Photochemistry

Photochemistry

Volume 23

A Specialist Periodical Report

Photochemistry Volume 23 A Review of the Literature published between July 1990 and June 1991 Senior Reporters D. Bryce-Smith, Department of Chemistry, University of Reading A. Gilbert, Department of Chemistry, University of Reading Reporters

N. S. Allen, Manchester Polytechnic A. Cox, University of Warwick R. B. Cundall, MRC Radiobiology Unit, Didcot M. Edge, Manchester Polytechnic W. M. Horspool, University of Dundee S. R. Meech, Heriot-Watt University S. T. Reid, University of Kent A. C. Weedon, University of Western Ontario, Canada

SOClETY OF CHEMISTRY

ISBN 0-85186-215-2 ISSN 0556-3860 Copyright 0The Royal Society of Chemistry 1992 All Rights Reserved N o part of this book may be reproduced or transmitted in any forrri or by any means -graphic, electronic, including photocopying, recording, taping or information storage and retrieval systems - without written permission from the Royal Society of Chemistry Published by The Royal Society of Chemistry, Thomas Graham House, The Science Park, Cambridge CB4 4WF

Printed in Great Britain by Billings & Sons Ltd., Worcester

Introduction and Review of the Year BY D. BRYCE-SMITH AND A. GILBERT

In the Introduction to Volume One of this series, the stated objective was to provide as broad as possible cover of the physical, inorganic, organometallic, and organic aspects of photochemistry in a unified way that would encourage interactions between workers in particular areas. Over the years, we introduced further topics, particularly polymer photochemistry and the chemical aspects of solar energy conversion. On the other hand, we have reluctantly been obliged to discontinue the coverage of gas phase and inorganic photochemistry. As a new feature, we have decided to introduce occasional reviews of specialised areas of photochemistry, starting with a review of photochemistry at surfaces by Dr.S.R.Meech. In selecting future topics for review, we shall aim for timeliness, as in the present case. We take this opportunity to invite suggestions for future occasional reviews of this type, and would be particularly pleased to hear from colleagues willing to accept the great honour of writing them! With these developments, we hope to return closer to the objectives originally set out in Volume One. As usual, we start with our personal and highly subjective assessments of important developments in the more physical areas of photochemistry. It is now evident that lasers have largely taken over from lamps as radiation sources in this area of the subject. This year, we note a somewhat increased interest in practise and decreased activity on the theoretical side. Femtosecond techniques are now well established.

The solvation dynamics for ion pairs in polar solvents can now be directly examined by the time dependence of fluorescence and by direct observation of photoinduced charge transfer (Carter and Hynes). We refer later in this review to the growing interest in photochemical charge transfer phenomena among organic photochemists. Sukowski et al. have described an important study of the energy transfer processes which occur during relaxation of vibrationally 'hot' molecules. There is growing interest in solvent friction phenomena (Simon and Su, inter alia). A number of interesting reports on fluorescence decay phenomena have appeared this year, including a Monte Carlo procedure for simulation and analysis of decay data (Chowdhury et ul.). Minami and Hirayama have described an interesting elliptical scan streak camera for the measurement of high quality fluorescence decay

vi

Introduction and Review of the Year

times and also lifetime imaging. Brochon et al. have shown that the maximum entropy method is suitable for recovery of fluorescence lifetime distributions. It is known that some insects are able to detect a single molecule of a pheromone. The reported detection of single molecules of a dyestuff by repetitive laser pulsing shows that photochemists are now catching up with insects (Shera et al.). Luminescence-activated barometry in wind tunnels represents an unusual application of photophysics (Kavandi et d.). Wild and Renn have provided a useful review of new ideas for high-density information storage. Adick et al. have described a chemical actinometer for light in the 670-795 nm range. The procedure is based on dye-sensitised photo-oxygenation of mesodiphenylhelianthrene. Mazsuzawa et al. have offered a simple but very useful tip for deoxygenation of solvents: add pieces of solid C02. Photochemists are now starting to get interested in Buckminsterfullerene (0). Various excited state assignments have been made (Whetten, Diedierich, and coworkers). Johnstone and Sodau have at last obtained evidence for the formation of triplet pyridine. Time-resolved fluorescence spectra of naphthalene in silica glasses provide evidence for a unique luminescent excimer derived from the ground state dimer (Yamanaka et d . ) . Hirayama et al. have used a pressure technique to provide evidence that the radiative lifetimes of certain S1 states in solution do not necessarily relate to the gas phase values, as has been previously accepted. Two groups have reported dual S 2 - 4 0 and S1 -So fluorescence from acenaphthylene (Samanta et al.) and silyl and germyl ketones (Wakasa et af.). Several detailed time-resolved studies of proton transfer processes have been described this year: see for example Masad and Huppert. Although examples of electron transfer along saturated chains are already known, it is interesting that a heavy atom effect has been observed over 13 o bonds (Basu et d.).Hirata and Mataga have achieved the remarkable feat of observing directly the generation of electron-cation geminate pairs in non-polar solvents as a function of the excitation wavelength. Brennecke et al. have described an important study of exciplex and excimer formation in supercritical carbon dioxide and ethene.

Introduction and Review of the Year

vii

Stilbene photochemistry continues to provide a mine of riches for the dedicated photochemist, in particular Hochstrasser, Fleming, and their groups. Metcalf et al. have described chiral discrimination in electronic energy transfer processes: enantioselective excited state quenching occurs. A study of singlet electronic energy transfer from cyclohexane to benzene appears to require revision of the benzene fluorescence efficiency in cyclohexane to 0.26i0.02 (Johnston and Lipsky). The use of methylated guanine as a luminescent probe in DNA has provided the first experimental evidence for electronic energy transfer along the double helix of a nucleic acid (Georghiou et al.). Radical cations of a,CiS-diphenylpolyenes trapped within zeolites have been spectroscopically characterised (Ramamurthy et uZ.). Merlo and Yager have described an interesting optical method for monitoring the concentration of anaesthetics and other small organic molecules in biologically interesting systems. The procedure enables phase transitions in lipid membranes to be detected. Hashimoto and Thomas have reported that upper triplet states of biphenyls in micelles ionise to produce hydrated electrons. 1-H-Indenylfuran and thiophene derivatives have been proposed as a new class of singlet oxygen sensitisers by D'Auria and Vantaggi. McLean and Truscott have reported that in triplet photosensitised singlet oxygen generation in benzene, the efficiency of exchange energy transfer between the triplet sensitiser and ground state oxygen correlated with the ionisation potential of the sensitiser. Ultrafast techniques are finding increasing applications in elucidating the mechanisms of photoreactions. For example, this powerful technique has been applied to photochemical ring-opening of cyclo-octatriene (Reed et ul. ). and the photo-cycloreversion of an aromatic endoperoxide (Emsting et al.). In the latter case, a C-0bond in the S3 state ruptures within 0.35 ps of excitation. We now turn to the more organic aspects of the subject. Studies of single electron transfer processes have become increasingly prominent. Ketone (1) gives the enol (2) by a Norrish Type I1 reaction. In the presence of (-)-ephedrine, asymmetric formation of the final product (3) occurs by enantioselective transformation of the enol (Henin et ul.). In the well known photocyclisation of o-alkyl substituted aryl ketones to cyclobutenols, it has now been shown that a dienol intermediate precedes the cyclobutenol: this resolves a longstanding disagreement (Wagner et a/.). In the ketones such as (4), a 1,5-biradical is

Introduction and Review of the Year

d

C

H

3

& C H3

xPh

H3C

R

(4) R = H or C2H5

(5) R' = CH3, R2 = H (6) H' = H, R2 = CH3

(7)

CH3

JyR

H3C\CHCHZ

/ H3C

H,C*CH3 CH3

(12) R=+

C02H

0 (13) R

=

q

C H3 0

(16) X = 0 or CH2 n =1,2,3,or4

(1 4)

Introduction and Review of the Year

ix

formed by hydrogen abstraction at the &-carbon: this cyclises to the two indanes (5) and (6) with high diastereoselectivity which is attributed to conformational effects in the triplet biradical (Wagner and Park). Cottet et al. have made interesting use of intramolecular 1$-hydrogen abstraction as a route to the synthesis of natural products such as the spiropeltogynoids. We have previously referred to the great interest in single electron transfer processes. A nice example of the application of such processes has been provided by Cossy and Leblanc: photocyclisation of the oxamide (7) in the presence of triethylamine gives the cyclic amide ( 8 ) , and thence iso-oxy-skytanthine (9). Garcia et al. have also used photochemical electron transfer to convert cyclic acetals and thioacetals into the corresponding ketones. Wan and Xu have shown that a-hydroxyarylacetic acids undergo rapid decarboxylation on irradiation in aqueous acetonitrile, in marked contrast with the unsubstituted arylacetic acids. Irradiation of the chloroketones (10) in aqueous acetone has been shown by Sonawane et al. to provide an efficient synthesis of carboxylic acids (1 1). This reaction has provided a convenient synthesis of the pain-killer Ibuprofen (12) from the ketone (13). A single electron transfer is again involved in formation of the ketone (14) together with the well-known intramolecular adduct (15) when carvone is irradiated in the presence of triethylamine (Givens et al.). In related work, Bischof and Mattay showed that the presence of triethylamine deflected the normal course of intramolecular photoaddition within the enones (16) to produce the spiro compounds (17) preferentially. Regio- and stereospecific adducts from cyclopentenones and the bicyclo[2.2.l]heptene (18) have been used by Salomon et ul. as a route to spatane diterpenes. Photoadditions of alkenes to enones can give oxetans and/or cyclobutanes. Cruciani et al. have reported that the use of acetonitrile as solvent favours the formation of cyclobutanes. The oxetanes may be formed via a contact ion pair whereas the cyclobutanes may arise from an exciplex.

Rather surprisingly, irradiation of 2'-deoxycytidine (19) and 2-carbethoxypsoralen (20) as a mixture of the dry solids gave the photoadducts (21) and (22) (Voyturiez et al.). The enol (23) has now been confirmed as an intermediate in the Nazarov reaction (Leitich et al.). Piva and Pete have described a technique for achieving high enantioselectivity in the photodeconjugation of an enone. The enone (24) has been shown to undergo a novel rearrangement on direct or sensitised irradiation to give the indanones (25) and (26) (Mori et al.). Pandey et al. have described an interesting procedure based on single electron transfer for conversion of endo Diels-Alder adducts of p-benzoquinone into the exo-isomer.

Introduction and Review of the Year

X

NH2

I

OH (21) endo -isomer (22) exo -isomer

WCH3

Introduction and Review of the Year

xi

Lewis et al. have described some interesting intramolecular cycloadditions of amines to alkenes: single electron transfer is involved. Pandey et al. have reported some related processes using dicyanobenzene as an electron-transfer sensitiser. Single electron transfer from triethylamine has been used for the ring-opening of cyclopropanes (Tomioka and Kanda). Chen et ul. have shown that irradiation in a chiral crystal lattice can give high enantiospecificity in a di-x-methane reaction. Gas phase irradiation of the dihydronaphthalene (27) at 254 nm gives the isomer (28) via the S2 excited state within which multiple 1,2-hydrogen migrations occur; but in the presence of butane as an inert buffer gas, the cyclobutene (29) is uniquely formed (Duguid and Morrison). Todd et al. have reported that the Nmethylacridinium ion (30) has considerable promise as a single-electron transfer sensitiser in solvents of low polarity. We come now to some of the more significant developments involving aromatic compounds. Although it has long been known that the formation of Dewar-benzene from benzene involves the S2 rather than the S1 state, it has now been shown that irradiation of benzene in a low temperature argon matrix produces the Dewarisomer, possibly by induced mixing of the S1 and S2 states (Johnstone and Sodeau). Although irradiation of tropone in acetonitrile normally yields dimers, the presence of an acid induces electrocyclic ring closure to give (31) (Cavazza et al.). This is one of several interesting acid-catalysed photoreactions to be described this year. meta-Photocycloaddition of alkenes to the benzene ring continues to attract interest: Wender and deLong have provided further examples of application of the intramolecular process in the construction of complex ring systems. The hitherto puzzling reluctance of cyclohexene to undergo meta-cycloaddition has now been explained on the basis of unfavourable conformational effects (Bryce-Smith et al.). Mattay et al. have described results consistent with this explanation. The natural product dactylol (32),has been synthesised by a route which includes intramolecular photocycloaddition of the alkenyltropone (33) to give the isomer (34) (Feldman et al.). Chow and Cheng have shown that P-diketones can be used as electron transfer sensitisers if they are treated with BF3 to give the complexes (35). These form exciplexes with benzenes leading to products of ortho addition to the benzene ring (Chow and Ouyang). Although little has previously been known of the photochemistry of indoles, five reports of cycloadditions to indole have appeared this year (Giesler et al., inter a h ) . Intramolecular photoaddition of an alkene to an N-acylindole has been used in synthesis of the alkaloid vindorosine (36) (Winkler et ul.). Irradiation of mesitylene

Introduction and Review of the Year

xii

(37) R = CH3or OCH3

& \ /

(39)

Introduction and Review of the Year

xiii

and other arenes in hexafluoropropan-2-01 leads to cyclohexadienyl cations such as (37) by protonation of the excited state of the arene (Steenken and McClelland). Photochemical dechlorination of organochlorine pollutants such as PCBs has been rendered somewhat more feasible by the discovery that they can be solublised to some extent in aqueous solutions of a vinylnaphthalene-styrene sulphonate copolymer (Nowakowska et al.). The pyrimido-pteridine N-oxide (38) can be used for the photohydroxylation of phenols in acetonitrile solution (Sako et d.). Wan and Wu have reported that irradiation of o-dialkoxybenzenes in aqueous acetonitrile containing sulphuric acid leads to replacement of the alkoxy groups by hydroxyl: the mechanism appears to involve protonation of the S1 arene to give a o-complex which then undergoes ipso attack by water. Fields has reported that diphenylmaleic anhydride photocyclises readily to the phenanthrene (39), and has corrected an earlier report that the product is a (2+2) dimer. Heller's pioneering work on photochromic fulgides is being followed up by Japanese and Russian workers in view of the potential applications in data storage systems: see e.g. Metelitsa et al., inter alia. Kimura et al. has reported a number of intramolecular examples of cinnamolyl photodimerisation in solution in which the presence of lithium ions can divert the course of the reaction. The applications of P-cyclodextrin in photochemistry continue to attract attention. These reactions are normally carried out in aqueous solution, but Pitchumani et al. have shown that irradiation of the solid complexes can give dramatically different results. J.A.Schmidt et al. have described a reaction which unusually proceeds from the of an acetophenone. It has previously been thought that benzocyclobutenols are not formed on irradiation of o-substituted phenyl ketones, but Wagner et al. have now shown that this is a misconception which has arisen because these compounds are thermally unstable and readily revert to the ketone. Several examples have appeared in which the normal photochemistry of ketones of this type is diverted, apparently by interception of an intermediate biradical by an unsaturated group present elsewhere in the molecule (Pandey et al., inter alia). S1 state

We come now to photochemical redox processes. Reinvestigation of the photoreaction between cyclohexanone and triethylamine by Schuster and Insogna has removed the necessity to postulate an intermediate triplet excimer of the enone. Selective photoreduction of aldehydes in the presence of a ketone has been achieved using cyclo-octane as H-donor and a rhodium complex as catalyst (Sakura et ul.).

xiv

Introduction and Review of the Year

Gebicki et al. have reported that 1H-3H-exchange in o-methyl-substituted phenyl ketones is a valuable tool for probing the formation of transient photoenols. Kraus et al. have described a useful direct photochemical route to benzofuranols and thence to aflatoxins. Dinitrogen has been photoreduced to ammonia in aqueous solutions containing colloidal transition metal catalysts (Nahor et af.). Kamogawa and Sat0 have observed redox photochromism in a crystalline 1,l '-diaryl-4,4'-bipyridiniumsalt. Electron donation normally involves x-electrons, but Fukuzumi et af. have shown that permethylpolysilanes can act as o-electron donors.

C-F bonds are normally difficult to reduce, but it has now been shown that such bonds a to the carbonyl group of alkyl perfluoroesters can be photoreduced in hexamethylphosphorotriamidein a reaction which involves electron transfer from excited phosphoramide (Portella and Iznaden). Several reports of the photoreduction of carbon dioxide have appeared: for photoreduction to methane see Yamase and Sugeta, and Diirr et af.; for reduction to formate see Lehn and Ziessel, and Matsuoka et al. Gollnick and Held have reported that mercurochrome is an efficient sensitizer for type I1 singlet oxygen photo-oxygenations. Suzuki et af. have reported that methane can be photo-oxidised to formaldehyde on silica-supported molybdena. Viswanathan et af. have shown that photocatalytic dehydrogenation of methanol on Pt/TiO2 generates metal clusters: this observation may have implications for the mechanism of catalytic activity in dehydrogenation reactions. Organosodium compounds (Na+-,R--)(R = naphthalene, etc.) have been converted to R+. by electron transfer to XeF2: subsequent combination of R-- and R+- gives excimers which decay with chemiluminescence (Bulgakov et af.). Pandey et al. have described a potentially useful single-electron N-demethylation of N-alkylN-methylanilines in alkaline methanol. Rokita et af. have identified key parameters that affect the photo-oxidation efficiency of DNA: these arise from association between DNA and a sensitizer. Bulatov et al. have proposed a new mechanism for photo-oxidation of H2S in the troposphere: the species HSO. may be involved. Guest selectivity in the cyclophane (40) can be controlled by E-Z photoisomerisation of the azo moiety (Shinkai et uf.). Aoyama et al. have provided the first example of the photo-regulation of membrane permeability. Pandey et al. have

lntroduction and Review of the Year

xv

described a particularly interesting double electron transfer process involving the sequence (41)-(42)-(43): dicyanoanthracene was the electron-transfer sensitizer. Polysilanes are important as photoresists. 1.M.T.Davidson et.al. have identified three pathways in the photodegradation of these species. Sekiguchi et al. have described a potentially valuable new photochemical synthesis of tetramethyldisilene (44)based on irradiation of its 1,4-adduct with benzene. An interesting contrast between silicon and carbon chemistry is provided by the photoisomerisation of the trisilacycloheptene (45)to the corresponding trans isomer (46)(Shimizu et al.). Photoelimination of N2 from 3H-pyrazoles gives intermediate vinylcarbenes: this reaction may have valuable applications in synthesis (Franck-Neumann et al.). Photochemists suffering from anosmia may be interested in the synthesis of 1,3selenaphospholes described by Burkhart et al. Maier and Fleischer have reported a new photochemical route to tetra-t-butyltetrahedrane. McCarthy et al. have described the photochemical formation of triptycene from compound (47): the reaction is remarkable in involving the loss of one carbon atom, and the mechanism remains obscure. Pasto and L'Hermine have described a novel and versatile method for the generation of alkyl radicals, based on the photolysis of alkyl 4-nitrobenzenesulphenates,e.g. (48). Tolbert et al. have described an interesting photochemical route to the highly strained cyclic allene (49). As usual, the chapter on polymer photochemistry has a considerable number of references although this year there are not so many as in the chapter concerned with the physical aspects of photochemistry. Various amides unexpectedly accelerate the photopolymerisation of methylmethacrylate in the presence of oxygen due to the formation of an oxygen-amide complex (Tskeuishi and Tao). There is much interest in the role of amine co-synergists in photoreduced polymerisation of acrylic monomers (Li et al.). In a related development, p-alkylaminobenzophenonesappear in most cases to be more effective photoinitiators of polymerisation than benzophenone itself (Allen et af.; Mateo et af.). Magnetic fields have been reported to promote syndiotactic photopolymerisation (Huang and Zhu).

Padmanaban et al. have described a new class of polymers containing a photosensitive disilane group in the main chain. Krongauz and Yohannan have developed an interesting non-destructive method for monitoring the kinetics of monomer transport during photopolymer formation.

introduction and Review of the Year

xvi

y

3

hv ____)

H3C0

H3C0

1

hv, DCA CH30H

H3C0 H3c0*

(44)

(43)

0ch3

Introduction and Review of the Year

xvii

Solid state and template photopolymerisations continue to attract interest: see e.g. Gerasimov; Jin et al.; Awasthi and Srivastava, infer alia. Vinyl ethers bearing pendant norbornadiene units have been cationically photopolymerised to give polymers containing quadricyclane units that may be reversibly isomerised using a cobalt complex (Hijikata and Nishikubo). Light stabilisers based on o-hydroxyphenylbenzotriazoles have been photografted on to polyolefins to improve durability (Lucki et al.). There has been considerable interest in the photochemical crosslinking of polyethylene: see e . g . Chen and Ramby; Zamotaev and Granchak, inter alia. Lu et al. have reported that the fluorescence intensities of Eu3+ and Tb3+ are markedly enhanced when they are bound on to a poly(acry1amide-acrylic acid copolymer). Dhake et al. have observed interesting differential effects of pressure on the fluorescence of a polymer and the corresponding monomer. Sterically hindered piperidines have for long been of interest as efficient photostabilisers of polymers. Gugumus has now reported that these stabilisers form charge-transfer complexes with molecular oxygen, thereby preventing the formation of such complexes with the polymer. In the field of solar energy conversion, Antonucci et al. report that mild reductive treatment of Ti02 surfaces leads to a series of oxides having varying properties including surface states having acid-based characteristics important in the photo-oxydative decomposition of water. Rophael et al. have made the interesting observation that aqueous sodium carbonate can be efficiently photoreduced to methanol using Ti02 coated with Fe2+ or Co2+-phthalocyanines. Okamoto et al. have described a GaAs solar cell fabricated on Si substrates capable of energy conversion efficiency of 18.3%. Bennett and Rajan have observed that the stability of multijunction amorphous silicon solar cells exceeds that of corresponding single junction cells: the stability increases with the number of junctions. Finally, we draw attention again to the inclusion in this year's Review of the chapter on surface photochemistry. This review is concerned mainly with developments during the past five years, though references to earlier work are also included. In view of the nature of this innovation, we have not felt it appropriate to draw attention to aspects of special importance.

Contents PART I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY Photophysical Processes in Condensed Phases Cundall

3

By R.B. I

1

General

3

2

Singlet Processes

9

2.1

2.2 2.3 2.4 2.5 2.6

Electron Reactions and Exciplexes Dyes Isomerization and Related Processes Electronic Excitation Energy Transfer Polymeric Systems Colloidal and Heterogeneous Systems

16 19

20 22 23

3

Triplet State Processes

27

4

Other Processes

3;I

4.1

4.2 4.3

Chemiluminescence Photochromism Photochemical Reactions

32 33 33 35

References ORGANIC ASPECTS OF PHOTOCHEMISTRY

PART I1 Chapter

13

1

Photolysis of Carbonyl Compounds

53

By W.M. Horspool

Chapter

1

Norrish Type I Reactions

53

2

Norrish Type I1 Reactions

56

3

Oxetane Formation

67

4

Miscellaneous Reactions

71

References

79

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool

84

Cycloaddition Reaction

84

Intramolecular Intermolecular

84 88

2

1

2

Rearrangement Reactions xix

106

Contents

xx

Chapter

Chapter

a, p-Unsaturated Systems p, Y-Unsaturated Systems

106 113

3

Photoreactions of Thymines, etc.

113

4

Photochemistry of Dienones

115

Cross-conjugated Dienones Linearly Conjugated Dienones

115 119

5

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

119

6

Quinones

128

References

131

Photochemistry of Alkenes, Alkynes, and Related Compounds By W . M . Horspool

141

Reactions of Alkenes

141

cis-trans Isomerization Addition Reactions Rearrangement Reactions Anion Reactions

141 141 145 145

Reactions involving Cyclopropane Rings

147

Reactions of Dienes, Trienes, and Higher Polyenes

162

[2+2] Intramolecular Additions

168

Dimerization and Intermolecular Additions

172

Miscellaneous Reactions

177

References

181

Photochemistry of Aromatic Compounds By A . C. Weedon

189

Introduction

189

Isomerlzation Reactions

192

3

4

Addition Reactions

196

Substitution Reactions

212

Intramolecular Cyclization Reactions

227

Dimerization Reactions

242

Lateral Nuclear Shifts

244

Peripheral Photochemistry

251

References

266

xxi

Contents Chapter

Chapter

5

Photo-reduction and -oxidation By A. Cox

282

1

Introduction

282

2

Reduction of the Carbonyl Groups

282

3

Reduction of Nitrogen-containing Compounds

287

4

Miscellaneous Reductions

290

5

Singlet Oxygen

291.

6

Oxidation of Aliphatic Compounds

293

7

Oxidation of Aromatic Compounds

297

8

Oxidation of Nitrogen-containing Compounds

303

9

Miscellaneous Oxidations

305

References

306

Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By S.T. Reid

320

Nitrogen-containing Compounds

320

Rearrangements Addition Reactions Miscellaneous Reactions

320 336 343

2

Sulphur-containing Compounds

343

3

Compounds Containing Other Heteroatoms

350

References

359

7

Photoelimination By S.T. Reid

369

1

Elimination of Nitrogen from Azo-compounds

369

2

Elimination of Nitrogen from Diazo-compounds

375

3

Elimination of Nitrogen from Azides

379

4

Photoelimination of Carbon Dioxide

383

5

Fragmentation of Organosulphur Compounds

385

6

Miscellaneous Decomposition and Elimination React ions

387

References

392

POLYMER PHOTOCHEMISTRY By N . S . Allen and M. Edge

403

Introduction

403

6

1

Chapter

PART I11 1

Contents

xxii 2

Photopolymerization

403

2.1 2.2 2.3

404 415 416

Photoinitiated Addition Polymerization Photografting Photocrosslinking

3

Polymer Luminescence

423

4

Photodegradation and Photooxidation of Polymers

436

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

436 438 439 440 442 445 445 446 446 448

Polyolefins Poly (vinylhalides) Poly (acrylates) and (alkyl acrylates) Polystyrenes Polyamides and Polyimides Polyesters Polyurethanes Rubbers Natural Polymers and Cellulose Esters Miscellaneous Polymers

5

Photostabilization of Polymers

449

6

Photochemistry of Dyed and Pigmented Polymers

453

References

455

PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By A . Cox

471

1

Introduction

471

2

Homogeneous Photosystems

471

3

Heterogeneous Photosystems

473

4

Photoelectrochemical Cells

474

5

Luminescent Solar Concentrators

475

References

495

ADSORBATE PHOTOCHEMISTRY By S.R. Meech

481

1

Introduction

481

2

Techniques

482

3

Mechanisms of Adsorbate Photochemistry

488

4

Mechanism: Molecular Factors

503

5

Conclusions and Perspectives

519

References

521

PART IV

PART V

AUTHOR INDEX

522

Part Z PHYSICAL ASPECTS OF PHOTOCHEMISTRY B y R . B. CUNDALL

Photophysical Processes in Condensed Phases BY R. B. CUNDALL The description of the subject follows the same pattern as in previous years. Most of the experimental work which is cited has used some form of laser technique and an increasing proportion of this involves the use of extremely short duration pulses. Very few investigations nowadays use lamps except as components of steady state luminescence spectrometers. It is curious to note the way that problems of study in science follow fashions. This is apparent in photophysics. What criteria dictate selection of these fashions would make an interesting study in the organisation and development of at least one area of science. Industrial developments arising from research in photophysics are again not very evident, except in field of radiation curing, and papers on theoretical topics seem to have declined in number during the year. No striking new developments in technique have emerged during the year but existing methodology has been applied to gain more precise data on selected systems. Femtosecond techniques are now well established and surely at least one find frontier must have been effectively achieved. 1. Generd During the year a number of specialized monographs of relevance to photophysics have appeared. Two of general interest deal with applications of time resolved optical spectroscopy' and luminescence techniques in chemical and biochemical analysis2. In view of the exciting research now in progress in the femtosecond time regime the review of Mukame13 which goes deeply into relevant theory is of general interest. A collection of sonie twenty six papers on various facets of ultrafast spectroscopy of chemical and biological processes is extremely useful4. All aspects, including experimental techniques and methodology are examined by acknowledged experts. Fleming and WolynesS have reviewed recently developed knowledge of chemical dynamics in solution in a very concise and authoritative account. Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example6. The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge

4

Photochemistry

transfer7. A theoretical treatment developed for understanding solvation structure and the interpretation of time resolved Stokes shifts in non-Debye solvents has been compared with experimentaldata obtained with systems in a variety of solvents*. The decay time of the fluorescence anisotropy of excited aniline is about 1 ps and femtosecond techniques have been used to elucidate the ultrafast rotational dynamics involved9. A comparison of subpicosecond, subnanosecond, and steady state studies of diffusion influenced fluorescence quenching has provided the basis for a considerableimprovement upon the classical models for the quenching process which are usually usedlo. A truly fundamental study of the energy transfer processes which occur during relaxation of vibrationally hot molecules generated by internal conversion in solution has made on So-S, transitions in vibrationally hot azulenell. A comprehensive theoretical analysis is applied to experimental data. Okamoto and Yoshihara'* provide a theoretical treatment of prolonged time resolution of resonance coherent anti-Stokes Raman scattering and the effect of ultrafast rearrangementon the electronically excited state transition dipole rotation. The profile of radiation scattering is affected since the latter process is very fast. Picosecond Stokes shift studies involving measurement of the time dependence of band shapes and the integrated intensity provide information on solvent friction at the microscopic level13. This paper is part of a collection on the subject of solvent friction in a dedicated issue Phv sics. Subpicosecond relaxation of the solvation of nonpolar of -mica1 electronic states has been examined by the transient hole burning technique for dimethyl-s-tetrazine in several solventsI4. Various details of the short time scale changes in interactions operating in these solvents have been studied. The validity of gas phase vibrational relaxation models, probably accepted intuitively by most photochemists, for reactions in the liquid state have been critically examined and correlated with the density dependence of bound electronic state lifetimes15. The "optical" Kerr effect induced by nanosecond laser pulses is another rapid photophysical effect measured and analysed for various fluidsl6. Techniques and equipment used to obtain data in this type! of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American ScientistI7. An apparatus for canying out pump-probe broad band spectroscopy by trrinsient absorption in the subpicosecond region has been described in detailI8. This equipment has been used to observe the photodissociation of bis-(p-aminophenyl)disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers19. Okamoto and Yoshihara20

I: Photophysical Processes in Condensed Phases

5

have used a femtosecond time resolved CARS system for measuring Raman scattering under various conditions of polarization. The problem of optimizing the sensitivity of detection when there is ground state depletion and photodestruction are particularly acute with direct laser excitation; this problem has been discussed in some detail by Mathies et a P . The measurement of fluorescence decay is an extremely important aspect of experimental photochemistry and attention needs to be given by all those who publish data to a working party recommendation on the data processing methods regarded as acceptable for analysis of fluorescence decay data22. Inevitably the report gives a consensus view and it is likely to require early revision as new or modified techniques are introduced. The use of an internal quantum counter which provides a standard for making lifetime measurements by the pulse method is recommended by Pelletier er a P . A Monte Car10 convolution method proposed for simulation and analysis of fluorescence decay data has been compared with other established methods". The theory of a priori analyses of fluorescence decay surfaces of excited state processes improves on some of the difficulties encounteredin the use of the important, and now widely used, global method25. An analysis of the effect of distributions of lifetimes, in the relaxation of excited states by fluorescence decays, which must exist for molecules in nonuniform media, shows for both uniform and Gaussian distributions that the decay profiles for such systems must be non-exponentia126. A method for fluorescence data reduction provides another example of work on this particular A powerful combination of different data analysis methods has been method~logy~~. applied to the resolution of heterogeneous fluorescence by making use of indirect z ~ ~ excitation decay together with spectral and principal factor analysis2*. F ~ s has considered problems involved in interpreting the fluorescence depolarization of fluorophores in solutions and ordered systems. Experimental equipment reports in this area of photophysics include a description of a picosecond single photon timing measurement made with a proximity type microchannel plate photomultiplier together with global analysis of data which It employed a laser source and the performance uses reference convol~tion~~. demonstrated with data obtained for several compounds. A particularly interesting paper presents evidence for the measurement of high quality fluorescencedecay times and also lifetime imaging which can be obtained using an elliptical scan streak camera recently developed by Hamatsu Photonics31. 1.09 ps channel-' resolution is shown in the paper but improved devices for better resolution are said to be possible. This technology should be very useful for imaging.

Photochemistry

6

A new review publication has appeared which sets out specifically to cover

the technique and applications of multidimensional lumine~cence~~. The alternative technique to the pulse method for lifetime measurements is the phase and modulation procedure recently reviewed by Bright, Betts, and Litwiler33. Exploitation of the modulation frequency dependence of the decay kinetics has considerably increased the power of the method. In a eulogy upon the technique the resolution of multicomponent emissions has been strikingly demonstrated34. A fluorescence lifetime resolution of spectra in the frequency domain has been achieved by niultiway analysis35. This clearly provides a valuable means of resolving different fluorescent components in mixtures. Data analysis by the maximum entropy method, already successfully used in the pulse technique, can be applied to frequency domain fluorometry and shown to be a suitable method for recovery of fluorescence lifetime distribution^^^. Morgan and Murray37 have published details of the design of a phase/quadrature correlator for determining fluorescence decay times by single photon counting. This has been used in a system with a fluorescence microscope rind shown to be applicable to the generation of time resolved images. Equipment for phase modulation fluorometry using frequency doubled pulse laser diode light source modulated at frequencies up to 2000 MHz has also been fully described38. Time resolved evanescent wave induced fluorescence spectroscopy is a powerful method for the investigation of dye molecules at interfaces. This technique has been used on studies on the popular photosensitizer aluminium phthalocyanine tetrdsulphonate absorbed a t fused silicdmethanol interface^^^. 2nd harmonic detection of sinusoidally modulated two photon excited fluorescence can also be used to obtain luminescence spectra40. A notable experimental achievement is the detection of single molecules of rhodamine 6G by the means of

it

system involving repetitive laser pulsing together

with time gated discrimination of fluorescence photons41. In electrically levitated microdroplets as few as 12 molecules of rhodamine 6G have been detected in glycerol-water solution42. An advantage claimed for this unusual system is that lifetimes can be measured with reduced impurity effects and also minimization of the Raman solvent background signal. A gated photomultiplier circuit which can be applied to the determination of phosphorescence lifetimes is also very usefu143. A comprehensive review of hole burning spectroscopy of species in glasses has been prepared by Haarer and S i l b e ~ ~Electric ~. field effects on the spectral holes for perylene in Shpol’skii matrices in n-heptane provide a useful extension of this t e c h n i q ~ e ~The ~ . limitaiions of the thermal lens method for the measurement of fluorescence yields have been examined and discussed46. It is pointed out that the

I: Photophysical Processes in Condensed Phases

7

method is not always accurate, especially when used with species, such as dyes, which have high triplet state yields. Imaging systems have been briefly mentioned earlier. Other examples of publications of this field of photophysical interest include accounts of a laser microfluorometer with intensified photodiode array detector and scan stage useful for imaging cytometry of cellular specimens47and a luminescence imaging system using biolumine~cence~~.Fluorescence energy transfer has been employed in eluant detection during capillary electrophoresis at attomole levels49. A photodiode array has also been employed as a fluorescence detector for HPLCS0. A Emote photoacoustic measurement technique for aqueous solutions using an optical fibre is Laser induced shown to be capable of considerable spatial optoacoustical effects combined with near IR emission have been successfully used to determine intersystem crossing yields of porphyrins5*. An unusual application of photophysics in measurement is a report on the use of the quenching of platinum octaethylporphyrin phosphorescence by oxygen for luminescence activated baromeuy in wind tunnels53. This may encourage other similar developments. The study of radicals in the condensed phase by fluorescence detected magnetic resonance is a procedure which is discussed in a paper by Werst and T r i f ~ n a c ~ ~ . A review of the putative technology of molecular computing discusses the possibilities for the high density storage of information by means of spectral hole burning55. Another approach through photophysics towards the technology of microelectronics is exploitation of light directed chemical synthesis of solid state chemistry of materials with photolabile protecting groups56. Light directed chemical synthesis with peptides has successfully produced a 1024 array. Re-examination of what are generally considered to be well established theories are needed for extension of photophysics into new areas. For example, the theory of concentration quenching of fluorescence in 1, 2 and 3 dimensional media has been examined by SienickiS7 and the time dependence of decay and fluorescence quenching in a one dimensional lattice analysed by Dudkiewicz and TwardowskP8. A stimulating paper deals with a revision of the familiar, and widely used, monomer-excimer kinetics by treating such systems as examples of reactions with time dependent rate c o n s t m P . The simple mathematical formulations usually employed in systems where excimers are involved are shown to be inadequate. No doubt future efforts will be directed to rectifying the situation, Strong transient effects arising from nonstationiiry diffusion which occur during excimer formation through reactions with time dependent rate coefficients have been used as a scheme to test different models used in convolution kineticsa. Time dependent excimer

8

Photochemistry

formation rate constants have also been considered in another detailed study of the reversible monomer-excimer kinetics". A mathematical model for the measurement of pseudo first order rate constants in laser flash photolysis has been put forward62. Another data treatment provides a method for determining quantum yields of reactions of the type A(+hv) = B(+hv,A) where only the spectrum of A is known, provides an extension of an earlier proposal for the analysis of such systems by Fischefi3. A chemical actinometer for light in the long wavelength range 670 to 795 nm iodide sensitized photooxygenation of uses 1,1',3,3,3',3'-hexaethylindotricarbocyanine mesodiphenylhelianthrene in CHC1364. Benzophenone-benzhydrol functions as an actinometric system applicable to practical reactors where there is complex dilution during operation65. This is similar to the situation which applies in a typical preparative organic photoreactor system. At 254 nm the photolysis of aqueous solutions potassium peroxydisulphate in the presence of t-butanol is recommended as a simple and convenient actinonieter system66. Colorimetric characterization of Magarai QS label (a commercial y-irradiation detector) can be conveniently used also for measurement of the intensity of UV radiation at levels such as those encountered under industrial condition^^^. Recommendations o n experimental methods for the determination and the compilation of molar absorption coefficients for transient species in solution have been proposed by Bonneau, Carmichael, and Hug68. Note should be taken of the proposals put forward. Structural transformations in N-isopropylcarbazole crystals have been monitored by pressure induced luminescence. The effect observed is related to the general phenomenon of piezol~minescence~9. Chemical and biological microstructures have been probed by means of arrays of excitable donor and acceptors whose spacing are measured by means of energy transfer70. The structures can be determined from the measured spread of separation distances. Intracellular sensitization of fluorescence has been applied to biological systems which are studied by a combination of microfluorimetry and fluorescence spectroscopy7'. A topical area of research i n chemical kinetics is the detailed study of chemical oscil1ators.A recent contribution from photochemistry is the study involving the wavelength dependent photoinhibition of oscillations in the reactions of phenol and aniline substrates72.

I: Photophysical Processes in Condensed Phases

9

A useful suggestion for general photochemistry is the addition of pieces of solid 0, to remove O,,which acts as a fluorescence quencher, from small samples

of solution73. Two more papers in their unique and extensive series on industrial photochemistry have been produced by the Nancy group. One is a Monte Carlo The other deals with modelling of light curing as applied to ph~tolithography~~. macroscopic transport effects on the performance of photochemicalreact01-s~~. A multi-author monograph on the chemistry of isolated species contains a number of articles of photochemical interest76. 2. Singlet State Processes Study of the photochemistry of 1,l-dichloroethanein xenon matrices shows that the nature of the environment affects crossing of potential energy surfaces and also hydrogen bonding geonietrie~~~. Out of plane deformation of the molecules in the S, state strongly influences the rate of internal conversion of s-trans-butadiene.

This accounts for the absence of fluorescencein this molecule78. Carotenoids are still highly topical systems for research. Both S, + So and S, + So electronic relaxation process in carotenoids with 7 to 11 conjugated double bonds have been subjected to very comprehensive study7!'. The lifetime of the S, state of P-carotene in CS,, meosured by a femtosecond absorption method, is found to

be 200-250 fs at room temperaturego. Fs time resolved CARS from p-carotene in solution shows the occurrence of ultra-high frequency (1 1THz) beating phenomena and sub-ps vibrational relaxation". The same technique has been used to observe solvent effects on the ag C=C stretching mode in the 2'A, excited state of gcarotene and two derivatives8*. A similar study has been made with several derivatives of carotene83. Buckminsterfullerene (C,,) has not escaped the attention of photochemists. , films has made some Determination of luminescence and absorption spectra of C excited state assignments possible for this moleculeg4. Some work has also been reported on the triplet state properties. Benzene still provides fruitful labour for the dedicated photochemist. A three photon excitation study of the S,(A,,) - S,(B,,) transition in the neat liquid shows that under this condition internal states exist which make the experimentally observed 3 photon allowed pathway85. A photochemical study of both benzene and pyridine shows that Dewar benzene structure formation involves S,-S, state mixing by irradiation at 253.7 nmN6. For pyridine in a xenon matrix all photochemical reactions are quenched and evidence for the formation triplet state of this molecule is produced for the first time. The photophysical significanceof PE surface geometries of the low

10

Ph otochemistry

lying singlet states of benzene, pyridine, and pyrazine has been examined in an ab initio studyS7. The S, state of benzene is the source of prefulvenic forms and the S, state is also involved in possible isomerization processes and Channel 3. Theoretical calculations on the nature of solvent effects which affect the n-x* blue shifts for pyrimidine, pyridazine, and pyrazine have been compared with the results of experimental observationss. A theoretical study of electronic spectra and photophysics of uracil derivativess9, the luminescence of 4-phenylpyridinegOand 1,4-bi~(heteroaryl)-l,3-butadiynes~~ are other reports on single ring aromatics. The application of two photon spectroscopy to antiaromatic molecules is exemplified by a study of biphenylene in which both 2A, and 2B,, states have been characteri~edg~. Vibrational analysis of fluorescence spectra of p-terphenyl cry~tals9~ and excimer formation kinetics in liquid crystalline alkylcyanobiphenyl~~~ are other spectroscopic studies. The technique for determining the total luminescence spectra of aromatic hydrocarbons in n-alkanes has been comprehensively describedg5. Absorption and emission properties have been examined for styrylstyrene and distyrylanthracene derivativesg6. The effect of high pressures (up to SGPa) on the crystal field induces mixing of exciton states in naphthalene crystals97. Time resolved fluorescence spectra of naphthalene doped in amorphous silica glasses shows the presence of the ground state dimer gives rise to a unique luminescent e ~ c i m e r ~ ~ . The effects of pressure on the natural radiative lifetimes of the S, states of five anthracene derivatives in solution show a dependence on the refractive index of the solvent. It is found that the solvent affected values do not relate to the value in gas phase through the relationship which is generally acceptedg9. This paper is of particular interest since it examines one of the widely accepted tenets of the photophysical discipline. The fluorescence spectra from 9,lO-dichloroanthracene in argon mamces have been compared with corresponding data from jet generated clusters1O0. Microscopic solvation of this molecule in heteroclusters with atoms of Ar, Kr, and Xe has been examined by observing the effect of these environments on the measured radiative lifetimesIo1. Derivatives of benz[a]anthracene metabolites can be detected by laser excited Shpol'skii spectrometrylo2. Anthracene and some derivatives have been used for both ns and ps timescale studies of probe molecule reorientation in o-terphenyl crystals103. The photophysics of ten substituted bis[2-(9-anthryl)ethyl] glutarates have also been reported'". The S, state of acenaphthylene emits fluorescence with a So transition lifetime of 40 ps in undergoing a S, + So transition whilst the S, occurs mainly through nonradiative internal conversion105. The electronic states of aceanthrylene have been analysed both experimentally and theoretically106. 2-

I: Photophysical Processes in Condensed Phases

11

Methylaceanthrylene shows a weak emission at about 410 nm which probably arises from an unassigned upper excited state. Pyrene excimer formation still continues to be of interest and importance as a model compound for various types of study. Recent re-examinationsof the kinetics have been referred to in the previous section. A non a priori analysis of experimentally determined fluorescence decay surfaces has been applied to the examination of intermolecular pyrene excimer formation107. The Kramers equation has been successfully applied to the formation of intermolecularexcimer states of 1,3di( 1-pyrenyl) propanelog. Measured fluorescence lifetimes fit the predictions of the Kramer equation very well. The concentration dependence of transient effects in monomer-excimer kinetics of pyrene and methyl 4-(1-pyrenebutyrate)in toluene and cyclohexane have also been studiedIw. Pyrene excimer formation in polypeptides carrying 2-pyrenyl groups in a-helices has been observed by means of circular polarized fluorescencellO. Another probe study of pyrene excimer has been employed in the investigation of multicomponent recombination of germinate pairs and the effect on the form of Stern-Volmerplots111. Pump-probe measurements using fluorescence show the formation of van der Wads dimers of fluorene in supersonic jets112. A single photon counting method has been used to observe intramolecular excimer formation dynamics in the case of bis(9-fluorenyl)methane113. Halogenated tetracene derivative photophysics114 and low temperature emission spectra of trapped tetracene pairs formed by the dissociation of ditetracene1I5have both been elucidated. Acridine in anthracene matrices has been shown by fluorescence to form dimers and to have I(n,n*) and ~ ( X , Z * )monomer states which are nearly degenerate116. Properties of 2,7-dimethyl carbazolates in various environments have been studied by both steady state and time resolved techniques117. The photophysics of aminophthalimides in solution118and temperature and acidity effects on solvent induced changes in N-alkylphthalimide~ll~ are properties of related systems which have also been investigated. The ground and electronicallyexcited states of o-hydroxybenzaldehydeand its non-hydrogen bounded photorotamer have been characterized in rare gas mamces at 12K12O. Absorption and fluorescence spectra and excited state lifetimes have been determined for S states of hydroxy- and amino-substituted naphthaquinones and anthraquinoneslzl. The absorption and emission spectroscopy as well as the photochemistry, of the important group of 1,lO-anthraquinones and its derivatives have recently been reviewed122. The fluorescence spectra of silyl and germyl ketones show that a dual fluorescence arises from both the S, + So and S, + S o transitionslZ3. Investigation

of the photorotamerization of methyl salicylate and related compounds in cryogenic

12

Photochemistry

matrices followed by luminescence indicates that emission arises from an excited state formed by intramolecular proton transferl24. Other related photophysical studies to be noted include solvation dynamics of 6-methoxyq~inoline~~~, conformational equilibria of trans-2-styrylquinoline'i6,solvation dynamics of 4-dicyanomethylene1,2,3,4-tetrahydromethylquinolinein alcoh0lsl2~,1-benzyloxy-2-pyridine and related compoundsI28, 4 - (1, 2, 4 - triazol - 1 - yl) - pyrimidin - 2 (1 H) ones129,complexation ~ ~ , relaxation effects influences on the electronic relaxation of excited a ~ r i d i n e ' solvent on 7-hydro~yflavonel3~,ps studies on 7-diethylamine-4-methyl c o ~ m a r i n l ~ ~ , solvation effects on biological active furocoumarin derivatives133, 6metho~yquinoline~~~, solvent effects on l-pyra~inyl-2-(4-quinolinyl)ethylene~~~, benzooxalolinone and related protropic low lying singlet states of a,w dithienylp~lyenes~~~, the concentration dependence of the luminescence of Pt" (4,7diphenyl- 1, 10-phenanthroline)(CN), indicating excimer formation138, and 1:1 van der Waals complexes formed by the S, state of ~ a n t h i o n e l ~ ~ . Indole photochemistry retains its attraction for research because of its supposed relevance to the biophysical properties of proteins. An interesting paper shows that the effect of temperature and viscosity on the excited states of indoles in The fluorescence polar solvents arises from vibronic mixing of 'Laand 'L, decay behaviour of indole in water has also been reported141. Simulations of the molecular dynamics of several indoles and tryptophan indicate details of processes which are involved142. 7-Azaindole in highly acidic and basic media has also been studied143. To approximate more closely to biological conditions suitable model compounds need to be examined. An example of one such molecule is 3-carboxy1,2,3,4-tetrahydro-2-carbolinewhich is rotationally c ~ n s t r a i n e d l ~ ~Two . decay processes in tryptophan derivatives which have been experimentally established are assigned to the involvement of different conformers145. The full power of photophysical techniques in examining molecular dynamics is demonstrated by the exemplary investigation which has been made of the anisotropy decays of indole, melittin monomer and tetramer by frequency domain fluorometry and the application of multiwavelength global The vibrational spectra of porphine embedded in an n-hexane-d,, Shpol'skii matrix have been published'47 and a ps absorption study of zinc octaethyl porphin n-monoanion shows that deactivation of the lowest excited state occurs with a lifetime of about 135 Other papers of some biochemical relevance cover the proximity effect on the photophysical behaviour of n ~ r h a r m a n e ' ~and ~ the quenching of carbazole fluorescence by tropanoic alkaloids through hydrogen bond formationI50.

I: Photophysical Processes in Condensed Phases

13

Fluorescence studies on compounds with unpaired electrons include investigations on some-anthraquinone-silyl radicals151 and a number of naphthaquinodimethanebiradi~als'~~. Proton transfer processes are specially important excited state properties, and several detailed time resolved studies have been reported. Time resolved fluorescence studies of excited 1-naphthol-3,6-disulphonateshows there is geminate recombination by proton transfer153. Another detailed study is the examination of proton transfer and solvent polarization dynamics in 3-hydro~yflavoneI~~. The dynamics of proton transfer using a geminate dissociation and recombination model has also been investigated with 8-hydroxypyrene-1,3,6-t1isulphonate~~~ and also with 2-naphthol, 2-methoxynaphthol, and 17P-dihydro~oequilenin~~~.Transient absorption and two step laser induced fluorescence has been used to examine excited proton transfer and relaxation processes in 3-naphthyl-8-hydro~ychromones~~~ and stimulated proton transfer effects in three chromonesI5*. Other topics which have been covered this year include the double exponential decay in the protonation equilibrium of 4-methoxy-N-[2-(l-pyrrolidine)ethyl]-1,8naphthalimide159, intramolecular proton transfer in 1-(acylamino)anthraquinones160, dual fluorescence of 2-(2'-pyridyl)ben~inidazole~~~, fs and ps studies on 2-(2'hydroxy-5-methylphenyl)benzotriazole in liquid and polymer environments show the presence of two ground states and keto-end tautomerization in the excited state162, intramolecular proton transfer in internal hydrogen bonded benzoxazole derivati~esl~~, excited state proton transfer reactions of "double" b e n z o x a ~ o l e s the ~~~, use of photoinduced intramolecular proton transfer to study dynamics of conformational changes in flexible molecules such as 2-hydroxy-1-(Nmorpholinomethyl)naphthalene~65,hydrogen bonding and phototautomerism of 3methyllumichrome166, and 248 nm laser flash photoprotonation of benzene derivatives to form cyclohexadienyl cationsl67. Cyclodextnn complexation also affects excited state proton transfer reactions16*. A theoretical analysis based on molecular orbital calculations has been made on intramolecular proton transfer processes in both singlet and triplet states of 3-hydro~yflavone~~~. 2.1 p 1 Quenching of anthracene fluorescence by iodide ions involves electron transfer. One study of this process in ethanol/propanol solutions has employed a Marcus type theory170 whilst another using methanol/ethanol media selects the Onuchic equation to take account of both electron transfer and diffusion effects. Oxygen quenchingI7' of the fluorescence of anthracene derivatives under hydrostatic pressures up to 700 MPa gives an insight into the role of complex formation in the

14

Photochemistry

interaction involved172. Exciplexes with anthracene have been shown to form with acetonitrile by comparison with the properties of anthracene carbon it rile^^^^. Insight into quenching by amino groups has been obtained by using lupinine, an aminoalcohol which involves intramolecular interactions through hydrogen bonding in the excited state174. Intramolecular CT transfer and thermal dissociation of the exciplex of p-aminobenzonitrile in toluene175and a study of dipole moments of planar and twisted nitroaniline analogues in ground and excited states176provide data on two molecules which serve as archetypal molecules. Fs-ps laser photolysis studies on excited state CT complexes have been made on other selected donor-acceptor systems in acetonit~ilel~~. Crossing from excited van der Waals states to exciplexes at low excess excitation energies is a process conveniently examined in supersonic molecular beamsl78. The effects of molecular structure and temperature dependence of radiative rates in twisted intramolecular CT and exciplex states have been investigated in some detail for 17 There have been two reviews of photoinduced electron transfer. The subject is dealt with generally in oneI8O whilst ultrafast photochemical charge transfer and excited state solvation are considered specifically in the otherI8l. Photoinduced electron transfer is a subject characterised, particularly at the present time, by papers with a strongly theoretical content. Solvent relaxation and electron back transfer following photoinduced electron transfer in an ensemble of randomly distributed donors and acceptors182,germinate recombination and spatial diffusion1s3, a comparison of theoretical models for forward and back electron transfer184,rate of translational modes on dynamic solvent effects185,forward and reverse transfer in nonadiabatic systernslp6,and a theory of photoinduced twisting dynamics in polar solvents has been applied to the archetypal dimethylaminobenzonimle in propanol at low ternperature~l~~ have all been subjects of very detailed study. The last system cited provides an extended model for dual fluorescence in which the effect of the time dependence of the solvent response is taken into account. The mechanism photochemical initiation of reactions involving electron transfer, with particular reference to biological systems, has been discussed by CusanovichIss. A particularly interesting contribution to the study of electron transfer is described in the published version of a lecture by VerhoevenIs9. This discusses electron transport through saturated hydrocarbon bridges and a resulting "exciplex" emission from flexible, rigid, and semiflexible bichromophores. A considerable number of papers deal with the details of specific systems. Only a selection of these can be mentioned; these are ground state anion formation and ps excitation dynamics of 3-hydroxyflavone in formamidelgOstyrylphenanthrene-

I: Photophysical Processes in Condensed Phases

15

amine systems in a~etonitrile~~l, self quenching in tertiary amines involving "non emissive" excimers192, stilbene-amine exciplexes193, rigidly linked anilinenaphthalene donor-acceptor s y ~ t e m s ~ 9 2-naphthol-amine ~, hydrogen bonded s y s t e m ~ l ~binary ~ J ~ ~and , ternary amine-anthracene system~19~, and indole-acrylamide interactionl98. Physicochemical aspects of electron transfer effects are closely analysed in a number of papers. Detailed examination of experimental Rehn-Weller plots for fluorescence quenching by electron transfer in acetonitrilelw and development of a model for bianthryl and related moleculesZmare both timely investigations. The free energy dependence of the rate constants of electron transfer for quenching of transstilbene fluorescence by electron deficient olefins has been studied by Angel and PetersZo1using ps spectroscopy. Ps optical calorimetry has been used to examine the solvent dependence of the twisted excited state energy of tetraphenylethylene evidence adduced for formation of a zwitterionic stateZo2. Japanese workers have been particularly involved in the deployment of fs-ps laser techniques to the detailed examination of photoproduced CT complexes. These investigations include work on 1,2,4,5-tetracyano-benzene-aromatk hydrocarbon complexes203J", p-phenylenedione in several alcohols205,a resonance CARS study of a long-lived species (cation?) generated by monophotonic excitation in acetone at 337 nm206,and ps dynamics of contact ion pairs from anthracene or bromoanthracene, and tetranitromethane207. Jet studies are useful complements to this type of experiment as exemplified by the examination of solvation effects in 1-cyanonaphthaleneZo8.Direct observation of the generation of electron-cation geminate pairs in nonpolar solvents as function of the excitation wavelength and measurement of the dependence of the subsequent thermalization is a considerable experimental achievement2". Another notable publication deals with fs timescale observation of intermolecular electron transfer in a diffusionless and weakly polar system for the dye nile blue together with either aniline or N,N-dirnethylaniline210. The ultrafast transfer rate process is nonexponential. Other papers deal with CT complex formation by disubstituted anthraquinones with aromatic hydn>carbonsZ1l,electron transfer in compounds with piperazine moieties212, nonradiative deactivation of p-(N,N-dialky1amine)benzylidene malonitrile2~3, styrene-amine exciplexes214, intramolecular CT fluorescence of aromatic amideP5, donor and acceptor substituted linear polyenes216, and examination of the energy gap dependence on the charge recombination process of ion pairs produced by excitation of 2,6,9,1O-tetracyano-anthraceneand methylsubstituted benzene CT complexes in acetonimle2l;. A study has been made of both naphthalene/trimethylamine exciplex and pyrene excimer formation in supercritical CO, and ethylene fluids2I8. This is a most

Photochemistry

16

unusual situation of considerable physical interest since the continuity of gaseous and liquid phases gives insite into specific effects of intermolecular interaction. Electron transfer effects in porphyrins are of biological relevance for a number of reasons, not least in the understanding of mechanisms involved in photodynamic therapy. Studies reported include fluorescence quenching of. porphyrins by oxidants such as p-benzoquinoneZ19,photoinduced electron transfer of porphyrin-acceptor molecules in solid statezz0, and ps experiments on quinone substituted monornetallic porphyrin dimer which show evidence for super-exchange mediated electron transfer in photosynthetic systemsz2'. Heavy atom effects on excited singlet state electron transfer reactions have also been analysed in some depthz22. Exciplex emission and photofragmentation reactions of contact ion pairs generated via quenching of cyanoaromatic singlet states by aminoalcohols is an example of a detailed study of an electron transfer reaction involving chemical change223. The effects of amine substitution on the photophysics and photochemistry of

2-(2'-hydroxypheny1)benzothiazoles must also involve electron transfer processeszz4. Salt-base equilibria and the effects this has on the fluorescence of dipynylmethenes in the presence of triethylamine have also been measured225. Photoemission from excited single states produced by photoionization of anthracene crystals occurs after two step laser e~citation226,z2~.Biphotonic excitation

of phenanthrene under 208 nm irradiation is a complex process involving both ionization and T-T annihilation228.Change transfer exciton band structures have been characterized with samples of crystalline tetra~ene~~9.Measurement of the photoionization efficiency in trans-stilbene crystals as a function of excitation energy shows that ionization occurs after rapid vibronic rela~ation23~. Multiphotonic photolysis of perylene and pyrene in liquid cyclohexane shows that ArCy+ ion pairs are involved23t. This piece of work is very relevant to understanding reactions which are involved in the allied discipline of radiation chemistry.

2.2 Experimental problems associated with the measurement of fluorescence yields of strongly absorbing fluorophores, typified by dyes, have been fully discussed232. The effects of both the strong absorption at high concentrations in changing the activated volume and self absorption of emission have been evaluated as a function of exciting wavelength and methods for the accurate determination of yields analyzed. Dyes are very convenient in their various properties for use as model

I: Photophysical Processes in Condensed Phases

17

compounds in photophysics. A typical example, coumarin-153 has been used to study static and dynamic electrolyte effects on the solvation of large excited dipoles in high dielectric constant solvents233. A large Stokes shift in the fluorescence spectrum of this dye produced by addition of LiClO, indicates that a new relaxation process is induced by the ionic atmosphere which is created by the presence of the electrolyte. The various ultrafast reaction techniques have inevitably been used in a number of the reported investigationson dyes. A study of the saturation absorption dynamics of a cyanovinyldiethylamine dye has yielded a measured lifetime of 3 f 1 ps, a value which is determined by very rapid internal conversion of the S, state234. The rapid decay of absorption of excited states on the ps time scale has been measured for pyrazolotriazole azomethine dyes235. The molecular orientation dynamics of polar dye probes in t-butanol-watermixtures have been determined by ps fluorescence depolarization spectroscopy236. Dyes studied in this investigation were the monocations nile blue and thionine, resorufin a monoanion, and nile red a polar but neutral molecule. A very detailed ps study of rotational diffusion of excited states of merocyanine-540in polar solvents, has also been reported237. Meech and Y0shihara23~ have used the time resolved 2nd harmonic generation technique they have developed to measure the isomerization of a number of adsorbed dyes occumng on the ps time scale. This is considered by the authors to provide a satisfactorytest of reliability of this particular technique for obtaining such data. The dynamics of Frenkel excitons in disordered molecular aggregates of pseudoisocyanine bromide and iodide have also been examined by fluorescence techniques239. Super-radiant emission and optical dephasing in J aggregates occurs with pseudoisocyanine bromide240. Also excitation energy transfer between J aggregates of cyanine dyes takes place when the aggregates are arranged in layer Dramatic changes in fluorescenceefficiency are observed for the laser dyes coumarin C6F and CI when coincluded with a variety of organic solvents in cy clodexmn~ 2 4 2 . Fluorimeuy has been used to measure various equilibrium constants and partitition coefficients for some pyronine dyes in organic solvent-water mixtures243. Photoinduced electron transfer involving monomers and dimers of triarylmethane dyes bound to polyelectrolytes is now also an established effect244. Ground state complexation of methylene blue with purine nucleotides is involved in electron transfer quenching of this dye245. Optical dephasing in organic glassy systems containing dyes has been studied by the hole burning method246.A correlation of this effect with excited state lifetimes of a free base porphin, dimethyl s-tetrazine, resorufin, and cresyl violet in glasses and polymer was established.

18

Photochemistry

Dyes are widely used as sensitizers and this provides the reason for a number of mechanistic studies. An example of such a study is the examination of the effect of pH on the photosensitising ability of e o ~ i n ~ ~ ~ . The photochemical stabilization of laser dye 7-amino-4-methylcoumarin (C120)by 1,4-diazobicyclo[2.2.2]octanearises from quenching of singlet oxygen generated in the systemz4*.This could be a valuable procedure. The rhodamine group of dyes have been especially fruitful subjects for research during the year. Molecular structure and solvent effects on the photophysics The influence of of rhodamines B, 3B,6G,and 19 have all been reported in structure on the lasing properties of rhodamines has also been examined by the same g r ~ ~ p zA~ correlation ~. of solvent structure (water-ethanol mixtures) with the photophysical properties of rhodamine B in acidic, basic, and ester forms has also been established251. Solvent polarity affects the rotational isomerization mechanism of rhodamine B in normal alc0hols2~~.Solvent polarity and viscosity both affect nonradiative processes. Very short time scale spectral diffusion has been observed in a low temperature glass by making a comparison of ps photon and stimulated echoes for rhodamine in PMMA253. Solute-solvent relaxation effects of electronically excited rhcdamines 3B and 101, as well as pyronine B chloride have been related to the results of theoretical predictionszs4. Ns time scale optical inhomogeneous broadening of spectra of rhodamine B in glycerol and propylene glycol at or near room temperature shows that the So-S, transition involves electronic excitation transfer255. A very detailed study of singlet excited states of rhodamines R19 and

R6G with monoethylamino groups in water-ethanol mixtures has also been publishedz56. The absorption and fluorescence spectra of rhodamine B molecules encapsulated in silica gel networks and the resulting changes in their thermal stability have also been studiedz5'. Aggregates of rhodamine 6G in aqueous surfactant solutions have also been characterized by photophysical methodsz58. The fluorescence properties of DCM, 4-cyanomethylene-2-methyl-6-pdimethyl aminostyryl-4H pyran, show that there is thermal equilibrium between the cis- and trans-isomers. There is no observable aggregation of this dye except in liquid membraneszsg. Polarized absorption and emission spectra of stilbazolium rnemyanines260 and the properties of pyrylium and thiopyrylium high efficiency laser dyesZ6lare topics covered other related publications. Cresyl violet in ethanol at 1.3K has been used to probe low temperature glass dynamics by the fast generation and detection of optical holesz62. Relaxation processes of the 3,3'-diethyloxadicarbocyanine iodide (DODCI) photoisomer have been characterised spectroscopically263.

I: Photophysical Processes in Condensed Phases

19

The spectroscopic behaviour of malachite green in sol-gel glasses depends on both the mode of preparation and ionic state of the dye264.A time resolved saturation absorption recovery in malachite green doped xerogel (SiO,/ZdI, mamx) has been made with loofs pulses265.Spectroscopicand redox properties of both the singlet and triplet states of cresyl violet have been examined%. A number of reports on phthalocyanines and porphyrins have been published. Spectral diffusion and thermal recovery of spectral holes burnt into phthalocyanine doped Shpol’skii systems has been examinedM7. An absorption, emission, and thermal lensing research on carboxylated zinc phthalocyanine shows the influence of dimerization on these propertiesz8. Fourier transformation of fluorescence and phosphorescence spectra of porphine in rare gas matrices has yielded much structural and electronic state data on this compoundM9. Exciton splitting is an effect which is A ps fluorescence study of the seen in the spectra of covalently linked porphyrin~~~~. semirigid zinc porphyrin-viologen dyad has provided evidence for two dyad ~onformers2~1. Spectral diffusion in organic glasses has been measured by observing the hole recovery kinetics over the time scale of 1 to 500 ms for zinc tetrabenzoporphyrinin PMMA272. The solvation dynamics of N-methylamides with experimental time resolution over the range up to 800 ps has been made by exploitation of the dynamic Stokes shifts of PRODAN and the dye coumarin-102z73.

. . 2.3 Jsomenzatimnd r e b d Drocessa Few reactions can have been studied in both extent and depth as the photochemical trans-cis isomerization of stilbene. An up to date review on the subject prepared by W a l d e ~ is k ~helpful ~ ~ and comprehensive. Hochstrasser and his groupus have made an impressive femtosecond laser study of energy dispersion in the solution phase isomerization of stilbene. The measurement of energy disposed shows that 1 ps is required for the cis-isomer to cross the barrier in contrast with the much longer times of 60 to 200 ps needed for rearrangement of the trans form. Equally detailed is a fluorescence upconversion study of cis-stilbene isomerization reported by Fleming and collaborator~~~6. In a variety of solvents they find that the isomerization occurs in less than one ps. The 2,2-dideutero-cis-stilbenecompound was investigated also. To assign the precise role of dihydrophenanthrenein the isomerization of stilbene measurements have made on the properties of 1,Zdiphenylcycloalkenes in a supersonic jet277.Some participation of this species as an intermediate seems to be established and the mechanism has been examined further by additional experimental and theoretical studies on the excited state dynamics of 1,2-diphenyl~ycloalkenes~~~.

20

Photochemistry

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene2’9. The efficiency of silver ions in this respect is compared with the effect of NaI which can only induce a heavy atom effect. Ag’ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives280. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals281. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-ben~-2,2-diquinoy12~~. The connection between alternative modes of isomerization and the shape of potential surfaces underlies understanding of the highly selective nature of the cistrans photoisomerization process of 1-pyrenylethylene283. Another specific example of the unique behaviour of an excited cis singlet state is the one way isomerization of 2-(3,3-dimethyl-l-b~tenyl)pyrene~~~. The shape of the PE surface involved has been examined from a study of the details of the fluorescence spectra. The photoisomerization dynamics of diphenylbutadiene in both liquid and solid alkane environments has been analyzed in paperzss which is one contribution to the complete journal issue on the role of solvents in liquid state reactions. Several related topics of photophysical interest are discussed in this collection of papers. Intramolecular heavy atom effects influence the photoisomerization The homogeneous acid catalysis derivatives of 5,5-diphenyl- 1,3-~yclohexadiene2~~ of the photoisomerization of trans-3-(2-hydroxy-benzylidene)-4,5-dihydrofuran2 ( 3 H ) - 0 n e ~and ~ ~ model mechanisms for isomerization of carbocyanines have both been analyzed2xx. The process of photoisomerization of the biologically important rhodopsin and bacteriorhodopsin has been examined by a theoretical ab initio study of retinal analogues289. 2.4 Electronic excitation energv transfer Radiative transfer is an unfortunate complication in many electronic energy transfer experiments and it is difficult either to eliminate or make satisfactory allowance for this effect. Martinho and d’Olveira29O have studied in detail the influence of radiative transport on observations of electronic excitation energy transfer. In particular they have analyzed the effects of radiative transport on measured fluorescence decay curves for concentrated solutions. An experimental study of the influence of radiative transport on energy transfer from excited fluorene to pyrene it occurs n-hexane relates closely with this work29*. Kawski et have

I: Photophysical Processes in Condensed Phases

21

analyzed the detailed mechanism of nonradiative electronic excitation energy transport in two component systems. Systems other than homogeneous solutions involving energy transfer are now of more than considerable interest. A special issue of Chemical Phvsics includes fifteen papers, most of which are of photochemical interest, dealing with energy transfer and relaxation processes in low dimensional systems*g3. In this compilation Dewey294discusses excitation transport in fractal aggregates. Energy transfer in solid solutions and on fractal polymer surfaces has been studied by Kost and Breuer2g5. Studies on polymer membrane films show these to be materials which display a vaiable apparent dimensionality2%. An analysis of donor fluorescence profiles by Sienicki*V for fluorophores dispersed in Langmuir-Blodgett multilayers which have asymmetric forward and reverse transfer rates and energy migration provides a challenge for a future experimental study. An example of a one dimensional singlet energy migration system is provided by columnar liquid crystals of a mphenylene derivative298. Nonradiative energy transfer with the simultaneous involvement of different mechanisms has been modelled by Rotman299 for solid state systems. Although his treatment is particularly directed towards inorganic systems it is generally relevant to organic systems also. The complex kinetics of sequential energy processes involving four species each with a t'I2dependence of rate coefficients has also been analyzed300. Simulated coherent energy transfer in a hydrogen bonded amide chain arising from Fermi resonance has been modelled by Clarke and Collins301. This interesting study is related to the Davydov soliton model which has been proposed for explaining energy transport in proteins. The role of similar nonlinear effects in simple organised chemical systems has yet to be established. Frequency domain fluorometry has been used to study end to end diffusion of flexible bichromophoric molecules by intramolecularenergy t r a n ~ f e r ~ ~ ~ l ~ ~ ~ . A particularly interesting paper deals with chiral discrimination in electronic energy transfer processes between dissymmetric metal complexes in solution3M. Time resolved luminescence measurements show that enantioselective excited state quenching occurs. Transfer of singlet electronic energy from cyclohexane to benzene and eventually to tetramethylphenylene diamine is a sequential process which has been studied in detai1305. An important detail reported in this work is that an upward revision of the benzene fluorescenceefficiency in cyclohexane to 0.26 f 0.02 appears to be required. Other experimental studies in this area are fluorescence quenching of excited perylene by Co2+ ions which occurs via energy transfer in viscous and nonviscous

22

Photochemistry

media306, migration modulated donor acceptor energy transfer in PMMA307,and a Forster energy transfer process in rhodamine-porphyrin mixture monitored by ODMR308. Ultrafast techniques have been used to observe energy transfer directly. For example, sub-picosecond time resolved intramolecular transfer examined in flexible bichromophoric coumarin molecules shows that exchange occurs within 1 to 20 ps depending polymethylene chain length3@. The distribution of interchromophoric distances in donor/acceptorcoumarin supermolecules has been measured analysis of data from time resolved energy transfer3*0. Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains31'. Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals312.Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals313. The dynamics of exciton trapping in P-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy314. Ps time resolved spectroscopy was the experimental technique used in this work. Energy transfer effects are of more than considerable interest in the area of biophysics. Examples of chemical interest from this field include a study of Forster energy transfer between dimethyldiazapero-pyrenium dication and ethidium intercalated in poly d(A-T)315.The R, value is 3.8 nm. Use of methylated guanine, which has a reasonably long fluorescent lifetime, as a luminescent probe in DNA has given the first experimental evidence of an electronic energy transfer along the double helix of a nucleic acid316. Long range electronic interactions in peptides have been established by the effect on excited state moieties of a remote heavy atom of bromine317. It is remarkable that the influence is effective even when the separation between tbe bromine atom and the excited state extends over 130 bonds. 2.5 Polvmeric svstems Supramolecular systems consist of suitably arranged molecular components and are becoming of increasing photochemical and photophysical interest. A survey of the area has been published by Balzani er a P 8 . More work in this field is certain

during the next few years. Polymers provide convenient media for controlling the behaviour of excited states. Examples involve studies of the differences in fluorescence decay characteristics of 9,9'-bianthryl in nonpolar and polar polymeric matrices at room

I : Photophysical Processes in Condensed Phases

23

temperature319and the concentration dependence of fading of the open form of 6 nitroindoleinospiropyran in a polymer matrix32O. Picosecond absorption studies of photoinduced charge separation in polyelectrolyte bound aromatic chromophores show that there is transfer of singlet excitation energy to methyl~iologen32~. Excited state dynamics have been measured in polysilane by ultrafast techniques322and site selective fluorescencestudies made with polysilylenes323. Polymer surfactant interaction has been examined by using sodium 2-(Ndcdecylamino)naphthalene-6-sulphonateas a probe3". Solute-solvent interaction of free base phthalocyanine has been examined in both polyethylene and polystyrene by the effect of pressure on spectroscopic hole burning3Z5. Fluorescence has been used to indicate the onset of aggregation in water soluble polymers326,the interaction of pyrenylmethylmbutylphosphonium bromide with single strand polynu~leotides~~~, and the interaction of indole compounds with synthetic polyele~trolytes~~~. Webber329has surveyed the photophysics of photon harvesting polymers. Electronic energy transfer and the role of intracoil excimer formation are aspects of the subject which are discussed in this review. Photoinduced processes and resonant third order nonlinearity in poly(3dodecylthiophene) has been studied by fs time resolved 4-wave mixing330. Similar work has been reported for the poly(p-phenylenevinylene) system331.Such materials have potential for the use as nonlinear optical switching devices. 11ie33~ has surveyed the properties and applications of photoresponsive polymers.

2.6 Colloidal and heteroeeneous systems This remains an active area although perhaps not as intensively so as in recent years in problems concerned with classical colloids. A basic problem which can be encountered in the application of photophysics to colloidal systems are difficulties involved in the measurement of true luminescence spectra and determination of luminescence quantum yields of molecules in light scattering media. Gade and Kaden333have produced a theory for this effect which can be used to take account of readsorption and re-emission effects in suspensions. A three dimensional extended dipole model which takes account of the interactions and alignment of molecules with carbazolyl chromophores in monolayer assemblies makes a useful contribution to the detailed understanding of the behaviour of layered A number of other interesting papers involve a nonlinear optical study of Frenkel excitons in LB films where there are J aggregates of pseudoisocyanine iodide335, an investigation of protonation equilibria and spectral

24

Photochemistry

properties of (aminostyry1)pyridiniumchromophore in solution, spread monolayers, and LB films336, and a ps time resolved fluorescence spectrum of the photochromic reaction of a spiropyran in LB films where there is interchange between spiropyran and merocyanir~e~~~. Fluorescence lifetimes of diphenylhexatriene in molecules located in both flat and bent bilayer liquid membranes show the effect of changes both in exposure to water and burial within the nonpolar membrane338.The effect of hydrostatic pressure on the system confirms the interpretation put forward to account for these effects. Photochemical electron transfer across surfactant bilayers has been shown to be mediated by the presence of 2,1,3-benzothiadiazole-4,7-dicarbonitrile339. Molecular assemblies in anionic environments influence the efficiency of fluorescence quenching by electron tran~fer3~0.Viseu and Costa341use a combination of steady state and time resolved fluorescence quenching data to evaluate partition coefficients of fluorescent molecules into micelles. The breakdown of rod-like micelles and light induced viscosity changes in micelles are effects that can both be induced by isomerization of azo-compounds in a variety of surf act ant^^^^. The fluorescence of p-toluidonaphthalene sulphonate (TNS) bound to triton X and sodium dodecyl sulphonate (SDS) has been used as a probe to study the effect of urea on m i ~ e l l e s ~The ~ ~presence . of urea increases the critical micellar concentration and also enhances the fluorescence efficiency by displacement of water from the region of the micelle. Polarized fluorescence emission measurements on TNS carried out on mixtures of 2-butoxyethanol, cetyltrimethylammonium bromide, and water has provided structural information on these systems344.Hydrophobic influences on both photophysical and photochemical effects, excimer fluorescence, and aggregate formation of long chain alkyl 4-(N,N-dimethylamine)benzoateshave been studied in water-organic solvent binary mixtures34s. Frequency domain spectroscopic studies of the effect of n-propanol on the behaviour of the fluorescent probe tetracene provide a measure the internal viscosity of SDS m i ~ e l l e s ~ Time ~ ~ . resolved fluorescence quenching of pyrene by N-hexadecylpyridinium chloride in mixed anionic micelles shows that this involves electron transfer interaction347. Aggregate numbers and micellar volumes can both be estimated by this effect. Protolytic photodissociation of hydroxyaromatic compounds in micelles and lipid bilayer membranes of vesicles is another photochemical which has been published during the year. Photophysical and photochemical studies recently reported describe the properties of rhodamine 6G in alcoholic and aqueous environment SDS m i ~ e l l e s ~ ~ 9 , psoralins dispersed in m i ~ e l l e s ~ fluorescence ~~, enhancement of tetrakis (sulphonatophenyl) porphyrin in homogeneous and micellar solution351,quenching of pyrene by dibutylaniline in interaction with double chained surfactants3s2, and a

I: Photophysical Processes in Condensed Phases

25

comparison of the kinetic and state behaviour of diphenylmethyl radicals in micellar solutions under the influence of magnetic fields353, Insight into the mode of anaesthetic action has been achieved by the examination of solubilization sites and acid-base forms of dibuccaine hydrochloride in neutral, anionic, and cationic micellar environments354. The latter offer some approximation to the structure of different possible sites for action of an anaesthetic in a living organism. Measurement of the influence of different micellar environments on proton transfer from excited states of 3-hydroxyflavone allows estimates to be made of micelle concentrationsfrom measurement of the tautomer emission yield355. Proton transfer reactions of benzimidazole excited singlet states have also been studied in ionic m i ~ e l l e s ~Magnetic ~~. fields are found to affect the behaviour of radicals generated by the photodissociation of benzil in micellar media357. The starburst dendrites which are formed by anionic macromolecules in interaction with both anionic and cationic surfactants have been examined by pyrene fluorescence358. Benzo[k]fluoranthrene fluorescence has served as a probe of the effects of metal salts on bile salt aggregati0n~~9.The incorporation and distribution of benzoquinone into liposomes containing amphilic Zn(I1) porphynn has been followed by its effect on the quenching of the excited state360. A comparison of the photochromism of spirobenzpyran derivatives in unilamellar surfactant vesicles and solvent cast surfactant films has also been reported361. The properties of reverse micelles are of considerable interest at the present time. Amongst photochemical studies reported in this area are the behaviour of indole alkanoic acids and tryptamine in sodium dioctyl s~ccinate~~*, fluorescence and phosphorescence studies of AOT/H,O/alkane systems using a variety of photoionization of alkylphenothiazine sulphonates in reversed m i ~ e l l e s ~and ~~, interfacial interaction of probes with AOT inverted m i c e l l e ~ ~ ~ ~ . Reverse micelles also form in supercritical fluids, as evidenced by changes in the fluorescence and absorption spectra of probes, in the two phase region366Fluorescence has been used by Kuykendall and Thomas to investigate the dispersion of both aqueous colloidal and pillared Solid surfaces are known to provide useful environments for carrying out photochemical reactions. An edited monograph on the subject has recently been p~blished3~9. Pyrene is a frequently used probe in photophysical studies of solid state surfaces. A report on the time resolved fluorescence spectra of pyrene adsorbed on calcinated Vycor glass s ~ r f a c e s 3is ~ ~one such study. Quenching of pyrene fluorescence by 0,, CH3N0,, and nitropropionic acid has been used to distinguish

26

Photochemistry

diffusional from nondiffusional behaviour in silica gel, and at liquid-solid and vacuum-solid interfa~es3~1.Excimer fluorescence of pyrene in sol-gel and broadening of excimer emission in ps time resolved spectroscopy in amorphous silica glasses, where a ground state dimer is involved373,are other examples of studies using this method. The structure of alkylated silica surfaces has been examined by its effect on the quenching of the fluorescence of covalently bound pyrene moieties374. Fluorescence lifetimes of the pyrene and rates of quenching reveal differences in interfacial polarity and degree of contact of the pyrene molecules with solvent. The size of alcohol molecules is a determining factor in the formation of ternary complexes of these species with pyrene and P-~yclodextrin~~s.Fluorescence spectroscopy and NMR were the experimental techniques used in this work. The restriction photoisomerization of stilbene at liquid-solid interface provides an interesting illustration of a situation where the influence of adsorption and the influence on reaction are coupled effects376. Meech and Y ~ s h i h a r ahave ~ ~ ~used the surface second harmonic generation method to study ps dynamics at solid liquid interfaces by total internal reflection for the dyes, rhodamine and malachite green. The same technique has also been used to follow the photoreaction of rhodamine 6G in monolayers adsorbed on q u a r t ~ 3 ~ ~ . The spectroscopy and laser action of rhodamine 6G in doped aluminosilicate glass (xerogels) shows that dyes can be very stable under these condition~3~9. Generation and trapping of radical cations of a,w-diphenylpolyeneswithin the channels of pentad zeolites provides an environment which allows these transient species to be spectroscopicallycharacteri~ed~~~. Similarly, complexation of xanthone in cyclodextrin has made it possible for the triplet state of this molecule to be fully characteri~ed~~~. Association and dissociation processes are related to the dipoles developed in the complex and in solution. A unimodal Lorentzian lifetime distribution for 2-anilinonaphthalene-6-sulphonate B-cyclodextran inclusion complexes have been recovered by multifrequency phase-modulation fluorometry in the presence of the quenchers Cu2+,acrylamide, and I-382. Both the fluorescence and phosphorescence spectra of benzo[ffquinolineadsorbed on P-cyclodextrin/NaClhave been determined as a function of temperat~re3~3. An optical method for monitoring the concentration of general anaesthetics and other small organic molecules in biologically interesting systems has been based on phase sensing of a fluorescent hydrophobic probe384. Phase transitions in lipid membranes are detected by this method. The technique shows promise.

I: Photophysical Processes in Condensed Phases

27

3. Triplet State Procesw Two comprehensive compilations of triplet-triplet absorption spectra and related references have appeared r e ~ e n t l y ~ ~ ~ . ~ ~ ~ . Resonantly enhanced heavy atom effects in organic glasses have been described and considered as an effect on intersystem crossing387. The direct observation of radiative triplet-triplet annihilation from measurements of the spectra of delayed luminescence has been reported for aromatic compounds in liquid solutions by Nickel and K a r b a ~ h ~Triplet ~ ~ . excitation energies of cyclic enones have been determined by means of time resolved photoacoustic calorimeny3*9; m* mplet energies are found to correlate with excited state lifetimes. The relationship between mplet lifetime and the photoacoustic waveform is a function that is determined by probe beam deflection. This has been established for the specific case of quinoxaline in b e n ~ n e ~ 9A~ .technique has been described for the measurement of quantum yields of triplet formation in polymer matrices. This is a time resolved thermal lens method used for measuring the dependence of the excitation wavelength dependence Analysis of the of $ix on excitation wavelength for N-methyl-p-nitr~aniline~~~. diffusion and non-diffusion control of triplet-triplet annihilation in anisotropic crystals392 and the spectral diffusion of mplet excitation energy in molecular glasses and doped polymer solids3g3are of general interest. The use of multifrequency cross correlation phase and modulation phosphorometry, based on a technology usually associated with fluorescence lifetime determination, has also now been applied to the measurement and analysis of triplet state decay times394. Observations of the room temperature phosphorescence of polycyclic aromatic hydrocarbons in micelles, which stabilize the mplet states by reduction of quenching, indicate that measurement of phosphorescence lifetimes can be a useful analytical parameter395. Room temperature phosphorescence and delayed fluorescence have been shown to occur with triplet states of a wide variety of organic molecules in silica sol-gel glasses3M. The T, state energies of cycloheptene derivatives, 1-phenylcyclohepteneand 1,3-~ycloheptadienehave both been measured by photoacoustic calorimetry397. S, and T, states of P-carotene have been generated by direct photoexcitation from alltrans, 9-cis-, 13-cis-, and 15-cis-isomersand studied by ps transient absorption and time resolved Raman spectroscopy398. No isomerization occurs with the S, states and the intersystem crossing efficiency is only about 103. Ns absorption kinetics and calorimetric studies on cyclopentenone in cyclohexane have characterised the T,state of this molecule and measured the rate of self-quenching and characterized the predimerization state of the generated biradicals3w.

28

Photochemistry

The most important photophysical properties of the triplet state of c6, (buckminsterfullerene) in benzene have been determined to be Ey157f19kj mole-' (ES=193kjmole-') and zT =, 4 0 f 4 p ~ ~The ~ .triplet state of c 6 0 seems to be formed in near unit efficiency from the first excited singlet state. This could have an important environmental consequence for atmospheric '0, formation since c60 might prove to

be an important component in soot formed from incomplete combustion of hydrocarbons. The conformational instability of the lowest triplet state of the benzene nucleus is an intriguing problem that continues to be of interest. For the unsubstituted molecule it has recently been shown that the 3Blustate is conformationally unstable due to vibronic coupling with the 3EIustate which is the next higher member of the manifold401. The influence of substituents has been examined by a correlating study with the corresponding excited states p-xylene402. Low temperature tunnelling of triplet excitation energy of p-dibromobenzene occurs in crystals doped with pdichlorober~zene~~~ and the character of triplet aggregates in these crystals404have both been reported. Magnetic field effects on the luminescence of a liquid polyphenoxy polymer excited in the VUV region show that highly excited singlet states of aromatic chromophores can relax to form two triplets405. A spectroscopic and ODMR study has been reported on the triplet state of N-methyl-p-nitr~aniline~~. Phosphorescence emission from 2,6-diacetylpyridine demonstrates the role of the N-heteroatom and also the effect of hydrogen bonding on the photo physic^^^^. The triplet state of 2,2'-bipyridine in aqueous solution has been produced by flash p h o t o l y ~ i s ~and ~ ~triplet , states are established and characterised in the photophysics and photochemistry of 4-dihydropyridinones409. Triplet state and Z/E isomerization of p-styrylstilbene induced by various sensitizers410and the T, potential energy curves and 'one way' photoisomerization (c -+ t only) of styryl aromatics411are the subject of two papers on the behaviour of triplets of this class of compound. Time resolved resonance Raman spectra of the triplet state and radical cation of 5-dibenzosuberenol has been used to study and examine the mechanism of photoisomerization of this compound4*2. Upper triplet states of biphenyls in micelles ionize to produce hydrated electrons413.Diffuse reflective laser flash photolysis has been used to characterise the triplet states of p-terphenyl generated in powder systems4I4. Other triplet states which have been characterised include those of naphthalene and acenaphthene in the solid state415, those involved in photoreactions of tetracene with anthracene and 9bromoanthrene4I6, phenanthrene and biphenylene417, highly excited triplet states

I: Photophysical Processes in Condensed Phases

29

participating in reactions of substituted anthracenes in polymer films418,and the thermally accessible coronene dication tiplet4I9. Cai and Lim4m maintain the validity of the earlier disputed assignment of observed photophysics the triplet excimer of naphthalene by studies on a pinacol type dimer. A careful comparison of structure and environmental effects is made of the 3nn* - 3m* level inversion which occurs in the multisite phosphorescence of 2,penones has been found to be affected by solvation and aggregation421.The position of the nitrogen atom influences both the triplet lifetimes and yields of the 3nx*states of six dipyridyl ketones42*. Photoinduced hydrogen abstraction reactions of quinoline triplet isomers in durene single crystals proceed by processes involving t~nnelling4~~. Heavy atom effects on free and hydrogen bonded complexes of excited 1,2,3,4tetrahydro-quinoline have been studied4z4. The quinoxaline triplet has been used to probe the molecular environment crystalline o - t e r p h e n ~ l ~The ~ ~ .solvation dynamics of the quinoxaline triplet has been examined in supercooled liquids4%. Triplet state properties of 2,2’-biquinolineand derivatives have been studied by ODMR4” and rate constants measured for quenching of 1-acetonaphthoneby various olefins which are involved in photo-Diels-Alder1eactions~2~.The trans + cis photoisomerizationof 1pyrazinyl-2-(4‘-quinolinyl)ethylene evidently occurs through the efficiently formed mplet state429. The mechanism of the quenching of mplet benzophenenone by both electron and hydrogen donors involves change transfer effects430. Diffuse reflectance laser flash photolysis studies of the reactions of mplet benzophenone with hydrogen atom donors on surfaces have also been reported431. A dual phosphorescence of benzophenone at 77K in H,O/EtOH in glasses indicates that in this environment a longer lived water benmphenone triplet complex is formed as well as the familiar and well established short lived 3nn*state432.A laser photolysis study has been reported on the pressure induced viscosity dependence of the primary process of photoreaction of the benmphenone mplet in alcohol solution433.The observed increase in the rate of this reaction with pressure shows that Ink, has a linear dependence on pressure linear up to 150 MPa. Enolization of triplet 5,8-dimethyl-1-tetralone by hydrogen transfer effects434, spin-lattice relaxation of pentacen-6,13-quinonemplets in a benzoic acid host crystal involves proton tunnelling435, and photoreduction of benzophenone and thioxanthen$)-one by amines436 are other researches of photochemical significance. Interactions of formate ions with triplet states of benzophenone-4-carboxylateand -bsulphonate, 1,4-naphthoquinone, and anthraquinone-2-sulphonate have been examined systematicallyby laser flash p h o t ~ l y s i s ~ ~ ~ .

30

Photochemistry

Other mplet state characterization studies involve phenazine phosphorescence in alkane solvents in the glass transition range438, establishment of the cis-keto form of the mplet 2-(2'-hydroxyphenyl)benzothiazole in nonpolar glass439, 1,5diaminoanthraquinone440 and l-acetylamin~anthraquinone~~~ have been studied by laser flash photolysis in solution. The formation of the triplet state of oxazine and the consequent delayed fluorescence in acetonitrile has been sensitized by benzophenone442. The intersystem crossing efficiency of molecules with small S,-T, energy gaps has been examined in the case of several aromatic thiones in s0lution~~3. Reverse intersystem crossing effects are also considered in this work. Circular polarization of the phosphorescence of 2,P-enones is affected by mplet-triplet This observation provides evidence for nondistorted 3m*states in these molecules. Protonation kinetics of the triplet state of sapranine in hydroxylic solvents445 the influence of external iodine atoms on the intersystem crossing rate of a cyanine iodide ion ~ a i I . 4 and ~ ~ , the triplet yield of mesocyanine 540 in water showing a wavelength dependence due to aggregation and photois~merization~~~, are some investigations which have general implications for understanding mplet state behaviour. A measurement of qT of 0.25 for merocyanine 540 is related to the chemotherapeutic activity observed with cyanine dyes448. Ionic strength effects on the ground state complexation and triplet state electron transfer reactions between rose bengal and methylviologen have also been reported449. In SDS micelles the eosin mplets do not undergo aggregation in contrast with the behaviour which is described for rose bengaI45O. The triplet states of 7-aminocoumarin laser dyes have been studied by ns pulse r a d i ~ l y s i s ~ ~The ' . effects of temperature and pH on the quenching of triplet luminflavin in the presence and absence of ferro- and fenicyanide have been reported by Naman452. It is not surprising that the triplet states of porphyrins attract interest. A comprehensive study of some purpurins with SJIV) substitution involving observations of absorption, fluorescence and triplet spectra and '0, formation453and the triplet state of sapphyrin dication (a large porphyrin like system) shows an unusual spin alignment in the monomer and spin delocalization in d i m e r ~are ~~~ systems of photophysical interest. The influence of external parameters on time resolved transient hole burning in porphyrins shows the effects of a triplet state bottleneck in the rate of state build up455. Studies on heterogeneous systems include diffuse reflection laser photolysis of the geminate recombination kinetics of triplet radical pairs generated from 2,4,6trimethylphenol adsorbed on microcrystallinecellulose4s6,and laser flash photolysis

I: Photophysical Processes in Condensed Phases

31

generation of geminate recombination of mplet radical pairs in ketone-phenol cyclodextran inclusion complexes457. Heavy atom effects on the decay dynamics of triplet state ion pairs have been studied in chlwanil-naphthalene 1:2 termolecular systems458. The triplet exciplexes and radical pairs involved in geminate recombination which occurs with the methylene blue triplet and p-iodoaniline have been examined for spin-orbit coupling induced magnetic field effects459. Quenching of both the fluorescenceand phosphorescence of indole by copper ions occurs at distances exceeding 14A460. The effects of cupric and cuprous ions have been compared. The same authors observed the influence of Cu(II), Cu(I), Ag(I), and Cd(I1) at coordination sites on the phosphorescence which arises from the ~ ~ .to end intramolecular quenching of triplet state tryptophan-48 in a ~ u r i n ~End aromatic ketone triplets in aqueous solutions of 2,6-di-(o-methyl)cyclodextrins has also been studied462. A limited number of examples of research on systems exhibiting energy transfer involving triplet states have been reported. Triplet-triplet and singlet-singlet energy transfer studies between trans-stilbene and 7-aminocoumarin laser dyes show that coulombic interactions are involved for the singlet states whilst an exchange interaction mechanism operates in the case of the mplet statesM3. Triplet energy transfer through the internal molecular system having benzophenone and dibenz[b,fjazepine at the ends of a methylene chain depends upon the intervening chain length4a. The efficiency decreases by about one tenth per CH, unit. The rate of triplet transfer is less than for the corresponding singlet case, Energy transfer from the triplet state of 2-acetylnaphthalene to Eu3+ ions occurs in micellar solution465. One dimensional triplet energy migration occurs in columnar liquid crystals of octasubstitutedphthalocyanines466. The measured hopping times of 0.4 to 68 ps show p i than it is in the crystalline state. the energy migration is m ~ ~efficient An interesting development in the technique is the use of the transient thermal lens method to measure directly the quantum yields of energy transfer from higher excited mplet states467. 2-Acetylphenanthrene with biphenyl in acetonitrile was the system selected as the illustrative example. zero-field splitting of the T, state Infra red spectra of triplet phenylnitrene~~~~, in biradicals measured by magnetic field effects on fluorescence and triplettriplet fluorescence and spin polarization of 1- and 2-naphthylphenylcarbne~~~~ are experimental studies reported on biradical species. Triplet sensitization is the usual and most convenient means of generating singlet oxygen. Singlet oxygen is photogenerated from both ionized and unionized derivatives of mplet rose bengal and eosin Y in dilute solutions471.The efficiency of

32

Photochemistry

the processes leading to 10, generation by 2-terthienyl triplets have been measured by optical absorption, optoacoustic calorimetry, and infra red luminescence472. 1-HIndenylfuran and thiophene derivatives have been proposed as a new class of '0, sensitizers constituting a development based upon the widely used 2 - t e r t h i e n ~ l ~In~ ~ . an investigation of mplet photosensitized '0, generation in benzene solution a correlation is established between the efficiency of exchange energy transfer between the mplet state of the sensitizer and 30, yielding 10, and the ionization potential of the sensitizer for a range of aromatic hydrocarbons474. A short, but useful, review covers the application of '0, reactions in chemistry as well as related mechanistic and kinetic investigation^^^^. A theoretical paper discusses the nature of solvent effects which affect the deactivation of 'A, 0,476. The effect of hydrostatic pressure on the radiationless ~ . perturbing effects of the deactivation of '0, in solution has also been ~ p o r t e d 4 ~The solventsH,O, D20, C,H, and C6H5CH3on the luminescence rate constant of '0, up to 100 atm provide evidence for participation of complexes involving both the ground and excited states of molecular oxygen478. The application of a collision complex model has been applied to the interpretation of photophysical quenching of '0, in

liquids by 4-amino-TEMW79. A published lecture discusses process involving 02(3C) and 02('A) which occur in microheterogeneous systems480. A distinction between physical and reactive quenching is drawn in a study of the interaction of 10, with indolic derivatives481. Kinetic studies on anthralin photoxidation show that '0, preferentiallyreacts with the uihydr~xyanion~~~. Direct spectroscopic measurement of lA,02 production in the thermal decomposition and 266 nm photolysis of benzaldehyde hydromoxide has been described by Chau et arg3. 4. Other P r o c u 4.1 Chemiluminescence Birks4" has recently prepared a monograph on chemluminescence and photochemical reaction detection in chromatography which is likely to be of particular interest to analytical chemists. Few papers on chemiluminescence have appeared in the mainstream chemical literature. A significant exception is a theoretical treatment of the mechanism of the chemiluminescent decompositon of 1,2-dio~etanes~~5. A series of phenyl N-alkylacridinium 9-carboxylates have been synthesis4 and their chemiluminescent properties

I: Photophysical Processes in Condensed Phases 4.2 m h r o b The number of papers on this topic is also small. The potential, limitations, and detailed comparisons with other types of systems have been discussed for the newly introduced i n d o l i ~ i n e s ~Another ~~. published lecture describes the use of ps pulses to study primary photochemical steps in unsubstituted ind~lino-spiropyrans~~~. A new photochromic system which has been described is mesoxaldehyde l-allyl-lphenyl-2-phenylosa~one~~9. Ps laser photolysis studies of the photochromism of a furylfulgide shows that photoinduced cyclization occurs within 10 ps490. The photochromic behaviour of long alkyl chain spiropyrans at air-water interfaces and in LB films has also been investigated4g1. 4.3 Photochemicalrac&mi A number of quantitative investigations of reaction mechanisms have been reported during the year, many using ultrafast techniques. A good example is the ps time resolved UV resonance Raman spectroscopy of photochemical ring opening processes in cyclooctatriene and 2-phellandrene492. Ring scission occurs within 11 to 12 ps. Time resolved Raman spectroscopy has also been used to examine the photorearrangement of o-niuobenzyl esters4g3. Picosecond optical absorption has been used to follow the geminate radical recombination kinetics of polyatomic free radical pairs produced by the photolysis of azocompounds in liquid alkanes494. A similar study has also been made for tetraphenylhydrazinealso in alkane solution495. The power of fast reaction techniques for examining the details of reaction processes is illustrated by studies made on the photocycloreversion of an aromatic endoperoxide4g6. Rupture of a single C - 0 band occurs in the reactive S, ( X P ) state within 0.35 ps of excitation. The generated biradical then decays with a lifetime of 1.6 f 05 ps at 22°C in CH2C12. Ps and ns laser photolysis applied to the reaction of excited benzophenone with 1,4-diazabicyclo[2.2.2] octane in acetonitrile solution shows that the benzophenone anion free radical abstracts a proton from the ground state of the amine497. Another study on benzophenone include a direct kinetic study of the radical transfer reaction Me,COH + Ph2C0 + Me2C0 + Ph,COH

The data show that the dynamics of this reaction involve hydrogen bonding effects498. Fs-ps laser photolysis has been deployed to follow the details of the photoreduction of excited benzophenone by N,N-dimethylanilinein acetonitrile solution499.Laser flash photolysis also has been used to examine the chromophore assisted peroxy-bond breakage in the case of a benzophenone peresterSm.

33

34

Photochemistry

The photochemistry of ethyl esters of 2-oxo-carboxylic acids has the participation of both singlet and triplet excited states501. Triplet state lifetimes have been measured and the occurrence of Norrish type I1 splits in these molecules established. Other flash photolysis studies reported deal with the photoinduced tautomerism of 2-hydroxyphenazinesm,3-methylisoxazolo[5,4-b]pyridine503,and the photolysis of 4,4'-biphenylbia~ide~~. The tautomerism and phototautomerization of 4(3H)-pyridinethione has been examined theoretically by the infra red isolation technique505. The effect of pressure on the photoinduced abstraction reaction of azanaphthalenesin mixed crystals of durene has also been studied5&. Photosolvolysis of arylmethanols also occurs in aqueous solutions of sulphuric acid507. Ultrafast photochemical events associated with the photosensitizing properties of squaraine dye in interaction with T i 4 colloids involves charge injection into the conduction band of the pigment within 18 ps of excitation508. The rapid charge recombination rate is 3.7 x 109 sl. Diffuse reflectance flash photolysis and product studies have been carried out on the reactions of diphenylmethyl radicals on zeolitess09. Oxirenes and ketocarbenes are metastable species obtained by photolysis of 2-dia~oketones~~O. Experimental data obtained in rare gas matrices is compared with stabilities and isomerization barriers estimated by theoretical calculation. Two mechanistic studies relevant to photocuring processes have appeared. One deals with the efficient photoinduced generation of radical cations in solvents of medium and low polarity511.These cations act as sensitizers of the polymerization of N-methylacridiniumhexafluorophosphate. The other is a study of the photochemistry of marylsulphonium salts5I2.

I: Photophysical Processes in Condensed Phases

35

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50

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m,

a,

s,

s, x,

m,

m,

m,

Part ZZ ORGANIC ASPECTS OF PHOTOCHEMISTRY

1

Photolysis of Carbonyl Compounds BY W. M. HORSPOOL

In recent years there has been a swing away from the photochemistry of simple compounds. The past year 1990-1991 with which this review is involved has followed this trend with only a few references to the photochemical reactions of the simpler ketones. Two

reviews

significance discussed

have to

focused

attention

areas

on

the organic photochemist.

some aspects of physical

One

of

general

by Turro'

has

organic photochemistry

of

relevance to the photochemistry of ketones and related compounds. Another

article

of

interest

has

reviewed

the

photochemical

reactivity of reaction intermediates such a s radicals, biradicals and carbenes.

2

There is a growing awareness in all areas of photochemistry of the control which can be exercised on the outcome of a reaction by

the

use

of

photochemical

single

electron

transfer

(SET)

reactions. Such a process has been applied to the reactions of propiophenones using 2.4.6-triphenylpyrylium

tetrafluoroborste

as the electron accepting sensitizer. Thus irradiation of the propiophenones ( 1 ) under SET conditions results in the generation of

radical

cation

intermediates

which

fission

to

benzophenone. benzoic acid. and benzaldehyde 3 well a s bringing about the formation of the enones ( 2 ) .

as

acetophenone.

1

undergo

Norrish Type I Reactions

The photochemical reactions of cyclobutanones has been a fruitful

54

Photochemistry

0 y o y P h

R’ Ph

Ph

(1) R’ = H, Me, or Ph R2 = H or OH

0

R’

4

Me

Me

(7)a; R’ = R2 = C02Me b; R‘ = H,R2 = C02Me c; R1= R2 = H

C02Me

C02Me

(9)R’ = R2 = C02Me

lII1: Photolysis of Carbonyl Compounds

55

synthetic reaction strategy for the synthesis of prostaglandins and other naturally occurring compounds.

The path described by

Roberts and his coworkers involves hydroxylation of the enone

(3). Photochemical two bond fission of the hydroxy ketone ( 4 ) results

in the formation

of

lactone

ketene. Further elaboration of the

(5) v i a an intermediate

lactone is then possible.

In larger ring ketones decarbonylation.

by a Norrish

Type

5

I

process, is often the result of irradiation. A review lecture has highlighted, among other routes. the use of photodecarbonylation as a path to o-xylylenes.' the

photochemical

pentanone

(6) in

An analogous reaction is involved in

decomposition the

gas

treatment yields propene. isomers.7 The

reaction

phase

of

trans-3,4-dimethylcyclo-

using

206 or

193 na. This

1,2-dimethylcyclobutane, and butene

proceeds by

the

formation of

a

1,4-

biradical. The reactions of this biradical, fission, ring closure and rearrangement, account for the products obtained. Irradiation of

the ketone

(7a) in methylene dichloride also brings about

Norrish Type I fission. The resultant biradical formed on loss of carbon monoxide undergoes transannular hydrogen sbstraction to yield the diester (8). The monoester (7b) also undergoes C-C bond fission on irradiation in methanol but decarbonylation does not result and instead the mixture of diesters (9) is obtained. The hydrogen transfer is totally intramolecular and the reaction is

partially

quenched

by

naphthalene.

Photolysis

of

the

unsubstituted ketone (7c) in methanol yields the monoester (10) on irradiation.

8

Photochemistry

56

Further work on the influence on constricting environments on the photochemical

behaviour of ketones has been reported.

In these

studies the influence of inclusion in Zeolites o n Norrish Type

I and Type

I1

processes

of

alkyl

benzoin

ethers

and

alkyl

d e o r y b e n ~ o i n and ~ the photochemical reectivity of the d . 1 . and meso-isomers faugasites

of have

2.4-diphenylpentan-3-one been

diastereoselectivity

In

studied." of

the

the

radical

in

a

latter

of

variety example

combination

the

reactions

following Norrish Type I cleavage were investigated. In another report the ketones ( 1 1 ) are readily converted i n high yield, 90%. into

the

para-cyclophanes

conditions.

The

reaction

resultant biradical. coupling

at

the

products. l1 This publication.

(12) is

on

an

irradistion

a-fission

under

process

such

and

the

in its constrained environment, undergoes

4-position report

of

the

extends

the

aryl

ring

results

affording

from

the

an earlier

12

Norrish Type IIBeactions

2

Norrish Type I 1 reactivity is often a common reaction path for ketones with available y-hydrogens. Hydrogen abstraction by the excited

carbonyl

group

results

in

the

formation

a

of

1,4-

biradical which can undergo either bond cleavage t o reform the carbonyl

group

and

an

alkene

or

bond

formation

to

yield

a

cyclobutanol derivative. The fragmentation path is followed by the ketone

(13). T h e interest in this reaction is the control

which can be exercised on the ketonization of the resultant enol (14).

Apparently

formation

of

the

in the presence final

product,

of

(-)-ephedrine asymmetric

(R)-2-methylindanone

(15),

57

M I : Photolysis of Carbonyl Compounds 0

QPh (11) n = 6,7,8,9,10

QMe OAPh

(22)

R' = OMe, R2 = CH3 R' = OCH2CH=CH2,R2 = Me R' = OCH2CH2CH=CH2,R2 = Me R' = CH2CH2CH=CH2,R2 = CF3 R' = H, R2 = H

(12)

58

Photochemistry

results by enantioselective transformation of the enol. There is solvent

dependency

and

the

enantiomeric

excess

21%

is

in

methylene dichloride and 47% acetonitrile. l3 Irradiation of the diketone results

(16.

in

Hydrogen

R

the

= H)

also follows the fragmentation path and

formation

abstraction

of

of

the

open

the axial

chain

7-hydrogen

instance and also with the ketone (16, R

=

(17).

diketone occurs

in this

Me). However, the

fission path is not followed in this latter case and instead the result

is cyclization

to a n intermediate cyclobutane

(18) a

reaction which is followed by ring opening t o yield the final product

(19). an apparent

migration, in 79%. The

1.3-benzoyl

change in reactivity observed with these compounds is thought to be due to conformational effects induced by the presence of the methyl substituent. Similar reactivity is seen with the ketone ( 2 0 ) which follows a n analogous path to give ( 2 1 ) in 36% yield. The observed reactions have been shown to arise from the triplet state. l a The photocyclization of o-alkyl substituted arylketones ( 2 2 ) to cyclobutenols ( 2 3 ) is also a result of Norrish Type I 1 hydrogen

abstraction

considerable

and

synthetic

is

a

well

potential.

studied

However,

reaction there

has

with been

disagreement on whether a dienol intermediate is formed from the biradical prior to cyclobutenol formation or if the dienol is formed

from

the

cyclobutenol.

derivatives such as

Studies

with

benzophenone

(22) shows unambiguously that the dienol

precedes the cyclobutenol. Additional to this is the fact that only the E-dienol is formed and that this then cyclizes to the final

product. The

long-lived

triplet

biradical

permits

the

formation of the dienol with the most stable geometry. Steric congestion reaction. l5

is The

also

important

irradiation

of

for

the

o-methyl

efficiency substituted

of

the

phenyl

llll: Photolysis of Carbonyl Compounds

59

ketones and their conversion into photoenols has been studied using

hydrogen-tritium

exchange. l6

some

In

examples

of

cyclobutenol formation high diastereoselectivtg can be observed such

as

observed

in

the

cyclization

of

the

benrophenone

derivatives ( 2 4 ) which yield the cyclobutenols ( 2 5 ) .17 One of the uses in synthesis to which the production of a dienol can be put is intramolscular trapping. Such a n example has been reported by and involves irradiation of the aldehyde (26)

H a s h i m t o et

which yields the dienol (27). This intermediate ( 2 7 ) subsequently undergoes intramolecular Diels-Alder addition to yield the adduct (28) which is accompanied by the C-4 epimer in a total yield of

64% (ratio of 99:l) .18 Another report describes the use of the photoenolization of 3,6-dimethoxy-2-methylbenzaldehyde a s a path to the 11-deoxyanthracycline skeleton. l9 T h e 1.3-diketones (29) undergo regiospecific hydrogen abstraction to afford a biradical

(30) whose methanol

fate

exhibits

the preferred

some

solvent

dependency.

Thus' in

cyclization occurs by addition of the

to the carbonyl

eroup at

C3 to yield

the

methylene

radical

biradical

(31) and gives the naphthalenone derivatives ( 3 2 ) a s

the predominant products (Scheme 1). The alternative cyclization mode, that of formation of a cyclobutenol. also occurs affording lower yields of the cyclobutenols (33). In hexane, however, the yields of the cyclobutenol are considerably higher . 2 0 The Norrish Type

I1

reactivity

irradiation at

of the ketones

> 340

diastereoisomeric

(34) has been studied

by

n m in methanol. T h i s treatment affords the

alcohols (35) and (36). These are formed by

cyclization of the initially formed 1.4-biradical to yield the carbene (37). Trapping of this by methanol yields the observed 21 products .

Photochemistry

60

Go OMe

HO

HOp J : ; e

OMe Ar

OMe

OH

0

R

(29) Me

(33)2

Et

trace

Pr

3 5

Pr' CHMeEt CHEt2 cyclohexyl BU'

12

37 4 3 Scheme 1

(32)80 66 85 50 67 41 52 36

IIl1: Photolysis of Carbonyl Compounds OH

CH,

R @

0

61

& qy

Me

0 '

\

/

\

R (311

(30)

R

(35)$ = 0.0041 6 = 0.004

Me

(34) R = H R=Me

R

J?

(36)$ = 0.017 $ = 0.012

(37)

gPh !&R

Me

Me OH R a ; P h

R (41)R = H (42)R = Et

(43)

(44)

Photochemistry

62 The

foregoing examples

involve

conventional

Norrish

Type

11

processes with 1,5-hydrogen transfer yielding a 1,4-biradical. Sauers and Huang22 have analyzed the reactivity of the cyclo-

(38) by molecular mechanics methodology. This showed

decanone that

the abstraction of a 7-hydrogen

would be

preferred to the abstraction

hydrogen

transfer).

irradiation of bicyclic

These

proposals

(38) at 300 nm

alcohol

(39)

in

in

40%

(1,5-hydrogen transfer) from

(1,7-

substantiated

by

t-butanol which yielded

the

yield

were

the €-site

from

the

y-hydrogen

abstraction path and the cyclodecanol ( 4 0 ) in 17% yield only from the

alternative

hydrogen

path. 22

abstraction

However,

can

take

in

place

specific from

environments,

other

sites

in

preference to 1,5-trensfer. Such is tbe case with the ketones (41. 42). Hydrogen abstraction occurs at the &carbon a 1.5-biradical.

to yield

This undergoes cyclization to yield the two

indanes ( 4 3 ) and ( 4 4 ) in ratios of 30:l and 4.3:1 respectively. The high diestereoselectivity observed for the cyclizstion is due to conformational effects o n the triplet 1.5-biradical. 23 This report

is related to earlier work which studied the 1.6-hydrogen

transfer within the acetophenone derivatives (45). on irradiation in the crystalline phase.24 The chemical yield of the products ( 4 6 ) is almost quantitative although there is some variation in the quantum studied

this

yield

a s shown. Wagner

process

in

and his

considerable

detail

have 8s

a

route

to

dihydrofuran derivatives. The irradiation of the ketones ( 4 7 , 48) in heptane afford the cyclized products (49-51) with good quantum efficiency. In some instances the cyclized product e . g . (49)from ( 4 7 ) is

accompanied

by

the

reduced

ketone

(52). Even

the

alkylketone ( 5 3 ) is reactive in this mode and yields ( 5 4 1 , albeit

IIll: Photolysis of Carbonyl Compounds

(45)

(46) $cYc

a;R'=H,R2=Me,R3=H b; R' = Me,R2= Me, R3 = H C; R' = R2 = R3 = Me d; R' = P f , R2 = R3 = Me e; R' = Ph, R2 = R3 = Me

WMe0

100% 67% 100% 94% 100%

R-0

0

1.0 0.05 0.24 0.12

0.03

(y /

(47)

(48)

(50) R = Me, R = Ph,

+=0.62 $=0.94

(49)

(51)

w: Me 'OH

phA#Me /

(53)

+ = 0.30

(54) 4=0.023

64

Photochemistry

with a much lower quantum yield. The reactivity of the various systems studied has been analyzed in terms of the conformations undergoing reaction and photo-physical

details of the various

processes are reported. 25 The reaction described above does not appear

to be

ketone

adversely affected

( 5 5 ) undergoes

biradical

by

disubstitution. T h u s the

1,g-hydrogen

transfer

(56). Following this two products

to

afford

( 5 7 ) and

the

( 5 8 ) are

in a ratio of 2:l and with the quantum yields shown

obtained

under the appropriate structures. T h e formation of the former ( 5 7 ) is the normal product of cyclization of the 1,5-biradical ( 5 6 ) . An n . m . r . investigation of the reaction has shown that the

enol (59) is formed during the reaction and that this is prone to acid

catalyzed cyclization to yield

the 1.3-dioxane

(58).

Other studies using deuteriated derivatives have shown that the enol (59) is formed from the biradical by disproportionation.

26

Other workers have also studied the effects of disubstitution and have shown that the keto ethers (60) react o n irradiation at 350 nm in benzene solution to yield the cyclized products ( 6 1 ) . The reaction again

involves

abstraction

of

hydrogen

the 6-

from

position by the triplet excited keto group. In the absence of a 6-hydrogen. as with (62). no cyclization takes place and instead fission

of

an

0-C

naphthalenones ( 6 4 , n a

afford

bond

=

1,5-biradical

yields

the

27

(63).

phenol

The

1 ) also undergo hydrogen abstraction to ( 6 5 ) which

derivative isolated as (66. n

=

can

cyclize

to

a

furan

1). However, the predominant path

for this series is conversion to the ketoelcohols ( 6 7 ) v i a the

oxirane (68). The outcome of the reaction is dependent on ring size a feature demonstrated o n irradiation of the series (64, n

=

2). Here cyclization yields predominantly the furan derivatives

(66.

n

=

2).

The

preference

for

this

path

is

apparently

1111: Photolysis of Carbonyl Compounds

65

(55)

(57)

0 = 0.42 (c&) $ = 0.71 (MeOH)

(58) $ = 0.21 (CsHs) $ = 0.14 (MeOH)

Go

(59)

Me

Me

Ph (60)

R’ = H; R2 = vinyl, ethynyl,CH =CM+ R‘ = Me; R2= ethynyl

(61)

Me

Go Go Me&CH

Me

Me

Ph

Ph (62)

(63)

Photochemistry

66 R

R

I

(65)

(64)

R H Me Et Pr' Ph

(66)

(67)

n =1 34 5 8 11

n =2

9

87

80 98 73 79

36 62 48 60 71

a; R' = Me, R2 = Me b; R' = Me, R2 = OAc c; R' = Me, R2 = OMe d; R' = C02Et, R2 = OM0

87% 20% trace 0%

OH

IIII: Photolysis of Carbonyl Compounds controlled by

67

conformational

e f f e c t s since

rotation within the biradical (65. n

=

it

is easier

for

2 ) to bring about bonding

28

rather then to form the oxirane intermediates.

The use of intramolecular 1.6-hydrogen abstraction has been made as a

products. Thus t h e

route to natural

carbohydrate derivatives spiro derivatives

irradiation

of the

(69) results in the formation of the

(70) in the yields shown. Some evidence for

substituent effects was observed in that the derivatives ( 6 9 ~ .d ) either yield only a trace of product or else fail to cyclize. The reaction a l s o occurs with an unsubstituted benzene derivative and irradiation of this ( 7 1 ) affords the product (72) in 41% yield. The transformation is described by the authors as an approach to the synthesis o f crombenin, a spiropeltogynoid.

29

Hydrogen abstraction at remote sites often provides a route t o large

ring

irradiation

compounds. of

Hasegara

ketoamine

et

(73)

al.30

undergoes

report such

a

that

the

process.

Irradiation populates the triplet state within which SET from the nitrogen occurs. It is this process which controls the site of attack eventually affording the two products (74) and ( 7 5 ) v i a the biradical (76).

Oxetane Formation

3

The addition of aldehydes and ketones to alkenes is a convenient high

yield

reaction

regiochemistry predicted

shown by

the

the

synthesis

of

addition process

the oxygen of

moiety.

Such

is

the excited

the

case

with

oxetanes.

formed by

carbonyl group the

The

can usually be

on the basis of the better biradical

addition of alkene

for

the

to the

photoeddition

of

Photochemistry

68

PhCOCOl;e Ph

d-0 0

k0

0

(80) 25% (2R, 3S7;7% (2S, 3s')

(79)

R'

hv

R2 $

O

H

+

f q R 2 L $ R 2

C02Me Me02C R' (81) R' = H, R2 = Me R' = H, R2 = Et

R' C02Me

R1

7%

6%

R' = Me, R2 = Et

4.7% 5.7% 3yo

+

4.5% 5.5% Scheme 2

6.5%

9.3% 11-3% 1 2%

1111: Photolysis of Carbonyl Compounds

69

acetophenone t o the alkene ( 7 7 ) affording the oxetane ( 7 8 ) with high diastereoselectivity. 31 Another

feature of photochemical

oxetane formation which has become of considerable importance is the

use

of

chiral

(S)-(79)

employing

dimethylbut-2-0110

auxiliaries in

the

afforded

as

control

cycloaddition the

two

features.

reaction

adducts

with (80)

Thus, 2,3where

cycloaddition has occurred preferentially from the re face.

32

Addition can also take place to other unsaturated compounds such as allenes ( 8 1 ) and the irradiation of benzophenone through Pyrex in

a

benzene

illustrated

in

solution scheme

affords (2).

It

low is

yields

of

interesting

the to

products note

that

cycloaddition occurs to both the electron-rich and the electrondeficient alkene moiety of the allene. 33 Photochemical addition of 1-naphthaldehyde

to 2.3-dimethylbut-2-ene

(83) and

affords the three

(84). 2-Naphthaldehyde 34 similar manner and yields analogous products.

products

(82),

reacts

in

a

Furan is a well-used substrate for oxetane formation. Typically the photochemical addition of the aldehydes ( 8 5 ) to furan affords the oxetanes (86). Hambalek and Just35 have developed a method for the chemicsl conversion of these adducts into tri-substituted monocyclic oxetanes which could be of value in the synthesis of naturally occurring compounds. Another example of synthetic value is the use of 2-methylfuran which with the aldehyde (87) affords the oxetane (88). This product w a s used as a starting point for the synthesis of racemic oxetanocin. 36 Other carbonyl compounds (89) also photo-add to furans in yields that can often be high (Scheme 3). In this series the endo-exo-ratio has been shown to be temperature dependent. However. in this system, when optically active addends were employed, only a low diastereoisomeric excess

70

Photochemistry Me Me Me

a M e

(83)

(84)

R-CHO

(85)R = Ph, Pri, CHPOTBDMS,or CH20Bz

PMe

P h c o o ~ o H

0

aoR +

R

hv

&*cN

0

0

30%

ratio

3.5:1

at-55"C

86%

R = Bu'

ratio ratio

8.9:l

89%

9.3:l

at ambient at -55°C

R = Ph

77% 95%

ratio ratio

3.7:l 5.3:l

at ambient at -55°C

Scheme 3

1111: Photolysis of Carbonyl Compounds

71

was observed.37

The silyl diene ( S O ) photochemically adds benzophenone via

I

SET

mechanism to yield the two oxetanes ( 9 1 , 18%) and ( 9 2 , 51%) on irradiation at 436 nm in acetonitrile solution. The oxetanes are accompanied by the (2+2)-dimer of the diene. The reaction appears to be efficient and can be carried out with a variety of diary1 ketones.

38

4 Irradiation

Miscellaneous Reactions of

the

diketone

( 9 3 ) results

in

products

from

reduction of both the carbonyl groups and also epiaerization at (216. The photoepimerization is thought to arise by a two step singlet to singlet energy transfer process. 39 Isomerization of a site adjacent to a carbonyl group is also observed with the trans-cyclopropyl

ketone

( 9 4 ) which

undergoes

photochemical

conversion into the cis-isomer (95). The position of the carbonyl group relative to the double bond is obviously important in this system in that irradiation of the trans-enone ( 9 6 ) results in the formation of the (2+2)-photodimer ( 9 7 ) with no evidence for the ring isomerization process.

40

SET processes can be used to effect ring opening within strained . , a ketonic systems. Thus Cossy et '

have reported ring fission

reactions of ketyl radical anions e . g . (98). These were obtained by the irradiation (254 n m ) of the bicyclic ketones (99-101) in acetonitrile in the presence of triethylamine (the SE donor). The resultant ring opened radical anion affords the final products (102-104). Several examples of the process were reported.

41

Photochemistry

72

eir

Ph Me'

Ph Me'

Me

Me

Ph Me'

PhMe,SiO"'

0

PhCH=HC'*-

PhCH=HC-

0 (94)

(97) Ar = p - MeOC6H4

R=HorMe

(98)

(95)

(99)

Me

IIII: Photolysis of Carbonyl Compounds

(100)

(101)

(102) 60%

(108) R’ CN H Me0

X = 0,R’ = Ph,R2 = H,Me, or CH2C02Et X = 0,R’ = Me, R2 = CH2C02Et X = S, R’ = Ph, R2 = H,Me, or CH2C02Et

73

(104) 45%

(1 03)50%

(1 10) 98%

37% 15%

Photochemistry

74

described the SET induced

Previously Pete and his coworkers4* photocyclization

of

alkynyl

triethylamine affording

oxoamides

high yields

in

the

presence

of

of cyclic alcohols. T h i s

process has been used by Cossy and Leblanc"

as a route to iso-

oxy-skytanthine (105). T h e starting material for this synthesis is the alkynyl oxoamide ( 1 0 6 ) and photocyclization of this in acetonitrile/triethylamine affords the cyclic amide (107) in 55%

yield. T h i s w a s subsequently transformed into the final product

(105). SET processes also control the outcome of the irradiation of the ketoepoxides ( 1 0 8 ) in the presence of tri-n-butylallyl

stannane. The resultant

radical

anion

ally1 radical yielding the products

( 1 0 9 ) is trapped by a n

( 1 1 0 ) . As can be seen the

yields are substituent dependent with the best obtained using an electron withdrawing substituent in the aryl ring. 4 4 Garcia and coworkers

45

have reported the photochemical conversion of cyclic

acetals and

thioacetals

using a n electron

( 1 1 1 ) into the corresponding

transfer process

ketones

from the acetal to 2 . 4 , 6 -

triphenylpyrylium tetrafluoroborate. T h e yields obtained using can be as high a s 88%.45 T h e

methylene dichloride as solvent fragmentation

of

the

ketone

( 1 1 2 ) can

be

brought

about

in

deuteriobenzene / water solution by a n electron transfer process using

9.10-dicyanoanthracene

as

the

electron

acceptor.

The

fragmentation process involves the radical cation ( 1 1 3 ) which ultimately affords a ketone and morpholine.

A

review of the photochemical

furanones

has

been

published.

undergoes

decarbonylation

from

46

reactions of The

former

the singlet

Z(3H)- and 2(5H)group

of

compounds

state. The

latter

group undergo a variety of reactions such as dimerization,

IIll: Photolysis of Carbonyl Compounds

75

cycloaddition, or hydrogen abstraction from the triplet state.

47

Decarbonylation also arises during the gas-phase irradiation of the ketene derivative ( 1 1 4 ) using A

>

220 nm. The decarbonylation

yields difluorovinylidene ( 1 1 5 ) which in the presence of alkenes such as cyclopentene can be trapped as the adducts (116). (117), and (118).48 The photochemical double decarbonylation of diary1 oxalate esters has been described a s an efficient path to aryloxy radicals. 49

Decarbonylation

of

quadricyclanone

(119)

on

irradiation at 300 nm in hexane affords a biradical (120) which

50

can be trapped by ethoxyethene as shown in scheme (4).

Aryl

acetic

acid derivatives are notable for their failure to

undergo photochemical decarboxylation easily. Wan and Xu51 have shown that decarboxylation does take place with the corresponding a-hydroxy

derivatives

acetonitrile/water corresponding

at

alcohols

(121)

254

on

nm.

after

irradiation This

only

of

treatment

5-20

min

these

in

yields

the

exposure.

The

mechanism, which has been substantiated by deuteriation studies. involves the production of a-hydroxy carbanions ( 1 2 2 ) following the decarboxylation step. 51 Fragmentation resulting in the loss of larger units can also take place such a s the loss of ketene and the formation of the new diketones (123) on irradiation of the ketolactones (124). T h e reaction presumably involves initial Norrish type

I cleavage

of the lactone C-0 bond followed by the

elimination process. Some evidence f o r this mechanism comes from the isolation of the spiro-diketone

( 1 2 5 ) as a minor product.

This must be formed by decarbonylation and cyclization within the initially formed biradical.

52

Photochemistry

76

F>c=c=o

:=)F

F

hv

OEt

Scheme 4

ArvoH

(123)n = 2 o r 3

(124)n = 2 o r 3

IIII: Photolysis of Carbonyl Compounds

77

The use of the Barton process involving the decarboxylation of derivatives

such

as

(126)

the

for

synthesis

of

vinyl

cyclopentanes ( 1 2 7 ) has been reported. The reaction is carried out in the presence of electron deficient alkenes ( 1 2 8 , R

=

CN,

COOEt. or COHe) and the yields of products ( 1 2 7 ) range from 4 0

A laser-flash study of the photochemical fragmentation

to

of the esters ( 1 2 9 ) has been carried out.

An

efficient

described

by

synthesis

of

Sonawane

carboxylic

and

54

acids

( 1 3 0 ) has

his

and

been

involves

irradiation (Pyrex filter) of the chloroketones ( 1 3 1 ) in aqueous acetone. The reaction is thought to involve heterolysis of the C-C1 bond followed by a 1,e-aryl migration and trapping of the resultant

carbocation by water. 5 5 This reaction has been the

subject of a patent application whereby irradiation of a series of a-haloalkylaryl

ketones

aqueous acetone affords

( 1 3 2 ) in the 200-800 nm

the a-arylpropionic

acids

range in

(133). T h e

reaction has some synthetic use a typical example of which is the conversion of dehalogenation

the ketone reactions

(134) into ibuprofen have

been

reported by Sket and his

( 1 3 5 ) . ~Other ~

described

such

as

that

involving the irradiation

of the fluorinated cyclic ketones (136) in cyclohexane solution. This results in defluorination by a free radical path. The extent of

the

conversion

cycloalkanone.

was

With

dependent the

upon

the

ring

size

of

the

corresponding

2-bromo-2-fluoro 57 derivatives both ionic and free radical reactions occurred.

The irradiation of a mixture of acetaldehyde and ally1 alcohol yields a variety of products such a s 5-hydroxypentan-Z-one, 2methyl-2-(4-oxopentyloxy)tetrahydrofuran,

and

7-hydroxs-4-

Photochemistry

78 Z

phs)

iiz

P-

R’

Z = CN,COOEt or COMe R’ = H; R2 = C5H9or PhCH2 R’ - ~ 2 = ( ~ ~ 2or) 4 (cH,)~ S Me ArACOOH

A r q C ‘

(130) 58% 84% 45% 32%

(131)

Me Ar=Ph Ar = p - MeC6H4 Ar = p - CIC6H4 Ar = p - MeOC6H4

ArCOCHClMe (132)

Ar = 6-MeO-2-naphthy1, phenyl, 3,4-diMe-phenyl, 3,4-diEt-phenyl, 3,4-diPr-phenyl, 3,4-diBu-phenyl, 3,4-diCI-phenyl MeCHArC02H (133)

fl 0 CI

Me

Me2CHCH2

d Me2CHCH2

J%@ (136)

n =1 n =2

C

0

2

H

IItl: Photolysis of Carbonyl Compounds

79

hydroxymethylheptan-2-one were obtained. The last compound can 58

be converted into the two products (137) and (138).

5.

1.

References

N. J . Turro, Photochem. P h o t o b i o l . , A , 1990, 51, 63 ( C h e r . A b s t r . . 1990, 113, 39538).

2.

J. C. Scaiano and L. J . Johnston, Org. Photochem., 1989. 10, 309.

3.

M. V .

Baldovi. H. Garcia, M .

A.

Miranda, and J .

Primo,

ldonatsh. Chem., 1990, 121, 371 (Chem. A b s t r . , 1990, 113.

114502). 4.

H. G. Davies, S. M. Roberts, B. J. Wakefield, and J. A . Winders, J . Chem. Soc., Chem. Commun., 1985, 1166.

5.

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Y.

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1990. 1661. 6

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a.

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9.

V.

10 *

N. D. Ghatlia and N. J . Turro, J . Photochem. P h o t o b i o l . A :

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Chem., 1991. 5 7 , 7 .

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N. Han, X. Lei, and N. J. Turro, J. Org. Chem.. 1991, 5 6 , 2927.

80 12.

Phofochemistry X . Lei, C. E. Doubleday, jun., W .

B. Zimmt, and N. J .

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T. Hasegawa,

M.

Nishimura, Y . Kodama, and

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16.

J . Cebicki,

W.

Reimschussel.

and

B. Zurawinska, J. Phys.

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L e t t . , 1 9 9 1 , 32. 2 6 3 . 18.

K , Hashimoto.

M.

Horikawa. and H. Shirahama. Tetrahedron

Lett., 1 9 9 0 , 3 1 , 7047. 19.

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M . Yoshioka, K. Nishikawa, M . Arai. and T. Hasegara. J. Chem. S O C . , Perkin T r a n s . 1 , 1 9 9 1 , 5 4 1 .

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J . I . Kravitz, P. Margaretha, and W . C.

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Lett., 1 9 9 1 . 32. 3 1 . 22.

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R. Sauers and S.-Y. Huang, Tetrahedron Lett.,

1990, 31,

5709.

23.

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24.

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30,

5389, 25.

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p . J . Wagner and C. Laidig. Tetrahedron Lett., 1 9 9 1 . 32, 895.

IIIl: Photolysis of Carbonyl Compounds 27.

81

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28.

T. Horaguchi, H. Iwanarni, T. Tanaka, E. Hasegara, and T. Shimizu, J . C h e m . S O C . , Chem. C o m m u n . , 1991, 44.

29.

F. Cottet, L. Cottier, and

G.

Descotes, C a n .

J.

Chem.,

1990, 68, 1251. 30.

T. Hasegara, K. Mukai, i d . Mizukoshi. and M. Yoshioka. B u l l . Chem. SOC. J p n . , 1990, 63, 3348.

31’.

S. A . Fleming and R . W . Jones, J . H e t e r o c y c l . Chem.. 1990, 27, 1167 (Chem. A b s t r . , 1990, 113, 191107).

32.

T. Oppenlander and P. Schoenholzer. H e l v . C h i a . A c t a . 1989. 72, 1792 (Chem. A b s t r . . 1990, 113, 78211).

33.

U. P. S. Ishar and R. P. Gandhi. T e t r a h e d r o n , 1991. 47, 2211.

34.

K.

M.

Shima,

Yasuda,

K. Tanabe, and

K. Nakabayashi,

Kogakubu Kenkgu Hokoku. 1990. 3 6 , 23 (Chem. A b s t r . , 1991, 114, 185157). 35.

R. Hambalek and G. Just, T e t r a h e d r o n L e t t . . 1990, 31, 4693.

36.

R . Hambalek and C. Just, T e t r a h e d r o n L e t t . , 1990, 31. 5445.

37.

C. Zagar and H.-D. Scarf, Chem. B e r . , 1991, 124, 967.

38.

S. Kyushin, Y. Ohkura, Y. Nakadaira, and M. Ohashi. J . Chem. SOC.

I

39.

2.-Z.

Wu

Chem. Commun., 1990. 1718.

and H. Morrison,

Tetrahedron L e t t . ,

1990, 31.

5865. 40.

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1989, 28B. 899 (Chem. Abstr., 1990. 112,

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J.

Cossy. P. Aclinau. V. Bellosta. N. Furet, J. Baranne-

Lafont, D. Sparfel, and C. Souchaud, T e t r a h e d r o n L e t t . ,

1991, 32, 1315.

82

Photochemistry

42.

J . Cossy, D. Belotti, and J.-P. P e t e , Tetrahedron Lett.,

43.

J . Cossy and C. Leblanc, Tetrahedron Lett., 1991, 32. 3051.

44.

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1987, 28, 4545 and 4547.

Tetrahedron Lett., 1991, 32, 2029.

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46.

W.

R. Bergmark and D. G. Whitten, J . Am. Chem.

S O C . , 1990,

112, 4042. 47.

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Uiranda.

Heterocycles, 1990,

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1990, 113.

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C. Bohme, R. Boch. and J . C. Scaiano. J . Org. Chem.. 1990, 55. 5414.

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H.

R.

Sonaaane.

D. G. Kulkarni, and

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Ayyangar,

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2

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY W. M. HORSPOOL

1 A

Cycloaddition Reactions

review dealing with

(2+2)-cycloaddition

reactions

has been

pub1 ished.

Intramolecular.- The photochemical conversion of carvone ( 1 ) into the adduct ( 2 ) w a s one of the first examples of intramolecular cycloaddition studied. Givens and his coworkers' enone

report that this

( 1 ) can be reduced o n irradiation in the presence of a n

electron donor. Thus the irradiation at 350 nm in methanol with triethylamine

affords

the

intramolecular

adduct

(2)

and

the

ketone ( 3 ) in a ratio of 1:6 (Scheme 1). T h e reduction occurs by a S E T process. Reduction is also reported f o r the steroidal enone

( 4 ) which yields the three products shown in scheme ( 1 ) under conditions similar to those used for the carvone experiment.

2

Acetone-sensitized irradiation of the ene-dione derivatives ( 5 ) results in the formation of the (2+2)-adducts ( 6 ) in the yields shown. T h e mechanism of the reaction is uncertain but there are similarities

between this and

the di-=-methane

bridging could afford a biradical

process. Thus

( 7 ) which could ring close to

the final products. T h e singlet state reactivity of ( 5 ) affording the

cyclopropane

biradical

derivatives

(8)

could

also

( 7 ) within which a hydrogen or proton

arise

from

the

transfer would

85

lIl2: Enone Cycloadditions and Rearrangements Me

MeOH

+

b-” MeA

(2)

Me

(3)

Scheme 1

J 5-10 Yo

OH

85-9Ooh

trace

Scheme 2

direct irradiation a; Me Me b; Me H c; H H acetone sensitized a; Me Me b; Me H c; H H

a; 17% b; 3% c; 3% a; 53% b; 37% C;

46%

a;

-

b; 5% c; -

86

Photochemistry

afford the cyclopropane derivatives

( 8 ) . It

is interesting to

note that in one case, the irradiation of ( 5 c ) in acetonitrile. 3 that a d i - w m e t h a n e product (9) is obtained albeit in low yield.

A laser flash investigation of the enones (10-12) has shorn that

the triplet states of these compounds is generally longer lived than those of the parent non-phenylated enones . 4 The interest in these compounds relates t o the formation of strained compounds by

intramolecular

(2+2)-cycloaddition. The cycloaddition with

enone (13a) yields the product in 58% yield arising from h e e d t o

tail addition (14a). With a shorter connecting chain in (13b) the cycloaddition also follows this route yielding predominantly the adduct

(lab). This is accompanied by the h e e d to h e e d product

( 1 5 ) in a total yield of 49% and a ratio of 2 : 1 . 5 Another study has addressed a similar problem with

the cyclohexanones (16).

Agein there is a chain length dependency o n the outcome of the reaction. Thus when the chain linking the two double bonds is four carbons, i.e. enone

( 1 6 ) with n

However, the enones (16). n

=

=

>

3 , the reaction fails.

1 or 2 , are reactive and yield the

adducts ( 1 7 ) in 46-48% yield by a ( 2 + 2 ) - h e a d t o tail addition.

6

Others have also shown interest in the problem of chain length and details from the photo-cycloaddition reactions of the chiral enones

(18) have been

effect

o f substituents o n the photochemical

reported.'

Other

features such as reactions of

the the

enones ( 1 9 ) towards intra-molecular cycloaddition have also been evaluated.8 Intramolecular cycloaddition within the enones (20) in acetonitrile solutions can be affected by SET processes with triethylamine as the electron donor. In three cases (20e-c) the formation of spiro compounds ( 2 1 ) competes favourebly with the normal (2+2)-cycloaddition yielding (22).There are substitution

IIl2: Enone Cycloadditions and Rearrangements

% $!L Ph

(13)a; n = 2 b;n = 1

Ph

ph&

87

(%

(14)a;n = 2; 58% (15) total yield 49% (16)a;n = 1 b;n = 1 ratio 2:l b;n = 2

(& (17)a;n = 1 b;n = 2

CHMe, Me%

h

R

M

e

Me Me

O

(18) R = (CH2),CH=CH2,n = 2-4

0

(19) R' = H, R2 = H or Me R' = F, F f = H or Me R' = Me, R2 = H or Me

0

(20) X n

a; 0 b; 0 C; CH2 d; 0 e; 0

2 4 2 1 3

c?, a; b; 6% 30% d; 0 e; 29%

C;

Photochemistry

88 e f f e c t s within undergo

the

this system

since

the

and

only

spiro additions

enones

(20d.e) fail

the normal

to

cyclobutane

derivatives are obtained. Clearly this modus operandi could be important in new synthetic routes involving enones.

a1.l'

report

on

the

use

of

vinylogous

amide

Winkler et

cycloaddition

reaction of enones in the synthesis of the alkaloid vindorosine

(23) via the key intermediate (24).l o T h i s vinylogous amide route has been used previously by the same group in the synthesis of the alkaloid mesembrine.

The pentacyclic photochemical

11

(25)

cage diones

by the

are readily prepared

cyclization of the diones

(26).l2 T h e cinnamate

derivative ( 2 7 ) undergoes (2+2)-photocycloaddition o n xanthone sensitization. This treatment

affords the 6-truxinate

(28) in

37.3 X yield. However, when the reaction is carried out in the presence of metal perchlorates such as lithium perchlorate the 6-truxinate is formed in only 10.3 X yield. The two new products formed from the reaction are the cyclobutane (29) in 48 X yield and the tetrahydronaphthalene derivative (30, 10.3 X ) .

13

Intermolecular.- cis-trans-Isomerization of the enone ( 3 1 ) occurs on irradiation and this is followed by formation of the adducts

(32) and (33) by a thermal cycloaddition of the trans-isomer of ( 3 1 ) to a ground

state cis-isomer.

cis-trans-isomerization

is

S u c h a mechanism involving

supported

by

the

fact

that

the

reaction does not show a solvent dependency and can be brought about equally efficiently in t-butanol or benzene.

The enone

(31)

the

type

of

4.4-

does

associated

not with

undergo the

diphenylcyclohexenone

1.2-phenyl

migrations

di-x-methane derivatives.

of

rearrangement The

regiospecific

IIl2: Enone Cycloadditions and Rearrangements

89

OAc

0 R R$ fo

0

Me

Me

H

H

(26)R = Br or CI

4

PPh h

00 (20 Ph Ph Ph Ph

(32)65%

Ph Ph

Ph Ph

(33)27%

Photochemistry

90

photodimerization of acetylacetone in non-polar solvent has been reported.

The

derivative

final product

(34) the

crystallography.

of

identity

The

route

the reaction

of to

which this

was

is the furanoid

verified

compound

by X-ray

involves

the

formation of the diaers (35) and (36) which subsequently undergo 15 rearrangement to the final product.

Toda" in

continues to report examples of specific photochemistry

inclusion

complexes.

Among

the

results

in

the

present

publication the successful syn-bead-to-tail direrization of the enones (37) a s a 2:l complex in high yields ( 8 2 - 9 0 X ) affording (38) has been demonstrated. Furan derivatives also undergo ( 2 + 2 ) -

dimerization.

Thus

the

furan

(39). o n

ester

benzophenone-

sensitized irradiation, yields the two dirers ( 4 0 ) and ( 4 1 ) in a ratio of

diacrylic

2:l and a total yield

acid

of

behaviour

dimer ( 4 5 ) is formed.

Weedon

20

furan-2.5-

(42) gives the cyclobutane derivative

irradiation as an aqueous slurry. photochemical

9 0 % ~ ' while

of the pyrone

A

( 4 3 ) on

reinvestigation of the ( 4 4 ) has shown that the

19

has reviewed the photochemical reactions in which enols

are formed or used in synthesis. T h i s particular reaction mode has been a successful photochemical route to many compounds over the years. Some publications relevant t o this have appeared in the past year such as the study of the photochemical addition of (46), the enol of a 1.3-diketone, to 2,5-dirnethylhexa-2,4-diene. This has shown that only a small amount of the anticipated adduct ( 4 7 ) is formed. This compound is produced

and

involves

conventional

v i a the triplet state

(2tZ)-cycloaddition and

retro-Aldol

ring opening of the cyclobutane. The dihydropyran derivatives

1112: Enone Cycloadditions and Rearrangements

91

H HO O ,Me V

HO”

Me” Me

Me

0

,

MeAO

HOCPh2CZCCPh20H Ph

--- R (38)

(37)

R = Ph, substituted Ph, or 2-naphthyl

?J=( COOMe

(-&f.#.OMe

0

COOMe

\

‘COOM~

(39)

(41)

WR COOH

d

^1.... A0 0

Ph-

Ph

(44)

-

*

R

P

(45) R = H or Me0

O

92

To

Photochemistry

C02Me

kH

Me

C02Me

Me

(46)

Me

Me

)M

Me& 02 :Me Me

Me

Me

Mel(

Mea

0 OH Y M e

2

(48)

Me@ Me

Me

(52) E and Z

(51) a; R’ = C02Me, R2 = CH2COMe b; R2 = C02Me, R’ = CH2COMe

vMe Me (54)

(53)

R

ph9F2 4

(56) R=CdHg

Ph

(55)

R = CH2=CHCH,CH3

R = Bu‘

(57)n = 1 n =2 n =3 n =4

M

e

1112: Enone Cycloadditions and Rearrangements

93

(48). ( 4 9 1 , and ( 5 0 ) also arise from the triplet state but are formed byalternativecycloaddition paths as are the oxetanes (51) and the three keto esters ( 5 2 1 , (53). and ( 5 4 ).21 Another report the addi tion of ( 4 6 ) to a-phellandrene. 22 Single

has examined

electron transfer processes are involved in the photo-chemical (2+2)-cycloaddition of

the boron derivative

( 5 5 ) . a n enol of

another 1 , 3-diketoneI to acyclic (56) and cyclic alkenes ( 5 7 ) in acetonitrile solution. The reaction follows the usual path end the diketones ( 5 8 ) and (59) respectively are formed in the yields 23 shown under the appropriate structures.

(2+2)-Intermolecular addition

between

cyclopentenone

and

the

alkene (60) results in regiospecific and stereospecific addition yielding

the

adduct

(61). Several

process were described starting

material

examples

and the adducts

for

the

of

this

type

of

( 6 1 ) were used a s the

synthesis

of

spatane

diterpene

d e r i v a t i v e ~ . ~ ' Some other examples of the synthetic utility of the process were also reported. 25 (2+2)-Cycloaddition is a common photochemical reaction which can make use of a variety of enone systems.

Thus

cyclobutane electron-rich

the

enone

derivatives

(62) with

in

its

both

triplet

state

yields

electron-deficient

and

alkenes. 26 Photochemical addition of isoprene to

the dione ( 6 3 ) affords the cyclobutane adduct (64). This adduct is thermally labile and can be converted into the hydroindole (65). T h e product

formed in this manner is regioisomeric with

that formed by the thermal addition of isoprene to the dione (63).27 Lange and Gottardo"

report

a synthetic route to the

bicyclic ketones (66). This involves the photochemical

(2+2)-

cycloaddition of cyclic alkenes t o the enones ( 6 7 ) which give the adducts ( 6 8 ) in good to high yield. These are then converted by

Photochemistry

94 0

Ph

(58) 78% 92% 69%

69% 23% 68%

+-F Ph

H

Me H

0 (63)

(61) 63%

Me02C M e o 2 c q ) n

-( f i i 0

0

(67)

(68) 78% 91 % 73% 63%

m =l,n =I m =l,n=2 rn=2,n =1 m =2,n = 2

Scheme 2

(66)

90% 60O/O 51yo 60%

1112: Enone Cycloadditions and Rearrangements

95

’RJf R2 R2

(70)

(69)

R’ = H or F; R2 = H or Me MeMe

4

R’ R1

R’

R2

(72)R’

H H Me

(744

R2 But Me But

0

0 MeMe

Me Me

R’

Me Me

H H

(78)68%

(79)7%

H H

(80)25%

(81)1%

Photochemistry

96 a

thermal

path

to the

ring expanded

(scheme 2).

compounds

28

Tetramethoxyethylene yields the oxetanes ( 6 9 ) by photochemical (2+2)-addition to the enones (70). A study of solvent effects found that

the use

of

acetonitrile

a s solvent

significantly

changed the reaction path to rield reasonable quantities of the cyclobutane derivatives

( 7 1 ) . The authorseg propose

that

the

oxetanes are formed via a contact ion pair while the cyclobutanes arise by way of a n exciplex.

29

The enones ( 7 2 ) photochemically react with tetramethylethylene o n irradiation at 350 nm. T h e products (73) and ( 7 4 ) formed from the

(75) formed by bonding

reaction arise by way of the biradical

between the alkene and the a-carbon of the enone. Conventional 1.4-cyclization yields the normal (2+2)-cycloadduct (73) while 1,5-addition ultimately yields the bicyclic products reaction

is

apparently

insensitive

to

changes

( 7 4 ) . The

in

C5

the

substitution of the ketone. 30 Changes to the alkene substituents does

alter

the

reaction

in

that

with

the

acetal

( 7 6 ) the

principal product formed is the cyclized compound ( 7 7 ) arising by the 1.5-reaction path.

Corey

and

his

coworkers

31

32

reported

many

years

ago

that

the

photocycloaddition of cyclohexenone to cyclopentene afforded only two

cycloadducts

temperatures. Schuster

In a

in

irradiation

was

reinvestigation of have

et

irradiation

when

isolated

acetonitrile

at

carried

out

at

low

this addition reaction

four products ambient

(78)-(81) from

temperatures.

In

a

detailed study the mechanism for the addition w a s discussed in terms o f intervention of planar and twisted triplet states of the

1112: Enone Cycloadditions and Rearrangements

97

cyclohexenone. The photochemical addition of the cyclic alkenes ( 8 2 ) to the cyclohexenones ( 8 3 ) have been studied with a view to establishing the repioselectivity of the process. All the alkenes undergo (2+2)-cycloaddition but increasing ring size brings about a reversal of the selectivity. T h e small ring alkene ( 8 2 , n favours head to head addition while the alkene ( 8 2 , n a preference

for head

to tail

=

=

1)

3 ) shows

reactivity and the results are

shown in scheme (3). T h e authors3'

reason that the reversal in

regioselectivity is not consistent with the dipolar interaction hypothesis. However, they suggest that the difference might be due to the relative stabilities of the intermediate free radicals which lead to the products.3' of

phenanthrene

A study of the (2t2)-cycloaddition

carboxylates

has

been

carried

out.

The

irradiation of a mixture of the phenanthrene ( 8 4 ) and the styrene

(85) in benzene affords the adduct (86) from the singlet excited state. A cyclobutane ( 8 7 ) with the same stereochemistry is also obtained in the intramolecular addition of the derivative ( 8 8 ) . However, this is the minor product and the principal product from this

reaction

separating

and

from others with

the phenanthrene

and

a

longer methylene

chain

the alkene components is the

oxetane derivative (89). T h e formation of products arises from a n exciplex and conformational restraints within this leads to the preference for oxetane formation.

35. 3 6

Photoaddition of ethyne to the hexenulose

(90) occurs stereo-

specifically to afford the adduct (91). The adduct can be readily 38 reduced to the cyclobutane derivative ( 9 2 ).37 Somekana et a l . have reported in considerable detail the outcome of cycloaddition reactions to some pyrones. T h e influence of the type of alkene undergoing addition was also assessed. Two examples from this

98

Photochemistry

8.

0

MeO&

+

(83) a; R = Me

+ (y$I)" R H ratio > 95:5 5050 11:89 >95:5 60:40 300 n r

derivative results

in

its

conversion

into

the

0-lactam

(259). Several

examples of this conversion were reported. T h e detailed study has suggested that the mechanism of the conversion t o the lactams involves

an

intramolecular

electron

transfer

from

the amine

nitrogen to the keto group affording the radical cation / radical anion. Bonding within this as shown in scheme ( 8 ) affords the final product. 97 The indandiones (260a) undergo Norrish Type I fission and recombination to afford the phthalides (261). When amine substituents were incorporated (260b) the reactions were less efficient

and gave a complex mixture of

endo-diketone

(262)

photoenolization

has

been

shown

into the exo-diketone

to

product^.'^ undergo

The

double

(263) o n irradiation in

the presence of triethylamine. A SET mechanism is thought t o be operative. 99 N-Methylphthalimide undergoes photochemical addition of allyltrimethylsilane to yield the two adducts (264) and (265). The former o f these products is formed from the singlet state while

the

latter arises

via

the trip1et.l"

A

study of

the

phthalimide derivatives ( 2 6 6 ) has provided another example of intramolecular energy transfer. The irradiation of the derivative (266,

n

=

1)

illustrates

that

Norrish

Type

I1

hydrogen

abstraction does take place but only to a small extent with the product

(267) formed in only 5% yield. The dominant

yields the alcohol

( 2 6 8 a ) and methyl

reaction

formate. These products

arise from 0-0 bond fission followed by elimination of methyl formate and the production of the radical (269a) which combines

Photochemistry

124

Ph2C=C=NC6H4--p

c??o&x

0 % :

-Me *

Or Ph2C=C=O

0

Ph

0

Ph

(254)

(256)

(255)

Mv*

h v1>300nm'

h v/800nm=

0

I

NO2

Mep 0

I

NHCOOBu'

Scheme 8

\

H

0

CH2R

(260) a; R = H, Me, Et or PhCH2 b; R = PhNH, MeNH or Me2N

Me3SiCH2

1

0 (264)

(261)

0

H o

lIl2: Enone Cycloadditions and Rearrangements 0

125

OOH

0

0

0

0

0

(268) a; n = 1, 72% b; n = 5,40%

0

(269) a; n = 1 b;n = 5

(270) a; n = 1 b;n = 5

0

E&%kN E

'

H

CN

0 H

0

H

0

(273) R' = R2 = H R' = H, R2 = Me R' = Me or MeO, R2 = H

(275)

(274)

0 0

0 (277)

0 (278) 46%

126

Photochemistry

with a hydroxy radical to afford the final product. T h e isolation o f N-methylphthalimide ( 2 7 0 a ) in 3% yield confirms the generation of

radical

(2698). When

a

longer

chain separates

the hydro-

peroxide and the phthalimide groups the reaction is analogous to the preceding and yields the corresponding alcohol (268b) and Npentylphthalimide (270b). The operation of the reaction in this instance is reasonable evidence that a n electron transfer process controls the outcome of the irradiation."'

T h e diester ( 2 7 1 )

undergoes (2+2)-cycloaddition to afford the cage compound ( 2 7 2 ) in

19% yield. The

formation

configuration within arrangement

in

the

it

of this compound

helped

original

to confirm compound. lo2

and

the

endo-

the stereochemical The

photochemical

reactivity of the trione ( 2 7 3 ) is dependent o n the substitution pattern.lo3

Irradiation of the anhydride ( 2 7 4 ) in nitrogen or

argon matrices has been studied. T h i s treatment gives the enyne nitrile ( 2 7 5 ) 0s the principal product arising by a two photon process. I t has been established that irradiation using nm yields the ketene ( 2 7 6 ) by loss of CO 2-position. Subsequent

irradiation of

2

(277)

undergoes

benzophenone sensitization

( 2 7 6 ) affords the final

photochemical

with the former being predominant.

a

study

of

In contrast, the dimerization

on

in dioxane as solvent using a high

pressure Hg arc lamp. Two products

The results of

340

preferentially from the

product by decarbonylation and rearrangement. lo' anhydride

>

( 2 7 8 ) and

( 2 7 9 ) are formed

105

the photochromic behaviour of

the

fulgide ( 2 8 0 ) and its conversion to (281) have been reported.

106

The photo-isomerization of the fulgide ( 2 8 2 ) brings about 2 to

E isomerization followed by cyclization

to

(283). While

this

compound is thermally stable in neutral media heating in base

IIi2: Enone Cycloadditions and Rearrangements

127

Me Me Me

Me

Me

(283)

(284)

(282)R = Me (285)R = H

Me

Me

Me

Me

(286)

(287)

0" R

,

$

,SiMe3

~

~

~

~ Rl&

e2

R

0 (291)a;R = Me

b; R = Ph c; R = But

0 (292)R1 = But, H or SiMe2SiMe3

OH

(293)

R , ~ k i M e 2

R2 R2

R2 = Me or Ph

0

(294)

Photochemistry

128

107 brings about isomerization to ( 2 8 4 ) by a 1.5-hydrogen shift. 108 have reported details of the structure and Metelitsa et a l . photochemical reactivity of the fulgide ( 2 8 5 ) and its conversion into the isomer (286).lo8 Only E-2-isomerization resulted o n the 109 irradiation of the indole fulgide (287).

6 The

Quinones

quinone

indenone

( 2 8 8 ) undergoes

( 2 8 9 ) . The

photochemical

reaction

is

thought

of

the

conversion to proceed

to the via

the

110 unlikely biradical (289).

The

photochemical

reduction

pquinones

(291)

with

triethylamine has been studied by the photo CIDNP technique. ( 2 9 2 ) is converted

The quinone

irradiation at

1 > 520

to a coloured

111

intermediate on

nm in hexane in the presence of ketones

such as acetone and benzophenone. Work-up of the reaction mixture affords

the

identified

adducts

as

Cyclization

of

( 2 9 3 ) . The

( 2 9 4 ) by

key

spectroscopy

the quinones

(295)

intermediate at

low

in anaerobic

has

been 112 temperature. conditions in

benzene using 410 nrn light yields the tricyclic compounds ( 2 9 6 ) as the principal products. Other minor products are also formed. This cyclization mode, a Norrish Type 1 1 analogue, has been used to yield

tetracyclic compounds using

(295) when R1-R2

is, for

example, part of a cyclohexene ring. Again the yields of product are h i g h .

113

The photochemical reaction of the naphthoquinone ( 2 9 7 ) with ally1 alcohol affords the cyclobutane derivative ( 2 9 8 ) whose structure was

determined by X-ray diffraction analysis. T h e final product

M2: Enone Cycloadditions and Rearrangements

129

(295)

R'

R2

Me Me Me H H

H Me Me Me Me

R3 H H Me H Me

80% 82% 79% 49% 77%

0

o

H " K R

0 (301) a; R = H

(300) R = n-CGHla, CH2CI, CHC12, or CF3

b; R = ( C H 2 12 a b

0

HO

*fJ$& 0 (302) quantitative

OH

0 (303)

0YCH-'

c; R = C

H

-2

D

130

Photochemkrry 0

0

AQCN,H

COMe I

(305)20%

!

AQCNH

NHCOMe

COMe

0

(306)17%

(307)26%

NHCOMe 1

IIl2: Enone Cycloadditions and Rearrangements is presumably

formed by way

of

131 the

(Z+Z)-adduct ( 2 9 9 ) which

subsequently undergoes a Norrish Type I 1 reaction with the alkoxy 114 group to yield the isolated product.

A

study

of

the

photochemical

1.5-proton

transfer

anthraquinone derivatives ( 3 0 0 ) had been reported.

in

the

115

The anthraquinone derivatives ( 3 0 1 ) are reactive in the triplet state and undergo dealkylation at the peri position to afford (302). The process involves the generation of a biradical (303) by a 1.6-hydrogen

transfer, reminiscent

of a Norrish Type I 1

process. The use of derivative ( 3 0 1 b ) allowed the isolation of o-nitrophenylpropanal

showing the fate of the eliminated alkyl

group. A detailed study of solvent and substituent effects using ( 3 0 1 c ) was also reported. '16 ( 3 0 4 ) in acetonitrile

under

T h e photochemical argon and

using

reactivity

copper

of

sulphate

filtered light has been studied. Three products (305). (306). and ( 3 0 7 ) are obtained and the authors"' involves

a n intermolecular

suggest that the reaction

hydrogen abstraction. T h e

radical

(308) produced by this subsequently undergoes rearrangement or 118 Skuratova fission processes to yield the isolated products.

'"

has

reviewed

the

photoreduction

reactions

of

anthraquinone

sulphonates

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G.

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M . Cavazza,

M . Zandomeneghi, and F . Pietra, J . C h e m . S O C . ,

C h e m . C o m m u n . , 1990. 1336.

87.

T. Fujinara. N. Nanba, K. Hamada, F. Toda, and K. Tanake, J . O r g . C h e m . , 1990. 5 5 , 4532.

88.

F.

A.

Amay-Guerri,

Rivera-Sagredo.

and

J.

Sanz.

J.

P h o t o c h e m . P h o t o b i o l . , A : C h e m i s t r y , 1990, 5 6 , 87.

89.

K . Maruyarna and Y. Matano, B u l l . C h e m . S O C . J p n . , 1990, 63, 2218.

90.

N.

A.

Al-Jalal,

Gopalakrishnan,

J.

hi.

J.

Ijem,

Photochem.

F.

Al-Omran.

Photobiol.,

A:

and

B.

Chemistry,

1991, 5 5 , 339. 91.

M . Kaftory, F . Toda, K. Tanaka, and M. Yagi, Mol. Cryst. Liq.

Cryst.,

1990, 186, 167

(Chem. A b s t r . .

1991, 114,

42402). 92.

D. R . Boate. L. J . Johnston, P. C. Kwong, E. Lee-Ruff, and J . C. Scsiano, J . Am. Chem. S O C . , 1990, 112, 8858.

93.

H. Frauenrath.

M.

Kapon. and M . B. Rubin, Isr. J. C h e m . .

1989, 29, 307 ( C h e m . A b s t r . , 1990, 113. 5754). 94.

T. Sano. Y . Horiguchi, K. Pharm.

58901 1 .

Bull.,

1990, 3 8 ,

Imafuku.

366

(Chem.

and

Y . T s u d a , Chem.

Abstr..

1990.

113.

IIl2: Enone Cycloadditions and Rearrangements 95.

T.

Sano,

Y.

Horisuchi,

139 K.

Imafuku,

M.

Hirose.

H.

Takayanagi, H. Ogura. and Y. T s u d a , C h e m . P h a r m . B u l l . ,

1990, 38, 370 ( C h e m . A b s t r . , 1990, 113, 58901). 96.

E. Terpetschnig, G. P e n n , C . Kollenz, K. Peters, E.-M. Peters, and H. G. von Schnering,

Tetrahedron,

1991, 4 7 .

3045. 97.

S. T . Perri, S. C. Slater. S. G . Toshe, and J . D. White, J . Org. C h e m . , 1990, 5 5 , 6037.

98.

P. Hrnciar, A. Caplovsky. J . Donovalova, and L. Marton. A c t e F a c . R e r u m N e t . V n i v . C o m e n i e n a e . Chim., 1990, 3 8 , 19 ( C h e m . A b s t r . . 1991, 114, 163912).

99.

B. Pandey. P. V . Dalvi, A . A . Athawole. B. G. Pent. and P. P. Kewale, J . Chem. Soc., C h e m .

Commun.,

1990. 1505.

100. Y. Kubo, E. Taniguchi, and T. Araki, H e t e r o c y c l e s ,

1989.

29, 1857 ( C h e m . A b s t r . , 1990, 112, 235122). 101. S. Matsugo and 1 . Saito. T e t r a h e d r o n L e t t . , 1991. 3 2 , 2949. 102. K. Ito. Y . Noro. K. Saito. and K. Takahashi, B u l l . Chem. SOC. J p n . , 1990. 63. 2573.

103. R. Singh and S. Lahiri.

J.

Chem. R e s . ,

Synop., 1991, 22

( C h e m . A b s t r . , 1991, 114. 142829).

104. H. H . Narn and G . E. Leroi, T e t r a h e d r o n L e t t . ,

1990, 31,

4837. 105. V. T. Hoffmann and H . Musso. C h e m . B e r . , 1991, 124, 103. 106. Y . Yokoyama, H . Hayata, H . Ito, and Y. Kurita, B u l l , C h e m . SOC. J p n . .

107. A .

1990. 6 3 , 1607.

V. Metelitsa, 0. T. Lyashik. N. V . Volbushko, E. A .

Medyantseva, M .

I. Knyazhanskii. and V . I . Minkin. Khim.

Geterosikl. Soedin.,

81456).

1990, 1137 ( C h e m . A b s t r . ,

1991, 114,

Photochemistry

140 108. A .

V.

Metelitsa.

N.

Kozina.

V.

Knyazhanskii. Geterosikl.

V.

0. T. Lyashik,

E.

Volbushko, I.

Soedin..

Minkin.

S. M . A.

and

1990, 3 3

Aldoshin. 0. A .

Medyantseva.

A.

0. Atovmyan.

(Chem. A b s t r . .

M.

1.

Khim.

1990. 113.

39667). 109. I .

Yu.

Grishin,

0. G .

M.

Rodin.

Yu.

Chunaev,

N.

Przhiyalgovskaye, V . F. Mandzhikov. S. M. Aldoshin. and

M. L.

0. Atovmyan, Zh. Obshch. K h i m . , 1990, 667 (Chem. A b s t r . .

1990, 113, 582793). 110. D . W . Jones and A. P o r n f r e t . J. Chem. S O C . , P e r k i n Trans. 1 . 1991, 13. 111. V.

I . Porkhun, B. D . Sviridov, and G . A . Nikiforov. Zh.

Obshch.

K h i m . , 1990, 6 0 ,

1607 (Chem. A b s t r . ,

1991, 114,

80813). 112. K. Sakamoto and H. Sakurai, J . A m . Chem. S O C . . 1991, 113, 1466. 113. H. Iwamoto and A . Takuwa, B u l l . Chem. S O C . J p n . , 1991, 64, 724. 114. G. A . Kraus. J . S h i , and D. Reynolds, S y n t h . Commun.. 1990. 2 0 , 1837 (Chem. A b s t r . , 1991, 114, 42417). 115. T. P. Smith, K. A . Zaklika, K. Thakur, and P. F. Barbare,

J.

Am.

116. R.

L.

Chem. S O C . , 1991, 113, 4035.

Blankespoor,

R.

L.

De

Jong,

R . Dykstra,

D.

A.

Houstra, D. B . Rozema, D. P. VanMeurs, and P. Vink, J . A m . Chem. S O C . , 1991, 113, 3507.

117. K . Maruyema. M . Hashimoto, and H. Tamieki, Chem. L e t t . , 1990, 2165. 118. s .

I.

Skuratova,

Zh.

Fiz.

A b s t r . , 1990. 112, 215770).

Khim..

1989. 63, 2577

(Chem.

3

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

1 cis-trans

Reactions of Alkenes

1sorerization.-

Some

research

associated

with

the

trans-cis-isomerization of stilbene and its derivatives has been published

during

the

year.

Saltiel

studied the fluorescence of

and

his

coworkers’

have

cis-stilbene in solution and have

shown that there is an adiabatic conversion of the cis-stilbene singlet

to

detected

the

is

trans-stilbene

that

of

the

singlet

and

trans-isomer.

the The

fluorescence photophysical

properties and photochemistry of fluorinated stilbenes such as

3,5,3’.5’-tetrafluorostilbene

and

1,2-diphenyl-l.Z-difluoro-

ethene has been studied in detail2 a s have the photocyclizations

of cis-di- (2-naphthyl )ethene and f luorinated derivatives.

The

polarity of the solvent is important in the photochemical E-Zisomerization of the stilbene analogue ( 1 ) . In contrast the Z-Eprocess is not influenced by this effect. T h e isomerization can be brought

about by direct

or biacetyl-sensitized

irradiation

both of which involve the triplet state of the alkene.

4

Addition Reactions.- A pulsed-laser induced SET from stilbene to dicyanoanthracenes has provided a general route to the production of stilbene cation radicals.’ efficient

alkylation of

some electron

reaction

involves

irradiation

the

have reported the

Mizuno et

of

deficient the

alkenes. The

alkenes

( 2 ) in

propionitrile as solvent with tetrabutyl stannane. SET generates

Photochemistry

142

xT NC

CN

(3)a;85% b; 20% c; 78%

(2)a; X = H b; X = O M e c; X = M e d; X = CI e; X = C N

d; 66% e; 61% Me

Me

I

Ph2CHCHNHR

+

I

Ph2CHCHCH2CN + Ph2CHCH2Me

(5)

(6)

R = H, 44% R = Pr', 65% R =(CH2)20H, 58%

+

Ph,CHCH=CH2

-

+

-

5% 3yo

2O/O

7%

1O/O

Ph2CH2

23% 6o/' 770

8%

10% Scheme 1

p Ph R

Ph H' 'R (7) R = Me, P t

Scheme 2

+J? R

143

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds

a butyl radical and the radical anion of the alkene. Combination of these affords the observed products ( 3 ) in yields ranging from good to excellent. I t is clear that substitution of the aryl ring can play a m a j o r role as shown by the methoxy derivative where yields of product are poor.

Electron transfer conditions using

dicyanobenzene as the sensitizer have also been used with the

( 4 ) . Irradiation in acetonitrile solution produces the

alkene

corresponding radical cation which reacts with ammonia or amines to yield the adducts ( 5 ) (Scheme 1 ) shown. Other reactions are also

involved

reduction, methane

.'

observed

such

as

addition

isomerization

and

Intramolecular on

(7, R

=

of

solvent

the

transfer

diphenyl

photochemistry

is

derivatives

(7). Thus

the intermediate

(8). This

amine

Me) yields

(6).

affording

fragmentation yielding

electron

irradiation

irradiation of

of

subsequently undergoes hydrogen transfer to afford either of the intermediates ( 9 ) and (10) when the substituent on the nitrogen is a methyl

group. These

biradicals

then react

to yield

the

products (11-13) shown in the Scheme(2). When the substituent on the nitrogen is other than methyl no evidence for the formation of pyrrolidine

product

( 1 2 ) is observed. The authors'

that the hydrogen transfer

step in these cases

(7,

suggest

R

=

i-Pr)

occurs regioselectively affording only the biradical ( 9 ). 8 A full account of the photophysical and photochemical processes involved has

supplemented

this original

material.

One aspect

reported

relates to steric factors and i t was shown that the derivative (7,

R = t-Bu) failed to cyclize and only gave a quantitative

yield

of

coworkers"

the

aldehyde

derived

from

(13).'

Pandey

and

his

have used intermolecular SET transfer with the smine

(14). Irradiation in the presence of dicyano-naphthalene as the electron accepting sensitizer is effective. Several examples of

Photochemistry

144

*Q

Ph

Ph

H’ ‘Me

Me

s?,.

(9) Me0

(14)

I

h v DCB

MeOyJJ

+

Me0 Meo*

v

Me0

(17)

OMe

(15)

Scheme 3 Me0

Me0

Scheme 4

0 hv

+

H

(1 9)

12%

33%

Scheme 5

45%

4%

lIt3: Photochemistry of Alkenes, Alkynes, and Related Compounds

145

the process were studied but the one shown in the Scheme ( 3 ) will suffice to illustrate the scope of the process. Thus irradiation of (14) at

1 >

280 nm brings about cyclization to (15). This is

stable to the initial exciting wavelength but i f

1 > 350 nm is

then used a second cyclization occurs v i a what is essentially a photo-Mannich

reaction

involving

the

(16).

intermediate

Cyclization within this and trapping by methanol yields the final 10 product ( 1 7 ) in 62% yield.

Rearrangement Reactions.- Both the c i s and the t r a n s isomers of the cyclobutene ( 1 8 ) undergo photochemical ring opening to yield the products shown in Scheme ( 4 ) . The yields from both compounds are similar and both show the absence of wavelength dependence in the range studied ( 1 8 5 , 193 and 214 n m ) . T w o mechanisms are thought

possible

to

account

for

the

non-stereospecificity

encountered. These are homolysis of the cyclobutene C-C bond or disrotatory opening to yield a diene in its excited state. The former mechanism

is thought

to be more probable."

A similar

conclusion w a s reached for the ring opening of the cyclobutenes ( 1 9 ) shown in Scheme ( 5 ) where non-stereospecific w a s suggested."

be

important

ring opening

I t would appear. however, that ring strain can

in the ring opening processes and

Scheme (6) l3 and Scheme (7)l'

do

two examples,

illustrate that reasonably

high stereospecificity c a n be observed and the major product in each case is that expected from the allowed disrotatory

ring

opening.

Anion Beections.- The anions (201, generated by deprotonetion of the corresponding alkenes. undergo photochemical conversion to the allenes (21) on irradiation in DMSO solution at

1 >

450 nm,

Photochemistry

146

cR3 H H

cq3

25% from cis 69% from trans

75% 31 yo

H H

Scheme 6 23% ex trans

76%

82% ex cis

11%

Scheme 7 R Ph&R

(20) R = H orPh

P

h

T

(211

R

0 1 Ph (22)

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds to

avoid

irradiation

of

the

parent

147

chloroalkene.

When

the

reactions are carried out in the presence of furan the adducts ( 2 2 ) are obtained

in good yield. T h e reaction appears to be

sensitive to ring size since the corresponding anion ( 2 3 ) fails A full account of the excited

to undergo the addition process."

state carbon acid behaviour of the alkene ( 2 4 ) has been reported on irradiation of in 70% D 0 / acetonitrile. T h e product (25) is 2

predominant but is accompanied by the di-=-methane product (26) in a ratio of 3 : l . This latter product (26) is the only product 16 obtained on irradiation of (24) in benzene o r acetonitrile.

2 Di-x-methane

Reactions involving Cyclopropane Rings reactivity

of

a

variety

of

compounds

has

been

reported in the past years. The substituted lactones ( 2 7 ) have the correct structure for di-u-methane

reactivity and undergo

photochemical conversion by a vinyl-aryl interaction into the two products (28) and (29). The influence of changes in substitution on

the

central

carbon has been

investigated."

A

vinyl-aryl

interaction is also found in the irradiation of the alkenes (30) under nitrogen in benzene and results in the formation of the cyclopropanes ( 3 1 ) . Subsequent irradiative ring opening of these compounds was studied using electron transfer conditions. Thus the irradiation of ( 3 2 ) in acetonitrile with dicyenoanthracene as the electron accepting sensitizer results in conversion t o the

alkenes reported

(33).18 Ring by

Hixson

opening

and

by

Xing.19

electron The

transfer

radical

cation

is

also

(34) of

benzobicyclo(3,1,O)hexene can be generated on irradiation (at 300

n m ) in methanol with dicyanobenzene

as the electron accepting

sensitizer. The radical cation undergoes nucleophilic addition of methanol

to afford radicals which either abstract hydrogen

Photochemistry

148 H, P h

ArlwPh *.I

Ar2

Ph

Ph

A$

(30)

(31) A?=A?=Ph A? = p -tolyl, A? = P h A? = p -tolyl = A? Ar' = p -MeOC6H4, A? = Ph A? = p -MeOC6H4= A?

d C H 2 O C H 3

(35)a;R = H, 7%

(34)

b; R = p -CNC&, Ph

Ph

N C A C N CN (37)

CN

14%

OMe

(36)a; R = H, 11% b; R = p -CNCsHd, 24%

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds

149

affording ( 3 5 a ) and ( 3 6 b ) or which add to dicyanobenzene to yield (35b) and (36b).19 Others have also used SET from triethylamine for the ring opening of cyclopropanes as in the conversion of the derivative ( 3 7 ) into the alkene ( 3 6 ) in 57 X yield. The influence of the number of cyano groups on the cyclopropane ring was also 20 evaluated. Photochemical excitation of horobenzvalene (39) results

in

formation of a biradical which in the presence of

tetracyano-ethylene affords the three adducts ( 4 0 ) - (42).21 Bond fission

also

dominates

the

photochemical

reactions

of

the

quadricyclane derivative ( 4 3 ) on irradiation at 300 nm in hexane solution.

The

acrylonitrile

biradical

( 4 4 ) can

be

trapped

by

O2

or

by

(Scheme 8 ) . In the absence of a trapping agent

dimerization results in the formation of two types of dimer shown 22 grossly as ( 4 5 ) and ( 4 6 ) .

In

last

year's

review

Shi

and

his

coworkers

reported

the

photochemical fission of the triarylalkane ( 4 7 ) in methanol using quartz apperetus. The reactions observed are shown in Scheme 9. Several processes

take place

but

the one of interest

is the

formation of the carbene ( 4 8 ) by the extrusion of biphenyl. The carbene is the precursor to the ether ( 4 9 ) and the slkene (50). The formation of the carbene is the result of a phenyl-phenyl interaction, a di-z-methane

process, followed by extrusion of

biphenyl.23 This year has seen the publication of a detailed account of this reaction.24 Other systems are also reactive in this mode and the alkynes ( 4 1 ) can be converted into biphenyl and its derivatives as shown in Scheme 1 0 for the reaction of (5la). The zwitterionic nature of the proposed bridged intermediate (52) is substantiated by substituent effects. Thus the irradiation of ( 5 1 d ) affords biphenyl (7.1%) and p-methoxybiphenyl

(30%) which

150 Ph

Photochemistry

Lhv ___t

6 (44)

(43)

Scheme8

Ph

p h l &

\

Ph

Ph

/

(46) 3%

(45) 49%

h vlMeOH

Ph (47)

(27%)

(50) 10%

Scheme 9

H (49) 16%

lIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds R2

R3

(511 a; R’ = R2 = R3 = H b; R‘ = R2 = H,R3= Me c; R‘ = R2 = R3 = Me d; R’ = R2 = H,R3= Me0 e; R’ = R2 = R3 = Me0 (a)

hv

I

MeOH argon atmos.

25%

24% Scheme 10

151

Photochemistry

152

is interpreted as showing a marked preference for bonding between a methoxy substituted aryl group and a phenyl group rather than between two phenyl groups.25 Further details of this have been published.26 An analogous path is followed by the alkenes ( 5 3 ) where biphenyl and the ethers (54) are obtained. In addition a 26 vinyl-aryl di-%-methane process affords the cyclopropanes ( 5 5 ) . This report supplements one already published."

T h e synthetic

utility of this reaction type has been extended further by the study of aryl triphenylmethane derivatives.

In these examples

irradiation brings about a di-x-methane reaction between two aryl groups with the subsequent extrusion of biaryls and the formation of a carbene intermediate. When the reaction is carried out in methanol

these

are

trapped as

the

corresponding

ethers. T h e

results of irradiation of (56) are shown in Scheme ( 1 2 ) where it can be seen that two types of bridging take place one between two phenyl groups and one between a phenyl and the p m e t h o x y p h e n y l group.

**

Analogous

react ions

were

reported

for

psridyl

derivatives such as (57). I t is apparent that this reaction type is

very

general

and

addition

path

ester

examples

(58) are

also

as is the phosphorus containing derivative (59).30 In

the di-z-methane

methyl

foregoing

reactive*'

case

its

the

acetic

former

and

to

triphenyl

the

acid

in

( 6 0 ) accounts for

the

formation of biphenyl and methyl 2-methoxyphenylacetete as shown in

Scheme

(13). The

two

other

products

isolated.

methyl

phenylacetate and methoxy diphenyl methane arise by a n oxa-di-xmethane (61) path.29 The phosphorus containing derivative is a l s o reactive but only traces of the carbene adduct (62)are formed. Other fragmentation processes are also found with this system and mixed biphenyls are formed by combination between phenyl groups

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

R=H R=Me R = Et

8% 16% 24%

153

5% 15% 16%

14% 8% 9%

Scheme 11 Ph g

j Ph

e

O

M

e

Ph-Ph 8%

+

(56)

17%

OMe 18%

Scheme 12 = p Jh+(

Ph (57)

Ph I

Ph-C-C02Me I

Ph

hv

+

-Ph-Ph MeOH

9.8%

OMe

PhCOeMe + PhAPh 11 Yo

Scheme 13

11.3%

C0,Me

+

1

Ph-CH 1

OMe 10.5%

154

Photochemistry

and the aryl groups o n phosphorus.30

Interest in the aza-di-x-methane reaction, a useful regiospecific reaction for the conversion of B,y-unsaturated aldehydes via the corresponding derivative,

imine has

oxime

or

continued.

acetate

The

into

present

e

work

cyclopropane describes

the

conversion by acetophenone-sensitized irradiation of the stable derivatives (63) of 2,2-dimethyl-4,4-diphenylbut-3-enel in the aza-di-%-methane

rearrangement

to

afford

the

cyclopropanes

(64).31 The chemical yields can often be high e . g . 86% yield end quantum yields

of 0 . 1 2 and

(65)

acetates

are

also

acetophenone-sensitized

above

can be attained.

photochemically

irradiation

are

reactive converted

The oxime and into

on the

corresponding cyclopropane derivatives ( 6 6 ) . 32 The first report of the successful aza-di-x-methane rearrangement with all elkyl substitution ( 6 7 ) affording

(68) was also described. A further

report has also approached

the irradiative cyclization of the

( 6 9 ) . Disubstitution (69a) appears to be necessary

derivatives for

the

success of

cyclopropanes

the aza-di-x-methane

( 7 0 ) . With

conversion

mono-substitution

at

C-5

into

the

(69b) the

cyclization fails a n explanation of which could be the operation of a deactivating free rotor effect although such a n explanation

is

speculative.”

The

influence

of

differing

substitution

patterns on the normal skeleton has also been investigated. Thus the irradiation o f the oxime acetate of 2.2-dimethyl-4-phenylbut-3-enal

(71) which undergoes conventional

aza-di-a-methane

rearrangement into the oxime acetate of 2,2-dirnethyl-3-phenylcyclopropane affording

carboxaldehyde.

only

the

This

trans-isomer

reaction (72).

is

stereospecific

Surprisingly

the

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

zi.x

Ph

P

h

(63)

155

xN-X I

(64)

X = NHCONH2, NHBz, or OBz

FR3

R1

R’ “OAc

(65) R’ Ph Ph Ph

R2 Me Ph Me

R3 H

H Me

F;

Me

3;

M

Me

R2

Me ‘OAc

R1 “OAc

Me

A:

-OAc

e

(69) a; R’ = Me, R2 = COzEt, CN, or CH20COMe b; R’ = H; R2 = C02Et, CN, CH20COMeor CH20Me

xN-OAC I

Photochemistry

156 irradiation

of

the

oxime

acetate

of

2,2-dimethyl-3-phenyl-

but-3-enal (73) follows a different reaction path and affords the oxime acetate of 3-methyl-2-phenylbut-2-enal

(74). The failure

o f this diene ( 7 3 ) to undergo the aza-di-x-methane be

due

to

a

preference

for

the

formation

of

is thought to a

stabilized

birsdical which c a n be produced by bonding between the methylene end of

the excited state alkene and

acetate. 34 Other methane

have

rearrangement

could

conversion.

by

direct

quinoxaline

derivative

be

or

the carbon of

suggested involved

sensitized

a n aza-di-x-

in the photochemical irradiation,

(76).

( 7 5 ) into

that

the oxime

They3'

of

suggest

the that

excitation to the triplet state initially affords the biradical ( 7 7 ) which subsequently unzips and rearranges to afford the final product

on

20%

irradiation of

yield.

Other

systems

( 7 8 ) results in a

are

also

reactive

and

(2+2)-cycloaddition yielding

( 7 9 ) . T h e study of this and other compounds does not negate the

involvement of the aza-di-%-methane

interaction. Another example

which could be construed as an aza-di-x-methane process involves the rearrangement o f the barrelene derivatives (80). The reaction is

subject

derivative

to

substituent

effects

and

(80a) affords two products

irradiation

of

the

(81) and (821s) by benzo-

vinyl and pyrazino-vinyl bridging respectively. With the dicyanoderivative (80b) only pyrazino-vinyl bridging occurs resulting

36 in the quantitative formation of ( 8 2 b ) .

A review has dealt with the di-x-methane type rearrangements of

a,B-unsaturated

organoboranes. 37

Related

to

this

is

the

publication3* which describes the photochemical behaviour of the borate

( 8 3 ) whereby

direct

and

a

sensitized

novel

di-x-borate

irradiation

to

process occurred on

give

the

norcaradiene

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

157

(76)20%

(75)

(77)

&&yJ

N-

I

(79) quantitative

(81) 42%

(80)a; R = H b; R = C N

&;f Pr

(82)a; R = H, 58% b; R = CN, 100°/o

[

Ph\

Ph

n B /' P h

]

-

Ph (84) 30% (direct) 20% (sensitized)

I

PhC=C-B-Ph Ph I Ph

(85)

1-

[

Ph, ,Ph

PhAPh

(86)50%

R

Photochemistry

158

derivative (84). A reinvestigation o f the photochemical behaviour o f the borate ( 8 3 ) to visible and U . V . irradiation failed to give evidence for the generation of diphenylborene. The product from the reaction is p t e r p h e n y l , 39 Details of this work have now been

i t is concluded, although, not unambiguously,

published where

that the reaction proceeds by a biphenylyl-phenyl Further examples of

the di-x-borate

interaction.

process, for example

40

the

conversion of (85) and (86) into (87) and (88) respectively in reasonable yields have been reported. The cyclopropyl derivative ( 8 9 ) is also reactive but which

nevertheless

readily

converted

fails to yield an isolable product

identified

was

into

as

(90).

This

compound

1.2-dideuterio-1,2-diphenylpropane

is

on

treatment with MeOD. Direct irradiation at 254 nm w a s found to be effective in all of the cases. T h e influence of substituents, 41 i . e . the corresponding dimethyl borates, was investigated.

The

adduct

formed by

[ a ,clcyclo-octene

the photochemical

dibenzo-

addition of

to the triazole ( 9 1 ) by a photo Diels-Alder

reaction undergoes secondary photolysis. This process is e di-xmethane reaction and affords the rearranged edduct direct irradiation ( A conversion

into the

semibullvalene converted further

>

(92). 4 2 The

290 n m ) of the triene ( 9 3 ) results in its

three products

( 9 4 ) is

also

shown in Scheme

photochemically

into the cyclo-octatetraene irradiation. T h i s

product

(14). The

reactive

( 9 5 ) in 75 X

( 9 5 ) is

the

sole

and

is

yield

on

product

obtained on the sensitized irradiation of the starting material (93). Labelling studies have shown that the semibullvalene (94) is formed process

by

two paths

involving

the

the major

of which

intermediate

is a di-x-methane

(96).

The

sensitized

irradiation of ( 9 3 ) does not involve a di-%-methane

process and

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

159

(88)60%

CN

CN

(93)

(95) 70%

(94) 5%

Scheme 14

(99) E = C02Me

14%

Photochemistry

160

the fission of the internal bond of the cyclobutane affords the product directly.

43

The influence of a chiral crystal lattice o n the outcome of the di-z-methane irradiation

reaction of in

the

crystalline

the latter is obtained

under

the best

Further

( 9 7 ) has

phase

been

gives

studied. The chiral

two

di-x-

( 9 8 ) and (99). T h e former of these is racemic

methane products but

achiral

in high enantiomeric -2OoC,

conditions, i . e . at

studies o n

the enantiomeric

excess which

approaches 100%.

control

44

of photochemical

reaction have been reported by Gudmundsdottir and Scheffer. 45 The study focused o n the photochemical beheviour of the acid salt

(100) using a n optically active amine such a s proline

(101).

Irradiation of the salt in the crystalline phase resulted in the formation of the two

products ( 1 0 2 ) and

(103).

The

enantio-

selectivity exhibited is shown below the appropriate structure. The results indicate that benzo-vinyl bridging at the carboxylate salt bearing carbon is favoured.

45

The ethenoanthracene ( 1 0 4 ) undergoes a n acetone-sensitized di-xmethane reaction to yield Direct

( 1 0 6 ) in a ratio of 4:l.

( 1 0 5 ) and

irradiation also affords these products but in addition

e different

reaction path

yields

the new

products

( 1 0 7 ) and

(108). The formation of these products, which involves a chlorine atom migration,

is

thought

to arise

via

the biradical

(109)

followed by chlorine atom expulsion and radical recombination. Toluene-sensitized

irradiation

of

the

optically

pure

46

vinyl

cyclopropane derivative (110) affords a diastereoisomeric mixture of

the

alcohols

respectively.

( 1 1 1 ) and

Similar

(112)

treotment

of

in

70

and

the acetate

30

X

yield

derivative

of

161

1113: Photochemistry ofAlkenes, Alkynes, and Related Compounds

COOY

COOY

0 (103)

(1 02) Y, R + 94% e.8. (- 80) Y, S - 94% 8.8. (+ 76)

4%' e.e. (0) 6% e.e. (0)

E = C02Et C02Me

I

F02Me

* /

Me02C

CH2

P

Me ,,OH

+,,.OH

H

Photochemistry

162

(110) also yields the same two products (111) and ( 1 1 2 ) a s the acetates except that the preference is reversed with ( 1 1 2 ) being 47 the predominant product.

3

of

Reactions

Dienes.

Trienes,

and

Higher

Po 1yenes

UHF MO study

An article has reported the results of an a b i n i t i o

on the reactivity of penta-1,3-diene in the triplet state.48 The photoisomerizetion of 1,4-diphenylbuta-1,3-diene in non polar lor viscosity solvents has been studied. at

15K

(e.g.

of

114)

several has

efficiently

dienes

been

despite

a

Photochemical ring closure

113)

(e.g.

carried

''

out.

barrier

to afford

The

ring

predicted

cyclobutenes

closure to

occurs

prevent

the

disrotatory closure. 50 The dienes ( 1 1 5 ) undergo cycloaddition on direct

irradiation

to yield

the tricyclic

isomers

(116). The

triplet reactivity of the dienes was found to be dependent upon the substituents at C-7 and C-8.

The dienes

( 1 1 7 ) photocyclize

51

efficiently

in benzene

solution

under air to yield the naphthalene derivatives (118)-5 2 The 1 , Q diaza-1.3-dienes

(119)

undergo

a

variety

of

photochemical

reactions. The processes encountered either involve cyclization and loss of a functional group to afford 2H-imidazoles (120) or an alternative

reaction

path

yielding the phenanthridine

The

involves a six-electron

process

53

(121).

tachysterol/ergosterol

system

continues

to

attrect

considerable interest. A typical result has shown that the triols ( 1 2 2 ) can

be

transformed

on

irradiation

in

ether/THF

into

Previtamin analogues which are thermally converted into the

IIt3: Photochemistry of Alkenes, Alkynes, and Related Compounds

(118) n = 1 n =2 n =3 n =4

Ph

RNP NPh PCOPh

(1 19)

(1 20)

R = Me2CH or PhMeCH

HO

(122)a; R’ = Me, R2 = H b; R’ = H, R2 = Me

57% 57% 74% 58%

163

Photochemistry

164 vi tamin D,

analogues

( 1 2 3 ). 5 4 Triplet

sensitized

irradiative

conversion of the diene ( 1 2 4 ) into the trienes ( 1 2 5 ) has been the subject of a patent. The reaction is highly efficient affording u p to 09% of the ring opened vitamin D3 analogue.55 The provitamins

(126)

can

be

photochemically

isomerized

into

the

corresponding previtamins using light filtered through solutions of

nitrophenols.56

A

study

new

of

sensitizers

for

the

isomerization of the tachysterol system has been published. A

method

for

assaying

isomerization of

the

ergosterol

success

of

the

photochemical

been described. 5 9 The

has

E-Z-

57, 58

laser-

induced isomerization of provitamin D analogues at -196OC has been reported.

60

A study of the influence of changes in methyl substitution on the

photoisomerization of retinal derivatives been

reported. 61 5-Di-cis-retinal

reactive.

( 1 2 7 ) and

isomers are photochemically

( 1 5 ) shows a result which

Scheme

(128) has

is typical of the

series involving the conversion of the tri-cis-retinal (129) into the all cis-retinal ( 1 3 0 ) in 46% yield o n irradiation in hexane solution.

'*

Other analogues ( 131) and ( 1 3 2 ) have been synthesized

also and irradiation of (131) brings about clean isomerizetion around

the

C-13

bond.

The

retinal

analogue

(132) behaves

similarly to retinal and affords a mixture of the 9-Z-, and the 13-2-isomers. 63 The photochemical

11-Z-,

isomerization of the

retinal acetate ( 1 3 3 ) has been described. Direct irradiation of this compound results in isomerization of the triply substituted double

bonds.

isomerization

Short of

the

irradiation C-9

bond

and

times longer

brings times

about results

the in

isornerization of the C-13 bond a s well as yielding anhydroretinol

165

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

(124) R’ = HI alkyl, acyl, trialkylsilyl or alkoxyalkyl (125) R2 = H, OH, acyloxy, alkoxy, trialkylsilyloxy or alkoxyalkoxy R3 = CH=CHMeCHMe2

&R3

\

R’O

(126) R’ = HI alkyl, acyl, trialkjlsilyl R2 = H, acyloxy, alkoxy, trialkylsilyloxy R3 = CH=CHCHMeCHMe2, CH2CH2CHR4CMe2R5 R4 = R5 = OH, R4-R5 = OCMe20

Me

Me Me

Me

Me Me

Me

CHO

Me Me

CHO

Me Me

Me

Me

CHO (129)

Scheme 15

166

Photochemistry C , HO

M&Me

Me

(132) R = Me or Et

(131)

Me

Me

Me

CHO

CHO

(136)R = H or CF3

wMe Me

Ph

(144)

(145)

(147) R = Ph, Me or But

167

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds (134). These processes

result from the singlet state and the

authors64 implicate ionic species as a result of polarization of the singlet state. Triplet-sensitized isomerization coworkersg5

of

the C-9

have

and

reported

irradiation brings about

the C-13

the

result

bonds.'l of

the

Liu and

his

irradiation of

retinal ( 1 3 5 ) entrapped in €3-lactoglobulin when a photostationary state enriched in the 11-cis-isomer was obtained. Other workers have

used

photochemical

methods

analogues. Thus the irradiation

in the

(1

synthesis

of

66

retinal

> 4 3 5 n m ) of the analogues

( 1 3 6 ) affords the corresponding cis-isomers.

Irradiation

at

254

nm

in

hexane

solution

of

the

dihydro-

naphthalene ( 1 3 7 ) results in the formation of the benzobicyclohexene ( 1 3 8 ) as the principal product with a quantum yield of -4 67 4.7 x 10 . Irradiation of the naphthalene ( 1 3 7 ) in the presence

of

Originally

acid it had

only been

brings

about

isomerization

reportedg8 that

the

to

(139).

formation of the

bicyclic product ( 1 3 8 ) did not occur o n irradiation at 254 nm but that a two photon process w a s required with irradiation at 280 nm bringing about ring opening and irradiation at 400 nm causing recyclization to the final product. Duguid and MorrisonCi9 reason that a phase

two

photon process is also involved at 254 n a . The eas

irradiation of the same naphthalene derivative w a s also

investigated. Here i t was found that irradiation at 254 n a gave the methyl derivative (140)vie the S excited state within which 2 multiple 1.2-hydrogen migrations took place. When a n inert buffer gas.

butane, w a s used in the gas phase a unique reaction yields

the cyclobutene thermally

(141). It is proposed

relaxed

solution phase

intermediates A

which

that this is formed v i a are

detailed

not formed

in

the

study of the photo-

Photochemistry

168 chemical behaviour of the dihydro-naphthalene

(142) has shown

that there is n o dependence o n the polarity of the solvent o n the outcome

the

of

reaction. Many

products

the major

were

irradiation and

ones

are

from

formed

identified as

the

( 1 4 3 ) and

(144). The influence of temperature on the outcome of the process was e ~ a l u a t e d . ~ 'The photochemical reactivity of arylalkenes has reviewed. 71 A

been

detailed

examination of

the photochemical

reactivity of the indenes ( 1 4 5 ) and ( 1 4 6 ) has been carried out. The

studs

involved

the

use

of

labelled

compounds

rhich

has

provided details of the mechanisms involved i n methyl migration reactions and ring carbon changes.

The

singlet

state

reactivity

72

of

the

diyne

( 1 4 7 ) has

been

studied. 73 Irradiation of the compounds ( 1 4 7 ) in acetonitrile / water

results

in the formation of

the

two ketones

( 1 4 8 ) and

( 1 4 9 ) . These are formed v i a hydration of the two charge separated

states ( 1 5 0 ) and ( 1 5 1 ) with the former being predominant. When the substituent R is alkyl the ketone ( 1 4 8 ) is formed from the triplet state of the diyne.73 When the photochemical reactions are carried out in methanol addition of the solvent also takes place. The products obtained from these reactions ere shown with the yields in Scheme (16). Again the addition involves a polarized charge o n C-1. A excited reaction

states was

proposal

are

with

suggest that the

excited state with the negative

that both

involved was

encountered

author^'^

the singlet and

made. the

An

analogous

phenyl

triplet addition

derivatives

(152)

74 a f f o r d i n g the adducts (153) and ( 1 5 4 ) .

4

[ 2 + 2 ] Intramolecular Additions

A description of the photochemical synthesis of naphthalenophanes

IIJ3: Photochemistry of Alkenes, Alkynes, and Related Compounds

169

a"O

R = Ph

O

M

/

e

+

7%

-

R = BU'

10%

11%

R=Me

12%

19%

R = TMS

14% Scheme 16

H

R (152)a; R = Ph b; R = Bu'

(153)a; 23% b; 27%

(154)a; 24% b; 28%

170

Photochemistry

by

the

intramolecular

cycloaddition

of

the

naphthalene

derivatives (155) has been reported.75 A SET process controls the conversion of the diene ( 1 5 6 ) into the cyclobutane ( 1 5 7 ) in 7 8 X

yield

on

irradiation

in

benzene

solution

with

dicyano-

naphthalene as electron transfer sensitizer. Various examples of the reaction were described giving the cyclized product in 54 69 X

yield. Benzene, or

an arene

solvent,

success of the reaction. When acetonitrile allylation of

the sensitizer results

for

is vital

-

the

is used a s solvent

in the formation of the

three products (158)-(160).76 An extension of the above process to

the

styryl

analogues

( 1 6 1 ) affording

(162) has

also been

reported. The use of other compounds containing metals such a s Germanium and Tin was also described.77 A further report on the synthesis

of

derivative

me

ta-cyclophanes

( 1 6 3 ) has

by

reported

cyclization

that

the

of

the

styryl

three adducts

(1641,

(165). and ( 1 6 6 ) are obtained in a total yield of 20%.

The

photochemical

formation

acetophenone-sensitized

of

the

quadricyclanes

78

( 1 6 7 ) by

irradiation of ( 1 6 8 ) has been reported.

The quadricyclanes were used as substrates in approach to the synthesis of 1.5-dehydroquadricyclane. 79 The doubly-bridged Dewar benzenes

( 1 6 9 ) are

photochemically

reactive

in

a

dependent fashion. Thus the irradiation of (169e) at

Wavelength

1 < 280

nm

results in aromatization and the formation of (170a). The same effect is observed for (169b) when irradiation at 250 nm yields (170b). However, yields

prismane

(169b) at

A

derivatives respectively.

>

with

longer

derivatives.

280

(171s) 80

nm and

wavelengths Thus

yields (171b)

(2+2)-cycloaddition

irradiation the in

of

( 1 6 9 e ) and

corresponding

15

and

30

prismane %

yields

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds

Si

171

@

SiMe

R' = Br, R2 = Br or CI

@zo2Me

(&C02Me n

C02Me

C0,Me (169)a; n = 1 b; n =2

(170)a; n = 1 b; n =2

(171)a; n = 1 b; n =2

Photochemistry

172 The adduct compound

( 1 7 2 ) is photochemically

( 1 7 3 ) in 3 8 X

yield

on

converted

irradiation

into the

cage

254 nm. T h e

at

cycloaddition gives proof for the original adduct being in the endo-conf ormat ion.

5

81

Direrization and Intermolecular Additions

Dimerization

of

cyclohexa-1,3-diene

under

aryl

nitrile

sensitization affords the four dimers ( 1 7 4 ) - ( 1 7 7 ) . The detailed results a r e interpreted as providing evidence for the involvement of two paths to products one involving a n exciplex and the other a radical ion pair.82 The search for a SET sensitizer of use in solvents of low polarity has shown that N-methylacridinium ( 1 7 8 ) has

considerable

sensitizer

was

value.

The

demonstrated

steady-state by

the

potential

dimerization

of

this

of

1,l-

dimethylindene as its radical cation ( 1 7 9 ) to afford the dimer ( 1 8 0 ) in 80% yield.

83

The photochemical dimerization of the alkene ( 1 8 1 ) in the solid state affords a dimer which o n oxidation yields the cyclobutane 84 dicarboxylic acid ( 1 8 2 ) . The optically active dimer ( 1 8 3 ) can be

formed by

irradiation of

a single

crystal of

the monomer

85 (184).

Kaupp and Ringer86‘ 87 have reported the photochemical addition of

some

photoexcited

stilbenes

to

caffeine

derivatives.

example, using 4,4’-dichlorostilbene and chlorocaffeine of

the

process

is

shown

in

Scheme

products are formed.86 A mechanistic

( 1 7 ) where study of

a

One

(185).

variety

of

the photocyclo-

addition of the indole derivative ( 1 8 6 ) to cyclopentene has shown that

a

triplet

biradical

is

involved.

The

analysis

of

the

IIl3: Photochemistry of Alkenes, Alkynes, and Related Compounds

173

& & Me

I

Me Me

Me

Me Me

N CO2H (182)

Eto2c*

-

C02Et N

Z C02Et

174

Photochemistry

Scheme 17

0-b N

Q--J NI H H

I H H

I

Me

R'

R'

0 (192) R' = Ph, Me or H

(193)a; R' = Ph 40%

b; R ' = Me38% c; R 1 = H 28%

0 (194)

lIl3: Photochemistry ofAlkenes, Alkynes, and Related Compounds

175

kinetics of the system indicate that this biradical reverts to starting material 84 X of the time with the remainder undergoing ring closure to afford the two products ( 1 8 7 ) and (188).88 A SET process is involved in the irradiation of the indole ( 1 8 9 ) and other derivatives in the presence of electron rich dienes whence the adducts ( 1 9 0 ) are obtained.

89

The indole derivatives ( 1 9 1 ) are photoreactive in the presence of the aryl substituted alkenes (192). (2+2)-Cycloaddition does not

result

from

the

irradiation

and

instead

products ( 1 9 3 ) are obtained. T h e author'' is not

involved

biradical

and

that

simple

the

alkylated

suggests that a

radical

addition

SET

affords a

( 1 9 4 ) within which a 1,3-hydrogen transfer completes

the formation of the products.

In a n earlier section irradiation of the diynes ( 1 9 5 , 196) in the presence of hydroxylic solvents resulted in their conversion into ketones

and

ethers.

These

diynes

are

also

reactive

in

the

presence of alkenes and undergo cycloaddition reactions. T h u s irradiation at 350 nm of the diyne ( 1 9 5 ) populates the triplet state which undergoes addition to 2.3-dimethylbut-2-ene affording the 2:l adduct ( 1 9 7 ) . The other product formed o n irradiation was identified as the cyclobutene adduct ( 1 9 8 ) which arises from both singlet and triplet states of the diyne

(195). A cyclobutene

( 1 9 9 ) is also formed on irradiation of the diyne ( 1 9 6 ) with the same alkane. 91 The diyne ( 1 9 5 ) also photochemically adds dimethyl fumarate to yield the (2+2)-adduct ( 2 0 0 ) v i a the triplet state of the diyne. Subsequent irradiation of this adduct ( 2 0 0 ) in the presence of excess ester brings about its conversion into two new products ( 2 0 1 ) and ( 2 0 2 ) .

92

176

Photochemistry

Ze Ph

Me

phYe

Me Me

Me

'I'

Ph

R = H, Ph, But or Me, Ar = l-naphthyl R = Ph; Ar = But

phR?ph

Me02C'

..-C02Me

C02Me

(197)

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds 6

177

Yiscellaneous Reactions

Ferris and Cuilleming3 have demonstrated that the irradiation of the alkyne ( 2 0 3 ) at 185 or 2 0 6 nm results in the formation of 1,3,5-tricyano

benzene.

This

product

is

also

formed

on

irradiation at 254 nm but i t is accompanied by 1.2-4-tricyano tetracyano cyclo-octatetraene. 93 The hepta cyclic

benzene and

compound ( 2 0 4 ) is photochemically reactive on irradiation at 254 nm at -2OC in ether and is converted into ( 2 0 5 , 45%) by a path which has yet

to be

(206) is converted irradiation.

identified.

Interestingly the cis-triene

quantitatively

into

the

isomer

(207) on

94

An efficient

cation-radical

photochemical

reversion

of

chain process

is involved

in the

( 2 0 8 ) into

the cage compounds

the

dienes (209). 9 5 The use of SET induced ring opening of the ethers ( 1 0 ) has

been

studied.

The

presence

of dicyanobenzene

reactions

are carried out

as sensitizer

and

in the

in a methanol /

acetonitrile mixture as solvent. The ring opening affords the corresponding

radical

cation

which

is

trapped

by

attach

of

solvent to afford (211). The presence of diphenyl substitution is critical since the reaction fails with the corresponding mono phenyl

derivatives.g6

A

detailed

report

decomposition of a,B-amino alcoholsg' reports.

of

the

SET induced

has supplemented earlier

98. 99

Following on earlier

work Krogh

and Wanloo

have

studied

the

photochemical behaviour of the alcohol ( 2 1 2 ) in aqueous methanol. The conversion to the ether ( 2 1 3 ) is extremely rapid and is 50 %

formed after only two minutes irradiation. Extended irradiation

brings about conversion to the hydrocarbon (214). The mechanism

178

Photochemistry

(208) Ar = Ph, p -MeOC6H4, p -MeC6H4,or p -CI%H4

(209)

Ph Ph

(211)a; 95% b; 50%

(210)a; R’ = H, R2 = OMe b; R’-R2 = OCH2CH20

&&(PJ.@pJ \ \ / \ (212)

(213)

A~-OH

Ar-OMe

(215) Ar = C&hj Ar = 2-naphthyl Ar = 1-naphthyl Ar = 9-anthracenyl

(216) 15% 19% 15% 14%

/

\

/

\

(2 14)

1113: Photochemistry of Afkenes, Alkynes, and Related Compounds

179

of hydroxy/ether exchange is presumed to involve heterolysis in

the S 1 state. loo The photosolvolysis (215) in aqueous sulphuric acid ethanol,

propan-2-01

of several arylmethanols

in the presence of methanol,

has reported 101 corresponding ethers (216). or

the

formation

of

the

A detailed study of the photoreactions of a series of vinyl bromides (217) has shown that E/Z-isomerization.

aryl migration

and nucleophilic attack occurs. The evidence collected supports that formation of a relaxed vinyl cerbocation intermediate by C-

B r heterolysis."*

Maier and his coworkers have reported o n the

photochemical isomerization of dihalomethanes on irradiation in Argon matrices. The rearrangement discovered is similar to that reported earlier for tetrahalomethanes under the same reaction conditions. lo3 Tetrechloromethane also undergoes photochemical rearrangement in an Argon matrix at 12K. The process is presumed to

involve

coworkers

C-C1 h o m o l y s i ~ . ' ~ Many ~ years ago Kropp and his reported

on

the

photochemical

behaviour

of

elkvl

bromides and iodides. The present paper reports details of this earlier

work

and

shows

that

irradiation

in

the

presence

of

ammonium hydroxide optimizes the cationic process. T w o examples 105 of the several described are illustrated in Scheme (18).

Photochemistry

180

A? (217) A? Ar' A? A? A?

X = Br X=I

X = Br X=I

= A? = Ar3 = p -MeOC&H4

= A? = p -MeOGH4, Ar3 = Ph = A? = p -MeOGH4, A? = Ph = p -MeOCeH4, Ar' = A? = Ph = Ph, A? = A? = p -MeOGH4

18% 1%

8%

-

Br

19% 19%

1 3%

-

5% 4%

32% 73%

27% 36%

Scheme 18

27% 26%

14% 20%

1113: Photochemistry of Alkenes, Alkynes, and Related Compounds 7 1.

J . Saltiel. A . Waller, Y.-P. Sing, and D. F. Sears, jun., J.

2.

181

References

Am.

Chem. S O C . , 1 9 9 0 . 1 1 2 , 4 5 8 0 .

K. A . Muszkat, N. Castel, A . Jakob, E. Fischer,

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Luettke,

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4

Photochemistry of Aromatic Compounds BY A. C.WEEDON

Introduction This year's report on the photochemistry of aromatic compounds follows the format of the previous three years with the material grouped in sections devoted to photochemical reactions which involve isomerisation of the aromatic ring, addition to the ring, substitution on the ring, intramolecular cyclisation of substituents onto the ring, dimerisation, and lateral nuclear rearrangements in which part of a substituent cleaves and rearranges to a new ring position. A final section deals with reactions of substituents on the periphery of the aromatic ring whose reactivity depends upon the presence of the aromatic system. In order to avoid unnecessary overlap with the contents of other chapters it has been this reporter's general policy this year, as in the previous three years, n o t to include in this chapter any discussion of literature reports dealing with the following four topics: 1.

Di-n-methane rearrangements ofhomo-conjugated arenes.

2.

Photo-oxidation reactions of the alkyl side chains of arenes; this includes light-induced autoxidation reactions as well as singlet oxygen oxidations. In general, singlet oxygen addition to arene rings is also omitted.

3.

Photochemical reactions of quinones.

4.

The reactions of nitrenes produced photoelimination reactions of aryl azides.

in

the

A number of reviews and texts whose topics include photochemical reactions of aromatic compounds have been published during the year and will be mentioned here. A new volume of the Organic Photochemistry series has appeared and contains a chapter' detailing the photochemical reactions of aromatic and heteroaromatic cations (e.g. cyclopropenium ions, tropylium ions, pyrilium ions and

190

Photochemistry

pyridinium ions). The same volume includes a review of photochemical hydrogen abstraction reactions and a section is devoted to photoenolisation processes of ortho-alkyl aromatic ketones. New examples of this reaction are also reported below in the Peripheral Photochemistry section. The photoenolisation process involves transfer of hydrogen from the ortho substituent to the excited carbonyl. Similar reactivity is well known for ortho-alkyl nitrobenzenes; here hydrogen abstraction is followed by transfer of oxygen from the nitro group to the ortho alkyl substituent, yielding ortho-(hydroxyalky1)nitrosobenzenes. The product is unstable if the ortho-alkyl group is substituted by a leaving group and this is the basis of a number of applications of the reaction to the protection and subsequent photochemical deprotection of alcohols and carboxylic acids. For example, in sugar chemistry diol functions can be converted to benzylidene acetals using ortho-nitrobenzaldehyde and later deprotected photochemically; this application has now been reviewed. The herbicide bentazone (1) is photolabile and its rearrangement to the isomer (2) was reported last year.4 The photochemical and other properties of (1) have now been re~iewed.~ The photochemistry of hydroxamic acids and their derivatives has also been surveyed. Their photoreactivity is frequently associated with homolytic cleavage of the N-0 bond and in the case of hydroxamic acid derivatives which are substituted by aromatic rings the fragments can recombine to give ring-substituted products. This reactivity is analogous to the photo-Fries rearrangement of anilides. For example, the N,O-diacylhydroxamic acid ( 3 ) yields the ortho and para acetoxy isomers of the anilide ( 4 ) . Much of the photochemistry of N-iminopyridinium ylids such as (5) results from the initial formation of cyclised intermediates such as (6). This area has now been reviewed.' Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems; a monograph has been published which surveys many of these rearrangements.8 A short review, written in Chinese, on the photodimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been rep~rted.~ Three monographs discussing chemical and theoretical aspects of photoinduced electron transfer have been published1''l2 and one of these'' is devoted to photoinduced electron transfer reactions of organic compounds, including many examples of aromatic systems.

IIl4: Photochemistry of Aromatic Compounds

?

0 Y+

NH-C-R

I

N-

A

(5)

Y

(7) X = Y = H, CI or Br; X = Br, Y = CI; X = Bu', Y = CI

H2C-CH2 I

Y

(9)

191

Photochemistry

192

Three general texts devoted to organic photochemistry have appeared.13-15 One of these is aimed at the synthetic organic chemist and classifies photochemical reactions according to their utility for functional group ~reparati0n.l~ The second of these, which is jointly authored by one of the senior reporters of this volume, covers the theory and reactions of organic photochemistry and should be an excellent teaching text for a full course in organic photochemistry.14 It has a complete chapter devoted to the photochemical reactions of aromatic compounds. The third text concentrates on a molecular orbital description of the detailed electronic nature of elementary photochemical reaction steps.l5 1.

Isomerisation Reactions

Benzvalene is the valence isomer expected from excitation of benzene into its S, state, while the formation of Dewar benzene requires irradiation with shorter wavelength ultra-violet light to produce the S2 state.16 It has now been found that long wavelength irradiation of benzene in a low temperature argon matrix also leads to the formation of Dewar benzene.17 In order to account for the change in product the authors suggest that the argon matrix induces mixing of the S1 and S2 states. A new study of the reverse process, isomerisation of hexamethyl Dewar benzene to hexamethylbenzene, has been described in a preliminary report" and in a full paper.lg The reaction has been investigated in non-polar solvents using 1,4-dicyanonaphthalene as sensitiser. On the basis of exciplex emission evidence along with a kinetic analysis it is proposed that the reaction proceeds adiabatically from a Dewar benzene/sensitiser exciplex to a benzene/sensitiser exciplex. The latter exciplex also appears to sensitise the isomerisation of the hexamethyl Dewar benzene, possibly via a triplex. The benzvalene ( 7 ) has been proposed as an intermediate in the photoisomerisation of the If the X-substituent in these cyclophanes ( 8 ) to yield ( 9 ) .*O compounds is halogen then a minor competing reaction pathway leads to the tricyclic product (lo), presumably by homolysis of the substituent followed by trans annular hydrogen abstraction and coupling. The photochemical isomerisation of 5-membered aromatic heterocycles is associated with either of two mechanisms depending on the structure of the heterocycle. One of these (scheme 1) involves

193

W4: Photochemistry ofAromatic Compounds

Scheme 1

Scheme 2

(11)

Scheme 3

RM

R

0

PR'

R = H, Me, OMe R' = H, OMe

194

Fhotochemistry

ring closure to a Dewar type system, which is followed by sigmatropic rearrangement and ring opening. The other (scheme 2) involves the reversible formation of a hetero-vinyl cyclopropene intermediate. For heterocycles in which the heteroatom is from the second row of the periodic table the possibility of d-orbital participation has allowed the formulation of a third mechanism (scheme 3) in which valence expansion of the heteroatom leads to an intermediate represented by structure (11). A semi-empirical M.O. calculation method (SINDO1) has been used to examine the rearrangement of thiophenes;21 the results of the calculations suggest that both of the processes shown in schemes 1 and 3 are viable, although that shown in scheme 3 may be the less favourable of the two. This calculation method has been used by the same group to probe the degree and nature of the coupling of electron and nuclear motion during internal conversion from the excited state to the ground state surface in the formation of Dewar type structures (scheme 1) from 2-cyanopyrrole and from pyrrole.2 2 The involvement of intermediates such as those shown in schemes 1 and 2 in the triplet sensitised photoisomerisation of the furan substituted tetralone (12) to yield the dihydronaphthofuran (13) has been discussed.23 The authors argue that a more likely route involves the formation and collapse of the intermediate (14). Ultra-violet light irradiation of tropone in acetonitrile yields dimers while 2-methoxytropone under the same conditions yields the bicyclic product (15). The course of the reaction is reported to be changed if an acid (either sulphuric acid or borontrifluoride Under these circumstances tropone also etherate) is present.24 undergoes electrocyclic ring closure to give (16), while t h e direction of ring closure of 2-methoxytropone is changed and (17) is obtained as the product. The photoisomerisations observed under acidic conditions may involve excitation of a tropylium ion. Isoxazoles undergo a variety of photoisomerisation reactions and the nature of the rearrangement depends upon the identity of the substituents on the isoxazole ring. However, all of the reactions appear to be initiated by N-0 homolysis. For example, it was reported several years ago25 that the isoxazolo-pyridine (18) rearranges to the isomer (19) upon irradiation with ultra-violet light. This rearrangement is thought to proceed by homolysis to give If a (20) which collapses to (19) via the intermediate (21). nucleophilic solvent such as an alcohol is used then (22) is also

195

M4: Photochemistry of Aromatic Compounds

0

(15) R' = H, R~ = OM^ (16) R' = R ~ = H

(17)R' = OMe,R2 = H

(25) R = Me,Ph

0

H (29)

(18)

(19)

(20)

196

Photochemistry

obtained as a product. Competitive rearrangement of (20) to the ketene imine (23) was proposed to explain this.25 This reaction has now been investigated by flash photolysis26 and two transient species observed. These had lifetimes in the millisecond range and were assigned structures (21) and (22). The analogous photoisomerisation of the series of acylated isoxazoles ( 2 4 ) to give oxazoles (25) has also been e~amined.~’ In most cases the azirine intermediates (26) are isolable and are converted to (25) on heating. Very low quantum yields were measured for the rearrangement and the authors argue that this implies that the major fate of the initial homolysis product from N-0 cleavage is reclosure to (24) rather than formation of the azirine (26). Some evidence for ketene imine formation was a l s o seen in this series.27 Light induced cleavage of the N-0 bond is also implicated in the photoisomerisation reactions of oxadiazoles. Last year several articles describing the photochemical rearrangement of acylamino-oxadiazoles such as (27) to give (28) were rep~rted~*-~’ and an additional paper has appeared in this series.31 Upon further irradiation the benzoylamino-oxadiazole system present in (28) also isomerises to yield quinazolones, e.g. (29), and this may proceed by N-0 cleavage and cyclisation of the nitrogen onto the benzene ring of the benzoylamino side chain. Similar cyclisation reactivity was observed for the phenyloxadiazoles (30) which yield quinazolones (31).31

2.

Addition Reactions

The photochemical cycloaddition reaction of alkenes with the singlet excited state of simple benzenes most commonly results in the formation of adducts in which bonding of the alkene termini has occurred either to adjacent aromatic carbons (to yield nlortholf adducts) , or to aromatic carbons which are situated 1,3 to each other (to yield llmetall adducts) Less commonly the alkene termini become bonded to the p a r a positions of the benzene ring. In recent years it has been shown that the extent to which ortho rather than meta adducts are produced correlates with the magnitude of charge transfer interactions between the alkene and the excited arene; a greater degree of charge transfer favours the production of o r t h o adducts over meta adducts, although if electron transfer between the alkene

.

lIJ4: Photochemistry of Aromatic Compounds

197

and the excited arene becomes exergonic, substitution or addition rather than cycloaddition can become the dominant reactions. In the meta addition of alkenes to the singlet excited state of substituted benzenes the orientation of addition is understood in terms of the existence along the reaction pathway of a polar species such as (32) which closes to the meta adduct (33) by bonding between the l-position and either the 3- or the 5-positions in (32). Species (32) may be an intermediate, or may correspond to a polarisation developed during the interaction of the excited arene and alkene along a concerted pathway to meta products. Species ( 3 2 ) is formed by preferential addition of the alkene to those meta carbons which place the arene substituent at positions it can most stabilise. Thus a benzene ring with an electron withdrawing substituent will produce meta adducts derived from closure of (32) in which the substituent resides at the 3- or 5-positions, while a benzene ring with an electron donating substituent will be attacked by an alkene to give products derived from (32) in which the donating substituent resides at the 1-position. Some years ago Cornelisse reported3* that deuteration of alkyl benzenes results in a deuterium isotope effect upon the quantum yield of the meta photocycloaddition reaction with alkenes. In a new report the same group33 has published an analysis describing how the observed isotope effect upon the reaction quantum yield can be ascribed to a kinetic deuterium isotope effect on the excited state reaction and distinguished from an effect upon the unimolecular photophysical modes of decay of the excited state. In addition, it is reported that when the quantum yield of meta photocycloaddition of cyclopentene to alkyl benzenes is measured using a mixture of deuterated and non-deuterated benzenes, the quantum yield is arene concentration dependent . 3 3 The authors argue that this arises from competition between cycloaddition and the formation of mixed excimers between deuterated and non-deuterated alkyl benzenes which dissociate to yield excited deuterated alkyl benzene and ground state nondeuterated alkyl benzene preferentially. The meta photo-addition of cyclic alkenes to benzene derivatives generally yields the endo stereoisomers such as (34) rather than the exo stereoisomers such as (35), even though formation of the latter would involve less steric interaction in the approach of the alkene to the arene. This preference has been attributed to a weak bonding interaction between the arene a-system and a C-H (i.e.

198

Photochemistry

allylic C-H) bonds of the alkene as the two approach to form the polarised species (32) .34-36 A full paper has now appeared summarising earlier work and describing how this interaction can be used to explain the low efficiency observed when cyclohexene rather than other simple cyclic alkenes are used in arene-alkene meta photocycloaddition reactions. 37 In addition it was shown that the intramolecular arene-alkene meta-photocycloaddition reaction of compounds (36) and (37) yields products derived from endo addition of the cycloalkene to the benzene ring; however, as with the intermolecular reaction, the photocycloaddition reaction quantum yield is lower when the alkene is a cyclohexene ring. On the other hand, only exo-products are sterically accessible in the intramolecular meta photocycloaddition reaction of (38) and (39), and the quantum yield for product formation is similar for both cycloalkene ring sizes. The authors suggest that the lower quantum yield for endo addition of the cyclohexene ring arises from an unfavourable steric interaction between the arene and the chair-like conformation of the alkene as they approach, whereas for other cycloalkenes the alkene conformation is such that endo addition can proceed with development of a bonding interaction between the arene and the allylic C-H bonds of the alkene but without simultaneous development of unfavourable steric interactions. When exo addition is forced upon the system, as in (38) and (39), then the conformation of the alkene has little effect upon the interaction of the alkene with the arene with the consequence that similar quantum efficiencies are seen for intramolecular cycloaddition of both cyclohexene and cyclopentene. Results consistent with these ideas have been reported by Mattay3' for the meta photocycloaddition of cyclopentene and the cycloalkenes (40)-(42) with anisole. With cyclopentene meta cycloaddition products with endo stereochemistry are obtained in high chemical yield; however with the alkenes (40) and (41) the yields are much lower. It is suggested that this is caused by the methoxy substituents of (40) and (41) which dictate that the orientation for the endo product forming reaction of the alkene and arene be that shown in structure (43). The methoxy substituents force a conformation on the cycloalkene which pushes the ring atom denoted by X (X=O or CH2) towards the arene and this inhibits the weak bonding interaction between the arene and the allylic C-H bonds of the alkene. When the cycloaddition is attempted with the alkene (42), in which the methoxy substituents are trans, the only meta addition

lIl4: Photochemistry of Aromatic Compounds

199

(38) n = 1 (39) n = 2

(36) n = 1 (37) n = 2

69 OMe

Me

OMe

OMe (40) X=CH2 (41) X = O

.-'Me

(42)

(44)

(43)

H

(47)

(45)

R3

(49) (50) (51) (52) (53) (54)

R' R2 R3 R4 H H OMe H OMe OH Me H Me H OMe H OH H H H OH Me H H OH H Me H

200

Photochemistry

products formed possess exo stereochemistry, presumably because one of the methoxy groups always disfavours the endo orientation of approach shown in ( 4 3 ) . The intramolecular meta photocycloaddition of 5-phenyl-lpentene derivatives has been applied very successfully by Wender and his group to the synthesis of a number of natural products. Some of their recent work in this area has been summarised in a paper based upon a lecture given at the XIIIth IUPAC Photochemistry Symposium.39 In addition, a communication40 has described the synthesis of the natural product subergorgic acid, (44), by a route which commences with intramolecular meta photocycloaddition of the phenylpentene (45). Ultra-violet light irradiation of (45) yields a mixture of (46) and (47). Formation of these can be rationalised by the intermediacy of the dipolar species (48) in which the orientation of addition is dictated by stabilisation of the developing positive charge by the C-2 methyl group, and the stereochemistry is determined by preferential placement of the side chain methyl group at C-2' in the less sterically hindering orientation. Products (46) and (47) are formed by bonding either between C-2 and C-4, or between C-2 and C-6 in (48) and were also found to interconvert photochemically under the reaction conditions by way of a light induced vinylcyclopropane rearrangement. The effect of substituents on the regiochemistry of the intramolecular meta photocycloaddition of the 1-(3-butenyl)indanes (49)-(54) has been examined by Keese and coworker^.^^ In the methoxy-substituted compounds (49)- (51) meta addition of the side chain alkene occurs across the 1,5-positions of the benzene ring while for (52)-(54) addition occurs across the 1,3-positions. The intramolecular photocycloaddition of the alkenyltropone (55) is reported to yield (56) which is then used as the starting material for a synthesis of the natural product dactylol, (57).42 The photocycloaddition reaction is presumed to involve intramolecular addition across the 2 ,7-positions of a tropylium intermediate.43 The photochemistry of P-diketones is dramatically altered if they are converted to their boron difluoride complexes (58). The reduction potential of the complex is lowered from that of the diketone or its enol so that their excited states can act as electron transfer sensitisers of alkene phot~chemistry~~ or will form exciplexes with benzene derivatives,45 leading to the formation of products which are apparently produced by o r t h o addition to the arene

W4: Photochemistry of Aromatic Compounds

201

Mi

(57)

(55)

H

(59) OMe

I

(61) R = R ’ = H (64)R = Me, R’ = H (66)R = H, R f = Ph (67) R = H, Rf = OEt

(62) R = R ’ = H (65) R = Me, R’ = H

+ OMe

flN

OMe CN

OMe

Ar

202

Photochemistry

ring. For example, ultra-violet light irradiation of benzene with the boron difluoride complex of acetyl acetone yields (59), probably by retro-aldol opening of a primary ortho photoadduct (60). With monosubstituted benzenes the addition occurs preferentially to the 3,4-bond of the arene but without selectivity in orientation of the diketonate complex in those cases where the latter is non-symmetrical (e.g. the boron difluoride complexes of 2-acetylcycloalkanones). A full paper has been published detailing substituent effects on the ortho addition of 3-substituted acrylonitriles to 4-cyanoanisole and various cyanomethylanisoles and cyanomethoxyIt was reported previously that ultra-violet light anisoles.46 irradiation of 4-cyanoanisole in the presence of acrylonitrile yields (61)-(63) in a ratio of 7:1:2, re~pectively.~~ The azocine (63) is thought to be formed by ortho addition of the cyano group to the arene followed by thermal electrocyclic opening of the primary adduct. With methacrylonitrile only the ortho adducts (64) and (65) were obtained (in an 8:2 ratio) and none of the corresponding azocine, while with trans-cinnamonitrile and trans-3-ethoxyacrylonitrile the adducts (66) and (67), respectively, were obtained exclusively. Introduction of a methyl group into the arene had little effect on the regiochemistry of acrylonitrile addition. However, the presence of a methoxy group in the 2-position of 4-cyanoanisole led to the preferential formation of products derived from (68), while the presence of a methoxy group in the 3-position of 4-cyanoanisole led to the exclusive formation of the product formed from (69) when the compounds were irradiated in the presence of acrylonitrile. Azocine formation by photocycloaddition of acrylonitrile to arenes is also observed when cyan~benzenes~'and ~yanonaphthalenes~' are irradiated with ultra-violet light in the presence of phenols. The products have the structure (70) and are presumed to be formed by thermal ring opening of the primary ortho adducts (71). The photocycloaddition of acrylonitrile to various methyl substituted naphthalenes has been reported.50 The reactions proceed in poor yield and the products generally result from 1,2-addition to the naphthalene ring. For example, 2-methylnaphthalene gives adduct (72) in 8% yield. This is in contrast to the photochemical reaction of acylnaphthalenes with a-substituted acrylonitriles, which mainly yield products of 1,4-addition of the alkene to the naphthalene ring. The reaction with a-morpholinoacrylonitrile is both regio-

M4: Photochemistry of Aromatic Compounds

203

and stereoselective, and with 1-naphthaldehyde, 1-acetonaphthone and The 1-naphthophenone the products are ( 7 3 ) - ( 7 5 ) , respectively. reaction proceeds from the triplet excited state of the acylnaphthalene and the rate constants for reaction of the triplet excited state with a-substituted acrylonitriles have been measured by flash photolysis.5 2 Various 6-cyanouracils, (76)-(78), have been irradiated with ultra-violet light in the presence of acenaphthylene or phenanthrene in the solid state and the results compared with those obtained in solution.53 With acenaphthylene the solid state reaction with ( 7 6 ) yielded adduct ( 7 9 ) exclusively, while in solution mixtures of ( 7 9 ) and the s y n and a n t i 2+2 photodimers of acenaphthylene were obtained. Yang and collaborator^^^ have established the structures of the photoadducts obtained by ultra-violet light irradiation of benz[alanthracene, ( 8 0 ) , benz[b]anthracene (also known as tetracene or naphthacene), (Sl), and dibenz[a,c]anthracene, (82), with 1,3-cyclohexadiene. The products result from a single mode of 4+4 cycloaddition. The fact that none of the alternative isomers which could be formed by 4nB+4as addition are observed is rationalised in terms of secondary orbital interactions between the LUMO's of the arene and diene. The formation of a photoadduct between 9-bromoanthracene and benz [ b] anthracene has been reported.55 Last year the photoaddition of alkenes to the furan ring of the vasodilator Khellin (86) to yield cyclobutane adducts was reported in this chapter.56 An earlier publication from the same group concerning this reaction has come to the attention of the writer. 5 7 The structurally related psoralen (87) reacts with alkenes upon irradiation with ultra-violet light to give products of 2+2 cycloaddition to the enone double bond of the coumarin ring.58 However, it is the furan ring of ( 8 8 ) which photocycloadds to the 2-deoxycytidine (89) to give ( 9 0 ) as a mixture of two stereoisomers possessing c i s - s y n - c i s and c i s - a n t i - c i s stereochemistry about the Intramolecular photoaddition has been cyclobutane ring formed.59 reported for (91) to yield (92); if the side chain is saturated, as in ( 9 3 ) , then intermolecular photocycloaddition with added 2,3-dimethyl-2-butene occurs instead at the enone double bond of the coumarin ring to give ( 9 4 ) .60

Photochemistry

204

(73)R = H (74)R = C H 3 (75)R = Ph

(76)R’ = R2 = Me (77)R’ = Me,R2 = H (78)R’ = R2 = Et

(79)

ZZl4: Photochemistry of Aromatic Compounds

205

oh(87) R = Bu'

OH

(89)

(88)R = Et

OH

(90)

Q$f) \ I

0 (95)

(91)R = CH&H=CH2 (93)R = CHzCHzCH3

4 \

' O

\ o

Photochemistry

206

The furan ring can also act as the alkene partner in Paterno-Buchi reactions and three new examples of this have been reported. Ultra-violet light irradiation of furan with furil, (95), or with diethyl oxalate yields oxetanes (96) and (97), respectively, while irradiation of furan with various aldehydes gives the e x o adducts ( 9 8 ) - (101) 62 The Paterno-Buchi products (102) and (103) are produced from photochemical reaction of furan with acylcyanides. The endo adduct is favoured while a chiral R-substituent resulted in little chiral induction.63 In comparison with the large literature describing the thermal chemistry of indole and its derivatives which has accumulated in the past century, the photochemical literature of indoles is exceedingly thin. However, during the period of coverage of this chapter five reports have appeared concerning photochemical cycloaddition Irradiation of the pyrilium reactions of the indole nucleus. 64-68 salt (104) in the presence of indole, 1,3-cyclohexadiene and acetyl chloride has been reported to give a good yield of the endo photoDiels Alder adduct (105). 6 4 t 6 5 The e x o adduct was also formed but in smaller amount. The reaction is thought to be initiated by electrcn transfer from the indole to the excited state of the pyrilium cation followed by coupling of the diene with the indole radical cation. Ultimately this leads to the indoline (106) which is trapped by the acetyl chloride to produce (105). Acetylation was found to be necessary otherwise the indoline (106) is too good an electron donor and inhibits the reaction by preferentially quenching the pyrilium excited state. The reaction was found to be regioselective; l-acetoxy-1,3-cyclohexadiene yielded (107) while 2-acetoxy-1,3cyclohexadiene yielded (108) 6 4 t 6 5 Ultra-violet light irradiation of N-acetylindole or N-benzoylindole in the presence of alkenes was shown some years ago to yield cyclobutane adducts by bonding of the alkene termini to the 2- and 3-positions of the indole n ~ c l e u s .The ~ ~mechanism ~ ~ ~ of the reaction has now been studied for the reaction of N-benzoylindole The quantum with cyclopentene which produces (109) and (110). 6 6 yield of adduct formation was found to vary with cyclopentene concentration and with concentration of added triplet quencher; the results are used to propose a mechanism in which the triplet excited state of N-benzoylindole reacts with cyclopentene to form stereoisomers of one or both of the triplet 1,4-biradical intermediates (111) and (112). These proceed to products or dissociate to starting

.

.

IIl4: Photochemistry of Aromatic Compounds

(98) (99) (100) (101)

R=CH3 R = mesityl R = a-naphthyl R = P-naphthyl Ph

207

(102) p2

& R3

(105) R’ = R2 = H, R3 = MeCO (106) R’ = R2 = R3 = H (107) R’ = OAc, R2 = H, R3 = MeCO

(108) R’ = H, R2 = OAC,R3 = H

0A P h

Me02C

208

Photochemistry

material following spin inversion. The reaction competes with photoFries rearrangement of the N-acyl substituent,67 although selectivity is possible by appropriate choice of solvent and irradiation wavelength. This is discussed further below in the section devoted to Lateral Nuclear Shifts. The intramolecular photoaddition of an alkene to an N-acylindole has been used for the formal synthesis of the alkaloid vindorosine (113) 68 Ultra-violet light irradiation of (114) yielded (115), which is presumed to arise from spontaneous opening of the adduct (116). The product was accompanied by a minor stereoisomer, presumably possessing structure (117). The stereoselectivity of the reaction was improved by using compound (118); the adduct obtained was converted to a previously reported vindorosine precursor. In recent years the technique of flash photolysis has been used very effectively to generate high energy, highly reactive ground state species and to monitor directly their rates of reaction. Included among the transients which have been generated are carbocations, carbanions, ketenes and enols. The generation of cyclohexadienyl cations such as (119) by excitation of arenes in hexafluoro-isopropanol has now been demonstrated and the rates of their reactions with nucleophiles studied.71 The cations are formed by protonation of the arene excited state by the solvent. In polar solvents the excited state of sufficiently electron deficient arenes will accept an electron from donors. The fates of the radical ion pairs produced include formation of products of addition to the arene ring. A new example of this mode of reactivity is the photochemical reaction of 1,4-dicyanonaphthalene with benzyl methyl ether in acetonitrile.72 This yields stereoisomers of the addition product (120). The reaction most likely involves electron transfer from the ether to the naphthalene excited state and subsequent ionisation of a proton from the benzyl ether radical cation. This produces a benzyl ether radical which adds to the naphthalene derivative. An analogous sequence is proposed to explain the photochemical formation of (121)-(124) from ultra-violet light irradiated solutions of naphthalene-1,2-dicarboxylic acid anhydride in methanolic benzene or acetonitrile containing isobutene, 2-butene or 2-methyl-2-butene.73 Here it is suggested that the alkene radical cation, formed by electron transfer to the excited state of the naphthalene, is attacked by methanol; deprotonation

.

1114: Photochemistry of Aromatic Compounds

209

I H

AOBut

0

H "QR

H

R

(121) R 1 , R 2 , R 3 = H o r M e

(123) R', R2, R3, = H or Me

(122) R', R2, R3, = H or Me

(124) R', R2, R3, = H or Me

210

Photochemistry

yields a methoxyalkyl radical which adds to the naphthalene at one of the carbonyl or ring carbons. The reductive dimerisation of N-methyl acridinium ion to give (125) proceeds under ultra-violet light illumination in aqueous methanol or aqueous acetonitrile in the presence of triphenylphosphine; here electron transfer from the triphenylphosphine to the excited state of the acridinium ion to produce the N-methylacridinyl radical is implicated.7 4 Hydrogen atom abstraction rather than electron transfer has been proposed to account for the photoreduction products (126) and (127) obtained from irradiation of phthalazine (128) in isopropan01.~~The relative amounts of the two products formed changes if heavy atom containing compounds are added; the effect is used to argue for a singlet excited state origin for (126) and a triplet excited state origin for (127). Ultra-violet light irradiation of quinoline, quinazoline and isoquinoline in crystals of durene (1,2,4,5-tetramethylbenzene) yields products which may also arise from hydrogen abstraction and photoreduction of the aza-arenes.7 6 f7 7 The exciplexes formed between excited arenes (e.g. phenanthrene, naphthalene, 2,3-dimethylnaphthalene) and the acceptor 1,4-dicyanobenzene in ether solutions containing l-propylamine are quenched by the addition of tetramethylammonium tetrafluoroborate, and products of addition, (129)-(131), are obtained.78 The authors suggest that under the relatively non-polar conditions the added salt promotes charge separation in the exciplex so that the amine can attack the arene radical cation. Deprotonation of rhodizonic acid (132) produces a dianion (133) which possesses aromatic character. Photolysis of this dianion in aqueous solution containing electron acceptors is found to yield croconate dianion (134),79 and the quantum yield of the reaction has been measured for various concentrations of the acceptors. The mechanism of the reaction is uncertain but may involve ring contraction following electron transfer and addition of water. Photolysis of (135) in aqueous solution is reported to give the highly fluorescent product (136). 8 0 This reaction has been shown to proceed by addition of water to the imidazole ring of (135); this is followed by ring opening to give the intermediate species (137). The formamide function present in (137) is acidic and its conjugate base can quench the excited state of (135) by electron transfer; loss

1114: Photochemistry of Aromatic Compounds

211

Me

Me

H

NHCH2CH2CH3

(130) R = H (131) R = M e

A

c

O

AcO OAc

~

~

J

AcO OAc

AcO OAc

212

Photochemistry

of the electron from (137) initiates cyclisation and ultimately leads to the isolated product (136) 8o

.

3.

Substitution Reactions

Photosubstitution reactions of arenes can occur by several mechanisms. One of these is the 1,S mechanism; this is a photoinitiated chain process which follows the general course shown in scheme 4. The reaction requires a polar solvent medium (e.g. liquid ammonia), a good nucleophile which is also a good electron donor (e.g. thiolate ions, enolate ions), and it requires that the arene possess a substituent which is a good leaving group (e.g. halide). The reaction has frequently been applied for synthetic purposes and a new report has appeared relating its use for the preparation of naphthylquinolines and naphthylisoquinolines. Several approaches were found to be successful; ultra-violet light irradiation of p-naphthoate with haloquinolines or isoquinolines in dimethyl sulphoxide gave, after alkylation with isopropyl bromide, the products (138), while the anion of hydroxyquinoline with halonaphthalenes similarly gave (139). The enolate of Q- or p-acetonaphthone underwent photostimulated reaction with 3-chloro-4-acetylpyridine or 2-chloro-3-acetylpyridine to give (140) or (141) which cyclised to (142) and (143), respectively. Isoquinolines were also prepared by photolysis of the enolate of a- or p-acetonaphthone with 2-bromobenzamide to yield (144), which cyclised and gave, following alkylation, the isolated products (145) An 1,S mechanism has been implicated in the photochemical reaction o f diarylsulphides (and the corresponding sulphoxides and sulphones) with the enolate of pinacolone, and with diphenylphosphiae anion and diethylphosphite anion. The products are derived from reaction of the anions with aryl radicals formed by cleavage of an aryl sulphur bond in a diarylsulphide radical anion intermediate. Thus (146) is formed from diphenylsulphide and the enolate of pinacolone. The effect of variation of solvent and concentration of reactants in the SRNl reaction between benzene selenate (PHSe-) and aryl halides has been studied.83 Evidence was found to suggest that the radical anion derived from combination of the benzene selenate anion and aryl radicals (see scheme 4) is formed reversibly and can also dissociate to generate phenyl radical and aryl selenate anion.

1114: Photochemistry of Aromatic Compounds

(ArX)*

+

Nu-

ArX-’ Ar’

+

Nu-

(Ar-Nu)-’

+

ArX

-

213 (ArX)-’ Ar’

+

+

Nu’

X-

(Ar-Nu)

-’

Ar-Nu

+

(ArX)-’

Scheme 4

(138) X = N, Y = CH or X = CH, Y = N

(140) X = N, Y = CH (141) X = C H , Y = N

(1 39)

(142) X = N, Y = CH (143) X = CH, Y = N

214

Photochemistry

Thus photolysis of phenyl selenate with 4-iodoanisole yielded diphenyl selenide and bLs-(4-methoxyphenyl) selenide, in addition to the expected product, phenyl 4-methoxyphenyl selenide. It was reported several years agoa4 that ultra-violet light irradiation of the anion of indole with 2-fluoropyridine yields N-(3pyridy1)indole. The authors have now examined the same reaction for 3- and 4-fluoropyridine.85 With 3-fluoropyridine some N- (3-pyridyl)indole was obtained; however, the photochemical coupling fails for 4fluoropyridine. The energetics of electron transfer between the nucleophile reactions have been examined and a and the aryl halide in S1, correlation found between reactivity and the LUMO energies of the donor and acceptor.86t87 The 1,S reaction requires a good electron donor in order to initiate the reaction. However, excitation of a highly electron deficient arene in the presence of moderately good electron donors in a sufficiently polar solvent medium can lead to full electron transfer to the arene excited state and formation of the arene radical anion and the donor radical cation. Depending upon the nature of the donor and the properties of the other species present, the fates of the radical ions can include the formation of arene substitution products by non-chain mechanisms. A new example of this mode of reactivity is seen in the formation of N-arylbenzotriazoles when 9,lO-dicyanoanthracene (DCA) is irradiated with ultra-violet light in the presence of benzotriazole and arenes such as biphenyl, naphthalene and anisole.88 The sequence of events in the reaction is shown in scheme 5; electron transfer from an arene donor to the DCA singlet excited state yields the arene radical cation which in turn accepts an electron from the benzotriazole to yield the radical cation of the latter. Like many radical cations, this is acidic; in this case ionisation gives the benzotriazole radical which ultimately produces the N-arylbenzotriazole by oxidative coupling with the arene. The authors present evidence to show that the arene radical cation is a necessary intermediate in the formation of the triazole radical cation since direct electron transfer from the benzotriazole to the excited DCA is endothermic. In the above reaction the DCA is present in catalytic amount and acts as an electron transfer sensitiser. It is not consumed during the reaction; instead the DCA radical anion is oxidised back to DCA and consequently the presence of oxygen was

IIl4: Photochemistry of Aromatic Compounds

215

found to be necessary for the reaction to proceed. In fact, in many applications using cyanoarenes such as DCA as a sensitiser, the cyanoarene is consumed and is converted to products resulting from substitution in the aromatic ring. A communication has appeared which indicates that the reactivity of the DCA radical anion is strongly dependant upon the medium in which it is generated.89 Specifically, if the radical anion is produced in the presence of tetrabutylammoniurn dihydrogen phosphate and triethylamine (as an electron donor) then it is stable in solution unless oxygen is present; in the absence of the salt the DCA is consumed, apparently by attack of solvent (MeCN) on the ring, and substitution products are obtained. Schuster has examined the photosubstitution reaction of 1,4-dicyanonaphthalene (DCN) with alkyl triphenyl borates (e.g. pH3pMe-) In acetonitrile solution the products obtained are 1cyano-4-alkyl- and 1-cyano-3-alkylnaphthalenes. The reaction is initiated by light induced electron transfer from the borate to the excited state of DCN, followed by dissociation of the boranyl radical to triphenyl borate and alkyl radical; the latter couples with the arene to yield the substitution products. The properties of cyanine borates were also e~amined.~’In these compounds the cyanine dye is the cation partner of the borate as well as the light absorber and the electron acceptor. For these salts it was found that the free energy of the light induced electron transfer reaction correlated with the rate constant for electron transfer in the manner predicted by Marcus theory. The same correlation has been reported for electron transfer from various arenes to the excited states of DCA.’l Light induced electron transfer reactions normally require a polar solvent medium. This is partly in order to ensure that solvent stabilisation can allow separation of the radical ion pair to compete with back electron transfer. The cyanine borates described above are an exception in that neutral species are produced by an electron transfer reaction between ions of opposite charge; consequently it becomes possible for the light induced electron transfer reaction to take place in relatively non-polar solvents.g0 A similar effect has been found with N-alkylacridinium salts; it has been shown that the excited states of these compounds will accept an electron from arenes in non-polar solvents to generate a neutral N-alkylacridinyl radical.92

216

Photochemistry

The reactivity described above for borates in the presence of DCN has also been observed for tetra-alkylsilanes, tetraalkylgermanes and tetra-alkylstannanes in the presence of dicyanobenzene and tetracyanobenzene,93 and in the presence of pyrilium salts.94 Thus ultra-violet irradiation of 1,4-dicyanobenzene in acetonitrile with one of tetrabutylsilane, tetrabutylstannane or tetrabutylgermane yielded 4-butylcyanobenzene,93 and excitation of 2,4,6-triphenylpyrilium tetrafluoroborate under the same conditions yielded the addition products (147) and (148). 9 4 Both reactions are thought to proceed by electron transfer from the tetra-alkyl metal to the excited state of the salt; the radical cation produced then dissociates to the trialkyl metal cation and an alkyl radical which couples with the cyanoarene or the pyrilium ring. A full report has been published describing the light induced reaction of naphthalene with carbon dioxide in DMF containing amines to give Q- and P-naphthoic acids.” The reaction most likely occurs by electron transfer from the amine to the naphthalene excited state followed by coupling of the naphthalene radical ion with carbon dioxide. In the past evidence has been presented to suggest that the photosubstitution of alkoxynitroarenes proceeds by an electron transfer pathway, either by electron transfer from the nucleophile to the excited state of the arene followed by coupling, or by electron transfer from the arene to an electron acceptor followed by attack of the nucleophile upon the arene radical cation produced. Further evidence for both sequences has appeared during the period of coverage of this chapter. The photolysis of 4-nitroveratrole (149) in aqueous acetonitrile containing hydroxide ion or amines to give products of substitution of one of the methoxy groups has been studied by e. s r. spectroscopy.96 Results obtained by this technique suggest that the first step in the reaction may be electron transfer from the nucleophile to the arene triplet excited state to give the veratrole radical anion along with a radical derived from the nucleophile. Attempts were also made to observe the o-complex formed by coupling of this radical anion with the nucleophile derived The radical by use of time resolved resonance Raman spectros~opy.~~ photo-Smiles rearrangement of (150) to give (151) has been investigated f~rther;’~t.he reaction is base catalysed and the reaction quantum yield dependency upon the concentration of base has now been determined. The authors conclude that the rearrangement

.

lIl4: Photochemistry of Aromatic Compounds

217

CN

-

(ArH) '

AM hv CH3CN

CN

-H+

c -

b

i, ArH

ii, - H I

Ar

Scheme 5

Ph\

NO2

-OH

NMe2

NO2

NMe2 (152) R = H (153) R = NH2 (154) R = CH2Ph

218

Photochemistry

commences with electron transfer from the side chain amine to an excited state localised in the arene ring and that this is followed by deprotonation of the amine radical cation and intramolecular coupling of the radical produced with the arene radical anion. Evidence for a photosubstitution reaction initiated by electron transfer in the reverse direction to that described above has been reported;98 thus photolysis of benzene solutions of 4-nitroethoxybenzene gives rise to line broadening and chemical shift changes in the 'H-NMR spectrum which have been attributed to the formation of the radical cation of the arene. Substitution of the nitro group of the triplet excited state of 2-nitrothiophene with a large number of nucleophiles has been studied.'' It is found that species which are poor ground state nucleophiles and poor electron donors are unreactive, while good ground state nucleophiles which are also good electron donors quench the arene excited state by electron transfer. However, rapid back electron transfer apparently prevents product formation. The only nucleophiles which yield substitution products are those which are good ground state nucleophiles and relatively poor electron donors (e.g. cyanide, cyanate, hydroxide and sulphite). The photochemical reactions of naphthalene and phenanthrene with hydroxide and cyanide in aqueous acetonitrile to give substitution products have been studied by flash photolysis and fluorescence spectroscopy.loo Evidence for the generation of the arene radical cation was obtained. Similarly, the conversion of 1,4dimethoxybenzene to 4-cyanoanisole by ultra-violet light irradiation of slurries of tungsten oxide or titanium oxide in aqueous acetonitrile has been shown by flash photolysis to involve the intermediacy of the dimethoxybenzene radical cation. Ultra-violet light irradiation of tetramethylphenylene diamine (152) in aqueous or methanolic solutions of alkyl nitriles With benzyl yields the ortho-amino substituted product (153) nitriles the ortho-benzyl product (154) is obtained. It is proposed that both products are produced by electron transfer from the excited state of the diamine to the nitrile. With alkyl nitriles the products are derived from solvolysis of the adduct formed by attack of the nitrile nitrogen on the arene radical cation, while with benzyl nitrile electron transfer from the excited amine gives the benzyl nitrile radical anion which ionises by loss of cyanide; the resulting benzyl radical then attacks the diamine.

.

iil4: Photochemistry of Aromatic Compounds

219

Fragmentation of benzylic species from radical cations and radical anions is a rapid process which produces reactive transients which have a variety of fates. These include the formation of products of substitution in an aromatic ring. For example, the photolysis of the bibenzyl (155) in the presence of tetranitromethane is reported to yield the radical cation of (155) and the radical anion of the tetranitromethane.lo3 The latter rapidly dissociates to trinitromethyl anion and NO2 radical while the former also dissociates to a benzyl radical and a benzyl cation in competition with generation of the substitution products (156) and (157). The competition between substitution and fragmentation, and the nature of the products formed from the benzyl fragments is dependent upon the identity of the substituent X in (155) and the identity of the solvent. Photolysis of the analogous bibenzyl (158) in acetonitrile containing an electron donor (tetramethylbenzidine) instead of an acceptor produces the bibenzyl radical anion which also dissociates to a benzyl radical along with a benzyl anion.lo4 In this case formation of substitution products does not compete with cleavage. The substitution products (159)-(161) are formed by ultraviolet light irradiation of (162) in the presence of styrene, amethylstyrene or 1,l-diphenylethene lo5 The photochemical literature of quinones and arylethenes would suggest that such a reaction might proceed by an electron transfer sequence. However, this is apparently not the case since it only occurs in non-polar solvents and the author calculates that light induced electron transfer from the ethene to the quinone would be endothermic. The photolytic removal of chlorine from chlorobenzene, chlorophenols and chlorinated biphenyls and dioxins continues to be examined as a potential solution to the problem of the destruction of these compounds when present as environmental pollutants. Guillet has reported that aqueous solutions of the copolymer of vinylnaphthalene and styrene sulphonate will solubilize to a small degree 2,2',3,3',6,6'-hexachlorobiphenyl, and that illumination of the solution with simulated sunlight leads to the formation of biphenyls with fewer chlorine substituents.lo6 It is suggested that the process involves the absorption of light by the naphthalene and exciplex formation with the biphenyl followed by electron transfer to the biphenyl. The chlorinated biphenyl anion radical would then be expected to expel chloride ion. The dechlorination of mixtures of variously chlorinated biphenyls (such as those typically used as

.

220

Photochemistry

*+J (155) R = H (156) R = NO2 (157) R = C(N03)3

I

(158) Y = Me or CN, X = H, OMe, CF3

0

R'

0

0

(159) R = Me, MeCH2CH2, R'= H (160) R = Me, MeCH2CH2, R'= Me (161) R = Me, MeCH2CH2, R'= Ph

(162) R = Me, MeCH2CH2

H H

Me

(166) X = C I (167) X = O H

IIl4: Photochemistry of Aromatic Compounds

221

electrical insulating fluids) and also of 4-chlorobiphenyl by ultraviolet light irradiation in aqueous methanol in the presence of sodium methyl siliconate (MeSi03Na3) has also been reported.lo' The authors' studies suggest that chloride ion is lost from the biphenyl radical anion which is produced by electron transfer from the siliconate to the chlorobiphenyl excited state. The photochemistry of 2-, 3-, and 4-chlorobiphenyl has been examined in aqueous solution in the absence of electron transfer agents. lo* For the 2- and 4-chloro-substituted compounds the products observed are the corresponding hydroxybiphenyls. The 3-chlorobiphenyl appeared to photoisomerise to the 2- and 4-chloro isomers before undergoing photohydrolysis to the corresponding hydroxybiphenyls. The most likely mechanism for this reaction is suggested to be homolysis of the halide followed by electron transfer to give chloride ion and the biphenyl cation which is quenched by water. A communication10g and full paper''' tell of the efficient photoreduction of 4-chlorobiphenyl to biphenyl by excitation of 9,lOdihydro-10-methylacridhe (163) or acriflavine (164) in aqueous A variety of alkyl acetonitrile containing sodium borohydride. halides, benzyl halides and chlorobenzenes were also reduced. The reaction proceeds by electron transfer from the excited state of the dihydroacridine to the chloroarene, chloride loss and hydrogen atom donation to the arene radical. Thus photoreduction of the arene is coupled with oxidation of the dihydroacridine to the acridinium salt; the latter is reduced back to the dihydroacridine by the borohydride. In iso-octane solution 2,3,7,8-tetrachlorodibenzodioxin (165) is destroyed by irradiation with ultra-violet light to give the trichlorodioxin and other unidentified products. The sunlight induced degradation of 1,2,3,4,7-pentachlorodibenzodioxin and 1,2,3,4,6,7,8-heptachlorodibenzodioxin in aqueous solution has also been reported, and the quantum yields of disappearance of 1,2,3,4-, 2,3,7,8-, and 1,3,6,8-tetrachlorodibenzodioxin in aqueous solution have been measured. '13 The photolysis of pentachlorobenzene in acetonitrile has been found to yield 1,2,3,5-, 1,2,4,5-, and lI2,3,4-tetrachlorobenzene;'I4 the results of quenching experiments, fluorescence studies and quantum yield determinations suggest that the products arise by three pathways: fission of a carbon chlorine bond in the triplet excited state, in the singlet excited state or in an excimer. The similarity of the product distribution with that obtained by

222

Photochemistry

reduction of pentachlorobenzene with lithium p,p'-di-tert butylbiphenyl radical ion is used to propose that the photochemical cleavage reaction proceeds by way of the pentachlorobenzene radical anion. Sunlight induced degradation of the diuretic furosemide (166) has been studied in aqueous solution.116 One of the products is identified as the phenol (167). The reaction proceeds more rapidly in acidic solution. Photolysis of 9-bromoanthracene in acetonitrile containing triphenylamine is reported to give a complex mixture of products which includes anthracene and 9-cyanoanthracene.'17 The disappearance of 2-chlorophenol from lake water under illumination with sunlight has been monitored."* The rate of disappearance of 4-chlorophenol has been determined in solutions containing an ultra-violet light illuminated slurry of the semiconductors titanium oxide, zirconium oxide and molybdenum oxide. The main product was hydroquinone, although 1,4-benzoquinone and 4-chloro-1,2-dihydroxybenzene were also detected. Evidence has also been obtained to suggest that in aqueous solution and in the absence of semiconductor the primary product of 4-chlorophenol photolysis is benzoquinone and that secondary photolysis of this yields hydroquinone and 2-hydroxyhydroquinone.120 The photochemical destruction of ortho, meta and paranitrophenols induced by ultra-violet light illumination of aqueous slurries of titanium dioxide has been monitored by electronic absorption spectroscopy; the products are said to have been identified as dihydroxynitrobenzene isomers by coupled gas chromatography-mass spectrometry although no details are supplied. Nitrophenols have a l s o been identified in the fog shrouding the University of Bayreuth. They are presumed to be t h e products of photochemical nitration and the possible precursors (phenol, cresol and nitrate) were also detected.'22. The pyrimido-pteridine N-oxide (168) is well known as an electron acceptor and oxygen atom transfer agent. Its use for the hydroxylation of phenols has now been described.123 Ultra-violet light irradiation of (168) in acetonitrile solution allows conversion of phenol to a mixture of catecol and hydroquinone, cresol to 4-methylcatecol, tyrosine methyl ester to dopa methyl ester (169), and the pain killer acetaminophen (170) to (171). The reaction is initiated by light induced electron transfer from the phenol to the

'"

223

llf4: Photochemistry of Aromatic Compounds NH2

OH

(170) X = H (171) X = O H

(169)

R

I

(173) X = S, 0, NH, NMe

R=H (175) R = M e

(174)

Me

(176) R = HgCl (177) R = P h

H

Me0

C02Me Me C02Me

224

Photochemistry

pteridine excited state and is followed by a sequence of steps resulting in transfer of oxygen from (168) to the phenol ring. The painkiller and anti-inflammatory ibuprofen (172) is also photooxidised in the presence of (168).124 The products arise from oxidation of the benzylic positions of the alkyl side chains of (172). This almost certainly occurs by light induced electron transfer from ibuprofen to (168) followed by proton loss from the radical cation to yield a benzylic radical which couples with the N-oxide oxygen of (168). A similar sequence has been invoked to rationalise the pteridine N-oxide mediated photo-oxidation of indole3-acetic acid to indole-3-carboxaldehydeI 125 the photochemical epoxidation of styrene126 and the photo-oxidation of alkenes.127 Hydroxylation of the aromatic ring of phenylalanine has been found to occur when aqueous solutions are subject to direct irradiation with ultra-violet light.12’ Similar quantities of the ortho, meta and para isomers were formed; the meta and para isomers were further hydroxylated to dopa. The reaction proceeds in the absence of oxygen and is inhibited by iodide, thiocyanate and thiourea, leading the authors to suggest that hydroxyl radical is involved. Ultra-violet light irradiation of 2-, 3 - , or 4-iodoquinolines with 5-membered ring aromatic heterocycles has been reported to yield the corresponding biaryls (173),I2’ while photolysis of bromopentafluorobenzene in benzene or toluene solution has been found to yield the biphenyls (174) or (175), respe~tive1y.l~~ Similarly, ultra-violet light irradiation of benzene solutions of the mercurated benzophenone (176) is reported to give para-benzoylbiphenyl. (177); in hydrogen atom donating solvents photoreduction of the ketone occurs instead.131 Two examples of the photo-arylation of Photolysis of the 3-bromocoumarins have been published.1 3 2 t 1 3 3 coumarin (178) in acetonitrile solutions containing carbocyclic or heterocyclic arenes gives 3-arylco~marins;l~~ with 5-membered ring heterocycles the 2-position of the heterocyclic ring becomes attached to the coumarin, while with naphthalene and phenanthrene it is the 1and 9-positions, respectively, which become attached to the coumarin. Photolysis of the coumarin (179) with monosubstituted benzenes (e.g. benzonitrile, fluoro- and chlorobenzene, toluene) yields 3-aryl coumarins in which coupling has occurred with both the ortho and the para positions of the benzene rings.133

1114: Photochemistry of Aromatic Compounds

225

An abstract of a report issued from the Lawrence Berkeley laboratory indicates that the excited state produced by photolysis of ortho, meta or para-chlorotoluene in a supersonic jet decays by homolysis and loss of a chlorine atom as a major reaction pathway,134 while two photon excitation of the van der Waals complex formed between ammonia and chlorobenzene in a supersonic beam results in photoionisation of an electron followed by substitution to give an anilinium ion. 135 A full paper expanding on the previously reported136 photochemical coupling of vindoline (180) with catharanthene (181) to give vinblastine (182) and vincristine (183) has been ~ub1ished.l~~ Photolysis of (180) and (181) in aqueous solution at pH 3.5 gives (182) in a surprisingly good yield of 25%; if oxygen is present then (183) is obtained instead in a yield of 35%. Ultra-violet light irradiation of acridine in the presence of carboxylic acids leads to decarboxylation and the formation of alkanes. 138 Alkyl radicals are intermediates and hydrogen bonded complexes between the acid and the acridine are also implicated in the reaction mechanism. Similar species are thought to be involved when 2-cyanoquinoline is irradiated with ultra-violet light in the presence of each of the enantiomers of 2-phenylpropionic acid to give In this system the 4-methyl-2-(1'-phenylethyl) quinoline (184) 13' effect of an external magnetic field and also of changing solvent upon the yield of the substitution product was investigated. Different results were obtained for the R and S enantiomers of 2-phenylpropionic acid. This work, especially the solvent effect, would seem to merit further investigation. Substitution of a cyano group is also observed when 2,4-dicyanopyridine and benzophenone are photolysed in aqueous isopropanol; the product isolated is 2-cyanopyridine when the reaction is performed under basic conditions, while under acidic conditions the coupled product (185) is obtained instead.140 The authors formulate a mechanism involving electron transfer from solvent to the triplet excited state of the dicyanopyridine to account for the products, although other routes involving benzophenone ketyl radical would also seem likely. The latter mode of reactivity is used to explain the formation of cyclohexylpentafluorobenzenewhen a cyclohexane solution of hexafluorobenzene and benzophenone is irradiated with ultra-violet light. The regiochemistry of the photoalkylation reaction was also

.

226

Photochemistry

Ph

(182) R = M e (183) R = C H O

kPh CN

Ph

S03H

OH

& N

0

(1 92) R = C02Me or CONH2

(193)

R = C02Me or CONH2

IU4: Photochemistry of Aromatic Compounds

227

investigated for pentafluorobenzene, pentafluoroanisole, octafluoronaphthalene and pentafluoropyridine. Photochemical formylation of carbazole,142 methyland di~henylaminel~~ has been observed when they are carbazoles,142 illuminated with ultra-violet light or sunlight in chloroform solution, while chlorination products of pyrene and perylene have been observed when these arenes are irradiated in carbon tetrachloride.144 A report of the light induced chlorination of pyridine by chlorine in the gas phase has also appeared.145 Whereas the ground state reactivity of arenes is characterised by attack of electrophiles upon the ring, this mode of reactivity is comparatively uncommon for the arene excited state. During the year of coverage of this chapter two reports of such a process have a ~ p e a r e d . ~ ~ ~The , ' ~photolysis ~ of the sodium salt of metanilic acid (186) in water yields aniline and the ortho and p a r a isomers of (186) (i.e. orthanilic acid and sulphanilic acid, respectively) The reaction is shown to occur from the triplet excited state and is most efficient in acidic solution. The authors argue for a mechanism in which the triplet excited state is protonated on the arene ring to yield a a-complex. This which can rearrange to produce the o r t h o and p a r a isomers after deprotonation, or it can lose the sulphonic acid group in a process which is essentially electrophilic substitution by a proton. Wan has reported that ultra-violet light irradiation of 1,2-dialkoxybenzenes and 2-alkylalkoxybenzenes in aqueous acetonitrile containing sulphuric acid yields products derived from substitution of the alkoxy groups by water. Evidence is presented to support a mechanism in which the singlet excited state of the arenes is protonated to give a a-complex which suffers i p s 0 attack by water.147 Ultra-violet light irradiation of tritiated naphthalene in methanol containing tertiary amines has been reported to lead to hydrogen-tritium exchange. 14* Electron transfer from the amine to the excited state of the arene and protonation of the resulting radical anion is implicated. 4.

Intramolecular Cvclisation Reactions

The cyclisation of the singlet excited state of cisstilbene systems is normally a concerted electrocyclic reaction which proceeds in a conrotatory fashion to yield the trans-dihydro-

228

Photochemistry

phenanthrene skeleton (187). The absorption and fluorescent emission spectra of cis-stilbene in a supersonic jet have been measured and interpreted, and an attempt made to assess the relative importance of cyclisation and cis-trans isomerisation as pathways for non-radiative decay of the excited state. Absorption and emission spectroscopy as well as flash photolysis have been used to examine the properties of fluorinated stilbenes;15' no evidence could be found for cyclisation to dihydrophenanthrenes if both the ortho positions of one or both of the phenyl rings of the stilbene were substituted by fluorine. The trans-dihydrophenanthrenes formed by stilbene photocyclisation are thermally unstable and revert to the stilbene unless an oxidant is present, in which case the phenanthrene is produced. In order to ensure that oxidation occurs the reaction is commonly conducted under aerated conditions in the presence of a catalytic quantity of iodine. However, it has been reported that the yields of the phenanthrene can be increased if a stoichiometric quantity of iodine and propylene oxide in the absence of air is used."' If an ortho substituent is present on a stilbene then it can direct the regiochemistry of the cyclisation reaction away from the position of the substituent either by sterically blocking that position or by preventing oxidation of the dihydrophenanthrene intermediate. Alternatively, an ortho substituent can direct cyclisation towards its position by acting as a leaving group for aromatisation of the dihydrophenanthrene intermediate, thus obviating the need for an oxidant. This year, for example, it has been reported that the ortho-chlorostilbene (188) photocyclises to (189),I5* and that the ortho-iodo aza analogue of a stilbene, (190) photocyclises to (191) Blocking rather than activation of an ortho position by a substituent is illustrated in the photocyclisation in the presence of air and iodine of (192) to give (193) The ortho-acetoxy substituent of (194) appears to direct the cyclisation to the alternative ortho position because in the presence of air the phenanthrene (195) is the reported product while in the absence of air, where the potential leaving group properties of the acetoxy group might be expected to encourage the formation of (196), alternative photochemistry arising from fragmentation of the vinylic acetoxy group is found.155 Fragmentation of the vinylic substituent is also seen in the photochemistry of the tetrazole (197) but not until after photocyclisation to a phenanthrene h t s occurred; thus

IIl4: Photochemistry of Aromatic Compounds

229

fi OAc R (194)

(195) R = OAC (196) R = H

t 197)

Ar Af

(204) R = O M e (205) R = H

(201) X = B r (202)X=OAc

(206) R = O M e (207) R = H

(208) (209) (210) (211)

R1 = R2 = R3 = H R1 = R3 = H, R2 = F R’ = F, R2 = R3 = H R2 = R3 = F, R’ = H

Photochemistry

230

(198) is formed and undergoes subsequent photoextrusion of nitrogen. 15' If a stilbene is constrained in a cis configuration, for example when the double bond is part of a small ring, the photocyclisation to a dihydrophenanthrene is normally very efficient. Fields has reported157 that diphenylmaleic anhydride photocyclises readily to the phenanthrene (199), and this corrects an earlier report which assigned the structure of the product as a 2+2 dimer.158 The cyclisation of the imidazole (200) to a phenanthrene has been observed anew159 and its cyclisation in a polymer film has been examined as a potential method of optical data storage.16* Photocyclisation to phenanthrenes competes with the photosubstitution reactions of the anisyl substituted arylvinyl bromides (201) however, with the corresponding thioanisyl arylvinyl bromides photosubstitution is the favoured reaction. The products of photosubstitution of (201) are the acetates (202); these also photocyclise to phenanthrenes16' and no products from fragmentation of the vinyl acetate (as seen for (194)155) were observed. Two examples of a 1,2-diarylethylene photocyclisation have been reported in which one or both of the arenes is a naphthalene. 163,164 The major product obtained by photocyclisation of (203) was (204) which results from cyclisation of the unsubstituted ortho position of (203) onto the l-position of the na~htha1ene.l'~ Products of the alternative modes of cyclisation, which would yield (205), (206), or (207), were formed in little or no quantity. A study of the photocyclisation the dinaphthylethylenes (208)-(211) concludes that no dibenzo-dihydrophenanthrenes are formed which derive from cyclisation onto a ring position possessing a fluorine substituent 164 The photocyclisation of diarylethylenes in which a positively charged nitrogen atom is present yields fused polycyclic azonia arenes; further examples of such reactions have appeared. Thus compound (212) upon ultra-violet light irradiation yields (213), presumably by way of sequential photochemical reactions in which (214) is an intermediate. If (213) is phenyl substituted then (215) is obtained al~0.I'~ The initial cyclisation of (212) is regioselective and none of (216) is formed. Analogous reactions proceed for the systems (217), which yields (218), 16' (219), which yields (220),167 and (221), which yields (222)."* A closely related reaction is reported for (223) which gives (224).16'

.

23 1

Hf4: Photochemistry of Aromatic Compounds

(212) R = H, Me, Ph

(213)

(214)

(217) R' = H, R2 = Me or R' = Me, R2 = H

& Me

Ph

232

Photochemistry

The photocyclisation of 1,2-diarylethylenes is also successful for compounds in which one or both of the aryl groups are 5-membered ring aromatic heterocycles, or their benzo fused analogues. This year systems have been described in which one of the aryl groups is a thiophene ring,170f171 an indole,I 7 I f172 an imidazopyridine,170 a benzothiophene,173 and a pyrr01e.l~~ In the last example, the diarylethylene (225) is converted to (226) and a nonoxidative dehydrogenation procedure for conversion of the In this procedure intermediate dihydro-compound is applied.174 instead of performing the reaction in the presence of iodine and air, the irradiation is conducted in refluxing acetonitrile which contains triethylamine and palladium on carbon. Synthetic routes to benz [a]a ~ r i d i n e s ' ~ ~ and have been described. The key step is photobenz [ c]acridine~'~~ cyclisation of a styryl quinolone; thus (227) yields (228) while (229) gave (230).176 The formation of (230) rather than (231) from (229) indicates that the presence of an ortho-chlorine substituent in (229) did not direct the cyclisation regiochemistry and obviate the need for an oxidant. It is not clear whether the photocyclisation of (227) and (229) proceeds from the quinolone or hydroxyquinoline tautomer; if it is the former then this reaction is not a diary1 ethylene cyclisation but rather an electrocyclic closure of an aryl butadiene. These rearrangements form the basis of the photochromic behaviour of the fulgides, which are formally condensation products of an aromatic aldehyde or ketone and of acetone with the active methylene positions of succinic anhydride or a succinimide. The photocyclisation is shown for (232) to give (233). The latter species can be stable or in thermal or photochemical equilibrium with the precursor, depending upon the nature of the aromatic ring and its substituents. These compounds have been the subject of many investigations because of their potential applications for optical data storage. During the past year the photochromic properties of a number of new fulgides have been described; the systems reported have been constructed with an aryl portion which is a derivative of an ind~le,'~~~'~' a pyrrole,179 a thiophene,lsO or a fu~-an.'~'-'~~ Among the last of these the photochromic behaviour of a furan based fulgide adsorbed on clay184 or encapsulated in a polymer film183 has been examined; in addition, a fulgide constructed from a furan and a succinimide possessing an acrylate group remotely attached to the succinimide nitrogen has been copolymerised"* and its photochromic properties

llt4: Photochemistry of Aromatic Compounds

233

Ph

Ph

Ph

(224)

(223)

&=-

Me0

H

I

0

CH2Ph

& \

I

I

H

%x

\

0

(230)X = C I (231)X = H

(233)

(236) n = 1,2,3, or 4

(234)

(237)n = 1,2,3, or 4

(235)

C02Me

Ph orochmisrry

234

studied in the polymer formed. What is claimed to be the first thioanhydride based fulgide, compound (234), has been prepared and its photochemistry examined;185 it undergoes oxidative photocyclisation to (235). A 671 electron oxidative photocyclisation of an arylbutadiene has been found to proceed for compounds (236) to give (237) la6 The reaction does not occur for 1-phenyl-1,3-butadiene and this may be due to the greater conformational mobility of the acyclic system. The photocyclisation of the indole (238) has been investigated.la7 In methylene chloride solution a major product is the cyanobenzocarbazole (239), which is presumably formed by oxidation of a dihydroaromatic intermediate. The authors speculate that the solvent is involved in the oxidation since (239) is formed even when oxygen is excluded from the reaction. The photochemical hydration of one of the triple bonds of naphthyl-1,3-butadiynes is reported to yield naphthyl substituted if the reaction is carried out in methanol rather than water then the primary products include (240) which result from addition of solvent or photoreduction. These compounds undergo secondary photochemistry and are found to photocyclise to the phenanthrenes (241).18’ A similar reaction occurs for phenyl substituted 1,3-butadiynes.18’ Photolysis of the phenyl iodonium ylid of dibenzoylmethane, i.e. (242), with terminal alkynes yields (243) which photocyclises in a similar manner to (240) to yield benzoylnaphthols (244) . l g o Amides in which both the nitrogen and the carbonyl group are attached to unsaturated groups can photocyclise to six membered lactams in a reaction which may be regarded as isoelectronic with the 67r electrocyclic closure of stilbenes to dihydrophenanthrenes. This cyclisation has been successfully applied to the synthesis of alkaloids, alkaloid analogues and novel heterocyclic systems, and several new examples have been published during the period of coverage of this chapter. Thus the quinoline-benzothiophene (245) photocyclises to (246) ,lgl and the tetrafluorinated derivative (247) yields (248).lg2 Similarly, the benzofuran (249) yields (250) which rearomatised under the reaction conditions or by subsequent treatment with palladium on carbon depending upon the identity of the X-substituent.lg3

.

IIl4: Photochemistry ofAromatic Compounds

235

R

R

(241) R = H or OMe, (240) R = H or OMe, R' = Ph, But, Me, SiMes R'= Ph, But, Me, SiMe3

H.

0

0

0 '

"0

OH

0

~fyprJ* \

/

I

\

Ill

7R

Ph

(245)

(247)

R

236

Photochemistry

0

0 (249)X = F, CI,Br

(250)

MeO% Me0

Me0 R

COPh

COPh

lIl4: Photochemistry of Aromatic Compounds

237

The application of the photocyclisation reaction of enamides of benzoic acid to the synthesis of alkaloids and their analogues is exemplified this year by the preparation of (251) from (252),lg4 (253) from (254),lg5 and (255) from (256) .Ig6 A 677 electrocyclic closure could be implicated in the photocyclisation reaction reported for the N-amino pyridinium ion (257) and some of its ring substituted derivatives to give (258).Ig7 This type of reactivity was also seen for compound (233).16' A summary of the previously published work and of new results for the 67r photocyclisation of photochromic ethylenes substituted by derivatives of cyclopentadiene anion and pyridinium cation has appeared. The basic skeleton involved in the rearrangement is shown in structure (259) which is in photochemical equilibrium with (260).

Carbazoles can be prepared by photocyclisation of diphenylamines and this has now been applied to the synthesis of ellipticine alkaloid precursors. The ultra-violet light irradiation of N-tosyl amines (261) in ethanol is also said to give Photolysis of the antithe corresponding carbazoles (262). inflammatory drug diclofenac (263) in water or methanol has been explored;200 the primary product is the carbazole (264), but on extended photolysis the second chlorine atom is lost to yield the photoreduction product (265) or the photosolvolysis product (266). An analogous reaction has been reported for the triflate esters of chlorobenzyloxyphenols such as the pesticide Irgasan DP300 which has structure (267) .201 Sensitised photolysis of (267) in acetone yields the trichlorodibenzofuran (268). The same product is obtained by sensitised photolysis of 2,2'4,4'-tetrachlorodiphenyl ether (269) 201 A dibenzofuran is the product of phenyl-1,4-benzoquinone photochemistry; the product has the structure (270) and the mechanism leading to its formation has been examined.202 Recently it was reported that ultra-violet light irradiation of the aryl vinyl sulphide (271) gave (272) if the irradiation was performed at low temperature (-70°C) and (273) when the irradiation was performed at elevated temperature (110OC).*03 A similar result has now been obtained for the derivative (274), although the temperature required to achieve (276) was lower.204 Analogous products were a l s o obtained if the naphthyl ring of (274) was replaced by a phenyl ring. Separate photolysis of (275) did not yield (276), which implies that (276) is formed by intramolecular

.

(264) X = CI (265) X = H (266) X = OMe or OH

(267) X = OSO2CF3 (269) X = C I

c c + ) ccj+

IIi4: Photochemistry of Aromatic Compounds

R

/

0 (271) R = H (274) R = C02Me

(277) R = H (278) R = M e

239

(272) R = H (275) R = C02Me

0 (273) R = H (276) R = C02Me

(279) R = H (280) R = Me

0

OH

0

240

Photochemistry

interception of an intermediate species involved in the formation of ( 2 7 5 ) from ( 2 7 4 ) . Ultra-violet irradiation of the benzoylquinazolines ( 2 7 7 ) and ( 2 7 8 ) in the presence of trifluoroacetic acid or toluene sulphonic acid is reported to give the highly coloured salts ( 2 7 9 ) and ( 2 8 0 ) , respectively.205 Photocyclisation has also been reported for the benzyl pyridinium salt ( 2 8 1 ) to give ( 2 8 2 ) . 2 0 6 r 2 0 7 A series of methoxy substituted indolines ( 2 8 3 ) have been prepared from P-arylethylamines ( 2 8 4 ) by ultra-violet light irradiation in the presence of dicyanonaphthalene. The authors propose that the amino group cyclises onto the radical cation of the arene ring which is formed by electron transfer from the excited arene to the dicyanonaphthalene.208 Kropp has examined the products of photolysis of l-bromo- and l-iodo-4-phenylbutane using hydroxide ion as a scavenger for hydrogen halide formed during the reaction. The products include tetralin, 209 which is thought to be produced by coupling of the phenyl ring with the radical formed by light induced homolysis of the carbon-halogen bond. The photo-Nazarov cyclisation of l-cyclohexenyl phenyl ketone ( 2 8 5 ) yields the hexahydrofluorenone ( 2 8 6 ) . Schaffner's group has examined the mechanism of this reaction and have found evidence for some novel intermediates.210 They propose that the excited state of ( 2 8 5 ) decays by cis-trans isomerisation of the cyclohexenyl double bond; the trans-cyclohexenyl derivative ( 2 8 7 ) cyclises to the oxallyl species ( 2 8 8 ) which collapses to the relatively stable enol ( 2 8 9 ) or reacts with accumulated ( 2 8 9 ) to give the isolable enol dimer ( 2 9 0 ) . When the ketone derived from ( 2 9 0 ) is photolysed it gives back ( 2 8 9 ) along with ( 2 8 6 ) . The enol ( 2 9 0 ) and its corresponding ketone were both isolated and characterised, while ( 2 8 7 ) and ( 2 8 8 ) were trappable with cyclopentadiene. The enol ( 2 8 9 ) was observable by 'H-NMR before it ketonised and gave the isolated product ( 2 8 6 ) . A photocyclisation reaction has been reported for ( 2 9 1 ) to give ( 2 9 2 ) . The reaction proceeds in benzene (thus implying that it does not involve an electron transfer process) and the product may arise by cyclisation of a Norrish Type I1 biradical.211 Ultra-violet light irradiation of cinnamoyl derivatives in the solid state produces truxinates resulting from 2+2 cycloaddition. In solution the dimerisation usually does not compete with rapid cist r a n s isomerisation. However, it has now been shown212 that intramolecular 2+2 cycloaddition to give truxinates occurs in solution

241

1114: Photochemistry of Aromatic Compounds

(292) R = H, CI, F

(291) R = H, F, CI

(293)

(295)

(294)

o -nono R=

-ouowo

onononono 0

0

I

I

W

(296) (297) R = CH =CH2

N

(299)

Photochemistry

242

when the cinnamoyl groups are tethered, as in (293), to give (294). More interestingly, when lithium ions are present in the solution the course of the reaction is diverted and the 2+4 cycloaddition product (295) is obtained. It would appear that the lithium cation complexes with the polyether tether in (293) and controls the orientation of the cinnamoyl groups f o r cycloaddition. 5.

Dimerisation Reactions

The photodimerisation of anthracene derivatives normally produces 4+4 adducts and if the 9-position of the anthracene is substituted then the product generally possesses the anti regiochemistry shown in (296). The adduct derived from 9-vinylanthracene conforms with this generalisation and has been assigned the structure (297). It has been characterised by ’H-NMR and 13C-NMR spectr o ~ c o p y ~and ’ ~ X-ray crystallography.214 A dimer structure (298) has been assigned for the product of ultra-violet light irradiation of (299).*15 The two compounds are in photochemical equilibrium and are proposed as a photochromic system; longer wavelength light converts (298) to (299), while shorter wavelength light reverses the reaction. The photochemistry of the surfactant anthracenes (300)- (303) has been investigated in methanol solution and in monolayers on an aqueous buffer solution.216 The authors report that the light induced disappearance of the anthracene is much faster in the monolayer than in solution and offer the reasonable interpretation that the monolayer ensures appropriate proximity and orientation of the anthracene rings. Unfortunately the products are only characterised by ultra-violet absorption spectroscopy and the evidence for the formation of dimers appears to rest upon the observation of nonconjugated benzene chromophores. Thus no information is available to determine whether the regiochemistry of the dimerisation is altered by the orienting effect of the monolayer; in addition, the possibility that faster reaction in the monolayer may result from reaction with oxygen, or singlet oxygen produced by quenching of the anthracene, is not discussed. The dimerisation of acenaphthylene has been used as a probe reaction to investigate the properties of the cavities of a zeolite.217 In solution acenaphthylene photodimerises to the syn and anti dimers ( 3 0 4 ) and (305); the singlet excited state yields the syn isomer exclusively, while the triplet excited state produces both the

243

1114: Photochemistry of Aromatic Compounds

R3 R' (300) (301) (302) (303)

R2

R3

n-C6H13 H (CH2)5C02H H H (CH2)llC02H n-CsH11 H H n-C4H9 (CH2)5C02H H

(304)

R'

NHS03-

I

NHR'O

Q

\

x (308) (309) (310) (311) (312)

X=H X=CH3 X = F,Cl,or Br X = NO2 X = OMe

0A

OH

(313) R = Me, Me0

0

R

(314) R = Me, M e 0 R'= H, AC

R

(315) R = Ph (316) R = OEt

H (319)

l!&

(318) X = H (320) X = I

Photochemistry

syn and anti dimers. It has previously been shown that when the photodimerisation is performed with the acenaphthylene adsorbed within a zeolite the syn-anti ratio is perturbed due to the restricted mobility of the acenaphthylene molecules within the zeolite, the probability that sites within the zeolite will be doubly occupied by the probe molecule, and the degree to which heavy cations in the zeolite structure alter the rate constant for intersystem crossing.218 New results have now been obtained using acenaphthylene adsorbed within a zeolite which is irradiated in a hexane slurry (in the previous work the irradiation was performed in the absence of a solvent).217 It is found that the presence of the solvent accentuates the effects of the zeolite upon syn-anti ratio of acenaphthylene dimers produced in the photochemical reaction. 6.

Lateral Nuclear Shifts

This section is concerned with the photochemical rearrangements of substituted arenes in which a substituent cleaves to give a fragment which recombines with the arene, usually at the ortho or para position of the ring. In the case of the rearrangement of phenyl esters to give acyl phenols, and the rearrangement of anilides to ring acylated anilines the reaction is commonly termed the photo-Fries rearrangement. The photoreactivity of phenyl benzoates in which the para positions of both rings are substituted has been examined in liquid crystalline media and compared with the results obtained in isotropic solution.219 The photoreactivity of phenyl esters of cyclohexane carboxylic acids in which the para position of the phenyl ring and the 4-position of the cyclohexane ring are substituted were also studied under the same conditions.219 The products were not identified but were assumed to arise from photo-Fries rearrangement based upon the development of absorption in the ultra-violet spectrum assignable to ortho-hydroxyphenyl ketones. The relative quantum yields of the rearrangements were correlated with the viscosity and order of the liquid crystalline phase. The photo-Fries rearrangement of polymers containing phenyl formate units (e.g. the polymer derived from para-formyloxystyrene) has been examined.220 The effect of complexation by /I-cyclodextrin on the photoFries rearrangement of benzene sulphanilide has been investigated.

1114: Photochemistry of Aromatic Compounds

245

In methanol or benzene solution the major photolysis product is aniline while para-aminodiphenylsulphone is obtained as a minor product. However, when the reaction is performed in aqueous solution in the presence of p-cyclodextrin relatively little aniline is produced and instead the major product is the para-aminodiphenylsulphone along with substantial quantities of the orthoisomer.221 Dramatically different results were obtained when the solid complex of the sulphanilide with j.3-cyclodextrin was photolysed in the absence of solvent; under these circumstances the ortho-aminodiphenylsulphone is the exclusive product. In previous years this chapter has reported several examples of the photo-Fries reaction being conducted in cyclodextrin cavities but this degree of selectivity is much higher than that seen before. Unfortunately, the authors do not provide details of how the solid complexes were prepared. The photochemistry of the para-substituted phenyl sulphamates (308)-(312) has been found to be substituent dependent. Photolysis of (308) and (309) in methanol gave photo-Fries products (i.e. the expected aniline sulphonic acids) as well as aniline, while the nitro derivative (311) was photochemically inert, and the halogen substituted sulphamates (310) all gave the photosolvolysis product (312).222 The phenyl anthranilates (313) and their N-acetyl derivatives undergo photo-Fries rearrangement to (314) and the products have been used as precursors for the synthesis of heterocycles.223 The wavelength and solvent dependence of the photo-Fries reactivity of N-benzoylindole (315) and N-ethoxycarbonylindole (316) has been explored.67 Quantum yields of rearrangement are higher when shorter wavelength light is used which may indicate reaction from an upper or vibrationally hot excited state. The photo-Fries rearrangement of N-acetylcarbazoles has been re-examined in The products are carbazole and 1-, 3 - , and 4-acetylcarbazole; traces of N,3-diacetylcarbazole (317) were also found. The photolysis of N,N-dipheny1benzami.de (318) in methanol is reported to give the ortho and para isomers of benzoyldiphenylamine (319) along with c a r b a ~ o l e . In ~ ~the ~ presence of iodine orthobenzoyldiphenylamine is subject to further photochemistry and is converted to 9-phenylacridine. When N,N-diphenyl-2-iodobenzamide

246

Photochemistry

(320) was photolysed an intramolecular cyclisation to give (321) took place in preference to photo-Fries rearrangement. The photolysis of dibenzylketones and related compounds can result in the formation of products from lateral nuclear shifts if Norrish Type I cleavage of the ketone is followed by attack of the acyl radical on the ring of the benzyl radical. Ramamurthy and Turro have both used the Type I reaction of benzyl ketones to probe the mobility of molecules, their excited states and the fragments resulting from their photodissociation in various constraining and ordered media. In the past year papers have been published in which the medium studied has been the pores and surfaces of zeoli t e ~ , ~ In ~ the ~ #case ~ of ~ 2-phenylalkanones ~ - ~ ~ ~ (322), Turro reports that Type I cleavage yields the expected biradical which either disproportionates to the alkenals (323) or rearranges to the cyclophanes (324) in which the acyl radical has migrated to the para position of the benzene ring.226 In solution the major product is the cyclophane (324) ; it is found that adsorption of (322) either into or onto zeolites does not greatly alter the product distribution, neither does performance of the reaction in aqueous solutions of cationic or anionic micelles. These results contrast with those previously reported227 when (322) was photolysed in the solid state as a complex with P-cyclodextrin. Ramamurthy has examined the photolysis of dibenzylketone adsorbed on zeolites.217 In solution the exclusive photoproduct is bibenzyl, which is formed by Type I cleavage followed by decarbonylation and recombination of the two benzyl radicals. On the zeolite surface the mobility of the radicals from a-cleavage is reduced so that recombination competes with diffusion apart and decarbonylation. Recombination gives back dibenzylketone or products of a lateral nuclear shift in which the acyl radical has recombined with the ring of the benzyl radical to give the ortho and para isomers of (325). The effect is magnified when the dibenzylketone-zeolite complex is photolysed as a slurry in pentane instead of in the solid state; it is assumed that the solvent molecules further impede the movement of the adsorbed radical intermediates on the zeolite surface. A similar effect is observed for a-alkyl benzyl ethers (326) where Type I cleavage leading to rearrangement products (327) competes with Type I1 photochemistry; in this case the demobilising effect of the zeolite promotes the Type 11 reaction.2178228 The a-alkyl deoxybenzoins (328) and the benzoin alkyl ethers (329) were also examined;

IIl4: Photochemistry ofAromatic Compounds

247

0

(323) n =11-15

(322) n = 11-15

qph & R

0

\

/

(324) n =11-15

R

(328) X=CH2 (329) X = O

(327)

(330) X=CH2 (331) X = O

q o (R = naphthalene, anthracene, pyrene) have been converted to R.+ by electrontransfer from the anion to XeF2, and this process is followed by combination of R - and R . + to give excimers ( R R ) * ,

which

decay

with

chemiluminescence.l44 The photooxidation of 9,lO-diethoxyanthracene by the diphenyliodonium cation has been studied in homogeneous media and in heptane/AOT/water reverse micelle solutions,l45 and it has also been reported that the redox photosensitized oxidation of methylarenes using 1,4dicyanobenzene, 9,1O-dicyanoanthracene, and chloranil gives principally the aromatic aldehyde in a process which occurs via the aromatic radical cation.146 Electron-transfer photooxygenation of the side chains of some electrondeficient aromatic compounds such as toluene, p-chlorotoluene, p cyanotoluene, and p-nitrotoluene have all been sensitized by 9,lOdicyanoanthracene and chloroanil, with added biphenyl as cosensitizer in certain cases. 147 The products are the corresponding substituted benzaldehydes and benzoic acids. Aromatics such as benzene, toluene, and anisole have been successfully photooxygenated by 1,3,7,9-tetrabuty~pyrimid0[5,4-g~pteridine~ 2,4,6,8(1H,3H,7H,9W)-tetrone 5-oxide in a reaction which occurs by

it

photoinduced single electron-transfer from the hydrocarbon to the N-oxide, followed by 0-atom transfer between the resulting radical ion pairs.148 These authors have also used this procedure to oxidise various polycyclic aromatic

298

Photochemistry

hydrocarbons, and in these cases, oxidation takes place at the most reactive position of the corresponding radical cations and occurs by the same mechanism; 149 similarly, various phenols undergo oxygenation using the same reagent and give dihydric phenols. 150 Alkylbenzenes have been photooxidised catalytically by (BuqN)4W10032 in a process whose initial step is a direct electron-transfer from substrate to catalyst.151 However, the related tungsten isopolyanions [VW5019]3- and [W6019]2-, are inactive as catalysts. Heteropoly acids such as [PMo12040]3- catalyse the photochemical oxidation of alkylbenzenes with oxygen to aldehydes and ketones,l5* and the same group of workers also reports that solutions of toluene in acetic acid are photooxidised in the presence of catalytic amounts of o-phenanthroline or a,a'-bipyridyl, in a transformation which is accelerated by FeC13.153 Aromatic hydrocarbons having an active methylene group undergo photooxidation in the presence of Fe(II1) in MeCN solution to a mixture of ketones, alcohols, and acetamides.154 For example, irradiation of a mixture of diphenylmethane containing Fe(N0)3.9H20 yields a mixture of benzophenone, benzhydrol, and N (diphenylmethy1)acetamide. A one-electron transfer mechanism has been suggested. Irradiation of mixtures of Ce(IV) ammonium nitrate and

0-,m-,

and p-cymene gives products arising from attack at a methyl group.155 The mechanism involves NO3. as electron-acceptor and suggests the importance of charge-transfer complexes and their conformation in H-abstraction processes from alkylaromatics. A selective procedure for benzylic oxidations based on a photochemical electron-transfer reaction, and using DCA as electron-acceptor and methylviologen as electron relay has appeared.156 The products are the corresponding hydroperoxides. In the presence of organic bromides such as bromobenzene, alkylbenzenes are photooxidised by molecular oxygen to produce acylbenzenes as the major product.157 The distribution of reaction products arising in the photooxidation of p-xylene under a variety of conditions in air has been studied,l58 and chemiluminescence intensity data have been used to estimate rate constants during the oxidative decomposition of AIBN in the presence of ethylbenzene.159 1,1,2,2-Tetraarylcyclopropanein

IIt5: Photo-reduction and -oxidation

299

trifluoroacetic acid undergoes photooxygenation to 3,3,5,5-tetraaryl- 1,2dioxolanes.160 The key step is a single electron-transfer from the cyclopropane to excited carbenium ions formed by ring opening and light absorption of protonated cyclopropanes. DCA-Photosensitized oxygenation reactions of a series of

1 , l -diaryl-2,2-diphenylcyclopropanes~6~ and of 1,1,2,3-

tetraarylcyclopropanesl62have been examined. In these electron-transfer induced oxygenations, product formation is governed by the resonance stability of the lY3-radical cations. Photooxygenation of some 1 -alkyl-2,3diarylcyclopropanes (10, R = 4-MeOC6H4, R' = Me, Pr, CH2Ph) in MeCN is reported to occur by electron-transfer with the formation of 4-alkyl-3,5diaryl- 1,2-dioxolanes. 163 In MeCN and using 9,lO-dicyanoanthracene as sensitizer photooxidation of 1-aryl-2,2-dimethylbicyclo[ 1,1,O]butanes gives a mixture

of

3,4-epoxy-4-methylpentan-1 -ones

and

3,4-epoxy-4-

methylpentanals,l64 of which the former arises in an electron-transfer process and the latter by a singlet oxygen reaction. Electron-transfer induced photooxygenations of aryl-substituted tricyclo[4,1 ,02,7]heptanes (1 1, R = 2thienyl, 4-tolyl) gives peroxides which rearrange in solution to epoxyketones (12) and epoxyaldehydes (13). 165 Photooxidation of Ph2C=CHR (R = Me, Et, Pry) using 9,lOdicyanoanthracene as sensitizer in MeCN gives benzophenone as major product together with some epoxides,l66 and styrene oxide and phenylacetaldehyde are reported to be the photooxygenation products of styrene in the presence of

pyrimido[5,4-g]pteridine 10-oxide.167 This latter process which involves successive single electron and oxygen atom transfer, mimics the haemincatalysed oxygenation mode. The primary light emitter in the chemiluminescent reactions of (E)-2-HOC6H4CH=CHC(jH4R-4(R = H , Me, Ph, NH2, NMe2, OMe, hal) with singlet oxygen is the stilbenol anion, and an electron-transfer mechanism has been invoked to rationalise the results.168 Cation radicals of cis- and trans-stilbene and of their ring-substituted derivatives, generated in solution by pulsed laser-induced direct electrontransfer to singlet cyanoanthracenes, are cleaved in the presence of oxygen to

300

Photochemistry

DCoR BCH R

0

Q-

CHR.(CH21nCH*
300 nm) in aqueous dispersions of Ti02 is greatly accelerated by Fe3+/Fe2+.173 This latter rate enhancement is caused by the ready decomposition of an intermediate peroxo species. Transition metal ions such as Fe3+ and Cu2+ affect the photocatalytic oxidation of phenol using H 2 0 2 in the presence of illuminated Ti02.174 The yield of semiquinone radicals formed in the U022+ sensitized oxidation of hydroquinone is reported to decrease in an external magnetic field.175 Increasing the field strength and solvent viscosity increases the magnitude of this effect, whereas its influence is reduced as the content of 235U falls. Photooxidation of single crystals of 2,6-

di-tert-butyl-4-methylphenoland 2,6-di-tert-butylphenol causes pair-wise trapping of triplet phenoxy radical pairs.176 In solution, time-resolved CIDEP observations for 2,4,6-tri-tert-butylphenol in C6F6 suggest that the phenol radical cation is initially formed from the photoexcited triplet state. In the 02(1Ag) mediated photooxidation of monochloro- and mononitrophenols, a kinetic study of solvent and substituent effects suggests the intermediacy of a complex having partial charge-transfer character as is the case with other phenols.177 Photooxidation of (-)-epicatechin using AIBN gives a pyranobenzopyran, 178 and [2+4]-cycloaddition of singlet oxygen to dihydroxynaphthalenes

substituted

in

the

1 -position

gives

a

monohydroxynaphtho-l,4-quinone;kinetic parameters have been determined by a Stern-Volmer analysis.179 The mechanism of the oxidation of benzyl alcohol in the presence of the UV-activated semiconductor catalysts TiOzPt/Ti02, has been discussed in terms of the surface characteristics of Pt/Ti02 and the one-photon action model,l80 and in contrast to an earlier

302

Photochemistry

report,l81 the photochemical oxidation of benzyl alcohol is claimed to yield stereoisomers

of

2,4,5-triphenyl-1,3-dioxolane rather

than

tetraphenyldioxanes.182 Some antimalarial 1,2,4-trioxanes have been prepared from the allylic alcohols 4-RC6H4CMe=CHCH20H (R = H, Me, MeO, F, Cl,)

Acetyl-3,4-dihydrocoumarin has via 4-RC6H4C(=CH2)CH(OOH)CH20H.l83 been photooxidised in air-saturated alcoholic solutions to 3-acetyl-3,4dihydrocoumarin and 3,3'-diacetyl-3,3',4,4'-tetrahydro-4,4'-biscoumarin.~~4 Photooxidation of the methyl group in the 7-(dialkylamino)-4methylcoumarins (14) (R = EtzN, R' = Me) to CHzOH, CHO, or C02H is observed to increase with increasing electron-donor capacity of R,185 and naphthalene photosensitization experiments show that the triplet state participates in this wavelength-independent reaction. Regioselective synthesis of 2-arylidenecoumaran-3-ones by the dye-sensitized photooxygenation of 2hydroxyphenyl styryl ketones has been achieved in the presence of anionic and cationic surfactants,l86 photooxygenation of 2-methyl-3-siloxybenzofurans (15, R = Me, CMe3) gives a-silylperoxyketones via isolable dioxetanes, keto-

ester cleavage products being ultimately formed,l87 and tetraphenylporphyrinsensitized photooxygenation of various benzofuran derivatives leads to the corresponding benzofuran dioxetans.188 6-Hydroxy-2H-pyran-3(6H)-oneshave been prepared by photooxidation of 2-furylcarbinols followed by reduction using PPh3,189 employing a procedure that has been applied to the synthesis of 6-undecyltetrahydro-2-pyrone, a pheromone of Vespa orientalis. A kinetic

study of the photocatalytic oxidation of furfuryl alcohol using an aqueous ZnO suspension to give 6-hydroxy-(2H)-pyran-3(6H)-oneshows that hydroxylation occurs in the homogeneous aqueous phase,l90 with 02(1Ag) making only a minor contribution. Selective monooxidation of the bisfurans (16; R = Me, SiMe3, R' = H, n = 1; R = Me, R' = OMe, n = 0-2) using 0 2 ( l A g ) followed by cycloaddition provides a useful stereoselective synthesis of decalins ( 1 7 ) w Dye-sensitized photooxygenation of 1-methoxy -3-carbomethoxy-5-phenylfuran has been reported,l92 and in the presence of Ti02 and hypochlorite, methanolic solutions of 2-furoic acid lead to endoperoxide formation.193

IIf5: Photo-reduction and -oxidation

303

8 Oxidation of Nitrogen-containinv ComDounds Aqueous solutions of ethylamine have been successively oxidised to ethanolamine and glycine using KrF excimer laser radiation.194 Some unsaturated amines such as 1-(dialkylamino)alka-2,4-dienes and 1-

(dialkylamino)-3-arylprop-2-enes,have been photooxidised by irradiating in air in the presence of a catalytic amount of iodine,l95 and various long-chain alkylamines (NR1R2R3, Rn = H, Me, C14H29, C16H33 or C18H37) undergo photosensitized oxidation using acetone in aqueous media.196 Photoinduced single electron N-demethylation of N-alkyl-N-methylanilinesin alkaline methanol has been reported.197 Salt effects on the N-demethylation of tertiary amines have also been studied.198 Thus in the absence of added salts, photosensitized oxidation using 9,lO-DCA as electron acceptor gives both norand N-formyl compounds, but in the presence of added LiC104 the norderivative is formed with high efficiency. The hydrophobicity of various dialkylviologens (Me to hexyl) is reported to affect their ability to quench photoexcited [Ru(bpy)3]2+.199 Viologens having shorter alkyl chains are most effective in poly(sodium styrenesulphonate) aqueous solution, whereas those with a hexyl substituent are most effective in styrene latex solution. The products of the acid-photosensitized oxygenation of N-furfurylbenzamide and N-furfurylacetamide have been investigated.200 Oxidation of 2- and 3,6dichloro-2-pyridinecarboxylic acid by Fenton's reagent is enhanced by sunlight in the presence of Fe(II),201 and the singlet state of the leucobase of malachite green has been shown to be important in its photooxidation.202 The first detectable products in the direct irradiation of 3-indoleacetic acid and methyl 3-indoleacetate under both aerobic and anaerobic conditions are the 3indolemethanols, their methyl ethers, and dimeric compounds.203,204 Under dye sensitization at pH 5 , the same primary products have been observed, and it is suggested that the primary species are two radicals and two peroxy forms from which all products can be derived by subsequent thermal reactions. Photooxidative decarboxylation of indole-3-acetic acid by pyrimido[5,4glpteridine N-oxide gives indole-3-carboxaldehyde in a process which may be

304

Photochemistry

of significance in plants.205 Photooxidation of theophylline, proline, pyridine, and piperidine over Ti02 gives a mixture of NH4+ and NO3-.206 The pathway leading to the formation of these species has not been closely defined and it is simply suggested that the transformations proceed through various organic and inorganic nitrogeneous intermediates. Photooxidation of tryptophan sensitized by hyprocrellin A, gives a mixture comprising NH3, C02, melanin, (18), and (19).158 The reaction appears to depend on 02(1Ag) and electron-transfer processes. The participation of 0 2 ( 1 A g )

in the photooxidation of

glycyltryptophan has been evaluated for a variety of sensitizers.208 Rose Bengal supported on an anionic resin, has been used as a heterogeneous photosensitizing agent for the

oxidation

of

9-methyl-l,2,3,4-

tetrahydrocarbazole to give a benzazonine derivative,209 and photooxidation of

meso-tetraphenylporphyrin dianion in ROH (R = H, Me, Et,) gives benzoylbilinone in a process occurring by addition of 02(1Ag), and followed by ring opening and addition of ROH.210 The key parameters affecting the photochemical oxidation efficiency of DNA arise from an association between DNA and sensitizer.211 They include increases in ionic strength or decreases in

the polarity of the sensitizer, both of which enhance the rate of reaction through formation of a local high concentration of sensitizer around the hydrophobic region of the DNA. Dye-sensitized photooxygenation of the imidazolin-2-ones (20; R = R' = H, Ph, Me, Bu, CH2CH=CH2; R = H, R' = Ph) using methylene blue gives the corresponding diacylureas, BzNRCONR'Bz, in a process which proceeds via the formation of zwitterionic perepoxides and dioxetans.212 Photooxidation of the bis(benzothiazo1-2-y1)methanes (21; R = H, R' = H, Me, Ph, 4-02NCgH4, 2-pyridinyl, benzothiazol-2-yl) gives methanols (21; R = OH, R' same) and ketones,213 and oxothiochrome (22) is formed by photochemical oxidation of aqueous solutions of thiochrome, using

0 2 ( lAg)

generated from Rose Bengal adsorbed on Sephadex G-25.214 Dialkyl phosphite inhibition of the photooxidation of 4-(p-tolylazo)- 1-naphthol with 0 2 ( 'Ag) has been employed as an indicator of reaction in the kinetic determination of dialkyl phosphites and some aromatic aldehydes.215

IIl5: Photo-reduction and -oxidation

305

9 Miscellaneous Oxidations The effects of common inorganic ions on the rates of photocatalytic oxidation of organic carbon over illuminated Ti02 have been discussed.2 16 Charge-transfer photooxygenation of substituted disiliranes and substituted oxadisiliranes have been studied spectroscopically in MeCN at 298 K and in a cryogenic matrix at 16 K.217 The products are dioxadisilolanes and trioxadisilolanes respectively. The same group also reports that in the singlet oxygenation of aryloxadisiliranes to give the corresponding trioxadisilolanes, a peroxonium ion intermediate may be involved.218 D C A - s e n s i t i z e d photooxygenation of silyl enol ether MegSiOC(Ar)=CHCHj (23; Ar = Ph, 4MeCgH4, 4-CICgHq) gives ArC02H by an electron-transfer mechanism.219 A dioxetan is produced by reaction of 0 2 - with (23.+) which cleaves to give trimethylsilyl benzoate; on subsequent hydrolysis, ArC02H is formed. Oxidative cyclisation of mono-tert-butyldimethylsilylated diols to give tetrahydrofurans is mediated by N-iodosuccinimide in a process in which the tert-butyldimethylsilyl moiety successfully controls both the direction of the cyclisation and ring size.220 The photooxidation of H2S has been studied in the presence of 0 2 and N02.221 In particular, the kinetics of the photolytic formation and decay of HSO' has been investigated and a new mechanism of H2S photooxidation in the troposphere has been proposed. Photooxidation of aqueous sulphite using 254 nm radiation gives s2062-and SO@- in the absence

-

of oxygen, but only SO42- (Q, 500) in its presence.222 Trapping experiments are described which suggest that S O 4 - - is the main chain carrier. Photooxidation of perhalofluoroalkyl sulphinates to give the corresponding perfluorohalocarboxylate is reported to proceed by photochemical electron-

transfer.223 In the reaction of sulphides with 0 2 ( l A g ) , persulphoxides and thiadioxiranes have been shown to be two important intermediates which are formed in competitive processes.224 Hydrogen bonding and coordination with solvents or additives serves to stabilise the dipolar persulphoxides, and the cyclic intermediates whose structure has been the subject of calculations, are readily converted to sulphones. Evidence has appeared suggesting that the

Photochemistry

306

photooxygenation of substituted diphenyl sulphides on irradiated Ti02 powders suspended in oxygenated acetonitrile, occurs via formation of a surface-bound cation radical as the primary photoprocess.225 Isopolytungstate, [W 10032]4-, has been shown to catalyse simultaneous oxidative C-H cleavage and reductive C-S cleavage in thioethers.226 The main oxidative process is abstraction of the hydrogens a to the S atoms of the thioether substrates and the main reductive process is reduction of these substrates by [WloO32]6-, the two-electron reduced form of the catalyst. Compelling evidence for a sulphurane intermediate in the photooxidation of y-hydroxy sulphides has emerged from a 1 7 0 tracer

study.227 Photooxidation

of

1-methylthianthrene

in

H20/MeOH/NaOH gives the 10-oxide and the 9,10-dioxide.228 An examination of the behaviour of a-terthienyl and related thiophenes in both homogeneous and micellar solution containing electron acceptors shows that the thiophenes can behave as excellent electron-donors,229 and a kinetic investigation of the photooxidation of leucothionine (DH2) by U022+ has enabled the ratio of thionine (D) to semithionine (DH.) emerging from the 3(UO2+, DH. ) cage to be determined.230 Photocatalytic oxidation of N02- on a Ti02 suspension in the presence of oxygen seems to occur electrochemically by a mechanism which probably involves oxidation of N 0 2 - by HO., formed as an intermediate in the anodic photooxidation of H20 at the catalyst surface.231 Fe2O3, Fe304, and FeO(OH), the most common iron compounds found in soil, are found to inhibit the sensitized photooxidation of bromaci1.232 References 1

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B Cai and D. Chen, Gaodeng Xuexiao Huaxue Xuebao, 1990, lJ, 1208.

181

R. E. Balsells and A. R. Frasca, Tetrahedron, 1982,38, 2525.

182

R. Fure, C. Roemming, and L. Skatteboel, Acta Chem. Scand., 1990, 44, 199.

m.,1 9 9 0 , a , 6901.

183

C. Singh, Tetrahedron

184

I. Petkov, A. Bozhilova, and P. Markov, Monatsh. Chem., 1990,121, 85.

185

E. A. Kalmykova, N. A. Kuznetsova, and 0. L. Kaliya,

a.Obshch.

Khim., 1990,60,946. 186

H. M. Chawla and S. K. Sharma, Tetrahedron, 1990,46, 1611.

187

W. Adam, E. Kades, and X. H. Wang, Tetrahedron

m.,1990,3l,

2259. 188

W. Adam, 0 Albrecht, E. Feineis, 1. Reuther, C. R. Saha-

Moeller,

P. Seufert-Baumbach, and D. Wild, Liebigs Ann. Chem. 1991, 33. 189

Y. H. Kuo and K. S. Shih, Heterocycles, 1990,& 1941.

190

C. Richard and J. Lemaire, J. Photochem. Photobiol.,

A 199O,Z, 127.

IIt.5: Photo-reduction and -oxidation

191

317

B. L. Feringa, 0. J. Gelling, and L. Meesters, Tetrahedron

m.,1990,

31.7201. 192

M. R. Iesce, M. L. Graziano, F. Cermola, G. Cimminiello, and R. Scarpati, Gazz. Chim. Ital., 1990,120, 629.

193

J. A. Navio, M. J. Fuentes, A. M. A. Pradera, and G. M. Garcia, J. Photochem. Photobiol., A, 1990,52,91.

194

T. Munegume, N. Nishi, K. Harada, J. Chem. SOC..Chem. Commun., 1990, 1689.

195

A. R. Gangloff, T. M. Judge, and P. Helquist, J. Org. Chem., 1990,55,

3679. 196

M. Valls, J. M. Bayona, J. Albaiges, and M. Mansour, Chemosuhere, 1 9 9 0 , a , 599.

197

G. Pandey, K. S . Rani, and U. T. Bhalerao, Tetrahedron

m.,1990,

31, 1199. 198

J. Santamaria, R. Ouchabane, and J. Rigaudy, Tetrahedron

m.,1989,

30, 3977. 199

T. Miyashita and M. Matsuda, Macromolecules, 1990,23, 2598.

200

Y. H. Kuo and K.-S. Shih, J. Photochem. Photobiol., A, 1989,4& 375.

201

E. G. Solozhenko, N. M. Soboleva, and V. V. Goncharuk, Ukr. Khim. Zh. 1990,& 439.

_.

202

K.Liu, S. Chen, Ganeuang Kexue Yu Kuang Huaxue, 1990,68.

203

F. Amat-Guerri, R. Martinez-Utrilla, and M. M. C. Lopez-Gonzalez,

J. Photochem. Photobiol., A, 1990,5J, 361. 204

F. Amat-Guerri and R. Martinez-Utrilla, J. Photochem. Photobiol., A,

205

Y. Maki, I. Oyabu, S. Ohara, M. Sako, Y. Kitade, and K. Kosaku,

1990,50, 377. Chem. Pharm. Bull., 1989,37,3239. 206

G. K. C. Low, S. R. McEvoy, and R. W. Matthews, Chemosphere, 1 9 8 9 , B , 1611.

207

Z. Diwu, M. Zhang, and L. Jiang, Chin. Sci. Bull., 1989,& 401.

Photochemistry

3 18

208

E. L. Lozovskaya and I. I. Sapezhinskii, Zh. Fiz. Khim., 1 9 9 0 , a , 1012.

209

R. J. Gonzalo and F. Temprano, React. Kinet. Catal. Lett., 1990,Q, 1.

210

J. A. S. Cavaleiro, M. G. P. S. Neves, M. J. E. Hewlins, and A. H. Jackson, J. Chem. SOC..Perkin Trans. 1, 1990, 1937.

21 1

S. E. Rokita, S. Prusiewicz, and L. Romero-Fredes, J. Am. Chem. SOC.,

1990,112, 3616. 212

H. M. Chawla and M. Pathak, Tetrahedron, 1990,46, 1331.

213

M. T. Ramos, C. Avendano, J. Elguero, and M. L. Jimeno, Bull. SOC. Chim. Belg., 1989,%, 497.

214

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Zimatkina, and G. N. Volkova, Khim. Prir. Soedin., 1989, 585. 215

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Ser. 2: Khim., 1989,30, 492. 216

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1990,94,6820. 217

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Chem. SOC.,1990,,lJ2,7804. 218

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1990,

31, 4177. 219

S. Wu and Z. Zhang, Chin. J. Chem., 1990, 271.

220

C. E. McDonald, T. R. Beebe, M. Beard, D. McMillen, and D. Selski, Tetrahedron

22 1

m.,1989,30,4791.

V. P. Bulatov, S. I. Vereshchuk, F. N. Dzegilenko, 0. M. Sarkisov, and V. N. Khabarov, Khim. Fiz., 1990,9, 1214.

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223

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224

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&., 1991,113,2677. 225

M. A. Foxe and A. A. Abdel-Wahab, Tetrahedron 4533.

m.,1 9 9 0 , a ,

M.5: Photo-reductionand -oxidation

319

226

R. C. Chambers and C. L. Hill, J. Am. Chem. SOC.,1990,112,8427.

227

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228

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229

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230

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Photoreactions of Compounds containing Heteroatoms other than Oxygen BY S. T. REID 1.

Nitrogen-containing Compounds

The photochemistry of nitrogen-containing compounds continues to attract attention although few reactions not easily predictable have been described in the year covered by this Report. Rearrangements.- Z,E-Isomerisation occurs readily on excitation of imines, azo compounds and related unsaturated species. Carbonnitrogen double bond photoisomerisations have been reported in Nmethoxy-l- (2-anthryl)ethanimine' and in 2- (arylhydrazo)-3 (2H)benzo [b]thiophenones. Irradiation of the E-isomer (1) similarly The affords the Z-isomer ( 2 ) in virtually quantitative yield. photochromism of the osazone (3), however, has been attributed to photochemically induced tautomeric conversion to the azo chromophore ( 4 ) rather than 2, E-isomerisat ion. An easily performed laboratory experiment which examines azo group Z,E-photoisomerisation in 4-anilin0-4~-nitroazobenzene has been d e ~ i s e d . ~In addition, examples of photochromisrn arising by isomerisation around the nitrogen-nitrogen double bond are common and the topic has been reviewed in detai1.6,7 An E-rZ isomerisation of this type is thought, for example, to be responsible for the Analogous photophotochromism observed in the formazans ( 5 ) . isomerisations have also been described in [2.2](4,41)azobenzenophane' and in [ 23](4,4 ) azobenzenophane,lo and E,E-2,19-dioxo [ 3.3 ] (3,31)azobenzolophanehas been converted by irradiation at 369 nm The cyclophane ( 6 ) into a mixture of E,E- , 2, E- and 2,Z-isomers. undergoes change in its cavity shape in response to a photochemically induced E-rZ isomerisation of the azobenzene moiety, thus controlling guest se1ectivity.l2 In the same way, molecular association can be controlled photochemically in an azobenzene-

lIJ6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

321

Q p - - O.Q YN-Nd -YNd N-H

*

H

,y

h

H

H\

0

0

(4)

(3)

(5) R = H, p -Me, p -OMe, p -Br, p -NO2, o -Me,

or o - ~ r '

2 CI-

$5

Ph

Ph

322

Photochemistry

modified 6-cyclodextrin, possibly because the non-planarity of the Z-isomer is unfavourable with respect to dimer formation.l 3 An azobenzene-modified y-cyclodextrin has also been used as a multiresponse type of host-guest sensory system.l4 Liquid crystal formation in polymers containing azobenzene moieties in the side chain can similarly be controlled by photoisomerisation.l5 Analogous photoresponsive peptide and polypeptide systems are being developed; the photochemical properties of poly [Np-p- (phenylazo)benzoyl-L-a,P-diaminopropionic acid] have been examined,l6 and the first example of photoregulation of permeability across a membrane has been achieved by the use of a new polyvinyl/polypeptide graft copolymer composed of a photoresponsive copolypeptide branch from 0-p-phenylazobenzyl L-asparate and P-benzyl L-asparate attached to a poly (Bu methacrylate) backbone. l7 Papain activity has similarly been regulated by anchoring a photo-responsive azo group to the enzyme backbone. A detailed study of phototautomerism in 3-methyllumichrome has been published, and light-induced intermolecular proton transfer from nitrogen to oxygen has been observed in m-bromo-Nt-(5-nitrofurfurylidene)benzohydrazide crystal hydrate. 2o Irradiation of the spirocyclic 4H-naphthalenone-perimidines ( 7 ) gave the quinone imines ( 8 ) .21 Examples of photorearrangement arising by 4n- and 6n-electrocyclic pathways have again been reported. The azepinone ( 9 ) is converted in this way into the bicycle (10) in 64% yield by a process which is thermally reversible.22 The analogous conversion of l-(methoxymethyl)-2(lH)-pyridinone into the corresponding azabicyclo[2.2.0]hexenone has been used to provide a synthetic route to lI4-oxazepinones and 1,4-diazepinonesI23 and the 1,4-benzoxazepines (11) undergo a similar 4n-photocyclization to give the thermally unstable dihydrobenzofuroazetes (12) 24 The formation of cis- and trans-3-ureidoacrylonitriles (13) on irradiation of the cytosine derivatives ( 1 4 ) can also be rationalised in terms of bicyclic intermediates as shown in scheme l.25 Photochemically induced 6n-electrocyclisation of the quinoneimine ( 1 5 ) , followed by elimination of HI, has been used in a synthesis of the marine alkaloid ascididemin (16) 26 Methylbenzo[a]quinolizinium salts have similarly been prepared by photocyclisation of the corresponding styrylpyridinium bromides in the

.

.

IIi6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

(11) R = H, MeO, or MeS

(1 2)

~q~~~ N p R 2

-o

OAN

A

A1

323

N\ R1

H

H

R10').('WY

0 R1

(13)

(14) R' = H, R2 = H or Me R' = Me, R2 = H or Me R' = ribosyl, R2 = H

Scheme 1

hv

Ph

I Ph

CH=CH-Ph

ClO,

Ph

2

324

Photochemistry

presence of iodine,27 and conversion of the pyridinium perchlorate (17) into the quinolizino[3,4,5,6-def]phenanthridinium cation (18) was achieved by irradiation in methanol.28 An analogous approach to the synthesis of the ll-azoniapyrene skeleton has been described.29 6n-Electrocyclisations are also responsible for the photochromism exhibited by many nitrogen-containing fulgides. Thus, the indole-containing fulgide (19) undergoes 2,E-isomerisation and cyclisation to the isomer ( 2 0 ) on irradiation.30 Photochromic 2pyrrylfulgides have also been described.31 Various attempts to prepare photochemically reversible but thermally irreversible 2- (1,2photochromic systems of this type have been reported.3 2 r 3 3 Dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)maleic anhydride undergoes cyclisation/ring opening reactions, for example, with high quantum yields; both isomers are thermally stable.32 Photochemically induced ring opening is responsible for the photochromism observed in indolinespiropyrans. Factors affecting the photostability of these ~ p i r o p y r a n sand ~ ~ their behaviour in organised molecular assemblies35 have been investigated, and new idolinespiropyrans with n-acceptor substituents in position 8' have Interest in the related indolinespiro-oxazines been prepared.3 6 appears to be increasing; the photochromism of spiro-oxazines has been reviewed,37 and photochromic indolinespiroanthro-oxazines38and the bisoxazine (21)39 have been prepared. Analogous photochromic behaviour in the spiroindolinebenzothiopyrans ( 2 2 ) 40 and in spirothia (selena)pyrans with condensed pyrazole nuclei41 has been reported. New applications of stilbene-to-dihydrophenanthrene photocyclization in nitrogen-containing systems have been reported. Oxidative photocyclisation of this type is implicated in the conversion of the diarylmaleimides (23) into the phenanthrene-9,lOdicarboximides ( 2 4 ) p 2 Identical synthetic approaches to certain phenanthrene alkaloids,43 to benz [ a] acridines,4 4 and to [ 13benzohave also been described, and the thieno[ 2,3-h] is~quinoline~~ pyrrole analogue ( 2 5 ) can be photochemically converted into the tricycle ( 2 6 ) using a dehydrogenation procedure.46 The same type of transformation can be achieved in halogen-containing compounds v i a 6n-cyclisation followed by elimination of HX. The benz[clacridines ( 2 7 ) have been prepared in this way from the

lII4: Photoreactions of Compounds containing Heteroatoms other than Oxygen Me

Me Me

Me

H

Me

H

MeMe

Me

(22) R' = Me, CHMe2, or octadecyl R2 = CH202CCMe=CH2 or CH202C(CH2)2,Me

R

Ar 0 I

H

I

H (24) R = H, 3-Me0, 2-Me0, 3-CI, 2-CI, 3-Br, or 3-F

325

326

Photochemistry

- hv HCI

*& \

/

0

(28)R = H,CI, or OMe

C02Me

(27)

R' hv

c

0 (29)R' = H,R2 = PhCH2, R3 = H R' = R2 = Me, R3 = H or Me

Me0

h D

R R

R

(31) R = H or M e 0

(32)

1116: Photoreactions of Compounds containing Heteroutoms other than Oxygen

327

chloro(carboxyphenyletheny1)quinolines as have phenanthro[ 9,lO-d] oxazoles from the corresponding 4,5-diaryloxazoles.48 The analogous photocyclisation of enamides is well known and Photocyclisation of the is of particular value in synthesis. enamides (29) to the furopyridones (30), for example, has been used in the preparation of key intermediates for alkaloid synthesis.49 Similar approaches have been employed in the synthesis of corynanthe alkaloidsSo and in the preparation of trans-hexahydroindolo[ 43-ablphenanthridines as selective dopamine D1 agonists.51 The benzoylmethylenebenzothiazepines (31) can be converted in the Enamide same way into the B-homo-5-thiaprotoberberines (32) 52 photocyclisation has also been employed in the syntheses of halobenzofuro[2,3-c]quinolines,53 benzo[h] [ 13- and benzo[f] [l]-benzothien0[2,3-~]quinolines,~~ and difluoro[ l]benzothieno[2,3-c]quinolines,55 whereas competing cyclisation and rearrangement pathways leading to the formation of phenanthridone and 9-arylacridines are observed on irradiation of N-aroyldiphenylamines.5 6 Further mechanistic studies of the known photocyclisation of 1benzamido-2-(l-cyclohexenyl)cyclohexene (33) intothe oxazabicyclo[4.4.0]decadiene (34) have been reported; an excited amide appears to be involved.57 The photochemically induced cyclisation of diphenylamines to carbazoles is well established in the chemical literature. Details of the first example of such a cyclisation in N,N-diarylsulphonamides have been p~blished,~'and a similar conversion of the Nphenylenamine (35) into the indoline (36) has been used in a new synthesis of oxindoles 59 Analogous cyclisations accompanied by the elimination of HX have also been reported;6 ot 6 1 the pyridinium bromide (37), for example, is converted in this way into the pyridoisoindolium bromide (38) 6o A photocyclisation related to that well established for dienones has been reported in the 2-benzoylquinoxaline (39) and (40) 62 yields the purple-coloured methylindoloquinoxaline Synthetically useful benzophenone derivatives (41) have been prepared by photo-Fries rearrangement of the anthranilates (42), and a detailed study of the same photorearrangement in N-acetylcarbazole has been described.6 4 Further studies of the aza-di-?r-methane rearrangement have been reported. The ease with which P,y-unsaturated oxime acetates

.

.

.

.

Photochemistry

328

Ye hv

hv - HCI

‘p?

H

0

(39)

(42) R’ = H or Ac R2 = Me or Me0 Me Me

(43) R = CN, COZEt, or CH20Ac

P

h

s N, (45)

(44)

hv

OAc

-

phpL, Me

Me

(46)

OAc

IIt6: Photoreactions of Compounds containing Heteroatoms other than Oxygen

329

undergo this photoreaction depends to a large extent on the presence and nature of substituents. Acetone-sensitised irradiation of the oxime acetates (43) of 4-ethoxycarbonyl-, 4-cyano-, and 4acetoxymethyl-2,2-dimethylpent-3-enal readily affords the cyclopropanes (44) by such a pathway, whereas the same derivatives of 2,2-dimethylbut-3-enal undergo only 2,E-photoisomerisation. 6 5 A yield of 86% was recorded for the aza-di-n-methane rearrangement of

2,2-dimethyl-4,4-diphenylbut-3-enal. 66 the oxime acetate of surprisingly, although the oxime acetate of trans-2,2-dimethyl-4phenylbut-3-enal undergoes an analogous stereospecific aza-di-nmethane rearrangement, the oxime acetate (45) is converted on irradiation into the isomer (46) by a pathway involving a novel l13-migration.67 The semicarbazone, the benzoylhydrazone, and the oxime benzoate of 2,2-dimethyl-4,4-diphenylbut-3-enal also undergo di-n-methane rearrangement to the corresponding cyclopropanes in good yield, 6 8 and various benzopyrazinobarrelenes are reported to undergo analogous photorearrangement to benzopyrazinosemibullvalenes. 69 The photorearrangement of five-membered heteroaromatic systems has attracted little attention in the year covered by this report. A detailed reinvestigation of the photoreactions of 4-acylisoxazoles has clarified some of the anomalies previously reported in the literature. Both oxazoles (47) and (48), expected on photorearrangement of the 4-benzoylisoxazole (49) v i a the 2H-azirine (SO),

have now been i~olated;~' evidence has also been obtained

from wavelength studies of the involvement of at least two distinct intermediates in these transformations.

In contrast, however,

photorearrangement of the 3-styryl-1,2,4-oxadiazoles (51) procedes by way of nitrogen-oxygen bond homolysis followed by cyclisation and affords the quinoline derivatives (52) 7 1 Initial nitrogenoxygen bond homolysis is also responsible for the tripletsensitised rearrangement of selected 5-phenyloxadiazoles (53) to the quinazolin-4-ones (54).72 Further examples of the photochemically induced conversion of isoxazolines into enamino aldehydes have been reported. The isoxazolines (55), for example, exclusively afford the aldehydes (56) v i a the stabilised biradicals (57) as shown in Scheme 2.73 Pyridyl ring-fused isoxazolines also

.

undergo the same photoreact ion. 7 4 Conversion of the dl-cyclomer (58) into the meso-stereoisomer

Photochemistry

330

Ph

Me

hv

*

R

R-C-N

I

H (51) R = Me or Ph

(52)

hv

XO" y aliphatic amino groups > alicyclic amino groups. This follows the ionisation potential of the amino groups. Both conventional and microsecond flash photolysis confirmed the involvement of intramolecular hydrogen atom abstraction and this is illustrated by the mechanistic processes in scheme 3 for an Nmethyl derivative and scheme

4

for a diethylamino derivative. Here

it is seen that the N-methylpiperazine derivative, does not undergo intermolecular hydrogen atom abstraction. Intramolecular hydrogen

Photochemistry

408

/

0; HG

C2H5

*CHCH3

o & o O - C H . J N 2 - N - Ci 2 H 5

I

0-CH2CH2-N-C2H5

I

+

x2

CHCH3 @!eO-CH2CH2-N-C2H51

e r G O - C H 2 C H 2 - N 72H5 I CHCH3 I

CHCH3 o/ i \ e O - C H 2 C H 2 - N

I

I C2H5

-

I l l : Polymer Photochemistry

409

atom abstraction to give a biradical product is the main process which like that shown for the N-diethylamino derivative gives a dimeric coupling product of mass 2M shown by (C) and not that for (D). Using benzene as a solvent both phenyl and alkylamino radicals

have been implicated in the photoinduced polymerisation of methyl methacrylate by 4-N ,N-dimethylamino-4 -isopropylbenz~phenone~~.

A novel monomer 2-acryloylthioxanthone has been prepared and found to undergo radical copolymerisation with a number of other m o n o m e r ~ ~ ~Of ' ~particular ~. interest is a conventional and laser flash photolysis study on the copolymers with methyl methacrylate where triplet formation was found to be significantly quenched compared with that of the monomer itself. Here intramolecular self quenching was shown to be important in the former case. Furthermore, whilst exciplex formation was evident in the presence of a tertiary amine with the monomer this was not the case for the copolymers due to steric hindrance by intra-molecular coiling of the polymer chains. Perester derivatives of benzophenone and fluorenone continue to be investigated by a number of methods42. In this work real time Fourier Transform infra-red spectroscopy and photodifferential scanning calorimetry were found to be related with the bis-tert-butylperester derivative of fluorenone being the most effective initiator. In all cases the results were consistent with the quantum yields of photolysis of the photoinitiators. In the photoinduced polymerisation of butyl methacrylate by benzophenone/dimethylamino ethyl methacrylate initiation is

associated with reduction of the benzophenone with the rate decreasing in polar solvents43. In non-polar benzene dark reactions continued. Another co-reactive monomer 2-hydroxy-3-morpholinopropyl methacrylate has also been prepared44 as have oligoethers with triazine groups and terminal acetylenic groups45 and a new

S-

410

Photochemistry

initiator 4,4'-azobis(dimethylaminoet~yl-4-cyanopentanoate

which

yield polymers with tertiary amine e n d - g r ~ u p s ~ ~

The effect of a magnetic field on photopolymerisation continues to attract interest.

In the polyvinylnaphthalene

photoinduced polymerisation of methyl methacrylate using azobisisobutyronitrile ( A I B N ) as initiator higher percentages of syndiotactic sequences of polymer were produced in the presence of a magnetic field47-49. Here the polymeric sensitiser acted as a cage around the A I B N which under the influence of the magnetic field had increased triplet yield

and consequently increased

intermolecular interactions and less entanglements. A magnetic field has also been found to enhance molecular weight conversions of polystyrene in emulsion polymerisation but only at low light intensitiesS0 Photocopolymerisation of monomers has been investigated in some detail. The rate of photocopolymerisation of styrene-acrylonitrile has been found to be much faster than either of the monomers alone due to exciplex formation with diradical initiation being predominantS1I 5 2 . Diradicals from single monomer units polymerised predominantly via cycloaddition. In the case of styrene-fumaronitrile a tetramethylene diradical was observed with photocycloaddition being favoured at 365 nm53,54. A 4vinylpyridine-zinc chloride complex has also been found to induce copolymerisation with styreneS5. The propagation rate constants for the photoinduced copolymerisation of styrene with alkylmethacrylates have been found to conform with the Mayo-lewis model 56 while block copolymers have been produced using polymer chains containing dithiocarbamate end-groups57. The polymer isation rates of various vinyl and acrylic monomers have been found to be dependent upon solvent polarity58 and high molecular weight polymers have been produced from butyl methacrylate using a

I l l : Polymer Photochemistry

41 1

quinoline-bromide charge-transfer complex as the phot~initiator~~. Polymerisation was also found to occur in the dark. On a more theoretical basis the chain lenght distribution and its moments have been calculated for a periodically interrupted photopolymerisation with termination by disproportionation and negligible chain transfer60. From the derived expressions the propagating rate constant could be evaluated from the chain length distribution of the polymer. The simultaneous effects of uv and

T-

irradiation of polymethylmethacrylate decreased the residual monomer concentration significantly with no effect on mechanical properties61. In a comprehensive study of charge-transfer complexes it has been found that strong and weak complexes favour copolymerisation62 while the photopolymerisation rates of vinyl monomers in the presence of p-substituted diphenyl disulphide increases in the order acrylonitrile > methyl methacrylate > styrene63. The ability of the monomer and sensitiser to form a complex determined the order while a styrylcoumarin compound has been found useful for monitoring the polymerisation kinetics of vinyl and acrylic monomers independently of the free volume changesd4.

Micelle formation has been studied for sodium salts of fatty acids containing terminal double bonds using electrical cond~ctivity~~. Here two critical micelle concentration points were observed of which the first point at 0.044 moles litres'l

was

critical in terms of the number average degree of polymerisation of the polymers produced. At concentrations up to the second point the molecular weight change was significantly smaller. The photopolymerisation of acrylamide in reverse micelles was found to be first order with respect to monomer concentration whilst the order was found to depend upon the oil concentration in the

412

Phoiochemistry

micelle66. When the oil used was toluene instead of benzene monoradical termination occurred indicating that degradative chain transfer is occurring to the former solvent system. Vinyl halides have apparently been photopolymerised in the presence of AIBN using light 750 nm through a monomer initiator charge-transfer complex67. One of the products of decomposition of the complex was determined to be hydrocyanic acid. Poly[4-(4-nitrobenzyloxy)styrene] has been synthesized with a high degree of etherification by the reaction of poly(4-hydroxystyrene) with p-bromomethylnitrobenzene although yields were relatively low in aprotic solvents68 whereas the photoinitiated polymerisation of 3,6-dioxa-1,8-octanedithiol

with

diallyl ether leads to poly(thi0ether.s) with an anti-Markownikov s t r u ~ t u r e ~In ~ .the latter case for a series of monomers whilst the rate was found to decrease in the order vinyl ether > ally1 ether > 1-alkene the thiol group was found to have no influence. High molecular weight polyvinyl alcohol has been prepared by a photoinduced polymerisation method70 and a two dimensional photopolymerisation has been carried out as a monolayer on gold71. Methyl viologen with sodium dithionite has been used in a two phase photoinduced polymerisation of methyl metha~rylate~~. Here the rate of polymerisation was found to be proportional to the square root

of the concentrations of initiator feeds. A cyclic phase transfer initiation mechanism was proposed for the mechanism based on a cationic radical disproportionation reaction involving the viologen species. Using photocalorimetry the photopolymerisation of acrylic esters has been found to be related to the glass transition temperature, oxygen permeability and the limiting conversion73 while the elementary steps in the photoinduced polymerisation of acrylamide in micelles has been considered with particular emphasis on the localisation of the initiator7*. Triphenylarsonium-pnitrophenacylide has been found to induce the photopolymerisation

111: Polymer Photochemistry

413

of methyl methacrylate by a free radical mechanism through scission of the arsenic-phenyl bond75 whereas poly( crownether) -picrate complexes have been found to release picrate groups on irradiati~n~~. The methacrylic acid ester of [ 1-( (4-phenylazo)phenylazo)]-2-naphthyl ester has been prepared and found to impair the photopolymerisation of p ~ l y s t y r e n ewhile ~~ acrylated monomers containing hydroxyl groups have been found to polymerise faster than corresponding monomers containing acetylated groups78. A new class of polymers containing a photosensitive disilane group in the main chain have been prepared with good solubility in most solvents as well as good thermal stability79. The photopolymerisation of tetrafluoroethylene has been found to be accelerated through the use of 7-irradiated hexafluoropropylene monomer which gives active trifluoromethyl radicals80. Of particular interest is the development of a direct and non-destructive method for monitoring the kinetics of monomer transport during photopolymer formation8'. This is carried out through the use of a thin poly(viny1acetate) film plasticised with a fluorescent probe N-vinylcarbazole. Thus, during the polymerisation reaction there is a monomer concentration gradient where the monomer diffuses across the film towards the illuminated surface. As polymerisation proceeds the fluorescence intensity increases and is directly related to the diffusion rate of the monomer and hence its polymerisation kinetics.

Solid state and template polymerisations continue to attract interest. The orientation of aliphatic tails in the photopolymerisation of diacetylenic liquids has been investigated and found to influence the lattice parameters of the polymers produced83. Two chiral structures were observed for the polymers which were assigned to differences in the tilt angles of the headgroups with respect to the polymer axis. Crystalline

Photochemistry

414

polyacrylamide has been prepared and found to contain large sphe-ru1itj.cstructuress4 while two glass transition temperatures have been found for poly(4,4’-cyanobiphenylyl-4(6acryloyloxyhexy1oxy)benzoate photopolymerised in an electric

f

The low temperature photocyclopolymerisation of

crystalline ethyl-p-phenylenediacrylate under high pressures gives rise to starins which hinder the lattice relaxations86 whereas phenylacetylene has been polymerised under laser irradiation to give a trans-rich double bond polymers7. The template photopolymerisation of methyl methacrylate has beencarried out on atactic (polyvinyl acetate)88 and diacetylenic phospholipids have been found to photopolymerise only in the presence of dinonanoyl pho~phatides~~. Living epoxides and episulphides have been prepared using zinc N-substituted porphyrin as a photoinitiatorgo.

Cationic photoinduced polymerisations continue to be investigated to some extent. The polymerisation kinetics of epoxy acrylates have been found to depend on the nature of the epoxy groups and their functionalityg1 and a series of non-toxic (4alkoxypheny1)phenyliodonium salts have been synthesized as cationic

photoinitiators for vinyl and heterocyclic monomersg2. Photoredox reactions using diphenyliodonium salts have been found responsible for polymerisation of divinylethersg4 with high conversions being observed at slower polymerisation rates. Vinyl ethers with pendant norbornadiene units have been cationically photopolymerised to give polymer containing quadricyclane units that may be reversibly isomerised using a cobalt tetraphenylporphyrinato complexg5. Tetrahydrofuran has been copolymerised with propylene oxide using a cationic photoinitiator with the co-monomer ratios controlling the molecular weight distributiong6 while a range of silicon containing epoxy resins have been prepared which are capable of cationic

111: Polymer Photochemistry

415

induced polymerisationg7. Other studies of interest include the photopolymerisation of surface active monomersg8, ~ - c a p r o l a c t o n e ~ ~ and monomers onto evaporated dye layerslo0.

2.2 Photoaraftinq The photochemical grafting of monomers onto substrates has diminished in activity over the last few years and this last year has been no exception. Again magnetic fields have attracted interest with the graft ratio of isoprene onto a

tetrafluoroethylene-propylene copolymer being three times higherlol. The surfaces of various materials have been modified by high energy uv irradiation for improved graftinglo2 while the uv grafting of acrylic acid onto polyethylene produces a polymer different from that obtained by other methodslo3. Benzophenone and Michler's

ketone have been used to photograft acrylonitrile onto

high density polyethylene to produce a graft yield of 2l%Io4 while the elasticity of cotton fabrics is increased by monomer graftinglo5 and the dyeability of polyester is enhanced in a similar fashionlod. Acrylamide has been grafted onto poly( vinylalcohol) using ceric ions as initiatorslo7 and block graft copolymers of butyl acrylate have been prepared with styrene by the photoinferter techniquelo8. Aliphatic a-diazo ketones have been used as photoinitiators for the grafting of fillers onto the surface of polyethylenelog while reverse osmosis membranes have been prepared by the photografting of monomers onto the surface of poly(methy1 vinyl ketone)llO. Light stabilisers based on the orthohydroxyphenylbenzotriazoles have been successfully photografted onto polyolef ins to impart improved stability to weatheringlll whereas methacrylic acid has been successfully photografted onto cotton using hydrogen peroxide as a catalyst with no effect on moisture regainll2.

Photochemistry

416

2.3 Photocrosslinkinq Unlike the field of photografting photocrosslinking remains to be a wide and actively growing subject of industrial and academic importance. Laser photolysis studies have been carried out on naphthoquinone diazides in Novalak films113. Time resolved spectra were obtahed to give three types of transients as shown in scheme 5. The important feature of the reactions appears to be the active

ketene intermediate shown by the second transient. In the photocrosslinking of phenol-formaldehyde resins using a copolymer of 2-diazo-1-naphthoquinone-5-sulphonyl chloride the ketone groups were observed to react with the hydroxyl groups of the pheno1114. High performance epoxy resins for microelectronics could be cured in thick layers by building them with intermediate layers of a cresol Novalak and curing cationically with a nulphonium salt115.

A

number of papers have concentrated on the polystyrenes with one interesting study on magnetic field effects on the gelation of styrene-vinylbenzyl azide using thioxanthone photoinitiators116I '17.

Here the increase in gelation was

interpreted in terms of a radical pair model of intermediate species formed by hydrogen atom abstraction of a triplet nitrene from the sensitiser. Photoreactions of poly[(4(trimethylsilylmethyl)styrene] with dicyanobenzenes produced

crosslinked polymers in high yields but in the presence of 1,2,3,4tetracyanobenzene a soluble polymer was produced118. In the latter case the trimethylsilyl group was replaced by a 2,4,5tricyanophenyl group. The photocrosslinking of polymer blends undergoing phase separation has been monitored using anthracene labelled polystyrene/poly( 2-chlorostyrene)

while the deblocking

of poly(tert-butoxystyrene) has been performed using

417

III: Polymer Photochemistry

Ydtransient

3rdtransient

0

HO

2

\C/OH

-@- @ R'

R'

1 Ql$H R'

Scheme 5

&

-A+

A+

Chain Scission

(3)

OH

' 7 +

Oxidation

(4)

+@

HO+

+H$ ' HO

(7) DHDMH (main prod.)

(8)

Photochemistry

418

trifluoromethylsulphonic acid in the presence of an iodinium

salt120.

Photosensitive polyimides have attracted much interest possibly because of their usage in electronic resists. A polyimide prepared from benzophenonetetracarboxylic dianhydride undergoes a primary photoreaction of hydrogen atom abstraction followed by rapid recombination to give high yields of crosslinking1211 122. Quantum yields of photoreaction were however, lower than that of benzophenone itself due to the charge-transfer nature of the imide groups. Other workers123-125 have developed photocrosslinkable polyimides containing perfluoromethyl groups which offer improvements in solubility and resolution. Those containing orthonaphthoquinone diazide groups undergo crosslinking reactions leading to insolubility which were associated with reactions between a ketene and a ketocarbene. Polyimides containing a diacetylenic functionality have also been prepared in a two step process involving oligomeric imide formation involving termination with l-amino-3-ethynylbenzene followed by oxidative coupling of the acetylenic groups with a bispropargyl ether of bisphenol A126.

A

polyamic acid has been synthesized for coating onto silicon wafers from cis-anti-cis-1,3-dimethyl-l-l12,3,4cyclobutanetetracarbxylic-l,2:3,4-dianhydride

and

4,4'-

oxydianiline which on heating is converted to the p~lyirnidel~~ ,128. Soluble polyimides have also been prepared with disilane units by the ring opening of 1,2-bis(4-aminophenyl)tetramethyld~s~lanewith 4

,4'-sulphonyldiphthalic dianhydride followed by irnidi~ationl~~. Ultraviolet autocurable semicrystalline prepolymers have been

acid synthesized from 3,3~,4,4~-benzophenonetetracarboxylic dianhydride-glycidyl acrylate-hydroxyethylacrylate-polycaprolactone

111: Polymer Photochemistry

419

dioll3O 131. The prepolymers were solids at room temperature with their physical properties being dependent upon the acrylic functionality. Liquid rubbers based on polyisoprene have been modified with maleic anhydride for photocr~sslinking~~~ while 6-[4(4-butoxyphenoxycarbonyl)phenoxy]hexyl methacrylate has been

photopolymerised in the presence of methylaluminium tetraphenylporphyrin to give a liquid crystalline polymer133. Heat and mass transfer reactions have been investigated during the curing of polyacrylamide gels134 while p-(N ,Ndimethy1amino)benzylidene malonitrile hasbeen used as a fluorescent

probe for monitoring the curing of epoxy resins135. The decoupling of electron donor and acceptor groups on the backbone of poly(viny1 cinnamates) markedly influences their reactivity136 and poly( vinyl cinnamoyl acetate) has been prepared and its sensitivity established137 as have derivatives of malonic with cinnamic acid138. Depth cure profiling has also been undertaken using Beer's law139 and in the curing of two piece cans high viscosity inks are claimed to be no problem140. New monoacrylates containing cyclic carbonates have been prepared and claimed to be more effective than triacrylates141 while during the curing of linear polyesters with a multifunctional acrylate monomer the glass transition of the system could be enhanced through heating due to a tight matrix of acrylate groups around the polyester units142. In the latter case although the phase was heterogeneous no phase separation was observed.

A

chitosan-ammonium dichromate film undergoes rapid crosslinking on irradiation due to the formation of acid dichromate ions143.

A

thermally irreversible novel photoresponsive polymer has been synthesized based on the monomer trans-2-[4[dimethylcarbamoyl)v~nyl]phenoxyJethyl methacrylate which on

polymerisation contains random cinnamimide moieties144. Solubility changes were observed on irradiation due to trans to cis

Photochemistry

420

conformational arrangements. Polymers containing N-phenylglycine units were found to undergo more rapid photocrosslinking when they were copolymerised with N-(2-hydroxy-3-methacryloxypropyl)-Nphenylglycine units145. The crosslinking of polyacrylamide gels has been monitored through the use of a pyrene fluorescent probe with dansyl chloride labels146 and the polymerisation of adamantyl-1acetic acid glycidyl ester produces a highly photosensitive polymer147 while alkoxysilane monomers have been shown to have different degrees of adherance to glass depending upon the crosslink density148. Diphenyliodonium salts have been found to accentuate the benzoin alkyl ether induced photocrosslinking of epoxypolysilicones149 while a highly photosensitive adduct has been produced from the addition of benzene to maleic anhydride150. Photocrosslinkable polyesters have been obtained by copolymerisation with N,N’bis (hydroxyethyl)benzophenonetetracarboxylic acid diimide151 and the rate of n-butyl acrylate has been studied in the presence of dithiodipropionic acid152.

A

series of tetraethylene glycol

dimethacrylates have been prepared and studied for applications in coating video disks153 while photocurable laquers have been investigated for coating various substrates such as wood154 155 whereas a series of cobaltamine complexes have been found to operate as effective deep curing initiators156.

A

series of

urethane-polyester-methacrylate oligomers have been made157 and the shelf life of radiation curable oligomers have been monitored by differential scanning calorimetry15*. In order to reduce the internal stresses in the photocrosslinking of epoxy resins acrylic polymers such as poly(n-butyl acrylate) are introduced and cured in-situl59.

111: Polymer Photochemistry

42 1

Many studies have appeared with regard to the photochemical crosslinking of polyolefins, particularly polyethylene. High density polyethylene has been crosslinked using

4-

chlorobenzophenone as a photoinitiator in the presence of triallyl cyanurate160. At high light intensities the crosslinking rate was first-order with time of irradiation and associated with the recombination of both ally1 and alkyl radicals. The thermal stability of such a photocrosslinked polymer was however, markedly reduced due to antioxidant depletion although the authors might well wishtoconsider the thermal activity of the residual initiator. Thioxanthone has been found to be a more effective photocrosslinking agent of polyethylene than xanthone and strongly dependent upon the light intensity161 while naphthaquinones have been found to be more effective than benzoquinones for the photocrosslinking of polyethylene162I 163 and combinations of aromatic photofragmenting initiators with the hydrogen abstracting types are synergistic164. The physical properties of polyethylene change markedly on photocrosslinking with a~etophenonel~~.

Rubber systems have been photocrosslinked but found to be dependent upon the solubility of the crosslinking agent in the rubber166 while caprolactone polyols have been found to impart superior properties to epoxy acrylate 01igomersl~~. The elimination of oxygen in uv curing has been found to be especially important16* and the photocrosslinking of 1,6-hexandiol diacrylate has been found to continue in the glassy state albeit in a self-decelerating rate169. Real-time infra-red spectroscopy has been used for monitoring ultra-fast photoinduced polymeri~ationsl~~ and the thermal decay of long-lived radicals produced in the photoinduced polymerisation of diacrylates has been monitored by EPR spectro~copyl~~. In the latter case both radical decay and double

422

Photochemistry

bond disappearance were found to be first-order coincident processes. Photocrosslinkable poly(viny1 alcohol) has been used to improve the wash fastness of cotton173.

The fluorescence of auramine 0 has been used to monitor local viscosity changes during the curing of an epoxy composite174 while acid fragment participation is believed to be involved during the irradiation of light sensitive layers of poly( itaconic acid)175 and the infra-red spectra of various radiation cured epoxy resins have been evaluated176. The rate of photopolymerisation of alicyclene epoxy resins is greater than that of cycloaliphatic types177 and the laser induced polymerisations of polyurethane-acrylates have been monitored by real-time FTIR spectro~copy’~~.

The photodegradability of poly(buty1 methacrylate) gels containing acyloxime units permitted their conversion from a crosslinked to a solubilized form179 whereas polyoxazolines gave hydrogels on irradiation with a level of swelling being dependent upon the degree of coumarin substitutionlaO. A unique wavelength dependence on the otherhand has been observed for a new thermotropic liquid crystalline polyether-siloxane-poly(ary1 cinnamate)lal. In the birefringent amorphous phase chromophores were observed to aggregate and undergo a preferential 2+2 cycloaddition reaction resulting in crosslinking. The unaggregated phase undergoes a photoFries rearrangement. Highly photocrosslinkable vinyl esters have been prepared containing a,& unsaturated carbonyl groupsla2. Temperature effects have been examined on phase transformations in thermosetting acrylicsla3 while network formation in vinyl terminated siloxane rubbers has been found to be dependent upon the vinyl content and particularly efficient when using benzoin initiatorsla4. Speciality glycidyl

III: Polymer Photochemistry

423

ethers have been prepared for cationic p ~ l y m e r i s a t i o n lwhile ~~ photocopolymers of ethylene glycol dimethacrylate with methyl methacrylate have been prepared as light focusing rods186. Intramolecular crosslinking of polyimides has been found to be influenced by the presence of oxygen187 while changes in the structure of polystyrene on crosslinking have been examined by integral and quasielastic light scattering techniques188. The uv curing of epoxysilanes has been discussed189 while the uv curing of monomers onto other polymer surfaces has been found to be a two phase process whereby the former diffuses into the surface layers of the substratelgo. Oligomers based on a hydroxy terminated acrylate have been found to cure rapidly with a high crosslink densityl’l

while new water soluble photoinitiators such as sodium

benzoylmethyl sulphonate have been found to be effective and nonyellowinglg2. Epoxy resins modified with poly( acrylates) have been found to reduce internal stresseslg3,lg4 whereas uv curing inks containing a trifunctional monomer were found to be lacking toughness and rheological behaviourlg5. Photodifferential calorimetry has been used to monitor the curing properties of 1,4butane diacrylatelg6 and a polyester urethane diacrylatelg7 and several new types of poly(perfluoroalkoxypropy1 methyl siloxanes) have been synthesized and their curing behaviour examinedlg8.

3

P O J l Y M E R S C E N a

A number of reviews of interest have appeared of both a specific

and general nature. These include photoresponsive polymers with the main emphasis on phase transitions, memory and shape retentionlg9’ 201, the kinetics of polyelectrolyte formation202, molecular modelling for excimer formation203, luminescent probes204 and molecular dynamics205. The uses of thermally stimulated emission

for monitoring radical decay206

and chemiluminescence of polymers

424

Photochemistry

for studying their degradation207 have also been reviewed and a model has been developed for the electronic excitation energy transport between chromophores on polymer chains using a polynomial approximation to the Gaussian distribution function for donor fluorescence decay208.

Radio, chemi and thermoluminescence studies on polymers continues to attract much interest as an analytical probe. The

7-

irradiation of pyrene and naphthalene doped polyethylene gives excimer emission in the range of the high temperature radiothermoluminescence peak209 whereas other workers have used the technique to establish changes in the glass transition of the polymer210. One particular feature of radiothermoluminescence is that it has been shown to originate from the surface layers of the polymer211 and is useful for monitoring the concentration ratios of dopants for thermal neutron activation212. Thermoluminescence has shown that the surface changes in polyethylene differ depending upon the nature of the prior treatment213 and in poly(p-phenylene) two thermally activated processes have been observed at two different temperatures with one of the processes involving the self-trapping of charge carriers via the formation of dimer cation radica1s2l4. Dielectric relaxation processes have been monitored in polybutadiene215. Some criticism has been made of work published on the decay kinetic analysis of the chemiluminescence of oxidised polypropylene as being invalid216 while the original author quotes the thermal oxidation of solid polymers as being similar to that of liquid phase hydrocarbon systems2I7. Whilst not wishing to become involved in such arguments the criticisms levied by the former authors are justified since such kinetic expressions would obviously require modification to account for differences in rates of diffusion. The technique has been discussed for characterising

I l l : Polymer Photochemistry

425

polyethylene218 and its degradation processes219 especially due to etching220 as well as critical micelle concentrations221. The emission from singlet molecular oxygen has been monitored subsequent to laser photolysis of polystyrene222 due to oxygenpolymer-charge transfer complexes on the surface of the polymer material. It is concluded that the method provides unequivocable evidence for the formation of such species in polymer systems and that even in pure polymers they are the major source of the initiation of degradation. The curing of polymer systems has been monitored by fluorescent probes such as epoxy resins223 224 and I

carbon fibre laminates225.

The luminescence of orientated poly(phenyleneviny1ene) has been shown to belong to the category of strong exciton-phonon coupled systems and has provided a valuable example of where transient free exciton luminescence is observable in time integrated spectra at room temperature226. The quantum efficiency of the luminescence was however, low due to large relaxations in the polymer. Charge carrier trapping in poly(epoxypropy1)carbazole is associated with both thermally activated processes and tunnelling with the latter being due to a photon type process227. Photoinduced infra-red absorptions have been observed in poly(acety1ene) at lower energies than those reported earlier due to charged solitons228 while the photoexcited states of poly( 3alkylthienylenes) have been c h a r a ~ t e r i s e d ~Intra-chain ~~. absorption across the

R-R*

gap has been observed as well as

photoluminescence from the radiative decay of the singlet exciton. The alignment of the polymer chains appears to control the stability of the photogenerated polarons. Charge delocalisation in poly(N-vinylcarbazole)/vinyl

acetate copolymers has been measured

by laser p h o t o l y ~ i sand ~ ~ ~found to be dependent upon the number of

426

Photochemistry

vinyl acetate units whereas the relaxation processes in poly(diacety1enes) have been monitored within the femtosecond time scale and are fully recovered within 1.5 picoseconds231. Poly( 2,5thienylene-vinylene) has been prepared as an optical film material which gives two transient absorption peaks which are assigned to

bipolar on^^^^. Pyrene terminated polymers continue to attract interest. The effect of pH has been found to be variable on the excimer fluorescence of pyrene end-tagged polyethylene glycols233 234 when bound onto silica. In the presence of poly(acry1ates) intermolecular pyrene excimer to monomer fluorescence is determined by intramolecular coiling of the polymer chains. The hydrophobic pyrene groups were found to complex with the alkyl groups on the poly(acry1ates) with similar effects being observed in the case of polystyrene latexes235. Mixed aggregates have also been observed in pyrene end labelled hydroxypropyl cellulose236 while using pyrene as an actual probe it was able to determine the surface morphology of carboxy containing dentrimers in surfact

ant^^^^.

Photochromism and isomeric conformational changes have been actively investigated. Polyacrylates containing trans-azobenzene mesogenic units have been shown to give rise to nematic mesophases and smetic modifications238 while vinyl and divinyl monomers containing azobenzene units as pendent groups undergo comparable rates of isomerisation whereas azo crosslinks isomerize much faster239. Photochemically induced phase transitions have been found to take place in polymers with less ordered nematic states such as homopolymers of 4-[(acryloyloxy)alkoxy]-4’-cyanobiphenyl doped with azobenzene or copolymerized with an acrylated a ~ o b e n z e n e ~Light ~ ~ . scattering methods have been used to monitor

111: Polymer Photochemistry

427

the photoinduced cis-trans isomerizations of azobenzene in liquid crystalline polymers for holographic applications241. Two relaxation processes are observed here both of which occur in the glassy state. The first is a fast process distributed over a wide range of relaxation times and is ascribed mainly to reorientation relaxation of the azobenzene while the second is a single exponential slower process ascribed to relaxation to the trans conformation and occurs within a time scale of 15,000 s . Different azo dyes produce different isomerization times. In this regard polyethylene has been observed to undergo photoinduced contractions when doped with azo dyes242. Azoaromatic polyaspartates containing octadecyl side chains have been observed to undergo reversible right to left handed helical changes with increasing azobenzene content with random coils being formed in trifluoroacetic acid243. Reversible cis-trans isomerization of azobenzene side chain units have been observed on the poly(viny1 alcohol) chain in thin films and at an air-water interface244. Here high cis conversions are observed compared with those in solution with the spacing of the units being a controlling factor. Optically active copolymers have been produced with racemic benzoin methyl ether moieties which undergo dissymmetric arrangements through short blocks of the ether245. Aggregates have been found to control helix-coil ~~~ transitions in poly[ 5-( 4-phenylazobenzyl)- L - g l ~ t a m a t ewhile photoreversible telomers have been produced from the polymerisation of 4-methoxy-4'-(6-methacry1oy1oxyhexy1oxy)azobenzene with a t h i ~ a c i d ~The ~ ~ permeability . of spirobenzopyran polymers has been found to be dependent upon the isomeric state of the c h r o r n ~ p h o r e ~ ~ ~ and polydiacetylenes with alkylurethane substituents have been found to exhibit photochromism with the length of the alkyl group playing an important role249. By utilizing the selective induction of the intramolecular heterophotodimerization of a photochromic

428

Photochemistry

molecule dispersed in a polymer matrix it has been found possible to generate and control the spatial distribution of the refractive index of glassy polymer films250. In another paper however, the alkyl chain length on spiropyran molecules embedded in polystyrene have been found to influence the annealing time of the polymer251 whereas lipophobic interactions between naphthalene terminated polyethylene glycol oligomers causes the chains to coil and form aggregates at low temperatures252. This was confirmed by the observation of strong excimer fluorescence and phosphorescence emissions at lower temperatures. Diastereomeric oligostyrenes have been found to exhibit excimer fluorescence in solid poly(methy1 methacrylate)253. Using fluorescence polarisation measurements at low temperatures rotation of the phenyl groups were not totally restricted in the solid state and the efficiency of energy migration was so high in these oligomers that fluorescence depolarisation could easily be induced. Using exact finite-size diagonalizations of extended Peierls-Hubbard Hamiltonians for trans and cis isomers of polyacetylenes consistent values for both onsite and nearest-neighbour correlation parameters for several electron-phonon couplings were determined254. Other papers of interest include nematic polyitaconates having 4-methoxyazobenzene units255, the photochromism of adamantylidene f ~ l g i d eand ~~~ modelling conformations in poly( acenaphthalenes)257.

Labelling and doping of polymers continues to attract interest especially with regard to metal ions and pH effects. Poly(sodium styrenesulphonate-CO-vinylpheny1)anthracene copolymers have been

prepared which appear to give compact conformations and are therefore able to solubilize large hydrophobic structures258. Polymer bound ortho-hydroxyphenylbenzotriazoles have been found to exhibit a temperature dependent red fluorescence emission from a

111:Polymer Photochemistry

429

proton transferred tautomer and a temperature insensitive blue emission associated with molecules in which this intra-molecular hydrogen bond is disrupted259. Water soluble polymers tagged with eosin and mercocyanine exhibit a pH dependent emission distinctly opposite to that normally observed in solution260. Electrostatic attraction and energy transfer between the two dye chromophores even at low concentrations is believed to account for this effect. The conformation of polymer chains has been examined on the reduction rate of poly[acrylatopentaaminecobalt (111)] by polymer bound ferrous chelates and excited states of tris( bipyridine)ruthenium (11)261. Inert and reactive cobalt species were observed with the former being shielded by the polymer chains whereas virtually no reaction was observed with the ruthenium complex. The chain configurations of polymer blends have been determined using fluorescence depolarisation262. Using anthracene labelled acrylates Flory ratios were obtained that were in good agreement with the literature and fluorescent dyes of xanthenol have been successfully reacted onto p ~ l y a c r y l a m i d e ~ ~ ~ . Poly(methy1 methacrylate) labelled with phenanthrene has been sterically stabilized with butyl rubber labelled with pyrene while both systems had also been similarly labelled with a fluorescent dye264. Quenching experiments on these systems consistent with a fractional quenching model demonstrating the diffuse nature of the rubber component when the particles are dispersed in aliphatic hydrocarbon media. Time resolved fluorescence spectroscopy has been used to investigate microdomains in solutions of p o l y ~ o a p s ~With ~~. pyrene as a fluorescent probe and the dodecylpyridinium ion as the quencher the microdomains were found to be highly dispersed and independent of the degree of polymerisation of the polymer although it is suggested that several microdomains may be produced from one high molecular weight polymer unit. Below the lower critical

430

Photochemistry

solution temperature pyrene labelled poly(N-isopropylacrylamidcl)is not quenched by nitromethane while at higher temperatures quenching is effective266 whereas the fluorescence of 8-anilinonaphthalene-lsulphonate is strongly enhanced by the addition o f vinyl polymers containing oligo(oxyethy1ene)cyclotriphosphazene derivatives due to binding reactions267. Similar results were obtained for fluorescent probes bound onto poly( N-vinylpyrrolidone)268 and pyrene bound into micellar clusters of a dihexadecyl ester of isophorone diisocyanate-poly( ethylene oxide) copolymer 269. In block copolymers o f styrene with ethylene oxide tagged with pyrene no excimer emission was observed due to large separation distances between the c h r o m o p h ~ r e s ~A~ ~simple . method has been found for labelling polystyrene with b e n ~ i l ~while ~ l the binding of platinum complexes with poly(acry1ic acid) has been found to be dependent upon the ionic strength of the solution272. A fluorescent probe showed the presence of a polymer metal complex. The fluorescence intensities of Eu3+ and Tb3+ ions are also markedly enhanced when bound onto a poly(acry1amide-acrylic acid)copolymer each of which is claimed to exist in two co-ordinated forms273. The complexation of poly(acry1ic acid) with poly(vinylpyrro1idone) is enhanced in the presence of copper ions274 and the fluorescence of europium complexes of polymethyl methacrylate with crown ethers is enhanced in the presence of zinc ions275. The emission quantum yield of a polyimide has been found to be

and dimeric model compounds

such as guaiacyl-R-D-glucopyranoside have been found to be better models for the phosphorescent chromophores in lignin than monomeric types277. Fluorescence quenching has been used to monitor the stabilisation of polystyrene particles by hydroxypropylcellulose through labelling the latter with ~ y r e n e and ~ ~a ~precise solution has been obtained for the relationship between the polarised luminescence and the nuclear Overhauser effect in polymers279.

111: Polymer Photochemistry

431

Energy migration and molecular confirmation are also subjects of interest. Long-lived charged excitons have been observed in poly(p-phenylene vinylene) due to intermolecular charge transport280 and a model has been developed for photoinduced structural changes in trans-polyacetylene281. The time dependence of a donor excited state in the presence of down chain electronic energy transfer between polymer segments with simultaneous trapping by a Foester dipole-dipole mechanism has been calculated282. Here the survival of the donor excited state was analysed in terms of an effective dimensionality with the objective of testing conformational models of polymers. In a similar regard energy transfer from excited donors to acceptors on chain-like polymers has also been modelled through the use of quasi-linear fractals283. Approximate expressions for the ensemble-averaged decay forms of the donor excitation were determined using the end-to-end distribution functions of random or self-avoiding walks. It was found that for the multipolar interactions the fluorescence decay obeys the Kohlrausch-Williams-Watts stretched exponential laws whereas for exchange type interactions the decay law follows exponential logarithmic patterns. The phosphorescence decay of 4-

methoxycarbonylbenzophenone in solutions of copolymers of methyl methacrylate and l-naphthylmethyl methacrylate has been measured and found to fit a model where diffusion of the photoexcited state of the berizophenone unit in the polymer coil is followed by a competition between reaction with the naphthalene groups and diffusion back into the solvent284. Energy transfer between chromophores on the ends of an isoprene-styrene block copolymer has been found to be sensitive to the miscibility with p ~ l y i s o p r e n e286. ~ ~ ~ The molecular weights of the blocks constituting the block copolymer and the molecular weight of the

432

Photochemistry

homopolymer were found to play a major role in controlling the phase behaviour of the system. Above the effective critical micelle concentration the block copolymer partitioned itself between the micelles and the homogeneous matrix phase. Furthermore the amount of free block copolymer increased as the total concentration of block

copolymer increased for concentrations at which the micelles

exist. The dynamics of the relaxation behaviour of poly(methacry1ic acid) and its complex with poly(ethy1ene oxide) have been found to be markedly influenced by pH using fluorescence a n i ~ o t r o p y ~The ~~. complex was found to be a very rigid species with a long-lived relaxation time constant. The fluorescence, phosphorescence and delayed fluorescence spectra of poly(9-vinylphenanthrene) have all been found to be very broad compared with those from an alternating copolymer with methacrylic acid indicating the presence of trap sites in both the singlet and triplet states288. Triplet exciton capture of the phosphorescence from carbazole containing polymers by molecular oxygen has been studied289 while aminobenzylidenemalonitrile has been used as a viscosity sensitive

fluorescent probe for monitoring the physical ageing of polystyrene290. In the latter case the fluorescence was found to increase with ageing time and was also dependent upon the microviscosity of the environment and glass transition temperature of the polymer. Structural changes have found to occur in the chromophore of poly(7-hydroxycoumarin-3-carboxylic acid) which induces higher intensity fluorescence than that observed from t h e monomer291. It was interesting to note that when the monomer was compressed the fluorescence increased while that of the polymer decreased due to changes in the internal structure which may be intra-molecular quenching. The polaron and bipolaron absorption energies of polythiophene have been found to be linear functions of the inverse of its chain length292. At infinite chain lengths

111: Polymer Photochemistry

433

polaron and bipolaron absorptions had similar energies implying that the latter was an important process in the solid state for inter-chain charge hopping.

Excimer formation and its potential applications for polymers continues to be widely studied. Poly(N-vinylcarbazole) continues to be investigated with the monomer exhibiting yet again a new emission below

400

nm assigned to a second excimer state similar to

that seen in the horn~polymer~~~. Poly( 3 6-di-tert-butyl-9I

vinylcarbazole) has been found to give unique sandwich excimer formation due to syndiotactic

while the effect of

hydrostatic pressure on intramolecular excimer formation for meso2,4-di-N-carbazolylpentane

dissolved in poly(propy1ene oxide) has

indicated that the compressibility of free volume cannot be considered to be c o n ~ t a n t ~ ~A ~new t ~scheme ~ ~ . has been proposed to account for diffusion controlled excimer formation between polymer chain ends297 which unlike the Birks scheme made no use of rate expressions for excimer formation and dissociation. In this case excimer formation was described as a natural consequence of chainend diffusion and produced curves which fitted those obtained experimentally. Diffusion coefficients for polymer chain ends could also be obtained by this method. Excimer formation has been used to investigate the concentration dependence of micelle formation in a hydrogenated isoprene-styrene block copolymer298. The solvent content in the micelle core markedly influenced excimer formation and the amount of free chains in the disperse phase could also be estimated from the lattice model by a lever-rule approach. Another theoretical model has also been developed to account for energy migration in aromatic polymers299. Here it was assumed that the photophysics of polymers could be modelled as a one dimensional system with excimers that could dissociate. From the numerical

434

Photochemistry

calculations it was deduced that the resonance mechanism of energy migration in polystyrene is very likely to be responsible for its photophysics. The excimer fluorescence from polyesters of 2,6naphthalene dicarboxylic acid has been found not only to depend upon the chain length of the glycol unit300 but also the odd-even effect of the methylene units. In homopolymers of 2-(10-alkyl-9anthry1)ethyl methacrylates excimer fluorescence was found to depend on the bulky nature of the alkyl groups301. Bulky groups reduced the number of traps and a kinetic model was also proposed for quenching by methyl nitrate. Intramolecular excimer formation in 1-naphthylmethyl methacrylate and its copolymers has been found to show a decay dependence on the copolymer composition302 while excimer formation of pyrene tagged hydrophobically modified poly(Nisopropylacrylamides) depends on the degree of solubilisation303. Further evidence has appeared for excimer emission from styrene which is enhanced when the ratio of this component is increased in acrylic acid-styrene copolymers304. Here the presence of metal ions had little effect on the excimer emission and energy migration was found to occur from isolated to non-isolated units. Segmental diffusion has been found to contribute to long-range excimer formation in poly( acenaphthylene) but not in poly( indene)305 while the extent of quenching of the

excimer fluorescence from

poly(vinylsu1phobetaines) is unaffected by the nature of the

presence of metal ions306. Another expression has been developed to account for the suppression of the effect of stray light on the intensity of fluorescence from weak polymeric emitters307. This relationship has been found useful for polymeric systems where the monomer and excimer fluorescence emissions overlap strongly. Excimer formation in flexible chain polymers such as poly(ethy1ene terephthalate) is controlled by the shrinkage of the polymer coils which increases with increasing polymer concentration308. Stacking

111: Polymer Photochemistry

435

of the terephthaloyl units is primarily responsible for the excimer emission. This is a rather questionable observation since poly(ethy1ene terephthalate) has only ever been reported to give ground-state dimer emission. The excimer fluorescence from poly(vinylnaphtha1ene) has been used to study the morphology of its blends with poly( cyclohexyl methacrylate)309. Here excimer forming sites are reduced on annealing the blends and the ratio of the excimer to monomer fluorescence gives an indication of blend miscibility. The fluorescence from methacrylic acid-2-(1naphthylacety1)ethyl acrylate copolymer has been found to increase markedly on increasing the pH which is associated with the expansion of the polymer coils3l0. Here copper ions were found to quench the emission due to ionic binding. Phosphazine polymers on the other hand have been found to exhibit three types of excimer forming sites3”.

In this case there is intramolecular excimer

formation between two aryloxy groups on the same phosphorus atom, adjacent phosphorus atoms and through intermolecular interactions. The latter interactions were responsible for both diffusive and deactivation processes. Excimer formation has also been found to be more effective in the liquid crystalline state and has been illustrated for polyesters of p-phenylenediacrylic acid with glycols312. Poly( allylamine) also gives excimer fluorescence when modified with 1,8-naphthalic anhydride provided the latter is u n s ~ b s t i t u t e d ~ The ~ ~ . ionic strength and pH of the solution had a dramatic effect on coil expansion and hence the excimer formation efficiency. For polyesters containing naphthalene units excimer formation has been found to exhibit an odd-even effect with the chain length of the glycol unit314 and excimer fluorescence has been reported from poly( pyridine-2,5-diyl) 315.

Photochemistry

436 4.

PHOTODEGRADATION AND PHOT0OXY;ZBITION OF PO J , Y M R E

This continues to be an active area of industrial and academic research with much emphasis on the mechanistic nature of the photochemical processes involved. Such processes are now more readily understood as polymeric systems become more amenable to analysis by modern methodologies. A number of reviews of topical interest have appeared with

degradable plastics making a come back. These include the role of plastics in the e n ~ i r o n m e n t ~ land ~ - agricultural ~~~ filrn3l9, ethylene carbon monoxide copolymers320 and their analysis321. Several other topic reviews of interest relate to the role of titanium dioxide

laser

and

p h o t ~ l y s i s ~degradation ~~, and stabilisation of polyolefins326, conformation defects in poly(viny1 chloride)327 and poly( ethylene terephthalate) p h o t o ~ x i d a t i o n ~ ~ ~ .

4.1 Polyolefins

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation329. Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer330. Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air331. These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments

III: Polymer Photochemistry

437

compacting into crystallites. Most of this information however, is not novel and the effects reported are well characterised. The outdoor weathering of polypropylene has been investigated and found to be controlled in the main by the light energy with humidity and temperature playing only a secondary role332. In the photooxidation of immiscible blends of low density polyethylene with nylon 6,6 the latter has been found to contribute in the main to the observed instability333. Here photooxidation starts in the nylon 6,6 phase and mainly at the boundaries with the polyethylene. Radicals from both polymers are claimed to react to form copolymers which act as compatibilizers. It is suggested that this process could be used to produce a commercial blend of the two polymers334. Ethylenepropylene copolymers are closely related to the polyolefins and their stability has attracted interest. Ethylene-propylene-diene terpolymer was found to be more stable than an ethylene-propylene copolymer and the model system dimethylhexane was evaluated in terms of understanding the detailed mechanistic processes involved335. Several products were identified from the photooxidation of this model as indicated in scheme 6 with one of the primary products being 2,5-dihydroxy-2,5-dimethylhexane (DHDMH).

Both oxidative fragments and coupling products were

produced in the scheme with oxidation products superseding the chain scission products followed by the coupling products. For the model the alcohols were the major products due to differences in radical diffusion rates from those in the solid polymer. Anthraquinone has been found to markedly sensitize the photooxidation of low density polyethylene with particular interest regarding the surface adhesion properties of the polymer336. Here adhesion strength increased with increasing oxidation rate. Ferric chloride has also been found, yet again, to markedly sensitize the rate of photooxidation of polypropylene achieving a maximum rate at

Photochemistry

438

0.5% w/w c ~ n c e n t r a t i o n ~Several ~~. mechanisms were proposed for the accelerating effect of the iron including the decomposition of hydroperoxides. The weathering of low density polyethylene in seawater has been found to be slower than that in air due to the cooling effect of the former environment338. Aromatic and aliphatic carbonyl groups are considered to be the major photoinitiators in ethylene-propylene-diene

t e r p 0 1 y m e r ~and ~ ~ are constantly being

regenerated by the presence of the norbornene unit. The same workers then proceed to consider that unsaturation is important in controlling the photooxidation kinetics of the polymer340. The benzophenone sensitized photodegradation of polypropylene continues to be investigated341. Using ESR spectroscopy the alkyl and methyl radicals were identified at low temperatures while at room temperature only the polyenyl radical was identified but at a much lower level than in polyethylene.

4.2 Polv( vinvl halides) Activation energies for the photooxidation rates of poly(viny1 chloride) have been determined in both air and nitrogen and was lower in the former case due to initiation by carbonyl groups342. Further work indicated that the lengths of polyenes do not change during photooxidation and that while ketonic groups increase in concentration the hydroperoxide groups decrease343. On the other hand other workers have concluded that in the absence of oxygen polyene growth is a major reaction344 while at short wavelength irradiation polyene growth and yellowing are dominant,bleaching occurs at longer wavelengths345. In the natural weathering of poly(viny1 chloride) photodegradation has been found, yet again, to occur on the near surface layers of the polymer346. In blends of polystyrene with poly(viny1 chloride) the formation of carbonyl groups and other chromophores stabilised the polymer to subsequent

III: Polymer Photochemistry

439

thermal d e h y d r o c h l ~ r i n a t i o n ~ ~Dielectric ’~. properties of polymers are also important and in this regard tan 6 increased with increasing irradiation time for a fluorinated polymer due to the formation of carbonyl and hydroxyl groups348.

4.3

Polv(acrv1a tesl and [alkvl acrvlatesl

Much of the research work on these polymer types has concentrated on laser ablation with little or no interest in mechanistic aspects of their degradation. One study on resistivity however, showed that

this property decreases for poly(methy1 methacrylate) with increasing irradiation time and is reversible349. Doping with dithizone reduced the effect further. Under monochromatic illumination it has been found that poly(methy1 methacrylate) exhibits photodegradation but not at wavelengths greater than 320 nm350. Maximum chain scission was observed at

280

nm and under low

light intensities while side chain scission was considered to be the major cause of the photoinduced main chain scission. Polymer anion radicals have been detected following the 7-irradiation of poly( alkyl methacrylates)351. On subsequent irradiation with ultraviolet light these species undergo electron transfer reactions. Films of poly(tert-butyl methacrylate) undergo more rapid photodegradation in nitrogen dioxide and sulphur dioxide atmospheres352. Laser ablation of poly( methyl methacrylate) has been found to depend upon the manufacturing history of the polymer353 as well as the nature of incubation pulses which do not initially ablate the polymer surface354. According to some workers the surface of polymer materials take the form of a rod-like structure with a molten pool of material underneath355. Using an interface model the kinetics of laser ablation have been evaluated taking into account a screening coeffi ~ i e n t and ~ ~ periodic ~ surface structures have been observed at low energies in areas greatly

Photochemistry

440

exceeding the coherence area of the laser itself357. On the other hand the etching of poly(methy1 methacrylate) by a pulsed uv laser can be increased more than three fold by overlapping both spatially and temporarily laser pulses of 308 nm along with 193 nm pulses358. The use of such combinations of laser light with increasing fluence removes the build-up of solid products359. In the actual ablation process gases are produced which are ejected from the polymer surface and carry the solid particles with them at high velocities360. In the laser ablation of poly(methy1 methacrylate)methacrylic acid copolymers mechanical stresses played an important role in the process361 while the incorporation of 1,3diphenyltriazine in poly(methy1 methacrylate) enhances the etch rate by up to a factor of ten362. Using a model di-methyl-2,2,4,4tetramethylglutarate for poly(methy1 methacrylate) an etching pattern similar to that for the polymer was obtained363. Side chain scission was the major process involved and the formation of double bonds with a quantum yield approaching 0.6 at 248 nm excitation. The photochemical Fries rearrangement for 2-naphthyl acetate has been studied in poly(methy1 methacrylate)364. Products were produced which acted as quenchers and prevented further product formation.

4.4 Polystyrenes

Much of the work on these polymer systems is with copolymers although the uv induced crosslinking of polystyrene has been studied by light scattering methods365. Here the rate of crosslinking was found to be influenced by both the diffusion properties of the polymer chains and the presence of isolated double bonds. The photodegradation of poly(o-propionylstyrene) has been found to undergo a typical Norrish type I fragmentation reaction as shown in scheme 7 to produce, phenyl and benzoyloxy

441

Ill: Polymer Photochemistry

#

#

@0c2H5

Scheme 7

Phorochemistry

442

radicals3G6. These reactions are however , limited by competition from photoenolization reactions via the carbonyl triplet shown in scheme

8.

Transient spectra indicated the presence of the syn and

anti-enols as shown. Poly(o-acetylstyrene) has been found to undergo similar reactions367. A 2: 1 poly( styrene-co-maleic anhydride) copolymer has been found to degrade more rapidly than a 1:l copolymer due to the instability of the styrene units368. The

anhydride absorption decreased rapidly during irradiation due to the formation of peroxy radicals shown in scheme 9 which will undergo an intramolecular hydrogen atom abstraction process. Polystyrene has been found to undergo a photohydration reaction in the presence of poly(sodium styrenesulphone-co-2vinylnaphthalene)369 while acids catalyse the photoFries rearrangements of poly( p-formyloxystyrene)370. A copolymer of 2- ( 4benzoyloxyphenyl)-2-phenylpropane with styrene has been found to

undergo several types of

photo reaction^^^^

and the

photodehydrochlorination of 2,2’,3,3’,6,6’-hexachlorobiphenyl

is

accentuated by the naphthalene antenna groups in poly(sodium s t y r e n e s u l p h o n a t e - c o - v i n y l n a p h t h a l e n e ) 372. The thermal and

photoageing of an impact modified polystyrene has also been investigated372 and the mass spectral analysis of photooxidised polystyrene has shown the presence of various hydroperoxide containing species373. Photolysis of copolymers of 9-

fluorenilideneimino-p-styrenesulphonatewith vinylpyridine has been shown to result in the formation of sulphonic acid and fluorenone azine species374.

4.5 Polvamides and Polvimides An analyt.ica1 and kinetic study of the photooxidation of Nsubstituted polyundecanamide has shown the formation of hydroperoxide and imidic groups as the major intermediates (scheme

443

111: Polymer Photochemistry

I,

h v ISC

ROT

ROT.

I,

eoH Anti

Scheme 8

Photochemistry

444

Scheme 9

-qH-N-CO-CH-NH-COI 02,polymer 0 2 , polymer

1

I

1

0 2 ,polymer

i

-CH(O0H)NH-COC6H13 I

-CO-N-CO-CO -NH-CO

-

Scheme 10

I

111: Polymer Photochemistry

445

10) with long wavelength light (>300 nm)375. The greater the degree

of substitution the nitrogen with n-heptyl groups the greater is the rate of hydroperoxidation. The tensile strength of nylon 6 has been correlated to its degree of chain scission on photo~xidation~~~. Photoablation studies have also concentrated on polyimides with the observation of yellow emission377 and enhanced ablation using a pulsed TEA carbon dioxide laser378. Ionic products have also been measured together with particle velocities, with the latter being ejected at high speeds due to Coulomb explosions on the surface of the polymer material and gas f o r m a t i ~ n ~ ~The ~'~~~. wavelength dependence of the photodegradation of nylon 6 has been investigated through ESR spectroscopy381.

4.6

Polyesters

All the studies on these polymers have involved some aspect of laser ablation. The minimum fluence for a

248

nm laser pulse to

etch the surface of poly(ethy1ene terephthalate) (polyester) film has been determined382 while in other work on aluminized polyester this has been found to be dependent upon the film thickness383. Fresnel patterns were obtained in biaxially stretched polyester films on laser ablation384 while other workers have found that the shock velocities approach the blast wave theory at fluences >1 J/cm2 385. The surface properties of carbon fibre filled polyester

has been studied following ablation by scanning electron microscopy386.

4.7 Polvuretham The photooxidation of diene containing urethanes has been found to proceed rapidly through a free radical chain mechanism involving high rates of bimolecular termination387. Visible light was found to contribute significantly to the degradation rate. The oligoester

446

Photochemistry

structure has been found to markedly influence the photooxidation rate of polyurethanes with polyesters being more stable than

pol yet her^^^^.

The weathering of polyurethane coatings has been

monitored using ESR spectroscopy389 and the morphology of stressed laser irradiated polyurethanes has been modelled390.

4 . 8 -

The photooxidation of butadiene copolymers is reduced by the introduction of petroleum resin391 while the photooxidation of 1 ,2polybutadiene involves radical addition to the 1 ,2 double bond392. The tertiary macroradicals formed are then oxidised to tertiary associated hydroperoxides which themselves will photolyze to saturated alcohols and ketones as illustrated in scheme 11. Strong competition between crosslinking and chain scission occurred with the latter being primarily associated with Norrish type I photolysis of the ketonic species. The long wavelength photolysis of polyisoprenes involves abstraction of hydrogen atoms in the allylic position of the 1,4 cis unit393. Again tertiary hydroperoxides are formed as the key species which then breakdown to form alcohols and ketones through the alkoxy radicals as shown in scheme 12. For 1,2 and 3,4 units chain scission is the main oxidative pathway.

4.9 Natural Polvmers and Cellulose Esters The photodegradation of painted wood panels has

been found to fail

at the wood paint interface394 while acid treatment of methylcellulose accentuates its rate of p h o t ~ d e g r a d a t i o n396. ~~~ Silk fibres have been found to form a roll-like structure after laser ablation with 193 nm

whereas the weatherability of

lacquer coated leather has been investigated in a number of

we at hero meter^^^^.

Poly(acry1i.c acid) up to 25% w/w has been found

I l l : Polymer Photochemistry

447

0-0-H

I

-CH2-C-

I I

CH2 CH2R

I

H20 +-CH2-C-

I I CH2 I

CH2R

Scheme 11

y

3

(4

-CH=CH-C-CH2I

9”/ y

3

-CH=CH-C-CHz-

OH CH3-C-CH2-

(b)

+*CH=CH-

Ii

0 -CH=CH-C-CH2-+ II

CH;

(C)

0

-CH=CH-C-

II

0 Scheme 12

CH3 +*CH2 -

(d)

Photochemistry

448

to increase the light stability of cellulose d i a ~ e t a t e ~At ~~. higher concentrations the blends were immiscible. Nitrocellulose has been found to be very sensitive to photooxidation giving rise to de-nitration and high levels of

hydro peroxide^^^'.

The

decomposition of the latter is accentuated by the presence of iron and oxygen was essential for crosslinking to occur.

4 .lOMiscellaneous Polvmers The triplet absorption spectrum of poly(viny1acetophenone) has been characterised which then decays through a Norrish type I scission process401. In hydrogen atom donating solvents the degree of chain scission is reduced and in poor solvents intramolecular photoreduction is predominant due to enhanced chain coiling. In the presence of bromine long wavelength irradiation where only the bromine absorbs light induces chain scission due to hydrogen atom abstraction processes402. Poly (organophosphazines) undergo rapid chain scission with the addition of benzophenone-phosphazines having no effect403 while diaryliodonium salts induce the depolymerisation of poly( chloroacetaldehyde)404. Dimer radical cations have been observed on laser flash photolysis of poly(3,6di-tert-butyl-9-vinylcarbazole) due to steric effects impairing

excimer formation405 while copolymers bearing acyloxyimino groups show a decrease in light stability with increasing unsaturation406. An increase in the double bond content reduces the glass transition of the polymer and allows greater access to oxygen diffusion. Transition metal substituted oligo and polymeric silanes have been found to be photochemically resistant depending on their metal while disilane polyamides have been found to be ph~todegradable~~~. Polymers containing sulphur atoms along the chain have been found to photodegrade rapidly with disulphide bonds being the more labile to p h o t o l y s i ~ ~Reversible ~~. photodegradation

III: Polymer Photochemistry

449

kinetics have been observed for blends of poly(1-vinylnaphthalene) and poly( methyl methacrylate)410 whereas alkyl viologen compounds undergo second order photoreduction kinetics in poly(viny1 alcohol)411. Random chain scission has been found to occur on irradiation of poly( alkylsilanes)412 and fluoroxy derivatives are produced on photodegradation of perfluorinated

pol yet her^^^^. Other

studies of interest include the photodegradation of impact modified polystyrene414, hole burning in crosslinked polymers415 and epoxy resins416 and the testing of the weatherability of coatings417.

5. PHOTOSTABILISATION OF POLYMERS The photostabilisation of polymers still maintains a high level of activity with the mechanistic action and performance of hindered piperidine compounds continuing to attract interest. Some interesting reviews have appeared with one very extensive and highly recommended paper on the mode of action of hindered piperidine light stabilisers418 while another has dealt with their manufacturing properties419. The kinetic reactions and photophysics of light stabilisers have been covered in another review420 as has the stabilisation of polya~etals~~l.

A hindered amine light stabiliser has been found to enhance the light stability of blends of low and linear low density polyethylene with the latter contributing linearly to the overall stability of the blend422. In coatings hindered piperidine light stabilisers are also effective especially when used in conjunction with a benzotriazole absorber423 while surface protection of styrene copolymers with 2-(2-hydroxy-5-vinylphenyl)benzotriazole requires a small amount of a hindered piperidine s t a b i l i ~ e r ~ ~ ~ . Polymeric hindered piperidine compounds on the other hand have been found to inhibit the singlet oxygen attack on p~ly(butadiene)~~~.

Photochemistry

450

Nitroxy radicals are the primary photoproducts of the oxidation of hindered piperidine compounds and these have been shown to be highly effective thermal stabilisers in polyolefins at high temperatures426. Acrylated hindered piperidine compounds have been found to be highly effective in polypropylene with their efficiency being directly related to their solubility in the polymer427. In the photostabilisation of rubber systems by hindered piperidine compounds peroxy radicals have found to react with the hindered nitrogen atom in the piperidine structure428. One novel aspect to this work was the finding that the hindered piperidine stabilisers appear to catalyse bimolecular termination of free radicals in the polymer before the formation of ketones. This could well account for the autoretarding effective of many types of hindered piperidine stabilisers. Coupled with this work is the finding that hindered piperidine compounds will form charge-transfer complexes with oxygen and thus quench the formation of such complexes with the polymer429. The termination of peroxy radicals close to the light stabiliser is therefore proposed as a likely mechanism in stabilisation.

Some interest continues in absorber systems. Silane and styrene monomers have been copolymerised with 2-vinylphenyl benzotriazole stabilisers in order to graft the stabilisers into the polymer chain430. When doped into plastics materials they were found to exhibit high surface activity. However, there is a conflicting report from other workers on similar structures where it is claimed that such polymeric stabilisers do not photoprotect the surface of polystyrene431. Poly( 2,6-dimethyl-l,4-phenylene oxide) has been effectively stabilised with an ortho-hydroxyphenyl benzotriazole ~ t a b i l i s e rwhile ~ ~ ~ in another study these compounds are claimed to be lost rapidly from polycarbonate~~~~. Other types

111: Polymer Photochemistry

451

of absorbers such as the naphthoylenebenzamidazoles have been found to function primarily as excited state q ~ e n c h e r swhile ~ ~ ~ a new absorber based on a-hexadecyldibenzoylmethane has been synthesized and found not to function effectively on the surface of polymers435. The photostability of silk has been found to be improved by treatment with tin chloride436 and poly( vinyl chloride) may be stabilised through the use of tin carboxylates with a hindered piperidine compound437. PhotoFries rearrangements of chlorophenyl esters of salicylic acid have been studied438 and analysis of in-chain copolymerised aromatic benzotriazole stabilisers has shown that they are statistically distributed along the polymer chain439. Metal acetylacetonates have been found to photoprotect polyurethanes with an efficiency which is determined by the nature of the central metal atom which in turn controls the hydrazide fragment structure on phot~lysis~~*. Certain types of metal chelate stabilisers have been found to function as effective light stabilisers through a radical scavenging process and also as effective flame retardants441. Poly( alkyl silanes) have been phot~stabilised~~~ as has poly (ethylene terephthalate) after ~. treatment with 1 , 3 , 5 - t r i p h e n y l p y r a ~ o l i n e ~ ~Conjugated

di(ketovinylpheno1s) have been found to act as both thermal and photostabilisers in polyethylene444. N-substituted maleimides have been found to markedly photostabilise poly( vinyl chloride)445. These compounds have been found to be more effective than the tin alkylmaleates. Here stabilkation efficiency has been found to depend upon the nature of the substituents on the aryl rings of the molecule and highly effective synergism is observed with phenyl salicylate. Initially, the chlorine radicals from the photooxidation of the polymer are believed to react with the stabiliser according to scheme 13. The stabiliser radicals will then react, with the polymer chains and graft in as shown. The

452

PVC CI* + HC=CH

CI-CH-CH-PVC I

O\/,'

I

+ CI*

hv

-

Photochemistry PVC*+CI* CI-CH-CH*

CI-CH-CH-PVC I

I

O/,cxNo YQO

N OcQO

Scheme 13

CI-

X

X

X IHCI

PVC

NH2.HCI

1

NH

X = H, CI, or PVC

Scheme 14

III: Polymer Photochemistry

453

maleimide bond will then break to produce aryl nitrogen radicals which themselves will stabilise the polymer further by reaction with the polymer radicals and/or polymer radicals as shown in scheme 14. Finally, the interactions between hindered piperidine compounds and hindered phenolic antioxidants use in thermal processing has been investigated using derivative uv spectroscopy446. Both synergistic and antagonistic effects were observed and were associated mainly with chemical interactions between the two types of compounds.

5. PHOTOCHRMISTRY OF DYED AND PIGMENTED POJiYMERS A number of review articles of interest have appeared. These include the synthesis and properties of laser dyes447, photocatalytic oxidation of polymers by pigments448, factors determining dye stability449 and the effect of additives450.

Some novel heptamethine pyrylium dyes have been synthesized and their structure and stability has been established451 while lipophilic indigo dyes give cis and trans isomers that undergo photoreduction reactions involving dealkylation of the amino substituents and oxidation452. Some lightfastness properties of new sulphonated naphthol azo dyes have been reported453 while the kinetics of the light fading of azo dyestuffs have been evaluated from their radical generating abilities in 2-propan01~~~. In the photofading of binary mixtures of vinyl sulphonyl reactive dyes on cellulose a positive concentration effect on the rate was observed when one of the dye partners was easily ~ x i d i s e d ~Here ~ ~ .a preferential surface fading model for the dyes was developed for wet cellulose. Both oxygen and moisture have been found necessary for the formation of Michler’s ketone on irradiation456* The nature of the polymer and the dye concentration influenced the quantum

454

Photochemistry

yield of dye fading. The sodium salt of D,L-mandelate has been found to act as a suitable model for cellulose to study the mechanism of fading of reactive vinyl sulphonate dyes457. In the presence of oxygen the dye faded through a singlet oxygen mechanism. Three common azo dyes have been found to undergo a photoreduction mechanism to give hydrazides as products458 while novel lO-piperidino/morpholino-9-(fl-hetarylethylene)anthracenes are highly fluorescent dyes with good light stability459. The crystallinity and glass transition temperature of polycaprolactam have been found to influence both the isomerisation and fading of azo dyes460 whereas the uv laser ablation of the surface of polyester materials has been found to enhance dye uptake461. In polyamide environments the photofading of acid dyes has been found to give mainly the leuco form of the dyes462 and the composition of an acrylic polymer has been found to influence the fading of copper phthal~cyanine~~~. Transition metal ions have been found to enhance the photofading of acid dyes in solution due to an electron transfer mechanism464 while a 2-hydroxybenzophenone absorber has been found to impair the photofading of dyed jute465.

I l l : Pcdyrner Photochemistry

455

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D.Y. Kim, C.S. Chang and W.J. Cho,

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W. Baeumer, Rontakte. (1989), 3, 42. W. Zhu, Tuliao Gonave. (1989), 3, 39.

22.

Y. Shirota in ''Photoinduced Electron Transfer", Elsevier Science Publishers, Amsterdam, (Editors M.A. Fox and M. Chanon) (1988), 441. H. Tomioka, Y. Takimoto, M. Kawabata, M. Harada, Z.P. Fouassier and D. Ruhlmann, J. Photochem. and Phot o b i U mem. Ed.. (1990), 53, 359. V.V. Shibanov, F.A. Mikhailenko, V.V. Klyushin, 1.1. Marshalok and V.L. Mizyuk, Ykr. K U . Zh., (1990), 56, 531. A.F. Maslyuk, S.F. Kercha and I.M. Sopina, Ukr. Khim. Zh.,

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73.

420. V.G. Plotnikov and A.A. Efimov, USD. Khk , (1990), 59, 1362. 421. F.R. Stohler and K. Berger, Fnuew Makromol. Chemie, (1990), 176, 323. 422. F.P. La Mantia and F. Gratani, m v m . Dea. 61 StabL,(19901, 30, 257. 423. H. Boehnke and E. Hess, Farbe Lack, (1989), 95, 715. 424. F.A. Bottino, A. Pollicino, A. Recca, D. Pawson, R.D. Short and D.T. Clark, Polvm. Dea. ti Stabil., (1991), 32, 71. 425. J. Pan, Gaofenzi Xuebao,,(1989), 6, 655. 426. W. Liang and X. Zhou, m v o u Huaaonq , (1989), 18, 547. 427. W. Liang, J. Qi, X. Hu and H. Xu, Polvm. Dea . & Stabil,. I (1991), 32, 39. 428. G. Geuskens, M.N. Kanda and G. Nedelkos, Bull. SOC. Chim. Bela., (1990), 99, 1085. (1990), 176, 241. 429. F. Gugumus, Anaew Makromol. Che 430. Y. Tsukahara, Y. Tsuruuta, K. Kohono and Y. Yamashita, Kobunshi Ronbunshy, (1990), 47, 361. 431. F.A. Bottino, G. Di Pasquale, A. Pollicino, A. Recca and D.T. Clark, Macromolecules. (19901, 23, 2662. 432. R.C. Anad, K. Chander and I.K. Varma, J. Polvm. Mater. , (1989), 6, 73. 433. D.R. Olson and K.K. Webb, Macromolecules, 91990), 23, 3762. 434. F.F. Niyazi, Yu.V. Chaiko and I.Ya. Kalontarov, Vvsokomol, Soedin. Ser. A. , (1990), 32, 484. 435. Y. Shi and S. Wu, Gaofenzi XUebaQ , 91989), 6, 748. 436. K. Hirabayashi and K.L. Chen, NiDDon Sanshiuaku Zasshi. (1990), 59, 151. 437. G. Capocci, J. Vinyl Techno1 (1989), 11, 195. 438. M.D. Madhury and V.B. Desai, Man-Made Text. India, (1989), 32, 8. 439. H. Pasch, K.F. Shuhaibar and S. Attari, J. Aml. Polvm. Sci* I (1991), 42, 263. 440. A.P. Grekov, Yu.V. Savel'ev, V.Ya Veselov and O.M. Fedorenko, Vvsokomol. Soedin. Sers B., (1990), 32, 499. 441. C.F. Cullis, A.M.M. Gad and M.M. Hirschler, W . Polvm. J * I (1990), 26, 919. 442. D.W. Kang, G.S. Yeom, J.W. Whang, J.K. Yang and J.R. Han, (1989), 26, 503. Ban'uuk Somvu ,-K 443. N. Iliskovic, M. Ristic and I. Tabakovic, Plast. G w , (1990), 10, 57. 444. T.F. Titova, A.P. Krysin, V.P. Rusov, V.N. Ovsyannik, L.A.

a,

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Ken'ko,

I.F. Sitnikova and M.V. Balykina, plast. Massy,

(1990), 7, 23. 445. M.W. Sabaa, M.G. Mikhael, N.A. Mohamed and A.A. Yassin, polvm. Dea. & Stabil,, (1990), 29, 291. 446. A.J. Chirinos-Padron, polurn. Dea. & S-11. , (1990), 29, 49. 447. N.R. Ayyangar and K.V. Srinivasan, Colouraue , (1989), 36, 39. 448. G. Kaempf, Anuew Makcomol. Cheh,(1990), 176, 1. 449. A. Luchian and 0. Toader, Usoara: Text.. Tricotaie, Confectii Text. , (189), 40, 584. 450. S. Hashizume and Y. Yamamoto, S,(1990), 38, 179. 451. W. Luo, 2. Zhu, Z. Yao and M. He, Collect. Czech. Chem. Commun., (1990), 55, 2066. 452. T. Kitao and J. Setsune, Kenkvu Hokoku-Asa GarKoavo Giiitsu Sh0reLk.U , (1989), 55, 73 453. J. Sokolowska-Gajda and H.S. Freeman, Dves and Piaments, (1990): 14, 35. 454. K.J. Sirbiladze, A. Vig, V.M. Anyisimov, O.M. Anyisimova, G.E. Krichevskiy and I. Rusznak, Dves and Piaments, (1990),

.

m.

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468

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Dves and Piqments, (1990), 14, 265. 456. T. Toda and M. Hida, Senti Gakkaishi, (1990), 46, 155. 457. Y. Okada, T. Kato, H. Motomura and Z. Morita, 5 i, (1990), 46, 346. 458. L.M. Gurdzhiyan, O.L. Kaliya, V.V. Karpov, E.I. Kashkovskaya, A.B. Korolev, L.A. Osmolovskaya and V.O. Stepaneko, Zh. Obshch. Khim., (1989), 59, 2600. 459. D.W. Rangnekar and D.D. Rajadhyaksha, Indian J. Text, Res.,(1989), 14, 135. 460. E. Dubini-Paglia, P.L. Beltrame, B. Marcandalli and A. Seves, J. A p l . Polvm. Sc., (1990), 41, 765. 461. D. Knittel, A. Eickmeier, T. Bahners and E. Schollmeyer, TPI. Text. Prax. Int., (19901, 45, 236. 462. H.S. Freeman and J. Sokolowska-Gadja, Text. Res. J., (1990), 60, 221. 463. T. Tsukamoto, S. Taguchi, T. Nakahira, S . Iwabuchi, K. Kojima and T. Sugiura, Polvm. Commun., (1990): 31, 108. 464. K. Seguchi and S. Yuasa, Mukouawa Josh1 Daiuaku Kivo Kaseiuakubu-hen, (1988), 36, 213. 465. F.I. Farouqui and I. Hossain, Text. Dver Printer, (1990), 23, 15.

Part ZV PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION ByA. COX

Photochemical Aspects of Solar Energy Conversion BY A. COX

1.Introduction Topics which have formed the subjects of reviews this year include energy conversion at the molecular leve1,l photochemical storage systems based on reversible valence photoisomerisation,* solar energy from photochemistry,3 high-efficiency multijunction solar cells based on GaAs, AlGaAs, and InGaAs,4 and on GeSi,S photoconversion of inorganics to methane,6 photoelectrochemical and photocatalytic methods of hydrogen production by water splitting? and artificial photosynthesis.8

2. Homogeneous Photosvstems Proton reduction arising from photolysis of water in the presence of a bipyridine catalyst is controlled by diffusion of the reacting species and by interactions of the catalyst and intermediates at the electrode surface.9 The hydrophobicity of various dialkylviologens ( Me to hexyl) influences their ability to quench photoexcited [ Ru(bpy)3]2+.10 Viologens with shorter alkyl chains are most effective in poly(sodium styrenesulphonate), whereas those with a hexyl substituent are most effective in styrene latex solution. A number of viologen-linked water-soluble zinc porphyrins of the form ZnPC3(CnV) (n = 2-6) and having methylene chains of various lengths between the porphyrins have been synthesised.11 Under steady-state conditions, photoinduced hydrogen evolution has been observed in a system containing NADPH, any one of these viologen-linked porphyrins, and hydrogenase. This suggests that these porphyrins can participate as both a photosensitizer and as an electron carrier in the same molecule. Disodium tetraphenanthroporphyrazine has been used as sensitizer in the photoreduction of methylviologen in solutions containing EDTA and

472

Photochemistry

cysteine as electron donors, together with an aqueous surfactant.12 Hydrogen is released from such a system in the presence of colloidal platinum in poly(viny1 alcohol), but its quantum yield is low compared with that of MV.+ production. Dissolution of either [Pt2(NH3)4L212(PF6)4, [Pt4(NH3)8L41(N03)2(PF6)3.5H20Or [P4(NH3)8L41(N03)6.2H20 (HL =

a-pyrrolidone) in water leads to evolution of hydrogen in a ground state process, and this observation has been utilised in the photochemical reduction of water to hydrogen.13 Thus [Pt4( NH3)L4]6+ and [Pt4(NH3)8L4]5+ have been incorporated into a model system comprising EDTA as sacrificial donor, [Ru(bpy)3]2+ as photosensitizer, and methylviologen as electron relay, and the quantum yields of hydrogen production are found to be 0.022 and 0.1 1 respectively. Reduction of K2PtC16 in aqueous cyclodextrin with either H2 or ultrasound gives a cyclodextrin-stabilised Pt-colloid. 14 This material has been successfully used for the generation of hydrogen from water when the aqueous system

EDTA/photosensitizer/a-CD-MV2+ is irradiated. In aqueous media, irradiation of aqueous tetrahydroxostannate( 11) liberates hydrogen with formation of hexahydroxostannate(1V) in a reaction whose rate can be increased by the presence of colloidal Pt.15 Photolysis of aqueous solutions of aliphatic alcohols containing Fe(II1) chlorocomplex ions generates hydrogen.16 The mechanism involves formation of C1* and H', of which the latter abstracts H from the alcohol. A study aimed at elucidating the mechanism by which radiolytically prepared IrOx.nH20 hydrosols catalyse the oxidation of water, has shown that spectral changes brought about by gradual oxidation of IrOx using various radiolytic and electrochemical techniques confirm the formation of identical species in these experiments. 17 A photogalvanic cell based on the photoreduction of xanthene dyes using riboflavin as a sensitizer in aqueous micellar solution has been described,l8 and dissolved oxygen is reported to accelerate the photodegradation of porphyrin photosensi tizers used in the conversion of

IV: Photochemical Aspects of Solar Energy Conversion

473

solar energy into chemical energy.19 The effects of substituents and complexation with divalent metals have also been examined.

3. Heterogeneous Photosystems Mild reductive treatment of Ti02 surfaces results in formation of a series of oxides having different properties.20 Among these, surface states with essentially acid-base characteristics are shown to be important in the photooxidative decomposition of water. Evidence has appeared for the incorporation of oxidised species such as H202 into the support during water cleavage on Rh/Ti02 under basic conditions.21 Small variations in hydrogen production with temperature have been observed and this is taken to imply changes in mass transfer of methylviologen radicals to the catalyst surface. The influence of the preparation temperature of colloidal Ru02 on the photosensitized reduction of H+ has been tested in an aqueous system consisting of [Ru(bpy)3]2+ as photosensitizer, methylviologen as electron relay, and EDTA as electron donor.22 Such incorporation accounts for the progressive decrease in the rate of H2 photoproduction after long irradiation of Rh/Ti02 suspensions, and for its regeneration when an inert gas is bubbled through the suspension. Hydrogen has been photogenerated from an intimate mixture of Ti02 and Mn02 in an alkaline medium, by a process in which Mn02 is oxidised to [Mn04]2-. It is suggested that, at least in principle, the redox reaction between Mn02 and [ Mn0412- can be used to photodecompose water.23 Sacrificial water

cleavage has been achieved using niobium-doped fine particles of Ti02 under band gap irradiation with reaction occurring in the presence of EDTA as hole scavenger.24 Visible light-induced hydrogen formation from water using various viologen dyes, of which pentylviologen is the most efficient, has been reported;25 the relative rate of hydrogen formation is highest for redox potentials near -0.65 V at pH 4.5. Heterogeneous photocatalysts with layered structures such as H+/K4Nb6017 and CdS/K4Nb6017 have proved effective in the photoreduction of MV2+ by various alcohols.26 The layer catalyst is successful using visible

Photochemistry

474

radiation. The same group of workers has also demonstrated that water splitting can proceed over a Pt-intercalated K4Nb60 17 photocatalyst without a reverse reaction.27 Irradiation of solutions containing HCO3/ C 0 2 , C2O42- in the presence of Pd/Ti02 gives formate and H simultaneously, and the kinetics of hydrogenation of aqueous s o h tions of H C 0 3 - / C 0 2 have been measured as a function of catalyst.28 Aqueous sodium carbonate has been efficiently photoreduced to methanol using rutile/titania pigments coated with Fez+ or Co2+-phthalocyanine dyes, by irradiating at 254 nm.29 Electrons from the conduction band trigger the reduction of CO32- to CH30H, HCHO, and HC02-, and optimum yields occur at 2% surface coverage. Atomic clusters containing only small numbers of Pt atoms have been used to catalyse the generation of hydrogen from H2O.30 Studies suggest that reaction of MV.+ on the Pt catalyst with

H+ to give H2 is first order. The effect of CdS and Rhox concentration, and of temperature, on hydrogen evolution by water photolysis in aqueous CdS dispersions, using Rhox as catalyst in the presence of S- as electrondonor, have been examined.31 In order to evaluate the rate-determining step in the photocatalytic production of hydrogen, measurements of photocurrent and dark current have been made using n-type and p-type Si electrodes having thin Pd overlayers. Changes in photocurrent with light intensity suggest that the location of the rate-determining step may be influenced by the magnitude of the combination rate constant.32 I n alcoholic CCl4 solutions, irradiation of dispersions of RCo6 (R = La, Sm) powders is reported to give metal hydrides by transfer-hydrogenation of the alloys with concomitant photodehydrogenation of the alcohols.33 4. Photoelectrochemical Cells

Solar energy conversion has been achieved using a photogalvanic cell incorporating methylene blue/diethylenetriaminepentaacetic acid, and the effects of conditions on the cell performance have been described.34 In a photogalvanic study of K3Mn(CN)6 in aqueous solutions of CN-, irradiation of the anodic compartment causes an increasing current in the

IV: Photochemical Aspects of Solar Energy Conversion

475

cathodic compartment.35 Papers have appeared on the effect of dislocations on the performance of GaAs solar cells,36 the effect of tilted crystallographically defined pyramids on light-trapping and reflection control,37 and on a V- grooved solar cell.38 A high efficiency GaAs solar cell, fabricated on Si substrates and capable of achieving an energy conversion efficiency of 18.3%,39 a light-trapping Si solar cell for space use40 and a single crystal Si solar cell with a textured surface have all been reported.41 In order to study the photoresponse of polycrystalline Si solar cells, a two-dimensional model has been proposed in which the contribution to photoresponse of preferential doping realised along grain boundaries, is introduced.42 The stability of multijunction amorphous Si solar cells exceeds that of corresponding single junction cells, and it has been shown that the stability improves as the number of junctions increases.43 The thickness dependence of light-induced effects in amorphous Si solar cells on prolonged irradiation has been investigated,44 and an electroluminescence cell based on the layered, ionically conducting solid HU02P04.4H20 as emissive medium has been described.45

5. Luminescent Solar Concentrators A high efficiency Si concentrator solar cell for use with prismatic covers has been described,46 and an LSC based upon PMMA doped with U022+ is reported to be suitable for use with solar cells.47 Efficiencies of 24.8% have been achieved using a GaAs-Fresnel lens concentrator solar ce11.48 References 1

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a, 766.

Part V ADSOR BATE PHOTOCHEMISTRY ByS. R. MEECH

Adsorbate Photochemistry BY S.R. MEECH

1

Introduction

S u r f a c e p h o t o c h e m i s t r y is b y n o m e a n s a n e w t o p i c . P h o t o c a t a l y s i s a n d p h o t o c h e m i c a l e n e r g y c o n v e r s i o n a t solid surfaces h a v e been studied for m a n y years. These areas h a v e been reviewed elsewhere.' A m o r e r e c e n t i n t e r e s t is i n t h e t e c h n o l o g i c a l l y i m p o r t a n t a r e a of p h o t o d e p o s i t i o n , t h e t e c h n i q u e of ' w r i t i n g ' m i c r o c i r c u i t s on s e m i c o n d u c t o r s b y m e a n s of t h e p h o t o c h e m i c a l d e c o m p o s i t i o n o f or g a n o m e t a l l i c corn p o u n d s b y focused laser beams.2 This w o r k h a s s t i m u l a t e d d e m a n d for a m o r e c o m p l e t e u n d e r s t a n d i n g of t h e m e c h a n i s m of s u r f a c e p h o t o c h e m i s t r y . R e c e n t i n v e s t i g a t i o n s h a v e c o n c e n t r a t e d on t h e a p p l i c a t i o n of c l a s s i c a l p h o t o c h e m i c a l t e c h n i q u e s t o t h e s t u d y of p h o t o c h e m i s t r y i n s u b m o n o l a y e r s of m o l e c u l e s a d s o r b e d on w e l l c h a r a c t e r i s e d solid s u r f a c e s . I t is t h i s a p p r o a c h t h a t i s n e w , a n d i t s r e s u l t s a r e t h e s u b j e c t of t h i s r e v i e w . I t w i l l b e s e e n t h a t s o m e standard methods are revealing new and unexpected mechanisms of p h o t o c h e m i s t r y i n a d s o r b a t e s . The basic m e a s u r e m e n t s a r e those that have long been familiar t o t h e photochemist; t h e i d e n t i t y of t h e p h o t o p r o d u c t s a n d t h e i r y i e l d a s a f u n c t i o n of intensity and excitation wavelength. However both the e x p e r i m e n t a l t e c h n i q u e s , which m u s t d e a l w i t h v e r y low s a m p l e d e n s i t i e s (typically less t h a n 1014 molecules/cm2), a n d t h e m e c h a n i s m of p h o t o c h e m i c a l a c t i o n m a y b e u n f a m i l i a r . For e x a m p l e d i i o d o m e t h a n e h a s a negligible dissociation yield on s i l v e r , t h o u g h t h e g a s p h a s e y i e l d is u n i t y . 3 C o n v e r s e l y Q o n a Pd( 1 1 1 ) s u r f a c e is dissociated by UV-visible radiation.4 T h e e x p e r i m e n t a l t e c h n i q u e s w i l l b e t h e s u b j e c t of t h e n e x t s e c t i o n . T h e p r e p a r a t i o n of c l e a n w e l l c h a r a c t e r i z e d s u b s t r a t e s is of c r u c i a l i m p o r t a n c e i n m a n y of t h e e x p e r i m e n t s d e s c r i b e d : h o w e v e r , w e w i l l n o t h e r e b e c o n c e r n e d w i t h a l l of t h e p a r a p h e r n a l i a of s u r f a c e s c i e n c e . D e t a i l e d r e v i e w s of t h i s s u b j e c t m a y b e f o u n d e l s e w h e r e . 5 T h e p r e s e n t r e v i e w will f o c u s on t h e strategies that have been employed to extract the important p a r a m e t e r s of y i e l d a n d r a t e . In t h e t h i r d s e c t i o n a g e n e r a l d e s c r i p t i o n of t h e d i f f e r e n t , o f t e n n o v e l , m e c h a n i s m s of s u r f a c e

482

Photochemistry

p h o t o c h e m i s t r y will b e g i v e n . T h e c o n d i t i o n s u n d e r w h i c h o n e or o t h e r of t h e m e c h a n i s m s - d i r e c t e x c i t a t i o n , a d s o r b a t e - s u b s t r a t e complex e x c i t a t i o n , s u b s t r a t e excitation - m a y d o m i n a t e will be d e s c r i b e d , a s w i l l t h e m e a n s of d i s t i n g u i s h i n g t h e m e x p e r i m e n t a l l y . The f o u r t h section details t h e d i f f e r e n t a d s o r b a t e s u b s t r a t e s y s t e m s , a n d is broadly divided by s u b s t r a t e (i n s u l a t o r, m e t a l , s e m i c o n d u c t o r ) . I m p o r t a n t t h e m e s a r e b r o u g h t t o g e t h e r in a f i n a l section. S e v e r a l r e v i e w s on a s p e c t s of s u r f a c e p h o t o c h e m i s t r y h a v e a l r e a d y a p p e a r e d . T w o e a r l y r e v i e w s by C h u a n g a r e s t i l l v e r y usefu1.6.7 Avouris and Walkup have reviewed the fundamental m ech a n i s m s of ad sor b a t e f r a g m e n t a t i o n , i n c l u d i n g b o m b a r d m e n t b y energetic particles.8 S h o r t r e v i e w s or r e v i e w s of s p e c i f i c s y s t e m s h a v e a p p e a r e d . 9 - 1 3 W h i l e t h i s w o r k w a s in p r e p a r a t i o n a c o m p r e h e n s i v e r e v i e w of m e t a l s u r f a c e p h o t o c h e m i s t r y , w r i t t e n from a slightly d i f f e r e n t view point, w a s p ~ b l i s h e d . 1 ~ 2

Techniques

A d s o r p t i o n c a n a l t e r c o n s i d e r a b l y t h e s t r u c t u r e s of m o l e c u l e s . T h e a d s o r b a t e m a y d e c o m p o s e or u n d e r g o l e s s d r a s t i c c h a n g e s in g e o m e t r y a n d e l e c t r o n i c s t r u c t u r e . A g e n e r a l d i s c u s s i o n of t h e f u n d a m e n t a l s of t h e a d s o r b a t e - s u b s t r a t e i n t e r a c t i o n c a n b e f o u n d i n Zangwill's b00k.15 O b v i o u s l y it i s i m p o r t a n t t o k n o w t h e s t a t e of t h e a d s o r b e d m o l e c u l e s p r i o r t o i r r a d i a t i o n . Optical spectroscopy is of l i t t l e h e l p , s i n c e t h e s a m p l e d e n s i t y i s s o l o w . F o r t u n a t e l y e l e c t r on s c a t t e r in g m e t h o d s c o n t a i n s i m i l a r in f o r m a t i o n , a l b e i t a t lower e n e r g y resolution.5$*5 Electron e n e r g y loss spectroscopy (EELS) i s m o s t i n f o r m a t i v e , a s d i s c u s s e d b y C h a k a r o v e t ~ 1 . 1 6 V i b r a t i o n a l ( o r high r e s o l u t i o n , HR) EELS g i v e s v i b r a t i o n a l l y r e s o l v e d i n f o r m a t i o n on t h e s t a t e of t h e a d s o r b e d m o l e c u l e , a n d m a y b e u s e d t o j u d g e t h e e x t e n t , if a n y , of d i s s o c i a t i o n . E l e c t r o n i c EELS p r o v i d e s t h e c r u c i a l i n f o r m a t i o n on t h e e n e r g y of a d s o r b a t e e l e c t r o n i c t r a n s i t i o n s , c l e a r l y i m p o r t a n t in t h e i n t e r p r e t a t i o n of a c t i o n s p e c t r a . F u r t h e r i n f o r m a t i o n on t h e a b s o l u t e e n e r g i e s of e l e c t r o n i c s t a t e s is a v a i l a b l e f r o m u l t r a v i o l e t p h o t o e m i s s i o n a n d i n v e r s e p h o t o e m i s s i o n s p e c t r o s c o p i e s (UPS, IPS). T h e p o s i t i o n s of ground and excited s t a t e s relative to t h e Fermi level contains i m p o r t a n t m e c h a n i s t i c i n f o r m a t i o n . A f i n a l m e a s u r e m e n t of g r e a t i m p o r t a n c e in i n t e r p r e t a t i o n of a c t i o n s p e c t r a i s t h e s u r f a c e w o r k f u n c t i o n , a,a n d i t s c h a n g e a s a f u n c t i o n of c o v e r a g e .

V:Adsorbate Photochemistry

483

A? GUN

(a)

T

EELS, UPS XPSJPS

LAMP F i g u r e 1 . ( a ) S c h e m a t i c of t h e a p p a r a t u s w h i c h m a y b e e m p l o y e d i n a s u r f a c e p h o t o c h e m i s t r y e x p e r i m e n t . T h e S a m p l e ( S ) is m o u n t e d on a t e m p e r a t u r e c o n t r o l l e d block in UHV. I t m a y b e c l e a n e d by a r g o n ion b o m b a r d m e n t . T h e a d s o r b a t e is d o s e d o n t o t h e s u r f a c e (D). I t s d a r k s t a t e is d e t e r m i n e d by T P D ( a s m e a s u r e d b y m a s s s p e c t r o m e t r y , M S ) , EELS, UPS e t c . I r r a d i a t i o n i s by an a r c l a m p via a m o n o c h r o m a t o r or f i l t e r s . P r o d u c t s a r e d e t e c t e d by t h e s a m e a n a l y s i s tools. ( b ) T h e o p t i c a l g e o m e t r y . P o l a r i s a t i o n is d e f i n e d r e l a t i v e t o t h e s u r f a c e ( p l a n e ( p ) or p e r p e n d i c u l a r (s) p o l a r i s e d a t an a n g l e of i n c i d e n c e Binc f r o m t h e s u r f a c e n o r m a l ) . T h e a n g l e of i n c i d e n c e m a y be a l t e r e d by c h a n g i n g t h e s u r f a c e o r i e n t a t i o n ( 0 ) . T h e reflected ( r ) and transmitted (absorbed) t fields a r e calculated f r o m t h e F r e s n e l e q u a t i o n s . T h e a d s o r b a t e o r i e n t a t i o n d e f i n e d in t h e x y z a x e s s y s t e m . Light p o l a r i s e d p h a s c o m p o n e n t s in t h e x z directions, s in the y direction.

484

Photochemistry

For t h e i n i t i a t i o n of p h o t o c h e m i s t r y t w o s o u r c e s a r e p o p u l a r . T h e m o s t w i d e l y u s e d i s t h e a r c l a m p . As w e l l a s b e i n g cheap and easy to operate arc lamps provide continuously tunable r a d i a t i o n f r o m t h e n e a r ir t o t h e UV. T h u s i t i s p o s s i b l e t o o b t a i n t h e a c t i o n s p e c t r u m , w h i c h y i e l d s t h e m o s t d i r e c t i n f o r m a t i o n on t h e m e c h a n i s m of a d s o r b a t e p h o t o c h e m i s t r y . A n o t h e r a d v a n t a g e i s t h a t t h e i n c i d e n t p o w e r i s r e l a t i v e l y low, so s u r f a c e h e a t i n g is negligible a n d t h e r e is n o competition f r o m t h e r m a l reactions. A d i a g r a m of a t y p i c a l s u r f a c e p h o t o c h e m i c a l e x p e r i m e n t i s s h o w n in f i g u r e 1 . T h e i m p o r t a n c e o f t h e c o n t r o l of p o l a r i s a t i o n a n d a n g l e of incidence will be described below. Pulsed lasers a r e essential for some experiments, a l t h o u g h t h e a d v a n t a g e of c o n t i n u o u s t u n a b i l i t y i s l o s t . An i n t e n s e s h o r t optical p u l s e can c a u s e a high d e n s i t y of p h o t o f r a g m e n t s t o be released from the surface into the gas phase. These may then b e s u b j e c t e d t o f u r t h e r a n a l y s i s . I f a m a s s s p e c t r o m e t e r is placed a k n o w n d i s t a n c e f r o m t h e s u r f a c e t h e t i m e of f l i g h t (TOF) distribution may b e measured. I f t h e s a m p l e position can be v a r i e d t h e a n g l e r e s o l v e d TOF d i s t r i b u t i o n i s o b t a i n e d . T h i s k i n d of m e a s u r e m e n t h a s p r o v i d e d a g r e a t d e a l of i n f o r m a t i o n c o n c e r n i n g t h e d y n a m i c s of s u r f a c e p h o t o c h e m i s t r y ( s e e e s p e c i a l l y t h e w o r k s b y P o l a n y i , E r t l , King, a n d c o - w o r k e r s c i t e d b e l o w ) . I t i s a l s o p o s s i b l e t o m e a s u r e b o t h t h e TOF a n d t h e l a s e r i n d u c e d f l u o r e s c e n c e s p e c t r u m of t h e d e s o r b e d p r o d u c t s . T h i s i s a c h i e v e d by t i m i n g a t u n a b l e p u l s e t o i n t e r s e c t t h e d e s o r b e d s p e c i e s a known distance from t h e surface. From t h e k n o w n flight p a t h and d e l a y t i m e b e t w e e n p h o t o l y s i s a n d a n a l y s i s p u l s e s t h e TOF i s obtained, which m a y again b e angle resolved. From t h e induced fluorescence excitation spectrum the internal q u a n t u m state d i s t r i b u t i o n of t h e p r o d u c t s i s d e t e r m i n e d . T h e l a t t e r q u a n t i t y is p o t e n t i a l l y a d e t a i l e d f i n g e r p r i n t of t h e d y n a m i c s of t h e p h o t o d e s o r p t ion or p h ot o r e a c t ion p r ocess.17 Of c o u r s e a n i n t e n s e l a s e r p u l s e w i l l b e a b s o r b e d b y t h e s u r f a c e a n d m a y c a u s e a significant t e m p e r a t u r e r i s e , l e a d i n g to photothermal reactions. S i n c e t h e m a i n i n t e r e s t i s in t h e p h o t o c h e m i s t r y it i s n e c e s s a r y t o s e p a r a t e t h e t w o c o n t r i b u t i o n s . F o r t u n a t e l y e x p r e s s i o n s for t h e t e m p e r a t u r e r i s e a t t h e s u r f a c e for a given laser pulse s h a p e h a v e been derived.18~19 Thus the t e m p e r a t u r e r i s e c a n b e c a l c u l a t e d a n d , if t h e t h e r m a l a c t i v a t i o n e n e r g y of t h e p r o c e s s u n d e r s t u d y i s k n o w n o r m e a s u r e d , t h e e x p e c t e d t h e r m a l l y e q u i l i b r a t e d v a l u e s of f l i g h t t i m e a n d i n t e r n a l

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s t a t e d i s t r i b u t i o n m a y b e calculated.20*2* T h u s t h e r m a l a n d non Of c o u r s e p h o t o t h e r m a l t h e r m a l p r o c e s s e s can b e s e p a r a t e d . d e s o r p t i o n h a s m a n y a p p l i c a t i o n s in s u r f a c e s t u d i e s , 2 0 - 2 2 b u t t h e s e a r e not d i s c u s s e d h e r e . A final problem with pu l s e d l a s e r m e t h o d s is t h a t t h e i n s t a n t a n e o u s p r o d u c t d e n s i t y j u s t a f t e r p h o t o l y s i s m a y b e q u i t e h i g h , a n d t h e r e is a d a n g e r of c o l l i s i o n s in t h e g a s p h a s e d i s t o r t i n g t h e q u a n t u m s t a t e a n d TOF d i s t r i b u t i o n . T h i s a s p e c t w a s c o n s i d e r e d by Polanyi.23 T h e m i n i m u m c o v e r a g e a n d p h o t o l y s i s e n e r g y c o m p a t i b l e w i t h good s i g n a l t o n o i s e s h o u l d b e u s e d . A f t e r d e t e r m i n i n g t h e d a r k s t a t e of t h e a d s o r b a t e a n d c h o o s i n g a m e a n s of i n i t i a t i n g t h e p h o t o c h e m i s t r y it i s n e c e s s a r y t o d e c i d e on a m e a n s of m o n i t o r i n g t h e p h o t o c h e m i c a l r e a c t i o n . For t h e s p e c i a l c a s e of p u l s e d l a s e r e x c i t a t i o n t h i s w a s d i s c u s s e d a b o v e . For e x c i t a t i o n w i t h a c o n t i n u o u s l a m p t h e m o n i t o r i n g t e c h n i q u e d e p e n d s on w h e t h e r t h e p r o d u c t s a r e d e s o r b e d i n t o t h e g a s p h a s e or a d s o r b e d on t h e s u r f a c e . For t h e f o r m e r c a s e t h e f a v o u r e d t e c h n i q u e is m a s s s p e c t r o m e t r y (MS). In t h e f i r s t p l a c e m a s s r e s o l u t i o n a i d s t h e p r o d u c t i d e n t i f i c a t i o n . S e c o n d l y MS p r o v i d e s a n i n s t a n t a n e o u s ( l i m i t e d by r e s p o n s e t i m e of e l e c t r o n i c s , p u m p i n g s p e e d , e t c . ) m e a s u r e of t h e d e n s i t y of p h o t o p r o d u c t a s a f u n c t i o n of i r r a d i a t i o n t i m e . T h i s is i m p o r t a n t a s it p r o v i d e s a d i r e c t r o u t e t o t h e cross section for t h e dissociation (or d e s o r p t i o n ) p ro c e s s . If d i s s o c i a t i o n l e a d s d i r e c t l y t o a d e s o r b e d p r o d u c t in a f i r s t o r d e r p r o c e s s t h e n d S / d t = - d [ A ] / d t = oF[A] w h e r e S i s t h e MS s i g n a l , [A] is t h e a d s o r b a t e c o n c e n t r a t i o n (cm-2). F t h e p h o t o n fluence ( p h o t o n s s - 1 c m - 2 ) , Q t h e c r o s s s e c t i o n ( c m 2 ) . T h u s OF can b e o b t a i n e d f r o m a p l o t of In S a g a i n s t t , a n d Q f r o m t h e f l u e n c e d e p e n d e n c e . I n s o m e c a s e s n o n e x p o n e n t i a l d e c a y of t h e MS s i g n a l h a v e been reported.24 The behaviour may arise from several factors including ( i ) a coverage d e p e n d e n t cross section (ii) an a l t e r a t i o n of t h e c r o s s s e c t i o n by a d s o r b e d p r o d u c t s , d u e t o c h a n g e s in a d s o r b a t e g e o m e t r y or s u r f a c e e l e c t r o n i c s t r u c t u r e (iii) m u l t i p l e e x c i t a t i o n m e c h a n i s m s ( i v ) s u b p o p u l a t i o n s of a d s o r b a t e s u n d e r g o i n g p h o t o c h e m i c a l reaction a t d i f f e r e n t r a t e s or with a d i f f e r e n t m e c h a n i s m . A t l e a s t f o r ( i ) a n d ( i i ) t h e s l o p e a t t = 0 will c o n t a i n u n d i s t o r t e d c r o s s s e c t i o n s . H o w e v e r , in g e n e r a l t h e r e is n o s u b s t i t u t e f o r m a k i n g c o m p l e m e n t a r y m e a s u r e m e n t s of p o p u l a t i o n on t h e s u r f a c e . One v e r y u s e f u l m e t h o d of m o n i t o r i n g a d s o r b a t e a n d p r o d u c t d e n s i t i e s on t h e s u r f a c e is HREELS. M o d e r n v e r s i o n s of HREELS h a v e r e a s o n a b l e t i m e r e s o l u t i o n . Ho h a s d e s c r i b e d a m u l t i c h a n n e l

486

Photo @hemistry

m e t h o d w i t h m i l l i s e c o n d t i m e r e s o l u t i o n .25 T h u s con t i n uou s m o n i t o r i n g of t h e r e l a t i v e p o p u l a t i o n of a d s o r b a t e ( o r p r o d u c t ) d u r i n g p h o t o l y s i s is p o s s i b l e . T h e c r o s s s e c t i o n i s d e t e r m i n e d a s d e s c r i b e d a b o v e . If c o n t i n u o u s m o n i t o r i n g is n o t possible s a m p l i n g of t h e a d s o r b a t e p o p u l a t i o n a s a f u n c t i o n of i r r a d i a t i o n t i m e is possible, provided the electron beam does not d a m a g e t h e surface. I f both a d s o r b e d a nd d e s o r b e d product f or m a t i o n k i n e t i c s a r e r e c o r d e d a r a t h e r c o m p l e t e p i c t u r e of t h e p h o t o c h e m i c a l p a t h w a y is o b t a i n e d . U n f o r t u n a t e l y t h e HREELS i n t e n s i t y i s n o t a l w a y s proportional to coverage, so careful calibration may be required. T e m p e r a t u r e p r o g r a m m e d d e s o r p t i o n (TPD) w i t h MS d e t e c t i o n can also yield t h e p h o t o lysis cross section, t h e a d s o r b a t e (or p r o d u c t ) d e n s i t y b e i n g o b t a i n e d f r o m i n t e g r a t i o n o f t h e TPD peak.26 T h e r m a l r e a c t i o n s of p h o t o p r o d u c t s h a v e t o b e a c c o u n t e d f o r , a n d c o n t i n u o u s m o n i t o r i n g is n o t a p o s s i b i l i t y . O t h e r t e c h n i q u e s w h i c h m a y b e u s e d in p o s t i r r a d i a t i o n a n a l y s i s of s u r f a c e s a r e X-ray photoelectron and Auger spectroscopy. I t s e e m s a p p r o p r i a t e t o i l l u s t r a t e t h e m u l t i p l i c i t y of t e c h n i q u e s which h a v e been e m p l o y e d in s u r f a c e p h o t o c h e m i s t ry w i t h a c a s e h i s t o r y . One e x a m p l e , a m o n g m a n y o t h e r s c i t e d b e l o w , is t h e s t u d y by So e t al.of N o o n Cu(111).24 I n i t i a l l y t h e s u r f a c e w a s c l e a n e d in u l t r a high v a c u u m a n d a s a t u r a t i o n c o v e r a g e of *5NO w a s d e p o s i t e d . TPD s p e c t r a w e r e r e c o r d e d a n d r e v e a l e d p e a k s or s h o u l d e r s , a s t h e s a m p l e w a s w a r m e d a t 2 - 3 K s - 1 , a t 1 0 6 , 1 4 0 , 2 2 0 a n d 2 6 0 K. T h i s w a s a c l e a r i n d i c a t i o n of m u l t i p l e s i t e s . At m a s s 4 6 (N20) a TPD p e a k w a s o b s e r v e d a t 1 0 6 K, i n d i c a t i n g a t h e r m a l r e a c t i o n . T h e n e x t s e t of e x p e r i m e n t s r e c o r d e d TPD s p e c t r a a s a f u n c t i o n of c o v e r a g e . I n i t i a l l y a single p e a k a t 1 5 5 K a p p e a r e d which s h i ft e d t o lower desorption temperatures as coverage increased. Further exposure r e v e a l e d a s e c o n d p e a k a t 1 2 5 K, w h i c h a l s o s h i f t e d t o l o w e r t e m p e r a t u r e a n d g a i n e d i n t e n s i t y w i t h i n c r e a s i n g c o v e r a g e . The p e a k s h i f t s s t o p p e d a t 1 0 6 a n d 1 4 0 K, b u t t h e 1 0 6 K p e a k g a i n e d i n t e n s i t y a t t h e e x p e n s e of t h e 1 4 0 K. Clearly t h e coverage d e p e n d e n c e w a s n o t s t r a i g h t f o r w a r d , a n d HREELS s p e c t r a w e r e r e q u i r e d f o r f u r t h e r i n t e r p r e t a t i o n . At s a t u r a t i o n HREELS r e v e a l e d t w o s t r o n g p e a k s w h i c h could b e a s c r i b e d t o NO s t r e t c h e s in a t o p a n d t w o fold b r i d g e s i t e s . Both p e a k s a p p e a r e d t o b e inhomogenously b r o a d e n e d . F u r t h e r p e a k s could b e assigned t o t h e t w o N O s p e c i e s a n d r e v e a l e d t h e a t o p NO t o b e o r i e n t e d a w a y f r o m t h e s u r f a c e n o r m a l . HREELS w e r e r e c o r d e d a s a f u n c t i o n of

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coverage. These revealed that t h e bridge bonded species was the o n l y o n e p r e s e n t a t low c o v e r a g e . H o w e v e r t h e a t o p s i t e g r e w in with increasing exposure and eventually began t o dominate t h e s p e c t r u m . N 2 0 a p p e a r e d a t i n t e r m e d i a t e , b u t n o t high c o v e r a g e . On s u b j e c t i n g t h e s a t u r a t e d s p e c t r u m t o a t h e r m a l cycle t o 1 8 0 K both NO s t r e t c h p e a k s d i s a p p e a r e d , b u t p e a k s w h i c h could b e This assigned t o atomic 0 and N adsorbates w e r e observed. i n d i c a t e d t h e r m a l d i s s o c i a t i o n . C o m p a r i n g TPD a n d HREELS it w a s apparent that the 106 K a n d 140 K T P D p e a k s corresponded to atop a n d b r i d g e b o n d e d NO r e s p e c t i v e l y . T h e a p p e a r a n c e of N 2 0 in TPD for a s a t u r a t e d s u r f a c e a t 106 K s h o w e d it h a d i t s origin i n a t h e r m a l r e a c t i o n of a t o p NO. H a v i n g m a d e a d e t a i l e d s t u d y of t h e d a r k s t a t e a n d t h e r m a l r e a c t i o n s of t h e C u ( l l l ) / N O s y s t e m So e t al. t u r n e d t o t h e p h o t o c h e m i c a l s t u d i e s . T h e f i r s t m e a s u r e m e n t r e p o r t e d w a s of t h e d e c a y of t h e p h o t o i n d u c e d d e s o r p t i o n s i g n a l ( a s m e a s u r e d by MS) a s a f u n c t i o n of i r r a d i a t i o n t i m e . I m m e d i a t e l y on c o m m e n c e m e n t of i r r a d i a t i o n t h e p h o t o d e s o r p t i o n s i g n a l j u m p e d t o i t s m a x i m u m v a l u e a n d d e c a y e d a w a y a s t h e p h o t o a c t i v e NO w a s d e p l e t e d . T h e immediate rise already indicated a photolytic mechanism; a t h e r m a l d e s o r p t i o n s i g n a l would r i s e w i t h t h e substrate t e m p e r a t u r e (moreover, the total temperature rise d u e to i r r a d i a t i o n w a s < 2 K). N 2 0 could a l s o b e d e t e c t e d ; i t f o l l o w e d t h e s a m e k i n e t i c s a s NO, T h e n e x t s t e p w a s t o m e a s u r e t h e TPD s p e c t r a recorded after different irradiation times. The main result w a s a d e c r e a s e in t h e i n t e n s i t y of t h e 1 0 6 K ( a t o p NO) p e a k w i t h i n c r e a s i n g i r r a d i a t i o n t i m e . H o w e v e r , t h e t o t a l NO c o v e r a g e ( i n t e g r a t e d TPD) did n o t d e c r e a s e t o z e r o f o r l o n g i r r a d i a t i o n t i m e s , i n d i c a t i n g t h a t n o t a l l NO w a s p h o t o a c t i v e . HREELS s h o w e d a d e c r e a s e in t h e a t o p N O s i g n a l a s a f u n c t i o n of i r r a d i a t i o n t i m e , a n d a s h i f t of t h e r e s i d u a l N O ( a t o p ) p e a k t o l o w e r e n e r g y . A p p a r e n t l y n o t e v e n a l l of t h e inhomogeneously b r o a d e n e d a t o p NO p o p u l a t i o n w a s p h o t o a c t i v e . H o w e v e r , t h e r e s u l t s c l e a r l y i n d i c a t e d t h a t only a t o p NO w a s p h o t o a c t i v e . For q u a n t i t a t i v e a n a l y s i s t h e l o g a r i t h m of t h e M S p h o t o d e s o r p t i o n i n t e n s i t y a n d HREELS NO a t o p i n t e n s i t y (with the photoinactive NOsignal subtracted out) were plotted as a f u n c t i o n of i r r a d i a t i o n t i m e . T h e s l o p e s , w h i c h w e r e in good a g r e e m e n t , g a v e t h e c r o s s s e c t i o n for p h o t o d e s o r p t i o n , t h e f l u e n c e b e i n g k n o w n . Finally t h e r e l a t i v e p h o t o y i e l d , a s d e t e r m i n e d f r o m t h e i n i t i a l (t=O) p h o t o d e s o r p t i o n s i g n a l , w a s m e a s u r e d a s a f u n c t i o n of w a v e l e n g t h , p o l a r i s a t i o n a n d a n g l e of i n c i d e n c e . P l o t s of t h e

488

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p e a k i n t e n s i t y a s a f u n c t i o n of f l u e n c e w e r e l i n e a r i n d i c a t i n g a s i n g l e p h o t o n p r o c e s s . T h e s e f i n a l m e a s u r e m e n t s p e r m i t t e d Su e t a1 t o d e t e r m i n e t h e m e c h a n i s m of t h e p h o t o r e a c t i o n r e v e a l e d in t h e p r e c e d i n g m e a s u r e m e n t s . D e t a i l s of t h e m e c h a n i s m of p h o t o l y s i s a r e g i v e n in t h e n e x t s e c t i o n . From t h e w o r k of r e f . 2 4 , a n d s e v e r a l o t h e r s c o u l d h a v e b e e n c h o s e n , it is c l e a r t h a t ( i ) a good u n d e r s t a n d i n g of t h e d a r k a n d t h e r m a l r e a c t i o n s a n d s t r u c t u r e of t h e a d s o r b e d l a y e r a r e r e q u i r e d prior t o a t t e m p t i n g t h e p h o t o c h e m i c a l e x p e r i m e n t a n d (ii) n o single m e a s u r e m e n t t e c h n i q u e is s u f f i c i e n t f o r a n a n a l y s i s of s u r f a c e photochemistry .

3

Mechanisms of A d s o r b a t e Photochemistry

Over t h e p a s t f e w y e a r s it h a s o f t e n b e e n o b s e r v e d t h a t t h e p h o t o c h e m i c a l b e h a v i o u r of a d s o r b e d m o l e c u l e s i s d i s t i n c t l y d i f f e r e n t t o t h a t of t h e i r g a s p h a s e c o u n t e r p a r t s . Even d i r e c t d i s s o c i a t i o n s of m o l e c u l e s physisorbed on i n s u l a t o r s u b s t r a t e s w e r e found to h a v e different dynamics to t h e analagous gas phase r e a c t i o n , a n d e x h i b i t e d a d e p e n d e n c e on t h e c o v e r a g e . T h i s n e e d s to be understood. For a d s o r b e d m o l e c u l e s a n e w k i n d of " d i s s o c a t i o n " is p o s s i b l e , n a m e l y d e s o r p t i o n . Ph o t o l y t i c (non t h e r m a l ) d e s o r p t i o n h a s b e e n r e p o r t e d f r o m a l l k i n d s of s u b s t r a t e . On m e t a l s u r f a c e s it i s o f t e n f o u n d t h a t t h e q u a n t u m y i e l d for a d i r e c t p h o t o d i s s o c i a t i o n r e a c t i o n is m u c h l o w e r t h a n in t h e i s o l a t e d molecule. This must b e accounted for. Finally, t h e o b s e rv a t i o n which h a s s t i m u l a t e d a g r e a t d e a l of r e s e a r c h in s u r f a c e p h o t o c h e m i s t r y , p h o t o l y s i s is o b s e r v a b l e a t e n e r g i e s w h e r e t h e gas p h a s e m o l e c u l e s a r e t r a n s p a r e n t . I t t u r n s o u t t h a t a l l of t h e s e i n t e r r e l a t e d e f f e c t s can b e i n t e r p r e t e d b y a d e l i c a t e i n t e r p l a y of e x c i t a t i o n m e c h a n i s m a n d t r a n s i e n t q u e n c h i n g . T h e f i n e d e t a i l s of c o u r s e d e p e n d on p a r t i c u l a r a d s o r b a t e - s u b s t r a t e s y s t e m s , which a r e d e s c r i b e d in s e c t i o n 4. ( a ) P r i m a r y P r o c e s s e s o n I n s u l a t o r s . - Direct e l e c t r o n i c e x c i t a t i o n of m o l e c u l e s p h y s i s o r b e d on i n s u l a t o r s u b s t r a t e s m i g h t be e x p e c t e d t o l e a d t o p h o t o c h e m i s t r y w i t h d y n a m i c s s i m i l a r t o t h a t of t h e g a s p h a s e . Polanyi and co-workers h a v e made a n u m b e r of d e t a i l e d s t u d i e s of p h o t o d i s s o c i a t i o n r e a c t i o n s on t h e LiF(OO1) s u r f a c e w i t h TOFMS detection.27.28 A n u m b e r of d i f f e r e n t photoinduced effects were observed. I n e a r l y w o r k on CH3Br p h y s i s o r b e d on LiF t h e y s h o w e d t h a t t h e t r a n s l a t i o n a l e n e r g y of

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489

t h e m e t h y l f r a g m e n t w a s v e r y close t o t h a t f o u n d in t h e g a s p h a s e . F r a g m e n t a t i o n could o n l y b e o b s e r v e d if t h e l a s e r w a s r e s o n a n t w i t h a n a d s o r b a t e e l e c t r o n i c transition.27 Even in t h i s c a s e t h e t r a n s l a t i o n a l e n e r g y d i s t r i b u t i o n did n o t r e s o l v e t h e s e p a r a t e p e a k s f o u n d in t h e gas p h a s e a n d a s s o c i a t e d w i t h p r o d u c t i o n of Br in g r o u n d a n d e x c i t e d states.23 T h i s i n d i c a t e s a p e r t u r b a t i o n of r e a c t i o n d y n a m i c s b y t h e s u r f a c e . I n a d s o r b e d HBr t h e r a t i o B r * / B r h a s b e e n f o u n d t o b e t h e s a m e a s in t h e g a s phase,28 i n d i c a t i n g u n p e r t u r b e d p o t e n t i a l e n e r g y s u r f a c e s . For CH3Br t h e fragment translational energy decreased as the coverage increased b e y o n d 1 ML, t h o u g h t h e d i s t r i b u t i o n r e m a i n e d narrow.23 This w a s a t t r i b u t e d t o d i s s o c i a t i o n of t h e t o p m o s t l a y e r of a n o r d e r e d CH3Br ice. A m o s t i n t e r e s t i n g o b s e r v a t i o n w a s t h a t t h e Br f r a g m e n t h a d t r a n s l a t i o n a l e n e r g i e s in e x c e s s of t h a t a t t a i n a b l e f r o m t h e p h o t o n e n e r g y . This w a s i n t e r p r e t e d a s follows: CH3Br w i t h Br o r i e n t e d a w a y f r o m t h e s u r f a c e w a s p h o t o l y s e d ; t h e sluggish e j e c t e d Br f r a g m e n t r e c e i v e d a s e c o n d i m p u l s e f r o m t h e r a p i d l y r e c o i l i n g m e t h y l f r a g m e n t a s it r e b o u n d e d f r o m t h e s u r f a c e ; t h e CH3 g r o u p s u b s e q u e n t l y b i n d s t o t h e surface.23~29>30 Observations at t h e adsorbate mass showed extensive molecular p h ot o d e s o r p tion w i t h low t r an s l a t i o n a l e n e r g y .23$2792833*$32 This w a s o b s e r v e d for s e v e r a l a d s o r b a t e s , a n d e l e c t r o n i c e x c i t a t i o n w a s not r e q u i r e d . I t w a s s h o w n t h a t t h i s w a s a p u r e l y s u b s t r a t e m e d i a t e d effect.31 T h e i n c i d e n t n a n o s e c o n d l a s e r p u l s e e x c i t e s F c e n t r e d e f e c t s in t h e s u b s t r a t e . These decay very rapidly launching phonons. If t h e p h o n o n s r e a c h t h e s u r f a c e w i t h sufficient energy they induce desorption; a shock w a v e effect. A second more energetic molecular desorption channel, termed p h o t o e j e c t i o n , w a s r e p o r t e d . 3 1 This b e c a m e a n i m p o r t a n t c h a n n e l for e l e c t r o n i c a l l y r e s o n a n t a d s o r b a t e s a t high c o v e r a g e . T h e e f f e c t w a s a s c r i b e d t o an e n e r g y t r a n s f e r f r o m e x c i t e d a d s o r b e d H2S t o g r o u n d s t a t e H2S in t h e u p p e r l a y e r , which a r e t h e n ejected.31 Fin ally in s e v e r a l c a s e s p h o t o r e a c t i o n s b e t w e e n p h o t o l y s i s p r o d u c t s a n d g r o u n d s t a t e a d s o r b a t e s w e r e observed.31.33 T h e s e a r e d i s c u s s e d in s e c t i o n 4 . ( b ) E x c i t e d S t a t e Q u e n c h i n g o n M e t a l S u b s t r a t e s . - On moving from insulator to m e t a l s u b s t r a t e s t h e first notable f e a t u r e i s a d r a m a t i c r e d u c t i o n i n t h e yield of s o m e p h o t o c h e m i c a l r e a c t i o n s . For e x a m p l e Mod1 e t a l . s t u d i e d CH212 o n A1 f i l m s a n d f o u n d a p h o t o l y s i s q u a n t u m yield of