Photochemistry Volume 29
A Specialist Periodical Report
Photochemistry Volume 29 A Review of the Literature Publishe...
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Photochemistry Volume 29
A Specialist Periodical Report
Photochemistry Volume 29 A Review of the Literature Published between July 1996 and June 1997 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 I.R. Dunkin, University of Strathclyde, Glasgow, UK A. Harriman, Ecole Europeenne Chimie Polymeres Materiaux, Strasbourg, France W. M. Horspool, University of Dundee, UK A.C. Pratt, Dublin City University Ireland
SOCIETY OF C HE MISTRY Information Services
ISBN 0-85404-415-9 ISSN 0556-3860 Copyright
8The Royal Society of Chemistry 1998
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 Patents Act, 1998, this publication 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 case of reprographic reproduction only in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization 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 page. 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 Part I
1
Physical Aspects of Photochemistry Photophysical Processes in Condensed Phases By Anthony Harriman
17
1
Introduction
17
2
General Aspects of Photophysical Processes
17
3
Theoretical and Kinetic Considerations
19
4
Photophysical Processes in Liquid or Solid Media 4.1 Detection of Single Molecules 4.2 Radiative and Nonradiative Decay Processes 4.3 Amplitude or Torisional Motion 4.4 Quenching of Excited States 4.4.1 Electron-transfer Reactions 4.4.2 Energy-transfer Reactions 4.5 Photophysics of Fullerenes
22 22 22 26 31 32 36 38
5
Applications of Photophysics
41
6
Advances in Instrument Design and Utilization 6.1 Instrumentation 6.2 Data Analysis
44 44 45
46
References Part II
Organic Aspects of Photochemistry
Chapter 1
Photolysis of Carbonyl Compounds By William M . Horspool
71
1
Norrish Type I Reactions
71
2
Norrish Type I1 Reactions 2,1 1,5-Hydrogen Transfer 2.2 Other Hydrogen Transfers
73 73
Oxetane Formation
79
3
V
76
Contents
vi
4
Chapter 2
Miscellaneous Reactions 4.1 SET Processes 4.2 Decarbonylation and Decarboxylation 4.3 Reactions of Miscellaneous Haloketones 4.4 Pho todeprotection 4.5 Other Fission Processes
83 83 86 89 90 90
References
93
Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool
95
1
Cycloaddition Reactions 95 95 1.1 Intermolecular Cycloaddition 95 1.1.1 Open-chain Systems 1.1.2 Additions to Cyclopentenones and Related Systems 95 1.1.3 Additions to Cyclohexenones and Related Systems 97 1.2 Intramolecular Additions 97 1.2.1 Intramolecular Additions to Cyclopentenones 100 1.2.2 Additions to Cyclohexenones and Related 100 Systems
2
Rearrangement Reactions 2.1 a$-Unsaturated Systems 2.1.1 Isomerization 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
105 105 105 106 108 110
3
Photoreactions of Thymines and Related Compounds 3.1 Photoreactions of Pyridones 3.2 Photoreactions of Thymines, etc.
112 112 114
4
Photochemistry of Dienones 4.1 Cross-conjugated Dienones 4.2 Linearly Conjugated Dienones
119 119 119
5
1,2-, 1,3- and 1,4-Diketones 5.1 Reactions of 1,2-Diketones 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
120 120 124 124 127 127
112
Contents
vii
6
Chapter 3
Chapter 4
Quinones 6.1 o-Quinones 6.2 p-Quinones
128 128 128
Refer en ces
131
Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool
135
1
Reactions of Alkenes 1.1 cis, trans-Isomerization 1.1.1 Stilbenes and Related Compounds 1.2 Miscellaneous Reactions 1.2.1 Addition Reactions 1.2.2 Electron Transfer Processes 1.2.3 Other Processes
135 135 135 136 136 137 139
2
Reactions Involving Cyclopropane Rings 2.1 The Di-n-rnethane Rearrangement and Related Processes 2.2 Other Reactions Involving Cyclopropane Rings 2.2.1 SET Induced Reactions 2.2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds
140
3
Reactions of Dienes and Trienes 3.1 Vitamin D Analogues
146 150
4
(2 +2)-Intramolecular Additions
151
5
Dimerization and Intermolecular Additions 5.1 Dimerization
154 154
6
Miscellaneous Reactions 6.1 Miscellaneous Rearrangements and Bond Fission Processes
155
140 143 143 144
155
References
160
Photochemistry of Aromatic Compounds By Alan Cox
164
1
Introduction
164
2
Isomerisation Reactions
164
3
Addition Reactions
172
4
Substitution Reactions
180
5
Cyclisation Reactions
183
6
Dimerisation Reactions
187
...
Contents
Vlll
Chapter 5
Chapter 6
Chapter 7
7
Lateral Nuclear Shifts
189
8
Miscellaneous Photochemistry
190
References
195
Photo-reduction and -oxidation By Alan Cox
204
1
Introduction
204
2
Reduction of the Carbonyl Group
204
3
Reduction of Nitrogen-containing Compounds
210
4
Miscellaneous Reductions
213
5
Singlet Oxygen
214
6
Oxidation of Aliphatic Compounds
216
7
Oxidation of Aromatic Compounds
219
8
Oxidation of Nitrogen-containing Compounds
224
9
Miscellaneous Oxidations
229
References
230
Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By Albert C. Pratt
239
1
Introduction
239
2
Nitrogen-containing Compounds 2.1 E,Z-Isomerisation 2.2 Photocyclisation 2.3 Photoaddition 2.4 Rearrangements 2.5 Other Processes
239 239 242 256 265 268
3
Sulfur-containing Compounds
289
4
Compounds Containing Other Heteroatoms 4.1 Silicon 4.2 Other Elements
297 297 30 1
References
305
Photoelimination By Ian R. Dunkin
316
Introduction
316
1
ix
Contents
2
3
4
316
Elimination of Nitrogen from Diazo Compounds and Diazirines 3.1 Generation of Alkyl and Cycloalkyl Carbenes 3.2 Generation of Aryl Carbenes 3.3 Photolysis of or-Diazo Carbonyl Compounds
320 320 322 324
Elinination of Nigrogen from Azides and Related Compounds 4.1 Aryl Azides 4.2 Heteroaryl Azides
326 326 328
Photoelimination of Carbon Monoxide 5.1 Photoelimination of CO from Organometallic Compounds
330
6
Photoelimination of Carbon Dioxide 6.1 Asymmetric Photodecarboxylations
332 334
7
Photoelimination of NO and NO2
334
8
Miscellaneous Photoeliminations and Photofragmentations 8.1 Photolysis of o-Nitrobenzyl Derivatives 8.2 Photofragmentations of 3- and 4-Membered Rings 8.3 Photofragmentations of Organosulfur Compounds 8.4 Photofragmentations of Organosilicon Compounds 8.5 Photofragmentations Involving Cleavage of Carbon-Metal Bonds 8.6 Other Photofragmentations
336 336 339 340 34 1
5
Part I11
Elimination of Nitrogen from Azo Compounds and An a1ogues
329
342 343
References
344
Polymer Photochemistry By Norman S. Allen
353
1
Introduction
353
2
Photopolymerisation 2.1 Photoinitiated Addition Polymerisation 2.2 Photografting 2.3 Photocrosslinking
353 354 361 361
3
Luminescence and Optical Properties
366
4
Photodegradation and Photooxidation Processes in Polymers 4.1 Polyolefins 4.2 Poly(viny1 halides) 4.3 Poly(acry1atesj and (alkyl acrylatesj
376 376 378 378
Contents
X
Part IV
Part V
4.4 Polyamides and Polyimides 4.5 Poly(alky1 and aromatic ethers) 4.6 Polyesters 4.7 Silicone Polymers 4.8 Poly(styrenes) and Copolymers 4.9 Polyurethanes and Rubbers 4.10 Polyketones 4.1 1 Photoablation of Polymers 4.12 Natural Polymers 4.13 Miscellaneous Polymers
378 379 379 379 379 380 380 380 380 38 1
5
Photostabilisation of Polymers
38 1
6
Photochemistry of Dyed and Pigmented Polymers
382
References
383
Photochemical Aspects of Solar Energy Conversion By Alan Cox
413
1
Introduction
41 3
2
Homogeneous Photosystems
413
3
Heterogeneous Photosystems
414
4
Photoelectrochemical Cells
416
5
Biological Systems
417
6
Luminescent Solar Concentrators
418
References
418
Artificial Photosynthesis By Anthony Hurriman
425
1
Introduction
425
2
Primary Photochemical Reactions 2.1 Photoinduced Electron Transfer in Molecular Dyads and Higher-order Analogues 2.1.1 Theory 2.1.2 Photoactive Dyads 2.1.3 Photoactive Triads 2.2 Light-induced Energy Transfer in Model Systems 2.3 Fullerene-based Molecular Systems
428
3
Environmental Effects on Rates of Electron Transfer
439
4
Energy Transfer in Pigment Clusters
440
5
Miscellaneous Photosystems
441
428 430 430 433 434 437
xi
Contents
6
Conclusions
442
References
442
Author Index
453
Introductionand Review of theyear BY ANDREW GILBERT
Readers should, as usual, use the Author Index to find the chapter and reference numbers of the papers cited in this Introduction and Review. As in the past two years, fullerenes continue to be a source of considerable photochemical interest and photoinduced electron transfer is a central theme of numerous studies. The field of molecular-scale electronic devices continues to promote interest in the photophysical properties of novel molecular architectures and such aspects of photoactive rotaxanes and catenates have been reviewed (Benniston; and Chambron et al.), while Harriman and Ziessel have outlined the design principles associated with the construction of photo-activated molecular wires. The elucidation of photophysical processes taking place at liquid-liquid interfaces is a comparatively new area. The dynamics of a variety of interfacial events such as, for example, photoinduced structural changes, interfacial photopolymerisation, and energy transfer across the interface have been studied using the technique of second harmonic generation spectroscopy (Eisenthal; and Long and Eisenthal). Studies of photoinduced electron transfer across such interfaces present appreciable challenges to photochemists but the phenomenon has seemingly attracted little attention to date despite its practical significance (see inter alia Dryfe et al.). Koshima and Matsuura have reviewed the photoreactions which occur at the interface between two crystallites, and Young has described the use of photophysical probes for monitoring the architectural deformations and determining the levels of residual strain in polymer composites. The distinction between through-space and through-bond electron-transfer processes can be difficult to determine unambiguously but using a series of donor-acceptor dyads of differing orientation, Gosztola et al. have studied this problem systematically. In recent years, selective laser excitation and ultrasensitive fluorescence detection has been elegantly used to study isolated and single molecules, and Lu and Xie report that recent advances in near-field and far-field fluorescence microscopy now allow the imaging of single molecules and the recording of their fluorescence properties at room temperature. The exceptional photostability of 2-(2’-hydroxypheny1)perimidine has been discussed by Catalan et al. in relation to the fluorescence of the enol, and the reported value of 240 ps for the fluorescence lifetime of adamantyldiazirine represents the first such record for a diazirine (Buterbaugh et d ) . Most phenanthrene derivatives do not usually exhibit Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998 1
2
Photochemistry
excimer fluorescence but Lewis and Burch have noted such emission from the lactams of phenanthrene-9-carboxylates and have proposed reasons why these derivatives should behave differently. Although there is a vast literature on reactions between excited state molecules and quenchers, unexpected results continue to be described from this area, and, for example, Ayala et ul. report that carbon tetrachloride quenches the fluorescence emission from alternant and nonalternant aromatic hydrocarbons to significantly different extents. The rate of electron transfer between a ruthenium(I1) tris (2,2'-bipyridyl) complex having an amidinium side-arm (donor) and a linked nitrobenzene derivative bearing a carboxylic acid function (acceptor) is reported to be remarkably affected (100-fold) by reversal of the salt bridge (Kirby et al.), and Hubig et al. have noted an ultrafast electron transfer between the excited singlet state of chloranil and an aromatic donor which competes with the fast intersystem crossing in the quinone. Again this year the photochemistry of porphyrinquinone systems has been the subject of a number of reports. The principal interest in this area lies in the use of such systems as models for solar energy conversion by photoinduced electron transfer (Gust et ul.). The rates of charge separation in polar and non-polar media have been measured for a series of zinc porphyrin-quinone units bridged by a cyclophane (Haeberle et al.). These data allow the first quantum mechanical analysis of the driving force dependence of charge separation alone and thus an experimental comparability between charge separation and recombination can be made. Comparison has been made between the rates of through-bond charge separation and recombination for two rigid dyads, which have a porphyrin chromophore, and either C6*or 1,6benzoquinone as the acceptor (Imahori et ul.). Remarkably, despite comparable separation distances and thermodynamic driving forces for the two dyads, the fullerene moiety facilitates both a faster charge separation and a slower recombination than the quinone system: this is rationalised by the C60 having a smaller reorganisation energy than the readily polarised quinone. A number of accounts have been published which describe aspects of the photochemistry of chiral systems. For example, Al-Kindy et al. have developed a chiral fluorescent agent for resolving enantiomers of carboxylic acids, and chiral calixarene derivatives can discriminate between the enantiomers of quenchers (Grady et a).). Several workers describe unique photocyclisation products from irradiation of chiral inclusion complexes in the solid state (Toda et al. and Fu et d.). New systems have been reported both for intercalation between base pairs in the DNA duplex (Bassini et ul., inter a h ) , and for photoswitchable materials in molecular-scale electronic devices (Tsivgoulis and Lehn). A methodology has been developed for synchronous scanning of dual-wavelength fluorescence which may allow the quantitative analysis of mixtures of known compounds (Hu et d.), and a versatile model for the evaluation of association constants from fluorescence data has been described which should have appreciable application in supramolecular chemistry (Lissi et ul.). Reports of the more organic aspects of photochemistry are now considered. Interestingly, while 2-benzoylcyclopentanonesare photostable in both solution and the solid state, when adsorbed on a silica surface and irradiated, they yield
3
I : Introduction and Review of the Year
products from a Norrish Type I process (Hasegawa et a].).Detailed studies of the photoreactivity of the large ring diketones (1) have been reported by Gudmundsdottir et al. and while the yields of the cyclised isomers (2) and (3) and the cleaved product (4), not surprisingly, vary with ring size, it is interesting to note that the preference for either (2) or (3) from (1) (rn = 11, n = 9) in the solid state is markedly dependent on the crystal form. The ratio of the cis:trans isomers of the dihydrobenzofurans (5) from irradiation of the benzophenone derivatives (6) is significantly influenced by the nature of the R-substituent (Kim and Park), and Kraus at al. propose that the minor isomer (7) (24% yield) from the diester (8) arises by a 1,12-hydrogen transfer. Photoexcitation of acetone in the presence of the alkene (9) yields (10) as the major addition product in what the authors (Chung and Ho) describe as a ‘photo-Conia’ reaction in which the a-keto radical, formed by hydrogen abstraction from acetone by an excited state acetone molecule, attacks the alkene.
Q; 6 (6) R = Me or CF3
HO
Photochemistry
4
e
A new and efficient route to endo-hirsutene has been described which uses, as the key step, the oxetane (1 1) from the intramolecular photocycloaddition of (12) acids can be synthe(Rawal et d.),and 3-deoxy-~-arabino-2-heptulopyranoson~c sised photochemically from Barton esters such as (13) (Barton and Liu). Interest in the development of photochemically removable protecting groups continues, and the irradiation of the 4-hydroxyphenacyl-protectedsystem (14) is reported to release ATP with a quantum yield of 0.37 (Givens and Park). Benzoylbenzoate esters of primary and secondary alcohols undergo cleavage in the presence of electron donor molecules and it is proposed that such esters could be effective photolabile protecting groups for alcohols and that thiols can be similarly protected (Jones et a].).
HO
OH
Contrary to earlier work, Caldwell et al. conclude from a detailed study of the 4,4-dimethylcyclohex-2-enone-l, 1-diphenylethene system, that the (2.n + 2.n) photocycloaddition arises from a triplet exciplex, and Crimmins et al. report that the enone (15) undergoes highly diastereoselective intramolecular photoaddition to give (16) which is a key intermediate in a new route to spiro[5.4]decanes. Similar intramolecular processes with other enones, followed by treatment with tributyltin hydride and AIBN, provide a convenient route to bicyclic compounds such as (17) in good yield (Crimmins et d.), and irradiation of the optically active silylallene (18) gives (19) with an ee in excess of 99% (Shepard and Carreira). Several accounts of (471+ 4x) intramolecular photodimerisation of tethered pyridones have appeared in recent years and the product from (20), having five stereogenic centres, is reported to have application towards a synthesis of taxanes (Sieburth et d.). A number of examples of intramolecular (2x + 2n) photocycloadditions involving pyrimidone units have been described within the year. The bichromophore (21) is reported to photoreact with chemo-, stereo- and regio-
"19.-
5
1: Introduction and Review of the Year
Me Me
0
Me
C02Me tN
* o
Photochemistry
6
specificity to give (22) exclusively (Saeyens et al.), and the major cyclobutane isomer from irradiation of (23) is claimed to be the first synthesis of a cis-syn furanoside (2.11 + 2x) photoadduct (Kobert and Essigmann). Irradiation of (24) induces singlet exciplex formation, and a novel [1,3]-sigmatropic shift from the singlet state occurs as well as (2x+ 27c) intramolecular cycloaddition from the triplet state (Ogino et al.). Chou et al. using the photoinduced ‘cage’ formation of enediones such as (25), have developed an approach to the synthesis of homosecoprismanes.
Me0 M
Ph
e
O
,
CI
Circularly polarised 190 nm radiation has been used to study the direct photoracemisation of the enantiomers of trans cyclo-octene (Inoue et al.), and new evidence has been reported for the 90” twisted intermediate on the S1 surface of stilbene (Gano et al.). Intramolecular charge-transfer excitation of 4-dimethylamino-4-cyanostilbene and 4-azetidinyl-4’-cyanostilbene is considered to involve at most an intermediate with a lifetime of less than 1 ps (Ilichev et a/.), and a novel synthetic route to S76-dihydro-4H-l,2-oxazines (26) from y,&-unsaturated oximes has been described by Armesto et al. The photorearrangement of the ammonium salts of the dibenzobarrelene (27) to the semibullvalene isomers has
Rs = ti, Me or Ph
“
(26)
been used to study energy transfer in the solid state and, interestingly, there is no reaction of these salts in solution (Gamlin et ul.). Under SET conditions, (1R,3S)(+)-cis-chrysanthemol (28) yields the dihydropyran (29) with retention of configuration (Herbertz and Roth), and in the presence of 1-cyanonaphthalene as the electron transfer sensitiser, the phenylcyclopropanes (30) are reported to undergo ring-opening stereospecifically with inversion of configuration at the reacting centre in what the authors (Dinnocenzo et al.) describe as ‘bona fide’ three electron SN2 reactions. Nevi11 et al. propose that the photorearrangement of the cyclopropane derivative (31) to the cyclobutane (32) (75%) is an example of a cyclopropyl-n-methane process involving an aromatic ring, and a number of substituted bicyclic dienes of type (33) have been shown to undergo efficient photoelectrocyclisation to strained cyclobutenes (Rigby et al.). Muller et al.
7
1: Introduction and Review of the Year
HOCH2 M %
Me
\
P
h
L
N
u
Me
(30) Me
I
Ph
Ph&e
4
Nu
Me Nu = MeO-, NC-, or HOMe0
\
,OMe
h
CHZPh
(33) X = H or OMe
A (34) R = OC12H25
8
Photochemistry
report that irradiation of the [ 181-annulene(34) leads to its dimerisation by (2x + 2x) photocycloaddition of the ethene bonds to give a complex novel cyclophane in 68% yield. The laser jet system of irradiation allows the observation of multi-photon processes and is becoming more popular. Under these conditions, the keto ether (35) in carbon tetrachloride solution undergoes two processes to give 4-phenylbenzaldehyde (one photon) and 4-phenylbenzyl chloride (two photons), while from ethanol solutions of (39, evidence is obtained to suggest that the formation of the ether (36) arises from a three-photon process (Adam and Schneider).
There continues to be a high level of interest shown in photochromic systems. The optically active l,l’-bi-2-naphthol gives an optically active intramolecular addition product (37) on irradiation, and on prolonged exposure, cycloreversion occurs which gives evidence for a photoequilibrium between the asymmetric molecules (Cavazza et al.). A number of reports within the year describe a variety of aspects of the well-known thermally-reversible spiroindoline-oxazine to photomerocyanine conversion. For example, the process is sensitised by triplet camphorquinone (Favaro et al.), the influence of complexation on indoline and phenanthroline spiropyrans with transition and rare earth metal ions has been described (Atabekyan et al.), and the photochromism of other derivatives in water using vesicles and y-cyclodextrin is reported to be faster than in methanol with the process most favoured in the vesicles (Ishiwatari et al.).
The intramolecular (4x + 47c) photoaddition of furan-pyran-2-one bichromophores [e.g. (38) to give (39)] is potentially useful as an approach towards the fusicoccane/ophiobolane skeleton (Chase et al.), and the formation of (40) from the irradiation of (41) constitutes the first example of a (4x + 4x) photocycloaddition of an enone to the benzene ring (Kishikawa et al.). Wender et al. continue to report elegant new applications of the intramolecular meta photocycloaddition of Sphenylpent- 1-ene derivatives and have described the synthesis of cis,cis,cis, trans-[5,5,5,5]-fenestrane (42) using the photoreaction of (43) as a key step. Park
1: Introduction and Review of the Year
02CCMe3
(38)
9
02CCMe3
(39)
et al. suggest that 9,lO-dihydrophenanthreneswhich may be difficult to access
by conventional routes, can be obtained by the photocyclisation of 2-aryl-lhalobenzenes, and the first reported synthesis of ( & )-solidaline uses photohydroxymethylation of berberinium salts (Suau et al.). Sunlight irradiation of 2styrylchromones provides a useful and convenient route to xanthones (Silva et ul.), and Funk et al. have described the photocycloaromatisation of a number of dialkynylarenes such as (44)which, they consider, could form the prototype for a new class of photocleaving agents. A novel photoassisted mild annulation process tolerant of a wide range of functional groups has been described by Zhu and co-workers, and the novel photocleavage of arylsulfonamides is considered to have applications in combinatorial chemistry for the solid-phase synthesis of amines (Garigipati and Adams).
The results in ordered media of the Norrish-Yang photochemistry of several phenones and of the photo-Fries reaction of 2-naphthyl myristate have been used to suggest how to choose and design ordered phases to influence radiationinduced transformations selectively (Baldvins et al.). A Norrish Type I1 process involving an uncommon 1,6-hydrogen abstraction to give a 1,5-diradical has been applied to synthesise the 2,8-dioxabicyclo[3.2.lloctane ring system (45) of the zaragozic acids (Freeman-Cook and Halcomb), and the single electron transfer induced cyclisation of (46) to (47) provides a key step in a route towards cephalotaxine, the parent of the family of cephalotaxus alkaloids (Lin et a].).
Photochemistry
10 H
Irradiation of dibenzothiophene sulfoxide in, for example, cyclohexane promotes oxidation of the solvent to give cyclohexene and cyclohexanol, possibly by way of atomic oxygen (Gregory et al.), and photofixation of carbon dioxide into a variety of organic substrates has been achieved by irradiation of a saturated solution of the gas in DMF in the presence of CdS nanocrystallites (Fujiwara et d.). Heinemann and Demuth report that in the presence of (-)-menthone as a chiral auxiliary, a highly diastereoselective cascade cyclisation of terpenoid polyalkenes occurs by photoinduced electron transfer, and using visible radiation in the presence of riboflavin 2’,3’,4’,5’-tetra-acetate,photoinduced electrochemical oxidation of benzyl alcohol to benzaldehyde has been achieved with 100% product selectivity and 100% current efficiency (Ishikawa et a/.). Novel cleavage of the pyrrole ring in (48) using singlet oxygen and followed by dehydrocyclisation to the imidazoline (49),provides a useful access to the derivative (50) which has acyl CoA:cholestrol acyltransferase activity (Li et a/.).
I
F (49)
R = H, X-X
= bond
(50) R = Me, X = X = H
Irradiation of the homochiral pyrazoline (51) leads to the elimination of nitrogen and the stereospecific formation of the cyclopropane (52) (Jimenez el a/.). The reaction proceeds by way of triplet diradical intermediates but evidently, closure to give the cyclopropane ring is faster than C-C bond rotation. The multiplicity of the diradical from the low-temperature photolysis of dihydropyrrolo[3,4-djpyridazines such as (53) is reported to be wavelength dependent and even at 77K, the singlet and triplet states do not equilibrate to a measurable extent (Bush et al.).The results reported by Motschiedler et al. from the triplet sensitised irradiation of 1,2-diaryldiazoethanes in methanol solution go some way to establishing that 1,2-H and 1,2-C shifts in a spin-equilibrated carbene compete
11
1: Introduction and Review of the Year Me
Me hv P
100%
H
(511 (52) with trapping of the species by an alcohol. The nonacarbene fonned from the 'starburst'-type polydiazo compound (54) has the highest spin multiplicity reported for a purely organic material (Matsuda e f al.), and the results from another approach to high-spin materials using manganese complexes of diazodi(4pyridyl)methane, lead Koga et al. to suggest that such species could be the basis of novel molecular optomagnetic recording devices as only the irradiated part of non- or weakly-magnetic material becomes strongly magnetic. The formation of
the azomethine ylide (55) and the spiroazepine (56) from irradiation of mesityl azide in the presence of tetracyanoethene provides the first example of competitive trapping of the singlet nitrene and didehydroazepine (Murata et al.), and by trapping diazacycloheptatetraenes,such as (57) with nucleophiles at room temperature, Reisinger and Wentrup have prepared the first stable monocyclic Nunsubstituted 1H- 1,3-diazepines. Intramolecular photoinduced electron transfer in (58) leads to decarboxylation to give hydroxylactams in good yield (Griesbeck et ul.), and irradiation of either the dexfro or leavo-rotatory crystals from the self-assembly of acridine and diphenyl acetic acid molecules, is reported to induce a decarboxylating condensation to give (+) or (-)-(59) with ees in the 35% region (Koshima et al,). The formation of ketones from the photolysis of nitrites is a low-yield process in
Photochemistry
12
I
Me
I
Me
I
Me
(55)
C02H
72Ohfor n = 11
solution and so the report by Kinbara et al. that irradiation of the steroidal nitrite (60) in the solid state (h>300 nm) yields the corresponding ketone in 50% is of appreciable interest. The novel class of compounds, 2,6-dithiabicyclo [3.1.O]hex-3-enes, is considered to be the intermediate in the photoconversion of the naturally occurring antibiotic red pigment (61) to the thiophene (62) (Block et al.), and Ouchi and Kogo report that o-quinodimethane is formed in a twophoton process from 1,2-bis(substitutedmethy1)benzenes (63). The various aspects of ‘polymer photochemistry’ continue to produce numerous publications representing a wide range of interests and describing a variety of applications: the selection of reports from this area for inclusion in this Review is, thus, very subjective. Large scale reflection holograms have been produced for the first time (Zhang et d.),a new real time FTIR monitoring system has been developed (Bradley), and new possibilities for storage display devices have been opened up by the observation of homeotropic alignment of the smectic phase in liquid crystalline polymers (Jain and Kitzerow). Itoh et a/. report that the reaction of [bis( 1-chloromethyl-2-vinyloxy)ethyl]terephthalatewith unsaturated carboxylic acids produces new hybrid monomers with both cationic and radical characteristics. A new template method has been developed for micro bead formation (Vorderbruggen et d.), and polymerisation rates can be monitored using a new fluorescent probe based on 4-(dimethylamino)-4’-nitrostilbene (Jager et d.).
I : Introduction and Review of the Year
13
phm H H
H
CH2R
(63)
R = PhO, PhS, or PhSe
Photocuring is a subject with many applications and water-based photocurable latexes with excellent physical properties have been prepared from polystyrenepoly(buty1 acetate-co-glycidyl methacrylate) together with allylamine for film forming characteristics (Odeberg et al.). A new method described by Santore et al. based on total internal reflectance fluorescence has been used to determine tracer diffusion processes in polymer systems, and novel 2-cyanophenyl alkyl ethers have been developed as sensors which are not prone to quenching reactions (Papkovsky). An acrylic polymer with phenoxazinyl units has been reported to show structural self-quenching effects (Yu et al.), and the colour of a new conjugated aromatic poly(azomethine) in solution is found to be dependent on the solvent polarity (Wang et al.). Fulgides have been synthesised which have a potential semiconductor laser compatibility (Tadros), and the fluorescence emission from poly(ethy1ene terephthalate) which was previously assigned to an excited ground state dimer, has been deduced to arise from an intermolecular excimer (Rufus et al.). Peng et al. have described a novel photoreactive polymer which has a ruthenium tris(bipyridyl) complex as the charge generating species, and a similar chromophore has been incorporated as a pendant group on a polysiloxane (Nagai). Hindered piperidine stabilisers (HALS) are widely used in polymer films and recent theories suggest that their oxygen charge-transfer complexes provide the principal inhibition effect (Gijsman and Dozeman): this is difficult to understand in view of the complexity of degradation processes in polymers Hunter and Hyde have described a new approach for the construction of supramolecular porphyrin assemblies which uses a co-operative co-ordination interaction to generate self-assembled chromophore arrays, and the presence of
14
Photochemistry
hydrogen sulfide as the electron donor in an aqueous solution of ruthenium trisbipyridyl and methylviologen is reported to increase hydrogen production by a factor of lo3 compared with the corresponding EDTA system (Werner and Bauer). A polymer surfactant template has been used in a colloidal platinum, zirconium viologen-phosphonate system (Gurunathan et d). The viologen is photoreduced to give a charge-separated species and hydrogen generation is increased five-fold compared with analogous microcrystalline systems in the absence of the template. It is a pleasure to welcome Tony Harriman back to the reporting team after a ‘break’ of some 10 years. Tony is now responsible for the Chapter on Photophysical Processes in Condensed Phases, and in the current Volume he has also written a timely and authoritative review on Artificial Photosynthesis. Ian Dunkin and Albert Pratt are also welcomed to the team and it is hoped that their first experience with the task will encourage them to stay with it for many years to come!
Part I Physical Aspects of Photochemistry By Anthony Harriman
Photophysical Processes in Condensed Phases BY ANTHONY HARRIMAN
1
Introduction
The scope and style of this chapter differs significantly from that used successfully in earlier years, although the literature coverage remains unchanged. More emphasis has been given to experimental methodology and to the advancements made in underlying theoretical concepts. Less consideration is given to the classical distinctions of singlet vs triplet excited states, and instead attention is focussed on the main types of photoprocesses occurring in condensed phases. The current trend in photochemical research is to seek applications for the work and this is reflected throughout the review. There is less coverage of biological systems, except where the biomaterial is simply a target for photoreagents - as with DNA visualization through fluorescent tags, since this subject requires its own chapter. While studies made in homogeneous solution continue to dominate the subject there is a steady migration towards heterogeneous media, with its more demanding analytical platform, and solid-state systems are being actively pursued by the more adventurous research groups. The proliferation of commercial software at affordable prices has ensured that relatively simple photosystems can be examined in great detail, with the inevitable consequence that the conclusions are often overstated. Analytical applications of standard luminescence spectroscopy, such as the routine detection of dissolved cations, have become so numerous that only a few examples can be highlighted herein. The chapter is organized to cover all important processes leading to deactivation of an excited state in a condensed phase. Special attention has been given to the various fullerenes because of the exceptionally high interest paid to these compounds over the last few years. Other sections consider theoretical concepts, instrumental methods for monitoring photophysical processes, and applications.
2
General Aspects of Photophysical Processes
Various aspects of excited state behaviour have been reviewed or highlighted during the relevant period. The tremendous contribution that flash photolysis has made to our understanding of excited states and free radicals has been exemplified Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998 17
18
Photochemistry
by one of the great pioneers of the field.' The general areas of luminescence spectroscopy and chemiluminescence have been reviewed2 while the photophysical properties of benzil have been described in detail3 The significance of exciplexes as reactive intermediates in polar media has been ~ t r e s s e d .Many ~ features of current concern in the field of light-induced electron-transfer reactions have been Particular attention has been given to energy- and electron-transfer reactions taking place in self-assembled porphyrin complexes.12*'3Interest in the emerging field of molecular-scale electronic devices has ensured that the photophysical properties of many novel molecular architectures, such as photoactive rotaxanes and catenates, have been while the design principles associated with construction of light-activated molecular wires have been outlined.I6 Much attention has been given to the photochemistry of supramolecular entities wherein complex multicomponent conjugates are assembled by non-covalent interactions between simple molecular species."-22 The photoinduced electron transport of macromolecular metal complexes in solution, at solid-liquid interfaces, and in the solid state has been described in a comprehensive report.23 Some consideration has been given24 to the design of multielectron photocatalysts for solar energy conversion. Several interesting reports have described the photophysical behaviour of conducting or conjugated polymer^^^-^^ and of light-induced energy and electron transfer occurring in polymeric media.28A relatively new area of photochemistry concerns trying to better define photophysical processes taking place at liquidliquid interfaces.29330 Using the technique of second harmonic generation spectroscopy to monitor the course of reaction it has been possible to measure the dynamics of numerous interfacial events. Such diverse processes as photoinduced structural changes, interfacial mass transport, rotational diffusion of molecules held at the interface, interfacial photopolymerization, and energy transfer across the interface have been studied. Photoreactions occurring at the interface between two crystallites have also been re~iewed.~' Some of the photophysical processes associated with organized molecular systems have been addressed using micelles and monomolecular layers to orient the molecules32 while the photochemistry that takes place between molecules oriented by external electrical fields has been c o n ~ i d e r e dA. ~detailed ~ review of the role of J-aggregates in spectral sensitization of photographic materials has appeared.34 The application of time-resolved luminescence spectrophotometry to distinguish between static and dynamic quenching processes has been i l l u ~ t r a t e dA . ~simple ~ methodology is outlined for defining the degree of static quenching in those cases where both static and dynamic quenching processes persist. Photophysical processes can be used to probe the local structure of organized systems that are difficult to characterize by conventional analytical protocols. Photophysical probes provide valuable structural information for monolayers assembled on metal supports36 and for polymeric composite^.'^ In the latter case, it has been possible to determine the level of thermal residual strain and to monitor architectural deformations. Fluorescence from single molecules continues to attract great attention3' while the dynamics of photodissociative processes have been reviewed, with emphasis given to experimental technique^.^^
I: Photophysical Processes in Condensed Phases
19
There is growing interest in the development of fluorescence-based molecular sensors4o and of photoswitchable system^.^' These photosystems involve the design of elaborate molecular receptors for binding adventitious guests in such a way that the complexation event causes a change in the photophysical properties of the host.
3
Theoretical and Kinetic Considerations
A comprehensive discussion has been made of the many factors that can control or modulate the rate of light-induced electron transfer in fluid solution.42 Particular emphasis has been given to the role of the solvent structure and to mutual donor-acceptor diffusion rates. It is shown that hydrodynamic effects can be very important at short separation distances, with the theory appearing to work especially well in viscous solvents. A related treatment extends the work to consider the rate of charge recombination under similar experimental condit i o n ~ It . ~has ~ proved necessary to introduce the concept of a time-dependent dielectric constant to properly account for the time-varying Coulomb potential that characterizes charge recombination between oppositely-charged ions. Oneand two-dimensional diffusion processes have been monitored for dye molecules entering into an intense laser beam.44A treatise has been presented that accounts for fluorescence quenching taking place at the diffusion-controlled limit with a time-dependent rate c0nstant.4~An explicit form of the rate constant is derived, together with examples of how to determine the dynamics of bimolecular quenching processes using combined time-resolved and steady-state measurements. A kinetic model has been described for the special case in which a gaseous or liquid reactant interacts with a polymer-bound fluor0phore.4~The model provides a quantitative description of the quenching behaviour and seems applicable to many such systems. Polymer-bound materials often possess chromophores existing in different environments that differ markedly in their ability to react with added substrates. The same situation abounds in biological media, especially high molecular weight proteins. It is interesting to note, therefore, that a protocol has been invoked to separate the emitting species according to their relative accessibility to q~enchers.~'A comparison of diffusion models relevant to fast time-scales has been made4*while the distance-dependence for bimolecular fluorescence quenching has been c o n ~ i d e r e dA. ~relationship ~ between the rates of diffusion and triplet-triplet annihilation in liquid solutions has been e~pressed.~' It is proposed that triplet-triplet annihilation occurs at the diffusion-controlled rate limit via formation of a weak exciplex-type intermediate. A theoretical description has been given to account for the kinetics of free radical initiated photopolymerization in solution, using Marcus theory as its basis.51The effect of temperature on the yield of charge-separated redox products has been analysed in terms of the dynamics of charge re~ombination.~~ It is concluded that only the inverted region of a Marcus-type rate YS. energy gap plot is seen and not the bell-shaped curve expected for genuine Marcus behaviour. A compilation has been made of the factors affecting adiabaticity in bimolecular
20
Photochemistry
electron-transfer reactions, with special reference to the difference between aromatic and aliphatic amines as electron donors.53 Since the HOMOS are different for these two types of donor, the disparate electron-transfer properties are ascribable to differencies in the size of the electronic coupling matrix elements. Diffusion-limited reactions occurring in one-dimensional systems have been reviewed. 54 A related study considers luminescence quenching taking place by way of a hopping mechanism and uses a Monte Carlo approach to analyse the relationship between luminescence yield and donor c~ncent rat i on. Kinetic ~~ models have been presented for bimolecular photochemical reactions occurring on solid surfaces56and for energy transfer between ions in solid-state materials.57 Considerable attention has been given to quantifying microscopic diffusion of probe molecules or of solvent molecules close to the probe. The effect of rotational diffusion on fluorescence spectra has been considereds8 in terms of the excited state dipole moment. The rotational dynamics of Coumarin-153 have been measured in a wide range of solvents and related to the structure of the solvent.59The importance of friction and dielectric-friction is discussed, together with the concept of solute-solvent coupling. In most cases studied, the rotational diffusional behaviour of the probe molecule differs markedly from that expected on the basis of hydrodynamical theory. Calculations of the time-dependent Stokes shift have been reported.60 Solvation dynamics have been measured for probe molecules dispersed in reverse micelles and, on the basis of time-dependent Stokes shift measurements, it is suggested that individual water molecules within the aqueous pool undergo slow (i.e. nanosecond) rotatiom6' The dynamics of solvation in non-dipolar solvents, such as benzene or 1,6dioxane, have been described, together with time-dependent Stokes shift measurements.62At present, there is no simple theory for modelling such solvation processes, unlike the case with polar solvents. Several studies have been concerned with improving our existing theoretical understanding of light-induced electron-transfer reaction^.^^-^^ Although most attention has been given to reaction in fluid solution, electron transfer occurring Frozen glasses present very in frozen media has been considered in some different properties to those provided by the same solvent in the liquid state, being characterized by a lower dielectric constant, higher refractive index, smaller solvent reorganization energy, and restricted solvation of polar species. The importance of vibrational dynamics for light-induced electron-transfer processes has been considered in light of time-resolved infrared spectral measurements made with fast temporal r e s o l ~ t i o n .Determination ~~ of inner-sphere7' and ~ o l v e n t ~ reorganization '-~~ energies is an important operation in the electrontransfer field and often limits the reliability of calculations. In particular, a simple and realistic methodology for calculating solvent reorganization energies would be a valuable addition to the subject since the outdated solvent dielectric continuum theory requires considerable patching in order to obtain viable numbers. The extent of electronic interaction between donor and acceptor pairs has concerned numerous research groups, especially in connection with covalently-linked molecular dyads. In several such cases, the rates of through-bond electron exchange have been measured for the same donor-acceptor pair sepa-
I: Photophysical Processes in Condensed Phases
21
rated by different spacer groups7480 and the results discussed in terms of the superexchange model. The conformation of the bridge, as well as the energy of localized HOMOS and LUMOs, is of particular importance in controlling the degree of through-bond interactions while flexible bridges also permit throughspace electronic coupling. The relationship between the rate of electron transfer and the thermodynamic driving force for the process has received further study.8 .82 Distinguishing between through-space and through-bond electron-transfer processes can be a hazardous task but this distinction has been studied systematically with a series of donor-acceptor dyads of differing ~ r i e n t a t i o n Charge .~~ separation and subsequent recombination can show different orientational behaviour to such an extent that in dyads possessing an oblique geometry these processes could take place via disparate routes. Other molecular dyads have been designed in order to separate electron transfer occurring at direct orbital overlap from superexchange-mediated electron transfer.84 In most photoactive dyads, charge recombination involves both reverse electron transfer to restore the initial ground state and intersystem crossing to form the corresponding triplet ion pair. The relative significance of these two processes depends on the molecular system, especially the lifetime of the radical pair, and the temperature. A careful study of this competition has now been made for charge recombination in rigid molecular dyads.85 It is shown that the spin multiplicity of the original localized excited state exerts a strong influence on the recombination pathway. Several photosystems have involved coupled electron-proton transfer events.sc88 Photophysical processes involved in the formation of twisted intramolecular charge-transfer (ICT) states continue to be of considerable interest and to receive intense investigation. It has been found that the efficiency for formation of the charge-transfer excited state can depend on the excitation wavelength, increasing with decreasing excitation energy.89Such systems are often characterized by dual fluorescence, emission arising from both the charge-transfer state and the locallyexcited (or Franck-Condon) state, that shows a marked solvent dependence. A stochastic model has been introduced to account for the fluorescence behaviour of an ICT system in glycerol.90An approximate description of the Stokes shift for ICT states has been given” and related to the dynamic properties of the surrounding polar solvent. Restricting the conformational mobility of the molecule affects its ability to attain the appropriate geometry to form an ICT state92while the conformation of the spacer used to separate donor and acceptor units controls the extent of electronic coupling along the molecular axis.y3 A comprehensive description of radiative and nonradiative decay processes in donor-acceptor complexes has been given.94The special case of intramolecular charge transfer in betaines has been considered in terms of the large change in molecular dipole moment expected to accompany the charge-shift rea~tion.’~ Such systems use an organic bridge to spatially isolate a positively-charged electron acceptor from its complementary negatively-charged electron donor. Light-induced, intramolecular charge transfer should generate a neutral species from a giant dipole and might have interesting applications for nonlinear optical spectroscopy or for second harmonic generation.
’
Photochemistry
22
The extent to which coupling to symmetric and anti-symmetric modes affects the spectral properties of intervalence charge-transfer complexes has been considered as a refinement to conventional Hush theory in the strong-coupling limit.96 Exciton trapping in crystals of weak donor-acceptor complexes has been reported9' and the effect of intermolecular coupling on spontaneous emission has received a t t e n t i ~ n . ~The ' use of resonance Raman spectroscopy to determine nuclear reorganization energies for ground-state donor-acceptor complexes has been reviewed.99 Monte Carlo methods for real-time path integration have been evaluatedI0' while a theoretical treatment of simple photoisomerization processes has been given.""
4
Photophysical Processes in Liquid or Solid Media
4.1 Detection of Single Molecules - The most elegant photophysical processes are undoubtedly those attributable to single or isolated molecules and such studies have become possible over the past few years by way of selective laser excitation and ultrasensitive fluorescence detection. A review has appeared recently that describes some of the experimental methodology currently being used for single molecule detection and illustrates the power of this approach by considering its application to flow-based, single DNA fragment sequencing and sizing."" The use of time-correlated, single-photon counting techniques to monitor fluorescence decay from single isolated molecules has also been reviewed. Recent advances in near-field and far-field fluorescence microscopy have made it possible to image single molecules and to record their fluorescence properties at room temperature. Improvements in data analysis facilitate distinguishing between similar molecules at the single molecule level.1os*'06 The significance of temperature-dependent linewidths has been stressed'"' while the cooperative nature of individual chromophores in single molecules of B-phycoerythrin has been established. Io8 Single molecule fluorescence has been observed from inhomogeneous environment^'^^ and the application of confocal microscopy provides an opportunity for two-dimensional single molecule imaging in solution."' Rhodamine dyes are attractive fluorescent labels for such 1 1 . 1 12 and detailed analysis of the fluorescence properties of isolated single molecules is beginning to provide important information on photophysical processes. 3-1
'
4.2 Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy' l6 the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins,' I and especially the tetraphenylsulfonates, l 8 have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = AII1',Gall', or Zn") form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are
I: Photophysical Processes in Condensed Phases
23
corresponding increases in the triplet quantum yields. I9 These compounds are inefficient sensitizers for production of singlet molecular oxygen in aqueous solution inasmuch as the reported quantum yields compare unfavourably with most other photosensitizers in current usage. The phosphorescence lifetime of vanadyl tetraphenylporphyrin in CHC13 has been measured as 60 ns and 2.2 ps, respectively, at 295 and 77 K.'" The spin-forbidden So-Tl absorption transition has been located in the far-red region of the spectrum. Spectroscopic properties have been recorded for antimony(II1) and (V) phthalocyanines,12' for free-base phthalocyanine at different temperatures, and for ytterbium porphyrins. I 23 The effect of substituents on the absorption and fluorescence spectral maxima of free-base phthalocyanine has been interpreted in terms of Hammett coefficients. 124 The photophysical properties of phosphorus(II1) tetrabenzotriazacorrole, a phthalocyanine-like dye, have been r e ~ 0 r d e d . IThis ~ ~ compound has particular interest because of its apparent ability to generate superoxide anions under illumination in aerated solution. The photophysical properties of free-base phthalocyanine tetrasulfonate (H2PcS4) and its corresponding zinc(I1) complex (ZnPcS4) have been studied in water and dimethyl sulfoxide using ultrafast transient spectroscopy. Fast internal conversion takes place in water because of extensive aggregation of the dyes but the monomeric pigments persist in dimethyl sulfoxide, where stimulated emission can be detected. The lowest-energy excited singlet states are reported to possess lifetimes of 370 and 460 ps, respectively, for H2PcS4 and ZnPcS4 but there are indications for an upper-lying singlet state that survives for cu. 10 f 4 ps. Related studies made with free-base phthalocyanines and porphyrins in CHCl, indicate that vibrational relaxation in the Sl level is complete within a few picoseconds.127 The fluorescence properties of 5,10,15,20-tetrakis(N-methyl-4pyridinium)porphyrin adsorbed onto silica have been described in terms of surface coverage and average orientation of the dye on the surface.12* Ultrafast transient absorption spectroscopy has been applied to study the excited singlet state of trans-urocanic acid in aqueous solution.129Excitation at 266 nm generates a x-n* excited state localized on the imidazole ring but excitation into the red edge at 306 nm populates a different electronic state. In acidic solution, the x-x* excited state can transfer a proton to the solvent. The nature of the electronic states of simple alkyl amines has been addressed and the importance of low-lying Rydberg states has been confirmed for this class of molecule. 3o The S2 level in N,N-dimethylanilino(pheny1)acetylene in n-hexane has a lifetime of ca. 20 ps and exhibits strong delayed fluorescence when excited into the S, 1evel.I3' It appears that the dimethylanilino substituent promotes internal conversion from S2 into S1 by increasing the density of vibronic states. Fluorescence from the S2 level of thioflavone has been observed132and a full quantitative description of the photophysical properties of this compound is now available. The photophysical properties of peropyrene, which has a fluorescence quantum yield of 0.93 in toluene, have been likened to those of ~ e r y 1 e n e . l ~ ~ Fluorescence from the enol form of 2-(2'-hydroxypheny1)perimidine has been explained in terms of why this molecule is exceptionally photostable. 134 The fluorescence properties of substituted polyphenyls have been recorded in
24
Photochemistry
liquid solvents and it has been noted that the longer analogues, having four or five phenyl rings, display very high fluorescence yields but sub-nanosecond lifetimes. The triplet state properties of related polyphenyls, accessed via benzophenone sensitization, have been described. 36 Laser flash photolysis studies made with a series of 1,8-bis(substituted methy1)naphthalenes 1-3 show that whereas X = 0 produces the triplet state of the naphthalene residue the higher molecular weight analogues X = S or Se undergo cleavage to give PhX and the corresponding carbon-centred radical. 37 The lifetimes of singlet nitrenes, which range from tens to hundreds of nanoseconds, produced by laser photolysis of polyfluorinated aryl azides are controlled by ring expansion and intersystem crossing to the lower energy triplet state.13*
'
X
X
1
x=o
2 x=s 3 X=Se
The photophysical properties of pyrene continue to attract attenti01-1'~~ and it has been shown that internal conversion from S6 into SI takes about 2-3 PS.'~() Vibrational cooling within the SI level, following laser excitation at 290 nm, is complete within 25-30 ps. Ultrafast transient spectroscopy has been used to monitor intersystem crossing in xanthone and to explore the effect of solvent polarity on the relative positioning of x-x* and n-x* excited singlet states.14' Assignment of these energy levels for both singlet and triplet states of 2naphthaldehyde has been made142but, in the vapour phase, the compound undergoes decomposition when excited into the S 2 (n-x*) level. The nature of the lowest-energy triplet excited state of 4-aminobenzophenone changes from n-n* in nonpolar solvents to charge-transfer in polar solvents while the triplet quantum yield decreases from 0.82 in cyclohexane to 0.1 in N,N-dimethylf~rmamide.'~~ Steric hindrance between the oxygen and halogen atoms causes structural deformation of cr-haloanthraquinones 4 and 5 and their triplet excited states are of mixed x-IT* and n-x* character with unusually short 1ifetimes.l4 The triplet state properties of 4-nitroacetophenone have been studied by conventional laser flash photolysis technique^'^^ whereas the orientation rules for photochemical nucleophilic substitution have been deduced by studying intramolecular photoreactions of molecules of type 6.146 The triplet energy of S-phenyl-1-thionaphthoate has been established147at 252 kJ mol-'. The first reported fluorescence lifetime of a diazirine has appeared14* and gives a lifetime for adamantyldiazirine of 240 ps at ambient temperature. A report has
I: Photophysical Processes in Condensed Phases
x
25
o
4 X = H, CI or Br; Y = H 5 X = H; Y = CI or Br
discussed the photophysics of 7-hydroxytetrahydroisoquinoline-3-carboxylic acid in terms of its relevance as a conformationally-restricted model for tyrosine. 149 Some triplet state properties of guanine have been describedlS0and the ability of certain methoxyflavones to dissipate excitation energy via rapid internal conversion has been d i s c ~ s s e d .These ' ~ ~ latter studies address the role played by natural flavonoids to protect higher plants from exposure to damaging UV illumination. The mechanism, and product distribution, for the photolysis of 4-chlorophenol in water has been elucidated by nanosecond laser flash photolysis studies.Is2 Photophysical investigations have been reported for rh~damine,'~' phenothia~ine,"~ ~ ketocyanineIs6dyes in solution. Fluorescence quantum yields and 55 ' and lifetimes have been recorded for some substituted 9-arylxanthyl and 9-arylthioxanthyl carbocationic dyes' " and it appears that the presence of electron-donating substituents promotes internal conversion for the xanthyl cations. In contrast, little or no substituent effects were found for the corresponding thioxanthyl cations. Phosphorescence lifetimes of 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine and benzo[f Iquinoline increase with decreasing temperature in glucose glasses.15gA series of articles has addressed the properties of the excited triplet state of 4H-1-benzopyrane-4-thione in perfluoroalkane solvents. '59-'61 Delayed S2-Sofluorescence arises via triplet-triplet annihilation. Biexponential fluorescence lifetimes have been found for thionine and phenosafranine dyes bound to poly(acrylaminoglyco1ic acid) or poly(methylolacry1amide) due to their localization in different domains created by the polymeric environment. 162 Time-resolved fluorescence studies have been made with 43diamino-2,7-diisopentylanthraquinonein nematic liquid crystals.163 Time-resolved diffuse reflectance and emission characteristics of biphenyl, naphthalene, and a few other aromatic molecules adsorbed in the cavities of NaX and NaY zeolites have been investigated as a function of sample loading.lbl At low surface coverage, biphotonic excitation causes ionization of the guest molecule, with the ejected electron being trapped by the zeolitic framework. As the surface loading increases, the radical anion of the guest is observed but it is proposed that this species arises, not from reaction with a zeolite-trapped electron, but, by direct electron transfer. The dimer cation of naphthalene can also be observed within the zeolite cage.Ibl A comparison has been made of the luminescence properties of phenanthrene and its perdeuterated analogue adsorbed onto different solid matrices over a wide temperature range.'65 A common problem encountered in photophysical investigations concerns
26
Photochemistry
aggregation or other nonlinear behaviour at high concentration of chromophore. The fluorescence dynamics of the diacid form of tetrasulfonatophenylporphyrin, which is well known to aggegate at modest concentration, have been studied in homogeneous solution. Somewhat different results are found according to whether excitation is to S l or Sz. A comparison has been made of the photophysical properties of aluminium(II1) phthalocyanines under aggregating and non-aggregating in an effort to better account for the performance of such dyes as sensitizers for photodynamic therapy. A fluorescent dimer has been reported for zinc(I1) tetrasulfonatophthalocyanine in aqueous a ~ e t o n i t r i l e , with ' ~ ~ the fluorescence maximum appearing at 7 10 nm. Excimer fluorescence has been detected for fluorenone in liquid solutions'70and also for lactams formed from phenanthrene-9-carboxylates.71 This latter finding is unusual since most phenanthrene derivatives do not show excimer emission and reasons are advocated as to why these lactams should behave differently. Excimer fluorescence has also been reported for acridine and diazaheteropolycycles in nonpolar media at 77 K. 72 Under similar conditions, 2,2-naphtho-l7-crown-S forms ground-state dimers that phosphoresce whereas the corresponding crown-6 shows only monomer phosphorescence. 173 Relaxation dynamics have been reported for dimers of silicon phthalocyanine. 74 Fluorescence from the dimer is red-shifted by ca. 4000 cm- with respect to the corresponding monomer and the fluorescence yield is decreased ca. 1000-fold7giving an estimated fluorescence lifetime of ca. 24 ps. Weakly fluorescent dimers have also been observed for rhodamine 6G in concentrated ethylene glycol solution'75 and the fluorescence lifetimes of some coumarin laser dyes have been measured over a wide concentration range. 17' Incorporating pyrene into silica xerogels causes its isolation and protection against excimer formation, when compared with ethanol solution. 77 Coumarin-4 gives highly complicated fluorescence behaviour when studied under similar conditions, especially when the gel is relatively acidic. 178
'
'
'
'
4.3 Amplitude or Torsional Motion - Light-induced conformational changes can provide a facile means by which to promote internal conversion. These geometry changes may be small, such as slight twisting around a connecting bond, or large-scale, leading to formation of a geometrical isomer. Such processes can be exceptionally fast, as is the case with photoisomerization of cis-stilbene where decay of the excited singlet state is on the femtosecond time-scale and little or no fluorescence can be detected.'79 In this case, internal conversion is so fast that vibrational relaxation is incomplete and it appears that there are several transition state geometries, in addition to the perpendicular configuration, at which fast decay can take place. In marked contrast, phosphorescence emission has been reported for sterically constrained, or so-called stiff, cis-stilbenes. No adequate theoretical model exists by which to explain the rates of torsional motion in quantitative terms and, in fact, many factors combine to influence the rate of photoisomerization. Even in supercritical fluids, Kramers theory does not properly explain the dynamics of photoisomerization of cis-to-trans 2-vinylanthracene. 18' Charge-transfer effects can be important in the simple conjugated
'
I: Photophysical Processes in Condensed Phases
27
dienes and trienes,18”’8’which tend to be easily oxidized. The most important parameter, however, concerns friction between the rotor and the surrounding solvent and several studies have addressed this issue with reference to the photoisomerization of DODCIi84”85and azobenzene.186-187 Merocyanine dyes have been studied in detail, partly because of their strong solvatochromicity, and the rates of light-induced isomerization are known to depend on both viscosity and polarity of the ~ o l v e n t . ’ ~The ~ ” ~rate ~ of isomerization of the polyene backbone also depends on the state of a g g r e g a t i ~ nand, ’ ~ ~ more importantly , on the presence of substituents in the bridge.’” The significance of intramolecular charge transfer for the rate of photoisomerization of stilbene derivatives has been stressed,’”-’94 while the site of attachment of electron-donating amino substituents is seen to exert a powerful effect on the overall photophysical behaviour.”’ A comprehensive report deals with photoisomerization of stilbene derivatives in which thiophene rings replace the phenyl ring(s).Ig6The size of the rotor affects the height of the barrier to rotation around the central double bond within the excited singlet state manifold such that polythiophene derivatives are strongly fluorescent and undergo efficient intersystem crossing in non-polar solvents. The height of the barrier to internal rotation drops in more polar solvents, however, due to a charge-transfer effect. The photophysical properties of 4-N,N-dimethylamino-4’-cyanostilbene (DCS) and 4-azetidinyl-4-cyanostilbene have been examined in considerable detail and activation energies for trans-to& isomerization have been measured in different solvents.I y 7 These compounds undergo intramolecular charge transfer under illumination such that the dipole moment increases upon formation of the Franck-Condon state with a further increase upon forming an ICT state. Thus, for DCS the dipole moment in the ground state was found to be 7 D but this increases to 13 D for the locally-excited singlet state and 21 D for the ICT state. Activation energies for internal rotation of 14.0 and 22.7 kJ mol-l, are found in hexane and acetonitrile solution, respectively. The increase in the barrier height with increasing solvent polarity, which is contrary to the case found for stilbene and for many cyanine dyes, suggests that the transition state for photoisomerization is less polar than the ICT state.’97 For 1-styrylanthracene, it has been found that cis-to-trans photoisomerization does not compete with photocyclization in non-polar solvents but occurs via a diabatic singlet state process in polar solvents. 19* The importance of rotation of the aminophenyl group for controlling relaxation of the SI level of 4-amino-9-styrylacridine has been deduced from a temperature dependence study. 199 This compound undergoes facile isomerization but intersystem crossing and fluorescence are negligible. The photoisomerization of several indole derivatives has been studied”’ while isomerization around the C=N double bonds of N-methoxy- 1-(1-pyrene)methimine has been described.20’In certain cases, the reversibility of the isomerization step depends on the spin multiplicity of the reacting state.’02 Several reports have been concerned with the photophysical properties of cyanine dyes promoted into upper excited state^.^*^-*^^ Indeed, for a series of cyanine dyes having different polyene backbones it was possible to establish that isomerization takes place
28
Photochemistry
from both first and second excited singlet states and a comparison has been made of the relevant relaxation rates.203Photoisomerization of certain rhodamine dyes was found to proceed from a higher-excited state.'M In contrast, excitation into an upper-triplet excited state of merocyanine 540 (MC540)is followed by rapid reverse intersystem crossing to the singlet manifold such that, in this case, isomerization occurs only from the first excited singlet state.'05 The photophysical properties of structurally-related derivatives of MC540 have been determined in fluid solution in order to identify ways to increase the CN
/
p
DCS
quantum yield for formation of the excited triplet state. It was found that replacing the heteroatoms in either or both benzoxazole and thiobarbiturate units allowed close control of the rate of intersystem crossing.206Although isomerization from the first-excited singlet state still occurred for the new derivatives, it was possible to bring about a substantial increase in the rate of intersystem crossing by using selenium-based dyes. A reasonable correlation was found between the triplet yield and the ability of the dye to cause photocytotoxicity of leukaemia cells but the nature of the heteroatom also affects the rate of assimilation of the dye into the intact biological cell. It was further considered that the triplet yield would be increased if isomerization from the singlet state could be inhibited by incorporating bulky substituents into the bridging polymethine chain.'07 These substituents tend to promote internal conversion, without full isomerization, because of slight structural distortion in the ground state. In fact, a nice correlation was obtained between the rate of internal conversion and the torsional angle around the isomerizing bond. Building the polymethine bridge into a cyclic structure stops isomerization but still does not give a high triplet yield.'07
I: Photophysical Processes in Condensed Phases
29
Because of the potential importance of photoisomerizing dyes as probes of local viscosity, especially in microheterogeneous environments, much attention has been given to understanding how interactions with the surroundings can influence the rate of isomerization. The dynamics of stilbene isomerization have been compared for surface-bound and isolated molecules.20s Related investigations have considered stilbene in a bilayer lipid membrane”’ or azobenzene in Nafion The photophysical properties of cyanine dyes in sol-gel matrices”’ and adsorbed onto microcrystalline cellulose surfaces213have been reported while isomerizing dyes have been intercalated between the layers of silicates.214The ability of bimolecular reactions, notably quenching of the firstexcited singlet state by carbon tetrachloride, to compete with isomerization of trans-stilbene has been considered in terms of diffusion.” Other studies have reported photophysical properties of molecules for which partial rotation around a connecting bond is sufficient to promote effective internal conversion into the ground state, without the need for formation of a photoisomer. Thus, ethenylene-bridged porphyrin dimers, such as trans- 1,2bis(meso-octaethylporphyrin)ethene, undergo fast internal rotation of the porphyrin planes within the first-excited singlet state but the corresponding cisisomer is not formed.216Such molecules are highly strained in the ground state without obvious indication of the two closely-spaced chromophores being strongly coupled; similar behaviour is found for twisted bis-coumarin derivatives.21 A thorough investigation, including molecular dynamics simulations, has been made of the torsional dynamics for the singlet excited state of 9,9’-bianthryl in nonpolar solvents.21s It has been shown that, unlike the situation found for coumarin derivatives, twisting of the amino or dimethylamino group does not affect the photophysical properties of carbostyril dyes in s~lution.~’’ The barrier height for inversion of the carbonyl group in cyclic ketones increases with the degree of ring angle strain at the ketone carbon atom and affects the photoA comparison has been made of the role of frame and physical rotor dynamics in deactivation of the SI state of 4- and 5-methylpyrimidines.”’ The photophysical properties have been recorded for a series of stericallyhindered derivatives of POPOP for which ortho substituents force the molecule out of planarity.222These molecules become more planar in the excited singlet state due to a structural modification having a low activation energy and there is a substantial increase in the Stokes shift. Rotational isomerization has been described for the phototautomer formed by proton transfer in the excited singlet state of 2,2’-bipyridin-3-01in hydrocarbon solvents.223 At room temperature both the primary phototautomer and its rotamer fluoresce, allowing the activation energy for internal rotation to be determined. Excited state tautomerization has also been described for camptothecin in acidic aqueous s ~ l u t i o n ”and ~ for derivatives of h y p e r i ~ i n . ’Light~~ icduced keto-enol tautomerism has been invoked to explain the fluorescence behaviour of certain benzimidazole compounds.226 Interconversion of conformers of constrained tryptophan derivatives takes place in the first-excited singlet state.227The excited state behaviour has been reported for conformationallydistorted porphyrin derivative^.^'^-'^^ The lifetimes of the S1 states of these
’
’
Photochemistry
30
ruffled porphyrins are extremely short compared with their planar counterparts in solution at room temperature but the lifetimes become more comparable at 77 K.’” Structural distortion also causes red-shifted spectra and a drop in yield for fluorescence from the upper-excited singlet state of the porphyrin.”’ In the corresponding nickel(I1) derivative, the lifetime of the lowest-energy singlet ( K , x * ) excited state is only ca. 0.7 ps.”‘O This state undergoes rapid internal conversion to give a (d,d) state localized on the central cation. An intramolecular vibrational relaxation occurring on the time-scale of CQ. 1 ps has also been reported for planar nickel(I1) porphyrins in t ~ l u e n e . ’ ~The effect of molecular conformation on the low-temperature photophysical properties of a set of methylated 2,2’bipyridines and their corresponding zinc(I1) complexes has been r e p ~ r t e d . ’The ~~ possible involvement of rotamers and/or taut omers in the photophysical properties of 2,2’-pyridi1,’33 7-hydo~yflavone,”~ and 1O-hydroxybenz[h]q~inoline”~has been considered. Internal rotation, proton transfer, and ICT have been implicated in the luminescence behaviour of hydroxy derivatives of 2,2’-bipyridine236.237 and pyridine.23x Molecules containing both electron donating and withdrawing substituents can undergo slight rotation after excitation into a Franck-Condon state to give a configuration suitable for intramolecular charge transfer (ICT).’39’-’4’ The photophysical properties of such molecules are often characterized by dual luminescence, with both Franck-Condon and ICT states emitting, that is strongly dependent on solvent p~larity’~’24s and t e r n p e r a t ~ r e . ’The ~ ~ conformational change needed to reach the ICT is usually too small for the photophysical properties to depend on bulk viscosity of the surrounding medium but there are strong indications for interaction with individual solvent molecule^.'^^^'^^ In certain cases, evolution of the ICT state is controlled by- solvation dynamics.’49 Many molecules that form an ICT state employ a dimethyamino group as electron donor and it has been concluded that an essential feature of the chargetransfer step concerns a change in configuration of the amino nitrogen atom from pyramidal towards planar while the occurrence of dual fluorescence requires that the energy gap between the two states remains modest.’47 Using time-resolved microconductivity techniques it has been observed that the triplet excited states of para-nitroaniline and its N-alkylated derivatives are considerably more polar than the corresponding ground states.250The electron donating or withdrawing properties of such dipolar molecules can be modified by protonation and, in some respects, the onset of ICT behaviour reflects pK transitions of the reactive g r o ~ p s . ” ~ . ~ ’The ~ ’ photophysical properties of a ‘pre-twisted’ donor-acceptor biphenyl have been investigated in liquid solv e n t ~ . ’By ~ ~comparison to the analogous flexible and rigid biphenyls it was concluded that pre-twisting introduces an additional relaxation pathway for the excited Franck-Condon state. Whereas the flexible compounds undergo rapid rotation towards planarity the pre-twisted biphenyls relax either towards a more twisted geometry or to the planar conformation. Steady-state and timeresolved fluorescence studies have been augmented by quantum chemical calculations.”’
’
I: Photophysical Processes in Condensed Phases
31
4.4 Quenching of Excited States - Reaction between excited states and appropriate quenchers is well known to provide important mechanistic information, although the intimate details are often obscured for bimolecular reactions occurring at the diffusion-controlled rate limit in fluid solution.254Such reactions have been very popular for the past three decades or so but they still succeed to provide surprising results, such as the finding that fluorescence from alternant and non-alternant aromatic hydrocarbons is quenched to significantly different extents by carbon tetra~hloride.’~~ In searching for novel systems where bimolecular quenching processes can provide valuable information, many researchers have resorted to using heterogeneous environments such as that employed for the quenching of pyrene fluorescence by amphiphilic nitroxide radicals embedded in cationic m i ~ e l l e s . ’Other ~ ~ approaches rely on using competitive kinetics wherein a quencher can react via several distinct routes according to its molecular structure. Thus, halogenated toluenes quench triplet benzophenone by both hydrogen-atom abstraction and spin-orbital coupling, with the significance of the latter process depending on the nature of the halogen.’57 There is growing usage of computer modelling to follow successive reactions in which primary products become q ~ e n c h e r sFavourite .~~~ reactions, such as the photoreduction of benzophenone, can be examined in ever-increasing detail by the application of sophisticated experimental technique^.'^^ For example, ”C-stimulated nuclear polarization techniques have been used to monitor exchange interactions between radical pairs in which the position of the I3C label was systematically varied.’60 The ubiquitous exciplex seems to be less popular nowadays but still draws great attention.26”262Again, the systems are becoming more complex, as adventitious species modify the ensuing chemistry of the or as the binding energy is derived from sources other than charge transfer or ~c-stacking.’~~ The most studied mode of quenching, as judged by the number of publications, concerns light-induced electron transfer - undoubtedly due to the success of Marcus theory - with energy transfer running a poor second. An additional but very important type of quenching process involves the photodissociation of weak bonds, such as found for tetraphenylhydrazine (Scheme 1) where the rate of dissociation into diphenylaminyl radicals shows a strong viscosity dependence.266This reaction has been considered in terms of the high damping regime of the Kramers-Smoluchowski theory and it appears that, in addition to the activated process, there is an activationless dissociative pathway that favours formation of free radicals at infinite viscosity.
Scheme 1
32
Photochemistry
The opposite behaviour, namely photodimerization, can take place when local concentrations of substrate are high, as for anthracene in Langmuir-Blodgett mono layer^.'^^ In fact, this is a common reaction pathway for anthracene derivatives and occurs by both bimolecular268and i n t r a m ~ l e c u l a rroutes. ~ ~ ~ The efficiency of intramolecular photocyclization depends on the length and flexibility of the connecting chain.’70 4.4.2 Electron-transfer Reactions - Light-induced electron-transfer processes continue to be studied in great detail and numerous investigations have been described during the review period. Often, it is not possible to definitively assign a quenching process to electron or energy transfer since the two pathways can be Switching between the two mechanisms can be achieved by changing the temperature or solvent polarity - in general, electron transfer is promoted in polar solvents at higher temperature and is frequently inhibited by freezing the solvent.273With covalently-linked donor-acceptor dyads the two processes run concurrently and their relative importance is set by local energetics and the nature of the connecting bridge. Electron transfer is not always reversible but can result in bond cleavage and elimination reactions leading to formation of free radicals.274 In fluid solution, it is often considered that electron transfer proceeds only at orbital contact, by way of a precursor complex, but recent models indicate that noncontact electron transfer cannot be neglected.275 Although most investigations refer to electron transfer in liquid solvents there is a growing awareness that similar reactions can proceed in the solid state and the use of layered materials as the medium for electron transfer between adsorbed guests is a particularly attractive alternati~e.”~ Zeolite cages provide a unique opportunity to control electron-transfer reactions and this is a rapidly expanding field.277 A challenging area of photochemistry relates to electron transfer across a liquid-liquid junction but little attention has been given to this subject, despite its practical ~ignificance.’~~-~** By varying the thermodynamic driving force for light-induced electron transfer, it has been possible to observe Marcus-type inverted behaviour for charge separation in a bimolecular system comprising various ruthenium(I1) tris(2,2’bipyridyl) complexes as chromophore and the phenolate anion as quencher.28’ Such behaviour is very rare, in marked constrast to charge recombination where Marcus inverted behaviour is The photoreduction of thionine by N-phenyl glycine in methanol has been while many other organic dyes are photoreduced by free radicals in solution.284 In this latter system, irradiation of benzophenone in the presence of an amine or alcohol is used to generate a source of reducing free radicals that can react with added substrates. If the substrate is coloured, reaction is conveniently followed by optical methods and bimolecular rate constants have been reported for numerous couples. Consideration has been given to the role of dimethylanisole as an electron donor in photochemical reactions with 2-nitrofluorene as a ~ c e p t o r . ” Studies ~ carried out in a molten salt and in acetonitrile solution have indicated that 9-methylanthracene, when illuminated in the presence of the 1-ethyl-3-methylimidazolium ion, can function as both electron donor and acceptor.286Both singlet and triplet
I: Photophysical Processes in Condensed Phases
33
excited states of 9-styrylanthracene enter into electron-transfer reactions with anilino donors and a complete mechanistic investigation has been de~cibed.”~ Similar dual state reactivity has been reported for quenching by onium salts’” whereas the importance of the nature of the lowest-energy triplet state has been stressed for electron-transfer reactions involving aromatic carbonyl comp o u n d ~ Thus, . ~ ~ ~the bimolecular rate constants for quenching the triplet excited state of aromatic carbonyl compounds by triethylamine do not depend on the Gibbs free energy change for electron transfer but reflect the type (i.e. n-n*, n-7c*, or charge transfer) of excited triplet state. Several studies have addressed the role of solvent polarity on the rate of bimolecular electron-transfer reactions and the importance of solvation of the emergent nradical ions has been Related investigations have shown that the yield of charge-separated products can be affected by pH, at least in the specific case of porphyridquinone redox couples.”’ Here, the main effect of protonating the quinone is to bring about a substantial increase in the driving force for charge separation but there are also important differences in the reactivity of the ion pair, as deduced by laser flash photolysis studies. Kinetic salt effects have been described for quenching the excited triplet state of ruthenium(I1) tris(2,2’-bipyridine) by various anthraquinones in acetonitrile solution.293In this system, charge recombination shows Marcus inverted behaviour. Similar reactions carried out in water with methyl viologen as quencher demonstrate specific anion which might reflect ternary complexes formed between donor, acceptor, and anion. Other studies have considered the effects of micelles, microemulsions, polyelectrolytes, and Langmuir-Blodgett layers on the kinetics of electron transfer between isolated The main effects of such environments are to concentrate the reactants and to modify thermodynamic driving forces for forward and reverse electron-transfer steps. The major drawback to these studies is that they provide little or no information about the electron-transfer reaction itself since the kinetics are dominated by diffusional processes and there is great uncertainty about the microscopic positioning of species. Electron-transfer quenching of excited states has also been described for fluorophores incorporated into polystyrene latex dispersion^,^" adsorbed onto macroreticular resins,303or built into conjugated polymer blends.304Attention has been given to electron-transfer processes in porous solids305307 and an analytical treatment has been given that takes account of the distribution of separation distances inherent to solid-state systems.308On the basis of fluorescence quenching studies made in solid matrices it has been concluded that the distance-dependence of the electron-transfer rate constant might exhibit a non-exponential form. The model presented can be used even in those cases where the experimental kinetic data are distorted by anisotropy and/or structural heterogeneity. In trying to establish fixed, or at least constrained, geometry for the donoracceptor pair many researchers have turned their attention towards supramolecular chemistry wherein the principles of molecular recognition are used to assemble suitable entities without covalent interactions. Thus, the photophysical properties of biacetyl imprisoned inside a hemicarcerand have been measured in the presence of about twenty different quenchers but the framework of the
34
Photochemistry
hemicarcerand permits only weak intermolecular interaction^.'^' Increased interaction between the redox partners can be realized by building the chromophore into a pseudorotaxane where the electron-affinic quencher is localized close to a phthalocyanine chromophore by way of n-stacking to an appended electron-rich ~ide-arm.~"In such systems, intracomplex electron transfer can proceed at low concentrations of quencher. A different approach to building supramolecular donor-acceptor pairs involves equipping the partners with complementary hydrogen bonding functionalities. Many systems of this type have now been constructed, usually having several hydrogen bonds for a more stable structure, and light-induced electron transfer within the complex has been demonstrated.31 For proper mechanistic investigations it is important to ensure that the hydrogen-bonded complex is sufficiently rigid to separate through-bond electron transfer from through-space interactions. Because of its significance to biological electron-transfer events, the general concept of whether electron transfer can proceed through a hydrogen bond, and if so at what price, is of great importance. Similar questions can be asked about electron transfer proceeding by way of a salt bridge and remarkable findings have been reported in this area.' 14.3 15 Thus, a system has been constructed for which the chromophore and electron donor is a ruthenium(I1) tris(2,2'-bipyridyl) complex bearing an amidinium side-arm while the electron acceptor is a dinitrobenzene derivative bearing a carboxylic acid function. Electron transfer proceeds through the salt bridge but is ca. 100-fold slower than the same reaction when the salt bridge is reversed.314 That is to say, the rate of electron transfer in the system D-(carboxylateamidinium)-A is much faster than for D-(amidinium-carboxy1ate)-A.Surprisingly, when the acceptor is N,N-dimethylaniline, giving rise to reductive rather than oxidative quenching, efficient electron transfer is found when the chromophore is attached to the amidinium side of the salt bridge.31sSuch studies provide a wonderful demonstration of the poweful role that photophysical investigations can play in supramolecular chemistry. Cooperative light-induced proton and electron transfer have been reported for a supramolecular ~ornplex.''~ Direct ion-pairing between oppositely-charged donor and acceptor functions has been used as a means by which to assemble photoactive a g g r e g a t e ~ . ~ l ' -An ~'~ interesting example of such systems involves complexation between a positivelycharged iron(II1) porphyrin and an alkylcarboxylate where illumination of the complex causes one-electron reduction of the metal centre.318For electrostatically-bonded pairs of zinc(I1) and magnesium(I1) porphyrins, the outcome of illumination depends on the nature of the peripheral ionic groups since these control the energetics for electron transfer.319Electron transfer upon illumination of ground-state electron-donor pairs is often extremely such that only the subsequent charge-recombination processes can be time resolved. An important extension to this field concerns the recent description of ultrafast electron transfer from the singlet excited state of chloranil to an aromatic donor, forming a singlet radical ion pair, in competition to fast intersystem crossing.322Observations made by several independent research groups over the last five years or so suggest that the rate of charge recombination in intimately-linked donor-acceptor complexes shows an exponential dependence on the energy gap and does not
I: Photophysical Processes in Condensed Phases
35
follow Marcus-type behaviour. The same situation has now been found for charge recombination within complexes adsorbed onto porous indicating that solvation is not responsible for the failure to comply with Marcus theory. Comparison between results collected at room temperature and at 77 K suggests the involvement of a low-frequency reorganization energy that is frozen at low temperature. Allowing for these new results, a comprehensive understanding of charge recombination taking place in strongly-coupled, donor-acceptor complexes has now been formulated that accounts for most of the observations made in recent years. Although demanding with respect to synthetic chemistry, the simplest way to learn more about photoinduced electron-transfer reactions is to employ molecular systems in which the donor and acceptor moieties are held at fixed positions via a rigid framework. This strategy permits the dynamics of electron (or electronic energy) transfer to be separated from diffusion and many such photoactive dyads, triads, and higher-order analogues have been prepared and studied over the past decade. It is from multicomponent supermolecules of this type that most of the important information about the factors that serve to control the rate of electron transfer has been obtained. Bimolecular systems are still interesting but they provide little real information about the electron-transfer event itself, and even then unsupported assumptions have to be made about the geometry of the encounter complex or ion pair. Some of the limitations of present porphyrin-quinone dyads, these being the most popular systems because of their relevance to natural photosynthesis, have been reviewed and a strategy to increase quantum yields for charge separation has been expounded.324 Building such multicomponent molecular systems requires careful consideration of how best to position individual modules so as to optimize the overall performance of the assembly for vectorial charge ~ e p a r a t i o n . ~It’ ~is interesting to note that slight changes in the local environment, such as adsorption onto porous glass, change the redox properties of certain molecules, thereby switching-on or -off the possibility of light-induced electron transfer across the Also, functions often used as inert spacer groups, such as anilide groups, can directly participate in electron-transfer reaction^,^'^ making it difficult to establish the reaction pathway with certitude. Many covalently-linked donor-acceptor systems have been synthesized having flexible spacer groups separating the reactant^.^'*-^'^ Such systems undergo lightinduced electron transfer but it is difficult to resolve the reaction pathway and, in particular, the possibility of through-space interactions can rarely be excluded. Much more useful are those systems in which the redox-active components are separated by rigid or constrained spacers so that the likelihood of through-bond electron transfer can be maximized and many organic-based systems have been reported recently.34c353The most common chromophores for such supermolecules are tetrapyrrolic pigments whilst quinones are the preferred electron acceptors. Illumination into the porphyrin chromophore causes fast electron transfer to form a radical ion pair that undergoes charge recombination. The rates of these processes depend on the energetics of the s y ~ t e m , ~ ~ ’ solvent --~~* polarity,343structural effect^,'^' and the nature of the connecting bridge.’53 In
36
Photochemistry
most cases, electron transfer involves the excited singlet state of the chromophore but several systems have been designed such that intersystem crossing to the triplet manifold is competitive with electron tran~fer.~~""'The triplet radical pair can be much longer lived than its singlet counterpart. A different strategy for restricting the importance of charge recombination is to add additional donors or acceptors to the dyad, arranged in such a way that a redox gradient can be established along the molecular axis. Several new triads have been described recently in which spatial separation of the charges has been established by laser flash spectroscopy~ 345,346.349.352 These systems are covered in more detail in Part V: Artificial Photosynthesis. Related molecular dyads have been constructed in which a metal complex, often ruthenium(I1) tris(2,2'-bipyridine) or similar, functions as chromophore and an appended organic moiety acts as redox Other systems36G364 have been built from two separate metal complexes. Each of these systems shows selective intramolecular electron transfer under illumination. Rates of charge separation and recombination have been measured in each case and, on the basis of transient spectroscopic studies, the reaction mechanism has been elucidated. The results are of extreme importance for furthering our understanding of electron-transfer reactions and for developing effective molecular-scale electronic devices. The field is open and still highly active. 4.4.2 Energy-transfer Reactions - The most intensively studied energy-transfer
reactions continue to be those involving singlet molecular oxygen, 0 2 ( 'Ag), and the status of the field has been reviewed recently.'65 The effect of solvent on the phosphorescence spectrum of 0 2 ( ' A g ) has been described"' and the selfquenching of 02('Ag) in carbon disulfide has been studied in some The product of this reaction appears to be O2('Cgt), formed by energy pooling, and subsequent studies showed that this latter species, which is itself a precursor of O2(lAg), transfers energy to (260. Interaction between 02(lAg) and a metal-free phthalocyanine causes delayed fluorescence from the macrocyclic dye and this process has been further explored using competition experiments.368The results suggest that it might be possible to design an experimental protocol to determine rate constants for quenching of Oz(lAg) by adventitious substrates simply by monitoring the yield of delayed fluorescence from the phthalocyanine. A comprehensive investigation has been made of the interaction between molecular oxygen and the excited singlet and triplet states of porphyrins and chlorins in different solvents and as a function of temperature.369 Under supersonic jet conditions there is no indication of a complex formed between oxygen and 9cyanoanthracene or anthra~ene.'~'The quatum yield for generation of 02( 'Ag) with chromium(II1) tris(2,2'-bipyridine) as sensitizer in D 2 0 has been established371as being 0.86 & 0.08. Quenching of the triplet excited state of zinc tetraphenylporphyrin adsorbed at a solid-gas interface by molecular oxygen results in delayed fluorescence from the porphyrin (Scheme 2).372It is suggested, on the basis of time-resolved diffuse-reflectance studies, that the first step in the quenching process involves efficient energy transfer from triplet porphyrin to ground-state 0 2 , forming 0 2 ( l A g ) . This is followed by faster energy transfer from
I: Photophysical Processes in Condensed Phases ZnTPP ZnTPPT
+
02
hv
-
Z~ITPP'~+ 02(1~g)
37 ZnTPP*T ZnTPP ZnTPP"
+ O#4) +
02
Scheme 2
02(*Ag) to a nearby porphyrin triplet, forming the excited singlet state of the porphyrin. No such reaction has been seen in solution phase. Molecular-orbital calculations have been made in an effort to predict the site and the validity of the of O2(IAg) addition to anthra[l,9-b~:4,1O]dichromene~~~ results has been confirmed by X-ray structural analysis of the reaction products. The methodology might find more general usage. Little attention has been paid to bimolecular energy-transfer processes374but there have been numerous investigations concerned with intramolecular singlet or triplet energy transfer. Several systems have used highly flexible connecting chains with the inevitable problem of evaluating separation distances and mutual orientations.3753376 Covalently-linked porphyrin-chlorin heterodimers connected through an ether linkage demonstrate very efficient Forster dipole-dipole exchange over the temperature range 77-293 K.377Similar behaviour is reported for hybrid zinc(I1) and free-base porphyrin dimers linked via a bis(phenylethyny1)phenylene spacer.378A series of dyads comprising a carotenoid and a pyropheophorbide have been shown to undergo Forster-type energy transfer from the shortlived excited singlet state of the carotene to the pyropheophorbide with vaying effi~iency.'~~ For certain zeaxanthin pigments, energy transfer from S1 to the pyropheophorbide does not take place because of unfavorable energetics but reasonably efficient transfer occurs from the S2 level, despite the very short lifetime of this excited state. A carotenoid-porphyrin-pheophorbidetriad has been synthesized380that undergoes two-step triplet energy transfer from the initially-excited pheophorbide (PHE) to the carotenoid (CAR) by way of the interspersed porphyrin (POR) (Scheme 3). The complete mechanism of this process has been evaluated by way of studying many relevant model compounds and the photosystem is a rare example of successive triplet energy-transfer steps. Triplet-triplet energy transfer has also been observed in crystalline organic salts.381 Several reports have considered triplet energy transfer from a ruthenium(I1) tris(2,2'-bipyridyl) complex to an appended aromatic hydrocarbon. The rates of
'(CAR)--POR--PHE
t -
Scheme 3
CAR--(POR)*--PHE
38
Photochemistry
energy transfer depend on the site of attachment”’ and on the triplet energy of the polycyclic a c ~ e p t o r . ”Related ~ work has shown that intramolecular triplet energy transfer between metal complexes is weakly dependent on the estimated separation distance at 77 K, where Forster dipole-dipole transfer is dominant, and almost temperature i n d e ~ e n d e n t . ~Changing ~‘ the nature of the connecting spacer group switches-on Dexter-type electron exchange. Other heteronuclear metal complexes have been d e s ~ r i b e d . Through-bond ~ ~ ~ * ~ ~ ~ singlet-triplet and triplet-triplet energy-transfer processes have been described for various organicbased dyads in which the nature of the spacer group can be varied.387A linear array of pigment molecules has been shown to demonstrate vectorial singlet energy transfer along the molecular axis with very high overall trapping efficiency by the terminal acceptor.388Changing the energy level of one of the interspersed pigments, by selective chemical oxidation, curtails propagation along the array, leading to creation of a molecular-scale optoelectronic gate. Energy transfer has been observed to take place along the backbone of b i o p o l y m e r ~across , ~ ~ ~ multifunctionalized scaffolds390and steroids,391and along short pep tide^.^" Other studies have reported electronic energy transfer in charge-transfer crystals,393between organic molecules trapped within zeolites,”‘ in crystals of double salts formed from oppositely-charged transition metal c~mplexes,”~and in mixed mono layer^.^^^ This latter system is especially interesting because polarized fluorescence spectroscopy confirms the orientation of the molecules, thereby facilitating analysis in terms of existing theoretical models. It is also possible to vary the mutual separation distance by increasing the surface pressure. Energy transfer has been reported for amphiphilic reagents bound in a bilayer membrane as a model of the photosynthetic process.397 Photophysics of Fullerenes - Interest in the synthesis, derivatization, and chemistry of the various fullerenes has almost reached the epidemic stage and these exotic molecules continue to attract the photochemist. There have been numerous publications concerned with the fundamental photophysical properties of fullerenes in solution and the solid state over the past five years or so but there are still some important issues to resolve. Lately, the characterization of fullerenes and related carbon nanotubes has been reviewed, along with the luminescence properties of C60.398The prompt and delayed fluorescence spectral characteristics of C70 have also been reviewed.399A more detailed investigation of delayed fluorescence from C70 has been made:’’ from which a reliable estimate of the singlet-triplet energy gap (AEST = 26 2 kJ mol-I) was derived from temperature variation studies. In fact, delayed fluorescence appears to be extremely important for this fullerene since, under appropriate conditions, the inherent fluorescence quantum yield can be increased by almost two orders of magnitude. Attention has also been given to the phosphorescence spectrum of C70 recorded at low temperature in glassy alkanes.‘” The absorption and fluorescence spectra of both c60 and C70 have been studied in a wide range of solvents at ambient temperature and it was found that the fluorescence spectra extend well into the near-infrared region.402 Fluorescence quantum yields of 3.2 x lo-‘ and 5.7 x lo-‘, respectively, for c6”and C70were measured while the
4.5
I: Photophysical Processes in Condensed Phases
39
ratio was noted to be solvent independent. Time-resolved and steady-state fluorescence polarization studies have been reported for c60 and C70 in lowtemperature organic glasses.403Whereas c60 is intrinsically unpolarized the main emission transition moment for C70 lies in the molecular xy plane, although there are complications from higher-energy transitions. Fluorescence properties of c60 molecules isolated in inert matrices have been described404while a generalized model for the photophysical properties of c60 has been presented.405 The photophysical properties of a higher fullerene, C76, have been recorded by laser flash photolysis techniques.406The lifetime of the excited singlet state was found to be around 1.7 ns and the excited triplet state, being formed in high yield, engaged in bimolecular reactions via both energy (to p-carotene) and electron (with N,N,N',N'-tetramethyl-para-phenylenediamineas donor) transfer. Spectral characterization of the triplet excited state and the radical cation of c60 has been made by various experimental technique^.^^^-^^^ The nature of the lowest-lying excited states of the fullerenes has been difficult to identify with much certainty. From Shpol'skii-type luminescence spectra recorded at 1.5 K it has been concluded that the first-excited singlet state in C70 is , of A 2 character.409The origins of the lowest energy transitions in C G ~namely SI(Tlg)and S2(G,),have been assigned on the basis of fluorescence and excitation spectra, supported by theoretical calculation^.^^^^^^ The luminescence properties and relaxation dynamics of single crystals of c60 have been described4123413 while related measurements have been made for solid films of C60.414Similar studies have reported the luminescence spectral properties of c60 trapped inside the cavities of NiY zeolite^.^' An analysis of the fine structure of electron-vibrational spectra has been made for c 6 0 and its derivatives in a solid toluene matrix.416The rate of triplet energy transfer between fullerenes in toluene solution has been measured as a function of temperature and used to derive thermodynamic parameters for the transfer process.417 Much of the interest in fullerene-based photochemistry arises because these compounds form a long-lived triplet excited state in very high quantum yield and, since they are readily reduced to the x-radical and the subsequent x - d i a n i ~ n , ~they ~ ~ ?participate ~~' in light-induced electron-transfer processes. In many respects, the simpler fullerenes exhibit photochemical properties that are not too dissimilar from those of aromatic carbonyl compounds.422Fullerene triplet states react with molecular oxygen to generate singlet molecular oxygen, 02('Ag), in high yield.423The triplet state is also quenched by a wide range of electron donors and acceptors due to electron transfer,42u27 forming the respective .n-radical ions, although complications can arise from competitive adduct formation428or from fragmentati~n.~'~ Attention has been given to the photoreactions of c6()adsorbed onto solid s ~ p p o r t s ~ and ~ ~ . in ~ ~polymer ' films.432Particular emphasis has been given to understanding the phototransformations and photopolymerization processes that take place when solid c60 is exposed to illumination.433435Incorporating c60 or C70into polymers provides a facile way to engineer photoconducting materials and such composites continue to attract attention.43u38 The photophysical properties of simple derivatives of c 6 0 have been recorded.43946
'
Photochemistry
40
Fullerenes are known to form weak charge-transfer complexes with certain electron donors in fluid solution and it has been shown that laser excitation of such complexes can result in efficient generation of the fullerene n-radical anion -447.448 The rates of light-induced charge separation and subsequent recombination have been measured for several complexes and analyzed in terms of Marcus theory, with charge recombination occurring well within the inverted region. Fullerenes are interesting reagents for use in photochemical electrontransfer reactions because they can be used as the c h r ~ m o p h o r 4s2 e ~ and/or ~~ as the electron acceptor.4s3Excitation of c60 or C70 in the presence of a phthalocyanine in polar solvent results in one-electron reduction of the fullerene, with C7,) giving the higher yield of n-radical anion, but in nonpolar solvents triplet energy transfer takes place to populate the lowest-energy triplet state of the phthalocyanine.4s4 The triplet excited state of Cbo reacts slowly with chloranil (CA) in benzonitrile solution to form the p-radical cation of the f~llerene,~” as monitored by Fourier transform EPR (Scheme 4). When perylene (PER) is added to the solution direct reduction of the chloranil acceptor is by-passed in favour of rapid triplet energy transfer from c60 to perylene but the triplet state of the latter species reacts quickly to reduce chloranil. Similarly, triplet c60 is readily reduced by tritolylamine (TTA) and the resultant n-radical anion transfers an electron to chloranil under the experimental conditions used.
Scheme 4
Fullerenes, especially c60, have been used as electron acceptors in covalentlylinked donor-acceptor dyads4s6-466and triads467* (see Part IV: on Artificial Photosynthesis). A critical comparison has been made between the rates of
I: Photophysical Processes in Condensed Phases
41
through-bond charge separation and recombination for a pair of rigid dyads having a porphyrin as chromophore and donor and either c60 or benzoquinone as acceptor.456 A remarkable finding to emerge from this study is that the fullerene facilitates faster charge separation and slower charge recombination, despite comparable separation distances and thermodynamic driving forces for the two systems. According to Marcus theory, these results are rationalized in terms of c60 possessing a much smaller reorganization energy than the easily polarized quinone. This conclusion is of extreme importance for the design of effective photochemical devices based on light-induced electron transfer since it allows optimization of the electron-transfer events without the need for a high driving force for charge separation. It should be noted that the nature and position of the spacer group (S) used to separate porphyrin donor (POR) from fullerene acceptor make important contributions to the rates of charge separation and recombination in such POR-S-Cso dyads (Scheme 5).464 Light-induced electron transfer is switched-off in nonpolar solvents because of unfavourable energetics,465and instead the first-excited singlet state of the porphyrin transfers excitation energy to the bound fullerene. This latter species is too short-lived to enter into energy- or electron-transfer reactions and undergoes intersystem crossing to the triplet manifold. The long-lived triplet state of the fullerene can transfer energy to the lowest-lying triplet state of the appended porphyrin, which is almost isoenergetic.
11 Scheme 5
5
Applications of Photophysics
The study of photophysical processes, especially time-resolved luminescence spectroscopy, provides unique opportunities to explore complex molecular systems, to selectively transfer information at the molecular level, to label biological materials, and to design analytical protocols. Perhaps the most popular application of photophysics concerns measuring the fluidity, polarity, electric field, surface charge, effective dielectric constant, or composition of microheterogeneous media.46948' Related photosystems have been designed to measure the water content of reverse AT m i ~ e l l e s the , ~ ~extent ~ of chain folding and critical micellar concentrations for various types of and the morphology of
Photochemistry
42
polymer micro sphere^.^^^ Related studies have addressed local organization in liquid crystals 486-488 and orientational order close to the surface in isotropic phases.489 Photophysical measurements have also formed the basis of experimental methods to determine the pressure in polymer films490or in s o l ~ t i o n s . ~ ~ ’ A fluorescence-based temperature sensor has been devised for use in organic solvents.492A fluorescence-based assay has been developed to monitor phase transitions between gels and liquid crystals.493 By measuring the nonradiative decay of the triplet excited states of organic phosphors it is possible to determine the heat capacity of solid matrices.4y4 More intriguing is the possibility to construct chiroptic switches as control elements for supramolecular functions.495 Modest amounts of steroselectivity have been found for the quenching of zinc myoglobin with optically-active v i ~ l o g e n s . ~ ~Helical ~ * ~ ’ chromophores ~ have been used for the photocatalytic asymmetrical synthesis of metal complexes498while chiral calixarene derivatives are able to discriminate between enantiomers of incoming q ~ e n c h e r sSimilar .~~~ studies have addressed the luminescence properties of different regioisomers included into P - c y c l o d e ~ t r i n sGeminate .~~~ recombination taking place within the pores of zeolites can be influenced by inclusion of chiral guest molecules into the supercage.50* Luminescent lanthanide complexes have been formed from chiral macrocyclic receptors.502 Photolysis of chiral inclusion complexes in the solid state has been found to give unique photocyclization p r o d ~ c t s . A~ ~chiral ~.~~~ fluorescent reagent has been developed for resolving enantiomers of carboxylic acids.505All these systems serve to indicate that proper design of the receptor molecule, functionalized with a fluorophore close to the binding site, allows some level of discrimination between optically-active guest molecules with the level of specificity being reflected by the fluorescence properties of the conjugate. The selective binding of luminescent compounds to DNA in aqueous solution is often accompanied by a change in emission properties, certainly there is a modification of the anisotropy and induced circular dichroism upon intercalation, that allows the binding process to be visualized. Many new reagents have been reported that intercalate between base pairs in the DNA duplex and their photophysical properties have been explored.506509In some cases, claims for site specificity have been made.5’0*51These luminescent stains, useful for imaging double-stranded DNA and discriminating against single-stranded polymer, operate in one of two ways. Highly-luminescent, electron-affinic dyes undergo light-induced electron abstraction from guanine upon illumination of the intercalated dye such that there is a strong attenuation of the luminescence yield and lifetime with respect to solution-bound dye. Alternately, the luminescence of certain dyes is quenched by water so that protection against contact with the aqueous solvent upon intercalation causes a marked amplification in the emission yield and lifetime. A photophysical investigation has been completed for one such example of the latter ~ a t e g o r y . ~ Continued ” claims that DNA provides an extremely effective medium for long-range electron tunnelling between remote but intercalated redox-active reagents513 are becoming tiresome and contradictory to all other experimental and theoretical studies. Binding of fluorescent water-soluble porphyrins to bovine serum albumin has been reported and, as for
’
I: Photophysical Processes in Condensed Phases
43
DNA, there is a substantial protective factor associated with binding to the protein.’I4 Again, light-induced electron transfer takes place between certain amino acids and a closely-spaced c h r ~ m o p h o r e . ~ ’ Sensitive analytical protocols can be devised by making use of these pronounced changes in emission probability and polarization that characterize association between dye and biological host. The methodology has been applied to design a fluorogenic assay for HIV-1 p r ~ t e a s e . ~ ” Simple model systems have been used to obtain an improved understanding of biological electron-transfer p r o c e ~ s e s , ’to ~ ~refine knowledge about the binding pocket in re~piration,”~ and to mimic photonuclease A simple fluorescence-based technique has been introduced to monitor protein dynamics in solution.52’ A common practice in biology is to employ fluorescent receptors to measure intracellular levels of calcium and, on the basis of picosecond laser spectroscopic studies, the mechanism by which these probes work in situ has been evaluated.523The importance of intramolecular proton transfer for deactivation of the excited states of the photoactive antiviral agent hypericin has been demonstrated by comparison with structurally modified derivatives.524An improved understanding of the mode of action of sunscreens that prevent photooxidative skin damage has been reached.525It is reported that certain flavonoids are excellent protective agents in cases where skin diseases are initiated or exacerbated by exposure to sunlight. The special protective properties provided by the cavity of P-cyclodextrin, especially its ability to enhance phosphorescence yields and to induce circular dichroism, has ensured numerous photophysical investigations of inclusion complexes with a cyclodextrin host.’2c536 Such systems, in addition to exhibiting interesting and varied photophysical processes, give important analytical opportunities in which the cyclodextrin functions as a size- and shape-selective receptor for aromatic molecules. Highly-sensitive systems, based on time-resolved luminescence detection, can be engineered in this way. An alternative methodology uses cyclodextrins, or other macro cycle^,^^^ functionalized with chromophores that can act as an antenna for a molecule trapped within the ~ a v i t y . ’ ~ ~This .~’~ approach has been demonstrated by way of the naphthalene-sensitized photoisomerization of a nitrone. Similar molecular-based systems have been designed ~ @ - these ~ ~ ~optical for detection of cations, including protons, in s ~ l u t i o n . ~While sensors often display high sensitivity they are not sufficiently selective for practical applications, except in rare cases. It is much easier, and certainly more practical, to design luminescence-based sensors for detection of dissolved oxygen. 548s49 Fluorescence spectroscopy has also been employed to measure the interlayer separation in two-dimensional vanadyl phosph~nates.’’~ Photoswitchable materials could be important components in molecular-scale electronic devices and several new systems have been r e p ~ r t e d . ” l - ’ ~There ~ have been other investigations of phototropic systems in which light is used to drive a reversible conformational change554557or, in the case of liquid crystals, a phase trans for ma ti or^.^^^ Such photosystems are of interest for the engineering of microscopic photoactive devices but the subject is still in its infancy and practical devices remain elusive. Much more subtle is the use of light to trigger a change in
’.’“
Photochemistry
44
spin of an ordered array of transition metal complexes and such photoresponsive magnets are beginning to appear.5593560 The field of second harmonic generation has been reviewed561while a critical comparison has been made of normal and electrogenerated fluorescence from some organic molecules.562
6
Advances in Instrument Design and Utilization
Photophysics depends critically on the availability of appropriate instrumentation and adequate computational protocols. To a large degree, progress in the field is limited by new developments in the type and scope of instrumentation, but the importance of a ready supply of pure and tailor-made molecules must never be underestimated. Improvements in the precision with which conventional experiments can be made and the opportunities to undertake new types of photophysical investigation continue to be reported; not all are expensive or subject to the simultaneous use of several sophisticated lasers. 6.1 Instrumentation - A steady-state luminescence spectrophotometer is probably the most useful and versatile instrument in the armoury of the photophysicist and can be applied to many different types of experimental science. A new experimental detector has been described for the measurement of emission from singlet molecular oxygen, OZ('Ag),in water where the radiative probability is very low. 563 Aluminium eriochrome black-blue R in propan- 1-01 has been proposed as a fluorescence standard for the 600-700 nm region.564The design of a Hadamard spectrometer for measuring very weak luminescence signals has been described in An improved version of an integrating sphere might be more useful for measuring emission quantum yields and a protocol for its use has been provided.566A methodology has been developed for the synchronous scanning of dual-wavelength fluorescence that could be valuable for the quantitative analysis of variable mixtures of known components.567Higher selectivity is attainable by using three-dimensional derivative fluorescence s p e c t r o p h o t ~ m e t r y .A~ ~tech~ nique for combining luminescence spectroscopy with spectroelectrochemistry has been outlined and shown to have particular virtues for monitoring electrogenerated l u r n i n ~ p h o r e s Endoscopic .~~~ fluorescence imaging systems for detection and visual examination of cancer are beginning to look promising techniques for rapid and noninvasive diagnosis.570357'The application of two-photon fluorescence techniques allows samples to be probed in greater detail than by conventional fluorescence spectroscopy572-575 and this technique, especially when used with a microscope,576 is extremely powerful. The virtues of multiple-photon imaging systems have been highlighted.575 A simple methodology has been outlined for using phase-modulated cw laser diodes to determine fluorescence lifetimes.577 Time-resolved luminescence spectroscopy complements the steady-state method578 and can provide essential kinetic information about the decay of excited states. Application of time-resolved fluorescence spectroscopy for analytical chemistry,579 where low concentrations might require the use of long
I: Photophysical Processes in Condensed Phases
45
pathlengths and scattering techniques,580 has been highlighted and reference made to the special case of constructing a miniaturized setup for monitoring dissolved oxygen.581Replacement of pulsed laser or flash lamps as excitation source with a sample of B-emitting "Sr has been recommended as being cheaper, more reliable, and compact and the design of an appropriate instrument has been reported.582Advanced detectors for monitoring emission decay on the picosecond time-scale have been described for microchannel plate photo tube^,^*^ streak cameras,584 time-gated image intensifier^^'^ or cameras,586and waveguides.587 For measurement of sub-picosecond fluorescence decay, upconversion spectroscopy remains the best ~ h o i ~ and e ~ can~be~used - ~to ~ measure ~ triplet quantum yields.59' The basic methodology used to make time-resolved fluorescence anisotropy measurements has been de~cribed.~~' A novel time-resolved total internal reflectance fluorescence spectrometer has been described and applied to the measurement of fluorescence from a dye held at a liquid-liquid interface.593 The application of various types of EPR spectrometers to detect and identify transient free radicals produced by flash photolysis has been reviewed in some The role of chemically-induced dynamic electron polarization in elucidating reaction pathways and the nature of precursor excited states is well illustrated. Attention is also given to explaining how the reaction field detected magnetic resonance and magnetic field effect studies can provide important information about the fate of free radicals. The first observation of CIDEP generated through interaction between a free radical and singlet molecular oxygen has been reported.595 Other studies have been concerned with the effects of applied magnetic59c602or e l e c t r i ~ a l fields ~ ~ ~ on - ~ the ~ ~ reactivity of free radicals or radical ion-pairs produced by laser photolysis. A thermal lens spectrometer has been designed to measure fluorescence quantum yields and to correct for innerfilter effects.608An improved design uses dual-beam techn~logy.~'~ A new instrument for making transient thermal lens experiments has been Transient grating spectroscopy has been applied to various problems6' '-'I4 and improved methodologies have been devised for this extremely useful and versatile technique. A photothermal beam deflection apparatus has been designed for measuring the rate of heat release from transient species on the microsecond tirne~cale.~'~ Several studies have been concerned with modifications to conventional hole-burning techniques616-6' and an instrument has been described620that can measure the rate of cooling of 'hot' ions by time-resolved photodissociation thermometry. Upper-excited singlet state lifetimes have been measured by a modified Z-scan approach.62' An experimental setup has been described that permits monitoring the competition between geminate recombination and solvation of polar radicals following bond cleavage.622The method has been tested by reference to ultrafast dissociation of bis(6aminophenyl)disulfide in polar solvents.
6.2 Data Analysis - Simple protocols have been reported for correcting commercial spectrofl~orimeters~'~ and for biasing against the inner filter Various ways, such as l ~ g - n o r m a l , ~three-dimensional:26 '~ or reciprocalspace,627have been outlined that allow presentation of luminescence spectra so as to better illustrate particular features of interest. An analysis of the transient
Photochemistry
46
effect in fluorescence quenching has been provided that allows expression of the rate constant for electron transfer as a function of separation distance.628 A versatile model has been presented for the evaluation of association constants from fluorescence data629and should prove to be extremely useful for those many researchers engaged in supramolecular chemistry. A different model permits calculation of aggregation numbers from steady-state and time-resolved fluorescence spectroscopy.630 Several studies have presented analyses of resonance fluore~cence.'~ Time-resolved fluorescence spectroscopy provides experimental data of excellent precision that justify detailed analysis. Several improvements or extensions to the methodology used to extract meaningful information from such decay profiles have been presented during the past year or so. In particular, a model has been described for the specific case of fluorescent probes in isotropic and ordered phases that undergo rotational d i f f ~ s i o n . The ~ ~ ~maximum . ~ ~ ~ entropy method for analysing fluorescence decay curves has been r e - e ~ a r n i n e dwhile ~ ~ ~ global analysis routines have been developed for dealing with large, correlated data sets.636Analysis of fluorescence from polymer films, a well-known nightmare, has been reconsidered in terms of the maximum entropy methodology.637 A path integral approach to fluorescence correlation experiments has been described638 and a detailed description has been given for the effect of intermolecular ipteractions on fluorescence decay Global analysis routines have been applied to the problem of two-dimensional singlet energy transferH0 and an extension has been reported for global analysis of unmatched fluorescence decay curves.641A further routine has been presented to account for global compartmental analysis of a system showing reversible intramolecular two-state excitedstate processes when a quencher is present.H2 It has been shown that Prony's algorithms can give superior performance to the commonly used Marquard counterpart for estimating exponential lifetimes.H3 An analysis has been given for fluorescence decay in aperiodic Frenkel lattices.644 A treatment for analysing the excitation and fluorescence multiwavelength polarized decay surfaces has been given for the case of a mixture of noninteracting species.H5An improved model for analysis of fluorescence anisotropy measurements has been presented.646Limitations to the use of intense excitation pulses in fluorescence and thermal lens spectrophotometers are discussed in terms of optical s a t ~ r a t i o n Such . ~ ~ artefacts can be eliminated by reference to the fluorescence quantum yield of Rhodamine 6G. A model has been given to describe spectral diffusion in time-resolved hole-burning spectroscopy.H8
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A. Jover, F. Meijide, E. Rodriguez Nunez, J. Vazquez Tato, and M. Mosquera, hngmuir, 1997,13, 161. M. B. Plenio, J. Mod. Opt., 1996,43,2171. G. Yeoman and S. M.Barnett, J. Mod. Opt., 1996,43,2037. L. Feltre, A. Polimeno, G. Saielli, and P. L. Nordio, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect A , 1996,290, 163. J . Kowalczyk, K. Wisniewski, and M. Wierzbowska, Chem. Phys., 1996,208,283. R. Swaminathan and N. Periasamy, Proc. Indian Acad. Sci., Chem. Sci., 1996, 108, 39. P. Stilbs and K. Paulsen, Rev. Sci. Instrum., 1996,67,4380. J. W . Hofstraat, H. J. Herman, J. W. Verhoeven, M. U. Kumke, L. B. McGown, E. G. Novikov, A. van Hoek, and A. J. W. G. Visser, Proc. SPIE-Int. Soc. Opt. Eng., 1996,2705 (Fluorescence Detection, I V ) , 1 10. J. Enderlein, Phys. Lett. A, 1996,221,427. S. Taen, Trends Phys. Chem., 1996,4,208. P. Ballet, M. Van der Auweraer, F. C. De Schryver, H. Lemmetyinen, and E. Vuoriman, J. Phys. Chem., 1996,100,13701. G. B. Dutt, M.Amebot, D. Bernik, R. M.Negri, and F. C. De Schryver, J. Phys. Chem., 1996,100,9751. G . B. Dutt, ,N. Boens, A. Kowalczyk, F. C. De Schryver, and M. Amebot, J. Phys. Chem. A, 1997,101,1993. Z. Zhang, K. T. V. Grattan, Y. Hu, A. W. Palmer, and B. T. Meggitt, Rev. Sci. Instrum., 1996,67, 2590. F . Domingeuz-Adame and E. Marcia, Phys. Rev. B: Condens. Matter, 1996, 53, 13921. J. J. Fisz, Chem. Phys. Lett., 1996,259, 579. C. K. Law, R. S. Knox, and J. H. Eberly, Chem. Phys. Lett., 1996,258,352. J . Georges, N. Amaud, and L. Parise, Appl. Spectrosc., 1996,550, 1505. J. M. A. Koedijk, R. Wannemacher, R. J. Silbey, and S. Voelker, J. Phys. Chem., 1996,100, 19945.
Part II Organic Aspects of Photochemistry
1 Photolysis of Carbonyl Compounds BY WILLIAM M. HORSPOOL
As in previous years there is a continued swing away from the more traditional
areas of study. This is obvious not only in this chapter but also in the other two compiled by this reviewer. Interest in organic photochemistry is clearly not on the wane, however, and there are still as many publications as in past years. This chapter is divided up in the same manner as in previous years with sections dealing with Norrish Types I and I1 reactions, oxetane formation and miscellaneous reactions relating to carbonyl compounds and related species.
1
Norrish Type I Reactions
More interest has been shown in the past year in the laser-induced processes involving organic molecules. One such study is the laser irradiation (193.3 nm) of the ketones (1-4). This study has shown that the Norrish Type I process is dominant resulting in or-fission and the formation of alkyl and acyl radicals. The ultimate products formed are alkanes and carbon monoxide.’ Norrish Type I reactivity is also observed in more complex molecules such as the carbohydrate derivatives ( 5 ) and (6). Irradiation of these in solution again brings about orcleavage to give the isomeric radicals (7).2 The control of the photochemical reactivity of molecules by their encapsulation in ‘cages’ has gained in importance over the years. Turro is one of the principal workers in this area and the text of a review by him of the photochemical phenomena within zeolites has been p~ bl i s hed.The ~ influence of the presence of water within zeolites has been shown to alter the composition of the products obtained from the irradiation of ketones within these cages. Several ketones were studied and the results obtained from diphenylacetone (8) are detailed in Scheme 1. Here it can be seen that three products (9-1 1) are obtained when the reactions are carried out in the dry. However, when water is included decarbonylation, an a-cleavage process, is observed as the most prominent reaction and leads to 1,ldiphenylethane as the dominant p r o d ~ c t Such . ~ studies have been extended to include the influence of cages, modified by a chiral guest molecule, on the irradiation of such chiral molecules as ( &)-benzoin methyl ether in zeolite super cages.5 The ketone (12) undergoes a-cleavage on irradiation in de-aerated water. The fission process arises from the triplet state. The acyl and the alkyl radicals formed Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
72
Photochemistry
(1)R'=R2=H (2) R' = H, R2 = Me (3) R' = R2 = Me (4) R' = vinyl, R2 = H
0
II
O-P(OEt)2
I
1
R' R'
= O;(OEt)Z, = H,
R2 = H
R2 = OP(OEt)2
R3 = TBDMS
II
0
O-P(OEt)2 I1
0
NaX
Kx
dry wet dry wet
17
55
26
100 40 98
0
0
16 1
40 1
Scheme 1
by this process gives rise to several products. Thus the alkyl radical is transformed into 34propylpyridine (13), 3-(2-propenyl)pyridine(14) and the dimer (15). The acyl radical reacts to form nicotinic acid (16). In addition to these products a polymer is produced. The reaction was studied in some detail and a slight solvent dependence on the outcome of the reaction was detected.6 The CIDEP-spectra of the transient radicals obtained from the laser irradiation (at 266 nm) of substituted benzaldehydes have been recorded.' A CIDNP study
IIII: Photolysis of Carbonyl Compounds
73
Me Me
@-hQ
QCo2H
of the production of biradicals from the irradiation of the ketone (17) has been carried out.' Norrish Type I cleavage is the principal photochemical reaction of the thiabicyclononanone (18). When the irradiations are carried out in t-butanol the ester (19, R = t-Bu) is obtained: this arises from the biradical(20) formed by a-cleavage. Hydrogen abstraction within the biradical affords a ketene which is trapped as the ester. The outcome of the reaction is solvent dependent and in methanol irradiation affords the two reduction products (21) and (22) with the endo derivative predominating. The authors' suggest that this is to be expected and reduction arises by way of the charge transfer interaction between the sulfur and the carbonyl group. Such single electron transfer processes (SETS)are not uncommon.
Me
Me
(1 7)
2
(18)
(19) R = But, Me or Pr'
Norrish Type I1 Reactions
2.1 1,SHydrogen Transfer - The typical reaction of this type is exemplified by the conversion of the arylketones (23) into the cyclobutanol derivatives (24) and (25)
Photochemistry
74
(Scheme 2)." Typically the reaction involves the triplet state of the aryl ketones, and is carried out in methanol using Pyrex-filtered light from a mercury arc lamp. The resultant 1,4-biradicals formed by the 1,5-hydrogen transfer bond to give the benzocyclobutenes (24) and (25). The carbohydrate derivative (26) is also reactive on irradiation in solution and undergoes a conventional 1,5-hydrogen transfer. The resultant biradical cyclizes to an azetidinol. Ring opening of this intermediate affords the final product (27)"
wR3 HCR'2 0
OH
MeOH,Pyrext light
R2
R' Me Me Me Me Me
/
R
?H
R3 CH=CHMe Et
Me Pr' Ph
OH
3 m
/ R2
R2 Pr' Pr' Pr' Pr' Pr'
OH
m
R
?H
3
/
R'
~1
R2
~1
R'
yield (YO)
22 35 34 37 25
31 2 27 19 37
Scheme 2
The two ketones (28) and (29) are known to undergo the Norrish Type I1 hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of (-)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the (-)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4:1.12 A detailed study of the photochemical reactivity of the large ring diketones (33) has been carried out. In solution irradiation brings about conventional y-hydrogen abstraction in every case except with the Clodiketone. The yields are
IIII: Photolysis of Carbanyl Compounds
75
opL43343c02Me C02Me
given in Table 1, and as can be seen, the products are dependent on ring size. Cleavage products (36) also result on irradiation. Similar reactivity or lack of it is exhibited in the solid state and it is interesting to note that the outcome of the irradiation of the C26ketone is dependent on the type of crystal. Thus the needle crystals yield 91% of (35) while the plates yield 97% of (34). A detailed X-ray analysis of all the crystalline compounds was carried out, From this and from the results obtained the authors" conclude that in the solid state the product distribution arises from the relative rate of the forward hydrogen transfer process. Apparently the biradicals react in a conformation specific fashion.
m
n
4
2
CH2 0
5 3 6 7 8
4 5 6
9 7 10
11
8 9
H
0
76
Table 1
Photochemistry
Yields (“A)“ of products obtained from the irradiation of ketones (33)
Ring Size (33)
m
12 14 16 18 20 22 24 26 26
4 5 6 7 8 9 10 11 11
n
cis-Product (34)
trans-Product (35)
84 (99) 65 (58) 22 (89) 17 (3) 10 (90) 10 (4) 15 (98) 14 (9) 14 (97)
0 (0) 25 (27) 35 (10) 42 (84) 23 (4) 34 (91) 27 (1) 33 (91) 33 (3)
Open chain (36)
Yields in brackets are those obtainedfrom solid state irradiation.
2.2 Other Hydrogen Transfers - Weigel and Wagner14 have studied the 1,4hydrogen abstraction reactions within the ketones (37) leading to the cyclopropanols (38). The study used wavelengths > 300 nm with benzene or methanol as the solvents. The yields of products are shown below the structures. Polar solvents appeared to have no effect on the outcome of the reaction.
R’
R2
H Ph PhCH2 H Ph PhCH2 Ph Ph
H H H H H H OMe H
yield (YO)
R3
C6H6
95 73 24
50 88 65 56
-
MeOH 0.15
42 68 c5 4
4 50
A 1,6-hydrogen transfer is observed on irradiation of aldehyde (39). The resultant 1,Sbiradical ring closes to afford the bridged compound (40) in 59% yield. The conditions which give the best results involve benzene solutions and quartz-filtered light. l 5 Kim and ParkI6 have reported that the photochemical cyclization of the benzophenone derivatives (41) results in the formation of dihydrobenzofurans (42). The most interesting result from this study is that there appears to be a dependence of the cis:trans ratio of the products on the substituent R. Thus when R is methyl the cis:trans ratio is 11:l but when R is
M I : Photolysis of Carbonyl Compounds
77
0 fJH
(41) R = Me R = CF3
R2
Ph
I
R’ R’ (43) a; H b; Et c; H
R’ R2 Me Me Ph
(44)
(45)
O
II
I
phaTs /NTs PhCCH2CH
Ts ,Me
\
0 Ph’
R
CO2Me
HO
C02Me
CH2C02CHzPh
(48) R = C02Me, C02CMe3 or CMe3 R ~ RHC ’
(49) R’ = R2 = Me or R’ = H, R2 = OTDS
trifluoromethyl the ratio is 2: 1. The authors16 suggest that a ‘captodative effect’ is operative within the l$biradical formed on hydrogen abstraction by the carbonyl group from the site adjacent to the oxygen. Wagner and ~oworkers’~ have studied the cyclization of biradicals produced by hydrogen abstraction in compounds such as (43). The cyclization affords the indanols (44) and (45). The
78
Photochemistry
present work has examined the question of the involvement of entropy versus enthalpy as the controlling factor in the cyclizations. The stereoselectivity is reduced by changing the hydrogen donating group from ethyl (43a) to benzyl (43b). A study of the resultant 1,5-biradicals formed on irradiation has shown that there is hydrogen bonding between the OH and the benzene ring. The product ratios reflect the equilibrium populations of the biradicals. Irradiation of the optically active ketone (46) gives the enantiomerically pure proline ester (47) but only with a low diastereoisomeric excess (de). This reaction involves the formation of a 1,5-biradical. The keto derivatives (48) afford a route to cis-3hydroxyproline esters. Irradiation of the ketone (49), however, leads to yhydrogen abstraction with the intermediacy of 1,4-biradicals and the formation of cyclobutanols as well as cleavage products from the normal Norrish Type TI reactivity. The photochemical behaviour of the dipeptides (50) has been studied, and involves a 1,7-hydrogen transfer. Bonding within the resultant biradicals produces the &lactams (51) and (52) with high regioselectivity. The major features that lead to this regioselectivity are the stability of the radicals and the conformation adopted by the substrate. The influence of substituents is also noted.” Remote photochemical hydrogen transfer by a reaction analogous to the Norrish Type I1 process is reported to convert 2-(dibenzy1amino)ethyl ethyl acetoacetate into eight-membered azalactones. Again the outcome of the reaction appears to be substitution sensitive.”
phYo 0 (50) Z = PhCH20C
F
(51)
R H Me Pr’ CH2Ph
(52) yield (YO)
35 48 59
63
35 22 8
a
The diester ( 5 3 ) undergoes photochemical conversion on irradiation at 350 nm in benzene solution. The two products (54) and (55) are formed by hydrogen abstraction paths. The major product (54, 37%) arises by a conventional Norrish Type I1 process involving the formation of a 1,4-biradical by a 1,5-hydrogen transfer. More interestingly, the minor product obtained in 24% yield was identified as the lactone (55): this is presumed to arise by a 1,12-hydrogen transfer. Analogous reactivity was observed for the ester (56) which gives (57) and (58) with the 1,12-product being formed in 28% yield.2’
IIII: Photolysis of Carbonyl Compounds
79
0
0
3
Oxetane Formation
Over recent years, Bach and his coworkers2*have published a number of papers dealing with the photocycloaddition of aldehydes to alkenes. The present report describes in detail the outcome of the addition of benzaldehyde to a series of enol ethers (59). The regioselectivity and the de obtained in these cycloadditions is recorded under the appropriate structures (60) and (61). The control of oxetane formation in constrained environments has also been reported using hydrotalcite clay as the supporting medium. The irradiation (1> 280 nm) of a mixture of 4benzoylbenzoic acid and 2-phenylethenylbenzoic acid in this environment leads to regioselective product formation. The oxetanes were identified as (62, 24%), the predominant regioisomer, and (63, 2%).*3A study of the addition of aldehydes to allenes has been reported. The reactions were brought about by irradiation through Pyrex under nitrogen. As illustrated in Scheme 3, there appears to be little regioselectivity in the reaction but reasonable yields of alkylidene oxetanes were achieved.24 The glyoxalate (64)undergoes oxetane formation with electron-rich alkenes. The reactions, as expected for an aryl ketone, take place from the triplet state and occur with high regio- and stereo-selectivity.Some examples of the alkenes and the yields of the oxetanes are illustrated in Scheme 4.*’ A study has described the results from the addition of a variety of ketones and diketones to the alkene (65).
Photochemistry
80 1
$:,
But
Ph
(59) R Et Pr' Ph But
yield(%)
regioselectivity dr
61 57 76 58
87:13 82:18 80:20 75:25
61:39 70:30 71 :29 95:5
RwR' Ph
Ph
p@R1 R2
(62) R'
=
R2=CsHdCOzH
(63)
R Et
(CH&&HCH2 Pr'
But
yield(%)
ratio
69 66 54 38
2:1 2:1 1:l 1:l
Scheme 3
The addition of benzophenone, for example, affords low yields of the four isomeric oxetanes (66, R' = Ph), (67, R' = Ph), (68) and (69). These addition reactions take place in competition with the conversion of the alkene into (70) and (71). The conversion to these alkenes presumably is the result of energy transfer from the excited ketone. The addition of methyl phenylglyoxalate to the alkene (65) affords a single adduct (72) in 70% yield while the addition of benzil to the same alkene yields the two adducts (66, R' = PhCO, 31Y0)and (67, R' = PhCO, YO).^^ Chung and Ho27 have reported the details of the photochemical addition of electron-deficient alkenes to the substituted adamantanones (73). All of the ketones undergo reaction from the singlet state and, for example, the addition to acrylonitrile affords the two adducts (74) and (75) in an anti:.syn ratio of 60:40 which is the approximate value for the products of all systems studied. The irradiation at 300 nm of the alkene (76) in neat spectral-grade acetone affords (77), referred to as a photo-Conia product, in 52% yield. A trace of the oxetane (78) was also found. In hexadeuterioacetone the ratio of the products was found to be 2:l
81
IIll: Photolysis of Carbonyl Compounds 0 Ph+O-
Et
0
x n o
4
Ph
P
0
0 4
OEt 920/0
h OEt
70%
q
P
0-
h
a
P
OEt
70%
0-
h
n
o
q
p
Ph 0
4 OEt
OEt 64%
Scheme 4
(72) 70%
81%; R'-R~ = CH2CH2CH =CH2 76%; R'-R2 = (CH& 659'0; R'-R2 = (CH&
82
Photochemistry
X
(73)X = F, CI, Br, HO or Ph
d
(77)
n (79)
(74)
1 2
3 3 3
R' H H H Me Me
(75)
(78) R = H or D
R2 H H H H D
14% 12?h 60%
17% 5%
trace trace trace 17 */o 32 O/o
Scheme 5
while in acetone the ratio was 1 1:1. This result indicates that a hydrogen from C2 of the acetone is the donor in the formation of the photo-Conia product (77). There is a solvent effect upon the reaction as well as that observed between acetone and deuteriated acetone in that the photo-Conia product is only formed in neat acetone. When other solvents are used this product is not obtained nor is an analogous product observed with other ketones. The exo-cyclic olefins (79) are also reactive in this system and the yields are shown under the structures in Scheme 5. The authors2* suggest that the photo-Conia products are obtained by the addition of an a-keto radical, formed by abstraction of a hydrogen from acetone by another excited state acetone molecule, to the exo-cyclic alkene. The use of the
IIIl: Photolysis of Carbonyl Compounds
83
deuteriated acetone demonstrates that the abstraction of deuterium is slow and under these conditions the normal oxetane formation competes favourably. Irradiation of the enone (80) at 282 nm brings about the formation of the oxetane (81), and this process has been used as a key step in the development of a synthesis of 2,7,9-trimethylenetricyclo[4.3 .O.#**]n0n-4-ene.~~ Such intramolecular additions are popular methods for the formation of polycyclic compounds that can be used as starting materials in syntheses. An efficient example of this from earlier years is the formation of the oxetane (82,909'0)from the intramolecular cyclization of the enone (83). The oxetane formed is a key intermediate in a new efficient route to endo-hirsutene.30A review of the application of the Diels-AlderIPaterno-Buchi reaction as an approach to di- and tri-quinanes has been p~blished.~'
4
Miscellaneous Reactions
SET Processes - For a number of years Cossy and coworkers3' have examined the electron-transfer-induced ring-opening reactions of acylcyclopropanes. In particular they have examined the control that can be exercised on which bond undergoes cleavage and whether ring expansion competes with other reaction processes. In the present study they32have examined the photochemistry of the isomeric tricyclic ketones (84) and (85). These are irradiated at 254 nm in acetonitrile/triethylamine in the presence of added lithium perchlorate. Under these conditions the compound (84) is converted into (86) in 65% yield after only 2 h irradiation. No ring expansion was detected with this system. The isomeric ketone (85) does undergo ring expansion and the principal product, 60% yield, was identified as (87) which is accompanied by (88) in 10% yield. The formation of (87) provides a route to the basic skeleton of the AB rings of the taxane system. A detailed account of the SET-induced ring opening of acylcyclopropanes has been published.33 Typical of the many reactions described, (89) undergoes ring opening to give a radical which is intramolecularly trapped by the pendant ally1 moiety to yield (90). The factors that control the outcome of the reaction are discussed in detail. Reductive photochemical cyclization occurs with (91) to give the oxabicyclononanol (92).34 The SET induced ring opening of the ketooxiranes (93) has been studied in the presence of the electron donor imidazoline (94). Irradiation in dry THF solution in the presence of (94) afforded the corresponding radical anion which gave only
4.1
84
Photochemistry
0
Me
:t Me-m:
AcO
AcO
H
“OH H
a low yield of the expected hydroxypropanone (95). The best solvent system for In a further the reaction was found to be aqueous THF or aqueous ben~ene.~’ study, both the P-diketone (96) and the P-hydroxy ketone (97) are formed from the trans-cyanochalcone epoxide (98) when the irradiations are carried out in the presence of amines as electron-donating sensitizers. SET processes are involved and the P-diketone is formed by P-hydrogen abstraction by the amine radical cation from the ring opened epoxyketone anion radical. The formation of the phydroxyketone is dependent on the proton-donating ability of the amine sensitizer. The effects of solvent on the ratios of products were also studied. Selected results are shown in Table 2.36 A series of steroidal systems have been synthesized to examine the possibility of singlet and triplet energy transfer between two functional groups held at a known distance apart. The derivative (99) is typical of the systems studied and its irradiation in acetonitrile using 266 nm light brings about reactions of the carbonyl group. Under these conditions the so-called antenna group, the phenylsilyloxy function, is the radiation-absorbing species. The energy is trans-
MI: Photolysis of Carbonyl Compounds
85
Table 2 Yields (%) of (96) and (97) obtained from ketooxirane (98).36 Electron donor
Solvent
DABCO DABCO Et3N Et,N Et2NH Et2NH (PhCH2hN (PhCHz)3N
MeCN Benzene MeCN Benzene MeCN Benzene MeCN Benzene
8 71 40 77 0 7 5 12
0 0 7 8 5 42 49 50
Me
0
Me (93) R' R2 Ph Ph Ph H Ph Pr' Ph Pr"
(94)
(95)
0
OH
0
ferred from this to the C17 keto group and reactions typical of this functionality are observed to give the alkenal(lO0) and the isomerized ketone (101). The ratio of these products from irradiation at 266 nm is 1:2.8 respectively. A similar ratio, 1:2.5, of products is obtained by irradiation at 308 nm when the carbonyl group is the absorbing specie^.^' The photoinduced SET reactions of small peptides such as glycylalanine in water have been studied.38
86
Photochemistry
4.2 Decarbonylation and Decarboxylation - The photochemical reactions between carboxylic acids such as formic and trifluoroacetic acid and silica surfaces have been studied.39The irradiation of diphenylacetic acid in acetonitrile with acridine results in the decarboxylation and the formation of the adduct (102). When the two achiral compounds, acridine and diphenylacetic acid, are cocrystallized a chiral two component molecular crystal is obtained in which the components are present in a 1:l ratio. Both (-)- and (+)- crystals can be obtained. This crystalline material is photochemically reactive and irradiation brings about decarboxylation and formation of the adduct (102) with an ee of 35%. This compound is accompanied by a low yield of (103) which is formed exclusively on irradiation of the reactants in acetonitrile ~olution.~'Photochemical decarboxylation can be brought about by the irradiation of benzilate (104)/ methyl viologen pairs.41
R % '
H R'
R3
/
R2
H R2
H R3
H R4
Me Me H H Me0 Me0 H H H H Me0 Me0
R4 (1 04)
Irradiation at 365 nm of the lactone (105) in benzene for seven days fails to yield products. However, the compound is reactive under laser-jet irradiation conditions and two products (106) and (107) are obtained in low yields. The authors4' suggest that the reactions occur from an upper triplet state. The route to the products involves C-0 bond fission to give the biradical(lO8). This species either reacts with benzene to yield (106) or decarboxylates and rearranges to give the cycloheptatriene(107) by the path shown in Scheme 6. Barton and have demonstrated that the Barton esters (109) can be used in the synthesis of the 3-deoxy-~-arabino-2-heptulopyranosonic acids (1 10) and (1 11): this is achieved by the addition of the radicals formed from irradiation of the esters (109) to the alkene (1 12). This process affords the intermediate compound (1 13) which is transformed into the final products. A tungsten filament lamp or ordinary laboratory lighting is sufficient to induce cleavage of the Barton esters (1 14), and this process affords moderate to good yields of the corresponding amides (1 15) by the addition of the alkyl radicals to acrylamide.4 Other workers have used the
MI:Photolysis of Carbonyl Compounds
(105)
(106) 2%
87
(107) 6%
Scheme 6
Barton ester system for the generation of specific radicals. In one example, the oxiranyl radicals (1 16) have been formed by the uranyl-glass-filtered irradiation of the substrate (1 17) and cyclize to yield (1 18) as the final product.45 A full report of the laser-induced enantio enrichment of tartaric acid, originally published in preliminary form,46has a~peared.~’ The photochemical fragmentation of 7-methyl-2,2,5-triphenyl1-oxa-5,6-diazaspiro-[2,4]-hept-6-en-4-one has been studied.48Interest in solid state photochemistry continues to burgeon. The present paper49discusses the problems associated with the proximity of the components of radical pairs or biradicals within the constrictions of the crystalline environment. This problem has been addressed by examining the photochemical reactivity of a series of cyclohexanone derivatives (1 19) whose solution-phase photochemistry is well known. The irradiations, using h = 350 nm, were carried out on microcrystals dispersed in potassium bromide. The influence of the conformations within the crystals and substitution were studied. The relative yields of the product, the corresponding cyclopentane, are shown beside the appropriate structure.49
Photochemistry
88
E;
PyS+%0CF3
0
II
*OCCF3 C02Et
0
R/yCONH2 SPY
(115) R = Ph(CH2)2, Pr', cyclohexyCC6H11, But or l-adamantyl
rH R' a ; P h b; Ph C; Ph d; OH e; Me f; Me
R2
eR' R3 R4
(119)
R3
R4 relativeyield
H H H Ph H H Ph OH H OH Ph Ph Ph Ph H Me Ph Ph
0 0.01 0.05 0.56 0.49 1.00
M I : Photolysis of Carbonyl Compounds
89
4.3 Reactions of Miscellaneous Haloketones - Irradiation of chloroketones brings about rearrangement and yields carboxylic acids. This process has been studied in some detai1.50,5’The fundamental reaction of the sequence is the photochemical transformation of (120) into (12 1) which is brought about by irradiation in acetone/water using propylene oxide as an acid scavenger. The carboxylic acid (121) formed by this route has been used in a synthesis of a - ~ u p a r e n o n e Other .~~ reports of the photochemical reactivity of chloroketones have focused on the production of free radicals. Thus, the irradiation of the chloroacetamide derivative (122) in acetonitrile brings about C-CI bond cleavage. Cyclization of the resultant intermediate radical affords the two products (123) and (124) in 26% and 39% yield, re~pectively.~~ The chloroacetamide (125) undergoes photochemical cyclization which involves attack at the C4 position of the indole and yields the lactam (126).54Sun lamp irradiation of the ketone (127) in benzene with added iodoethane and bis(tributy1tin) produces an a-carbonyl radical which ring closes by cyclization with the pendant alkyne. The product (128) is obtained in 73% yield.55 A short review of the photochemical oxalyl chloride chlorocarbonylation of cage compounds such as cubane has been p~blished.’~ 0
1
Me Me
Me Me
‘OH Me OMe
0
WMe OMe
Photochemistry
90
me \
I
I
Me02C
&& -Me
I
Me
4.4 Photodeprotection - Jones et al.57 have described the photochemical reactivity of a series of keto esters (129) in propan-2-01. Irradiation of (129) affords the radical (130) which either dimerizes to yield (131) or picks up hydrogen to afford (1 32) which lactonizes liberating the long-chain alcohols. The formation of these alcohols is highly efficient. The use has been made of the benzil functional group as a photochemically activated protecting group for amines. A selection of the compounds reported are illustrated in (133).5s Many applications of the photolability of a benzoin protecting group have been devised over the years. The one described in reference 59 deals with a system where the benzoin group is attached to a solid phase as in (134) and (135). This system permits the facile isolation of the compounds once they have been liberated from the protecting group by irradiation. The photolysis of the system is brought about at 350 nm in T H F / r n e t h a n ~ l .Considerable ~~ interest continues to be shown in the development of new photochemical active protecting groups. One example is based on the photochemistry of the p-hydroxyphenacyl system which is described as an efficient phototrigger for ATP. The quantum yield for its release has been determined as 0.37 (Scheme 7).60
4.5 Other Fission Processes - The results of the irradiation of benzyl acetate and 3,5-dimethoxybenzyl acetate have been analysed using membrane introduction mass-spectrometry.61Irradiation of the acid derivative (1 36) under nitrogen in methanol brings about the formation of the corresponding ester. The authors62 suggest that this process is the result of CO bond fission and the liberation of a hydroxy radical. Subsequently this is trapped by methanol as the ester (137).
91
IIII: Photolysis of Carbonyl Compounds OH Ph
Ph%
C02R
OH
/
P
h
C02R
OH
C02R
/M
/
R = n-C12H25,cholestenyl, geranyl OCONHCeHI1
(133) R1 H
R2
H
3,5diMeo H 3,5diMe0 4-Me0 3,5diMe0 4-SMe 3,5diMe0 2-naphthyl 3,WiMeO H 3,WiMeO
R3 H H H H H H 3,5diMeOC&
NHFMOC
"HFMoC 0
0 OH
eoyJY, Ph
(135) R = cholesterol
000
II II II I l l 0-o-o-
O-POPOP-
OH OH
Scheme 7
Photochemistry
92 0
0
II CHzCOH
II
CH2COMe
R'
SePh
Further proof for the intermediacy of such a radical is obtained by its trapping with oxygen to give the adduct (1 38). Irradiation of the cyclopentanecarboxylatederivatives (139) results in cyclization to afford the spiro compounds (140). This process arises from Se-C bond cleavage, addition of the resultant radical to the pendant alkene group, and readdition of the PhSC radical. Several examples of the process are described as a synthetic path to novel bakken~lides.~~ Irradiation of the stannane (141) in benzene brings about C-Sn bond cleavage and the formation of the tripropyltin radical.@ The irradiation of telluro derivatives such as (142) can be used as a source of a variety of alkyl and acyl radicals formed by the fission of the C-Te bond.65
IIt1: Photolysis of Carbonyl Compounds
93
References 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. 27. 28. 29. 30. 31. 32. 33.
J. Pola, Y. Koga and A. Ouchi, Tetrahedron, 1997,53,3757. S. Peukert and B. Giese, Tetrahedron Lett., 1996,37,4365. N. J. Turro, Nato ASI Ser., Ser. C, 1996, 480 (Crystallography of Supramolecular Compounds), 429 (Chem. Abstr., 1996,125,518034). Z. Zhang, N. J. Turro, L. Johnston and V . Ramamurthy, Tetrahedron Lett., 1996, 37,4861. N. A. Kaprinidis, M. S. Landis and N. J. Turro, Tetrahedron Lett., 1997,38,2609. E. Fasani, M. Mella, S. Monti, S. Sortino and A. Albini, J. Chem. Soc., Perkin Trans. 2, 1996, 1889. Q.-X.Meng, Y. Sakaguchi and H. Hayashi, Mol. Phys., 1997,90, 15 (Chem. Abstr., 1997,126,211779). 0 . B. Morozova, A. N. Yurovskaya, Y. P. Tsentalovich and H.-M. Vieth, J. Phys. Chem A, 1997,101,399. 0 . Muaoka, B. 2. Zheng, M. Nishimura and G. Tanabe, J. Chem. Soc., Perkin Trans. 1 , 1996,2265. M. Yoshioka, K. Iida, E. Kawata, K. Maeda, S. Kumakura and T. Hasegawa, J. Org. Chem., 1997,62,2655. C. E. Sowa and M. Stark, Synlett, 1996,227. G. Sundarababu, M. Leibovitch, D. R. Corbin, J. R. Scheffer and V . Ramamurthy, J. Chem. Soc., Chem. Commun., 1996,2159. A. D. Gudmundsdottir, T. J. Lewis, L. H. Randall, J. R. Scheffer, S. J. Rettig, J. Trotter and C. H. Wu, J. Am. Chem. Soc., 1996,118,6167. W. Weigel and P. J. Wagner, J. Am. Chem. Soc., 1996,118, 12858. K. D. Freeman-Cook and R. L. Halcomb, Tetrahedron Lett., 1996,37,4883. T. Y. Kim and B. S. Park, Bull. Korean Chem. Soc., 1997, 18, 141 (Chem. Abstr., 1997,126,27735 1). P. J. Wagner, A. Zand and B. S. Park, J. Am. Chem. Soc., 1996,118,12856. A. Steiner, P. Wessig and K. Polborn, Helv. Chim. Acta, 1996, 79, 1843 (Chem. Abstr., 1997, 126, 19198). C. Wyss, R. Batra, C. Lehmann, S. Sauer and B. Giese, Angew. Chem. Int. Ed. Engl., 1996,35,2529. T. Hasegawa, N. Yasuda and M. Yoshioka, J. Phys. Org. Chem., 1996, 9, 221 (Chem. Abstr., 1996,125,57662). G. A. Kraus, W. Zhang and Y. Wu, J. Chem. Soc.. Chem. Commun., 1996,2439. T. Bach, K. Jodicke, K. Kather and R. Frohlich, J. Am. Chem. Soc., 1997, 119, 2437. T. Shichi, K. Takagi and Y. Sawaki, Chem. Lett., 1996,781. A. R. Howell, R. Fan and A. Truong, Tetrahedron Lett., 1996,37,8651. S . K. Hu and D. C. Neckers, J. Org. Chem., 1997,62,564. M. Christ1 and M. Braun, Liebigs Annalen, 1997,1135. W.-S. Chung and C.-C. Ho, J. Chem Soc., Perkin Trans. 2, 1997,553. W.-S. Chung and C.-C. Ho, J. Chem. SOC.,Chem. Commun., 1997,317. T. Herb and R. Gleiter, Angew. Chem. In?.,Ed Engl., 1996,35, 2368. V. H. Rawal, A. Fabre and S. Iwasa, Tetrahedron Lett., 1995,36,6851. V. H. Rawal, A. Eschbach, C. Dufour and S. Iwasa, Pure Appl. Chem., 1996,68,675 (Chem. Abstr., 1996,125, 58767). J. Cossy and S. BouzBouz, TetrahedronLett., 1997,38, 1931. T. Kirschberg and J. Mattay, J Org. Chem.. 1996,61, 8885.
94 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.
45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55.
56. 57. 58. 59. 60. 61. 62. 63. 64. 65.
Photochemistry
J. Cossy and S. Ibhi, Carbohydr. Rex, 1996, 291, 189 (Chem. Abstr., 1997, 126, 8386). E. Hasegawa, T. Kato, T. Kitazume, K. Yanagi, K. Hasegawa and K. Horaguchi, Tetrahedron Lett., 1996,37,7079. E. Hasegawa, K. Ishiyama, T. Fujita, T. Kato and T. Abe, J. Org. Chem., 1997,62, 2396. S. A. Jiang. C. H. Xiao and H. Morrison, J. Org. Chem.. 1996,61,7045. R. R. Hill, G. E. Geffs, F. Banaghan, T. McNally and A. R. Werninck, J. Chem. Soc., Perkin Trans. 2, 1996, 1595. E. J. Lee, T. W. Bitner, J. S. Ha, J. M. Shane and M. J. Sailor, J. Am. Chem. Soc., 1996,118,5375. H. Koshima, K. L. Ding, Y. Chisaka and T. Matsuura, J. Am. Chem. Soc., 1996, 118,12059. T. M. Bockman, S. M.Hubig and J. K. Kochi, J. Org. Chem., 1997,62,2210. R. M. Wilson, K. A. Schnapp, M. Glos, C. Bohne and A. C. Dixon, J. Chem. SOC., Chem. Commun., 1997,149. D. H. R. Barton and W . 3 Liu, Tetrahedron Lett., 1997,38,367. D. H. R. Barton and W.4. Liu, Tetrahedron Lett., 1997,38,2431. F. E. Ziegler and Y. Wang, Tetrahedron Lett., 1996,37,6299. Y. Shimizu and S. Kawanishi, J. Chem. Soc., Chem. Commun., 1996,819,1333. Y. Shimizu, J. Chem. SOC., Perkin Trans. I , 1997, 1275. M. S. Amine, Y. A. Issa, I. Hegazy and A. F. M. Fahmy, Egypt J. Chem., 1996,39, 215 (Chem. Abstr., 1996,125,221696). T . Choi, K. Peterfy, S. I. Khan and M. A. Garcia-Garibay, J. Am. Chem. SOC.,1996, 118, 12477. H. R. Sonawane, N. S. Bellur, D. G. Kulkarni and R. N. Ayyangar, Tetrahedron, 1994,50, 1243. H. R. Sonawane, N. S. Bellur and G. M. Nazeruddin, Tetrahedron, 1995,51, 11281. S . P. Chava, T. Ravindranathan, S. S. Patil, V. D. Dhondge and S. W. Shubhada, Tetrahedron Lett., 1996,37,2629. I. M. Sudarma and J. B. Bremner, ACGC Chem. Res. Commun., 1995,3,45 (Chem. Abstr., 1997,126,238527). M. L1. Bennasar, E. Zulaica, A. Ramirez and J. Bosch, Heterocycles, 1996,43, 1959 (Chem. Abstr., 1996, 125, 301296). C. K. Sha, R. T. Chiu, C. F. Yang, N. T. Yao, W. H. Tseng, F. L. Liao and S. L. Wang, J. Am. Chem. SOC.,1997,119,4130. A. Bashir-Hashemi and G. Doyle, Aldrichimica Acta, 1996, 29, 43 (Chem. Abstr., 1996,125, 300477). P. B. Jones, M. P. Pollastri and N. A. Porter, J. Org. Chem., 1996,61,9455. J. F. Cameron, C. G. Wilson and J. M. J. Frechet, J. Am Chem. SOC.,1996,118,12925. A. Routledge, C. Abell and S. Balasubramanian, Tetrahedron Lett., 1997,38, 1227. R. S. Givens and C. H. Park, Tetrahedron Lett., 1996,37,6259. P. S . H. Wong, N. Srinivasan, N. Kasthurikrishnan, R. G. Cooks, J. A. Pincock and J. S. Grossert, J. Org. Chem., 1996,61,6627. A. B. Wu, H. W. Cheng, C. M. Hu, F. A. Chen, T. C. Chou and C. Y. Chen, Tetrahedron Lett., 1997,38,621. T. G. Back, P. L. Gladstone and M. Parvez, J. Org. Chem., 1996,61,3806. A. I. Kruppa, M. B. Taraban, S. A. Svarovsky and T. V. Leshina, J. Chem. Soc., Perkin Trans. 2,1996,2151. M. A. Lucas and C. H.Schiesser, J. Org. Chem., 1996,61,5754.
2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY WILLIAM M. HORSPOOL
1
Cycloaddition Reactions
1.1 Intermolecular Cycloaddition - Reviews pertinent to the area discussed in this chapter have commented upon photocycloaddition reactions' and also have highlighted methods for the synthesis of macrocyclic ring systems.2 1.1.1 Open-chain Systems - Chow and his have carried out further studies on the cycloaddition reactions of dibenzoyl(methanato)boron difluoride (1) to alkenes and dienes. This present work has examined the mechanistic details of the process. Previously they have reported the addition of the same reactant to enol ethers.3bThe influence of the chiral centre in the alkene (2) on the cycloaddition of a,P-unsaturated nitriles such as acrylonitrile has been evaluated. The products obtained from these (2+2)-photoaddition reactions are the azabicyclooctanes (3).4 F.,
CONHCHMePh
F
oB , )o , Ph-Ph
UCONHCHMePh (1 1
I
@--.. I H PhCH2
CH2Ph (2)
(3)
Apparently the presence of a fluorine atom in a styrylcoumarin changes the regiochemistry of photochemical dimerization in the solid state. Usually dimerization of non-fluorinated styrylcoumarims leads to centrosymmetric photodimers. The photodimerization of (4), however, affords the dimers identified as (5).5 The dimerization of methyl 3-(2-furyl)acrylate can be brought about by irradiation in acetonitrile using benzophenone as the triplet sensitizer. An investigation of the stereo and regiochemical control was carried out? The (2+2)photocycloaddition that takes place between p-nitrostyrene and 2,3dimethylbuta- 1,3-diene has been suggested for an undergraduate experiment.' 1.1.2 Additions to Cyclopentenones and Related Systems - (2+2)Photocycloaddition has often been used as the key step towards an important
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
Photochemistry
96
(4) R’ R2 F H H F
intermediate in the synthesis of naturally occurring compounds. Lange and Organ’ report that the cyclopentenone (6) adds efficiently to the cyclohexene derivative (7) to yield the two adducts (8) and (9) in a ratio of 1:l. A single adduct (10) was formed in 35% yield when cyclopentenone was added to the silylated cyclohexene (1 1). These adducts were aimed at a synthetic approach to norasteriscanolide. Calculations have dealt with the (2+2)-photocycloaddition reactions of the enones to a variety of alkenes.’” The results demonstrate that the regioselectivity is determined within the initially formed biradicals and partitioning of these radicals. The results are in general agreement with those of other researchersgb who examined the problem from a product analysis standpoint. Benzophenone acts as an electron-accepting sensitizer when it is irradiated in the presence of the amine (12). The outcome of this event is the production of the radical (1 3) that undergoes regio- and stereo-selective addition to the lactone (14). The intermediate radical is thought to be (15) and this combines with a diphenylhydroxymethyl radical to yield (16), the final product.”
IIl2: Enone Cycloadditions and Rearrangements
97
1.1.3 Additions to Cyclohexenones and Related Systems - Caldwell and his coworkers' have studied the photochemical addition of 1,I-diphenylethene to 4,4-dimethylcyclohex-2-enone. The products from this reaction, carried out in cyclohexane, are shown in Scheme 1. Although other evidence (see reference 9b above and references cited therein) has suggested that an exciplex is not a key interaction in such (2+2)-photocycloaddition reactions Caldwell et al. conclude from their detailed study of this system that a triplet exciplex is involved. Schuster and his coworkers" have reported that a variety of cyclic enones (17) (19) add photochemically to fullerene. The yields of the adducts vary but with some of the less heavily substituted enones the yields can be reasonable as shown by the data under the appropriate structure. Suginome et a1.13 report the synthesis of the cycloadducts (20) by the photochemical addition of various ethenes to the enone (21). The adduct (22) was also synthesized by photochemical addition of methoxycyclohexene to the enone (20). An examination of the fluorescent behaviour of 7-hydroxycoumarin (umbelliferone) has been reported. The authors suggest that dimerization may result on irradiation of solutions of high concentration. l4 A conformational analysis has been carried out on the C4 photocycloadducts formed from 4,SY8-trimethylpsoralen. l 5
'
1.2 Intramolecular Additions - The truxinic acid derivative (23) is formed on irradiation of (24) in methanol using a Pyrex filter. The reaction is quantitative and has a quantum yield of 0.55. Similar reactivity is observed when crystals of (24)are irradiated.16 The use of a benzene ring as a constraint has provided a method for the formation of a single stereoisomer in high yield: this is seen best of all with the yield: this is cinnamate ester (25). Irradiation of (25) for 21 h in methylene chloride solution affords an 88% yield of (26). A less efficient reaction
0
Photochemistry
98
+
,,APh
cyc'ohexane,
gPh bPh+ +
Qh+ ,
H
Ph
H
Ph
/
\
Scheme 1
a Me
R*
(17)
R'
0
R'
R2
R3
H H H Me H Me Me H Bun H H
2
H H H Me
yield(%) of adduct 6 56 53 3 11
(18) yield of adduct 1YO
OAc
&
0
(19) yield of adduct 12%
I Ph
(20) R' Me OEt OAc
R2 Me H Me
is observed with the diyne derivative (27) when a 68% yield is obtained of (28). The influence for the benzene constraint is seen to advantage with the enediyne (29). Here irradiation brings about efficient cis,trans-isomerization. Such isomerism results in the placement of the cinnamate groups at some distance from each other, and only an 8% yield of the cycloaddition product (30) is formed.I7
IIl2: Enone Cycloadditions and Rearrangements
99
C02H
f i C O z H
C02H
0
(27)
0
0
PhHC02)
Ph
(31)R' R2 Me0 Me EtO Me Me Me0 Me EtO
PhQC02)
'CO2
Ph'
C02
100
Photochemistry
Another example of control over the (2+2)-photodimerization of cinnamates has been published. In these examples the conformation tether is a dioxane system as shown in (31). The irradiation of these affords mixtures from which cyclobutane derivatives (32), (33), (34) and (35) can be isolated.’* 1.2.1 Intramolecular Additions to Cyclopentenones - The enones (36) are photochemically reactive and undergo (2+2)-photocycloaddition to afford the tricyclic ketones (37).19 The competition between (2+2)-cycloaddition and hydrogen abstraction reactions in the enones (38), (39) and (40) has been studied. The reactions are brought about by irradiation at 366 nm in toluene as solvent. The reaction type that the molecules undergo is dependent upon the nature of the substituent on the nitrogen side-chain. Thus, with two ally1 groups the products obtained are (41) and (42) in 74% and 9%, respectively. With the t-butyl system (39) the cycloadduct (43) is obtained in 50% yield while the hydrogen abstraction process yields the spiro derivative (44).The chain length separating the pendant ethenyl group from the nitrogen was also varied and here cycloaddition usually occurred in competition with hydrogen abstraction from the N-Me group. The results from this study are shown in Scheme 2.20 The enone (45) undergoes a highly diastereoselective cycloaddition when it is irradiated in hexane solution. The product (46) is obtained in 95% yield and is a key intermediate in a new synthesis of spir0[5.4]decanes.~’A review has been published dealing with the synthesis of fenestranes, and photochemical synthetic approaches to such systems are also included. A typical example of the (2+2)-cycloaddition used is the transformation of (47) into (48).22
I .2.2 Additions to Cyclohexenones and Related Systems - The temperature dependence of the intramolecular photochemical cycloaddition of the enone derivatives (49) and (50) (h > 300 nm hexane) has been studied.23 Crimmins et al.24report the use of intramolecular cycloaddition of the enones (51t(53) for the synthesis of the tricyclic compounds (54)-(56), respectively. These adducts can then be ring opened using tributyltinhydridelAIBN in benzene to yield the bicyclic compounds (57)-(59). The yields obtained are high, up to 80%. The synthesis of tricyclic compounds formed by intramolecular cycloaddition reactions has also been studied by Pete and his coworkers2’ This reaction path involves photocycloaddition within the diesters (60)-(62). The cyclizations are influenced by conformational restrictions and attempts to bring about the (2+2)intramolecular cycloaddition of (60) failed; in this case only the dimer (63) was formed. The compounds (61) and (62) do, however, undergo the intramolecular cycloaddition in reasonable yields. Compound (61) affords the two adducts (64) and (65) in yields of 27% and 49%, respectively. This reaction affords products of both head-to-head and head-to-tail with a predominance of the head-to-head process. With the shorter chain length in (62) only head-to-head adducts (66, 62%), (67, 16%) and (68, 1%) were obtained. A study of the photoreactions of the enone (69) has shown that four products (70)-(73) are formed. The reactions involved the use of the pure 2-isomer of (69) and irradiation at wavelengths > 330 nm. Temperature dependence studies permitted an analysis of the reaction
M2: Enone Cycloadditions and Rearrangements
101
(36) R = H, X 5 CH2 R = Me, X = 0
yield (YO) 28 2
39
-
58 26 45
-
36
30
4
Scheme 2
(41)
0
Me
4-
(42)
(43)
(44) 12%
0
(47)
system and the authors propose that only two biradicals (74) and (75) are involved in the formation of the products.26The lifetime of these radicals is sufficiently long to permit the ring opening of the cyclopropane substituent. The regio- and stereoselective intramolecular cycloaddition of the enones (76) and (77) to afford the adducts (78) and (79), respectively, has been reported. The cycloaddition reactions are efficient but y-hydrogen abstraction does occur in competition with the
102
Photochemistry
cycloaddition processes.*’ Enantioselection in (2+2)-photocycloadditions is an important synthetic goal and has been realised in the intramolecular (2+2)cycloaddition of the optically active silylallene derivatives (80) in cyclohexane solution using Pyrex-filtered radiation. Selected results are shown in Scheme 3 and ees of 92”/0 are achieved. One of the best examples of enantioselection observed in this study was the cyclization of (81) to yield (82) which takes place with an ee > 99%.28 A study has examined the effects of chain length, solvent, viscosity and flexibility of the linking groups upon the (2+2)-cycloaddition reactions of coumarin groups in long chain polyether corn pound^.^^ 0
(49) R’
H But H Me H
(50) R‘ H H Me H Me H But
R2 H H But H
Me
OCH20Me
(52)
&Me M e O C H 2 0 w (54)
(57)
R2 H H H Me H Me H
R3 H But H H But But H
OCH20Me
(53)
103
1112: Enone Cycloadditionsand Rearrangements
0
0
-Me
'Me
Photochemistry
104 0
0
i R3 (77) R’
H Me Me H H H
R2 H H H Me Me H
R3 CH2Ph CH2Ph (CH&Me CH2Ph (CH2)sMe CH2Ph
R’
R’
0
(79)
n 1 2 2
X 0
0 NBOC
yield(%) 67 86
77 Scheme 3
88
z
78% E, 76% 86% E, 97% 2 85% E
X
Y
bond CH2 bond CH2 bond CH2 CH2
CH20rCHzCH2
CH2 CH2
CH2 CH2
M2: Enone Cycloadditionsand Rearrangements 2
Rearrangement Reactions
2.1
ct,&Unsaturated Systems
105
2.1.1 Isomerization - Kaupp and Haak3’ have reported an atomic force field microscopic investigation of the phase changes in the crystal of (83) when it is irradiated at 365 nm. The photochemical process is the isomerization around the ethene bond to afford the isomer (84). The E,Z-isomerism of the exocyclic ethene bond in the natrexone derivatives (85) can be carried out photo~hemically.~~
(85) R = Ph, 4-PhCsHd 1- and 2-naphthyl 9-anthracenyl
trans,cis-Isomerism is the dominant reaction on irradiation of the p-toluidides of cinnamic (86) and crotonic acid (87). Cyclization is also detected with the crotonic acid derivative and this yields the dihydroquinolone (88). No evidence for the photo-Fries reaction (i.e. photofission of the N-C bond) was detected with these systems. However, the toluidides of methylcrotonic and phenylpropiolic acids did undergo this well-documented rearrangement.32 The unsaturated ester (89) undergoes photochemical isomerization into the Z-isomer. This process brings the ester and the hydroxy function into close proximity and as a result thermal cyclization occurs with the formation of the furanone (90), (R)-(+)~ m b e l a c t o n eBroad-band .~~ UV irradiation of the bis cinnamate salt (9 1) results in the formation of a photostationary state comprising the three isomers (91), (92) and (93) in a ratio of 69:3:28, re~pectively.~~ A further report of the photochemical behaviour of (94) has been published,35 and complements the details given last year.36 Direct irradiation of the substituted trienes (94) in chloroform solution results in rapid isomerization and an equilibrium is established between the E,E,E and the Z,E,E isomers in a ratio of 62:28 respectively. The authors36 suggest that the outcome of the reaction is the result of a twisted transition state with zwitterionic character. This transition state is stabilized by the presence of the two carbonyl groups. The authors35 report that in non-polar solvents a one-way cis, trans,trans to trans,trans,trans process takes place. Huang et aL3’ have described the facile photochemical ring opening of the cyclobuteneones (95). The process results from cleavage of the C-C bond labelled ‘a’, and is induced by irradiation at 350 nm in deuteriated chloroform at 5°C. The yields of the allenylketenes (96) formed by this reaction process are almost
106
Photochemistry
0
J$
Me
Me\
Me
Me
quantitative. Interestingly, the ketenes have relatively long half-lives at room temperature. 2.1.2 Hydrogen Abstraction Reactions - A review has highlighted the photodeconjugation processes of or$-unsaturated en one^.^^ Such reactions are typical of a Norrish Type I1 hydrogen abstraction process. Irradiation of (97) at 308 nm using an excimer laser also follows this path and brings about hydrogen transfer from the phenolic OH with the formation of the keto enol (98).39Norrish
1112: Enone Cycloadditions and Rearrangements
I07
R'
R2
(95) R' C02Et Ph Ph Me3Si
H
CI
R2 H H Me H
H
q;O). 0
CI (99)
eoMe
n = 1 or 2, Ar = Ph
OMe
n = 2, Ar =
In
(100)
R=H, n=l,20% R = OMe, n = 2,20%
OMe
(102) n = 1 , 2 o r 3
(103) n = 1,50% n=2,10% n = 3, 12940
108
Photochemistry
Type I1 hydrogen abstraction within the. chromone derivatives (99) provides the route to the spiropyran derivatives (100). In these examples the hydrogen abstraction occurs from the allylic position on the pendant ring substituent. The result of this is the formation of the biradical (101) which cyclizes and loses hydrogen to afford the final products. The yields are modest at best. With the thiophenyl derivatives (102) the radical generated by the hydrogen abstraction process adds to the thiophene ring in an analogous manner to the reactions encountered with (99). In this instance, however, rearomatization does not occur and the reduced products (103) are formed.40 2.1.3 Rearrangement Reactions - The aziridine derivatives (104) are photochemically reactive and irradiation transforms them into the pyrroles ( 105).4' Toda et al.42 have demonstrated that the photochemical conrotatory cyclization of the enone (106) into the dihydrobenzothiophene derivative (107) can be brought about with an ee of 37%. This cyclization is achieved by the irradiation of aqueous solutions of powdered crystals of the 1:l complex of the enone with the chiral auxiliary (-)-(108). The success of this type of control was demonstrated further by the successful cyclization of the enones (log), under analogous conditions, into the derivatives (1 10). An investigation into the photochemical reactivity of the enone derivatives (1 1 1) has been reported. This work was aimed at examining the influence on the photochemical reaction of the medium in which the irradiation was carried out. The reactions encountered in benzene solution are summarized in Scheme 4. The photoprocesses are typical of such systems and yield a phenol by 1,2-phenyl migration, rearrangement to the linearly conjugated dienone (1 12) and in some cases to the enone (1 13). This enone is a new product that arises by secondary photolysis of the ketene formed by ring opening of the enone (1 12). The behaviour in a crystalline environment is radically different and the principal products are the phenols (1 14) and (1 15)
IIl2: Enone Cycloadditions and Rearrangements
109
shown in Scheme 5. The influence of stereochemistry was also studied and, for example, the enone (1 16) yields the two products (1 17) and (1 18) regardless of the stereochemistry of the starting material.43
R H Me MeO
yield (%) 80 73 57 63 12
20 27 43 37 38 40
But PhCHO Ph2CHO
48 low conversion 60 high conversion
0
Scheme 4
A
R
P
h Ph
light
crystal
,&..
+
RJ@
Ph (114)
Me Me0 BU~
+
Ph
b0 Ph
Ph
Ph
(115) yield (%) 40 100
R
H
'
60 0 100 42
0 8
Scheme 5
?H
+
@
NC
Photochemistry
110
A review has discussed the photochemical cyclization of a variety of aryl- and heteroaryl-prop-2-enoic acids.44 Irradiation (h > 340nm) of the cyclohexenone (1 19) in argon purged benzene solution affords the cyclized derivatives (120) as a 3: 1 mixture of diastereoisomers. The formation of the cyclopropyl group arises via the carbene (121) and insertion into a C-H bond of a neighbouring methyl group. This carbene is formed, presumably, from the biradical(l22) which arises by addition of the alkene to the excited state of the enone. Further evidence for the carbene intermediate comes from a reaction in methanol when the diastereoisomeric mixture of the ethers (123) and (124) is obtained.45
1,3-Migrations occur on brief irradiation ( 5 min) of the enone (125) in cyclohexane solution under argon using unfiltered light from a mercury vapour arc lamp. The products isolated were identified as (126, R = H, 53%; R = OAc, 56%) and (127, R = H, 12%; R = OAc, 15%).46
- -R
--R
(125) R = H or OAc
(126)
- -R
(127)
2.2 P,y-Unsaturated Systems - Irradiation (1> 280 nm) in methanol solution for 24 h of the amide (128) brings about Z,E isomerization: as well as this process, both a 1,3- and a 1,5-acetyl migration take place. The result of the 1,3migration is the formation of the azetine (129). The 1,s-migration from the Zisomer results in the formation of the isoquinoline (130). The formation of this
IIl2: Enone Cycloadditionsand Rearrangements
111
product is proposed to arise via the intermediate (131).47A 1,3-aroyl migration, a photo-Fries like process, occurs on irradiation at 254 nm of the amides (132, n = 1) to give (133). The outcome of the reaction appears to be dependent upon ring size. Thus, irradiation of the derivatives (132, n = 2) results in cyclization to the benzene ring to afford the tetrahydroquinolinone derivatives (134).48
n 1 1 1 2 2 2 2 2 2
R1 OMe H H OMe H H H H H
R2 OMe H Me OMe CF3 H Me F OMe
(133)
It is well known that irradiation of stilbene induces a 61~-photocyclisationfrom the singlet excited state. Interestingly, irradiation of the stilbene moiety of (135) does not undergo this reaction to yield a substituted phenanthrene. The compound is photoreactive, however, and two products are obtained in low yield. These were identified as the rearranged ketone (136) and the (2+2)-cycloadduct (137). Ketone (136) is formed by a 1,3-acyl migration. The authors4' argue that this singlet state reactivity of the enone must be brought about by energy transfer from the stilbene moiety. The singlet state of the enone affords the 1,3-migration product while the triplet state leads to the cycloadduct. The photochemical rearrangement, brought about by irradiation through Pyrex in toluene solutions, of the bicyclic, (P,y-unsaturated enones (1 38) has been studied in detail. Typically these systems undergo decarbonylation and rebonding to afford the cyclopropane derivatives (139)."
Photochemistry
112
OMe
2.2.1 The Oxa Di-z-methane Reaction and Related Processes - A detailed examination of the triplet and singlet state reactions of a series of P,y-unsaturated enones has been reported. Many examples are cited and are typified by the conversion of the enone (140) into (141) on sensitized irradiation. This reaction is a typical example of the well known oxa-di-n-methane process, which fundamentally involves a 1,Zmigration of the acyl group. Direct irradiation populates the singlet state of the enone (140) and this yields (142) by a 173-acyl migration.” The oxa-di-n-methane reaction of chiral bicyclo[2.2.2]oct-5-en-2-one systems has been used as a path to tricyclic ketone^.'^
3
Photoreactions of Thymines and Related Compounds
3.1 Photoreactions of Pyridones - Previously Sieburth and his colleague^^^ have demonstrated that the photochemical cyclization of tethered pyridones such as (143) undergo the normal head-to-tail cycloaddition. The present study has examined the photochemical reactivity of the symmetrically tethered compound (144). Here it is expected that the ‘unnatural’head-to-head cycloaddition should result. Irradiation of (144) in methanol solution under nitrogen is slower than the head-to-tail process but after 12 h two products were identified as the head-tohead adducts (145) and (146). These were obtained in a ratio of 1:1.54 Other
1112: Enone Cycloadditions and Rearrangements
113
examples of this type of addition have been examined. Thus the photocyclization in methanol of the tethered compounds (147) and (148) results in good yields of
0
Me
Me
(143)
(144)
OMe
Me I Me
I Me (148)
(147)
OTBS
Me
114
Photochemistry
the adducts (149) and (150), respectively. The additions yield compounds with five stereogenic centres. These products can be reacted further to provide a path towards a synthesis of t a x a n e ~ . ~ ~ The X-ray crystal structures of a series of Dewar pyrimidones (15l), obtained by the irradiation of the pyrimidones (152), have been determined.56 The photochemical reactivity of I -methyl-4,6-diaryl-2(1H)pyrimidinone derivatives in the presence of various thiols has been studied.57
3.2 Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type I1 hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant I ,6biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154).58 The structure of this product was verified by X-ray diffraction techniques. The Norrish type I1 reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 %
As a consequence of photodamage to DNA there is still considerable interest in the photochemical dimerization of pyrimidine derivatives. Thus, the synthesis of the pyrimidine dimers (157), (158) and (159) has been carried out by irradiation of the pyrimidine derivative (160).61 Sugiki et aZ.62 have studied the photochemical dimerization of the long-chain substituted thymine derivatives (161). The results of the measurement of the reduction potentials of thymine and cytosine cyclobutane dimers have been reported. In addition the electron-transfer-induced ring cleavage by electron donation from suitable sensitizers of the adducts (162) and (163) has also been examined.63 Calculations have been carried out on the photo-induced repair mechanism in DNA both by direct irradiation@ and by the use of SET induced phot~cleavage.~~ Irradiation of (164) in a matrix of frozen benzene and THF brings about different photochemical results from those obtained from irradiation in solution phase. The solution phase reaction was carried out in benzene with added trifluoroacetic acid. This treatment yielded only the cyclooctatriene derivative (165). In the frozen system meta addition to the benzene ring results in the formation of the three products (166), (167) and (168).66 The photochemical reaction of the same chlorouracil (164) in the presence of p-xylene results in the
IIl2: Enone Cycloadditions and Rearrangements
R02C
\C02R
115
R02C
CH2CH2
0
Me
Me
formation of the pentacyclic adduct (169). The mechanism of the addition process is discussed in some detail. An analogous addition reaction occurs with rn-xy~ene.~~
Me
I Me
Me
H
116
Photochemistry
Intramolecular addition to aromatic systems also occurs and De Keukeleire and coworkers6’ have described a fascinating example of such a process. In their example the pyrimidone unit adds to the 1,2-positions of a benzene ring. This reaction takes place in the molecule (170) and occurs with full chemo-, stereoand regio-selectivity affording the single stable adduct identified as (171). Intramolecular addition is also reported for the pyrimidine derivative (172) which on irradiation at 300 nm in acetone/acetonitrile yields two adducts in a ratio of 4:l of the general structure (173). The major isomer was isolated by transesterification and was identified as (174). This resutt is claimed to be the first synthesis of a cis-syn-furanoside (2+2)-cy~loadduct.~~
Me0
Interest in features that can control (2+2)-intermolecular cycloadditions is of considerable interest. Mori et ~21.’’ have examined the influence that hydrogen bonding can have on such reactions. The system studied is the cycloaddition between the substituted coumarin (175) and the pyrimidone (176). Irradiation
IIl2: Enone Cycloaddirions and Rearrangements
117
through Pyrex for 8 h of this system in acetonitrile solution gave the two adducts (177), the cis,syn adduct, and (178), the cis,anti adduct in 15 and 8%, respectively. In the absence of a tether that can complex with the approaching addend, as in the cycloaddition between acetoxycoumarin and the same pyrimidone, only the cis,anti adduct is formed. The authors7' argue that molecular recognition between (175) and (176) is important in determining the outcome of the cycloaddition reaction. 0
(175)
(176) R = Bu'
0
A semiempirical study of the relative energies of the oxetane (179) and the socalled 6-4-product (1 80), obtained from the photochemical reaction of thymine, has shown that these products are more mutagenic than the thymine dirne~-.~' An intramolecular example of this addition has also been reported. Thus irradiation (at 254 nm) in water/acetonitrile solution of the linked pyrimidine system (181) yields the product (182, 20%) which presumably arises by formation of an oxetane of type (179) by addition of a pyrimidone carbonyl group to the 5,6double bond of the other pyrimidone unit; this is followed by ring opening of the ~ x e t a n eThe . ~ ~thiothymidine (183) undergoes an analogous (2+2)-photoaddition to adenosine (184) on irradiation at 360 nm in water to give (185): this is formed presumably by way of a thiaoxetane that ring opens ultimately to yield the final product. 73 A study of the photochemical reactivity of 5-iodouracil(lS6) and its bearing on cross-linking of nucleic acids has been carried
Photochemistry
118
S
NH2
Mefx.o
I
f iOH
OH
OH
0
119
1112: Enone Cycloaddilions and Rearrangements
4
Photochemistry of Dienones
4.1 Cross-conjugated Dienones - The cross-conjugated cyclohexadienone (187) undergoes non-regioselective photoisomerization on irradiation at 366 nm in benzene. The two products were identified as (188) and (1 89), and are reported to be sensitive to acid and rearrange readily on work-up. The products are photochemically reactive and further irradiation brings about isomerization to benzenoid derivative^.^^ The spirocyclohexadienone (190) undergoes isomerism on irradiation to afford the derivative (191). The process involves bond cleavage with the formation of the cyano-stabilized biradical (1 92). The derivative (193) appeared to be photochemically unreactive since any simple process is degenerate. However, an analogous biradical to that from (190) is formed under the same irradiation conditions. This biradical species can be trapped by the addition of a suitable alkene. Thus, when 2,3-dimethylbut-2-ene is used the adduct (1 94) was isolated.76
b
C
0
2
Me C02Me
Me3Si
M
\
Me3Si \
H
\
Me SiMea
,
Me
0
0
0'
0
0
Me0
boz
e
Me0 NC
4.2 Linearly Conjugated Dienones - The photochemical reactivity of linearly conjugated dienones has been studied for many years. The ring opening reaction to afford ketenes has synthetic utility. This type of reaction has again been
Photochemistry
120
exploited in a synthetic approach to a m i d e ~Thus . ~ ~ 2,4,6-trimethylcyclohexa-2,4dien- 1-one (195) undergoes facile photochemical ring opening reactions, using visible light, in the presence of a,o-diamines to afford symmetrical amides. Barton et aL7* have previously demonstrated that irradiation from a tungsten lamp of the cyclohexadienone (196) in diethyl ether in the presence of amines provides a simple and efficient route for the synthesis of the amides (197). The reactions involve the opening of the dienone into a ketene that is trapped by the amine.
Me (195) R = H (196) R = CH2SMe
5
Me
LS,Me
(197)
1,2-, 1,3- and 1,4-Diketones
5.1 Reactions of 1,ZDiketones - The photochemical behaviour of some 1,2diketones adsorbed on a silica gel surface has been studied. The reactions are brought about by nn* excitation using a high pressure mercury arc lamp. The principal products, as shown in Scheme 6, arise by a-cleavage. This process yields either a carboxylic acid or an aldehyde dependent upon the nature of the acyl radical produced by the @-cleavage.In two examples the dimeric compounds (198) are formed: these arise by cyclization within the excited state to afford the biradical (199). Subsequent re-aromatization and dimerization yields the final product (1!18).~~ Photocyclization of a 1,Zdiketone is also reported by Nakatani et L Z Z . ~ ~ * * ' Irradiation (366 nm in acetonitrile/water) of the diketone (200) has been shown" to give the furan derivative (201) in 73% yield with a quantum yield of 0.06. In other solvents the same type of reaction occurs and yields the products (202) from reactions carried out in methanol, propan-2-01 and t-butanol. The mechanism of the cyclization involves the carbene (203) which is protonated and the resultant cation is trapped by the solvent. The involvement of the carbene has been demonstrated further by irradiation in acetonitrile/D*O when the appropriately labelled product was obtained. Intermolecular trapping can also occur and this is illustrated by the conversion of the diketone (204) into the bis-furan (205).8 1,2-Diones also undergo photochemical intramolecular hydrogen abstraction reactions. Thus irradiation of (206) induces isomerization to the pyran derivative (207). This rearrangement is the result of Norrish Type I1 hydrogen abstraction to afford the biradical(208). Ring opening within the biradical yields an alkenal that cyclizes to give the hemiacetal (207).82 A further mode of reaction is (4+2)cycloaddition which is demonstrated by the photochemical addition of the 1,2-
IIl2: Enone Cycloadditions and Rearrangements
121
diketones (209) to 2,3-dimethylbut-2-ene. This results in the formation of the dioxene derivatives (210).83The indantrione (21 1) is photoreactive when irradiated in degassed alcoholic solutions (methanol, ethanol and propan-2-01). The products formed from this treatment are (212) and (213) in the yields shown under the appropriate structures. The paths to these products are thought to involve C-C bond fission to yield the biradical(214) which then transforms into the anion (215) which yields (212), while a carbene (216) is the precursor to (213).84
light
R'CHO
+
R2C02H
0
+ 0
Me (30%)
Scheme 6
Ph
(202) R = Me 76% R = Pr' 62Y0 R = Bu' 49%
R3
Photochemistry
122
H R
H R
H R
(209) R = Me or R-R = (CH2)3
@:
*o 0 (211)
H C02R (212) yield(%) 57 54 35
0
g
o H OR
(213) yield(%) 21 12 5
R Me Et Pr'
The photochemical reactions of ethyl phenylglyoxalate (21 7) in benzene have been re-examined. The three new products (218), (219) and (220) have been isolated from the reaction mixture. The quantum yields for the formation of the products are dependent on c ~ n c e n t r a t i o n Irradiation .~~ of the phenylglyoxalate derivatives (221) results in conversion into the lactones (222) from (221, R = H) and (223) and (224) from (221, R = Me). The reactions are proposed to involve an intramolecular electron transfer process forming a zwitterionic biradical. This leads to activation of the methylenes adjacent to the sulfur atom. A similar effect is observed with the nitrogen analogue (225) which affords (226) as the
IIl2: Enone Cycloadditions and Rearrangements
123
photochemical product. The SET process was substantiated by a laser-flash study of the intermediates produced during the reaction. The oxygen analogues only undergo conventional Norrish Type I1 reactions.86 Glyoxalate derivatives (227) with sulfur substituents having varying chain lengths separating the thio group from the ester moiety have also been studied. Irradiation of (227) affords products by two reaction paths. All the compounds except (227, n = 11) undergo an SET process that leads to the zwitterionic biradical (228). From this radical catiodradical anion species cyclization affords the lactones (229) in good to modest yields. The alternative reaction path involves intermolecular hydrogen abstraction by the excited carbonyl group from the methylene function adjacent to the oxygen in another molecule. This process yields the radicals (230) and (231). The radical (230) leads to the dimeric products (232) while bond fission in (231) affords (233)." The cycloaddition of the diene (234) to the furan-2,3-dione derivatives (235) affords the cycloadducts (236).88 The reaction takes place efficiently with high regio- and stereo-selectivity.
phYo
P
0
R
0
P HO h - c J
Me (221) R = H or Me
0 Me Ph+/-'
h
0
0
0
n
d, Me
?-
4 . .
qPh
124
Photochemistry OH
(229) n 2 3 4 5 6 7
yield (YO) n 100 8
96 89
9 10
71
11
yield (YO)
32 30 25
-
ii
53 43
(232) n 5 6 7 8 9 10 11
yield (YO) 19 30 25 14 15 35 41
(235)
5.2 Reactions of 1,SDiketones - The benzoylcyclopentanones (237) are photochemically unreactive on irradiation in the solid or in solution in inert solvents. However, when these compounds are irradiated on a silica gel surface products from Norrish Type I reactivity are observed.89
5.3 Reactions of 1,rl-Diketones - An X-ray crystallographic study of the triketone (238) has shown that exocyclic keto groups are oriented differently in the solid state and both the anti-syn (239) and the syn-anti (240) forms are present. Thus, as a result of the constrained environment within the crystal the phenyl groups of the keto functions are differently aligned. This difference in the keto groups results in different photochemistry and when crystalline (238) is irradiated only the rearrangement product (241) is obtained. This must arise from the anti-syn arrangement (239). In methanol solution, however, the two
125
IIl2: Enone Cycloadditions and Rearrangements
products (241) and (242) are obtained on irradiation at 300 nm in a yield of 55% and a ratio of 1:4, respectively, illustrating that under these conditions the syn-anti form is more reactive.” Jabbar and Banerjee” report that the various I ,4-diketo-derivatives (243)-(246) exhibit colouring on irradiation in benzene solution.
;qph ;qH
% ;
COPh
0
COPh
Ph
Ph
0
C02Me
C02Me
OPh H Ph (241)
(242)
dcH (245)
The photochemical reactions of homoquinones of the type illustrated by (247) have been studied. The reactions are brought about by irradiation in the presence of amine electron donors. When triethylamine is the donor derivative (247a) undergoes ring opening to yield the quinone (248). A different reaction is observed when (247a) is irradiated in the presence of N,N-dimethylaniline. Under these conditions the addition product (249) is formed.92Direct irradiation of the benzoquinone derivatives (250) in methylene chloride results in their efficient conversion into lactones (251). The reactions are proposed to involve the formation of a triplet nitrene.93
126
Photochemistry
R' (247)
a; Me b; c;
d;
Me H Me
R2 H H Me Me
X Br Me Me Me
(250) R' M e0 EtO Me0 Me0 NMe2
(249)
R2 Ph Ph pBrCsH4 Me Ph
(251) yield (YO) 85 82 06 75 76
X NH NH NH 0 NH
An approach to the synthesis of homosecoprismanes has also been developed using intramolecular photocycloaddition reactions of the enediones (252) and (253). The irradiation of these in benzene affords the cycloadducts (254) and (255), respectively in 80-90% yield.94 CI M e0
M
e
O
G
Meo CI Meo
CI
0
o
iIi2: Enone Cycloadditionsand Rearrangements
127
5.3.I Phthalimides and Related Compounds - G r i e ~ b e c khas ~ ~ reviewed the photochemical reactivity of phthalimide derivatives that have amino acids incorporated into the nitrogen side chain. The photochemistry of the phthalimide derivatives (256), in benzene solution under nitrogen, is dominated by electron transfer processes involving ring opening of the cyclopropane system. Irradiation of (256a) affords the products (257) and (258). The key intermediate to the formation of these products is the biradical (259, R' = Ph, R' = H). Cyclization within this intermediate affords (257) while hydrogen transfer yields the ether (258). The isomeric compounds (256b) behave analogously and yield (260), (261) and (262). Again a biradical(259, R' = H, R' = Ph) is key to the formation of the products.96
0
0 (257) n = 1,7% n=2,61%
(256) a; R' = Ph, R2 = H, n = 1 or 2 b;R'=H,R2=Ph,n=10r2
g:kR' 0
0
(258) 7%
(259)
pjp"
#>)" \ \
0 (260) n = 1 n=2
0 (261) n = 1 n-2
& \
0 (262)
5.3.2 Fulgides and Fulgimides - The influence of conformational effects on the photochromism of the furylfulgide (263) has been examined.97The effect on the absorption maximum of the coioured form of (264) by the introduction of methoxy substituents to the indole ring has been rep~rted.~'
128
w;
Photochemistry
R'
R2
MeN
(264) R' H Me0 H Me0
6
R2 H H Me0 Me0
Quinones
6.1 o-Quinones - The photoreactions of tetrachloro- and tetrabromobenzo- 1,2quinone with 1,6diphenylbut- l-en-3-yne have been described.99 Dihydrodioxins are formed from these reactions as well as compounds that are precursors to 9phenylphenanthrene derivatives. The photochemical cyclization of the o-quinone derivatives (265) provides a good route to the chrysene quinones (266).'" A review lecture has discussed the chemistry of perylenequinones.lo' 0
0
Ar
Ar
(265)Ar Ph pCtC~jH4 pMeCsH4 pMeOCGH4
R H CI
Me Me0
(266)yield (%) 97 86 80 38
6.2 p-Quinones - Quinone methides are produced on irradiation of p-benzoquinones in the presence of the eneyne (267) in methylene chloride as solvent. 1,4Naphthoquinones also afford quinone methides but unstable spirooxetenes were also detected.lo* The photochemical addition of phenanthraquinone and acenaphthenequinone to eneynes has also been studied. Io3 1,6Benzoquinones undergo addition to electron donating alkenes to yield spirooxetanes. The resultant cyclohexadienones are themselves photochemically reactive and irradiation converts these into dihydrobenzofurans via a dienone-phenol rearrangemen t. lo4
129
IIl2: Enone Cycloadditions and Rearrangements
An examination of the photo-induced electron transfer processes between the quinone (268) and multifunctional porphyrins has been carried out. ' 0 5 Chloranil has been used as an electron accepting sensitizer in the presence of the silylenols (269) and (270). The reactions can be carried out in both methylene chloride and acetonitrile and result in the conversion of the enols into the corresponding ketones.Io6 Other studies on the same systems have shown that the outcome of the irradiation at h > 380 nm can be influenced by solveit polarity and by added salt."' Time-resolved spectroscopy has been used to study the excited singlet state of p-chloranil.'** R' I
0
Me0
OSiPh3
OSiMe2
OMe
(5
6"
A report describes the results of irradiation of the quinone (271) to yield three (2+2)-cycloadducts.lo9 The quinone (272) undergoes four-centre photodimeriza-
0
I
(273)
(274)
Photochemistry
130
tion in the crystalline phase. Photodimerization is also observed with the quinone (273) and in the crystalline phase this yields a centrosymmetric cyclobutane derivative. l o Femtosecond spectroscopy has been used to study electron transfer processes in the duroquinone system (274).' Suginome and coworkers' I have described the photochemical (2+2)-cycloaddition of alkenes to 2-acetoxynaphtho- 1,4-quinone. The resultant adducts can be converted into the corresponding cyclobutanols which react with mercury(I1) oxidehodine to afford a cyclobutanoxyl radical. A laser-flash study has examined the photochemical behaviour of vitamin K3.This investigation sought to provide details for the hydrogen atom abstraction reactions in this system.'I3 The anthraquinone (275) undergoes intercalation with DNA. Irradiation brings about an electron transfer process that effectively cleaves the DNA.'I4 A study of a 1,4-hexyl-bridged anthraquinone derivative has shown that intramolecular hydrogen abstraction occurs from the singlet biradical produced on excitation. I The photochemical reactivity of anthraquinone with p-chlorothiophenol in the solid state at room temperature has been investigated.l16 Mori et ~1."' report that the azulene quinone (276) in methylene chloride solution undergoes photochemical dimerization on irradiation using a 400 W mercury arc lamp. Three products were isolated from this process and were identified as (277), (278) and (279) in 6, 10 and 24%, respectively. The outcome of the reaction shows some dependence upon the solvent used for the irradiations. A time-resolved single photon study of the photophysics of hypericin and its methylated analogues has been reported.
'
'
0
0
OMe I
1112: Enone Cycloadditions and Rearrangements
131
References 1. 2. 3.
4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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C. Meyer, 0. Piva and J.-P. Pete, Tetrahedron Lett., 1996,37,5885. M. T . Crimmins, Z. Wang and L. A. McKerlie, Tetrahedron Lett., 1996,37,8703. M. Thommen and R. Keese, Synlett, 1997,231. D. Becker and Y. Cohenarazi, J. Am. Chem. SOC.,1996,118,8278. M. T . Crimmins, S. J. Huang and L. E. Guisezawacki, Tetrahedron Lett., 1996, 36,
17.
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44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59.
Photochemistry
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133
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Photochemistry
134
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3 Photochemistryof Alkenes, Alkynes and Related Compounds BY WILLIAM M. HORSPOOL
1
Reactions of Alkenes
cis,trans-Isomerization - A study of both the singlet and the triplet sensitized irradiation of cis-cyclooctene has sought to clarify the effects of multiplicity of the photochemical reaction upon the optical purity of the product.' Circularly polarized light of 190 nm obtained from a synchrotron source has been used to study the direct photoracemization of (a-cyclooctene. The results indicate that (R)-( - ) and (S)-(+)-cyclooctene are enantioselectively isomerized by this circularly polarized light into achiral (Z)-cyclooctene.2 In a related study Inoue and coworkers3have reported the results of a detailed study on the use of a variety of electron-acceptingsensitizers used to effect enantiodifferentiation in the Z,E isomerization of cycloocta-1,3-diene. Among the sensitizers used (-)-menthylbenzene hexacarboxylate was found to give the highest enantiomeric excesses (10% and 18% in pentane at 25 "C and 40 "C, respectively). Irradiations in polar solvents were less effective and gave lower ees. Reversible cis,trans-isomerism has been reported following the irradiation of micellar aggregates of the photochromic surfactants (1) in water.4 1.1
1.1. 1 Stilbenes and Related Compounds - Irradiation of the stilbene derivative (2) brings about Si-Si bond cleavage. A detailed photophysical study of the system was carried out.' Lewis and Yang6 have reported the results of a study of the excited state behaviour of a series of amino substituted stilbenes such as (3), (4) and (5). The photochemical reactivity of the sterically congested stilbenes (6) has been e ~ a m i ned.Both ~ of these alkenes are planar about the ethene bond but to overcome the congestion, the phenyl groups are rotated 90" out of plane. The decay processes, following excitation, have been analysed in great detail and the authors7 suggest that there are distinct paths for the photochemical isomerizationprocesses. The highly selective truns,cis-isomerism of (7) has been described by Arai and Maeda.*The reaction is completely one-way when it is carried out in benzene. The reason for the selectivity is the presence of the 'strong' hydrogen bond in the cisisomer (8). The photostationary state is to some extent solvent dependent. Thus in acetonitrile the ratio of (7):(8) is 4:96 while in methanol the ratio is 84: 16.
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
136
Photochemistry
.i' Fi'+
R1 X-
(3)
(6)a; R = H b;R=Me
R
(7)
1.2
MiscellaneousReactions
1.2.1 Addition Reactions - The ally1 ketone (9) is the product formed on photohydration of the diyne (10). The photohydration step arises from the triplet excited state of (10). The polarization within the excited state is the key to the outcome of the reaction and is such that the protonation occurs at C L Shim" ~ has published a short review dealing with the hydration of such systems.
8
pCH=C=CH-CCMe3
Me
Me
Me
1113: Photochemistry of Alkenes, Alkynes and Related Compounds
137
Direct irradiation of the arylalkenes (1 1) results in their conversion into the cyclic ethers (12) and (13). These cyclizations are the result of proton transfer within the excited state and cyclization of the resultant zwitterion. Using 2,4,6triphenylpyrylium tetrafluoroborate as an electron-accepting sensitizer the compounds (1 1, n = 1, R = CH3) and (1 1, n = 1, R = CH30) undergo oxidative cleavage even when the reactions are carried out under argon.'
'
flRo0p / \
\
\
n (11)
(12)
7 ; 1 1
(13)
Me Me0
Smith and Richards'' report that there is doubt over the formation of the products claimed by Padwa et a l l 3 from the irradiation of compounds such as (14). In the earlier report it was claimed that products, the result of ring contraction, were formed by a two-photon path but this has now been shown to be unlikely and the product formed on irradiation of (14) is merely that of addition of methanol to the ethene bond affording the adduct (1 5). l 2
I
H
Ph Ph
1.2.2 EZectron Transfer Processes - A study of the benzophenone/acetonitrile/tbutylamine system has shown that electron transfer occurs from the amine to the excited state benzophenone. Hydrogen abstraction processes within this system ultimately yield the 'CHZCN radical. When this species is generated in the presence of a diarylethene such as that shown in Scheme 1 then addition affords reasonable yields of the adducts (16). The mode of addition is dictated by the stability of the radical formed on addition to the ethene bond. Addition can also occur to alkenes such as (17) when the adduct (18)is obtained.14 SET processes also provide a novel synthetic path to 5,6-dihydro-4H-l,2-oxazines.These reactions involve the DCA-sensitized transformations of the y,&-unsaturated oximes (19).The reactions are carried out in acetonitrile solution with irradiation through Pyrex for no longer than 30 min. This treatment yields the products (20) in reasonable yields. The likely mechanism for the process involves the formation of the intermediate (21)which cyclizes to yield the final products. The reaction is to some extent substituent dependent and when the oxime (19,R' = H, R' = R3 = R4 = Me) is irradiated under the same conditions for 3 h no reaction is
138
Photochemistry
detected.I5 Clearly the presence of at least one phenyl group on the alkene moiety is important for the success of the cyclization process. Ph
Ph Ph2CO MeCN/ButNH2 R
R
(16) R = H, Ar = Ph, 77% R = Me, Ar = Ph, 40%
Scheme 1
(18) 46%
(17)
R2 R2
R' Me H H
R2 H H Me
R3 Ph Ph Ph
R4 Ph Ph H
R' Me H H
R2 H H Me
R4 yield (%) Ph 53 Ph 21 H 52
As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers'6 have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)-(25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation.'6 A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers. " Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (R)-(+)-a-terpineol and (R)-(+)limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed."
139
IIi3: Photochemistry of Alkenes, Alkynes and Related Compounds
4 +
Me2C=CH2
phenanthrene MeOHlMeCN
-$+
CN
\
6 \
CN
CN
52%
2940
Scheme 2
(26) n P 1 or 2
Pyrrole and N-methylpyrrole form exciplexes with a variety of arylalkenes and arylalkynes. When pyrrole is irradiated in the presence of styrene the adduct (27) is formed in 67% yield. The reaction is brought about by an electron-transfer process with the amine as the donor and the alkene as the acceptor. Ultimately coupling affords the final product. An analogous addition is seen with 1methylstyrene when the adduct (28, 52%) is produced. Other examples of such additions were also described.l9
1.2.3 Other Processes - The involvement of the upper singlet state in the photochemistry of some 1,l -diarylethenes such as (29) has been assessed.20 Irradiation of (30) at 365 nm in chloroform solution brings about a 1,3-hydrogen
140
Photochemistry
migration to yield the final products (31). No evidence for C-Br bond fission was reported. The reaction shows some dependence on ring size since irradiation of [30,n = 3, R = CH(CH3)Et] fails to afford a product.2' Sensitized-irradiation of the phenylallyl phosphites (32) and (33) brings about population of the triplet state of the alkene moiety. This excited state is converted into the biradical, e.g. (34), from which the final products (35) and (36) are formed.22
phY
(aBr SeR
(29)
(32)
(35)
2
(30)R = Et. Pr' or C(Me)HEt; n = 1
(33)n = 1 or 2
(34)
(36)n = 1 or 2
Reactions Involving Cyclopropane Rings
2.1 The Di-n-methane Rearrangement and Related Processes - Armesto and his coworkers23have reported more examples of systems that undergo the aza-di-nmethane rearrangement. This account gives the details of work that was reported in preliminary form some time The paper describes the photochemical reactivity of a series of 1-iminobutene derivatives (37)-(39). The reactions are all carried out under sensitized conditions. Typical of the results obtained is the transformation of the oximes (37a) and (37b) into the cyclopropane derivatives (40, 19%) and (41, lo%), respectively. More surprising than the aza-di-x-methane reactivity is the remarkable conversion into dihydroisoxazoles. Thus oximes (37a) and (37b) are also converted in low yield into the products (42, R' = Ph, R2 = H, 15%) and (42, R' = H, R2 = Ph, 8%). This reaction mode is again brought about under sensitized conditions. It is proposed that the oxime, in its triplet state, undergoes intramolecular electron transfer to afford a zwitterionic biradical that cyclizes and ultimately yields the final products. Analogous reactivity is seen with the hydrazone derivatives (38) that also undergo cyclization to yield the dihydro1,2-diazole derivatives (43). The yields are variable although (38, R' = Ts, R2 =
IIl3: Photochemistry of Alkenes, AIkynes and Related Compounds
141
Me) cyclizes efficiently to give only (43, R’ = Ts, R2 = Me) in 75% yield. In competition with this cyclization mode the aza-di-n-methane process occurs to yield the cyclopropanes (44)as shown in Scheme 3. Remarkably, the hydrazone derivatives (45) only undergo the aza-di-n-methane rearrangement to yield (46) efficiently.
fiR’
Ph
R2
‘OH
R’
R2
(37) a; Ph b;H
(38)
R’ Ts Ts
R2 Me
Br
H H
Ph
(43)yield (%)
f441 yield (%)
-
75 18 11 11
H
Ac
H
7 22 68
Scheme 3
fi)n
P h - “ q ,
“OH
Ph
(39) n = 1 n=3
h f’-;&,
“OH
N‘OH
pR2 @ (40) 19%
(41) 10’30
R2
R2%R1
O-N
(42)
NxR1
/
\
R’
/
\
R’ (45) a; NHTs
b;NHTs
R2 Me H
(46) a; 68% b; 46%
Steady state and laser-flash photolysis have been used to study the photochemical transformation of the barrelene derivative (47). Irradiation of this in a
Photochemistry
142
variety of solvents (benzene, acetone or methanol) results in its efficient converThe transformation arises by way of a sion into the pentalene ketone (48,70?40).~~ semibullvalene derivative involving the di-lc-methane transformation of the starting material (47). The dibenzobarrelene (49) has been shown previously to be converted into the cyclooctatetraene (50) when subjected to direct irradiation in acetonitrile. Conversely when the photo-reaction of (49) is sensitized the two semibullvalenes (51) and (52) are formed in a ratio of 72:28. This clear differentiation in the outcome of the reaction dependent on the excited state has been used to study energy transfer in the solid state. The compound (49) was converted into ammonium salts by reaction with the three amines (53), (54) and (55). Irradiation of these in the solid state where only the amine derivative absorbed light (at h > 330 nm) failed to yield the cyclooctatetraene. Instead the two semibullvalenes(51) and (52) were obtained in ratios of 6:1, 5:l and 154, indicating that triplet energy and been transferred successfully. Irradiation of the salts in methanol was not effective.26A short review has dealt with some reactions affording cyclopropane ring compounds and also with the photochemistry of such systems.27 0
COPh
H’(?LO (47)
Ph
(0
(49)
fopco2H
ropC02H
IIl3: Photochemistry of Alkenes, Alkynes and Relared Compounds
2.2
143
Other Reactions Involving Cyclopropane Rings
2.2.I SET Induced Reactions - (1R,3S)-(+)-cis-Chrysanthemol (56) undergoes conversion into the dihydropyran (57) on irradiation in the presence of the 1,4dicyanobenzene/phenanthrene system. The reaction is carried out in acetonitrile and the significant outcome is that retention of configuration results. The authors” suggest that the cyclization arises by intramolecular attack of the hydroxyl group on the radical cation of the vinylcyclopropane moiety. Photochemical ringopening of cyclopropanes such as those shown in Scheme 4 can be brought about by intermolecular electron transfer to sensitizers such as 1-cyanonaphthalene. The nucleophiles used to attack the radical cation of the cyclopropane are methanol, water or cyanide among others. The reactions are all stereospecific and occur with inversion of configuration of the centre undergoing attack. The a ~ t h o r s * ~ * ~ ~ describe the reactions as ‘bona fide’ three electron SN2 reactions. Such a mechanism has been supported by detailed studies of the kinetics of the reactions.
I
I
Me
Me
Nuc = MeO, HO or CN
PhJe
P h A N u c
A
Me
Scheme 4
The details of the electron-transfer processes involved on irradiation of mixtures of tetrachloro-p-benzoquinonewith the cyclopropane derivatives (58) and (59) have been reported. The intermolecular electron-transfer processes lead to the formation of the radical cation of the cyclopropanes. Ring-opening of the cyclopropanes results from these specie^.^
’
kAr R’
(58) R’
H, Ar = Ph, @VkC&, /I-bkOCsH,+ 1-naphthyl or 2-naphthyl
(59) R’ = Ar = Ph or pMeOC6H4
Photochemistry
144
2.2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds Pincock et aZ.32 have observed the photochemical conversion of the cyclopropane derivative (60) into the cyclobutane (61, 75%). This reaction is proposed as an example of a cyclopropyl-n-methane process involving an aryl OMe
CH2Ph
(60) OMe
OMe
I
(64) R’ H Ph
R2 Me Me H
CH20CO
R3 Me
Me Me
R5
(66) R4 Me Me
H
R5 H
H Me
yield (YO)
79 73 71
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
145
ring. The conversion is suggested to take place via the biradical (62) which transforms into biradical (63) prior to formation of the final product. No evidence was obtained for cleavage of the ester moiety which is common in other similar systems. Direct irradiation of the cyclopropenes (64)at 366 nm in benzene under argon fails to bring about a reaction. However, using benzophenone or thioxanthone as sensitizer, at the same wavelength, brings about excitation of the cyclopropene to its triplet state. Addition of this biradical to the pendant alkene moiety affords the biradical (65). Hydrogen abstraction within this species yields the final products (66) in the yields shown below the structure.33 The direct irradiation at 254 nm of the allenes (67) in hexane gives the molecule in its triplet state. Bridging within this state affords the intermediate biradical (68). Bond formation then yields the major products (69) and (70). Minor products (71) and (72) are also formed. Irradiation at h > 280 nm fails to yield products. However, acetophenone sensitization is effective. The authors34suggest that in this instance the vinyl moiety is the chromophore which is excited and that intramolecular energy transfer is involved. The yields obtained from the reactions are shown in Table 1.
- -R CaMe (68) (69)
(67) a; R' = R2 = C02Me b; R' H, R2 = C02Me
Table 1 Yield (%) of products obtainedfrom the allenes (67) Wavelength (nm)
Compound Conditions (71)
254 254
(67a) (67b)
> 280 > 280
(67a, b) (67a)
direct direct direct sensitized
4
1 0 12
Products formed (69) (70)
15 25 0
3
18 14 0 0
(72)
4 2 0 0
146
Photochemistry
A two-photon process has been identified following the flash photolysis of 1,3dichloro- 1,3-diphenylpr0pane.~’The 1,3-biradical (73) yields 1,2-diphenylcyclopropane.
Samarium iodide has been used to bring about reductive photo-dehalogenation of 1,I-dichlorocyclopropane.The results and the eficiency of the reaction are shown in Scheme 5. The process, a Barbier reaction, is brought about using visible light. The yields of product are enhanced by the addition PhSH as a hydrogen donor.36
26%
46%
8%
Scheme 5
3
Reactions of Dienes and Trienes
Zimmerman and Hofacker3’ have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol(77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81).38 DCA-sensitized reactions of the dienes (82) and E,E-(83) and the bicyclohexane (84) have been studied.39At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and E,E-(83) in ratios that are independent of temperature. The photochemical ring opening of the isomers of the bicycloalkenes (8 5 ) has been studied in detail. The results demonstrate that the ring opening to the diene products arises by di~rotation.~’Takahashi and coworkers41have shown that cyclobutenols (86) undergo ring opening when irradiated under electron-transfer conditions. The reaction is equally efficient with tetracyanobenzene or 2,6,9,10tetracyanoanthracene as the electron-accepting sensitizer. This treatment affords the o-quinodimethane intermediate (87). The result of the reaction is the formation of the ketone (88) and the arylated product (89). The X-ray irradiation of the
lIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
(78) 70% 75%
(77)
Ar
(79) 6% 5%
147
(80)
(81)
Ar
derivative (90) in the crystalline state brings about conversion into the cycloocta1,5-diene(91) without destruction of the crystal?*
(
a::
(85) I/
1,2,3,4
148
Photochemistry
A number of years ago Armesto and his colleagues4' reported the photochemical Mannich reactions of the dienes (92) in the presence of perchloric acid. This reaction brought about conversion into the isoquinolinones (93) efficiently via the photocyclization of the protonated species (94). The mechanism for the transformation was proposed to involve an intramolecular SET process as a key event in the reaction. The present report gives details of quantum yield measurements on a series of derivatives that support this mechanistic postulate.44 Kojima and coworkers45 have studied the dicyanobenzene-sensitized reactions of the dienes (95) and (96) in acetonitrile/water/ammonia systems. Products (97) and (98) are R3
R'
R'
(95)
R' H Me H Me H
R2 H H Me0 Me0 Me0
R3 H H H H Me
(98) yield (YO)
(97) yield (46) 89
0
58
11
60
0 0 13
50 58
R5 I
R2
(99) (96)
R' H Me H H H H
R2 Me H Me H
Me H
R3 H H Me Me H Me
R4 H Me H Me H Me
R5 H H H H MeO Me0
yield (%) 26 28 30 37 77 85
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
149
formed from addition of NH3 to the radical cation of (99, and (99) from addition to the radical cation of (96). Calculations have shown that the distribution of the positive charge within the radical cation is in good agreement with the ratio of products obtained. have studied the photochemistry of a series of furyl-oSindler-Kulyk et divinylbenzene derivatives ( 100). The derivatives (1OOa,b) undergo photo-conversion in high yield into the adducts (lOla,b) and are accompanied by a low yield of adduct (102a,b). The principal products are thought to arise by cyclization within the biradical (103). The accompanying products (102) arise by an intramolecular Diels-Alder process within the intermediate (104). The other derivatives (100c,d) give traces of the adducts corresponding to (101) and (102) but the main reaction is merely trans, cis-isomerism about the ethene bond.
(100)a; R = H b;R=Me c;I3..CN d; R = pCH3CsH4
(101) R
E
H or Me R
(102)R = ti or Me
Calculations have been carried out on the (4+4)cycloaddition reactions of b~tadiene.4~ Calculations have also been carried out on the photochemical reactivity of octa- 1,3,4,6-tetraene under cool-jet irradiation conditions and suggest that adiabatic isomerization cannot occur:* This contrasts with an earlier report by K ~ h l e r A . ~series ~ of tetraenes [e.g. (lOS)] have been synthesized. The photochemical reactivity of these was also studied and, for example, irradiation of (105) using Rose Bengal as the sensitizer affords the product (106) by a 1,7-hydrogen migrati~n.~’The isomerization of (107) in a mixture of acetone and benzene can be brought about by irradiation with wavelengths of 300 nm. This ethene bond isomerization affords (108), racemic stobilurin E, as the product.” The photochemical isomerization of 11-cis,-13-cis-retinoic acid to the isomer (109) can be effected by irradiation in methylene chloride/acetonitrile with Rose Bengal as the sensitizer.s2 A detailed study of the photochemical isomerization of (1 10) into (1 11) using light in the band 420-570 nm has been reported. 53
I50
Me
Photochemistry
{**o ‘
0 ‘
R’
h (110)
R’
R2
H But
Me Me
\
Me02C
OMe
R’
I
3.1 Vitamin D Analogues - A patent covering the photochemical conversion of the diene (1 12) into the triene (1 13) within what is called a microreactor has been published. The microreactor system is a zeolite with the appropriate size of the The use of the 2,7cavity to provide stereochemical control of the rea~tion.’~ dimethyl-3,6-diazacyclohepta1,6-diene tetrafluoroboratdbiphenyl filter solution has allowed the double wavelength irradiation (290-300nm and h > 330 nm) of procalcitriol as a route to 1a,25-dihydroxycholecalciferol. A study of the control that can be exercised upon the reaction by changes in temperature was carried The results of a study of the influence of intensity on the picosecond laser irradiation of provitamin D have been published.56 Other research has been aimed at the examination of the photochemical behaviour of previtamin D3.57
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
4
151
(2+2)-Intramolecular Additions
Calculations have dealt with the energetics of the photoisomerization within the norbornadiene/quadricyclane system.58The cyclization of norbornadiene to quadricyclane is a well-studied process and over the years many examples of the process have been described. Only a few have been reported in the past year. Thus, for example, the quadricyclane (1 14) is formed on irradiation of the corresponding norbornadiene. The reaction can be reversed thermall~.~'The kinetics of the photochemical isomerization of the norbornadienes (1 15) into the corresponding quadricyclanes have been measured. The quantum yields for the processes were found to be in the range of 0.18 to 0.36. The authors6' suggest that the results are in agreement with the involvement of a radical cation mechanism for the cyclization. Irradiation ( h > 300 nm) of the norbornadiene derivative (1 16) results in excitation of the androstene carbonyl group. Apparently this affords the triplet excited state that transfers triplet energy by a through-bond mechanism to the norbornadiene. This undergoes cyclization to the corresponding quadricyclane. The energy transfer occurs with 18.6% efficiency.6' A full report of the photochemical activity of the silaheptadiene (1 17) has been r e p ~ r t e d . ~ * . ~ ~
0 mesityl,Si,mesityl
& -
(117)
Gleiter et a1.@ have demonstrated that Dewar-benzene derivatives such as (1 IS) undergo efficient conversion into prismanes on irradiation at h > 280 nm in ether solution. The reactions of these systems are substituent dependent and derivatives such as (1 19) are unreactive.
k (118) R = H R = (CH2)20H
S02Bu' (119) R = H, Me, PhCH2 or Ph
Photochemistry
152
(2+2)-Photocycloadditions are a useful method for the synthesis of cage compounds. This reaction has been used in an approach to the synthesis of homosecoprisrnanes, and involves the (2+2)-cycloaddition of the polycyclic diene (120). The irradiation of (120) in acetone, presumably involving the triplet state, gives an almost quantitative yield of the cycloadduct (l2l)? Interest is still being shown in the photochemical addition reactions between ethene bonds and -N=N- bonds. An example of this process reported in the past year is the quantitative conversion of (122) into (123) by irradiation at h > 320 nm in deuteriochloroform.66 Other examples of such cyclizations have also been published. Thus the cycloadditions of (124) affording (125) can be brought about using irradiation of h > 320 nm.67The -N=N- bond can also be added to cyclopropane moieties providing that these substituents are suitably placed within the molecular framework. The photochemical production of cyclophanes continues to be of interest. The (2+2)-photocycloaddition reactions of the distyryl benzene moieties attached peri to a naphthalene unit (126) can be brought about to afford (127) on irradiation using Pyrex-filtered light in benzene solution. Irradiation for only 30 min affords the adduct (127) in 90% yield. The isomeric compound (128) is also reactive under these conditions, affording (1 29) in 85% yield.68
&
CI
Me0
Me0
CI
OAc (121)
?2
(124)
R’
R2
H CN H CN
H H
OSiMe2But OSiMeZBu’
0
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
OH
HO
Me H&-
153
Me#;H
MH e
a
-e;
H- -
\ /
1_1
(131) X = H, 74% X = MeO, 79%
(132) X = H, 16% X = MeO, 13%
'-ZOH
Ho J + \ /
-
HH@
HH@
(134)X = H, 53%
( 133)
(JpD Me02C' 'Me
-
wo
X = MeO, 68%
CHO
''20H
(135) X = H, 27% X = MeO, 0%
Me02C M e (136)
(137)
Photochemistry
154
Several examples of isomerism of heavily substituted dienes into cyclobutenes have been reported. The reactions are brought about by the use of quartz filtered light. The reactions are quite efficient and, for example, the bicyclo[4.4.llundecadienes (130) can be converted efficiently into the isomeric products (131) and (132) in the yields shown below the appropriate structure. Ring strain does not adversely affect the reaction since it is also possible to bring about ring closure of the bicyclo[4.2.llnonadienes (133) to afford the tricyclic products (134) and (1 35).69 The (2+2)-photocycloaddition of the lambertianate derivative (136) results in the formation of the cyclobutane adduct (137).70
5
Dimerization and Intermolecular Additions
5.1 Dimerization - As reported in Part 11, Chapter 1 the control of photochemical reactions in the constrained environment of a hydrotalcite clay as the supporting medium has been examined. This particular study examined the irradiation (A > 280 nm) of a mixture of 4-benzoylbenzoic acid and 2-phenylethenylbenzoic acid in this environment. While the regioselective formation of oxetanes was observed, dimerization of the phenylethenylbenzoic acid also takes Ph ph+R2
Ph R* Ph
(138) R' = R2 = pCeH4C02H
(139)
Ph
Ph
Ph Selectivity (YO)
Site distance
(A) X = e-C O 2 H X=C02H
5.4 6.2 8.2 5-4 6.2 8.2
67 59 42 66 39 14
0.8 4.9 11 1.6 2.9 18
30 34 42 0 0 0
Scheme 6
Phm \
P\
h
(140) a; R' = H = R2 b; R' CN, R2 = H C; R' = CN, R2 = Me I
Ph (141)
0.8 1.6 3.6 58 71 68
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
155
place and yields the cyclobutanes (138) and (139) in a total yield of 45%." The same group has demonstrated that the stereoselectivity of dimerization of 2phenylethenylbenzoic acid in clays is dependent upon the site distances. These distances can be controlled by varying the fraction of A13+ in the clay. The controlled dimerization of cinnamic acid was also studied. Typical results for the selectivity observed are shown in Scheme 6.7' The three crystalline dienes (140) all show red-shifted fluorescence. Only (140b) is photochemically active, however, and irradiation yields the cyclobutane derivative ( 141).73 Irradiation of the [18]-annulene derivative (142) results in the formation of a complex novel cyclophane in 68% yield.74The synthesis of pentalene (143) has been achieved by the irradiation of the dimer (144) in an argon matrix. The process occurs in two steps.75
6
6.1
Miscellaneous Reactions
Miscellaneous Rearrangements and Bond Fission Processes - Chan and have carried out calculations to confirm that the two electrons in the biscation (145) are delocalized three-dimensionally. A review has given details of specific photoreactions of acyclic and cyclic saturated hydrocarbons that can provide paths for their f~nctionalization.~~ A study of the SET-induced photochemical bond fission processes in the series of 2-alkoxyphenylethers (146), (147) and (148) has been studied. The reaction involves bond cleavage in the inter-
Photochemistry
I56
mediate radical cation. The research was aimed at investigating the features that controlled the bond cleavage proce~ses.~*
L
(145)
Photochemical carbon acid dissociation has been studied in considerable detail by Wan and his coworkers.79The current work has examined the photochemical behaviour of the dibenzocycloheptene derivatives (149). The influence of substituents on the process was studied. A review has highlighted the photochemical methods for the formation of carbanions." Within the general framework of photodeprotonation of benzylic systems Wan and his coworkers" have also reported the results of a study into the photochemical deprotonation of the thioxanthenium' salts (1 50)-(153). The reactions are carried out by irradiation at 254 nm in dry acetonitrile under an argon purge. Prolonged irradiation of compound (1 50), for example, results in the production of (1 54) in 60% yield. The authors" reason that this product is formed via the intermediacy of the R'
(151) X = 8F4(153) x = c104-
(149) R' = H, R2 r 0-9 R'=Me, 7 R' = H, 3 R'=Ph, 8
D.
D
Me
I
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
157
thiaanthracene (155) as a result of loss of a proton from the starting material. The thiaanthracene is presumed to be the species that provides the coloured solution on irradiation of (150). Indeed coloured solutions (Lax = 388 and 497 nm or compound 150) are obtained on irradiation of all the thioxanthenium salts (150)-(153). Prolonged irradiation, (i.e. a second photon process) is presumed to bring about rearrangement to the thioxanthene (154). Interest has also been shown in the formation and photochemistry of cations such as those produced from (156). These species are generated by the photolysis The reaction of the of 9-fluorene in 1,1,1,3,3,3-hexafluoro-i-propylalcohol.s2 cations with aromatic compounds has been studied. H OH
The paracyclophane (157) undergoes fission of the ethano bridge C-C bond when irradiated in an argon matrix at 10 K. Similar behaviour is reported for the analogue (158).s3 Carbon-carbon bond fission has also been studied within the epoxide (159). Irradiation of this at 266 nm brings about conversion into the trans-ylide (160). The quantum yield for the process was measured as 0.099.84A patent has described a method for the photochemical isomerization of a cis,transmixture of 1-(4-propylphenyl)-4-pentylcyclohexane to give the trans-isomer in 98% p ~ r i t y . ' ~
A,
Ph
Ph
Shi and Wans6 report that the three biarylmethyl alcohols (161), (162) and (163) undergo conversion into the corresponding pyrans (1 64),(165) and (166) on irradiation at 254 nm in acetonitrile or acetonitrile/water. The reaction involves the conversion of the diols into the corresponding quinone methide; for example, (167) is formed from (161) by formal loss of water. Cyclization of this quinomethide intermediate affords the final products. Previously Wan and his coworkers have described this reactivity for biphenyl derivative^.^^*^^ The quantum
Photochemistry
158
yield for the conversion of the highly twisted derivative (1 63) is reasonable at 0.17. The singlet excited state is thought to be involved in these transformations. Laserjet studies are very much in favour these days. This system of irradiation allows for multi-photon processes to occur. Adam and S ~ h n e i d e rhave ~ ~ studied the behaviour of the keto ether (168) in carbon tetrachloride under such irradiation conditions. Two processes were detected; one involving one-photon absorption leads to the aldehyde (169), while a two-photon process yields the chloro compound (170). Evidence has been gathered to suggest that a three-photon process leads to the formation of the ethoxy ether (1 71) when (168) is irradiated in ethanol. Nevi11 and Pincockgohave studied the photocleavage of the naphthalene derivatives (172). Within these the presence of radical clock systems has permitted a study of the lifetime of the species formed following C-0 bond cleavage.
P
f?
h
0
(172) R = CH2Br,CHzSMe, H 2 C - ge M e ,
e
C
H
O
TPh, Tph
IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
159
A two-photon process has been identified following the flash photolysis of 1,3dichloro-l,3-diphenylpropane,and this treatment yields the 1,3-biradical (1 73).35 Low intensity irradiation of the propane at 254 nm in cyclohexane permits trapping of intermediates and gives the products (1 74)-( 177). Clearly these arise
(178) X = Br or CI
Scheme 7 OH
Scheme 8
wcl CO/SmI,
* 0
light
\
*
Scheme 9
5
7
\
160
Photochemistry
from radical species. Adam and coworkers9' have described the stepwise photochemical formation of the biradical by two-photon laser-induced reactions of the halomethylnaphthalenes (178) (Scheme 7). Chloro compounds can also react photochemically in the presence of samarium diiodide. An example of this is shown in Scheme 8. The radicals obtained by the irradiation can be trapped by a ketone. Both the cyclized compound (179) and the open chain system (180) are formed in this reaction in a ratio of 29:71.36 Another example of the use of samarium iodide in photochemical reactions is that of alkyl halides with carbon monoxide at 50 atmospheres. The reactions are brought about using wavelengths > 400 nm. The outcome of the process is the formation of the ketones outlined in Scheme 9. The reactions are thought to proceed by way of acyl samarium species that dimerize. One of the acyl groups is reduced to a methylene to afford the final
product^.^' Fission of the Sn-C bond in the stannanes (181) takes place on irradiation in the presence of 10-methylacridinium salts. The process is induced by a single electron transfer and this affords the cation that leads to (182). The other product obtained in the reaction is (183).93
I
Me
Me
yield (YO) relative (18*)
(183) X = H X=Me X=CI
46 45 81
54
55 19
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161
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162
Photochemistry
42.
A. Mori, N. Kato, S. Morishita and H. Takeshita, Mol. Cryst. Liq. Cryst. Technol.,
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IIl3: Photochemistry of Alkenes, Alkynes and Related Compounds
163
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1869.
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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 theoretical studies of the photochemistry of thiiranes,’ photoaddition of amines to aryl olefins and arenes,2 the synthesis of heterocyclic compounds,3 photoamination directed towards the synthesis of heterocycles: selective addition of organic dichalcogenides to carbon-carbon unsaturated bonds,5 photocyclisation mechanisms of cis-stilbene analogues,6 synthetic utility of the photocyclisation of aryland heteroarylpropenoic acids,7 photochromic diarylethenes,8 spiropyrans,’ clophanes,” and polycondensed aromatics,’ photochromic organic media, photophysics and photochemistry of P-~arbolines,’~ and the photochemical synthesis of macrocycles. I4 Chirality switching by light has also been described. l 5
?;
2
Isomerisation Reactions
The photoinduced isomerisation of cis-stilbene has been studied by quasi-classical molecular dynamics using a procedure which includes all internal coordinates. l 6 A multidimensional view of the microscopic reaction mechanism is obtained and this reveals two channels one of which leads to trans-stilbene and the second of which leads to dihydrophenanthrene. Studies have been described of the photoisomerisations of styryl dyes having a benzodithia-18-crown-6fragment [I; X = s, R = (CH2)&02H],” and also of the crownophane (2a) to the corresponding cis isomer (2b).” Unlike (2b), (2a) is capable of binding Ag+ and this system, therefore, fulfils the basic requirements of a binary memory device. EIZ Photoisomerisation of the 7-arylidenenaltrexones (3; R = Ph, 4-PhC6H4, 1-,2-naphthyl, 9-anthryl) may have implications for opioid receptors.” The efficiency of the biacetyl-sensitised photoisomerisation of (Z,E)-3,3”,5,5”-tetra(t-butyl)-4’-styrylstilbene to (E,E)-3,3”,5,5”-tetra(t-butyl)-4‘-’styrylstilbene is greatly enhanced in the presence of anthracene and is explicable in terms of a more efficient energy transfer within a quantum chain process.2*Solvent polarity and viscosity effects on the photophysics and photochemistry of trans-3-styrylpyridineand the kinetics and mechanism of competitive deactivation processes in the singlet manifold have been discussed,2 and reversible cis-trans photoisomerisation of the photochromic surfactants 1-alkyl-4-(4‘-alkylstyryl)pyridinium halides (C,StzRX; n = 0,4,6,8; R Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
1Il4: Photochemistry of Aromatic Compounds
165
= Me, Et, CH2CH20H, n-Bu, n-CgH19;X = Br, I) has been observed for the micellised material.22 On prolonged irradiation, the thermodynamically least stable syn-head-to-head and anti-head-to-head dimers are formed. Excited cisstilbene decay rates show a linear dependence on solvent polarity and polarisability parameters, and solvent-dependent dipole moments have been calculated for an approximate stilbene transition state geometry by the polarisable continuum modeL2’ In polar solvents, the twisted intramolecular charge transfer states of 4-pe~uorooctylsulfonyl-4‘-N,N-dimethylaminostil~ne (PFSDS) and its bridged derivatives (PFSDS-023 and PFSDS-N34) are the lowest states,24 new evidence has been announced for the 90” twisted intermediate on the S1 surface of stilbene, and for synlanti isomerisation as a new rea~tion;~and quantum yields of trans-cis photoisomerisation and photophysical effects have been measured in styrenes containing such residues as anthracene, phenanthrene, pyrene, and chrysene.262-[2-(2-Pyrrolyl)ethenyl]quinoxalineis reported to undergo a one-way trans-cis photois~merisation.~~ Irradiation of cis,trans- or trans,trans-l,4-diphenylbuta-l,3-diene in the presence of 9,lO-dicyanoanthracene promotes geometric isomerisation, and in benzene or acetonitrile solution the photostationary state is composed of more than 94% of the trans,trans isomer.28In benzene, the reaction proceeds via a triplet state, but by contrast in acetonitrile a cation radical mechanism is involved. A study of the photophysics of 4-dimethylamino-4’cyanostilbene and 4-azetidinyl-4‘-cyanostilbenereveals that the intramolecular charge transfer excitation involves at most an intermediate whose lifetime is less than 1 ps.29 Evidence is presented which suggests that the photoisomerisation transition state is less polar than the charge transfer state. cis-trans-Isomerisation and cross-linking reactions of 2,3,7,8,12,13,17,18-octakis[(4-methoxycinnamoyl)oxyalkylthio]tetraazaporphyrins having polymethylene chains of different lengths (3,6, 8, or 11) have been investigated in solution and in spin-coated films, and it has been found that the efficiency of the photoisomerisation and crosslinking of the cinnamoyl group is lower with shorter chain lengths.30 Recent observations suggest that in nematic liquid crystals having cinnamate side-chains, the mechanism of azimuthal reorientation induced by linearly polarised light results from polarised EIZ photoisomerisation of these cinnamate side chain^.^' truns,trans,trans- 1,6-Bis(p-formylphenyI)hexa-1,3,5-trienes are stereoselectively photoisomerised to the cis,trans,trans-isomer, and decreases in solvent polarity also cause the quantum yields of the reaction to fall.32 However, those of the reverse reaction increase, and this leads to the cis,trans,trans- to trans, trans,transisomerisation being ‘one-way’ in nonpolar solvents. Adsorption of a,o-disubstituted a,o-diphenylalkanes within acid ZSM-5 and mordenite zeolites leads to the formation of 1,3-diphenylpropenyliumand 1,5-diphenylpentadienyliumcations.33 Irradiation of these species induces to cis-trans isomerisation as the only observable process. Reports have appeared of the photoirradiation of cis-1-(4‘-propylphenyl-4-pentylcyclohexanewhich gives the trans isomer with 98% of the triplet sensitised ZIE isomerisation of N-methoxy- 1-(1-pyrenyl)methanimine about the C=N bond, for which a mechanistic discussion is presented in terms of a dual two-way and one-way mode,35and of the irradiation-induced isomerisation of trans,trans-[2,2][2,2’]azobenzenophane(4)to the trans,cis- and cis,cis-is~mers.~~
Photochemistry
I66
HO & R
(3)
0
Details of the photoinduced electron transfer isomerisations of 3,3-dimethyl1,1,5,5-tetraarylpenta-1,4-diene~,~~ of the photoconversion of the colourless photochromic cyclophanediene (5) into the green dibenzannulene (6),38 and of the electron transfer electrocycloreversion of benzocyclobutenols to o-quinodimethane intermediates which themselves are equilibrated with benzophenones have all been announced.39Direct or acetone-sensitised irradiation of substituted 1,1-diarylspiropentanes produces various methylidenecyclobutanes probably via the intermediacy of diradi~als.~'It has also been reported that charge transfer band excitation of the donor/acceptor complexes formed between the substrates and tetracyanoethylene similarly induces skeletal rearrangement with production .0z7']deca-4,8-dien-3of TCNE adducts. Irradiation of 8,9-diphenyltricyclo[5.2.1 one, a compound possessing both cyclopentenone and stilbene chromophores, induces singlet exciplex formation and a novel [ 1,3]-sigmatropic rearrangement via a singlet state as well as [2+2] intramolecular cycloaddition via a triplet state.4' 1 l,12-Dibenzoyl-9,10-dihydro-9-(hydroxymethyl)-lO-methoxy9,lO-ethenoanthracene (7a) in benzene or methanol is photolysed to the dibenzopentalene ketone (8a) via a dibenzosemibullvalene, and 11,12-dibenzoyl-9,10dihydro-9-(1-hydroxyethyl)-lO-methoxy-9,10-ethenoanthracene (7b) which exists in equilibrium with its cyclised form gives a mixture of dibenzopentalene ketone (8b) and a dibenzopentalenopyran d e r i ~ a t i v e . ~ ~ In the solid state photorearrangement of cis-l,2-dibenzoylalkenes,intramolecular carbon to oxygen phenyl migration is reported to be controlled by synlanti conformational constraint^:^ and in the photolysis of trans-2,3-diphenyloxirane, quantum yields for formation of the trans ylid, cis-2,3-diphenyloxirane,benzaidehyde, and deoxybenzoin have been measured.& This reaction gives both orbital symmetry-allowed and -forbidden products. Photolysis of 2,3-diaroyiaziridines
167
IIl4: Photochemistry of Aromatic Compounds
*H
Me0
-
R
I
&Ph
and 2-aryl-3-aroylaziridines induces ring opening and formation of an azomethine ylid, which in the presence of DMAD is followed by stereospecific formation of 3-pyrroline derivative^:^ and the vinylaziridine (9) has been Related isomers also undergo this transformation phototransposed into ( and regioselective cyclisation of the ylid is observed when the trifluoromethyl group is attached to the aziridine ring. In the presence of methoxide or diethylamine, photolysis of 1-(2-azido-6-chloropyridin-4-yl)-3-phenylureainduces nitrene formation followed by ring enlargement to give a 1,3-diazepine, addition of one molecule of nucleophile, and nucleophilic substitution of the C1 atom.47 The absorption and emission spectra of 2-(2'-hydroxyphenyl)benzimidazole are different in water and in ethanol and are also dependent upon the acidity of the medium?' At pH 7, there is thought to be a conformational equilibrium between the cis-enol which displays intramolecular H-bonding and the trans-enol which shows H-bonding to the solvent. Excitation of the cis-enol gives the excited keto tautomer by intramolecular proton transfer. The mechanism of the photoinduced reactions of oxime 0-ethers derived from N-substituted 3-acyl-l,2-dihydrocinnoline-l,2-dicarboximides has been shown to involve N-N cleavage of a polar excited state followed by skeletal rearrangement, and in particular (1 1; X = H, C1, MeO; R' = Me, Pr; R2 = Me, Et) gives (12; same X, R',R2).49Irradiation of the (2)-3-(2-phenylimidazo[1,2-a]pyridin-3-y1)- 1,3-diphenylprop-2-en-1-ones (13; R' = H, Me, Ph; R2 = H, Me, I; R3, R4 = H, Me) gives the (E)-isomers and (E,Z)mixtures of N-(pyridin-2-yl)-[(3,5-diphenyl-2-furanyl)phenylmethylene]amines (14; same R' - R4), but only furanoic derivatives are obtained from 5-Me, 7-Me, 8-Me, and 5-Ph derivatives of (2)-3-(2-phenylimidazo[1,2-a]pyridin-3-y1)-1,3diphenylprop-2-en-1 The mechanism of this photoisomerisation has been discussed in terms of semiempirical quantum calculations. Direct irradiation
168
Photochemistry
of 3-alkoxy[2,l]benzisoxazolequinones(15; R' = MeO, R' = Ph, X = NH) and the 3-N,N-dimethylamino[2,l]benzisoxazolequinone (1 5; R' = Me2N) causes rearrangement to the corresponding y-cyanoalkylidenebutenolides (16; same R , R2) in a transformation which in some cases proceeds highly stereoselectively and in which a triplet state is thought to be involved.52Irradiation of 2-[2-(2-vinylpheny1)ethenyllfuran and 5-methyl-2-[2-(2-vinylphenyl)ethenyl]furan gives 9,1O-dihydro-4,9-methano-4H-benzo[4,5]cyclohepta[ 1,2-b]furan and 9,l O-dihydro2-methyl-4,9-methano-4H-benzo[4,5]cyclohepta[ 1,2-b]furan re~pectively,~~ and in alcoholic solution 10-hydroxyanthrone undergoes both phototautomerisation to anthrahydroquinone and photoreduction to 9-hydro~yanthracene.~~ By contrast, in hydrogels neither is formed and the only transformation is photooxidation to anthraquinone. Studies of ethylene-bridged porphyrin dimers such as trans-1,2bis(meso-octaethy1porphyrin)ethene imply that quenching of the first excited singlet state of symmetric close-spaced porphyrin dimers induces internal rotation of the porphyrin planes, and leads to a photoisomerisation-like photophysics; no cis-trans isomerisation is observed.55 Irradiation of S-(0-tolyl) o-benzoylbenzothioate (1 7 ) in the solid state gives optically active 3-phenyl-3-(o-t01ylthio)phthalide (18) by a concerted or stepwise mechanism involving a zwitterionic intermediate (19) and which collapses by phenyl m i g r a t i ~ n . ~ ~ a-Cuparenone (20) has been synthesised using the photoconversion of the a-halo arylketone p-MeC6H4COCHC1Meto 1-methyl-1-(p-toly1)acetic acid as the key step.57 A potential energy surface of the singlet state for the interconversion of 2-hydroxypyridine and 2( 1H)-pyridinone has been obtained using ab initio methods.58The results confirm that the photoinduced dissociation mechanism is probably responsible for the excited state tautomerisation in this system. Fluorescence kinetics have been reported for the effect of solvent polarity on the photoinduced long-range intramolecular proton transfer from the hydroxyl group in 7-hydroxy-8-(N-morpholinomethyl)quinoline to the ring N atom,59 and ab initio methods have been applied to the methylimine to vinylamine isomerisation as a model for the proton transfer in the photoinduced isomerisation of 2(2',4'-dinitroben~yl)pyridine.~' Excitation of benzil in the presence of 3-hydroxyflavone in fluid solution leads to formation of the triplet state of the flavone from which a green fluorescence can be observed and which is associated with the lowest singlet state of the phototautomer.6' Photoinduced intramolecular proton transfer from HO to N in 2-(2-hydroxyaryl)pyridinesoccurs in planar conformers at 77 K and has been studied by semiempirical quantum chemical methods,62 pK, values of intramolecular proton transfer reactions of 2-(2-hydroxyphenyl)benzimidazole have been studied in the ground and excited states and at differing concentrations of SDS, HTAB, and TX- 100,63 and photoinduced non-linear effects in organic photochromics have been discussed.@ In particular, an analytical expression describing two-wavelength light propagation through an organic photoreversible photochromic azo- and stilbene-like molecular medium has been derived. Considerable interest is currently being shown in photochromism. This phenomenon has been observed in the cyclised products (21), (22), (23), and
'
Iil4: Photochemistry of Aromatic Compounds
169
Ph
Ph
(11)
(9)
Men S
“OR2
Me
I
Ph (17)
0
Me
(24),65 and irradiation of optically active l,l’-bi-2-naphthol gives an optically active intramolecular addition product which on further irradiation reverts to the original compound; this provides evidence for the existence of a photochromic equilibrium between asymmetric molecules.66Photoinduced proton transfer in 2(2’,4‘-dinitrobenzyl)pyridine and derivatives proceeds with formation of a blue
170
Photochemistry
Phfifo Ph
mo CH2C02H
0
‘Ph
H Ph
Ph
(23)
C02H
(24)
tautomer which has the ‘enamine’ structure, and for the phenanthroline analogue stabilisation of the corresponding phototautomer is sufficient to make it thermally accessible.67 Some new Schiff bases derived from hydroquinone and having internal hydrogen bonds are reported to be photochromic,68 and in an investigation of salicylideneaniline (25) and the more rigid phenylimine of 7-hydroxyindanine-1 (26) in liquid solvents, excited state intramolecular proton transfer is followed by twisted intramolecular charge transfer.69 Unlike (25), (26) exhibits no photochromism and it is suggested that the photocoloured form of (25) originates from a concerted diabatic process competing with anomalous Stokes shift fluorescence. The spiropyranindolines (27; R’ - R4 = H, alkyl; Rs, R6 = H, alkyl, nitro; X = 0; Y = alkylene) have been prepared and [27; R’ = R2 = R3 = R4 = R5 = R6 = H; Y = (CH2)3; X = 01 are p h o t o c h r ~ m i c .The ~ ~ thermoreversible photochromic reaction, spiroindoline-oxazine to photomerocyanine is photosensitised by diffusion controlled triplet energy transfer from camphorquinone using visible r a d i ati~n ,~’the photochromism of spiropyrans prepared from 1,2,3,3tetramethylindole iodide has been described,72 the effects of complexation of indoline and phenanthroline spiropyrans with transition and rare earth metal ions on the photochromic and other properties of spiropyrans have been reported,73and the chromenes annulated with a furan [28 R’= (CH2)4, R2 = Me; R’ = (CH2)4, R2 = Ph; R’= (CH2)5, R2 = Me; R’ = (CH2)5, R2 = Ph] have been found to be p h o to ~hr om i c.A ~ ~number of reversible photochromic naphthopyrans have been studied which are substituted at position 3 of the pyran ring by an aryl group, or by a phenyl residue having a 5- or 6-membered heterocyclic ring fused at carbons 3 and 4 of the phenyl ring, and at position 6 of the naphthyl portion of the naphthopyran with a N-containing het er~cycl e.Comparison ~~ of the photochromic properties of spiro[2,3-dihydronaphtho[1,8-de][1,3]thiazine2,2’-[2H]-chromenes, (29; R = H, 7’-NEt,, 8’-OMe) with those of (30) and (31) has shown that whereas in alcoholic solution the latter two compounds are largely converted into their coloured form, (29) is present to only about 3% in its coloured form.76 2-Phenyl-2-(2-naphthyl)(naphtho[1,2-b]pyran) derivatives and their heterocyclic analogues have been prepared and are found to be useful as photochromic compounds.77 The photochromism of 1’,3‘,3’-trimethyl-6-nitrospiroE2H- 1-benzopyran-2,2’-indoline has been examined in water using vesicles and in y-cyclodextrin, and it has been found that the photochromic reaction is slower in the presence of y-cyclodextrin than in vesicles, but that it is faster in methanol.78 Prolonged irradiation induces simultaneous photoreaction and polymerisation leading to an unknown side reaction. Photochromism and photo-
171
IIl4: Photochemistry of Aromatic Compounds R' Ph
isomerisation of derivatives of 4-aryl- and 4-methyl-2,3,4,5,6-pentaphenyl-4Hthiopyrans and 4-aryl- and 4-methyl-2,4,6-triphenyl-3,5-dimethyl-4H-thiopyrans have been studied.79 The first group of compounds show no photochromic properties; neither do they photoisomerise to the isomeric 2H-thiopyrans, whereas the second group gives isomeric 2H-thiopyrans and are photochromic. Photochromic behaviour of substituted 6-aroyl-3,5-diarylspiro[cyclohexa-2,4dien-l,2'-indolines] is analogous to that of spiropyrans," and a laser flash photolysis study of the photochromism of the bispyrryl-substituted ethenes 2,3bis( 1-p-methoxyphenyl-5-phenyl-2-methyl-3-pyrryl)but-2-ene (32) and 2,3-bis( 1p-bromophenyl-4-phenyl-2-methyl-3-pyrryl)but-Zene (33) reveals that the photocyclisation occurs by an excited triplet state, that cis-trans isomerisation occurs by an excited singlet, that (32) undergoes both photocyclisation and cis-trans isomerisation, and that (33) reacts mainly by photocyclisation.81 Substituents exert a significant influence on the photochromic processes of pyrryl-substituted fulgides and fulgimides8* Steady state kinetics show that the photodecoloration of 7,7a-dihydroindole is first order, and that structures having substituents at the 2- or 3-position have larger rate constants for decoloration than non-substituted compounds. Indole fulgides having an adamantylidene fragment have been prepared as 2 and E isomers, and 2-E photoisomerisation of these products is accompanied by cyclisation to a coloured product.83 The kinetics of photochromism of some spirooxazines (34; R' = C18H37, R2 = H, R3 = H; R' = C18H37,R2 = C02CH3, R3 = H; R' = C18H37, R2 = H, R3 = Br)) have been studied and laser flash photolysis shows that cleavage of the CO bond in (34) proceeds with formation of the photomerocyanines (35) after 1ps.84 Molecular mechanics and PPP-CI semiempirical quantum chemical calculations of the photochemical ring opening of spirooxazines indicate that it is the carbonyl structure which is
Photochemistry
172
formed.85In the photochromism of pyrryl substituted fulgides the main excited state is S1 but the triplet state is also known to participate, whereas in the case of fulgides the triplet state plays no role.86The 2’-substituent in spirooxazines exerts a large effect on the formation of the photoproduct as well as on the mechanism, by causing an increase in steric hindrance at this position, and leading to a consequential decrease in quantum yield for photochromic merocyanine. Spiro[indoline-naphth[1,2-b]oxazines]have been prepared by reaction of Fischer’s base with oximes of 4-sulfanyl substituted- 1,2-naphthoquinones,” the indolyfulgide having a 1-( 1,2-dimethyl-3-indolyl)-2,2,2-trifluoroethylidene group on the succinic acid moiety exhibits high resistivity to repetitive photochromic transformations in toluene and PMMA films,88 and a pyrryl substituted photochromic fulgide has been synthesised and used to prepare optical discs by spin coating.89
3
Addition Reactions
Photoaddition of cyclopentene to Schiff bases has been reported to be catalysed by the heterogeneous semiconductor CdS.90 In this reaction the powder suspension of CdS is thought to behave as an electrochemical cell, but with the reductive and oxidative reactions of the adsorbed substrates occurring simultaneously at a single crystal particle. Irradiation of 3-phenylisochromene (36; R = H) in methanol promotes solvent addition to the C3-C4 double bond to give (37), but under similar conditions, (36; R = Me, Ph) gives (38).91This is rationalised in terms of the importance of steric interactions between the substituents at C3 and C4 which influence the photoinitiated ring opening reaction. Polycrystalline mixtures of diphenylamine and trans-stilbene form a C-N bonded photoadduct and a C-C bonded photoadduct by reactions thought to occur at component crystallite interfaces; in the case of phenanthrene, C-N and C-C bonded adducts are also formed.92 Photolysis of 2-phenylseleno-l,3-dithianein the presence of electron-deficient alkenes produces addition products, and under these conditions for example, butyl acrylate gives (39).93 Irradiation of mixtures of meso- and rac- 1,2-diethyl-1,2-dimethyldiphenyldisilane and isobutylene gives (R,S)- and (S,R)-2-(isobutylethylmethylsilyl)- 1-(ethylmethylphenylsilyl)benzene with high diastere~specificity.~~ Other related observations are reported together with theoretical studies on PhSiHzSiH3. Photohydration of l-(m-nitrophenyl)-5,5dimethylhexa-l,3-diyne gives the allenyl ketone (40)which itself can be thermally
M4: Photochemistry of Aromatic Compoundr
173
hydrated to the corresponding P-dicarbonyl compound.9s The mechanism of the photochemical step involves a synchronous addition of water in which protonation occurs at C1 implying a reverse polarisation in the triplet excited state compared with the p-nitro analogue. Methoxy-, methyl-, and chloro-substituted benzonitriles are reported to photoadd to naphthalen-1-01giving as substitution products the 2-(aminophenylmethylene)-l(2H)-naphthalenones (41; R = H, MeO, Me, C1) by attack of the cyano group on the aromatic ring of naphthalen-1-01.~~ The efficiency of the reaction depends on the position of the substituents, and the p-substituent is reported to give the highest yields.
9-Anthraquinonecarboxamide in its SI state will photoadd two molecules of water.” The photoreactions of diphenylhomobenzoquinones in the presence of amine donors have been studied.” 1-Bromo substituted diphenylhomobenzoquinone irradiated in the presence of triethylamine induces opening of the cyclopro1,6benzoquinone, and pane ring with formation of 2-diphenylmethyl-5-methylwith dimethylaniline, a mixture of an aminated bicyclic dione and bis(p-dimethylaminopheny1)methane is produced. Irradiation of boron difluoride complexes derived from 1,3-diketones can lead to a number of different types of process, in particular exciplex formation and slow cy~loaddition.~~ Evidence has now been advanced to suggest that excitation of these complexes leads directly to an exciplex without the participation of an encounter complex, and that this solvolyses to free ions. A range of 1:1 adducts together with an acetylene reduction product is formed when 3-phenyl-4-phenylethynylcyclobut-3-en-tr~ns1,2-dicarboxylic acid dimethyl ester is irradiated in Evidence is presented to show that the
Photochemistry
174
reduction product arises from the triplet state, and that the other products arise via a pathway which involves a singlet excited state, an exciplex, and a carbocation. Visible light irradiation of alkylbenzenes in the presence of the 10methylacridinium ion in aqueous acetonitrile gives 9-alkyl- 10-methyl-9,lO-dihydroacridine. lo' By contrast, in the presence of perchloric acid 10-methyl-9,lOdihydroacridine is formed along with benzyl alcohols derived from oxygenation of the substrate. Photophysical studies reveal an electron transfer reaction from the alkylbenzenes to the excited singlet state of the acridinium ion to give the radical cation of the alkylbenzene. Irradiation of silyl enol ethers and 2'-nitroand 2,2'-dicyanostyrenes causes diastereoselective Michael addition, but apunsaturated carbonyl compounds are, however, only cis-trans isomerised.lo' The transformation occurs by regiospecific addition of the silyl enol ether to the excited Michael acceptor to give a zwitterion. Reports have also appeared of the mechanism of the CdS-catalysed photoaddition of 2,Sdihydrofuran to azobenzene, lo3 model studies involving additions to pyrimidine dimers,Io4 and the synthesis of fullerenes using laser pyrolysis of benzene in a system containing O2 with SF6 added as energy transfer agent. A stereocontrolled approach to substituted cyclopentanones has been described involving an intramolecular [2+2]photocycloaddition and in which the product cyclobutane can be converted to cyclopentenone.Io6 Intramolecular photocycloaddition of dihydropyrones to alkenes (42)followed by fragmentation, aromatisation, and annulation gives (43)and this constitutes a useful pathway to fused ring phen01s.l~~ Irradiation of the methanolic pyran-2-ones (44;R = Me, Ac, Me3CC0, tosyl, CF3S02) connected to a pendant furan by a three-carbon tether gives endo and exo [4+4]cycloadducts. lo8 This transformation is potentially useful as an approach to the fusicoccane/ophiobolane skeleton, for example in the conversion of (45) into (46). The 2-methoxyoxazolo[5,4-b]dihydroazocine (47;R' = R2 = H;R' = H, R2 = Me; R' = Me, R2 = H) has been prepared by regio- and stereospecific cycloaddition of acrylonitrile and cis- and trans-but-2enenitrile to 2-methyloxazolo[5,4-b]pyridinefollowed by ring opening. '09 18-Crown-6-ether trans-styryl dyes and the 15-crown-5-ether analogues in the
m
175
IIi4: Photochemistry of Aromatic Compounds
presence of Ca(C10)z and in acetonitrile solution will participate in competing reactions, including cis-trans photoisomerisation, formation of ion-capped complexes, and [2+2] autophotocycloaddition, to give cyclobutanes. l o The regioand stereoselectivity as well as the efficiency are determined by the type of preorganisation of the dye trans-isomers brought about by the presence of the Ca2' atoms. Solid state irradiation of 2,5-bis(vinyl)-l,4-benzoquinone (48; R = Ar, ester) (49; R = Ar, ester) promotes [2+2] photopolymerand 2-vinyl-l,4-benzoquinone isation for R = o-tolyl only, although if R = COOEt, photooligomerisation occurs. Bis-cinnamates will photocycloadd to give the macrocycles (50) and (51) as single stereoisomers although analogous reactions of cis-enediyne biscinnamate are less efficient.lI2 Irradiation of 14GP (naGP and nPGP = a and p anomers respectively of 1,2,3,4,5-penta-O-(trans-3,4-dialkoxycinnamoyl)-(D)-glucopyranose; n, the number of C atoms in the alkyl chain, is 6 - 8, 10, and 14) in dilute solution induces [2+2] cycloaddition between cinnamoyl double bonds. l 3 Two solid phases are known and in one of these there appears to be a preference for intramolecular cycloadditions, whereas in the second solid phase unbranched oligomers are formed. Oligomer formation is discussed with reference to inter-/ intramolecular and inter4ntracolumnar competition for [2+2] cycloadditions. Oxetanes are formed regioselectively by irradiation of 4-benzoylbenzoate (52) and 4-(2-phenylethenyl)benzoate in the presence of hydrotalcite clay, but by contrast (52) and cinnamate give p-truxinate.' l4 These observations have been rationalised in terms of favourable C=O/C=C distances by packing in clay interlayers. The same workers also report that carboxylates undergo [2+2] photocycloaddition and control of the stereoselectivity can be exercised by changing the site distances in the hydrotalcite clay interlayers."5 The pseudogem-cinnamophane dicarboxylic acid bis-4,15-(2-hydroxycarbonylvinyl)[2.2]paracyclophane (53) undergoes a stereospecific[2+2] cycloaddition in the solid state to give 100% yield of the corresponding truxinic acid,Il6 and irradiation of a series of bis-coumarinyl derivatives having alkyl chains of various lengths in the presence and absence of P-cyclodextrin has shown that photodimerisation and excimer formation are possible in the respectively favourable environments.' I 7 The [2+2] photocycloaddition of the olefin moieties in the amide derivatives of p-phenylenediacrylic acid leading to polymerisation of the acrylic monomer has been controlled by hydrogen bonding in the solid state."' No reaction is observed in either 'ribbon', 'sheet', or 'tape' networks, but in planar conformations cycloaddition can occur between adjacent networks. [2+2] Photocycloaddition of ap-enoates using (54) as chiral auxiliary has enabled (55) and (56) to be prepared and this methodology has also been used for asymmetric dimerisations leading to such products as (57) and (58).'19 Irradiation of 3-unsubstituted benzoxazole-2-thione in the presence of alkenes gives of 2-alkylated benzoxazoles; these reactions proceed via the intermediacy of aminospirothietanes which arise from [2+2] photocycloaddition of benzoxazole and the double bond of the alkenes.12* Allenylsilanes will photocycloadd intramolecularly to give high enantiomeric excesses of exo-methylenecyclobutanes which are capable of being protodesilylated; for example (59) has been prepared. l z l
'
'
aR
176
R
Photochemistry
\
0 (48)
0 (49)
PPh2 PPh2
OMe
I
:
An examination of the conical intersections and radicaloid intermediates on the potential energy surface of the photocycloaddition of ethylene to benzene has shown that one carbon-carbon bond is formed to give a low energy conical intersection and that the reactivity is controlled by this single conical intersection whose geometry is a distorted tetrahedron.' 22 The decay dynamics of this conical intersection indicate that one may form either ortho, meta, or, para cycloadducts. In a study of the photochemistry of Dewar benzenes bridged at the 1,4- and 5,6positions and substituted at the 2,3-positions, it has emerged that simple alkyl substitution leads to minor quantities of prismanes.'23 However, phenyl substitution at positions 2 or 2,3 gives doubly bridged prismanes in high yield.
IIl4: Photochemistry of Aromatic Compounds
177
Intramolecular meta photocycloaddition of 2-MeC6H4CH2CMe2CH2CH:CH2 provides appropriate tetracyclic precursors from which the key tricyclic framework required for the synthesis of (+)-ceratopicanol can be obtained.'24 This constitutes an example of a typical holosynthon. N-Benzoyl-N-benzylcinnamamides and related compounds will photocycloadd in the presence of benzil to give 3-azatricyclo[5.2.2.01~5]undeca-8,10-dien-4-ones with high stereoselectivity, and this constitutes the first example of a photochemical [4+2] cycloaddition of an enone to a benzene ring.'25 For example, irradiation of (60; R' = H, alkyl, phenyl; R2 = H, Me; X = H, OMe, C1, etc.) gives (61; same R', R2, X). cis,cis,cis,trans-[5,5,5,5]-Fenestrane (62) has been synthesised by an arene-alkene photocycloaddition-radicalcyclisation cascade involving (63) which leads to the formation of five new rings, 126 and regioselectivity in the intramolecular cycload' ~ ~ Photodition of double bonds to triplet benzenes has been d i s c ~ s s e d . [6+2] cycloaddition has been observed in systems containing a 2,3-diazabicyclo[2.2.lJhept-2-ene,as for example the conversion of (64;X = CHZ) into (65; same X),but not in systems with a 2,3-diazabicyclo[2.2.2]oct-2-ene unit, and this has been attributed to several factors, including the hypochromicity of the nn* state in the former and the higher ground state energy of this compared with the latter. I 28 Cyano-substituted phenols will undergo 2,6-photocycloaddition of cyclopentene to produce 2- and 4-cyanobicyclo[3.2.l]oct-2-en-8-onesbut with low selectivity.12' 3-Trifluoromethylphenoland cyclopentene give stereoisomeric and the specificity and stabi1-hydroxy-2-trifluoromethyldihydrosemibullvalenes lity are a consequence of intramolecular H-bonding between the HO and CF3 groups. 1,l -Diphenylethylene quenches 4,4-dimethylcyclohexenone triplets to give a transient whose physical data indicate it to be the triplet exciplex of 1,ldiphenylethylene and 4,4dimethylcyclohexenone.I3O It is suggested that this transient is involved in the photocycloaddition. Photosensitised Diels-Alder reaction of alkenylcyclopentadienes and a styrene-like dienophile linked by an alkyl chain proceeds by a [4+2] reaction if the chain length is three carbons or more in length, but in the case of chains of only two carbons, cyclisation is preceded by a [1,5] hydrogen shift.I3'
178
Photochemistry
Irradiation of 2-alkoxy-3-cyano-4,6-dimethylpyridine with methacrylonitrile 32 gives 6-alkoxy-3,5-dicyano-2,5,8-trimethyl-7-~abicyclo[4.2.O]octa-2,7-diene, and photoequilibration of benzo/pyridazino cyclobutane-photo[6+6]cycloadducts has been established, and occurs through a syn-periplanar arrangement. 1 3 3 The products are sufficiently stable for pagodane formation through cycloaddition of dienophiles. Photocycloaddition of furo[2,3-c]pyridin-7(6H)-one and its N-methyl derivative to acrylonitrile gives an adduct at the carbonyl oxygen atom and 8-cyano-8,9-dihydrofuro[d]azocin-7(6H)-one~134 and under similar conditions, 2,5-dihydro- 1H-pyrrole-2-thiones reacts with alkenes to form spiroaminothietanes which decompose to 2-alkylated p y r r ~ l e s . ' ~The ~ cis-syn 2carbomethoxypsoralen furan-side thymidine monoadduct (66) has been synthesised in which the key step is an intramolecular [2+2] photocycloaddition whose stereochemical outcome is biased in favour of the desired cis-syn product. 136+137 In the presence of thymine, Kemp's imide-linked coumarin (67) leads to the formation of the cis-syn (68) and cis-anti (69) cross adducts whose yields and ratios are much higher in benzene than in acetonitrile.'38 Use of 7-acetoxycoumarin gives only the cis-anti cross adduct. Irradiation of benzoylthiophene at hirr > 300 nm in the presence of mono- and dimethylmaleic anhydride promotes [2+2] cy~loaddition,'~~ and 3-acetylbenzo[b]thiophene will photochemically add 2morpholinopropenenitrile regioselectively but not stereoselectively to give the adduct (70); the isomeric adduct (71) is similarly formed from 2-acetylbenzo[b]thiophene and two isomeric [2+2] photoadducts are also produced using Me3CSC(CN)CH2.I4O Photocatalytic co-cyclisation of benzonitrile and acetylene has been achieved in a solar photoreactor to give 2-phenylpyridine.14' Irradiation of 2-[(alkenyloxy)methyl]-1-naphthalenecarbonitriles (72; R' - R3 = H, Me) in the presence of Eu(hfc)3 gives [2+2] intramolecular cycloaddition products at the 1,2-position in good yields in a process which proceeds via intermediate exciplexes.142 Diels-Alder reaction of norbornadiene and 1-cyanonaphthalene leads to the formation of four [2+2] cycloadducts in which addition occurs to the exo face of the norbornadiene on either ring of the electron acceptor.'43 In more polar solvents the reaction is more selective as characterised by enhanced anti-addition to the substituted ring, and this is rationalised by collapse of short-lived encounter complexes of differing geometries. Mixtures of furan and 1-naphthalenecarbonitrile irradiated in a Pyrex container give the [4+4]-endo-cycloadduct (73) and the [2+2]-syn-cycloadduct (74), the latter of which is thought to arise by preferential formation of the [4+4]-exo-cycloadduct (75) over (73) as a consequence of secondary orbital interactions in the singlet state, followed by a facile Cope rearrangement.'& These conclusions have been confirmed by studies at low temperature. [2+2]-Photocycloaddition of 4-hydroxy1-phenyl[1,8]naphthyridin-2(1H)-one with various alkenes in methanol gives the corresponding head-to-tail adducts regioselectively.'45Photolysis of their hypoiodites promotes regioselective scission of the non-ring junction bond of the corresponding alkoxy radicals to give substituted 3,9-dihydro-9-phenylfuro[2,3b][1,8]naphthyridin-4(2H)-ones and other products. Solutions of c6()and anthracene in benzene irradiated at wavelengths in excess of 500 nm give a 1 : 1 cycloadduct which is formed by a triplet e ~ c i p l e x ,and ' ~ ~ details have appeared of
179
1114: Photochemistry of Aromatic Compounds
2 0
\ /
the [2+2]photocycloadditions of tetrahydrodianthracene to alkenes and cycloalkenes to give bianthraquinodimethanes. '41 3,6-Divinylphenanthrene and 4,4dimethoxy-3,3'-divinylbiphenylare photoconverted into [2.2](3,3')-biphenylo(3,6)phenanthrenophane, a compound in which emission from the biphenyl fragment is quenched by intramolecular energy transfer to the phenanthrene fragment.'41 CN
& CN
Electron-rich olefins such as vinyl ethers and stilbenes will photocycloadd to various 1,6benzoquinones to give a spirooxetane, which following dienonephenol rearrangement, gives a dihydrobenzofuran.'49 Irradiation of o-benzoqui-
180
Photochemistry
nones with vinyl ethers gives 2-alkoxy-7-hydroxy-2,3-dihydobenzofurans (76) via a regioselective [3+2] photocycl~addition,'~~ and in the presence of 1,4-diphenylbut- 1-en-3-yne, tetrachloro- or tetrabromo-o-benzoquinone gives 9-phen ylphenanthrenes or their precursors such as C6C14(Ph)(CCPh)-1,2. ''I Photoaddition of p-quinones such as naphthoquinone to 1,bdiphenylbut- 1-en-3-yne forms a spirooxetene which following rearrangement to a quinone methide produces an oxidative photocyclisation product.'52Quinones such as 2,3dichloronaphtho-1 ,$-quinone and anthrmne-9,lO-quinone give another type of quinone methide. 2-Acetoxynaphtho- 1,4-quinone undergoes [2+2] photocycloadFollowing hydrolysis dition to 2-methylpropene to give a cyclobutanol a~etate.''~ to the cyclobutanol and conversion into the hypoiodite, irradiation in the presence of Hg(I1) oxide-iodine/benzene promotes scission of the cyclobutanoxyl radical to 2,3-dihydronaphtho[2,3-b]furan-4,9-dioneand its [1,2-b]furan-4,5dione isomer. Kim et al. also report on the photocycloaddition of 9,lOphenanthrenequinone and acenaphthenequinone to some conjugated molecules involving double and triple bonds.lS4 The reaction pathways seem to be governed by steric and electronic factors as well as by considerations of bond angles. OH
4
Substitution Reactions
Pseudosaccharin 3-alkyl ethers undergo solvent photosubstitution to substituted pseudosaccharin alkyl ethers whereas the analogous 3-ally1 ethers suffer homolysis of the ally1 ether-oxygen bond resulting in products of nucleophilic substitution of the allyloxy group by solvent.'" C-Arylation of ketones, nitriles, and esters has been achieved by direct hydrogen nucleophilic aromatic photosubstitution of m-dinitrobenzene in a process which is promoted by fluoride ion.'56 A study has been made of factors controlling the regiochemistry of photo-nucleophile-olefin combination reactions of 1,Cdicyanobenzene in which the nucleophile and olefin are methanol and 4-methylpenta-1,3-diene respectively. This transformation proceeds by photoelectron transfer to the aromatic, and the major adduct arises from attachment of methanol to the end of the alkene or diene giving the more stable intermediate radical. The observations are consistent with the results of ab initio MO calculations. In some related work, irradiation of a mixture of 1,4-dicyanobenzene and 2,3-dimethylbut-2-ene promotes allylation of the aromatic ring, but in the presence of nucleophiles such as water or methanol, however, a nucleophile olefin combination-aromatic substitution
IIl4: Photochemistry of Aromatic Compounds
181
occurs. 158 This reaction probably involves competing deprotonation and nucleophile addition to the olefin radical cation followed by coupling with the radical anion of the dicyanobenzene. Irradiation of 3,4-dibromobiphenyl in acetonitrile solution gives 3-bromobiphenyl and 4-bromobiphenyl. "' AM1 calculations on the product-determining radicals rationalise the observations in terms of excimer and radical anion intermediates. Photodehalogenation of 1,2,3,4-tetrachlorodibenzo-p-dioxinis described both with and without triethanolamine, in the presence of a triplet qucncher, and in the presence of p,p'-di-tert-butylbiphenylidesuggesting that the mechanism is analogous to that of the photodechlorination of pentachlorobenzene. I6O The sensitivity of the regiochemistry to mechanism is also demonstrated. Irradiation of 2-benzyl-1-halobenzene gives diphenylmethane, as photoreduced product, together with fluorine, the photocyclised product. 16' The reaction proceeds by a radical mechanism, and the overall process is useful for the preparation of 9,l O-dihydrophenanthrenes which are otherwise accessible only with difficulty. Perfluoroalkyl aromatics have been prepared by irradiating azo compounds of the type RN=NR (R = peduoroalkyl), such as peduoroazooctane, in the presence of 5-membered ring heterocycles. 162 A similar transformation has been carried out using benzene itself. Irradiation of aqueous methanolic solutions of phenols in the presence of 1, I , 1-trichloroethane gives o-acetylphenols; there is also spectroscopic evidence for the formation of o-chloroethylphenols.163The reaction occurs by elimination of HCl followed by recombination of the resulting phenoxy-dichloroethyl radical pairs. Photonitration continues to attract attention. Irradiation of aromatic compounds in the presence of 1,1, 1,3,3,3-hexafluoropropan-2-01(HFP) gives primary aromatic radical cations and secondary radical cations from such aromatics as pentamethylbenzene, durene, some naphthalenes, and 2,3-dimethylanisole.164 The long lifetimes of these radical cations is a consequence of the stabilising effect of the nucleophile by the solvent, a view which is supported by preparative work on 1-methoxynaphthalene and 1,4-dimethylnaphthalene in which cases pathways incorporating the trinitromethanide ion are suppressed in HFP. Irradiation of the phenanthrenehetranitromethanecharge-transfer complex leads to the formation of phenanthrene'+, N204, and tetranitromethanide, which on recombination at 20 "C in methylene dichloride gives 9-nitrophenanthrene together with variously nitrated dih~dr0phenanthrenes.l~~ In the presence of nitrite or nitrate ions a range of phenols has been found to undergo nitrosation, nitration, and oxidation. Hydroxylation is thought to involve HO' generated by photolysis of nitrite and nitrate ions, and nitrosation and nitration occur via the nitrogen oxides, NO2, N203,and N204.Excitation of the charge-transfer complex formed between dibenzofuran and tetranitromethane in acetonitrile solution gives a variety of products some of which arise from reaction of the trinitromethide anion with the dibenzofuran radical cation. '15' 1,2,3,4-Tetramethylbenzene(TMB) and tetranitromethane (TNM) similarly give a triad consisting of TMB'+, NOz, and TNM-.I6* Recombination of this triad produces numerous products including the epimers of l,2,3,4-tetramethyl-3-nitro-6-trinitromethylcyclohexa1,4-diene, a nitroalkene cycloaddition product, a nitro cycloadduct, and two
182
Photochemistry
products of elimination. The photoreaction between 1,2,4,5tetramethylbenzene and tetranitromethane gives the epimeric 1,3,4,6-tetramethyl-3-nitro-6-trinitromethylcyclohexa-l,4-dienestogether with other materials. '69 Under the same conditions, 1,2,3,5tetramethylbenzene gives trans- 1,3,5,6-tetramethy1-6-nitro-3trinitromethylcyclohexa- 1,4-diene. Low temperature irradiation of the chargetransfer complex of tetranitromethane and pentamethylbenzene leads to the formation of the labile epimeric 1,2,3,4,6-pentamethyl-3-nitro-6-trinitromethylcyciohexa- 1,4-dienes.17* At room temperature in dichloromethane, photoexcitation of the same charge transfer complex gives products which probably arise from rearrangement of the labile epimeric 1,2,3,5,6-pentamethyl-3-nitro-6-trinitromethylcyclohexa-1,4-dienes. The regiochemistry of adduct formation in the attack of trinitromethanide ion on radical cations in the photochemical reactions of 2-methyl-, 2,3-dimethyl-, and 2,4-dimethylanisoles has been described, and 4-t-butyl-2,5-dimethoxynitrobenzeneis formed in the photoreaction of 2,5-ditert-butyl- 1,4-dimethoxybenzene by a possible addition-elimination pathway involving decomposition of transients in which trinitromethyl and NO2 have been added across the aromatic ring."* Irradiation of tris(4-bromopheny1)amine and tetranitromethane in solution gives 4-nitrophenylbis(4'-bromophenyl)amine almost exclusively, and the simple regiochemistry seems to arise from an additionelimination mechanism in which attack occurs at the carbon atom ips0 to the nitrogen function. 73 Photolysis of the 4-fluoroanisole/TNM charge-transfer complex at - 20 "C gives a wide variety of products, for example, l-fluoro-4methoxy-5-nitro-6-trinitromethylcyclohexa-1,3-diene, which in contrast to many other substrates are the result of photoaddition; 4-fluoro-3-methylanisolegives a similar range of products. 174 Substituted dibenzofurans have also been photonitrated using tetranitromethane. '75 Irradiation of methylarenes in chloroform solution is reported to give good yields of the corresponding dichloromethyl analogues,176and berberinium salts have been photohydroxymethylated at C-8 using methanol to give tetrahydroprotoberberine derivatives; this procedure has been used in the first reported preparation of ( & )-solidaline. 177 The kinetics of the imidazole-catalysed decomposition of bis(2,4,6-trichlorophenyl) oxalate and bis(2,4-dinitrophenyl) oxalate have been shown to be pseudo-first order, leading to the release of two molecules of substituted phenol with formation of 1,l'-oxalyldiimidazole for both esters.17*These results have implications for interpreting the initial steps in the peroxyoxalate chemiluminescence. Irradiation of 1,1-diarylethene in the presence of benzophenone as sensitiser and tert-butylamine in acetonitrile gives 4,4-diphenylbutanenitrile, but other amines, however, appear to be ineffecThis cyanomethylation is also successful with other diarylethenes. The photohydroxylation of salicylic acid by hydrogen peroxide is reported to occur by the same intermediate irrespective of which reactant is excited,'*' and in neutral or acidic media photolysis of selected ketones in the presence of 4cyanopyridine fails to lead to the expected products of biradicals and gives pyridine derivatives. *
''
'
Hl4: Photochemistry of Aromatic Compounds
5
183
Cyclisation Reactions
Intramolecular photorearrangement of (77; R' = H, Ph; R2 = H, Me, Et; R3 = H, Me) has been rationalised in terms of the intermediacy of a 1,4-biradical which has an option either to give the product of a [2+2] cycloaddition, or in cases of high ring strain, to undergo hydrogen transfer and cyclise to the S-oxaspiro[2.4]heptan-4-ones (78; same R'; R2, R3 = H, Me).'82 Dihydrobenzofurans and dihydrobenzopyrans are formed on direct irradiation of o-cinnamylphenols along with their cis-is~mers.'~~ In the presence of 2,4,6-triphenylpyrylium tetrafluoroborate as sensitiser, however, the methyl- and methoxy- derivatives react by single electron transfer, and for example, (E)-2-[2-(4-methylphenyl)ethenyl]phenol cyclises to 2-(4-methylphenyl)benzofuran.Intramolecular photochemical cyclomerisation of a series of polymethylene bis-coumarinyloxyacetates is reported to lead to the syn-HT configurated products only, and in yields which decrease with increasing chain length.'84 Daylight irradiation of the 2-styrylchromone (79; R' = Me, Et, hexyl, H; R2 = H, CMe3, C1) gives the (2) isomer which undergoes photooxidative electrocyclisation to the 12H-benzo[a]xanthene-12-0ne(80;same R',R2) in a useful synthesis of xanthones.'" The unsubstituted [7]phenacene (81; R = H) along with 2,13-di-n-pentyl[7]phenacene(81; R = n-pentyl) and 2,13-di-tbutyl[7]phenacene (8 1; R = t-butyl) have been prepared by stilbene-like photocyclisations. Photodehydrogenation of di-(2)-naphthylethylene solutions in the presence of iodine gives 3,4-5,6-dibenzphenanthrene,1,Zbenzotetraphene, and 1,12-benzperylene, and their rates of formation have been interpreted in terms of complexation between the &isomer of the substrate and the iodine molecule.187 The cycloreversion of 1 -(1-naphthyl)-5-phenylbicyclo[3.2.0]hept-6-ene into 1-(1naphthyl)4-phenyl- 1,3-~ycloheptadiene,using benzophenone sensitisation and direct irradiation, has been reported together with details of the effect of substituents in the naphthalene moiety,'" and a number of dialkynylarenes of the general type (82) undergo cycloaromatisation and form the prototype of a new class of photocleaving agents.'"
R3
(77) R'
184
Photochemistry
As part of a study of the preparative potential of the pyridinium photoelectrocyclisation-nucleophilic bicyclic aziridine ring opening reaction sequence, Nalkylpyridine perchlorates were photoelectrocyclised to give 6-alkyl-6-azabicyclo[3.1.0jhex-2-en-4-yl alcohols and ethers which were subsequently transformed by the nucleophilic solvent into cyclopent-3-en-1-amines. 190 Irradiation of 2pyridyl phenyl ketone in a range of H-bonding organic solvents induces cyclisation together with reduction via the 3 ( n ~ *state ) of the carbonyl group, and it is the H-bonding character of the solvent which determines the relative proportions of the two pathways. Solutions of DL-N-acetyl-2-chlorotyrosine methyl ester in methanol undergo intramolecular photocyclisation into yield methyl 1-acetyl6-hydroxy-2-indolinecarboxylateand methyl 6-hydro~y-2-indolecarboxylate.'~~ This transformation involving selective reaction of an almost non-nucleophilic amide over the nucleophilic solvent methanol, suggests control by steric factors. Photocyclisations of N-arylenamides give spirotricyclic &lactams which are easily convertible to spirocyclohexylisoquinolines, and which show important structural relations to the Amarylliduceae alkaloids, galanthamine and lycoramine. 193 Irradiation of (2)-2-(acetylamino)-N-butyl-3-(4-chlorophenyl)-2-propenamide (83) promotes two reactions one of which gives the isoquinoline (84) while the other leads to the cis- and trans-1-azetine derivatives (85).'94 These transformations occur by a 1,5-acetyl shift from the (Z)-isomer and 1,3-acetyl migration from the (E)-isomer. Photocyclisation of the chloroacetamide (86) gives the lactam the chloroacetamide (88) in aqueous acetonitrile gives the new heterocyclic system (89), together with and 2,4,6-triphenylpyridine Nazomethines (91) give tricyclic salts (92) which are further transformed into the pentacyclic salts (93).'97 This last reaction occurs by an intramolecular [131sigmatropic proton shift followed by an intermolecular proton transfer. A novel photo-assisted mild annulation, tolerant of a wide range of functional groups, has been reported for the synthesis of 6,7-dichloro-l,3-dihydro-2H-imidazo[4,5b]quinolin-2-one,19* berberine derivatives with a nitro group in ring D have been synthesised by irradiation of (94),'99 and the substituents R' and R2 have been found to influence ring closure of the merocyanine form (95)and consequently its rate of decoloration, as well as the kinetics of helical conformational changes in the polypeptide backbone.2M)A general parallelism has been revealed between the relative rates of helix to random coil conformational change for various R' and R2 and the rate of decoloration of unbound dyes. Photocyclisation of 2styrylbenzo[a]quinoIizinium derivatives (96) gives azoniahelicene and azoniaperylene salts in a ratio which depends on the steric hindrance between the two substituents in the 2- and 13-positions of the helices,20' and the conversion of azastilbene into dihydroazaphenanthrene has been reported as a key step in the synthesis of 6-aza-1,lO-phenanthroic anhydride.202As part of an examination of the behaviour of azobenzene in the solvent-swollen acid form of Nafion membranes it has been found that where water is the solvent, irradiation causes cyclisation and formation of benzo[c]cinnoline together with ben~idine.~'~ The product distribution depends upon the number of azobenzene molecules in each water cluster of Nafion, ( i e . the occupancy number) and if this exceeds 2 then about equal quantities of both products are formed, whereas if it is only 1 then
IIl4: Photochemistry of Aromatic Compounds 0
185
@
NHBu NHCOMe
Me+CONHBu
Bu
Q
Me
\
CI
CI
CI
(85)
(84)
(83)
CH2Ph
Me02C
Q-&e
Me PhCH2'
'COCH2CI Me
Ph
NI
C104Ph
Ph%
HN
II CHR
/
c10,-
H(L-Glu),,OH (94)
(95)
& /
/
R
Photochemistry
186
benzocinnoline is produced exclusively. These observations have been rationalised in terms of the water swelling the Nafion-H’ which allows the protons to participate in the photochemical and photophysical processes. Photolysis of imidoylbenzotriazoles is reported to produce 1,Zdisubstituted benzimidazoles in a synthetically useful reaction.204Solid state irradiation of 2-arylthio-3-methylcyclohex-2-en-l-ones (97; R = H, p-Me, o-Me, p-C1, o-Cl, p-Br, o-Br) gives the corresponding dihydrobenzothiophenes from these prochiral compounds when complexed in chiral hosts,205 and irradiation of some ally1 derivatives of indanethiones leads to thietane formation.*06 Numerous sulfur heteroarenes of the type (98) have been prepared by double photocyclisation of the corresponding tetraaryl-substituted ethenes and form products that are isoelectronic with the dibenzo[g,p]chry~ene.~~~ Photolysis of (99) in the presence of iodine and air produces (100) and (10 1) together with 10-methoxy-2-methy1-6H-benzo[b]naphth0[2,3-d]pyran-6-one,’~~and in aerated benzene, solutions of 2-(naphthalen-1 -yl)-3-(thien-2-yl)propenoic acid containing iodine give phenanthro[2,1 -b]thiophene- 1O-carboxylic acid, phenanthro[2,1 -b]thiophene, and naphtho[ 1,8-cde]thieno[3,2-g][2lbenzopyran. The novel polycyclic heterocycles, thieno[3’,2’:4,5]thien0[2,3-~]naphtho[2,1 -f Iquinoline and thieno [3’,2’:4,5]thien0[2,3-~]naphtho[ 1,2-g]quinoline have been synthesised by a photocyclisation,210and the same authors also describe the preparation of the related heterocycles thieno[2’,3’:4,5]thieno[2,3-c][ 1,lOlphenanthroline and thieno [3’,2‘:4,5]thieno[2,3-c][1,lOlphenanthr~line.~~ Oxidative photocyclisation of 1chloro-N-( 1-naphthyl)naphtho[2,1 -b]thiophene-2-carboxamide leads to the formation of the novel polycyclic heterocyclic ring systems benzo[h]naphtho[ 1’,2’:4,5]thieno[2,3-~]quinolineand benzo[h]naphtho[ 1’,2’:4,5]thieno[2,3* and photoinduced dehydrogenation of E,Ec][1,2,4]triazo10[4,3-a]quinoline,~’ bis(carboxystyry1)furan (102; X = 0) and E,E-bis(carboxystyry1)thiophene (102; X = S) gives 2-(2-carboxystyryl)-6-phenyl-7a-methoxy-3a,7a-dihydrofuro[3,2blpyran-5-one and 6-phenyl-2,3-epoxy-2,3-dihyrofuro[3,2-b]pyran-5-0ne.*~
’
0
H02C
C02H
IIl4: Photochemistry of Aromatic Compounds
187
Photocyclisation of 8-alkoxy-l,2,3,4-tetrahydro1-naphthalenones and 4alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones gives naphtho[ 1,8bclfurans and cyclohepta[cd]benzofuransrespectively, and conformational and substituent effects of 1,5-biradicalsin the cyclisation process are disc~ssed."~ The same authors also describe substituent effects on the photocyclisation of ethyl 2(8-0~0-5,6,7,8-tetrahydro-l-naphthyloxy)acetates and ethyl 2-(5-oxo-6,7,8,9-tetrahydro-5H-benzocyclohepten-4-yloxy)acetates to give naphtho[ 1,8-bc]furans and cyclohepta[c,d]benzofurans re~pectively."~Also reported are cyclisations involving photogenerated radical cations of unsaturated silyl enol ethers, fragmentation cyclisations of unsaturated or-cyclopropyl ketones which occur by photoelectron transfer and give polycyclics, and kinetic and theoretical studies of [2+3] cycloadditions of nitrile ylids2l 6 These reactions have been studied mechanistically and their synthetic potential investigated. Cyclic monoketals of a- and j3-substituted quinones are photocyclised in acidic media, to substituted alkenes in a substituent-dependent process via the corresponding cyclopropane-oxyallyl cation which is subsequently ~olvolysed."~In another di-n-methane rearrangement, various aryl substituted penta-l,4dienes undergo a photosensitised regioselective cyclisation using either dicyanonaphthalene or dicyanoanthracene, via radical cation intermediates.21 For example, in C6H3CN) on irradiation is converted into (104) Scheme 1, (103; Ar = C6H4, which cyclises to (105). Ab initio and semiempirical computations are found to be in accordance with radical cation and triplet regioselectivity.
(104)
Scheme 1
6
Dimerisation Reactions
Photolysis of 1-phenylcyclopentene (106) and 1,4-dicyanobenzene in acetonitrile gives [2+4] cycloadducts based upon the tetralin skeleton, and a 2:l adduct derived from the substrate^.^'^ However, although 1-phenylcyclohexene (107) leads to 1-cyano-2-phenyl-cyclohexane,no 1:1 products are formed. Radical cations are seen which have been shown by the PM3 method to be fully planar (106'+), and which will dimerise by reaction with (106); in contrast, the optimised structure of (107'+) has a chair-like structure. Styryl dyes containing both a crown ether group and a heteroaromatic residue with a sulfoalkyl N-substituent in acetonitrile solution and in the presence of Mg2+ ions will undergo photodimerisation.220Only one cyclobutane dimer is produced and this is interpreted
188
Photochemistry
as self-organisation of the trans isomers of the substrate by the magnesium ions. Photocyclodimerisation of various substituted cinnamic acids in micelles, vesicles, and microemulsions formed by dodecyl and hexadecyldimethylamine oxides proceeds most efficiently in vesicles with formation of head-to-head dimers.*” The regio- and stereochemical control in the photodimerisation of methyl 3-(2fury1)acrylate in acetonitrile using benzophenone as sensitiser has been studied using the AM1 semiempirical method.”’ This shows that regiocontrol is determined by the frontier orbitals and that stereochemical control is determined by the stability of the dimers. An examination of the photochemistry of crystalline p-phenylenediacrylic acid monoesters reveals that whereas the ethyl derivative is photostable, the n-decyl derivative forms p-cyclophanes and oligomer~.”~ It is believed that the crystal structure is influenced by the disparity between the alkyl chains and the parent acid with the result that it is only the n-decyl ester which is correctly oriented for photocycloaddition. The desorption and photochemistry of trans-stilbene on A1203(OOOl)have been studied using temperature programmed desorption (TPD).224Following irradiation at 313 nm, TPD provides evidence for 1,2,3,4-tetraphenylcyclobutane, and the data are consistent with dimerisation occurring through an excimer intermediate. Photodimerisation of a,co-biscoumarin long chain polyethers has been examined with particular reference,to the effect various parameters have on the substrate c o n f ~ r m a t i o n ~and ‘ ~ 0-,m-, and p-bis(3-methyl-1-thyminylmethy1)benzenesare photodimerised when irradiated at 280 nm.226 Irradiation of hexane solutions of N-(cis-4-t-butylcyclohexyl)-1naphthylamine and iodoform gives bis-1 ,1’-[4,4‘-(N-cis-4-tert-butylcyc1ohexyl) aminonaphthyl]methylium triiodide in a process, which following initiation by electron transfer from the amine to the haloform, proceeds by a series of radical reactions.”’ 1-Acetylanthracene photoreacts to form dimers in the ratio 2: 1:1:1:1 comprising the symmetrical dianthracenes as well as the dissymmetric head-totail dimers which arise by [4+4] processes involving 1,4- and 9’,10-p0sitions.’~~ In the presence of anthracene itself, 1-acetylanthracene gives 9,lO-9’, 1 0 and 1,4,10 [4+4] cycloadducts in the ratio 3:l. Benzene solutions of the chiral anthracenes AnCH(0Ac)R (An = 9-anthryl; R = Me, Et, i-Pr, t-Bu, CF3) can be photoconverted into two head-to-tail d i m e r ~ In . ~ ~the ~ solid state, however, the structure is substituent dependent and this has been explained in terms of the crystal structure. 9-Anthracenecarboxylic acid as its double salt with various diamines has been photodimerised in the solid state to give a head-to-tail and an unsymmetrical product, along with decarboxylation and reduction.230The same authors have also shown that regiocontrol in the photodimerisation of anthracene-9-propionic acid can be exercised by use of its double salt with cyclohexane1,2-diamine as the supramolecular linker, and in this way the head-to-head photodimer has been obtained.231 Photodimerisation of 12-(9-anthroyloxy)stearic acid in cetyltrimethylammonium chloride and polyoxyethylene(10) lauryl ether micelles has been examined in order to ascertain the effects of the distribution of the reactant by the micelles on the kinetics.232Observations suggest that inhibition of the photodimerisation is a consequence of reactant concentration within the mean displacement distance of the excited species, as well as being dependent upon
IIl4: Photochemistry of Aromatic Compounds
189
the depletion of the available reactants in the non-homogeneous medium as the reaction proceeds. Tetracene on dry Si02 at low surface coverage is photoconverted into syn and anti dimers, and under aerated conditions tetracene5,12-endoperoxide is formed in a process mediated by 02('Ag)-233 At higher surface coverage, the degree of dimerisation is increased. Photolysis of 3methoxyazulene- 1,5-quinone produces various dimers whose structural distribution depends on the polarity of the solvent, polar solvents giving head-to-head dimers, and non-polar solvents favouring head-to-tail d i m e r ~ Microemul.~~~ sions of Aerosol OT, isooctane, and water exhibit percolation as revealed by the onset of conductivity at 33 0C.235Addition of acridizinium bromide causes an increase in percolation temperature to 38 "C which following irradiation is found to rise further, enabling the system to be switched from conducting to non-conducting using light. Irradiation of the 1:1 inclusion complex formed between trans-2-styrylbenzoxazoleand 1,1,6,6-tetraphenylhexa-2,4-diyn-l ,6-diol as host gives the syn-head-to-tail dimer.236
7
Lateral Nuclear Shifts
Irradiation of the p-toluides of crotonic and cinnamic acids produces their cis isomers together with 4,6-dimethyl-2,4-dihydr0-2-q~inolone.~~~ Under the same conditions, however, the corresponding p-toluides of methylcrotonic and phenylpropiolic acids undergo a photo-Fries rearrangement to the o-aminophenones. In the presence of potassium carbonate greater selectivity towards acyl migration is observed. An investigation of the photo-Fries reaction of 2-naphthyl myristate in ordered phases of three isomeric alkyl alkanoates shows that the photochemical reaction is effectively controlled within the ordered media.238These results also suggest how to design and choose ordered media to effect other photochemical transformations selectively. Irradiation of (108; R' = OCOR, R2 = H, Me, Ph; R' = H, R2 = OCOR) within zeolites promotes a reaction whose selectivity is cationdependent and analogous to that in isotropic solution,239and the same authors have also discovered a remarkable product selectivity for both the photo-Fries and photo-Claisen rearrangements using zeolite NaZSM-11, in which the cagefree volume of the radical pair intermediates and cation intermediates effects are important.240Rearrangement of the 1-hydroxy-2-methoxyanthraquinone(109; R = Me, Ph, tolyl, EtO, Me2N) (Scheme 2) has been shown to be adiabatic and to occur on a triplet potential energy surface."'
Photochemistry
190 0
WoM
OCOR
RCOO
0
Qj$-JJoMe 0
0 (109)
8
Scheme 2
Miscellaneous Photochemistry
Irradiation of [2.2]paracyclophan-ene and benzo[2.2]paracyclophan-ene under conditions of matrix isolation in argon at 4 K induces cleavage of the ethano bridges and formation of (1 10) and (1 11) respectively.242
Aroylnitrenes have been generated by photoelectron transfer and a study made of their reactions with x-bonds in various alkenes and carbonyl compounds; diastereoselectivity was observed with substituted cyclic enol ethers.243 The azidophenylcyanoguanidines [112; X = C1, I; R = t-Bu, CMe2Et, 1-(mfluorophenyl)cyclobutyl] are effective as photoaffinity probes,244and a study has been made of substituent effects on the lifetimes and reactivities of N-t-butyl-(2acetyl-4-substituted)phenylnitreniumions derived by photolysis of 5-substitutedN-tert-butyl-3-methylanthraniliumi0ns.2~'The major decay pathway is reaction of the aromatic ring with nucleophiles (Scheme 3). Irradiation of the (9-phenanthrylmethy1)amines [1 13; R = phenylpropyl, phenyl, phenylmethyl, (methoxycarbonyl)methyl] in the presence of N-methylaniline promotes cleavage of the CH2-N bond and formation of substituted N-methy1a11iline.s.~~~ The 9fluorenyl cation has been photogenerated from 9-fluorenol in 1,1,1,3,3,3-hexafluoroisopropyl alcohol and this has enabled the reactions of the benzylic-type cation to be followed directly by flash phot~lysis.'~~ In the presence of electronrich aromatic hydrocarbons growth of the cyclohexadienyl cation can be observed along with decay of the fluorenyl cation, and this has made it possible to observe both cationic components of the Friedel-Crafts alkylation in the same experiment. Irradiation of ethyl phenylglyoxylate (1 14) leads to the formation of ethyl 0benzoylmandelate (1 1 9 , ethyl 0-acetylmandelate, and biphenyl triketone (1 16)
IIl4: Photochemistry of Aromatic Compounris
191
X
Scheme 3
and a mechanism involving both y-hydrogen abstraction and intermolecular hydrogen abstraction has been suggested, for which rate constants have been measured using methyl phenylgly~xalate.~~~ In alcoholic solution, indan- 1,2,3trione is photolysed to 3-alkoxycarbonylphthalides(1 17) and 3-alkoxyphthalides (1 18) in a process which may involve the spirooxiranone (1 19),249and mechanistic studies on the photolysis of substituted benzyl acetates and pivalates, and of 1naphthylmethyl carbonates and carbamates have been reported.250Irradiation of a donor-acceptor pair in the presence of substituted dibenzyl ketones as precursors of benzyl radicals leads to one-electron oxidation and one-electron reduction of the benzyl radicals.25’ Selective formation of carbocations or of carbanions was achieved using some donor-acceptor pairs and the implications are discussed in terms of recent theories of electron transfer rates. Methyl w(phenylse1eno)acetate and related compounds in the presence of an alkene and under an atmosphere of CO can be photolysed to acyl selenides by group transfer ~arbonylation.~’~ The mechanism involves addition of a (methoxycarbony1)methyl radical to the alkene and trapping of the alkyl free radical by carbon monoxide. Photolysis of 2-benzoylbenzoate esters of primary and secondary alcohols in the presence of isopropanol as hydrogen donor or primary amines as electron donor gives alcohols, and it is concluded that these esters are effective photolabile protecting groups for primary and secondary alcohols.253It is suggested that thiols can be protected in an analogous way. 3,3-Diphenyl1H,3H-naphtho[cd][2]pyran-1-one (120) has been decarboxylated from an upper triplet state to a mixture of (121) and (122) using high intensity, laser-jet pho t~chemistry.~’~ Fluoresence quenching of 1,4-dimethoxynaphthalene( 123) and 1,s-dimethoxy-
Photochemistry
192
0
0
Mo
@o
\
H
a
0
\
/
H02C
0
0
OR
CPhS
c y 3
naphthalene (124) by tetraphenylporphyrin, 9,1O-diphenylanthracene,and 3cyano-4-phenyl-6-(p-tolyl)pyridin-2-one occurs by resonance energy transfer such that the rate of quenching of (124) is less than that of (123) for the same acceptor.255This implies that steric effects dominate those of ionisation potential in all systems. Charge transfer dominates when 7,7,8,8-tetracyanoquinonedimethane is used as quencher. Either C-0 or C-S bond cleavage has been shown to be the primary step in the photolysis of cyanomethyl-1-naphthyl ether, cyanomethyl-2-naphthyl ether and some related c o m p o ~ n d s . 'Both ~ ~ the lowest S1 and TI states are photoactive and bond cleavage is caused by thermally activated crossing from Sl(oo*) and Tl(oo*) into dissociative states. Using laser irradiation at 308 nm, aryl vinyl thioethers (125) and aryl vinyl ethers form ylid intermediates (126) whose decay kinetics imply the participation of more than one ylid species.257In benzene solution, ring closure occurs to give a dihydrothiophene (127) and a dihydrofuran respectively. The vinyl ether (128; X = S) may undergo cyclisation to (129) or for (128; X = 0)[3+2]cycloaddition may occur to give (130). The thioethers 1,9-bis(methylthio)dibenzothiophene, 1,9-bis(ethylthio)dibenzothiophene, and 1,9-bis(isopropylthio)dibenzothiophene in which t+e 1,g-dithia substituents lie within the van der Waals S-S contact distance of 3.70 A, have been
or*
IIl4: Photochemistry of Aromatic Compounds
193
thermolysed and photolysed, and their reactivities c ~ m p a r e d . 'Derivatised ~~ solid supports having nitrophenol ethers attached as photolabile linking groups at synthesis initiation sites have been described,259and photolysis of 3-hydroxy-2,3dihydro-2, l-benzisoxazole and derivatives reported to lead to 2-acetylaniline derivatives via photoformation of arylnitroxyl radicals which themselves subsequently photoreact.260A study has been made of the oxidation of podophyllotoxin (131) by sodium persufate using laser flash photolysis and this has enabled rate constants to be determined for the formation and decay of transients.'61
a X
% 0
M%e
X
Me
(125)
Photocleavage of benzyl-S bonds appears to proceed neither by an electron transfer pathway nor with participation of an exciplex.262A meta effect seems to operate in these reactions as evidenced by the influence of 3-methoxy and of 3cyan0 substituents on the efficiency of the cleavage process. Photolysis of 1,2bis(phenoxymethy1)-, 1,2-bk(phenylthiomethyl)-, and 1,2-bis(phenylselenomethy1)benzene induces a two-photon process to give o-quinonedimethane, which in the presence of dienophiles undergoes a cycloaddition reaction,263and 4,8,1O-trithiadibenzo[cd,ij]amlene%oxides gives the corresponding aldehydes The two and ketones together with 4,8,9-tnithiacy~lopenta[def]phenanthrene.~~
194
Photochemistry
substituted benzo[b]thien0[3,2-~]quinolones 9-(3-dimethylaminopropyl)benzo[b]thienyl[3,2-c]quinolin-6(5H)-one [132; R = H, R’= NH(CH2)3NMe2]and 5-N(3 - dimethylaminopropyl) - 10 - carbomethoxybenzo[b]thienyl[3,2-clquinolin-6-one [ 132; R 1= (CH2)3NMe2, R’ = OMe] have been prepared using a procedure which involves a photodehydrohal~genation.~~~
In the presence of 2,4,6-triphenylpyrilium tetrafluoroborate as sensitiser, 4-(pisopropylphenyl)-2-nonyl-1,3-dioxolane will undergo a photodeacetalisation to give n-decanal and p-isopropylbenzaldehyde.266The reaction proceeds by electron transfer followed by attack by water on the radical cation. Irradiation of 1-methoxy-4-trinitromethylnaphthaleneinduces a nitro-nitrito rearrangement followed by loss of NO2, with ultimate formation of the corresponding naphthalenecarboxylic acid; spin trapping has shown the presence of 1 H)-thiones can be photocleaved Ar COO’.267N-(Pent-4-enyl-l-oxy)pyridine-2( to alkoxy radicals, which in the case of 1-phenyl derivatives leads to substituted thioethers, and in the case of 4-phenyl derivatives leads to endo cyclisation.268 Self-sensitised photolysis of N-(1-naphthoy1)-N-phenyl-0-(benzoyl-substituted benzoy1)hydroxylamines induces triplet-triplet energy transfer from the benzoyl to naphthoyl moieties of about unit efficiency, and is more likely to occur by a through-space rather than a through-bond mechanism.269 The products of fragmentation in methanol at room temperature are N-phenyl-1-naphthalenecarboxamide, benzophenone, and benzoyl-substituted benzoic acids. All of these compounds seem to arise from the triplet state of the naphthoyl chromophore, and in some cases a mechanism may be operating which involves a vibrationally excited triplet radical pair whose relaxation is slow compared to decarboxylation of the caged benzoyl-substituted benzoyloxy radical in 1,2-dichloromethane. The same authors have also investigated the direct photolysis of 0-(p-anisoy1)-N,Ndibenzoylhydroxylamine which proceeds in both polar and non-polar solvents to give N-benzylidenebenzylamine and anisic acid.270 Cleavage occurs from the singlet state of the substrate and kinetic evidence suggests that decarboxylation of the anisoyloxyl radical ensues after it has left the triplet cage. Trichloroacetic acid is reported to enhance the yield of 2-hydroxyazobenzeneproduced on irradiation of solutions of a~oxybenzene.~’~ In some related work, 4-methyl-ONN-azoxybenzene will photoisomerise to 4-methyl-NNO-azoxybenzene (133) and 2-hydroxy4‘-methylazobenzene can arise from (133) intramolecularly. Some new photoprotecting groups have been described. These include nucleoside derivatives having photolabile protecting groups such as [134; R’ = H, NO2, CN, OMe; R2 = H, OMe; R3 = H, F, C1, Br, NO2; R5 = H,
195
W 4 : Photochemistry of Aromatic Compounds
NCCH2CH20PN(R7)2,p-02NC6H4CH2CH20PN(R7)2;R7 = alkyl; R6 = H, OH, alkoxy, alkenyloxy, or acetal, silyl ether protecting groups; B = (protected) adenine, cytosine, guanine, thymine, uracil residue^]."^ Irradiation of the glycine derivative (135; R ' = H, R2 = COz-', R ' = CH2CH2C02-, R2 = COz-', R' = H, R2 = CH2C02-) releases glycine in poor yield?73In the presence of large amounts of ascorbate the yields are much enhanced, and it has been suggested that a decarboxylative side-reaction may be occurring in which the ascorbate acts as reducing agent. A novel photochemical cleavage of aryl sulfonamides of the type P-A3-A2-A* -S(0)2NR'R2(R' and R2 are substituted alkyl, arylalkyl, or heteroarylalkyl groups, A' is an aryl or heteroaryl moiety, A2 is a linking agent, and P is a solid support) is reported to have application in combinatorial chemistry for the solid phase synthesis of a r n i n e ~ . ~ ~ ~ OMe
I
R'
The effects of substituents on the carbon acidity of SH-dibenzo[a,d]cycloheptatriene (suberene) have been studied, and deprotonation has been shown to approach the diffusion controlled limit in acetonitrile solution in the presence of primary a m i n e ~ However, . ~ ~ ~ secondary and tertiary amines facilitate photoreduction of the substrate.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
R. D. J. Froese, Diss. Abstr. Int., B, 1996,57,2567. F. D. Lewis, Adv. Electron Transfer Chem., 1996,5, 1. S . Tanaka and K. Seguchi, Yuki Gosei Kagaku Kyokaishi, 1996,54,212. M. Yasuda, T. Yamashita, R. Kojima, and K. Shima, Heterocycles, 1996,43,2513. A. Ogawa and N. Sonoda, Yuki Gosei Kagaku Kyokaishi, 1996,54,894. H. Petek, Kikan Kagaku Sosetsu, 1996,28,25. Y. Tominaga and R. N. Castle, J. Heterocycl. Chem., 1996,33,523. K. Uchida and M. Irie, Senryo to Yakuhin, 1996,41,37. A. Miyashita, Kikan Kagaku Sosetsu, 1996,28,51. M. Usui, Kikan Kagaku Sosetsu, 1996,28,129. S . Tokita, Kikan Kagaku Sosetsu, 1996,28, 135. V . A. Barachevsky, Proc. SPIE-Int. SOC.Opt. Eng., 1997,2968. M. Balon, M. A. Munoz, P. Guardado, J. Hidalgo, and C. Carmona, Trends Photochem Photobiol., 1994,3, 117.
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18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.
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Iil4: Photochemistry of Aromatic Compounds 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.
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106. 107. 108. 109. 110. 111.
112.
IIl4: Photochemistry of Aromatic Compoundr
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IIl4: Photochemistry of Aromatic Compounds
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202 209. 210.
Photochemistry
Y. Tominaga, L. W. Castle, and R. N. Castle, J. Heterocycl. Chem., 1996,33, 1017. J.-K. Luo, R. F. Federspiel, R. N. Castle, and L. W. Castle, J. Heterocycl. Chem., 1996,33, 185. 21 1. A. P. Halverson, L. W. Castle, and R. N. Castle, J. Heterocycl. Chem., 1996,33, 119. 212. J.-K. Luo, R. F. Federspiel, and R. N. Castle, J. Heterocycl. Chem., 1996,33,923. 213. M. Bajic and G. Karminski-Zamola, Croat. Chem. Acta, 1996,69,261. 214. E. M. Sharshira, H. Iwanami, M. Okamura, E. Hasegawa, and T. Horaguchi, J. Heterocycl. Chem., 1996,33, 17. 215. E. M. Sharshira, H. Iwanami, M. Okamura, E. Hasegawa, and T. Horaguchi, J. Heterocycl. Chem., 1996,33, 137. 216. J. Mattay, E. Albrecht, M. Fagnoni, A. Heidbreder, S. Hintz, Th. Kirschberg, M. Klessinger, J. Maehlmann, I. Schlachter, and S. Steenken, J. InJ Rec., 1996,23, 23. 217. M. C. Pirrung and D. S . Nunn, Tetrahedron, 1996,52,5707. 218. H. E. Zimmerman and K. D. Hoffacker, J. Org. Chem., 1996,61,6526. 219. M. Kojima, A. Kakehi, A. Ishida, and S. Takamuku, J. Am. Chem. Soc., 1996,118, 2612. 220. M. V. Alfimov, S. P. Gromov, 0. B. Stanislavskii, E. N. Eshakov, and 0. A. Fedorova, Izv. Akad. Nauk, Ser. Khim., 1993, 1449. 221. K. Takagi, T. Nakamura, H. Katsu, M. Itoh, Y. Sawaki, and T. Imae, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1996,277,495. 222. M. D’Auria, Heterocycles, 1996,43,959. 223. N. Feeder and F. Nakanishi, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1996, 277, 537. 224. R. M. Slayton, N. R. Franklin, and N. J. Tro, J. Phys. Chem., 1996,100, 15551. 225. T.-J. Liu and S.-K. Wu, Gaodeng Xuexiao Huaxue Xuebao, 1996,17, 1754. 226. N. Thonai, M. Miyata, and Y. Inaki, J. Photopolym. Sci. Technol., 1996,9,63. 227. K. Maeda, Y. Horikoshi, M. Hayashi, Y. Mori, and H. Nagano, J. Chem. Soc., Perkin Trans., I , 1996,789. 228. H.-D. Becker, H.-C. Becker, and V. Langer, J. Photochem. Photobiol., A, 1996, W, 25. 229. Y. Mori, Y. Horikoshi, and K. Maeda, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1996,277,399. 230. Y. Ito, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A , 1996,277. 231. Y. Ito and G. Olovsoon, J. Chem. Soc., Perkin Trans., I , 1997, 127. 232. M. J. Moreno, I. M. G. Lourtie, and E. Melo, J. Phys. Chem., 1996,100, 18192. 233. R. Dabestani, M. Nelson, and M. E. Sigman, Photochem. Photobiol., 1996,64, 80. 234. A. Mori, H. Kawakami, H. Takeshita, and T. Nozoe, Chem. Lett., 1996,985. 235. D. Nees, U. Cichos, and T. Wolff, Ber. Bunsen-Ges., 1996,100, 1372. 236. T. Kang, Y. M. Wang, and J. B. Meng, Chin. Chem. Lett., 1996,7,641. 237. C . Barbera, H. Garcia, M. A. Miranda, and J. Primo, An. Quim., 1995,91,95. 238. J. E. Baldvins, C. Cui, and R. G. Weiss, Photochem Photobiol., 1996,63,726. 239. K. Pitchumani, M. Warrier, C. Cui, R. G. Weiss, and V. Ramamurthy, Tetrahedron Lett., 1996,37,6251. 240. K. Pitchumani, M. Warrier, and V. Ramamurthy, J. Am. Chem. Soc., 1996, 118, 9428. 241. N. P. Gritsan, A. Kellman, F. Tfibel, and L. S . Klimenko, J. Phys. Chem., A , 1997, 101,794. 242. R. Marquardt, W. Sander, T. Laue, and H. Hopf, Liebigs Ann., 1996,2039. 243. W. Abraham, K. Buck, and K.-U. Clauss, J. In$ Rec., 1996,22,389. 244. R. C . Gadwood and V. E. Groppi, U.S. US 5,525,742.
IIl4: Photochemistry of Aromatic Compounh
245. 246. 247. 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259. 260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274, 275.
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R. A. McClelland, F. L. Cozens, J. Li, and S. Steenken, J. Chem. Soc., Perkin Trans. 2, 1996, 1531. S. Hu and D. C. Neckers, J. Org. Chem., 1996,61,6407. J. Tatsugi, T. Hara, and Y . Izawa, Chem. Lett., 1997,177. P. J. Macleod, Diss. Abstr. Int., B, 1996,57,3208. K. Ishiguro, T. Nakano, H. Shibata, and Y . Sawaki, J. Am. Chem. Soc., 1996, 118, 7255. I. Ryu, H. Muraoka, N. Kambe, M. Komatsu, and N. Sonoda, J. Org. Chem., 1996, 61,6396. P. B. Jones, M. P. Pollastri, and N. A. Porter, J. Org. Chem., 1996,61,9455. R . M. Wilson, K. A. Schnapp, M. Glos, C. Bohne, and A. C. Dixon, Chem. Commun (Cambridge), 1997,149. M. A. El-Kemary and S. A. El-Daly, Monatsh. Chem., 1996,127,593. G. Pohlers, H. Dreeskamp, and S. Grimme, J. Photochem. Photobiol., A, 1996, M, 41. J. P. Dittami, Y. Luo, D. Moss, and W. G. McGimpsey, J. Org. Chem., 1996, 61, 6256. T. Kimura, Y. Izumi, E. Horn, and N. Furukawa, Heteroat. Chem., 1996,7,313. C . Holmes, PCT Int. Appl. WO 96 00,378. S. Srivastava and D. E. Falvey, Tetrahedron Lett., 1996,37,2895. S . Wang, S. Yao, Z. Zhang, M. Zhang, and Y. Jiang, Sci. China, Ser. B: Chem., 1996,39,425. S . A. Fleming and A. W. Jensen, J. Org. Chem., 1996,61,7040. A. Ouchi and Y. Kogo, Chem. Commun. (Cambridge), 1996,2075. T. Fujii, H. Kasunagai, andN. Furukawa, Chem. Lett., 1996,655. J. Dogan, G. Karminski-Zamola, and D. W. Boykin, Heterocycl. Commun., 1996,2, 213. P. Gavina, N. L. Lavernia, R. Mestres, and M. A. Miranda, Tetrahedron, 1996, 52, 491 1. L. Eberson, M. P. Hartshorn, F. Radner, and J. 0. Svensson, Acta Chem. Scand, 1996,50,885. J. Hartung, M. Hiller, and P. Schmidt, Chem. Eur. J., 1996,2, 1014. H. Nagakubo, G. Kubota, K. Kubo, T. Kaneko, T. Sakurai, and H. Inoue, Bull. Chem. SOC.Jpn., 1996,69,2603. T. Nishijima, T. Ohishi, K. Kubo, T. Sakurai, and H. Inoue, Nippon Kagaku Kaishi, 1996,882. T . Issiki, H. Miyagawa, H. Sasaki, and J. Yamamoto, Nippon Kagaku Kaishi, 1997, 207. W. PAeiderer and H. Giegrich, Ger. Offen. DE 4,444,996. J. E. T.Corrie and G. Papageorgiou, J. Chem. Soc., Perkin Trans. I , 1996,1583. R. S. Garigipati and J. L. Adams, PCT Int. Appl. WO 9630337. D. Budac and P. Wan, J. Photochem. Photobiol., A, 1996,98,27.
5 Photo-reductionand -oxidation BY ALAN COX
1
Introduction
Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research,’ dynamics of bimolecular photoelectron transfer reactions,2 photophysical properties of functionalised fullerene derivat i v e ~ carbon-carbon ,~ bond formation via radical ions,4 photoinduced electron transfer processes in ketone, aldehyde, and ester ~ynthesis,~ photochemical reactions between arenenitriles and benzylic donors,6 photo-oxidation of conjugated dienes,’ photoredox reactions of aromatic nitro compounds,8 electron transfer-mediated photochemistry of some unsaturated nitrogen-containing comp o u n d ~ ,reactions ~ of 0 2 ( IAg),l0 carbon dioxide activation by aza-macrocyclic complexes,’ and photochromism of chalcone derivatives. j 2 A general kinetics model for all photochemical reaction orders,13 and a photosystem designed to harvest visible light as a source of electrons for use in redox reactions applicable to organic synthesis have also been described. l4
2
Reduction of the Carbonyl Group
In pure cyclohexane photolysis of benzophenone gives a mixture consisting principally of benzpinacol and benzhydrol, but in the presence of zeolite NaX the main product is 1-cyclohexyl-1,l -diphenylmethanol.’ Evidence is presented to show that this is formed from collapse of a radical pair derived by hydrogen abstraction from cyclohexane by excited benzophenone, and it is also suggested that the benzophenone and cyclohexane share the zeolite supercages in a close packed arrangement. The excited benzpinacol radical generated using high intensity laser jet radiation causes reaction in the p-position and gives benzophenones by head-to-tail coupling, together with benzhydrol as the major twophoton product.16 These observations stand in sharp contrast to the ground state reactions of the hydroxydiphenylmethyl radical which can lead either to benzpinacol by head-to-head coupling, or to diols by cross-coupling. 1 ,CDioxane, a small amphiphile, has been observed to affect the radical recombination and escape rates of the H-abstracted product of 3Ph2C0 in SDS micelles, and this implies the ability of dioxane to form ‘mixed’ micelles.” Photoreduction of
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
1115: Photo-reduction and -oxidation
205
benzophenone by diphenylamine in micellar solution gives ketyl and diphenylamine radicals which react to form 4-phenylaminophenyldiphenylcarbinolalong with other triarylmethane dyes." Observed magnetic effects on the reaction are negative and imply an enhanced radical escape owing to a rise in lifetime of the triplet radical pairs. Magnetic field effects have also been observed in triplet biradicals derived from the irradiation of benzophenone containing chain-linked hydrogen donor moieties." These effects have been explained in terms of spinlattice relaxation due to hyperfine anisotropy and to g tensor anisotropy. The nature of the light-absorbing transients (LATs) formed in the photoreduction of benzophenone by various hydrogen donors has been examined, and for propan-201 the branching ratio for dimethylketyl and diphenylketyl radicals has been extracted by computer simulation.20The LAT could be an efficient H-donor, and from the differential partitioning of the products it has been concluded that both propan-2-01 and THF are about an order of magnitude more efficient than alkanes. Kinetics for both inter- and intramolecular abstraction of phenolic hydrogen atoms by the excited states of various aromatic ketones, including alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone have appeared.2' Information on geometric requirements suggests that for n,n* triplets and donor-substituted benzophenones which have n,x* triplets, the transformations may proceed via a hydrogen-bonded exciplex which decays to radicals by sequential electron- and proton-transfer. A study has been reported of a substrate-enzyme interaction involving a series of 4'-substituted benzophenones as models in which the carbonyl group of the aromatic ketone is oriented such that following excitation it is capable of participating in a 3'-hydrogen abstraction from the enzyme.22 These investigations may have implications for an understanding of the mechanism by which ribonucleotide reductases catalyse the 2'reduction of ribonucleotides. Rate constants have been measured for electron transfer from triethylamine to the triplet state of aromatic ketones in acetonitrile solution and the results show that the reactions are kinetically controlled with the activation energy being determined by the electrostatic charge distribution in the carbonyl triplet state.2' Irradiation of benzophenone with amines and alcohols produces ketyl and a-amino radicals respectively, and it has been found that these nucleophilic radicals will transfer an electron to cationic dyes such as methylene blue.24 Electron transfer rates have been determined for a range of dyes, and those of the ketyl radical are found to be smaller than those of the a-amino radical; it has also been found that if dye mixtures are used, stepwise electron transfer from one semi-reduced dye to another can occur. Photoinduced electron transfer reactions of chalcone derivatives (1) in the presence of triethylamine gives cis- and trans-2-benzoyl- 1,3,4-triphenylcyclopentanols,meso- and ( f )-l,3,4,6tetraphenylhexan- 1,6-ones and 5-benzoyl-1,3,4-triphenylpent-1-en-3-01? These products arise by radical addition to neutral chalcone by the chalcone radical anion and the ketyl radical which arises by electron transfer from the amine. Ar d \ A (1)
r
206
.
Photochemistry
Irradiation of bibenzylic donors in the presence of chlorinated benzoquinones promotes electron transfer with mesolytic scission of the C-C bonds in the donor.26 From the fragmentation rates and other data, the dynamics of backelectron transfer can be deduced. Photoinduced electron transfer from 1-anaphthyl-3-oxa-alkanes (2; n = 1, 8, 12, 16) to 2-alkyl-3,5,6-trichlorobenzo-l,4quinones (3; n = 8, 12, 16) is preceded by pre-association indicating that the quenching is static.27A chain-length, and possibly also a chain-foldability effect are observed as well as a solvent aggregating effect. Various acceptors based upon benzoquinone and TCNQ have been used to study photoinduced electron transfer from a series of conjugated polymers such as poly[2-methoxy-5-(2ethylhexy1oxy)-p-phenylenevinylene] and poly[3-(2-(3-methoxylbutoxy)ethyl)thiophene], and a comparison has been made with CG0as an acceptor.28An ESR examination of electron transfer from zinc tetrakis(4-sulfonatopheny1)porphyrin (ZnTPPS) to duroquinone (DQ) in CTAC micellar solution reveals a spin correlated radical ion pair and a spin polarised radical anion.29Two radical pairs from the same donor-acceptor complex are produced during the photolysis of frozen solutions of spatially hindered quinones containing tertiary aliphatic amine~.~' The first of these arises from deprotonation of the amine cation radical within the cage, and the second involves interaction with excess amine after the radical pairs have left the cage. The report of a study of the electron transfer quenching of triplet duroquinone by Ph3N co-adsorbed on silicas having different pore sizes (PS) has appeared.31The effect of PS is discussed in terms of fractal effects in restricted geometries, static quenching is adequately described by the Perrin model, and the conclusion is drawn that molecular mobility is faster on small PS silicas. 0
Irradiation of a system consisting of 2-methyl- 1,4-dihydroxynaphthaleneand benzophenone leads to formation of benzophenone ketyl radical and 2-methylnaphthosemiq~inone,~'and 1a97a-dihydro-1a-methyl-I, 1-diphenyl-1H-cyclopropa[b]naphthalene-2,7-dione derivatives (4) in the presence of Mg(C104)2 give indenonaphthoquinones (5) (Scheme l), by photoinduced intramolecular electron transfer in almost quantitative yield.33 In agreement with Rehm-Weller predictions, the rate constants for electron transfer between the triplet states of various substituted naphthoquinones and N,N-dimethylaniline (DMA) in acetonitrile solution were found to be diffusion ~ o n t r o l l e d Using . ~ ~ this information, it has been shown that the efficiencies of formation of naphthalene'- and DMA'+ are unity. 1,4-Hexyl-bridged anthraquinone undergoes intramolecular photoinduced hydrogen atom transfer on the picosecond time-scale to form the biradical methide
207
IIIS: Pho to-reduction and -oxidation X
I
0 (4)
Scheme 1
1,4-hexylidene-bridged9-hydroxyanthracen-lO-one,which in aprotic solvents is converted into 1,4-hexylidene-bridged anthraquinone, and in ethanol to 1,4hexyl-bridged 1,4-diethoxy-9,10-dihydroxyanthracen ~ 1 Photoelectron . ~ ~ transfer has been studied using dyad and triad model compounds based upon the chlorophyll derivative, 3'-hydroxyphytochlorin, and 2-(hydroxymethy1)anthraq ~ i n o n eand , ~ ~in their excited state, dibutyl and diethyl esters of 2,1,3-benzothiadiazole-4,7-dicarboxylicacid in CTAB will accept an electron from morpholine ethene sulfonic acid and subsequently transfer it to 1,5-anthraquinonedisuIfonate." This transference will also occur across vesicle bilayers of dioctadecyldimethylammonium bromide, and the rate determining step may be charge compensating diffusion of H' across the bilayer. A photochemical synthesis of a spirobenzopyranochromone has been described which proceeds via intermediate (6) formed by abstraction of an allylic H atom by the excited state of the carbonyl group in ( 7 1 . ~ ~
Photoinduced electron transfer in rigid porphyrin-quinone dyads having strong interactions between the chromophores has been examined for its use as a model for solar energy conversion by photoinduced electron transfer.39 The rates of charge separation have been measured for a series of cyclophane-bridged zincporphyrin-quinone systems in polar and non-polar media, and data obtained allow the first quantum mechanical analysis of the driving force dependence of charge separation alone, and hence an experimental comparability of charge separation and rec~mbination.~'Quantum efficiencies of charge separation between zinc meso-tetra(4-sulfonatopheny1)porphine acting as donor, and 2,3dimethoxy-5-methylbenzo1,4-quinoneas acceptor have been improved by proto-
208
Photochemistry
nation, and this may influence both the nature of the ion pair as well as the competition between free ion separation and back electron t r a n ~ f e r . Irradiation ~ of the biomimetic photosynthetic model compounds 10,15,20-tritolylporphyrins linked to quinones by cyclohexylene bridges in reversed Triton X-100 micelles yields EPR spectra which are indicative of strong electron spin polarisation effects.42 The polarisation pattern can be attributed to a radical triplet pair mechanism between two photoactive species in one micelle. The porphyrinspacer-benzoquinone molecule (8; R = H, Ph) has been prepared and the electron transfer rates for charge separation as deduced from fluorescence lifetimes are found to be the same for both substitution patterns.43 This suggests that the inserted mystem is not involved in electron transfer pathways, and that in the present molecule at least, electron transfer occurs by the ‘through-bond’ rather than the ‘through-space’ pathway. Photoinduced electron transfer in various carotenoid-porphyrin-quinone molecular triads has been examined and steric hindrance by methyl groups situated at the p-pyrrolic positions found to reduce the rate of electron transfer.& In addition, amide-containing donor-acceptor linkages with a nitrogen atom attached to the porphyrin at the meso position of the aryl ring has an electron transfer rate some thirty times larger than that of similar links with the amide reversed. Me
Me
The nanosecond kinetics of e-aq generated by excitation of phenothiazine in the presence of idebenone (IDB) and CoQ, separately in SDS micelles show that IDB, a quinone possessing a long side-chain, gives IDB’-.45 By contrast, CoQ, which does not possess a long side-chain is reduced by the two processes of quenching of e-aq and direct electron transfer from the triplet. Deuterium substitution effects on intermolecular electron transfer from solvents such as deuterated and normal versions of aniline and dimethylaniline to similarly isotopically substituted (S1) coumarin dyes, have been examined on the femtosecond timescale!6 Differences in driving forces are observed that are attributable to solvent structural effects arising from intermolecular bonding in the solvent, and good correlations are observed with theoretical predictions from the twodimensional electron transfer model. Irradiation of 2’-alkylthioethyl phenylglyoxalates and 2’dimethylaminoethyl phenylglyoxalates induces intramolecular elec-
III5: Photo-reduction and -oxidation
209
tron transfer from the heteroatom to the carbonyl function, and following proton transfer and ring closure of the biradicals, 7- and 5-membered ring lactones are formed.47 In thiodialkyl phenylglyoxalates, intramolecular self-quenching leads only to an inefficient reaction. The product distribution in the photoinduced electron transfer reactions of peptides has been accounted for in terms of participation of the peptide bond.48Thus irradiation of glycyl-DL-alanine gives (9; R' = H, R2 = CH3) as intermediate which breaks down to give acetamide, acetaldehydeand other products. R'
0
R2
investigations of the Norrish-Yang photochemistry of isomeric p-alkyl alkanophenones, p-propyl nonadecanophenone, p-pentyl heptadecanophenone, and poctyl tetradecanophenone, and of the photo-Fries reaction of 2-naphthyl myristate in ordered phases of three isomeric alkyl alkanoates, have shown that the photochemical reaction is effectively controlled within the ordered These results also suggest how to design and choose ordered media to effect other photochemical transformations selectively. Enantioselectivity has been achieved in the Norrish-Yang reaction of ketones (10; X = C02Me) and (1 1;X = C02Me) by using chirally modified zeolite^.^' Irradiation of the chiral N-(2-benzoylethy1)N-tosylglycine esters PhCOCH2CHRNTsCH2C02CH2Ph(12; R = C02Me, C02Me3,CMe3) gives cis-3-hydroxyproline esters with asymmetric induction by the C( 1')-substituent, whereas the a-amino-y-keto ester (S)-PhCOCH2CHC02MeNTsMe leads to enantiomericially pure 4-hydroxy-4-phenyl-L-prolineesters having low de." However, irradiation of (12; R = CH20TDS, CHMe2), bearing y-H atoms with respect to the keto function, forms cyclobutanols in addition to the preferred Norrish Type I1 cleavage product. 2-(Dibenzylamino)ethyl and 2(N-benzyl-N-methy1amino)ethylacetoacetate undergo photocyclisation to the
210
Photochemistry
corresponding 8-membered ring azlactone, and a Stern-Volmer investigation suggests that two excited states are involved.52 P-(p-Dimethylaminopheny1)propiophenones are photocyclised to 2-(p-dimethylaminophenyl)-1-phenylcyclopropanols in a reaction in which transfer of a P-hydrogen to the carbonyl group is promoted by either strong charge and/or electron transfer to the benzoyl 2-0xabicyclo[3.4.0]nonan-7-ols have been produced in the photoinduced cyclisation of unsaturated C-glycosyl ketones and aldehyde^.^^ For example, 5-(acetyloxy)-5,6-dihydro-a,6-dimethyl-2H-pyran-2-methylene-propana1(13) gives (14). The photochemistry of a series of cyclic diketones having between 10- and 26membered rings has been studied in the solid state and in solution.55 In the solid state there is stereoselective cyclobutanol formation in which the cisltrans ring fusion of the products is determined by the conformation of the diketone in the crystal. Abstractions occur through boat rather than chair conformations and distance and angular parameters have been determined for the reactive yhydrogen atoms. Estimates of geometrical parameters for ring closure of biradicals indicate that alignment is good for closure but poor for cleavage. In solution, the reactions are ring size-dependent and this arises from the relative conformational freedom of the intermediate l ,4-biradicals. The 2,8-dioxabicyclo[3.2.lloctane ring system (15) of the zaragozic acids has been synthesised by a Norrish type I1 reaction which occurs by an uncommon 1,6-H abstraction to give a 1,5b i r a d i ~ a lRelative .~~ quantum yields for the Norrish type I1 elimination of formic acid from a range of formate esters have been determined, and semi-empirical and ab initio calculations of the excited state hydrogen abstraction process perf~rmed.~’ Hydrogen and deuterium transfer have been monitored in EPA at 77 K for the excited states of 2-, 3-, and 4-methylben~ophenone.~~ 2-Methylbenzophenone shows an inhomogeneous decay which implies that the internal Habstraction rate is dependent upon molecular conformation, and spectroscopic evidence for the appropriate o-quinodimethane enol is reported. Irradiation of aroyl substituted epoxides in the presence of 2,3-dihydro-1,3-dimethy1-2-phenylbenzimidazole promotes electron transfer to the carbonyl group of the aroyl moiety with subsequent ring opening to structures such as (16), and ultimate formation of an a i d 0 1 . ~ ~ H
3
Reduction of Nitrogen-containing Compounds
An examination of the photoelectron transfer in covalently -(CH2) 10- linked eosin-butylviologen has shown that the intramolecular process is more efficient
1115: Photo-reduction and -oxidation
211
than the intermolecular transfer.60Addition of cyclodextrin or amylose enhances the efficiency of the intramolecular process and this is believed to be due to the formation of an inclusion complex. Rate constants have been determined for the oxidative quenching of *[Ru(bpy)3l2+by MV2+, the subsequent charge recombination reaction, and the cage escape yield as a function of added electrolytes.61 A study of the pH and SDS micellar effects on the quenching of * [ R ~ ( b p y ) ~ ] ~ + by N-alkyl-4,4-bipyridinium ions (RBPY'+; R = Me, octyl, dodecyl, benzyl) shows that the quenching rate at pH 2 is greatly enhanced in SDS, but that at high pH it is retarded.62This is interpreted as indicating the deep embedment of the RBPY+ into the hydrophobic hydrocarbon region. The quantum yield of the photocatalytic reduction of MV2+ by [Cu(dmphen)L2]+, (dmphen = 2,9dimethyl-I ,lo-phenanthroline, L = PPh,(C6H40Me-p)3-, rises with the order of the phosphine donating ability, and this has been attributed to an increase in the lifetime of the excited state and to the charge separation step becoming more facile.63 Photoelectron transfer from myoglobin, reconstituted with Zn porphyrin, to viologen is regulated by electrostatic interactions between the artificially constructed anion site and the cation receptor enabling the velocity of the electron transfer to be measured.64 The photosensitised reduction of MV2' by various tetrakis[(4-trimethylamino)phenyl]metalloporphyrins [MeTPI, Me = H2, Zn(II), Ni(II), Cu(II), Mn(III), Co(III)] is influenced by substituent groups and by axially coordinated groups.65In particular, Zn porphyrins emerge as the best series of photosensitizers, and aromatic organic bases increase the photosensitivity by axial ligation especially if they are positively charged. Irradiation of the zinc phthalocyanine-viologen-bisphenolA system (ZnPcAV2+)promotes electron transfer and it is found that both the excited singlet state and the fluorescence of the phthalocyanine are quenched.66 Laser flash photolysis shows that the viologen is reduced to the radical cation. In the linked [Ru(bpy)3I2+/ dimethylviologen, [Ru(bpy)2(4-(2-( l'-methyl-4-4'-bipyridinium-I -yl)propyl)-4'methyl-2,2'-bipyridine)I4+,photoexcitation does not lead to emission from the MLCT state, suggesting that this is due to quenching by MV2+,67and viologenzinc porphyrins linked by a chain consisting of between three and six methylene units have been synthesised, and the quenching of the porphyrins by the binding viologen has been investigated.68 One-electron reduction of MV2' intercalated into zeolite-Y has been achieved using carbonylmanganate donors such as [C+Mn(C0)4L- ] [L = CO, P(OPh),] in a process in which the available evidence suggests that the transference only occurs at the zeolite periphery.69 A LFP investigation of MV2+ adsorbed on various zeolites has shown the existence of MV'+ as a transient, and is the first firm evidence that zeolites can behave as single electron donors.70 Irradiation (h > 300 nm) of solutions of the Schiff bases N-benzylidenebenzylamine and N-benzylideneaniline in propan-2-01 in the presence of either platinised titanium(1V) oxide or platinised CdS induces transfer hydrogenation to give the corresponding secondary amines, dibenzylamine and N-benzylaniline respect i ~ e l y . ~ Donor-acceptor ' compounds of the type N,N-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine, N,N-dicyanobenzo[b]naphtho [2,3-el[ 1,4]oxathiin-6,11-quinonediimine (DCNQI), and derivatives are reported
212
Photochemistry
to undergo a photoinduced intramolecular electron transfer from the HOMO in the donor to the LUMO located on the DCNQI fragment7' This transition induces planarisation of the donor-acceptor moiety. Irradiation of 1,4,8,11,15,18,22,25-octabutoxyphthalocyanineor its copper derivatives in degassed ethanol containing triethanolamine causes its reduction by a mechanism which involves a higher excited state of the substrate,73 and a plot of the log of the rate constants for the photoinduced intramolecular electron transfer in a supramolecular assembly of Zn and Au porphyrins bridged by diphenylphenanthroline has been found to decrease linearly with diminishing energy difference between the HOMO and LUMO of the participating orbitals.74 Intermolecular photoreduction of the 3n7t* state of the azo chromophore in (1 7) to give the corresponding hydrazine has been observed using donors such as tributyltin hydride, cyclohexa-1,4-diene, benzhydrol, and amines, and the mechanism and photoreactivity of this chromophore have been found to be comparable with those of keto groups.75 Cephalotaxine, the parent of the family of cephalotaxus alkaloids, has been prepared by a route which involves single electron transfer induced cyclisation of (18) into ( 19).76 Excitation of 4,4'bipyridine gives the '(nn*) state which subsequently undergoes fast intersystem state, and in various alcohols there is rapid formation of crossing to the '(m*) the N-hydro radical from the S1 state.77 This process is activated by the protic character of the solvent and not by its H atom donor ability. Various pyrrolidine derivatives Of the form C ~ O C ~ H ~C60CgH N O ~ ,13NO4, C60CgH17NO4, C60C9H 1 IN, c60cII H ~ ~ C60C9H1~N02, N ~ , and C ~ O C ~ H fluoresce ~N with an intensity up to four times that of pure c60, and the quenching of these materials can be decreased by the presence of electron-donor substituent groups.78Resazurin (20)triplets are photoreduced by aliphatic amines to resorufin in a process which is inhibited by molecular oxygen.79
IIl5: Photo-reduction and -oxidation
4
213
Miscellaneous Reductions
The relaxation processes of the C60-DMA adduct (DMA = N,N-dimethylaniline) in solution in cyclohexane and in benzonitrile, and the relaxation processes of C60-OQD-DMA (OQD = o-quinodimethane) have been compared.80Relaxation of C6,-OQD-DMA occurs intermolecularly mainly by exciplex formation, and although intramolecular quenching of C60-DMA in cyclohexane is made difficult as a consequence of an unfavourable molecular structure, in benzonitrile quenching occurs largely intramolecularly from the S I state by a through-bond mechanism. Functionalised poly(propy1ene imine) dendrimers are reported to be more efficient electron donors in photoelectron transfer to c60 in solution than either tripropylamine which resembles the interior of the dendrimer, or N-(tertbutoxycarbony1)-L-phenylalanine which resembles the shell of the dendrimer." The dynamics of the ion pairs C70-/aminef generated by excitation of the donor-acceptor complexes formed between C70 and a range of tertiary amines such as N,N,N',N,-tetramethyl-p-phenylenediamine,p-methoxydimethylaniline, p-methyldimethylaniline,N,N-dieth ylaniline, N,N-dimethylaniline, and triphenylamine have been studied in chlorobenzene solution.82The radical ion states arise both by excitation of the charge transfer complexes and by quenching of the excited singlet state of the C70 by the amine; a comparison has also been made with the C6damine system. Investigation of the photoreduction of the functionalised fullerenes C60[C(C02Et)2]n and related compounds such as eqCm[C(C02Et)& (n = 2), trans,-C6o[C(Co,Et),j, (n = 2), r r a n ~ ~ - C ~ [ C ( C 0 ~ E t ) ~ ] , , (n = 2), and eq-Cm[C(C02Et),13reveals that perturbation of the x-system inhibits the ease of the process, and that increasing the number of bis(ethoxycarbony1)methylene groups shifts the redox potential of the triplet state from +1.01 to 0.64.83*84 Irradiation of o-quinodimethane adducts of c60 in DMSO leads to the formation of monoanion radicals,85and DNA is cleaved by c60 in water in the presence of poly(vinylpyrrolidone), a reaction which is enhanced if the c60 adduct derived from the acridine 9-(4-azidomethylphenyl)acridine is used.86 The intramolecular photophysical properties of systems in which c60 is linked to the mesophenyl ring of zinc tetraarylporphyrin at the p-, m-, or o-positions by an amido group have been In particular, the size effect of fullerenes in electron transfer has been investigated using zinc porphyrin-linked quinone dyads, and the observed faster charge separation and slower charge recombination rates in the fullerenes are interpreted in terms of the smaller reorganisation energy in c60 as compared with that of benzoquinone. Irradiation of a triad consisting of a diaryrporphyrin (P) covalently linked to a carotenoid polyene (C) and C60 in 2-methyltetrahydrofuran induces electron transfer to give C-P'+-c60' which evolves into C"-P-C' -, and which subsequently decays to the carotenoid triplet state.89 Electron transfer at low temperature also occurs, and these observations may have relevance for photosynthetic reaction centres. The radical anion of c60 photogenerated in micellar media is reported to have a lifetime of the order of minutes." Valence isomerisations of some norbornadienes have been photosensitised using N-methylcarbazole and evidence presented supports the view that single
Photochemistry
214
electron transfer makes a greater contribution to the photoisomerisation than does triplet energy transfer." Reductive photoelectron transfer of various bicyclic a-cyclopropyl-substituted ketones by tertiary amines leads to regioselective cleavage of the cyclopropyl ring with formation of the exocyclic radical with an endocyclic e n ~la te .' ~Bicyclic ketones having unsaturated side-chains give bicyclic, spirocyclic, and tricyclic products from radical cyclisation. Photoredox reactions have been observed in self-assembled monolayers of 8-chlorooctyl disulfide adsorbed in gold.93 Oxidation occurs at the goldsulfur interface and reduction at the chlorine/air interface, and evidence is presented to show that these photoredox reactions are mediated by the gold surface. Photolysis of dibenzothiophene sulfoxide in a variety of solvents promotes oxidation of the solvent by a mechanism which may involve atomic oxygen [O(3P)]or a closely related non-covalent complex." Cyclohexane, for example, gives cyclohexene and cyclohexanol, and benzene gives phenol. Carbon dioxide has been photoreduced to carbon monoxide using [Re(bpy) (CO)3(P(OEt)3)]' as catalyst, and the quantum yield and turnover found to be dependent on both the intensity and wavelength of the radiation, with low intensities leading to higher quantum yields.95796 Evidence is presented to show that a one-electron species is important in the system. Photolysis of iron tetraphenylporphyrin and derivatives such as trimethyl-, dichloro-, and pentafluorophenyl gives the Fe'P state, and in the presence of C 0 2 , CO and (C0)Fe"P are formed.97 Related studies in aqueous media have been carried out on the Fe'P state of tetrakis(N-methyl-3-pyridy1)porphyrinand tetrakis(Nmethyl-4-pyridy1)porphyrin. In the presence of triethanolamine as a hole scavenger, irradiation of carbon dioxide in acidic solution and in the presence of metal phthalocyanine adsorbed onto a Nafion membrane gives formic acid,98 and photofixation of carbon dioxide into organic substrates such as benzophenone, acetophenone, and benzylic halides to produce benzilic acid, atrolactic acid, and phenylacetic acid respectively has been achieved by irradiating a saturated solution of the gas in DMF in the presence of CdS nanocry~tallites.~~ Spin coupling studies show that COz'- is produced, and the reaction is thought to occur by bimolecular coupling on the surface of the catalyst. The colloidal monodispersed nanocrystallite ZnS-DMF is effective for the photoreduction of carbon dioxide to formate and carbon monoxide in the presence of triethylamine, and MO calculations suggest that there is preferential absorptive interaction of C 0 2 with a zinc atom in the vicinity of a sulfur vacancy on hexagonal ZnS. loo Primary, secondary, and tertiary alkyl chlorides as well as aryl chlorides undergo reductive dechlorination using Sm12, and in the presence of CO ketones are formed by photocarbonylation."'
5
Singlet Oxygen
5,10,15,20-Tetrakis(heptafluoropropyl)porphyrin (TPFPP) in a fluorous biphasic medium is effective for 02('Ag) generation and possesses good stability for
M5: Photo-reduction and -oxidation
215
preparative photooxygenations. Io2 Evidence has been provided supporting the view that the coloured open forms of photochromic compounds will act as photosensitisers for 02('Ag) generation as shown by their ability to convert cisand trans-a,a'-dimethylstilbenesinto a hydroperoxide. '03 The yields of 02(' Ag) formed using a series of alkyl ether derivatives of chlorophyll-a and analogues as sensitisers have been detailed. '04 Both stereochemical and steric factors cause differences in the sensitising activity, and it is found that in general analogues of pyropheophorbide-a are more active than the related chlorin e6 derivatives. A range of ketones has been used to induce the photocatalysed decomposition of peroxymonosulfuric acid to Oz(lAg), and the kinetics of the process have been elucidated.' 0 5 Examination of the efficiency of O2('Ag) production using laser excited fullerene derivatives suggests that it is independent of the addend types, that it is decreased by increasing numbers of addends, and that proximate addends lead to greater decreases than do remote addends.'06 The generation of 02( 'Ag) from chlorine dioxide, hypericin, and buckminsterfullerene derivatives has been studied photophy~ically,'~~ and it has also been demonstrated that the iron-sulfur centre in ferredoxin is capable of acting as a photosensitiser for O2('Ag)generation; the quantum yield is estimated to be 0.0047.'0s Quenching of 02(a'A$ by 02(a'A& in CS2 solution has been examined spectroscopically and it appears that energy pooling to form 02(b'C,+) occurs.1o9The quantum yield of 02('Ag) generated from molecular oxygen and [Cr(bpyM'+ in D20 has been determined as 0.86.'1° Energy transfer is the main quenching pathway but physical quenching by spin deactivation may be an additional minor route. The quenching of 02(' A,)-sensitised delayed fluorescence (SOSDF) of tetra-tert-butylphthalocyanine by p-carotene, a-tocopherol, 1,4-diazabicyclo[2.2.2]octane,2,6-di-tert-butyl-4-methylphenol, and lauric acid is reported to occur by 02('A&-quenching only.' SOSDF is emerging as a highly sensitive method for the measurement of rate constants for 02(]Ag)quenching. The quenching kinetics of O2('Ag) by the colourless trap cyclohexa-l,3-dien1,4-diaceticacid disodium salt, used for the determination of 02('Ag) in aqueous solution, show that pure chemical quenching is involved, and that the products are a stable endoperoxide and a hydroperoxide. ' I 2 Sulfenate and sulfinate have been used to trap the perepoxide formed from adamantylideneadamantane, and these functionalities are claimed to be the most potent trapping agents yet reported.' l 3 Photooxidation of acyclic chiral olefins can be directed threoselectively by intramolecular H-bonding in the threo-A exciplexes of allylic alcohols, and the results have been compared with the diastereoselectiveepoxidations of heteroatom-substituted acyclic olefins using mCPBA. l4 Both processes are similarly influenced by allylic and homoallylic substituents. The mechanism of 02( 'Ag) sensitised luminescence of tetra-t-butylphthalocyanineand the photooxidation of a new antidiabetic drug, troglitazone, have been described,'I5 and an investigation of the quenching of various biomolecules, such as glycyrrhetic acid, isoliquiritigenin, and their glycosides glycyrrhizic acid and licurazid, has shown that the rates at which the glycosides are quenched exceed those of the corresponding aglycones by an order of magnitude.' l 6
''
'
216
6
Photochemistry
Oxidation of Aliphatic Compounds
Open chain, cyclic, and polycyclic alkanes can be photofunctionalised in the presence of 1,2,4,5-benzenetetracarbonitrileby an electron transfer process,' and the same group of workers also reports that irradiation of chloranil in the presence of alkanes such as 2,3-dimethylbutane, cyclohexane, norbornane, and adamantane leads to unselective hydrogen atom abstraction by the triplet state of the quinone and functionalisation of the alkane.' Catalytic oxygenation of hydrocarbons by carbonyl[5,10,15,20-tetrakis(pentafluorophenylporphyrinato)]ruthenium(I1) and 2,6-dichloropyridine N-oxide gives, a high conversion for hydroxylation in the cases of cis- and trans-decalin and of adamantane, for epoxidation in the case of oct-1-ene, and for benzoquinone formation from benzene.'" These reactions can be photostimulated at hirr > 560 nm and are thought to involve a Ru(II1) porphyrin complex which is oxidised to a reactive 0x0 species in the rate determining step. Irradiation of c60 in toluene gives both 3 C 6and ~ c60'+, and in the presence of Ph4P' Ph4B- c60'- is formed.'20 Both the radical cation and the radical anion arise by disproportionation of 3C60.Photooxidative functionalisation of c&) has been achieved using 3,4-benzo-1,2-disilacyclobutene and gives a 1:l cycloadduct, and similar reactions have also been successful using cyclotetrasilane and cyclotetragermane.l 2 Addition of allylic stannanes to c60 occurs on irradiation of their benzene or acetonitrile solutions to form allylic fullerenes and use of a pinene-derived stannane produces an enantiopure fullerene terpenoid. 22 [2+2] Cycloaddition of N,N-diethyl-4-methylpent-3-en-1-yn-1 -amine to c60 forms a stable C6o-fused cyclobutenamine which is reported to undergo self-sensitised photooxidation to a dihydrofullerenone. 123 Reaction of c60 with aryl azides produces triazolinofullerenes which can be photolysed to aziridinofullerenes, and the synthesis of 1,2-(3,4-dihydro-ZHpyrrolo)-[60]fullerenes by a [3+2] cycloaddition of photogenerated nitrile ylides is also reported.124Irradiation of [6O]-fullerene in the presence of RNMe2 (R = Ph, Me) gives an adduct which arises from addition of an a-C-H bond of the amine to the 6/6 ring junction of [60]fullerene (Scheme 2).'25It has also been reported that dialkoxy disulfides of the form ROSSOR (R = Me, Et, i-Pr, t-Bu, i-Bu, tBuCH2) can be photolysed to RO', ROS', and [ROS:O]', of which RO' is found to undergo addition to c60 to produce ROC60'.126 Photosensitised oxidation of alkenes in the presence of azides and molecular oxygen gives 1,2-azidohydroperoxides, and a study has shown that the reaction is initiated by electron transfer to produce a sensitiser radical anion and azidyl radicals.127Subsequently these add to the alkenes to give carbon radicals which are then trapped by molecular oxygen. Investigation of the reaction profile of isotopically substituted trimethylethylenes has shown that formation of the perepoxide is rate determining. 28 Reaction of 5-isopropylcyclohexa-1,3-diene with 02( 'Ag) promotes endoperoxide formation by preferential attack on the sterically more hindered face of the diene whereas attack on the less hindered face leads to an ene reaction.'29 This suggests that both the ene reaction and the cycloaddition occur through a common intermediate which may be a perepoxide. (-)-Menthone has been used as a chiral auxiliary to induce a highly diastereose-
'
'
'
III.5: Photo-reduction and -oxidation
c60
+
217
NMe2R
3:+ b H
-.
c60
CH2NMeR
H&
+.
-
NR
+
H2Cso
Scheme 2
lective cascade cyclisation of terpenoid polyalkenes by photoinduced electron transfer. 30 It is supposed that the chiral induction is achieved by chiral folding of the polyalkene chain (21). Electron transfer photoreactions of (E)-3,7-dimethylocta-2,6dien-l-o1 in methylene dichloride solution in the presence of 9,lOdicyanoanthracene leads to the formation of cis-2-(2-propenyl)-trans-5-methylcyclopentanemethanol, and in acetonitrile in the presence of 1,4-dicyanobenzene, two trans-fused 3-oxabicyclo[3.3.0]octanes are generated. 1 3 ' Radical cations lie on the reaction co-ordinate and these react by C-C cyclisation to give di-tertiary 2a-bifunctional methylidenecyclopentyl radical cations before undergoing back electron transfer and intramolecular hydrogen transfer. Cyclohexene in acetonitrile has been photooxygenated to cyclohex-Zen01 and 3,3'-bicyclohexenyl using dimethoxy-coordinated tetraphenylporphyrinatoantimony(v)as sensitiser A
218
Photochemistry
and potassium hexachloroplatinate(1V) as electron acceptor. ‘ 3 2 The mechanism involves hole-transfer from Sb(V)TPP+ to give the cyclohexene radical cation which then undergoes nucleophilic attack by water and dimerisation. Irradiation of optically pure (1 R,2S)-(+)-ci~-chrysanthemol(22) in the presence of dicyanobenzene as an electron acceptor leads to the formation of (R)-5-(1-(pcyanophenyl)-l-methylethyl)-2,2-dimethyloxacyclohex-3-ene (23; Ar = p-cyanophenyl).”’ The success of this reaction depends upon the relief of ring strain which can be rationalised in terms of nucleophilic attack of the radical cation on the terminal carbon of the vinyl group, and simultaneous replacement of the isopropyl radical as an intramolecular leaving group in an SN2’reaction. H
Me
H’
L
Anaerobic photooxidation of a range of aliphatic carboxylic acids using iron(II1) tetra(2-N-methylpyridy1)porphyrin has been described and the primary step reported to be photoreduction of the Fe(II1) atom by the axial carboxylate ligand to give a solvent caged [RC02’ Fe(1I)porphyrinl species.’34The rate of the reaction is determined by competitive reactions of RC02’ in the solvent cage. Irradiation of the bichromophoric system consisting of 4-methoxyaniline as electron donor and N-alkylnaphthalimide as electron acceptor produces a singlet state charge transfer complex which intersystem crosses to give (D-3A), and in which the triplet spin is located on the naphthalimide residue.13’ In more polar solvents such as dibutyl ether this species equilibrates with 3(D’-A-), and in even more polar solvents only 3(D’-A-) is detectable This observation is rationalised in terms of retardation of the singlet charge separation process leading to effective competition by the triplet pathway. The ether (24;X = H2) has been oxidised to ester (24; X = 0) by irradiation in the presence of benzil as sensitizer in a transformation which may proceed by a mechanism involving a benzoylperoxy radical. 136
Irradiation of 2-phenylcyclopropanone methyl tert-butyldimethylsilyl acetal in the presence of either phenanthrene or pyrene as redox photosensitiser and dimethyl fumarate as radical acceptor, gives a mixture of the cross-coupled
IIL5: Photo-reduction and -oxidation
219
product and the P-carbonyl radical dimer. 137 Selenoglycosides undergo photoinduced electron transfer using aromatics as acceptors. 38 The phenylselenyl radical cation produced in this process then cleaves to a glycosyl cation which subsequently reacts with various alcohols to give the 0-glycosides. Photolysis of ring-substituted phenacyl bromides gives ketyl radicals which in the presence of alcohols Ieads to the corresponding carbonyl compounds. 39 Two different chain mechanisms seem to operate and in the presence of methanol, oxidation predominantly occurs by hydrogen transfer whereas in the presence of isopropanol, acetone is formed mainly by an electron transfer process.
'
'
7
Oxidation of Aromatic Compounds
A theoretical study of atmospheric photooxidation of aromatics has been undertaken in which the energies of the intermediates are obtained by the semiempirical UHFPM3 geometry optimisation method combined with ab initio calculations. l4O Full mechanisms for hydroxyl radical initiated transformations are obtained in some cases. Photodegradation of organic pollutants in water has been carried out using metal-supported Ti02 catalysts prepared by the sol-gel technique,I4' the photocatalytic behaviour of 2,4,6-trinitrotoluene in Ti02 systems studied,'42 and the role of aromatic hydrocarbons in O2(*A,) formation and its part in the initial stages of the photochemical cracking of gas-oil fractions established. '43 Benzene is oxidised by H202 in the presence of U022+as photocatalyst to give phenol, and toluene is converted to a mixture of benzaldehyde, benzyl alcohol, and cresols.144In this transformation, *UO$+ is quenched by H202. Perylene undergoes photooxidation in aqueous solutions of some polymeric photocatalysts, and the ability of 1,4-naphthoquinone to function as electron acceptor has lead to its being postulated as a participating entity within a naphthalene-based catalyst.145Observations suggest that the polymer has a number of key functions, including solubilisation of the hydrophobic molecule, energy migration through the polymeric coil, energy transfer, and enhancement of oxidation by photoelectron transfer. The atmospheric photooxidation of alkylbenzenes using 0(2,3,4,5,5-pentafluorobenzyl)hydroxylamine has been studied as a GC/MS method for the detection and identification of carbonyl products, '46 and naphthalene has been photooxidised on a titanidFe203 catalyst in a process which may have some relevance for the fate of micropollutants in the at m 0~phere. I~~ Irradiation of a mixture of 1,5-dimethoxynaphthalene,ascorbic acid, and organoselenium compounds such as R2CHSePh causes electron transfer to the selenium and reductive activation of the selenium substrate.'41 Cleavage of R2CHSePh' gives a carbon-centred radical and PhSe-', the synthetic utility of this process is discussed. Room temperature irradiation of a deoxygenated melt consisting of 9methylanthracene (9-MeAn), l-ethyl-3-methylimidazoliumchloride (EMI), and A1Cl3 leads to the formation of an anti [4+4] dimer together with six other products. 149 These latter materials arise by electron transfer from the singlet excited state of the substrate to the substituted imidazolium salt to give 9-
Photochemistry
220
MeAn" and EMI'. Evidence is presented to suggest that both the radical cation and radical anion of 9-MeAn are present at different stages of the reaction. An examination of the rate constants for the fluorescence quenching of a series of meso-substituted anthracene derivatives by molecular oxygen suggests that charge transfer is involved in the quenching process.'50 However, at rates less than the diffusion limit, there appears to be little charge transfer involvement. Irradiation of oxygenated solutions of alkylbenzenes in chloroform gives high yields of the corresponding acids. 15' Visible light irradiation of alkylbenzenes in the presence of the 10-methylacridinium ion in aqueous acetonitrile leads to 9' 5 2 By contrast, irradiation in perchloric alkyl- 10-methyl-9,lO-dihydroacridine. along with oxygenation of acid gives a mixture of 10-methyl-9,lO-dihydroacridine the substrate to benzyl alcohols. Quenching studies reveal an electron transfer reaction from the alkylbenzenes to the excited singlet state of the acridinium ion to generate the radical cation of the alkylbenzene. In the presence of Fe(C104)3, an aqueous emulsion of cumene will undergo photooxidation by air to form acetophenone as the predominant product. Autoxidation of diphenylmethane in DMSO containing t-BuOK causes chemiluminescent emission from the triplet state of benzophenone.' 54 In the presence of 9,lO-dibromoanthracene as activator, fluorescence from the 9,lO-anthrasemiquinone radical anion is seen, and may arise in the degradation of the dioxetan formed by substitution of a diphenylmethylperoxy anion for bromine in 9,lO-dibromoanthracene. Benzophenone is reported to be the product of peroxyls from the chemiluminescent oxidation of diphenylmethane, and the absolute yields for the excitation of triplet benzophenones have been measured. 55 In conjunction with MNDO calculations on some intermediate structures and for the energy profile of their decay, the conclusion has been reached that excitation of the 3n51* state occurs in the region between the transition state of the intermediate Ph2CHOOOOCHPh2 and Ph*C0000', it is this biradical which subsequently decays to benzophenone and oxygen. The carbanion generated from diphenylmethane in the presence of tBuOK in DMSO solution is oxidised by molecular oxygen and the solution found to chemiluminesce.'56 The origin of this emission has been ascribed to relaxation of an excited state of benzophenone which has been formed as an excited species by degradation of a peroxide anion. Sensitised photoelectron transfer of (pheny1azo)triphenylmethane using dicyanoanthracene gives a mixture of benzenetriphenylmethane, 3,3,3-triphenylpropionitrile, and [4-(diphenylmethyl)phenyl]acetonitrile in the same proportions as the direct irradiation.' 57 Use of 2,4,6-triphenylpyrylium tetrafluoroborate, however, gives 9-phenylfluorene as the principal product. Observations suggest that this arises by photoreaction of the trityl cation formed on decomposition of the starting material, and that in these transformations the rate of back electron transfer between the radical anion of the sensitiser and the trityl cation is important. Chiral 1,2-dihydronaphthalenes undergo a strongly stereoselective photooxidation suggesting that a common intermediate with perepoxide geometry exists on the reaction coordinates,' 58 and photooxidation of 3,3'- and 4,4'-dimethylbiphenyls has been reported to occur in aqueous media without the use of a photosensitiser to give mainly the corresponding aldehydes and carboxylic
'
II/.5: Photo-reduction and -oxidation
22 1
acids.159Irradiation of tetracene on dry SiOz at low surface coverage produces syn and anti dimers.160 Under aerated conditions tetracene-5,12-endoperoxideis formed in a reaction which is mediated by OZ(lAg),and at higher surface coverage the degree of dimerisation is increased. Irradiation of the vinylbiphenyl (25) at low temperature and under an oxygen atmosphere in the presence of TPP gives (26), which in DMSO solution has been found to luminesce.’‘I Optimum conditions have been elucidated for the photooxidation of lignin model compounds having both carbonyl and ring-conjugated double bonds using hydrogen peroxide. 162 The results show that oxidative cleavage of the C& double bond as well as demethylation occurs. Irradiation of styrene in acetonitrile solutions of aromatic nitro compounds gives the corresponding nitrones. 163 Under similar conditions, however, 4-methyl-Sethenylthiazole gives a pyrrole analogue of thianthrene, and 1,l-diphenylethylene gives benzophenone. These observations have been rationalised on the basis of AM 1 semiempirical calculations involving the frontier orbitals of the reagents. Under photoelectron transfer conditions using 9,1O-dicyanoanthracene, 2,6diary1 substituted hepta- 1,6-dienes and 2,7-diaryl substituted octa- 1,6-dienes are mainly converted into the corresponding annulated cyclobutanes, and in cases of Direct photooxidation electron-rich substrates, endoperoxides are produced. of RR’C=CHCH*SiMe3(R = Ph, R’ = H, Me, Ph; R = 1-naphthyl, R’ = H) gives RR’CO (same R,R’) and RR’C=CHCHO (R = Ph, R’ = Me, Ph),’65and stilbenes are reported to be photooxidised more efficiently when included within zeolites than in isotropic organic solvents, in a reaction whose efficiency depends upon the cation and the presence of intra-zeolite water.166
2-Amino-3-(4’-nitrophenyl)propan-1,3-diol in its optically active form has been selectively photooxidised to the corresponding ketone,167 and a study of the oxidation of podophyllotoxin (27) by sodium persulfate using laser flash photolysis has enabled rate constants to be determined for the formation and decay of transients. 168 Photoinduced electrochemical oxidation of benzyl alcohol to benzaldehyde has been achieved with 100% product selectivity and 100% current efficiency using visible radiation in the presence of riboflavin 2,3’,4,5’-tetraacetate. Photoelectron transfer oxidation of various phenols in the presence of 2nitrofluorene has been examined in both acetonitrile and cyclohexane solution. 70 Although no charge transfer donor-acceptor pairs are present in the ground state, a contact exciplex is apparent in cyclohexane, and in acetonitrile the anion radical of 2,6-dimethylphenol has been observed as the final product. Irradiation of
222
Photochemistry
Me0
OMe
Me0
(27)
neutral aqueous solutions of [CO(NH~)~N#+ containing m-phenyl and p-phenylphenols releases Nj* and the m-phenylphenoxyl radical which reacts efficiently to give phenolic dimers as the major p r ~ d u c t . ' ~ The ' same authors also discuss the photochemistry of solutions of [Co(NH3)5N3I2' containing 2,6-dimethylphenol,'72 and rate constants have been measured for photoinduced electron transfer between the hydroxyl groups of a non-covalent assembly of a calix[4]arene (28)-substituted Zn(I1) metalloporphyrin and benzophenone. 173
The photokinetics of electron transfer within the exciplex acenaphthenone/ 9,lO-dicyanoanthracene in which the ketone acts as donor have been measured, and show that the forward rate exceeds the rate of the reverse process and other decay processes. 74 Photoredox reactions of acids of the type ArXC02H (X = CH2, OCH2, SCH2, SOCH2, and S02CH2)have been examined on Ti02 and the catalyst shown to act site-selectivelyin that the phenylacetic and phenoxyacetic acids undergo oxidative decarboxylation, whereas the arylthioacetic acids are oxidised to the corresponding sulfinylacetic acids. 175 Photoinduced singlet intermolecular and intramolecular electron and energy transfer within four dyads, comprising 9anthranoic acid and different xanthene dyes linked by identical flexible bridges, have been studied.'76 The results can be interpreted in terms of the conformations of the dyads. Irradiation of charge transfer complexes of naphthaleneacetic acids and 1,2,4,5-tetracyanobenzenein the crystalline state is reported to cause their decarboxylation, but the behaviour of 3-indoleacetic acid under similar conditions is ineffi~ient.'~'
IIl5: Photo-reduction and -oxidation
223
Excitation of the charge transfer band of the complex N,N-dirnethyl-4,4'bipyridinium (PQ2')-- 1,4-dimethoxybenzene (DMR) gives (PQ'+).-(DMB'+), and it is also reported that similar radical ion pairs have been obtained on excitation of complexes involving crown ethers. 178 The rates of charge recombination have been interpreted using the Fermi-Golden rule. Titanium dioxide modified by a bipyridinium monolayer, possesses enhanced activity for the degradation of ethers such as lY4-dimethoxybenzeneand 1,2-dimethoxybenzene, as well as for indole and eosin.'79 This observation implies the formation of a supramolecular donor-acceptor complex having the bipyridinium units at the semiconductor interface. The photooxidation of cypennethrin, a pyrethroid, is sensitised by various dyes, and a kinetic study confirms the mechanism of Oz(lAg) photooxidation. 18* Furfural can be photooxidised in methanol using uranyl salts as sensitiser to give furan-2-carboxylic acid in a reaction which proceeds via U(V) as intermediate,l8I and photooxidation of the alditol derivative of furan [(S)-29; R' = alditol group] with 02('Ag) gives (30; same R'), a specific example of which is the conversion of (3 1) into (32). '82 Selective oxyfunctionalisation of the complexed furans (33; R = H, Me) in methanol followed by reduction gives the furanone (34) and the ZIE-enediones (35); the precursors are the furan endoperoxides which are trapped by methanol.'83 As part of an investigation into the synthesis of the zaragozic acidsqualestatin backbone, (36) has been made by sequential CHO I
CH I1
Y
CH I
c=o
CHO CH II1
O
I
MeCO2-1
c=o
HO
R
H
M e C O zRi
(29)
(30)
(33)
+ ' Me Me
Me
(31)
(34)
Photochemistry
224
treatment of (37) with OZ('Ag), sodium borohydride, and triisopropylsilyl triflate.Ig4 Using an analogous procedure, the target compound (38) has been synthesised by a comparable photooxygenation. Photosensitised oxidation of hedychenone, a furanoid diterpene, leads to the formation of several products in which the furan ring has been oxidised.'85
TIPSO
OTIPS
Some substituted dibenzo-7-silabicyclo[2,2,l]hepta-2,5-dienesare reported to phototransfer an electron to TCNE using 2,4,6-triphenylpyrylium tetrafluoroborate as sensitizer to give difluorosilane and anthracene as products.'86 The MOPAC MP3 method has been used to examine the electronic features of the radical cation of the parent silane.
8
Oxidation of Nitrogen-containing Compounds
Anchoring of a linear, rigid molecular dyad comprising triethylamine as electron donor linked to [Ru-bis(terpyridine), S] as acceptor to a nanocrystalline TiOz electrode leads to charge transfer to give D'-S I (e-)TiO:! which decays under i l l ~ m i n a t i o n . 'Photoelectron ~~ transfer reactions of N-allylamines such as Nallylpiperidine in the presence of methyl crotonate or other ap-unsaturated esters and catalysed by anthraquinone, give lactams.'81 The reaction probably involves u-aminoallyl radical formation, which following rearrangement to the a-aminoalkyl radicals reacts with the ester. Allylic amines also undergo oxidation by Oz('A,) in a process whose chemoselectivity and pathway is determined by the character of the allylic nitrogen atom.Ig9In particular the secondary amine (39; R = neopentyl, p-anisylmethyl) gives imines by a-oxidation, the primary amine
IIl.5: Photo-reduction and -oxidation
225
(39; R = H) and amides (40) undergo 1,Zaddition of 02(’Ag), and the imide (41) follows an ene reaction. The lactam (42) undergoes both an ene reaction and 1,2‘Ag). Irradiation of the cyanine dye, anhydro-1’,1’-diethyl-3,3’addition of 02( disulfobutyl-5,5’-dicyanimidazolocarbocyaninehydrochloride, adsorbed on to the surface of a Ti02 colloid causes electron injection into the conduction band of the Ti02 from the excited singlet state of the dye,’90and single electron transfer of ap-silylamino-enones and -ynones using DCA as sensitizer gives hydropyridine rings by 6-endo type a-amino radical cyclisations in a process which is under regio- and stereocontrol control.’” A study of the effect of Ca2+ on the rate constants for photoelectron transfer in the donor-donor and acceptor-donor stilbene-crowns (43; R 1= CN, Me2N; R2 = H, CN) using time-resolved transient absorption spectra with sub-picosecond excitation has been reported. I 9 l Photoinduced intramolecular charge separation in (44;R = H, Me, MeO) has been d i s c ~ s s e d . ’For ~ ~ (44;R = H), local emission is observed, and for (44;R = Me, MeO) there is formation of a charge separated state with extended conformation and electron transfer across three o-bonds. These observations have been interpreted in terms of intramolecular radiationless transition theory. The efficiency of aniline photooxidation by colloidal CdS is enhanced by doping with Ag+.194At concentrations in excess of about lo%, the effect ceases to be apparent and this suggests that Ag2S is formed on the surface of the catalyst preventing photogenerated holes from interacting with adsorbed aniline. This catalysis implies the occurrence of radiation-induced charge transfer to the adsorbed redox species at the CdS-Ag2S interface. Excitation of the charge-transfer band of 4aminonaphthalene- 1,g-diimide induces a very efficient two-step electron transfer process, and in the linear dyad formed when aniline is attached to p-methoxyaniline by a piperazine bridge, electron transfer occurs to give a 99% yield of the ion pairs. 195 Attachment of 1,8:4,5naphthalenediimideusing a 2,5-dimethylphenyl spacer produces a dyad in which the same high yield of charge separation can occur. Methylviologen is photocatalytically oxidised by air in aqueous suspensions of Ti02 and addition of hydrogen peroxide is found to cause an increase in the initial rate of the r e a ~ t i 0 n . lIt~is ~ suggested that the routes for the oxidation of the methylviologen in the presence and in the absence of hydrogen peroxide are independent, and that they involve adsorbed methylviologen and the same forms of primary photogenerated oxidants. S-Containing amino acids and methionine-containing di- and tri-peptides have been photooxidised in aqueous solution using 4-carboxybenzophenone as sensitiser. 197 Quenching constants have been determined and the primary photochemical step was shown to be electron transfer from sulfur to the triplet state of the ketone. Photooxidation of N-furfurylphthalimide under an oxygen atmosphere using a sensitiser such as Rose Bengal, Cb0, or coronene gives 5-phthalimido-4oxopentenoic acid which can be converted into 5-aminolaev~linic.’~~ Using dicyanoanthracene as sensitiser, photoelectron transfer oxidation of 6,6-diphenylhex-5-en-2-one oxime, 5,5-diphenylpent-4-enal oxime, and 2,2-dimethyl-5phenylpent-4-enal oxime induces cyclisation to 3-methyl-6-diphenylmethyl-5,6dihydro-4H- 1,2-0xazine, 6-diphenylmethyl-5,6-dihydro-4H1,2-0xazine, and 6benzyl-4,4-dimethyl-5,6-dihydro-4H-1,2-oxa~ine,~~~ and irradiation of oxyge-
Photochemistry
226
OMe OMe (39)
nated and oxygen-free suspensions of Ti02 containing tetranitromethane causes degradation initiated by conduction band electrons or by superoxide anions.200 Further redox reaction gives nitrate and ammonium ions, and carbon dioxide, formate and formaldehyde. 2-Mercaptopyridine (PySH) is photocatalytically oxidised to the corresponding disulfide in the presence of heptylviologen in a process which occurs by electron transfer within a complex.201Attempts to apply this transformation to produce 2oxazolidinone from carbon dioxide and L-phenylalaninol (2-amino-3-phenylpropan-1-01) in the ground state, however, fail, but irradiating (kirr>300 nm) the reaction mixture promotes the transformation successfully. This is rationalised in terms of the higher reducing power of excited Pys- even in the absence of HV2’F‘yS- complex formation. Examination of the photophysical properties of the two bicoumarins (45; R = H, CN) bridged by a flexible single covalent bond, reveal solvent dependent spectral shifts which imply that the lowest excited state of both dyes is intramolecular charge transfer in nature.202Quinoline has been used to probe the basic mechanisms of degradation of aromatic compounds in water by TiO2, and the results show that the oxidative steps do not involve only the hydroxyl radical, but that activation by hole transfer is also important.*03Direct irradiation of 2,2’-indolylindolines in air produces fluorescent 2,2’-biindole~,’~~ and the reaction of 0 2 (‘Ag)with 2-phenylindole and 1-methyl-2-phenylindoleis a
IIi.5: Photo-reductionand -oxidation
227
key step in the synthesis of 1,2,2-trisubstituted l,2-dihydro-3H-indol-3-ones.20s The same authors also report that photooxidation of various 2-arylindoles in the presence of methylene blue gives 2,2’-diary1-[2,3’-bi-lH-ind01]-3(2H)-ones.*~~ The charge separated state generated by excitation of poly(N-vinylcarbazole) coabsorbed with 1,2,4,5-tetracyanobenzeneas acceptor on the microreticular resin Amberlite XAD-8 has been shown to possess a long lifetime.207This has been interpreted in terms of hole migration along the polymer chain, and it appears that the trap sites are not simply related to carbazole dimer cation sites but also to carbazole moieties in the vicinity of polar ester groups in the absorbent.
Photooxidation of 5-methyl-2’-deoxycytidineusing menadione (2-methyl-l,4naphthoquinone) as a type I sensitizer produces several stable products including
5-(hydroperoxymethy1)-2’-deoxycytidine,
5-(hydroxymethyl)-2’-deoxycytidine,
and 5-formyl-2’-deoxycytidine in which the methyl group has been oxidised.208 Such compounds may arise by deprotonation of the 5-methyl-2‘-deoxycytidine radical cation generated in the photoreaction. The photocatalysed oxidation of thymine using an aqueous suspension of Ti02 has been shown to involve HO. and H’ and to lead to thymine glyc01,2~~ and the thienopyrimidine (46) and has been photooxygenated to (47).210
Sensitised photooxidation of tert-butyl 3-methoxypyrrole-2-carboxylatein methanol gives tert-butyl 3,5-dimethoxypyrrole-2-carboxylatetogether with a bipyrrolic oxidative coupling product (48).21A novel ring cleavage of 4,5-bis(4fluorophenyl)-a,a-bis(trifluoromethyl)-lH-pyrrole-2-methanamine (49) using 02(’Ag) followed by acid catalysed dehydrocyclisation leads to the 4,4-bis(trifluoromethy1)imidazoline (50) which can be derivatised into (51), a compound which has acyl CoA:cholesterol acyltransferase activity.2122-Imidazolidinethione
’
228
Photochemistry
undergoes oxidation by excited UO~”/HC104 to give 2-imidazolidone, sulfur, sulfur dioxide, and sulfuric acid.” The transformation occurs via exciplex formation and U(V) has been identified spectrophotometrically in the reaction mixture. 2-Imidazolidinethione in SDS micelles is photooxidised in the presence of methylene blue to 2-imidazolidine-2-yl-sulfinic acid as the primary product in a process which is mediated by 02(iA,).2i4 High concentrations of SDS promote enhanced energy/electron transfer within the MB-302/MB-imidazolinethione pairs, and an ionic mechanism is found to operate at higher imidazolinethione concentrations. Dye-sensitised photooxidation of the 2,4-disubstituted imidazoles (52; R = Ph, Me, 4-O2NC6H4,4-MeC6H4) using haematoporphyrin in methanol, leads to the formation of the 3-imidazolin-5-ones (53) and the 2imidazolin-4-ones (54),”’ and photooxygenation of 3,7-dihydroimidazol[1,2alpyrazin-3-one at - 78 “C gives two peroxidic products (1,2-dioxetanes), an observation which has implications for the photooxygenation of coelenterate luciferin.’ l 6
Me0 C02CMe (48)
F
Ph
ph\
2kR OMe
(53)
R (54)
HIS: Photo-reductionand -oxidation
229
cis-4-Amino-L-proline derivatives having different redox-active carboxylic acids as side chains have been incorporated into a helical oligoproline possessing in linear array a phenothiazine (donor), [Ru(bpy)312' (chromophore), and anthraquinone ( a ~ c e p t o r ) .Irradiation ~'~ of this triad at 460 nm causes formation of phenothiazine" and anthraquinone' - . A study of the effect of a magnetic field on the lifetime of the triplet biradical generated by photoinduced intramolecular electron transfer in an a-cyclodextrin inclusion complex of a phenothiazineviologen chain-linked compound in aqueous media, has shown that magnetic fields up to -1 T dramatically increase the lifetime, but that beyond this point a decrease becomes apparent.218This reversal has been attributed to a spin-lattice relaxation mechanism which is induced by anisotropic Zeeman interaction. Weak charge transfer complexes are formed between C60and phenothiazine or methylphenothiazine, and an investigation of the charge separation-recombination and ion dissociation reactions in the solid phase following charge transfer excitation have shown that C60'- is formed.219The effects of solvent polarity on charge recombination have been probed. An examination of photoelectron transfer within multifunctional porphyrin and ubiquinone analogues linked by H-bonding interactions which serve to hold the donor and acceptor in a co-facial orientation, has been made, and is the first example of electron transfer between such structures which is complete within the picosecond range.220Irradiation of colloidal syn-tetraresorcinolporphyrinhaving eight alkylphosphocholine side-chains (octopus porphyrin), and of the corresponding Zn(I1) complex in the presence of hydrophobic phenyl-p-benzoquinone or hydrophilic naphtho-l,2-quinone4-sulfonate, induces electron transfer to the quinone; electron transfer to MV2+ is also reported.221A study has been made of photoinduced electron transfer within dyads consisting of electron donating porphyrins covalently linked to accepting porpyrins in various solvents.222A correlation has been established between the electron rate constants and the solvent static dielectric constants, and the results stand in contrast with those obtained using porphyrin-quinone dyads. Photoelectron transfer from a polyconjugated porphyrin-containing polymer to bromanil occurs by a two-quantum mechanism.223Long wavelength photoexcitation induces intramolecular electron transfer to the porphyrin fragments of the macromolecule, and the second quantum promotes intermolecular electron transfer from the porphyrin radical anion to the neutral polymer followed by dark electron transfer to the acceptor. Visible light irradiation of the cerium(1V) complexes of octaethyl- and tetraphenyl-bisporphyrins in the presence of triethylamine as electron donor causes their photoreduction, and irradiation in carbon tetrachloride solution induces n-cation radical formation.224
9
MiscellaneousOxidations
Sodium sulfide has been photooxidised using Zn and A1 complexes and the metal-free derivatives of tetrasulfophthalocyanine in oxygen-saturated alkaline solutions to give the sulfate.225 Irradiation of self-assembled n-dodecanethiol
Photochemistry
230
monolayers (SAMs) on Au( 111) in air leads to the corresponding sulfonate by a process in which although the ordered structure of the SAM is destroyed, the underlying gold surface is not exposed.226Observations have been reported which suggest that the mechanism of oxidation of diethyl sulfide with O2('Ag) is consistent with persulfoxide formation followed by reaction with methanol to give a hydroperoxy-methoxy s ~ l f u r a n e .Short ~ ~ ~ wavelength irradiation of aliphatic disulfides, sulfides, and butyl thiol in the presence of molecular oxygen principally gives sulfonic acids, but sulfinic acids and thiosulfonate have also been detected as intermediates, and thiyl radicals may be involved.228Photooxidation of 3~-acetoxy-5a-hydroxy-6~-sulfanylpregnan-2O-one with HgO-I2 leads via a sulfenyl iodide to the 5,lO-secopregnane having a sulfur bridge between C6 and C,0,229and irradiation of alkyl aryl sulfoxides induces cr-homolysis to give sulfinyllalkyl radical pairs whose reactions partition themselves between a number of processes including disproportionation to the corresponding olefin and arylsulfenic acid.230A CIDNP study of the photoreactions of H-Cys(R)-OH (55; R = Me, Et, CMe3, CH2C02H, CH2CH2C02H) in the presence of 4carboxybenzophenone in D20 has shown the quenching mechanism to be photoelectron transfer from the S atom.231Following decarboxylation of (55"), [RSCH2CHNH2]' is formed. The enol silyl ether (56) undergoes a photoinduced reaction in the presence of chloranil to give cyclohexenone and the adduct (57) and the currently available evidence suggests that the reaction proceeds by electron transfer to the photoactivated chloranil to give (56)*c.232A photophysical study has appeared of the 'chromophore-quencher' compounds [Au(CCPh)(L')] (L' = 1 0-[(diphenylphosphino)methyl]anthracene) and [Au(CCPh)(L2)]BPh4(L2 = 1-[2-(diphenylphosphinoxy)ethyl]pyridinium], 1-[2-(diphenylphosphinoxy)ethyl]-4-methyl pyridinium, 1-[2-(diphenylphosphinoxy)ethyl]-4-tert-butylpy~dinium, and 1-[2-(diphen y lphosphinoxy)ethyl]triethylammonium in which the Au(CCPh) chromophore is linked by a flexible tether to the acceptors.233
Cl
(57)
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6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen BY ALBERT C. PRATT
1
Introduction
Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes,' photochemical synthesis of macrocycles,2 phototransformations of phthalimido amino acids,3 photoaddition reactions of amines with aryl alkenes and arenes? photoreactions between arenenitriles and benzylic donor^,^ photostability of drugs,6 polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids,7 photoreactions of pyrroles,' photoamination reactions in heterocyclic synthesis? switching of chirality by light," photochromic diarylethenes for molecular photoionics' and solid state bimolecular photoreactions. l 2 Reports of single electron transfer (SET) processes were again prominent, as were examples of the use of photochemical approaches to chiral synthesis.
'
2
Nitrogen-containing Compounds
2.1 E,Z-Isomerisation - E-Z photoisomerisation is frequently observed for compounds containing a n-bond, the efficiency of the process for the stilbene, Nbenzylideneaniline and azobenzene chromophores being markedly dependent on the presence of electron-donor and electron-acceptor substituents at the 4- and 4'positions. l 3 Solvent viscosity and polarity effects on the E - 2 photoisomerisation of E-4-dimethylamino- and E-4-azetidinyl-4'-cyanostilbenes have been reported. I 4 * l 5 Aggregate formation, structure and properties in bilayer vesicles and Langmuir-Blodgett films have been studied using stilbene- and azobenzenederived phospholipids, for example (1). I 6 Relatively small integral numbers have been established for aggregate sizes, and chiral cyclic pinwheel structures have been proposed. Clean E - 2 photoisomerisation of the azobenzene phospholipids occurs and in water the 2-products appear not to be aggregated. The azobenzene aggregates in dipalmitoyl phosphatidyl choline vesicles containing an entrapped reagent promote efficient release of the reagent from the vesicles on irradiation with wavelengths absorbed by the azobenzene chromophore. Whereas E-stilbene and E-azobenzene are nearly planar, N-benzylideneanilines generally have a nonPhotochemistry, Volume 29 @ TheIRoyal Society of Chemistry, 1998
Photochemistry
240
= H, Y = NO2 X = NMe2,Y = H X = NMq, Y = NOn
(2) X
planar conformation due to conjugation between the aniline-derived ring and the nitrogen lone pair electrons. In concentrated sulfuric acid the N-benzylideneanilines (2) are fully protonated at the imino nitrogen lone pair and their uv absorption spectra are similar to one another and to that of E-stilbene. Protonation prevents delocalisation of the nitrogen lone pair, with consequent coplanarity arising from .n-conjugation between the aniline-derived ring and the carbon-nitrogen double bond and strongly suppresses the fast thermal 2 - E isomerisation characteristic of unprotonated 2-N-benzylideneanilines.On irradiation, protonated (2) undergo reversible E - 2 photoisomerisation and a photostationary state is reached.I7 The ring substituents have little effect on the E-2 and Z-E photoisomerisation quantum yields and a photoreversible process occurring through a common excited state, probably the short-lived K-K* S1 state, is suggested. A series of E-7-arylidenenaltrexone opioid antagonists has been converted to the less-accessible 2 isomers,’* and 15-2 photoisomerisation occurs for the photochromic benzodithiacrown ethers (3) and their complexes with Hg2+ and Ag+ cations.’’ Strong intramolecular hydrogen bonding in 2-2[2-(2-pyridyl)ethenyl]indole (4) in both polar and non-polar solvents accelerates deactivation of the 2 singlet excited state, thereby suppressing 2 to E interconver-
(3)
R = Me, (CH&COzH (n = 3,4)
x=o,s
(5)
H
(4)
IIl6: Photoreactionsof Compounds Containing Heteroatoms Other than Oxygen
(7) R’
241
= H; R2, R3 = alkyl; X = 0
(9) R’ = 3-, 4- or 5-OMe, 4-NEt2
X
= 0, S; NR2R3= Leu-Leu-OMe
sion, whereas E to 2 photoisomerisation around the carbon-carbon double bond occurs from the E singlet excited state.20In contrast, triplet excited (4) undergoes mutual EIZ interconversion, with each process originating from a specific rotamer.21 The influence of intramolecular hydrogen bonding is solvent dependent in the case of the pyrrolylethenylquinoxaline (5),22with direct irradiation in non-polar or polar aprotic solvents leading to high ZIE ratios (benzene 99:l; acetonitrile 24:1) whereas in methanol, in which intermolecular hydrogen bonding can occur, the E isomer predominates at equilibrium (ZIE ratio 15). A detailed investigation of the influence of ground state conformation and intramolecular hydrogen bonding on the photophysical and photochemical behaviour of E- and Z-ethyl 3-(2-indolyl)propenoates has been reported.23 The E-ester exists predominantly in the s-trans enone conformation (6) and the rate constant for singlet excited state isomerisation of this conformer has been determined. The Z isomer possesses a relatively strong intramolecular hydrogen bond and photoisomerises inefficiently in non-polar solvents, resulting in high enrichment by the 2 isomer at the photostationary state. The corresponding N,N-dimethylamides exhibit analogous behaviour. 2-Hydroxycinnamides act as photosensitive protecting groups for amines. Irradiation converts them to the Z-isomers (7)which readily lactonise to form coumarin (8) with release of the a m i x ~ e .A~ ~similar strategy has been used for release of cytotoxic L-leucyl-L-leucine methyl ester from derivatives (9).25Irradiation of the E-enaminoketones (10) causes photoequilibration with the 2 isomers; rapid thermal reversion occurs in the dark.26 The related E-enaminoketones (11) do not undergo E-2 photoisomerisation, presumably for steric reasons. Both the E and 2 isomers of N-methoxy-l-(1pyreny1)methanimine (12) undergo geometrical isomerisation on triplet sensitisation and a mechanism involving rotation, rather than inversion, around the carbon-nitrogen double bond is pr~posed.~’ Irradiation of E,E-azobenzenophane
(10) R’ = H; R2 = Ph, 4-EfOC6H4, PhCH2 (1 1) R’ = Me; R2 = Ph, 4-EtOC6H4, PhCH2
242
Photochemistry
.aH (14) Ar = Ph, 3-pyridy1, 3-KalkyIpyridinium
(15) Ar = Ph, 4-pyridyl, 4-Nalkylpyridinium
(13) results in geometrical isomerisation about the nitrogen-nitrogen double bonds, yielding a photoequilibrated mixture of the E,E-, E,Z- and Z,Z-isomers.** The E-3-azopyridinium salts (14) undergo E - 2 photoequilibration on excitation whereas the corresponding 4-substituted isomers (15) are inactive.29
Photocyclisation - A wide variety of ring-forming reactions has again been reported. Irradiation of azepine derivative (16) results in 4-n-electrocyclisation to a mixture of the corresponding e m and endo c y c l o b ~ t e n e s 6-n-Electrocyclisa.~~ tion has been employed in a scaled-up synthesis (>300g) of 6-aza- 1,lO-phenanthroic anhydride (18) from the stilbazole (17).3' E-Azobenzene, incorporated into water-swollen acid-form Nafion fluorocarbon membranes, exists as the protonated form (19) and exhibits ambient temperature fluore~cence,~~ previously
2.2
q \
&x+
0
0
0
\
N
N
observed only at low temperatures. When water clusters in the membrane microenvironment contain only a single azobenzene molecule, oxidative photocyclisation yields only benzo[c]cinnoline (20).33 For occupancy numbers greater than 2, equimolar amounts of benzidine (22) are also formed. A disproportionation mechanism is proposed, involving reaction of dihydro intermediate (21) with protonated azobenzene (19) to produce benzo[c]cinnoline (20) and protonated hydrazobenzene (23) which subsequently rearranges efficiently to benzidine (22). A series of 1,2-disubstituted-3,4-dihydronaphthalenes (24), carrying an aziridine function, has been designed to undergo efficient covalent reaction with a specific cysteine residue near the C-terminal of the human estrogen receptor. Oxidative photocyclisation of the benzyl ether analogues (26) yields the corresponding azaphenanthrenoids, for example (25). The 2-and 4-pyridyl cyclised products exhibit intense fluorescence, suggesting that the aziridines (24) may be well suited as sensitive fluorescent probes for the estrogen receptor.34Photocyclisation of Nazomethine pyridinium salts (27) yields (28) and then (29).3s
I M : Photoreactions of Compoundr Containing Heteroatorns Other than Oxygen
243
H H Ph,
+,H N=N, Ph
+
PhNH-NH2Ph (23)
O
W
o-ocH2ph
X
2-,3- or 4-pyridyl (24) X =
~3
(26) X = OCH2Ph Ph
I
Ph
I
A
There has been much interest in recent years in the development of photochromic fulgides and fulgimides with high fatigue resistance. A possible mechanism for photodegradation involves excited state intramolecular abstraction of hydrogen from an adjacent allylic site by a carbonyl group. Indolyl fulgide (30), containing a trifluoromethyl group, has been reported to show much greater photodegradative resistance than its methyl analogue (31) to repeated alternating irradiation cycles with uv and visible light, involving interconversions between its colourless (30)and coloured forms (32). In addition it exhibits high resistance to thermal conversion of the coloured form.36 A 'saccharide tweezers' with a photoswitch has been rep~rted.~' The flexible diarylethylene(33) binds to glucose through its boronic acid receptor sites. The binding capacity is reduced by irradiation with 313 nm light, (33)being converted to the more rigid ring-closed form (34)in which the boronic acid groups are well separated and unable to bind
244
Photochemistry
do 'Me
F;'
Me Me Me
Me
(30)X = F (31)X = H
to glucose. Irradiation with 533 nm light reconverts (34)to (33), fully restoring the binding capacity. Several systems based on 1,2-diarylhexafluorocyclopentene have been ~atented.~'The photochromic behaviour of a series of pyrryl f ~ l g i d e s ~and ~ . ~1,2-dipyrrylethylenes4* ~.~ has been reported.
(34)
HO'
Interest continues in the photochromic behaviour of the spiropyran and related systems both in the patent43 and the basic research literature. Nanosecond laser kinetic spectroscopy has been used to investigate the intermediates involved in the interconversions between the colourless spiropyrans (35) and (36)and the coloured merocyanines present in solution at room temperature as the E-isomers (37)and On excitation (530 nm) of nitro derivative (37)in acetone, two transients were observed, the triplet excited state (lifetime 6ps) and the 2-isomer (lifetime 0.3ms). The same transients were observed on excitation (353 nm) of the spiropyran form (35). Naphthalene or benzophenone sensitisation of the photo-
IIJ6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
245
dO 9 R
Me Me
I
Me
Me
R'
dMe
(35) R = NO2 (36) R = H
(37)R = N 0 2 (38) R = H
k2
(39) Ri,R2 = H,OMe, benzo (40) R' = H, R2 = NEt2
coloration of both (35) and (36) indicates that one pathway for this transformation involves triplet excited states. As part of a study aimed at understanding the role of radical species in the degradation of photochromic spiropyrans, compounds containing a spin trap or a spin label have been synthesised and their photochromic performance inve~tigated.4~ The naphthothiazine spiropyrans (39) and (40) are photochromic in solution, flash excitation in toluene and ethanol giving rise to visible region absorption. In ethanol the photoisomerisation is a singlet excited state process, though the triplet participates in toluene solution.46 Effects due to the presence of geminal N,N-dimethylaminophenyl groups at the sp3-carbon of 3H-naphtho[2, l-blpyrans and 2H- 1-benzopyrans have been studied by comparison of the photochromic behaviour of (41), (42) and (43).47A series of new 2,2-diphenyl-2H-chromenes (45) and (a), annulated with different sixmembered azaheterocycles, has been prepared and their photochromic behaviour compared to that of the corresponding naphthopyrans. Both phenyls are required at the pyran ring sp3-carbon for room temperature photochromism and the isoquinoline fused systems exhibit increased colourabilities and bathochromically shifted spectra for the coloured forms.48 Pyran ring steric strain energies, calculated by the MM2 method for a series of benzoannulated 2,2-diphenyl-2H~ h r o m e n e s ,appear ~~ to correlate with photochromic activity. However, (44) though it has high strain energy, is non-photochromic, possibly because steric hindrance by the 4-phenyl group may prevent E-2 isomerisation during the photochromic process. Fluorescence from the coloured merocyanines (50),
(41) R' = R2 = R3 = H
R4 = 5,Gbenzo (42) R' = NMe2, R2 = R3 = H
R4 = 5,6-benzo (43) R' = NMe2, R2 = R3 = R4 = H (44) R' = R2 = H, R3 = Ph
R4 = 5,bbenzo
(45) A = CH, N B = CH. CMe, CPh C = CH, N,CPh D=CH.N
-
ph73 Ph
Photochemistry
246
R4
” Me Me
R3
I
Me
Me
(47) R’, R2= H, Me; R3 = H, piperidino; R4 = H (48) R’ = R2 = R3 = H; R4 = H, Me, Et, P f (49) R’ = R2 = Me; R3 = R4 = H
present in equilibrium with colourless spironaphthoxazines (47) in solution, has been studied by time-correlated single photon counting.5*Signal decays could be understood in terms of two or three components corresponding to the contributions from the different near-planar merocyanine isomers/conformers possible from rotations around the C=C, C-N and N=C bonds in the acyclic portion of merocyanine (50). Nanosecond laser kinetic spectroscopy has been used to investigate the intermediates formed on excitation of spirooxazines With a methyl group at the imino carbon, two photoproducts are reported, the expected merocyanine (51) and an intermediate with lifetime of 1.7ps, suggested to be a twisted charge separated metastable state.” As the steric demand at the imino carbon is increased, the relative yield of photoproduct (51) decreases due to increasing steric difficulty encountered in achieving the necessary planarity, and the lifetime of the intermediate decreases because of increasing ease of return to the spirooxazine form. The role of singlet oxygen or superoxide ion is negligible in the photodegradation of the model photochromic spirooxazine (49). Rather, reaction of its singlet excited state with solvent results in hydrogen abstraction and degradation by a radical mechanism.52 Further studies on the use of triplet sensitisation of the spirooxazine to merocyanine conversion have been reported for a series of structurally diverse spiroo~azines.~~ Camphorquinone is a useful sensitiser, not only shifting the usable excitation wavelength region into the visible but also in increasing the yield of coloured form produced. The limiting quantum yields of the sensitised interconversions are approximately unity, significantly higher than those generally encountered on direct excitation of spirooxazines. The photophysical properties of a series of quinolinodihydroindolizines (52) and of their open-chain betaine forms have been reported.54 The
& R’
-
\
,
C02MeC02Me
R3
Me Me
Ar’
IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
247
photochromism of a series of novel cyclohexadienes (53)has been r e p ~ r t e d . ’ ~ ’ ~ ~ Excitation in cyclohexane results in conrotatory ring opening of singlet excited (53) to give the corresponding red hexatrienes. In acetonitrile the process is much less efficient and intersystem crossing to triplet (53), which decays to ground state without product formation, dominates. Visible light irradiation of the ringopened compounds regenerates the original spirocyclohexadienes, though in fluid solution decomposition competes with photorecyclisation. Behaviour in polymer matrices seems more promising. Thermal reconversion of the coloured form to the cyclised form is very slow.
(54) X = H, H; R’ = OMe R2 = Me, n = 2
(55) X = 0; R’ = H R2 = CH*Ph, n = 1,2,3
The arylenamide photocyclisation protocol continues to provide convenient access to intermediates of interest in alkaloid synthesis. Spirocyclic &lactam (55) has been prepared from cycloalkylenamide (54) and irradiation of enones (56) yields the corresponding spiroketones (57).57Analogous photocyclisations have led to nitro-substituted berbine derivative^.^^ The piperidine arylenamides (59) are efficiently photocyclised to the corresponding benzo[b]quinolizine-6-ones (60), providing convenient access to benzoquinolizidine derivatives. In contrast,
(59) n = 2 (61) n - 1
allvl
248
Photochemistry
the pyrrolidine arylenamides (61) fail to undergo the analogous ring closure, possibly to avoid the extra ring strain involved in closure to a 5-membered ring. However, they do undergo efficient photo-Fries rearrangement to give enaminoketones (58) by 1,3-aroyl migration.59 Crystalline state absolute asymmetric induction is of wide interest and photoreactions of achiral molecules occurring within chiral crystals have potential for the synthesis of homochiral products. The achiral N-ally1 furan-2-carboxanilide (62) forms 1:1 and 2: 1 inclusion -)-chiral host (63). Within the crystals the molecules of complexes with (R,R)-( achiral (62) are fixed in chiral conformations such that irradiation of crystals of the 1:l complex yields (-)-trans-dihydrofuran (64) (50% yield, 96% ee) whereas similar irradiation of the 2:1 complex yields the enantiomeric product, (+)-transdihydrofuran (64) (86% yield, 98% ee).60 Cyclisatioddehydrohalogenation also continues to provide useful routes to novel ring systems. Irradiation of amide (65) produces (66) (87Y0),~'and amides (67) undergo analogous cyclisations.62Photocyclisation of chloroacetamides onto activated aromatic rings is useful for formation of medium-sized lactams, with C-3 of the indole nucleus being normally its most reactive position. However, irradiation of (68) in aqueous methanol yields 10-membered lactam (69) rather than the anticipated 8-membered system.6' Papavine derivative (70) yields alcohol (71)64whereas chloroacetamide (72) in aqueous acetonitrile cyclises to (73) and (74).65
Earlier reports66 that pyridinium cation (75) can follow an excited state electrocyclisation pathway have been developed into a versatile method for .O]hex-2-en-4-y1 alcohols and ethers (77). synthesis of 6-alkyl-6-azabicyclo[3.1 Irradiation of (75) under basic conditions in hydoxylic solvents results in capture of cation (76) to yield (77). These may be converted regio- and stereoselectively to trans, trans-3,5-disubstituted-4-alkylaminocyclopentenes 67 or to cis-fused cyclopenteno-P-lactams.68 The structures of a series of valence bond photoisomers of 4(3H)-pyrimidinones (78) have been confirmed by X-ray crystallography to be the corresponding Dewar pyrimidin~nes.~~ Photolysis of chloro-iminoenamines
1116: Photoreactions of Compound Containing Heteroatoms Other than Oxygen
I Me
249
I Me
(69)
(68)
Me
I
OMe
Me0
OMe
R (70)X = CI,R = H (71)X = H, R = OH
0
(79) produces almost quantitative yields of the chloroquinolines (SO)." The p toluidides of crotonic and methylcrotonic acids yield predominantly quinolones (81) and (82) respectively on irradiation." The 1 -(2-nitrophenyl)buta-1,3-diynes (83) are converted, via their singlet excited states, to isatogen derivatives (84) in high yields.72
Photochemistry
250
(81) R = H (82) R = Me
(83) R = Ph, Bu'
(84)
Photochemical routes to enantiomerically pure proline derivatives have been developed. The homochiral glycine derivative (86), from chiral auxiliary 1,3:4,6di-O-benzylidene-2,5-dideoxy-2,5-imino-D-iditol, gives the L-proline derivative (85) in high yield on photolysis." The related homochiral esters (87), from (5')-Ntosylaspartic anhydride, are also converted in good yield, via &hydrogen abstraction, and with complete diastereo- and enantioselectivity, to the 3-hydroxyproline derivatives (90).74*75 The high selectivity may be due to intramolecular hydrogenbonding interactions between the hydroxyl group and the ester carbonyls within the intermediate biradicals. Compounds (88) and (89), which in addition have reactive y-hydrogens, show somewhat different behaviour, forming diastereomeric cyclobutanols rather than proline derivatives. The substituted dipeptides (92) (Z = benzyloxycarbonyl) also undergo stereoselective photocyclisation, forming diastereomeric peptidomimetic cis ti-lactams (91) in approximately 70% yield in which chiral induction arises from the constituent cr-amino ester Ph
Ph
('7'
(--c
L
R- -
N
Ts
Ts
p c o x Ph
-(g------JPh
r9 (86) R = H, X = "\
(90)R = C02Me, C02But
(87) R = C02Me, C02Bu'; X = OCH2Ph (88) R = CH2OSiMepBu'; X = OCH2Ph (89) R = CHMe2; X = OCH2Ph
R'
C ' 0,R
(92)n = 2 (93)n = 1 R = Me, CH2Ph; R' = Me, CHMe2; R2 = H, alkyl; R3 = H, alkyl
IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
25 1
function. 76*77 The high regioselectivity may be due to the relative stabilities of the intermediate 1,6-biradicals and the alternative 1,5-biradicals, calculations suggesting the operation of unfavourable steric interactions in the latter. The photocyclisation is glycine-selective; however, if the tether to the benzoyl group is shortened, for example as in (93), cyclisation takes place to give y-lactams (94), alkyl-substitu ted at the a-carbon.
O=N-(CH2),--C=O
+
O=N-(CH2),-CHO
1
n=3,5
O=N-(CH~),TN-(CH~),--CHO I
0 0'
(99)
The Barton reaction" provides a useful method for remote functionalisation. The alkoxy radical generated by photolysis of a nitrite ester abstracts a hydrogen intramolecularly and the resulting alkyl radical combines with the nitric oxide liberated during photolysis to yield ultimately an oxime. The reaction has been utilised as a key step in a practical partial synthesis of myriceric acid A from oleanolic acid.79For a small number of nitrites (some steroidal 17-nitrites,bornyl and isobornyl nitrites), hydroxamic acids have also been obtained from a ring expansion process." EPR studies of the photolysis of bornyl and isobornyl nitrites have confirmed the formation of lactam-1-oxyl radicals.*' The use of wavelengths at which the aldehydic and C-NO groups are inactive, and only the 0-NO bond cleaves, has suggested the pathway by which ring expansion occurs. Radical (98) is formed from cyclopentyl nitrite by cyclisation of (97),generated by hydrogen abstraction from product (W)by initially-formed alkoxy radical (95).82From cyclobutyl and cyclohexyl nitrites, open-chain acyl nitroxide radicals (W)are formed by an intermolecular acyl radicahitroso group reaction. Photolysis of pent-4-enyl thiohydroxamic acid esters (100) yields reactive pent-4-enyl-1oxy radicals whose cyclisation is regioselective, 5-exu-trig closure to (101) being strongly favoured for phenyl substitution at positions 1, 2, 3 and 5, whereas a phenyl substituent at position 4 leads to 6-endu-trig cyclisation to stabilised
252
Photochemistry
(105) R = S P y r (106) R = H
radical (102). Cyclisations are stereoselective except for substitution at position 1.83 Oxiranyl radicals undergo stereoselective intra- and intermolecular reaction more rapidly than they rearrange. For example, (103) is converted on photolysis to (105) (42%) or, in the presence of n-Bu3SnH, to (106) (47%) via the intermediacy of oxiranyl radical (104).84Cyclopropanols are formed by hydrogen abstraction in the photocyclisation of a~ylpyrazines.~~ Cyclopropyl substituted acylpyrazine (109) yields cyclopropanol (108) as major product.86 The relative amounts of (108) and minor product (110), coupled with the known rate constant for opening of a secondary cyclopropyl radical such as (107), provides an estimate ~ s-I. The of the rate constant for collapse of biradical(lO7) to (108) of 2 2 . 8 108 5-t-butyl- and 5-i-propyl-2’-deoxyuridine derivatives (111) photocyclise to yield 1,2-dihydrocyclobuta[d]pyrimidin-2-ones(113), presumably by dehydration of the cyclobutanols (112).87Similarly irradiation of 5-t-butyl-1-methyluracil (114) in water gives high yields of (115).88 1,4-Dicyan0-2,3,5,6-tetramethylbenzene/biphenyl sensitised SET cyclisation of isoprenoid polyalkenes with dicyanomethylene end groups, for example (116), occurs with five-membered, rather than six-membered, ring formation to give the
IIl4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(111)
R’ = deoxyribosyl R2 = H, Me
(114)
R’ = R2 = Me
253
(112)
more favoured malonodinitrile radical (117).89Minimisation of methyl-methyl interactions in the transition state leading to (117) determines the syn relationship between the dicyanomethyl and the vicinal angular methyl group in the product, which is formed by attack by water at C-3 and the addition of a hydrogen at the radical centre. Excited state reaction of iminium salt (118) gives spirocyclic amine (121), a key conversion in a potential synthesis of cephalotaxine, via intramolecular SET, cyclisation and de~ilylation.’~ Cyclisation of enone (119) yields the isomeric indolizidine alkaloid-type intermediates (120) on sensitisation with either 1,6dicyanobenzene (DCB) or benzophenone.” Acetoacetates (122), carrying a tertiary amine function, undergo efficient photocyclisation to the eight-
PhC,H2 @CN
*
‘
Me
Me (116)
R3si0-Q0 MeO- -
5N,
PhCH2
- R3si05k MeO- -
RCOO*A
membered azalactones (123),92by intramolecular SET from nitrogen to the acetyl group, followed by a-proton transfer and cyclisation. Cyclisation occurs from both singlet and triplet excited states. The 2-dimethylaminoethyl phenylglyoxalates (124) and the 2-alkylthioethyl phenylglyoxalates (125) similarly yield Conforintramolecular photocyclisation products (126) and (127), re~pectively.~~ mational differences imposed by the different heteroatoms on the intermediates leading to a-proton transfer are suggested as being responsible for the different
Photochemistry
2 54 I t
Ph
rn
(124) Y = NMe2 (125) Y = SCH2R (R = H, Me)
CH-R
(126)
ring-size products observed. The primary photoreaction of p-N,N-dialkylaminopropiophenones (128) involves an electron-transfer/proton-transfersequence which produces hydroxy- 1,3-biradical (129) which cyclises to the 2-aminocyclopropanol (130) with minimisation of repulsive interactions between the aryl group and the substituents (R'/R') at the a- and P-positions resulting in high diastero~electivity.~~ Appropriately substituted 2-aminocyclopropanols may be isolated. In the absence of such substitution, hydroxy-l,3-biradical (129) may be trapped by oxygen to ultimately yield isolable p-enaminone (131). The course of the photoinitiated SET reactions of geraniol depends on the electron-acceptor system used and the free energy of electron transfer. Contact radical ion pairs (CRIP) arise for systems with marginal driving force and cis-fused monocyclic products are formed. Irradiation of 9,lO-dicyanoanthracene (DCA) and geraniol in dichloromethane produces cis-2-(2-propenyl)-trans-5-methylcyclopentanemethanol, cyclisation of geraniol radical cation (133) occurring so as to minimise
IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
255
R/XIR’ pR’ R2 R2
R3
R2 R2
0’
*
/ HO’ +-
Ph
Ph
interference with the complexing DCA radical anion, favouring cis-fused cyclised radical cation (132). In contrast systems resulting in exergonic SET lead to solvent separated radical ion pairs (SSRIP) and transfused bicyclic products predominate. Thus irradiation of geraniol and DCB in acetonitrile forms mainly trans-fused bicyclic ethers, cyclisation of the unencumbered geraniol radical cation (133) occurring to form thermodyamically more stable transfused radical cation The radical cations (135), formed by DCA-sensitised irradiation of y,&unsaturated oximes, cyclise and subsequently yield 1,2-oxazines (136).96The substituent dependence of the quantum yield for photoconversion of the protonated 2-aza-1,3-dienes (137) to the isoquinolinones(139) has been reported and is consistent with intramolecular SET within the twisted protonated diene.97 Radical coupling yields (138) and subsequent rearomatisation, hydrolysis and oxidation produces (139).
___)
clod-
ArCOO Ar’
ArCOO Ar’
Ar’
Intramolecular cyclisation of N-substituted phthalimides continues to attract interest. Product formation from a series of N-phthaloylcysteinederivatives (141) has been shown to be multiplicity de~endent.’~ y-Hydrogen abstraction occurs predominantly from the singlet excited state to yield products derived from the intermediate azetidine (140). In contrast, triplet excited (141) initiates SET from the sidechain sulfur to the excited carbonyl. Deprotonation at the &-carbonand cyclisation produces thiazinoisoindoles (142). N-Phthalimido-o-amino carboxy-
256
Photochemistry
(143)
(144)
lates undergo intramolecular SET on irradiation and decarboxylation yields radical anion (144),for which the ratio of cyclisation to reduction is influenced by the presence of water.99At low water concentrations a second SET can dominate, generating carbanion (145) and reduction product (146). At higher water concentrations, rapid protonation of radical anion (144) minimises this process and cyclisation to (143)results."' Photoinitiated intramolecular SET between the phthalimide and phenylcyclopropane unit occurs within N-alkylcyclopropylphthalimides to yield, for example, (147).1°' Intramolecular cyclisation yields bridged ethers (148)and (149).
gN& -yJyPh.$H
0
0-
'
(148) n = 1, 2
+
eoT ==3
\
0 (147)
0
N
0 (149)
n= 2
2.3 Photoaddition - Micellar aggregates of alkylated E-stilbazolium salts in water form only head-to-head photodimers, with a preference for the syn-isomer, suggesting a significant degree of order within the aggregates. Preferential
IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
257
association of the hydrophobic moieties may be sufficient to overcome the intermolecular like-charge repulsions present. Io2 The fluorescent dye E- 1,2-bis[2(5-phenyloxazo1y)lethene does not undergo geometrical isomerisation but is reversibly converted to the syn photodimer in 1,4-dioxane.Io3 E-2-Styrylbenzoxazole is unreactive when irradiated in benzene. However, its crystalline 1:l inclusion complex with 1,1,6,6-tetraphenylhexa-2,4-diyn1,6-diol is quantitatively converted to photodimer (150), hydrogen bonding with the diol host resulting in favourable alignment of the reacting double bonds within the 1atti~e.I'~The photodirnerisation of thymine derivatives for possible use in photoresist materials has led to investigations of the behaviour of long chain carbamate derivatives in amorphous and crystalline film states. lo5 Crystalline 1-Fyano- 1,4-diphenylbutadiene gives a [2+2]photoadduct, despite a reported 5.04 A separation between the reacting double bonds. '06 Ethyl N-benzyl-l,4-dihydronicotinatereacts concertedly with E- and 2-methacrylonitrile to give two [2+2]adducts in each case.'" The phenanthrene ester (151), lactone (153) and lactam (154) exhibit fluorescence self-quenching whereas its absence for amide (152) is attributed to the presence of an orthogonal amide group, which sterically hinders excimer formation. lo8 Selfquenching of (151) results in formation of a non-fluorescent excimer which undergoes efficient photodimerisation to yield the syn head-to-tail dimer whereas the increased steric demand for dimerisation involving the more heavily substituted n-bond in (152) or (154) is suggested to account for the absence of photodimerisation and the observation of excimer fluorescence.
(151) X = O M e (152) X=NMe2
(153) X = 0 (154) X = N M e
The total synthesis of psoralen-thymidine monoadducts is of considerable importance in studies of nucleoside damage arising from radiation, chemical carcinogens and chemotherapeutic agents. Within helical DNA, psoralens (155) photoreact with thymidine to form a cis-syn [2+2]adduct. Outside the DNA helix, achievement of the required cis-syn addition has been very difficult and is complicated by the ease of psoralen photodimerisation. Efficient routes to a psoralen-thymidine adduct have used benzofuran derivative (156) which provides a tethered arrangement to ensure the correct cyclobutane stereochemistry. lrradiation in acetonitrile containing acetone, followed by methanoVsilica gel
Photochemistry
258 R2
fJ&
0
-yOAc
‘0
How Me02C H
‘i
U
treatment of the crude photoproduct, yields the precursor (157) to the psoralen adduct. ‘09,’ l o A molecular recognition approach for control of the stereochemistry of coumarin-thymine photoaddition has been reported. Kemp’s imide, carrying a covalently-linked coumarin, complexes with 1-butylthymine by hydrogenbonding to give the cis-syn and cis-anti complexes. Because of the more favourable alignment of the thymine and coumarin double bonds, the cis-syn complex is more favourable for cycloaddition than the cis-anti complex. Irradiation of a mixture of the host coumarin and 1-butylthymine in benzene strongly favours the cis-syn cycloadduct over the cis-anti cycloadduct by a factor of 24: 1. Intramolecular [2+2jphotocycloaddition involving a thymine derivative and its side-chain aromatic substituent gives a labile adduct. I Product outcome from irradiation of N-alkenoyl-P-enaminones is determined by the length of the tether separating the side-chain alkene group from the enone carbon-carbon double bond. ‘ I 3 For short chains cycloaddition yields polycyclic lactams with cis-syn stereochemistry as the sole products. For example, the cyclopentenes (158) give tetracyclic compounds (159) in high yields (65 - 95%). For longer chains conformational and entropic effects militate against [2+2]cycloaddition and intramolecular hydrogen abstraction intervenes. The competition between intramolecular [2+2]cycloaddition and hydrogen abstraction is also observed in the photochemistry of 2carboxamidocyclopent-2-enones. l4 Added Eu(II1) or Mg(I1) salts have a marked effect on the regioselectivity of intramolecular photocyclisation of cyanonaphthalene derivative (160), accelerating formation of the 1,2-~ycloadduct(161) and suppressing both formation of the 3,4-cycloadduct and cycloreversion of the 1,2adduct.’I5 Coordination of the metal cation to the cyano and ether groups in (160) and (161) may be responsible for the observed effects. 1-Cyanonaphthalene reacts with norbornadiene in benzene to yield four [2+2]adducts. Addition occurs exclusively to the exo-face of norbornadiene but product formation occurs at both rings of 1-cyanonaphthalene, involving the 1,2- or 7,8-positions with a limited preference for anti vs. syn addition.lI6 In more polar solvents the reaction becomes more selective, favouring anti-addition to the substituted ring. E-Head-to-head dimers are formed by SET sensitisation of 2-aminopropenenitriles by DCA, provided their half-wave oxidation potentials exceed 1.2V vs. SCE in acetonitrile. A 1,6biradical, possibly formed by interception of an initial ion pair by a second alkene molecule, may be involved. Alkenes with lower
’
I M : Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
259
oxidation potentials yield biradicals which experience such effective captodative stabilisation that the activation barrier for cleavage to monomer is lower than for cyclisation to dimer. I 2-Methyloxazolo[5,4-b]pyridine(162) reacts with acrylonitrile in acetonitrile to give three photoadducts, derived from initial regioselective addition to the 3 a 4 4 - 5 and 5-6 bonds. l 8 6-Methylfuro[2,3-c]pyridin-7(6H)one similarly reacts with acrylonitrile in methanol to yield three adducts arising from additions to the six-membered ring. l 9 Benzil sensitised intramolecular photo[4+2]cycloaddition between an enamide and a benzenoid ring occurs with high stereoselectivity, compound (163) for example yielding (164) via the less sterically crowded trans biradical. I2O The photo[4+4]dimerisation of 2-pyridones proceeds almost exclusively head-to-tail, providing a highly efficient route to eight-membered rings. In a study directed at taxane synthesis, quantitative intramolecular photocycloaddition of (165) yielded a single photoproduct, the
'
tN
Sens.
Me
I
I
MeN@o
Me
Me
MeN
260
Photochemistry
unsymmetrical tethering reinforcing the ‘natural’ head-to-tail regioselectivity.I’ * However a symmetrical tether of appropriate length permits intramolecular photoaddition only via the ‘unnatural’ head-to-head regiochemistry, compound (166) giving a mixture of the cis- and trans-isomers of adduct (167) ( l : l , 98% yield). 22 The cycloaddition of 1-cyanonaphthalene to furan has been reexamined. Adducts (168) and (170) are found as major and minor primary photoadducts respectively at - 70 “C. On warming to room temperature, (170) undergoes Cope rearrangement to dihydronaphthalene (169) which, on irradiation, rapidly reverts to 1 -cyanonaphthalene. Room temperature irradiation of 1 -cyanonaphthalene and furan yields adduct (168) as major isolable product, as previously r e p ~ r t e d ’ ’ ~ and consistent with its buildup during the thermal conversion of (170) to (169) and subsequent photoremoval of (169). CN
Examples of arene-azo [6+2]addition have been reported for a series of 2,3diazabicyclo[2.2.l]hept-2-enes (DBH) whereas closely related 2,3-diazabicyclo[2.2.2]oct-2-enes (DBO) fail to cyclise. 125 Compounds (171) are smoothly converted to (172), whereas compounds (173) fail to cyclise, the different strains experienced by an azo group within a DBH system compared to one within a DBO system being responsible for the differences in behaviour. Two more examples of cyclobutane formation involving arene/arene [6+6]photocycloadditions in situations where rigid poylcyclic frameworks ensure a face-to-face orientation of the two chomophores at less than van der Waals separations have been reported. Irradiation of (174) and (175) at 254 nm in acetonitrile causes their
(171)
I?= 1,2
(173)
n = 1,2
IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(174) R = P h (175) R = CF3
26 1
(176) R = Ph (177) R=CF3
photoequilibration with cycloaddition products (176) and (177), respectively. A cyclooctatetraene derivative and pentacyclic compounds are amongst those isolated following irradiation of 6-chloro-1,3-dimethyluracil in p-xylene in the presence of trifluoracetic acid. Analogous products are obtained in rn-xylene'27 and in benzene. 12' 1-Methyl-4,6-diaryl-2(1H)-pyrimidinones are recovered unchanged when irradiated alone in benzene whereas in the presence of thiols they give 2: 1 pyrimidone:thiol adducts. In contrast, 1-phenyl-4,6-dimethyl-2( 1H)pyrimidinone gives 1:l adducts of the Dewar pyrimidinone and the thiols.129 Semiconductor-catalysed photoadditions to Schiff bases are reported. N-Benzylideneaniline and N-benzylidenebenzylamineare photoreduced in propan-2-01 by platinised titanium(IV)oxide, 30 and irradiation of a series of arylideneanilines in the presence of suspended cadmium sulfide in methanol containing excess cyclopentene gives good yields of y,&unsaturated amines accompanied by hydrodimers. A mechanism involving electron-hole pair trapping at reactive surface sites is proposed.13' Cadmium sulfide catalyses the photoaddition of 2,5-dihydrofuran to a z0 b en ~en e . I ~ ~ Low temperature irradiation of the glycoside thiohydroxamates (178) in the presence of 2-nitropropene gives clean formation of adducts (179). Conversion of the geminal nitro thioether function to a ketone and removal of the chiral auxiliary yields the free aldol product (180) in >90% enantiomeric excess.133 Single stranded aminoethylglycine dimers (181) and (182) differ only in whether OR2
R20
==?"
Rl^-rii S (178)
* Rl
a
NO2 s Me p
y
R'
NOn
(179)
(180)
R' = alkyl, R2 = sugarderived chiral auxiliary
the 4-thiothymine unit is at the N- or C-terminal end. Irradiation of (181) in aqueous solution at 366 nm yields a mixture of three different adducts, whereas isomer (182) is converted to a single a d d ~ c t .All ' ~ ~the adducts derive from initial hydrogen abstraction from the thymine methyl group by the excited thione group, followed by subsequent reactions of the resulting biradical. The different outcomes are consistent with different preferred backbone conformations, with
262
Photochemistry
respect to the N- and C-ends, modifying the interactions between the two adjacent bases. Irradiation of 2-phenylbenzimidazole in the presence of a large excess of methyl acrylate gives 2-(2-phenylbenzimidazol-1-yl)propionate (75%), by a sequence involving SET, proton transfer and radical coupling. 35 Acridine is converted to 9-(N-formyl-N-methyl-aminomethyl)acridineby 357 nm irradiation or y-radiolysis in dimethylformamide in the absence of oxygen. The product is strongly chemiluminescent when treated with strong base in DMF in the absence of oxygen. 136 Dimethylaniline and trimethylamine photoadd to [60]fullerene to give adducts (183) and (184) respectively via an SET, proton-transfer, coupling sequence. Though (184) is stable under similar conditions, photolysis of (183) in the presence of [60]fullerene yields pyrrolidine derivative (185) and 1,2-dihydro[60]fullerene.13’ a-Naphthol and substituted benzonitriles form photoadducts (186).I 38 X
Y
(181) X = 0, Y = S (182) X = S, Y = 0
(183) R = M e (184) R = Ph
(186) X = H, CI, Me, OMe
Arylsubstituted 1,3-dienes (187) give photoaminated products (188) and (189) following SET from singlet excited (187) to DCB. Attack by ammonia on the radical cations of (187), followed by deprotonation and reduction of the resulting aminated radicals by DCB radical anion, yields aminated anions which on protonation yield the final products.’39 Alkylbenzenes add efficiently to singlet excited 10-methyacridinium ion (190) in deaerated acetonitrile containing water to yield adducts (191) via an SEThadical coupling mechanism (path a), the water assisting in deprotonation of the alkylbenzene radical cation (193). 1O-Methyl-
Ar
AR4 A NHdDCB
R’
R3
NH2
Ar
~1
R3 R4
R2
IIl6: Photoreactions of Compounh Containing Heteroatoms Other than Oxygen
+
PhCHMe2
I
Me
Me
(190)
(191)
Me
(lg2)
H20(
263
1
Me
(195)
PhCMe2
(195) path c.1 02
I-
PhCMe200
(196)
+ PhCMe200Me
(190) H, ,CH2Ph
9,1O-dihydroacridine (197) and the corresponding benzylic alcohols (200) are alternatively produced in the presence of added perchloric acid (path b). Protonation of the initially-formed radical (192) gives radical cation (194). Reaction with benzylic radical (195) yields (197) and benzylic cation (198). In the presence of oxygen the benzylic radicals (1%) are converted to peroxy radicals (196) (path c). Back electron transfer between radicals (192) and (196) regenerates 1O-methylacridinium ion (190) and protonation of the accompanying peroxy anion yields hydroperoxide (199). I4O Benzyltributylstannane radical cations cleave at the carbon-metal bond to yield exclusively a benzyl radical and tributylstannyl cation. When generated in dry acetonitrile by excited state SET to 10-methyacridinium ion (190), coupling of benzyl radical (201) to the 10-methyacridine radical (192) accounts for formation
264
Photochemistry h,le
Me //
Me
of the sole acridine derivative (202). 14’ 1,l -Diarylethylenes react with ‘alkylating agents’ such as acetonitrile, proprionitrile, dichloromethane, chloroform or acetone on photosensitisation by benzophenone in the presence of t-butylamine. The t-butylaminyl radical (t-BuNH *), produced by an SET-NH proton transfer process involving benzophenone triplet and t-butylamine, generates the attacking radical from the ‘alkylating agent’ by hydrogen a b ~ t r a c t i 0 n . IThe ~ ~ SET acceptor-cosensitiser pair DCB/phenanthrene has been used to generate the vinylcyclopropane radical cation (204) from cis- and trans-chrysanthemols (203). Intramolecular trapping of (204)yields a radical which reacts with DCB radical anion to yield product (205).’43Chalcone derivatives (206),on irradiation in the presence of triethylamine, form chalcone radical anions which add to the neutral chalcone to give, on protonation, radicals (207)and (208). Subsequent endo-trig cyclisation and/or reduction reactions yield the observed products. A method for 1,2-N,O-functionalisationof alkenes has been developed based on their irradiation in methanol in the presence of azide anion, oxygen and a sensitiser such as rhodamine B. SET from azide ion to the sensitiser produces azidyl radicals which add regioselectively to the alkene, for example limonene (209), producing the more stabilised carbon radical (210)which is trapped by oxygen to yield a peroxy radical. SET and protonation yield the 1,2-azidohydroperoxide (21l), readily reducible to the corresponding a-aminoalchohol. 145
OH I Ar’CH-CH=CA?
ArlCH=CHCOA?
I
EtN hv
+
OH I Ar’CH=CH-CA? A?CO$H-iHAr1
Ar’ CH- CHCOA?
Me, N3
M
A
A
Me
6**” A
. i, 0 2
,
OOH
ii, Sens.-’ iii, H+
Me
1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
265
2.4 Rearrangements - Triplet sensitisation of P,y-unsaturated oximes (213) may result in either aza-di-x-methane (ADPM) rearrangement to cyclopropyl oximes (212), via Cl-C3 bridging (215), or intramolecular SET to give dihydroisoxazoles (214),via zwitterionic biradicals (216)and C3-0 bridging. A gradation in partitioning of the reactivity of (213)has been reported,’46 the complementarity of the hydroxyimino group as an electron acceptor and the vinyl unit as an electron donor determining the reaction outcome. The combination of an effective electron-donating diphenylvinyl group and an effective phenyl ketoxime electron-accepting group results solely in the SET process, with (214) being formed from (213a). Oxime (213b) in contrast undergoes solely the ADPM process, possessing a combination of groups which does not facilitate intramolecular SET. Analogous conclusions apply to the behaviour of the related hydrazone derivatives (217). X-Ray crystallography has confirmed that the cyanodibenzobarrellene (218)yields the 8b-cyanosubstituted dibenzocyclopropaCrystalline salts involving pentalene (219)and not the 4b-cyano regioi~omer.’~~ 9,lO-ethenoanthracene derived anion (220), and in which the cations (221) contain a triplet sensitising group, exhibit cation to anion triplet energy transfer FOPh
COPh
+
(a) PhCO(CH2)2NHMe2
+
(b) ~ - M ~ C O C G H ~ N ( C H ~ C H ~ ) ~ N H (c) 4-MeCOC5H4hH (2211
266
Photochemistry
as evidenced by formation of characteristic triplet state di-.n-methane rearrangement products from (220).14*Competing reactions are found for the dibenzobarrelene amides (225)-(227). All yield the corresponding di-n-methane rearrangement products (222) on irradiation either in solution or in the solid state.'49 In solution the N, N-diethyl and N,N-di-i-propyl amides (226) also undergo an intramolecular hydrogen abstraction to produce amides (223). Excited state transfer of hydrogen from the carbon a to nitrogen via a six-centred transition state to the P-carbon of the enamide system (228)and subsequent bond cleavage within (229) forms ketene (230) and imine (231). Amine (232), from hydrolysis of the imine, then adds to the ketene to produce (223). No analogous products are found in the solid state, presumably because of restrictions arising within the rigid crystal environment. The major product in solution from N,Ndibenzylamide (227)is the p-lactam (224),from cycloaddition of ketene (230)to imine (231). CH2Ph I
CONR2
I
\
(224)
\
J-
H2°
(225) R = H, Me (226) R = CHpMe, CHMe2 (227) R = CH2Ph
(228)
A series of enamide derivatives has been subjected to singlet excited state rearrangement to yield dihydropyrromethenones. For example, irradiation of (233) in the presence of piperylene as triplet quencher, yielded [3,5]sigmatropic rearrangement product (234)(6O%), a synthon for linear tetrapyrroles related to phytochrome.15' Imidazo[1,2a]pyridines of type (235) are converted to imines (236) on irradiati~n."'*'~*The alkoxyimino-substituted triazolinedione derivatives (237)undergo rearrangement involving a ring expansion-ring contraction process to give (239).'53 [2.1]Benzisoxazoloquinones(238) undergo high yield photorearrangement via triplet nitrenes, resulting in stereoselective formation of
267
1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
hv
I
3:
piperylene
Me
(234)
Me
(235) R', R2, R3, R4 = H, Me, Ph, I
(236) Ph
NOR
H
(237) X = H, CI, OMe; R = alkyl
"OR
I X
/
O
NOMe
R = Ph, 4-BrC6H4, Me X=NH,O R' = OMe. OEt. NMe7. Me
268
Photochemistry
2-butenolides (240).'54 Photolysis of methyl 2-phenoxybenzohydroxamate (241) in methanol yields three primary photoproducts. A Norrish Type I1 process yields 2-phenoxybenzamide. Norrish Type I cleavage of the carbon-nitrogen bond and cyclisation of the resulting radical yields xanthone. The third product, methyl N-phenyl-2-hydroxybenzohydoxamate(242), is formed via phenyl migration, possibly involving intramolecular SET and proton transfer, cyclisation and ring-opening processes. 2-Hydroxybenzanilide, previously reported as the sole product from (241), arises from triplet excited (242) by a Norrish Type I1 process and involves sensitisation of (242) by (241).
Other Processes - Allylation of DCB occurs on irradiation in the presence of 2,3-dimethylbut-2-ene (DMB) in acetonitrile. In the presence of nucleophiles the nucleophile-olefin combination aromatic substitution (NOCAS) process occurs. 56 Further study has provided additional information about the sequence of steps involved and additional minor products have been identified (Scheme 1). About 98% of the radical ion pairs formed undergo back electron transfer (path a). The free radical cation (path b) undergoes deprotonation, and subsequent ally1 radical/radical ion coupling and loss of cyanide yields allylated products (243) and (244). Involvement of the cyanomethyl radical yields minor product (245). In the presence of a nucleophile (methanol, water, trifluoroethanol) addition to the DMB radical cation occurs (path c), leading to NOCAS product (247). A small amount of in-cage coupling (path d) yields minor product (246).'57 Irradiation of DCB in acetonitrile-methanol containing 4-methyl- 1,3-pentadiene yields (248) as the major NOCAS product. The regiochemistry is determined by addition of methanol at C-4 of the diene radical cation to preferentially form the less alkyl-substituted allylic radical. High level MO calculations for a series of radical cations, and possible intermediates derived by reaction of these with methanol, confirm that the major adduct arises from attachment of methanol to the end of the alkene/diene which yields the more stable intermediate radical, but that the more stable radical may not necessarily be the more heavily alkylsubstituted one.I5* The N,N-dimethylaminomethylarenes(249) undergo photoreaction by benzylic carbon-nitrogen bond cleavage, yielding the corresponding methylarenes and 1,2diarylethanes from the arylmethyl-dimethylaminyl radical pair produced. Photophysical investigation suggests that dissociation occurs from the localised triplet excited state of the carbon-nitrogen o-bond, populated via the charge-transfer state (250).'59Irradiation of N-(9-phenanthrylmethyl)-N-alkylamines(251) in the
2.5
'
1116: Photoreactionsof Compounds Containing Heteroatoms Other than Oxygen
DCB-'
+
>( CN GH3CN
DCB 98%
1
Dk*'
I
6:2c*}4-
DMB
(243)
LBlT CN
(244)
CH2CN
CH2CN
patha
DCB-'
4.1%
+
269
?\
CN
path c
pathd
DCB"
+
CN CN (247)
CN (246)
Scheme 1
Me Me ArCH2-NMeZ
+*
A k H 2-NMe2
Ar (248) Ar = 4cyanophenyl
(249)
(250)
Ar = 2-, 3-or 4-styry1, 1-naphthyl, 9-anthryl, 9-phenanthryl
presence of N-methylaniline results in cleavage of the CH2-N bond and formation of (254). SET from N-methylaniline to (251), followed by proton transfer within the radical ion pair, yields (252) and (253). Cleavage of (252) and coupling of the phenanthrylmethyl radical with (253) yields the products. 160 Ground state nucleophilic aromatic replacement of hydrogen formally requires displacement of hydride ion from the substrate (255) and may occur by oxidation of the anionic a-complex (256). Deprotonation of the resulting cation (257) yields substitution product (258). However yields are often poor and the process lacks generality. Oxidation of (256) to (257) may involve substrate (255), typically a nitroaromatic. In the basic medium normally used to generate the nucleophilic anion, protonation of the nitroarene radical anion is very inefficient and back electron transfer
270
Photochemistry
+
PhenCH2-NHR
+
PhenCHTNLZR --
hv
PhNHMe
Me+ - - J
(2511
(252)
(253)
J
PhenCHTCsH4NHME 2- and 4-isomers (254)
ArH
%[
Arc Nu
(255) NuH
1-
oxidation
[
(256)
+
H Ar< Nu
]
+
(257)
ArNu
(258)
hv
1,3-DNB
4-NU-1,3-DNB (+ 2-NU-1,3-DNB)
may occur. Utilising fluoride ion to activate nucleophiles with acidic hydrogens, the resulting hydrogen fluoride to protonate the nitroaromatic radical anions and exploiting the fact that an excited state is easier to oxidise than the corresponding ground state, a photochemical method for effecting such substitution has been reported. Irradiation of 1,3-dinitrobenzene (DNB) in the presence of tetrabutylammonium fluoride (TBAF) and methylene compounds as carbon nucleophiles accompanied (NuH) provides useful yields of 4-substituted-l,3-dinitrobenzenes, in some cases by lower yields of the 2-substituted isomers.'61 Excitation of the charge transfer complex (259) between an aromatic compound and tetranitromethane generates the aromatic radical cation (261), trinitromethyl anion (260), and nitrogen dioxide (262). Under standard conditions (CH2C12 at - 60 "C or MeCN at -40 "C) path (a) is fast and accumulation of radicals usually too low to permit their detection by EPR. In the presence of added trifluoroacetic acid (TFA), anion (261) is quenched and the slower operation of path (b) allows sufficient buildup of radical species for radical cation (261) or a derived radical cation to be detected by EPR. Under these conditions only nitroarenes (264) result, whereas in the absence of TFA, adducts (263) are also formed. The use of hexafluoropropan-2-01 has made possible room temperature studies of ArH/
(259)
(260)
(261)
(262)
VaTb
27 1
IIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
C(N02)4photolyses. Many aromatic radical cations are detectable by EPR under these conditions, path (a) is eliminated, and the beneficial effect is probably due to strong stabilisation of nucleophiles by hexafluoropropan-2-01.'~~ The photoreactions of tetranitromethane with dibenzofuran,'63 1,2,3,4-tetramethylben1,2,3,5-tetrarnethylbenzenes,166 ~ e n e , ' ~~h ~ en a n thr ene, '~ ~1,2,4,5- and pentamethyl- and hexamethylbenzenes, 67 2,5-di-z-butyl-l,4-dimethoxybenzene, 4-fluoro- and 4-fluor0-3-rnethylanisole,~~~ 2-methyl-, 2,3-dimethyl- and 2,4-dirnethylanis0le,'~~ and tris(4-bromophenyl)amineY 71 also 2,8-dimethyl- and 1,3,7,9-tetramethyIdibenzof~ran'~~ have been reported. Photoassisted decomposition of I -methoxy-4-trinitromethylnaphthalene rapidly yields 4-methoxynaphthoic acid (29%) and 1-methoxy-4-nitronaphthalene(68%), whereas 2trinitromethyl-4-chloroanisole yielded 5-chloro-2-methoxybenzoicacid. 73 Irradiation of the charge-transfer complex between pentachloronitrobenzene (PCNB), or 3,4,5-trichloronitrobenzene,and thiophenol in acetonitrile through Pyrex results only in nitro group photoreduction to the corresponding amines. Pentafluoronitrobenzene is converted to 4-(phenylthio)tetrafluoroaniline.174
'
'
a-Keto carbamates (265) derived from 3',5'-dimethoxybenzoin undergo nearquantitative photocleavage to give free amine and the corresponding substituted benzofuran photocyclisation product (266) and may find use in imaging systems requiring photogenerated base.'75 A synthetic strategy for the synthesis of p-2'deoxyribonucleosides has been developed, based on the deoxygenation of the 3trifluoromethylbenzoate (267) of a secondary alcohol. N-Methylcarbazole photosensitised SET to (267) followed by protonation of its radical anion yields radical (268). p-Scission and hydrogen atom abstraction gives the deoxygenated product (269). 76 Photolysis of (270) through Pyrex in degassed aqueous propan-2-01 containing N-methylcarbazole yields the dibenzoyl derivative (271) of the required P-2'-deoxyribonucleoside.177 The strategy has resulted in the stereocon-
'
272
Photochemistry
trolled synthesis of the anticancer nucleosides 5-fluoro-2’-deoxyuridine and 5trifluoromethyl-2’-deoxyuridine.2-Methyl-1,4-naphthoquinone sensitisation of aqueous 5-methyl-2’-deoxycytidinegenerates the pyrimidine radical cation. Hydration and deprotonation yield a radical which, following reaction with molecular oxygen and subsequent electron transfer and protonation, is converted to a hydroperoxide. Its decomposition yields cis-(5R,6R)-dihydroxy-5,6-dihydro-5rnethyl-2’-deoxy~ytidine.’~~ 2-Phenyl-N,N-dimethylbenzimidazoline is an effective SET agent (Eox = +0.32 V vs. SCE) for reduction of ar,P-epoxy phenyl ketones to aldols (PhCOCH2CHROH) in either tetrahydrofuran or benzene containing about 1% of water.’79
&
CN
The photoreactions of all six dicyanopyridines with a range of alkenes have been reported. In non-polar solvents @so-substitutionof cyano groups a or y to the heterocyclic nitrogen predominates, involving abstraction of an allylic hydrogen from the alkene by the singlet excited dicyanopyridine. In polar solvents, addition of the alkene between a cyano group carbon and an adjacent ring position predominates, resulting in formation of a cyclopenta[b or clpyridine derivative and involving SET from the alkene to the triplet dicyanopyridine. In benzene solution for example, 2,4-dicyanopyridine reacts with cyclopentene to give (272) and (273) (1.2:1, 40%) whereas in acetonitrile (274) (70%) is obtained following aqueous acidic workup.’80 5-Amino-1,2,4-oxadiazoles on irradiation in the presence of primary aliphatic amines yield 1,2,4-triazolin-5-0nes (277) via NO bond cleaved intermediate (276). In an analogous process, 5-alkyl-l,2,4oxadiazoles yield 5-alkyl-1,2,4-triazoles (275). A 2-aminophenyl substitutent at position 3 on the ring results in intramolecular trapping of the ring-opened intermediate.
’
Ph
0(275)X = Me, Ph
Ph
Pk
(276) X = NH2, NHMe, NMe2 X = Me,Ph
R’
(277)
IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
273
Typically photolysis of aziridines yields azomethine ylides as reactive intermediates. The products obtained from a series of aziridinyl enones and fumarates (278), variously substituted by trifluoromethyl groups, are derived from subsequent intramolecular reactions of imine (281), the primary photoproduct. The E- and Z-benzoylaziridines (282) photorearrange to the ( 9 - 1-(alkylamino)-1,2dibenzoylethylene (280) as major product via triplet excited state carbon-nitrogen bond cleavage to biradical (279). In the presence of dimethylacetylenedicarboxylate (DMAD) trapping of the intermediate azomethine ylides (283), formed by carbon-carbon bond cleavage of (282), yields adduct (284). Product evolution from (282) occurs much more rapidly in the presence of DMAD and it is suggested that the major process on excitation of (282) is carbon-carbon bond cleavage and that, in the absence of a trapping agent, the ylides (283) revert to starting material and the minor carbon-nitrogen bond cleavage process yields the radical-derived product (280). Theoretical results are consistent with spectroscopic observation of an azomethine ylide species during conrotatory photocyclisation of a-anilino-P-methylstyrene because of the relatively large energy separation between its So and TI excited states.lS4
'R2
'R2
An SET process involving formation of unstable intermediates (285) is suggested to account for the products observed on irradiation of certain nitroarenes in the presence of styrene, 1,l -diphenylethylene or E-stilbene. High yields of nitrones (286) are isolated when styrene is used. Nitrobenzene, or 4iodonitrobenzene, in the presence of 1,l -diphenylethylene give the nitrone (287)
274
Photochemistry
in modest yield, the main product being benzophenone. For other nitroarenes the nitrones are not formed, benzophenone again being the major product, presumably derived from (285). In these cases alkenes (288) are also obtained, involving the unusual replacement of a nitro group by an alkene.I8' 0 Ar-N:
2R2
CHO
R'
0
Ph
Ar-l&Ph I
ArLl&Ph I
0-
0-
Ph
Ph
The diazoketones (289) from protected a-amino acids undergo smooth Wolff rearrangement to the corresponding ketenes. When generated in the presence of N-benzylbenzaldimine (290), the ketenes yield only trans-p-lactams (291) and (292) in 54-90% yield and with diastereomeric ratios (304:305) ranging from 59:41 to 93:7.'86 Photodecompositon of truns-3-(2-t-butylcyclopropyl)-3H-diazirine gives 3-t-butylcyclobutene (293) and t-butylethylene (294). In the presence of tetramethylethylene (TME) adduct (297) is also formed, from carbene (296), its yield increasing in line with the concentration of TME. The yield of the cyclobutene (293) is not decreased by the presence of TME, consistent with the operation of two product-forming pathways, and is proposed to arise from diradical (295), as is the fragmentation to t-butylethylene (294), also unaffected
-6-
3-t-buty lcyclobutene
(293) +
H t-buty lethylene
(294)
N\ij
by the addition of TME.18' The n,x* triplet excited state of (298) is efficiently photoreduced to the corresponding hydrazine in the presence of hydrogen donors such as 1,4-~yclohexadiene,tributylstannane and benzhydrol. Photoreduction also occurs by SET in the presence of a variety of amines containing an a carbon-
275
IIl6: Photoreactionsof Compounds Containing Heteroatoms Other than Oxygen
0
hv
5-&2
O
H’ w
‘R2 1
(299) R’ = NHCbz, R2 = C02Me R’ = C02Me, R2 = NHCbz
P
hydrogen bond. 88 The pyrazolines (299) are quantitatively and sterospecifically converted to the corresponding cyclopropanes (300)on sensitisation by benzophenone in dichlor~methane.’~~ Irradiation of bisdiazene (301) results only in nitrogen extrusion, whereas monoxide (302),in addition to products arising from loss of nitrogen, yields some metathesis isomer (303).The dioxide (304)produces a high degree of metathesis, yielding 30% of dioxide (3O5).l9OIrradiation of azoalkanes (306)in non-polar solvents yields predominantly housanes (308)via singlet excited state carbon-nitrogen bond a-cleavage and intermediate diradicals (307).In contrast in polar protic media the aziranes (310)predominate, exclusively triplet excited state derived via carbon-carbon bond p-scission and intermediate diradicals (309).19’ Decomposition of (pheny1azo)triphenylmethane (312)yields essentially the same distribution of products (315)-(319),irrespective of whether the process occurs by thermolysis, direct photolysis or SET sensitisa-
276
Photochemistry
(306)R = H, Me
\ pcieavage
tion by DCA, and is interpreted as involving the trityl radical (313) as the common key intermediate. SET sensitisation by 2,4,6-triphenylpyrilium tetrafluoroborate (TPP) however gives a different product distribution, with 9phenylfluorene (314) as one of the main products, produced by photoreaction of the trityl cation (311). Laser flash photolysis experiments failed to detect (311) in the DCA-sensitised reaction whereas in the TPP'-sensitised process (311) could be detected, consistent with a much smaller rate constant for back electron transfer from the TPP radical than from the DCA radical anion.'92Photolysis of 5,5-dibenzyl-A3-1,3,4-0xadiazolines (320) in the presence of dimethyl acetylenedicarboxylate gives a complex product mixture. Most of the primary photoproduct (321) is trapped as the adduct (322) which is converted to secondary photoproducts by loss of nitrogen.'93 Laser flash photolysis of 5,5-dialkyl-A3-l,3,4-
+.
+
TPP' TPP'
\ /
Ph&-N=N--Ph
-Ph36 or A
+
N2
PhH
+
(314)
Ph'
+ Ph&H +
(315) (316)
Ph&
+
(317)
Ph3CCH2CN + 4-Ph2CHCeH4CH2CN
(318)
(319)
oxadiazolines in the presence of carboxylic acids has been used to measure the rate constants for protonation of the generated dialkyldiazomethanes (321).'94 2,3-Diazabicyclo[2.2.2]oct-2-ene(DBO) undergoes singlet excited state deazatisation to bicyclo[2.2.2]hexane and triplet excited state conversion to 1,5-hexadiene7 and has been used as a reporter molecule for enhanced intersystem crossing
1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
277
through spin-orbit coupling due to external heavy atom additives. Direct photolysis of DBO adsorbed in NaY zeolite containing adsorbed xenon resulted in a 24% increase in triplet product relative to direct photolysis in n-octane solution. A similar effect is found in the absence of xenon when the Na' cation of the zeolite is exchanged by Cs+ cation.'91 Laser flash and dual wavelength photolysis of 3,4-diaza-2,2-dimethoxy-1-oxa[4.5]spirooct-3-ene (323) has been used to produce diazocyclobutane and hence cyclobutylidene (324).'96 The imidoylbenzotriazoles (325)lose nitrogen on irradiation in ethanol and intramolecular cyclisation of the resulting biradical yields 1,Zdisubstituted benzimidazoles (326).19'
The D-hex-2-ulopyranosyl azides (327) lose nitrogen on photolysis and are converted to a mixture of two labile imidates. The intermediate nitrenes rearrange by either C2-C3 cleavage (major pathway) or C1-C2 cleavage (minor pathway) followed by carbon-nitrogen bond formation to yield the imidates (328)and (329) respectively, irrespective of the anomeric configuration of the azides studied. 98 The E,E-, E,Z- and 2,Z-isomers of 2,6-di(4'-azidobenzylidene)cyclohexanone do not interconvert on irradiation either in the crystalline state or when adsorbed on silica gel but decompose to yield nitrene-derived products. 199 The main decomposition products of the azides of terephthalic and isophthalic acids are reported
+ -Me
Me (327)
Me+O I
Me (328) Major
Me (329) Minor
278
Photochemistry NHCONHPh
NHCONHPh
"W0 Me
\ /
!J+
Y (331) X = Y = OMe X = NEt2, Y = OMe X = Y = NEt2
But
(332)
Ic
xaco ..i B " '
Me
Me
(335)
(334) (333)
to be isocyanates produced by a concerted mechanism rather than via an intermediate nitrene.200Irradiation of 2-hydroxyphenylazide in an argon matrix at 10K results in formation of an EIZ mixture of 6-imino-2,4-cyclohexadien-1one. 2-Aminophenylazide undergoes a similar transformation to the corresponding diiminocyclohexadiene. Whereas the iminocyclohexadienone is photoconverted to the cumulene O=C=CH-CH=CH-CH=C=NH, the corresponding electrocyclic process for the diiminocyclohexadiene does not occur.20'-202The photolysis of 1-(2-azido-6-chloropyrid-4-yl)-3-phenylurea(330), a potential photoaffinity labelling reagent for probing cytokinin-binding proteins, in the presence of methoxide ion or diethylamine results in high yields of 1,3-diazepines (331).203Irradiation of 2,4,6-trimethylphenylazide in the presence of tetracyanoethylene in acetonitrile yields the azomethine ylide (333) by carbon-carbon bond cleavage in the aziridine obtained by cycloaddition of the singlet aryl nitrene to TCNE. The major product, the spiroazepine (334), is formed by TCNE trapping of trimethyldidehydroazepine, the ring-expanded isomer of 2,4,6-trimethylphenylnitrene.2MLaser flash photolysis of 4-substituted tetrafluorophenylazides in acetonitrile containing sulfuric acid produces the corresponding singlet nitrenium ions, by protonation of the first-formed n i t r e n e ~ . ~The ' ~ nitrenes formed on irradiation of 4-alkoxyphenylazides in aqueous solution are sufficiently basic to undergo protonation by water and the resulting 4-alkoxynitrenium ions, observed by laser flash photolysis, are hydrolysed to benzoquinone.206 Photolysis of N-(diarylamino)-2,4,6-trimethylpyridinium tetrafluoroborates results in product formation via the corresponding diarylnitrenium ions (Ar2N+).207 Photolysis of 5-substituted-N-t-butyl-3-methylanthraniliumion (332)produces transient nitrenium ions (335) which react predominantly by addition of nucleophiles to the aromatic ring.208 Photolysis of ethyl 5-0x0-2phenyl-2,5-dihydroisoxazole-4-carboxylate(336) at 300 nm in the presence of
IIl4: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen Ph\
Ph I
Ph I
N
'1X H "")I
'?CO2Etm
0
Co2Et
(336)
(337)
Et02C
(338)
X
279
X = OR, NHAr, SAr,
OCMsCHCOMe, C(C02Et)=CMeNH2 SCMe=CHCOMe,
phenols, enols, enamines, anilines, aryl thiols and thioenols affords enamines (338),via imino carbene (337).209 The photolabile diagnostic agent metapyrone (339)undergoes triplet state 6cleavage. In addition to anticipated coupling and disproportionation reactions, the resulting radicals recombine in a major process to give (340) which is converted to a The photodegradation of the acylanilide fungicides metalaxyl, benalaxyl and furalaxyl has been reported," also that of the triazine pesticides atrazine, propazine, simazine, terbuthylazine, ametryn and atraton.212 The herbicide benzthiocarb yields 4-chlorobenzaldehyde and 4-chlorobenzoic acid as major photodegradation products.2' The main photodegradation product from the antimicrobial ciprofloxacin (341)is amine (342).2'41,4-Dihydropyridines (343) with a 2-nitro group are particularly photolabile whereas halogeno derivatives display low light sensitivity. Complexation with P-cyclodextrin considerably improves the stability of the 2-nitro derivative, whereas in other cases it accelerates photodecomposition.2'5 The photolabile therapeutic agent indomethacin (344)is reported to yield the unusual product (345) when exposed to sunlight in rnethanoL2l6 4-Chloroaniline is the most photolabile of the
n
(341)X = N-NH (342)X = NH2
0
(343)
(345)
280
Photochemistry
haloanilines, the quantum yield for hydrogen chloride formation being close to unity in solvent mixtures containing 20% or more of water or alcohols. The primary photochemical step is suggested2” to be the one-step elimination of hydrogen chloride, analogous to the process previously proposed2I8 for 4chlorophenol. Hydrogen abstraction by the 4-iminocyclohexa-2,5-dienylidene (346) yields the anilino radical (PhN’), the precusor to the isolated photoproducts, aniline and benzidine. A series of strongly-absorbing triaminotriazines, developed for sunscreen applications, exhibit low disappearance quantum yields to 1 x lop4) in aerated methanol. Structures have been proposed for some of the isolated photo product^.^'^ A kinetic investigation of the photoisomerisation and photodegradation of the E- and 2-oximes of 2-hydroxy-5-methylbenzophenone has been reported.220 The derived model is related to their use in industrial metal extraction processes. RN-NO
I
.. RN-NO
CI
(346)
(347)R = CH2C02H (348)R = Me
0 NR
(349)
Nitric oxide is an important messenger molecule in brain and in blood vessels and acts as a cytotoxic agent. Two highly efficient caged nitric oxides, photosensitive precursors to NO, have been reported. Water-soluble (347) and lipidsoluble (348)release NO by a stepwise mechanism on irradiation, each quantum of light absorbed releasing two NO molecules and generating quinoneimine (349).221 Laser flash photolysis of the S-nitroso derivative of glutathione has been applied to investigation of the homolytic photogeneration of nitric oxide by sulfur-nitrogen bond cleavage. Subsequent reactions of the glutathione thiyl radical stimulate further release of nitric oxide from the precursor.222Photolysis of N-methyl-N-nitrosoanilinesand N-methyl-N-nitroanilinesinvolves N-N bond fission as the primary process, homolytic in aprotic solvents and heterolytic in methanol.223 Photolabile linkers are valuable in polymer-supported organic synthesis where mild methods of release are required. The kinetics of photocleavage of a series of model 2-nitrobenzyl linkers have been reported, leading to the choice of (350)as a peptide-polymer linker (351) in the generation of combinatorial libraries of An efficient synthesis of (350) has been published.225An analogous strategy has been applied to the release of cytotoxic L-leucyl-L-leucine methyl ester from a 2-nitrobenzyl d e r i ~ a t i v e A . ~ liquid ~ phase method for the efficient synthesis of C-terminal peptide N-methylamides under mild conditions utilises a modified PEG support (352). Assembly of a peptide on the methylamino site gives the PEG-protected derivative (353) which, on phototriggered cleavage, releases the peptide N-methylamide. For example, the assembly of a complex
III4: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
28 1
NO2 PEG-NHCO&Ct+-NMe
MeCH&OwCOX RO I
RI
OMe (350) R = H, X = OMe (351) R = CO-peptide X = NH-polymer
(352) R = H (353) R = CO-peptide
nonapeptide N-methylamide with several side-chain protecting groups was achieved in 80% yield.226The photochromism of 2-(2’,4’-dinitrobenzy1)pyridine (356) arises from intramolecular proton transfer. The different tautomers (354), (355) and (356) are considered to be involved. Deuterated (356) and model compound (357) have been used in identification of the coloured form as the NH tautomer (355) using IR, UV and NMR spectroscopy. For the phenanthroline derivative (358) the lifetime of the coloured form in toluene at ambient temperature is increased by a factor of 5 x lo3 compared to (356), and is attributed to the extra stabilisation imparted by hydrogen bonding between the transferred proton in the NH form analogous to (355) and the extra adjacent nitrogen atom.227The photochromism of (356) also occurs within the crystal lattice, though this does not hold for some similar systems, for example (358).228It has been suggested that for photoinert crystals x-stacking of the 2,4dinitrophenyl group with aromatic rings of neighbouring molecules may provide for faster deactivation of the excited state than the proton-transfer process. A nitro group oxygen atom appears to be responsible for the photoinduced abstraction of the benzylic hydrogen, with the pyridine nitrogen playing largely an inductive role.229 A quantum chemical analysis of the proton transfer processes led to a similar conclusion and suggests that an intermediate related to the hydroxy tautomer (354) may be involved in the interconversion between the CH form (356) and the NH form (355).230
282
Photochemistry
A crystalline material exhibiting a novel type of photochromism, involving cooperative proton-electron transfer, and displaying paramagnetism, has been reported. The yellow 1:1 charge transfer complex (359) between bis(4-cyanobenzy1idene)ethylenediamine and meso- 1,2-bis(4-dimethylarninophenyl)-1,2-ethanediol changes to a black paramagnetic material on irradiation. The former state is regained on tempering the material in the dark at 60°C and the behaviour is reversible. It is proposed that SET from the dimethylaminophenyl- to the cyanophenyl-substituted ring, on irradiation into the charge-transfer band of the yellow material, is accompanied by proton transfer from a hydroxy group to an imine nitrogen, thus stabilising the radical-ion pair state.’31 The high photostability of 2-hydroxybenzophenone and of the 2’-hydoxyphenylbenzotriazole (360) arises from extremely efficient reversible deactivation associated with their strong intramolecular hydrogen bonds. Laser excitation in hexane does not generate any transients whereas in dimethyl sulfoxide under the same conditions strong transients due to the corresponding phenoxide ions are observed. The strong intramolecular hydrogen-bonding in non-polar solvents is replaced in dimethyl sulfoxide by strong intermolecular hydrogen bonding to solvent (361). Excitation results in the observed deprotonation by the ~olvent.’~’In aqueous solution, 6-hydroxyquinoline undergoes rapid protonation (15 ps) of its first excited singlet state at nitrogen. Rapid deprotonation (40 ps) from oxygen then yields excited species (362) which undergoes intramolecular charge transfer to yield fluorescent quinonoid species (363).2’3The rates of excited state deprotonation of 2-oxybenzylidene-4’-aminodiphenylamineand 2-oxybenzylidene-4-aminobenzanilide have been reported.’34
H-0
I
I
I
I
I
n
(360)
CN
n
-9 (359)
Me
IIl6: Photoreactions of CompounA Containing Heteroatoms Other than Oxygen
(363)
(364) A (365) A (366) A (367) A
= NH2, B = H, X = N, Y = NPh = NH2,B = Me, X = N, Y = NPh = NMe2, B = H, X = CH, Y = S = NMe2, B = H, X = N, Y = S
283
(368)
Cationic organic dyes find diverse applications, for example as laser dyes, textile dyes and as photopolymerisation initiators. They are often employed in conjunction with triplet sensitisers since typically they undergo intersystem crossing with low efficiency. Phenosafranine (364),safranine T (365), thiopyronine (366)and methylene blue (367)are efficient quenchers of the triplet states of benzophenone, xanthone, thioxanthone, benzil and N-methylacridone, generating the triplet excited dyes by energy transfer. However in the case of dimethylamino substituted sensitisers, such as Michler's ketone, 4-(dimethylamino)benzophenone and 3,6-bis(dimethylamino)thioxanthone,triplet quenching occurs predominantly by electron transfer in polar solvents such as acetonitrile, whereas energy transfer predominates in less polar solvents such as dichloromethane.235 In the absence of a hydrogen donor, crystal violet (368) reacts directly with benzophenone triplets at a diffusion-controlled rate, predominantly by energy transfer. In addition, crystal violet, an aromatic amine, acts as a hydrogen donor generating ketyl and crystal violet radicals. In the case of anthracene and naphthalene, only energy transfer to form triplet crystal violet occurs.236The or-amino and ketyl radicals, produced by reaction of benzophenone triplets with amines and alcohols, respectively, react efficiently by SET with cationic organic dyes such as phenosafranine (364), thiopyronine (366),methylene blue (367) and crystal violet (368).The a-amino radicals are more reactive than the ketyl radicals, In dye mixtures used with ketone-amine UV photoinitiator systems, SET from an initially formed semireduced dye to a molecule of another dye occurs. The initially formed crystal violet radical, for example, reacts very rapidly with phenosafranine, forming semireduced phenosafranine and regenerating crystal violet.237 In a steroid derivative selective excitation of a 17P-benzidine chromophore results in isomerisation of a 3P-norbornadiene unit to quadricyclane. The major pathway involves long-range triplet energy transfer, though a significant contribution arises from SET from singlet benzidine to the norbornadiene. Both processes are believed to proceed via a through-bond mechanism.238Photophysical studies on bichromophoric compound (369) indicate the presence of an additional rapid nonradiative process, k > 2.5 x 10" s-', when compared to a model dimethoxynaphthalene system, possibly associated with long-range energy or electron transfer processes.239Photoinduced SET processes have been studied in the bichromophoric donor-acceptor system (370).240The donor-bridgeacceptor piperidines (371) undergo harpooning, involving long-range photoinduced charge transfer followed by electrostatically driven conformational
Photochemistry
284 Me I OMe 2BF4-
OMe Me
(369)
folding. Charge transfer within the locally excited state (373)yields the extended charge transfer state (374)which, under the influence of electrostatic attraction in non-polar solvents, change to the compact structure (375); in polar solvents dielectric screening reduces the driving force for folding and allows the extended CT state (374)to survive. In contrast to the piperidines (371),the piperazines (372)do not undergo harpooning, the weaker trialkyl substituted nitrogen donor becoming involved in the CT process because of its much shorter distance from the acceptor Excitation of the bichromophoric system (376)results in intramolecular SET from the anilino to the cyanonaphthalene chromophore and strong fluorescence results. The fluorescence responds sharply to changes in medium polarity and polarizability and provides a probe for monitoring microscale events. The derivative (377)behaves quite differently and is essentially nonfluorescent. However, on reaction with amines or thiols, the resulting adducts (378)become highly fluorescent, confirming that substrate labelling has occurred and avoiding the necessity to remove unreacted (377)from the mixture.242
ZIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
285
CN
(376)R = H
(379)n = 1 , 5
0
3 I
(377)R = N
0
0
(378)R = N p s : 0
0 NpNHR' 0
Compounds (379) have been developed as second-generation fluorescent PET pH sensors. In acidic solution, irradiation at 367 nm mainly excites the anthracene chromophore whereas both fluorophores emit in comparable proportions. As the pH is increased the anthracene emission decreases sharply due to intramolecular SET quenching whereas that from the 3-amino-1$-naphthalimide unit is less affected. The ratio of the two emissions permits the environmental pH to be monitored.243 Multichromophoric b-cyclodextrins have been used as microphotoreactors to induce unimolecular photoreactions. A detailed mathematical model has been developed which takes account of the different possible excitation routes by which transformation of substrate into product may occur.244The photoisomerisation of ot-(4-dimethylaminophenyl)-N-phenylnitrone to N-(4-dimethylaminophenyl)formanilide in the cavity of a water-soluble P-cyclodextrin bearing seven naphthoyl chromophores as light-harvesting antennae has been investigated using this model. In the presence of the modified cyclodextrin, the overwhelming portion of the reaction arises from energy transfer from the antennae to the substrate within the cavity. Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1:1 pseudorotaxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the 'thread' (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs.245 The enhanced photoreactivity of rneta- vs. para-substituted benzenoids has been rationalised in terms of selective transmission of electron withdrawal or donation.246A new example of the meta effect has been reported for the excited
Photochemistry
286
HO
u o u o state behaviour of the E-isomers of a series of aminostilbene derivative^.'^' SET from borate anions to the triplet excited benzoyl group has been shown by nanoand picosecond laser flash photolysis to occur on excitation of the benzophenone chromophore in the tetraalkylamrnonium triphenyl-n-butylborate (382). Dissociation of (383) and back electron transfer is proposed to yield tributylarnine and radical (384) which yields coupling products (385) and (386). The corresponding gallate salt behaves similarly.248Nanosecond laser flash photolysis techniques
P h - r e C H 2
-
J
t
+ +' NBu3-
Ph3B BET
+
f
Bu
M6: Photoreactionsof Compoundr Containing Heteroatoms Other than Oxygen
-
l-naphthyl-CONHPh
1-naphthyl-CON-OCO I (387) Ph
(388)
+
287
Ph2C0
(389)
+ (HO&CEH&OPh) COPh = 2-, 3-or 4-benzoyl
(390)
H3Gygpo 0
PhCON'BUt I
0' PhANOCOPh
O-
Me
have been used to investigate magnetic field effects on the photochemical SET reactions of 10-methylphenothiazinewith DCB and tetrafl~oro-DCB.~~~ Direct photolysis of O-p-anisoyl-N,N-dibenzylhydroxylamine in both polar and nonpolar solvents gives N-benzylidenebenzylamineand p-anisic acid in a 1:1 stoichiometric ratio from the singlet excited state and involving disproportionation of the in-cage radical pair produced by homolytic N-0 bond cleavage. Triplet sensitisation yields anisole as an additional product, resulting from out-of-cage decarboxylation of the anisoyloxy radical.250Photolysis of the N-( 1-naphthoyl)-N-phenylO-(benzoylbenzoy1)hydroxylamines (387) in methanol gives (388), (389) and (390), whereas the benzoylbenzoic acids (390) are not formed in 1,2-dichloroethane or acetonitrile. In 1,Zdichloroethane relaxation of the vibrationally excited radical pair resulting from N-0 bond cleavage is slow compared to decarboxylation of the benzoylbenzoyloxy radical and (390) is therefore absent from the products. In methanol hydrogen-bonding to the vibrationally hot radical pair promotes its relaxation, reducing the extent of decarboxylation and yielding (390) as major product.251N-Chlorobenzotriazole and N-t-butyl-aphenyl nitrone react together photochemically to form spin adducts which are subsequently converted to E- and Z-benzotriazolyl phenyl ketone O-benzoyloximes (391) and the N-benzoyl-N-t-butyl aminoxyl radical (392).252Photolysis of aqueous glycylalanine produces acetamide, glycine N-ethylamide and the two diastereomeric N,N'-diglycylbutane-2,3-diamines, in addition to ammonia, carbon dioxide and acetaldehyde. The product outcome can be rationalised in terms of initial intramolecular SET from the carboxylate group to the peptide bond to give intermediate (393).253Irradiation of benzaldehyde, furan-2-carboxaldehyde and thiophene-2-carboxaldehydein the presence of pyrrole gives the aryl dipyrrylmethane derivatives (394) in good yield.254The achiral compounds, acridine and diphenylacetic acid, form chiral molecular crystals in which the components are assembled in a 1:1 ratio by hydrogen bonding. Irradiation of the crystals results in decarboxylation and formation of chiral compound (395) in ca. 35% enantiomeric excess from the diphenylmethyl and hydroacridine radicals. Radical coupling to give (395) occurs within the shortest distance of 5.1A
Photochemistry
288 H
OH (394)
(395)
(396)X = S (397)x = 0
CN
CN
(399)
between the two preradical carbon atoms in the crystal lattice to form the major enantiomer. T6e minor enantiomer can be produced by coupling over a longer distance of 6.8A, resulting in a ca. 2:l ratio of enantiomers, i.e. about 35% ee.255 The photochemistry of N-hydroxypyridine-2( 1H)-thione (396) is pH dependent and influenced by solvent polarity. It cannot be considered as a clean photolytic source of hydroxyl radicals, since simultaneous generation of several highly reactive intermediates also occurs.256However N-hydroxy-2(1H)-pyridone (397) is a much simpler and more specific generator of hydroxyl radicals, the accompanying 2-pyridyloxy radical being relatively unreactive and removed mainly by bimolecular radical reactions.2s7Irradiation of some ihiocarbazone derivatives in the presence of small amounts of hexabutylditin yields adducts derived from ring opening of a cyclobutyl iminyl radical followed by transfer of an iminodithiocarbonate group. For example, nitrile (399) (97% yield; 35% cis, 65% trans) was produced from (398).25sIrradiation of the 2-dehydrophenylalanine derivative (400) yields the E-isomer (402). Further irradiation yields (401) and (403). Azetidine (403) arises from a sequence involving 1,3-acetyl migration, imine reduction and cyclisation whereas the isoquinoline (401) arises from 1,5-acetyl migration followed by cyclisation and dehydration.259 Laser flash photolysis studies have shown that, unlike phthalimide, the triplet excited state of 1,8-naphthalimide does not undergo hydrogen abstraction reactions with propan-2-01, consistent with a lowest energy n,n* triplet in the latter case. Electron-donors such as triethylamine and triphenylamine quench this triplet by an SET mechanism.260N-Arylmaleimides, substituted in the 4-position of the aromatic ring, quench the fluorescence of N,N-dimethyl-p-toluidine (DMT) by charge transfer, the values of the Stern-Volmer constants (kqz) paralleling the Hammett 0 values for the substituents. Irradiation of mixtures of the N-arylmaleimides and DMT in cyclohexanone yields homopolymers.261 Cyclopentadienyl-cyclooctadiene-cobalt(1)may be used as a photocatalyst for the reaction of nitriles with ethyne to give substituted pyridines, for example
IIl4: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
289
2-phenylpyridine from benzonitrile and ethyne. The reaction can be driven by sunlight since maximum light absorption by the catalyst is in the 400-450 nm range and water may be used as the reaction medium.262
3
Sulfur-containing Compounds
E-Stilbene analogue (404)undergoes singlet excited state EIZ isomerisation. The presence of two heteroaromatic rings in (405) enhances spin-orbit coupling and leads to mixed singlet and triplet derived isomerisation in non-polar solvents. The condensed rings in (406)and (409) increase the torsional barrier to singlet excited state twisting, reducing the isomerisation quantum yields, and these therefore deactivate mainly through fluorescence and intersystem crossing, resulting in predominantly triplet state EIZ photoisomerisation. The activation barrier to twisting is reduced in polar solvents, favouring singlet excited state isomerisat i ~ nOxidative . ~ ~ ~ photocyclisation of a series of thienylpropenoic acids, thiastilbene analogues, yields the anticipated polycyclic heterocycles as main produ~ts.~ New ~ , ~sulfur ~ ~ heteroaromatics have been prepared by double oxidative photocyclisation of appropriate tetraaryl substituted ethenes. The first step proceeds readily, though the subsequent second closure is only efficient when one of the cyclising substituents is thienyl. For example, irradiation of (407) yields (408),whereas only (411) results from irradiation of (410).266 Achiral thioamide (412) crystallises in a chiral space group and irradiation of the powdered crystals leads to exclusive formation of optically active P-thiolactam (413) with high enantiomeric excess by a mechanism involving intramolecular transfer of the appropriately-oriented benzylic hydrogen and cyclisation with minimal molecular motion within the S-(o-Toly1)-o-benzoylbenzothioate also crystallises in a chiral space group. Irradiation of a solid sample of a single enantiomorphic modification yields optically active 3-phenyl-3-(o-tolyl-
Photochemistry
290
hv
(404) Ar = Ph (405) Ar = 2-thienyl (406) Ar =
(407)
Th = 2-thienyl
thio)phthalide. The sterochemical outcome is consistent with minimum molecular and atomic motion for phenyl migration and involving bond formation between the thioester oxygen and the benzoyl carbon.268 Photocyclisation of 2-arylthiocyclohexenones(414) yields racemic dihydrobenzothiophenes (416) via a thiocarbonyl ylide (415). The prochiral compounds (414) also form 1:l inclusion complexes with the homochiral host (63) and irradiation of crystals of these complexes yields the dihydrobenzothiophenes (416) in 32-83% enantiomeric excess, optically pure samples being easily accessible by further recrystalli~ation.~~~ The decay kinetics for ylides (415), produced by laser flash photolysis of the aryl vinyl sulfides (414), indicate the presence of more than one ylide speciesin each case.27o
lIl6: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
29 1
(419) (420) (417) R' = H, R2 = Ph (418) R' = H, Me; R2 = 4-MeCsH4, Me(CH&, PhCH=CH, 2-fury1 R', R2 = (CH2)" (n= 3-6)
The sulfur-sulfur interatomic distance in 9-phenyl-4,8,lO-trithiadibenzo[cd,ij]azulene-8-oxide (417) is significantly shorter than the sum of their van der Waals radii and photolysis of (417) and (418) yields (420) and aldehydes or ketones (421) quantitatively. The unstable primary photoproduct (419) may be isolated and photolyses to (420) and (421).271A similarly short sulfur-sulfur interatomic distance is found in a series of naphtho[ 1,8-ef][1,4]dithiepins and direct irradiation yields naphtho[ 1,8-cd][1,2]dithioles quantitatively with analogous S-S bond formation and alkene elimination.272Photolysis of compounds (422), (424) and (426) gives the dimerised disulfides (423), (425) and (427) in 45%, 17% and 1% yields respectively.273Irradiation of the tetraalkyl-2H-thietes (428) at 254 nm leads to a photostationary mixture containing the purple enethiones (429) (25%). Exposure of the mixture to 300 nm radiation induces almost complete reversion to (428).'74
But
(428) (422)R =CHMe2 (424) R = Et (426) R = Me
(423) R =CHMe2 (425) R = Et (427) R = Me
(429)
R = Me, CH2CHMe2, CH2Ph
Thiobarbiturate (430) undergoes [2+2]photocycloaddition with alkenes (2,3dimethylbut-2-ene, ethyl vinyl ether, a-methylstyrene and acrylonitrile) to give the thietane (433) and, by secondary photolysis involving extrusion of either thioacetone or thioformaldehyde, the methylene derivative (434). Dithiobarbiturate (431) and trithiobarbiturate (432) undergo analogous reactions, though at the thioamide rather than the thioimide sulfur. For trithiobarbiturate (432) a competing conversion to dithiohydantoin (435) occurs in all cases, possibly involving a-cleavage adjacent to the gem-dimethyl group, reclosure to form a
Photochemistry
292
Me Y
0
Me
(430) X = Y = 0 (431) X = 0, Y = S (432) X = Y = S
I Me
Me R' (433)
(434)
(435)
(436) R = H, alkyl (439) R = (CHZ)&H~CH=H~
sulfur heterocycle and subsequent carbon-carbon bond formation with extrusion of Benzoxazole-2-thiones (436), with alkenes, yield products whose formation may be rationalised in terms of intermediate (437), from ring-opening of an aminothietane, formed by regioselective [2+2]photocycloaddition of the carbon-sulfur double bond to the alkene double bond. The subsequent reaction outcome depends on the presence or absence of a substituent at nitrogen and on the alkene used. Compounds (439) undergo an intramolecular version of the process to yield lactam analogues (UO).276 In the presence of alkenes, irradiation of N-aryl-2,5-dihydro-1H-pyrrole-2-thiones yields pyrrole analogues of (438). Indoline-2-thiones and alkenes give the corresponding indole derivatives. In undergo the presence of triethylamine, N-aryl-2,5-dihydro-lH-pyrrole-2-thiones
D
C
O
M
e
+
4' '-CN
(441) COMe
3%
1116: Photoreactions of Compoundr Containing Heteroatoms Other than Oxygen
293
N H
\
NH2
(444)
(445)
(443)
desulfurisation to pyrroles, whereas the N-alkyl substituted analogues are photoreduced to pyrrolidine-2-thiones. SET, coupling, elimination sequences have been proposed.277Similar desulfurisation occurs when indoline-Zthiones are irradiated in the presence of t r i b ~ t y l a m i n e .Photoexcited ~~~ 2-acetylbenzothiophene (441) adds to 2 - morpholinopropenenitrile regioselectively head-totail and stereoselectively to give (442) as sole adduct. 3-Acetylbenzothiophene adds to 2-morpholinopropenenitrileand to 24 t-buty1thio)propenenitrile regioselectively to form both steroisomeric head-to-tail products.279Both regioisomeric anti-adducts are formed when 2-benzoylthiophene undergoes [2+2]photocycloaddition to methylmaleic anhydride. The corresponding anti-adduct is also formed with dimethylmaleic anhydride.280Irradiation of benzene solutions containing thiobenzamide and aldehydes (RCHO) produces 2-substituted-4,5-diphenylimidazoles (443)in low yields.28' Modification of the reaction conditions facilitates subsequent oxidative photocyclisation of (443)to the corresponding phenanthroimidazoles in moderate yields. Irradiation of arenethioamides (ArCSNH2) and 2trimethylsilyloxyfuran (444)gives arene-fused aminobenzoates (446) in good yields following workup with acidified possibly involving initial formation of thietane (445). Radical cations generated by photoinduced SET from u-stannyl sulphides (447) to cyclohexenone in methanol cleave to u-alkylthio radicals which couple with the cyclohexenone radical anion to subsequently yield 3-substituted cyclohexanones (448). The corresponding intramolecular coupling process proceeds more smoothly in the presence of added 1,4-dicyanonaphthalene as SET sensitiser, for example (449) cyclises to (450).283 Steric interactions in the transition states leading to photoproducts and which affect the ability of the aryl group to delocalise charge are suggested to account for the differences in behaviour
(447)R = Bu, Me; R' = Ph, Me; R2 = H,SMe
(448)
(449)
(450) 84%
294
Photochemistry
reported for variously substituted 2,3,4,4,5,6-hexasubstituted4H-thi0pyrans.”~ Conversion of the red dithiines (451) to thiophenes (454) involves the singlet excited state and may occur via ring-opening to (452), followed by conformational change, ring-closure to episulfide (453) and extrusion of The main products isolated from either photolysis or pyrolysis of 4-substituted-2,s-diamino3,5-dicyano-4H-thiopyransmay be rationalised in terms of subsequent reactions of the biradical (455) formed by initial carbon-sulfur bond cleavage, to yield the 2-amino-3,s-dicyanopyridinethiones (456) and acrylonitrile derivatives.286
(457)X = Y
H
I
(458) X = H, Y = I
(459)
The photolysis of alcohols with hypoiodite-forming reagents can lead to products derived from alkoxy radicals, typically involving intermolecular cyclisation or oxidative fragmentation. Treatment of a 6P-sulfanylpregnane (457) with mercuric oxidehodine reagent in carbon tetrachloride and irradiation yielded the sulfur-bridged product (459), believed to involve initial formation of sulfenyl iodide (458) followed by C-5 alkoxy radical formation, C5-C10 bond fragmentation and carbon-sulfur bond formation.287Irradiation of thermally stable thiiranes (460) causes their rapid isomerisation to the more extensively conjugated thiiranes (461), possibly involving a thiatrimethylenemethane intermediate.288 Bisthiocamphor (463), by analogy with thiocamphor, might have been expected to yield thiol (462) on irradiation. However only mercapto enethione (464)is obtained, suggested to involve 1,5-hydrogen abstraction followed by intermolecular disproportionation and a 1,5-hydrogen shift.2899-Thiabicyclo[3.3.llnonan3-one (465) forms Norrish Type I product (467)on photolysis in t-butyl alcohol whereas it has been previously reported290that 8-thiabicyclo[3.2.l]octan-3-one
IIl6: Photoreactions of Compound Containing Heteroatoms Other than Oxygen
295
(469) yields thiolactone (468).The different chemoselectivities arise from the different reactivities of biradicals (466) and (470). For biradical (466)the
stereochemistry facilitates intramolecular hydrogen abstraction from the acyl radical side chain by the alkyl radical centre to form the intermediate ketene which is captured by solvent to yield (467). In contrast the tetrahydothiophene ring conformation in biradical (470) increases the distance between the alkyl radical and the a-hydrogen to be extracted and this, along with relief of the greater ring strain in the five-membered ring upon cleavage of the carbon-sulfur bond of (470), favours formation of thiolactone (468) rather than the corresponding ketene.291
J2
Me
, ‘0 (465) n = 1 (469) n = 0
(466) n = 1 (470) n = 0
CH~CO~BU‘
(467)
Benzyl phenyl sulfides undergo photocleavage through direct homolytic cleavage of the benzyl-sulfur bond. The meta derivatives show more significant variances in quantum yield than the para derivatives, and this can be explained”’ on the basis of the ‘meta effect’.246The reactions of some benzylphenyl sulfide radical cations, formed on irradiation of the sulfides in oxygenated acetonitrile in the presence of DCA, have been reported. Three main reaction pathways are available. Benzylic products arise from carbon-sulfur bond cleavage whereas benzylic carbon-hydrogen bond cleavage leads to benzaldehydes, and oxidation
296
Photochemistry
at sulfur forms the corresponding s ~ l f o x i d e .Photoinduced ~~~ SET from two dithienothiophenes to either 1,6dinitrobenzene (DNB) or carbon tetrachloride has been used to generate the radical cations of the condensed thiophene trimers. Diffusion controlled SET to DNB occurred from both the singlet and triplet excited states of the thiophenes. In cyclohexane, radical cation generation did not occur, the thiophene triplet excited states being deactivated by energy transfer.294 The photolysis of alkyl aryl sulfoxides occurs via homolysis of the sulfur-alkyl bond. The sulfinyYalkyl radical pair partitions between recombination, formation of sulfenic esters, disproportionation to an alkene and benzenesulfenic acid, and formation of radical escape products. No evidence for product formation by hydrogen abstraction pathways was found.295 Dibenzothiophene sulfoxide produces high chemical yields of dibenzothiophene and oxidised solvent, though with low quantum efficiency. Typically benzene is oxidised to phenol, cyclohexane to cyclohexene and cyclohexanol, and cyclohexene to 2-cyclohexenol and epoxycyclohexane. The active oxidising agent may be ground state atomic oxygen O(3P).296Direct irradiation of S-phenyl-1-naphthoate in acetonitrile results in diphenyldisulfide and 1,l '-binaphthyl as main products. External magnetic field effects are consistent with product formation by a triplet excited state process involving carbonyl-sulfur bond cleavage.297 Titanium dioxide acts as a siteselective photocalalyst for sulfur-containing carboxylic acids, with reaction occurring at the sulfur. Whereas phenylacetic acid (471) and phenoxyacetic acid (472) undergo oxidative decarboxylation to toluene and anisole respectively, arylthioacetic acids (473) in oxygen-saturated methanol are oxidised to the corresponding sulfinylacetic acids (474). Phenylsulfinylacetic acid (474) in methanol under nitrogen is photoreduced to phenylthioacetic acid (473) and photoreduction also occurs for arylsulfonylacetic acids (475) yielding mainly the sulfinylacetic acids (474) and the thioacetic acids (473).298The first examples of irreversible excited state deprotonation of a carbon-hydrogen bond have been reported.299Irradiation of the thioxanthenium salts (476) in dry acetonitrile gives the stable orange thiaanthracenes (477). Efficient deprotection of 2-methyl-2-tbutyl-l,3-dithiolane (478) and of the corresponding oxathiolane (479) to the ketone occurs on sensitisation by catalytic amounts of SET sensitisers in oxygen saturated acetonitrile. The reaction involves interception of the substrate radical cation by oxygen or superoxide anion but is unsatisfactory for protected aldehydes. The dioxolane (480)undergoes carbon-carbon bond fragmentation rather than depr~tection.~"2,2-Di-p-tolyl-l,3-dithiolane is efficiently deprotected using sunlight and a triphenylpyrilium salt.301 ArXC02 (471) X = CH2 (472) X = OCH2 (473) X = SCH2 (474) X = SOCH2 (475) X = S02CH2
H
H
A
x-
(476) R = Me, Ph; X = BF4,C104
H
I R
(477)
1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
297
Electron transport across organised bilayers is an integral part of biological energy storage systems such as photosynthesis and provides a means of controlling back electron transfer and of separation of the products of redox reactions. Esters of 2,1,3-benzothiadiazole-4,7-dicarboxylic acid (481) have been used to study the transfer of electrons from 2-(morpholino)ethanesulfonic acid (MES) to 1,5-anthraquinone disulfonate in micelles or across vesicle bilayers. The esters absorb the light, accept an electron from MES and transfer it to the q~inone.”~
C02R
I
Me3c0 Me
(478) X = Y = S (479) x = s, Y = 0 (480) X = Y = 0
C02R
(4811
An approach to the photorelease of caged a-amino acids involves formation of intermediate (482) by intramolecular electon transfer, triggering elimination of the required a-amino acid from the water-soluble sulphonamide precursor. However in the absence of a large excess of added ascorbate as reducing agent much of the photoreleased a-amino acid suffers decarboxylation, probably by incage oxidation of the carboxylate group by a radical site in the cleaved residue and subsequent decarboxylation of the carboxyl radical.303Thioxanthones are commonly used as photoinitiators for the polymerisation of unsaturated substrates. Investigations of their photophysics and photoreactions with other components present, such as tertiary amines and vinyl monomers,3o4and of the incorporation of thioxanthone initiators and amine synergists into polymer ~ t r u c t ~ r ehave s , ~ been ~ ~ reported.
4
Compounds Containing Other Heteroatoms
4.1
Silicon - Direct irradiation of E-4-cyano-4‘-(pentamethyldisilanyl)stilbene
(483) in dichloromethane yields the 2-isomer. Both fluorescence and isomerisa-
tion are quenched in the presence of added methanol, due mainly to a solvent polarity induced change from a locally excited to a charge-transfer process, competitive with fluoresence and E,Z-photoisomerisation. Reaction of the CT state with methanol gives E-hydrosilane (484a) inefficiently, deuterium incorporation from methanol-OD giving (484b), consistent with regioselective nucleophilic cleavage of the silicon-silicon bond.306 SET cyclisation of 3-(3-butenyl)-l-trimethylsilyloxy-1-cyclohexene in acetonitrile containing DCA yields the 6-end0 products (489) (25%) via radical cations (485) and (486).In acetonitrile/alcohol mixtures the overall yield is enhanced and the reaction selectivity favours 5-exo
298
Photochemistry
(483) R = SiMe3 (484) (a) R = H (b) R = D
(485)
i
a3 H
Bu,'
Me3Si
,Bur (493)
+ MesSi
GeMes2
products (488),the alcohol causing desilylation to ol-keto radical (487)prior to 5ex0 cyclisation in accordance with Baldwin's rule.307The allenylketenes (491) are formed essentially quantitatively on photolysis of the 2,3-bis(trimethylsilyl)methylenecyclobutenones (490).308The siladigermirane (492) undergoes regioselective photocleavage to give the germasilene (493) and the germylene (494), which may be trapped as their methanol or water ad duct^.^'^ The [2+2]photocycloaddition of 5-arylfuran-2,3-diones (496) to trimethylsilyloxyethylenes (495) occurs with high regio- and stereoselectivity to efficiently yield polyfunctionalised cyclobutanes (497).310A convenient four step stereoselective synthesis of ( & )-norasteriscanolide (500) has used the [2+2]photoadduct (499) from 2-cyclopentenone and the trimethylsilyl enol ether (498) to assemble the 5/8 ring system present in many terpen~ids.~ * 2-Naphthaldehyde undergoes regioand stereoselective [2+2]photoadditon to the cyclic ketene silyl acetals (501) to
'
X
-
+x%oh
/f
MeaSiO
RjrOSiMe3
0
0 (495) R = vinyl, Ph
(496) X = H, Me, OMe, CI, NO2
0
0 (497)
299
M6: Photoreactionsof Compounh Containing Heteroatoms Other than Oxygen 0
(499)
(498)
ButMe2s;y;r
-
H20
___)
R'
'R2
R'
7FA
R'
(5011
k 2
R'
(502)
(503)
R' = H, Me; R2 = H, Me; Ar = 2-naphthyl
give the bicyclic orthoesters (502) which, on treatment with water, yield the aldoltype products (503). Ground state Mukaiyama-aldol condensations of 2naphthaldehyde with (501) gave (503)with much reduced diastereosele~tivities.~~ 2,2-Dialkylcyclopropanone silyl acetals (505) have been used to regioselectively introduce a three-carbon unit containing an ester group to an electron-deficient alkene or aromatic. For example, irradiation of an acetonitrile solution of (505) containing an electron-deficient alkene, such as 1,l -dicyano-2-phenylethene or 1cyano- 1-methoxycarbonyl-2-phenylethene, or an electron-deficient aromatic, such as DCB or 1,2,4,Stetracyanobenzene,in the presence of phenanthrene gives high yields of alkene addition product (504) or aromatic substitution product (506), re~pectively.~'~ Scheme 2 illustrates the role of phenanthrene and its radical
CN R'
'Phen' X C@R3 (504)
Phen+' D+' A- '
..
A
D
+ A-'
Phen+*
Phen
-
+
D+'
Me3Si+ + R'6R2-CH2C02R
+ R'CR2CH2C02R3
-
Addition or Substitution
..
(506)
A = electron deficient alkene or aromatic D = dialkylcyclopropanone silyl acetal
Scheme 2
Photochemistry
300
Me I Me I
p H s i Mer
Q - L - C Me IH M ~ ~
Et-Si-Si-Et I
I
Ph Ph
Ph-Si-Et I Me
Ph-Si-Et I Me
(509) X = CH2 (510) X = 0
(507)
cation in the reactions. Photolysis of the meso and racemic disilanes (507) in the presence of isobutene gives the adducts (509) by a diasterospecific ene reaction of the rearranged silenes (508) with isobutene, consistent with a concerted me~hanism."~In the presence of acetone the adducts (510) are formed.315 Hexakis(t-butyldimethylsilyl)tetrasilacyclobutene (511) yields the less thermo~'~ dynamically stable tetrasilabicyclo[1.1.O]butane (512) on p h o t o l y ~ i s . Irradiation of acyltrimethylsilanes (513) gives the corresponding a-siloxycarbenes (514). Reaction of the carbenes with acidic non-nucleophilic alcohols such as trifluoroethanol to give carbocations (515) and the subsequent reactions of these with nucleophiles may be followed by laser flash p h o t ~ l y s i s . ~ ' ~ R
0 II Ar-C-SiMe3
(513)
h
Ar-E-OSiMe3
(514)
I
CF3CH20H
Ar6HOSiMe3 (515)
Steroid 6f3-dimethylphenylsiloxy(DPSO) groups have been used as antennae to harvest 266 nm photons and subsequently to photoactivate ketone functions at either the steroid 3- or 17-positionsby intramolecular singlet energy transfer. The monoketones (516) and (517) both exhibit DPSO-sensitised photochemistry at the carbonyl groups, whereas diketone (518) gives products which arise from 17ketone activity, indicative of facile singlet energy transfer from the 3-keto to the 17-keto The nature of the monoatomic bridge in the dimethylsilylene and isopropylidene compounds (519) has a major effect on photoinduced chargetransfer between the two aromatic systems. CT is thermodynamically more favourable for the SiMe2 bridge, a consquence of the lowering of the cyanophenyl reduction potential by the silicon SET sensitisation of silanorbornadiene (520) by 2,4,6-triphenylpyrilium tetrafluoroborate yields anthracene and difluorodimesitylsilane (521), involving rapid transfer of fluoride ion to the radical cation of (520).320
1116: Photoreactionsof Compoundr Containing Heteroatoms Other than Oxygen
30 1
ODPS dimethylphenylsiloxy P
Disilyl carbenes (522) and (523), generated by photolysis of the corresponding disilyldiazomethanes, rearrange to silenes, for example (524), by 1,Zshift of substituents from one of the silicon atoms to the carbene centre. The relative migratory aptitudes of substituents for both of these c a r b e n e ~ is ~ ~opposite ' to that reported322for monosilyl carbenes. Photolysis of benzyltrimethylsilane in ethanol glass at 77K produces benzyl radicals via the singlet or a higher triplet excited state whereas in 3-methylpentaneglass a-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations are formed via the lowest triplet state.323 Mercury-sensitised photolysis of dimethylsilane results in hydrogen-abstraction from the silicon-hydrogen bond with unit quantum yield. The dimethylsilyl radical disproportionates by two different reactions to give 1-methylsilaethene and dimethy~ilylene.~~~
Other Elements - Though most of the focus on photoinitiated SET processes remains on the mesolytic reactions of radical cations, organoselenium radical anions, produced by SET sensitisation by 1,5-dirnethoxynaphthalene, show synthetic promise as intermediates leading to unimolecular group transfer radical sequences. Thus, irradiation of (525) in the presence of 1,5-dirnethoxynaphthalene and ascorbic acid as sacrificial electron donor in aqueous acetonitrile
4.2
302
Photochemistry
X = CH2, R = H X = C(C02Et)2, R = H X = 0, R = OEt
containing dissolved oxygen leads to formation of cyclised products (526) in good yields. Cleavage of the radical anions produces the corresponding carbon-based radicals and phenylselenyl anion. Radical cyclisation and reaction with diphenyldiselenide, produced from phenylselenyl anion, yields the products.325 In the presence of an alkene and carbon monoxide, photolysis of methyl a-(phenylseleno)acetate (527) and related compounds results in formation of acyl selenides (528) by group transfer carbonylation. Addition of a methoxycarbonylmethyl radical to the alkene, trapping of the resulting alkyl radical by carbon monoxide and termination of the reaction by phenylselenyl group transfer from (527) yields the products.326Reversible photoepirnerisation of C-phenylselenated deoxyribonucleoside (529) in the presence of diphenyldiselenide occurs by cleavage of the C-Se bond and reaction of the intermediate radical with diphenyldi~elenide.~~~ The 2-bromovinylselenides (530) undergo regiospecific 1,3-hydrogen migration to yield the vinylselenides (531) q ~ a n t i t a t i v e l y . ~ ~ ~
A steady state photoequilibrating mixture of germacyclopentadieneisomers is formed by photodecomposition of 1,l -diazido-1-germacyclopent-3-ene (532) in an argon matrix at 11K.329
Aryltelluroformates (533), accessible from the corresponding chloroformates, undergo cleavage of the acyl-tellurium bond on photolysis. In the presence of diphenyldiselenide, the oxyacyl radical (534) is trapped as the corresponding alkyl (phenylse1eno)formate (535) in excellent yield.330 Irradiation of aryl bromides or iodides (536) with potassium tellurocyanate (537) in dimethyl sulfoxide, as both solvent and methyl source, yields aryl methyltellurides (538) in modest yields (9-34%).33'
M6: Photoreactionsof Compounds Containing Heteroatoms Other than Oxygen 0
0
II
RO-C
(533)
RO-C-SePh
(534)
+
KTeCN
(536)
(537)
ArX
0
II
II
RO-C-TeAr
303
(535)
hv
DMSO
Ar-TeMe
(538)
X = Br, I; Ar = Ph, 1-naphthyl, 2-naphthyl,2-MeCONHCsH4
Photostimulated addition of t-butylmercury halides to 1,6-dienes (539) and enynes yields cyclised primary alkylmercury halides (540) from 5-exo cyclisation of the intermediate adduct radicals. In some related cases, unfavourable rotamer populations militate against efficient c y ~ l i s a t i o n . ~ ~ ~ CH2HgHal
X = Y = CH2; X = CH2, Y = C(C02Et)z X = CH2, Y t 0; X = CH2,Y = N-ally1 X = P(O)OH, Y = 0; X = P(0)O-allyl,Y = 0
0 II
PhO-P(OPh);!
0 II
R
R
I
hv MeOH
0
II 2-PhCeHaO-Y-OPh OH (542)
(544)
+ PhPh (545)
9,1O-Dihydro-9,10-ethenoanthracene-11,12-bis(diphenylphosphine oxide) (541) forms a variety of crystalline solvent inclusion complexes. Those involving ethanol, propan-1-01 and propan-2-01 crystallise in a chiral space group and irradiation of single crystals of these produces the chiral photoproduct (542) in high enantiomeric excess.333Irradiation of triphenylphosphate (543)in methanol under argon yields (biphenyly1)phenylphosphate (544) and biphenyl (545). Oxygen quenching experiments are consistent with (544) arising through a singlet
Photochemistry
304
Ar,
/p
R’*R2* R‘
R R2
(550)
O ,R
Ar,
’
h
R
R’
R2
-
2
OR Ar-P
I
I OH
====
OR I Ar-PH I1
Ar-P=O
0
(552)
(553)
(5511
excited state and (545) through an intramolecular excimer. Increasing concentrations of water in the methanol increase the proportion of (545) markedly, possibly due to intramolecular aggregation of the non-polar phenyl groups of the phosphate with accompanying increase in phenyl-phenyl interactions and increased rate of Triplet-sensitised photorearrangement of the 2phenylallyl phosphites (546) occurs to yield the phosphonates (548) with complete regioselectivity, as evidenced by deuterium labelling at the allylic carbon. The styryl triplet, with a primary alkyl radical-like centre, cyclises to triplet biradical (547) and subsequent p-scission generates product (548). The 3-phenylallyl phosphite (549) undergoes triplet sensitised geometrical isomerisation but reduced reactivity at the terminal benzylic radical-type centre of triplet (549) prevents rearrangement to the corresponding p h o ~ p h o n a t eThe . ~ ~rate ~ of formation of H-phosphinates (552) on irradiation of 7-phosphanorbornene 7-oxides (550) is dependent on the concentration of added alcohol and the P-2,4,6-triisopropylphenyl derivative reacts more slowly than the P-phenyl derivative. Both observations are consistent with a mechanism involving initial formation of a five-coordinate species (551) followed by fast fragmentation to give (552), rather than initial elimination of a two-coordinate species (553) and later addition of the The epimeric phosphate-containing radicals (554) and (555), produced by Norrish Type I cleavage of the corresponding t-butyl ketones, yield identical mixtures of products in methanol containing tributylstitnnane. A common reaction intermediate, radical cation (556), produced by SNl type loss of the phosphate group from (554) and (555) is proposed.33731P-, 13C- and ’H-NMR CIDNP have been used in a study of the formation and reactions of phosphorus-
IIl6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen 0 0 0 1 1 II II Arc-P-CAr
I
fz
A&-CAr
Ph
(557)
(558)
0 0 0 0
II II I1 I1 ArC-P-P-CAr I I Ph Ph
0
0
II
Arc-0-P-CAr
It
I
Ph
(559)
(560)
305
0 0 0 I1 It II Arc-P-0-P-CAr I I Ph Ph
(5611
Ar = 2,4,6-trirnethylphenyl
4-MeCOC&14CH20P(OMe)2
4-MeOC6H4dH2
(562)
(563)
(MeO)$=O
(564)
(MeO)2P(O)Br
(565)
centred radicals obtained from monoacyl- and bisacylphosphine oxide photoinitiators which undergo triplet excited state cleavage of the carbonyl-phosphinoyl bond. The bisacylphosphine oxide (557), for example, yields radical combination products (558), (559), (560) and (561). In the presence of a model acrylate, phosphorus-centred radicals are more reactive than their benzoyl counterparts.338 4-Acetylbenzyl dimethylphosphite (562) produces radicals (563) and (564) on photolysis in benzene and the mechanism of intersystem crossing of this triplet radical pair is influenced by the large 31P hyperfine coupling constant. 31P CIDNP studies show that the net polansation of dimethyl phosphorobromodite (565), formed by cage-free trapping of phosphonyl radical (564) by radical scavengers such as benzyl bromide or bromotrichloromethane, is magnetic field strength dependent. Thus the net absorption observed at 58.8 kG is due to the commonly observed To-S mechanism for intersystem crossing in the triplet radical pair, whereas the net emission at 18.8 kG arises from involvement of the much more rarely observed T--S mechanism.339
References 1. 2. 3. 4.
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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 moleculessuch as nitrogen, carbon monoxide or carbon dioxide. Photodecompositions which produce two or more larger fragments and other miscellaneousphotoeliminations are reviewed in the final section. Photofragmentations of carbonyl compounds by Norrish Type I and I1 processes, are discussed in Part I1,Chapter 1. Research into photoelimination reactions continues to receive stimulus from applications such as photoaffinity labelling, photoresist technology and chemical vapour deposition. At the fundamental level, studies of photoelimination reactions are being carried out with the benefit of improved computational methods and experimental techniques, including time-resolved studies on the femtosecond scale. Advances in studies of photodissociation dynamics over the past 30 years have been reviewed,' and a book on both theoretical and experimental aspects of unimolecular reaction dynamics has been published.2 Papers have appeared on the subject of bond-selective electronic excitation3and vibrational state control of selective bond breaking.4 Flash-photolysis studies of ylides derived from carbenes' and of carbenium and nitrenium ions have been reviewed;6 while a chapter on photochemistry and radiation chemistry is included in a treatise on the chemistry of triple-bonded functional groups, including diazo compounds and a ~ i d e s Another .~ area of increasing research activity is the study of photofragmentations of molecules adsorbed on surfaces. Reviews of various aspects of this work have appeared.*'13
2
Elimination of Nitrogen from Azo Compounds and Analogues
High level ab initio molecular orbital calculations have been carried out for the ground state and several excited states of trans-azomethane.I4 In the FranckCondon region, the ordering of states (in increasing energy) is So, TI, SI,T2, T3, S2 and S3, and the 350 nm absorption is assigned to the So -+ Sl transition. Photoelimination of Nz from the Sl state is predicted to occur by sequential breaking of the two C-N bonds, via an intermediate diazenyl radical (CH3N2*),
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998
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rather than by a concerted mechanism. This explains experimental observations that the two methyl radicals generated have very different translational and internal energies. An ab initio study of the mechanism of photolysis of 2,3diazabicyclo[2.2.2]oct-2-ene(DBO) and 2,3-diazabicyclo[2.2.l]hept-2-ene(DBH) also indicates sequential cleavage of the C-N bonds.I5 Experimentally, the two compounds behave quite differently: DBO produces bicyclo[2.2.0]hexane and hexa-1,Sdiene with low quantum yields and in a ratio that depends on the spin state of the excited species, while DBH produces bicyclo[2.1 .O]pentane with near unit efficiency from ether singlet or triplet excitation. The proposed model rationalizes these observations. In the photolysis of DBO adsorbed on zeolites, the formation of hexa-l,Sdiene, which derives from the triplet manifold, is substantially increased by the heavy atom effect of Xe, Ca2+ (isoelectronic with Xe) and other heavy ions.I6 Phenyl- and heteroaryl-substituted cyclopentane-1,3-diyl triplet diradicals (2) (Scheme 1) have been generated at 77 K in 2-methyl-THF glasses by photolysis of the tricyclic azoalkane precursors (1) and observed by ESR spectroscopy.17 The measured zero-field D parameters provide an indication of the spin-delocalizing ability of each of the heteroaryl substituents relative to phenyl. The related tetracyclic azoalkanes (3) (Scheme 2) are unusual in that both singlet and triplet n,z* photoprocesses compete, and the main products formed are either housanes (4), from C-N cleavage and loss of N2, or aziranes (5), from C-C cleavage and rearrangement. l 8 In quenching experiments, the latter products have been shown to arise exclusively from the triplet manifold. A series of these azoalkanes (3a-f) have been photolysed at 350 nm in a range of solvents. The product distributions for the triplet-excited azoalkanes (3a,b) depend markedly on the solvent, with polar protic solvents strongly favouring the azirane products (5a,b). In contrast, azoalkanes (3c,d), which undergo efficient loss of N2 from the singlet-excited states, give the corresponding housanes (4c,d) exclusively and independently of the solvent. Compounds such as (6) have been synthesized to investigate whether diazene-diazene [2+2] photocycloaddition can compete with expulsion of NZ.l9 Photolysis of the monoxide (6) results mainly in loss of N2, but also yields a small percentage of the metathesis isomer (7), which could arise via the [2+2] cycloadduct. Analogous dioxides give higher proportions of metathesis isomers. Photoelimination of N2 from homochiral pyrazolines such as (8) (Scheme 3) yields the corresponding cyclopropanes (9) stereospecifically.20 Sensitization with benzophenone and flash photolysis show the intermediacy of triplet diradicals, but ring
Scheme 1 Ar = Ph;2-, 3-furyl; 2-, 3-thienyl; 2-, 3-, 4-pyridyl; 2-, 3-, 4-pyridylium; 3-, 4-N-oxypyridyl
Photochemistry
318
R' (R2)
b
c
:,)-(5)
a
R2
H Me Ph Ph Me H Me Me Ph CH20Ac CH20H
d
e
(5)
$,
Scheme 2
0-
p-
closure to the cyclopropane is clearly faster than loss of stereochemistry by rotation about C-C bonds. 3-Oxabicyclo[3.1.O]hexan-2-ones have been prepared by photolysis of pyrazolines obtained by cycloaddition of diazoalkanes to 5methoxy-2-(SH)-f~ranone.~ The low-temperature photolysis of dihydropyrrolo[3,4-d]pyridazines has yielded very interesting results.22 In contrast to N-methyl or N-pivaloyl analogues, which give singlet diradicals at all wavelengths, the products observed from N-arenesulfonyl derivatives such as (10) (Scheme 4) are wavelength dependent. Photolysis of (10) at longer wavelengths (328-381 nm) gives the diradical (11) in its singlet state, characterized by a blue colour (hmax590 nm) and no detectable
'
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ESR signal; while photolysis at 265 nm yields a different species with a triplet ESR signal, and which is identified as triplet (11). Surprisingly the two spin states of (11) do not equilibrate at 77 K to a measurable extent. It is postulated that the two spin states of (11) differ from each other conformationally, and that the equilibration is impeded when the species are held rigid in low-temperature glasses.
triplet (11) ESR active
77 K No interconversion in > 30 days
singlet (11) no ESR signal hmax 590 nm
Scheme 4
The hitherto elusive nitrilimine (13) (Scheme 5) has been generated in noble gas matrices by photolysis of the tetrazole (12), or 1,2,3- or 1,2,4-triazole,and also by flash pyrolysis.23 On further photolysis, nitrilimine is transformed into other CH2N2isomers, including diazomethane, and a novel complex between NH and HCN. The identification of the species involved was aided by isotopic substitution and high-level ab initio calculations. 1,3-DiphenylnitriIimine,generated by photolysis or thermolysis, undergoes 1,3-dipolar cycloaddition to [60]fullerene, forming two or more adducts with A*-pyrazoline s t r u c t ~ r e s . ~ ~ H I
N ,,
'i-6
hv(185,193nm)
Ar, 12K
- +
HC=N=NH
Scheme 5
Photodecomposition of azo compounds can be brought about by single electron transfer. Thus sensitization of (14) (Scheme 6) by 9,l O-dicyanoanthracene (DCA) results in the formation of the trityl cation and a phenyl radi~al.~' Photocleavable surfactants have several potential applications, e.g. in waterbased paints and coatings and in advanced drug-delivery systems. The azo group can be used to provide a photocleavable link between the tail and headgroup of a surfactant molecule, and a number of such surfactants, based on the azosulfonate functionality, have been synthesized (15, R = hexyl, octyl, decyl and dodecyl).26 When aqueous solutions of these surfactants are photolysed, N2 and the sulfite
320
hv
DCA
Yh
Ph-C-N=N-Ph
I
Photochemistry '[DCA]*
DCA 2
FTh
+'
Ph-C-N=N-Ph
I
Ph
Ph
-
Yh
Ph-C+
I
+ N2 + Ph.
Ph
(14)
Scheme 6
anion are eliminated, and the corresponding phenols (R-CsH4-OH) or arenes (RPh) are formed. The surface active properties are destroyed in the process. Complexes between surfactants and polyelectrolytes have been synthesized and studied, in which diazosulfonate groups are incorporated in either the surfactant or the p~lyelectrolyte.'~ The complexes are insoluble in water, and change their characteristics on photodecomposition of the azosulfonate groups. Copolymers of methyl methacrylate and N-aryl acrylamides bearing azosulfonate groups are water-soluble when the azo content is sufficiently high, but become insoluble on photolysis.28Such materials have potential applications as photoresins.
3-Alkyl-l,6pentadiazenes(Ar-N2-NR-N2-Ar) and related hexadiazenes (ArN2-NR-NR-N2-Ar) have been synthesized by reaction of aryldiazonium salts with amines or h y d r a ~ i n e s . ~The ~ - ~ use ' of arylenebis(diaz0nium salts) leads to analogous polymers, which undergo irreversible chain degradation when photolysed. The photosensitivity of these high-nitrogen compounds can be adjusted to some extent by changing substituents, particularly on the aromatic rings. 3
Elimination of Nitrogen from Diazo Compounds and Diazirines
3.1 Generation of Alkyl and Cycloalkyl Carbenes - Photolysis or thennolysis of a series of alkylchlorodiazirines (16) (Scheme 7) in the presence of alkenes, such as tetramethylethene, results in 1,2-H shifts, giving the corresponding vinyl chorides (18), in competition with additions of the carbenes (17) to the alkenes, yielding cyclopropanes (19).32The mechanism of these reactions is discussed in the light of results obtained from photoacoustic calorimetry, and the ratio of vinyl chloride to cyclopropane seems to depend on the excited states of the carbene precursors and also on carbene-alkene complexes. Similar reactions of related diazirines have been investigated by flash p h o t o l y ~ i s . ~ ~ Adamantyldiazirine is the first diazirine for which a fluorescence lifetime has been measured.34 Primary decay occurs with a lifetime of about 240 ps at ambient temperature, and increases at lower temperatures. The results support
W 7 : Photoelimination X-CH2qN CI
--% X-CH2. - N2
CI
32 1
-
H
x +C=C, 1 1
H
X = H, CI, Et, Ph, 4-CI-C6H4, 4-Me-C6H4
CI (19)
Scheme 7
earlier calculations that predicted an energy barrier on the S , surface between the excited diazirine and formation of the corresponding carbene or diazo compound; the experimental value for this barrier for adamantyldiazirine is 11- 12 kJ mol - I . The carbene, 2-methylcyclohexanylidene,has been generated by photolysis of the corresponding diazirine (20) in a-, p- and y-cyclodextrins, to determine what effect such constraints would have on the reaction pathway.35 Only products from 1,2-H shifts were observed (1- and 3-methylcyclohexene), and encapsulation of the diazirine in this way seemed to have little effect on product ratios. Rate constants have been measured by flash-photolysis techniques for the reactions of cyclopentadienylidene and fluorenylidene with alcohols and other q ~ e n c h e r s .The ~ ~ rates increase with increasing acidity of the alcohol, and alcohols react faster than likely ylide-forming reagents, such as pyridine or THF. In contrast, tetrachlorocyclopentadienylidene reacts most rapidly with the least acidic alcohol studied, and reacts more rapidly with pyridine and T H F than with methanol. Photolysis of the silacyclobutyl diazomethane (21) in t-butanol gave three different t-butoxy-substituted (silylalkyl)silanes, e.g. (Me3CO)Me2SiCHPhSiMe3.37 The migratory aptitudes of substituents from the Si atoms to the carbene centre were determined as Ph:Me = 3.8: 1.0.
The photolytic pathway for A3-1 ,3,4-oxadiazolines (22) (Scheme 8) begins with C-N cleavage to form diradicals (23), followed by ester elimination and generation of diazoalkanes (24).38 Further photolysis gives the corresponding carbenes. In a study of (22b) and (22c), flash-photolysis and two-wavelength continuous
322
Photochemistry
photolysis were employed to examine the reaction pathway and the fate of the carbene, cyclobutylidene, generated.39 This carbene could be trapped by added pyridine or tetramethylethene, but otherwise the products were cyclobutene and methylenecyclopropane, from 1,2-H or 1,2-C shifts, respectively. Upper and lower limits for the rate of 1,2-C migration were estimated. In a related study, (22a,c-e) were photolysed in protic media, and estimates were obtained for the rates of protonation of the corresponding diazoalkanes ( 2 4 a , ~ - e )5,5-Dibenzyl .~~ analogues have also been in~estigated.~'
3.2 Generation of Aryl Carbenes - Flash photolysis at 308 nm of phenyldiazomethane and pentafluorophenyldiazomethane generated phenylcarbene and pentafluorophenylcarbene, respectively, which formed nitrile ylides with the solvent, acetonitrile, or pyridine ylides in the presence of ~ y r i d i n e . ~Absolute ' rate constants for the reactions of the carbenes with acetonitrile and pyridine were determined, and estimates for the singlet-triplet splittings were deduced to be 9.6 and 13 kJ mol- for phenylcarbene and its pentafluoro derivative, respectively. Triplet-triplet (TI -+ To) fluorescence spectra and decay curves for diphenylcarbene, generated by photolysis of diphenyldiazomethane, have been recorded in organic glasses at 77 K, in the absence and presence of dye molecules such as Rhodamine B.42Without dye, the fluorescence exhibits biexponential decay with lifetimes of -30 and -140 ns, which are attributed to independent TI --+ To emission from individual T I sublevels. In the presence of dye molecules, the observed fluorescence decay agrees well with Forster-type resonance-energy transfer. Triplet sensitization of two 1,2-diaryldiazoethanes in methanol solution gave rise to stilbenes, by intramolecular 1,2-H shifts, and ethers, by reaction of the corresponding carbenes with methan01.~~ This method of generating the intermediate carbenes bypasses the singlet excited diazo compounds, and thus eliminates the possibility that the products arise by direct reaction of these excited species. The results go some way towards establishing that 1,2-H and 1,2-C shifts can compete with alcohol trapping of a spin-equilibrated carbene. A study of aryl(acyloxy)carbenes, generated photolytically from diazirine
IIl7: Photoelimination
323
precursors, has shown that 1,2-acyl migrations occur efficiently in most cases, to give high yields of 1,2-diketone~.~ In the presence of alkenes, phenylacetoxycarbene undergoes alkene addition in good yields, in lieu of the 1,2-acyl shifts. Visible light photolysis of diazo(3-thieny1)methane (25) (Scheme 9) in argon matrices generated a methylenecyclopropene as the s - 2 conformer (27), which isomerized to the s-E form (28) on further photon a b ~ o r p t i o nThe . ~ ~intermediate carbene (26) was not observed directly, but it could be trapped by oxygen in 0 2 doped matrices. A second intermediate in the reaction pathway, 1H-2-thiabicyclo[3.1.O]hexa-3,5-diene, was also postulated, and a few weak absorptions in the matrix IR spectra may have belonged to this species. S hv (A35 nm) Ar, 10 K
Arylcyclopropylcarbenium ions (32) (Scheme 10) were generated by flash photolysis of 2,3-diphenylaziridinimines (29) of cyclopropyl ketones in 2,2,2trifluoroethanol, and rate constants were obtained for their unimolecular decay and bimolecular reactions with methanol.46 The carbenium ions are thought to arise via the corresponding diazo compounds (30) and carbenes (31). The results show that the cation stabilizing abilities of cyclopropyl and phenyl groups are similar in magnitude, e.g. cyclopropyl is superior to phenyl but inferior to 4methoxyphenyl.
Ar&'"''ph
4 hv
- PhCH=CHPh Ar
\ N2
hv
~
A
r
R
O ~L
A
r
L
(29)
In explorations of molecules with high spin states, 'starburst'-type polydiazo compounds such as (33) have been synthesized and photolysed at low temperatures to give poly~arbenes!~.~~ The nonadiazo compound (33) gave a nonacarbene which, from magnetic measurements, seemed to have a ground state with S = 9, the highest spin multiplicity so far reported for a purely organic material. On the other hand, a similar dodecadiazo compound gave rise to a polycarbene with a lower spin multiplicity, seemingly owing to antiferromagnetic interactions. In another approach to high-spin materials, complexes of diazodi(4-pyridy1)-
324
Photochemistry
methane with manganese have been prepared.49 Irradiation of the crystalline materials at low temperatures produced ferromagnetic chains. It is suggested that this could become the basis for novel molecular optomagnetic recording devices, since only the irradiated portion of the non-magnetic or weakly magnetic material becomes strongly magnetic.
\
Q
N2
N2474-3N2
/
(33)
A number of biochemical probes containing diazirine moieties as arylcarbene precursors have been synthesized. These include a photolabile oligodeoxyribonu~leotide,~' a photoactivatable antagonist for the binding site of the tachykinin NK2 r e ~ e p t o r , and ~ ' a biotinylated lactose derivative as a photoprobe for GM3 synthase.'*
3.3 Photolysis of cr-Diazo Carbonyl Compounds - The mechanism of the photochemical Wolff rearrangement of diazomalonates (34, R = Me, Et) (Scheme 11) has been studied in low-temperature mat rice^.'^ In Ar matrices, the Wolff rearrangement products (36) are formed, but in CO or CO-doped matrices the intermediate carbenes (35) react with CO to give the dialkoxycarbonylketenes (38). The latter ketenes were identified positively by independent generation from (37), and were also shown to undergo some photolytic loss of CO, to give (36) via the carbenes. These results complement those from flash-photolysis studies at room temperature, and confirm that the Wolff rearrangement of diazomalonates occurs via dialkoxycarbonyl carbenes (39, and is not concerted. Evidence has been gained for a propadienone intermediate, following Wolff rearrangment, in the photolysis of 2-diazoindan- 1,3-di0ne.~~ Wolff rearrangement is suppressed in the photolysis of diazofluoranthenones, such as (39), presumably on account of the strain which would be present in the
IIl7: Photoelimination
325
'0
(35)
(34)
(36)
colth
- Ro-s hv
- N2
Ro< 0 (37)
RO<'
kc=o 0
(38) Scheme 11
resulting, ring-contracted ketenes.55 Photolysis in benzene leads only to products deriving from insertion of the oxocarbenes into the C-H bonds of the solvent. Interestingly, the addition of t-butylamine promotes [3+2] cycloadditions of the oxocarbenes to benzene, in which, for example, (39) yields (40).The photolysis of several a-diazoacetophenones (ArCOCHN2) has been studied in isopropanol solutions.56 These reactions take place on the triplet manifold, the Wolff rearrangement (a singlet process) is therefore bypassed, and the products are the parent acetophenones (ArCOCH3), obtained from the triplet carbenes by hydrogen abstraction from the solvent.
A stereoselective synthesis of aminoalkyl-substituted p-lactams (41) has been developed, the key step of which is [2+2] cycloaddition of imines to ketenes generated photochemically from diazoketones (42), which are derived from protected a-amino acids.57 A number of steroidal diazoketones related to progesterone have been synthesized as potential photoaffinity labelling reagents for the mineralocorticoid receptor.58 Novel o-quinone diazides have been the subject of several patents for applications as positive photo resist^^^-^^ or in lithography.64 Thin-film composite membranes with pendant diazoketone groups have been synthe~ized.~~ The resulting membranes can be modified photochemically after fabrication.
Photochemistry
326
PG’ , PG2 = protecting group Bn = CeHsCH2
4
Elimination of Nitrogen from Azides and Related Compounds
The majority of recent research in the photochemistry of azides and the generation of nitrenes has involved aryl rather than alkyl azides. Amongst the latter, however, is an interesting study of the photodissociation of methyl azide at wavelengths from 292 to 325 nm.66 The photodissociation dynamics of methyl azide are apparently complex, with the predominant pathway producing CH2=NH by concerted 1,2-H shift and N2 extrusion. Triplet methylnitrene (CH3N) was also observed by emission spectroscopy, but seems to arise by a minor, spin-forbidden pathway. A number of anomeric D-hex-2-ulopyranosyl azides have been synthesized and their photochemistry examined.67For both CL- and P-azides, the major photoproducts arise from cleavage of the C-2C-3 bond and migration of the C-3 carbon to the nitrene centre. Decomposition reactions of a glycidyl azide polymer have been induced by pulsed laser infrared pyrolysis and UV photolysis of thin films at 17-77 K and monitored by IR spectroscopy.68 The initial step is elimination of N2 and formation of imines, which decompose on warming, possibly with secondary polymerization. UV irradiation of 1,1-diazido-1-germacyclopent-3-eneisolated in low-temperature matrices generates the corresponding cyclic germylene (1-germacyclopent-3ene-1,I -diyl) through loss of three molecules of nitrogen.69 Under the conditions of the experiments, the germylene undergoes secondary photochemistry, and rearranges to 1H-germole (1-germacyclopenta-2,4-diene)and lesser amounts of 3H- and 2H-germole. 4.1 Aryl Azides - Semiempirical (AMI) calculations have been carried out to provide potential-energy surfaces for the photodissociation of phenyl a ~ i d e . ~ ’ The direct photolysis of an aryl azide usually produces the corresponding singlet nitrene, which can then undergo either ring expansion to a didehydroazepine or intersystem crossing to the triplet nitrene. The ring expansion is disfavoured by di-ortho substitution, however. Irradiation of mesityl azide (43) (Scheme 12) in the presence of TCNE gives a mixture of two adducts: the azomethine ylide (a), and the spiroazepine (45).7’ This appears to be the first report of competitive trapping of the singlet nitrene and didehydroazepine. The photolysis of 2,6difluorophenyl azide and pentaflurophenyl azide in solid argon at 10 K has been investigated, and the corresponding nitrenes characterized by W and IR spectro-
IIl7: Photoelimination
(43)
327
(45)
(44)
(46)
Scheme 12
copy.^^ Subsequent irradiation with h = 444 nm produced the bicyclic azirines (46,R = H, F), but no further rearrangement to didehydroazepines occurred. Instead, on 366 nm irradiation, the azirines rearranged back to the nitrenes. Such azirine intermediates have not previously been detected directly in the matrix photolysis of monocyclic aryl azides, but have been observed, for example, in the matrix photolysis of naphthyl azides. Time-resolved IR studies of the photolysis of 2-(methoxycarbony1)phenylazide in solution at room temperature showed that the didehydroazepine (47) was the sole intermediate, at least on the ps time-~cale.~~ This contrasts with photolysis of the same compound in matrices at 10 K, where the nitrene, iminoketene (48) and azetinone (49) were observed as well as (47). Matrix photolysis of 2-hydroxyphenyl azide gives at least three major products, all of which are photointer~onvertible.~~ Two of these are identified as the EIZ mixture of iminodienones (50), while the third is the ring-opened compound (51), existing as a mixture of conformers. 2-Aminophenyl azide behaves in a similar manner. Rapid H-transfer from the ortho hydroxy or amino group to the nitrene centre in each case appears to suppress ring expansion completely.
NOMe
'OMe
-C=NH
-c=o
Matrix photolysis of several cyano-substituted phenyl azides with plane polarized light, combined with qualitative linear dichroism measurements in the IR spectra of the resulting matrices, has been used to deduce information about
328
Photochemistry
the conformation of the azido group with respect to the benzene ring and s u b ~ t i t u e n t s . ~Despite ~.'~ some difficulties of interpretation, the results confirm that the azido group is planar or nearly coplanar with the benzene ring in matrices, as well as in crystals, and prefers an anti conformation with respect to a single ortho methyl group. Diazide precursors of quinonoidal dinitrenes have been prepared with various chain lengths: 1,6diazidobenzene, 4,4'-diazidobiphenyl, and (52, n = 1, 2, 4). These were photolysed in 2-methyl-THF glasses at 77 K, and the resulting diradicals characterized by ESR spectro~copy.~~ According to the most advanced level of theory, the diradicals have singlet ground states, and the observed ESR spectra are due to low-lying excited states. The observed zero field splitting parameters were consistent with a spin-polarized model of the electronic structures. The photolysis of a cross-conjugated diazide, 2,6-di(4-azidobenzylidene)cyclohexanone, in both the crystalline state and adsorbed on silica, has also been reported. 78
Nitrenium ions have been observed following flash photolysis of 4-ethoxy- and 4-methoxyphenyl azides in aqueous solution79 and a series of polyfluorinated phenyl azides in acidic media, such as acetonitrile containing sulfuric acid." Measured lifetimes of the phenylnitrenium ions varied from -1 ps up to milliseconds. The phenyl azide functionality continues to be a popular component of photoaffinity reagents and systems for functionalizing materials, such as polymers. Recently reported photoaffinity probes based on aryl azides include azidophenylcyanoguanidines,*'azido derivatives of semotiadil for investigation of calcium channels,82 perfluoroarylazido derivatives of oligoribonucleotides,83 1251-labelledphotoaffinity analogues of rapamy~in,'~and a series of glycopeptides.85 [60]Fullerene has been functionalized by [3+2] cycloaddition of aryl azides, to give isolable triazolinofullerene derivatives, which lose N2 on photolysis to yield aziridinofullerenes.86Examples of trifunctional reagents containing the perfluorophenyl azide group have been s y n t h e s i ~ e d ;and ~ ~ a range of phenylazido-derivatized substances have been used in the photochemical modification of polymer surfaces.88 Derivatives of perfluorophenyl azide have been developed for the modification of biopolymers.R9~90 4.2 Heteroaryl Azides - 2-Pyridyl azides exist in equilibrium with bicyclic tetrazoles, with the latter predominating under normal conditions. A number of trifluoromethyl-substituted derivatives of tetrazoles, e.g. (53), have been synthe-
IIi7: Photoelimination
329
sized and photolysed in matrices at 12-18 K with IR and ESR monitoring." In most cases, clean conversion into the corresponding 1,3-diazacycloheptatetraenes was observed, e.g. (54) from (53), but 2-pyridylnitrenes were also detectable by ESR. With the 3,5-bis(trifluoromethyl) tetrazole, the corresponding 2-pyridylnitrene was observed by both ESR and IR spectroscopy, and was converted into the diazacycloheptatetraene on further UV irradiation. A number of stable 1,3diazepines have been prepared at room-temperature by trapping diazacycloheptatetraenes such as (54) with n~cleophiles.~~ These are claimed as the first stable, monocyclic N-unsubstituted 1H- 1,3-diazepinesto have been reported.
Photochemical studies of azido-3-phenylcoumarinsin solution and in polymer layers have been described.93A derivative of NAD' and the corresponding ATP analogue have been synthesized in which a spin label is attached to the N-6position and an azido function to the C-2 atom of the adenine ring.94 Both compounds were shown to be active coenzymes or substrates for various enzymes, and could be covalently incorporated into the enzymes upon photolysis.
5
Photoelimination of Carbon Monoxide
During the period covered by this review, the majority of studies of photodecarbonylations have been either of small molecules such as ketene and phosgene, or of metal carbonyls. The photodissociation of ketene has received a good deal of detailed study, both theoretical and experimental. Rotational energy release in the dissociation of singlet ketene has been compared with statistical and dynamical theorie~?~ and the question of K-conservation in this process has been in~estigated.~~ The internal energy distributions of CH2 from ketene photolysis at 351 nm9' and at 308 nm98have been estimated from metastable time-of-flight spectroscopy of the ejected CO. There has also been a study of ketene dissociation using an IR-UV double resonance photolysis set-up, in which laser induced fluorescence of the methylene fragment was observed.99 Similarly detailed studies have been made of the photodissociation of isocyanic acid (HNCO). With this molecule, three reaction channels compete: fragmentation into NH and CO, with the NH in either of two electronic states, and fragmentation into H and NCO. Experiments in which HNCO was excited to the S,state in expansionjets have shown that the competition depends on the photon energy and the excitation conditions.loo*'o' The photodissociation dynamics of
330
Photochemistry
HNCO at 248 nm have been investigated and, at this wavelength, the predominant reaction channel is direct formation of H atoms.'02 Photolysis of DNCO at 193 and 148 nm has also been examined as a comparison with HNC0.'03 Semiempirical (AM 1) calculations have provided a potential-energy surface for dissociation of PhNC0.70 Other small molecules for which photodecarbonylation studies have been made lo' include phosgene in both the gas phase'04 and as a s ~ l i d , ' ~acetyl ~ . ' ~chloride, ~ and oxalyl chloride;"* though extrusion of CO is only one of the pathways observed for these species. Photodissociations of acetyl and propionyl radicals have been studied by IR emission spectroscopy of the vibrationally excited CO prod~ced,'~' and an ab initio study of the photodissociation of the formyl radical (HCO) has been made. l o Trifluoroacetylthiol (CF3COSH) and trifluoroacetylsulfenyl chloride (CF3COSC1) have been photolysed in noble gas matrices, to produce CF3SH and CO, and CF3SC1,CO and SCO, respectively.' When 2-phenyl-4-alkylidene-5(4H)-oxazolones(55, R = Me, Et) (Scheme 13) were irradiated with a medium pressure mercury arc through a Pyrex filter, only cis-trans isomerization was observed; without the Pyrex filter, however, decarbonylation occurred.' I 2 In ethanol, propan-2-01 or t-butanol, the sole products (obtained in >90% yield) were identified as (57, R' = Me, Et; R2 = Et, i-Pr, t-Bu), apparently arising by trapping of the intermediate ketenimines (56) by the solvent. One of the ketenimines (56,R = Me) was probably detected by NMR when (55, R = Me) was irradiated in CD3CN, but it could not be further characterized owing to its instability.
'
'
R'
5.1 Photoelimination of CO from Organometallic Compounds - Simple metal carbonyls have been studied by a variety of state-of-the-art experimental techniques. For example, photodissociation of Fe(CO)5 in the gas phase has been investigated using femtosecond laser pulses at 400 and 800 nm, and the parent and fragment molecules observed with a time-of-flight spectrometer.' l 3 Loss of four CO ligands, leading to FeCO, takes place in about 100 fs; while the subsequent fragmentation of FeCO to Fe and CO occurs on a longer time-scale of 230 fs. The IR spectrum of FeCO, produced by 193 nm photolysis of Fe(CO)5 has been observed by time-resolved IR diode laser spectroscopy, and contains triplet lines due to spin-spin interaction, which confirm that FeCO has a electronic ground state. 'I4 The photochemistry of Fe(CO)5 adsorbed on the Au(l11) surface at -90 K has also been examined.'I5 At relatively long wave-
'1
331
IIl7: Photoelimination
lengths (300-360 nm), photodecomposition is enhanced in the vicinity of the surface, but at wavelengths below 260 nm the photolysis yield increases with increasing distance from the surface, suggesting significant quenching of the excited states near the surface. Time-resolved IR and UV spectroscopy have been used in a study of the photolysis of M(C0)6 (M = Cr, Mo and W) in supercritical noble gas and C 0 2 solutions,'16 and this allowed the observation of compounds containing Kr and Xe, M(CO)SL (M = Cr, Mo, W; L = Kr, Xe), and one argon complex w(CO)5Ar], as well as complexes with C02. Photoinduced loss of CO from W(C0)sL-L (L-L = bipyridine) has been studied in mixtures of supercritical C 0 2 and benzene to determine medium effects on the activation volume of the resulting ring-closure reaction to give W(CO)4(L-L).' A similar study of chelate-ring formation involving photoelimination of CO from Cr(CO),( 1,lOphenanthroline) has also been reported;' I 8 quantum yields have been determined for photosubstitution of CO by phosphine ligands in Cr(CO),( 1,lo-phenanthroline);' l 9 and photolysis of Cr(C0)4(bypyridine) has been investigated by timeresolved IR spectroscopy.I2' The photochemistry of Ru(C0)3(dmpe) (dmpe = Me2PCH2CH2PMe2)has been studied both in matrices at 12 K and by flash photolysis at ambient temperature, with UV-visible and IR detection.12' Photolysis of the matrix isolated compound results in CO loss and formation of complexes between R~(CO)~(dmpe) and the matrix hosts (Ar, Xe, CH4). The electronic absorptions of these complexes show the largest dependence on matrix host so far reported for any matrix solvated species. Photolysis in heptane in the presence of hydrogen or triethylsilane yields R~(CO)~(drnpe)H~ and Ru (C0)2(dmpe)(SiEt3)H, respectively. The formation of both Mn(CO)S and the CO-loss product, Mn2(C0)9,from UV photolysis of Mn2(CO)" has been investigated by ultrafast IR spectrosDensity functional calculations have been performed on the ground and excited states of MnCl(CO)S,as an aid to understanding the photochemistry of MX(C0)S complexes (M = Mn, Re; X = C1, Br, I).'24 The lowest energy excitations are to the Mn-Cl CT*orbital, and these states are dissociative for both axial and equatorial CO loss, but not for Mn-Cl bond homolysis. The photochemistry of manganese carbonyl complexes (58, R' = Me, benzyl; R2= i-Pr, p tolyl) depends critically on the R' group.'25 CO is released with R' = Me, but with R' = benzyl there is efficient homolysis of the Mn-R' bond .
''
Photolysis of the dinuclear cobalt complex (59) in low-temperature matrices has been investigated using IR spectroscopy.'26The primary process is loss of CO
332
Photochemistry
to give the CO-bridged complex (a), which thermally back-reacts with CO to regenerate starting material. Secondary photolysis of (60)produces further free CO and a complex which is identified as the double-CO loss product (61), which can also back-react with CO. Photochemical CO-loss reactions of (q6C6H6)Cr(C0)3 in n-heptane have been studied by high-pressure photoacoustic ca10rimetry.l~' The kinetics of the reactions with H2 and N2 to form (q6C6H6)Cr(C0)2L(L = H2, N2) were measured by time-resolved IR spectroscopy. Rates of addition of various trisubstituted silanes to photochemically generated (q5-C5Hs)Mn(CO)2have been measured at 70-125 K in the neat silane or 1:l mixtures of the silane and methylcyclohexane.128 Decarbonylations of a number of dimolybdenum complexes have been investigated, in which the pathways followed by the thermal and photochemical reactions are markedly different.' 29 The silyliron(I1) complex ( $ - C S H ~ ) F ~ ( C O ) ~ Sloses ~ M ~CO ~ and gives mixtures of germylene-bridged diiron complexes, when photolysed in the presence of p tolylgermane.I3O
There has been a report of a study of the photodissociation of carbonmonoxyiron(I1) porphyrin complexes. 31 The reactions were monitored by Mossbauer spectroscopy, which showed a transition from the low-spin to the high-spin state accompanying loss of CO, and a pronounced solvent effect on recombination rates.
6
Photoelimination of Carbon Dioxide
A triplet-sensitized photodecarboxylation reaction, initiated by intramolecular electron transfer, has been developed as a key step in the synthesis of a number of hydroxy 1 a ~ t a m s .The j ~ ~ring sizes available by this method range from 4 to 26 atoms; an example is the ring closure of (62) to (63) (Scheme 14), which occurs in 72% yield. The efficiency of ring closure for the larger ring sizes is thought to be due to ground-state template formation. A1though photodecarboxylations of esters and carboxylic acids are widely known, loss of C 0 2 from lactones is usually only a minor photochemical pathway. When pyrone (64) (Scheme 15) was irradiated in benzene solution with 356 nm light under conventional, low-intensity conditions, no products were formed, but under high-intensity, laser-jet conditions, the carboxylic acid (66) and cycloheptatriene (67) were obtained. 133 The products can be accounted for, as shown in Scheme 15, on the assumption of initial C - 0 bond cleavage to
IIl7: Photoelimination
333
hv
..____)
WO3 acetone
H20
Scheme 14
Scheme 15
diradical(65), which can then react with benzene to give (66),or lose COZ to give (67) via two ring closures followed by a 1,7-shift of a phenyl group. Elimination of COz from carboxylate anions can be brought about by visiblelight irradiation of complexes of the anions with iron(II1) tetra(2-N-methylpyri d y l ) p ~ r p h y r i n . ' ~Spectroscopic ~-'~~ studies show that the primary reaction is photoreduction of iron(II1) to iron(I1) by the carboxylate ligands, and consequent formation of the corresponding acyloxy radicals. The reactions have been studied under both aerobic and anaerobic conditions. In the presence of air, re-oxidation of iron(I1) to iron(II1) can occur, allowing the system to become photocatalytic in
Photochemistry
334
iron porphyrin. Similar decarboxylations of a range of phenylacetate anions have been studied on the femtosecond time-scale, with methylviologen as the electron acceptor. 13' Photogenerated acyloxy radicals, ArCH2CO2*,lose C02, with rate constants in the range 1-2x lo9 s - ' . In contrast, C-CO2 bond scission in ArZC(OH)C02*occurs within only a few picoseconds (k = 2-8x 10' s-I).
6.1 Asymmetric Photodecarboxylations - Acridine and diphenylacetic acid are both achiral molecules, but despite this fact the two compounds self-assemble to form dextro- and laevo-rotatory chiral mixed crystals from acetonitrile solution. Irradiation of either the dextro- or Zaevo-rotating crystals causes a stereospecific decarboxylating condensation, which yields excess of (+)- or (-)-(68), respectively, with about 35% ee.137 H H
An efficient method has been claimed for the enantiomeric enrichment of racemic tartaric acid by irradiation with highly intense, circularly polarized light from an excimer laser. 13* The enrichment is presumed to occur by enantioselective decarboxylation, and further degradation, of the tartaric acid molecules. All previous attempts to achieve such enantioselection with circularly polarized light have produced very low enantiomeric excesses, as predicted by theory. The highly intense laser beam increases the probability of two-photon absorption, and this could result in higher enantioselectivity than attained with lower power sources. It seems advisable, however, to await further confirmatory results before this approach to the resolution of racemic mixtures should be considered viable.
7
Photoelimination of NO and NO2
Elimination of NO and NOz is observed in the photochemistry of a range of nitro and nitroso compounds, nitrites and nitrates. Femtosecond photodissociation dynamics of nitroethane and 1-nitropropane have been studied in the gas phase and in solution by resonance Raman spectroscopy, with excitation in the absorption band around 200 nm.'39 At such short time-scales it is possible to detect changes in the two N-0 bond lengths in the Franck-Condon region, prior to C-N bond cleavage. Photolyses of nitroalkanes at 193 nm have been monitored by photoionization of the fragments and time-offlight mass spectrometry.140Both C-N and N - 0 bond dissociation pathways are observed; and, under the conditions of free jet expansion, primary products such as pentyl and hexyl radicals are stabilized and can be detected. An extensive investigation of photochemical nitrations with tetranitromethane,
IIl7: Photoelimination
335
involving research groups in New Zealand and Sweden, has produced a number of new reports on aromatic nitrations within the period covered by this r e v i e ~ . ’ ~ ’ - The ’ ’ ~ basic mechanism of the reactions involves electron transfer within a charge transfer complex between the aromatic substrate (ArH) and tetranitromethane, resulting in formation of a radical cation (ArH*+),NO2 and the trinitromethanide anion. The radical cations derived from 1-methoxynaphthalene and 4,4‘-dimethoxy- 1,l‘-binaphthalene during such nitrations have been directly observed by ESR spectroscopy.14’ Typically, both substitution and addition products are obtained. Nitration of 2,8-dimethylbenzofuran, for example, yields the 3-trinitromethyl (69) and 3-nitro (70) derivatives as major products, together with minor amounts of the 4-nitro derivative (71) and the adduct (72). 142 Me
Me
Me
Me
NO*
Photolyses of N-methyl-N-nitrosoanilinesand N-methyl-N-nitroanilineshave been studied in solution.lS3 The primary photochemical process involves N-N bond fission, which is homolytic in aprotic solvents but heterolytic in methanol. The reaction dynamics of NO produced after excitation of 2-chloro-2-nitrosoproIn research related to the biological role of NO, pane have been inve~tigated.”~ the mechanism of the photochemical release of NO from S-nitrosoglutathione (GSNO) has been studied by flash photolysis.*55The initial step is homolytic cleavage of the S-N bond to give NO and glutathione thiyl radical (GS-), which subsequently reacts with ground-state GNSO, producing additional NO and GSSG. Alternatively the radical GS* can react with oxygen to give the peroxy radical, GSOO., which also reacts with ground state GNSO, producing NO and GSSG. Radical species from the photolysis of cycloalkyl nitrites have been identified by ESR spectroscopy in a continuous flow ca~ity.’’~ The processes following initial 0 - N bond cleavage can be quite complex. For example, cyclobutyl nitrite (73) (Scheme 16) initially loses NO, giving the cyclobutoxy radical, which then undergoes ring opening and recombination with NO, yielding 4-nitrosobutanal
Photochemistry
336
(74). The dimer of (74) is isolated as one of the products. In the flow ESR spectra, nitroxides (76) and (77) can be recognized. These are proposed to arise by Habstraction from (74) by a cyclobutoxy radical, giving (79, which either reacts with a further molecule of (74), yielding (76), or undergoes 1,5-exo ring closure to (77). The steroidal nitrite (78) (Scheme 17) yields 50% of the ketone (79) and 16% of the alcohol (80) when photolysed in the solid state (h > 300 nm), but only 5% of (79) and 52% of the Barton-type product (81) when photolysed in toluene solution.157It is usual for ketones to be produced only in low yields from photoreactions of nitrites in solution, and so the promotion of this reaction pathway in solid-state photolysis is of considerable interest. Similar results were obtained for the solid-state photolyses of a number of other steroidal nitrites, but nitrites prepared from acyclic alcohols showed much less selectivity in favour of the corresponding ketones.
Scheme 16
'*
Photodissociation dynamics of alkyl nitrites adsorbed on MgFz surfaces' and on the Ag( 111) surface159have been studied. The laser photodissociation and thermal pyrolysis of poly(glycidy1 nitrate) have been investigated.68 Such highenergy polymers have been proposed for use as binders in solid rocket motors.
8
Miscellaneous Photoeliminations and Photofragmentations
8.1 Photolysis of 0-Nitrobenzyl Derivatives - The cleavage of o-nitrobenzyl derivatives is one of the relatively few classes of photofragmentations in which the two fragments can both bear complex functionality. The protection of alcohols as o-nitrobenzyl ethers is well known, and provides a photochemical method of deprotection, but other applications include the design of photocleavable polymers and photochemically active links for molecules synthesized on polymer beads.
337
lIl7: Photoelimination C8H17
9
AcO
0
(80)16%
(79) 50%
(78)
h > 300 nm toluene solution
I
MCBH” (79) 5%
+ AcO
(81) 52%
OH
Scheme 17
o-Nitrobenzyloxycarbonyl and a number of related functional groups have been examined for the photolabile protection of nucleoside 5’-hydroxyls.16’ Deprotection of the resulting carbonates (82) (Scheme 18) follows a pathway similar to that established for o-nitrobenzyl ethers, beginning with H-atom transfer from the a-carbon to the nitro group, followed by the formation of an o-quinonoid intermediate and final cleavage to o-nitrosobenzaldehyde (83), C02 and the free alcohol. Rates of photodeprotection and quantum yields were determined for a range of protected thymidines (% which I)included , examples of o-nitrobenzyl (84, n = 0) and the homologous, o-nitrophenylethyl derivatives (84, n = 1). The latter were found to cleave at rates comparable to those of the o-nitrobenzyl analogues, and a mechanism was proposed, similar to that shown in Scheme 18, but resulting in o-nitrostyrene products rather than o-nitrobenzalde-
Photochemistry
338
(84)
OH
R' = H, Me, o-nitrophenyl R2 = H, F, CI, Br, I, NOp R3 = H, Me0 R4 = H,CI, Me0 n =0,1
hydes. These protected thymidines are the subject of a patent,16' as are also similar photolabile protected nucleotide cyclic phosophotriesters. 162 The 2,4-dinitrobenzyl diglyceride (85) has been synthesized as a means of photoactivating protein kinase C (PKC).'63 PKC is inactive in the cytoplasm until its primary activator, a diacylglycerol, is produced by phospholipase C. Upon photolysis (85) releases dioctanoylglycerol, and thus provides an approach to the controlled activation of PKC.
Photo-initiated selective C-terminal cleavage of peptides bound to polyethyleneglycol(86) (Scheme 19) has been reported.Ia This methodology has potential
IIP: Photoelimination
339
NO [E+NH!!&CHO
-
+
MeNH7,f-
0 Scheme 19
in the controlled release of biologically active peptides for therapeutic applications. Model studies for a variety of novel photolabile linkers based on onitrobenzyl photocleavage have been reported, in which substituent effects on the rates of cleavage were assessed.16’ Curing of epoxy resins can be initiated photochemically by release of imidazole from photolabile protected derivatives, such as N-(2-nitrobenzyloxycarbonyl)imidazole.166 Positive working polyimide resists derived from 2-nitro-p-xylyleneoxyamine have been synthesized and studied;I6’ and 2-nitrobenzyloxy derivatives are included in a formulation for submicron imaging.’68
8.2 Photofragmentations of 3- and 4Membered Rings - Flash photolysis of endo-7-chlorodibenzo[a,c]bicyclo[4.1.O]heptane (87) generates phenanthrene and chlorocarbene (CHC1).169The carbene can be trapped with pyridine, to give an ylide with a strong absorption at 374 nm and a lifetime of many tens of microseconds. Lifetimes of the carbene in several solvents were estimated from the yields of this ylide at various concentrations of pyridine.
The preparation and photolysis of the siladigermirane (88) have been reported.’” The photochemistry of (88) involves initial cleavage to form dimesitylgermylene (Mes2Ge:) and t-Bu*Si=GeMesz. The latter could be trapped with methanol or water, but attempts to trap the germylene were unsuccessful. In flash photolysis studies, however, a transient absorption assigned to dimesitylger-
340
Photochemistry
mylene was observed, and absolute rate constants measured for the reaction of the germylene with oxygen, triethylsilane, ethanol and 1-bromohexane. 7 1 The UV spectrum and decay kinetics of transient 1-methylsilene (H2C=SiHCH3)have been determined from 193 nm flash photolysis of 1-methyl-1-silacyclobutane. Gas-phase polymerizations of silenes generated by laser photolysis of silacyclobutanes have been studied.'73 The photochemistry of the benzo-l,2-disilacyclobutene (89), which can involve either an o-quinonoid intermediate or fragmentation to a silene, has been reviewed. 174
8.3 Photofragmentations of Organosulfur Compounds - The photochemistry of ethylene episulfoxide has been studied, using laser-induced fluorescence to probe the SO fragment p r 0 d ~ c e d . lA~ ~time-of-flight mass spectrometry study of the photofragmentation of dimethyl sulfoxide has led to the detection of CH3S0 radicals as well as CH3 and SO.'76 Dibenzothiophene sulfoxide undergoes unimolecular S - 0 cleavage, when photolysed in solvents such as benzene, cyclohexanol, cyclohexane and cyclohexene.17' The products are dibenzothiophene and solvent oxidation products, e.g. phenol from benzene or cyclohexanol and cyclohexene from cyclohexane. Transient absorption spectroscopy on the picosecond time-scale has been used to follow S-S bond cleavage in bisb-aminophenyl) disulfide and the fate of the resulting geminate p-aminophenylthiyl radical pairs. 78~179The photocleavage of the S-S bond in the dithiobis(tetrazo1e) (90) has been studied by nanosecond flash photolysis. The resulting radicals react rapidly with conjugated dienes by addition, with formation of an S-C bond, suggesting that the unpaired electron is localized mainly on the S atom. Thiarubrine A (91), a naturally occurring, red, antibiotic pigment, yields the thiophene (92) when irradiated with visible light; and the mechanism of the photodesulfurization of this and three related 1,2dithiins has been studied in solution and in low-temperature glasses.'" After initial S-S bond cleavage, the reactions proceed via 2,6-dithiabicyclo[3.1.0]hex-3enes, a novel class of compounds.
Ml7: Photoelimination
34 1
Photocleavage of the N - 0 bond in N-hydroxypyridine-2(1H)-thione generates hydroxyl and 2-pyridylthiyl radicals.'81 In this reaction several types of highly reactive intermediate are generated, which is a disadvantage for a photochemical source of hydroxyl radicals. On the other hand, N-hydroxy-2(1H)-pyridone has a simpler photochemistry and seems a more specific generator of hydroxyl radicals. Reactions of purines with hydroxyl radicals generated from 4-mercaptopyridine-N-oxides have also been studied.184 A series of W(pent-4-enyl-loxy)pyridine-2(1H)-thiones (93) have been synthesized and photolysed. These undergo N-0 cleavage on irradiation, liberating 2-pyridylthiyl and the corresponding alkoxy radicals.
R4
R2
Efficient deprotection of thioketals to the corresponding ketones can be induced by photosensitized single electron transfer to .rr-acceptors such as 9,lOdicyanoanthracene. 86 Similar reactions are observed for oxathiolanes, but the method is unsatisfactory for protected aldehydes. 8.4 Photofragmentations of Organmilicon Compounds - Mercury-sensitized photolysis of dimethylsilane produces the Me2(H)Si radical and a hydrogen atom in the primary step. Thereafter, Me2(H)Si. radicals undergo a combination reaction and two kinds of disproportionation, leading to dimethylsilylene and l-methylsilaethene. The photochemistry of benzyltrimethylsilane has been studied in 3-methylpentane and ethanol glasses at 77 K, revealing a remarkable solvent effect.18* In 3-methylpentane, a-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations were produced via the lowest triplet state, while in ethanol, benzyl radicals were produced via the excited singlet state or a higher triplet state. Direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane(94) afforded a mixture of reactive intermediates, which were detected and identified by flashphotolysis techniques and trapped as methanol adducts in continuous irradiation experiment^.'^^ Amongst the intermediates identified was the 1-silaallene (95), which, although a minor photoproduct, was readily observable owing to its relatively long lifetime. The stereochemistry of the addition of carbonyl compounds to silenes generated by photolysis of rneso- and rac- 1,2-diethyl-1,2dimethyldiphenyldisilane has been investigated.190 SiMe3 Me3SiCrCSiMe2SiMe3
Me2Si=C= C; SiMe3
(94)
(95)
342
Photochemistry
Photofragmentations of silacyclobutanes, a disilacyclobutene and a siladigermirane have been mentioned in Section 8.2. 8.5 Photofragmentations Involving Cleavage of Carbon-Metal Bonds - Because of its importance in chemical vapour deposition, the decomposition of dimethylcadmium is receiving widespread attention, and this includes photofragmentation processes. Thus the photochemistry of dimethylcadmium has been studied on GaAs( 1 10) and CdTe( 110) ~ u r f a c e s ' ~and ' on quartz surfaces,i92using time-offlight mass spectrometry, and on Si(lOO), fused quartz, crown glass and stainless steel, using laser-desorption mass ~pectrometry.'~~ Photolysis of diethylthallium bromide in cyclohexane is a radical process involving cleavage of the thallium-carbon bond, which yields ethylcyclohexane and dicyclohexyl, as well as other products. 194 Photoelectron transfer from benzyltributylstannanes to 1O-methylacridinium ion results in cleavage of the metal-carbon bond, to give the corresponding benzyl radicals, rather than benzyl cations. i95 Photochemical homolysis of Re- and Ru-alkyl bonds in Re(alky1)(C0)3(diimine) and Ru(I)(alkyl)(CO)2(diimine) complexes has been studied by Fourier transform ESR.196 In related manganese complexes, Mn(R)(C0)3(diimine), elimination of CO is the predominant pathway when R = methyl, but Mn-alkyl homolysis occurs when R = benzy1.125 Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97).i97 It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(I1) porphyrin and the free carbene.'91 The carbenes can be trapped in high yield with a variety of alkenes. OH
0=.=(
:
R'
I
0
(97) Scheme 20
Cationic cyclopentadienyl iron fragments have been generated by photolysis of [(C,H,)Fe(C,H6)]'[PF~]- and shown to form transient cationic triple-decker Similar photolytic removal sandwich complexes with [(~-BU~C,P~)F~(~-BU~P~)].'~~ of cyclopentadienyliron moieties has been utilized as a key step in the synthesis of ether and thioether building blocks for polymer synthesis.200 Photoinduced elimination of carbon monoxide from metal carbonyls has been treated in Section 5.1.
III 7: Pho toeliminat ion
343
8.6 Other Photofragmentations - [4+4] Cycloadducts of an thracenes and furan are both formed and cleaved photochemically."' o-Anthrylpolystyrenes have been shown to undergo reversible photodimerization, allowing photochemical control of molar mass.*02 The cycles can be conducted more than ten times without detectable degradation of the polymers. polyurethane^^'^ and polyethers204containing coumarin components exhibit similar reversible photopolymerization and cleavage. Photocleavage of the N-N bond in tetraphenylhydrazine has been investigated in alkane solvents at pressures of 1-4500 bar, by pump-probe absorption spectroscopy and time-correlated single-photo counting.205A strong viscosity dependence was found for the pressure-dependent rate constants, indicating a barrier-crossing process in the dissociation. Photolysis of the pyridinium salts (98, R = H, Me) generates diarylnitrenium ions (Ar2N+)by N-N cleavage.206The major products of the reactions depend on the presence of various added nucleophiles, such as electron-rich alkenes, and arise by addition to the ortho and para positions of the aromatic rings in the nitrenium ions.
o-Xylylene (o-quinodimethane) is formed in a two-photon process in the laser photolysis of 1,2-bis(substituted-methyl)benzenes(99, R = PhO, PhS, PhSe).207It reacts with dienophiles, such as maleic anhydride and dimethyl acetylenedicarboxylate. The photochemical formation of o-xylylene from (99, R = C1, Br, Me3Sn) and also from 2-indanone has been investigated spectroscopically in cyclohexane solutions at room temperature.'" Studies of the generation of hydroxyl radicals by photocleavage of the N - 0 bonds in N-hydroxypyridine-2(1H)-thione,ls2 and N-hydroxy-2(1H)-pyridone' 83 have been reported. Alternative photochemical sources of hydroxyl radicals are 4-mercaptopyridine-N-oxides.84 Conformational effects on the photosensitized carbon-carbon bond cleavage of P-phenethyl ether radical cations have been in~estigated.~'~ Generally, bond cleavage is found to occur if the bond dissociation energy is less than 55 kJ mol-' and if there is significant overlap between the singly occupied molecular orbital and the vulnerable C-C bond. The photodeacetalization of dioxolane (loo),using 2,4,6-triphenylpyrylium tetrafluoroborate as sensitizer, has been studied to explore the possibility of achieving sunlight-controlled release of polymer-bound chemicals.*" The reaction produces mainly n-decanal and
'
Photochemistry
344
p-isopropylbenzaldehyde, and can be compared with similar photodeprotection of thioketals and oxathiolanes’86 mentioned in Section 8.3, which did not work well for protected aldehydes.
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IIl7: Pho toelimination 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128.
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E. A. Wade, H. Clauberg, S. K. Kim, A. Mellinger and C. B. Moore, J. Phys. Chem. A, 1997,101,732. A. Mellinger, M. A. Hall, E. A. Wade and C. Moore, Bradley, Ber. Busenges. Phys. Chem., 1997,101,465. C. G. Morgan, M. Drabbels and A. M. Wodtke, J. Chem. Phys. , 1996,105,4550. C. G. Morgan, M. Drabbels and A. M. Wodtke, J. Chem. Phys., 1996,104,7460. C.-K. Ni, E. A. Wade, M. V. Ashikhmin and C. B. Moore, J. Mol. Spectrosc., 1996, 177,285. T. Droz-Georget, M. Zyrianov, A. Sanov and H. Reisler, Ber. Busenges. Phys. Chem., 1997,101,469. A. Sanov, T. Droz-Georget, M. Zyraniov and H. Reisler, J. Chem. Phys., 1997,106, 7013. R. A. Brownsword, T. Laurent, R. K. Vatsa, H.-R. Volpp and J. Wolfrum, Chem. Phys. Lett., 1996,258, 164. R. A. Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa and H.-R. Volpp, J. Chem. Phys., 1997,106,4436. M. Jager, H. Heydtmann and C. Zetsch, Chem. Phys. Lett., 1996,263,817. Q.-S. Xin and X.-Y. Zhu, J. Chem. Phys., 1996,104,7895. Q . - S . Xin and X.-Y. Zhu, J. Chem. Phys., 1996,104,7904. B. Rowland and W. P. Hess, Chem. Phys. Lett., 1996,263,574. M. Ahmed, D. Blunt, D. Chen and A. G. Suits, J. Chem. Phys., 1997,106,7617. H. Li, Q. Li, W. Mao, Q. Zhu and F. Kong, J. Chem. Phys., 1997,106,5943. A. Loettgers, A. Untch, H.-M. Keller, R. Schinke, H.-J. Werner, C. Bauer and P. Rosmus, J. Chem. Phys., 1997,106,3186. S . E. Ulic, K. I. Gobbato and C. 0. Della Vkdova, J. Mol. Struct., 1997,407, 171. B. Jung, H. Kim and B. S. Park, Tetrahedron Lett., 1996,37,4019. L. Baiiares, T. Baumert, M. Bergt, B. Kiefer and G. Gerber, Chem. Phys. Lett., 1997,267,141. K. Tanaka, K. Sakaguchi and T. Tanaka, J. Chem. Phys., 1997,106,2118. S . Sat0 and T. Suzuki, J. Phys. Chem., 1996,100, 14769. X.-Z.Sun, M. W. George, S. Kazarian, S. M. Nikiforov and M. Poliakoff, J. Am. Chem. SOC.,1996,118, 10525. Q. Ji, C. R.Lloyd, E. M. Eyring and R. van Eldik, J. Phys. Chem. A, 1997,101,243. S . Oishi, H. Inui and Y.Kuriyama, Chem. Lett., 1997, 115. F. Wen-Fu and R.van Eldik, Inorg. Chim. Acta, 1996,251,341. I. G. Virrels, M. W. George, J. J. Turner, J. Peters and A. VlEek, Jr., Organometallics, 1996,15,4089. M. K. Whittlesey, R. N. Perutz, I. G. Virrels and M. W. George, Organometallics, 1997,16,268. J. C. Owrutsky and A. P. Baronavski, J. Chem Phys., 1996,105,9864. J. C. Owrutsky and A. P. Baronavski, Springer Ser. Chem. Phys., 1996,62,243. M. P. Wilms, E. J. Baerends, A. Rosa and D. J. Stufkens, Inorg. Chem., 1997, 36, 1541. B. D. Rossenaar, D. J. Stufkens, A. Oskam, J. Fraanje and K. Goubitz, Inorg. Chim. Acta, 1996,247,215. E. M. Mitchell, T. A. Barckholtz and B. E. Bursten, Inorg. Chim. Acta, 1996, 252, 405. E. F. Walsh, M. W. George, S. Goff, S. M. Nikiforov, V. K. Popov, X.-Z. Sun and M. Poliakoff, J. Phys. Chem., 1996,100,19425. B. J. Palmer and R. H. Hill, Can. J. Chem., 1996,74, 1959.
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G. Garcia, M. E. Garcia, S. Melon, V. Riera, M. A. Ruiz and F. Villafafie, Organometallics, 1997,16,624. A. El-Maradny, H. Tobita and H. Ogino, Organometallics, 1996,15,4954. H. Winkler, C. Ober, A. X.Trautwein, L.Prevot and R. Weiss, Nuovo Cimento SOC. Ital. Fis. D, 1996, 18, 353. A. G. Griesbeck, A. Henz, W. Kramer, J. Lex, F. Nerowski, M. Oelgemoller, K. Peters and E.-M. Peters, Helv. Chim. Acta, 1997,80,912. R. M. Wilson, K. A. Schnapp, M. Glos, C. Bohne and A. C. Dixon, J. Chem. Soc., Chem. Commun., 1997, 149. B. C. Gilbert, J. R. Lindsay Smith, P. MacFaul and P. Taylor, J. Chem. Soc., Perkin Trans. 2, 1996,511. B. C. Gilbert, G. R. Hodges, J. R. Lindsay Smith, P. MacFaul and P. Taylor, J. Chem. Soc., Perkin Trans. 2 , 1996,519. T. M. Bockman, S. M. Hubig and J. K. Kochi, J. Org. Chem., 1997,62,2210. H. Koshima, K. Ding, Y. Chisaka and T. Matsuura, J. Am. Chem. Soc., 1996,118, 12059. Y. Shimizu and S. Kawanashi, J. Chem. SOC.,Chem. Commun., 1996,819. W. M. Kwok, M. S. Hung and D. L. Phillips, Mol. Phys., 1996,88,517. P. L. Ross, S. E. Van Bramer and M. V. Johnston, Appl. Spectrosc., 1996,50,608. L. Eberson, M. P. Hartshorn and 0.Persson, Angew. Chem., Znt. Ed. Engl., 1995,34, 2268 (Angew. Chem., 1995,107,2417). C . P. Butts, L. Eberson, M. P. Hartshorn, F. Radner and B. R. Wood, Acta. Chem. Scand., 1997,51,476. C . P. Butts, L. Eberson, M. P. Hartshorn, W. T. Robinson and B. R. Wood, Acta Chem. Scand., 1996,50,587. C . P. Butts, L. Eberson, K. L. Fulton, M. P. Hartshorn, G. B. Jamieson and W. T. Robinson, Acta Chem. Scand., 1996,50,735. C . P. Butts, L. Eberson, K. L. Fulton, M. P. Hartshorn and W. T. Robinson, Aust. J. Chem., 1996,49,469. C . P. Butts, L. Eberson, K. L. Fulton, M.P. Hartshorn, W. T. Robinson and D. J. Timmerman-Vaughan, Acta Chem. Scand., 1996,50,991. L. Eberson, M. P. Hartshorn and D. J. Timmerman-Vaughan, Acta Chem. Scand., 1996,50, 1121. J. 0. Svensson, Acta Chem. Scand., 1997,51,31. C . P. Butts, L. Eberson, M. P. Hartshorn and W. T. Robinson, J. Chem. SOC., Perkin Trans. 2, 1996, 1877. C. P. Butts, L. Eberson, M. P. Hartshorn, W. T. Robinson and D. J. TimmermanVaughan, Acta Chem. Scand., 1997,51,73. L. Eberson, M. P. Hartshorn and J. 0. Svensson, Acta Chem. Scand., 1997,51,279. L. Eberson, 0. Persson and F. Radner, Res. Chem. Intermed., 1996,22,799. B. G. Gowenlock, J. Pfab and V. M. Young, J. Chem. Soc., Perkin Trans. 2, 1997, 915. R. Uberna, R. D. Hinchliffe and J. I. Cline, J. Chem. Phys., 1996,105,9847. P. D. Wood, B. Mutus and R. W. Redmond, Photochem. Photobiol., 1996,64,5 18. L. Grossi, Tetrahedron, 1997,53,6401. K. Kinbara, H. Takezaki, A. Kai and K. Saigo, Chem. Lett., 1996,217. C . J. S. M. Simpson, P. T. Griffiths, H. L. Wallaart and M. Towrie, Chem. Phys. Lett., 1996,263, 19. H. G. Jenniskens, W. van Essenberg, M. Kadodwala and A. W. Kleyn, Chem. Phys. Lett., 1997,268, 7.
lIJ7: Photoelimination 160. 161. 162.
163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191.
349
A. Hasan, K.-P. Stengele, H. Giegrich, P. Cornwell, K. R. Isham, R. A. Sachleben, W. Meiderer and R. S. Foote, Tetrahedron, 1997,53,4247. W. Meiderer and H. Giegerich, German Patent, 4 444 96, 1996 (Chem. Abstr., 1996, 125,143236s). V. Hagen and U. B. Kaupp, German Patent, Forschungsverbund Berlin, E.V.; Forschungszentrum Juelich GmbH, 19 529 025, 1997 (Chem. Abstr., 1997, 126, 186325h). R. Sreekumar, Y. Q. Pi, X. P. Huang and J. W. Walker, Bioorg. Med. Chem. Lett., 1997,7, 341. A. Ajayaghosh and V. N. R. Pillai, Tetrahedron Lett., 1996,37,6421. C. P. Holmes, J. Org. Chem., 1997,62,2370. T. Nishibuko, A. Kameyama and Y. Toya, Polym. J., 1997,29,450. K. Feng, T. Matsumoto and T. Kurosaki, Chem. Muter., 1997,9, 1362. Y. Tani and M. Sasako, Japanese Patent, Matishita Electric Ind. Co. Ltd., 08 82 933, 1996 (Chem. Abstr., 1996,125,71913k). M. Robert, J. P. Toscano, M. S. Platz, S. C. Abbot, M. M. Kirchoff and R. P. Johnson, J. Phys. Chem., 1996,100,18426. G . M. Kolleger, W. G. Stibbs, J. J. Vittal and K. M. Baines, Main Group Met. Chem., 1996,19,317. N. P. Toltl, W. J. Leigh, G. M. Kolleger, W. G. Stibbs and K. M. Baines, Organometallics, 1996, 15, 3732. R. K. Vatsa, A. Kumar, P. D. Naik, H. P. Upadhyaya, U. B. Pavanaja, R. D. Saini, J. P. Mittal and J. Pola, Chem. Phys. Lett., 1996,255, 129. J. Pola, Radiat. Phys. Chem., 1996,49, 151. M. Ishikawa, A. Naka and S. Okazaki, Prog. Organosilicon Chem., (Jubilee Int. Symp. Organosilicon Chem.], 10th 1993,1995,309 (Chem. Abstr., 1997,126,18916h). F. Wu, X. Chen and B. R. Weiner, J. Am. Chem. Soc., 1996,118,8417. H.-Q. Zhao, Y . 3 . Cheung, D. P. Heck, C. Y. Ng, T. Tetzlaff and W. S. Jenka, J. Chem. Phys., 1997,106,86. D. D. Gregory, Z. Wan and W. S. Jenks, J. Am. Chem. Soc., 1997,119,94. T. Bultmann and N. P. Emsting, J. Phys. Chem., 1996,100, 19417. Y. Hirata, Y. Niga, S. Makita and T. Okada, J. Phys. Chem. A , 1997,101, 561. M. M. Alam, A. Watanabe and 0.Ito, Int. J. Chem. Kine?., 1996,28,405. E. Block, J. Page, J. P. Toscano, C.-X. Wang, X. Zhang, R. DeOrazio, C. Guo, R. S. Sheridan and G. H. N. Towers, J. Am. Chem. Soc., 1996,118,4719. B. M. Aveline, I. E. Kochevar and R. W. Redmond, J. Am. Chem. Soc., 1996, 118, 101 13. B. M. Aveline, I. E. Kochevar and R. W. Redmond, J. Am. Chem. Soc., 1996, 118, 10124. A. J. S. C. Vieira, J. P. Telo and R. M. B. Dias, J. Chim. Phys., 1997,94, 318. J. Hartung, M. Hiller and P. Schmidt, Chem. Eur. J., 1996,2, 1014. E. Fasani, M. Freccero, M. Mella and A. Albini, Tetrahedron, 1997,53,2219. C. Kerst and P. Potzinger, J. Chem. SOC.,Faraday Trans., 1997,93, 1071. H. Hiratsuka, Y. Kadokura, H. Chida, M. Tanaka, S. Kobayashi, T. Okutsu, M. Oba and K. Nishiyama, J. Chem. SOC., Faraday Trans., 1996,92,3035. C . Kerst, C . W. Rogers, R. Ruff010 and W. J. Leigh, J. Am. Chem. SOC.,1997, 119, 466. J. Ohshita, H. Niwa and M. Ishikawa, Organometallics, 1996,15,4632. P. J. Lasky, P. H. Lu, K. A. Khan, D. A. Slater and R. M. Osgood, Jr., J. Chem. Phys., 1997,106,6552.
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Part 111 Polymer Photochemistry By Norman S.Allen
Polymer Photochemistry BY NORMAN S. ALLEN
1
Introduction
The field of radiation curing and photopolymerisation continues to dominate the scene in polymer photochemistry as many new applications find their way onto the commercial market. Photolithography continues to be developed, particularly with regard towards designing systems for molecular devices. There has been a very marked increase in interest in active ionic initiators and radicalhonic processes while the photocrosslinking of polymers is attractive in terms of enhancing the physical and mechanical properties of materials. The luminescence 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 as well as Commercial implications in light emitting diodes and monitoring radiation curing systems. On the more commercial front, the photooxidation of polymers continues to attract attention with a continued special interest in natural materials. Bio- and photodegradable plastics are important for agricultural usage. The same applies to polymer stabilisation where commercial applications dominate significantly with much emphasis on the synergistic behaviour of stabilisers. For dyes and pigments stability continues to be of major concern.
2
Photopolymerisation
Interest and 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 over thirty articles to date. An extensive review has appeared on the function of different types of photoinitiators and their future development' as well as a short overview2 and advances in terms of developments in polymeric type ~ensitisers.~ A number of articles have targeted interest in photosensitive polymer^^.^ with particular emphasis on the design of p ~ l y i m i d e s Recent . ~ ~ trends in radiation curing have been discussed,".' crosslinking of polymers,'2 curable powder coatings," phase ~ e p a r a t i o nand ' ~ self-ordering' in crosslinkable blends and the ecological advantages of adhesives.l 6 Compo~ites,'~ membranes" and electronic packaging materials" are also important areas of development as are reactive
'
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998 353
Photochemistry
354
adhesives,20 water thinnable furniture coatings" and polyurethane prepolymers.22 Specialised topics include kinetic processes in bulk systems,23photodimerisation of diolefin crystals,24 donor-acceptor processes,25poly(fullerenes),26anisotropic gels2' and dichroic elliptical reflectors.28Ionic polymerisation processes have attracted interest in a number of areas such as in surface modification^,^^ epoxy reaction^,^' acid generation processes3' and photolytic processes.32Gravity has been found to influence the molecular weight of photopolymerised poly(methyl m e t h a ~ r y l a t e )and ~ ~ for the first time large scale reflection holograms have been produced.34 A new real-time FTIR monitoring system has been developed3' and a new series of block copolymers for lithography have been developed based on t-butyl methacrylate with silicon based methacrylates as shown in the reaction sequences in Scheme 1.36 ,CH3
TAS' HF2-
n CH2=C\
L
c=o
7.5 hlroom temp.
d I
(?H2)3 H3C-Si- CH3
0 H3C- Si- CH3 CH3
TAS' HF2-
,CH3 mCH2=C\
+ (1)
c=o
12 hl- 25 "C
*
-(- CH2-
CH2
Q
d I
H3C-C-
{c=o & -{c=o &
(?H2)3
CH3
CH3
H3C-Si-CH3 I 0
Q
H3C-C-CH3 CH3
I
H3C-Si- CH3 CH3
Scheme 1
2.1 Photoinitiated Addition Polymerisation - There continues to be an interest in research dealing with the use of luminescent molecular probes for monitoring photopolymerisation processes as will be seen throughout this article. Recent applications include the use of a fibre optic system coupled to a fluorimeter for analysing pyrene doped a ~ r y l a t e s .Transitions ~~ in fluorescence intensity were found to be markedly dependent upon the crosslinked density and flexibility of the curable system. The presence of a sulfur atom in polyacrylates has been found to enhance conversion rates significantly over a very wide temperature range.38 The sulfur is found to effectively scavenge oxygen at higher temperatures. Post polymerisation reactions in UV cured poly(methy1 methcarylate) have been
Ilk Polymer Photochemistry
355
shown to be dependent upon the rheological characteristics of the resin.39 Active polymer radical ends inside globules are postulated to surface during rearrangement and initiate chain growths causing marked post polymerisations. Significant changes in absorption spectra have been observed during the photopolymerisation of oligomer carbonat~methacrylates~~ while some attention has been centred on the photoconversions of acrylate monomers trapped in the pores of silica gel.4'*42Homogeneous transparent materials were obtained coupled with surface grafting reactions on the silica sites. The stereoregularity of poly(methacry1ate) salts has been found to be highly syndiotactic due to repulsive interactions between the anions promoting the racemic placement of adding monomer.43 In another studyu the homogeneous homo- and co-photopolymerisation of methacrylate monomers has been found to yield polymers with a higher degree of syndiotacticity than conventionally thermally cured polymers at the same temperature. The triplet state of the initiator is considered to have a major influence over the orientation of propagating chains. Diazosulfonate surfactants have been found useful for the emulsion photopolymerisation of methyl m e t h a ~ r y l a t e ~ ~ while liquid crystalline polydienes have been found to undergo a photocyclic polymerisation!6 The fluorescence of copolymers of N-(4-N,N-dimethylaminopheny1)-maleimide with vinyl acetate made by photopolymerisation has been found to be quenched by an intermolecular charge-transfer i n t e r a ~ t i o n Elec.~~ tron transfer occurs from the amine to the ketone in the vinyl acetate moiety. The kinetics of iniferter photopolymerisation of methacrylate monomers have been a ~ c e r t a i n e dby ~ ~ DSC. Viscosity changes are crucial in controlling the rate determining processes. Dithienothiophenes give cation polymeric radicals capable of further copolymer addition49 while polystryene with a narrow polydispersity has been prepared through the use of an end-capped photoactive anthryl group.50Large differences in radical termination rates have been found to be responsible for the marked variations in molecular weights of polymer from the UV flash polymerisation of has been found to 1 , 3 - b ~ta d ien e .~'trans-l,2-bis(5-Phenyl-2-oxazolyl)ethene *~~ exhibit low laser conversion efficiency due to preferential dimerisations3 while thermally activated patterns can be formed on the surface of poly(methy1 methacrylate) by coating with photodimerisable 9-anthraldehyde.54 Several photocopolymerisations have been undertaken. Polyethylene and propylene oxides have been made with hydrogel propertied5 while copolymers of polyethylene glycol with acrylamide have good flocculation proper tie^.^^ Functional monomers have been prepared based on dimethacryloyl isocyanate with b i ~ ph en o l- AFas ~~have ring opening co-monomers of 4-methylene-2-phenyl-1,3dioxolane.58 Fluorescence and dilatometry correlate well in the photocopolymerisation of di(ethy1ene glycol)bis(allylcarbonate) and 2-naphthyl methacrylateS9 while difunctional maleimides and vinyl ethers have been found to undergo photopolymerisation without the need of a photoinitiator.60 At less than 30% w/w of plasticiser butadiene-styrene copolymers preserve their heterophase structure6' while in the addition of dithiols to bis(alkoxyal1enes) the sulfur adds to the double bond62 forming a highly reactive centre. Poly(styrene oxide) has been found useful in the photoinitiated formation of copolymers63and the kinetics of
356
Photochemistry
radical copolymerisation of N-vinyl caprolactam with propargyl a ~ r y l a t eand ~~ 2-acrylamido-2-methyl-1-propanesulfonic acid with acrylamide N-vinylpyrrolidone65have been determined. Copolymers with silanes have attracted much interest. These include a range of methacrylic acid copolymers with d i ~ i l a n e s , ~secondary ~ , ~ ~ silanes,6s phenylsilane69 and fumarate terminated poly(dimethylsiloxanes).70 A platinum(I1) bis(acetylacetonato) catalyst has been found to be highly effective for enhancing the activities of hydride and vinyl polymer end groups in step polymerisation reactions.71 The photopolymerisation of liquid crystals has attracted some interest. Liquid crystals with acrylate end groups have been found to exhibit a wide discotic nematic phase coupled with a spontaneous homeotropic alignment between s~bstrates.~' Transparent, thermally stable films are produced with a negative birefringence. Novel biphenyl compounds with diene end groups have also been synthesised that form polymers with a uniaxial molecular alignment.73 Dimer formation during the photopolymerisation of diacetylene crystals has been associated with relaxation processes from higher excited states74 while homeotropic alignment of the smectic phase in liquid crystalline polymers has opened up new possibilities for storage display devices.75 Chain kinetics during photopolymerisation have been found to be longer for fluorinated difunctional monomers in the smectic phase due to a decrease in termination rates76associated with diffusion controlled processes. Polymeric vesicles of a diallyldioctylammonium salt have been made that are highly orientated and retain the permeation and entrapment abilities of liposome^.^^ Thin films of vacuum deposited conjugated aromatic compounds have been possible via scanning tunnelling microscopy.78 On gold surfaces monolayers were found to parallel the step direction of the substrate. Topochemical polymerisation was not possible due to the strong intermolecular forces between the polymer molecules. The reactivity of polymerisable amphiphiles in two-dimensional supramolecular assemblies has been found to be dependent upon the mode of initiation, the polymerisable group and the position of the reactive group in the a m ~ h i p h i l e .Using ~ ~ a novel lipid with two different polymerisable diene groups, ladder-like polymers were achieved. Polymerisation reactions in monomer droplets have been monitored via Raman spectroscopy." This process of optical levitation allowed the in-situ measurement of single particles. The molecular weight of emulsion polymerised n-butyl acrylate has been found to be independent of the emulsifier types'-83 but was strongly reduced by the addition of quenchers such as anthracene and naphthalene. A photosensitive surfactant based on natural rubber has been developed for emulsion polymerisations4 whereas highly monodisperse polymer particles have been prepared by the photopolymerisation of monomer droplet^.'^ Complex water soluble acrylamideacrylic acid copolymers with surfactants have been found to be effective catalysts for the emulsion polymerisation of methyl methacrylate, providing improved stability, polarity and stronger aggregation.86 Photopolymerisation of Langmuir-Blodgett films continues to attract interest. Polymeric monolayer films of octadecyl methacrylate with methacrylamide con-
III: Polymer Photochemistry
357
taining an azo dye NLO group have been prepared.87 Thermal treatment of the layers of polymer was found to enhance the second harmonic generation intensities of sub-layers of films. Resonance Raman scattering has been used for measuring conformational changes in the polymerisation of diacetylene monolayers.88An increase in the blue to red conformations was associated with the tilt of the alkyl groups altering the interplanar distances between the polymer chains. A similar analysis has been undertaken on blue-green transformations of polydiacetylene layer^.^^*^' In this case the resonance conjugation of the chains rather than the intermolecular packing was responsible for the colour changes. Novel supramolecular surface assemblies have been made from tree-like molecules composed of phenylazido groups and tris(hydroxymethy1)aminoethane groups as leaves.” These bimolecular assemblies were found to have a wellstructured molecular organisation with a high degree of packing. Very different morphological anisotropies have been obtained in thin amorphous films of poly(viny1-4-methoxycinnamate)and 12-8(poly(diacetylene)) on glass.92The films were found to become smooth upon irradiation along the direction of polarisation of the light. Mixed monolayers of a xanthate disulfide have been formed on a gold-coated quartz crystal balance in order to measure self-assembly processes through frequency shifts.93The photopolymerisation of methacrylic acid on the surface layer was found to exhibit a marked pH dependence. Photoinitiator formulations and their mode of action play a major role in determining the rates and efficiencies of photocuring processes. Novel acrylamido and acryloxyanthraquinone derivatives have been made and their photoactivities compared with model acetyl and acetylamido derivative^.^^ In the absence of an amine cosynergist the order in photocuring activity was found to follow triplet nx* > xx* > C-T state. This follows the ability of the triplet state of the quinone to abstract a hydrogen atom from the environment. In the presence of an amine cosynergist the order was reversed, indicative of the ease of the quinone to accept an electron from the amine. In a later study, these reactions were confirmed using nanosecond laser flash photoly~is.’~ Exciplex formation with an amine was found to be stronger with anthraquinones with low lying triplet C-T states. Furthermore, derivatives of anthraquinone with halo groups were found to undergo additional reactions yielding halo radicals. This reaction is shown in Scheme 2 for the 2-dibromomethylene derivative. The bromine radical can induce further polymerisation or form an acid via hydrogen atom abstraction to induce potential cationic reactions. Thioxanthones also continue to attract interest in terms of their activity. The 2,4-diethyl derivative of thioxanthone has been found to be effective in curing96and the mode of action of 1-chloro-4-n-propoxythioxanthone discussed.97 Thioxanthone chromophores have been found to be incorporated into the chains of photocured polymers98 and three component thioxanthone/ amineldye systems have been found effective for curing photo polymer^.^^ In terms of discolouration reactions, yellowing in post-cured formulations has been found to be associated with the presence of N-methylamino groups in either the initiator or amine cosynergist. loo White pigmented formulations have been found to be effectively cured using a triphenylphosphine oxide”’ photoinitiator, and trans- 10,ll -dibromodibenzenosuberone has been found to be effective for curing
Photochemistry
358
trimethylolpropane triacrylate. lo2 Polymerisable quaternary phosphonium salts have been synthesised by reaction of triphenylphosphine with ally1 bromide' O3 while the dye Safranine T synergises with triethanolamine in the photopolymerisation of 2-hydroxyethylacrylate.' 0 4 Mono- and bis-functional maleimides are effective photoinitiators for a~rylates'~'as are N-alkylphenothiazines'06 while arylalkyl sulfoxides photolyse in the singlet-state to give sulfur and alkyl radicals. lo7 On the other hand S-(6benzoyl)phenyl thiobenzoates photolyse via the triplet state. lo8 The latter, interestingly, do not photoinduce the polymerisation of styrene. Coumarin ketones have been found to induce the photodecomposition of bisimidazoles generating active free radicals for photocuring. lo9 T
Aromatic ring substitution
/ Br-
-
Polymer
+
H I ,Br
0
I
0
1'"
H-atom abstraction Scheme 2
The activities of various alkanolamines have been examined in conjunction with the 2-(2-chlorophenyl)-4,5-diphenylimidazolylradical and benzophenone. l o In the case of the former the ability of the radical to abstract a hydrogen atom from the amine determines the extent of the cure while for benzophenone it is the degree of hydrogen atom abstraction, and hence cure is the same. Polymeric and monomeric sensitisers have been compared' and the influence of initiator
''
III: Polymer Photochemistry
359
''
structure on hydrogel properties of 2-hydroxyethyl acrylate determined. Polymers bearing tertiary amino groups have been synthesised and their fluorescence spectra found to be significantly quenched' l 3 while maleic anhydride' l4 and cyclododecanones'l5have been found to be effective initiators of the photopolymerisation of styrene. Poly(methylphenylsi1ane) is also an effective photoinitiator for styrenes and acrylates via a photolytic process to give silyl radicals."6 Iron oxalate is also an effective photoinitiator for acrylate monomers' l 7 while a theoretical description of the kinetics of free radical dye-initiated polymerisation via an electron transfer process has been proposed.'" Using the Marcus theory it has been shown that the rate of electron transfer can affect the rate of initiation. Oligomeric photoinitiators have been prepared based on poly(propy1ene glycol) and benzoin hydrogen sulfate' l 9 while several novel benzoate ester derivatives of Et-hydroxymethylmethacrylatehave been made that undergo very rapid photopolymerisation.120 A polymerisable photoinitiator with both an aromatic ketone and aromatic tertiary amine based on 2-(N-acridonyl)ethyl methacrylate has been synthesised12' and found to be more reactive in its polymeric than its monomeric state. In the former case strong self-quenching of the monomer fluorescence was observed. TEMPO-capped initiators have been made for producing living polymers'22 as have polybutadiene based photoiniferters based on sodium diethyldithiocarbamate.123 The latter were useful for making styrene-butadiene-acrylicterpolymers. The bulk photopolymerisation of methacrylates has been undertaken using bis(N,N-diethyldithiocarbamate) as iniferter124 while N ,N-diethyldithiocarbamoylacetamide has been used as an iniferter for the polymerisation of bulk styrene.12' In the latter case, although the polymer was not strictly a living system, block copolymers could be made. Azo-containing poly(dimethylsi1oxanes) can initiate the photopolymerisation of methyl m e t h a ~ r y l a t e ' ~with ~ . ' ~strong ~ deviations in the polymerisation rate due to primary radical termination processes. The metallocene induced photopolymerisation of methyl methacrylate has been found to be dependent upon the nature of the metal ion,I2* while in the photopolymerisation of dicyanate esters induced by tricarbonyl cyclopentadienyl manganese complexes cyclotrimerisationtakes place involving the substitution of of the carbonyl groups in the initiator by the cyanate Polymeric photoinitiators of N-(4-benzoylphenyl)maleimide with dimethylamino ethyl methacrylate are more effective than the corresponding monomers' 30 and cyclisation of allylthiourea has been proposed in its photoinitiation of N,N-methylenebisacrylamide.13' The latter monomer has also been successfully photolymerised in the presence of a 2-propanol-Ag salt. 32 Trimethyl-P-dicarbonyl platinum complexes are effective for the photoinduced hydrosilation of silicone polymers'33 and methylaluminoxane has been found to induce the photopolymerisation of methyl metha~ry1ate.l~~ In the latter case no evidence for living polymers was found. Interest in cationic photopolymerisations has increased dramatically over the last year. Novel crown ether styryl dyes have been developed for 2+2 cycloaddition reactions'35 while a tris(2,2'-bipyridyl)ruthenium complex has been successful in inducing the photopolymerisation of aniline. 36*137 New hybrid vinyl
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360
Photochemistry
monomers with both cationic and radical characteristics have been made by reaction of { bis[1-(chloromethyl)-2-vinyloxy]ethyl} terephthalate with unsaturated carboxylic acids.138Sulfonium salts were found to be highly effective photoinitiators. Syrene sulfonate-vinyl carbazole copolymers have been made' 39 and diepoxide reactivites have been examined using model systems.140 Naturally occurring epoxidised triglyceride oils have been made from plant extracts141 and a variety of novel cyclic ketene acetals have been ~ y n t h e s i s e d 'while ~ ~ DSC has been used to monitor the photopolymerisation of pentaerythritol tetraglycidyl ether.143 Bisphenol-A has been found to quench the laser induced cationic polymerisation of cycloaliphatic epoxy resins by 2,6-di-(4-methoxypheny1)4phenylpyrilium hexafluoroantimonatelU while for a series of vinyl ethers anthracene derivatives has been found to exhibit the reverse effect. 145 Poly(methy1 methacrylate) with azo groups have been found to play a useful role in the ordering of cationically cured liquid crystal diglycidyl ethers'46 and a series of novel monomers based on 5,6-epoxy-,13-dioxepane have been found to be highly reactive. 147 A new template method has been developed for microbead format i ~ n and ' ~ ~2,4,4-trimethylpentylphosphine oxide has distinct advantages for photocuring pigmented systems.149 In the cationic photopolymerisation of cyclohexene oxides by coumaridonium salts no initiator fragments have been found in the polymer chaini5' while for the same monomer using triphenyl sulfonium hexafluorophosphate vinyl radicals are first formed which then oxidise to the cation.151*'52 Raman spectroscopy has been used for monitoring the cationic photopolymerisation of divinyl ethers. 537154 The technique proved useful in view of the fact that it provided much useful information on various structural features of the resin during cure. Extensive work has been undertaken on the use of triarylsulfonium and iodonium salts. This includes the photopolymerisation of silicone epoxies,155novel 1-propenyl ether siloxanes, 58 spiroorth~esters,'~~ hexamethylcyclotrisiloxane,160 epoxypolysiloxanes,16' cyclic ketenes,162 vinyl ethers163 and several cyclic vinyl Redox mechanisms have been examined in the photofragmentation processes of di- and triarylsulfonium salts166*167 as well as iodonium salts.'68The kinetics of their photopolymerisations have also been measured169and excited state quenching examined by phosphorescence quenching processes.I7O A tridimensional dual polymer network has been produced in the cationic polymerisation of rubber with pendant epoxy and acrylate groups. 71 Diacrylates have been shown to enhance the ring opening of epoxidised p ~ l y i s o p r e n e land ~~ the influence of epoxidation has been examined on their rates of ring opening.173 Metallocenes are active cationic initiators with ferrocene being reported as the most effective system174and the cyclopentadienyl(iron)arene hexafluorophosphate being commercially available. 75 The addition of polyols accelerates their initiation rate'76 as do dyes via an electron transfer process.177A poly(alky1ene carbonate ketone) has been produced by the cationic induced double ring 178 Dialkyldiopening of 2-methylene-7-phenyl-1,4,6,9-tetraoxaspir0[4,4]nonane. phenyl borates have excellent photoinitiation a ~ t i v i t y ' ~ ~as- ~do ~ ' pyridinium salts.'82-'86In the latter case polymerisation is highly dependent upon the nature of the counter-ion,'82 with thioxanthones acting as effective synergists. 83 Block
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'
'
'
111: Polymer Photochemistry
361
copolymers are shown to be produced by different mechanisms184and zwitterionic processes may be involved.'85 A new fluorescent probe based on 4(dimethylamino)-4'-nitrostilbene has been developed for monitoring the polymerisation rates.186 2.2 Photografting - Photografting for modifying the surfaces of materials continues to attract interest. Modification of polymers such as ethyl cellulose has been undertaken with 4-~inylpyridine'*~ while acrylic acid can be photografted to polyethylene using benzophenone. 188*189 In the same way N-isopropylacrylamide can be photografted to polyethylene via hydrogen peroxide."' Vitamin K has also been photografted onto the surface of polypropylene using benzophenone as an initiator. 191 Mineral acids are claimed to enhance the photografting yields onto polyolefin substrate^'^^ and for acrylamide onto ABS a reverse effect was observed between graft yield and butadiene content in the resin.'93 In the latter case some evidence for copolymer formation was found. A water soluble thioxanthone photoinitiator has been used to graft acrylamide onto the surface of a styrene-ethylene block copolymer.194Here the presence of a hydrogen atom donor such as 2-propanol was necessary for the effective grafting to occur. Photografting onto silica has been possible using azo molecules195and hydrophilically modified poly(styrene-co-butanedionemonoxime vinylbenzoate) has been possible via photografting with 2-hydroxyethyl methacrylate. Acrylic acid has been photografted onto the surface of silicone rubber in the vapour phase'96 while acrylated monomers of hindered piperidine light stabilisers can be photografted onto rubber197and poly(organophosphazines).198 N,N-diethyldithiocarbamate can be used to control the architecture of surface grafting of monomers such as N,N-dimethylacrylamide onto polymer surfaces'99while copolymers have been made by grafting acrylonitrile onto mixed poly(alkylacrylamides).200The latter had pendant amino groups which synergised with the benzophenone photointiator. Poly(viny1 acetate) has been successfully photografted onto poly(organophosphazines)2'1 and polysiloxanes have been made with perfluorinated and been grafts.202PVC has been grafted with N,N-diethyldithi~carbamate~'~ found effective for initiating the photopolymerisation of methyl methacrylate.2M Heterogeneous grafting of acrylic monomers has been undertaken on polymer surfaces such as poly(propy1ene) and nylon for producing microfiltration memb r a n e ~ . ~ " - Here ~ ' ~ benzophenone and bromine have been used as initiators with the latter behaving as a charge-transfer complex with the monomer. 2.3 Photocrosslinking - Studies on photocrosslinking reactions are numerous and widespread in terms of the types of systems investigated. The kinetics and catalysts play a major role in this regard208and numerous studies have concentrated on monitoring network formation via sensors, Using an optical waveguide, polarised absorption measurements can be made that coincide with e1lipsomet1-y~'~while fluorescence intensities have been found to increase in the case of hexaarylbiimidazoles.21' In the latter case a number of studies have appeared using fluorescent probes for measuring cure rates and coating properties through C-T probes.21 The type of transitions in terms of the matrix change
362
Photochemistry
has been considered in radiation-cured polymers216 as well as the production of 3D objects and laser ~ r i t i n g . ~ ' ~Inks - ~ ~in* flexography222and coatings in furniture223are also important areas of development and here the outgassing of the resin is a crucial factor.224 During curing the resin shrinkage affects the properties of the article225although in the case of di- and tri-functional oxetanes Encapsulated microsensors have been this problem appears to be made using UV curable resins227while curing has been measured by real-time infra-red spectroscopy228and photodynamic mechanical analysis.229 Poly(phosphazine) membranes have been developed by crosslinking with ben~ophenone.~~' These polymers can be functionalised via phenoxy groups. Poly(acy1oxyimino) systems can be crosslinked using p-benzoquinone as ini t i a t ~ r while ~ ~ ' polymers with selenocyanato groups are highly p h o t o s e n s i t i ~ e . ~ ~ ~ A new mono azide photoinitiator has been found to be highly reactive for photocrosslinking polymers233while trianthrylene based films have been prepared that undergo a 4+4 cycloaddition reaction.234Highly crosslinked networks have been made from sebacic acid with maleic anhydride for orthopaedic applicat i o n whereas ~ ~ ~ ~benzoylformylated novolaks are useful for lithographic applicat i o n ~ .An ~ ~acid ~ catalysed rearrangement has been used to measure the migration rates of acids in cured films237while the disruption of hydrogen bonds in a Novolak resin causes a dissolution inhibition process in the resin.238Dual tone photoresists have been prepared from polymeric systems with P-ketosulfone Hybrid type photocrosslinkable resins have been developed from photobase generators and quinoid compounds240while pigmented resists have been found useful as colour filters.2417242 Coumarin-biimidazoles are useful systems for laser initiation243and a series of hyper-branched polyesters have been made based upon pentaerythritol and 1,2,4-benzenetricarboxylic anhydride.244 The latter systems have higher photocuring rates than simple unsaturated polyesters. Several studies of a related nature have appeared on the photocrosslinking of polyethylene by benzophenone identifying benzopinacol as the main product by FT NMR a n a l y ~ i s . ~ ~Benzoin ~ - ~ ~ 'ethers have been used for the same reaction25' while in polypropylene little damage was found in terms of the morphological structure of the polymer.252 Poly(viny1 alcohol) (PVA) systems are of particular importance because of their water solubility. Photosensitive systems based on 1-[methyl-4-[2-4-formalphenyl]ethenyl]pyridinium m e t h o s ~ l f a t e ~have ~ ~ ' significantly ~~~ greater sensitivity than dichromated PVA while PVA grafted with styrylpyridinium groups exhibit high uptake of moisture.256Acrolein modified P is also highly reactive as a photoresist257as is that with N-imine The liquid crystalline characteristics of cinnamate esters have been examined259 and novel azo bearing systems have been made based on 4-[N-ethyl-N-(2methacryloyloxyethy1)lamino-4-(2-cinnamoyloxyethylcarbonyl)-2'-(ni troazobenPoly[4-(2-methacryloylethoxy)azobenzene]has been found to be valuable ~e n e )?~' for the photocontrol of liquid crystalline alignment261as have polymers with benzylidene phthalimidine side chains.262 Poly(methy1 methacrylates) with 0cinnamate side chains undergo polarisation isomerisation resulting in photoalign-
363
111: Polymer Photochemistry
ment263while polymers with [4'-(cinnamoyloxy)biphenyl-4-yloxy]alkyl groups264 exhibit a nematic phase which is dependent upon the spacer length. Crosslinked bifunctional liquid crystalline acrylates exhibit thermal rever~ibility~~' while anisotropic networks have been produced by mixing monomers with reactive double bonds with liquid crystals.266The prealignment of cinnamate groups in a liquid crystalline polymer results in a temperature independent photocrosslinking267and mesoscopic ordering has been found in binary polymer mixtures."' Diphenyl dibenzoates give liquid crystalline polymers with 3D control269 and a laser-equipped photo-DSC has been used to monitor turbidity during phase changes in acrylate copolymers.270 A number of studies have developed novel siloxane based systems. Poly(divinylsiloxyethylene glycol) is an excellent p h o t o r e s i ~ t ~as~ 'are siloxanes with t h i o l e n e ~ Photocrosslinked .~~~ siloxanes have useful applications for coatings on poly(methy1 m e t h a ~ r y l a t e )while ~ ~ ~ epoxysilicones are useful control release agents274and novel silsequioxanes with methacrylate resin give hard abrasion reistant Photocrosslinking also stabilises liquid crystalline polysiloxanes giving them a homeotropic texture277whereas trimethylsiloxanes have been ring opened and crosslinked to provide hard coatings.278Silicone acrylates on polypropylene surfaces. have been investigated by phot~calorirnetry~~~-~' Oxygen diffusing through the polypropylene inhibits termination of crosslinking, which was measured by Raman spectroscopy. A series of star burst dendritic methacrylated polyesters have been prepared with a high degree of structural symmetry, branching and terminal f ~ nct i onal i t y. ~'~-~'~ The glass transition temperatures of these polymers increase with terminal functionality and exhibit higher curing rates than conventional linear analogues. CI3 FT NMR has been used to determine the network structure of polyol a ~ r y l a t e s ~and ' ~ crosslink density was able to be followed with laser curing. Using EPR P-methylene radicals adjacent to the main central propagating carbon-centred radical have been identified as the major species formed during photocuring reactions.286In this interesting study, a secondary propagating radical abstracts a hydrogen atom from the polymer chain to form a tertiary radical, which is flanked by two isotropically coupled methylene groups. Alternating heterogeneous line widths were associated with static orientation distribution of the methylene protons with respect to the axis of the orbital of the odd electron. In the photopolymerisation of multifunctional acrylated prepolymers real-time techniques have been used to examine the temperature dependence of curing.287 Epoxy based resins have widespread applications and these have been extended to microelectronics through the use of silica fillers.288 A photocrosslinked epoxycinnamate ester has been found to exhibit good NLO properties with no while temperature fluctuations during relaxation over a period of 500 the photocuring of an epoxy resin caused variations in flow characteristic^.^^^ A series of epoxy resins have been made that can withstand several hours in boiling ate?^' and thermally stable epoxides have been made using fluorinated ally1 ethers.292Tetrahydrofuran forms charge-transfer complexes with epoxy resins293 and effective curing for epoxides has been found using 2,4-diiodo-6-butoxy-3A ~ o r o n e .Epoxy-polyimides ~~~ undergo ring-opening reactions with cationic
'
364
Photochemistry
initiators295and can be used for negative resists. Quartz filled epoxy resins exhibit enhanced strength for electronic boards296and epoxy copolymers with enriched surface layers of sulfonyl fluoride have been ~ y n t h e s i s e d Epoxy . ~ ~ ~ resins with poly(glycidy1 methacrylate) have been cured with d i t h i 0 1 s ~while ~ ~ curing with ketone-amines has been found to be dependent upon the concentration of the latter.299Amine formation in the photolysis of o-acyloximes has been found useful in the photocuring of epoxy resins300and novel a,o-methacryloyl terminated epoxides have been synthe~ised,~"giving networks with low volume shrinkage. Polyimides for negative resists have been synthesised by crosslinking with dinitrenes302while the properties of polyimides are influenced by the nature of the photosensitive groups, which, in turn, influence the degree of molecular disordering in the resin.303Novel polyimides with azo and epoxy groups have been s y n t h e ~ i s e d ~ "that * ~ ~have ~ higher photocrosslinking activity than similar resins with benzophenone. A distribution of solvent removal steps has been found in crosslinked p~lyimides~'~ while the number of chemical crosslinks increases the refractive index of the resins and colour. Isomidisation has been monitored in the photocuring of polyamic acids307 and bisimide models have been used to measure the kinetics of cycloaddition and photopolymerisation in negative type polyimide resist^.^" Dicyclopentadiene acrylate undergoes rapid photocrosslinking using ketone l o while multiethylene glycol dimethacrylates have been used as an initiators30993 experimental framework for measuring factors influencing complex network^.^' Methacrylate copolymers with anthryl groups undergo photocrosslinking by dimerisation3'* as do polymers with methylcoumarin Here the rates are dependent upon the side group concentrations and their rates of reaction can be measured by UV spectroscopy. The reactivity of ethyl-a-(hydroxymethy1)acrylate is important314 as are dimetha~rylates.~'~ Polymers with low levels of methacryloyl groups can be photocrosslinked rapidly316 while the copolymerisation of a difunctional with a tetrafunctional monomer decreases structural heter~geneity.~'Polymeric microspheres of multifunctional acrylates have been photocrosslinked in suspension by laser irradiation3I8 whereas for a series of diacrylate liquid crystalline monomers rates of polymerisation have been found to be higher in the more ordered regions.319In the copolymerisation of methyl methacrylate with ethylene glycol dimethacrylate strong bimodal curves were obtained in the vicinity of the gel point,320indicating coagulation processes to be important. Radical inhibition of copolymerisation has been observed in the photocuring of oligoester acrylates with alkyl met ha cry late^^^' and a sensitive methacrylate has been prepared with benzoin ether side The mechanical properties of photocured polyacrylate networks have been related to their homogeneity as measured by solid-state C13 FT NMR323while a series of novel alkyl carbodiimides have been prepared for crosslinked powder coatings.324The reactivity ratios of a series of novel 4-methylcoumarin copolymers have been measured and determined by FT NMR325while steady-state fluorescence has been used to examine the sol-gel state in the photocuring of methyl methacrylate with ethylene glycol d i m e t h a ~ r y l a t eIn . ~ the ~ ~ latter case activation energies were
III: Polymer Photochemistry
365
measured using pyrene as a fluorescent probe. Water based photocurable latexes with excellent physical properties have been prepared from polystyrene-poly(butyl acetate-co-glycidyl m e t h a ~ r y l a t e )together ~~~ with allylamine to give it film-forming characteristics. The presence of a sulfide group in dimethacrylates enhances their photocuring reactions in the presence of oxygen3'* while photodimerisable methacrylate monomers have been prepared with or,p-unsaturated ketone side groups.329The mobility of radicals produced during photocuring of multifunctional acrylates is dependent upon the viscosity of the matrix.330 A series of poly(tetramethy1ene ether)glycol a,o-acrylates have been prepared which on photocuring gave amorphous-rubbery materials.33' A series of novel comb-like polyamides have been prepared with cinnamoyl side chains332that have high reactivity as have polyesters with norbornadiene residues.333In the latter case the quadricyclane groups released large amounts of thermal energy. Acrylate monomers have been found to markedly accelerate the ring opening reactions in epoxidised r ~ b b e r s and ~ ~the ~ *activation ~ ~ ~ energy of photocrosslinking for a thiol-ene polymer has been determined.336Copolymers of 4-vinylbenzyl selenocyanate and thiocyanate with styrene undergo a lower degree of hydrogen atom abstraction than that with benzyl bromide monomer.337 Cellulose polymers with photocrosslinkable cinnamate groups have been made338as have diblock nanofibres 25 nm thick from ethylene glycol cinnamate methacrylate-styrene diblock copolymers.339Branched polyethylene glycols with cinnamate groups have been found to undergo rapid photocrosslinking.340The degree of gel formation was controlled by the extent of substitution on the glycol chains giving polymers with antithrombogenic properties. Highly mobile liquid crystals have been prepared from 6-(4-cinnamoylbiphenyloxy)hexyl methacrylate.34' Polymers made with N-cinnamoyloxymethy1)maleimide groups give excimer formation which decreases due to cycloaddition reactions on photocrosslinking.342A mesogenic dicarboxylic acid has been made from 4-amino-cinnamic acid and trimellitic anhydride343with semi-crystalline characteristics. Copolymers of 4-cinnamoylphenyl methacrylate with methyl methacrylate have been crosslinked with benzoyl peroxide344while the control of a solid-state polymerisation has been achieved using hydrogen bonding to direct crystal packing.345The latter was achieved through amide derivatives of p-phenylenediacrylic acid. Polymers of 4-hydroxyvinylether and 5-hydroxypentylmaleimideare claimed to have achieved 100%conversion on irradiation.346Anisotropic networks have been obtained from the cationic photocuring of mixtures of mono and di-functional vinyl ethers347 while the catalysed isomerisation of ally1 and crotonyl ethers has been undertaken with base and transition metal catalysts.348Polyphosphoramide esters with divanillylidene cycloalkanone groups have been prepared with high thermal stability.349 These monomers photocrosslink via the usual 2+2 cycloaddition reaction, the rate decreasing with an increase in ring size. Maleic into fumaric ester conversion has been achieved using polyester resins with allylic monomers35owhereas polyethers with azo links undergo cis-trans isomerism on exposure to light351to produce tough films. Isopropyl substituted poly(pheny1ene ether ether ketones) also exhibit high thermal stability as well as good contrast exposure for resists.352Long-lived cationic centres have been observed in pentaerythritol tetraglycidyl ethers.353
366
3
Photochemistry
Luminescence and Optical Properties
A number of reviews of specialised interest have appeared. A few articles have concentrated on photoresponsive polymers with emphasis on cyclic systems,354 polyamine dendrimer~,~’~ p ~ l y i m i d e s ~ ’ ~ and . ~ ’ ~ poly(hydro~yalkanoates).~~~ Light scattering and flash photolysis techniques are reviewed for investigating polymer transformation processes359as have photografted blends360and ringopening metathesis polymer is at ion^.^^' Time-resolved luminescence analysis is important as a dynamic tool362while orientation processes in composites are reviewed using photoreactivity Fluorescent probes continue to have applications3@ as do mono and multilayer processes.365 Of all the polymers under investigation in this field the most widely studied are the poly(aryleneviny1e n e ~ due ) ~ to~ their ~ light emitting diode characteristics as is the cure monitoring of resins using fluorescent sensors367and photoinduced processes in amphiphilic polyele~trolytes.~~~ The use of luminescence as a polymer sensor continues to attract interest although its implementation and application on an industrial scale remains to be firmly established. Fibre optic probes have been used to measure cure rates of 4,4’-bismaleimidodiphenylaminewith o,o’-dially bisphen01-A~~~ and viscosity sensitive probes such as pyrene have been used to monitor the bulk polymerisation of methyl metha~rylate.~~’ Diffusion coefficients during gel formation in methacrylate and dimethacrylates have been monitored with a pyrene probe37’.372while during the drawing of PVC, polarisation effects have been observed in the spectra of 0xazine-17~~~ due to ordered aggregation effects. Fluorescence has been used to measure polyurethane formation and correlated to FTIR changes374while in composites stress-induced fluorescence shifts were observed upon tensile d e f ~ r m a t i o n . ~Novel ~’ 2-cyanophenyl alkyl ethers have been developed as sensors that are not susceptible to quenching processes376 whereas a new method based on total internal reflectance fluorescence has been employed to measure tracer diffusion processes in polymer Three types of photochemical switching devices have been identified in polymer systems379and curing has been monitored during moulding processes.380The oxygen sensitivity of porphyrin complexes is useful as sensors of oxygen for biomedical application^^^' whereas dye-epoxy systems have exhibited fast wavelength shifts as particle detectors.382 Excimer formation and fluorescence quenching processes continue to attract some interest in relation to polymer structure. In polysiloxanes bromohydrocarbons have been shown to follow Stern-Volmer k i n e t i ~ s ~in ~ ~quenching - ~ ~ ’ the fluorescence of cyanoanthracenes by a charge-transfer process. Excimer formation has been observed from polysiloxanes with 4-cyanobiphenyl side Here non-radiative decay processes increased with increasing concentration of side groups due to an increase in mesogenic interactions. Living polymer systems have been prepared from the ring opening metathesis of benzophenone containing p o l y n ~ r b o r n e n e sand ~ ~ ~their photophysical characteristics examined while a study has been undertaken on the influence of different polymer characteristics on diffusion processes using phosphorescence q~enching.~” Am-
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367
phiphilic methacrylate polymers with carbazolylalkyl groups exhibit reduced fluorescence emission in aqueous media due to energy transfer processes389 whereas phosphorescence emission has shown that additives reside in the polystyrene phase of butadiene-styrene block copolymers.390In the latter case oxygen quenching was found to be higher in polystyrene than that of the copolymer system. Photoinduced electron transfer has been investigated in poly(methacry1ic acid) tagged with porphyrin and benzoquinone moieties391 while photoinduced charge sepaprations have been measured in polynorbornyl triblock copolymers bearing electron relay groups such as dicyanobenzenephenanthrene and dimethylaniline groups. 3927393 No excimer emission was observed in these copolymers indicative of a rigid molecular backbone. Very long-lived charge-separation states have been observed in poly(N-vinylcarbazole) coadsorbed with 1,2,4,5-tetracycanobenzeneon a macroreticular resin.394This process was ascribed to both a hole-migration process and a hole-trapping process causing large interionic distances. Charge-transfer processes have been observed in c60 with N,N-dimethylamino~tyrene~~~ while in polyimides with 9,1O-bis(p-aminophenyl)anthraceneexciplex emission has been observed that can be efficiently quenched by an external electric field.396A novel acrylic polymer with phenoxazinyl moieties exhibits a structural self-quenching effect397while photoelectron transfer in polyconjugated porphyrin polymers containing an electron acceptor occurs via a two-quantum process.398Fullerene doped poly(pphenylene ethynylene) exhibits enhanced polaron and triplet exciton reson a n c e ~while ~ ~ ~singlet exciton transport processes in polystyrene have been observed to be temperature dependent?00 It is claimed that 70% of excimer formation in polystyrene is due to subsequent geminate ion recombination processes. Phosphorescence quenching has been developed as a method for measuring oxygen diffusion4" whereas the room temperature fluorescence quenching of polysiloxanes by chlorohydrocarbons is due to dynamic quenching4'* at concentrations below the critical quencher concentration, The adsorption isotherms of polymethine dyes on calcium fluoride crystals have been found to be non-Langmuir in typem3 while intramolecular hydrogen atom transfer processes in bridged polymeric benzotriazole polymers have been shown to be matrix i n d e ~ e n d e n tPhotoinduced .~~ electron transfer processes have also been measured in polybenzothiazole blends.405 There continues to be extensive interest in latexes and micellar systems. The structure of acrylic latex particles has been investigated by non-radiative energy transfer by labelling the co-monomers406 with fluorescent acceptor-donor systems. Phase separations could also be measured in this way. Excimer fluorescence has been used to measure the critical micelle temperature in diblock copolymers of polystyrene with ethylene-propylene and the results agree well with dynamic light scattering measurement^.^'^ Fluorescence anisotropy has been used to measure adsorption isotherms of labelled polymers to silica408 as well as segmental relaxation processes in solutions of acrylic p0lymers.4~~ In the latter case unusual interactions were indicated between the polymers and chlorinated hydrocarbon solvents. Fluorescence analysis of hydrophobically modified cellulose have shown the operation of slow dynamic processes410while fluorescence
Photochemistry
368
analysis of end-capped poly(ethy1ene oxide) has shown that association occurs through its hydrophobic end groups.411In the latter case a more hydrophobic surfactant promotes the formation of large networks4l 2 whereas for a non-ionic surfactant small clusters of micelles form within the polymer coil leading to its e ~ t e n s i o n . ~At ~ high concentrations of surfactant, the coiVmicellar cluster complex co-exists with free non-ionic micelles in the solution. The average number of micelles increases at about 1.2% w/w of surfactant to a maximum. In the binding of pyrene to cyclodextrin polymers, fluorescence analysis indicates that the probe exists in a more exposed hydrophilic environment in the polymer possibly through the glyceryl linker units.414 For hydrophobically modified poly(styrene sulfonates) excimer fluorescence has been critically dependent upon micelle concentration4 while rotational motions in partly crosslinked polystyrene grafted to poly(ethy1ene glycol) are highly dependent upon the degree of solvation by the solvent.41 Using 1-(dimethylamino)naphthalene sulfonic acid as a probe, chain dynamics were slower in non-solvating hydrocarbon solvents. The fluorescence from poly(2-naphthol) with 4-ethylphenol can be tuned by varying the copolymer composition in an oil-microemulsion system4I7 whereas fluorescence quenching effects of pyrene in sodium alkylcarboxylate solutions showed assembly changes from micelles to vesicles!'* Polyionenes with aromatic hydrocarbons have been found to exhibit an open conformational arrangement whereas with aliphatic hydrocarbons they form microdomains in aqueous media.419 The latter was evidenced by an increase in excimer fluorescence, emission. Poly(N-ethyl-4-vinylpyridinium)has been observed to shift into different environments within a polyelectrolyte phase420 using fluorescence, and similar interactions have been observed in poly(acry1onitrile) electrolyte butyl methacrylate-styrene block copolymers,422poly(2-naphth01),~~~ anthracene polymers,424 poly(2-~inylpyridine)~~~ and general polymer systems.42u28 Pyrene tagged poly(acry1ic) acid is claimed to be a useful sensor of pH429while interdiffusion processes between latex particles can be investigated through pyrenehaphthalene-labelled poly(alky1 met ha cry late^).^^^-^^' The pyrene fluorescence increased with an increase in the rigidity of the core micelles. A similar study using a [4-( 1-pyrene)butyl]trimethylammonium bromide probe showed pH dependent micro-viscosity changes.432Interactions of non-ionic surfactants with poly(ethy1ene oxides) have been shown to comprise clusters of micelles sited in the polymer Pyrene probes have again been shown to be sensitive to pH swelling effects of acrylic copolymers434while in poly(N-isopropylacrylamide) fluoro-substituted pyrenes have been found useful as probes of the microdomains for the polymer in fluorocarbon solvents.435Pyrene is also a useful probe for investigating nanoscale environments in sol-gel reactions436 and several other block copolymer systems.437 Homopolymers and copolymers made from 3-vinylpyridinium salts form compact coils in solution media438while different isomers of dimethylaminostilbazolium dyes exhibit different emitting states with a temperature dependent population.439 Using fluorescence depolarisation the relaxation time of poly(styrene) chains has been found to decrease with expansion of the chain coils.44o
'
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369
In high viscosity solvents networks of chains were formed through intermolecular interactions. Non-radiative energy tranfer studies have been undertaken in naphthalene- and pyrene-labelled poly[acrylamide-co-sodium11-(acrylamido)und e c a n ~ a t e ] .Excimer ~’ emission increased with increasing polymer concentration due to intermolecular associations. At low polymer concentrations the naphthalene was found to sensitise the pyrene emission while an increase in ionicity reduced the excimer emission due to a reduction in interpolymer forces. Phenylpropiophenone has been found, using reflectance laser spectroscopy, to be nonhomogeneously bound onto cellulose.M2 Copolymers with pendant carbazole groups exhibit strong self-quenching effects that compete with efficient energy transferM3 while pyrene groups on a polyethylene oxide chain gave useful information on cyclisation kinetics.eq4Anthracene has been substituted anionically onto living vinyl polymers giving a relationship between relaxation times and polymer ~tereoregularity.~’ Fickian diffusion processes have been monitored in rubbery poly(iso-butyl methacrylate) using non-radiative energy transferu6 as have end-chain interactions via phosphorescence q u e n ~ h i n g Intramolecular .~~ associations effect polymer drag reductiona8 while the local motion in anthracene-labelled vinyl polymers is controlled by the barrier height of the local motions.449Conformational changes in conjugated polymers are discussed critically with the appearance of a J-like band in the low temperature fluorescence of p~lydiacetylene~’~ while pyrene containing dendrimers have been prepared from methyl-3,5-dihydro~ybenzoate.~~~ Transient stimulated emission has been measured from oligothiophene~~’~ where the dynamics are associated with an intraband population relaxation due to internal energy redistribution processes. This leads to the formation of quinoid planar conformations. Blue and red type dual fluorescence emissions have been resolved using picosecond time-resolved fluorescence techniques from 9-anthryl 0ligothiphenes.4’~Intramolecular conformational arrangements and solvent effects control these emissions with the red component being completely solvated in polar solvents. Photoexcitation and relaxation of exciton bleaching in polymers have been simulated454while the inhomogeneous broadening of electronic transitions is associated with a distribution of solvent-solute orientation^.^^' Using non-radiative energy transfer for rubbery polymer films a theoretical formalism has been developed which directly relates the small molecular translational diffusion coefficient to changes in the energy transfer effi~iency.~’~ The infra-red emission from the surface of fractured poly(methy1 methacrylate) contains two main components457whereas exciton migration and annihilation dynamics have been investigated in poly(viny1 sulfate) containing a pseudoisocyanine d ~ e . 4 ’In ~ the latter case analysis of the decay profiles demonstrates that the decay dynamics are consistent with exciton migration limited to finite molecular domains. Anthryl-labelled poly(styrene) gives fluorescenceanisotropic decay curves that can be analysed by time-resolved fluorescence dep~larisation~’~ while the interdiffusion of polymer molecules has been examined during the annealing of latex Isomerism, optical activity and liquid crystalline materials exhibit intersting photophysical features where luminescence is often a valuable probe. Fluorescence analysis of thermotropic biphenyl liquid crystalline polyesters has shown a
370
Photochemistry
high excitation wavelength dependence of various intermolecular ground-state complexes that depends upon different degrees of overlap between mesogenic moieties.46 Thermo tropic polyglu tamates with pho tochromic azobenzene side groups form Langmuir-Blodgett multilayer assemblies in which the rod-like macromolecules are orientated in the dipping direction.462Their lamellar order and in-plane anisotropy are irreversibly lost on irradiation. Reversible alignments have been observed between homeotropic and planar modes for copolymers of methylhydrosilsequioxane with 4-hexyloxy-4-undecyloxyazobenzene463 while a series of viologen copolymers have been made that are fatigue resistant.464The mechanism of azimuthal reorientation of nematic liquid crystals induced by polarised light irradiation of thin films of cinnamate ester based polymers has been investigated.465 Photoalignment was found to be induced by polarised photoisomerism. Fluorescent complexes have been formed between potassium sulfonate and a liquid crystalline polyester4@' which show fluorescence changes in the lyotropic phase. Chiral, isosorbide, spacers have been positioned in a polyester chain to stabilise the liquid crystalline phase467while liquid crystalline polymers with cinnamoyl biphenyl mesogens exhibit high temperature phase changes.468 In solution the latter polymers undergo both 2+2 cycloaddition reactions and photo-Fries rearrangements. Highly fluorescent naphthalene imide dye co-monomers have been synthesised which give polymers yielding orthogonal smectic phases that were fast switching.469 A new conjugated aromatic poly(azomethine) has been made and characterised exhibiting a range of colours in solution depending upon the solvent polarity.470 Photodichroism has been measured in poly(viny1 alcohol) films doped with azo dyes47' and polymeric Schiff bases exhibit pH dependent fluorescence emiss i o n ~ Monoalkyl .~~~ phosphates have been observed to induce large conformational changes in collagen than the sulfate473and optically active poly[3,4-di(S)-2(methylbutoxy)thiophene] has been ~ y n t h e s i s e d A . ~ new ~ ~ methacrylate polymer with terthiophene pendant groups has also been made which exhibits electrochromic properties.475Epoxy polymers with norbornadiene moieties have been observed to undergo rapid isomerism on irradiation in the presence of benzophenone s e n ~ i t i s e r s These . ~ ~ ~ polymers are quite effective thermal energy storage devices. The introduction of branching points in n-hexyl substituted poly(sily1ene) has been found to reduce its ~rystallinity.~'~ These polymers exhibit thermochromic behaviour at which point a marked drop in carrier mobility is observed. The branch sites appear to act as dispersion points for exciton and charge-carriers. A series of photochromic diarylnaphth~pyrans:~~ ~pironaphthoxazines~~~~~~~ pen taN-(4-methoxysalicylidene)anilines48' and s p i r o o ~ a z i n e s ~ ~ ~ , ~ ~ ceneq~inones~~' have also been made. Fulgides have been made with potential semiconductor laser compatibility484while the photochromism of polysulfone systems required a high degree of covalent binding to be effective.485Polydiacetylenes with ambient thermochromic temperatures are made by substituting with ester or carbonate groups!86 Polysiloxanes with azobenzene side groups on the other hand exhibit photochromic properties that are independent of the polymer structure.487Similar polymers with spirobenzopyran groups also exhibit photochromic properties.488
III: Polymer Photochemistry
371
Here the introduction of polar substituents disrupts the chromic effects due to strong association of these groups with chromic units. Acrylic polymers with azobenzene side groups489and liquid crystalline polyi m i d e ~ ~have ~ ’ been synthesised. In the latter case the depolarisation of linearly polarised light on brushed surfaces influenced the reorientation and consequently the irradiation direction became the new order axis after irradiation. The photochromic characteristics of azobenzene containing polymers of copper phthal~cyanine:~’ p ~l yal l yl am i ne~ and ~ ~poly(methy1 m e t h a ~ r y l a t e have ) ~ ~ ~been made. In the latter case laser Raman spectroscopy was used to probe intramolecular orientational processes. Crowned ionic azobenzene polymers exhibit ionic switching processes494while monolayers of azobenzene in poly(viny1 alcohol) gave photoresponses that were dependent upon the packing responses of the c h r o m ~ p h o r e s .Polyglutamates ~~~ with azobenzenes groups exhibited similar properties496and were found to be markedly dependent upon surface pressures. Helical conformation racemisation has been observed in trityl methacrylate copolymers with azobenzene and new synthetic methodologies have been found for photoresponsive methacrylate copolymers with dimethylaminozobenzene Phenylazothioxanthenes are also highly p h o t o r e s p ~ n s i v eas~ ~ ~ are polyi~ocyanides.~~ The ferroelectric properties of azobenzene systems can be manipulated via photomechanical i~ornerisrn.’~~ A fluorescence microscope has been used to monitor the dyeing of wool with a fluorescent brightening agent502and a similar technique has been used to monitor the winding of polyester fibres.503 The normal fluorescence emission from poly(ethy1ene terephthalate) has been assigned to an intermolecular excimer5@’ which is in conflict with previous assignments to it being a ground-state dimer. The fluorescence from polyester has also been found to depend upon its casting temperat~re.~’’In this case the emission is assigned to some phenylene exciplex although undefined. Time-resolved fluorescence analysis has also been applied to polyesters.506 The anomalous fluorescence emission from bisphenol-A based polcarbonates has been assigned to an excimer507while polyesters with benzimidazole units exhibit strong blue luminescence suitable for light emitting diode app~ications.~’~ Silicone polymers with cyanostyrylacrylic side groups have been prepared that exhibit true crystalline order509 while ferroelectric polysiloxanes’ l o with room temperature switching have been prepared with naphthilimide groups. The photoluminescence of polysilanes has been found to decay by a power law, the exponent changing as the transmittance of the films increase^.^' Polythiophenes are also good optical switching devices5I2 and exhibit strong luminescence in good non-polar Blends of polythiophenes with polyethylenes exhibit strong anisotropic emission spectra after drawing due to orientation of the polymer aggregates514whereas in another study polythiophenes have also been shown to undergo rapid degradation and crosslinking on irradiation”’ in an oxygen atmosphere. In the case of poly(sily1thiophenes) chain scission is reduced with a consequent increase in the silyl Nonadiazo starburst compounds have been made with m-phenylene connected groups that undergo intramolecular crosslinking reactions517 while other star-
372
Photochemistry
burst dendrimers are good fluorescent probes.’I8 Polyamide-imide films have been used to produce an aligned linear nematic liquid crystal519 whereas polystyrene latexes with thienylpyridine groups have been made with controlled levels of surface charges.520 The photoactivity of chloroarylheterylamines has been found to vary with the nature of the polymeric e n ~ i r o n m e n t ~and ~ ’ Nalkylaryl phthalimides are useful models for p ~ l y i m i d e sPolyesters .~~~ with both 9,lO-diethynylanthraceneand pyromellitic diimide exhibit strong charge-transfer type luminescence at 600 nm523whereas a series of novel metallopolymers have been prepared that exhibit emission spectra which are solvent insen~itive.’~~ Other strongly fluorescent polymers include p~ l y( ar yl enes),polymers ~~~ with 2,5distyrylpyrazine groups,526 pentaa~adienes,’~~ acrylics with phenothiazine groups,528polymers with 3-alkoxybenzanthrone groups,529poly[2,6-(p-phenylsulfany1)-4-phenylq~inoline],~~~ p~ly(allylarnine),~~’ pyridinium d i a l k y n y l ~ , ~ ~ ~ thiophenoperylenedicarpoly[( 1-methylene-2-rnethylnaphthalene)-N-pyrr0le],~~~ b ~ x i m i d e s ’and ~ ~ dibromo4-( 10-undecynoy1oxy)styrene.535 Single nanocrystallites have been examined in ~ l i g o e t h e r while s ~ ~ ~CH3+ ions have been desorbed from poly(methy1 metha~rylate).’~~ Polystyrene fluorescence is dependent upon the pressure of carbon dioxide when used as a supercritical while the emission behaviour of polysilanes is dependent upon oxygen-induced crosslinking.539Polysiloxanes containing dansyl groups also exhibit emission spectra dependent upon the composite matrix540whereas in thiophene based polymers the emission intensity is dependent upon spectral diffusion and non-nearest neighbour interaction^.^^' The phosphorescence from poly(3,6-dibromocarbazolyloxiranes) is associated with shallow trapping levels542while the fluorescence of paper depends upon the recycled lifetime.543The luminescence characteristics of lignin pulp have also been i n v e ~ t i g a t e d . ~ ~ Viscosity changes in polymer systems continue to be monitored through fluorescent dansyl groups545whereas diffusion coefficients can be measured in polyamidoamine cascade polymers using fluorescein isothiocyanate. 546 In the latter, case dynamic light scattering depended strongly on the addition of salts. Anionically modified poly(N-isopropylacrylamide) has been used as a molecular probe547while the transport of electrons and ions has been demonstrated through poly(pyrr~le).’~~ Reabsorption effects of fluorescent whitening agents has been examined in cellulose fibres549whereas in polymer blends laser confocal fluorescence microscopy has been found to be useful for examining the nature of microspheres in blends of poly(methy1 methacrylate) with p~ly(styrene).’~~ Similar studies have been undertaken on blends of polyolefins” where particles of polyethylene were dispersed within a polypropylene matrix. Maleated polyethylene on the other hand was freely dissolved in the polypropylene matrix. The spatial requirements for quenching processes are important in diblock copolymer micelles of poly[styrene-anthracene-poly(methacry1icacid)]?52Here the interfacial region of the micelle was not fully protonated even at high pH, causing a heterogeneous distribution of quenching sites. Large spherulites of a polysilane have been prepared with a high fluorescence quantum yield553and cyclodextrin forms a 1:1 inclusion complex with 2-chl0ronaphthalene.”~ In the latter case ternary complexes are formed upon the addition of KI. The aggregation of two
’
III: Polymer Photochemistry
313
chains has been observed in polymer blends using picosecond fluorescence depolarisationSSSand an electric field has been found to quench the fluorescence of a ethylcarbazole/dimethyl terephthalate exciplex in poly(methy1 methacrylate).556A novel photoreactive polymer has been developed with a tris(bipyridy1) ruthenium complex557as the charge generating species while transfer at a liquidsolid interface in a turbulent flow has been developed based on pyrene fluorescence quenching immobilised on silica surfaces.558Double bridged viologen compounds have been developed in polymer films559and the properties of fluorescent dye solar concentrators have been examined in polyethylene especially with regard to photo~tability.~~' Fluorescent aryl azides have been used to modify the surface of polyurethane^.^^' Here the degree of binding to the surface is measured by the fluorescence intensity. The addition of alkaline earth metal ions to polymers bearing naphtho-18crown-6 moieties quenches the fluorescence emission of the polymers by heavy atom spin orbit Some competition from cation complexation was also found. Fluorescence analysis has also measured the binding of Te(II1) ions to polyelectrolyte~~~~ with the binding of water playing an important competitive role. Ionic microenvironments in perfluorinated polymers have been investigated . ~ ~ phenanthroline complexes have also been through Ru(I1) c o m p l e ~ e s Ru(1I) labelled with nitroxyl radicals565as molecular probes while a novel polysiloxane with pendant Ru(I1) bipyridyl groups has been s y n t h e ~ i s e dand ~ ~ ~found to be highly sensitive to oxygen quenching. The photoactivity of Ru(I1) complexes with imidazolyl groups has been found to be dependent upon the alkyl chain length of the imidazolium The diffusion of the viologen radical appears to be important for charge separation. Four types of fluorescent lanthanide diketone polymers have been made, the stability of which depends upon the molecular E ~ r o p i u m ~ ~and ~ - terbium572 ~~' complexes of poly(methacry1ates) have also been made with self-quenching being dependent upon the ion concentration distribution. Tb complexes have been used to monitor the polymerisation of dienes on a Ziegler-Natta catalyst573while in another study their fluorescence increases markedly upon p h o t o l y s i ~The . ~ ~presence ~ of p-benzoic acid poly(acrylamide) markedly enhances the fluorescence of Eu3+ polyacrylic complexes via the formation of a ternary complex.575There is a competition between water molecules and Tb3' ions in polyacrylic acid solution576 causing changes in fluorescence intensity while in the same polymer thallium cations quench the fluorescence of bound ~ h e n a n t h r e n e Tungsten .~~~ phenanthroline complexes are also excellent fluorescent probes for polyacrylate films.578Transparent resins of methacrylic acid with bound lanthanides have also been prepared.579 Novel fluorescent crown-ether dyes have been prepared580as have terbium complexes of triphenylcarbonium radical^.^' * Anthracene is highly sensitive as a molecular probe for composites582as is a stilbene c o m p l e ~ . ' ~ Photoinduced ~-~~~ strain measurements have been used to detect forbidden transitions in conducting polymers586while poly(methy1 methacrylate) adsorbs strongly to a silica surface compared with that of polystyrene due to the polar ester groups.5s7 In the latter case fluorescence distributions of pyrene were used as a molecular probe. Fluorescence changes in an anthracene probe have been related to free volume
374
Photochemistry
changes in poly(n-butyl m e t h a ~ r y l a t e ) .Fluorescence ~~~ has been used to study chain exchange kinetics in m i ~ e l l e s ~while ' ~ the molecular structures of zinc porphyrin aryl ether dendrimers can be interpreted through fluorescence lifetime measurement~.~~' The fluorescence properties of laser dyes have been examined bound to polymers in s o l ~ t i o n ~ "while *~~~ fluorescent labelling on polymer surfaces can provide useful information on structural Various relaxation processes in polymers have been surveyed by luminescence596whereas single fluorophores can be observed via a mode-locked laser technique.597 Phosphorescence quenching in polymers has been investigated via Taylor dispersion in solution where free volume effects appear to be probe dependent.598 Fullerenes have been found to be partly crosslinked into poly(methy1 methacrylate)599while triphenyl methane dyes give long-lived phosphorescence in PVC films.600Thermally induced phase separations have been monitored in polyether blends with anthracene as a fluorescent probe6'* and quenching kinetics in phenanthroline labelled polyacrylates are dependent upon counterion selectivity.602Room temperature phosphorescence has been observed from xanthene dyes on filter paper.603 Numerous articles have appeared on poly(ary1ene vinylene) systems, acetylenes and related polymers. The temperature dependence of the photoluminescence from poly(p-phenylene vinylene) (PPPV) has been examined in terms of its influence on diffusion rates604while a magnetic field has been found to enhance luminescence intensity.605A series of poly(alkoxypheny1enes) have been synthesised with high luminescence quantum yields606T607 whereas vapour deposited poly(p-phenylene vinylene) has been found to have an unidentifiable defect structure when annealed at high temperatures.608 Stilbene-amine doped polystyrene has been found to exhibit electronic properties not dissimilar to those of PPPV609 while some novel oligo(p-phenylene) sequences have been copolymerised with vinylene units to give soluble polymers with quantum yields of emission of unity.610Highly phenylated poly(p-phenylene vinylenes) have been made6' and photoinduced charge-transfer has been disThe decay dynamics in poly(p-phenylenes) indicate that stimulated emission and photoinduced absorption processes originate from different specie^.^" Pure ladder-like poly(p-phenylenes) with high quantum yields of emission have been made6147615 with a high intra-chain order.616Novel bimane acetylenes have been prepared6I7 with high fluorescence quantum yields, whereas some poly(diacety1enes) have been found to exhibit photochromism.618Hot selftrapped excitons and a positively charged, spatially confined soliton-antisoliton pair have been spectrally resolved in p~ly(acetylenes).~'~ Structure-property relationships are described for arylenevinylene polymers620while novel poly( 1,4naphthalenediylvinylenes)have been made with high conductivity62' and electroluminescent poly(3,6-N-2-ethylhexylcarbazolylvinylene) properties are reported.622Doping of poly(p-phenylene vinylenes) with fullerenes quenches the polaron resonance due to photoinduced ~harge-transfer~'~ while excimer emission The polymerhas been observed from poly(2,3-di-p-tolylquinoxaline-5,8-diyl).624 isation of 4-tert-butyl-4'-(4-vinylstyryl)-trans-stilbeneby nitroxyl radicals gives a highly luminescent polymer625as do triphenylamine based oligo(ary1eneviny-
III: Polymer Photochemistry
375
lenes)626 and poly(hydro~yaminoethers).~~’Picosecond transient studies on poly(p-phenylene benzobisthiazoles) show the presence of delocalised radical ion pairs and diion pairs suitable for p-n junctions.628 The photoluminescence of PPPV is independent of the number of phenyl 01igomers~~~ and more dependent upon the nature of the excitation wavelength and energy, indicating the presence of inhomogeneous sites. Only excimer sites in cyano substituted poly(p-phenylene vinylenes) luminesce630while field quenched excited states lead to carrier generation with non-exponential decays.631Intermolecular interactions are considered important in terms of the luminescence intensity of PPPV.632In one case the electroluminescence efficiency is increased through the use of bilayers of film forming a heterojunction that confines the charge and inducing electron-hole capture.633A threshold energy has been found for excitation of these polymers634 while the solid polymer exhibits two transitions, solutions exhibit only 0ne.635*636 Dielectric interfaces can also influence the emission intensity.637Pure PPPV has been found to give rise to only one emitting centre.638The effect of orientation on the luminescence of PPPV has been studied639while a polymer with biphenyl groups undergoes a 2+2 cycloaddition.640The vibronic structure in the absorption maxima of PPPV is removed by alkoxy substitution@*while photoexcitation processes are influenced by c60 addition.642Novel alkylthio substituted PPPV has been made643while a Monte Carlo study of PPPV provides a complete picture of the dissociation dynamics of the polymer.644Thermochromic reactions of the octyloxy derivative of PPPV give rise to two emission component^,^^ one due to single strand elements and the second to aggregated segments. Oxidation centres in PPPV can quench the intra-chain excitons responsible for the emissionM6while polymers with sterically hindered side chains are essentially amorphous, giving a higher degree of phot~luminescence.~~ Orientated PPPV gives low emission quantum yields due to the formation of non-emissive excimers. Chain length and temperature effects are also important factors in this Other factors include solution concentration where luminescence quantum yields decrease with increasing con~entration~’~ while oxidation causes a reduction65’ as does increasing pressure.652The emission from PPPV has also been controlled by using a m i c r ~ c a v i t yand ~~~ short chain sequences will enhance the luminescence inten~ i t yIn. thin ~ ~ films ~ of PPPV selective laser excitation can be undertaken655while the fluorescence of n-conjugated systems has been linked to the parity of the lowest singlet excitation.656 PPPV systems with different supramolecular structures have been made657as have sougel composites.658In the latter case the quantum yields of emission are higher. Thiophene containing polymers exhibit intense luminescence659while others are quenched when formed into heterogeneous layers.660-662 Aggregates in poly(p-pyridyl vinylenes) exhibit strong red-shifted while large Stokes shifts in thiophene polymers are associated with spectral relaxation of excitons within an inhomogeneously broadened decay of states.665The electroluminescent characteristics of poly(p-pyridyl vinylene) have also been investigated.666 Chemiluminescence continues to be a topical probe for investigating oxidation processes in polymers. A number of imaging techniques are now available for
376
Photochemistry
probing diffusional oxidation processes directly in polymer samples.667 The chemiluminescence from polypropylene correlates with peroxide levels and their kinetics of decay.668*669 The chemiluminescence also correlates well with antioxidant levels in the p ~ l y m e r . ~ ~ 'With - ~ ~ 'an imaging technique oxidation is seen to occur between the crystallites in polypropylene673whereas secondary emitting species in the polymer have been found to be a,p-unsaturated ketones.674The plasma induced luminescence of polypropylene is associated with radical processes675.676while poly(methy1 methacrylate) has been used as a matrix for monitoring the decay kinetics of dioxetane chemilumine~cence.~~~ Stabiliser performance has been measured by chemiluminescence analysis of stabilised ABS678while in polyamides the kinetics of decay of the chemiluminescence reflect the morphology of the matrix679and the nature of the hydroperoxides.680The chemiluminescence from crosslinked epoxides is independent of their conversion6" while butyl and ethylene propylene rubbers have been found to be thermally stable.68' In hard wood substrates on the other hand the chemiluminescence has been directly related to the phenolic lignin concentration^^^^ while a diode-like chemiluminescence has been observed from poly[ruthenium(vinylbipyridy1)(PF6),] films.684Polymers with chemiluminescent releasable acridinium groups have been made.685 Blue electroluminescence has been observed from polyacetylenes686 while under ionising irradiation high density polyethylene (HDPE) gives initially thermoluminescence which then transforms into chemilum i n e s ~ e n c e .Yellowing ~~~ in aromatic polyesters is a major problem, especially from a recycling point-of-view. Here luminescence analysis has shown that oxidation of a polyester backbone results in hydroxylation to give fluorescent hydroxyaromatic esters.685The latter will then oxidise to give phosphorescent quinones. Decarboxylation of the ester groups can also result in the formation of coloured stilbene quinones as shown in Scheme 3.
4
Photodegradationand Photooxidation Processes in Polymers
Outdoor applications of most polymers are severely restricted owing to their instability toward sunlight induced oxidation processes. The topic continues to attract much interest with a notable increase, again, in the stability and yellowing of wood related materials. Numerous reviews and general articles of interest have appeared. These include fire damage,689degradability of styrene foam,690agricultural ma t e r i a ~ s ,*692 ~ ~ 'b i ~ d e g r a d a t i o n ~ ~ and ~ - ~photodegradable ~' materials.696 Several articles have appeared on photooxidation processes in polymer^,^^^-^" wavelength effect^,^'^-^^ predictions of outdoor durability with artifical fatigue b e h a v i o ~ r , ~excimer '~ laser treatrnent~,~'~ durabi1ity7O9and recycling of polyarylene s~lfide.~" Specialist reviews include Tefol modification by an excimer l a ~ e r , ~organo-tin ' p~lymers,~''silicone^,^'^ p ~ l y k e t o n e s and ~ ' ~ light resistance of star shaped polymers for paint^.^"
'
4.1 Polyolefins - Polyolefins such as polypropylene and polyethylene continue to attract widespread interest. A number of interesting papers have appeared on
III: Polymer Photochemistry
311
1-
CH2CH2 or HOCH2CH20H OH
-0-c
tjc-0 HO HO
b
b
-2H
1 1
l
-2C02
-2C02
[0]
Scheme 3
the mechanistic features of photooxidation processes. The long-term ambient storage of polypropylene has been shown to have a marked influence on the thermal and light induced oxidation of the p~lyrner.”~ High concentrations of hydroperoxide and luminescent chromophores are produced. Removal of the oxidation products by extraction with cold hexane improves the light stability of the polymer. Light scattering data showed that oxidation occurs primarily in the
378
Photochemistry
amorphous regions of the polymer where the chains fragment and reorganise into smaller crystallites. A number of articles have dealt with the degradability of 7-721 Here the hydrophilicity of the polymer polyolefins, especially p~lyethylene.~ appeared to be a crucial factor in terms of their abiotic degradation as well as the inclusion of predegraded material. Atomic force spectroscopy has been used to measure and analyse low molecular weight material on the surface of UV exposed p~lypropylene~~’ while the distribution of radicals in photooxidised polypropylene is important when considering the kinetics.723Talc filled polypropylene is photooxidised faster during the early stages of i r r a d i a t i ~ n ~and ’~ random chain scission occurs during the photooxidation of polypropylene copolymer^.^'^ Moulding temperatures significantly influence the mechanical properties of irradiated p~lypropylene~’~-~’~ while morphological changes preceed photooxidation of polypropylene during the induction period.728Oxidation processes in polypropylene packaging are examined729and oxidation processes are considered to be so heterogeneous as to completely distort any kinetic evaluation^.^'^ Consistent with this observation is the weight loss during irradiation associated with a backbiting process.73’ The lifetimes of y-irradiated polypropylene samples have also been measured.732 Irradiation of thick sections of polyethylene causes increases in crystallinity and elastic modulus73’ and ethylene carbon monoxide copolymers degrade behind window glass.734The latter observation places considerable doubt on the usefulness of such materials in terms of indoor durability. Surface sweat influences the photoageing of polyethylene copolymers735as do different types of metal oxide pigments.736Polyethylene has been found to be sensitive to light wavelengths over the range 260-420 nm with 330-360 being the most while stress is also an important The photolysis of low molecular weight ketones has been undertaken and compared with solid polyethylene chromoph~res.~~~
4.2 Poly(viny1 halides) - The photocatalysed oxidation of PVC has been undertaken in the presence of titanium dioxide and zinc oxide pigments and the extent of dehydrochlorination measured.74’ Acetic and formic acids were major products along with carbon dioxide. Copper(I1) dialkyldithiocarbamate complexes are also sen~itisers.~~’ Photodegradable PVC has also been developed by grafting with benzophenone c h r o m ~ p h o r e s . ’Plasticised ~~ PVC also degrades and discolours on irradiation but this is due mainly to the p l a ~ t i c i s e r . ~ ~ 4.3 Poly(acry1ates) and (alkyl acrylates) - The p-relaxation time of poly(methy1 methacrylate) (PMMA) is broadened by UV irradiation, indicating that whilst chain scission is occurring depolymerisation is minimal. 745 The presence of polybutadiene accelerates the photooxidation of PMMA746 as does poly(viny1 acetate)747when used as blends. 4.4 Polyamides and Polyimides - Thermal cyclodehydration of polyamic acid occurs during irradiation748whereas molybdenum-polyamides undergo photolysis at the Mo-Mo bonds.749Irradiation of new rod-like polyimides generates an
III: Polymer Photochemistry
379
anion radical of the diimide moiety750while polyimides with cyclobutane rings undergo ring cleavage when an electron donating phenyl group is attached to the ring.751Substitution with cyclohexane rings prevents cleavage and improves stability. Fluorinated polyimide films are also light stable75' while others irradiated under vacuum give stable carbon centred radicals.753The latter were assigned to oxygen centred radicals which were slowly depeleted on photolysis.
4.5 Poly(alky1 and aromatic ethers) - Fluorinated polyethers have been investigated7549755 and found to undergo relatively slow photooxidation reactions when compared with simpler polyethers. Secondary carbon centred C-H bonds adjacent to CF2 groups were found to be higly stabilised toward hydrogen atom abstraction reactions. Here tertiary centred C-H sites were the only ones prone to attack. Stericlrepulsion effects of the fluorine groups were considered to play a key role in inhibiting the approach of radical species. Oxidation products such as formic acid and methyl formate were identified by mass spectrometry. The photoproducts of poly(pheny1ene ether) have also been a n a l y ~ e dand ~ ~in ~ the case of poly(2,6-dimethyl-1,bphenylene oxide) 1,5-hydrogen atom transfer occurs from the methyl group of one ring system to the ipso-carbon of an adjacent monomer unit.757Dimethylphenoxy radicals have also been observed during irradiation of this polymer system758that are stable in the dark. Radical cations and superoxide anions are also formed with the consequent formation of quinone type products.
4.6
Polyesters - Aliphatic crosslinked polyesters undergo chain scission and crosslink breakage during photo~xidation~'~ while using ESCA phenolic hydroxyl groups have been identified in photooxidised poly(ethy1ene terephthalate).760The presence of an optical brightener has been found to enhance the surface stability of poly(ethy1ene tere~hthalate).~~'
4.7 Silicone Polymers - Laser flash photolysis studies on poly(sily1enes) generates radical cations along with silyl radicals762and polysiloxane composities for the space shuttle have been found to be stable to far UV light exposure.763Linear polysiloxanes have been found to be more unstable than branched or crosslinked polymers764while the transparency of poly(methylphenylsi1ane) increases with light exposure.765Photooxidised polysiloxanes doped with iodine are converted into semiconductors.766 4.8 Polystyrenes and Copolymers - Polystyrene-hinderedpiperidine copolymers have been found to be more stable to 274 nm radiation than at 318 nm.767 However, this is not surprising in view of the total absorption of the polymer below 300 nm. Crosslinked polymers of 4-allyloxystyrene are stable768whereas fluorinated polystyrene is less stable giving rise to perfluorocarbon centred radicals on irradiati~n.~~' Irradiation of poly(ch1oro- and bromo-styrenes) give corresponding hydrogen halides and monomer77owhile ferric dimethyldithiocarbamate is a powerful photosensitiser for p~ly(styrene).~~' Property changes during the irradiation of high impact polystyrene have been measured772and in
380
Photochemistry
ABS the butadiene rubber component is the primary photosensitiser accelerating oxidation of the poly(styrene) component.773 4.9 Polyurethanes and Rubbers - Hydroperoxidation of EPDM has been undertaken using irradiated anthracene as a dopant and then grafting the rubber with vinyl monomers.774In poly(butadiene) rubber there are two successive steps in its transformation during irradiation.775The first involves the initial diffusion of oxygen giving rise to hydroperoxides and a,p-unsaturated ketones and then carboxylic acid. The second involves crosslinking which depletes the oxygen diffusion, causing further crosslinking and cracking. Cleavage of hydrogen bonds in polyurethanes is claimed to be primarily responsible for their instability.776 4.10 Polyketones - Acyloyl acetophenone polymers undergo phototransformation reactions to give pendant amino with the keto groups acting as energy transfer sensitisers. Structural effects on the photolysis of new polyketones have been examined778while new synthetic routes have been found to making degradable poly(keto esters).779New p-methoxy-carbonyl-substituted 2-methyl1-phenylprop-2-en-1-one copolymers have enhanced photo~ensitivity~~' as does poly(3,5-dimethyloxyacrylophenone).781 In the latter case de-methoxylation and the formation of ketones are important processes.
4.11 Photoablation of Polymers - Mass losses produced during the laser ablation of polyimides are associated with small gaseous fragments produced within the bulk of the polymer close to the ablation threshold.782At higher fluences, stresses caused by these fragments facilitate the photophysical ablation. In the presence of water, hydroxyl radicals are produced which attack the surface of the polyimide making it hydrophilic, thus enhancing its adhesion characteristic^.^'^ Hydrogen peroxide is also an accelerator of p h ~ t o a b l a t i o n ~as' ~are dyes785and tert-butyltetraa~aporphine.~'~ Supersonic time of flight mass spectrometry has identified slow degradation processes in the photoablation of p o l y ( ~ t y r e n e s )and ~ ~ ~using quadrupole mass spectrometry slow moving species during the laser ablation of PMMA have been assigned to methanol, carbon monoxide and methyl f ~ r m a t e . ~Poly(acry1ics) ~' doped with porphyrin dyes undergo laser induced hole filling due to the non-site selective excitation of the dye.789PMMA shows the highest thermal stability of hole profiles during burning with no local relaxation processes.790Irreversible hole broadening in polymers is controlled mainly by the local motion of the host matrix.
4.12 Natural Polymers - Numerous articles have appeared on this subject. Several have dealt with paper and pulp bleaching with peroxides and the influence of long-term irradiati~n.~'~-''~ This includes blocking of photoyellowing by a~etylation,~~' light intensity,792 singlet oxygen,793 colour c o - o r d i n a t e ~ , ~ ~ ~ . ~ ~ bleaching p r o c e ~ s e s ~and ~ ~copolymerisation ~'~ techniques.'02 Tensile properties of wood are also importantso3 while the infra-red changes in collagen are primarily unsaturation and carbonyl groups.804DABCO?" hexadienolso6 and ascorbic acid with a 2,4-dihydroxybenzophenone molecule are all effective
III: Polymer Photochemistry
38 1
inhibitors of photoyellowing.807Detailed studies have appeared on the mechanistic features of lignin d e g r a d a t i ~ n . ~ ' ~Photooxidation -~'~ of the phenolic species in the wood are primary contenders for the discolourations08-8'4 as well as stilbene~''~ and aromatic ketones as products.816Resinols have been identified in sap wood^^'^ and syringic derivatives in Eucalyptus wood.'18 The surface analysis of wood has been undertaken using FTIR spectro~copy.~'~ Two types of cystine reactions have been identified in the photolysis of wool keratin depending upon the wavelength of light820and for nitrocellulose this has been established at 280300 nm with shorter wavelengths causing strong yellowing.s2' 4.13 Miscellaneous Polymers - In the photooxidation of melamine clear coats oxidation of the melamine ether link is the prime site for attack822 while in poly(pheny1ene vinylene) singlet oxygen is an important intermediate in the hydroperoxidation reactions.823The products of photolysis of poly(p-xylylene) have been measured by mass spe~trometry'~~ and the yellowness of polycarbonate assessed.825Cu(I1) complexes with poly(ethy1ene oxide) behave as accelerators of photooxidation826 while 3-alkyl-1,Cpentazadienes undergo reversible chain degradation on i r r a d i a t i ~ n . Anthraquinones ~~~.~~~ are claimed to be photosensitisers of singlet oxygen in poly(~tyrene)~~' whereas phosphine azide precursors generate stable phenoxy radical sites in poly(pheny1ene ~i nyl ene). ~~'
5
Photostabilisationof Polymers
Activity in this field appears to have diminished in terms of publications with much of the attention still centring on hindered piperidine stabilisers. Several of polymers while general reviews have appeared on photostabilisation83'~840 more specialised articles include polysiloxane-hindered amine light ~tabilisers,'~' recycling,842flame retardants,843 automotive coatingss4 and electrochemical coatings.845The natural and artificial ageing of stabilised PVC formulations have been found to give the same effectss46 while a number of new stabilisation packages have been considered for packaging film materials.847 In the more general areas of stabilisation while phosphite formulations are useful for polypropylene fibres848the addition of c6()stabilises PPPV polymers.849 Fluorescent sunlamps have been found to give good predictive stabilisation for high density p~lyethylene~~' and several stabilisation methodologies are available for elastomers.s51The coloured stilbene phototransformation products of butylated hydroxytoluene are light stabilisers in low density polyethylene852whereas pyrene has been found to synergise with phenolic antioxidants.853 Stabilised polyethylene formulations have been designed in order to maximise accelerated ageing conditions854while chemically induced dynamic nuclear polarisation has been found useful for measuring the hydrogen atom transfer constants of phenolic antioxidants.855p-Benzoquinones give good stability to paper pulp856 but photorevert to coloured cyclodienones while a ~ e t y l a t i o nand ~ ~ ~sodium hydroxymethylpho~phinate~~~ treatments are more permanent stabilisers. Mercaptotriazinone derivatives are good stabilisers for plasticised PVC859 as are
382
Photochemistry
macrocyclic dyes for polymer fibres860and triazinylaminobenzoates with polystyrene.861 Luminescent additives such as p-terphenyl have been found to photoprotect poly(methy1 methacrylate)s62 whereas benztriazole stabilisers coated in PMMA on wool act as effective stabilisers.s63 Hindered piperidine stabilisers (HALS) still rank highly in the field of thermoplastic stabilisation. Recent theories suggest that oxygen charge-transfer complexes with the HALS is the main process of inhibition.864Such a mechanism is difficult to grasp given the complexity of polymer degradation processes. Hydroperoxide decomposition has also been proposed865while their diffusion rates also play an important role.866The HALS are quite effective in but antagonised by aromatic p h ~ s p h i t e sThe . ~ ~key ~ reaction with HALS is their conversion into a stable intermediate nitroxyl radical"' with peroxy radical interactions being ruled out. They are claimed to be effective with brominated flame retardents when bound to a polymeric siloxane link.87'.s72Binding to the polymer surfaces73and copolymerising with methyl methacrylate monomer874results in enhanced stabilisation. HALS migration is also important in terms of stabilisation efficiency.875In acrylic paint emulsions based on methyl and butyl acrylates the incorporation of dialkylacrylamides has been found to be highly effective for photostabilisation and they out-perform co-reactive acrylated HALS.876
6
Photochemistry of Dyed and Pigmented Polymers
Dye and pigment effects on polymers as well as their inherent stability are still subjects of considerable complexity and interest. Additive effects on the stability of dyed systems have been reviewed877as has general dye p h o t ~ f a d i n g . ~ ~ ~ In terms of pigment systems carbon black plays a major role in influencing the light stability of p o l y c h l ~ r o p r e n e while ~ ~ ~ for titanium dioxide pigments the trends in photoactivity parallel those occurring during thermal oxidation.880In the former case, the crystal size and nature of the surface treatment play an important role. Thus, under long wavelength near UV excitation all the pigments behave as photostabilisers while under far UV irradiation they behave as photostabilisers. Titania pigments are also photoactive in alkyd resins"' and their photoactivity is enhanced through doping with lanthanides.882 Fluorescent brighteners based on stilbene fade in cellophane, but are stabilised by treatment with sodium carbonate.883Fat liquors influence the dye fading on leatherss4while phototropic fading of eosine has been observed on p o l y a r n i d e ~ . ~ ~ ~ A salicylate benzyl ester inhibits the photofading of acid dyes on silkss6 whereas for disperse dyes on polyester the most active wavelength for photofading is 320 nm1.887 It is interesting that this wavelength corrsponds with the absorption cutoff point for polyester. An iron(II1) hydroxy complex influences the photofading of reactive dyes in solutionss8while the photofading of metallised formazan dyes depended upon the nature of the central metal atom.889Cobalt complexes were found to be highly effective singlet oxygen quenchers. Benzophenone absorbers impaired the photofading of azo dyes when tagged to cy~lodextrin.~~' A combination of halotriazine and vinylsulfone fibre reactive groups has been
111: Polymer Photochemistry
383
found to inhibit the photofading of reactive dyesp9' and stable free radicals have been identified in the powders of all azo dyes.89' This ESR signal increases on irradiation and appears to be unreactive. The photofading and fluorescence characteristics of dyes have also been examined893and related to their behaviour on cotton.
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Part IV Photochemical Aspects of Solar Energy Conversion ByAlan Cox
Photochemical Aspects of Solar Energy Conversion BY ALAN COX
1
Introduction
Topics which have formed the subjects of reviews this year include homogeneous and heterogeneous catalysts for water oxidation,' polymers of norbornadiene functioning as solar energy storage systems,2 hydrogen production using photobiological technique^,^ and photosynthetic ba~ t er i a.Photoharvesting ~ on twodimensional vesicular assemblies has also been de~cribed.~
2
Homogeneous Photosystems
Nitrilotriacetic acid has been used as sacrificial electron donor for hydrogen production in a system consisting of [Ru(bpy)3I2', methylviologen, and colloidal platinum as catalyst.6 2,2':5',2"-Terpyridine and 2,2':5',2":5",2"'-quaterpyridine have been synthesised and will photocatalyse the evolution of molecular hydrogen from water efficiently in the presence of triethylamine as donor, and RuC13 or KlPtCl, as source of Ru', Pt', or Pto colloid^.^ The mechanism seems to involve production of anion radicals which transfer electrons to protons on metal colloids. Methylene blue has been used as a photosensitiser in photogalvanic cells for solar energy conversion in conjunction with glucose as electron donor.' Photoinduced electron transfer in rigid porphyrin-quinone dyads having strong interactions between the chromophores has been studied as a potential model for solar energy conversion; the use of proton transfer reactions to retard charge recombination has also been examined.' A new approach to constructing supramolecular porphyrin assemblies has been described which uses co-operative coordination interactions to generate self-assembled chromophore arrays. This may have implications for photoinduced energy and photoelectron transfer. The kinetics of the elementary processes of electron transfer in the photochemical hydrogen evolution with hydrogenase using cytochrome c3 and methylviologen as carriers have been reported." The results indicate that cytochrome c3 is reduced by reduced methylviologen (MV.+), and that this is followed by rapid electron transfer from c3 to hydrogenase. Molecular hydrogen has been obtained
''
Photochemistry, Volume 29 0The Royal Society of Chemistry, 1998 413
414
Photochemistry
from an aqueous solution comprising [Ru(bpy)3],'+ methylviologen, and hydrogen sulfide as electron donor in a process which performs best at pH 5 after about 4 hours irradiation.I2 This system produces more hydrogen than analogous solutions containing EDTA by a factor of about lo3 in catalyst-free solutions. Photolysis of [N3Co(chelat)B] [chelat = dimethylglyoxime, N,N'-o-phenylenebis(salicylideneimine), N,N'-ethylene-bis-salicylideneimine, B = pyridine] promotes Co-N3 homolysis, and in the presence of thiophenol and other thiols the corresponding disulfides and dihydrogen are formed catalytically. A photocatalytic cycle describing the generation of dihydrogen has been proposed. Irradiation of aqueous iodide in the presence of Pt-loaded Ti02 leads to the evolution of hydrogen and formation of iodine.I4 Enhanced reactivity at low pH has been shown to arise from the greater adsorptivity of I- on TiOz. Norbornadiene-2,3-dicarboxylicacid will undergo photoisomerisation using either Acridine Yellow or Acridine Orange as sensitizer by a process in which fluorescence quenching occurs by electron transfer; this system is promising for solar energy s t ~ r a g e . 'In ~ the copolymer and homocopolymer P3 which contain norbornadiene and carbazole pendants, the norbornadiene fragment is valence isomerised to quadricyclane by an electron transfer mechanism when irradiated by visible light.I6 The back isomerisation has also been investigated and the system found to be capable of storing solar energy. The norbornadienes [l; R = H, PhCH2, p-MeOC6H4CH2, P - M ~ O C ~ H ~ ( C Hwhose ~ ) ~ ] ,substituents have electron-donor and electron-acceptor properties have been synthesised, and the photoisomerisation kinetics for the equilibration of (1) and quadricyclene in various solvents measured and the quantum yields shown to be high.I7 A radical cation mechanism has been suggested and these observations may have relevance for light energy photochemical storage. Polyesters containing norbornadiene residues have been prepared and evaluation of their properties in the solid state shows that their rate of isomerisation is greatly increased by addition of 4,4'bis(diethy1amino)benzophenone as photosensitiser." The photoirradiated polymers release about 90 kJ mol-' of thermal energy. Irradiation of 2-(trifluoroacetyl)-3-phenylbicyclo[2.2.l]hepta-2,5-diene (2) produces quadricyclane (3) which can be thermally reconverted into (2) in the presence of trifluoracetic acid."
3
Heterogeneous Photosystems
Photochemical energy conversion using semiconductor electrodes has been discussed.20Semiconductor based particles in a dual-bed configuration have been used for the solar photocatalytic production of hydrogen from water.21 In one vessel an oxidative transformation occurs by means of which molecular oxygen is
I Y: Photochemical Aspects of Solar Energy Conversion
415
formed, and in a second vessel a reductive process generates molecular hydrogen. This arrangement converts the energetic requirement for photodecomposition of water into a two-photon process and also permits separate production of molecular hydrogen and molecular oxygen. Methanol and hydrogen have been obtained from methane and water by irradiating over a doped tungsten oxide semiconductor catalyst.** Hydroxyl radicals are produced, and these react with methane to give methyl radicals which are subsequently quenched by water to give methanol and hydrogen. The spectral dependencies for the simultaneous photogeneration of dihydrogen and carbon dioxide from 1M sulfuric acid using Pt/Ti02 as photocatalyst have been evaluated and the observed differences attributed to two different phases of Ti02, namely anatase and r ~ t i l e . *Photo~ decomposition of water over Zr02 to produce dioxygen and dihydrogen displays a remarkable increase in activity and stability of gas evolution when carbonate, either in the form of NaHC03 or Na2C03, is added to the cataly~t.'~ The carbonate may play an important role in the desorption of molecular oxygen via the carbonate radical. Water has been successfully photolysed on cerium oxide as photocatalyst to give a rate of evolution of hydrogen which is high and also dependent upon Ce loading.*' The product distribution is a function of the adsorption properties of both molecular hydrogen and molecular oxygen. Hydrogen has been generated using Sn02powder loaded with Pt and Ru02, and sensitised by [Ru(bpy)312' or an organic dye such as acriflavin, eosin blue, rhodamine B, Rose Bengal, or fluorescein; the use of CdS instead of the dye has been examined.26 Hydrogen has also been obtained from the photocatalytic decomposition of water on Cs-loaded metal oxide and alkali-treated metal compound catalysts,27and self-sensitised photochemical oxidation of water has been achieved using AgCl layers on SnOz-coated glass plates in the presence of excess Ag' in water, for which a mechanism has been suggested.** &Ta,Nb6-,OI7 (A = K or Rb, x = 2, 3, 4 and 6) exhibits considerable photocatalytic activity for water splitting when intercalated with nickel metal particles, and is active even without m ~ d i f i c a t i o nIt. ~is~ suggested that there are two kinds of interlayer spaces which are essential for the process to occur. The same authors have also described a photocatalyst active in the visible region which has the general structure AB2Nb3-,M,010 (A = H andor an alkali metal; B = an alkaline earth element and/or Pb; M = V, W, andor Mo; 0 < x < 3) and which will decompose water into hydrogen and oxygen.30Three classes of layered (A = alkali metal, M = alkaline earth perovskites of the form AMn-1Nbn03n+l metal, and 2 5 n 5 7) will photocatalytically decompose water to generate hydr~gen.~' RbPbzNb3010is also reported to decompose water photocatalytically when irradiated with wavelengths in excess of 420 nm. Hydrogen and oxygen have been generated by irradiating water containing the layered titanium oxides A2Tin02n+l.mHz0(A = Na, K, Rb, Cs: n = 2 - 5; m = 0 - 4) or the lepidocrocite-typelayered C ~ , T i ~ - ~ ~ n / 4 0 4 . m(x H=2 00.65 - 0.75; m = 0 - 4; is pore); the catalysts may be loaded with transition metals such as Ni, Pt, Ru, Pd or their oxides.32In some closely related studies, water has been irradiated in the presence of Ti02 or its derivatives having a layered and tunnel structure.33These materials include structures of the form A6+dTi16035+d,2(A = K, Rb; d = - 1.0 to
416
Photochemistry
+1.0) in which all or a part of the intercalating cation may be exchanged by H+, NH4', metal ions, or organic compounds, and the structure may in addition be loaded with transition metals such as Ni, Pt, Ru, Pd, or their oxides. Nanocomposites of the form H2Ti409/Cd0.8Zn0.2S promote efficient hydrogen evolution on irradiation with visible light in the presence of Na2S as sacrificial donor.34The hydrogen production is enhanced by doping Pt with Cdo.8Zno.2Sin the interlayer, but is depressed by depositing Pt on the outer surface of the H2Ti409/Cd0.8Zn0.2S nanocomposite. The sol-gel method has been used to prepare BaTi409.35In combination with Ru02, the photocatalytic activity for water decomposition rises with calcination temperature of the tetratitanate, and its activity results from the high efficiency of charge-separation. Thin film photoelectrodes of CuInSe2 for hydrogen and oxygen production have been evaluated,36and a study of the efficacy of WS2 having photodeposited Pt(IV), Ru(III), Rh(III), Pd(II), Cu(II), or Ni(I1) for photocatalytic production of molecular hydrogen from water in the presence of MV2+ as electron relay, has shown Rh(II1) to have the maximum effect.37Colloidal Pt particles and zirconium viologen-phosphonate are both reported to interact with a polymer surfactant template, and a method is described enabling the metal phosphonate to be grown.38The viologen in these compounds can be photoreduced to yield a charge separated state, and this system is capable of hydrogen generation with a yield five times that for analogous microcrystalline systems without the polymer surfactant template. The structure of Z~~(PO~)(O~PCH~CH~(V~O CH2CH2P03)X3.3H20(X = halide) consists of inorganic lamellae bridged by phosphono-ethylviologen groups, and exchange of the free halide contained within its pores with [PtChl2-, followed by reduction of the anion to finely divided metal particles, gives a compound which is capable of producing dihydrogen from water photo~hemically.~~ Platinum catalysts will increase the efficiency of the photosensitized decomposition of water in heterogeneous systems,40and both methanol and water have been successfully photolysed over a titanosilicate catalyst; for a WTi ratio of 5, activities are similar to those achieved using catalysts comprising Pt-Ti02.41 The photophysical behaviour of monometallic complexes of Ru, Re, and Pt has been studied in relation to their MLCT states, and also with respect to modifying them to give supramolecular arrays!2 Photophysical and photoredox processes at polymer-water interfaces have been discussed with particular reference to poly(ethy1ene oxide) which has been endtagged with anthracene and pyrene, polystyrene-poly(vinyldipheny1anthracenepolymethacrylic acid) tri-block polymers, and fullerene-styrene copolymer^.^^
4
Photoelectrochemical Cells
Details have appeared of a photovoltaic cell having a high conversion efficiency in which the output efficiency is enhanced by use of a phosphor film containing a terbium bipyridine complex,44and of a photorechargeable battery incorporating a conducting polymer of polypyrrole deposited on carbon fibres coated with ferric chloride.45 The irradiated electrode is thought to possess semiconductor
IV: Photochemical Aspects of Solar Energy Conversion
417
properties. Solar photocatalytic hydrogen has been obtained from water using a dual-bed photosystem in which the modules contain different photoparticles and catalysts.46 One module brings about oxidative water splitting, and the other promotes reductive water splitting. A series of redox mediators have been tested. The same authors have also described hydrogen-evolving solar cells incorporating semiconductor powders in which the catalyst has been deposited side-by-side on a conductive substrate such as Au/Pd, carbon cloth, carbon paint, or In-Sn02.47 The PLS3 (phospholuminescence surface state spectroscopy) technique is reported to be useful for characterisation and optimisation of the fabrication processes of silicon and compound semiconductor solar and a new method for the estimation of surface recombination velocity in compoundsemiconductor solar cells has been reported.49 The effectiveness of phosphorus diffusion gettering in silicon wafers for solar cell substrates by the photoluminescence technique has been ~haracterised,~'and interference effects on room temperature photoluminescence spectra of GaAs/Ge solar cells de~cribed.~' The optimum structure for multiple quantum well solar cells has been determined by simultaneous measurement of the photocurrent and photoluminescence in GaAs solar cells, and shows that shallow wells are essential to prevent degradation of the cell performance and improve conversion effi~iency.'~ Reports have appeared of an IR reflecting cover glass for Si and GaAs solar cells,53the characterisation of AlGaAdGaAs heterojunction solar cells using ph~to-ellipsometry,~~ and of the effects of solvent and dopant impurities on the performance of liquid phase epitaxy (LPE) Si solar cells.55Photoluminescence analysis of Ino.5Gao,5Psolar cells grown on GaAs and Si substrates has been carried out and compared with the properties of the Ino.5Gao.5Psolar cell^.'^ This has enabled I ~ O . ~ G Qcells .~P on GaAs of high efficiency to be fabricated. Persistent photogenerated voltages have been observed in ion-conducting amorphous Ag-As-S cells.57Improvements in a-Si solar cell performance fabricated by Hg-sensitised photochemistry have been made using the hydrogen-dilution method in layer p r e p a r a t i ~ n and , ~ ~ hotspot minimisation in solar cells has been achieved on GPS solar arrays using SABER m~delling.'~ All-weather solar cells,60 and the effect of post-plasma enhanced chemical vapour deposition photo-assisted annealing on multicrystalline silicon solar cells have been discussed.6'
5
Biological Systems
A stable three-stage system for solar energy conversion into hydrogen has been described.62The stages are photosynthetic starch accumulation and dark anaerobic starch fermentation followed by further conversion using photosynthetic bacteria. A combination of the marine alga, Chlumydomonas sp. MGM 161 and the marine photosynthetic bacterium Rhodopseudomonus sp. W-1s are used in the photobiological process. Heat treated extract of sewage sludge has been shown to be a useful substrate for hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides strain RV, and continuous hydrogen production has
418
Photochemistry
also been investigated using a two-culture system.63Details have been given of a photobioreactor incorporating Spirulina platensis which can achieve a photosynthetic efficiency of 6 . 8 3 Y 0 . ~The effects of glycogen and glucose on current outputs of biofuel cells separately incorporating either the cyanobacteria Anabaena variabilis M-3 or Synechocystis sp. M-203 along with 2-hydroxy-l,4naphthoquinone have been found to lead to an increase in current in the case of Synechocystis sp. M-203 .65 The performance of biofuel cells incorporating immobilised cyanobacterium Anabaena variabilis M-3 cells has been compared with that of biofuel cells using freely suspended cells,66and a photobioreactor has been designed to determine the effect of Halobacterium halobium on the electrochemical production of hydrogen in salt s~lution.~’ The results imply an enhancement of hydrogen production in the presence of light and the Halobacterium halobium, compared with the Halobacterium halobium-free system. Carbon dioxide has been fixed and ethanol produced using microalgal photosynthesis and intracellular anaerobic fermentation.68 Results are presented which suggest that intracellular ethanol production is simpler and requires less energy than conventional ethanol fermentation processes. Details have been announced of a new plate loop reactor suitable for photobiological production of hydrogen using immobilised Rhodobacter capsulatus cells,69 the use of genetic engineering of micro-organisms as an approach to enhance hydrogen production using photosynthetic ba~teria,~’and of a highly efficient solar induced and diffused photobi~reactor.~~
6
Luminescent Solar Concentrators
Efficiency yields of re-absorption and re-emission processes have been studied as part of an attempt to understand the problem of photon transport in luminescent solar concentrators. The results suggest that the mean photon transport length is proportional to the square root of the surface area of the concentrator for all geometries.72
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Part V Artificial Photosynthesis By Anthony Harriman
Artificial Photosynthesis BY ANTHONY HARRIMAN
1
Introduction
Artificial photosynthesis, which is a subdivision of the general area of solar energy conversion and storage, is primarily concerned with duplicating the reaction centre complexes found in natural photosynthetic organisms. Because the structure of the reaction centres isolated from certain photosynthetic bacteria has been elucidated by X-ray crystallography'-2 and the sequence of events that follows from absorption of a photon has been resolved in great detai1,3+4 most attention has been directed to such entities. The primative bacteria lack the capacity to oxidize water to molecular oxygen but retain the ability to store sunlight in the form of chemical potential. Understanding the rationale employed by Mother Nature in constructing these reaction centres has been the real motivation for making these investigations, which have been on-going now for about 25 years or so. Research into photosynthetic systems has run along three parallel and intersecting paths that relate to (i) the study of isolated components from natural systems, (ii) improvement of theoretical concepts that explain the light-induced events occurring within natural organisms, and (iii) modelling these events with synthetic molecular systems. The last subject, which forms the basis of the current review, has successfully combined elegant organic synthesis with advanced spectroscopic methodology and many beautiful model systems have emerged.5-7 In general, these model systems bear some [slight] resemblance to the natural organisms but they are never as elegant nor as effective as the living photosynthetic units. The main strength of such model systems is that they allow detailed evaluation of theoretical concepts since both structural and thermodynamic parameters can be varied in a systematic manner. Structural modifications to the natural organisms can be made by way of mutagenesisg but this is a demanding task. The most significant weakness of current model systems is that they tend to be isolated supermolecules dissolved in polar solvents. As such, no information can be derived about the critical role played by the protein scaffold in the natural units and, by necessity, covalent bonds replace non-covalent interactions. The main reactions that occur within the reaction centres of illuminated photosynthetic bacteria are energy- and electron-transfer processes between remote but closely-spaced components. A series of light-harvesting complexes collect incident photons and, by a succession of rapid steps, transfer the photon
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around an array of protein-bound pigments.' Such energy migration occurs much faster than radiative or nonradiative deactivation of individual pigment molecules and serves to effectively delocalize the photon over the entire pigment array. These arrays tend to be cyclic and organized within the membrane so that excitation energy can be transferred between arrays with the energetics of each array between determined simply by interaction of the protein scaffold with the pigment. Eventually, the photon is transferred from the light-harvesting apparatus to the reaction centre complex itself, which is located inside one of the cyclic arrays of pigments. Energy transfer induces a series of electron-transfer events that provide for charge separation across the membrane. The co-factors that participate in the charge-separation event are tetrapyrrolic pigments and quinones. A set of four haeme molecules, also embedded in protein, removes the positive charge generated during charge separation from the reaction centre while the promoted electron is used for production of chemical fuel. In green plants, there is a second photosystem that uses the positive charge to oxidize water to molecular oxygen. Since high resolution structural information exists for both light-harvesting complexes and the reaction centre, there has been tremendous activity focussed on understanding why these various components have been selected and why they are arranged so carefully by the protein matrix. Nothing is left random; everything is put firmly into place in order to optimize the overall light conversion process with minimum loss of energy. Individual steps are often extremely fast, if there is a competing undesirable side-reaction, and unidirectional. The challenge for artificial photosynthesis is to design and construct model systems that can perform in a similar fashion. Progress in the field of artificial photosynthesis began with the realization that it is necessary to know the mutual positioning of all the components in the system. The rates of energy- and electron-transfer processes depend critically on separation distance and orientation of the donor with respect to the acceptor. Randomly arranged donor-acceptor pairs provide little real information about the process under investigation. This realization resulted in the synthesis of rigidly-linked, or at least constrained, molecular dyads for which the rate of lightinduced energy or electron transfer could be studied as a function of thermodynamic driving force, solvent polarity, temperature, and orientation. The concept of through-bond electron exchange, virtually unacceptable until the 1980s, came of age and allowed detailed investigation of superexchange interactions in which intramolecular electron transfer proceeds via LUMOs or HOMOS localized on the bridging unit." The next major advance came with understanding that the efficient charge separation is most easily achieved by providing a cascade of electron-transfer events in which the charges are separated in space.' A variety of donor-chromophore-acceptor triads appeared that more closely resembled the natural systems. Adding a light-harvesting antenna to the triad, although a challenge for the synthetic chemist," increases the scope of the supermolecule and brings the system even closer to the genuine photosynthetic units. Many subtleties have been incorporated into advanced model systems and it has been possible to both engineer and resolve rapid intramolecular energy- or electron-transfer steps. Attention has turned from the fundamental issue of
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ifcia1 Photosynthesis
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achieving fast vectorial electron transfer to seeking a better understanding of how the local environment, not simply the solvent polarity, influences the rate of a light-induced process.I3 Model systems become ever more elaborate as the more elusive features of photosynthesis are exposed to microscopic investigation. In the following section we review recent progress made in the area of artificial photosynthesis and we note that there is much in common with this subject and the emerging science of molecular-scale electronic devices. Although this review is not intended to include material relevant to photochemical energy storage or catalytic activity we highlight two important areas where substantial progress has been made. First, several articles have reported on the reduction of carbon dioxide. Reductive photosystems have been designed with nickel(I1) tetraaza-macrocycles as electron-transferring relay^'^^'^ while successive reduction of iron porphyrins in DMF or acetonitrile solution under photochemical or radiolytic conditions can also lead to reduction of carbon dioxide.I6 The mechanism of the latter process has been considered in some detail. The photoreduction of C 0 2 by metallophthalocyanines adsorbed onto Nafion membranes has been describedI7 while the photocatalytic reduction of COz to methane is reported" to occur with water as the substrate under illumination of Ti02 anchored within the pores of a zeolite and with deposited Pt serving as catalyst. Secondly, considerable attention has been paid to identifying and mimicking the water-oxidation catalyst found in green plants. This enzyme, which successfully couples the one-electron photochemistry with the four-electron oxidation of water, is known to contain manganese and to operate in a multistep fashion.l9 Of late, several new bi- and tetranuclear manganese complexes have been tested for their ability to function as artificial enzymes.2c22It will be a long time, however, before the intimate secrets of the natural catalytic cycle are revealed. In trying to mimic the coupling of photochemistry and catalysis a number of binuclear chromophore-manganese dyads have been ~ y n t h e s i z e d . ~Such ~,~~ systems operate in a very simple fashion in that illumination of ruthenium(I1) tris(2,2'-bipyridine) in the presence of methyl viologen results in oxidation of the metal complex due to a bimolecular electron-transfer reaction. A manganese(I1) complex is attached covalently to the chromophore and reduces the so-formed ruthenium(II1) tris(2,2'-bipyridyl) complex due to intramolecular electron transfer, which occurs within a few ps. The ground-state system is restored by diffusional encounter between reduced viologen and the manganese(II1) complex. Thus far, the model systems are unable to undergo multiple electron-transfer steps or to stabilize the charge-separated state. Other reviews or updates have appeared during the past year. A comprehensive discussion of current electron-transfer theory, with emphasis on the more puzzling features, has been reported.25Related aspects of bimolecular electrontransfer reactions have been d i s c ~ s s e dbut ~ ~in , ~much ~ less detail. Other authors have reviewed the special case in which light-induced electron transfer is followed by bond fragmentation28or bond formation.29In recent years there has been a virtual revolution in synthetic organic chemistry that has resulted in the construction of a wide range of exotic molecular architectures. Such systems have not
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escaped the attention of the photochemistry community and close attention has been given to the possible application of photoactive rotaxanes and catenates in molecular electronic devices3' and as models of the photosynthetic reaction centre complex.31Related work has focussed on ways to ensure strong electronic coupling between terminal electron donors and acceptors such that rapid through-bond energy or electron transfer occurs under i l l ~ m i n a t i o n . ~ Photo~'~~ induced electron transfer in conducting polymers has been and the ability of certain mixed-metal complexes to function as multielectron photocatalysts has been i l l ~ s t r a t e d Several .~~ reviews of so-called 'supramolecular photochemistry' have appeared3740 which cover many aspects of electron transfer in self-assembled multicomponent systems. Photoinduced energy- and electrontransfer processes occurring in small clusters of tetrapyrrolic pigments has received special Similar reactions taking place in polymeric or polymerizable systems have likewise been re~iewed.4~ Some of the more unusual aspects of light-induced electron transfer have been Electronic energy transfer in natural& and artifi~ial"~ photosystems has been considered. A brief comparison of the conformation of synthetic and naturally-occurring tetrapyrrolic pigments has been attern~ted"~ while the outstanding contributions made to photosynthesis research by W.A. Arnold have been e ~ p o u n d e d . " ~The -~~ photoreactions of isolated bacterial reaction centre complexes re-dispersed in polymeric media have been ~onsidered.~"
2
Primary Photochemical Reactions
The key photoprocesses occurring in natural and artificial photosynthetic systems under illumination are (i) energy migration between identical chromophoric units, (ii) directed energy transfer between pigments, and (iii) vectorial electron transfer between co-factors. Most model systems are designed to display only one of these events in order to explore the parameters that allow optimization of that particular process. Few systems have attempted to combine the primary events into a single molecular entity but such multifunctional supermolecules will become more common in the near future and there are several instances where the reaction mechanism is switched between energy and electron transfer by minor perturbation of the While the real interest lies with using pigments and electron-transferring relays that bear a marked structural resemblance to those used by Nature (e.g. metalloporphyrins, quinones and carotenes), many recent model systems have employed alternative components. In particular, numerous photosystems based on derivatized fullerenes have been described. These non-classical systems are included in this review for the sake of completeness and for their relevance to the field of photoactive molecular electronic devices.
2.1 Photoinduced Electron Transfer in Molecular Dyads and Higher-order Analogues - Light-induced electron transfer from a donor to an acceptor, followed by successive transfer of the redox equivalents across the membrane, has
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been selected by Nature as being the best way to convert and store sunlight as chemical potential. In previous years, there has been a plethora of reports dealing with charge-separation and recombination occurring between suitable donors and acceptors dispersed in a polar solvent. Little useful mechanistic information can now be gained by studying bimolecular electron-transfer processes where the important events are obscured by diffusional motion of the reactants or products. Such studies have long been popular, because of their simplicity, but they no longer serve a useful purpose with respect to building viable artificial photosynthetic models. Instead, it is necessary to organize the system such that the donoracceptor pair are already positioned for electron transfer prior to excitation and there is no need for large-scale translational motion. This permits visualization of the electron-transfer process itself and allows exploration of the many factors that combine to control the rates of charge separation and recombination. With the reactants kept outside the range of orbital overlap it becomes possible to investigate how the rate of electron transfer depends on the nature of the material that separates donor from acceptor. Although many studies using donor-spaceracceptor architectures have addressed this issue there are still areas of uncertainty and new spacer units appear regularly. Of particular importance to this subject are those model systems in which additional donor or acceptor functions are attached to the photoactive dyad, forming species of the general type: donor-spacer-donor-spacer-acceptor, where more than one spacer might be involved. These systems provide more realistic models of the natural photosynthetic units and allow examination of the directionality of the electron-transfer event. The natural apparatus has evolved by ensuring the forward electron-transfer reaction is always much faster than recombination or other energy-wasting steps. In this way, the quantum yield for charge separation across the membrane approaches unity. The model systems have to use a similar approach and ensure that the rates of successive electrontransfer steps are competitive with all those processes that combine to deactivate the system. This balancing of the rates has to be achieved without losing too much of the initial photon energy while th? final charge-separated state needs to have the redox pair positioned at least 30 A apart. The latter demand means that the multicomponent model system must be conformationally constrained into a more-or-less linear arrangement. Once this has been done the light-harvesting antenna can be attached. Building appropriate model systems is, of course, a serious endeavour, especially if they are to compare favourably with earlier versions. The synthesis and proper characterization of multicomponent arrays of this general type requires considerable commitment and expertise. Few genuine models appear each year and most attention is given to the study of simpler fragments, such as molecular dyads, where the rates of electron transfer can be optimized before attempting to construct higher-order analogues. These dyads provide invaluable information about how to piece together complex molecular systems and enable theoretical treatments to the updated. The photoinduced charge-transfer processes occurring within intimately-coupled donor-acceptor pairs (e.g. Mullikentype charge-transfer complexes) or that lead to twisted intramolecular charge-
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transfer states are relevant to artificial photosynthesis but are covered elsewhere in this volume. Recent developments in the field of bimolecular electron-transfer reactions are also described in an earlier chapter. - Improvements to current theoretical treatments of electrontransfer events, especially those occurring very rapidly, continue to be In the photosynthetic reaction centre complex, individual co-factors are coupled to the protein scaffold so that the dynamics of electron-transfer steps are controlled, to some extent, by the level of this secondary interaction. Recent theoretical studies have tried to modify the more conventional Marcus and Jortner-Bixon theories to better describe coupling with the Although such coupling might be weak it can exert a significant influence on the rate and needs to be better understood. Continuous attention has been given to the problem of calculating inner-6' and outer-sphere62 reorganization energies since the classical method of using dielectric continuum theory and treating the . ~ ~ importance reactants as isolated species provides only a crude e ~ t i m a t i o nThe of solvation dynamicsa in fast reactions occurring in nondipolar solvents has been addressed. An analysis of how the degree of electron delocalization in mixed-valence complexes can affect the rate of electron transfer has been made65 and related to biological electron-transfer processes. Such theoretical descriptions are likely to acquire increased importance in the near future because of the rapid development of molecular dyads in which the redox-active components are maintained in strong electronic communication. Distinguishing between throughbond and through-space charge transfer is often difficult in model systems and ways to make such discrimination have been considered.6668 Finally, a method to calculate rates of charge recombination from emission spectra has been proposed for the special case of MLCT-based sensitizer^.^^ Special attention has been given to understanding the role of the bridge, or spacer, used to connect the donor with its complementary acceptor. The bridge plays the dual part of being a conduit by which to promote effective throughbond electronic interaction and a girder that has to maintain structural integrity of the assembly. A major aspect of this work, therefore, concerns how the length of the bridge affects the rates of electron transfer and of any competing sidereactions.70771 The conformation of the bridge can influence the magnitude of electronic coupling between terminal donor-acceptor moieties,72as does the state of hybridization of carbon bridges.73This latter study compared the electronic coupling properties of short alkane, alkene, and alkyne bridges by measuring electron delocalization, hole transfer and rates of electron transfer in binuclear metal complexes. The roles played by electronic and nuclear factors in controlling rates of electron transfer taking place in rigid systems have been explored.74
2.1.1 Theory.
- Many new molecular dyads, comprising donorspacer-acceptor systems, have been described in the recent literature. These systems are intended to reproduce the essential electron-transfer steps occurring in natural photosynthetic organisms by eliminating as many components as possible. The main events in the natural apparatus involve light-induced electron
2.2.2 Photoactive Dyads.
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transfer from a tetrapyrrolic pigment to a quinone, and most model systems are intended as crude replicants. In some cases, only synthetic details are available at present for molecular systems intended to explore the electronic coupling properties of the connecting spacer function. Other dyads have employed flexible spacers that provide for conformational mobility, and thereby complicate data Often the donor and acceptor can approach each other to such an extent that through-space interactions outweigh through-bond electronic coupling. The more interesting dyads, having constrained or rigid geometries, can be sub-divided into organic, mixed-metal, or hydrid systems according to the composition of the redox-active components. The most popular molecular dyads consist of a tetrapyrrolic pigment as the chromophore and electron donor with a quinone as the electron acceptor. More than 100 such systems have been reported during the past decade and new analogues appear regularly. An examination of the rates of charge separation and recombination in a series of cyclophane-bridged porphyrin-quinone dyads has given a satisfactory quantum mechanical analysis in terms of how the rate depends on driving force.88 The synthetic strategy of attaching the quinone through one of the pyrrole rings, rather than via a meso-phenyl group, has been highlighted.89.90Other systems have replaced the porphyrin with a chlorin9’ or used quinones of unusually high oxidative power.92Photoactive dyads can also be constructed from phthalocyanine and viologen component^.^^ Light-induced charge separation between dissimilar porphyrins is of direct relevance for modelling the primary electron-transfer steps that take place in natural photosynthesis. The rates of charge separation and recombination in such porphyrin-based dyads have been studied as a function of solvent polarity94 and compared with conclusions drawn from porphyrin-quinone studies. There appears to be a marked difference between the two types of dyads, with the porphyrin dimer being much less sensitive to changes in solvent polarity. An interesting effect of how the overall spin multiplicity affects the lifetime of charge-separated states has been described for bichromophoric systems comprising an N-alkylnaphthalimide acceptor and an anilino donor?’ In cyclohexane solution, efficient charge separation takes place within the excited singlet state manifold, with charge recombination populating the excited triplet state of the naphthalimide residue. In a slightly more polar solvent, the local triplet state exists in equilibrium with a triplet ion pair, which survives for ca. 1 ps. In more polar solvents, only the triplet radical pair is detected. Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(I1) tris(2,2’-bipyridyl) complexes have been synthesized bearing appended a n t h r a q ~ i n o n eor~ t~y r o ~ i n e ~ ~ functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(II1) tris(2,2’-bipyridyl)complex
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with a rate constant of ca. 5 x lo4 s-'. Related dyads have used viologen, as the electron acceptor, or l0-(4-picolyl)phenothiazine, as the donor, bound to a ruthenium(I1) tris(2,2'-bipyridyl) complex.98 Similarly, photoactive dyads have been formed by attaching methyl viologen to a copper(1) complex and the photochemistry has been contrasted with that found for related compounds based on ruthenium(I1) tris(2,2'-bipyridyl) complexes.99Peripherally-molybdated porphyrins have been prepared"' that undergo fast charge separation upon excitation into the first-excited singlet state of the porphyrin, despite the absence of a thermodynamic driving force for light-induced electron transfer. The resultant radical pair survives for 300-400 ps but the exact nature of the reduced molybdenum species remains unclear. A porphyrin has also been covalently linked to an oxomolybdenum(V) complex. lo' In this case, light-induced electron transfer from the porphyrin to the appended molybdenum centre has been confirmed by transient resonance Raman spectroscopy. Photoinduced electron transfer has been found to occur rapidly between ruthenium centres linked via cyanide bridges.102,103Such polynuclear complexes are easily assembled into dimers and short oligomers for which the energetics of each metal centre can be varied. These systems are particularly amenable to full mechanistic interpretation and are readily evaluated by electrochemical methods. The rates of intramolecular electron transfer between adjacent metal centres tend to be very fast. Polynuclear complexes have also been formed by attaching a zinc porphyrin to a metal bis-terpyridyl complex.'" A rich variety of intramolecular energy- and electron-transfer processes can be resolved in these systems, according to which subunit is excited. Closely related molecular systems have ruthenium(I1) and rhodium(II1) bis-terpyridyl units linked directly or through one-to-three p-phenylene groups. lo5 As expected, the rate of intramolecular electron transfer depends upon the number of interspersed phenylene rings while the directly-linked system exhibits pronounced electronic coupling between the subunits. One of the many fascinating aspects of natural photosynthesis is that, in common with most other biological systems, a limited number of basic components is used to assemble the entire structure. Functionality is achieved by minor structural modification, such as inserting a magnesium cation into the centre of a tetrapyrrolic pigment, and by careful attention to the mutual arrangement of the components. This situation contrasts sharply with the idea of constructing artificial photosynthetic models by the stepwise acretion of individual units via covalent attachments. Attempts are being made, however, to design a suitable modular approach in which dyads are assembled by noncovalent linkages. Over the past few years, several porphyrin-quinone dyads have been built from components that interact via multiple hydrogen bonding.'".''' Other systems have been pieced together by way of n-stacking between appropriate derivatized modules108*'09or by using a salt bridge to connect together the donor and acceptor units. Supramolecular systems based on molecular-recognition principles have also been self-assembled into organized entities."'+"2 Attention has been given to using helical peptides as the spacer group. * 3 ~ l 14
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2.1.3 Photoactive Triads. - Several new molecular triads have been described over the past yearr115.'16in certain cases the final products could be separated into the component diasteroisomers. The efficiency of sequential light-induced electron transfer in carotenoid-porphyrin-quinone triads has been shown to depend on the way in which the supermolecule is assembled.''7 In many such porphyrin-based systems, electron donors or acceptors have been connected to the porphyrinic chromophore via a phenyl ring attached at one of the meso sites. When the porphyrin also possesses alkyl substituents at the p-pyrrolic positions, steric hindrance prevents the bridging phenyl ring from rotating and this has invoked the notion that the rate of through-bond electron transfer might be affected by the stereochemistry of the connector. It has now been shown that substituents at the P-pyrrolic positions cause a 5-fold drop in the rate of charge separation. These studies have further shown that the rate of electron transfer in amide-linked donor-acceptor pairs depends critically on linkage isomerization. Thus, after allowing for changes in thermodynamic factors, the rate of charge separation is ca. 30-fold higher for the case in which the meso-phenyl ring of the porphyrinic donor is attached to the N atom of the amide compared with the case with the linkage reversed. This is a remarkable result that confirms the importance of making systematic investigations of apparently minor effects. The photochemistry of other porphyrin-based triads has been described. In particular, a carotene-porphyrin-imide system, where the imide acceptor is a 4 3 dinitro-1,8-naphthalenedicarboximide derivative, has been shown to undergo stepwise electron transfer under excitation into the porphyrin chromophore. l8 This system compares favourably with the better known carotene-porphyrinquinone systems and has the advantage that the one-electron reduced form of the imide shows a characteristic absorption transition in the visible region. In benzonitrile solution, the final charge-separated state is formed with a quantum yield of 0.32 and survives for ca. 430 ns. A somewhat related porphyrin-based triad has a naphthoquinone as acceptor and N,N-dimethylethanolamine as donor,' l 9 while triads utilizing fluorescein as the chromophore and the electron donor have been reported.120.'2'These latter systems are impressive in respect of the high quantum yield for the initial charge-separation step in which an electron is transferred from the excited singlet state of fluorescein to carbazole. Secondary electron transfer occurs to anthraquinone or viologen residues used as ancillary acceptors, and again the yields are high. The solvent dependence found for the rates of electron transfer in triads of the type donor-spacer-donor-spacer-acceptor has been reported and compared with the behaviour found for the corresponding dyads studied under supersonic jet conditions.'22These latter studies are especially interesting because they refer to the isolated molecule, free from solvent effects, and because of the excellent quality of spectroscopic data attainable with the supersonicjet. Similar triads have been constructed with a ruthenium(I1) tris(diimine) complex as the c h r ~ m o p h o r e 'and ~ ~ where the geometry of the triad can be varied. This strategy allows comparison of the photochemistry of a series of closely-related positional isomers. Imides and diimides have been used as the electron acceptor in other triads
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which represent variations on the usual theme. In one such example, a diimide has been used as a rigid spacer that separates ruthenium(I1) and osmium(II1) bis(2,2'-bipyridine) complexes. 24 Here, excitation into the ruthenium(I1) complex induces a fast, two-step electron-transfer reaction leading to reduction of osmium(II1) with a quantum yield in excess of 0.7. A series of rigid triads has been constructed and shown to undergo directional two-step electron transfer reactions leading to the formation of charge-separated states.12' The chromophore used in these studies possesses a charge-transfer excited state arising from intramolecular electron transfer. The initial excited state, and the two ion pair states that are formed by subsequent electron transfer, have been resolved in transient spectroscopic studies while the final charge-separated state survives for ca. 300 ns in toluene solution. An added bonus found with these systems is that the intermediate ion pair undergoes radiative charge recombination. This permits a more detailed analysis of the energetics and reaction mechanism than is normally possible.
'
2.2 Light-induced Energy Transfer in Model Systems - Photons collected by the protein-bound light-harvesting complexes of natural photosynthetic units are transferred to the reaction centre complex where the primary energy acceptor is a pair of tetrapyrrolic pigments held in a face-to-face orientation. Considerable attention, therefore, has been given to understanding and mimicking the role of this so-called 'special pair', which is also the primary electron donor for bacterial photosynthesis. A major problem with regard to constructing appropriate model systems that resemble the special pair is that the surrounding protein is able to generate an asymmetric electrostatic field that effectively allows the two chemically-identical pigments to reside in slightly different environments. 26 Thus, the special pair looks more like a heterodimer than a homodimer. There have been many attempts to synthesize cofacial dimers formed from tetrapyrrolic pigments, although X-ray crystal structural data are rare. 127 Electronic interaction between adjacent chromophores can cause rapid nonradiative deactivation of the excited state. 12* The photophysical properties of closely-spaced, side-by-side porphyrin dimers have been r e p ~ r t e d . ' ~ ~ Methods -'~' other than covalent bonding have been used to self-assemble dimers or aggregates of tetrapyrrolic pigments132as a simple means to generate photosynthetic models but the lack of structural information about the final entity is an obvious drawback. The use of peptides as scaffolds by which to construct cofacial dimers is more attractive and makes use of the helical nature of the linker.133In other systems, amide-amide hydrogen bonding has been used as a scaffold by which to construct edge-to-edge arene d i r n e r ~ . Some ' ~ ~ of these dimers show exciton splitting in both solution and the solid state. The mechanism of exciton coupling has been considered. 135 It is not often that the triplet state properties of cofacial dimers are reported but, in an interesting variation on the normal theme, this has been done for a series of naphthalenophanes. 136Here, the triplet state is accessed via sensitization with benzophenone and it is shown that the excited state behaviour depends markedly on the conformation of the superstructure. Both monomer and excimer states are apparent in the transient absorption records, according to the extent of
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orbital overlap between the two naphthyl rings. In certain cases having highly constrained geometries, triplet energy transfer to the naphthalenophanes is followed by bond dissociation to relieve the strain. A great advantage for the detailed investigation of electronic energy transfer, as opposed to the study of light-induced electron-transfer reactions, is that the event can usually be monitored by time-resolved luminescence spectroscopy. This can be an extremely sensitive technique, capable of very high precision, and, in recent years, we have seen the introduction of powerful analytical protocols that permit the meaningful interpretation of complex systems.137*138Even quite simple molecular dyads can present a problem for data analysis when rotational diffusion competes with intramolecular energy transfer and the standard Forster-type theory has to be modified.'39 The problems are magnified when attempting to study energy migration in pigmented arrays, as is well known to researchers engaged in studying energy transfer in polymeric matrices. 140 However, theoretical models are beginning to appear that deal specifically with the case of energy transfer to acceptors distributed randomnly around the donor. 14' Consideration has also been given to a special case of energy transfer in which two identical donors D are covalently attached to a central target molecule so as to form tripartite molecules of the type: D-T-D.142The supermolecule is so designed that energy transfer from donor to target does not take place. At high light intensity, however, it becomes possible to simultaneously excite both the donor groups, thereby opening up the possibility for a two-photon excitation energy-transfer process. This is an interesting example of non-linear optical effects but it demands careful design of the system, especially if reverse energy transfer from the excited target molecule is to be avoided. Electronic energy transfer from an excited state donor to a suitable acceptor is a fundamental process in photochemistry and is of prime importance in artificial photosynthesis. A variety of mechanisms exist by which energy migration and transfer can take place143and such processes have been used to create the socalled 'antenna effect'. In particular, much research has concentrated on designing molecular systems in which an organic host is used to bind a lanthanide cation in such a way that photoexcitation of the host results in intramolecular energy transfer to the bound cation. This provides a simple means by which to offset the poor absorption spectral properties of the lanthanide and to obtain a long-lived, emissive state in solution, Clearly, there are marked similarities with the light-harvesting complexes used by Nature to collect and direct sunlight to the reaction centre complex. Other guest mo!ecules can be considered.'45 Electronic energy transfer has been described for bichromophoric molecules dispersed in micelles, along polymer chains, 147-149 in sol-gel glasses,150and in supersonic jets.I5' In order to mimic electronic energy transfer from the light-harvesting complex to the special pair of the reaction centre complex, a variety of bichromophoric molecules have been synthesized in which terminal donor and acceptor units are separated by a transparent spacer group. Provided the geometry of the dyad is known, especially the separation distance and mutual orientation, it becomes possible to consider the rates of intramolecular energy transfer in terms of
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theoretical models. With a long spacer group that holds the chromophores at remote sites there will be little electronic interaction between the reactants but energy transfer can proceed via a through-space mechanism. A fine example of such a system is provided by attaching naphthalene and protoporphyrin residues to short linear peptides. Excitation into absorption transitions localized on the naphthalene residue is followed by singlet energy transfer to the porphyrin bound further along the peptide on the time-scale of a few nanoseconds. The rate of transfer depends on the helical periodicity of the peptide, which exists as an a-helix, due to an orientation effect. The same chromophores bound to poly(Llysine) dissolved in water undergo light-induced electron transfer from porphyrin to the excited singlet state of naphthalene. The difference in the two systems is attributed to changes in molecular topology. If the chain is flexible, such that the two chromophores can come into orbital contact, there are difficulties to separate energy transfer from excimer f o r ~ n a t i o n ~ and ’ ~ ~ a’ ~discussion ~ of the transfer mechanism becomes hazardous. Excitation of the excimer can induce a chargetransfer reaction leading to formation of a closely-coupled radical pair’” that bears some resemblance to the special pair found in bacterial reaction centre complexes. Singlet-singlet energy transfer has been studied in covalently-bonded tetrapyrrolic-based heterodimers.i5671’7 It is necessary to ensure that the two chromophores are not sufficiently coupled to create a single Ir-electron system but, at the same time, the separation must be kept reasonably short for energy transfer to be fast and quantitative. Unlike electron transfer, the rate of singlet-singlet energy transfer is not much affected by temperature and usually proceeds in a low temperature glass. ’6 Particularly relevant models for artificial photosynthesis are those comprising a carotene residue closely bound to a tetrapyrrolic pigment and such systems are known to demonstrate fast energy transfer to the first excited singlet state of the porphyrin.’’* The short lifetime of the excited singlet state of the carotene demands that the reactants are closely spaced or maintained in strong electronic communication. In turn, this means that several reaction mechanisms might contribute to the transfer process. Related molecular triads have been synthesized having a carotenoid-porphyrin-pyropheophorbidebackbone’’9 for which triplet energy transfer occurs from the terminal pyropheophorbide to the carotene by way of the triplet state localized on the interspersed porphyrin residue. Such systems provide excellent examples for a cascade of energy-transfer events since the energy levels of each intermediate state can be determined and the overall process can be followed by time-resolved transient spectroscopy. Other examples of energy transfer between organic molecules have appeared recently; including studies made for molecules trapped inside zeolitesi6’ and for crystalline salts. Such studies have included both dyads’62 and triads. 163 Forster-type dipole-dipole energy transfer has been resolved for individual sublevels of the first excited triplet state of diphenylcarbene in a low temperature glass.’@ In this latter case, the rates of triplet energy transfer to organic dyes, such as Rhodamine 6G, are quite different according to which sublevel of the donor triplet is responsible for transfer.
’’*
’
V: Artificial Photosynthesis
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Triplet-triplet energy transfer has been demonstrated for dyads in which a ruthenium(I1) tris(diimine) complex is covalently bound to an aromatic hydrocarbon, such as pyrene,l6' naphthalene,'66 or anthracene.lM In the case with pyrene as the energy acceptor, the hydrocarbon is attached close to one of the coordinated diimine ligands giving conformationally constrained complexes. It is proposed that electronic coupling between the two units remains weak, despite their close proximity, and triplet energy transfer is slower than nonradiative deactivation of the triplet state of the metal complex. The naphthalene- and anthracene-based systems are more flexible and the photophysical properties depend on the relative triplet energies. Fluorescence from the pendant aromatic units is completely quenched due to fast singlet-singlet energy transfer to the metal complex. For the naphthyl derivative, the lowest-energy triplet state is localized on the metal complex whereas in the anthryl system the lowest-energy triplet state is localized on the aromatic hydrocarbon. The same system with pyrene as energy acceptor gives rise to dual exponential decay profiles at 77 K and the two triplets are essentially isoelectronic. Numerous systems have been described for which both the donor and the acceptor are metal complexes and for which intramolecular triplet energy transfer can be very fast. Thus, intramolecular energy transfer occurs between ruthenium(I1) tris(diimine)-based components which differ with respect to their mutual energy p ~ s i t i o n i n g . ' ~ ~ Related -l~~ systems have used r h e n i ~ r n ( I ) , ' ~os~ rnium(II), 171-174chr~mium(III),'~~ f e r r ~ c e n e , or ' ~ ~a metall~porphyrin'~~ as the energy acceptor. The strength of these studies is that they allow exploration of the effect of the connecting linkage on the rate of through-bond electron exchange. ifxi
2.3 Fullerene-based Molecular Systems - Since their conception and popularization, the various types of fullerenes have caught the imagination of researchers involved in many areas of chemistry and materials science. These compounds exhibit a rich and varied photochemistry and they have been used in numerous artificial photosynthetic units. Fullerenes are readily reduced and can function as electron acceptors in photoinduced electron-transfer reactions where their relatively large volume and spherical shape provide interesting alternatives to the more commonly used acceptors. In fact, comparison of the properties of Cm and benzo- 1,4-quinone as electron acceptors in covalently-linked dyads with a porphyrin playing the dual roles of the chromophore and the donor has concluded that the porphyrin-fullerene dyad shows accelerated charge separation and attenuated charge recombinati~n.~'~ This is a very important finding, made with dyads of comparable separation distance and thermodynamic factors, that alludes to possible future applications for novel fullerene-based photosystems. The most likely explanation for the improved performance of the porphyrinfullerene dyad relative to its more classical counterpart is that the fullerene possesses a substantially smaller reorganization energy. This would cause charge separation to fall closer to the apex of a Marcus rate vs. energy gap plot whilst pushing subsequent charge recombination deeper into the inverted region. Numerous studies have been concerned with refining our level of under-
438
Photochemistry
standing about the basic photophysical properties of c60, C70, and, to a lesser extent, C76 in fluid ~ o l u t i o n . ' ~ ~Despite - ' ~ ~ the enormous interest in these materials, there are still several unresolved issues that need to be clarified before full agreement is reached regarding the photophysics of the isolated compounds. Similarly, attention has been given to simple derivatives that are more soluble in polar solvents. 185-189The photophysical properties of fullerenes bound to solid surface^^^^*'^' and entrapped within zeolitic cages"' have also received attention. The photochemical reduction of fullerenes has been investigated in solution by several research groups using a wide range of electron donors. The motivation for many of these studies is derived by the realization that there are many important similarities in the photochemistry of fullerenes and aromatic carbonyl compounds, especially the high triplet yield and ease of reduction of the corresponding triplet state. 193 Fullerenes are readily reduced to the x-radical anion'94 and the x - d i a n i ~ n . ' ~Rates ~ . ' ~and ~ yields of photoreduction have been measured by laser flash photolysis methods for several bimolecular reactions initiated by triplet fullerene.197-203 and for complexes formed between C70 and ternary amines in nonpolar solvents.2w Similar studies have reported fluorescence from welldefined fullerene complexes in the solid state.'05 The complex formed by binding bis(triphenylphosphite)platinum(O) to buckminsterfullerene - C ~ O [ P ( O P ~-) ~ ] ~ exhibits a long-wavelength metal-to-ligand, charge-transfer transition centred around 770 nm.*06 Illumination into this absorption band causes facile dissociation of the complex, enabling the products to be trapped by added substrates. Photoinduced charge-transfer reactions of various fullerenes have been explored in ad duct^,^'^ polymers,208-2"bilayers,'12 and the solid Illumination of fullerene-bridge-donor systems, where the donor is either a ruthenium(I1) tris(2,2'-bipyridine) derivative2I5or a functionalized f e r r o ~ e n e , ~ ' ~ ~ ' ~ results in fast charge separation due to electron transfer to the appended fullerene. In the latter system, the influence of the rate of charge separation has been followed as a function of solvent polarity and the nature of the connecting It has been found that the reaction pathway, that is to say throughbond or through-space interactions, depends on the structure of the bridge. On the basis of flash photolysis studies it was concluded that electron transfer originated from the excited singlet state of the fullerene. Particular attention has been given to understanding the photoreactions of fullerene-based donor-acceptor dyads having a porphyrin as chromophore. Such systems closely resemble the well-studied porphyrin-quinone dyads with the fullerene operating as an electron acceptor. In these molecular dyads, light-induced electron transfer occurs from the excited singlet state of the porphyrin to the bound fullerene, followed by slower charge recombination.220-223 The rate of electron transfer depends on the type of bridging unit and on the polarity of the solvent, while the yield of the charge-separated state can approach unity. Interestingly, direct excitation into the fullerene chromophore generates the corresponding excited singlet state which undergoes rapid intersystem crossing to the triplet manifold.222The triplet state localized on the fullerene can transfer excitation energy to the appended porphyrin. These results confirm the fact that fullerenes are important modules for the construction of organized multicomponent arrays.
V: Artijcial Photosynthesis
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In fact, the work has already been extended to produce fullerene-porphyrincarotene triad^.^^^*^*' Here, the first-excited singlet state of the porphyrin, produced under illumination, transfers an electron to the fullerene before the carotene reduces the resultant x-radical cation of the porphyrin. The ultimate charge-separated state is formed with a quantum yield of ca. 0.14 in 2-methyltetrahydrofuran and decays with a lifetime of ca. 170 ns by way of populating the triplet excited state of the carotene. Remarkably, charge separation still occurs in a frozen glass and, again, decays to form the triplet excited state of the carotene. Such behaviour is rare among model systems, especially the ability to operate at low temperature in the solid state, and closely mimics the natural photosynthetic unit.
3
Environmental Effects on Rates of Electron Transfer
The importance of the protein matrix for construction and working of the photosynthetic reaction centre complex should not be overlooked. The protein provides an integral 3-dimensional arrangement able to fix exactly the position of each component in the array, to control the local environment, and to perturb the energy levels of certain pigments by coordination. It provides much more than a mere scaffold on which to assemble the reaction centre complex. Perhaps the most important aspect of the protein matrix, and certainly the most difficult to mimic with model systems, concerns the ease with which the local polarity can be changed. This is done simply by selecting the type of amino acid to position next to a particular co-factor and this simple feat is extremely difficult to engineer in model systems. Almost as important is the realization that the solid-state structure is characterized by having an uncommonly small reorganization energy that hardly changes with temperature. As a consequence, light-induced charge separation requires only a small thermodynamic driving force to sit at the apex of a Marcus rate vs. energy gap profile. The rate of electron transfer, therefore, is fast and almost activationless. In contrast, charge recombination is certain to fall deep into the Marcus inverted region where the rate will be decreased. By transferring electrons across a membrane, the natural photosynthetic unit adds a further complication to challenge the imagination of mortal researchers. These subtleties have to be incorporated into model systems if they are to be effective and if they are to truly mimic the natural system. There have been many reports of how the nature of the surrounding solvent22" 229 or the temperat~re*~'-~~' affects the rate of electron transfer in solution. Mostly these variations modify the thermodynamic parameters, especially the solvent reorganization energy, and can be well understood in terms of current theoretical models. The importance of the solvent polarity increases for throughspace interactions while the viscosity has to be considered when the dyad is linked via a flexible connecting chain. In all cases, the free energy change accompanying electron transfer has to be known with some accuracy if comparison between different systems is to be made in a meaningful manner. An extreme example of how the medium affects the rate of light-induced electron transfer is provided by
Photochemistry
440
the system in which a ruthenium(I1) tris(diimine) complex, used as electron donor, is coupled to a dinitrobenzene acceptor by way of a ‘salt bridge’. It is rep~rted’~’ that the rate of electron transfer is ca. 100-fold slower when the system comprises D-(amidinium-carboxy1ate)-A compared with D-(carboxylateamidinium)-A. This is a remarkable kinetic difference that could be exploited to design advanced artificial photosynthetic units. In order to more closely resemble the membrane-bound, protein-encased reaction centre complex, several model systems have been studied in microheterogeneous media. Thus, the reaction sequence that follows from laser excitation of a donor-acceptor-acceptor triad has been followed in liquid crystalline media,233 where the significance of charge recombination to form the triplet excited state of the terminal acceptor is stressed. Vectorial electron transfer and subsequent recombination have been investigated in organized Langmuir-Blodget b i l a y e r ~ . ’ ~ ~ In this system, the donor was covalently attached to the hydrophobic chains of the phospholipid whilst the acceptor was adsorbed at the polar surface. Other studies have considered electron-transfer reactions occurring in m i ~ e l l e s , ’ ~ ~ vesicles, 236 and dendrirner~.~~’ Although extremely difficult, with regard to both the experiment and the underlying theory, electron transfer across an immiscible liquid-liquid 238*239or p~lymer-liquid~~’ interface is under active consideration. The solid-state structure of the photosynthetic reaction centre complexes has inspired several studies of light-induced electron transfer in solid media. A particularly useful medium is provided by porous glass241-243which facilitates rapid electron-transfer reactions without the involvement of polar solvents. Solid matrices suitable for light-induced electron-transfer processes are also provided by ~ i l i c a , ~ ~ z e o l i t e s , ~clays.246 ~ ~ a n d A theoretical description has been reported for dealing with the distribution of separation distances between donor and acceptor that is often found in the solid state.247 A long-held ambition is to selectively control the rates of electron transfer by external perturbation of the system. Continued attempts to engineer appropriate systems have used strong magnetic248P252 or fields to provide the perturbation. Quite pronounced electric field effects have been reported, although a detailed interpretation is difficult.
4
Energy Transfer in Pigment Clusters
Photosynthetic organisms make use of light-harvesting complexes to collect incident sunlight and, via a series of energy-migration steps, to direct absorbed photons to the special pair of the reaction centre complex. As such, an important feature of artificial photosynthesis concerns building a light-harvesting antenna into the model system. Some success has been achieved in this area, although the number of chromophores included in the antenna is very much restricted. Following the course of energy migration among identical pigments, which usually occurs by random motion throughout the array, is a severe experimental challenge, especially in solution where rotation takes place. Even so, attention is slowly turning to this area and model systems are beginning to appear.
V: ArtiJicial Photosynthesis
44 1
Research is proceeding along two directions. Firstly, synthetic systems are being developed which are intended to interconnect a series of chromophores in a well-defined pattern, often linear, so as to form an organized array of pigments. Particular attention is given to porphyrin-based c h r o m o p h ~ r e but s ~ ~other ~~~~ systems, such as phycocyanins260or simple aromatic hydrocarbons,261are under investigation. Energy migration can occur throughout the array, usually by way of Forster coulombic interactions, with the rate being determined by the nature of the spacer group used to hold individual pigments in place. To be useful such systems must be rigid and equipped with a shallow trap for the absorbed photons. Secondly, light-absorbing anntenae can be constructed by attaching numerous chromophores to a platform, such as that provided by a macrocyclic receptor. The chromophores direct absorbed photons to a guest bound at the macrocycle and can also enter into energy-migration processes in the absence of a trap. Many such systems have been described during the past year.262-268 A consequence of positioning several identical chromophores in close proximity is that multiphoton events can be expected at high incident light intensities. Such processes cause exciton annihilation to compete with energy transfer to the trap and are an important feature of the natural photosynthetic apparatus. Little attention has been given to these reactions in artificial photosynthetic model systems but the application of exciton annihilation to measure properties of molecular aggregates and natural light-harvesting complexes has been reviewed.269
5
Miscellaneous Photosystems
Most artificial photosynthetic model systems are designed such that light-induced electron transfer occurs via a through-bond, or superexchange, mechanism because this allows for greater control over the individual rates of transfer. This control, or fine-tuning, is achieved by modulating the extent of orbital interaction between adjacent components that make up the array. Several attempts have been made to build molecular systems in which a conjugated spacer group is used to actively promote through-bond electron exchange between redox-active termi n a l ~ . ~ ~Other ' ~ ~ ~systems ' have sought to modify the rate of intramolecular electron transfer by complexation of ~ a t i o n s * ~ ~neutral *~~~ organic o r species274to one of the reactants so as to modify its relative electron density Oxidation or reduction of one of the components in a linear array of redoxactive units can affect the ability of the supermolecule to undergo vectorial transfer of energy or electrons. A particularly clear example of this concept utilizes a B-dipyrromethene dye, as the light absorber, covalently-linked to a series of porphyrins arranged according to their singlet energy levels.275Thus, photons collected by the B-dipyrromethene dye are directed first to an adjacent zinc porphyrin, then to an interspersed magnesium porphyrin, and finally to a terminal free-base porphyrin. The system is so well constructed that essentially all the photons absorbed by the B-dipyrromethene dye are transferred to the freebase porphyrin, which represents the trap. However, selective oxidation of the
442
Photochemistry
magnesium porphyrin, which can occur without damage to the other components, decreases the efficacy of the transfer process. Reduction of the oxidized magnesium porphyrin restores the system to its original state.
Conclusions
6
The state-of-the-art has progressed over the past decade to such an extent that it is now difficult to make rapid progress. Theoretical models, although far from perfect, give an adequate representation of experimental data and permit predictions to be made with reasonable accuracy. There are still severe difficulties associated with understanding the electron-transfer reactions occurring within intimately-linked donor-acceptor complexes and for calculating reorganization energies. Even so, Marcus theory provides the necessary foundation from which the synthetic chemist can launch new and advanced prototypes. Such molecular systems, which need to provide for a cascade of energy- and/or electron-transfer steps along the molecular axis, become more sophisticated and complex as they seek to better match the natural reaction centre complex. As more components are integrated into the supermolecule it becomes important to distinguish between competitive processes and this requires precise information regarding the positioning of individual components. It has also been realized that each active component in the supermolecule must possess a characteristic signature by which to monitor its entry in the overall transfer process. The main challenge to come concerns incorporating highly-sophisticated supermolecules into an organized macroscopic structure. Replacing the protein with a polar solvent is no longer acceptable and ‘artificial proteins’ have to be designed. Whether this can be done by way of supramolecular chemistry involving noncovalent association of components remains an unknown factor. Little attempt has been made to span a membrane with a photosynthetic model system, although some models have the necessary length, and this is essential. The principles involved in generating a long-lived, charge-separated state under illumination with visible light have been more-or-less established and to construct an appropriate supermolecule is simply a matter of finding the required funding. The intellectual challenge has passed. Using this charge-separated state in a constructive way is a quite different problem and will begin to dominate the subject in the coming years. To position and protect a supermolecule inside an organized network is not a simple task. Clearly, the field of artificial photosynthesis will merge with molecular electronics since the goals are becoming less distinct.
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W. Jaeger, S. Schneider, and J. W. Verhoeven, Chem. Phys. Lett., 1997,270,50. K. Hara, D. S. Bulgarevich, and 0. Kajimoto, J. Chem. Phys., 1996, 104,9431. M. B. Zimmt, Chimia, 1997,51,82. C. Burgdorff, H.-G. Loehmannroeben, and T. Sander, J. Chem. SOC., Faraday Trans., 1996,92,3043. D. Wiesenfeld, M. Bachrach, T. M. McCleskey, M. G. Hill, H. B. Gray, and J. R. Winkler, J. Phys. Chem. B, 1997,101,8823. J. P. Kirby, J. A. Roberts, and D. G. Nocera, J. Am. Chem. SOC.,1997,119,9230. K. Hasharoni, H. Levanon, S. R. Greenfield, D. J. Gosztola, W. A. Svec, and M. R. Wasielewski, J. Am. Chem SOC.,1996,118, 10228. G. Caminati, G. Gabrielli, R. Ricceri, C. Turro, and N. J. Turro, Thin Solid Films, 1996,284,7 18. H.Aota, S. Araki, Y. Moriscima, and M. Kamachi, Macromolecules, 1997,30,4090. J. Otsuki, N. Okuda, T. Amamiya, H. Araki, and M. Seno, Chem. Commun., 1997, 31 1. R. Sadamoto, N. Tomioka, and T. Aida, J. Am. Chem. SOC.,1996,118,1978. M. K. DeArmond and A. H. DeArmond, Liquid-Liquid Interfaces, eds. A. G. Volkov and D. W. Deaner, CRC, Boca Raton, FL, 1996, p. 255. R. A. W. Dryfe, Z. Ding, R. G. Wellington, P. F. Brevet, A. M. Kurnetzov, and H. H. Girault, J. Phys. Chem. A , 1997,101,2519. A. Eckert, T. Martin, T. Cao, and S. E. Webber, Brookhaven Natl. Lab; [Rep. J BNL, 1995,61733,89. S . Kotani, H. Miyasaka, A. Itaya, Y. Hamanaka, N. Mataga, S. Nakajima, and A. Osuka, Chem. Phys. Lett., 1997,269,274. H. Miyasaka, S. Kotani, A. Itaya, G. Schweitzer, F. C. De Schryver, and N. Mataga, J. Phys. Chem. B, 1997,101,7978. B. W. Pfennig, P. Chen, and T. J. Meyer, Inorg. Chem., 1996,35,2898. A. M. Eremenko, V. P. Kondilenjo, 0. I. Lyaksutova, N. P. Smirnova, I. G. Tarasov, T. A. Kikteva, and V. M. Ogenko, Proc. - Indian Acad. Sci. Chem. Sci., 1995,107,779. M. Sykora and J. R. Kincaid, Nature, 1997,287, 162. R. A. Schoonheydt, ‘Comprehensive Supramolecular Chemistry’, eds. G. Alberti and T. Bein, Elsevier, Oxford, 1996, p. 337.. E. B. Krissinel and S. I. Mackarchuck, Chem. Phys., 1996,208,259. N. Kh. Petrov, V. K. Borisenko, M. V. Alfinov, T. Fiebig, and H. Staerk, J. Phys. Chem., 1996,dOO, 6368. K. Nishuawa, Y. Sakaguchi, H. Hayashi, H. Abe, and G. Kido, Chem. Phys. Lett., 1997,267,501. U. Till and P. J. Hare, Mol. Phys., 1997, 90,289. Y. Fujiwara, T. Aoki, K. Yoda, H. Cao, M. Mukai, T. Haino, Y. Fukazawa, and Y. Tanimoto, Chem. Phys. Lett., 1996,259,361. H. Cao, Y.Fujiwara, T. Haino, Y. Fukazawa, C.-H. Tung, and Y. Tanimoto, Bull. Chem. Soc. Jpn., 1996,69,2801. N. Ohta, M. Koizumi, S. Umeuchi, Y.Nishimura, and I. Yamazaki, J. Phys. Chem., 1996,100,16466. N. Ohta, M. Koizumi, Y. Nishimura, I. Yamazaki, and Y. Hatano, J. Phys. Chem., 1996,100, 19295. E. Galoppini and M. A. Fox, J. Am. Chem. Soc., 1996,118,2299. A. Otsuhiro and H. Shimidzu, Angew. Chem, Int. Ed. Engl., 1997,36, 135. T. Shimidzu, Synth. Met., 1996,81,235.
452
Photochemistry
258.
K. Susumu, T. Shimidzu, K. Tanaka, and H. Segawa, Tetrahedron Lett., 1996,37,
259.
260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271, 272. 273. 274. 275.
8399. J.-S. Hsiao, B. P. Krueger, R. W. Wagner, T. E. Johnson, J. K. Delaney, D. C. Mauzerall, G. R. Fleming, J. S. Lindsey, D. F. Bocian, and R. J. Donohue, J. Am. Chem. SOC.,1996,118, 11181. R. E. Ritter, M. D. Edlington, and W. F. Beck, J. Phys. Chem. B, 1997,201,2366. W. Li and M. A. Fox, J. Am. Chem. SOC.,1996,218, 11752. X . Song, C. Geiger, S. Vaday, J. Perlstein, and D. G. Whitten, J. Photochem. Photobiol. A, 1996,102, 39. M. R. Shortreed, Z.-Y. Shi, and R. Kopelman, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1996,283,95. S . Pellet-Rostaing, J.-B. Regnouf-de-Vains, and R. Lemartine, Tetrahedron Lett., 1996,37,5889. A. Beeby, D. Parker, and J. A. G. Williams, J. Chem. SOC.,Perkin Trans. 2, 1996, 1565. M. R. Shortreed, S. F. Swallen, Z.-Y. Shi, W. Tan, 2. Xu, C. Devadoss, J. S. Moore, andR. Kopelman, J. Phys. Chem. B, 1997,101,6318. H. E. Toma and T. E. Chavez-Gil, Inorg. Chim. Acta, 1997,257, 197. L. Jullien, J. Canceilli, B. Valeur, E. Bardez, J.-P. Lefevre, J.-M. Lehn, V. MarchiArtzner, and R. Pansu, J. Am. Chem. SOC.,1996,118,5432. V. Sundstroem, J. Aggregates, 1996, 199. V. Grosshenny, A. Harriman, F. M. Romero, and R. Ziessel, J. Phys. Chem., 1996, 100,17472. R. B. Tykwinski, M. Schreiber, R. Carlos Perez, F. Diederich, and V. Gramlich, Helv. Chim. Acta, 1996, 79, 2249. S. Delmond, J.-F. Letard, R. Lapouyarde, R. Mathevet, G. Jonusauskas, and C. Rulliere, New J. Chem., 1996,20, 861. G . Jonusauskas, R. Lapouyarde, S. Delmond, J.-F. Letard, and C. Rulliere, J. Chim. Phys. Phys.-Chim. Biol.,1996,93,1670. F. D’Souza, G. Deviprasad, and Y.-Y. Hsieh, Chem. Commun., 1997,533. R. W. Wagner, J. S. Lindsey, J. Seth, V. Palaniappen, and D. F. Bocian, J. Am. Chem. Sac., 1996,118,3996.
Author Index
In this index the number in parenthesis is the Part and, whoi appropriate, the Chapter nunrber of the citation nnd this isfollowed by the refirence number or numbers ofthe relevant citalions, c.g., (2.2) 137 represenls Part 11, Chapter 2, ReJkrence 137. Aakcrmark, B. (1) 355;(5) 23,24, 58,97 Aaron, J.-J. (2.3)60;(3)532;(4) 17 Aarlsma, T.J. (5) 46 Abdie, M.J.M. (3) 102 Abbalc, G.(1) 163 Abbot, J. (2.5)162;(3)814 Abbot, S.C. (2.7)169 Abdclaal, M.Y.(3) 193 A M cl-Aziz, A.S. (2.7)200 Abdcl-Bq, E.M.(3) 187 Abdcl Fattah, A.M. (2.5)210 Abdcllah, L.(3)203 Abdcl Motti, F. (2.5)210 Abdel-Razik, E.A. (3) 187,193 Abdclrazzaq, F.B. (3) 58 Abdcl-Salm, N.M. (3) 187 Abdou, M.S.A. (3)5 15 Abc, A. (3)576 Abc, H.(1) 598;(5) 249 Abe, K.(1) 267,496;(3)794 Abc, M.(2.5)137;(2.6)3 12 Abc, S.(2.6)204;(2.7)71 Abc, T.(2)36 Abc, Y.(2.7)64;(3) 488 Abcl, Y.(1) 344;(5) 90 Abcll, C. (2)59 Abiblad, G.(3)667 Abidnejad, M. (3) 370,438 Abonia, R (2.4)204;(2.6)197 Abraha, P.A. (3) 218,220,221 Abraham, W.(2.4)243 Abuin, E.B.(1) 473 Abuwrova, G.R.(2.5)116 Achim, C.(1) 78;(5) 65 Acrce, W.E., Jr. (1) 480
Adachi, G.-y. (4)44 Adam, W. (2.3)89,91;(2.5)16, 75, 114,183;(2.6) 188,191; (2.7)17,18 Adamczak, E.(3)175 Adamowicz, L.(1) 238;(2.4)58 Adams, J.L.(2.4)274 Adclinc, M.-T. (2.6)134 Adcnair, A. (1) 188 Admasu, A. (2.7)41 Aciyach, S.(3) 532 Agbaria, RA. (1) 533;(3)414 Agosta, W.C.(2.2)45;(2.6)85,86 Agostini, G. (3) 599 Aguilar, G. (3)594 Ahmad, C.M. (3)792 Ahmcd, M.(2.7)108 A h , J.S. (1) 522 Aida, T.(5) 237 Ainbund, M.R.(1) 583 Airinci, A. (2.7)GO Ajayaghosh, A. (2.6)226;(2.7) I64 Akagi, T.(3) 526 Akai, T.( I ) 562 Akano, T.(4)62 Akhmadccv, R G . (3) 581 Alihymbetova, G.D.(3)64 Akinioto, S. (1) 262,267,584; (2.4)125;(2.6)120 Akimoto, Y.(1) 120 Akiiis, D.L.(1) 166, 167 Akila, Y.(2.4)3 1; (3)263,465 Akiyama, H. (2.4)31;(3) 146, 26 1,263,465 Akiyania, T. (1) 456,463;(2.5) 88;(5) 178,220,221
453
Akkara, J. (3)4 17,423 Alin, A. (3)438 Alam, M. (1) 137;(2.7)180;(5) 199 AIami,E.(3)411,412 Al-Ansari, I.A.Z. (1) 226 Albcrs, R.(2.7)45 Albcrt, A. (2.5) 72 Albini, A. (2)6;(2.3) 16;(2.4) 158;(2.5)6, 117, 118,203; (2.6)5,157,210,300;(2.7) 186 Albrccht, E. (2.4)216 Aldoshin, S.M. (2.4)69,83,87; (2.6)49 Alcjski, K.(2.6)220 Alfimov, M.V.(1) 264,597;(2.4) 17,110,220;(2.6)19;(3) 135, 136,580;(5) 248 Alfonso, A.M. (1) 255 Ali, M.M. (3) 193 Al-Jalal, N.A. (2.4)96;(2.6)138 Al-Kindy, S. (1) 505 Allacv, Zh. (3) 64 Allcn, N.S.(2.6)304;(3) 1,44,94, 95,688,716,846,876,880 Allonas, X.(1) 53,82;(5) 81 Al-Malaika, S. (3)717,718,869 Almcida, Y.M.B.(3) 102 Almgrcn, M.(1) 471;(3)41 I, 412; (5)23,24,58 Aloisi, G.G. (1) 192,209,287; (2.4)26 Altomarc, A. (3) 489 Alvaro, M. (2.5)70 Aly, A.S. (2.5)210 Aly, H.A.H. (2.5)210
454 Aniada, 1. (2.2) 113;(2.5)32,34 Amadclli, R. (3)758 Amalric, L.(2.5)147 Amamiya, T.(I) 359;(5) 236 Amao, Y.(2.5)68;(5) 83 Amaroli, N.(5) 104,170,173,
174,177 Ambrosc, W.P. (1) 38, 105, 108,
112 Amebot, M. (1) 641,642 Amcrz, J. (5)46 Amcta, R. (4)8 Amcta, S.C.(4) 8 Aminc, M.S. (2)48 Amo, J.F.R (2.5)182 Amougay, A. (2.2)27;(2.6)113 Anancva, T.D. (3) 424 Andcrs, C.(1) 342;(2.5)40;(5)
88 Andcrson, D.G. (2.6)305;(3)96,
98 Andcrson, J.L. (2.5)107 Andcrson, J.T. (2.6)133 Andcrson, M.A. (2.5)141;(2.6)
195;(2.7)16 Andersson, H.(3) 347 Andcrsson, M. (5)23,24 Andino, J.M. (2.5) 140 Ando, W. (2.5)121;(2.6)322 Andrade, C.T. (3)84 Andrady, A.L. (3)702,734 Andrawcs, F.F.(3)867 Andrc, J.C. (3)370,558 Andrcis, C.(2.6)54 Andrcopoulos, F.M. (3)340 Andrcw, D.(2.2)9 Andrcws, J.C. (5) 19 Andrews, M.C. (3) 375 Andrzcjcwska, E.(3)38,328 Angeloni, L.(1) 193;(2.4)2 I Angiolini, L.(3)3 Angioni, A. (2.6)21 1 Anylo-Sanchcx, J.L. (3) 854 Ania, F. (3)347 Anikcev, A.V. (3) 627 Anisimov, S.(3) 782 Anhcr-Mylon, S.(1) 10; (5) 44 AnLy, H.(2.5)99 Annapoorani, R. (2.5)209 AAon, E. (2.6)70 Ansclh, K.S. (3)235.3 1 1 Antic, D. (2.6)329;(2.7)69 Antonictti, M. (2.7)27 Antoulinakis, E.G. (2.2)75 Anufricva, E.V. (3)424 Ao, S.(3) 824 Aoe, K. (2.2)50,67;(2.6)127
Photochemistry Adri, H.(3)271 Aoki, M. (1) 463,465;(2.5)87,
88;(3)783;(5) 178,220,221 Aoki, S.(2.2)49;(2.4)4I Aoki, T. (1) 600;(2.5)218;(5) 25 1 Aoshima, S. (3) 601 Aota, H.(1) 299,301;(5)235 Aoudia, M. (2.5)104 Aoyama, K. (4)70 APP, M. ( 5 ) 18 Arai, N.(2.6)283 Arai, S.(2.4)201 Arai, T.(1) 169,201,202;(2.2)
Asano, M. (3)303,357 Ashikhmin, M.V. (2.7)99 Ashok, A.K.(3)538 Ashton, P.R. (1) 356,555;(2.2)
58-60;(2.6)87,88,245;(5) I00 Askilrov, M.A. (3) 64 Askdin, C.-P. (2.6)214 Asmus, K.-D. (1) 418,441,442,
444;(2.5)3,83,84;(5) 186, 187,189 Asokan, C.V. (2.3)25;(2.4)42 Aspari, P. (1)289;(2.5)23 A s p , A. (1) 473 39;(2.4)27,35;(2.5)63,73; Aspncs, D.E.(1) 627 Atabckyan, L.S. (2.4)73 (2.6)20-22,27,221 Arai, Y.(3)224;(4)45 Atalla, A.A. (2.6)286 Arakanc, K.( I ) 424;(2.5)106 Atasbcliyan, L.S. (2.6)44 Arakawa, H.(4)24,29 Athcrlon, S.J. (2.6)218;(5) 169 Arakc, Y.(2.6)79 Atkinson, H.(2.6)51 Araki, H.(1) 359;(5) 236 Atvus, T.D.Z. (3) 596 Araki, K.(1) 354;(5)96 Au, G.A. (2.3)13 Araki, S.(1) 301;(5) 235 Aubard, J. (2.6)48 Ariunakc, S.(2.6)5 I Aubv, J.-M. (2.5)I12 Aramcndia, P.F. (1) 296 Audicrc, J.-P. ( I ) 560 Aranyosi, P. (3)89 1 Audouin, L. (3) 680, 681,730, 73 1 Arathi, R.S.(1) 170 Archiba, Y.(1) 430 Augustyniak, w. (1) 48 Arct, J. (3)78 1 Ausnian, K.D. (1) 4 17 Argaui, R. (I) 360;(5) 102 Austin, B.C.N. (4)46 Argyropoulos, D. (3)8I2 Austin, M.A. (2.3)15, 19;(2.5) Arimitsu, K. (3)237 199;(2.6)96 Arimura, J. (2.4)188 Avar, L. (3) 107 Arimwa, T.(1) 19,312;(2.5)173; Avci, D. (3)33, 120,314 (5) 38, 107 Avclinc, B.M. (2.6)256,257;(2.7) Arisc, T.(3) 90 182,183 Aristov, A.V. (1) 204 Avcrdung, J. (2.5)124;(2.7)86 Arlihangclski, I.V. (5)190 Avlascvich, YuS. (1) 341 Arkin, M.R. (1) 5 12 Avramcnko, L.F. (2.7)93 Armand, X.(2.4)105 Awazu, K.(2.3)2 Armaroli, N.(1)362,556 Ayala, J.H. (1) 255 Anncnlfout, R.S.(3)547 Aydcmir, A. (4)67 Armcsto, D. (2.2)93;(2.3)15,23, Ayyagari, M.(3)423 24,43,44;(2.4)52;(2.5)9, Ayyangar, RN.(2)50 199;(2.6)96,97,146,154 Azaroual, N.(2.5)1 12 Amlitage, B. (2.2)114 Azuma, C. (3)84 Armstrong, D.(1) 573 Arnaud, N. (1) 647 Amaud, R. (2.6)219;(3)717,718 Baba, A.I. (5) 169 Arnold, D.R. (2.3)17, 18,76,78; Baba, M. (1) 590 (2.4)157;(2.6)156,158;(2.7) Babu, B.S. (3) 13 1 209 Bach, H.(1) 107 Amold, N. (3) 782 Bach, RD. (2.3)58 Arsu, N.(3) 100, 185 Bach, T.(2)22 Artcnicv, N.( I ) 4 I3 Bachilo. S.M. (1) 341 Artyukhov, V. (1) 375;(5) 153 Bacluach, M. (1) 358,361;(5) 103,231 Asada, Y.(4)3,70
Author Index Baciocchi, E. (2.6)293 Back, T.G. (2)63 Badr, M.Z.A. (2.6)286 Back, E.K. (2.4)95;(2.6)72 Bacr, T.(2.7)2 Baerends, E.J. (2.7) 124 Bacsslcr, H. (3)608,609,644,654 Bacucrlc, D.(3)782 Bacurich, Ch. (3)798 Bacycns, W.R.G. (2.4)97 Baczold, M. (3)403 Bagchi, S. (1) 156 Bagnich, S.A. (1) 259 Bagii, E.I. (2.3)77 Bagryanskaya, E.G. (1) 260 Bai, F. (1) 241;(3)383-385,402 Bai, Y.(3)582 Baigcnl. D.R.(3) 633,634 Baillct, G.(2.5)103 Baindl, A. (2.7)29,30;(3) 827 Baincs, K.M. (2.6)309;(2.7)170, 171 Bajic, M. (2.4)213 Bakac, A. (2.5)145 Bakcr, T.L.(3)706 Bakkcr, B.H. (2.3)53 Bakshi, P.K. (2.3)17 Balasubramanian, S.(2)59 Baldovi, M.V. (3) 122 Baldvins, J.E. (2.4)238;(2.5)49 Balkan, A. (2.2)60;(2.6)87 Balkc, S.T. (3)550 Ballardini, R (1) 555;(2.6)245; (5) 145 Ballct, P. (1) 640 Bally, T.(2.3)75 Balo, M.C. (1) 470 Balon, M. (2.4) 13 Balzani, V. (I) 17, 18,309,362, 554-556;(2.6)245;(5)33,37, 57, 104,145,172,177 Barnbury, R.E. (3) 70 Bimaghan, F. (2)38;(2.5)48;(2.6) 253 Baiiares, L. (2.7)113 Bandermann, F.(3)51,52 Banejcc, D.(1) 156 Banerjec, S. (2.2)91;(2.4)65;(3) 417,423 Banhanr, P. (3)795 Banks,J.T. (2.7)54 Bansal, A. (2.4)63 Bao, R.(3)545 Barachcvshy, V.A. (2.4) 12 Baralotto, C. (2.4) 124 Baran, A.Z. ( I ) 32 Barashkov, N.N.(3)59,382,508
-,
455 Barbara, P.D.(1) 5 12 Bazan, G.C. (3) 648,650 Barbara, P.F. (1) 5;(2.5)I; (3) Bazhcnov, A.V.(1) 445;(5)205 458;(5)25 Beard, R.E. (3) 191 Barbcra, C. (2.2)32;(2.4)237; Bcarpark, M.J. (1) 179;(2.3)47; (2.6)71 (2.4)122 Barbiou, V. (2.7)60 Bcbcra, P.K. (1) 472 Barche, J. (2.6)285 Beck, K.(2.4)128;(2.6) 125 Barckholtz, T.A. (2.7)126 Bcck, M. (1) 553 Bard, A.J. (2.5)67 Bcck, W.F. (5)260 Bardcz, E. (I) 539;(5)268 Bccker, D. (2.2)23,26 Bardwell, D.A. (5) 170, 174 Bccker, H.-C. (2.4)228 Barigcllcti, F. (1) 362,547;(5) Bcckcr, H.-D. (2.4)228 104,170,172-174,177 Bccker, RS.(1) 196;(2.6) 263 Barklcy, M.D. (1) 227 Bcddard, G.S. (1) 356;(5) 100 Barman, S.P. (3) 55 Bcddoes, RL.(5)22 Barnctt, S.M.(1) 632 Bcdlck, J.M. (1) 157 Barnctt, V. (3)795 Bccb, R (3) 309,310 Baronavski, A.P. (2.4)23;(2.7) Bccby, A. (I) 119,537;(2.5)90; 122,123 (5)265 Barrios, H. (2.2)109 Bccr, A. (3)469,s10 Barrois-Oudin, N. (3) 879 Bchar, D. (2.5)97;(5) 16 Barlberger, M.D. (5) 148 Bchcra, G.B. (1) 472 Barth, K. (1) 618 Bchmcr, M. (3)277 Bartning, H. (3) 837 Bclmisch, J. (3)593 Bartocci, G.(I) 193, 198;(2.4)21 Bclv, H.(2.6)288 Bartolini, P.(1) 163 Bckkcr, C.H.W. (2.6)242 Barton, D.H.R (2)43,44;(2.2) Beletski, N.I. (1) 383;(5) 171 77,78;(2.6)78,80 Belficld, K.D. (3) 58,158,159 Barton, J.K. (1) 510,512,513 Bclik, P. (1) 420;(5) 194 Barlon, T.J. (3)659 Bcllingcr, V. (3)681 Bartroli, J. (3)227 Bcllnicr, D.A. (2.5)104 Baruah, S.D.(3) 1 17 Bcllobono, 1.R (3) 17 Basche, T.(1) 106, I 13 Bcllur, N.S. (2)50,51 Baselga, J. (3) 37 Bclscr, P. (5) 172 Bashir-Hashcmi, A. (2)56 .Bclyakov, V.A. (2.5) 155 Bashtanov, M.E. ( I ) 380;(5) 159 Bcnassar, M.-L. (2.6)63 Basioli, P.(3)844 Bcndcr, J.A. (2.4) 108 Basnak, I. (2.2)58-60;(2.6)87,88 Bcndig, J. (1) 390;(5) 142,163 Bassani, D.M. (1) 509 Benfmmo, N. (3)486 Bcnjclloun, A. (3)438 Bassclls, RE. (3) 115 Bassctt, D.R. (3)324 BcnGo, E.M. (3) 793 Bastic, M.B. (2.6)234 Bcnnasar, M.L. (2)54;(2.4)195 Basu, P. (1) 357;(5) 101 Bcnnctau, B.(2.4)24 Bcimislon, A.C. (1) 14,75,206, Basu, S.( I ) 145 207;(2.5)178;(2.6) 1; (5) 13, Batchelder, D.N. (3) 652 Batchclder, T.L. (5)114 30,73 Batra, R. (2) 19;(2.6)76 Benoist d'Azy, 0. (1) 249 Baucr, C.(2.7)110 Bcnoit, M. (2.6)289 Baucr, I. (3) 869 Bcnsasson, R.V. (5) 1 1 Baucr, M.P. (2.7)94 Bcn-Shoshon. M.(3)617 Baucr, R.(4)6, 12 Bcnhdc, W.G. (2.3)22;(2.6) 335,339 Baumann, W. (I) 58 Bcra, P.K. (1) 261 Baumert, T.(2.7)I13 Baumgartcn, M. (I) 420;(5) 194 Bcrberan-Santos,M.N. (1) 55,399, 400,403;(5) 179,181 Bay, E.J. (2.6)66 Baycr, A. (3)503 Bcrclitold, G.A. (2.6)290. Bcrcznilskii, G.K. (3)119 Baycr, E.(3)4 I6
456 Berg, A. (1) 3 19 Berg, K. (5) 23 Berg, M. (1) 616 Bcrgbrciter, D.E. (3) 594 Bcrgcr, H. (1) 4 13 Bcrghahn, M. (3) 467 Berglund, H. (I) 355; (5) 23,24, 97 Bergmann, R. (4) 55 Bergstrocm, H. (3) 544 Bergt, M. (2.7) 113 Bcrkcsscl, A. (2.5) 22 Berlanga-Duarle, M.L. (3) 854 Bermcjo, M.R.(5) 22 Bernardi, F. (1) 179; (2.3) 47,48; (2.4) 122 Bcrnardi, R. (2.4) 181; (2.6) 180 Bcrnardinclli, G. (2.6) 68 Bcrnik, D. (1) 64 1 Bcrnstein, E.R. (1) 130 Berrada, M.(3) 308 Bcrson, J.A. (2.7) 22 Bcrtolotti, S.G. (1) 265; (2.5) 79 Bertram, G. (2.3) 5 1 Bcssho, K. (1) 593 Beurskens, P.T. (1) 3 13 Bcvcridge, K.A. (2.3) 87 Beyer, M. (3) 798 Bhatnagar, A. (3) 35 1 Bhattacharyya, K. (1) 6 1 Bhnttacharyya. S. (1) 173 Bhogal, N. (2.7) 5 1 Bi, G. (2.5) 180 ,Bib, N. (1) 375; (5) 153 Biczok, L. (1) 86,455; ( 5 ) 198 Bicdron, T. (3) 152 Bicnians, H.A.M. (1) 250 Bicnvcnu, C. (2.5) 208; (2.6) 178 Bignozzi, C.A. (1) 360; (5) 102 Bikbaeva, G.G. (3) 573,581 Billig, F. (2.6) 285 Billing, R. (4) 13 Billingham, N.C. (3) 668,669 Bindhu, C.V. (1) 609 Biondic, M. (3) 115 Birch, D.J.S. (1) 209,586 Bircher, E. (1) 199 Birkett, P.R (1) 443; ( 5 ) 188 Birks, J.W. (2.4) 178 Bishop, S.M. (1) 119 Bisht, P.B. (1) 485 Biskupic, S. (1) 420; ( 5 ) 194 Bitncr, T.W.(2) 39 Bittersmann, E. ( 5 ) 12 Bittman, E. (3) 288 Bitto, B. (1) 221 Bityurin, N. (3) 782
Phofocheniisfry Blachford, J.W. (3) 663,664 Blagoi, Yu.P. (1) 389 Blanchard, L. (1) 153 Blasioli, P. (3) 84 1 Biinn, E.L.( 5 ) 15 Block, E. (2.7) 181 Blumstcngcl, S. (3) 605 Blunt, D. (2.7) 108 Bobrowski, K.(2.5) 197 Bocian, D.F.(1) 388; (5) 259,275 Bock, A. (1) 174; ( 5 ) 128 Bock, K. (2.7) 85 Bockman, T.M.(1) 322; (2) 4 1; (2.2) 106-108;(2.5) 232; (2.7) 136 Bodunov, E.N. (1) 55 Bocffcl, C. (3) 490 Boehm, A. (3) 640 Bohm, D. (2.7) 199 Bohm, S.(2.4) 50,51; (2.6) 151, 152 Bochncr, R. (3) 226 Boens, N. (1) 642; (5) 138 Bocrio-Goaks,J. (1) 494 Bocrjc, A. ( 5 ) 23,24 Bocsc, R (1) 3 16; (2.6) 23 1 Bocttchcr, C. (2.5) 221 Boganov, S.E. (2.6) 329; (2.7) 69 Bogilo. V.I. (1) 56 Bohnc, C. (2) 42; (2.4) 254; (2.7) I33 Bohrowski, K. (1) 5 16 Boillot, M.-L. (I) 560 Bojarski, C. (1) 175 Bojarski, P. (I) 175 Bojinov, V.B. (3) 529 Bolin, D.G. (2.2) 82 Bolivar, R.A. (2.4) 139; (2.6) 280 Bollc, B. (3) 620 Bolln, C. (3) 178 Bologncsi, A. (3) 514 Bolte, M. (2.5) 171, 172 Boltc, 0.(2.6) 288 Bominacr, E.L. (1) 78; (5) 65 Bonchio, M. (2.5) 119 Bong, P.-H. (2.5) 165 Bongiovanni, G. (3) 33 1,54I, 665; ( 5 ) 140 Bonhotc, P. (2.5) 187 Bonina, F. (1) 525 Bonneau, R (2.7) 32.33 Bonnebat, C. (3) 759 Bonnett, R. (1) 369 Bonnevie, C. (2.6) 198; (2.7) 67 Boots, H. (3) 270 Borg, R.M. (2.6) 156 Borghi, R. (2.5) 126
Boriscnko, A.A. (2.3) 59; (4) 19 Boriscnko, V.N. (1) 248,264,597 Boriscvich, Yu.E. (3) 472 Borisscvitch, I.E. (1) 5 14 Born, R. ( 5 ) 109 Borsarclli, C.D. (1) 265 Borshich, S.A. (1) 78; (5) 65 Borst, W.L. (3) 382 Bortolini, 0. (1) 360; ( 5 ) 102 Bortolus, P. (2.4) 191; (2.6) 50 Borzalta, V. (3) 199 Bos, M.A. (1) 128 Bosch, J. (2) 54; (2.4) 195; (2.6) 63 Bosch, P. (3) 41 Bosnyak, C.P. (3) 707 Bossek, U. (5) 2 1 Bossmann, S.H.(3) 429,565 Boswvcll, G.A. (2.5) 212 Bolhur, A. (2.7) 23 BotoshansLy, M. (2.6) 229 Botta, C. (3) 665 Bottino, F.A. (3) 761 Bouas-Laurcnt, H. (2.2) 16, 17; (2.3) 74; (2.4) 112, 116 Boule, P. (2.4) 166 Bourcier, S. (2.4) 105 Bourdclandc, J.L. (2.6) 189; (2.7) 20; (3) 111 Boutcvin, B. (3) 203,292 BouzBouz, S.(2) 32 Bowman, C.N. (3) 11,48,124, 317,319 Bowman, RM.(1) 141 Boyd, M.K.(1) 157 Boyd, P.D.W. (1) 466; (5) 222 Boyd, S.E.(1) 555; (2.6) 245 Boykin, D.W.(2.4) 265; (2.6) 62 Bradley, D.D.C. (3) 629,653 Bradley, G. (3) 35,110,502 Bracucldc, C. (1) 1 13 Brana, B.M. (2.2) 82 Brand, U. (2.3) 66,67; (2.4) 128; (2.6) 125 Bratov, A. (3) 227 Bratslavsky, S. (3) 157 Brauct, H.-D. (2.5) 105, 150 Braun, A. (1) 563 Braun, A.M. (3) 429 Braun, C. (1) 10; ( 5 ) 44 Braun, D. (I) 89-9 1 Braun, M.(2) 26 Bravo, J. (1) 376; (3) 37; (5) 154 Brcdas, J.L. (1) 437; (3) 366; (5) 208 Brcdc, 0. (2.4) 80; (2.6) 55,56; (3) 852
Airlhor Index Brcdchorn, J. (2.6)117 Brehcret, E.(1) 249 Brcmncr, J.B. (2)53;(2.4)196; (2.6)64,65 Brcn, V.A. (2.6)26 Brcncman, C.M. (3) 148 Brcnnan, P.J. (3)705 Brcvet, P.F. (1) 279;(5) 239 Brczova, V.(1) 418,420;(2.5)85; (5) 194 Bright, F.V. (3) 539 Brinker, U.H. (2.7)35 Brodilova, J. (2.4)180 Brocr, D.J. (3)269 Broska, R (3)677 Brousmiche, D.(2.3)8 1; (2.6)299 Brouwcr, A.M. (1) 347,352;(2.5) 135;(2.6)240,241;(5)95,122 Brown, C.T. (I) 312;(2.5)173;(5) 107 Brown, P.J.N. (2.7)55 Brown, R.G.(1) 152 Brown, S.B.(1) 116 Brown, W.(3)411-413,433 Brownsword, RA.(2.7)102,103 Bruenker, H.-G. (2.5)I14 Bruin, P.(3)551 Brusentscva, M.A. (3)627 Brzozowski, Z.K.(3)233 Buck, K.(2.4)243 Buckley, P. (I) 571 Budac, D. (2.3)79,80;(2.4)275 Buddhu, S.C. (2.6)150 Budrucv, A.V. (2.6)200 Bucvich, A.V. (2.4)110; (3) 136 Buisine, J.M. (3)509 Bulgakov, R.G. (3)573,581 Bulgakova, L.M. (3)59 Bulgarcvich, D.S.(1) 491;(5) 228 Bullita, D. (2.6)2 1 I Bultmann, T. (1) 622;(2.7)178 Buncl, C. (3)315 Bunkcr, C.E. (I) 438;(5) 21 1 Bunker, G.M. (3)48 Buntcn, K.A. (3) 538 Buntinx, G.(2.5)77 Buranda, T. (I) 292,357;(2.5)41; (5) 101 Burch, E.L. (1) 171;(2.6)I08 Burdcn, D.L. (1) 582 Burgdorff, C. (I) 337;(5) 230 Burger, B. (1) 435;(5) 210 Burger, U. (2.6)68 Burgers, M. (3)852 Burgct, D. (1) 82;(3) 133;(5) 81 Buriscn, B.K. (3)796 Burke, J. (3) 768
457 Burkc, N.A.D. (2.5)144 Burland, D.M. (3) 363 Bum, P.L. (3) 629 Bums, C.L. (2.4)I13 Bums, S.E.(3)637 Burrows, H.(2.5)171 Burshtcin, A.I. (1) 65,275;(5) 226 Burstein, E.A. (1) 47,625 Bursten, B.E. (2.7)126 Burlon, G.(2.5)229;(2.6)287 Buruiana, E.C. (2.7)60 Bwuma, R (3)498 Buscemi, S.(2.6)181 BusGcld, W.K.(3)739 Bush, L.C. (2.7)22 Bushkov, A.Ja. (2.4)69 Busson, R. (2.2)68 Buterbaugh, J.S.(1) 148;(2.7)34 Butler, K.E. (2.4)186 Butlcr, L.J. (1) 39;(2.7)1 Buttafava, A. (3) 697 Buttcrficld, M.T. (1) 533 Butts, C.P. (2.4)165, 167-169, 171, 174, 175;(2.6)163-166, 169, 170, 172;(2.7)142-146, 149,150 Butuc, E. (2.7)60 Buxton, P.I. (3)46 Bycrs, J.H. (2.4)93 Byrd, H.(4)39 Byrom, K.J. (5) 170 Caballero, 0. (2.6)146 Cabras, P. (2.6)21 1 Cadet, J. (2.5)208;(2.6) 178 Cadogan, J.I.G. (2.7)55 Cai, H. (3) 47 Cai, L.(4)GI Cai, Q.(3)25 1 Cai, S.X.(2.7)87 Cakmak, Y.(3) 56 Caldwcll, R.A. (2.2)11; (2.4)130 Califano, S. (1) 140 Callicr-Dublanchct, A.-C. (2.6) 258 Calogcro, G. (5) 170,173, 174 Calm, P. (2.5)200 Calzafcrri, G. (4)28 Cambron, RT.(2.3)22;(2.6)335 Cameron, J.F.(2)58;(2.6)175 Camcron, T.S. (2.3)17;(2.6) 156 Caminati, G.(1) 300;(3)5 18;(5) 234 Camino, G. (3) 697 Campagna, S.(5) 167 Campbell, B.H. (3)867
Campbcll, E.E.B. (1) 43 ;(5) 191 Campbell, I.H. (3)629 Campos, A. (3)425 Campos, P.J. (2.6)70 Camprcdon, M. (2.6)45 Canalcs, J. (2.5)1 1 Canceilli, J. (1) 538,539;(2.6) 244;(5) 268 Cano, F.H. (2.5)72 Canpolat, M.(3)430,460 Cao, H. (1)101,600,601;(2.5) 218;(2.6)238;(5) 251,252 Cao, J.R. (2.2) 12 Cao, T.( I ) 280;(3)437;(4)43; (5) 240 Cao, W. (3) 106,114 Cao, X.-Z. (2.5)65 Cao, Y.(2.5)91;(4)15, 16 Capck, I. (3)81-83 Carassiti, V.(3)758 Cardin, C.J. (2.2)10 Cardmet, C. (3)879 Carell, T.(1) 521;(2.2)61 Carcra, L.C.M. (3) 102 Carctti, D.(3)3 Carey, J.H. (3)796 Carcy, M. (3)362 Carlini, C. (3)3 Carloni, P.(2.6)252 Carlos Pcrcz, R (5)271 Carlson, K.D.(3) 141 Carlson, K.E. (2.6)34 Carlsson, J. (3) 544 Cannona, C. (2.4)13 Carofiglio, T.(2.5) 119 Caronna, T.(2.4)18I; (2.6)180, 181 Carpcnter, B.K. (2.5)129 Carpintcro, M.(2.3)15;(2.5)199; (2.6)96 Carrara, F. (3) 759 Carre, M.C. (3)438,558 Carrcira, E.M. (2.2)28;(2.4)121 Canicrc, F. (3)308 Cartcr, D.S.(2.5)204 Cartcr, T.P. (3) 145 Casals, P.F. (3)778 Castcllan, A. (3)81 1 Castelli, F. (1) 525 Castellucci, E. (1) 193;(2.4)21 Castineiras, A. (5) 22 Castle. L.W. (2.4)208-21I; (2.6) 264,265 Castle, N.R (2.2)44 Castle, RN.(2.4)7,208-2!2; (2.6) 7,61,264,265 Castncr, G.D. (3) 297
458 Catalan, C.A.N. (2.2) 46 Catalan, J. (1) 134 Catalina, F. (2.6) 304; (3) 94,95, 359,716
Catchings, R.M. (2.4) 113 Caucheticr, M. (2.4) 105 Caudan, A. (2.2) 17; (2.4) 112 Cavaleiro, J.A.S. (2.4) 185 Cavalcri, J.J. (1) 14 1 Cavam, M. (2.4) 66 Ccbc, P. (3) 306,s 19 Cebulska, Z. (2.2) 41; (2.4) 46; (2.6) 182
Ceccarclli, A. (3) 78 1 Cecil, T.L. (2.4) 189 Ccdcrslrom, E. (2.5) 191 Ccdillo-Garga, R (3) 72 1 Ccglie, A. (5) 54 Cejka, J. (2.4) 51; (2.6) 152 Cclani, P. (2.3) 48 Ccrda-Garcia-Lopez, C.M. (2.2) 46 Ccrfontain, H. (2.3) 53 Ccrioni, G. (2.5) 126 Cermcnati, L. (2.5) 203 Ceroni, P. (1) 4 19 Ccrvera, M. (2.4) 156; (2.6) 161, 174
Cervoise, I. (2.5) 112 Chaban, A.N. (I) 602; (3) 605 Chabcrt, N. (1) 386; (5) 176 Chachaty, C. (5) 11 Chachisvilis, M.(1) 129, 130,216; (2.4) 55
Chac, K. (3) 196 Chac, W.K. (2.4) 131 Chac. Y.S.(2.3) 8; (2.4) 95; (2.6) 72
Chaffy, C.E. (3) 550 Chai, S.(2.3) 75 Chaiko, Yu.V.(3) 860 Chakravoti, S.(I) 233 Chambau, A. (3) 735 Chambcrlain, D.T. (3) 101, 149 Chambron, J.-C. (1) 15; (5) 3 1 Chan, M.S.W. (2.3) 18,76; (2.4) 157; (2.6) 158
Chang, B.H.(3) 779 Chang, C.T. (3) 116 Chang, D.-R. (4) 27 Chang, S.T. (3) 821 Chang, T.C. (3) 126, 127 Chang, W.P. (3) 658 Chang, W.S.(3) 342 Chang, 2.(3) 65 Changenct, P. (1) 244 Chanon, M. (2.4) 124 Chao, C. (3) 606,649
Photochenristry Chaplin, R.P. (3) 192 Chapman, G. (3) 695 Chapman, O.L. (2.2) 7 Chapman, W.H.(2.5) 26 Charlcsworth, J.M. (1) 549 Chasc, C.E. (2.4) 108 Chattopadhyay,N. (1) 155 Chau, F.-T. (1) 124, 125 Chaudhuri, R. (1) 484 Chauvct, J.-F. (1) 481 Chavan, S.P. (2) 52; (2.4) 57 Chavcz-Gil, T.E. (5) 267 Chc, J. (3) 712 Chcn, C. (2.5) 130 Chcn, C.-F. (2.5) 25; (2.6) 144 Chcn, C.-L. (2.5) 162 Chcn, C.Y. (2) 62; (2.6) 216 Chcn, D. (2.7) 108; (3) 383-385, 402
Chcn, F.A. (2) 62; (2.6) 2 16 Chcn, G.-Z. (1) 528,529,53 1, 534,568,626
Chcn, H.(1) 187; (3) 65 Chcn, H.B. (3) 116,126, 127 Chcn, J. (2.6) 121 Chcn, J.H. (2.2) 55 Chcn, J.L. (2.2) 55 Chcn, J.M. (3) 484 Chcn, J.P. (3) 497,500 Chcn, J.S. (2.5) 128 Chcn, K.-H. (2.7) 203 Chcn, L. (3) 421,531,551 Chcn, L.X. (3) 628 Chcn, M.-F. (2.5) 113 Chcn, P. (1) 66,325; (2.6) 332; (5) 123,243
Chen, R.-L. (2.3) 50 Chen, S. (3) 606 Chcn, S.A. (3) 649 Chen, S.J. (3) 113 Chcn, T. (2.6) 196; (2.7) 39 Chen, W. (3) 484 Chcn, X. (2.2) 116; (2.5) 27; (2.7) 175
Chcn, X.L.(1) 304 Chcn, Y.(2.4) 38; (2.7) 203,204; (3) 3 13,325
Chcn, Y.P. (3) 886 Chcn, Y.Z. (2.5) 185 Chen, Z. (3) 77,720 Cheng, C . 4 . (2.5) 125; (2.6) 137 Chcng, G. (1) 415; (5) 192 Cheng, H.-M. (2.6) 2 13 Chcng, H.W. (2) 62; (2.6) 216 Chcng, J. (2.5) 180 Cheng, N. (3) 583 Chcng, S.Z.D. (3) 750
Chcong, S.H. (3) 719 Chcrgui, M. (1) 404 Chcrnook, A. (1) 12, 13; (5) 41,42 Chcung, Y . 4 . (2.7) 176 Chi, B. (3) 824 Chibisov, A.K. (2.4) 73; (2.6) 44 Chida, H. (2.6) 323; (2.7) 188 Chicn, L.C. (3) 266 Chikunov, O.V.(3) 457 Childs, RF. (2.7) 65 Chimosc, M. (3) 60 Chin, R (1) 191 Chin, V.L.(4) 55 Chiou, B.S.(3) 336 Chipalkatti, M.H.(3) 825 Chirvcny, V.(5) 129, 130 Chirvony, V.S. (1) 216; (2.4) 55 Chisaka, Y.(2)40; (2.5) 177; (2.6) 25% (2.7) 137
Chiu, Y.S. (3) 116, 126, 127 Chnicla, S.(3) 198,729,774 Cho, D.W. (1) 251 Chohan, S. (3) 717,718 Choi, C . 4 . (I) 354; (5) 96 Choi, J.K. (2.5) 108 Choi, T. (2) 49 Choi, Y.K.(3) 174 Choib, S.K. (3) 661 Chong, S.P. (3) 374 Choong, V.(3) 660,662 Chou, P.-T. (1) 235 Chou, T.C. (2) 62; (2.2) 94; (2.3) 65; (2.6) 216
Choun, S. (1) 386; (5) 176 Chovino, C. (3) 43 Chow, Y.L.(1) 321; (2.2) 3; (2.4) 99
Chowdhury, M. M.(1) 261; (2.5) 17
Christcnscn, J.B. (2.4) 207; (2.6) 266
Christl, M. (2) 26 Christou, G. (5) 20 Chruscinska, A. (1) 565 Chuang, K.R (3) 649 Chudnovsky, A. (3) 707 Chucv, 1. (2.6) 49 Chugrccv, A.V. (1) 416 Chui, R.T. (2) 55 Chun, H.J. (3) 191 Chung, K.-H. (4) 25,27 Chung, S.K. (2.2) 77,78 Chug, W . 4 . (2) 27.28 Chupka, A.E. (3) 543 Chupka, E.I. (3) 543 Church, J.S.(3) 820 Churio, M.S. (1) 258; (2.5) 20
459 Ciardelli, F. (3) 489 Cichos, U. (2.4) 235 Cinco, RM. ( 5 ) 19 Cizmcciyan, D. (2.5) I5 Claramunt, R.M. (1) 134 Clark, C.D. ( I ) 73,294,295; (2.5) 61 Clark, C.G. (1) 52 Clark, S.(3) 60 Clark, S.C. (3) 105,346 Clark, T. (3) 757 Clauberg, H.(2.7) 95 Claudc, J.P. (1) 68; (5) 69 Claws, B. (3) 503 Clauss, K.-U. (2.4) 243 Clayton, A.H.A. (1) 11,80; (5) 47, 74 Clcarfcld, A. (4) 39 Clcary, R.L. (5) 170 Clcij, T.J. (3) 477 E.L. (1) 365; (2.5) 10, C~CM~W, 113,227 Clcrc, C. (1) 4 13 Clcvc, B. (3) 644 CliKord, S.( I ) 179; (2.4) 122 Clinc, J.L. (2.7) 154 Clivio, P. (2.2) 73; (2.6) 134 Clovis, D. (4) 21 Cody, R.D. (3) 832 Coe, B.J. (5) 98 Cogdcll, R.J. (5) 9 Cohcn, G.G. (I) 254 Cohcn, S.G. (2.6) 236 Cohcniuazi, Y.(2.2) 23 Coker, M. (3) 7 17 Colbow, K.M. (1) 436; (5) 209 Cole-Hamilton, D.J. (2.5) 37; (2.6) 302 Colcs, H.(3) 469 Coil, J.B. (3) 381 Collar, E.P. (3) 716 Collin, G.J. (1) 101 Collin, J.-P. (I) 362,363,547; ( 5 ) 104,105,177 Collins, M.J. (3) 324 Collins, S.(3) 502 Collison, C.J.(3) 632 Colofemmina, G.( 5 ) 54 Colwcll, K. (3) 4 14 Commereuc, S.(3) 740 Compton, R.N. (1) 405; (5) 180 Coimolly, T.J. (3) 122 Connor, D.A. (2.3) 17 Constantine, S.(2.5) 104 Consuclo, J.M.(2.4) 183 Conwcll, E.M. (3) 612,630, 650; Cooks, R.G. (2) 61
Coombridgc, B.A. (3) 857 Coons, L.S.(3) 18 Coopcr, T.M. (2.4) 200 Coquerel, X. (3) 509 Corbil, J. (1) 437; ( 5 ) 208 Corbin, D.R. (2) 12; (2.5) 50 Corclli, E. (3) 3 Cornil, J. (3) 366 Cornwall, E.M. (1) 27; (5) 35 Cornwcll, P. (2.7) 160 Corralcs,T. (2.6) 304; (3) 44,359, 716 Comic, J.E.T. (2.4) 273; (2.6) 303 Corti, S.(I) 140 Corvaja, C. (3) 599 Corval, A. (2.4) 67; (2.6) 227 Cosa, J.J. (1) 265 Cossy, J. (2) 32,34; (2.5) 5,54 Costa, C.V. (2.5) 20 (1) 372; (2.5) 31; Costa, S.M.B. (3) 442 Costamanga, J. (2.5) I 1 Coursan, M.(2.7) 202; (3) 50 Courlncy, S.H.( I ) 427 Coury, A.J. (3) 55 Couturc, A. (2.2) 48; (2.6) 59 Cowan, F.J. (3) 857 Cowan, M.E. (3) 448 Comd, R.A. (3) 259 Cozens, F.L. (2.3) 82; (2.4) 247 Craig, D.C. (2.6) 239 Craig, D.P. (1) 98 Cramcr, C. (2.6) 207; (2.7) 206 Crcdi, A. (1) 555,623; (2.6) 245 C r d , D.(1) 25; (3) 259,522 Crescenzi, C. (2.6) 293 Crim, F.F. (2.7) 4 Crimmins, M.T.(2.2) 2 1,24 Cristcnson, R.L. (3) 409 Cristoply, E. (2.5) 57 Cri~~l10, J.V. (3) 140-142, 147, 156, 157, 161-163, 166, 167, 173,348,446 Cronshaw, D.R. (3) 857 Crooks, E.R.(2.5) 90 Crosslcy, M.J.(1) 313; (5) 71 Crystall, B. (3) 513,632 Csepregi, 2.(3) 891 Cui, C. (2.4) 238,239; (2.5) 49 Cui, Q. (2.7) 14 Cui, W.G.(3) 204 Cui, Y.(3) 583 Culiicr, R.I.(1) 87 Cull, B. (3) 92 Cwnpson, B.H. (3) 823 Cunningham, K.L. (1) 272; ( 5 ) 56 Curry, S.L. (1) 382; ( 5 ) 165
Cvijin, I.V. (2.3) 46;(2.4) 53 Cywar, D. (3) 286 Czajlik, 1. (3) 143,353 Czamik, A.W. (1) 54 I Czcmcy, P. (1) 199 Dabcstani, R. (2.4) 233; (2.5) 160 Dabin, P. (3) 7 17,7 18 Dachuc, S.(1) 2 12 Dahmiwal, N.R.(2.5) 214 Dai, G. (2.6) 297 Dai, S.(5) 150 Dnihavie, J.-0. (1) 362; (5) 104, 177 Dainty, J.C. (1) 585 Dakin, V.I. (3) 216 Danicldross, S.(2.2) 52 Daniclson, E. (2.5) 217; (5) 113 Dantalc, S.W.(2.4) 57 Dmlsker, D. (2.4) 64 Daraio, M.E. (1) 296 D s , K.(1) 61 Das, P.K. (2.4) 45; (2.6) 183 Das, S.(1) 61; (2.3) 25; (2.4) 42; (2.5) 188 Das, S.K. (2.4) 63 Das,T.K. (2.7) 56 da Silva, A.P. (1) 544-546 Date, M. (3) 93 Dau, H. ( 5 ) 19 Daub, J. (1) 553; (3) 379,585 D'Auria, M. (2.2) 6; (2.4) 176, 222; (2.5) 151, 163; (2.6) 8, 185,254 Dausse, A. (1) 560 David, E. ( 5 ) 109, 1I5 Davidson, RS. (2.6) 305; (3) 35, 96,98,100,110,502 Davioud, E. (2.7) 58 Davis, E.J. (3) 318 Davis, F.J. (2.6) 13 Davis, K.M. (2.5) 129 Davydov, R ( 5 ) 23,24 Dawson, B.S.W. (3) 857 Dc, R. (1) 273,285; (2.5) 170 Dc Alvarenga, E.S.(2.2) 10 DcArmond, A.H. (I) 278; (5) 238 DcArmond, M.K. (I) 278; (5) 238 Dc Backcr, S.(1) 122 dcBoer, B. (3) 123 Debrcczcny, M.P. (1) 379; (5) 158 Dc Bnryn, A. (2.6) I12 Dechcr, G. (3) 277 DcCian, A. (2.6) 228 Dcck, RT.(1) 574 Dcckcr, C. (3) 171, 172,208,228,
Phobchemisrry
460 280,287,334,335,346,868 Deckcr, D. (3) 172,228,287 Dc Cola, L. ( 5 ) 172 dc Denus, C.R.(2.7) 200 Dc Feyter,S.(1) 3 13; ( 5 ) 138 Deftieux, A. (2.7) 202 Deflandrc, A. (2.6) 219 de Gcldcr,R. (1) 313 DcGrado, W.F. ( 5 ) 133 DeGraziano, J.M. (1) 343; (2.5) 222; (5) 12,94 Degtgarcva, A.A. (3) 293 Dciblc, C.R. (3) 340 Dcisenhofer, J. ( 5 ) 1,3 Dc Kcukclcirc, D. (2.2) 68; (2.6) 47,112 dc Koning, G. (3) 358 Delaney, J.K. ( 5 ) 259 Dc Langc, 8.(I) 495; (2.4) 15; (2.6) 10 Delgado, J. (2.3) 11; (2.4) 183 Dcligeorgicv, T. (3) 742 Dc Lisjer, H.J.P. (2.3) 78; (2.7) 209 Della Monica, M. ( 5 ) 54 Della Vcdova, C.O. (2.7) 111 Dclmond, S.(2.5) 192; (5) 272, 273 Del Monte, F. (3) 4 1 delong, M.A. (2.4) 126 Dclor, F. (3) 879 Delormc, P. (3) 759 Dclouis, J.F. (1) 249 Dclprat, H. (2.5) 147 Dclprat, P. (3) 730,73 1 Dc Luca, E. (2.6) 254 DcLucca, I. (2.5) 212 del Valle, J.C. (1) 134 Dcmanchc, L.J.(5) 12 Dcmandt, R.C.J.E. (3) 823 Dc Marc, G.R.(1) 101 DcMcllo, A.J. (3) 513 Dc Mcllo, J.C. (1) 566 Demcnl'ev, LA. (1) 383; ( 5 ) 171 Dcmulh, M.(2.5) 130; (2.6) 89 Dcng,G. (1) I88 Dcng, N. (3) 888 Dcng, Q. (3) 436 Dcng, S.(1) 46 Dcng,W. (3) 824 Dcniau, E. (2.2) 48; (2.6) 59 Dcnisov, N.N. (1) 449; (2.4) 146; (2.5) 82; (5) 204 Dcnizligil, S.(3) 164 Denti, G.( 5 ) 167 DcOrazio, R. (2.7) 18 1 Dc Paor, L.R. (1) 26,304; (3) 405,
628; ( 5 ) 34 Dc Pasqualc, G.(3) 76 I Dcpcrasinka, 1. (1) 320 Dcppc, D.D. (3) 456 Dc Raaff, A.M. (3) 180,294 Dcrbeyshire, H. (3) 803 Dc Rossi, U. (1) 212 de Sainte Clairc, V. (2.3) 69;(2.6) 30 Dc Schiyver, F.C.(1) 122,3 13, 323,640-642; (5) 138,242 Descoles, G. (2.6) 198; (2.7) 67 debilva, A.P. (2.6) 243 Dc Silvcstri, S.(3) 452,613 DEstc, J.R. (4) 22 Dcstri, S.(3) 541,665; (5) 140 Dcsvcrgnc, J.-P. (2.2) 16, 17; (2.3) 74; (2.4) 112, 116; (2.7) 202; (3) 50 Deumal, M.(2.3) 47 Deusson, M.(3) 609 Dcvadoss, C. ( 5 ) 266 Dcviprasad, G.(5) 274 Dey, J. (1) 224,243 Dhami, S. (1) 168 Dhancnjcyan, M.R.(2.5) 209 Dhawan, S.N.(2.2) 40; (2.5) 38 Dhinojwala, A. (3) 456 Dhondge, V.D.(2) 52; (2.4) 57 Diachun, N.A. (3) 555 Diamond, D. (1) 499 Dianov, E.M. (1) 561 Dias, M. (2.4) 47; (2.6) 203 Dias, R.M.B.(2.7) 184 Diaz, E. (2.2) 109 Dickcns, R.S.(1) 502 Dickinson, T.J. (3) 506 Dickson, J.M. (2.7) 65 Diederich, F.(5) 27 I Dichl, H. (1) 175 Dictlikcr, K.(2.6) 338; (3) 855 Dictrich-Buchcckcr, C.O. (1) 15; ( 5 ) 31 DiMagno, S.G.(2.5) 102 Dimitru, M. (3) 682 Dimotikali, D. (2.6) 136 Ding, K. (2.5) 177; (2.6) 255; (2.7) 137 Ding, K.L. (2) 40 Ding, S. (2.5) 167 Ding, W. (1) 415; ( 5 ) 192 Ding, X. (3) 712 Ding, 2. (I) 279; ( 5 ) 239 Dinia, N.M.(3) 203 Dinnoccnzo, J.P. (2.3) 29,30 Dinse, K.-P. (1) 420; (5) 194 Dion, C.F. (1) 130
Di Paolo, R.E. (1) 185 Dishncr, M.H.(2.5) 226 Dismukcs, G.C. (4) I Dittami, J.P. (2.4) 257; (2.6) 270 Dixon, A.C. (2) 42; (2.4) 254; (2.7) 133 Djalilov, A.T. (3) 103 Dmisoumi, 2.(3) 40 I Dmitricv, Yu.A. (3) 736 Doane, J.W. (3) 266 Dobo, J. (3) 143,353 Dobrikov, M.I.(2.7) 89,90 Dobrin, S.(2.4) 68 Dockery, K.P. (2.3) 22; (2.6) 335 Dochling, A. (1) 342; (2.5) 40; ( 5 ) 88 Dijpp, D. (2.4) 140; (2.6) 117,279 Dccmcr, T. (2.2) 83 Doetschman, D.C. (3) 286 Dogan, J. (2.4) 265; (2.6) 62 Dogra, S.K.(2.4) 63 Dohmaru, T. (3) 5 1 1,765 Doizi, D. (5) I1 Dolczal, C. (1) 58 1 DOlivcira, J.-C. (2.5) 147 Domcn, K. (4) 29-33 Domingcuz-Adame,F. (I) 644 Domingucz, C. (3) 227 Donati, D. (2.2) 4; (2.4) 109; (2.6) 107,118 Dong, J.H. (2.6) 261 Dong, X. (1) 332; ( 5 ) 86 Donnclly, D. (2.7) 5 1 Donohuc, RJ. (5) 259 Dorc. T.M. (2.4) 126 Dorfman, RC. (3) 602 Doron, A. (1) 557 Doroshenko, A.O. (1) 222 Dosovilskaya, I.E.(3) 302 Dossow, D. (3) 183 Doumc, L. (2.6) 219 Doughcrly, J.A. (3) 446 Dougherty, T.J. (2.5) 104 Doumenq, P. (2.5) 159 Dowling, K. (1) 585 Doylc, G. (2) 56 Dozcman, A. (3) 864 Drabbcls, M. (2.7) 97,98 Drager, G. (2.6) 288 Dragostinova, V. (3) 471 Drain, C.M. (1) 230 Drake, C.R (3) 409 Draxler, S.(1) 579,587 Drccskamp, H. (2.4) 256 Dregcr, Z.A. (1) 490 Drcsncr, J. (I) 320 Drcss, M.J.(3) 229
A urhor hdex Drickamcr, H.G. (1) 490 Drobizhev, M.A. (3)655 Drovetskaya, T.(1) 466;(5) 222 Droy, C. (2.5)77 Droz-Georgct, T.(2.7) 100,101 Drummond, S.(2.5)212 Druzhhh, S.I. (3) 580 Dtyfe, R.A.W. (1) 279;(5) 239 DSouza, F. (5) 274 Du,D. (1) 270;(2.2)1 16 Du,D.M. (2.4)117 Du,F.S.(3) 121 Du, H. (3)582 Du,M.-D. (2.4)184 Du,W.(3)483 Du,X.(1) 529 Du, X.4. (1) 528,534 Du,Y. ( I ) 415;(5) 192 Dubest, R (2.6)48 Dubey, G.C. (4) 8 Dubin,P.(1)413 Dubois, C. (3)735 Dubonosov, A.D. (2.6)26 Dubs, P. (3)855 Dudko, RY. (2.7) 89,90 Dudlet, V. (3)671,673,674 Diirr, H. (2.6)54;(5) 109,115 Duff, M.E. (2.4)93 Dufour, C.(2)31 Duhamel, J. (3) 506 Duldi, D.M. (I) 418 Dunbar, R C . (1) 620 Dunford, C.L. (1) 617 , Dungcr, S.(2.6)285 Dunk, W.A.E.(3)876 Dunkers, J.P.(3)380 Dunkin, 1.R (2.7)26,75,76 Dunn, K.M. (2.7)14 Duo, K. (1) 612 Dupray, L.M. (3) 524 Duran, N.(3)683 Durand, A.-P. (1) 152 Durand, G. (1) 489 Durncll, C.(I) 334;(5) 85 Durochcr, G. (1) 153 Dussault, P.H.(2.5)102 Dutor, J. (I) 134 Dutt, G.B. (1) 641,642;(5) 138 Dutta, A.K. (I) 61,396,572 Dutta, R.(2.5)17 Dutton, P.L. (5) 133 Duysens, L.N.M. (5) 53 Dvcdova, N.N.(1) 380;(5) 159 Dwojanyn, P.A. (3) 192 Dymott, J.P.M. (1) 573 l&tmacv, K.M. (3)302 Dzliafarova, R.A. (2.5) 143
461 Endeward, B.(I) 45 1; (5) 201
Eapen, D. (4)36 Eastoc, 1.(2.5)90 Ebcrhardt, W. (1) 436; (5) 209 Eberly, J.H. (1) 646 Ebcrson, L. (2.4) 164, 165, 167-171,173-175,267;(2.6) 162-167,169-173,252;(2.7) 141-147,149-152 Ebisuno, T. (3)256 Eckberg, R.P.(3)274 Eckcrt, A. (1) 280; (4)43;(3)437, 552;(5) 240 Eckcrt, G. (1) 288,29I Eckhardt, A. (3)762 Eckhardt, G. (3)20 Eckwert, J. (2.7)23 Edge, M. (3)44,94,95,688,716 Edlington, M.D. (5) 260 Edman,L.(1)111 Edman, P.(5) 139 ECremkin,A.F. (2.5) 18;(3)390 Efremova, O.A. (1) 369 Egclhaal; H.J.(3) 416 Egg-, R (1) 100 Uycnskin,G.W. (3)288,296
Eichcn, Y.(2.4)67;(2.6)227-229 Eichhom, H. (2.4)30 Eiger, G. (5) 91 Eiscle, G. (3)309.3 10 Eiscnbcrger, H. (1) 159-161 Eisenthal, K.B.(I) 29,30 Eklund, P.C. (1) 434;(5) 214 El Anba-Lurot, F. (2.5)159 El Bachiri, A. (3)292 El-Daly, S.A. (2.4)255 Elias, C.A.B. (3) 536 Elich, K. (1) 2I8 Elisecv, A.V. (2.2)34 Elisci, F. (1) 192, 196, 198,287; (2.3)19;(2.4)26, 191;(2.6) 263 El-Kcniary, M.A. (2.4) 255 Ellison, E.H.(3) 587 El-M~adny,A. (2.7)130 Elmino, P.(3) 5 14 Eloy, D. (2.6)52 El Shafce, E.(3)745 El-Shishtawy, R.M.A. (4)71 Elvcry, J.J. (2.6)305 Elzaouk, B.(3) 228 Emclymcnko,V.I.(1) 625 Emmert, 0.(2.7)17 Encinas, M.V. (2.6)304;(3) 104, 1 I5 Endcrlcin, J. (1) 103, 105, 112, 638
Endo,T.(3)62,184,694 E n h , U.(2.4) 103;(2.6) 132 Encmark, J.H. (1) 357; (5) 101 English, D.S. (1) 225,524;(2.2) I I8 English, RJ. (3) 336 Enriqucz, RG. (2.2) 109 Eom,H.S. (1) 23 1 Epp, K.M. (2.7)200 EPP. 0. ( 5 ) 1 Epplc, R (I) 521;(2.2)61 Epstcin, A.J. (3)663,664,666 Erddalanc, A. (3) 99 Erdmann,C. (3) 226 Erdmann, R (1) 103,105 Ercmcnko, A.M. (1) 306;(5) 244 Ercmdo, V.V. (1) 97 Ercntova, K.(2.7)3I Erickson, B.W. (2.5)217;(5) 113 Emsting, N.P. (1) 622;(2.7)178 Eroglu, I. (4) 67 Ershov, A.Yu. (1) 383;(5) 171 Escfibach, A. (2) 31 Escn, C.(3) 80.85 Eshakov, E.N. (2.4)220;(3) 435 Esin,S.A. (3) 321 Espinos, A. (2.3) 11; (2.4) 183 Esposito, V.(2.5) 163;(2.6) 185, 254 Essigmann, J.M. (2.2)69;(2.4) 136, 137;(2.6) 109, 110 Esteghamatian,M.(3) 631 Evans, A.C. (2.4) 186 Evans, RA. (2.7)91 Evcry, J.J. (3)96.98 Evlcth, E.M.(1) 101 EVSCCV, A.V. (3)219 Evstaticv, M.(3)742 Evstignecva,RP. (1) 33 1; (5) 119 Emcr, K. (2.6)190;(2.7)19 Eyal, A. (1) 57 Eyring, E.M. (2.7) 117 Fabian, J. (2.6)I84 Fabrc, A. (2)30 Fagart, J. (2.7)58 Faponi, M.(2.4)216 Fahmy, A.F.M. (2)48 Fajer, 1. (1) 228 Falruda, K. (1) 485 Falvey, D.E. (1) 615;(2.2)63; (2.4)245,260;(2.6)207,208; (2.7)206 Fambri, L. (3)202 Fan,A. (2.5) 123
462 Fan, G. (2.4) 8 I ; (2.6) 42 Fan, M. (2.4) 81,82, 86,89; (2.6) 39-42 Fan, R. (2) 24 Fan, X. (1) 177 Fang, D. (2.2) 37; (2.6) 308 (2.7) 70 Fang, G.-H. Fang, J.-Y. ( I ) 88 Fang, P. (3) 86 Fang, Q.(3) 435 Fang, S.B. (3) 575 Fang, T. (3) 888 Fann, W.S. (3) 649 Fansu, R. (1) 48 1 Fantc, N.D. (3) 728 Fara, L. (3) 560 Farago, G. (2.6) 53 Farahat, M.S.( 5 ) 135 Farid, S.( I ) 7; (2.5) 2; (5) 27 Farina, F. (2.7) 21 Farinha, J.P.S. ( 5 ) 141 Farkas, E. (4) 72 Farkas, Z. (4) 72 Farmcr, P.J. (1) 361; ( 5 ) 103 Famikova, M. (1) 426 Farrant, E. (2.6) 91 Fasani, E. (2) 6; (2.3) 16; (2.4) 158; (2.5) 6; (2.6) 5 , 157,210, 300; (2.7) 186 Fasslcr, D. (3) 403 Fahi, A. (2.6) 284 Faucitano, A. (3) 697 Faure, J. (1) 48 1 Faurc, S.(2.2) 25 Fauve, A. (3) 717,718 Favaro, G. (I) 189; (2.4) 71, 191 Favre, A. (2.2) 73 Favre-Nicolin, C.D. (3) 72 Fayer, M.D. (1) 42,43; (3) 540, 555; ( 5 ) 75,78 Fedcrova, G.F. (2.5) 155 Fcderova, O.A. (2.6) 19; (3) 580 Fcderspiel, R.F. (2.4) 210,212; (2.6) 61 Fcdor, G.R (3) 705 Fedorov, A. (1) 403; (5) 18 1 Fcdorov, Y.E. (2.6) 19 Fedorova, I.A. (1) 4 16 Fedorova, O.A. (2.4) 17, 110,220; (3) 135, 136 Fedorova, Y.V. (2.4) 17 Fccdcr, N. (2.4) 118,223; (3) 345 Fcchan, T.M. (I) 356; (5) 100 Fehcr, F.J. (2.5) 226 Fehlncr, J.R. (2.3) 54 Fcilcr, L. (3) 533 Feitosa, E. (3) 4 13,433
Pholochentis@ Fcld, W.A. (3) 61 1 FcldcrhoK, M. (1) 3 16; (2.6) 23 1 Fcldman, J. (3) 639 Fcll, J. (2.6) 196; (2.7) 39 Feltrc, L. (1) 633 Fcng, K. (2.7) 167 Fcng, W. (3) 7,307 Fcng, X. (3) 106, I 14 Fcng, X.W. (2.7) 22 Fcng, Y. (3) 714 Fcng, Z. (2.5) 180 Fenick, D.J. (2.2) 63; (2.4) 104 Fcnyvcsi, E. (3) 890 Fcoklistov, L.G. (3) 845 Fcrcncc, A. (5) 128 Fcrguson, A.I. (1) 573 Fcringa, B.L. (1) 495; (2.4) 15; (2.6) 10 Fcmandcz, A.M. (3) 8 19; (4)36 Fcrnandcz, F. (1) 470 Fcrnccz, A. (1) 174 Ferraris, J.P. (3) 508,629 Fcrraudi, G. (2.5) 11 Fcrrcira, E. (1) 309; (5) 57 Fcrrcira, L.F. (1) 372; (2.5) 3 1 Fcrri, D.C.(2.5) 19I Fcrri, T. (2.5) 15 I Fcstag, R. (3) 639 Fetzcr, J.C. (1) 480 Fidlar, V. (1) 334; (5) 85 Fiebig, T. (1) 597; (5) 248 Ficsscr, G. (3) 640 Filhol, J.-S. ( I ) 538; (2.6) 244 Filipciiko, O.S. (2.4) 87; (2.6) 49 Filippov, RV. (2.7) 83 Finacnova, E.V. (2.7) 194 Findlay, J.B.C. (2.7) 5 1 Findscn, E.W.(1) 357; (5) 101 Fimbcrg, D. (2.3) 54 Fischcr, E. (2.4) 207; (2.6) 216 Fischcr, J. (2.6) 228 Fischcr, K. (3) 798 Fischer, M. (1) 608 Fischcr, P. (3) 404 Fischcr, Th. (3) 462 Fischcr, W. (3) 6 I4 Fishcr, S. (1) 5 17 Fishcr, T.A. (3) 653 Fishwick, C. (2.7) 5 1 Fisz, J.J. (1) 592,645 Flagan, R.C.(2.5) 140 Flamigni, L. (1) 362,363,547, 556; ( 5 ) 104, 105, 170, 172, 173,177 Flanagm, M.F. (2.4) 189 Flcming, G.R. (1) 334; (5) 85,259 Flcrning, S.A. (2.4) 262; (2.6) 292
Floin, S.R. (3) 849 Florcs, C. (4) 5 I Flynn, K.M. (3) 380 Foggi, P. (1) 140; (2.5) 77 Folcy, M.S.C. (1) 119 Folting, K. (5) 20 Foniina, L. (3) 523,534 Fominc, S. (3) 523,534 Font, J. (3) 11 1 Font-Sanchis, E. (2.3) 35 Foolc, C.S. (1) 365; (2.5) 10, 11 1, 123, 128 Foolc, RS. (2.7) 160 Ford, F. (2.6) 196; (2.7) 39 Ford, W.T. (2.5) 225 Fork, D.C. (5) 50 Forsskahi, 1. (3) 800 Forsytli, T.P. ( 5 ) 131 Forsythc, J.S. (3) 752,753 Forlunak, J.M.D. (2.4) 202; (2.6) 31 Fossum, R. (3) 387 Fouassicr, J.-P. (1) 147; (2.6) 297; (3) 2, 107, 133, 164, 177,309, 3 10 Fourniigut, M.(2.4) 207; (2.6) 266 Fourrcy, J.L. (2.2) 73; (2.6) 134 Fox, M.A. (1) 36,607; (3) 387, 392,393; (5) 114,255,261 Fox, RJ. (5) 114 Foylc, V.P. (2.4) 54 Framjc, J. (2.7) 125 Franccsco, S.(3) 199 Franchy, R. (2.7) 13 Francois, J. (3) 41 1 Frank, C.W. (3) 407,540 Frank, I. (2.4) 60; (2.6) 230 Frank, M.(5) 172 Frank, R (I) 293 Frailkcvich, E.L. (1) 602; (3) 605 Frankland, S.J.V. (1) 62; ( 5 ) 64 Franklin, N.R. (2.4) 224 Frantsuzov, P.A. (1) 65 Frazicr, A.A. (1) 469 Frcccero, M.(2.5) 6,117, 118; (2.6) 5,300; (2.7) 186 FrCchct, J.M.J. (2) 58; (2.6) 175 Frcdcrick, J.H. (2.4) 16 Frccman, P.K. (2.4) 159, 160 Frccnian-Cook, K.D.(2) 15; (2.5) 56 Frccr, A.A. ( 5 ) 9 Frcnigan, D. (3) 144 Frcnch, P.M.W. (1) 585 Frcnke, A. (1) 390; (5) 163 Frcy, H. (3) 178 Fricnd, R.H. (1) 566; (3) 633,634,
Author Index 637,638 Fricscn, D.A. (2.5) 217; ( 5 ) 98, 113 Fritz, H. (3) 416 Fritz, R. (3) 588 Frrihlich, R. (2) 22; (2.6) 307 Froesc, R.D.J. (2.4) 1 Frolov, A.N. (3) 521 Frolov, S.(3) 586,642 Fronczck, F.R. (1) 227; (2.6) 33 1 Fry, B.E.(3) 71 Fry, D. (2.6) 150 Fu, D.K. (3) 663,664,666 Fu, T.Y.(1) 504; (2.6) 333 Fu, X. (3) 882 Fu, Y. (1) 368; (2.5) I 1 1 , l I5 Fuchs, C. (1) 64 Fudim, E.V.(3) 2 17 Fucrniss, S.J. (3) 578 Fucrstncr, A. (2.4) 18 Fugami, K. (3) 685 Fuhrhop, J.-H. (2.5) 221 Fuhs, M. (5) 91 Fujihara, K. (4) 14 Fujii, T. (2.4) 264; (2.6) 271,272 Fujii, Y. (5) 18 Fujiki, Y. (4) 32,33 Fujimori, K. (2.6) 221 Fujishiro, Y. (4) 34 Fujita, M. (2.4) 101; (2.5) 152; (2.6) 140 Fujita, T. (2) 36; (2.4) 56, 132; (2.5) 45; (2.6) 267,268 .Fujitsuka, M. (1) 406; (2.6) 294; (3) 49; ( 5 ) 184 Fujiwara, H. (2.5) 99 Fujiwara, M. (2.7) 208 Fujiwara, Y. (I) 120,600,601; (2.5) 19,218; (2.6) 232; (5) 25 I, 252 Fukatsu, K. (4) 62 Fukazawa, Y. (1) 600,601; (2.5) 218;(5)251,252 Fukawmi, S. (I) 423; (5) 193 Fukinishi, K. (3) 48 1 Fukuda, H. (3) 236 Fukuda, R (3) 492,495 Fukuda, T. (3) 3 12 Fukumara, H. (1) 164 Fukunishi, K. (3) 499 Fukushima, K. (2.5) 33 Fukushima, T. (1) 505 Fukuzumi, S . (1) 421,422; (2.3) 93; (2.4) 101; (2.5) 122, 152; (2.6) 140, 141; (2.7) 195; ( 5 ) 195,196 Fulton, K.L. (2.4) 165, 168, 169;
463 (2.6) 164-166; (2.7) 144-146 Funasaka, H.(1) 406; ( 5 ) 184 Fung, Y.K.(3) 266 Funk, R.L. (2.4) 189 Funken, K.H. (2.4) 141 Furukawa, N. (2.4) 258,264; (2.6) 27 1-273 Furuta, T. (2.5) 138; (2.6) 25 Furuya, Y. ( I ) 20 I ; (2.4) 35; (2.6) 27 Furya, H. (3) 576 Fusi, S. (2.2) 4; (2.4) 109; (2.6) 107, 118 Futami, Y. (2.5) 157; (2.6) 192; (2.7) 25 Gabor, A.H. (3) 36 Gaboury, L. (1) 153 Gabriclli, G. (1) 300; (5) 234 Gadwood, RC. (2.4) 244; (2.7) 8 1 Gacl, V.I.(I) 377; (5) 156 Gagarin, S.G. (1) 81; (2.5) 74 Gai, C. (3) 579 Gaillard, E.R.(1) 8; ( 5 ) 28 Gajdck, P. (1) 196; (2.6) 263 Galenticr, E. (3) 750 Galiauo, G. (1) 287 Galili, N. (2.2) 26 Galili, T. (1) 3 19,486 Galindo, F. (2.4) 163 Gallardo, I. (2.6) 174 Gallcgo, M.G. (2.3) 43; (2.5) 9 Galli-Stanipino,L. (2.7) 85 Gallo, R. (3) 728 Galoppini, E. (1) 607; ( 5 ) 255 Galvin, M.E. (3) 604,646 Gamagc, N.J.W. (3) 192 Gamboa, S.(4) 36 Gamlin, J.N. (1) 381,394; (2.3) 26; (2.6) 148; ( 5 ) 160, 161 Gan, L.-B.(1) 440; (2.5) 78; ( 5 ) I85 Gan, T.H. (1) 549 Ganapathy, S. (2.3) 22; (2.6) 335, 339 Gandc, M.E.(3) 870 Gandolfi, M.T. (1) 555; (2.6) 245 Gangopadhyay, S.(3) 382 Ganguly, T. (1) 273,285; (2.5) 170 Gano, J.E. (2.3) 7; (2.4) 25 Gao, C. (1) 167,535 Gao, F. ( 5 ) 12 Gao, H. (1) 270; (2.4) 117 Gao, J. (3) 497 Gao, J.P. (3) 500
Gao, Q. (3) 130 Gao, Q.U. (3) 528 Gao, W. (3) 824 Gao, X. (3) 247 Gao, Y. (3) 61 I, 660,662 Garau, V.L. (2.6) 21 1 Garavelli, M. (2.3) 48 Garcia, A. (4) 59 Garcia, C.R.-A. (2.5) 182 Garcia, G.(2.7) 129 Garcia, H. (2.2) 32; (2.4) 33,237; (2.6) 7 1 Garcia, J.M.M. (1) 400 Garcia, M.E.(2.7) 129 Garcia, M.S.(2.5) 182 Garcia, R.M. (3) 425 Garcia, S. (2.4) 33; (2.5) 70 Garcia-Garibay, M.A. (2) 49; (2.5) 15,58; (2.7) 43 Garcia-Montcagudo, J.C. (5) 22 Gard, G.L. (3) 297 Gardocki, J.A. (1) 59,62; (5) 64 Gardctte, J.L. (3) 754,755 Garigipati, R.S. (2.4) 274 Garner, P. (2.6) 133 Garnctt, J.L. (3) 192 Garry, P.A. (2.3) 7; (2.4) 25 G a s m v , K.G.(2.5) 143 Gasyna, Z. (1) 617 Gavacrt, M. (1) 432 Gaviiia, P. (2.4) 266; (2.7) 210 Gavrilova, A.A. (3) 573,581 Gay, C. (2.6) 52 Gcbcl, RC. (2.2) 19 Gcddc, U.W. (3) 265,347 Gcdck, P. (3) 756,757 Gccrlings, J.D. (2.4) 59 Gcffs, G.E.(2) 38 Gch, L.J. (3) 3 13,325 Gehlcn, M.H. (2.6) 304; (3) 432 Gcigcr, C. (2.6) 16; ( 5 ) 262 Gcigrich, H. (2.4) 272 Gclinck, G.H.(3) 645 Gcntcmann, S. (1) 228 George, G.A. (3) 129,752,753 George, M.V. (2.3) 25; (2.4) 42, 45; (2.5) 188; (2.6) 147,183 George, M.W. (2.7) 73, 116, 120, 121,127 Gcorgcs, J. (1) 608,647 Gcorgiadou, Y. (1) 563 Gcrard, J.D. (3) 426 Gcrbcr, G. (2.7) 113 Gcnnain, C. (1) 489 Gcmian, E.D.(1) 72; (5) 63 Gcrosa, P. (3) 2 1 Gcscithucs, U. (3) 88 I
464 Gcuskcns, G. (3) 194,732,865 Gfellcr, N. (4) 28 Gfcllcr, S. (1) 22 1 Gharvi, A.R (3) 557 Ghatlia, N.D. (2.6) 339 Ghiggino, K.P. (1) 11,80; (2.6) 239; (3) 838; ( 5 ) 47,74, 143 Gholamkhass, B. (1) 77,364; ( 5 ) 79, 124 Ghoncim, N. (1) 143,289; (2.5) 23 Ghorai, M.K.(1) 9; (2.5) 14; ( 5 ) 29 Ghosh, H.N. (1) 154,448,476; (2.5) 219; ( 5 ) 197 Ghosh, N.R (2.7) 56 Ghosh, S. (1) 173; (2.4) 106 Ghuman, M.A. (3) 792 Giacopcllo, S.(3) I 15 Giannotti, Ch. (2.5) 224 Giasson, R. (1) 153 Gibson, C.L. (2.2) 33 Giegerich, H. (2.7) 160, 161 Giese, B. (2) 2, 19; (2.6) 73,76, 77,327,337 Gijsen, H.J.M. (2.5) 184 Gijsman, P. (3) 864 Gilbcrt, A. (2.4) 129; (2.6) 13 Gilbert, B.C. (1) 318; (2.5) 134; (2.7) 134, 135 Gilcad, D. (3) 69 1 Gilman, J.W. (3) 763 Ginai, N.A. (3) 792 Girard, D. (1) 153 Girault, H.H. (1) 279; (5) 239 Gircrd, J.-J. (1) 78; ( 5 ) 65 Girois, S.(3) 730,73 1 Girolanii, G.S. (1) 340 Gittingcr, A. (2.7) 26 Giusti, G. (2.5) 103; (2.6) 45 Giustini, M.( 5 ) 54 Givens, R.S. (2) 60 Gladstone, P.L. (2) 63 Glagolcv, N.N.(3) 479 Glarner, F. (2.6) 68 Glasbcck, M. (1) 237,588 Gleiter, R. (2) 29; (2.2) 83; (2.3) 64; (2.4) 123 Gleria, M.(3) 199,202 Gliesing, S. (2.3) 55 Glos, M. (2) 42; (2.4) 254; (2.7) 133 Gninguc-Sall, D. (3) 532 Gnoj, 0. (2.6) 80 Gobbato, K.I. (2.7) 11 1 Goddard, W.A. (2.5) 140 Godipas, K. (3) 5 18 Goebcl, L. (2.4) 80; (2.6) 55,56
Photocheniistry Gorncr, H. (1) 172, 192; (2.4) 26; (2.6) 44 Gocz, M. (1) 288,291; (2.5) 23 1 Goff, S.(2.7) 127 Goldmann, D. (1) 488 Goldschlcgcr, N.F. (2.4) 146 Gomcs, S.A. (1)431; ( 5 ) 191 Gomcz, C.M.(3) 425 Gomez-Mpcz, M. (1) 555; (2.6) 245 Gong, G. (1) 46 Gong, Z. (1) 540 Gonschior, M. (2.3) 55 Gonzalcz, D.G. (2.5) 182 Gonzalez, F.D. (1) 473 Gonzalez, M.F. (5) 174 Gonzilcz, R (2.4) 139; (2.6) 280 Gonzalcz, V. (1) 255 Gonzalcz-Cantu, M.C. (3) 854 Gonzalcz-Gutienez, L. (2.2) 109 Goodman, J.L. (2.7) 32 Goodman,S. (1) 334; (5) 85 Goodson, T. (3) 289 Goodwin, P.M. (1) 38, 102, 105, 108
Gopidas, K.R. (2.4) 45; (2.6) 183 Gorbunov, A.V. (5) 205 Gorbunov, B.N. (3) 845 Gorbunov, Z.V. (1) 446 Gord, J.R. (1) 148; (2.7) 34 Gordicnko, V.P. (3) 736 Gorccki, T. (1) 191 Gormin, D.A. (2.3) 3 1 Gosncy, I. (2.7) 55 Goswami, A. (3) 117 Gosztola, D. (1) 83,327,348,349; (2.5) 195; ( 5 ) 67, 125,233 Goto, S. (3) 473 Goto, T. (1) 139; (2.2) 97 Goto, Y. (2.5) 71; (2.6) 130 Goubitz, K. (2.7) 125 Go&, M. (3) 389 Gould, I.R. (1) 7; (2.5) 2; ( 5 ) 27 Gouni, I. ( 5 ) 12 Gowcnlock, B.G.(2.6) 223; (2.7) 153 Gouclino, G. (3) 33 1 Grabncr, G. (1) 526 Grabowska, A. (2.4) 68 Grady, T. (1) 499 Gractzcl, M. (2.5) 187 Graf, U. (1) 370 Gramain, J.-C. (2.4) 193; (2.6) 57 Gramain, P. (3) 43 Gramlich, V. (I) 521; (2.2) 61; (5) 27 1 Granipp, G. (1) 577
Grandclaudon, P. (2.2) 48; (2.6) 59 Grangcr, D.W. (3) 297 Gratani, F. (3) 84 1, 844 Grattan, K.T.V. (1) 643 Graupncr, W. (3) 613,616 Gray, D.G. (3) 858 Gray, H.B. (1) 358,532; ( 5 ) 23 1 Gray, RL. (3) 843,844,871,872 Grazulcvicius, J.V. (3) 542 Grcci, L. (2.6) 252; (3) 697 Grcen, M.A. (4) 55 Grccncn, E.J.J. (1) 409 Grccnficld, S. (1) 10,327,348, 349; (5) 44 Greenfield, S.R(2.5) 195; ( 5 ) 125, 233 Grecnham, N. (3) 637 Grccr, A. (2.5) 227 Grcgoiy, D.D. (2.5) 94; (2.6) 296; (2.7) 177 Greiner, A. (3) 608,620,626,639 Grciving, H. (2.4) 116 Grela, M.A.(1) 258; (2.5) 20 Grelicr, S.(3) 8 11 Griesbcck, A.G. (2.2) 2,95; (2.4) 14; (2.5) 127, 164; (2.6) 2,3, 98-100, 145; (2.7) 132 Grieving, H. (2.2) 16 Griflin, A.C. (3) 259 Grifiths, J. (1) 116; (3) 877 GrifiLhs, O.J. (2.3) 15; (2.5) 199; (2.6) 96 Grilliths, P.T. (2.7) 158 Grigorcnsko, T.F. (2.7) 93 Grigojcv, N.N. (1) 364 Grilkova, S.E.(1) 33 1; ( 5 ) 119 Grimme, J. (3) 654 Grirnmc, S.(2.4) 60,256; (2.6) 230 Grimsdalc, C.A. (3) 634 Grishanin, B.A. (1) 208; (2.4) 16 Grishko, V.V. (2.3) 70 Grissom, C.B. (2.6) 195; (2.7) 1G Grilsai, Yu.V. (3) 54 Gritsan, N.P. (2.4) 241 Grobys, M. (1) 247 Grodkowski, J. (2.5) 97; (5) 16 Grocn, C.P.(1) 347; (2.5) 135; (2.6) 240; (5) 95 Gromov, S.P. (1) 264; (2.4) 110, 220; (3) 135,136,580 Gromov, S.S.(2.4) 17; (2.6) 19 Gromova, R.A. (3) 424 Groppi, V.E.,Jr. (2.4) 244; (2.7) 81 Gross, R (1) 420; (5) 194 Grossert,J.S. (2) 61
Author It idex Grosshcnny, V. (1) 75; (5) 73,270 Grossi, L. (2.6) 81,82; (2.7) 156 Grovcs, J.T. (2.5) 119 Grubb, C.J. (2.5) 37; (2.6) 302 Grubbs, S. (3) 614 Grubcr, A. (1) I15 Grubcr, H. (3) 3 1 6 Grubs, R.H. (3) 621 Grud, R. (1) 576 G m t , U.-W. (1) 199 Grundcn, B. (3) 212 Grushko, Yu.S. (1) 416 Gryczynski, I. (1) 49 Grzcskowiak, K. (1) 10; (5) 44 Gu, G.( I ) 415; (5) 192 Gu,X. (3) 38 1 Guan,A. (3) 77 Guan, J.J. (3) 204 Gum, J.-Q. (1) 186,210,211; (2.4) 203; (2.6) 32,33
Guardado, P. (2.4) 13 Guardigli, M. (1) 547 Gudmundsdottir, A.D. (2) 13; (2.5) 55; (2.7) 41 Gucgcl, A. ( I ) 420; (2.5) 85; (5) 194
Gucmra, K. (3) 778 Gucnowi, P. (3) 4 15 Gucnther, C. (3) 5 1,52 Guerrcro, D.J. (3) 508 Gucskn, H. (2.5) 150 Gugliclmctti, R (2.4) 74,87; (2.5) 103; (2.6) 45,4749
Guymus, F. (3) 83 1 Gulia, S. (3) 484 Guharay, J. (1) 484 Guiliano, M. (2.5) 159 Guillard, C. (2.5) 147,203 Guillaumc, D. (2.6) 134 Guillet, J.E. (2.4) 54; (2.5) 144; (3) 391,780
Guisezawacki, L.E. (2.2) 24 Guldi, D.M. ( 1) 44 1,442,444, 458-462; (2.5) 3,83,84; (5) 186,187,189,215-219
Gulliya, K.S. ( I ) 206 Gunaratna, H.Q.N. (I) 544-546; (2.6) 243
Gundlach, E.M. ( 5 ) 15 Gunduz, U. (4) 67 Gun'kin, 1.F. (2.7) 194 Gunnlaugsson, T. (I) 544-546; (2.6) 243
Gunthcr, G.(1) 629 Guo, A. (3) 339 Guo, C.(2.7) 18 1 Guo, H.Q. (3) 182
465 Guo, J.X. (3) 858 Guo, L. (3) 434 Guo, Y. (2.5) 230; (2.6) 295 Gupta, N. (1) 455; (5) 198 Gupta, P.K. (2.5) 214 Gupta, S.C. (2.2) 40; (2.5) 38 Gurau, M. (3) 467 Gurunathan, K. (4) 26.37 Guss, W. (3) 608 Gust, D.(1) 324,343,345,346, 380,464,466,468; (2.5) 39, 44,89,222; (4) 9; (5) 5, 11, 12, 89,94, 117, 118, 159,222-225 Gustafson, T.L. (I) 148; (2.7) 34; (3) 663,664 Guth, M. (2.6) 3 17 Guymon, C.A. (3) 11,3 19
Ha, H.Y. (2.5) 141 Ha, J.-H. (1) 23 1 Ha, J.S. (2) 39 Haacke, G.(3) 867 Haag, D. (2.2) 18; (2.4) 119 Haak, M. (2.2) 30 Haarcr, D. (2.6) 228 Habcrhaucr, G. (2.3) 64; (2.4) 123 Habiclicr, W.D. (3) 869 Habuchi, H. (5) 213 Hachimori, A. (3) 389 Hackcthal, M. (4) 69 Hadad, C.M.(1) 148; (2.7) 34 Hadd, A.G. (2.4) 178 Haddad, N. (2.2) 26; (2.4) 107 Hadziioannou, G. (3) 123,625 Hacbcrlc, T. (1) 342; (2.5) 40; (5) 88 Haesslcr, R (3) 20 Hafner, A. (3) 361 Haga, M.(I) 364; (5) 124 Hagcn, V. (2.7) 162 Hagiwara, K. (1) 456,463,465; (2.5) 87,88; (5) 178,220,22 1 Hailcy, P.(I) 499 Hain, C.L. (1) 340 Haino, T. ( I ) 600,601; (2.5) 218; (5) 25 I, 252
Haitjcma, H.J. (3) 498 Hajra, S.(1) 9; (2.5) 14; (5) 29 Hakushi, T. (2.3) 1,2 Halcomb, RL. (2) 15; (2.5) 56 Hall, A.W. (3) 46 Hall, D.O. (4) 64 Hall, M.A. (2.7) 96 Halvcrson, A.P. (2.4) 21 1 Ham, H.S. (3) 66-69, 128,196 Hama, J. (2.4) 70
Haniada, T. (2.5) 63 Hamaguchi, H. (1) 215 Hamai, S.(1) 240; (3) 554 Hamana, T. ( I ) 262; (2.2) 115; (2.5) 35 Hmanaka, Y. (1) 326; (5) 24 1 Hamano, T. (I) 424; (2.5) 106 Hamanouc, K. (1) 144,262; (2.2) 1 15; (2.5) 35 Hamasaki, A. (4) 62 Hambright, P. (2.5) 97; (5) 16 Hmiid, H.S. (3) 12 Hanmarstrocm, L. (1) 355; (5) 23, 24,58,97
Hanuncs-Schiflcr, S.(1) 88 Hamor, T.A. (1) 356; (2.2) 58,59; (2.6) 88; (5) 100 Hamplova, V. (1) 426 Han, J.-H. (2.5) 165 Han, Y.S.(3) 253,254 Hanabusa, K. (1) 3 10; (3) 567; (5) 108
Hanaishi, R (2.5) 29 Handa, S.(2.2) 33 Hannachi, Y. (3) 8 11 Hannah, D.T.B. (3) 466 Hanson, J.E. (3) 45 1 Hanson, K.M. (1) 129 Hansson, P.(1) 471; (3) 410 Hao, J. (3) 720 Hao, X.J.(2.5) 185 Hara, H. (3) 90 Hara, K.(1) 181,491; (5) 228 Hara, M. (4) 45 Hara, T. (1) 496,497,614; (2.2) 84; (2.4) 77,249; (2.6) 43; (3) 520 Harada, A. (3) 268 Harada, H. (4) 57 Harada, K.(3) 780 Harada, M. (2.6) 283 Harada, Y. (3) 537 Haramoto, Y. (3) 482 Harcourt, R.D. (1) 79 Harc, P.J. (1) 599; (5) 250 Harcr, J. (3) 757 Harcs. J.D. (1) 585 Harit, G.(2.6) 47 Hiuilal, S.S.(1) 609 Harima, Y. (1) 120 Haring, B.P. (3) 609 Hwoutounian. S.A. (2.6) 34 Hmcr, H.M. (2.7) 17 Hmiman, A. (1) 15, 16,75,206, 207,360; (2.5) 178; (2.6) 218; (5) 7, 13,31,32,73, 102,270 Harris, F.W.(3) 750
466
Harris, J.M. (2.3) 22; (2.6) 335 Harris, S.J. (1) 499 Harrison, I. (2.7) 12 Harrison, N.T. (3) 634,638 Harrit, N. (2.4) 207; (2.6) 266 H a l , F. (1) 347; (2.5) 135; (2.6) 240; ( 5 ) 95 Hartmann, U. (1) 429; (2.7) 24 (1) 500,532 Hartmann, W.K. Hartmcicr, W. (4) 69 Hartnunpf, M. (3) 503 Hartschuh, A. (1) 35 1 Hartshorn, M.P. (2.4) 165, 167-171, 173-175,267; (2.6) 163-167, 169-173; (2.7) 141-147, 149-151 Hartung, J. (2.4) 268; (2.6) 83; (2.7) 185 Harvey, L.C. (3) 806,807 Haryono, A. (3) 7 10 Hasagawa, K. (3) 525 Hasan, A. (2.7) 160 Hasc, W.H. (2.7) 2 Hasegawa, E. (2) 35,36; (2.4) 214, 215; (2.5) 59; (2.6) 179,301 Hasegawa, H. (4) 48 Hascgawa, K.(2) 35; (2.5) 59; (2.6) 179 Hascgawa, M. (3) 24,256,356 Hascgawa, T. (2) 10,20; (2.2) 79, 89; (2.5) 52; (2.6) 92 Hasclbach, E. (2.5) 23 Hasharoni, K.(1) 348; (5) 233 Hashimoto, A. (4) 49 Hashimoto, K. (3) 195,210 Hashimoto, M.(2.7) 52 Hashimoto, S. (1) 164,305; (2.2) 70; (2.4) 138; (2.6) 111 Hassan, A.A. (2.4) 140; (2.6) 279 Hassan, N.A. (2.5) 210 Hassclbach, E. (1) 289 Hassoon, S.(1) 274; (2.6) 248; (3) 181 Hatakcnaka, K.(2.3) 63; (2.5) 186; (2.6) 320 Hatako, K. (3) 783 Hatanaka, Y. (2.7) 52 Hatano, Y. (1) 606; ( 5 ) 254 Hatashida, Y. (2.3) 68 Hatcnaka, K. (2.3) 62 Hatlevig, S.A. (2.4) 160 Hatton, T.A.(1) 492 Haug, P. (2.6) 198 Haugcn, C.M. (2.4) 159 Haun, M. (3) 683 Haung, P. (2.7) 67 Havlicek, V. (2.4) 50,5 1; (2.6)
Photochemisiry 151, 152 Hawthonilhwaitc-Lawlcss,A.M. (5) 9 Hayakawa, M. (3) GOO Hayasc, S . (3) 535 Hayashi, H. (1) 598; (2) 7; (2.6) 249; (3) 5 11,765; (5) 249 Hayashi, M.(3) 649 Hayashi, N. (3) 257 Hayashi, S. (3) 553 Hayashi, T. (1) 31 1; (2.2) 105; (2.5) 64,220; (5) 106 Hayashi, Y. (2.4) 3 1; (3) 237, 261-263,465 Hayashida, Y. (1) 447; (2.5) 80; ( 5 ) 207 Hayes, G.R.(3) 635,636,638 Hc, B.L. (2.4) I17 Hc, J. (1) 317 Hc, J.H. (3) 168 Hc, J.-j. (2.5) 176 Hc, X.H. (3) 47 Hc, Y. (3) 591 Hcbcckcr, A.(1) 92,247 Hccht, S. ( 5 ) 142 Hcck, D.P. (2.7) 176 Hcckcr, C.R.(1) 272; ( 5 ) 56 Hcdayatullah, M. (3) 532 Hedvig, P.(3) 143,353 Hceg, M.J. (2.3) 69; (2.6) 30 Hccgcr, A.J. (1) 439; (2.5) 28; (3) 366; ( 5 ) 212 Hcclis, P.F. (2.2) 7 1 Hcgazy, I. (2) 48 Hcgcr, A.J. (1) 437; (5) 208 Hcidbrcdcr, A. (2.4) 216 Hcincmam, C. (2.5) 130 Hcinemann, F. (2.7) 199 Heinrich, T. (2.3) 30 Hcislcr, L. (2.3) 5 ; (2.6) 306 Hcitclc, H. (1) 342; (2.5) 40; (5) 88 Hcitncr, C. (3) 797, 805 Hcitz, V. (1) 15,362; ( 5 ) 31, 104, 177 Hcilz, W. (3) 639 Hclaja, J. (1) 328; (2.5) 36; (5) 82, I16 Hcllcntin, P. (3) 544 Hcllcr, B. (2.4) 141; (2.6) 262; ( 5 ) 8 Hcller, C.M. (3) 629 Hclm, S. (1) 390; (5) 163 Hcnuningcr, J.C. (2.5) 226 Hcndcrson, B.W. (2.5) 104 Hcndricks, C.G. (3) 504 Hcndrickson, D.N. (5) 20
Hcnkcl, G. (2.4) 140; (2.6) 279 Hcnnig, H. (4) 13 Hcnning, H.-G. (2.6) 94 Hcnz, A. (2.2) 2; (2.4) 14; (2.6) 2, 99, 100; (2.7) 132 Hepuzcr, Y. (3) 152 Hcrb, T. (2) 29 Hcrbcctz, T.(2.3) 28; (2.5) 133; (2.6) 143 Hcrbich, J. (1) 94 Hcrbst, H. (3) 842 Hcrdcwijn, P. (2.2) 68; (2.6) 112 Hcrges, R. (2.4) 147 Hcrlin, N. (2.4) 105 Herman, H.J. (1) 637 Hcrmans, E. (5) 138 Hcrmant, R.M.(1) 352; ( 5 ) 122 Hcrpich, R (2.2) 110; (2.4) 111 Hcrtcl,I.V.(1)431;(5) 191 Hcrzig, C. (3) 272,280,281 Herzschuh, R (2.6) 285 Hcscmann, P. (3) 620 Hcss, W.P. (2.7) 107 Hcslcr, R.E. (2.4) 17; (2.6) 19 Hcydmann, H. (2.7) 104 Hcys, J.R (2.7) 84 Hibino, H.(3) 7 15 Hicks, P.J. ( I ) 586 Hidaka, H. (3) 741 Hidalgo, J. (2.4) 13 Hidiui, K.4.P.J. (2.7) 52 Hicfijc, G.M.(1) 582 Hiel, G. (2.2) 53 Hien, L.X.(3) 172 Higashi, N. (3) 93 Higashida, S . (1) 84; (2.5) 43; ( 5 ) 68 Higgins, D.A.(3) 458 Hikmct, R.A.M. (3) 27 Hilbercr, A. (3) 625 Hilbcrt, M. (4) 72 Hilinski, E.E. (2.3) 3 1 Hill, D.J.T. (3) 752,753 Hill, J. (3) 722 Hill, M.G. (1) 358; (5) 23 1 Hill, R.H. (2.7) 128 Hill, RR (2) 38; (2.5) 48; (2.6) 253 Hillcnkamp, M. (2.7) 103 Hillcr, M. (2.4) 268; (2.6) 83; (2.7) 185 Hinchcliffc, RD. (2.7) 154 Hinouc, T. (1) 603 Hintz, S.(2.4) 216; (2.6) 307 Hiraishi, T. (4) 11 Hiraniatsu, H. (2.2) 50, 67;'(2.6) 127
A irlhor Index Hiramoto, H. (3) 357 Hirano, A. (4) 68 Hirao, T. (2.3) 36,92; (2.5) 101 Hiraoka, T. (3) 535 Hirashima, H. (3) 856 Hirata, Y. (1) 131; (2.7) 179 Hiratsuka, H. (2.6) 323; (2.7) 188 Hirayama, S.(1) 370,485 Hirobe, M. (1) 424; (2.5) 106 Hirohashi, R. (3) 137 Hirokami, S.(2.2) 56; (2.6) 69 Hiromi, H. (1) 223 Hirota, N. (1) 61 I, 614 Hirsch, J. (1) 342; (2.5) 40; (5) 88 Hirsch, T. (1) 35 I Hirso, K. (1) 589 Hirt, J. (2.2) 2; (2.4) 14; (2.6) 2, 98,99 Hiruta, K.4. (2.5) 19 Hiryama, S. (4) 68 Hisamatsu, T. (4) 50 Hisamatsu, Y. (3) 74 1 Hizal, G. (3) 183 Ho, C.-C. (2) 27,28 Ho, S.Y. (3) 116, 126, 127 Ho, W. (2.7) 9 Ho, Y.-P. (1) 620 Hoang-Van, C. (2.5) 147 Hobbs, S.E.(1) 582 Hochstrasscr, R. (I) 509 Hochslrirtc, D. (2.6) 190; (2.7) 19 Hodges, G.R. (I) 318; (2.5) 134; (2.7) 135 Hpockcr, H. (3) 863 Hocrmann, A. (1) 5 12 Hof, M. (1) 482 Hoffackcr, K.D. (2.3) 37; (2.4) 37, 218 Hoflinan, M.Z. ( I ) 52,73,294, 295; (2.5) 61 Hoffmann, K.(3) 842 Hoflinm, R. (1) 450; (5) 200 Hofstraat, J.W. (1) 637; (2.6) 242 Hogg, G. (1) 573 Hoggan, E.N. (3) 3 I 9 Holcomb, N.R. (3) 297 HoldcroR, S.(3) 5 15 Holdcn, J.M. (1) 434; (5) 214 Holdsworth, D. (3) 716 Hollaender, A. (3) 593 Hollandc, S.(3) 744 Hollcman, I. (I) 409 Holnian, R. (3) 100,222 Holmes, A.B. (3) 5 13,632-634, 637 Holnics, AS. (1) 209 Holmcs, C. (2.4) 259
467
Holmcs, C.P. (2.6) 224; (2.7) 165 Holmlin, R.E. (1) 5 10,5 I3 Holt, D.A. (2.7) 84 Holtcn, D.(1) 228,230; (5) 8 Homma, H. ( I ) 505 Honda, M.(5) 18 Honda, T. (2.7) 82 Hong, B. (3) 393 Hong, E. (2.4) 100 Hang, L.Y.(3) 66*69,128 Hong, 2.( I ) 177 Hopf, H. (2.2) 16; (2.3) 83; (2.4) 116,242 Hopmeicr, M.(3) 639 Horaguchi, K. (2) 35 Horaguchi, T. (2.4) 214,215; (2.5) 59; (2.6) 179 Horata, K. (3) 5 16 Hori, H. (2.5) 95,96 Horibc, 1. (2.6) 79 Horic, K.(3) 295,304,305,461, . 466,785,789,790 Horic, Y. (4) 45 Horiguchi, Y. (2.2) 88; (2.6) 3 10 Horii, T. (3) 488 Horikoshi, S.(3) 741 Horikoshi, Y. (2.4) 227,229 Horinaka, J. (3) 449,601 Horn, C. (2.2) 17; (2.4) 112 Horn, E.(2.4) 258; (2.6) 273 Horn& M.-L. (I) 59,62; (5) 64 Horspool, W.M. (2.3) 15,23,24, 43,44; (2.5) 9, 199; (2.6) 96, 97, 146; (2.7) 7 Hoshino, H. (2.3) 34 Hoshino, M. (2.5) 136 Hosokawa, H. (2.5) 100 Hosomi, A. (2.4) 120; (2.6) 276 Hosotte-Darnc, R (3) 558 Hossain, M.D.(I) 364; (5) 124 Hosscnlopp, J.M. (2.5) 57 Howard, J.A.K. (1) 502 Howc, L. (I) 126 Howcll, A.R. (2) 24 Howcll, B.F.(3) 294 Howgatc, G.J.(3)35 Hoylc, C.E. (I) 25; (3) 60,76,105, 165,259,346,504,507,522, 776 Hrivik, A. (3) 866 Hmcir, D.C. (2.2) 1I; (2.4) 130 Hsiao, J.-S. (5) 259 Hsich, B.R. (3) 61 1,660,662 Hsich, Y.-Y. (5) 274 HSU,C.-P. (1) 60 Hsu, J.S. (3) 649 Hu, A.T. (3) 480
Hu, C.M. (2) 62; (2.6) 216 Hu, D. (1) 5 12; (2.7)199 Hu, kI. (3) 385 Hu, J. (1) 567 Hu, K. (3) 767 Hu, S. (2.4) 248; (2.5) 47; (2.6) 93 Hu, S.K. (2) 25; (2.2) 85-87 Hu, X. (3) 189 Hu, X.Z. (3) 874 Hu, Y.( 1) 643; (3) 436,44 1,547 Huang, B. (3) 421 Huang. C. (2.5) 78 HUUQ, C.-G. (2.3) 87 Humg, C.-H. (1) 440;(5) 185 Huang, D. (2.4) 85 Huang, H. (2.6) 187; (3) 421 Huang, H.W. (3) 461 Huang, J. (1) 405; (5) 180 Huang, J.B. (3) 4 18 Huang, S.(I) 612 Huang, S.J. (2.2) 24 Humg, T.-H. (I) 62 1 Huang, W. (2.2) 37; (2.6) 308 Huang, X.(1) 529 Huang, X.P.(2.7) 163 Huang, X . 4 . (1) 528,534 Huang, Y. (3) 816 Hubcr, H. (3) 615 Hubcr, R (5) I, 4 Hubig, S.M. (I) 322; (2) 41; (2.2) 108; (2.7) 136 Huck, N.P.M. (1) 495; (2.4) 15; (2.6) 10 Hiinig, S.(2.3) 66,67; (2.4) 128; (2.6) 125 Hug, G.L. (1) 5 16; (2.5) 197 Huizer, A.H. (2.4) 59 Hulk, A. (3) 265,347 Hummcl, H. (5) 21 Hununcl, K. (3) 232,337 Huniphry-Baker, R (2.5) 187 Hundertmark, T.(2.5) 127; (2.6) 145 Hung, M.S. (2.7) 139 Hung, S.-C. (1) 345; (2.5) 44; (5) 12,117 Hurklcr, D. (2.4) 133; (2.6) 126 Hunter, C.A. (4) 10; (5) 111, 112 Hunter, D. (3) 722 Hurtubisc, RJ. (1) 158,165,494 Hush, N.S. (I) 96; (5) 60 Husscin, A.M. (2.6) 286 Husscy, D.M. (3) 555 Hwmg, C.-g. (2.4) 161 Hwang, D. (3) 643 Htv;uIg, D.-F. (2.6) 213 Hwang, D.H. (5) 149
468
Hwang, E. (1) 427 Hwang, K.C. (I) 407;(2.5)120 Hydc, RK.(4) 10; (5) 11 1 Hydc, S.C.W. (1) 585 Hynnincn, I. (2.5)36 Hynninen, P.H.(5) 116
Photocheniistry Inaki, Y.(2.2)62;(2.4)226;(2.6)
A.(3)220 J. (3)48 1 Indclli, M.T. (1) 363;(5) 105 110, K. (3)23 1,300,777 Inokuchi, K.-j. (2.5)63 110, M. (3) 257 Inokuchi, Y.(5) 155 Ito,O. (1) 137,406,425,450,452, Inouc, H. (2.2)47;(2.4)142, 194, 454;(2.6)294;(2.7) 180;(3) 246,269,270;(2.5)132;(2.6) 49;(5) 184,199,200,202 115, 160,250,251,259,313; 110, S.(3)440,449 Ibhi, S. (2)34;(2.5)54 (3)278,488 Ito, T.(1) 267 Ibuka, S.(4)50 Inouc, K. (2.7)47,48;(3) 5 17 Ito. Y.(2.3)41;(2.4)39,230,231; Ibusuki, T.(2.5)95,96 Inouc, s.(4)44 (3)443 Ichida, M.(1) 4I2 Inouc, T.(3) 624 Itoh, C.(3) 74 Ichihashi, Y.(5) 18 Inouc, Y.(2.3)1.2; (4)35 Itoh, H. (3) 138 Ichikawa, K.(2.7)62 Insuasty, B. (2.4)204;(2.6)197 Itoh, K.(2.4)49;(2.6) 153; (3)88 Ichimura, K.(2.4)31; (3) 146,237, Inui, H. (2.7)118 Itoh, M.(1) 223,374;(2.4)61, 261-263,465,492,495 Inui, T.(2.5)71;(2.6)130 221;(2.7)42;(5) 164 Idc, N. (3)312 Ion, R M . (3)560 Itoh, S. (2.3)93;(2.6)141;(2.7) Iida, 1. (2.4) 120;(2.6)276 Ipsalc, S.(3)728 195 Iida, K. (2) 10 Iqbal, S.(2.6)245 Itoh, T.(I) 142;(5) 213 liqbal, S.(1) 555 Irk, M. (2.4)8; (2.6)I I, 37,38; Itoh, Y.(3)389 lizawa, T. (4)2 Ivanov, M. (3)471 (3)4 Ikada, E. (3)743,771 Irngiulingcr, H.(2.2)83, 110; (2.3) Ivanov, V.B.(2.5) 18; (3) 61,390, Ikawa, H.(2.5)189 64;(2.4)I 1 1, 123 593 Ikeda, A, (3)6 Isaacs, N.W.(5) 9 Ivanova, E. (3)707 Ikeda, E.(4)56 Isham, K.R.(2.7)160 Ivanova, T.M.(2.7) 83 Ikcda, H. (2.3)31,39,41;(2.4)39, Ishibashi, D.(2.5)132 Ivanlsov, A.A. (I) 32 40 Ishida, A. (1) 421,422;(2.3)39; Ivcrson, B.L.(2.5)67 Ikcda, M. (2.6)312 (2.4)101,219;(2.5)122, 152; Iwai, S.(2.2)72 Ikeda, N. (1) 384;(5) 168 (2.6)140; (4)7;(5) 195, 196 Iwai, T. (2.2)97 Ikcda, T.(1) 558 Ishida, T. (2.4)134;(2.6)119 Iwaizumi, M. (2.5)29 Ikchara, T. (3)78,239 Ishiguro, K. (2.4)25 1 Iwamoto, H. (2.2)100; (2.4)150 Ikchira, H. (2.6)176 Ishihara, A.(3)893 Iwiunoto, T.(2.6)316 Ikcno, T.(2.6) 283 Ishihara, K.(2.2)1 1 1 Iwmura, H. (1) 559;(2.7)47-49; Ikcyama, T.(I) I80 Ishihara, T. (2.7)59 (3)517 Ikcyama, Y.(3)499 Ishii, C.(3) 87 Iwamura, M. (2.5)138;(2.6)25 Ikuno, H. (3)883 Ishii, T.(3)525 Iwanami, H. (2.4)214,215 Ihta, Y.(4)62 Isliii, Y.(2.7)208 Iwasa, S.(2)30,31 Il'ichcv, Yu.V.(1) 92, 197,247; Ishikawa, M. (2.4)94;(2.5)100, lwasaki, F.(2.3)62,63;(2.5)186; (2.4)29;(2.6)14,I5 169;(2.6)314,315;(2.7)174, (2.6)320 Ilk, A.(3) 143,353 190;(3)516 Iwasaki, T.(1) 202;(2.6)20,2I Il'ymova, D.D.(3) 64 Ishikawa, T.(3) 197 Iwata, K. (1) 215 Imada, M.(2.2)79.89 Ishimaru, Y.(1) 559;(2.7)49 Izawa, Y.(2.2)84;(2.4)249 Imac, I. (3)475 Ishitani, 0.(2.5)95.96 Izwiii, Y.(2.4)258;(2.6)273 Imac, T.(2.4)22I Ishitani, Y.(3)278 Izwmdov, V.A.(3)420 Imahashi, S.(3) 177 Ishiwatari, T.(2.4)78 Iniahori, H.(I) 456,463,465; Ishiyama, K.(2)36 (2.5)87,88;(5) 178,220,221 Islam,Q.(2.4)202;(2.6)31 Jabbar, S. (2.2)91;(2.4)65 Imai, K. (1) 505 Isobc, M.(2.5)216 Jackson, J.E.(2.7)5 Imai, T,(3) 764,766 Isracl, G.(2.6)285 Jacobi, P.A. (2.6) 150 Imai, Y.(3)526 Israeli, Y.(3) 713 Jacqucmin, R (1) 436;(5)209 Imanishi, Y.(1) 397 Issa, Y.A.(2)48 Jacqucs, P. (1) 53,82;(5) 8 1 ~ma~aka, T. (3)787 Issac, R.C.(1) 609 Jacquct, L. ( 1) 386;(5) I76 Imasato, H.(1) 5 14 Issiki, T. (2.4)271 Jagcr, M.(2.7) 104 Imasc, Y.(2.2)79.89 Itagaki, H. (3) 505 Jacgcr, W. (1) 74,93,338,495; Imgartingcr, T.(1) 107 Itaya, A. (1) 303,323,326,433; (2.4) 15;(2.5) 193;(2.6)10;(3) In, Y.(2.4) 134;(2.6)119 (2.5)207;(3)394;(5) 241,242 186,213,214;(5) 66,72,'227 Inagaki, Y.(3)505 Ilho, N.(3)236 Jain, S.C.(3) 75 105
110, 110,
AufhorI d e x
469
Jamcson, D.N. (1) 508 Jamicson, G.B.(2.4) 168;(2.6) 164;(2.7) 144 Jana, P.(1) 262 Jmg, D.-J. (2.6)233 Jang, J.H. (4) 58 J a g , J.-S. (2.4) 159 Janietz, D.(I) 488 Jankicwicz, S.V.(3) 192 Janota, H.(3) 108 Janscn, J.F.G.A. (1) 453;(2.5)8 1; (5)203 Janscn, RA.J. (1) 453;(2.5)28, 8 1;(3) 474;(5)203 Jao, T.C. (3)55 1 Jaquinod, L. (1) 228;(5) I3 1 Jardon, P. (2.6)52 Jariwala, C.(3)76 Jasso, A.R (3)44 Jayanthi, S.S.(1) 282 J ~ a n C, . 4 . (2.7)204 Jcffcry, J.C. (5) 170, 174 Jcffs, G.E. (2.5)48;(2.6)253 Jcliakowz, B. (3) 742 Jende, T.(2.6)285 Jendrhc, S.A. (1) 26,304;(3)405, 628;(5)34 Jcnka, W.S.(2.7)176 Jenkins, R.D.(3)434 Jcnks, W.S. (2.5)58,94,230;(2.6) 295,296;(2.7) 177 Jcnneskens, L.W. (1) 353;(2.6) 3 19;(3)477 Jcnniskcns, H.G. (2.7) 159 Jcnscn, A.W. (I) 427;(2.2)12; (2.4)262;(2.6)292 Jcnscn, K.F. (3)823 Jcnscn, T.(2.7)85 Jcnusauskas, G.(1) 523 Jeong, Y.T.(3)67-69 Jcoung, S.C. (1) 231;(5) 149 Jcsscn, S.W. (3)663,664 Jctt, J.M. (1) 38 Ji, J. (2.7)65 Ji, L.-N. (1) 506,507 Ji, Q.(2.7) I17 Jiang, D.(3)590 Jiang, J.-B. (1) 478 Jiang, M. (3)435 Jiang, S.(2.5)7 Jiang, S.A. (1) 391;(2)37;(2.6) 318;(5) 162 Jiang, T.(3)160 Jiang, X.-K. (2.5)27 Jiang, Y. (1) 529;(2.4)261;(2.5) 168;(3)248 Jiang, Y.-B. (1) 528,534 '
Jiang, 2. (2.5) 174;(2.6)174 Jikci, M.(3)526 JimCncz, C.F. (3)523 Jimcncz, J.M.(2.6)189;(2.7)20 Jimencz, M.C. (2.3)1 I; (2.4)163 Jin, C.(1) 612 Jin, L.(3) 259 Jin, M.-G. (I) 478 Jin, R.(3)590 Jin, S.H.(3) 661 Jin, S.M. ( I ) 414 Jin, T.(4)44 Jin, W.(1) 535 Jin, X.(I) 382;(5) 165 Jing, H.(1) 527 Jipa, S.(3) 672,682,853 Jo, J.C. (3)641 Jo, K.D. (3) 157 Jobst, G.(3)3 I6 Jockusch, S.(I) 284;(2.5)24; (2.6)232,235,237;(3)5 18 Jodickc, K. (2)22 Joensson, J.E. (3) 327 Joensson, S.(3) 60,105,165,346 Johansscn, L.B.-A. (5) 139 Johansson, C.I. (1) 321;(2.2)3; (2.4)99 John, V.T.(3) 417,423 Joluicn, E. (5) 91 Johnson, A.E. (2.7)3 Johnson, F.P.A. (2.5)96 Johnson, J.E. (2.6) 155 Johnson, M.L. (1) 49 Johnson, P.Y.(2.6)290 Johnson, R.P. (2.7)169 Johnson, T.E.(5)259 Johnston, B.(3) 876 Johnston, B.K. (3) 822 Johnston, L.(2)4 Johnston, M.V.(2.7)140 Johnstone, A. (2.7)55 Jollifl'c, K.A. (2.6)239 JOIICS,A.S. (3)506 Joncs, C.J. (1) 356;(5) 100 Joncs, D.T.(2.4)129 Joncs, G.(3)592;(4)53 Joncs, G.,I1 (I) 327 Joncs, P.B. (2)57;(2.4)253 Joncs, R.H.(2.6)135, 149 Jo~lknian,A.M. (1) 512,588 Jonusaukas, G.(2.5)192;(5) 272, 273 Josclcvicb, E. (2.5)179;(5) 109 Joxph-Nitlhan, P. (2.2)46 Jossi, S.(3) 536 Jovanic, S.V.(3)810 Jovcr, A. (1) 630
Julia, L. (1) 426 Julliard, M.(2.4)124 Jullicn, L. (1) 538,539;(2.6)244; (5)268 J u g , B. (2.7) 112 J u g , J. (2.5)108 Junk, T.(2.6)33 I Juskowiak, B.(5) 146 Jusuja, R.(1) 508 Kabnlhova, N.N. (2.5)116 Kabanov, V.A. (3)420 Kabasakaiian, P. (2.6)80 Kabcta, K. (3)764,766 Kabra, B.V. (2.5)213 Knbulo, C. ( I ) 180;(2.6)316 Kaczniarcli, H.(3)746,758,784, 826 Kacmarck, L.(2.4)68 Kadodwala, M. (2.7)159 Kaddura, Y. (2.6)323;(2.7)188 Kacllcbring, B.(5) 129,130 Kafifi, Z.H. (3)849 Kagancr, E.(5) 109 Kdima, N.(3)485 Kai, A. (2.7) 157 Kai, R.(2.4)150 Kai, S.(2.5)215 Kai, Y. (3)817 Kaiscr, T. (3)80 Kaiku, Y.(1) 395;(5) 175 Kajihara, Y.(2.2)97 Kajimoto, 0.(1) 491;(5)228 Kajiwvara, A. (3) 182 Kajiyama, T.(3) 89 Kajizuka, Y.(2.2)112;(2.4) 153 Kakchi, A. (2.4)219 Kakimdo, M. (3)526 Kako, M. (2.3)62.63 Kakuda, H.(2.2)56;(2.6)69 Kakuma, S.(2.3)62.63; (2.5)186; (2.6)320 Kalisch, W.W. (5) 127 Kallcbring, B. (I) 216;(2.4)55 Kalonlarov, I.Ya. (3) 860 Kaluschc, G. (I) 1 10 Kani, S.M. (2.7)35 Kaniachi, M.(1) 299,301;(3) 182, 577;(5)235 Kaniachi, T.(2.5)68;(4) 11; (5) 83 Kamada, M. (I) 413 Kaniat, P.V.(1) 432 Kamab, M. (2.6)301 Kamalh, M. (3) 338 Kanibc, N.(2.4)252; (2.6)326
Photochemistty
470 Kamcoka, I. (2.2) 100 Kamcyama, A. (2.7) 166; (3) 138, 298,333,476; (4) 18 Kamcycv, S.V. (3) 219 Kaminska, A. (3) 746,747,804, 826 Kamiya, H. (2.2) 72 Kamiya, K. (2.6) 267; (3) 537 Kanmcrmcicr, S. (2.4) 147 Kan, Y.Z. (2.2) 114 Kanaglingam, S. (1) 474 Kanaoka, Y. (2.7) 52 Kanatzidis, M.G. (3) 470 Kanazawa, K. (3) 2 10 Kanbara, T. (3) 525 Kanda, K. (3) 260 Kanda, M.N.(3) 865 Kancda, T. (1) 84; (2.5) 43; ( 5 ) 68 Kancko, J. (1) 496,497 Kancko, M.(I) 23; (3) 386,566; ( 5 ) 45 Kancko, T. (2.4) 269; (2.6) 25 1 Kancko, Y. (1) 169; (2.5) 73 Kancniatsu, Y. (1) 522 Kancmoto, K. (1) 503; (2.6) 60 Kancmoto, M. (2.5) 99, 100 Kang, H.G. (3) 68 Kang, I.-N. ( 5 ) 149 Kang, T. (2.4) 236; (2.6) 104 Kang, W.B. (3) 24 I Kannan, P. (3) 349 Kanncr, G.S. (3) 586 Krumurpatti, R.A. (3) I I, 48, 124, 3 17 Kapcller, H. (3) 337 Kaplan, D. (3) 417,423 Kaplan, L. (2.6) 66 Kapoor, M. (2.2) 40; (2.5) 38 Kapplcr, P. (3) 754 Kaprinidis, N.A. (2) 5; (2.2) 12; (2.6) 232 Kapturkiewicz, A. (1) 94 Kapustin, G.V. (3) 396 Karachcviscv, V.A. (1) 97 Karascv, V.E.(3) 570,571,574 Karatsu, T. (2.5) 157; (2.6) 192; (2.7) 25 Karg, S. (3) 644 Karimov, F.Ch. (3) 859 Karminski-Zaniola,G. (2.4) 213, 265; (2.6) 62 Karpachcva, G.P. (2.5) 223; (3) 398 Karpiuk, J. (I) 94; (2.4) 68 Karprinidis, N.A. (1) 501 Karukstis, K.K. (1) 469 Karyakina, L.N.(2.6) 199; (2.7) 78
Kasai, C. (3) 305 Kasatani, K. (1) 203 Kasatani, R. (2.6) 176 Kasc, Y . 4 (2.5) 63 Kashinia, C. (2.4) 206 Kashiwagi, H. ( 5 ) 126 Kashiwagi, K.(3) 298 Kaspcrczk, J. (3) 536 Kasscm, M. (1) 369 Kastliurikrishnan, N. (2) 61 Kasunagai, H. (2.4) 264 Kaspova, E.I. (3) 860 Katagiri, N. (3) 256 Kataowlia, F. (3) 391 Katayama, S. (2.2) 50 Kathcr, K. (2) 22 Kato, M. (1) 497; (2.2) 57; (2.6) 129,320; (3) 87,260,27 1 Kato, N. (2.3) 42 Kato, T. (2) 35,36; (2.5) 59; (2.6) 179 Katrit;r.y, A.R. (2.4) 204; (2.6) 197 Katsnclson, A.A. ( 5 ) 190 Katsu, H. (2.4) 22 I Katsuda, N. (3) 887 Kabuki, A. (2.5) 45 Katz, E. (I) 557 Katzcncllcnbogcn, J.A. (2.6) 34 Kaufman, M.J. (3) 377 Kaupp, G. (2.2) 30 Kaupp, U.B. (2.7) 162 Kavash, R.W.(2.5) 76; (2.6) 90 Kawahara, Y. (2.4) 134; (2.6) I19 Kawahata, S.(1) 378; ( 5 ) 157 Kawai, A. (1) 595 Kawai, S. (3) 8 17 Kawai, T. (3) 66 1 Kawakanii, H. (2.2) 117; (2.4) 234 Kawamura, F. (3) 8 17 Kawamura, S. (I) 42 1,422; (5) 195,196 Kawanashi, S. (2.7) 138 Kawanislii, S.(2) 46; (3) 708 Kawasaki, K.4. (2.4) 150 Kawasaki, M. (2.7) 192 Kawaski, A. (3) 584 Kawasugi, T. (4) 4,63 Kawata, E. (2) 10 Kawata, K. (3) 300 Kawata, Y. (3) 553 Kawabuki, N. (3) 264,34 1,468 Kawazaki, S. (4) 7 1 Kawski, A. (1) 135, 175,246 K a d o v , D.V. (2.5) 116 Kazakov, S.P. (2.4) 187 Kazakov, V.P. (2.5) 154, 156; (3)
373 Kazarian, S.(2.7) 116 Kaz'min, A.G. (2.3) 59; (4) 19 Kcyschi, C. (5) 128 Kcana, J.F.W. (2.7) 87 Kcck, J. (3) 404 Kccnc, F.R (1) 325; (5) 123 Kccsc, R (2.2) 22 Kcglcvich, G. (2.6) 336 Kcllcr, H.-M. (2.7) 110 Kcllcr, L. (3) 555 Kcllcr, M. (2.4) 133; (2.6) 126, 190; (2.7) 19 Kcllcr, RA. (1) 38, 105, 108, 112 Kcllerhals, M. (3) 358 Kellman, A. (2.4) 241 Kclly, C.A. (5) 15,99 Kclly, R.N. (2.4) 97 Kclly, S.O. (3) 45 1 Kcinnitz. K. (1) 583 Kcnnis, J.T. ( 5 ) 46 Kcrcha, S.F.(3) I19 Kcrn, W. (3) 232,337 Kcrrigan, P.K. (5) 12 Kcrscy, I.D. (2.7) 5 1 Kcrst, C. (2.6) 324; (2.7) 187, 189 Kersting, R (3) 644 Kcssab, L. (3) 8 11 Kctskemcty, 1. (4) 72 Kcttncr, R (1) 106 Kcycs, T.E. (5) 167 Khabashcsku, V.N. (2.6) 329; (2.7) 69 Khairutdinov, R F . (I) 190 Khalimskaya, L.M.(2.7) 90 Khamacv, Kh.V. (3) 859 Khan, K.A. (2.7) 191 Khan, M.A. (3) 192 Khan, S.A. (3) 336 Khan, S.I. (2) 49 Kliaritonov, A.P. (3) 769 Kliarlanov, V.A. (2.4) 62,197; (2.6) 35 Kliasanov, S.S.(1) 446; ( 5 ) 205 KIiatami, H. (3) 880 Khatib, S. (2.6) 229 Khim, S.-K. (2.5) 191 Klunclinsliil, LV. (1) 48 Khranovskii, V.A. (3) 119 Khusainova, A.I. (2.5) 154, 156 Khyulishto, V.N. (1) 369 Kido, G. (1) 598; ( 5 ) 249 Kido, N. (1) 302 Kidowvaki, M. (3) 146 Kicfcr, B. (2.7) 113 Kicslingcr, D. ( I ) 587 Kikai, A. (3) 341
Author hidex Kiktcva, T.A. (1) 306; (5) 244 Kikuchi, H.(3) 89 Kikuchi, K. (1) 182,430; (2.4) 28 Kikuchi, S.(2.2) 42; (2.4) 205; (2.6) 269 Kilimnik, A.B. (3) 845 Kilpelainen, 1. (1) 328; (2.5) 36; (5) 82, 116 Kilzcr, F. (1) 113 Kim, A.R (2.2) 99, 103; (2.4) 151, 154 Kim, B.C. (3) 254 Kim, C.S. (2.5) 108 Kim, D. (I) 23 1,25 1,4 14; (2.6) 123; (2.7) 201; (3) 641; (5) 149 Kim, G . 4 . (2.4) 100, 144 Kim, H. (2.7) 112 Kim, H.J. (2.4) 155 Kim, H.K. (3) 622 Kim, J.H. (2.2) 15; (2.4) 155 Kim, J.J. (3) 65 1 Kim, J.L. (3) 530 Kim, K. (3) 583 Kim, K.A. (3) 530 Kim, K.C. (2.7) 32 Kim, S.K.(1) 414; (2.7) 95 Kim, S.S. (2.2) 99, 102, 103; (2.4) 151,152, 154 Kim, T.Y. (2) 16 Kim, W.G. (3) 66,157,174 Kim, W.S. (3) 342 Kim, W.Y. (5) 169 Kim, Y.H. (1) 251; (2.4) 161 Kim, Y.M.(1) 627; (2.2) 77 Kim, Y.N. (2.5) 62 Kim, Y.-R. (1) 23 1 Kim, Y.S. (3) 212 Kim, Y.W. (2.3) 7; (2.4) 25; (3) 64 1 Kimoto, S. (2.4) 246; (2.6) 160 Kimura, C. (I) 496 Kimwa, K. (3) 494; (4) 63 Kimura, M. (1) 3 10,497; (3) 567; (5) 108
Kimura, S. ( I ) 397 Kirnura, T. (2.4) 258; (2.6) 273; (3) 260 Kinbara, K. (2.7) 157 Kincaid, J.R. (1) 307; (3) 338; ( 5 ) 245 King, N.R. (2.6) 13 Kinhiut, W.J. (3) 97 Kinoshita, K. (1) 584 Kira, M.(2.6) 3 16 Kirby, J.P. (1) 314,315; (5) 110, 232 Kirchoff, J.R. (1) 569
47 1 Kirchoff, M.M. (2.7) 169 Kircchcnko, G.N. (3) 862 Kirichcnko, A.V. (1) 222 Kirk, M.L. (1) 357; ( 5 ) 101 Kirmaicr, C. (I) 230; (5) 8 Kirmsc, W. (2.6) 317; (2.7) 46 Kirschbcrg, T. (2) 33; (2.4) 216; (2.5) 92 Kirsch-Micsmackcr, A. (1) 386; ( 5 ) 176 Kisch, H. (2.4) 90, 103; (2.6) 131, 132 Kiscrow, D.J. (3) 443 Kishi, K. (3) 299 Kishikawa, K. (2.3) 33; (2.4) 125, 182; (2.6) 120 Kishimoto, S. (1) 589 Kishorc, K. (3) 63 Kisilowski, B. (2.2) 45 Kitahama, Y. (1) 530 Kitahara, N. (1) 373 Kilamura, A. (2.5) 157; (2.6) 192; (2.7) 25 Kitano, Y. (2.5) 189 Kitatani, T. (4) 52 Kitazawa, M. (I) 2 18 Kitazuinc, T. (2) 35; (2.5) 59; (2.6) 179 Kitchen, C. (4) 53 Kityk, I.V. (3) 40,536 Kitzcrow, H.S.(3) 75 IGuchi, F. (2.2) 88; (2.6) 3 10 Kiyota, A. (2.5) 157; (2.6)192; (2.7) 25 Klhicr, F.-G. (2.6) 190; (2.7) 19 Klamann, J.D. (3) 835 Klavctter, F. (3) 823 Klce, J. (3) 301 Klcin, C. (1) 526 Klcin, M.P.( 5 ) 19 Klcmchuk, P.P. (3) 870 Klcssingcr, M.(2.4) 216 Kleverlaan, C.J. (2.7) 196 Klcyn, A.W. (2.7) 159 Klimcnko, L.S. (2.4) 24 I Klok, D.A. (2.3) 70 Kloosterbocr, H. (3) 270 Klosc, E. (1) 103 Knobloch, G. (3) 855 Knoch, F. (2.6) 131 Knccr,G. (I) 121 Knorr, A. (1) 553; (3) 379,585 Knowlcs, D.B. (2.4) 75; (3) 478 Knox, R.S. (1) 646; ( 5 ) 5 1 Knyazcva, T.E. (3) 321 Knyazhruiskii, M.I.(2.4) 62,69, 83, 197; (2.6) 35
Knyuhshto, V.N. (I) 377; ( 5 ) 156 KO,H.S. (1) 627 Kobayashi, E.(3) 601 Kobayashi, H. (2.5) 100, 189 Kobayashi, K. (2.2) 13, 112; (2.4) 145,153 Kobayashi, M. (2.7) 59 Kobayashi, N. (3) 137,505 Kobayashi, S. (2.6) 323; (2.7) 188 Kobayashi, T. (3) 6 19,686 Kobayashi, Y. (2.6) 25; (2.7) 59 Kobertz, W.R. (2.2) 69; (2.4) 136, 137; (2.6) 109, 110 Kobori, Y. (1) 595 Kobryanskii, V.M. (3) 605,655 Koch, B. (5) 15 Koch, T.H.(2.2) 74 Kochcvar, I.E.(1) 205; (2.6) 256, 257; (2.7) 182, 183 Kochi, J.K. (1) 322; (2) 41; (2.2) 106-108; (2.5) 69,232; (2.7) 136 Kodaka, M.(2.5) 201 Kodaka, T. (1) 120 Kodymova, J. (1) 426 Kocbcrg, A.T. (2.3) 53 Kocdijk, J.M.A. (1) 648 Kochlcr, G. (1) 245,248,526 Kocllner, M.(1) 110 Kocnig, B. (2.2) 17; (2.4) 112 Koepping, G.G. (3) 615 K o ~ ~N. I , (1) 559; (2.7) 47-49; (3) 517 Koga, Y. (1) 137; (2) 1; (2.2) 56; (2.3) 91; (2.4) 66 Kogo, Y. (2.4) 263; (2.7) 207 Koliler, B.E. (2.3) 49 Kohnioto, S. (2.3) 33; (2.4) 125, 182; (2.6) 120 Kohno, M.(4) 35 Kohtani, S. (1) 374; (2.7) 42; ( 5 ) 164 Koikc, K. (2.5) 95,96 Koinuma, Y. (3) 685 Koishi, M.( I ) 584 Koizumi, M. ( I ) 605, 606; (3) 556; ( 5 ) 253,254 Kojcr, R (1) 320 Kojima, K. (1) 178; (3) 257 Kojima, M. (2.4) 219; (2.5) 198 Kojima. R (2.3) 14,45; (2.4) 4, 179; (2.6) 9,139,142 Kolb, U. (5) 2 1 Kolczak, U. (2.6) 338; (3) 855 Kolcsnik, S.N.(1) 4 16 Kolcsnikov, V.A. (3) 627 Kolle, C. (1) 581
472
Kollegcr, G.M. (2.6) 309; (2.7) 170,171
Kolniakov, A. ( I ) 4 13 Koniaki, M. (3) 883 Komatsu, M. (2.4) 252; (2.6) 326 Komatsu, T. (2.5) 22 1 Kombcr, H.(3) 255 Konagai, M. (4) 58 Konami, H. (1) 454; (5) 199 Konarcv, D.V. (1) 446; (5) 205 Kondilenjo, V.P. (1) 306; (5) 244 Kondo, S . (2.7) 64 Kondo, T. (2.4) 78; (5) 14 Kondoh, T. (3) 74 Kondratcnko, P.A. (2.7) 93 Konc, A. (3) 532 Kong, D. (3) 568 Kong, F. (1) 127; (2.7) 109 Konishi, S . 4 (2.4) 142; (2.6) 115 Konoikc, T. (2.6) 79 Konstantinova, T.N. (3) 529, 861 Konya, K. (2.3) 35; (3) 808,810 Kooijman, H. (1) 353; (2.6) 3 19 Kook, S.K. (3) 174 Koolc, L.H. (3) 561 Kopclrnan, R. (1) 44; (5) 263,266 Kopiskc, C. (2.4) 18 Kopping-Grcm, G. (3) 614 Koytyug, I.V. (2.6) 339 Kopylova, R.(5) 153 Kopylova, T. (1) 375 Korall, P. (1) 355; (5) 23,24,97 Koreck, L. (3) 892 Komclyuk, A.I. (1) 5 17 Komcr, S. (3) 869 Korotkov, V.I. (4) 40 Korovin, Yu.V. (1) 123 Kosa, Cs. (1) 3 Koscki, N. (2.2) 88; (2.6) 3 10 Koscki, S. (2.6) 201; (2.7) 74 Koshirna, H.(1) 31; (2) 40; (2.4) 92; (2.5) 177; (2.6) 12,255; (2.7) 137 Kosower, E.M.(3) 617 Kossanyi, J. (2.6) 54 Kostcnko, L.I. (3) 627 Kostic, N.M. (1) 5 18 Kosugi, M. (3) 685 Kotani, S.(1) 303,323,326; (2.5) 207; (3) 394; (5) 241,242 Kolkov, S.Yu. (I) 208 Kotov, B.V.(3) 396 Koudounias, E. (1) 426 Kougo, S.(2.7) 50 Kovarova, J. (3) 852 Kowalczyk, A. (1) 642 Kowalczyk, J. (1) 634
Phoiocheniistry Kowalczyk, T. (3) 233 Kounacki, K.(2.4) 68 Koyania, T. (2.7) 63 Koyano, 1. (2.3) 33; (2.4) 182 Kozankicwicz, 8.( I ) 393 Kozlccki, T. (2.3) 3; (2.4) 22; (2.6) I02 Kozlovski, V.F. (5) 190 Kozlovskii, D.A. (1) 204 Kragol, G. (2.3) 46; (2.4) 53 Krahl, R. (I)103 Krajnovicli, D.J. (3) 788 Krakovyak, M.G. (3) 424 Kratncr, E. (3) 834 Kramcr, H.E.E. (3) 404 Kramcr, M.C.(3) 44 I Kramcr, W. (2.6) 100; (2.7) 132 Krasa, J. (1) 426 Krasnovsky, A.A., Jr. (1) 368,380; (2.5) 11 1; (5) 159 Kratochvil, B.(2.4) 5 1; (2.6) 152 Kraus, G.A. (1) 225,524; (2) 21; (2.2) 118
Kraus, S. (1) 436; (5) 209 Krausz, E. (1) 6 17 Krcig, M. (1) 205 Krichcldorf, H.R.(3) 343,467 Kricgcr, C. (1) 342; (2.5) 40; (5) 88
Krihak, M. (1) 475 Krishtia, T.S.R.(2.3) 73; (2.6) 106 Krishnasamy, V. (3) 329 Krissinci, E.B. (1) 308; (5) 247 Krocze, E. (3) 123 Krogh-Jespcrscn, K. (2.7) 44 Krohnkc, Ch. (3) 67 I Kron, A. (3) 668,669,687 Krongauz, V. (3) 485 Kroon, R. ( I ) 54 Kruegcr, B.P.(5) 259 Krucgcr, C. (2.4) 18 Krull, U.J. (1) 474 Kruppa, A.I. (2) 64 Kryschi, C. (1) 174 Kryszcwski, M. (3) 419 Krzossa, B. (2.7) 46 Krzyzanowska, E. (2.6) 220 Kuan, D.P. (2.2) 53 Kubat, P. (1) 118,426 Kubic, R. (2.4) 50,51; (2.6) 151, 152
Kubicki, A. (1) 135 Kubisa, P. (3) 152 Kublickas, R. (2.7) 27 Kubo, K. (2.2) 47; (2.4) 194,269, 270; (2.6) 250,25 1,259
Kubo, T. (4) 60
Kubomura, K. (2.5) 132 Kubota, G. (2.4) 269; (2.6) 25 I Kubota, H. (3) 190 Kuciauskas, D. (1) 345,346, 466-468; (2.5) 44,89; (5) 117, I 18,222,224,225 Kucybala, Z. (1) 5 1 Kudashcva, D. (3) 186 Kudo, K. (2.4) 31; (3) 146,237, 26 1-263.465 Kudo, T. (3) 24 1,242 Kuchnlc, W. (1) 92, 197,247; (2.4) 29; (2.6) 14 Kuklinski, B. (1) 175; (3) 584 Kuldova, K. (2.4) 67; (2.6) 227 Kulcshov, S.P. (3) 573,581 Kulkami, D.G. (2) 50 Kuniachcva, E. (3) 434,550 Kumaliura, K. (2.2) 89 Kumdwa, S . (2) 10 Kumilr, A. (2.5) 114, 194; (2.7) 172 Kumar, J. (3) 486,618 Kuniar, J.S.D.(2.5) 188 Kumar, S.(2.5) 194; (3) 92 Kmar, S.A. (2.3) 25; (2.4) 42 Kumazaki, S.(1) 31 1; (2.2) 105; (2.5) 220; (5) 106 Kunictsova, R.(1) 375; (5) 153 K u d c , M.U. (1) 637 Kwnmcr, S. (1) 106, 113 Kwiai, K. (3) 5 16 Kungl, A. (3) 588 Kunihiro, T. (2.5) 157; (2.6) 192; (2.7) 25 Kunishi, N. (4) 71 Kunkcly, H.(I) 457; (5) 206 Kunugi, Y. (1) 120 Kunzc, A. (1) 288; (3) 272,280, 28 1 Kuranioto, N. (3) 878 Kurando, T. (3) 5 11,765 Kurauchi, M. (2.4) 61 Kurdikar, D.L. (3) 23 Kurcishi, Y. (5) 132 Kuriaki, H. (4) 63 Kurihara, S. (3) 73 Kurita, H.(4) 56 Kurita, Y. (2.2) 97 Kuriyama, K. (3) 89 Kuriyama, Y. (2.7) 118 Kurmitsky, V.A. (1) 377; (5) 156 Kuroda, R.(2.2) 42; (2.4) 205; (2.6) 269 Kurosaki, T. (2.7) 167 Kurrcck, H. (2.5) 42; (5) 92 Kurrcck, M. (5) 91
Atifhorhidex Land, E.J. (5) 1 1 Landgraf, S. (1) 577 Landis, M.S.(I) 501;(2)5; (2.6) 339 Lane, P.A. (3)399,623 Lang, A. (2.7)30;(3) 827 Lang, J. (3)406,426 Lang, M.J. (1) 334;(2.4) 180;(5) 85 Langc, A. (2.5) 105 Langc, C. (2.3)55 Langc, G.L. (2.2)8;(2.6) 3 1 I Langcr, R. (3)235 Langcr, V, (2.4)228 Langcvcid, B.M.W. (3)474 Langford, S.J. (2.6)239 Langhals, H.(3)533 Lankicwicz, L. (1) 149 Lanza, M.(I) 525 Lanzalunga, 0.(2.6)293 Lanmi, G. (3) 452,613 Lapouyadc, R.(1) 253,523;(2.4) 24;(2.5)192;(5)272,273 Larcginie, P. (2.4)87 Larscn, J. (2.4)207;(2.6)266 Larscn, R (1) 292,508;(2.5)4 I Larsson, S. (1) 216;(2.4)55;(5) 129, 130 L a m , J.C. (4)53 Laschewshy, A. (1) 482 Laska, L.(I) 426 Laanc, J. (1) 220 Laski, J.L.(3)825 Labarthet, F.L. (3) 493 Lashy, P.J.(2.7)191 Lacaze, P.C. (3) 532 Lahioor, E.C. (2.5)21 Latimcr, M.J. (5) 13 Lacey, D. (3)46 Laccy, D.J. (3)671,673,674 Lato, S.(1) 487 Lacombc, S.(2.5)228 Latowski, T. (2.6)2I7 Lacostc, J. (3) 198,713,725,729, Lattcrini, L. (I) 287;(2.3)19 740,774,879 Lau, W.W. (3) 874 Laguitton-Pasquicr, H.(1) 481 Lauc, T. (2.3)83;(2.4)242 Lahamksi, C. (3)509 Laumkonis, A. (1) 383;(5) 166 Lahiri, S.(2.2)90; (2.4)43 Laurcncin, C.T.(3)235 Lahmani, F. (1) 249 Laurcnt, A.J. (2.2)41;(2.4)46; Lahrahar, N. (2.2)16; (2.4)116 (2.6)182 Lahli, P.M. (2.7)77;(3) 830 Laurcnt, C. (3)675,676 Lai, Y.C.(3)70, 112 Laurcnt, E.G.(2.2)4 1; (2.4)46; Lai, Y.L. (3) 142,161 (2.6)182 Laibinis, P.E. (1) 492 Laurcnt, J.L. (3)744 Lainc, RM.(3)275,276 Laurcnt, T. (2.7)102, 103 Lakowicz, J.R. (1) 49 Laurich, B.K. (3) 629 Lakshmi, S.(2.5)209 Lautcslagcr, X.Y.(1) 74;(2.6) Lam,J.H.W. (3) 166,167 241;(5) 66 Lam, T. (4)59 Lavcrnia, N.L. (2.4)266;(2.7)210 Laman, D.M. (1) 6 15; (2.4)245; Lavoic, H. (I) 396 (2.6)208 Lavrov, V.V. (5) 190 Lamcrs, P.H. (3)822 Law, C.K.(1) 646 Lan, W.G. (3)454 Law, K.-Y.(5) I35
Kuntmada, T. (3)839 Kurz, H. (3)609,644 Kusaba, M. (4)7 Kusanagi, H. (2.6)271 Kusba, J. (I) 49 Kusch, Ch. ( I ) 431;(5) 191 Kushida, T. (I) 522 Kusuliawa, T.(2.5)121 Kuthan, J. (2.4)50,s1; (2.6)15 I, I52 Kuucybala, Z.(3) I 18 Kuvatov, Yu.G. (3) 58 1 Kuwabara, T. (3)482 Kuwana, Y.(4)7 Kuwano, K. (3)7 15 Kuzina, S.I. (3)769 Kuzmany, K. (1) 435;(5) 2 I0 Kuzmenko, 1. (2.4) 107 Kuzmin, M.G.(1) 4 Kumclsov, A.M. (1) 72,279;(5) 63,239 Kvcdcr, V.V. (1) 446;(5) 205 Kwan, W. (3) 849 Kwiatck, J. (3)779 Kwock, E.W. (3) 646 Kwok, W.M.(2.7)3, 139 Kwon, H.C. (1) 475 Kwon, H.-J. (2.6)233 Kwon, T.W. (2.2)77,78
473 Lawson, J.M. (1) 80;(5) 74 Lazdina, B. (3)802 Lcbauby, P.(3) 315 Lcbertois, J.P. (3) 3 15 Lcbniann, C.W. (1) 502 Lc Brcton, H.(2.4)24 Lcbrun, C.(2.6)178 Lcbrun, S.(2.2)48;(2.6)59 Lccamp, L. (3)315 LcdibLi, G. (3)616 Lcdncv, I.K. (2.4)17;(2.6)19 Lcc, B.L. (3) 624 Lcc, C.(1) 286;(2.5)149 LCC,E.J. (2)39 LCC,G.A. (2.3)13 Lcc, G.J. (3) 641 Lcc,H.(1) 268;(3)480,796 Lcc, J.H. (3)622 Lcc, J.1. (3)641;(5) 149 Lcc, K.(1) 437;(5)208 Lcc, R.E. (3)843,844,871,872 Lce,s.(3)444 Lee,S.G. (2.3)22;(2.6)335 Lee,S.H. (2.4)131;(3) 719 Lcc,S.J. (2.4)155;(5) 12 Lee, S.N.(I) 220 Lee, S.T. (2.4)95 h, T.(2.3)8 Lcc, T.S. (2.3)8 Lccs, A.J. (3)578 Lcczcnbcrg, P.B. (3)540 Lcfcvrc, J.-P. (I) 403,539;(5) 181,268 LeGoff, E. (3)470 Lclunan, W.R(5) 11 Lchmann, C.(2) 19;(2.6)76 Lchman, T.E. (2.5)22 L c ~ J.-M. , (1) 538,539,551,552; (2.4)67;(2.6)227,228,244; (5)268 Lchr, B.(3)416 Lcibovitch, M. (2) 12;(2.5)50 Leigh, W.J. (2.3)40;(2.5)21; (2.7)171,189;(3)815 Leising, G.(3) 613-616 Lcm, G.(1) 427 Lemairc, J. (2.6)219;(3)709,713, 717.71 8,725,759,836 Lcrnartinc, R. (5)264 Lcmmcr, u.(3)644 Lcmmetyincn, H. (I) 640 Lenoir, D.(2.3)7;(2.4)25 Lcplyanin, G.V.(3) 39,862 Leppard, D.(3)855 Lcrmontov, S.A. (2.3)59;(4) 19 Lcshina, T.V. (2)64 Lcslic, R.(2.6)133
Photochemistry
474 Lctud, J.-F. (1) 523; (2.4) 24; (2.5) 192; ( 5 ) 272,273 Lcuc, S. (2.2) 17; (2.4) 112 Lcung, S.H. (1) 228 Lupin, W. (1) 509 Levanon, H. (1) 3 l9,348,45 1, 486; ( 5 ) 20 1,233 Lcvcnfcld, B. (3) 37 Levin, P.P. (1) 372; (2.5) 3 1 Levin, S.J. (1) 624 Lcvina, A.S. (2.7) 90 Levinson, E.G. (1) 377; ( 5 ) 156 Lcvitsky, LA. (1) 389 Lcvy, D. (3) 4 1 Lcw, C.S.Q. (2.6) 194; (2.7) 38 Lcwis, F.D. (1) 171, 195,200, 202; (2.3) 6; (2.4) 2; (2.6) 4, 20,21,23, 108, 159,247; (5) 134 Lewis, J.E. (1) 343; (2.5) 222; (5) 94 Lcwis, J.T. (3) 815 Lewis, N.S.(4) 20 Lcwis, T.J. (2) 13; (2.5) 55 Lex,J. (2.6) 100; (2.7) 132 Lc Xuan, H. (3) 334,335 Ley, K.D. ( 5 ) 148 Li, B. ( I ) 129 Li, B.S. (3) 575 Li, C. (2.6) 332; (3) 109,243 Li, F. (3) 7,395,397,53 1 Li, F.M. (3) 25,47, 113, 121,528 Li, G. (3) 383-385,402 Li, G.W. ( I ) 493 Li, H. (2.7) 109; (3) 818 Li, H.-C. (1) 150 Li, H.-Y. (2.5) 212 Li, J. (2.4) 247 Li, L. (1) 507; (2.2) 14; (2.3) 5; (2.6) 306; (3) 109,243,444, 550,55 1,603 Li, L.-D. (1) 479 Li, M. (1) 317; (3) 65,435,539 Li, P. (3) 720 Li, Q. (2.7) 109; (3) 247 Li, R.-H. ( I ) 507 Li, S. (1) 332; ( 5 ) 86 Li, T.(3) 582 Li, W. ( 5 ) 261 Li, X. (1) 46,277; (2.5) 166 Li, Y. (1) 590; (2.3) 61; (2.5) 167; (2.6) 238 Li, Y,-Q. ( I ) 528,534 Li, 2. (3) 7,307 Li, Z.C. (3) 47, 1 13,528 Li, Z.-M. (1) 194 Lian, 2.(I) 408
Liang, K. (5) 135 Liang, K.K. (3) 649 Liang, L. (1) 270 Liang, L.J. (2.4) 117 Liang, W. (3) 818; ( 5 ) 19 Liang, Y. (2.4) 86; (2.6) 41 Liang, Y.Q. (I) 493 Liang, Y.Y. ( 5 ) 169 Lianos, P. (1) 482 Liao, F.L. (2) 55 Liaw, D.J. (3) 57 Libby, E. ( 5 ) 20 Lichtcnhan, J.D. (3) 763 Lichtcnlhaclcr,J. (2.4) 11 1 Liddcll, P.A. (1) 343,345,380, 467,468; (2.5) 44,89,222; (5) 11,94,117,159,224,225 Lidzcy, D.G. (3) 653 Licbcrman, D.R. (2.3) 29 Licgard, A. (3) 855 Lightcntliaclcr, J. (2.2) I10 Ligncr, G. (3) 875 Likhtcnshtcin,G.I.( I ) 76; ( 5 ) 76 Lilkin, A.I. (2.4) 73 Lim, H. (2.7) 201 Lim, K.S.(4) 58 Lin, A.L. (1) 44 Lin, C. (3) 787 Lh, C.-H. (2.2) 53 Lin, G.-H. (2.2) 94; (2.3) 65 Lin, L.B. (3) 663,664 Lin, N. (1) 408; (2.4) 84 Lin, N.-Y. (1) 150 Lin, S. (1) 292,345,346,466; (2.5) 41,44; (3) 649; (5) 117, 118,222 Lin, V. (3) 815 Lin, W. (1) 408; (3) 454 Lin, X. (2.5) 76; (2.6) 90 Lin, Z. (3) 22 Lin, Z . 4 . (1) 568 Linden, L. (3) 175 Linda, L.A. (3) 826 Lindsay Smith, J.R. (1) 3 18; (2.5) 134; (2.7) 134, 135 Lindscy, J.S. ( I ) 388; ( 5 ) 259,275 Ling, K.-Q.(2.5) 205,206 Ling, P. (2.7) 68 Ling, R. (2.4) 190; (2.6) 67 Linker, T.(2.5) 158 Linkous, A. (4) 21 Linkous, C.A. (4) 46,47 Linschitz, H. (1) 86,455; ( 5 ) 198 Lion, C. (2.3) 60; (4) 17 Lion-Dagm, M. (1) 557 Liou, K.-F. (2.5) 125; (2.6) 137 Lippa, B.S. (2.4) 198
Lippitsch, M.E. (1) 579,587 Lipsliy, S. (3) 415 Lipson, M. (2.3) 84; (2.4) 44 Lissi, E.A. (1) 473,629; (3) 1 15 Liu, C. (1) 53 1,535 Liu, D. (3) 463 Liu, F. (3) 582 Liu, G. (2.3) 5 ; (2.6) 306; (3) 339, 431,589 Liu, H. (3) 129 Liu, J. (I) 124, 125,515,542,612 Liu, L.I. (3) 382 Liu, M.T.H. (2.7) 32,33 Liu, Q. (2.4) 85 Liu, R. (2.7) 14; (3) 568 Liu, R.S.H. (2.3) 50 Liu, S.B.(2.2) 71 Liu, T.-J. (2.2) 29; (2.4) 225 Liu, W. (2.7) 44; (3) 824 Liu, W.4. (2) 43,44 Liu, X. ( I ) 187; (3) 767 Liu, Y.(1) 127; (2.4) 84 Liu, Y.-C. (2.5) 25; (2.6) 144 Liu, Z. (1) 504; (2.6) 333 Liu, Z.-L. (1) 32 1; (2.2) 3; (2.4) 99 Liu, 2.-X. (1) 117 Livorcil, A. (1) 15,556; (5) 3 1 Lloyd, C.R. (2.7) 117 Lobach, AS. (2.4) 146 Lochon, P. (3) 438 Lochmmoebcn,H.-G. (1) 133, 183,337; ( 5 ) 230 Locttgcrs, A. (2.7) 110 Lohdcn, G. (3) 156,162 Loluncycr, S. (3) 689 Lokshin, V. (2.4) 74,87; (2.6) 49 Long, F.H. (1) 30 Lopcz, A. (5) 12 Lopcz, A.F. (1) 239 Lopcz, A.I. (1) 239 Lopcz, A.T. (I) 239 Lopcz, C. (1) 470 Losev, A.P. (1) 341 Lou, J. (1) 492 Lougnot, D.J. (3) 107 Lounis, B. (1) 114 Lourtic, I.M.G. (2.4) 232 Lowingcr, T.B. (2.2) 82 Lown,J.W. (2.2) 101 Lowrey, A.H. (2.4) 23 Lozinskaya, E.I. (3) 472 Lu, B. (3) 225 Lu, F. (3) 249 Lu, H.P. (1) 104 Lu, H.S.M. (2.7) 22 Lu, M. (2.4) 85 Lu, P.H. (2.7) 191
Atifhorhidex Lu, Q. (2.6)86 Lu, Q.Y. (2.2) 12 Lu, S. (3)48, 124 Lu, Z.(1) 527 Lub, J. (3) 72 Lubovskaya, R.N. (I)446 Lucas, M.A. (2)65;(2.6)330 Luccionc-Hod, B.(2.5) 103; (2.6)45 Ludhyi, K.(2.6)336 Lucsscm, G. (3) 608,639 Lujan-Upton, H.(3)563,572 Lukac, I. (1) 3 Luk'yanchuk, B. (3)782 Lumbcr, D.C.J. (3)408 Lunak, S. (2.4) 180 Lunazzi, L. (2.5)126 Lungu, A. (3) 179,213,285,323 Lunin, V.V. (3)793 Luo, C.(2.5)78 LUO,C.-P. (1) 440;(5) 185 Luo. F. (5) 15 LUO,J.-K. (2.4)210,212;(2.6)61 Luo, T.(I) 507 Luo, Y.(2.4)257;(2.6)270 Luogo, D.(2.6) I80 Lurgina, V.N.(1) 331;(5) 119 Luria, E.(1) 57 Lushchik, V.B. (3) 424 Lusztyk, J. (2,6) 194;(2.7)38 Luurull, D.K.(5) 12 Luk, T.G.(3)477 Luzati, S.(3)5 14 Lwason, G.E.(3)26 Ly, D.(2.2)114 Lyaksutova, 0.1(1) . 306;(5)244 Lyalin, G.N.(3)543 Lyashik, O.T.(2.4)83 Lynch, P.L.M. (1) 545 Lynch, P.M. (2.6)243 Lyubovskaya, RN.(5) 205 Ma, B. (1) 402,438; (3)26;(5) 211
Ma, H.(3) 824 Ma,J. (1) 616 Ma,W.(I) 567 Ma, X.C. (5) 12 Maafi, M.(2.3)GO; (4) 17 Maaroufi, A. (3) 718 Maartcnsson, J. (5)23,58 McArdlc, C. (3) 164,768 McAuliffc, C.A. (5) 22 McAvoy, C.A. (1) 207 McBranch, D.W. (1) 439;(5)212 McBridc, J.S. (3) 496
475 McCaffeferly, D.G. (2.5)217;(5) 113 McCany, B.E. (2.7)65 McCaskill, J. (1) I 10 McClclland, R.A. (2.3)82;(2.4) 247;(2.6)206;(2.7)6.79 McClcskcy, T.M. (1) 358;(5) 23 1 McClcvcrty, J.A. (1) 356;(5) 100 McCoimcll, H.M. ( 5 ) 10 McCormick, C.L. (3)436,441, 448,547 McCoy, C.P. ( I ) 546 MacCrailh, B.D.(I) 548 McDcmtt, G. (5) 9 MacDiannid, A.G. (3)663,664, 666 McDonagh, C.M. (1) 548 MacDonald, B.C. (1) 624 McDonald, W.A. (3)688 McEvoy, A.K. (I) 548 MacFaul, P. (1) 3 18;(2.5)134; (2.7)134, 135 McGany, P.F. (2.6)232 McGill, R.A. (2.4)23 McGimpscy, W.G. (I) 205;(2.4) 257;(2.6)270 McGown, L.B.(1) 2,637 Machado, F.(2.4)166 Machado, R. (2.6)280 Macharchuck, S.I.(5) 247 Machida, H.(3)461 Machida, K.4. (4)44 Machida, M. (2.6)275,28 1 Machida, S.(3)466,789,790 Machinck, R.(2.4)36;(2.6)28 Macicjcwski, A. (1) 132 Mclntyrc, S.(3)722 McKaigc, G.T.(4)46 Mackarchuck, S.I.(1) 308 McKcchncy, W.M. (2.3)29 McKcrlic, L.A. (2.2)21 Mackie, P.R. (5) I3 M c K i ~ c l l D. , (2.2)58-60;(2.6) 87.88 McLauchlan, K.A. (1) 594 McLaughlin, M.L. (1) 227 Macleod, P.J. (2.4)250 McMahon, L.P. (I) 227 McManus, K.A. (2.3: 17, 18;(2.4) 157;(2.6)158 McMillin, D.R (1) 272;(5)56 McNally, T. (2)38;(2.5)48;(2.6) 253 Macpherson, A.N. (1) 343;(2.5) 222;(5) 12.94 McPhcrson, G.L. (3)4 17,423 Madcorc, F. (I) 174;(5) 128
Madhcswari, D. (3)344 M a d k o ~A.E.-E. , (2.4) 127 Madscn, H.G. (2.4)207;(2.6)266 Macda, A. (1) 178 Macda, K.(I) 269;(2) 10;(2.4) 227,229 Macda, Y. (2.4)27;(2.6)22;(3) 4G 1 Machlmann, J. (2.4)216 Mackawa, S.(2.2)80 Macshima, Y.(3)893 Macslri, M. (I) 309;(5) 57 Mac&, N.(2.5)184 Magdc, D. (2.6)50 Magdincts, V.V. (3) 293 Maggini, M. (I) 458-462;(3) 599; (5) 215-219 Magnani, L. (3)513 Magner, J.T. (2.2)7 Magnonc, A.G. (2.3)52 Mapuson, A. (1) 355;(5)97 Mah, Y.J. (2.2)99;(2.4)15 1 Mahapalra, G.S.(1) 578 Mahrt, R.F.(3)644 Maicr, G.(2.7)23 Maislrcnko, G.Ya. (2.5)154,156 Maiti, N.C. (1) 229 Maiti, S.(2.7)56 Maiwald, B. (1) 291 Maiya, B.G. (I) 520 Mnji, D. (2.2)90;(2.4)43 Mnjiina, Y.(3)535 Mak, C.H. (1) 100 Makarcwicz, A. (3)45 1 Makarova, N.I.(2.4)62,197;(2.6) 35 Makita, S.(2.7)179 Mako, M. (2.5) 186 Maksimovic, L. (2.7)22 Maksimuk, M.Yu. (1) 446;(5) 205 Makuuchi, K.(3)670 Malachcshy, P. (4)59 Malatesta, V.(2.4)71;(2.6)50,53 Malccva, N.A. (3)793 Malik, J. (3)866,875 Malik, R (3) 142, 161 Malkin, S.(5) 50 Mallardi, A. (5)54 Mallory, C.W. (2.4)186 Mdlory, F.B. (2.4)186 Mahqvist, L.(3)544 Malthcte, J. (1) 489 Mal'tscv, E.I. (3)627 Malucelli, G.(3) 33 I Malyshcv, A. (3)782 Mrunantov, G.(I) 286;(2.5)149 Mamcdov, A.P. (2.5) 143 '
Photochemisfty
476 Man, M. (3) 55 Man, S.-Q. (2.7)3 Manaji, M. (1) 136 Manako, S.(5) 14 Manchcno, M.J. (2.3)24;(2.5)9; (2.6)146 Mancini, V.( I ) 198 Mandal, B.K.(3) 338 Mancss, K.M.(3)684 Mani, R.(3)725,873 Maniloff, E.S. (1) 439;(5) 212 Mann, J. (2.2)10;(2.6)91 Mmicrs, 1. (3)381,401 Mansilla, D.H. (3)683 Miuisri, A. (3) 778 Mao, W. (2.7) 109 Mao, Y. (2.5)145 Marcaccio, M. (I) 4 19 Marchi-Artmr, V. (I) 539;(5) 268 Marcia, E. (1) 644 Marcinid, B.(1) 5 16;(2.5) 197, 23 1 Marconi, G. (1) 193,526;(2.4)21 Marcus, A.H. (3)555 Marcus, R.A. (1) 60 Marcncclli, M. (1) 62;(5)64 Marcvtscv, V.S. (3)479 Margarctha, P.(2.2) 19,45;(2.6) 274 Margaritondo, G. (I) 4 13 Margolin, A.L. (3) 723 Marguct, S.(I) 488 Mariano, P.S. (2.4) 190;(2.5)76, 191;(2.6)66,67,90 Maricnficld, M.L.(3)706 Marin, M. (3)523 Marinic, 2.(2.3)46;(2.4)53 Marino, T.L.(3) 180,294 Marion, P. (3)426 Markhrun, K.R. (I) 15 1 Markovitsi, D.( I ) 488 Maroncclli, M. (I) 59 Marot, G.(3) 730 Marquardt, R.(2.3)83;(2.4)242 Marquct, A. (2.7)58 Marquct, J. (2.4)156; (2.6)161, 174 Marqucz, F. (2.5)70 Marmcci, L.(1) 163 Marsau, P. (2.2) 16;(2.4)116 Marsclla, M.J. (3)666 Martcllaro, M.A. (1) 500 Martello, D.V.(4)22 Martin, E.(1) 242 Martin, H.-D. (2.4) 128;(2.6)125 Martin, J.C. (I) 38, 112
'
Martin, M.M. (1) 244 Martin, M.V. (2.7)21 Martin, N. (2.5)28,72 Martin, R. (3) 855 Martin, S.C. (1) 6 I3 Martin. T.(I) 280;(3)437,422, 552;(4)43;(5) 240 Martincz, J.G. (3)846 Martinho, J.M.G. (5)141 Martino, D.M. (2.7)I96 Martins, S.F.(3) 596 MarLinyu, 2h.M.G. (1) 55 Marlula, D.S.(1) 469 Marl'yanov, I.N.(2.5)196 Maflynova, V.P.(2.4)76;(2.6)46 Marutliamutliu, P.(4)26,37 Marzin, C. (1) 386;(5) I76 Masaki, K. (4)34 Mashiko, S.(1) 424;(2.5) 106 Mashino, T.(1) 424;(2.5)106 Maslali, P. (2.5)26 .Masly&, A.F. (3) I19 Masoumi, 2.(3)38I Massincs, F. (3) 675,676 Mastcrson, M. (3)768 Masuda, T.(3) 686 Masuliara, H.(I) 164 Mataga, N.(I) 323,326;(5) 241, 242 MatczuIi, A. (I) 175 Mateo, J.L. (3) 41 Mathcvct, R. (1) 523;(5) 272 Matlicw, D.T.(2.4)42 Mathcw, T.(2.3)25;(2.4)133; (2.6)126 Mathias, L.J.(3)33,76,120,314 Mathis, H.(1) I 10 Mathis, P. (5) 1 1 Motisova-Rychla, L. (3)677,680 Matsuda, H.(3) 87,90,260 Matsuda, K.(2.7)47,48;(3)5 17 Malsuda, T.(2.7)88;(3)91,200 Matsuda, Y.(2.5)169 Matsui, A.H. (1) 139 Matsui, M. (I) 496 Matsukawa, K. (3)278 Matsuni, Y.(2.7)192 Matsuiioto, H.(4)62 Matsuiioto, M. (2.3)68;(2.5)161, 189;(5) 155 Matsumoto, S.(I) 422;(2.5)122; (5) 196 Matsumoto, T.(2.2)92;(2.4)98; (2.7)167 Malsuniura, M. (1) 562;(3) 548; (4) 14 Mahuiinga, D.(1) I69
Matsuoka, M. (1) 590 Matsura, T.(1) 3 I Matsushima, R (2.5)12 Malsushima, Y.(1) 139 Matsushita, T.(2.6)201;(2.7)74 Matsuura, T.(2)40;(2.4)92;(2.5) 177;(2.6)12, 176,255;(2.7) 137 Mattay, J. (1) 450;(2)33;(2.4) 216;(2.5)92, 124;(2.6)307; (2.7)86;(5) 200 Matticc, W.L. (1) 376;(5) 154 Mattincn, J. (2.6)214 Matusche, P.(2.7)28 M ~ uA.W.-H. , (1) 383;(5) 166 M~UCIIII~M-DUCII, H. (3)640 Mauricllo, G.(2.4)176;(2.5) I5 1, 163;(2.6) 185,254 Mauritz, K.A. (3)436 Maus, M. (1) 253 Maurnall, D. (5)52,259 Maxwell, P.RS. (1) 546 Maya, M. dcl RP. (2.6)289 Mayashi, M. (2.4)227 Maya, B.(1) 526;(2.4) 128;(2.6) 125 Maycr, G. (1) 375;(5) 153 Maya, H.-D. (2.3)67 Maycr, T. (3)133 Mayoral, E.P. (2.6)146 Mays, J.W. (3)415 Mazcpa, A.V. (1) 123 Mazitov, A.K. (3)859 Mazmcchi, P.H. (2.2)96;(2.6) 101 Mauucato,U. (1) 193,196,198; (2.3)19;(2.4)21;(2.6)263 Mcdforth, C.J.(1) 228,230 Mcdyiuitscva, E.A. (2.4)83 Mccch, S.R. (3)455 Mcggitt, B.T.(I) 643 Mcghdadi, F. (3)614 Mchdom, F.(3) 593 Mchlcnbachcr, RC.(3)286 Mchta, G. (1) 520 Mci, W. (3) 109,243 Mcicr, H. (2.3)74 Mcicr, H.R.(3)855 Mcijcr, A.E. (1) 453;(5)203 Mcijcr, E.W. (1) 250;(2.5)81;(3) 474 Mcijcr, G. (1) 409 Mcijidc, F. (1) 630 Mciklyar, V.(1) 451; (5)201 Mcindi, K.(3) 45 Mcinjohanns, E.(2.7)85 Mciscnhcimcr, P.L. (2.2)74
Author Itdm Mcjcritskaia, E.(5) 15 Mcldal, M.(2.7) 85 Mclcndcz-Rodriguez, M. (2.2) 46 Mclla, M. (2) 6; (2.3) 16; (2.4) 158; (2.5) 117, 118; (2.6) 157, 210,211,300; (2.7) 186 Mcllinger, A. (2.7) 95,96 Mcl'nikov, M.Ya. (3) 42,698-701 Mclo, E.(2.4) 232 Mclon, S. (2.7) 129 Mcnag, J. (2.2) 116 Mcndez, J.M. (3) 523 Mendicuti, F. (1) 376; (5) 154
Mcndoza, V.S.(3) 168 Mcng, J.B. (I) 270; (2.4) 1 17, 184, 236; (2.6) 104
Mcng, Q.-X. (2) 7 Mcng, X. (2.4) 86; (2.6) 41 Mcnzcl, H. (3) 462,496 Mcnzhercs, G.Ya. (3) 293 Mcrcicr, M.F. (3) 735 Mcrcicr, R. (3) 308 Mcrgcdiagcn, T. (2.6) 274 Mcrritt, C.D. (3) 849 Mcrtz, E.L. (I)72; (5) 63 Mcrtz, 1. (3) 597 Mcrvinskii, R.I. (3) 40,536 Mcslres, R (2.4) 266; (2.7) 210 Mctcalf, D.H. (5) 150 Metelitsa, A.V. (2.4) 69,83 Mctclko, B. (2.3) 46; (2.4) 53 Mcts,U.(I) 111 Mcunicr, H. (1) 1 19 Mcuris, H.G. (5) 144 Mcycr, A. (I) 266; (2.7) 205 Mcycr, C. (2.2) 20; (2.6) 114 Meyer, D.U. (3) 453 Meyer, G.J. (5) 99 Mcycr, T.J. ( I ) 5,66,68,325; (2.5) 1,217; (3) 524,684; (5) 25,69,98, 113, 123,243 Mcycr, U. (2.2) 16; (2.4) 1 I6 Mcycr, Y.H. (1) 244 Mezger, T. (3) 45 Mialocq, J.-C. (5) 11 Miao, Y.J. (3) 648,650 Micliacl, S.(1) 45 1; (5) 201 Michalak, J. ( I ) 138; (2.6) 205; (2.7) 80 Michcl, H. (5) 1,3 Michcl-Bcycrlc, M.E. ( I ) 342; (2.2) 64; (2.5) 40; (5) 88 Michl, J. (2.6) 329; (2.7) 69 Michnicwicz, A. (I) 149 Midoux, N. (3) 558 Miclcarck, J. (2.6) 215 Migita, T. (2.6) 322
477
Mignani, G. (3) 133 Miycl, A. (2.5) 58 Mihalcca, I. (3) 682 Mikanii, K. (2.5) 122 Mikami, M. (1) 42 I, 422; (5) 195, 196
Mikhailov, A.I. (3) 769 Miki, K. (5) 1 Miki, S.(2.2) 1 15; (2.5) 35; (3) 48 I , 499 Miki, T. (2.6) 321; (2.7) 37 Miliani, C. (2.4) 71; (2.6) 53 Millar, K.R (3) 796 Milk, G. (2.5) 159 Millcr, C. (3) GO Millcr, E.K. ( I ) 437; (5) 208 Millcr, E.R. (3) 803 Millcr, R.D. (5) 138 Millcr, T.M. (3) 646 Millington, K.R. (3) 820 Milk, J. (3) 733 Minami, M. (4) 63 Minato, M. (2.7) 77 Mincniura, J. (4) 52 Mincro, C. (2.5) 200 Mines, H.A. ( I ) 27; (5) 35 Ming, Y. (2.4) 82, 86,89; (2.6) 39-42
Minkin, V.I. (2.4) 74,83; (2.6) 26 Minowa, T. (I) 447; (2.5) 80; (5) 207
Minskcr, K.S. (3) 859 Minti, S. (2.4) 191 Minto, F. (3) 202 Mir, M. (1) 536 Miranda, M.A. (2.2) 32; (2.3) 11, 35; (2.4) 163, 183,237,266; (2.6) 71; (2.7) 210 Mirochnik, A.G. (3) 570,57 1,574 Mironov, A.F. ( I ) 369,377; (5) I56 Misawa, Y. ( I ) 310; (5) 108 Miscv, L. (3) 149 Misliima, K. (2.7) 208; (3) 2 18, 220 Mislva, A. (1) 472 Mislva, B.K. (1) 472 Mislva, L. ( I ) 354; (5) 96 Misra, A. (I)261; (2.5) 17 Misra, T.N. ( I ) 572 Missoum, A. (2.4) 193; (2.6) 57 Mita, T. (2.4) 148 Mitani, M. (3) 685 Mitchcll, A. (2.6) 80 Mitchcll, E.M. (2.7) 126 Mitchcll, G.R. (2.6) 13 Mitchcll, R.H. (2.3) 53; (2.4) 38
Mitina, V.G. (I) 222 Mitrofanova, A.N.(3) 793 Mitsuda, M. (3) 256 Mibuhashi, T. (3) 352 Milsui, M. (1) 595 Mikuishi, M. (2.4) 78 Mittol, J.P. (1) 154,443,448,476; (2.5) 213; (2.7) 172; (5) 188, 197
Mittcnzway, K.H. (I) 580 Mitzncr, R. (1) 431; (5) 191 Miura, Y. (4) 62 Miwa, T. (3) 6 Miyagawa, H.(2.4) 27 1 Miyagawa, N. (2.5) 157; (2.6) 192; (2.7) 25
Miyahara, T. (1) 3 11; (2.2) 105; (2.5) 220; (5) 106
Miyaji, T. ( I ) 497 Miyakc, J. (4) 70 Miyakc, M.(4) 70 Miyamoto, H. (1) 503; (2.2) 42; (2.4) 205; (2.6) GO, 269
Miyamoto, K. (2.3) 41; (2.4) 39 Miyamoto, T. (3) 3 12 Miyanari, S. (2.5) 198 Miyasaka, H. (I) 303,323,326, 433; (2.5) 207; (3) 394; (4) 62; (5) 241,242 Miyashi, T. (1) 182; (2.3) 3 1,39, 41; (2.4) 28,39,40 Miyashita, A. (2.4) 9 Miyata, M. (2.4) 226 Miyala, N. (2.5) 86 Miyala, S. (5) 132 Miyawaki, N. (2.7) 82 Miyazaki, T. (4) 45 Mizcs, H.A. (3) 612 Mizoguchi, T. (4) 62 Mizuno, H. (2.5) 12 Mizuno, K. ( I ) 139; (2.4) 142; (2.5) 4; (2.6) 115,313; (2.7) 208 Mizuno, Y.(2.2) 98 Mizutani, H. (2.5) 63 Mo, B. (1) 46 Mo, D. (3) 454 Mo, Q. (3) 322 Mo, Y. (1) 24 1; (3) 383-385,402 Mobius, K. (1)451; (5) 91,201 Mochida, K. (1) 452; (5) 202 Mochizuki, K. (5) 14 Mocmcr, W.E.(3) 363 Mogilnyi, V.V. (3) 54 Mohan, D. (I) 176 Mohnn, H. (I) 443; (5) 188 Mohanty, D.K. (3) 351 '
Pholocheniislry
478 Mohr, G.J. (3) 376 Mohtat, N. (3) 122 Moloncy, J. ( I ) 502 Momota, J. (2.4) 77; (2.6) 43 Monbclli, A. ( I ) 143 Mondini, S. (5) 215 Monjol, P. (3) 308 Monjushiro, H. (I) 364; ( 5 ) 124 Monson, E. ( I ) 44 Montalti, M. (5) 145 Montanari, I. (5) 177 Montcncgro, L. ( I ) 525 Montcscrin, M.C. (2.6) 287 Montforts, F.-P. (I) 344; (5) 90 Monthcard, J.P. (3) 778 Monti, S. (I) 526; (2) 6; (2.6) 50, 210 Montscrin, M.C. (2.5) 229 Moon, K.J. (3) 643 Moorc, A.L. (1) 324,343,345, 346,380,464,466,467; (2.5) 39,44,89,222; (4) 9; (5) 11, 12,89,94, 117, 118, 159, 222-225 Moorc, C.B. (2.7) 95,9699 Moorc, J.A. (3) 75 1 Moorc, J.N. (2.4) 17; (2.6) 19 Moore, J.S. (5) 266 Moorc, R.B. (3) 436 Moore, T.A. (1) 324,343,345, 346,380,464,466-468; (2.5) 39,44,89,222; (4) 9; ( 5 ) 5, 11, 12,89,94, 117, 118, 159, 222-225 Moorjani, S.K. (3) 169, 170 Moors, R. (3) 355 Moorthy, J.N. (2.5) 75; (2.6) 188, 191; (2.7) 18 Moradpur, A. (1) 489 Morais, J. (3) 592 Moralcs, G.A. (1) 227 Moran, R.J. (2.6) 207; (2.7) 206 Moratti, S.C. (3) 513,632-634, 637 Moravskii, A.P. (1) 449; (2.5) 82; ( 5 ) 204 Morawietz, J. (2.6) 202; (2.7) 72 Morawski, 0. ( I ) 247 Morel, F. (3) 287,346 Morcno, M.J. (2.4) 232 Morgan, C.G. (2.7) 97,98 Morgcnthalcr, M.J.E. (3) 455 Mori, A. (2.2) 117; (2.3) 42; (2.4) 234 Mori, H. (2.5) 100; (4) 41 Mori, K. (2.2) 70; (2.4) 138; (2.6) 1I I ; (4) 45
Mori, M. (2.3) 62; (2.5) 186; (2.6) 320 Mori, Y. (1) 269; (2.4) 120,227, 229; (2.6) 276; (3) 567 Moriguchi, E. (3) 341 Moriizumi, S. (2.4) 56; (2.6) 268 Morimoto, M. (4) 71 Moriscima, Y. ( 5 ) 235 Morishima, S . 4 (2.4) 40 Morishirna, Y. ( I ) 299,301; (3) 182,368,577,602 Morishita, H. (3) 257 Morishita, S.(2.3) 42 Morita, H. (3) 210 Morita, T. (1) 397; (2.5) 169 Moriwaki, H. (2.2) 92; (2.4) 98; (2.5) 33 Morlct-Savary, F. (1) 147; (2.6) 297; (3) 99, 177 Morlcy, C.P. (2.3) 2 I; (2.6) 328 Mornct, R. (2.4) 47; (2.6) 203 Morokuma, K. (2.7) 14 Moroni, M. (3) 625 Morozova, O.B.(2) 8 Morrison, H. ( I ) 391; (2) 37; (2.6) 318; ( 5 ) 162 Morrison, M.E. (3) 602 Morrocchi, S. (2.4) 181; (2.6) 180 Mortlock, S.V. (3) 688 Moscr, I. (3) 3 16 Moscr, J.E. (2.5) 187 Mosingcr, J. (1) 1 18 Moskvin, Yu.L. (3) 769 Mosqucra, M. (1) 630; (2.4) 48 Moss, D. (2.4) 257; (2.6) 270 Moss, R.A. (2.7) 44 Mosslcr, H. (5) 91 Mostafa, G. (2.2) 90; (2.4) 43 Molschicdlcr, K.R. (2.7) 43 Moudjoodi, A. (2.6) 284 Mouillat, C.G.J. (3) 35 Moulik, S.P. (1) 298 Moura, J.C.V.P. (3) 877 Mouradzadcgun, A. (2.4) 79; (2.6) 284 Mouritsen, S. (2.7) 85 Moussa, K. (3) 776 Moylan, C.R(3) 363 MU,L.-J. (1) 1 17 Muaoka, 0. (2) 9 Muchado, R. (2.4) 139 Muchlbach, A. (3) 361 Muelhaupt, R. (3) 178 Mucllcn, K. (3) 234,629,640, 654,660,662 Mullcr, S.N. (2.6) 73 Mucllcr, U. (1) 288,271; (3) 272,
279-28 1 Mucnck, E. (1) 78; ( 5 ) 65 Mukai, I. (I) 267 Mukai, M. (1) GOO; (2.5) 2 18; (5) 25 1 Mukasa, M. (2.5) 177 Mukliamcdgalicv, B.A. (3) 103 Mukhopadhyay, D.(3) 656 Mukl;amda, R. (2.4) 113 M~dikhcqcc,D.C.(I) 298 Muldcr, P. (3) 816 Mullancy, K. (4) 53 Mullcr, K. (2.3) 74 Mulyon, K.E. (3) 370 Munccr, M. (2.4) 45; (2.6) 147, 183 Munoz, J. (3) 227 Munoz, M.A. (2.4) 13 Munoz, T. (2.2) 11; (2.4) 130 Mura, A. (3) 541,665; (5) 140 Muraliara, M. (3) 71 1,783 Murai, H. (1) 530 Murai, 0. (2.2) 70; (2.4) 138; (2.6) 111 Murai, S.(3) 535 Murakanii, H. (3) 562 Murakami, Y. (1) 445; (5) 183 Murakoshi, K. (2.5) 99, 100; (4) 7 Murali Krishna, C. (1) 145 Murao, A. (2.2) 56; (2.6) 69 Muraoka, H. (2.4) 252; (2.6) 326 Muraoka, 0. (2.6) 29 1 Murasc, S.(3) 445 Murata, S. (1) 628; (2.6) 201,204; (2.7) 7 1,74 Murata, Y. (3) 787 Murayama, T. (2.4) 4 1 Murinov, Yu.1. (2.5) 1I6 Murphy, W.R.(3) 45 1 Murray, K.A. (3) 5 13 Murray, R.W. (3) 684 Murtagh, M.T. (I) 475 Murugavcl, S.C.(3) 349 Mwuyama, T. (2.2) 49 Mutai, K. ( I ) 146; (2.5) 8 Mulhusamy, S. (1) 520 Mutoh, T. (1) 164 Mutus, B. (2.6) 222; (2.7) 155 Myashita, A. (2.4) 70 Myazawa, S.(2.7) 61 Mycrs, A.B. (I) 99 Mynott, R.(2.4) 18
Nadtochcnko, V.A. (I) 449; (2.5) 82; (2.4) 146; (5) 204 Nadzliafova, M.A. (2.5) 143
Author Index Nacmura, K. (2.6) 80 Nagai, K.(3) 386,566 Nagai, S.(2.6) 301 Nagai, T.(2.2) 92, 104; (2.4) 98, 149; (2.5) 33 Nagai, Y. (2.7) 52 Nagakubo, H. (2.4) 269; (2.6) 25 1 Nagami, F. (2.2) 42; (2.4) 205; (2.6) 269 Nagamoto, I. (3) 5 16 Nagiunura, T. (3) 209 Nagano, H. (2.4) 227 Nagano, T. (1) 424; (2.5) 106 Naganwna, T.(3) 600 Nagao, K. (3) 241,242 Nagaoka, S.(2.2) 11I Nagas&, Y. (3) 271 Nagasawa, J. (3) 258 Nagasawa, M. (3) 715 Nagasawa, Y. (2.5) 46 Naghash, H.J.(3) 320 Nagira, A. (2.2) 100 Naguib, Y.M.A. (1) 254; (2.6) 236 Naik, P.D. (2.7) 172 Najbar, J. (1) 71; (5)62 Najera, F. (2.4) 177 Naka, A. (2.7) 174 Nakabayash, K. (2.4) 188 Nakace, P.(2.6) 45 Nakada, E. (4) 70 Nakadaira, Y. (2.3) 62,63; (2.5) 186; (2.6) 320 Nakagaki, R. (1) 146; (2.5) 8, 19 Nakagawa, K. (2.5) 45 Nakagawa, M. (2.6) 29 Nakagawa, T.(4) 48 Nakajima, K. (2.4) 201; (3) 78 Nakajima, R (3) 520 Nakajima, S. (1) 326; (5) 241 Nakajima, T. (3) 883 Nokajima, Y. (2.2) 104; (2.4) 149 Nakamoto, T.(3) 2 18,220.22 1 Nakaniura, A. (1) 412 Nakamura, H.( 5 ) 126 Nakamura, K. (2.4) 94; (2.6) 3 14 Nakamura, N. (2.7) 47,48; (3) 5 17 Nakamura, S. (4) 18 Nakamura, T.(2.4) 162,221 Nakmura, Y. (1) 447; (2.2) 70; (2.3) 68; (2.4) 138, 148; (2.5) 80; (2.6) 11 1; (5) 207 Nakanishi, F.(2.4) 118,223; (3) 258,345 Nakanishi, H. (3) 87,90,260 Nakano, T. (2.4) 25 1 Nakano, Y. (3) 535,553 Nakao, R. (3) 488
479 Nakashima, K. ( I ) 302 Nakasshima, N. (4)7 Nakalani, K. (2.2) 80.8 I Nakayama, N. (3) 5 I 1,760 Nakayama, T. (1) 144,262; (2.2) 115; (2.5) 35; (3) 352 Nakayama, Y. (3) 200,765 Nakazaki, M. (2.6) 80 Nakhhacv, A.1. (2.3) 77 Naniiki, S.(2.6) 221 Nampoori, V.P.N. (1) 609 Nanasawa, M.(3) 482 Nanjo, Y. (3) 24 I, 242 Nankc, T. (2.3) 92; (2.5) 101 Naqaniinc, K. (4) 70 Narasaka, K. (2.6) 283 Nardcllo, V. (2.5) 112 Narita, K. (2.7) 50 Narymbctov, B.Zh. (1) 446; ( 5 ) 205 Nascimcnto, E.A. (3) 799 Nasccm, A. (3) 792 Nash, B. (1) 467,468; (2.5) 89; ( 5 ) 224,225 Natarajan, L.V.(2.4) 200 Natarajan, P. (1) 162 NaUi, D. (1) 26 I; (2.5) 17 Nau, W.M. (2.5) 75; (2.6) 188, 191; (2.7) 18 Navaraham, S. (3) 94,95 Nazcruddin, G.M.(2) 5 1 Nechwatal, A. (3) 885 Ncckcrs, D.C.(1) 274; (2) 25; (2.2) 85-87; (2.4) 248; (2.5) 47; (2.6) 93,248; (3) 71, 179-181, 186,211,213,214,285,323, 545 Ncdclkos, G. (3) 732 Ncdoshivin, V.Yu. (3) 479 Nccs, D.(2.4) 235 Ncfcdov, O.M.(2.6) 329; (2.7) 69 Ncgri, F. ( I ) 401,410,411; ( 5 ) 182 Ncgri, R.M. (1) 64 1 Nchcr, D. (1) 174; (5) 128 Neilands, 0. (1) 95 Nckrasova, T.N. (3) 424 Nclcn, M.I. (2.2) 34 Ncl'soii, D.K. ( 1) 4 16 Nclson, E.W.(3) 145, 153, 154, 170 Nclson, M. (2.4) 233; (2.5) 160 Nclson, N.Y. ( I ) 228 Nelson, W.L. (2.2) 31; (2.4) 19; (2.6) 18 Ncmath, G.A. (5) 11 Ncnioto, N. (3) 386
Ncrowski, F. (2.6) 100; (2.7) 132 Ncta, P. (2.5) 97; (5) 16 Nctto-Fcrrcira, J.C.(3) 442 Ncucnschwandcr, M. (2.3) 75 Ncumann, M.G. (1) 265; (2.5) 79; (2.6) 304; (3) 104,432 Ncumiuk, D.M. (I) 39; (2.7) 1 Ncvill, S.M. (2.3) 32,90 Ng, C.Y. (2.7) I76 Nguycti. D. (1) 467,468; (2.5) 89; (5) 224,225 Nguycn, T. (3) 171 Ni, C.-K. (2.7) 99 Ni, S. ( 5 ) 147 Nickcl, B. (1) 159-161 Nickcl, S.(3) 296 Nickolcit, M. (5) 142 Nicolai, M.(3) 885 Nic, J. (3) 65 Nicckarz, G.F.(3) 749 Nicgcr, M.( 5 ) 172 Nicnian, R.A. ( 5 ) 12 Niclhanmcr, D. (2.5) 42; (5) 92 Nicuwcnhuizen, M. (1) 544 Niga, Y. (2.7) 179 Nijakowski, T.R(3) 6 I 1 Nikiforov, S.M. (2.7) 116, 127 Nikith, A.N. (3) 219 Niko, A. (3) 614 Nikokavouras, J. (2.6) 136 Nikolacv, A.B. (1) 204 Nikolov, P. (1) 172 Nikolova, L. (3) 471 Nikol'ski, A.B. (1) 383; ( 5 ) 17 1 Nikowa, L.(1) 266; (2.7) 205 Nikura, H.(1) 370 Nimal, H.Q. (1) 546 Nislu, N. (5) 155 Nishi, 0. (1) 139 Nishi, T.(2.7) 82; (3) 78 Nishibuko, T. (2.7) 166; (3) 138 Nishidc, H. (3) 830 Nisliigaichi, Y. (2.2) 100; (2.4) 150 Nishijima, N. (3) 481 Nishijima, T.(2.4) 270; (2.6)250 Nishijo, C.K.(1) 508 Nishikawa, K. (2.7) 59 Nishikawa, Y. (1) 522 Nishikubo, T. (3) 298,333,476; (4) 18 Nishiinoto, M. (3) 672,853 Nishimoto, S.(2.5) 71; (2.6) 130 Nishimura, 1. (3) 476 Nishimura, J. (1) 447; (2.3) 68; (2.4) 148; (2.5) 80; (5) 136, 207 Nishimura, M. (2) 9; (2.6) 291
Phoiochetnistty
480 Nishimura, Y. (1) 378,605,606; (3) 556; ( 5 ) 157,253,254 Nishio, I. (2.7) 63 Nishio, T. (2.2) 57; (2.4) 120, 135, 206; (2.6) 129,276-278 Nishioka, N. (2.3) 3 1; (3) 278 Nishishiro, T. (4) 63 Nishiyama, K. (2.6) 323; (2.7) 188 Nishiyama, T. (2.6) 3 13 Nishizawa, K. (1) 598; ( 5 ) 249 Nisoli, M. (3) 452,613 Nitta, S. ( 5 ) 2 13 Niu, Y. (2.5) 57; (3) 291 Niwa, H. (2.6) 3 14,3 15; (2.7) 190 Niwa, M. (3) 93 Niwa, N. (2.4) 94 Niyazi, F.F. (3) 860 Nizova, G.V. (2.5) 153 Noccra, D.G. (1) 3 14,315, 500, 532,550; ( 5 ) 110,232 Noccti, R.P. (4) 22 Noda, R. (3) 48 1 Noguchi, Y. (2.2) 66; (2.6) 128 Noh, T. (1) 268; (2.4) 144; (2.6) 123; (2.7) 201 Nohara, K. (3) 74 1 Nojinia, M. (2.5) 137; (2.6) 3 12 Nojiri, T. (1) 454; ( 5 ) 199 Noll, B.C. (2.3) 84; (2.4) 44 Noltc, R.J.M. (1) 313 Nonia, N. (3) 475 Noniiyama, T. (4) 45 Nomoto, T. (1) 13 1 Nonaka, T. (3) 73 Nonomura, S.( 5 ) 2 13 Nordio, P.L. (1) 63,67,90,9 1, 633; ( 5 ) 77,80 Norikanc, Y. (2.2) 39 Norrby, T. (5) 23,24,58 Norris, C.L. (2.2) 74 Noss, L. (1) 343; (2.5) 222; (5) 94 Nourmamodc, A. (3) 8 1 1 Novikov, E.G.(1) 637 Novikov, M.M. (3) 219 Novikova, T.S.(3) 59,382,508 Novotny, L. (1) 109 Nowacki, J. (1) 94 Nowakowska, M. (2.4) 54; (3) 139, 391 Nowosiclski, T. (3) 584 Nozaki, K. ( I ) 70,77,364,384; ( 5 ) 61, 79, 124, 168 Nozaki, Y.(3) 241,242 Nozawa, T. (5) 46 N o m , T. (2.2) 117; (2.4) 234 N u n , D.S. (2.4) 2 17 Nurco, D.J. (1) 228,230
Nuykcn, 0. (2.7) 27-3 1; (3) 45, 226,827,828 Oba, M. (2.6) 323; (2.7) 188 Obcr, C. ( I ) 5 19; (2.7) I3 1 Obcr, C.K. (3) 36 Obcrski, J.M. (3) 620 Obi, K. (1) 595 Obi, M. (1) 202; (2.6) 2 1 OBrien, D.F. (3) 79 Ochi, M. (3) 548 OConnor, S. (3) 230 Oczlrowski, H.L. (1) 565 Oda, K. (2.2) 96; (2.6) 101,281 Odaka, M. (2.6) 25 Oddos-Marccl, L. ( I) 174; ( 5 ) 128 Odcbcrg, J. (3) 327 Odcns, H.H. (2.7) 197 O’Donncll, J.H. (3) 752, 753 Oechcl, A. (3) 207 Ocda, Y. ( I ) 139 Ocgc, T. (3) 501 Ochmc, G.(2.4) 141; (2.6) 262 Oclgeniollcr, M. (2.6) 100; (2.7) 132 Oclknrg, D. (3) 4 16 Ocncn, A. (3) I85 Ocscr, T. (2.3) 64; (2.4) I23 Ogalc, A.A. (3) 229 Ogaoshi, H. (2.2) 105 Ogasawara, M. (1) 28; ( 5 ) 43 Ogata, N. (2.6) 29 Ogata, T. (4) 7 Ogawa, A. (2.3) 36,92; (2.4) 5 ; (2.5) 101 Ogawa, K. (2.2) 97 Ogawa, M. (1) 2 14 Ogawa, T. (3) 523,534 OgClkO, V.M.( I ) 306; (5) 244 Ogi, T. (4) 65, 66 Ogilby, P.R. (1) 367; (2.5) 109 Ogino, H. (2.7) 130 Ogino, T. (2.2) 49; (2.4) 4 1 Ogitani, S.(3) 6 Ogoshi, H. ( I ) 3 1 I ; (2.5) 64,220; (5) 106 ogura, s. (4) 35 Oyshi, Y. (4) 68 Ohaku, H. (2.3) 3 1; (2.4) 40 Ohashi, H. (3) 8 17 Ohashi, K. ( 5 ) 155 Ohashi, M.(2.7) 192 Ohashi, Y. (2.5) 177 Ohba, Y. (2.5) 29 Ohislii, T. (2.4) 270; (2.6) 250 Ohkita, H. (3) 75 1
Ollkubo, K. (1) 498 Ohkubo, M. (4) 49 Olkura, K. (2.2) 66,157; (2.6) 127, 128 Ohlbach, F. (2.3) 64; (2.4) 123 Olmori, H. (3) 237 Ohmori, M. (4) 56 Oluio, T. (1) 70,77,364,384; (3) 548; (4) 14; ( 5 ) 61,79, 124, 168
Ohono, T. (3) 87 Ohsedo, Y. (3) 475 Ohshinia, K. (2.4) 41 Ohshita, J. (2.4) 94; (2.6) 314, 315; (2.7) 190; (3) 516 Ohla, H. (1) 498; (3) 73 Ohta, K. (1) 396 O h , N. (1) 267,596,605,606; (3) 556; ( 5 ) 253,254 Ohtani, B. (2.5) 71; (2.6) 130 Olitsuka, E.(2.2) 72 Ohya, S . (2.3) 36,92; (2.5) 101 Oislii, K. (3) 893 Oishi, S . (2.7) 118 Oka, K. (3) 5 1 1,765 Oka, M. (2.6) 278 Okada, K. ( I ) 257 Okada,T. (1)84, 131,218,463, 465; (2.5) 43,87,88; (2.7) 179; ( 5 ) 68, 178,220,22 1 Okmioto, S.(3) 576 Okamoto, T. (3) 783 Okamoto, Y. (2.6) 334; (3) 563, 572,576,590 Okamura, M. (2.4) 214,215 Okiui, A. (3) 151 Okay, 0. (3) 320,326,372 O k a d , H.(3) 242 Okazaki, S . (2.7) 59. 174 Okazaki, T. (1) 6 11 Okimoto, H. (2.5) 169 Oku, A. (2.5) 137; (2.6) 321; (2.7) 37 Okuda, K. (1) 424; (2.5) 106 Okuda, N. (1) 359; (2.4) 206; ( 5 ) 236 Okuda, S. (3) 520 Okumura, R. (1) 603 Okuni, M. (3) 883 Okura, I. (2.5) 68; (4) 11; (5) 83 Okuku, T. (2.7) 188 Okuwaki, A. (4) 34 Olayan, H.B. (3) 12 O k a y , P. (1) 58 1 Olcinik, A.V. (2.6) 199,200; (2.7) 78 Olckhnovich, E.P. (2.4) 62
Airlhor Itidex Olcshko, V.P. (2.4) 17; (2.6) 19 Oliva, C. (3) 330 Olivcira, A S . (I) 213 Olivcira-Campos, A.M.F. (3) 877 Olivcr, A.M. (1) 85; ( 5 ) 70 Olivcra, V.A. (3) 432 Olivcros, E. (1) 563 Olivcto, P.E. (2.6) 80 Olivucci, M. ( I ) 179; (2.3) 47,48; (2.4) 122 Ollivicr, J. (2.5) 228 Olovsson, G. (1) 38 1,504; (2.3) 26; (2.4) 23 1; (2.6) 148,333; (5) 161 Olson, D.R. (2.7) 36 Olson, E.J.C. (1) 5 12 Olszanowski, A. (2.6) 220 Ommer, H.J. (3) 332 Ornote, T.(3) 8,9 Omwa, T. (3) 887 Ondrias, M. (1) 292,357; (2.5) 4 I; (5) 101 Ono, K. (3) 440,445,449,601 Onoda, M. (3) 661 Onuki, H.(2.3) 2 Oppcmiann, W. (3) 503 Opplingcr, K.D. (2.6) 155 Orhiopoylos, M. (2.5) 128 Organ, M.G. (2.2) 8; (2.6) 3 11 Orlandi,G. (1)401,410,411;(5) 182 Orlmann, U. (1) 103 Orrit, M. (1) 114 Orti, E. (2.5) 72 Ortica, F. ( I ) 189; (2.4) 191 Ortiz, B. (2.2) 109 Ortiz, M.J. (2.2) 93; (2.3) 23,24, 43,44; (2.4) 52; (2.5) 9; (2.6) 97, 146,154 Ortner, J. (2.4) 141 Ortuiio, R.M.(2.6) 189; (2.7) 20 Osalii, H. (2.7) 62 Osanai, K. (2.7) 82 Osawa, Z. (3) 672,853 Osc, Y. (2.6) 321; (2.7) 37 Osgood, R.M., Jr. (2.7) 191 Oshima, K. (2.2) 49 Oshima, T. (2.2) 92, 104; (2.4) 98, 149; (2.5) 33 Osipov, V.V. (1) 56 Oskam, A. (2.7) 125, 196 Osokina, N.Yu. (3) 42 Ossipyan, Yu.A. (1) 446; ( 5 ) 205 Ostalihov, S.S. (3) 373 Osuka, A. (1) 326,378,379; (5) 157,158,241 Ota, M. (3) 794
48 1 Otakc, K. (3) 887 Otsuliiro, A. ( 5 ) 256 Otstikn. T.( I ) 395; ( 5 ) 175
Otsuki, H. (3) 672,853 Olsuki, J. (I) 359; ( 5 ) 236 Ottaviani, M.F. (3) 5 18,565 Ou, D.L. (3) 57 Ouchi, A. (1) 137; (2) 1; (2.3) 91; (2.4) 66,263; (2.7) 207 Oucllcttc, A.J.A. (4) 46 Ouyang, X. (2.5) 25; (2.6) 144 Ovcrholt, B.F. (1) 57 I Owcn, E.D. (3) 12 Owcn, H. (3) 469 Owcns, J. (3) 60 Owcns, W. (2.3) 13 Owrutsky, J.C. (2.4) 23; (2.7) 122, 123 Oyama, N. (3) 298 Oyama, 0. (1) 223 Ozcclik, S. ( I ) 167 Paavo, H. (1) 328; (5) 82 Pac, C. (1) 487; (2.5) 73; (2.6) 124 Pace, M.D. (1) 428 Padowski, J. ( I ) 5 1; (3) 118 Paddoo-Row, M.N.( I ) 1 I, 80, 85; (2.6) 239; ( 5 ) 47,70, 74 Padwa, A. (2.3) 13 Page, D. (3) 8 16 Page, J. (2.7) 181 Pagii, R.M. (1) 286; (2.5) 149 Pahl, A. (3) 16 Paik, H.J. (3) 369 Pal, A.J. (1) 572 Pal, H. (2.5) 46 Pal, P. (1) 153 Palaniappan, V. (1) 388; ( 5 ) 275 Palauo, G. ( 5 ) 54 Palit, D.K. ( I ) 154,443,448; (2.5) 219; ( 5 ) 188, 197 Pallcschi, A. (1) 392; ( 5 ) 152 Palm, W.U. (2.6) 212 Palnicr, A.W. (1) 643 Palnicr, B.J. (2.7) 128 Palmer, R.B. (2.2) 31; (2.4) 19; (2.6) 18 Palsule, C.P. (3) 382 Palto, S. (1) 489 Pan, J. (2.4) 84 Pan, J.Q. (3) 874 Pan, X. (3) 807 Panchcnko, V.Ya. (3) 219 Panda, M. (1) 472 Pandcy, C.A. (1) 25; (3) 522 Pandcy, G. (1) 9; (2.5) 14, 148;
(2.6) 325; (5) 29 Pandcy, R.K. (2.5) 104; ( 5 ) 131 Pandcy, S. (I) 480 Pang, L. (2.5) 58 Pang, Y. (3) 659 Pang, Z. (3) 381,401 Panjchpour, M. (1) 57 1 Pankascni, S.(1) 25; (3) 522 Pansu, R. (1) 539; ( 5 ) 268 Paolucci, F. (1) 419 Papadimitrakopoulos, F. (3) 604, 646 Papadopoulos, K. (2.6) 136 Papageorgiou,G. (2.4) 273; (2.6) 303 Paparo, D. (1) 163 Papathomas, K.I. (3) 578 Papiz, M.Z. ( 5 ) 9 Papkovsky, D.B. (3) 376 Pappas, S.P. (3) 144 Paqucttc, L.A. (2.2) 82; (2.5) 184 Paradisi, C. (1) 4 19 Pardo, A. (1) 242 Parikh, S.S.(3) 678 Parise, L. (1) 647 Park, B.S. (2) 16, 17; (2.7) I12 Park, C.H. (2) 60 Park, D.-C. (4) 25,27 Park, H.R. (3) 67-69 Park, J. (1) 4 14 Park, J.W. (2.5) 62; (3) 622 Park, J.Y. (3) 66-69, 128 Park, L.S. (3) 253,254 Park, M. (2.6) 177 Park, S.-K. (2.2) 102, 103; (2.4) 152, 154 Park, S.Y.(3) 530 Park, Y. (3) 660,662 Park, Y.S.(2.5) 69 Park, Y.T. (2.4) 161 Parkanyi, C. ( I ) 188 Parker, A.W. (1) 119 Parker, D. (I) 502,537; (5) 265 Pamion, V.N. (2.5) 196; (4) 23 Parnachcv, A.P. (1) 260 Panias, RS. (3) 215,367,380 Parola, A.J. ( I ) 309; (5) 57 Paron, J.-J. (1) 188 Parson, W.W. (5) 2 Parthsiuthy, T. (3) 132 Parvcz, M.(2) 63 Pasimcnti, L. (3) 599 Passcrini, M.(1) 287 Pate, M.A. (3) 653 Path&, C.P. (3) 55 Patil, S.S.(2) 52; (2.4) 5 7 . Paton, R.M. (2.7) 55
482 Patonay, G. (1) 191 Patra, D. (2.4) 106 Pattcrson, H. (1) 624 Pauck, T. (3) 654 Paul, B.K. (1) 298 Paulscn, K. ( I ) 636 Paulsson, M. (3) 79 1,809 Pavanaja, U.B. (2.7) 172 Pavlik, J.W. (2.6) 66 Pawlowski, G.(3) 24 1,242 Pcacock, R.D. (1) 502 Pcarlstcin, J. ( 5 ) 135 Pcbalk, D.V. (3) 396 Pcchcrz, J. (3) 419 Pechy, P. (2.5) 187 Pcckan, 0. (3) 326,371,372,430, 460,527 Peclcr, A.M. ( I ) 25 Pcgram, J.E. (3) 734 Pei, Q. (I) 437; ( 5 ) 208 Pciffcr, R.W. (3) 11 Pcinado, C. (2.6) 304; (3) 359,716 Pclizzelti, E. (2.5) 200; (3) 741 Pcllct-Rostaing, S.( 5 ) 264 Pcn, H. (3) 25 1 Pencdo, J.C. (2.4) 48 Peng, C. (3) 155 Pcng, S.-M. (2.5) 233 Peng, Z. (3) 557 Penomaryov, O.A. (1) 222 Pcnturk, H.B. (3) 56 Penzkofcr, A. (1) 184,59 1 Pcppas, N.A. (3) 23 Pcreira, E.Zh.N. ( I ) 55 Pcrcz, E. (3) 406 Perez-Pricto, J. (2.3) 35; (2.4) 33 Pcrgushov, V.I. (3) 42,699 Pcriasamy, N. (1) 635 Perlstcin, J. (2.6) 16; (3) 630; ( 5 ) 262 Pernkis, R. (3) 802 Pernot, P. (1) 48 1 Pcrrott, A.L. (2.3) 78; (2.7) 209 Pcrsson, 0.(2.4) 164; (2.6) 162; (2.7) 141, 152 Pcrutz, R.N. (2.7) 12I Pete, J.-P. (2.2) 20,25,27,38; (2.5) 5 ; (2.6) 113, 114 Petek, H. (2.4) 6 Petcrfy, K. (2) 49 Pctcrs, E.-M. (2.5) 114; (2.6) 98, 100; (2.7) 132 Pcters, J. (2.7) 120 Pctcrs, K. (2.4) 128; (2.5) 114; (2.6) 98, 100, 125; (2.7) 132 Pctcrs, K.S. (2.3) 84; (2.4) 44 Pcterscn, A.K. (2.5) 21 1
Pholochemistry Petharan, P. (1) 25 Pclkov, I. (3) 742 Pctrich, J.W. (1) 225,524; (2.2) I18 Pctrochcidiova, N.V. (3) 570,571, 574 Pctrov, N.Kh. (1) 264,597; ( 5 ) 248 Peulicrt, S.(2) 2; (2.6) 337 Pcycrimhoff, S.D. (2.4) GO; (2.6) 230 Pczacki, J.P.(2.6) 194, 196; (2.7) 38,39 Pfaadt, M. (3) 490 Pfab, J. (2.6) 223; (2.7) 153 Pfacndcr, R. (3) 842 Pfanncr, K. (4) 28 Pfcifcr, L. (1) 583 Pfcniiig, B.W. ( 5 ) 243 Pfleidercr, W. (2.4) 272; (2.7) 160, 161 Philipart, J.L. (3) 754,755 Phillips, D. (1) 119, 168; (3) 362, 409 Phillips, D.L. (2.7) 3, 139 Phillips, R.T. (3) 635,636,638 Philouze, C. ( 5 ) 23,24 Philp, D. (1) 555; (2.6) 245 Pi, Y.Q. (2.7) 163 Piaggi, A. (3) 541,665; ( 5 ) 140 Piantanida, I. (2.3) 46; (2.4) 53 Piccinni, P. (2.5) 200 Pichat, P. (2.5) 147,203 Pichko, V.A. (2.4) 62 Picrce, M.E.(2.4) 202; (2.6) 3 1 Pietrzak, M. (1) 51; (3) 118 Pictschinann, N. (3) 176 Piguct, C. (1) 543 Pikranicnou, 2. (2.4) 67; (2.6) 227 Pilichowski, J.F. (3) 740,774 Pillai, V.N.R. (2.6) 226; (2.7) 164 Pilloud, D. (1) 143 Pin, L. (3) 6 14 Pina, F. (1) 309; ( 5 ) 57 Pincock, A.L. (2.3) 32 Pincock, J.A. (2) 61; (2.3) 32,90 Pintauro, P.N. (3) 230 Piotrowiak, P. ( I ) 382; ( 5 ) 165 Pirclahi, H. (2.4) 79; (2.6) 284 Pirisi, F.M. (2.6) 21 I Pirotta, M.(1) 604 Pimmg, M.C. (2.4) 217 Phyetinskii, A.Yu. (1) 574 Piryatinskii, Yu.P. (1) 574 Pisaiio, P.J. (2.5) 57 Pispisa, B. (1) 392; (5) 152 Piszczck, G. (1) 135,246
Pitchumani, K. (1) 381,394; (2.3) 26; (2.4) 239,240; (2.6) 148; ( 5 ) 160, 161 Piton, M. (3) 773,775 Piva, 0. (2.2) 20,27; (2.6) 113, 1 I4 Piva, S.(2.2) 25 Piva-LcBlanc, S.(2.2) 25 Pkutsu, T. (2.6) 323 Pla, F. (3) 370 Placucci, G. (2.5) 126 Platz, M.S.( I ) 138, 148; (2.6) 187, 196,205; (2.7) 5,34,36,39, 41,80,169 Plaza, P. (1) 244 Plenio, M.B. (1) 63 1 Plunitallo, A. (2.5) 126 Podlcch, J. (2.6) 186; (2.7) 57 Pocllingcr, F. (1) 342; (2.5) 40; (5) 88 Pogi, G. (2.6) 180 Pohar, c. (3) 549 Pohlcrs, G. (2.4) 256 Poiricr, M. (2.6) 289 Poizat, 0. (2.5) 77 Pokhrcl, M.R. (3) 429 Pola, J. (2) 1; (2.7) 172, 173 Polanyi, J.C. (2.7) 11 Polborn, K. (2) 18; (2.5) 51, 164; (2.6) 74 Pole, D.L. (2.6) 196; (2.7) 39 Poliakoff, M. (2.7) 116, 127 Polirncno, A. (1) 63,67,90,91, 633; (5) 77,80 Pollaslri, M.P. (2) 57; (2.4) 253 Pollicino, A, (3) 761 PoloncAy, V.V. (1) 32 Polykarpov, A.Y. (1) 274; (2.6) 248; (3) 180,lS I Porncry, P.J. (3) 752,753 Ponuncrchne, J. (3) 608 Poncc, A. (I) 361,532; ( 5 ) 103 Ponomareva, RP. (3) 829 Ponticclli, F. (2.2) 4; (2.4) 109; (2.6) 107, 118 Ponyacv, A.I. (2.4) 76; (2.6) 46 Poojary, D. (4) 39 Popielarz, R (3) 213 Popov, V.K. (2.7) 127 Popovic, Z.D. (3) 63 1 Porcar, I. (3) 425 Porinchu, M. (2.2) 5 1 Port, H. (I) 35 1,602; (3) 404,453 Porter, N.A. (2) 57; (2.4) 253 Portcr, T. (1)517 Portnoy, M. (1) 557 Posada, F. (3) 754,755
AuIhor Index Postigo, J.A. (2.3) 40; (3) 8 15 Postnikov, L.M. (3) 679 Pottcr, W.R. (2.5) 104 Potts, c. (5) 59 Potzingcr, P. (2.6) 324; (2.7) 187 Powcr, P. (2.5) 2 I I Pozzo, J.L. (2.4) 74; (2.6) 48,49 Pradclla, F. (3) 758 Pracfcke, K. (1) 488 Pragcr, R.H. (2.6) 209 Praly, J.-P. (2.6) 198; (2.7) 67 Prasad, P.N. (3) 583 Prater, K. ( I ) 141 Prathapan, S.(2.6) 85 Prato, M. (1) 419,458-462; (3) 599; ( 5 ) 215-219 Prcccc, J.A. (1) 555; (2.6) 245 Prcmachandriui, R. (3) 4 17,423 Prcmkuniar, J.R. (2.5) 98; ( 5 ) 17 Prcvitali, C.M. (1) 265; (2.5) 79; (3) 104 Prcvot, L. (I) 5 19; (2.7) 131 Price, N.J. (2.5) 93 Priddy, D.B. (3) 690 Primo, J. (2.2) 32; (2.4) 237; (2.6) 71 Prinzbach, H. (2.4) 133; (2.6) 126, 190; (2.7) 19 Priola, A. (3) 33 1 Priou, C. (3) 30 Pritchard, R.G. ( 5 ) 22 Probst, N. (3) 343,467 Prochorow, J. (1) 320 Prodi, L. (1) 555,623; (2.6) 245; (5) 145 Proskwina, M.V. (2.3) 59; (4) 19 Proskuryakov, T.V. (3) 829 Pryalihin, A.N. (3) 793 Prysdun, V.V. ( 5 ) 190 Przegictka, K.(1) 565 Pu, L. (3) 62 1 Puchala, A. (I) 245 Pucntcner, A. (3) 884 Puglisi, C. (1) 525 Pullcn, G.K.(3) 94 Pubam, R. (1) 494 Pykhtccv, D.M. (1) 123 Qi, W. (1) 527 Qi, X. (2.5) 190 Qi, Y. (3) 29 1 Qian, G. (1) 177 Qiao, L. (3) 339 Qiu, J. (3) 113,528 Qiu, K.Y. (2.6) 261; (3) 125 Qu, B. (3) 245-250
483 Qu, X. (3) 155, 160,250 Qu, Z. ( I ) 527 Qui, 1. (3) 47,395 Qui, T.C. (3) 14 Qui, T.R.(3) 15 Qui, Y. (3) 201 Quiclct-Sirc, B. (2.6) 258 Quillcn, S.L. (2.6) 66 Quin, L.D. (2.6) 336 Quinn, E.T. (3) 70, 1 12 Rabanal, F. ( 5 ) 133 Rabck, J.F. (3) 175,826 Rabcllo, M.S. (3) 724,726 Raclianiin, M. (1) 3 19 Racioppi, R. (2.6) 254 Radcmachcr, J.T. (1) 546 Rdncr, F. (2.4) 164, 175,267; (2.6) 162, 172, 173; (2.7) 142, 152 RadAi, S.(2.5) 224 (2.7) 58 Rafcstin-Oblin, M.-E. Ragauskas, A.J. (3) 806,807 Rahmani, H. (2.4) 79; (2.6) 284 Rajagapal, S.(1) 281,297 Rajakovic, L.V. (2.6) 234 Rajaram, S.(3) 550 Rajcndiran, N. (1) 252 Rajcndran, T. (1) 28 1,297 Rajcswari, S.(2.6) 150 Raju, B.B. ( I ) 217; (2.5) 202 Raldugin, V.A. (2.3) 70 Ram, A. (3) 733 Riunachiuidram, B.(2.6) 260 Ramaiah, D. (2.3) 25; (2.4) 42,45; (2.6) 183 Ramaniurthy, K. (3) 772 Ranirundiy, P. (1) 281,282,297 Ramaniurlhy, V. (1) 277,38 1,394; (2) 4, 12; (2.3) 26; (2.4) 239, 240; (2.5) 50, 166; (2.6) 148; ( 5 ) 160,161 Raniaraj, R. ( I ) 28 I , 297; (2.5) 98; ( 5 ) 17
Raniasiuny, S.M.( I ) 165,494 Raniclspcrgcr, M. (1) 184 Rami, A.V.R (3) 329 Ramircz, A. (2) 54; (2.4) 195; (2.6) 63 Rarnircz, E.V. (3) 846 Riuilkumar, D. (2.4) 102 Ramos, A. (2.3) 23; (2.5) 9; (2.6) 146 Ramsdcn, C.A. (2.6) 135 Ranby, B. (3) 188,244-246, 282-284
Randall, L.H. (2) 13; (2.5) 55 Rangarajan, B. (3) 18, 169 Ranjit, K.T. (2.5) 179 Rao, A.M. (1) 434; (5) 214 Rao, C.N.R. (1) 398 Rao, K.N. (3) 131,132 Rao, K.S.(2.5) 148; (2.6) 325 Rao, K.V.N. (2.5) 148; (2.6) 325 Rao, R.T.A. (I) 170 Rapoport, Yu.M. (3) 845 Rapp, W. (3) 416 Rapta, P. (1) 418; (2.5) 85 Rasala, D. (1) 245 Rasbirin, B.S.(1) 416 Rasoul, F.A. (3) 752,753 Rassing, J. (3) 327 Rah, N.P. (2.3) 25; (2.4) 42,45; (2.6) 147, 183 Rabicr, J. (3) 485 Ratncr, M.A. (I) 5; (2.5) 1; (5) 25 Rattray, A.G.M. (2.6) 149 Rau, H. ( I ) 293 Ravichandran, J. (3) 329 Ravikanth, M. (1) 229 Ravindran, K. (2.2) 55; (2.6) 121 Ravindranathan, T. (2) 52; (2.4) 57 Rawal, V.H. (2) 30,3 1 Rawtins, K.A. (3) 578 Ray, J.G. (1) 234 Ray, S.(2.2) 90; (2.4) 43 Razafilrimo, H. (3) 61 1 Razumov, V.F. (2.4) 187 Rcbar, V.A. (3) 378 Rcbicn, F. (2.5) 158 Rccca, A. (3) 761 Rcchthaler, K. (1) 245,248 Rcddingcr, J.L. (3) 607 Rcddy, A.V.R (3) 344 Rcddy, S.S.(3) 134 Rcdiiond, RW. (1) 205; (2.6) 222, 256,257; (2.7) 155, 182,183 Rcc, S.J. (1) 627 Red, C.A. (1) 466; (5) 222 Rcck, J.N.H. (1) 3 13 Rccs, L.H. ( 5 ) 174 Rccsc, R.S.(1) 36 Rcgm, C.J. (3) 876 Rcgcv, A. (1) 486 Rcgismond, S.T.A. (3) 428 Rcgnouf-de-Vains, J.-B. (5) 264 Rchabcr, H. (2.6) 159 Rcibcl, 1. (3) 277 Rcichcnbacchcr, M.(2.3) 55 Rcid, P.J. (3) 458 Rcidel, M. (3) 205 Rcimcrs, J.R (1) 96; ( 5 ) 60 Rcindl, S.(1) 591
Photochemistry
484 Rcinhoudt, D.N. (5) 144 Rciningcr, F. (1) 581 Rcis, H. ( I ) 58 Rcis, K.P. (4)39 Rciscnaucr, H.P. (2.7)23 Rcisingcr, A. (2.7)92 Rcisler, H.(2.7)100, 101 Rcitbcrgcr, T. (3) 667,668 Rcmi, E.(3) 890 Rcmmcrs, M. (3) 610 Rcmpcl, U.(1) 12, 13;(5) 41,42 Rcn, Y. (3)34 Rcnak, M.L. (3)648 Rcnaud, J. (2.5)139 Rcnault, T.(3) 229 Rcnganathan, R. (2.5)209 Rengc, 1. (1) 619;(3)786 Rcngcl, H.(I) 174;(5) 128 Rcnn, A. (I) 107,604 Rcpccv, Yu.A. (2.3)56,57 Rcpclskii, E.(I) 574 Rcpkova, M.N. (2.7)83 Rcsul, R. (3) 164 Rctbcrgcr, T.(3) 687 Rcttig, A.J. (2.6)149 Rcttig, S.J. (2) 13; (2.5)55 Rcttig, W. (1) 89,236,244,248, 253; (2.4)24;(3) 439,588;(5) 137 Rcttschnick, R.P.H. (I) 93;(2.5) 133;(5) 72 Rcy, M. (5) 22 Rcycs-Arcllano, A. (1) 3 16; (2.6) 23 1 Reynolds, J.R. (3) 607 Rcynolds, L.(1) 62;(5) 64 Rliarbi, Y. (3)434 Ribciro, F.R. (I) 372 Ribitsch, V.(3) 549 Ribou, A . 4 . (1) 22 Ricccri, R. (1)300; (5) 234 Rice, J.K. (2.4)23 Rice, T.E.(1) 546 Rich, D.C.(3) 306,519 Richard, H. (2.6)219 Richards, N.G.J. (2.3)12;(2.4)91 Riche, C.(2.2)73 Richommc, P.(2.4)47;(2.6)203 Richtcr, F.(3) 756,757 Richtcr, W.(1) 618 Rickctts, H.G.(1) 555;(2.6)245 Rico, R. (2.4)177 Ricdcl, J.H. (3) 863 Riclcy, H.(2.5)93 Ricra, N.(2.7)129 Rics, M.(2.6)288 Ricscn, H. (1) 6 17
Rokicki, G. (3)350 Rollins, H.W.(1) 438;(5) 21 1 Roniani, A. (2.4)71;(2.6)53 Romano, S.(2.3)43,44;(2.6)97 Romcro, F.M. (5) 270 Ronipcl, A. (5) 19 Rong, J. (3) 160 Rosa, A. (2.7)J24 Rosch, N. (2.2)64 Rosc, H.L. (2.6)135 Rosc, K. (3)273 Roscnbcrg, M.G. (2.7)35 Rosmus, P. (2.7)110 Ross, P.L.(2.7)140 Rosscnaiu, B.D. (2.7)125 ROB, H.D. (2.3)28,38;(2.4)143; (2.5)131,133;(2.6)95,116, 143 Rohbcrg, L.J. (3)604,646 Rotkicwicz, K.(1) 245,248 Rotman, S.R. (1) 57 Rotschova, J. (3) 852 Routlcdgc, A. (2)59 Roux, C.(1) 560 Row, T.N.G. (2.2)5 Rowan, A.E. (1) 313 Rowland, B. (2.7)107 Rowlcy, N.M. (1) 356;(5) 100 Roy, A.K. (I) 145 Rozcnbcrg, L.P. ( I ) 446;(5) 205 110,232 Rozwadoski, J. (2.5)23 1 Robcrtson, G.( I ) 573 Rtishchcv, N.I. (3) 521 Robcrtson, M.A.F. (3)564,595 Ruan, F.(1) 46 Robichaud, J. (2.6)289 Rubin, M.B. (1) 387 Robinson, C.H.(2.6)80 Rubtsov, I.V.(1) 449;(2.5)82;(5) Robinson, K.E. (2.6)85 204 Robinson, W.T. (2.4)165, Ruckcrt, D.(3) 194 167-169,171, 174;(2.6) Rucckcmann, A. (1) 342;(2.5)40; 163-166,169, 170;(2.7) 143-146,149, 150 ( 5 ) 88 Ructhcr, M. (3)496 Rocha, H. (3) 84 Ructtingcr, W.(4) 1 Rockcnbaucr, A. (3) 892 RuiTolo, R. (2.7)189 Rodgcrs, M.A. (2.5)104 Rodgcrs, M.A.J. (1) 274;(2.6)248 Rufs, A.M. (3) 104 Rufus, I.B. (3) 504,507 Rodrigucz, G.S. (3) 846 Rughooputh, D.D.V.S. (3)450 Rodrigucz, M.A. (2.6)70 Ruhlandt-Sengc, S.(5) 127 Rodriguez, M.C.R (2.4)48 Ruiz, M.A. (2.7)129 Rodriguez, P.F. (2.4)48 Rullicrc, C.(1)523;(2.5)192;(5) Rodriguez-Morgadc, S.(2.2)93; 272,273 (2.4)52;(2.6)154 Rumbles, G. (3)5 13,632 Rodrigucz Nwcz, E. (1) 630 Runiyanscv, B.M.(3) 396 Roelofs, T.A. (5)19 Rocst, M.R. (1) 85,353;(2.6)319; RusaIov, M.V. (3)580 Russell, A.A. (2.2)7 (5) 70 Russcll, G.A. (2.6)332 Rofia,S.(1)419 Russcll, S.(5) 21 Rogcrs, C.W.(2.7) 189 Russo, P.S. (3) 546 Rohatgi, A. (4)6I Ruth, A.A. (1) 161 Roj, A. (1) 149
Ricsfcld, R. ( 1) 212 Ricss, W.(3)644 Rigby, J.H. (2.3)69;(2.6)30 Righhini, R. (1) 140 Riglcr, R. (1) I I 1 Rigny-Bourgcois, V. (3)735 Rikukawa, M. (2.6)29 Rilb,J.M. (3) 451 Rillcma, D.P. (1) 24;(4)42;(5) 36 Rincc, S.M. (5) 9 Rincon, G.A. (3)770 Rios, M.A. (I) 470 . Rist, G. (2.6)338 Rist, P. (3) 855 Rittcr, H. (3)332 Ritkr, K.(4) 13 Rittcr, R.E. (5) 260 Rivaroh, C.(1) 265 Rivas, C.(2.4)139;(2.6)280 Rivaton, A. (3) 773,775 R i m , C.J.(2.6)177 Robb, I.D. (3) 408 Robb, M.A. (1) 179;(2.3)47,48; (2.4) 122 Robbins, R.J. (2.4)245;(2.6)208 Robcrson, M.J. (2.7)I5 Robcrt, M.(2.7)169 Robcrt-Banchcrcau, E. (2.5)228 Robcrts,J.A. (1)314,315;(5)
Author hidex Rutherford, T.J. (1) 325; (5) 123 RuUdiosliy, M. ( 5 ) 99 Rutloh, M. (2.4) 30 Ruzsnak, I. (3) 890-892 Rychly, J. (3) 677,680 Rytz, G. (3) 404 Ryu, A. ( I ) 424; (2.5) 106 Ryu, 1. (2.4) 252; (2.6) 326 Rzadck, P. (3) 855
'
Sabadini, E.(3) 596 Sachlcbcn, R.A. (2.7) 160 Sadamoto, R. ( 5 ) 237 Sadcrholm, M.J. (2.5) 217; (5) 113 Sadlck, 0. (2.5) 164 Saegusa, K. (1) 299 Sacycns, W. (2.2) 68; (2.6) 47,112 Saf, R (3) 337 Safonova, L.A. (3) 573,581 Sagcar, P.A. (1) 220 Sagisaka, T. (2.2) 98 Saglcev, RZ.(1) 260 Sagun, E.I. (1) 369 Salia, M. (2.7) 56 Sahlcn, F. (3) 265,347 Saielli, G. (1) 90,91,633 Saigo, K. (2.7) 157 Saika, T. (3) 5 12 Sailor, M.J. (2) 39 Saini, A. (2.2) 40; (2.5) 38 Saini, R.D. (2.7) I72 St. John, M.R. (3) 805 Saintomc, C. (2.2) 73 Sl. Picrrc, M.J. (2.5) 2 1 Saito, A. (3) 88 Saito, 1. (2.2) 65,80,81; (2.6) 176 Saito, M. (1) 562 Sailo, S. (3) 548; (4) 14 Sailoh, H. (2.4) 134; (2.6) I19 Saitoh, T. (2.2) 88; (2.6) 310; (4) 48,54 Saitoh, Y. (3) 624 Saiz, E.(1) 376; ( 5 ) 154 Saja, A. (1) 525 Saji, T. (3) 491 Sakaguchi, K. (2.7) 114 Sakaguchi, Y. (I) 598; (2) 7; (2.6) 249; ( 5 ) 249 Sakai, K. (2.3) 41; (2.4) 39 Sakai, M. (1) 412; (2.6) 281 Sakaki, S. (2.5) 63 Sakamizu, T. (3) 238 Sakamoto, M. (2.4) 56, 132; (2.6) 267,268 Sakashita, S. (3) 34 1,468 Sakata, T. (2.5) 100
485 Sakata, Y. (1) 84,456,463,465;
(2.5) 43,87,88; ( 5 ) 68, 178, 220,22 1 Sakhno,T.V. (3) 59 Sakuma,T. ( 5 ) 126 Sakuragi, H. ( I ) 169; (2.5) 73 Srtkurai, H. (2.6) 124 Sakuni, T. (2.2) 47; (2.4) 194, 269,270; (2.6) 250,251,259; (3) 476 Salama, P. (2.6) 283 Salazar,N. ( I ) 191 'Salcedo, R. (3) 523,534 Salcmi-Delvaux,C. (2.5) 103 SalclsAy, A.M. (1) 32 Salimgarceva, V.N. (3) 39,862 Salimoncnko, D.A. (3) 457 SaIlah, M. ( I ) 188 Sallassc, C. (1) 396 Salmmovo, Ch.K. (2.5) 143 Sallhiunmcr,T. (3) 223 Samajdar, S.(2.4) 106 Samanta, A. (1) 219,483; (2.6) 260 Samal, A. (2.4) 74,87; (2.6) 47,48 Samsonova, L. (1) 375; (5) 153 Samuel, I.D.W. (3) 5 13,632, 634-637 Samuels, S.B. (3) 832 Sanai, Y.(2.7) 52 Sanchcz-Obrcgon, R. (2.2) 109 Sanchcz-Valdcs,S. (3) 721 Sanda, F. (3) 694 Sandc, S.A. (2.5) 13 Sandcr, R. (3) 620,626 Sandcr, T. (1) 337; ( 5 ) 230 Sandcr, W. (2.3) 83; (2.4) 242; (2.6) 202; (2.7) 45,72 Sandcrs, B.M. (3) 843 Sandcrson, A. (1) 209,586 Sandcz, M.I.( I ) 470 Sandman, D.J. (3) 486,618 Sandri, M. (3) 76 I Sancyoshi, M. (2.7) 50 Sangcn, 0. (3) 264,34 1,468 Sanghi, S.(1) 176 Sangster, D.F. (3) 192 Sankaraman, S.(2.4) 102 Sankaran,N.B. ( I ) 219 Sannikova, N.S.(3) 39,862 Sano,K. (4) 63 Sano, T. (2.2) 88; (2.4) 132; (2.6) 310 Sanov, A. (2.7) 100, 101 San Roman, E. ( I ) 296 Sansorcs, E. (3) 523 Santa, T. ( I ) 505
Santamalo, E.(I) 163 Santiago, S.L.(3) 846 Sanlorc, M.M. (3) 377,378 Smui, K.(2.6) 29 Sanz, D. ( I ) I34 Sanz. J.M.B. (2.5) 182 S ~ OG.-Q. , (2.7) 70 Sapozlmikov, M.N.(3) 655 Saprc, A.V. (1) 154,448,476; (2.5) 219; ( 5 ) 197 Sapunov, V.V. (1) 50 Saraja, G. (1) 483 Sarakha, M. (2.5) 171,172 Sarhm, A.A. (3) 187,193 Sariciltci, N.S. (2.5) 28 Sarkar, A. (1) 233,274 Sarkar,M. (1) 234 Sarkar, N. (1) 61; (3) 873 Sarkas, H.W. (3) 849 Sarker, A. (2.6) 248; (3) 179, 180 Sarovo, G.(3) 742 Sasabc, H.(1) 22 Sasabc,M. (3) 90 Sasaki,H. (2.4) 271 Sasaki, K. (2.2) 47; (2.4) 194; (2.6) 259; (3) 209 Sasaki, M. (3) 520 Sasaki, N. (2.3) 85; (2.4) 34 Sasaki, T. (3) 445,670; (4) 32,33 Sasaki, Y.(1) 450,452; (5) 200, 202 Sasako, M. (2.7) 168 Sasanioto, T. (3) 760 Sasc, M. (2.2) 113; (2.5) 32 Sassara, A. (1) 404 Sasse, W.H.F. (1) 383; (5) 166 Saslre, J.A.L. (2.5) 182 Sastri, M.V.C. (4) 26 Sato, E. (3) 62 Sato, H. (1) 203 Sato, K. (2.4) 201; (4) 34 Sato, M. (1) 180 Sato, S. (2.7) 115 Sato, T. (2.6) 294; (3) 49,577; (4) 34 Sato, Y. (3) 473 Satonov, I. (1) 427 Sauer, K.( 5 ) 19 Saucr, M.H.M. (1) 5 1 1 Saucr, S.(2) 19; (2.6) 76,77 Sauvagc, J.-P. (1) 15,362,363, 547,556; ( 5 ) 7,31, 104,105, 177 Savinov, E.N.(2.5) 196 Sawaki, Y. (2) 23; (2.3) 71,72; (2.4) 114, 115,221,251 * Sawayama, S. (4) 65,66
486 Sayama, K. (4) 24,29 Scaiano, J.C. (2.3) 35; (2.4) 33; (2.5) 70,75, 139; (2.6) 188, 191; (2.7) 18,54; (3) 122,808, 810 Scandola, F. (1) 360,363,554; (5) 102,105 Scanncll, M.P. (2.2) 63 Scarlata, C. (2.3) 38; (2.5) 131; (2.6) 95 Schadt, M. (3) 92 Schacfcr, 0. (3) 608 Schael, F. (1) 183 Schsrcr, D.(2.6) 68 Sclianzc, K.S. ( 5 ) 148 Scharf, H.D. (2.2) 18; (2.4) 119 Schatz, P.N.(1) 6 17 Schatz, T.R (1) 382; (5) 165 Schaumann, E. (2.6) 288 Scheffer, J.R. (1) 381,394,504; (2) 12, 13; (2.3) 26; (2.5) 50, 55; (2.6) 148, 149,333; (5) 160,161 Scheidlcr, M. (3) 644 Schercr, A. (2.3) 5 1 Schercr, C. (2.7) 29,3 1; (3) 828 Schcrer, T. (1) 93; (2.5) 193; (5) 72 Scherf, U. (3) 654 Scherl, M. (2.6) 228 Scherowshy, G. (3) 469,5 10 Schicsscr, C.H. (2) 65; (2.6) 330 Schike, H. (2.3) 58 Schillcr, S. (2.6) 94 Schimctta, M. (3) 6 15 Schindlcr, W. (2.4) 90; (2.6) 131 Schinke, R. (2.7) 110 Schlachtcr, I. (2.4) 216 Schlegel, H.B. (2.3) 58 Schlicsscr, J. (2.3) 7; (2.4) 25 Schlitzcr, D.S.(3) 763 Schmehl, R.H. ( I ) 382; ( 5 ) 165, I69 Schmclling, D.C. (2.5) 142 Schmidt, C. (2.7) 23; (3) 205,608, 639 Sclunidt, P. (2.4) 268; (2.6) 83; (2.7) 185 Schniidt, R. (1) 366 Schmitt, C.A. (2.5) 79 Schnabel, H.-J.(1) 284 Schnabcl, W. (2.5) 24; (2.6) 235, 237; (3) 32, 150, 151, 183,359, 762 Schnapp, K.A. (2) 42; (2.4) 254; (2.7) 133 Schneider, G. (2.5) 225
Photochemistry Schncidcr, K. (2.3) 89 Schncider, S. (1) 74,93,338; (2.5) 193; (2.6) 159; (3) 756,757; (5)66,72,227 Schncidcr, U.(2.5) I14 Schnctz, A. (1) 576 Schnurcr, A.U.(3) 297 Schoencckcr, B. (2.3) 55 Schocvaars, A.-M. (1) 495; (2.4) 15; (2.6) 10 Schoficld, J. (1) 116 Scholcs, G.D.(1) 11,79; ( 5 ) 47, 143 Schoonlicydt, R.A.(1) 276; (5) 246 Scluauwcn, C. (3) 370 Scbcibcr, M. (1) 64; ( 5 ) 59,271 Schroedcr, J. (I) 266; (2.7) 205 Schrolh, W. (2.6) 285 Schuddcboom, W. (1) 250,353; (2.6) 3 19 R.M.(2.5) 183 Scl~~hni;llm, Schulman, S.G. (2.4) 97 Schultz, A.G.(2.2) 75,76 Schultz, H. (3) 176 Schultz, J.A. (2.5) 102 Schulz, A. (1) 5 I 1 Schulzc, M. (3) 610 S ~ h ~ l ~ - E k G. l ~ (2.5) f f , 225 SchuDbaucr, W. (2.6) 159 Schustcr, D.I.(1) 427; (2.2) 12 Schuster, G.B. (2.2) 114 Schustcr, J. (1) 115 Schwartz, G. (3) 467 Schwarzcr, D.(1) 266; (2.7) 205 Scliwccr, J. (3) 5 I, 52 Schwcigcr, G. (3) 80,85 Schwcitzcr, G. (I) 323; (5)242 Schwitter, U.(2.6) 327 Scoponi, M. (3) 758 Scorrano, G. (1) 458-462; ( 5 ) 215-219 Scott, G.(3) 692,717,718 Scranton, A.B. (3) 18, 145, 153, 154,169,170 Scribner, A.W. (2.6) 34 Scrim, R. (3) 199 Scurlock, R.D.(I) 367; (2.5) 109 Scarlc, N.D.(3) 734 Scbastian, P.J. (4) 36 Scbck, P. (2.2) 43 Sediroglu, V. (4) 67 Scely, G.R (1) 343,345,466; (2.5) 44,222; ( 5 ) 12,94, 117, 222 Scgawa, H. ( 5 ) 258 Seguchi, K. (2.4) 3,49; (2.6) 153 Scgura, J.L. (2.5) 72
Schanobish, K. (3) 707 Scidcl, C.A.M. (I) 5 11 Scidcl, G.(2.4) 18 Scifcrt, H. (5) 137 Scilcr, M. ( 5 ) 115 Scinfcld, J.H. (2.5) 140 Scitcr, M.S. (2.7) 94 Sckhcr, P. (2.3) 7; (2.4) 25 Scki, H. (2.5) 136 Scki, K. (2.2) 66,67; (2.6) 127, 128 Seki, T. (3) 365,492,495 Sekiguchi, A. (2.6) 322 Sckiguchi, H. (3) 308 Sckiguchi, T. (3) 794 Sckitani, T. (3) 537 Sckizawa, H. (3) 492 Sclkc, M. (2.2) 7 Sclli, E. (2.6) 17; (3) 330 Scllinger, A. (3) 275,276 Scmcl~ikov,Yu.D. (3) 32 1 Scmcnova, O.M.(3) 321 Scmin, D.J. (1) 112 Scnbuko, H. (2.2) 13,112; (2.4) 145,153 Scnge, M.O. ( 5 ) 48, 127, 131 Scngupta, P.K. (1) 234,484 Scno, M. (1) 359; ( 5 ) 236 Sco, K.H. (3) 342 Scoanc,C. (2.5) 72 Scra, A. (2.4) 49; (2.6) 153 Scrbutovick, C. (3) 270 Scrdob, M.V. (2.5) 30 Scrf, U. (3) 6 13 Scrgan, T. (1) 489 Scropegina, E.N.(3) 42,698-700 Scrponc, N. (1) 190; (3) 741 Scrrano, B. (3) 37 Scrroni, S. (5) 167 Scsha, P. (2.5) 148 Scshadri, R. (1) 398 Scsslcr, J.L. (1) 312; (2.5) 173; (5) 107 Scth, J. (1) 388; ( 5 ) 275 Sclhuram, B. (3) 131,132 Sctncscu, R. (3) 682 Sctncscu, T. (3) 682 Scto, K. (2.5) 136 Scvcrini, F. (3) 728 Scybold, P.G. (3) GOO Sgarabotto, P. (2.6)252 Sha, C.K. (2) 55 Shah,H. (3) 507,776 Shah, M. (3) 95,716 Shahccn, A.A. (3) 792 Shduiari, M.R (1) 475 Shaik, S. (3) 630
Author h i e x Shakirov, M.M. (2.3)70 Shalyacv, K. (2.5) 119 Shan, Z.(3)882 Shane, J.M. (2) 39 Shancr, B.E.(3)850 Shang, H.(2.7)66 Shannon, R.J. (5) 112 Sharapova, L.I. (3)573,581 Sharipov, G.L.(3)573,581 Sharma, R P . (2.5)181,213 Sharma, S. (2.2)40;(2.5)38 Sharshira, E.M.(2.4)214,215 Shashtri, V.R. (3) 235 Shc, W.(3) 155,160 Shchcrd, B.D. (3) 274 Shcn, C.(1) 5 18 Shcn, H.(3)247 Shcn, J. (3)579 Shcn, J.C. (3) 204 Shcn, P.-W. (1) 117 Shcn, S.-Y.(1) 271;(5)55 Shcn, T. (1) 290,329,330,333, 335;(2.5)60,176;(5) 84,87, 120,121 Shcn, Y.(3) 175 Sheng, D.(3)76 Shcntu, B.(3)25 1 Shcpard, M.S. (2.2)28;(2.4)121 Shepelenko, E.N.(2.6)26 Shcreshovcts, V.V. (2.5) 116 Shcrf, U. (3) 616 Sheridan, R.S. (2.7) 181 Shcrrington, D.C. (2.7)26;(3)466 Shcvchenko, S.M.(3)801 Sheveleva, T.V. (3)424 Shi, F. (3)22 Shi, J.-L. (2.5)27 Shi, S.-Q.(1) 117 Shi, W. (3)244,248,282-284 Shi, Y.(2.3)86,88;(3) 86,92 Shi, Z.(4)55 Shi, Z.-Y. (5)263,266 Shibaeva, R.P. (5) 205 Shibano, T. (2.7)82 Shibanov, V.V. (3) 61 Shibata, H.(2.4)25 1 Shibirin, Y.V. (3)252 Shibita, J. (3) 197 Shichi, T.(2.3)71,72;(2.4) 114, I15 Shiclii, T. (2)23 Shich, S.(3)470 Shields, C.J.(2.7)75,76 Shigcru, Y. (3)23 I, 240,300 Shii, D.(2.7)82 Sliikc, A. (2.5)12I Shil'ga, A. ( I ) 12
487 Shim, H.K. (3) 641,643 Shim, S.C. (2.2)99;(2.3)8, 10; (2.4)95, 100, 151, 154;(2.6) 72;(3)772 Shim, S.S.(2.2)103 Shinia, K. (2.2)1; (2.3)14,45; (2.4)4, 179, 188;(2.6)9, 139, I42 Shimada, R. (1) 178 Shimazaki, K. (2.2)97 Shimidzu, H. (5) 256 Shimidzu, T.(2.6)294;(3)49;(5) 257,258 Shimizu, M. (2.2)72 Shimizu, T.(2.5)136,198;(3)783 Shimizu, Y.(2)46,47;(2.7)138 Shinio, T. (2.2)9 Shin, E.-J.(1) 414 Shin, H.-K. (5) 149 Shina, K. (5) 136 Shinar, J. (3)399,623 Shindo, Y.(3) 256 Shinkai, S.(1) 41 Shinoda, H.(2.2)56 Shinoda, S. (I) 379;(5) 158 Shinohara, Y.(4)30 Shioji, N. (4)62 Shiotani, S.(2.4)134;(2.6)119 Shioya, T.(1) 593 Shiraganii, T. (2.3)45;(2.5)132; (2.6)139 Shirai, H. (1) 3 10;(3)567;(5) 108 Shirai, M.(3)31,239,562 Shirai, Y.(3)195 Shiraishi, H.(3)238 Shirakawa, M.(1) 456,465;(2.5) 88;(5) 178,221 Shirodai, Y.(2.6)312 Shiromiuu, H.(1) 430 Shirota, Y. (3) 475 Shinrka, H. (1) 257 Shishkin, G.V. (2.7)89.90 Shishlov, N.M. (2.5)154 Shivaraniayya,K. (2.5)I88 Shivshrmkar, V. (3)486.6 18 Shizuka, H.(1) 136,447;(2.2) 113;(2.5)32,34,80;(5) 136, 207 Skilcv, V.P. (I) 56 Shokhirev, N.V. (1) 275;(5) 226 Shortrccd, M.R. (5)263,266 Shouji, E. (3) 710 Shrcdcr, K. (2.5)67 Shrive, J.D.A. (1) 474 Shu, A.Y.L.(2.7)84 Sliubhada,S.W. (2)52 Sbui, L. (I) 227,363
Shukia, D. (2.2) 107;(2.3)81; (2.5)232;(2.6)299 Shul'ga, A.M.(1) 216,377;(2.4) 55, 146;(5)42, 129, 130, 156 Shul'pin, G.B. (2.5)153 Shunichi, T.(1) 421;(5) 195 Shushin, A.1. (1) 602 Shuto, K. (3) 764 Si, K. (3)201 Siaclli, G.(1) 63;(5) 77 Siamchai, P. (4)58 Sibaeva, R P . (1) 446 Siclicl, E.K. (3)306,519 Sidorov, L.N. (5) 190 Sicburlh, S.(2.6)121, 122 Sicburth, S.McN. (2.2)53-55 Sicdshlag, C. (I) 450;(5) 200 Sicgcl, B. (2.2)54;(2.6) 122 Siclcr, J. (2.6)285 Siggcl, U. (2.5)221 S i p a n , M.E.(2.4)233;(2.5)160 Sikkcma, K. (3) 690 Sikorski, M.(1) 45,536 Silbcy, R.J. ( I ) 648 Siling, S.A. (3)472 Silva, A.M.S. (2.4)185 Siniburger, E. (4)59 Simon, J.A. (I) 382;(5) 165 Simon, J.D. (1) 129 Simonov, A.P. (2.7) 193 Siinons, J. (2.7)I5 Simonson, R (3) 791,809 Simpson, C.J.S.M. (2.7) 158 Simpson, T.R (2.3)29,30 Sindlcr-Kulyk, M. (2.3)46;(2.4) 53 Singer, D. (1) 488 Singh, A.K. (2.3)73;(2.6)106 Singh, N.(1) 613 Singh, R. (2.2)90;(2.4)43 Singh, R.P. (3)725,873 Singh, V. (2.2)5 1 Sinha, S.(1) 273,285;(2.5)17, I70 Sinibaldi, M.-E. (2.4)193;(2.6)57 Sinn, G.(1) 580 Sionkowska, A. (3) 804 Sirbiladzc, K.(3) 892 Sirish, M.(1) 520 Sirovalka, K.J. (3) 170 Sisido, M.(4)5 Sisson, T.M. (3)79 Sitck, F.(3) 842 Sitncr, E.Ya. (3) 845 Sivarani, S.(3) 134,725,873 Skokan, E.V. (5) 130 Skolling, 0. (3) 165
488
Skolnick, M.S. (3) 653 Skurski, P. (1) 149 Slam, C.N. (3) 848 Slatcr, D.A. (2.7) 191 Slattery, D.K. (4) 46,47 Stavin, V.V. (I) 97 Slayton, R.M.(2.4) 224 Sluggett, G.W. (2.6) 339 Smirnov, S. (1) 10; ( 5 ) 44 Smirnova, N.P. (I) 306; (5) 244 Smith, B.R. (2.4) 122 Smith, C.K. (3) 43 I Smith, D.L. (3) 629 Smith, C.(1) 205 Smith, G.J. (1) 151 Smith, J.N. (2.5) 140 Smith, K.M. (1) 228,230; (2.5) 104; ( 5 ) 48, 131 Smith, R.E. (2.3) 12; (2.4) 91 Smith, T.L. (2.5) 93 Smyth, M.R. (1) 499 So, M.H. (2.2) 102; (2.4) 152 So, S.M. (2.5) 165 Sobhanadri, J. (1) 170 Sobolcwski, A.L. (1) 238; (2.4) 58 Sodcrberg, B. (2.7) 197 Socdcrman, 0. (3) 4 10 Sohm,H.S. (2.2) 15 Soice, N. (1) 292; (2.5) 4 1 Sokolova, I. (1) 375; ( 5 ) 153 Sokotowska-Gadja, J. (3) 889 Solaro, R. (3) 489 Solorzil,0. (4) 36 Solovjcva, L.A. (1) 364 Solov'yov, K.N. (1) 34 1 Somasundaram, N. (2.5) 175; (2.6) 298 Somckawa, K. (2.2) 9 Sonawanc, H.R. (2) 50,5 I Sonc, M. (3) 46 I Song, J.C. (3) 21 1,545 (1) 150 Song, Q.-H. Song, S. (1) 5 15 Song, S.I. (3) 530 Song, S.J.(3) 66 Song, X. (1) 60, 124, 125,542; (2.4) 72; (2.6) 16; (5) 262 Sonnichscn, L.B. (2.5) 15 Sonoda, N. (2.3) 92; (2.4) 5,252; (2.5) 101; (2.6) 326 Sonoda, Y.(2.2) 35,36; (2.4) 32 Soper, S.A. (1) 2 Sopina, I.M. (3) 119 Soria, M.L. (2.7) 2 I Soria, V. (3) 425 Sortino, S. (2) 6; (2.6) 210 Sosnowski, S. (3) 550
Ptiolochcmistry Soti, K.(4) 72 Souquc, A. (2.7) 58 Sour, A. ( I ) 363,547; ( 5 ) 105 Sourisseau, C. (3) 493 Sousa, A. (5) 22 Sousa, L.R. ( I ) 173 Soutar, 1. (3) 408,409 Sowa, C.E. (2) 1 I Spalck, 0. (1) 426 Spallctti, A. (1) 193, 196, 198; (2.4) 21; (2.6) 263 Spears, K.G. (1) 69 Speiscr, S. (1) 387; ( 5 ) 15 1 Spciss, H.W. (3) 490 Spck, A.L. (1) 353; (2.6) 3 19 Spencer, N. ( I ) 356; (2.2) 58-60; (2.6) 87.88; (5) 100 Spiller, W. (2.5) 225 Spitzner, R. (2.6) 285 Sprcitzcr, H. (I) 553; (3) 379 Sprik, R. (1) 54 Springer, J. (3) 588 Springs, S.L.(1) 312; (2.5) 173; ( 5 ) 107 Srcckuniar, R. (2.7) 163 Srikrishna, A. (2.2) 52 Srinivasan, C. (1) 28 1,297; (2.5) 175; (2.6) 298 Srinivasan, N. (2) 6 1 Srisiri, W. (3) 79 Srivastava, S. (2.4) 260 Staab, H.A. (2.5) 40 Stachowiak, K. (1) 149 Stadlcr, B. (2.2) 110; (2.4) I 1 1 Stachelin, C. (2.6) 77 Stachler, K. (1) 477 Staerk, H. (1) 597; (5) 248 Stamfer, S.(3) 296 Stana, K.K. (3) 549 Stanislavskii, O.B. (2.4) 220; (3) 135 Stanticvitch, V. (1) 413 Staredubov, D.S. (1) 561 Staring, G.J.E. (3) 645,823 Stark, J. (2.5) 225; (3) 486 Stark, M. (2) I 1 Stmdiliin, A.N. ( I ) 416 Staiko, A. (1) 41 8,420; (2.5) 85; (2.7) 31; (5) 194 StautTcr, M.T.(3) 340 Stavinoha, J.L. (2.6) 66 Stccl, C. (1) 254; (2.6) 236 Stecmers, F.J. ( 5 ) 144 Stecnliea, S. (2.3) 82; (2.4) 2 16, 247; (2.6) 3 17; (2.7) 46 Stccr, R.P. (1) 132, 159, 161 Stcger, J.R.(3) 441
Stcglich, W. (2.3) 5 I Stchnicl, B. (3) 146 Stchniel, V. (3) 146 Stcin, S. (3) 234 Stcincr, A. (2) 18; (2.5) 5 1; (2.6) 74,75 Stcincr, H.(2.6) 73 Stcinman, E.A. ( I ) 446; (5) 205 Stcinmctz, M.G. (2.3) 5; (2.6) 306 Slcinwascher,I. (2.5) 127; (2.6) I45 Stcllcr, 1. (1) 3 16; (2.6) 23 1 Stclzcr, F. (3) 615 Stcnip, E.D.A. (1) 510,512,513 Stcnbcrg, B. (3) 667,668,687 Stcngclc, K.-P. (2.7) 160 Stcnhagcn, G. ( 5 ) 58 Stcp, E.N. (3) 870 Stcpanova, S.O. (4) 40 Stephens, A. (4) 55 Stcren, C.A. (1) 455; (5) 198 Stcm, C.L. (5) 134 Stcveiison, D.R (3) 848 Stibbs, W.G. (2.6) 309; (2.7) 170, 171 Stilbs, P. (1) 636 Stipa, P. (2.6) 252 Stitzcl, D. (2.4) 200 Stoddart, J.F. (1) 555; (2.6) 245 Stocva, V. (3) 401 Stonc, B.M. (3) 513 Stonc, S. (1) 345,346; (2.5) 44; ( 5 ) 117, 118 Stratakis, M. (2.5) 128 SLraub, H.A. (1) 342; ( 5 ) 88 SLravrcv, K.K. (2.4) 23 Strchlcr, B.L. (5) 49 Strchmcl, B. (5) 137 Strcib, W.E. (5) 20 Strckowski, L. (1) I9 1 Strcltsov, A.M. (5) 46 SLrcl'Lsova, Z.O. (3) 252 Strcniel, B. (3) 439 Slrcnhagcn, G. (5) 23 Stmad, T. (2.4) 51; (2.6) 152 Strobcl, M. (3) 722 Strohl, D.(2.6) 285 Studzinski, A.P. (1) 570; (3) 829 Stucmpflen, V. (3) 626 Stukcns, D.J. (2.7) 124, 125, 196 Stumpe, J. (2.4) 30; (3) 462 Styrcz, S. (1) 245 Styring, S.(1) 355; (5) 23,24,97 Su, J. (1) 147; (2.6) 297 Suarcz, A. (1) 470 Suau, R. (2.4) 177 Subranimian, K. (3) 63,329,344
489 Subramanian, P. (3) 522 Sudarma, 1.M. (2) 53; (2.4) 196; (2.6) 64,65 Sucmatsu, H. (1) 445; ( 5 ) 183 Sucnobu, T. (1) 422; (2.5) 122; ( 5 ) I96 Sucyoshi, S.(2.5) 86 Sugawara, T. (2.7) 88 Sugi, S. (3) 764,766 Sugiki, T. (2.2) 62; (2.6) 105 Sugimoto, A. (2.4) 246; (2.6) 160 Suginome, H. (2.2) 13, 112; (2.4) 145,153 Sugioka, T. (2.6) 124 Sugila, C. (1) 374; (2.7) 42; ( 5 ) 164 Sugita, K. (3) 696,780 Sugiura, K.4. (2.5) 43 Sugiura, K . 4 . (1) 84; (5) 68 Sugiyama, H. (2.2) 65 Sugiyama, K. (2.3) 34 Suguwara, T. (3) 9 1 Suh, D.H. (3) 262 Suh, M.H. (3) 719 Suh, Y.D. (1) 414 Suhadolink, J. (3) 504 Suliling, K. (1) 586 Suishi, T. (2.2) 9 Suits, A.G. (2.7) 108 Sukhai, P. (2.6) 206; (2.7) 79 Sumida, J.P. (1) 343,467,468; (2.5) 89,222; ( 5 ) 94,224,225 Sumida, T. (3) 305 Sumimoto, M. (3) 856 sumino, Y. (2.3) 92; (2.5) 101 Sumlin, A.B. (2.5) 104 Sun,B.J. (3) 648,650 Sun, J. (3) 568,714 Sun, L. (1) 355; (5) 23,24,97 Sun, L.-Q.(2.5) 184 Sun, Q. (3) 155, I60 Sun,R. (3) 686 Sun, X.(3) 464,559 Sun,X.D.(3) 374 Sun,X.-Z. (2.7) 73, 116, 127 Sun,Y. (3) 812 Sun, Y.A. (3) 26 Sun, Y.-P. (1) 402,438; (2.5) 162; (3) 814; ( 5 ) 21 1 Sundahl, M. (2.4) 20 Sundarababu, G. (2) 12; (2.5) 50 Sundararajan, P.R. (3) 354 Sundcll, P.E.(3) 60, 165 Sundstrocm, V. (1) 2 16; (2.4) 55; (5) 129, 130,269 SUng, c . (3) 486,618 sung, C.S.P. (3) 212
Sung, D.D. (2.4) 161 Sung,N.H. (3) 369 Suppan, P.(1) 143,289; (2.5) 23 Susmic, K. (2.7) 199 Suslick, K.S. (1) 340; (2.7) 198 Sustmann, R. (1) 3 16; (2.6) 23 1 Susumu, K. ( 5 ) 258 Sutin, N. (1) 6; (5) 26 Sulo, K. (1) 136 Suyama, K. (2.7) 50; (3) 777 Suzuki, A. (4) 50 Suzuki, 1. (1) 433 Suzuki, M. (2.2) 13; (2.4) 145; (2.5) 215; (3) 567 Suzulii, N. (3) 708 Suzulii, S.(1) 430 Suzulii, T. (2.4) 40; (2.7) 115; (3) 197,257 Suzulii, Y. (2.2) 35,36; (2.4) 32; (3) 741,780 Svarovshy, S.A. (2) 64 Svcc, W.A. (1) 348,349; (2.5) 195; ( 5 ) 125,233 Svccluiikov,N. (1) 413 Svcnsson, J.O. (2.4) 172, 173,267; (2.6) 168, 171, 173; (2.7) 148, 151 Swagcr, T.M. (3) 663,664,666 Stvallen,S.F. (1) 42,43; ( 5 ) 75, 78,266 Swaminallian,R (1) 635 Swaminatyhan,M. (1) 252 Swanson, L. (3) 408,409 Swanson, M.J. (3) 29 Swanson, P. (3) 19 Swanson, R (2.6) 66 Swanson-Vclhiunuthu,M. (3) 4 13, 433 Swart, R. (3) 94 Swcdck, B. (3) 583 Swiatck, M. (3) 746,747 Sykora, M. (I) 307; ( 5 ) 245 Syromyatnikov,V.S. (3) 233 Szajdzinska-Pictck, E. (1) 256 Szczcpanik, B. (2.6) 217 Szelinslii, H. (2.5) 42; (5) 92 Szymanski, M.(1) 132 Taba, P. (3) 739 Tabak, M.(1) 5 14 Tabct, M. (2.3) 22; (2.6) 335 Tabohashi, K. (2.3) 41; (2.4) 39 Tachi, H. (3) 240 Tachikawa, T. (1) 373 Tachiya, M. (1) 628 Tackocs, J.M. (3) 289
Tada, K. (3) 661 Tadros, M. (3) 484 Tacn, S.(1) 639 Tajima, M. (4) 50 Tak, H.Y. (3) 608 Takada, T. ( 5 ) 126 Takagi, K.(1) 33; (2) 23; (2.3) 71, 72; (2.4) 114, 115,221 Takagi, M. ( 5 ) 146 Takagislii, T. (3) 887 Takahashi, A. (1) 584 Takahashi, H. (3) 466 Takahashi, K.(2.4) 88; (2.6) 36, 79; (3) 760 Takahashi, M. (2.4) 56, 132; (2.6) 267,268; (3) 830 Takahashi, N. (2.6) 3 13 Takahashi, 0.(2.6) 272 Takahashi, Y. (1) 178,182; (2.3) 31,41; (2.4) 28,39,40; (3) 526 Takahira, 0. (2.5) 19 Takamiya, N. (3) 386,566 Takamoto, T. (4) 56 Takamuko, S. (1) 422; (2.3) 39; ( 5 ) I96 Takamuku, S. (2.4) 101, 188,219; (2.5) 152; (2.6) 140,334; (4) 7 Takano, H. (3) 520 Takasaki, T. (2.3) 39 Takashima, M. (2.2) 13; (2.4) 145 Takata, T. (2.4) 142; (2.6) 1 I5 Takatani, K. (3) 341 Takatsuka, H. (3) 264 Takcchi, H. (2.6) 275 Takcda, 1. (5) 14 Takcda, K. (1) 595 Takemoto, T. (3) 239 Takcmura, K. (1) 232 Takcnaka, S. (5) 146 Takcshima, M. (1) 139 Takcshita, H. (2.2) 117; (2.3) 42; (2.4) 234 Takcshita, M. (2.6) 37 Takeuchi, K. (2.5) 95, 138 Takcuchi, Y. (3) 780 Takeye, H. (2.5) 198 Takczaki, H. (2.7) 157 Takimoto, Y. (3) 99 Takimura, T. (2.5) 64 Taktani, K. (3) 468 Takuwa, A. (2.2) 100; (2.4) 150 Talhayini, M. (3) 596 Tom, C.K. (3) 434 Tani, H.Y. (3) 483 Taniai, K. (2.7) 208 Tamaki, T. (3) 492,495 Trunarat, Ph. (1) I 14
Photochentisky
490
Tamiaki, H. (5) 132 Tamma, T. (2.4) 23 Tammilehto, S. (2.6) 214 Tamura, K. (4) 52 Tan, C.-Q.(2.6) 70 Tan, H. (2.5) 78 Tan, Q. (1) 346; ( 5 ) I 18 Tan, W. (5) 266 Tanabc, G. (2) 9; (2.6) 29 1 Tanabc, K. (2.2) 80,81; (2.3) 14, 45; (2.4) 179; (2.6) 139, 142 Tanaka, A. (4) 30,3 1 Tanaka, K. (2.7) 114; (3) 5 16,537; (4) 57; ( 5 ) 258 Tanaka, M.(2.6) 323; (2.7) 188 Tanaka, S. (2.4) 3,49; (2.6) 153; (3) 790 Tanaka, T. ( I ) 136; (2.7) 1 14 Tanaka, Y. (1) 84; (2.5) 43; (5) 68 Tang, C.W. (3) 660 Tang, T.C. (3) 662 Tang,X. (3) 582; (4) 38 Tang, Y. (I) 5 15; (2.4) 72; (2.7) 66; (3) 463,824 Tani, T. (I) 34 Tani, Y. (2.7) 168 Taniguchi, S.(1) 84,456,465; (2.5) 43,88; ( 5 ) 68, 178,221 Tanikaga, R (5) 132 Tanimoto, Y. (1) 120, GOO, 601; (2.5) 19,218; (2.6) 238; (2.7) 208; ( 5 ) 25 1,252 Taniniura, K. (3) 74 .Tao, J. (3) 720 Taraban, M.B. (2) 64 Tarasov, LG. (1) 306; ( 5 ) 244 Tarasov, V.V. (1) 602 Tarka, R.M.(3) 628 Tasch, S.(3) 614 Tashiro, M.(2.3) 53 Tashiro, Y. (4) 49 Tashrifov, K. (3) 748 Tatsugi, J. (2.2) 84; (2.4) 249 Tatsuno, T. (2.6) 334 Tauber, A.Y. (1) 328; (2.5) 36; ( 5 ) . 82,116 Taucr, E. (2.4) 36; (2.6) 28 Tavarcs, H.R (2.4) 185 Taveras, A.G. (2.2) 76 Tavcmicr, H.L. (I) 42,43; ( 5 ) 75, 78 Taylor, C.E. (4) 22 Taylor, D.P. (1) 130 Taylor, J.W. (3) 324 Taylor, P. (1) 3 IS; (2.5) 134; (2.7) 134,135 Tcharlihtchi, A. (3) 680,68 1
Tcherkasskaya, 0. (5) 147 Twguc, S.J.(2.6) 225 Tcissedre, G. (3) 198,740 Tejedor, M.A.S. (2.5) 182 Tcllcz-Rosas, M. (3) 44 Tclo, J.P.(2.7) 184 Tcmplc, K. (2.2) 37; (2.6) 308 Tcmplin, M. (2.5) 144 ten Brinke, G. (3) 123 Tcramae, N. (1) 593 Terashima, M. (2.2) 67; (2.6) 127 Tcrazima, M. (1) 610, 61 1,614 Tercnctskaya, I.P. (2.3) 56,57 Tercntiev, A.G. (1) 5 I7 Tcrctani, F. (3) 817 Tcro-Kubota, S. (2.5) 45 Tcnill, R.H. (3) 684 Tcrsclius, B. (3) 667 Tcshinia, K. (3) 137 Tctzlaff, T. (2.7) 176 Tcysscdrc, G. (3) 675,676 Tezuka, Y. (2.7) 52 Tfibcl, F. (2.4) 24 I Thanasckaran, P. ( I ) 28 I Thckla, J.M. (1) 128 Then, E.T.H. (3) 669 Thigpen, K. (3) 33, 120,3 14 Thirunaniachandran, T. (1) 98 Thi Vie1 Nguyen, T. (3) 334,335 Thomas, J.K. (3) 400,587 Thomas, R.W. (3) 85 1 Thomnicn, M. (2.2) 22 Thompson, D.W. (5) 98 Thompson, M.E.(4) 38,39 Thonai, N. (2.4) 226 Thornton, S.R. (2.6) 68 Thummel, R.P. (1) 382; (5) 165 Thurau, G. (2.7) 205 Thurcsson, K. (3) 4 10 Tian, H.J. (1) 27 I, 336; (2.5) 66; (3) 531; ( 5 ) 55,33 Tian, P. (2.5) 42; ( 5 ) 91,92 Tian, S. (2.5) 180; (3) 888 Tidwcll, T.T. (2.2) 37; (2.6) 308 Ticmblo, P. (3) 675,676 Tiera, M.J. (3) 432 Tikhoniirov, V.A. (2.5) 155 Till, U. ( I ) 599; ( 5 ) 250 Tiiilliicrmaii-Vaughan, D.J.(2.4) 169-171; (2.6) 166, 167, 170; (2.7) 146, 147, I50 Timo, G.L. (4) 5 I Timpc, H.-J. (2.5) 24; (2.6) 235, 23 7 Tinone, M.C.K. (3) 537 Tirclli, N. (3) 489 Tirrcll, M. (3) 4 I5
Tittcl, J. (1) 106 Tiyabhom, A. (1) 37 1; (2.5) 1 10 Tobita, H. (2.7) 130 Tobita, S. (1) 447; (2.5) 80; (5) 207 Tocho, J.O. (1) 185 Tock, P. (3) 847 Toda, F. (1) 503; (2.2) 42; (2.4) 205; (2.6) 60,269 Toda, J. (2.2) 88; (2.6) 3 I0 Toda, S.(2.7) 61 Todd, W.P. (2.3) 30 Todorv, T. (3) 471 Tohnai, N. (2.2) 62; (2.6) 105 Tolmo, S. (3) 520 Toirov, A. (3) 748 Tojo, S. (2.3) 39 Tokc, L. (3) 891 Tokita, M. (3) 461 Tokita, S. (1) 373; (2.4) 11 Tokuda, M. (2.2) 112; (2.4) 153 Tohxda, T. (3) 271 Tokuhisa, H. (3) 494 Tokuniara, K. (1) 169,201,202, 223; (2.4) 35 Tokuniaru, K. (2.5) 73; (2.6) 20, 21,27 Tokumura, K. (2.4) 61 Tolcdano, E.(1) 387 Tollcy, M.S. (I) 555; (2.6) 245 Tolstikov, G.A. (2.5) 116 Tolt1,N.P. (2.7) 171 Tonia, H.E. (5) 267 Toinaino, A. (1) 525 Tonialia, D.A. (3) 5 18,565 Tomaschewski, G. (1) 429; (2.7) 24; (3) 255 Tomikawa, A. (2.7) 50 Tomikawa, M. (3) 303,750 Tominaga, K. (2.5) 46 Toniinaga, T.T.(1) 5 I4 Toniinaga, Y. (2.2) 44; (2.4) 7, 208,209; (2.6) 7,264,265 Tomioka, H. (2.6) 201,204; (2.7) 71,73,74 Toniioka, J. (2.7) 62 Tomioka, N. (5) 237 Toniisc, S. (2.3) 46 Tomova, N. (3) 471 Tomoyosa, Y. (3) 590 Tomsic, S. (2.4) 53 Tong, A.-J. (1) 479 Tong, L. (3) 727,738 Tonoi, T. (2.5) 122 Too, D.Y.(2.2) 103 Topchiev, D.A. (3) 103 Torgcrson, M.R. (1) 550
Atrlhor Iiidex
Torikai, A. (3) 703,704 Torkclsson, J.M. (3) 388,447, 456,598 Torrnos, R. (2.3) 11; (2.4) 163, 183 Torniaincn, K. (2.6) 2 14 Torrc, R. (1) 163 Torres,T. (2.2) 93; (2.4) 52; (2.6) 154 Toniani, R (2.3) 16; (2.4) 158; (2.6) 157 Toscano, J.P. (I) 148; (2.6) 196; (2.7) 34,39,43, 169, 181 Toshida, S. (2.5) 136 Tolh, G. (2.5) 158 Toth, L.M. ( 5 ) 150 Touwslagcr, F. (3) 270 Towcrs, G.H.N. (2.7) 181 Townlcy, E. (2.6) 80 Townscnd, L.B. (2.4) 198 Towrie, M. (2.7) 158 Toya, Y. (2.7) 166 Toyoda, E. (3) 5 I6 Tramm-Wcrncr, S. (4) 69 Tran, A. (5) 23 Tran-Cong, Q. (3) 268,360 Trautwcin, A.X. (1) 5 19; (2.7) I3 1 Trcadway, J.A. (1) 325; (5) 123 Trcinin, A. (1) 86 Trettnak, W. (1) 58 1 Treuschnikov, V.M. (3) 32 1 Trcvors, J.T. (3) 796 Tricbcl, M.M. (3) 602,605 Trieu, N.T.V. (3) I72 Tripathy, S.K. (3) 486,618 Tro, N.J. (2.4) 224 Trollss, M. (3) 265 Trombetta, D. (1) 525 Trommcr, W.E. (2.7) 94 Tromnisdorff, H.P. (1) 174; (2.4) 67; (2.6) 227; (5) 128 Trottcr, J. (1) 381,504; (2) 13; (2.3) 26; (2.5) 55; (2.6) 148, 149,333; (5) 161 Tmong, A. (2) 24 Trushinski, B.J. (2.7) 65 Try, A.C. ( 5 ) 71 Trznadcl, K. (I) 587 Tsai, H.-L. (5) 20 Tsai, Z. (3) 486 Tscng, W.H. (2) 55 Tscntalovish, Y.P. (2) 8 Tsivgoulis, G.M. (1) 55 1,552 Tsnooka, F. (3) 459 Tsnooka,M. (3) 5 , 10,31,231, 239,240,300,562,777 Tsubanic, T. (3) 2 10
49 1 Tsubata, A. (3) 333; (4) 18 Tsuboi, Y. (1) 433 Tsubokawa, N. (3) 195 Tsuchida, A. (3) 75 1 Tsuchida, E.(2.5) 221; (3) 7 10, 830 Tsuchihashi, N. (2.5) 45 Tsuchimori, M. (3) 715 Tsuchiyn, Y. (3) 238 Tsuda, K.(3) 267 Tsuda, Y. (2.2) 88; (2.6) 310 Tsue, H. (1) 84; (2.5) 43; ( 5 ) 68 Tsujii, Y. (3) 312 Tsujita, H. (2.6) 28 I Tsukahara, K. (1) 496,497; (4) 65, 66 Tsukamoto, Y. (2.4) 134; (2.6) I19 Tsuncishi, H. (2.3) 1,2 Tsuno, T. (2.3) 34 Tsunoda, S. (2.5) 34 Tswuta, N. (3) 6 Tsutsui, K. (3) 77 1 Tsutsunii, 0. (1) 558 Tsuhunii, S. (4) 44 Tsypyslieva, T.A. (3) 58 1 Tuan, D.Q.(3) 866 Tubino, R. (3) 452,54 1,665; ( 5 ) 140 Tuinman, A.A. (1) 405; (5) 180 Tung,C.-H. (1) 186,210,211, 263,601; (2.3) 61; (2.4) 203; (2.6) 32,33,238; ( 5 ) 252 Twkcr, L. (4) 67 Turkka, RM. (1) 304 Tumcr, J.J. (2.7) 120 Turncr, P.J. (3) 35 Two, C. (1) 300; (3) 565; (5) 234 Turr0,N.J. (1)20,21,284,300, 501; (2) 3-5; (2.5) 24; (2.6) 232,235,237; (3) 518,565, 870; ( 5 ) 39,40,234 Tuter, M. (3) 56 Twcig, R.J. (3) 363 Tygcr, W.H. (3) 822 Tykwinski, R.B. (5) 27 I Tylcr, D.R(3) 749 Tylcr, T.L. (3) 45 I Ubcrna, R. (2.7) 154 Uchida, K. (1) 178; (2.4) 8; (2.6) 37 Uchida, T. (1) 593 Uchimura, M. (3) 90 Uddin, R. (1) 283 Ucclii, T. (4) 63 Ucda, K.(3) 445
Ueda, M. (3) 352 Ucda, N. (3) 488 Ueda,R (4) 68 Ucda, T. (3) 5 I6 Ucki, H. (2.5) 198 Ucno, A. (1) 40 Ucno, N. (3) 537,780 Ucno, Y. (4) 71 Ucta, H. (3) 893 Uhl, A. (5) 142 Wshima, S. (3) 526 Ulbricht, M. (3) 205-207 Ulic, S.E.(2.7) 11 I Ulrich, T. (3) 255 Unicchi, S.(3) 556 Umcniura, M. (2.7) 59 Unieuchi, S.(1) 605; (5) 253 Unctt, D.J. (2.2) 11; (2.4) 130 Unni, A. (1) 286 Untch, A. (2.7) 110 Untcrlcchncr, C. (3) 639 Upadhyaya, H.P. (2.7) 172 Uprichard, J.M. (3) 857 Uplliagrovc, A.L. (2.2) 3 1; (2.4) 19; (2.6) 18 Urai, Y. (3) 88 Urban, G. (3) 3 16 Urizar, S.(3) 819 Ursini, M. (2.4) 181 Usai, S. (1) 498 Usanii, K. (2.5) 216 Ushakov, E.N. (2.4) 110; (3) 136 Ushiki, H. (3) 459 Usui, M. (2.4) 10 Utcnishev, A.N.(2.4) 83; (2.6) 49 Utinans, M. (1) 95 Utsunomiya, K. (3) 386 Uzhinov, B.M. (3) 580 Uznanski, P.(3) 4 19 Vacar, D. (1) 439; ( 5 ) 212 Vachcv, V.D. (2.4) 16 Vaday, S . (2.6) 16; (5) 262 Vaidya, V.K. (2.5) 181,213 Vaillant, D.(3) 729 Vainer, A.Ya. (3) 302 Valat, P. (2.6) 54 Valcntino, M.R (1) 157 Valcur, B.(1) 403,538,539; (2.6) 244; (5) 18I, 268 Vallabhan, C.P.G.(1) 609 Vallkc-Goyct, D. (2.4) 193; (2.6) 57 Van, 0. (1) 105 Van Bramer, S.E.(2.7) 140' Van Danunc, M. (2.7) 28
Phoiocheinis(ry
492 van den Bergh, H.E. (1) 570 Vandcn Bout, D. (1) 6 16 Vnri dcr Auwcracr, M. (1) 122,640 Van dcr Eyckcn, J. (2.2) 68 van dcr Hcidcn, A.P. (3) 561 van dcr Laan, G.P. (3) 477 van dcr Mculcn, P. (1) 237,588 van dcr Schaaf, P.A. (3) 361 van dcr Tol, E.B. (5) 144 van dcr Wolf, L. (5) 21 van Dijk, S.I. (I) 347; (2.5) 135; (2.6) 240; (5) 95 van Ekcnstcin, A.G.O.R.A. (3) 498 van Eldik, R. (2.7) 117, 1 19 van Esscnbcrg, W. (2.7) 159 Vancz-Florcs, J. (3) 72 1 Van Gcmcrt, B. (2.4) 75; (3) 478 Van Haarc, J.A.E.H. (1) 453; (2.5) 8 1; (5) 203 van Hock, A. (1) 637 van Mcurs, C. (3) 358 Van Mingroot, H. (1) 122 Vannokov, A.V. (3) 627 Vanossi, M. (2.2) 3 Van Parys, I. (2.6) 47 van Stam, J. (1) 122 van Stum, J. ( 5 ) 138 Van Vrankcn, D.L. (2.5) 204 van Walrcc, C.A. (1) 353; (2.6) 3 19; (3) 477 Van Willigcn, H. (1) 455; (2.7) i96; ( 5 ) 198 Varakin, V.N. (2.7) 193 Varas, J.M. ( I ) 473 Vardcny, Z.V. (3) 586,642 Vargas, F. (2.4) 139; (2.6) 280 Vargas, J. (2.5) 11 Varicr, G.K. (1) 609 Varlcmann, U. (3) 140 Varnia, C.A.G.O. (2.4) 59 Vasilcscu, M. (3) 413 Vasilcts, V.N. (2.4) 146 Vasil'cv, R.F. (2.5) 155 Vasil'cva, L. (4) 70 Vasil'cva, N. (1) 375; (5) 153 Vasqucz, R. (3) 8 19 Vatsa, R.K. (2.7) 102, 103, 172 Vaudiey, E. (1) 289; (2.5) 23 Vazada-Martin, R. (3) 72 1 Vazqucz, P. (2.2) 93; (2.4) 52; (2.6) 154 Vazqucz Tato, J. (1) 630 Vcdcrnikov, A.I. (2.4) 17; (2.6) 19 Vcla, M.A. (1) 227 Vclciro, A.S. (2.5) 229; (2.6) 287 Vclmani, N. (4) 37 Vcnanzi, M. (1) 392; (5) 152
Vcikatachalapathy, B. (1) 28 1 Vcnkatcsan, K. (2.2) 5 Vcnturi, M.(1) 555,556; (2.6) 245 Vcn'yaminova, A.G. (2.7) 83 Vcrboom,W. (5) 144 Vcrdu, J. (3) 680,68 1,730,73 1 Vcrcknnikov, A.V. (2.4) 187 Vcrhcy, H.J. (2.6) 242 Vcrhocvcn, J.W. (1) 74,85,93, 338,347,352,353,637; (2.5) 135,193; (2.6) 240-242.319; (5) 66,70,72,95, 122, 144, 227 Vcrmccrsch, G. (2.5) I12 Vcslwcbcr, H. (3) 608 Vidnl-Ganccdo, J. (2.5) 72 Vicira, A.J.S.C. (2.7) 184 Vicra Fcrrcira, L.F. (1) 213,372; (3) 442 Vict, T. (3) 171 Victh, H.-M. (2) 8 Vig, A. (3) 890-892 Villafiliic, F. (2.7) 129 Villcncuvc, L. (1) 153 Viltrcs Costa, C. (1) 258 h o g o p a l , K. (1) 432 Vinogadov, A.V. (3) 679 Vinogradova, L.V. (1) 4 16 Viriot, M.L.(3) 370,438,558 Virrcls, I.G. (2.7) 73, 120, 121 Virucla, P.M. (2.5) 72 Viscontc, L.L.Y. (3) 84 Vishnwnurthy, K. (2.2) 5 Visscr, A.J.W.G. (1) 637 Visscr, P. (2.7) 53 Viswanathan, K. (1) 162 Vitlal, J.J. (2.6) 309; (2.7) 170 Vittimbcrga, B.M. (2.6) 180 Vivona, N. (2.6) I8 1 Vlachopoulos,N. (2.5) 187 VIEck, A,, Jr. (2.7) 120 Vlicstra, E.J. (3) 477 Vo-Dinh, T. (1) 571 Vodzinskii, S.V. (1) 123 Vwgtlc, F. (3) 355; (5) 172 Voclkcr, A. (1) 296 Voclkcr, S. (1) 648 Vogcl, M. (1) 1 15 Vogcl, P.D. (2.7) 94 Voglcr, A. (1) 457; (5) 206 Vogt, A. (2.6) 285 Voicu, 1. (2.4) 105 Voit, B. (2.7) 27-29,31; (3) 828 Voityuk, A.A. (2.2) 64 Volkovicli, D.I. (I) 341 Vollmcr, F. (1) 236 Voloshin, A.I. (2.5) 154, 156; (3)
373 VOIPP.H.-R. (2.7) 102, 103 von Bcrlcpsch, H. (1) 477 von Borcgskowski, C. (1) 12,13, 115;(5)41,42 von dcr Haar, Th. (I) 92,247 von Raumcr, M. (1) 289; (2.5) 23 Von Sclmcring, H.G. (2.4) 128; (2.5) 114; (2.6) 98, 125 von ZclcwsLy, A. (5) 172 Vordcrbruggcn, M.A. (3) 148 Vos, J.G. (5) 167 Vrcvcn, T. (1) 179; (2.3) 47 Vulto, S.I.E. (5) 46 Vuoriman, E.(1) 640 Vyprachtick.y, D.(3) 576 Wackcrnagcl, F. (2.6) 327 Wada, F. (2.2) 49; (2.4) 41 Wada, R (2.7) 192 Wadir, T. (1) 22; (2.2) 62; (2.6) 105 Wada, Y. (2.5) 99, 100; (4) 7 Wadc, E.A. (2.7) 95,96,99 Wagaman, M.W. (3) 621 Wagewijs, B. (1) 352; (5) 122 Waglcr, P. (2.4) 141; (2.6) 262 Wapcr, A.H. (3) 832 Wagncr, B.D.(2.6) 194; (2.7) 38 Wagncr, G. (5) 128 Wapcr, J.R. (2.5) 208 Wagncr, M.J. (3) 647 Wagncr, M.W. (3) 614 Wagncr, P.J. (2.1) 14, 17; (2.5) 53 Wagncr, R.W. (1) 388; (5) 259, 275 Wagnicrcs, G.A. (1) 570 Wainwright, M. (1) 1 16 Wakabayashi, T. (1) 430 Wakaniatsu, K. (1) 182; (2.4) 28 Wakamatsu, S. (3) 766 Waldcr, L. (2.5) 187 Walkcr, J.W. (2.7) 163 Walkcr, R.T. (2.2) 58-60; (2.6) 87, 88 Wall, C.G. (2.5) 217; ( 5 ) 113 Wall, M.H., Jr. (1) 357; (5) 101 Wall, S.T.( I ) 3 15; (5) 110 Wallaart, H.L. (2.7) 158 Wallace, L. (1) 24; (4) 42; (5) 36 Wallace, S.C.(I) 613 Walls, F. (2.2) 109 Walscr, D.A. (3) 563 Walsh, E.F. (2.7) 127 WalBcr, B. (2.5) 16 Wallon, R. (5) 7 1
493
Walzok, M.J. (3) 722 Wan, P. (2.3) 79-8 1.86-88; (2.4) 275; (2.6) 299 Wan, Z. (2.5) 94; (2.6) 296; (2.7) 177 Wang, B. (I) 83; (2.6) 24; (5) 67 Wang, C. (3) 25 I, 470 Wang, C.H. (3) 289 Wang, C.-X.(2.7) 18 1 Wang, D. (3) 106 Wang, E.(I) 3 17; (3) 65,86 Wang, F. (1) 187 Wag, G. (1) 270; (3) 410 Wang, G.C.(2.4) I17 Wang, H. (I) 86; (2.2) 12; (2.6) 103; (3) 53,454,663,664,666 Wang, J. (1) 158; (3) 225,805 Wang, J.Z. (3) 545 Wang, K. (1) 47 I Wang, L. (1) 506; (3) 7,307 W a g , L.-T. (1) 568 Wang, M.(1) 177; (2.6) 103; (3) 53 Wang, P. (3) 175 Wang, P.F. ( I ) 538; (2.6) 244 Wang, P.Y. (3) 886 Wan& Q.-M. (2.5) 65 Wang, S. (2.4) 261; (2.5) 168; (3) 225,824; (5) 20 wang, S.L.(2. I) 55 wang, s.4. (2.2) 3 Wang, W. (3) 230 Wang, W.H. (3) 480 Wang, X. (2.5) 91; (4) 15 Wang,X.S.(4) 16 Wang, Y. (I) 270; (2. I) 45; (2.2) 116; (2.6) 84, 105; (3) 487 Wang, Y.H. (2.2) 62 Wang, Y.M. (2.4) 117,184,236; (2.6) 104 Wang, 2.(1) 127,241; (2.2) 21; (3) 385,42 1,659 Wan& Z.Y. (3) 500 Wanncniachcr, R. (I) 648 Warabisako, T. (4) 52 Warashins, M. (4) 50 Ward, M.D. (5) 170,173,174 Ward, P. (2.7) 5 1 Warkentin, J. (2.6) 193, 194, 196; (2.7) 38-40 Warman, J.M. ( I ) 93,250,353; (2.5) 193; (2.6) 3 19; (3) 645; ( 5 ) 72 Warncr, I.M.(1) 2,224,243,533; (3) 4 14 Warntjics. J.B.M. (1) 409 Warricr, M. (2.4) 239,240
Warshawshy, A. (3) 485 Waraxha, K.-D.(2.6) 89 Wasiclcwski, M.R (1) 10,83,327, 348,349,379; (2.5) 195; (5) 6, 44,67, 125,158,233 W;lsscrman, H.H. (2.5) 21 I Watahiki, S. (4) 52 Walanabc, A. (1) 406,450,452, 454; (2.6) 294; (2.7) 180; (3) 49; (5) 184,199,200,202 Watanabe, J. (3) 461; (4) 32.33 Watiuiabc, M. (1) 498,584; (2.3) 14; (2.4) 179; (2.6) 29, 142, I76 Wahabc, S. (2.4) 56,132; (2.6) 267,268 Watanabe, T. (I) 373 Watanabc, Y. (2.7) 82; (4) 64 Watarai, H. (1) 603 Walkins, D.M. (3) 392 Walkinson, M. (5) 22 . Waynct, D.D.M. (3) 816 Wcisk, R (3) 230 Wcavcr, M.S.(3) 653 Wcavcr, W.L. ( I ) 148; (2.7) 34 Wcbb, W.W. (3) 597 Wcbbcr, S.E.(1) 35,280; (3) 422, 427,437,443,552,602; (4) 43; (5) 240 Wcbcr, S.G. (3) 340 Weber, W. (2.3) 5 I Wcbstcr, S. (3) 652 Wcddcll, 1. (3) 94 Wcdcr, C. (3) 647,657 Wccdon, A.C. (2.2) 9 Wcgewijs, B. (1) 93; (2.5) 193; (2.6) 24 1; (5) 72 Wegmnnn, G. (3) 609 Wcper, G. (1) 174; (3) 610 Wchnncistcr, T. (3) 660,662 Wci, C.-Y. (1) 235 Wei, J.-H. (2.5) 25; (2.6) 144 Wci, T.-H. (1) 621 Wci, X. (3) 642 Wci, Y. ( I ) 535 Wcichart, B. (3) 496 Wcidcmaicr, K. (I) 42,43; (5) 75, 78 Wcidncr-Wells, M.A. (1) 350; (2.2) 96; (2.6) 101 Wcigand, R ( I ) 242 Wcigcl, W. (2.1) 14; (2.5) 53; (2.6) 94 Wciner, B.R (2.7) 175 Wcir, N.A. (3) 78 I Wcis, J. (3) 280,28 1 Wcisnian, R.B.(1) 417
Wciss, R (1) 5 19; (2.7) 13 1 Wciss, R.G.(2.4) 113,238,239; (2.5) 49 Wclch, E.(I) 586 Wcllington, R.G.( I ) 279; (5) 239 Wcn, X. (I) 69 Wcndcr, P.A. (2.4) 126 Wcndorff, J.H. (3) 626,639 Wcn-Fu, F. (2.7) 119 W a g , H. (2.3) 38; (2.4) 143; (2.5) 131; (2.6) 95, 116 Wcng, M. (1) 290; (4) 69 WC~& Y.-X.(2.5) 233 Wcnnerstrocm,0.(2.4) 20 Wcntrup, C. (2.7) 53,91,92 Wcnzel, U. (I) 133 Wcrdclin, 0.(2.7) 85 Wcrkhovcn, T.M. (I) 128 Wcrncr, H.A.F. (4) 6.12 W a r , H.-J. (2.7) 110 Wcmer, T.C. (3) 414 Weminck, A.R (2.1) 38; (2.5) 48; (2.6) 253 Wcssig, P. (2.1) 18; (2.5) 5 1; (2.6) 74,75 Wcsslcn, B. (3) 327 Wcst, F.G. (2.4) 108 Wcsl,R(3)511 Wcstemiark, U. (3) 791,809 Wcsthd, P.-0.(5) 139 Wcyhcrmuellcr,T. (5) 2 1 Whdc, E.A. (2.6) 13 Whang, W.T. (3) 658 Wlulcs, J.A. (1) 469 Wliite, A.J.P. (1) 555; (2.6) 245 While, C.M.(5) 173,174 Whitc, J.G.(1) 575 Whitc, J.M. (2.7) 10 While, J.R(3) 724,726,727,738 While, RC. (2.6) 155 Whitc, T.H.(1) 356; (5) 100 Whitchcad, C.C. (2.4) 93 Wliittakcr, D.M. (3) 653 Whittalicr, P. (2.7) 26 Whittd, RM. (3) 444 Wliittcn, D.G. (1) 8; (2.6) 16; (5) 28,135,262 Whittle, C.E.(5) 148 Whittlcscy, M.K.(2.5) 37; (2.6) 302; (2.7) 121 Wick, M.T. (I) 159 Wicks, B.S.(1) 357; (5) 101 Wiczk, W. (1) 149 Widnm, J.F.(3) 3 18 Wieghardt, K. (5) 21 Wierzbowska, M.(1) 634 Wicscnfcld, D. ( I ) 358; (5) 231
494 Wight, C.A. (2.7) 68 Wightman, M.R. (3) 684 Wiitc, F.M. (3) 13 Wild, U.P.(I) 107,604,619; (3) 786 Wilcs, R. (3) 688 Wilk, A.K. (2.3) 3; (2.4) 22; (2.6) 102 Wilkinson, F. (1) 48, 152,213,536 Willcmse, R.J. (1) 352; (5) 122 Williams, C.M. (2.6) 209 Williams, D.J. (I) 555; (2.6) 245 Williams, D.S. (1) 68; (5) 69 Williams, J.A.G.( I) 537; (5) 265 Williams, R.M. (2.4) 189 Williamson, B.E. (1) 6 17 Williamson, S.E. (3) 165 Willncr, 1. (I) 557; (2.5) 179; ( 5 ) I09 Willson, C.G. (2.6) 175 Wilnis, M.P.(2.7) 124 Wilson, B.E. (3) 506 Wilson, C.G. (2.1) 58 Wilson, G.J. (1) 383; (5) 166 Wilson, N.H. (2.7) 55 Wilson, R.M. (2. I) 42; (2.4) 254; (2.7) 133 Wilson, S.R.(1) 427; (2.2) 12 Wilzbach, K.E. (2.6) 66 Winiarz, J. (3) 583 Winkler, H. (1) 519; (2.7) 131 Winkler, J.R (1) 358,361; (5) 103,23 1 Winnik, F.M. (3) 428 Winnik, M.A. (3) 381,401,434, 444,506,550,551; (5) 141, 147 Winston, T. (1) 286; (2.5) 149 Wintcr, B. (1) 431; ( 5 ) 191 Wintcr, J. ( I ) 435; ( 5 ) 210 Wintgcns, V. (2.6) 54 Wirp, C. (2.5) 150 Wirz, J. (1) 509; (2.6) 338 Wisniewski, K. (1) 634 Wisnudel, M.B. (3) 388,447,598 Witliolt, B. (3) 358 WilLmann, F.H. (1) 566 Wittmann, M. (1) 184 Wodtke, A.M. (2.7) 97,98 Wochrle, D. (I) 23; (2.4) 30; (2.5) 225; (5) 45 Wocrdcman, D.L. (3) 215,367, 380 Wokaun, A. (3) 45 Wokosin, D.L. (I) 575 Wolf, H.C. (1) 351; (3) 453 Wolf, M.O. (I) 36
Photochemisfry Wollbcis, S.O. (3) 376 WoliT, D. (3) 5 10 Wolff, T. (2.4) 235; (2.6) 184 Wolfrum, J. (2.7) 102 Wolszczak, M. ( I ) 256 Wong, C.C. (4) 54 Wong, M.W.(2.7) 53,91 Wong, P.A. (1) 532 Wong, P.S.H. (2.1) 61 WOO,H.G. (3) 66-63, 128, 174 Woo, J.C.(1) 627 Wood, B.R (2.4) 167, 175; (2.6) 163, 172; (2.7) 142, 143 Wood, P.D. (2.6) 222; (2.7) 155 Woods, C. (1) 24; (4) 42; ( 5 ) 36 Woods,J. (3) 768 Woolard, J.McK.R (2.6) 193; (2.7) 40 Wornall, D. (1) 152,213 Wortmann, R. (1) 2 I8 Wrachtrup, J. ( I ) 1 I5 Wrighton, M.S. (3) 647,657 Wnysuzynski, A. (3) 108 Wu, A. (3) 526 Wu, A.B. (2.1) 62; (2.6) 216 Wu, B. (3) 267 Wu, C.H. (2. I ) 13; (2.5) 55 Wu, F. (2.7) 175 Wu, H. (3) 8 18 Wu, J. (I) 506,507 Wu, K. (3) 148,423 WU,L.Z. (I) 263 Wu, M. ( I ) 38, 108 Wu, Q.(4) 70 Wu, S.(1) 147; (2.5) 174; (2.6) 297; (3) 175,364,707 WU,S.-K. (1) 194; (2.2) 29; (2.4) 225 Wu, W. (1) 187 wu, Y. (2.1) 2 1 WU,Y.-G. (I) 479 Wu, Y.H. (2.2) 12 Wu, Y.W. (2.3) 22; (2.6) 335 Wudl, F. (1) 437; (2.5) 28; (5) 208 Wyss, C. (2.1) 19; (2.6) 73,76,77 Xi, F. (3) 712 Xia, Y. (2.7) 66 Xiao, C. (1) 391; (2.1) 37; (2.6) 3 18; (5) 162 Xiao, H. (2.5) 233 Xiao, K. (3) 882 Xiao, X. (3) 818 Xic, J. (1) 53 1 Xie, P. (3) 463 Xic, X.S. ( I ) 104
Xin, Q.3. (2.7) 105, 106 Xing, X. (2.6) 89 Xingzhou, H. (3) 737 Xiong, Y.-M.(4) 54 xu, c. (3) 597 Xu, G. (2.5) 113; (3) 63 1 Xu, G.Z. (2.6) 261 Xu, H. (1) 332; ( 5 ) 86 XU,H.-J. (1) 271,336; (2.5) 66; (5) 55,93 Xu, J. (1) 531; (3) 824 XU,J.-G. (1) 478,568,626 XU,J.-H. (2.5) 25; (2.6) 144 XU,J.-Q. (2.5) 65 Xu, W. (3) 109,243; (5) 150 Xu, X. (2.5) 67 XU,Y. (3) 245-250,290 Xu, Z. (5) 266 Xuan, L.H. (3) 171 Xuc, R. (3) 42 1 Xuc, S.(2.7) 44 Yabc, A. (2.4) 162 Yabushita, S.(3) 887 Yachandra, V.K. (5) 19 Yacgashi, S.(2.2) 47; (2.4) 194; (2.6) 259 Yagami, T. (2.5) 86 Yagata, N. (2.4) 61 Yagci, Y. (3) 151, 152, 164, 183-185,320 Yagi, M. (1) 232 Yagi, T. (3) 670 Yagishita, T. (4) 65,66 Yajimr, T. (I) 412 Yajima, Y. (3) 553 Yakovlev, V.N. (2.5) 154,156 Yaliunin, V.P.(3) 219 Yallce, RB. (3) 375 Yaniabc, T. (2.4) 94; (2.6) 3 14 Yanada, A. (4) 58 Yanirda, K.(2.3) 33; (2.4) 125, 182; (2.5) 221; (2.6) 120; (3) 137 Yaniagishi, T. (2.4) 201 Yaniaguchi, A. (1) 593 Yaniaguchi, H. (3) 241,242 Yaniaychi, K. (2.4) 56, 132; (2.6) 267,268; (3) 21 8,220,22 1 Yamaguchi, M. (4) 56 Yaniaguchi, R. (3) 520 Yaniaguchi, T. (2.7) 50 Yaniaguchi, Y. (1) 310; (5) 108 Yaniaji, M. (1) 257; (2.2) 113; (2.5) 32,34; (5) I36 , Yamakoshi, Y.N. (2.5) 86
495 Yamamoto, A. (4) 49 Yamamoto, J. (2.4) 271 Yamanioto, K. (1) 406; (3) 710; (5) 184 Yamamoto, M. (2.3) 33; (2.4) 125, 182; (2.6) 120; (3) 190,440, 445,449,601,751 Yamamoto, N. (2.3) 47.48 Yamanioto, R (3) 90 Yamamoto, T. (3) 264,34 1,468, 525,624 Yamanouchi, H.(3) 49 1 Yamaoka, M.(2.5) 19 Yamaoka, T. (3) 8 Yamashita, D.S. (2.7) 84 Yamashita, H. (5) 18 Yamashita, K.( I ) 120; (3) 177, 267 Yamashita, T. (2.3) 14,45; (2.4) 4, 179; (2.6) 9, 139, 142; (3) 295, 304,305,461,466,790 Yamauchi, M. (2.2) 50 Yiunauchi, S.(2.5) 29 Yaniauc, T. (3) 661 Yamazaki, 1. (I) 262,267,378, 584,605,606; (3) 556; ( 5 ) 157, 253,254 Yamazaki, Y. (2.2) 79 Y a , D.-X. ( I ) 327 Yan, G.-Q. ( I ) 440; (5) 185 Yan, M. (3) 604,646 Yan, Q. (3) 189 Yan, Y. (1) 568,626 Yanagi, K. (2.1) 35; (2.5) 59; (2.6) 179 Yanagida, S.(2.5) 99, 100; (4) 7 Yanai, G. (3) 733 Yang, B. (2.4) 204; (2.6) 197; (3) 173,579 Yang, C.C. (I) 407; (2.5) 120 Yang, C.F. (2.1) 55 Yang, D.-D.H. (2.4) 192 Yang, D.K.(3) 266 Yang, G. (1) 147,506; (2.6) 297; (3) 130 Yang, G.X.(3) 528 Yang, G.-Y. (2.5) 65 Yang, H. (3) 395 Yang, J. (1) 567; (3) 322 Yang, J . 4 . ( I ) 195,200; (2.3) 6; (2.6) 23; (5) 134 Yang, J.-Y. (2.6) 247 Yang, M.-J. (4) 56 Yang,N.-C. (1) 334; (5) 85 Yang, N.-c.C. (2.4) 192 Yang, P.Z. (3) 886 Yang, S. (1) 415; (2.2) 14; (2.5)
93; ( 5 ) 192 Yang, S.I. (1) 414 Yang, S.Y. (3) 128 Yang, W. (3) 109,243 Yang, W.J. (3) 204 Yang, W.T. (3) 188 Yang, X.M.(3) 125 Yang, Y. (3) 109,243,322,464, 559 Yang, Y.S. (3) 174 Yang, Z. (2.4) 86; (2.6) 4 1 Yruiin, A.M. (3) 32 I Yao, F. (3) 55 Yao, G. (2.5) 78; (3) 397 Yao, G.Q. (3) 528 Yao, N. (4) 38 Yao, N.T. (2.1) 55 Yao, S.(1)408; (2.4) 81,84,261; (2.5) 168; (2.6) 42 (1) 150 Yao, S.-D. Yao, 2. (3) 531 Yarovaya, N.V. (3) 293 Yasuda, M. (2.2) 1: (2.3) 14,45; (2.4) 4, 179, 188; (2.6) 9, 139, 142 Yasuda, N. (2.1) 20; (2.5) 52; (2.6) 92 Yasuc, T. (3) 389 Yasui, K.(2.3) 93; (2.6) 141; (2.7) I95 Yasui, M. (2.3) 62,63; (2.5) 186; (2.6) 320 Yasunori, S.(2.5) 29 Yatcs, B. (2.5) 162; (3) 795, 814 Yazawa, Y. (4) 52 Yc, J. (2.5) 174 Yc, Q. (2.4) 89; (2.6) 39 Yc, 2.-Q.(2.4) 199; (2.6) 58 Ycager, H.L.(3) 564,595 Ych, S.R. (2.2) 63 Ych, Y.-L. (2.2) 94; (2.3) 65 Yckta, A. (3) 381,401; (5) 141 Ycllowlccs, L.J. (1) 356; ( 5 ) 100 Ycoinan, G. (1) 632 Yic, J.E. (3) 719 Yilniaz, Y. (3) 326,371,372 Yin, M. (3) 482 Yin, Q. (3) 395 Ying, L. (2.7) 66 Ying, S.(3) 266 Yitzhaki, N. (1) 57 Ylitao, D.A. (3) 407 Yoda, K. (1) 600; (2.5) 218; (5) 25 1 Yoh, K. (4) 48 Yokoi, M. (3) 492,495 Yokoyama, M. (3) 494
Yokoyama, Y. (2.2) 97,98; (2.4) 88; (2.6) 36 Yokozawa, T. (3) 62 Yoncmura, H. (2.5) 2 18 Yoo, D.J. (2.2) 99; (2.4) 151, 154 Yoo, S.D. (1) 627 Yoo, T. (3) 147, 163 Yoon, B.A. (2.6) 20, 159 Yoon, B.H. (3) 813 Yoon, B.S. (3) 719 Yoon, C. (3) 643 Yoon, C.H. (3) I96 Yoon, H.J.(2.4) 13I Yoon, K.B. (2.5) 69 Yoon, M. (1) 25 1 Yoon, S.H.(3) 7 19 Yoon, U.C.(2.4) 155; (2.6) 66 Yoon, Y.J. (3) 813 Yohi, F. (3) 670 Yoshida, K.(2.5) 136 Yoshida, M.(2.4) 190; (2.6) 67 Yoshida, N. (4) 57 Yoshihara, K. (1) 3 11; (2.2) 105; (2.5) 46,220; (3) 455; (5) 106 Yoshii, T. (2.4) 94; (2.6) 314 Yoshimura, A. (1) 70,384; (5) 61, 168 Yosliino, K. (3) 623,661 Yoshioka, M. (2.1) 10,20; (2.2) 79,89; (2.5) 52; (2.6) 92 You, H. (3) 128, 174 Young, E.RR (2.4) 189 Young, M.A. (I) 254; (2.6) 236 Young, RJ. (1) 37; (3) 375 Young, V.M. (2.6) 223; (2.7) 153 Youscf, B. (3) 203 Yousscf, B. (3) 3 I5 Yu,H. (2.4) 89; (2.6) 39,233 Yu, H.S.(3) 295,304 Yu, H.-T. (1) 227 Yu, J. (1) 612; (2.5) 146; (3) 649 Yu, K. (3) 546 Yu, L. (2.4) 82,86,89; (2.6) 39-41; (3) 316,557 Yu, L.-X. (2.5) 65 Yu, Q. (2.5) 176 Yu,s. (3) 397 Yuccl, M. (4) 67 Yukino, T. (3) 840 Yurovskaya, A.N. (2.1) 8 Yustc, F. (2.2) 109 Zaballos-Garcia, E. (2.4) 87 Zachariassc, K.A. (1) 92, 197,247; (2.4) 29; (2.6) 14,15 Zadkov, V.N. (1) 208; (2.4) 16
Pholochemistry
496 Zadrozna, 1. (3) 233 Zahir, K.O.(1) 37 1; (2.5) 110 Zaichcnko, N.L. (3) 479 Zaikov, G.E.(3) 859 Zakccruddin, S.M. (2.5) 187 Zakharcnko, V.S. (4) 23 Zakharova, G.V. (2.4) 73 Zakhs, E.R.(2.4) 76; (2.6) 46 Zalcnt, B. (1) 49 Zaliznaya, N.F. (2.5) 223; (3) 398 Zamotacv, P.V. (3) 252 Zana, R. (1) 477 Zand, A. (2.1) 17 Zandonicncghi, M. (2.4) 66 Zanoco, A.L. (1) 629 Zanotti, G. ( I ) 392; (5) 152 Zapotoczny, S.(3) 139 Zard, S.Z. (2.6) 258 Zarcmbowitch, J. (1) 560 Zarkliin, L.S. (3) 457 Zaros, M.C. (5) 99 Zefirov, N.S. (2.3) 59; (4) 19 Zchnacker-Renticn, A. (1) 249 Zciri, Y. (2.7) 11 Zelentsov, S.V. (2.6) 200 Zelt'scr, L.E. (2.6) 234 Zcng, H. ( I ) 153,369; (3) 591 Zcng, I. (3) 882 Zeng, T.-X. (1) 506,507 Zcnkcvich, E. (1) 12,377; (5) 42, 156 Zcnneck, U. (2.7) 199 Zcnova, A.Yu. (2.3) 59; (4) 19 Zcntcl, R. (3) 277,501 Zcrbctto,F. (1)410,411;(5) 182 Zcrner, M.C.(2.4) 23 Zcrza, G. (1) 404 Zctzsch, C. (2.6) 212; (2.7) 104 Zezin, A.B. (3) 420 Zgonnik, V.N. (1) 4 I6 Zhili, H.B. (1) 138; (2.6) 205; (2.7) 80 Zhan, Y. (1) 529 Zhang, B. (2.4) 192; (2.5) 78,91; (4) 15 Zhang, B.W.(4)16 Zhang, C. (3) 109,243,275 Zhang, D. (1) 24 1;(2.2) 14 Zhang, F. (1) 124,125,335,515; (2.4) 72; (2.5) 60; (3) 130; ( 5 )
84 Zhang, G. (3) 34,158,159,400 Zhang, H. (1) 237,329,330,333, 588; (2.2) 116; (3) 34; ( 5 ) 87, 120,121 Zhang, H.-P. (2.4) 184 Zhang, 1. (1) 187,408; (2.4) 192; (2.5) 65; (3) 130,487,575 Zhang, J.4. (1) 150 Zhang, J.X. (3) 528 Zhang, J.Z. (1) 126 Zhang, L. (2.3) 6 1; (3) 289 'Zhang, L.-P.(2.6) 238 Zhang, M. (1) 290,329,330,333, 335; (2.4) 261; (2.5) 60, 168; ( 5 ) 84,87, 120, 121 Zhang, P. (3) 114 Zhang, Q. (1) 127; (3) 291,487, 569 Zhang, R. (1) 69; (3) 463 Zhang, S.(1) 415; ( 5 ) 192 Zhang, S.-L. (I) 334; ( 5 ) 85 Zhang, W. (1) 225,524; (2.1) 21; (2.2) 118; (2.6) 103; (3) 53; (4) 55
Zhang, X. (2.4) 86; (2.5) 123; (2.6) 41; (2.7) 181; (3) 322,582 Zhang, Y. (1) 528,534; (2.5) 57; (3) 245-249,435 Zhang, Z. (1) 540,643; (2.1) 4; (2.4) 261; (2.5) 168; (3) 189 Zhang, Z.F. (3) 874 Zhao,D. (3) 77 Zhao, F. (1) 124, 125; (2.4) 72 Zhao, G.X. (3) 4 18 Zhao, H.-Q. (2.7) 176 Zhao, J. (1) 612 Zhao, Q. (2.5) 185 Bao, X. (2.7) 66 Zhao, Y. (1) 124, 125,515,542; (3) 603 Zhcng, A. (2.6) 24 Zhcng, B.Z. (2.1) 9; (2.6) 291 Zhcng, C. (3) 720 Zhcng, D.Y.(3) 47 Zhcng, G.F.(4) 55 Zheng, K.C. (2.3) 40 Zhi, Y. (3) 497 Zhilina, Z.I. (1) 123 Zhong, X. (3) 670
Zhou, B. (2.5) 190 Zhou, D.(2.5) 78 D.-J. Z~OU , (1) 440; ( 5 ) 185 Zhou, G. (1) 567 Zhou, J. (2.4) 72 J.-Y. Z~OU , (1) 507 Zhou, Q.-F. ( I ) 27 1,336; (2.5) 66; (5) 55,93 Zhou, Q.-T. (2.4) 199; (2.6) 58 Zhou, X. (1) 270; (3) 712 Zhou, X.-L. (2.7) 10 ZIIOU,X.-Z. (2.4) 184 Zhou, Y. (3) 568 Z~OU Y,-Q. , (1) 1 17 Zhu, D.(1) 24 1;(2.4) 84 Zliu, H. (3) 22 Zhu, H.-R. (1) 167 Zhu, L. (1) 527 Zliu, P. (3) 7,307 Zhu, Q. (1) 127; (2.7) 109 Zhu, Q.Q.(3) 150,183 Zhu, X.-Y. (2.7) 8, 105, 106 Zhu, Z. (2.3) 75; (2.4) 198 Zicba, 1. (3) 583 Zicglcr, C.J.(2.7) 198 Zicglcr, F.E. (2.1) 45; (2.6) 84 Zicsscl, R. (1) 16,75; ( 5 ) 32,73, 270 Zinuncrman, H.E.(2.2) 43; (2.3) 27,37; (2.4) 218; (2.6) 246; (3) 593 Zinmicnnm, T. (2.4) 80; (2.6) 55, 56
Zinunt, M.B. (1) 339; ( 5 ) 229 Zlatkcvich, L.(3) 678 Zoos, Z.G.(3) 656 Zorinyants, G.E.(1) 602; (3) 605 Zou. C. (2.5) 185 Zucva, T.A. (3) 32 I Zuhsc, R. (2.7) 53 Zuilhof, H. (2.3) 29,30 Zulaica, E.(2. I) 54; (2.4) 195; (2.6) 63 Zunier, S.(3) 266 Zuo, 2.(2.4) 81; (2.6) 42 ZUO,Z.-H. (1) 150 Zurkowska, G. ( 1) 175 Zyrianov, M. (2.7) 100, 101 Zyung, T. (3) 65 1
