ADVANCES IN PHOTOCHEMISTRY

Volume 19

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC.

New York Chichester Brisbane Toronto Singapore

ADVANCES IN PHOTOCHEMISTRY

Volume 19

ADVANCES IN PHOTOCHEMISTRY

Volume 19

Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GUNTHER VON BUNAU Physikalische Chemie, Universitat Siegen, Germany

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC.

New York Chichester Brisbane Toronto Singapore

This text is printed on acid-free paper.

Copyright 0 1995 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 101 58-001 2.

Library of Congress Cataloging in Publication Data:

Library of Congress Catalog Card Number: 63- 13592 ISBN 0-47 1-04912-3

Printed in the United States of America

1 0 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

David L. Boucher Department of Chemistry The University of Toledo Toledo. OH 43606

Julian A. Davies Department of Chemistry The University of Toledo Toledo, OH 43606

Jimmie G. Edwards Department of Chemistry The University of Toledo Toledo, OH 43606

Helmut Gorner Max-Planck-Institut

fur Strahlenchemie D-45413 Mulheim an der Ruhr,

Germany

Gerd Kaupp Organic Chemistry I University of Oldenburg P.O. Box 2503 D-26111 Oldenburg, Germany

Hans Jochen Kuhn Max-Planck-Institut

fur Strahlenchemie D-45413 Mulheim an der Ruhr,

Germany

Y ing Wang Central Research and Development Du Pont Co. P.O. Box 80356 Wilmington, DE 19880-0356

V

PREFACE

I t is with a sense of some apprehension that 1 address our readers as the new senior editor of Advances in Photochemistry. The series is of such long standing, it seems a very large responsibility. David Volman remains as a series editor, but has turned over the details of final editorial decisions and of interacting with the publisher to me. Dave did a terrific job as senior editor and I am pleased to express gratitude to him on behalf of the community.

Our editorial policy remains steadfast to that which was established by the original editors of the series: J. N. Pitts, Jr., G. S. Hammond, and W. A. Noyes Jr., and 1 quote:

Volume 1 of Aduancrs in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sense have the articles been simple literature surveys, although in some cases they may have also fulfilled that purpose.

I t is our editorial plan to publish a volume of Adcances each year. David Volman and I are pleased to welcome Giinther von Biinau as co-editor with the publication of Volume 19. Gunther replaces one of the founding editors, George S. Hammond, and we are delighted that Gunther is willing to do so. We look forward to a long association.

We also note that, after 29 years, George Hammond has decided that he should retire with the publication of Volume 18. George, in his letter of retirement dated August 14, 1991, said:

vii

vii i PREFACE

In my retirement (from Allied Signal), I find myself perpetually snowed under with unfinished work. I am trying to concentrate on things which I really enjoy, doing fewer of them. I have derived both pleasure and pride from the series, but after 29 years have decided to call enough “enough.”

Try as we might, neither David nor I could convince George that this decision was anything but final. So, in this, the first volume of Aduanres post H a m m o n d , we recognize his many contributions. Several years ago, we held a symposium in George’s h o n o r a t the Center for Photochemical Sciences. Below, in part, is the biographical sketch prepared for that symposium.

George Simms Hammond was born on May 22, 1921 in Auburn, Maine. He received degrees from Bates College (B.S., 1943) and Harvard University (Ph.D.. 1947) where he worked with Paul Bartlett. After postdoctoral work with Saul Winstein at UCLA, he accepted a faculty position at Iowa State University in 1948. Ten years later he moved to the California Institute of Technology, where he was appointed Arthur Amos Noyes Professor of Chemistry and became chairman of the Division of Chemistry and Chemical Engineering. In 1972 he accepted the post of vice chancellor for natural sciences at the University of California at Santa Cruz. In 1974 he relinquished this position to spend half time as foreign secretary of the National Academy of Sciences while continuing his professional research and teaching. Following completion of his term as foreign secretary in 1978. he accepted a position as associate director of corporate research for the Allied Chemical Corporation, where he remained until his retirement several years ago. Sine 1989 he has been spending part of his time at the Center for Photochemical Sciences as Senior McMaster Fellow. His primary concern in that capacity is the devel- opment of a new program in Materials Science at the University.

Among the many honors Hammond has received are the American Chemical Society Award in Petroleum Chemistry (1961), the James Flack Norris Award in Physical Organic Chemistry (l967), the Danforth Foundation Award for Gifted Teaching (1971), the American Chemical Society Award in Chemical Education (1972), and the Priestley Medal (1976). He was elected to the National Academy of Sciences (1963), the American Academy of Arts & Sciences (1965), the American Institute of Chemists (1968), the Indian Nation- al Science Academy (1976), and the Society of Chemical Industry (1980). He has received honorary doctorates from Bates College, Wittenberg University, the University of Ghent, Bowling Green State University, Georgetown Uni- versity, and the Weissman Institute.

George Hammond began his independent research career as a physical organic chemist studying the benzidine rearrangement. He studied the effects of remote substituents in solvolytic displacement reactions and was one of the first to point out the existence of systematic deviations from the Hammett equation. He also undertook studies of the reactions of free radicals and

PREFACE ix

suggested an interesting but short-lived theory about the mode of action of antioxidants.

During his years at Iowa State, Hammond began the study of the mechanisms of photochemical reactions, although he did not publish in the field until 1959. He went on to produce more than 50 papers on the subject. The underlying theme in his study was the study of the chemical behavior of energy-rich molecules, particularly the transfer of electronic excitation energy from one molecule to another. In later years, his work in photochemical energy transfer extended into the field of high-energy radiation chemistry, and he also initiated a program for the study of the kinetics and mechanisms of reactions of metallic compounds with molecular oxygen.

As I have reviewed the early volumes of Aduances, I realize just how important Professor Hammond was not only to the success of the series, but also to insuring that contributions were more than just reviews. Many chapters in the early volumes of Advances pose important questions which subsequently led to further advancement of a then developing field. Who would have surmised in 1963 that photochemistry would have the enormous commercial impact that it has assumed today? In no small part, and through the foresight of the original editors, Adcancrs in Photocheniistry contributed to that development. George’s contributions will be missed, but he has given so much that it is truly time to give George a break. Thanks, George, on behalf of the community.

Finally we note, with sadness. the passing of Professor Klaus Gollnick in Munich on October 27, 1993. Professor Gollnick served as series editor for Volumes 9-15. His editorship of Advances extended from 1973 through 1989.

During his distinguished career first as a student and research associate of Professor Gunther Schenck’s at the Max Planck Institut fur Strahlenchemie in Mulheim, and later at the University of Munich, Klaus established a distinguished research record in fields ranging from photooxygenation reactions to cycloaddition reactions of organic sulfur compounds. He collaborated with workers at the Max Planck Institut in early work on photoacoustic spectroscopy, being one of the early workers in this field. A detailed description of Klaus’ many research contributions appears in the November, 1993 issue of the European Photochemistry Association (EPA) Newsletter.

We extend sincere condolences to his friends and family.

D. C . NECKERS Bowling Green. Ohio

CONTENTS

cis- trans Photoisomerization of Stilbenes and Stilbene-Like Molecules 1 HELMUT CORNER AND HANS JOCHEN KUHN

AFM and STM in Photochemistry Including Photon Tunneling 119 GERD KAUPP

Photophysical and Photochemical Processes of Semiconductor Nanoclusters 179

Y ING WANC

The Question of Artificial Photosynthesis of Ammonia on Heterogeneous Catalysts 235

JULIAN A. DAVIES, DAVID L. BOUCHER, AND JIMMIE G. EDWARDS

Index 31 I

Cumulative Index, Volumes 1 - 19 32 1

xi

CIS- TRANS PHOTOISOMERIZATION

OF STILBENES

MOLECULES AND STILBENE-LIKE

Helmut Corner and Hans Jochen Kuhn Max-Planck-Institut fur Strahlenchemie, D-45413 Mulheim an der Ruhr,

Germany

CONTENTS

I . Introduction 11. Direct cis $ trans photoisomerization

A. B. Quantum yields

Ground state properties and absorption spectra

1. Effects of substitution 2. Effects of solvents 3. 4. Effects of quenchers

Effects of temperature and viscosity

Advances in Photochemisrrg, Volume 19, Edited by Douglas C. Neckers, David H. Volman, and Giinther von Biinau ISBN 0-471-04912-3 cc’ 1995 by John Wiley & Sons, Inc.

1

2 C I S - T R A N S PHOTOISOMERIZATION OF STILBENES

111. Sensitized c i s e trans photoisomerization A. B. Quantum yields C. Effects of quenchers Excited state properties of 1,2-diarylethylenes A. Singlet states

Effects of sensitizers on the photostationary state

1V.

1 . Fluorescence spectra 2. Fluorescence quantum yields 3. Activated processes 4. Fluorescence quenching 5. 6. Potential energy curves

1. 2. Triplet-triplet absorption spectra 3. Triplet lifetimes 4. Triplet yields 5. Effects of quenchers 6. Transients upon sensitized excitation

Fluorescence lifetimes and picosecond dynamics

B. Triplet states Energy of the lowest triplet state

C. Categories of stilbenes V. Competing reactions

A. Photocyclization B. Photoreduction C. Photoinduced electron transfer Mechanisms of cis e trans photoisomerization A. Singlet mechanism

V1.

I . History 2. 3. 4. Effects of substitution

1. 2. 3. Other mechanisms of trans + cis photoisomerization 1. Upper excited triplet pathway 2. Mixed singlet-triplet mechanism 3. Double activated mechanism

Acknowledgment Glossary References

Pathway for truns + cis photoisomerization Pathway for cis -+ trans photoisomerization

B. Triplet mechanism Sensitized cis e truns photoisomerization Direct truns + cis photoisomerization Direct cis 4 trans photoisomerization

C.

D. Mechanistic classification of 1,2-diarylethylenes

INTRODUCTION 3

I. INTRODUCTION

At first sight, the cis e trans photoisomerization of stilbene seems to be nothing but a simple example of a fundamental photochemical process: twisting about the C=C double bond. The first model for cis s trans photoisomerization was proposed by Olson as early as 1931 [I], only a few years after the formulation of quantum mechanics. Within the last 60 years, however, it became more and more obvious that stilbene photoisomerization is indeed a rather complex reaction and exhibits many intriguing features. Just about every step of the reaction mechanism may be influenced by various parameters, for example, sensitizers [2,3], additives [4], solvents [S], concentration [6], irradiation wavelengths [S], temperature [7- 1 I], and viscosity [9,12] (external factors); by substitution with diverse functional groups, for example, electron-pulling and/or pushing groups [ S , 7,131, heavy atoms [ 141, or deuterium [ 10,151; or by introduction of steric hindrance [ 161 (internal factors). Today, on the basis of a vast amount of experimental results, the essential features of the cis= trans photoisomerization reaction have been identified, widely accepted reasonable mechanistic models developed, and lively controversies settled. However, numerous issues have not yet been resolved right down to the last detail.

Stilbenes are extremely versatile model compounds. They provide the classic example for the now well-known fact that selective preparation of less stable isomers of higher energy content is one of the outstanding advantages of photochemistry [ 17, IS]. Even in photobiology, understanding of photoisomeriz- ation has proved highly productive [19]. Stilbenes have found many technical applications-as radiation detectors in neutron or 7-spectroscopy, as ingredients of photographic emulsions and resists, as fluorescence brighteners for textiles and paper, as nematic phases, or in optoelectronics [20,2 11. Besides photoisomeriz- ation and thermal or catalytic isomerization, there are at least six different competing types of photoreactions 1,2-diarylethylenes may undergo (Section V).

Aryl -C /H

Aryl’ H’

Cis truns photoisomerization of olefins has been reviewed repeatedly. Ross and Blanc [22] reported on photochromism of several classes of compounds containing C=C and N=N double bonds. Fischer [23,24] highlighted certain aspects of photochromism and reversible photoisomer- ization. Hammond [2] and his group put the understanding of the sensitized isomerization of stilbene on a firm basis. Saltiel et al. [25-281 presented several reviews about the mechanism of cis e trans photoisomerization of 1,3-dienes, alkenes, and stilbenes. Mazzucato [29] compared the properties

4 C I S - 7 R A N S PHOTOISOMERIZATION OF STILBENES

of stilbenes and stilbene-like compounds ( eg , aza analogues) and, together with Momicchioli [30], reviewed the rotational isomerism. The photochemi- cal features of 4-nitrostilbenes, such as 4-nitro- and 4,4'-dinitrostilbene, as well as 4,4'-NMS, 4,4'-NAS, and 4,4'-NDS were summarized by Schulte- Frohlinde and Gorner [31,32]. Tokumaru and co-workers reviewed the

O C H ~ 4,4'-NMS R: NH2 4,4'-NAS

N(CH3)2 4,4'-NDS photochemical properties of novel l-aryl-2-alkylethylenes, for example, l-aryl-3,3-dimethyl-l-butenes (ADBs) [33].

H ADB Awl-C'

'C-B"' H'

This chapter deals with the properties of the excited states of 1,2-diarylethy- lenes in general, with the effects caused by specific variation of reaction conditions on photochemical cis s trans isomerization in condensed phase, and with the reaction mechanisms. In particular, the influence of substitution on the properties of excited states involved in cis* trans isomerization in solution is examined. Besides various substituted stilbenes (which have been most extensively studied), styrylpyridines (StPs, azastilbenes) including some of their positively charged derivatives (quaternary stilbazolium salts), dipyridylethylenes (DPEs), styrylnaphthalenes (StNs), their pyridine analogues (NPEs), and some related compounds, such as dinaphthylethylenes (DNEs), are surveyed. Results on photochemical c i s 6 trans isomerization of stilbenes and other 1,2- diarylethylenes under direct (Section 11) and sensitized (Section 111) irradiation conditions are summarized, as well as their photophysical excited singlet and triplet state properties (Section IV) and some selected side reactions (Section V). The mechanistic section (Section VI) describes several photochemical isomeriz- ation routes. Characteristic photophysical and photochemical aspects of specific classes of substituted stilbenes are discussed and mechanistic schemes are critically examined with reference to their experimental basis.

4,4'-DPE

1,4'-NPE

1,1'-DNE

DIRECT CIS= T R A N S PHOTOlSOMERlZATlON 5

11. DIRECT CIS= TRANS PHOTOISOMERIZATION

A. Ground State Properties and Absorption Spectra

The electronic structure and energy levels of trans- and cis-stilbene and several related compounds have been calculated using various methods [34-421. The result for the ground state of cis-stilbene is a propeller-shaped conformation with out-of-plane twisting of the phenyl rings by 30-60" [34,43], while trans-stilbene should show only a very small deviation from planarity [44]. Experimental results confirm that cis-stilbene is nonplanar in the gas and liquid phase with an angle between the planes of the central double bond and a phenyl ring (dihedral angle) of 30-500 [34,45-471. For trans-stilbene an almost planar structure in the solid state has been determined by X-ray analysis [48], whereas in liquid solution out-of-plane distortion was observed by Raman spectroscopy [49]. For the gas phase a propeller-like conformation was found for trans-stilbene on the basis of electron diffraction results [SO] , while So -+ S , jet spectra of isolated mol- ecules [Sl] yielded evidence that trans-stilbene is planar. Recent results from analysis of the rotational structure of the fluorescence excitation spectrum [52] confirm that in the collision-free environment of a molecular beam, trans-stilbene is indeed planar (a rigid asymmetric top) in the zero-point vibrational levels of both its So and S, states.

Owing to the larger dihedral angle of cis-stilbene, the energy of the cis ground state is higher than that of trans-stilbene. The measured energy difference between the stilbene isomers ranges between 10 and 20 kJ m o l ~ depending on the method used [53-551. The free energy difference between cis and trans isomers (Eq. I),

has been determined from the equilibrium constant K = [cis]/[trans] in the presence of a catalyst such as atomic iodine. Table 1 shows that the energy difference is only slightly influenced by the nature of the substituent [56-581. Steric interactions, however, reduce the free energy difference and hence shift the equilibrium towards the cis isomer.

The enthalpy difference of cis- compared with trans-stilbenes (Table 2) has been determined for a few cases [55 , 59-64]. For stilbene an energy difference of 10-13 kJ mol-' has been reported, the result of a reexamina- tion [63] is 19kJ mol-I. This value has to be added to the barrier for the thermal cis + trans isomerization in order to obtain the energy of the twisted ground state.

6 ( ' I S T R A N S PHOTOISOMERIZATION OF' STILBEN ES

TABLE 1 Thermal Equilibrium Constants and Free Energy Differences

Temperature [cis] - AG" Compound Medium ("C) [trans] (kJ mol ~ ' ) Ref.

S ti I bene Solution 21 0.002 15.5 55 Neat 200 0.042 13 53

a,/]-Difluoro- Solution 27 0.067 6.8 55 a-Chloro- Neat 200 -240 0.020 53 a,l-Dichloro- Neat 200 3 - 4.4 55 4-Chloro- Solution 27 <0.005 > 13 55 4-Bromo- Solution 27 0.0023 15 55 r-Methyl- Solution 27 0.25 3.5 55 a./bDimethyl- Neat 200,210 0.82 1.0 54,55 4.4-Dimethyl- 0.042 58 2,4,6-Trimethyl- Solution 27 0.075 6.4 55 4-Methoxy- Solution 27 0.0024 15 55 4-Nitro- Solution 21 0.0009 18 55 a-Carbox y- Ac,O, Et,N > 100 4.26 56 a-Methoxycarbonyl-

4.4-dimethyl- > 100 58 n-Carboxy-4.4-

dimethyl- 5.25 58

TABLE 2 Enthalpy Differences Between cis and trans Isomers

- A H O Compound Medium (kJ mol- ' ) Ref.

S t i l bene Gas phase MCH Benzene Toluene Acetic acid

a- Methyl- MCH 4-Nitro- Solid 4,4-Dinitro- Solid

13 59 12 55 19 63 9.6 55

21, 24 60,62 2.1 55

29 61 19 61

The thermal cis -, trans isomerization of stilbenes has been reviewed by Cundall [65] and Laidler and Loucks [66]. For the energy barrier between cis- and trans-stilbene ground states values of 172- 193 kJ mol- ' have been determined in the gas phase (Table 3a). For liquid solutions an activation

DIRECT C I S $ T R A N S PHOTOISOMERIZATION 7

TABLE 3a cis + trans Isomerization

Activation Energies and A-Factors for Thermal

Ea Temp. Range Compound (kJ mol-') log A ("C) Ref.

Stilbene

m-Chloro- cr,fl-Dichloro- 4-Methoxy- 4-Amino- 4-Nitro- 4,4'-NMS 4,4'-NAS 2-StP 3-StP 4-StP D"

154 172 179 193 155 143 149 65

142 121 72

114 153 163 88

10.4 12.2 12.8

11.1 11.0 10.2

10.2 8.1 5.0 7.1

10.3 12.3

214-233 212-236 3 20 -405 300-440 226 - 246 175-200 272-300 40 - 80

210-272 210-293 97- 138 200-240 212-240 192 - 240 4.6--19

53 69

59,68 67 53 53 64 70 64 64 64 69 69 69 71

"D: cis- 1-(2,2-dimethyl-l-tetralinylidene)-2,2-dimethyltetralin

energy (E,) of approximately 170 kJ mol- and an A-factor of the order of lo', s- * have been reported [53, 67-69]. Both structure and substitution strongly influence the thermal cis + trans isomerization [70,7 11. Substitu- tion of stilbene by OCH,, NO,, and NH, in the 4-position reduces E, [64,70]. New experiments would be highly desirable; exclusion of isomeriz- ation catalysts could lead to higher A-factors and E, values.

For StPs, A-factors and E , values are smaller than for stilbene and are influenced by the position of the N-atom [69]. Gusten and Schulte-Froh- linde have shown that quaternary cis-styrylquinolinium salts are suitable compounds for kinetic studies because of their low Ea values and a sufficiently large rate constant of isomerization at room temperature [ 13,72, 731. Variation of the substituent in 4-position of the styrene and a change of the anion have a moderate effect on E, (83-93 kJ mol-' in acetone) and the A-factor (Table 3b), but the rate constant for thermal &-+trans isomerization increases strongly with increasing polarity of the solvent [73]. A correlated solvatokinetic behavior is known for certain merocyanines (cf. [74]). The StNs and DNEs are thermally stable at room temperature, indicating that E , is larger than 80 kJ mol- '.

8 CIS-TRANS PHOTOISOMERIZATION OF STILBENES

TABLE 3b Activation Energies and A-Factors for Thermal cis + trans Isomeriz- ation of Quaternary Salts of 4-R-Styrylquinolines"

R Solvent X- E a

(kJ m o l ~ ') log A

H Acetone OCH, Tetramethylurea

Dichlorornethane Acetone

n-Propanol O H Acetoile + HCI NCH,) , Acetone + HCI NO2 Acetone

I - I - 1- I - CH,SO; I - 1- I - 1-

93 I 5 85 88 83 83 90 90 86

12.0 9.4

11.4 11.2 11.1 9.3

11.3 12.3 11.5

"Taken from refs. 13, 73

Some useful concepts have been developed concerning the relationship between substituent and electron distribution [75-771. Calculated and experimentally determined activation energies agree quite well. The activa- tion energy and the n-bond energy of the central bond are influenced by interactions between the two n-electrons and the side groups which decrease the energy required for bond twisting. The activation energy is necessary for bond opening and formation of a twisted intermediate with minimum overlap of the n-orbitals.

The UV-absorption spectrum of trans-stilbene exhibits two main bands with maxima near 280nm (A-band) and 230nm (B-band) [22, 54, 78, 791. The A-band of cis-stilbene is attributed to a (n,n*) transition [SO]. As compared to trans-stilbene it is blue-shifted, much broader, and has a considerably lower molar absorption coefficient (6,) [24]. For the B-band, c, is greater than E , (Figure 1); the origin of the absorption transition in cis-stilbene is red-shifted relative to the same transition in trans-stilbene [Sl]. Two-photon excitation spectra ( < 5 x 104cm- '), together with theor- etical calculations, indicate seven excited singlet states [38].

DIRECT CIS* TRANS PHOTOISOMERIZATION - V x 1 ~ - 3 Icrn-’]

9

Figure 1. Absorption spectra for truns (-, stilbene (left) and 4,4’-NDS (right) in cyclohexane.

and cis (----, .-) isomers of

The spectra of trans- and cis-stilbenes are different owing to the steric hindrance of the cis isomer [82-871 (Table 4). Although it is easy to choose an appropriate irradiation wavelength to arrive at a photostationary state which is either cis- or trans-rich, the pure isomers of stilbenes can only be obtained by subsequent separation techniques. An increase of the twisting angle (e.g., by steric crowding of substituents, such as alkyl or halogen in the a- or 2-position), results in a blue-shift of the long wavelength band for both isomers [12, 54, 791. On the other hand, introduction of a 4-alkyl substituent has a bathochromic effect [79], which is stronger with phenyl or polar substituents such as OCH,, NO,, NH,, or N(CH,), [12, 22, SO]. As an example, the absorption spectra of trans- and cis-4,4’-NDS are shown in Figure I .

The potential energy diagram of stilbene is shown schematically in Figure 2. Apart from the two stable ground states it contains three

10 CIS-TRANS PHOTOISOMERIZATION OF STILBENES

TABLE 4 Maxima of Absorption Spectra of trans and cis Isomers"

1, 4 Compound Solvent (nm) c,' (nm) 6th Ref.

Stilbene

3-Fluoro- 4-Fluoro- 3-Chloro- 4-Chloro- 3-Brorno- 4-Bromo- 3-Methoxy- 4-Met hoxy- 3-Amino 4-Amino- 4,4'-Diamino- 3-Cyano- 4 -Cyan o - 4,4-Dicyano-

3-Nitro- 4-Nitro- 4,4'-Dinit ro-

4,4'-CMS

4,3'-NMS 3,4'-NM S 4,4'-NDS I-StN 2-StN

Cyclohexane n-Pentane Isohexane Benzene

Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane C yclohexane C yclohexane Acetonitrile Cyclohexane C yclohexane Ethanol Methanol C yclohexane

DMF Benzene Cyclohexane Benzene n-Hexane n-Hexane

t-Butyl a l ~ ~ h o l

MCH-IP

276

270

273 212 279 283 211 279 278 290 276 302

277 292

315 260 320 320 320

418 298 30 1

1.05

1 .oo

0.90 0.94 0.96 1.18 0.75 1.20 0.94 1.31 0.85 1.47 0.86 0.93 1.19

1.2 1.65 1.40 I .56 0.96

0.88 1 .oo 1.36

297 292, 306

290 299, 31 1 294, 306

294 293 299 30 I 296 298 297 306 296 316 347 294 323

326, 343 337 297 340 368 357 318 432 320 316

2.82 2.80, 2.45

2.6 2.67, 2.64 2.75, 2.5

2.69 2.53 2.68 3.02 2.79 3.15 2.46 2.91 2.57 2.86 3.2 2.73 3.53

3.9 2.38 3.20 3.78 2.53 2.76 2.99 2.07 3.94

82 4

22 4 4

82 82 82 82 82 82 82 82 82 82 84 82 82 85 83 82 22 5 5

86 5

87 87

"At RT . "In units of M ~ I cm ~ x lo4.

perpendicular conformations and four excited configurations as the mini- mum number of distinguishable states. In this respect styrenes [37] and stilbenes [88] are analogous. An asterisk denotes excited states, and the superscripts 1 and 3 refer to singlet and triplet states, respectively. The symbols t, p, and c represent the trans, perpendicular, and cis configurations, respectively. There are particular cases known for which additional states have to be assumed, apart from the ground state, the lowest triplet state, and the lowest excited singlet state (see Section VI). The results supporting

DIRECT C I S = T R A N S PHOTOISOMERIZATION

I -!, ,.‘a.

11

Figure2. Schematic potential energy curves for stilbenes, that is, energy as a function of the angle of twisting about the double bond.

the involvement of trans, twisted, and cis configurations of several electronic states, methods for their determination, and mechanistic models for several stilbene-like molecules are presented in the following sections.

B. Quantum Yields

The direct photochemical cis + trans isomerization of 1,2-diarylethylenes leads to a photostationary state if side reactions (Section V) are negligible. The percentages of trans and cis isomers in the photostationary state, (YO trans),, and (YO cis)ps, respectively, are directly related to @‘t-c and a,,, and the molar absorption coefficients c , and c, at the wavelength of irradiation (Iwi,,). Both @I+c and OC+, depend on several parameters e.g., on properties of the solvent, temperature, viscosity, additives and, less frequent- ly, concentration and ,Iirr [89-1381. In the simplest case, that is, if photo- cyclization and self-quenching can be neglected, the photostationary trans/ cis ratio is given by Eq. (2).

Since the absorption spectra of trans- and cis-stilbenes differ considerably, the isomeric composition in the photostationary state changes from mainly

12 CIS-TRANS PHOTOISOMERIZATION OF STILBENES

TABLE 5a Quantum Yields of cis +i trans Photoisomerization and Photostationary State for Stilbene"

'irr

Solvent a),,, a,,, (% w,, (nm) Ref.

n-Pentane

n-Heptane n-Hexane

C yclohexane

MCH MP

MCH Perfluoro-MCH Isooctane Benzene

MCH-IH

GT Glycerol Dichloromethane

Acetonitrile EtOH-MeOH Methanol

0.52 0.62 0.42 0.61 0.59 0.41 0.27 0.79 0.90 0.30 0.32 0.50 0.67 0.50 0.50 0.46 0.35 0.36 0.46 0.42 0.42 0.52

0.56 0.50 0.48

0.35 0.26 0.29 0.28 0.32

0.30

0.35

0.32 0.32

0.33 0.22 0.19 0.24

0.35 0.32

48 92

58 93

69

95 93

90,93

46 55 81 92

94

254 313 313 254 254 313 313 313 254 313 313

300-313 313 333 313 313 313 254

313 40Sh 313 313 254 264 290 313 337 313 313

300- 3 13

26, 104, 105 25, 104, 105

101 106 92

2, 92, 99 97 98

100 107 93

100 108 97

12, 108 110 1 1 1 95

100 2, 13, 25

115 7 7

109 109 109 109 102

I , 12 13

"In deoxygenated solution (in most cases) at RT. 'Using 140 atmospheres 0,.

rrans t o mainly cis under suitable conditions (e.g., ,Iirr, environment). Although a photostationary state results even on irradiation with a broad spectrum, monochromaticity is a prior condition for the precise determination of quantum yields. Zimmerman and co-workers introduced

DIRECT C I S = T R A N S PHOTOISOMERIZATION 13

a standard method [89] and some simplifications were developed later [13, 90, 911.

Determination of quantum yields for stilbene is prone to several experimental difficulties and systematic errors [92-991. A back-reaction correction has been introduced by Lamola and Hammond [139] and modified by Saltiel [ 1051. Earlier, Smakula [94] found higher values for stilbene in hexane on irradiation at 313nm than at 265nm, but later studies showed that ,Iirr has no effect (within experimental error) on and @c+l, in contrast to the case of azobenzene [89]. Values for @,,,, and (%cis),, of stilbene in various solvents at room temperature are listed in Table 5a. In general, accepted at+, values in a variety of solvents are around 0.5 and are somewhat lower (0.3) for @c,t [13, 93, lOlLl05].

1. Effects of Substitution. The quantum yields and the position of the photostationary state of halogenated stilbenes (Table 5b) and various other substituted stilbenes (Table 5c) at room temperature are quite similar to those of the parent compound. A previously reported strong influence of the position of bromo substituents on @t+c and mC+, [116] has been carefully reexamined by Saltiel and his group [26, 104, 1051. A small increase of and @c+l is generated by 4-bromo substitution; the introduction of a bromine atom in the 3-position has practically no influence on the quantum yields.

In most cases studied, the cis isomer predominates in the photoequi- librium. There exist, however, some cases in the series of sterically hindered stilbenes where the cis isomers are thermodynamically the more stable isomers. For example, a,/?-dichloro- and a-carboxystilbene contain larger amounts of cis than of trans isomers [12, 53, 55, 561 and only the cis isomer is known for a-nitrostilbene [58 ] . Maciaszek and co-workers [58] were also interested in the photochemical preparation of the “less stable isomer” of several stilbenes bearing bulky substituents in the a-position: Photochemical isomerization was found impracticable for cis-a-nitro- and a-CONH2-4,4- dimethylstilbene. With a-carboxy- and a-methoxycarbonyl-4,4’-dimethylstil- bene in benzene, however, a considerable amount of less stable trans isomer has been observed under steady state conditions.

For nitrostilbenes (Table 5d) in nonpolar solvents at ambient tempera- ture the quantum yields of isomerization are comparable to those of stilbene itself. However, 4-nitro-4-R-stilbenes show a different dependence on sol- vent properties and temperature. On increasing the solvent polarity, decreases strongly for polar substituents; for example, R = OCH,, NH,, and N(CH,),, 4,4‘-NMS, 4,4-NAS, and 4,4’-NDS, respectively. and Q C + of 4,4‘-NDS in nonpolar solvents (cyclohexane and benzene) are

14 C I S - T R A N S PHOTOISOMERIZATION O F STI LBENES

practically independent of ,Iirr (3 13-436 nm) [5] even when different absorp- tion bands are excited (Table 5d).

The effect of concentration on the photoisomerization has been studied with 4.4'-NMS [6,13]; CD,,, in a-methylnaphthalene is constant (0.2) within the range of 10pM to 1 mM and decreases above 10pM. A similar concen- tration dependent decrease of @,+, from 0.32 [(Yocis),, = 80.51 at 10 p M to @,-E = 0 at 1 M was found for 4,3'-NMS [13]. The trans +cis photoisomer- ization of 4,4'-NMS follows normal first-order kinetics independent of the concentration. However, for the cis + trans photoisomerization a deviation from the first-order law was observed at concentrations larger than 10 mM,

TABLE 5b Quantum Yields of cis ~t trans Photoisomerization and Photostationary State for Halogenated Stilbenes"

Compound Solvent @c-l (YO cis),, Ref.

3-Fluorostilbene 4-Fluoro- a,fl-Difluoro- 3-Chloro- 4-Chloro-

3,3'-Dichloro- a,P-Dichloro- 4-Bromo-

4,4'-Dibromo- 3-Bromo-

3,3'-Dibromo-

3-IOdO-

Cyclohexane Cyclohexane

Cyclohexane Cyclohexane

MCH-IH

MCH-IH GT Glycerol n-Hexane

n-Pentane Cyclohexane

Glycerol Ethanol n-Hexane n-Hexane n- Pen tane Cyclohexane n-Hexane n- Pentane n-Hexane

MCH-IP

MCH-IH

0.39 0.34 0.42 0.40 0.35 0.45 0.40 0.23 0.41 0.2 1 0.47" 0.60 0.45 0.55 0.3 0.33 0.53 0.51 0.25 0.4 1 0.14 0.53 0.44 0.39 0.35 0.35 0.16 0.52 0.27 0.5 0.3 0.4 1 0.20 0.46 0.18 0.56 0.34 0.38 0.40 0.53 <0.05 0.56 0.24 0.40 0.13

91 89 80 90 91

94 53 89 88

90 94 89 89

100 95 95

93 93 12 93 93

102 7 , l o x

1 I4 7

1 I6 12

104, 105 93

7 , 108 7

1 I4 1 I6 1 I6

104, 105 93

116 104, 105

116

" I n deoxygenated solution (in most cases, not ref. 93) at RT, i,,, = 313nm. *i,,r = 337 nm.

DIRECT C I S + T R A N S PHOTOlSOMERlZATION 15

TABLE 5c State for Substituted Stilbenes"

Quantum Yields of cis e trans Photoisomerization and Photostationary

'irr

Compound Solvent O,,, Oc+, (% cis),, (nm) Ref.

a-Methylstilbene

3-Methyl- 4-Methyl- 2,4,6-Trimethyl-

2,2',4,4',6,6'- hexamethyl-

3-Methoxy- 4-Methoxy-

4-Acetyl-

4-Benzoyl-

4-Phenacyl- 4,4'-Diphenyl- 4-Amino- 4-dimethy lamino-

3-Cyano- 4-Cyano-

4,4'-Dicyano- 4,4'-CMS

4,4'-CDS

Isohexane Glycerol Cyclohexane n-Hexane MCH-IH Glycerol

MCH-IH Cyclohexane Cyclo hexane

Ethyl acetate

Ethanol

MTHF

MCH Cyclohexane

G T EtOH-MeOH Cyclohexane Cyclohexane Benzene Cyclohexane Cyclohexane Ethanol Cyclohexane Benzene

MCH-IH

MCH-IH

MCH-IH

MCH-IH

MCH-IP

0.48 0.48 0.40 0.55 0.47 0.5

0.48 0.3 1 0.40 0.46 0.58 0.5 0.48 0.54 0.52 0.5 0.0025 0.49 0.52 0.44

0.39 0.46 0.42 0.45 0.45 0.42 0.50 0.50

0.58 0.1 0.2 1

0.39

0.40 0.19 0.29 0.25

0.3 0.31 0.27 0.32 0.3

0.30 0.22 0.26

0.44 0.4 1 0.45 0.35 0.40 0.40 0.3 0.3

63

92

92

94.5 77 85 89

79

80

81 94

86 83 81 80 83 85 86 82 78

313 313 313 337 313 313

337 313 313 313 313 313 366 313 365 313 365 313 313 313 313 366 366 366 366

12 7

93 102 12 7

12 93 93 12

102 12

121 12

121 12

110 93 12 7

12 93 93 85 85 85 85 85 85

"In deoxygenated solution at RT.

the reaction becoming faster during irradiation; 0, + t increases above unity upon increasing the amount of trans isomer. An autosensitized photoreac- tion involving a short-lived intermediate has been suggested by Schulte- Frohlinde and Giisten [ 6 ] . A related phenomenon has been reported for thioindigo dyes [ 1401.

16 CIS-TRANS PHOTOlSOMERlZATlON OF STILBENES

A dependence of @,,, on concentration has been reported for 2-StN in n-hexane or benzene, contrasting with the case of the 1-analogue [87,122, 1273. An energy transfer mechanism implying formation of an excimer between molecules in the excited and the ground state has been suggested [87]. With 2-StNs, Hammond and co-workers [124] found that with increasing wavelength @,,, increases distinctly while @,-, decreases, also causing a change in the shape of the emission spectra. The authors postulated that higher excited singlet states may have been populated, leading to different conformation and decay characteristics. A similar interpretation was offered for another case of pronounced wavelength dependence: The cyclization of cis-DNEs is considerably favored over formation of the trans isomer if irradiated at 366nm instead of 334 or 313nm [123].

A large number of papers deal with the direct cis & trans isomerization of arylethylenes with larger chromophores, such as anthracene [33, 141, 1421, fluoranthene [143,144], pyrene [145], or related compounds [33, 146- 1511. In several papers Tokumaru and his group have shown that @,+, increases with increasing cis concentration of ADBs, where the aryl group is 2-anthryl [ 1421, 8-fluoranthenyl [33, 143, 144, 1461, or 1-pyrenyl [145].

2. Effects of Solvents. The trans -+ cis photoisomerization of stilbenes and related compounds in the liquid phase is influenced by solvent properties, for example, the external heavy atom effect [117], temperature and viscosity (Section II.B.3), polarity, or the pH value [152-1891. However, no pro- nounced effect of solvent properties on @,,, has been found for stilbene in liquid solution. This indicates that long-lived intermediates are not involved in the major cis-trans pathway. The change of @,,, with solvent polarity has been observed almost exclusively with polar stilbenes and with azastilbenes. Stilbenes bearing polar ring substituents of different electron-attracting power undergo several solvent-solute in- teractions which strongly influence their photophysical and photochemical behavior.

The effect of solvent polarity on @,,, can be interpreted qualitatively: Because the highly polar excited states enter into strong interaction with polar solvents, their tendency toward geometric isomerization (which re- quires complete reorganization of the solvate structure) is naturally very low. Data from Schulte-Frohlinde and co-workers [5] show substantial and different @,+, values (0.44 and 0.20, respectively) for 4,4’-NAS and the even more polar 4,4‘-NDS in cyclohexane. In benzene the difference is still more pronounced (0.10 and 0.016, respectively) while in polar solvents, for example, N,N’-dimethylformamide (DMF) or ethanol, @,-, is zero; the photostationary state is correspondingly shifted to the trans side [ 1521. With