxygen in the air, its spin state and proton concentration in the solution. Meanwhile, in order to theoretically confirm the new mechanism of O i generation therein we also examined the reaction by means of calculations and ESR detection in a self-constructed artificial system. Another research interest was to experimentally determine which reactive species was mainly responsible for the photo-induced inhibition and destruction among various ROS. Accordingly, we have mainly probed the photo-damage effects of superoxide and hydroxyl radicals that can be responsible for the inactivation of water splitting complex in PS particles, the degeneration D1 and D2 proteins and the pigments photobleaching during irradiation of visible light. 1. INTRODUCTION
Photosystem (PS 11) is a large membrane complex that performs the unique chemistry of water splitting. It catalyzes the light-driven electron transfer from water to plastoquinone to give rise to the generation of molecular 0 2 . The chemical reactions involved are intrinsically dangerous by the reason that oxygen can readily form reactive oxygen species (ROS) that can attack proteins and other components of PS 11. Singlet oxygen is formed by the reaction between P680 triplet state and oxygen when PSII is subjected to acceptor side induced photodamage[ 1-41. Singlet oxygen has been suggested to be responsible for the chlorophyll bleaching and protein degradation and crosslinking[5-81. Despite the arguments for singlet oxygen as a main damaging source of PSII, other ROS may be involved. By means of cytochrome c reduction[9], voltammetric detection[lO] and spin - trapping EPR [ 11-12] superoxide was found to be actively produced by PS 11fragments Corresponding author. Tel. ++86-10-62571074; Fax. ++86-10-62559373; E-mail.
[email protected] 430
under strong illumination. Although superoxide dismutase has been detected in the PSII fragments preparation indicating a physiological demand for disproportion in PSII, the exact role of superoxide in PSII reaction center remains to be established since inconsistent phenomena were found that SOD alone could slow down the photodamage in chloroplast[ 131 and PSII fragments preparation[ 141, but neither in PSII core nor in PSII RC preparation[ 141. Hydrogen peroxide also is one of ROS generated by PS I1 under illumination. Several researches have shown the light-induced H202 production at donor side of PSII when the function of the water-splitting was disturbed[ 15-17]. Besides, large amounts of hydrogen peroxide can be able to arise from the disproportioning of superoxide catalyzed by SOD in chloroplast. Under physiological conditions, hydrogen peroxide can further produce hydroxyl radical by the transition metal-catalyzed reduction[ 181. hydroxyl radical had been detected in the illuminated PS 11[191, however, like the situation of superoxide, the exact destructive role of hydroxyl radical in PS 11is not clear enough. PS RC complex prepared by the method of Nanba and Satoh (1987) is composed of D1, D2 and cytochrome and only contains three electron transfer intermediates: primary donor Tyr, P680 and primary acceptor Pheophytin. It is proved to be an ideal material for studying many basic phenomenon of photosynthesis. In this study we just experimentally and theoretically investigated formation of ROS, especially for superoxide anion radicals, and the corresponding photooxidative damage in PS I1 particles.
2. MATERIALS AND METHODS 2.1. Isolation of PSII and Its Reaction Center Complex PSII fragments were prepared from market spinach by the procedure of Kuwabara and Murata (1982)[20]. PSII RC complex were obtained from PSII fragments by a method based on that of Nanba and Satoh (1987)[21] and Chapman et al. (1988)[22] as modified by Miyao (1994)[23]. PS I1 RC preparation was further concentrated by the centrifugal filter device (YM-30) from Millipore.
2.2. Spin Trapping-ESR Measurements For ESR measurement, the PSII RC complex (10 pg Chl /mL) was suspended in 1mM DM, lOmh4 NaC1, 2mM DETAPAC, 0.lmM ubiquinone, 50mM Mes-NaOH (pH 5.0) and spin trap DEPMPO(25 mM) or DMPO(75 mM). ESR spectra were recorded on a Bruker ESP 300 instrument operating at X-band. Photoinhibitory illumination was performed by continuous He-Ne laser (25mW, 663nm) that could give strong irradiance without heating the sample. The ESR spectra were recorded simultaneously with the illumination at ambient temperature (20°C). The instrument settings used were as follows: modulation amplitude, 2.5G; time constant, 0.3 S; modulation frequency, 100 kHz; microwave power, 1OmW; microwave frequency, 9.76 GHz.
2.3. Photo-Damaging Measurements To monitor the pigments bleaching, PS I1 RC complex (10 pg Chl /mL) was suspended in ImM DM, l O m M NaCI, 2mM DETAPAC and 50mM Mes-NaOH (pH 5.0). The sample was placed into a 10x10 quartz cell. Photoinhibitory illumination was performed as same as that for ESR measurement. Absorbance spectra were recorded by a PC2000-UV-VIS fiber optic spectrometer (Ocean Optics, Inc., Dunedin, FL) simultaneously with the illumination. Parts of sample were reserved at certain illumination time for SDS-PAGE. SDS-PAGE was carried out in a 13.75% gel containing 6 M urea.
43 1
2.4. Theoretical Calculations: Gibbs free energy is theoretically expressed by =
+
+
-
-
Here,
was obtained by means of geometric optimization; Zero point vibration energy heat content and entropy S were obtained with frequency calculations. Gaussian 98 was utilized throughout the present work. Marcus cross electron transfer theory[24] provided a good way to calculate the electron transfer reaction between semiquinone anion radicals and oxygen molecular by the formula of
k,, = (kl,kzXl&1'2W12 where k,,and kIzare the rate constant of two self-exchange reactions; K12is the equilibrium constant;fand W,,are often close to unity[25-281. Rate constants of two electron self-exchange reactions, k,,and kz2,can be obtained by [24]
k,, = K
exp(-AG*/RT)
where K is the transmission coefficient or averaged transition probability for electron transfer per passage of the system through the intersection region; 2 is effective collision frequency for the reaction in solutions; AG* is the free energy of activation.
3. RESULTS AND DISCUSSION 3.1 Evidence in Oz-' Production Spin trapping-ESR evidence of superoxide radicals ( 0 2 was obtained by ESR measurements in PSII particles, as exhibited in the upside of Figurel-B. The ESR signal can be obviously enhanced by tetracyanoethylene (TCNE) that acts as an inhibitor of superoxide and inhibited by dismutase (SOD). on 0 2 . formation Another observation on the reduction of cytocrome c upon the irradiation of PS particles further supported the ESR evidence. Similarly, decreasing in the rate of Cyt c reduction induced by SOD and its increasing by TCNE could be also observable. When using other new phosphorylated spin traps, such as pyrroline-N-oxide (DEPMPO) and 5-diethoxy
4
o o - l . , 5
.
, 10
, 15
.
,
.
25
Illumination Time(min)
Figure 1 Production of of Cyt c(B).
02-'
in PSII indicated by spin trapping-ESR(A) and reduction
432
Tablel. ESR hyperfine splitting constants for spin adducts of DMPO, DEPMPO, DEPDMPO and DEPPEPO from the photo-irradiated PS 11particles spin T~~~~
DMPO
Oxidative Radicals
aN (mT)
ESR hyperfine splitting constants aH'? (mT) aHY (mT)
a~(mT)
1.53
1.53
1.43
1.17
0.125
____ ____
'OH
1.40
I .30
0.027(3H)
4.74
02.'
1.32
1.03
0.09(1 H), 0.043(6H)
4.85
'OH
1.46
___.
0;'
1.34
.__.
'OH
1.40
1.37
0;
1.36
1.14
'OH
_.__
-i
DEPMPO
DEPDMPO
DEPPEPO
____ ____ ____ ____
4.68 5.05 5.11 5.28
(DEPPEP0)[29] to trap ROS in PSII, the signals were more persists comparing with DMPO, and when using (DEPDMP0)[30] the ESR picture became more simplified. ESR parameters of the corresponding phosphorylated spin adducts were listed in Table 1.
3.2. Oxygen and Oz-' Formation There are two possibilities for the 02-' generation inside PSII. One pathway is the oxidation of water, which only occurred in donor side of the photoinduced primary ET process; another is the reduction of oxygen that can happen in the acceptor side of the ET chain. Correlation between concentration of superoxide spin adduct and oxygen components in PSII particle of spinach under strong illumination was illustrated in Figure 2-A, which clearly indicated that the 0 2 - ' was generated from the consumption of oxygen The further observation in Figure2-B indicated that the generation of superoxide was
Figure 2. Oxygen and Formation. A, Reaction system: PSWDEPMPO. the samples were saturated by pure oxygen (I), by air(II) and by high purity Ar(1II) respectively; B, Reaction system: PSII/DEPMPO in H20 buffer(I), in D20 buffer(1I) and in H20 buffer but containing histidine.
433
Figure 4. ESR evidence of the photo-initiated 0; generation in the UQOreconstituted PSII reaction center. a. PSII RC/DEPMPO in aerobic; b. U Q n S I I RC/ DEPMPO in anaerobic; c. UQdPSII RC/DEPMPO in aerobic.
Figure 3. Inhibiting effects by TEMED and DCMU on the electron transfer (ET) activity and OF formation.
increased under D20 environment that prolonged the half-life of singlet oxygen and was decreased when histidine existed as a scavenger of singlet oxygen. It is highly possible that the superoxide generated in phototsystem particle originates from singlet oxygen.
3.3 Quinone and Oz-' Formation The inhibiting effects of two ET inhibitors, TEMED and DCMU, on the formation of have been examined in Figure 3. There was no observable inhibiting effects on superoxide radical formation by TEMED, which might indicate that the formation of 02-' not directly linked with donor side of the ET chain. As contrast, only DCMU, an electron transfer inhibitor between two plastoquinones, QA and QB,could exclude about 50% superoxide production. Therefore, the generation of superoxide could be located at plastoquinone site. Further evidences have been presented with the detection of superoxide formation by the artificial quinone reconstituted PS I1 reaction center preparation. Several lines of ESR spectrum (in Figure 4-c) have indicated that the semiplastoquinone which located at the QA site of PS directly participated in superoxide formation. 3.4. pH Effects on Oz-' Generation As the results shown in Figure 5, superoxide production by PS has been found to be proton dependent, which indicated that the protonation of semiqinone was the key step for the generation of 0; in PS 11fragments.
-
*
100-
Y
.
-*
80-
-2
60-
2
(0-
5
20-
0-
Figure 5 Effects of different pH value on the generation of superoxide in PSII particles. The PSII particles were suspended either in Mes-NaOH (pH6.0, pH6.5) or in Hepes-NaOH (pH7.0, pH7.5, pH8.0) during the mearurement. , 6.0
6.5
7.0
7.5
8.0
434
3.5. Theoretical Calculations on Semiquinone Induced Oz-' Formation and Their Chemical Mimicry Table 2. Ab calculations on the rate constants in reaction 1 and 2 km
Reaction Model
Q
a 1
BQ
2.23~10-l~
2
2.88
1
UQo
C
2 . 2 5 1~0-14
1 . 7 910-14 ~
2.38
1.87
1.58xIO-~~
1.40~10-~3
2
b
18.0
1.54~10-13
16.0
15.9
Reaction models for the theoretical calculation are listed as following
+ 302-~ Q + '02-Q Q H + 302-QH+ QH + IO~-QH+ QH' + 3 0 2 - ~ Q H + 302-Q Q
+
0;
Reaction- 1
+
O$
Reaction-2
+ +
+
0;
Reaction-3
+
02"
Reaction-4
02H
Reaction-5
OzH
Reaction-6
As shown in Table 2 , rate constants of cross ET processes in reactions 1 and 2 were calculated via the rate of the corresponding self-exchange reactions and the equilibrium Table 3. Ab Q
BQ
UQo
calculations on the thermodynamic constants AGO and Keq in reaction 1-6. AGO(kJ/mol) b
Keq b
Reaction Model
a
1 2
209.4
209.2
210.5
2x10-37
42.34
43.30
45.56
3.8~10-8 2.6~10-8 1.6~10-8
3
783.6
766.2
766.1
4 5
616.7
609.6
609.4
42.59
41.25
39.50
3.4~10~8 6.0~10-8 1.2~10-8
6
-124.4
-124.6
-126.4
6.1~102'
1.4~1022 6.8~1022
1
203.3
202.7
202.8
2x10-36
3x10-36
2
36.28
36.86
36.90
4.4~10-7
3.5~10-7 3.5~10-7
3 4
696.6
697.1
687.6
529.65
521.1
521.7
56.57
55.19
50.79
-110.4
-110.7
-115.2
5 6 h:
C
a
1x10-'0
2x10-37
2xIOlO
C
1x1037
3x10-36
1.0~10-9
2.2~1019 2.4~1019 1.5~1020
1 .
A
Photoinhibimn lime(min)
0, incibation time(min)
Figure 6. Damage of water splitting complex induced by photoinhibition(A) and exogenous superoxide radicals. constants Kq according to Marcus theory. We use both benzoqinone and ubiquinone-0 as the model compounds of plastoquinone in PSII of high plants. Obviously, the singlet oxygen can kinetically benefits the reactions of superoxide generation. 0 Data of the thermodynamic calculations on AG and Keq of 6 models are listed Table 3. As the results, we find that only the reaction 6 can be thermodynamically performed for both quinines. Fortunately, The theoretical result on reaction model is totally in accordance with the experimental estimation as described before. The existences of protonated quinone and singlet oxygen are key factors for the generation of superoxide anion radicals. In addition, could also be generated in an irradiated solution composed by TPP, UQO and DEPMPO. The reaction system was an artificial model of theoretical estimation on the formation. Singlet oxygen came from irradiation of TPP and protonated semiquinone from the irradiation of UQ0 in the protonated solvent.
3.6. Photo-damageInduced by ROS The inactivation of water splitting complex in PSII particles can be comparatively obtained by photoinhibition and exogenous superoxide as indicated in A and B of Figure 6. The exogenous superoxide anion radicals came from redox couple of XOROD. To make sense of the other destructive roles of superoxide and its derivatives generated by the illuminated PS RC complex, the pigments photobleaching inside PSII RC complex under photoinhibitory illumination were also monitored (Figure not shown). The results indicated that the existence of ubiquinone could inhibit the formation of singlet oxygen and the hydrogen peroxide or hydroxyl radical might play an important role in the further destruction of chlorophyll in presence of SOD. In contrast, a similar situation in the protein damage of D1 and D2 of PSII RC can be observed by means of SDS-PAGE method. 4. ACKNOWLEDGEMENTS The study has been financially supported in part by the State Key Plan for National Natural Science (G1998010100) and in part by the National Natural Science Foundation of China (No. 39890390 and 39870208)
436
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EPR in the 21’ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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EPR and theoretical investigations of [NiFe] hydrogenase: Insight into the mechanism of biological hydrogen conversion W. Lubitz3b,M. Brechtb, S. Foersterb,M. Steinb,Y. Higuchi‘, T. Buhrked and B. Friedrichd aMax-Planck-Institut
Strahlenchemie, Stiftstr. 34-36,45470 MiilheindRuhr, Germany
bMax-Volmer-Laboratoriumf%rBiophytsikalischeChemie, Technische Universitsit Berlin, Str. d. 17. Juni 135, 10623 Berlin, Germany ‘Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan dInstitut Biologiehlikrobiologie, Humboldt-Universitatzu Berlin, Chausseestr. 1 17, 10 1 15 Berlin, Germany* Advanced EPR techniques are applied to study the paramagnetic intermediates in the enzymatic cycle of [NiFe] hydrogenase. The g-tensor magnitudes and orientations were obtained for Ni-A/B, Ni-C and Ni-L by experiments performed on hydrogenase single crystals. Pulse ENDOR and ESEEM experiments aided in the determination of electronnuclear hyperfiie coupling constants of the magnetic nuclei in these species. Density functional theory calculations were performed on a large geometry-optimized model cluster for the active center and gave magnetic resonance parameters in good agreement with experimental data. Based on the experimental and theoretical data, the structure of the intermediates has been deduced and a reaction mechanism proposed for this enzyme.
1. INTRODUCTION Hydrogenases catalyze the reversible heterolytic cleavage of molecular hydrogen: H 2 e Hc+ H-. The most commonly found enzymes [l] contain two major subunits with a hetero-dinuclear NiFe center the active site in the large subunit and three iron-sulfur centers electron transfer components in the small subunit. The structure of the active center and its unusual ligation sphere is shown schematically in Figure 1 . The Ni is coordinated by four cysteines, two of them act bridging ligands between Ni and Fe. The six-coordinated Fe, in addition, carries three small inorganic diatomic ligands. Another bridging ligand X (0 or S) is present in the oxidized NiFe cluster and is removed in the reduced state. Although the geometrical structure of the [NiFe] hydrogenase is known from X-ray crystallography for the oxidized [2,3] and reduced forms [4], details of the related electronic structures are still not fully understood *
Supported by Deutsche Forschungsgemeinschq7(Sfb 498, TP
and C2).
43
CYs
-
Figure 1. Schematic structure of the active center of [NiFe] hydrogenase from [2]. 2 CN and 1 CO ligand at the Fe were determined by FTIR [6]. In the oxidized enzyme X is O(S), in the reduced enzyme no bridging ligand is detected in the X-ray structure M.
I)\ Fe ICN CO cys-s / \;/ C N ‘ Cys-S
-Ni -X-
CYs
The [NiFe] hydrogenase cycles through various states in the catalytic process Ni-AA3 -+Ni-Si -+ Ni-C -+ Ni-R
(1)
The isolated’ oxidized forms Ni-A and Ni-B and the reduced form Ni-C are paramagnetic and can thus be studied by EPR techniques. The Ni-C state is believed to be directly involved in the catalytic turnover and to carry the substrate hydrogen [5]. This state is light-sensitive and can be converted reversibly at low temperatures into the paramagnetic Ni-L state which is spectroscopically different from Ni-C. Although much work has been devoted to the investigation of the [NiFe] hydrogenases [5] many open questions remain. These concern the oxidation states of the Ni and Fe in all intermediate steps of the reaction cycle, the electron spin and charge density distribution of the NiFe center, the binding site of the substrate hydrogen and, finally, the detailed mechanism of the hydrogen conversion. In this paper we show a way how such questions can be answered by a combined approach of advanced EPR techniques [7] and density kctional theory (DFT) calculations. 2. MATERIALS AND METHODS Experiments have been performed on the “standard”[NiFe] hydrogenases of Desulfovibrio vulgaris Miyazaki F (DvH) and the regulatory hydrogenase from Ralstonia eutropha (RRH) which exhibit very similar Ni-C and Ni-L EPR characteristics in their reduced states. The isolation and purification of the hydrogenases have been described earlier [8,9]. In single crystals of DvH [3] the Ni-AA3 well the Ni-C and Ni-L states were generated and angular dependent EPR experiments were performed described [lo-121. X-band pulse EPR and ESEEM measurements were done on a Bruker ESP 380 E spectrometer equipped with a Bruker dielectric ring cavity (ESP 380-1052 DLQ-H) an Oxford CF 935 liquid helium cryostat. Pulse ENDOR experiments were carried out on the same instrument by use of a Bruker ESP 360 D-P ENDOR system. For data analysis of samples in frozen solutions and single crystals simulation and fit programs were used that were previously described [13]. DFT calculations were performed by using the ADF program (SCM, Vrije Universiteit, Amsterdam).
439
3. RESULTS AND DISCUSSION 3.1. Determination of g-tensors of Ni-C and Ni-L The determination of the complete g-tensors for the oxidized states Ni-A and Ni-B by Xband EPR spectroscopy in oxidized single crystals of DvH and the assignment to the molecular structure of the complex has been described by us earlier [10,11]. Here we focus on the reduced active state Ni-C and the light-induced paramagnetic state Ni-L of DvH. The X-band EPR spectra obtained in frozen solutions and the related principal values gl, g2 and g3 of the rhombic-g-tensor are shown in Figure 2. g-value 24
2.3
22
2.1
20
19
I
I
I
I
I
I
exp
I
the0
9 -
1
2
2.144
2.097
3 2.010 2.001 9 -
1 2
3
Figure 2. Left: EPR-spectra in frozen solutions of reduced (Ni-C) and illuminated (Ni-L) [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F (DvH). Experimental conditions: X-band 9.6 GHz, T = 50K, microwave power 1 mW, field modulation 100 kHz, modulation amplitude 1 mT. Right: g-tensor principal values of the [NiFe] hydrogenase determined by simulations of the EPR spectra and DFT calculations (see text). These values have been used starting parameters.in the analysis of the angular dependent EPR spectra of these species in DvH single crystals (Figure 3). Both states Ni-C and Ni-L are present in the crystals (space groups P212121, 4 protein molecules/unit cell). A simultaneous fit of the effective gz values of all four sites yielded the g-tensor magnitude and orientation. In a next step, an assignment of the g-tensor axes to a particular site (one of the four possible sites in the unit cell) had to be made. This became possible due to additional structural information derived from ENDOR and ESEEM spectroscopy (see below). In Figure 4 our final assignment of the g-tensor axe$ in the molecule is depicted. Note that in case of Ni-C the g3 axis (smallest g-value) - which is close to points to the empty sixth coordination site of the Ni, whereas the largest gl value is directed to the (open) Ni-Fe bridge. From the g-tensor magnitude and orientation a formal Ni(1II) with a d, ground state seems to be likely. For the Ni-L state significantly different g-values are obtained indicating a different ground state - although the direction of the tensor axes is similar to those of the Ni-C state. The question of the correct formal oxidation states of the paramagnetic Ni centers and the electronic ground states can be solved by a comparison of the data with those obtained from density functional calculations.
-
440 g-value 2.3
2.2
2.1
2.0
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1800
1600
1400
1300 1zoo
1100
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800 700
600
500
.loo 200
100
00
300
320
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340
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Figure 3. Left: Angular dependence of EPR transitions of single crystals of the reduced DvH. Rotation of the sample in an arbitrary plane about an angle perpendicular to the applied magnetic field, Bo. The signal at g = 2.00 results from reduced methyl viologen (for details see reference 12). Experimental conditions: X-band 9.6 GHz, T = 50 K, microwave power 1 mW, field modulation 100 MIz, modulation amplitude 1 mT. Right: Angular dependence of g’ values. The dots represent experimental values derived !&omthe spectra. The curves show the theoretical resonance positions calculated by a simultaneous fit of all four sites. 3.2. DFT calculations of the g-tensors of Ni-C and Ni-L DFT calculations were performed on a large (41 atoms) model cluster of the active site of DvH. Based on the X ray coordinates [2-41 a 111 geometry optimization has been performed with the BP86 functional. For the Ni-C and Ni-L states various oxidation states of the Ni and Fe were investigated, calculations were also performed with different bridging ligands X (Figure 1). For these models the charge and spin density distributions were calculated [14] well the g-tensor magnitudes and orientations using a relativistic Hamiltonian [14-161. The g-tensor principal values of Ni-C could be reproduced well with a Ni(II1) oxidation state and a bridge X = H-. This shows that the bridge in the reduced state is occupied by a hydride which results from the heterolytic splitting of hydrogen by the enzyme. This is not detectable by Xray structure analysis. Best agreement for Ni-L was obtained a formal NiO) state and an empty bridging position. This shows that upon illumination the Ni-C state looses a proton from the bridge which leads to a conversion Ni(II1)H+ Ni(I) In all states the iron remains in a low spin Fe(I1) state which is caused by the three strong inorganic diatomics. The Fe is thus
441
Figure 4. Stereo views of g-tensor orientations of Ni-C and Ni-L derived from analysis of the single crystal EPR-spectra of the reduced (Ni-C) and illuminated (Ni-L) enzyme (top). Bottom: g-tensor orientation of the respective states obtained from DFT calculations on geometry-optimized structures. not redox active and seems to be only indirectly involved in the catalytic process by providing a coordination site for the bridging ligand. The calculated g-tensor principal values of Ni-C and NI-L are given in Figure 2 (table), the g-tensor orientations are shown in Figure 4; they compare well with the experimental ones. A population analysis of the DFT calculations supports a predominant d, ground state for Ni-C, whereas for Ni-L a larger contribution of a dx2-p state is obtained. In Ni-L 75% of the spin is localized at the nickel, whereas in Ni-C it is 50%. The remaining spin density is distributed over the sulfurs of the cysteine ligands with the largest part on one of the sulfur bridges (see Figure 5, bottom). There is virtually no spin at the iron.
-
-
Further corroborationof the proposed structural model of the Ni-C and Ni-L states, which is deduced here from the g-tensor magnitudes and orientations, can be obtained from experiments that are able to directly address the interaction of the electron spin with the magnetic nuclei. Consequently, a series of ENDOR and ESEEM measurements were carried out on the Ni-C and Ni-L states. Some of the experiments are briefly described in the following section.
3.3. ENDOR spectroscopy of Ni-C and Ni-L The following ENDOR studies were done on the Ni-C and Ni-L signals of the RH of eutropha (RRH) since this enzyme shows no spin-spin coupling between the Ni and the proximal 4Fe4S center observed in DvH [17]. Figure5 shows a comparison of the orientation-selected 'H ENDOR spectra obtained at the smallest g value (g3) of RRH for both states. The spectra indicate a significant change of the hyperfine couplings (hfcs) of all protons. The assignment indicated by dotted lines is the result of an extensive orientationselection ENDOR study (not shown) that aimed to measure all the major 'H hfcs of both states [17]. The measured and assigned hfcs yield information about the spin density
442
Figure 5. Top: 'H Pulse-ENDOR spectra of Ni-C and Ni-L recorded at g3, the special ENDOR-pulse sequence [7] used is also shown (top). Exp. cond.: 7tm = 96 ns, ntr = 16 ns, = 8 ps, = 400 ns, rep. rate 100 Hz. Bottom: Distribution of unpaired spin density obtained from DFT calculations. Contour plots at a value of 0.003 e/(a.~)~.Table: total atomic spin populations obtained from a Mulliken analysis.
lMHzl Figure 6. High frequency part of 'H Pulse-ENDOR spectra of Ni-C in H2O (H2 activation), upper trace, and DzO (Dz activation) lower trace. Both spectra were recorded at g3. The shaded region in the upper spectrum indicates the contribution of the "exchangeable" proton to the spectrum.
443
distribution in the cluster. Clearly, the extent of spin delocalization onto the cysteines is smaller in Ni-L than in Ni-C. Figure 5, bottom, also shows the spin density distribution for both states calculated by DFT methods. A direct detection of the substrate hydrogen (hydride) in the bridge of Ni-C should be possible by replacing this nucleus with deuterium. Consequently, the oxidized enzyme in D20 buffer has been activated with D2. In Fig. 6 the 'H ENDOR spectrum of this sample is compared with the one activated with H2 in H20. The broad resonance between 20 and 21 MHz is reduced for DdD20 showing that at least one strongly coupled hydrogen has been exchanged (hfc = 11 MHz at g3). An analysis of further orientation-selected ENDOR experiments at different field positions suggested an anisotropic hfc tensor of (+20, -6, -14) MHz. Note that the DFT calculation yields (+19, -7.5, -1 1.5) MHz for the bridging hydrogen [18]. The exchanged hydrogen in Ni-C should also be detectable in the 'H ENDOR spectrum. However, such experiments are difficult to perform at X-band frequencies owing to the small 'H Larmor frequency and the reduced hfcs. An alternative approach to the problem is to perform ESEEM experiments, as shown below. 3.4. ESEEM spectroscopy of Ni-C and Ni-L In Figure7 a 4-Pulse ESEEM experiment (HYSCORE) [7] on RRH activated by D2 in D20 is shown, taken at the g3 edge of the EPR spectrum. In the Ni-C state a 'H hfc of = 1.7
Figure 7. Left: HYSCORE (4-Pulse ESEEM) spectra of Ni-C and Ni-L recorded at g3 in the 2H regime show the removal of the hfc of the "exchangeable"deuteron from the bridging position after illumination. The 'H Larmor frequency (- 2.2 MHz) is indicated by arrows. Experimental conditions: x = 16, z = 120 rep. rate 100 Hz, AT1 = AT2 = 24 ns. Right: Structural model for the conversion f'rom Ni-C to Ni-L.
444
\ proton transfer channel Figure 8. Model for the heterolytic splitting of Hz by [NiFe] hydrogenase. The activation of the oxidized enzyme is believed to remove the (oxygenic) bridging ligand via a protonation step (shown here as water) [181. MHz is clearly visible which corresponds to the exchangeable 'H hfcs of 11 MHz observed by 'H ENDOR along this g direction (Figure 6). Upon illumination of the sample at 77 K the 2Hhf splitting is removed indicating that the respective deuteron is photodissociated from the complex as depicted in Figure 7 (right). This shows that it is the single exchangeable hydrogen in Ni-C that is photolabile and lost upon conversion to Ni-L. It is interesting to note that in Ni-L a 2H hf coupling can still be detected but it is about one order of magnitude smaller than in Ni-C. Annealing of the Ni-L sample at 200 K in the dark fully restores the Ni-C signal showing that the process is reversible. Orientation-selected 2HESEEM and 'HENDOR experiments finally allowed the determination of the full hf tensor of the bridging photolabile hydrogenic species [17]. In agreement with the EPR and DFT results the hydride is located in the bridge between Ni and Fe with a distance of 1.8 +O.l to both metals which agrees with the DFT calculations.
-
4. CONCLUSIONS In the turnover of the [NiFe] hydrogenase, the H2 entering the oxidized enzyme is obviously heterolytically cleaved. H- remains in the complex whereas leaves via a proton transfer channel (see Figure 8). The EPR, ENDOR and ESEEM experiments described here together with DFT calculations showed that the reduced Ni-C state is carrying the hydride (H-) in the bridging position between Ni and Fe. Ni-C can be formally described by a Ni(II1) species with a dZ , ground state. The bridging hydrogen species is lost upon illumination and transfered to a nearby proton acceptor. This leaves the complex in a formal Ni(1) oxidation state with an empty bridge (Ni-L). This process is reversible. Based on the structure of these intermediates, insight into the reaction cycle of the [NiFe] hydrogenase is obtained [ 15-181.
REFERENCES 1. P.M. Vignais, B. Billoud, J. Meyer, Microbiol. Rev. FEMS, 25 (2001) 455.
2. A. Volbeda, M.H. Charon, C. Piras, E.C. Hatchikian, M. Frey, J.C. Fontecilla-Camps, Nature, 373 (1995) 580. 3. Y. Higuchi, T. Yagi,N. Yasuoka, Structure, 5 (1997) 1671. 4. Y. Higuchi, H. Ogata, K. Miki, N. Yasuoka, T. Yagi, Struct. Fold. Des., 5 (1999) 549. 5. R. Cammack, R.L. Robson, M. Frey (eds.), "Hydrogen as a Fuel", Taylor and Francis, London, 2001. 6. R.P. Happe, W. Roseboom, A.J.Pierik, S.P.J. Albracht, K.A. Bagley,Nature, 385 (1997) 126. 7. A. Schweiger, G. Jeschke, "Principles of Pulse Electron Paramagnetic Resonance", Oxford University Press, 2001. 8. T. Yagi, K. Kimura, H. Daidoji, F. Sakai, S. Tamura, H. Inokuchi, J. Biochem. (Tokyo), 79 (1976) 661. 9. A.J. Pierik, M. Schmelz, 0. Lenz, B. Friedrich, S.P.J. Albracht, FEBS Lett., 438 (1998) 231. 10. C. GeBner, 0. Trofanchuk, K. Kawagoe, Y. Higuchi, N. Yasuoka, W. Lubitz, Chem. Phys. Lett., 256 (1996) 518. 11.0. Trofanchuk, M. Stein, C. GeBner, F. Lendzian, Y. Higuchi, W. Lubitz, J. Biol. Inorg. Chem., 5 (2000) 5 . 12. S. Foerster, M. Brecht, M. Stein, Y. Higuchi, W. Lubitz, in preparation. 13. C.GeBner, PhD thesis, Technische Universitlit Berlin, 1996. 14. M. Stein, W. Lubitz, Phys. Chem. Chem. Phys. 3 (2001) 2668. 15. M. Stein, E. van Lenthe, E.J. Baerends, W. Lubitz, J. Am. Chem. SOC.123 (2001) 5839. 16. M. Stein, PhD thesis, Technische Universitlit Berlin, 2001. 17. M. Brecht, PhD thesis, Technische Universitlit Berlin, 2001. 18. M. Stein, W. Lubitz, Phys. Chem. Chem. Phys., 23 (2001) 51 15.
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EPR in the 21RCentury A Kawarnori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
EPR studies on free radical generation by the reaction of methylglyoxal with amino acids and protein Hyung-Soon Yim", Cheolju Leea, P. Boon Chockb, Moon B.
and Sa-Ouk Kanga
aLaboratory of Biophysics, School of Biological Sciences, and Institute of Microbiology, Seoul National University, Seoul 151-742, Korea bLaboratory of Biochemistry, NHLBI, National Institutes of Health, Bethesda, Maryland 20892, U. S. A.
We studied the reaction between a three-carbon a-dicarbonyl compound, methylglyoxal, and amino acids. This reaction generated yellow fluorescent products as formed in some glycated proteins. In addition, three types of free radical species were also produced, and their structures were determined by EPR spectroscopy. These free radicals are the cross-linked radical cation and the methylglyoxal radical anion as the counterion. Time course studies suggest that the cross-linked radical cation is a precursor of yellow fluorescent glycation end products. Glycation of bovine serum albumin by methylglyoxal generated the protein-bound free radical, probably the cation radical of the cross-linked Schiff base in the reaction of methylglyoxal with N"-acetyl-L-lysine. The glycated bovine serum albumin showed increased electrophoretic mobility suggesting that the basic residues, such as lysine, were modified by methylglyoxal. The glycated protein catalyzed the oxidation of ascorbate in the presence of oxygen, whereas the protein free radical signal disappeared. These results indicate that glycation of protein generates active centers for catalyzing one-electron oxidation-reduction reactions. This active center, which exhibits enzyme-like characteristic, was suggested to be the cross-linked S c M b a d t h e cross-linked Schiff base radical cation of the protein.
1. INTRODUCTION Glycation reaction (nonenzymatic glycosylation; Maillard reaction), which produces brown fluorescent compounds, is a chance event that may occur when a protein is in solution with a reducing sugar, such as glucose. In this reaction, free amino groups of protein react slowly with the carbonyl groups of reducing sugars to yield Schiff-base intermediates, which undergo Amadori rearrangement to stable ketoamine derivatives. These Amadori products subsequently degrade into a-dicarbonyl compounds, deoxyglucosones. Schiff bases can also be fragmented to glyoxal). These compounds are more reactive than the parent sugars with respect to their ability to react with amino groups of proteins. Thus, the a-dicarbonyl compounds or a-ketoaldehydes are mainly responsible for forming inter- and intramolecular cross-links of proteins, known as advanced glycation end products (AGES)' The AGES, which are irreversibly formed, accumulate with aging, atherosclerosis, and diabetes mellitus,
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especially associated with long-lived proteins such as collagens, lens crystallins, and nerve proteins (4-8). The a-dicarbonyl compounds are produced in a variety of ways. Fenton reaction-mediated oxidation of sugars, lipids, and proteins produces various a-dicarbonyl compounds. Accordingly, the transition metal ion-catalyzed oxidation of glucose is suggested to be a more important factor in glycation than the formation of the Amadori product of glucose itself (911). The a-ketoaldehydes, such as methylglyoxal, are also found as a normal metabolite in mammals and microorganisms. The methylglyoxal is formed by the non-enzymatic or enzymatic elimination of phosphate from triose phosphate and by the oxidation of hydroxyacetone and aminoacetone (12-14). The increased formation of methylglyoxal was observed in hyperglycemia associated with diabetes mellitus (15, 16). In addition, it was shown that methylglyoxal-modified albumin underwent receptor-mediated endocytosis by macrophage, which may suggest the involvement of methylglyoxal in pathophysiology (17). it WAS suggested that cellular oxidant stress or free radicals are generated by AGEs themselves (18, 19) or as a consequence of the AGEs interactionwith their receptors (20,21). Several AGEs were identified from the products formed during the reaction of methylglyoxal with model compounds and proteins. These species include N"(carboxyethy1)lysine (22), imidazolone compounds (23), and imidazolium cross-link species, methylglyoxal-lysine dimer (24-26). In addition to these AGEs, several investigations have also shown by electron paramagnetic resonance (EPR) spectroscopy that unidentified protein free radicals were produced during the reaction of methylglyoxal with proteins, such as bovine serum albumin (BSA) and casein (27,28). In this report, the free radical is assigned to be the radical cation of the cross-linked Schiff base on the basis of the detailed analysis of EPR spectra observed from the reaction mixture containing methylglyoxal and alanine. We also suggest that the cation radical sites in the cross-linked proteins could serve as reactive centers for one-electron oxidation-reduction with appropriate substrates. These reactions will produce free radicals for a long duration and contribute to accelerating oxidative modification of the biological macromolecules (29). And we use methylglyoxal-modified bovine serum albumin (MG-BSA) as a model protein to study the free radical generating properties of glycated proteins and their capacity to catalyze oxidative modifications of macromolecules (30).
2. EXPERIMENTAL PROCEDURES 2.1. Materials Methylglyoxal, diethylenetriaminepentaacetic acid (DTPA), N"-acetyl-L-lysine, N"-acetylL-arginine, BSA, Cu,Zn-superoxide dismutase from bovine erythrocyte, catalase from bovine liver, and cytochrome c from bovine heart were obtained from Sigma. Cu,Zn-superoxide dismutase from bovine erythrocytes was obtained from Boehringer Mannheim, nitro blue tetrazolium (NBT) was from Calbiochem, and Chelex 100 resin (sodium form) was from BioRad. Stable isotope-enriched alanines ("N-, l-13C-, 2-13C-, and 3-13C-labeled), D4-alanines, and D2O were purchased from Cambridge Isotopes. Ascorbic acid was purchased from Merck. Commercially supplied BSA was purified by gel filtration chromatography on a column of Superdex 200 HR10/30 (10 x 300 mm; Vo:8.2ml; Amersham Pharmacia Biotech). The eluent was 20 mM sodium phosphate buffer (pH 7.4) containing 0.15 M sodium chloride, and the
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flow rate was 0.4 ml/min. The eluted sample was desalted and lyophilized to dryness. The catalase was also purified by gel filtration chromatography (GFC) to remove contaminating superoxide dismutase. Commercial catalase dissolved in Tris glycine buffer (pH 8.0) containing 1 mM EDTA and 0.3 M NaCl was fractionated using the same column. The buffers used for the reaction of BSA with methylglyoxal were treated with Chelex 100 resin (BioRad) to remove traces of transition metal ions.
2.2. Modification of BSA with methylglyoxal BSA was reacted with methylglyoxal in 0.1 M phosphate buffer (pH 7.4). Unless otherwise indicated, the concentrations of BSA and methylglyoxal were 20 mg/ml(O.3 mM) and 30 mM, respectively. After modification, samples were repeatedly filtered though PM-10 ultrafiltration membrane (Amicon) using 20 mM phosphate buffer (pH 7.4) and further desalted with Fast Desalting Column H10/10 (Amersham Pharmacia Biotech).
2.3. Characterization of modified protein The effect of methylglyoxal modification on the net charge, adduct formation, and oligomerization of BSA was investigated by polyacrylamide gel electrophoresis (PAGE) with/without urea, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing, cation exchange chromatography, and gel filtration chromatography. PAGE was performed on 9% polyacrylamide gel with or without 6 M urea, and SDS-PAGE was on 5 2 0 % linear gradient polyacrylamide gel. Isoelectric focusing was carried out at 4 "C under constant power of 7 watts, using a polyacrylamide gel (5% T, 3% C) containing 2% Bio-Lyte (Bio-Rad), 5% glycerol, and 6 M urea at a pH range between 3.8 and 9.4. GFC was performed using a Superdex 200 HR10/30 on fast protein liquid chromatography system (Amersham Pharmacia Biotech) with 20 mM phosphate buffer (pH 7.4) containing 0.15 M NaCl as eluent. In ion exchange chromatography, a Protein Pak SP 5PW column (Waters) was used on a Waters Delta Prep 4000 system. The mobile phase was 50 mM acetate buffer (pH 4.5) at a flow rate of 5 ml/min. Proteins were eluted isocratically for 15 min, and then the eluent was shifted linearly to 50 mM acetate buffer containing 1 M NaCl for 10 min.
2.4. Degradation of ascorbate by MG-BSA Ascorbate was incubated with MG-BSA in 20 mM phosphate buffer (pH 7.4) at 30 "C. The concentration of the unreacted ascorbate in the reaction mixture was determined by the high performance liquid chromatography. An aliquot of the sample solution was loaded onto an ODS Hypersil column (4.6 x 100 mm) (Hewlett-Packard) in a Waters HPLC 600s equipped with a Hewlett-Packard 1100 UV detector. Mobile phase was 0.04% trifluoroacetic acid, and the eluent was monitored at 254 nm. 2.5. EPR spectroscopy EPR spectra were recorded on a Bruker ESP EPR spectrometer. For some analysis, samples were frozen at 77 K using liquid nitrogen. The operating conditions were as follows: microwave frequency, 9.44 GHz; microwave power, 10 milliwatts; sweep width, 100 G; conversion, 40.96 ms. Modulation amplitude was set to 4.00 G at 77 K or 1.47 G at room temperature.
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3.
3.1. EPR spectra of the cross-linked free radical cation The first derivative EPR spectrum shown in Figure (upper spectrum) was obtained with the anaerobic reaction mixture of methylglyoxal and natural-abundance L-alanine in carbonate buffer at pH 9.5. A similar spectrum also appeared in phosphate buffer at pH 7.5 with a much slower rate and a weaker signal amplitude. For the assignment of hyperfine coupling constants (hfc) and structural identification of this radical, similar experiments were carried out in various isotope-enriched reaction mixtures. The upper spectrum in Figure lB, obtained from the reaction mixture prepared with ~-[N]alanine,exhibits a different hyperfine splitting pattern compared with the spectrum in Fig. This alteration is entirely caused by changes of nitrogen nuclear spins, N (I = 1/2) in place of 14N (I = l), and their nuclear moments, p(1sN)/p('4N) = 1.40. With this information, the experimental spectra in Fig. 1 (A and B, lower spectra) were simulated. MO+Ala
I
3440
,
,
,
I
,
3480
.
.
I
Figure 1. EPR spectra obtained from the reaction mixture containing methylglyoxal (0.2 M) and various isotope-enriched L-alanines (0.2 M) in carbonate buffer (0.5 M) at pH 9.5 (upper spectra) and simulated spectra (lower spectra). A, methylglyoxal and naturalabundance L-alanine; B, methylglyoxal and L[lSN]alanine;C, methylglyoxal and ~-[2-'~C]alanine.
,
3520
Gauss
From the spectrum in Fig. 1C and the spectra obtained with the reaction mixture containing L-[2-13C]alanine,~-[l-'~C]alanine,and ~-[3-'~C]alanine in place of natural-abundance alanine (29), respectively, they exhibited extra hyperfine interactions due to C (I = 1/2) nuclei. Although we have obtained a large number of hfc constants from this cross-linked radical, exact structural assignment for this radical is difficult because of asymmetry in hfc constants. Two equivocal structures are shown in Figure 2 (structures a-I and a-2). In the case of structure a-1, the asymmetric nature of the spin distribution may be caused by the effect of the methyl group of methylglyoxal on the singly occupied molecular orbital, most likely the orbital, which includes two N=C bonds. On the basis of experimental hfc constants, structure a-1' is assigned to the conformations of two alkyl groups with respect to the p-orbitals of the two nitrogen atoms. In this conformation, carboxyl, and methyl carbons will have one small and one large I3C hfc constant in each carbon group because of the different dihedral angles to the p-orbitals of the nitrogen atoms (one carbon is close to I% and the other is
450
closer to 0') and the cos20 dependence of these hfc constants. The assignment of C hfc constants to individual carbons will be as follows: for the N-1 side, 8.52 G for 1-C, 4.10 G for 2-C, and 0.3 G for 3-C, and for the N-2 side, 0.3 G for 1-C, 0.2 G for 2-C, and 3.0 G for 3-C. This assignment gives a ratio of 3.7 for the total carbon spin densities between the N-1 and N2 sides. This value is closest, among several possible assignments, to the value of 3.2, the ratio of spin densities on N-1 and N-2 atoms (NA). The other possible structure of this radical is shown in Figure 2 (structure a-2). A protonation of a nitrogen in the cross-linked Schiff base will produce a triene-type compound, which may lose an electron to form the crosslinked radical cation. In this structure, the observed large C hfc constants will originate entirely from one alanine molecule in the cross-linked radical. In addition, we expect to detect two sets of methyl hydrogen hfc constants if the radical has this structure, in contrast to the experimental observation of only one set of methyl hydrogen hfc constants. It may be possible, however, that AH(3) of one set is smaller than the line width, which may arise from the canceling effects of spin delocalization (hyperconjugation) and spin polarization in the spin transfer to the s-orbitals of the methyl hydrogens from the delocalized x-center. Although we prefer structure a-1 as the structure of the cross-linked radical, structure a-2 cannot be ruled out at this time. It is certain, however, that the radical formed due to the cross-linking reaction contains two amino acids and one methylglyoxal. To find whether the Schiff base is the precursor of this radical, the base was reduced with NaCNBH3, which is known to reduce Schiff bases selectively and to inhibit the subsequent reactions. When NaCNBH3 (1.0 M) was added to the reaction mixture, the EPR signal of the cross-linked radical and the yellow color were not detected. The effect of NaCNBH3 may indicate that methylglyoxal dialkylimine, -02C(CH3)is the intermediate for the formation of this cross-linked radical. nac
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H
-i;$a-(dNp n' (+')
a21
2 a-
'
Figure 2. Chemical structures of the cross-linked radical cation and the radical anion observed in this reaction and identified by EPR.
3.2. Observation of the enediol radical anion of methylglyoxal The EPR parameters obtained from the EPR spectra using isotope-enriched 2,3,3,3-D4alanine are identical to those of the cis-form of the methylglyoxal radical anion (see Figure 2, structure b) (29). This result suggests that the cross-linked radical observed in this reaction is a cation formed with the methylglyoxal radical anion being the radical counterion.
3.3. Modification of BSA by methylglyoxal BSA was incubated with methylglyoxal in 0.1 M Chelex-treated phosphate buffer (pH 7.4) at 37 "C for 5 days. Further incubation had little effect on the modification of BSA as indicated by PAGE analysis. The native PAGE of the reaction products (30) demonstrated that the electrophoretic mobility of MG-BSA increases with increasing concentration of
45 1
methylglyoxal used for the glycation. This result indicates the progressive loss of the positive charge in the MG-BSA during the glycation reaction. When a high concentration of methylglyoxal (30 mM or higher) was used for glycation, additional bands appeared with lower electrophoretic mobility. The species with lower mobility may represent the higher aggregates of MG-BSA. At 600 mM methylglyoxal, the aggregate formed precipitates (30). The result obtained from the denaturing PAGE (6 M urea; data not shown) also supports this conclusion. The identical results were also obtained with the reaction products prepared anaerobically. Isoelectric focusing revealed similar results, suggesting that glycation of BSA by methylglyoxal introduced net negative charges into MG-BSA adducts, although at a higher concentration of methylglyoxal (30 mM or higher) some species possessing net positive charges were also formed (data not shown). In accordance with the above observation, the SDS-PAGE data (30) indicate the presence of additional protein bands for samples obtained with higher concentration of methylglyoxal at the positions corresponding to the molecular mass of the dimer and tetramer of BSA. Chromatographic analysis also gave similar results. Cation exchange chromatography showed that, as BSA incubation with methylglyoxal proceeded, its net charge becomes more negative. Gel filtration chromatographic analysis of MG-BSA gave peaks that correspond to the dimer and tetramer. The basic lysines and arginines are known to be the residues modified and responsible for forming Schiff bases for intra- and intermolecular cross-links during the glycation reaction. Thus, the net charges of MG-BSA adducts may be shifted to more negative values. Previous investigations of the reaction of methylglyoxal with protein suggested that, in addition to the modification of protein, the treatment of methylglyoxal fragmented the protein (17). In contrast, we did not detect any protein fragments during the modification of BSA by methylglyoxal. The difference may have arisen from the fact that we treated all the solutions with Chelexresin to remove traces of transition metal ions, such as ironand copper, known to cleave protein in the presence of oxygen. Thus, the characteristics of the glycated protein observed in this study are caused mostly by the covalent binding of the methylglyoxal with few effects from protein fragmentation or reaction products of the fragments. 3.4. Cross-linking residues and free radical formation We examined the residues responsible for cross-linking by investigating various substituted amino acids for their ability to undergo the glycationreaction and form the crosslinked radical cation. The EPR spectrum was obtained from the reaction mixture containing 0.2 M N"-acetyl-Llysine and 0.2 M methylglyoxal in carbonate buffer at pH 9.5 (30). This spectrum was compared with the obtained from alanine-methylglyoxal system, which was identified as the radical cation of the cross-linked Schiff base, the methylglyoxal dialkylimine radical cation (29). Thus, &-amino groups of two N"-acetyl-L-lysines form a cross-linked Schiff base with a methylglyoxal, which lose one electron to form the cross-linked radical cation. With other N"-blocked amino acids, including N"-acetyl-L-arginine, no detectable EPR signal was observed (30). This result is in agreement with previous reports showing that hP-acetyl-L-arginine did not produce a Schiff base (23). Methylglyoxal is also known to react with arginine and cysteine residues in proteins, yielding stable imidazolone and hemithioacetal compounds, respectively (23). Several investigators have shown, however, that lysine residues are the main cross-linking sites in MG-treated proteins by forming imidazolium cross-links between two lysines (24-26). These observations together with our results showing that the cross-linked radical cations were
452
detected only with N"-acetyl-L-lysines suggest that the lysine residues are likely the main cross-linking sites for forming protein free radicals in MG-BSA adducts.
3.5. Effect of reducing ascorbate on the protein free radical The effect of the reducing agent ascorbate on the protein free radical of MG-BSA (30). Addition of ascorbate to MG-BSA, which originally displayed spectrum A, quenched the EPR signal as shown in spectrum B. This result indicates that ascorbate reduces the protein-radical cation of MG-BSA to the non-radical species. This reduction of radical cation was accompanied by the degradation of ascorbate (30). Under anaerobic conditions, a small portion of incubated ascorbate was consumed, which reached a plateau with time when 0.1 mM ascorbate was consumed. The controlled experiments showed that unmodified BSA failed to degrade ascorbate, and metal chelator, DTPA, had no inhibitory effect on the ascorbate degradation. The latter experiment indicates that adventitious metal ions were not the cause of the observed ascorbate consumption. These results suggest that the ascorbate was directly oxidized by the protein free radical cation in MG-BSA according to Reaction 2, and the concentration of the protein free radical cation in MG-BSA used in this experiment is approximately 0.1 mM. In the presence of oxygen, however, the oxidation of ascorbate continued to reach far beyond 1 molar ratio of the degraded ascorbate to MG-BSA (30). The degradation of ascorbate increased with increasing MG-BSA concentration. The initial rates obtained from these degradation data showed that the oxidation of ascorbate proceeded linearly with respect to the MG-BSA concentration. The straight line obtained from the concentration-dependent plot, however, intercepts at 0.02 mM/h, indicating that the initial rate observed for the ascorbate degradation contained a small MG-BSA independent term. The oxidation of ascorbate as a function of time obtained by using various concentrations of ascorbate and a fixed concentration of MG-BSA (7 mg/ml; 0.1 mM BSA) (30). The initial rates of the ascorbate oxidation determined from the degradation curves increased as a saturation function with respect to the ascorbate concentration. The double-reciprocal plot of the initial rates yielded a K,of 1 mM for ascorbate and a k,, of 3.3h if the MG-BSA used contained0.l mM protein free radical cation (30). Furthermore, superoxide dismutase, but not catalase, exerts partial inhibition on the degradation of ascorbate (approximately 20% inhibition by 0.4 pM Cu, Zn-superoxide dismutase in the reaction between 0.4 mM ascorbate and 2 mg/ml MG-BSA at pH 7.4). The fact that this inhibition is only partial indicates that 0 2 , but not is directly involved in this catalytic reaction. The partial inhibition, however, suggests that superoxide radical anions are produced during this reaction, and they play a role in ascorbate degradation, probably via the superoxide-scavenging reaction by ascorbate. Together these results indicate that MG-BSA behaves as an enzyme, which has an ability to catalyze the oxidationof ascorbate in the presence of oxygen to produce superoxide radical anion and semi-dehydroascorbate radical. This reaction is initiated by the protein-radical cation of MG-BSA. 4. DISCUSSION
Free amino groups in protein react with the carbonyl groups of reducing sugars or ketoaldehyde, which has been implicated as the onset of glycation. Previous investigations have shown that free radicals were produced in the reaction of methylglyoxal with proteins
(27, 28). We also detected protein free radical from the reaction between methylglyoxal and BSA. The results obtained in this study are summarized in the reaction scheme shown in Figure 3. The structure of the free radical of the MG-BSA is most likely to be the radical cation of the cross-linked Schiff base (Figure 3, species 0)on the basis of our previous results obtained with alanine a model system (29). When methylglyoxal was reacted with various N"-acetyl substituted amino acids, the free radical signal was observed only with fl-acetyl-L lysine. In addition, previous investigations have shown that lysine residues are the main crosslinking sites in MG-treated proteins (24-26). These results together suggest that methylglyoxal cross-links inter- or intramolecular lysine residues of the protein to form cross-linked Schiff bases (Figure 3, species A and B). This cross-linked Schiff base of MG-BSA can donate an electron to methylglyoxal to produce the radical cation of the cross-linked Schiff base (29). During these processes, the Schiff bases or the protein free radicals may be oxidized to form N"-(carboxyethy1)lysine (22) or matured to imidazolium cross-links (24-26) and other products such as imidazolysine (23). In the presence of electron-donating ascorbate, however, the radical cation of the crosslinked Schiff base accepts electron from ascorbate to produce the cross-linked Schiff base and semi-dehydroascorbate radical (see Figure 9) (30). These reactions can proceed even in the absence of oxygen. Moreover, in the presence of oxygen, MG-BSA behaves as an enzyme, which is capable of catalyzing the oxidation of ascorbate (K, = 1mM). We do not know at this time the exact mechanism for this catalytic reaction. However, the reaction catalyzed by MG-BSA in the presence of oxygen is similar to the transition metal ion-catalyzed oxidation of ascorbate. In a transition metal ion-catalyzed oxidation system such as Fe3+/ascorbate/Oz, the reaction is initiated via the one-electron reduction of Fe3+by ascorbate, whereas in the MG-BSA/ascorbate/Oz system, it is initiated via the one-electron reduction of the protein free radical cation by ascorbate. In both reactions, superoxide radical anions are generated by the oxidation of ascorbate.
-
CEL+
q o
- -"* - --
I+N
NHz
I+N
+
H/Gc,
1
+ N p NH2
0
N,
;C-C
CY
H
InlnnwlBCular
CY
H
C
intermolecular
.-.eSr/yKF
HzN
NHz
Imdazolium
;C-
[lWl. After the reaction is completed, the EPR spectrum of BSA bound dansyl-piperidine nitroxide can be observed. The intensity of this EPR signal increases with BSA concentration. In addition, at higher ionic strength, the effect becomes stronger. At 1 mM BSA (and above 1 mM) and physiological ionic strength (0.15), the reaction is very low and the EPR spectrum shows complete binding of dansyl-piperidine to BSA. Apparently, when BSA is present at low concentrations, molecules of and ascorbate bind to the same BSA molecule. The sites of the primary binding are close, and therefore, the probability of collision increases and the reaction proceeds faster. At high concentrations of BSA, the number of available sites on BSA for dansyl-piperidine and is large. Some of the dansylpiperidine and molecules are located at remote sites of the BSA or are even bound to different BSA molecules. Thus, they are spatially separated from each other and the reaction is slowed down. This result suggests that three types of probe are present in the system: 1 . not adsorbed (free FN in the solution); 2. loosely bound FN to the BSA (this fraction is in fast equilibrium with the free FN, and therefore their rates of reaction with ascorbate are equal); 3. tightly bound FN (FN reacts slowly with the ascorbate either inherently or due to low exchange rate with the other types of the probe). The observed rate is a sum of rates of the at least, four reactions involving: free probe and free ascorbate, free probe and bound ascorbate, bound probe and bound ascorbate and bound probe and free ascorbate. Since the EPR signals of free and loosely bound probe decay with the same observed rate, it can be concluded that a fast exchange exists between bound and free probes and hx>> kobs. Therefore, the observed rate is independent of the binding of the probe to BSA: kobs = akeee + pkbo,,,,d, where k,, is the observed rate of the reaction with free ascorbate and kbo,,,,d is the observed rate of the reaction with bound ascorbate. and p denote the contribution of kh, and kbound to kobs. Following the study of the ascorbate binding to BSA, the contribution of kbound to kobs was examined as a function of pH and ionic strength on the binding process. At p = 0.15, kpp values were calculated to be 14, 87, 42 M-' s-' at pH 4.0, 5.0 and 7.4, respectively. In the absence of BSA, the kinetics of the reaction between the probe and ascorbate was only slightly affected by changes in pH. Since ascorbate binding decreases with increasing the contribution of the kbound to kobs was also found to decrease (Figure 3). The apparent rate constant kaPpwas found to be 82.1 (at p = 0.0015); 41.5 (at = 0.048); 18.0 (at = 0.15) and 8.0 (at p = 0.8) M-' At p 0.8, the effect of albumin was completely was completely k& = kee,. Thus, it can be concluded that dansyl-piperidine is a convenient tool for the determination of unknown ascorbate concentrations when pH and p are known. For the determination of ascorbate in a biological system, one should use a calibration curve obtained under physiological conditions at pH 7.4 and p 0.15. Moreover, under each condition, the binding constant of ascorbate to BSA can be easily calculated. Since the dependence of k& on the ascorbate concentration is linear (see Figure 3), it can now be concluded that at [BSA] = constant, the a / p ratio is constant, i.e the alp ratio is independent of the A H concentration. As [ M I 0> [BSA], this observation shows that each molecule of albumin has a large number of available sites at which might
476
be adsorbed, in accord with earlier reports Therefore, [BSAIo might be considered as a constant. Thus, the ratio between free and adsorbed AH.BSA is constant:
K[BSA] =
[AH-aBSA] [AH1
0 10
/
Figure 3. Dependence of observed rate on ascorbate concentration in the presence of 0.1 mM BSA.
Therefore, the slope of a plot of [ A H - BSA]/[AH-] versus [BSA] yields the equilibrium constant K, which was found to be 3.0*105, 2.3*104 and l.2*103 M' at 0.0015, 0.048 and 0.15, respectively. The ratio of bound and free ascorbate is expressed as a l p and can readily be calculated. k,, =7 M' s" is derived from the kinetic measurements performed in buffer in the absence of BSA, while kbo,,,,d is obtained under conditions of complete binding of ascorbate to BSA. It was experimentally found that at pH 7.4 and = 0.0015, the ascorbate binding is maximal and kapp='kbo,,,,d = 82 M-' sd. Thus, k, = 82a + 78. Since a + p = 1, these parameters are easily extracted for each kpp.
REFERENCES 1. B. Halliwell, Medical Pharmacology, 37 (1988) 569-571. 2. X. M. He, D. . arter, Nature, 358 (1992) 209-214. 3. D. . arter, X. M. He, S. H. Munson, P. D. Twigg, K. M. Gernert, B. Broom, T. Y. Miller, Science, 4 (1989) 1195-1198. 4. B. Frey, L. England, B. N. Ames, Proc. Natl. Acad. Sci. USA, 86 (1989) 6377-6381. 5. V. , Okore, Arzneim.-Forsch. Drug Res, 44 (1994) 671-673. 6. K. R. Dhariwal, W. O.Hartzell, M. Levine, Am. J. lin. Nutr. 54 (1991) 712-716. 7. G. I. Likhtenshtein, V. R. Bogatyrenko, A. V. Kulikov, K. Hideg, 0. H. Hankovsky, N. V. Lukianov, A. I. Kotelnikov, B. S. Tanaschelchuk, Dokl. Akad. Nauk SSSR, 253 (1980) 481-484. 8. T. . Farrar and E. D. Becker Academic press, New York. 1971. 9. E. Lozinsky, A. Novoselsky, A. I. Shames, 0. Saphier, G.I. Likhtenshtein, D. Meyerstein, Biochim. Biophys. Acta, 1526 (2001) 53-60.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
477
Electron magnetic resonance study on the effect of radioactive radiation on the photosynthesis of chlorophyll in lipid bilayers Y. S. Kang, a D. K. LEE, a S. M. Parkb and K. W. Seoa aDepartment of Chemistry, Pukyong National University, Pusan 608-737, Korea* bCooperative Laboratory of Pukyong National University, Pusan 608-737, Korea
Photoinduced
electron
transfer from chlorophyll a through the interface of (DPPC) headgroup of the lipid bilayers was studied with electron magnetic resonance (EMR). The photoproduced radicals were indentified with electron spin resonance (ESR) and radical yields of chlorophyll a were determined by double integration ESR spectra. The formation of vesicles was identified by measuring h,,, value change from diethyl ether solution to vesicles solutions indirectly, and observed directly with SEM and TEM images. When the systems were y-irradiated with 100 Gy at room temperature, the photoyields were decreased about 30%. This was identified with the destruction and decomposition of chlorophyll a and vesicles molecules.
1. INTRODUCTION Molecular assemblies such as micelles and vesicles may be used as model systems for the storage of light energy [ 1,2]. These self-forming molecular assemblies compartmentalize the electron donors and acceptors relative to the solvent, typically water. Although the structures of vesicles and micelles are not as complex as natural membranes, photochemical studies of chlorophyll in such organized assemblies have proved to be relevant to the determination of fundamental properties of chlorophyll in photosynthetic system. The objective of this research is to study on the effect of radioactive y-ray on photosynthesis. Charge separation may be partially controlled by various factors such as the vesicle surface charge, headgroup variation, and alkyl chain length variation [3-71. The addition of slightly water soluble, surface active compounds such as alcohols and cholesterol modify the assembly interface. It has been shown that charge separation may be partially controlled by the addition of such intercalating agents [8,9]. In the current investigation, we have studied the photoionization of chlorophyll a in vesicles with or without a radiation of radioactive y-ray.
2. EXPERIMENT Chlorophyll a was extracted from fresh spinach leaves by the conventional method. Its purity was determined to be 96% from its extinction coefficient in diethyl ether at 660 nm vs
478
the literature value of 8.6 x M" cm-l [lo]. DPPC was purchased from Sigma Chemical Co. and were used without further purification. Buffer solutions were prepared with sodium phosphate, sodium pyrophosphate and sodium ethylene diaminetetraacetate (EDTA) from Aldrich Chemicals.
2.1. Sample preparation and Y-irradiation DPPC vesicle solutions of chlorophyll a were prepared by the method developed by Huang [ l l ] and modified by Norris et al. [12]. After the chloroform solutions of DPPC containing chlorophyll a were evaporated, the resulting film was sonicated in aqueous buffer solutions with a Fisher Model 300 sonic dismembrator operated at 30 W with a 4 mm 0.d. microtip for 1 h at 55 'C (above the liquid crystal gel phase transition temperature). Phosphate buffer solutions contained 0.1 M sodium phosphate, 0.1 M sodium pyrophosphate, and 1 mM EDTA in triply distilled water and were adjusted to pH 7.0 with sulfuric acid. Tris-HC1 buffer in triply solutions were prepared by dissolving 0.5 M distilled water and were adjusted to pH 7.0 with hydrochloric acid. The formation of DPPC vesicles containing chlorophyll a was identified by measuring value change from diethyl ether solution to vesicles solutions indirectly. Optical absorption spectra were measured in 1 cm path length quartz cell with a Varian CARY 1C UV-vis spectrophotometer at room temperature. The morphologic analysis of vesicles was carried out by scanning electron microscopy (SEM), with a Hitachi S-4200 FE-SEM, after coating the sample with Pt in vacuum chamber and transmission electron microscopy (TEM), with Jeol JEM-2010, after staining of the sample with uranyl acetate (2%), respectively. DPPC vesicle solution of chlorophyll a was y-irradiated at room temperature with a dose of 100 Gy in 6oCoGammacell-220 from Korea Atomic Energy Research Institute (KAERI). 2.2. Electron spin resonance experiments Photoirradiation at 77 K was performed with 300-W Cermax xenon lamp with a power supply form ILC Technology. The light was passed through a 10-cm water filter and a Corning 5030 band pass filter for blue light irradiation (300 nm < hlrr< 558). ESR spectra were recorded at X-band on JEOL JEX-FX 2000-300. Mn2+in MgO was used as a magnetic field marker. The efficiency of photosynthesis in model system was determined by measuring the amount of chlorophyll radical yields which were obtained from integration of spectra.
400
500
600
Wavelength (nm)
Figure I . Optical absorption spectra of DPPC vesicle and chlorophyll a in diethyl ether and DPPC vesicles.
479
RESULTS AND DISCUSSION
Optical absorption spectra of DPPC vesicle and chlorophyll a in diethyl ether and DPPC vesicles is shown in Figure 1. There is no optical absorption band in DPPC vesicle solution. Some shift of the absorption bands of chlorophyll in the vesicle solution from the bands in the organic solvents might be caused by different environmental interactions of chlorophyll a [13]. In DPPC vesicle solutions of chlorophyll a an absorption band at 685 nm has been assigned to aggregated chlorophyll a [141. The formation of hydrated chlorophyll polymer has been reported to give an absorption band at 740 nm [15]. Since there are no absorption bands at 740 nm or 685 nm in our preparations, we conclude that chlorophyll is solubilized in its monomeric form in our samples. Chlorophyll a solubilized into a phospholipid vesicles, causes a 10 nm shift to the red region compared to diethyl ethyl solution in which hmax= 660 nm. This red shift is still persisted in DPPC vesicle solution with hmax= 670 nm [16]. This red shift indicates the chlorin ring being located in a polar environment near the surfactant headgroup region and possibly exposed to the aqueous environment.
Figure 2. Scanning electron micrographs of DPPC vesicles (left) and DPPC vesicles containing chlorophyll a (right). The size of vesicles is 80 -1 80 nm.
15
[Chloraphyll]l[DPPC]
Figure 3. Dependence of the normalized photoyield of chlorophyll a upon the molar ratio of DPPC in DPPC frozen vesicles. Photoirradiation was carried out with bluelight for 20 min at 77K.
45
Irradiation Time (mm)
Figure 4. Dependence of the normalized photoyield of chlorophyll a upon the bluelight irradiation in DPPC frozen vesicles at 77K. [Chlorophyll a]/[DPPC] molar ratio is 0.03.
480
Scanning electron micrographs of DPPC vesicles (left, ~15,000)and DPPC vesicle containing chlorophyll a (right, ~100,000)were represented in Figure 2. The size of vesicles is 80 - 180 nm. The vesicles were formed almost homogenously in both DPPC vesicles and DPPC vesicles containing chlorophyll a.
B
A
C
Figure 5 . Transmission electron micrographs of DPPC vesicles containing chlorophyll a (A) and DPPC vesicles containing chlorophyll a y-irradiated with 100 Gy at room temperature (B and C). The samples were stained with uranyl acetate. 0 8 ,
(u
Y)
400
Wavelength (nrn)
Figure 6. Optical absorption spectra of chlorophyll a in DPPC vesicles (solid line) and chlorophyll a in DPPC vesicles y-irradiated with 100 Gy at room temperature (dot line). The photosynthesis of chlorophylls in vesicles was studied with ESR. No radical formation was observed in DPPC vesicle solutions not containing chlorophyll after blue-light irradiation at 77K. After blue-light irradiation of chlorophyll a in DPPC vesicle solutions the ESR singlet was observed. This ESR singlet did not decay within several days at 77K. The photoproduced cation radical of chlorophyll a was identified as g = 2.0026 and broad singlet in frozen state at 77K. Dependence of the normalized photoyield of chlorophyll a upon the molar ratio of DPPC in DPPC frozen vesicles was shown in Figure 3. Photoirradiation was carried out with blue-light for 20 min at 77K. The normalized photoyield increases linearly with the Chlorophyll dDPPC molar ratio up to 0.03 and then is constant. Dependence of the
48 1
normalized photoyield of chlorophyll a upon the blue-light irradiation in DPPC frozen vesicles at 77K was shown in Figure 4. The normalized photoyield increases rapidly with blue-light irradiation up to 30 min and then increases slowly after 30 min. Transmission electron micrographs of DPPC vesicles containing chlorophyll a (A) and DPPC vesicles containing chlorophyll a y-irradiated with 100 Gy at room temperature (B and C) were shown in Figure 5 . TEM images show that the round shaped DPPC vesicle before yirradiation (Figure 5 A) is destructed by y-irradiation (Figure 5 B and C). Optical absorption spectra of chlorophyll a in DPPC vesicles (solid line) and chlorophyll a in DPPC vesicles y-irradiated with 100 Gy at room temperature (dot line) were shown in Figure 6. The optical absorption bands of chlorophyll a in DPPC vesicles in both before and after y-irradiating show no significantly change. This result can be interpreted as the chorin ring of chlorophyll a molecular is not almost decomposed by y-irradiation with 100 Gy at room temperature. First derivative X-band ESR spectra of chlorophyll a in DPPC vesicles (left) and chlorophyll a in DPPC vesicles (right) y-irradiated with 100 Gy at room temperature was shown in Figure 7. The spectra were recorded at 77K after 90 min blue-light irradiation. The 2+ . sharp lines on both sides of the spectra are form Mn in MgO used as a magnetic field marker. The efficiency of photosynthesis in model system was decreased about 30% by yirradiation with 100 Gy at room temperature. This is interpreted as with the destruction of DPPC vesicle system the photoinduced cation radicals of chlorophyll a were decayed due to back electron transfer to water. And the decomposed radical was disappeared at room temperature as increased temperature from 77K.
1
I ' g=20026
Figure 7. First derivative X-band ESR spectra of chlorophyll a in DPPC vesicles (left) and chlorophyll a in DPPC vesicles (right) y-irradiated with 100 Gy at room temperature. The spectra were recorded at 77K after 90 min blue-light irradiation. The sharp lines on both sides of the spectra are form M? in MgO used as a magnetic field marker.
CONCLUSION The formation of DPPC vesicles containing chlorophyll a was identified by a 10 nm shift to the red region compared to diethyl ethyl solution in which hmax= 660 nm indirectly. From
482
SEM images we could confirm that the vesicles were formed almost homogenously in both DPPC vesicles and DPPC vesicles containing chlorophyll a with a size of 80 - 180 nm. After blue-light irradiation of chlorophyll a in DPPC vesicle solutions, the ESR singlet was observed. The photoproduced cation radical of chlorophyll a was identified as g = 2.0026 and broad singlet in frozen state at 77K. From TEM images we could observe that the round shaped DPPC vesicle is destructed by y-irradiation. Due to the decay of the photoinduced cation radicals by back electron transfer to water, the efficiency of photosynthesis in model system was decreased about 30% by y-irradiation with 100 Gy at room temperature. In the furthermore future study, the destruction of systems and decomposition of chlorophyll and vesicle molecules will be studied and the degree of destruction and decomposition versus dosage of y-radiation will be studied. Acknowledgments This project was supported by Ministry of Science and Technology (MOST) as a part of the Nuclear R&D Program.
REFERENCES 1. 2. 3. 4. 5. 6.
J.H. Fendler, Acc. Chem. Res., 13 (1980) 7. J.K. Hurley and G. Tollin, Sol. Energy, 28 (1982) 197. P.A. Narayana, S.S.W. Li and L. Kevan, J. Am. Chem. SOC.,104 (1982) 6502. E.-M. Rivara, P. Baglioni and L. Kevan, J. Phys. Chem., 93 (1998) 2612. M.P. Lanot and L. Kevan, J. Phys. Chem., 93 (1989) 998. T. Hiff and L. Kevan, J. Phys. Chem., 93 (1989) 2069. 7. P.J. Bratt, Y.S. Kang, L. Kevan, H. Nakamura and T. Matsuo, J. Phys. Chem., 95 (1991) 6399. 8. P. Baglioni and L. Kevan, J. Phys. Chem., 91 (1987) 2016. 9. I. Hiromitsu and L Kevan, J. Am. Chem. SOC.,109 (1987) 4501. 10. H.H. Strain and W.A. Svec, The Chlorophylls, Academic Press, New York, 1966. 11. G.H. Huang, Biochemistry, 8 (1969) 344. 12. W. Oettmeier, J.R. Norris and J.J. Katz, Naturforsch, 31 (1976) 163. 13. G. R. Seely and R. G. Jensen, Spectrochim. Acta, 21 (1965) 1835. 14. A. G. Lee, Biochemistry 14 (1975) 4397. 15. R. G. Brown and E. H. Evans, Photochem. Photobiol., 32 (1980) 103. 16. T. Trosper, D. Raveed and B. Ke, Biochem. Biophys. Acta, 223 (1970) 463.
EPR in the 21" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
483
Effects of tannin compounds on metal ion-hydrogenperoxide systems* A. Nakajima,a Y. Ueda) N. Endoh,c K. Tajima,c and K. Makinod a Department of Chemistry, Miyazaki Medical College, Kiyotake, Miyazaki Japan Department of Psychiatry, Miyazaki Medical College, Kiyotake, Miyazaki Japan C Department of Applied Biology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto Japan d International Innovation Center, Kyoto University, Yoshida, Sakyo-ku, Kyoto Japan
When (DMPO) and hydrogen peroxide (H202) were added into a uranyl solution, ESR signal of DMPO-OH was observed. No DMPO-OH signal was observed in U022+-DMPO solution without H202. The ESR signal intensity of DMPO-OH in U022+-DMPO-H202 solution gradually increased with time. Based on these results, it may be proposed the following mechanism for DMPO-OH formation; (a) reduction of U022+ to U02+, (b) disproportionation of U02+ to U022+ and UO2+, (c) Fenton-like reaction, and (d) spin-trapping reaction. Persimmon tannin affected peculiarly on the DMPO-OH formation in U022+-DMPO-H202 solution compared with those in other metal solutions. The addition of constituent compounds of PT, especially epigallocatechin (EGC), enhanced the DMPO-OH formation by their reduction abilities. Thus, the behavior of PT in the U022+-DMPO-H202 solution reflected the metal-reducing effect of these components of PT.
1. INTRODUCTION Polyphenol compounds, such as tannins, are well known to act as anti-oxidants and free radical scavengers.1-3 In metal ion-H~O2solution, these compounds affected the generation of hydroxy radical by the chelation with metal ion as well the direct scavenging of hydroxy radical.4.5 The authors previously investigated the interaction between these metal ions and tannins, and found that persimmon tannin has high affinities for metal ions, such as
* This work was supported by the Grant-in-Aid for ScientificResearch, the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the REIMEI Research Resources of Japan Atomic Energy Research Institute.
484
uranyl, ferric and vanadyl ion^.^-^ Recently, Hamilton et a1 reported that hydroxy radical was generated a U022+-DMPO-H202 solution.10 It is, therefore, very interesting to analyze the rise and fall of hydroxy radical in U 0 2 2 + - D M P O - H 2 0 2 - ~ nsolution for elucidation of not only the radical scavenging effect of tannins but also the interaction between uranyl ion and tannins. In this paper, the mechanism of hydroxy radical generation in the solution and the effects of tannin compounds on the system were examined in comparison with those in Fe2+-DMPO-H202and V02+-DMPO-H202 solutions.
2.1 Materials Purified persimmon tannin powder used throughout this study was obtained from Tomiyama Shoten Ltd., Kyoto, uranyl nitrate hexahydrate, UO2(NO3)2 6H20, from Merck Chemical Industries, Ltd., 5,SDirnethyl- 1-pyrroline-N-oxide(DMPO), from DOJINDO Ltd., Kumamoto, Japan, and other chemicals used in this study, from Nacarai Tesque, Inc., Kyoto and Wako Pure Chemical Industries, Ltd., Osaka.
2.2. Electron paramagnetic resonance measurements Uranyl nitrate (final concentration 1 mM), DMPO (final concentration 100 mM), H202 (final concentration 100 mM), and tannins (final concentration0 - 0.625 mg/ml) were mixed. Then, the mixture was sucked up into a capillary tube and quickly measured ESR spectrum using X-band ESR spectrometer (JEOL JES TE-100) under the conditions of microwave frequency, 9.44 magnetic field, 334.5 mT; field amplitude, 5 mT; field modulation, 100 kHz; modulation width, 0.079 mT; microwave power 2 mW, and the response time, 0.1 sec, and sweep time 1 min.
3. RESULTS AND DISCUSSION
3.1. Reaction mechanism of hydroxy radical formation in U0z2+-DMPO-H202 solution When 5,5-dimethyl-1-pyrroline-N-oxide(DMPO) and hydrogen peroxide (H202) were added to a uranyl solution, ESR signal with four lines (intensity ratio, 1:2:2:1) was appeared. From its g-value, g = 2.0066, and hyperfine constants, = 1.49 mT, aH = 1.49 mT ,the signal was identified to be that of DMPO-OH (Fig. la). The addition of methanol and dimethyl sulfoxide (DMSO) into the system decreased DMPO-OH signal and DMPO-CH3, and DMPO-CH20H radicals were appeared, respectively (Fig. l b and c). It is, therefore, confirmed that the DMPO-OH was resulted by the trapping of hydroxy radical as reported by Hamilton et a l l 0 A signal of DMPO-OCH3 was also observed in U022+-DMPO-H202 solution with 20 % of methanol (asterisks in Fig. lc). These results suggested that the nucleophilic addition of water molecule was also occurred in these solutions, being similar Fe3+-DMPO-H202, and Cu2+-DMPO-H202 solution^.^>^ However, no DMPO-OH
485
signal was observed in U022+-DMPO solution without H202. The ESR signal intensity of DMPO-OH in V02+-DMPO-H202 solution decreased with time, and that in Fe2+-DMPOH202 solution increased rapidly in the initial stage and then decreased in a manner similar to that in V02+-DMPO-H202 solution (Fig. 2), as reported by Mizuta et a112 and Sakurai et al.13 On the other hand, DMPO-OH signal intensity in U022+-DMPO-H202 solution gradually increased with time, which is quite different from other metal ions. When hydrogen peroxide was added to the uranyl solution, U02(H202)2 (or UO4.2H2O) was precipitated,14 which should retard the DMPO-OH formation. Hamilton et al. suggested that three reaction steps were assumed for hydroxy radical generation in U022+-DMPOH202 solution,1° namely reduction of U022+ to U02+, disproportionation of the resulting U02+ to UO2+ and U022+, and generation of hydroxy radical through the Fenton-like reaction. As no signal of DMF'O-OH or DMPOX was observed in the U022+-DMPO solution without H202, the possibility of the oxidation of DMPO by metal ion, such as Fe3+,11was omitted. Thus, H202 should be only an agent for the reduction of U022+ to U02+. It is therefore possible to propose the following reaction scheme for DMPO-OH formation; (a) reduction of U022+ to U02+ ( 2U022+ + H202 2U02+ + 2 H+ + 0 2 7 ), (b) disproportionation of U02+ to U022+ and U02+ ( 2 U02+ + H20 U02+ + U022+ + 20H- ), (c) Fenton-like reaction ( UO2+ + H202 U022+ + OH + H+ ), (d) spintrapping reaction ( DMPO + * OH DMPO-OH ). +
+
+
+
332
334 336 Magnetic field
338
Figure 1. ESR spectra of free radicals in U0z2+-DMPOH202 solutions. (a) UOz2+ (1 mM), DMPO (100 mM), H202 (100 mM) water were mixed for 1 h. (b) (a) in 20 % MeOH, (c) (a) in 20 % DMSO. Asterisk indicates the signal of DMPO-OCH3.
3.2. Effect of tannins on the DMPO-OH in U022+-DMPO-H202 solution When persimmon tannin (PT) was added to U022+-DMF'O-H202solution, the DMPOOH signal rapidly increased in first few minutes, followed by a slow increase and slow
486
decrease (Fig. 2). On the other hand, PT caused the rapid decrease of DMPO-OH signal in V02+-DMPO-H202 solution. In Fe2+-DMPO-H202solution, PT gave extremely high DMPO-OH signal intensity within first few minutes, and then the signal came back to that of control with time. Matsuo and Itoh reported that PT consists of catechin, catechin-3gallate, gallocatechin and gallocatechin gallate at the ratio of 1:1:2:2 (chemical formula weight 2248).15 It contains two 1,2-dihydroxyphenyl(catechol) groups and seven 1,2,3trihydroxyphenyl (pyrogallol) ones, acting as hard bases, in a chemical formula unit. As V@+, Fe3+, and UOz2+ions are classified into hard acids, they form stable complexes with both hard bases.16.17 Thus, PT couples strongly with VO2+ ion, which suppress the Fenton-like reaction. Ferrous (Fe2+)ion, being an intermediate acid, could not combine very strongly with PT, so that the apparent effect of PT was not observed in Fe2+-DMPO-H202 solution, except an initially high DMPO-OH signal. As PT has a high reduction ability,*y9 it should reduce Fe3+ produced by air oxidation to Fez+, which leads to the high DMPO-OH signal. On the other hand, the behavior of the DMPO-OH signal in U022+-DMPO-H202PT solution was quite peculiar compared with those of other metal solutions. Thus, the effect of PT on the DMPO-OH signal in U022+-DMPO-H202 solution was examined carefully in comparison with its constituent compounds, such as gallic acid (GA), catechin (CA), epicatechin (EC), epigallocatechin FGC), and epigallocatechin gallate (EGCG).
b
l4
20
I
1-
8 0
2
4 E
10
0 0
20
40
60
Reaction time (min) Figure 2. Effect of persimmon tannin (PT) on DMPO-OH signal in metalDMPO-H202 solutions. Metal ion (1 mM), DMPO (100 mM), PT (0.625 mg /ml) and H202 (100 mM) were mixed. 0 : U(VI), 0 : U(VI)-PT, : Fe(II), A : Fe(I1)-PT, : V(IV), : V(1V)-PT.
20
0
40
60
Reaction time (min) Figure 3. Effect of components compounds on DMPO-OH signal in U022+-DMPO-H202 solutions. U022+ (1 mM), DMPO (100 mM), tannins (0.153 mg /ml) and H202 ( 100 mM) were mixed. 0 : control, 0 : PT, A : GA, : CA, H : EC, : EGC, 0 : EGCG.
+
487
As shown in Fig. 3, in GA, CA and EC solutions, DMPO-OH signal increased gradually in similar manner that of control. In EGCG solution, the signal increased slightly rapidly in the first stage, and then gradually increased up to that in EC solution. In EGC solution, the signal increased very rapidly up to six-times of that in control, and then gradually decreased. As a whole, the DMPO-OH signals were enhanced by the addition of these compounds. The order of magnitude of the enhancement effect was EGC >> EGCG > PT, EC > CA, GA. Both the free radical-scavenging effect and the metal-chelation effect5 of these compounds were omitted in the present discussion, because they reveal as the suppression of DMPO-OH formation. As tannin compounds have high abilities to reduce metal ions,*>9these compounds could reduce U022+ to U02+, which precedes the formation of hydroxy radical through processes (b) and (c) in the previous section. According to this line of reasoning, EGC has the hghest reducing ability. Summarizing these results, the anomalous behavior of PT in the U022+-DMPO-H202 solution reflected the metal-reducing effect of EGC group in PT.
1. K. Kondo, M. Kurihara, N. Miyata, T. Suzuki, and M. Toyoda, Free Radical Biol. Med., 27 (1999) 855. 2. H. Yoshioka, H. Kurosalu, and H. Yoshioka, J. Radioanal. Nucl. Chem., 239 (1999) 217. 3. M. Noferi, E. Masson, A. Merlin, A. Pizzi, and X. Deglise, J. Appl. Polymer Sci., 63 (1997) 475. 4. M. Kashima, K. Saitoh, Y. Higashi, Y. Tsujimoto, and M. Yamazaki, Magnetic Resonance in Medicine, 10 (1999) 89. 5. H. Yoshioka, Y. Senba, K. Saito, T. Kimura, and F. Hayakawa, Biosci. Biotechnol. Biochem., 65 (2001) 1697. 6. T. Sakaguch and A, Nakajima, Sep. Sci. Technol., 29 (1994) 205. 7. A. Nakajima and T. Sakaguchi, J. Radioanal. Nucl. Chem., 242 (1999) 623. 8. A. Nakajima and T. Sakaguchi, J. Chem. Technol. Biotechnol., 75 (2000) 977. 9. A. Nakajima, unpublished results. 10. M. M. Hamilton, J. W. Ejnik, and A. J. Carmichael, J. Chem. SOC.Perkin Trans. (1997) 2491. 11 K. Makino, T. Hagiwara, A. Hagi, M. Hishi, and A. Murakami, Biochem. Biophys. Res. Commun., 172 (1990) 1073. 12. Y. Mizuta, T. Masumizu, M. Kohno, A. Mori, andL. Packer, Biochem. Mol. Biol. Int., 43 (1997) 1107. 13. H. Sakurai, M. Nakai, T. Miki, K. Tsuchiya, J. Takada, andR. Matsushita, Biochem. Biophys. Res. Commun., 189 (1992) 1090. 14. J. J. Katz, G. T. Seaborg, and L. R. Morss, The Chemistry of the Actinide Elements, Chapmann and Hall Ltd., New York (1986) Vol. 1, Part 1. 15. T. Matsuo and S. Itoh, Agric. Biol. Chem., 42 (1978) 1637. 16. R. G. Pearson, J. Am. Chem. SOC.,85 (1963) 3532. 17. R. D. Hancock and A. E. Martell, Chem. Rev., 89 (1989) 1875.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
488
The [2Fe-2S] cluster in sulredoxin from the thermoacidophilic archaeon strain 7, a novel water-soluble Rieske protein* Toshio Iwasakia, Asako Kounosua and Sergei A. Dikanovhc aDepartment of Biochemistry and Molecular Biology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo 113-8602, Japan bIllinois EPR Research Center and Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. CInstitute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia The [2Fe-2S] cluster surrounding in reduced sulredoxin from the thermoacidophilic archaeon strain 7 was examined by one- and two-dimensional electron spin echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra revealed two coordinated nitrogens assigned to two histidine ligands and the lines from several nonexchangeable protons. No strongly coupled protons involved in hydrogen bonds near the reduced Rieske center were detected. These results are discussed in light of the structural, redox, and spectroscopic characteristics of this ubiquitous electron transfer protein family. INTRODUCTION
The eponymous Rieske iron-sulfur protein (ISP) is an intrinsic constituent of complexes from mitochondria, chloroplasts, and bacteria, which acts to transfer reducing equivalent from a quinol bound at the Q,-site of cytochrome (cyt) b to cyt c, off[l-3]. The Rieske [2Fe-2S] cluster has a high midpoint potential of +150--k490 mV [3,4] and characteristic optical and EPR spectra that are distinctively different from those of the conventional cluster bound to four cysteine residues. The Rieske center comprises the asymmetric iron-sulfur core environment, with the Sy atom of the two cysteine residues coordinated to one iron site and with the Nfiatom of the two histidine residues coordinated to the other iron site [5,6]. Recent crystal structures [7-lo], along with others evidence [l l-131, indicated a conformational movement of the extrinsic domain of the Rieske subunit that bridges long electron transfer distances between the electron-donating and electron-accepting cofactors in the complex, apparently in association with the occupancy of the Q,-site. *This investigation was supported in part by Grants-in-aid from the Ministry of Education, Science, Sports and Culture of Japan (no. 11 169237 to T.I.) and by a grant of Cooperative Research under the Japan4.S. Cooperative Science Program from JSPS (BSAR-507 to T.I.) and NSF (INT-9910113 to S.A.D.). S.A.D. thanks the Illinois EPR Research Center (NIH grant RR01811) for assistance.
489
The [2Fe-2S] clusters in several ferredoxins involved in bacterial oxygenase systems also have properties analogous to those of the respiratory Rieske iron-sulfur proteins, and are called the "Rieske-type" ferredoxins [2,14,15]. Their redox properties are considerably different from those of the ISPs, despite the common structures of the cluster and ligands [16]. These bacterial Rieske-type ferredoxins have midpoint redox potentil below 0 mV, which are probably independent of pH, while the ISPs involved in the bcllbdcomplexes have midpoint potential well above 0 mV, which decreases upon deprotonation due to a pK on the oxidized form [3]. This has a special significance in mechanism, because the pK,,, on ISP allows it to act as a H-carrier (instead of electron carrier generally assumed), and thereby transfer both an electron and a proton away from the catalytic site. Archaeal "sulredoxin" is a novel water-soluble and high-potential Rieske-type [2Fe-2S] protein isolated from the hyperthermoacidophile Sulfolobus strain 7 (formerly Sulfolobus sp. strain 7; optimal growth conditions, pH 2.5-3 and 80 "C) [17]. It possesses = +188 mV; pK,,,I of -6.2, tightly linked ionization affecting the redox properties (&(low pK,,,2 of -8.6), which is similar to those found for the ISPs in the bcllbdcomplexes [IS]. Deprotonation of one of the two putative coordinated histidine imidazoles in sulredoxin, having the pKaloX2 of -8.6, causes a decrease in the midpoint redox potential, the change in the optical and circular dichroism spectra, and the appearance of a new Raman transition at 278 cm-1, without major structural rearrangement of the overall protein conformation [ 181. The redox potential of each particular cluster is determined by its protein surroundings and the factors that influence its value need to be considered individually for each protein at the atomic level. A variety of factors have been proposed to control the redox potential of iron-sulfur cluster including hydrogen bonding, solvent accessibility, ligand residue arrangement, the proximity and orientation of proton-based dipoles and charged residues. EPR spectroscopy plays an important role in the characterization of iron-sulfur clusters, providing direct data about their type, oxidation state, and nearest surroundings. In this work, we have applied ESEEM spectroscopy to examine the Rieske [2Fe-2S] cluster in archaeal sulredoxin, particularly to probe histidine coordination and hydrogen-deuterium exchange around the cluster. We then investigated the possible structural reorganization accompanying the reduction of the Rieske center, based on the comparison of EPR spectra of the [2Fe-2S] center reduced chemically at room temperature and radiolytically at 77 K.
2. RESULTS AND DISCUSSION
2.1. Structural features The structural gene sak coding for an archaeal sulredoxin from strain 7 has been cloned and sequenced (to be published). The deduced amino acid sequence does not contain potential signal sequence or transmembrane region at the N-terminus. The cluster binding site of sulredoxin involves total of four cysteine and two histidine residues arranged in the same motif as observed for regular Rieske iron-sulfur proteins found as a part of cyt bc complex. The sequence feature at the cluster-binding site of sulredoxin is in line with the redox properties of the protein [18]. Thus, archaeal sulredoxin can be regarded as a new hyperthermostable water-soluble redox module of the cyt bcj-associated high-potential Rieske protein as they share the same redox site cysteine and histidine motif.
490
0.00
2.S0
5.00
7.50
10.00
MHz
Figure 1. Superimposed plot of a set of three pulse ESEEM spectra of the sulredoxin stored 24 hours in excess D,O as a modulus of the Fourier transformation along T axis. Parameters: magnetic field 362.7 mT, microwave frequency 9.697 GHz; the initial is 88 ns in the nearest trace, increased by 16 ns in successive traces. F2:[MHz] I
FI:[MHz]
Figure 2. HYSCORE spectrum of reduced Rieske cluster in the sulredoxin recorded at g, = 1.91 where the EPR intensity is maximum. The time ‘c between first and second pulses 176 ns; magnetic field 362.3 mT, and microwave frequency 9.688 GHz. 2.2. EPR spectrum In all previously reported cases, the reduced Rieske [2Fe-2S] clusters exhibit a characteristic g tensor (gl = 2.01-2.02, g2 = 1.90-1.92, g3 = 1.76-1.8) with larger anisotropy and considerably lower g, =: 1.91 than those of the plant ferredoxin-type cluster (g, =: 2.042.05, g2 = 1.96, g3 = 1.87-1.88) or adrenal ferredoxin (gl =: 2.025, gy= 1.932), both with g,
49 1
= 1.96 [2]. Dithionite-reduced sulredoxin shows an EPR spectrum in the form of anisotropic line with a rhombic g tensor having principal values of gl = 2.01, g2 = 1.91, g3 = 1.79 (g, 1.90). They are typical of Rieske-type clusters [17] (Fig. 3A).
=
2.3. Nitrogen ESEEM Figure 1 shows a stacked plot of three-pulse ESEEM spectra of dithionite-reduced sulredoxin in the region 0-8 MHz. Two peaks from the spectra with maxima at 6.3 and 7.5 MHz are the characteristic indication of the Rieske type cluster reported in all previous ESEEM studies. They belong to double-quantum transitions (dq), vdq+,of two directly coordinated histidine nitrogens [ 15,191. In accordance with the properties of nitrogen nuclear frequencies, the dq transitions from the opposite electron spin manifold, (vdq.),have smaller values, with a frequency difference less than 4v,. The assignment of these transitions from three-pulse spectra is not straightforward. The direct way to determine the paired (vdq+,vdqJtransitions is using HYSCORE spectra. These transitions produce easily recognizable cross-peaks with coordinates corresponding to frequencies (vdq+)and (vdq.) in the (+-) quadrant of the HYSCORE spectra with the frequencies 7.5, 3.4 MHz and 6.3, 2.8 MHz (Fig. 2). The hyperfine coupling and quadrupole coupling parameter K = K2 (3 + q2)(K = e*qQ/h and qasymmetry parameter) can be estimated from the following expression vdq+= 2 [
.t A12)’
+ K’ 1’”
The application of Eq. (1) to two pairs of dq-frequencies gives the values A, = 5.0 MHz and A,= 3.6 MHz that is in a good agreement with the values 4.6-5.5 MHz and 3.6-4.5 MHz reported for the two nitrogens coordinated with the iron-sulfur cluster of other Rieske centers [2]. The K’ parameter constrains the quadrupole coupling constants for coordinated nitrogens to K, = 0.55.tO.04 MHz and K, = 0.65k0.04 MHz. These constrains follow the noted tendency for K of a coordinated imine nitrogen to decrease from that of an uncoordinated imidazole residue. They also agree well with the K values 0.65 and 0.57 MHz from the nitrogen of the histidine ligands in phthalate dioxygenase [19]. Thus, our ESEEM data confirm the coordination of the [2Fe-2S] cluster in the sulredoxin with two histidine nitrogens. These nitrogens belong to His-44 and His-46 because they are only histidine residues present in the sulredoxin sequence. The comparison of the nitrogen ESEEM spectra of sulredoxin with the available spectra of respiratory Rieske proteins and Rieske-type ferredoxins shows their individual character for each protein. Comparative analysis of these spectra provides us with information about either differences resulting from variation of the ligand geometry or from other groups in the local environment that influence the electronic structure of the cluster and affect the hyperfine interaction with nitrogen.
2.4. Proton and deuterium ESEEM HYSCORE spectra of the Rieske center in the sulredoxin resolve cross-peaks from three groups of nonexchangeable protons with couplings around 2-5 MHz. These couplings, however, were less then the maximum couplings 8-10 MHz observed for [2Fe-2S] cluster with four cysteine ligands in ferredoxin [20]. We tentatively assign these lines to the pprotons of cysteine ligands, because recent NMR studies of Rieske type proteins report the
492
paramagnetic shifts of these protons, which are several times larger then the protons shifts of imidazole residues [2]. This observation opens the way for the determination of cysteine ligand conformation in the sulredoxin using the approach developed on [2Fe-2S] cluster in ferredoxin [20]. This has not been studied previously by magnetic resonance. Figure 1 shows three-pulse ESEEM spectrum of the sulredoxin stored 24 hours in excess D,O. The line at -2.2 MHz is from deuterium nuclei that have replaced exchangeable protons ( i e . protons of hydrogen bonds, if present, and solvent molecules). However the intensity of this line is relatively low, especially taking into account its partial overlap with nitrogen lines. The ratio of two- and three-pulse echo envelopes of the sulredoxin prepared in D,O and H,O only shows the line at the Zeeman frequency of deuterium from weakly coupled nuclei. Also, exchangeable protons with significant coupling in HYSCORE spectra were absent. This indicates a low accessibility of solvent to the cluster and the absence of the strongly coupled protons involved in hydrogen bonds with sulfur atoms of the cluster. 2.5. Radiolytic reduction of the Rieske center in sulredoxin The traditional technique for the preparation of the reduced iron-sulfur cluster is chemical reduction in solution at room temperature by dithionite or ascorbate, followed by freezing. Another method of cluster reduction is low temperature radiolytic reduction. Since molecular motion is limited at 77 K, the cluster produced by cryogenic reduction are trapped in a constrained non-equilibrium state with ligand coordination similar to that of the initial oxidation state [21-221. Radiolytic reduction of the Rieske center in sulredoxin at 77 K, brought no significant change in the EPR spectrum as compared with the chemically reduced form (Fig. 3). The same results have been reported for monooxygenase [22]. It led to the conclusion that reduction produces negligible effects on the ligand field of the Fe” ion, which brings in the major contribution to the g-tensor anisotropy. The redox-linked deprotonation of the Rieske proteins occurs only in the oxidized state and the reduced state generated at room temperature is always protonated. Therefore, if radiolytic reduction produces the Rieske cluster with the ionizable group(s) responsible for the redox-linked ionization still in the deprotonated state, the EPR characteristics of this state might be indistinguishable from the characteristics of protonated state. On the other hand,
*
I
2.01
330
1.91
360 Magnetic Field [mT]
Sdx
390
Figure 3. X-band EPR spectra of dithionite-reduced (A) and the radiolytically-reduced sulredoxin at 77 K (B). For radiolytically reduced sample, only the g, and g, features are shown because of masking of the g, signal by strong radical signals also formed during the irradiation.
493
the interaction of the Rieske cluster with occupants of the Q,-site in the cyt complex realized over hydrogen bond formation with the ITH fragment of one histidine ligand produces noticeable changes in the EPR spectra [2,12,23]. This implies that one would expect to see a difference if the reduced form is deprotonated, and that Rieske center reduction may initiate its reversed protonation upon strong irradiation of the frozen aqueous system even at 77 K. In this case, significantly higher pH (well above pK,,,,) or lower temperatures might be required for the stabilization of the deprotonated reduced state.
REFERENCES 1. B.L. Trumpower and R.B. Gennis, Annu. Rev. Biochem., 63 (1994) 675. 2. T.A. Link, Adv. Inorg. Chem., 47 (1999) 83. 3. E.A. Berry, M. Guergova-Kuras, L.-S. Huang and A.R. Crofts, Annu. Rev. Biochem., 69 (2000) 1005. 4. M. Brugna, W. Nitschke, M. Asso, B. Guigliarelli, D. Lemesle-Meunier and C. Schmidt, J. Biol. Chem., 274 (1999) 16766. 5. S. Iwata, M. Saynovits, T.A. Link and H. Michel, Structure, 4 (1996) 567. 6. C.J. Carrell, H. Zhang, W.A. Cramer and J.L. Smith, Structure, 5 (1997) 1613. 7. D. Xia, C.-A. Yu, H. Kim, J.-Z. Xia, A.M. Kachurin, L. Zhang, L. Yu and J. Deisenhofer, Science, 277 (1997) 60. 8. Z. Zhang, L.-S. Huang, V.M. Shulmeister, Y.-I. Chi, K.K. Kim, L.-W. Hung, A.R. Crofts, E.A. Berry and S.-H. Kim, Nature, 392 (1998) 677. 9. S. Iwata, J.W. Lee, K. Okada, J.K. Lee, M. Iwata, B. Rasmussen, T.A. Link, S. Ramaswamy and B.K. Jap, Science, 281 (1998) 64. 10. H. Kim, D. Xia, C.-A. Yu, J.-Z. Xia, A.M. Kachurin, L. Zhang, L. Yu and J. Deisenhofer, Proc. Natl. Acad. Sci. U.S.A., 95 (1998) 8026. 11. H. Tian, S. White, L. Yu and C.-A. Yu, J. Biol. Chem., 274 (1999) 7146. 12. M. Brugna, S. Rodgers, A. Schricker, G. Montoya, M. Kazmeier, W. Nitschke and I. Sinning, Proc. Natl. Acad. Sci. U.S.A., 97 (2000) 2069. 13. E. Darrouzet, M. Valkova-Valchanova, C.C. Moser, P.L. Dutton and F. Daldal, Proc. Natl. Acad. Sci. U.S.A., 97 (2000) 4567. 14. B. Kauppi, K. Lee, E. Carredano, R.E. Parales, D.T. Gibson, H. Eklund and S. Ramaswamy, Structure, 6 (1 998) 57 1. 15. S.A. Dikanov, L. Xun, A.B. Karpiel, A.M. Tyryshkin and M.K. Bowman, J. Am. Chem. Soc., 118 (1996) 8408. 16. C.L. Colbert, M.M.-J. Couture, L.D. Eltis and J. Bolin, Structure, 8 (2000) 1267. 17. T. Iwasaki, T. Isogai, T. Iizuka and T. Oshima, J. Bacteriol., 177 (1995) 2576. 18. T. Iwasaki, T. Imai, A. Urushiyama and T. Oshima, J. Biol. Chem., 271 (1996) 27659. 19. R.J. Gurbiel, C.J. Batie, M. Sivaraja, A.E. True, J.A. Fee, B.M. Hoffman and D.P. Ballou, Biochemistry, 28 (1989) 4861. 20. S.A. Dikanov and M.K. Bowman, J. Biol. Inorg. Chem., 3 (1998) 18. 21. R. Davydov, S. Kuprin, A. Graslund and A. Ehrenberg, J. Am. Chem. Soc., 116 (1994) 11120. 22. S.A. Dikanov, R.M. Davydov, L. Xun and M.K. Bowman, J. Magn. Reson. Ser. B, I12 (1996) 289. 23. M. Guergova-Kuras, R. Kuras, N. Ugulava, I. Hadad and A.R. Crofts, Biochemistry, 39 (2000) 7436.
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EPR in the 21' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
EPR and saturation recovery investigations of spin probes in dispersions of hydrogenated castor oil Kouichi Nakagawa Research Center, Fukushima Medical University, 1 Hikarigaoka, Fukushima, 960-1295, Japan Rotational correlation times and electron spin-lattice relaxation times (T,J of spin probes in 10 wt % dispersions of poly(oxyethy1ene) hydrogenated castor oil (HCO) were investigated using continuous wave (CW) and saturation recovery (SR) electron paramagnetic resonance (EPR) spectroscopy. The TI,, and activation energy (EJ were obtained in the system for the first time. The E, calculated from the correlation times in the HCO vesicle phase was -6.8 [kcal/mol] for TEMPO and DTBN. TI, was measured using SR as a function of temperature (22-50 "C) for spin probes in the aqueous and vesicle phases. TI, is about the same for TEMPO in the vesicle, TEMPO in the aqueous phase, and DTBN in the aqueous phase. T,, was shorter for DTBN in the vesicles than in the aqueous phase. Shorter TI, of DTBN in the vesicle phase can be attributed to location of DTBN within the vesicle. For both TEMPO and DTBN, there was no significant difference between T,, obtained in H,O and D,O solutions. 1. INTRODUCTION Nonionic surface active agents have been of great interest in the fields of cosmetics well as pharmaceuticals (1). In drug encapsulation, vesicle formation of the surfactant is necessary. In addition, nontoxic and naturally degradable substances are desired. Recently, a poly(oxyethy1ene) hydrogenated castor oil (HCO) was found to be a natural nonionic surfactant that forms vesicles (2, 3). Initial investigation of HCO vesicle formation in the aqueous dispersions was made by Horiuchi and co-workers (3). A stable mutilamellar vesicle with an average diameter of -500 nm was observed. Additional physical characterization of the vesicle for the nonionic amphiphilic compound HCO could lead to detailed understanding of the bilayer structure as well its physical behavior. Electron paramagnetic resonance (EPR) techniques in conjunction with spin probe methods are useful for characterization of micellar and bilayer structure well (4-5). Spin probe methodology is a powerful technique for studying molecular dynamics in biological and physical sciences on timescales from nanoseconds to milliseconds (4). The lineshape of the EPR signal can be analyzed to determine the rotational correlation time and electron spin-lattice relaxation times (TI,) provide insight concerning processes by which spin probes exchange energy with the surrounding membrane (6). Measurement of TI, is useful for monitoring Heisenberg exchange between oxygen and spin labels in biological systems (7-10).
E-mail:
[email protected] The effective Ti, mechanism leads to a description of membrane fluidity, including translational diffusion. Furthermore, theoretical understanding of the longitudinal relaxation mechanism is still controversial (10-12). In the present study, rotational correlation times and longitudinal relaxation times (T,J of various spin probes in aqueous solution containing a dispersion of poly(oxyethy1ene) hydrogenated castor oil were investigated for the first time. The behavior of spin probes in both the aqueous and vesicle phase a function of temperature was studied. The correlation times of spin probes are also discussed in relation to Tie. 2. MATERIALS AND METHODS 2.1. Samples Poly(oxyethy1ene) hydrogenated castor oil (termed HCO) was donated by Nikko Chemicals Co. Ltd. (Tokyo, Japan) and used as received. The HCO had about 10 moles of oxyethylene moieties per mole of oil. The chemical structures of HCO and the spin probes used in this study are depicted in Figure 1. The spin probes, di-tert-butyl nitroxide 0 o.(cn,cn,o).H (DTBN), 2, 2, 6, 6-tretramethylpiperidine-1~2~o-(cn2CHzo) o-(cn2cn20)p oxyl (TEMPO), 4-hydroxy-2, 2, 6, C H ~ O ( C H ~ C H ~ O o.(cti,cn20),n (TEMPOL), 4cI n , . o - ~ c H , c H , o ) , - P I 1-oxyl Oxyethylene Group = I + m + n + x + y + z (TEMPONE), and 3-(aminomethyl)-proxy1 - 10 (CTPO) were obtained from Aldrich Chemical Fi ure 1. Molecular structure of Co. and used as received. poTy(oxyethy1ene) HCO. A weighed amount of HCO was dissolved in a few milliliters of chloroform (13). The spin probe was dissolved in -0.3 milliliters of chloroform and mixed with the HCO solution. After evaporation of the chloroform on a rotary evaporator, a 10 wt % dispersion of HCO/spin probe in distilled water was prepared. The final concentration of the spin probe was approximately 10 micro molar for CW EPR and 100 micro molar for saturation recovery. 2.2. Deoxygenation For CW EPR, the sample solutions were degassed about 15 minutes in an AtmosBag (Aldrich, USA) and the solutions were put into capillaries (I.D., 0.9 mm; O.D., 1.4 mm; Nippon Rikagaku Kikai Co. Ltd., Japan). The sample capillary was inserted in a 3 mm EPR tube (JEOL Datum) in the AtmosBag and taped around the tube cap. Degassing for SR measurements was carried out using TPX gas-permeable plastic capillaries (7-10, 14). We used Teflon tubes to achieve degassing in the following manner. Two Teflon tubes were inserted side-by-side in a 4 mm EPR tube. One Teflon tube (I.D., 0.96 mm; O.D., 1.56 mm) contained the sample solution. Nitrogen was passed through the second Teflon tube (I.D., 1.5mm; O.D., 2 mm) to purge oxygen from the solution. The degree of degassing of an aqueous CTPO (0.2 mM) solution after 90 minutes was estimated to be -98% using the equation provided by Hyde and co-workers (14). 2.3. CW EPR Measurements EPR measurements were made with a 9 GHz JEOL FE 1XG spectrometer with a TE,,, cylindrical cavity. Sample temperature was controlled by nitrogen gas flow through the
)
~
-
~
496
Dewar using JEOL ES-DVT system. EPR signals were digitized using a Scientific Software Service System (Illinois, USA). The microwave frequency was measured using an EMC-14 X-band microwave frequency counter (Echo ElectronicsCo., Ltd., Japan). 2.4. Rotational Correlation Time Various methods have been developed for determining the correlation time for molecular motion based on changes in the amplitude, position, and widths of EPR lines (15, 16). Rotational correlation time (zR) of the order of 10"' sec can be estimated from spectra of a nitroxyl spin probe using equation (1) (17-20),
where p, is the electron Bohr magneton, R is Plank's constant, I is peak amplitude and the subscripts, +1, 0, -1, are nuclear quantum numbers for I4N. is the peak-to-peak line width of the centerline. Values of g,,, = (1/3)(gn + g, +gJ and b = (2/3)[AA,- 0.5(An +A,)] were calculated from the parameters for an immobilized spin probe. Calculations of for TEMPO and DTBN were made using the magnetic principal values (21). The values of g's and A's were assumed to be appropriate. The parameters I+l, I,, I+ and AH,,, obtained from the experimental spectra were used to calculate zR.
2.5. Saturation Recovery (SR) Measurements Electron spin-lattice relaxation time (TI,) was measured on a home-built SR spectrometer at the University of Denver (22). A 5-loop-4-gap resonator (LGR, Medical Advances model XP-0201 (Milwaukee, Wisconsin)) was used (23). The klystron was locked via an AFC circuit to a high-Q cylindrical external reference cavity. The EPR signal was amplified by a lownoise GaAsFET amplifier and detected by use of a double-balanced mixer. No magnetic field modulation was used. Sample temperature was controlled using a Varian V-6040 with nitrogen gas passed over the LGR. Sample temperature was monitored with a thermocouple positioned immediately above the resonator. The magnetic field for recording the recovery signal was set on the high-field nitrogen hyperfine line for the probe in the vesicle or aqueous phase. Artifacts were removed by subtracting an instrumental background response that was measured with the magnetic field set 100 G higher than for the signal. The signal was amplified and then digitized with an EG&G 9825 in a Pentium PC. Usually -3 lo5 recovery signals of about 1000 digitized points were averaged. The digitized signal was fitted to a single exponential. The data were recorded with pump times of 5 ps, which are long relative to the recovery time constants and relative to the tumbling correlation times. Under these conditions the contribution to the recovery due to spectral diffusion is minimized and the time constant is assigned as TI,. 3. RESULTS AND DISCUSSION
3.1. CW EPR of Spin Probes CW EPR spectra of spin probes DTBN and TEMPO in aqueous HCO dispersions consist of two overlapping triplets with slightly different g and hyperfine values presented in
491
Figure 2. The contributions from the two triplets are best resolved for the high-field hyperfine line. EPR lines from the probes in the vesicle phase are broader than those from the aqueous phase. Molecular motions of the spin probes in the vesicle are somewhat restricted. In addition, the EPR spectrum obtained from DTBN/HCO/H,O shows relatively sharp EPR lines. The linewidth difference between the aqueous and vesicle phase is more significant than the one for TEMPO. Relative partitioning of spin probes between the aqueous and vesicle phases was studied a function of temperature. Focusing on the high-field lines, the relative amplitude of the signal from a probe in the vesicle increases as temperature increases. The EPR signals from the vesicle phase becomes dominant above -40 "C. In order to verify the partitioning of spin probes, the amplitudes of the aqueous peaks were analyzed indicated in Figure 2. Linewidth change at half-height in the aqueous phase was minimal throughout the temperature studied. The normalized intensities of the aqueous peaks are plotted in Figure 3. The magnitude of the intensity reduction for DTBN was more significant than that of TEMPO. The results suggest that the temperature dependence of DTBN partitioning is more efficient in comparison with TEMPO. When TEMPONE and TEMPOL were examined in dispersions similar to those examined with DTBN and TEMPO, there were no distinguishable vesicle peaks for TEMPONE and TEMPOL at any temperature studied. The EPR hyperfine splitting suggests that both TEMPONE and TEMPOL might remain in the aqueous phase or might have very small gvalue and hyperfine differences in the two phases. In order to analyze the molecular motion of spin probes in the vesicle, the rotational correlation time (zd was calculated using eq. (1). EPR spectra of the vesicle phase were obtained by subtracting two spectra at the same experimental conditions, but with different partitioning of the probe between the vesicle and aqueous phases. The subtraction eliminated the aqueous peak. Based on the EPR spectrum of the vesicle phase, was obtained. In addition, an Arrhenius analysis of the rotational correlation time in the vesicle phase gave the activation energy. The activation energies obtained for TEMPO and DTBN are 6.6 0.4 and 7.1 0.4 [kcaVmol], respectively. Both activation energies in this phase are similar. The calculated for TEMPONE in TEMPONE/HCO/H,O is approximately 5 x lo-" s and is independent of temperature in the 20-50 "C range. The estimated from the spectra for TEMPO and DTBN in the aqueous phase was close to this value. Similar values were obtained for TEMPOL over the same temperature range. The correlation times of TEMPOL and TEMPONE in water were one order of magnitude shorter than for TEMPO in the vesicle phase.
~~~~
1:: P
0 5
10
20
40
50
Temperature ("C)
Figure 2. CW EPR s ectrum of DTBN obtained in aqueous gspersion of HCO.
Figure 3. Normalized intensity of aqueous phase for DTBN and TEMPO in dispersion of HCO.
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3.2. Spin Lattice Relaxation Time (Tie) of Spin Probes Direct observation of the interaction between spin probes and lattice (membrane) was accomplished. TI, for the probes in the aqueous and vesicle phases was measured at the magnetic field that corresponds to the maximum intensity in the first-integral spectrum for each of the components of the high-field hyperfine line. The SR signal from the vesicle peak in DTBN/HCO/H,O system at 22 "C is presented in Figure 4. A single exponential fit to the experimental data is shown in the dotted line. At this temperature the signal-to-noise ratio of the SR signal from the vesicle phase is poorer than that for the aqueous phase because of weaker EPR intensity. The value of T,, for TEMPONE in H,O is very similar to that reported previously (10). With reference to the values obtained, TI, of DTBN in the vesicle phase is close to that reported for DTBN in paraffin oil. T,, of TEMPO in the HCO vesicle is slightly shorter the one for TEMPO in paraffin oil. TI, of the probes in the vesicle and aqueous phases showed little temperature dependence in the range examined (Figure 5). The relaxation times for TEMPO and DTBN in the aqueous phase are similar and consistent with a shorter in the phase. The TI, of TEMPO is similar in the aqueous and vesicle phases. The TI, of DTBN in vesicle phase is slightly shorter than the one for aqueous phase. The difference in TI, obtained from direct SR measurements can be due to the interaction between the probe and the environment. Thus, a shorter TI, for DTBN in the vesicle phase implies that DTBN is located within the vesicle. In addition, shortening of TI of DTBN towards the terminal methyl group was shown by NMR (24). The relative resistance might account for the results obtained. On the other hand, the and longer TI, in the vesicle suggest that TEMPO may locate slightly different region from DTBN.
=
ps
Time I p s
Figure 4. Saturation recovery signal from DTBN in H,O dispersion of HCO. The dotted line indicates a single exponential fit to obtain T,,.
Temperature /"C
Figure 5. Plot of TI for DTBN (aqueous: 0 , vesicle: H) and YEMPO (aqueous: 0, vesicle: +) in H,O dispersion of HCO as a function temperature.
Furthermore, we made measurements in D,O dispersions. The T,, values of DTBN and TEMPO for the aqueous and.vesicle phase are similar to the case of H,O. The present SR measurements on T,, did not show clear evidence of dehydration of the membrane based on the present results regarding the dispersion of HCO in H,O or D,O. It may be due partly to location of the spin probes not near the oxyethylene moiety.
Acknowledgment. Author thanks Profs. Sandra S. Eaton and Gareth R. Eaton for the use of saturation recovery apparatus, and fruitful discussion concerning the results.
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REFERENCES 1. Y. Okahata, S. Tanamachi, M. Nagai, and Y. Kumitake, J. Colloid Interface Sci., 82 (1981) 401. 2. M. Tanaka, H. Fukuda, and T. Horiuchi, J. Am. Oil Chem. SOC.,67 (1990) 55. 3. T. Honuchi and K. Tajima, Yukagaku, 41 (1992) 1197. 4. L. J. Berliner (ed.), Spin Labeling, Theory and Applications, Academic Press, New York, 1976. 5. W. K. Subczynski, A. H. Lewis, R. N. McElhaney, R. S. Hodges, J. S. Hyde, and A. Kusumi, Biochemistry, 37 (1998) 3156. 6. G. R. Eaton and S. S. Eaton, In Biological Magnetic Resonance Vol. 19, G. R. Eaton, S. S. Eaton, and L. J. Berliner (eds.), Chapter 2, Plenum Press, New York, 2000. 7. W. K. Subczynski and J. S. Hyde, Biochim. Biophys. Acta, 643 (1981) 283. 8. W. K. Subczynski, J. S. Hyde, and A. Kusumi, Biochemistry, 30 (1991) 8578. 9. A. Kusumi, W. K. Subczynski, and J. S. Hyde, Proc. Natl. Acad. Sci. USA, 70 (1982) 1854. 10. J. S. Hyde and J. B. Feix, In Biological Magnetic Resonance Vol. 8, L. J. Berliner and J. Reuben, J. (eds.), Chapter 6, Plenum Press, New York, 1989. 11. B. H. Robinson, D. A. Haas, and C. Mailer, Science, 263 (1994) 490. 12. B. H. Robinson, A. W. Reese, E. Gibbons, and C. Mailer, J. Phys. Chem. B, 103 (1999) 5881. 13. R. R. C. New, (ed.), In Liposomes, A Practical Approach, Chapter 2, Oxford University Press, Oxford, 1990. 14. C-S. Lai, L. E. Hopwood, J. S. Hyde, and S. Lukiewicz, Proc. Natl. Acad. Sci. USA, 79 (1982) 1166. 15. D. Kivelson, J. Chem. Phys., 33 (1960) 1094. 16. R. Wilson and D. Kivelson, J. Chem. Phys., 44 (1966) 154. 17. N. D. Chasteen, and M. W. Hanna, J. Phys. Chem., 76 (1976) 3951. 18. A. L. Buchachenko, A. L. Kovarskii, A. M. Vasserman, In Advances in Polymer Science, Z . A. Rogovin (ed.), p. 26. Wiley, New York, 1974. 19. D. J. Schneider and J. H. Freed, Biol. Magn. Reson., 8 (1989) 1. 20. K. Nakagawa and K. Tajima, Langmuir, 14 (1998) 6409. 21. L. J. Berliner (ed.), Spin Labeling, Theory and Applications, Appendix 11, p. 565. Academic Press, New York, 1976. 22. R. W. Quine, S. S. Eaton, and G. R. Eaton, Rev. Sci. Instrum., 63 (1992) 4251. 23. G. A. Rinard, R. W. Quine, S. S. Eaton, G. R. Eaton, and W. Froncisz, J. Magn. Reson. A, 108 (1994) 71. 24. J. A. Dix, D. Kivelson, and J. M. Diamond, J. Membr. Biol., 40 (1978) 3 15.
Section 6 Medical Sciences
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EPR in the 21'' Century A Kawarnori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
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Advances in the spin labeling method Lawrence J. Berliner Dept. Chemistry & Biochemistry, University of Denver 2190 E. Iliff Avenue, Denver, CO 80208, U.S.A. 1. INTRODUCTION
The spin labeling technique, coined by Harden McConnell in the early 1960s, was modeled after the "reporter group" approach of placing a spectroscopic probe in a biological system such as an enzyme or a protein in order to monitor spectroscopic variables to which the biosystem alone is transparent. This might be a fluorophore, NMR isotope such as 'F or I3C, resonance Raman probe, a stable paramagnetic nitroxyl molecule, etc. The spin label technique has flourished since then with several texts devoted specifically to the subject. Spin labels have now enjoyed a history of close to twenty-five years in demonstrating the applicability of paramagnetic nitroxides to biochemical problems of structure and function in enzymes, membranes, cells and animals (1-4). These have been employed traditionally as reporter groups; that is, the nitroxide spin label serves as a physical probe that reports aspects of its structure and environment at a localized molecular site. It has been quoted many times that this reporter group must "report the news" not "make the news". That is, it is important to insure that the sometimes bulky nitroxide spin label, does not perturb the macromolecular system under study. Although the method is technically not restricted to just nitroxyl compounds (most authors use the term nitroxide for these structures) it is fair to say that more than 99.5% of all papers using ESR reporter group techniques employ nitroxyl (nitroxide) spin labels. These molecules are classified as spin when a covalent linkage to the biological system is employed (i.e. with proteins, polymers and nucleic acids) while a spin probe involves noncovalent interactions (cells, membranes, liquid crystals and some polymer systems). 2. WHAT THE SPECIFIC BIOLOGICAL SYSTEMS/APPLICATIONS OF SPIN LABELS?
The answer to this question is literally everything large and small: proteins and enzymes, lipids and membranes, nucleic acids, pharmaceuticals, drug-receptor interactions, cells and cell membranes, polymers, animals. A number of examples are presented here.
5 04
2.1. Stoichiometry and specificity A major obstacle for reporter group (labeling) studies is obtaining a unique, specific covalent modification of a protein or enzyme that yields a 1:l nitroxyl label to macromolecule complex. That is, the labeling must be to one amino acid and not, e.g., 0.3 here, 0.4 there, and 0.3 spread over several others. Otherwise the spectral results will be ambiguous and any eventual distance measurement will weight the “closest” spins inordinately. The most ideal candidate is the thiol side chain (-SH) of the amino acid cysteine (Cys). However, it is frequently rare to find free cysteines in proteins since they are mostly always coupled to another cysteine in a disulfide (cystine) bridge. On the other hand, most thiol (cysteine) reagents are very specific, particularly the methanethiomethane sulfonate analogs. The technique coined as “site-directed spin labeling” (SDSL) offers unique local probing of the environment at a specific amino acid residue in a protein or protein complex of interest (2). It has been especially powerful in discerning membrane bound vs. exposed segments, distinguishing aspects of secondary structure (alpha- vs. beta vs. random structure), identifying helix-helix interfaces and facilitating intramolecular distance measurement s within a protein. The details of the method involve site directed ‘cysteine scanning’ or specific incorporation of cysteine residues at targeted regions of interest on the protein molecule. The technology allows one to conformationally examine several different positions in a protein representing a significant fraction of the amino acid sequence. The Cys residue is subsequently spin labeled with specific, reversible covalent probes which are analogs of methanemethylthiol thiosulfonate, R-S-SOZ-CH~, where R is a pyrrolinyl, pyrrolidinyl or piperidinyl nitroxyl group. The second critical advance involved utilizing molecular biological techniques to incorporate cysteine groups at different selected positions in a protein. Hubbell first demonstrated this methodology with membrane-bound protein bacteriorhodopsin (5). 2.2. The reporter group approach -who makes the news? While spectroscopists are ‘purists’ in that they observe natural spins (nuclei) without altering the physical or chemical environment, spin labels and fluorescent probes are by necessity exogenously introduce, sometimes bulky structures. Consequently, spin labeling is constantly accused of making the news instead of reporting the news. In fact, the fluorescence community occasionally accuses the EPR community, while neglecting the typically bi- and tricyclic aromatic molecules employed in luminescence studies. When substituting a nitroxyl side chain in a specific position in a protein, some type of structural perturbation may be produced, but the evidence continually shows that the majority, including those at internal positions in a protein, do not result in debilitating structure/function changes while the thermal stability of the protein may be altered somewhat. The fact is that one must slightly perturb or interact with the macromolecular environment in order to report meaningful information.
2.3. Site directed spin labeling (SDSL) - how is it done? Briefly, you must specifically incorporate one (or occasionally two) thiol groups in a protein sequence and subsequently covalently label it with a uniquely specific reactive, reversible spin label. Berliner and coworkers (6) introduced the MTSL label and demonstrated its efficacy with a highly reactive thiol protease, papain. This was followed by the groundbreaking work of Hubbell and coworkers mentioned earlier (5). The following
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protocols and parameters, taken in part from the chapter of Feix and Klug (7), summarizes the SDSL paradigm and what we can learn. 2.3.3. The SDSL paradigm
no
by 2.3.4. SDSL parameters 0 2
-
-
The real success of this method has been the possibility of describing protein structure almost entirely from the ESR results. That is, one can determine local rigidity and proximity to other structural elements, accessibility to solvent or the nonpolar interior and quantitative conformational movements form double labeling studies. In fact, the trends and cycles in increasing and decreasing exposure (accessibility) can trace an alpha-helical backbone or beta-sheet structure. By utilizing MTSL labels of varying tether length and rigidity one can confirm these results unambiguously. 2.4. Spin labeled DNA - structure, dynamics and DNA sequence analysis The major challenge in probing DNA structure and conformation was introducing the probe at sensitive sites (ie., purine or pyrimidine bases) which sense local structure, global motion, DNA wobbling, etc. A selected sample of a range of modified pyrimidine bases are shown in Figure 1. These labels offer properties from very rigid, restricted tumbling volumes to very flexible (8). Figure 2 depicts a motional model of the various features Thai contribute to the EPR spectrum. Needless to say, the plethora of modified basis, the methodology to incorporate them into various polynucleotides, etc., is a great feat.
506
UUMHT
y;-%y-. R t*?. yij+o DCAVAP I
O l NI
DCAT
Jy;-y. I DCAP
Figure 1. A selection of structures of spin labeled pyrimidine bases that can be incorporated into nucleic acid structures (adapted from reference 8 with permission).
Figure 2. The nitroxide (reporter group) monitors DNA dynamic processes according to the degree of coupling to each mode. The nitroxide ring is couple the base motion through a tether linkage. The labeled bas is couple to another base which together experience base pair motion. This base pair is couple to the collective bending and twisting of all base pairs in the helix. Collective motions are also coupled to the global tumbling (adapted from reference 8 with permission).
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2.5. pH Sensitive Spin Labels EPR has the advantage of being a noninvasive, nondestructive technique. Recently, Volodarsky and Khramtsov have synthesized and tested imidazoline nitroxides which can be protonated (9). These labels have been tested over an array of pH ranges, each one shown in Figure 3 acting as a pH range probe. Where the results have been unusually impressive are with in vivo studies where the stomach pH of a laboratory animal was determined noninvasively. In addition, one could covalently attach a pH sensitive imidazoline spin label on a protein, perhaps based on a dithio label or a methylthiolsulfonate analog (ie a pH sensitive MTSL). Consequently, the local pH near the labeling site and/or the environment of the labeled protein could be determined.
16.2
8
z
15.4
S
0
14.6
0
2
4
8
1
0
1
2
1
4
Figure 3. Imidazoline nitroxide protonation and a plot of pH induced changes in the hyperfine splitting for eight different imidazoline nitroxides (adapted from reference 9 with permission). 3. SPIN LABELS AND PHARMACOLOGY - IN VIVO ESR/EPR
There has been a plethora of studies on the metabolic fate of nitroxyl compounds in vivo, which in all cases lead to the reduced hydroxylamine product. Various research groups have taken advantage on the pharmacokinetics to learn something about the redox state of the suborganelles where the nitroxide localizes. This has been particularly instructive with the use of EPR imaging or localized spectroscopy. However, this is not technically the case of a ‘spin labeled’ animal since the nitroxyl is simply circulating throughout the tissue and is not a new covalent product. On the other hand some intriguing results have come form the study of nitrosobenzene and its analogs with living mice and rats. 3.1. Nitrosobenzene Nitrosobenzene is a potent, toxic mutagen. Hence, we want to avoid exposure to even trace levels. One should note however, as shown in Figure 4 that there are various oxidation
508
states of nitrogen substituted aromatics going from aniline through to nitrobenzene and that there are hepatic and intestinal enzymes capable of converting anilines and nitrobenzenes to the nitroso form. Our studies focused on the radical products from nitrosobenzene both vitro and vivo.
Aniline
phenylhydroxylarnine Nitrosobenzene
Nitrobenzene
Figure 4. Oxidation states of nitrogen substituted aromatics Typical X-band spectra of nitrosobenzene in either hemolyzed blood or in NADH or vitamin C are shown in Figure 5. This spectrum was simulated as the phenylhydronitroxide radical from a one-electron reduction of nitrosobenzene. This spectrum will persist only if continuous, stoichiometric oxygen is supplied (lo). The overall scheme depicting this chemistry is shown in Figure 6. Although most biological redox chemistry is two electron, this radical can be produced from either the one electron "back oxidation" of the hydroxylamine by oxygen (yielding also superoxide) or as a result of the disproportionation between nitrosobenzene and
Phen) Ihydronitrode radical
Figure 6. Reduction pathway of nitrosobenzene Figure 5. X-band EPR (A) Nitrosobenzene and excess NADH (also found with red blood cell hemolysates and 3 mM nitrosobenzene); (B) Sample A lus yeast glucose oxidase which depletes a\ oxy en (C) Sample A with continuous oxygen. EPR conditions were microwave power, 5 mW; modulation amplitude, 0.1 auss; time constant, 0.1 sec; scan rate, 5 8 gauss/min. From reference 10 with permission.
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3.1.1. Pathway of nitrosobenzene in-vivo A 15-20 g mouse was injected with an ethanol or liposome suspension of nitrosobenzene and the L-band in vivo EPR spectrum measured after 20min (Figure 7). As the control, the animal was injected with the (liposome suspension or ethanol) vehicle alone (1 1). Note the broad three line spectral component in Figure 7a, which is an immobilized nitroxyl radical, but a slowly tumbling phenylhydronitroxyl radical, such as seen in Figure 5. This spectrum remained persistent for many hours (in fact this broad-line EPR spectrum remained days after death!). If buttocks muscle tissue was excised from an anesthetized mouse within 5 minutes after injection, on observed a multiline phenylhydronitroxide radical spectral component which disappeared with time, leading to the broad three-line nitroxyl component shown in Figure 6. Most likely, the early multiline spectral component appeared since hemolysis occurs during tissue excision. The precise chemical nature of this adduct was suggested most convincingly by observing this excised muscle tissue at high microwave power and comparing it with the radical adduct obtained from mixing nitrosobenzene with oleic acid or any other fatty acid (Figure 7). The product in muscle tissue was spectrally identical with that from a pure oleic acid sample. The mechanism of this adduct formation is shown in Figure 8, which is commonly called an “ene-addition” or pseudo Diels-Alder reaction (Sullivan. 1966), resulting in a migration of the double bond, then formation of an hydroxylamine intermediate which is easily oxidized to the nitroxyl adduct (nitroxide radical I).
10 G
Figure 6. (a) L-Band EPR spectra of an anesthetized (25g) mouse at room temperature. (a) 20 min. after an ip injection of O.lml of 50mM nitrosobenzene (in a liposome suspension); (b) control injection without nitrosobenzene. Spectrometer conditions: frequency, 1.128GHz; microwave power, 10mW; modulation, 1.9G; applied magnetic field, 390G; sweep width, 100G; sweep rate, 25G/min.; time constant, 1.O. From ref. 11 with permission.
Figure 7. X-Band EPR spectra of excised muscle tissue after in-vivo i.m. administration of 0. lml 50mM nitrosobenzene (in ethanol) in the mouse buttocks: (a) 20 min. or (b) 80 min. after injection, and (c) 200 min. after injection. The muscle tissue was removed within 5 minutes after injection. Spectrometer conditions were: microwave power, 10mW; kequency, 9.25GHz; modulation, 0.5G; applied magnetic field, 3400G; sweep width, 100G; sweep rate, 25G/min.; time constant, 0.128s. From ref. 11 with permission. In summary, nitrosobenzene form persistent radical adducts with unsaturated fatty acids in fat tissue or membranes. It is considerably less accessible to bioreduction, yielding a strong, stable nitroxyl EPR spectrum. Recall that there are metabolic pathways for converting anilino- and nitroaromatics to the nitroso analog (see Figure hence one may not be able to avoid nitroso compounds that may further lead to stable nitroxyl radical adducts in fatty tissue. Lastly, these adducts have been shown to catalyze lipid peroxidation (12).
Ph-N=O
i
or 0,-.+
H H N Ph'
nitroxide radical (1) Figure 8. Pseudo Diels-Alder "ene-addition" reaction scheme
511
Both the nitroxyl radical and its hydroxylamine are potentially capable of reducing trace femc to ferrous ion with possible initiation of lipid peroxidation (13). 3.2. Nifedipine: a pharmaceutical that forms nitrosoaromatic intermediates Nifedipene dimethylester) is a very popular slow calcium channel blocker or calcium ion antagonist. It lowers blood pressure by inhibiting the transmembrane influx of Ca(I1) into cardiac and smooth muscle and by inhibiting calcium ion flux across the cell membrane (14). Additionally, nifedipene can be converted by high power illumination, or by prolonged normal room light, to its nitroso-fonn under (15-17) as shown in Figure 9. Extensive exposure to the sun may result in severe adverse skin inflammation (18).
Yifedipine
COOCHj
H3C00C
-
H3C00C
/
COOCHj )OCH3
,
H3C' I
H3C
CH3 33
NitrosoNifedipine
H
Figure 9. Nifedipene photochemically converted to its nitroso-form. A 100 mM nifedipine solution (in DMSO), illuminated under 15 W visible light for hr. was injected as a 100 pL bolus i.m. in a 15g anesthetized mouse. The animal was then placed in an L-band EPR loop gap resonator 15 minutes later (17). A broad three-line spectrum was observed, which persisted for more than 1 hr. shown in Figure 10. Excised tissue observed at X-band 15 min after injection yielded'an even stronger signal, especially in the liver (Figure 10).
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gauss
Figure 10. L-band in-vivo EPR spectra of an anesthetized (15g) mouse at room temperature 15 min after an im injection of 100 mM illuminated nifedipine (100 pL in DMSO). Spectrometer conditions were similar to those noted in Figure X-band ex-vivo EPR spectra of excised liver tissue and buttocks muscle. Spectrometer conditions were similar to those noted in Figure 6. Nifedipine solutions were illuminated by placing solutions in a 5-mL (8 mm i.d.) glass test tube on a 15 W common household fluorescent tube for 24 hrs. Adapted from ref. 11 with permission. Since the liver contains nitroreductases, conversion of nifedipine to its nitroso analog could potentially occur Figures 11A and B compare X-band EPR spectra of freshly excised livers treated with illuminated and non-illuminated nifedipine, respectively. the two spectra are superimposable, suggesting that non-illuminated nifedipine was converted to its nitroso analog by hepatic nitroreductases. The relative level of radical adduct was 10% in the non illuminated nifedipine case, however, this is still a quite
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respectable level (17). Nifedipine analogs which are meta-substituted (i.e. nimodipine, nicardipine) do not show radical adducts when illuminated. They appear to be less susceptible to conversion to the nitrosoforms (15).
Figure 11. X-band EPR spectra of liver tissue exposed to (a) non-illuminated and (b) illuminated nifedipine. Nifedipine solutions were freshly prepared and soaked with liver tissue for 5 min at room temperature in the dark. Illuminated nifedipine was prepared as described in Figure 10. Adapted from ref. 11 with permission. Epidemiological studies showed that patients have a 40-60% higher susceptibility to myocardial infarction with nifedipine than with other classes of slow calcium channel blockers (19,20). Furthermore, as mentioned earlier, nitroxyl radical adducts of unsaturated fatty acids can support lipid peroxidation (12). Both the radical and the hydroxylamine are potentially capable of reducing trace Fe(II1) to Fe(I1) with possible initiation of lipid peroxidation (1 3). Considering the many controversies over free radicals, reactive oxygen species, and lipid peroxidation in heart disease, the radical adducts found with nifedipine are worth further study. 4. SYNOPSIS
The spin label technique offers some unique advantages over other biophysical methods of probing conformation in biological systems. There are no real molecular weight limits; optical transparency or physical homogeneity of the sample is not required; sensitivity is much higher than, e.g., NMR, intramolecular distances can be assessed and the tools of molecular biology now allow one to virtually label any position in a protein or enzyme. This technology leaves great promise for future studies of proteins and enzymes.
5 14
REFERENCES 1. L.J. Berliner (ed.), Spin Labeling: Theory and Applications, Academic Press, New York, 1976. 2. L.J. Berliner (ed.), Spin Labeling 11: Theory and Applications, Academic Press, New York, 1979. 3. L.J. Berliner and J. Reuben (eds.), Spin Labeling: Theory and Applications, Biological Magnetic Resonance, Volume 8, Plenum., New York, 1989. L.J. Berliner and J. Reuben (eds.), Spin Labeling: The Next Millenium, Biological Magnetic Resonance, Volume 14, Plenum., New York, 1998. 5. C. Altenbach, T. Marti, H.G. Khorana and W.L. Hubbell. Science, 24 (1990) 1088. 6. L.J. Berliner, J. Grunwald, H.O. Hankovszky and K. Hideg, Anal. Biochem., 119 (1982) 450 7. J. B. Feix and C.S. Klug, Biol. Magn. Reson., 14 (1998) 251. 8. R.S. Keyes and A.M. Bobst, Biological Magnetic Resonance 14 (1998), 283. 9. V.V. Khramtsov and L.B. Volodarsky, Biological Magnetic Resonance 14 (1998), 109. 10. H. Fujii, and L.J. Berliner, Free Radical Research Commun. 21 (1994) 235. 11. H. Fujii, B. Zhao, J. Koscielniak and L.J. Berliner, Magn. Reson. Med., 3 1 (1994) 77. 12. L. J. Sammartano and D. Malejka-Giganti,Chem. Biol. Interact., 77 (1991) 63. 13. G. Minotti and S. D. Aust,. J. Biol. Chem., 262 (1987) 1098. 14. K. A. Lamping and G. J. Gross, J. Cardiovasc. Pharmacol., 7 (1985) 158. 15. V. Misik, A. Stasko D. Gergel and K. Ondrias, Molec. Pharmacol., 40 (1991) 435. 16. A. Stasko, V. Brezova, S. Biskupic, K. Ondrias and V. Misik, Free Rad. Biol. Med., 17 (1994) 545. 17. H. Fujii and L.J. Berliner, Magn. Reson. Med., 42 (1999) 691. 18. S. E. Thomas and M. L. Wood, Brit. Med. J. 292, (1986) 992. 19. B.M. Psaty, S.R. Heckbert, T.D. Koepsell, D.S. Siscovick, T.E. Raghunathan, N.S. Weiss, F.R. Rosendaal, R. N. Lemaitre, N.L. Smith, and P.W. Wahl, J. h e r . Med. Assoc., 274 (1995) 620. 20. B.M. Psaty, N.L. Smith, D.S. Siscovick, T.D. Koepsell, N.S. Weiss, S.R. Heckbert, R.N. Lemaitre, E.H. Wagner, and C.D. Furberg, J. Amer. Med. Assoc., 277 (1995) 739.
EPR in the 2 Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Recent progress and future prospects of free radical imaging by PEDRI David J. Luriea, Margaret A. Fostera, Wiwat Youngdeea, Valery V. Khramtsovb, Igor Grigor’evc aDepartment of Bio-Medical Physics and Bio-Engineering, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK* bInstitute of Chemical Kinetics and Combustion, Institutskaja 3, Novosibirsk 630090, Russia ‘Institute of Organic Chemistry, Lavrent’eva 9, Novosibirsk 630090, Russia PEDRI is a double-resonance method for imaging free radicals which relies on the Overhauser effect. A proton M R image of the sample is recorded while an EPR resonance of the free radical is irradiated. In this way high resolution images of the free radical distribution can be obtained. Field-cycled PEDRI improves the sensitivity and reduces the RF power deposition by switching the magnetic field during the pulse sequence. Applications of PEDRI and Field-cycled PEDRI include the pharmacokinetics of exogenous free radical contrast agents as well as the determination of local oxygen concentration and pH via their effect on exogenous agents. The detection and imaging of endogenous free radicals is more challenging, but may become feasible by virtue of improvements in techniques.
1. INTRODUCTION PEDRI (Proton-electron double-resonance imaging) is a technique for imaging free radicals in animals or biological samples which is based on the Overhauser effect [l].It offers high sensitivity and high spatial resolution which is independent of the linewidth of the paramagnetic sample under study. In addition, it can image free radicals in large animals, potentially even in humans.
1.1. The Overhauser effect In the Overhauser effect (also known as dynamic nuclear polarization, DNP), the signal of a solution is observed (often the proton NMR signal in water) while an EPR resonance of the free radical under study is irradiated. Provided there is efficient coupling (usually dipole-dipole) between the solute’s unpaired electrons and the solvent protons, a transfer of polarization from the unpaired electrons to the protons can occur, resulting in a change in amplitude of the observed NMR signal. This work was funded by the UK EPSRC and by INTAS (project 99-1086). WY received a studentship from the Thai government. We are grateful to Klaes Golman of Nycomed Innovation, Malmo, Sweden, for the kind gift of TAM radical.
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e T E P R RF
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0.02
0.04
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0.08
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-Av-
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Figure 1. NMR FIDs obtained at field strength of 10 mT. Sample was 2 mM aqueous solution of TEMPOL. (a) Without EPR irradiation; (b) with 1.6-watt EPR irradiation at 288 MHz; (c) with 10-watt EPR irradiation at 288 MHz.
Figure 2. Typical PEDRI pulse sequence. In this case a gradient-echo spin-warp pulse sequence is shown. It is preceded by a period of EPR irradiation, lasting approximately 3 times the NMR TI (typically, TEpR= 500 ms).
The enhancement factor is defined as the ratio ZdZo, where and are the measured NMR signals with and without EPR irradiation. The enhancement factor is given by:
where “/s and ‘/I are respectively the electron and proton gyromagnetic ratios, the absolute value of their ratio being equal to 658. p is the coupling factor (0 I p I l), which depends on the extent and nature of the coupling between the unpaired electrons and the protons. For dipole-dipole interactions, p = %. is the leakage factor (0 I f I l), which accounts for the fraction of nuclear relaxation caused by the presence in solution of the paramagnetic solute. In a concentrated free radical solution, with few competing relaxation mechanisms, will approach unity. The saturation factor, s (0 s 5 l), measures the amount of saturation of the EPR resonance. It depends on the unpaired electron’s relaxation times and on the strength (power) of the EPR irradiation. Under conditions of complete saturation (high irradiation power, or narrow EPR linewidth), s = 1. Finally, n is the number of lines in the free radical’s EPR spectrum. With stable nitroxide radicals, n = 3. Taking all the factors in equation (1) into account, the maximum achievable enhancement signal undergoes a phase reversal factor is = -329. The minus sign means that the upon EPR irradiation. With a nitroxide free radical, the presence of the hyperfine triplet reduces this to -1 10. In practice, however, complete saturation is not achieved. This, together with the relatively low concentration of free radicals in many experiments (f. 1) means that the observed enhancement factor is often no more than -10. Figure 1 shows NMR FID signals obtained from a nitroxide free radical solution at a field strength of 10 mT, without EPR irradiation and with irradiation at two different power levels. With 1.6-watt irradiation a reversal in phase of the NMR signal occurs, together with a 25% reduction in amplitude. This represents an enhancement factor of With an irradiation at 10-watt incident power an enhancement factor of -8 is obtained.
517
R F EPR
Y
.v E switched
Signal 1
Time (s)
Figure 3. NMR FIDs obtained 10 mT, showing build-up of Overhauser enhancement after switching on 288 MHz EPR irradiation. Sample was 2 mM TEMPOL solution. Solid line was calculated on the basis of a TI value
Polarization
I -
A
. . u n
Evolution
Detection
Figure 4. Field-cycled PEDRI pulse sequence. EPR irradiation is applied during the evolution period at field strength BoE.
of 500 ms.
2. TECHNIQUES 2.1. PEDRI PEDRI is essentially the imaging version of the DNP experiment [2]. Instead of just measuring the NMR signals, they are used to generate an image, by using pulsed magnetic field gradients in the usual way. EPR irradiation causes an enhancement in the NMR signal from parts of the sample containing free radicals, and these regions have different intensity in the final image, showing the distribution of the free radical. A typical PEDRI pulse sequence is shown in Figure 2. Sometimes an interleaved PEDRI pulse sequence is used to obtain images with and without EPR irradiation simultaneously: each line of k-space is recorded twice, once with and once without EPR irradiation. Subtraction of the with-EPR and withoutEPR data sets yields a 'difference' image showing only the distribution of the free radical under study. As seen in Figure 3, after switching on the EPR irradiation the DNP enhancement builds up with a time constant equal to the longitudinal NMR relaxation time TI. Therefore the duration TEPR of the EPR irradiation in a PEDRI experiment should be at least 3 x TI in order to achieve over 95% of the available enhancement.
2.2. Field-Cycled PEDRI (FC-PEDRI) PEDRI is normally implemented at low field (S20 mT) so that the EPR irradiation can penetrate into the sample and not cause excessive heating. Despite the use of low field strengths, excessive specific absorption rate ( S A R ) in biological samples may still be problematic. In principle extremely low field strengths mT) could be used, in an attempt to reduce the EPR irradiation frequency and hence lower the SAR. However, the use of such of the NMR low field strengths would severely compromise the signal-to-noise ratio experiment (despite the DNP enhancement). What is needed is to irradiate the EPR at very
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low magnetic field (hence low frequency and low SAR) and to observe the NMR signals at a much higher field (to preserve SNR). This conflict can be resolved by FC-PEDRI [3]. In an FC-PEDRI experiment the field strength Bo is switched between three levels during the pulse sequence, as shown in Figure 4. The pulse se uence commences with the . 3-5 mT) for the polarization period at BZ. The field is then reduced to BoB (typically evolution period, during which the EPR irradiation is applied at low frequency (-100 MHz), with correspondingly good penetration and low power deposition (SAR). The Overhauser enhancement occurs during the evolution period, affecting the proton magnetization. Next, the field is ramped up to the detection value, BoD, for the application of the NMR detection pulse(s) and imaging gradients. The SNR of the experiment is increased by virtue of the higher value of BoD.To ensure that as little enhancement as possible is lost, the time to ramp the field from BoE to BoDshould be shorter than the T I relaxation time of the sample. We have constructed an FC-PEDRI imager with a large field-cycling magnet [4] which makes use of the field-compensation method of field cycling. A human whole-body sized ferrite permanent magnet provides the vertically-oriented detection field (BoD) of 59 mT. A resistive magnet is mounted coaxially within the bore of the permanent magnet. When energised, it partially cancels the field from the permanent magnet, so that field cycling can be accomplished by controlling the current in the resistive magnet coil. With this system the magnetic field can be ramped from 5 mT to 59 mT (or vice-versa) in 40 ms. Provided coil noise is dominant over sample-induced noise, it can be shown that the and therefore the sensitivity and image quality, is proportional to the detection magnetic field strength BoD. For experiments with small animals (e.g. rats), this will hold true up to at least 0.5 tesla. This means that as high a detection field as possible should be used. Recent theoretical and experimental studies have shown that the optimum value of the evolution field BoE lies in the range 3 - 6mT, depending on the sample size, the applied EPR irradiation power and on the EPR linewidth of the free radical under study [ 5 ] . 2.3. Field-Cycled DNP spectroscopy In studies of free radicals in biological samples it is often useful to obtain information on the EPR spectrum of the paramagnetic species under study. In an FC-PEDRI system, this can be achieved by field-cycled DNP (FC-DNP) spectroscopy [6]. Figure 5 shows the FC-DNP pulse sequence. In order to obtain an Overhauser-detected EPR spectrum the pulse sequence is repeated a number of times, always with the same EPR irradiation frequency, but each time incrementing the value of the evolution field strength BoE. After each evolution period the magnetic field is ramped to BoD and an NMR detection pulse is applied to generate an FID, which is measured. When an EPR resonance is encountered the Overhauser effect causes an enhancement of the NMR signal and its amplitude is altered. A plot of NMR signal amplitude versus BoE shows the positions of the EPR resonances. The relative amplitudes of the peaks provide information on the EPR line intensities, as well as on the electron-proton coupling.
2.4. Rapid FC-PEDRI Even with field-cycling, it is still possible that excessive non-resonant power deposition may result from the EPR irradiation. This might occur, for example, when large animals are imaged (e.g. rabbits), because the SAR increases quadratically with the size of the conductive sample under study. In ‘conventional’ PEDRI or FC-PEDRI one period of EPR irradiation is applied per phase-encoding step (in other words, one EPR irradiation period per line of kspace). Another way of reducing the total energy deposited per is to reduce the number
519
4
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Figure 5. FC-DNP pulse sequence. It is repeated a number of times, incrementing B$ in order to obtain a DNP-detected EPR spectrum.
--
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Figure 6. Hybrid FISP FC-PEDRI pulse sequence for rapid imaging of free radicals.
of EPR irradiation periods necessary to record a free radical image. An extreme example of this was described by Puwanich et al. [7], where only one period of EPR irradiation is applied (at field BoE), followed by rapid acquisition of all the image data at field strength BoD. This ‘snapshot FC-PEDRI’ method was successful, but suffered from some image blurring and relatively low sensitivity, due to the decay of the Overhauser-enhanced proton magnetization during the data acquisition period. We have recently developed a new pulse sequence called Hybrid FISP FC-PEDRI [8]. The method lies somewhere between conventional FC-PEDRI (one EPR irradiation per line of kspace) and snapshot FC-PEDRI (one EPR irradiation per image). In the hybrid FISP FCPEDRI method k-space is subdivided into M segments, each of N lines, with M x N = n, the image matrix size. A period of EPR irradiation precedes the rapid collection of M lines of kspace. So, for example, with n = 64 we might use a pulse sequence with M = 2 and N = 32, i.e. 2 periods of EPR irradiation, each followed by the rapid application of 32 NMR detection pulses to acquire 32 lines of k-space. Figure 6 shows the pulse sequence. Experimentally, we have demonstrated that image quality is only slightly degraded relative to the conventional method when 4 periods of EPR irradiation are used per image.
3. APPLICATIONS 3.1. Imaging exogenous free radicals Perhaps the most straightforward application of PEDRI or FC-PEDRI is the imaging of exogenous free radical ‘contrast agents’ injected into living animals. A variety of experiments of this sort have been carried out. The first example, in 1990, involved the injection of a solution of Fremy’s salt into the peritoneal cavity of rats [9]. PEDRI images were recorded at a field strength of 6.8 mT with an EPR frequency of 197 MHz. Although Fremy’s salt has the benefit of narrow EPR lines which are easy to saturate, its toxicity is too high for it to be injected intravenously. However, it was found that the nitroxide free radical proxy1 carboxylic acid (PCA) could be administered in this way, allowing PEDRI images of PCA in the rat to be obtained [lo]. These experiments were carried out at 10 mT with EPR
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Figure 7. Projective coronal FC-PEDRI images of prone, anaesthetized adult Sprague-Dawley rat. Image on left was obtained before injection of radical. Image on right was obtained 1 min after injection of Nycomed OX063 TAM radical (0.5 mmol/kg). Sequence paramaters were: B$ = 4.25 mT, BOD= 59 mT, TR = 1200 ms, TEPR = 400 ms, FOV 120 mm, EPR irradiation at 120.7 MHz, 20-W incident power. irradiation at 237 MHz. Time series of PEDRI images allowed the pharmacokinetics of PCA in the rat to be studied, in order to elucidate its excretion mechanism fkom the kidney [l 11. While nitroxide fkee radicals such as PCA are non-toxic, they are not ideal for use with Overhauser detection methods because of their relatively wide EPR line-widths. Broader l i e s mean that more irradiation power must be used to achieve a given enhancement. the applied power will be limited by considerations, this means that the observed enhancement is reduced. Another disadvantage is the presence of the triplet of hyperfine lines in the nitroxide’s EPR spectrum, further reducing the enhancement. An ideal ‘contrast agent’ for PEDRI would have a single, narrow EPR line. The triaryl-methyl (TAM) compounds developed by Nycomed Innovations (Malmo, Sweden) fulfil this characteristic [12]. It has been used in experiments on anaesthetized rats at 9.5 mT with EPR irradiation at 263 MHz [131 and in mice at 20 mT with EPR irradiation at 564 MHz [141. TAM has also been used in FC-PEDRI experiments: Figure 7 shows FC-PEDRI images fkom our laboratory, of an anaeasthetized rat which had been given a dose of TAM radical through a jugular cannula. The image collected before injection of TAM shows very little contrast, but the later image shows clearly the distribution of the compound in the animal’s heart, lungs, blood vessels and kidneys.
3.2. Oximetry Images of the distribution of exogenous fkee radicals in the body can provide pharmacokinetic data by measuring the time course of the agent through different organs. In addition, it is possible to obtain information on other useful parameters fkom PEDRI or FCPEDRI images. One of these parameters is the concentration of dissolved oxygen. Because molecular oxygen is paramagnetic, it will broaden the EPR linewidth of a fi-ee radical contrast agent present in solution. In an Overhauser experiment the presence of oxygen in solution will reduce the observed enhancement, because the oxygen-broadened EPR lines will exhibit a lower degree of saturation at a given EPR irradiation power. Therefore, regions of the sample or animal with high oxygen concentration will exhibit lower enhancement values than regions with low oxygen concentration. The enhancement will also be influenced by the concentration of the fkee radical agent itself, and the main difficulty of oximetry by PEDRI is to separate out
52 1
Evolution Field (mT)
Figure 8. FC-DNP spectra of 15 ml 2.5 mM HMI solutions. Top: at pH 6.6, bottom: at pH 2.3. TR = 1200 TEPR = 400 17-W EPR irradiation at 121 MHz.
Evolution Field (mT)
Figure 9. FC-DNP spectrum of anaesth-etized 225-gram rat following gavage of 3 ml of 5 mM HMI into its stomach. TR = 1200 ms, TEPR = 400 ms, 20-W EPR irradiation at 120.7 MHz.
these effects. Golman et have suggested collecting PEDRI images at two different EPR irradiation power levels, together with an image obtained without EPR irradiation. An image collected with high EPR irradiation power, close to saturation of the electron spins, will be less affected by oxygen-induced changes in EPR linewidth than will an image obtained at a lower irradiation power. Pixel-by-pixel analysis of the data from the three images allows the calculation of separate radical concentration and oxygen concentration images.
3.3. pH measurement pH is another important physiological parameter which can affect the EPR spectrum of certain types of stable free radicals, in particular the imidazoline and imidazolidine nitroxides The hyperfine splitting in the EPR spectrum of these agents is affected by the local pH, and EPR spectroscopy and EPR imaging [18] studies on these agents have been carried out in recent years. We have recently conducted and FC-DNP and FC-PEDRI experiments using (HMI). The a variety of such agents, including range of pH which can be sensed using this agent is approximately 2 pH units, centred on its pK value of (the pK is the pH value at which half of the free radical molecules are (top) and protonated). Figure 8 shows FC-DNP spectra of aqueous solutions of HMI at pH pH 2.3, representing the two extremes of the range. At near-neutral pH this agent is unprotonated, and the splitting of the high and low-field lines is mT. At low pH the free radical is completely protonated and the splitting is reduced to mT. Anaesthetized adult Sprague-Dawley rats (body weight g) were given a gavage (intubation into the stomach) of a neutral-pH solution of HMI. Stomach acid caused a lowering of pH which was detectable by FC-DNP as a splitting of mT (Figure FCPEDRI was also used to confirm the position of the HMI solution in the animal’s stomach, as shown in Figure
522
Figure 10. Coronal projective FC-PEDRI images of supine, anaesthetized rat in vivo, following gavage of 3 ml of 5 mM HMI into its stomach. B," = 5.5 mT, FOV = 120 mm, TR = 1200 ms, TEPR = 400 ms. Lefk image obtained without EPR irradiation. Middle: image obtained with 20-W EPR irradiation at 120.7 M E . Right: 'difference' image, showing the HMI solution in the animal's stomach.
4. FUTURE PROSPECTS 4.1. Techniques The main impetus for the further development of PEDRI must lie in attempts to increase the sensitivity, to enable lower concentrations of free radicals to be detected and imaged. As mentioned above, in an FC-PEDRI experiment the SNR, and hence the sensitivity, is Hence, one way to determined largely by the strength of the detection magnetic field improve the sensitivity is to build an imager with a higher value of BoD. In our laboratory we are doing just this, and construction of a new instrument with a detection field of 0.5 tesla is underway. As with our existing imager [4], the new instrument will use a double magnet system. This time, however, the 0.5 tesla detection field will be generated by a whole-body sized superconducting magnet. Again, a resistive field-offset coil will be used, mounted coaxially inside the superconducting magnet. In order to avoid eddy-currents in the metallic structure of the superconducting magnet, the field-offset coil will employ an active shield, in a manner exactly analogous to that used for several years in actively-shielded gradient coils. Due to the high power-density of the resistive offset coil, the inner bore of our new imager (inside the shield and offset coils and the gradient and shim coils) will be only 12 cm, suitable for imaging rats and mice. Nevertheless, the instrument should exhibit an order of magnitude greater sensitivity than our existing 59-mT FC-PEDRI imager, and so should be able to detect free radicals in vivo at sub-micromolar concentrations. 4.2. Applications As with EPR imaging, the greatest challenge to PEDRI and FC-PEDRI is the imaging of naturally-occurring fiee radicals. Endogenous ftee radicals are involved in normal metabolism, and it is widely believed that changes in their concentration can occur a result of many types of disease. However, naturally-occurring radicals such as hydroxyl (.OH) or nitric oxide (.NO) are very short-lived in vivo, and normally exist at very low concentrations. Nevertheless, spin-trapping may be used to stabilize these moieties and, for example, spintrapped nitric oxide has already been imaged by EPR in mice administered with lipopolysaccharide to stimulate a septic-shock reaction [191. The improved sensitivity of our new imager mentioned in the previous section should allow similar studies to be achieved
523
using FC-PEDRI, with the benefit of the improved spatial resolution afforded by this method. In fact, the use of an exogenous spin-trap may not even be necessary, as *NO produced in the body can form long-lived paramagnetic compounds by complexing with haem proteins, and it has already been shown that these are detectable by FC-DNP and FC-PEDRI [20].
5. CONCLUSIONS It is encouraging to see that the number of research groups working on PEDRI and FCPEDRI is gradually increasing. One obstacle to the increase of activity in this area has been the non-availability of a commercial PEDRI or FC-PEDRI imaging system (although a FCPEDRI system was briefly available from the manufacturer Philips). However, virtually any resistive-magnet MRI system could be used for PEDRI, with the addition of relativelystraightforward hardware [ 141. Overhauser-based free radical imaging methods will always be complementary to the more conventional EPR imaging techniques (PEDRI cannot, for example, detect unpaired electrons in solid materials such as oxygen-sensitive lithium pthalocyanine crystds). On the other hand, PEDRI and FC-PEDRI do have the advantage that the spatial resolution is entirely determined by the NMR resonance, and is therefore independent of the EPR linewidth. Furthermore, PEDRI by definition produces a proton M R image of the sample or animal under study, and this can be very useful in determining the underlying anatomy in experiments. With improvements in technology, it is likely that the applications described above, namely pharmacokinetic, oximetric and pH-sensitive imaging, will become more widespread and these, together with the detection of spin-trapped endogenous radicals, will be of genuine use in biomedical research.
REFERENCES 1. D.J. Lurie, in: “In Vivo EPR (ESR): Theory and Applications”, Biological Magnetic Resonance, Vol. 18, ed: L.J. Berliner, Kluwer / Plenum, New York., In Press (2001). 2. D.J. Lurie, D.M. Bussell, L.H. Bell and J.R. Mallard, J. Magn. Reson., 76 (1988) 366. 3. D.J. Lurie, J.M.S. Hutchison, L.H. Bell, I. Nicholson, D.M. Bussell and J.R. Mallard, J. Magn. Reson., 84 (1989) 431. 4. D.J. Lurie, M.A. Foster, D. Yeung and J.M.S. Hutchison, Phys. Med. Biol., 43 (1998) 1877. 5 . W. Youngdee, P. Planincic and D.J. Lurie, Phys. Med. Biol., 46 (2001) 2531. 6. D.J. Lurie, I. Nicholson and J.R. Mallard, J. Magn. Reson., 95 (1991) 405. 7. P. Puwanich, D.J. Lurie and M.A. Foster, Phys. Med. Biol., 44 (1999) 2867. 8. W. Youngdee, D.J. Lurie and M.A. Foster, Proc. ISMRM 9th Scientific Meeting, Glasgow, (2001) 940. 9. D. Grucker, Magn. Reson. Med., 14 (1990) 140. 10. D.J. Lurie, I. Nicholson, M.A Foster. and J.R. Mallard, A333 (1990) 453. 11. I. Seimenis, M.A. Foster, D.J. Lurie, J.M.S. Hutchison, P.H. Whiting and S. Payne, Magn. Reson. Med., 37 (1997) 552. 12. J.H. Ardenkjaer-Larsen, I. Laursen, I. Leunbach, G. Ehnholm, L.G. Wistrand, J.S. Petersson and K. Golman, J. Magn. Reson., 133 (1998) 1.
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13. K. Golman, I. Leunbach, J.H. Ardenkjaer-Larsen, G.J. Enholm, L.-G. Wistrand, J.S. Peterson, A. Jiirvi and S. Vahasalo, Acta Radiologica, 39 (1998) 10. 14. D.J. Lurie, H. Li, S. Petryakov and J.L. Zweier, Magn. Reson. Med., In Press (2001). 15. Golman, J.S. Peterson, J.-H. Ardenkjaer-Larsen, I. Leunbach, L.-G. Wistrand, G. Ehnholm and K. Liu, J. Magn. Reson. Imaging, 12 (2000) 929. 16. V.V Khramtsov, LA. Grigor’ev, M.A. Foster, D.J. Lurie and I. Nicholson, Cell. Mol. Biol., 46 (2000) 1361. 17. B. Gallez, K. Mader and H.M. Swartz, Magn. Reson. Med., 36, (1996) 694. 18. A. Sotgiu, K. Mader, G. Placidi, S. Colacicchi, C.L. Ursini M. Alecci, Phys. Med. Biol., 43 (1998) 1921. 19. T. Yoshimura, H. Yokoyama, S. Fujii, F. Takayama, K. Oikawa, H. Kamada, Nature Biotech., 14 (1996) 992. 20. A. Mulsch, D.J. Lurie, I. Seimenis, B. Fichtlscherer and M.A. Foster, Free Radical Biology and Medicine, 27 (1999) 636.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
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Electron paramagnetic resonance in medicine R.Saifoutdinov Chair of Therapy of Medical Academy, 420012, Mushtary str.,l 1, Kazan, Republic of Tatarstan, Russia
The own results on electron paramagnetic resonance (EPR) investigation of biological fluids from the organism of normal individuals and patients with different diseases have been presented in this lecture. The EPR spectra of blood and blood components, gastric contents, synovial fluid and some other tissues and biological fluids have been considered. A qualitative and quantitative analysis of paramagnetic centers in the organism of volunteers and patients suffering from various diseases is presented in this lecture. Much attention is given to the use of the EPR method diagnostics, investigation of metabolism, free-radical reactions in the development of different pathologies.
1. PREFACE Pathologic states are caused by not only disorders of lipid, hydrocarbon, protein and mineral metabolism. Alterations in redox processes are also of importance. Paramagnetic centers present in many biological tissues and fluids can serve as indicators of these changes owing to the presence of free radicals and cations of transition microelements. These centers present in different organic and inorganic biomolecules. Electronic paramagnetic resonance spectroscopy presents an objective and accurate method of recording paramagnetic centers widely accepted in chemistry, biochemistry, biology and medicine [11. The EPR method was discovered in 1944 by Evgeny Konstantinovich Zavoisky in the city of Kazan. In the middle fifties, nearly simultaneously in the United States and the Soviet Union, EPR spectroscopy was first used for the investigation of tissues and fluids in human and animals. In that time, owing to low sensitivity of EPR spectrometers, the method of tissue lyophilic drying was used. The development of low-temperature registration of biological tissues in the early seventies extended the scope of application of EPR spectroscopy in biochemistry and medicine. By the present time a high sensitivity and precision of this method has been achieved, which allows investigation of even native water-containing tissues. 2. CONTENTS OF STOMACH AND DISEASES ONE
In the stomach contents the following paramagnetic centers have been found: a sixcomponent Mn2+EPR signal, a Hem-NO signal with line half-width 7.5 mT and maximal at
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g = 2.1,2.07,2.007 and 1.98 with a triplet structure and centers at g = 2.007 and splitting of 1.5-1.6 mT [2]. The application of EPR spectroscopy Hem-NO studies has been considered in a review [3]. The Hem-NO signal was fixed in tumor cells and in the blood of animals exposed to nitrites and other toxic compounds [4]. This signal was also present in the spectra tumors of human liver, large intestine and stomach. It is most often found in necrotic area of tumor 151. The nitrosyl complexes can form cytochromes, peroxydases, catalases and other enzymes. The signal shape is specific for each type of hemoproteins which allows them to be distinguished. For instance, a Hem-NO signal was recorded in the EPR spectrum of human hemoglobin [6]. In the stomach contents, in the presence of hem-containing groups (hemoglobin, myoglobin, catalase, cytochromes, Hem-NO can form nitrates or nitrites, nitrate- and nitrite-reducing enzymes or microorganisms containing the above enzymes. Hem-NO also appears at pH above 3.0, under reduced acidity, which promotes the development of microflora. Maximum Hem-ON concentration is recorded in the sample taken from the empty stomach. Thus, it is likely that nitrate- and nitrite-reducing microorganisms act as NO-transporters to the human stomach. There is a close relation between the Hem-NO level and the content of microorganisms in the stomach. From the empty stomach portion the following bacteria were sown out in concentrations ranging from lo4 to lo7 cells in 1 ml: Thus, Hem-NO appears in the stomach contents owing to the reproduction of nitrate- and nitrite-reducing microorganisms which form NO bound to hemcontainin groups [2]. The Mn$+ signal having a characteristic six-component superfine structure can be detected in the spectrum of stomach contents only in the presence of duodenal-gastric reflux [7]. The development of gastric and duodenal chronic diseases is accompanied by some changes in the host paramagnetic centers content. In the stomach contents of patients with gastritis, gastric and duodenal ulcer EPR Hem-NO signal was observed. Its concentration in healthy individuals is higher than in patients with chronic gastritis and peptic ulcer (71 and 66%, respectively) [8]. Hem-NO exerts an activating effect on guanylate cyclase thus increasing tissue c-GMP and stimulating cell proliferation. This supports the gastric wall integrity and prevents ulceration [9]. C-GMP inhibits thrombocyte aggregation and produces a vasodilatory effect. The above diseases are characterized by a reduced blood flow and the appearance of thrombocyte aggregations. Certain amount of carcinogenic nitro- and nitroso-compounds are brought into the gastro-intestinal tract with food. The activity of nitrate- and nitritereducing microorganisms is most likely to be compensatory reaction preventing the development of oncologic diseases. Malignant tumors are often met in patients with chronic gastritis and peptic ulcers. This suggests Hem-NO to be a compulsory component of gastric contents. Hem-NO is supposed to be involved in the development of chronic gastritis and peptic ulcer by the following pathogenic mechanism. Changes in the stomach microflora, in particular, a decrease in the content of nitrate- and nitrite-containing microorganisms leads
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to a drop of the Hem-NO level [lo]. This inhibits cell proliferation, decreases mucous wall restoration, strengthens thrombocyte aggregation and reduces vascular tonus regulation.
3. SYNOVIAL FLUID AND DISEASES OF JOINTS Synovial fluid resulted mainly from plasma dialysis acts as synovia. Only small and, on rare occasions, slightly larger size molecules penetrate synovial fluid due to gel-filtration through sinovial cells. The synovial fluid viscosity is ensured by hyaluronic acid. The major synovial fluid properties are dependent on various pathological conditions. This allows differentiation of dystrophic and pathologic diseases of joints. Investigation of the synovial fluid composition provides an informative method in the diagnostics of various diseases of joints. In the synovial fluid of patients with rheumatoid arthritis and deforming osteoarthrosis paramagnetic centers of three es were observed. They correspond to the following EPR signals: g = 4.3 (fe3'), 2.05 (Cu ) and free radical with g 2.0024-2.0029, AH 0.6-0.8 mT of unidentified nature [l 11. It is suggested that the g = 4.3 signal is caused by transferrin Fe3+since the g factor value and form (characteristic splitting in the upper maximum and a jag on the descending line) are coincident with the Fe3+ spectrum. The g = 2.05 signal is most likely to correspond-to Cu2' atoms incorporated into a protein of ceruloplasmin type. This suggestion was confirmed using an EPR spectrum of rheumatoid synovial fluid (1 ml) after addition of 0.2 ml of 0.01M FeS04 solution. It turned out that the amplitude of the EPR signal at g = 4.3 having a characteristic spectral form of three valence iron on transferrin had increased more than ten-fold, whereas that of the signal at g = 2.05 decreased 4 times. This shows that Fe2+is oxidized to Fe3+by Cu2' of the rheumatoid synovial fluid. Besides, this is also indicative of the presence in the fluid of a considerable amount of protein of transferrin type unbound to iron. Quite a different picture is observed in the rheumatoid synovial fluid (1 ml) drawn from deforming osteoarthrosis patients. The addition of 0.2 ml of 0.01M FeS04 solution did not cause such an intensive rise of the Fe3+level. This indicates either poor ferroxidase activity of the fluid or the presence of a decreased number of transferrin molecules, or saturation with Fe3+[ 111. In rheumatoid arthritis patients the synovial fluid Fe3+content is four times lower than in deforming osteoarthrosis patients. The Cu" content inversely changes in these diseases [121. Table 1 Synovial fluid paramagnetic centers content in rheumatoid arthritis (RA) and deforming osteoarthrosis (DOA) patients (M*m) RA DOA Indexes Fe", (mcmol/l) 4.5k0.38 13.5k0.86 cu2+,( l o 5 M) 2.24k0.11 1.20+0.10 FR, (10-6M) 0.5k0.03 0.4k0.03
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The analysis of synovial fluid of 18 rheumatoid arthritis patients was carried out on admission and after 3 weeks of stationary treatment. The positive dynamics in the patient state was characterized by a statistically reliable decrease in the Cu2+level and an increase in the Fe3+signal amplitude in EPR spectra [121. 4. FECES The EPR spectra of normal human feces display the following signals: at g = 6.0 corresponding to Fe3+of MetHb and its derivatives, at g = 4.3 arising from low-spin Fe3+, Cu2+ with characteristic SFS, a six-component spectrum of Mn", Hem-NO, and free radicals [7]. To elucidate the source of paramagnetic centers in feces the large intestine microorganisms were isolated. In feces anaerobes of the Bacteroides genus as well E.coli and Citobacter aerobes were mainly found. In the EPR spectra of anaerobes the Mn2+signal is fixed whereas Fe3+signal is absent. In anaerobes, on the contrary, the Mn2' content is very low (Mn2' is observed only under maximum amplification of the spectrometer), the Fe3+content being high. Colon bacillus (E.coli) contains significant amount of Fe in both heme and non-heme forms. Recombinant human microsomal heme oxygenase-2 contained in coIi was studied using EPR spectroscopy. At pH = 7 the ferric heme is six coordinate high-spin and six coordinate low-spin at pH > 7 (p& = 8.5). The reaction with hydrogen peroxide converts the heme of the heme oxygenase-2 fragment complex into a verdoheme-like product. The oxidation of 3-chloroperbenzoic acid leads to an oxofenyl derivative. As the spectroscopic properties of heme oxygenase-1 and heme oxygenase-2 are similar their catalytical mechanism seems to be identical [131. Thus, in the EPR spectrum of feces main contribution to the Mn" signal is made by anaerobes, whereas aerobes are responsible for the Fe3+ signal. It should be noted that anaerobes show higher FR concentrations [ 141. In feces Hem-NO is formed of nitrates, nitrites and heme brought to the intestine with food, water, bile and saliva. In the intestine there are nitrate- and nitrite-reducing microorganisms. In diarrhea clearly defined signals at g = 6.0 and Hem-NO appear in the EPR spectrum the Cu2' and Mn2+contents are decreased. This seems to be related to changes of the intestinal microflora and increased water content in feces [12].
5. PERITONITIS
Peritonitis is peritonium inflammation, the morbidity and death rate of which remain very high. In surgery, there is no consensus of opinion on the mechanism of pathologic reactions and the cause of endogenous intoxication in peritonitis. The EPR method was used to study paramagnetic centers in blood, urine, peritoneal exudate, intestinal contents of 61 patients with general purulent peritonitis. The vast majority of these was represented by men (71%). In these patients, peritonitis was caused by
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surgical invasion and traumas of peritoneal cavity organs. In 16% of the patients an intermediate, severe and extremely severe course of peritonitis was observed [151. For EPR examination, the patients' blood was taken from the subclavian vein and centrifuged (2000 rev/min, +4OC). Peritoneal exudate was withdrawn from drainages installed during surgery in the subdiaphragmatic and subhepatic spaces and in small pelvis. The intestinal content was taken by means of stable vasointestinal probe of Miller-Ebbot type. Blood, peritoneal exudate, gastrointestinal tract contents were taken three times every 48 hours. Blood and urine nitrate levels were determined by the photoelectrocolorimetry technique [161. In postoperational peritonic patients the samples of blood, peritoneal exudate and intestinal contents were taken every 24, 48 and 72 hours. It was found, on average: (26%), (23%), (12%), (14%), (12%). Simultaneously, the nitrate-reducing capability of peritoneal and intestinal microorganisms was investigated by EPR spectroscopy. Most gastro-intestinal microflora are able to reduce nitrates to nitrites. This process is initiated by a nitrate reductase enzyme inhabiting these microorganisms. Nitrites, in turn, react with blood hemoglobin to form methemoglobin. The study of 9 species of the above bacteria has shown 7 of these (80%) to exhibit nitrate-reducing capability [151. As shown by EPR spectroscopy, erythrocyte methemoglobin content in peritonic patients depends on the duration and severity of disease. 20 volunteers served as donors (methemoglobin concentration is 0.6kO.1 dl). The highest methemoglobin concentration is observed on the 10-1lth day after the onset of disease. In peritonic patients with extremely severe course of disease the methemoglobin level is increased with the very first days and further reaches the highest values [161. In donor plasma no methemoglobin is fixed. In blood plasma of peritonic patients the methemoglobin level depends on the severity of state. The presence of methemoglobin in patients plasma may be due to its absorption from the abdominal cavity by lymphatic vias. A linear correlation (r = +0.4) between methemoglobin levels in plasma and peritoneal exudate is observed [17]. The highest plasma methemoglobin concentration in peritonic patients appears on the 2 - 5 ~ day and later it is decreased. This may be explained by the fact that during this time purulent exudate character, its biochemical composition, concentration and microflora virulence point out to a clearly-expressed peritonitis. Methemoglobin and Hem-NO have been detected in peritoneal exudate by the EPR method. In the course of treatment and abdominal cavity assanation, MetHb inestinal content is decreasing independently of the severity and duration of peritonitis. The highest peritoneal exudate MetHb level is observed in peritonic patients in extremely severe state. This is explained by exudate character and contamination degree. A Hem-NO signal, detectable in the peritoneal exudate and intestinal content EPR spectrum, corresponds to the hemoglobin-nitric monoxidecomplex. Hemoglobin oxidation to Hem-NO occurs in the following manner. Interacting with blood, nitrites find access into erythrocytes and get involved in the reaction with hemoglobin. In the results redox reactions desoxihemoglobin is oxidized to methemoglobin, the nitrite ions being reduced to nitric oxide. Hb2++NO, +2H -4MtHb +NO + H,O
Nitric oxide reacts with the reduced hemoglobin to form stable Hem-NO complexes [16]. The Hem-NO signal was detected in peritoneal exudate EPR spectrum in 20% of patients. In 75% of the patients the hem-NO signal was detected in the exudate displaying a clearlyexpressed hemorrhagic component. In patients with extremely severe peritonitis the HemNO signal appears in the spectrum late in the disease [181. On the 2-3d day after the onset of disease, the intestinal methemoglobin concentration in peritonic patients does not differ much depending on the severity of illness and decreased by the 4-5th day. This due to intestinal probe evacuation of congested intestinal contents, detoxication measures and correcting therapy [191. In patients with severe and extremely severe peritonitis the methemoglobin level increases proportionally with time. This is related to intestinal trophic and motor changes, which amplify fermentation, ammonia formation, intestine mucus necrotic changes accompanied by hemorrhage, growth of microflora showing nitrate reductase capability. In peritonitis the Hem-NO signal detected in the intestinal EPR spectrum does not change significantly in the first 1-3 days independently of the severity of illness. On the 4-5th day the Hem-NO level decreases in patients with intermediate and severe peritonitis and increases in patients in the extremely heavy state. On the 6'h or 7'h day the patients with severe peritonitis show a tendency towards decreasing the intestinal Hem-NO level. The same is observed for next 2-3 days. In patients with extremely severe peritonitis, the intestinal Hem-NO concentration is increasing in the period from 6 to 11 days since the onset of disease and the tendency towards decreasing is observed only on the 12" day [19]. Comparison of time-dependent peritoneal exudate methemoglobin level to the intestinal one allows a conclusion that the former exhibits a tendency to decrease, whereas the latter is increasing. These suggests that the paretically affected intestine and its contents, and not the peritoneal exudate, for a long time remains the main source of intoxication [20]. In peritonic patients the paretically affected intestine provides favorable conditions for extensive growth of microorganisms and alteration of necrotic cells due to harmful agents and disorder in blood circulation. The formation of methemoglobin enhances tissue hypoxia. The iron-containing enzymes present in tissues are bound to nitric oxide and become switched out of the respiratory system. This intensifies disorder in intracellular metabolism and may lead to the formation of new low molecular nitrogen compounds, including nitric oxide. Nitric oxide contributes to microcirculation disorders and thrombosis followed by trophic alterations in tissues. Nitric oxide acts as a secondary mediator in intracellular signaling system owing to its effect on Ca-mobilizing cell system. This property of subcellular structures may be of special value in the development of pathologic processes occurring on the background of ischemia and tissue hypoxia. This also results in increased NO binding by heme and non-heme iron with the formation of methemoglobin and Hem-NO. In nitrite methemoglobinemia-induced tissue hypoxia, not only globulin transport h c t i o n s are broken, but cellular iron-containing enzymes are blocked. According to modern concepts, lipopolysaccharides of bacterial origin interact with neutrophils and macrophagues thus inducing accumulation of free radical oxygen forms. Free radical-mediated oxidative processes facilitate the formation of nitric oxide and ammonia conversion to nitrites and nitrates. Nitric oxide is the intermediate of these processes.
1
Microflora of the host organism plays an important role in nitrate metabolism. This is mainly due to nitrate reductase activity and the ability to synthesize nitrites and nitrates from ammonia. Investigation of methemoglobin and Hem-NO in biological fluids of patients with peritonitis reveals wide possibilities of this method in clearing up pathogenic aspects in the course of the disease at a molecular level. These results suggest the study of paramagnetic centers using EPR spectroscopy to be of value as supplementary diagnostic tests as well as criteria in assessment of the severity of clinical course of many diseases and the efficiency of treatment. Summing up the above said the EPR method is suggested to extend our knowledge of the role of transition microelements in vital activities of man, to provide an understanding of thin mechanisms of pathogenic processes of various diseases and to offers a great promise for new therapeutic and prophylactic measures. This all demonstrates the unlimited possibilities of the use of EPR spectroscopy in biochemistry and medicine.
REFERENCES 1. D.J.E. Ingram (ed.), Biological and Biochemical Applications of Electron Spin Resonance, Adam Hilder LTD, London, 1969. 2. R.G. Saifoutdinov (ed.), Paramagnetic Centers in human biological fluids and their diagnostic and pathogenetic role in some internal diseases. Paramagnitnye tsentry biologicheskikh zhidkostei cheloveka i ikh diagnosticheskaya i patogeneticheskaya rol' pri nekotorykh zabolevaniyakh vnutrennikh organov, Thesis (Doct. biolog. sci.) Tomsk, 1989 (Russ.). 3. G. Bemski, Mol. Biol. Rep., 24 (1997) 263. 4. A.F. Vanin, L.V. Vakhnina, A.G. Chetverikov, Biophysics, 15 (1970) 1044. 5. M.C. Symons, I.J. Rowland, N. Deighton, K. Shorrock, K.P. West, Free Radic. Res., 21 (1994) 197. 6. H. Rein, 0. Ristau, W. Sheler, FEBS Letts, 24 (1972) 24. 7. K.R. Sedov, R.G. Saifoutdinov (eds.), Electron Paramagnetic Resonance in internal diseases clinics, Irkutskii dom pechati, Irkutsk, 1993 (Russ.). 8. R.G. Saifoutdinov, EPRNewslette, 4 (1992) 3. 9. Wennmalm, B. Lame, A.S. Petersson, Analytical Biochem., 187 (1990) 359. 10. V.P. Kryshen, M.F. Nesterova, Vracheb. Delo, 9 (1982) 16 (Russ.). 11. R.G. Saifoutdinov, E.V. Popova, Yu.A. Goryaev, EPR Newsletter, 3 (1991) 6. 12. R.G.Saifoutdinov, L.I.Larina, T.I.Vakul'skaya, M.G.Voronkov (eds.), Electron Paramagnetic Resonance in Biochemistry and Medicine, Kluwer Academic/Plenum Publishers, New York, 2001. 13. Ishikawa, N. Takeuchi, S. Takahashi, K.M. Matera, M. Sato, S. Shibahara, D.L. Rousseau, M. Ikeda-Saito, T. Yoshida, J. Biol. Chem. 270 (1995) 6345. 14. R.G. Saifoutdinov, International Conference on Bioradicals Detected by ESR Spectroscopy; Yamagata, 1994. 15. L.A. Sadokhina (ed.), A study of NO endogenous synthesis markers in diffuse abscess peritonitis, Thesis Ph.D. Irkutsk, Medical University, 1998 (Russ.).
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16. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev, Russian journal of gastroenterology, hepatology and coloproctology, 6 (1996) 60 (Russ.). 17. R.G. Saifoutdinov, L.A. Sadokhina, Russian journal of gastroenterology, 4 (1997) 131 (Russ.). 18. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev Investigated of paramagnetic centres of the intestinal content by the method of ESR at the patients with the peritonitis. The ninth annual conference of the international society for Environmental Epidemiology; 1997 August 17-20; Taipei. Academia Sinica International Center, Taiwan, Republic of China, 1997. 19. R.G. Saifoutdinov, L.A. Sadochina, E.G. Grigorjev, J. Inorganic Biochem., 1997. 20. R.G. Saifoutdinov, L.A. Sadokhina, E.G. Grigorjev, An EPR study of MetHb intestinalcontent in peritonitic patients. International Workshop Fal'ka; 1996 June 20; SanktPeterburg. Sankt-Peterburg, (1996) 72.
EPR in the 2 1* Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
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Development of in vivo ESWspin probe technique for oxidative injuries H. Utsumi, J.-Y. Hana, and K. Takeshitab Graduate School of Pharmaceutical Sciences, Kyushu University, Higashi-ku, Fukuoka 8 12-8582, Japan Present address; aFaculty of Pharmaceutical Sciences, Seoul National University, Seoul, Korea and bBioregulation Research Group, National Institute of Radiological Sciences, Chiba 263-8555, Japan
Free radicals are reportedly involved in various oxidative diseases, but the kind, amount, and origin of radical generation have yet to be clarified. Recent development of an free radical ESR spectrometer enables us non-invasive and direct measurement of reactions in living organism. This non-invasive method can be utilized widely for animal disease model to investigate the mechanism of oxidative injuries and the effect of antioxidant drugs. In the present paper, DEP-induced lung injury is precisely introduced to demonstrate ESWspin probe technique for non-invasive evaluation of ROS generation.
1. INTRODUCTION Free radicals such as reactive oxygen species (ROS) and Reactive nitrogen species (RNS) are believed to be very essential and functional compounds in various biological systems [ 13. Free radicals are also reportedly involved in various diseases involving, but the kind, amount, and origin of radical generation have yet to be clarified. Non-invasive measurement of free radical reactions is very important to understand the role of free radicals in diseases and to evaluate antioxidant activity, since oxygen concentration in tissues is much lower than that in experiments and there are many substances and reaction pathways, which influence radical reactions.
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Recently, ESR spectroscopy has developed and enabled non-invasive measurement of radicals in whole animals [2-61. Figure I demonstrates our ESR-CT system. This non-invasive method can be utilized widely for animal disease model to investigate the mechanism of oxidative injuries and the effect of antioxidant drugs. As shown in Figure 1 b, combination of the ESR measurement with MRI makes possible to visualize radical distribution and degree of free radical reaction in living organism. The sensitivity of an ESR spectrometer is unfortunately insufficient to detect trace amount of endogenous radicals. Thus, we utilize nitroxyl radicals as a probe for free radical reaction and reactive oxygen generation. The nitroxyl probe is reported to react with OH radical, causing loss of the paramagnetism of the probe [7-91. We have used this character of nitroxyl radicals to analyze free radical reactions and reactive oxygen species (ROS) generation in experimental animal disease model, including hyperoxia [ 10,1I], muscular ischemia-reperfusion [ 121, streptozotocin-induced diabetes [ 131, liver damage induced by iron-overload [ 141, diesel-exhaust-particles (DEP) induced lung injuries [ 151, etc. In the present paper, DEP-induced lung injury is precisely introduced to demonstrate ESWspin probe technique for non-invasive evaluation of ROS generation. DEP are one of the main air pollutants in urban areas, and they are increasing in quantity because of the rising number of diesel-engine-powered cars. DEP are composed of carbon nuclei, many adsorbed organic compounds, and trace heavy metals, including iron and copper [16]. DEP have been reported to reach the alveoli pulmonis easily during inhalation and to cause pulmonary tumors, fibrosis, and edematous change in experimental animals [17, 181. Thus, air pollution by DEP is a cause for serious concern about our health, and the toxicological mechanism of DEP should be clarified as soon as possible to reduce the risk of DEP injuries. In order to provide the first direct evidence of ROS generation in the lung after exposure to DEP and to clarify the mechanism of DEP toxicity, combined technique of ESR spectroscopy with a nitroxyl probe was applied to mice treated with DEP.
Figure 1.
ESR-CT system (a) and combined system of ESRI with MRI (b)
2. EXPERIMENTS Experimental procedure is demonstrated in Figure 2. As a spin probe, 4- trimethyl(CAT-I) was used for this experiment because of its membrane-impermeability. DEP were the kind gift of Dr. M. Sagai (National Institute for Environmental Studies, Japan). DEP were suspended in 50 mM phosphate-buffered saline (pH 7.4, PBS) containing 0.05 % Tween 80 and sonicated for 3 min using a bath-type ultrasonic disrupter. Mice (ddY, female, 2.5 weeks old) were administered with SO pL of DEP suspension through an intratracheal cannula into the lungs of mice under anesthesia with 4% halothane. Vehicle-treated animals received the same volume of PBS containing 0.05 % Tween 80. One day after DEP and vehicle challenges, mice were anesthetized with intramuscular injection of 1.8 g / k g b.w. of urethane (Aldrich Chemical Co., Inc.). After 100 pL of a sterilized solution of 10 mM CAT-1 (Molecular Probes Inc.) in PBS was administered through an intratracheal
cannula into the lungs of the mice under anesthesia with urethane. L-band ESR spectra were immediately recorded at mid-thorax of the mice with a JEOL JES PE-1X spectrometer equipped with a L-band unit and a loop-gap resonator (33 mm i.d., 5 mm long). The microwave frequency was 1.1 GHz, and the power was 5.0 mW. The amplitude of the 100-kHz field modulation was 0.16 mT. The external magnetic field was swept between 40 and 45 mT at a scan rate of 5 mT/min. Experimental Method
(1.8mglgb.w.: i.m.)
14 V
InlmlrBCheal administration
cimrn
CH.
CAT-1
Figure 2. Experimental procedure for DEP-induced lung injuries
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3. RESULTS An aqueous solution of CAT-1 was intratracheally administered into the lung of mice treated with DEP one day before ESR measurement, and then L-band ESR spectra of CAT-I were recorded at the mid-thorax of living mice. Figure 3 shows a typical L-band ESR spectrum of CAT- 1 recorded at the mid-thorax of a mouse treated with 0.05 mg of DEP. The spectra consist of narrow triplet lines with 1.67 mT of hyperfine splitting constant, indicating that CAT-I is freely tumbling in a hydrophilic atmosphere in the lung. The ESR signal decreased gradually with time in the living mice. A semi logarithmic plot of the signal intensity against time showed a linear relation (Figure indicating the first-order reaction kinetics for the signal reduction of nitroxyl probe in the lung. The signal decay in a DEP-treated mouse was clearly faster than that in the vehicle-treated one, while DEP themselves induced no decay of the CAT-1 signal (data not shown). The enhancement of the signal decay in the mice occurred in a DEP dose-dependent manner, and a significant difference (p 140 GHz) of transition-metal ions Mn" (S=5/2), Fe3' (S=5/2), and Ni2+ (S=l), with spin S > %, are discussed to demonstrate the virtues of very high frequency EPR. It is shown that a considerable simplification of the spectra, and thus their interpretation, is achieved in the high-field limit even for large zero-field splittings. On the other hand, broadening of lines at higher frequencies may hinder their observation. Simulated spectra for various spin systems at Xband (-9.5 GHz) and VHF (-249.9 GHz), will be shown. Two important special cases: (i) EPR of non-Kramers ions with integral spins at various frequencies, and (ii) the advantage of using multi-frequency approach in estimating SHP, are highlighted, in particular. Proposed VHF EPR study of single crystals and powder samples of four metalloproteins of biological interest is presented. The details of the iron centres involved in these metalloproteins, namely, non-heme mononuclear iron, Fe(II), 2Fe-2S cluster, diiron centre, and mononuclear Fe(I1) are provided.
1. INTRODUCTION Recently, there has been a resurgence of activity in very-high-frequency (VHF > 140 GHz) EPR due to advances in magnet and millimeter-wave technology [I]. However, there have been reported very few cases of single-crystal studies at VHF in contrast to those involving powders. In addition, most VHF research has involved organic systems in amorphous states, for which preparation of single crystals is not easy. In this paper are described advantages of using single-crystal EPR over powder EPR. The virtues, as well as disadvantages, of VHF EPR are illustrated by simulations and use of X-band EPR results for three ions, including both Kramers and non-Kramers ones.
2. SINGLE CRYSTAL VERSUS POWDER (POLYCRYSTALLINE) EPR Single-crystal EPR has distinct advantages over powder EPR. (i) It enables a more precise determination of spin-Hamiltonian (SH) parameters [ 1,2]. This is because there are available many more EPR line positions by rotation of a single crystal with respect to the external magnetic field (B) to fit to SH parameters [2], unlike that for a powder (polycrystalline) sample for which one only observes broad averages over all orientations at each field value with a concomitant loss of spectral resolution. (ii) Single-crystal EPR lines are much narrower than those of powder lines, which can be very broad, e.g. in the range of
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m
1.0-
crystal
v
5
0.50.0-
5
I
-1.0
7J
.-
Magnetic field (kG)
Figure la. Simulated single-crystal spectrum for Mn" in ZnzVz07 at 249.9 GHz.
.-
LI
Magnetic field (kG)
Figure Ib. Simulated powder spectrum for MnZ+in ZnzVz07 at 249.9 GHz.
0.1 -lT in the presence of either large zero field splittings (zfs) and/or large g-tensor strain or anisotropy [3-51. This reduces seriously the signal-to-noise ratio of a powder spectrum over that of a single-crystal. (iii) The resonant susceptibility of a single-crystal sample is enhanced by the cooperative effect of many spins at a given orientation intensifying the transition amplitude of a single line as opposed to a powder sample, where the absorption is averaged over all orientations as noted above, and partitioning of spectral intensity among all transitions reduces the achievable signal-to-noise ratio for any one transition. (iv) Single crystals have a higher effective spin concentration of the crystalline lattice as opposed to that in the looser packing of polycrystalline powders. The effects of (ii) - (iv) can be illustrated by simulating the single-crystal fine-structure spectrum, as shown in Figure la, and then comparing it with the powder simulation, as shown in Figure 1 b for Mn2+(electron spin S=5/2) in ZnVz07 using the parameters listed in Section 5.2. It is seen from Figures la and lb that the powder spectrum is generally much broader, thereby reducing both signal sensitivity and spectral resolution. In particular, whereas all 5 fine-structure lines for MnZ+are prominent, and of comparable amplitude in the single-crystal spectrum, only the central line (-1/2 1/2) is not severely broadened in the powder. The non-central transitions in the latter are only a few percent of the amplitude of the central line which itself is less intense by about a factor of 3 than the single-crystal line. Given the typical expected signal-to-noise ratio of the experiments, the non-central transitions fine-structure transitions would not be observable unless there exist very high spin concentrations as those in neat samples, rendering it impossible to determine the zero-field splitting. This observation is consistent with previous studies on powder samples of MnZ+at 250 GHz [3,4].
3. VIRTUES OF VHF EPR 3.1. Determination of large zero-field splittings One requires to use an appropriately high frequency to observe the allowed fine-structure (AM = +1) transitions for the case of large zfs. For example, Mn3+was considered to be
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1
3
4
5
Magnetic field (T)
Figure 2. A plot of the eigenvalues of the Kramers Fe3+ion in YCaA104, calculated using the indicated parameters, for values of B up to 5T showing all expected AM = 1 transitions at X (9.79 GHz) and Q (35.69 GHz) bands. The inset shows the eigenvalue plot for values of B up to 15T showing all expected AM = 1 transitions at 249.9 GHz.
“EPR silent” due to its large zfs, but its EPR signal was recently reported at VHF by Goldberg et al. [5]. High-frequency EPR is especially suitable to measure large zfs, which is more difficult at low frequencies. It is noted that at low frequencies, it may be possible to observe the forbidden AM = 0 transitions with the use of a dual-mode cavity [6]; however, being forbidden, they have much smaller intensities compared to those of AM = 1 transitions. 3.1.1. Case of Kramers ions (half-odd integral spins) For the Kramers case of the Fe3+ion in YCaA104, this can be seen in Figure 2, which shows the energy level scheme as well as those AM = 1 transitions that can be observed at 9.5 GHz. However, one notes from Figure 3, showing the angular variation of EPR line positions that it is possible to observe all possible AM = 1 transitions at 249.9 GHz with an external magnetic field that can be varied up to about 13T. At least one non-central AM = 1 transition, that is other than -1/2 %, is required to estimate the values of the zfs parameters accurately. In addition, when one tries to determine the absolute sign of D, and thus those of all the spin- Hamiltonian parameters since the least-squares fittingmatrix-diagonalization
734
h
v v)
Q) ii=
8
200-
Q
8 100-
-120 -90
-60 -30
0
60
90
120 150 180 210 240
Orientation of magnetic field (degrees) Figure 3. Angular variation of Fe3+ EPR line positions at 9.79 GHz in YCaA104 single crystal using the spin-Hamiltonian parameters listed in Section 5.3; the insets show angular variations at 35.69 and 249.9 GHz. procedure yields correct relative signs of all the parameters [2], this requires comparing the intensities of the two extreme lines belonging to the whole set of AM = 1 transitions 3.1.2. Case of Non-Kramers ions (integral spins) The result for the non-Kramers ions NiZ' (S=l) is shown in Figure 4, showing the energy levels versus the magnetic field. One can observe AM = k 1 transitions at X-band values up to about 40 GHz, from which D may be extracted from a rotational set of spectra. On the other hand, when D is significantly larger than 40 GHz, VHF EPR becomes essential, as was the case for another non-Kramers ion Mn3+with S = 2 ( D 66 GHz) [5].
-
3.2. Simplification of EPR spectrum at VHF At high-enough frequencies where the Zeeman interaction dominates, the EPR spectra are considerably simplified [3,4]. This will be seen in the spectra simulated in Section 5. 3.3. More accurate determination of g-tensor More precise determination of g-tensor is obtainable at VHF due to increased size of the Zeeman term in the spin Hamiltonian with respect to the ZFS terms. 3.4. Determination of absolute sign of Spin-Hamiltonian parameters Another virtue of single crystal studies is the determination of the absolute sign of spin-
735
100
1
.
.
.
.
.
. ' .
t
L
0 0 0.2 0.4 0.6 0 8 1.0 1.2 1.4 1.6
D=-30.97 GHz : dotted lines (site II)
- 249.9 GHz I
I
I
4
6
Magnetic field (T)
I
I
a
Figure 4. A plot of the eigenvalues of the Ni*' ion in NizCdCls . 12Hz0, calculated using the indicated parameters, versus the intensity of the exernal magnetic field (B) for values of B up to 11 T showing all expected AM = 1 transitions at 249.9 GHz. The inset shows the eigenvalues versus B plot for values of B up to 1.5 T showing all expected AM = 1 transitions at X-band (9.498 GHz).
*
Hamiltonian (SH) parameters [8] at a relatively higher temperature than that required at low frequencies, because the Boltzmann factor, which governs population differences, is enhanced considerably at VHF frequencies, decreasing exponentially with the microwave frequency. 3.5. More precise determination of spin-Hamiltonian parameters: multifrequency approach A more precise determination of spin-Hamiltonian parameters can be accomplished by combining low and high frequency EPR studies since, in general, the fine-structure zero-field splitting and hyperfine structure parameters A,B, are determined more precisely at lower frequencies [9], while the g-tensor is determined more precisely at higher frequencies.
4. DISADVANTAGES OF VHF EPR
As for disadvantages, the EPR linewidth increases at higher frequencies, which can mask small splittings between lines, such as those due to the hyperfine interaction. It is well known that there occurs enhancement of inhomogeneous broadening due to distribution of g- values in the crystal at higher frequencies. It can be overcome by using low-frequency EPR to study the hyperfine interaction (9). Furthermore, g-strain broadening can be another impediment at
736
Ni,CdCl, H20 249 9 GHz D=-3097 g,=2 244
00-
g
00
05:
; e
B.0;
05
N$CdCIe.1ZH20 919GHz
80
D= 97 GHz q=2.244 9,=2 207
15
1.0
8.5
7.0
,
.
0.m
.
~.
.
,
.
0.m
0
.
I
85
OOJ
0.m
,
.
,
,
.
.
,
.
M
0
Orientakn d magneticfield(degrees)
.
,
w
Owntabon d magnstr field (degrees)
Figure 5a. The simulated ular variation EPR line positions for the' N ! ion in NizCdC16.12HzO at 249.9 GHz calculated using the indicated parameters.
Figure 5b. The simulated an lar variation of EPR line positions for the Ni'ion in NizCdCl6-12HzOat X-band (9.6 GHz) calculated using the indicated parameters.
VHF. However, one may gain if the exchange-narrowing effect ("10/3 effect") at higher frequencies (6) becomes effective at the same time. 5. ILLUSTRATIVE EXAMPLES
-
5.1. Ni2+in NiZCdCla 12H20
NiZ'is a non-Kramers S = 1 ion. In the single-crystal host NizCdCls * 12Hz0, its EPR has been studied in detail at X-band (9.6 GHz) by Misra et al. (10). The spin-Hamiltonian, characterized by axial symmetry, is, in the usual notation:
H = p&llBzSz
+ a(BxSx+ BySy)]+ D [S?
-
S(S+1)/3] + E (Sx' - S:)
(1)
There were found two magnetically-inequivalent NiZ' ions, I and 11, characterized by the following values ofthe SH parameters at 295 K (10): Ion I: gll = 2.236, g~=2.291, D = -31.12 GHz; Ion 11: gll = 2.274, 2.323, D = -6.81 GHz. For ion I, with much larger zfs, it is still possible to get an observable transition at Xband, although the Zeeman interaction is small compared to the zero-field splitting of 31 [Above about 45 GHz it would not be possible to observe any transition at 9.6 GHz for magnetic field values below 1 TI. However, at VHF (249.9 GHz), the allowed transitions are well into the high-field regime where the Zeeman term dominates as seen from Figure 4. Angular variation of EPR line positions were calculated using the above parameters for rotations about an axis perpendicular to the crystallographic c-axis, as shown in Figures 5a and 5b at 9.6 and 249.9 GHz, respectively. Since the Zeeman interaction dominates the zerofield splitting, these spectra are very easy to interpret at 249.9 GHz. In fact, as seen from Figure the magnitudes of the zero-field splitting can be directly measured from the VHF spectra, where the splitting of the two lines is 2D for B along the magnetic z-axis, unlike the case for X-band spectra. The linewidths at VHF for this crystal are expected to experience the compensating
737
effects of two competing mechanisms: (i) g-distribution which increases linewidth with increasing frequency; and (ii) exchange narrowing, (i.e. the 10/3 effect) which decreases the linewidth with increasing frequency. [See [S], and references therein]. One can expect to obtain greater accuracy in determination of g-values at VHF, in this case, by an additional significant figure from VHF spectra at 249.9 GHz from that at X-band. Since, at 249.9 GHz, the g-tensor effects (Zeeman term) are greater by a factor of about 25 as compared to that at X-band. 5.2. Mn2+in a-ZnzV207 single crystal
There were observed two magnetically inequivalent, but physically equivalent, Mn" ions [ll]. Due to the 55Mnnuclear spin I = 5/2 (g,, = 1.382), each fine-structure line splits into six hyperfine lines. Both sets of EPR lines were described by the same set of SH parameters. The EPR line positions were fitted to a spin Hamiltonian appropriate to a monoclinic site symmetry in the usual notation [12]:
The values of the SH parameters at 295 K, as estimated using the least-squaredmatrixdiagonalization procedure [2], are: g = 2.008; b:= 5.75 GHz, b: = 1.09 GHz. Since this is a case of isotropic g-tensor, VHF EPR can only offer limited improvement in the accuracy of the average g-value. The simulated angular variations of the MnZCfine-structure line positions for one magnetically inequivalent MnZ+ion, with the above spin-Hamiltonian parameters are shown in Figures 6a and 6b, respectively, at 9.6 and 249.9 GHz. Again, it noted that there is greater simplicity of the simulated angular variations of the line positions at 249.9 GHz shown in Figure 6b versus those at 9.6 GHz in Figure 6a; see also References [3,4]. Fe3' in YCaA104 single crystal
Fe3+ EPR studies at X-band (9.79 GHz) were carried out on a YCaA104 single crystal doped with 0.2% Fe3', substituting for AI3+ion [14]. The ion is characterized by tetragoanl site symmetry in this crystal. The magnetic z-axis is parallel to the crystal c-axis. The spin Hamiltonian applicable to this crystal, characterized by tetragonal symmetry, has the same form and definitions as those given by Equation (2), with m = 0,4, n= 2,4 (m < n), and with no hyperfine terms. There are two main problems in the determination of the spin-Hamiltonian parameters from X-band EPR line positions in this crystal: (i) The EPR lines are almost completely broadened out for magnetic field values higher than 0.35 T at Xband; (ii) Full angular variation was observed only for the central +1/2++-1/2 transition; only the EPR line positions for this transition for the orientation of B near the X (Y) magnetic axis can be used to estimate b: , because for B near the X(Y)-axis the dependence
of the central transition on is in first order, due to the transformation of the spin operators [8,13]. Finally, the SH parameters determined at X-band are: gll = 1.991; = 2.021; = 34.7 GHz.
738
Zn,V,O,: Mn” 249.9 GHz
Orientation of magnetic field (degrees)
Figure 6a. Simulated angular variation of EPR line positions for the Mn” ion in ZnzVz07 single crystal at 9.6 GHz.
Orientation of magnetic field (degrees)
Figure 6b. Simulated angular variation of EPR line positions for the Mn2+ion in ZnzVz07 single crystal at 249.9 GHz.
The EPR line positions corresponding to the non-central allowed fine-structure transitions, required to estimate the zero-field splitting parameter b; (=D) precisely, can only be observed at sufficiently high frequency, e.g. 249.9 GHz. It is noted that at room temperature the non-central allowed transitions may not be observable at VHF due to their fast spin-lattice relaxation; one may have to lower temperatures to observe them. The simulated angular variation of EPR line positions at 9.79 and 249.9 GHz are shown in Figure 3. 6. METALLOPROTEINS
Metalloproteins, primarily in the disordered state, are biologically and medicinally interesting systems. Moreover, their EPR data are amenable to sophisticated computational analysis [ 17,181. A number of biologically interesting enzymes, including a variety of oxygenases, use bound Fe@I)in catalysis. The study of the mechanisms of these enzymes has been hampered by the lack of a suitable spectroscopic probe for Fe(I1). It is well known that the Fe(II) ion possesses large zero-field splitting (ZFS) in metalloproteins, and temperatures below 10 K are required for observation of its EPR signal The local order around the Fe (11) ion can be probed more sensitively by VHF EPR, thus providing information on the role that the iron ion plays in some metalloenzymes. This can be accomplished by exploiting the recently developed computational techniques [ 16,171 to simulate and fit EPR spectra of mononuclear Fez+and exchange coupled FeZC-Fe3+ centres in these amorphous materials. The electronic environment of the paramagnetic centre in the protein and the perturbations induced by ligand binding will be defined by the values of the spin-Hamiltonian (SH) parameters and the shape of the spectrum. We propose to study four specific Fe-containing proteins by VHF EPR at Cornell University’s Advanced Center for Electron Spin Resoanance Technology (ACERT) directed by Professor J. Freed: (i) biphenyl dioxygenase (BPDO) [IS], (ii) 2,3-dihydroxybiphenyI 1,2dioxygenase (DHBD) 11191, (iii) BphF, and (iv) phenol hydroxylase (PH) [20]. The samples will be prepared by Ms. N. Imbault and Professor J. Powlowski (Concordia University), and Professor L. Eltis (University of British Columbia). BPDO contains two metallocentres: a mononuclear Fe(I1) (S = 2) and a “Rieske-type” 2Fe-2S cluster with one Fe bound by two cysteines and the second by two histidines. The
739
reduced Rieske center has a net charge of I+ and the Fe(I1) and Fe(II1) ions are exchangecoupled so that the net spin is = %. Since the mononuclear iron center is believed to be the site of oxygen activation and insertion, EPR will be used to characterize its environment when different chlorinated biphenyl substrates and uncouplers are bound. DHBD enzyme is a type I Fe(I1)-dependent extradiol dioxygenase [21] that catalyzes the extradiol cleavage of 2,3-dihydroxybiphenyI (DHB), incorporating both atoms of dioxygen into the product, (HOPDA). DHBD is a limiting step in the aerobic degradation of PCBs by the bph pathway as it is competitively inhibited or suicide-inhibited by a number of chlorinated catechols that are produced during PCB-transformation, such as 3-chlorocatechol. The enzyme is thus the subject of directed evolution experiments to overcome these metabolic blocks and to engineer microorganisms for bioremediation. EPR data are expected to provide novel insights into substrate activation of the reaction by DHBD and related Fe(I1)-dependent enzymes. BphF is the Rieske-type ferredoxin associated with biphenyl dioxygenase. The 2Fe-2S cluster of BphF is structurally and electronically very similar to that of BPDO. The recently determined crystal structure of BphF, refined at 1.6 8, resolution, reveals that the protein has the same fold as other Rieske proteins, including the Rieske cluster domain of BPDO [22]. High field EPR studies of reduced and oxidized BphF will greatly facilitate studies of BPDO. The phenol hydroxylase enzyme (PH) [I91 is similar to the well-studied methane monooxygenase in terms of type of iron center, and the requirement for interaction with both a reductase protein and an activator protein. Progress on characterization of PH has been hampered by the lack of an x-ray structure, since the protein is not easily crystallized. It is precisely for this reason that EPR studies of phenol hydroxylase in different redox states and with various phenolic ligands should provide greater detail of the environment around the iron centres, and how it changes during binding of substrates, binding of interacting proteins, and during steps in the catalytic cycle. This type of information is not readily accessible using other techniques. In addition, a Mn-substituted form of phenol hydroxylase is available,which will also be examined using the EPR technique. CONCLUDING REMARKS
Single-crystal VHF EPR is a potentially powerful technique to study biological systems, e.g. metalloproteins, which contain transition metal ions characterized by large zero-field splitting parameters D, E, e.g. Fe2+, Mn3+. A multi-frequency approach has been found usehl to accurately estimate all of spin-Hamiltonian parameters by simultaneously fitting all resonant line positions observed at various frequencies. The g-values are determined more accurately at higher frequencies, whereas the fine- and hyperfine-structure parameters require lower frequencies for more precise determination. It is important to use complete and accurate diagonalization of the spin Hamiltonian matrix [2,4], rather than less accurate perturbation methods [3,6], and to fit all EPR line positions simultaneously. In a forthcoming publication by Misra et al. [ 1 I] experimental 249.9-GHz EPR studies on the samples discussed in Section 5 are reported. Acknowledgments The author is grateful to the Natural Sciences and Engineering Research Council of Canada for partial financial support, and to Professor J. Freed for many useful discussions. Computations were facilitated by the Cornell Theory Center.
740
REFERENCES 1. J.H. Freed, Annu. Rev. Chem., 51 (2000) 655. 2. S.K. Misra, J. Magn. Reson., 23 (1 976) 406. 3. W.B. Lynch, R.S. Boorse, J.H. Freed, J. Am. Chem. Soc., 115 (1993) 10909. 4. R.M. Wood, D.M. Stucker, L.M. Jones, W.B. Lynch, S.K. Misra, J.H. Freed, Inorg. Chem., 38 (1999) 5384. 5. D.P. Goldberg, J. Tesler, J. Krzystek, A.G. Montalban, L.-C. Brunel, A.G.H. Barrett, and B.M. Hoffman, J. Am. Chem. Soc., 119 (1997) 8722. Their powder of microcrystals aligned in the strong magnetic field, produced an effective single-crystal spectrum with the magnetic z-axis parallel to B in a VHF study of Mn3+ in Mn(TPP)Cl, Mn(ODMAPz)Cl, Mn(ODMAPz)DTC, and Mn(DD-IX-DME)CI at frequencies of 200 GHz and higher. 6. W.R. Hagen, in Advances in Inorganic Chemistry, 38, (1992) 165; as an illustration spectra for Mo-nitrogenese is recorded for both B1 B (no signal) and B1 11 B (a signal) configurations; W.R. Hagen, Coord. Chem. Revs., 190-192 (1999) 209. 7. S.K. Misra, New Methods of Simulation of MnZfEPR Spectra: Singli Crystals, Polycrystalline and Amorphous (biological) Materials, Vol. 18: Instrumental Methods in Electron Magnetic Resonance, Eds. C. Bender and L. Berliner (Kluwer AcademicPlenum Publications, 2002), in Press. 8. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, (Clarendon Press, Oxford, 1970). 9. S.K. Misra. PhysicaB, 240 (1997) 183. 10. S.K. Misra, L.E. Misiak, and P. Chand, Physica, B202 (1994) 31. 11. S.K. Misra, S. I. Andronenko, K.A. Earle, and J.H. Freed, Appl. Mag. Reson. (In press). 12. S.K. Misra and C. Z. Rudowicz, Physica, B147 (1988) 677. 13. S.K. Misra, in Handbook of Electron Spin Resonance, C.P. Poole, Jr. And H.A. Farach, Eds. (A.I.P. Press, Springer, New York, 1999), Chap. VII, p. 115. 14. S.K. Misra and S. I. Andronenko, Phys. Rev. B (In press) 15. S.K. Misra, Physica, 121B (1983) 193. 16. S.K. Misra, J. Magn. Reson., 137 (1999) 83-92. 17. S.K. Misra, J. Magn. Reson., 140 (1999) 179-188. 18. N.Y.R Imbeault., J.B. Powlowski, C.L.Colbert, J.T.Bolin, L.D Eltis, J. Biol. Chem.,. 275 (2000) 12430-12437. 19. J.T Bolin and L.D. Eltis “2,3-Dihydroxybiphenyl 1,2-Dioxygenase”. In Handbook of Metalloproteins (Eids. Messerschmidt et al.), 02000, Wiley. 20. J.B. Powlowski, J. Sealy, V. Shingler, and E. Cadieux, ”On the Role of DmpK, An Auxiliary Protein Associated with Multicomponent Phenol Hydroxylase from Pseudomonas sp. strain CF600”, J. Biol. Chem., 272 (1997) 945-951. 21. Eltis L.D. and Bolin J.T, J Bacteriol, 178 (1996) 5930-5937. 22. C.L. Colbert, M.M.-J. Couture, L.D. Eltis, and J.T. Bolin, Structure Fold. Des., 8 (2000) 1267-1278.
EPR in the 21" Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
74 1
EPR evidence of onset of the quantum critical point in CuGe03:Fe S.V.Demishevasb,R.V.Buntinga, H.Ohtac, S.Okubo', Y.Oshimad and N.E.Sluchankoa aL~w Temperatures Laboratory, General Physics Institute, Vavilov street, 38, 117942 Moscow, Russia bVentureBusiness Laboratory, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan 'Molecular Photoscience Research Center and Department of Physics, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan dGraduate School of Science and technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan Influence of doping by iron impurity on spin-Peierls state in CuGe03 is studied. EPR measurements for the frequencyltemperature domain 60-450 GHdl.8-300 K show that insertion of 1% of Fe completely destroy both spin-Peierls and antiferromagnetic orders. Damping of long-range magnetic order is accompanied by onset at K of +ewer asymptotic for magnetic susceptibility with the index a=0.35. This effect is characteristic to the limit of strong disorder for doped CuGe03 and may reflect onset of a quantum critical point in CuGeO3:Fe. Discovery of inorganic spin-Peierls compound CuGeO3 opened an opportunity to study influence of doping and disorder on the spin-Peierls state. Numerous experiments and theoretical studies have been carried out in this field up to now. However, from the theoretical point of view, most of the available data correspond to the limit of weak disorder when density of states have a pseudogap, i.e. a spin-Peierls gap filled by disorder-induced states [l]. In this case the expected temperature-concentration T-x phase diagram consists of uniform state, spin-Peierls state, antiferromagnetic state and the region, where antiferromagnetic and spin-Peierls orders coexist [l]. This structure of phase diagram was observed experimentally for Zn, Si, Ni, Co, Mg and Mn impurities [l-lo]. In the limit of a strong disorder the ground state of CuGeOs is gapless and the density of states diverges at E=O: P(E) cc I [l]. As a consequence the temperature dependence of magnetic susceptibility acquires the form [111
742
where a 4 . From the theoretical point of view the non-Curie asymptotic behavior of magnetic susceptibility reflects the onset of the Griffiths phase (GP) which thermodynamic properties are controlled by relatively rare spin clusters correlated more strongly than average [12-151. The GP appears in various spin systems below some critical temperature if the magnitude of random potential is strong enough to destroy transition to magnetically ordered phase [ 12151.
As long as any type of the long-range magnetic order in GP at finite temperature is smeared, the aforementioned physical situation is often discussed in terms of a quantum critical point. In this ansatz the disorder shifts the temperature of magnetic transition to absolute zero and in the vicinity of the “critical point” susceptibility diverges in accordance with Equation (1). A non-Curie type behavior of have been first reported for CuGeO3 doped with Zn [4]. However, experiments in Ref.4 were carried out for extremely low Zn concentrations corresponding to the weak disorder limit, and the observed deviations from the Curie law can not be related to the case of strong disorder. The aim of the present work consists in providing evidence of experimental realisation of a strong disordered limit in doped CuGe03. We argue that insertion of 1% of iron in CuGe03 matrix completely damp both spin-Peierls and antiferromagnetic transitions and gives rise to onset of a quantum critical point. Single crystals of Cuo99Feo.olGe03were obtained by self-flux technique [16]. The quality of crystals have been controlled by X-ray and Raman scattering data; the actual contents of Fe in crystals was determined by chemical analysis. The structure of the samples studied coincided with the structure of pure CuGe03 and the effect of doping on the Raman spectra confirmed that iron impurity substitute cooper [17]. Two types of experimental facilities were used to study magnetic properties of CuGe03:Fe. Magnetoabsorption lines for the frequencies up to 450 GHz were studied with the help of the magneto-optical facility at Kobe University. In this experiment we measured transmission through the sample as a function of magnetic field up to 16 T at fixed frequency for liquid helium temperatures 4.2 K and 1.8 K. Simultaneously the reference transmittance of a thin layer of DPPH powder have been recorded and both high-frequency EPR spectra of CuGe03:Fe and DPPH were analyzed quantitatively. Temperature dependence of the EPR spectrum in the range 1.8-140 K was measured using 60 GHz cavity spectrometer in the General Physics Institute [17]. For each temperature studied the accuracy of the temperature stabilization was better than 0.01 K. All magnetoabsorption experiments reported below were carried out in B I I a geometry. Typical magnetoabsorption spectra in transmission experiment are shown in Figure 1. Up to o / 2 x = 450 GHz neither additional “impurity” lines, nor AFMR modes have been detected and the spectrum for CuGe03:Fe consists of a single lorenzian EPR line. The corresponding g-factor value is close to that for Cu2+ions in the case B I I (see below). The resonant field for this line scales linearly with frequency, i.e. g-factor is frequency independent (see inset in Figure 1). Transmission data obtained at various frequencies were used to calculate EPR integrated intensities for CuGe03:Fe and DPPH. The main problem in the quantitative analysis of high frequency EPR line is a possible violation of the widely applied approximation [18]: . Indeed, in general case the integrated intensity for arbitrary electromagnetic wave frequency and resonant magnetic field B,es is given by [191
743
h
% 0.2 0
100
200
300
400
Frequency (GHz) Figure 1. Magnetoabsorption spectra for CuGeO3:Fe in transmission experiment.
Figure 2. Frequency and resonant field dependence of magnetic moment and ESR line width AB for CuGe03:Fe at K. Curve 1 correspond to best fit of using Equation (5).
and the deviations from Curie law deduced from may reflect a non-linearity of magnetic moment in strong magnetic field. It follows from Equation (2) that for each frequency
where indexes 0 and 1 denote characteristics of ESR lines for DPPH and CuGeO3:Fe respectively. Assuming that magnetic moment of DPPH is given by Brillouin function it is possible to calculate field or frequency dependence of magnetic moment = for CuGe03:Fe with the help of Equation (3). The result gives (Figure 2) that linear region Mi cc lasts up to = 6 T (w/271 e200 GHz). At higher resonant fielddfrequencies magnetic moment of CuGe03:Fe tends to saturate and above = 1 1 T (o/271 w 350 GHz) Mi starts to decrease with field (Figure 2). Along with the integrated intensity the width AB of the EPR line in CuGeO3:Fe was calculated. Contrary to the frequency independent g-factor this parameter demonstrates a considerable frequency dependence. It follows from Figure 2 that AB increases two times
744
when frequency is varied from o / 2 n = 60 GHz to w/271: = 450 GHz. Experimental data at T=1.8 K can be modeled by expression +C,
(4)
which is characteristic to Raman relaxation mechanism for S=1/2 ion [20]. The line 1 in Figure 2 correspond to best fit parameters in Equation (4) A =(3.2f0.4).10-7 T/GHz2 and =(0.063+_0.004)T. It is interesting, that attempts to model by expression for the direct process cc /tanh(Aw/2kBT) [20] have failed as long as the theoretical frequency dependence was too strong to fit the experimental data in Figure 2. Therefore it is possible to conclude that at low temperatures the dispersion of the relaxation time in CuGeO3:Fe is controlled mainly by the Raman process. Temperature measurements of the EPR in the 60 GHz cavity spectrometer were carried out on the same crystal as was investigated in the quasi-optical transmission experiment. For the precise determination of the g-factor a DPPH crystal was placed in the cavity together with the CuGe03:Fe sample. The results of the previous section indicate that at o/271: = 60 GHz the CuGeO3:Fe remains in the region of linear magnetic response and consequently a relation cc = is valid. At all temperatures studied a single absorption line of lorenzian shape was observed (Fig. 3), that is in agreement with the results of the quasi-optical experiment. Data in Figure 3 were used to calculate temperature dependences of the g-factor g(T), line width M ( T ) and integrated intensity (see Figure 4). For >20 K the value of the g-factor is g m2.15 and characteristic to Cu2' ions in CuGeO3 structure for geometry B I I a 1211. Below T=20 K g-factor starts to increase with lowering temperature and reach the value g=2.19 at T=1.8 K (Figure 4). It is worth to note, that in the case of Fe-doped crystal no giant changes of the gfactor like in Ni-doped CuGeO3 [S] are observed. The temperature dependence of the line width is non-monotonic: when temperature is lowered the first decreases, passes through a minimum at T=10-20 K, and finally starts to increase again. It is interesting that in pure CuGe03 the width of the EPR line decreases gradually with lowering temperature and the magnitude of at Figure 3. Evolution of the ESR absorption line with temperature measured in cavity spectrometer T=100 K is about 6 times smaller ( 0 / 2 n =60 GHz, mode TEoll, quality factor Q=104). than in Fe-doped crystal [21].
.-u)
L
. 0.1
i
Figure 4. Temperature dependences of the integrated intensity line width and g-factor obtained in cavity experiment.
The decrease of temperature makes a difference in more dramatic: at T=l.S K the line width for the Fe-doped CuGe03 is 200 times bigger than in pure crystal (see Figure 4 and data from [21]). The significant difference between pure and Fe-doped CuGeO3 is visible in the temperature dependence of the integrated intensity. It follows from Figure 4 that in the Fedoped crystal spin-Peierls transition is completely damped. For 70 K integrated intensity obeys Curie law (Figure 4, curve 1). In the temperature range 2570 K saturates, and at lower temperatures a power law asymptotic behavior with the index
a=0.35+0.03 is observed (Figure 4, curve 2). Summarizing experimental results of the present work, we wish to mark that observed low temperature behavior of the ESR line reflects intrinsic properties of Cu2' chains modified by Fe impurity rather than impurity paramagnetism caused by Fe ions. This conclusion can be deduced from the g-factor values characteristic to Cu2' (Figure 4) and the observation of the line width frequency dependence given by Equation (4). Indeed, for the Fe2+ion substituting Cu2+ion in S=1/2 chain a spin state with S=2 may be expected [22]. For the integer impurity spin the term proportional to in expression for the line width should vanish and will be frequency independent [20]. The expected "impurity behavior" for Fe2' contradicts to experimental data (Figure 2) and magnetic properties of CuGe03:Fe system are controlled by disordered Cu2' chains. The presence of the disorder in magnetic subsystem follows from the strong broadening of the ESR line with respect to the pure crystal (Figure 4 and Reference 21) and agrees with the results of the structural studies [17]. As long as the measured integrated intensity at 0 / 2 n = 60 GHz for CuGeO3:Fe is proportional to magnetic susceptibility the latter quantity diverges at TE,>E,,,. However, the V - 0 distance along the V-01 direction of AP phase is slightly shorter than that of the HP phase [2,3] which means the pyramids are strained along the a axis in AP phase. Therefore, the energy difference of E4 - E, and E,,, - E, will be increased. I98 I91 1 96 Iu
2
?
195
I 94 I 93 I 92
0
50 100 150 200 Angle (degree)
250
Figure 1. Angular dependence of g-value at room temperature, the single crystal was rotated around (a) the c axis and (b) the b axis.
773
Using spin Hamiltonian approximation, g-values are obtained as the following expressions,
where is g-factor of free electron, h is spin-orbit coupling, and El is the i-th energy level as denoted above. Therefore, the g-values for AP phase are larger than those of HP phase. On the other hand, the VO, pyramids of AP phase are inclined each other from the a-axis which means the observed g-value should be smaller than the principle g-value estimated by the averaging of exchange interaction. However, we consider qualitatively that the increase of g-value for AP phase caused by the distortion of VO, pyramid is still larger than the decrease of g-value caused by the inclination of VO, pyramid. The line shape and angular dependence of linewidth did not show any one dimensional characteristic, which may suggest that the exchange interaction is dominant in this system. However, the temperature dependence of integrated intensity is consistent with the magnetic susceptibility results which reveal broad maximum around 80K Both results show typical temperature dependence of antiferromagnetic chain. We also performed the temperature dependence high field ESR measurements for AP phase. Figures 2(a) and 2(b) show absorption spectra of c-axis at various temperatures and frequency-field diagram, respectively. A simple EPR absorption line was observed as shown in Figure 2(a) and the g-values for each three axes were obtained as ga=l.934(8), gb=l.972(4), g,=1.976(1) as shown in Figure 2(b). These values are consistent with the result of X-band ESR considering the error bars. Below 15K, the g-values for three axes increase with decreasing the temperature as shown in Figure 3. The positive g-shifts for H parallel to a and b axes, which are perpendicular to the chain direction, are inconsistent with the typical behavior of one-dimensional antiferromagnet [lo]. However, the width increases below 15K which is the typical behavior of one-dimensional antiferromagnet. 4. CONCLUSION
X-band and high field ESR measurements of AP-VOPO single crystal have been performed.
-m
g
la)
Y
70 4K
60 IK
-
300
b-axis ---c-axis -e
50 1 K
Figure 2. (a) The temperature variation of absorption spectra of AP phase sample observed at 160GHz. Applied magnetic field is parallel to the c-axis. (b) Frequency-field diagram at 4.2K for three axes.
774 2.00 199 198 197
2 1.96 I95
I .94
1.93 I .92
10 20 10 40 50 60 70 80 Temperature ( K )
Figure 3. The temperature dependence of g-values at 16OGHz. The g-values are determined to be ga=l.929(6), gb=l.975( I), g,=1.974(6). It turned out that the AP g-values are slightly larger than HP one.The temperature dependence of high field ESR result is discussed in connection of the one dimensionality. ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (A) (Novel Quantum Phenomena in Transition Metal Oxides), and Scientific Research on Priority Areas (B) (No. 13130204 "Field-Induced New Quantum Phenomena in Magnetic Systems") from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1 T. Saito, T. Terasima, M. Azuma, M. Takano, T. Goto, W. Utsumi, P. Bordet and D. C. Johnston, Journal of Solid State Chemistry., 153 (2000) 124. 2 M. Azuma, T. Saito, Y. Fujishiro, Z. Hiroi, M. Takano, F. Izumi, T. Kamiyama, T, Ikeda, Y. Narumi, K. Kindo, Phys. Rev., B60 (1999) 10145. 3 Z. Hiroi, M. Azuma, Y. Fujishiro, T. Saito, M. Takano, F. Izumi, T. Kamiyama and T. Ikeda, Journal of Solid State Chemistry., 146 (1999) 369. 4 P. T. Nguyen, R. D. Hoffman, A. W. Sleight, Mater. Res. Bull., 30 (1995) 1055. 5 A. W. Garrett, S. E. Nagler, D. A. Tennant, B. C. Sales and T. Barnes, Phys. Rev. Lett., 79 (1997) 745. 6 T. Yamauchi, Y. Narumi, J. Kikuchi, Y. Ueda, K. Tatani, T.C. Kobayachi, K. Kindo and K. Motoya, Phys. Rev. Lett., 83 (1999) 3729. 7 M. Motokawa, H. Ohta and N . Makita, Int. J. Infrared & MMW., 12(2) (1991) 149-155. 8 S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W. J. Jang, M. Hasegawa and H. Takei, Int. J. Infrared & MMW., 17(5) (1996) 833-841. 9 N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura and H. Ohta, Int. J. Infrared & MMW., 19(2) (1998) 167-176. 10 K. Nagata and Y. Tazuke, J. Phys. Soc. Japan., 32 (1972) 337.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
775
ESR measurements on triangular antiferromagnets CsCu,,Co, C1, Toshio Onoa, Hidekazu Tanaka", Hiroyuki Nojirib and Mitsuhiro Motokawa" "Department of Physics, Tokyo Institute of Technology, Tokyo 152-855 1, Japan bDepartment of Physics, Okayama University, Okayama 700-8530, Japan 'Institute for Materials Research, Tohoku University, Miyagi 980-8577, Japan CsCuCI, is a ferromagnetically stacked S = 1/2 triangular antiferromagnet with the weak easy plane anisotropy, and undergoes a field-induced magnetic phase transition due to the competition between the easy plane anisotropy and the quantum fluctuation. From our previous magnetic measurements, it was found that a new ordered phase appears in the ion substituted system CsCu,,Co,C1, with small Co2+ion concentration x. In order to investigate the spin dynamics, high field and high frequency ESR measurements were performed on CsCu,,Co,CI, for the samples with x = 0.015 0.032. Experimental results are discussed in comparison with the results for pure CsCuC1,.
-
1. INTRODUCTION
In frustrated antiferromagnetic systems, manifold degeneracy of the ground state often remains, when they are treated within the classical mean-field theory. In such cases, the quantum fluctuation plays an important role in the determination of the ground state. Theoretical studies [ 1,2] predict that a magnetization plateau appears at one-third of the saturation magnetization M, in the S =1/2 2D or ferromagnetically stacked triangular antiferromagnets in the isotropic limit. CsCuC1, is a ferromagnetically stacked S = 1/2 triangular antiferromagnets with weak planar anisotropy [3,4]. For the magnetic field parallel to the c-axis, this compounds undergoes a phase transition from the 12O0-structurein c-plane to a new coplanar structure, which is stabilized by the quantum fluctuation [2,5]. Since the energy gain due to the quantum fluctuation increases with increasing magnetic field, and then it overcomes the classical anisotropy energy, so that the field-induced phase transition occurs 121. Therefore, it is very interesting to study how the phase transition changes, when the macroscopic anisotropy of CsCuCI, is varied continuously. With this motivation, we have investigated the phase transitions in the mixed system CsCu,,Co,CI, with a few percent of Co2+ions, which is expected to produce the easy-axis anisotropy. In order to investigate the phase transitions in CSCU,~CO,CI,,we measured the temperature and the field dependence of magnetization for the samples with 0.015 Ix 5 0.032 [6]. We found two ordered phases in the present systems. Figure 1 shows the phase diagram of CsCu, ,Co,CI, (x = 0.015, 0.023 and 0.032) for H II The small amount of Coz+ion dopant produces a new ordered phase in the low-temperature and low-field region. The area of phase is enlarged with increasing Co2+ion concentration. Magnetic structures in the ordered phases I and I1 were determined by the neutron scattering experiments [7]. In phase I, the magnetic structure is identical to that in the ordered state of CsCuCl,, i.e., spins form 120"-
776 0tt
I
I
+I
I
I "1 V l ,
cscu .xcOxC13
CL
Hllc
c
t r
A
x=0.015
E *L
10
Figure 1. Magnetic phase diagram of cscu,,co,c1,.
Figure 2. Oblique triangular antiferromagnetic structure realized in phase 11. Angle ydenotes the half angle between S, and S,. structure within the c-plane. The phase I1 is an oblique triangular antiferromagnetic phase, in which the spin plane spanned by the spins on a triangular lattice is tilted from the c-plane as shown in Figure 2. For the sample with 0.03, tilting angle $ was estimated as $ = 44" at T = 1.6 K. The microscopic mechanism leading to the successive phase transitions has not been clear. In order to investigate the spin dynamics of the present system, we carried out the high frequency ESR measurements for the frequency 95 5 s 525.8 GHz:In this paper we report the results.
-
2. EXPERIMENTS Single crystals of CsCu,,Co,Cl, were prepared by the vertical Bridgman method from a melt of CsCuC1, and CsCoCI,. The details of the sample preparation have been described in Reference [ 61. The ESR measurement was performed at Institute for Materials Research, Tohoku University, using a multilayer pulse magnet, which produces magnetic fields up to 30 T. Optical-pumped far infrared gas lasers (525.8 - 326.1 GHz), backward traveling wave tubes (- 200 - 240 GHz and 330 - 380 GHz), and Gunn oscillators (- 95 - 190 GHz) were used as light sources. The Faraday configuration was taken in the measurement. The transmitted light power was detected by an In-Sb detector. ESR signals were taken at T = 1.6K in the magnetic field parallel to the c-axis.
-
3. RESULTS AND DISCUSSION Figure 3 shows the ESR absorption spectra observed at T = 1.5 K for the samples with = 0.015, 0.023 and 0.032. Arrows indicate the resonance fields. The vertical dashed lines denote the magnetic phase transition fields H, detected by the magnetization measurements. For all samples, each ESR signal is composed of a broad strong absorption line and a weak critical resonance at For the samples with x = 0.023 and 0.032, resonance fields increase monotonously with increasing frequency. One of the noticeable features of the absorption
777
5
j
zE s
c
0
5
10
15
0
5
10
15
0
5
10
15
I
Figure 3. ESR spectrum of CsCu,,Co,CI, for the samples with x = 0.015 (a), 0.023 (b) and 0.032 (c). Dashed vertical lines indicated by H, denote the phase transition field determined by the magnetization measurements. spectrum is that the line width broadens significantly, when the resonance field is located at around H,. Figure 4 shows the temperature variation of the resonance field for the sample with x = 0.023. Dot dashed line indicated by TNdenotes the phase transition temperature detected by magnetization measurements. With increasing temperature, the resonance field shifts abruptly toward higher field side, when the temperature passes through TN.Below TN,the resonance field reaches a value corresponding to g = 2.06. Similar results were obtained for other systems. The resonance data obtained for x = 0.032 (see Figure 3(c)) are summarized in the frequency versus field diagram as shown in Figure 5. In this diagram, the magnetic field is
h
!
I
I
10
I
I
50
T
Figure 4. Temperature dependence of the resonance field for H II c and = 135 GHz for the sample with x = 0.023. Dot dashed line indicates the phase transition temperature determined by the magnetic measurements.
5
H
m
15
Figure 5. Frequency versus field diagram for the sample with x = 0.032 measured at T = 1.6 K for H II c-axis.
778
Table 1. Magnetic planar anisotropy fields obtained by fitting of the data for CsCu,. xCoxCI,and CsCuC1, [4]. Other magnetic parameters were fixed as HE,,= 21 [TI, HE1= 11 [TI and HDM= 1.9 [TI.
HA [TI
x = 0.015
x = 0.023
x = 0.032
cscuc1,
0.203
0.174
0.174
0.18
normalized by the g-factor. Above H > 5 T, the resonance point approaches gradually the EPR line with increasing frequency. Solid curves labeled by and LO. denote the ESR modes calculated for the 120"-structure within the c-plane as observed in CsCuCI, [4]. When the effective fields due to the Dzyaloshinsky-Moriya interaction HDMand the planar anisotropy HA are much smaller than the exchange fields due the interactions along the chain (H,) and between the chains (&), the resonance conditions for the and modes are expressed as H, We fix the values of H,, HE1 and HDM to H , = 21 [TI, HEI= 11 [TI and HDM= 1.9 [TI, which are the same as those in CsCuCI, 141, since the concentration of Coz' ion is sufficiently small, and the period of helical spin arrangement is close to that in CsCuCI, [7]. Therefore, only the anisotropy fields HA are treated as the adjustable parameter. We see that the experimental results for H 5 T are well described by equation (1). Table 1 shows the values of HA obtained by fitting the data for H L 5 T to equation (I). On the contrary to our expectation, there is no significant change in HA. The resonance point below H < H, deviates from the theoretical curves for the and modes. The ESR signals obtained at lower field region H < 4 T are supposed to be the antiferromagnetic resonance modes, which are characteristic of the oblique triangular antiferromagnetic structure. Further investigations such as mode calculation for the new magnetic structure in phase I1 or the experiments for the lower frequency region v 5 95 GHz are needed.
REFERENCES 1. A.V. Chubukov and D. I. Golosov, J. Phys.: Condens. Matter, 3 (1991) 69. 2. T. Nikuni and H. Shiba, J. Phys. SOC.Jpn. 62 (1993) 3268. 3. K. Adachi, N. Achiwa and M. Mekata, J. Phys. SOC.Jpn., 49 (1980) 545. 4. H. Tanaka, U. Schotte and K. D. Schotte, J. Phys. SOC.Jpn., 61 (1992) 1344. 5. H. Nojiri, Y. Tokunaga and M. Motokawa, J. Phys. (Paris) 49 Suppl. C8, (1988)1459. 6. T. Ono, H. Horai and H. Tanaka, J. Phys.: Condens. Matter, 12 (2000) 975. 7. T. Ono, T. Kato, H. Tanaka,A. Hoser, N. StuBer and U. Schotte, Phys. Rev., B 63 (2001) 224425.
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
779
Gyrotron ESR in CsFeCls up to 40 T M. Chiba", K. Kitai*, S. Mitsudob, T. Ideharab, S. Ueda" and M. Todad 'Department of Applied Physics, Fukui University, Fukui 910-8507, Japan bResearch Center for Development of Far Infrared Region, Fukui University, Fukui 9108507, Japan "Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan dResearch Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
More than ten years ago, an anomalous magnetization was observed in CsFeC13 under the magnetic field exceeding 33 T. Usually, the spin state of Fez+ in CsFeCl3 is treated the fictitious spin S = 1 with the singlet ground state due to the crystallographic single ion anisotropy. The magnetization saturates at the magnetic field of about 10 T applied parallel to the crystal c-axis. The anomalous magnetization can not be expected within the framework of the fictitious spin S = 1. In order to study the relation between the anomalous magnetization and the excited spin multiplet = 2, an ESR experiment was performed under magnetic fields up to 40 T by use of a pulse magnet. The submillimeter wave was supplied by Gyrotron FU IV at Research Center of Far Infrared Region, Fukui University. The ESR absorption lines were observed corresponding to the anomalous magnetization jump. 1. INTRODUCTION
An ABX3 type hexagonal compound CsFeC13 has been attracted an attention relating to the magnetic frustration caused by the triangular-lattice antiferromagnetism. Further, the Fe2+spin in this material is treated the fictitious spin S = 1 and has the characteristics of the singlet ground state [l].At zero magnetic field the spin states are composed of the singlet ground- and doublet excited states separated by due to the crystallographic single-ion anisotropy. The material does not have any long range order at zero magnetic field reflecting the nature of the singlet ground state. On the other hand under a magnetic field B applied parallel to the crystal c-axis, ( B // c-axis), one of the states of the doublet excited state comes down to cross the ground state at 7.5 T. The long range magnetic order is reported to occur around the level-cross field below 2.5 K [2,3]. More than ten years ago a high field magnetization was measured at 1.3 K with Bllcaxis [4]. An anomalous magnetization has been observed under high magnetic field around 33 T [4]. The anomalous magnetization suggests the appearance of a new magnetic
780
a m
CsFeCl
4-
B I/ c-axis T=1.3K
10
20
30
(T)
Figure 1. Crystal structure of CsFeCl3.
Figure 2. High field magnetization in CsFeC13. After ref.[4].
structure, which can not be explained within the framework of the fictitious spin S = 1. In order to clarify the nature of the above-mentioned anomalous magnetization, an ESR experiment was performed under magnetic fields up to 40 T. The ESR experiment has an advantage that a direct observation is possible on the behavior of the electronic spin states. A preliminary results for the operating frequency of 301 GHz have been already reported [ 5 ] . 2. HIGH FIELD MAGNETIZATION
The present ESR experiment in CsFeC13 under high magnetic fields is motivated by an anomalous magnetization jump around 33 T observed previously by one of the authors (41. Here we briefly survey the high field magnetization observed at 1.3 K with B//c-axis. The crystal structure of CsFeCla is shown in Figure 1. At that time, in order to determine the g-factor from the saturation magnetization, a high field magnetization has been measured up to 40 T. The magnetization curve is shown in Figure 2. Under the magnetic field from zero to T the magnetization is weak with a slight linear increasing. The weakness of the magnetization is consistent with the fact that the ground state is singlet. The slight increasing in the magnetization with increasing magnetic field is considered to be due to the Van Vleck paramagnetism. The magnetization increases rapidly with increasing magnetic field from 4 to 11 T. The linear increasing of the magnetization in this region suggests the appearance of the magnetic order around the field of the ground state crossover. These features are consistent with the theory by Tsuneto and Murao [6]based on the exchange coupled S = 1 singlet-ground state system. The magnetization seems to saturate at 11 T. Afterwards, up to about 32 T the magnetization increases gradually affected by the Van Vleck paramagnetism. Thus below 32 T the framework of the fictitious spin = 1 has been found to work well for the explanation of the magnetization. However, under the magnetic field around 33 T an anomalous magnetization has been observed. This behavior in the magnetization can not be explained in the framework of the fictitious spin S = 1.
781
sz = sz = * l
sz = * z
cubic
I
Spin Energy Level of F e in CsFeCll 40
100
yr)
spin mbll
Figure 4. Calculated electron spin energy level diagram of Fez+ ion in CsFeC13. B//c-axis. After ref.[4].
Figure 3. Spin state of Fe2+ in CsFeC13.
SPIN STATE OF Fe2+IN CsFeC13 The free ion state of 3d6 is 5D. The energy levels are split by the effects of the crystalline field and the spin-orbit interaction as is indicated in Figure 3. At low temperatures under low magnetic fields, only the lowest levels, namely, singlet ground- and doublet excited states, are taken into account and are treated as the fictitious spin S = 1. Within the framework of fictitious spin = 1, the Hamiltonian describing a single ion spin state under the configuration of B//c-axis is given by ?-I
=
+ gpuBBS,,
(1)
where is the single ion anisotropy energy, g the g-factor and p~gthe Bohr magneton. Here, 0) brings a singlet ground- and doublet excited states. The octahedrons of C1- ions surrounding a Fez+ ion make a chain along the c-axis. The exchange coupling between Fe2+ ions along the chain is ferromagnetic, while the interchain coupling is antiferromagnetic, where 0.05 171. Since the effect of overcomes in the case of CsFeCl,, the material does not exhibit any long range order at zero magnetic field. The 3D-long range order is realized only under the applied field where the ground state crossover occurs. In order to consider the possibility that one of the levels in the excited spin multiplet = 2 crosses the ground state under the applied field, the single ion energy level scheme of Fe2+ is calculated with the following Hamiltonian 181, N
?-I = -kX1
.
-
- 2/3)
+
+
(2)
Here X is the spin-orbit coupling energy, k the orbital reduction factor, b the magnitude of the trigonal distortion. The result is shown in Figure where the following parameters = -2 are used: k = 0.9, I X I= 103 cm-‘ and I 6 / k X I= -2 191. The excited state comes down with increasing magnetic field but never crosses the ground state = -1 up to 100 T. Thus as far as the calculated energy level scheme in Figure 4 is concerned, no anomalous effect is expected under the available magnetic field up to 40 T. One of the possible
782
LY Sr= 1
sz =
CsFeC13 B // c-axis T=
K
sz= -
01 0
'
I
,
I
10 Bcl
20
B (T)
Figure 5 . High field ESR in CsFeCla. magnetization appears.
,
I
t
,
i
I
40
Bc2
Bc2 is the field where anomalous
mechanisms of the anomalous magnetization is an intrachain (along c-axis) ferromagnetic coupling. The ion Fez+ sees a large molecular field added to the applied magnetic field, = -2. which will enhance the lowering of the energy level of 4.
GYROTRON ESR EXPERIMENT
An ESR experiment was performed under pulsed magnetic fields up to 40 T with operating frequencies of submillimeter region. The experiment was performed at Research Center for Development of Far Infrared Region, Fukui University. The submillimeter wave was supplied by Gyrotron FU IV operating in the superconducting solenoid up to 12 T. A typical cw operation mode of the gyrotron at the frequency of 301 GHz was TEo3 mode with the output power of 20 W. The submillimeter wave was guided to the specimen by a light pipe. The absorption signal was detected by an InSb hot electron detector operating at the liquid helium temperature. The magnetic field up to 40 T was generated by a pulse magnet driven by a capacitor bank of 30 kJ. The magnet is immersed in the liquid nitrogen. The bore where the magnetic field is generated was 12 mm in diameter. An insert Dewar was settled inside the magnet for cooling down the specimen by use of the liquid helium. The inner diameter of the insert Dewar at the center of the magnet was 7.6 mm. The experiment was carried out at 4.2 K. The magnetic field was applied parallel to the c-axis of a single crystal CsFeCls. 5 . EXPERIMENTAL RESULTS AND DISCUSSION
In the experiment of ESR several branches of absorption spectrum were observed as is denoted by A, B, C, D and E in Figure 5 . The branches A and B are consistent with
783
the experiment carried out by another group [10,11]. Other branches C, D and E are the spectrum observed for the first time. Now we consider the branches A, B and C in the framework of fictitious spin = 1. The branches A and B correspond to the transitions from the state = 0 to = -1, and from the state = 0 to = 1, respectively. = 7.5 T appears the Under the magnetic field exceeding the first level cross field = 1 to = 0. The branch C which corresponds to the transition from the state slope of the straight lines in Fig. 5 corresponds to g = 2.6, which is consistent with that determined from the saturation magnetization [4]. The origin of the groups of the resonance lines, which belongs to neither A nor B, are not interpreted so far. One of the possible mechanisms is the contribution from some collective spin mode. According to the results of high field magnetization [5] the value of the anomalous magnetization is a little larger than 4 pB/Fe2'. By assuming that g is about 2, the = -2 at the field above 33 T. How the energy level of = possible ground state is -2 comes down? The branches D and E composed of strong absorption spectral lines were observed around 33 T coinciding the field where the anomalous magnetization jump appears, namely, the second level cross field The straight lines in branches D and E are drawn to fit the experimental data. If we formally determine the g-factor from their slopes, we obtain g = 3.7 for both branches of D and E. The value is too large considered from the calculated energy level diagram in Fig. 4. This fact means the rapid decreasing of the energy level = -2 in = 2 spin multiplet enhanced by the strong ferromagnetic intrachain coupling as has been proposed by Hori et al. [12]. The detailed qualitative discussion will be presented in a separate paper. References 1. H. Yoshizawa, W. Kozukue and K. Hirakawa, J. Phys. Soc. Jpn., 49 (1980) 144. 2. T. Haseda, N. Wada, M. Hata and K. Amaya, Physica, 108B (1981) 841. 3. M. Chiba, S. Ueda, T Yanagimoto, M. Toda, and T. Goto, Physica B, 204-208 (2000) 284. 4. M. Chiba, T. Tsuboi, H. Hori, I. Shiozaki and M. Date, Solid State Commun., 63 (1987) 427. 5. M. Chiba, A. Aripin, K. Kitai, T. Idehara, S. Ueda and M. Toda, Physica B , 294-295 (2001) 64. 6. T. Tsuneto and T. Murao, Physica, 51 (1971) 186. 7. M. Steiner, K. Kakurai, W. Knop, B. Dorner, R. Pynn, U. Happek, P. Day and G. McLeen, Solid State Commun., 38 (1981) 1179. 8. N. Suzuki, J . Phys. Soc. Jpn., 50 (1981) 2931. 9. W. B. Euler, C. Long, W. G. Moulton and B. B. Garrett, J . Magn. Resonance, 32 (1978) 23. 10. H. Ohta, N. Makita, K. Yoshida, T. Nanba and M. Motokawa, Int. J. Infrared Millim. Waves, 13 (1992) 457. 11. H. Ohta and M. Motokawa, Recent Advances in Magnetism of Transition MetalCompounds, (eds. A. Kotani and N. Suzuki, World Scientific, Singapore, 1993), p.316.. 12. H. Hori, I. Shiozaki, M. Chiba, T. Tsuboi and M. Date, Physica B, 155 (1989) 299.
784
EPR in the 21'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
Magnetic properties of Fe12 ring : ESR and magnetization measurements Y. InagakP*,T. Asano", Y. Ajiroa, Y. Narumib,', K. KindoCsb,H. Nojirid,", M. Motokawad, A. Cornia" and D. Gatteschif "Department of Physics, Kyushu University, Fukuoka, 8 12-8581, Japan bCREST JST Corporation, Kawaguchi, Saitama 332-0012 Japan 'KYOKUGEN, Osaka University, Toyonaka, Osaka 560-853 1 Japan kstitute for Materials Research, Tohoku University, Sendai, 980-8577, Japan "Department of Chemistry, University of Modena, Modena, Italy Qepartment of Chemistry, University of Florence, Firenze, Italy The results of ESR and magnetization measurements on the powder sample of Fe12 molecular magnetic ring are reported. A discrete structure of energy levels of the system in lower energy part is revealed directly through magnetization measurements up to 55 T at 0.1 K. The ESR spectrum shows anomalous temperature- and frequency-dependent behaviors which may be attributable to the formation process of magnetic ring.
1. INTRODUCTION Recent progress on the synthesis technique of assembled metal cluster has provided a rich variety of substances, enable to investigate the fundamental properties of molecular magnets. Recently, dodecanuclear Iron(II1) ring cluster [Fe(OCH,),(dbm],, (abbreviated as Fe12), where dbm = dibenzoylmethane, was synthesized successfully [11. The crystal structure is monoclinic (space group P2,ic) and each molecule is composed of twelve number of Fe3' (S=5/2) ions forming a ring with chemically equivalent bridging rigands. The data of magnetic susceptibility shows broad maximum around 150 K and was well fitted by the S=5/2 quantum antiferromagnetic chain model, except for the temperature regime below about 10 K, in which decreases suddenly, reflecting the discrete quantum energy levels with singlet ground state characteristic to this finite size system. In this paper, we report the results of high-field magnetization and electron spin resonance (ESR) measurements performed on powder samples of this interesting molecular magnet, Fe12.
Present address; *Graduate School of Science and Technology, Kobe University, Kobe 6578501, Japan. **Department of Physics Okayama University, Okayama 700-8530, Japan.
785
2. MAGNETIZATION High-field magnetization was measured by induction method, using pulsed high magnetic field up to 55 T, at the temperature down to 0.1 K. FIGURE 1 shows the magnetization curve for increasing field H up to 55 T at 0.1 K, together with the field derivative of magnetization Five distinct magnetization steps are clearly seen; the magnetization is zero at low field below about 10 T and it then increased by about 2 p, at each step with the field separation of about 10 T. The behavior is characteristic of the system with discrete energy levels with different total spin states I&+, 1, 2, ...30>, which is one important aspect of the quantum nature. In increasing each of the Zeeman level crossings raises the value of quantized magnetic moment in the ground state by one unit. The fact that the crossovers occur at regular interval implies that the lowest energy levels for every states are given by a Lande interval rule. We can determine the energy gap AE between the singlet ground state /ST=O>and the lowest excited state IST=l> to be 13.6 K, directly from the first transition field at about 10 T, using A = g p B q , and g = 2.02 from ESR. Using the relation of AE = with N = 12, the exchange interaction within a ring is also determined to be 40.7 K. These values of A E a n d J should be compared to the values of 10.7 K and 3 1.9 K estimated from the susceptibility data [ 11. In principle, the present determination is more direct and therefore more accurate in view of that the finite size effect should have to be considered in the analysis of susceptibility data. It is of interest to notice that the dM1dH curve is somewhat unusual in a sense that second peak is higher and narrower than others as seen in FIGURE 1. Besides, we found that the magnetization curve at 1.3 K exhibits some unusual features with an anomalous hysteresis for increasing and decreasing field; the magnetization change associated with each step occurs at two stages leading to a characteristic winged hysteresis loop with a plateau region These anomalous behaviors are discussed in detail elsewhere [2-41. h
12 . 0 , . ,
. , .
I . ,
.
I
.
2.08
1000
,
CJ
. 2
'-2.06
v
+! 0 0 0 0 0
x x x x x
f
g-value 0
10 20 30 40 50 60 Field (T)
FIGURE 1. curve and its field derivative curve observed at 0.1K.
-
- 0
0
2.04 .
: ::2.02 s -
2.00
1.98 50 100 150 200 250 300 Temperature (K)
FIGURE 2. Temperature dependence of signal intensity line width DH and g-value observed at X-band frequency.
3. ESR Submillimeter wave ESR measurements were performed up to about 800 GHz and in pulsed magnetic field up to about 30T at the temperature down to 1.7 K. Additional ESR measurements were performed at X-band, using a standard equipment. FIGURE 2 shows the temperature dependence of the signal intensity Z, g-value and line width at X-band (9.0 GHz) frequency. The signal intensity decreases rapidly accompanying with the increasing line width below 150 K which corresponds to the maximum of , and the resonance position shows a marked g-shift below about 20 K which corresponds to the sudden drop of These temperature-dependent behaviors are attributable to the formation process of magnetic cluster. At high temperature, the ESR signal is nothing but the transition of the exchange coupled paramagnetic Fe3' ions in a sense that the total spin states are obscured in the thermal window. As the system becomes to correlate below = 200 K, the character of signal gradually changes. In the intermediate temperature regime, the observed signal is the collection of various transitions within the excited low-lying multiplets which are sufficiently populated by thermal excitations. In this situation, however, there still exist thermally induced transitions between the different multiplets which is sufficient to cause the average of spectrum and therefore only the single averaged spectrum is observed. This averaging effect diminishes with decreasing temperature, causing the broadening of the line width. At the same time, the signal intensity also decreases because of a depopulation of higher energy states with large total spins. At sufficiently low temperatures where magnetic cluster is formed to give a singlet ground state and a characteristic energy gap becomes to be well defined in the thermal window, we expect to observe eventually a resolved structure. However, at these temperatures, only the first excited state is populated, giving rise to a single line without averaging effect. This explains the reason why the g-shift occurs below about 20 K. Around this temperature, the transitions within = 2 multiplet lose intensity and the triplet signal within = 1 state moved from the averaged position to its own position without the effect of other transitions. It is noteworthy that the observed temperature dependence of ESR intensity is not the same as that of susceptibility; the intensity decreases more rapidly than the susceptibility, suggesting that the transitions within some multiplets could not be detected practically due to the broadening. FIGURE 3 shows the temperature dependence of ESR transmission spectrum at 190 GHz. The main absorption line at g = 2 disappears around 10 K with decreasing temperature, accompanying with the g-shift. At the same time, a weak signal appears at low field as indicated by the solid triangle. It is now clear that the main line comes from the excited state resonance because of the loss of intensity with decreasing temperature. FIGURE 4 shows the frequency dependence of ESR spectrum at 1.7 K. A very broad absorption line with a typical powder pattern due to the anisotropy of about 0.8 K are clearly seen at high field where the ground state is ST=2multiplet. The broad line is attributable to the transition within the quintuplet state. Interesting enough, the low-field mode mentioned above coincides with the g = 4 line at high field, however, it seems to have a gap at zero field as indicated by the dotted line. This gap mode appears simultaneously with disappearing of main absorption. This behavior suggests that the origin of this mode is related to the cluster-
787
60K 55K 50K 45K 40K 35K 3OK 25K 20K 15K 1OK 4.2K 3.5K 3.OK 2.5K 2.OK
A
m
.3
? P
3
v
s
.-
m
c="
6 9 Field (T) Field (T) FIGURE 3 . Temperature dependence of ESR FIGURE 4. Frequency dependence of transmission spectrum at 190 GHz. Weak sig- ESR spectrum at 1.7 K nal indicated by the solid triangle is observed at low temperatures. 0
3
ing within a ring. In summary, we performed the magnetization and ESR studies on the powder sample of Fe12 over wide field and temperature range. From the magnetization at very low temperature, the discrete energy level of the system was revealed and the gap AE was estimated to be 13.6 K directly. From the ESR measurements, we discussed the formation process of magnetic cluster with decreasing of temperature. ACKNOWREDGEMENT
The authors would like to thank H. Nakano and S. Miyashita for useful discussions. This work was performed at KYOKUGEN, Osaka University and IMR, Tohoku University, and supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Y.I. was financially supported by Japan Society for the Promotion of Science (JPSJ). REFERENCES
Caneschi, Cornia, A. C. Fabretti and D. Gatteschi: Angew. Chem. Int. Ed. Engl. 38 (1999) 1295 2. H. Nakano and S. Miyashita: J. Phys. SOC.Jpn. 70 (2001) 2151. 3. H. Nakano and S. Miyashita: To be published in J. Phys. Chem. Solids. 4. Y. Inagaki et al. to be published. 1.
EPR in the 2 1'' Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
788
High magnetic field ESR measurements of ACu2(PO& (A=Ba, Sr) Masayuki Hataa, Seitarou Mitsudob, Toshitaka Ideharab and Mamoru Mekataa "Faculty of Engineering, Fukui University, Fukui 910-8507, Japan bResearch Center for Development of Far Infrared Region, Fukui University Bunkyo 3-9-1, Fukui 910-8507, Japan High magnetic field ESR measurements of BaCu2(PO& have been performed using a Gunn oscillator (120 GHz) at temperatures from 4.2 K to 140 K. Three resonance lines have been observed below 20 K, while only one resonance line has been observed above 20 K. Above 20 K, ESR integrated intensity was in good agreement with 4 spin cluster model. The best fit of ESR integrated intensity data was obtained with
= 25 K and
= 55 K. High magnetic field ESR measurements of SrCu2(PO& have been performed
using a Gunn oscillator (120 GHz) and Gyrotron (301 GHz) at temperatures from 4.2 K to
77 K. Both integrated intensity and magnetic susceptibility have a broad maximum at 40 K and in the best fitting of both integrated intensity and magnetic susceptibility is obtained with = 30 K and = 40 K for 4 spin cluster model.
1. INTRODUCTION It is known that A C U ~ ( P O (A=Ba, ~)~ Sr) crystallizes into a triclinic structure and belongs to the P-1 space group. Cu2+ions occupy four crystallographically different sites forming zigzag chains in the a-b plain [l].The A2+ ions separate Cu2+ chains and the magnetic coupling along the c-axis is diminished due to PO4 tetrahedra. The spacing of the Cu2+ions in a chain has sequence of -3.80A-3.60A-3.47A-3.58A-3.80A-
so
that the four spins form a relatively isolated group. Based on the results of x-ray diffraction, it is expected that Sr compound has the same crystallographic structure as Ba compound. Results of magnetic susceptibility measurements of both Ba and Sr compounds suggest that both of them have a singlet ground state. A broad maximum in magnetic susceptibility was observed at 60 K for Ba and 40 K for Sr compounds. Experimental results for Ba
789
compound were best fitting using the ladder model with energy gap of 33 K. On the other hand, experimental results of Sr compound were best fitting using 4 spin cluster model = 30 K and = 40 K The differences in the magnetic properties of these with two compounds have been investigated by ESR technique.
2. EXPERIMENTS ESR measurements were performed using powder samples of Ba and Sr compounds. The equipment used in experiments allowed to perform ESR measurements with pulsed magnetic field up to 35 T. Measurements could be performed in 4.2 K to 140 K temperature range. Experiments were performed at a fixed frequency of 120 GHz provided by a Gunn oscillator, and at 301 GHz provided by a gyrotron. A gyrotron is a electro magnetic wave radiation source with an output power in millimeter and submillimeter wave range. Output frequency of gyrotron can be step varied allowing for certain tunability, and its output power can be from several 10 W to several kW
I
'
I
~
I
'
I
'
I
'
w
77 K .1
66 K
h . I
E
?
60 K
-_
x - _ - _ _
-56K
* '
m
v
48 K
a . I
. I
E
-
,
-
34K
,
~~
a
-
_-
-
25 K
m
20 K
b
11 K I
4.2
DPPHl I
,
1
,
I
,
1
1
,
I
,
Figure 1. Temperature dependence of the ESR spectrum of BaCu2(PO&
790
3. RESULTS AND DISCUSSIONS
3.1. BaCuz(PO4)z High magnetic field ESR measurements of BaCuz(PO& have been performed using a Gunn oscillator (120 GHz) at temperatures from 4.2 K to 77 K, as is shown in Figure 1. Three resonance lines have been observed below 20 K, while only one broad resonance line has been observed above 20 K. It has also been found that the temperature increase from 20 K to 60 K leads to a slight shift of the resonance position toward the lower fields side and increasing temperature further from 60 K to 77 K results in the resonance position shift back towards higher field side. The results of g-value estimation from experimental data for temperatures above 20 K are shown in Figure 2. It turns out that g-value has a sharp peak at 60 K which coincides with the temperature of a broad maximum of magnetic susceptibility. Figure 3 shows the temperature dependence of integrated intensity for BaCu*(PO&. The circle point in Figure 3 shows the integrated intensity of ESR resonance lines, above 20 K. The temperature dependence of integrated intensity shows a broad maximum around 60 K. Below 60 K, as temperature decrease, it toward to zero at 0 K. These features are in agreement with the results obtained from the magnetic susceptibility measurements [ 2 ] . The line of (a), (b), and are the fitting lines using ladder model, alternating chain model, and 4 spin cluster model, respectively [4-61. The best fit of experimental data was obtained with Jl/kB = 25 K and J2/ks = 55 K in 4 spin cluster model. The triangle point in Figure 3
*
2.101
il 2.05
i
1.95 0
20
40 60 80 100 120 14 Temperature (K)
Figure 2. Temperature dependance of g-value for BaCuz(PO&.
0
(a) : ladder (b) : alternating chain (c) : 4 spin cluster (d) : Curie term
20 40 60 80 100 120 140 160 180 20(
Temperature (K)
Figure 3. The temperature dependence of integrated intensity for BaCu2(P04)2.
79 1
shows the integrated intensity of ESR resonance lines, below 20 K. These points are in agreement with calculated value using Curie term (d).
3.2. SrCuz(PO& High magnetic field ESR measurements of SrCu2(PO& have been performed using a Gunn oscillator (120 GHz) and Gyrotron (301 GHz) at temperatures from 4.2 K to 77 K. Since similar results were obtained at both frequencies, only results at 120 GHz are shown in Figure 4. Only one resonance line ( P I ) was observed at 77 K. Also, g-value of P I dose not depend on temperature and is equal 2.20. Below 40 K, two new resonance lines ( P 2 , P3) appeared. Figure 5 shows the temperature dependence of integrated intensity for SrCu2(PO4)2. The circle point in Figure 5 shows the integrated intensity of ESR resonance lines of PI. The temperature dependence of integrated intensity shows a broad maximum around 40 K. Below 40 K, as temperature decrease, it toward to zero at 0 K. This behavior is in agreement with the result obtained from the magnetic susceptibility measurements. The solid line is the fitting lines using 4 spin cluster model. The best fit of ESR data was obtained with JIIkB = 30 K and J 2 l k ~= 40 K. The triangle point in Figure 5 shows the integrated intensity of ESR resonance lines of
P2
and
P3,
below 40 K. These points are in
agreement with calculated value using Curie term (dotted line).
0 3
20
30 40 50 Temperature (K)
70
80
4 5 Magnetic field (T)
Figure 4. Temperature dependence of the ESR spectrum of SrCu2(PO4)2 at 120 GHz.
10
Figure 5. The temperature dependence of integrated intensity for SrCu2(P04)2.
792
4. CONCLUSION Integrated intensity of ESR absorption for both Ba and Sr compounds showes good agreement with 4 spin cluster model. It is consistent with crystallographic symmetry. However, the values of exchange interaction constants
obtained form the 4 spin
cluster model are different. Sr compound has smaller alternating exchange interaction in 4 spin cluster model than Ba compound one. In order to clarify the reason for experimentally observed differences between Ba and Sr compounds it is necessary to further investigate the crystal structure of the Sr compound. And also at 4.2 K, the difference of ESR spectrum between Ba and Sr compounds may be come from these differences.
REFERENCES 1.A. Moqine, A. Boukhari, and J. Darriet, J. Solid State Chem. , 107 (1993) 362. 2. M. Mekata, T. Hanabata, K. Nakaya, and Y. Ajiro, to be published in J. Magn. Magn. Mater. , 2001. 3. T. Idehara, I. Ogawa, S. Mitsudo, M. Pereyaslavets, N. Nishida, Y. Yoshida, IEEE Trans. Plasma Sci. , 27 (1999) 340. 4. M. Troyer, H. Tsunetsugu, and D. Warts, Phys. Rev. B, 50 (1994) 13515. 5. W. E. Hatfied, J.Appl.Phys., 52 (3) March 1981. 6. M. Hase, K. M. S. Etheredge, S. Hwu, K. Hirota, and G. Shirane, Phys. Rev. Lett. , 70 (1993) 3651.
EPR in the 21” Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
793
ESR transmission experiments on P’-(ET)2SF5CF2S03and (ET)2SF5NN02, investigations of spin-Peierls systems. I.B. Rutela, J.Brooks”, B.H. Ward’, D. VanDerveer“, M.E. Sitzmannd, J. Schluetef, R.W Winter‘, G. Gardf aDepartment of Physics, FSU/NHMFL, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310, USA ’School of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 323 10, USA ‘School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA dNaval Surface Warfare Center, Indian Head, MD 20640, USA ‘Chemistry and Materials Science, Argonne National Laboratory, Argonne, IL 60439, USA fDepartment of Chemistry, Portland State University, Portland, OR 97207, USA An anisotropic temperature dependent ESR investigation of the organic systems (ET)2SF5CF2S03and (ET)2SF+J”N2 has led to the indication of a spin-Peierls transition at TSP= 33, and 5 K, respectively. We present the details of our investigation and place our data in the context of the general spin-Peierls theory. We also show the dc susceptibility SQUID measurements revealing novel behavior in the second material. Note that to access the high field regime, large fields and high frequencies are needed.
1. INTRODUCTION This paper discusses two organic materials, and the investigation of whether the spinPeierls transition describes the behavior of each using magnetic resonance techniques. Following the introduction, data collected on the two compounds P’-(ET)zSF&SSO3 (CF2), and (ET)2SFsNNOz (”Oz), (where ET is will be presented in the general context of spin-Peierls theory. Crystals of both salts were grown by electrocrystallization methods previously described. The spin-Peierls (SP) transition is a second order magnetic transition that is defined by a structural distortion and subsequent dimerization of the lattice. The transition is best described as a one-dimensional (1D) chain of Heisenberg antiferromagnetic spins that at a given temperature, TSP,progressively dimerize into spin singlet pairs. Consequently, both the anisotropic behavior in the susceptibility is seen, as well as the susceptibility vanishing with
*
794
decreasing temperature. This is easily distinguished from a three-dimensional (3D) antiferromagnetic ordering by the decreasing of the susceptibility to zero along all three crystallographic axes in the SP material as opposed to just the easy axis in an antiferromagnetic material.2 Higher temperature behavior, antiferromagnetic fluctuations, is best described by Bonner-Fisher theory (BF), from which the coupling constant, can be determined.3
2. EQUIPMENT AND TECHNIQUES Three techniques were used in characterizing the materials, a cavity resonance transmission technique and a direct transmission technique. The cavity resonance technique was used in conjunction with a Millimeter-wave Vector Network Analyzer (MVNA), with a continually tunable (8-18 GHz) YIG source. Schottky diodes frequency multiply and mix the microwave source to frequencies ranging from 29-220 GHz. These signals then propagate through waveguides to a resonant cylindrical cavity that is 9.55 x 9.31 mm (height x diameter) exciting the fundamental mode (TEoll) of 41 GHz4,5 . ,Ouxlo* The experimental set-up is nearly 95
5
identical for the direct transmission technique, but allows 85 for measurements, which are impractical 75 with resonant cavities, since the cavity inn isn 2511 3uu "> l/T [KI] volume decreases [Kl inversely with Figure 1. a) SQUID data for the CF;, (open diamonds) material with frequency. The probe the BF fit. b) SQUID data for both samples plotted on an Arrhenius has the mixer diode plot, showing the possible onset of two separate transitions, in the replaced with an NZ "02 sample (filled diamonds) and CF2. cooled detector, a Gunn oscillator multiplies the diode output, and the sample is placed in the waveguide allowing only radiation that passes through the sample to be detected. The resonant cavity is replaced with two 45" mirrors that direct the radiation to the detector. =1
YO
I
02
03
"4
"3
795
A dc SQUID measurement was also carried out on the "02 material, using a Quantum Interference Design SQUID, where the field was held at a constant 1 T, and the temperature varied from 350 to 1.7 K.
I
I
.M
3.
5
5 4000A The ~ ' - ( E T ) Z S F ~ C F ~isSaOshiny ~ 0055 s . + black rectangular crystal with a p' packing 0 3000 crystal structure. Previous investigation of 0 ; 2 l CF2 has been performed to determine the A 0 nature of the observed magnetic transition - -0.05 2000 at 33K4. Previous dc susceptibility measurements, SQUID, indicate the -0.10 1000 susceptibility vanishes with exponential T [K] . +-Hllc-axis behavior (Figure la). Further Hlla-axis investigation was performed in order to I determine the characteristics of the 100 l o T[K] transition. ESR experiments were performed on Figure 2. The temperature dependence of the all three crystallographic directions EPR integrated absorption for the resonance providing the data in Figure 2 . The runs line at 67.5 GHz for all three orientations were performed between 10 and 50 K. The (Hlla,Hllb, and Hllc ) for CF2. Areas show spectra contain two distinct absorption anisotropic intensities due to coupling lines corresponding to the ET and DPPH differences of the crystal with the cavity mode signals, where the DPPH was used as a as the sample was rotated. The top inset marker to calibrate the magnetic field. shows a perspective view of the ET cation The spectra were analyzed by fitting layer in the crystal looking down the long axis the ESR absorption signal to a Lorentzian of the molecule. The bottom insert shows the line shape and then integrated. The g-shift normalized by the g value at 9 K. The integrated area, which is proportional to largest change is seen along the b axis the magnetic susceptibility, is then plotted showing direct support for the ET molecules against temperature in Figure 2. The being dimerized and forming a 1D chain susceptibility clearly displays a vanishing along the b axis. Symbols correspond to axis exponential behavior along all three orientation in for both graphs. Open triangles directions showing the characteristic spinindirnte dntn t n k m nt 3 3 8 5 GH7 Peierls behavior. Measurements of both increasing and decreasing temperature show no hysteretic behavior (i.e. 1St order behavior). The activated gap values and relating transition temperatures were found using an exponential function, fit over the transition, ranging from 3 to 33 K6. This calculation yields a gap (A,,) and TSPof 114 (*21) K and 33 (*7) K respectively, where the uncertainty arises from fitting over slightly different ranges of temperatures. The transition onset seems to be lower than 33K, but our value fits well within experimental error. A BF analysis was also performed in 4-
Y
--
* -
796
the high temperature regime of the SQUID plot yielding a magnetic exchange constant JBFof 257 K (Figure 3b). In the inset of Figure 2, we investigate the temperature dependent g-shift of the CF2 material. The largest change in the g-value occurs along the b-axis (-0.12%), where the baxis is along the plane (bc) of dimerized ET molecules. The J3' crystal packing symmetry results in the initially dimerized ET planes. These ET dimers then show long-range antiferromagnetic correlation and singlet ordering, consistent with spin-Peierls theory. This leaves the ET dimers paired with neighboring ET dimers, predominantly in the b direction (some distortion in the a direction is reported by Pigos et al.7, evident in the inset by a small change in the a-axis g-shift as well). Overall, the lattice distortions have dimerized quartets of ET molecules, which a detailed Xray analysis can confirm.8 For completeness we now turn to the Figure 3. The temperature dependence of higher frequency (i.e. field) investigation. The the EPR integrated absorption for the high field runs were carried out using a direct resonance line at 79.9 GHz for all three transmission probe with frequencies >220 orientations (Hlla, Hllb, and Hllc ) for GHz. By performing experiments at these " 0 2 . The dashed line is an exponential frequencies we are able to probe the field vs. fit to the data along orientation 1. The temperature phase diagram, the high field top inset shows the orientation of the regime, and the universality of the spin-Peierls sample with respect to the magnetic field transition. (i.e. Hlllong axis (a) for orientation 3). Our experiments were carried out at 228.5 The second inset is the crystal structure GHz and -1 1 T. Our data show (open triangles of the "02 material, stacking is along in Figure 2) the transition temperature moving the a axis shown in perspective, into the with the increase in field. We have also seen a page. The final inset is the temperature broadening of the line width with decreasing dependent g-shift, where the onset of 3D temperature, which may suggest the onset of a lattice distortion is evident as a first order transition at 22 K as well, providing percentage change from the g-value at 30 evidence of a possible intermediate high field K, for all 3 directions. Note the feature phase. (dip) in Orientation 2, see text for details. A similar analysis has been camed out on the (ET)2SFsNN02 materialg, and can be seen in Figure 3. The sample has been placed in a cavity varying the position to line up each crystallographic axis with the magnetic field. This material is a dark brown whisker like crystal with a primitive space group, and preliminary dimensions of 6.52, 15.91, and 15.97 8, for a, b, and c respectively, with angles of 97.2",
797
90.3", and 90.2" for and A view of the crystal structure is provided in the inset of Figure 3. We are uncertain of the packing symmetry due to the preliminary nature of the crystallography. The sample was placed in the same cavity from the CF2 investigation, and the ESR data was collected at a frequency of 79.87 GHz corresponding to a field of 2.54 T, from 1.5 to 50 K. The material shows a precipitous drop in the susceptibility at 8 K. This drop is seen along all three of the principal axes and is confirmed to be second order in nature (from similar hysteresis arguments as above), but has not been observed to reach zero (as opposed to the CF2 sample). The signal is weakest when the field is oriented along the stacking direction, aaxis, and when normalized has the largest error, seen in the plot scatter. The transition temperature and gap value of TSP= 4.6 K, and A. = 8.07 K respectively, has been calculated using a linear fit on a plot of SQUID data of In vs. inverse temperature (i.e. an Arrhenius plot) as shown in Figure lb. A coupling constant of JBF= 341 K was also determined using BF theory, but does not fit well in the high temperature regime. Analysis of the g-shift shows marked distortion along all principle axes below 8 K. This is consistent with the precursor lattice distortion seen just prior to the onset of the spin-Peierls transition. It appears that the quasi ID chain axis is not confined to any one of the principle crystal axes. Also of note, is the g-value shift in the second orientation, which distorts in a non-monotonic manner at just above the precursor onset of the transition; suggesting a possible frustration in the lattice from a higher temperature ordered state. Evidence of this state is seen in the SQUID data (Figure 2b) which clearly shows a second exponential transition at 53 K. This high temperature order is unexpected, and not seen in the CF2 material, or other spin-Peierls systems and deserves further investigation. 4. DISCUSSION
A universal phase diagram has been suggested for spin-Peierls materials1°-13. The two well understood phases are the uniform (U),and Dimerized (D) phases shown in Figure 4. The (U) phase has the spins equidistant, and each spin has equal magnetic coupling (i.e. a single Jvalue). In the Figure 4. Plot of Phase Diagram for Dimerized phase, the spins are distorted and now both CF2 and " 0 2 samples. alternate coupling constants between intra-dimer and Universality arguments are inter-dimer interactions.2. Guided by previous data introduced with broken lines for presented for compounds like CuGeOa, we present both compounds. Figure 4, and the proposed universality arguments for both compounds described here. The final phase is the (I) phase, and has an intermediate, or incommensurate state. Given a BCS-like transition a general rule for the relationship between critical temperature for = 0, and the critical field for = 0 is approximately
798 = kB/b (i.e., the ratio of the Boltzmann constant to the Bohr magneton). We might expect that would be of the order 35 and 4 T, for CF2 and "02 respectively. High field and frequency measurements will be carried out to better understand the universality of both compounds and the nature of the (I) phase itself.
5. CONCLUSION Two different organic class materials show strong evidence of spin-Peierls behavior. The ESR data show that these materials behave in a Bonner-Fisher like manner in the high temperature regime, which is then followed by an exponential isotropic decrease in the susceptibility with temperature around the transition. We note the details of the crystal structure are not well determined, and there is also a feature in the higher temperature susceptibility. Due to the high temperature of the spin-Peierls transition and the ESR up to 11 T, it seems evident that further high magnetic field ESR will be necessary to compare these materials with others, already studied. Acknowledgement. Work at NHMFL-FSU was supported by NSF Grant No. DMR-99-71474 and NHMFL/MRP 500/503 1. The NHMFL is supported through a contractual agreement between the NSF through Grant No. NSF-DMR-95-27035 and the State of Florida. Work at ANL is supported by US-DOE, Office of Basic Energy Sciences, Division of Materials Sciences under contract W-31-109-ENG-38. Research at PSU is supported by NSF grant No. CHE-9904316 and the Petroleum Research Fund ACS-PRF 34624-AC7. Thanks also to Sergei Zvyagin and Fivos Drymiotis.
REFERENCES B. H. Ward, J. A. Schlueter, U. Geiser, Chem. Mat. 12, 343 (2000). J. W. Bray, L. V. Interrante, I. S. Jacobs, in edited by J. S. Miller (Plenum Press, New York, 1982), Vol. 3, p. 353. J. C. Bonner and M. E. Fisher, Phys. Rev. 135,640 (1964). B. H. Ward, I. B. Rutel, J. Brooks, J. Phys. Chem. B 105, 1750 (2001). S. Hill, J. S. Brooks, J. S. Qualls, Physica B 246-247, 110 (1998). 6 A. C. Rose-Innes and E. H. Rhoderick, (Pergamon Press, New York, 1988). J. M. Pigos, B. R. Jones, J. L. Musfeldt, Chemistry of Materials 13, 1326 (2001). J. A. Schlueter, et al. to be published . B. H. Ward, et al. crystal data to be published. l o J. Northby, H. Groenendijk, and L. Jongh, PRB 25,3215 (1982). V. Kiryukhin, B. Keimer, and D. Moncton, PRL 74, 1669 (1995). l 2 U. Ammerahl, T. Lorenz, B. Buchner, Zeitschrift Fiir Physik 102, 71 (1997). l3 T. Hijmans, H. Brom, and L. Jongh, PRL 54, 1714 (1985).
EPR in the 2 1" Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
799
High field ESR measurements on molecular oxygen Shojiro Kimura and Koichi Kindo KYOKUGEN, Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-853 1 Japan
The high field ESR measurements have been performed on solid molecular oxygen in the frequency region from 94 to 1400GHz using the pulsed magnetic field up to 55T at 4.2. It turned out that the observed antiferromagneticresonance modes can be explained by the molecular field theory for the two-sublattice model.
INTRODUCTION Among the simple diatomic molecules, molecular oxygen 0 2 is of particular interest because it has the magnetic moment. The ground state of the molecular oxygen is state with While the molecular oxygen shows simple paramagnetic nature in the gas and the liquid phases, the solid molecular oxygen below the melting temperature Tmp=54.4K shows two successive structural phase transitions at T p ~ 4 3 . 8 Kand Tap=23 . 9 K accompanied with drastic changes of the magnetic properties. It is known that the antiferromagnetic ordering of the magnetic moments realizes in the low temperature phase below Tap, whereas there is no long range order in the p and the y-phase above Tap. The crystal structure of a-oxygen is monoclinic, space group C2/m [ 11. There are two molecules in the unit cell and the figure axis of the molecule is parallel to the c*-axis. The magnetic properties of a-oxygen have been investigated by the magnetic susceptibility, neutron scattering, high field magnetization and the far infrared (FIR) spectroscopy measurements [2-61. The neutron scattering measurement shows that, the magnetic structure of a-oxygen is the two-sublattice type, and the easy axis turned out to be the b-axis [21. The spin-flop transition at &=6.9T was observed by the high field magnetization measurement [ 3 ] , and it was revealed from the
spectroscopy
800
Figure 1 . Block diagram of our experimental setup. measurements, that two antiferromagnetic magnon modes exist in far infrared region at 190GHz and 810GHz [4-61. However, the field dependencies of these magnon modes are not so clear at this time. In this study, to clarify the antiferromagnetic resonance (AFMR) modes of a-oxygen , we have performed the high field ESR measurements on solid molecular oxygen in the frequency region from 94 to 1400GHz using the pulsed magnetic field up to 55T.
2. EXPERIMENTAL The high field ESR measurements on solid molecular oxygen were performed using the far-infrared laser (FIR laser) and the 94GHz Gunn oscillator, and the pulsed high magnetic field up to 55T at 4.2K. The magnetically enhanced InSb detector made by
QMC instruments was used. Figure 1 shows the block diagram of our experimental setup. The light from the laser and the Gunn oscillator is guided by the light pipe to the sample, which is located at the center of the magnet, and the transmitted light through
80 1
.............................
1182.OGHz
H(T) 10
20
30 40 H(T)
50
C
Figure 2. ESR absorption lines of a oxygen observed at 4.2K.
Figure 3. Frequency-field diagram of oxygen observed at 4.2K. Solid circle and open square show this study and the results by the FIR spectroscopy measurement [6], respectively. Solid lines correspond to the theoretical lines obtained from the molecular field theory.
the sample is reflected by the mirror just below the sample. The reflected light passes through the pin hole on the 45" mirror above the cryostat, and is detected by the magnetically enhanced InSb detector. In the measurement on the molecular oxygen, Si02 glass plated with silver is used for the light pipe of the cryostat. The oxygen gas is condensed on the bottom of the light pipe at liquid nitrogen temperature, and then slowly cooled down to 4.2K. Obtained solid molecular oxygen is a polycrystal.
3. RESULTS AND DISCUSSION Figure 2 shows the frequency dependence of the ESR absorption lines of a-oxygen observed at 4.2K. The ESR absorption lines accompanied with several peak structures, indicated by the arrows, are observed. Even using the polycrystalline sample, one can distinguish the resonance absorption lines for the field applied along the magnetic principal axis as such structures, because the density of state of the magnon for the direction of the each principal axis is large [7]. Two absorption lines B and C are observed near the resonance field of g=2. With increasing the frequency, the resonance
802
fields of those increase, and new absorption line A appears above 847.OGHz. The resonance field of the absorption line A approaches to that of the B, as the frequency is increased. The broad structure, observed at 1182.OGHz and 1392.8GHz in the high field region, may come from the interference effect and is considered to be not intrinsic. The origin of a broad one between the absorption lines A and B is, however, not clear at the moment. Figure 3 shows the frequency-field diagram of the observed ESR absorption lines. Open squares and dotted line in Figure 3 show the frequencies of the antiferromagnetic magnon at zero magnetic field, observed from the FIR spectroscopy measurements [4-61, and the spin-flop field Hc [3], respectively. The obtained AFMR modes are consistent with the results of the far infrared spectroscopy measurements, and are typical of the two-sublattice antiferromagnet with the orthorombic anisotropy. As shown in Figure 3, the AFMR modes can be explained by the conventional molecular field theory for the two-sublattice model using the parameters which were determined in ref. [3] from the high field magnetization and the FIR spectroscopy measurements. More detailed discussion will be published elsewhere.
ACKNOWLEDGMENTS This work was supported by Reseach Fellowship of the Japan Society of the Promotion
of Science for Young Scientists.
REFERENCES 1. C. S. Barrett, L. Meyer and J. Wasserman, J. Chem. Phys., 47 (1967) 592. 2. R. J. Meiyer and R. B. Helmholdt, Phys Rev., B 29 (1984) 1387. 3 . C.Uyeda, K. Sugiyama and M. Date, J. Phys. SOC.Jpn., 54 (1985) 1107. 4. T. G. Blocker, M. A. Kinch and F. G. West, Phys. Rev. Lett., 22 (1969) 853. 5. E. J. Wachel and R. G. Wheeler, Phys. Rev. Lett., 24 (1970) 233. 6. R. J. Meier, J. H. P. Colpa and H. Sigg, J. Phys. C: Solid State Phys., 17 (1984) 4501. 7. H. Ohta, N. Yamauchi, T. Nanba, M. Motokawa, K. Kawamata and K. Okuda, J. Phys. SOC.Jpn 61, (1992) 149.
EPR in the 21“ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
803
High field ESR measurements of Cu,(C,H,,N,),Cl, under high pressure M. Saruhashi”, T. Sakurai”,’, K. Kiritaa,b,T. Kunimoto’, S. Okubo‘,‘, H. oh tab^', H. Kikuchid, Y. Uwatoko” ’The Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan bVenture Business Laboratory, Kobe University, Kobe 657-8501, Japan “Molecular Photoscience Research Center and Department of Physics, Kobe University, Kobe 657-8501, Japan dDepartment of Applied Physics, Fukui University, Fukui 91 0-8507, Japan eInstitute for Solid State Physics, University of Tokyo, 5-1 -5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan We have performed high field ESR measurements of Cq(C,H,2N,)2C14single crystal under ambient pressure and 2 k bar pressure for H//g,. The measurement is performed at 80 GHz, and temperature region is from 1.8 to 6.0 K. The integrated intensity of the absorption lines decreases rapidly as the temperature is decreased for ambient pressure, while the integrated intensity shows the upturn below 4 K for 2 k bar. On the other hand, the temperature dependence of the linewidth under ambient pressure increased as the temperature is decreased, while that under 2 k bar was almost independent of the temperature.
1. INTRODUCTION Low-dimensional antiferromagnets are interesting system where quantum fluctuations play a important role. An important class of such systems are the S=1/2 spin-gap systems and
Cu,(C,H,,N,),CI, (abbreviation as CHpC) is one of such system. Crystal structure of CHpC takes the monoclinic system and space group P2,/c [l]. The Cu dimer formula units in CHpC stack to form double chains along the [loll direction. Therefore, CHpC is proposed to be a S=1/2 spin ladder system [2]. Correspondingly, the magnetic interactions between Cu spins are thought to yield a spin ladder with a spin Hamiltonian written as
,=,
where
J,
,=I ,=I
and
J,,
are the exchange constants along the rung and the leg, respectively, and s, ,
804
is an S=112 spin on the i-th leg and the j-th rung. was evaluated to be about five by the analysis of the magnetic susceptibility and the magnetization data [3]. The temperature dependence of the magnetic susceptibility shows a maximum around 8 K and it tends to zero below 8K which shows that CHpC has the singlet ground state. The energy gap of one dimensional Heisenberg antiferromagnet (1D HAF) between the singlet ground state and the first excited state can be suppressed under a certain applied magnetic field Hcl. From the magnetization measurements at 0.42 K, the magnetization appears above the critical field Hcl=7.5 T and it saturates at H,,=13 T. Our previous high field ESR measurements at 1.8 K revealed that there exists a magnetic phase transition not only at Hcl but also at Hm=lO.l T.141. We also found the direct transition between the singlet ground state and the excited state below 1 K 151. The mechanism of these high field ESR results are still under discussion. Recently the magnetic susceptibility measurements of CHpC under pressures have been performed [6]. Under the pressure, CHpC has organic structure. The magnetic properties changes are expected in CHpC. An upturn was observed at several k bar as the temperature is decreased below 8 K. The upturn of the magnetic susceptibility disappears when the pressure is released to ambient pressure. Therefore, it cannot be considered as a simple paramagnetic impurity centers formed by the pressure, and the origin of these behaviors under pressure is not clear at the moment. Recently we have developed high field ESR system under high pressure using the pulsed magnet [7j. The main aim of this study is to clarify the origin of the above anomaly of the magnetic susceptibility under high pressure. We performed high field ESR measurements of CHpC single crystal under ambient pressure and 2 k bar pressure.
2. EXPERIMENTAL Single crystals of CHpC were synthesized by means of.the slow evaporation method from a methanol solution of CHpC. Detail of synthesis was described in Reference [ 11 and [4]. The millimeter wave ESR measurements of CHpC single crystal under ambient pressure and 2 k bar pressure are performed using high pressure ESR system of Kobe university. High pressure is generated by the piston cylinder type cell. Detailed describe of the high pressure ESR system can be found in Reference [7]. The applied magnetic field is perpendicular to the crystal plane which corresponds to Hllg, 141. The millimeter wave ESR measurements are performed using the pulsed magnetic field up to 16 T and Gunn oscillators of 80 GHz as light sources . The measured temperature range is from 1.8 K to 6.0 K [8,9,10]. 3. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence of the ESR spectra under the ambient pressure for 80 GHz. The intensity of absorption lines decrease with decreasing the temperature. This correspond to the temperature dependence of the magnetic susceptibility which has a peak at 8 K [2j. Figure 2 shows the temperature dependence of the ESR spectra under 2 k bar pressure for 80 GHz. The intensity of absorption lines increase distinctly with decreasing the temperature. It is clearly seen as compared with the paramagnetic behavior of DPPH signal.
805
1.8K
v ,
2.5
.
.
.
.
B
#
.
.
.
.
I
.
.
.
.
3.5
2
2.5
3.5
3 B (T)
Figure 2. The temperature dependence of the Figure 1. The temperature dependence of the absorption lines under lbar for 80GHz. absorption lines under 2kbar for 80GHz.
.
3.0: 2.5: 2.0-
25 :
0
20 :
.
15 :
1.5: 10:
1.0:
.
0
.
0.5 : n ' .
I
2 3 4 5 6 7 8 T (K) Figure 3. The temperature dependence of normalized integrated intensity.
"0
1
2 3 4 5 6 7 8 T (K) Figure 4. The temperature dependence of linewidth.
"0
1
For 2.0 K ESR spectra of Figure 1 and Figure 2 deformed back ground are observed. It comes from the unstable bubbling of liquid He because the sample is immersed in liq. He in the pumping cryostat. The temperature dependence of the integrated intensity of the absorption lines under ambient pressure and 2 k bar are shown in Figure 3. The integrated intensity under ambient pressure decreased rapidly as the temperature is decreased below 5 K, while that under 2 k bar showed the upturn below 4 K. This behavior seems to be consistent with the results of Reference [6]. The temperature dependence of the g-value for ambient pressure takes almost constant value of about 2.047 above 6 K, and increases below 6 K as the temperature is decreased [I I]. On the other hand, the temperature dependence of the g-value for 2 k bar pressure seems to stay constant around 2.047. We would like to point out that we did not observe new impurity resonance, and the pressure affected only the temperature dependence not the g-value. Moreover, the temperature dependence of the linewidth under
806
ambient pressure increased as the temperature is decreased, while that under 2 k bar was almost independent of the temperature (Figure 4). The results of the temperature dependences of g-value and linewidth seem to suggest that the short range ordering in the low dimensional antiferromagnet is suppressed by the pressure in CHpC. The origin of this mechanism remains as a future problem. 4. CONCLUSIONS
The high field ESR measurements of CHpC have been performed in the temperature region from 1.8 K to 6.0 K under ambient pressure and 2 k bar pressure. The temperature dependence of the integrated intensity under pressure is consistent with the magnetic susceptibility and we observed no new impurity resonance. However the temperature dependence of the g-value and linewidth under pressure suggested the suppression of the short range ordering in CHpC. ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research (B) (No. 10440109), and Grant-in-Aid for Scientific Research on Priority Areas (A) (No.11136231, 12023232 Metalassembled Complexes, No. 12046250 Novel Quantum Phenomena in Transition Metal Oxides) and (B) (No. 13130204 “Field-Induced New Quantum Phenomena in Magnetic Systems”) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES 1. B. Chiari, 0. Piovesana, T. Tarentelli and P. F. Zanazzi, Inorg. Chem., 29 (1990) 1172. 2. G. Chaboussant, P. A. Crowell, L. P. Levy, 0. Piovesana, A. Madouri and D. Mailly,
Phys. Rev. B, 55 (1997) 3046. 3 . M. Hagiwara, Y. Narumi, K. Kindo, T. Nishida, M. Kaburagi and T. Tonegawa, Physica B, 246-247 (1998) 234. 4. H. Ohta, T. Tanaka, S. Okubo, S. Kimura, H. Kikuchi and H. Nagasawa, J. Phys. SOC.Jpn., 68 (1 999) 732. 5. H. Ohta, Y. Oshima, T. Sakurai, S. Okubo, T. Tanaka, K. Koyama, M. Motokawa, H. Kikuchi, H. Nagasawa and J. P. Boucher, J. Magn. Magn. Mater., 226-230 (2001)439-440. 6. M. Mito, Private communication and Meeting Abstracts of the Phys. SOC.of Japan (2001). 7. H. Ohta, S. Okubo, T. Sakurai, T. Goto, K. Kirita, K. Ueda, Y. Uwatoko, T. Saito, M. Azuma, M. Takano and J. Akimitsu, Physica B, 294-295 (2001) 624. 8. M. Motokawa, H. Ohta and N. Makita, Int. J. Infrared & MMW, 12(2) (1991) 149-155. 9. S. Kimura, H. Ohta, M. Motokawa, S. Mitsudo, W-J. Jang, M. Hasagawa and H. Takei, Int. J. Infrared & NMW, 17(5) (1996) 833-841. 10. N. Nakagawa, T. Yamada, K. Akioka, S. Okubo, S. Kimura, and H. Ohta, Int. J. Infrared &NMW, 19(2) (1998) 167-176. 11 H. Ohta, S. Okubo, S. Kimura, T. Sakurai, S. Takeda, T. Tanaka, H. Kikuchi and H. Nagasawa, Applied Magn. reson., 18 (2000) 469.
EPR in the 2 l XCentury A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Published by Elsevier Science B.V.
807
ESR at ultra-low temperatures and observation of new mode in Cu-Benzoate T. Sakona, H. Nojirib, K. Koyama", T. Asano', Y. Ajiro' and M. Motokawa" a
b
Institute for Materials Research, Tohoku University, Sendai, 980-8577 Japan Department of Physics, Okayama University, Okayama, 700-8530 Japan Department of Physics, Kyushu University, Fukuoka, 8 12-8581 Japan
The development of the ESR measurement system at ultra low temperature using a 3He4He dilution refrigerator and a vector network analyzer has been achieved. For the first experiment, the ESR was measured on a quantum spin chain Cu-Benzoate down to 160 mK. 1. INTRODUCTION
Low dimensional quantum spin systems have attracted much attention by interesting quantum phenomena. A magnetic resonance measurement is powerful tool to observe the dynamical physical properties in spin system such as quantum fluctuations. NMR or pSR measurements have been carried out at ultra low temperatures and many notable results were achieved [l]. On the contrary, as for ESR measurement, there are hardly any experiments using dilution refrigerator. This is because of the technical difficulties such as heat leakage through the wave-guide, or heating by microwave power. In this study, the development of the ESR measurement system at ultra low temperature by using a 3He-4He dilution refrigerator and a vector network analyzer, which has ultra-high sensitivity, has been achieved. As the first application of our equipment, we have studied low temperature behavior of Cu benzoate, that is one of the ideal one-dimensional = 112 quantum spin systems. Many ESR and other studies have been done since the beginning of 1960s by Date and his collaborators [2 - 41. As for low temperature ESR studies, Oshima et a1 performed pioneer studies using 3He cryostat more than two decades ago [2]. In addition to the paramagnetic resonance, they observed a new resonance lines at low temperature and they interpreted as an appearance of the long-range antiferromagnetic order (LRO). In the specific heat measurement, however, no obvious transition to antiferromagnetic state was observed. Recently a broad peak was found when magnetic fields were applied [ S ] , but this was not a clear indication of the long range magnetic order. On the other hand, Oshikawa and Affleck [6, 71, and Essler and Tsverik [S, 91, proposed a theory that a gap is induced by an applied magnetic field caused by breather excitation and a crossover occurs from the paramagnetic state to the breather gap state in magnetic field.
808
Sample
Cavity
Figure 1. The schematic picture of the
system using a 3He-4Hedilution refrigerator
This theory was confirmed by specific heat and neutron scattering investigations According to this theory, a precise study was performed and a clear anomaly that considered to be due to the field induced breather gap was found at 0.5 K [lo]. This experiment was done applying external magnetic field parallel to the c-axis, that is the direction of the largest field-induced gap. In order to confirm this phenomenon, similar experiment must be done in another direction. In the case of H//b-axis, however, the magnitude of the gap is very small even in a magnetic field conventionally available and 0.5 K is not a sufficiently low temperature to observe the breather mode. This is the motivation of this experiment. 2. APPARATUS
Figure 1 shows a schematic picture of the ESR system. ESR measurements have been performed using a vector network analyzer (Al3 millimetre Co., Ltd.) in the frequency region between 54 GHz and 111 GHZ using a 20 T superconducting magnet. Using the vector network analyzer, full tunability of frequency enabled us to purge a mechanical tuner to adjust the length of a cavity, which is very difficult to do at ultralow temperatures. High-sensitive measurements are possible using a cylindrical resonant cavity. The loaded @value is 10000 at lowest temperature. In order to achieve a good thermal contact between the sample and the cavity, the sample was fixed directly on the endplate of the cavity. [ 1I ]
809
The cavity is installed in a 3He-4Hedilution refrigerator (Kelvinox System, Oxford Co., Ltd.). The cooling power is 25 pW at 100 mK. The cavity holder was fixed directly on the Cu plate, which is connected to the mixing chamber by a copper pipe covered with a stainlesssteel pipe. Between the cavity and the analyzer, rectangular wave-guides WR 22 made from Cu-Ni alloy of low thermal conductivity were used for the microwave transmission. In order to suppress the heat leakage from the room-temperature area, several heat anchors were attached at the still heat exchanger and the mixing chamber. Moreover, a sapphire plate and black paper were inserted as a cold filter, as shown in Figure 1. In order to cool the sample more efficiently, meshed Cu wires were connected between the cavity and the mixing chamber. The temperatures were measured by a ruthenium oxide thermometer (Scientific Instruments Co. Ltd.) attached to the cavity. The lowest temperature of the cavity was 155 mK. The power of the microwave in the cavity is estimated to be one pW, which is much smaller than the cooling power of the dilution refrigerator. Varying the input power by more than 40 dB, we checked the heating of the sample. The heating due to the microwave was only 2 or 3 mK at the lowest temperature. When the ESR measurement was carried out, the field sweep speed was very slow, about 0.05 TI min, in order to avoid the Eddy current heating of the cavity. All measurements were performed in the Faraday configuration, where a propagation vector of the radiation is parallel to the external magnetic fields. Single crystals grown by the diffusion method and rectangular-shaped samples with typical dimensions of 0.5 x 0.5 x 0.1 mm3 were used for the measurements. The sample quality was confirmed by X-ray diffraction and magnetic susceptibility measurements.
3. RESULTS AND DISCUSSTION
The temperature dependence of the ESR spectrum for b is shown in Figure 2. The microwave frequency was 57.423 GHz and the lowest temperature of the sample was 160 mK. Although the microwave power was very weak, clear ESR absorption line was detected. According to the theoretical prediction, it changed by varying temperature. At high temperature around 4.2 K, a sharp absorption line due to a conventional paramagnetic resonance was observed. By lowering temperature, the absorption line became broader and weaker. At low temperature around 0.6 K, a new absorption line appeared. However, the line is very broad. The absorption became stronger and sharper as the temperature decreased. The crossover temperature from paramagnetic region to low temperature region for was same as that obtained by the specific heat measurement as well as the case of [5]. Then the behavior for was identical with that of previously reported and the new mode was considered to be the gap mode (breather excitation). Further we measured the frequency dependence of the ESR spectrum at = 220 mK. The peak field increased with increasing frequency, and the frequency dependence of the resonance field was nonlinear. Theoretically, the field dependence of the resonance field is written as [ 121,
810
800 mK
mK
W
-
c
-500 mK 400 mK
250 mK
B B
mK 190 mK
160 mK
1.9
2.0
Magnetic Field (T)
Figure 2. The ESR spectra at the frequency of 57.423 GHz for
b
where, is a energy gap of the breather excitation, v is a ESR frequency, g is a g-value in a paramagnetic state = 2.06). Figure 3 shows the field dependence of the energy gap which was obtained from ESR resonance. The is well explained by the function of denoted by solid line, which is consistent with theoretical prediction, The gap obtained from the specific heat measurement (reference is as same value as that obtained from this ESR investigation.
81 1
30 25
20
15 10 5
0
Figure 3. The field dependence of the breather energy gap for at Solid circles obtained from this ESR investigation and solid line is a fitting line.
=
220 mK.
4. SUMMARY
In summary, an ESR measurement system at ultra-low temperatures has been developed by using a 3He-4He dilution refrigerator and a Vector Network Analyzer. We used a cylindrical resonant cavity with high-Q value for ESR measurement. In order to cool the wave-guide and avoid the heat leak efficiently, some thermal anchors and filters were used. Moreover, to decrease the power of the microwave, attenuators were installed. The lowest temperature was 160 mK. We have studied the ESR experiment on Cu benzoate. Due to cooling down the sample to ultra-low temperatures, a well-defined breather excitation has been observed for as well as for in previous study [lo]. The field dependence of the energy gap of the breather excitation agrees well with the results of the specific heat measurements. ACKNOWLEDGEMENT The authors would like to thank to Mr. M.Yoshida for helping our experiments. This investigation has been performed at the High Field Laboratory for Superconducting Materials, Tohoku University. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
812
REFERENCES 1. NMR: H. Okura, K. Ishida, Y. Kawasaki, Y. Kitaoka, Y Yamamoto, Y. Miyako, T. Fukuhara and K. Maezawa, Physica B, 281-282 (2000) 61. pSR: A. Koda, W. Higemoto, R. Kadono, K. Ishida, Y. Kitaoka, C. Geibel and F. Steglich, Physica B, 281-282 (2000) 16. 2. K. Oshima, K. Okuda and M. Date, J.Phys. Soc. Jpn., 44 (1978) 757. 3. M. Date, M. MotokawaandH. Yamazaki, J. Phys. Soc. Jpn , l 8 (1963) 911. 4. M. Date, H. Yamazaki, M. Motokawa and S. Tazawa, Suppl. Prog. Theor. Phys., 46 (1970) 194. 5. D. C. Dender, P. R. Hammar, D. H. Reich, C. Broholm and G. Aeppli, Phys. Rev. Lett., 79 (1997) 1750. 6. M. Oshikawa and I. Affleck, Phys. Rev. Lett., 79 (1997) 2883. 7. I. Affleck and M. Oshikawa, Phys. Rev. B, 60 (1999) 1038. 8. F. H. L. Essler and A.M. Tsverik, Phys. Rev. B, 57 (1998) 10592. 9. F. H. L. Essler, Phys. Rev. B, 59 (1999) 14376. 10. T. Asano, H. Nojiri, Y. Inagaki, J. P. Boucher, T.Sakon, Y. Ajiro and M. Motokawa, Phys. Rev. Lett., 84 (2000) 5880. 11.G.Griiner (eds.), Topics in Advanced Physics 74, Millimeter and Submillimeter Wave Spectroscopy of Solids, p 127 (Springer-Verlag, Berlin, Heidelberg, 1998). 12. M. Oshikawa, I. Affleck, Phys. Rev. Lett., 82 (1999) 5136.
EPR in the 21‘ Century A Kawamori, J Yamauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved
813
ESR spectrometer using frequency tunable gyrotrons as a radiation source Seitaro Mitsudo”, Kazuaki Kanazawa”, Masayuki Hata”, Isamu Ogawab, Tomohiro Kanemaki” and Toshitaka Idehara” ”Research Center for Development of Far Infrared Region, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan bCryogenic Laboratory, Faculty of Engineering, Fukui University, Bunkyo 3-9-1, Fukui 910-8507, Japan
The Gyrotron FU series in Fukui University is being developed to provide high frequency source for far-infrared spectroscopy and plasma diagnostics. The series has achieved the frequency step tunability in wide range from 38 GHz to 889 GHz by many single mode operations at fundamentals and the second and third harmonics of electron cyclotron frequency. Gyrotron FU IVA in the series has achieved maximum frequency 889 GHz and has most wide frequency tunable range. Also, Gyrotron FU IV has achieved complete cw operation. Submillimeter wave ESR spectrometer using Gyrotron FU series as an electromagnetic wave radiation source and pulse magnet for high field up fo 35 T has been developed. The ESR spectrometer has been successfully applied to several ESR measurements.
1. INTRODUCTION Electron spin resonance (ESR) spectroscopy in the millimeter to submillimeter wave region using high magnetic fields is one of the most powerful tools for study of the magnetic properties of materials. However, there are many technical problems that we have to overcome, namely: 1) as the frequency is increased, the power of radiation sources and sensitivity of available detectors are decreased rapidly. 2) High magnetic field with intensity up to several tens Tesla is necessary. Klystrons and solid state oscillators have been used up to now as millimeter wave sources, whereas frequency multiplied solid state source, backward traveling wave oscillator (BWO) and FIR laser were employed as submillimeter wave sources. Gyrotrons are new sources in the millimeter to submillimeter wave region. Internationally, gyrotron development is being directed mainly towards the efficient generation of high-power millimeter waves for the electron cyclotron heating of thermonuclear plasmas. The gyrotron programs at the Fukui University have different aims. The goal is to develop moderately high power sources tunable over broad frequency ranges in millimeter to submillimeter wave region [l]. Such sources have many advantages for applications to various fields including the far-infrared spectroscopy, the measurement of material properties, polarization-enhanced NMR spectroscopy [2] and plasma diagnostics [3,4].
814
In order to construct a new ESR device, we have developed a pulse magnet with a maximum field intensity of 35 T. The development of high power, high frequency ESR spectrometer using a frequency tunable gyrotron for the radiation source and pulsed high magnetic fields, is described in this paper. This high frequency and high power technique enables us to measure an ESR spectrum during one pulse of magnetic field, instead of the integration technique using a lock-in amplifier.
2. SUBMILLIMETER WAVE GYROTRONS Gyrotrons included in Gyrotron FU series are frequency step-tunable sources covering a wide frequency range from millimeter to submillimeter wave region. The output powers are several hundreds watt to several tens kilowatt in fundamental operation and from several tens watt to several kilowatt in second harmonic operation. This is not so high, compared with the output power of other high power millimeter wave gyrotrons. However, these powers are much higher than other radiation sources employed for ESR experiments in this frequency region. Gyrotrons usually operate near the fundamental of electron cyclotron frequency, w - - ,eB0 (1) where and e are the electron rest mass and charge, and is the relativistic factor. Because the maximum frequency that can be obtained is limited by the maximum available magnetic field. Operation at second harmonic offers the possibilities of doubling the frequency while using the same superconducting magnet. For a 17 T superconducting magnet, the frequency is
Vacuum layer
Sample
Figure 1. The schematic diagram of ESR spectrometer using Gyrotron FU IVA as a radiation source and pulse magnet with maximum field intensity of 35 T.
815
limited below about 440 GHz at fundamental operations of the electron cyclotron resonance, but harmonic operations can extend the range dramatically. Gyrotron FU IVA in the Gyrotron FU series has achieved maximum frequency 889 GHz at the second harmonic operation and has most wide frequency tunable rage. It consists of a magnetron injection type electron gun, a drift tube, a resonant cavity, an output waveguide and an output window. It is installed in the 70 mm room temperature bore of the 17 T superconducting magnet. We are improving the operation performances of Gyrotron FU series. Gyrotron FU IV has achieved many advantages as a high quality radiation source, for example, complete cw mode operations, modulations in frequency and amplitude, high stabilities of frequency and amplitude, and so on. When the gyrotron is applied to any measurements, these advantages are very convenient. Specially, the complete cw operation with high stabilities of frequency and amplitude is useful for the application to ESR measurement. The operation condition is limited by the heat capacity of the tube. As the consequence, the output power is several tens of watts.
3. ESR SPECTROMETER Figure 1 shows the schematic diagram of our ESR spectrometer using Gyrotron FU IVA as the electromagnetic wave radiation source. The electromagnetic wave from the Gyrotron FU IV A is transmitted by an oversize circular waveguide system with three quasi-optical bends, and fed on a sample located at the center of the pulse magnet. The power transmitted through the sample is measured by an InSb hot electron detector. The pulsed magnetic field is produced by discharging a capacitor bank of 30 kJ into the magnet coil. When the capacitor
Gyrotron 301 GHz cw
B 11 c-axis
theory B 11 c-axis theory8 l e - a x i s
Gunn 114.7 GHz
0
5 10 15 Field intensity B (T)
Figure 2. AFMR spectrum of MnF2 single crystal.
Figure 3. Frequency versus field diagram of MnF2.
816
(a) 610 GHz, TE,,, Mode, 2nd harmonics
(b) 323 GHz, TEo3Mode, fundamental
? 20
10
B (Tesla)
Figure 4. Typical ESR spectrum of standard sample of DPPH. Gyrotron FU IVA was operated in (a) TE93 mode (2nd harmonics) and (b) TEo3 mode (fundamental)
Table 1. Operating parameters of Gyrotron FU IVA. Operating mode Harmonics Frequency (GHz) Beam current (mA) Cathode voltage (kV) Anode voltage (kV) Magnetic field (T)
TEo3
TE93
1st
2nd
323
610
500
400
-9.9
-9.8
11.705
11.177
bank is charged up to 5 kV (30 kJ), the field intensity is increased up to the maximum of 35 T and decreased. The field intensity pattern is similar to a half sinusoidal one with the width of 2.5 ms. The signals from the pickup coil for measurement of magnetic field intensity and from the InSb detector are recorded in digital oscilloscope as functions of time. Thereafter, the computer arranges automatically both signals for constructing an ESR spectrum as a function of magnetic field intensity. The temperature of the sample can be varied from about 100 K down to the liquid helium temperature by the heater surrounding the sample The temperature is measured by the FeAu 0.007 at.% - Ag thermocouple. The Gyrotron FU IVA is operated in pulse mode with pulse width of 4 ms. A single pulse triggers the operation of a high voltage supply for gyrotron, while the condenser bank for magnet is triggered by a delayed pulse. Then, the pulsed magnetic field can be synchronized with the gyrotron operation. The ESR spectrometer using the Gyrotron FU IV and Gyrotron FU IVA as radiation source has been applied to ESR measurements[5-71. Typical results of the experiments are introduced. In the first case, Gyrotron FU IV was used as radiation source. The gyrotron is operated in cw mode. The operation conditions are as follows. The cathode potential and the beam current are varied from -14 to -16 kV and from 50 to 100 mA, respectively. Figure 2 shows typical ESR spectrum of well-studied antiferromagnetic compounds MnF2 single crystal. Pulsed magnetic fields were applied along c-axis, which is antiferromagnetic easy axis. The measurements temperature was 4.2 K. At the frequencies 114.7 GHz and 120 GHz, the Gunn oscillators were used as radiation sources with output powers 25 mW. When Gyrotron was used as radiation sauce, the output power was attenuated to avoid increasing temperature of sample. As seen from Figure 2, Gyrotron output has enough stability for ESR measurements. Figure 3 shows the frequency versus magnetic field intensity diagram. AFMR resonance line
817
at each frequency is in good agreement with theoretical value. In the second case Figure 4 shows typical ESR spectrum of standard sample DPPH (its g-value is 2.0036) measured by this system. The ESR absorption line of DPPH is indicated by an arrow. the frequency of 610 GHz in Figure 4(a), a Gyrotron FU was operated in TEg3mode at second harmonics. The sharp and strong absorption is caused by DPPH. Other week and broad dips of transmission power come from fluctuation of the output power from gyrotron, because the output power of second harmonics operation is sensitive to operation parameter, mainly high-voltage power supply. However, we can distinguish between signal and noise by comparing a pattern in the phase of increased magnetic field with one of decreased field. The operating parameters of Gyrotron FU are shown in table 1.
REFERENCES 1. T. Idehara, I. Ogawa, S. Mitsudo, M. Pereyaslavets, N. Nishida and Y. Yoshida, IEEE Trans. Plasma Sci., 27 (1999) 340. 2. D. Hall et al., Science, 276(1997) 930. 3. I. Ogawa, M. Iwata, T. Idehara, K. Kawahata, H. Iguch and A. Ejiri, Fusion Eng. and Des., 34-35 (1997) 455. 4. I. Ogawa, K. Yoshisue, H. Ibe, T. Idehara and K. Kawahata, Rev. Sci. Instrum., 65 (1994) 3145. 5. S. Mitsudo, Aripin, T. Matsuda and T. Idehara, Int. J. Infrared and Millimeter Waves, 21 (2000) 661. 6. Aripin, S. Mitsudo, T. Shirai, K. Matsuda, T. Kanemaki, T. Idehara, T. Tatsukawa, Int. J. Infrared and Millimeter Waves, 20 (1999) 1875. 7. M. Chiba, Aripin, K. Kitai, S. Mitsudo, T. Idehara, S. Ueda and M. Toda, Physica B, 294-295 (2001) 64.
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EPR in the 21%Century A Kawamori, J Yarnauchi and H Ohta (Editors) 2002 Elsevier Science B.V. All rights reserved.
High-frequency (W-band) EPR studies of biological samples KBichi Fukui? Tomohiro Itob and Hiroaki Ohya"" "Regional Joint Research Project of Yamagata Prefecture, Yamagata Public Corporation for the Development of Industry, Matsuei 2-2-1, Yamagata 990-2473, Japan bGraduate School of Science and Engineering, Yamagata University, Yonezawa, Japan. 'Institute for Life Support Technology, Yamagata Public Corporation for the Development of Industry, Matsuei 2-2-1, Yamagata, Japan This paper reports a new W-band (-94 GHz) EPR spectrometer which is constructed for measurements of raw or fresh biological samples. Measured biological samples include plant seeds, leaves, and bacterial pellets, and their size is typically lmm x lmm x Imm. The setup and performance of the spectrometer are described, and some remaining problems are discussed. Results for a raw seed, a freshly cut leaf, and a TEMPO-absorbed leaf are presented. The former two exhibited well-resolved six-line signals due to trace Mn", which were not observed or only weakly observed in X-band spectra The latter one exhibited a spectrum where two signals from TEMPO in aqueous media and lipid media are clearly separated, which demonstrates good g resolution of W-band EPR. 1. INTRODUCTION
Electron paramagnetic resonance (EPR) is a powerful technique for investigation of radical and metal-iodcomplex species in biological systems. In fact, EPR with low microwave frequency such L band or even lower frequency band has been shown to be usehl for in-vivo measurements. In contrast, applying high-frequency (HF) EPR to biological samples, in particular raw or fresh biological samples, is still a difficult task because of severe dielectric loss of such samples. HF EPR, however, has several unique advantages such as (i) high sensitivity because of larger Zeeman splitting, and (2) better g resolution [ 11. Furthermore, particular usefulness in investigating mononuclear high-spin Mn(I1) iodcomplexes has been pointed out [2]. Hence, it is valuable to develop a HF EPR system that is easily applicable to raw (hopefully live) biological samples. We have been developing a W-band (-94 GHz) EPR spectrometer for measurements of raw or fresh biological samples. Specifically, biological samples which we are interested in are, for example, raw plant seeds, freshly cut leaves, and bacterial pellets. Although there have been published a considerable number of reports on HF EPR so far [1-4], it seems that no HF-EPR experiments have been performed for such samples. In this paper, we report details of our newly built W-band EPR spectrometer, and some of the results obtained using the spectrometer.
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2. EXPEIUMENTAL 2.1 W-band EPR spectroscopy
A block diagram of our W-band spectrometer is shown in Figure 1. The milliwave supplied with a cavity-stabilized Gunn oscillator (Keycom Co., CSO-01) is divided into two, where one is given to a mixer (MRI Co., model BMRlW) as Local signal and the other is lead to a Fabry-Ptrot cavity (fabricated in Keycom Co.). The Fabry-Perot cavity is made up of one concave mirror (upper mirror) and one flat mirror (lower mirror). The support of the lower mirror is connected to three stepping motors (Suruga Seiki, Co.), which allows threedimensional position adjustment. Around the cavity a coil is wound to provide field modulation (100 kHz). Dielectric waveguides (Keycom Co., DW110A Series) are used to connect the cavity and the milliwave main circuit, which allows flexible arrangement of the circuit. The output signal from the cavity and the local signal are mixed at the mixer to provide a DC output (modulated by 100 kHz owing to the field modulation). The DC output is first amplified by a low-noise preamplifier (NF electronic Instruments, SA430F5), and subsequently by a lock-in amplifier (NF electronic Instruments, 561OB). A sample is placed at the center of the lower (flat) mirror of the Fabry-Perot cavity. The position of the lower mirror is adjusted to achieve the optimal resonance condition. The actual frequency of the milliwave and the power of the output from the cavity are monitored using a spectrum analyzer (Hewlett Packard 8562E equipped with a 11970 harmonic mixer). Magnetic field is applied by a wide-bore superconducting magnet (Suzuki Shokan Co., A960077; bore diameter, 100 mm). The superconducting magnet was directly swept for EPR measurements, and the superconducting current is monitored with a digital multimeter (Iwatsu Co., VOAC7513). For calibration of the magnetic flux density, Mn2+ in MgO is used standard. All measurements were performed at ambient temperature.
._ Mixer
Oscillator
-5 I
Connector
--- ^-plifier
Superconducting Magnet 1-5T Fabry-PBrot Cavity
100 kHz Modulation
\
I Ill
I
Moduhtiom Coil Amp.
Sample Stage
Figure 1. Schematic diagram of our W-band EPR spectrometer
Computer
I
820
2.1 Samples Brown rice, white radish sprouts, and gypsophila seeds were purchased from local stores. Cotoneaster leaves were taken from cotoneasters planted in a laboratory yard. Gypsophila seeds were of the size of 1 mm x 1 mm x 1 mm, so that one whole seed was used as a sample. After EPR measurements, gypsophila seeds were watered for six days. Cotyledon leaves thus germinated were cut off, and measured.
-
3. RESULTS AND DISCUSSION 3.1 Instrument We have employed a Fabry-Perot cavity resonator in this construction of our W-band spectrometer, although cylindrical cavity resonators seem to be usually used in recent W-band spectrometers. A defect of the cylindrical cavity for W band is that it has very limited sample space and thus is not suited for measurements of biological samples such as leaves and seeds. The Fabry-Perot cavity, on the other hand, provides larger sample space and is expected to accept much wider range of samples. In our spectrometer, one only needs to put a sample on the flat mirror and place it on a support for measurements. The sample sizes eligible for our spectrometer are typically lmm x lmm for leave samples, lmm x lmm x lmm for seed samples, and 1 yl for aqueous solution samples. Of course, much larger samples can be measured when the samples are dried or dissolved in organic non-polar solvent. Our Fabry-PCrot cavity has two holes on the upper mirror, where one is used for milliwave input and the other is for output. Since the input and output lines are separated, we need not use a circulator in the milliwave circuit. This also helps the milliwave circuit to keep simple and inexpensive. In typical settings, the input power is 7 dBm and the output power under unloaded condition is - -8 dBm. Hence the power loss due to the (unloaded) cavity is 15 dB. As long as the sample size did not exceed the above-descried ones, the loss did not increase so drastically, still staying I 2 0 dB. However, when the size of a watercontaining sample became larger than these, the loss increased drastically and EPR measurements became difficult. Another factor which influences the output milliwave is the shape of the sample. When the sample is flat and thin, the phase of the EPR signal is expected. For example, solution samples, which were spread on the flat mirror and covered with a thin glass for measurements, generally provided correct-phased signals. However, when the sample was not flat and had an irregular shape, as may be usual for biological samples, the signal was a mixture of the dispersion and absorption signals. The phase of the signal could not be corrected with a phase shifter. This admixture most likely occurs because the irregular-shaped sample severely interfered the milliwave electromagnetic field. At present, we digitally correct the phase of the signal when it is severely distorted. The sensitivity of our spectrometer (minimal detectable spin number) was estimated Nmin 2x10" spins/gauss 1 Hz from room-temperature spectra of TEMPOL in H20. This value may be compared with that for X-band spectrometer (-10" spindgauss). Values of N,in = lo7-lo9 spins/gauss were reported in literature for sensitivities of W-band EPR spectrometers [1,2]. However, these values were obtained from data of nonaqueous samples. Very few data seem to be available in literature for W-band EPR spectra of aqueous samples. One example was nevertheless reported by Wang et al. [2], who estimated Nmin = 2.4~10" spindgauss for their W-band spectrometer from spectra of TEMPOL in HzO. Compared with their value, the sensitivity of our spectrometer is one-order worse. However, this
-
-
-
82
difference most likely comes from simply the difference in the cavity employed. Their spectrometer employs a cylindrical cavity, which is advantageous with respect to sensitivity. We have adopted a cavity-stabilized oscillator as a milliwave source of the W-band EPR spectrometer. This oscillator provides milliwave of (i) negligible frequency drift and (ii) very sharp frequency profile. The half-height-full-width peak width of the milliwave was -0.2 MHz as measured with a spectrum analyzer. This peak width corresponds to a linewidth of 0.007 mT, which is negligibly small in usual experiments. (In the spectrum of TEMPOL in H20, the sharpest line (lowest-field line) had a peak-to-peak width of = 0.17 mT, which is practically the same as the linewidth in X-band spectra. This shows that inhomogeneity in magnetic field is also negligible in usual experiments) One drawback of our system is that we can not vary the frequency of the oscillator. Accordingly, to achieve the resonance condition, we adjust the spacing between the two mirrors of the Fabry-PCrot cavity, where the lower (flat) mirror is constructed to be movable using stepping motors. A problem around this tuning method is that it is difficult to include automatic-tuning control (ATC) system in it (in fact, at present, our spectrometer is equipped with ATC system). One solvent may be to use piezoelectric actuators to control the mirror position, or to include an additional oscillator for frequency adjustment and modulation. Improvement of our spectrometer in this point is under way. Nevertheless, it was reported that automaticfrequency control (AFC) may disturb EPR signals when narrow signals (such as nitroxyl signals in fast-motional regime) are concerned, and that such signals may have to be recorded without AFC after careful tuning [5]. Hence, the absence of AFC (or ATC) in our spectrometer would not affect the quality of our data itself, which were of course measured after careful tuning.
-
3.2 Mn(I1) in plants One of the useful applications of W-band EPR is a study of mononuclear high-spin Mn" ionskomplexes. In W-band EPR, intensities of the forbidden transitions and the effects of zero-field splitting are greatly reduced, which makes the Mn" six-line signal much more clearer than in X-band EPR [2]. Mononuclear high-spin Mn" species are in fact abundantly found in plants, and the coordination environments and the roles of the Mn" species have been a subject for investigation. It is therefore expected that W-band EPR provides invaluable information about these questions. Figure 2 shows W-band EPR spectra of a raw seed and a fresh cotyledon leaf of gypsophila. X-band EPR spectra are also shown for ~
~
W
'0
Seed
3335
3350
ImT
3365
3380
3-..~ 3 '
315
330
345
360
375
ImT
Figure 2. W-band and X-band EPR spectra of one whole gypsophila seed and freshly cut cotyledon leaf measured at ambient temperature. W-band: v = 93.994 (seed), 93.996 (cotyledon) GHz. X-band: v = 9.442 GHz.
822
Table 1 g and HFC parameters of Mn(I1) species Samples g Gypsophila Cotyledon Leaf 2.0007 MnS04 in H20 2.0008 Cotoneaster Leaf, Fresh 2.0008 Brown Rice Embryo 2.0009 Gypsophila Seed 2.001 1 ConcanavalinA 2.0009 Mn2+in MgO 2.0010
A(55Mn)/mT 9.57 9.56 9.56 9.39 9.32 9.25 8.71
Ref. This work This work This work This work This work 6 1
comparison. From the W-band spectra, the g values and the "Mn hyperfine coupling parameters A("Mn) were determined (Table 1). EPR parameters determined from W-band data for some other Mn" species are also listed in the table. It should be noted again that the determination of the EPR parameters from X-band data for these samples is very difficult or impossible because only a broad or no signal was observed from X-band EPR. The EPR parameters of the leaves are very close to those of MnSOs in H2O. Hence the Mn signal is most likely due to a free Mn" ion (Mn(H20):+). For the seed and rice embryo, on the other hand, the HFC parameters are remarkably different from that of MnS04 in H2O. This may indicate that the Md' ions in these samples are bound to some peptides or proteins. 3.3. Spin probes in biological samples Another advantage of W-band EPR is that it has better g resolution than X-band EPR. Figure 3 well demonstrates this advantage. The figure shows a W-band EPR spectrum of a TEMPO-absorbed white radish cotyledon leaf. The spectrum clearly displays two sets of three-line signals; one is a relatively sharp signal with g = 2.0058 and A('%) = 1.73 mT, and the other is a relatively broad signal with g = 2.0063 and A(I4N) = 1.55 mT. As is wellknown, the g and A('%) values of nitroxyl radicals depend on the hydrophobicity / hydrophilicity of medium, where the ,q value increases and the A(I4N) value decreases with the increase- of hydrophobicity of-the medium [7]. Hence, the sharp signal is attributed to TEMPO in aqueous phase and the broad peak is attributed to TEMPO in lipid phase. To our knowledge, this is the first clear observation that spin probes such as TEMPO are actually distributed in leaves to more than one type of matrix. We also performed X-band EPR measurements for the same sample. However, the two 6lmT signals were almost overlap in the X-band Figure 3. W-band EPR spectrum for a white spectrum with the highest-field line only radish cotyledon leaf. The leaf was floated split slightly (not shown). Also notable on a 5 mM TEMPO solution for a few hours = is the difference in linewidth between the before measurement. Conditions: two signals. Analyses of the linewidth 93.996 GHz, room temperature.
823
may provide further information concerning the difference between the environments around the two types of TEMPO. 4. SUMMARY
We have described our new W-band EPR spectrometer that is constructed for biological applications. The biological samples we have measured are raw plant seeds, freshly cut leaves, bacterial pellets, etc. One of the powerful biological applications would be investigation of Mn(I1) species in plants. It is known that there are many Mn-related biological activities such Mn excess and Mn deficiency symptoms of plants [8], lignin synthesis and decomposition [9], sugar-binding activities of lectins [6]. Furthermore, Wband EPR may be used with spin probes to investigate the lipid content and viscosity (or also cytosol viscosity) of living cells and tissues.
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9.
0. Burghaus, M. Rohrer, T. Gotzinger, M. Plato, and K. Mobius, Meas. Sci. Technol., 3 (1992) 765-774. W. Wang, R. L. Belford, R. B. Clarkson, P. H. Davis, J. Forrer, M. J. Nilges, M. D. Timken, T. Walczak, M. C. Thurnaucer, J. R. Norris, A. L. Morris, and Y. Zhang, Appl. Magn. Reson., 6 (1994) 195-2 15. R. T. Weber, J. A. M. Disselhorst, L. J. Prevo, J. Schmidt, and W. Th. Wenckebach, J. Magn. Reson., 81 (1989) 129-144. M. J. Nilges, A. I. Smirnov, R. B. Clarkson, and R. L. Belford, Appl. Magn. Reson., 16 (1999) 167-183. T. I. Smirnova, A. I. Smirnov, R. B. Clarkson, and R. L. Belford, J. Phys. Chem., 99 (1995) 9008-9016. E. Meirovitch, Z. Luv, and A. J. Kalb, J. Am. Chem. SOC., 96 (1974) 7538-7546. C. F. Polnaszek, S. Schreier, K. W. Butler, and I. C. P. Smith, J. Am. Chem. SOC.,100 (1978) 8223-8232. Z. Rengel, A. Sigel and H. Sigel (eds.), Metal Ions in Biological Systems. Vol. 37, pp. 57-88, Marcel Dekker, New York, 2000. (a) G. Engelsma, Plant Physiol., 50 (1972) 5 9 9 4 0 2 . (b) B. Halliwell, Planta, 140 (1978) 81-88.
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825
A ABREGO, F.C., 614 AJIRO, Y., 784, 807 ALLAYAROV, S.R., 373,378 AMMERLAAN, C.A.J., 33 ANANDALAKSHMI, H., 225 ANZAI, K., 562 213,219 ARAKAWA, M., ARAO,S., 466 ASANO, T., 784,807 ASANO-SOMEDA, M., 355 AZUMA, M., 747 B BABA,M., 282 BABA,M., 361 614,624 BAFFA, O., BAKHANOVA, E.V., 628 BELTRAN-LOPEZ, V., 63 BERLINER, L.J., 503 BHAT, S.V., 145 BOROVYKH, I.V., 659 BRECHT, M., 437 BROOKS, J., 793 BUHRKE, T., 437 BUNTING, R.V., 741 C CAI, J., 93 CHANDRASEKHR, A.V., 575 CHEN, S., 389 CHEN,Z., 421 CHERNYSHEVA, T.E., 378 CHIBA, M., 93,779 CHOCK, P.B., 446 CHOH, S.H., 288 CHOI,D., 288 CHUMAK, V.V., 628 CORNIA, A,, 784
D DEMISHEV, S.V., 741 DHANASEKARAN, R., 139 DIKANOV, S.A., 488 DINH, N.L., 412 DINH, P.T., 412 DOMRACHEVA, N.E., 710
DREW, S.C., DZUBA, S.A., E EBISU, H., EHARA,M., ENDO, T., ENDOH, N., EST, A.V.D.,
39 669 213,219 567 133, 139, 145, 157 483 355
F FOSTER, M.A., 515 437 FOERSTER, S., FRANCAF, H.K., 585 FREED, J.H., 719 FRIEDRICH, B., 437 FUJII, K., 688 552 FUJISAWA, M., FUJITA, T., 247 302 FUJIWARA, Y., FUKUDA, A,, 97 FUKUDA, Y., 700,706 FUKUI, K., 384,817 706 FURUE, S., FURUKAWA, S., 79 FURUNO, N., 264 FURUTA, H., 306 FURUTA, M., 306 FUSE,T., 349 G GAST,P., 659 GARD,G., 793 GATTESCHI, D., 784 GEIFMAN, I.N., 694 GLASER, R., 471 GOLOVINA, I.S., 694 GORDON, D.A., 373 GRAEFF, C.F.O., 624 GRIGOREV, I., 5 15 GRUN,R., 613 GUDENKO, S.V., 127 H HA,V.M., 412 HAGIWARA, M., HAN, J.Y., 533
73
826
HARA,H., 679 HARADA, Y., 93 HARIMA, H., 306 HASHIZUME, A., 133 HATA, M., 788,813 HAYASHI, Y., 556 HAZUKI, K., 747 HIDA, A.I., 552 HIGASHI, N., 151 HIGUCHI, Y., 437 HIRAO,T., 306 HIRATA, T., 93 333 HIROTA, N., HIROI, Z., 767 264 HIROTSU, K., HOA, T.T.K., 259 HOFF, A.J., 659 HONDA, M., 201 HONDA, S., 168 HORI,H., 259 69 HOSOKOSHI, Y., I IDEHARA, T., 168,779,788,813 784 INAGAKI, Y., INOUE,K., 69 IONNIDIS, N., 466 ISHIHARA, K., 89 ITO, T., 818 ITOH, K., 133, 145 133, 139, 145, 157 IWASAKI, S., IWASAKI, T., 488 J JIN,G., 389 JIN,S., 389 JINMON, A., 355
K KAGOMIYA, I., 755 KAINARA, V.V., 293 KAIzU,Y., 355 KAJI, H., 186 KALAUOVA, A.S., 595 KALE, R.D., 145 KALINOVSKAYA, I.V., 276 KAMADA, H., 556
KAMEKAWA, M., 552 KAMIKAWA, T., 759,763 KANAZAWA, K., 168,813 KANEMAKI, T., 813 446 KANG, S.O., KANG, Y.S., 477 KANG,K., 288 KARASEV, V.E., 276 KASAHARA, M., 322 KASSAB, L.R.P., 585 KATO,N., 403 KATO, R., 197,312 KATO,Y., 93 KATOH,K., 69 KATSUTA, N., 679 KATZ, E.A., 174 KAWABATA, T., 552 KAWAI,A., 349 KAWAI, J., 384 KAWAKAMI, K., 767 KAWAMATA, N., 201 KAWAMORI, A,, 466,679 KAWAMOTO, T., 302 KAWASHIMA, T.,., 306 KEMPINSKI, W., 174 KEVAN,L., 105 KHAN,%, 125 KHRAMTSOV, V.V., 515 KIKUCHI, H., 755,759,763,803 KILIBAYEV, M.B., 640 KIM, S.S., 288 KIMURA, S., 799 KWDO, K., 784,799 KINOSHITA, A., 614,624 KIRITA, K., 69, 803 KITAI,K., 779 KLENINA, I.B., 659 KOHMOTO, H., 133, 139,145,157 KOHMOTO, T., 700,706 KOHN,K., 755 KOIDE,T., 302 KOJIMA, K., 247 KOJIMA, Y., 264 378 KONOVALIKHIN, S.V., KOTAKE, Y., 322 KOUNOSU, A., 488 KOYAMA, K., 807
827 KOYAMA, R., 298 KUANG, T., 429 KUDAIBAYEV, K., 595,640 KULIK, L.V.,669 KUNIMOTO, T., 298,759,763,771 803 KUNITOMO, M., 700,706 KURODA, S., 79, 113,367 KURYAVYI, V.G., 276 KUSPANOVA, B.K., 595,640 KUWABARA, M., 59 L LATTERMANN, G., 710 LEE, C., 446 LEE, D.K., 477 LEE, M.C., 562 LI,T., 133 LI,Y., 326 471 LIKHTENSHTEIN, G.T., LIU,K., 429 LILT,Y., 429 LILT, Z.L., 421 LOS, S., 174 LOZINSKY, E., 471 437 LUBITZ, W., LURIE, D.J., 515
M MAKINO, K., 483 MANABE, T., 79 MARUMOTO, K., 79,367 MATSUBARA, A., 97 MATSUDA, K., 168 MATSUOKA, H., 264 MEKATA, M., 788 MEYERSTEIN, D., 471 MIKUNI, T., 567 MILOV, A.D., 647 MINO,H., 684 MINO, M., 85,89 MIROCHNITCHENKO, O., 542 MISAKI, Y., 192 731 MISRA, SK., 168,779,788,813 MITSUDO, S., MITSUYAMA, Y., 556 MIYAMOTO, R., 3 16
27,48 MIYASHITA, S., MIYAZAKI, H., 562 MIZUSAKI, T., 97 MORI,M., 213 MORII, T., 567 MORITA, Y., 384 MOTOKAWA, M., 775,784,807 MURAKAMI, K., 542 MURALI, A., 242 N NAGASAKA, Y., 771 NAIDENOVA, I., 456 NAJIMA, H., 567 494 NAKAGAWA, K., NAKAJIMA, A,, 483,556 NAKAMURA, M., 151 NAKAMURA, T., 63,270 NAKAMURA, T., 192, 197 NAKASUJI, K., 384 NAKAUE, Y., 145 NAKAYAMA, H., 298 NAKAYAMA, K., 706 NAKAZAWA, S., 384 562 NAKAZONO, K., 207,253 NARASIMHULU, K.V., NARUMI, Y., 784 NASIROV, R.N., 595,640 NEERAJA, P., 225 NGUYEN, V.T., 412 NISHIDA, S., 384 NISHIDATE, I., 282 NOGAMI, Y., 192 NOJIRI, H., 751,775,784,807 NOVOSELSKY, A., 471
0 OGASAWARA, A,, 27,48 OGAWA,A., 93 OGAWA, I., 813 OGURA,F., 247 OHMI,T., 97 OHNISHI, T., 54 OHTA, H., 59,69, 197,298,302 312, 741, 747, 755,759 763,767,771,803 OHTA,K., 282
828
OHTA,M., 270 OHYA,H., 818 OHYA-NISHIGUCHI, H., 556 OKADA,S., 552 OKAZAKI,N., 54 OKUBO, S., 197,298,302,741 747,755,759,763 767,771,803 ONO,T., 684 ONO,T., 775 OSHIKAWA, M., 15 OSHIMA, Y., 312,741 OVCHNNIKOV, I.V., 7 10 OZAWA, T., 562 P PARK, I.W., 288 PARK, S.M., 477 PETKOVA, G., 456 PETROULEAS, V., 466 PHAM, T.V., 412 PHU, N.H., 259 PIEKARA-SADY, L., 174 PILBROW, J.R., 39 PING,Z., 389 634 PIVOVAROV, S., PRAKASH, P.G., 242 PRASUNA, C.P.L., 253 PROSKURYAKOV, I.I., 659 PUNNOOSE, A,, 162 R RAGOGNA, P., 355 RAJENDIRAN, T.M., 225 RAJU, B.D.P., 207 RANGUELOVA, K., 395 RAO, J.L., 207,242,253 RAO, P.S., 225 RAO, T.V.R.K., 253 RAVIKUMAR,R.V.S.S.N., 575 REDDY, B.J., 157,575,589 REDDY, G.S., 589 589 REDDY, R.R.S., REDDY, S.L., 589 REDDY, Y.P., 575 ROMANOWSKI, W.R., 282 ROMANYUKHA, A.A., 603
ROWLANDS, C.C., 63 RUDOWICZ, C., 3 RUKHIN,A., 634 RUTEL, I.B., 793
S SAIFOUTDINOV, R., 525 SAKAI,T., 54 SAKON, T., 807 SAKURAI, T., 69, 197,747,803 SAKURAI, Y., 157 SALIKHOV, K.M., 678 201 SAMEJIMA, K., SARATANI, H., 139 SARJONO 282 SARUHASHI, M., 803 SATO,H., 316 SATO, K., 264,384 SCHAURE, D.A., 603 SCHLUETER, J., 793 SEEHRA, M.S., 162 SEKI,K., 344 SEO, K.W., 477 SEREDAVINA, T., 634 SHAHABUDDW, M., 133 SHAMES, A.I., 127, 174,471 SHARMA, P.K., 125 SHEN, J.R., 466 SHEN,Y., 389 SHIBUYA, K., 349 SHIMIZU, H., 151 SHIMOYAMA, Y., 180, 186 SHIOMI, D., 264,384 SHIRATORI, K., 755 SHOJI, H., 562 SHOLOM, S.V., 628 174 SHTUTWA, S., 614 SILVA, N.A., SINGH, R.J., 125 SINLAPADECH, S., 105 SITZMANN, M.E., 793 SKRYLNIK, P.G., 236 SLUCHANKO, N.E., 741 SOKOLSKA, I., 282 STEHLIK, D., 678 STEIN,M., 437 316 SUDOH, S.,
829
SUEISHI, Y., 322 SUN, J., 429 SZAMAROV, S.S., 595
V VANDERVEER, D., VENKATESAN, R.,
T TADA, M., 145, 157 TAGUMA, K., 361 TAI, N.T., 259 TAJIMA, K., 483 TAKAHASHI, N., 63 TAKAMI, M., 270 TAKANO, M., 747,767 TAKEDA, Y., 302 TAKEMURA, S., 54 TAKENAKA, H., 97 TAKESHITA, K., 533 TAKEUCHI, H., 213,219 TAKEUCHI, N., 367 TAKEYAMA, T., 63 TAIU,M., 259 TAKUI, T., 264,384 79,213 TANAKA, H., TANAKA, H., 775 TANAKA, K., 192 TANIGUCHI, M., 192 TATSUTA, M., 567 TATUMI, S.H., 585 TOBISAKO, H., 322 TODA,M., 779 TOKI,M., 755 TSUCHIHASHI, Y., 306 TSUKAMOTO, M., 85,89 TSUKUDA, H., 85 TSVETKOV, Y.D., 647 TURANOV, A., 710
W WADA,N., 247 WARD, B.H., 793 WAKI,Y., 97 WAKUTA, M., 139 WANG, J.S., 326 WANG,Q., 326 WELLS, J.-P.R., 201 WINTER, R.W., 793 WU, G.S., 326 WU,L.M., 421
U UEDA,A., 767 UEDA,S., 779 UEDA, Y., 483,556 UEDA,Y., 770 231 UEKI, S., 139 UTHAYAKUMAR, S., UTSUMI, H., 533,542,548 UWATOKO, Y., 803
793 225
Y YAGI,M., 344 YAKUBOVSKY, A.Y,, 127 YAMADA, J., 157 YAMADA, S., 466 YAMAGA, M., 201 YAMAGUCHI, K., 133 YAMAMOTO, H.M., 312 YAMAMOTO, K., 247 322 YAMAMOTO, S., YAMAOTO, I., 344 YAMASAKI, M., 133 YAMASHITA, M., 79 YAMAUCHI, J., 231,361,575,688 YAMAUCHI, T., 771 YAMAZAKI, H., 85,89 YAMAZAKI, M., 247 YANG,L., 421 YASUKAWA, K., 548 YIM, H.S., 446 YIM,M.B., 446 YOKOYAMA, H., 556 YONEMITSU, K., 59 YORDANOV, N.D., 395,456 YOSHIDA, H., 747 151 YOSHIDA, K., YOSHIDA, A,, 306 298,302 YOSHIKAWA, J., YOSHIMURA, T., 403 YOSHINO, F., 562 YOUNGDEE, W., 515
830
Z ZADOROZHNAYA, A.N., 276 ZECH, S.G., 678 ZNANG, Q., 429 ZHDANOV, A., 634 ZHOU,B., 421 ZHU, H., 421 ZIATDINOV, A.M., 236,293