Photochemistry (SPR Photochemistry (RSC)) (Vol 31)

Photochemistry Volume 31

A Specialist Periodical Report

Photochemistry Volume 31 A Review of the Literature Published between July 1998 and June I999 Senior Reporter A. Gilbert Department of Chemistry, University of Reading, UK Reporters

N.S.Allen Manchester Metropolitan University, UK A. Cox , University of Warwick, UK

1. Dunkin University of Strathclyde, Glasgow, UK A. Harriman University of Newcastle upon Tyne, UK W.M. Horspool University of Dundee, UK A.C. Pratt Dublin City University, Ireland

RSeC

ROYAL SOCIETY OF CHEMISTRY

ISBN 0-85404-425-6 ISSN 0556-3860 Copyright 0The Royal Society of Chemistry

All rights reserved Apart from any fair dealingfor the purposes of research or private study, or criticism or review as permitted under the terms of the UK Copyright, Designs and Putents Act, 1988, this publicution may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the cuse of reprographic reproduction only in accordance with the terms of the licences issued by the uppropriute Reproduction Rights Organizution outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this puge. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK For further information see our web site at www.rsc.org Typeset by Computape (Pickering) Ltd, Pickering, North Yorkshire, UK Printed and bound by Athenaeum Press Ltd, Gateshead, Tyne and Wear, UK

ISBN 0-85404-425-6 ISSN 0556-3860 Copyright 0The Royal Society of Chemistry

All rights reserved Apart from any fair dealingfor the purposes of research or private study, or criticism or review as permitted under the terms of the UK Copyright, Designs and Putents Act, 1988, this publicution may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the cuse of reprographic reproduction only in accordance with the terms of the licences issued by the uppropriute Reproduction Rights Organizution outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this puge. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK For further information see our web site at www.rsc.org Typeset by Computape (Pickering) Ltd, Pickering, North Yorkshire, UK Printed and bound by Athenaeum Press Ltd, Gateshead, Tyne and Wear, UK

Contents

Introduction and Review of the Year By Andrew Gilbert

1

Physical Aspects of Photochemistry

13

Photophysical Processes in Condensed Phases By Anthony Harriman

15

1 Introduction

15

2 General Aspects of Photophysical Processes

15

3 Kinetic and Theoretical Considerations

17

4 Photophysical Processes in Liquid or Solid Media 4.1 Detection of Single Molecules 4.2 Radiative and Non-radiative Decay Processes 4.3 Amplitude or Torsional Motion 4.4 Quenching of Excited States 4.4.1 Electron-transfer Reactions 4.4.2 Energy-transfer Reactions 4.5 Photophysics of Fullerenes

18 19 19 20 21 21 22 23

5 Applications of Photophysics

24

6 Advances in Instrument Design and Utilization

26

References

28

Organic Aspects of Photochemistry

45

Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspool

47

Part I

Part I1

1 Norrish Type I Reactions Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 V

47

vi

Contents

2 Norrish Type I1 Reactions 2.1 1,5-Hydrogen Transfer 2.2 Other Hydrogen Transfers

50 50 55

3 Oxetane Formation

57

4 Miscellaneous Reactions 4.1 SET Processes 4.2 Decarbonylation and Decarboxylation 4.3 Reactions of Miscellaneous Haloketones and Acid Chlorides 4.4 Other Fission Processes

61 61

67 69

References

71

Chapter 2 Enone Cycloadditionsand Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool

64

75

1 Cycloaddition Reactions 1.1 Intermolecular Cycloaddition 1.1.1 Open-chain Systems 1.1.2 Additions to Cyclopentenones and Related Systems 1.1.3 Additions to Cyclohexenones and Related Systems 1.2 Intramolecular Additions 1.2.1 Intramolecular Additions to Cyclopentenones 1.2.2 Additions to Cyclohexenones and Related Systems

75 75 75

2 Rearrangement Reactions 2.1 a,P-Unsaturated Systems 2.1,l Isomerisation 2.1.2 Hydrogen Abstraction Reactions 2.1.3 Rearrangement Reactions 2.2 P,y-Unsaturated Systems 2.2.1 The Oxa Di-n-methane Reaction and Related Processes

86 86 86 86 87 88

3 Photoreactions of Thymines and Related Compounds 3.1 Photoreactions of Pyridones 3.2 Photoreactions of Thymines etc. 3.3 Miscellaneous Processes

89 89 90 93

76 80 81 83 83

88

vii

Contents

4 Photochemistry of Dienones 4.1 Cross-conjugated Dienones 4.2 Linearly Conjugated Dienones

93 93 95

5

96

1,2-, 1,3- and 1,4-Diketones 5.1 Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds 5.2 Reactions of 1,3-Diketones 5.3 Reactions of 1,4-Diketones 5.3.1 Phthalimides and Related Compounds 5.3.2 Fulgides and Fulgimides

96 99 100 101 103

6 Quinones 6.1 o-Quinones 6.2 p-Quinones

104 104 104

References

106

Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool 1 Reactions of Alkenes 1.1 cis,trans-Isomerisation 1.1.1 Stilbenes and Related Compounds 1.1.2 The Dithienylethene System and Related Compounds 1.2 Miscellaneous Reactions 1.2.1 Addition Reactions 1.2.2 Electron Transfer Processes 1.2.3 Other Processes

112

112 112 113 116 120 120 122 122

2 Reactions Involving Cyclopropane Rings '2.1 The Di-n-methane Rearrangement and Related Processes 2.1.1 The Aza-di-n-methane Rearrangement and Related Processes 2.1.2 SET Induced Reactions 2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds

127

3 Reactions of Dienes and Trienes 3.1 Vitamin D Analogues

128 132

4 (2+2)-Intramolecular Additions

133

124 124 125 125

...

Contents

Vlll

5 Dimerisation

134

6 Miscellaneous Reactions 6.1 Reactions Involving Cations and Radicals 6.2 Miscellaneous Rearrangements and Bond Fission Processes

137 137

References

140

Chapter 4 Photochemistry of Aromatic Compounds By Alan Cox

138

145

1 Introduction

145

2 Isomerisation Reactions

145

3 Addition Reactions

154

4 Substitution Reactions

163

5 Cyclisation Reactions

165

6 Dimerisation Reactions

171

7 Lateral Nuclear Shifts

175

8 Miscellaneous Photochemistry

176

References

182

Chapter 5 Photo-reduction and -oxidation By Alan Cox

193

1 Introduction

193

2 Reduction of the Carbonyl Group

193

3

Reduction of Nitrogen-containing Compounds

202

4

Miscellaneous Reductions

205

5 Singlet Oxygen

209

6 Oxidation of Aliphatic Compounds

21 1

7 Oxidation of Aromatic Compounds

217

ix

Contents

8 Oxidation of Nitrogen-containing Compounds

220

9 Miscellaneous Oxidations

225

References

226

Chapter 6 Photoreactionsof CompoundsContaining Heteroatoms Other than Oxygen By William M.Horspool and Albert C. Pratt

234

1 Introduction

234

2 Nitrogen-containing Compounds 2.1 E,Z-Isomerisations 2.2 Photocyclisations 2.3 Photoadditions 2.3.1 Intramolecular Processes 2.3.2 Intermolecular Processes 2.3.3 Other Addition Reactions 2.4 Rearrangements 2.5 Other Processes

235 235 238 244 244 245 245 25 1 256

3 Sulfur-containing Compounds

27 1

4 Compounds Containing Other Heteroatoms 4.1 Silicon and Germanium 4.2 Phosphorus 4.3 Other Elements

277 277 280 283

References

285

Chapter 7 Photoelimination By Ian R Dunkin

2w

1 Introduction

297

2 Elimination of Nitrogen from Azo Compounds and Analogues

297

3 Elimination of Nitrogen from Diazo Compounds and Diazirines 3.1 Generation of Alkyl and Alicyclic Carbenes 3.2 Generation of Aryl Carbenes 3.3 Photolysis of a-Diazo Carbonyl Compounds

299 299 300 302

X

Contents

4 Elimination of Nitrogen from Azides and Related Compounds 4.1 Aryl Azides 4.2 Heteroaryl Azides

5 Photoelimination of Carbon Monoxide and Carbon Dioxide 5.1 Photoelimination of CO and CO;! from Organometallic Compounds

Part 111

303 304 307 307

309

6 Photoelimination of NO and NO;!

312

7 Miscellaneous Photoelimination and Photofragmentations 7.1 Photoelimination from Hydrocarbons 7.2 Photoelimination from Organohalogen Compounds 7.3 Photofragmentations of Organosilicon and Organogermanium Compounds 7.4 Photofragmentations of Organosulfur and Organoselenium Compounds 7.5 Photolysis of o-Nitrobenzyl Derivatives 7.6 Other Phot ofragmentations

3 14 314 314

References

324

Polymer Photochemistry By Norman S. Allen

333

1 Introduction

335

2 Photopolymerisation 2.1 Photoinitiated Addition Polymerisation 2.2 Photocrosslinking 2.3 Photografting

335 336 340 345

3 Luminescence and Optical Properties

346

4 Photodegradation and Photooxidation Processes in Polymers 4.1 Polyolefins 4.2 Poly(viny1halides) 4.3 Poly(acry1ates) and (alkyl acrylates) 4.4 Polyamides and Polyimides 4.5 Poly(a1kyl and aromatic ethers) 4.6 Silicone Polymers 4.7 Polystyrenes and Copolymers 4.8 Polyurethanes and Rubbers

357 357 358 359 359 359 360 360 360

317 3 19 321 322

xi

Contents

4.9 4.10 4.1 1 4.12 5

Part IV

Polyesters Photoablation of Polymers Natural Polymers Miscellaneous Polymers

Photostabilisation of Polymers

360 360 36 1 36 1 362

6 Photochemistry of Dyed and Pigmented Polymers

363

References

364

Photochemical Aspects of Solar Energy Conversion By Alan Cox

393

1 Introduction

395

2 Homogeneous Photosystems

395

3 Heterogeneous Photosystems

396

4 Photoelectrochemical Cells

398

5 Biological Systems

399

6 Luminescent Solar Concentrators

400

References

400

Author Index

403

Introduction and Review of the Year BY ANDREW GILBERT

The chapter and reference numbers of the reports cited in this Introduction and Review can be found by using the Author Index. The study of the photophysical processes occurring in transition metal complexes, particularly those of potential use as light-activated molecular scale devices, continues to attract much attention. The photo-induced electron transfer in metal-organic dyads (Schanze et al.) and in supramolecular assemblies (Willner et al.) has been reviewed, and the photophysical properties of several structurally modified porphyrins have been measured in order to identify new sensitisers for photodynamic therapy (Srinivasan et al.) Metalloporphyrins are important building blocks for the assembly of photo-active dyads, triads and higher order arrays and are known to have relatively longlived upper excited states. Indeed several studies reported in the year (e.g. Andersson et al.) have been concerned with energy or electron transfer from the S2 state of porphyrin-based dyads: these observations provide new possibilities for the design of advanced systems having high selectivity and wavelength-dependent mu1ti photoevent s. Recent advances in the theoretical and experimental understanding of ultrafast solvation processes, with particular reference to the role of high frequency vibrational modes, have been highlighted by Bagchi and Gayathri. A new mathematical expression has been evolved to explain the kinetic processes inherent to particular photochromic systems (Ottavi et al.) and a new power law dependence has been proposed by Kim et al. for the long-time behaviour of reversible diffusion-influenced reactions. The Rehm-Weller model for bimolecular electron transfer reactions has been of great value in aiding the understanding of photoprocesses and has recently been the subject of a critical comparison with encounter complex models (Takeda et al.). The photophysical properties of individual molecules under a wide variety of conditions are currently attracting considerable interest. Spatial photoselection of single molecules on surfaces has been reported (Watson et al. and Lerner et al.) and the importance of the triplet excited state for single molecule detection has been emphasised by Kilin et al. and Brouwer et al. Interest in the study of higher energy states has increased in recent years and weak fluorescence has been observed from the S2 level of anthracene crystals using a two-step excitation approach (Katch et al.). Ultrafast relaxation from higher-lying excited states has been reported for several systems and the wellPhotochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 1

2

Photochemistry

known case of azulene has been comprehensively investigated (Tetredult et al.). A number of puzzling features have emerged from the study of Brocklehurst and Young on the rotational relaxation times of a fluorophore over a vrlide range of viscosity, and a new model to account for the time-dependent evolution of products from fast photoisomerisations has been described by Lochbrunner et al. There are various potential applications of photophysical phenomena in analytical chemistry. The relatively short lifetimes of most excited states, however, is a serious drawback to the construction of practical devices but studies which focus on finding ways to extend triplet lifetimes have now been described by Harriman et al. Kneas et al. have examined new types of luminescent sensor on polymer supports, and both Neurauter et al. and Marazuela et al. have designed sensors based on the ruthenium(I1) polypyridine complex for the detection of carbon dioxide. A system, based on the formation of twisted intramolecular charge transfer states, has been devised for measuring the molecular weight of polymeric matrices (Al-Hassan et al.), and the chemical reactivity at the interface of self-assembled monolayers has been assessed using fluorescence spectroscopy (Fox et aZ.). The long-standing difficulty of determining the quantum yield of a heterogeneous photocatalytic process is reported to be overcome by an experimental protocol (Schiavello et al.), and a new approach to alleviate the previous difficult and tedious process of the collection of quantitative data from commercial fluorescence microscopes has been published (Leabeck and Ladds). A new design has been suggested by Pack et al. for a photon-counting fluorescence spectrometer which allows the rapid accumulation of decay data, and in situ measurements of photochemical processes occurring in microdrops has been described by Bhanti and Ray. The reports in the organic sections of this review are now considered. Irradiation of valerophenone is well known to yield both acetophenone and cyclobutanols by a Norrish Type I1 process but Zepp et al. report that the latter product (cis : trans ratio 2.4 : 1) is more efficient in aqueous systems than hydrocarbons. Such ketones as 1 readily undergo the Type I1 process in the solid phase and from a detailed study involving the use of chiral auxiliaries as counter ions of its carboxylate derivative, Leibovitch et aZ. conclude that the ‘ionic chiral auxiliary approach’ is a viable general method for asymmetric synthesis. Crystals of the ketone 2 are apparently photostable at room temperature but when finely ground or at elevated temperatures intramolecular hydrogen abstraction and formation of the benzocyclobutene 3 occurs (It0 et al.), and the same workers also note that irradiation of S-4 at 4°C in the solid state and at 34% conversion gives the SS product 5 with a diastereoselectivity of 99%. The formation of oxetanes from the irradiation of benzaldehyde and furan is reported to occur with a diastereoselectivityof 212 : 1 for the exo-6 and endo-7 isomers respectively (Griesbeck et al.) and similar reaction of this aldehyde with the enamine 8 favours the oxetane 9 over 10 in what the authors describe as an unprecedented facial diastereoselectivity (Bach and Brummerhop).

Introduction and Review of the Year

3

I

0

(1) R = CN, C02Me, Me, OMe or C G H

I

H

0

(6) R' = H, R2 = Ph (7) R' = Ph, R2 = H

C02Me (8)

C02Me (9) R' = H, R2 = Ph (10) R' = Ph, R2 = H

Mehta and Ravikrishna have described the ready formation of monosubstituted semibullvalenes 11 by photodecarbonylation of the polycyclic ketones 12 in methanol solution, and by a similar procedure, meta and para [2.2] cyclophanes are obtained by bisdecarbonylation of 13 and 14 respectively (Isaji et al.).

R

R

B (12) R

(1 1)

R = C02Me, CN, Ph or C H e M e

4

Photochemistry

Recently the photochemistry of 1,2-dithienyletheneshas attracted considerable attention as a result of their largely fatigue-free photochromic properties. Indeed Irie et al. observe no noticeable spectral changes following 800 of the 15 -+ 16 + 15 cycles, and the same group (Uchida et al.) note that the

Ph

Ph

hv

Me Me

Ph

Me

efficiency of the photocyclisation reaction in these systems increases markedly with increase in the size of the 2,2'-substitutents on the thiophene units. The disulfide 17 is readily transformed photochemically into the isomer 18 which is 0

S/Ph

(17) R = (18)

R=

'

SPh

the key intermediate in a new synthetic pathway to quinanes (Usui and Paquette), and during a study into the biogenesis of functionalised lactones from a Caribbean gorgonia, Rodriguez et al. have observed that the photoconversion of 19 gives 20 as the major product which appears to involve a 1,3sigmatropic migration with retention of configuration at the migrating carbon

Me

hv

Me

Introduction and Review of the Year

5

atom. Maier and Bothur report that irradiation (270 nm) of cis 3,4-dichlorocyclobutene induces conrotatory ring opening which is that normally favoured in the thermal process, and Ackermann et aZ. have noted that DCA-sensitised (419 nm) cyclisation of the enol ether 21 yields the cis ketone 22 with high selectivity.

Paddlanes such as 23 can be readily obtained in good yields from di+ 2n) photocycloaddition of the linked 1,3-divinylbenzenes24 (Inokuma et uZ.), and in the crystalline state the enamides 25 give head-to-tail dimers 26 in yields exceeding 87% (Song et aZ.). The diastereoselectivity of the (2x+27t) photocycloaddition of vinylene carbonate to homochiral furanones 27 is observed to be very dependent on the nature of the R group (Gregori et uZ.) and these adducts have been used in a synthetic route to carbohydrate derivatives. (271

(23) n = 2 , 3 o r 4

(24)

eNXAI 0

(27)

e

o

A

r

Ryy

R = H,OAc, -OCOPh, -0COCMe3, -0SiPh2Bu'

Photoinduced electron transfer initiated addition occurs between N-substituted pyrrolidines and electron deficient ethenes such as acrylonitrile or

6

Photochemistry

furanones. In the latter case, the facial stereoselectivity of the addition to 28 to give 29, has been exploited in a synthetic route to alkaloids of type 30

(Bertrand et al.). The adduct 31 from irradiation of the chromone 32 in the presence of ethene is a key intermediate in a synthetic strategy towards tricothecene analogues (Ma1 and Venkateswaran), and the acetone-sensitised photoaddition of the enone 33 to 2,3-dimethylbut-2-ene to give 34 is suggested

by Schwebel et al. to arise from an upper excited triplet state in a stepwise sequence. Formation of the novel 3-azabicyclo[3.l.l]heptan-2-one35 from 36 has been used by Tsujishima et al. in a route to new glutamate analogues, and a synthetic application has also been recognised for the oxa-di-mmethane rearrangement of enones such as 37 into the isomer 38 (Singh et al.)

Me Me

0

It is reported by Meth-Cohn et al. that, in contrast to earlier claims, the ketone-sensitised reaction of a-azidocinnamates gives a high yield of the intermediate pair of aziridinoimidazoline dimers, and Hoffmann et al. have used the irradiation of aziridines such as 39 to access novel compounds of type

Introduction and Review of the Year

7

40. Photoisomerisation of certain diazaphospholes proceeds by an unprecedented contraction of a five-membered ring to a four-membered ring (Manz et al.) and the 1,3-diaryl-1,2-dihydropentalene 41 undergoes a novel rearrangement on irradiation to give the corresponding 1,5-isomer 42 (Nair et al.).

Ph

Q

Ph

Q

Timmermdns et al. report that diastereoselectivity can be induced in the intramoleuclar meta photocycloaddition of ethenes to the benzene ring as a result of minimisation of steric interactions between substituents on the linking tether of the bichromophore and a methoxy group at the 2-position of the arene unity: this type of photoprocess has also been used as a key step in a formal synthesis of crinipellin B (Wender and Dore). New polycyclic cage compounds 43 have been obtained by irradiation of the [3.3.3] (1,3,5) cyclophane 44 (Sakamoto et al.) and Kubo et al. have described the intramolecular [3 + 21 photocycloaddition of bichromophores such as 45 which gives rise to nine- to eleven-membered ring systems 46.

Dibenzo[b.floxepins and dibenzo[b,flthiepins have important medicinal uses and can be prepared by irradiation of halogeno-substituted acetophones in liquid ammonia in the presence of bases (Nagaoka). A novel 1,9-hydrogen atom abstraction occurs on irradiation of 47 to give ultimately the keto ether 48 (Mizuno et al.) and previously unknown polycyclic ring systems such as 49,

8

Photochemistry

(45) n = 3 , 4 , 5

(46)

for example, can be obtained by an oxidative photocyclisation reaction (Luo et al.).

Several reports published within the year describe new photocleavage protecting groups. The accounts by Misetic and Boyd, and Giegrich et al. outline such mechanisms for primary alcohols and in nucleosidehucleotide systems respectively, while the work of Akerblom et al. is relevant to the synthesis of combinatorial chemical libraries on solid phases and that of McGall et al. is of use in the solid phase synthesis of polypeptides and oligonucleotides. Scaiano et al. report that 1-azaxanthone has greater reactivity towards hydrogen atom abstraction in polar media than other aromatic ketones and has also been suggested for use as a probe in radical ion reactions. 4-0x0-4phenylbutanoyl amines yield the corresponding 6-lactams following &-hydrogen atom abstraction with a diastereoselectivity exceeding 99% (Lindemann et al.) and irradiation of 50 gives the seven-membered lactam 51 by charge-transfer interaction and 6-hydrogen abstraction with no direct y- or &abstraction by the excited state carbonyl group being observed (Hasegawa and Yamazaki).

As in recent previous years, there has again been a considerable number of publications within the review period describing various aspects of the photo-

9

Introduction and Review of the Year

chemistry of C a fullerene. Stasko et aZ. report the photoreduction of C60 by triethylamine to give C60H- which decays to Cm’- on cessation of irradiation. Visible irradiation of c60 in toluene in the presence of methyl 2-furoate yields oxides, CWOn (n 2 5) which are the highest oxides produced to date photochemically, and calculations indicate that the epoxide groups in these oxides are in close proximity on one side of the fullerene core (Tajima et aZ.). The photocyclisation of enones of type 52 to give the trans fused product 53 is well documented. However, in apparent contradiction of the WoodwardHoffmann rules, 54 is now reported to give the cis fused product 55 (Pascal et af.). Near quantitative yields of 56 have been obtained from the photocyclisation of imines 57 on Ti02 (Park and Jun). Aziridines can be formed photochemically from pyridinium salts and in the presence of a nucleophile these give a potentially useful access to aminocyclopentane derivatives as is well illustrated by Ling and Mariano with their application of this procedure to the synthesis of (+)-mannostatin 58 from pyridinium perchlorate. The formation of the naphthisoxazoles 59 by irradiation of oximes 60 can be rdtionalised by the unprecedented 1,3-addition of a nitrile oxide moiety to an aryl ring (Barnes et af.), and a novel approach to the synthesis of dihydrquinoline derivatives 61 from the irradiation of rn-nitrocinnamic acid in the presence of Ti02 and alcohols has been described by Park et af.

ad-& QRfi 0-b ‘

N

I

R

(52)

YR H

(53)

N

N

I

I H Me (55) R = Hor Me

M8 (54) R = H or Me

It was earlier reported that the photo-Wolff rearrangement of diazoacetic acid gave phenylhydroxyketene. However, new experiments described by Kresge show that this is only a minor route and the major pathway appears to involve the carboxycarbene which undergoes the hydration to yield the enol of

10

Photochemistry 3r

Me Me

I

H (61) R' = H or alkyl

mandelic acid. The reactions of compound having two non-equivalent diazo groups are only rarely described. Interestingly, the studies with 62 indicate that the dependence of product formation on wavelength of irradiation arises from differential photoinduced decomposition of the 2- and 4-diazo groups (Murata et al.). Irradiation of powered crystals of aryl azides gives azo compounds in yields greater than 97% and from ESR monitoring of the reaction, it is evident that the intermediate arylnitrenes have extremely long half-lives compared to those formed in solution or the gas phase, and that there is a marked influence on the reaction pathway by the crystalline environment (Sasaki et al.). The photolabile compounds 63 chelate metals via the two amino groups but the photoactive azide unit is then shielded from the steric and electronic effects of the metal by the linking ester. In such cases, the nitrene produced on irradiation has a very high C-H insertion efficiency and the potential usefulness of this feature has been demonstrated by the chelation to the diagnostic radionuclide gmnTcand then attached to human serum albumin by photoactivation (Pandurangi et al.).

Knolker et al. have described a novel and convenient procedure for the demetallation of tricarbonyliron-diene complexes such as 64 which involves

Introduction and Review of the Year

11

initial photoinduced exchange of the CO ligands with acetonitrile, and Hwang et al. have reported the first example, albeit in low yield, of photoelimination of NO from a furoxan. The results of gas phase photolysis of silacyclopent-3ene have led Pola et al. to suggest that the resulting clean extrusion of silylene to give butadiene is suitable for the chemical deposition of Si/C/H films. A variety of aspects of ‘polymer photochemistry’ continue to attract wide interest and activity in the area of polymeric light emitting diodes has increased considerably in the past year or so (see Part 111, Section 3). New photochromic polymers based on spiropyrans with polymerisable groups which are sensitive to the heterogeneity of the polymer, are suggested to be suitable for development as optical storage media (Lyubmov et a2.). Photochemical ‘command’ effects have been designed as a new method for the development of planar or homeotropic alignment of photochromic polymers (Stumpe et al.) and a novel method has been reported for tunable emissions in smart gels (Vaganova and Yitzchaik). Allen et al. have described a series of novel derivatives of benzophenones which are highly effective photoinitiators and which give alkyl and thio radicals by side chain scission, and for photocrosslinking, several new amine co-synergists have been prepared having poly(ethy1eneoxy) groups with a high reactivity and low extractability (Anderson et al.). Fast curing of paper coatings can be effected using new cationic initiators synthesised from the reaction of diaryliodonium salts with lithium tetrakis(pentafluoropheny1)borate (Priou), and a new process of photografting has been developed using dendritic polyesters (Ranby). The photochemistry related to solar energy conversion continues to tax the skill and ingenuity of a number of research groups. The investigation by Jiang and Aida of some dendrimer porphyrins having different numbers of fivelayered dendron subunits has shown that the excitation energy is able to migrate over the array of the chromophoric units that surround the energy trap: this finding provides a new strategy for the design of light-harvesting materials. The further study by the same group (Aida et al.) of cis-trans interconversions using infrared radiation (1597 cm- I ) of layers of large aryl ether azodendrimers may have similar implications for new approaches to light harvesting. Fujihara et al. have described a system capable of continuously photosplitting water into hydrogen and oxygen based on a combination of two photocatalytic reactions on the surface of Ti02 particles, and several workers comment on new photocatalysts for such applications (On0 et al. inter alia).

Part I Physical Aspects of Photochemistry By Anthony Harriman

Photophysical Processes in Condensed Phases BY ANTHONY HARRIMAN

1

Introduction

The format of this chapter follows that used in previous volumes. Coverage is given to the multifarious routes by which an electronically excited state may undergo deactivation in solution or solid phase. Additional attention is given to instrumental methods used to detect photophysical processes and to the application of photophysics in contemporary analytical chemistry. The huge literature accompanying these subjects precludes a thorough review of each important development and it is regretted that, given page restrictions, all relevant publications cannot be covered. 2

General Aspects of Photophysical Processes

A database of absorption and fluorescence spectra for some 125 photoactive compounds has been established, together with accompanying routines for calculating various photophysical events. The numerous scientific achievements of J. and F. Perrin, Vavilov, Levshin and Pringsheim have been documented2 and a historical overview of fluorescence analysis has been ~ o m p i l e d .A~ review of luminescence techniques and instrumentation has a ~ p e a r e dSeparate .~ reviews have covered most areas of contemporary luminescence spectroscopy, including general theoretical aspect^,^ photoluminescence,6 ionoluminescence,7 thermoluminescence,* different forms of sonol~minescence,~ mechanoluminescence,lo bioluminescence, and chemiluminescence. Critical reviews of single bubble sonoluminescence have appeared12913 while further attention has been given to understanding the emission properties of complex molecules in solution, crystals and thin solid films.l4 The kinetics of fluorescence quenching, including fast bimolecular reactions, have been reviewedlS while an overview of the effects of complexation on emission properties has been presented.l 6 This latter study is addressed primarily towards the use of non-radiative energy transfer between lanthanides and appropriate chelating functions. Recent trends in the analytical applications of chemiluminescence have been reviewed l 7 while the technique of laser flash photolysis has been summarized.'* Many important aspects of electron-transfer reactions have been reviewed Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 15

16

Photochemistry

and current topics of research in this field have been highlighted. The possible application of electron-transfer processes in solar energy conversion has been c~nsidered'~ and a comprehensive theoretical description of ultrafast electron transfer has been presented.*O This latter review covers most areas of electron transfer in condensed phase. A comparison of through-space and throughbond electron transfer has been made21 while additional interest has been given to the effects of separation distance on the rates of intramolecular electron-transfer events.22The effect of the conformation of the spacer group on the rates of through-bond electron transfer has been considered for o-bonded systems in solution.23The similarity of electron transfer and exciplex chemistry has been noted.24The theory of electron-transfer processes has been applied to ultrafast intermolecular reactions occurring in polar solvents.25 Light-induced charge separation has been reviewed for small clusters,26 bichromophoric molecules in the gas phase,27and solvent-free donor-bridgeacceptor systems.28 The use of emission spectral data to estimate electrontransfer barriers and rate constants has been described in some detail.29 A comprehensive review has considered the importance of coherence and adiabaticity in ultrafast electron transfer30 while the effects of vibrational dynamics on the rates of non-radiative deactivation have been ~ o n s i d e r e d . ~ ~ The stereodynamical aspects of light-induced bimolecular reactions, as studied by way of Doppler-resolved polarized laser pump-probe techniques, have been reviewed.32 Recent advances in theoretical and experimental understanding of ultrafast solvation processes have been highlighted with special reference to the role of high-frequency vibrational modes.33 The photoinduced electron-transfer reactions of cyclopropane derivatives have been described34while the time scales for electron-transfer processes occurring in J-aggregates formed at AgBr surfaces have been analysed in terms of theoretical models.35 Various types of artificial antenna systems have been described,36 the photochemical reactions of stilbenes and related heterocyclic derivatives have been reviewed,37and the photophysical properties of tetrapyrrolic pigments have been ~ u m m a r i z e dSpecial . ~ ~ attention has been given to the photodissociation of NO from nitrosyl metallop~rphyrins.~~ Considerable effort continues to be expended in the study of the photophysical processes taking place in transition metal complexes, especially with regard to the possible construction of light-activated molecular-scale devices. The photo-induced energy- and electron-transfer processes occurring in rigidly-linked Ru/Os complexes have been reviewed in considerable detail.40 Similar attention has been given to the light-induced electron-transfer reactions taking place in metal-organic dyads41 and in related supramolecular assemb l i e ~ . ~The * rational design of molecular devices based on luminescent transition metal complexes has been considered43 while the photophysics of transition metal complexes bound to nucleic acids have been reviewed.44This latter study has concentrated on trying to identify new types of specific luminescent probes for DNA. A comprehensive review deals with pressure tuning of the photochemical properties of transition metal complexes in solution.45

I: Photophysical Processes in Condensed Phases

17

The technique of transient grating spectroscopy has been reviewed, with particular emphasis on its application to monitoring non-radiative deactivaunified theory of time-resolved fluorescence anisotropy and Stokes t i ~ nA. ~ ~ shift spectroscopy has appeared.47 A separate review has considered the chemical and photophysical events occurring from upper excited states as accessed by multiphoton absorption technique^.^^

3

Kinetic and Theoretical Considerations

The measurement of kinetic parameters remains a very important part of photochemistry and there have been several recent attempts to improve our understanding of the dynamics of complex processes occurring in condensed phase. A new mathematical expression has been formulated to explain the kinetic processes inherent to certain types of photochromic systems.49 A treatment has been advanced to account for reversible diffusion-influenced complex formation, as studied by laser flash p h o t o l y ~ i sThe . ~ ~ line shapes of a two-level resonance fluorescence system, subject to stochastic collisions, have been analysed in terms of the Rabi freq~ency.~' A theory has been proposed to explain photo-induced nucleation in one-dimensional system^.'^ The effect of quantum beats on the recombination kinetics of radical ion pairs has been ~ o n s i d e r e d Various .~~ algorithms based on Prony's method have been proposed for the determination of individual lifetimes from dual-exponential decay curves.54 This non-iterative method has been applied to the real time study of quasi-distributed temperature sensors that operate by way of timeresolved fluorescence spectroscopy. A new power law dependence has been suggested for the long-time behaviour of reversible diffusion-influenced reacmodel has been introduced to account for rapid tion~A . ~ theoretical ~ dephasing relaxation effects? The observed non-exponential kinetics for delayed fluorescence in Langmuir-Blodgett films has been analysed in terms of a percolation while reversible fluorescence quenching has been treated through a series on non-Markovian generalised kinetic expression^.^^ A model has been developed to explain the non-exponential decay kinetics associated with ultrafast electron transfer in bimolecular systems.59Kinetic models have been expressed to account for fluorescence quenching in sol-gel xerogel transitions,6O doped glasses,61 and inter-particle interactions.62 A kinetic analysis has been made63of the fluorescence behaviour of rigid and non-rigid dye molecules under lasing conditions. The transport kinetics governing triplet-triplet annihilation in solid media have been described64 while the kinetics of fluorescence polarization in solid bichromophore molecules under intense illumination have been explored.65This latter study attempts to relate the excited state lifetime to the intensity of the excitation pulse. A unified theory for the kinetics of bimolecular photoionization followed by geminate recombination has been proposed.66 Fluorescence quenching rate constants have been analysed under conditions where a hopping mechanism is expected

18

Photochemistry

to play an important role67and the heats of formation of radicals in solution have been discussed in terms of a solvation A critical review has assessed the value of quantum dynamical simulations for modelling ultrafast processes in polyatomic molecule^.^^ A semi-classical regime exists for the dynamics of vibrational relaxation following excitation by an ultrashort laser pulse.70The molecular mechanics valence-bond method has been applied to the problem of understanding molecular structure and photophysical properties of polyatomic species in solution.71Theoretical descriptions have been proposed for energy pooling among three-centre systems,72for the suppression of resonant dipole-dipole interaction^,^^ and for intramolecular and electron-transfer p r o c e ~ s e s .Rate ~ ~ constants for electron transfer in protein matrices have been analysed in order to derive more information about the electronic factor and about the importance of nuclear t ~ n n e l l i n gA . ~theory ~ has been advanced for electron-transfer processes that involve multi-dimensional solvation dynamics.78 A generalized theory for superexchange interactions has been proposed and applied to exchangecoupled pairs.79 Theoretical studies have addressed problems related to lightinduced charge separation,80 the electronic factor in photo-induced electron transfer,81and re-organization energies accompanying electron t r a n ~ f e r .A~ ~ - ~ ~ critical comparison has been made of the Rehm-Weller and encounter complex models for bimolecular electron-transfer reactions.86A new methodology for describing distance dependence effects in radical pair reactions has been proposed.87 Semi-empirical studies have addressed the issue of solvation dynamics associated with light-induced charge-separated state^.^*-^' Other theoretical studies have examined the emission properties of charge-transfer s t a t e ~ ~and ' 9 ~exchange ~ interactions in transition metal complexes.93 The theoretical basis of sonoluminescence has been r e v i e ~ e d and ~ ~ . a~ ~ method has been presented to predict the absorption spectral band shape of polar dye molecule^.^^ A related treatment can be used to explain the fluorescence spectral profiles of aromatic amino acids in different states of ionization.97Theoretical calibratrion curves have been compiled for typical fluorescence-based temperature sensors.98 Theoretical studies have also addressed various issues relating to molecules that undergo a large change in dipole moment under i l l u m i n a t i ~ n . Electronic ~ ~ - ~ ~ ~ energy levels have been calculated for cis-stilbene,lo3 substituted benzofurans, lo4 isoquinolinium cations,lo5 and small aromatic heterocycles.lo6 Numerous theoretical calculations have been directed towards understanding the role of torsional motion in controlling the rates of isomerization and/or non-radiative deactivation. * 07-' 4

Photophysical Processes in Liquid or Solid Media

A tremendous number of publications continue to appear that report photophysical properties of individual molecules measured under a wide variety of conditions. Many reports follow standard lines and present little or no new properties - merely being concerned with examining a new derivative of a well-

19

I: Photophysical Processes in Condensed Phases

known family. Page restrictions preclude coverage of such work and require that attention be focussed on emerging trends in photophysics.

4.1 Detection of Single Molecules - The spatial resolution needed to isolate and detect fluorescence from single molecules has been reviewed119 and a theory for single molecule detection has been advanced that accounts for a simple two-state system.120A Monte Carlo simulation of single molecule fluorescence has been presented. Fluorescence from single molecules embedded in host crystals has been described for several different system^'^^-'^^ and the essential experimental methodology needed to monitor single molecule fluorescence has been improved and revised.125-127 Spatial photoselection of single molecules on surfaces has now been r e ~ 0 r t e d . lThe ~ ~ importance '~~~ of the triplet excited state for single molecule detection has been s t r e ~ s e d '31~ ~ ~ ' and the effects of photobleaching have been described.132Spontaneous emission from a single molecule has been considered in terms of a Monte Carlo approach133while the reasons for fluorescence intensity fluctuations associated with single molecule detection have been explored.134-136

'

4.2 Radiative and Non-radiative Decay Processes - The dynamical response of a trans-polyacetylene chain to excitation with an ultrashort laser pulse has been interpreted in terms of a charged soliton having a lifetime less than 250 fs. 37 Related studies have explored the photophysical properties of diphenylacetylene and diphenylbutadiyne, with emphasis being placed on the importance of the S2-S1 energy gap. The photophysics of substituted p ~ l y e n e sand l~~ diphenylhe~atriene'~'have also been studied by ultrafast spectroscopy. The photophysical and lasing properties of several types of laser dye have been studied in polar solvents.142-145 Vibrational relaxation within the S1 level of azulene has been monitored146and the photophysics of aromatic thioketones supported on cellulose have been probed147by laser flash photolysis. The photophysical properties of several derivatives of tyrosine have been recorded in solution and interpreted in terms of theoretical conformational studies.148 There is considerable interest in developing new fluorescent reagents based on the benzofuran nucleus and the photophysical properties of several analogues Photophysical data have been described for 9,lOhave been reported. anthraquinone-2-sulfonate, 5 1 4-chlorotoluene, 52 ethidium bromide in micellar media,153 angelins and t h i ~ a n g e l i n s ,and ~ ~ ~ 2-aminooxanthone.l 55 Excimer emission has been reported for pyrene-derived cyclophanes' 56 and for microcrystallinepolycyclic aromatic hydrocarbons dispersed in water. 57 Metalloporphyrins and related compounds continue to attract attention as photosensitizers and as building blocks for construction of artificial lightharvesting arrays. The photophysical properties of several structurally modified porphyrins have been measured with a view to identifying new sensitizers for use in photodynamic therapy.158The triplet state properties of porphyrins adsorbed onto the outer surface of vesicles have been describedIs9 and the fluorescence spectral properties of some amphiphilic porphyrins have been recorded. Similar studies have been carried out with halogenated tetraar1387139

1499150

'

'

Photochemistry

20

ylporphyrins. The acretion of individual porphyrin units into larger arrays has been achieved in several cases and the photophysical properties of the final assembly have been described.16*-la Environmental effects, especially changes in solvent polarity, on the photophysical properties of dyes have been described.165-167 Likewise, solvation dynamics have been measured for dyes that undergo a substantial increase in dipole moment following excitation. Light-induced intramolecular proton transfer is an important route for non-radiative deactivation of an excited state and has been studied extensively in recent time^.'^^''^^ Double 180 and for proton transfer has been reported for [2,2’-bipyridyl]-3,3’-diol 7-amindole. The photophysical properties of singlet molecular oxygen, 02( Ad, have been recorded in a variety of solvents182and the importance of charge-transfer interactions on the rate of radiative decay has been noted.183An attempt has been made to analyse these solvent effects’84and also to account for solventinduced variations in the efficiency of O#A& production with phthalocyanine-based sensitizer^.'^^ In this latter system, the energy gap between 0 2 ( Ag) and the lowest-lying triplet state localized on the sensitizer is very small, allowing accurate assignment of the triplet energy of free-base phthalocyanine. 86 The availability of ultrafast laser spectroscopic techniques has resulted in an upsurge in the study of higher-energy excited states. Thus, weak fluorescence has been observed from the S2 level of anthracene crystals using a two-step Ultrafast relaxation from higher-lying excited states excitation approach. has been recorded for Coumarin 48 1in cyclohexane,’89 for certain phenazine derivatives,lW and for azulene.191The latter case is very well known but a thorough investigation of this system has now been completed. Fluorescence from an organic radical cation has been detected in liquid solution at room temperature.192 1879188

4.3 Amplitude or Torsional Motion - Time-resolved, single-photon counting techniques coupled with synchrotron radiation have been used to measure the fluorescence anisotropy of a variety of aromatic hydrocarbons in alkane solution. By varying the temperature it was possible to measure rotational relaxation times for the fluorophore over a wide range of viscosity. A number of puzzling features have emerged.193*194 The rates of internal twisting and charge shift in an auramine dye have been measured by ultrafast spectrocopy'^^ and a model has been proposed to account for the wavelengthdependent fluorescence decay times. The rates of internal conversion of several aromatic compounds have been measured and considered in light of the twist angle of appropriate substituents.196~197Internal rotation can be influenced by external effects such as pressure,lg8 solvent interactions, local environment,201or substitution pattern.202 It is now well known that certain molecules form a twisted intramolecular charge-transfer (TICT) state upon excitation, provided the molecule is equipped with suitable electron donating and accepting functionalities. Many 1939194

1991200

I: Photophysical Processes in Condensed Phases

21

such systems have been examined in recent years, especially those that emit from the TICT state. The effect of twist angle on the ability of substituted biphenyls to form a TICT state has been examined.203Multiple fluorescence bands observed for 9,9'-bianthryl derivatives in solution have been interpreted in terms of TICT formation.204Similar effects have been noted in benzoxazole derivatives205and in substituted benzanilides.206Pressure and temperature effects have been used to elucidate the electronic and conformational pathways associated with deactivation of donor-acceptor substituted biphenyls in sohThe effect of solvent on the ability of 4-dimethylaminocinnamicacid to undergo TICT formation has been traced to conformational changes.208 Enhanced TICT formation is found when this latter molecule is included into P-cyclodextrin. The rates of relaxation of certain TICT states is strongly dependent on solvent viscosity2o9and on the degree of internal strain.210TICT formation has been observed in small clusters,2'I for inclusion complexes,212 and in rnicelle~.~'~ Photoisomerization represents an important form of non-radiativate deactivation of excited singlet states and numerous such studies have been reported during the review period.214-225In most cases, the rate of isomerization depends on the local environment, temperature, and structure. Photoisomerization can be extemely fast in certain cases226and a new model has been introduced to account for the time-dependent evolution of products. Chargetransfer interactions can play an important role in photois~merization~~~ while the products might complicate fluorescence spectral patterns.228The influence of restricted space, as imposed by incorporating the species inside a zeolite cavity229 or in a glass,23o on the rates of photoisomerization has been considered. Complexation of metal ions can also affect the efficiency of photois~merization,~~~ as can p r ~ t o n a t i o nThe . ~ ~rates ~ of photoenolization of certain reagents have been studied by laser flash photolysis techniques.233

4.4 Quenching of Excited States - A major area of photochemistry concerns the quenching of excited states by adventitious reagents, either free in solution or closely associated in some way with the chromophore, so as to drive a particular reaction. Several recent studies have reported on how nitroxide r a d i ~ a l s or ~ ~~ ~a r* b~e~n ~e saffect ~ ~ ~ the photophysical properties of triplet excited states. Ever increasing attention is being given to the possible construction of photo-active and to the study of long-range interact i o n ~ In . ~many ~ ~ cases, it is not possible to properly distinguish between quenching processes occurring by way of electron or energy but in other cases the mode of quenching is clear.

4.4.I Electron-transfer Reactions - Light-induced electron transfer from a donor to a suitable acceptor has been described for numerous bimolecular

systems. The reagents have been dispersed in a polar at microscopic or macroscopic interface^,^^^^^^^ in latex dispersions,2s9~260 in nematic liquid crystals,26' in reverse micelles,262in vesicles,263and in lipid bilayer membranes.264Additional studies have been concerned with electron transfer

22

Photochemistry

occurring in self-organized superstructure^,^^^ dendrimers,266complexes,267 hydrogen-bonded assemblies,268and supramolecular units formed via cation c h e l a t i ~ n .These ~ ~ ~ various ? ~ ~ ~ studies provide valuable information by which to measure how the local environment influences the rate of electron traiisfer but it is often difficult to correct for the effects of diffusion. Several studies have considered how the rate of intramolecular electron transfer is affected by large-scale changes in molecular c ~ n f o r m a t i o n . ~ These ~ ~ - ~investigations ~~ involve donor-acceptor units linked by flexible spacers whose average conformation can be modified by external effects, such as protonation. More complex systems have been devised wherein light-induced electron transfer occurs between weakly associated s p e ~ i e s . ~ ~ ~ - ~ ~ ~ In order to learn more about the electron-transfer event, isolated from the effects of diffusion, it is necessary to use rigidly-linked donor-acceptor dyads and several such systems have been studied. Different types of spacer group have been used, the resultant systems often being far from rigid, and rates of forward and/or reverse electron transfer measured in s o l ~ t i o n .Particular ~~~-~~~ aspects of the electron-transfer mechanism have been probed using these systems. Thus, the importance of conformational exchange has been studied using covalently-linked pyropheophytin-anthraquinone dyads.288The effect of chain length for flexibly-linked dyads has been investigated by incorporating the system inside the cavity of P-cy~lodextrin.~~~ Light-induced electron transfer has been studied in face-to-face, donor-acceptor dyads built from cycIophane~.~A ~ - *simple ~ ~ system has been devised that demonstrates unidirectional electron transfer along a particular pathway.293 The special effect of using a negatively-charged spacer to separate a donor-acceptor pair has been stressed294while competition between through-bond and throughspace electron transfer has been considered for U-shaped dyads.295Similar studies have been carried out with various triads296-298 in which an additional donor or acceptor is built into the system. Here, sequential electron-transfer processes can take place, leading to spatial separation of the charges and a relatively slow rate of charge recombination. Metalloporphyrins, because of their relevance to natural photosynthesis, are important building blocks for the assembly of light-active dyads, triads, and higher-order arrays. These compounds are also known to exhibit a relatively long-lived upper-excited state. Indeed, several studies have reported energy- or electron-transfer reactions occurring from the S2 level of porphyrin-based These findings open-up new possibilities to design advanced dyads demonstrating high selectivity and multiple photoevents according to the choice of excitation wavelength. A somewhat related study has reported fluorescence from the S2 level of a charge-transfer complex.302 4.4.2 Energy-transfer Reactions - Electronic energy transfer is an important component of the overall mechanism for certain natural processes and there has been an intensive and prolonged effort to duplicate some of the more interesting features with artificial models. Indeed, recent work has shown that the energy-transfer step in DNA photolyase can be mimicked with flavin- and

I: Photophysical Processes in Condensed Phases

23

deazaflavin-based model compounds.303 There have been several reports of intermolecular energy transfer taking place in fluid s o l ~ t i o n in , ~crys~ ~ ~ ~ ~ t a l ~ in , ~double-complex ~ ~ salts,307 in Langmuir-Blodgett films,308 and in d e n d r i m e r ~ . These ~ ~ ~ .latter ~ ~ ~studies are aimed at generating artificial lightharvesting complexes. Several investigations have addressed the issue of intramolecular triplet-triplet energy transfer taking place in covalently-linked Related studies have reported on singlet-singlet energy transfer in dyads.3* porphyrin-based d i m e r ~ . ~ Through-bond ~~-~~* energy transfer has been described for a series of naphthalene-anthracene and naphthalene-acridine dyads in solution.319This latter study included the effects of chain length and mutual orientation on the rates of intramolecular energy transfer in flexiblylinked systems. Energy transfer along the backbone of short peptides has been reported,320 with the results being considered in terms of both separation distance and internal rigidity. Ultrafast energy transfer has been observed in donor-acceptor substituted fulgides where the rate of energy transfer can be controlled by the conformation of the photochromic f~lgide.~*l On-off switching of the energy-transfer event is made possible by irradiation into the fulgide. Sequential energy transfer over distances of ca. 80 has been achieved in a triad.322 A new area of energy-transfer research involves the use of strongly-coupled donor-acceptor moieties. Such systems are capable of displaying extremely fast excitation delocalization over several pigments.323 Similar studies have considered ultrafast energy relaxation in closely-coupled porphyrin d i m e r ~ . ~ ~ ~

A

Photophysics of Fullerenes - As in recent years, there have been many reports of the photochemistry and photophysics of substituted fullerenes, with the fullerene acting as either chromophore or electron a ~ c e p t o r . Con~~~,~~~ siderable attention has been given to identifying suitable derivatives of c 6 0 or C70 that can be used in photochemical processes. A particular problem is the need to produce soluble compounds and the photophysical properties of numerous new analogues have been r e p ~ r t e d . ~ Related ~ ~ - ~studies ~ ~ have addressed the photoreactions of fullerenes in mixed solvents334and attached to polymers.335The triplet excited state of C70 has been examined in a zero-field while delayed fluorescence has been observed from several fullerene derivatives.337An unusual system comprises c60 spin-labelled with a TEMPO The role of fullerene negative ions has been r e ~ i e w e d . A~ ~ ~ , ~ ~ ~ separate in~estigation~~l has considered the mechanism for formation of C600 under illumination of c 6 0 in aerated solution. It appears that reaction occurs only when O2(IAg) reacts with triplet c60. Numerous studies have described light-induced electron-transfer reactions that follow from UV or visible light irradiation of fullerene derivatives in solution containing a reducing agent.342,343In most cases, the one-electron reduced form of the fullerene can be detected by laser flash photolysis technique^^^-^^^ and kinetic parameters have been m e a ~ u r e d . ~In~ ~certain -~~' cases, reduction of the fullerene is followed by alkylation reactions.352Detailed studies have been carried out with mixtures of c 6 0 or C70and zinc(I1) meso4.5

24

Photochemistry

tetraphenylporphyrin in polar solvent.353The reaction involves overall oneelectron reduction of the fullerene, regardless of which reactant is illuminated, but the efficacy of the process depends on the nature of the initial excited state. Pulse radiolysis studies have been used to monitor interaction between primary radicals and c60 derivatives354while the photoreaction between pyrene and c 6 0 has been described.355 A number of investigations have been reported where light-induced electron-transfer reactions occur within covalently-linked dyads containing a fullerene derivative.356A variety of secondary reactants has been used and different types of anchor have been employed, although in almost every case the second reactant is held close to the fullerene. On the basis of EPR studies it has been shown that efficient light-induced electron transfer occurs from a tethered tetrathiafulvalene unit to c60 at low temperature.357Rapid charge separation has also been reported for fullerene derivatives attached to ruthe~ ~to* ~ f e~r ~ r ~~c e n e Several . ~ ~ ~ porphyrin nium(11) polypyridine c ~ m p l e x e s or derivatives have been synthesized bearing two fullerenes and the photophysical properties of the resultant arrays have been measured.361 Self-assembled porphyrin-C60 modules have been formed via axial coordination and found to undergo fast intramolecular electron transfer from the first excited state of the p ~ r p h y r i nSimilar . ~ ~ ~ results have been reported for closely-related porphyrinc 6 0 dyads363-368 and there are remarkably close analogies between these dyads and those formed from porphyrin-quinone modules. A phthalocyanine-C60 dyad has been reported369 and studied by EDESR spectroscopy. Some corresponding triads have also been ~ r e p a r e d . ~ ~ A O -full ~~* investigation has been completed of the photophysical processes occurring within a Cso-based triad covalently linked to a c a r ~ t e n o p o r p h y r i n .A~ ~long-lived ~ charge-separated state is formed by way of a two-step process with the kinetics depending on solvent polarity. A related triad has C60 as electron acceptor, anthracene as primary donor and quinquethiophene as secondary donor.372This system has been studied by EPR spectroscopy in low temperature matrices and clear evidence has been obtained to show the transient formation of the chargeseparated redox pair. 5

Applications of Photophysics

Molecular photophysics, especially the use of steady-state and time-resolved luminescence spectroscopy, have many important applications and there has been a progressive emergence of a new field of analytical chemistry based on these principles. It has been known for many decades that the excited state properties of certain molecules are highly sensitive to the local environment but it is only recently that a concerted effort has been made to use this sensitivity in a practical way. The main approaches to employing variations in photophysical properties as an analytical tool can be divided into two areas; namely, (i) development of luminescent probes that respond to changes in the environment and (ii) identification of molecular systems for which the emission

I: Photophysical Processes in Condensed Phases

25

is quenched selectively by certain solutes. This latter field is becoming very popular and there have been some well-designed supermolecular systems aimed at detection of specific targets. A serious drawback to the construction of practical devices concerns the relatively short lifetime of most excited states and attention has focussed on finding ways to prolong triplet lifetimes.373 A relatively easy target for quantitative analysis by way of photophysical investigation concerns the detection of trace quantities of molecular oxygen in solution or vapour phase. A variety of proposals has been put forward, including the use of phase-modulation techniques, to improve the signal-tonoise ratio of conventional instruments.374A mathematical treatment has been devised for improving calibration curves375and the use of a photobleaching strategy for measuring in situ oxygen concentrations has been devised.376 Many oxygen-sensing systems are based on quenching of the phosphorescence emitted by transition metal complexes and the choice of medium can be of considerable importance. Preferences for the carrier medium include various sol-gel hosts,377porous siloxane films,379polymeric matrices380and filter paper.381New types of luminescent sensor have been identified and tested on copolymer supports.382Similar sensors have been designed for the detection of carbon d i o ~ i d e ,again ~ ~ ~the , ~luminescent ~~ chromophore of choice being a ruthenium(I1) polypyridine complex. Certain organic molecules are also attractive targets for luminescent sensors and systems have been designed to monitor i m m u n o - r e a g e n t ~natural , ~ ~ ~ p h y t ~ p l a n k t o n DNA,387-389 ,~~~ sacchari d e ~polycyclic , ~ ~ ~ hydrocarbons in seawater39* and c a t e c h ~ l s . ~ ~ ~ Many molecular sensors have been designed to detect cations, including protons, in s o l ~ t i o n . ~These ~ ~ - ~systems ' work by registering a change in luminescence yield and/or lifetime upon binding a cation to an appended chelating function and selectivity is set by the nature of the coordination site. Corresponding systems have been designed to monitor in situ concentrations of anionic substrates,402including bi~arbonate.4~~ Luminescence techniques have been applied to the problem of measuring radiation dosimetry, in both direct404 and retrospective modes.405 In the former case, it is proposed to develop simple systems based on the bleaching of a fluorescent dye that can be used for monitoring the dose during radiation therapy. In the latter case, the idea is to determine the level of radiation exposure delivered during an accidental release of a high dose. These systems represent extremely important opportunities for photophysical applications to real-world problems. A related, but less pressing, issue concerns the use of delayed luminescence techniques to indicate the quality of tomato juice.406It has been noted that there is significant difference in delayed luminescence yield according to the maturity of harvested tomatoes. A new solvatochromic probe, based on 3,6-diethyltetrazine, has been proposed that extends the solvent acidity scale to highly acidic organic solvents.407A co-polymerizable dansyl monomer has been suggested as an indicator of solvent polarity,408 since the fluorescent TICT state is very sensitive to this parameter. A method his been developed to prepare fluorescent labelled natural sediment for use as a measure of sediment transport

26

Photochemistry

rates .409 Applications of this approach have involved studying bedload distributions from the Jordan river to Lake Kinneret. A two-photon fluorescence-based system has been devised for measuring penetration depths in turbid biological samples410and a different system has been proposed to locate the position of fluorophores in model membrane systems.41 The macromolecular chain dynamics occurring in polymeric systems have been studied by time-resolved fluorescence spectroscopy.412A related study has been applied to monitor translational mobility in polystyrene-polyethyleneglycol mi~robeads.4'~ Several systems have been designed to measure viscosity by virtue of environmental effects on f l u o r ~ p h o r e s . ~ ' ~ - ~ ' By making use of TICT formation, it has been possible to engineer a system for measuring the molecular weight of polymeric matrices.416Fluorescence correlation spectroscopy has been applied to the determination of polydispersity of suspensions4' while fluorescence techniques have been developed to A monitor swelling and slow release kinetics of disk-shaped polymer fluorescent probe has been described that responds to the degree of flocculation of silica particles.419 Photophysical measurements have been shown to provide a meaningful estimate of the size of reverse m i ~ e l l e sand ~ ~of ~ the aggregation number in aqueous adipic acid.421Fluorecent dyes have been reported for probing the interiors of hydrophobic cavities in both chemical and biological s y ~ t e r n s Other .~~~ fluorescent ~ ~ ~ ~ reagents can be used to monitor temperat ~ r e solvent , ~ ~ polarity,425 ~ and the interface between water and polyelectrolytic m i ~ e l l e s The .~~~ chemical reactivity at the interface of self-assembled monolayers can be measured by fluorescence spectroscopy.427The application of 4-aminophthalimide as a general reagent for monitoring the local environment has been reviewed.428Finally, the use of fluorescent molecules to measure the degree of curing of bone cement has been described.429

6

Advances in Instrument Design and Utilization

An experimental protocol has been devised that facilitates determination of the quantum yield of a heterogeneous photocatalytic process.430This is a longstanding problem because, in the past, it has been difficult to determine the number of absorbed photons. The present work tries to address this issue by considering both absorbed and reflected photon densities.430Attention has been given to how and when the term 'average' fluorescence parameters should be used, especially in the context of mixed static and dynamic fluorescence quenching.431A method for deriving association constants from static fluorescence quenching effects has been proposed432 and the importance of ionpairing on photochemical processes has been stressed.433A global methodology has been devised that corrects for the effects of self-association in solution.434A generalized correlation analysis has been applied to fluorescence spectral data recorded for binary mixtures of f l u o r ~ p h o r e sA. ~new ~ ~ approach

I: Photophysical Processes in Condensed Phases

27

has been proposed for the collection of quantitative data from commercial fluorescence microscopes,436this being a difficult and tedious process. A new set of standards for measuring fluorescence quantum yields has been proposed.437The reference compounds are photostable and their emission yields are relatively insensitive to the presence of dissolved oxygen. Procedures for correcting spectral data for changes in refractive index have been proposed for both solutions438and suspensions.439Likewise, problems arising from inner-filter effects have been reconsidered in light of corrections to SternVolmer constantsm and improved cell design.441 A method has been outlined that permits determination of the volume change associated with photophysical processes, notable light-induced electron transfep2 or charge s e p a r a t i ~ nThe . ~ ~approach is based on the use of timeresolved optoacoustic spectroscopy to monitor the course of reaction. Transient grating spectroscopy has been used to monitor translational motion at a solid-liquid interfacew and also to measure enthalpy changes and reaction volumes for various photochemical processes in solution.445 Both studies provide detailed analytical procedures for data analysis. A procedure has been introduced to calculate the average photon number for delayed single-photon coincidence measurement of fluorescence lifetimes.a6 Various models have been proposed to account for fluorescence interference noise in a two-site system, especially with regard to excitation transfer.447This theoretical study suggests that the detailed analysis of fluorescence interference fluctuations as a function of light intensity might provide a powerful diagnostic tool for probing the dynamics of energy transfer. A simple but reliable method has been proposed for calibration of the time scale of time-correlated, single-photon counting system.a8 The approach involves insertion of an optical delay line into one of the startktop channels. A method has been reported that allows recording of the high-resolution Sphol’skii fluorescence spectrum for the S2 state of non-alternant polycyclic hydrocarbon^.^^ Descriptions have been given for the effect of light quenching on fluorescence anisotr0py,4~~ for fluorescence imaging in microcolumns,45 and for photobleaching fluorescence microscopy.452Two-photon excitation spectra of xanthene dyes have been measured453while a model has been presented that accounts for molecular dynamics in multiphoton microscopy.454 Chirped femtosecond laser pulses have been used for quantum control of population transfer in proteins under high light intensity.455Two-photon induced anisotropy measurements have been applied to the recording of the rotational relaxation time of perylene in alcohol solution.456This approach provides additional information to that obtained from conventional methods. A method has been suggested for measurement of electronic transition moments by using orientation filters.457The real-time observation of singlettriplet dephasing has been reported.458An investigation has been made into the effects of dichroism in photoelectron fluorescence coincidence spectroscopy of rotating linear molecules.459The quadrature squeezing spectra produced by resonance fluorescence in a two-state system have been investigated.460The reorientation of a nematic liquid crystal formed from discotic molecules by

Photochemistry

28

light-induced space charge effects has been described.461The underlying theory for molecular electronic spectral broadening in liquids and glasses has been considered.462 A low cost fluorescence lifetime apparatus, based on the phase-modulafron technique, has been d e s ~ r i b e d while ~ ~ ~ av new ~ ~ design has been suggested for a photon-counting fluorescence spectrometer that allows rapid accumulation of decay data.465A variety of improvements have been suggested for fluorescence m i ~ r o s c o p y , 4 ~especially ~ " ~ ~ with regards to the design of two-photon A picosecond near-field microspectrometry system has been designed and applied to the study of fluorescence from a microcrystalline charge-transfer state.475 Methods for enhancing room temperature phosphorescence continue to be r e p ~ r t e d .Based ~ ~ ~ on , ~two-photon ~~ absorption techniques, a non-linear fluorescence spectrometer has been described.478The main advantages and disadvantages of upconversion luminescence spectroscopy have been reviewed.479The use of high-intensity chirped laser pulses to probe microscopic chemical environments has been described480 while the efficient detection of surface generated fluorescence has been ~ o n s i d e r e d . ~ ~ Novel methods for fluorescence sensing have been proposed482 and other variations on the usual method for recording fluorescence spectral properties have been ~ ~ ~ e r eThed technique . ~ ~ ~of -fluorescence ~ ~ ~ depletion spectroscopy has been applied to the study of organometallic radicals488while an approach to increasing the spatial resolution of a scanning fluorescence microscope has been reported.489 A design has been proposed for a computer-controlled nanosecond laser flash photolysis setup.490Time-resolved CIDNP spectroscopy has been applied to the study of light-induced charge separation in rigid bichromophoric molecules491and the influence of pressure on the photophysical properties of a TICT state has been explored.492In situ measurement of the photochemical reactions occurring within microdrops has been reported493 and transient spectral hole burning spectroscopy has been used to monitor the breaking of hydogen bonds.494Ways to monitor ultrafast vibrational relaxation have been ~onsidered~ and ~ ~ the . ~ ~importance ~ of external field effects has been n 0 t e d . 4 ~ ~The 7 ~ ~influence ~ of an applied magnetic field on photophysical processes has been reported for numerous

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378. M, P. Xavier, D. Garcia-Fresnadillo, M. C. Moreno-Bondi and G. Orellana, Anal. Chem., 1998,70, 5184. 379. M. L. Bossi, M. E. Daraio and P. F. Aramendia, J. Photochem. Photobiol., A , 1999, 120, 15. 380. G. Di Marco, M. Lanza, A. Mamo, I. Stefio, C. Di Pietro, G. Romeo and S. Campagna, Anal. Chem., 1998,70,5019. 381. S . M. Ramasamy and R. J. Hurtubise, Talanta, 1998,47,971. 382. K. A. Kneas, W. Xu, J. N. Demas, B. A. Degraff and A. P. Zipp, J. Fluoresc., 1998,8,295. 383. G. Neurauter, I. Klimant and 0.S. Wolfbeis, Anal. Chim. Acta, 1999,382,67. 384. M. D. Marazuela, M. C. Moreno-Bondi and G. Orellana, Appl. Spectrosc., 1998, 52, 1314. 385. P. Onnerford, S. Eremin, J. Emneus and G. Marko-Varga, J. Immunol. Methods, 1998,213, 3 1 . 386. Y. Z. Yaobi, V. Gerhardt, Y. Gonen-Zurgil and A. Sukenik, Water Res., 1998, 32,2577. 387. S. Arounaguiri and B. G. Maiya, Inorg. Chem., 1999,38, 842. 388. R. E. Holmlin, J. A. Yao and J. K. Barton, Inorg. Chem., 1999,38, 174. 389. R. H. Terbrueggen, T. W. Johann and J. K. Barton, Inorg. Chem., 1998,37,6874. 390. C. R. Cooper and T. D. James, Chem. Lett., 1998,883. 391. S. M. Rudnink and R. F. Chen, Talanta, 1998,47,907. 392. G. R. Gollapalli, B. Keshavan and D. D’Souza, J. Chem. Soc., Perkin Trans. 2, 1999,3133. 393. D. Parker, P. K. Senanayake and J. A. G. Williams, J. Chem. Soc., Perkin Trans. 2, 1998,2129. 394. L. Gobbi, P. Selier and F. Deiderich, Angew. Chem., Int. Ed., 1999,38,674. 395. U. Kosch, I. Klimant, T. Werner and 0. S. Wolfbeis, Anal. Chem., 1998, 70, 2156. 396. L. Fabbrizzi, F. Gatti, P. Pallavicini and L. Parodi, New J. Chem., 1998, 22, 1403. 397. L. Prodi, F. Bolletta, M. Montalti and N. Zaccheroni, Proc. SPIE-Int. SOC. Opt. Eng., 1 999,3602 (Advances in Fluorescence Sensing Technology I V ),202. 398. F. Barigelletti, L. Flamigni, G. Calogero, L. Hammarstroem, J.-P. Sauvage and J.-P. Collin, Chem. Commun., 1998,2333. 399. L. Prodi, F. Bolletta, M. Montalti and N. Zaccheroni, Eur. J. Inorg. Chem., 1999, 455. 400. K. Kubo, E. Yamamoto and T. Sakurai, Heterocycles, 1998,48,2133. 401. 0. J. Rolinski and D. J. S. Birch, Meas. Sci. Technol., 1999,10, 127, 402. L. Fabbrizzi, M. Licchelli, L. Parodi, A. Poggi and A. Taglietti, J. Fluoresc., 1998,8,263. 403. R. S . Dickins, T. Gunnlaugsson, D. Parker and R. D. Peacock, Chem. Commun., 1998, 1643. 404. A. Kovacs, M. Baranyi, L. Wojnarovits, W. McLaughtlin and S. D. Miller, Radiat. Technol. Conserv. Environ., Proc. Symp., 1997, (Pub. 1998), 475. 405. L. Botter-Jensen and H. Jungner, Acta Phys. Pol., A , 1999,95,275. 406. A. Triglia, G. La Malfa, F. Masumeci, C. Leonardi and A. Scordino, J. Food Sci., 1998,63, 512. 407. J. Catalan and C. Diaz, Eur. J. Org. Chem., 1999, 885. 408. B. Ren, F. Gao, Z. Tong and Y. Yan, Chem. Phys. Lett., 1999,307,55. 409. B. Steinman, T. Berman, M. Inbar and M. Gaft, Isr. J. Earth Sci., 1997,46, 107.

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V. Daria, 0. Nakamura, C. Palmes-Saloma and S. Kawata, Jpn. J. Appl. Phys.,

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I: Photophysical Processes in Condensed Phases

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446. 2. Wang, T. Xu and C. Wu, Nucl. Instrum. Methods Phys. Res. Sect. A , 1998, 419, 154. 447. V. Szoes and H. F. Kauffmann, J. Chem. Phys., 1998,109,7431. 448. K . Dobek, J. Karolczak, D. Komar, J. Kubicki, M. Szymanski, T. Wrozowa, M. Ziolek and A. Maciejewski, Opt. Appl., 1998,28,201. 449. C. Gooijer, 1. Kozin, N. H. Velthoerst, M. Sarobe, L. W. Jenneskens and E. J. Vlietstra, Speclrochim. Acta, Part A , 1998, 54, 1443. 450. I. Gryczynski, Z. Gryczynski and J. R. Lakowicz, Photochem. Photobiol., 1998, 67,641. 451. J. Johansson, T. Johansson and S. Nilsson, Electrophoresis, 1998, 19,2233. 452. R. I. Ghauharali, J. W.Holstraat and G. J. Brakenhoff, J. Microsc. (Oxford), 1998,192,99. 453. T . Wakebe and E. Van Keuren, Jpn. J. Appl. Phys., Part I , 1999,38,3556. 454. J. Mertz, Eur. Phys, J., D, 1998,3, 53. 455. C. J. Bardeen, V. V. Yakovlev, J. A. Squier and K. R. Wilson, J. Am. Chem. SOC.,1998, 102, 13023. 456. S. W. Pauls, J. F. Hedstrom and C. K. Carey, Chem. Phys., 1998,237,205. 457. M. Gil, J. Marczyk, S. Dobrin, P. Kaszynski and J. Waluk, J. Mol. Struct., 1999, 475, 141. 458. T. Fukuju, H. Yashiro, K. Maeda and H. Murai, Chem. Phys. Lett., 1999, 304, 173. 459. S. Sen, Indian J. Phys., B, 1999,73,223. 460. P. Zhou and S. Swain, Phys. Rev. A: Mol. Opt. Phys., 1999,59,841. 461. R. Macdonald, P. Meindl, G. Chilaya and D. Sikharulidze, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A , 1998,320, 1 15. 462. A. B. Myers, Annu. Rev. Phys. Chem., 1998,49,267. 463. P. Harms, J. Sipior, N. Ram, G. M. Carter and G. Rao, Rev. Sci. Instrum., 1999, 70, 1535. Opt. Eng., 1999,3602 (Advances 464. P. D. Harms and G. Rao, Proc. SPIE-Int. SOC. in Fluorescence Sensing Technology,I V ) , 52. 465. S. D. Pack, M. W. Renfro, G. B. King and N. M. Laurendeau, Opt. Lett., 1998, 23, 1215. 466. Q. S. Hanley, P. J. Verveer and T. M. Jovin, Appl. Spectrosc., 1999,53, 1. 467. J. Ying, F. Liu and R.R. Alfano, Appl. Opt., 1999,38,224. 468. J. R. Lackowicz, I. Gryczynski, L. Tolosa, J. D. Dattelbaum, F. N. Castellano, L. Li and G. Rao, Acta Phys. Pol., A , 1999,95, 179. 4169. M. Straub and S. W. Hell, Appl. Phys. Lett., 1998,73, 1769. 470. Q. S. Hanley, P. J. Verveer and T. M. Jovin, Appl. Spectrosc., 1998,52,783. 471. G. Merritt, E. Monson, E. Betzig and R. Kopelman, Rev. Sci. Instrum., 1998, 69, 2685. 472. G. T. Shubeita, S. K. Sekatski, M. Chergui, G. Dietler and V. S. Letokhov, Appl. Phys. Lett., 1999,74, 3453. 473. J . Enderlein, T. Ruckstuhl, F. Loescher, M. Boehmer and S. Seeger, Proc. SPIEInt. SOC. Opt. Eng., 1999, 3602 (Advances in Fluorescence Sensing Technology I V ) , 94. 474. L. K. van Geest, F. R. Boddeke, P. W. van Dijk, A. F. Kamp, C. J. R. van der Oord and K. W. J. Stoop, Proc. SPIE-Int. SOC. Opt. Eng., 1999, 3605 (ThreeDimensional and Multidimensional Microscopy: Image Acquisition and Processing VI), 55. 475. K. Sasaki and H. Masuhara, Chem. Phys. Lett., 1998,293, 185.

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Photochemistry

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Part II Organic Aspects of Photochemistry

I

Photolysis of Carbonyl Compounds BY WILLIAM M. HORSPOOL

Some studies have been reported dealing with the photochemical behaviour of cyc1opropylketones.l In particular AM1 studies have been carried out on the three ketones (1-3). The calculations show that when the carbonyl group is adjacent to the cyclopropyl moiety as in (1) ring opening is the dominant reaction. In the other two ketones, (2) and (3), Norrish Type I cleavage is the dominant reaction path. Another report has also given further details of calculations relating to the activity of ketones (1) and (2).* Other studies have investigated the electron-transfer photochemistry of some cyclopropyl ketones3 The electron transfer to the ketone from added triethylamine results in a cyclopropylcarbinyl-homoallyl rearrangement. The photophysics of the 0, m and p-derivatives (4) have been e ~ a m i n e d . ~

1

Norrish Type I Reactions

A study of the reaction kinetics of the photodissociation of jet-cooled benzaldehyde has been r e p ~ r t e d The . ~ t-butylketone ( 5 ) undergoes Norrish Type I fission on irradiation in water and this yields the radical (6) that was studied spectroscopically.6 The use of Norrish Type I cleavage of t-butyl keto groups has also been utilized by Peukert and G i e ~ eThe . ~ reaction described is another example of a photoactive protecting group for carboxylic acids and is based on the reactivity of the tethered pivalyoyl systems (7). Irradiation of (7) brings about Norrish Type I fission and the formation of the radical (8). Collapse of the radical liberates the carboxy radical from which the acids are formed. The yields of the pure acids derived from (7) were high. A review has highlighted the use of photocleavable protecting groups.* Benzoin and its derivatives (9) undergo Norrish type I cleavage on irradiation to afford a phenacyl radical and a a-hydroxy or a-alkoxy radicalegThe Photochemistry, Volume 3 I (6 The Royal Society of Chemistry, 2000 47

48

Photochemistry

But

quantum yields for these processes have been measured. Infrared techniques have been used to identify the photoproducts obtained from the irradiation of benzoin. l o Another study has focussed on benzoin related compounds that have been used as photoinitiators.l' Thus the Norrish Type I fission of the ketones (10) and (1 1) has been studied by laser-flash photolysis. The bond fission in these compounds, which are used as polymerization initiators, affords acyl radicals and a-hydroxy and oc-amino radicals. The rates of reaction of these species with oxygen and with acrylates were measured. A study of the Norrish Type I fission of dibenzylketone in vesicles consisting of dioctadecyldimethylammonium chloride has been reported. l 2 The product studies of this system showed that there were substantial cage effects and the results from laser flash studies indicate that no geminate reactions occur even at room temperature. Radical coupling products are formed from the irradiation of phenol and di-t-butylketone in cyclohexane as s01vent.l~t-Butyl ethers are the major products. 0 OR' II I PhC- C- Ph

I

R2

(9) R' = R2 = H

R' = H, R2 = Ph R' = Me, R2 = Ph

A r q N AW O OH (10) R = Me or (CH2)5

R*

(1 1) R' = R2 = Me; R' = Me, R2 = allyl; Ar = Ph or pMeSC6H4

The well-known ring expansion of cyclobutanes to tetrahydrofuranyl carbenes has been exploited in an approach to the synthesis of disa~charides.'~

49

IIII: Photochemistry of Carbonyl Compounds

Irradiation of (12) affords the carbene (13) which can be trapped by carbene insertion into the OH bond of a protected monosaccharide such as (14).14 Umbricht and co-workers’ have described the photochemical ring expansion reactions of a series of substituted cyclobutanones. Some of the examples cited are shown with the conversion of (15 ) into (16) and (17) into (18). The yields are often high and although various nucleophiles were used, acetic acid was found to be the best for trapping the intermediate carbene. The ring expansion process takes place with retention of all the stereo centres present and occurs on irradiation using 350 nm light of both the cis and trans isomers of the diketone (19). With isopropanol as the solvent, ring expansion via a carbene intermediate affords the tetrahydrofuranone (20). Evidence was collected that suggests that the carbene formed in this ring expansion process is more readily trapped by thiophenol than by isopropanol.

(16)R = M e (8OY01:l) R = AC (89’301:l)

(18) R = Me (70% 3:1), EtCO (66% 2:1), Bu’CO (60% 31)

Another process typical of the Norrish Type I reaction is also reported to occur with the ketone (19).16 Thus the biradical formed on bond cleavage undergoes decarbonylation. Rebonding within the biradical yields the indanones (21) as cis and trans isomers. There is also evidence for further fission to yield a ketene and rebonding to give a cyclopropane. a-Cleavage is also the dominant reaction of the larger ring ketones (22).17 The outcome of the process is dependent on the ring size and how the resultant biradicals formed by Norrish Type I fission can react. Thus with the larger ring systems (n = 7 or 8) the sites of the radical centres within the biradicals are sufficiently far apart that attack on the p-position of one of the aryl rings is possible and the

50

Photochemistry

cyclophanes (23) are formed in 50% and 27% yields respectively. Decarbonylation also occurs resulting in the formation of the cyclophanes (24). There is no evidence in these larger ring ketones of the formation of unsaturated aldehydes. Such products are, however, formed from the ketone for n = 1 in (22). Thus the aldehyde (25) (n = 1) is produced in 76% yield and (25) (n = 3) is formed in 91% yield from the ketone (22) (n=3). Decarbonylation and the formation of the cyclophane (26) are competing processes for these ketones. A detailed examination was carried out of the nature of the biradicals formed and the effect of magnetic fields on their lifetimes. A CIDNP study of the photochemical reactions of the ketodiol(27) has been carried out. l8 Irradiation leads to Norrish Type I fission followed by decarbonylation to yield the biradical(28).

(22) n = 7 o r 8 ; n = 1; n = 3

(23) n = 3 o r 4

(24) n = 2 or3

Ph (25) n = 1 or 3

t3 (28)

2

Norrish Type I1 Reactions

2.1 1J-Hydrogen Transfer - Irradiation of valerophenone (29) in aqueous solution has been studied.19 The reaction follows the same path as that in hydrocarbon solution and yields acetophenone and cyclobutanols. The reaction in water arises from the triplet state. Interestingly, the formation of the cyclobutanols (cis: trans ratio is 2.4 : 1) is more efficient in the aqueous system than in hydrocarbons. Cyclobutanols are also formed on irradiation of the butanoate derivatives (30).20 Hydrogen abstraction by the triplet excited state carbonyl group occurs from the alkyl groups on C2 of the butanoate chain.

M I :Photochemistry of Carbony 1 Compountis

51

0

Ph\l(yCox O R (29)

(30) X = NH2 or OR; R = alkyl

Pincock and his co-workers21have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (3 1a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) andp-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish Type I1 process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over TiO2, have been reported.22Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate.23 A Norrish Type I1 hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D.

as

b,

C,

d, 8,

R’

R2

R3

H H H D H

H H H H D

H 3-Me0 4-Me0 H 4-Me0

Yield (YO)

52 84 44

2 10

16 3 1

7 2 7

A detailed study of the photochemical reactions of the ketones (38) and (39) in the solid phase has been reported.24 Both of these systems readily undergo Norrish Type I1 hydrogen abstraction in solution and it was this fact that attracted the authors to the systems. One of the facets of the work focussed

52

Photochemistry

upon was the use of chiral auxiliaries as counter ions of the carboxylate examples in (38a) and (39a). The authors conclude that the 'ionic chiral auxiliary approach' is a viable general method for asymmetric synthesis. The irradiation of the biphenyl ketoamide (40) at 340 nm affords the two products (41) and (42) via the conventional Norrish Type I1 hydrogen abstraction process.25 When the reaction is carried out in the presence of an antibody microenvironment the reaction follows a different route and yields the tetrahydropyrdzine derivative (43). The authors reason that there is interplay between conformational control and chemical catalysis that results in this high specificity.

(38)a, R1=H, R 2 = H

b, R' = H, R2 = F c, R' = Me, R2 = H

d, R' = Me, R2 = F

(39) a, R' =Me, b, R1 = Me, c, R' = Me, d, R' = Me, 8, R' = Me,

R2 = CN R2 = CQH R2 = CGMe R2 = Me R2 = OMe

Ar =

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported.26The irradiations involved the crystalline materials using h > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-( -)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type I1 hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the ciscyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trans-cyclobutanol (47). A detailed examination of the photochemical behaIrradiation viour of a series of large ring diketones (48) has been carried in both the solid phase and solution were compared. Norrish Type I1 reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive

a

53

IIl1: Photochemistry of Carbonyl Compounds

+

H

(44) (9-(-)-Malic (R)-(+)-Malic (2R,3R)-(+)-Tartaric

+

H

(45)

Yield (YO) 24 45 24

(47)

(46)

2 0 0

32 24 0

YOConversion 60 69 24

Scheme 1

0

under the conditions used. The yields of product formed are shown below the structure of the product. Those in brackets refer to the solution phase reactions. Marked differences between the solution phase and the solid state reactions were frequently observed and the authors argue that these are related to the crystal structure of the compound under study. Photoenolisation is an area of study that remains of interest. The irradiation of (52), where a hydrogen is transferred from the methyl group adjacent to the carbonyl function, has been studied and the influence of the solvent (methylcyclohexane, ethanol and 2,2,2-trifluoroethanol) on the efficiency of photoenolisation has been assessed.28 The heat of reaction for intramolecular hydrogen abstraction in 1,3-dimethylanthrone has been measured.29 The ketone (53) is reported to be stable to irradiation in the solid state but for finely ground crystals, or when the irradiation is carried out at elevated temperatures, the normal cyclization reaction to yield (54)is observed.30The authors have shown that rigorous deoxygenation is required for the success of the reaction. If this is carried out evidence for the presence of the 1,4-biradical (55) can be obtained. In the absence of efficient de-oxygenation the resultant biradical undergoes facile trapping by oxygen. The crystal structures of the and some substituted derivatives have parent 2,4,6-tri-i-propylbenzophenone been determined in an attempt to identify the features that prevent cyclization

54

Photochemistry

(53) R - C a M e

within the 1,4-biradical formed on irradiation in the solid state.3' Related to these observations is the outcome of the irradiation of S-(56) in the crystalline state at 4 "C which results in cyclization to a cyclobutenol with high diastereoselectivity (de).32Thus at 34% conversion the SS product (57) is obtained with 99% de. Higher conversions result in a lower de. When S-(58) is irradiated in benzene solution the conversion to (57) is 100% but the product exhibits no de. Other examples of this cyclization in the crystalline phase also occurs with high de. The keto esters (59) are also photochemically reactive and undergo Norrish

IIII: Photochemistry of Carbonyl Compounds

55

Type I1 hydrogen abstraction by the keto group from the adjacent aryl methyl group to give the biradical (60) from which both products (61) and (62) are formed.33The reaction is solvent dependent. In cyclohexane solvent only the cyclobutanols (61) are formed, but in methanol a mixture of (61) and the new diketone (62) are obtained in a ratio which is dependent upon steric factors. The photochemically induced proton transfer in 3-methyl-6-hydroxy-rnphthalic acid has been reported.34

2.2 Other Hydrogen Transfers - The anticipated 1,5-biradical is formed on irradiation of the cyclopropyl ketone (63, X=CH2) in benzene using Pyrex filtered light.35A good chemical yield of the final product (64)is obtained with a quantum yield of 4 = 0.14. There is no evidence for ring opening of the cyclopropyl group in this reaction but when the oxirane (63, X = O ) is irradiated the final product is (65) which arises by the reaction path shown in Scheme 2. Support for this mechanism has been obtained from labelling studies. A 1,5-biradical is also implicated in the photochemical cyclization of (66) into (67) on irradiation in the crystalline phase.36 The outcome of the reaction is controlled by the matrix effect within the crystal lattice and irradiation affords (67) as the main product with a de of 97%. A review has highlighted the area of solid state photochemistry where single crystals of starting material are transformed into single crystals of product.37

wph -9 -

-

(63 X -0)

Ph

H

Scheme 2

@ \ Ph X

(63)X = CH2 or 0

P\

P

(64)

OH h

(65)

Po

H

O

'

OH (65)

H

have reported the photochemical reactivity of some dipeptides. Sauer et To exemplify the reactivity of such systems the dipeptide (68) has been chosen. This, on irradiation, undergoes conversion into the products (69) and (70) in the ratios shown. The reaction involves a 1,6-hydrogen abstraction and rotation within the resultant 1,5-biradical is hindered. Temperature effects

56

Photochemistry Ph OH BocHN M‘’‘cNvC02Bn

BocHN

br’

0

Pri

0

(“I

0

(70)

8 : 1 20°C MeOH 16 : 1 -16°C MeOH 4 : 1 0°C CH&

Pri

were also studied and the results from these are also shown below the appropriate structures. The reactions occur with retention of configuration at the optical centre in the alanine unit. Cyclization to (71) is the result of irradiation of the aspartic acid derivatives (72).39This reaction occurs by a 1,7hydrogen transfer and the formation of a 1,6-biradical which cyclises giving the observed products. When the amine is not symmetrically substituted as in (72c) irradiation gives a mixture of products (73) and (74) in a ratio of 25 : 75. Transfer of hydrogen from a ring carbon to the carbonyl group oxygen in (75) also results in the formation of a 1,6-biradicale40Cyclization within this species Ph I

AcHN

AcHN

0 (71) R5 = C02Me (75%) R 5 = Ph (90%)

(72)a, R3 = CH2C02Me, R4 = CH2C02Me b, R3 = R4 = PhCH2 c, R3 = Me, R4 = CH2C02Me

8::

AcHN = CH2C02Me, R2 = H = Me, R2 = C02Me

(73) R’ (74)R’

NHTfa Ph

TfaHN 0

(75) n

)n

TfaHN

0 (76)

0 (77)

52 74 53 72

28 22 15 9

Yield (YO)

0

(78) R’ R’ R’ R’

=

Ph, R2 = CH2Ph

= R2 = Ph =

H, R2 = Me R2 = Ph

= H,

37 51 44 38

ds

67 77 78 89

36 25 44 38

M I : Photochemistry of Carbonyl Compounds

57

yields the two products (76) and (77). The diastereoselectivity (ds) increases with increase in ring size. The keto esters (78) undergo conversion into the The reactions are quite lactones (79) and (80) on irradiation in ~olution.~' specific and no evidence for y-hydrogen abstraction is observed. The 1,8hydrogen transfer that occurs on irradiation is explained on the basis that a charge transfer state is involved and this ensures that the reaction is regiospecific with cyclization of the resultant biradicals affording the final products. Reactions involving 1,8-hydrogen abstraction processes have been reviewed.42 Several products are formed from the irradiation ( h > 280 nm) of the ester (81) in benzene under an atmosphere of argon.43 The major product (48%) from this was identified as the cyclic ketone (82) which is presumed to arise by a hydrogen abstraction path involving the ester carbonyl group and affording the 1,8-biradical (83). Cyclization of this species and loss of methanol affords

the product. Other cycloaddition products (84) and (85) are also formed in yields of 6 and 13% respectively and the fragmentation product (86) is produced in 13% yield. When acetonitrile is used as the solvent the fragmentation reaction becomes dominant. A single electron-transfer process is used to account for this process and the dependence of the reaction upon the substitution pattern is demonstrated by the failure of the related ester (87) to yield a cyclic ketone. The only reaction detected in this case is a (2+2)cycloaddition that gives a product in low ~ i e l d . 4 ~ C02Me

3

Oxetane Formation

BachM has reviewed photochemical (2 + 2)-cycloaddition reactions including oxetane-forming processes and the stereochemical aspects of the reactions are highlighted. Earlier studies by the same author45 reported the results of irradiation of the alkene (88) in benzene with benzaldehyde to give the oxetane (89). These 3-oxetanols have been subjected to further study and have been

58

Photochemistry

J4

- -pri

Ph

H H H Me Me Bn Me Pr OBu' Me OBu' Bn OCH2CH2TMS Bn CH2CH2CH

Scheme 3

OH

74 58 81 71 56 77 74 82

71129 7912 1 8911 1 >90110 90110 87113 >go11 0 88/12

shown to undergo ring opening to yield diastereoisomerically pure 1,2-di0ls!~ Full details of the photochemical addition of benzaldehyde to the alkenes (90) have been reported (Scheme 3).47 The results of irradiation ( h> 290 nm) of a series of aldehydes and ketones (91) in the presence of the silyl acetals (92) have been reported.48The reactions are both solvent and silyl group dependent and the best results are obtained when the solvents used are n-hexane, THF, diethyl ether or benzene and with the silyl group TBDMS. The products are the oxetanes (93) and the silylmigrated product (94) in a ratio greater than 95 :5 respectively. There is no evidence for the formation of the isomeric oxetane. Other studies from this research have examined the photochemical addition of a series of aryl aldehydes (95) to the cyclic silyl alkenes (96) brought about by irradiation at h > 290 nm in methylene chloride solution. The additions encountered take place with regio and ex0 selectivity as shown by the yields and ratios of the products (97). The photoaddition of aldehydes or ketones to furan has been reported over the years. Griesbeck and his co-w~rkers'~ have established that the diastereoselectivity of the addition of benzaldehyde to furan is 212 : 1 for the formation of the exo and endu products (98) and (99). The study was extended to the addition of other carbonyl compounds (100) and the ratio of products from these additions is shown under the appropriate structure (101) and (102). L-Ascorbic acid and some of its derivatives (103) also undergo photochemical addition of aromatic aldehydes and ketone^.^' With benzaldehyde and benzophenone the products obtained are the mixture of (104) and (105) with a preference for the formation of the former. The stereochemistry of the addition of the excited state carbonyl compound to ascorbic acid favours the path where the phenyl and the alkoxy groups are cis on the resultant oxetane. Benzaldehyde adds photochemically to the enamine (106) when the mixture is irradiated in acetonitrile solution.52 Three products are

IIIl: Photochemistry of Carbonyl Compounds

59 OSiR3

ii,

,YiR3 $OMe R

Ar (911 Ar

R

(92) SiR3

2-naph 4-CNCpH4 4-MaeH4 C6H5 Ph Ph

H H H H Ph Me

TMS TES TBDMS DMEDMS DMEDMS DMEDMS

Ar

(93)

(94)

OTBDMS

T B D M S O . ~ArH

I

8,

R2--

R2

Ar (95) Ar=a, 2-naph b, 1-naph c.d. 6-MeO-2-na~h

R3 (97)

(96)

I

R'

R2

Yield (YO)

R3

ca

( 100) R' =Ph R2 =

(101) 21211 4911 H Me

(102) 3.7:l 1:9 1:19 1:49 CN C02Me OMe C@R

X

\

O H 0

Me0 OR (103) R Ar

Me Bn Bn Bn Bn

R'

Ph H Ph H 4-CICeH4 H 4-MeOzCCsH4 H Ph Ph

60 57 40 42 33

25 28 20 23 65

8x0:

endo

90: 10 95: 5 86 : 14 87: 13 91: 9

60

Photochemistry

formed, one of which is a 2-aminooxetane that could not be isolated, but the other two were identified as the diastereoisomeric compounds (107) and (108) in yields of 53% and 12%, respectively. According to these authors the facial diastereoselectivity is unprecedented as normally in such systems the pres6nce of a bulky side-chain favours addition to give the product currently obtained as the minor one. The major product (107) was chemically transformed into (+)-preussion (109).

Q IC 9 H 1 9

C9H19

C02Me

Photocycloaddition of benzaldehyde or benzophenone to the alkene (1 10) follows the usual path and affords the oxetanes (111) in moderate to good yields.53 With the simple cyclopropyl substituted alkenes the biradical intermediate in the addition does not undergo cyclopropane ring opening, but with an appropriately substituted cyclopropane ring opening does occur. Thus irradiation of benzophenone with the alkene (1 12) yields the bis-adduct (1 13) where a second addition of benzophenone to the ethene bond in (114), the primary product, has occurred. Addition of aldehydes to (1 12) is not complicated by this second addition and the primary products obtained were identified as the tetrahydrooxepins (1 14). OSiMe3

The primary photochemical product formed from the irradiation of the 2-thiones (1 15) in the presence of alkenes is the oxetanes (1 16). The reaction conditions use Pyrex filtered light in benzene solution. Under these conditions the initial product is unstable and reacts further either by C-0 or C-S bond fission which leads to the isolated products (117) and (118) in the yields shown. 54

IIII: Photochemistry of Carbonyl Compounds

(115)a Me Me Me Me Me

Me Me Me Me CN

H Me Me Me H

H H Me CH=CMe2 H

(115)b Me

59 61 7 28 63

Me Me Ph

Me Me H

H Me H

-

Me Ph

4

61

55

-

10 52 31 53 51

Miscellaneous Reactions

4.1 SET Processes - Irradiation of the enone (1 19) under electron-transfer conditions (triethylamine/acetonitrile) results in a 55% yield of the bicyclic alcohol (120) which has been used as a precursor in a synthesis of isoafricanal? A study of the reductive cyclization of some cyclopropyl ketones has been carried out under SET condition^.^^ The reactions are initiated using h > 300 nm in acetonitrileltriethylamine and the resultant radical anions undergo ring opening and cyclization. Thus the ketones (121) and (122) are converted in moderate yields into the bicyclic ketones (123) and (124) respectively. The influence of position of the alkyne substituent on the outcome of the reaction was studied and (125) can be converted into (126). Again the yields are modest. An aryl group as in (127) can replace the alkyne moiety, but even lower yields are observed in this example giving products (128). The ketocyclopropane derivatives (129) are photochemically reactive on irradiation in acetonitrile with added trieth~larnine.~~ Again this treatment results in electron transfer photochemistry and regioselective bond fission of

62

Photochemistry

the cyclopropane ring occurs to afford (130a and b) in 15% and 25%, respectively. The ketone (129b) also yields another rearrangement product, identified as (131).

3: i:cl 8 . b

(121) R - M e

H

(122)

(123) 24%

R-H

(124) 23%

45%

(127) R = H R = OMe

(126) 27%

(128) 8% 5%

Rvo 0

(129) a, R = Me b, R = Pr’

(130) a, R = M e b, R = Pr’

Arylalkylcarboxylicacids can be decarboxylated readily by irradiation in the presence of HgO with the resultant arylalkyl radicals undergoing dimerisat i ~ n . ~Another * report on decarboxylation by the same authors59 has used Hg2F2 as the catalyst for the process: this efficiently converts the acids (132) in acetonitrile into the dimers (133). The reaction follows an electron transfer R’

R’ R’

I

I

Ar-C-C02H

I I R2 R2

A2

(132) Ar

I

Ar-C-C-Ar

(133) R’

R2

Yield (%)

Ph Ph Me Et H H H H

H

83

Me H H H H H H

73 66 72 63 78 73 68

IIII: Photochemistry of Carbonyl Compounds

63

,,YCOfPhBun4N+ ( 134)

(135) R = CI or Me0

(136)

path with the formation of the carboxyl radical which decarboxylates to give the alkyl radicals that are the precursors to the dimers. Mercury is formed as a byproduct. Other workers have reported electron transfer induced decarboxylation of carboxylate salts.60The counterions in this study were either tetra-nbutylammonium or K+/18-crown-6 and this work has demonstrated the decarboxylation of the salts (134- 136) using wavelengths > 300 nm in THF or benzene as solvent. Mariano and his co-workers6*have carried out a detailed study of the electron transfer photochemistry of a-anilino carboxylates, P-anilinoalcohols and a-anilinosilanes. The rates of decarboxylation of anilinium carboxylate radicals have been measured and the base induced retroAldol fragmentations of the radical cations formed from the P-anilinoalcohols and the influence of substituents on the nitrogen on the desilylation of the or-anilinosilanes were also investigated. In addition, the synthetic potential of some of the electron transfer photochemistry of the carboxylate salts (137) and (138) has been examined. Here irradiation, using DCA in methanol or acetonitrile as solvents, leads to decarboxylation and the formation of an alkyl radical. These cyclise to (139) and (140), respectively, in yields of 55-77%. Similar cyclizations were carried out for some phthalimide derivatives e.g . the conversion of (141) into (142).

(139)

0

@J

R

\

0 (142) R = Me or Ac

Banerjee and have demonstrated that it is possible to use SET processes for the elimination of protective groups. The molecules studied were the phenacyl esters (143) that could be converted into the free acid. Several electron-donating sensitisers (144- 150) were used and the yields and wavelengths used are shown under the appropriate structure in Scheme 4.

64

Photochemistry 0

0

I1 II PhCCH20CR (143)

-

M e 2 N e N M e 2

RC02H + PhCOMe

I Me

1-naphthyl oxide

(144)

( 146)

(149) R = H (150) R = M e sensitizer (144) (145) (146) (147) (148) (149) (150)

76 88 70

83

h>320mm h>320mm 350mm h>390mm 350mm h>390mm hM0mm

86 90 97 Scheme 4

4.2 Decarbonylation and Decarboxylation - A detailed investigation of the photochemical decomposition of ketene by excitation at 230 nm has been reported.63 A further study of this molecule has examined the decomposition on its singlet energy surface: this was carried out using a two-step IR and UV approach.64 Bisketene (151) undergoes loss of CO when irradiated in an argon matrix using h = 254 nrn? The resultant mono-ketene (152) is also photochemically sensitive. And irradiation in the 420-680 nm region brings about a second decarbonylation and the formation of the biradical (153) which rearranges to afford (1 54).

Mehta and Ravikrishna66 have demonstrated that the monosubstituted semibullvalenes (1 55) can be readily prepared by photodecarbonylation of the polycyclic ketones (156). The reaction is best carried out in methanol solution.

IIII: Photochemistry of Carbonyl Compounds

65

Irradiation of (157) through Pyrex brings about decarbonylation and the formation of the tetraene (158) which on further irradiation using h > 220 nm gives a low yield of the (4 + 4)-adduct ( 159).67The photodecarbonylation of endo-tricycle[5.2.2.02-6]undecadienoneshas been reported.68 0

72% 65% 70% 60%

(155)

(156) R = C02Me R=CN R = Ph R = CH20Me

An efficient route for the synthesis of [2.2]cyclophanes has also been described which involves the photochemical double decarbonylation of the diketones (160), (161) and (162).69 The reactions are carried out in argondegassed benzene solution and give high yields of products efficiently. In the case of the meta systems (160) both mono (163) and double decarbonylation products (164) are formed, but with (161) and (162) only the bisdecarbonylation is observed affording (165) and (1 66) respectively.

(164) X = CH (78%) X = N (56%)

(165) 94%

(166) 97%

66

Photochemistry

Irradiation of the oxazolone derivative (167) in acetonitrile results in decarbonylation and the formation of the imine ( 168).70In the presence of ally1 alcohols, trapping (a thermal reaction) of (168) results in the formation of the ethers (169) which undergo Norrish Type I1 hydrogen transfer and ?he formation of the isomeric compounds (170). These isomeric compounds readily undergo a Claisen rearrangement to afford the second product (171) isolated from the initial irradiation. "'/C

x,

Ph

Ph

~~

H H Pr" Me H

H Pr" H

Me H

H H H H Me

48 29 79 60 58

(171) Yield (YO)

28 42

trace 19 24

Photochemical decomposition of malonic acid by irradiation in solution has been reported.71 Some of the radical species produced by this treatment are identical to those formed by the Ce4+ decomposition of malonic acid in the Belousov-Zhabotinsky reaction. The (2 + 2)-cycloadducts (172) can be readily prepared by irradiation of mixtures of the corresponding enone and alkene, and these adducts can conveniently be converted into the hydroperoxide (1 73) by irradiation at 366 nm in the presence of air and acridine in toluene.72The decarboxylation occurs by a free radical pathway and treatment of the hydroperoxide with dimethyl sulfide brings about formation of the ringexpanded ketones or lactones (174).

(172) X = CH2 or 0

IIII: Photochemistry of Carbonyl Compounds

67

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 1OK using 254 nm light.73 Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylat i ~ nand , ~this ~ is reported to take place from an upper triplet excited state.75A study of the transient photochemistry of 5-@-toluy1)-1-methyl-2-pyrrolylacetic acid has been reported.76 Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed.77 This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (18 1) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), re~pectively.~~ H

A detailed study of the kinetics of ring opening of cyclopropylcarbinyl radicals has been reported.79The radicals, with a variety of substituents, were formed by irradiation of the Barton esters (184). Irradiation of other Barton esters ( 185) has been used to generate P-(phosphatoxy)alkyl and P-(acyloxy) alkyl radicals.80Laser flash photolysis brings about bond fission and decarboxylation yields the radicals (186) which undergo rearrangement to yield (187). The xanthate derivatives (188) undergo S-CU bond fission on irradiation with visible light and the resultant radical decarboxylates to afford an alkyl radical.81Recombination with the sulfur radical affords the products (189) and several examples of this type of reactivity have been described. Intramolecular trapping has also been demonstrated using the xanthate ( 190): here, the acyl radical (191) decarboxylates and cyclises to yield the radical (192) which also cyclises and is trapped as (193) by recombination (Scheme 5). In other examples [e.g. (194)] decarboxylation is suppressed and the final products were identified as (195).

4.3 Reactions of Miscellaneous Haloketones and Acid Chlorides - Photochemical chlorocarbonylation of the polycyclic tetradecane (196) results in the formation of mixture of isomeric acid chlorides which, on treatment with

68

Photochemistry

Rvo,Np

Ar

Me Me S (185) R = P(O)(OPh)2, P(O)(OEt)P, COMe, COCF3; Ar = Ph R = P(O)(OEt)2; Ar = pMeOC6H4

(184) R' = R2 = H R' = Me, R2 = H R1=R2=Me R' = H, R2 = C02Et R' = Me, R2 = C02Et R' = H, R2 = Ph

Ar

0

s

S R-SKSEt

R,OASKOEt

0S ' W0R4

( 194)

(195)

R'

R2

R3

R4

H H ally1

H H H

H PhCH2CH2 H

Et neopentyl neopentyl

Yield (YO) 84 67 42

IIl1: Photochemistry of Carbonyl Compounds

69

methanol, affords the two esters ( 197).82 The irradiation of chloropropiophenone (1 98) has been previously described, and recent work has focussed on the photolysis of the optically active forms of the compound.83 The irradiations were carried out in trifluoroethanol and give good yields of the corresponding ester (199) by a 1,2-phenyl migration. The evidence suggests that the photolysis proceeds by heterolysis of the C-Cl bond affording a cation in which the migration occurs. Partial racemisation is observed with either the (9-or the (R)-enantiomers, but there is evidence that irradiation of the (3form affords the (3-ester with around 47-50% ee while the ( R ) ketone gives the (R)-ester with an ee of 40-5OY0.

(196)

(197) R’ = H, R 2 = C02Me R’ = C02Me, R2 = H

(198)

4.4 Other Fission Processes. - The photochemically induced fission reactions of a series of naphthylmethyl alkanoates have been described.84Some factors that control the ease of bond cleavage within the esters were identified. Irradiation of (200) in a NaY zeolite using 308 nm light results in C - 0 bond fission and the formation of an acetyl radical.85 Irradiation of perfluoroacetyl fluoride at 254 nm in the gas phase brings about fission of CO-F bond with the formation of fluorine atoms and perfluoroacetyl radicals.86

V

I

Neckers and his co-workersg7have investigated the photochemical reactivity of ammonium borate salts both in solution and tethered to a polymer substrate as a means of producing free tertiary amines. The reactions are carried out in acetonitrile solution on the benzoylbenzyl molecules (20 1) using wavelengths > 300 nm. Irradiation of (202) results in the formation of the products shown in Scheme 6. More detailed studies examined the influence of different borate salts as the counterions to the ammonium salts. Some of these results are shown for the decomposition of the salt (201). From these data, it can be seen that the most efficient quantum yield for the release of tri-n-butylamine is obtained with the triphenylbutylborate counterion.

Photo chemistry

70 Ph

l

O

C

-

Bu Br Ph4B Me PhsBBn Bu P

t

l

~

~

~

~

;

+

(202)

H

R1 X2-N+--R2 I I

R3

0.06 0.38 0.98

Bu Bu Me Me Bu Bu

NBu3 ~ h+ Ph-Ph ; ~

+

+P h x e h 4 e

C8Hl8 + Ph

+

Scheme 6

Laser-flash photolysis of (203) in solution affords the ketene (204) and this is the first time that this ketene has been observed using time-resolved IR spectroscopy,88Although other workers have previously reported the formation of this species. The present work reports the kinetics of the reaction of the ketene with water, methanol and diethylamine. The products obtained from the irradiation of a series of N-acetoacetyl-a-amino acids have been identified by gas chromatography and chemical ionization mass s p e c t r ~ m e t r y . ~ ~ A

The aldehydes (205) undergo conversion into the corresponding acyl radical when irradiated in aromatic solvents (chlorobenzene or benzene) or acetonitrile with benzophenone as the hydrogen abstracting species.90 The resultant radicals undergo facile addition to thioalkenes such as (206) and the adducts obtained were used as precursors in the synthesis of indanone derivatives. In a further study of this reaction system Ogura et aL9' have described the photochemical transformations of the sulfonyl alkenes (207) into the derivatives (208). The reactions again involve the electronic excitation of benzophenone as the key step. The aldehydic hydrogen is abstracted from the aldehyde substrate and the resultant radical adds to the alkene to afford the adducts. The reaction occurs with high syn selectivity as can be seen from the figures cited under the products. The irradiation of 2-pyridyl phenyl ketone in sodium dodecyl sulfate micelles gives no evidence for a hydrogen abstraction path, but instead a rapid intramolecular cyclization takes place.92The study of the photophysics of the has shown that ketone nabumetone, 4-(6-methoxy-2-naphthyl)butan-/3-2-one a naphthalene like triplet state is formed upon irradiation in a ~ e t o n i t r i l e . ~ ~

IIII :Photochemistry of Carbony l Compounds

71

0

(207) R' H Ac Ac Ac H H Ac Ac Me Me

Me

R2 Me

F7

Me Me Me Me Me H Ac Ar

?Me

(208) Yield (YO) syn:an?i 92 83:17 77 91: 9 89 96: 4 96 89:11 84 84:16 68 82: 18 86: 14 PClOHPl 90 Pr' 60 9o:lO Ph 64 83:17 Ph 86 93: 7

R3 Et Et Et Me Pr" Bu'

Ph

A6

97. A

References 1. 2. 3. 4.

5. 6. 7. 8.

A. Bhattacharyya, E. J. P. Malar and S. Subramanian, J. Mol. Struct., 1998,434, 101 (Chem. Abstr., 1999,160289). A. Bhattacharyya, E. J. P. Malar and S. Subramanian, THEOCHEM, 1998,434, 101 (Chem. Abstr., 1998,451329). P. Schmoldt, Th. Kirschberg, M. Fagnoni and J. Mattay, J. In$ Rec., 1998, 24, 249 (Chem. Abstr., 1999,25 1962). A. C. Bhasikuttan, A. K. Singh, D. K. Palti, A. V. Sapre and J. P. Mittal, J. Phys. Chem. A, 1998,102,3470. C. R. Silva and J. P. Reilly, J. Phys. Chem. A , 1997, 101,7934. J. H. Horner, M. Newcomb, M. Lucarini and G. F. Pedulli, Tetrahedron Lett., 1998,39,3947. S . Peukert and B. Giese, J. Org. Chem., 1998,63,9045. R. S . Givens, J. F. W. Weber, A. H. Jung and C.-H. Park, Methods Enzymol., 1998,291 (cage compounds), 1 (Chem. Abstr., 1999,130,209853).

10.

N. K. Shrestha, E. J. Yagi, Y. Takatori, A. Kawai, Y. Kajii, K. Shibuya and K. Obi, J. Photochem. Photobiol., A , 1998, 116, 179. Y. Feng, Z. Chen and C. Xia, Dame Huaxue, 1998, 13,44 (Chem. Abstr., 1998,

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S . Jockusch and N. J. Turro. J. Am. Chem. SOC.,1999,121,3921. M. H. Kleinman, T. Shevchenko and C. Bohne, Photochem. Photobiol., 1998,68,

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M. P. Angelini and E. Lee-Ruff, Tetrahedron Lett., 1998,39,8783. G. Umbricht, M. D. Hellman and L. S. Hegedus, J. Org. Chem., 1998,63,5173. H. M. Zang and D. C. Neckers, J. Org. Chem., 1999,64,2103.

15.

16.

117, 17,

72

Photochemistry

19.

1. Suzuki, R. Tanaka, A. Yamaguchi, S. Maki, H. Misawa, K. Tokumaru, R. Nakagaki and H. Sakuragi, Bull. Chem. SOC.Jpn., 1999,72, 103. 0. B. Morozova, Y. P. Tsentalovich, A. V. Yurkovskaya and R. Z. Sagdeev, J. Phys. Chem. A , 1998,102,3492. R. G. Zepp, M. M. Gumz, W. L. Miller and H. Gao, J. Phys. Chem. A, 1998,

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U. Lindemann, D. Wolff-Molder and P. Wessig, J. Photochem. Photobiol. A-

17. 18.

102,5716.

21.

Chem., 1998,119,73. D. P. De Costa, A. K. Bennett and J. A. Pincock, J. Am. Chem. SOC.,1999, 121,

22.

Z.-F. Cui, Y.-2. Wang and C.-Y. Liu, Youji Huaxue, 1999,19, 166 (Chem. Abstr.,

23. 24.

J. G. Ling, 2. Z. Tian and D. H. Guang, Tetrahedron Lett., 1999,40, 13 1. M. Leibovitch, G. Olovsson, J. R. Scheffer and J. Trotter, J. Am. Chem. Soc.,

25. 26.

M. J. Taylor, T. Z. Hoffman, J. T. Yli-Kauhaluoma, R. A. Lerner and K. D. Janda, J. Am. Chem. SOC.,1998,120, 12783. E. Cheung, M. R. Netherton, J. R. Scheffer and J. Trotter, J. Am. Chem. Soc.,

27. 28.

H. Ihmels and J. R. Scheffer, Tetruhedron, 1999,55,885 A. Gamarnik, B. A. Johnson and M. A. Garcia-Garibay, J. Phys. Chem. A , 1998,

29.

T. Omori, T. Suzuki and T. Ichimura, Chem. Phys. Lett., 1998, 293,436 (Chem. Abstr., 1998,129, 302268). Y. Ito, S. Yasui, J. Yanauchi, S. Ohba and G. Kano, J. Phys. Chem. A, 1998,

3785.

1999,268603).

1998,120,12755.

30.

1999,121,2919.

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102,5415.

31. 32. 33. 34. 35. 36. 37. 38.

S . Fukushima, Y. Ito, H. Hosomi and S. Ohba, Acta Crystallogr., Sect. B: Struct. Sci.,1998,54, 895 (Chem. Abstr., 1999, 33093). 'L. Ito, G. Kano and N. Nakamura, J. Org. Chem., 1998,63, 5643. M. Saito, Y. Kamei, K. Kuribara, M. Yoshioka and T. Hasegawa, J. Org. Chem., 1998,63,9013. R. Das, S. Mitra, D. Guha and S. Mukherjee, J. Lumin., 1999, 81, 61 (Chem. Abstr., 1999, 2 15279). D. J. Chang, E. Koh, T. Y. Kim, B. S. Park, T. G. Kim, H. Kim and D.-J. Jang, Tetrahedron Lett., 1999,40,903. H. Irngartinger, P. W. Fettel and V. Siemund, Eur. J. Org. Chem., 1998,2079. A. E. Keating and M. A. Garcia-Garibay, Mol. Supramol. Photochem., 1998, 2(0rganic and Inorganic Photochemistry), 195, (Chem. Abstr., 1999,130, 58927). S. Sauer, A. Schumacher, F. Barbosa and B. Giese, Tetrahedron Lett., 1998, 39,

3685. A. G. Griesbeck, H. Heckroth and H. Schmickler, Tetrahedron Lett., 1999, 40, 3137. 40. U. Lindemann, D. Wulff-Molder and P. Wessig, Tetrahedron: Asymmetry, 1998, 9,4459 (Chem. Abstr., 1999, 130, 223549). 41. T. Hasegawa and Y. Yamazaki, Tetrahedron, 1998,54, 12223. 42. T. Hasegawa, Y. Yamazaki and M. Yoshioka, Trends Photochem. Photobiol., 1997,4,27 (Chem. Abstr., 1999,130, 31015). 43. K. Mizuno, S. Konishi, Y. Yoshimi and A. Sugimoto, Chem. Commun., 1998, 1659. 44. T. Bach, Synthesis, 1998,683. 39.

1111: Photochemistry of Carbonyl Compounds

45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77.

73

T. Bach, K. Jodicke, K. Kather and J. Hecht, Angew. Chem. Int. Edn. Engl., 1995,34,2271. T. Bach and F. Eilers, Eur. J. Org. Chem., 1998,2161. T. Bach and J. Schroeder, J. Org. Chem., 1999,64, 1265. M . Abe, Y. Shirodai and M. Nojima, J. Chem. SOC., Perkin Truns. 1, 1998,3253. M. Abe, M. Ikeda and M. Nojima, J. Chem. Soc., Perkin Truns. I , 1998,3261. A. G. Griesbeck, S. Buhr, M. Fiege, H. Schmickler and J. Lex, J. Org, Chem., 1998,63,3847. S . R. Thopate, M. G. Kulkarni and V. G. Puranik, Angew. Chem., Int. Edn. Engl., 1998,37, 1 1 10. T. Bach and H. Brummerhop, Angew. Chem., Int. Edn. Engl., 1998,37, 3400. C. Y. Gan and J. N. Lambert, J. Chem. SOC., Perkin Trans. 1, 1998,2363. T. Nishio, J. Chem. SOC.,Perkin Trans. 1 , 1998, 1007. J. Cossy, S. Bouz-Bouz and C. Mouza, Synlett, 1998,621. M. Fagnoni, P. Schmoldt, T. Kirschberg and J. Mattay, Tetrahedron, 1998, 54, 6427. B. C. Maiti and S. Lahiri, Tetrahedron, 1998, 54,9111. M. H. Habibi and S. Farhadi, J. Chem. Res., Synop., 1998,776. M. H. Habibi and S. Farhadi, Tetrahedron Lett., 1999,40, 2821. H. Yokio, T. Nakano, W. Fujita, K. Ishiguro and Y. Sawaki, J. Am. Chem. SOC., 1998,120, 12453. Z. Y. Su, P. S. Mariano, D. E. Falvey, U. C. Yoon and S. W. Oh, J. Am. Chem. SOC., 1998, 120, 10676. A. Banerjee, K. Lee, Q. Yu, A. G. Fang and D. E. Falvey, Tetrahedron Lett., 1998,39,4635. M. Castillejo, S. Couris, E. Lane, M. Martin and J. Ruiz, Chem. Phys., 1998,232, 353 (Chem. Abstr., 1998,393 928). M. V. Ashikhmin, A. Mellinger and C. B. Moore, Proc. SPIE-Int. SOC.Opt. Eng., 1998, 3271 (Laser techniques), 64 (Chem. Abstr., 1998,383373). H. H. Wenk and W. Sander, Eur. J. Org. Chem., 1999,57. G. Mehta and C. Ravikrishna, Tetrahedron Lett., 1998,39,4899. T. D. Golobish, J. K. Burke, A. H. Kim, S. W. Chong, E. L. Probst, P. J. Carroll and W. P. Dailey, Tetrahedron, 1998,54,7013. V. Singh and B. Thomas, J. Indian Chem. Soc., 1998,75,640 (Chem. Abstr., 1999, 244080). H. Isaji, K. Sako, H. Takemura, H. Tanemitsu and T. Shinmyozu, Tetrahedron Lett., 1998,39,4303. P. S . Park, C. M. Oh, K. H. Chun and J. 0. Lee, Tetrahedron Lett., 1998, 39, 971 1 . I. Szalai, H.-D. Foersterling and Z. Noszticzius, J. Phys. Chem. A, 1998, 102, 31 18. S. Piva-Le Blanc, S. Henon and S. Piva, Tetrahedron Lett., 1998,39,9683. G. Maier and J. Endres, Eur. J. Org. Chem., 1998, 1517. M. A. Miranda, J. Perez-Prieto, A. Lahoz, I. M. Morera, Z. Sarabia, R. Martinez-Manez and J. V. Castell, Eur. J. Org. Chem., 1999,497. S . Encinas, M. A. Miranda, G. Marconi and S. Monti, Photochem. Photobiol., 1998,68, 633. S. Sortino and J. C. Scaiano, Photochem. Photobiol., 1999,69, 167. F. L. Cozens, M. L. Cano, H. Garcia and N. P. Schepp, J. Am. Chem. SOC., 1998, 120,5667.

74

Photochemistry

78. 79.

H.-M. Steuhl and M. Klessinger, J. Chem. Soc., Perkin Trans. 2, 1998,2035. J. H. Homer, N. Tanaka and M. Newcomb, J. Am. Chem. SOC., 1998, 120, 10379. S.-Y. Choi, D. Crich, J. H. Horner, X. Hudng, M. Newcomb and P. 0. Whitted, Tetrahedron, 1999,55, 33 17. J. E. Forbes, R. N. Saicic and S. Z. Zard, Tetrahedron, 1999,55, 3791. A. P. Marchand, S. Alihodzic, I. N. N. Manboothiri and B. Ganguly, J. Org. Chem., 1998,63,8390. S . Usui, T. Matsumoto and K. Ohkubo, Tetrahedron Lett., 1998,39,9755. Y. Itoh, M. Gouki, T. Goshima, A. Hachimori, M. Kojima and T. Karatsu, J. Photochem. Photobiol., A , 1998,117, 91. S. Vasenkow and H. Frei, J. Am. Chem. Soc., 1998,120,4031. K. L. Bierbauer, M. S. Chiappero, F. E. Malanca and G. A. Arguello, J. Photochem. Photobiol., A , 1999,122, 73, A. M. Sarkar, A. Lungu, A. Mejiritshi, Y. Kaneko and D. C. Neckers, J. Chem. SOC., Perkin Trans. 2, 1998,23 15. R. C. Y. Liu, J. Lusztyk, M. A. McAllister, T. T. Tidwell and B. D. Wagner, J. Am. Chem. SOC., 1998,120,6247. H. Budzikiewicz, P. Dallakian, A. G. Griesbeck and H. Heckroth, J. Mass Spectrom., 1998,33, 1256 (Chem. Abstr., 1999, 18042). K. Ogura, T. Arai, A. Kayano and M. Akazome, Tetrahedron Lett., 1998, 39, 905 1. K. Ogura, T. Arai, A. Kayano and M. Akazome, Tetrahedron Lett., 1999, 40, 2537. F. Ortica, F. Elisei and G. Favaro, J. Phys. Org. Chem., 1999, 12, 21 (Chem. Abstr., 1999, 115909). L. J. Martinez and J. C. Scaiano, Photochem. Photobiol., 1998,68, 646.

80. 81. 82.

83. 84. 85. 86. 87.

88. 89. 90. 91.

92. 93.

2 Enone Cycloadditions and Reamangements: Photoreactions of Dienones and Quinones BY WILLIAM M. HORSPOOL

1

Cycloaddition Reactions

1.1

Intermolecular Cycloaddition

1.1.1 Open-chain Systems - Chalcones such as (1) undergo photodimerisation when they are irradiated in the molten state. Heating the crystalline material to 60 “C and irradiating the melt with light from a 400-watt mercury vapour lamp for 24 h results in the formation of the racemic anti-head-to-head dimers (2) exclusively. trans-Cinnamic acid has been irradiated in a bilayer with the ammonium bromide surfactant (3).2 Films of this mixture were cast and irradiated at h > 280 nm which gave the cis-cinnamic acid, the syn head-to-head dimer as the major product and a trace of the syn-head-to-tail dimer. Heating the cast film followed by irradiation brings about a decrease in the amount of the syn head-to-head dimer, previously the major product. This change is thought to be the result of change of order within the film. The authors reason that the formation of the major product arises from the fact that hydrogen bonding within the film holds the cinnamic units parallel to each other. An additional report by the same authors3 has commented upon the highly selective formation of the syn-head-to-head dimer of cinnamic acid from irradiation of cinnamic acid as a composite bilayer with the same surfactant. The solid state dimerisation of 4-methylcinnamicacid can be brought about phot~chemically.~ The mechanism of this process has been studied using Raman spectroscopy and the reaction is proposed to be topochemically controlled. The photophysics of 4-dimethylaminocinnamicacid have been studied in a variety of environments? The photoisomerisation of (E)-N-iso-propylcinnamidehas been shown to be wavelength and solvent dependent.6

Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 75

Photochemistry

76

The photochemistry of a phenyldiacrylic acid derivative has been studied in Langmuir-Blodgett filrnsa7The crystal structure of the major dimer formed on perdeuterioacetone-sensitized irradiation of t-butyl-2,5-dihydro-5,5-dimethyl2-oxo-1H-pyrrole-1-carboxylate has been determined.* Styryldicyanopyrazines undergo topochemical dimerisation when they are irradiated in the crystalline phase.9 The photocycloaddition of chloroprene to methyl 2,4-dioxopentanoate (4) has been reported.'* Only two de Mayo style products were obtained from this process and these were identified as the adducts (5) and (6) arising from the two paths of addition of the enol (7) to the diene. Precise kinetic data has been obtained for the photochemical dimerisation of the cyclopentanone derivative (8).'

'

42

C02Me

C02Me

h

A review has highlighted the photocycloaddition reactions of alkenes with aromatic esters and nitriles.l 2 Cycloadditions occur by a (3+2)-mode and provides a path to medium size ring systems. When the cinnamic acid derivative (9) is irradiated at 359 nm in ethanol with added Ti02 the product (10) is formed in 30% yield.13 Analogous products are formed from other straight chain alcohols such as (11) from propan-1-01. Benzonitrile can be photochemically hydrated in the presence of oxophosphorus porphyrins. l4

I. 1.2 Additions to Cyclopentenones and Related Systems - The photophysics of ~ in methylene a series of cyclopentenones (12) has been ~ t u d i e d . 'Irradiation (13) and the alkene (14) results in the chloride of a solution of the enone 5) in 47% yield. The reaction is best carried out at formation of the adduct (1 temperatures around 0°C. The adduct (15) has been converted into the

IIl2: Enone Cycloadditionsand Rearrangements

(12) n = 1, n=2, n = 1, n = 2,

77

R = Ph R=Ph R = 1-naphthyl R = 1-naphthyl

diketone ( 16).16 A report has focused on the photocycloaddition reactions of vinylene carbonate to the homochiral furanones (1 7).17 These cycloadditions give reasonable yields of adducts such as (18) and (19). More importantly the diastereoselectivity (de) of the processes rises from 40% de with (1 7a) to almost 92% de for (17e). These adducts obtained in the previous study have been developed further as a synthetic path to some carbohydrate derivatives.18 A review has focused upon the photochemical reactions of 2(5H)-furanones.l 9

Me R'

Me

yy R2

(16)

(17) a, b, c, d, 8,

R1 = R2 = H R' = OAC, R2 = H R' = OCOCMe3, R2 = H R' = OSiPh2But, R2 = H R'=OCOPh, W = H

The photochemical cycloaddition of ethene to the bis-butenolides (20) has been examined in an attempt to establish the influence of the ether-protecting groups of the diol system.20Generally only two adducts are formed as can be seen from the results shown for the appropriate structures. The most effective ether protecting group is the trimethylsilyl function and here the facial selectivity yields predominantly the anti,anti adduct (2 1). With the unprotected systems (20, R = H), there is virtually no selectivity and in this case the three adducts (21), (22) and (23) are formed. Irradiation of the butenolides (20a) and (20b) in the absence of ethene leads to intramolecular hydrogen abstraction (a Norrish Type I1 process) with the formation of the products (24a) and (24b) in 79% and 76%, respectively. A further example of photochemically induced addition to the enone double bond in (25) has been reported.21In this example irradiation using benzophenone as the radical producing agent in methanol results in a 51% yield of the adduct (26). Pete and his co-workers2*have reported the sensitized addition of tertiary amines such as (27) to the furanone double bond in (28). The reaction involves electron transfer from the amine to the sensitiser which ultimately

78

Photochemistry

a,

Et PhCH2 TMS H f?-R=C(Me)2

b,

30 5 83

44 51

10

-

2 38 14

0

0 (24) a, R' =Me (79%) a, R1 =Ph (76%)

yields a carbon-centred radical that adds efficiently to the enone double bond. The sensitisers used are aryl ketones such as benzophenone and the best results are obtained with 4,4'-dimethoxybenzophenone when a mixture of the products (29) and (30) is obtained in 94% yield. The same type of addition is also

observed between the amines and electron-deficient alkenes such as acrylonitrile or methyl a ~ r y l a t eThe . ~ ~facial stereoselectivity of this addition has been exploited in synthetic paths to some alkaloids. Thus the addition of the radical derived from the pyrrolidine (31, R = Me) to the furanone (28) retains the (3s)configuration. The transformation of the diastereoisomer (32) from this addition into (+)-laburnine (33) can be carried out smoothly and in good overall yield. Similar transformation of the diastereoisomer (34) from the addition of (31, R = But) to (28) affords (-)-isoretronecanol (35). The photo-

79

M2: Enone Cycloadditionsand Rearrangements

reaction is equally successful with piperidine based amines such as (36) which adds to the same enone to afford (37). In this instance only the one diastereoisomer appears to be formed.

(,) cNIR N Me

(31) R = Me R 3- Bu'

~o y Omenthyl

~

~ &OH

"

"\

Bu'

0

(32)

&OH

(33)

0 (34)

Me (36)

(37)

N (35)

0

A single electron transfer mechanism is involved in the phototransformation of the enones (38) into (39) in the presence of p h ~ s p h i t e sThe . ~ ~ reactions are carried out in acetonitrile and proceeded by the triplet enone to which an electron is transferred from the phosphite to give the radical catiodradical anion pair (A). Collapse of radical cation component of (A) gives (B) which then reacts by addition to the enone radical anion. The products (39) are isolated after hydrolysis of the corresponding silyl ethers. The influence of ring size and substituents was also examined and these results are given in Scheme 0

(38)

n 1 2 3 1 2 3

R1

R2

R3

H H H H H H 8 H H H 8 Me H H H M e H H H Me

/

(A)

R

(39) Yield (YO) = Me R=Et 92 9 1 86

82 91 78 87

5

57mixtureof products

Scheme 1

OSiMe3 I P(OR)2

Me3Si+/-P(OR)2

(B)

(40) Ar = ptolyl, 2,4,6-trimethylC6H2,2,4,6-tri-PriC,jH2

80

Photochemistry

1. Entries 5 and 6 show that only low yields of product are obtained when heavily substituted enones are used. Mase et al.25 have demonstrated the outcome of the addition of free radicals to the enone (40). The radicals were formed using excited benzophenone as the hydrogen-abstracting reagent. By this method radicals generated from 1,3-dioxolane added to (40, Ar = 2,4,6-triPriC6H2) to yield a single diastereoisomer. Further reports on the addition of 2,3-dimethylbut-2-ene to enones such as (41) have been made.26

1.1.3 Additions to Cyclohexenones and Related Systems - Irradiation of 3-methylcyclohexenone in methanol solution in the presence of the ester (42) results in the synthesis of adduct (43) in moderate yields.27This product was used as the starting material for an approach to the synthesis of trichodiene. The enone (44)undergoes photochemical reaction with 2,3-dimethylbut-2-ene in benzene or acetonitrile solution and using 350 nm light.28The four products were identified as (49, (46), (47) and (48). The formation of the oxetane (45) follows the conventional route and the cycloadduct (48) arises via the biradical (49). Hydrogen abstraction can also occur within this species to give (47). The major product is also formed via intermediate (49). The structural rearrangement involves interaction with the carbonyl group to afford the new biradical (50) which subsequently undergoes ring contraction to yield (51) from which (46) is produced.

(44)

(45) 18%

(46) 36%

1112: Enone Cycioadditions and Rearrangements

81

The chromone (52) undergoes photochemical addition of ethene.29 The primary product from this cycloaddition, presumed to be (53), is photochemically reactive and is converted into (54) and (55). The former of these is a key intermediate in a synthetic strategy to tricothecene analogues. Both (54) and (55) arise via the Norrish Type I1 reactivity of (53). Thus hydrogen abstraction from the methoxy substituent by the excited carbonyl group results in a 1,4-biradical that either ring closes to (54) or fragments with the loss of methanal to yield the enol of (55).

Acetone-sensitized addition of 2,3-dimethylbut-2-ene to the enone (56) affords both the (2 + 2)-cycloadduct (57) and the cyclopentanobenzofuran (58).30 The latter product arises by a stepwise addition of the alkene to the enone ethene bond and the attached cyano group. The authors suggest that this mode of addition arises from an upper excited triplet state and similar behaviour is observed with (59) to give the adduct (60).

Photodimerisation of (61) affords the cis,anti,cis-head-to-tailproduct (62).3 The position of the fluoro substituent appears to play some part in the outcome of the reaction since irradiation of the isomer (63) affords the cis,syn,cis head-to-head dimer. Further studies of the influence of fluoro substituents on the dimerisation of the styrylcoumarins (64)have also been reported.32 Intramolecular Additions - The predominant photochemical reaction of the allene derivatives (65) is (2 + 2)-cycloaddition yielding the housanes (66).33 The reaction occurs in a variety of solvents (e.g. hexane, acetonitrile or acetone) and the triplet excited state of (65) is implicated. In hexane and 1.2

82

Photochemistry

(62) R4

acetonitrile the cyclopropane derivative (67) is formed on irradiation of (65, R' = R2= H) but only in 10% yield. Similar reactivity is seen for the less heavily substituted derivatives (68) but in toluene the cyclopentenes (69) and (70) are formed which suggests that a cyclopentane-1,3-diyl radical is involved. The enone (71) undergoes a photochemical (2 + 2)-cycloaddition to afford the novel 3-azabicyclo[3.1.1Jheptan-2-one (72) which has been used as a synthetic route to new glutamate analogues.34

4

C02Me

(68) R = HorMe

(69)

1112: Enone Cycloadditions and Rearrangements

83

Winkler and his c o - ~ o r k e r shave ~ ~ previously reported the vinylogous amide photocycloaddition. In their present account, the highly diastereoselective cyclization of (73) to afford (74) is described. This product affords a basis for a synthetic strategy towards the manzanine alkaloids.

1.2.1 Intramolecular Additions to Cyclopentenones - The enones (75) fail to undergo (2 + 2) cycloaddition when irradiated.36 The only photochemical reaction observed is reduction of the remote double bond. The authors suggest that the failure of the cyclization is a result of interaction between the nitrogen lone pair and the ethene bond. When the interaction is minimized by the acylation of the nitrogen, normal (2 + 2)-cycloaddition becomes efficient giving high yields of the cage compounds (76).

R ".' (75) R'

=

H, R2 = CyClOheXyl

R1= H,

R2 = benzyl R1 = R2 = morpholino

\

6

R2

R1 Bn Bn Bn

0

(76) R2 Yield (YO)

COMe C(0)OMe COCH2Ph C02CH2Ph

99 67 91 81

1.2.2 Additions to Cyclohexenones and Related Systems - Two modes of initial bonding are possible in most (2 + 2) photocycloaddition reactions of enones. Thus both 1,6- or 1,5-ring closure can occur. The intramolecular cyclizations involving the enone (77, X=CH2) have been examined both in solution and absorbed in zeolite^.^^^^* For the latter case, the authors report that there is an increased yield of products from the 1,6-ring closure path compared with solution phase chemistry. A further observation reported was that within the 1,5-ring closure path the amount of cis-fused products formed is greater from (77, X=CH2 or 0) when they are in the zeolites. It is suggested that the influence of the zeolites on the outcome of the reaction is controlled by binding of the enones with the cations within the zeolite cages.

84

Photochemistry 0

(77)X = CH2 or 0

The intramolecular cycloaddition of the enone derivative (78) affords the adducts (79) in yields of around 58%.39 The advantage of this compound for synthetic purposes is that the alcohol group is functionalized by a novel protecting group. The adducts (79) can be converted into derivatives such as (80). Four intramolecular cycloaddition products (82) and (83) are formed on irradiation of the naphthalenone derivative (8 1).& Changes in the regiochemistry are observed when the irradiations are carried out on silica surfaces.

k & O

qWH

? Q

0

R

R

(79) R = H or Me

(82) R' = Ph, R2 = H R' = H, R2 = Ph

R (80)

(83) R' = Ph, R2 = H R' 5 H, R2 = Ph

The unsaturated 6-lactone (84) undergoes intramolecular photochemical (2+2)-cycloaddition to give (85).41 The scope of the process has been evaluated and the 6-lactones (86-88) all behave similarly, affording the products shown in Scheme 2. The lactones (86) and (87) both cyclise in the two possible modes. Intramolecular cycloaddition reactions within polymethyldiaminebis(4-methyl-7-coumarinyl)oxyacetamides have been described.42 Zhu and Wu43have reported that a biscoumarin system linked by a phenanthroline unit does not undergo photodimerisation. Instead, intramolecular addition of a coumarin unit to the phenanthroline occurs. Photoinduced electron transfer from the enone (89) to DCA in acetonitrile results in opening of the cyclopropane ring.44 The resultant radical (90) undergoes cyclization to the enone moiety to give the isomeric compounds (91)

IIJ2: Enone Cycloadditionsand Rearrangements

-

85

A q0

AcO

Me0

+

85%

0

MeO

0

It

75% Scheme 2

0

0

in a total yield of 63%. Another example of this cyclization utilised the substrate (92) which gives (93) in a low yield. Radical intermediates arising from C-Br bond fission are reported in the photocyclizations observed with the enaminone (94).45 The two products identified from the reaction are the debrominated starting material and the pyridoindolone (95). The photocyclization of the enone derivatives (96) occur enantiomerically in aqueous suspensions of their inclusion compounds with optically active diols such as (97).46

86

Photochemistry

(94) R = Br or Me

2

Rearrangement Reactions

2.1 a,fl-Unsaturated Systems - The photochemical behaviour of acryloyl chloride (98) in an argon matrix at 10 K has been studied in Irradiation at h > 310 nm results in a 1,3-chlorine migration with the formation of the chloroketene (99). CI

/

2.1.1 Isomerisation - The influence of aryl substituents upon the photoisomerism of methyl-a-phenylcinnamates has been studied,48 and reversible trans,cis-isomerism of p-methoxycinnamic acid is reported to occur on irradiation in polar solvents.49 Irradiation of the cinnamides (100) in methanol induces efficient trans,cis-isomerism.

(100) R' R' R' R' R'

= PhCH2, R2 = H = CsH11, R2 = H = Pr", R2 = H = PhCH2, R2 = Me =

R2 = Et

2.1.2 Hydrogen Abstraction Reactions - The taxine derivatives (101) undergo smooth photochemical conversion into the tetracyclic derivatives (102) in a reaction which involves hydrogen abstraction by the a-carbon of the excited state enone from the transannular site labelled 'a'? The resultant biradical ring closes to afford the final products.

IIl2: Enone Cycloadditionsand Rearrangements R4Q

4 H

87

OR3

R4Q

u I I

OR1

OR3

’ I

OR’

PhCH=CHCO H PhCO PhCO H

H PhCO H PhCO

AC

AC

AC

Ac PhCH=CHCO PhCH=CHCO PhCH=CHCO PhCH=CHCO PhCH=CHCO PhCH=CHCO H

AC

AC

AC

AC

AC

AC

AC AC

Ac

AC AC

H H

H

II

VI I

AC

H

2. I . 3 Rearrangement Reactions - Previously Matsumoto et aL5* had reported that a,P-unsaturated compounds such as (103) were photochemically reactive. Irradiation with a high-pressure mercury lamp in benzene solution results in their efficient conversion (4 = 0.1) into 1,4-diketones and crossover experiments have demonstrated that the rearrangement is truly intramolecular. Further work on this system has been reported and deals with the conversion of the enones (103) into fur an^.^^ Thus irradiation of (103) in benzene for 6 h affords the dihydrofuran (104) which on elimination of methanol gives (105) in moderate to good yields. Irradiation at 254 nm of 2-pivaloylcyclohex-2enone in propanol brings about its conversion into tetrahydrobenzo[c]furan4-0ne.~~ 0

OMe

Me Ph Me pMeC6H4 Me pMeOC6H4 Me pCIC6H4 Ph Me

72

87

54 52

55

The dienones (106) are well known to undergo photochemical conversion into the cyclopentenones (107). Fleming et aZ.55have shown that irradiation of the enones (106) in methanol and ethanol yields the two products (108) and (109). This transformation involves a Norrish Type I1 hydrogen abstraction within the cyclopentenone derivatives ( 107). The 1,4-hydrogen transfer results in the formation of the biradical(ll0) which cyclises by two paths to give (108) and (109).

88

Photochemistry

Me

Me

Me Me

Me

Me

Et

H

Me

Me Me

2.2

Et

MeOH EtOH MeOH EtOH MeOH EtOH

30 45 23

35

20 33

29 19 14 14 17 14

P,y-Unsaturated Systems

2.2.1 The Oxa Di-n-methane Reaction and Related Processes - Acetonesensitized irradiation of the enones (111) and (112) results in smooth and efficient conversion into the products (1 13) and (1 14) re~pectively.~~ Such rearrangement products are of value as starting materials in the synthesis of naturally occurring compounds and in the present example they were used in a new synthesis of capnellene. Rearrangement of the P,y-unsaturated enone (115) on irradiation through Pyrex in benzene solution has been reported.57 The product (1 16) (42% yield) is considered to be a useful starting material in the synthesis of phorbol and arises by a 1,3-acyl migration during which decarbonylation occurs. The photochemical rearrangement of (1 17) has been described.

Me Me H H MevMe

47 53

IIl2: Enone Cycloadditions and Rearrangements

89

The enones (118) undergo both Norrish Type I fission and an oxa-di-nmethane process on direct irradiation in benzene solution.59The decarbonylated product (1 19) predominates and the oxa-di-n-methane product ( 120) is minor under these conditions. However, the oxa-di-x-methane product (120) becomes the dominant reaction mode when irradiation is carried out under acetone sensitisation. The decarbonylation product (1 19) arises by Norrish Type I fission, decarbonylation and intramolecular SET to afford the zwitterion (121) which is trapped by water. Elimination of methanol then affords the final product. XVR

Me02C

3

Me0 OMe

t'

H R

C02Me

C02Me

OMe OMe

Photoreactions of Thymines and Related Compounds

3.1 Photoreactions of Pyridones - The pyridone derivative (122) undergoes ready (2 + 2) head-to-head photochemical addition when irradiated in acetone.60The reaction appears to be very facile and requires only 16 minutes irradiation at 5 "C to give a 79% yield of the adduct (123) which has been used as the starting material in a total synthesis of ( -)-perhydrohistrionicotoxin ( 124).

Pyrex-filtered irradiation of methanol solutions of the pyridone (125) results in the formation of the (2 + 2) cycloadduct (126).61This photoisomer is not, however, the primary photochemical product and the route to (126) is thought to involve (4 + 4)-photocycloaddition to yield the adduct (127). This adduct is thermally unstable and undergoes a facile Cope rearrangement to yield (126).

90

Photochemistry

Irradiation of methanol solutions of mixtures of the pyridones (128) and (129) result in the formation of cycloadducts.62The methoxypyridone (128) does not dimerise but will undergo a cycloaddition with (129) and this leads to the adducts (130) and (131). The pyridone (129) does dimerise and in competition with the cycloaddition affords the two dimers (132) and (133). The cycloadduct (130) was the product of the greatest interest and the best yields (51%) were obtained using a ratio of 7 : 1 of (128) to (129) respectively. Further synthetic studies have been carried out with adduct ( 130).63 OMe

Bun\

U0

0?N\Bu"

3.2 Photoreactionsof Thymines etc. - A detailed study of the photoreduction of thymine and uracil to (134) on irradiation at 254 nm in the presence of hypophosphite has been reported.64Addition of methanol occurs when (135) is

irradiated at 254 nm. The initially formed products such as (136) are unstable and readily, either thermally or photochemically, eliminate HF to give (137) or CH30F to yield (138). A further product (139) is also obtained.65A study of the outcome of irradiation at 302 nrn of 5-iodouracil containing deoxyoctanucleotides has been carried out.66 6-Chloro-1,3-dimethyluracilundergoes 1,2-Addition to benzene on irradiat i ~ n A. ~1,3-addition ~ path has been discounted. Other studies by the same group have demonstrated that the irradiation of the pyrimidine dione derivative (140) in acidic media (a large excess of trifluoroacetic acid) brings about its conversion into the tricyclic product (141)?*

IIl2: Enone Cycloadditions and Rearrangements

(140)

91

(141) R = CF&O2, OH Of Ph

Single crystals of thymine derivatives with long alkyl-chain substituents are photochemically reactive and undergo (2 + 2) photodimerisation to yield solely the trans-anti dimer.69 In solution, however, the photoreaction affords the usual four cycloadducts. Irradiation of the bis-thymine PNA dimer (142) brings about intramolecular cycloaddition to give the adduct (143) in 50% yield.70The reaction is carried out in water using irradiation at 254 nm. The results from a study of the photochemical cycloaddition within the thymidilyl system (144) has been reported.71 Photoadducts have been obtained from the

H

n

92

Photochemistry

irradiation of caffeine in the presence of some hydroxyflavylium salts.72The reactions of caffeine, theobromine and theophylline with benzophenone in ethanol solutions have been described.73 Photoadducts are produced on irradiation of DNA in the presence of chl~rpromazine.~~ Photochemical monomerisation of the cyclobutane dimers (145) can be brought about effectively using tetra-O-acylriboflavins as the sensiti~er.~~ The reaction is efficient when carried out in aqueous solution with surfactants such as sodium dodecyl sulfate and sodium hexadecyl sulfate. A review has highlighted the many methods available for the photocleavage of nucleic acids.76 I

MxJj-f-x;

0

MeH H Me (145) R = Me or H

The thymidine derivatives (146) and (147) undergo cleavage of a C-C bond on i r r a d i a t i ~ n .These ~ ~ reactions are typical Norrish Type I processes and provide a route to study C-3’-DNA radicals. Hydrogen abstraction by the radicals yields a 1 : 1 mixture of the threo and erythro derivatives (148). The reactions from the p-isomers (146) are generally more efficient than from the a-isomer (147). A study of the photochemical reactivity of the deoxyuridine derivative (149) has been reported.78 This novel compound is an electronaccepting nucleo base. It has been used as a means of cleaving DNA. The photochemical fission occurs specifically at the 5’-G of SGG3’ sequences.

$xo

&

DMTrO

DMTrO

fxo

H$yRCO

RCO

HO (146)

0

R = Me, Ph or Bu’

HO

(147)

Yield (YO) Me Ph

Bu‘

Hd

$xo

@

DMTrO

20 35

79

(149)

(148) 65

33 92

IIl2: Enone Cycloadditions and Rearrangements

93

The photophysical behaviour of a series of methylated angelicins has been recorded using flash photochemical technique^.^^ Irradiation of the complex formed between 4,6-dimethyltetrahydrobenzoangelicinand DNA results in the formation of cycloadducts.80These arise by addition between the pyrimidine bases, thymine and cytosine and the furan ring of the angelicin.

3.3 Miscellaneous Processes - The formation of cis-dimers is reported to occur when 1,4-dihydropyridinederivatives are irradiated in solutionmgl 4

Photochemistry of Dienones

4.1 Cross-conjugated Dienones - The cross-conjugated dienone (150) undergoes photochemical cyclization to afford the product (1 51) in low yield.82This study is a repeat of earlier work in which the reaction was claimed to be more efficient.83

‘The dependence of the photochemical rearrangement of the dienones (1 52) on wavelength has been assessed.84 Direct irradiation of (152) affords the rearrangement products (153) and (154) and measurement of the quantum yields of product formation for both the direct and the sensitised irradiation shows that a triplet excited state is involved. The products (153) and (154) are formed in a photostationary state and are interconverted by way of a cyclopropane bond fission process. Both (153) and (154) are converted into the phenol (1 55) on prolonged irradiation. Some years ago West and his co-workers studied the photochemical behaviour of pyrones such as (156) and reported that the ring contracted

4 0

Me0

R’ R2

&R2 Me0

P

Me0

.H

d,

Me

Me

R

.H

’

Me0

Photochemistry

94

bicyclic zwitterion (157) was formed. When a suitable functional group is present intramolecular trapping results in the formation cyclopentenone derivatives. Several examples of this are shown in Scheme 3. The reactions are efficient and are stereospecific and the present work has also shown that reduction of the products (158) to (159) is also efficient.85Furthermore, (159) can be transformed by thermal means to yield medium ring ketones (160) and bicyclic ethers (161). Other examples of the cyclization and reduction path are shown in Scheme 4.

n

R

a, H

Yield (YO)

(158)

2 62 2 75 Me 1 67 Scheme 3 Me

RQ2"c 0

R--H&fl

d

( 159)

(157)

Me

o

OH

OMe

a (160) 20%

-

a OH (161) 20%

50

Me--

Scheme 4

o

M$

OH

Other researchers have also investigated the involvement of zwitterions in the cyclization of the dienone (162) to give bicyclic products in the presence of electron-rich ethenes.86The key reaction is the cis,trans-isomerism of the enone to afford the highly reactive dienone (163) which cyclises to an oxyallyl intermediate that reacts with the ethenes (e.g. vinyl ethers). Cyclization within the resultant intermediate, possibly a zwitterion (e.g. l a ) , can account for the formation of both the tricyclic ether (165) or the bridged ketone (166). The yields obtained are shown for the appropriate structures. The reaction also takes place with alkenes and, for example, using 2-methylpropene the adduct (167) is formed.

IIl2: Enone Cycloadditions and Rearrangements

OEt H H H H OMe Me H H OEt OEt H Me OMe OMe Me OMe OMe OMe OMe

95

51 39 49 30 23

19 34 24 14 45

The photophysical data for the furanochromones (168) have been measured .87

R

(168) R = H or Me

4.2 Linearly Conjugated Dienones - The quinonemethide (169) has been prepared by flash vacuum pyrolysis.88 Irradiation at 7.6 K of the quinonemethide (169) at h > 340 nm results in ring closure and the formation of the 4,6-dimethylbenzoxete (170).

Irradiation through Pyrex of crystalline mixtures of the pyrone (171) and maleimide (172) results in the formation of the (2 + 2) cycloadduct (173) which is different from the solution phase behaviour when products arising from a (2 + 4)-cycloaddition mode are obtained.89 The (2 + 4)-cycloadduct is unstable under the experimental conditions and rearranges into the (2 + 2)-cycloadduct

96

Photochemistry

(174). In addition decarboxylation of the (2 + 4)-adduct occurs to give a diene that is trapped as (175) by a second addition of maleimide. A further report by the same group states that the cycloaddition of maleimides with 2-pyrone carboxylates in the solid-state yields endo ad duct^.^^ This is in contrast to the sensitized cycloaddition that leads exclusively to the ex0 products.

T-f HoYNH

MenoMe 0Y I?

QH

0

(171)

0 (172)

0 (173)

Me

Me

Me0 o+$

Me

NH

0 (175)

Irradiation of the tropolone ether (176) in unmodified NaY zeolite results in the formation of the racemic cyclopentenone (177).91When a chiral auxiliary is used in this system, enantioselectivityis observed. A variety of chiral auxiliaries have been used with the best results being achieved with (-)-norephedrine and RbY zeolite to give an ee of 40%. The authors suggest that the results imply that there is a three-point interaction between the tropolone, the zeolite and the chiral auxiliary and that this induces a preferential absorption of the starting material from a single prochiral face. This selects one of the two modes for the cyclization of (176) affording a system rich in either (178) or (179). Changing the chiral auxiliary to (+)-norephedrine, for example, yields a system rich in the other enantiomer. Four products (180), (181), (182) and (183) are formed when tropone is irradiated in the presence of DCA in benzene as the solvent using h > 400 nm.92An electron-transfer mechanism is proposed to account for product formation.

5

1,2-, 1,3- and 1,4-Diketones

5.1 Reactions of 1,2-Diketones and Other 1,2-Dicarbonyl Compounds - The photoreduction of methyl phenylglyoxalate has been reported.93Oxygen does not have an adverse effect on the disappearance of starting material during the irradiation of phenylglyoxalate esters with y-hydrogens. Under these conditions the usual 1,4-biradicals are formed but these are trapped by oxygen thereby giving products different from those obtained under anaerobic condi-

1112: Enone Cycloadditions and Rearrangements

97

A

OH

I

I

NC

ti on^.^^ The photochemical reactivity of a series of alkyl thiopheneglyoxalates

and alkyl furanylglyoxalates has been studied.95 The photodecomposition processes are inefficient and only traces of Norrish Type I1 products were obtained. The excited states involved in these compounds are thought to be n,n* in character. Cycloaddition reactions could be carried out with electronrich alkenes and oxetanes were obtained. The photochemical decarboxylation of oxalic acid in the presence of TiOz has been r e p ~ r t e d . Irradiation ~ ~ ? ~ ~ at 351 nm of oxalyl chloride results in population of the singlet excited state and cleavage of the C-Cl bonds occurs. This fission of the C-C bond occurs from the second singlet state which is populated by irradiation at 248 nm.98 In another study both oxalyl chloride and butan-2,3-dione have been shown to undergo decomposition on irradiation at 193 and 248 nm.99Again the likely mechanism for the reaction is fission of the central C-C bond to afford acyl radicals. The photochemical decarbonylation of the bis-ketenes (184) is favoured when electronegative substituents are present. loo The decarbonylation affords the cyclopropenones (185) which themselves undergo loss of CO to yield the corresponding alkyne. Details of the kinetic behaviour of the bis-ketene (186) formed by irradiation of the diketone (187) have been reported.lol A further study of photo doubledecarbonylation of 1,2-diketones has examined, amongst others, the behaviour of (188) and (189). The diketone (188) readily undergoes decarbonylation with the formation of the corresponding diene (190) but interestingly the diketone (189) behaves like a P,y-unsaturated enone and isomerises to the diketone (191) by a 1,3-migration pathway. The diketone (191) is readily decarbonylated by irradiation at 436 nm. (2 + 2)-Photocycloaddition to (192) followed by a 1,3-acyl migration has

98

Photochemistry

been used as essential the steps in a new synthesis of (193) which is a useful intermediate in the synthesis of homoerythrinan alkaloids.lo3 Photoaddition reactions of simple alkenes (195) to the enones (194) have been described.lW (2 + 2)-Photocycloaddition results in the formation of the adducts (196) and phenylacetylene also undergoes cycloaddition to this substrate. Interestingly when the substituent on the nitrogen in (194, X = NR) is large (R = Ph or pMeC6H4) a different reaction mode is observed and products such as (197) from ethoxyethene are formed. The cycloadditions are visualised as arising via a two-step process involving radical intermediates. Thus, the path to (197) is suggested to occur by bonding within the intermediate biradical (198) to yield (199). Ring opening of (199) followed by a 1,2-acyl shift yields the final product.

(194) X = 0, S, NH or NMe

(193)

(1 95) R = OAc, Ph or OEt

(196)

99

IIl2: Enone Cycloadditions and Rearrangements

The irradiation of the 6-methoxybenzofuran-2,3-dione (200) in the presence 5H)-furanone as of styrene affords 3-(2-hydroxy-4-methoxyphenyl)-4-phenyl-2( one of the products.1052,3-Dimethylbut-2-ene and 2-methylpropene undergo photoaddition to the C9-ClO double bond of 2H,8H-benzo[1,2-b : 3,4b]dipyran-2,8-dione. *06 The indanetrione (201) yields an oxetane when irradiated in the presence of the ethene (202) or 2-methylbut-2-ene, but hydrogen abstraction reactions predominate when the alkene is 2,4,4-trimethylpent-1e~e.~O~

5.2 Reactions of 1,3-Diketones - The carboximide derivative (203) undergoes photochemical rearrangement on irradiation in acetonitrile using Pyrex filtered light.lo8 The reaction is considered to proceed by a zwitterionic intermediate such as (204). When the reaction is carried out in the presence of P-ketoesters [e.g. (205)] with added triethylamine, adducts are formed in yields of 40-60% and have been identified as (206). The intermediacy of (204) is supported by the formation of (207).

Me0 EtO (205) R = Ph, M e or CH2CaEt

0

(206) Y =

ye C=NOMe

Ph (207)

The 1,3-diketones (208) isomerise on irradiation at 300 nm in benzene solution.'09 The products are the lactones (209) and they are formed in high yield as a mixture of the two possible isomers (209a) and (209b). The reactions are proposed to occur from a short-lived singlet state since sensitisation was ineffective. When acetophenone was used as a sensitiser the dione (208d) did

100

Photochemistry

rearrange but the reaction was complicated by the addition of the sensitiser to the ethene bond to yield the oxetane (210) as the main product. The sunscreen Parsol 1789 is converted into a 1,3-diketone from its enol form on irradiation in dilute solution. l o

OQ-. R' R2 (208) R'

R'

Me Me Me Et Ph Me d -(CH2)4e, -(CH2)6f, -(CH2)12a, b, c,

B$ R2

R2

(209a) R2 Yield (YO)

I

92 95 96 96 25 92

R'

(209b)

3:2 3:2 3:2

The iodonium salt (2 1 1) adds alkenes on irradiation. I The photochemical reactions are carried with the salt as a suspension in acetonitrile or methylene chloride using a 400-watt lamp. The products obtained were identified as the dihydrofuran derivatives (212) and the best yields were obtained with electronrich ethenes such as the enol ethers (Scheme 5).

R'

R2

R3

Ph PhCH2 Ph

EtO

H

H

H

H

58 22 37 86 60

aCH2 70

CHzCHCHp OCH2CH2

H H

H

Yield (YO)

74 68

40 -(CH2)4OCH2CH2CH2

26 60

Scheme 5

5.3 Reactions of 1,4-Diketones - Brief irradiation of the imide derivatives (213) in acetonitrile leads to their conversion into bicyclic (214) or tricyclic

IIl2: Enone Cycioadditionsand Rearrangements

101

'*

products (219.' Full details have been published describing the photochemical addition of alkenes to the anhydride (216).' l4 Diels-Alder adducts can be formed in high yield by the irradiation of mixtures of maleic anhydride The efficiency of the photoor maleimides in the presence of anthra~ene."~ reactions and the dependence on the concentration of dienophile have been measured and an electron-transfer path is proposed. 13y1

RgNR

R'

R'

0 (213) R ; . R ' = H , n = 3 R=Me, R'=H, n = 3

;& ;Q--. 0 (214) R = H (510/0) R = M e (99%)

0 (215) R = H, R' = R2 = (CH& 50% R = Me, R' = R2 = (CH& 83% R-R = (CH2)4 89%

The solid state irradiation of the o-aroylbenzothioates (2 17) yields the phthalides (218). l6 This rearrangement involves a 1,4-aryl migration from the aroyl group to the thio moiety and the yields of products are high at low conversion. Some substrates, (2 17a-c), undergo rearrangement with a level of enantiomeric excess which can be quite high as shown by the data under the appropriate structure.

5.3.1 Phthalimides and Related Compounds - A detailed study of the formation of the ylide (219) from irradiation of the phthalimide derivatives (220) has been reported.' l 7 Suau et a1."* have examined the irradiation of phthalimide in the presence of a low concentration of hydroxide ion and alkenes (221). The

Photochemistry

102 0

OR

result of this treatment is addition of the phthalimide moiety to the alkene. A SET mechanism is proposed and in general the yields of adducts (222) range from good to excellent as shown by the data under the appropriate structure. In the case of cyclohexene, the initially formed adduct (223) undergoes secondary photolysis and is converted into the ring-expanded product (224) by a Norrish Type I1 hydrogen abstraction path. The irradiation of phthalimides (225), derived from a variety of amino acids in the presence of carboxylates such as potassium propanoate has been described. l9 The process brings about decarboxylation of the potassium propanoate to afford an ethyl radical which adds to the phthalimide derivative to give (226) in yields ranging from 51 to 89%. The photodecarboxylation of some a-phthalimido carboxylates has been developed as a path to macrocyclic ring systems.120 R'

I

R

3

4Ph

R2

H

H

H H

Ph H

R+ : $ - ' f

H

H

0

H H OMe

53 90 44 71 70

C02Me

0

(225) (Gly, Ala, Val, Leu, Ile, Phe, Phg, Asp, Glu)

The naphthalimide (227) undergoes a SET process with arylalkenes such as p-xylene.1 2 1 The initial process yields a radical cationhadical anion pair within

Ul2: Enone Cycloadditions and Rearrangements

103

which a proton is transferred from the alkyl group of the xylene giving the radical pair (228). This species then reacts to afford the two products (229, 23”/0) and (230, R = H, 31%). If methanol is present this second product is produced as the methyl ether (230, R = Me). A study of the electron transfer processes in the naphthalimide derivatives (231) has been reported in Me I

Me

Me

Me I

Me O

\

*.

(229)

N

0

OR \ (230) R I H or Me

Me

(231) R’ = H, R2 = N M e R’ = NH(CH2)3NMe, R2= NMe2 R’ H, R2= +NMe3 R’ = NH(CH2)3+NMe3 r, R2 = +NMe3 W=H, R ~ = M ~

5.3.2 FuZgides and Fulgimides - A theoretical study of the photochromic compound 2,3-bis(2,4,5-trimethyl-3-thieny1)maleic anhydride has been reported.123The photochromic properties of the indyl fulgide (E)- I-benzyl-2methyl-3-indylethylindene(isopropylidene) succinic anhydride have been studied.124The photochromism of (232) and (233) has been in~estigated.’~~ The cyclizations are reversible and the materials show reasonable fatigue resistance.

y phA fCN

Me Me Me /’

OMe Me

(232) R

=H

‘

or CH=CH2

R

Me

(233)

Ph

Photochemistry

104

6

Quinones

6.1 o-Quinones - A report has dealt with the photochemical addition reactions of a series of o-benzoquinones with 1,3-diket0nes.l~~ The products from this process were identified as keto-oxetanes and these ring-open to afford 1,5-diketones. The photochemical addition of diphenylacetylene to oquinones affords two isomeric quinomethanes. The reactions occur by cycloaddition of the alkyne to a carbonyl group and the resulting oxetene thermally eliminates benzaldehyde to yield the 2 and E isomers of the quinomethane.

6.2 p-Quinones - Electron transfer photochemistry of quinones (234), (235) and (236) has been reported.'** A detailed examination of the photochemical reaction of p-chloranil with trans-stilbene which gives the spiro-oxetane (237) has been r e ~ 0 r t e d . In l ~ a~ further examination of this reaction several stilbene derivatives (238) were employed. All of these form oxetanes and, for example, (239, 440/0) and (240, %YO)are formed by addition of chloranil to 4-chlorophenylstilbene. The irradiations are carried out using wavelengths > 480 nm in dioxane solution which specifically excites the CT band. Single electron transfer is a dominant process and this yields the singlet ion radical pair. This work has established that an electron transfer is the first step within the cycloaddition of alkenes to the chloranil. When the carbonyl chromophore of the quinone is excited specifically the results are the same as for CT excitation.I3* Coupling products are formed when p-chloranil is irradiated in the presence of 3P-methoxycholest-5-ene in acetonitrile solution. An electron transfer mechanism is thought to be operative.

CI

GC1 0

(234)

Me

Ph

I

105

IIl2: Enone Cycloadditions and Rearrangements

The vinyl groups of the quinones (24 1) undergo (2 + 2)-cycloaddition on irradiation in the crystalline phase. 132 There is no evidence for the involvement of the quinone ethene bonds in the formation of the dimers and oligomers produced on irradiation. Iwamoto and co-workers' 33 have described the photochemical reactivity of the quinone derivatives (242) which undergo cyclization to the biradical (243). Subsequent hydrogen transfer affords the benzofuran derivatives (244) quantitatively in most cases. This reaction path is followed exclusively when there are no abstractable hydrogens. When the derivatives (245) were investigated only low to moderate yields of (246) were obtained. This decrease in efficiency is thought to be due to a hydrogen abstraction path leading to the biradical (247) which occurs in competition with the cyclization via the biradical(243). *co*R

fl::

$f:: I 1

I I

I I

R02C \

0

(241) R = Me, Et, Pr, CHMe2, Bu or PhCH2

0

0 (242) R' a, Ph b, Ph

Ph

c,

d, Me 8, Me 1, Me

/

OH (243) (244) R2 Yield (YO) H quantitative Me quantitative Ph quantitative H 46 Me 21 Ph 28

The vinyl quinones (248) undergo different photochemistry. When these are irradiated a quantitative yield of dimers (249) is obtained. A report has been published dealing with the photocyclisation reactions of benzoyl benzo- 1,4q ~ i n 0 n e s .Al ~laser ~ flash study at 248 nm of vitamin K3 has been reported and

JyAr 0

0

(248) (249) Ar = a, Ph or b, CICeH4

'

106

Photochemistry

the results have indicated that this vitamin is an effective electron-transfer agent.135 Irradiation of the anthraquinone (250) brings about a 1,6-hydrogen migration from one of the carbon atoms adjacent to the nitrogen in the piperidine s ~ b s t i t u e n t . This ' ~ ~ process results in the formation of the ylide (251). Group migration involving the p-Bu'C6h moiety takes place in two modes upon irradiation of the anthraquinone (252).137The first mode yields the 1,lOquinone (253) by migration to the quinone carbonyl group and the second path yields the hydroxyquinone (254) by migration to the amino substituent. A photophysical study of methyl and dimethylanthraquinone has been reported.*38The anthraquinone (255) can act as a photochemical DNA cleaving reagent. 39 Electron transfers within porphyrin-quinone cyclophanes have been studied. 1407141

BU'

I

A nanosecond flash study of the hydrogen abstraction processes of acenaphthenequinone has been carried out. 14*

References 1. 2. 3. 4. 5. 6.

F. Toda, K. Tanaka and M. Kato, J. Chem. Soc., Perkin Trans. I , 1998, 13 15. T. Nakamura, K. Takagi and Y. Sawaki, Buff. Chem. Soc. Jpn., 1998,71,909. T. Nakamura, K. Takagi and Y. Sawaki, Mof. Cryst. Liq. Cryst. Sci. Technol., Sect. A , 1998,313, 341 (Chem. Abstr., 1998,441677). M. Ghosh, S. Chakrabarti and T. N. Misra, J. Raman Spectrosc., 1998, 29, 262 (Chem. Abstr., 1998,292739). P. R. Bangal and S. Chakravorti,J. Photochem. Photobiol., A , 1998, 116, 191. X. Coqueret, J. Photochem. Photobiof., A, 1998, 115, 143.

IIf2: Enone Cycloadditions and Rearrangements 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17.

107

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18.

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2806.

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I10

Photochemistry

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3

Photochemistry of Alkenes, Alkynes and Related Compounds BY WILLIAM M. HORSPOOL

1

Reactions of Alkenes

1.1 &,trans-Isomerization - ab initio Calculations have been carried out dealing with the photochemical isomerism of ethene. Irradiation of the chiral alkene (1) using wavelengths > 280 nm affords the corresponding cis-isomer in a one-way isomerism. Irradiation of this isomer affords a photostationary state (55:45) of the cis and the other trans-isomer (2).* A study of the quenching of the fluorescence of two naphthylindonylethenes has been r e p ~ r t e dAliphatic .~ amines were effective in the quenching studies. Both polar and non-polar solvents were used. The influence of the quenching on the isomerism of the alkenes was examined.

Further studies on the photoisomerization of cis-cyclohexene and cycloocta1,3-diene have been r e p ~ r t e dAgain .~ the work has focused on enantiodifferentiation. In this case a series of optically active chiral sensitisers (3) have been used under conditions where solvent and temperature have been varied. Some of the o-disubstituted and tetra substituted amide sensitisers afford mixtures with enantioisomeric excesses of 14%. The influence of pressure and temperature on the asymmetric photochemistry of cyclooctene has been reported? A variety of chiral sensitisers were used. Some of these are shown in (4). Other work has shown that aromatic phosphates, phosphinates and phosphines (e.g. 5-8) can also sensitise the isomerism of cyclo~ctene.~ Moderate stationary-state ratios were obtained. ~~~

~~

~

~

Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 112

111'3: Photochemistry of Alkenes, Alkynes and Related Compounds

I13

co*w I

(4) X = H or 3,4 or 5 CQR'

X

Results from an examination of energy transfer within steroidal molecules have been pre~ented.~ This work has examined the derivatives (9) and (10) among many others. Triplet-triplet energy transfer is detected by the isomerism of the remote alkene in ring D following excitation of the DPSO group and detailed kinetic studies were carried out.

1.1. I Stilbenes and Related Compounds - Isomerism of stilbene continues to be an area for detailed examination, The uses of photochemically active stilbene systems have been reviewed.* Studies have been directed towards the isomerism of stilbene within the constrained environment of zeolite^.^ The results show that irradiation at 254 nm in cyclohexane affords a cis: trans ratio of 76:24 whereas within the zeolite there is always a preference for the transisomer. Thus in NaY zeolite the cis: trans ratio is 5 : 9 5 and 24:76 in KY

Pho tochernistry

114

zeolite. At 313 nm the irradiation in cyclohexane gives a cis: trans ratio of 96:4 and there is still an influence of constraint in the zeolites at this wavelength as the ratios are 45 :55 in NaY and 24 :75 in KY. Clearly environment is important and can determine the outcome of stilbene isomerism.'O Thus, while stilbenes immobilised on a quartz surface do undergo photoisomerism the rate of isomerism is three or four times less than that in the free state. The photophysics of a series of trans-stilbenes (1 1) have been reported.' Energy transfer between Erythrosin B and some stilbene derivatives has been examined in detail. The intramolecular electron transfer encountered on irradiation of the stilbene derivative (12) in methylene chloride has been studied. Irradiation populates a charge transfer state that undergoes E,Zisomerism. When methanol is added to the system irradiation not only brings about isomerism but also converts the trisilanyl group into the silane (13). The results of a study of the photochemical behaviour of a series of halostilbenes have been published.l4

'

'

Me I Me3Si-Si-SiMe3 I

Me

I

Me3Si-SiH

I

Irradiation of the oligostilbene (14) at 254 nm in methanol transforms it into the corresponding cis-isomer.l5 The one-way photochemical cis to transisomerism of the stilbene analogue (15) is affected by solvent.16The results of a study of the photochemical transformations of trans-2-styrylpyridine( 16) have been published and it is reported that irradiation in acetonitrile solution results only in conversion into the corresponding cis-isomer, reaching a stationary state composition of 97% cis.17 When the styrylpyridine is encapsulated in y-cyclodextrin and irradiated in the solid state little isomerism occurs (7% cis) and the principal reaction is the formation of the (2 + 2)-cyclodimer (17, 50%). The photoisomerism of (18) has been described.'* Direct irradiation of the alkene (19) at 366 nm brings about geometrical is~merism.'~ The formyl group

IIt3: Photochemistry of Alkenes, Alkynes and Related Compounh

115

(14) R 1 = H o V

Ph

*NO2

l! / I

Q

Ph (17) Pyr-

I

enhances the intersystem crossing efficiency of (19) and the photostationary state obtained is solvent dependent (t : c = 12 : 88 in benzene, 21 : 79 in acetonitrile and 26 :74 in methanol). The cis, trans-isomerism of bis(2-benzoxazoly1)stilbene has been reported and again a solvent dependence has been observed.20 In particular the addition of ethylene glycol leads to enhanced isomerism which is thought to be the result of hydrogen bonding. Calculations have been carried out dealing with the photochemical isomerism of pyazinylquinoxalinylethylene.21The results indicate that there is extensive mixing between the m* and the nn* states.

116

Photochemistry

A review has highlighted the principal photochemical reactions that stilbenes and heteroanalogues undergo.22 1.1.2 The Dithienylethene System and Related Compounds - The dithienylethene system is an area of study that has become of great interest in the last few years. A review has highlighted the photochromic properties of such diarylethenes (20).23Since the original discovery of the photochromism in such molecules many changes have been brought about to modify the photochromic properties of the system. Irradiation of (21), for example, brings about ring closure to The efficiency of both the forward and the back reactions has been measured and these values are given under the appropriate structures. The fatigue resistance was also measured and (21a) is reported to undergo 800 cycles without noticeable change in its spectrum. For (21b), however, the stable photoproduct (23) was obtained on prolonged irradiation. The influence of the phase in which the isomerism is carried out has also been assessed.25 Irradiation of the triene (24) as a single crystal converts it into its cyclized coloured form. Bleaching, the reverse reaction, can be brought about by irradiation with wavelengths > 450 nm.26

(21) a, R = Me (gcy~l-0.46 &,,an b, R = H (gwcl a 0.68

= 0.013) 3

0.015)

Other groups have also synthesized the related dithienylethenes (25).27All these derivatives exhibit photochromism by conversion into the isomeric compounds (26). For example, irradiation of (25d) at 313 nm results in its conversion into the closed form (26d) which has an absorption band at 583

IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds

117

nm: reversion occurs thermally. With the less conjugated systems (25a-c) a yellow colouration results on irradiation. A further report has described a useful general synthesis for such molecules.28

R

(25)

a, R = H b, R = CI c,R=Me d, R = HCO e, R = PhMeCHN=CH f, R = (CN)&=CH

The basic structure of this photochromic system has remained as a dithienylalkene but there are many ways by which the periphery of the molecules undergoing reaction can be altered. Typical of the changes that have been made is the use of benzothienyl groups as shown in (27).29The quantum yields of cyclization of these systems (27) using 3 13 nm radiation have been measured to assess the influence of substituents and is reported to be more efficient with bulky groups at the 2,2’-positions. Thus with isopropyl groups &ycl is 0.52 while with methyl groups &l is 0.35. With the benzoylthienyl groups it is also possible to incorporate substituents on the benzene rings to modify the reactivity of the basic molecule. In addition the added functional groups can permit the inclusion of the compounds into constrained environments. The effect of these changes on the photochromism can then be examined. A typical example of this is the study of the incorporation of the analogue (28) in cyclode~trins.~~ The effects on the quantum yield of photocyclisation have been assessed and the authors report that there is enrichment of the antiparallel

(27) R = Me or Me2CH-

(28)

118

Photochemistry

conformation. This enrichment causes an increase in the photocyclisation quantum yield. Other studies have focused on changes in the photochemistry of the dithienylethene (29) in y-cy~lodextrin.~ A further example (30) in this photochromic series has been reported bydrie and c o - ~ o r k e r s The . ~ ~ coloured isomer (3 1) obtained by photochemical ring closure of (32) has been resolved into its enantiomers using HPLC techniq u e ~The . ~ incorporation ~ of other side-chains has also been of interest. Recent work has examined the effects of including carotenoid side chains in (33). The quantum yield for the forward and back reactions was found to decrease markedly with the increase in chain length.34

F2

6

F2

/

F2

CHO

OH

OH

F2

Me Me

The influence of alkali metal perchlorates on the photocyclisation of (34) has also been studied.35The quantum yield for cyclization is reduced from 0.21 for the free system to 0.17 for irradiations in the presence of sodium perchlorate. The effect is even more dramatic with potassium and rubidium perchlorates when 4 is reduced to 0.02. The crown ether systems are obviously important since the overall shape can be controlled by the photochemical ring closure.

1113: Photochemistry of Alkenes, Alkynes and Related Compounds

119

Towards this end, (35) has been used as a method for the extraction of alkali metal ions.36Another variant based upon the triene system represented in (36) has been studied.37

Me Me

(36)n = 1 o r 2

In other studies the photochromism of some bis(2-thienyl)perfluorocyclopentenes has been examined. The efficiency of the photochromism is dependent to some extent on the position of attachment of the thienyl ring. Substitution on the thienyl ring is also an important factor and suppression of the photochromism is observed when a 4-(N,N’-dimethylamino)phenylsubstituent is attached to the thienyl 5 - p o ~ i t i o nOther . ~ ~ aryl rings can be used within the system and this has been demonstrated with (37) which has been shown to be photochromic.39

Photo chemistry

120

(37)R’

= OEt, R2 = Me R1 = Me, R2 = Ph

Benzo[1,2-b: 4,3-b’]dithiophene is one of the products formed on irradiation of cis- I ,2-di(2-thienyl)ethene both in degassed and aerated solution. The analysis of the system suggests that a singlet excited state is involved.40

1.2 Miscellaneous Reactions - The stannane (38) is converted into the isomeric compound (39) when it is irradiated in aerated benzene.41 The presence of a radical trapping agent appears to be critical for the success of this intramolecular 1,3-stannyl migration and several examples of the reaction using (40) have been described. The efficiency of the rearrangement varied dependent upon substitution. Usui and Paquette4* have reported the photochemical transformation of the sulfide (41) into the isomeric product (42). This 1,3-phenylthio migration can be brought about using Sun-lamp irradiation in carbon tetrachloride solution. The product (42) was used as a key molecule in a new synthetic path to diquinanes.

Ph 3 S n w Ph (38)

(39)

R3Sn-Ar (40) R = Ar = Ph R=Me, Ar=Ph R = B u , Ar=Ph R = Ph, Ar = pyren-l-yl

KSP (42)

SPh

1.2.1 Addition Reactions - An examination of the photochemical behaviour of styrenes encapsulated in zeolites has shown that both oxidation and hydration take place and one of the major reactions encountered is the formation of 2-phenylethan01.~~ Irradiation ( h> 300 nm) of the allenes (43) and (44) in deuteriochloroform solution in the presence of a 1 : I molar ratio of (PhS);?and (PhSe)* provides a convenient method for thioselenation and affords good yields of adducts such as (45, 99%, 22 : 78 E : 2 ratio) and (46, 75%, 40: 60 E : z ratio), The telluroglycoside (47) undergoes C-Te bond fission on irradiation in benzene solution at 100°C to give the glycosyl radical (48).45 The radicals

1113: Photochemistry of Alkenes, Alkynes and Related Compounds

But L o

0

=

(43)

(44)

But

121

SePh

"c,,,

(45) 99% (22:78, €2)

(46) 75% (40:60, E :Z)

produced in this manner can be trapped by alkynes (49) to yield the alkenyl derivatives (50) by regiospecific addition of the glycosyl radical to the alkene and trapping of this radical by combination with the TeAryl radical.46 Addition of the glycosyl radicals also occurs to isonitriles such as (51) to give the imine (52).47

AcO (48)

(50)

Te-ptolyl

Yield (%)

€:Z

93 78 48 38

7525 80:20 79:21 69:31 74:26

11

Benzophenone and other diarylketones have been used along with tbutylamine as a means of generating free radicals from acetone, dimethyl sulfoxide and a l k y l a m i d e ~In . ~ ~this case, it is suggested that abstraction of a hydrogen atom by the excited ketone from the amine results in the formation of corresponding amine radical which then abstracts a hydrogen atom from the alkylating agents. The resultant radicals undergo addition to alkenes such as 1,l-diphenylethene and 10,ll -dihydro-5-methylene-5H-dibenzo[a,d]cycloheptene. The conversion of O-ally1 substituted saccharide units (53) into the products (54) can be brought about by irradiation in the presence of cysteamine.49

122

Photochemistry

Medium ring cycloalkenes undergo carbonylation when they are irradiated over a Co(acac)z catalyst in the presence of CO and methanol and the yields of methyl cycloalkanecarboxylatesare high.50 1.2.2 Electron Transfer Processes - A single electron-transfer mechanism is involved in the cycloaddition of alkenes, such as 2-methylpropene, to 1,2dicyanonaphthalene. Reaction of the alkene radical cation with the radical anion of the sensitiser results in the products shown in the Scheme l.51 Incorporation of solvent to give (55) occurs as one of the main products in addition to what are essentially photo-NOCAS products (56).

(56)17%

Scheme 1

(55) 17%

4%

The photo-NOCAS process has also been reported with P-myrcene (57) as the reactant.52The resultant radical cation, generated using dicyanobenzene as the sensitiser, affords the five products (58-62) shown and cyclization within the myrcene radical cation is an essential feature of this reaction sequence. SET photochemistry of aliphatic electron donors can provide a source of radicals. Thus irradiation of donors such as (63), (64),(65) and (66) results in bond fission and the formation of alkyl radicals which undergo addition to alkenes (e.g. 67) or alkynes (e.g. 68) to give the adducts (69) and (70), respectively. p-Dicyanobenzene has been used as an electron accepting sensitiser in the photochemical addition of ammonia to some naphthylpropene derivative^.^^ For example, irradiation of (71) results in the formation of the adducts (72). Prolonged irradiation brings about bond cleavage within the products (72) and yields 1-methylnaphthalene. Other positional isomers have also been examined and (73) can be converted under the same conditions into the adducts (74). Again prolonged irradiation converts (74) into 2-methylnaphthalene which reacts further to give the addition product (75). The influence of substituents on the naphthalene moiety on the outcome of the reaction was also studied. 1.2.3 Other Processes. The photochemical behaviour of dicyanodiacetylene has been reported.55 Irradiation of ethyne in argon or xenon matrixes results in

IIl3: Photochemistry of Alkenes, Alkynes and Related Compounh

123

Bu4Sn (63)

C02Me

I

‘I‘

C02Me (68)

(71) a, R1 = R2 = Me b, cis R’ = H, R2 = Ph C, trans R’ = Ph, R2 = H

(72) a, 45% b, 67% C, 40%

H

H

h

R

C02Me

MeQC

CaMe

(69) (70) R = Bu or Bu3SN (27%) R = But or Bu$nMe3 (70%)

(73) a, R’ = R2 = Me b, cis R’ = H, R2 = Ph C, trans R’ = Ph, R2 = H

(74) a, 64% b, 54% C, 2940

C-H bond fission and the production of ethynyl radicals.56 Butadiyne and vinyl acetate are formed when the ‘T’-shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied.57The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen.57A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical.58 Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm.59 An examination of competition between homolysis and heterolysis of the C-Br bond of the haloalkenes (76) has been described?O Solid-state NMR techniques have been used in a study of the photochemical behaviour of

124

Photochemistry

At

Br

(76)Ar = Ph or o-MeOCeH4

trichloroethylene on Ti02 surfaces.61The photofission of a C-Cl bond of 1,1,2trichloroethene can be brought about by irradiation at 193 nm.62The photodissociation of trifluoroethene at 157 nm has also been studied.63The photochemical rearrangement of propargyl bromide into allenyl bromide in an argon matrix proceeds by way of C-Br bond homolysis and thus involves a radical mechanisrnaMThe primary photodissociation at 266 nm of 1-bromo-2chloroethane involves fission of the C-Br bond to yield a chloroethyl radical.65 Silacyclopropene intermediates are produced when the 1-(o-allyloxyphenyl)-2pentanemethyldisilanylethyneis irradiated in methanol and addition products are formed under these reaction conditions.66 An examination of the possible biogenesis of some highly functionalized lactones obtained from a Caribbean gorgonian, Pseudopterogorgia bipinnata, has been carried An example of this lactone system (77) is shown in Scheme 2. The authors suggest that the biogenesis path involves a photochemical step and have supplied proof for their postulate by the irradiation of (77). The reaction takes 1 h in acetonitrile solution and affords the three products shown in Scheme 2 in a ratio of 120 : 1 :6. The conversion of (77) into the major product (78) appears to involve a 1,3-sigmatropic migration with retention of configuration at the migrating carbon.

+

Me

Scheme 2

2

Reactions Involving Cyclopropane Rings

2.1 The Di-E-methane Rearrangement and Related Processes - Zimmerman and co-workers68have reported results of calculations carried out on the well

IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds

125

known photochemical rearrangement of enones such as (79). Previously, chemical studies had failed to resolve the problem of which phenyl group the endo or the exo underwent migration. The calculations have sought to resolve this problem within crystal lattices. 0

2.1.1 The Aza-di-n-methane Rearrangement and Related Processes - Armesto

and co-workers have reported the photochemical transformations encountered with the azadienes The transformations are initiated by electron transfer to the DCA sensitiser. This treatment generates the corresponding radical cation of the dienes that undergoes bridging to produce the intermediate [e.g. (8l)l which undergoes ring opening and cyclization to give low yields of the cyclopropane derivatives (82).

Ph ) I* ' = R3 = Ph, R2 = H b, R' = Ph, R2 = H, R 3 = OMe C, R1 = Ph, R2 = H, R3 = OAC d, R 1 = R2=Ph, R 3 = O A c

(80)a, R'

+&No Nxph

(82)

Ph

Ph I (811

2.1.2 SET Induced Reactions - Further studies on ring opening reactions of 1,2-diarylcyclopropanes have focused on compounds (83) bearing an acetyl functional All of the cyclopropane derivatives studied show efficient cis,trans isomerism with reasonable quantum efficiency. The isomerism reaction involves an acetophenone-like triplet state with lifetimes shorter than 1 ns. The formation of ring opened products also takes place but inefficiently to give a mixture of the two alkenes (84) and (85). A study of the electron-transferinduced photochemical reactivity of the cyclopropane (86) has been reported in the co-sensitisation system (bi~henyl/phenanthrene/DCA).~' An electron transfer strategy has been used in the irradiation of the cyclopropylamine derivatives (87) and other related molecules.72The reactions are carried out in acetonitrile/water with DCB as the sensitiser and the radical cation of (87) formed ring opens to afford a radical cation of the type illustrated as (88). Within this intermediate only cyclization occurs and there is no evidence for a hydrogen transfer path. There are two modes of cyclization that occur and both are 5-exo-radical in type giving the products shown in Scheme 3 in good yield.

126

Photochemistry

*

Ar

Ar’

Ar

Ar

Ar AAr

(86) Ar = pM0OCsH4

(87) n = 1,2

n = 1 (60%)

n = 2 (32%)

\

Ph

\

I

i ”

Ph

60%

The photochemical reactivity of vinylcyclopropane under electron-transfer conditions has been studied.73 The reactions were carried out using DCB/ phenanthrene as the sensitiser system and in a solvent mixture of acetonitrile/ methanol (3: 1). The three products (89)-(91) result from ring opening and trapping by solvent. The radical cation of (92) can be formed photochemically using DCB as the electron-accepting ~ e n s i t i s e rWhen . ~ ~ the reaction is carried out in acetonitrile/methanol as the solvent mixture the two products (93) and

IIl3: Photochemistry of Alkenes. A lkynes and Related Compounds

127

(94) are obtained in 27% and 24%, respectively. There is no evidence for ring opening of the cyclopropane moiety.

Miyashi et aL75 have previously demonstrated that the isomerism and ring opening of the methylene cyclopropanes (95) is initiated by SET processes. The present study has examined in detail the steps within the system.76 SET photochemistry of (95) can be brought about by the use of DCA, TCNB or NMQ+ BF4- as sensitisers and the formation of the radical cation (96) was demonstrated as was the formation of the biradical (97) which is formed from (96) by a back electron-transfer step.

2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds Armesto and co-workers* have reported the photochemical behaviour of the vinylcyclopropane derivatives (98) under m-methoxyacetophenone sensitisation in methylene chloride as the solvent.77A variety of products are formed, the nature of which is dependent upon the substituent on the cyclopropyi ring. The key intermediate is believed to be the biradical (99) that is formed following energy transfer. A study of the effect of wavelength on the outcome of the irradiation of (100) has been reported.78At short wavelengths (250 and 254 nm), it is thought that the S2 state of (100) is populated and this results in the formation of the three products (101), (102) and (103) in a ratio of 6:l:l. A report by Radzig et aZ.79has given details of the control exercised by silica surfaces on the photochemical conversion of the ally1 radicals (104) into the isomeric cyclopropyl radicals. Free radicals are involved in the photoreaction of phenylcyclopropane with trifluoromethylsulfenyl chloride and many products are obtained.80

Photochemistry

128

Ph (98) R = a, C02Et; b, C02H; c, CHO; d, COMe; e, CH=NOH; f, CHpOH

\/ Ph

(99)

Me$3i-(CH2),-cH-CH=CH2 (104)

3

n = 0, 1 or 3

Reactions of Dienes and Trienes

A study of the photochemically induced absorption of some butadienes on Ti02 surfaces has been carried out.8' Calculations have dealt with the photochemical cyclization of the bulky substituted diene (105) and the results have provided a mechanism for the cyclization of such dienes into bicyclobutanes such as (106).82When the cyclobutene (107) is irradiated in a xenon matrix at 270 nm ring opening occurs by a conrotatory mode which is the usual thermal path.83 The authors explain this change in behaviour as a result of light being absorbed by the xenon and not directly by the cycloalkene. A detailed discussion of the possible mechanism for the reaction is given and it is suggested that a path involving radical cations cannot be excluded. B u t x But

""h But

CI

Some aspects of the photochemical behaviour of 'terpene' have been investigated to evaluate some of the source materials in air pollution.84 A computational study of the 2,E-isomerism of buta-l,3-diene, hexa-l,3,5-triene and related compounds has been reported.85 An ab initio study of the photocyclisation paths of buta-l,3-diene has been carried out.86 Calculations have

IIl3: Photochemistry of Alkenes, Alkynes and Related Compounh

129

been reported dealing with the (4 + 4)-photocycloaddition reactions of dienes such as buta-1 ,3-diene.87The effects of pressure on the triplet sensitised or electron-transfer controlled dimerisation of cyclohexa-1,3-diene have been studied.88In the DCA induced reactions there is evidence that different types of solvated ion pairs are involved in benzene and in acetonitrile. UV irradiation of the diazaanthracene derivatives (108) in the presence of cyclopentadiene affords a single cycloadduct in each which was identified as (109).89 Irradiation of mixtures of butadiene and the dicyanophenanthrene (110) in acetonitrile brings about an electron transfer to yield the radical cation of the diene and the radical anion of the electron acceptor.90 Three products result from the interaction within this radical catiodradical anion pair and at high concentrations (1 M) of the diene the dominant product is (1 11, 34”/) and the two (2 + 2) adducts (1 12) and (1 13) in 23 and 9% yields respectively. At lower concentrations (0.13 M) of the diene the cis-(2 + 2)cycloadduct (112, 37y0) becomes dominant with the others (111, 10%) and (113, 14”/0)less so.

Mattay and co-workers have reported the cyclization of the enol ethers (1 14) into the ketones (1 15) in modest yields.91The reactions are brought about in acetonitrile solution generally using DCA as the sensitiser and with irradiation at 419 nm. A study of the influence of the sensitiser upon the outcome of the reaction was also reported. The cyclization within the resultant radical cation is highly selective as shown by the cyclization of (114, R’ =Ph, R2=Me, R3= H) to yield the &ketone (1 16). The effect on the cyclization upon the position of substitution of the isolated double bond was also investigated. Thus irradiation of (1 14, R’ = Ph, R2 = H, R3= Me) affords the three products (117), (118) and (119) in 27%, 10% and 1%, respectively. Other SET cyclizations have been carried out on a series of related non-conjugated dienes such as (120) using DCA as the electron accepting sensitiser and this results in the formation of the radical cation of the diene:92this undergoes cyclisation (120, R = H) to give the cyclic ketone (121) in 25% yield. The reaction is solvent sensitive and when a mixture of acetonitrile/propan-2-01 is used the three products (121, 30%), (122, 11%) and (123, 9%) are obtained. The reaction has

130

Photochemistry

some considerable synthetic potential and the effect of chain length and substituents on the reaction has been evaluated. Thus (124) is converted into (125, 11?h) while (120, R = Me) affords an isomeric mixture of products (126). Tricyclic products such as (127) can also be obtained in moderate yields (26%) from the cyclization of (128).

(1 14) R’ = But, R2 = R3 = H (23%) R’ =CeHll, R2= R 3 = H (37%) R’=Ph, R 2 = R 3 = H (38%) R’ = pMeOCeH4, R2 = R3 = H (26%) R1 = Ph, R2=Me, R 3 = H R’ = Ph, R2 = H, R 3 = Me

‘ H lire

Me&‘?

H (125)

H (126) R’ = Me, R2 = H Yield 9% R’ = H, R2 = Me Yield 11%

The photochemical transformation of the dienes (129), (130) and (1 3 1) under DCA sensitisation has been studied in detail.93 Electron-transfer-induced cyclization of the dienes (132)-( 134) occurs in the phenanthrene/DCB/acetonitrile system with irradiation at wavelengths > 334 nm.94The electron transfer takes place from the 1,l-diphenylalkenyl moiety and results in the formation of the radical cation [e.g. (135)] which cyclises to give the radical cation (136) and this undergoes electrophilic aromatic substitution to ultimately afford (137). Cyclization is chain length dependent and the first two dienes (132) and (133) cyclise while (134) fails but undergoes double bond migration. The efficiency is best with (1 32), and (137) is formed in 71% yield. Shortening the chain linking the two chromophores reduces the efficiency and (133) affords a mixture of stereoisomers (138) and (139) in yields of 25 and 5%, respectively.

IIi3: Photochemistry of Alkenes, Alkynes and Related Compounds

Ar

Ar

131

Ar

Me

I

The fluorenone-sensitized irradiation of the all trans-triene (140) gives a phot 0sta tionary mixture composed of the trans-cis- trans, cis-trans-trans and cis-cis-trans isomers.95The potential energy surfaces of the ground and excited state of the triene (140) have been mapped,96 and a study of the fluorescence from this triene in lipid bilayers and isotropic solvents has been carried The photophysical properties of the dithienylpolyenes (141) and (142) have been measured.98The photoisomerism of all-trans-retinal to the 11-cis isomer brought about by honeybee retina enzyme photoisomerase has been described.99

Ph

Ph (141) n = 1 , 2 , 3 or4

(142) n = 3 o r 4

When the triene (143) is irradiated under electron-transfer conditions using DCB in methanol or water the products obtained are (144,4%), (145, 9%) and (146, 12%).'O0 The results were used in an attempt to correlate the homoconjugated structure of the radical cations with its reaction towards nucleophiles. In a similar fashion the reactivity and selectivity of the reactions of alkenes (147) and (148) have been studied. With the parent compound (147) addition of

132

Photochemistry

methanol to the double bond yields product (149). This type of addition is also observed with the benzo derivative (148). . ,Ph

\\

Ph.

,P

H

Crystalline mixtures of the dimers (150- 152) with tetracyanoquinodimethane ( 153) undergo photoinduced intermolecular electron transfer when irradiated at h > 350 nm.Io1 Cleavage of the C-C bond and the formation of the monomers then occurs. The photochromic properties of the chromene (154) have been studied.lo2 The ring opening kinetics using 357 nm were measured and ring closure was effected by irradiation at 422 nm. The quantum yields of the forward and back processes were also measured. Kaneko and coworkers have studied the photochemical cyclization of the enediyne (155) which forms the aromatic compound (156).lo3 The best yields are obtained using hexane as the solvent. Me

Me (150) a, X = NMe b,X=O

Ph

:% Ar t (151)

(152) a, R = H, Ar = jl-MeOC&4 b, R = HO, Ar = pMeOCsH4

NCKCN

3.1 Vitamin D Analogues - A complex mixture of products is formed on irradiation of a new 19-phenylsulfonylprovitamin D analogue. Io4

IIl3: Photochemistry of Alkenes, Alkynes and Related Compoundr

4

133

( 2 + 2)-IntramolecularAdditions

The norbornadiene derivatives (157) undergo efficient ring closure to give the corresponding quadricyclanes (15 8 ) . ’ 0 5 The ester functions of these Goducts were elaborated by reduction and ether formation to provide a path to socalled tentacle molecules.

High yields of intramolecular (2 + 2)-cycloadducts are obtained from irradiation of (159).lo6 The success of the intramolecular additions is reputedly due to the flexibility of the ether linkages. The use of the (2 + 2)-photocycloaddition reactions in the synthesis of so-called paddlanes (160) has been explored. lo7 Irradiations of (161) are carried out through a Pyrex filter and are best in cyclohexane as the solvent. The yields of the adducts with two cyclobutane moieties (160) is very good. These products are accompanied by small amounts of the mono-cycloaddition product (162).

(159) n = 1-4

(160) 84%

92%

0

0

(161) n = 2

n=3 n-4

(162) 6% 6%

The cyclophane (163) undergoes cyclization when irradiated in solution in the presence or absence of water and a low yield of the cycloadduct (164) is obtained which itself is also photochemically reactive and undergoes a further

134

Photochemistry

(2 + 2)-cycloaddition.lo* Several products are formed on irradiation of the tricyclooctadienes (165).'09

R R (164) 17%

(165) R = Me or Et

The acetophenone-sensitised irradiation of non-conjugated dienes such as (166) has been studied. Typically irradiation of (166) affords the exo-adduct (167) where facial selectivity is observed.' lo The diene (168) is also reactive and yields the adduct (169) in 77% yield. Perfect facial diastereoselection is exhibited in the more rigid diene (170) which affords (171) with a selectivity of > 95 :5. The disilanyl alkyne derivative (1 72) rearranges on irradiation at 300 nm in benzene solution to give (173) in modest yields."' The authors suggest that the reorganisation involves the conversion of (172) into the intermediate (174) as the first step.

(166) R = benzyloxycahonyl

(167) 53%

(173) R = H (25%) R = Me (18%)

5

Dimerisation

Irradiation of the styrene (175) in a NaY zeolite brings about (2+2)dimerisation to yield (176) and (1 77).' l 2 Oxidation of the styrene also occurs during this treatment. Radical cations of the alkenes (178), (179) and (180)

IIl3: Photochemistry of Alkenes, Alkynes and Related Compoundr

135

can be formed by electron transfer from sensitisers such as DCN, MCN, MCA and N-methylacridinium within NaX zeolites. l 3 Thermal reactions were detected when NaY zeolites were used. The radical cations formed from the alkenes undergo dimerisation within the constrained framework of the zeolites. A comparison of the results obtained with those from solution is given below the appropriate alkene. The enamides (18 1) crystallise with short intermolecular distances between the alkene moieties and on irradiation at 350 nm the crystals yield head-to-tail dimers (182) in high yield.114 This result is contrary to that observed on irradiation of (181) in solution when only cis-trans-isomerism of the ethene bond occurs. Dimerisation is also observed on irradiation of crystals of (183) which gives the dimer (184) in 89% yield.

Sensitizer

(178) R = H or Me

(179)

(180) R = H or Me

H.H syn H,H anti

syn anti

anti

55 55 65 63

95 95 95 95

DCN MCN MCA NMethylacridinium

4 66 45 55

96 33 55 45

45 45 35 37

Irradiation of an acetone solution of the azetine (185) affords (2 + 2)-dimers and although four dimers are possible, only a 1 : 1 mixture in 52% yield of the head-to-head dimers (1 86) and (187) was obtained.

136

Photochemistry

The mechanism of the electron transfer-sensitised dimerisation of acenaphthylene (188) has been studied in considerable detail. l6 Two dimers (189) and (190), formed from the radical cation of acenaphthylene, are obtained in addition to (191) and (192) which incorporate the TCNE sensitiser. Fumaronitrile behaves in a similar fashion yielding the dimers (189) and (190) as well as cycloadducts. A study of the energetics of electron transfer in acenaphthylene charge transfer photochemistry has been carried out.' l7

H H

The photochemical dimerisation of (193) results in the formation of some major products, one of which has been identified as the adduct (194).Il8 The photochemical dimerisation of single crystals of (E)-2,6-di-But-4-[2-(4-methylpheny1)-ethenyllpyryliumsalts has been studied. I9

'

The results of a study of the catalytic cycloreversion of the benzene dimers (195) and (196) have been published.120

IIf3: Photochemistry of Alkenes, Alkynes and Related Compounds

6

137

Miscellaneous Reactions

6.1 Reactions Involving Cations and Radicals - A study of the electron transfer photochemistry of 1,l-diarylalkanes [e.g. (197)] using chloranil as the sensitiser has been reported.12’ The work sought to differentiate between homolytic and heterolytic processes involved in C-H bond cleavage. A CIDNP study of the photochemistry of the pinacols (198) in deaerated chloroform has shown that the process involves fission of the central C-C bond and the production of radicals.122 These disproportionate to yield the products (199) in high yield. Other pinacols such as 9,9’-bifluorene-9-9’-dio1(200) have also been studied. The irradiation affords fluorenone, a reaction mode similar to that reported for (198), and the spiroketone (201). When the reaction is carried out in acetonitrile the ratio of the two products is 85 : 15, but when care is taken over de-aeration this changes to 76:26. In methanol a further change is observed and a ratio of 67 :33 is reported. It is suggested that the products are formed from the cation (202). This is produced by 0 - C bond fission as the first photochemical process.

McGarry and S ~ a i a n o ’have ~ ~ measured the absolute kinetics for the addition of free radicals to the propellane (203). The photodecomposition of ethylene oxide has been examined.’25

Perfluoroalkenes can be formed photochemically from perfluoroalkanes using decamethylferrocene as the sensitiser. 26 The reaction involves an electron transfer process from the ferrocene to the perfluoroalkanes and the tertiary C-F bonds are cleaved preferentially. Thus perfluoromethylcyclohexane can be converted into perfluoromethylcyclohex-1-ene. Further studies on the photochemical reactivity of the 1,3-dichloropropane (204) have been

138

Photochemistry

reported. 127 Irradiation of (204) in 2,2,2-trifluoroethanol at 266 nm yields the cation (205) from C-CI bond heterolysis, and subsequent loss of HCI produces the propenyl cation (206). There is no evidence from this study that a two

photon process is involved. Irradiation at 193 nm of a series of small haloalkanes has been studied. 12* A comparison between the photochemistry and the sonochemistry of bromotrichloromethane has been reported. 29 Irradiation of bromotrichloromethane failed to yield the dimer, hexachloroethane which is also formed using the sonochemistry system. However, with added oct-1 -ene further differences were noted. Thus irradiation gave the dimer accompanied by 1,2-dibromooctane and the product from the addition of bromotrichloromethane to the alkene and although these products were also formed using sonochemistry the yields were lower. 6.2 Miscellaneous Rearrangements and Bond Fission Processes - The X-ray crystal structure of the biphenylmethanol (207) shows that it is highly crowded with a dihedral angle of 80" between the biphenyl ring system.130In acetonitrile solution or in the solid state, (207) cyclises efficiently to give the pyran (208). In solution, the mechanism of the reaction involves intramolecular proton transfer from the phenolic OH to the benzyl alcohol function but in the solid state the proton transfer occurs intermolecularly. The search for photochromic optical triggers continues. In recent work the binaphthylpyran (209)hinaphthylene (210) pair has been studied.I3' A quinone methide is a key intermediate in the transformation of one into the other. This mechanism precludes the use of such a system for consideration as an optical trigger.

Evidence for both C-0 and N - 0 bond fission has been found following the irradiation of the pyridone derivatives (211).132 Irradiation in methanol purged with nitrogen and using wavelengths > 340 nm results in the formation of five products identified as the pyridone (212), the corresponding alcohol (213), the aldehyde (214), the ether (215) and the hydroxypyridone (216). The first three products arise by N - 0 bond homolysis while the remaining two are the results

IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds

139

of C - 0 bond heterolysis. The aromatic ethers (217) are photochemically reactive and undergo rearrangement to the cyclohexadienones(218) on irradiation in benzene solution. 33

H or Me

(218)

R

Gravel and Bordeleau have demonstrated that the cyclohexanediol (2 19) can be converted into the deoxysugar (220) by irradiation in the presence of benzophenone, acetonitrile and t h i ~ p h e n o l . 'The ~ ~ conversion of (219) into (220) involves the formation of the aldehyde (221) that is converted into the acetal, i.e. the deoxysugar. An extension of this work has demonstrated that deoxyazasugars can also be formed using the same ~ 0 n d i t i o n s . lThus ~ ~ irradiation of (222) gave the aldehyde (223) which can then be cyclized by the same path used for the formation of (220). The conditions used were irradiation at 350 nm in acetonitrile solution with xanthene and thiophenol. HO

SPh

Leigh and his co-workers have studied the photochemical decomposition of the silacyclobutanes (224). 36 In hydrocarbon solution with added methanol, (224) undergoes decomposition and the formation of the alkoxysilanes (225). These are formed via the intermediacy of the silenes (226) formed by photochemical elimination of ethene.

Photochemistry

140 Me I

Me\ R'OSiMe Si=CH2 I R (225) (226) (224) R = Me, Ph, H, Et, ButSiMe3,CH2SiMe3, CH=CHp, C S H , SiMe3, OMe, OSiMe3

"-TI

d

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4

Photochemistry of Aromatic Compounds BY ALAN COX

1 Introduction Topics which have formed the subjects of reviews this year include reactions of stilbenes and their heterocyclic derivatives,*recent photochemical transformations of 2(5H)-f~ranones,~ photochemical nitration by tetranitr~methane,~ selectivity in inter- and intramolecular photocycloaddition of ethenes to benzenoid compound^,^ photocycloaddition of aromatic esters and nitriles to a l k e n e ~photocatalysed ,~ synthesis of pyridine,6 a photochemical alternative to some Friedel-Crafts reaction^,^ the synthesis of indoloazepines and indoloazocines by the photo-Friedel-Crafts reaction,* molecular arrangement and photochemical reactions in the layered inorganic minerals hydrotal~ites,~ the photochemistry of layered inorganic-organic nanocomposites,'O the use of 1,2diphenyl-2-oxoethyl (desyl) and p-hydroxyphenacyl phosphate and carboxylate ester protecting groups, photolabile protecting groups for carboxylic acids and neurotransmitters, l 2 trends in chromism, photochromic polymer systems,14photochromic compounds, and the preparation of photochromic compounds by incorporation of spiropyrans into inorganic matrices with organic compounds. Recent advances in organic photochromic storage materials,l7 and photochromic switching,l8 have also been described.

2

Isomerisation Reactions

An ab initio study of ethylene using a multi-electronic state and molecular dynamics has shown that cis-trans photoisomerisation begins within 50 fs of optical excitation and starts with stretching of the C :C bond.19 Return to the ground state occurs from an ionic species and proceeds through a conical intersection which is achieved by pyramidalisation of a methylene group. The role played by intermolecular hydrogen bonds in solid state photoisomerisation has been attributed to the changes they can induce in the electronic state of the reactive group, and to the way in which migration of the group may be inhibited as the reaction proceeds.20 These bonds may also strengthen the crystal lattice and accelerate the photoreaction. The complex photochemistry of derivatives of the penta-atomic heterocycles, furan, thioPhotochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 145

146

Photochemistry

phene, pyrrole, isoxazole, imidazole, and pyrazole has been analysed using PM3-RHF-CI semiempirical calculations and a unitary description of their behaviour offered.*l In an attempt to reduce the computational costs for molecular dynamics simulations, an interpolation scheme using a quantum chemical potential energy function has been described.22Application of this procedure to the photoisomerisation of cis-stilbene in super-critical argon has demonstrated its feasibility, and the accuracy and efficiency have also been evaluated. The photoisomerisation of cis- and trans-stilbenes adsorbed in a zeolite super-cage has been shown to occur by singlet states for NaY and by triplet states for KY; a new potential energy surface for the photoisomerisation has been sugg e ~ t e dNon-resonant .~~ two-photon excitation of trans-stilbene in the presence of an excess of tetramethylethylene induces cis-trans isomerisation by a process which occurs from the Ag state, but under the same conditions one-photon excitation gives the [2 + 23 c y c l ~ a d d u c t . ~ ~ Irradiation of 1Z,3E- 1-cyanodiphenylbuta-1,4-diene and 12,32-1-cyano-3methyldiphenylbuta-1,4-diene promotes preferential isomerisation of the double bond substituted with the cyan0 group in a one-photon-one-bond process.25The effects of substituents on the potential energy surface of the S1 states of the a,o-diphenylpolyenes, and of the role of zwitterionic dipolar species on the photoisomerisation processes of linearly conjugated polyenes are discussed. All-trans-1,6-diphenylhexa-1,3,5-triene (trans,trans, trans-DPH) has been interconverted under fluorenone sensitization with its trans,cis, trans-, cis, trans, trans-, and cis,cis,trans-DPH isomers, and quantum yields in degassed solutions are strongly concentration dependent owing to quantum chain processes.26Evidence is available which suggests that equilibrated isomeric planar triplets are the quantum chain carriers and it is speculated that chain carriers in other systems may be planar triplets in equilibrium with the usually dominant twisted triplets. In some related work, a Japanese group has shown that the quantum yield of the trans, trans, trans -,cis, trans, trans photoisomerisation of p,p’-disubstituted 1$-diphenylhexa- 1,3,5-triene increases with increasing substituent polarity irrespective of the electron-donating or -withdrawing nature of the substituent, whereas the quantum yield of trans, trans, trans + trans,cis, trans photoisomerisation increases with increasing electrondonating nature of the s u b ~ t i t u e n t sDiphenyl .~~ diselenide has been used as an isomerisation catalyst to map the potential energy surfaces of the ground and triplet state of isomers of 1,6-diphenylhexa-1,3,5-triene.28The results show that, in the triplet state, the substrate exists as an equilibrium mixture of the all-trans; trans,cis,trans; cis,trans, trans; and &,cis, trans isomers. Points on the potential energy surfaces of the ground and triplet states corresponding to the equilibrium geometries of these isomers have been determined, and at these geometries the two surfaces are approximately parallel to each another. The use of diphenyl diselenide as an alternative to iodine as a catalyst for the EIZ photoisomerisation of carotenoids has been described and conditions reported for the stereomutation of zeaxanthin, violaxanthin, canthaxanthin and fucoxant hi^^.^^ Enhanced R to S stereomutation is also reported for the photo-

IIl4: Photochemistry of Aromatic Compounds

147

chemical allenic isomerisation of allenes where this same reagent is used instead of molecular iodine. 2-[2-(2-Pyrrolyl)ethenyl]phenanthroline will undergo a one-way trans-to-cis photoisomerisation, and the cis isomer also exhibits an intramolecular hydrogen atom transfer in its excited singlet state.30The same workers have also shown that the presence of a formyl group in 2-[2-(2-pyrrolyl)ethenyl]pyridine (1) has the effect of enhancing both the efficiency of intersystem crossing (S1 -.TI)of the substrate to give (cis-1), as well as the efficiency of H atom transfer within the S1 state, which occurs from the H-bonded species (cis-2),to produce the product (~is-3).~'The kinetic parameters of the competitive

relaxation properties of the lowest excited singlet and triplet states of some trans-stilbene-like molecules having 2- or 3-ring N- or S-heterocyclic groups have been determined, and this has provided information about the heteroatom effect on the photoisomerisation mechanism.32The n,n* states exert a deactivating effect through vibronic coupling and subsequently by internal conversion into the ground state, and a heavy atom effect arises from the thiophene ring promoting enhanced intersystem crossing. An increased torsional barrier is caused by the polycondensed rings. An investigation of the structures of the lowest excited states of pyrazinylquinoxalinylethylene using PM3-UHF-CI and a molecular mechanics force field has shown that there is extensive mixing of the (n,n*)and (n,n*) states, and that the (n,n*) transition band is absent from the spectrum but appears to be hidden by the more intense (n,n*)bands.33 The effects of methanol and acetone present as clathrates in crystals of N-[3,5- bis( 1-methyl-1-phenylethyl)salicylidene]-4-tritylaniline have been examined on the solid phase photoisomerisation of the Schiff base,34 Laser flash photolysis has shown that at low temperatures the cis forms of 4-aminoazobenzene and 4-dimethylaminoazobenzene are photoconverted to the corresponding trans forms via short lived intermediates which are believed to be z ~ i t t e r i o n sThe . ~ ~ copper hydroxy layered hybrid Cu2(OH)3X (X = 8-([p(phenylazo)phenyl]oxy)octanoate) undergoes an irreversible trans to cis photoisomerisation in a process in which there is little change in magnetic properties.36 Kinetic and thermodynamic parameters of the trans-cis photoisomerisation of tran~-4-(4'-alkylphenylazo)phenyl derivatives (pCnHn + 1CbH4N = N-pC6H4-X; n =4, 8; X = SO3, C02Na) at birr 366 nm have been measured in homogeneous and micellar solutions,37 and a study has revealed that the phototransformations of chalcone phthalamide derivatives and chalcone-containing polyimides involve either trans-cis isomerisation or [2+ 21 cy~loaddition.~~

148

Photochemistry

The photophysics of 4-dimethylaminocinnamic acid have been studied in different environments and the computed excited state dipole moments in different twisted geometries suggest that a twist of 90” of N(CH3)2 produces a minimum energy state and maximum dipole moment change.39 Quantum mechanical calculations have been performed with the AM 1 Hamiltonian to find the actual geometry causing the intramolecular charge transfer state. The EIZ photostationary state compositions of a series of methyl-a-arylcinnamates and derivatives p-substituted with electron withdrawing or electron donating groups are similar and about 2.8.40 However, the pss values for the o,odichloro substituted arylcinnamate are found to be only 0.5, and this has been discussed in terms of factors influencing the excited state potential energy surface for arylcinnamate photoisomerisation. In non-polar solvents, the cis and trans forms of the 9-anthrylethylene derivatives 9-AnthCH = CHC02CH3, 9-AnthCH = CHCH202CCH3, and 9-AnthCH = CHCH20H exhibit dual emission at 340 nm and 460 nm, and a correlation has been established between the solvent and excitation wavelength dependencies of the trans -,cis quantum yields.41These and other observations strongly suggest that the 340 nm emission originates from the S2 states of the cis form, and that the S1 state is the only singlet excited state possessing a large enough charge transfer character to facilitate the photoisomerisation. 4,4’-Diazidodibenzylideneacetone which is formed as a mixture of three stereoisomers has been irradiated in both its crystalline state and adsorbed on silica gel; the products which arise result from stereoisomerisation and degradation of the azido groups into nitrene~.~* The first example of fluorescence enhancement following EIZ isomerisation of an N : N double bond has appeared.43 Irradiation of (4; Ar = 1-naphthyl, 2-carboxyphenyl) at 330 nm is accompanied by a fluorescence enhancement which has been rationalised in terms of inhibition of photoinduced electron transfer, and which in turn arises as a consequence of the non-planar geometry of this isomer by reducing effective overlap of the lone pair electrons with the ~r:electrons of the fluorophore. A study of the new photoresponsive mono-crown-6-azobenzocalix[4Jarene(5) has shown that the trans isomer is phototransposed into the corresponding cis-(5),and that this in turn can be thermally isomerised to the trans isomer.44 The cis isomer complexes Cs+ and Rb+ better than does the trans isomer. Irradiation of trans-4-(dimethylamino)azobenzenein a 4,4‘,4”-tri(N-carbazoly1)triphenylamine glass causes cis-trans isomerisation.45 Removal of the radiation source following attainment of the photostationary state is followed by a return to the original composition and this is thought to be due to a thermal cis + trans isomerisation process. The results have been discussed in terms of relaxed and strained cis isomers and the microstructure of the glass. The photoresponsive Z-3,3’-dialkanoyloxyazobenzenes(6) are reported to be compatible with a nematic liquid host as well as with the cori-esponding Eisomers, and this implies that there is a small alteration in the interchange free energy parameters during photois~merisation.~~ For R = Me, an increased compatibility is evident suggesting that stable rod-like conformations provide effective steric hindrance. cis-trans Isomerisation of two azo com-

IIl4: Photochemistry of Aromatic Compounds

149

pounds has been carried out in poly(methy1 methacrylate) using birr 405 and hirf 546 nm, and it has been observed that excitation at the shorter wavelength produces the more reactive cis particles.47This has been rationalised partly in terms of the failure of some trans molecules to isomerise at longer wavelengths under these conditions, partly in terms of the greater rate constant of ‘dark isomerisation’, and also by the greater quantum yield of back photoisomerisation at birr 546 nm. The photoisomerisation kinetics and other properties of the 1 : 1 inclusion complexes formed between aromatic derivatives of norbornadiene and p-cyclodextrin have been measured.48 (S)- or (R)-2-Chloropropiophenoneaffords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photoinduced rearrangement via what is probably an ion or radical intermediate,4g and (a-N-substituted benzoyl-a-dehydrophenylalaninessuch as (7) are photoisomerised to 1-metidine derivatives (8) by a 1,3-acyl migration.50Irradiation of 9,9’-bifluorene-9,9’-diol (9) gives a mixture of fluoren-9-one and spiroE9Hfluorene-9,9’(10-H)-phenanthrenl-10’-one (10) whose composition is solvent dependent with the more polar solvents favouring (lo)? Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (1 l), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-

150

3

Photochemistry

Me

8 Me

(7)

\

CONHBu

CI

(8)

/

(9)

ment of (1 1) competes with nucleophilic quenching. Vinylcyclopropanes (12; R = C02Et, CHO, CH :NOH, Ac) have been photorearranged to the cyclopentenes (13; same R) or other heterocycles such as the furanol (14),52 and a computational approach capable of evaluating competing transition state structures applicable to photorearrangement in crystal lattices has been d e ~ c r i b e d . The ~ ~ method has been successfully applied to the photorearrangement of 6,6-diphenylbicyclo[3.1.O]hexen-2-ones and 5-ethyl-4,4-diphenylcyclohex-2-en-1-one, and a preference established for endo phenyl migration in bicyclo[3.1 .O]hexen-2-ones. In contrast to earlier claims, it has now been reported that ketone photosensitization of a-azidocinnamates gives a high yield of the presumed intermediate diastereomeric pair of aziridinoimidazoline d i m e r ~and , ~ ~irradiation of the aziridines (15 ) and (16; 16 = 15 having a C14-CI5 double bond) gives the new compounds 1,2-seco-l,21-cyclovincadifformine (17) and 1,2-seco-1,21-cyclotabersonine (18; 18 = 17 having a C14-Ci 5 double bond)?

I

C02CH3

Ul4: Photochemistry of Aromatic Compounds

151

Variously substituted tetrazolo[1,5-a]pyridines (19) and 2-azidopyridines (20) are photolysed to 2-alkoxy-1H- 1,3-diazepines (2 l), 2-dialkylamino-5H1,3-diazepines (22), 2,3-dihydro-1H- 1,3-diazepin-2-0nes (23), and 2,4-diazabicyclo[3.2.0]-hepten-3-ones(24); the relative stabilities of the 2-alkoxy- and

2-dialkylamino-1,3-diazepines are in accordance with ab initio energy calculat i o n ~Excitation .~~ converts 4-arylazopyrazolin-5-onesinto the corresponding hydrazo derivatives, and occurs by reduction of the azo group and subsequent rearrangement of the pyrazolinone ring.57 The reaction may proceed by a-cleavage of the -CO-N-Ph moiety in the pyrazolinone ring to give a diradical component which undergoes a subsequent cyclisation. Photoisomerisation of 5-alkylidene-4,5-dihydro-3H1,2,4(h3)diazaphospholes proceeds by an unprecedented 5 + 4 ring contraction to a semicyclic azomethineimine to give ultimately amino-(imidoy1)-phosphanesor 2-hydrazinobenzo[b]phosphole,depending upon the starting material.58 In aqueous solution, p-nitrosobenzaldehyde has been photoisomerised to p nitrosobenzoic acid in a process which occurs by rapid formation of the intermediate aci-nitroketene, followed by its r e a r r a ~ ~ g e m e nAt . ~ ~ of the study phototransposition of p-, m-,and o-methylbenzonitrile reveals that any one of these substrates is converted into the other two in the primary photochemical step by way of either a 1,2- or a 1,3-isomerisation, though with different reactivities.60 The transformation proceeds via an excited singlet state, and labelling indicates that only the cyano-substituted carbon undergoes migration. A light induced change in a cholesteric pitch in the liquid crystal phase of 4'-pentyl-4-biphenyl carbonitrile doped with (R)-( +)-1-pyrenyl-4-tolyl sulfoxide has been observed, and has been ascribed to photoracemisation of the sulfoxide.61 Irradiation of the P,q-unsaturated ketones, bicyclo[2.2.l]hept-5-en-2-one and bicyclo[2.2.2]oct-5-en-2-oneincluded within MY zeolites promotes an oxa-di-n-methane rearrangement.62 The transformation is thought to occur by a triplet state which has its origin in the heavy cations present within the supercage. Use has been made of a range of aromatic phosphorus compounds such as phosphates, phosphonates, phosphinates, and phosphine oxide as sensitizers in the photoisomerisation of (2)-cyclooctene to the highly strained (E) alkene.63 EIZ Ratios of about 0.15 have been obtained, and investigations show that a mixed singlet/triplet mechanism operates and that sensitizers incorporating either of the chiral groups ( -)-menthy1 or ( -)-bornyl achieve enantiomeric excesses of 5%. The novel photoinduced rearrangement of the 1,3-diaryl-l,2dihydropentalene (25) to the corresponding 1,5-isomer (26) and their subse-

152

Pho tochernistry Ph

Q

(25)

OMe

Ph

(26)

OMe

quent reaction with maleic anhydride have been described,64 and although 6-methyl-5-nitroquinoxaline,2,3,6-trimethyl-5-nitroquinoxaline,and 1,6-dimethyl-5-nitroquinoxalinium perchlorate have not been observed to exhibit photochromism under time resolved conditions, under continuous photolysis they are reported to undergo a nitro-nitrite rearrangement to quinoxalinol derivative^.^^ Irradiation of peri-phenoxy-5,12-naphthacenequinones,aza-substituted at the 1, 2, and 10 positions, promotes the solvent-sensitive arylotropic interconversion of, for example, (27) and (28), and this is thought to account

for their photochromism.66 Computer simulation of the tautomerism and fluorescent properties of 2,3-dicyano-5-methylpyrazine and 2,3-dicyano-6hydroxy-5-methylpyrazine,as well as their spectral properties by MO and molecular mechanics methods indicate that they have strong intramolecular charge-transfer chromophoric systems and high solvatochromism.67 Exhausin the tive photolysis of 3,3,3’,3’-tetramethy1-4,5-diphenyl-4,5‘-bi-3H-pyrazolyl presence of Rh6(CO)16 gives 2,7-dimethyl-3,6-diphenylo~ta-2,6-diene-4-yne.~~ However, at shorter irradiation times 3,3-dimethyl-5-(3,3-dimethyl-2-phenyl-lcyclopropenyl)-4-phenyl-3H-pyrazoleis produced. Irradiation of methanolic 1-ethoxy-2-phenylindole is reported to give 3- and 6-etho~y-2-phenylindoles.~~ In the absence of light, 3,4-bis[4-(dimethylamino)phenyl]-1,2-dithiete (29) exists in stable equilibrium with its ring opened valence isomer (30), and although the 1,2-dithiin (31) is stable in the dark, irradiation induces isomerisation to 2,5-bis[4-(dimethylamino)phenyl]thiophene-3-thiol (32).70 This transformation may proceed via 4-Me2NC6H4C(S)CH : CHC(S)C6H4-4-NMe2and a dihydrothiophene episulfide. A study of the stereochemistry of the photoArbuzov rearrangement of the benzylic phosphite trans-(R,R’)-(33) to the corresponding phosphonate (34) has shown that the reaction occurs with predominant retention of configuration at the stereogenic migrating carbon atom of configuration R’ in the ~ u b s t r a t e . ~ ~

IIl4: Photochemistry of Aromatic Compounds

0

Using the AM1 method, a study of intramolecular barriers to proton transfer in the ground and excited states of perylenequinone has revealed that although such transfers do occur, the rate of transfer in the excited state is much higher than that in the ground state.72 Excited state proton transfer has been observed in 2-(2-hydroxyphenyl)pyridine, IO-hydroxy-5,6-dihydrobenzo[h]quinoline and 1O-hydroxybenzo[h]quinoline, and the role of intramolecular rotational mechanisms and structural flexibility discussed.73 Two deactivation channels are thought to be available, one of which is dependent upon twisting and the other related to n,n* quenching. A time-resolved fs study of photoexcited [2,2’-bipyridyl]-3,3’-diolin solution has shown the involvement of two simultaneous processes, a concerted double protontransfer process (l-step ) occurring in
154

3

Photochemistry

Addition Reactions

Attempted photohydration of (36; R = H , Me), (37), and (38; R = H , Me) is reported to be efficient only for (36; R = H ) , (37), and (38; R = H ) implying that excited state intramolecular proton transfer is the primary photochemical step. 165 Observations also suggest that trimeric water deactivates the excited singlet state, and ns flash photolysis indicates that the primary photochemical step results in formation of the m-quinone methides. In the presence of tertbutylamine, aromatic ketones will sensitize the alkylation of diphenylethene and 10,ll -dihydro-5-methylene-5H-dibenzo[a,d]cycloheptenewith a variety of alkylating agents. 166 Dimethyl sulfoxide has been used to alkylate stilbene, 1-methyl-l,2-diphenylethene, and anethole. The reaction is believed to occur by abstraction of a hydrogen atom from the amine by the triplet state of the ketone sensitizer to give the t-BuNH radical, which in turn abstracts a hydrogen atom from the alkylating agent. The halogenoheterocycles (39; X = S , Y = I , EWGzCOCH3; X = S , Y = I , EWG=CHO; X = S , Y = I , EWG = N02, X = 0, Y = Br, EWG = CHO) undergo photoaddition to (40) and in some cases to (41), and (42; X = Ph, Y = COCH3; X = 2-thienyl, Y = CHO; X = 2-thienyl, Y = COCH3) when irradiated in the presence of phenylacetylene gives 1-phenylnaphthalene. 67 N-Acylbenzoxazole-2-thiones (43; R = Me, CHMe2) will photoadd to alkenes of the type R1R2C:CR3R4 (R1=Me, Ph; R2= H, Me, CN, Ph; R3 = H, Me; R4= H, Me, CH : CMe2, Ph) regiospecifically to give the benzoxazoles (44) and, depending upon the nature of R-R4, the iminothietanes (45).

In the presence of 9,1O-dicyanoanthracene, irradiation of methanolic 1,4diphenylbuta-l,3-diyne yields four MeOH adducts. 169 Time-resolved studies indicate that a photoinduced electron-transfer process is involved. Irradiation

IIl4: Photochemistry of Aromatic Compounds

155

of o-quinones at 300 nm in the presence of diphenylacetylene produces two isomeric o-quinomethane adducts, of which the E-isomers are capable of undergoing photoisomerisation. 170 A mixture of benzonitrile and 2,2,2-trifluoroethanoI has been photoconverted into the four stereoisomers of 6-cyano-2-(2,2,2-trifluoroethoxy)bicyclo[3.1.O]hex-3-ene in a singlet state process which occurs by initial formation of the biradical or zwitterion of 6-cyanobicyclo[3.1.O]hex-3-en-2,6-diyl which subsequently collapses by ex0 and endo protonation. 71 Labelling studies show that the cation of 6-cyanobicyclo[3.1.O]hex-3-en-2-y1 undergoes a 1,4-sigmatropic rearrangement involving inversion of configuration at the migrating carbon followed by nucleophilic attack by the solvent. Aromatic aldehydes have been reported to undergo 1,l-addition to the ketene dithioacetal S,S-dioxides (46; R = alkyl) in either the presence or absence of benzophenone to give adducts (47).17* The transformation is initiated by H abstraction from the aromatic aldehyde by the n,n* excited state of the sensitizer or substrate aldehyde to form the aroyl radical (48) as intermediate. These adducts may be useful synthetic precursors of indanones.

'

0

(46)

SMe

(47)

5,5-Dimethyl-6-oxocyclohex1-ene- 1-carbonitrile will cycloadd 2,3-dimethylbut-2-ene to give hexahydroindeno- 1,2-0xazole selectively, and either 2-methylbut-2-ene or isobutene to give mixtures of tricyclic 1,2-oxazoles and 2-oxobicyclooctane-1-carbonitriles.173 Structural evidence suggests that two isomeric triplet 1,4-biradical intermediates are produced, and that these decay to the corresponding oxazoles and bicyclooctanes. Paterno-Buchi reaction of indan- 1,2,3-trione in the presence of 2,3-diphenyl-1,4-dioxene leads to the corresponding oxetane. 74 Use of 2-methylbut-2-ene, however, gives in addition to the cycloadduct, products arising from allylic hydrogen abstraction, and use of 2,4,4-trimethylpent-1-ene generates a single allylic hydrogen abstraction product only. Strong steric effects are apparent on the reaction coordinate, and these control the regioselectivity. Aromatic aldehydes will regio- and em-selectively photocycloadd cyclic ketene silyl acetals to form bicyclic 2-alkoxyoxetanes (49; R = 1-naphthyl, 2-naphthyl, 6-methoxy-2naphthyl, Ph, 4-cyanophenyl; R' =Me, H; R2 = Me, H, Ph; R3= H, Me).175 Following aqueous hydrolysis, aldol-type adducts (50) are produced with threo selectivity. Various N-alkenyl substituted maleimides such as (5 1) and (52) undergo photocycloaddition producing (53) and (54) respectively, and similarly, tetrahydrophthalimides give 1-azabicyclo[5.3.0]dec-3-enes. 176 It is hoped that these studies will aid the development of methods which will be applicable to aromatic analogues. The yellow form (55) of bis(2-phenyletheny1)dicyano-

156

Photochemistry

N

Me& Me

0

(52)

0 (53)

':f& H0

'H

(54)

pyrazine is reported to undergo a [2+2] cycloaddition in solution, in vapour deposited thin films, and in single crystals to form the syn head-to-tail dimer.177By contrast, the orange crystal is found to be unreactive under all of these conditions. It has been suggested that the selective topochemical photocycloaddition of (55) can be rationalised in terms of lattice contraction and dynamic movements of the molecular packing arrangements. Phenyliodonium 2,6-dioxocyclohexylide photochemically cycloadds various alkenes and dienes

to form dihydrofuran derivatives such as (56).17* Irradiation of Z-vinyl-2H1,4-benzothiazin-3(4H)-one-2-spirocyclopropanes in the presence of catalytic amounts of diphenyl dichalcogenide produces 1,2-dioxolanes, although diphenyl diselenide is found to be a more effective radical source.179In the presence of electron-deficient alkenes, photoinduced [3 + 21 cycloaddition occurs to give spirocyclopentanes, and alkynes are found to react similarly to generate spirocyclopentenes. Photolysis of sulfine (57) in the presence of cyclooctyne gives a mixture of (58), (59), and fluoren-9-one,180and photochemical benzannulation of pentacarbonyl(cc-exo-benzylidene-2-oxacyclohexy1idene)chromium with various alkynes leads to the formation of 6-chromanols.lS1

IIf4: Photochemistry of Aromatic Compounds

157

In an attempt to probe the face-to-face stacking interaction between phenyl and perfluorophenyl groups, the solid state packing structure and reactivity of some mono-ethenes and di-ethenes substituted with these groups have been investigated.18* Photoinduced [2 + 21 addition has been observed and an analysis of the structure of the olefin precursors and of the product stereochemistry is of value in interpreting the stacked interaction between the phenyl and perfluorophenyl groups. In order to determine whether the high yields of the intramolecular [2 + 21-photocycloaddition of (60; n = 1-4) are a conse-

$6

quence of the electronic effects of the oxygen atoms located at the para positions, or whether they arise from steric interactions resulting from the flexibility of the oligooxyethylene linkages, the photoreactions of analogues in which the oxygen atoms are present only at the end of the chain have been examined.Ig3 The quantum yields of these reactions clearly show that the controlling factor is the flexibility of the linkages. trans-2-Styrylpyridine when photolysed as a complex with y-cyclodextrin in the solid state gives the synhead-to-tail dimer in contrast to its behaviour in solution.*84Comparison of the results of a molecular mechanics study of the regio- and stereoselectivity of [2 + 21-photocycloaddition in complexes containing crown ether styryl dyes and alkaline earth metals with experimentally obtained data shows that the quantum yield is governed by the relative energies of the dimeric complexes and product cyclobutanes, and also by the mutual arrangement of the dye molecules in the dimeric complexes.185These observations are an indicator of the supramolecular control of regio- and stereochemistry of cation-dependent [2 + 21-photocycloadditions The products of irradiating 3-alkenyloxy-5hydroxybenzoic and the corresponding 3-alkenyloxy-5-methoxybenzoicacid derivatives (61; X = H, CH,; R = H, CH3) in the presence of sulfuric acid are the highly functionalised alkoxyenones (62, 63, 64; same X, R; Scheme 1) which are thought to arise in an acid catalysed transformation of the primary [2 + 21 photocycloadducts.ls6 A series of 5-(2-methoxyphenyl)pent-l-enes(65), substituted at the a-, p-, or y- positions with either an OH or OSiMe3 group, undergo an intramolecular meta photocycloaddition reaction to give a product (66) having endo stereochemistry at the substituted carbon atom. lg7 The diastereoselectivity is a consequence of minimisation of steric interactions between the side chain substituent and the ortho-Me0 group of the arene unit. A molecular mechanics study of the regio- and stereoselectivity of cation-dependent [2 + 21-photo-

158

Photochemistry

ox

0

MeOWH+

A

(61)

(63)

R

OH

+

Scheme 1

(64)

cycloaddition of crown ether styryl dyes and alkaline earth metal cations has appeared. * 88 The reaction is governed by the relative conformational energies of the dimeric adducts and the resulting cyclobutanes, and the geometry of the dimeric complexes. Irradiation of the cinnamyl derivative (67) leads to a mixture of the cycloadducts (68), (69), (70) and (71).lg9The proportions of these photoproducts are altered by conducting the reaction in the presence of silica gel, and a model has been proposed which accounts for these variations in terms of surface control of biradical formation.

The new polycyclic cage compounds (72), a chlorinated tri-bridged hexacyclododecene, and its hydroxy analogue (73) have been obtained by irradiating the multi-bridged [3.3.3](1,3,5)cyclophane (74) using short wavelength 1ight,lg0 and a formal synthesis of crinipellin B (75) has been reported starting from triethyl phosphonopropionate and 3-nitro-2-methylbenzoicacid and involving as the key step an arene-alkene rneta-photocycloadditionof (76) to give (77).19' 6-Chloro-1,3-dimethyluracil will photoreact with frozen benzene to give an

IIl4: Photochemistry of Aromatic Compounds

159

ortho cycloadduct and this is succeeded by photochemical disrotatory cleavage of the cyclobutene moiety and with intramolecular photo-Diels-Alder reaction of the resulting cyclooctatetraene.192 Intramolecular [3 + 21-photocycloadditions of methyl naphthalene- 1,4-carboxylates which contain remote alkene moieties such as isobutene (78) or a-methylstyrene (79) give nine- to eleven-membered ring systems (80; n=3,4,5; and 81; n=3,4,5) in addition to five-membered rings.193 The oxetane structure (82) is produced for n = 2 . These workers have also reported that [3 + 21-photocycloaddition of 1,2-, 1,3- and 2,3-dicyanonaphthalenes to alkenes occurs at the 1,8-, 43- and 1,8-positions of the dicyanonaphthalene rings respectively.194 So, for example, 1,2-naphthalenedicarbonitrile and butadiene give hydroacenaphthene (83), and 1,3-naphthalenedicarbonitrile and hex- 1-ene give hydroacenaphthalene (84) and cyclobutenotetrahydronaphthalene (85). These reactions have been rationalised in terms of the spin and charge densities of the radical anions of dicyanonaphthalenes. The same group further record that photoreaction of N-methyl-1,8-naphthalimide with methylbenzenes such as xylenes, mesitylene, and durene in acetonitrile solution produces [3 + 31 adducts as well as waterincorporated addition^.'^^ An electron-transfer mechanism has been proposed. Photocycloaddition of RC02(CH2CH20)nCOR (n = 4, 5, 10, 12; R = 2-naphthyl) (N-P,N), RC02(CH2)nCOR (n = 3, 5, 8, 10) (N-M,N), R1CH202C(CH2)mC02CH2R1 (R' = 9-anthryl; m =4, 8) (A-Mm-A), and RC02(CH2CH20)nCOCH2CH2R1 (n = 3, 4, 5 ) (N-Pn-A) within an NaY zeolite leads to intramolecular photocyclodimers only. 196 These observations have been rationalised in terms of the compartmentalisation of the guest within the zeolite, and points the way to the use of micropores of zeolites for the synthesis of large-ring compounds.

160

Photochemistry

CN

&N \ CO&le

CN

\

Bu

yyJ

CN

BU

The [4+2]-cycloaddition of O*(lAg) to rubrene has been shown to be a simple method for the determination of oxygen concentrations in organic solvents.lg7 Irradiation of benzene solutions of tropone (86) in the presence of 9,lO-dicyanoanthracene leads to the formation of four products, (87), (88), (89) and (90), and in acetonitrile-dichloromethane there is also an [8 + 4]7c adduct (91) produced.198It has been suggested that this latter compound arises by coupling of the radical cation of (86) with the radical anion of the dicyanoanthracene (Scheme 2). Solvent-dependent quenching of the lowest excited state of 9,l O-dibromoanthracene by 2,5-dimethylhexa-2,4-dienehas been studied and appears to proceed by an e~cip1ex.l~~ Flash photolysis investigations have shown that a neutral radical species is an intermediate in the formation of the [4 + 21 adduct which is a dibenzobicyclo[2.2.2]octadienetype compound. Irradiation of 9,l O-dicyanophenanthrene (DCA) in the presence of buta-1,3-diene gives a mixture of the product of [3 + 21-photo-

IIl4: Photochemistry of Aromatic Compounds DCA

(86'+) + DCA-

h

-

161 'DCA*

(91)

Scheme 2

cycloaddition (92), and a pair of stereoisomers (93) and (94) which arise from the singlet and triplet states of DCA respectively.200

Phot olysis of (E)- 1-buty l-2,3-diphenylaziridine, (E)-2,3-diphenylaziridine, and 1-butyl-2192-diphenylaziridine,1-(~1-methoxyphenyl)-2-phenylaziridine, phenylaziridine produces azomethine ylides, and the rate constants of their [3 + 21 reactions with electron-deficient alkenes to give pyrrolidines have been determined.201Further investigations have shown that the transition state for protonation of the carbanionic site is linear. In the presence of 2-pyridones lacking a methoxyl group such as N-butylpyrid-2-one, 4-methoxypyrid-2-one will undergo a [4 + 41 cross photocycloaddition to give trans-( 1~t,2P,5P,6a)-ene-4,8-dione.~~~ This is a high yield reaction producing a tricyclic product having four stereogenic centres and four distinct functional groups. The same authors have also illustrated the synthetic potential of these pyrid-2-one photocycloaddition products by generating a cyclooctene as a single isomer having functionality at seven carbons and five stereogenic centres.203Irradiation of benzene solutions of 3-cyano-2-methoxypyridines (95) containing furan gives a mixture of the 1 : 1 adducts 1l-cyanol0-methoxy-8-methyl~-oxa-9-azapentacyclo[5.4.O.O2~6.O3~11.OS~g]undec-9-ene (96), 10and 1O-cyano-9-methoxy-7-methyl-5-oxa-8-azatricyclo[5.4.0.02~6]undeca-3,8, triene together with the transpositional pyridine, 5-cyano-2-methoxy-6-methylpyridine and the pyridine dimer.204[2 + 21 Photocycloaddition of isoquinolin-

162

Photochemistry

l(2H)-one to electron-deficient ethenes is reported to be an efficient process and to proceed regiospecifically with high stereoselectivity;cycloreversion (kirr 254 nm) of the acrylonitrile adduct occurs with rearrangement to an N-vinylis ~ i n d o l i n e Benzannulated .~~~ isoquinolinones such as benzoxazolo[3,2-b]Iso11-one, benzimidazoquinolin-11-ones (97), benzothiazolo[3,2-b]isoquinolin[ 1,2-b]isoquinolin-11-ones and isoquino[2,3-a][3,l]-benzoxazine-5,12-dioneare reported to phot ocycloadd to electron deficient a1kenes.206

In the solid state, 2-pyrone-5-carboxylates undergo an endo [4 + 21 photocycloaddition with maleimides to give (98; R=Me, Pr; R ' = H , Ph, 4-MeOCbH4, 4-02NC6H4), whereas the sensitized photoreactions give the corresponding ex0 [4 + 21 cycloadducts stereoselectively.207 2H,8HBenzo[ 1,2-b: 3,4-b']dipyran-2,8-dione will participate in a [2 + 21 photocycloaddition to 2,3-dimethylbut-2-ene or 2-methylpropene at the C(9)C( 10) double bond to afford 8a,9,10,1Oa-tetrahydro-2H,8H-cyclobuta[c]pyrano[2,3-f][l]ben~opyran-2,8-diones.~~~ A comparison of the diastereoselectivity of the photocycloaddition of benzaldehyde to furan and related substrates with the Paterno-Buchi reactions between cycloalkenes and cyclic enol ethers suggests that the increased steric demand of the a-substituent in benzoyl compounds plays a crucial role in determining diastereoselectivity for the furan adducts.209These stereoselectivities are accounted for in terms of different ISC-reactive conformers with enhanced spin-orbit coupling. A highly diastereoselective [2 + 21 photocycloaddition of homochiral 5-alkyl2(5H)-furanones (99; R' = H, R2= H; R' = OCOCH3, R2 = H; R' = OCOC(CH3)3, R2 = H; R*= OSiPh2C-(CH3)3, R2 = H; R' = OCOPh, R2 = H; R' = OCOC(CH3)3, R2 = CH3) to vinylene carbonate has been observed to give (100); this has been attributed to n-stacking of the enone and the carbonyl moieties.2lo Irradiation of 4-methoxy-6-methyl-2-pyrone with maleimide in the solid state forms the endo-[2 + 21 cycloadduct stereospecifically.21

'

(99)

IIl4: Photochemistry of A roma tic Compounds

4

163

Substitution Reactions

An ab initio computational search for conical intersections carried out for the 3,5-dimethoxybenzyl cation and the 3,5-dimethoxybenzyl radical has revealed a degeneracy in the cation whose geometry is only slightly perturbed from that of the S1 minimum.212Similar calculations for the corresponding radical indicate that the closest approach of the excited- and ground state surfaces is relatively large, and the reaction hypersurface of the geometries of the minimised excited-state species was found to provide an excellent decay route for the meta ion pairs to ground state. Aromatic radical anions produced during nucleophilic aromatic photosubstitutions are capable of generating 0 2 - in non-deoxygenated polar aprotic solvent^.^' This superoxide anion is able to displace CN- from MeCN and PhCH2CN, EtO- from EtOAc, and MeSO- from DMSO; these observations may have implications for the use of non-deoxygenated polar aprotic solvents in nucleophilic aromatic photosubstitution reactions. Irradiation of cyanoarenes and azacyanoarenes in the presence of a donor reagent leads to the replacement of the cyano group by the donor.214 The mechanism involves photoelectron transfer or photodehydrogenation in the case of azacyanoarenes followed by reaction of the semiquinone radical with the radical form of the donor. The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction between methanol, 7-methyl-3-methyleneocta1,6diene and 1,4-dicyanobenzene gives the five 1 : 1 : 1 adducts cis-2-(4-cyanopheny1)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane, trans-2-(4cyanopheny1)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane, 1-(4-cyanophenylmethy1)-4-(1-methoxy-1-methylethyl)cyclohexene, 4-[4-methoxy-3,3dimethylcyclohex-(E)- 1-ylidenyl]methylbenzonitrile, and 4 4 1-vinyl-4-transmethoxy -3,3-dimethylcyclohexyl)benzonitrile. Studies show that the cyclisation is 1,6-endo with both the radical cation and the intermediate P-alkoxyalkyl radicals, and that the initial electron transfer involves the trisubstituted monoalkene fragment. These suggestions are supported by high-level ab initio MO calculations. Arene radical cations have been generated by photoelectron transfer from excited methoxy-substituted arenes to ground state 1,4-dicyanonaphthalene, and then allowed to react with silyl enol ethers leading to an effective intramolecular or-arylation of the corresponding ketone.216 This procedure permits the construction of five-, six-, seven- and eight-membered benzannulated as well as benzospiroannulated compounds. Naphtho- 1,4quinones and coumarins will photoreact with diphenylmercury in acetonitrile solution to give 2-phenylnaphtho- 1,4-quinonesand 4-phenylcoumarins respect i ~ e l y , ~and ' ~ irradiation of tetrazolo[ 1,5-a]pyrirnidine with benzene and substituted benzenes in the presence of trifluoroacetic acid gives 2-(2-, 3- and 4-substituted anilino)pyrimidines.218Heavy atom solvent effects show that some products arise from singlet and others from triplet species, and the intermediacy of the 2-pyrimidylnitrenium ion is indicated by use of a Hammett plot and by the effects of solvent nucleophilicity and counter-anions. Irradiation of a suspension of Ti02 in acetonitrile containing maleic anhydride and

164

Photochemistry

4-methoxybenzyl(trimethyl)silane leads to the formation of benzylated succinic acid.21 1,2,2-Triphenylbromoethene,1-phenyl-2,2-bis(o-methoxyphenyl)-1-bromoethene, and P-bromostyrene have been used to study the competition between photoheterolysis of the C-X bond to give vinyl cations, and photohomolysis to give vinyl radicals.220Involvement of the SRNl process has been demonstrated in the presence of reducing nucleophiles such as enolate ions of ketones, and the participation of vinyl cations which arise through the heterolytic path has been verified if weak electron-donor anions such as NO2-, N3- and CI- are added. 1,2,4-Trichlorobenzene is formed as the only product at high conversions by irradiating 1,4-dichlorobenzene in the presence of molecular chlorine in zeolite NaZSM-5 with green or blue light.221Such selectivity exceeds that obtainable by other methods, and the higher reaction rate observed is rationalised in terms of an enhanced electrophilic character of the chlorine molecule following electronic excitation. Sunlight irradiation of a,a-difluorotoluene together with N-chlorosuccinimide or N-bromosuccinimide gives a,a,achlorodifluorotoluene and a,a,a-bromodifluorotoluene respectively in a high yield, in a low byproduct transformation,222and the photochlorination of the methyl group of methylbenzenes has been described.223 3-Acetylcoumarins have been photohalogenated in a process which forms a facile route into 3-(2amino-4-thiazolyl)coumarins and which itself may be converted to 3-[(2,5dimethylpyrrol- 1-yl)thiazol-4-yl]coumarins.224Irradiation of trans-o-bromo-4phenoxymethylstyrene in the presence of sodium benzenethiolate leads to replacement of both the bromine atom and the phenoxy group in a process which occurs by a radical anion substitution mechanism in which the radical anion derived from the substrate dissociates along two competing pathways.225 Dibenz[b,f]oxepins and dibenzo[b,f]thiepins have been prepared photochemically from halo-substituted acetophenones by irradiation in liquid ammonia in the presence of bases.226The products have important medicinal use. Photonitration of 4-methoxystyrene with tetranitromethane gives the nitrotrinitromethyl adduct, 2-(4-methoxyphenyl)-1-nitr0-2-trinitromethylethane.~~~ Similar reactions with trans- 1-(4-methoxyphenyl)prop-1-ene give stereoisomeric nitrotrinitromethyl adducts. A radical chain mechanism is thought to operate, and some kinetic studies of base catalysed reactions of the products are reported. In some related work, the same authors have studied the photoreactions of tetranitromethane with 4-methylstyrene, styrene, 4-chlorostyrene, 3-chlorostyrene and 4-acetoxystyrene, and in these cases two stereoisomeric isoxazolidines 2-(2-nitro-1-X-phenyl)ethoxy-3,3-dinitro-5-(X-phenyl)isoxazolidine (X=4-Me, H, 4-C1, 3-Cl or 4-Ac0) and a nitronic ester, 3-nitro-5-(Xphenyl)-2-isoxazoline N-oxide (X = 4-Me, H, 4-C1, or 4-Ac0) are formed.*’* In these reactions, the first step is formation of the triad [substrate+ N02(N02)3C-]. These workers have also examined the reaction of 2-phenylpropene with tetranitromethane to give 1-nitro-2-phenyl-2-trinitromethylpropane and the two diastereoisomers of 5-methyl-2-(2-nitro-1-phenyl)ethoxy-3,3dinitro-5-phenylisoxazolidine.229This reaction occurs slowly in the dark but

IIl4: Photochemistry of A romatic Compounds

165

1-(2-Methylprop-1-enyl)naphthalene will undergo selective photoamination with ammonia in the presence of p-dicyanobenzene at the alkenyl group to give 1-(2-amino-2-methylpropyl)naphthalenein a process which shows some generality.230 The transformation occurs by nucleophilic addition of ammonia to the radical cation of the aromatic substrate generated following electron transfer to the p-dicyanobenzene. The distribution of positive charge in this radical cation and the stabilities of the aminated radicals as calculated by the PM3-UHF method are reflected in the regioselectivity of the products. Concentration-dependent isotope effects (IEs) have been measured in the competitive photocyanation of naphthalene and perdeuterionaphthalene, and have been shown to be influenced by the concentration of the reagents naphthalene, cyanide and oxygen.231Together with semiempirical PM3 calculations and other measurements, the variation of the IE with naphthalene concentration is suggested to be ascribable to excited state equilibration; a detailed mechanism to explain the observed IEs is proposed. The photo-Reimer-Tiemann reaction of phenols with chloroform in the presence of P-cyclodextrin has been reported to produce 4-hydroxybenzaldehydes with high selectivity.232 5

Cyclisation Reactions

Potassium and rubidium perchlorates have been observed to decrease the photocyclisation quantum yield of 1,2-bis(2,4-dimethylthien-3-yl)perfluorocyclopentene having two benzo-15-crown-5 ethers (101), and this has been ascribed to increases in the ratio of the photoinactive parallel conformational analogue of (101) by intramolecular interaction of the two crown ether moieties with a metal ion.233

Irradiation of asymmetrically substituted N-benzyl-N-isopropyl-aP-unsaturated thioamides (102) in benzene solution induces hydrogen abstraction by the alkenyl carbon from the benzyl and isopropyl groups to give a P-thiolactam (103) and 1,3,5-dithiazinane (104) as products.234In the solid state, however, photolysis causes hydrogen abstraction from only the isopropyl group to give the isomeric P-thiolactam (105).

166

Photochemistry

Irradiation of 1,l-diphenylhepta- 1,6-diene in the presence of 1,4-dicyanobenzene and phenanthrene triggers a photo-Ritter reaction with formation of cis-1-acetamido-3-(diphenylmethyl)cyclohexane (106) with high stereoselect i ~ i t yPhotocyclisation .~~~ of solid 2,4,6-triisopropylbenzophenonescontaining phenylalanine as a chiral substituent at the 4’- position (107) produces (108) with high diastereomeric excess.236However, only lower diastereomeric excesses are obtained for the related substrates in which the same substituent is placed at the 3-position, and for the esters (109; X = (R)-C02CHMePh, (R)C02CHMeEt). Photocyclisation of 4,5-diaryl-2-phenyl-1,3,2-dioxaborolesand 2,3-diaryldioxenes followed by oxidative hydrolysis leads to the formation of 9,lO-phenanthrenequinoneshaving substituents at the 3- and 6-p0sitions.*~~ A range of methylated azobenzenes have been photacyclodehydrogenated to the corresponding benzo[c]cinn~lines.~~~ Of particular interest is the synthesis of compounds possessing methyl groups having various degrees of steric compression since such structures are of theoretical significance in the context of intramolecular dynamics and quantum tunnelling effects. Irradiation of benzene solutions of methyl 2-(2,3-dimethylbut-2-enyloxymethyl)naphthalene1-carboxylate (1 10) promotes a novel 1,9-hydrogen abstraction by the ester carbonyl followed by intramolecular photocyclisation to give the keto ether (11l).239 The previously unknown polycyclic ring systems, benzo[h]thieno[3’,2’ :4,5]thieno[2,3-~]quinoline(1 12), benzo[f]thieno[3’,2’:4,5]thieno[2,3clquinoline (113), benzo[f]thieno[3’,2’:4,5]thieno[2,3-~]tetrazolo[ 1,5-a]quinoline (1 14; X = N), and benzo[f]thieno[3’,2’:4,5]thieno[2,3-c][1,2,4]triazolo[4,3alquinoline (114; X = C H have been synthesised by an oxidative photocycli~ation.~~~ CHMe2

Me2CH Ph

(106)

(107)

C02Me

IIl4: Photochemistry of Aromatic Compounds

167

Irradiation of the cinnamylnaphthols (1 15; R'R2 = (CH :CH)2, R3 = R4= H,

R5= Ph; R' = R2= H, R3R4= (CH :CH)2, R5= H) promotes photocyclisation

from the naphtholic singlet excited states with formation of the five- and sixmembered ring products (1 16) and (1 17).241By contrast, formation of (1 17) occurs by a proton transfer mechanism.

The photocyclisations of aroyl- 1,4-quinones have been studied as possible routes to anthracyclones and heteroanthracyclones, in particular the conversion of benzoylbenzo-1,4-benzoquinones into xanthones and phenyl gentisate esters.242Daylight-induced photooxidative cyclisation of some (E)-2-styrylchromones is reported to give 12H-benzo[a]xanthene-1 2 - 0 n e s , ~and ~ ~irradiation of various silyl enol ethers and silyloxy-2H-chromoneshaving an olefinic or silylacetylenic side chain induces oxidative photoelectron transfer followed by regioselective ring closure to produce perhydrophenanthrenones or benzannulated xanthenones.244The synthesis of 6-endo products by radical cationic pathways and 5-ex0 ring-closure by radical intermediates has been achieved, and the conditions for these processes have been optimised. N-(Aminoalkyl)-2-stilbenecarboxamidesin which the amine is secondary are reported to undergo regioselective intramolecular photoaddition to give 9- or 1O-ring a ~ a l a c t a m s and , ~ ~ irradiation ~ of 1-(o-allyloxyphenyl)-2-pentamethyldisilanylethynes (2-R2C=CHCH20C6H4CCSiMe2SiMe~(R = H,Me) leads to the novel cyclisation product ( 118; same R).246

168

Photochemistry

Flash photolysis studies of 2-pyridyl phenyl ketone in sodium dodecyl sulfate submicellar and micellar solutions have revealed that a fast intramolecular photocyclisation occurs followed by addition of water, and that hydrogen abstraction from the surfactants by the triplet ketone is absent.247Addition of the water takes place predominantly from the intramicellar water molecules. Time-dependent H NMR examination of the photoinduced 1,6-n-electrocyclisation of 3-vinyl-functionalised N,N'-dimethyl-2,2'-bisindolylshas enabled the educt transformations to be observed.248Irradiation of N-arylenaminones derived from cyclohexan-1,3-diones induces a stereoselective kinetically controlled conrotatory ring closure to give trans-hexahydrocarbazol-4-onesin conformity with the Woodward-Hoffmann orbital symmetry rules.249 However, the analogous substrates derived from cyclopentan- 1,3-dione give cis-cyclopent[b]indol-3-ones, products whose stereochemistry is contrary to that predicted by these rules. It is suggested that this apparent anomaly is explicable in terms of thermodynamic control of a dark reaction which occurs following a photocyclisation, the stereochemistry of which is in agreement with the predictions of Woodward and Hoffmann. AM1 calculations for the photoelectrocyclisations are consistent with these suggestions. Derivatives of N-methyl-N-phenyl-3-aminocyclohex-2-en1-one (1 19; R' = H, R2 = Me; R' = Me, R2 = H), as an aqueous suspension of a 1 : 1 inclusion complex with an optically active derived host related to tartaric acid, undergo an enantioselective photocyclisation in an aqueous suspension to give the corresponding N-methylhexahydrocarbazol-4-one( 120).250Although two kinds of diamorphous crystals are produced by 3-(N-methylanilino)-2,5,5-trimethylcyclohex-2en-l-one (1 19; R' = Me, R2 = Me), it is found that only one of these gives an optically active carbazolone derivative. 2-Methyl-6,7,8,9-tetrahydropyridino[2,3-b]indol-9-one ( 121) and some related structures have been synthesised by irradiation of the arylenaminone (122) in the presence of t r i e t h ~ l a r n i n e . ~ ~ ~ Three mechanistic pathways have been suggested for this transformation, namely photoelectron transfer from the added triethylamine to give (123), direct cleavage of the C-Br bond to give (124), and photoelectron transfer followed by direct cleavage of the same bond.

1114: Photochemistry of Aromatic Compounds

169

The (a-isomers of N-acyl-a-dehydro( 1-naphthy1)alanines (125; n = 2,3; R = Me, Ph; R' = Me, Et, Pr) produce 1,2-dihydrobenzo[fJquinolines (126) by and (2)-isomers produce minor an electron-transfer reaction, and the amounts of the benzo[f]isoquinoline (127) and 1-azetidine derivatives respectively by an intramolecular p h ~ t o a d d i t i o nEvidence . ~ ~ ~ suggests that the bulky diisopropylamino donor and the N-benzoyl group enhance the relative yield of (127) to (126).

(a-

1-N-Benzoyl-4-amino-1-butadiene is reported to be photocyclised to new 3H substituted quinolines which may be of significance in the fields of anticancer and antimalarial medicine.253The introduction of truns-azobenzene into the 8 channels of an aluminophosphate framework AlP04-5 causes it to be protonated despite the low Bronsted acidity of the framework, and photolysis of such an assembly produces benzo[c]cinnoline and benzidine.254Since the number of such sites is few, the yields of these products is low, and the bulk of the photoreaction consists in a reversible cis + trans photoisomerisation. A new preparation of substituted benzoquinolines (128; R' = Ph; R2 = Me; R3 = Ph, p-MeC6H4) involves the cyclisation of 3-(naphthylamino)-2-alkenimines(129; same R', R2, R3) as the key step.255 This transformation has been extended to include polycyclic aza compounds containing four rings, and includes for example the preparation of (130) from (131), and (132) from (133) The same authors also report a synthesis of variously substituted benzo-, dibenzo, and naphthoquinolines by irradiation of 3-naphthyl- and 3-phenanthrylalk-2-ene imines which may be important for the preparation of four-ring aminoazapolycycles of significance to medicinal chemistry.256 Photocyclisation of 1-benzylidene-2-formyl-6-methoxy-7-benzyloxy-3,4-d~hydroisoquinoline is the key step in the synthesis of bharatamine, a natural racemic protoberberine alkaloid,257and thermolysis of 1-biaryl-5-morpholinov-triazolines (134) gives the amidines (135) which will undergo photocyclisation to the 6-alkylphenanthridines (136) following morpholine eliminat i ~ nThese . ~ ~6-alkylphenanthridines ~ can also be synthesised, but with lower

A

170

Photochemistry

Ph HN

(133)

yield, on direct photolysis of the triazolines (134). N,N'-Diphenyl-l,5dihydroxy-9,lO-anthraquinonediimine (137; X' = X2= OH) has been photolysed to acridine condensed compounds (138; same XI, X2)and (139;same X', X2) in a process which occurs by excited state intramolecular proton t r a n ~ f e rPhotocyclisation . ~ ~ ~ ~ ~ ~ of 3-(o-halocarboxanilido)quinolin-2(1H)-ones is reported to give dibenzo[c,f][2,7]-naphthyridine-6,7(5H,8H)-diones,which can be readily converted into 6,7-dichlorodibenzo[c,f][2,7)-naphthyridine~,~~~ and photolysis of N,N'-o-phenylenebis(salicylideneaminato)]diaquamanganese(II1) gives 2-(2-hydroxyphenyl)benzimidazole in a sequence of reactions involving an intramolecular one-electron redox process, base hydrolysis of the oxidised Schiff base, and cyclisation of the hydrolysate.262

q \

o m N

(135)

\ Ph/

'

x2

IIl4: Photochemistry of Aromatic Compounds

171

Irradiation of chiral crystals of S-(o-tolyl), S-Ph, and S-(m-tolyl) 2-benzoylbenzothioates (140) causes intramolecular cyclisation to give the optically active 3-phenyl-3-(arylthio)phthalide(141) with high chemo- and enantioselect i ~ i t yThese .~~~ transformations seem to proceed by a 1,4-aryl migration (142). N-Phthaloylcysteine derivatives (143) are reported to be photoconverted into a variety of products including (144), (149, (146) and (147).264An investigation of the spin selectivities reveals that the excited singlets are prone to elimination and q-Habstraction, and that the triplets cyclise to thiazinoisoindoles. This behaviour can be rationalised in terms of the efficiencies of the forward and back electron-transfer steps as opposed to homolytic hydrogen abstractions. Irradiation of arenecarbothioamides and 2-vinylfurans in benzene solution leads to tetracyclic indole systems via diarylethylene intermediate^,^^^ and it has also been reported that the hydroindole sulfoxide (148), which has been prepared from the dioxopyrroline (149), can be converted by Pummerer-type cyclisation into the cyclohomoerythrinan (150).266This is a potential intermediate for the preparation of the homoerythrin alkaloids

c 0

6

- Po

MeQC

Dimerisation Reactions

trans-Cinnamic acid irradiated within a cast film of dimethyldioctadecylammonium bromide (DODAB) leads to the syn-head-to-head dimer with high selectivity, implying that within the film the substrate monomers are aligned parallel to one a n ~ t h e r . ~ X-ray ~ ~ ~diffraction ~~* and differential scanning

172

Photochemistry

calorimetry studies of the film support this suggestion, but mixtures of cinnamic acid and DODAC show less and non-selective photodimerisation indicating a disordered molecular rearrangement. Excitation of the contact charge transfer band formed between the 1,l-diarylethylenes (151; Ar = 4-MeOC6H4, 4-MeC6H4, Ph) and molecular oxygen produces the corresponding 3,3,6,6tetraaryl-1,2-dioxanes(152), and subsequently the diarylketones (153).269It has been shown that (152) arises from (151'+)whereas (153) is formed by autoxidation of (151) by neutral radical species which may have their origin in reaction of the monomer cation radicals ( 15 1'+) with superoxide anion radical.

Irradiation of crystals of tertiary enamides (154) and (155) which are characterised by short intermolecular distances of 3.0-4.0 between the alkene carbon atoms affords the head-to-tail dimers (156) and (157) respect i ~ e l y . ~These ~ * observations stand in sharp contrast to the photopromoted E -+ 2 isomerisation of enamides (154) and (155) which is the exclusive reaction of these substrates in solution. One of the major products of the photodimerisation of 2-[2-(2-methylphenyl)ethenyl]naphth01[2,1-b]furan has been shown to be the new fused cyclobutane-naphthofuran derivative ( 158),271 and following irradiation of r- 1, c-2, t-3, t-4- 1,3-bis(4-methylphenyl)-2,4-bis(4pyridy1)cyclobutane using short wavelength light, trans- 1-(2-methylphenyl)-2(4-pyridy1)ethene together with some of the corresponding cis isomer are formed.272A degree of control of the trans to cis ratio could be exercised by varying the wavelength of the irradiating light.

A

A

0

IIl4: Photochemistry of Aromatic Compounds

173

Photodimerisation of 7-fluoro-4-methylcoumarin (159) yields the topochemically expected anti-HT photodimer (160) whereas 6-fluoro-4-methylcoumarin (161) produces the related syn-HH photodimer (162), but in higher yield.273This apparent anomaly has been rationalised in terms of formation of the syn-HH photodimer by reaction at defect sites. Moreover, the failure of the parent coumarin to undergo the [2 + 21 photocycloaddition which is observed for the fluorine derivative illustrates the importance of substitution of H by F.

F

Irradiation of naphthalene- 1-carbonitrile (163; X = CN) or methyl l-naphthoate (163; X = C02Me) at low temperature produces the corresponding syn-[2 + 21 cyclodimer exclusively (164; X = CN, C02Me), but under the same conditions this transformation is reported to fail with l-methylnaphthalene (163; X = Me), 1-methoxynaphthalene (163; X = OMe), and naphthalene (163; X = H) itself.274Evidence is cited to support the view that the excited singlet state of the substrate undergoes 1,4-4',1' dimerisation to the ex0-[4 + 41 cyclodimer (165; X = CN, C02Me) as primary product which X

subsequently participates in a Cope rearrangement to produce the syn-[2+ 21 cyclodimer in the presence of a triplet sensitizer. The photodimer of methyl 2-naphthoate can be efficiently isomerised to its monomer in the presence of appropriate sensitizers.275Use of singlet sensitizers such as 1-cyanonaphthalene and 9,1O-dicyanoanthracene trigger a cation-radical chain process in acetonitrile whereas in dichloromethane or benzene an exciplex or partial charge transfer mechanism seems to be involved. By contrast, in the presence of the triplet sensitizer chloranil, a highly efficient radical-cation chain process is promoted in all solvents. A study of the mechanism of the photodimerisation of acenaphthylene in the presence of tetracyanoethylene has focussed on the role of radical cation formation.276Although selective excitation (Lirr > 500

174

Photochemistry

nm) of the charge transfer band causes no reaction, excitation (Xjrr > 400 nm) gives both a cisoid and trartsoid dimer together with other adducts. It is suggested that the dimers arise by direct excitation of the acenaphthylene to give a triplet solvent-separated ion pair, followed by formation of the dimeric radical cation of acenaphthylene which finally leads to the products. High resolution 13C NMR has been used to investigate the photodimerisation of 9-meth~lanthracene.~~~ In the solid state, only the trans dimer is formed whereas in benzene solution both the trans and cis dimers are produced. Studies show that the maximum domain size of the minor component is about 0.3 pm, and that the reaction occurs at crystal defects in the monomer. Photodimerisation of the 9-substituted anthracenes AnCH2N+Me3Br- (166), AnCH2C02-Na+ (167), AnCH20H (168), AnCOMe (169) and AnCH3 (170) (An = 9-anthryl) in homogeneous solution, and also (170) in Nafion gives the h-t photodimers; however, h-h photodimers of (166) (169) are produced in Nafion membranes.278These observations have been rationalised in terms of a pre-orientation of the substrate molecules in the inverse micelle-like clusters of Nafion. s-Dipentacene has been produced by irradiation of solutions of pentacene, and photodecomposition of this dimer, dispersed in a poly(methy1 methacrylate) host matrix, to pentacene has been studied.279This retro-dimerisation involves a trapped intermediate which can be regarded as a ’broken dimer’ of two pentacene molecules. Irradiation of some 1,4-dihydropyridine derivatives leads in some cases to the formation of cis-dimers and in others to oxidation products,280and an enhanced regioselectivity has been observed for the [4 + 41 photodimerisation of 9-aminoacridizinium perchlorate (171; R = NH2, X = C104-) as compared with (171; R = H , X = Br-). The head-to-tail products syn (172) and anti (173) are produced exclusively.281Styryldicyanopyrazines undergo a selective topochemical photodimerisation in the solid state by a process whose reactivity and stereochemistry are controlled by differences in their molecular stacking.282

The mechanism of the photodehydrodimerisation of 2,5-dihydrofuran on suspended ZnS powders has been investigated using a variety of techniques.283 Both mono- and multilayer adsorption participate, and the substrate appears to be adsorbed perpendicularly to the surface at all of the available zinc sites. Dissociative electron transfer occurs from the adsorbed substrate to a reactive hole affording a proton and a dihydrofuryl radical. Urocanate esters have been dimerised in the presence of benzophenone to

M4: Photochemistry of Aromatic Compounds

175

mixtures of dimethyl or diethyl c-3,t-4-di-(1H-imidazol-4-yl)cyclobutane-r1,t2-dicarboxylate and dimethyl or diethyl t-3,c-4-di-(1H-imidazol-4-y1)cyclobutane-r- 1,t-2-di~arboxylate.~~~ Frontier orbital interactions have been used to account for the regiochemistry and in all cases the most stable dimers were obtained. Irradiation of crystalline O-methyl N-(2,2-dimethylbut-3-enoyl)-N-phenylthiocarbamate (174; R*R2 = -(CH2)4-) which exists in chiral space group P2 promotes intramolecular [2 + 21 thietane formation (175; same R1R2)followed by rearrangement to give the optically active y-thiolactone (176; same R R2).285

7

Lateral Nuclear Shifts

Evidence has been made available to show that the intramolecular 1,3hydrogen shift in the photo-Fries rearranged intermediate of phenyl acetate, and the 1,2-hydrogen shift in the photo-rearranged intermediate of N-acetylpyrrole using methylcyclohexane as solvent occur by tunnelling processes.286 The same authors have also carried out a kinetic study on the 1,3-sigmatropic hydrogen shift in (177), the intermediate in the photo-Fries rearrangement of 2,4-dimethoxy-6-(p-tolyloxy)-s-triazine ( 178) to 2,4-dimethoxy-6-(2-hydroxy-5methylpheny1)-s-triazine (179), and the rates were found to be enhanced by base catalysis arising from the solvent.287Intramolecular [ 1,3]-H and [1,3]-D shifts in the rearranged intermediates are shown to occur by quantum mechanical tunnelling at two vibrational levels, and it has been further established that the migrating hydrogen atom is transferred to the carbonyl oxygen intramolecularly without any catalysis from the adjacent triazine ring. Selectivity has been reported for some photo-Fries reactions carried out within a Nafion membrane.288 For example, under such conditions photoirradiation of PhCH2Co2C6H4R (R = H, 4-Me, 2-Me) produces the o-hydroxyphenones 2-HOC6H4COCH2Ph with high regioselectivity. The photochemical Fries

176

Photochemistry

reactions of [l,l’-biphenyl]-4-01 acetate, [ l,l’-biphenyl]-4-01 benzoate, [1,l’biphenyl]-2-01 acetate, and [ l,l’-biphenyl]-2-01benzoate have been examined in order to investigate a possible carbonyl transfer reaction.289As part of an evaluation of the use of X, Y,and Beta zeolites as catalysts for the photo-Fries rearrangement, it has been shown that the major product of the rearrangement of acetanilide is o-arninoacetophen~ne.~~~ The suggestion has been made that the selectivity for the ortho isomer rises with decreasing acidity. Photochemical rearrangement of 2-arylamino-1-(4-tert-butyIphenoxy)-9,10anthraquinones involves migration of the tert-butylphenoxy group either to the peri carbonyl oxygen atom to produce 2-arylamino-9-(4-tert-butylphenoxy)-1,lO-anthraquinones, or to the nitrogen atom to give 2-aryl(4-tertbutylpheny1)amino-1-hydroxy-9,1O-anthraq~inones.~~’ 8

Miscellaneous Photochemistry

In the solid state though not in solution, the halide ions in the hydrogen halide are salt of 11,12-bis(diethylaminomethyl)-9,10-dihydro-9,10-ethenoanthracene observed to cause heavy atom effects.292 Rate constants have been measured for the abstraction of a hydrogen atom from a range of alkylaromatics by alkoxy radicals and excited state ketones, and are found to be similar for the two abstractors.293Differences are largely accounted for in statistical terms. Photolysis of 1-(o-toly1)-1-benzoylcyclopropane and 2-(o-tolyl)-2-benzoyloxiranepromotes hydrogen transfer to give a 1,5-biradical intermediate, but an analogous reaction is not observed with cc-(o-tolyl)isobutyrophenone.294The rate constant for rearrangement of a substituted oxiranylcarbinyl radical has been determined from kinetic studies on these reactions. An investigation has shown that the photodecomposition of the [3]rotaxane (180) is slower than the naked dumb-bell; this may be of significance for a new approach to insulated molecular wires.295 The pseudorotaxane formed between (9-anthrylmethyl)methylammonium hexafluorophosphate and dibenzo-24-crown-8 can be unthreaded by addition of NBu4+ CIA, and rethreaded by addition of NHBu3+ PF6- .296 Such modifications cause profound changes in their luminescence spectra, and it has been suggested that such a system has potential use as a sensor for chloride ions. Following photolysis of methyl 2-diazo(2-naphthyl)acetate (1 8 1) singlet 2-naphthyl(carbomethoxy)carbene is formed, and this has been found to undergo intersystem crossing faster than Wolff rearrangement to the corresponding ~ a r b e n e . *Flash ~~ photolysis studies on aqueous 4X-C6H4N3 (X = MeO, EtO, i-Pro, t-BuO, C6H50,4-MeOC6H40, F, CI) and 4-methoxy1-naphthyl azide have been carried out .298 The products are p-benzoquinone or naphtho-1,4-quinone and arise by a pathway from the initially formed singlet arylnitrene through a nitrenium ion. The lifetimes of these nitrenium ions have been determined except those derived from the 4-halophenyl azides which are thought to be too short for detection on the ns timescale. Variousp-

IIl4: Photochemistry of Aromatic Compounds

177

substituted phenyl azides bearing a dimethylpyrazolyl group in the 2-position have been photodecomposed at low temperature, and some substituents found to allow intramolecular trapping of the singlet nitrene to give pyrazolobenzotriazoles; didehydroazepine can also be trapped by diethylamine to give 5Hazepines and subsequently 3 H - a ~ e p i n e sThis .~~~ provides information on how substituents affect the phenylnitrene S-T gap in relation to the barrier to ring expansion. Irradiation of 2,4-bis(diazo)-l,2,3,4-tetrahydronaphthalene1,3-dione (I 82) in methanolic benzene solution at birr > 300 nm gives the spironorcaradiene (183) along with methyl 3-0x0-2-diazoindan-1-carboxylate(1 84) in a ratio of These 2 : 1, but at hitr > 420 nm the amount of (183) produced is very observations suggest that long wavelength light is incapable of causing extrusion of molecular nitrogen from the 2-diazo group. 2'-Deoxyribonolactonecontaining oligodeoxyribonucleotides have been prepared by photolysis of nitroindole-containing oligodeoxyribonucleotides (1 85; R' = GC, R2 = TA).30' Irradiation of pyridinium perchlorate in dilute perchloric acid (birr = 254 nm) is reported to give an amino diol which can be acetylated to the corresponding amido-diacetate (1 86; R = Ac), and which itself can be converted into the a-mannosidase inhibitor (+)-mannostatin A (187).302The photochemical rearrangement of 1-acetyl-1,2-dihydroquinoline-2-carbonitriles to 3,l-benzoxazines and cycloprop[b]indoles has been described,303and photo-

178

Photochemistry

N2

lysis of the triazepines (188; R' = Ph, 2-MeC6H4, 4-MeC6H4; R2 = H, Me) produces 2,2-dimethylpropanenitrile and the 1H-pyrazoles (189; R' = Ph, 2-MeC&14, 4-MeC6H4; R2= H, Me) in high yield.3042,4-Bis(2-nitropheny1)-6methyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate can be photolysed to a mixture of pyrimido[1,6-b]- and -[1,2-b]indazoles (190; R = Me, R' = NO2) and 1,2,3,4(19 1) respectively, and similarly 2-phenyl-4-(2-nitrophenyl)-6-methyltetrahydropyrimidine-5-carboxylates yields pyrimido[1,6-b]indazoles (190; R = Me, Et; R1 = H).305 34 1-Naphthyl)-2-(1-naphthalenemethy1)oxaziridine undergoes a benzophenone sensitized ring-opening to produce 1aaphthaldehyde and N-( 1-naphthoyl)- 1-naphthalenemethylamine.306The reaction probably proceeds through a triplet state which decays by N - 0 bond cleavage as well as by N - 0 and C-N bond fission in the three-membered ring. The various tautomers of 3-amino and 3-hydroxyisoxazol-5(2H)-oneseach give a discrete photolysis and photoreaction of isoxazolone with indole-2,3-dione 11-benzazepine and 3-phenylisoxproduces 4,5-dioxo-3-phenylisoxazolo[5,4-b][ azolo[5,4-b]quinoline-4-carboxylicacid.308Photolysis of l-benzoylamino-4,5diphenyl-l,2,3-triazole gives 4,5-diphenyl-1(2)H-1,2,3-triazole via the 1,2,3triazole radical along with benzamide (192) and 1,2-bisbenzoylhydrazine (193).309Products (192) and (193) arise by hydrogen atom abstraction from, and dimerisation of the benzoylamino radical respectively. On photolysis of the cycloadduct ( 194) of 1,2-dihydrophosphine oxide and N-phenylmaleimide in the presence of protic species such as alcohols (ROH), the corresponding

IIt4: Photochemistry of Aromatic Compounds

179

phosphorylated derivatives (195) of the alcohol are produced and it has been suggested that fragmentation occurs by the eliminatiodaddition and addition/ elimination mechanisms concurrently. lo The intramolecular OH-ITinteractions of the phenolic and ethene chromophores in o-RCH=CH(CH2),C6H40H (R = H, Ph; n = 1,2) in the ground and excited states have been investigated by various procedures including ab initio HF MO calculations and gas-phase FTIR, and it has been concluded that these interactions are crucially important in the photochemistry of this ~ubstrate.~' In the presence of pyridine, phatolysis of benzocyclobutene-1,2-dione (196) leads to the formation of pyridine ylide (197), and this is thought to arise by reaction with bisketene formed from the substrate dione rather than from an oxacarbene (198).3 3-(2-Hydroxy-4-methoxyphenyl)-4-phenyl-2(5H)-furanone, one of the photoproducts of 6-methoxybenzofuran-2,3-dioneand styrene, has now been ~ y n t h e s i s e d . ~ ~ ~ 0

The synthesis of a coumarin C-ribofuranoside (199; R = Q1*2* 3, has been described and may be of value as a structurally novel photophysical probe for use in the study of ultra-fast DNA dynamics, hence providing some understanding of the sequence-dependentconformation of the DNA double helix on an ultra-fast t i m e ~ c a l e . ~ ~ ~ Photolysis of benzene solutions of l-methoxycarbonyl-2-naphthylmethyl 2,6-di-methyl substituted phenyl ethers induces C - 0 cleavage with formation of 2,4-cyclohexadienone intermediates which are subsequently photorearranged into meta substituted phenols.31 In methanol, 9-anthrylmethoxyundergo photoheterolysis to give pyrid-2-one or 1-pyrenylmethoxypyrid-2-one the C - 0 heterolysis products 1-hydroxypyrid-2-one and the arylmethyl methyl ether, together with 2-pyridone, aryl-substituted methanol and aryl aldehyde Evidence shows that an intraderived from homolysis of the N - 0 molecular exciplex plays a crucial role in C - 0 bond heterolysis.

180

Photochemistry

Visible light irradiation of sulfides using methylene blue as sensitizer induces electron transfer to the dye followed by fragmentation of the resulting phenylthio radical cation and formation of the corresponding o-quinone rnethide~.”~ These methide intermediates can be trapped by alkenes to give chromans. In the presence of a water-soluble 1,5-dialkoxynaphthalene as light absorber and electron donor, aqueous N-arenesulfonyl amino acids (200; R1= O(CH&OP03*-) have been photocleaved at the sulfonamide with formation of the corresponding amino acid (201).318This transformation probably occurs by electron transfer to give the sulfonamide radical anion (202) followed by its decarboxylation. Irradiation of arene carbothioamides and methanolic

OMe

furan leads to the formation of 2-arylpyrroles in a process which has been applied to the synthesis of pentagonal di- and tri-heterocyclic and some trithiobarbiturates undergo a photopromoted ring contraction with production of thiohydantoins and imidazolinothiophenes; treatment of the thiohydantoins with molecular iodine also gives imidazolinothiophene derivatives.320 Phenacyl esters (PhCOCH202CR) are reported to release carboxylic acids when irradiated in the presence of photosensitizers that are good excited state one-electron donors.321Such information provides the basis for a procedure that can be used to control the wavelength of the light required to trigger the release. Primary alcohols will react with 9-chloro-9-phenylxantheneto give the

181

IIl4: Photochemistry of Aromatic Compounds

corresponding 9-phenylxanthyl (pixyl) derivative from which on irradiation the alcohols are regenerated, and it has been suggested that these pixyl derivatives are a novel photocleavable group for primary alcohols.322 The 2-(2nitropheny1)ethoxycarbonyl and 2-(2-nitrophenyl)ethylsulfonyl groups are reported as new photolabile protecting groups in nucleoside and nucleotide chemistry.323The influence of substituents on the phenyl ring has been detailed and a new photocleavage mechanism involving a photoinduced p-elimination process has been suggested. Imidazole-1-sulfonates (imidazylates) of carbohydrates are reported to be photocleaved in the presence of triethylamine to give high yields of the deprotected sugar from which the imidazylate was originally produced,324 and a series of o-nitrophenyldioxolanes (203; R1 - R4=H; R 1 = R 3 = R 4 = H , R2=Me; R' - R3=H, R4=Me, O2CMe; R 1 = R 2 = H , R3 = R4 = Me; R1= 02CMe, R2 - R4 = H) have been prepared and the kinetics of their photodecomposition examined with a view to their use as protecting groups for pheromones.325A number of new linkers (204; R = NH2, OH, Br, NHMe, 02COC6H4N02-4, NHC02C6H4N02-4) for use in the synthesis of combinatorial chemical libraries on solid-phases and which are capable of undergoing mild photolytic cleavage have been described.3264-Hydroxy-2nitrobenzoic acid residue (205) on a solid support has been used as a photolabile linker for the solid phase construction of the dodecasaccharide (206; TBDPS = t-butyldiphenylsilyl, Bz = benzoyl, Bn = benzyl) and may be of use generally for the construction of diverse combinatorial libraries of oligosac~ h a r i d e s The . ~ ~ tosyl ~ group is an effective protector of the amino group in thymidine derivatives and has been reported to be capable of being removed photo~hemically.~~~ This technique has found successful application in the

TBDPS

I Bn

-

BZ

(206)

OH

Photochemistry

182

synthesis of 5'-amino analogues of 3'-azido-3'-deoxythymidine (AZT). A new family of photoprotecting groups suitable for use in the solid phase synthesis of oligonucleotides and polypeptides and having the general formula ArC(R1)(R2)-0-C(0)- (Ar = polycyclic, aryl, or heteroarom group; R', R2 H, alkyl, alkenyl, alkynyl, aryl, or heteroarom group) has been reported.329The indocarbocyanine and benzindocarbocyanine dye-linked phosphoramidites (207; R = H, trityl, 4-monomethoxytrityl, 4,4'-dimethoxytrityl, or acyl groups and R may be used as a protecting group or as an H; Ra = a phosphoramidite; Ria, Rlb = H, lower alkyl; R4, R5= H, lower alkyl, acyl, -CH = CHCH = CHor (CH2)p1CO2(CH2)qMe;where pl, q = a n integer from 0 to 4; m, n = a n integer from 0 to 10; p = 1, 2, or 3; X- = a negative ion) may be useful for fluorescent, non-radioactive labelling of oligonucleotides.330

\

ORa

(207)

'ORa

A large combinatorial library of octahydrobenzoisoxazoles (208; R = 2-1, 3-1, 4-1, 2-R4CC, 3-R4CC, 4-R4CC; R = alkyl, cycloalkyl, arylalkyl; R2= alkyl, cycloalkyl, aryl, arylalkyl, heteroaryl; R3 = NH2, CH2CONH2, (CH&CONH2; R4 = alkyl, aryl, arylalkyl) has been prepared using a procedure whose last step involves photochemical cleavage from the resin.331 The biphenylcarboxamide of (209; R = Me; R' = 4-PhC6H4CO; P = Tentage1 S) is reported to be readily photocleaved (kirr 365 nm), but in contrast to (209; R=Me) it is stable to acid, base, and Lewis acidlamine combinat i ~ n . ~ ~ ~

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267. T. Nakamura, K. Takagi and Y. Sawaki, Bull. Chem. Soc. Jpn., 1998,71,909. 268. T. Nakamura, K. Takagi and Y. Sawaki, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1998,313, 341. 269. M. Kojima, A. Ishida and S. Takamuku, Bull. Chem. SOC.Jpn., 1998,71,2211. 270. F. Song, J. H. Snook, B. M. Foxman and B. B. Snider, Tetrahedron, 1998, 54, 13035. 271. I. V. Cvijin, 2. Marinic and M. Sindler-Kulyk, Spectrosc. Lett., 1998, 31,989. 272, W.-Q. Zang, M.-J. Zhang, J.-X. Wang, X.-R. Yang, S. -L. Wang, Q. Jiang and Y. An, Huaxue Xuebao, 1998,56,612. 273. K. Vishnumurthy, T. N. G . Row and K. Venkatesan, Tetrahedron, 1998, 54, 11235. 274. T. Noh, Y. Jeong and D. Kim, J. Chem. SOC.,Perkin Trans. I , 1998,2501. 275. C. H. Tung, Y.-M. Ying and Z.-Y. Yuan, J. Photochem. Photobiol., A , 1998,119, 93. 276. N. Haga, N. Nakajima, H. Takayanagi and K. Tokumaru, J. Org,. Chem., 1998, 63, 5372. 277. K. Takegoshi, S. Nakamura and T. Terao, Solid State Nucl. Magn. Reson., 1998, 11, 189. 278. C.-H. Tung and J.-Q. Guan, J. Org. Chem., 1998,63,5857. 279. 0. Berg, E. L. Chronister, T. Yamashita, G. W. Scott, R. M. Sweet and J. Calabrese, J. Phys. Chem., A , 1999, 103,2451. 280. H. R. Memarian, M. M. Sadeghi and H. Aliyan, Indian J. Chem., Sect. B: Org. Chem. Incl. hied Chem., 1998,37,219. 281. H. Ihmels, Tetrahedron Lett., 1998,39, 8641. 282. J. H. Kim, M. Matsuoka and K. Fukunishi, J. Chem. Res., Synop., 1999,132. 283. G. Horner, P. Johne, R. Kunneth, G. Twardzik, H. Roth, T. Clark and H. Kisch, Chem. Eur. J. 1999,5,208. 284. M. D’Auria and R. Racioppi, J. Photochem. Photobiol., A , 1998,112, 145. 285. M. Sakamoto, M. Takahashi, T. Arai, M. Shimizu, T. Mino, S. Watanabe, T. Fujita and K. Yamaguchi, Chem. Commun. (Cambridge), 1998,23 15. 286. H. Shizuka and S. Tobita, JAERI-Con$ 1998,98,76. 287. Y. Kimura, N. Kakiuchi, S. Tobita and H. Shizuka, J. Chem. SOC.,Faraday Trans., 1998,94,3077. 288. C.-H.Tung and X.-H. Xu, Tetrahedron Lett., 1999,40, 127. 289. S . H. KO and W. K. Chae, Bull. Korean Chem. SOC.,1998,19,513. 290. K. J. Balkus, A. K. Khanmamedova and R. Woo, J. Mol. Catal. A: Chem., 1998, 134, 137, 291. I. Ya. Mainagashev, L. S. Klimenko, N. P. Gritsan, Russ. Chem. Bull., 1998, 47, 2437. 292. H. Ihmels, B. 0. Patrick, J. R. Scheffer and J. Trotter, Tetrahedron, 1999, 55, 2171. 293. S. T. Belt, S. Rowland and J. C. Scaiano, Mar. Chem., 1998,61, 157. 294. D. J. Chang, E. Koh, T. Y. Kim, B. S. Park, T. G. Kim, H. Kim and D.-J. Jang, Tetrahedron Lett., 1999,40, 903. 295. S . Anderson, R. T. Aplin, T. D. W. Claridge, T. Goodson, A. C. Maciel, G. Rumbles, J. F. Ryan and H. L. Anderson, J. Chem. SOC.,Perkin Trans. I , 1998, 2383. 296. M. Montalti, Chem. Commun. (Cumbridge), 1998, 146 1 . 297. J.-L. Wang, I. Likhotvorik and M. S. Platz, J. Am. Chem. SOC.,1999, 121,2883. 298. P. Ramlall and R. A. McClelland, J. Chem. Soc., Perkin Trans. 2, 1999,225.

192

Photochemistry

299. A. Albini, G. Bettinetti and G. Minoli, J. Am. Chem. Soc., 1999, 121, 3104. 300. S. Murata, J. Kobayashi, C. Kongou, M. Miyata, T. Matsushita and H. Tomioka, J. Am. Chem. Soc., 1998,120,9088. 301. M . Kotera, A.-G. Bourdat, E. Defrancq and J. Lhomme, J. Am. Chem. SOC., 1998,120, 11810. 302. R. Ling and P. S. Mariano, J. Org. Chem., 1998,63, 6072. 303. M. Ikeda, S. Matsugashita, C. Yukawa and T. Yakura, Heterocycles, 1998, 49, 121. Chem. Fr., 1997, 304. P. Bach, U. Bergstrasser, S. Leininger and M. Regitz, Bull. SOC. 134,927. 305. K. Goerlitzer and C. Heinrici, Pharmazie, 1998, 53, 847. 306. Y. Ohba, K. Kubo and T. Sakurai, J. Photochem. Photobiol., A, 1998, 113,45. 307. J. Khalafy, R. H. Prager and J. A. Smith, J. Chem. Res., Synop., 1999, 70. 308 R. T. Pardasani, P. Pardasani, S. Muktawat, R. Ghosh and T. Mukherjee, J. Heterocycl. Chem., 1999,36, 189. 309. C . P. Hadjiantoniou-Maroulis, A. Ph. Charalambopoulos and A. J. Maroulis, J. Heterocycl. Chem., 1998,35, 891. 310. G. Keglevich, K. Steinhauser, K. Ludanyi and L. Toke, J. Organomet. Chem., 1998, 570,49. 31 1 . M. T. Bosch-Montalva, L. R. Domingo, M. C. Jimenez, M. A. Miranda and R. Tormos, J, Chem. Soc., Perkin Trans. 2, 1998,2175. 312. S . Oishi and J. Ozaki, Chem. Lett., 1998, 1071. 313. V. P. Kamat, R. N. Asolkar and J. K. Kirtany, Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem., 1998,37,1269. 314. R. S . Coleman and M. L. Madaras, J. Org. Chem., 1998,63, 5700. 315. Y. Yoshimi, A. Sugimoto, H. Maeda and K. Mizuno, Tetrahedron Lett., 1998, 39,4683. 316. T. Sakurai, K. Kubo, S. Kojima, T. Shoro and H. Inoue, Tetrahedron Lett., 1998, 39,9747. 317. K. Chiba, Y. Yamaguchi and M. Tada, Tetrahedron Lett., 1998,39,9035. 318. G. Papageorgiou and J. E. T. Corrie, Tetrahedron, 1999,55,237. 319. K. Oda, M. Sakai, K. Ohno and M. Machida, Heterocycles, 1999,50,277. 320. H. Takechi, H. Takahashi and M. Machida, Heterocycles, 1999,50, 159. 321. A. Banerjee, K. Lee, Q. Yu, A. G. Fang and D. E. Falvey, Tetahedron Lett., 1998,39,4635. 322. A. Misetic and M. K. Boyd, Tetrahedron Lett., 1998,39, 1653. 323. H. Giegrich, S. Eisele-Buhler, Chr. Hermann, E. Kvasyuk, R. Charubala and W. Meiderer, Nucleosides Nucleotides, 1998, 17, 1987. 324. S. Duan, E. R. Binkley and R. W. Binkley, J. Carbohydr. Chem., 1998,17,391. 325. L. Ceita, R. Mestres and A. Tortajada, Bol. SOC.Quim. Peru, 1998,64, 5 5 . 326. E. V. Akerblom, A. S, Nygren and K. H. Agback, Mol. Diversity, 1998,3, 137. 327. K. C. Nicolaou, N. Watanabe, J. Li, J. Pastor and N. Winssinger, Angew. Chem. Int. Ed. Engl., 1998, 37, 1559. 328. W. Urjasz and L. Celewicz, J. Phys. Org. Chem., 1998, 11, 618. 329. G. H. McGall, N. Q. Nam and R. P. Rava, PCT Int. Appl. WO 98 39,348. 330. C. K. Brush and E. D. Anderson, U.S. US 5,808,044. 331. D. S. Tan, M. A. Foley, M. D. Shair and S. L. Schreiber, J. Am. Chern. SOC., 1998,120,8565. 332. S . M. Sternson and S. L. Schreiber, Tetrahedron Lett., 1998,39,7451.

5

Photo-reduction and =oxidation BYALAN COX

1

Introduction

Topics which have formed the subjects of reviews this year include electron transfer dynamics, organic reactions involving exciplexes and radical ions,2 solvent effects on formation and decay of singlet ex~iplexes,~ catalytic control of photoinduced electron transfer dynamics: supramolecular structural and magnetic control of photoinduced electron transfer reaction$ spin dynamics in photochemistry,6 the nodal-plane model in excited-state intramolecular proton t r a n ~ f e r the , ~ use of computational chemistry in elucidating the photochemical reaction pathways of 2-~arbonylstyrenes,*the photophysics and photochemistry of aromatic ketones and a-diketones in s o l ~ t i o n ,photo~ cyclisation via remote hydrogen transfer to ketone carbonyl oxygen,lo mechanistic aspects of saturated hydrocarbon photooxidation induced by hydrogen atom abstraction, excited state processes of fullerenes and functionalised fullerenes,l 2 photophysical processes of fullerenes,* alkylation reactions of c60 by photoinduced electron transfer, l4 intermolecular [2 + 21 photocycloaddition reactions of a l k e n e ~ , ' [2 ~ + 21-photocycloaddition reactions of c60, l 6 and photocatalytic reactions of porphyrin-based multi-electron transfer sensitizer~.~~ The role of solid state reactions controlled by crystal lattices'* has also been discussed.

2

Reduction of the Carbonyl Group

The ground state structure of the photochemical reaction site of benzophenones has been examined by the MO method and IR absorption spectrometry, and comparisons made with fluorenone.l9 An assessment of experimental observations concerning the influence of butyl methacrylate on the primary photochemical processes occurring in the reduction of benzophenone by triethylamine suggests that they may point to the participation of ternary exciplexes.20Such species may be of the form [donor + acceptor + alkene] and may play an important role in the electron transfer processes. Studies have been reported which characterise the triplet states of 0-, m-,and p-hydroxybenzophenones.21In non-hydrogen bond Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 193

194

Photochemistry

forming solvents, the excited triplet states of m- and p-hydroxybenzophenone have an nx* configuration and will undergo hydrogen abstractions to form ketyl and phenoxy type radicals. However, in those solvents which promote intermolecular hydrogen bond formation, the triplet state is too short lived to participate in H-abstraction processes. o-Hydroxybenzophenone is intramolecularly hydrogen bonded, and as a consequence internal conversion is highly efficient leading to a very short lived singlet state with the result that the triplet yield is low. Cyclobutanones in acetic acid undergo a regiospecific photoconversion into 2-acetoxy-5-alkoxytetrahydrofuranswith retention of configuration at the migrating a-position,22and an investigation of ultrafast relaxation processes in N,N-dimethylaminobenzylideneindan- 1,3-dioneas a molecular film has shown that formation and vibronic relaxation of the exciton states occurs in less than 100 fs.23Equilibration of the two trapped exciton states seems to occur within 20 ps. An examination of the photochemistry of 1-azaxanthone has shown that its reactivity toward hydrogen atom abstraction in polar media exceeds that of other aromatic ketones, and is a consequence of the unaltered n,n* triplet character which in turn stems from the presence of the pyridine ring.24 It has been suggested that this substrate may find use as a probe in radical pair reactions. The same authors have also considered the behaviour of 1-azaxanthone in aqueous solution and conclude that under these conditions the lowest triplet state possesses predominantly n,n* character.25 It is also observed that although there is high reactivity of 1-azaxanthone toward photoreduction in organic solvents, the generation of ketyl radicals in micellar systems is inefficient. Rate constants have been measured for the reductive quenching of benzophenone triplets by the sterically hindered amines, 2,2,6,6-tetramethyl- and 1,2,2,6,6-~entamethylpiperidine in a range of solvents of various polarities.26 The contact ion pairs decay by proton transfer forming the benzophenone ketyl radical, followed by back electron transfer and charge separation. This decay is observed to be solvent dependent and is rationalised in terms of the solvent dependence of the back electron-transfer process. A detailed investigation of the photochemistry of valerophenone has been carried out as a function of temperature, pH, and excitation wavelength in aqueous solution.27 Quantum yields were determined for the Type I1 process and for cyclisation to cyclobutanols, and the triplet lifetime estimated. The slower rate of H abstraction in aqueous media can be interpreted in terms of stabilisation of the excited ( ~ , n * state ) by water, and vibronic mixing and some degree of inversion of the reactive (n,x*) triplet state and the unreactive triplet (x,x*) state. Benzophenone moieties, suitably positioned relative to allylic and doubly-allylic H-atoms within a lipid environment can trigger peroxidation of the lipid via the triplet state of the aromatic ketone.28 If the structures of the targets are well authenticated, the reaction can be highly selective, and to assure the better positioning of the crucial chromophore, benzophenone-4-heptyl-4’-pentanoic acid has been used. Spectroscopic studies have shown that within alkali metal

1115: Photo-reduction and -oxidation

195

cation exchanged ZSM-5 zeolites, benzophenone exists in both protonated and hydrogen bonded form depending upon the cation in question.29Irradiation of such systems gives benzhydrol and benzpinacol, but in yields which strongly depend upon the kind of cations exchanged. In particular, the protonated species appears to fulfil a prominent role. The two rigidly linked porphyrin-naphthoquinone dyads (1) and (2), whose quinone carbonyl groups lie at different distances from the porphyrin macrocycle, have been prepared.30The rate constants for the photoinduced electron transfer in (1) appear to be independent of solvent dielectric constant, whereas (2), the quinone carbonyl groups of which are further from the porphyrin, Me

\

(1)

Me

he Me

displays photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. These molecules may find application as components in complex molecular devices. Quantum mechanical calculations have been performed at the semiempirical level for porphyrin- bridge-quinone systems and at the ab initio level for CH2-bridge-CH2 systems; in both cases the bridge comprises a number of aromatic saturated or mixed units.31 The electronic factor (A) for photoinduced electron transfer was obtained for the quinone systems, and for thermal reaction in the second systems. For perylene and staffane units, the dependence of A with distance is non-exponential for photoelectron transfer, but exponential behaviour is observed with the other bridges. Irradiation of a solid state mixture of indole and naphtho- 1,4-quinone gives SH-dinaphtho[2,3-a: -2’,3’-c]carbazole-6,11,12,17-tetroneand 2-(3indoly1)napht ho- 1,4-quinone.32 Triplet sensitized electron transfer from thymine to 9,1O-anthraquinone-2,6disulfonate in aqueous solution leads to ‘the anthraquinone radical anion and the deprotonated thymine-1-yl radical, both of which are spin polarised by the

196

Photochemistry

CIDEP triplet mechanism and pair radical pair mechanism.33 The radical anion dominates in weakly acidic solution, and the deprotonated thymine-1-yl radical displays two different radical pair polarisation patterns. This feature has been attributed to two different states in the primary radical pair. At 77 K, the lowest excited singlet and triplet states of 1-piperidinoanthraquinone give the N-ylide and the reduced compound at room temperature re~pectively.~~ In the latter process, indirect population of the excited state from a higher singlet state plays an important role. The absorption spectrum of the addition compound (i-C3H7C5H4)2WH2-9, 10-phenanthrenequinone shows an outersphere charge-transfer transition, excitation of which induces hydride transfer and as part from the hydride to the quinone to give 9,10-~henanthrenediol,~~ of an investigation of the photochemistry of 1-azaxanthone, it has been shown that, although there is high reactivity towards photoreduction in organic solvents, in micelles the formation of ketyl radicals is ineffi~ient.~~ The distance dependencies of photoinduced electron transfer rates have been examined in anthracene-spacered porphyrin-quinone c y ~ l o p h a n e sand ,~~ the same authors have also discussed the distance dependencies of photoinduced electron-transfer rates in benzene-, naphthalene-, and anthracenespacered porphyrin-quinone cyclophanes and biphenylene-spacered porphyrin-quinone c y c l o p h a n e ~ .Photoelectron ~~ transfer reactions of the porphyrin-quinone cyclophanes (3) and their zinc complexes have been examined, and in some cases at least interaction of the quinone carbonyl group with the zinc atom may be an alternative to through-space electron transfer.39 A study of intramolecular photoinduced electron transfer for the quinone-porphyrin cyclophane type (4) containing the especially strong acceptor 7,7,8,8-tetracyanoquinodimethane(TCNQ) has appeared.40 The distance dependence of the TCNQ and porphyrin is of particular interest, and to this end the corresponding 2,8-naphthalenediyl-TCNQ-porphyrin has been synthesised. The AM1 method has been used to calculate parameters such as HOMO and LUMO levels and spin density distributions in perylenequinone ( 5 ) and derivatives, and in combination with experimentally obtained data these results have made possible the elucidation of some photochemical and photophysical characteristics which may have mechanistic imp~rtance.~' In acetonitrile solution, the fluorescence of the polyhydroxylated perylenequinone, hypericin (6), can be quenched both by electron donors such as N,N-diethylaniline, and by the electron acceptors, methylviologen and a n t h r a q ~ i n o n e .It~ ~ has been suggested that hypericin may participate in ground state charge transfer complex formation with anthraquinone. Excitation of hypericin or its 0-alkylated derivatives in lipid vesicles results in excited state regioselective transfer of a proton to the substrate from one of the peri-hydroxyl groups of the pigment.43Addition of Ba2' and some other cations to calix[4]diquinones such as (7) which have pendant 2,2'-bipyridines, one of which is complexed with [Ru(bp~)2]~' has been found to increase their phosphorescence yield, and this has been rationalised in terms of a fall in the electron-transfer kinetics arising from an electrostatically driven conformational ~ h a n g e . ~ ~ . ~ ~

197

IIl.5: Photo-reduction and -oxidation

Ei

(4)

Et

198

Photochemistry

Irradiation of N,N,N-tributyl-N-(4-methylene-7-methoxycoumarin)ammonium borates promotes electron transfer from the borate anion to the singlet excited state of the coumarin, and in acetonitrile separation of the radical pair is efficient? Measurements have shown that electron donors will quench the excited state of triplet methyl phenylglyoxylate in excess of three orders of magnitude more rapidly than will hydrogen donors.47 The photophysics of a number of aromatic thioketones including 4H-1benzopyran-4-thione, xanthione, thioflavone, and Michler's thione have been determined as adsorbates on cellulose, in homogeneous solution of various polarities, and as P-cyclodextrin inclusion complexes.48Evidence is presented which shows that on cellulose the thioketones are bound at polar sites, that their lowest triplets are of (7c,n*)character, and that they have a wider range of decay constants than in P-cyclodextrin. 2,4,6-Triisopropyl-4'-(methoxycarbonyl)benzophenone has been reported to photocyclise normally to give the corresponding benzocyclobutenol (8;

X =p-C02Me) if its solid-state photolysis is carried out after grinding, after mixing with 2,4,6-triisopropyl-4'-(ethoxycarbonyl)beophenone), or at elevated temperature^.^^ X-Ray studies of the substrate show that although the distances between the carbonyl oxygen atom and the methine hydrogen atoms of the o-isopropyl group are small enough for hydrogen abstraction to occur, either a small cavity or the compact crystal packing around both of the o-isopropyl groups may interfere with the photocyclisation. Irradiation of o-benzylbenzophenone generates cis-1,2-diphenylbenzocyclobutenol. 50 Although this compound is stable at temperatures below O'C, it has been found that at room temperature an equilibrium is established with its E,E 0xylylenol precursor and this has been trapped with maleic anhydride. 2-Methyl-4-oxo-4-phenylbutanoyldimethylamineand 2-methyl-4-oxo-4-phenylbutanoate esters undergo a photocyclisation to the corresponding cyclobutanols in a process whose diastereoselectivity is dependent upon the carboxyl and y-sub~tituents.~' A competitive &-hydrogentransfer operates for 2-methyl-4-oxo-4-phenylbutanoylpyrrolidine in parallel with y-hydrogen abstraction. Irradiation of 4-0x0-4-phenylbutanoyl amines gives the corresponding &-lactamswith a diastereoselectivity > 99% following &-hydrogen ab~traction.~~ An examination of the photoreaction of valerophenone in aqueous solution as a function of temperature, pH, and wavelength has appeared.53Quantum yields have been measured for the Norrish Type I1 reaction, as well as for the cleavage to acetophenone and propene, and for cyclisation to two cyclobuta-

IIl.5: Photo- reduction and -0xida tion

199

nols. Quenching studies indicate a slower observed rate of H abstraction in aqueous solution which is attributable to stabilisation of the (n;,n;*) state by water, and to slight inversion of the reactive 3(n,n;*) and unreactive 3(x,n;*) states. The NorrisWang Type I1 photoreactions of a range of ketones all possessing a cis-4-tert-butyl-1-benzoylcyclohexane or 2-benzoyladamantane structure have been investigated in solution and in the solid state, and it emerges that in all cases ketones bearing methyl substituents a to the benzoyl group undergo stereoselective Yang photocyclisation to give endo-arylcyclob ~ t a n o l s ?These ~ transformations have been shown to proceed by an efficient triplet process. In one particular case, the steric course of the reaction has been mapped crystallographically enabling the y-hydrogen which is abstracted to be unequivocally identified. A comparison of the role of intramolecular hydrogen abstraction reactions in pentan-2-thione and pentan-2-one in their lowest triplet states using the AM-1 semiempirical MO method has shown that tunnelling of hydrogen is less significant to sulfur than to oxygen? Hydroxyketones (9) selectively rearrange to 1,4-dicarbonyl compounds (lo)? This transformation is initiated by a 194-H-abstractionand occurs intramolecularly (Scheme 1). The heat of reaction of intramolecular hydrogen abstraction by an excited carbonyl group has been determined for 1,3-dirnethylanthrone (1 1) as being 131 & 6 kJ mol-', and the suggestion has been made that the inter~ of mediate is the enol (12) rather than the corresponding b i r a d i ~ a lA. ~study the kinetics of hydrogen abstraction by acenaphthenequinone and 1-acenaphthenone triplets from a range of substrates, including cyclohexa-1,4-diene, propan-2-01, and phenols, has appeared.58 Although molecular oxygen has been reported to have no significant effect on the rate of photolysis of alkyl

O

H

OH

OH

OH

R' v R hv_ 2 R 1 W 2 0 H

I Scheme 1

OH

200

Photochemistry

phenylglyoxylates incorporating a reactive y-hydrogen atom, the products formed are different.59 This observation has been rationalised in terms of trapping of the 1,4-radical intermediate by ground state oxygen following triplet state y-hydrogen atom abstraction. The ornithine lactams (13; R = F3CC0, PhCHZOCO; n = 1-4; for R = F3CC0, racemic products are obtained; for R = PhCH20C0, products of absolute stereochemistry are obtained) and (14; same R) have been diastereoselectively synthesised.60 A mechanism involving &-hydrogenabstraction followed by cyclisation of the corresponding 1,6-biradicals seems to operate. Irradiation of 2-(N,Ndialky1amino)- and 2-(N-alkylanilino)ethyl benzoyl formates promotes regioselective c-hydrogen abstraction by the carbonyl oxygen through charge transfer states (15) and (16) to give the seven-membered ring lactone (17).61 Neither y- nor &hydrogen transfer are observed, and this is taken to imply that the rate of charge transfer interaction greatly exceeds direct y-hydrogen abstraction by the excited carbonyl oxygen atom. Photolysis of N-arylcarbonyl-N’-arylthiourea containing an o-halo substituent attached to the nitrogen of the thioamide triggers both Norrish Type I and Norrish Type I1 processes, the latter giving a benzothiazole by an electron-transfer reaction.62 In the absence of the halogen atom the nature of the products is observed to be solvent-dependent. R’

R’

6’

O O *’J

Time resolved studies have shown that following excitation of 2-methylbenzophenone, its lowest excited cis triplet undergoes intramolecular hydrogen atom transfer from the methyl group followed by decay to the ground state dienold3 At 77 K the trans dienol is found to be stable, and evidence is presented to show that the non-emissive lowest excited singlet or triplet state of the trans dienol gives the keto form of dihydroanthrone. In some related work by the same authors, both the lowest excited singlet and triplet states of 1-methyl and 1,4-dimethylanthraquinonesundergo intramolecular hydrogen atom transfer followed by closure of the corresponding excited biradicals to give 9-hydroxy-l , 10-anthraquinone-1-methide and 9-hydroxy-4-methyl-1,lOanthraquinone-1-methidemM Studies also reveal that 1-methyl-9,lO-dihydroxyanthracene is formed irrespective of the exciting wavelength, but that 1,4dimethyl-9,lO-dihydroxyanthracenearises at 313 nm only. It is suggested that the lowest excited triplet state of 1-methylanthraquinone abstracts a hydrogen atom from ethanol to generate the semiquinone radical which collapses to 1-methyl-9,lO-dihydroxyanthracene.An examination of the effects of electronic structure on excited state intramolecular proton transfer in 1-hydroxy-2-

IIl.5: Photo-reductionand -oxidation

201

acetonaphthone (18) and related compounds such as 1-hydroxy-2-naphthaldehyde (19) and methyl 1-hydroxy-2-naphthoate (20), suggests that (18) and (19) form a long lived keto-tautomer, and that the relaxation properties of excited (18) and (19), as distinct from those of (20), reflect differences in the relative stabilities of the enol and keto forms in their excited singlet state which are in turn influenced by electronic effects of the substituent on the carbonyl group? The effect of solvents on the photoenolisation of o-methylanthrone has been studied at low temperatures? In particular, the reactivities of 1,4dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d8 (1,4-DMAT) have been measured in methylcyclohexane, ethanol and 2,2,2-trifluoroethanol, and the predominantly lowest 3(n,7c)* states in non-polar solvents become the less reactive 3(?c,7t)* configuration in polar solvents. Below 90 K, there is a lack of thermal equilibrium between the two triplets, and deuterium transfer has been shown to occur by quantum mechanical tunnelling. Direct evidence has been obtained for the S1S1' photoenolisation of 3',4'-benzo-2'-hydroxychalcone by detection of the Sl'Si tautomer fluorescence spectra.67 The enolate anions of 2-acetylthiophene and 2-acetylfuran have been arylated under photochemical conditions in the presence of t-BuOK and good electron donors such as acetone enolate (entrainment reaction) to give the corresponding benzyl 2-thienyl and 2-furanyl ketones respectively.68 Use of FeBr2 as initiator in a dark reaction gives good yields of the substitution products without the need for added nucleophiles, and it is suggested that these arylation processes occur by an SRNl mechanism. 2-Alkoxyoxetanes(21) are formed regioselectively on irradiation of aromatic ketones (22) in the presence of electron-rich ketene silyl acetals (23), together with the silyl-migration product (24).69The product ratio is dependent upon both solvent and the structure of the silyl group, and correct choice of these variables enables the 2-alkoxyoxetane to be obtained exclusively. Regio- and stereoselective synthesis of protected cis-aminooxetanes has been achieved by photocycloaddition of aliphatic and aromatic aldehydes to the corresponding enamides or ene~arbamates.~~ Diastereoselectivity is high for the aromatic aldehydes. The chiral dihydropyrrole (25) has been used in an unprecedented facial diastereoselective Paterno-Buchi reaction for the synthesis of (+)-preussin (26),71 Paterno-Buchi reaction of L-ascorbic acid with benzaldehydes and benzophenone gives the corresponding oxetane lac tone^,^^ and 1,5diketones have been produced photochemically from o-quinones and 1,3diketones via keto-oxetanes.73 ?SIR3

Ar

The concentration profiles of the products formed on oxidation of the triplet state of 4-carboxybenzophenone by a series of substituted methionines and

202

Photochemistry

three methionine-containing dipeptides have been obtained, enabling branching ratios of the competing processes of back electron transfer, proton transfer and radical escape to be determined.74This has enabled the relative importance of these processes to be discussed in terms of proton transfer potential for sulfur-radical-cationic species. In sodium dodecyl sulfate submicellar and micellar solutions, the photochemistry of 2-pyridyl phenyl ketone is largely fast intramolecular photocyclisation followed by photoaddition of water; hydrogen abstraction from the surfactants by the triplet ketone does not compete.75 3

Reductionof Nitrogen-containing Compounds

Ab initio calculations have been used to predict the molecular structures, the energetic properties (proton affinity, electron affinity, bond dissociation energy and rotational barrier height) and the vibrational properties (harmonic wavenumbers, force fields, and potential energy distributions) of species likely to be involved in the photoreduction of various isotopomers of 4,4'-bi~ y r i d i n e These . ~ ~ species are the three isoelectronic closed-shell systems, and the three isoelectronic open-shell systems of their reduced forms. Photoinduced electron transfer from pyrene to methylviologen is reported to be enhanced in polystyrene latex dispersions, and back electron transfer is strongly suppressed; these effects result in a charge separation which is highly effective.77 The linkage length-dependence of intramolecular photoinduced electron transfer reactions in aromatic donor-viologen acceptor molecules connected by polymethylene bridges (27; R = 1-naphthoxy, n = 3, 6, 8, 10; R = 2-naphthoxy, n = 3-10, 12; R = 2-dibenzofuryloxy,n = 3,6, 8, 10) has been reported.78Such a length-dependence is only marginal, and the presence of P-cyclodextrin disrupts the formation of the intramolecular charge transfer complexes. An efficient photoenergy-harvesting and electron-transfer system has been described and consists of a bilayer membrane composed of two amphiphiles (28; ECz = N-ethylcarbazoyl) and (29; Ant = anthryl), and an electron accepting viologen group (30).79Examination of the fluorescent lifetimes of carbazoleviologen linked compounds having long spacers (2 nm) possessing a biphenyl unit and incorporated into two a-cyclodextrins has shown that compared with cases involving a simple alkyl spacer, reductions of about 2 ns are observed.80 This has been rationalised in terms of a contribution of the super-exchange mechanism to long-range electron transfer from the carbazole to the viologen. Carboxy and ester groups in photochromic 4,4'-bipyridines are capable of shifting reversible photoinduced electron transfer towards the formation of the stable radical cations which are responsible for colouring the crystal.8* Photoexcitation is thought to generate only monomeric radical cation species, and observed decreases in the sensitivity along the series Cl-, Br-, I- may be due to an internal heavy atom effect. A study of the oxidative quenching of photoexcited *[Ru(bpy)3I2' reveals that at pH 5, reaction with methylviologen (MV2+) occurs much more slowly than with 1,l-bis(2-~arboxyethyl)-4,4'-

111.5:Photo-reduction and -oxidation

203

.

(27)

a

Ant

yH2 I

Me It H H II Me-N+-(CH2)5-C-N-C-C-N, I Me 0 0 (29)

(28)

/C18H37 C18H37

Et -N

x

N

+

R I

yH2

/C18H37

0

Cl8H37

- (CH2)5- CII H N- CH C-N\ I1

0

(30) R = Ant, ECz or H

bipyridinium (BCEBP2+).82These and other differences may be accounted for in terms of charge effects on the diffusion and diffusional dissociation of an exciplex and an encounter complex. An examination has been made of the efficiencies of charge separation by electron transfer from [Ru(bpy)3I2+ to methylviologen via semiconductor TiOz xerogel particles.83From this it has been concluded that electron injection into the conduction band of TiOz is photosensitised, and that this electron subsequently reduces the methylviologen which is also incorporated into the support. A donor/acceptor system consisting of Ru(phen)bps (phen = 1,lo-phenanthroline, bps = disulfonated 4,7-diphenyl-1,lO-phenanthroline) and 4,4’-diheptylviologen has been used to evaluate the bimolecular electron-transfer rate constant from a donor in the aqueous phase to an acceptor anchored on the micellar surface.84 In aqueous media, ground state aggregation of the components causes biexponential decay of the emission in the presence of quencher with a rate constant similar to that observed with methylviologen itself. However, in conditions under which aggregation does occur, the quenching rate constant is observed to be much less, and the long- and short-lifetime components have been attributed to reaction of aqueous Ru(phen)bps with 4,4’-diheptylviologen in the aqueous phase or bound to the surface of the SDS micelles. Quenching and subsequent cage-escape efficiencies have been measured for the methylviologen [Ru(bpy),(4-methyl-4’-(2-arylethyl)-2,2’-bipyridine)l2+ (aryl = 2-naphthyl, l-pyrenyl, and 9-anthryl) showing that the overall yield of MV.+ depends partly upon specific solvent effects and partly upon the nature of the quenched excited state of the bichromoph~re.~~ A solvent dependency is apparent and this has been related to the cage-escape efficiency. In the bisviologen linked Ru(I1) complexes Ru(bpy)2(dcbpy)CmV~CnVB (m = 2, n = 3; m = 3, n = 4; dcbpy = 4,4-dicarboxy-2,2’-bipyridine)having different methylene chain lengths between the Ru complex and the viologen, no interaction exists between the Ru(I1) complex moiety and the bis-viologen.86However, in the

204

Photochemistry

excited state the Ru(bpy)z(dcbpy) grouping is oxidatively quenched by the bound bis-viologen, and intramolecular electron transfer is possible. Efficient photoinduced intramolecular electron transfer has been observed in the rigid U-shaped tetrad (31; Ar = 3,5-di-tert-butylphenyl)having terminal porphyrin and viologen units,87 as well as in the non-covalently linked donor and acceptor components of the [2]catenane complex (32; L = 4,4'-dimethyl-2,2'bipyridine).88 A comparison of the chemical and photochemically induced reduction of some 2(4),5-dihydro-1,2,4-triazines and aromatic 1,2,4-triazines has appeared.89

The photoreduction of organic nitro compounds over Ti02 has been studied in the presence of methanol or isopropanol as sacrificial donor and in the absence of molecular oxygen.9o Secondary radicals generated from oxidation of the alcohol do not feature in the reduction kinetics, and the by-products occur in the sequence RN02 -+ RNO -+RNH2.p-Nitroacetophenone has been photoreduced in propan-2-01 in a triplet state process to p-hydroxyaminoacetophenone, p-aminoacetophenone, and 4,4'-diacetyla~obenzene.~' Under similar conditions, p-hydroxyaminoacetophenone is photoreduced to p-aminoacetophenone and 4,4'-diacetylazobenzene, and it has also been observed that on irradiation monomeric p-nitrosoacetophenone affords acetophenone. The photoredox reactions of a variety of 1 : 1 charge transfer crystals prepared from dinitrobenzoic acids as acceptor and N-alkylcarbazoles as donor lead to a-oxidation of the N-alkyl groups.92 These transformations are much more efficient in the solid state than in solution, and a correlation has been found to exist relating the C - 0 distances between the carbazole N-a-carbon atom and the appropriate nitro oxygen atom. The dynamics of the photogenerated styrylpyridinyl radical and its dimer radical cation with the styrylpyridinium cation has been studied by fs flash p h o t ~ l y s i s . ~ ~

III.5: Photo-reduction and -oxidation

4

205

MiscellaneousReductions

Irradiation of 5-amino-substitutedendo-tricyclo[5.2.1.02.6]deca-4,8-dien-3-ones (33; R' = H, R2 = cyclohexyl; R1= H, R2= benzyl; R*= R2= morpholino) promotes an electron-transfer process which results in photoreduction of the norbornene c&9 double bond to give (34; same R', R2).94The anticipated [x2 + x2] photocyclisation to give the 4-amido-substituted 1,3-bishomocubanones (35; R=CH3, OCH3, benzyl) can, however, be achieved by irradiating the corresponding N-acylated analogue.

[C,-j~]Fullerene has been photochemically reduced by Et3N to give Cm-- and subsequently to C a H - , probably by protonation of Cm2- intermediate^.^^.^^ Following cessation of irradiation, the C60H- decays to Ca--. In polar solvents, mixtures of [C6o]fullerene and [C7o]fullerene and tetrakis(dimethy1amino)ethylene exist as radical ions, and in less polar solvents as radical ion pairs.97 Following excitation, photoinduced electron transfer occurs via the triplet state of the fullerenes to give the radical anions, the decay kinetics of which in polar solvents suggest that they are solvent-separated species; in less polar solvents the kinetic evidence indicates that ion pairs are present. The excited triplet state of [Cm]fullerene will undergo one-electron reduction to c60'- using the NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the corresponding dimer [(BNA)2]with a limiting quantum yield 0, = 1.3?8 Initial electron transference from (BNA)2 to 3Cm* triggers C-C bond cleavage in the resulting (BNA)2'+ to give BNA' and BNA+, and subsequently a second electron transference from BNA to Cm yields BNA+ and c60'-. Use of 4-t-butylated BNAH (t-BuBNAH) gives t-BuCm-, and selective two-electron reduction of [Cm]fullerene to 1,2-dihydr0[60]fullerene (1,2-C60H2)has been achieved using the NADH analogue 10-methyl9,lO-dihydroacridine under visible light. The observed differences between the selective one-electron and two-electron processes are accounted for in terms of the differences in redox and acid-base properties of NADH and the dimer analogues. A laser flash photolysis examination of [Cm]fullerene in polar solution in the presence of aromatic thiols, phenols, and disulfides with amino substituents shows that decay of 3C6< is accompanied by formation of Cm'-, and evidence is obtained suggesting that the corresponding monoadducts of [C60]fullereneare formed by electron transfer followed by consecutive radical coupling and protonation reactions.99 Photoinduced electron transfer to water-soluble mono-adducts and bis-functionalised [C6o]fullerene derivatives (36-4 1) have been subject to ESR spectroscopic examination in heterogeneous

206

Photochemistry

bold bonds indicate>C(CO2Na), functionalisation

water/propan-2-ol/Ti02 and in aqueous ascorbic acid, and this reveals signals that can be ascribed to the mono anions of C ~ O C ( C ~ ~ N ~ )and ~/~-CD C ~ ~ ( C ~ H ~ O N + ) /of ~-C which D , the latter undergoes conversion into two further radicals.loo The three water-soluble anionic bis-adducts (39-41) give a single line spectrum corresponding to a mono-anion only. A comparison has been made between the time resolved photolysis and steady state luminescence of the supramolecular Ru(I1)-Cm donor-bridge-acceptor dyad and the model complex Ru(II)(bpy)2(bpy-R).Io1 Decreases in the luminescence yield of the Ru(II)-Ca dyad are observed, and are attributed to intramolecular quenching of the MLCT state, and ps studies of Ru(II)-C60 reveal that this state rapidly transforms into (Ru(III)-Cm*-). The same authors also report a flash photolysis study of the effect of orientation on the photoinduced electron-transfer processes involving a fdlerene derivative covalently linked to aniline through a flexible chain.lo2 The electron-transfer processes of the novel-fullerene based dyad [Ru(CO)(TPP)(L)] (H2TPP = tetraphenylporphyrin, L = 42) have been investigated and the complex shown to undergo eight reduction- and two oxidation-processes,103~1@' and an examination of the photophysical properties and electrochemical redox potentials of the [60]fullerene-porphyrin dyad (43) reveals pronounced electronic interactions between the two z-7~stacked chromoph~res.'~~ A time-resolved study of photoelectron transfer reactions of the pyrrolidinofullerenes (C60-cyclo-CH2NMeCHR; R = H, cbH4N02-p C&CHO-p, C6H5, C6H40Me-p and C6H4NMe2-p) with N,N-dimethylaniline suggests that the rate constants for electron transfer via the triplet states in polar solvents are less than those involving [C60]fullereneitself.Io6 Substituent effects become marked in less polar solvents. Examinations on the picosecond resolved timescale of a tris(2,2'-bipyridine)-C~-ruthenium(II)-fullerenopyrrolidine dyad has shown that, following excitation of the ruthenium centre, rapid intramolecular electron transfer to the fullerene occurs with concomitant quenching of the ruthenium MLCT excited state. Io7 Time-resolved EPR using TEMPO free radical linked to fulleropyrrolidine has been used to probe photoelectron transfer involving the triplet state of the fullerene derivative as

III.5: Photo-reduction and -0xida tion

(42)

207

(43)

acceptor and ferrocene as donor. lo8 N-Methylfulleropyrrolidinium (Cm(C4HloN+)/I-) has been photoreduced by suspensions of Ti02 in organic media to the n-radical anion (C,.-)(C4HloN+), and proceeds analogously to unsubstituted fullerene (C60).log By contrast, in aqueous solutions no reduction is apparent, and it has been suggested that electron transfer is suppressed by the formation of fullerene clusters. Such clustering may be a general feature of the behaviour of [C6o]fullerene in aqueous media. Application of the ns laser flash photolysis technique to photoinduced electron-transfer reduction of [C~]fullerene/[C~]fullerene using zinc tetraphenylporphyrin (ZnTPP) in polar solvents has shown that under normal conditions C66-/c70'- is generated by photoelectron transfer from the ground state of ZnTPP to the triplet states 3C60f/3C70f.110 However, if the solution has a high concentration of ZnTPP, 3ZnTPP* donates an electron to the ground state of C&70, and it is this last process which is the more efficient. A study of the electron-transfer dynamics in a covalently linked carotenoid (C), porphyrin (P), and [C60]fullerene molecular triad (c'p'c60), and in the related dyad ( P - o ) , has shown that, following photoexcitation, electron transfer to the fullerene occurs to give P+c6().-.111 This state collapses to the first excited singlet state in toluene, whereas in polar solvents the carotenoporphyrin-fullerene triad forms the long-lived c'+-P'c60'-charge separated state. An examination of the effect of substituents on the quenching of the singlet excited state of various substituted anthracenes by indole derivatives reveals that those indoles bearing a methyl group at the N atom are much less effective than those possessing a hydrogen atom.112The differences in the quenching capacity are attributed to charge transfer interaction followed by proton transfer; a non-reactive decay route is involved with N-CH3 derivatives. The oligoheterocycles (44; X, Y =CH, N; Z = S, 0) are reported to have an enhanced electron acceptor character compared with the parent compound (44; X = Y = CH; Z = S), and the claim is made that oxidation of the oligomer is more difficult.Il3 Spectroscopic estimation of the HOMO/LUMO energy difference has been correlated with values obtained from semiempirical

208

Photochemistry

calculations. Photoinduced electron transfer has been reported to occur between indolic compounds such as tryptamine and tryptophane as donors, and 1-pyrenemethanol as acceptor.' l4 Compared with aqueous homogeneous solution, the transfer from tryptamine, but not tryptophane, is greatly enhanced in polystyrene latex dispersions, and this has been interpreted as reflecting an effective absorption of the tryptamine on the latex particles. Evidence is presented showing that quenching of the fluorescence of the 1-pyrenemethanoloccurs by a static mechanism. A study of the photoinduced electron-transfer reactions between excited singlet cyanoaromatics as acceptors and arylalkenes entrapped within zeolites has been shown to occur by a static process, but that between cyanoaromatics and either trans-anethole or 4-vinylanisole occurs by electron transfer to yield long-lived radical cations. l 5 These observations suggest that a zeolite environment may be valuable for controlling energy-wasting back electron-transfer steps. An investigation of the dynamics of the intermediate formed following quenching of 9,lO-dicyanoanthracene by various electron donors in solution has shown that charge recombination takes place predominantly within the exciplex. Photocatalytic reduction of carbon dioxide to formate and carbon monoxide on CDS particles, with and without modification by various thiols, has been shown to be solvent dependent,l17 and formic acid and formaldehyde have been photocatalytically produced from carbon dioxide in aqueous media using neutral red-coated TiO,; a tentative mechanism has been suggested for the process."* Photocatalytic reduction of carbon dioxide using Ti02 powders in supercritical fluid carbon dioxide followed by addition of water generates formic acid.' l9 The formic acid arises by protonation of reactive intermediates produced on the Ti02 powders. This procedure may be of value for efficient carbon dioxide conversion and fixation, and for solar energy storage and the production of industrial raw materials. Carbon dioxide adsorbed on the surface of zirconium oxide is photoconverted into formate in the presence of hydrogen.120 Bulk ZrO2 is not the photoactive species, rather it is a material formed by adsorption of C 0 2 on the 2 1 - 0 2 surface. Carbon dioxide has been photoreduced with water on titanium oxides anchored within zeolites.12' *122 In the gas phase at 328 K, there is high selectivity for formation of methanol by a process in which the charge transfer excited state of the highly dispersed titanium oxide is important. By contrast, use of the octahedrally coordinated Ti02 species produces methane with high selectivity. The same authors also report XAFS studies which indicate that the titanium may exist in either a tetrahedrally or an aggregated co-ordinated form. 123 Of these, the former species is highly selective for methanol production and the latter species for the production of methane. The photocatalytic reduction of high pressure C02

'

111.5: Photo-reduction and -oxidation

209

using Ti02 powders in isopropyl alcohol as positive hole scavenger has been reported to give methane,124and mixtures of carbon dioxide and nitrate have been photoreduced in various solvents using Ti02 nanocrystals embedded in S O 2 as photocatalysts to give formate and carbon monoxide, and ammonia respectively.125 Selectivity of these reactions is greatly influenced by the dielectric constant of the chosen solvent. Carbon dioxide has been photoreduced using CDS nanocrystallites which have been prepared in N,N-dimethylformamide.'26 Surface studies and MO calculations point to the existence of a correlation between the photocatalysis and the formation of sulfur vacancies, and experimental evidence suggests involvement of the C02 radical anion. The results have been useful in achieving the photofixation of CO2 into benzophenone, acetophenone, and benzyl halides. The surface characteristics of ZnS nanocrystallites such as ZnS-DMF(0Ac) in relation to their use as potential photocatalysts for the reduction of C 0 2 in the presence of triethylamine has been discussed. 27 Photoinduced charge separation processes in the supramolecular triad systems D*-A-A, D+-A--A and D+-A-A- have been investigated using three potential energy surfaces and two reaction coordinates by the stochastic Liouville equation to describe their time evolution.12*A comparison has been made between the predictions of this model and results involving charge separation obtained experimentally from bacterial photosynthetic reaction centres. Nitrite anion has been photoreduced to ammonia in aqueous media using [Ni(teta)l2' and [Ru(bpy)312+adsorbed on a Nafion membrane.12' 5

Singlet Oxygen

Quantum yields of 02( A& generation using different phthalocyanines and [Ru(bpy)3I2+ have been determined in DMF and aqueous CTAC micellar solution by both luminescence and chemical (1,3-diphenylisobenzofuran) methods. 30 Photochemical 02(lA& generators having cationic substantivity modifiers have been reported,131 and the same authors have also described systems possessing enhanced 02( 'A& generation resulting from their having tethered aromatic molecules. 32 Recent observations suggest that phenalenone (perinaphthenone), a sensitizer of O,('aJ, suffers significant photodegradation under steady-state irradiation in air-equilibrated 1,4-dioxan and N,N'dimethylacetamide caused by hydrogen abstraction from the solvent by its triplet excited state.133 It has also been shown that at least one of the degradation products contributes to 02( lA& sensitization. The ability of (45; R' = C(: O)P1, P1 = 46, R2 = H, X = C6H4-Me-4; R 1= COCH20P*, R2= H, X = C6H4-Me-4; R1= H, R2 = COCH20P1, X = C6H4-Me-4) and (47; R3= COCH20P', X = C6H4-Me-4) to generate 02( 'Ag) has been investigated. 34 Photodissociation of O3using 335 -352 nm radiation leads to the formation of 02(b1Z,+), and studies indicate that absorption of only a single photon is involved. 35

'

210

Photochemistry

BzNH

0

Ph

OR'

X

The absolute phosphorescence quantum yield (@p) of 02( lAg) sensitized by phenalenone and its lifetime have been determined in a range of different solvent^,^^^^' 37 and from measurements of the radiative rate constant and lifetime of 02(lAg) in various solvents, it has emerged that charge transfer interaction is the main factor responsible for removing the prohibition on the radiative spin-forbidden transition lAg -,3C,-.*38 Low temperature photolysis of 1,2,3,4-tetramethylnaphthaleneendoperoxide leads to a geminate radical pair capable of separating into the triplet and singlet states of 1,2,3,4tetramethylnaphthalene together with molecular oxygen, and which is able to quench both of these states.139Spectroscopic techniques have been used to probe both the pair distribution function and its time dependence. A study of the quenching of 02('Ag) by a large cross section of amines and aromatic hydrocarbons has shown that it occurs via an exciplex and involves partial electron transfer.'& From the data obtained it appears that in solvents which are difficult to oxidise such as toluene-dg and mesitylene, charge transfer is important in the non-radiative decay of O#Ag), and that although, in D20, N,N,N',N'-tetramethyl-p-phenylenediamine quenches 02( 'A& by full electron transfer, in other solvents electron transfer is not apparent. The establishment of an equilibrium for triplet energy transfer between 02(lA& and both 2,3,9,10,16,17,23,24-octahexadecylphthalocyanineand tetra-t-butylphthalocyanine indicates that these non-metalated phthalocyanines have identical triplet energies, and these observations also strongly support a mechanism of 02(' A&-sensitized phthalocyanine luminescence involving triplet energy transfer from 02('Ag) to the phthalocyanine triplet, 141 An intermediate has been detected in the 02(lA&sensitized delayed fluorescence using tetra-tertbutylphthalocyanine as fluorescer and C60 as sensitizer.142Modelling of the kinetics of the intermediate suggests either an exciplex or a triplet phthalocyanine mechanism, with the latter being the more probable. The quantum yields of O2(lAg) production obtained using a number of regioisomers of [6O]full-

111s: Pho to-reduct ion and -0xidation

21 1

erene-o-quinodimethane bis-adducts as well as the electronic structures of these substrates in the ground and excited states are reported to be dependent upon their addition pattern.143 Nile Blue A has been used to study the photophysical properties of the singlet oxygen dimolecule in solution, particularly the (lAg)2 + (3Eg-)2 transition; the radiative lifetime in CDC13 has been deduced to be 1.2 f 0.3 x lo3 s-l.l4 A new apparatus for evaluating the efficiency of a 02(lAg) quencher has been described and utilises a method which is based upon determining the ratio of the luminescence obtained from a photosensitizer and the emission from O2('Ad. 145 O&Ag) is reported to be produced when a donor-acceptor pair consisting of curcumine and thionine, immobilised on cellulose acetate film, is photoexcited in the presence of molecular 0 ~ y g e n . This l ~ ~ process may have relevance for the scavenging of oxygen. Measurements of the overall rate constants for the quenching of 02(IAg) by P-glycerrhizic acid and derivatives suggest that these cause decreases by up to three orders of magnitude,*47and a spectroscopic examination of the inclusion complex calix[8]arene-Ca in the solid state has shown its similarity to y-cyclodextrin-C60.14* In contrast to y-cyclodextrin-C60, however, this new complex photosensitizes the formation of O#Ag).

6

Oxidation of Aliphatic Compounds

Methane and ethane have been photooxidised to the corresponding aldehydes using a solid-supported vanadium oxide catalyst, V20s/Si02-IW (incipient wetness) at elevated temperatures.149 Both processes are highly sensitive to reaction temperature and to the method by which the catalyst is prepared. Linear alkyl ethenes have been photooxidised to epoxides by their irradiation under an oxygen atmosphere in the presence of Ti02 powder.150Rates of formation of the epoxide products are strongly dependent upon the type of catalyst used. The same authors have also examined the epoxidation of hex-2ene by passing it as a stream with gaseous oxygen over Ti02 powder, and find that the main product is 2,3-epoxyhexane.151 Under these conditions, the epoxidation is stereospecific, and evidence is presented to show that, on this photocatalyst, the trans-hex-2-ene diastereoisomer is more reactive than the cis. The relative order of activity of a series of porphines and oxotitanium(1V) and peroxotitanium(1V) porphyrinates as sensitizers for the 0 2 ( lA$ ene reaction has been shown to be H2(P)> 0 :Ti(P) 2 Ti(02)P, with the highest stability occurring for those porphyrins and porphyrinates which have fluorine sub~tituents.'~~ Some hydrocarbons have been selectively photooxidised in zeolites by molecular oxygen.153 For example, irradiation of 2,3-dimethylbut-

212

Photochemistry

2-ene loaded onto a dehydrated NaY zeolite produces 2,3-dimethyl-3-hydroperoxobut- 1-ene with greater than 90% selectivity, and, using dehydrated BaY zeolite, toluene gives benzaldehyde as the final product. Reaction of the adamantylidene substituted allylic alcohol (48) in CDC13 with 02(lAg) occurs diastereoselectively to give a mixture of the threo dioxetane (49), and the dioxolane (50). 154 Under analogous conditions, the acetate of (49) yields only the ene product. Hydrogen bonding in the exciplex between O#Ag) and the hydroxy group is invoked to account for the threo-selective hydroxy group directivity, and the interplay of conformational strain and hydrogen is used to rationalise the competition between ene, [4 + 21, and [2+ 21 modes of reaction. In the presence of tetraphenylporphine or methylene blue as sensitizers of 02( * A& limonene gives 1-methylidenyl-4-(1-propen-2-y1)-2,3’-dihydroperoxycyclohexane and 3-methyl-6-(l-propen-2-yl)-3,3’-dihydroperoxycyclohexene, which on further photolysis in benzene give 1-methylidenyl-4-(1-propen-2-yl)2,3-dihydroxycyclohexane and 3-methyl-6-(1-propen-2-y1)-3,3’-dihydroxycyclohexane respectively. 55 Sensitized photooxygenation of cycloalkenes using 2,4,6-triphenylpyrylium tetrafluoroborate leads to the formation of allylic hydroperoxides by an unusual electron-transfer process. 56 A study of the photooxygenation of 2,4-dimethylpenta-l,3-diene in a range of polar and nonpolar solvents has revealed that the differences in solvent-polarity effects on the [4 + 21 cycloaddition which forms an endoperoxide, and on the ene reaction which leads to an allylic hydroperoxide, can be accounted for in terms of a competition between the concerted and the perepoxide mechanisms. 57 Sensitized photooxygenation of 2,5-dimethylhexa-2,4-dieneproduces a perepoxide (51) which on rearrangement to an open biradicaloid zwitterionic intermediate (52) can give a dioxetane (53) and methoxy adduct (54).15*In aprotic solvents, the ene pathway to (55) becomes predominant as collapse of the zwitterionic intermediate to starting materials is a significant process.

Irradiation of oxygenated solutions of a-terpinene (56) in the presence of perylene diimide or 9,lO-dicyanoanthracene leads to the formation of p-cymene (57) in a process which occurs by an exciplex rather than by 02(lAg) pathways.159Hydroperoxy radicals may be involved in a quantum yield amplification process, and it is speculated that 0 2 quenching of the exciplex, or proton transfer within the exciplex, followed by trapping of the semi-reduced perylene diimide by molecular oxygen are possibilities.

M.5: Photo-reduction and -oxidation

213

Photosensitized oxygenation of the twisted 1,3-dienes (58; R' = H, Me, Et, Ph, CCH, R2= SiEt3; R' = CH : CH2, R2 = SiMe3, SiEt3) and the acyclic derivatives (+Me$ :CRCH : CHCMe2OSiEt3 (59; RH, Me, CHMe2, CMe3, CMe2-OH) have been studied, and the vinyl hydrogen Hafound to be more reactive than the allylic hydrogen Hb. For example, (58; R*= Ph, R2 = SiEt3) and (59; R=CMe3 selectively give (60; R=Ph) and HOCMe2C(CMe3) : C :CHCMe20H instead of the allylic alcohols (61; R = Ph) and

rnR? Me

Me Me

R

Me Me

Me

H2C :CMeC(CMe3)(OH)CH:CHCMezOH, which would have arisen from allylic hydrogen abstraction. 160 This implies that the higher reactivity of the vinyl hydrogen can be rationalised in terms of the large o*-n orbital interaction between the vinyl C-H bond and a second double bond in 1,3-dienes, which HOMO electron density calculations show to be significantly twisted. A study of the photosensitized oxidation using 9,lO-dicyanoanthracene of trans, trans1,4-diphenylbuta-1,3-diene, trans-stilbene, and a-pinene incorporated within Nafion membranes has shown that either O#AJ or superoxide radical anionmediated products are formed, and that the choice is a function of the status and location of the substrate and sensitizer in the medium.'61 These observations stand in sharp contrast to oxidations in homogeneous solution: the products under these conditions are derived by energy transfer and electrontransfer routes, and this behaviour is interpreted in terms of isolation of the sensitizer from the substrate during irradiation, with the consequence that electron transfer is suppressed. Chloranil will photoreact with 3P-methoxycholest-5-ene to give coupling products between the benzoquinone and steroid and tetrachloro-p-hydroat the 7-position, 3~-methoxycholest-5-en-7-o1, quinone. 16* These processes are thought to occur by a photoelectron transfer process. Photoinduced electron-transfer reactions between 1-cyanonaphthalene and norbornadiene have been examined.'63 In polar solvents such as methanol the norbornadiene radical cation is generated, and this is stereospecifically attacked by the nucleophilic methanol to give the em-adduct.

214

Photochemistry

[2 + 21-Cycloadducts originate on the exo-face of the norbornadiene in less polar solvents, and in non-polar solvents a 1 : 1 : 1 adduct is produced from the 1-cyanonaphthalene, the norbornadiene and the methanol. These observations have been rationalised in terms of the trapping of encounter complexes having different geometries. Various hydrocarbons including alkenes, alkanes, and alkyl substituted benzenes undergo a selective oxidation when irradiated within the cavities of alkali or alkaline earth ion-exchanged zeolites.164 An electron-transfer mechanism seems to operate and the large electrostatic field within the zeolite may be responsible for stabilising the highly polar chargetransfer states of a hydrocarbon-02 collisional pair. Alkenes such as trans,trans-1,4-diphenylbuta-1,3-diene, trans-stilbene, and 2,3-dihydro-y-pyran have been photooxidised while occluded within the framework of NaZSM-5 zeolites, and in the presence of 9,lO-dihydroanthracene and hypocrellin A as sensitizers. Since the molecular sizes of the sensitizers and solvents are great compared with the channels within the zeolites, electron-transfer oxidation cannot occur, and only products derived from 02(lAJ are detected. Dye sensitized photooxygenation of 3,4-dihydro-2H-pyran,(62) 5,6,7,8-tetrahydrochroman, and 2-oxabicyclo[4.6.0]dodec-1(6)-ene gives the corresponding 1,2dioxetanes.166 In the presence of acetaldehyde, (62) is additionally photooxygenated under these conditions to the cis-fused epimeric 1,2,4-trioxanes. Electron-transfer photooxidation from tricyclo[4.3.1.0'*6]deca-2,4-diene(63) to dicyanobenzene gives the corresponding radical cation which has been captured in a regiospecific, though not stereospecific, nucleophilic attack by methanol at the 2- and 5-po~itions.l~~ Substitution of the radical anion of the dicyanobenzene by this allylic radical occurs at the 3-position in a process compatible with a theoretical model capable of correlating radical cation reactivity with the spin density of the corresponding triplet state. From fluorescence lifetime measurements, it appears that following excitation of diazabicyclo[2.2.2]oct-2-ene,quenching may occur at least in part by an electron transfer mechanism.16*

The five hexa-functionalised c60 derivatives, CmC16, C60Ph5C1, CmPhSH, and two isomers of CmPh50H, have been prepared, and evidence is presented to show that on excitation electronic transfer occurs from the cage to the functional group. 69 Sunlight irradiation of solutions of fullerenes in chlorinated solvents leads to fullerene photooxygenation with the formation of fullerene epoxides.170 It is suggested that fullerene photooxygenation may be more complex than direct oxidation by 02( 'A&. Evidence has been provided to show that photochemical formation of CmO occurs by reaction of the lowest triplet state of C60 with 02(lAg) rather than from the ground state of the

IIl.5: Photo-reduction and -oxidation

215

organic substrate,I7l and visible irradiation of toluene solutions of [60]fullerene in the presence of methyl 2-furoate gives their oxides CmOn (n 2 5.172These are the highest oxides of fullerene which have so far been obtained photochemically, and calculations suggest a structure in which the epoxy groups lie in close proximity to each other on one side of the fullerene core. [60]Fullerene has been irradiated in the presence of various electron-transfer photosensitizers, and it is found that on addition of H-donors, such as alcohols, N,N-dimethylformamide, 1,3-dioxalane, aldehydes, and methyl formate, 1-substituted 1,2-dihydr0-[60]fullerenesare formed.*73The fullerene radical cation is involved. Irradiation of [60]fullerene at 420 nm in the presence of 9,lO-dicyanoanthracene and biphenyl produces the fullerene cation radical which reacts with alcohols, MeOCMe3, and propanal to give the 1 : 1 adducts H-Cm-R, 74 and mixtures of acetylglycine and [60]fullerene in methanolic solution will undergo photolysis to 1,4-dimethoxy-1,4-dihydrof~llerene.'~~ Tetramethyl ethylenediaminetetraacetate will photoreact with [60]fullerene to yield the EDTA-containing fullerene monoadduct C60(Me02CCH2)2NCH2CH2N(CH2C02Me)2together with other monoadducts. 176Pentamethyl dimethylenetriaminepentaacetatebehaves similarly, and both types of transformation are thought to proceed by a radical mechanism. [60]Fullerene has been photofunctionalised by irradiating in the presence of various tert-butyl substituted disilanes to give 1,16-adducts by silyl addition to the f ~ l l e r e n e , ' ~ ~ and 9-methylanthracene and [60]fullerene will undergo a selective Diels-Alder reaction by a photoinduced electron-transfer process in the solid state.17* A range of [2 + 2 + 21 bicyclic hexadiene derivatives (64; X = C(C02Me)2, C(C02Et)2, C(COMe)2, CH2, 0, NSO2-p-C6H&H3, C(S02Ph)2, and (65)) in which cycloaddition to [60]fullerene has occurred at the 6,6-ring junction of the fullerene have been prepared by treatment of [60]fullerene with terminal 1,6-diynes (66) in the presence of a nickel ~ a t a 1 y s t . lAll ~ ~ derivatives are oxidised by molecular oxygen in the presence of light to give peroxide (67) which undergoes conversion into the dialdehyde (68). Low temperature (20 K) excitation of a molecular triad consisting of a carotenoid polyene (C) covalently linked to a porphyrin (P), and which is further covalently linked to a fullerene derivative (C60), affords C-1P-C60.180This species decays by photoinduced electron transfer to c'P+'C60'- which itself evolves to c+'P-c60.and which subsequently decays to the carotenoid triplet. The charge separated - has been studied using time-resolved EPR. state C.+-P-C600

Co-adsorption of aliphatic and aromatic aldehydes and molecular oxygen on reduced Ti02 surfaces, followed by UV irradiation at low temperature,

216

Photochemistry

results in the formation of stable surface radicals identified as the peroxyacyl species (RC03*), and which are stabilised on the semiconductor surface. * Photooxidation rates of propan-2-01 in aqueous Ti02 suspensions are reported to be increased by ultrasound radiation, an observation which has been rationalised in terms of mass transport of the substrate and activation of the solid catalyst. 18* The value of the newly described photochemical rearrangement of 2-phenylthio-1,3-cyclohexanediols such as (69) to deoxysugars (70) which are in equilibrium with the closed form (71) has been illustrated by its application to the synthesis of (+)-cis-rose oxide (72),lS3and the same authors have also described the regioselective photorearrangement of 2-phenylthio-3aminocyclohexanols (73) to deoxyazasugars (74); this has proved to be useful in the synthesis of various piperidines (79, amino-sulfones, -sulfoxides and -acids.184 Hydroxy(a1koxy)methyl radicals have been generated by photoinduced electron transfer. 185

qoMe ?*

R3Y7R1 H

OMe

(74)

(75)

Photoinduced electron transfer from donors such as prenyl acetate, geranyl acetate, all-trans-farnesyl acetate, and all-trans-geranylgeranylacetate to 1,4dicyano-2,3,5,6-tetramethylbenzene,1,4-dicyanonaphthalene, and 9,1O-dicyanoanthracene in the presence of 1,l’-biphenyl as co-donor in acetonitrile produces the radical cation of biphenyl and the radical anion of the electron acceptors.186 Geranyl acetate is observed to photocyclise, and the mechanism of this process which involves reaction of its radical cation with water is discussed. Ally1 glycosides (76) can be photodeprotected to give (77) via (78) by irradiating with di-t-butyl peroxide in the presence of bromotrichloromethane. 187

IIl.5: Photo-reduction and -oxidation

217

Hypotaurine has been oxidised to taurine (2-aminoethanesulfonic acid) by which has been photochemically produced using methylene blue as sensitizer.188In the presence of azide, a well known quencher of 0 2 ( l A g ) , rather than the expected inhibition, an activating effect is observed, and this has been attributed to the generation of the azidyl radical. It is supposed that this radical participates in a strong one-electron abstraction process with the hypotaurine resulting in its oxidation. 02(IAg)

7

Oxidation of Aromatic Compounds

Benzene has been photoxygenated in supercritical COZ (kirr 248 nm) to give phenol as principal product plus smaller amounts of dihydroxybenzenes in a one photon absorption process.189Excitation of tri- 1-naphthyl phosphate and related compounds in the presence of 9,lO-dicyanobenzene produces 1,l'binaphthyl and the corresponding biaryl.lgo Following electron transfer which seems to occur at the diffusion controlled rate, the radical cation of tri-lnaphthyl phosphate forms an intramolecular mdimer radical cation with faceto-face interaction between the two naphthyl groups within 8 ns of the electron pulse. The rate constant for elimination of the 1,l'-binaphthyl radical cation from the intramolecular n-dimer radical cation is k, = 5.3 x lo5 s-l. In an investigation of the effect of solvents and substituent groups on the photooxidation of fluorene, it has been reported that the solvent effect is directly dependent upon the solubility of molecular oxygen in the media used.'9* 1-Methyifluorene has a stability comparable to that of fluorene itself, and 2-nitrofluorene, although largely unaffected by direct oxidation, does undergo a replacement reaction in the presence of a chlorine source. Kinetic isotope effects have been measured for hydrogen atom transfer pathways in the photooxidation of various toluenes by photoactivated quinones (Q*).19*The effect of added salts on the yields of the cation radicals ArH*+indicate that, following electron transfer to form the ion-radical pair [ArH-+,Q*-], proton transfer occurs. Photooxygenation of 4,5,6,8,16-pentamethyl[2.2]metacyclophane gives the corresponding [2.2]metacyclophane endoperoxide, a product which is stable to thermal deoxygenation.193 Photoinduced electron transfer from alkylpyrene derivatives in cationic dioctadecyldimethylammonium chloride (DODAC), neutral dipalmitoylphosphatidylcholine (DPPC) and anionic dihexadecylphosphate (DHP) vesicles to interface-water is dependent on the alkyl chain length of the alkylpyrenes and the interface charge on the ve~ic1es.l~~ The photoyield of the pyrene cation radical decreases with increasing alkyl chain length as well as with interface charge, and this has been attributed to increases in the electron transfer distance between pyrene as donor and the interface-water as acceptor. Tetraphenyiporphine-sensitized photooxygenation of (E,E)- or (E,Z)-1-arylpenta- 1,3-dienes gives most of the cis-endoperoxides, cis-3-aryl-6-methyl-1,2dioxacyclohex-4-enes, in a process which occurs by exclusive addition of 02( 'A& to the (E,E)-dienes formed by photoisomerisation of the (E,2)-dienes,

Photochemistry

218

the rates of both transformations being enhanced by the presence of electrondonating groups in the aryl ring.Ig5 It is thought that exclusive formation of the cis-endoperoxides suggests the participation of a concerted [4 + 21 cycloaddition. Irradiation of alcoholic solutions of trans rn-nitrocinnamic acid in the presence of Ti02 leads to a number of products including the 4-alkoxy1,2,3,4-tetrahydroquinoline(79; R = Et, n-Pr, n-Bu) and the new 7-membered ring lactone (80; same R).196These latter products may arise from (79). Following irradiation of a mixture of triarylstibines and styrenes, air oxidation gives the 2-aryl-l-phenylethanols (81; R = H, Me) in a transformation which has been rationalised in terms of valence expansion of the oxygen-antimonystyrene complex and subsequent reductive coupling.Ig7

& 0

&2R

R

H

CHiR

H

CH2R

Ar

OH

Irradiation of the contact charge transfer complex formed between transstilbenes and oxygen molecules in a zeolite NaY matrix at 313 nm leads to generation of the corresponding benzaldehydes in an electron-transfer process from which stilbene cation radicals and superoxide anion radicals arise.19* By contrast, excitation at 254 nm induces isomerisation and phenanthrene production, but without formation of any oxygenation products. Lead dioxide will oxidise 4,4'-(trimethylene)bis(2,6-di-t-butylphenol)leading to formation of a dispiro-compound by intramolecular cyclisation at the 4,4'positions, and irradiation of this compound in a methylcyclohexane matrix at - 150"C gives 4,4'-( trimethylene)bis(2,6-di-t-butylphenoxy) diradical as a stable triplet species.lg9 Rose Bengal photooxidation of 2,3-dihydroxynaphthalene and 2,7-dihydroxynaphthalenein basic aqueous solution gives the 1,l'-dimeric products by coupling of the radicals formed by electron transfer either to the excited sensitizer or to 02(1A&.200The dimer which originates from 2,7-dihydroxynaphthalene is subject to further oxidation to 6,7dihydroxyperylene-1,12-quinone. Irradiation of the dimethoxyphenol (82) under constant current electrolysis leads via (83) to formation of the substituted tricyclo[5.4.0.0'~5]undec-8-ene(84) which can be converted into (?)isoitalicene (85; R' = Me, R2 = H).201 The benzyl alcohols XC6H4CH20H (X = 4-Me0,4-Me, H, 3- and 4-C1 and CF3) have been photooxidised to the corresponding aldehyde by Ti02 in MeCN containing Ag2S04, and kinetic evidence is presented which suggests that in some cases there is a change-over in the electron abstraction site from the aromatic group to the hydroxyl group.202This has been rationalised in terms of preferential adsorption of OH on TiO2. 2,2'-Diiodobiphenyl reacts with arenediazonium hexafluorophosphate by a

IIl.5: Photo-reduction and -oxi& t ion

219

Meo% 0

Me0 Meo%

R'

MeO&-H

0

radical chain process initiated by PAIBN (phenylazoisobutyronitrile)and involving the cyclic diaryliodine which can be trapped as the iodonium salt This demonstrates that the lifetimes of the diaryliodine intermediates are sufficiently long to allow such trapping to occur in bimolecular processes.

Photooxygenation of natural visnagin (87) affords a variety of products including 4,9-dihydro-4,9-epidioxo-4-methoxy-7-methylidenylfuro[3,2-g]benzopyran-5,6-dione and 9-hydroperoxy-4-methoxy-7-methyl-5H-furo[3,2gIbenzopyran-5-0ne.~~ Similar studies have also been reported on the 4-hydroxyl derivative.

0

OMe

Theoretical studies of the photoinduced intramolecular electron-transfer reorganisation energies of a series of electron donor-acceptor systems, comprising 9,lO-dimethoxyanthraceneas donor and a benzyl group substituted with different electron withdrawing groups as acceptors linked by bisphenol A (88), have appeared.205A FTIR investigation of the autosensitized photooxygenation kinetics of hypocrellin in a range of chlorinated solvents has been reported,206and the same authors have also shown that decay to unchanged starting material is a first order process.207Photolysis of the alkyl phenyl sulfides (89; R 1= R2 = R3 = H; R 1= R2 = H, R3 = Ph; R1= H, R2= R3= Ph; R1= R2 = R3 = Ph) in the presence of tetranitromethane leads to the production of a mixture of oxidation and fragmentation products whose

220

Photochemistry

composition depends on the substrate structure.208 Benzyl ethyl sulfide is oxidised by 02(lAg) to benzaldehyde together with some of the sulfone, and, in the presence of a proton source, the corresponding sulfoxide is the principal Under aprotic conditions, the transformation involves an exciplex which undergoes intramolecular hydrogen abstraction to an ylide and which itself decays in a concerted or radical process to the product aldehyde. A second intermediate, possibly a persulfoxide, may also be present, and it is this which may be responsible for formation of the sulfoxide. Time-resolved transient absorption measurements of photoinduced electron transfer from 1-, 2- and 9-isomers of anthracenecarboxylic acid to either anatase or amorphous crystals of Ti02 have shown that the rates of the forward (dye-to-semiconductor) and the reverse (semiconductor-to-dye) reactions are a function of both the structure of the dye, and of the method by which the nanoparticles have been synthesised.210The polynuclear sulfurcontaining heterocycles, dibenzothiophene, thioxanthone, thianthrene and thioxanthene undergo Ti02-mediated photocatalytic oxidation in aerated acetonitrile, and it is suggested that an electron-transfer mechanism operates in which an electron-hole pair is photogenerated on the surface of the Ti02.21 8

Oxidation of Nitrogen-containing Compounds

The intramolecular charge separation and charge recombination processes of aminophenyl(pheny1)acetylene and N,N-dimethylaminophenyl(pheny1)acetylene have been studied in solvents of various polarities using ps lifetime measurements.212Protic solvents promote more quenching of the intramolecular charge separated state than do aprotic polar solvents, and triplet yields are correspondingly lower. This effect is rationalised in terms of increases in the radiationless transition probability to the ground state, and attributed partly to structural deformation around the nitrogen atom and partly to solvation effects. Radical ion formation from the quenching of the pyrene-N,M-dimethylanilineexciplex has been examined in reverse micelles of benzylhexadecyldimethylammonium chloride (BHDC) dispersed in benzene or chlorobenzene, and the quantum yield of their formation has been determined as a function of BHDC concentration and the amount of water dispersed.213 The results are interpretable in terms of the properties of the micellar surface and the fast exchange of material produced by collisions between reverse micelles. It has been shown that photoinduced electron transfer in both (90) and (91) originates from the dialkylamino unit attached to the 4-amino

1115: Photo-reduction and -oxidation

4’ @

221

0

\

/

position, but a major difference exists whic,, has its origins in the relative orientation of the photogenerated electric field of the 4-aminophthalimide excited state towards a given receptor.214This suggests a rationale for the unidirectionality of the photoelectron transfer process. A study of photoinduced charge separation has been made on systems of the type, (D-SITi02) in which S, the sensitizer, is a ruthenium(I1) bis-terpyridine complex anchored to the semiconductor surface by a phosphonate group, and D is either a 4-(N,Ndi-panisy1amino)phenyl group linked to the 4’-position of the terpyridine either directly or by a benzyl ether interlocking group, or alternatively D may 1 -ph~sphonate.~ Laser be a 3-[4-(N,N-di-p-anisylamino)phenoxy]propylflash photolysis has established the charge separation sequence, and has enabled lifetimes of intermediates to be extracted. Aromatic amines, thianthrene, and thiocyanate are reported to be photooxidised by [Wl0032]~-in an electron-transfer process, and under these conditions alkanes and butan-2-01 undergo hydrogen atom transfer.*I6Events occurring on the nanosecond time scale have been investigated, and the reactive species in these photocatalysed oxidations have been revealed as a long-lived ligand-to-metal charge transfer intermediate [w10032]4-. Photooxygenation of natural piperine (92) using a tungsten lamp gives 1-N-piperidino-1-ox0-5-(3-formyl-4-hydroperoxypheny1)penta-2,4-diene (93) and 1 -N-piperidino-1-oxo-5-(4-formyl-3-hydroperoxyphenyl)penta-2,4-diene (94).2*7By contrast, use of a sodium lamp gives both (93) and (94) along with a cyclic peroxide. In the presence of Rose Bengal as sensitizer and under either aerobic or anaerobic conditions, proline is photodecarboxylated to A’-pyrroline, and under similar conditions the methyl ester yields an equimolar mixture of A*and As-pyrroline-2-carboxylicacid methyl esters.218 These observations suggest that the reaction proceeds by a Type I photooxidation. A ns laser flash photolysis study of peptides composed of alanine (Ala) and tryptophan (Trp), modified with the (nitro)pyrenesulfonyl chromophore (Pyr and NPyr), reveals the existence of a triplet excited state local to the pyrene

222

Photochemistry

0

9

CHO

HOO

N

OOH

chromophore (3pYr).219The local triplet is not apparent for those cases in which there is a stronger electron acceptor group at the N-terminus (e.g. NPyrAla-TrpOEt and NPyr-Ala-Ala-TrpOEt), but has been replaced by the radical anion, NPyr--, which is thought to result from intramolecular electrontransfer quenching of excited NP species by pendant groups such as the indole ring of tryptophan. Dye-sensitized photooxidation of Tyr and Tyr di- and tripeptides (H-Tyr-Tyr-OH and H-Tyr-Tyr-Tyr-OH) using 02( IAJ in alkali media occurs through a polar intermediate encounter complex.220Collapse of this complex proceeds in an entropy-controlled step which may be solvent polarity dependent. Intramolecular electronic interactions between the styrylpyridinium cation and the styrylpyridinyl radical have been studied by examining photoinduced electron transfer within a structure consisting of two 4-(4-nitrostyryl)pyridines connected by methylene chains of different length.221 When a trimethylene chain is used as link, a new absorption peak is observed which is attributed to the intramolecular dimer radical cation for which two models have been proposed. This absorption peak is not observed in those cases in which the dimer model is linked by a tetramethylene or hexamethylene chain. In a study of the 02('A&mediated photooxidation of some monohydroxylated nitrogen-containing heterocycles, the nature of the hydroxy group plays a key role in determining the outcome of the transformation.222 In those substrates such as 2- and 4-hydroxypyridines, 4-hydroxyquinoline, and 4-hydroxypyrimidines in which the carbonyl function features mainly in its 0x0 form, interaction with the 02(lAg) reagent is severely suppressed. For 3-hydroxypyridine and 8-hydroxyquinoline in which the oxygen is present as the tautomeric hydroxy form, the photooxidation proceeds smoothly, and it has been suggested that a mechanism involving a charge-transfer complex may be operating. Using Rose Bengal as sensitizer, isoquinoline-1,3-diones have been oxidised by 02('AJ to such products as isoquinoline-1,3,4-triones, methyl 1-hydroxy-3-oxoisoindole-1-carboxylates, and 3-hydroxy-3-alkyl(ary1)aminocarbonylbenzoisofuran-1-ones, depending upon the solvent chosen.223 0 2 ( AJ generated using tetraphenylporphyrin has also been used, in which

HIS: Pho t o-reduction and -oxi& tion

223

cases the oxidation products from the 4-alkylated isoquinoline-1,3-diones are

4-alkyl-4-hydroxyisoquinoline1,3-diones, 4-alkyl-4-hydroperoxyisoquinoline1,3-diones, and the 3-alkyl-3-hydroxy-benzoisofuran1-ones. Sensitized photooxygenation of the benzannelated isoquinolinones (95; X = 0, S; R = H , MeO, Me, Cl) and (96; R=H,Me, Ac, CH2CH20H) occurs by initial attack of 0 2 ( 'A& on the enol ether-enamine C = C bond, and proceeds via zwitterionic and endoperoxidic intermediate^.^^^ In acetonitrile, (97) is formed exclusively, and in methanol (97) is produced along with the solvent trapped product (98).

2-Methylindole is reported to form a charge transfer complex when irradiated on the surface of colloidal CDS particles.225 Under aerated conditions, irradiation of a mixture of 2-methylindole and colloidal CDS using visible light produces 2-methyl-3-indolinone together with 2-acetamidobenzaldehyde which arises by oxidative cleavage of a C-C bond in the pyrrole ring. A general mechanism for the CDS-induced oxidation of indoles is presented. A time resolved CIDEP study of radical pair systems derived by electrontransfer photooxidation of carbazole derivatives using maleic anhydride has revealed that the singlet state of the radical ion-pair has an energy which is greater than that of the triplet state.226From this evidence the conclusion is drawn that the sign of the apparent exchange interaction is positive. The photooxidation of oxopurines such as caffeine, theophylline, theobromine, and 1,3,7-trimethyluricacid using Rose Bengal as sensitizer occurs by a type I1 mechanism.227 3-Methyl-5-(methylamine)-l,5-dehydrohydantoin has been characterised as a reaction product, and evidence is presented which suggests that the initial exciplex formed between O2(lAg) and the oxopurine evolves into a zwitterionic transition state. A post-column photochemical reaction detection system based on the reaction of 3-substituted pyrroles with OZ(*Ag), and which can be used in conjunction with HPLC, has been described.228 The method consists in irradiating the HPLC analytes (02( 'A&-sensitizers) in the presence of either added tert-butyl 3,4,5-trimethylpyrrole-2-carboxylateor added tert-butyl

224

Photochemistry

N-benzyl-3-methoxypyrrole-2-carboxylate,and searching for loss of the pyrrole. Photoinduced intramolecular electron transfer in porphyrin triads of cthe type [zinc octaethylporphyrin-4,4'-bipyridinium-tetraphenylp0rphyrin]~+ occurs either from the singlet state of the zinc porphyrin (lA) or from the corresponding free base (lB) with formation of A.+V. or B.+V.229 An energy level diagram has been proposed and the efficiencies of the various transfer processes discussed. The bolaamphiphilic tetraresorcinolporphyrins having eight long side-chains (octopusporphyrins) and their metal complexes (99; R = 100, 101, 102) will also undergo a photoinduced electron transfer process and this occurs between the porphyrin centre and the hydrophobic quenchers as well as the hydrophilic q ~ e n c h e r s An . ~ ~exam~ ination of the oxidation of various meso-tetraphenylporphyrins by 0 2 ( Ag) shows that those substrates having a 2,6-disubstitution pattern display a high degree of stability in contrast to porphyrins possessing a different substitution pattern.23* Steric effects seem to dominate electronic effects, however, and this appears to be the case even for electron donors at the 2,6-positions. Strong parallels exist with observations made with oxygenated donors. The novel parachute-shaped Ca-porphyrin diad (103) shows strong quenching of the excited singlet state of the porphyrin by the attached f ~ l l e r e n e The . ~ ~ ~quenching process occurs by electron transfer. Excitation of a series of molecular triads, (C-P-Q), consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a naphthoquinone (Q), leads to formation of the C-'P-Q singlet state.233This yields the charge separated state C-P+-Q.- which decays to C.+-P-Q*- in competition with fast electron-hole recombination. Evidence is presented to show that in those quinones which feature an internal hydrogen bond, the lifetime of the corresponding P + is longer.

(99) (100) R = -OCOCMe2(CH2)l~OP(02-)O(CH2)2N+Me3 ( 1 01) R = -OCO(CH2), 1)OCO(CH2)2C02CH2CH[O(CH2)1 7Me]CHpOP(02-)O(CH2)2N+Me3 (1 02) R = -OCO(CH2)10-N+

3

IIl.5: Photo-reduction and -oxidation

225 Et

9

Et

Miscellaneous Oxidations

Electronic excitation of 1,2-bis[4-(N,N-dimethylamino)phenyl]ethane1,2-diol in aerated chloroand 2,3-bis[4-(N,N-dimethylamino)phenyl]butane-2,3-diol form induces photoelectron transfer to give the chloroform radical anion followed by its subsequent dechlorination, and also a retro-pinacol reaction in the substrates.234 This latter process leads to 4-(N,N-dimethylamino)benzaldehyde and 4-(N,N-dimethylamino)acetophenone. Irradiation of dicyanonaphthalene and thioanisole in acetonitrile solution leads to radical dimer cations with Amax 470 and 800 nm.235 Similar intramolecular complexes are formed for 1,n-bis(pheny1thio)alkanes (n = 3 and 4), but with bissulfides (n=2, 6, and 8) different radical cation spectra are produced. It has been suggested that the dimer radical cations (Amax 460-500 nm) are the a-type of complexes having a sulfur-sulfur threeelectron bond, and that the complexes showing Am,, 800 nm are of the 7c-type. An investigation of the photophysical properties of 3,3’-bridged 2,2’bithiophenes {dithieno[3,2-b:2’,3’-d]-thiophene (104),4-~yclopenta[2,1 -b : 3,4b’ldithiophene (109, and 4H-dithieno[3,2-b;2’,3’-d]pyrrole( 106)) and 2,2’bithiophene (107) has been reported, and photochemical generation of the radical cations has been confirmed.236The mechanism of the one-electron photooxidation of N-methionyl peptides has been discussed.237This research has shown that sulfide radical cations photolytically generated in Met-Met and Met-Met-Ala using triplet carboxybenzophenone intramolecularly form sulfur-sulfur three-electron-bonded radical cation complexes of the type [R2SSR2]+,and that these react to yield the disulfoxides Met(0)-Met(0) and Met(0)-Met(0)-Ala.

Photochemistry

226

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207. Z. Zhang, I. Hu, X. Hu, S. Sun, W. Lu, Q. Zhou, Q. Wang and Y. Hu, Guangpuxue Yu Guanpu Fenxi, 1998,18,57. 208. W. Adam, J. E. Argueello and A. B. Penenory, J. Org. Chem., 1998,63,3905. 209. S . M. Bonesi, M. Mella, N. d’Alessandro, G. G. Aloisi, M. Vanossi and A. Albini, J. Org. Chem., 1998,63, 9946. 210. I. Martini, J. H. Hodak and G. V. Hartland, J. Phys. Chem. B, 1998,102,9508. 21 1. A.-M. A. Abdel-Wahab and A. E.-A. M. Gaber, J. Photochem. Photobiol., A , 1998,114,213. 212. Y. Hirata, T. Okada and T. Nomoto, J. Phys. Chem. A , 1998,102,6585. 213. C. D. Borsarelli, J. J. Cosa and C. M. Previtali, Photochem. Photobiol., 1998,68, 438. 214. A. Prasanna de Silva and T. E. Rice, Chem. Commun. (Cambridge), 1999,163. 215. P. Bonhote, J.-E. Moser, R. Humphry-Baker, N. Vlachopoulos, S. M. Zakeeruddin, L. Walder and M. Graetzel, J. Am. Chem. SOC.,1999,121, 1324. . 216. D. C. Duncan and M. A. Fox, J. Phys. Chem., A , 1998,102,4559. 217. S . N. Ayyad and E. M. Elgendy, Heterocycl. Commun., 1998,4,449. 218. K. Endo, K. Hirayama, Y. Aota, K. Seya, H. Asakura and K. Hisamichi, Heterocycles, 1998,47, 865. 219. G. Jones and L. N. Lu, J. Org. Chem., 1998,63,8938. 220. S . Criado, A. T. Soltermann, J. M. Marioli and N. A. Garcia, Photochem. Photobiol., 1998,68,453. 221. S . Kashihara, H. Kawai and T. Nagamura, Shizuoka Daigaku Denshi Kogaku Kenkyusho Kenkyu Hokoku, 1997,32,7. 222. A. Pajares, J. Gianotti, E. Haggi, G. Stettler, F. Amat-Guerri, S. Criado, S. Miskoski and N. A. Garcia, J. Photochem. Photobiol., A , 1998,119,9. 223. L. Ke-Qing, J. Gang, C. Hu and X. Jian-Hua, Tetrahedron Lett., 1998,39,2381. 224. K.-Q. Ling, H. Cai, J.-H. Ye, J.-H. Xu, Tetrahedron, 1999,55, 1707. 225. A. Kumar and S. Kumar, J. Phys. Org. Chem., 1998,11,277. 226. A. Sekihara, H. Honma, T. Fukuju, K. Maeda and H. Murai, Res. Chem. Intermed., 1998,24, 859. 227. D. H. Murgida, P.F. Aramendia and R. E. Balsells, Photochem. Photobiol., 1998, 68,467. 228. K. Denham and R. E. Milofsky, Anal. Chem., 1998,70,4081. 229. M. El Baraka, J. M. Janot, L. Ruhlmann, A. Giraudeau, M. Deumie and P.Seta, J. Photochem. Photobiol., A , 1998, 113, 163. 230. E. Tsuchida, T. Komatsu and J.-H. Fuhrhop, Polym. Adv. Technol., 1998,9,569. 231. A. M. S. Silva, M. G. P. M. S. Neves, R. R. L. Martins, J. A. S. Cavaleiro, T. Boschi and P. Tagliatesta, J. Porphyrins Phthalocyanines, 1998,2,45. 232. P . Cheng, S. R. Wilson and D. I. Schuster, Chem. Commun. (Cambridge), 1999, 89. 233. S.-C. Hung, A. N. Macpherson, S. Lin, P. A. Liddell, G. R. Seely, A. L. Moore, T. A. Moore and D. Gust, Adv. Chem. Ser., 1998,254,177. 234. W. Zhang, L. Yang, L.-M. Wu, Y.-C. Liu and Z.-L. Liu, J. Chem. Soc., Perkin Trans., 2, 1998, 1 189. 235. H. Yokoi, A. Hatta, K. Ishiguro and Y. Sawaki, J. Am. Chem. Soc., 1998, 120, 12728. 236. M. Fujitsuka, T. Sato, F. Sezaki, K. Tanaka, A, Watanabe and 0. Ito, J. Chem. SOC., Faraday Trans., 1998,94,3331. 237. B. L. Miller, K. Kuczera and C. Schoeneich, J. Am. Chem. Sac., 1998,120,3345.

6

Photoreactions of Compounds Containing Heteroatoms Other than Oxygen M. HORSPOOL AND ALBERT C. PRAlT ~

BY WILLIAM

1

~~~

~

~

~

~

~~

~~

Introduction

Several review articles have been published that deal with areas pertinent to the material described in this chapter. Albini and Fasani' have reviewed the photochemistry undergone by drugs. In particular the problems of handling, packaging and labelling of drugs were discussed. Others have reviewed the photoreactivity of quinoline based antimalarials such as mefloquine and primaquine.2 Addition reactions have also been of interest and the dimerisation of acridinium salts in the solid state is one of the areas covered in a detailed review of photochemical reactions in the solid state.3 A review has highlighted the photochemical syntheses that can be used for the construction of a variety of pyridine derivative^.^ The processes are based on 2+2+2cycloadditions between alkynes and nitriles. An article has discussed the mechanism for the photosubstitution of cyano groups in aromatic compounds in general and one section has detailed the mechanism for such photosubstitution in pyrazine and quinolines.' Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review.6 Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc7 and the photoinduced electrontransfer reactions involving organosilicon compounds.* Several publications have dealt with electron transfer and exciplex photochemistry such as the review by Mataga.9 Catalytic processes brought about by photoinduced electron transfer have been reviewed'* and the application of spectroscopic methods for the determination of the barriers to electrontransfer processes have been highlighted. Other articles have been concerned with processes involving intramolecular transfer over long range or close contact,I2 long-range charge separation in donor-bridge-acceptor systems1 and solution phase electron transfer through sigma bonds.14 A short review has highlighted some aspects of the reactions encountered with photo-activatable caged peptides.

'

Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000

234

IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen

2

235

Nitrogen-containing Compounds

2.1 E,Z-Isomerisations - An examination of the influence of solvent and wavelength on the photochemical reactions of (a-N-isopropylcinnamide has been reported? The efficiency of the isomerism using 254 nm or 312 nm irradiation was dependent on the solvent composition and because the trans and the cis isomers have different absorption characteristics. The quantum yields for isomerisation using 312 nm radiation with a water/THF mixture of 30 : 70 were @E,Z = 0.19 and @z,,= 0.15 and with a water/THF mixture of 80 :20 were OE,z= 0.38 and = 0.35. The photochemical trans,cis-isomerism of the indolylethenes (1, 2) is observed even in the presence of amines.l7 A study of the photochemical reactivity of pyrazinylquinoxalinylethene has shown that there is extensive mixing between the nlc* and the m* excited states.'* trans,cis-Isomerism of the ethene bond in (3) occurs on irradiation and the influence of intramolecular proton transfer on the isomerism process has been in~estigated.'~ Proton transfer, which is a 1,5-process, is also observed in salicylideneaniline and takes place from the phenolic OH to the nitrogen resulting in the formation of the keto form.2oA computational investigation of the photoisomerisation of salicylideneaniline has been carried out. Other workers have reported that the colouration process observed with salicylideneanilinesis due to a keto-enol tautomerism.*' Intramolecular proton transfer has also been studied in 2-(2-N-palmitoylaminophenyl)benzimidazole.22A detailed study of the cis,trans-isomerism of the protonated Schiff base of retinal (4) has sought to unravel the mechanisms which control the selectivity of product formation.23Calculations have been carried out on the protonated Schiff bases (5) and (6) to establish the minimum energy paths in the S1 and So states.24

The principal photochemical process on irradiation of the benzoxazole A photostationary state is derivative (7) is reversible trans, cis-isomeri~m.~~ established comprising 69% of the cis isomer. The trans,cis-isomerism of bis(2-

Photochemistry

236

benzoxazoly1)stilbene has been reported.26 The ability of merocyanine dyes to recognise leukaemia cells ensures continuing interest in this area. One of the problems associated with such systems is which of the three double bonds is the most prone to undergoing photochemical isomerisation. The molecule (8) has been synthesised in an attempt to identify which bond is involved. In this molecule there is an ethene bond in a small ring which prevents the isomerisation at this site but does not increase the efficiency of the fluore~cence.~~

The photophysics of the thiacarbocyanines (9) have been assessed.28Particular attention was paid to dimerisation and other studies with related compounds have examined the complexation of ions with trans and cis-isomers of These crown ethers showed high selectivities for heavy ions such as Ag' and Hg2+. Another report has described the influence of ethene bond isomerism on the ability of the crown ethers (1 1) to complex Mg2+.30These crown ethers undergo trans,cis-isomerism on irradiation at 436 nm and the complexes of the cis-isomers have been demonstrated to be more stable than those of the trans.

I

R ' X

\

R'

(9) R = R" = H, R' = (CH2)3S03-r X = ~ N + - H

R (10) R = Et

The photochemical behaviour of trans-azobenzene in an aluminophosphate framework has been reported.31 The principal reaction on irradiation is formation of the cis-isomer, a common process in azo-benzene derivatives. However, in the aluminophosphate environment protonation of the azo group occurs and this affords another photochemical reaction mode. Thus, irradiation of this protonated form leads to the formation of benzo[c]cinnoline and benzidine. Usually the photochemical E,Z-isomerism of azobenzene derivatives doped in liquid crystals brings about mesophase changes, but a study with the derivatives (12) containing long alkanyloxy chains on the 3,3' positions has shown that this does not occur even at high (20%) dopant

IIl4: Photoreactions of Compounh Containing Heteroatoms Other than Oxygen

231

concentration^.^^ A further examination has shown the same effect with acetoxy substituents (12, R = COCH3).33The photochromism of 4-[bis(4methylphenyl)amino]azobenzene in an amorphous glass film has been studied and demonstrated to be due to trans, cisi~omerism.~~ Other photochromic azobenzene derivatives have also been synthesised and, for example, the azobenzene (13) exhibits large optical rotation changes on i r r a d i a t i ~ nThe .~~ photochromism exhibited by the azobenzene derivative (14) has been attributed to an intramolecular charge transfer process.36 Isomerism of the azobenzene moiety in oligonucleotides (15) occurs on irradiation at 300-400 nm in water.37The process is reversible and using wavelengths > 400 nm the trans-

(12) R=C&3, X = H R = C&13, X 3: Me R = COC3H7, X = H R = COCSH11, X = H R = COC11H23, X = H R = COC5H11,X = Me

0

A

(15) DMT = 4,4'dimethoxytrityl

isomer is reformed. The processes can be repeated several times without deterioration. Interestingly the physicochemical properties of the oligonucleotides are changed with the isomerism of the azobenzene unit.37 Flash photolysis with visible light of 4,4'-nitroanilinoazobenzeneaffords the thermally unstable ci.~-isomer.~*-~~ The examination of this azobenzene was carried out in both homogeneous and microheterogeneous media. Irradiation at 335 nm of the azobenzene derivatives ( 16) brings about facile E,Z-i~omerisation.~' Photostationary states are established with 2 : E ratios of 14 :86 for (16a) and 27 :73 for (16b). Furthermore, the isomerisation is accompanied by fluorescence enhancement resulting from the absence of an electron-transfer process which

238

Photochemistry

is inhibited by the non-planarity of the 2-isomer. Irradiation of phenylazonaphthalenes at 380 nm in chloroform brings about conversion into the corresponding ~is-isorner.~~ The cis-isomers are thermally unstable and revert to the starting material in the dark. An investigation of the trans,cis-isomerism of the related two azo compounds (17) and (18) in poly(methy1 methacrylate) has indicated that not all the trans-isomer isomerises at long wavelength (564 A study of the photoisomerisation of some azo-dyes in nm) nematic liquid crystals has been reported.45 Further work on the construction of photoresponsive dendrimers has been reported wherein three azobenzene units are attached to the central core.& The azobenzene units appear to react independently of each other on irradiation. A multiphotonic mechanism is suggested for the isomerism observed on IR irradiation of the dendritic antenna system based on the azobenzene molecule (19).47 Other photoresponsive dendrimer systems have been based upon the azobenzene derivatives (20).48 The photoresponsive behaviour of these dendrimers has been shown to be identical to the azobenzenes themselves. Isomerism can be brought about by irradiation at 350 nm, but thermal reversion to the trarzsisomer occurs over a few hours. The details of the trans& (irradiation at 365 nm) and cis,trans (irradiation at 436 nm) -isomerism of the azobenzene moieties in the some catenanes have been measured.49 /=NAr

8

(16) a, Ar = l-naphthyl b, Ar = 2carboxyphenyl OMe

w

(20)X = OH, COCi or NH2

2.2 Photocyclisations - The six-electron conrotatory cyclisation of enones such as (21) is well established. The outcome of the reaction is usually the formation of the trans-fused compound shown as (22). One example of this cyclisation has now been reported in which the final product is c i s - f u ~ e dThis .~~ involves photocyclisation of the enones (23) to yield (24). The formation of the cis-fused product is suggested to be in contradiction of the Woodward-

1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

239

Hoffmann rules for such reactions and the authors propose that the outcome is a result of thermodynamic control of the dark reactions during which the hydrogen migrations occur. Other workers have described the photocyclisation of related amino enones as inclusion compounds with the optically active host compounds (25).5

I

R

R

I Me (23) R = H or Me

I H Me (24) R = HorMe

The photoreactions are carried out as suspensions of the complexes in water and the irradiation times vary from 11.5 h to 100 h. The cyclisations of (26) to afford the carbazolones (27) take place with ee's as high as 97%. Substituent effects have been observed in the photocyclisation of the aminoenones (28).52 Irradiation of (28, R = Br) in benzene/methanol/triethylaminegave only recovered starting material, but the enone (28, R = CH3) was reactive on irradiation in acetonitrile/triethylamine and gave a low yield of the cyclised product (29) accompanied by the debrominated compound (30). The mechanism for the transformation could involve an electron-transfer process or direct C-Br bond fission. Photocyclisation of (31, R=Et) is also observed on irradiation in benzene/methanol and the route to product (32) is a typical 6e cyclisation (like 21 above) with a 1,4-hydrogen migration and oxidation. Interestingly the

(27) R' = H, R2 = Me R' = Me, R2 = H R1 = R2 = Me

H (29)

Et (32)

240

Photochemistry

secondary amine (31, R=H) is unreactive under the reaction conditions employed. Irradiation of the enones (33) brings about cyclisation to afford the naphthyridine diones (34) in moderate yieldsaS3 The reaction involves fission of the C-halogen bond on the pendant aryl group. This process generates mineral acid and the reaction needs to be carried out in the presence of triethylamine as a trap for the acid. The photocyclisation of inclusion compounds of (35) with the host molecules (25) gives high ee yields of optically active products is high although the chemical yields are

(34) R' = R2 = R3 = H

(33)

R'-MeO, R 2 = R 3 = H R 1 = R 3 = H R2=Me0 R' = Me, R* = R3 = H R 1 - R 3 = H , R2=Me R' = R2 = H, R3 = Me

An electron-transfer mechanism is proposed to account for the photochemical cyclisation of the fluorinated aryl amines (36) and (37).55 Photochemical oxidative cyclisation has also been reported as an efficient path to some benzo[h]thienothienoquinolines.56A study of the photochemical reactivity and degradation of diclofenac and meclofenamic acid has been publ i ~ h e d The . ~ ~ bi.s(diphenylamino)butane (38) is photochemically reactive and irradiation at 313 nm in an air-saturated solution brings about cyclisation which yields the biscarbazole (39).58 It is clear from the results obtained that the reaction is stepwise with initial cyclisation affording (40). The quantum yields for the cyclisation of (38) to (40) is 0.3 while that for (40) to (39) is 0.02. Me

Fy-JFn aFJ..?J N H

N

N H

Me

I M : Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen

24 I

In the second step the reaction is less efficient because of quenching of the excited state of the diphenylamine component by the adjacent carbazole. Other cyclisations such as the conversion of (41)into (42)by irradiation in acetic acid have also been reported.59 As mentioned earlier cis-azobenzene derivatives cyclise under conditions where the nitrogens are protonated or complexed. A series of heavily substituted azobenzenes has been shown to undergo photochemical cyclodehydrogenation on irradiation in methylene dichloride solution with added SnC14.60The reaction has been shown to be an efficient path to the heavily methylated cinnolines in good yield.

The photocyclisation of a series of imines (43)on irradiation at 350 nm on Ti02 has been carried The reaction involves an electron-transfer process to the Ti02 from the imine to yield the corresponding radical-cation which undergoes cyclisation to yield (44)in almost quantitative yields. Oxidative cyclisation results on irradiation (benzene solutions through Pyrex) of the anthraquinone diimines (45).62The cyclisation is analogous to that of stilbenes which yield phenanthrenes and dibenzacridine derivatives are formed from the present system. Photocyclisation within l-(2-chloro-4-benzyloxy-5-methoxy)1 H)-isoquinoline carboxbenzylidene-6-methoxy-7-benzyloxy-3,4-dihydro-2( aldehyde provides a path to the protoberberine alkaloid govadine, obtained as a r a ~ e m a t eA. ~further ~ ~ report has used an analogous method for the synthesis of racemic bharat amine.63b

The photochemical cyclisations of the imines (46)is a typical 6e process and irradiation in the presence of HBF4 gives conversion into the corresponding cyclic compounds.64The yields obtained from the processes are moderate and there is little doubt that cyclisation involves the protonated imine. Several products are formed on irradiation of the naphthylalanine derivatives (47).65

242

Photochemistry

The initial reaction brings about 2,E-isomerism of the ethene bond and prolonged irradiation converts this into the major product identified as (48, ca. 78%). The route to the major product involves an intramolecular electron transfer with the formation of a zwitterionic biradical and it is within this that the transformation to product occurs.

Several reports have been published during the reporting period that deal with photochromism in spiro systems. One report has provided a mathematical analysis of rate equations for such systems? Evidence has been collected to suggest that the photoisomerization of the spiro-indoline (49) occurs via an intramolecular charge transfer.67The inclusion of the indoline spiropyran (50) into a-cyclodextrin has been reported and it is argued that the spiropyran head group is too large to fit within the cavity of the CD and must lie outside?* The irradiation of the spiropyran in that environment shows the usual photochromism and a careful analysis of this system suggests that a dimer is involved in the photochromic process. A detailed study of the behaviour of the

IIl6: Photoreactions of Compounh Containing Heteroatoms Other than Oxygen

243

merocyanine forms of the spiro-pyrans (51) has indicated that there is rapid exchange between the trans, trans, trans and the trans, trans,cis The photochromic properties of the spiropyrans (52) attached to poly(L-glutamic acid) have been studied.70 Other reports have dealt with the photochromism of the acridine spiropyrans (53)71 and of several chromene derivatives, (54) and (55).72 In some instances the photochromic properties are poor. The influence of heteroatoms and ring size upon the photodegradation of some [2H]-chromenes has been in~estigated.~~ A study of the photochromism of some 2,2-diarylsubstituted pyridoannelated [2H]-chromeneshas been published.74Photochromism is also exhibited by the indolino-naphthoxazine (56) in toluene.75 The photochromicity of 1,3,3-trimethylspiro[indoline-2,3’-[3H]-naphth-[2,1 -b][ 1,4]oxazine] has been studied in the presence of transition metal ions.76An X-ray diffraction investigation of indolino-naphthoxazine has examined the influence of substituents on the CO and CN spiro bond lengths.77Photodegradation of some photochromic spirooxazine derivatives has been investigated under conditions of constant i r r a d i a t i ~ n Several .~~ patents have been lodged for a variety of indolino-naphthoxazine derivative^.^^-^^ R’

(53)R = H, 8-OMe, 6-OMe, 6-Br

(54) X = 0,R1= Me, R2 = H X = 0.R’ = Me, R2 = C,.,H13 X = S , R’=Me, R 2 = H

(55) X = O , R’ =Me X 0. R’ = C2H5 X = NCH3, R’ Me X-Se, R1=Me

-

The Zn2+ complex of the merocyanine system (57) releases the zinc when it is irradiated with visible light and this results in the formation of the colourless closed spiropyranindoline (58, R = H).83 When the irradiation is stopped the Zn2+ complex reforms but this does not happen with the nitro derivative (58, R = NOz) in which it is thought that the nitro group stabilises the phenoxide ion in the open form. Other workers have also studied the complexation of spiropyran based merocyanines with transition and rare earth metal ions.84 An investigation of the influence of Lewis acids (hexafluoropropanol, trifluoroethanol and 2-fluoroethanol) on the stability of the coloured form of spiropyran and spirooxazines has been rep~rted.~’ Protonation of the open system produces a form that is photochemically inert and the behaviour of these acids is markedly different from that of acetic acid with such systems.

244

3

Photochemistry

R

Q.

I

;zF

' ,

(57)

2ct

'R

The merocyanine form of the spirooxazine (59) is produced after flash irradiation of the dye in a polymer film and the thermal decay of the open form has been studied in The influence of solvent on the photochromic properties of a series of spirooxazines related to (59) has been reported.87The photochromism of 2-(2,4-dinitrobenyl)pyridine has been studied in polymer films.88

2.3 Photoadditions 2.3.1 Intramolecular Processes - The enones (60) fail to undergo (2+2)cycloaddition when irradiated and the sole photochemical reaction encountered is reduction of the remote double bond.89It is suggested that the failure of the cyclisation is a result of nitrogen lone paidethene bond interaction since when this interaction is minimised by the acylation of the amino group, normal (2+2)-cycloaddition becomes efficient giving high yields of the cage compounds (61). The intramolecular photocycloaddition within (R,R)-(62) affords (63) as a 7 : 1 mixture and is brought about by brief irradiation in acetone so presumably involves the triplet state of the enone system.90 The principal adduct obtained from this reaction is a key intermediate in a new approach to ( - )-perhydrohistrionicotoxin. Winkler and co-workers have previously reported several examples of what is referred to as a vinylogous amide photocycloaddition and in the present account the highly diastereoselective cyclisation of (64) to afford (65) has been d e ~ c r i b e d This . ~ ~ access to the skeleton in (65) provides the basis of a synthetic strategy towards the manzanine alkaloids. Intramolecular cycloaddition in coumarin derivatives affords (2+2)-cycloadducts.92

III6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

245

2.3.2 Intermolecular Processes - Encapsulation of trans-2-styrlpyridine in y-cyclodextrin radically alters its photochemical behaviour. Solution phase irradiation brings about solely trans, cis-isomerism and irradiation of the alkene in the solid phase affords only a low yield of the (2+2)-dimer (66). However, irradiation of the solid complex gives the dimer (66) in 50% yield.93 Two modifications, one yellow and one orange, of the cyanopyrazines (67) are produced on crystallisation. Only the yellow form is photochemically reactive and irradiation of this yields a head-to-tail dimer analogous to the cis,anti,cisdimer (66)?4 The derivative (68) is also photochemically reactive in the solid state and affords the (2+2)-dimer (69).95The two dimers formed on irradiation of (70) have been identified.96Head-to-tail dimers (72) are formed on irradiation (at 350 nm) of the crystalline enamides (71).97Apparently the distances between the two alkenyl units are short within these crystals and cycloaddition occurs in good yields and this reaction mode is in contrast to the behaviour observed in solution phase when only trans,cis-isomerism results. 2.3.2 Other Addition Reactions - The photochemical addition in the gas phase of ammonia to a,P-unsaturated nitriles has been studied.98The reactions are carried under conditions where all, or most, of the light is absorbed by the ammonia and this results in N-H bond fission and the production of NH2 radicals. These radicals add to the a-position of the substrate to yield 2-aminopropionitrile from acrylonitrile and analogous products are formed from crotononitrile, methacrylonitrile and 1-cyclohexenecarbonitrile.Additions also occur to but-2-yne nitrile which yields the 2,E-isomers of 3-aminocrotononitrile with the E-isomer being predominant. The SET-induced amination (using 1,4-dicyanobenzene as the sensitiser) of alkenylnaphthalene derivatives affords products of addition to both the alkene and the naphthalene skeleton.99Suau et al. have examined the irradiation of phthalimide in the presence of low concentrations of hydroxide ion and alkenes.lOO The result of this treatment is addition of the phthalimide moiety to the alkene. A SET

NcI;

246

Photochemistry

p HR ' NC

NcxN; NC

H

R2 R' = R2 = H

N

(67)

(68)

R' = But, R2 = Et

NC

\

/

CN

ao pAr Ar (71) a, Ar = Ph b, Ar = Pchlorophenyl c, Ar = 3chlorophenyl

D

0

(72) a, Ar = Ph (87%) b, Ar = 2chlorophenyl (92Y0) c, Ar 3chlorophenyl (90%)

-

mechanism is proposed and in general the yields of adducts such as (73), formed by addition to 2-methylstyrene, range from good to excellent. The adduct formed from cyclohexene undergoes a secondary photolysis by which it is converted into the ring-expanded product (74). This reaction path is a conventional Norrish Type I1 process.

Q 0

0 (73)

H

(74)

Irradiation of the carbothioamides (75) in the presence of furan or some 2-substituted derivatives results in the formation of the pyrrole derivatives (76) with the R group being the original 2-substituent on the furan.lO'The yields of products are variable and are shown below the structures. The key step in the

IIl6: Photoreactions of Cornpounh Containing Heteroatoms Other than Oxygen

247

reaction is suggested to be a (2+2)-photocycloaddition of the carbothioamide group to the furan to yield the intermediate (77) which undergoes subsequent loss of HZS and rearrangement to give (76). Addition of the same carbothiamides to 2-furanacrylic acid yields the pyrrolinone (78). lo2 (2+2)-Photocycloadditions have also been reported between methyl phenanthrene-9carboxylate and aminopropene nitriles.lo3 Head-to-tail adducts (79) are formed with high regioselectivity from this reaction. A temperature dependent (2+2)-photochemical cycloaddition giving (81) has been described for the reaction of acrylonitrile and its derivatives with the isoquinolinone (80) triplet state.*04 Photocycloaddition of electron deficient alkenes to the isoquinolinones (82) can be brought about by irradiation at 334 nm in benzene solution.lo5 The adducts formed were identified as conventional cyclobutane derivatives (83) and are produced in almost quantitative yield. The likely mechanism for product formation has been shown to involve single electron transfer. O5

X 0 S NH NH NH NH

(82) X = O o r S

R H H H Ph

Me0 OPh

Yield (YO)

68 65 48 44 39 44

(83) R = CN or C02Me

Photoaddition reactions have also been described with (84) as the substrate.lo6 (2+2)-Photocycloadditionof simple alkenes to these enones results in the formation of the adducts (85) and phenylacetylene undergoes similar addition to this substrate. A substituent effect has been detected in these

248

Photochemistry

addition reactions. Thus, when the substituent on the nitrogen in (84, X = NR) is large (R = Ph or p-MeC6H4) a different reaction mode is encountered and, for example, with ethoxyethene the adducts (86) are formed. These latter cycloadditions are visualised as arising via a two-step process involving bonding within the intermediate biradicals (87) to yield (88) which on ring opening followed by a 1,2-acyl shift yields the final product.

so &

R x 0 (85)X = 0,R = AcO

(84)

Ho

X = S , R-OAC X=S, R=Ph X=NMe, R-OEt

0

Ph

Ph& hr: -

Ph

Ph

0400 OEt

Ph

Ar (86)

Ar

(87)

Ar

(88)

Photocycloaddition of 2-TMSO-buta-l,3-diene to enone (89) affords the adduct (90) which is a key intermediate in an approach to the synthesis of homoerythrinan alkaloids. Io7 The principal product obtained by irradiation (298-310 nm) of the dione (91) in the presence of the isoxazolone (92) has been identified as (93, 38%).'08

CSPh

Me0 MeO%O

TMSO C02Me

0

R

H

(91)R = Br or NO2

~

o&ph ' 0

(92)

o

H

(93)

Pyrex-filtered irradiation of methanol solutions of the pyridone (94) results in the formation of the (2+2)-cycloadduct (95).lo9 The route to (95) is thought to involve (4+4)-photocycloaddition to yield the adduct (96) which is thermally unstable and undergoes a facile Cope rearrangement to yield the cyclobutane isomer (95). A full account has been published of the photoinduced mixed addition between (97) and (98).lI0 A 1 : 1 ratio of these compounds yields the two adducts (99) and (100) in a 6 :22 ratio. A 1 :4 ratio of the reactants gives the same products but in a respective ratio of 21 :6.

IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen

249

Several other minor adducts were also detected including dimers of (98). This work was originally published in note form. Further investigations into possible transformations of the adducts has highlighted the synthetic potential of such compounds. l2

/\\/\\/\

$yo

Me

0

(94)

OMe

N

O

1 H

Qo

(4+4)-Cycloadditions are also reported on irradiation of 9-aminoacridizinium perchlorate and the two adducts (101) and (102) are obtained in a ratio of 1 : 1 from deoxygenated methanol or acetonitrile solution^."^ The photochemical addition of furan to the pyridines (103) can be brought about in benzene solution using wavelengths > 290 nm.' l4 Two types of crossed adducts are formed and identified as (104) and (105). It is likely that the adducts are formed by a two step mechanism initially affording the (4+4)adduct (106) which undergoes cage formation on further irradiation. 12+

2+

2c10*-

L

H2"

"&RxeoR3

(101)

N

\

R30 R' H H H Me Me

R2

H

H H H Me

he R3 Me Et Pri Me Me

R2 Me

250

Photochemistry

Solid state NMR spectroscopy has been used to analyse the dimerisation of the 1,4-dihydropyridine derivative (107) that yields the two dimers (108) and (109) on irradiation.' l5 This report complements an earlier account of the same dimerisation. The reaction is claimed as a useful synthetic entry into derivatives of 3,9-diazatetraasteranes.l6 The photodimerisation of the thymine derivative (110) does not take place to any great extent on direct irradiation in solution but when acetone-sensitisation is used all four possible (2+2)-cycloadducts are obtained.' l7 Only one of these dimers is obtained from the irradiation in the crystalline phase and this was identified as the cis,anti,cis dimer (1 11). Interestingly, when the thymine (1 10) is crystallised from acetonitrile both needles and plates are obtained but only the needle form of the crystalline compound gives the dimer (1 1l).l l 8 R3

R3

(I

R'

R3

OR2

R20

k'

k3

Crystals of thymine obtained by the evaporation of an aqueous solution to dryness are photochemically non-reactive but freeze-drying of an aqueous frozen solution gave crystals of thymine that afforded the three dimers identified as (112), (1 13) and (114). l 9 This behaviour is attributed to a change in the arrangement of the thymine molecules during freeze-drying, which is, apparently, contrary to expectation. Transfer of energy has been studied within the cis,syn,cis-dimer (115) and this research has sought to emulate the energy-transfer step observed in Type I1 DNA photolyases.120 The photoligation of oligodeoxyribonucleotides using the photochemical reactivity of

M6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

25 1

4-thiothymidine has been described. 121 One-electron reduction of thymine derivatives results in the formation of dihydrothymine dimers.'22

R

Intramolecular photocycloaddition occurs with thymine derivatives and related compounds. Thus, the bis thymine dimer (1 16) is formed on irradiation at 254 nm of (1 17).123 Zinc complexes of 1,4,7,1O-tetraazacyclododecane inhibit the intramolecular photodimerisation of the thymidilyl thymidine (1 18) and the same complexes are active in cleaving cyclobutane systems (119).*24 Conventional (2+2)-cycloaddition does not occur on irradiation of (120) but instead the main product is the cytosine hydrate accompanied by the (6-4)photoproduct (121).125 Dimerisation is reported to occur on irradiation in an acidic medium.126*127 2.4 Rearrangements - Studies dealing with the ring isomerisations of heterocyclic compounds have been published recently. Thus the photochemical isomerisation of the dimethylpyridines (122) has been investigated in the gas phase.12* The reactions were studied under irradiation at 254 nm and a pressure of 2-5 torr with 15-21 torr of nitrogen as a diluant. Under these conditions the interconversion of 2,3- and 2,5-dimethylpyridines is enhanced and occurs at the expense of other processes that are observed when the nitrogen diluant is omitted. An example of these results and the effect of nitrogen on the process is shown in Scheme 1 for 2,3-dimethylpyridinewhere it can be seen that demethylation also takes place. When the nitrogen diluant is not present 3,4-dimethylpyridine is also formed. The phototransposition chemistry of the isothiazoles (123) has been described in some detail.129The reactions are brought about by irradiation in the presence of base and this apparently plays a part in enhancing the formation of the transposition product. Thus irradiation of (123a) yields (124, ca. 70%) but in the absence of base a fission process is the dominant reaction and affords products of ring opening. Other work related to such transpositions has suggested that there is no common description for the processes undergone by furans, thiophenes and related heterocyclic compounds. 30*131

252

Photochemistry

Fluorescent complexes are formed between trifluoroacetic acid and 2-, 3and 4-phen~lpyridine.'~~ A study of the pH dependence of the photodegradation of amiloride hydrochloride (1 25a) has been carried out. 33 This

'

I I 6 : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen Me

Me

253

254nm

with Nitrogen diluant

Me

1.8YQ

Me

Scheme 1

Me 1.5%

0.7%

Me 1.8%

Ph H (123) R' = Ph, R2 = H R' = Ph, R2 = D R' = PhCH2, R2 = H R' = PhCH2, R2 = D R' = Me, R2 = H

work has shown that the dominant reaction is dechlorination with the formation of (125b) as the principal product and a mechanism involving a radical-cation has been proposed. An electron-transfer process has also been 1,2-diol codetected within the fluorescent phenazine/l,2-diphenylethanecrystal system.134 The fluorescence of phenosafranine (126) can be quenched by electron donors, such as halogenated benzene derivatives amongst others and again an electron-transfer mechanism is proposed to account for this. 135

(125) a, R - CI b,R=OH

1-Azaxanthone (127) does not exhibit polarity-induced shifts in its absorption spectra even although the excited state is m* in character.136It does, however, undergo hydrogen abstraction reactions from suitable substrates and in methanol solution affords the radical (128). Apparently 1-azaxanthone is a far superior hydrogen abstracting ketone than other simple aromatic ketones and this reactivity is enhanced as a result of the influence of the pyridine ring. Further studies of this compound have examined its behaviour in micellar environments and in water where it is seen that the hydrogen abstraction reaction of the ketyl system is ineffi~ient.'~~ 0

OH

A laser flash study of the acridinethione (129) has shown that the triplet state is populated on irradiation at 355 nm and electron transfer from

254

Photochemistry

tetramethylbenzidine to (129) was also in~estigated.'~~ The results of a study of the photophysical properties of 2-(7-dimethylamino-3-coumarinyl)-5-oxo-4methyl[l]benzopyrano[3,4-~]pyridine in both polar and dipolar media have been reported. 139 The photoreactions of caffeine, theobromine and theophylline with benzophenone in ethanol solutions have been identified." Energy transfer from the laser dye (130) to rhodamine 6G has been studied.14' S

A laser flash photolysis study of the aziridines (131) has shown that the intermediates formed are the azomethine ylides (132) which result from fission of the C-C bond within the ring.142The reactions of these intermediates with oxygen, alcohols and acrylonitrile were also investigated and the last cycloaddition processes provide a convenient path for the synthesis of pyrrolidines. The rate constants for the cycloaddition reactions of the azomethine ylides formed on the photochemical ring opening of such phenylaziridines have been measured.143 The aziridine ring in the hexacyclic molecule (133) is photochemically reactive and unlike the foregoing reactions, in this instance, fission of a C-N bond results on irradiation.14 The rearrangement within the biradical formed by this process yields the structurally reorganised molecule (134).

Aziridines are important intermediates in the photochemical reactions of pyridinium salts. Thus irradiation of salts (135) in the presence of a nucleophile (nuc) affords the aziridine (136) that readily undergoes ring opening to yield the aminocyclopentane derivatives (137). 145 The reaction can be applied to the synthesis of (+)-mannostatin A (138) starting from pyridinium perchlorate. Other examples of the synthesis of such aminocyclopentitols have been reported by Acar et ~ Z Z . ' For ~ example, the irradiation of (139) affords the substituted aziridines (140) in moderate to excellent yields. Further reactions

M4: Photoreactionsof Compounh Containing Heteroatoms Other than Oxygen

255

of these intermediates with benzoic acid in chloroform provide a path to molecules such as (141). Other nucleophiles are equally efficient in ring opening the aziridine intermediates and the process has also been used as a synthetic path to the cyclopentenone ketals (142)from irradiation of (143)in methanol.'41

R I

OCt (139)

R = -(CH2)20Me, -CH20(CH2)20Me,

NH(CH2)aOH

OH

-CHzCHz<~]

QF

H P h C a - -&--OH

N Me I

( 140)

,-(CH2)30H or -(CH&C@H

QOM* I

Me BF4-

Armesto and co-workers have reported the novel photochemical transformations encountered with the azadienes (144).14* The transformations are carried out using electron transfer photochemistry using DCA as the sensitiser. This treatment generates the corresponding radical cation of the dienes, e.g. (145),that undergoes bridging to produce the intermediate e.g. (146).This radicalcation undergoes ring opening and cyclisation to afford low yields of the cyclopropane derivative (147).

(144) R' = R3 = Ph, R2 = H

R1 =Ph, R2=H, R 3 = M e 0 R' = Ph, R2 = H, R3=Ac0 R' = R2= Ph, R3 = AcO

Di-n-methane reactivity is reported as a key process in the photochromism of the dihydropyridine (148).149 Irradiation (313 nm) of this compound brings about irreversible 1,2-phenyl migration and formation of the bicyclic isomer (149).Hydrogen migration occurs in competition with this reaction and yields

256

Photochemistry

(150). Direct irradiation through quartz of the dibenzobarrelene (15 1) in pentane or benzene results in the formation of the cyclooctatetraene (152) and this process is not affected even when the amino functions are converted into the HC1 or HBr salts.I5*In the solid state, however, irradiation of the bis salt yields a semibullvalene (153), as the main product and the authors suggest that this change in reactivity is due to a change in the excited state that is operative. Thus in the solid the chloride or bromide ions act as heavy atoms that enhance the intersystem crossing within the excited state of (151) but apparently this does not occur in solution.

Ph

Ph'

Me

.I

I"

-H \ Ph

Irradiation (with a tungsten lamp) of the oximes (154) under oxidative conditions ( P ~ I ( O A C ) ~in- Idichloromethane) ~ yields the adduct (155). 151 The reaction can also be brought about using HgO-12 and irradiation with a medium pressure Hg arc lamp. The formation of the naphthisoxazoles (I 55) can be interpreted as the addition of a nitrile oxide to an aryl ring. Such a 1,3addition is unprecedented under such conditions. Ar

R Ar

'OH

0-N

Other Processes - The phototautomerism of N-salicylideneanilines has been studied at 77 K.ls2 A study of the fluorescent behaviour of the unprotonated n-butyl Schiff base of retinal in alcohols has been reported. 153 The photochromic properties of the diimines (156) have been studied with attention being paid to the influence of substituents upon the various processes encountered. lS4 Excited state proton transfer within the hydroxyphenylpyridine (157) has been reported and the influence of restricted 2.5

IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

257

rotation on proton transfer has been assessed using the derivatives (158) and (159).lS5

The phototautomerism within derivatives of [2,2'-bipyridyl]-3,3'-diolhas been described and the influence of substituents close to the reactive hydrogens has been assessed.156 Intramolecular photochemical proton transfer in the imidazoles (160) and (161) has been studied.157Other workers have reported on the photochemical behaviour of the imidazole (162) and the influence exerted by change of solvent.15* Proton transfer occurs within the excited state of the benzazole derivatives (163),159 and details of excited state proton transfer within the flavylium salt (164) have been published.160

H (160) X = CH or N

H

R (163) R = H or R-R = -(CH=CH)2-

(161)

(164)

The photochemical reactivity of the drug acetazolamide (165) has been studied using 300 nm under nitrogen and 337 nm using Rose Bengal as a sensitizer.I6' The products were identified as (166) and (167) and are formed under both irradiation conditions. The 1,2,3-triazolyl radical is formed on irradiation of (168a) and this radical is the precursor to the triazole (168b). Benzamide and benzoylhydrazine are formed from the PhCONH radical.162 The photochemical decomposition of the diones (169) has been studied as a comparison with the decomposition of the herbicide metribuzin. 163 The dependence of the reactions encountered in oxygenated solutions of varying

258

Photochemistry N-N

N-N

MeCONAs%SQNH2 I H

N-N

MeCONAs-((s%NHCOMe I

(165)

(166)

phHph

so 0 MeCO I Nq FSaNH2 H N=N

N+ ,N-R N (168) a, R = PhCONH b,R=H

NHp (169) X = O o r S

pH has been assessed and the main reaction pathways are side chain degradation which leads to deamination and decarboxylation among other processes. The pyrazolinone ring in (170) undergoes a-fission of the N-CO bond on irradiation and the resultant biradical rearranges via (171) followed by rebonding to give the pyrazolinone (172) isomeric with the starting material. 164 A reinvestigation of the photochemical reactions of the dihydroquinoline derivatives (173) has been r e ~ 0 r t e d . This I ~ ~ new investigation has shown that the major product obtained is (174) and not the azete as was proposed originally.166The irradiation of (173, R = H) in ethanol affords a complex mixture while (173, R = Me), under the same conditions, gives (174, R = Me) and (175). The route to products is thought to involve the biradical (176) formed by N-C bond fission. Photoreduction results on irradiation of the heterocyclic compound (177) in propan-2-01.'~~ Ring opening and rebonding also occurs under these reaction conditions and affords a mixture of the pyrazole (178) and the imidazole (179). Miranda and co-workers have reported on some cross-coupling reactions between tiaprofenic acid (180) and 2,6-di-tbutylphenol.16* Irradiation at 254 nm of the imidate (Scheme 2) in aqueous solutions results in the formation of 4-cyanophenol and N-isopropylbenzamide.169 The photochemical step involved in the formation of the products has been shown to be C-0 bond fission with the production of the phenoxide and the benzonitrilium ion. Mariano and co-workers have reported a detailed study of the electrontransfer photochemistry of a-anilino carboxylates, P-anilinoalcohols and a-anilinosilanes.170 This study has measured the rates of decarboxylation of aniliniumcarboxylate radicals. The base induced retro-Aldol fragmentations of the radical-cations formed from the 9-anilinoalcohols and the influence of substituents on the nitrogen on the desilylation of the a-anilinosilaneswas also investigated. In addition, the synthetic potential of some of the electrontransfer photochemistry of the carboxylate salts (181) and (182) has been examined. In these examples irradiation, using DCA in methanol or acetoni-

IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen

259

(173)

OH

NC H

Ph

N'

I

Me (174) R = H or Me

I

COMe

COMe (176)

(175)

NaN

CN Ph'NAPh (1 77)

H

O

e

C

N

254 nm

Ph-C=N-Pr +

+N

C

-O

V

N-Pri

0 + NCD

O

H

Scheme 2

trile as solvents, leads to decarboxylation and the formation of an alkyl radical which cyclises to (183) and (184), respectively, in yields of 55-77%. Similar cyclisations were carried out for some phthalimide derivatives, e.g. the conversion of (185) into (186). Other workers have gathered evidence for the reduction in C-N bond strength with increasing phenylation in the amines (187).17' As a result there is enhancement of the quantum yield for radical formation by C-N bond fission. Bond fission is also the predominant process on irradiation of phenylazotriphenylmethane in solvents such as methylene chloride or acetonitrile. This bond fission results in the formation of trityl radicals and trityl cations. 72 Deprotection of the amino function of (188) can be brought about by irradiation using h >300 nm in aqueous acetonitrile solution.173 The reaction is a SET process and depends on the presence of 1,5-dimethoxynaphthalene as the electron donor. The photodissociation of the ammonium borate salts (189) and (190) has been reported. 174 Irradiation brings about homolysis of the N-C

260

Photochemistry

0

~N-N0cH2c02Et N 3 0

R I

\

0

(186) R = Me or COMe

R' I

Ph-CNHPh k2

(187) R1 = R2 = H R' = Ph, R2 = H R1 R2 = Ph

Tos I

R'

>

N

*

ThY

H (188) R = Me or Et

0

X(189) X- = Ph3BBu, Ph4B or BF4

hR2 bMe 4

(190)

Ph

R'

R'=Ph, R 2 = H c

Me

BMe4-

the 75% yield

Me

bond to yield the corresponding acetyl radicals which undergo dimerisation and a single electron-transfer process is thought to be involved. A further examination of the photochemical reactivity of 4-benzoylbenzylammonium tetraphenylborates has shown that quantitative yields of biphenyl as well as amines and methylbenzophenone are obtained. 175 Other studies with organoborate salts have been carried out with pyridinium species as the counterions.176 Again photoinduced electron transfer is a key step in the photochemical reactions and this results in alkylation of the pyridinium compounds as shown in Scheme 3. Intramolecular electron transfer has been

I M : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

261

detected following irradiation of 1,3-bis[4-(4-nitrosyl)pyridinium]propanetetraphenylborate. 77 Ultrafast electron transfer has been studied in the styrylpyridinyl radical and tetraphenylborate anion system. 78 Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191).179 The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described.I8O a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature.lgl A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described.182 The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts.Ig3The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (h> 340 nm) of (193) in methanol were identified as N-hydroxy-2-pyridone and (194) from the fission of the C-0 bond.lg4 Other products were 2-pyridone, (195) and (196) that arise from 0-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-0 bond heterolysis.

?

Ph*N--NHC-Ph R

MeO2C

(191) R = H, Ph or Me

(192)

I

OCH2Ar (193) Ar = 9-anthryl or l-pyrenyl

ArCH20Me ArCH20H (194) (195)

ArCHO (196)

The pyridinethiones (197) are a useful source of alkoxy radicals upon i r r a d i a t i ~ n . The ' ~ ~ bond fission processes are brought about by irradiation at 350 nm and the resultant radicals have been demonstrated to react effectively with guanine. A study of the isomeric N-hydroxypyridine-4-thionehas also been reported.* 86 N-0 bond fission has been studied in N-phenylhydroxylamine.lg7Derivatives of this type have been used frequently as sources of free radicals. For example, the Barton esters (198) have been used to form a-amido radicals. The kinetics of rotation around the C-N bond within these have been measured.lg8 Other studies have examined cyclisations within radicals produced from Barton esters.189

I OR (197) R = H, Pri, But or PhCO

(198) R = H or Me

262

Photochemistry

Irradiation (quartz filtered) of the oxazolone derivative (199) in acetonitrile results in decarbonylation and the formation of the imine (200). When this imine is formed in the presence of ally1 alcohols, trapping (a thermal reaction) results in the formation of the ethers which are also photochemically reactive and are transformed by a Norrish type I1 hydrogen abstraction process into the isomeric compounds (201). Oxy-Cope rearrangement of (201) yields the second product (202) isolated from the initial irradiation.

Apparently, the isoxazolones (203) can exist in different tautomeric forms and these compounds are photochemically reactive and yield different products from the different tautomeric Some of the reactions encountered are shown in Scheme 4. Sigmatropic migrations are used to explain the formation of the products 3-ethoxy-2-phenylindole and 6-ethoxy-2-phenylindole from irradiation of 1-ethoxy-2-phenylindole in methanol.lg2 The deethoxylated compound 2-phenylindole is also formed. The photostability of the novel antibiotic (204) has been assessed.lg3 R

R

HO

+

Q N

I \

N T NC HaPEht

NHPh

Scheme 4

IIl4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

263

N-Cl bond fission takes place on irradiation of the N-chloropyrrolidinone derivatives.lg4 The resultant radicals undergo exo-cyclisation and a typical example of the reaction is illustrated in Scheme 5.

Irradiation at 313 nm or sunlight irradiation is effective in bringing about fission of the oxadiazole oxides (205).195This process primarily yields the nitrosocarbonyl derivatives (206) and when the reactions are carried out in the presence of cyclohexa-1,3-diene the adducts (207) are obtained in good to excellent yields. The bicyclic nonadienes (208) are also adducts of this same type and undergo bond fission on irradiation to give zwitterions which in general undergo a further bond fission with the elimination of nitrosobenzene and the formation of the azepine derivatives (2O9).Ig6 Bond fission in the related 3,4-bis-2(-chlrorphenylfuroxanis brought about on irradiation at 254 or 300 nm and this yields NO and bis-2-chl0rophenylethyne.~~~ C-C bond fission occurs on irradiation of the azetidinones (210) in acetone solution and the resultant biradicals undergo cyclisation to yield the pyrimidinone derivatives (21

Bn

H Ph Me H

Ph 70 Ph quant. Me quant. OMequant.

Radical polymerisation can be induced using the charge transfer complex formed between N-ethoxy-p-cyanopyridinium hexafluorophosphate and 1,2,4-

Photochemistry

264

trimetho~ybenzene.'~~ A study of free radical and ionic pathways in the photochemistry of 2-(4-methoxynaphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine derivatives has been carried out.200 Heterolytic C-F bond fission is reported on the irradiation of the fluoroquinolone antibiotics.201The SRN 1 reactivity of enolates of 2-acetylthiophene and 2-acetylfuran has been studied.202 The irradiation of acetdnilide in the cavity of zeolites (X, Y and p) results in the formation of o-aminoacetophenone as the principal product .203 A laser flash study of the kinetics of rearrangement of the triazine derivative (212) into the isomerised product (213) has been reported.204 In particular the 1,3hydrogen migration was studied to establish the degree of quantum mechanical tunnelling involved. The results obtained indicate that tunnelling at two vibrational levels is involved.

A laser flash photolytic study of the reaction between 2,2'-dipyridyl and tryptophan has been described.205The primary photochemical step has been demonstrated to be pH independent and involves an electron transfer from the tryptophan to the dipyridyl triplet state. The triplet excited state of some peptide conjugates is produced on irradiation by a nanosecond laser flash.206 C-C Bond cleavage is the result of irradiation of the pinacols (214) in chloroform. This yields the corresponding aldehydes.207The mechanism of the cleavage process has been shown to involve single electron transfer with chloroform as the electron acceptor. A study of intramolecular charge separation in aminophenyl(pheny1)acetyleneand N,N-dimethylaminophenyl(pheny1)acetylene has been reported.208 OH OH

M e $ l G +t+wIt?z R R (214) R = H or Me

Irradiation of (215) in the presence of strong base not only brings about cleavage of the C-C bond liberating the t-butyl group but also the loss of the methyl group from the nitrogen.2w Photochemical electron transfer induced reduction of C ~ has O been described using NADH and NAD dimer analogues as the donors.210A photoinduced electron-transfer mechanism is involved in the oxidation of the dihydropyridines (216) in CC14 solution and affords the radical-cation of (216) and the radical-anion of CC1+211These decay to yield

IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

265

the corresponding dihydropyridinyl radical and the trichloromethyl radical. Hydrogen abstraction then affords the final products (2 17) in quantitative yields. H But

H R C02Et

Me

Me

H

Me

The results from a detailed study of the solvent effect on the SET induced photochemical reactions of phthalimides and related maleimides have been reported.212 For example, irradiation of the phthalimide (218, n = 4 ) in acetonitrile gives a mixture of the three products (219, n = l), (220) and (221, n = 4). When a watedacetonitrile (35 :65) mixture is used as the solvent the reaction is very selective and affords the benzoindolizidine (221, n = 4 ) in 94% yield. The same selectivity is observed with the phthalimide (218, n = 5) although in this example the change in efficiency is not as dramatic. The results from a study of the formation of the ylide (222) by irradiation of the phthalimide derivatives (223) have been reported.2

#:-(CH2),,SiMe3

@:-(CH2),SiMe3

0

0

(218) n = 4 o r 5

0

(219)

n = 1 or2

Intramolecular single electron transfer occurs within the supramolecular systems (224).2'4 The electron transfer only occurs from the nitrogen function attached to the 4-amino substituent. A comparison between redox potentials and the photophysical behaviour of the phthalimides (225) has been made.215 Other work dealing with electron transfer in phthalimide systems has examined the behaviour of (226).216The singlet excited state of (227) is fluorescent and this can be effectively quenched by f e r r ~ c e n e . ~ ' ~ The intramolecular electron transfer within rigid U-shaped tetrads such as (228) has been demonstrated to be highly efficient.218The photophysics of some pentahelicene systems has been studied with particular attention being

266

Photochemistry

R’

(227) Fc = ferrocenyl H

H

CH&Hz-Na

3-OMe

CH$HZ-N~

4-OMe

Et

4-OMe

C H & H ~ - N ~ 4-NH2

Et

3-NH2

paid to the fluorescence and to electron-transfer processes.219Other studies have examined intramolecular charge transfer in a series of 4-(dialky1amino)benzonitriles that exhibit dual fluorescence.220The excited states of 4-(N,Ndimethy1amino)benzonitrile have been reported.221An intramolecular charge transfer is observed for 4-(dimethy1amino)benzonitrile. The effect of changes in pressure on this has been evaluated.2222-Amino-5-hydroxybenldehyde is formed on irradiation of 3-hydroxybenzonitrile in water.223 OMe

IIl4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

267

Weak electronic coupling is involved in excited state charge transfer in some donor-acceptor biphenyls (229).2249225A study on conformational pathways available to such twisted biphenyls has been published.226 The long-range electron transfer within the carbazole-viologen linked system (230) and related molecules has been dernon~trated.~~’ Donor-acceptor crystals containing dinitrobenzoic acid and carbazoles exhibit charge transfer.228The quenching of the singlet excited state of anthracene by indoles has been investigated. Evidence for intermolecular single electron-transfer processes within this system was gathered.229

N-(CH2)16-

+

N

s

N

+

- (CH2)2Me

2Br (230)

A novel approach to dihydroquinoline derivatives (231) has been described by Park et aZ.230The reaction involves the irradiation (350 nm) of m-nitrocinammic acid in the presence of alcohols and TiO2. The products are shown in Scheme 6. Irradiation of o-nitrobenzaldehyde in solution affords the aci-nitro species by a hydrogen transfer process and the determination of the pK, of this species has been made.231 The major products formed on irradiation of the oxiranes (232) have been identified as the diketones (233) and the isomeric ~ x i r a n e s 2-Nitrosobenzoic .~~~ acid is also formed in 10Y0 yield and this is suggested to arise from fission of the ylide formed by ring opening of the oxirane. Such a reaction would yield 2-nitrobenzoic acid that readily undergoes conversion into 2-nitrosobenzoic acid.

268

Photochemistry

The photochemical decomposition of several dioxolane derivatives (234) has been reported.233The reactions are brought about in cyclohexane with Pyrexfiltered light and the work is aimed at producing protected derivatives of aldehydes that can be liberated by photolysis and uses the well-established hydrogen abstracting power of a photoexcited o-nitro Pirrung et al. have described a new photochemically removable protecting group based on the reactivity of an o-nitro This system uses the diene function in (235) as a means of trapping the reactive nitroso group formed following the hydrogen abstraction. Thus the irradiation of (235) affords the cycloadduct (236) in high yield. The key step is the abstraction of the hydrogen atom adjacent to the hydroxy group. Several derivatives of (235) were examined and it was shown that the protected molecule, the group R (Scheme 7) could be recovered in good yield.

& NO2

+RH

0

Me R = PhCO, PhCH2, C6H13, CsHllNHCO, PhCHzCO, Ph Scheme 7

Both the o-nitrobenzyl and the o-nitrophenethyl derivative of 2-deoxyglucose (237) are photochemically reactive.2352-Deoxyglucose is liberated on irradiation but the latter derivative is the more efficient. 2-Nitrobenzyl ester groups have been used as photocleavable protecting groups on some phospholipids, e.g. (238).236All the compounds studied undergo photocleavage with the deprotection of the carboxylic acid groups but the photolysis rates vary and are dependent on the substitution pattern. Irradiation of the so-called ‘prodrug’ (239) liberates the deoxyuridine (240) and acts as a deprotection of an acid group.237The deprotection of the amine moiety in (241) is brought about by irradiation of a chlorofordmethanol solution and this gives the free

M6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

269

(237) R'sAc, R2=H R' PAC, R2=Me

0

0

Me0

(239)

OH

amino derivative (242).238The photoprotecting group in the nucleotides (243) can be readily removed by irradiation at 365 nm to afford the liberated amines in > 94%.239Nitrobenzyl esters have also been used as part of a solid support synthesis of oligonucleotides.240New photolabile linkers have been synthesised that permit the coupling of building blocks with functional groups such as carboxy, amino, hydroxy and s ~ l f o n y l .Photocleavable ~ ~ ' ~ ~ ~ ~ nitrobenzyl esters have also been used in a strategy for the synthesis of new catalysts from RNA libraries.243The flash photolytic study of the ester (244) has examined the behaviour of the intermediate radicals and, according to this study, these make . ~ intermediate ~ detected is proposed to be the up 10% of the reaction f f ~ xThe radical-anion of the nitroaryl group.

270

Photochemistry

Several groups have reported the photochemical conversion of the dihydropyrimidines (245) into the indazoles (246).245-247The reaction, like the preceding processes, involves the hydrogen abstracting ability of the o-nitro group. The results from a detailed study of the photochemical reactions of (247), musk ambrette, in cyclohexene have been published and a variety of reactions were uncovered, most of which are reactions of the aryl nitro groups."* Thus reduction to nitroso and amino moieties occurs and hydrogen abstraction and cyclisation processes are also observed. Both reduction of the nitro group and substitution of the cyano moieties occur on broad band irradiation of the malonic acid derivatives (248) in benzene solution.249 A study of the photochemical reactivity of the nitroquinoxalines (249) has been reported.250The photochemical reactivity of the pesticide (250) in the presence of 2,4,6-triphenylpyrylium ion in a zeolite cage has been studied.25' Other research has examined the formation of the radical-cations of amines and their conversion into nitroxyl r a d i ~ a l s . 2The ~ ~ photochemical reaction of nitrate (by irradiation at h > 290 nm) with several aromatic heterocyclic compounds has demonstrated that many products are formed resulting from both oxidation and nitration.253For example, 11 nitration products and 2 oxidation products are formed from quinazoline while isoquinoline yields 32 products.

(248) R = CQEt or CN

(249) R = H or Me

IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen

3

27 1

Sulhrrlcontaining Compounds

Usui and Paquette have reported the photochemical transformation of the sulfide (251) into the isomeric product (252).254This 1,3-phenylthio migration can be brought about using sunlamp irradiation in carbon tetrachloride solution and the isomerised compound is used as a key intermediate in a new synthetic strategy for diquinanes.

Gravel and co-workers have demonstrated that the cyclohexanediol (253) can be converted into the deoxysugar (254) by irradiation in the presence of benzophenone, acetonitrile and thiophen01.~~~ The conversion of (253) into (254) involves the formation of the aldehyde (255) which is transformed into the acetal, i.e. the deoxysugar. An extension of this reaction has demonstrated that deoxyazasugars can also be formed using the same condit i o n ~ . *Thus ~ ~ irradiation of (256) gave the aldehyde (257) which can then cyclise by the same path as for the formation of (258). The conditions utilised were irradiation at 350 nm in acetonitrile solution with xanthene and thiophenol.

OH

-OH

?&

NHPG

SPh

The involvement of o-and n-type dimeric radical-cations in one electron transfer to the aromatic sulfides (259) has been assessed.257A laser-flash study of the behaviour of p-nitrobenzenethiol has shown that S-H fission is the dominant reaction with the formation of the corresponding thiyl radical. This occurs particularly when the irradiations are carried out in nonpolar solvents. The reactions encountered in polar solvents are different. Under these conditions the triplet state of p-nitrobenzenethiol is involved and this undergoes ready d e p r o t ~ n a t i o nThe . ~ ~ amino ~ acid derivatives (260) can be desulfurised by irradiation using triethylboron and triethylph~sphite.~~~

272

Photochemistry

(259) R = H, CI, Me or OMe

(260) R1 = R2 = H R1 = R 2 = M e

The photochemical reaction between the sulfine (261) and cyclooctyne affords the three products shown in Scheme 8.260The enone and (262) are also formed from the thermal reaction of the sulfine and cyclooctyne and the route to these products is proposed to involve 1,3-addition of the sulfine to the cyclooctyne with the formation of (263) as the key intermediate. Fluorenone is proposed to arise from the oxathirane (264) which is formed photochemically from (261). The photochemical fragmentation of dimethyl sulfoxide in the gas phase has been studied and the primary fragments were identified as arising from C-S bond fission.261

s//o

The influence of the sulfur substituent on the addition of radicals to the enones (265) has been assessed.262The radicals are produced by hydrogen abstraction from the alcohols using photoexcited benzophenone and the products formed were identified as (266) and (267). High yields of the 1,2-dioxolanes(268) are obtained on irradiation (tungsten lamp) of the benzothiazinone (269a).263The reaction is catalysed by either diphenyl disulfide or diphenyl diselenide with the latter catalyst being more efficient and the likely reaction path involves free radical intermediates. The results from a study of the fluorescence behaviour and the inversion of the optically active sulfoxides (269b) have been published.264Aryl methyl sulfoxides can be readily photooxidised using Ti02 as the cafalyst to give sulfones as the main products.265 Aromatic thioketones are recommended as useful compounds to measure

1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

0

Me

(269)a R1 = R2 = Me R' R2 = H R1 Me, R2 = H 4-

P

273

(269b) R' = R2 = R3 = R4 = H R'-R2 = -fH=CH)z-, R3 = R4 = H R'-R2 R R4 = -(CH=CH)25

the kinetics of fast biomolecular reactions that can be studied by fluorescence quenching.266The solid state irradiation of the o-aroylbenzothioates (270) yields the phthalides (27l).267This rearrangement involves a 1,4-aryl migration from the aroyl group to the thio moiety and the yields of products are high at low conversion. The cyclisations of (270a-c), undergo rearrangement with the development of some enantiomeric excess and in a few cases this can be quite high as shown under the appropriate structure.

(270) (2711 R = H, Ar' = c+MeC6H4, A? = Ph (30% ee) R = H, Ar' = A? = Ph (35% ee) R = H, Arl= mMeGH4, A 6 = Ph (23% ee) R-Me, A r ' = A ? = P h R = H, Ar' = Ph, A? = pMeC6H4

Irradiation of the thioamide (272a) in benzene yields a mixture of (273a) and (274a) but irradiation in the solid state yields (274a) exclusively.268The reaction is substituent dependent and with (272a) in benzene the analogous products (273b) and (274b) are formed but in the solid (272b) affords (275, 88%) and (274b, 12%). The reactions are proposed to occur from the singlet state and involve hydrogen abstraction reactions. The site from which

274

Photochemistry

hydrogen abstraction occurs is dictated by the crystal packing. The formation of (274) must arise by addition of a biradical such as (276) to a thioketal formed by a bond fission process and clearly this implies that crystal disruption must occur on irradiation to permit such an addition reaction.

a, R'

(272)

(274)

= Me, R2 = H

(275),

b, R1 = # = M e

Sakamoto et al. have also studied the photochemical behaviour of the thiocarbamate (277).269Irradiation of a benzene solution affords quantitative conversion into the tricyclic thietane (278) and irradiation of (277) in the solid phase was effective both at 0°C and -78°C. A marked difference was observed with the derivative (277b) and in benzene solution gave the rearranged product (279). The mechanism for the formation of (279) is thought to involve ring opening of the thietane (278) formed initially and this leads to the zwitterion (280) which can ring close to the product (279). In the solid state this photoreaction affords the product with an ee of 20%. Photo-SET from the water soluble 1,5-dialkoxynaphthalenesto the sulfonamides (281) in aqueous solution affords the corresponding radical-cation radical-anion pair.270The radical-anion of the sulfonamide undergoes bond fission and releases glycine. R2

OMe I Ph (277) a, R1-R2 = (CH2)4 b, R' = R2 H

Me

y0

OMe

P

R2aS02NHCH2CO&l R' (281) R' =C&H, R 2 = H R' = CH~COZH,R2 = H R' = OCH2C@H, R2 = M e

The aromatic aldehydes (282) undergo addition to the ketene dithioacetals (283) on irradiation in chlorobenzene or other solvents.27' The reaction proceeds via radical intermediates that are produced using excited benzophenone as the hydrogen abstractor. The yields of products are high and there

M6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

R

Et Me Me Me Me Me

Ar

275

Yeld(%) dr

Ph pAaH4 pMeOQH4 pMGH4 M e H 4 o'FC~H~

75 57

60 83

67 69

54:a

50:50 51:49 56~44 54M 56:44

scheme9

is some diastereoisomeric control as can be seen from the results quoted in Scheme 9. Several products are formed when phenylcyclopropane is irradiated in acetonitrile with added trifluoromethylsulfenyl The phenylpropane derivative (284) is one of the products and its formation can be rationalised by attack of a chlorine atom, formed by S-Cl bond fission, on the cyclopropane to yield the radical (285) which combines with a trifluoromethylthio radical to give the product. cis-Stereoselective addition of benzenesulfonyl bromide occurs to the tricycloheptane (286) on irradiation and the two products (287) and (288) are formed in a ratio of 3 : 1.273

A photochemical study of the electron-transfer reactions of a sulfonium salt, 4-cyanobenzylmethylphenylsulfoniumtetrafluoroborate, has been reported to give phenyl methyl sulfide q ~ a n t i t a t i v e l y .9,l ~ ~O-Dimethylanthracene ~ and naphthalene have both been used in the sensitised reactions of triphenylsulfonium hexafluoroantimonate and the reactions encountered involve the singlet states and produce the radical-cation of the sensitiser and phenyl radicals.275The phenyl radicals are formed by bond fission within the neutral triphenylmethyl radical. The reactions of some aromatic sulfonium compounds have been patented for use in resin corn position^.^^^ Irradiation of 3-methylthiophene in hexafluoropropan-2-01 with added methanesulfonic acid brings about single electron-transfer oxidation and this yields a species that has been identified as the bis-protonated radical-cation of (289).277Irradiation of 2-acetyl-5-iodothiopheneleads to cleavage of the C-I bond and the resultant radical adds to acrylonitrile to afford the adduct (290) in moderate yield.278 Reactions of this type have also been described for iodofuran and iodopyrrole derivatives and in the latter case 4,Sdiiodo- 1Hpyrrole-:!-carboxaldehyde reacts photochemically with thiophene to give

276

Photochemistry

Nc*COMe

*ph C02H

0

I

4-iodo-5-(2-thienyl)-1H-pyrrole-2-carbo~aldehyde.~~~ Tiaprofenic acid (291) undergoes facile photochemical decarboxylation280and this is reported to take place from an upper triplet excited state.281The photoluminescence and thermal stability of 1,6-dithienylhexa-1,3,5-triene has been studied,282and the photochemistry of a-terthiophene in a variety of micellar systems has been reported.283 The Ti02 catalysed photooxidation of dibenzothiophene, thioxanthone, thiaanthrene and thioxanthene gives mixtures of the corresponding sulfoxides and s ~ l f o n e s . ~ ~ ~ Irradiation of the thiopyran derivative (292a) results in extrusion of HNCS and the formation of the pyridine (292b, 63.3%) as the major product.285The photochemical reactivity of the pyranthione (293) is concentration dependent in 3-methylpentane as solvent.286 At low concentrations the thione reacts with the solvent but at higher concentrations the main reaction is the production of thiyl radicals. Laser flash photolysis has been used to identify that the triplet excited state of (294) is involved in addition reactions which occur to electron deficient alkenes such as acrylonitrile giving (295), for example.287Other compounds related to thiourea are also photochemically reactive. Thus the photochemical cyclisation of (296) to afford (297) has been reported.288

Irradiation of the dithiin (298) brings about rearrangement and forms the thiophene derivative (299).289An extrusion of sulfur also takes place and yields the thiophene (300). The rearrangement is proposed to occur via the ring opened dithione (301) and the episulfide (302). Sulfur-sulfur interactions appear to be a vital component in the photochemical reactions of dithiin-l-oxide (303) to give benzaldehyde and the dithiin (304).290Thio analogues of (6 -4) pyrimidine-pyrimidinone photodimers have been ~ynthesised.~~'

1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

4

271

Compounds Containing Other Heteroatoms

4.1 Silicon and Germanium - The silacyclopropene (305) undergoes photochemical extrusion of the silylene (306) which is readily proven by trapping experiments with triethylsilylethene or 2,3-dimethylbuta-1,3-diene to give the adducts (307) and (308), respectively.292In the absence of trapping agents, the dimer (309) is formed. The matrix isolated isomeric silylenes (310) and (311) undergo photochemical conversion into the isomer (3 1 2).2939294

A study of the rates of insertion of silylene into the C-H bonds of methane has been reported.295 2,2-Diphenylhexamethyltrisilane (3 13) is photochemically reactive when it is irradiated (low pressure Hg lamp) in ethanol (1 M) and hexane and this gives the products (314) and (315), both of which are ethanol adducts of diphenyl~ilylene.~~~ At higher concentrations of ethanol (13.6 M) the third product (316) is formed. A study of temperature and solvent effects on the formation of diphenylsilylene from (3 13) was also reported. Leigh and co-workers have investigated the photochemical behaviour of the a-silylketenes, e.g. (317).297The silylene initially formed by this process rearranges to give the silene (318) and the reaction of these species with alcohols was studied. Laser flash photolysis of the silacyclopenta-3-ene in the gas phase results in extrusion of ~ilylene.~~*

278

Photochemistry

Me3Si,/SiPh2 Me3Si (313)

-

Ph2Si/\OEt +hP;& (314) 24%

I

H OEt + Ph2SiSiMe3 I

SiMe3 (315) 21%

(316) 16%

0

9

Intramolecular electron transfer occurs on irradiation of the stilbene derivative (3 19) in methylene chloride.299Irradiation populates a charge transfer state that undergoes E,Z-isomerism and when methanol is added to the system not only is isomerism, observed but also the trisilanyl group in (319) is converted into the silane (320). The photochemical reactivity of the vinyldisilanes (321) has been examined and direct irradiation of (321a) in cyclohexane is reported to yield two products identified as the (2+2)-dimers (322) and (323).300Irradiation of (321c) in cyclohexane with added methanol afforded the methoxy addition product (324). Both these results are in agreement with the intermediacy of silenes such as (325) and (326) in these experiments and

R2 R' I 1 MeSi-SiMe2

Nc

A3

(319) R = SiMe3 (320) R = H

.CH2SiMe3

M*J-;M* Me3SiH2C

(321) a, R' = R3 = H, R2 = CH=CHp b, R' = R2 = CHzCH2, R3 H C, R' = H, R2 = R3 = CH=CH2 I

pMe

MeSi-CH2CH2SiMe3 (323)

CH=CH2 I (324)

-\

Me\

Me/siySiMe3 (325)

Me

Si=\ b3Mee (326)

confirmation of the intermediacy of such compounds was gained from low temperature spectroscopy. The photochemical addition of ethanethiol to vinylethynylsilanes has been reported.301The disilanyl alkyne derivative (327) rearranges on irradiation at 300 nm in benzene solution and leads to the The authors suggest formation of the products (328) in modest yields.302~303 that the reorganisation involves the conversion of (327) into the intermediate (329) as the first step. The intramolecular charge-transfer emission in a series of phenyldisilanes has been studied.304

IIf6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen Me

Me, ,Me

R

(327) R = H or Me

279

(328)

OH _ ..

dSi ' O

A (329)

R

SET induced cyclisations have been carried out on a series of nonconjugated dienes one component of which is a silyl ether.305Irradiation in acetonitrile of dienes such as (330, R = H) using DCA as the electron accepting sensitiser results in the formation of the radical-cation of the diene which on cyclisation affords the cyclic ketone (331) in 25% yield. The reaction is solvent sensitive and in a mixture of acetonitrile/propan-2-01 yields three products which are identified as (331, 30%) and two minor products (332, 11%) and (333, 9%). The reaction is suggested to have some considerable synthetic potential and the effect of chain length and substituents on the reaction has been evaluated. Thus (334) is converted into (335, 11%) while (330, R = Me) affords an isomeric mixture of products (336). Other studies with silyl ether derivatives have examined the electron transfer-induced ring opening processes encountered with the cyclopropane derivatives (337).306

(335)

(336) R' =Me, R2 = H YO) R1 = H, R2 = Me (11%)

(337)n = 1, R'

= R2= H 17-1, R 1 = R 2 = M e n=2, R 1 = R 2 = H

Poly(phenylsi1oxanes) are formed on irradiation (COZ laser) of liquid 1,3diphenyldisilo~anes.~~~ The polymerisation is the result of extrusion of PhHSiO and its insertion into the disiloxane. Dimethoxydisilanes are formed on irradiation of the bicyclodienes (338) under electron-transfer conditions.308

(338) R = Me or PIi

The photophysical properties of a series on metalloles (339) have been

280

Photochemistry

reported.3w The single electron-transfer induced photochemistry of the digermoxanes (340) has been described and leads to the formation of radicalcation of the germoxane which in CCldCH3CN results in chlorination by a free radical path.310 The germatetrasilacyclopntanes (341) undergo ring contraction reactions on irradiation at 254 nm in cyclohexane (see Scheme Irradiation of the norbornadiene derivatives (342) results in the extrusion of organogermylenes. l 2

Ph

-A,->, R2

R'' \ R2

(

(339) R' = R2 = Me or Ph

~

1

~

o

(340) R1 = R2 = Me R1 = R2 = Et R' = R2 = P P R1 = Ph, R2 = Me

R' R' t

(341)

R' = Me3SiCH2,-R2= Pr' R' = Ph. R2 = Pri R' = Phi R2 = Bu'CH2

.

n22Si/-i/+

other Si products

Scheme 10

(342) R = Me or Pr'

4.2 Phosphorus - The photo-Arbusov rearrangement involves the rearrangement of phosphite to phosphonate and has been studied with a view to establishing the stereochemistry of the process.31 Thus irradiation through quartz at 254 nm in acetonitrile solution of the trans-(It,@-phosphite (343) affords the cis-(R,R)-phosphonate (344) as the predominant product which shows that the rearrangement has occurred with retention at the 1-phenylethyl centre. The photo-Arbusov rearrangement is also the principal reaction mode of the phosphite (345, R = H) which in cyclohexane solution yields the phosphonate (346, R = H) in 81 YOyield.314The reaction is efficient and occurs with a quantum yield of 0.43. Other products such as bibenzyl and dimethyl phosphite are minor suggesting that there is little radical diffusion with (345, R=H). The reaction is thought to arise from short-lived singlet free radical pairs and there is a marked difference with the p-acetylbenzyl phosphite (345, R = COCH3) in which irradiation affords mainly products of radical diffusion. These products are the dimer (347), dimethylphosphite and p-acetyltoluene and experiments have demonstrated that the reaction of (345, RzCOCH3) occurs via the triplet state of the phosphite. A SET mechanism is involved in the phototransformation of the enones

IIl4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

28 1

Ph

(348) into the products (349) in the presence of the phosphites (see Scheme 1l).3'5 Irradiation affords the triplet enone and an electron is transferred from the phosphite to the enone resulting in the radical-catiodradical-anion pair (350). Collapse of the radical-cation component of (350) gives (351) which then reacts by addition to the enone radical-anion. The products (349) are isolated after hydrolysis of the corresponding silyl ethers and the influence of ring size and substituents was also examined as can be seen from the results illustrated in Scheme 1I , Entries 5 and 6 in this scheme show that only low yields are obtained with heavily substituted enones. 0

0

(348)

In 1

2 3 4 5 6

1

2 3 1 2 3

R' H H H Me H H

R2 H H H H Me H

R3 H H H H H Me

(349)O Yield (YO) R = Me R = Et 92 82 89 91 81 78 86 87

5

57 mixture of products

The DCA-sensitised reactions of the phosphate (352) in acetonitrile using h > 355 nm results in formation of the corresponding radical-cation which undergoes bond fission with the formation of the binaphthyl (353) and the phosphate (354).316An analogous reaction takes place with (355) to give (353) and the phosphate (356). The yields of product are low as are the quantum yields and interestingly the phosphates (357) are unreactive under these conditions. A CIDNP study of recombination of radicals following irradiation of the

282

Photochemistry

8

0

1 \

(EtO)2!-0

0 II Me-P-OH I

OH

phosphonic acid derivative (358) has been p ~ b l i s h e d . ~ High ' ~ rate constants have been established for the addition of radicals derived from the phosphine oxides (359) to acrylates and these data explain why these compounds are good photoinitiators for p o l y m e r i ~ a t i o n . ~ ~ ~

R'

(359)

Ph &,But

Ph &But

R2

R2

R3

Me0

H

Me0

H

Me

Me

Me

Me

Fragmentation of the phosphabicycloctene derivatives (360) occurs with 254 nm radiation in mixed solvent (ROH/CH3CN).319This process affords good to excellent yields of the phosphinic esters (361). The mechanism is thought to involve the elimination of a methylene phosphine oxide which is trapped by the alcohol.

0

0 (360) R1 = H, R2 = Me R' = Me, R2 = H

II Ph-P-Me I OR (361) R = Me, Et, Pr", Pri, Bunor H

Irradiation ( h> 300 nm) of the sterically hindered phosphiranes (362) results in extrusion of the alkene moiety to give the phosphinidene (363)

IIl4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

283

which undergoes insertion into the neighbouring t-butyl group to afford the phosphaindane (364).320 The diphosphirane (365) also undergoes a photoextrusion reaction and yields (364). A low-temperature matrix-isolation study provides evidence for the formation of phosphinidene intermediates in these reactions.

ArO P i P h

362 A

r

-

(363)

(364)

(365)

4

4.3 Other Elements - Irradiation (( > 300 nm) of the allene (366) in the presence of diphenyl diselenide affords a high yield of the adduct (367) as an El 2 mixture (28:72).321 Diphenyl disulfide affords a complex mixture of products with the same allene while diphenyl ditelluride does not react. The difference between the sulfide and the selenide is due to the lower ability of diphenyl disulfide to react with carbon radicals. When a mixed system is used [(PhS), :(PhSe)* as a 1 : 1 mixture] mixed addition occurs. Thus with the allene (366) an almost quantitative yield of (368) is produced and other allenes (369) are also reactive under these conditions, affording (370).32'The use of diphenyl selenide as a catalyst for the photochemical isomerism of some carotenoids has been described.322

Me H

H Me

Pr" C5Hll

71

28

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in The products are the 3-substituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (A > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377).324The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy ally1 radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for

284

Photochemistry

addition at the less hindered site and the reaction occurs mainly without loss of the configuration of the double bond in the ailyl unit : examples are shown in Scheme 12.

B u 3 S n A O R 1 (374) R’ = Me or TBDMS

a,

Ph b, flNC6H4 c, Ph

Bz H Ph

Scheme 12

2-Aryl-1-phenylethanol derivatives (378) are formed from the irradiation of styrenes (379) with triarylstibines (380) in the presence of oxygen.325The yields of the products are modest (14-48%) and the mechanism for product formation involves a stibine/oxygen/styrene complex (38 l) which reportedly undergoes valence expansion to (382) followed by rearrangement to (383) and hydrolysis to yield the isolated alcohols.

The telluroglycoside (384) undergoes C-Te bond fission on irradiation in benzene solution at 100°C to give the glycosyl radical (385).326The radicals produced in this manner can be trapped by alkynes (386) to yield the alkenyl derivatives (387).327The process involves regiospecific addition of the glycosyl

IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen

285

radical to the alkene and trapping of this radical by combination with the Tearyl radical. Addition of the glycosyl radicals also occurs to isonitriles such as (388) to give the imine (389).328

g:o.*

A:Eo*Te-ptdyl AcO

AcO

AcO

(384)

AcO

(385)

Yield (YO) E : Z

93 78 48 38 11

=-Q

7525 80:20 79:21 69:31 74:26

* ; ;A AcO Te-ptolyl

(387)

Te-ptolyl

(389)

Irradiation of diphenylmercury in the presence of quinones and coumarins results in p h e n y l a t i ~ n . ~ ~ ~

References

5 1.

2. 3. 4. 5. 6.

7. 8. 9. 10.

A. Albini and E. Fasani, Spec. Publ. - R. SOC.Chem., 1998, 225 (Drugs; Photochemistry and Photostability), 1. H. H. Tonnesen, S. Kristensen and K. Nord, Spec. Publ. - R. Sac. Chem., 1998, 225 (Drugs; Photochemistry and Photostability), 87. A. E. Keating and M. A. Garcia-Garibay, Mol. Supramol. Photochem., 1998, 2, 195. B. Heller, Nachr. Chem. Tech. Lab., 1999, 47, 9 (Chem. Abstr., 1999, 130, 1101 12). A. N.Frolov, Russ. J. Org. Chem., 1998,34,139. M.Kira, Chem. Org. Silicon Compd., 1998, 2 (pt 2), 131 1 (Chem. Abstr., 1999, 130,8 1534). A. G.Brook, Chem. Org. Silicon Compd., 1998, 2 (pt 2), 1233 (Chem. Abstr., 1999,130,81533). K. Mimno, T. Tamai, A. Sugimioto and M. Akira, Adv. Electron Transfer Chem., 1999,6,131. N. Mataga, Adv. Chem. Phys., 1999, 107 (Electron Transfer: From Isolated Molecules to Biomolecules, Pt l), 431. S. Fukuzumi and S. Itoh, Adv. Photochem., 1999,25,107.

286 11.

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13. 14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31. 32. 33. 34.

35. 36.

37. 38.

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7 Photoelimination BY IAN. R. DUNKIN

1

Introduction

This chapter deals with photoinduced fragmentations of organic and selected organometallic compounds, in particular reactions accompanied by loss of small molecules such as nitrogen, carbon monoxide or carbon dioxide. Photodecompositions which produce two or more larger fragments and other miscellaneous photoeliminations are reviewed in the final section. Photofragmentations of carbonyl compounds, taking place by Norrish Type I and I1 processes, are discussed in Part 11, Chapter 1. A number of papers and reviews have appeared which are of general relevance to photoelimination chemistry. There have been two reviews of the applications, merits and disadvantages of the transient grating method for investigating photochemical processes in solution. Fluorescence studies of photofragments from vacuum-UV photolysis have also been reviewed, covering a range of species, e.g. CC12 from C2C16 and CC14, CH and C2 from C2H2, OH and CH from C~HSOH, CDCH3 from CD(CH3)2, and CF3, CF2 and CF from CF3C1.3 The MINDOC-CI approach for determining spin-orbit coupling surfaces has been compared with previous ab initio calculations for a series of organic biradi~als.~ Applications of laser photofragmentation and fragment detection for the analysis of gas-phase mixtures have been de~cribed.~ s2

2

Elimination of Nitrogen from Azo Compounds and Analogues

The syn and anti stereoisomers of the spiroepoxy diazene (1) (Scheme 1) have been synthesized, separated chromatographically, and distinguished spectroscopically by the significant NOE between phenyl and the e m hydrogens exhibited by the syn isomer.6 Photolysis of (1) in benzene at 15 "C with an argon-ion laser yields the oxabicycloheptene (2) exclusively (>95%), independently of the stereochemistry of the diazene. Triplet sensitization by benzophenone gives the same result. The initially formed diradical (3) thus appears to undergo exclusive C-0 bond cleavage to give (4); there is no evidence for the housane (S), even at -98 "C. In the presence of MeOH, the adducts (6) and (7) are formed instead of (2). Control experiments showed that (2) reacts with MeOH to give exclusively (6); so the adduct (7) seems to arise from ~

Photochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 297

298

Photochemistry

trapping of diradical (4), in accord with its expected dipolar (zwitterionic) character, with localized negative charge on oxygen and allylic stabilization of the positive charge.

an&( 1)

Although the diazene (8) (Scheme 2) decomposes thermally by loss of N2 to give the bicyclobutane (9) and its ring opened isomer (10) as the major products, photolysis with Pyrex filtered UV light gives mainly the rearranged bicyclic compound (1 1) and an unidentified hydrocarbon mixture of empirical formula C22H22.7

140°C

*N

-2

Ph

-

Ph

+

phy

Photolysis of the bipyrazole (12) in the presence of Rh6(C0)16 results in smooth loss of N2.* The cyclopropene derivative (13) can be isolated after short reaction times, but this loses a second N2 molecule on further irradiation, to yield 2,7-dimethyl-3,6-diphenylocta-2,6-dien-4-yne as the main product. A series of biaryl-5-morpholinotriazolines(14; R = Me, Et, Pr, Ph; Y = CH, N) have been synthe~ized.~ On direct photolysis they lose N2 and produce the phenanthridines (1 5; Y = CH) or analogues (15; Y = N). Better yields of these products are obtained, however, by first inducing loss of N2 thermally, followed by photocyclization of the resulting intermediate amidines.

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The photolysis of phenylazotriphenylmethane in CHzCl2 and acetonitrile solution at 347 nm and at 23°C results in formation of both trityl (triphenylmethyl) radicals and trityl cations.’* The ions are assumed to derive from electron transfer between radicals, and their yield is increased in solutions containing also a pyridinium salt. Trityl ions generated in this way are capable of initiating the cationic polymerization of cyclohexene oxide. The photolyses of poly(ary1azophosphonates) and model monomeric analogues (ArN=NP(0)(OMe)2) have been studied to assess the suitability of the polymers as materials for laser ablation lithography. In this application the azo group functions as a design fracture point, and the nitrogen released during photolysis acts as a driving gas for ablation. 3

Elimination of Nitrogen from Diazo Compounds and Diazirines

The photolysis of diazo compounds and the isomeric diazirines, and investigations of the resulting carbenes, are research areas with continuing high levels of activity, ranging from fundamental and theoretical studies to applications such as photoaffinity labelling. The rearrangements of singlet carbenes by 1,2hydrogen migration, and particularly kinetic studies of these reactions utilizing laser-flash photolysis, have been reviewed.l 2 3.1 Generation of Alkyl and Alicyclic Carbenes - Photolysis (350 nm) of dipropyldiazirine in CH2C12 at 4 “C produces a mixture of E- and 2-hept-3-ene in 81% yield and with an EIZ ratio of 1.8.13 The intermediate dipropylcarbene can be trapped with piperidine as a carbene-amine adduct, in competition with the formation of hept-3-ene; but a detailed analysis of the kinetics suggested that direct carbene formation from the diazirine is not the only reaction pathway and that a significant proportion of the heptene arises from the diazo isomer and possibly also from an excited state of the diazirine. A laser-flash photolysis study of the generation of trans-2-tert-butylcyclopropylcarbene(17) from trans-2tert-butylcyclopropyldiazirine ( 16) has been reported.l4 The carbene can be trapped by pyridine as an ylide, the rate of formation of which is linearly dependent on the pyridine concentration. The lifetime of carbene (17) at ambient temperature was found to lie in the range 15-27 ns in cyclohexane, cyclohexane-d12,pentane, CF2ClCFC12 and acetonitrile, and the disappearance of (17) seems to occur mainly as a result of reaction with the

300

Photochemistry

solvent, and only to a lesser extent by rearrangement to the corresponding cyclobutene.

The generation of 2-adamantylidene (18) from the corresponding diazirine has been reinvestigated by two groups. * *6 As a result, previous anomalously high estimates for the lifetime of this carbene (ca. 2.2 ps in benzene at ambient temperature) and anomalously low rate constants, e.g. for reaction with pyridine, have been revised and brought more into line with those of other simple carbenes. Adamantylchlorocarbene (19) has been generated in Ar matrices at 10 K by 366 nm photolysis of the diazirine pre~urs0r.l~ Under these conditions, a substantial amount of the diazo isomer of the diazirine is also formed. On further irradiation with visible light, both the diazo compound and carbene (19) are photolysed, and chlorohomoadamant-3-ene (20) is produced. The matrix reactions were monitored by UV-visible and IR spectroscopy, and (19) and (20) were identified with the aid of DFT calculations of their IR spectra, and by trapping of the carbene with HCl. 53

CI

/

Visible light irradiation ( h > 475 nm) of quinone diazide (21) isolated in Ar matrices at 10 K produces the corresponding carbene quantitatively. l8 Further UV photolysis (h > 360 nm) of this carbene results in formation of a labile species, which is identified as the diradical (22). The identification of (22) was supported by ROSS-BLYP/6-31G(d,P) calculations, and confirmed by deuterium labelling. 0

3.2 Generation of Aryl Carbenes - A review has been published of studies of the fluorescence spectroscopy and the determination of zero-field splitting

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301

parameters of the excited triplet states of aromatic carbenes generated in Shpolskii matrices.19 The heats of reaction and reaction volumes for the formation of ethers by photolysis of diphenyldiazo compounds in methanol have been measured by the transient grating method.20 Laser flash-photolysis investigations have been carried out for benzylchlorocarbene, phenylchlor ocarbene,methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes.21These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range lo4 to 1O'O M-' s-' at 22"C, but there is no evidence for the formation of ylides from carbene attack on the heteroatom donors. Singlet (biphenyl4yl)chlorocarbene has been generated from (biphenyl-4-yl)chlorodiazirinein a laser flash-photolysis study, and its reactions with nitriles, to give nitrile ylides, and also with 2,3dimethylbut-2-ene, were investigated.22 This carbene forms nitrile ylides in equilibrium processes; with equilibrium constants for addition to propionitrile and pivalonitrile estimated to be 0.45 and 0.37 M-l, respectively, at 295 K. The carbenes (23; R = H, Me) have been generated in Ar matrices at 10 K, by visible light irradiation of the diazoanthrone precursor^.^^ The carbenes were identified by trapping with 02, comparison of IR and UV-visible spectra with those of the parent anthronylidene, and by comparison of the IR spectra with spectra calculated by DFT methods. Further irradiation of the carbenes with visible or UV light induces hydrogen migration and rearrangement to the quinone methides (24; R = H, Me).

Me

(23)

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-n0radamantyl@henyl)diazomethane.~~ This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with 0 2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable tert-butylaminoxyl and Ullman's 'nitronyl nitroxide' radicals, e.g. (25), have been prepared by photolysis of the parent diazornethane~.~~ Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected.

302

Photochemistry

A new photo-cross-linking deoxyuridine analogue, containing an aryl(trifluoromethy1)diazirine moiety has been synthesized, incorporated into a synthetic oligonucleotide, and used in the photoaffinity labelling of EcoRII and MvaI restriction-modification enzymes.26The grafting characteristics of photoactivatible reagents containing diazirine groups, designed for carbene-mediated grafting onto solid supports such as silicon, silicon nitride and diamond, have been examined by time-of-flight secondary ion mass spectrometry and by xPs.27*28 3.3 Photolysis of a-Diazo Carbonyl Compounds - Laser flash-photolysis studies of carbonyl carbenes have been reviewed, with discussion of singlet vs. triplet and conformational effects on the Wolff rearrangement .29 An earlier report that flash photolysis of diazoacetic acid produces the enol of mandelic acid by hydration of phenylhydroxyketene, itself arising from the photo-Wolff reaction of the diazo acid, has been challenged by new experiments, which show that this is only a minor route.30 The major pathway appears to be a novel enol-forming reaction involving hydration of the carboxycarbene. The bisdiazo dione (26) is one of very few reported compounds with two inequivalent diazo group^.^' Photolysis of (26) in benzene containing 1% MeOH with light of h > 300 nm produces the spironorcaradiene (27) and diazoketone (28) in yields of 21% and 9%, respectively. At h > 420 nm, however, the product ratio (27):(28) is dramatically reduced to 0.06:I. Since (27) arises from initial loss of N2 from the 2-position, and (28) from initial loss of N2 from the 4-position7the wavelength dependent product yields suggest that long wavelength light can decompose the 4-diazo group but not the 2-diazo group. It is suggested that with shorter wavelength light excitation of (26) can give rise to an excited state higher than S1.

2-Naphthyl(methoxycarbony1)carbene (29) has been investigated in low temperature matrices,32 and in flash-photolysis e x p e r i m e n t ~ . ~In~ 9each ~ ~ of these complementary studies the carbene was generated from the a-diazo ester, methyl 2-diazo-(2-naphthyl)acetate. Matrix photolysis of the precursor at 450 nm yields primarily the triplet ground-state carbene, which has been

IIl7: Photoelimination

303

characterized by UV-visible, IR and ESR spectroscopy, as well as by trapping with 0 2 and C0.32 Bleaching of the visible bands of this species results in disappearance of the ESR signal and the appearance of a new optical spectrum, which is assigned to the singlet carbene, an assignment supported by quantum chemical calculations. The original spectra of the triplet are recovered in the dark at 12 K. Flash-photolysis studies of the same ester precursor showed that the singlet carbene is generated initially, but this decays within 350 ps - 1 ns to the lower energy triplet, much faster than the Wolff rearrangement to the corresponding ketene.34 Time-resolved IR studies confirmed that in this case the ketene is formed almost exclusively from the carbene rather than an excited state of the diazoester, and transient IR bands for both singlet and triplet carbene were detected. (Trialkylsilyl)vinylketenes, which are useful diene components in DielsAlder reactions, have been generated by photochemical Wolff rearrangment of a’-silyl-a’-diazo-a,P-unsaturated ketones.35 Electron-donor additives increase the quantum yield of photodecomposition of diazonaphthoquinone sulfonate in photoresists, thus increasing resist ~ensitivity.~~ Photoaffinity probes containing diazoacetyl and trifluorodiazopropionyl groups have been developed for labelling PKC i~oenzymes.~~ The photo-Wolff reaction of an a-diazoketone in a self-assembled monolayer, and in the presence of methanol as a ketene trap, has been used to modify the surface of a gold electrode.38It is suggested that such photoreactive templates have potential for a range of surface modifications. 4

Elimination of Nitrogen from Azides and Related Compounds

The photochemistry of a-azidocinnamates (30; R = Me, Et; Ar = Ph, 2-MeCsH4, 4-ClCsH4, 2,6-Cl&H3, 3,4,5-(MeO)&H2) has been reinvestigated.39 It had previously been reported that photolysis of an cc-azidocinnamate in quartz produces a single stereoisomer of a trimer (32), arising from the initially formed azirine (31). The new study has shown that use of Pyrex filters or ketone sensitization leads to high yields of a diastereomeric pair of aziridinoimidazoline dimers (33), which are formed by the 1,3-dipolar addition of an azirine molecule and the azomethine ylide formed by C-C bond cleavage of another azirine molecule. The dimers had been presumed as intermediates in the formation of the trimer, but not previously identified. A curious further observation is that only a mixture of the two stereoisomeric dimers gives the trimer on further photolysis; irradiation of either pure dimer leads only to

304

Photochemistry

decomposition. It was concluded that, while both dimers will be in photochemical equilibrium with the parent b r i n e , one of the dimers must ring-open more readily than the other. Photolysis of triphenylsilyl azide (34; R = Ph) in Ar matrices at 15 K gives triphenylsilanimine (35; R' = R2 = Ph), identified by UV and IR spectroscopy and comparison of experimental and calculated IR spectra.@Similarly, matrix photolysis of diphenylsilyl azide (34; R = H) gives two silanimines from 1,2phenyl migration (35; R' = H, R2= Ph) and 1,2-hydrogen migration (35; R' = Ph; R2 = H). In the latter case, further photolysis leads to elimination of benzene, generating phenylsilaisonitrile (PhNSi) presumably via phenylsilanitrile (PhSiN), and also possibly one or more silylenes.

Photocrosslinking of glycidyl azide polymers, supposedly via nitrene intermediates, has been ~tudied.~'Addition of an unsaturated hydrocarbon polymer accelerates the crosslinking process, and this is thought to be due to the formation of aziridine rings by nitrene addition to the carbon-carbon double bonds. Aryl Azides - The dissociation of aromatic azides in the ground and lower excited states has been studied theoretically using the PM3 method with correlation corrections, which has previously been shown to reproduce ab initio minimum energy pathways for the dissociation of HN3.42 Several fluorophenyl azides have been photolysed in the presence of diethylamine at room temperature, and a study made of the competition between ring expansion, yielding azepine products, and nitrene insertion into the N-H bond of Et,NH, yielding h y d r a z i n e ~Pentafluoro.~~ and 2,6-difluorophenyl azides give hydrazines as the major products, but fluorophenyl azides with at least one unsubstituted position ortho to the azide group give azepines. Photolysis of (36) in an argon matrix at 10 K generates the corresponding triplet nitrene, recognized by its typical UV-visible absorption spectrum; this nitrene seems essentially photostable under these conditions.44 On the other hand, photolysis of (36) at ambient temperature in acetonitrile containing diethylamine yields two products, which have been identified as the ringexpanded methyleneazepines (37; R = OPh) and (37; R = The 9-(azidopheny1)acridine (38) and corresponding N-methylated acridi4.1

+

H).44945

N-

PhO

N3

OPh

NEB

1117: Photoelimination

305

nium salts have absorption bands in the near-UV and visible regions. Photodissociation quantum yields for 365 nm photolysis of these compounds are 0.88 for (38) and < for the acridinium salts.& The photochemical reactivity of (38) and relative inertness of its quaternized derivatives were accounted for by MNDO-PM3 analysis of the changes in bond order, NNN bond angle, and charge redistribution following excitation to the lowest excited singlet states.

A study of substituent effects on the photodecomposition of para-substituted 2-pyrazolylphenyl azides (39; R=H,F, C1, Me, CF3, OMe, NMe2) at 295 and 90 IS in ethanol has been carried The results suggest that electronic effects do not hinder the equilibrium between the singlet nitrene and its cyclic isomers, but do influence which of the intermediates decays to stable end products. The electron-donor group NMe2 has the second effect of greatly increasing the triplet nitrene energy, thus reducing the S-T energy gap; so that the singlet and triplet nitrenes are in equilibrium along with the corresponding benzazirine. The triplet nitrenes were detected at 90 K by their characteristic electronic absorptions, In the presence of 0 2 the nitrenes form nitroso oxides (ArNOO) which then convert into nitroso and nitro products in good yields.

UV photolysis of the powdered crystals of several aryl azides has been found to give azo compounds, from dimerization of the nitrenes, mostly in yields of ~ 9 7 % The . ~ only ~ exceptions noted were p-(N-methy1acetamido)phenyl azide, which also gives a product from insertion of the nitrene into a C-H bond of the methyl group, and 2-azidobiphenyl, which gives comparable amounts of the azo product and carbazole. Monitoring these solid-state photoreactions by ESR revealed that the arylnitrenes have extremely long half-lives, compared with those of nitrenes in the gas-phase or solution, and clearly the crystalline

306

Photochemistry

environment exerts a profound influence on the course of reaction. Photolyses and several other aromatic diazidessO of 4,4’-diazidodibenzylidenea~etone~~ have also been investigated in the solid state; while photolysis of 4,4’diazidobiphenyl in solution has been noted to deposit thin layers of a branched and crosslinked polymer, containing azo, hydrazo and azepine units.5 A series of p-aryloxy- and p-alkoxyphenylnitrenium ions have been generated in aqueous solutions by photolysis of the parent azides, whereupon the resulting nitrenes are p r ~ t o n a t e d ?Hydration ~ of these cations at the para position leads via hemiacetal or halohydrin intermediates to quinone imines, which finally hydrolyse to the ultimate quinone products. In flash-photolysis studies of these reactions it was shown that nitrenium ion hydration occurs on the ps timescale, hemiacetal or halohydrin breakdown on the MS timescale, and the final imine hydrolysis over minutes. A photoreactive poly(ethy1ene glycol) derivative has been synthesized, incorporating a terminal 4-azidobenzoyl This material was photografted onto polysulfone ultrafiltration membranes, thus increasing their hydrophilicity. The advantages of perfluorination of aryl azides in increasing the efficiency of nitrene insertion have been exploited in the design and synthesis of new multifunctional crosslinking agents for probing proteinprotein interaction^.^^ In an extension of this work, the efficiency of C-H insertion by photolabile chelating agents, such as (40), has been correlated with the lifetimes of singlet nitrenes, determined by flash phot~lysis.’~ In

Me

molecules such as (40), chelation of metals occurs through the two amino groups, but the azide group is shielded from the electronic and steric influences of the chelated metal centres by the ester (or similar amide) bridges. Very high C-H insertion efficiencies have thus been achieved, e.g. up to 93% with cyclohexane. As an example of the applications of photolabile chelating ~ covalently agents, (40) was chelated to the diagnostic radionuclide 9 9 m Tand attached to human serum albumin by photochemical activation. This should allow probing at tracer level concentrations. An azidoisoleucine derivative has been synthesized as a molecular probe for receptors and binding proteins involved in stress signalling of plants, and appears to bind significantly to a model protein on photoactivati~n.~~ Photoaffinity labelling analogues of a - t o ~ o p h e r o l ,the ~ ~ marine sponge metabolite i l i m a q ~ i n o n e ,25-hydroxy~~ vitamin D3,59and isocarbacyclin derivativesm have also been reported, all containing azidophenyl substituents.

IIJ7: Photoelimination

307

4.2 Heteroaryl Azides - Despite previous fears to the contrary, the synthesis of diazepines by photolysis of tetrazolo[lY5-a]pyridines(4 I) (which usually exist in equilibrium with the isomeric 2-azidopyridines (42)) (Scheme 3), now seems to be reasonably general?' The reaction has been studied for a range of starting materials (R1= H, Me; R2= H, CF3; R3= H, C1, Me, CF3; R4 = H, Cl, Me, CF3) and nucleophiles (NuH = MeOH, EtOH, t-BuOH, H20, HNMe2, HNEt2, HN(i-Prh). It proceeds by ring expansion of the initially formed nitrene to a carbodiimide (43), which in unsubstituted form (R'= R2= R3 = R4 = H) has been detected in Ar matrices at 10 K by virtue of its characteristic v(NCN) IR absorption at 1978 cm-'.In solutions containing nucleophiles, the photoproducts are 1H- lY3-diazepines(44)or 5H-lY3-diazepines (45), which in many cases can be isolated in yields of 39-94%. With tBuOH and H20, however, secondary loss of isobutene or hydrolysis lead to diazepinone products.

NuH

or

R*

R4 R 3 & :

NU

Photoreactions of the analogous tetrazolo[lY5-a]pyrimidinewith benzene and substituted benzenes in the presence of trifluoroacetic acid seem to involve the intermediacy of the 2-pyrimidylnitrenium ion.62 5

Photoelimination of Carbon Monoxide and Carbon Dioxide

Laser photodissociation of ketene at 230 nm has been investigated in molecular beams.63 The experimental rovibrational population distribution has been compared to predictions from phase-space theory for the channels leading to CO + CH2(ii1A~)and CO + CH#B1). The calculations are not compatible with the latter channel, suggesting that it does not contribute significantly to the dissociation process. The photodissociation of singlet ketene by two-step IR + UV excitation has been studied using state-selective detection of CH2 by laser-induced fluorescence, and the results compared with

308

Photochemistry

phase-space theory." Fragmentation on the triplet surface of ketene has been the subject of a high level theoretical Competing channels in the photodissociation of HNCO have been examined by the photofragment ion imaging technique.66Excitation with light of 193280 nm accesses S1. At the longer wavelengths (260-280 nm) 3NH + CO are the only products, and these were thought to be formed in the pathway S1-SO- Tl +3NH + CO. At shorter wavelengths, the processes forming H + NCO and 'NH + CO are also observed. In a full mechanistic discussion, dissociation from SO and S1 are considered for the singlet channels, and a refined triplet dissociation mechanism is now proposed, involving competition between the S l - T p T l and Sl+So+Tl pathways. At 222 nm, photofragmentation dynamics of HNCO and DNCO have been studied at room temperature by a pwnp-probe technique.67The quantum yields for H and D atom production were measured as 0.33 (kO.04) and 0.42 (f0.09),respectively, and dissociation from S1 is proposed as the prevalent reaction channel. Other studies of CO loss from small molecules include the photolysis of OCS on Ag clusters, in which an odd-even dependence on the number of Ag atoms in the cluster was discovered.68 OCS desorbs non-dissociatively from even numbered clusters, but dissociates to CO and Ag,S on odd numbered clusters. This alternation correlates with the ionization potentials of the naked metal clusters, and the photoreactivity pattern can be explained in terms of a charge-transfer mechanism. A numerically efficient algorithm has been developed in the theory of ladder-climbing and IR multiphoton dissociation, and has been applied to HC0.69 Decarboxylation of arylacetic acids can be induced photochemically in the presence of Hg070 or Hg2F2.71 The major products are arylethanes from dimerization of the resulting radicals, and with reported yields in the range 52-92%, this photo-Kolbe synthesis is a potentially useful synthetic method. Related photodecarboxylations of carboxylate complexes of iron(@ tetra(2-Nmethylpyridy1)porphyrin pentachloride have also been in~estigated.~~ The observed substituent and deuterium isotope effects on the rates of reaction indicate the formation of a discrete carboxyl radical as an intermediate. Photoinduced decarboxylative additions of formate and alkyl carboxylates, as well as a-keto carboxylates, to N-substituted phthalimides and N-phthaloyl amino acid esters have been reported to take place in moderate to high yields.73 In nearly all cases the products are hydroxy phthalimidines or hydroxy phthalimidine methyl esters, probably arising from initial radical attack on one of the phthalimide carbonyl groups. For the amino acid ester derivatives, the chemoselectivity of the photoreaction is much higher than for the analogous Grignard additions. Theoretical studies of the photodecarboxylation of cycloheptatriene and cyclopentadiene carboxylate anions suggest that the same carbanion intermediates are involved in 'these processes and the photodeprotonations of cycloheptatriene and ~yclopentadiene.~~ It is concluded that both types of process occur on the HOMO-LUMO excited energy surfaces with A ' symmetry. Irradiation of imidazole-2-carboxylicacid (46) at 254 nm in an Ar matrix at

IIi7: Photoelimination

309

10 K induces decarboxylation, and a complex between the carbene (47) and COZ is formed.75 Further irradiation of this carbene at 254 nm does not produce detectable amounts of imidazole; while irradiation at 193 or 185 nm results in decomposition to acetylene, acetonitrile, methyl isocyanide, the ylide H2C-NCH, and HCN.

Photolyses of 1,2:4,5- and 1,2:3,4-benzenetetracarboxylic dianhydrides (48) and (49) have been carried out in low temperature matrices with selected wavelengths, in the hope of detecting and characterizing 1,4- and 1,3benzdiyne.76 Sequential decarboxylation and decarbonylation from one anhydride moiety in each of (48) and (49) was indeed observed, yielding benzocyclopropene and benzyne intermediates; but continued photolysis produced 1,3,5-hexatriene as the final product, without any further intermediates being detected. It is possible, however, that the benzdiynes which were being sought may have participated as intermediates in the formation of the triene product, even though they were not detected. Similar matrix photolysis of 1,2naphthalenedicarboxylic anhydride gives 1-naphthyne (1,2didehydronaphthalene) (50), which was identified by comparison of experimental and calculated IR spectra.77

5.1 Photoelimination of CO and C 0 2 from Organometallic Compounds Volume 177 of Coordination Chemistry Reviews was devoted to metal-toligand charge-transfer excitation, and contains useful reviews of metal-CO photodissociation in transition metal complexes,78quantum chemical investigations of a-diimine transition metal carbonyl complexes, 79 time-resolved IR studies of transition metal complexes containing CO and other ligands,*O the mechanistic roles of metal-to-ligand charge-transfer excited states in oganomet a l k photochemistry,81and other topics. The experimental techniques available for investigating short lived intermediates in organometallic photochemistry have also been discussed.82 Photochemical studies of simple mononuclear metal carbonyls have been relatively few in the period under review. The ultrafast photodissociation dynamics of Cr(C0)6 following excitation at 200 and 267 nm have been

310

Photochemistry

examined, and reveal that the first loss of a CO ligand occurs within 140 fs.83 The weakly bound CrCO species generated by IR multiphoton decomposition of Cr(CO)6 has been detected by laser induced fluore~cence.~~ Tungsten and molybdenum hexacarbonyls, when photolysed with Cm in solution, yield q2complexes of the metal pentacarbonyls with fullerene, which are precursors of metal f u l l e r i d e ~ .The ~ ~ hexacarbonyls M(CO)6 (M=Cr, Mo, W), as well as W(CO)sCS and two mesitylene complexes, have been photolysed in polyethylene matrices containing H2 and N2 at low temperatures under high pressure.86 With N2, the hexacarbonyls give initially M(CO)SN2, followed by more highly substituted species on longer photolysis. With H2, M(C0)5(q2-H2) and cisM(C0)4(q2-H& are formed, which were previously unknown for Mo and W. Photochemical studies of Fe(CO)S on silver surface^*^^^^ and on evaporated Fe films89have also been reported. The tricarbonyl( 1-hydronaphthalene)manganese complex (51) reacts readily with trimethyl phosphite to give the complex (52;L = P(OMe)3), in which there has been a simultaneous q5-q3hapticity change (Scheme 4).90 UV irradiation of (52) selectively removes one CO ligand to afford (53), thus effecting overall ligand exchange by addition-elimination rather than the more usual elimination-addition. In similar fashion (53) can undergo exchange of a further CO by t rimethyl phosp hite.

OC-Mn

oc’ %o

(52)

Scheme 4

(53)

The chromium mediated [3 + 2 + 11 cycloaddition of alkyoxy(ary1 or vinyl)carbenes, alkynes and CO - usually known as the Dotz annulation - provides convenient access to highly substituted oxygenated arenes from chromium complexes such as (54). It is normally a thermal reaction, but two precursors (54; R’= H, Ph) have been found to be surprisingly inert under the usual conditions. Nevertheless, benzannulation can be achieved by photoactivation, presumably via initial loss of CO. Thus photoreactions of (54) with symmetrical alkynes (R2C= CR2) give dihydrobenzofuranols (55) in moderate to good yield^.^' Unsymmetrical alkynes generally produce both possible regioisomers. A novel and convenient procedure for demetallation of tricarbonyliron-diene complexes such as (56) has been described.92 The first step is photolytically induced ligand exchange of the CO ligands by acetonitrile at low temperature; and demetallation is finally achieved by aerial oxidation. Manganese tricarbonyl complexes with alkyl halide side chains (57; X=Br, I) undergo CO loss on photolysis to form complexes in which the halo atom is coordinated to manganese (58; X = Br, I).93 Hydrido stannyl complexes containing the q 2-H-SnPh3 ligand and bis-stannyl compounds containing two SnPh3 ligands have been synthesized from (q6-arene)Cr(C0)3 complexes and

31 1

IIl7: Photoelimination

HSnPh3 by photoinduced CO exchange.94 Dimeric ruthenium(r1) complexes [RuL(CO)C&, where L is 2,2'-bipyridine or a related diimine, have been synthesized from the mononuclear complexes RuL(C0)2C12 by photochemical monode~arbonylation.~~ Binuclear rhenium complexes have been similarly obtained by photolysis of indenylrhenium tricarbonyl in h e ~ a n eUV . ~ ~irradiation of the rhenium complexes Re(q5-C5R5)(C0),(R = H, Me) in the presence of C6F5H or 1,2,4,5-CbF4Hz results in C-H activation, generating hydrido complexes as the main products.97

R'

/

(54)

Reactive intermediates in the photodecarbonylation of the cyclopentadienyl complex (59) and its indenyl analogue have been studied by time-resolved IR and optical spectroscopy, and the competitive reaction dynamics of methyl migration to the metal centre vs. trapping by CO and trapping by other ligands determined.98Solvated species formed after initial loss of one CO ligand are the most likely intermediates. The solvated species (q6-arene)Mo(C0)2(Sol) and M(CO)5(Sol) (M = Cr, Mo, W) have been generated photochemically in a range of alkane solvents (Sol), and kinetic parameters determined for solvent displacement by C0.99 The results indicate that solvent displacement by CO involves an interchange mechanism for the Cr complexes, but for Mo and W the mechanism is more associative in nature. In a sub-picosecond IR study,loO the reactive intermediate in C-H bond activation in cyclohexane by photoinduced reaction with CpRh(C0)z (Cp = q5-C5H5)has been identified as the cyclohexane solvate CpRh(CO)(C6H12).

O OC C - VMe - O

The photolyses of a series of binuclear molybdenum carbonyl complexes containing alkyne ligands have been studied in frozen Nujol matrices at 77 K. lo' IR spectra indicated the formation of two isomeric carbonyl-loss products in each case, one of which seems to be formed by photolysis into the

312

Photochemistry

low energy electronic transitions of the starting material, while the other results either from secondary photolysis of the first product, or by photolysis into the high energy charge-transfer bands of the starting material. Solution photolysis of ring-coupled binuclear complexes such as (60; M = Mo, W; R = H , Me) in the presence of PPh3 or PMe3 results in simple substitution of one or two CO ligands, with the phosphine ligands located trans to the M-M bond. lo2 There is no evidence for disproportionation processes, such as those found for similar reactions of non-ring-coupled complexes.

Photolysis of the trinuclear complex R U ~ ( C Owith ) ~ ~hydrogen halides (HCl, HBr and HI) has been shown to give high yields of (pH)(pX)Ru3(CO)IO (X = CI, Br, I).lo3 With pyrazole, R U ~ ( C O gives ) ~ ~ a substitution product H R U ~ ( C O ) ~ ~ ( C ~with H~N structure ~) (6 I), but Os3(CO)12 gives initially an ortho-metallated species with structure (62), which rearranges to the substitution product on heating.lW Photolysis of O S ~ ( C Owith ) ~ ~ AuPPh3Cl affords the bimetallic cluster compound (pL-AuPPh3)(p-Cl)Os3(CO)lo. lo5 The control of the photochemistry of R u ~ ( C O and ) ~ ~Os3(CO)12 by variation of solvent has been examined.lo6 Diethyl ether, ethyl acetate and acetonitrile, when used as photolysis solvents, suppress photofragmentation, thus promoting photosubstitution reactions.

6

Photoelimination of NO and NO2

Elimination of NO and NO2 is observed in the photochemistry of a range of nitro and nitroso compounds, nitrites and nitrates. The photochemical reactions of aromatic compounds with tetranitromethane and other related reagents have been reviewed. lo7 These reactions proceed in the simplest cases by addition of the elements of tetranitromethane to the arene, e.g. to give nitro trinitromethyl adducts, but large numbers of other products can be formed by a diverse range of multistep sequences.

313

1117: Photoelimination

Methyl nitrite excited to the S2 state by 125 fs laser pulses near 200 nm undergoes direct dissociation with a decay time of 25 (215) fs, and producing NO in its electronic ground state.lo8 Irradiation of methyl nitrite adsorbed on Ag(ll1) surfaces at 248 or 351 nm leads to ejection of NO with three translational energy components. lo9 In the proposed mechanism, the fast molecules are directly ejected, the intermediate speed molecules have undergone one collision, and the slow molecules have experienced several collisions. Isotope-selective IR multiphoton dissociation of nitromethane has been demonstrated in experiments utilizing a free electron laser. lo Irradiation into the frequency region of the NO2 group stretching vibrations of nitromethane with the natural content of 15N (0.4%) produces NO with a 15N content varying within 0. l -O.6%, depending on the precise laser frequency. The 355 nm photodissociation dynamics of NO ejection from jet-cooled methyl thionitrite (MeSNO) have been examined by polarized laser-induced fluorescence. Energy partitioning in the 193 nm photodissociation of tert-butyl nitrite has been investigated using a new experimental set-up for photofragment imaging. l 2 The results differ markedly from those of previous photodissociation studies of aliphatic nitrites at lower photon energies, because a different photodissociation channel is taken. The first laser-induced fluorescence spectrum of the ?err-butoxy radical has. been observed following laser photolysis of tert-butyl nitrite. l 3 The temperature dependencies of the reaction of this radical with NO and of its decomposition to acetone and methyl radicals were also determined in this study. There is a significant wavelength dependence in the photochemistry of tert-butyl nitrite adsorbed on Ag( 111). l 4 Desorption of NO is observed at 532, 355 and 266 nm, but the results suggest that excitation into the S1state occurs at 355 nm and excitation into the S2 state at 266 nm. The photoinduced release of NO from S-nitrosoglutathione has been investigated on the picosecond timescale by IR spectroscopy. l 5 Formation of solvated NO and geminate recombination were both observed. The So+& absorption spectrum of nitrosobenzene in a supersonic jet has been measured indirectly by monitoring yields of photoeliminated NO. l 6 Photolysis of the furoxan (63) (Scheme 5) generates two molecules of NO and the acetylene (64).'17 At 17%, the yield is low, but this is the first example of the photoelimination of NO from a furoxan to have been reported. A method for detecting nitrobenzene has been developed in which laser photolysis is combined with laser-induced fluorescence of the resulting NO fragments. * A similar approach to detecting 2,4,6-trinitrotoluene in soil and groundwater has also been described.Il9 CI /

CI'

Scheme 5

(64)

314

7

Photochemistry

Miscellaneous Photoeliminations and Photofragmentations

7.1 Photoelimination from Hydrocarbons - Methane doped into Kr crystals has been dissociated by synchrotron irradiation with the production of carbon atoms and CH radicals, both of which were recognized by their VUV absorption and emission spectra. 120 VUV photochemistry of methane and deuteriated isotopomers has also been investigated in the gas phase.I2' In this study, two distinct dissociation channels for formation of CH3 + H were distinguished, one involving a perpendicular-type transition in absorption and leading to the triplet surface, the other involving a parallel-type transition followed by internal conversion into the electronic ground state, from which dissociation occurs. A theoretical study, using density functional theory, has been made of methane photodissociation on Pd and Ni(ll1) surfaces, and the results compared with a previous study for Pt( 1 11) surfaces.122 Laser irradiation of acetylene at 193 nm in Ar matrices yields ethynyl radical (C2H) and C2; whereas similar irradiation of acetylene in Xe matrices results in formation of Xe-C2 as the only detectable p r 0 d ~ c t . lThe ~ ~ same species is formed in Xe matrices with 248 nm irradiation, even though acetylene does not absorb at this wavelength; suggesting that, under these conditions, acetylene decomposition is promoted indirectly through absorption by the matrix material. When acetylene is promoted to excited electronic states with trans-bent geometry, additional rovibrational excitation greatly enhances the H photofragment yield.124Site and isotope effects on molecular hydrogen elimination from ethylene at 157 nm have been investigated in crossed beam experiments.125 Three distinct elimination processes were identified: 1,1-, 1,2cis- and 1,2-trans-elimination, each showing significantly different translational energy distributions. Two theoretical papers on ethylene photodissociation have been published. The photodissociation of the vinyl radical (C2H3) at 243 nm in molecular beams has been studied by velocity-map imaging.128 The primary product is singlet vinylidene (H2CC), or singlet acetylene at energies where there is facile interconversion between the H2CC and HCCH geometries. A minor contribution, assigned to triplet acetylene, is also seen. Reports on the 193 nm photodissociation of propyne and allene have appeared from two laboratories. The primary channels for both molecules lead to C3H3+ H and C3H2 + H2. In both studies, it was shown that these two molecules dissociate by different mechanisms; thus dissociation occurs before complete isomerization. In one of the studies,130 the product radicals were distinguished by measurements of photoionization-efficiency curves. From this it was concluded that the C3H3 product from propyne is the propynyl radical (CH3CC), while from allene it is the propargyl radical (CH2CCH). The predominant C3H2 product from both reactants is propadienylidene (H2CCC). There have also been two reports on the photodissociation dynamics of the ally1 radica1.131y132 1267127

1297130

7.2 Photoeliminations from Organohalogen Compounds. The A-band photodissociation of methyl iodide to CH3 and I fragments has been studied in detail

1117: Photoelimination

315

by velocity mapping.133~134Somewhat different dynamics are observed for methyl iodide in small clusters, probably arising from shifts of electronic energy levels and caging of the excited specie^.'^^^^^^ Photolysis of methyl iodide at 254 nm in solid parahydrogen produces only the methyl radical, whereas with 185 nm radiation both the methyl radical and methane are formed.137It seems that the methyl radical in its ground state does not react with p-H2 molecules but can absorb a 185 nm photon to generate an excited state, which decomposes to singlet methylene. Reaction of singlet methylene with p-H2 is the source of methane. UV-induced fragmentations of CH31 adsorbed on Ti02(110) surfaces'38 and of CH3Br on CaF2(111) surfaces'39 have also been examined. Photodissociation of CH3Cl and CHD2Cl by excitation of their fourth C-H stretch overtones has been investigated by detection of the C1, H and D atomic fragments.'@ The yield of Cl fragments from CHD2Cl is significantly less than for CH3C1, suggesting differences in wave-function amplitudes along the dissociation coordinates in the two vibrationally excited species. Femtosecond dynamics of the 312 nm photoelimination of I2 from CH212 are consistent with an asynchronous concerted elimination process. 14' It is concluded that, while breaking of the two C-I bonds and formation of the 1-1 bond happen in a single kinetic step, one of the C-I bonds breaks more quickly than the other. A concerted mechanism is also proposed for the photoinduced formation of CF2, I and 12 from CF212, following a similar femtosecond study. 142 The photoinduced molecular detachment of halogens X2 (X = C1, Br, I) from halogenated alkanes CH2X2 and CH3CH2CH2CHI2has been observed on the femtosecond time~ca1e.l~~ The dissociations are fast (<50 fs), with no evidence of intramolecular vibrational redistribution. Multiphoton dissociation of CH3Br at 248 and 193 nm generates electronically excited CH(A 2A);144and the quenching of this species by alcohol, alkane, 0 2 and ethylene molecules has been studied at different temperat u r e ~ . Product '~~ branching ratios for the reaction of NO2 with CH relaxed to its 2rI electronic ground state by Xe buffer gas have also been determined.146 The complex dissociation of CF212 following nanosecond excitation at 248, 266 and 304 nm has been investigated by the resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique.147 The primary bondbreaking processes all occur on a sub-picosecond timescale, apparently from excited states that are exclusively of B1 symmetry. The carbene CFBr, generated by pyrolysis of CHFBr2, has been jet-cooled and excited into its A( 'A") state by irradiation at 385-435 nm. 148 Photodissociation dynamics for the process CFBr+CF + Br were studied. Direct dissociation from the A" state takes place over a barrier with a height of 3360 cm-l, but where insufficient vibrational energy is available the molecule crosses to either the or a" state and undergoes a slower dissociation. Isotope-selective IR multiphoton photodissociation of CF31 has been examined as a means of achieving 13C enrichment, and about 400-fold enrichment has been reported following a single laser pulse in a short-path gas dynamic flow ~ y s t e m . ' ~ Multiphoton ~-~~' IR dissociation of CHClF2 has been shown to

316

Photochemistry

lead to 13C-enriched product (C2H4) and 13C-depleted reactant. 152 With a multipass dissociation cell, enrichments of 14 g of 13Cup to 50% at 4-5 mmol h-' and of 11 mol of 12C up to 99.993% at 70-90 mmol h-' have been demonstrated. The isotopically selective IR dissociation of CF3Br is subsfantially enhanced by the presence of NO, apparently owing to the radical trapping ability of the latter.153 The photodissociation of 1-bromo-2-chloroethene at 266 nm has been investigated in molecular beams. 154 The primary step leads exclusively to CH2CH2Cl radicals and Br atoms, but some CH2CH2Cl radicals may undergo secondary dissociation into C1 atoms and ethylene. The 13C-selective fR multiphoton dissociation of CF3CH2Cl has been studied by analysis of the distribution of 13C concentrations in the main products, CF2=CHCl, CF2=CH2, CF2=CFH, C2F6, and in two trace products.155No significant radical-molecule reactions take place to degrade the intrinsic I3C dissociation selectivity. Photodissociation dynamics of 1,l-difl~oroethylene'~~ and of trifl~oroethylene'~~ following 157 nm excitation have been investigated. Elimination of HF, H atom elimination, F atom elimination and double bond breaking were identified as reaction channels for both molecules. In addition, 1,l-difluoroethylene also undergoes elimination of molecular hydrogen. Evidence has been presented which indicates that C-Cl bond fissions during 193 nm photolysis of trichloroethylene occur by two independent reaction routes, one from an excited state and the other on the electronic ground state after internal conversion.158 1,3-Dichloro-l,3-diphenylpropane(65) (Scheme 6) has been the subject of a flash-photolysis study in the polar solvent, 2,2,2-trifluoroethanol.159 Kinetic evidence suggests that irradiation at 266 nm induces heterolysis, giving a y-chloropropyl cation (66), which is in equilibrium with the cyclic chloronium ion (67). Subsequent thermal loss of HCl gives the 1,3-diphenylprop-2-enyl cation (68). The possibility of (68) being formed in a two-photon process was ruled out by studies at different laser intensities. 3-Chloro-1,3-diphenyIprop-1ene (69) also gave (68) when photolysed at 266 nm, but is not apparently an intermediate in the formation of (68) from (65). In contrast, both radical and ionic intermediates are implicated in the solution photolysis of 3- and 4-chlorobenzyl chlorides.lm Product analyses show that the first step involves homolytic cleavage of the benzylic C-Cl bond, but the ensuing steps depend on the solvent. In cyclohexane and THF, the products are formed from the benzyl

III 7: Pho toeliminat ion

317

radical alone, but in acetonitrile both the radical and benzyl cation appear to be intermediates. Studies of the photodissociation dynamics of chlorinated benzene derivatives have been reviewed. Photodissociation of chlorobenzene at 266 nm has been investigated by the crossed laser-molecular beam technique,162and a hot molecule mechanism is considered probable. Similar studies have been carried out for bromobenzene 163 andp-bromotoluene,lMwhich show that for each of these molecules the dissociation is fast and the transition dipole moment is almost perpendicular to the C-Br bond. In deoxygenated aqueous solutions, 254 nm photolysis of chlorobenzene yields phenol and chloride ions as the main products, along with benzene, phenylphenols and biphenyl.165 Iodobenzene adsorbed on sapphire(0001) at 110 K undergoes C-I bond cleavage when irradiated at 193 nm. 66 Photolysis of 5-chloro-2-hydroxybenzonitrile (70) in aqueous solution gives the triplet carbene (71), which can be detected by transient absorption spectroscopy (hmax at 368 and 385 nm).167 The carbene was recognized by its reactions, e.g. with 02,to produce the corresponding carbonyl 0-oxide (h,,, at 470 nm), and with propan-2-01 to give 2-cyanophenoxyl radical. In deoxygenated solutions the main stable products are 2,5-dihydroxybenzonitrileand two substituted biphenyls. Within the context of cleaning up environmental contamination by organohalogen compounds, the photolytic dechlorination of chlorobiphenyls168and debromination of decabrorn~biphenyl'~~ have been investigated. The photosensitization mechanism of the antiinflammatory drugs diclofenac (72; R' = CH2C02H, R2 = H) and meclofenamic acid (72; R' = CO2H; R2 = Me) has been probed in a study of the photochemistry of the model compound, 2,6dichlorophenylamine.170 The photochemistry involves cyclization to monohalogenated carbazoles, and the results of the model study suggest that this does not occur by prior C-Cl bond cleavage and radical cyclization, but by via a very rapid initial 6n-electrocyclization. 7.3 Photofragmentationsof Organosilicon and Organogermanium Compounds reviews have been published on the photochemistry of organosilicon compounds171and the mechanisms of these reactions.172 The photolysis of disilane at 193 nm has been investigated by time-resolved mass spectrometry and laser-induced fluorescen~e.'~~ The primary photoproduct was confirmed as Si2H2, with no observation of signals due to SiH3, Si2H3,Si2H4 or Si2H5. In addition, however, SiH2 was observed by laser-induced fluorescence as a minor product, which was found to react rapidly with the starting material to give Si3H8. A detailed study of the photophysics and photochemistry of 2,2-diphenylhexamethyltrisilane (73) (Scheme 7) has been made.174In polar solvents (73) exhibits an intense fluorescence attributed to intramolecular charge transfer, and this solvent effect is also reflected in the photochemistry. In non-polar solvents extrusion of diphenylsilylene and 1,341~1migration to give (74) are the major photoreactions; but in ethanol-hexane solvolytic cleavage of the - General

318

Photochemistry OH

silylene exlrusion

+ 7PhnSi:

Me~SiSiMe3

Me3Si

(73)

SiPhSiMe3

Scheme 7

(74)

Si-Si bond occurs to a significant extent. The charge-transfer state is apparently responsible for the latter reaction. Gas-phase photolysis of silacyclopent-3-ene results in clean extrusion of silylene, yielding buta- 1,3-diene, which undergoes secondaiy photolysis. 75 It is suggested that this process is suitable for the chemical vapocr deposition of Si/ C/H films. The influence of an external magnetic field on the yields of the photodecomposition products of the 7,7-dimethy1-7-silanorbornadienederivative (75) has been investigated in laser-pulse photolysis experiments.176 These experiments reveal the intermediacy of paramagnetic species (biradicals and dimethylsilylene).

(75)

Bis(diisopropy1amino)silylene has been generated by photolysis of the precursor (76), and trapped chemically by reaction with triethylvinylsilane and 2,3-dimethylbuta-1,3-diene.'77 Contrary to theoretical prediction, its dimer seems to have a disilene structure and not a bridged structure.

The photogeneration of germylene (GeH2) from phenylgermane was previously reported to give anomalously low reaction-rate constants in comparison with data for germylene generated from the alternative precursor, 3,4-

IIl7: Photoelimination

319

dimethylgermacyclopentene. A re-examination of germylene formation from phenylgermane, however, has now given rate constants which are consistent for both precursors. 178 Organogermylenes have been generated by photolysis of 7-germanorbornadienes and cyclohexagermanes, and a study made of their reactions with organic halides and trimethyltin chloride.179 The organogermylenes react quantitatively with the polyhalomethanes C C 4 and CBrC13 and with benzyl bromide, to give halide abstraction products together with polyhaolethanes and bibenzyl, respectively. With trimethyltin chloride, insertion products are obtained. Irradiation of permethyloligogermanes, Me(Me2Ge),Me (n = 2-5), in CC14-CH3CNin the presence of 9,lO-dicyanoanthracene (DCA) yields the corresponding chlorogermanes and hexachloroethane.180 The fluorescence of DCA is quenched by the oligogermanes, and a mechanism involving oligogermane radical cations is proposed for the Ge-Ge bond cleavage. Similar chlorinative cleavage of digermoxanes (R3GeOGeR3), initiated by photo-induced electron transfer, has also been reported.I8l

7.4 Photofragmentations of Organosulfur and Organoselenium Compounds A combined theoretical and experimental study of polarized fluorescence of polyatomic fragments produced through photodissociation has been reported, in which the generation of identical radicals from the photodissociation of several disulfides was investigated. 182 The results support the free recoil model of photodissociation, in which the photoproducts experience no torque and fly apart freely, as against the impulsive model, in which the fragments are subjected to instantaneous torque owing to the rupture of the chemical bonds of the parent molecules. The photodissociation dynamics of the methylthio radical and its perdeuterio isotopomer have been investigated using fast radical beam spectroscopy of two electronic absorption bands. 183 At all energies, only one major channel producing CH3 + S was observed. Photofragmentation pathways of dimethyl sulfoxide in a molecular beam have been studied with irradiation at 210 and 222 nm.184The primary fragments are CH3 and CH3S0, while CH3 and SO are identified as secondary fragments. Secondary decomposition is minor (ca. 10%) for 222 nm photolysis, but increases to about 30%with 210 nm photolysis. The thermal and photochemical reactions of fluorenethione S-oxide (77) with cyclooctyne have been compared.185The thermal reaction proceeds by 1,3-dipolar cycloaddition of (77) to cyclooctyne, followed by extrusion of sulfur, to yield dithiin (78) and enone (79). In contrast, on the basis of previous studies of the reaction of (77) with transcyclooctene, the photochemical reaction was expected to proceed via the 1,2oxathiole (80), to give (78) and fluorenone. In fact, the photoreaction of (77) proved somewhat surprising, because the expected pathway via (80) and the 1,3dipolar cycladdition route are both followed. The dithiin oxide (8 1) undergoes facile elimination of benzaldehyde in a sequence of steps to give the disulfide (82).186 Similar elimination of benzaldehyde was also observed for two analogues of (8 1). These reactions appear to involve significant sulfur-sulfur interaction, and ab initio calculations suggest that the S . S distance becomes shorter when the dithiin oxide is excited to the S1state.

320

Photochemistry

n

Near-UV irradiation of N-arensulfonyl amino acids such as (83; R1= C02H, CH2C02H, OCH2C02H; R2=H, Me) in aqueous solution in the presence of a dialkoxynaphthalene light absorber and a single-electron source results in cleavage of the sulfonamide linkage.187The release of the intact amino acid, however, is very sub-stoichiometric, owing to decarboxylation accompanying the photocleavage. Photoremoval of the tosyl protecting group from the amine function of thymidine derivatives has been successfully applied in the synthesis of novel 5’-amino analogues of 3’-azido-3’-deoxythymidine (AZT).18* The photodecomposition of triphenylsulfonium hexafluoroantimonate ([Ph3S]’[SbF6]-) sensitized by naphthalene or 9,lOdimethylanthracene has been studied in pseudo-steady-state and time-resolved CIDNP experiments.189 Key intermediates are radical pairs consisting of the sensitizer radical cation and the phenyl radical, which are formed by photoinduced electron transfer followed by cleavage of the resulting neutral onium radical. R * ~ S O ~ N H C H ~ C & H

(83)

R’

The reactive intermediate o-quinodimethane (84) has been generated in room-temperature solutions from 1,2-bis[(phenylseleno)methyl]benzene (85) by sequential, two-colour laser irradiation.190 A third laser was then used to induce photorearrangement of (84) to benzocyclobutene. Owing to the high thermal reactivity of (84), room-temperature photoreactions of (84) have not previously been observed. Diphenyl diselenide (Ph2Se2)has been shown to be an effective catalyst - an alternative to iodine - for the photochemical stereoisomerization of carotenoids. 191 Unlike iodine, Ph2Se2 tolerates the presence of Hunig’s base in the isomerization of acid-sensitive carotenoids. In

II/7: Photoelimination

32 1

the report of this work, the catalysis mechanism is not discussed, but it presumably involves Se-Se cleavage and reversible addition of one or two PhSe fragments to the double bond which is isomerized.

7.5 Photolysis of o-Nitrobenzyl Derivatives - The protection of hydroxyl and other functionalities by the photocleavable o-nitrobenzyl group continues to find many applications. Two mechanistic studies related to this reaction have been published recently. In one of these, the pK, of the aci-nitro intermediates from 2-nitrobenzaldehyde and its analogue (86) have been estimated from structural volume changes following photoexcitation. lg2 The values obtained are 2.1 and 2.57, respectively. In the second mechanistic study, transient free radicals were observed, in addition to the mi-nitro intermediate, following flash photolysis of the P3-1-(2-nitrophenyl)ethylester of ATP (87) in the presence of dithi~threitol.'~~ The kinetics of formation of the radicals suggest that they arise by single-electron transfer to the triplet excited state of the nitroarene, and it is proposed that they are radical anions of the nitroaryl group.

HO

(87)

(86)

OH

In research aimed at studying the photorelease of different water-soluble materials in biochemical systems, several phospholipids of general structure (88) have been synthesized.lg4 The photocleavage of the 2-nitrobenzyl ester

I

R2

R'

0

II Me3&-(CH2)2-O-P-O-CH2 I 0(88)

322

Photochemistry

linkage appears to be promoted by methyl substitution at the benzylic position

(R'= Me). Liposomes of these phopholipids containing the fluorescent dye

calcein in the inner aqueous layer were prepared, and it was shown that UV irradiation resulted in fast release of the entrapped dye. Reactive functional groups in modified oligonucleotides have been selectively revealed by 365 nm irradiation of the appropriate o-nitrobenzyl based protecting groups. 195 Under these conditions yields are high (commonly >90%) and the biopolymer remains undamaged; so this is an attractive way of introducing structural diversity into oligonucleotides. New photolabile protecting groups for nucleosides and nucleotides have been found in the 2-(2nitropheny1)ethoxycarbonyl and the 2-(2-nitrophenyl)ethylsulfonyl groups. 196 The photoinduced release of carbonyl compounds from o-nitroaryldioxolanes has been examined, as part of a study of the protection of pheromone^.'^^ 2-Nitrobenzyl groups have been incorporated into caged pep tide^'^^ and caged hydroxyphenylpropionic acids for the fluorescent detection of peroxidase.199 New photolabile linkers for solid phase synthesis, based on the 2-nitrobenzyl functionality, have been reported, which allow coupling to building blocks with carboxy, amino, hydroxy and sulfonyl groups.200,201 On photocleavage, these building blocks will give libraries with carboxy, amido, methyamido, amino, ureido, hydroxy, aminocarbonyloxy and aminosulfonyl terminal groups. A photolabile o-nitrobenzyl peptide linker has been developed which, in contrast to previous related systems, is also stable towards acids and bases.202The photolabile 2-nitrobenzyl group has also been utilized in the solid phase synthesis of oligonucleotides containing 3 ' - p h o s p h a t e ~ and , ~ ~ ~of oligosaccharides. 2049205

7.6 Other Photofragmentations - Photodissociation of tert-butyl hydroperoxide at 266 nm gives OH radicals with dynamics which are similar to those found for OH from H202, and which are consistent with dissociation via a repulsive excited state.206 Rates of p-scission of the tert-butoxy radical to acetone and methyl radicals have been determined in flash-photolysis experiments by monitoring its transient UV absorption207 and its laser-induced fluorescence. Photolysis of formaldoxime (CH2NOH) at 193 nm in solid Ar at 17 K results in elimination of water, yielding complexes of HCN and HNC with the water molecules.208The reactive ketene (89) has been generated by photoelimination of benzaldehyde from (90) in laser flash-photolysis experiments, and the kinetics of its reactions with H20, MeOH and Et2NH determined.209 The pinacols (91; R = H, Me) undergo cleavage of the central C-C bond when photolysed in chloroform, in a retropinacol reaction.210CIDNP and fluorescence-quenching experiments showed that the reaction proceeeds via a photoinduced electron transfer from the excited pinacol, with very fast dechlorination of the chloroform radical anion and fragmentation of the pinacol radical cation. The imidate ester (92) undergoes photoheterolysis in water, producing 4-cyanophenoxide anion and the N-isopropylbenzonitrilium ion, which was detected directly in flash-photolysisexperiments.2'

'

323

IIl7: Photoelimination 0

The heterocyclic compound 1,3,5-triazine generates highly translationally excited HCN molecules when photolysed at 266 nm, and these have been studied by collisional excitation of C02 and monitoring by high-resolution diode-laser absorption spectroscopy.212A novel approach to the photochemical generation of benzylic polyradicals has been reported.213The diradical (93), for example, was produced by reduction of the corresponding dication with Na-Hg. Subsequent photolysis of the diradical in methyltetrahydrofuran at 10 K resulted in elimination of two molecules of methyl pyridine-4-carboxylate, and formation of rn-xylylene, which was detected by ESR. 1,s-Naphthoquinodimethane was generated in an analogous photoelimination. The phosphabicyclo[2.2.2]octenes (94; R’= H; R2 = Me and R’= Me; R2 = H) eliminate the methylene phosphine oxide PhP(O)CH2 when irradiated at 254 nm, and are thus useful in the light-mediated phophorylation of protic species such as alcohols.214A mechanistic study of this reaction with several alcohols, however, has revealed that two concurrent processes operate: elimination-addition and addition-elimination. In the latter, the first step is addition of the alcohol to the phosphoryl group, followed by elimination of the alkyl phosphinate product. Photochemical cyclodehydrogenation of azobenzenes to benzo[c]cinnolines has been exploited in the synthesis of compounds with sterically hindered methyl groups.21

(94)

0

Glycosyl radicals are formed photochemically and thermally from telluroglycosides,2’6 and addition of this type of radical to alkynes provides a

324

Photochemistry

synthetic route to vinylic C - g l y c ~ s i d e sAlkyl . ~ ~ ~ radicals are generated photolytically by homolysis of the metal-carbon bonds in Re(alkyl)(a-diimine) and Ru(alkyl)(a-diimine)complexes.218A marked alkyl-dependent difference in the photochemical behaviour of certain Ru(I)(R)(CO)2(a-diimine)complexes has been noted.219 With R=Me, the primary process is CO loss, but with R=benzyl, homolysis of the Ru-benzyl bond occurs. This difference in reaction pathway is attributed to the involvement of different excited states. Coordinated ethene in the rhodium complex Rh(qS-CsHs)(PPh3)(C2H4)can be displaced photochemically by, e.g., trialkylsilanes or hexafluorobenzene.220 Ruthenium(I1) sandwich complexes of the type [Ru(q6-arene)2]*' undergo photosubstitution of one arene ring by solvent, and half-sandwich complexes of the type [Ru(q6-arene)(L)3I2' (L = MeCN, NH3) undergo competitive photosubstitution of arene and L by solvent.221The photodecomplexation of nitroferrocenyl complexes such as (95) yields 2-nitrocyclopentadienyliden-1,3oxazolines, e.g. (96).222The vacuum UV photodissociation of ferrocene has been studied by time-of-flight photoionization mass spectrometry.223Sequential elimination of CsHs ligands is the main reaction channel, but the concerted loss of both ligands also takes place to a lesser degree.

8 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 I. 12. 13. 14. 15.

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Part 111 Polymer Photochemistry By Norman S. Allen

Polymer Photochemistry BY NORMAN S. ALLEN

1

Introduction

The field of polymer photochemistry continues to play a major role in photochemistry with many new areas of academic interest and industrial development. Photolithography continues to be developed particularly with regard toward designing systems for molecular devices. Interest in active ionic initiators and radicalhonic processes continues while the photocrosslinking of polymers is attractive in terms of enhancing the physical and mechanical properties of materials. The optical properties of polymers, particularly the use of probes and excimer formation continues to be an active area as a means of studying their macromolecular structure, energy migration and molecular mobility. Polymer interactions and behavioural features in micellar media provide a valuable probe for determining molecular sizes and forces in surfactant systems. In fact, over fifty articles have been devoted to this topic in the last review period. In terms of growth, interest in polymeric light emitting diodes has increased at a phenomenal rate, since there are obvious commercial implications. Within the last review period there have been over eighty articles dealing alone with this topic. Further developments in terms of expansion have seen a major shift toward photochromic materials and liquid crystalline polymers. The photooxidation of polymers on the other hand continues to decline in attention although there is special interest in natural cellulosic-based materials. Bio- and photodegradable plastics are important for agricultural usage although interest here is again in decline. The same applies to polymer stabilisation where commercial applications dominate very much with much emphasis on the practical use of stabilisers. For dyes and pigments stability continues to be a major issue.

2

Photopolymerisation

Activity in a field is often a reflection of the number and variety of papers that have appeared of a topical or review nature. This last year has seen less than twenty articles to date, slightly less than the previous review period. An extensive review has appeared on the function of different types of photoPhotochemistry, Volume 3 1 0The Royal Society of Chemistry, 2000 335

336

Photochemistry

initiators and their future developmentl. A number of articles have targeted interest in photosensitive polymers2,novel highly catalytic systems3,cyclisation systems4,vinyl ether materials5, stereolithography6, ~rganometallic~ and redox initiators8. Ring opening metathesis by ruthenium complexesg has been reviewed for the formulation of positive tone high resolution microresists'O as have electron photoejection processes' and volatile initiator fragments12. Cationic photoinitiation has been covered in depth13as have vinyl polymerisations14, sulfonium initiators' and polymeric initiators with benzophenone side groupsF

'

2.1 Photoinitiated Addition Polymerisation - Many new photoinitiator systems continue to be developed. Three novel water soluble copolymers with pendant benzil groups have been synthesised and characterised17. The polymeric systems were found somewhat more reactive for photoinducing polymerisation than the corresponding monomeric structure. Ketyl radical formation by hydrogen atom abstraction was the prime mechanism with the radical anion being formed through a triplet exciplex in the presence of an amine cosynergist.

Me

+H2+

c=o

P I

+2-yt,

Me

I

COO-CH2-CH2-N+-Me, I

CT

Me

Bz-CO-

CI

Me -fCH2-t-f;; -(CH2-?Hh Me I c=o I CO-NH-C-CH2-SO3-, I Me

0

Bz-co-SOSNa

Bz-co-AAm

Na+

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Novel structural derivatives of thioxanthone continue to be developed with a number of 1-chloro-4-oxy derivatives having been synthesised.' Alkoxy subsitution in the 4-position of the molecule enhances the rate of photoinduced polymerisation in visible light with the 4-hydroxy derivative exhibiting least activity. The 1-chloro group was also implicated in the photoreactions in undergoing direct photolysis and forming active chlorine radicals. This gave rise to high polymerisation activity in the presence of oxygen. A number of novel water soluble derivatives of thioxanthone have also been de~eloped'~~~*. Those with hydroxyethylaminopropoxy groups were found to be effective in the absence of an amine co-synergist. A series of novel alkyl and phenylthio derivatives of benzophenone have been found to be highly effective photoinitiators through side chain scission to give alkyl and thio radicals21-22. Those with sulfoxide groups, however, were found to be less effective. Dialkyldithiocarbamate derivatives have been found to give rise to living free radical polymer is at ion^^^ using butyl acrylate as an example while a series of thiobenzoate derivatives show high activity dependent upon the nature of the s ~ b s t i t u t i o n ~A~ .series of polymer bound hydroxamic dithiobenzoic anhydride compounds have been found to be very effective heterogeneous photo initiator^^^. The photopolymerisation of vinyl monomers has been successfully carried out using a two-phase solvent system with tetrabuylammonium chloride-KSCN system in carbon tetrachloride26.In the solid phase on the other hand vinyl monomers can be photopolymerised on photocatalytic surfaces with CDSZ7.In the camphorquinone initiated polymerisation of acrylates oxygen has been found to accelerate the rate possibly through some type of oxidation complex assisting the process2! Thionine requires an amine for photoinducing polymer is at ion^^^ with a maximum rate at 0.3 M. Mixtures of benzophenone with hexachloro-p-xylene exhibit powerful synergism for photopolymerising styrene30while an a-hydroxyketone system has been described which is not only a powerful initiator but also gives little odour3*. Phenothiazine initiators have been found to graft onto the polymerising as did the use of TEMPO to provide stable polymeric radicals34. The use of 4-[diphenyl(trimethylsilyl)methyl]benzophenone also gives rise to grafted polymers with two types of silyl moieties35.At low initiator concentration a living polymer was obtained. Carbonate radicals generated by light from sodium carbonate have been found to successfully polymerise pyrr01e~~ while in polar media the application of a magnetic field has been found to influence the molecular weight and yield of poly(methy1 methacrylate)37.Phenylazotriphenylmethane gives trityl radicals on irradiation formed by election transfer38that are apparently capable of inducing the polymerisation of cyclohexene oxide. Dye aggregation influences the photoinitiation activity of Rose Benga139 while morpholine-sulfur dioxidem and bromine41 complexes initiate the photopolymerisation of methyl methacrylate. Camphorquinone is claimed to photoinduce the ring opening polymerisation of 4-methylenedio~olanes~~ while carbocyanine borate salts induce photopolymerisation via an electron-transfer step43. Irradiation of polybutadiene with o-tolualdehyde has been shown to produce random copolymers4 whereas

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xanthates induce the photopolymerisation of methyl methacrylate to give polymers with a narrow p~lydispersity~~. The polydispersity was found to be independent of initiator concentration and the polymer chains were found to be capped with reactive ‘macroiniferters’. Oligooxypropylene-p(benzoy1)benzoyl chloride copolymers with benzophenone end groups have been synthesised& as have photoredox systems based on N-(4-benzoylphenyl)itaconimide and N,N-dimethylaminoethyl m e t h a ~ r y l a t e ~The ~ . latter is claimed to be a powerful photointiator system that also grafts into the polymer chains. Macroazo initiators have been found to induce short chain photopolymerised fragments when compared to an equivalent thermally induced polymerisawhile correlations have been established between the photochemical activity and electronic structure of aromatic a ~ i d e s ~ Polystyrenes ~. with a narrow molecular weight distribution have been made using a surfactant photointiator based on [4-(4‘-tert-butyldioxycarbonylbenzoyl)benzyl]trimethyl ammonium chloride5*.Here initiation occurs at the interface with the latex via the perester radicals. Pyridinium chlorochromate forms a complex with vinyl monomers5 giving free radicals on irradiation which induces vinyl monomer polymerisation with non-ideal kinetics. Titanocene is also an effective photointiator in visible light5* whereas ruthenium complexes give low yields of polymer at low pH53. Tetrahydrofurfuryl acrylate monomer has been photopolymerised using butyltriphenylborates as initiator^^^. Aromatic carbonyl initiators were found to act as effective sensitisers via an electron-transfer process with the borates. Iodonium butyltriphenylborate salts have been found to be more effective visible photointiators than the corresponding tetraphenylborate salt? Bis(cyclopentadieny1)titanium dichloride has been found to give poly(methy1 methacrylate) on irradiation with mixed solubility characteristicss6 while organocobaloximes give rise to living polymers with different end fun~tionalities~~ having star or block architectures. Attempts have been made to identify the nature of the active species formed in metathesis polymerisation using tungsten hexacarbonylS8. Some studies have appeared on photoiniferters. Tetraphenylbiphosphine has been used as a photoiniferter with methyl methacrylate monomer59 where termination still occurred through the diphenylphosphine radicals. Benzyl phenyl selenide induces the photopolymerisation of styrene giving M. and CI) chain ends60 while with methyl methacrylate the use of a piperidino-dithiocarbamate iniferter with a disulfide transfer agent increased the living character of the growing chains61. Block copolymers of epichlorohydrin with styrene and methyl methacrylate have also been made using HBF4 as the initiator and N,N-di-ethyldithiocarbamate as the terminator62. A polymer with thiuram disulfide end groups was obtained. Some aspects of the photopolymerisation kinetics of different monomers have been investigated. The photopolymerisation rate of methyl methacrylate is accelerated in the presence of oxygen when triethylamine is p r e ~ e n t ~This ~.~~. enhanced rate is associated with the usual oxygen-amine complex which can form a variety of species such as oxygen radical anions or hydrogen peroxide to give reactive hydroxyl radicals. The rate of photopolymerisation of methyl

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methacrylate has been controlled using N-cetylpyridinium thiocyanate to give only 01igomers~~whereas with furfuryl methacrylate degradation chain transfer has been observed via the furan ring? Here activation energies were found to be higher where allylic radicals were concerned. Free radical propagation rate constants have been measured for different cycloalkyl methacrylate monomers67. The lowest rate using a pulsed laser system was observed for isobornyl methacrylate. A suitable procedure has been developed that allows calculation of the chain length distributions of polymers prepared by periodic modulation of the initiation process taking into account concomitant continuous initiation68. During the laser pulse the contribution from thermally induced reactions was found to be minimal at peak chain length distributions. In related work pulsed lasers have been used to ascertain the chain length dependence of the termination rate during styrene p~lymerisation~~. Microscopic flows of liquid polymer during laser exposures have also been measured70as have the growth of spherical polymeric micro particle^^^. Various new monomers include those for space application^^^, photonic crystals73,suspension grade C6074,monlayers of azobenzene on 10,12-pentacosadiyonic acid75and multimonomers of poly(acryloyloxyethy1methacrylate) and poly(methacryloyloxyethy1 metha~rylate)~~. Polymers of a-methoxy-3,6have been found to endomethylene-1,2,3,6-tetrahydrophthaloyl-5-fluorouracil exhibit anti-cancer activitie~~~. Radicals produced in the solid state photopolymerisation of octadecyl sorbate are long-lived due to the production of ally1 radicals78.In the solid state 5,4 structures only were obtained whereas in chloroform both 5,4 and $2 structures were produced. Fumarate esters with abstractable hydrogen atoms have been found to copolymerise on irradiation in presence of electron donor Mixtures with N-vinyl formamide had higher exotherms than those with N-vinylpyrrolideone and vinyl ethers. In the photocopolymerisation of maleimides with vinyl ethers electron transfer occurs to give both cis and trans conformerss0. Triplet states have been identified via laser flash photolysis for N-maleimidess1*82.The same workers have also shown that planar N-arylmaleimides are ineffective initiators compared to ortho substituted twisted structuress3.N-Substituted maleimides have also been found to form highly effective complexes with thioxanthone and benzophenone initiator^^^. The properties of a number of novel polymers have been described including biodegradable p~lyanhydrides~’,hyperbranched polyesterss6, dimethacrylatesilicate composities for yarnsg7, thermally stable polydihydrofuranyl compoundss8, light stabilised systemss9, gold-polydiacetylene nanocompositiesgO, evolutive dissipative structures in polyacrylate filmsg1,and photopolymerised micro sphere^^^. The cycloaddition of 2,3-dimethyl-1,3-butadiene (DMB) to acrylonitrile was found to be independent of an initiator93.A 1:l adduct was obtained consisting mainly of cyclobutane moieties. The triplet state of the DMB is supposedly involved. Copolymers of furfuryl methacrylate and N,Ndimethylacrylamide have been made94as have copolymers of methyl acrylate with vinyl acetate using an aniline-benzophenone initiator mixture9’. The acetate groups were then subsequently hydrolysed to give a PMMA-vinyl

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alcohol copolymer. The template polymerisation of methacrylic acid in the presence of poly(viny1 pyrrolidone) has been studiedg6and bulk copolymers of acrylamide with maleic anhydride have been made using benzoyl peroxide as the initiatorg7. The copolymerisation rate in the latter case was found to increase with increasing acrylamide and benzoyl peroxide concentration. Cationic photoinduced polymerisation continues to attract some interest. The oxidative quenching of a pyrrole-substituted ruthenium complex by a diazonium salt results in the formation of a metall~polymer~~. Electron transfer within the complex oxidises the pyrrole moieties. Two novel a-terpineols have been photopolymerised via diaryliodonium saltsg9 as has 2,7dioxabicyclo[3.2.lloctane via N-ethoxy-2-picolinium hexafluorophosphate O0. The radical reaction of trimethylphosphite with 4,4'-di-tert-butyldiphenyliodonium hexafluorophosphate gives trimethoxyphosphonium cations that can bring about the polymerisation of vinyl ethers'O'. Phenothiazine with antimony hexafluoride anions is highly effective for the polymerisation of cyclohexene oxide'0'. The photobleaching of the salt was a good indicator of the polymerisation rate. The influence of the counterion type has been examined on the initiation activity of diphenylamine diazonium salt^'^^^'^^ and polyacrylamide has been prepared via a methylene blue/sodium toluene sulfonate/diphenyl-iodoniumchloride complex '04. An electron-transfer reaction between an excited fluorane leuco dye and a diaryliodonium salt resulted in the formation of a ring-opening colourant to give active radicals capable of inducing the polymerisation of epoxy acrylate rnon~rners'~~. Colour formation was a good measure of the conversion rate. A linear relationship was found between the conversion rate and Mn for the cationic induced polymerisation of THF by iodonium salts'%. This reaction produced a living polymer that could add further monomer units such as N-2-(hydroxyethyl)ethylenmeimine. In the presence of 1,2-ethandiol the cationic polymerisation of 1,2-epoxy-6-(9carbazolyl)-4-oxahexane proceeds via the activated monomer rne~hanisrn'~~. Acylferrocene derivatives induce the photopolymerisation of ethyl-2-cyanoacrylate108*109, butyl glycidyl ether' lo and cyclohexene oxide' ' I . The latter workers have also found that the reaction promotes the activity of organic peroxides' 12. Styryl dyes with dimethylphenylacylsulfonium butyltriphenyl borate form donor-acceptor complexes that induce the visible laser photopolymerisation of acrylate monomers' 3. Crystal violet lactone undergoes a ring opening reaction to form the coloured cation in the presence of a phenyliodonium salt' 14. The reaction is induced by the acid release mechanism which breaks the lactone ring. Butenyl and pentenyl ethers have also been cationically photopolymerised' while the anionic polymerisation of butyl acrylate has been undertaken with phosphazine base' 16.

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2.2 Photocrosslinking - A number of novel initiator systems/packages have been developed for photocuring. The vapour deposition of p-benzoquinone onto o-acryloylacetophenone oxime-styrene copolymers induced crosslinking117 while a covalent type microgel has been formed by treating a divinylbenzene-dimethylaminomethylstyrene-styrene copolymer with 3-chloro-

III: Polymer Photochemistry

34 1

2-hydroxypropyl methacrylate and methacryloxyethyl sulfonate' 18. Microgels having a high number of cation groups on the particle surface showed high sensitivity due to the formation of exciplexes with the amino groups. Covalent type microgels were also found to be more reactive than ionic types. Photoinitiators have also been successfully grafted onto resin structures in order to minimise volatility and migration' l9 while the photolysis of acyloxyimino groups pendant in a styrene copolymer gives rise to amino capped chains'20. An a-hydroxyketone photointiator, Esacure KIP 150, has been found to give no volatile aldehyde photoproducts after curing121.Monoacylphosphine oxide initiators have been found highly effective for initiating the photocuring of 123. A range of novel amine co-synergists have been pigmented coatings122. made with poly(ethy1eneoxy)groups124with high reactivity and low extractability. Amine complexes with p-nitr~aniline'~~ and ruthenium bipyridyl complexes126 have 'also been reported. In the latter case the rate of photopolymerisation is independent of initiator concentration and only participates in the actual initiation step. a-Alkylaminoacetophenone initiators have been found to be highly effective for photocuring pigmented inks when used with a longer wavelength absorbing initiator of the same structure127.Apparently, the hydrogen atom abstracting photoinitiators were not complementary. Copolymers with epoxy and oxime urethane groups have been synthesised128 that can cure through the evolution of photogenerated amines. This has interesting possibilities but may not be acceptable from a commercial point-ofview in terms of toxicity. Polymers bearing imino sulfonate groups generate acid upon irradiation that causes c r ~ s s l i n k i n g and ' ~ ~ again, in a similar way, some borate esters have been made that release amine on irradiation13*. Poly(3- and 4-vinylphenyl selenocyanates) have been synthesised and found to undergo crosslinking upon 254 nm irradiati~nl~'. Diphenyldiselenide was the main photoproduct along with phenylseleno and cyano radicals formed from the SeCN groups. Fullerene Cm has been used to polycondense f ~ r a nrings '~~ that are attached to a methacrylate backbone. The light absorbing characteristics of tetraphenylborates have been enhanced through the substitution of chromophoric groups' 33. Cationic photocuring has also attracted interest. For cyclic enol ethers substitution of an a-methyl moiety enhances reactivity while methyl groups in p positions decreases it134.The addition of triethylene glycol divinyl ether has been found to enhance the reactivity of epoxy resin curing with an iron-arene complex135while the crosslinking of disiloxanes via iodonium salts is dependent upon the media and sensitiser structure136.Crotyl glycidyl ether undergoes a regioselective cationic ring opening polymerisation to give a p ~ l y e t h e rand ' ~ ~ epoxidised castor oils have been found by the same group to functionaform excellent low cost resins for cationic p h o t ~ c u r i n g *139. ~ *Epoxy ~ lised polyisoprene has been crosslinked via cationic intiation". Vinyl ethers were found to accelerate the process with inter and intramolecular interactions taking place. Polyepoxyacrylates have also been prepared via cationic photocuring with triphenylsulfonium141 and iodonium 142 salts. Dihydrofuran and pyrans have also been photocured using iodonium salts143.Low conversions

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Photochemistry

were observed at first that slowly increased in the dark due to the presence of living polymer. New cationic initiators for silicone release coatings have been prepared by reacting diaryliodonium salts with lithium tetrakis(pentafluor0phenyl)borate'"? They induce fast cure for paper coating productions. Diazonium resins form complexes with sodium dodecyl sulfate and retain their high photointiation activity for coatings'45. Diphenylamine aryl cations have been found to be the dominant intiation species in the photocuring of a condensate formed from a diphenylamine-4-diazonium salt and paraf~rmaldehyde'~~. 3-Methoxydiphenylamine-4-diazoniumsalts have also been prepared and their activity examined in diazo resins'47. Solid state photocuring and (2+2) cycloaddition processes have important applications in resist technologies. Hydropolysilanes have been made where the aryl derivatives undergo slower photobleaching than the corresponding alkyl derivative^'^^. For different functionalities photobleaching rates were found to be in the order p-cyanophenol > p-chlorocinnamic acid > pnitrophenol. Muconic acid derivatives on irradiation in the solid state gave essentially the (E,E)-isomers or tritactic polymers while ammonium derivatives were found to have very high phot~reactivity'~~. Phthalimido chalcones undergo a (2+2) cycloaddition' 50 while in mixtures of poly(viny1 cinnamate) with poly(viny1 phenol) it has been possible to measure the separate types of non-bonded and hydrogen bonded double bonds15'. Crosslink distribution in poly(viny1 cinnamate) has been examined 52 while functionalised vinyl cinnamate monomers have been prepared with hydroxyethyl acrylate groups' 53. The anisotropic photo-orientation behaviour of poly(viny1 cinnamate) derivatives are influenced by the chemical nature of substitutents at the ends of the side chains'54. On the other hand the face to face stacking interactions between phenyl and perfluorophenyl groups in (2+2) cycloaddition reactions is emerging as a common noncovalent i n t e r a ~ t i o n ' ~Changes ~. in surface morphology have been determined in the (2+2) cycloaddition of poly(viny1-4-methoxy inn am ate)'^^. The method of preparation controls the size of di-Et-(2,Z)muconate crystals'57 while crystal-crystal interactions have been observed in the polymerisation of bis(benzy1ammonium muconate) 58. The solid state polymerisation of the dimethyl ester of p-phenylenediacrylic acid is heterogeneous and does not form a solid solution159 and various (2+2) cycloaddition reactions have been discussed 60. Photocrosslinking of solid thermoplastics is also a subject of some interest. Blends of poly(2-chlorostyrene) and poly(viny1 methyl ether) have been successfully crosslinked through the photodimerisation of anthracene moieties labelled on the polymer chains161. It was found that the reaction kinetics approximate to the mean field kinetics inside the spinoidal region, resembling the behaviour of the crosslink-reaction performed in the miscible region at relatively low crosslink densities. EPDM has been photocrosslinked using buckminsterfullerenes'62 as has PVC using a triacrylate resin and diphenylketone p h ~ t o i n i t i a t o r ' ~Remaining ~. with PVC, the chlorine atoms have been partially replaced with dithiocarbamate groups that undergo photocrosslinking in order to reduce the migration effect of

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the plasticisers164. Poly(ethy1ene oxide)has been photocrosslinked using a tetraalkylammonium salt'65 and a triacrylatehenzophenone mixture'? Polystyrene with pendant benzoyl groups undergoes photocrosslinking to give resins with variable porosities dependent upon their c~ncentration'~~. Photocrosslinked polyethylene becomes severely oxidised on the near surface IayerP. Photocuring to produce enhanced property requirements for polymers and coatings is a wide topic of interest. These include fibre-reinforced composities and l a r n i n a t e ~ ~ ~ ~photomoulding -'~', of polyesters172, abrasion resistant polyester a ~ r y l a t e s ' ~ enhanced ~, poly(viny1 alcohol) resists'74, degradable netw o r k ~ production ~~~, of nanoparts' 76, composite membranes177-179, microparticle encapsulation180, polyelectrolytes'81, hot meltslg2 and optical filters' 83. Several studies have appeared dealing with polyimide systems. These include fluorine containing derivatives' 84- 186, polysiloxane derivat i v e ~ *chalcone ~~, derivatives188and resist s e n s i t i ~ i t y ' ~ ~Electro-optical -'~~. devices have been made through doping of coatingslg2as have self-assembled diazo resinsIg3. A novel thermal curing reaction has been found for use in ~ ~ have novel photogeneration of free amines, thiols and i m i d a ~ o l e s 'as isomerisable 4-vinylphenyl cyanatesIg5. A number of studies have dealt with the properties of resins during and after photocuring for multifunctional met ha cry late^'^^-^^^, epoxy resins200, polyfunctional urethanes201,202,crown ethers203, styrene-maleic anhydride2w and polyesters205. Conductive studies have been undertaken on poly(viny1 ketones)206 and curing studies undertaken on pigmented systems207 and water based coatings208.Amphiphilic diblock copolymers of poly(viny1 alcohol) have been made209as have cyclised isoprene rubbers with acid labelled tert-butyl carbonate groups210for negative resists. Silicone polymers have also been made with vinyl2" and carbino1212 terminal groups and phenyldiacrylate derivatives213. The latter exhibit microphase separation of the siloxane and organic phases. Other studies include diacetyleneic-thiolmono layer^^^^, tri-n-butylstannyl methacrylate-ally1chloroacetate copolymers2' and acylphosphine oxides for inorganic pigmented coatings216. Monitoring cure kinetics continues to attract much interest. Fluorescence ranks high on the list for monitoring the cure rates and appears to be growing in interest. Such studies include stilbene, oxazolyl and biphenyl molecules in methyl methacrylate2I7, pyrenetetrasulfonic acid for microemulsion polymerised polyacrlaonitrile218, pyrene for cyclohexylmethacrylate219, phenyl glyoxylate for diacrylate monomers220y221, phenanthroline organometallic complexes in epoxy acrylates222,phenoxazone in vinyl esters223,Schiff bases in epoxy resins224and organometallic complexes in aromatic cyanate esters225y226. Other related methods include charge-recombination luminescence227,pigmented inks228,fibre optic and general curing a p ~ e c t s ~ The ~',~~~. kinetics of di- and tetra-functional monomers have been studied and postpolymerisation radicals monitored via ESR233-235.The functionality of the resin had a major controlling influence in the nature of the termination reactions. Dielectric loss has also been found to be a useful measure of cure

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kinetics236indicating that the morphology of the resin is a function of light intensity. The kinetics of the 3-dimensional photopolymerisation of dimethacrylate monomers is described237and different monofunctional monomers give rise to different kinetic responses238.No double gel effects have been observed in the copolymerisation of acrylate and dimethacrylate monomers239while monomers with several double bonds give heterogeneous networks240.Kinetic cure models have also been developed for dimethacrylate monomers in dental curing241and ~tereolithography~~~. Methods of photocuring are variable. The photocuring of acrylates has been monitored by photoDSC in the presence of alumina suspensions243. Apparently, the filler has no effect on the rate. Of particular interest is the attempt to p hot opolymerise 4-vinylbenzoate and p-phenylenediacrylates in hydrotalcite interlayers2? Whilst the 4-vinylbenzoate photodimerised to give the polymer the 4-phenylenediacrylates gave only oligomers. The state of aggregation of the monomers in the pores was dependent upon the anion charge in the clay. Monolayers of octadecyl acrylate give stereoregular material on irradiation245and UV curable composities have been found to be as tough as equivalent thermally cured systems246.The photomoulding of resins is not without problems in terms of release247 as are hot melt adhesives248.Conductivity methods have been found useful for measuring the photocuring of resins249while ion mobility spectrometry has been found useful for measuring extractable components in UV cured coatings250.Deep photocuring in filled composities has been overcome25 as have temperature variations during cure252. Carboxylate counterion interactions have been monitored in diacetylene carboxylate monolayers via reflective infrared spectroscopy253.In the presence of certain divalent ions the coordination mode changes from a bridging state to a bidentate state. Standards in rates of UV polymerisations have been assessed by using visible laser systems with little success254.Hydrogen sesquioxane has been rendered photopatternable by spinning onto glass surfaces255and ring opening mechanisms for epoxypolyamides have been examined by FT NMR256.Ink curing processes have also been monitored257. The photopolymerisation of liquid crystals is attracting significant interest. The liquid crystal phase and the temperature have been found to markedly influence the photopolymerisation kinetics if acrylate monomers258. For ferroelectric liquid crystal possessing a chiral moiety polymerisation has been found to be highest in the smectic phase in the absence of an applied external electric field259.Thus, in the initial polymerisation stages molecular alignment was more important whereas during the later stages diffusion rates dominated. The application of an electric field immobilised the smectic phase. Similar studies have been undertaken on reverse mode polymer stabilised cholesteric textures260.The photopolymerisation of triphenylene acrylates in the mesophase has been found to be influenced by small amounts of residual initiator261.Defect sites are created reducing the carrier mobility by one order of magnitude. Divinyl ether networks have also been prepared by cationic polymerisation262while the retention of molecular orientation during curing

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of liquid crystalline systems is crucial for the optimisation of anisotropic mechanical and physical properties263.The solid state polymerisation of diethyl cis,cis-muconate gave crystals, the size of which depended upon the molecular weight of the p01ymel.2~~. A model has been developed to define growing spherulites during the irradiation of polymer dispersed liquid cryst a l ~ Under ~ ~ ~intense . UV liquid crystal droplets are formed while under low intensity irradiation the growth of spherulites occurs in a circular shape to give 3D plates. A nematic liquid crystal has been made with a high birefringence from mercapto and olefinic compounds266,while large amounts of a liquid crystalline polymer have been found to reduce the photopolymerisation rate of a mixture of a divinyl ether and a bi~rnaleimide~~~. Benzanthrone derivatives have been utilised as luminophores for liquid crystals268whereas the pretilt angle of photoreactive polymers is influenced by exposure to polarised light269.Second order non-linear optical activity has been observed in photocrosslinked polymers of glycidyl methacrylate with 4-nitro-4'-hydroxy ~ t i l b e n e The ~ ~ ~anisotropic . properties of discotic liquid crystals are stabilised by in-situ photop~lymerisation~~~. However, using X-ray diffraction studies diacrylate systems showed a decrease in order with increasing polymerisation temperature. Similar studies have been undertaken on difunctional reactive Schiff bases whereas Cu(I1) ions inhibited their p ~ l y m e r i s a t i o n ~Photocrosslinkable ~~*~~~. polymers have been developed based on cinnamoylethoxybiphenyl where anisotropy increases with increasing irradiation t e m ~ e r a t u r e ~ Intramolecular ~ ~ - ~ ~ ~ . photoreactions dominated at the smectic temperature. Other work on biphenyl containing polymers has developed phase diagrams for the various transitions277whereas cholesteric polyesters based on cinnamic acid undergo (2+2) cycloaddition causing stabilisation of Grandjean textures278.

2.3 Photografting

- The photografting of monomers onto polymer substrates continues to attract interest for property modifications. A review using Ce(1V) ions has appeared279while the hydrophilicity of polysulfone membranes has been enhanced through photografting of poly(ethy1ene glycol) with 4-azidobenzoyl-methoxy groups280.Polyacrylonitrile has also been successfully photografted onto poly(sulfopropyl acrylate)28* . Reactive maleic anhydride sites have been photografted onto both polypropylene powder282 and polystyrene surfaces283.For polyolefins successful photografting has been performed with methyl methacrylate v a ~ o u r ~ hydroxypropyl ~~, a~rylate~~~ and acrylamide286.Poly(N-isopropylacrylamide) has been photografted onto poly(viny1 while acrylonitrile has been photografted onto starch288. A new process of photografting has been developed through dentritic polyesters289while cellulosics have been photografted with N-isop r ~ p y l a c r y l a m i d eand ~ ~ ~4-vinylpyridine and iso-butyl rnetha~rylate~~'. Polyester fibres have been photografted with acrylic acid292whereas Fe-Si bonds have been grafted onto oligoorganosiloxanes293.N-Isopropylacrylamide has been photografted onto glass294 and acrylic acid onto PTFE to improve adhesi~n~~~.~~~.

346

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Photochemistry

Luminescence and Optical Properties

The field of polymer luminescence and general optical properties continues to grow at an alarming rate. Significant interest centres on polymers for LED applications and photochromic materials. This last year has seen an exponential growth in papers in both areas with much emphasis on the poly(pheny1ene vinylenes) for LEDs. A number of specific and general reviews of interest have appeared. Topics of interest include conjugated p ~ l y n i t r i l e s ~polymer ~~, blends298,interfacial membranes299, memory effects in polymers300, dentrimers301,chemical femtosecond studies303, rare earth polymers304, chemiluminescence of elastomers305,‘glo polymers’306,photoprobes for microstructural analysis307,polymer d e g r a d a t i ~ nand ~ ~ chemiluminescence ~?~~~ for monitoring d e g r a d a t i ~ n ~A~general ~ ~ ~ ~review312 ’. has appeared while another author questions whether or nor ‘polarons’ exist313. Three reviews have appeared on isomerism with azo p ~ l y m e r s ~ while ~ ~ - ~others l ~ show that luminescence is a valuable tool for monitoring the molecular behaviour of polymer^^^^^^^ *. Several reviews have appeared on LED conjugated polymers319-324 Chemiluminescence analysis continues to attract much interest in the field especially with regard to polymer oxidation processes. Studies on polyamides have shown that the chemiluminescence source is primarly associated with cyclic hydrogen bonded lactam hydroperoxide with the o-aldehyde of the amide325while thermoluminescence has associated the emission with electron detrapping and recombination processes326.Imaging chemiluminescence continues to be reported as a valuable method for examining the hetergeneous oxidation of polymers such as rubbers327.Other workers have clearly shown the usefulness of the methods for monitoring the performance of stabilised polymers328. Here it has been found that the migration of the stabilisers influences the chemiluminescence intensity. For filled rubber a direct correlation has been found between DSC analysis and chemiluminescence329.The chemiluminescence of different polyolefins has been related to the methyl group content330.One can assume that this relates to the ability of reactive hydroxyl radicals to abstract labile tertiary hydrogen atoms generating more hydroperoxides in the polymers. In y-irradiated polymer the chemiluminescence is associated with free hydroper~xides~~l while in another study various additives were found to prevent the y dose effects332. Other studies on polyolefins include kinetic i r r e g ~ l a r i t i e sand ~ ~ combined ~ stress334.The use of chemiluminescence for monitoring the stability of coatings is not yet viable although can be used to screen clear In polysilanes the electroluminescence is associated with energy transfer and carrier generation p r o c e ~ s e s ~ ~ ~ ~ while other studies has dealt with the chemiluminescence of polymeric f l u o r ~ p h o r e and s ~ ~free ~ radical measurements340. In terms of general polymer luminescence there have been a number of novel reports. Colour contaminants have been identified in the manufacture of terephthalic acid as a precursor to polyester341.Using fluorescence analysis the primary contaminant formed by oxidation was 4-carboxybenzaldehyde. This

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product gives rise to biphenyl, fluorenone and anthraquinone products on further degradation. The luminescence from chitosan is associated with an intramolecular hydrogen bond342 while photostimulated emission from poly(methy1 methacrylate) is dependent upon chain mobility343. Mirror imaged fluorescence is associated with the rod like structure of p o l y s i l ~ x a n e s ~ ~ while a tetraphenylsilane polymer is significantly more fluorescent than a hexaphenyl structure345.Fluorescent plasma polymers have been made from aromatic hydrocarbons346 as have fluorescent poly(ary1 ether ketones)347, aromatic p ~ l y a m i d e s ~oligomeric ~~, esters of 3-thienylglutaric acid349 and polysulfones350. The fluorescence from wood and paper pulps is complex. Thus, whilst the emission from solid wood is independent of excitation wavelength, extracts show a dependence indicating the presence of many, possibly hidden component^^^'. Fluorescence is also claimed to be useful in identifying the pulp source352-354while fluorescence from biphenyl components has been found to be highly dependent upon the torsional angle of the molecules in the pulp355.Polysiloxane films give yellow luminescence, the intensity of which is dependent upon the energy fluences of He, C or Au ions356. For a series of diethynyl-2,2’-bipyridineRe(C0)3CI polymers the fluorescence has been found to decrease with increasing Re content along the chain357.It is possible that the Re atoms are acting as exciton traps. A novel electron transporting polymer has been synthesised from mesitylene borane and 1,g-di~yanoanthracene~~~. The polycyclodiborazane emits strongly at 494 nm and is highly thermally stable. The plasma induced luminescence from polypropylene is found to be dependent upon its crystallinity359while the electroluminescence for crosslinked polyethylene is dependent upon its state of degradati~n~~. Photochromic polymers have seen a major growth especially those based on azobenzene and spyropyran chromophores. New photochromic polymers have been developed based on the spyropyran unit with polymerisable The groups were found to be sensitive to the heterogeneity of the polymer and had potential for the development of optical storage information media. Metal ions bound to photochromic naphthoxazines gave highly fluorescent species362 while the presence of zinc 1-hydroxy-2-naphthoate has been found to markedly improve the lightfastness of ~ p y r o p y r a n sAzobenzene ~~~. bound to poly(methy1 methacrylate) (PMMA) exhibited gas permeation changes when light switched from the trans to the cis f 0 1 - m ~while ~ solvent dilation changes have been similarly observed in azo tagged poly(viny1 Polyacetylene derivatives with phenylazo groups exhibit smectic liquid crystalline properties366 while photoinduced alignment in azobenzene-methacrylate copolymers decreased with increasing367strength of donor-acceptor groups attached to the 4,4 positions of the azo chromophore. This was associated with the higher enthalpic stability of the mesophase and the decreased concentration of cis-azo groups. The ferroelectricity of surface stabilised aligned films of photochromic azo-benzene polysiloxanes is reversible on UVhisible light irradiation368. Polarised absorption spectroscopy indicates that this light switching controls the degree of order, orientation and EZ photoisomerism of the chains. Chiral

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inductions have been reversed in isocyanate-azo and optical switching in vinyl copolymers with crowned azobenzene groups has been found to induce changes in ionic conductivity371.Ionene polysoaps bearing azobenzene groups can be optically switched to adsorb polyelectrolytes at different layers372while cationic stilbene amphiphiles can be optically switched to control microviscosity effects373.Copolymers with polar ester and azobenzene groups can be switched to control b i r e f r i n g e n ~ ewhile ~ ~ ~orientation ~~~~ effects in Langmiur-Blodgett films of azobenzene tagged polyamic acids have been studied via second harmonic generation376.The time dependence of photoinduced isomerism in azobenzene doped PMMA has been monitored via real time infrared377 whereas a novel palladium catalysed homocoupling process has been developed for preparing azocoulped polymer materials378. Strongly visible absorbing naphthopyrans have been developed379along with copolymers with diphenylthiocarbazonylmercury groups38o, bis-spiropyrans via ultrasound38l , poly(2-methyl-oxazoline)382,polymers of azobenzene and holes sterol^^^^^^^, photochromic crown ether styryl dyes386,amphiphilic phenylazonaphthalene~~~~, azobenzene PMMA systems388, bis-spironaphthoxazine~~~~, dithienylethene pendant p o l y a c r y l i c ~ ~polypropyl~~, v i o l o g e n ~ ~ ~acridine ~, ~ p y r o p y r a n s ~ ~poly(S)-4-(2-methacryloyloxy~, propanoyloxy)azobenzene393and spyran doped PMMA394.A general overview on many types of structures has also been presented395.Cis-trans isomerism in polyamides provides information on molecular constraint^^^^.^^^ while photomechanical motions have been measured in azo doped poly(viny1 alcohol) films at air-water interfaces398.A mean field model of photoinduced surface reliefs in dye substituted polymers has been developed399as has the dynamic properties in azo doped acrylics for optical storage dataw. Urethane substituted diacetylene films have been grown onto Ag films and found to exhibit a dielectric constant comparable to those of orientated filmsN1.Polyesters with norbonadiene units have been made and found to undergo highly efficent photosensitised transformations yielding large amounts of thermal energye2. Azobenzene-succinimide polymers on the other hand gave rise to optical birefringenceN3 whereas photochromic hybrid organic-inorganic materials have been developed that undergo marked changes in refractive indexm. Poly(ary1 ether ketones) have been made with azobenzene groups and the cistrans isomerism dependence on molecular weight measureda5. Spyropyrans have been grafted onto PMMA and found to exhibit useful solvent permeation effects on isomerismN6. Polymers with benzylidenephthalimidine side chains undergo (2+2) cycloadditiona7 while the cis-trans isomerism of Disperse Red 1 dye doped in PMMA can be fitted to the time dependence of the macroscopic chain dynamicsN8.A stochastic model has been developed for azo side chain polymersN9 and random association processes have been measured in polymers with spirobenzopyran molecules410.Here an increase in solvent quality or screening by co-ions suppresses photoassociation. A common feature in this process was the formation of large clusters with short irradiation times followed by a plateau when the photostationary state is achieved. The kinetics of cluster dissolution were, in fact, found to be twice as slow as the relaxation

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time of the spontaneous photochromic conversion of the spyropyran moieties. Spiroindolinonaphthaloxazine groups have been incorporated into ormocer groups via the sol-gel route and found to be relatively thermally stable41 while 2-phenylphenanthroimidazoledimers exhibit piezochromic behaviour412. Following on from this there are numerous related articles on liquid crystalline materials with a major review on optical storage media413. Poly(Nvinylcarbazole) doped with amino-dicyanostyrene is a high performance photorefractive polyrnefiL4 as are functionalised acrylate composities with 1,4,:5$-naphthalenediimide groups415. Photopolymerisation of this medium creates an anisotropic gel like material in which the liquid crystal is free to reorientate in the presence of a space-charge field. Transient periodic stripe domains have been observed in copolymer vesicles416and the effect of various dyes examined on the properties of liquid crystalline materials for colouration purposes417.The fluorescence dynamics of cyanobiphenyl in a liquid crystalline composite has been examined4I8.Here surface excitation was undertaken at 266 nm while bulk excitation was performed at 320 nm. In the former case short-lived excimers were formed while in the bulk long-lived excimers dominated the decay profile due to nematic molecular associations. Fluorescence shifts have also been monitored from polyesters containing 4,4'-biphenyldicarboxylate moieties41g. Various fluorescence patterns are observed during the heating cycles. Fluorescence analysis has also been used to examine the aggregates in rod-like polyesters420formed from pyromellitic anhydride and 4'4'-biphenyl units. Ground-state charge-transfer complexes are formed between both moieties having an alternating lateral alignment inside a layer. Two layered crystals are seen with different lateral packing distances which are shown to match exactly the electron-donating and electron-accepting units in the adjacent chains. It is suggested that these types of charge-transfer interactions contribute to the organisation of the spatial arrangements of the different phase structures. Metal(I1) p-styryl octadecanoates have been found to form an inverted hexagonal lyotropic phase at ambient temperature with the exception of Cu(I1) ions421.Photochemical crosslinking of these monomers gives rise to polymer networks with phase retention. Photoinduced 'command' effects have been designed as a new method for the development of planar or homeotropic alignment of photochromic polymers422and three types of fluorescence emission have been observed from substituted benzanilides in the crystalline state associated with different states423.The level of photoinduced LC alignment in polymethacrylates with benzylidenephthalimidine side chains has been found to be enhanced by p-methoxy substitution at the benzylidene residue424.With these polymers photodimerisation under controlled polarised light irradiation markedly enhanced the thermal stability of the LC alignment due to the formation of the crosslinks. Diethyl (Z,Z)-2,4-hexadienedioate undergoes polymerisation to give a high molecular weight, highly stereoregular polymer"25 while poly(viny1fluorocinnamate) undergoes liquid crystal alignment perpendicular to the direction of p ~ l a r i s a t i o n ~Several ~ ~ . cholesteric polysiloxanes have been synthesised that can be r a c e m i ~ e d as ~ ~have ~?~~~ smectic d i a ~ r y l a t e st~h~i ~~p, h e n e s and ~ ~ *poly(2,5-didecyloxy-1,4-phenylenebu-

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t a d i ~ n y l e n e )Several ~ ~ ~ . azo polymers have also been made with liquid crystalline textures. These include polymethacrylates with p-nitroazobenzene g r o ~ p s as ~ ~well ~ -as~ azobenzene ~ ~ g r o ~ p s ~ and ~ ~ -1,4-butanediol ~' diacrylate-4-(2-acryloyloxyethoxy)azobenzenecopolymers442. The technique of time resolved photomodulation has been used to examine the excitation dynamics in luminescent Si-bridged polythiopheneM3as has the luminescence in cyclosiloxanes'? Electropolymerised indole monomers gives a cyclic trimer polymerM5 with long wavelength shifted fluorescence while polyimides give excimer fluorescence due to head-to-tail intermolecular 0 v e r l a p ~ ~ 9Ground ~ ~ . state complexes were also observed due to various packing conformations. The application of an electric field has been found to decrease the luminescence from polymer blends"? This is due to intra- and intermolecular dissociation processes. Quantum mechanical calculations have been used to investigate the influence of interchain interactions on the absorption and emission spectra of n-conjugated systemsM9.These data have enabled guidelines to be established in terms of maximising the parameters that control the luminescence intensity from solid films. Electronic relaxation processes in polydiacetylenes have been found to be sensitive to changes in chain conformation450whereas 3D photoplasticity studies have been undertaken on poly~arbonate~~l. Lasing action in conducting polymers has been examined452while UV induced changes have been examined in poly(pyridinium salts)453. Articles dealing with LED polymers based on poly(p-phenylene vinylene) (PPV) have grown exponentially in the last year. Polymer modification has been undertaken through various routes in order to enhance the LED properties in relation to the emission quantum efficiencies, electrical conductivity and photoconductivity. PPV with perfluoro groups exhibit maximum emission in the blue region454 while fluorinated oligomers of PPV exhibit a reduced emission efficiency with increasing solvent polarity455. PPV with aromatic amine groups exhibit red-orange while material with sulfinyl oxidation centres have restricted conjugation but increased emission intensitf 5 7 . A1terna ting poly [@-pheny leneethyny1ene)-ah-(2,5-thienyleneethyny lene)] has been made and found to exhibit fluorescence quantum yields of between 0.4-0.5 with relatively good solvent as have polymers with 2,6-pyridylene for green and blue emission. Cyano substituted PPVs gave emissions that are dependent upon the excitation wavelength460 whereas polymers of 2,2'-bipyridine and phenyldiacetylene have an emission quantum yield of close to unity461.PPV with in-chain and pendant 9,lOdiphenylanthracene groups have been synthesised and found to exhibit combinations of spectra in electro and photoluminescence that are dependent upon the nature of the c h r ~ m o p h o r e ~For ~ ~ .example, PPV with in-chain and pendant chromophores exhibited electroluminescence only from the in-chain chromophores whereas the fluorescence originated from the pendant groups. Blue emission has been observed from silicon containing P P V S while ~ ~ ~PPVs with diphenyl substitution are more stable since these groups act as conjugation [email protected] and didodecyl-PPVs exhibit aggregates and reduced

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35 1

emission i n t e n s i t i e ~while ~ ~ ~a. number ~ ~ ~ of soluble phenylate PPV’s have been made that emit strong green emission467.Polymers with 2’5-dialkoxy groups have also been made and exhibit strong emission^^^^^^^^ while alkylated products are reported to give an emission intensity of 65%470.The alkyl chain length has an important bearing here471 with non-aromatic substitutions giving polymers with blue shifted emission spectra472.PPV has been prepared by CVD473and has also been examined by ultrafast spectroscopic techniq u e ~ including ~ ~ ~ t i-m e~- ~~f - f~l i g h tfor ~ ~ ~carrier mobility, picosecond laser flash p h o t ~ l y s i sto ~ ~determine ~ the excitation intensity dependence of the emission, femtosecond spectroscopy to measure excited state and the use of PPV’s themselves as polymer laser diodes478-479. The prospects for producing electric pumped solid-state polymer diode lasers using PPVs is discussed in the context of low-threshold gain narrowing in sub-micron thick films. Structural effects on luminescence efficiency are important. Apparently, an increase in the flexible components in the PPV chain enhances the fluorescence efficiency480while two annihilation processes have been identified that hamper the emission481. Electron withdrawing substitutents apparently reduce the of PPV’s whereas fast fluorescence decays are observed at high excitation densities which apparently disappear at low f l u e n c e ~Here ~ ~ ~dipole. dipole interactions between spatially extended photoexcitations is suggested as a mechanism for the observed bimolecular decay. Liquid crystalline PPV’s exhibit fluorescence with a dependency on chain length484as do phenylene polymers under pressure485. Vinylene bonds also influence the oscillator strengths in P P V ’ S while ~ ~ ~ blends of PPV with a red emitter poly(pery1ene-codiethynylbenzene) undergo effective energy transfer processes4s7. Triplet state efficiency can be examined by time-resolved thermal lensing488while a general strategy has been developed for the construction of ordered nanaocomposities with hexagonal symmetry using polymerisable lyotropic liquid crystals489. Using ion-exchange procedures amphiphilic polymers are obtained with significantly different emissions from that of bulk PPV. Poly( 1-phenyl-1butene) has been found to be more emissive than poly(phenyla~ety1ene)~~~ whereas the emission from PPV is significantly reduced on photodegradati~n~~’. Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes492while c60 doping appears to be ~ a r i a b l e ~Doping ~ ~ - ~with ~ ~ electron . transport materials such as oxadiazoles give polymers with balanced properties for hole transport497.The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important49sas are head to head and tail to tail chain sequences in thiophene based polymers499.Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV’s are claimed to be good chemosensors500and broad emission is observed from titania doped PPVSo1. Electron rich dopants enhance the emission in the red region502 while electro and photoinduced infrared bands from PPV are similar503. Electric field induced fluorescence quenching on a series of poly(p-terpheny-

352

Photochemistry

lene vinylenes) follows a strictly quadratic dependence on the applied field amp1itude5O4.Here quenching occurs predominantly at higher emission energies, causing a distinct blue shift between the electro modulated and photoinduced spectra. In a pair of coupled donor-acceptor conjugated polymer chains it is possible for an exciton which is photoexcited on either polymer to decay into a hole in the donor polymer’s valence band and an electron in the conduction band of the acceptor polymerM5. A processable PPV has been developed via m-phenylene units506.Here the chains are able to adopt a coil like conformation. Unfortunately, a second emissive band is observed in the red region. Photochemically converted PPV has been c h a r a ~ t e r i s e dwhile ~~~ those with biphenyl units are stacked and exhibit LC properties508.Head-totail sequences in poly[(p-phenylene ethynylene)-alt-(2,Sthienylene ethynylene) give rise to enhanced fluorescence509as do alkoxysulfonated PPVs which can be self-assembled into LED’s510. A number of other LED polymers have been made such as poly(l,4l , polymers with 2-benzylidene-4,5-dicyano1,3naphthalene ~inylenes)~ dithiole512,coordinate silicon units513,poly(alky1phenyla~etylene)~~~ all exhibiting strong green emission. Poly(benzimidazole-4,7-diyl)sare highly electroluminescent but also exhibit electrochromism515. Fluorene is a strong excimer 7, poly(4,4’-biphenylene pyroquencher5 in LEDs while p~ly(p-pyridine)~ mellitimide)518, arylethylene disylylene polyrners5l9, polymers of distyrylbenzene520, polyamides from tetra(4-biphenylyl)-4,4‘”’-diamino-pquinq~ephenyl~~l and poly(~yanoterephtha1ylidene)~~~ exhibit strong blue emissions. Polymers with boron atoms are highly rigid structures with an intense blue emission523as are those based on poly(amic There has also been a surge of interest in thiophene polymers. Dual luminescence from polythiophene films has been assigned to exciton trapping at different local environments525while b i n a ~ h t h y land ~ ~ ~o x a d i a z 0 1 ~ ~ ~ 9 ~ ~ ~ thiophenes exhibit a variety of coloured emissions. Poly(alky1 esters) exhibit orange emission529while tetra and sexithiophenes exhibit blue emission530.The kinetics of luminescence decay in hexamethylsexithiophenes are controlled by excess energy redistributions via vibrational and torsional c o ~ p l i n g ~ ~ ~ ~ Polythiophenes with aniline units exhibit high electrical conductivity533while quinquethiophenes exhibit long-lived emissions due to aggregates and physical defects534. In poor solvents the emission characteristics of poly(3-hexylthiophene) are similar to those of the solid film535.Silylene polymers also exhibit dual emissions536whereas poly(dialky1thiophenes) with 4,4’-dicarboxylate groups exhibit chain twisting with an orange fluorescence emission and red electroluminescence537.Relaxation processes have been examined in transp ~ l y a c e t y l e n e ~showing ~ ~ - ~ ~that ~ relaxation processes are twice as fast as charge-transfer. High excitation energies also gave rise to the formation of neutral long-lived states absorption edge excitation leading to a higher probability of chain relaxation into a deformationally neutral state with a long lifetime. Polymer blend studies have attracted much less interest. Strong excimer formation in blends of polystyrene with 1,4-bis(4-a-cyano-styry1)-2,5-dio-

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ctyloxybenzene correlates with enhanced photocond~ctivity~~~ whereas morphological changes during the phase separation of polyamide/polysulfone blends are effectively measured via a fluorene probe542.Excimer fluorescence can also be used to measure miscibility in polyolefidpolymethacrylic acid blends543.Shear force microscopy can also measure topographical differences between blends of conjugated polymerssM while photoisomerism has been used via trans-stilbene moieties for blends of polystyrene-vinyl methyl ether545. In the latter case variations in reaction and annealing rates results in elastic stress caused by reaction inhomogeneitieswhich play a role in the formation of lamellar like morphology. Polystyrene size is measured in blends of polystyrene and poly(viny1 methyl ether) using co-2-vinylnaphthalene probes546while for blends of poly(viny1 alcohol) with poly(viny1 acetate) phase separation via fluorescence microscopy was evident by the use of anthracene and fluores~ e i n Here ~ ~ ~ the. rich poly(viny1 alcohol) phase was evident through green fluorescein emission while for the poly(viny1 acetate) the blue anthracene emission dominated. Polyamic acid and polyimide mixtures are monitored via The aromatic amide groups in perylenetetracarboxydiimide the polyamic acid were found to function as quenchers. A few articles have appeared dealing with dendrimers. A four generation dendrimer has been made with a rubicene moiety549that contains two emitting conformations. Here dentrons close to the core exhibit a different decay process to that on the outer sphere. Dendrimers with a crown ether receptor moiety and a hydrophobic dentritic sector can be optically switched via a photoresponsive azobenzene groupss0. Electron-transfer quenching in dendritic macromolecules has been found to exhibit an unusually large quenching constant with DABCOS5'. Here ground-state charge-transfer complexes and exciplexes were formed with the higher generation dendrimers. Dendrimers with a calixC41arene core and azobenzene skeletons has been synthesisedSs2as have dendrimers with central azobenzene linkers553*554. Metallic based dendrimers have also been synthesised as multielectronic catalysts555. Energy transfer and excimer formation continue to attract widespread interest in terms of molecular dynamics. The mobility in polystyrene-poly (ethylene glycol) microbeads has been ascertained through a pyrenebutyric acid probe? Excimer to monomer intensity ratios were found to be in accordance with the solvation capacity of a liquid phase. Only in the solid dry beads is aggregated excimer emission observed. The addition of anionic surfactants has been found to control the interpolymer aggregation in naphthalene labelled styrene/N,N-dimethyl maleimido propylammonium sulfate copolymer557.The pH induced expansion of polyacrylic acid labelled with pyrene and naphthalene has been followed by a reduction in excimer formation558.The energy transfer distance between the chromophores also increases. Poly(n-vinylcarbazole) and its copolymers exhibit two types of traps559. The first is associated with a conventional sandwich structure of overlapping aromatic rings while the second type is due to species involving two partially overlapped carbazole substituents. In the copolymer intermolecular interactions are high even when the carbazole content is low. At low

354

Photochemistry

temperatures the contribution from the second type of trap increases. Implicit in this work is the finding that the high concentration of excimer forming traps mitigates against significant energy migration between the carbazole groups which might otherwise populate the excimer traps. In free radical polymerisation the rate of photoelectron transfer has been found to be much lower than the rate of diffusion controlled processes560. Thus, for processes controlled by diffusion, the reactivity of free radicals formed as a result of electron transfer limits the rate of initiation. For a series of vinyloxy polymers and copolymers with benzonitrile groups substitution with electron rich groups caused significant q u e n ~ h i n g ~Spectral ~ ~ - ~ ~broad~. ening was also caused by interaction chromophores whereas isolation on the chain resulted in spectral narrowing. Mixed systems of polyhexyl and diphenylsilanes exhibit intermolecular energy transfer the efficiency of which is dependent upon the mixing Complex triplet energy migration has been observed in methacrylate copolymers with 9-phenanthryl as have relaxation processes in benzophenone doped polystyrene567.No evidence for excimer formation has been found in carbazole containing oligomeric ethers568. Intramolecular energy migration was shown to occur in these systems but trapping was inefficient. Interfacial interactions at blends of latexes have been monitored through the use of donor-acceptor chromop h o r e ~ Here ~ ~ ~interfacial . contact and surface area controlled the efficiency of the energy transfer process. The presence of a surfactant reduced the energy transfer by increasing the interfacial barrier. The presence of pendant benzophenone groups enhances the isomerism of norbornadiene groups tagged to epoxy resins570 while hole recombination at dimeric sites gives delayed exterplex emission in perylene doped polystyrenes7'. Polyelectrolyte chemistry and micellar interactions continue to attract widespread interest. The dynamic anisotropy of Sulforhodamine 101 at a water/ phthalate ester interface has been found to be restricted to the X-Y plane572. This was explained in terms of different adsorption modes of the dye on the interface and the chemical structure of the ester. In ethylene-methacrylic acid ionomers using pyrene as a hydrophobic probe the critical micelle concentraBelow the CMC unimeric tion (CMC) was determined to be 0.02% micelles were identified that are in equilibrium with the larger aggregates. For a naphthalene tagged copolymer of poly(dimethy1 sulfate acrylamide/N,Ndimethylaminopropylmaleimide) the hydrodynamic diameters were found to decrease with increasing salt concentration due to chain Enhanced compartmentalisation of the naphthalene labels were observed at high salt concentrations. Fluorescence decay profiles have been generated by the Monte-Carlo technique during the interdiffusion of donor-acceptor spheres for mimicking latex film formation575.Microdomains in amphiphilic monomers of N,N-diallyl-N,N-dialkylammonium chloride have been found to be ordered576 while non-radiative energy transfer from naphthalene to pyrene groups has been found useful for detecting hydrophobic associations in copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonateand N-dodecylmethacrylamide577.Discrepancies between different techniques for measuring CMCs

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have been ascertained578while in methyl methacrylate-methacrylic acid fractions with vinylanthracene groups Ca(1I) and Tb(II1) ions have been found to bind strongly and enhance the fluorescence of the c h r ~ m o p h o r e ~An ~~. ethylene oxide-propylene oxide-ethylene oxide triblock copolymer does not micellize at concentrations below 5% w/wSS0whereas amphiphilic cationic porphyrins are stabilised by polyallylamine at pH values close to the pKa of the amineS8'. Dansyl labelled polyanions exhibited a high response to alkali metal cations582while photoassisted poling and depoling has been measured in biaryl tagged zwitterion polymers583.The viscoelasticity of micellar cetyltrimethylammonium bromide is reduced by the addition of ionene polyelectrolytes due to the shortening of the worm like m i ~ e l l e s ~Excitation ~~. energy migration in micellar pores has been modelled585 and amphiphilic ruthenium(I1) polypyridine complexes have been investigated on interfacial surf a c e ~Microviscosity ~~~. effects in Nafion-Na+ membranes have been measured through pyrene probes587indicating that the probes themselves are located at the fluorocarbodwater interface. Hydrophobic microdomains have been measured in modified p o l y e l e ~ t r o l y t e s ~vinyl ~ ~ - ~polymer ~~, latexes593,cetyl pyridinium chloride with metal complexes of 8-hydroxyquinoline-5-sulfonicacid594 poly(dimethy1amino)alkyl methacrylate-block-sodium m e t h a ~ r y l a t e ~ poly(~~, benzyl glutamate)/poly(ethylene oxide) copolymers596, azo initiated poly(bromide598, methyl m e t h a ~ r y l a t e ) ~ ~ ~dodecyltrimethylammonium , p~ly(ethylene-co-methacrylate)~~~ and non-ionic cellulose derivativesm. In the latter case, using pyrene and perylene probes, microviscosity effects were dependent upon the CMC with increasing polymer hydrophobicity giving rise to an increase in the rigidity of the polymer-surfactant aggregates. The nanosecond dynamics of molecular complexes have been discussed in depthm1 while J-aggregates in dyes have been examined by pressure dependent absorption and fluorescence measurementsm2. Other studies on polymer labelling include the use of pyrene labelling for methyl methacrylate latexes where the excimer emission is related to annealing effectsm3. The polymerisation rate of hydroxyethyl methacrylate has been measured in the presence of triethylamine and excited pyrenebutyltrimethylammonium ionsw. Electron transfer was found to be highly dependent upon the nature of solvent used in the polymerisation process. In the case of pyrene labelled poly(ethy1enimine) excimer emission increases with decreasing pH due to chain coilingm5.The absorption and fluorescence characteristics of perylene tagged poly(ethy1ene oxide) have been found to be independent of the polymer chain length in dilute solutionm6whereas the fluorescence of a new Schiff base of o-phenylenediimidocellulose is highly dependent upon pH607y608.Fluorescent probes have been found useful for the determination of the functionalisation of atactic polypropylenem9 while c60 has been covalently linked to alkylsulfonate groups610. The relaxation time of anthryl groups on a poly(ethy1ene oxide) (PEO) chain increases with increasing chain length61 up to a maximum of 4000. The PEO chain has a much higher local mobility than vinyl polymers such as styrene. The fluorescence properties of dialkylamino modified polysiloxanes depended upon the crosslink density612 whereas the

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Photochemistry

fluorescence of poly(N-isopropylacrylamide) shows that the polymer undergoes a conformational transition from a flexible coil to a globular structure, followed by collapse to give aggregates6I3.The fluorescence from porphyrin tagged methacrylate copolymers decreases with increasing porphyrin groups6* while the same chromophore exhibits marked changes in absorption when polymerised into poly(N-isopropylacrylamide)615. Polymer doping experiments continue for various applications with emphasis on dye chromophores. Tetraphenylporphene has been used as a dissolved oxygen sensor in plastic fibres616 while the fluorescence quenching of 1,2benzanthracene in poly(vinylbutyra1) has been described by a fractal cluster mode1617.Coumarin 6 has been observed to form aggregates and is also highly sensitive to the pH of the environment6'*. This molecule was able to detect Lewis acidity in certain types of zeolite that were otherwise thought to be nonacidic. The kappa number in single wood fibres has been measured using Acridine Orange as a molecular probe6I9while acridine has been found to be a useful molecular probe in polyamines620.Using 9,lO-diphenylanthracene as a molecular probe in PMMA the effective thickness of a degradation front is greater and radical propagation rate slower in glassy polymers621.Benzonitrile derivatives have been used as molecular weight detectors622,static quenching observed in p ~ l y s i l a n e smolecular ~~~, alignments determined in PMMA624-626, colourants determined in Japanese woodblock prints627and the association of fluorocarbons measured in poly(N-isopropylacrylamide)628. Dimethylketene has been identified as the photoluminescence precursor in plasma polymerised films of methyl methacrylate with tetramethyl- 1,3-~yclobutanedione~~~. Rhodamine 6G in PMMA changes from isolated dye molecules to nanocrystallites at high concentrations on plasma irradiatiod30. Rhodamine B in poly(acry1ic acid)631and benzylidenemalonitriles in PMMA632 exhibit pressure sensitive fluorescence while Rhodamine 6G has been found useful for measuring spherical polymer particles633.Temperature profiles are claimed to be measured during processing634as are melting and crystallisation processes in pyrene doped polyethylene635.Dyes which have geometrical asymmetry in their molecular structure are useful for anisotropy A novel route has been developed to tunable emission in smart gels637while chitosan self-association has been monitored through a flavone molecular probe638.Rhodamine B has different conformers when grafted to cellulose639 while 9-methylanthracene is an effective dye for monitoring internal stresses in coatingsM0.The reaction kinetics of 9-hydroxymethyl-10-[9-(naphthylmethoxy)methyl]anthracene reflects the rearrangements of local free volume in PMMAM1whereas surface concentrations of rhodamine 6G have been measured on derivatised Fluorescence studies show that divinylbenzenestyrene gels continue to grow after gellationM3while the presence of metal ions interfere with the measurement of dyes on fibresw. Dyes based on pyrene and perylene has measured temperature profiling in the curing of resinsM5as have scalar behaviour in bulk fluidsM6.The compound 1,l-dicyano-4-(4'-dimethylaminopheny1)-1,3-butadiene is a valuable rotor for polymersa7. Gellation effects in PMMA were easily monitored. The botanical source of amylose can

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be measured by tagging the sugar with 2-aminopyridineU8 and N-pheny1-Nnaphthylamines can measure probe depths in poly(N-isopropylacrylamide)649. The local dynamics in heterogeneous polymerisations have been measured using pyrene probes where termination was noted by an enhanced excimer emission650.Perester initiators, however, were powerful quenchers. Pyrene has also been used to measure gellations in methcarylate polymers651 *652, cholester~l-methacrylates~’~, and p o l y b u t a d i e n e ~ ~Micrometric ~~. measurements have been undertaken on labelled polysiloxanes cast onto steel surfaces655, charge-transfer processes in amino-substituted boron dipyrromethane dimethylaminophenylphenanthrene probes in thermosensitive N-isopropyla~ r y l a m i d and e ~ ~polyelectrolyte ~ behaviour using fullerene probeP8. Rare earth complexes have been investigated in some depth. The electrochemical redox processes for europium complexes can be switched in PEO Europium complexes with giving rise to different types of polyacrylic acid are different when compared to those in copolymers6603661 and cellulose662where the fluorescence is higher. Europium(II1) ions form complexes with CMCs, the fluorescence intensity of which is higher in the solid state than in solution663.Europium naphthoate complexes give intense red emission in polystyrene6a and highly conductive fluorescent metal-porphyrazines have been made6? Strong fluorescence has been observed from Tb(II1) complexes in poly(N-oxides)666as do samarium complexes in PMMA667. Europium ternary complexes have also been made with triphenylarsine and triphenylstilbene668and photochemically induced charge separation has been electrostatically constructed in organic-inorganic multilayer c o m p o ~ i t i e s ~ ~ ~ . Silver ion complexes have been observed in ethanol following y - i r r a d i a t i ~ n ~ ~ ~ while alkyl substituted porphyrin polymers are useful optical sensors671.Tb salicylate complexes exhibit strong fluorescence in PMMA672while the time resolved emission from polypyridylruthenium derivatised polystyrene is dependent upon the Ru(I1) content673. 4

Photodegradation and PhotooxidationProcesses in Polymers

The photooxidation and photodegradation of polymers continues to attract some interest but is not as widespread as in previous years. Review articles have appeared dealing with poly(2,6-dimethyl-1,4-phenylene photocatalyst fibres675, polymers with azo links676 and accelerated weathering specification^^^^. Other articles of interest include the design of an integrating sphere for repeatability in polymer ageing678 and the use of FTIR for monitoring the photostability of clear coat^^^^.

4.1 Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene680.Photooxidation processes in blends of polypropylene with poly(buty1ene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester681.This effect is shown in Scheme 1.

358

with

-

Photochemistry

= - O - ~ ~ ~ - O - ( C & ’ . -

0

PBT

0

8

“+-C-O-(Ct-&-

/\

Coloured conjugated structures

--

I

Oxidation products or PBT

I

Scheme 1 Photooxidation mechanism for PP/PBT polymers

Conjugated chromophores develop from the PBT and these will screen the photoinduced decomposition of the hydroperoxides in PP. This effect results in an accumulation of hydroperoxides from the PP. Iron diethyldithiocarbamate has been shown to exhibit an initial stabilisation effect on polyethylene followed by sensitisation682as does a n t h r a q ~ i n o n e ~Trace ~~9~~~. volatiles have been measured during the photooxidation of polyethylenes685 while ferric stearate/cerium(III) mixtures686 and starch materials are good degradants687. 4.2 Poly(viny1 halides) - Photodehydrochlorination occurs in PVC with a foam backing688as determined by Raman spectroscopy. For PVC stabilised with Zn/Ca stearates phooxidation indicates that dehydrochlorination dominates over oxidation at the inner layers beyond 100 micron^^^^-^^*. The effect of polyene formation also appears to be a function of light intensity as well as oxygen diffusion rate.

359

I l l : Polymer Photochemistry

4.3 Poly(acry1ates) and (alkyl acrylates) - Fluorinated acrylics are inherently p h ~ t o s t a b l ewhile ~ ~ ~ the vacuum UV photolysis of PMMA results in de-esterification and double bond formation694. The stereostructure of PMMA also has an important influence on the product distribution695. X-Ray and UV exposure of PMMA was fingerprinted via FT NMR analysis696. 4.4 Polyamides and Polyimides - The photodegradation of a polyamidehydroxyurethane is dependent upon the light flux697 and is sensitised by the addition of Fe3+ ions698 and r i b ~ f l a v i n e ~The ~ ~ .latter indicated the possible role of singlet oxygen in the photooxidation although this was not substantiated. Carbonyl and hydroxyl products have been monitored in the photooxidation of nylon 6,ti7O0 and dye fixation is reduced7O1. Fractal kinetics have also been applied to the photooxidation of polyamides702.703allowing an estimation of homogeneous and non-homogeneous factors. 4.5 Poly(alky1 and aromatic ethers) - Using laser flash photolysis poly(2,6dimethyl-1,4-phenylene oxide) undergoes scission at the phenolic link to give phenoxy radical^^^. Poly(viny1 methyl ether) has been shown to undergo a complex series of photoprocesses as shown in Scheme 2.705

-CH2-C-

H I I OMe

hv, 0 2 Autoxidation

*

$1

-CH2-C-

360

Photochemistry

The major product is the ketodiester with methanol as a major volatile. The photoyellowing is associated with polyconjugation formed via the dehydration of hemiacetals.

4.6 Silicone Polymers - Laser flash photolysis of poly(methylphenylsily1ene) gives a transient absorption associated with exciton states706while fullerene reduces bond scission in p o l y ~ i l a n e s ~ ~ ~ .

4.7 Polystyrenes and Copolymers - Rates of singlet oxygen reactions in polystyrene have been examined by time resolved spectroscopy708.Solute diffusion coefficients were considered to have an important influence on the rate constants and these, in turn, varied with the type of reactant. Irradiation of styrene-butadiene-styreneblock copolymer gave alcohols and epoxides as products which were not observed to be formed in high impact polystyrene7o9.The presence of sulfur dioxide and nitrogen dioxide significantly accelerate the photodegradation of polystyrene710while ESR has identified the formation of ally1 and alkyl radicals71 The photodissociation of peroxide in polystyrene is primarily associated with the evolution of water and C02712. 4.8 Polyurethanes and Rubbers - The photooxidation of polyether-polyurethanes exhibits sensitivity due to the ether segments713.Formates were the primary products of photorearrangement. The addition of styrene-butadiene copolymers to polyolefins significantly enhances their susceptibility to photooxidation via the butadiene component714.Horizontal attenuated FTIR spectroscopy has been found useful for detecting the products of photooxidation of rubbers715.

4.9 Polyesters - Aliphatic polyesters undergo rapid outdoor degradation and are claimed to be viable as environmentally friend1y7l6. The photodegradation of polyester coatings has been investigated in depth7I7 while for poly(buty1ene succinate) depth profiling on photooxidation to 3 12 nm irradiation showed a gradual decrease in chromophore concentration whereas for poly(ethy1ene terephthalate) chromophore development did not go beyond 10 microns7 8. 4.10 Photoablation of Polymers - Laser ablation remains a topical subject. Phenylhydrazine has been found useful in aiding the excimer ablation of PTFE719 as are bithiophene compounds720.An anomalous increase in the spectral diffusion of hole burning in PMMA samples has been observed when doped with Zn tetraben~oporphyrin~~' while large and small changes in microenvironments have been noted when hole burning PMMA and polyethylene722.Excimer laser ablation of polysilane films has been successfully carried as has PTFE using amine based charge-transfer comp l e ~ e s The ~ ~ ~photochemical . hole burning in poly(viny1 alcohol) gave more stable holes when doping with unimolecular micelles than with tetraphenyl-

111: Polymer Photochemistry

361

p ~ r p h y r i n ewhile ~ ~ ~polymers with photolabile azo links undergo microexplosions on laser ablation726. Vacuum UV excimer laser ablation of a fluorinated polymer film gave good ~ e t t a b i l i t ywhereas ~ ~ ~ a polyurethane film has been ablated in the presence of a polysaccharide for hydrophilic packaging applications728. 4.11 Natural Polymers - Amino acids are shown to participate in the photoinduced oxidation of silk with protein yellowing being inhibited by the presence of a UV a b ~ o r b e r ~Monomeric ~ ~ . ~ ~ ~o-quinones . are considered to be the major chromophores responsible for the photoyellowing of paper pulps731. In another study, the formation of p-stilbene-phenols are found to be the major yellowing c h r o m ~ p h o r e s ~In~ ~sapwood . various resinols have been identified and formed via quinonemethide intermediate^^^^. Transient radicals produced from a-guaiacoxy-P-propioveratroneare intermediates in lignin p h o t ~ y e l l o w i n gand ~ ~ ~these have been monitored via ESR735. The colour changes in softwoods have been monitored736while the production of peroxyformic processes in the production of pulps has been improved737and their effects examined on p h o t o y e l l o ~ i n g ~Phenoxy ~ ~ * ~ ~radicals ~. have been measured in situ on paper following laser flash p h o t o l y s i ~and ~ ~ biotreatment of spent liquors examined for bleaching741. The photoinduced formation of enzymic polymers produced from coniferyl alcohol form large assemblies called ‘supermodules’742. Other studies include the photodegradation of populus g r a n d i ~ thiol ~ ~ ~stabilisation , of hardwood744and photodegradation of milled wood l i g n i x ~ Using ~ ~ ~ . Raman spectroscopy the rate of S-S cysteine bond scissions can be monitored in wool fibre746. 4.12 Miscellaneous Polymers - The photooxidation of an epoxy cured resin increases through the layer at a constant rate747while photobiodegradable starches have been synthesised by grafting vinyl ketone monomers748.Vinyl ketone copolymers give vinyl end groups and crosslinking on irradiation749.A new method has been developed for monitoring the out-door ageing of dispersed coatings750and anhydride cured epoxy resins have been monitored via FTIR analysis75*.Iron doped poly(ethy1ene-co-acrylic acid) acts as a s e n s i t i ~ e rwhile ~ ~ ~ the decay of radicals has been measured in PMMA glasses753.Bivoltine silk fibres undergo an initial process of chain scission followed by extensive crosslinking on irradiation754.The photoyellowing of coatings has been measured755as has the photodegradation of clear and polymer metal complexes7s7. The bio and photodegradation rates of poly(hydr0xybutyrate-co-hydroxyvalerate) have been measured and controlled758as have organotin macromolecules759.Poly(alkylaryldiazosu1fides) undergo photorearrangement from the E to the 2 while thermal blankets have been made for spacecraft from a perfluoropropylene-tetrafluoroethylene copolymer761.Using laser desorption mass spectrometry triterpenes undergo p ~ l y m e r i s a t i o n Polyperoxides ~~~. degrade energetically763while the photodegradation of a 4-nitrostilbene polymer is highly wavelength dependent764.New findings into the yellowing of coatings have been made76sand the

362

Photochemistry

water barrier of paints examined by FTIR766. Polydiacetylene undergoes random chain scission followed by depolymerisation on irradiation767whereas polyanhydrides undergo rapid c r ~ s s l i n k i n g ~Polyphenylene ~~. ether only undergoes surface degradation protecting its bulk through self screening769while the incorporation of side groups into PPV's enhances their photolytic stabi1 i t ~ ~ Nitro ~ O . groups have been found to be particularly effective in this regard. Chain scission reactions in different acrylic melamine clearcoats have been examined771as have photoreactions in perfluorinated polymers772and nanosized p ~ l y a c r y l a t e s ~ ~ ~ .

5

Photostabilisationof Polymers

A number of reviews on polymer photostabilisation have as well. as the stabilisation of ceramic paints781,clear coat^^^^, wood finishes783 and car body paints784-786.Hindered piperidine stabilisers (HALS) have also been reviewed in depth787 and the surface stabilisation of polystyrene covered788. The dielectric strength of polyethylene has been measured as a function of additive concentration789while in stabilised polymers the depth of degradation was found to be uniform7w. HALS have been found to be effective stabilisers for UV cured coatings and do not influence the cure rate791.Bleached wood pulps can be effectively stabilised by ascorbic acid792and wool by hydroxybenzotriazoles and HALS793. Hydroxybenzotriazole stabilisers also protect wood pulp794and polyurethanes by ~ o - r e a c t i o nThe ~ ~ ~spectroscopic . properties of monomeric and polymeric benzotriazoles have also been compared796. UV absorbers are effective in clear coats797and when grafted to Dihydroxybenzophenone stabilisers inhibit the chain scission in the photodegradation of poly(methoxyacrylophenone)799 while tin stabilisers have also been found to photostabilise PVC8O0. Several studies have dealt with HALS stabilisers. A number of novel naphthyl and naphthoate-HALS derivatives have been synthesised and evaluated as thermal and light stabilisers in polyolefinssOl.The naphthalene moiety is found to induce good thermal and light stability with the tertiary structures operating more effectively in the latter case. In another study it was found that naphthalene adducts with HALS are less effective as stabilisers802.Polymer bound HALS have been found to be more effective than either the monomeric or polymeric types in stabilisation803.The y-ray grafting of HALS to polyolefins gives rise to improved performance8@' as did partially grafted HALS to polyurethane^^^^. Polymeric HALS are also effective when grafted into PMMA806 and other polymers807*808 as well as when doped into polysiloxanes809.The time evolution of nitroxyl radicals produced in the photo-oxidation of polypropylene has been monitored via ESR810. The migration of HALS during irradiation has been shown to be effective only when oxidation products in the polymer build-up to an effective concentration8

111: Polymer Photochemistry

6

363

Photochemistry of Dyed and Pigmented Polymers

A review has appeared on the photodegradation of dyes8'* and another article on carbon black technology813.Of particular interest is the observation that zinc 1-hydroxy-2-naphthoate photoprotects photochromic dyes814 as does a UV absorber for azo dyed silk8? Benzanthrone dyes enhance the photostability of PMMA816 and testing methods for colour fastness have been assessed for some reactive dyes817. Enzymatic processes for the degradation of dyes have been examined8I8while diffuse reflectance spectroscopy is useful for examining the triplet states of dyes in situ on polymer fabrics819. Singlet oxygen has been found to be dynamically quenched on cotton fibres. The photofading of reactive azo dyes in various bound and unbound forms has been assessed. Dyes with the reactive substituent were more photostable while a methoxy substituent reduced stability820.Covalently fixed dyes were also more photostable to perspiration effects821*822. The addition of fluorescent brightening agents alters the hue of dyes after washing823 whereas the presence of ferric citrate reduces dye l i g h t f a ~ t n e s sPhotobleaching ~~~. processes for fluorescent rhodamine dyes have been examined under conditions where a two step photolysis is involved825 while methylene blue undergoes photochemical reduction with an amine826. The light stability of triarytlmethane dyes is reduced when bound to bovine albumin and this correlates with an enhanced fluorescence quantum yield827. Using ESR dichlorofluorescein is shown to undergo a one electron transfer process to give a semiquinone radical that is immediately oxidised on admitting oxygen828.A rhodacyanine dye photofades via a self-sensitised reaction with singlet oxygen829and photobleaching reactions have been examined for a fluorescein dye in cotton830. Much of the work on pigments centres on titanium dioxide pigments. Calcium carbonate has been shown to behave as a stabiliser in p~lyethylene~~! Titanium dioxide filler particles become agglomerated after photooxidation in PVC-rubber mixtures832whereas in PVC itself titanium dioxide pigment induces subterranean polyene formation833.Stabiliser interactions have been studied with uncoated rutile and anatase pigments in the photooxidation of Rutile is found to be synergistic with phenolic antioxidants and HALS but antagonistic with hydroxyaromatic absorbers. In the presence of anatase pigment strong antagonism is observed especially in the presence of mixtures of stabilisers. Polymeric HALS are found to perform well against the photocatalytic effect of the anatase pigment. Microwave dielectric spectroscopy has been developed to describe the charge-carrier dynamics in titanium dioxide pigments835.Shifts in the resonant frequency and microwave power are related to the population of free carriers and photocatalytic activity of the pigment. Photoinduced electron transfer from bound anthracene carboxylic acid dye to titanium dioxide (titania) nano-particles depends upon the synthetic method for the particles836.Facile electron transfer was observed for the anatase modification. Apparently, silica alone on the surface of titania particles gives poor coatability whereas mixtures with alumina are superior837.The

364

Photochemistry

surface activity of nanoparticulate anatase was blocked to reduce its activity for acrylic paints838.Degradation processes in PTFE depend on the nucleating efficiency of the titania particles839.Wood pulps are effectively bleached using titania pigments8& while carboxylic acid formation has been found to be high in titania filled rubber and atactic p ~ l y p r o p y l e n e Titania ~ ~ ~ . films have been sputtered onto the surface of polyester films842and found to photocatalyse the decomposition of azo dyes843-845in solution and Encapsulating titania particles in crosslinked polymers reduces their p h o t o a ~ t i v i t y ~ ~ ~ .

7 1. 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.

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842. 843. 844. 845.

Part IV PhotochemicalAspects of Solar Energy Conversion By Alan Cox

Photochemical Aspects of Solar Energy Conversion BYALAN COX

1

Introduction

Topics which have formed the subjects of reviews this year include photochemical conversion and storage of solar energy, photocatalysts for water splitting using ion-exchangeable layered materials,2 solar cells,3 solid state organic solar cells,4 photoenergy utilisation for solving energy and environmental issue^,^ and green energy? The use of solar energy in applied photo~hemistry,~ and the future of hydrogen as an energy carrier,* have also been described. 2

Homogeneous Photosystems

Unsaturated hydrocarbons undergo a dye sensitized photooxidation which may have implications for the photochemical conversion of solar energy.9 Formation of hydroperoxides in these photoreactions has been reported. A series of newly characterised water-soluble viologen-linked zinc porphyrins having different methylene chain lengths (n = 3 -6) between the porphyrin and viologen, ZnP(CnV)4 has been characterised.lo These compounds have been used in the photoproduction of hydrogen using a system containing nicotinamide-adenine dinucleotide phosphate and hydrogenase. The excited state of the viologen-linked ruthenium(I1) complex, Ru(bpy)2(dcbpy)CnVCH3, is oxidatively quenched by the binding viologen in an electron-transfer process, and under steady state illumination photoinduced hydrogen evolution occurs in a system containing nicotinamide-adenine dinucleotide phosphate and hydrogenase. Investigation of some dendrimer porphyrins ((L5),P, n = 1 -4) possessing different numbers of five-layered dendron subunits (L5) has shown that following excitation, the energy is able to migrate over the dendrimatic array of the chromophoric building units that surround the energy trap.12 This observation is relevant to the energy transduction events in wheel-like arrays of chromophores in a purple synthetic bacterium, and provides a new strategy for molecular designs of light-harvesting materials.

Photochemistry, Volume 3 I 0The Royal Society of Chemistry, 2000 395

396

3

Photochemistry

HeterogeneousPhotosystems

A search for the electron source in photoinduced hydrogen production on Pt-

supported Ti02 particles has found that stoichiometric molecular hydrogen and molecular oxygen formation does not occur with this photocatalyst, but only molecular hydrogen production is observed.l3 Investigations suggest that Ti02 itself behaves as an electron donor to produce molecular hydrogen. The splitting of water has been successfully achieved using a combination of two photocatalytic reactions on the surface of Ti02 particles. l 4 This process consists of reducing water to hydrogen by bromide ions and oxidising water to oxygen using Fe(II1) ions in separate compartments, and combining the reactions with platinum electrodes and cation-exchange membranes. Such an arrangement is capable of continuously splitting water into hydrogen and oxygen. A study has been made of the adsorption-desorption of ammonia on cadmium sulfate (3CDS04-8H20) with particular reference to the formation of cubic phase CDS on reaction with H2S? These observations are of importance in the use of CDS as photocatalyst for the production of hydrogen from water. Cadmium sulfide suitable for photochemical water cleavage to give hydrogen has been prepared by a number of techniques, and its activity shown to correlate with its semiconducting (n- or p-type) behaviour.16 Prepared by gas-solid reaction, CDS possesses an excess of interstitial CD2+in the lattice, and this is reflected in its n-type behaviour and its superior activity. It has been reported that some semiconductor fine powders mixed with platinum black and thinly coated with polymer are excellent photocatalysts for the production of hydrogen from liquid water, and function corrosion free.l7 Examples include encapsulated amorphous Ti02 particles with poly(butyl acrylate) or poly(methy1 methacrylate), and encapsulated rutile Ti02 or CDS particles with potassium poly(viny1 sulfate)-glycol chitosan. Some closely related work details descriptions of novel photocatalysts effective for water photolysis include Pt-carrying Ti oxide or CD sulfide encapsulated within vinyl polymers such as poly(K vinyl sulfate)-glycol chitosan or poly(K vinyl sulfate)-methylglycol chitosan. Hydrogen and oxygen have been produced by irradiating an aqueous system consisting of agarose gel containing CDS or Ti02 particles with Rh as a ~ata1yst.l~ It is also reported'that use of alcohol as electron donor gives hydrogen and the corresponding aldehyde. A CDS-ZnS photocatalyst supported on Li20, and CaO-doped MgO and Ce02-doped y-Al2O3 have been found to be effective for the production of hydrogen in a S2-/S032- mixture.20 The same authors have also examined a CDS-ZnS photocatalyst supported on a range of basic oxides for hydrogen production using a sulfide/sulfite mixture.21 Hydrogen formation was most successful using a 30% by weight Li2O-MgO support, and it is suggested that the super basic properties of the support play a crucial role in the efficiency of the process. A photocatalyst of the form Pt/Zn[M]S (M = Co, Fe, Ni) has been described, and is photoactive in the visible region and capable of being used to generate hydrogen in high yield in the absence of any oxygen-containing compound as

'

I V: Photochemical Aspects of Solar Energy Conversion

397

hydrogen promoter.22 Ru02 has been prepared by thermal decomposition of R U ~ ( C O in ) ~ air ~ on zeolite Y to give a product whose morphology is dependent upon t e m p e r a t ~ r e .At ~ ~200°C, fibres are produced which are capable of functioning as an efficient catalyst for the oxidation of water to oxygen in the presence of [Ru(bpy)3I3'. These observation suggest that since Ru02 of a specific morphology is necessary for a catalyst to be effective, particular faces of the RuO2 may act as catalytic sites. The preparation of a new catalyst comprising an oxide of Ni, Pt, Ir or Ru, and supported on a layered compound containing alkali metals, rare-earth metals and Group IVB elements has been published, and details of its use for the photolysis of water to produce hydrogen given.24 Devices capable of inducing photocatalytic decomposition of water over zeolites and in which the mass variation of the zeolite can be measured during the photolysis have been d e ~ c r i b e d and ,~~ a comparative study has appeared of the preparation of hydrogen by the photodecomposition of water using a range of heteropolyates having the Keggin structure.26 The conclusion is drawn that the efficiency with which the hydrogen is formed is higher with heteropolyates having a half-wave potential between -0.15 and -0.35 V.Details have been given of a new photocatalyst for the production of hydrogen from water, which consists of a reduced Cr03graphite intercalation compound in potassium naphthalenide anhydrous THF solution.27 The photocatalytic reduction of high pressure COZ using Ti02 powders in isopropyl alcohol as positive hole scavenger has been reported to give methane.28 Formic acid is produced by photocatalytic reduction of supercritical fluid carbon dioxide using Ti02 powders followed by addition of water, and arises by protonation of reactive intermediates formed on the Ti02 powders.29This procedure may be of value for efficient carbon dioxide conversion and fixation, and for solar energy storage and the production of industrial raw materials. A report of solar energy storage using photoinduced electron transfer in a polymer containing norbornadiene and carbazole pendants has appeared.30 Photoisomerisation of the norbornadiene pendants has been achieved using wavelengths above 350 nm, and a photoinduced electron-transfer mechanism is suggested to occur on the basis of measurements involving fluorescence quenching and chemically induced dynamic nuclear polarisation. Some new hydrophilic immobilised photosensitizers based on porphyrin moieties bound on poly(methy1 methacrylate) have been described, and have been found to produce 0 2 ( l A g ) at a rates significantly in excess of those observed using Rose Bengal immobilised on Merryfield polymer.31The photochemical conversion of solar energy by photooxidation of unsaturated hydrocarbons to give organic hydroperoxides has also been described.32 cis-trans-Isomerisation has been observed when large aryl ether azodendrimers, arranged such that the number of aromatic layers exceeds four, are excited using IR radiation (1 597 cm- 1).33 This dependence of the isomerisation on distance may imply a multiphotonic isomerisation mechanism and may have implications for a new approach to light harvesting.

398

4

Photochemistry

Photoelectrochemical Cells

The use has been described of transition metal phthalocyanine complexes as photosensitizers for dye solar cells? For example, (Me8Pc)RuLz (MegPcH2 = 1,4,8,11,15,18,22,25-octamethylphthalocyanine, L = pyridine-3,4dicarboxylic acid) has been prepared, absorbed on a Ti02 film, and its photocurrent action spectrum recorded. A new type of photovoltaic cell has been discussed.35The operation of this cell, which can be tuned through the visible spectrum, is based upon dye-sensitization of thin nanocrystalline films of Ti02 nanoparticles in contact with a non-aqueous liquid electrolyte. Solar cells containing polyhalide ions such as 1~~ + deposited on a porous Ti02 film have been c ~ n s t r u c t e d ,and ~ ~ photocatalysts for use in solar cells have been fabricated by forming catalyst electrodes composed of alternate laminates of Ti02 thin layers and SnO2 thin layers.37Such photocatalysts have been found to be useful for the generation of hydrogen in a range of applications. A photochargeable lithium battery has been described which incorporates as cathode a lithium-containing transition metal compound such as LiMoS2, LiTiS3, or LiNbS2, and a hard carbon lithium intercalating anode, together with a lithium conducting electrolyte between the electrode^.^^ The absorptivities for band to band transitions of some solar cells have been obtained from measurements of electroluminescence and a CDTe thin film solar cell having a glass/CDS/CDTe/Cu-doped carbon/Ag structure has been characterised by low temperature photoluminescence measurements.40 Studies have appeared of the depth-dependence of photoluminescence from thin film CDS/CDTe solar cells,41and of the junction photoluminescence in CDTe/CDS solar cells which shows that back-contact application causes a large qualitative change in the junction photoluminescence spectrum.42A highly efficient CDTe thin film solar . cell having a glass/ITO/CDS/CDTe/Cu-doped carbon/Ag structure has been prepared by close-spaced s ~ b l i m a t i o n .A~ ~CDS,Te I - ~ mixed crystal layer has been shown to be formed at the CDS/CDTe interface. Detailed photoelectric measurements on ZnO/CDS/Cu(In,Ga)Se2 solar cells have shown that in the region 1.2-2.3 eV, such cells exhibit a conversion efficiency of > 15%, and may act as broad band analysers of linearly polarised light.44In the first phase of a project aimed at demonstrating the validity of the concept of the next generation of CuInl~,Ga,(S,Sel-y)~(CIGS) based solar cells, particular emphasis is placed upon a theoretical analysis of potential tandem-junction devices.45 Solar cells have been fabricated by mercurysensitized photo-CVD using a-Si:H films which have been modified by SiH2C12 addition and H2 dilution, and a high stabilised efficiency has been achieved.46 Measurements of the temperature dependent current-voltage (J-V) characteristics of p a-Sic : H/n c-Si heterojunction solar cells with different doping levels in the p a-Sic : H layer have been made, and it is reported that as long as the p a-Sic : H layer in these heterojunction cells is highly doped, collection problems do not occur under normal operating condition^.^^ In some related work by the same authors, a study has been reported of the current-voltage

I V: Photochemical Aspects of Solar Energy Conversion

399

characteristics of p+-type hydrogenated amorphous silicon carbide (a-Sic :H)/ n-type crystalline silicon (c-Si) heterojunction solar cells under different condition^.^^ Space silicon solar cells are known to suffer anomalous degradation in regions of high fluence, and a photoluminescence investigation has attributed this phenomenon mainly to a complex of interstitial carbon and interstitial oxygen (C1-0,).49A large area flexible solar cell based upon the soluble polymer (3,7-dimethyloctyloxymethyloxypoly(phenylenevinylene)) and the highly soluble fullerene derivative, 1-(3-methoxycarbonyl)propyl-1 -phenyl[5,6]Cb1has been successfully constructed, and takes advantage of the known ultrafast electron transfer from the conjugated polymer to the fullerene as acceptor Coating solar cells with a fluorescent colouring agent has been reported to increase their energy conversion efficiency by reducing reflection of incident Solar cells with anti-fouling covers have been described.53 These transparent covers comprise a transparent layer of powdered photocatalytic oxide particles, silicone or amorphous SiO2, and a hydrophobic polymer on the surface of the cover. A design for self-cleaning solar cells has appeared in which the cell surface is covered successively with an ethylene-vinyl acetate copolymer layer, a fluoropolymer layer, and the photocatalyst layer which contains Ti02 mixed with a fluorop~lymer.~~ Some screen devices comprising a flexible solar cell sheet possessing thin film solar cells on both sides of a substrate, which is then wound on a roller, have been d e s ~ r i b e d and , ~ ~ the range of applicability of the reciprocity theorem has been extended to all types of solar cells, including metal-insulator-semiconductor-type and electrochemical.56 5

Biological Systems

An analysis of the thermodynamic efficiency of photosynthesis has appeared.57 The green algae Neochloris sp TS4F is reported to be capable of fixing C02 by photosynthesis, and also of producing lipids when grown under nitrogen s t a r v a t i ~ n .The ~ ~ performance of photosynthetic electrochemical cells using Anabaena variabilis M-3 immobilised within alginate beads has been investigated over a sequence of discharge and culture cycles.59Comparison with continuous discharge operation reveals that the current output can be extended by a factor of 10 under certain specified conditions. Hydrogenproducing photosynthetic Rhodopseudomonaspalustris R- 1 has been isolated, and hydrogen formed by adding a culture of this bacterium to waste water obtained from food manufacturing plants containing a variety of materials including acetic acid and ethanol; this mixture is then irradiated.60 Rhodobacter spheroides RV cells have been examined for phototrophic hydrogen evolution from municipal solid wastes and found to be capable of producing stable rates of hydrogen evolution over a number of days? Hydrogen formation using co-immobilised photosynthetic and fermentable bacteria in high concentration organic wastewater treatment has been discussed,62 and

400

Photochemistry

irradiation of either standard media culture or lactate-containing waste waters in a photobioreactor with Rhodospirillum fulvum and Rhodobacter spheroides cells immobilised on porous glass is reported to be capable of generating hydrogen.63 A photosynthetic reactor comprising a light energy converter and culturing photosynthetic microorganisms has been describedaa Shading the reactor with energy converters capable of transforming solar energy into electrical or thermal energy, but without lowering the production rates of hydrogen release by photosynthesis, results in high energy efficiency. As part of an examination of the conversion of thermal energy to biomass, a study has been made of a cone-shaped helical tubular photobioreactor incorporating Chlorella sp. under outdoor conditions for the photofixation of C02,65and details have appeared of a bioreactor whose design is directed towards the efficient fixation of C 0 2 in the form of biomass using microorganisms.66 In particular the excretion of porphyrins by purple non-sulfur photosynthetic bacteria (Rhodopseudomonaspalustris strain No. 7) has been discussed.

6

Luminescent Solar Concentrators

The photophysical properties and photostability of oxazine dye molecules for use in luminescent solar concentrators have been reported.67 Tests have shown that the optical efficiencies of luminescent solar concentrators prepared using a single dye in a liquid methyl methacrylate polymer are as efficient as those fabricated using the same single dye dissolved in methyl methacrylate, and which is then thermally polymerised.68

7 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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Author Index

In this index the number in parenthesis is the Part a d when appropriate, the Chapter number of the citation and this is followed by the refirence number or numbers of the relevant citations, e.g., (2.2) I37 represents Part II, Chapter 2, ReBrence 137. Aardahl, C.L.(3) 180 Aaron, J.-J. (1) 101;(2.4)48 Abadie, M.J.M. (3) 15,211 Abbiati, G.(2.6) 198 Abdel-Bary, E.M.(3) 291 Abd-El-Malak, N.A. (3)202 Abdel-Razik, E.A. (3) 291 Abdel-Salam, N.M. (3) 291 Abdel-Wahab, A.-M.A. (2.5)21 1; (2.6)284 Abe, K. (3) 592 Abe, M.(2.1)48,49;(2.2)44; (2.4)175;(2.5)69;(2.7)6 Abe, T.(1) 329;(4)13 Abraham, G.(3)365 Abraham, W.(3)612 Abramova, N.Yu. (3) 302 Abreu, I. (1) 178;(2.6)160 Abu-Abdoun, 1.1. (3)102 Acar, E.A. (2.6)146 Acar, M.H. (2.6)172;(2.7)10;(3) 38 Achimsky, L. (3) 680 Ackermann, L.(2.3)91 Ackley, D.E.(1) 377 Acree, W.E.(1) 166 Adachi, M.(2.6)312;(2.7)179 Adachi, N.(3)257 Adachi, T.(2.4) 193 Adam, W.(2.4)180;(2.5)154, 208;(2.6)138,185-187,260; (2.7)6,185 Adam, J. (3) 218,650 Addo, J.K.(2.7)59 Adelene, J. (1) 334 Adeniyi, J.B. (3) 800 Adhami, I.M.(3) 826

Adriaensens, P.J. (3) 457 Adronov, A. (1) 309 Advaoti, S.G. (3) 229 Agarrabeitia, A.R (2.3)69,77; (2.4)52;(2.6)148 Agarwala, V.S.(3)224 Agback, K.H.(2.4)326;(2.6) 242;(2.7)200 Ageeva, V.V. (3) 46 Aglietto, M.(3) 693 Agostini, G.(2.5)180 Aguirre, G.(2.6)25 1 Ahlblad, G.(3) 327 Ahmed, M.(2.7)128;(3)291 Ahn, K.D. (3)403 Ahn, K.I. (2.2)13;(2.5)196 Ahrenkiel, R.K.(4)42 Aida, T.(2.2)14;(2.6)47;(3) 301;(4)12,33 Aihara, S.(3) 336,564 Airinei, A. (2.7)5 1; (3) 434 Ait-Lyazidi, S.(1) 204 Aito, H.(2.4)223 Ajayagosh, A. (3)45 Akabane, T.(2.6)3 1 1 Akagi, K.(3)484 Akamatsu, K.(2.4)103 Akamatsu, T.(3) 401 Akasaka, T.(2.5)99 Akazomc, M.(2.1)90.91; (2.4) 172;(2.6)271 Akerblom, E.B. (2.4)326;(2.6) 241,242;(2.7)200,201 Akerman, B.(1) 419 Akimoto, S.(1) 323 Akinaga, Y.(2.7)122 Akira, M.(2.6)8 403

Akiyama, H. (2.2)7;(2.4)101 Akiyama, K.(1) 34;(2.3)76 Akutagawa, K.(2.6)314 Alam, M.M. (1) 242,344,345; (2.5)99;(2.6)138, 186,258, 287;(3) 54 Albano, G.(1) 243 Albertin, L.(3) 181 Albin, D.S.(4)42 Albini, A. (2.3)53;(2.4)219, 299;(2.5)209;(2.6)1,201; (2.7)47 Alderson, V.(3) 558 Aldoshin, S.M. (2.4)131, 147; (2.6)74,77 Alegret, S. (3) 302 Alcssio, E.(1) 327;(2.5)103, 104 Alexander, A.J. (1) 32 Alexander, U.N. (2.7)178 Alexander-Katz, R.(3) 839 Alexe-Ionescu, A.L. (3) 152 Alfano, R.R(1) 467 Alfimov, M.V. (2.4)80, 185, 188; (2.6)29,30;(3)386 Alford, T.L.(3) 189 Al-Ghamdi, A.A. (1) 485 Al-Has~an,K.A. (1) 416,423;(3) 622 Al-Hazmy, S.M.(2.6)141 Ali, M.M.(3) 291 Alibes, R.(2.2)17, 18; (2.4)210 Alihodzic, S.(2.1)82 Alison, L.(3) 744 Aliwi, S.M. (3) 8 Aliyan, H.(2.2)81;(2.4)280 Al-Jalal, N. (2.4)205;(2.6)I04 Allen,N.S. (3) 17, 18,21,22,

404 341,801,834,835 Allen, W.D. (2.7) 65 Allender, D.W. (3) 269 Allonas,X. (2.5) 116 Almeida, P. (3) 639 Aloisi, G.G. (1) 154; (2.2) 79; (2.5) 209 Altamirano, M. (3) 604 Altomare, A. (3) 393,436 Aha, K.S.(3) 396,658 Alvarez, J. (3) 126,597 Alvaro, M. (2.6) 25 1 Amann, C.M. (2.2) 85 Amano, S.(3) 61 1 Amao, Y. (4) 10 Amat-Guem, F. (1) 142,422; (2.5) 200,222 Ameerunish, S.(1) 219; (2.4) 43; (2.6) 40 Ameta, S.C. (2.5) 118; (3) 845 h e y , D.M. (2.4) 4 Amimoto, K. (2.4) 34 Amin, N.(4) 40,43 h i t , B. (2.6) 127 Amler, E. (1) 141; (2.3) 97 Amoozadeh, A, (2.5) 184; (2.6) 256 Amori, L. (2.5) 202 Amornsakchai, T. (3) 284 An, Y.(2.3) 18; (2.4) 272 Anachenko, G.S. (2.6) 317 Anders, C. (1) 290; (2.5) 39 Anderson, D.G. (3) 124 Anderson, E.D.(2.4) 330 Anderson, H.L.(2.4) 295 Anderson, S. (2.4) 295 Andersson, M. (1) 299,300 Ando, H. (3) 253 Ando, T.(2.3) 129 Ando, W. (2.5) 177 Andrassay, M. (3) 701 Andrawes, F.F. (3) 89 Andresen, P. (2.7) 112 Andresen, S. (2.4) 173 Andrews, D.L. (1) 72 Andruzzi, L. (3) 436 Andrzejewska, E. (3) 28,232,240 Andrzejewska, M. (3) 232,240 Angelini, M.P. (2.1) 14 Anghel, D.F. (3) 558 Angiolini, L.(3) 393 Anguille, S. (3) 379 Anilkumar, G. (2.4) 64 Anisimov, V.M.(3) 621 Anne, A. (2.6) 209 Anon, E.(2.4) 255,256; (2.6) 64 Anpo, M. (1) 144; (2.5) 29, 121123; (4) 3

Anseth, K.S. (3) 85, 198,768 Antonio, M.E. (3) 266 Anton-Prinet, C. (3) 689-692,833 Antonucci, A. (2.5) 188 Anufieva, E.V. (3) 601 Aoi, Y. (2.3) 29; (2.4) 94 Aoki, S. (2.2) 71; (2.6) 124; (3)

157, 158,425 Aoki, T. (2.3) 93 Aota, Y. (2.5) 218 Aoyama, H. (2.5) 195 Aparicio-Lara, S. (2.3) 69; (2.6) 148 Aplin, RT.(2.4) 295 Appella, D. (2.2) 86 Arabindooo, B. (3) 846 Arai, S. (3) 670 Arai, T.(2.1) 90,91; (2.3) 19; (2.4) 30,31, 172,285; (2.6) 19,269,271 Arai, W. (2.4) 234; (2.6) 268 Arakawa, R (1) 352; (2.5) 14 Araki, K.(1) 275,434 Aramendia, P.F. (1) 379; (2.2) 73; (2.4) 153; (2.5) 227; (2.6) 86, 140 Aramoto, T.(4) 40 Aranyosi, P. (3) 820-822 Arc4 3. (3) 799 Arda, E.(3) 603 Areso, (3) 609 Argueello, J.E. (2.5) 208 Arguello, G.A. (2.1) 86 Argyropoulos, D.S.(3) 354 Arif, A. (2.4) 71; (2.6) 313 Arkhangelskii, I.V. (3) 74 Armaroli, N.(1) 313,314,363 Armesto, D. (2.3) 69,77; (2.4) 52; (2.6) 148 Armitage, B. (2.2) 76, 139 Armor, J.N. (2.3) 43 Arnaud, R (3) 840 h u t , L.G. (1) 77 Amold, D.R (2.3) 52; (2.4) 215 Arora, M.K.(4) 15,16 Arounaguiri, S. (1) 387 Arsen'ev, A.S. (2.4) 80 Artal, M.C. (3) 262 Arunagiri, T.N.(2.7) 85 Arvanitopoulos, L.D.(3) 57 Asada, y. (4) 64 Asakawa, M. (1) 238; (2.6) 49 Asakura, H.(2.5) 218 AsaneSomeda, M. (1) 3 15 Asanov, A.N. (1) 441 Asanuma, H.(2.6) 37 Asaoka. S. (2.5) 92: (2.6) 228 Asbury; J.B: (2:7) 100

s.

I

Photochemistry Ashi, Z. (2.4) 44

Ashenhurst, J. (3) 5 16 Ashihara, Y. (3) 707 Ashikhmin, M.V. (2.1) 64; (2.7) 64

Ashkenazi, G. (1) 56 Ashton, P.R (1) 238; (2.6) 49 Asmus, K.-D. (1) 343; (2.5) 100, 109

Asolkar, RN.(2.2) 105; (2.4) 313 Asomova, R (4) 40,46 Asouti, A. (2.2) 111; (2.4) 178 Atalla, A.A. (2.6) 285 Ataz, E.M.(1) 422 Atkinson, J.K. (2.7) 57 Atobe, M. (2.5) 182 Atvars, T.D.Z. (3) 547,567,635 Aubard, J. (2.4) 128, 132, 148; (2.6) 72,73,78 Audouin, L. (3) 680,689492,833 Augoliaro, V. (1) 430 Auner, N. (3) 444 Aust, E.F. (3) 44 1 Avar, L. (3) 803,807 Avlasevich, Yu.S. (3) 615 Awaga, K. (2.4) 36 Axten, J. (2.2) 35; (2.6) 91 Aycard, J.P. (2.2) 47 Ayodele, E.T.(1) 225; (2.3) 16 Ayyad, S.N. (2.5) 217

Babaqi, A.S. (2.6) 141 Bach, P. (2.4) 304 Bach, T. (2.1) 44-47.52; (2.3) 110; (2.5) 15,70,71

Bachilo, S.M.(1) 265; (2.6) 153 Baciocchi, E. (2.3) 121 Backer, M. (3) 444 Baczynski, A. (1) 96 Badr, M.Z.A. (2.6) 285 Bae, J.Y.(3) 376 Baerends, E.J. (2.7) 78 Baessler. H. (3) 474 Bag, D.S.(3) 37 Bagatur'yants, A.A. (2.4) 185, 188 Bagchi, B. (1) 33 Bagnich, S.A. (1) 64 Bagryanskaya, E.G.(2.6) 3 17 Bagryansky, V.A. (1) 53 Bahadur, P. (3) 580 Bai, D. (2.4) 257; (2.6) 63 Bai, F. (3) 493 Bai, G.(3) 45 1 Baigel, D.M. (1) 186; (2.5) 141 Baikerikar. K.K.(3) 25 1 Bain, A.J. (1) 485 Baitoul, M. (3) 507

Author Inder Baker, J. (2.6) 244; (2.7) 193 Baker, W.E.(3) 412 Bakker, M.P. (1) 61 Bakkeren, F.J.A.D.(2.2) 36; (2.5)

Baselga, J. (3) 219,635 Bashore, C. (2.7) 17 Bashtanov, M.E. (1) 185 Baskin, 1.1. (2.4) 80, 185, 188;

Balakrishnan, S. (3) 847 Balashova, T.A. (2.4) 80 Balkus, K.J., Jr. (2.4) 290; (2.6)

Bassiony, A.H. (4) 67 Bassler, H (3) 541 Basta, R (2.2) 55 Bad, Z. (2.6) 298; (2.7) 175 Batchelder, D.N. (3) 544 Batocci, G.(2.4) 32 Batog, O.P. (3) 141 Baud, G.(3) 841 Bauerle, P. (2.5) 113 Bauman, D. (3) 417 Baumeister, U. (2.6) 115,116 Bawngartner, K.M. (3) 346 Bwgartner, M.T.(2.5) 68; (2.6)

94; (2.6) 89

203

Ballardini, R (1) 178; (2.6) 160 Bally, T. (2.7) 32 Balsells, RE.(2.5) 227 Baltina, L.A. (2.5) 147 Baltova, S. (3) 815 Balzani, V. (1) 43,238,243; (2.4) 134, 135; (2.6) 49; (3) 98,555 Bandyopadhyay, T. (1) 78 Banejee, A. (2.1) 62; (2.4) 321 Banford, H.M.(3) 350 Bangal, P.R (1) 172,208; (2.2) 5; (2.4) 39

Bankowsky, H.H. (3) 175 Banu, H.S.(2.3) 17; (2.6) 93 Bao, Z. (3) 497 Baptista, M.S. (3) 827 Bar, I. (2.3) 128; (2.7) 118, 124 Baran, P.S. (1) 341; (2.5) 171 B m y i , M. (1) 404 Barashkov, N. (3) 348,459 Baraton, M.I. (3) 501

Barberi, R (3) 152 Barbet, F. (1) 111 Barbibi, D.C.(3) 671 Barboiu, V. (2.7) 51 Barbosa, F. (2.1) 38; (2.6) 277 Barbosa, V.C. (3) 547 Barckholtz, T.A. (1) 488 Bardeen, C.J.(1) 455,480 Bardeny, Z.V.(3) 324 Barigelletti, F. (1) 313,314,398 Barltrop, J.A. (2.6) 128 Bama, E. (1) 76 Barnes, J.C. (2.6) 151 Barnes,M.D. (1) 129,132 Baronavski, A.P. (2.4) 88 Barrash-Shdh, N.(1) 165 Barns& c. (3) 375 Barrio, J.R (1) 414 Barrois-Oudin, N. (3) 715 Barsy, M.A. (2.6) 249 Bad, A. (1) 349; (2.5) 95,96 Bartocci, G. (2.3) 98 Barton, J.K. (1) 388,389 Barton, T.J. (1) 239; (3) 443,458 Bartroli, D. (3) 302 Barwich, J. (3) 182 B q k , D. (3) 744 Basche, T. (1) 135

(2.6) 30

202

Bar, J. (3) 510 Baxter, B.C. (3) 429 Baxter, G.W.(1) 98 Bayer, E. (1) 413; (3) 556 Bays, J.T. (2.7) 101 Bazhin, N.M. (2.5) 139 Bazin, M. (1) 249 Bearpark, M.J.(1) 71; (2.3) 87 B d e , M.S. (3) 405 Beaumont, P.C.(1) 145 Becerra, R (2.6) 295 Becher, J. (1) 357 Bechmanq W. (2.6) 253 Bechtold, K. (3) 807 Beck, C. (2.7) 69 Beck, E. (3) 175,208,216 Becker, I. (1) 26 Becker, R.S. (2.2) 87; (2.3) 98 Beckert, D.(2.5) 33 Begisheva, V.P.(3) 170 Beh, J. (2.2) 102 Beitz, T. (2.6) 253 Belfield, K.D. (3) 42 Beljonne, D. (3) 464 Bell, T.D.M. (1) 295; (2.5) 87; (2.6) 218

Belletete, M. (1) 202; (3) 530 Beloliptseva, G.M.(3) 594 Belser, P. (1) 40 Belt, S.T. (2.4) 293 Belu, A.M. (3) 785 Belyavskiy, S. (3) 348 Ben, AS. (3) 841 Benasson, R.V. (1) 326; (2.5) 12 Benezech, V. (2.2) 45; (2.4) 25 1; (2.6) 52

Benfaremo, N. (3) 520 Bennett, A.K. (2.1) 21 Bennett, D.M. (2.7) 35

405 Benniston, A.C.(2.6) 27 Ben-Nun, M. (1) 110,232; (2.3) 1; (2.4) 19; (2.6) 23

Bens, A.T. (2.3) 34; (2.4) 92 Bentrude, W.G.(2.2) 24; (2.4) 71; (2.6) 313-315

Bera, S.C. (2.4) 35 Berberan-Santos,M.N. (1) 67, 337

Berezhkovskii, A.M. (1) 120 Beremitski, G.K. (3) 46 Berg, 0. (2.4) 279 Berg, U. (2.4) 229 Bergman, RG. (2.7) 100 Bergmann, K.(2.7) 147 Bergo, P. (1) 338; (2.5) 108 Bergstrasser, U. (2.4) 304 Bennan, P.R. (1) 130 Berman, T. (1) 409 Bemard, P. (3) 10 Bernardi, F. (1) 71, 108, 115,116; (2.3) 82,87; (2.6) 24

Berrehar, J. (3) 450 Bertault, M. (2.4) 160 Bertolotti, S.G.(1) 253 Bert~and,S. (2.2) 22,23 Benveger, C.D.(1) 118; (2.4) 22 Bestiuc, I. (3) 434 Bethke, J. (2.2) 106; (2.4) 208 Bettinetti, G. (2.4) 299; (2.7) 47 Betzig, E. (1) 471 Beuerrnann, S. (3) 67 Beyene, K. (1) 254 Bhanthumnavin, W. (2.4) 71; (2.6) 3 13

Bhanti, D. (1) 493 Bhathan, J. (3) 456 Bharathi, P. (1) 266; (3) 551 Bhasikutkm, A.C. (2.1) 4; (2.5) 21 BhattacharWa, A. (2.1) 1.2; (3) 36

Bhattacharyya, K. (1) 153,171, 22 1

Bianco, A. (1) 360 Biasutti, A. (2.5) 200 Bielesch, U. (2.7) 152 Bierbauer, K.L.(2.1) 86 Bigger, S. (3) 811 Biju, P.R. (1) 63 Biju, V. (1) 356; (2.5) 102 Billa, E.(3) 352-354 Bin, Z. (3) 292 Bindig, U. (3) 581 Binkley, E.R (2.4) 324 Binkley, RW. (2.4) 324 Birch, D.J.S.(1) 401 Birge, R.R (1) 162 Birkett, P.R (1) 330; (2.5) 169

406 Birkmire, R.(4) 45 Bise, R.T. (2.7) 183 Bishop, T.(3) 755 Bisio, A. (4) 57 Bisio, G. (4) 57 Biskupic, S.(1) 349; (2.5) 95,96 Biteau, J. (2.4) 158; (3) 404 Bitit, N. (1) 204 Bitterwolf, T.E. (2.7) 101, 102 Bixon, M. (1) 19,20; (2.5) 1 Blache, Y.(2.2) 45; (2.4) 25 1; (2.6) 52

Blackstock, S.C. (2.3) 72 Blanch, H.W. (1) 486 Blanchard, G.J. (1) 421; (3) 80, 223

Blanchard, P. (2.6) 282 Blanche, C. (3) 435 Blanco, M. (3) 17 Blanco-Pinar, M.(3) 21,22 Blasinska, A. (3) 76 Blatter, F. (2.5) 153, 164 Blau, W.J. (3) 475 Bleyl, I. (3) 261 Blitz, M.(2.7) 113 Blokhin, A.P. (2.7) 182 Bloor, D. (3) 767 Blough, N.V. (1) 235 Blume, F. (2.3) 74; (2.5) 167 Bobrovsky, A.Yu. (2.4) 163 Bobrowski, K.(2.5) 74 Boccara,A.C. (3) 685 Bochu, C. (2.4) 118 Bocian, D.F. (1) 161, 162, 164 Bockman, T.M.(2.5) 192 Boddeke, F.R (1) 474 Bodunov, E.N.(1) 67 Boehmer, M. (1) 473 Boerner, H.G.(3) 116 Bogomolov, V.N. (1) 123 Bohne, C. (2.1) 12 Boiko, N.I. (2.4) 163 Boilot, J.-P, (2.4) 158 Boils, D. (3) 510 Bokabza, L. (3) 654 Boland, W. (2.7) 56 Bolcskei, H.(2.4) 8 Bolcu, c. (3) 795 Bolivar, P.H. (3) 481 Bolle, Th. (3) 335 Bolletta, F. (1) 397,399 Bol'shakov, B.V. (1) 217; (2.4) 47; (2.6) 43,44

Bolte, M. (1) 247 Boltova, S.(3) 729,730 Bonafoux, D. (1) 203; (2.6) 224 Bond, S.G.(3) 749 Bonefant, S.(3) 375

Bonesi, S.M.(2.5) 209 Bonhote, P. (2.5) 215

Bo*,

c. (1) 101

Bonneau, R (1) 269; (2.7) 12, 16 Bonnichon, F. (2.6) 223; (2.7) 167 Bonora, P.L. (3) 766 W y , F.P. (2.5) 170 B o o k ~ - M i l b uK.I. ~ (2.2) 112114; (2.4) 176

Bordeleau, J. (2.3) 134, 135; (2.5) 183; (2.6) 255

Borges, M.L. (2.2) 87 Borges, R (2.2) 87 Borisov, RA.(3) 73 Borissevitch, I.E. (1) 440 Bormann, D.(I) 111 Born, R (1) 276 Borovkov, V.I. (1) 53 Borowicz, P. (1) 179; (2.6) 156 Borsarelli, C.D. (1) 262,442; (2.5) 112,213; (2.6) 229

Bortolus, P.(2.3) 3; (2.6) 17 Boss F. (2.6) 57; (2.7)170 Bosch, P. (3) 233,234 Boschi, T. (2.5) 23 1 Bosch-Montalva, M.T. (2.4) 3 I I b e , R (2.6) 134 k s i , M.L.(1) 379 Bossrnann, S.(1) 237,276; (2.5) 88, 133; (2.6) 29

Botelho do Rego, A.M. (3) 639 Bothur, A. (2.3) 83 Botter-Jensen, L. (1) 405 Bottino, F.A. (3) 788 Bouas-Laurent, H.(1) 269 Boudreaux, G.M. (2.7) 119 Boukherroub, R.(2.3) 136; (2.6) 297

Boule, P. (2.6) 181 Bourdat, A.G. (2.4) 301 Bourdelande, J.L. (2.2) 17, 18; (2.4) 210; (2.5) 148

Bourson, J. (1) 484 Bousseau, J.N. (3) 838 Boutevin, B. (3) 61 Bouz-Bouq s. (2.1) 55 Bowen, C.M. (2.6) 83 Bowers, J.S. (3) 84 Bowman, C.N. (3) 5, 196-198, 258 Bowman, P.B. (2.6) 193 Boyd, M.K. (2.4) 322 Brabec, C.J. (3) 494; (4) 50 Bradaric, C.J. (2.6) 300 Bradely, M.(3) 535 Bradley, D.D.C. (3) 475,544 Brady, G.A. (3) 243 Braeuchle, C. (1) 135,170

Photochemistry Braeuer, M. (2.4) 75; (2.6) 157 Braga, M. (I) 81; (2.5) 31 Brakenhoff, G.J. (1) 452,480 Braacaleon, L. (2.3) 113; (2.5) 1I5 Brand, S.(2.3) 109 Braslavsky, S.E.(1) 442,443 Bratcher, S.(3) 414 Bratov, A.V. (3) 302 Brauer, B. (1) 165 Brauer, H-D.(2.4) 197 Braun, A.M. (2.5) 133; (2.6) 29 Braune, R (2.2) 102 Bravo, J. (3) 219,635 Bredas, J.L. (1) 330; (2.5) 169; (3) 449,464

Brede, 0. (1) 227; (2.4) 140; (2.5) 26; (2.6) 67,252

Breheny, C.J. (2.7) 99 Brem, B. (3) 704 Brembilla, A. (3) 647 Brendel, R (4) 56 Breneman, C.M. (2.6) 98 Brcnner, M.P. (1) 12 Brevnov, M.G.(2.7) 26 Brewer, K.J. (1) 285 Breyne, 0. (2.4) 121, 122 Bremva, V. (1) 343,349; (2.5) 95,96,100,109

Brigham, E.S.(3) 669 Brink, M. (1) 113; (2.3) 85 Brisset, H. (2.6) 282 Brockelhurst, B. (I) 193,194 Bmkmann, W.(3) 248 Broer, D. (3) 213,266 Bronsveld, M.V. (1) 336 Brook, A.G. (2.6) 7; (2.7) 171 Brouard, M. (1) 32 Brousmiche, D. (2.3) 113; (2.5) 115

Bmwer, A.C.J. (1) 13 1 Brovko, I.Yu. (1) 11 Brown, C.I.(2.6) 49 Brown, C.L. (1) 238 Brown, E.J. (2.7) 143 Brown, L.A. (2.7) 68 Brown, R.G. (1) 102, 177,305; (2.4) 73; (2.6) 155

Brownsword, RA. (2.7) 67 Bruhn, C. (2.4) 70; (2.6) 289 Brummerhop, H.(2.1) 52; (2.5) 71 Bnm, P. (3) 379 Brunel, C. (1) 133 Brush, C.K. (2.4) 330 Bwt-Friedrich, A. (2.2) 77 Bnozowski, Z.K.(3) 205 Bucher, G. (2.7) 18,44,45

Author Inder Bucher, 0. (2.7) 27,28 Buchholz, V.(2.3) 119 Budinski-Simendic, J. (3) 742 Budyka, M.F.(2.6) 58; (2.7) 42, 46; (3) 49

Budzikiewicz, H. (2.1) 89 Buehner, RW.(3) 23 1 Buerssner, D. (1) 296 Buevich, A.V. (2.6) 30 Buffeteau, T.(3) 375,377,408 Buffle, J. (1) 417 Buhr, S.(2.1) 50; (2.4) 209 Buika, G. (3) 107 Buisson, J.P. (3) 507 Buist, A.H. (1) 480 Bulinski, A.T. (3) 360 Bullock, E.R (1) 285 Bullot, J. (3) 507 Buncel, E.(3) 412 Bunel, C. (3) 252 Bunfield, D.H. (1) 121 Bunker, C.E. (1) 328 Bur, A.J. (3) 634,636,645 Buratto, S.K. (1) 128,134 Burdeniuc, J. (2.3) 126; (3) 724 Burger, U.(2.6) 146 Burgess, K.(2.7) 105 Burke, J.K. (2.1) 67 Burkoth, A.K. (3) 768 Burlett, D.J.(3) 329 Burnham, K.S. (2.3) 13 1; (2.4) 119

Burrows, H.D. (3) 579 Burshtein, A.I. (1) 66 Burshtein, E.A. (1) 97 Burtin, D.(3) 61 Buruiana, E.(3) 434 Buruiana, T.(3) 434 Bushan, K.M. (1) 225; (2.3) 16 Butler, D.N.(2.3) 89 Butts, C.P.(2.4) 229 Byrd, W.E. (3) 678 Bystryak, S.(3) 605 Bytheway, I. (2.2) 88 Cabanelas, J.C. (3) 219 Cab&, P.V. (3) 639 CaEeri, S.(2.2) 80 Cai, H.(2.5) 224; (3) 563 Cai, 2.(3) 270,495,625 Cairns, S.M. (2.6) 3 14 CalaMk, I. (3) 62 Calabrese, J. (2.4) 279 Caldwell, R.A. (2.2) 106; (2.3) 70; (2.4) 208

Calle, P. (3) 233,234

Callis, J. (3) 619

407

Calogero, G. (1) 398

calzaferri,G.(1) 36

Campagna, S. (1) 294,380; (3) 555 Campbell, B.H. (3) 89 Campbell, S.(3) 427,428 C a m p , P.J. (2.4) 255,256; (2.6) 64 Campredon, M. (2.4) 118 Cano, M.L. (2.1) 77; (3) 273 Canpolat, M.(3) 603 Cantor, S.E.(3) 192,230 Canva, M.(3) 764 Cao,J. (3) 147 Cao, S.(3) 146 Cao,W.X. (3) 9,103, 145,147, 193 cao, x. (1) 200 Cao, Y. (4) 30 Capocci, G. (3) 786 Catamella, P. (2.6) 195 Carano, M. (1) 327; (2.5) 103, 104 carbooera, D.(2.5) 180 Carcelli, M.(2.6) 231; (2.7) 192 Cardinet, C. (3) 715 Caretti, D.(3) 393 Carey, C.K. (1) 456 carlini, (3) 393 Carlos Lima,J. (1) 178 Carlsson, L.(3) 238 Caro, 1. (3) 364 Carpenter, B.K. (2.4) 12 Can, C.I. (3) 746 Cam, RW.(2.7) 155 Carrascoso, M.(2.5) 200 Carre,M.C. (3) 647 Carrell, H.L.(1) 23; (2.6) 14 Carrell, T. (1) 303; (2.6) 120 Carroll, C.A. (2.2) 39 Carroll, P.J. (2.1) 67 Carson, P.J. (1) 128 Carter, G.M.(1) 463 Carter, RO.(3) 756 Carter, RT. (2.7) 147 b e y , C.P. (2.7) 93 Caperson, J. (3) 414 Castell, J.V. (2.1) 74; (2.6) 168, 280 Castellan, A. (3) 355,732,737, 839 Castellano, F.N.(1) 468 Castella-Ventura, M.(2.5) 76 Castelvetro, V.(3) 693 Castillejo, M.(2.1) 63; (2.7) 63 Castle, RN. (2.4) 240; (2.6) 56 Castleman, A.W., Jr. (2.7) 136 Catalan,1. (1) 407 Catalina, F. (3) 17, 18,21,22,

c.

714,779,834 Cattaneo,M. (3) 12,121, I22 Caw, M.P. (1) 357 Cadeiro, J.A.S.(2.4) 243; (2.5) 23 1 Cavallini, D.(2.5) 188 Cazeau, P.(1) 204 Cazeau-Dubroca, C. (1) 204 Ceita, L. (2.4) 325; (2.6) 233; (2.7) 197 Cehni, P. (1) 115; (2.3) 82 Celewicz, L.(2.2) 65; (2.4) 328; (2.6) 173; (2.7) 188 Centurioni, E.(4) 47 Cemenati, L.(2.4) 219 Ceroni, P. (1) 327; (2.5) 103, 104, 107 Cerullo, G. (3) 476 Cemantes-Ue, J.M. (3) 83 1 Ctmera, M.(2.4) 213 Cha, J.K.(2.3) 72 Chadziiannos, G.(3) 320 Chae, K.H. (3) 128,186 Chae, W.K. (2.4) 289 chai, z. (2.2) 64 Chakrabarti, S. (2.2) 4; (3) 159 Chakrapani, S.(3) 138,139 Chakravoh, S.(1) 172,208; (2.2) 5; (2.4) 39 Chambaudet, A. (3) 747 Chamontin, K.(2.4) 146 Chan, H.S.O. (3) 471,533 Chan, K.-C. (1) 248 Chan, T.-T. (2.5) 91 Chan, Y.P. (2.4) 121, 122, 125127 Chandra, A.K. (2.5) 55 Chandra, B.P. (1) 10 Chandrasekhar, M.(3) 485 Chandrasekhar, R.(3) 485 Chanhhekar, T.K. (1) 158 Chang, A.H.H. (2.3) 57 (2.7) 126, 127 Chang, C.H. (3) 123 Chang, D.J. (2.1) 35; (2.4) 294 Chang, H.(3) 154 Chang, J.A. (2.2) 126; (2.5) 73 Chang, J . 4 . (2.5) 121 Chang, J.Y. (3) 186 Chang, S.C. (3) 456 Chang, Z. (3) 270 Changenet, P. (1) 195 C h o n , M.(2.5) 134 Chapat, J.-P. (2.2) 45; (2.4) 25 1; (2.6) 52 Chaput, F. (2.4) 158 Charambopoulos, A.Ph. (2.4) 309; (2.6) 162

Photochemistry

408

Chartoff, RP.(3) 263 Charubah, R (2.4) 323; (2.7) 196 Chignon, 0. (2.2) 45; (2.4) 251; (2.6) 52

Chawla, c.(3) 755 Che, C.-M. (1) 248 Cheatum, C.M. (2.6) 261; (2.7) 184

Chee, C.K. (3) 613 Chemela, S. (3) 802 Chen, C. (2.7) 145 Chen, C.H. (3) 347,492,587 Chen, D. (3) 623,723 Chen, F.Q.(3) 418 Chen, H. (3) 297 Chen, J. (3) 269 Chen, J.F.(2.2) 135 Chen, J.Y.(3) 103 Chen, J.-Z. (2.4) 154 Chen, K.(1) 286,287; (2.6) 2 16, 217; (3) 389,842

Chen, K.C. (2.2) 122 Chen, K.-X. (2.4) 154 Chen, L.(2.3) 50; (2.5) 159; (3) 593

Chen, L.C. (3) 277 Chen, P. (1) 29; (2.6) 11; (2.7) 132

Chen, Q.(2.4) 15 Cheri, R.F.(1) 391 Chen, R.M. (3) 536 Chen, S. (1) 241; (3) 588 Chen, S.A. (3) 466 Chen, S.-C. (2.5) 9 Chen, S.H. (3) 430 Chen, T. (3) 87 Chen, W. (3) 443,804,806 Chen, W.D. (3) 516 Chen, X.(3) 111,383,384,665 Chen, X.-M. (2.5) 32 Chen, X.-T. (2.2) 82 Chen, X.-Y. (2.4) 206; (2.6) 105 Chen, Y.(2.7) 145; (3) 20,39, 111, 112, 135,374

Chen, Y . 4 . (1) 181,182; (2.5) 144

Chen, Y.T.(2.3) 59; (2.7) 129 Chen, 2.(2.1) 10 Cheng, C.-H. (2.5) 179 Cheng, P.(1) 368; (2.5) 232 Cheng, Y.(3) 537 Chennattucherry, G.C. (1) 63 Cheong, B.-S.(2.2) 123; (2.4) 96 Cheong, J . 4 (4) 19 Cherche, T. (1) 76 Chergui, M.(1) 472 Chemega, A.N. (2.5) 81 Cheshnovsky, 0. ( I ) 26

Chesko, J.D. (2.7) 131 Chesnokov, E.N. (2.7) 110 Chesta, C.A. (1) 250 Cheung, E. (2.1) 26 Cheung, K.-K. (1) 236 Chevolot, Y.(2.7) 27,28 Chi, A.Y. (3) 813 Chiantore, 0. (3) 693 Chiappero, M.S.(2.1) 86 Chiba, K.(2.4) 16,3 17 Chiba, M.(1) 364 Chibisov, A.K. (1) 231; (2.4) 110; (2.6) 28,84

Chichos, F. (1) 102 Chien, H. (3) 344 Chien, L.C. (3) 427,428 Chignell, C.F. (3) 828 Chihara, M.(4) 58 Chikaoka, S. (2.6) 276 Chilaya, G. (1) 461 Chin, J.W. (3) 678 Chirio-hbnm, M.4. (1) 74 Chisaki, Y.(2.7) 168 Chela, S. (3) 805 Cho, C.H. (3) 186 Cho, C.S. (3) 596 Cho, H.(3) 106 Cho, H . 4 . (2.2) 123; (2.4) 96 Cho, J.H. (3) 339 Cho, M. (1) 73 Cho, S.(3) 106 Cho, S.J. (2.6) 212 Cho, W.J. (3) 77,748,758 Chodorowski-Kimmcs, S. (1) 282 Choi, H. (2.7) 183 Choi, J. (1) 228,362; (2.4) 41 Choi, K.(2.2) 37,38 Choi, L . 4 . (2.6) 83 Choi, M.M.F. (1) 375 Choi, S.-Y. (2.1) 80; (2.6) 189 Choi, W.M. (3) 77,748,758 Chong, S.W. (2.1) 67 Choo, D.J.(3) 403 Chou, C.-C. (2.3) 62; (2.7) 158 Chou, P.-T. (1) 181, 182; (2.5) 144

Chow, M.(3) 239 Chowdhury, M.J. (3) 201 Chronister, E.L. (2.4) 279 Chrywchoos, J. (1) 254 Chu, B.W.-K. (1) 236 Chu, G.S.(2.2) 135, Chua, S.J.(3) 527 Chuang, K.R (3) 466 Chuev, 1.1. (2.4) 131, 147; (2.6) 74,77

Chujo, Y.(3) 358,382,523 Chun, K.H.(2.1) 70; (2.6) 190

Chung, A.M. (3) 218 Chung, C.H. (3) 160,403 Chung, H.W.(3) 823 Chug, S.J.(3) 462 Chug, W . 4 . (2.3) 88 Chung, Y.(2.7) 223 Chumkov, A.V. (2.6) 29 Ciardelli, F. (3) 436,693 Cifrain,M.(3) 195 Cik, S.G.(3) 349 Ciobanu, C. (3) 697499 Cirak, J. (3) 349 Cires, L. (1) 150 Ciringh, Y.(1) 162 Clara, S.(3) 98 Claridge, T.D.W. (2.4) 295 Clark, S.C. (3) 81, 82 Clark, T. (2.4) 283 Clarke, D.A. (2.4) 123, 124 Clivio, P. (2.2) 70; (2.6) 123 Clough, RL. (3) 708 Coates, G.W. (2.4) 182; (3) 155 Coffey, M.J.(2.6) 261; (2.7) 184 Coheur, P.-F.(1) 330; (2.5) 169 Coleman, D. (3) 236 Coleman, M.M.(3) 151 Coleman, R.S.(2.4) 3 14 Colin, R.(2.5) 169 Collar, E.P.(3) 609,714 Collin, J.-P.(1) 3 13.3 14,398 Collin, R. (1) 330

Collins, G.E.(2.6) 83

Collins, S.F.(I) 98 Collinson, C.J. (3) 460 Combellas, C. (3) 583 Comins, D.L. (2.2) 60; (2.6) 90 Commereuc, S. (3) 8 10 Compton, RG.(3) 830 Compton, R.N.(1) 340 Cong, N.H. (1) 5 1 Conger, B.M.(3) 430 Conlin, R.T. (2.6) 300 COMOIIY,B.A. (2.6) 291 Connolly, T.J. (3) 618 Constable, E.C.(1) 243 Constien, R.(2.2) 28 Conti, F. (I) 338; (2.5) 108 Contractor, K.(3) 580 Conwell, E.M. (3) 321,505 Cook, C. (3) 744 Cooke, W.D. (3) 746

Coons,L.S.(3)

172

Cooper, C.R. (1) 390 Cooper, T.M. (1) 140; (2.4) 145; (2.6) 70

Cooper, V.A. (3) 679 Coppeta, J. (3) 646 Coqucret, X.(2.2) 6; (2.6) 16

Author Index Corbin, D.R. (2.2)91 Cordry, S.M.(1) 9 Corkan, L.A. (1) 1 Comelisse, J. (2.4)187,231 Carnil, J. (1) 330;(2.5)169;(3) 449 Corns, S.N.(2.4)123, 124 C o d a , T.(3) 17, 18,21,22, 834 C o m t , S.(3)618 Corrie, J.E.T. (2.4)318;(2.6)244, 270;(2.7)187, 193 Corvaja, C. (1) 338;(2.5) 108, 180 Cosa, J.J. (1) 262;(2.5)112,213; (2.6)229;(3)604 Cossy, J. (2.1)55 Costa, M.(2.5)188 Costa,S.M.B. (1) 420 Costcla, A. (1) 142;(3) 125 Costin, N.J. (2.2) 112;(2.4)176 Cotrait, M.(3)355 Cotzur, C. (3) 21 1 Courcoux, P. (1) 435 Couris, S.(2.1)63;(2.7)63 Coutterez, C. (3) 181 Couve, J. (3) 15 Covell, C. (2.4)205;(2.6) 104 Cowell, J.K. (2.2) 113, 114 Cozens, F.L.(2.1)77 Crabtree, R.H.(2.3)126;(3)724 Craita, C. (1) 150 Credi, A. (1) 238;(2.6)49;(3) 98 Cremer, D. (2.7) 18 creutz,s.(3)595 Creuzburg, M.(2.7) 120 Criado, S.(2.5)220,222 Crich, D. (2.1)80 Crim, F.F. (2.7) 184 Crimmins, M.T.(2.2)39 Crisci, L.(2.4)150;(2.6)82 crispin, x. (3)449 Crivello, J.V.(3)72,99,115,137139 Crooks, RM.(3) 214 Crowder, J.M.(1) 114;(2.3)96; (2.4)28 Cruise, G.M.(3) 179 Crum, L.A. (1) 9 CSespEgi, 2. (3)820-822 Cugaoli, C. (2.3)99 Cui, H.(3)84 Cui, J. (3) 495 Cui, Y.(3) 16,33,47 Cui, 2.-F. (2.1)22 Cullum, N.R.(3) 124 Cumpston, B.H. (3)770 Cunko, R.(3)701

Curtis, F.W. (3)453 Curto, M.J.M.(1) 337 Cvijin, I.V.(2.3)118;(2.4)271 Czaplcwski, C.(1) 148 czerwinski, CJ. (2.7)93 Dabrio, J. (3) 125 D'Addario, E.(4)61 Dachae, S.(3)602 Dagdigh, P.J. (2.3)128;(2.7) 140 Dahlberg, A. (1) 419 Daik, R (3)464 Dailey, S.(3) 517 Dailcy, W.P. (2.1)67 Dainty, R.F. (2.2) 112;(2.4)176 Dairaio, M.E.(1) 379 Dairiki, K.(4)46 Dale, MJ. (3)762 d'Alessandm, N.(2.5)209 Dalinkevich, A.A. (3) 683,684 Dall'Aqua, F.(1) 154;(2.2)79 Dallakian, P. (2.1)89; (2.4)264 Daltrom, E.(2.6) 141 D'Amico, J. (4)45 Danheiser, RL.(2.7)35 Daniel, C. (2.7)79 Daniel, M.H.(2.6)75 Daniels, 1. (3) 341 Danielson, E. (3)673 Danishefsky, S.J.(2.2)82 Dantus, M.(2.7)141,143 Daoust, B. (2.6)194 Daria, V. (1) 410 Darmanyan, A.P. (1) 183;(2.5) 138,140 Da Ros, T. (1) 327;(2.5) 103, 104 Danacq, B. (3) 404 Das, R (1) 176;(2.1)34 Das, S.K. (1) 171;(4)23 da Silva Perez,D. (3)839 Datta, A. (1) 171,221 Dattelbaum, J.D.(1) 468,482 Daub, E.(4)39 Daub, 1. (3)656 Dauben, W.G. (2.2)84 Daugey, N.(2.7) 118 DAuria, M.(1) 106,(2.4)21, 167,284;(2.6) 130,131,278, 279 Dauw, X.L.R (1) 336 Dave, P.R. (2.3)115 Davey, A.P. (3)475 David, E.(1) 42,276 Davidcnko, N.(3) 66,94 Davidson, K. (3)559 Davidson, RS.(3) I24

409 Davihoa, J. (1) 299,300 Davies, W.D. (3) 119 Davis, E.J. (3) 180 Davis, L.M.(1) 121 Davis, T.P. (3)254 D a y , J. (1) 478 Day, A.C. (2.6) 128 De,A. (3)36 De, A.K. (1) 244 Deacon, G.B. (2.7)95 Deb, S.K. (4)35 Dc Backer, S.(3)549 Dcbacrdemacker, T. (2.5)113 De ReIda, G.(1) 310 De C a m w Moleiro, P. (3)818 Decker, C. (3) 140,285,791 Declue, M.S.(3)414 De Cola, L.(1) 40 Dc costa,D.P. (2.1)21 De Costa,MF.(3)578 D c d o & - ~ c.(1) 21 1 Deck F.W. (1) 170 Deflaadn, A. (2.2) 110 Deflorian, E.(3)766 Defianq, E. (2.4)301 de Gelder, R (2.2)36;(2.5)94; (2.6)89 Dcglise, X. (3)783 Dc G d e q L.S.(3)381 DegntT, B.A. (1) 382 Dehcn, (3)549 Deheza, M.F. (2.5)166 De Jong, M.J.M. (3)448 Dektt, C. (3) 81 De Kok, M.M. (3)457 DeLapasse, G.(3)777 Delbacre, S.(2.4)11 8 dcl Barrio, J.I. (2.7)153 Del Giacco, T. (2.3) 121;(2.5) 202 Deligeorgiev, T. (2.4)79, 149; (2.6)76;(3)363 Dclor, F. (3)715 Dc Luca, E.(2.6)279 de Lucas, N.C.(2.2)142;(2.5)58 Dcmpchi, Y. (3)367,440 De Msresmadcer, A.K. (1) 44 de Mar4 P. (2.2)20 Dtmu,J.N. (1) 382 Dtmttn, A. (1) 197 de Morpes, S.G. (3) 818 Demuth, M.(2.5) 186 Den& F. (2.6)132 Den& H. (3)489 Den& Z.B.(3)536 Denham, K. (2.5) 228 Darhrd, P.(3) 507 Dcnnison, SM.(A) 175

w.

410 Dent04 D.D. (3)629 De Oliveira, V.A. (3)579 Depew,M.C.(3)734 Deronzier, A. (3)98 Desai, S.(2.7)54 De Schryver, F.C. (1) 119,310; (2.6)283;(3) 549,595 de Silva, A.P. (2.6)214 De Silvestri, S.(3)476 Desmedt, K. (3)549 Dessent, C.E.H. (1) 26 Deumal, M. (2.3)87 Deumie, M. (1) 424;(2.5)229 Devadoss, C. (1) 266;(3)551 Devau~,M.-F. (1) 435 Devenny, M. (1) 279 Deviprasad, G.R (1) 362 De Waal,B.F.M. (3)213 De Wit, C.(2.7)169 Deyd, H.4. (2.7)132 Dhal, P.K. (3)658 Dhamodharan, R (3) 167 Dhujati, M.S.K. (2.6)300 Diachun, N.A. (3)546 Diaq C. (1) 407 Diaz-Garcii M.A. (3)414,478 Dibbern-Brunclli, D. (3)547 DiCcsarc, N.(1) 202;(3)530 Dick, B. (2.7)116 Dickins, RS. (1) 403 Dickinson, J.T. (3)726 DiDonna, B.A. (1) 503 Diederich, F. (1) 363,394 Dicppedale, M. (3)840 Dierking, I. (3)260 Dietel, E.(1) 365;(2.5)105 Dialer, G. (1) 472 Dilung, 1.1. (2.5)20 Di Marco, G. (1) 380 Dimicoli, I. (1) 211 Dimitru, M.(3) 332 Dina, I. (1) 76 Dindch, L.(1) 339 Ding, L. (3) 13 Ding, M. (3)342 Ding, Y.(1) 239 Di Pasquale,G. (3)788 Di Pietro, C. (1) 380 Di Valentin, M.(2.5)180 D W , M. (1) 204 Dneprovskii, A.S. (2.4)225 Dobek, K. (1) 448 Dobrin, S.(1) 457 Dobrodumov, A.V. (2.4)38;(3) 150 Doaq K.-H. (2.4) 181;(2.7)91 hgariu, A. (3)483 Dogra, S.K. (1) 205,213;(2.6)22

Dohno, C. (2.2)78 Dohno, R (2.5) 190;(2.6)316 h i , M.(2.3)44;(2.6)321 Domen, K. (4)2,24 Domingo, L.R (2.4)31 1 do Monte, S.A. (1) 8 1;(2.5)3 1 Domrachev, G.A. (3) 293 Donat-Bouillud, A. (1) 202;(3) 530 Dong, S.(3) 391 Donovan, RJ. (2.5)135 Dopp, D. (2.6) 103 Dore, T.M. (2.4)191 Dorion, c.(3) 647 Dorojkina, G.N.(3)73 Doslic, N. (1) 112 Dosovitskayrs I.E.(3) 184,210 das Santos, D.A.(1) 330;(2.5) 169;(3)449 Dotcbeva, D.(3) 166 Dotcbcva, M. (3) 166 Dou, S.-X. (4)30 Doughs, W.E.(3) 5 13 Douhal, A. (1) 422 Doyle, M.E. (2.2)15 Doytcheva, M.(3)165 Doz01,J.-F. (2.4)44 Dreger, Z.A. (1) 198;(3) 631,632 Drew, M.G.B. (2.2)21 Drickamer, H.G.(1) 198;(3) 631, 632 Driessen-Holscber,B. (2.5)152 Droz4eorgct, T. (2.7)66 D'Souza, D. (1) 392 DSouza, F. (1) 362 Du, C. (3) 664,672 Du,D.M. (2.2)42;(2.5)32;(2.6) 92 Du, F.S.(3)32,561,562 Du, H. (1) 1 D m C.-Y. (1) 283 Duan, S.(2.4)324 Dubest, R (2.4)128, 132, 148; (2.6)72,73,78 Dubi4 P.L. (3)590,591 Dubois, C. (3) 747 Dubois, F. (3) 762 Dubois, J. (3)691 Dubois, M.(2.2)110 Duddu, R.(2.3)115 Dudler, V. (3) 335 Duerr, H. (1) 237,276;(2.5)88 Duholke, W.K.(2.6)193 Dulicu, B. (3) 507 Dulog, L. (3)710 Dumitrescu, I. (3) 812 Dumont. M.(3)394 Dunc&D.C.- (2.5)216

Photochemishy Dunn, A.R (2.4) 182;(3) 155

Dunsch, L.(1) 349;(2.5)95,%, 173 Dunwoody, N. (3)225,226 Duran, N. (3)818 Durnell, C. (1) 23;(2.6)14 Durocher, G. (1) 202;(3) 530 Durose, K.(4)41 Dun, H.(1) 42 Durrant, J.R (4)6 Durn H.D. (2.3)80;(2.6)272 Dusbiber, T.G. (3)679 D u s t c d d , U.(3) 182 Dutcurtrt, X. (3)715 Dutta, A.K. (1) 411 Dutta. P.K. (2.4)254;(2.6)31;(4) 23 A.B. (3)34 Dvomva, D. (1) 343;(2.5) 100 Dyakonov, V. (3)494;(4)50 Dyumacv, K.M.(3) 184,210 Dzuba, S.A. (1) 87 Ebara, S.(2.6)324 Ebdon, J.R (3)749 b i d , E.M.(2.6) 141 Ebcrlein, J. (2.7)120 Ebers~n,L.(2.4)3,227-229;(2.6) 277;(2.7) 107 Ebinhara, T. (3) 296 Echegoyen, L.(1) 363 Eckberg, RP. (3)212 Eckert, G.(2.6)275;(2.7)189 Eckhardt, A. (3) 565 Ecoffet, C.(3) 176 Eddaoudi, H.(1) 335 Ederle, Y.(1) 335 Edge, M. (3) 18,2422,341,834, 835 Edlund, U. (2.6)30 Eduardo, A. (3) 381 Effenberger, F. (1) 372 Egbe, D.A.M. (3)461 Egclhaaf, H.J. (1) 413;(3) 556 Eggelin& C. (3)825 Eggleston, J.M. (4)41 Egorov, M.P. (1) 53; (2.7)176 Eguchi, N. (2.4)105 Eha, S. (2.5) 121 Ehlbom, B. (1) 239 Eichbom, E. (1) 365;(2.5) 105 Eilers, F.(2.1)46 EisclaBiihler, S.(2.4)323;(2.7) 196 Ekelof, R.(3) 227 Ekhorutomwen, S.A. (3) 148 El Baraka, M.(1) 424;(2.5)229

Author index Elder, D.L. (3) 5 11 El-Fayoumy, A.Z. (3) 202 Elgendy, E.M. (2.5) 155,204,217 El-Ghayoury, A. (I) 278 Elguero, J. (2.4) 243 El-Hamouly, S.H. (3) 202 Eliasson, B. (1) 210; (2.6) 30,139 Elisei, F. (1) 154; (2.1) 92; (2.2)

79,87; (2.3) 98, 121; (2.4) 32, 247; (2.5) 75 El-Kemary, M. (1) 350 El Latif, F.M.A. (2.6) 249 Elliott, C.M. (1) 296 El Maghraby, M.A. (2.6) 249 El Osman, A. (3) 394 El-Sayed, N.M. (4) 67 Elsembaumer, R.L. (3) 537 E l - S h y , E.H. (3) 202 El'tsov, A.V. (2.4) 65; (2.6) 250 Embree, E. (3) 678 Emel'yanenko, V.I. (1) 97 Emiroglu, S.E. (2.6) 199 Emneus, J. (1) 385 Encinas, M.V. (3) 126,597,604 Encinas, S.(2.1) 75; (2.6) 57,281; (2.7) 170 Enderlein, J. (1) 125,473,481 Endisch, C.(3) 581 Endo,'K. (2.5) 218 Endres, J. (2.1) 73; (2.7) 75 Enenkel, P.(3) 175,208,765 Engelborghs, Y.(1) 43 1 Engleitner, S.(I) 59 Enkelmann, V. (2.3) 119 Eppink, A.T.J.B. (2.7) 133, 134 Epple, R.(1) 303; (2.6) 120 Epshein, P.Z. (3) 210 Era,M. (1) 476 Erba, E. (2.4) 258; (2.7) 9 Erdelen, C. (3) 261 Eremenko, A.M. (1) 60.62 Eremin, S.(1) 385 Ermisch, K.(2.7) 144 Ermolaev, V.L. (1) 16 Emst, L.(2.6) 247 E ~ ~ a - B a l ~R. d h(2.2) , 73; (2.6) 140 Enan, A. (3) 652 Escandar,G.M. (1) 477 Ben, C. (3) 71,92,633 Espanet, A. (3) 176 Esser, P.E.(2.5) 152 Etori, H.(3) 5 15 Etzbath, K.H. (3) 261 Evans, C.H. (2.2) 48; (2.4) 40; (2.6) 283 Evans, P.D. (3) 798 Evertsson, H.(1) 415; (3) 600

Ewing, K.J.(2.6) 83 Ezhova, M.B.(2.6) 300 Fabbrizzi, L. (1) 396,402 Fabich, E. (3) 736 Fabre, J.M.(3) 61 Fagnoni, M. (2.1) 3,56; (2.3) 53 Faherman, V.B. (1) 224 Fak, H.(2.5) 43 Falvey, D.E. (2.1) 61,62; (2.2)

117; (2.4) 321; (2.6) 170,213

Fan, L. (3) 824 Fang, A.G. (2.1) 62; (2.4) 321 Fang, D.C. (2.2) 100 F w P.-H.(2.3)64 Fang, Q.(3) 628 Fang, T . 4 . (2.5) 9 Fann, W.(3) 466 Fann, Y.C.(3) 828 Fanni, S. (1) 294 Farhadi, S.(2.1) 58,59; (2.7) 70, 71

Farley, K.A. (2.6) 193 Farmanara, P. (2.7) 108, 142 Farona, M.F.(3) 44 Farooqui, Z.H.(3) 97 Fananeh, F. (2.4) 62 Fasani, E. (2.6) 1,201 Fascetti, E. (4) 61 Fatkulbayanov, RM.(2.7) 46 Fato, M. (2.3) 82 Faust, D.(4) 3 1 Favaro, G. (1) 49; (2.1) 92; (2.3)

102; (2.4) 130,247; (2.5) 75; (2.6) 66 Favre-Nicolin, C.D. (3) 271 Fayed, T.A. (1) 425; (2.6) 25 Fayer, M.D.(3) 546 Fe, H.(3) 723 Feast, W.J. (3) 464 Federov, A.S.(4) 63 Federspiel, R.F.(2.4) 240; (2.6) 56 Fedorov, Yu.V. (2.6) 29; (3) 386 Fedorova, O.A. (2.4) 80; (2.6) 29, 30; (3) 386 Fedrizzi, L.(3) 766 Feeder, N. (2.2) 11 Feld, W.A. (3) 467,470 Feng, H.Y. (3) 668 Feng, J. (3) 569 Feng, K. (2.6) 175; (3) 55, 188 Feng, S.J. (3) 52,385 Feng, W. (3) 187,824 Feng, X.(3) 9,279,294 Feng, Y.(2.1) 10 Ferguson, M.W.(1) 145

41 1

Feringa, B.L. (2.3) 27,28; (2.4) 81,84

Femun, J. (2.6) 309 Fenrandez G b r , R (2.7) 153 Femmte, C. (1) 170 Ferraris, J.P. (3) 459 Ferreira, L.F.V. (3) 639 Femr, L.O.(2.2) 54 Ferrere, S. (4) 35 F A , T. (2.4) 167; (2.6) 278 Fems, J.P. (2.6) 98 Ferry, J.L.(2.5) 90 Ferry, L.(3) 839 Fettel, P.W. (2.1) 36 Fetten, M.(2.3) 8 Fieberg, J.E.(2.7) 109 Fiege, M.(2.1) 50; (2.4) 209; (2.5) 157

Figueredo, M.(2.2) 20 Figueroa, I.D. (1) 424 Fijiwara, Y.(2.7) 22 Filipenko, O.S. (2.4) 131, 147; (2.6) 74.77

Findeisen, M.(2.5) 187 Fink, B.K. (3) 229 Finke, H.(2.7) 112 Finkelmann, M.R (3) 416 Fischer, B. (3) 368 Fischer, H. (1) 491; (3) 213 Fischer, I. (2.7) 132 Fischcr, M. (2.4) 165 Fischer, T. (3) 368,422,438 Fischer, U.C.(1) 135,136 Fischer, W.M. (3) 489 Fisher, P.V. (2.2) 55,85 Flamigni, L.(1) 313,314,363, 398

Fleischer, G. (2.6) 38 Fleitz, P.A. (1) 140 Fleming, G.R (1) 23; (2.6) 14 Fleming, M. (2.2) 55 Fleming Grim, F. (2.6) 261 Floridi, S.(2.3) 98 Foerstcrling, H.-D. (2.1) 71 Foley, M.A. (2.4) 331 Fonash, S.(4) 45 Fong, R (1) 367 Foaseca, T. (1) 337 Font, J. (2.2) 17, 18,20; (2.4) 210; (2.5) 148

Fontan, E. (3) 714 Font-Sanchis, E. (2.3) 127; (2.7) 159

Foote, C.S. (2.5) 142 Forbes, J.E. (2.1) 81 Ford, J.V. (2.7) 136 Ford, P.C. (1) 45; (2.7) 98 Formosinho, S.J. (1) 77

412

Fornes, V. (3) 6 18 Forsythe, E.W.(3) 470 Fort, T. (3) 245 Fostcr, J. (2.4) 171 Fouassier, J.P.(3) 1,24 Fouracre, RA. (3) 350 Fox, M.A. (1) 427; (2.5) 216 Foxman, B.M. (2.3) 114; (2.4)

Fujimoto, K. (1) 156 Fujimoto, M. (2.6) 96; (4) 38 Fujisawa, J.4. (1) 234 Fujisawa, K. (2.2) 78 Fujisuka, M. (1) 339 Fujisuka, S. (1) 352 Fujita, M. (2.3) 129 Fujita, T. (2.2) 116; (2.4) 234,

Fmbboni, B. (2.3) 82 Fmga, L.M. (3) 714 F d e y , M.E. (2.7) 93 Framk, C.W.(3) 612 Francis,R (3) 45 Frank, A.J. (4) 35 Fraalrcvich, E.L. (1) 239 Franld, J. (3) 820-822 Franzke, D. (3) 760 F m a , A. (2.6) 209 Fmzier, D.O. (3) 183 Frechet, J.M.J. (1) 309 Frederick, J.H. (1) 31 Frei, H. (2.1) 85; (2.4) 221; (2.5)

Fujita, W. (2.1) 60;(2.4) 36 Fujitsuka, M. (1) 347,348,350-

270; (2.6) 97

153,164

Freidzon, A.Ya. (2.4) 185, 188 Flcisen, D.A. (3) 673 F m h , K.M.(2.6) 129 Frewcrt, D. (2.3) 34; (2.4) 92 Friend, RH (3) 319,464 Fritz, A. (3) 400

Ft(ihliag, B.(2.4) 180; (2.6) 260;

(2.7) 185

Froehner, S.J. (3) 578 Frolov, A.N. (2.4) 214; (2.6) 5,55 Frolov, S.V.(3) 443,452,496 Frommel, J. (3) 584 Froute, C.(2.6) 75 Fryxell, G.E.(3) 294 Fu, F.S. (3) 563 Fu, R (3) 539 Fu, RT. (1) 342 Fu, S. (2.4) 14 Fu, W. (2.7) 98 Fu, W.F. (2.3) 92; (2.4) 244; (2.6) 305

Fuchs, H. (1) 135, 136 Fuh, R4.A. (1) 1 Fuhmop, J.-H(1) . 258; (2.5) 230; (3) 581

Fuhnna~,J. (3) 218,409,650 Fujibuchi, T. (2.6) 159 Fuji-, N. (1) 307 Fujihara, K. (4) 14 Fujii, T. (2.3) 106; (2.4) 183; (2.6) 290; (2.7) 186; (4) 66

Fujii, Y. (2.4) 262; (2.5) 121-123 Fujiki, M. (3) 344 Fujimoto, E.(3) 718

263,285; (2.6) 114,267-269

352; (2.5) 14,97,98, 106, 173,236; (2.6) 210; (2.7) 180; (3) 525 Fujiwara, H (2.5) 126, 127 FujiR (2.2) 133 Fujiyoshi, S. (1) 188 Fukuda, D. (3) 48 Fukuda,T. (2.4) 262 Fukui, M. (3) 401 Fukuju, T. (1) 458; (2.5) 226 Fukumun, H, (3) 726 Fukunaga, S. (4) 60 Fukunishi, K. (2.2) 9; (2.4) 177, 282; (2.6) 94,95 Fukuoka, F. (3) 817 Fukushima, S. (2.1) 31; (2.5) 92; (2.6) 228 FJNShima, T. (1) 149; (2.5) 18 Fukuzumi, S. (2.5) 4, 14,98; (2.6) 10,210 Fun, H.-K. (2.4) 206; (2.6) 105 Funabiki, T. (2.5) 120 Funada, H (3) 611 Funken, K.-H. (4) 3 1 Furher, T. (3) 5 16 Furlan, L. (2.2) 27 Funman, C.E.(1) 87 Furuhama, A. (3) 2 Furukawa, H. (2.2) 83 Fu~kawa,N. (2.6) 290; (2.7) 186 Furukawa, S. (3) 336 Furukwa, Y. (3) 3 13 Furusyo, M. (1) 156 Furutani, H. (3) 726 Fuscya, N. (2.2) 121; (2.4) 195 Fushitani, M. (2.7) 137 Fuss, W.(1) 196,226; (2.7) 152

Gabbug C.D. (2.4) 123,124 Gaber, A.E.-A.M. (2.5) 211; (2.6) 179,284

Gabowska, A. (2.6) 156

Gadosy, T.(2.3) 123; (2.4) 51; (2.7) 21 Gaebert, C.(2.4) 201; (2.6) 142, 143

Photmhemistry Ga~cs-Baitz,E. (2.4) 8 Gael, V.I. (1) 3 17 Ga& M. (1) 409 Gaillard, E.R (2.5) 159 Galabova, H.G. (3) 269

Galan, J.C.(3) 272 GaliaUn, G. (2.3) 3; (2.6) 17 Galili, T.(1) 261 Galindo, F.(2.1) 13 Gallego, M.H. (2.5) 68; (2.6) 202 Galli, C.(2.3) 60; (2.4) 220 GalloN, R (4) 47 Galvin, M.A. (3) 497,498 Gamamik, A. (2.1) 28; (2.5) 66 Gamlh, I. (2.6) 212 Gan,C.Y.(2.1) 53 Gan,L.(1) 346; (2.5) 176 G m L.-B.(2.5) 175 Ganapathy, S. (2.6) 3 14 Gancan, R (1) 2 16; (2.4) 37 Gandini, A. (3) 181 Gang,1. (2.5) 223 Ganguly, B. (2.1) 82 Gaquly, T.(1) 240,244 Ganot, Y.(2.7) 124 Gao, D. (2.3) 50 Gao,F. (1) 408; (3) 52 Gao,G.H. (3) 465,468 Gao,H. (2.1) 19; (2.3) 125; (2.5) 27,53; (3) 643; (4) 30 Gao, J. (3) 342,528 Gao, Q.(3) 16,32,33,47 Gao,Y. (3) 467 Gaponenko, S.V.(1) 123 Garapon, C. (3) 839 Garavelli, M. (1) 107, 108, 115, 116; (2.3) 82; (2.6) 24 Garavito, M.R (1) 23; (2.6) 14 Garcia, c.(2.2) 74 Garcia, H.(2.1) 77; (2.6) 25 1; (3) 618 Garcia, I. (2.3) 8 1 Garcia, J.M.M. (1) 337 Garcia, N.A. (1) 25 1; (2.5) 200, 220,222 Garcia, R (3) 783 Garcia-Fresdillo, D. (1) 378 GarciaGaribay, M.A. (2.1) 28, 37; (2.5) 66; (2.6) 3 Garcia-Martinez, J.M. (3) 609 Garcia-Moreno, I. (1) 142; (3) 125 Garcia-Ocboa, I. (1) 422 Garcia Sanchez, F. (1) 483,487 Garcia-Segura, R. (2.2) 118; (2.6) 100 Gardette, D. (2.4) 249; (2.6) 50 Garrison, M. (3). 612 Gaspard, P. (1) 70

AufhorIndex Gasper, S.M. (2.2) 139 Gas&P. (1) 87 Gatti, F. (1) 396 Gavrishova, T.N. (2.6) 58

Gay, M.(3) 689492,833 Gayathri, N. (1) 33 Ge, X.W. (2.2) 135 Geactinov, N.E. (2.2) 74 Geahigan, K.B. (2.5) 203 Gebert, H. (1) 432 Gee, K.R. (2.4) 12 Geerts, Y.(3) 487 Geimer, J. (2.5) 33 Geirson, J.K.F. (2.2) 48; (2.4) 40 Geiss, P.L. (3) 248 Gelan, J.M. (3) 457 Gelin, M.F. (2.7) 182 Gellerstedt, G.(3) 73 1

Genniui, G.(2.3) 3; (2.6) 17

Gensch, T. (1) 310 Gentemann, S. (1) 161 Gentili, P. (2.3) 60; (2.4) 220 Georg, A. (2.7) 90 George, B. (3) 167 George, B.M. (2.5) 203 George, G.A. (3) 227,761 George, M.V.(1) 356; (2.5) 102 George, M.W. (2.7) 80,86 George, T.F. (I) 342 Georgescu, L. (1) 76 Geraghty, N.W.A. (2.2) 40; (2.4) 189

Gerber, R B . (1) 69 Gerhardt, V. (1) 386 Gerlock, G.L. (3) 771 Gerlock, J.L. (3) 679 German, E.D. (1) 85 Gescheidt, G. (2.6) 277 Gessert, T.A. (4) 42 Geuskens, G. (3) 81 1 Ghandi, M. (2.4) 62 Ghauharali, R.I. (1) 452 Ghazi, H. (2.3) 123; (2.4) 5 1 Ghiggino, K.P. (1) 295; (2.5) 87; (2.6) 218

Ghiglione, C. (3) 674 Ghose, D. (1) 7 Ghosh, A. (3) 792 Ghosh, H.N. (2.7) 100 Ghosh, M. (2.2) 4; (3) 159 Ghosh, P. (3) 40,51 Ghosh, R. (2.2) 134; (2.4) 242,

308; (2.6) 108 Giacometti, G. (2.5) 180 Gianotti, J. (2.5) 222 Gibson, J.E. (3) 5 Giegrich, H. (2.4) 323; (2.7) 1% Gierschner, J. (3) 482

413

Giertz, K. (2.4) 152; (2.6) 87 Giese, B. (2.1) 7,38; (2.2) 77 Gigante, B. (1) 337 Gijsman, P. (3) 717 Gil, M. (1) 457 Gilard, P. (2.2) 110 Gilardi, R (2.3) 115 Gilat, S.L. (1) 309; (2.4) 88 Gilbert, A. (2.4) 4,205; (2.6) 104 Gilberts, J. (3) 213 Gilch, P. (1) 500 Gillbro, T. (2.6) 153 Gille, K. (1) 214,215; (2.6) 3840 Gin,D.L. (3) 421,429,489 Gindin, V.A. (2.7) 7 Giocondo, M. (3) 152 Giorgini, L. (3) 393 Giraudeau, A. (2.5) 229 Giraud4irard, J. (1) 112 Girois, S. (3) 680 Giroldini, W. (2.4) 150; (2.6) 82 Gita, B. (3) 58 Giusti, G. (2.4) 148; (2.6) 78 Givens, R.S.(2.1) 8; (2.4) 11 Giz, A.T. (3) 575 Glapski, C. (2.2) 22 Glamer, F. (2.6) 146 Glasbeek, M. (1) 180, 195; (2.4) 74

Glaze, W.H. (2.5) 90 Glazer, A.N. (1) 320 Gleiter, R (2.3) 109 Glenn, D. (2.2) 100 Glybina, N.S. (3) 210 Gobaru, K. (2.3) 15 Gobbi, L. (1) 394 Godoy, F. (2.7) 97 Godshall, D. (3) 172 Goehde, W. (1) 135, 136 Goerlitzer, K. (2.4) 305; (2.6) 245-247

Goerner, H. (1) 23 1; (2.3) 11;

(2.4) 59, 110; (2.5) 186; (2.6) 28,84,219 Goesmann, H. (2.7) 92 Goettinger, H.A. (2.6) 252 Goez, M. (2.6) 275; (2.7) 189 Goff, S.E.J.(2.7) 86 Gogotov, I.N. (4) 63 Golic, M. (2.3) 89 Gollapalli, G.R. (1) 392 Golobish, T.D. (2.1) 67 Gomez, A.M. (2.2) 41 Gomez-Elvira, J.M. (3) 359 Gonen-Zurgil, Y. (1) 386 Gong, B. (2.6) 237 Gong, M.S. (3) 339 Gong, S.H. (3) 475

Gonsalves, K.E. (3) 501 G o d e z , A. (2.6) 259 Gonzaleq D. (2.3) 120 Gonzalez, L. (1) 112 Gonzalez, S.A. (3) 467 Gonzalez-Benito, J. (3) 2 19 G o d e z De LossS.E.A. (2.4) 113

Gonzalez-Moreno, R. (2.5) 148 Goodby, J.W. (3) 259 Goodman, J.L. (2.3) 76, 93 Goodman, S. (1) 23; (2.6) 14 Goodson, T.(2.4) 295 Goodwin, T.E. (3) 467 Gooijer, C. (1) 449 Gopal, V.R. (1) 225; (2.3) 16 Gorbatsevich, S.K. (1) 65 Gorelik, S.R. (2.7) 110 Gorman, A.A. (1) 186; (2.5) 141 Gormin, D.A. (2.3) 95; (2.4) 26 Gosh, P. (3) 41 Goshima, T. (2.1) 84 Gosse, B. (3) 75 1 Gosztola, D. (1) 270 Goto, M. (2.6) 3 11 Goto, R (2.3) 47; (2.6) 328

Gouki, M.(2.1) 84 Grabchev, I. (3) 268

Grabner, G. (2.7) 167 Grabowska, A. (1) 179 Gracia, M.M. (3) 579 Graefcnstein, J. (2.7) 18 Graetzel, M. (2.5) 215; (4) 34 Gramain, J.-C. (2.2) 45; (2.4) 249, 251; (2.6) 50,52

Gramain, P. (3) 61 Granchak, V.M.(2.5) 20 Grandell, D. (1) 288 Grant, J.T. (3) 225 Grattan, K.T.V. (1) 54,98 Graupner, W. (3) 476,477,485, 487,494

Gravel, D. (2.3) 134, 135; (2.5) 183, 184; (2.6) 255,256

Gray, D.H. (3) 421 Gray, R.L. (3) 809 Graarlevicius, J.V. (3) 107,568 Grebcnkin, S.Yu. (1) 217; (2.4) 47; (2.6) 43,44

Green, A. (3) 18,21,22 Greenbcrg, M.M. (2.6) 239,240; (2.7) 195,203

Greenhill, D.C. (3) 819 Greenland, P.T. (1) 13 Gregg, B.A. (4) 35 Gregoire, G. (1) 211 Gregori, A. (2.2) 17, 18; (2.4) 210 Greier, S. (3) 737

414 Greiner, G. (2.5)168 Grelier, S.(3)355,732,738,839 Grenier, S.(1) 411 Greuel, M.P. (3) 57 Gribkovskii, V.P.(1) 5 Griesbeck, A.G. (2.1)39,50,89; (2.2) 119, 120;(2.4)209,264; (2.5)157;(2.7)73 Grigoryants, V.M. (1) 501 Grimm, G.M. (2.6) 138, 185, 186 G r i m e , S.(2.2)102 Grineva, L.G. (2.5)81 Grishina, A.D.(2.7)36 Gritsan, N.P.(2.2) 137;(2.4)291; (2.7)207 Gritsenko, O.M. (2.7)26 Grivin, V.P.(2.7)176 Grobys, M. (1) 492;(2.6)222 Groenen, E.J.J. (1) 131,336 Gromov, S.P.(2.4)80,185,188; (2.6)29,30;(3) 386 Gromova, E.S.(2.7)26 Gronowitz, S.(2.6)277 Gross, P. (1) 498 Grosso, V.N.(1) 250 Grubbs, R.H. (2.4)182;(3) 155, 511 Grube, G. (1) 372 Gruber, H.J.(2.5)43 Grupa, W.(3)417 Grupp, A.(1) 372 Gryczynski, I. (1) 209,450,468, 482 Gryczynski, Z. (1) 209,450,482 Grzegorzewski, P.(2.3)104 Gu, H. (3) 282 Gu, J.-D. (2.4)154 Gu, L. (3)385 Guan, J.-Q. (2.4)278;(2.5)161; (3)587 Guang, D.H. (2.1)23 Guamieri, A. (2.3)60; (2.4)220 Gudipati, M.S. (2.5)157 Gudmundsdottir, A.D.(2.7)15 Guglielmetti, R.(2.4) 128, 132, 146-148;(2.6)72-75,77,78; (3)379 Guha, D.(1) 176;(2.1)34 Guha, M.(2.2)15 Guha, S.(3) 485 Guharey, J. (1) 175 Gui, H. (3) 286 Guido, J.E. (2.6)193 Guillaume, D. (2.2)70;(2.6)123 Guillaumont, D. (2.7)79 Guillemin, J.-C. (2.6)98 Guiotto, A. (2.2)80 Gulbinas, V. (2.5)23

Photochemistry Hamad, A.-S.S. (2.2)19;(2.4)2 Guldi, D.M. (1) 325327,343, Hamada, F. (2.4) 109 356,358-360;(2.5) 12, 100Hamada, K.(2.2)133 104,107,109 Gum,M.M. (2.1)19;(2.5)27,53 Hamada, T.(1) 256;(2.5)82 Hamaguchi, H.(2.4)259,26p; Gunduz, N.(3) 241 (2.6)62;(2.7)33 Gunnlaugsson, T.(1) 403 Hamanoue, K. (1) 174;(2.2)136, Gunter, M.J. (1) 42 138;(2.4)199;(2.5)3,34,63, Guo, J. (1) 84;(3) 468 64 Guo, J.S.(3) 465 Hamadci, T. (2.2)75 Gurge, RM.(3)502 Hamblett, I. (1) 186;(2.5)141 Gunadyan, G.G. (1) 479 Hamed, G.R. (3) 162 Guseva, L.R (3) 110, 170 Hammach, A.H.(2.2)35;(2.6)91 Gust, D. (1) 371;(2.5)30, 1 1 1, Hammarstroem, L. (1) 299,300, 180,233 398 Gustafson, R. (3)619 Hamplova, V. (2.5)170 Gustavsson, T. (2.5)23 Han, J.S.(3) 403 Gutenberger, G.(2.5)185 Han, J.Y. (3) 479 Gutierrez, A.R. (I) 101 Han, K.-L. (2.3)65; (2.7)154, Guy, D.M.H. (3)5 13 162-164 Guymon, A.C. (3)258 1% S.H. (3) 376 Guyot, G. (2.7)167 Han, S.J. (2.7)223 Guzman, J. (3) 234 Hanabusa, K.(1) 274;(3) 500 Guznturk, K.S.(3) 575 Hanafusa, A. (4)40 Hanashiro, I. (3)648 Hancock, G.(3) 308 Ha,C.S. (3) 77,748,758 Handjieva, S.(3)815 Ha,J.D. (2.3)72 Hanemann, T.(3) 173,247 Haacke, G. (3) 89 Hanley, Q . S . (1) 466,470 Haarer, D.(3) 261,721 Hanninen, P.E.(1) 489 Haba, 0.(3) 185 Hansen, R.L.(3) 642 Habibi, M.H. (2.1)58, 59;(2.7) Hanson, P.(2.7)72 70,71 Habicher, T. (1) 363 Hara,M. (4)2 Hara, T. (2.4) 133 Habuchi, S.(3) 572 Hara, Y. (4)24 Hachimori, A.(2.1)84;(3)592 Harabagiu, V. (3) 21 1 Hackbarth, S.(1) 365;(2.5)105, Hamda, K. (1) 275 130 Harada, M. (2.6)323 Hackett, J. (2.5)84 Harada, N.(2.3)2 Hadjiantoniou-Maroulis, C.P. Harada, Y.(2.5)201;(3)694 (2.4)309;(2.6)162 Haraguchi, M. (3)401 Hadjiarapglou, L.P.(2.2)1 1 1; Haraldsson, T.(3)83,238 (2.4)178 Harie, G. (2.6)74 Haenni, W. (2.7)28 Harigaya, K. (3)486 Hafiz, H.R (2.2)7 Harkness, B . R (3)255,420 W e r , A. (3)10 Haga, N.(2.3)116, 117;(2.4)276 Harms, K. (2.3)110 Harms, P.D.(1) 463,464 Hageman, H.J. (2.6) 181, 182 Hamman, A.(1) 271,272,278, Haggi, E. (2.5)222 301,373;(2.5)13,44,45, Hahn, C. (2.7) 1 1 132;(2.6)27 Hahn, J.R (2.7)223 Harris, J.M. (3)642 Hahn, J.W. (2.7)206 Harrison, N.T. (3) 464 Hahn, P. (3) 618 Hamson, R.J. (2.2)21 Halim, M. (3)517 Harsui, T.(2.2)10 Hall, G.E. (2.7)147 Hartl, I. (1) 496 Hall, H.K. (3)93 Hartland, G.V. (2.5)210;(3)836 Halliday, D.P.(4)41 Hartmann,P.(1) 376 Halloran, J.W. (3) 243 Hartog, F.T.H. (1) 61 Ham, S.K. (3) 426

Author Index

Hartshorn, M.P.(2.4)3,227-229; (2.7)107 Harwood, H.J. (3)57 Hasato,A. (2.7)60 h e , H. (3.)670 Hasegawa, K.(3)620 Hasegawa, M. (3) 160,548 Hasegawa, S.(3) 694 Hasegawa, T.(2.1)33,41,42; (2.5)10,61;(3)401 Hashem, A.I. (2.2)19;(2.4)2 Hashemzadeh, M. (3)550,553 Hashida, I. (2.3)71 Hashipchi, T.(2.2) 10 Hashimoto, M. (2.6)96 Hashimoto, S.(1) 259,260;(2.5) 77, 114 Hashimoto, T. (3)542 Hashizume, D.(2.3)94 Haslinger, E. (2.2)58 Hassan, M.E. (2.6)249 Hasselgren, C.(3)83,238 Hatakenaka, K.(2.6)308 Hatakenaka, S. (2.6) 159 Hatch, RK.(1) 341;(2.5) 171 Hatta, A. (2.5)235;(2.6)257; (2.7)88,89 Hatta, H. (2.6)122 Hauck, R.(2.2)140;(2.5)37 Haupl, T.(1) 227;(2.4)140;(2.6) 67 Hausch, F. (2.6)243 Hausselt, J.H. (3) 173,247 Haussling, L.(3)208 Hay, C.M. (2.7)105 Hayakawa, M. (4)53 Hayashi, H. (2.6)308,312;(2.7) 179;(3)401,515 Hayashi, M. (3) 542 Hayashi, S.(2.5)195 Hayashi, T. (3)27 Hayashi, Y.(3) 407 Hayataka, K.M. (3) 696 Haylett, N.(3)464 h i , L. (1) 37;(2.3)22;(2.4)1 He, B. (2.5) 83;(3) 391,413,432, 643 He, B.-L. (2.4) 100;(3)433 He, G.-H. (2.3)65 He, G . Q . (4)30 He, G.-Z. (2.7) 154, 162-164 He, J. (2.4)222;(3) 662,663 He, Q.(3)593 He, X.(1) 160;(2.5)42 He, Y.(3) 105, 127,297,326,451 Heaton, S.N.(2.7)220 Hecht, J. (2.1)45 Hecht, S.(2.2)84

Heckroth, H. (2.1)39,89 Hedstrom, J.F. (1) 456 Heeger, A.J. (3) 322,478,483, 528 Hegarty, A.F. (2.6)169;(2.7)21 1 Hegedus, L.S. (2.1)15;(2.5)22 Heidbreder, A. (2.3)91 Heikkilii, A. (2.7)208 Heilmann, A.(3)630 Heinemann, F.W. (2.6) 116 Heinrici, C.(2.4)305;(2.6)245247 Heinz, K.J. (1) 437 Heit, G.(2.5)133 Heitner, C.(3)745,792 Heitz, W.(3) 116 Held, G.A. (3)260 Hell, S.W. (1) 469,489 Heller, B. (2.4)6;(2.6)4 Hellman, M.D.(2.1)15;(2.5)22 Hellrung, B. (2.7)16 Helms, A.M. (2.3)70 Helms, J.H. (3) 688 Hempel, G.(2.6)115 Henkel, G. (2.6)103 Henling, L.M. (2.4)182;(3) 155 Hennig, L. (2.5)187 Henon, S. (2.1)72 Henrich, M.(1) 321 Henry, D.(2.4)121,122,164 Hepworth, J.D. (2.4)123,124 Herbelin, S.E. (1) 235 Herbertz, T. (2.3)73,74;(2.5) 167 Herczegh, P. (2.2)41 Herden, V. (3) 565 Herges, R.(2.3)105 Hering, P. (2.7)152 Herman, P. (1) 141;(2.3)97 Hermann, A. (1) 135,310 Hermann, C.(2.4)323;(2.7)196 Hermann, R.(1) 227;(2.4)140; (2.6)67 Hermetter, A. (3)487 Heron, B.M. (2.4)123,124 Herpich, R.(2.2)132 Hershberger, J.F. (2.7)146 Hertel, D.(3) 474,541 Hervet, H.(3) 655 Herzig, C.(3) 136 Hess, G.P. (2.4)12 Heyns, A.M. (3) 837 Hichour, M.(2.2)45;(2.4)251; (2.6)52 Hidayat, R (3) 5 14 Hide, F. (3) 478 Hierle, R.(2.6)282 Higashida, S.(1) 280,361;(3)

415 610 Higgitt, C.L. (2.7)97 Hikida, T.(1) 212 Hild, M. (2.4) 197 Hilgenfeldt, S.(1) 12 Hilgeroth, A. (2.6)115, 116 Hill, D.J.T. (3) 761 Hill,S.C. (1) 129, 132 Hill,T.J. (1) 186;(2.5)141 Hillenkamp, M. (2.7)67 Hintz, S.(2.3)92;(2.4)244;(2.6) 305 Hippler, R. (1) 7 Himi, K.(2.7)24 Hiraishi, T. (2.5)86;(4)11 Hirama, M. (2.3)103 Hiramatsu, T. (4)49 Hirano,T. (2.3)94, 100;(2.4) 235;(2.5)201 Hirao, K.(2.6)264;(2.7)122 Hirao, T.(2.3)44;(2.6)321 Hirasaka, T.(1) 352;(2.5)14,98; (2.6)210 Hirata, I. (3) 727 Himta, M.(3)295,296 Hirata, Y.(1) 138, 139,252;(2.5) 212;(2.6)208 Hiratsuka, H. (1) 152 Hirayama, K.(2.5)218 Hirohashi, R (3) 53 Hirohata, M. (3)5 14 Hirokawa, R.(2.6)235 Hiromitsu, I. (1) 369 Hiroshi, K. (3) 610 Hirota, K.(2.2)5 1 Hirota, N.(1) 444;(2.7)20 Hirota, Y.(3) 203 Hirsch, A. (1) 365;(2.5)105 Hirsch, R.(2.6)240;(2.7)203 Hirt, J. (2.4)264 Hisada, K. (3) 566 Hisamichi, K.(2.5)218 Hisbeth, M.V.(2.6)161 Hishigaichi, Y.(2.6)324 Hissler, M.(1) 271,272,278,301, 373;(2.5)44,45 Hitomi, K.(2.6)125 Hizal, G. (2.6)199 Hoang, M.(2.3) 123;(2.4)5 1 Hobisch, J. (3) 131 Hobley, J. (2.6)69 Hochstrasser, R.M.(1) 324 Hodak, J.H. (2.5)210;(3) 836 Hoecker, H. (3) 793 Hoegemann, C.(2.5)116 Hoff, A.J. (1) 87 Hoflinan, T.Z.(2.1)25 Hoffman, Z.(1) 247

416

Hof€mann, B. (3) 41 1 Hoflinann, K. (3) 364 Hoffmann, N.(2.2) 22,23; (2.4) 55, 186; (2.6) 144

Hofkens, J. (1) 3 10 Homer, D. (2.2) 58 Hofstotter, M.(3) 195 Holderich, W.F. (2.3) 43 Holme, N.C.R (3) 399 Holmes, A.B. (3) 460,535 Holmlin, R.E. (1) 388 Holstraat, J.W. (1) 452 Holten, D.(1) 161, 162, 164 Holzer, W. (3) 475 Homilius, F. (3) 630 Homma, H. (1) 149 Honda, K. (2.2) 11 Hong, B. (1) 3 12 Hong, S.I. (3) 529 Hong, S.W. (2.3) 21; (2.4) 33; (2.6) 18

Hong, X.(3) 7 Hong, 2.(3) 480 Honma, H. (2.5) 226 Hontani, M.(4) 60 Hopkinson, A.C. (2.3) 123; (2.4) 51

Hoppmeier, M. (1) 192; (3) 504 Horai, T.(3) 500 Hore, P.J. (1) 87; (2.6) 205 Horhold, H.H. (3) 541 Hori, T.(2.4) 16 Hone, K. (2.4) 61; (2.6) 264; (3)

307,317,419,420,446,447, 722,725 Horinaka, J. (3) 611 Horn, E. (2.6) 290; (2.7) 186 Homek, G.(4) 31 Homer, G.(2.4) 283 Homer, J.H. (2.1) 6,79, 80; (2.6) 188, 189 Homfcldt, A.-B. (2.6) 277 Homyak, G. (1) 37; (2.3) 22; (2.4) 1 Horsburgh, L.E. (3) 5 17 Horspool, W.M. (2.6) 151 Horvath, A. (2.5) 89; (2.6) 167 Hoshi, Y.(2.3) 93 H o s h i ~H. , (2.4) 50 Hoshino, M.(1) 3 11; (3) 670 Hosoda, K.(1) 329 Hosoda, N. (2.3) 33; (2.4) 90 Hosokawa, H. (2.5) 127 Hosomi, H. (2.1) 3 1 Hotta, Y.(1) 306 HOU,D.-F. (2.3) 123; (2.4) 51 HOU,J.-Y. (4) 45 Hou, L. (3) 411

Houari, S.(1) 111 Houk, K.N. (2.2) 35; (2.6) 91 Houle, C.E.(3) 267 Houmam, A. (2.3) 76 Howard, J.A.K. (2.3) 130; (2.4) 54; (2.6) 29; (2.7) 39

Hoyle, C.E. (3) 79,81-84 Hozumi, Y. (2.3) 19; (2.4) 31; (2.6) 19

Hrdovic, P.(3) 310,315,316,

676,802 Hsaio, T.-Y. (2.5) 179 Hsieh, B.R. (3) 467,470 Hsu, J.H. (3) 466 Hsu, T.C.(2.3) 63; (2.7) 157 HSU,W.-L. (2.3) 64 Hu, B. (3) 502,506 Hu, C. (2.5) 223 Hu, G.G.(2.2) 139 Hu, H. (3) 623,741 Hu, I. (2.5) 207 Hu, J. (2.4) 14 Hu, Q.S. (3) 526 Hu, S. (2.2) 93-95; (2.5) 47.59; (3) 133,220,221,228 Hu, X.(2.5) 206,207 Hu, Y.(2.4) 257; (2.5) 207; (2.6) 63; (3) 151 Hu, Y . 4 . (1) 237; (2.5) 88 Huang, B. (2.4) 144; (3) 543 Huang, C.(1) 346; (2.5) 176 Huang, C.C. (3) 557,574 Huang, C.-H. (1) 347; (2.5) 106, 175 Huang, D. (2.5) 83 Huang, H. (2.7) 14; (3) 374 Huang, H.W. (3) 317,419,420 Huang, J. (3) 95,200,209 Huang, J.D. (2.3) 59; (2.7) 129; (3) 443 Huang, K. (3) 623 Huang, S.(3) 200,288 Huang, S.P.D.(2.3) 67 Huang, S.Y.(4) 35 Huang, W. (3) 413,432,527,528 Huang, W:Q. (2.4) 100; (3) 433 Huang, X.(2.1) 80; (3) 95,209, 374,625 Huang, X.-Y. (2.4) 206; (2.6) 105 Huangpu, L. (3) 342 Hubbell, J.A. (3) 179 Huber, J.R. (2.7) 142, 147 Hubig, S.M.(1) 86; (2.2) 115, 128-130; (2.5) 192 Huch, V. (1) 237; (2.5) 88 Hudlicky, T.(2.3) 120 Hug, G.L. (1) 490; (2.5) 74 Hugel, G.(2.4) 55; (2.6) 144

Photochemistry Hughes, F.J. (2.4) 112 Hummel, K. (3) 131, 195 Hummelen, J.C. (3) 494; (4) 50 Humphry-Baker, R (2.5) 215 Hung, S.-C. (2.5) 233 Hunt, J.K. (1) 159,263; (2.4) 152; (2.6) 87

Hurtubise, R.J. (1) 381 Hussein, A.M. (2.6) 285 Hussey, D.M. (3) 546 Hutchinson, I. (3) 119 Hutchinson, R.A. (3) 67 Hutchison, K.(1) 245 Hvilsted, S.(3) 399 Hwang, C.H. (2.7) 223 Hwang, D.W. (2.3) 63; (2.7) 125,

127, 156, 157 Hwang,H. (3) 154 Hwang, K.C.(1) 355 H w g , K.-J. (2.6) 197; (2.7) 117 H w g , M.-L. (2.3) 64 Hwang, S.J. (2.3) 61 Hwang, Y.L.(1) 355 Hy, Y.(2.5) 206 Hyder, A. (3) 153 Hynd, G. (2.6) 151 Hynninen, P.H. (1) 288 Hyslop, A.G. (1) 284

Ibraev, N.Kh. (1) 57; (3) 617 Ibrahim, Y.A.(2.4) 57; (2.6) 164 Ichihashi, Y. (2.5) 121-123 Ichikawa, M.(1) 364 Ichimura, K. (2.4) 46, 101; (2.6)

32,33; (3) 75,3 14,398,407, 424,442 Ichimura, T.(1) 152; (2.1) 29; (2.5) 57; (2.7) 161 Ichinose, N. (2.3) 70,71 Icil, H.(2.5) 159 Icil, S.(2.5) 159 Idada, Y.(3) 727 Igarashi, T.(2.2) 14 Iguchi, M. (2.4) 259,260; (2.6) 62 Ihm,H. (2.7) 166 Ihmels, H. (2.1) 27; (2.4) 28 1, 292; (2.6) 113. 150, 187 Ikada, E.(3) 716 Ikawa, T.(3) 640 Ike, T.(3) 3 Ikeda, A. (3) 201 Ikeda, H. (1) 34; (2.3) 76.93 Ikeda, M. (2.1) 49; (2.4) 175,303; (2.6) 165, 166 Ikeda, S. (4) 24 Ikeda, T.(3) 259,367,439,440 Kkeda, Y.(2.7) 168

Author Index

Ikegaya, K.(3) 118 Ikeue, K.(2.5) 121-123 Ikoma, K.(2.5) 160 Ilharm, L.(1) 60 Il’ichev, Y.V.(2.6) 220

Imasawa,A. (3)

161 Imahori, H. (1) 270,280,361, 366; (3) 610 Imai, K.(1) 104,149 Imai, Y.(2.3) 94; (3) 382 Imakubo, K.(2.5) 136, 137 Imakubo,T.(2.4) 259,260; (2.6) 62 Imams, F. (2.6) 283 Imamura, K.(2.4) 107 Imamura, M. (3) 70 Imamura, T. (1) 364 Imanishi, Y.(1) 246; (2.5) 79; (3) 406 Imase, T. (2.3) 30; (2.4) 91 Imayasu, T. (3) 620 Imbusch, G.F. (1) 4 Inaki, Y. (2.2) 69; (2.6) 117, 118 Inamura, 1. (2.3) 51, 90; (2.4) 193, 194,200 Inbar, M. (1) 409 Indig, G.L. (3) 827 Ingram, D.W. (2.5) 131, 132 Inokuma, S. (2.3) 106, 107; (2.4) 183 Inoue, H. (2.3) 132; (2.4) 316; (2.6) 184 Inoue, T. (1) 126 Inoue, Y. (2.3) 4-6; (2.4) 63 Inouyc, M. (1) 156 Ion, R.M.(4) 32 Ionescu, A.Th. (3) 152 Iovane, M. (3) 152 Ine, M.(2.2) 125; (2.3) 23-26,2932,35,36,38,39; (2.4) 13, 82, 83,85-87,89,91,93-95, 161,233; (2.6) 35; (3) 3,300, 312,390 Iriyama, K.(1) 308 Irngartinger, H.(2.1) 36; (2.2) 132 Isaji, H. (2.1) 69 Isela, G.M.(1) 251 Ishibashi, H. (2.6) 166 Ishida, A. (2.4) 269 Ishiguro, H. (2.6) 196 Ishiguro, K.(1) 332; (2.1) 60; (2.3) 101; (2.5) 143,235; (2.6) 257 Ishii, H. (2.3) 94, 100; (2.4) 235 Ishii, J.I. (3) 548 Ishii, T. (3) 775 Ishii, Y. (2.4) 252; (2.6) 65 Ishikawa, M.(1) 91; (2.6) 304

417

Ishikawa, T. (2.3) 39; (2.4) 85 Ishino, K.(2.4) 107 Ishizaka, S.(3) 572 Ishizaka, W. (3) 336,564 Ishizu, Y.(3) 78 Isoda, J. (3) 296 Ito, H. (2.7) 168 Ito, J. (2.3) 15 Ito, 0. (1) 242,339,344,345,

347,348,350-353; (2.5) 14, 97-99, 106,110, 173,236; (2.6) 138, 186,210,258,287; (2.7) 180; (3) 54 Ito, S. (3) 566,611 Ito, T. (1) 369; (2.4) 179; (2.6) 37, 122,263 Ito, Y.(2.1) 30-32; (2.4) 236; (2.5) 49,92; (2.6) 228; (3) 406 Itoh, K. (2.6) 96; (3) 253 Itoh, M. (1) 497 Itoh, S. (2.5) 98; (2.6) 10, 210 Itoh, Y. (2.1) 84; (3) 592 Ivanchenko, A.G. (3) 614 Ivanenko, M.(2.7) 152 Ivanov, V.B.(3) 30,621,753 Ivanov, V.V.(3) 110,62 1 Ivanov, Y.V.(2.7) 176 Iwai, K.(3) 657 Iwai, S. (1) 302; (2.6) 125 Iwama, T. (2.4) 179; (2.6) 263 Iwamatsu, S. (1) 329 Iwamoto,H. (2.2) 133; (2.6) 324 Iwamoto, T. (3) 706 Iwamura, H.(2.7) 25 Iwamura, M.(2.6) 235 Iwasaki, F. (2.3) 94; (2.6) 308 Iwata, K.(2.4) 259,260; (2.6) 62 Iyengar, R (3) 797 Izuoka, A. (2.7) 48

Jaaskelainen, T. (2.4) 162 Jackiw, J. (3) 5 10 Jackson, C. (3) 67,405 Jackson, W.M. (2.3) 57.59; (2.7) 129

Jacobson, A.F. (1) 414 Jaquet, M. (3) 841 Jain, S. (2.6) 126, 127; (3) 156 Jakubek, V.(3) 225,226 James, T.D. (1) 390 Jancar, J. (3) 235 Janda, K.D.(2.1) 25 Janes, R. (3) 835 J a g , D.-J. (2.1) 35; (2.4) 294 Jang, M.S.(3) 454,479,491 Jankova, K.(2.7) 53; (3) 280

Jannach, S.H. (2.3) 81 Janossy, I. (1)218; (2.6) 45 Janet, J.-M. (1) 330,335; (2.5) 169,229; (3) 610

Jansen, L.M.G. (3) 819 Jansen, RA.J. (4) 50 Jao, T.C. (3) 773 Jardon, P.(2.5) 140; (3) 98 Jaschke, A. (2.6) 243 Jayaknshnan, A. (3) 164 Jayan, C.N. (2.4) 64 Jayanthi, G. (2.5) 62; (2.6) 288 Jayanthi, S.S. (2.6) 135 Jayaseham, J. (3) 763 Jaycox, G.D.(3) 405 Jazwinski, K.(3) 4 17 Jeandon, C.(2.2) 28 Jefford, C.W.(2.5) 166 Jeffreys, B.(2.5) 131, 132 Jelczko, F. (1) 124 Jcliazkova, B.(2.4) 149; (2.6) 76; (3) 363

Jen, A.K.Y. (3) 456 Jeng, G.-Y. (2.5) 91 Jenkins, C.A. (2.5) 181 Jenkins, RD. (1) 72 Jenkins, S.I.(2.6) 297 Jcnks, W.S. (2.5) 140 Jenneskens, L.W. (1) 449 Jennings, P. (3) 445 Jenniskcns, H.G. (2.7) 114 Jensen, K.F.(3) 473,770 Jeong, J.I. (3) 596 Jeong, Y. (2.4) 274 Jeoung, C.S. (3) 479 Jeremic, M. (3) 742 Jerome, R. (3) 595 Jessop, J.L.P. (3) 223 Jewel, J.T. (3) 226 Ji, R.-Y. (2.4) 154 Jiang, B. (3) 671 Jiang, C.(3) 672 Jiang, D.-J. (2.6) 47 Jiang, D.-L. (4) 12,33 Jiang, H.-L. (2.4) 154 Jiang, J. (1) 346; (2.5) 176 Jiang, M.(3) 628 Jiang, Q.(2.3) 18; (2.4) 272 Jiang, X.D.(3) 24 Jiang, Y.(2.3) 20; (2.6) 26 Jiang, 2.(2.2) 131; (2.5) 162 Jian-Hua, X. (2.5) 223 Jianji, A, (3) 162 Jimenez, M.C. (2.4) 241,311 Jimeno, M.L.(2.4) 243 Jimincz, F.D.(2.2) 113, 114 Jin, J. (3) 462 Jin, K.(2.2) 86

418

Jin, M.-Z. (2.6) 211 Jin, S.(1) 346 Jin, X.(3) 593 Jing, X.(3) 480 Jiongxin, 2.(3) 292 Jipa, S.(3) 330-332, 334 Joao Melo, M. (1) 178 Jockusch, S. (2.1) 11; (2.6) 318 Jocys, G.J. (2.7) 38 Jodicke, K.(2.1) 45 Joe, H.S.(2.2) 13 Joekes, I. (3) 547 J o h , T.W. (1) 389 Johansson, J. (1) 451 Johansson, L.B.A. (1) 437 Johansson, T. (1) 45 1 Johnasen, P.M. (3) 399 Johne, P.(2.4) 283 Johnson, A. (2.7) 94 Johnson, B.A. (2.1) 28; (2.5) 66 Johnson, M.A.(1) 26 Johnson, M.R(1) 42 Johnson, R.E.(3) 224 Johnson, S.A. (3) 669 Johnson, T.J. (3) 180 Johnsonbaugh, D.S. (3) 645 Johnston, L.J. (2.3) 113, 123; (2.4) 51; (2.5) 115; (3) 740

Jolif'fe, K.A. (1) 295; (2.5) 87; (2.6) 218 Jones, A.C. (3) 445 Jones, D.C. (3) 746 Jones, G., I1 (2.5) 219; (2.6) 206 Jones, J.R (3) 688 Jones, P.G.(2.3) 105 Jones, S.E.(1) 145 Jones, W.E. (3) 671 Jonson, H.(1) 113; (2.3) 85 Jonsson,G. (2.7) 53; (3) 280 Jonsson, S. (3) 79, 81-84,238, 267

Jonusauskas, G. (2.6) 225 Joo, H.S.(2.5) 196; (2.6) 230 Jordan, G.K. (3) 44 Jorg. J.F.W. (2.4) 11 Jortner, J. (1) 19,20; (2.5) 1 Joselevich, E. (1) 42 Joseph, J.C. (2.6) 98 Jost, P. (1) 27 1,272; (2.5) 44,45 Jouvet, C. (1) 21 1 Jovin, T.M. (1) 466,470 Joy, A. (2.2) 91 Juan, S.H.(3) 668 Juha, L.(2.5) 170 Julia, L. (2.6) 277 Julliard, M. (2.5) 134 Jun, K. (2.2) 13; (2.5) 196; (2.6) 61

Jun, P.(3) 117 Jung, A.H. (2.1) 8; (2.4) 11 Jung, I.D. (3) 748 J u g , J.-A. (2.5) 194 J u g K.-H. (2.7) 206 Jung, S.D.(3) 491 Jung, T. (3) 123, 169,174,207 Junge, D.M.(2.6) 46,48; (3) 553, 554

Jungner, H.(1) 405 Jungwirth, P.(1) 69 Juris, A. (1) 281; (3) 555 Jursenas, S.(2.5) 23

Kabalnova, N.N. (2.5) 147 Kabatc, J. (3) 43,560 Kable, S.H.(2.7) 148 Kabuto, C. (3) 345

Kaczmarek L.(1) 179; (2.6) 156

Kadashchuk, A. (3) 337 Kado, Y. (2.5) 182 Kadodwala, M.(2.7) 114 Kadokawa, J . 4 (2.4) 16 Kador, L. (1) 127 Kaeyama, A. (3) 6 Kaganer, E.(1) 42 Kahl, J.D. (2.6) 239; (2.7) 195 Kahq 0. (1) 112

Kai,Y. (2.2)69;(2.6) 117, 118 Kaiho, A. (3) 442

Kaimori, Y. (1) 369 Kaise, T. (3) 649 Kaiser, M.(3) 346 Kaizu, Y.(1) 306,307,315 Kajii, Y.(1) 68; (2.1) 9 Kajino, Y.(3) 769 Kajita, T. (3) 673 Kajzar, F. (3) 583 Kakareka, J.P. (2.6) 309 Kakihana, M.(4) 24 Kakimi, S. (3) 397 Kakiuchi, N. (2.4) 287; (2.6) 204 Kakiuchi, T. (1) 484 Kako, M. (2.6) 308 Kakuma, S.(2.6) 308 Kakusawa, N. (2.5) 197; (2.6) 325 Kalaus, G. (2.4) 8 Kalechits, 1.1. (3) 3 18 Kalinin, A.V. (3) 683,684 Kaliteevskaya, E.N. (1) 222 Kallitsis, I. (3) 320 Kalogerakis, K.S.(1) 32 Kalosha, 1.1. (1) 123; (2.7) 182 Kaluzny, B.D.(2.6) 193 Kam, N. (3) 336 Kamachi, M.(3) 653 Kamachi, T.(2.5) 86; (4) 10, 11

Photochemistry Kamada, T. (2.2) 44 h e , T. (3) 425 Kamat, P.V. (1) 326,356; (2.5) 12,102 Kamat., V.P.(2.2) 105; (2.4) 3 13 Kamata, N.(3) 564 Kambe, S.(2.3) 25 Kamei, Y.(2.1) 33 Kamerling, J.P. (2.3) 49 Kameyama, A. (3) 402,570 Kamino, S.(2.4) 226 Kamiya, K.(2.4) 234; (2.6) 268 Kamp, A.F. (1) 474 Kamphaussen, C.(2.6) 152 Kampmeier, J.A. (2.6) 274 Kamusewitz, H.(3) 281 Kanada, T. (1) 497 Kanatomi, H.(2.4) 34 Kanaue, T. (2.3) 41 Kanaawa, A. (3) 259,367,439, 440 Kanbara, T. (3) 5 15,620 Kaneco, S.(2.5) 119, 124; (4) 28, 29 Kaneda, T. (1) 280,361; (3) 610 Kaneko, M.(4) 13 Kaneko, T. (2.3) 103 Kaneko, Y.(2.1) 87; (2.2) 95; (2.5) 46; (2.6) 174; (3) 114, 133 Kanemoto, K. (2.6) 54 Kang, E.T. (3) 557,574 Kang, H.(2.7) 223 Kang, J.S. (3) 5 18,524 Kang, T.J. ( 1 ) 190; (3) 529 Kang, Y.N.(1) 220 A.R (3) 196-198 Kano, G. (2.1) 30,32; (2.4) 236; (2.5) 49 Kantor, M.M.(2.7) 46; (3) 49 Kao, S. (1) 441 Kar, A.K. (3) 5 13 Karakostas, N.(2.6) 171 Karasawa, T. (3) 265 Karasev, V.E. (3) 660,661 Karasz, F.E. (3) 502,506 Karatsu, T. (2.1) 84; (3) 39 Karaymi, K. (3) 598 Karg, S.(3) 516 Kami,Y.(1) 310 Karolczak, J. (1) 448 Karthikeyan, M.(2.4) 216 h b e , I. (4) 58 Karyakina, L.N. (2.4) 42; (2.7) 49, 50 Kasatani, K.(2.3) 25 Kashihara, S. (2.5) 221; (2.6) 177 Kasim, R.K.(3) 537

AufhorIndex Kassab, E. (2.5) 76 Kaszynski, P. (1) 457 Kataoka, T.(2.4) 179; (2.6) 263 Katch, R (1) 187 Kather, K. (2.1) 45 Kato, C.(2.5) 19 Kato, M.(2.2) 1 Kato, N. (2.2) 92; (2.3) 26,30;

(2.4) 83,91, 198 Kato, S.(3) 418 Katogi, S.(3) 79 Katoh, R (1) 188 Katsuhara, S. (3) 627 Katsumura, S. (2.5) 160 Kauffmann, H.F. (1) 447 Kaur, S. (3) 115 Kautek, W. (3) 728 Kavaliunas, R (3) 107 KavP.V. (2.2) 15 Kawabata, Y. (2.4) 133 Kawada, A. (4) 36 Kawaham, S.(1) 322 Kawahata, T.(2.5) 189 Kawai, A. (2.1) 9 Kawai, H.(2.5) 93,221; (2.6) 177, 178; (3) 829 Kawai, K. (2.2) 66 Kawai, S. (3) 733 Kawai, S.H. (2.3) 37; (2.4) 97 Kawai, T.(1) 357 Kawakami, M.(3) 829 Kawakami, Y.(1) 157; (2.4) 77 Kawaminami, M. (2.2) 89; (2.4) 211 Kawamura, F.(3) 733 Kawasaki, M.(2.7) 135 Kawasaki, 0. (4) 49 Kawashima, T.(2.4) 136 Kawashima, Y. (2.6) 264 Kawata, M. (4) 58 Kawata, S.(1) 410 Kawato, T.(2.4) 34 K a ~ t s u k i N. , (3) 274-276 Kawauchi, S. (2.3) 30; (2.4) 91 Kawazoe,N.(3) 161 Kawazu, M. (3) 586 Kawazumi, H.(1) 126 Kawski, A. (1) 209 Kayano, A.(2.1) 90,91; (2.4) 172; (2.6) 271 Kayukshto, V.N. (1) 317 Kazarin, L.A. (1) 267 Ka;tmaier, P.M. (3) 412 Kaz'mitski, V.A.(1) 3 17 Ke, Y. (2.2) 124; (2.4) 155 Keating, A.E. (2.1) 37; (2.6) 3 Kebede, N.(2.6) 128 Keck, J. (3) 796

419

Keglevich, G. (2.4) 310; (2.6) 319; (2.7) 214

Keil, E. (3) 175,216 Keim, W. (2.5) 152 Keller, S.W.(3) 669 Kelley, J.M.(1) 44 Kellogg, RM. (2.3) 27,28; (2.4) 81,84

Kelly, J.F.D. (2.2) 15 Kelly, J.M.(2.7) 99 Kende, A.S. (3) 430 Kenmotsu, N. (2.6) 3 11 Kennedy, G.R. (2.7) 11 1 Kennedy, K.C.(3) 87 Kensy, U. (2.7) 116 Kepert, C.M.(2.7) 95 Ke-Qing, L. (2.5) 223 Kern, W. (3) 131,195 Kerst, C. (2.3) 136; (2.6) 297 Kerth, J. (2.4) 58 Keshavan, B. (1) 392 Kessler, A. (2.7) 116 Kevan, L. (2.5) 194 Keyes, T.E. (1) 294 Keyser, S.A. (3) 768 Khairutdinov, R.F. (1) 159,263; (2.4) 152; (2.6) 87

Khdafy, J. (2.4) 307; (2.6) 191 KhaIeel, A. (2.7) 94 Khalizov, A.F. (2.5) 147 Khan, A.R (3) 97 Khan, A.U. (1) 341,367; (2.5) 171

Khanfer, M.F. (1) 423 Khanmamedova, A.K. (2.4) 290; (2.6) 203

Kharlamov, B.M. (3) 721 Kharlanov, V. (1) 177; (2.4) 73;

(2.6) 155 Khatyr, A. (1) 373 Khavina, E.Yu. (3) 30 Khayatpoor, R (2.7) 96 Khazova, G.O. (2.7) 36 Khelifa, B. (1) 111 Khoa, D.X.(1) 5 1 Khodykin, O.V.(3) 721 Kholer, D.R. (3) 328 Kholi, P. (3) 80 Khoudyakov, D.V. (2.3) 12 Khranovskii, V.A. (3) 46 Khriachtchev, L. (2.7) 208 Kiatkamjornwong, S. (3) 687 Kido, Y. (2.2) 125; (2.4) 87 Kiguchi, M. (3) 798 Kijima, M.(3) 431 Kilin, D. (1) 298 Kilin, S.Y. (1) 130 Kim, A.H.(2.1) 67

Kim, A.R (2.2) 127; (2.4) 170;

(2.5) 73 Kim, B.D. (2.4) 169 Kim, D. (1) 161, 162, 164; (2.4) 274; (3) 154,479 Kim, D . 4 . (2.7) 206 Kim, H.(1) 55; (2.1) 35; (2.4) 294 Kim, H.K. (3) 472,522 Kim, H.S.(3) 178, 376 Kim, J. (1) 201 Kun, J.H. (2.2) 9; (2.3) 21; (2.4) 33, 177,282; (2.6) 18,94,95 Kim,J.J. (3) 178 Kim, J.M. (3) 403 Kim, J.S. (2.5) 194 Kim, K.D. (3) 472,522 Kim, K.J. (3) 277 Kim, M.S. (2.7) 165 Kim, 0.-K. (2.4) 88 Kim, S.(3) 510 Kim, S.H. (2.7) 138 Kim, S.I. (3) 518,524 Kim, S.K. (2.6) 197; (2.7) 117 Kim, S.S.(2.2) 126, 127; (2.4) 170; (2.5) 73 Kim, S.W. (2.2) 13; (2.5) 196; (2.6) 230 Kim, T. (3) 154,214 Kim, T.G. (2.1) 35; (2.4) 294 Kim, T.Y. (2.1) 35; (2.4) 294 Kim, W.H. (3) 758 Kim,Y. (1) 273; (3) 154 Kim, Y.H.(3) 479 Kim, Y.-S. (2.4) 248 Kimura, E. (2.2) 71; (2.6) 124 Kimura, H. (3) 397 Kimura, J. (3) 295 Kimura, K. (1) 274; (2.4) 104; (3) 37 1 Kimura, M.(3) 500; (4) 64 Kimura, S. (1) 246; (2.5) 79 Kimura, T. (2.3) 129 Kimura, Y. (2.4) 287; (2.6) 204 Kinbara, A. (2.4) 204; (2.6) 114 Kinbara, K. (3) 160 King, A.H.(2.3) 59 King, B.M. (3) 57 King, G.B. (1) 465 King, K.D. (2.7) 178 King, R.A. (2.7) 65 Kino, T. (3) 725 Kinoshita, I. (3) 43 1 Kinoshita, T, (3) 203 Kira, M.(2.4) 24; (2.6) 6,292, 296; (2.7) 172, 174, 177 Kirkby, S.J. (2.4) 221 Kirkpatrick, S.M.(1) 140 Kirschenberg, Th.(2.1) 3,56

420

Kirtany, J.K. (2.2)105;(2.4)313 Kisch, H. (2.4)283 Kishore, K. (3) 763 Kispert, L.D.(1) 357 Kitagawa, T. (2.7)24 Kitagawa, Y.(1) 93 Kitamura, N. (3)572 Kitano, M. (1) 369 Kitao, 0.(1) 80 Kitaura, K. (2.5)160 Kiuchi, K. (2.3)51;(2.4)194 Klabunde, K.J.(2.7)94 Klaerner, G.(3)5 16 Klahn, A.H.(2.7)97 Klapshina, L.G. (3)293 KIamer, F.G.(2.3)91 Klauss, R.(2.7)92 Klein, S.(1) 115 Kleinekathofer, U.(1) 298 Kleinman, M.H. (2.1)12 Klemm, E.(3) 461 Klessinger, M. (2.1)78;(2.7)4, 74 Kletskii, M.E. (2.6)20 Kleverlaan, C.J. (2.7)218,219 Kleyn, A.W. (2.7) 114 Kliesch, H.(2.5) 130 Klimant, I. (1) 383,395 Klimenko, L.S. (2.2)137;(2.4) 291 Klink, M. (2.6)141 Klooster, W.T. (2.6)309 Klop, J.M. (3) 696 Klopffer, M.H. (3) 654 Klubek,.K.P. (3)492 Kluge, T.(2.5)26 Klumpp, T. (1) 296 Klunder, A.J.H. (2.2)36;(2.5)94; (2.6)89 Kneas, K.A. (1) 382 Knepp, P.T. (2.7) 148 Knight, A.W. (1) 17 Knochenmuss, R (3) 762 Knijlker, H.-J. (2.7)92 Knoesen, A. (3) 441 Knoles, D.B. (2.4) 117 Knoll, H. (1) 214,215;(2.6)3840 Knoll, w.(3)441 Knor, G.(2.5)17 Knorr, S. (1) 372 Knyazhanskii, M.1. (2.6)20 Knyukshto, V.N. (3)615 KO,D.-H. (2.3)95;(2.4)26 KO, S.H.(2.4)289 KO,W.J. (4)24 Kobatake, S.(2.3)24,26;(2.4) 83,89

Kobayashi, J. (2.4)300;(2.7)3 1 Kobayashi, K. (2.4)259,260; (2.6)62 Kobayashi, M. (3) 425 Kobayashi, N. (1) 234;(3) 53 Kobayashi, S.(2.3)60;(2.4)220 Kobayashi, T. (3) 303,534 Kobryanskii, V.M. (3) 540 Koch, A.T. (3)464 Koch, R.(2.4)56; (2.7)61 Kochevar, I.E.(2.2)74 Kochi, J.K. (1) 86;(2.2)115, 128130;(2.5) 192;(2.6)176 Koda, S.(2.5)189 Koda, T.(3) 401 Kodaira, T.(2.4)105 Kodatis, K. (2.3)34;(2.4)92 Koeberg, M.(1) 21,491 Koehler, G. (1) 92;(2.6)221 Koehler, M.(3) 31, 169,174,207 Koenig, B.(1) 269 Koga, Y.(2.6)298,307;(2.7) 175, 190 Koh, E.(2.1)35; (2.4)294 Kohlbacher, P. (1) 376 Kohler, G.(1) 89,90 Kohler, J. (1)131 Kohler, M. (3) 123 Kohno, Y.(2.5)120 Koike, K. (4)5 Koinuma, H. (3)508 Kojima, M.(1) 229;(2.1)84; (2.3)9, 112;(2.4)23,269; (2.5)198 Kojima, R.(2.3)48,54;(2.4)166, 230;(2.6)99 Kojima, S.(2.3)132;(2.4)316 Kokubo, K. (2.5)92;(2.6)228 Koladiewicz, I. (2.3)104 Kolendo, O.Y.(1)155 Kolev, A.K.(3) 363 Kolev, K. (2.4)149;(2.6)76 Kollenq G. (2.2)104;(2.6)106 Kollmannsberger, M. (3) 656 Kolos, R (2.3)55 Kol'tsov, Y.I.(2.7)36 Komar, D.(1) 448 Komatsu, T.(1) 258;(2.5)230; (3)581 Korniyama, M. (2.6)37 Kornpa, K.-L. (1) 196,226;(2.7) 152 Konagai, M.(4)40,43,46 Konak, C.(3) 410 Kondilenko, V.P. (1) 60,62 Kondo, F.(3) 397 Kondo, J.N. (4)2 Kondo, K. (2.5) 142

Photochemistry

Kondo, M. (2.2)46;(2.4)250; (2.6)5 1 Kondo, S.(3) 60 Kong, F.(2.2)98,99 Kongou, C.(2.4)300;(2.7)Tl Konishi, K. (2.2)14 Konishi, S.(2.1)43;(2.4)239 Konishi, T. (1) 35 1 Kono, A. (2.3)93 Kono, T.(2.7)83 Konopasek, I. (1) 141;(2.3)97 Konovalov, V.V. (1) 357 Konovalova, T.A. (1) 357 Konstantinov, H.(3) 816 Konstantinova, T.(3) 816 Konya, K. (2.3)127;(2.7)159 Kopecek, J. (3)410 Kopeckova, P. (3) 410 Kopelman, R.(1) 471 Kopf, J. (2.2)8,26,30 Kops, J. (2.7)53;(3)280 Korncr, S.(2.2)77 Komev, A.N. (3) 293 Korobov, M.S. (2.6)154 Korolev, V.V. (2.5) 139 Koroteev, N.I. (3) 73 Korppi-Tommol, J. (1) 300 Korshunova, G.A. (2.7)26 Korth, H . 4 . (2.7)44 Kompoju, S.R(I) 219;(2.4)43; (2.6)40 Kosanic, M.M. (2.2)96 Kosbar, L.L.(3) 260 Kosch, U.(1) 395 Koshi, M. (2.7) 173 Koshiba, M.(2.4)50 K ~ ~ h i h a rS.-Y. a , (2.4)24;(2.6) 296;(2.7) 174 Koshima, H. (2.6) 119 Koshino, K. (1) 52 Kosikova, B.(3) 743 Kosloff, R (1) 56 Kosmrlj, B.(2.3)14;(2.7) 160 K o s s m a ~H. , (3) 750 Kostarev. K.G. (3) 110 Kostryukov, S.G. (2.6)273 Kotani, M. (I) 187,188 Kotelevskiy, S.1. (1) 438 Kotera, M.(2.4)301 Kdov, N.A.(1) 257 Koukos, E.G. (3)352-354 Koumura, N. (2.3)2 Kounosc,N.(1) 152 Koussathana,M.(3) 598 Koutsoula. E.(3)352,353 Kovacs, A. (1) 404 Kovalenko, A.M. (3) 184 Koyama, H.(2.4)34

Author Index Koyano, K. (1) 144;(2.5) 122, 123 Kotaki, M.(2.4) 107;(2.5)199; (2.6)183;(2.7)213 Kozankiewicz, B.(2.7)19 Kozenkov, V.M. (3) 73 Kozikowski, A.P. (2.6)238 Kozin, 1. (1) 449 Kozlecki, T.(1) 216;(2.4)37;(3) 373 Kozlowski, J. (2.6)286 Kozubek, H.(2.5)74 Kraeger, J. (2.6)38 Kraft, K.M. (3) 678 Kraka, E. (2.7)18 Krakovyak, N.M. (3) 601 Krallafh,A. (1) 1 1 1 h e r , H.E.A. (3)7% Kramer, R.(2.5)187 Kramer, W. (2.2)120;(2.4)264 Kranzelbinder, G.(3) 476,477 Krasnovskii, A.A. (1) 185 W r , B.(1) 292;(2.2)141; (2.5)38 Kraus, G.A. (2.4)7 Krawinkel, T.(3) 278 Kreiter, C.G.(2.7)90 Kremer, F.(3) 368 Krenceski, M. (3) 212 Kresge, A.J. (2.7)30 Kretzschmar, W.(1) 432 Kreyeneschmidt, M. (3)5 16 Kricheldorf, H.R (3) 278 Krieger, C.(1) 290;(2.5)39 Krihak, M.(1) 377 Krijtova, K.(1) 426;(3)589 Krishna, T.S.R (1) 223;(2.4)25 Krishnamoorthy, G. (1) 205,213 Krishnan, V. (1) 316;(4) 1,7 Krissinel, E.B. (1) 296 Kristensen, S.(2.6)2 Kristiansen, M.(3) 708 Kritzenberger, J. (3)760 h h n k e , C.(3)328 Kroto, H.W. (1) 330;(2.5)169 Kruchenok, J.V. (1) 439 Kruger, J. (3) 728 Kruppa, A.I. (2.7) 176 Krylova, I.V. (3) 343 Kryschi, C. (2.3)34;(2.4)92 Krystkowiak, E.(1) 15 Kuang, W.(3)267 Kubat, P.(2.5)170 Kubicki, J. (1) 448 Kubin, J. (2.6) 132 Kubo, K. (I) 400; (2.3) 132;(2.4) 50,252,306,316;(2.6)65, 184

Kubo, Y.(2.2)12, 121;(2.3)51, 90; (2.4)5, 193-195,200 Kubota, H. (3)284,290 Kuchuk, I.D.(2.7)7,8 Kuciauskas, D.(1) 371;(2.5)1 1 1, 180 Kucybala, Z. (3)560 Kuczera, K. (2.5)237 Kudo, T. (1) 91;(2.6)304 Kudryavtsev, V.V.(2.4)38;(3) 150 Kuehnle, W. (1) 197;(2.6)220 Kuhn, A. (2.7)40 Kujita, T.(2.4)204 Kukhto, A.V. (1) 14 Kulakov, T.A. (3) 540 Kulik, L.V. (2.7)207 Kulinkovich, O.G. (3) 615 Kulkami, M.G. (2.1)51;(2.5)72 Kulkarni, S.G.(2.7)41 Kultgen, S.G. (2.4)245 Kumagai, T. (2.3)108;(2.4)190 Kumar, A. (2.4)117, 120, 137, 151;(2.5)225;(3) 685 Kumar, B.A. (2.5)156 Kumar, G.A. (1) 63 Kumar, J. (3)396,520,658 Kumar, M.R(1) 158 Kumar, P.(2.2)109 Kumar, S.(2.5)225 Kumari, V.D. (4)20 Kumbhar, C.G. (2.7)41 Kumble, R.(1) 324 Kummer, M.(2.2)102 Kunefke, A.J. (2.7)119 Kung, A.H.(2.7)129 Kung, C.-Y. (1) 129 Kunisada, H. (3) 60 Kunkely, H. (2.5)35 Kunneth, R (2.4)283 Kuntz, R.R.(2.7)54,55 Kunz, A. (3)818 Kunz, T.(2.7)11; (3) 726,760 Kunze, A. (3) 136 Kunze, M.(3)409 Kupcik, J. (2.6)307 Kurahayashi, N.(1) 173;(2.5)65 Kuribarq K. (2.1)33 Kurihara, S.(3) 287 Kurihara, T. (3) 63,64 Kurimoto, H. (2.5) 119;(4)29 Kurita, J. (2.5)197;(2.6)325 Kurita, S.(2.4)105, 106;(2.6)88 Kuriyama, Y.(1) 229;(2.3)9; (2.4)23;(2.5)198 Kuroda, K. (2.4) 10 Kuroda, M.(3)534 Kurosaki, T. (3) 188

421 Kurz, H.(3) 481 Kusanagi, H. (2.6)290;(2.7)186 Kusano, S.(2.5)80;(2.6)227 Kusukawa, T. (2.5)177 Kusumi, A. (2.6)236;(2.7)194 Kusumoto, K. (2.3)90;(2.4)200 Kusumoto, M. (1) 230;(2.4)45, 103 Kusunoki, J. (2.6)159 Kusy, R.P.(3)87 Kutal, C.(2.7)221;(3) 108,109 Kutsenova, A.V. (3) 753 Kutyrkin, L.P.P.(3) 753 Kuwahara, T. (2.4)16 Kuzaev, A.I. (3)614 Kuzina, S.I.(3)711,712,735 Kuz'menko, N.E. (3) 3 18 Kuzmin, M.G.(1) 257 Kuz'mina, L.G. (2.6)29 Kuznetsov, A.M. (1) 85 Kuznetsov, M.A. (2.4)68;(2.7)7, 8 Kumetsov, S.V. (1) 249 Kvaran, A. (2.2)48;(2.4)40 Kvasnicka, P. (1) 141;(2.3)97 Kvasyuk, E.(2.4)323;(2.7)196 Kwak, J. (3) 516 Kwak, Y . 4 . (2.2)35;(2.6)91 Kwei, T.K. (3) 666 Kwon, €3. (2.2)38 Kwon, H.C.(1) 377 Kwon, S.K. (3) 748 Kwon, T.S. (3) 60

4 F. (3) 469 Labarthet, F.L. (3) 377,408 Lackowicz, J.R (1) 468 Lacoste, J. (3) 715,805,808,810, 840 Ladds, R.A. (1) 436 Ladsous, P. (3) 840 Laenen, R.(1) 494,495 Lafosse, X.(2.4)121, 122,164 Lahamer, A.S. (1) 340 Lahi,P.M. (3) 502 Lahiri, s.(2.1)57 Lahoz, A. (2.1)74;(2.6)168,280 Lahti, P.M. (2.2)86;(3) 469 Lai, Y.H. (3) 527 Lakowicz, J.R. (1) 450,482 Lakshmi, S.(3) 164 Lalitha, A. (2.3)17;(2.4)184; (2.6)93 La Malfa, G.(1) 406 Lambert, H.M. (2.3)128;(2.7) 140 Lambert, J.N. (2.1)53

422

Lane, E. (2.1) 63; (2.7) 63 Lane, P.A. (3) 324,496 Lang, K. (2.5) 170 Lang, M.J. (1) 23; (2.6) 14 Lange, G.L. (2.2) 16,27 Lange, J. (3) 66,227 Langford, S.C.(3) 726 Laagford, S.J. (1) 295; (2.5) 87;

(2.6) 218 Languna, 0. (3) 609 Lankieewicz, L. (1) 148 Lanska, B.(3) 325 Lanza, M.(1) 380 Lanzalunga, 0. (2.3) 121 Lanzani, G. (3) 476,53 1,532 Lapck, L. (3) 235 Lapersonne-Meyer, C. (3) 450 Lapouyade, R. (1) 203; (2.6) 224, 225 Laregenie, P. (2.4) 147; (2.6) 77 Larson, S.L. (1) 296 Lamon, A. (1) 419 Laterini, L. (2.2) 79 Latonin, V.A. (1) 57 Latterhi, L. (1) 154,310; (2.2) 87 Laukhina, O.D.(2.6) 58 Launikonis, A. (1) 277; (2.5) 85 Laurendeau, N.M. (1) 465 Laurent, C. (3) 359 Laurinavichene, T.V. (4) 63 Lautz, C. (2.3) 56; (2.7) 123 Lavabre, D. (2.6) 75 Lavallee, R.J.(2.7) 221 Lavielle, L. (3) 91 Lank, N.L.(1) 502 Lavrov, V.V. (3) 74 Lawley, K.P.(2.5) 135 Lawrance, W.D. (2.7) 178 Lawson, J.M. (1) 2 1 Lazauskaite, R. (3) 107 Leabeck, D.H. (1) 436 Leach, D.M.(3) 799 Leach, S.(1) 330; (2.5) 169 Leadbeater, N.E.(2.7) 82, 103106 Leal, P. (2.4) 241 Lebaudy, P. (3) 252 Lebel, A. (3) 15 Lecamp, L. (3) 252 Leclerc, M. (1) 202; (3) 530 Lecuiller, R (3) 450 Ledwith, A. (3) 102 Lee,B. (2.7) 98 Lee,B.A. (1) 289; (2.5) 78 Lee, B.H.(3) 426 Lee, C.(3) 529 Lee,C.W. (3) 339 Lee,H.R. (3) 768

Lee,1.Y.S. (3) 624

Lee, J.D. (1) 285 Lee, J.G. (3) 376 Lee, J.I. (3) 479 Lee, J.O. (2.1) 70; (2.6) 190 Lee, J.W. (2.3) 72 Lee,J.Y. (3) 752; (4) 19

Lee, K.(2.1) 62; (2.4) Lee,K.W. (2.7) 206

321

Lee, M. (1) 20 1 Lee, N.J. (3) 77 Lee, R.E. (3) 787,809 Lee,S. (1) 50; (3) 529 Lee,S . 4 . (4) 19 Lee, S.M. (2.6) 212 Lee, S.T.(3) 536 Lee,S.Y. (1) 289; (2.5) 78; (3) 403

Lee,T.B.(3) 522

Lee, T.G. (2.7) 139

Lee,T.-H. (2.2) 59

Lee, T.S. (3) 396 Lee,V.Y. (3) 441 Lee, Y.-J. (2.3) 62; (2.7) 158 Lee, Y.-R. (2.3) 62; (2.6) 234; (2.7) 158

Lee, Y.T. (2.3) 63; (2.7) 125-127,

131, 156,157 Leecharoen, R.(2.7) 115 Lee-Ruff, E. (2.1) 14; (2.3) 123; (2.4) 5 1 Lees, A. (3) 225 Lees, A.J. (3) 222,226 Lefebre, M.A. (2.2) 110 Le Gourrierec, D.(1) 177,299; (2.4) 73; (2.6) 155 Legrand, F. (3) 655 Lehmann, M. (2.3) 8 Lehn,J.-M. (2.4) 88; (3) 404 Lehr, B. (1) 413; (3) 556 Lei, H. (2.7) 57 Lei, Q. (3) 326 Lei, Z. (2.4) 254; (2.6) 3 1; (3) 543 Lei, Z.Q. (3) 668 Leibovitch, M. (2.1) 24; (2.5) 54 Leigh, W.J. (2.3) 136; (2.6) 297, 300 Leiner, J.P. (1) 376 Leininger, S. (2.4) 304 Leising, G. (3) 476,477,48 1,485, 494,503 Leiva, A.M. (1) 279 Lemaire, J. (3) 715, 838 Lemmetyinen, H. (1) 288 Le Nest, J.F. (3) 181 Lenghaus, K. (2.2) 88 Lengweiler, U. (2.2) 35; (2.6) 91 Leo&d, D.(2.7) 27,28

Photochemistry Leonard, c. (1) 406 Leplyanin, G.V. (3) 217 Lepoint, T. (1) 95 Lepoint-Mullie, F. (1) 95 Leppancn, V.P. (2.4) 162 Lemer, N. (I) 129, 132 Lemer, R A . (2.1) 25 Les, A. (1) 179; (2.6) 156 Leshenyuk, N.G. (2.4) 129; (2.6) 71; (3) 392

Leshina, T.V. (2.7) 176 Lessard, J. (2.6) 194 Letokhov, V.S. (1) 472 Letts, S.A. (3) 245 Levanon, H. (1) 26 1 Levi, D.H. (4) 42 Levinson, E.G.(1) 3 17 Levitus. M. (2.4) 153; (2.6) 86 Levy, D.H. (1) 27 Levy, Y. (3) 404 Lewanowicz, A. (1) 189; (2.6) 149

Lewis,D.M. (3) 746 Lewis, F.D.(2.4) 245 Lewis, J. (2.7) 103-105 Lex, J. (2.1) 50; (2.4) 209 Ley, K.D.(3) 357,488

Leyva, E. (2.7) 43 Lhomme, J. (2.4) 301 Li, B. (3) 135,722 Li, B.G. (3) 121 Li, C. (1) 247; (2.2) 124; (2.4) 155; (3) 432

Li, C.-X.(2.4) 100; (3) 433 Li, F. (1) 162; (3) 33,47,493,562 Li, F.M. (3) 32,561,563 Li, G. (3) 723 Li, H. (2.2) 98,99; (2.6) 36; (3) 463,519,643

Li, J. (1) 1; (2.4) 327; (2.7) 204;

(3) 127, 187,458,499,506, 509 Li, J.C. (2.3) 115 Li, L. (1) 468 Li, L.D. (3) 52 Li, M. (3) 127,628 Li, Q. (2.2) 98,99; (3) 446,447 Li, R. (3) 200 Li, S. (1) 82; (2.5) 205; (3) 495 Li, T. (1) 160 Li, W. (1) 427; (3) 168,391,664 Li, W.S. (3) 190 Li, X.(2.2) 97; (3) 682 Li, X.C.(3) 456 Li, X.-Y.(1) 318 Li, Y. (1) 320; (2.3) 50; (2.4) 17, 108; (2.6) 21; (3) 50,395, 493,63 1

Author Index Li, Y.X.(2.3) 122 Li, Y.N.B. (2.6) 133 Li, Z.(1) 82; (2.5) 205; (3) 187, 792

Li, Z.C.(3) 32,561,563 Liaaen-Jensen, S.(2.4) 29; (2.6) 322; (2.7) 191

Liable-Sands, L. (2.7) 102 Lian, T. (2.7) 100 Liang, L. (3) 294,391 Liang, W.(3) 607,608,662,663, 74 1

Liang, X.(3) 374 Lianos, P. (3) 598 Liao, C.-C.(2.2) 59 Liaw, D.J. (3) 557,574 Li Bassi, G. (3) 12 Licchelli, M. (1) 402 Liddell, P.A. (1) 371; (2.5) 30, 111, 180,233

Lidzey, D.G. (3) 544 Lievin, J. (1) 330; (2.5) 169 Liflca, T. (2.3) 24; (2.4) 89 Ligner, G. (3) 774,780,803 Likhotvorik, I. (2.4) 297; (2.7) 34 Likhtenshtein, G.I. (2.3) 10, 12 Lim, B.(3) 154 Lim, C.(2.3) 108; (2.4) 190 Lim, J.C.(3) 426 Lim, S.Y. (4) 22 Lima, J.C. (2.6) 160 Limoges, B.R.(1) 2% Limpouchova, Z.(3) 585 Lin, C.-H. (2.2) 62,63; (2.4) 202, 203; (2.6) 110-112

Lin, F.(3) 453 Lin, F.-T. (2.4) 111; (2.6) 85 Lin, J. (2.4) 138; (3) 468,559 Lin, J.J. (2.3) 63; (2.7) 125, 156, 157

Lin, L. (3) 380 Lin, N. (1) 333 Lin, N.Y. (1) 151; (2.2) 135 Lin, S. (2.2) 86; (2.5) 30,233 Lin, S.H.(2.3) 57; (2.7) 126, 127 Lin, S.-M. (2.3) 62; (2.7) 158 Lin, V.S.-Y. (1) 324 Lin, W.(1) 333; (3) 374 Lin, W.-Z. (1) 15 1,354 Lin, Y.(3) 19,277,427,428 Lin, Y.-N. (2.5) 91 Lindeman, A.V. (2.4) 80 Lindemann, U.(2.1) 20,40; (2.5) 51, 52, 60

Linden, L. (3) 28,240 Lindner, J. (2.7) 144 Lindsey, J.S. (1) 1, 161, 162, 164 Ling, J.G. (2.1) 23

423

Ling, K . 4 . (2.4) 206; (2.5) 224; (2.6) 105

Ling, Q. (3) 304,667 Ling, R (2.4) 302; (2.6) 145 Linnertz, H.(1) 141; (2.3) 97 Linsenmann, M. (1) 296 Lion, C. (2.4) 48 Liou, K.F. (2.5) 179 Lipinski, J. (1) 189; (2.6) 149 Lippa, B. (2.6) 59 Lippert, T. (3) 726 Lipsky, S.(1) 50 1 Lissi, E.A. (3) 126,597,604 Litty, J.L. (3) 757 Liu, B.-J. (2.5) 117, 125 Liu, C.(2.4) 24; (2.6) 296; (2.7) 174; (3) 16

Liu, C.-Y. (2.1) 22 Liu, F. (1) 467; (2.3) 125; (3) 391 Liu, G. (2.3) 13; (2.6) 299 Liu, J. (2.6) 121; (3) 294 Liu, K.(2.7) 121 Liu, M.T.H. (2.7) 12, 16 Liu, P. (2.4) 7 Liu, R (2.6) 36 Liu, R.C.Y.(2.1) 88; (2.7) 209 Liu, R.S.H.(1) 191 Liu, S. (3) 99 Liu, S.S.(3) 137 Liu, W.(2.7) 139 Liu, X.(2.6) 21; (3) 33,50 Liu, X.Q.(3) 32 Liu, Y. (3) 288,469,619 Liu, Y.-C. (2.5) 234; (2.6) 207, 211; (2.7) 210

Liu, Y.Q.(3) 456 Liu, Z. (3) 605,773 Liu, Z.-L.(2.3) 122; (2.5) 234;

(2.6) 207,211; (2.7) 210 Liwo, A. (1) 148 Lobkovsky, E.B.(2.4) 182; (3) 155 Lochbrunner, S.(1) 226 Lodder, G. (2.4) 187,23 1 Loddo, V.(1) 430 Lodygin, S.K.(3) 184 Loeb, B.(1) 279 Luebach, J.L. (2.7) 35 Loescher, F.(1) 473 Lohse, D. (1) 12 Lokai, M. (3) 175,216,765 Lokshin, V.(2.4) 128, 131, 146, 147; (2.6) 72,74,77 Long, C.(2.7) 99 Long, X.(3) 475 Longordo, E. (3) 89 Lopez, A.F. (1) 142, 167 Lopez, A.I. (1) 142, 167

Lopez, A.T. (1) 142, 167 Lopez, J.C. (2.2) 41 Lopez, M.-A.D. (1) 422 Lopez, R.(1) 279 Lopez-Munoz, M.J. (1) 430 Losev, A.P. (3) 615 LOU,N.-Q.(2.3) 65; (2.7) 154, 162-164

Lougnof D.J. (3) 91,176 Lounis, B. (1) 124, 133 Lovell, L.G. (3) 197 Lowe, A.C. (3) 260 Lozano, G.M.J. (2.4) 113; (3) 381 Lu, c. (1) 333 Lu, c.-Y. (1) 354 Lu, F.(3) 204 Lu, J. (3) 271 Lu, L.N. (2.5) 219; (2.6) 206 Lu, N.(3) 347 Lu, Q.(3) 191 Lu, W. (2.5) 206,207 Lu, Y. (2.4) 7 Lu, Z. (3) 95,209 Lu, Z.-L. (1) 283 Lub, J. (3) 262,266 Lucarini, M. (2.1) 6 Lucas, L.N. (2.3) 27,28; (2.4) 81, 84

Lucchini, V. (2.2) 80 Luccioni-Houze, B.(2.4) 118 Lucero, M.J. (2.2) 35; (2.6) 91 Luc-Gardctte, J. (3) 681,705,713 Luchian, M. (3) 832 Lucht, S.(1) 206; (3) 423 Luchuk, I.D. (2.4) 68 Lucia, L.A. (2.5) 159 Ludhyi, K. (2.4) 3 10; (2.6) 3 19; (2.7) 214

Ludi, A. (3) 10 Lue, M. (2.5) 176 Luftmann, H.(1) 33 1,339; (2.5) 173, 174

Luh, T.Y. (3) 536 Luiz, M. (2.5) 200 Lukac, I. (3) 17,805 Luk'yashina, V.A.(2.4) 38; (3) 150

Lundell, J. (2.7) 208 Lungu, A. (2.1) 87; (3) 171 Luo, C.(1) 347, 348; (2.3) 13; (2.5) 97, 106; (2.6) 299

Luo, H. (3) 145 LUO,J.-K. (2.4) 240; (2.6) 56 Luo, Q. (1) 283 LUO,X.-M. (2.4) 154 Lusiak, P. (2.7) 54.55 Lusztyk, J. (2.1) 88; (2.7) 15,21, 209

424 Lyons, C. (3) 520 Lyubimov, A.V. (3) 361 Ma, B.(1) 328; (2.7) 65 Ma, D.(3) 480 Ma, H.(3) 723 Ma, J. (3) 206,381 Ma, J.-H. (1) 151 Ma, L. (3) 664,672 Ma, P.H. (2.7) 155 Ma, X.(2.7) 145; (3) 13 Maail, M. (2.4) 48 Maas, G. (2.4) 58 Maas, H.J. (3) 213 Mabji, S.S.(3) 360 McAllister, M.A. (2.1) 88; (2.7) 209

Macanita, A.L. (2.2) 87 McClelland, R.A. (2.4) 298; (2.6) 169; (2.7) 52,2 11

McCormick, A.V. (3) 239 McCormick, C.L. (3) 576 Macdonald, R.(1) 46 1 McElhanon, J.R (3) 553 McFarlane, D. (3) 81 1 McFarlane, K.L.(2.7)98

McGall, G.H.(2.4) 329 McGany, P.F. (2.3) 124; (3) 792 McGimpsey, W.G.(1) 48; (2.2) 48; (2.4) 40 McGinness, J. (3) 785 McGrane, S.D.(I) 501 McGrath, D.M. (3) 554 McGrath, D.V. (2.6) 46,48; (3) 550,553 McGrath, J.E. (3) 241 Machado, A.E.H. (3) 737,738, 839 McHale, J.L.(1) 200 Machida, M. (2.4) 265, 3 19,320; (2.6) 101, 102; (4) 53 Machida, S.(2.4) 61; (2.6) 264; (3) 722,725 Maciejewski, A. (1) 448; (2.2) 65; (2.6) 266,286 Maciel, A.C. (2.4) 295 McKay, T.J. (1) 478 McKean, W. (3) 619 McKerrow, A. (1) 427 MacKinnon, A. (2.3) 130; (2.4) 54; (2.7) 39 MacKinnon, M.C. (2.2) 27 Mchghtlin, W. (1) 404 Mackad, P.J. (2.4) 60 McMahon, R.J. (2.7) 32 McManus, K.A. (2.3) 52; (2.4) 215

Photochemistry McMinn, D.L. (2.6) 240; (2.7) 203

McMinn, J.H. (3) 67 McMuny, T.B.H.(2.2) 15 Macpherson, A.N. (2.5) 30,233 Madaxas, M.L. (2.4) 3 14 Madey, J.M.J. (2.7) 110 Madhusoodnan, K.S.(2.4) 99 Maeda, H.(2.3) 133; (2.4) 7,315; (2.5) 16

Maeda, K.(1) 458,476; (2.5) 226 Maeda, S.(2.4) 78 Maeham, M. (2.3) 40 M.ae.ima, E. (3) 626 Maestri, M. (1) 178,243; (2.4) 134, 135; (2.6) 160

MaEezzoli, A. (3) 242 Magaretha, P. (2.2) 54, 106 Magdinets, V.V. (3) 141 Maggini, M.(1) 325,338,358360; (2.5) 101, 107, 108

Magnani, L. (3) 535 Magnitskii, S.A. (3) 73 Mah, S.(3) 106 Mah, Y.J.(2.2) 127; (2.4) 170 Mahajan, S. (1) 158 MahB, L. (2.7) 48 Mahran, A.M. (2.4) 57; (2.6) 164 Mahrt, R.(3) 504 Maier, G.(2.1) 73; (2.3) 56,83; (2.6) 293,294; (2.7) 75, 123

Mailhot, B. (3) 705 Mailhot, G. (1) 247 Mainagashev, LYa. (2.2) 137; (2.4) 291

Maiti, B.C. (2.1) 57 Maiti, S.(3) 37 Maiya, B.G. (1) 387; (2.5) 134 Majima, T. (2.5) 190; (2.6) 3 16 Mak, T.C.W. (1) 283 Makarov, G.N. (2.7) 149-151 Maki, S.(2.1) 17; (2.3) 94; (2.4) 235; (2.5) 201

Makimoto, M. (3) 376 Makino, H.(2.3) 84 Makino, M. (2.4) 252; (2.6) 65 Mal, J. (2.2) 29 Malakov, D.V. (3) 73 Malanca, F.E. (2.1) 86 Malar, E.J.P.(2.1) 1,2 Malatesta, V.(2.3) 102; (2.4) 130, 150; (2.6) 69

Maledant, P.RL. (1) 309 Malik, J. (3) 774,780,803 Malinovsky, D.E.(2.7) 149,151 Mallouk, T.E. (3) 669 Malo, M.C. (2.4) 255,256; (2.6) 64

Maloba, F.W. (2.5) 146

Malpert, J.H.(3) 455 Malta, O.L.(1) 75 Maltesat, V. (2.6) 82 Malyshcva, E.V. (2.4) 80 Mamaev, A.L. (2.5) 139 Mamo, A. (1) 380 Manboothiri, I.N.N.(2.1) 82 Mancheno, M. (2.2) 117; (2.6) 213 Mandal, D. (1) 153,171,221 Mandal,P.C. (3) 36 Mandal, T.K.(3) 159 Mandravei, C. (4) 9,32 Mang, x.(3) 375 Man& D.J. (3) 23 Manivannan, R.(3) 58 Mankman, A.P. (3) 517 Mann, C. (2.4) 139 Mann, J. (2.2) 21 Mansour, A.F. (4) 67,68 Mantocon, S. (2.2) 41 Manz, B.(2.4) 58 Mao, W.(2.2) 98 Maoquan, Y.(3) 292 Marazuela, M.D. (1) 384 Marchand, A.P. (2.1) 82 Marchcsi, E.(3) 828 Marchiori, M.L.P.de F.C. (2.4) 237 Marchiori, R.(2.4) 237 Marciniak, B.(2.5) 74 Marconi, G.(1) 363; (2.1) 75; (2.6) 281 Marczyk, J. (1) 457 Mareeva, S.A. (3) 210 Marevtsev, V.S. (3) 361 Margaretha, P. (2.2) 8,26,28,30; (2.4) 173,208 Mariano, P.S. (2.1) 61; (2.2) 117; (2.4) 302; (2.6) 145, 170,212, 213 Marinic, Z. (2.3) 118; (2.4) 271 Marino, H.L. (3) 55 Marioli, J.M. (2.5) 220 Marko-Varga, G. (1) 385 Markovic, D. (2.5) 28 Marks, D. (1) 180; (2.4) 74 Marks, R (2.3) 10 Marks, V. (2.7) 57 Marois, C.(3) 404 Maroulis, A.J. (2.4) 309; (2.6) 162 Marquardt, S.(2.6) 185 Marquet, J. (2.4) 213 Marszalek, T. (1) 96,117 Marte, A. (3) 98 Marti, C.(2.5) 133 Marticz,T.J. (2.4) 19

Author Index Martin, D. (2.6) 175; (3) 55 Martin, E.(2.6) 215 Martin, G.E.(2.6) 193 Martin, H.-D. (2.2) 102; (2.3) 34; (2.4) 92

Martin, J.W. (3) 678 Martin, M. (2.1) 63; (2.7) 63 Martin, M.M. (1) 195 Martin, N.(1) 370 Martindill, M. (3) 306 Martinez, C.(2.5) 133 Martinez, J.T.(2.3) 1 Martinez, L.J.(2.1) 93; (2.5) 24, 25,36; (2.6) 136, 137

Martinez, S.S.(1) 181 Martinez, T.J.(1) 232; (2.6) 23 Martine~-Mane~, R (2.1) 74; (2.6) 168,280

Martinho, J.M.G. (1) 60,67 Martini, I. (2.5) 210; (3) 836 Martino, D.M. (2.7) 218 Martins, RR.L. (2.5) 23 1 Martrenchard, S. (1) 2 11 Martynova, V.P. (2.4) 129; (2.6) 71; (3) 392

Marudachalam, M. (4) 45 Marutani, K.(4) 24 Maruyama, T.(4) 51,52 Marvet, U.(2.7) 141,143 Masashi, S.(2.3) 40 Mase, N. (2.2) 25; (2.6) 262 Masetti, F. (1) 154; (2.2) 79; (2.4) 32

Maslyuk, A.F. (3) 46 Mason, R.P.(3) 828 Massines, F. (3) 359 Mast, A.P. (3) 717 Mastrangelo, J.C. (3) 430 Masuda, T. (3) 5 14 Masuhara, H. (1) 475; (3) 726 Masumeci, F.(1) 406 Masuzaki, Y.(2.7) 168 MaN. (1) 24,47,100,168, 268; (2.6) 9

Mataka, S. (2.5) 193 Mateo, J.L. (3) 233,234 Mathews, O.A. (1) 238 Mathey, G. (3) 583 Mathieu, H.J. (2.7) 27,28 Mathis, C. (1) 335 Mathivanan, N. (3) 740 Mathur, R. (3) 229 Matisova-Rychla, L. (3) 325 Matlin, A.R. (2.2) 86 Matoba,R. (4) 66 Matoba, Y.(3) 582 Matohara, K.(2.3) 108; (2.4) 190 Matsubam, C. (2.3) 112; (3) 649

Matsuda, K.(2.7) 25 Matsuda, S. (4) 49 Matsugashita, S.(2.4) 303; (2.6) 165,166

Matsui, H. (2.7) 173 Matsui, S. (3) 177 Matsui, Y.(3) 706 Matsukawa, R (4) 58 Matsumi, N.(3) 358,523 Matsumi, Y.(2.7) 135 Matsumoto, A. (3) 78,149,157, 158,264,425

Matsumoto, H. (2.4) 179; (2.6) 263

Matsumoto, K.(2.7) 173 Matsumoto, M. (2.3) 93 Matsumoto, N. (3) 657 Matsumoto, S.(2.2) 52,53; (2.5) 56,178

Matsumoto, T. (2.1) 83; (2.4) 49; (3) 188,552

Matsumoto, Y.(2.7) 88, 89 Matsumura, M. (2.5) 150; (4) 14 Matsumura, T.(2.7) 168 Matsuo, T.(2.5) 80; (2.6) 227; (3) 586,624

Matsuoka, H.(4) 18 Matsuoka, I. (2.5) 195 Matsuoka, M. (2.2) 9; (2.4) 67,

177,282; (2.6) 94,95; (4) 3 Matsuoka, T. (3) 120 Matsurada, M. (2.4) 78 Matsushima, E.(2.3) 5 Matsushima, R. (1) 233; (2.5) 67 Matsushita, T.(2.4) 300; (2.7) 31 Matsushita, Y.(1) 212 Matsuura, K. (3) 670 Matsuura, S.(2.6) 311 Matsuura, T. (2.5) 32; (2.6) 119 Matsuzaki, A. (2.4) 159; (3) 720 Matsuzaki, M. (4) 54 Matsuzaki, Y.(4) 43 Matsuzawa, S.(2.5) 191 Mattay, J. (1) 33 1,339; (2.1) 3, 56; (2.3) 91,92; (2.4) 201, 244; (2.5) 173, 174; (2.6) 142, 143,219,305,306 Matthews, O.A. (2.6) 49 Matuszewska, J. (3) 96 Mau, A.W.-H. (1) 277; (2.5) 85 Mauriello, G. (2.4) 167; (2.6) 278, 279 Maus, M. (1) 203,207,3 10; (2.6) 224-226 Mauzerall, D. (1) 264 May, B. (1) 305 Mayer, B. (2.2) 102 Mayer, C.(3) 195

425

Mayer, J.W. (3) 189 Mazzanti, L.(1) 141; (2.3) 97 Mazzucato, U. (2.3) 98; (2.4) 32 Meagher, D.A. (2.5) 203 Mebel, A.M. (2.3) 57; (2.7) 126, 127

Medvedeva, N. (2.3) 12 Medvesevskikh, Yu.(3) 237 Meetani, M.A. (1) 416; (3) 622 Mcggers, E.(2.5) 185 Mehring, M. (1) 372 Mehta, G. (2.1) 65 Mei, X. (3) 824 Meier, G. (3) 422 Meier, H.(2.3) 8 Meier, J.G. (3) 368 Meijer, E.W.(3) 213 Meindl, P. (1) 461 Mejiritshi, A. (2.1) 87; (3) 171 Melchior, A. (2.3) 128 Melekhov, A. (2.4) 7 Mella, M. (2.3) 53; (2.5) 209; (2.6) 195

Mellinger, A. (2.1) 64;(2.7) 64 Mel'nikov, M.Ya. (2.3) 79 Melo, M.J. (2.4) 134; (2.6) 160 Mclzig, M. (2.4) 139 Memarian, H.R.(2.2) 8 1; (2.4) 280

Meng,H. (3) 7,527,528

Meng, J.-B. (2.2) 42; (2.5) 32; (2.6) 92

Mennig, M. (3) 4 11 Mcnzcl, H. (3) 438 Menzel, K.(3) 208,765 Menzel, R.(1) 143 Merchan, M. (1) 103 Merhari, L. (3) 501 Merica, A. (2.2) 16 Merkulov, A. (4) 40,46 Merlin, A. (3) 783 Memtt, G. (1) 47 1 Mert, J.H. (3) 130 Mertz, E.L. (1) 85 Mertz, J. (1) 454 Metres, R (2.4) 325; (2.6) 233; (2.7) 197

Metelista, A.V. (2.6) 154 Metha, G. (2.5) 134 Meth-Cohn, 0. (2.4) 54; (2.7) 39 Metiu, H. (1) 128 Metzger, R.M. (1) 357 Meudf A. (2.6) 293,294 Meunier, M. (3) 772 Meyer, S.(3) 369,370 Meyer, T.J. (1) 29,279,282; (2.6) 11; (3) 673

Meyer-Roscher, B. (3) 182,248

426

Meziane, D.(1) 204 Mi, H. (1) 367 Mialocq, L-C. (2.5) 23 Micewicz, R.(1) 148 Micheau, J.C. (2.6) 75 Michel-Beyerle, M.E. (1) 109, 500

Michelet, V.(2.6) 83 Michniewicz, A. (1) 148 Migirdicyan, E.(2.7) 19 Migler, K.B. (3) 634,645 Mihalcea, I. (3) 332 Mikami, K.(2.2) 52,53; (2.5) 56, 178

Mikami, T. (2.6) 323 Mikes, F.(1) 412 Mikhailov, A.I. (3) 711, 712, 735 Mikhnevich, S.Yu. (1) 65 Miki, S.(2.2) 138; (2.5) 64 Miki, T. (2.2) 44 Mikkelsen, K.V. (1) 184 Mikroyannidis, J.A. (3) 521 Milewski, M. (2.6) 286 Miller, B.L. (2.5) 237 Miller, C. (3) 82,84 Miller, C.G. (2.4) 145; (2.6) 70 Miller, C.W. (3) 79,83 Miller, I.J. (2.2) 49 Miller, RD.(3) 441,516 Miller, S.D. (1) 404 Miller, T.A. (1) 488 Miller, T.S.(2.7) 119 Miller, W.L. (2.1) 19; (2.5) 27,53 Millini, R (2.6) 69 Milofsky, R.E.(2.5) 228 Milov, A.A. (2.6) 20 Milow, B. (4) 31 Mimura, K. (2.5) 193 Min, H.(1) 220; (4) 62 Minamoto, Y.(3) 720 Minkin, V.I. (2.4) 152; (2.6) 87, 154

Minkovska, S.(2.4) 149; (2.6) 76;

(3) 363 Mino, T. (2.4) 204,234,285; (2.6) 114,268,269 Minoli, G. (2.4) 299; (2.7) 47 Miolo, G. (2.2) 80 Miranda, M.A. (2.1) 13,74,75; (2.3) 127; (2.4) 241,311; (2.6) 57, 168,280,281; (2.7) 159, 170 Mirochnik, A.G. (3) 660,66 1 Mironov, A.F. (1) 3 17 Mironov, V.S.(1) 79 Misawa, H.(2.1) 17 Misetic, A. (2.4) 322 Miskoski, S.(2.5) 222

Misra, T.N. (2.2) 4; (3) 159 Mitchell, S. (2.2) 55 Mitra, S.(1) 176; (2.1) 34 Mitschke, U. (2.5) 113 Mitsudo, T.-a. (2.5) 149 Mittal, J.P. (2.1) 4; (2.5) 21 Mitzner, R.(2.6) 253 Miura, N. (1) 80 Miyachi, M. (3) 733 Miyagawa, N. (3) 118 Miyahara, N. (2.4) 193 Miyake, A. (4) 64 Miyake, J. (4) 63 Miyamoto, H. (2.2) 46; (2.4) 250; (2.6) 5 1.54

Miyamoto, T. (1) 259,260; (2.5) 77, 114

Miyamoto, Y.(3) 120 Miyasaka, H. (1) 24 Miyashi, T. (1) 34; (2.2) 117;

(2.3) 75,76,93; (2.5) 18; (2.6) 2 13 Miyashita, K.(4) 13 Miyata, M. (2.2) 69; (2.4) 300; (2.6) 117, 118; (2.7) 3 1 Miyatake, Y.(3) 670 Miyazawa, T.(2.4) 24; (2.6) 296; (2.7) 174 Miyazoe, H. (2.3) 45-47; (2.6) 326-328; (2.7) 216,217 Miyoshi, A. (2.7) 173 Mizukoshi, T. (2.6) 125 Mizuma, H. (3) 27 Mizuno, K.(2.1) 43; (2.3) 70, 71, 133; (2.4) 239, 315; (2.5) 2, 16; (2.6) 8 Mizuno, T. (2.5) 119, 124; (4) 28, 29 Mizusaki, M. (3) 558,577,590, 591 Mizutani, R. (2.4) 104 Mo, 0. (1) 112 Moad, G. (3) 69 Mochida, K.(2.6) 3 10,3 12; (2.7) 179-18 1 Mochizuki, A. (3) 185 Mochizuki, E.(2.2) 69; (2.6) 117, 118 Moeini-Nombel, L. (2.5) 191 Moerner, W.E. (3) 414 Mohamed, N.R(2.4) 57; (2.6) 164 Mohamed, S.K.(2.6) 179 Mohney, B.K. (2.7) 115 Moiroux, J. (2.6) 209 Mol, G.N.(3) 266 Molin, Yu.N. (1) 53 Molina, V.(1) 103

Photochemistry Momirlan, M. (4) 25 Momoda, J. (2.4) 133, 157 Momose, T. (2.7) 137 Monaghan, M.J. (2.2) 40; (2.4) 189

Mondini, S. (1) 358, 359; (2.5) 101, 107

Moneva, I. (3) 268 Monneric, L. (3) 654 Monney, L. (3) 747 Monnier, M.(2.2) 47 Mom, M. (1) 211 Monson, E.(1) 471 Montalti, M. (1) 397,399; (2.4) 296

Montanan, L. (2.6) 69 Montefoschi, G. (2.5) 188 Montginoul, C. (3) 6 1 Monti, S.(2.1) 75; (2.6) 281 Monts, D.L. (2.7) 119 Moore, A.L. (1) 37 1; (2.5) 30, 111, 180,233

Moore, B.C. (2.7) 64 Moore, C.B. (2.1) 64 Moore, D.E. (2.6) 133 Moore, J.A. (3) 695 Moore, J.S.(1) 266; (3) 55 1 Moore, T.A. (1) 371; (2.5) 30, 111, 180,233

Moratsi, S.C. (3) 460,535 Morawski, A.W. (4) 27 Morawski, 0. (1) 197 Mordaunt, D.H.(2.7) 183 Morel. D.(4) 45 Morel, F. (3) 81 Morel, S. (3) 705 Moreno-Bondi, M.C. (1) 378,384 Morera, I.M. (2.1) 74; (2.6) 168, 280

Morgan, S.(2.7) 15 Mori,A. (2.2) 92; (2.4) 198 Mori, H. (2.5) 126, 160; (2.6) 183; (2.7) 213

Morii, H.(2.4) 27 Morikawa, E.(3) 694 Morimoto, Y.(2.4) 107; (2.5) 199 Morini, S.(2.4) 46 Morino, S. (3) 398,442 Morisato, T. (2.7) 89 Morisawa, M. (3) 616 Morishima, Y. (1) 280; (3) 590, 591,610,653,725

Morishimo, Y.(3) 558,577 Morishita, T. (2.2) 83 Morita, M. (2.7) 168; (4) 65 Morita, T. (1) 246; (2.5) 79 Morita, Z. (3) 8 17 Moriwaki, K. (1) 230; (2.4) 45,

427

Author Index 98, 102, 103; (2.6) 34 Moriyama, H.(1) 344,345,351 Moriyama, M. (2.4) 77; (2.7) 76, 77 Morki, T.L. (2.6) 297 Morlet-Savarey, F. (3) 24 Morlino, E.A. (1) 39 Morozov, S.V.(2.5) 139 Morozova, O.B. (2.1) 18; (2.6) 205 Moms, J.C.(1) 245 Morrison, H.(2.3) 7,78 Monycki, J.W. (2.3) 104 Moser, J.-E. (2.5) 215 Moses, D. (3) 323 Mosquera, M. (2.4) 75; (2.6) 157, 158 Motoyoshiya, J. (2.5) 195 Mouanda, B. (3) 435 Moucheron, C.(1) 44 Mount, A.R. (3) 445 Moutet, J.C. (3) 98 M o w C. (2.1) 55 Muellen, K.(1) 135, 136 Mueller, M. (1) 480 Mueller, N. (2.4) 238; (2.6) 60; (2.7) 215 Mueller, U. (3) 427,428 Muggli, D.S. (3) 85,768 Muhlebach, A. (3) 10 Mukai, K.(2.6) 159 Mukai, T. (2.3) 75 Mukaigawa, M. (3) 659 Mukherjee, S. (1) 176; (2.1) 34; (2.4) 35 Mukherjee, T. (2.2) 134; (2.4) 76, 242, 308; (2.6) 108 Mukhtar, E. (1) 299 Muktawat, S.(2.2) 134; (2.4) 242, 308; (2.6) 108 Mullen, K.(1) 3 10; (3) 477,485, 487 Muller, A. (1) 127 Muller, D. (2.5) 187 Muller, J. (3) 721 Muller, U.(3) 136 Muller-Plathe, F. (1) 118; (2.4) 22 Mullin, J.L. (2.6) 309 Munavalli, S. (2.3) 80; (2.6) 272 Muneer, R (3) 101 Munmaya, K.(3) 14 Munoz de la Pena, A. (1) 477 Mur, G. (3) 689492,833 Murai, H.(1) 458; (2.5) 6,226 Murakoshi, K. (2.5) 126, 127 Mumta, S. (1) 302; (2.4) 300; (2.6) 310; (2.7)31, 180, 181 Murgida, D.H. (2.2) 73; (2.5) 227;

(2.6) 140

Murinov, Yu.1. (2.5) 147 Murozono, M. (4) 40 Murphy, D.M. (2.5) 181 Murphy, S.T.(2.5) 142 Murray, J.B. (2.6) 291 Murray, RW. (3) 606 Murrer, B.A. (4) 34 Murtagh, M.T. (1) 377 Murugan, A. (2.4) 216 Murugesan, V. (3) 846 Mum, O.M. (2.6) 188 Musewald, C. (1) 500 Musick, K.Y. (3) 526 Mutai, M. (1) 499 Muthusamy, S. (2.5) 62, 134; (2.6) 288

Muthyals, R.S.(1) 191 Muto, S. (3) 616 Muto, T. (1) 274 Mwabuma, D.(3) 277 Myers, A.B. (1) 462 Mykowska, E.(3) 4 17 Mylonas, Y. (3) 598 Nada, A.A. (2.4) 57; (2.6) 164 Nagahara, T.(2.2) 138; (2.4) 199; (2.5) 63,64

Nagai, N. (1) 174; (2.2) 136; (2.5) 34

Nagai, S. (3) 48 Nagamura, T. (2.5) 93,221; (2.6) 177,178

Nagaoka, H. (2.4) 226 Nagaoka, S.(2.6) 159 Naga~ka,S.4. (2.5) 7 Nagaosa, K.(1) 233; (2.5) 67 Nagarajan, E.R (3) 256 Nagamnjan, R (3) 84 Nagasaki, T. (3) 552 Nagasawa, Y.(1) 83 Nagase, Y.(3) 440 Nagashima, H.(1) 329 Nagashima, U.(2.5) 7; (2.6) 159 Nagoh, H.(2.4) 133,157 Nagoshi, K.(4) 13 Nagy, J. (2.5) 89; (2.6) 167 Nah, J.W. (3) 596 Nair, V. (2.4) 64 Naito, I. (2.7) 22 Naito, S. (2.5) 136,137 Najafi, H.M. (2.4) 62 Najbar, J. (1) 297; (2.5) 128 Naka, K.(3) 358,382,523 Nakabeya, K.(2.5) 150 Nakadaira, Y, (2.6) 308 Nakagaki, R.(1) 499; (2.1) 17

Nakagawa, K.(3) 65 Nakagawa, M. (2.4) 46 Nakagawa, T. (3) 70,177 Nakaham, H.(2.4) 136; (3) 299 Nakai, T. (2.4) 159 Nakajima, H. (2.3) 116; (2.4) 161 Nakajima, N. (1) 444; (2.4) 276 Nakajima, S. (2.2) 121; (2.3) 51, 90; (2.4) 193-195,200

Nakamura, M.(2.5) 190; (2.6) 3 16

Nakamura, N. (2.1) 32; (2.2) 75; (2.4) 236

Nakamura, 0. (1) 410 Nakamura, S.(2.3) 29; (2.4) 94, 277

Nakamura, T. (2.2) 2,3,78; (2.3) 76; (2.4) 267, 268

Nakamura, Y. (1) 173,332; (2.3)

106, 107; (2.4) 183; (2.5) 65, 143 Nakanami, H.(3) 784 Nakanishi, F. (2.2) 7; (3) 203 Nakano, A. (1) 163 Nakano, H.(1) 230; (2.4) 45,98, 102, 103; (2.6) 34,264 Nakano, T. (2.1) 60 Nakano, Y. (2.4) 262 Nakashima, H. (2.4) 93,95; (3) 390 N h h i m a , K.(1) 259,260,308; (2.5) 77, 114 Nakata, E. (4) 64 Nakatani, K. (2.2) 78; (3) 572 Nakatsuki, H. (1) 152 Nakatuji, K. (2.5) 199 Nakayama, T. (1) 174; (2.2) 136, 138; (2.4) 199; (2.5) 34,63, 64 Nakayama, Y.(3) 707 Nakazumi, H. (1) 156; (2.6) 35 Nakozono, T. (2.2) 75 Nalli, T.W. (2.5) 203; (3) 101 Nam, N.Q. (2.4) 329 Nanba,M.(4) 58 Nandy, S.K.(1) 244 Nanshcng, D. (3) 824 Narasaka, K.(2.6) 323 Narayanan, S.J.(1) 158 Nmyanan, V. (3) 246,25 1

Narita, T. (3) 640 Narliker, A.V. (3) 156 Nartin, C.H. (1) 162 Nasrullah, J.M. (3) 167 Natansohn, A. (3) 375 Natamjan, L.V. (1) 140; (2.4) 145; (2.6) 70

Natsir, N. (3) 201

Photochemistry

428

Nau, W.M. (2.5) 168; (2.7) 6 Naumann, W. (1) 58 Navamtnam, S. (1) 145 Navas Diaz, A. (1) 483,487 Nayak, P.L.(3) 658 Nazeeruddin, M.K.(4) 34 Neckers, D.C.(2.1) 16,87; (2.2) 93-95; (2.5) 46,47,59; (2.6) 174, 175; (3) 55, 114, 130, 133,220,221,228,455 Nefedov, O.M.(1) 53; (2.7) 176 Negri, F.(1) 107 Nehe, N. (3) 504 Nelson, H.H. (2.4) 88 Nemeth, K. (3) 736 Nemoto, T. (3) 769 Neppolian, B.(3) 846 Neri, C.(3) 787,809 Nerowski, F. (2.2) 120 Nespurek, S.(2.4) 160; (2.6) 149; (3) 337,565 Netherton, M.R (2.1) 26 Netto-Ferreira, J.C.(2.2) 107, 142; (2.4) 174; (2.5) 58 Neudeck, A. (1) 349; (2.5) 95,96 Neugebauer, H.(3) 503 Neumann, M.G.(2.4) 197; (3) 29, 579 Neumark, D.M. (2.3) 58; (2.7) 130, 183 Neuner, A. (2.5) 133 Neurauter, G. (1) 383 Neves, M.G.P.M.S. (2.5) 231 Newcomb, M.(2.1) 6, 79, 80; (2.6) 188, 189 Newton, S.P.(1) 238; (2.6) 49 Ng, S.C. (3) 471,533 Ngoc, T.H. (3) 140 Nguyen, M. (2.5) 146 Ni, C.-K.(2.3) 59; (2.7) 129 Ni, Y. (3) 792 Nian, N.-Y. (1) 354 Niaz, N.A. (3) 201 Nichinaka, Y. (2.2) 75 Nichols, M.E. (3) 771 Nickel, B. (1) 2 Nicodem, D.E.(2.4) 237 Nicolaescu, T. (1) 150 Nicolaou, K.C.(2.4) 327; (2.7) 204 Nieckan, G.F.(3) 757 Nierengarten, J.-F. (1) 363 Niggennann, J. (2.3) 49 Niidome, Y.(3) 624 Niimura, Y.(2.2) 103; (2.4) 266; (2.6) 107 Niino, H.(2.7) 77; (3) 719,728 Niki, M. (3) 657

Nikolaev, V.N. (3) 198 Niku-Paavola, M.L.(3) 340 Nile, T.A. (1) 245 Niles, D.W. (4) 42 Nilsson, S.(1) 415,451; (3) 600 Ning, N.-L. (2.7) I 1 I Ninomiya, M.(2.6) 308 Ninomiya, S.(3) 707 Nishibori, A. (3) 564 Nishibu, S. (3) 257 Nishigaichi, Y.(2.3) 41 Nishiguchi, H.(2.5) 29 Nishikubo, T. (3) 194,402,570 Nishimoto, S. (2.6) 122 Nishimura, I. (3) 570 Nishimura, J. (1) 332; (2.3) 106, 107; (2.4) 183; (2.5) 143 Nishimura, M.(1) 144; (2.5) 29 Nishimura, Y.(1) 163,366 Nishio, A. (4) 38 Nishio, S.(2.4) 159; (3) 720 Nishio, T. (2,l) 54; (2.4) 168 Nishiyama, I. (3) 259 Nishiyama, K.(1) 280; (3) 610 Nisoli, M. (3) 476 Nivorozhkin, L.E.(2.6) 154 Niwa, H.(2.3) 94, 100; (2.4) 235; (2.5) 201 Niwa, M.(2.3) I5 Nizovtsev, A.P. (1) 130 No, K.T. (3) 522 Noack, M.(3) 364 Noda, H.(2.7) 199 Noda, Y.(3) 627,644 Nogi, N. (3) 129 Noguchi, A. (3) 552 Noguchi, Y.(2.2) 67,68; (2.4) 192 Noh, T. (2.2) 37,38; (2.4) 274; (3)

90

Nojima, M.(2.1) 48,49; (2.4) 175; (2.5) 69 Nojiri, T. (1) 353; (2.5) 1 10 Nolan, T.F. (2.7) 86 Noma, T. (4) 38 Nomoto, T. (1) 138, 139, 168, 252; (2.5) 212; (2.6) 208 Nomura, J. (4) 24 Nonaka, T. (2.5) 182; (3) 287 Nonell, S.(2.5) 133, 148 Nooijen, M. (1) 92; (2.6) 221 Nor, H.M. (3) 749 Norambuena, E. (1) 279 Nord, K. (2.6) 2 Nosova, G.I. (2.4) 38; (3) 150 Noszticzius, 2.(2.1) 71 Nourmamode, A. (3) 355,732, 737

Novaira, A.I. (2.5) 112; (2.6) 229 Novales, B.(1) 435 Novikova, 0.0.(1) 155 Novosa, G.I. (3) 150 Nozik, A.J. (4) 35 Nunzi, J.-M. (2.4) 79 Nuyken, 0. (3) 88, 100,134,760 Nutillard, J.-M. (2.4) 55; (2.6) 144 Nygren, A.S. (2.4) 326; (2.6) 242; (2.7) 200 Nyitrai, J. (2.5) 89; (2.6) 167

Obata, T. (2.2) 89,90; (2.4) 207,

21 1 Obayashi, R.(2.3) 44; (2.6) 321 Obermueller, R.A. (2.5) 43 Obi, K.(1) 68; (2.1) 9 Obsil, T. (1) 141; (2.3) 97 Oda, H.(2.4) 114, 115; (3) 362, 8 14 Oda, K.(2.4) 265,319; (2.6) 101, 102 Oda, M.(2.4) 107; (2.5) 199 Odani, T. (3) 149, 158 Odochian, L. (3) 832 ODonncll, J.H. (3) 761 Oelcken, B. (2.7) 97 Oelcken, S. (1) 479 Oelgemiiller, M. (2.2) 119, 120; (2.7) 73 Oelkrug, D. (1) 413; (3) 482,556 Oertli, A.G. (3) 776 Offenberg, H.(1) 150 Ogawa, A. (2.3) 44; (2.6) 321 Ogawa, K.(3) 829 Ogawa, M.(2.4) 10 Ogawa, T. (1) 52, 126 Oge, T. (3) 368 Ogi. T. (4) 59 Ogilby, P.R(1) 184; (3) 708 Ogino, K. (3) 552 Ogiri, S.(3) 259 Ogul'chansky, T.Y. (1) 155 Oguma, J. (3) 484 Ogura, K. (1) 157; (2.1) 90.91; (2.4) 172; (2.6) 271; (3) 592 Ogurok, D.D. (2.7) 149, 151 ogurtsov, V.I. (1) 374 Oh,C.M.(2.1) 70; (2.6) 190 Oh,S. (3) 366 Oh,S.W. (2.1) 61; (2.2) 117; (2.6) 170,212,213 Oh,S.Y.(3) 366 Ohashi, H.(3) 733 Ohashi, M. (2.3) 94, 100; (2.4) 235

Author I d x Ohashi, Y. (1) 307; (2.2) 46; (2.4) 20,250; (2.6) 5 1

Ohba, S.(2.1) 30,31; (2.5) 49,92; (2.6) 228

Ohba, Y. (1) 234 Ohe, C. (3) 253 Offine, Y. (2.2) 34 Ohira, M. (2.2) 44 Ohira, R (2.3) 54; (2.4) 230; (2.6) 99

Ohishi, F. (3) 3 11 Ohishi, S. (2.4) 105 Ohkawa, K. (2.6) 276 Ohkubo, K. (2.1) 83; (2.4) 49 Ohkura, K. (2.2) 67,68; (2.4) 192 Ohno, H.(3) 659 Ohno,K. (2.4) 319; (2.6) 101 Ohno, T.(2.5) 150; (4) 14 Ohta, K. (1) 190; (2.5) 119, 124;

(4) 28,29 Ohta, N. (1) 497 Ohta, T. (3) 545 Ohta, Y. (1) 233; (2.5) 67 Ohtsuka, H. (1) 256; (2.5) 82 Ohyama, H. (4) 40 Oishi, M.(3) 65 Oishi, S. (1) 229; (2.2) 101; (2.3) 9; (2.4) 23,312 Ojars, 0. (1) 99 Oka,K. (3) 707 Okada,A, (1) 78; (3) 640 Okada, I. (2.4) 78 Okada, K. (2.4) 107; (2.5) 199; (2.6) 183; (2.7) 213 Okada, S.(3) 720 Okada, T.(1) 138,139,252,268, 366; (2.5) 212; (2.6) 208 Okada,Y. (3) 817 Okamoto, S. (2.5) 29 Okamoto, T. (3) 401,641; (4) 40, 43 Okamoto, Y. (2.6) 47; (3) 666; (4) 33 Okamura, M.(3) 720 Okazaki, S. (2.5) 145 Okazaki, T. (2.7) 24 O'Keefe, P.K. (2.5) 135 OKeefe, S. (2.7) 99 Okita, K. (3) 4 Oku,A. (2.2) 44; (2.7) 22 Okubo, T. (4) 24 Okubo, Y. (2.2) 52; (2.5) 56, 178 Okuda,Y. (2.5) 195 Okudaira, K.K. (3) 694 Okura,I. (2.5) 86; (4) 10, 11 Okutsu, T. (1) 152 Olaj, O.F.(3) 68 Olayemi, J.Y. (3) 800

429

Oldham, P.B. (1) 441 Oldziej, S. (1) 148 Oleinik, A.V. (2.4) 42; (2.7) 49, 50 Oliveira, A.S. (3) 639 Oliver, A.M. (1) 491 Oliveros, E.(2.5) 133 Olivos, H.J. (3) 459 Olivucci, M. (1) 71, 107, 108, 115, 116; (2.3) 87; (2.5) 168; (2.6) 24 Ollier-Dureault, V. (3) 75 1 Ollikainen, 0. (1) 122 Olovsson, G.(2.1) 24; (2.5) 54 Olszowski, A. (2.6) 149 Omberg, K.M. (1) 29,279; (2.6) 11 Omenat, A. (3) 262 Omori, T. (2.1) 29; (2.5) 57 Onen, A. (3) 34 Onnetford, P. (1) 385 Ono, K. (1) 484 Ono, T. (4) 17, 18 Onu, A. (3) 697-699 Onuki, H. (2.4) 226 Ooike, T. (2.4) 106; (2.6) 88 Oosterhoff, P. (2.6) 181 Orellana, G. (1) 378,384 Orfhnopoulos, M. (2.5) 158 Oriol, L. (3) 272,273 Onit, M.(1) 124,133 Ortica, F. (1) 49; (2.1) 92; (2.4) 247; (2.5) 75; (2.6) 66 Ortiz, M.J. (2.3) 69,77; (2.4) 52; (2.6) 148 Ortiz, RA. (3) 801 Ortner, J. (4) 31 Osako, S. (4) 52 Osano, Y.T. (2.3) 33; (2.4) 90 Osawa, M. (1) 3 11 Osawa, S.(2.5) 172 O S ~ WZ.~(3) , 330-332,334 Osbome, V.A. (2.7) 72 O'Shea, K.E. (2.3) 81 Oshima, Y. (2.3) 15; (2.5) 189 Oskam, A. (2.7) 218 Osokina, N.Yu. (2.3) 79 Ostap, E.M. (2.6) 244; (2.7) 193 Ostapenko, N. (3) 337 Osteritz, E.M. (2.5) 113 Osuka, A. (1) 163,268,323 Otega, J.V. (1) 312 Othmen, K. (2.6) 181 Otomo, J. (2.5) 189 Otsuji, Y. (2.3) 70 Otsuka,J. (1) 275 Otsuka, S. (4) 60 Otsuka,T.(1) 306,307

Ottavi, G.(1) 49; (2.3) 102; (2.4)

130; (2.6) 66 O ~ ~ O S C.-H. S O ~ (1) , 113; (2.3) 85 Ouchi, A. (2.6) 298,307; (2.7) 175, 190 Owens, J.W. (1) 38 Owrutsky, J.C. (2.4) 88 Ozaki, J. (2.2) 101; (2.4) 312 Ozaki, M. (3) 443,452,496 Ozakia, Y. (1) 308

Pabunruang, T. (3) 687 Pac, C. (2.2) 75; (3) 418 Pacaud, B. (3) 838 Pack, S.D.(1) 465 Paczkowski, J. (3) 43,560 Paddon-Row, M.N. (1) 21,295,

443,491; (2.5) 87; (2.6) 218

Padias, B.A. (3) 93 Padinger, F. (4) 50 Painter, P.C. (3) 151 Pajares, A. (2.5) 222 Pal, G.(3) 40,41,51 Pal, H. (1) 83 Pal, S.K.(1) 153,171,221 Palamaru, M. (3) 697-699 Palanichamy, M.(3) 846 Palese, S. (1) 324 Palit, D.K.(2.4) 76; (2.5) 21 Pallavicini, P. (1) 396 Palm, v. (1) 122 Palmer, A.W. (1) 54,98 Palmer, B.J. (3) 108, 109 Palmes-Saloma, C. (1) 410 Palmisano, L. (1) 430 Palti, D.K. (2.1) 4 Pan, C.(3) 623 Pan, H. (3) 200 Pan, J. (2.5) 176 Pan, Y.V. (3) 629 Panda, S.P.(2.7) 41 Pandey, G.C.(2.4) 216; (3) 685 Pandey, K.K. (3) 35 1 Pandey, S. (1) 166 Pandian, RP. (1) 158 Pandurangi, R.S.(2.7) 54,55 Pang, Y. (3) 458,499,506,509 Panse, P. (4) 45 Panzer, 0. (1) 136 Paolucci, F. (1) 327,359; (2.5) 103, 104, 107; (3) 98

Papageorgiou, G. (2.4) 3 18; (2.6) 270; (2.7) 187

Papper, V. (2.3) 10, 12 Paquet, D.A. (3) 67 Paquette, L.A. (2.3) 42; (2.6) 254 Parashschuk, D.Yu. (3) 540

430

Pardasani, P. (2.2) 134; (2.4) 242, 308; (2.6) 108 Padasani, RT. (2.2) 134; (2.4) 242,308; (2.6) 108

Park, B. (3) 376 Park, B.S.(2.1) 35; (2.4) 294 Park, C.-H. (2.1) 8; (2.4) 11 Park, D.C. (4) 22 Park, D.R (2.5) 121-123 Park, H.K.(3) 524 Park, H-R. (2.7) 165 Park, J. (1) 220; (2.2) 37; (2.7) 212; (3) 758

Park, J.G. (3) 77 Park, J.S. (3) 186,522 Park, J.W. (1) 289; (2.5) 78 Park, K.(2.6) 234 Park, K.H.(2.2) 13; (2.5) 196; (2.6) 61,230; (3) 518

Park, M.S. (2.2) 13; (2.5) 196; (2.6) 230

Park, P.S. (2.1) 70; (2.6) 190 Park, S.-E. (2.5) 121 Park, S.I.(2.3) 111 Park, S.K.(2.3) 66; (2.4) 246;

(2.6) 302,303 Park, Y.S. (3) 406 Park, Y.-T. (4) 19 Parkanyi, C. (1) 101 Parker, D.(1) 393,403; (2.7) 133, 134 Parker, W.O.N., Jr. (2.6) 69 Parkhomyuk, P. (2.3) 10 Parkkinen, J.P.S. (2.4) 162 Parkkinen, S.(2.4) 162 Parmar, J.S.(3) 778 Parnas, RS.(3) 229 Parodi, L. (1) 396,402 Parola, A.J. (2.4) 134 Parsons, B.J. (1) 145 Parsons, S.(2.7) 97 Parsons, S.J. (2.7) 220 Partee, J. (1) 239 Partigianoni, C.M. (1) 282 PartingtOn, S.M.(2.4) 123,124 Partridge, M.G. (2.7) 220 P a r ~ ~ eA.B.J. l, (1) 88-90,92, 105; (2.6) 221 Pascal, Y.(2.4) 249; (2.6) 50 Pascuzzi, N.(3) 115 Pashhek, V.Yu. (3) 141 Pasheni, L. (1) 327; (2.5) 103 Pasquale, T. (2.7) 57 Pastor, J. (2.4) 327; (2.7) 204 Patel, D.(2.2) 112; (2.4) 176 Patel, H. (2.2) 86 Patrick, B.O. (2.4) 292; (2.6) 150 Pa& M.(2.5) 98; (2.6) 210

Pauls, J. (2.2) 86 Pauls, S.W. (1) 456 Paulson, M. (3) 739,794 Paulus, W. (3) 261 Pautov, V.D. (3) 601 Pavlik, J.W. (2.6) 128, 129 Peacock, R.D.(1) 403 Pecci, L. (2.5) 188 Pecka, J. (1) 412 Peckan, 0. (3) 575,603,65 1,652 Pedersen, H.B. (2.7) 183 Pederson, T.G. (3) 399 Pedma, A.M. (2.2) 118; (2.6) 100 Pedulli, G.F. (2.1) 6 Pei, D.(2.6) 237 Pei, J. (3) 527 Peinado, C. (3) 17,21,22 Pekcan, 0. (1) 418 Pelagat&i, P. (2.6) 23 I; (2.7) 192 Pelegrini, R.(3) 8 18 Pelka, M.(1) 269 PeUunann, C.(2.3) 110 Pellet, J. (3) 808 Peltola, T. (1) 300 Penenory, A.B. (2.5) 208 Peng, 2.(3) 497,498,665 Peniche, C.(3) 94 PeAett, C.S.(2.6) 147 Penn, B.G. (3) 183 P e w S.(3) 693 Penzkofer, A. (3) 475 Pepe, G. (2.4) 131; (2.6) 74 Pepe, I.M. (2.3) 99 Peralta-Zamora, P. (3) 8 18 Perez, D.S. (3) 737,738 Perez, E. (1) 417 Perez-Lustres, J.L. (2.4) 75; (2.6) 157

PB~z-PrietO,J. (2.1) 74; (2.3)

127; (2.6) 168,280; (2.7) 159

Pergushov, V.I. (2.3) 79 Pernisz, U.(3) 444 Perrones, M.G.H. (3) 737 Persson, 0. (2.4) 227-229; (2.6) 277

Perutz, RN. (2.7) 97,220 Pesce, A. (2.4) 167; (2.6) 278 Pete, J.-P. (2.2) 22,23; (2.4) 186 Peterka, D.S.(2.7) 128 Peters, E.-M. (2.2) 104; (2.6) 106 Peters, K.(2.2) 104; (2.6) 106 Peters, 0. (3) 800 Petersen, J.D. (1) 245 Petit, M.A. (3) 583 Petkov, I. (2.4) 79 Petric, A. (1) 414 Petrochenkova, N.V. (3) 660,66 1 Petrov, A.K. (2.7) 110

Photochemistry Petrov, E.P. (1) 123,439 Pettersson, M.(2.7) 208 Pettus, T.RR (2.2) 82 Petucci, C. (2.3) 61 Peukert, S.(2.1) 7 Pezacki, J.P. (2.7) 15,21 Pezeli, E.(3) 701 Pezolet, M. (3) 375,377 Pfab, J. (2.7) 111 Pfeffer, N. (3) 504 Pfleiderer, W. (2.4) 323; (2.7) 196 Phillipe, L.(2.7) 114 Piau, J.M. (3) 655 Pichandi, S.(3) 42 Pichon, N. (3) 841 Pickett, T.E. (2.7) 222 Piel, J. (1) 146 Pierini, A.B. (2.5) 68; (2.6) 202 Pietri, N.(2.2) 47 PiM. (3) 43,560 Pilichowski, J.F. (3) 805,808 Pilling, M.J. (2.7) 113 Pimicnta, V. (2.6) 75 Pina, F. (1) 178; (2.2) 72; (2.4) 134, 135; (2.6) 160

Pincock, A.L. (2.4) 60,171 Pincock, J.A. (2.1) 21; (2.4) 60, 171

Pindur, U.(2.4) 248 Pines, R.(1) 165 Pinghini, R (4) 47,48 Phi, E.(2.6) 198 Pinol, M. (3) 272,273 Pinto, D.C.G.A. (2.4) 243 Pirowsika, K.(1) 297; (2.5) 128 Pirrung, M.C. (2.6) 234 Piryatinsky, Y.P. (1) 155 Pisulina, L.P. (2.4) 66 Piszcek, G. (1) 209 Pitchumani, K.(2.3) 17; (2.4) 184; (2.6) 93

Pittman, C.U. (2.4) 222 Piva, S.(2.1) 72 Piva-Le Blanc, S. (2.1) 72 Pivin, J.C. (3) 356 Pizolet, M. (3) 408 Pizzocaro, C. (1) 247 Pla, F. (3) 647 Plantard, J. (1) 133 Platz, M.S. (2.4) 297; (2.7) 13-15, 19,34,55

Pliva, C.N. (2.5) 24; (2.6) 136 Plonka, A. (3) 573 Plyusnin, V.F.(2.7) 176 Plzak, Z. (2.6) 307 Pocar, D. (2.4) 258; (2.7) 9 Podina, C. (3) 331,332 Poga, C. (3) 504

Author In&x Poggi, A. (1) 402 Pohlers, G. (2.6) 200; (3) 618

Poindexter, B.D. (3) 688 Pokorna, V. (1) 412 Pokorna, 2.(2.5) 170 Pola, J. (2.6) 298,307; (2.7) 175 Poliakoff, M. (2.7) 86 Polishchuk, A.P. (2.5) 81 Polishchuk, I.Yu. (2.5) 8 1 Poljansek, I. (3) 59 Pollet, A. (1) 150 Pollicino, A. (3) 788 Pollinger-Dammer, F.(1) 500 Polowinski, S. (3) 96 Polton, A. (3) 23 Polubisok, S.A. (2.7) 182 Polyani, J.C. (2.7) 139 Pornerantz, M. (3) 537 Pomogailo, A.D. (3) 614 Ponterini, G. (1) 433 Ponyaev, A.I. (2.4) 129; (2.6) 71; (3) 392

Popielarz, R (3) 220,221,228 Popovici, D. (3) 772 Popp, B. (2.2) 140; (2.5) 37 Porinchu, M. (2.2) 56 Porsch, M.J. (3) 378 Port, H. (1) 321 Postnikov, L.M. (3) 333, 702,703 Poteau, X. (1) 305 Poth, L. (2.7) 136 Pottier, E. (2.4) 132; (2.6) 73 Poulsen, T.D. (1) 184 Powell, D.R.(3) 5 19 POZZO, J.-L. (2.4) 128, 131; (2.6) 72

P d e e p , T. (2.7) 85 Pradhan, D. (2.6) 126 Prager, R.H.(2.4) 307; (2.6) 191 Prasad, A.V. (3) 709 Prasanna de Silva, A. (1) 293; (2.5) 214

Prasassarakich, P. (3) 687 Ptathap, S. (2.2) 56 Prato, M.(1) 74,327,338,360; (2.5) 101, 103, 104, 108

Premila, M. (1) 334 Pretre, P. (3) 441 Prevenslik, T.V. (1) 94 Previtali, C.M.(I) 250,253,262; (2.5) 112,213; (2.6) 229

Prinzie, Y. (3) 549 Priou, C. (3) 144 Priyadashy, S. (2.4) 111; (2.6) 85 Priyawan, R.(3) 201 Probst, E.L. (2.1) 67 Prochazka, K. (1) 426; (3) 585, 5 89

43 1

Prodi, L. (1) 397,399 Prohno, A. (2.6) 201 Prospito, P. (1) 180; (2.4) 74 Pryce, M.T. (2.7) 99 Pus L.(3) 526 Puget, F.P. (2.2) 107; (2.4) 174 Pugh, M.L. (2.2) 85 Pumnik, V.G.(2.1) 5 1; (2.5) 72 Purtov, P.A. (2.6) 317 Purushothaman, E. (3) 365 Pushkarsky, M.B. (1) 488 Pushpan, K.S.(1) 158 Pyo, S.M. (3) 5 18,524 Qi, F. (2.3) 125 Qi, G.(3) 19 Qian, J. (3) 495 Qian, S. (3) 495 Qiao, G.G.-H. (2.2) 88 Qiao, L. (2.6) 238 Qin, J. (2.6) 42; (3) 387 Qin, 2.(3) 665 Qiu, K.Y.(3) 9,279 Qu, B. (3) 168 Quadrelli, P. (2.6) 195 Quathucci, J. (2.6) 309 QuazzOtti, S. (1) 292; (2.2) 141; (2.5) 38 Quick, A. (1) 238 Quinones, E. (1) 424

Rabe, J.P. (3) 437 Rabck, J.F. (3) 28,240 Racioppi, R (2.4) 167,284; (2.6) 278,279

Radeke, H.S. (2.7) 58 Radhakrishnan, T.S. (1) 334

Radloff,W.(2.7) 108, 142 Radotic, K. (3) 742 Rady, E.A. (2.6) 249 Radzig, V.A. (2.3) 79 Raether, RB. (3) 88, 100,134 Raferty, D. (2.3) 61 Ragauskas, A.J. (3) 739,744,794 Rahman, M.S. (1) 362

Raithby, P.R (2.7) 103-105 Rajan, K.S. (3) 224 Rajan, P. (2.5) 129 Rajasakhafan, K.N. (2.6) 232 Rajendran, S.P. (2.4) 261; (2.6) 53 Rajesh, K. (3) 156 Rajeswari, N.(3) 256 Raju, B.B.(1) 210,420; (2.6) 139 Rakosa, R.C. (3) 736 Ram, M.K. (3) 156 Ram, N.(I) 463

Ramachandra, P. (3) 754 -dram, B.(1) 428 Ramachandran, N.(3) 183 Ramakrishnan,V.T. (2.5) 62; (2.6) 288

Ramamurthy, P. (1) 255; (2.6) 135 Ramamurihy, V. (2.2) 91; (2.3)

113; (2.5) 115

Ramani, R (3) 754 Ramanujam, P.S. (3) 399 Ramaraj, R (2.5) 129 Ramasamy, S.M.(1) 381 Ramgopal, G.(3) 754 W a l l , P. (2.4) 298; (2.7) 52 Rammert, K.F.(2.5) 8 Ramsey, J.M. (1) 129, 132 Ranby, B. (3) 289,327

Ranganathaiah, c. (3) 754 Rangarajan, B.(3) 172 Ran Shin, D.(2.5) 194 Rao, G.(1) 463,464,468 Rao,J.V. (2.5) 115 Rao, P. (2.5) 118 Rao, P.D. (2.2) 59 Rao,V. (2.5) 55 Rao, V.J. (1) 225; (2.3) 16, 113 Rao, V.R (2.4) 224 Rao, (3) 539 Rapp, W. (1) 413; (3) 556 Rappoport, 2.(2.3) 60; (2.4) 220 W e n , M. (2.7) 208 M e , U. (2.6) 163 Rash, V.V. (2.6) 273 Rasoul, F.A. (3) 761 Rath, J.K. (4) 47 Rath, M.C. (2.4) 76 Rath, N.P. (2.4) 64 Rathi, RC. (3) 410 Ratner, M.A. (1) 19,56; (2.5) 1 Ratner, V. (2.3) 10 Rau, H. (2.5) 168 Rau, J. (1) 170 Rau, U.(4) 56 Raulerson, P. (2.2) 86 Rauscher, C.(1) 494 Rava, RP. (2.4) 329 Ravichandran, R (2.4) 232 Ravikrishna, C. (2.1) 65 Ray, A.K. (1) 493 Ray, R. (2.7) 59 Raya, 1. (2.2) 20 Raymo, F.M. (1) 238; (2.6) 49 Raper, D.M.(2.7) 68 Razumov, V.F. (3) 614 Razumova, T.K. (1) 222 Rebane, K. (1) 122 Rebizak, R. (3) 647 Recca, A. (3) 788

x.

43 2

Re&, G. (2.5) 52 Reddy, G.D.(2.3) 120 Reddy, M.A. (1) 228;(2.4) 41 Reddy, RS.(2.2) 109 Redke, C.J. (1) 486 Redy,G.S.(4) 20 Ree, M. (3) 518 Rees, M.T.L. (3) 254 Ree&, I. (3) 14 Regenstein, W.(1) 432 Regitz, M.(2.4) 304 Reich, W.(3) 208,765 Reichert, D. (2.6) 115 Reichow, S. (2.2) 26 Reilly, J.P.(2.1) 5 Reimann, G.(3) 247 Reis, M.J.(3) 639 Reisenauer, H.P. (2.6) 293,294 Reisinger, A. (2.2) 88; (2.4) 56; (2.7) 61

Reisla, H.(2.7) 66 Reitberger, T.(3) 327 Remillard, J.T. (3) 688 Remmers, M. (3) 504 Rempala, P. (2.7) 17 Rernpel, U.(1) 102,265; (3) 630 Ren, B.(1) 408 Renfro, M.W. (1) 465 Renner, R (3) 236 Rennie, M.-A. (2.7) 103 Reschgenger, U.(3) 656 Resmi, M.R (2.7) 85 Rettig, W.(1) 177,203,207; (2.4) 73; (2.6) 155,224-226

Reyes, J. (3) 818 Reynier, N. (2.4) 44 Reynisson, J. (2.2) 48; (2.4) 40 Reynolds, J.R (3) 510 Rhan, A.E. (2.2) 126 Rhee, S.B.(3) 529 Rheingold, A.L. (2.7) 102 Ribbe, A.E. (3) 542 Ricci, J.S. (2.6) 309 Ricco, A.J. (3) 214 Rice, T.E. (1) 293; (2.5) 214; (2.6) 214

Richard, C. (2.6) 223; (2.7) 167 Richards, C.J. (2.7) 222 Richardson, M.F. (3) 576 Richau, K.(3) 28 1 Richert, R. (1) 169 Richter, C.(2.4) 219 Richter, F. (3) 704 Rick, c. (2.2) 102 Ridley, T.(2.5) 135 Riedle, E.(1) 146 Rieke, P.C. (3) 294 Rieker, A. (1) 365; (2.5) 105

Riela, S.(3) 732 Rieumont, J. (3) 66 Rigler, R (3) 825 Rim, K.T.(2.7) 146 Rinderghagen, H. (2.6) 306 Riou, A. (2.6) 282 Ritter, T. (1) 269 Rittig, F. (2.6) 38 Rivaton, A. (3) 681 Rixmaa, M.A. (3) 512

Rizzoli, R (4) 47,48 Roa, P. (3) 845 Robb, M.A. (1) 71,108, 115, 116; (2.3) 82.87; (2.6) 24

Robertiello, A. (4) 61 Robertson, S.H.(2.7) 113 Robins, M. (1) 38 Robins, R.H. (2.6) 193 Robinson, E. (3) 510 Robinson, J.C. (2.3) 58; (2.7) 130; (3) 835

Robinson, R (1) 38 Rochon, P.(3) 375 Rockett, A. (4) 45 Roder, B. (1) 365; (2.5) 105, 130 Rodgers, M.A.J. (1) 39 Rodica, M. (4) 9 Rodighiero, P. (1) 154; (2.2) 79 Rodrigues, M.R. (3) 29 Rodriguez, A.D. (2.3) 67 Rodriguez, M.A. (2.4) 255,256; (2.6) 64

Rodriguez, M.C.R (2.6) 158 Rodriguez-Prim, F. (2.4) 75; (2.6) 157, 158

Roeder, B.(I) 479 Roesch, J. (3) 773 Roest, M.R. (1) 2 1 Rofia, S. (1) 327,359; (2.5) 103, 104, 107; (3) 98

Rogers, C.(3) 646 Rogowski, J. (2.7) 55 Rohr, U.(3) 487 Rohrbaugh, D.K. (2.3) 80; (2.6) 272

Rojas, J.K. (2.6) 161 Rokitskii, RI. (3) 540 Rol, C. (2.5) 202 Rolinski, O.J.(1) 401 Roman, J.S. (3) 94 Romeo, G.(1) 380 Romero Sdas, E.A. (3) 801 Roncali, J. (2.6) 282 Rong, T.-W. (1) 333,354 Rontani, J.-F. (2.5) 11 Roomy, M.L. (2.5) 146 Roos, B.O.(1) 103 Ropot, M. (3) 211

Photochemistry Rosario, 0. (1) 424 Rosch, N.(1) 109 Rose, D.H. (4) 42 Rosenwaks, S.(2.3) 128; (2.7) 118, 124

Ross, A. (2.7) 78 Rossi, E. (2.6) 198 Rossman, D.I. (2.3) 80; (2.6) 272 Rota, C. (3) 828 Roth, H.D.(2.3) 73-75; (2.4) 283; (2.5) 163, 167

Roth, S.C.(3) 636 Rouge, N. (3) 747 Row, T.N.G. (2.2) 3 1.32; (2.4) 273

Rowland, S. (2.4) 293

Royall, D.(3) 341 Royer, D. (2.4) 55; (2.6) 144 Rtishchev, N.I.(2.4) 38,65; (2.6) 250; (3) 150

Ruane, P.H. (2.6) 169; (2.7) 211 Rubin, M.B.(1) 319; (2.2) 102 Rubinelli, F.A. (4) 47,48 Rubinov, A.N. (1) 439 Rubner, M.F. (3) 5 10,520 Rubtsov, I.V. (1) 199 Rucando, D. (2.2) 62,63; (2.4) 202,203; (2.6) 110, 112

Ruckemann, A. (2.5) 39 Ru&tuhl, T. (1) 473,48 1 Rud, V.Yu. (4) 44 Rud, Yu.V. (4) 44 Rudnink, S.M. (1) 391 Rudschuck, S.(3) 650 Rueckemann, A. (1) 290,29 1 ; (2.5) 40

Rueckcrt, I. (1) 197 Ruffin, B.(3) 732 Rufs, A.M. (3) 597,604 Ruggiero, R (3) 737,738,839 Ruhlmann, L. (2.5) 229 Ruhmann, R. (3) 364,388 Ruiz, A.M. (3) 262 Ruiz, J. (2.1) 63; (2.7) 63 Rulliere, C.(2.6) 225 Rumbles, G.(2.4) 295; (3) 3 19, 460,535

Ruprecht, R. (3) 173,247 Rurack, K.(3) 656 Rusakov, V.S.(3) 293 Ruslim, C. (2.4) 46; (2.6) 32,33 Russell, G.T.(2.7) 99; (3) 254 Rusu, E.(2.7) 5 I RUS& I. (3) 820-822 Rutloh, M.(1) 206; (3) 400,422, 423

Ruua, A.A. (3) 578 Ruzzi, M.(1) 327; (2.5) 103

Author Index Ryan, J.F. (2.4) 295

Rychly, J. (3) 325 Rytov, B.L. (3) 621 Rytz, G. (3) 335 Rzaev, Z.M.O. (3) 215,759 Saadioui, M. (2.4) 44

Sacher, E. (3) 772

Sadafule, D.S. (2.7) 41 Sadeghi, M.M.(2.2) 81; (2.4) 280 Sadeghpoor, R (2.4) 62 Saeva,F.D. (2.6) 274 Sagdew, R.Z. (2.1) 18; (2.6) 317 Sagisaka, T. (2.4) 18 Sagredo, R (2.7) 43 Saha, P. (3) 235 Saha-Moeller, C.R (2.5) 154; (2.6) 138, 185 Sahely,N. (2.7) 95 Sahu,N. (4) 15 Sahu, S.K.(2.7) 41 Saicic, R.N. (2.1) 8 1 Said, Z.F.M. (1) 416; (3) 622 Saifbl, I.S.M. (1) 234 Saijo, H.(3) 338 Saiki, H.(4) 65 Saile, V. (3) 694 St.John Manley, R (3) 745 Saito, I. (2.2) 66,78 Saito, K.(2.6) 196 Saito, M. (2.1) 33 Saito, S.K.(1) 312 Saitoh, T. (3) 649 Sakai,M. (2.4) 265,3 19; (2.6) 101,102 Sakai,S. (2.3) 86 Sakai,T. (3) 345 Sakaki, S. (1) 256; (2.5) 82 Sakamoto, F. (2.4) 262 Sakamoto, K.(2.6) 292; (2.7) 177 Sakamoto, M. (2.2) 116; (2.4) 204,234,263,285; (2.6) 114, 267-269 Sakamoto, Y. (2.3) 108; (2.4) 190 Sakata, T. (2.5) 126 Sakata, Y. (1) 280,361,366; (3) 610 Sako, K.(2. I) 69 Sako, M. (2.2) 5 1 Sakthivel, S. (3) 846 Sakumgi, H.(2.1) 17 Sakumgi, M. (2.4) 27 Sakumi, H. (2.6) 296; (2.7) 174; (3) 345 Sakurai, N. (1) 344,345 Sakumi, T. (1) 400; (2.3) 132; (2.4) 50,252,306,3 16; (2.6)

65, 184; (3) 649 Salatelli, E. (3) 393 Salazar, F.A.(1) 337 Salem, A.E.(4) 67 Salemi-Delvaux, C. (2.4) 132, 148; (2.6) 73,78 Salesse,C.(1)411 Salimgareeva, V.N. (3) 217 Salleh, N.G.K. (3) 18 Salter, R (2.7) 222 Saltiel, J. (I) 114; (2.3) 95,96; (2.4) 26,28; (2.6) 29 Samanta, A. (1) 428 Samanta, B. (2.2) 57 Samanta, S.B.(3) 156 Samat, A. (2.4) 128, 146, 147; (2.6) 72,74,77 Sampei, A. (3) 402 Samuel, I.D.W. (3) 319,460,517, 535 Sanchez, C. (2.2) 118; (2.6) 100 Sander, W. (2.1) 65; (2.7) 18,23, 40 Sanders, B.M. (3) 787 Sandman, D.J. (3) 512,520 Sanford, M. (3) 724 Sang, H.C.(3) 557 Sang, S.S. (3) 178 Sangen, 0. (3) 274 Sanji, T. (3) 345 Sanjuan, A. (2.6) 25 1 Sankarapandran,M. (3) 241 Sannikova, N.S.(3) 217 Sano,T. (2.2) 103; (2.4) 266; (2.6) 107 Santa, T. (1) 104,149 Santhosh, K.C. (2.5) 179 Santos, c. (1) 337 Santos, H.(1) 178; (2.6) 160 Santos, L. (I) 422 Santos, S.F. (3) 578 Santra, S. (2.6) 22 Santus, R (1) 249 Sapich, B.(3) 278,400,437 Sappok-Stang, A. (1) 101 Sapre, A.V. (2.1) 4; (2.5) 21 Sapunov, V.V. (1) 304 Sarabia,2.(2.1) 74; (2.6) 168, 280 Saran, A.A. (3) 291 Saravanan, J.S.(1) 255 Saravanan, K.(2.2) 109 Saricifici,N.S. (3) 494,503; (4) 50 Sarker, A.M.(2,l) 97; (2.5) 46; (2.6) 174; (3) 130, 133,455 Sarobe, M. (1) 449 Saroja, G. (1) 428

Sasabe, H. (3) 90 Sasai, R. (2.4) 9

433

Sasaki, A. (2.7) 48 Sasaki, C. (2.3) 33; (2.4) 90 Sasaki, K. (1) 475 Sasaki, S. (2.6) 320 Sasaki, T. (2.3) 107 Sasaki, Y. (1) 351,364 Sass, K. (3) 216 Sasst, W.H.F. (1) 277; (2.5) 85 Sastre, R (1) 142; (3) 66,94,125 Sasuga, T. (2.3) 71 Sato, H. (2.4) 159; (3) 177,720 Sato, K. (2.6) 183; (2.7) 213 Sato, M. (2.4) 262; (2.5) 99 Sato, N.(3) 253 Sato, R (3) 63,64 Sato, S. (2.7) 87 Sato, T. (2.5) 236; (2.7) 76,77; (3) 56,525 Satoh, H. (2.3) 107 Sattlegger, M. (4) 3 1 Sauder, D.G. (1) 429 Sauer, S. (2.I) 38 Saupe,G.B. (3) 669 Sausa, RC. (2.7) 5 Sauter, J. (2.6) 141 Sauvage, J.-P.(1) 313,314,398 Saveant, J.-M. (2.6) 209 Sawada, T. (2.5) 193 Sawaki, Y. (1) 332; (2.1) 60; (2.2) 2.3; (2.3) 101; (2.4) 267,268; (2.5) 143,235; (2.6) 257 Sawan, S.P. (3) 148 Sawayama, S. (4) 59 Saygh, A. (2.7) 102 Scaiano, J.C. (2.1) 76,93; (2.3) 124, 127; (2.4) 241,293; (2.5) 24,25,36, 168; (2.6) 136, 137,200,251; (2.7) 159; (3) 618 S M , G.M. (3) 470 Schadowski, R (3) 793 Schaedeli, U. (3) 10 Schaefer, H.F., 111 (2.7) 65 Schaefer, K. (3) 793 Schael, F. (1) 3 19 Schafer, J. (2.7) 152 Schafer, M. (4) 3 I Schambony, S.B. (2.5) 154 Schamschule, R. (1) 89.90 Schanze, K.S.(1) 41; (3) 357, 488,510 Scheffer, J.R (2.1) 24,26,27; (2.2) 91; (2.4) 292; (2.5) 54; (2.6) 150 Scheirs, S. (3) 810 Schellenberg,B. (3) 797

434 Schenk, G.(1) 3 Schepp,N.P. (2.1)77;(3)740 Scherf, U.(3) 474,477,485,487 Schiavello, M.(I) 430 Schikarski, T.(1) 196 Schilling, M.L.M. (2.3)75 schim, S.C. (3) 454 Schinke, R (2.7)69 Schiichthorl, G. (4)35 Schiichting, P.(3)487 Schlick, S.(3)573 Schmickler, H. (2.1)39,50;(2.4) 209 Schmid, R.P. (2.7)124 Schmid, W.E. (1) 196,226 Schmidt, H. (3)411 Schmidt, H.W. (3)261 Schmidt, J. (1) 131 Schmitz, C.(4)31 Schmitz, G.(4)8 Schmoldt, P.(2.1)3.56 Schnabel, W.(2.6)172;(2.7)10; (3) 38, 143,565 Schneider, F.W. (1) 105 Schneider, S.(3) 704 Schnorpfeil, C.(2.3)8 Schnurpfeil, G. (2.5) 130 Schock, H.W. (4)44 Schoeneich, C.(2.5)237 Schoenle, A. (1) 489 Schonhols, A. (3)400 Schoonover, J.R (1) 245,279 Schott, M.(3) 450 Schreiber, E.(2.7) 142 Schreiber, K.P.(2.2)86 Schreiber, M. (I) 298 Schreiber, S.L.(2.4)33 1,332; (2.7)202 Schroeder, D.J.(4)45 Schroeder, J. (2.1)47;(2.5)70 Schroeder, R. (3) 195 Schroer, F.(2.2)36;(2.5)94; (2.6)89 Schropp, R.E.I. (4) 47,48 Schroth, W. (2.4)70;(2.6)289 Schiiler, G. (2.7)56 Schuetz, G.J. (2.5)43 Schultz,A.R.(3)241 Schultz, J.W.(3)263 Schumacher, A. (2.1)38 Schuster, D.I. (1) 341,367,368; (2.5) 171,232 Schuster, G.B. (2.2)139;(2.3) 131;(2.4) 119 Schwack, W. (2.6)248 Schwalm, R.(3) 208,765 Schwiutz, B.J. (3)478 Schwartz, M. (3) 750

Schwartz, S.D.(1) 498

Schwan, 0.(1) 237;(2.5)88 Schwebel, C.(2.2)30 Schweig, A. (1) 192 Schweigcr, G.(3) 71,633 Schweitzer, B. (3)541 Schweitzer, G.(1) 3 10 Schwendner, P.(2.7)69 Schwoegler, A. (I) 291;(2.5)40 Scigalski, F. (3) 560 Scoponi, M.(3)674 Scordino, A. (1) 406 S c o r n , G.(1) 338,358-360; (2.5)101,107, 108 Scott, G.W. (2.4)279 Scdt, J.C. (3)5 16 Scott, J.D.(3)696 Scranton, A.B. (3) 80,172,223, 246,251 Scriven, L.E. (3)239 Scurlock, RD.(3)708 Seakins, P.W. (2.7) 113 Sears, D.J., Jr. (2.6)29 Sebastiani, G.V. (2.5)202 Sebek, P. (2.3)68;(2.4)53 Sebenik, A. (3) 59 Seeger, S.(1) 473,481 Seel,M.(1) 59 Seely,G.R (2.5)30,233 Segawa, H.(3) 525 Seguchi, K.(2.2) 108 Segura, J.L.(1) 370 Seibold, M.(1) 321 Seide, C.A.M. (3) 825 Seino, H.(3) 185 Sekatski, S.K.(1) 472 Sekhar, B.B.V.S. (2.2)24;(2.6) 315 Seki, K.(2.2)67,68;(2.4)192; (3)694 Seki, S. (3)706 Seki, T.(3) 75,398 Sekihara, A. (2.5)226 Sekikawa, J. (2.2)83 Sekine, A. (1) 307;(2.4)20 Sekine, N.(2.2) 116;(2.4)263; (2.6)267 Sekinwa, H. (3) 398 Sekkat, Z.(3)441 Seko, T.(1) 157 Selier, P. (1) 394 Selitrenikov, A.V. (2.4)65;(2.6) 250 Sellstrocm, U. (2.7)169 Selvam, T.(2.6)187 Semenov, V.V. (3)293 Sen, A. (1) 316 Sen, S.(1) 459

Photochcmistry

Senanayake, P.K.(1) 393

Sendova-Vassileva,M. (3) 356 Seng, H.P. (3)250 Sengupta, P.K.(1) 175 Senning, A. (2.2)19; (2.4)2 Seno, M.(3) 56 Seong, H.(3) 462 Seraphin, A. (2.2)26 Serrano, B. (3)219 Serrano, J. (3) 219,233,234,272, 273 Serre, F. (3)681 Sermni, S. (3)555 Sertova, N. (2.4)79 Seta, P.(1) 330,335;(2.5) 169, 229;(3) 610 Seth, J. (1) 161, 162,164 Setnescu, R.(3) 330-332 Setnescu, T.(3) 33 1,334 Seung, M.(3) 366 Seus, P.(2.3)8 Seya, K.(2.5)218 Sezaki, F. (2.5)236 Shabnari, M.R (1) 377 Shade, J.E. (2.7)102 Shah, M.(3) 18 Shair, M.D. (2.4)331 Shan,F.(3) 495 Shang, C.(3)283 Shanks, RA. (3) 81 1 Shao,L. (3)82-84 Shao,Y.(3)695 Shapiro, V. (2.3)120 Shaplty, J.R. (2.7)96 Sharma, B.K. (2.5)118;(3) 845 Sharp, D.S.(3) 179 Sharpe,A. (2.2)112-114;(2.4) 176 Shayira, B.H. (2.4) 184 She, W.(3) 142 Shekar, B.B.V.S.(2.6)314 Sheldon, P. (4)42 Shen, T.(1) 160,241;(2.5)42 Shen, Z.X.(3) 190 Sheng, L. (2.3)125 Shereshovets, V.V. (2.5)147 Sheridan, R.S.(2.7)17 Shevchenko, T.(2.1) 12 Shi, H. (3) 112 Shi,J. (3) 342,492 Shi,J.G. (2.3)67 Shi,M. (2.3)4,6;(2.4)63 Shi, S.K. (3) 24 Shi, W.(3)86, 168 Shi, X. (3) 638 Shi, Y.(1) 346;(2.3)130;(2.5) 176 Shibaev, V.P. (2.4) 163

Author Index

Shibasaki, M. (2.7)37 Shibuya, K. (1) 68;(2.1)9 Shichi, T.(3)244 Shida,T. (2.7)137 Shiga, T. (3) 640 Shigemizu, H.(2.4)34 Shigeri, Y. (2.2)34;(2.6) 15 Shigesato,Y.(2.7)198 Shiina, S.(2.3)100 Shilov, LA. (3) 735 Shim, A.K. (3) 57 Shim, H.K. (3) 454,479,491 Shim, S.C.(2.3)66, 11 1; (2.4) 169,246;(2.6) 197,302,303; (2.7) 117 Shima, K. (2.2)75;(2.3)48,54; (2.4)166,230;(2.6)99 Shimada, S.(3) 26 Shimadzu, T. (3) 525 Shimakage, T.(2.6)236;(2.7) 194 Shimamoto, K. (2.2)34 Shimamura, T.(2.3)15 Shimizu, H.(2.4) 179;(2.6)263 Shimizu, K. (4)55 Shimizu, M.(2.4)285;(2.6)269 Shimizu, 0.(2.5) 136, 137 Shimizu,Y. (2.5) 119, 124;(4) 28.29 Shimo, T. (2.2)89,90;(2.4)207, 21 1 Shimomura, A. (1) 366 Shimoyama, S.(3) 627,644 Shin, J.H. (3) 106 Shin, K.J. (1) 50,55 Shin, P.S. (2.2) 13; (2.5) 196; (2.6)230 Shin, T.J. (3) 518,524 Shinar,J. (1) 239 Shindo, Y.(2.3)24;(2.4)89;(3) 548 Shinmyom, T. (2.1)69;(2.3)108; (2.4) 190 Shinohara, H.(2.6)122 Shinohara, S.(2.4)105,106;(2.6) 88 Shinoya, S. (1) 6 Shinotaki, K. (1) 306 Shintani, H.(3)201 Shinwell, V.(2.2)28 Shinyashiki, Y.(4)51,52 Shiobara, N.(3)290 Shiono, H.(2.7) 199 Shiono, T.(3)259,367,439,440 Shipp, D.A. (3) 69 Shipway, A.N. (1) 238;(2.6)49 Shiragami, T.(2.2)75;(2.3)48, 54;(2.4)166,230;(2.6)99

435 Simeonidis, K. (1) 494,495 Simeonsson, J.B. (2.7)5 Simionescu, B.C.(3) 21 1 Simons, J.P.(1) 32 Simonsen, K.B. (1) 357 Simpson, I.D.(2.6) 147 Sims, S.M.(2.6)193 Sindler-Kulyk, M. (2.3)1 18;(2.4) 271 Sing, V.(2.1)68 102, 103;(2.6)34 Singh, A.K. (1) 223;(2.1)4;(2.4) Shishido, T.(3) 829 25,99;(2.5)21 Shizuka, H.(1) 91, 173,332;(2.4) Singh, D.L. (1) 162 77,286,287;(2.5)65,143; Singh, J.P.(2.7)119 (2.6)204,304 Singh, M.S. (2.6)180 Shkunov, M.(3)452 Singh,R.P. (3) 700,709,778 Shobha, H.K. (3)241 Singh, V.(2.2)56,57 Shoro, T.(2.3)132;(2.4)3 16; Sinha, A.S.K. (4)15,16 (2.6)184 Sinha, S.(1) 240,244 Shrestha, N.K. (2.1)9 Sinibaldi-Troin, M.E.(2.2)45; (2.4)25 1; (2.6)52 Shroeder, J. (3) 216 Shshegolinkhina, 0.1. (3)293 Sinke, W.C. (4)4 Shtykov, S.N.(3) 594 Sinta,R (2.6)200 Shu, J. (2.7)118;(3) 389 Sipior, J. (1) 463 Shubeita, G.T. (1) 472 Siskos, M.G. (2.6)171;(3)35 Shuele, D. (3)236 Sivachenko, A.Yu. (1) 66 Shui, X.Q.(2.2)139 Skelton, B.W. (2.7)95 Shuiga, A.M. (1) 265 Sket, B. (2.3)14;(2.7)160 Shukla, D.(3)740 Skokan, E.V.(3) 74 Skowrolnski, J.M.(4)27 Shul'ga, A.M. (1) 317 Sliwinska, E.(2.6)149 Sicinski, R.R (2.3)104 Siddiqui, J. (3) 605 Sloan, D.(2.7)166 Sidorov, L.N.(3)74 Sluch, M. (3) 5 17 Sieburth, S.M.(2.2)61-63;(2.4) Sluggett, G.W. (2.6)300 202,203;(2.6) 109-112 Smet, M. (3) 549 Siedschlag, C. (1) 339;(2.5)173, Smith, B.R (1) 71;(2.3)87;(3) 414 174 Siegela, S.J. (3) 414 Smith, C.A. (3) 679 Siegner, C. (2.4)201;(2.6)142, Smith, D.A. (3) 544 143 Smith, D.R (1) 243 Sieiro, C. (3)233,234 Smith, E.R.(3) 830 Siemensmeyer, K. (3) 261 Smith, G.J. (2.2)49 Smith, J.A. (2.4)307; (2.6)191 Siemiarczuk, A. (3)569 Siemund, V.(2.1)36 Smith, J.RL. (2.7)72 Siggel, U.(1) 258;(3) 581 Smith,R (1) 38 Smith, RC.(3)489 Sigrist, H.(2.7)27,28 Smith, T.A. (3)69 Sigwalt, P. (3) 23 Smith, W.E. (2.6) 154 Sijbesma, R.P. (3) 213 Sikharulidze, D.(1) 461 Smitha, K. (2.7)85 Sikorski, M. (1) 15, 147;(2.5)48; Smolka,T.(2.6) 134 (2.6)266 Smolyanskii, S.A. (3) 343 Silbey, R.J. (1) 61;(3)449 Snapper, M.L. (2.7)58 Silen, A. (1) 431 Snider, B.B.(2.3) 114;(2.4)270; Silva, A.M.S. (2.4)243;(2.5)231 (2.6)97 Silva, C.R (2.1)5 Snook, J.H. (2.3)114;(2.4)270; (2.6)97 Silva, F.RG. (1) 75 Snowwhite, P. (3) 755 Silva, M.T. (2.2)107; . (2.4) . . 174 Sobczak, M. (2.5)50 Silva P.F. (2.2)-87 Shirai, H.(1) 274;(3)500 Shirai, K.(2.4)67;(3) 201 Shirai, M. (3) 129,582 Shiraishi, Y. (3) 534 Shirakawa, H.(3) 431,484 Shiratori, H. (I) 268,323 Shiratori, N.(4)18 Shirodai, Y.(2.1)48;(2.5)69 Shirota, H.(1) 83, 199 Shirota, Y. (1) 230;(2.4)45,98,

436 Sobkowiak, M. (3) 151 Soboleva, I.V.(1) 257 Sodeoka, M.(2.7)37 Soederstroem, G. (2.7) 169 Sogoshi, N.(2.7)137 Sohm, S.H.(2.6)18 Sohmiya, H.(2.3) 129 Sohn, S.H.(2.3)21;(2.4)33 Sokolowski, M. (2.5)113 Sokolyuk, N.T. (2.4)66 Solaro, R.(3) 393,436 Solera, P. (3)786 Solgadi, D.(1) 21 1 Solomon, D.H.(2.2)88;(3)69 Solov'ev, K.N. (3) 615 Solovskaya, A.N. (3) 150 Solovskaya,N.A.(2.4)38 Soltermann, A.T. (2.5)200,220 Somasundaram, N. (2.6)265 Somei, M.(2.4)69;(2.6)192 Sornekawa, K. (2.2)89,90;(2.4) 207,211 Sone, M. (3) 420 Song, A. (1) 241 Song, B.(3)451 Song, F. (2.4)270;(2.6)97 Song, F.B. (2.3)114 Song, H.B.(3) 128 Song, J.M. (1) 126 Song, K. (3)426 Song, X.(2.4)108 Song, Z.(3) 432 Son& N. (2.3)44; (2.6)321 Sonoda, T. (1) 329 Sonoda, Y.(2.4)27 Sonoki, H.(1) 3 11 Soo Kang, Y.(2.5) 194 Soong, C.F. (2.3)35;(2.4)233 Sophiamma, P.N. (3)25 Sopina, I.M. (3) 46 Sortino, S. (2.1)76 Soujanya, T. (1) 225,428;(2.3) 16 Sourisseau, C. (3) 377,408 Soutar, I. (3) 559,568 Sowarck, L.(1) 140 Spalletti, A. (2.3)98;(2.4)32 Spangler, C.W. (1) 140 Specht, K.G. (3) 221,228 Speiser, S.(1) 319 Spencer, N.(1) 238;(2.6)49 Spiccia, L.(2.7)95 Spiecker, H.(2.7)112 Spiliopoulous,I.K. (3) 521 Spiller, W.(2.5)130 Spindler, C.E.(3) 88, 100, 134 Spiricva, A. (3) 816 Spitler, M.T. (1) 35

Spitz, C. (3)602 Spitzner, R (2.4)70;(2.6)289 Springer, J.B. (2.6)234 Sprunger, P.T. (3)694 Squier, J.A. (1) 455,480 Sreekumar, K.(3)25 Sridevi, B.(1) 158 Sridhar, M.(2.5)156 sfidharan, v.(1) 334 Srinivasan, A.(1) 158 Srinivasan, C. (2.3)17;(2.4)184; (2.6)93,265 Srinivasan, K.S.V. (3) 153 Srinivasan, S.(3) 503 Srinivasan, V.V.(3) 35 1 Staab, H.A.(1) 290-292;(2.2) 140, 141;(2.5)37-40 Stachowiak, K. (1) 148 Stagira, S.(3)476 Stagnaro,P. (3) 181 Staikos, G. (3) 598 Stair, P.C. (2.7)138 Stalmach, U.(2.3)8 Stamenova, R.(3) 165,166 Stammel, C. (2.6)219 Starchev, K. (1)417 Starck, F. (2.3)105 Staromlynska,J. (1) 478 Stasko, A. (1) 343,349;(2.5)95, 96, 100, 109;(3)235 Staton, A.W. (3) 214 Stauffer, M.T. (2.4)116 Steckhan, E.(2.5)185 Stecnken, S.(2.4)201;(2.6)142, 143, 169,171;(2.7)211 Steer, RP. (1) 15, 147,191;(2.5) 48 Stefio, I. (1) 380 Stegeman, G. (3) 764 Steilema~,M.(2.3)43 Steiner, U.E.(1) 296,500 Steinhauser, K. (2.4)3 10;(2.6) 3 19;(2.7)214 Steinhuber, E.(3)482 Steinman, B.(1) 409 Steinmetz, M.G. (2.3) 13; (2.6) 299 Stemmler, M.(2.7) 131 Step, E.(2.6)200 Stepanek, M.(1) 426;(3) 589 Sterk, H.(2.2)104;(2.6)106 Sternson, S.M. (2.4)332;(2.7) 202 Stert, V.(2.7)108, 142 Stettler, G.(2.5)222 Steuhl, H.-M. (2.1)78;(2.7)74 Stibor, I. (2.5) 170 Stocker, W. (3)437

Photochemistry

Stoddart, J.F. (1) 238;(2.6)49

Stoleriu, A. (3) 832 Stone, K. (3) 535 Stoop, K.W.J. (1) 474 Stottmeister, U.(2.6)163 Stracener, L.L.(2.7)32 Sh.achan, J.-P. (1) 161 Straker, A.M. (3) 502 Strand, A. (2.4)29; (2.6)322; (2.7)191 Stranges, D. (2.7)131 Strashnikova, N.(2.3)10 Stratakis, M.(2.5)158 Straub, K.D. (2.7) 110 Straub, M.(1) 469 Sbaurnanis, A. (2.2)86 Strehmel, B.(3) 130,455,612 Striplin, D.(1) 279,282 Stnaka,K.(1) 141;(2.3)97 Stuber, G.J. (3)7% Stufkens, D.J. (2.7)218,219 Stumpe, J. (1) 206;(2.6)152;(3) 278,368,400,422,423,437, 438 Su, H. (2.2)98 Su, N. (2.4)196 Su, Y.(2.5)176 Su, Z.Y.(2.1)61;(2.2) 117;(2.6) 170,213 Suau, R (2.2)1 18;(2.6)100 Subdunanyam, M.(4)20.21 Subramanian, S.(2.1)1,2 h b r t , J. (2.6)298;(2.7)175 Suchiro, C. (3) 276 Suen, 2.(3)200 Suenobu, M.(2.5)98 Suenobu, T. (1) 352;(2.5)14; (2.6)210 Sugane, A. (3) 817 Sugawara, K.4. (2.7)84 Sugawara, T.(2.7)48 Sugimoto, A. (2.1)43;(2.3)133; (2.4)3 15;(2.6)8 Sugimoto, R. (3) 443 Sugimura, C.(2.2)71;(2.6)124 Sugino, H.(1) 157 Sugiyama, H.(2.2)66 Sugiyama, K.(2.2)33;(3)5 15 Suh, D.H.(3)407,424 Suh,M.C.(3) 454 Sui, Q.(2.4)143, 144;(2.6)68 Sui, W.(3) 686 Suishi, T.(2.2)90 Suishu, T. (2.4)207 Suits, A.G. (2.3)58;(2.7)128, 130, 131 Sukcnik, A. (1)386 Sulekha, A. (2.6)232

AufhorInalew Sumida, J.P. (2.5) 30 Summonte, C. (4) 47

Sun,C.(3) 638 Sun,D. (2.2) 115,129, 130 Sun,G.(3) 536 Sun, H.(2.5) 153, 164 Sun, J. (3) 193 Sun,K. (1) 264; (2.6) 297

Sun, L.(1) 299; (3) 191,741

Sun,S. (2.5) 206,207; (3) 168 Sun,T. (1) 54,98 Sun,W.(2.3) 58; (2.7) 130 Sun, X. (1) 342; (3) 539 Sun,Y.(3) 193

Sun,Y.-M. (2.7) 166 Sun,Y.-P. (1) 328 sundar, C.S. (1) 334

S u n h j a n , G. (3) 58 Sundermann, K.(1) 112 Sung,J. (1) SO Sung, K.S. (2.2) 100 Sunkara, H.B. (3) 183 Sutapaneni, R (2.3) 115 Sustmann, R (2.6) 134 Sutherland, RL. (1) 140 Suyama, K. (3) 120 Suyama, Y. (2.3) 84 Suzki, M. (2.7) 60 Suzuki, E. (4) 13 Suzuki,H.(2.2) 5 1; (2.6) 3 11 Suzuki,I. (2.1) 17 Suzuki, M. (3) 829 Suzuki,R (2.2) 83 Suzuki,T. (1) 152; (2.1) 29; (2.3) 93; (2.4) 104, 111; (2.5) 18, 57; (2.6) 85 Suzuki, Y. (2.4) 27, 156; (2.5) 122; (2.6) 79-81; (2.7) 88,89 Svenshkova, E.B. (1) 16 Svobodova, J. (1) 141; (2.3) 97 Swain, S.(1) 460 Swanson, L. (3) 559,568 Swartzlander, A. (4) 42 Sweet, RM. (2.4) 279 Sworakowski, J. (2.4) 160; (2.6) 149 Syper, L. (3) 373 Syromyatnikov, V.G. (1) 155 Szabados, L.(1) 218; (2.6) 45 Szabo, A. (1) 120 Szajdzinska, E. (3) 573 Szalai, I. (2.1) 71 Szantay, C. (2.4) 8 Szarmes, E.B. (2.7) 110 SZoeS, (1) 447 Szollosy, A. (2.5) 89; (2.6) 167 Szulczewski, G.J.(2.7) 109 Szymanska, E.(3) 205

v.

Szymanski, M.(1) 448; (2.6) 266

Tabla, F.M.G. (2.7) 153 Tabuchi, K.(3) 26,65 Tabu4 F. (2.6) 166 Tachi, H. (3) 117 Tachibana, J. (1) 364 Tachikawa, H (2.6) 276 Tachikawa, M. (3) 255 Tachiya, M. (1) 78,302 Tada, K. (3) 514 Tada, M. (2.4) 3 17 Tada, N. (2.7) 173 Tada, S. (3) 249 Tae, E.L. (2.7) 13 Taeishi, M. (3) 397 Tagaki, K. (3) 244 Tagawa, S. (3) 706 Tagaya, H. (2.4) 16 Tagliatesta, P. (2.5) 23 1 Taglietti, A. (1) 402 Taguti, M. (3) 670 Taha-Bouamri, K. (1) 101 Taintor, R.J. (2.5) 203 Tajima, Y. (2.5) 172; (3) 132 Taka, S. (3) 39 Takada, Y. (3) 648 Takag~,K. (2.2) 2,3; (2.4) 9,267, 268

Takaguchi,Y. (2.5) 195 Takaham, S. (3) 27, 118 Takahashi, A. (1) 137; (3) 538 Takahashi, H.(2.4) 320; (3) 420; (4) 24

Takahashi,M. (2.2) 116; (2.3)

103; (2.4) 204,234,263,285; (2.6) 114,267-269 Takahashi, N. (1) 307 Takahashi, 0. (2.6) 290; (2.7) 186 Takahashi, T. (3) 118; (4) 17 Takahashi, Y. (1) 34; (2.2) 117; (2.3) 75,93; (2.6) 54,213; (3) 249,257 Takamuku, S. (2.4) 269 Takasaki, T. (2.3) 93 Takashima, K. (2.4) 223 Takasu, D.(3) 301 Takasu, Y. (1) 484 Takata,T. (4) 2 Takatani, K. (3) 275 Takatori, Y. (2.1) 9 Takatsuka, H. (3) 274 Takaya, Y. (2.3) 15 Takayanagi, H. (2.3) 116, 117; (2.4) 276 Takechi, H. (2.4) 320 T a k a J. (2.4) 105, 106; (2.6) 88

43 7

T a k a K. (1) 68; (3) 401,769 Takcgoshi, K. (2.4) 277 Takeishi, M. (3) 63,64 Takekuma, K. (2.6) 310; (2.7) 181 Takemura, H. (2.1) 69 Takenaka, Y. (2.6) 54 Takeo, H. (2.7) 84 Takcshi, Y. (3) 265 Takeshita, H. (2.2) 10,92; (2.4) 198

Take~hiti~, M.(2.3) 30-32,35,36, 39; (2.4) 82,85,91,233

Taketsugu, T. (2.7) 122 Takeuchi, H. (2.4) 218; (2.7) 62 Takeuchi, K. (2.5) 172; (2.7) 24; (3) 4,255; (4) 5 Takeya, H. (1) 229; (2.3) 9; (2.4) 23; (2.5) 198

Takezawa, M. (2.3) 107 Takezoe, H. (3) 376 Taki, M. (1) 332; (2.5) 143 Takigawa, H. (3) 344 Takui, T.(2.6) 183; (2.7) 213 Takuwa, A. (2.2) 133; (2.3) 41; (2.6) 324

Taleuchi, K. (3) 132 Tahavini, M. (3) 567 Talipova, I.V.(4) 63 Tamada, K.(2.2) 7 Tamagaki, S. (3) 552 Tamai, N. (1) 168,308 Tamai, T. (2.3) 71; (2.6) 8; (3) 161

Ta~naki,K. (1) 366 Tamara6 P. (1) 124, 133 Tamashima, T. (2.2) 46; (2.4) 250; (2.6) 5 1

Tambwekar, S.V.(4) 20,21 Tamura, Y. (2.6) 166 Tan, C. (2.4) 253 Tan, C.-Q. (2.4) 255,256 Tan, D.S.(2.4) 33 1 Tan, G. (2.6) 36 Tan, J. (3) 723 Tan, X.S.(2.4) 262 Tambe, T. (2.7) 88, 89 Tahaguchi, S. (1) 268 Tanaka, A. (1) 144; (4) 2,24 Tanaka, F. (1) 47, 100, 168 Tanaka, J. (1) 157 Tanaka, K. (2.2) 1; (2.5) 236; (2.6) 54,3 I 1; (3) 75,525 Tanaka, N. (2.1) 79 Tanaka, R (2.1) 17 Tanaka, S. (2.2) 108; (2.7) 87; (3) 722

Tanaka, T. (2.3) 71; (2.5) 120; (3) 305,309

438 Tanaka, Y. (2.7) 135 Tanaka, Y.K. (3) 586 Tanemitsu, H.(2.1) 69 Tang, C.W.(3) 492 Tang, J. (3) 127 Tang, S.H.(3) 190 Tang, Y. (2.4) 143; (2.6) 68 Tani, M. (2.4) 159 Tanida, S. (1) 259,260; (2.5) 114 Tanimoto, Y. (2.7) 22 Taaizawa, T. (2.4) 133, 157 Tanta, M. (3) 332 Taraban, M.B. (2.7) 176 Tarasishin, A.V. (3) 73 Tarasov, I.G.(1) 60,62 Tarasyuk, A.Yu. (2.5) 20 Tardi, M. (3) 23 Targowski, P. (1) 96 Tamovskii, A.N. (1) 222 Tasch, S. (3) 487,503 Tashiro, K.(2.4) 107; (3) 425 Tashiro, M. (2.5) 193 Tasis, D.A. (3) 35 Tatarq E.(3) 205 Tatikolov, A.S. (1) 433 Tatsu, Y. (2.6) 15; (2.7) 198 Tatsuham.K.(4) 37 Tatsumi, T. (1) 144; (2.5) 122, 123

Tattani, B.N. (2.6) 133 Tauber, A.Y. (1) 288 Tauer, E. (1) 197 Tavares, H.R. (2.4) 243 Taylor, J . 4 . (2.6) 121 Taylor, M.J. (2.1) 25 Taylor, R (1) 330; (2.5) 169 Taylor, V.L. (3) 708 Tedoradze, M.G. (2.7) 36 Tench, A.J. (2.2) 21 Teraguchi, M. (3) 514 T e d , Y. (2.4) 118 Tern, T. (2.4) 277 Terazima, M. (1) 46,444,445; (2.7) 1,2,20

Terbrueggen, R.H. (1) 389 Tero-Kubota, S. (2.3) 76 Terpetschnig, E. (2.2) 104; (2.6) 106

Terrones, M.G.H. (3) 839 Terselius, B. (3) 327 Terunuma, D. (3) 564 Terzi, R. (3) 242 Teshima, K. (3) 53 Testa, A.C. (2.6) 132 Tetreault, N . (1) 191 Teulade, J.-C. (2.2) 45; (2.4) 251; (2.6) 52

Teyssedre, G. (3) 359

Photochemistry Thakur, J.V. (2.7) 41 Thamam, T.J. (2.6) 193 Thanki, P.N. (3) 700 Thapliyal, P.C. (2.4) 217; (2.6) 329

Therien, M.J. (1) 284,324 Thiel, E. (1) 143 Thiemann, T. (2.5) 193 Thieme, C. (3) 368 Thom, V. (2.7) 53; (3) 280 Thomas, B. (2.1) 68 Thomas, D.D. (2.6) 244; (2.7) 193 Thomas, J.K. (3) 571 Thomas, K.(1) 356 Thomas, K.G. (2.5) 102 Thomas, M.D. (1) 490 Thomas, N.C.(2.7) 95 Thompson, B.E. (2.6) 193 Thompson, D.W.(1) 245 Thompson, F. (3) 835 Thompson, K.A. (2.4) 60, 171 Thompson, M. (2.6) 128 Thopate, S.R (2.1) 5 1; (2.5) 72 Thorson, G.M. (2.6) 261; (2.7) 184

Tian, H.(1) 286,287; (2.2) 122;

(2.6) 216, 217; (3) 389,842844 Tian, H.4. (1) 3 18 Tian, Z.Z. (2.1) 23 Tidwell, T.T. (2.1) 88; (2.2) 100; (2.6) 297; (2.7) 209 Tieke, B. (3) 372 Tiemblo, P. (3) 359 Tiera, M.J.(3) 579 Tiercet, P. (2.2) 110 Tilman, H.(3) 541 Timberlake,L.D. (2.3) 7 Timmel, C.R (1) 87 Timmermans, J.L.(2.4) 187 Timpu, D. (2.7) 5 1 Tirelli, N. (3) 436 Tittel, J. (1) 135 Tkachenko,N.V. (1) 288 Toba, Y. (1) 351; (3) 54,113 Tobita, S. (1) 91, 173,332; (2.4) 77,286,287; (2.5) 65, 143; (2.6) 204,304 Toda, F. (2.2) 1,46; (2.4) 250; (2.6) 5 1,54 Toda, J. (2.2) 103; (2.4) 266; (2.6) 107 Todd, J. (1) 110 Todini, 0. (4) 61 Todo, T. (2.6) 125 Toffoletti, A. (1) 338; (2.5) 108 Togashi, D.M. (2.4) 237 Tohnai, N. (2.2) 69; (2.6) 117,

118

Toke, L. (2.4) 310; (2.6) 319; (2.7) 2 14; (3) 820-822

Tokita, M. (3) 419 Tokita, S. (2.4) 136 Tokuhisa, H.(3) 371 Tokumanr, K. (2.1) 17; (2.3) 116, 117; (2.4) 276; (3) 39

Tokumura, K.(1) 233; (2.5) 67 Tokunaga, K. (2.7) 24 Tolkachev, V.A. (2.7) 182 Tolosa, L.(1) 468 Tolstikov, G.A. (2.5) 147 Tolvaj, T. (3) 743 Toma, H.E.(1) 434 Tomcik, P. (3) 349 Tominaga, K. (1) 83,190 Tomioka, H. (2.4) 300; (2.7) 24, 31

Tomita, T. (2.6) 196 Tompert, A. (3) 482 Tong, Y. (1) 333 Tong, Y.-P. (1) 354 Tong, 2.(1) 408 Tongcharoensirikul, P. (2.6) 129 Toniolo, C. (1) 360 Tonnesen, H.H. (2.6) 2 Tonoi, T. (2.5) 178 Tonokura, K.(2.7) 173 Topin, A.N. (2.7) 26 , Torii, Y. (1) 174; (2.2) 136, 138; (2.5) 34,63,64

Torijas, M.C. (1) 483,487 Torimoto, T. (2.5) 117, 125 Tormos, R (2.1) 13; (2.4) 241, 311

Torres-Garcia,G. (1) 33 1,339; (2.5) 173

Tortajada, A. (2.4) 325; (2.6) 233; (2.7) 197

Tom, T. (2.2) 25; (2.6) 262 Toscano, J.P. (2.7) 29,33 Toubartz,M. (2.4) 201; (2.6) 142 Toussaint, P. (3) 364 Toutianoush, A. (3) 372 Townsend, L.B. (2.6) 59 Toyama, M.M. (1) 434

Toyoda, S. (3) 344 Toyotama, H.(1) 157 Tracy, H.J. (2.6) 309

Trammell, S.A. (1) 282 Tran-Cong, Q. (3) 161,298,545, 64 1

Treadway, J.A. (1) 282 Trentham, D.R. (2.6) 244; (2.7) 193

Trichet, V. (3) 732 Triglia, A. (1) 406

Author Index Trimarco, P. (2.4) 258; (2.7) 9 Tripathy, S.K.(3) 396,520,658 Trommsdorff, H.P. (2.3) 34; (2.4) 92

Trompette, 0. (1) 301 Troscher, G. (2.5) 133 Trotter, J. (2.1) 24,26; (2.4) 292; (2.5) 54; (2.6) 150

Trout, N.A. (2.7) 178 Trupke, T. (4) 39 Trushin, S.A. (1) 196; (2.7) 84 Tscherny, I. (2.6) 103 Tsentalovich, Y.P. (2.1) 18; (2.6) 205; (2.7) 207 Tsivgoulis, G.M.(2.4) 88 Tsnooka, M. (3) 26, 117, 120, 129 Tsubota, H.(2.7) 168 Tsuchida, E.(1) 258; (2.3) 29; (2.4) 94; (2.5) 230; (3) 581

Tsuchiya, T. (2.5) 197; (2.6) 325 Tsuda, Y.(2.2) 103; (2.4) 266; (2.6) 107,236; (2.7) 194

Tsuji, K.(2.6) 320 Tsujishima, H.(2.2) 34 Tsujita, H.(2.4) 265; (2.6) 102 Tsujita, Y. (3) 203 Tsukagoshi, R. (1) 173; (2.5) 65 T~ukahara,K.4. (4) 59 Tsukamoto, K. (4) 49 Tsukamoto, M.(1) 256; (2.5) 82 Tsuneizumi, T. (2.4) 223 Tsuno, T. (2.2) 33 Tsunooka,M. (3) 582 Tsurutani, Y.(2.4) 61; (2.6) 264 Tsushima, M. (3) 188 Tsutsui, S.(2.6) 292; (2.7) 177 Tsutsui, T. (1) 476 Tsutsumi, C. (3) 65 Tsutsumi, 0. (3) 367,440 Tsvetanov, CbB. (3) 165,166 Tsygankov, A.A. (4) 63 Tuchinsky, A. (2.7) 205 Tuchkin, A.I. (2.4) 225 Tucker, S.A. (1) 166 Tuinman, A.A. (1) 340 Tulock, J.J. (1) 42 1 Twg, C.-H. (2.4) 196,275,278, 288; (2.5) 161, 165

Tunik, S.P. (2.4) 68; (2.7) 8 Tupy, M.J. (1) 486 Tuck, C. (3) 91 Turner, J.J. (2.7) 80 Turoti, M. (3) 800 Turovski, A. (3) 237 Turro, C. (2.5) 84 Turro,N.J. (2.1) 11; (2.6) 3 18 Turton, T.J. (3) 790 Tutovan, E.(3) 697-699

439

Twardzik, G.(2.4) 283 Twieg, R.J. (3) 414 Tyler, D.R. (3) 757 Tysklind, M.(2.7) 169 Tzeng, B . 4 . (1) 248

31

Vainer, A.Yu. (3) 184,210 ValdezGodez, A. (3) 83 1 Vdencia, G. (2.6) 259 Valet, A. (3) 123,207,782 Vdeur, B. (1) 484 Valkunas, L. (2.5) 23 Uchida, K. (2.3) 23,24,26,29, Valli, L.(2.5) 104 38,39; (2.4) 13, 83,85-87, Valtcheva, E. (3) 729 89,94 Valverde, S.(2.2) 41 Uchimaru, T. (1) 322; (2.7) 76 Van Aert, H.A.M. (3) 213 Uchiyama, S.(1) 104, 149; (4) 60 van Breemen, A.J.J.M. (3) 457 Uchoda, K.(2.2) 125 van Cleef, M.W.M. (4) 47,48 Udal’tsov, A.V. (1) 267 van der Mer, H.J. (1) 195 van der Oord, C.J.R (1) 474 Uddin, F. (3) 826 van der Schaaf, P.A. (3) 10 Ueda, M. (3) 185 Van Der Sluis, P. (3) 271 Ueki, M.M.(3) 789 van der We& W.F. (4) 47,48 Uemura, S.(3) 53 Vandenande, D.J. (3) 457 Ueno, N. (3) 694 van Dijk, P.W. (1) 474 Ueno, Y. (3) 287 van Dixhoom, A. (3) 457 Ugarova, N.N. (1) 11 Uhlenbusch, J. (2.7) 152 van Dyk, A.C. (3) 837 Ujike, T. (2.4) 98, 102; (2.6) 34 van Me, P. (1) 70 Ulbricht, M. (2.7) 53; (3) 280,28 1 van Eldik, R. (1) 45; (2.3) 92; (2.4) 244; (2.6) 305; (2.7) 98 Ullett, J.S. (3) 263 Ulmer, L. (1) 33 1 van Esch, J. (2.3) 27,28; (2.4) 81, 84 Ulrich, G. (2.7) 25 van Geest, L.K.(1) 474 Ulrich, K.(3) 10 Umbricht, G. (2.1) 15; (2.5) 22 Vangel, M.G. (3) 645 van Gelderen, F.A. (2.4) 23 1 Umeda, M. (3) 564 Umenoki, T. (3) 56 Van Gemert, B. (2.4) 117, 138 van Gunstercn, W.F. (1) 118; Unnikrishnan, N.V. (1) 63 (2.4) 22 Uotso, K.(2.7) 37 Vanhanen, J. (2.4) 162 Upadhyay, A. (2.6) 126, 127 Van Keuren, E. (1) 453 Upadhyay, S.N. (4) 15, 16 Upreti, N.K. (3) 35 1 van Loyen, D. (1) 237,276; (2.5) urakawa,0. (3) 545,641 88 Vannikov, A.V. (2.7) 36 Urano, T. (3) 39 Urbanova, M. (2.6) 298; (2.7) 175 van Oijen, A.M. (1) 13 1 Vanossi, M.(2.5) 209 Ujasz, W.(2.2) 65; (2.4) 328; Van Slageren, J. (2.7) 218 (2.6) 173; (2.7) 188 van Stam, J. (2.6) 283; (3) 595 Ushakov, E.N.(2.4) 80; (2.6) 30 Van Willigen, H. (2.7) 218 Ushiyama, H. (1) 80 Vardeny, Z.V. (3) 443,452,496 Ustynyuk, L.Yu. (2.3) 79 Vardhan, V.A. (2.4) 224 Usui, S.(2.1) 83; (2.3) 42; (2.4) Vardia, J. (2.5) 118; (3) 845 49; (2.6) 254 Varga, V. (3) 736 Usui, Y. (3) 54, 113 Vargas, F. (2.6) 161 Utenyshev, A.N. (2.4) 147; (2.6) Vasenkov, S. (2.5) 164 74,77 Vasenkow, S.(2.1) 85 Utinans, M. (1) 99 Vasile, C. (3) 832 Uzuyama, C.(2.7) 199 Vasilets, V.N. (3) 727 Vasin, V.A. (2.6) 273 Vassileva, V. (3) 729,730,8 15 V a w , D. (3) 483 Vassilikogiannakis, G.(2.5) 158 Vachev, V.D. (1) 3 1 Vatsa, RK. (2.7) 67 Vaeth, K.M.(3) 473 Vatulev, V.M. (3) 141 Vaganova, E.(3) 637 Vauthey, E. (2.5) 116 Vaidyalingam, A. (2.4) 254; (2.6)

440 Vecer, J. (1) 141 Vedernikov, A.I. (2.6) 29,30; (3) 386

Veedaldi, D. (2.2) 80 Vegh, D. (3) 349 Veith, M.(1) 237; (2.5) 88 Velazqueq S. (2.2) 41 Veleva, L. (3) 831 Velthoerst, N.H.(1) 449 Venkatesan, K.(2.2) 3 1,32; (2.4) 273

Venkateswamn, RV. (2.2) 29 Venneri, P.(2.6) 300 Ventura, B.(1) 3 14 Venturi, M.(1) 281; (3) 555 Venugopal, C. (1) 63 Verbeek, J. (2.6) 181 Vercer, J. (2.3) 97 Verdu, J. (3) 680,689492, 833 Verheijen, W. (3) 549 Verhoeven, J.W. (1) 21,22,28, 491; (2.6) 12, 13

Vermeersch, G. (2.4) 118 Vema, A.L. (1) 308 Verney, V. (3) 810 Veroni, P.(1) 359 Verveer, P.J. (1) 466,470 Viaene, L.(2.6) 283 Viappiani, C. (2.6) 23 1; (2.7) 192 Vicens, J. (2.4) 44 Viduna, D. (3) 585 Vie], P.(3) 435 Vig, A. (3) 820-822 Vigier, G.(3) 839 Vigil, M.R (3) 635 Vignean, E.(1) 435 Vij, D.R (1) 8 Vijayalakshmi, S. (2.4) 261; (2.6) 53 Vinas, M.H.(1) 422 Vinhg, W.J. (2.6) 309 Vinogradov, A.V. (3) 333,702, 703

Viriot, M.L. (3) 647 Visconti, M.(3) 12, 121, 122 Vishnoi, G. (3) 616 Vishnumurthy, K. (2.2) 3 1,32; (2.4) 273

Vissenberg, M.C.J.M. (3) 448 Viswanathan, K.(3) 82 Viswanathan, S. (3) 256 Vix, A. (3) 437 Vlachopoulos, N. (2.5) 2 15 Vladimir, I. (1) 374 Vladimirsky, 0. (3) 696 Vladimirsky, Y. (3) 696 Vlasov, Yu.G. (3) 302 Vlasova, N.N. (2.6) 301

Photochemistry Vlcek, A., Jr. (2.7) 81 Vliegenthart, J.F.G. (2.3) 49 Vlietstra, E.J. (1) 449 Vogler, A. (2.5) 35 Voityuk, A.A. (1) 109 Volker, S. (1) 61 Volkert, W.A. (2.7) 55 Volkova, O.S. (2.7) 176 Volksen, W. (3) 441 Voloshanovskii, I.S.(3) 614 Voloshin, N.A. (2.4) 152 Voloshina, E.N.(2.4) 152; (2.6)

Walton, G. (3) 119 Waluk, I. (1) 457 Warnelink, M.P. (2.4) 187 Wan, J.K.S. (3) 734 Wan, P. (2.3) 130; (2.4) 165 Wan, W.C. (3) 467 Wandel, H. (2.7) 18,23 Wang, B.(2.2) 50 Wang, C.(2.7) 115; (3) 347,5 13 Wang, D. (3) 480 Wan& E. (3) 105, 127 Wang, F.(2.6) 21; (2.7) 145; (3)

Volpp, H.-R (2.7) 67 Von Borczyskowski, C.(1) 102,

Wang, F.W. (1) 429 Wang, G. (3) 643 W a g , G.-J. (2.3) 65; (2.7) 154,

87

130,265; (3) 630

Voronkov, M.G. (2.6) 301 Vorsa, V. (2.7) 83 Vos, J.G. (1) 294 Vosch, T. (1) 3 10 Vovk, O.M.(3) 74 Vrana, L.M.(1) 285 Vreven, T. (1) 115 Vyprachticky, D. (1) 4 12; (3) 666 Waali, E.E.(3) 696 Wada, K. (2.5) 149 Wada, T. (2.3) 5; (3) 90,420 Wada, Y. (2.5) 126, 127 Wadayama, T,(2.7) 88, 89 Wade, S.A. (1) 98 Wagaman, M.W. (3) 5 11 Wagner, B.D. (2.1) 88; (2.7) 209 Wagner, P.J. (2.5) 50 Wagner, R.(2.5) 157 Wagner, R W . (1) 164 Wakabayashi, T.(2.7) 137 Wakamatsu, T. (3) 108, 109 Wakasa, K.(3) 201 Wakasa, M. (2.6) 308,3 12; (2.7) 179

Wakatsuki, Y. (1) 311 Wakayama, T. (4) 64 Wakebe, T. (1) 453 Walder, L. (2.5) 215 Walker, G.C. (2.7) 115 Walker, M.(3) 346 Wall, S.(1) 419 Walla, P.J. (1) 124 Wallace, S.(2.6) 309 Walsh, J.L. (1) 245 Walsh, M.M. (2.7) 99

Walsh,R.(2.6) 295 Walter, T. (4) 44

Walten, K.A. (1) 41; (3) 488 Walther, M.(3) 416 Walton, D.RM. (1) 330; (2.5) 169

480

162-164

Wang, H.(2.5) 165; (3) 93,53 1, 532,804,806

Wang, J. (3) 19,501,745 Wan& J.B. (2.5) 166 W a g , J.-H. (2.7) 121 W a g , J.-L. (2.4) 297; (2.7) 34 W a g , J.-X. (2.3) 18; (2.4) 272 Wan& J.Z.(3) 347 Wang, K. (2.2) 64 Wang, L.(1) 160; (2.6) 207; (3) 50

Wang, Q.(2.5) 206,207; (3) 187 Wan& R.M. (3) 668 Wan& S.(1) 114; (2.3) 95,96;

(2.4) 26,28; (2.5) 135 Wmg, S.-L. (2.3) 18 Wang, W. (1) 333; (2.2) 50; (2.4) 142; (3) 142,283 W a g , W.-F. (1) 151,354 Wang, X.(2.6) 274; (3) 686 W a g , X . 4 . (4) 30 Wang, Y. (1) 84; (2.4) 108,143; (2.5) 184; (2.6) 68,256; (2.7) 33; (3) 383,384,686 W a g , Y.-M. (2.2) 42; (2.5) 32; (2.6) 92 Wang, Y.P.(3) 668 wag, Y.-z. (2.1) 22 Wang, 2.(1) 287,446; (2.4) 15; (2.6) 217; (3) 191, 193,665 Ward, G.P. (2.7) 104 Warkentin, J. (2.7) 15.21 Warman, J. (3) 835 Warren, M.A. (2.6) 291 Warrener, R.N. (2.3) 89 W a n e ~ h aK.-D. , (2.5) 186 Wasgcstian, F. (2.2) 97 Wasielewski, M.R (1) 261,270; (3) 415 Wasternack, C.(2.7) 56 Watanabe, A. (1) 339, 353; (2.5)

Author Index 99, 110, 173,236; (2.6) 235 Watanabe, H. (2.6) 3 10.3 11; (2.7) 180, 181; (3) 118 Watanabe, J. (2.3) 30; (2.4) 91; (2.5) 136, 137; (3) 419,420 Watanabe, K.(2.4) 218; (2.7) 62 Watanabe, M. (2.3) 48; (2.4) 166 Watanabe, N. (2.4) 327; (2.7) 204 Watanabe, S.(2.2) 116; (2.4) 204, 234,263,285; (2.6) 114,267269 Watanabe, T. (2.4) 136 Watanabe, Y.(2.2) 25; (2.5) 149; (2.6) 262; (2.7) 60; (4) 65 Watson, K.D. (1) 128 Waugh, T. (2.3) 78 Wayne, K.(1) 30 Wayner, D.D.M. (2.3) 76 Webber, S.E.(1) 426; (3) 589 Weber, J.F.W. (2.1) 8 Weber, M.(3) 542 Weber, S.G.(2.4) 111, 116; (2.6) 85 Weber, W.H. (3) 679,688 Webster, S.(3) 544 Wegewijs, B. (1) 28,443; (2.6) 13 Wegmann, G.(3) 481 Wegner, M.(1) 491 Weh, K. (3) 364 Wei, C.-Y. (1) 181, 182; (2.5) 144 Wei, X.(3) 324 Wei, Y.(2.6) 237; (3) 286,665 Wei, 2.(3) 282 Weigand, R (2.6) 215 Weigand, U.(2.4) 139 Weikard, J. (1) 291; (2.5) 40 Weinkiitz, S.(2.4) 180; (2.6) 260; (2.7) 185 Weir, N.A. (3) 799 Weis, J. (3) 136 Weiss, G.H.(1) 120 Weitz, E.(2.7) 138 Weixiao, W. (3) 146 Weldon, D. (2.5) 24; (2.6) 136 Welland, A.D. (1) 105 Wells, A.J. (2.2) 39 Welzel, P.(2.5) 187 Wen, J. (3) 374 Wen, M.(3) 239 Wen, Z. (2.5) 32 Wender, P.(2.4) 191 Wendorff, J.H. (3) 409 Wendt, H. (1) 169 Weng, H. (2.5) 163 Weng, Y.-X.(1) 248 Wenk, H.H. (2.1) 65 Wentrup, C.(2.2) 88; (2.4) 56; (2.7) 61

Wenzl, F.P. (3) 487 Werner, G. (2.6) 163 Werner, T. (1) 395 Wery, J. (3) 507 Wessig, P.(2.1) 20,40; (2.5) 51, 52,60

44 1 Wilson, S.R(1) 368; (2.5) 171, 232

Wimmer, R.(2.4) 238; (2.6) 60; (2.7) 215

Winkler, J.D. (2.2) 35; (2.6) 83, 91; (3) 830

West, F.G. (2.2) 55,85 West, R (3) 463,5 19,707 West, T.K.(3) 5 19 Weston, K.D.(1) 134 Wetzel, D.L. (3) 756 Weyerhausen, B. (2.4) 181; (2.7)

Winnik, F.M.(3) 558 Winnik, M.A. (3) 569,605,773 Winograd, N. (2.7)83 Winssinger, N.(2.4) 327; (2.7)

White, A.H. (2.7) 95 White, A.J. (1) 238; (2.2) 113;

Win, J. (1) 18; (2.7) 16 Wis, M.L. (2.4) 150; (2.6) 82 Witten, T.A.(1) 503 Woerner, M.(2.6) 29 Wohlgenannt, M. (3) 487 Wojnarovits, L.(1) 404 Wokaun, A. (2.7) 11; (3) 726,760 Wolf, H.C. (1) 321 Wolfbeis, O.S. (1) 383,395 Wolff, C. (1) 339; (2.5) 173, 174 Wolff, T. (3) 584 Wolszczak, M.(3) 573 Wong, K.(3) 490 Wong, K.S.(3) 531,532 Wong, Y . 4 . (2.5) 91 Wongvisetsirikul, N.(3) 687 Woo, D.H. (2.7) 223 Woo, H.S. (3) 376 Woo, K.W.(3) 599 Woo, R.(2.4) 290; (2.6) 203 Wood,J. (3) 441 Wood, P.D. (2.7) 15,21 Woodcock, S.R(1) 312 Woods, L.M.(4) 42 Wooton, M. (1) 427 Wootton, A.B. (3) 677 Worboys, M.R (3) 767 Workentin, M.S. (2.7) 38 Wormell, P.(2.5) 146 Wostratzky, D. (3) 174,782 Wrachtrup, J. (1) 130 Wright, D. (3) 414 Wright, M.E. (3) 378 Wrobe, M.N. (2.2) 8 Wrozowa, T. (1) 448 Wu, B.(2.7) 155 Wu, C. (1) 446; (2.4) 141, 142;

91

(2.6) 49

White, J.M.(2.7) 109, 166 White, J.O. (1) 198; (3) 63 1,632 White, J.R (3) 790 White, R.C. (2.6) 187 Whitehead, J.B. (3) 267 Whitehead, J.C. (2.7) 3 Whitesell, J.K.(1) 427 Whitted, P.O. (2.1) 80 Whittcn, D.G. (2.5) 159 Whitten, W.B. (1) 129, 132 Wiczk, W. (1) 148 Widengren, J. (3) 825 Widman, J.F. (3) 180 Wiederrecht, G.P. (1) 261; (3) 415 Wienk, M.M. (4) 4 Wiest, 0. (2.3) 120 Wihrle, D. (2.5) 130 Wilberg, C. (3) 238 Wilde, H. (2.6) 163 Wilhelm, C.(3) 713 Wilhelm, R.(2.7) 144 Wilhelm, T. (1) 146 Wilk, K.A. (1) 216; (2.4) 37; (3) 373

Wilkes, I.P. (3) 8 19 Wilkinson, F.(1) 147; (2.5) 48; (3) 819

Willert, A. (1) 265 Willey, A.D. (2.5) 13 1, 132 Willey, K.F.(2.7) 83 Williams, D.J. (1) 238; (2.6) 49 Williams, J.A.G. (1) 313,314, 393

Williams, L.D. (2.2) 139 Williams, M.E. (3) 606 Williams, N.J.R (2.4) 54; (2.7) 39 Willig, F. (1) 35 Willner, I. (1) 42,276 Wilson, G.J. (1) 277; (2.5) 85 Wilson, K.R. (1) 455,480 Wilson, P.(1) 478 Wilson, R.S.(I) 367

204

Wipff, G. (1) 271,272; (2.5) 44, 45

(3) 292,628

Wu, J.W. (3) 376 WU,L.-M. (2.3) 122; (2.5) 234; (2.6) 207,211; (2.7) 210

WU,L.-Z. (2.4) 196 Wu, M.W. (3) 505 Wu, Q.(3) 432 Wu, S.(2.2) 43, 13 1; (2.3) 20;

442 (2.5) 162; (2.6) 26; (3) 638, 741 Wu, S.M.(2.7) 156 Wu, S.P.(2.2) 92; (2.4) 198 Wu, T. (1) 24 1; (3) 193 Wu, X. (2.4) 141, 142; (2.6) 175 Wu, Y. (1) 346; (2.5) 176; (3) 367,439 WU,Y.-W. (2.3) 64 Wu, Z. (3) 105,347,608 Wuelfert, S.(3) 762 Wuestneck, R. (1) 224 Wulff-Molder, D.(2.1) 20,40; (2.5) 5 1,52,60 Wurche, F.(2.3) 91 Wurfel, P.(4) 39 Wuner, A.J. (1) 146 Wusterfeld, R. (3) 182 Wynne, A.M. (2.4) 208 Wysacki, S. (3) 350

Xavier, M.P. (1) 378 Xheng, J.Z. (3) 190 Xia, C. (2.1) 10 Xia, G. (2.5) 42 Xiao, D.(1) 375 Xie, H.Q. (3) 465,468 Xie, M.(2.4) 17; (3) 395,723 Xie,'Z. (3) 374 Xiong, J. (3) 607,608,662,663 Xiong, Y. (3) 326 Xiuyu, Z. (3) 682 Xu, H. (1) 82; (2.5) 205 XU,H.-J. (1) 3 18 XU,J.-H. (2.4) 206; (2.5) 224; (2.6) 105

Xu, J.M. (3) 471 Xu, J.Q. (3) 52 Xu, L.G. (3) 533 Xu, T. (1) 286,446; (2.2) 122; (2.6) 216

Xu, W.(1) 382 XU,X.-H. (2.4) 288 Xu, Y. (3) 70,168,508,664,672 Xuan, W. (3) 326 Xue, Q.B. (3) 383,384 Yabe, A. (2.7) 76,77; (3) 719 Yagashita, T. (4) 59 Yagci, Y. (2.6) 172, 199; (2.7) 10; (3) 14,34,38

Yagi, E.J. (2.1) 9 Yagi, T. (2.4) 204; (2.6) 114 Yakovlev, V.V. (1) 455,480 Yakura, T. (2.4) 303; (2.6) 165 Yakushujin, K.(2.2) 83

Yam, V.W.-W. (1) 236 Yamada, A. (4) 40,43,46 Yamada, H. (2.5) 149 Yamada, I<. (1) 270; (2.4) 69;

(2.6) 192; (3) 295,2%, 564

Yamada, S. (2.5) 80; (2.6) 227; (3) 249,257,586,624

Yamada, T. (2.3) 26; (2.4) 83 Yamagah, H. (3) 781; (4) 66 Yamago, S. (2.3) 4547; (2.6) 326-328; (2.7) 216,217

Yamaguchi, A. (2.1) 17 Yamaguchi, K.(1) 93; (2.2) 116;

(2.4) 204,234,263,285; (2.6) 114,236,267-269; (2.7) 194 Yamaguchi, S. (2.7) 20 Yamaguchi, T. (2.2) 125; (2.4) 87; (2.6) 35 Yamaguchi, Y. (2.4) 317; (3) 108, 109,641 Yamamoto, E. (1) 400 Yamamoto, H. (3) 577 Yamamoto, M.(1) 91, 173; (2.5) 65; (2.6) 304; (3) 253,566, 61 1,657 Yamatnoto, T. (2.6) 3 11; (3) 274276,508,5 15 Yamamura, S. (2.5) 201 Yamaoka, M.(1) 499 Yamaoka, R (2.3) 94 Yamaoka, T. (3) 39,118 Yamasaki, Y. (3) 201 Yamashiro, A. (1) 137; (3) 538 Yamashita, H. (1) 144; (2.5) 29, 121-123 Yamashita, K.(2.3) 41; (2.6) 119 Yamashita, N. (3) 675 Yamashita, S.(3) 244 Yamashita, T. (2.3) 48; (2.4) 166, 279; (3) 420,725 Yamashita, Y. (2.3) 40; (4) 24 Yamatake, K.(2.7) 88 Yamato, T. (2.5) 193 Yamauchi, J. (2.5) 49 Yamauchi, S. (1) 234 Yamazaki, I. (1) 163,323,366, 491 Yamazaki, T. (1) 163,323 Yamazaki, Y. (2.1) 41,42; (2.5) 10,61 Yan, X. (3) 625 Yan, Y. (1) 408; (2.6) 237 Yanagida, S. (2.5) 126, 127 Yanagimoto, T. (1) 258 Yanashima, C. (2.4) 136 Yanauchi, J. (2.1) 30 Yang, A. (3) 534 Yang, B.X. (3) 145

Photochemistry Yang, C. (2.2) 13 1; (2.5) 162 Yang, D.B.(3) 108, 109,267 Yang, G. (3) 33,47,63 1 Yang, G.Q. (3) 24 Yang, G.X. (3) 32 Yang, I.J. (2.7) 165 Yang, J. (3) 20 Yang, K.(3) 658 Yang, K.Z.(3) 383,384,520 Yang, L.(2.3) 122; (2.5) 234; (2.6) 211; (2.7) 210; (3) 145

Yang, M.(1) 55 Yang, M.C. (2.7) 223 Yang, N.-C. (1) 23; (2.6) 14 Yang, S. (3) 485 Yang, S.I.(1) 161, 162, 164 Yang, S.W. (3) 671 Yang, W. (2.4) 14 Yang, X.(2.3) 63; (2.7) 125, 131, 156, 157

Yang, X.-M. (2.7) 126, 127 Yang, X.-R. (2.3) 18; (2.4) 272 Yang, Y. (2.4) 30; (3) 456,588 Yang, Y.Y. (3) 52 Y~o,D.-D. (1) 15 1 Yao, G. (2.7) 17 Yao, H. (3) 27 Yao, J.A. (1) 388 Yao, S. (1) 333; (3) 200,288 Yao, S.D.(1) 354; (2.2) 135; (2.4) 142

Yao, Z. (2.6) 36 Yaobi, Y.Z.(1) 386 Yap, G.P.A. (2.7) 102 Yarosh, O.G.(2.6) 301 Yashchuk, V.M.(1) 155 Yashima, E.(2.6) 47; (4) 33 Yashiro, H.(1) 458 Yasuda, M. (2.2) 75; (2.3) 48,54; (2.4) 166,230; (2.6) 99

Yasui, M. (2.6) 308 Yasui, N. (2.2) 69; (2.6) 117, 118 Yasui, S. (2.1) 30; (2.5) 49 Yasuike, M.(3) 113 Yasuyuki, K.(3) 439 Ye, H. (3) 539 Ye, J. (3) 607,608 Ye, J.-H. (2.5) 224 Ye, P.-X. (4) 30 Yen, G.-F. (2.5) 91 Yeo, S.I. (3) 599 Yeom, M.O. (3) 178 Yeston, J.S. (2.7) 100 Yi, J. (2.3) 50 Yilmaz, Y. (1) 418; (3) 651,652 Yin, J. (3) 7 Ying, J. (1) 467 Ying, Y.-M. (2.4) 275; (2.5) 165

Author 1 . x Yitzchaik, S. (3) 637 Yli-Kauhaluoma, J.T. (2.1) 25 Yoda, K. (3) 577 Yokoi, H. (1) 332; (2.1) 60; (2.3)

101; (2.5) 143,235; (2.6) 257

Yokoi, K. (3) 78, 149, 157,264, 425

Yokota, R. (3) 446,447 Yokoyama, K. (2.3) 58; (2.7) 130 Yokoyama, M. (2.4) 104; (3) 371 Yokoyama, Y. (2.3) 33; (2.4) 18, 90, 158

Yonekura, Y. (4) 37 Yonemoto, E.H.(3) 669 Yonemura, H.(2.5) 5,80; (2.6) 227

Yoneshima, R. (1) 268 Yoneyama, H. (2.5) 117,125 Yoneyama, M. (1) 308 Yong, F. (3) 326 Yoo, H.J. (3) 752 Yoo, J.W. (2.5) 121 Yoon, C.S.(3) 479 Yoon, H.S.(3) 823 Yoon, M. (1) 228,273; (2.4) 41 Yoon, U.C. (2.1) 61; (2.2) 117; (2.6) 170,212,213

Yoshi, K.(3) 307 Yo~hida,J. (2.3) 45-47; (2.6) 326328; (2.7) 216,217

Yoshida, K. (3) 591; (4) 24 Yoshida, S. (2.5) 120 Yoshida, Y. (3) 201 Yoshifiji, M. (2.6) 320 Yoshihara, K. (1) 25,83, 190,199 Yoshikawa, S. (2.4) 102; (2.7) 198

Yoshmi, Y. (2.1) 43; (2.3) 133;

(2.4) 239, 315 Yoshimizu, H. (3) 203 Yoshimoto, S. (2.2) 89; (2.4) 211 Yoshino, K. (3) 443,452,496, 5 14 Yoshioka, M. (2.1) 33,42; (2.4) 194; (2.5) 10; (3) 201 Yoshioka, Y. (1) 93 Yoshizawa,E.(1) 270 Yoshizoe, B. (2.6) 3 11 Yosioka, M. (2.3) 5 1 You, D. (3) 106 YOU,X.-Z. (1) 283 Young, J.B.(1) 503 Young, J.S.(3) 85, 196 Young, R.J. (2.2) 21 Young, R.N. (1) 193,194 Yousheng, Y. (3) 204 Youssef, B. (3) 252 Yousufiai, M.A. (3) 826

443

Yrai, Y. (3) 253 Yu, C. (2.2) 139 Yu, D.(4) 26 Yu, F. (3) 288 Yu, Q. (2.1) 62; (2.4) 321; (3) 270,625

Yu, W.L. (3) 527,528 Yu, W . 4 . (1) 181 Yu, X. (2.6) 36 Yu, Y. (3) 467,495 Yuan, D.(1) 305 Yuan, RX. (3) 347 Yuan, Y. (2.3) 50; (3) 470 Yuan, 2.(3) 380 Yuan, Z.-Y. (2.4) 196,275 Yudina, T.M. (3) 110 Yueh, F.-Y. (2.7) 119 Yukawa, C. (2.4) 303; (2.6) 165 Yukawa, H.(4) 66 Yuki, Y. (3) 60 Yumoto, N. (2.2) 34; (2.6) 15; (2.7) 198

Yurkovskaya, A.V. (2.1) 18; (2.6) 205; (2.7) 207

Yusa, S. (1) 280; (3) 725 Yusa, S.I.(3) 610,653 Yuzawa, T. (2.7) 15,33 Zaban, A. (4) 35 Zacchcroni, N. (1) 397,399 Zacharias, P.S.(1) 219; (2.4) 43; (2.6) 40

Zachariasse, K.A. (1) 197,492; (2.6) 220,222

a f a r , s. (4) 45 Zagladko, E.(3) 237

Zagrodzki, B. (1) 179; (2.6) 156

Zahora, E.P.(3) 755 Zahouily, K. (3) 285,791 Zaichenko, N.L.(3) 361 zaika, (3) 337 Zaiov, G. (3) 237 Zakardis, A.K. (2.6) 171; (3) 35 Zakeeruddin, S.M.(2.5) 215 Zakharova, G.V.(2.6) 28 Zakhs, E.R (2.4) 129; (2.6) 71; (3) 392 Zalupsky, P. (3) 349 Zammit, M.D.(3) 254 Zang, H. (3) 55 Zang, H.M. (2.1) 16 Zang, S.G. (2.5) 122 Zang, W.-Q. (2.4) 272 Zanin, M.(3) 789 Zantte, D.(3) 578 Zard, S.Z. (2.1) 8 1 Zauls, V. (1) 224

v.

Zehani, U. (2.7) 205 Zeigler, A. (3) 438 Zeng, C. (3) 112 Zen& X. (3) 283 Zeng, 2.(3) 20 Zenkevich, E.I.(1) 265,3 17 Zenobi, R (3) 762 Zentel, R. (3) 368-370 Zepp, R.G. (2.1) 19; (2.5) 27,53 zhan, H.(3) 842-844 Zhang, A. (2.6) 42; (3) 387 Zhang, B.-W. (4) 30

Zhang, D.(3) 389 Zhang, F. (2.4) 108; (2.6) 109; (3) 47 Zhang, F.N. (2.2) 61 Zhaug, G. (3) 571 Zhang, G.P.(1) 342 Zhang, H. (1) 180, 195; (2.3) 65; (2.4) 72,74; (2.7) 154, 162164; (3) 413,432,643 Zhang, H.J. (2.2) 50 Zhang, H.-Q. (2.4) 100; (3) 433 Zhang, H.-Y. (2.5) 41 Zhang, J. (2.5) 83; (2.6) 21; (3) 33,47,383,384 Zhang, J . 4 . (4) 30 Zhang, J.X.(3) 32 Zhang, L. (3) 270,374,73 1 Zhang, M. (1) 160,241; (2.5) 42; (3) 283 Zhang, M.-J. (2.3) 18; (2.4) 272 Zhang, M.W. (2.2) 135 Zhang,Q.(1) 84; (2.5) 83; (2.7) 141, 143; (3) 764 Zhang, Q.Z.(3) 383,384 Zhang, S.G.(2.5) 121, 123 Zhang, S.-L. (1) 23; (2.6) 14 Zhang, W. (1) 346; (2.3) 122; (2.5) 175, 176,234; (2.6) 175, 207; (2.7) 210; (3) 304,667 Zhang, W.J. (3) 347 Zhang, W.-Q. (2.3) 18 Zhang, X.(1) 84; (3) 163 Zhang, Y. (2.3) 125; (2.6) 90; (3) 204,283,628 Zhang, Y.M. (2.2) 60 Zhang, Y.X. (3) 628 Zhang, Z. (2.4) 72; (2.5) 206,207; (3) 282,286,391 Zhang, Z.C. (2.2) 135 Zhang, Z.Y. (1) 54.98 Zhanga, J.S. (3) 193 Zhanga, Z.4. (1) 308 Zhao, C.(3) 19,103, 146,147 Zhao, H. (3) 543 Zhao, J. (2.2) 7 Zhao, X.(2.6) 248; (3) 297,480

444

Zhao, Y. (1) 286;(2.6)216;(3)

672

Zhao, Y.B. (2.2) 122

Zhdanov, A.A. (3) 293 Zheltikov, A.M. (3) 73 Zhen, 2.(2.6)21 Zheng, A.L. (2.2)50 Zheng, M.(3)493 Zheng, X. (2.6)90 Zheng, X.L.(2.2)60 Zheng, Y. (4)62 Zhila, G.Yu. (2.6)301 Zhong, Q. (2.7)136 Zhou, H.W.(3)347 Zhou, J. (2.4) 108, 143, 144,(2.6) 68;(3) 104 Z~OU, J.-Y. (1) 283 Zhou, L.(1) 340 Zha, L.-P. (2.2)42;(2.6)92 Zhou, P.(1) 460 Zhw; Q.(2.4)257;(2.5)206, 207;(2.6)63;(3)297 Zhw, W.(3) 105

Zhou, W.-L. (2.4) 154 Zhou, Y.(1) 160;(2.5)42;(3)412 Z ~ O UZ.-Y. , (1) 283 Zhu, A. (2.2)43 Zhu, D.(2.6) 176;(3) 493 Zhu, P.(3) 187 Zhu, Q.-F. (1) 318 Zhu, Q.Q. (3) 143 Zhu, R . 4 . (2.3)65;(2.7)154, 162-164 Zhu. s.(3) 593 Zhu, 2.(23)68;(2.4)53;(2.6) 59;(2.7)32;(3) 191 Zhuang, X. (3) 112,135 Zhunusbekov, A.M. (3)617 Zieg, H. (1) 269 Ziegenbalg, J. (2.2)30 Ziegs, F.(1) 339;(2.5)173 Ziessel, R.(1) 271,272,278,281, 301,373;(2.5)44.45 Zifferer, G.(3) 68 Ziller, J.W.(2.4) 182;(3) 155 Zimmer, K.(1) 192

Photochemistry Zimmering, B.L. (3)685 Zimmerman, H.E. (2.3)68;(2.4) 53,212 Zimmennann, F.(2.7)220 Zimmermann, T.(1) 227;(2.4) 140;(2.6)67 Zinth, W.(1) 59,496 Ziolek, M. (1) 448 Zipp, A.P. (1) 382 Zlatkevich, L.(3) 329 zou, Y.(3) 374 ZQU, Y.L.(3) 189 Zuilhof, H. (2.4)23 1 Zuloaga,F.(1) 279 Zumbuehl, S.(3) 762 Zvetkov, V.(3) 165 Zwanenburg, B.(2.2)36;(2.5)94; (2.6)89 Zwarc, M. (3) 1 1 Zyrianov, M.(2.7)66 Zyubina, T.S.(2.7)42;(3) 49 Zyung, T.(3) 472,491