Photochemistry

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Photochemistry Volume 36

A Specialist Periodical Report

Photochemistry Volume 36 A Review of the Literature Published between July 2003 and June 2004 Editor I. Dunkin, University of Strathclyde, Glasgow, UK

Authors N.S. Allen, Manchester Metropolitan University, UK N.W.A. Geraghty, National University of Ireland, Galway, Ireland A. Gilbert, University of Reading, UK W.M. Horspool, Dundee, UK

If you buy this title on standing order, you will be given FREE access to the chapters online. Please contact [email protected] with proof of purchase to arrange access to be set up. Thank you.

ISBN-13: 978-0-85404-450-4 ISSN 0556-3860 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2007 All rights reserved Apart from any fair dealing for the purpose of research or private study for non-commercial purposes, or criticism or review as permitted under the terms of the UK Copyright, Designs and Patents Act, 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org Typeset by Macmillan India Ltd, Bangalore, India Printed by Henry Lings, Dorchester, Dorset, UK

Contents

Cover An energy level diagram overlaid on the sun. Background image reproduced by permission of NASA.

Introduction and Review of the Year By Ian R. Dunkin

1

Photolysis of Carbonyl Compounds By William M. Horspool

9

1 2

Norrish Type I Reactions Norrish Type II Reactions 2.1 1,5-Hydrogen Transfer 2.2 Other Hydrogen Transfers 3 Oxetane Formation 4 Miscellaneous Processes 4.1 Decarboxylation and Decarbonylation 4.2 Reactions of Miscellaneous Haloketones and Acid Chlorides 4.3 Other Processes References v

10 11 11 15 15 17 17 18 19 20

vi

Contents

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool 1

Cycloaddition Reactions 1.1 Intermolecular Cycloadditions 1.2 Intramolecular Cycloadditions 2 Rearrangement Reactions 2.1 a, b-Unsaturad Systems 2.2 b, g-Unsaturated Systems 3 Photoreactions of Thymines and Related Compounds 3.1 Photoreactions of Pyridones 3.2 Photoreactions of Thymines, etc 4 Photochemistry of Dienones 4.1 Cross-conjugated Dienones 4.2 Linearly Conjugated Dienones 5 1,2-, 1,3- and 1,4-Diketones 5.1 Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds 5.2 Reactions of 1,3-Diketones 5.3 Reactions of 1,4-Diketones 5.4 Fulgides and Fulgimides 6 Quinones 6.1 p-Quinones References

Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool 1

2

3 4 5 6

Reactions of Alkenes 1.1 cis,trans-Isomerization 1.2 Miscellaneous Reactions Reactions involving Cyclopropane Rings 2.1 The Di-p-methane Rearrangement and Related Processes 2.2 Miscellaneous Reactions Involving three-membered Ring Compounds Reactions of Dienes and Trienes (2p + 2p)-Intramolecular Additions Dimerization and Intermolecular Additions Miscellaneous Reactions 6.1 Reactions of Halo Compounds

23

23 23 29 31 31 34 35 35 36 39 39 41 42 42 43 44 46 48 48 49

55

55 55 65 70 70 72 74 77 79 82 82

vii

Contents

6.2

Miscellaneous Rearrangements and Bond Fission Processes References

Photochemistry of Aromatic Compounds By Andrew Gilbert 1 Introduction 2 Isomerization Reactions 3 Addition Reactions 4 Substitution Reactions 5 Cyclization Reactions 6 Dimerization Processes 7 Lateral Nuclear Shifts 8 Miscellaneous Photochemistry of Aromatic Systems References

Photooxidation and Photoreduction By Niall W. A. Geraghty 1 2 3 4 5

Introduction Reduction of the Carbonyl Group Reduction of Nitrogen-containing Compounds Miscellaneous Reductions Singlet Oxygen 5.1 Singlet Oxygen 5.2 Oxidation of Aliphatic Compounds 5.3 Oxidation of Aromatic Compounds 6 Other Oxidation Methods 6.1 Oxidation of Aliphatic Compounds 6.2 Oxidation of Aromatic Compounds 7 Oxidation of Nitrogen-containing Systems 8 Miscellaneous Oxidations References

Photoelimination By Ian R. Dunkin 1 2

Introduction Elimination of Nitrogen from Azo Compounds and Analogues

84 84

91

91 91 94 98 101 119 121 124 128

133

133 134 136 142 146 146 149 153 155 155 163 175 190 196

205 205 205

viii

Contents

3

Elimination of Nitrogen from Diazo Compounds and Diazirines 3.1 Generation of Alkyl and Aryl Carbenes 3.2 Photolysis of a-Diazo Carbonyl and Related Compounds 4 Elimination of Nitrogen from Azides and Related Compounds 5 Photoelimination of Carbon Monoxide and Carbon Dioxide 5.1 Photoelimination of CO from Organometallic Compounds 6 Photoelimination of NO and NO2 7 Miscellaneous Photoeliminations and Photofragmentations 7.1 Photoelimination from Hydrocarbons 7.2 Photoeliminations from Organohalogen Compounds 7.3 Photofragmentations of Organosilicon Compounds 7.4 Photofragmentations of Organosulfur Compounds 7.5 Photolysis of o-Nitrobenzyl Derivatives and Related Compounds 7.6 Other Photofragmentations References

Polymer Photochemistry By Norman S. Allen 1 2

3 4

Introduction Photopolymerization 2.1 Photoinitiated Addition Polymerization 2.2 Photocrosslinking 2.3 Photografting Luminescence and Optical Properties Photodegradation and Photooxidation Processes in Polymers 4.1 Polyolefins 4.2 Polystyrenes 4.3 Poly(Acrylates) and -(Alkyl Acrylates) 4.4 Poly(Vinyl Halides) 4.5 Polyamides and Polyimides 4.6 Poly(Aromatics) 4.7 Silicone Polymers 4.8 Polyurethanes and Rubbers 4.9 Polyesters 4.10 Photoablation of Polymers

207 207 210 212 215 217 218 219 219 220 223 224 225 227 228

232

232 232 233 236 243 243 262 263 263 263 264 264 264 264 264 264 265

Contents

4.11 Natural Polymers 4.12 Miscellaneous Polymers 5 Photostabilization of Polymers 6 Photochemistry of Dyed and Pigmented Polymers References

ix

265 266 266 267 268

Introduction and Review of the Year BY IAN R. DUNKIN Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, G1 1XL, Glasgow, UK

This is a very special volume of Photochemistry, because it is to be the last in the current format. It has been apparent in recent years that we are experiencing a profound change in the way in which information of all types is disseminated. In particular, more-or-less universally available on-line searching for scientific papers and data has greatly speeded up the task of searching the literature for material relevant to any area of interest. So much so, that the inevitable time delay inherent in the production of review serials, such as the Specialist Periodical Reports, now seems excessive. Moreover, the nature of some other RSC publications has changed in the past few years. Chemical Society Reviews, for example, has moved from being a vehicle for the comprehensive or historical coverage of selected topics to being more of a collection of updates on the research of individual groups. This has left a niche in the review coverage of the RSC, and the intention is to fill this with a new style of Specialist Periodical Reports. In the future, from Volume 37 onwards, Photochemistry will aim at providing a critical analysis of recently published research in photochemistry, with less of an attempt at comprehensiveness. It also is intended to emphasize applications of photochemistry in, for example, synthesis, fabrication of new devices and materials, medicine, pollution control, etc. At the same time, we must bid farewell to three reporters who have contributed indefatigably to the publication over many years. First of all, William Horspool, who has written the first three chapters on organic photochemistry – covering carbonyl compounds, enones and alkenes – for many volumes. The sum of William’s contributions represents an enormously valuable and continuous review of these topics, which will, I am sure, continue to be of benefit for many years to come. Secondly, Norman Allen has carried out the Gargantuan task of reviewing the field of polymer photochemistry, also for many years. His contribution to the present volume, containing 949 references, is no exception and well illustrates the size of this job. I am pleased to acknowledge the sustained and enthusiastic contributions made by both William and Norman, and thank them for their support during the brief time I have assumed this responsibility of editorship.

Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 1

2

Photochemistry, 36, 2007, 1–8

Deserving special recognition for his contribution is Andrew Gilbert, the third of our current contributors who is leaving at this time. Not only has he written the chapters on aromatic photochemistry and photo-oxidation and reduction for a good many years, but was also the Senior Reporter until I took over for Volume 34. He has now passed over the photo-oxidation and reduction chapter into the capable hands of Neil Geraghty, but continues with his contribution on aromatic photochemistry in the current volume. Most of all, though, Andrew merits our gratitude for maintaining both the quality and breadth of coverage of Photochemistry in his long-held position of Senior Reporter. As I am beginning to discover, finding the right authors, with the necessary expertise and the willingness to commit to writing a review annually for a considerable period, is no mean task, one which Andrew has always managed with characteristic tact and persistence. I speak therefore not only for myself but also on behalf of the RSC in thanking him for all the years of dedication which he has brought to the direction of Photochemistry. Having, with great pleasure, made these preliminary acknowledgements, I move to my selection of photochemical highlights of the period reviewed in this volume, which is as subjective and personal as ever. I continue to believe that photochemistry needs to ‘earn its living’ in the chemical world and not be just a subject of interest for a relatively closed group of specialists, a point of view which will inform the new format Photochemistry in future volumes. I have therefore looked especially to highlight work in which photochemistry has accomplished something of real utility: efficient syntheses, the manufacture of commercially useful devices, and biological, medical or environmental applications, for example. As usual, the chapter and reference numbers of the publications cited in this review can be found by using the Author Index, but I have also included the chapter numbers to aid those who wish to scan for chemical structures. A good example of a high-yielding photoreaction of synthetic utility is provided by the photoenol of the benzaldehyde (1), which can be trapped in very good yield (81%) by methyl 2-ethylacrylate, acting as a dienophile, to give the tetrahydronaphthalene derivative (2) (Nicolaou and Gray, Chapter 1). Related, intramolecular reactions of photoenols have provided routes to polycyclic carbon frameworks, such as the conversion of (3) into (4) (Nicolau et al., Chapter 1).

OMe Me

OMe OH CHO

MeO

Me OMe (1)

Me

OMe

MeO

O OMe (2)

3

Photochemistry, 36, 2007, 1–8 O

OH X

H

X H H

(3) X = CO2Et, COMe, CN

(4)

Scheffer et al. (Chapter 1) have shown how the use of ionic chiral auxiliaries as a means of immobilizing ketoacids (5) within crystals results in excellent yields of cyclized products upon irradiation, and with high ee. With (S)-(þ)1-phenylethylamine as the salt-forming amine, for example, a quantitative yield of (6) with 98% ee was obtained. The corresponding R amine similarly gave a quantitative yield of the enantiomer of (6) with an ee of 97%, and other chiral amines were also tried with success. R R

O

OH

(5) R = CO2 H3NR*

(6)

The photocycloaddition of aldehydes to the 5-methoxyxoxazole (7) has been developed by Griesbeck et al. (Chapter 1) as a path to esters of erythro-a-aminob-hydroxycarboxylic acids. The photoaddition, giving the intermediate bicyclic oxetanes (8: R ¼ Ph, 2–naphthyl, BnCH2, Et, i–Pr, i–Bu), occurs with excellent exo-diastereoselectivity (dr 4 98:2) and chemical yields are 485%. An intramolecular 2 þ 2 photocycloaddition has been exploited as the key step in the synthesis of pentacyclic terpene systems, such as the formation of (10) from (9) (de la Torre et al., Chapter 2). The yield of 63% for this example seems reasonable in view of the complexity of the system. Nishimura and co-workers (Chapter 3) have described the synthesis of a series of interesting crownopaddlanes by intramolecular photocycloadditions of the compounds (11: n ¼ 2–4), while new crownophanes (12: n ¼ 2–4) were prepared by irradiation of the corresponding divinyl derivatives. Pyridine analogues of (12) were also prepared. O N

H

H

H R

N

R

O Me

O (7)

OMe

Me

O (8)

OMe

4


O

O

H H

H (9)

(10)

HO2C O

O

CO2H O

O O

O n

n

(11)

(12)

The irradiation of CBr4 in MeOH has been reported as an efficient photochemical means of removing protecting groups (Chen et al., Chapter 3). Examples are the release of (13b) from (13a) in 89% yield, and (14b) from (14a) in 86% yield.

RO RO

OAc O

OR O

O (13) a: R-R = CMe2 b: R = H

AcO AcO

O AcO OMe

(14) a: R = Tf b: R = H

Potentially useful synthetic reactions may not always proceed in high yields. The intramolecular meta photocycloadditions of (15: X ¼ CH2, SiMe2; R1, R2, R3 ¼ H, Me), for example, gave the corresponding 1,6-bridged dihydrosemibullvalene adducts (16) in yields of only 7–19%, but starting with relatively simple precursors provide an approach to complex molecular systems not readily accessible by conventional means (Penkett et al., Chapter 4).

5

Photochemistry, 36, 2007, 1–8 R1 X

O

O

X

O

O R

3

R1 R2

R3 R2 (15)

(16)

Photocyclizations of mono- and distyryl substituted polynuclear arenes in the synthesis of helicenes have been reported by El Abed et al. (Chapter 4). In this work, irradiation of toluene solutions of (17: R ¼ H, OMe, CN, Me, OH) and (18: R1, R2 ¼ H, OMe, Me) with iodine in the presence of propylene oxide as an HI scavenger, afforded 62–96% and 70–90% of the [5]- and [7]carbohelicenes (19) and (20), respectively, depending on the substituents. A remarkable enhancement has been observed for the photocyclizations of 1allyloxy- and 1-allylamino-2-halogenoarenes, giving dihydrofurans and indoles respectively, when an enolate ion is added as an entrainment reagent (Vaillard et al., Chapter 4). The dihydrofuran (22), for example, was obtained in 55% yield from irradiation of (21) but this was increased to 91% when the CH3COCH2 ion was present; and similarly under the latter conditions the yield of (24) from (23) was greater than 96%.

R1

R

Br (17)

R2 (18)

R1 Br R R2 (19)

(20)

6

Photochemistry, 36, 2007, 1–8 Me Br

hν O

O (21)

(22)

Me Br

hν N

N

(23)

(24)

A microreactor has been described, based on SiO2 capillaries with the inner walls covered with TiO2-coated colloidal SiO2 particles (Li et al., Chapter 5). The reduction of methylene blue was carried out by injecting a solution of the reactant through the microcapillary using a syringe pump, while irradiating at 254 nm, and a 150-fold increase in the reduction rate was obtained relative to a comparable batch system. The potential of using ceramic-based nanoparticles as carriers of photosensitizers for photodynamic therapy (PDT) has been described (Roy et al., Chapter 5). As an example, the anticancer drug 2-devinyl-2-(1-hexyloxyethyl) pyropheophorbide (25) has been encapsulated in the non-polar core of micelles by hydrolysis of triethoxyvinylsilane. The resulting particles were uniform in size, having an average diameter of 30 nm, and were stable in an aqueous medium. The uptake of the nanoparticles by tumour cells, and significant cell death following 1O2 generating irradiation at 650 nm, was then demonstrated in vitro. The photooxygenation of trans-8-(acetyloxy)bicyclo[4.2.0]octa-2,4-dien-7-yl acetate (26) was a key step in the stereospecific synthesis of a new inositol analogue (27) (Kara and Balci, Chapter 5). Me

O(CH2)5CH3 Me

Me

Me NH

N

N

HN

Me

Me

O CO2H (25)

7


OH H

OAc 1O

H

2,

CCl4

O

OAc

H

OAc

HO

H

OAc

HO

H

OH

H

OH

O HO

(26)

(27)

The trifluoromethyl group has long been considered as having a low tendency to migrate to a carbene centre, and this seemed to explain why, for example, photolysis of the diazoester (28) results in hydrogen abstraction to give (29), rather than rearrangement. An investigation of the photolysis of a 13C-labelled isotopomer of (28), however, has now shown that an estimated 93.5% of the rearranged product (30) was produced by trifluoromethyl migration (Haiss and Zeller, Chapter 6). Meldrum’s acid (31) undergoes isomerization to the corresponding diazirine when irradiated at 355 nm, but loss of N2 and Wolff rearrangement at 254 nm; the two processes clearly occur from two different electronic excited states (Bogdanova and Popik, Chapter 6). O

O

O

H

H

F3C N2

O F3C

* C

OEt

(28)

(29)

CF3 (30)

O O N2 O O (31)

A very good example of the enormous benefits that DFT computations of IR spectra have brought to matrix-isolation studies – giving the technique a veritable new lease of life in the study of organic reactive intermediates – has been provided by a study of the photolysis of a-pyrone (32) and its 4,6-dimethyl derivative (Breda et al., Chapter 6). The photochemistry of a-pyrone was the subject of some of the earliest matrix-isolation studies of organic species, but the use of DFT computations has now allowed a virtually complete identification of the individual rotamers of the ring opened aldehyde-ketene (33)–(36).

8


C O

O

O

O

C

(33)

O

C

O

(34)

O

C O

(32) O O (35)

(36)

Wavelength dependent differential release of compounds from a solid-phase resin has been demonstrated, using beads bifurcated with nitroveratryl (37) and pivaloyl glycol (38) photo-linkers (Ladlow et al., Chapter 6). A narrow bandwidth tuneable pulsed laser was used for photolysis, and it was found that the nitroveratryl linker undergoes cleavage over a wide range of wavelengths, with maximum cleavage rates at 320 and 340 nm, while the pivaloyl glycol linker is photo-stable at wavelengths above 340 nm. O

O HO

O

MeO (37)

OTMS

NO2 OH

O HO

O

OTBDMS

(38)

Finally, it must be remarked that, as is so often the case, the real highlight in polymer photochemistry (Chapter 7) for the review period is the sheer volume of work in this field. Polymer photochemistry continues to be an active area of applied photochemistry, with many topics growing in commercial and industrial importance. New materials are constantly emerging from studies of photopolymerization, photocuring and photocrosslinking, e.g. polymers with useful mechanical and electronic properties and liquid crystalline materials. Last year, for example, saw a virtual literature explosion in LEDs (light emitting diodes), and in this year’s review it still represents one of the largest specialized topics in photochemistry and photophysics.

Photolysis of Carbonyl Compounds BY WILLIAM M. HORSPOOL Dundee, UK

There is little doubt that photochemically induced single electron-transfer (SET) processes are becoming of greater importance as the years pass and our understanding of them increases. As a consequence, the area has attracted exponents to compile reviews such as those by Hasegawa,1 who has reviewed the application of photochemically induced SET reactions to organic molecules, and Mattay and co-workers,2 who have reviewed the subject from preparative and mechanistic standpoints. Another area that attracts much attention is that of photochemical reactions within constrained media, and reviews of this directed at reactions within zeolites have been published.3,4 The behaviour of benzyl radials formed by the photochemical decomposition of dibenzylketone in NaY zeolites has been monitored using 4-(3-hydroxy2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl free radical as a probe.5 Cyclodextrins also provide a useful constraining medium for reactions, and b-cyclodextrin has been used in a study of the photochemical behaviour of variously substituted ketones (1) attached to peptide links. The results show that carbon-carbon bond formation is the outcome.6

The use of the XeCl excimer 308 nm radiation system has been described in its application to scalable photochemical reactions of ketones.7 Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 9

10

1


Norrish Type I Reactions

a-Fission processes are commonly brought about by the irradiation of aldehydes and ketones. The simplest example of these, formaldehyde, is no exception, and studies have shown that the rate of photodissociation of formaldehyde in a xenon matrix is greater than that in other matrices at a variety of wavelengths.8 a-Fission into methyl and formyl radicals results on irradiation of acetaldehyde at 308 nm in the gas-phase.9 Calculations have been used to analyse the photofragmentation of crotonaldehyde.10 The Rydberg state of acetone was studied by irradiation at 195 nm.11 An evaluation of the photodissociation of acetone at a series of wavelengths has been published.12 Wu and co-workers13 have reported the results of calculation on the low-lying pathways for the photodissociation of hydroxyacetone. The reactions encountered are Norrish Type I processes and involve the fission of either of the a-bonds. Albini and his co-workers14 have studied the Norrish Type I activity of the hydroxyalkyl ketone moiety of triamcinolone (2).

As mentioned earlier in this chapter, irradiations within zeolites continue to be of interest, and one study has shown that the irradiation of 1-naphthylacylates in NaY zeolites results in the formation of a single product. The selectivity in this reaction is in contrast to the reaction of the same ketones in solution when many products are formed. The authors suggest that the selectivity is due to restriction around the biradical formed initially.15 A review article has highlighted the control that can be exercised with cations on the photochemistry of ketones and other substrates contained within zeolites.16 The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone.17 Calculations have shown that decarbonylation of cyclobutanone occurs from the np* triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.18 Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed.19 Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5).20


11

Irradiation of the ketone (6) in argon-degassed cyclohexane brings about Norrish Type I fission. In this case decarbonylation does not result and the triplet biradical formed by the fission affords the aldehyde (7). The enol (8) is also a product of this irradiation.21 An aldehyde is also the principal product on irradiation of (9) in benzene. The a-fission affords the aldehyde derivative (10) in 90% yield.22 Analogous behaviour is observed on irradiation of bicycloheptanone (11) to afford an aldehyde that was a key intermediate in the synthesis of dimethyl secologanoside.23

2

Norrish Type II Reactions

2.1 1,5-Hydrogen Transfer. – Both experimental and theoretical approaches have been used to study the reactivity of n-butyrophenone included in alkalimetal-exchanged zeolites. The results indicate that with smaller cations the Norrish Type I process is enhanced over the Norrish Type II reaction.24 Others have reported that the photochemical decomposition of n-butyrophenone in a variety of solvents follows first-order kinetics.25 a-Chlorovalerophenone undergoes a normal Norrish Type II cyclization path on irradiation. Interestingly, the corresponding a-bromovalerophenone undergoes only C–Br fission on irradiation.26 The ketone (12) undergoes

12


Norrish Type II hydrogen abstraction to afford the usual biradicals, which can cyclize into cyclobutanols. Both the cis-(13) and the trans-isomeric forms are possible. This particular investigation has studied the influence of antibodies (12B4, 20F10 ad 21H9) on the cyclization reaction. The authors observed that the most reactive antibody, 20F10, catalyses the formation of the cis-product (13).27

The benzaldehyde derivative (14) undergoes Norrish Type II hydrogen abstraction with the formation of a photoenol. This enol can be trapped efficiently (81% yield) using methyl 2-ethylacrylate as the dienophile, to afford the tetrahydronaphthalene derivative (15).28 A detailed analysis of intramolecular versions of the addition to photoenols has been described. The method provides a path to polycyclic carbon frameworks such as the conversion of (16) into (17). Examples are also reported using a four-carbon chain separating the dienophile from the photoenol.29 Bach and co-workers30 have demonstrated that irradiation of the aldehyde (18) affords the corresponding o-quinodimethane derivative by a Norrish Type II process. 1,4-Biradicals are formed on irradiation of [4-(11-mercaptoundecyl)phenyl](2-methylphenyl)methanone as a monolayer. The biradicals collapse to yield photoenol intermediates that can be trapped in a Diels-Alder reaction.31 A study of the photochemical behaviour of the pyridyl aldehydes (19) has reported that irradiation brings about colour changes. Only the derivative (19e) undergoes Norrish Type II hydrogen abstraction with formation of the corresponding cyclobutene derivative.32


13

The Norrish Type II reactivity of the acetophenone derivatives (20) has been exploited as a new photoremovable protecting system for carboxylic acids. The irradiation affords the usual 1,5-biradicals that then release the acids (21) in the yields shown in parenthesis. Irradiation times are short.33 Klan and coworkers34 have described the photochemical reactivity of 1,5-dimethylphenacyl phosphoric and sulfonic esters.

Chong and Scheffer35 have examined the photochemical and thermal reactions of the ketonic carboxylates (22). The photoreactions of the carboxylate salts are carried out in the crystalline phase, and the hydrogen abstraction reaction and ring opening of the cyclopropyl ring results in the formation of (23) with almost quantitative ee. The thermal reactions also afford the same product but with much lower ee. Scheffer et al.36 have also described the use of ionic chiral auxiliaries as a means of immobilizing ketoacids (24) within crystals. The irradiation of these gives excellent yields of either of the cyclized products (25) and (26). Thus, using (S)-(þ)-1-phenylethylamine as the amine salt affords a quantitative yield of (25) with 98% ee. The corresponding (R)amine again gives a quantitative yield of product but this time the products is (26) with an ee of 97%. The same enantiomer (26) is obtained using (1S,2R)(–)-1-amino-2-indanol with an ee of 96%, while (1S,2S)-(þ)-2-amino-3methoxy-1-phenyl-1-propanol affords (25) with an ee of 95%.

14


Norrish Type II hydrogen abstraction is the predominant reaction on irradiation of the silylated ketones (27). This affords the dealkylated product (28). There is some Norrish Type I reactivity that results in the formation of the isomerized product (29) and the two ring-opened products (30) and (31). The ratio of the two reactions varies with the silyl group, with a 32:1 ratio of (28):(29) obtained from (27, R ¼ Me) and a 13:1 ratio from (27, R ¼ Ph or R3 ¼ MePh2).37

15


2.2 Other Hydrogen Transfers. – A full account of the photochemical reaction of ketones with leaving groups adjacent to the carbonyl function has been published.38 This study provides a route to a variety of di- and tri-substituted cyclopropyl ketones. Calculations have been carried out on the photobehaviour of a-substituted butyrophenones to establish a mechanism whereby cyclopropane systems can be formed.39 The photochemical behaviour inducing a hydrogen transfer reaction of 2-(o-tolyl)benzofuran-3-one has been studied.40

3

Oxetane Formation

Griesbeck41 has published an account of how stereoselectivity in single and triplet addition reactions to afford oxetanes is linked to spin selectivity. A study of the stereo- and regioselectivity of oxetane formation by addition of aldehydes to substituted furans (Scheme 1) has been carried out. The total yield of adducts is high at 95%. The reactions are regio-random but stereoselective, affording an exo:endo ratio of 97:3 for the addition of benzaldehyde. The authors42 argue that the 1,4-biradicals formed in the addition reaction are the key to explaining the photochemical addition. D’Auria et al.43 have shown that the addition of benzaldehyde to furan affords the exo-adduct selectively. This selectivity is explained on the basis of adduct stability. The cycloaddition of aldehydes to the 5-methoxyxoxazole derivatives shown in Scheme 2 has been developed as a path to esters of erythro-a-amino-b-hydroxycarboxylic acids. The photoaddition occurs with excellent exo-diastereoselectivity. The dr obtained is 498:2 and chemical yields are 485%.44 Intramolecular oxetane formation has been examined in the sclareolide system. Typical examples are the photocyclizations of the substituted keto derivatives (32) and (33). The yields obtained are high in this series.45

Scheme 1

16


Scheme 2

The sensitizer dependency for the cycloreversion of trans,trans-2,3-diphenyl4-methyloxetane has been studied. When chloranil is used as the sensitizer, the reaction proceeds via the radical cation of trans-b-methylstyrene, while with pyrylium salts the trans-stilbene radical cation is involved.46 Other work in this area has examined the cycloreversion of the oxetanes (34) using (35) or chloranil as the sensitizers.47


4

17

Miscellaneous Processes

A mixture of carbohydrates is formed on irradiation of formaldehyde at 77 K. The authors48 suggest that hydroxymethylene formation is the key to this and that addition of this intermediate to formaldehyde yields glyoxaldehyde. The photodecomposition of 4-(6-methoxy-2-naphthyl)butan-2-one (nabumetone) in water probably involves the formation of the nabumetone radical cation. This leads to the formation of 6-methoxy-2-naphthalene carboxaldehyde.49 Further study has examined the photodegradation of this ketone in n-butanol where it was shown that a first-order degradation took place. An excited singlet state is involved, and the author50 proposes that both concentration and hydrogen bonding are important in this solvent. Irradiation of 4-methyl-5-p-anisyl substituted N-alkoxythiazolethiones brings about N–O bond homolysis with the formation of alkoxy radicals. This technique has been applied to the synthesis the compound (36).51 The aldehyde (37) undergoes addition to dimesitylsilene when it is irradiated at –571C in hexane. The product, obtained in 76% yield, was identified as (38).

4.1 Decarboxylation and Decarbonylation. – The potential energy surfaces for the dissociation of formic acid have been determined by ab initio methods.52 The photochemical dissociation of simple amides such as formamide, acetamide and N-methylacetamide has been investigated using CASSCF/MRSDCI single point calculations.53 Mori et al.54 have studied the decarboxylation of (39) under a variety of conditions and have found that the conversion to (40) occurs without the involvement of radicals. They suggest that the process is a concerted cheletropic extrusion via the s-cis conformation. A further study55 has examined the photodecarboxylation of the (S)-ester (39) in unstretched-polyethylene films. The decarboxylation affords (40) with complete retention of the stereochemistry. The yield of product is 98% and the ee is 498%. The photochemical behaviour of the ester in other confining media such as cyclodextrins indicates that cage-escape products are also formed. The irradiation of grandifloric acid (41) at 254 nm in acetonitrile brings about decarboxylation with the formation of epimers. In methanol a different reaction occurs that results in the conversion of the C-methyl group into a carbomethoxy substituent.56

18


The photoreactivity of indoprofen is centred on the propionic acid moiety within the molecule.57 The salt of ketoprofen (42) is known to undergo decarboxylation to afford the anion (43). The present study has demonstrated that, under carefully controlled conditions in THF, the lifetime of the carbanion can be extended to many hours.58 Roberts and Pincock59 report that a carbocation is formed on irradiation of the acetate (44) in 2,2,2-trifluoroethanol and methanol.

Pyruvic acid undergoes elimination of an OH radical on irradiation at 193 nm, a process involving the T1 excited state.60 Calculations have shown that photodissociation of formylcyanide is unlikely to occur from the excited singlet state.61 4.2 Reactions of Miscellaneous Haloketones and Acid Chlorides. – UV irradiation of fluorocarbonyl iodide results in the formation of (OCIF). . .I and (OCFI). . .F complexes,62 while ab initio methods have been used to interpret the photoreactivity of bromoacetyl chloride.63 A further study on the latter system has shown that the C–Cl bond ruptures on irradiation at 248 nm.64


19

a-Iodoketones (45) undergo facile conversion to the corresponding ahydroxyketones (46) in the yields shown.65 Morrison and his co-workers66 have described evidence for the interaction between the ketone and the CBr moieties in 17a-bromo-3a-(triphenylsiloxy)-5a-androstan-6-one.

4.3 Other Processes. – The quantum yield for the release of benzoic acid from 2,5-dimethylphenacyl benzoate is temperature dependent in benzene solution. At room temperature f ¼ 0.22, while at 501C the value rises to 0.28. A much greater effect is observed in methanol or ethanol, when there is a threefold increase in the quantum yield. The authors67 suggest that the reaction in heated methanol enhances the E-photoenol population. Givens and Lee68 have reviewed the use of the p-hydroxyphenacyl moiety as a photoprotecting group for biological substrates. 1- and 2-Naphthoyloxyl radicals can be formed by irradiation of the 2pyridone derivatives (47) and (48), respectively. Apparently the presence of a methoxy group in the ring prevents decarboxylation.69

The SET-induced ring opening of the cyclopropyl moiety in (49) results in its conversion to the mixture of products (40:3) shown in Scheme 3. The best results are obtained using a mixture of MeCN/5 equivalents Et3N/1 equivalent LiClO4. These conditions were also applied to (50) that ring-opens to yield (51).70

20


Scheme 3

References 1. (a) E. Hasegawa, Kokagaku, 2003, 33, 220; (b) E. Hasegawa, Chem. Abstr., 2003, 139, 36091. 2. P. Schmoldt, H. Rinderhagen and J. Mattay, Mol. Supramol. Chem., 2003, 9, 185. 3. J. Sivaguru, J. Shailaja and V. Ramamurthy, in: Handbook of Zeolite Science and Technology, ed. S.M. Auerbach, K.A. Carrado and P.K. Dutta, Marcel Dekker, New York, 2003, p. 515. 4. S. Hashimoto, J. Photochem. Photobiol. C: Photochem. Rev., 2003, 4, 19. 5. (a) A. Aspee, L. Maretti and J.C. Scaiano, Photochem. Photobiol. Sci., 2003, 2, 1125; (b) A. Aspee, L. Maretti and J.C. Scaiano, Chem. Abstr., 2004, 140, 198945. 6. V. Jullian, F. Courtois, G. Bolbach and G. Chassaing, Tetrahedron Lett., 2003 44, 6437. 7. (a) A.G. Griesbeck, N. Maptue, S. Bondock and M. Oelgemoeller, Photochem. Photobiol. Sci., 2003, 2, 450; (b) A.G. Griesbeck, N. Maptue, S. Bondock and M. Oelgemoeller, Chem. Abstr., 2003, 139, 149181. 8. K.J. Vaskonen and H.M. Kunttu, J. Phys. Chem. A., 2003, 107, 5881. 9. M. Cordeiro, D.S. Natalia, E. Martinez-Nunez, A. Fernandez-Ramos and S.A. Vazquez, Chem. Phys. Lett., 2003, 375, 591. 10. (a) M.-C. Wang, H.-l. Guo and B.-Z. Yan, Beijing Huagong Daxue Xuebao, Ziran Kexueban, 2003, 30, 59; (b) M.-C. Wang, H.-l. Guo and B.-Z. Yan, Chem. Abstr., 2004, 140, 286875. 11. W.-K. Chen, J.-W. Ho and P.-Y. Cheng, Chem. Phys. Lett., 2003, 380, 411. 12. E. Martinez-Nunez, A. Fernandez-Ramos, M.N.D.S. Cordeiro, S.A. Vazquez, F.J. Aoiz and L. Banares, J. Chem. Phys., 2003, 119, 10618. 13. Y. Wu, D. Xie and Y. Yue, J. Comput. Chem., 2003, 24, 931. 14. G. Miolo, A. Ricci, S. Caffieri, L. Levorato, E. Fasani and A. Albini, Photochem. Photobiol., 2003, 78, 425. 15. M. Warrier, L.S. Kaanumalle and V. Ramamurthy, Can. J. Chem., 2003, 81, 620. 16. V. Ramamurthy, J. Shailaja, L.S. Kaanumalle, R.B. Sunoj and J. Chandresekhar, J. Chem. Soc., Chem. Commun., 2003, 1987.


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53. 54. 55. 56.

X.B. Chen, W.H. Fang and D.C. Fang, J. Am. Chem. Soc., 2003, 125, 9689. T. Mori, H. Saito and Y. Inoue, J. Chem. Soc., Chem. Commun., 2003, 2302. T. Mori, Y. Inoue and R.G. Weiss, Org. Lett., 2003, 5, 4661. S. Buscemi, S. Rosselli, M. Bruno, L. Scaglioni, N. Vivona and F. Piozzi, J. Photochem. Photobiol. A: Chem., 2004, 163, 381. V. Lhiaubet-Vallet, J. Trzcionka, S. Encinas, M.A. Miranda and N. Chouini-Lalanne, Photochem. Photobiol., 2003, 77, 487. M. Laferriere, C.N. Sanrame and J.C. Scaiano, Org. Lett., 2004, 6, 873. J.C. Roberts and J.A. Pincock, Can. J. Chem., 2003, 81, 709. S. Dhanya, D.K. Maity, H.P. Upadhyaya, A. Kumar, P.D. Naik and R.D. Saini, J. Chem. Phys., 2003, 18, 10093. (a) W.-J. Ding, W.-H. Fang and R.-Z. Liu, Chem. Phys. Lett., 2003, 369, 570; (b) W.-J. Ding, W.-H. Fang and R.-Z. Liu, Chem. Abstr., 2003, 138, 368357. M.S. Chiappero, G.A. Arguello, P. Garcia, H. Pernice, H. Willner, H. Oberhammer, K.A. Peterson and J.S. Francisco, Chem.-Eur. J., 2004, 10, 917. M.-C. Bacchus-Montabonel, N. Vaeck, B. Lasorne and M. Desouter-Lecomte, Chem. Phys. Lett., 2003, 374, 307. B. Lasorne, M.-C. Bacchus-Montabonel, N. Vaeck and M. Desouter-Lecomte, J. Chem. Phys., 2004, 120, 1271. C.A. Horiuchi, A. Takeda, W. Chai, K. Ohwada, S.J. Ji and T.T. Takahashi, Tetrahedron Lett., 2003, 52, 9307. W.S. Li, L. Torun and H. Morrison, Can. J. Chem., 2003, 81, 660. J. Literik, S. Relich, P. Kulhanek and P. Klan, Molecular Diversity, 2003, 7, 265. (a) R.S. Givens and J.-I. Lee, J. Photosci., 2003, 10, 37; (b) R.S. Givens and J.-I. Lee, Chem. Abstr., 2004, 140, 217043. T. Najiwara, J. Hashimoto, K. Segawa and H. Sakuragi, Bull. Chem. Soc. Jpn., 2003, 76, 575. N. Tzvetkov, M. Schmidtmann, A. Muller and J. Mattay, Tetrahedron Lett., 2003, 44, 5979.

57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70.

Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY WILLIAM M. HORSPOOL Dundee, UK

1

Cycloaddition Reactions

Kakiuchi1 has reviewed some recent advances in enantioselective (2 þ 2)- and (2 þ 4)-photocycloaddition reactions in solution. Particular attention is given to the use of chiral host molecules and chiral auxiliary groups. 1.1 Intermolecular Cycloadditions 1.1.1 Open-Chain Systems. The influence of hydrogen bonding on the outcome of photochemical reactions between hydroxy derivatives of substituted chalcones has been studied in the crystalline phase.2 The reaction in Scheme 1 has been examined to establish the difference between exciplex and CT excitation. The results obtained from the two excitation paths are quite different, with different yields and different de values. For example, direct irradiation involving the exciplex in toluene at 501C yields the two products in 6% and 5% yields, respectively. The de values are 53 and þ1. Charge-transfer excitation, however, at the same temperature affords yields of 4% and 10% with de values of þ52 and 9.3

Scheme 1

The photochemical (2 þ 2)-dimerization between trans-cinnamates contained within a photocrosslinkable dendrimer has been reported. The cycloaddition is accompanied by trans-cis isomerism.4 The b-form of trans-2,4-dichlorocinnamic acid dimerizes on irradiation to give the b-truxinic acid derivative in a first-order kinetic process.5 Others6 have studied the influence of substituents on the aryl ring of cinnamic acid, with a view to control crystal engineering. Ito et al.7 have reported the photodimerization in the solid state of cinnamic acids (1) as their Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 23

24


ammonium salts. This treatment yields mainly the b-(2) and d-(3) truxinic dimers. Imidazole salts are also reactive in this process. The control of the dimerization of cinnamates has been demonstrated using 5,5-dihexylbarbituric acid as a template. A marked efficiency of dimerization was observed.8

The photochemical addition of methyl 2,3-dioxopentanoate to terpinolene provides (2 þ 2)-cycloadducts that can undergo retro-aldol ring opening to afford cyclohexane derivatives.9 The enol of acetylacetone undergoes photoaddition to the isocarbostyril (4) resulting in the formation of two (2 þ 2)cycloaddition products that undergo retro-aldol ring opening (Scheme 2). The first of these ring-opened compounds can be cyclized to yield the enone (5). An intramolecular version of this photocycloaddition has also been studied using the derivative (6). Here again two adducts (7) and (8) are produced on

Scheme 2

25


irradiation, and the former (7) can also be cyclized to yield the galanthan ring system shown as (9). The (2þ2)-photodimerization within crystals of 5-benzylidene-2-(4-chlorobenzyl)cyclopentanone brings about changes within the crystal structure as the dimerization progresses.10 (2þ2)-Cycloaddition takes place on irradiation of the enone (10) providing (11), which has been used as a key intermediate in a synthesis of pentacyclic terpene systems.11

Photocycloaddition of dimethyl cyclobut-1-ene dicarboxylate to cyclohexene affords the two products shown in Scheme 3. The former of these adducts, obtained in 44% yield, is thermally unstable and undergoes ring opening to yield (4Z,10Z) dimethyl cyclodeca-4,10-diene-1,4-dicarboxylate.12

Scheme 3

1.1.2 Intermolecular Additions to Cyclopentenones and Related Systems. Mehta and co-workers13 have described the photochemical addition of 1,2dichloroethene to the enone (12). This affords the adduct (13), which can be transformed into the protoilludane-type molecule shown in Scheme 4. A further study related to the (2 þ 2)-cycloaddition reactions of compounds such as (14) and (15) has determined the crystal structures of these compounds. The

26


Scheme 4

structures obtained are claimed to be in good agreement with the diastereofacial selectivity observed for these compounds.14

1.1.3 Intermolecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 þ 2)-cycloaddition reactions of cyclohex-2-enone.15 Diastereoselective (2 þ 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%.16 The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at 781C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%.17 Addition of 1,1-diethoxyethene to the cyclohexenone (18, R ¼ Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cis/trans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R ¼ Ph) in acetone at 31C, a c/t ratio of 4.4 is obtained, and this changes to 2.1 at 401C. In acetonitrile the c/t ratio is only 1.9 at the same temperature. With the other derivative of (18), the c/t ratio is 1.3 in acetone and 0.8 in acetonitrile.18 Photoaddition of ethene to the enone carboxylate [21, R* ¼ ()-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at 781C. The diastereoselectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene.19


27

Margaretha and co-workers20 have described the cycloaddition of some cyclohex-2-enones to acrylonitrile. The addition reactions show moderate regioselectivity and form mixtures of exo- and endo-5-oxobicyclo[4.2.0]octane-7-carbonitriles. Madhavan and Pitchumani21 have reported the dimerization of 2-cyclohexenone confined in clay interlayers (cation-exchanged bentonite). The reaction is remarkably regioselective and affords the headto-head dimer almost exclusively. The cyclic alkene (24) undergoes photochemical addition to enones to afford the adducts (25) and (26) in the yields shown.22

28


A patent has been lodged dealing with the photochemical addition of styrene to the substituted benzopyranone (27, W ¼ CO2Et, R1–R4 ¼ H), which results in the formation of the adduct (28) in high yield. This is part of a larger study of the cycloaddition of such compounds.23 Lehn and co-workers24 have described the dimerization of some coumarins (29) as 2:1 complexes with receptor molecules such as (30). The photochemical dimerization within these complexes affords, for example, the dimer (31), selectively. The photodimerization of the 6-alkylcoumarins (32) has been examined in solvents and micelles. With the 6methyl derivative the anti head-to-head dimer (33) was formed exclusively on triplet sensitization in non-polar solvents. The syn head-to-head dimers (34) were formed in ionic micellar conditions. Coumarin derivatives with longer alkyl groups at C-6 also lead to the photo-dimers.25 The triplet-state properties of coumarin and a series of 6-alkyl derivatives have been studied.26 The coumarin moieties attached to a pentacyclo[9.5.1.13,9.15,15.17,13]octasilane with eight coumarin groups undergo intermolecular (2 þ 2)-additions, resulting in the linking of the monomeric units.27 7-Chlorodimethylsilanoxy-4-methyl coumarin undergoes (2 þ 2)-photocycloaddition on irradiation at 310 nm within a copolymer containing styrene units.28 Flavone derivatives have been shown to undergo conversion into 2,2 0 -biflavanones on irradiation in the presence of triethylamine.29


29

1.2 Intramolecular Cycloadditions. – (2 þ 2)-Photocycloaddition within the chalcones (35) results in the formation of cyclobutane derivatives. The process can be brought about using sunlight irradiation of solutions in chloroform, but better yields (ca. 85%) can be achieved using l ¼ 350 nm.30

1.2.1 Intramolecular Additions to Cyclopentenones and Related Systems. The unsaturated lactones shown in Scheme 5 can undergo photocycloadditions

30


Scheme 5

under the conditions described, to afford the intramolecular adducts. The yields of product are all reasonable.31 Acetone-sensitized irradiation of (36) affords the adduct (37) in 70% yield. This product is suggested as a key synthetic intermediate in a synthesis of the cyclobutanone core of solanoeclepin A.32

1.2.2 Intramolecular Additions to Cyclohexenones and Related Systems. 3– Styrylcyclohex-2-enone undergoes photochemical cyclization to afford 2,3dihydro-[1H]-phenanthrene-4-one.33 Work carried out on the photostability of curcumin has also examined the photolability of the chalcone (38). This undergoes cyclization to (39) on irradiation.34


2 2.1

31

Rearrangement Reactions a,b-Unsaturated Systems

2.1.1 Isomerization. Sunlight irradiation of E-octyl p-methoxycinnamate results in conversion to the Z-isomer.35 The photochemically induced conformational changes of enolic (trifluoroacetyl)acetone have been studied in detail.36 The ring-opening reactions of (40) and (41), in the presence of chloranil and (42) as electron-transfer sensitizers, has been studied. Enone (40) reacts only under chloranil sensitization and affords the two products (43) and (44) after 10 min irradiation in methylene chloride. Compound (41) affords (45) with chloranil and (46) under sensitization with (42). It is clear from these studies and from the types of products formed that radical cation intermediates are involved.37,38

The photoisomerization of retinal has been studied using super-short light pulses,39 and the neutral and protonated forms of the retinal analogue (47) have been examined by time-resolved methods.40 The irradiation of b-ionone as a complex with b-CD in water leads only to the formation of retro-g-ionone.41

Time-dependent functional theory has been used to study the gas-phase isomerism of urocanic acid (48).42 The photoisomerism of b-nor-5,10-secosteroidal ketones has been studied.43

32


2.1.2 Hydrogen Abstraction Reactions. The photo-deconjugation of the diacetone D–glucose derivatives (49) results in reactions that generally provide products (50) and (51) with high de.44

Quantum mechanical studies have been carried out on the hydroxychalcone (52) in an investigation of the one-way isomerization of the system.45 Kaneda and Arai46 have described the photochemical hydrogen-transfer reactivity of the enone (53).

2.1.3 Addition and Cyclization Reactions. Benzocyclobutenone undergoes ring opening on flash photolysis, to afford 6-methylene-2,4-cyclohexadienylketene.47 A report has given details of the highly selective sequential addition and cyclization reactions illustrated in Scheme 6. The reactions in Scheme 6 show the results with t-butyl acrylate, but other acrylate derivatives are also effective.48 Hoffmann and Goerner49 have studied the electron transfer from

Scheme 6


33

N-methylpyrrolidine to a number of ketones. When these intermediates are generated in the presence of (5R)-5-menthyloxyfuran-2[5H]-one addition takes place to the double bond of the furanone. The adducts obtained from the addition of vinylene carbonate to such homochiral furanones have been used in a new stereoselective synthesis of butyrolactones of lyxofuranose.50 2.1.4 Miscellaneous Reactions. A study of the photodissociation of ketene under jet-cooled conditions in the 193–215 nm wavelength range has shown that bond fission to afford H atoms is a principal reaction.51 Acryloyl chloride undergoes C–Cl bond fission on irradiation at 193 nm.52 The photodecarbonylation of the cyclopropenones (54) occurs quantitatively on irradiation in methanol solution to yield the corresponding alkynes. The quantum yield for decarbonylation for the alkyl substituted derivatives is in the range 0.2–0.3, while for the aryl derivatives f ¼ 0.7.53 A determination of the reaction dynamics for the photodissociation of diphenylcyclopropenone has shown that the dissociation occurs from the S2 state.54

Albini and co-workers55 have described the photochemical behaviour of bicyclo[3.1.0]hexenones such as (55). The irradiation brings about the ringopening of the three-membered ring, and the resultant zwitterions are trapped by the adjacent hydroxyl group to afford (56). Several examples were described.

Several derivatives of (57) have been studied as phototriggers for alcohols and phenols. All of them are reactive, with varying quantum yields for the release of the alcohol component from decomposition of the carbonate moiety. The best of these was found to be (58), which releases phenol with a quantum yield of 0.067.56 Dore and co-workers57 have described the photochemical activity of the related coumarin derivative (59). Irradiation brings about release of the aldehyde or ketone unit from the side chain affording (60).

34


2.2 b,c-Unsaturated Systems 2.2.1 The Oxa-Di-p-methane reaction and Related Processes. Interestingly, some b,g-unsaturated ketones do not undergo 1,3-acyl shifts or the oxa-di-pmethane rearrangement. An example of this is compound (61), which on irradiation undergoes only a Norrish Type II hydrogen abstraction.58 The direct irradiation of the bicycloenones (62) results in a 1,3-acyl shift followed by decarbonylation.59 The photochemical and thermal reactivity of so-called o- and p-acylcyclohexadienones has been studied in a further attempt to examine the mechanism of the photo-Fries process.60

Acetophenone-sensitized irradiation of the quinuclidinones (63) brings about an oxa-di-p-methane rearrangement to afford (64) in 70% yield.61 Singh and Lahiri62 have reported the odd photochemistry of the enone (65, R ¼ H, R1 ¼ Et). On irradiation of this compound at 300 nm in acetone, only a low yield of


35

the oxa-di-p-methane product (66) is obtained. The dominant product is (67), formed by a 1,3-acyl migration. Interestingly, in the irradiation of (66) again in acetone but using 254 nm light, the yield of the oxa di-p-product rises to 48%. The rearrangement is also observed with (65, R ¼ R1 ¼ Me), which yields the corresponding product (66). The rearrangement product (68) is formed on acetone-sensitized irradiation of the bicyclo[2.2.2]octenone (69). The rearrangement is a standard 1,2-acyl shift of the oxa-di-p-methane type. The product is obtained in 42% yield and has been used in an approach to the synthesis of hirsutic acid.63 The oxa-di-p-methane rearrangement of the enediones (70) has been studied in detail. The reactions shown in the Scheme 7 are typical of such rearrangements, and the regiospecificity shown is determined by the stability of the intermediate biradicals.

3

Photoreactions of Thymines and Related Compounds

3.1 Photoreactions of Pyridones. – The a-fission of the pyrimidone (71) has been studied in a low temperature matrix. Irradiation under these conditions leads to the formation of a conjugated isocyanate.64 A study has reported on the control by hydrogen bonding of dimerization of pyridones such as (72).65a A patent has been lodged dealing with the dimerization of 2-pyridone as an inclusion compound with diphenic acid. This results in the formation of the trans-anti-dimer (73) in 92% yield.65b The pyridone derivatives (74) also undergo photochemical (4 þ 4)-dimerization to (75) on irradiation in the solid

36


Scheme 7

phase. Contrary to this behaviour is that observed in benzene when the rearrangement product (76) is formed.65c

3.2 Photoreactions of Thymines, etc. – The photochemical decay paths for some pyrimidines (thymine, 1,3-dimethylthymine, 1,3-dimethyluracil) in the gas phase following laser excitation have been studied. Apparently, in contradiction to how they behave in aqueous solution, the excited state molecules do not decay back to the ground state immediately. The molecules exist in a dark state for tens


37

to hundreds of nanoseconds.66 Further study on the same systems has examined the behaviour of the thymine-water complexes in the gas phase.67 Acetone-sensitized irradiation of (77) in acetonitrile solution affords the intramolecular adduct (78) in 36% yield. The cycloaddition is quite specific and affords the [5S,6S,7S]-lactone.68 Acetone-sensitized irradiation of the thymine derivative (79) results in the formation of the cis,syn-dimer (80).69 A patent has been filed dealing with the synthesis of (2 þ 2)-adducts such as (81).70 A study of the conformational effects within the sugar moieties of (82) has been investigated. Systems such as (82) are photochemically reactive and undergo (2 þ 2)-cycloaddition to afford the adduct (83) on irradiation at 254 nm.71

38


Photodimerization of thymine in the crystalline form has been reported.72 A study has examined theoretically the dimerization of pyrimidine units in DNA. The results illustrate that thymine-thymine dimerization is more likely than other combinations.73 Ethenobenzoquinazolines are obtained on irradiation of 5-fluoro-1,3-dimethyuracil in the presence of 1-acetonaphthone. The product has the acetyl group at the C–10 bridgehead carbon atom.74 The same uracil derivative undergoes addition to naphthalene75 and other polyarenes. 6-Chloro-1,3dimethyuracil undergoes photochemical addition to various polycyclic arenes and, for example, the addition to phenanthrene affords the adduct (84) where the initial product has undergone elimination of HCl.76 6-Chloro-1,3-dimethyuracil also undergoes 1,2-cycloaddition to naphthalenes to yield naphthocyclobutapyrimidines. A 1,4-addition is involved in the photoaddition of 1,3-dimethylthymine (85; R ¼ H) and some derivatives (R ¼ OMe, CF3 and CH2OMe) to naphthalene. An ethenobenzoquinazoline (86) is formed by this reaction.77

Mixtures of coumarin and thymine undergo cycloaddition on irradiation in the presence of a molecular recognition catalyst. The reaction affords two cross adducts identified as the cis,syn- and the cis,anti-adducts.78 1,3-Dimethyluracil is reported to form oxetanes on irradiation in the presence of ketones.79 An electron transfer process can be used to regenerate the uracil from the oxetane. A study of the triplet-state induced splitting of cyclobutane thymine dimers has


39

been carried out using 9,10-anthraquinone-2-sulphonate as the sensitizer. An electron-transfer mechanism is involved.80

4

Photochemistry of Dienones

4.1 Cross-conjugated Dienones. – The bicyclic dienone (87) undergoes a reversible colour change on irradiation as inclusion crystals.81 Cross-conjugated cyclohexadienones provide a good path to the corresponding ketenes on irradiation. When this is carried out in the presence of a nucleophile, the ketene can be trapped as the corresponding amide. A series of linked, cross-conjugated bis-cyclohexadienones have been studied. These compounds undergo ring opening to afford the corresponding bis-ketenes, which again can be trapped efficiently with amines, etc.82 Albini and co-workers83 have investigated the photochemical reactivity of the dienones (88). The outcome of the reactions is wavelength dependent. Irradiation at 254 nm or 366 nm converts (88) into the corresponding lumi-products (89). Irradiation at 310 nm, however, brings about loss of the C–20 side chain by an efficient a-cleavage. This yields products such as (90).

40


The marine deoxytridachione (91) converts to the rearrangement products (92) in a reaction that is thought to be photochemically activated. In an attempt to justify this hypothesis, the polyenes (93) were synthesized and their photochemical behaviour examined. Irradiation demonstrated that there was an initial E,Z-isomerization to afford (94), which then cyclizes to (95). This product can then be photochemically converted into the bicyclic product (96).84 Such reactivity is in support of the original hypothesis. The photochemical decomposition of nalidixic acid (97) in methanol solution in the presence and absence of air has been studied.85

The irradiation of the bromotropones in the presence of 1,10-dicyanoanthracene affords novel products. The conditions used for the irradiations excite the anthracene and are carried out in methylene chloride or methylene chlorideacetonitrile solution. The compound (98a) affords the two adducts (99) and (100), while (98b) gives (101) and (102) in the yields shown.86 The polyhalogenated tropones (103) undergo the usual ring closure to afford (104) on


41

irradiation in solution.87 Colchicine (105) also undergoes this photochemical transformation to yield the b-(106) and the g-(107) isomers.88

4.2 Linearly Conjugated Dienones. – Two reaction modes have been detected following a study of the photoreactivity of 4,6-dimethyl-a-pyrone in a matrix. Irradiation with l 4 315 nm brings about both ring-opening to yield a ketene and valence isomerization to produce 1,5-dimethyl-2-oxa-3-oxobicyclo[2.2.0]hex-5-ene.89,90

42


A further study has examined the photobehaviour of a-pyrone on irradiation at wavelengths 4285 nm in a matrix. The most efficient reaction observed is a Norrish Type I ring-opening to afford the usual conjugated aldehydoketene. Further irradiation brings about isomerization of the double bonds to afford the Z-isomer. Ring-opening is accompanied by isomerization to the corresponding Dewar structure, while irradiation at wavelengths 4235 nm gives evidence for the formation of cyclobutadiene.91

5

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

5.1 Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds. – 1,2Dicarbonyl compounds such as glyoxal and biacetyl have been irradiated under direct sunlight.92 A study has examined the wavelength dependence for fission of glyoxal into the CHO radical on irradiation in the 290–420 nm range.93 aFission is the principal photochemical reaction of vicinal cyclic tricarbonyl compounds such as indanetriones.94 ab initio Calculations on the photochemical decomposition of oxalyl chloride have indicated that, in the first excited state, the first fission is the release of a chlorine atom,95 but irradiation of oxalyl chloride at 193 nm results in decarbonylation.96 The irradiation of dipropargyl oxalate at 193 nm provides a new path to propargyl radicals in the gas phase.97 Both H abstraction and desilylation compete on irradiation of the ketoamide (108, M ¼ Si). Some of the products formed are illustrated in the Scheme 8. Irradiation of the stannyl derivative (108, M ¼ Sn) involves a SET-destannylation pathway.98 The nature of the substituent on the aryl group in (109) appears to control the outcome of the photochemical reaction. Thus for (109, Y ¼ CN or CF3), irradiation brings about release of the phenol moiety, while for (109, Y ¼ Me or MeO) rearrangement occurs to yield (110).99 Irradiation of the complex ketoamide (111) in methanol brings about cleavage of the N–CO bond. Several model systems were also studied.100

Scheme 8

43


Triplet 1-acetyl isatin (112) undergoes hydrogen abstraction from a variety of aldehydes. Some of the products, shown in Scheme 9, result from coupling of the radicals (113) formed by the hydrogen abstraction path.101

Scheme 9

5.2 Reactions of 1,3-Diketones. – A tuneable laser has been used to study the photoisomerization of acetylacetone in a nitrogen matrix at 10 K.102 Organero and Douhal103 have reported a temperature effect on non-radiative decay

44


processes within 1-hydroxy-2 0 -acetonaphthone. Viscosity and polarity effects were also studied. 5.3 Reactions of 1,4-Diketones. – The indenylidenedione (114) is photochromic on irradiation in the crystalline phase. When the dideuterio isomer is used, the photochromic material is extremely slow to revert to the dione in the dark.104 The diones (115) undergo efficient conversion into the isobenzofuranones (116) on irradiation.105 Irradiation of (117) at 308 nm in a nitrogen matrix brings about CO and CO2 loss to afford the benzyne (118).106

A route to cyclic peptide mimetics has been developed using the SET-induced photochemical cyclization of the phthalimide derivatives (119). This process affords the cyclized compounds (120) in excellent yields (85–91%).107 Hydroxyphthalimidines are the products formed from the photochemical decarboxylation of a series of alkyl carboxylates in the presence of N-substituted phthalimides.108 The photochemical addition of a-silyl electron-donor systems, such as Et2NCH2SiMe3, to N-methylthiophthalimide takes place in polar solvents such as acetonitrile.109

45


Thio ether N-substituted 4,5-dimethoxyphthalimides undergo strong fluorescence quenching by an intramolecular electron transfer from the substituents on the nitrogen.110 Suau et al.111 have studied further the SET-induced photochemical reactions of the phthalimide anion. Under conditions of high NaOH concentration, the predominant reaction is (2 þ 2)-cycloaddition to afford products such as (121). At lower NaOH concentrations, N-attachment is the dominant reaction, affording (122). This reactivity is shown in Scheme 10. The N-attachment reaction has been utilized to synthesize a series of derivatives (123). This provides a reasonable route to the synthesis of phenylethylamine derivatives. Biczok et al.112 have studied the pathways available for energy dissipation from the excited ion pairs formed between N-methyl-3-hydroxynaphthalimide and 1-methylimidazole. Some important fragmentation reactions of aminium radicals have been discussed.113

Scheme 10

46


A (2 þ 2)-photoadduct is formed on irradiation of 2,3-dimethylmaleic anhydride with 3,4-dimethyl-1-phenylphosphole.114 The photocycloaddition of maleic anhydride derivatives (124) to the alkene moiety in (125) affords the adducts (126) by a straightforward (2 þ 2)-addition.115 The (2 þ 2)photoadducts (127) are readily produced by the dimerization of the corresponding maleic anhydrides in ethyl acetate solution.116 The intramolecular photochemical cyclization of the succinimide derivative (128) is a key step in the new synthetic approach to the tetracyclic fragment of neotuberostemonine.117

A detailed account of the (2 þ 2)-photocycloaddition reactions of homoquinones has been published. The addition reactions afford regio and exo,endo isomers of tricyclic diones. The major product is thought to arise from the more stable triplet 1,4–biradical intermediate.118 A review has reported on the addition of alkenes and alkynes to the enone system of homobenzoquinones.119 Dibromo-9-(3-oxo-3-phenylprop-1-ynyl)-[9H]-fluoren-9-yl-3-fluoren-9-ylidene1-phenylpropenone is photochromic in the crystalline state.120 5.4 Fulgides and Fulgimides. – Calculations have been carried out in an examination of the mechanism of fulgide photochromism121 and have dealt with the absorptivities of photochromic indolylfulgides.122 A patent has been lodged dealing with the synthesis of photochromic indolylfulgides.123 The oxalylfulgides (129) have been prepared and studied. These photochromic yellow dyes undergo ring closure in the conventional manner. The quantum yields for the process and the solvent dependency results are shown below the structures.124 A double Stobbe condensation of 3,5-dimethoxybenzaldehyde with succinic anhydride affords the fulgide (130). This is photochemically reactive and undergoes cyclization to afford a pink cyclic form on irradiation at 366 nm.125


47

Some new pyrrylfulgide derivatives have been studied as potential optical storage materials.126 The photochromic properties of (Z)-4-oxazolylfulgimide{(Z)-1-benzyl-4-isopropylidene-3-[1-(2-aryl-5-methyloxazolyl)ethylidene]tetrahydrofuran-2,5-dione}, a new class of fulgimide, have been examined.127 A patent application has described the synthesis and photochemical reactivity of the fulgimide derivatives (131).128 Fulgimides can be prepared by conversion of the fulgides (132) and (133). The resultant products shown photochromic properties.129 Some new derivatives of fulgimides have been reported with thieno[3,2-b]pyrrole fragments attached to the maleimide.130

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6


Quinones

6.1 p-Quinones. – Goerner131 has demonstrated that the yield of semiquinone radicals is low in the absence of hydrogen donating solvents. He notes that the quantum yield for decomposition is substantial in aqueous solution. A detailed account of the photohydroxylation reactions of 1,4-benzoquinone in aqueous solution has been published.132 The photochemical cyclization of some DielsAlder adducts of benzo-1,4-quinones has been described. This has provided a path to complex molecules such as 3-bromotetracyclo[5.3.1.02,6.04,8]undec10(12)-ene-9,11-dione.133 A biradical has been identified as the key intermediate in the photocyclization of phenylbenzoquinone.134 A charge-transfer complex is formed initially on irradiation of 2-chloro-5-methoxybenzo-1,4-quinone in the presence of triethylamine and various solvents.135 2-Chloro-5-methoxybenzo-1,4-quinone also undergoes addition to arylalkynes to afford oxetenes. The mechanism of this addition reaction was studied.136 A hydrogen abstraction is involved in the photochemical reactions of chloranil with b-diketones.137 The rate of photochemical transformation of naphtho-1,4-quinone into 1,4dihydroxynaphthalene and 5- and 7-hydroxynaphthoquinone is high in acidic media. The rate of transformation is slowed dramatically at higher pH.138 A study of the bichromophoric quinostilbene derivatives (134) has examined the donor-acceptor properties of the molecules.139

Studies of electron transfer between 1,4-dihydroxy-9,10-anthraquinone and aromatic amines has shown that quenching of the quinone fluorescence is the result.140 The azaquinone imine (135) undergoes photoarylotropic rearrangement on irradiation.141


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81. K. Tanaka, T. Watanabe and M. Kato, J. Chem. Res., 2003, S, 535. 82. (a) T.W. Kwon, S.J. Song, U. Yong and S.K. Chung, Bull. Korean Chem. Soc., 2003, 24, 229; (b) T.W. Kwon, S.J. Song, U. Yong and S.K. Chung, Chem. Abstr., 2003, 139, 28552. 83. A. Ricci, E. Fasani, M. Mella and A. Albini, J. Org. Chem., 2003, 68, 4361. 84. S. Bruckner, J.E. Baldwin, J. Moses, R.M. Adlington and A.R. Cowley, Tetrahedron Lett., 2003, 44, 7471. 85. H. Park, O. Park, H. Lee, J. Seo and K. Bark, Bull Korean Chem. Soc., 2003, 24, 1618. 86. A. Mori, H. Kawakami, N. Kato, S.-P. Wu and H. Takeshita, Org. Biomol. Chem., 2003, 1, 1730. 87. Y. Lou, Y.G. He, J.T. Kendall and D.M. Lemal, J. Org. Chem., 2003, 68, 3891. 88. L. Bussotti, I. Cacelli, M. D’Auria, P. Foggi, G. Lesma, A. Silvani and V. Villani, J. Phys. Chem. A, 2003, 107, 9079. 89. S. Breda, L. Lapinski, I. Reva and R. Fausto, J. Photochem. Photobiol. A: Chem., 2004, 162, 139. 90. (a) S. Breda, L. Lapinski, R. Fausto and M.J. Nowak, Phys. Chem. Chem. Phys., 2003, 5, 4527; (b) S. Breda, L. Lapinski, R. Fausto and M.J. Nowak, Chem. Abstr., 2004, 140, 76745. 91. S. Breda, I. Reva, L. Lapinski and R. Fausto, Phys. Chem. Chem. Phys., 2004 6, 929. 92. (a) B. Klotz, F. Graedler, S. Sorensen, I. Barnes and K.-H. Becker, Ber.-Berg. Univers, Physikal. Chem., 2000, 60; (b) B. Klotz, F. Graedler, S. Sorensen, I. Barnes and K.-H. Becker, Chem. Abstr., 2003, 139, 237532. 93. Y.Q. Chen and L. Zhu, J. Phys. Chem. A, 2003, 107, 4643. 94. (a) J. Tatsugi, Kokagaku, 2002, 33, 22; (b) J. Tatsugi, Chem. Abstr., 2003, 139, 36086. 95. (a) N. Dai and Z. Li, Sci, China, Ser. B: Chem., 2003, 46, 19; (b) N. Dai and Z. Li, Chem. Abstr., 2003, 139, 21794. 96. C.-Y. Wu, Y.-P. Lee and N.S. Wang, J. Chem. Phys., 2004, 120, 6957. 97. B.R. Giri, H. Hippler, M. Olzmann and A.N. Unterreiner, Phys. Chem. Chem. Phys., 2003, 5, 4641. 98. R.T. Wang, C.F. Chen, E. Duesler, P.S. Mariano and U.C. Yoon, J. Org. Chem., 2004, 69, 1215. 99. C.C. Ma and M.G. Steinmetz, Org. Lett., 2004, 6, 629. 100. M.A.R.C. Bulusu, P. Waldstatten, T. Tricotett and G. Schulz, Tetrahedron Lett., 2004, 45, 2523–2527. 101. Y. Zhang, L. Wang, Y. Zhu and J.-H. Xu, Eur. J. Org. Chem., 2004, 527. 102. (a) S. Coussan, C. Manca, Y. Ferro and P. Roubin, Chem. Phys. Lett., 2003, 370, 118; (b) S. Coussan, C. Manca, Y. Ferro and P. Roubin, Chem. Abstr., 2003, 138, 409191. 103. (a) J.A. Organero and A. Douhal, Chem. Phys. Lett., 2003, 381, 759; (b) J.A. Organero and A. Douhal, Chem. Abstr., 2004, 140, 110994. 104. K. Tanaka, Y. Yamamoto, H. Takano and M.R. Caira, Chem. Lett., 2003, 680. 105. M. Kapoor, S.N. Dhawan, S. Mor, S.C. Bhatia, S.C. Gupta and M.S. Hundal, Tetrahedron, 2003, 59, 5027. 106. T. Sato, H. Niino and A. Yabe, J. Am. Chem. Soc., 2003, 125, 11936. 107. U.C. Yoon, Y.X. Jin, S.W. Oh, C.H. Park, J.H. Park, C.F. Campana, X.L. Cai, E.N. Duesler and P.S. Mariano, J. Am. Chem. Soc., 2003, 125, 10664. 108. (a) M. Oelgemoller, P. Cygon, J. Lex and A.G. Griesbeck, Heterocycles, 2003, 59, 669; (b) M. Oelgemoller, P. Cygon, J. Lex and A.G. Griesbeck, Chem. Abstr., 2003, 139, 85195.


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109. (a) U.C. Yoon, S.W. Oh, S.C. Moon and G. Tae, J. Photosci., 2002, 9, 17; (b) U.C. Yoon, S.W. Oh, S.C. Moon and G. Tae, Chem. Abstr., 2003, 138, 385246. 110. A.G. Griesbeck and S. Schieffer, Photochem. Photobiol. Sci., 2003, 2, 113. 111. R. Suau, R. Garcia-Segura, C. Sanchez-Sanchez, E. Perez-Inestrosa and A.M. Pedraza, Tetrahedron, 2003, 59, 2913. 112. (a) L. Biczok, P. Valat and V. Wintgens, Photochem. Photobiol. Sci., 2003, 2, 230; (b) L. Biczok, P. Valat and V. Wintgens, Chem. Abstr., 2003, 139, 44099. 113. U.C. Yoon and P.S. Mariano, J. Photosci., 2003, 10, 89. 114. F. Vargas, C. Rivas, A. Fuentes, K. Carbonell and R. Rodriguez, J. Photochem. Photobiol. A: Chem., 2004, 162, 63. 115. (a) H. Suzuki, S. Tsuji, H. Nawata and T. Nihei, Jpn. Kokai Tokkyo Koho. JP, 2003, 119, 199; (b) H. Suzuki, S. Tsuji, H. Nawata and T. Nihei, Chem. Abstr., 2003, 138, 321271. 116. (a) H. Suzuki, Y. Otsuka and M. Ishikawa, Jpn. Kokai Tokkyo Koho. JP, 2003, 192, 685; (b) H. Suzuki, Y. Otsuka and M. Ishikawa, Chem. Abstr., 2003, 139, 101519. 117. K.I. Booker-Milburn, P. Hirst, J.P.H. Charmant and L.H.J. Taylor, Angew. Chem. Int. Edn., 2003, 42, 1642. 118. (a) K. Kokubo and T. Oshima, Kokagaku, 2002, 33, 28; (b) K. Kokubo and T. Oshima, Chem. Abstr., 2003, 139, 36087. 119. (a) K. Kokubo and T. Oshima, Yuki Gosei Kagaku Kyokaishi, 2003, 61, 360; (b) K. Kokubo and T. Oshima, Chem. Abstr., 2003, 139, 149354. 120. K. Tanaka, A. Tomomori and J.L. Scott, CrystEngComm., 2003, 5, 147. 121. S. Li, Z. Li and W. Feng, Beijing Huagong Daxue Xuebao, Ziran Kexueban, 2003, 30, 62. 122. R. Zalesny, W. Bartkowiak and J. Leszczynski, J. Lumin., 2003, 105, 111. 123. P.M. Rentzepis and A. Dvornikov, US 6693201. 124. R. Matsushima, H. Morikane and Y. Kohno, Chem. Lett., 2003, 302. 125. A.M. Asiri, J. Photochem. Photobiol. A: Chem., 2003, 159, 1. 126. M. Lei, B. Yao, Y. Chen, H. Yi, W. Yong, M. Yingli, N. Menke, Y. Zheng, C. Wang, M. Fan and G. Chen, Chin. Sci. Bull., 2003, 48, 1548. 127. J. Xiao, Y. Han, Y. Chen, W. Li and M. Fan, Chin. J. Chem., 2004, 22, 100. 128. M. Fan, L. Zhao, Y. Ming and W. Zhao, CN 1,353,162; Chem. Abstr., 2003, 139, 92825. 129. B. Otto and K. Ruck-Braun, Eur. J. Org. Chem., 2003, 2409. 130. M.M. Krayushkin, V.N. Yarovenko, S.L. Semenov, I.V. Zavarzin, A.Yu. Martynkin and B.M. Uzhinov, Russ. Chem. Bull., 2003, 52, 1814. 131. H. Goerner, J. Phys. Chem. A, 2003, 107, 11587. 132. J. von Sonntag, E. Mvula, K. Hildenbrand and C. von Sonntag, Chem.-Eur. J., 2004, 10, 440. 133. U. Sudhir, B. James, S. Joly and M.S. Nair, Res. Chem. Intermed., 2003, 29, 523. 134. N. Chattopadhyay, C. Serpa, L.G. Arnaut and S.J. Formosinho, Indian J. Chem., Sect. A., 2003, 42A, 1827. 135. (a) H.-L. Guo, B.-Z. Yan and X.Y. Guo, Youji Huaxue, 2003, 23, 288; (b) H.-L. Guo, B.-Z. Yan and X.Y. Guo, Chem. Abstr., 2003, 139, 68839. 136. (a) X.Y. Wang, B.Z. Yan and T. Wang, Chin. Chem. Lett., 2003, 14, 270; (b) X.Y. Wang, B.Z. Yan and T. Wang, Chem. Abstr., 2003, 139, 149234. 137. (a) H.-l. Guo, B.-z. Yan and M.-c. Wang, Gangguang Kexue Yu Guang Huaxue, 2003, 21, 95; (b) H.-l. Guo, B.-z. Yan and M.-c. Wang, Chem. Abstr., 2003, 139, 84912.

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

1

Reactions of Alkenes

A review of photochemistry carried out in ionic liquids1 and a monograph dealing with a variety of aspects of photochromism have been published.2 Reviews have also highlighted the photosensitized enantiodifferentiating isomerization of cycloalkenes other than cyclooctene3 and the area of asymmetric photochemistry in general.4 Mechanisms for the photosensitized isomerization of alkenes have been discussed5 and calculations have been carried out to assess the electron-transfer processes in the tetramethylethene-tetracyanoethene system.6

1.1 cis, trans-Isomerization. – The influence of sudden polarization on the photochemical behaviour of ethene has been studied.7 Changes in the absorption spectra of the styrenes (1) are reported as evidence for the hula-twist isomerization of the compounds. The reactions are carried out at 1961C in an EPA glass.8 Lewis and Zuo9 have described the application of non-linear fitting of lifetime and quantum yield data for styrene derivatives such as 2-vinylbiphenyls and 1,3-diphenylpropenes, and they10 have shown that syn-2-vinylbiphenyls undergo conformer-specific photochemical cyclization to afford dihydrophenanthrenes. A highly selective isomerization of some naphthylidene derivatives has been reported. The molecules in question have both electron donating and electron withdrawing groups that lead to a CT excited singlet state.11 The components of the fluorescence spectra of trans-1-(2-naphthyl)-1phenylethene in benzene solution have been obtained by using variation of the excitation wavelength.12

Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 55

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Two related publications have dealt with the so-called molecular motors (2). The unidirectional motion observed with such molecules occurs on exposure to visible light.13,14 A further example (3) of a chiroptical switch has been synthesized and studied. The isomerization of the (3S,3 0 S)-trans derivative (3) affords a mixture of the cis and trans isomers.15

1.1.1 Stilbene and Related compounds. As a result of calculations on the isomerization of ethene, it has been suggested16 that ‘there is a need for the reconsideration and refinement of the photoisomerization mechanism of stilbene’. Both a semi-classical approach17 and simulations18 have been employed to study the cis,trans-isomerization of stilbene. Simulations have examined the photoisomerization of stilbene, and ab initio quantum chemistry has also been utilized to study this system.19 Donor-acceptor stilbene systems with N,N-dimethylamino groups as the donor and pyridine, thiophene, quinoline and other aryl groups as acceptors exhibit electrogenerated chemiluminscence by intramolecular charge transfer.20 The influence of N-substituents (methyl and phenyl) on the photophysical behaviour of trans-4-aminostilbene has been evaluated.21 Calculations have been carried out on trans-4-dimethylamino-4 0 -cyanostilbene, and the torsion within the central ethenic moiety was calculated. The isomerization of such systems was discussed.22 Liu and co-workers23 have shown that irradiation of the 2,2 0 -dimethylstilbene isomers in glassy 2-methylpentane brings about isomerization by a hula-twist mechanism. A hula-twist mechanism to afford the trans isomer is also involved in the isomerization of a complex (1 : 1) of cis-3,3 0 -bis(diphenylhydroxymethyl)stilbene and acetone.24 The conformational dynamics


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of stilbenes such as (4) within dendrimers have been studied.25 Lewis et al.26 have reported the use of the stilbene derivative (5) as the linker in DNA hairpins. The photochemical isomerization of resveratrol27 (3,5,4 0 -transtrihydroxystilbene) and trans-3,3 0 ,5,5 0 -tetramethoxystilbene has been examined. Photocyclization of the cis isomer of the latter stilbene to a phenanthrene occurs readily.28 Cyanoaromatics have been used as sensitizers for the one-way isomerization of 1,1-diaryl-2-t-butylethene.29

The (Z)-stilbene (6) does not form a rotaxane with dibenzo-24-crown-8 but, when the stilbene is isomerized (sensitized by benzil) to the E form, a pseudorotaxane is formed.30 Lewis and Crompton have reported that the photoacidity of the stilbene derivatives (7) and (8) is dependent on the position of the hydroxy group on the stilbene, and as a consequence the meta derivative undergoes isomerism.31 The influence of the nitrogen atoms on the photochemistry of the 1,4-distyrylbenzene analogue (9) and related compounds has been assessed.32 The photophysics and conformational analysis of the stilbene dimer (10) have been studied.33 Modelling of the optical properties of the stilbenophane (11) has been reported.34

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1.1.2 The Dithienylethene System and Related Compounds. The area of diarylethenes has been the subject of a review.35 Calculations have been carried out dealing with the photochromism of such compounds.36 A description of the preparation of diarylethene nanoparticles has been published, and a decrease in photocyclization was attributed to aggregation.37 Some novel photochromic diarylethenes have been synthesized. The photochemistry exhibited by these compounds suggests that they could be of use as optical switches.38 Tian and Yang39 have reviewed the area of diarylethene photochromic switches. 1,2Bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene is at the basis of a new photochromic liquid crystal system.40 Yokoyama and his co-workers have patented details for such photochromic materials.41 Single crystals composed of the three diarylethenes (12)–(14) have been demonstrated to undergo a variety of colour changes dependent upon the wavelength used.42 Other studies have also investigated photochromism within the crystals of diarylethenes.43 The presence of the alkynyl substituents in the diarylethenes (15) has been shown to enhance the cycloreversion of the derivatives,44 and some new photochromes have been synthesized by Kobatake and Irie,45 in an examination of the influence of alkyl groups at the reacting carbons and also the effect of extended conjugation to


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heteroatoms. The irradiation of (16) at 350 nm brings about the usual cyclization to yield a coloured product. The influence of the changes can be seen in the data recorded under the structure.45 Interesting new systems have been developed based on the cyclization of the bisthienylalkene (17) to the closed form (18).46 The latter was then converted chemically into the cyclophane (19), which underwent ring opening to yield the open form (20). The quantum yield for the cyclization of (20) to (19) was 0.67. Interestingly, the closed form (19) is thermally stable up to 1001C. The absolute asymmetric photocyclization of the diarylethene derivatives (21) and (22) has also been described. The compounds (21) and (22) do not interchange on irradiation. Thus, in the crystal, (21) is converted to the R,R closed form while (22) gives the S,S form.46 The chiral diarylethene (23) undergoes diastereoselective cyclization on irradiation in the crystal.47 The influence on photochromism of hydrogen bonding in the related carboxylic acid derivatives (24) has been detailed.48

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61

The two pentafluoroaryl derivatives (25) form a 1 : 1 co-crystal of alternate layers of the different molecules. Reversible colour changes were observed on irradiation.49 Kobatake and Irie50 have described the photochemical reactivity of the dimeric system (26). Irradiation at 313 nm brings about purple blue colouration of the hexane solution of (26). Continued irradiation changes the colour to blue. The colour changes are due to the formation of compounds with one closed form and two closed forms. The dimer (27) undergoes a single ring closure on irradiation at 350 nm in hexane solution.51 Two cyclization modes are also described for the irradiation of (28).52

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The dynamics for the ring opening of the diarylethene derivative (29) has been shown53 to be a one-step process with a reaction rate of 1.7 1010 s1. Reversible changes in the optical rotation have been observed on irradiation of (30) in stretched DNA-quaternary ammonium ion complex films.53 The influence of cyano substituents has also been quantified for the derivative (31). Apparently, the cycloreversion quantum yield for the closed form of (31) is 30 times larger than for alkyl substituted derivatives.54 Other factors can control the quantum yield of cyclization and, for the cyclization of 1,2-bis-(2-methyl-5phenyl-3-thienyl)perfluorocyclopentene, when compared that of 1,2-bis-(2methyl-1-benzothiophen-3-yl) perfluorocyclopentene in the crystalline phase, the initial geometries of the alkenes are important.55 Irradiation of the racemic bisthienylethene (32a) at 313 nm in hexane affords two diastereoisomeric cyclized products. The de was established as 87%. The closed compounds return to the open form on irradiation at 510 nm.56 Yokoyama and coworkers57 have studied the photochromism of the derivative (32b) in a DNAquaternary ammonium ion complex film. Under these conditions, however, no photochromism was observed owing to the ethene (32b) being in the wrong conformation. Yagi and Irie58 have reported the photochromic behaviour of indole derivatives (33) based on the usual hexafluoroethene moiety. Irradiation at 366 nm converts the open form to a coloured closed form, which can be ringopened using wavelengths 4480 nm. The derivative (34) is also photochromic and affords a highly thermally stable closed ring form.59


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Interest in photochromic systems other than those based on the hexafluorocyclopentene moiety continues to grow. The photochemical reactivity of the two photoswitches (35) is similar, and irradiation is efficient with conversions of 85% and quantum yields of around 0.6.60 The novel photochromic systems (36) undergo reversible ring closure in a reaction analogous to that observed in the bisthienyl system.61 Qin et al.62 have studied the novel pyridyl substituted cyclopentene system (37). This undergoes photocyclization with an enhanced quantum yield when the reactions are carried out in the presence of a metal. The pyridine units are capable of co-ordinating with the metal. The photochromic dithienylethene unit tethered to b-cyclodextrin (38) has been used as a photoswitch to control the uptake of porphyrin.63 A series of new photochromic molecules (39) have been synthesized and studied. These exhibit the usual cyclization on irradiation.64 The terthiophene derivatives (40) exhibit reversible photochemical cyclization (at 313 nm) and reversion (at wavelengths 4460 nm) reactions. The cycles can be carried out many times

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without apparent degradation.65 A novel bisthienylethene photochrome (41) has been synthesized and studied. The compound, in a polymer film, undergoes ring closure on irradiation at 254 nm and the product ring opens at 450 nm.66


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1.2 Miscellaneous Reactions. – Miranda and his co-workers67 have investigated the formation of o-quinone methides by the photochemical reactions of the phenol derivatives (42). Excitation of these brings about proton transfer to afford the zwitterions (43), which then undergo ring opening of the cycloalkyl moiety to afford the quinone methides. There is a ring-size effect, and the best results are obtained using the cyclohexenyl derivatives. The authors69 also observed that the presence of the CF3 group accelerated the formation of the methide, whereas the MeO group slowed down the reaction. Both 1,2,4triphenylbenzene and the binaphthalene derivative (44) sensitize the aminations illustrated in Scheme 1. The reactions involve electron transfer, and produce radical cations as the key intermediates. Apparently, a key step is the hole transfer from the radical cations to the sensitizer.68

Wan and his co-workers69 have reported the photohydration and photosolvolysis of the alkenes (45). The reactions involve the intermediacy of quinonemethide type intermediates. Flash photolysis of coniferyl alcohol (46) and isoeugenol (47) in acetonitrile has supplied evidence for the formation of the corresponding radical cations. These transient species react readily with water and other hydroxylic solvents to afford 4-vinylphenoxyl radicals.70

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Scheme 1

The vacuum-UV photodissociation of ethene has provided evidence for the formation of CH fragments.71 Eleven photofragments have been detected following the photodissociation of propene, where the principal reaction channel followed is the formation of ethyne, methyl radicals and hydrogen atoms.72 The elimination of HCl from vinyl chloride has been studied using irradiation at 193 nm, and a concerted mechanism is suggested from the results obtained.73 Vinyl bromide undergoes conversion to the vinyl radical on irradiation at 193 nm using an excimer laser.74 A detailed study has examined the mechanism for the formation of a primary vinyl cation from the styrene (48).75


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The influence of substitution on the alkenes (49) has been assessed. The study has shown that styrenes with o-methyl and a-methyl groups adopt orthogonal geometry. This results in a blue shift in the pp* transition.76 The ocimene derivative (50) is formed on irradiation of the pinene derivative (51). There is no evidence for [2 þ 2]-cycloaddition, and the rearrangement occurs on irradiation in benzene or methanol. Excitation results in triplet energy transfer from the benzene moiety to the pinene. The ring opening affords the cis isomer (50), but continued irradiation affords a ratio, trans:cis, of 52 : 48.77

1.2.1 Addition Reactions. A patent has described a method for the formation of perfluoroalkyl iodides. This consists of irradiating (254 nm) a mixture of perfluoroethyl iodide with tetrafluoroethene at elevated temperatures and pressures.78 Methanol addition results on irradiation of 2,3-dimethylbut-2-ene in a mixture of 1,4-dicyanobenzene and phenanthrene in the presence of acrylonitrile or methyl acrylate.79 Inoue and co-workers80 have studied the enantiodifferentiating addition of alcohols to the 1,1-diphenylethene derivatives (52). The reactions are sensitized by the naphthalenecarboxylates (53) and (54), where the R* groups are saccharide moieties. The ee of the products (55) is influenced by steric, electronic and solvent effects. Efficient addition of water to 3-hydroxystilbene can be brought about on irradiation in acetonitrile-water mixtures.81 Pincock has highlighted the importance of the discovery in 1973 of the formation of the radical cation of 1,1-diphenylethene.82 Grainger and Patel83 have described a new photochemical approach to cuparene (56). The reaction involves the electron-transfer induced cyclization of the styrene

68


derivative (57), which affords the two cycloadducts (58) and (59) in a ratio of 6 : 1. The major isomer (58) was converted to racemic cuparene. The incorporation of an optically active amine affords the styrene (60). This also undergoes photochemical cyclization on irradiation at 265 nm in hexane. Four diastereoisomers are obtained and one of the major products is (61), which can be converted to (–)-cuparene.

1.2.2 Reactions of Alkynes. Propyne undergoes fission on irradiation at 193 d nm. This treatment affords the propargyl radical (CH2CCH) and ethyne. Irradiation (at wavelengths 4120 nm) of cyanoacetylene in a matrix at 10 K gives evidence for the formation of three different isomers.84 A C2HO radical is formed on irradiation (at 193.3 nm) of ethyl ethynyl ether. This fragmentation path amounts to 37% of the overall processes.85 The propargyl radical is formed on irradiation of propargyl chloride at 193 nm or propargyl bromide at 248 nm.86,87 Others88 have also investigated this process. Vinyl cations can be formed by flash irradiation of the alkynes (62).89 Hydrosilylation of alkynes has been achieved using irradiation at 350 nm in the presence of a Pt(II) catalyst.90


69

A silacyclopropene intermediate is involved in the photochemical cyclization of 1-(3-hydroxy-2-pyridyl)-2-(pentamethyldisilanyl)ethyne.91,92

The Bergman cyclization of the copper complex (63) or its zinc counterpart involves the triplet state of the enediyne. The cyclization affords the derivative (64).93 Irradiation of the diethynylsulfides (65) at 300 nm in the presence of a hydrogen donating additive (cyclohexadiene or alcohols) results in cyclization. This reaction is of the photo-Bergman type and affords modest yields of 3,4subsituted furans. The diphenyl-substituted product (66) is obtained in 16% yield and is accompanied by phenylacetylene as a by-product.94 Irradiation (at 254 nm in n-hexane) of the cyclodecatriene derivatives (67) results in cheletropic extrusion of the bridge as a carbene. This rearranges into the triynes (68) obtained in the yields shown. An additional reaction process is rearrangement of the skeleton of (67) to afford the products (69).95 The photon-absorption properties of the macrocycles (70) have been measured.96

70

2


Reactions Involving Cyclopropane Rings

2.1 The Di-p-Methane Rearrangement and Related Processes. – Armesto and co-workers97 have reviewed recent advances in di-p-methane photochemistry. Scheffer and Vishnumurthy98 have described the photoconversion of the enone (71) into the cyclopropane derivative (72). This occurs by a 1,2-phenyl migration in an example of a di-p-methane rearrangement. The reaction is very efficient, and the authors100 speculate that steric acceleration of the migration of phenyl from the triphenylmethyl group occurs. The photochemical reactivity of compounds related to 1,1-dicyano-3-phenylpropene has been studied. This work was aimed at comparing the di-p-methane reactivity with cyclization involving hydrogen migration and bond formation. Apparently, electron donating groups on the aryl ring and polar solvents decrease the di-p-methane pathway but have little or no effect on the other cyclization mode.99


71

A study of the di-p-methane reactivity of the barrelene derivatives (73) in zeolites has been published. The reaction in a slurry affords a 77 : 23 mixture of (74) and (75); when the reaction is carried out in a zeolite the cyclooctatetraene product is suppressed and the two products are obtained in a ratio of 1 : 99. This enhancement of the di-p-methane reactivity occurs with Li1- and Na1exchanged zeolites.100 Liao and co-workers101 have reported new reactivity of some barrelenes. The reactions encountered are sensitive to substitution pattern. Thus, the irradiation of (76) with electron withdrawing groups follows the di-p-methane route, to yield (77) and (78) predominantly. The less heavily substituted derivative (79) behaves differently, and irradiation affords (80) and (81) by the aza-di-p-methane rearrangement, with (82) formed only in small amounts by the alternative di-p-methane path. Calculations have been used to examine the mechanism of the barrelene-semibullvalene isomerization. These results indicate that two biradical intermediates are involved in the T1 state.102 Other calculations on the di-p-methane rearrangement of barrelene substantiate the Zimmerman mechanism for the sensitized rearrangement.103

72


Zimmerman and Novak have continued the search for further examples of the tri-p-methane process. Some of the systems studied are illustrated by (83). Sensitized irradiation of these compounds brings about the di-p-methane reaction affording (84) and (85). The tri-p-product (86) is not formed under sensitized conditions, and direct irradiation is needed to produce this product. A secondary product (87) is also formed. The regioselectivity of the process was studied in detail.104

Armesto and his co-workers105 have reported further on their interesting new reactions of azadienes. Acetophenone-sensitized reactions of the imines (88)–(92) results in the formation of a variety of products. These, with yields, are shown in Scheme 2.

2.2 Miscellaneous Reactions Involving Three-Membered Ring Compounds. – Asymmetric induction within 1,2-diphenylcyclopropane on photolysis in b-cyclodextrin has been studied.106 Recent research has shown that cis-2,3diphenylcyclopropane-1-carboxylic acid does not undergo ISC on direct irradiation. The reaction encountered is isomerization to the corresponding trans isomer via a 1,3-biradical intermediate.107 Exothermic bond cleavage is the dominant reaction within radical cations of cyclopropylamines formed by SET to DCA.108 The photoheterolysis of 9-cyclopropyl-9-fluorenol has been studied in non-acidic zeolites. The rate of formation of the resultant cation is dependent upon the alkali metal counterion.109 A detailed spectroscopic and calorimetric study of the photorearrangements of (93) has shown that both zwitterionic and biradical species are involved in the isomerization.110 DCA-sensitized irradiation of the bicyclopropenyl compounds (94) results in conversion into Dewar benzene derivatives (95) and (96) and eventually into the corresponding benzene derivatives (97). The irradiation

73


Scheme 2

of (94, R ¼ Me) affords the three products (95), (96) and (97) in ratios of 24 : 16 : 46, and prolonged irradiation gives (95) and (97) in a ratio of 28 : 72. Compound (94, R ¼ H) affords the three products in ratios of 18 : 41 : 35.111

74


Laser irradiation at 266 nm of the cyclopropylbutene system (98, R ¼ H or CO2Et) results in the formation of the radical by C–Se bond fission. This radical cyclizes to afford (99a,b), which ultimately undergoes cyclization to (100). This study was aimed at proving the mechanism of the a-methyleneglutarate-mutase-induced rearrangement of 2-methyleneglutarate to 3-methylitaconate.112

The two-bond photochemical cleavage product, 1,5-hexadiene, obtained from bicyclo[3.1.0]hexane arises from the first excited singlet state of the starting material. This proposal has been substantiated by calculations at the CASSCF level.113 Both cis- and trans-9-diphenylvinylidene-bicyclo[6.1.0]nonanes undergo isomerization on irradiation in the presence of Michler’s ketone.114

3

Reactions of Dienes and Trienes

Irradiation of the cyclobutene derivatives (101) at either 214 nm or 228 nm affords mixtures of dienes with a greater preponderance of the product arising from disrotatory ring opening. The authors115 suggest that these results indicate that Rydberg-derived ring opening is dependent upon the ability of the molecules to twist around the central bond as ring opening occurs.


75

Reviews of the hula-twist photoisomerization of dienes have been published.116,117 New calculations dealing with the isomerism of s-cis-butadiene have been reported.118 Molecular dynamic simulations of the photochemical behaviour of butadiene have shown that cis-trans isomerism occurs.119,120 The regioselectivity observed in the photochemical isomerism of trans,trans-1fluorohexa-2,4-diene is proposed as due to electrostatic control.121 Direct irradiation of the fluorinated dienes (102) results in preferential conversion to the 1Z,3E isomers (103).122 DCA-sensitized irradiation of the butatrienes (104) in various alcohols afford the adducts (105). The simpler alcohols give moderate yields of product, while t-butanol affords only a trace of the adduct.123

Miyazaki and Yamada124 report that irradiation of cyclopentadiene in an argon matrix with a super-high-pressure Hg lamp results in the formation of bicyclo[2.1.0]pent-2-ene. By irradiation at shorter wavelengths the initial product is transformed into allylacetylene and vinylallene. The photochemical ring opening of the cyclohexadiene moiety within the triterpenoids (106) yields the corresponding trienes.125 The tricyclo[4.3.1.0]dienes (107) have been used as a source of some fluorocarbenes: irradiation affords the carbenes with indane produced as the by-product.126

76


The divinyl benzene molecule (108) is reported to undergo intramolecular addition to yield an exo-endo mixture of (109) in 10% yield. Other products such as phenanthrenes are also formed.127 Saltiel et al.128 have reported a study of the photoisomerizsationof all-trans-1,6-diphenylhexa-1,3,5-triene in acetonitrile. Their observations show that the isomerization is mainly via the singlet excited state, since there is low intersystem crossing within this triene. Interestingly, the presence of oxygen enhances terminal bond isomerization. The mechanism for the interaction is thought to involve a charge transfer between the triene and oxygen, leading to either a zwitterion or a biradicaloid intermediate. Irradiation of the cycloheptatriene (110) brings about two processes identified as photoheterolysis and hydrogen migration. The first process results from loss of methoxide and affords the corresponding tropylium methoxide, while a 1,7-hydrogen migration occurs in competition, with the exclusive formation of 5-(4-dimethylaminophenyl)-1-methoxycyclohepta-1,3,5-triene.129

The photochemical isomerization of trans-vitamin D3 to the cis isomer has been achieved under conditions where the vitamin was tethered to a silica gel substrate.130 A continuous photoreactor has been described for the photochemical conversion of ergosterol into vitamin D2.131 Saltiel et al.132 have studied the isomerization within the 25-hydroxytachysterol and the 25hydroxyprevitamin D3 systems shown in Scheme 3. The study used a twostage irradiation at 254nm to 313 nm, and this results in conformer-specific photoconversion.

77


Scheme 3

4

(2p þ 2p)-Intramolecular Additions

The complex of (111) with copper triflate undergoes photochemical conversion into the adduct (112). The yields are high for this process. A similar cyclization can be carried out with (113). When the bis-dienes (114) are used, cyclization affords the adducts (115). These cyclizations provide new routes to bicyclo[3.2.0]heptanes and bicyclo[6.3.0]undecanes.133

78


The donor acceptor norbornadiene derivative (116) exhibits high quantum yields for valence isomerization using blue light.134 Intramolecular electron transfer is involved in ring opening of the quadricyclane derivative (117) to the corresponding norbornadiene. The study has indicated that electron transfer occurs from the quadricyclane moiety to the BF2 chromophore.135 Further work on such systems has shown that intramolecular triplet energy transfer from the carbazole to the norbornadiene moiety also occurs in the molecule (118).136

Nishimura et al.137 have described a path to crownopaddlanes by the (2 þ 2)-intramolecular cycloadditions of the compound (119). The sensitivity of the resultant compounds towards ions was assessed. The new crownophanes (120, n ¼ 2-4) have been synthesized by irradiation of the corresponding divinyl derivatives.138 The vinylpyridine derivatives (121) are also reactive, and afford the (2 þ 2)-adduct (122) in yields that can be as high as 84% (n ¼ 4 and 8).139 Nishimura and his co-workers have reviewed this area of research.140


5

79

Dimerization and Intermolecular Additions

Calculations have studied the (2 þ 2)-photocycloaddition of ethene, either under direct conditions141 or on a Si (100) surface.142 Inokuma et al.143 have reported a further example of additions with crown ether derivatives. In this instance the additions are intermolecular and involve the dimerization of the vinylbenzene derivative (123) to afford the two adducts (124) and (125). The ion-complexing capabilities of the adducts were assessed. A layered ternary solid is formed between 1,2-dihydroxybenzene and trans-1-(2pyridyl)-2-(4-pyridyl)ethylene. Within this, the stilbene is held in a head-to-tail arrangement. Irradiation brings about the formation of a cyclobutane identified as (R)-cis,trans,trans-1,3-bis(2-pyridyl)-2,4-bis(4-pyridyl)cyclobutane.144 An extension of this work to the use of 5-methoxyresorcinol as the template has demonstrated that quantitative yields of ladderanes (126) can be obtained by irradiation of the solid-state units represented as (127).145 The diazastilbene derivative (128) readily forms complexes with the tetra-acid (129). This acts as a supramolecular template and holds the ethene systems close enough for photochemical dimerization.146

80


The styrylpyrilium salt derivatives (130) undergo (2 þ 2)-cycloaddition to afford the corresponding dimers. The magnetic properties of the radical cations formed from these dimers were compared with those formed from (130).147 Styryl dyes of the type shown as (131) undergo E-Z isomerization on irradiation at 436 nm. The dyes align themselves in the pattern shown in (132, where the filled blob represents the crown ether complex). These undergo dimerization on irradiation to afford compounds (133), from which the magnesium ions can be removed.148


81

Cross-photodimerization of (E)-2-(2-phenylethenyl)benzoxazole and (E)-4[2-(4-chlorophenyl)ethenyl]-pyridine in sulfuric acid solution results in the formation of a product identified as (R)-cis,trans,trans-1-(2-benzoxazolyl)-2(4-chlorophenyl)-4-phenyl-3-(4-pyridinyl)cyclobutane.149,150 Nishimura and his co-workers151 have reported the synthesis of the calixarane derivatives (134). These are formed by photoaddition of ethene to the corresponding alkene derivative.

Photodimerization of acenaphthylene in cation-exchanged bentonite clays has shown that the cis dimer (135) is formed preferentially with smaller cations. Heavier ions favour the formation of (136).152

82

6


Miscellaneous Reactions

6.1 Reactions of Halo Compounds. – Di-(p-methoxyphenyl)methyl chloride undergoes a variety of processes when irradiated with a three-laser system at 308 nm, 355 nm and 495 nm. This treatment leads to the formation of the ground-state radical, an excited state radical and the corresponding cation. Irradiation at 308 nm brings about the fission of the C–Cl bond, while 355 nm raises this radical to its excited state and also affords the cation.153 The benzyl chloride derivatives (137) undergo photolysis in aqueous acetonitrile to yield the corresponding benzyl alcohol and the reduction product 4RC6H4C(CF3)H.154

The photobehaviour of methyl iodide clusters is dependent on the mode of excitation. Thus, in a supersonic jet iodine molecules are formed,155 while a visible laser forms ions.156 Pulsed photolysis has been used to generate iodine atoms from trifluoromethyl iodide.157 The allyl-d2 radical is formed on irradiation of allyl-d2 iodide at 193 nm.158 Allyl radicals are also formed by irradiation of allyl alcohol at the same wavelength. The dissociation in this instance occurs to form a triplet state following pp* excitation.159 The results obtained from the irradiation at 235 nm of vibrationally excited dichloromethane suggest that there is higher non-adiabaticity for vibrationally excited molecules.160 Laser irradiation at 355 nm of methylene chloride and methylene bromide brings about ionization.161 A simple C–Br bond fission is the key step in the photodissociation of methylene dibromide at 248 nm.162 Ionization of methylene bromide in the 10–24 eV range has been reported.163 Irradiation of methylene iodide in water has provided evidence for the formation of iodine atoms and protons.164 Further work on the photochemical behaviour of methylene iodide has reported that vibrationally relaxed CH2I I reacts with cyclohexene to afford norcarane.165 The photodissociation of bromoform at 193 nm has been studied using dispersed fluorescence.166 Bromoform also undergoes fission on irradiation at 234 and 267 nm.167 A series of n-alkylbromides has also been studied on irradiation at the same wavelengths.168 Prompt C–Br bond fission occurs on irradiation of bromoform at 248 nm.169 The isomerization of bromoform into isobromoform occurs on irradiation in water. Insertion of OH into isobromoform results in the formation of the final products, HBr, CO and formic acid.170

83


Bromine atoms are formed on irradiation of difluorobromochloromethane at 267 nm.171 Photodissociation of trifluoroiodomethane at 248 nm has been studied using a high-resolution photofragment translational spectrometer.172 Chen et al.173 have reported the use of irradiation of CBr4 in methanol as a selective reagent for the removal of protecting groups. Many examples were given, but two will suffice to illustrate what can be achieved. Thus, the conversion of (138a) into (138b, 89%) can be brought about by irradiation for 24 h. A more facile reaction is the conversion of (139a) into (139b, 86%) in 6 h.

The photodissociation of 1-bromo-2-chloroethane at 193 nm demonstrated that there were no stable bromoethyl or chloroethyl radicals formed. Dissociation of these radicals is rapid and affords ethene.174 1-Chloro-1,1,3,3,3pentafluoropropane can be made regioselectively by the photochlorination of 1,1,1,3,3-pentafluoropentane.175 Irradiation of 2-iodooctane in methanol at 254 nm results in the formation of many products. The ones containing oxygen arise from the 2-octyl carbocation. Laser irradiation at 266 nm of the same compound in methanol affords only three products, octane and alkenes are formed. The authors176 suggest that excitation produces a short-lived ns* excited state, which dissociates into a radical pair; and it is from this that the products are formed. Sun et al.177 report that carbonylation of alkyl halides can be brought about using CO and suitable catalysts. Harato et al.178 have also reported the carbonylation of alkyl chlorides using SmI2 as a catalyst and CO at 50 atm. These authors consider that an acyl samarium species might be involved in the reaction. Cobalt acac catalysis has been used to carboxylate alkenes at ambient conditions.179 The use of InCl3-MgCO3-Co(acac)2 in methanol-acetone mixtures gives improved yields of methoxycarbonylated alkanes from halocycloalkanes. Some of the results are shown in Scheme 4.

Scheme 4

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6.2 Miscellaneous Rearrangements and Bond Fission Processes. – Methacrylonitrile undergoes C–CN bond fission on irradiation at 193 nm.180 Substitution of the bromo group by the anion of nitromethane occurs when 1-bromo-, 2-bromo-, 1,3-dibromo- and 1,4-dibromoadamantane are irradiated in DMSOammonia solution.181 Photodissociation of jet-cooled propan-1-ol and propan2-ol at 193.3 nm shows that O–H bond fission is the dominant reaction.182 Albini and his co-workers have described the generation of alkyl radicals from cyclopentane, cyclohexane and cyloheptane. This reaction is carried out by irradiation in the presence of tetrabutylammonium decatungstate. The resultant radicals can be trapped efficiently (63%) by electrophilic alkenes such as acrylonitrile.183 Zwitterionic intermediates are generated by O–C bond fission on irradiation of the pinacols (140) in methanol solution.184

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161. P. Sharma, R.K. Vatsa, D.K. Maity and S.K. Kulshreshtha, Chem. Phys. Lett., 2003, 382, 637. 162. Y.-R. Lee, C.-C. Chen and S.-M. Lin, J. Chem. Phys., 2003, 118, 10494. 163. S.-Y. Chiang, Y.-S. Fang, K. Sankaran and Y.-P. Lee, J. Chem. Phys., 2004, 120, 3270. 164. Y.-L. Li, C. Zhao, W.M. Kwok, X. Guan, P. Zuo and D.L. Phillips, J. Chem. Phys., 2003, 119, 4671. 165. A.N. Tarnovsky, V. Sundstrom, E. Akesson and T. Pascher, J. Phys. Chem. A., 2004, 108, 237. 166. B.J. Petro, E.D. Tweeten and R.W. Quandt, J. Phys. Chem. A., 2004, 108, 384. 167. Y. Tang, L. Jei, B.-F. Tang, R.-S. Zhu, S. Zhang and B. Zhang, Wuli Huaxue Xuebao, 2004, 20, 344. 168. R.-S. Zhu, B.-F. Tang, L. Jei, Y. Tang, S. Zhang and B. Zhang, Optics Commun., 2004, 235, 325. 169. P. Zou, J.N. Shu, T.J. Sears, G.E. Hall and S.W. North, J. Phys. Chem. A., 2004, 108, 1482. 170. W.M. Kwok, C.Y. Zhao, Y.L. Li, X.G. Guan, D.G. Wang and D.L. Phillips, J. Am. Chem. Soc., 2004, 126, 3119. 171. J. Huang, D. Xu, J.S. Francisco and W.M. Jackson, J. Chem. Phys., 2003, 119, 3661. 172. X. Wang, Z. Tian, T. Shi, X. Shi, D. Yang and Q. Zhu, Chem. Phys. Lett., 2003, 380, 600. 173. M.Y. Chen, L.N. Paktar, M.D. Jan, A.S.Y. Lee and C.C. Lin, Tetrahedron Lett., 2004, 45, 635. 174. Y.-R. Lee, C.-C. Chen and S.-M. Lin, J. Chem. Phys., 2004, 120, 1223. 175. N.K. Nair, M. van der Puy, D. Nalewajek, T.R. Demmin, A.J. Poss, D.E. Bradley, I.R. Shankland and M.E. Cheney, U.S. US 6,551,469; Chem. Abstr., 2003, 138, 320904. 176. F. Gao, R.N. Compton and R.M. Pagni, J. Chem. Soc., Chem. Commun., 2003, 1584. 177. (a) Y. Sun, J.M. Yin, D.B. Gao, G.Y. Zhou and Y.P. Jia, Chin. Chem. Lett., 2003, 14, 575; (b) Y. Sun, J.M. Yin, D.B. Gao, G.Y. Zhou and Y.P. Jia, Chem. Abstr., 2003, 139, 381169. 178. (a) N. Harato, Y. Tomisaka and A. Ogawa, Kidorui, 2003, 42, 68; (b) N. Harato, Y. Tomisaka and A. Ogawa, Chem. Abstr., 2004, 140, 181138. 179. (a) J. Yin, D. Gao, J. Hu, G. Zhou, Y. Jia and X. Wang, Chinese Sci. Bull., 2003, 48, 2180; (b) J. Yin, D. Gao, J. Hu, G. Zhou, Y. Jia and X. Wang, Chem. Abs, 2004, 140, 163351. 180. C.Y. Oh, S.K. Shim, H.L. Kim and C.R. Park, J. Phys. Chem. A., 2003, 107, 4333. 181. (a) A.N. Santiago, C.A. Toledo and R.A. Rossi, J. Phys. Org. Chem., 2003, 16, 413; (b) A.N. Santiago, C.A. Toledo and R.A. Rossi, Chem. Abstr., 2003, 139, 323181. 182. W. Zhou, Y. Yuan and J. Zhang, J. Chem. Phys., 2003, 119, 7179. 183. D. Dondi, M. Fagnoni, A. Molinari, A. Maldotti and A. Albini, Chem.-Eur. J., 2004, 10, 142. 184. G. Mladenova, G. Singh, A. Acton, L. Chen, O. Rinco, L.J. Johnston and E. Lee-Ruff, J. Org. Chem., 2004, 69, 2017.

Photochemistry of Aromatic Compounds BY ANDREW GILBERT University of Reading, Reading, UK

1

Introduction

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids1,2 and in isotropic and anisotropic media,3 organic synthesis utilizing photoinduced electron-transfer reactions,4 heteroatom-directed photoarylation processes,5 photochromism,6 and photochemical molecular devices.7 Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of fivemembered heteroaromatic azoles,8 the photocycloaddition of benzene derivatives to alkenes,9 Diels-Alder additions of anthracenes,10 advances in the synthesis of polycyclic aromatic compounds,11 diarylethene-based photochromic switches,12 the photo-Fries rearrangement,13 and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds.14 A number of chapters in the two recently published handbooks of photochemistry and photobiology15,16 and in the revised edition of the text on photochromism17 are also pertinent to the current subject matter.

2

Isomerization Reactions

The photoinduced cis-trans interconversions of 1,2-diarylethenes are reviewed in Chapter 3 of this Volume. It is, however, pertinent to note here the study into the geometric isomerization of cis 2,3-diphenylcyclopropane-1-carboxylic acid derivatives.18 Such compounds, contrary to what is known about 1,2-diphenylcyclopropanes, are now reported to have triplet energies of approximately 311 kJ mol1, to undergo the less common adiabatic isomerization to the trans isomers, to display emission from the electronically excited 1,3-diradical intermediates, and not to undergo intersystem crossing on direct excitation. A new mechanism involving Dewar benzene intermediates has been shown to operate in the photoinduced electron-transfer promoted rearrangement of 2,2 0 ,3,3 0 -tetraphenylbicyclopropenyls (1).19 Thus irradiation (l 4 410 nm) of Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 91

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(1) and 9,10-dicyanoanthracene (DCA) in degassed CH2Cl2 yields the two Dewar isomers (2) and (3) and the arene (4) in time-dependent ratios, from which it is deduced that, while (2) is stable under the reaction conditions, (3) is the precursor of (4) following electron transfer to DCA. These results differ from those reported earlier for such systems in acetonitrile solution.20

The vapour-phase photochemistry of 3,4,5-trideuteriopyridine is reported to yield the positional isomers (5) and (6) by way of a route involving two successive [1,3] sigmatropic shifts of nitrogen and an equilibrating mixture of the azaprefulvenes (7) and (8), rather than by interconverting Dewar pyridine intermediates, which would have lead to the (unobserved) 2,3,5-d3 pyridine.21 This work provides the first direct evidence that phototransposition of ring atoms occurs in pyridine vapour. Earlier work on the photoisomerization of 1-methylpyrazoles indicated that the imidazole products arose from a competition between mechanisms involving N1–N2 bond cleavage (termed P4 pathway) and electrocyclic ring closure to Dewar isomers (termed P6 and P7 pathways). Further studies on these systems and monodeuterio derivatives have revealed that the presence of a trifluoromethyl substituent promotes the P4 pathway, resulting in, for example, the imidazoles (9) and (10) from (11) in a respective ratio of 2:1 at 15% conversion.22 While both imidazoles are primary products, (9) does undergo photoconversion to (10) via (12) under the reaction conditions. Pyrazole photoisomerizations have also been the subject of an ab initio study, from which it is concluded that the S1 state can yield the Dewar isomer and the corresponding triplet state, which then forms the 1,2-diradical leading to isomerization.23 While this behaviour is deduced for 1,5-dimethylpyrazole, the T1 state of 1-methyl-5-phenylpyrazole cannot seemingly yield the corresponding diradical, and isomerization occurs only from the Dewar isomer.


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Both aniline and 4-methylpyridine are reported to undergo photorearrangement on 193 nm irradiation, to give seven-membered ring isomers, which then undergo internal H-atom migration and rearomatization prior to dissociation.24 Using similar experimental conditions, the same research group has also shown that azulene isomerizes to naphthalene and eventually undergoes dissociation by way of the H-atom elimination channel.25 Several previous reports have described various features of the reversible photoisomerization of 1,8a-dihydroazulene dicarbonitriles to vinylheptafulvenes. The first time-resolved investigation of this ring-opening reaction has now been reported and, using sub-30 fs transient absorption spectroscopy, prominent workers in this area have determined that the ring opening of (13) to (14) occurs within 1.2 ps, which is followed by internal conversion of S1 (14) to S0 (14) in 13 ps.26

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3


Addition Reactions

The intramolecular meta photocycloaddition of ethenes to the benzene ring has proved to be a useful key step in the synthesis of a number of complex molecular skeletons. It is well appreciated that the photoreactivity of these bichromophores is maximized for non-conjugated systems, such as derivatives of 5-phenylpent-1ene, in which the two interacting moieties are separated by three molecular units in the tether. The synthetic scope of the procedure would be significantly improved if the reaction could be extended to efficient intramolecular cyclizations for bichromophores having longer tether units and to systems with chemically reactive tethers for subsequent elaboration or removal. Both of these aspects have been examined with the bichromophores (15), but, although intramolecular meta photocycloaddition was observed for (15, X ¼ CH2 or SiMe2), the chemical yields of the expected 1,6-bridged dihydrosemibullvalene adducts (16) were low (7–19%).27 As the authors note, however, these reactions with four units and removable tethers do have merit, as they provide an approach to complex molecular systems not readily accessible by conventional means. The molecular constraints to approach of the aryl and alkenyl chromophores in the 2-alkenylnaphtha-4-chromanones (17) are severe, but, although a complex mixtures does result on their irradiation (l 4 330 nm) in methanol solution, the highly functionalized polycyclic compound (18) was isolated in 20% yield.28 Interestingly, as outlined in Scheme 1, the formation of (18) can be envisaged by either 1,2- or 1,4- intramolecular cycloaddition to the naphthalene moiety. The sole process from irradiation (l 4 330 nm) of cyclohexane solutions of 1,3-dimethylthymine and naphthalene is reported to arise from 1,4-addition giving (19), with high stereoselectivity, and in 72% yield at 46% consumption of the pyrimidine.29 The same research group has also described the photoreaction between 6-chloro-1,3-dimethyluracil and phenanthrenes or pyrene.30 Both (2p þ 2p) cycloaddition and substitution occur, giving from phenanthrene, for example, the products (20) and (21), respectively, in ratios which are dependent on the solvent. Thus, while there is no reaction in cyclohexane solution, the respective yields of (20) and (21) in benzene are 10 and 30%, which change in the presence of small amounts of trifluoroacetic acid to 55 and 5%. Similar addition at the 9,10-positions of methyl 9-phenanthrenecarboxylate has been observed for cholesteryl cinnamate (22).31 The photoadduct (23) is isolated in 74% yield (de 8%) along with 9% of the cis isomer. The photodimerization of cholesteryl 9-phenanthrenecarboxylate, giving solely the head-to-tail syn isomer (24) in 55%

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yield (de 8%) is also described in this report. Highly regioselective (2pþ2p) cycloaddition has been reported for 4-methyl-1,2,4-triazoline-3,5-dione at the cis-1 [6,6] ethene bond of C60 and several fullerene derivatives from 420 nm irradiation of solutions in tetrachloroethane.32

Scheme 1

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The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution.33 Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (8pþ4p) photoaddition to 9,10-dicyanoanthracene in benzene–methanol (9:1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26).34 In contrast, 2-bromotropone and the anthracene in CH2Cl2 solution afford the substitution products (27) (62%) and (28) (25%).

97


Scheme 2

The photocycloaddition chemistry of pyridines substituted with electrondonor and electron-acceptor groups at the 2- and 3- positions continues to be exploited. The results of irradiation of such pyridines in the presence of 2-cyanofuran have now been described.35 The yields of the (4pþ4p) cycloadducts (29) and (30), the pyridine dimer (31) and the transposition isomer (32) are dependent on the level of methyl substitution on the heteroarene and are given in Scheme 2. Other photocycloadditions to heteroarenes reported within the year include the reactions of benzodithiophene (33) with butadiyne derivatives and dimethyl acetylene dicarboxylate, giving low yields of (34) and (35) respectively, the latter from photorearrangement of the primary adduct (36).36 The (2pþ4p) photocycloaddition of indoles (37) to cyclohexa-1,3-dienes (38) is sensitized by the aromatic ketones (39), and yields (14–46%) of the exo and endo isomers of the adduct (40) in ratios which are dependent on the substituents on the addends.37

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The radical addition cyclization which occurs on 350 nm irradiation of mixed acetonitrile solutions of N,N-dimethylaniline and menthyloxyfuranone (41) is reported to be diastereoselective and is initiated by photoinduced electron transfer from the amine to the semiconductors TiO2, SnO2, ZnS or SiC.38 The major and minor adducts are proposed to be formed by the route outlined in Scheme 3 and are accompanied by products derived from the radical produced on partial reduction of (41). Irradiation of the enantiomorphous co-crystals obtained from mixed solutions of 9-methylbenz[c]acridine and diphenylacetic acid is reported to yield the photodecarboxylation-addition product (44) as essentially a racemic mixture, along with diphenylmethane and 1,2-tetraphenylethane.39 The lack of selectivity in the formation of (44) from the co-crystals is considered to arise from the molecular arrangement of the reactants in the lattice, and the same products are also formed from irradiation of the acridine and the acid in acetonitrile solution.

4

Substitution Reactions

The effects of variation of pH, nucleophile concentration and light intensity on the photoinduced substitution of m-nitroaniline by nitrite ion have been reported, and a tentative mechanism has been proposed for the process.40 Good yields (39–93%) of 2,4,6-tribromoanilines can seemingly be obtained by irradiation (l 4 340 nm) of nitrobenzene and some derivatives in concentrated hydrobromic acid.41 The quantum yield for the reaction with nitrobenzene is 0.16, and this is little affected by a 3-carboxy group, but the presence of a 3- or 4-hydroxy substituent has a detrimental effect on the reaction, which leads the authors to suggest that electron transfer to the nitroarene np* triplet from the bromide ion, is the primary process. Photonucleophilic substitution reactions of nitroanisoles continue to attract interest, and the research group that earlier

99


Scheme 3

described the exclusive formation of methoxyphenols from irradiation of 2- and 4-nitroanisoles in the presence of cyclodextrins,42 now report that, using the same experimental conditions, photoamination of 1,2-dimethoxy-4-nitrobenzene by ammonia, methylamine or hexylamine leads to regioselective displacement of the methoxy group which is para to the NO2 substituent, in contrast to the solution reaction which predominantly yields the 3-amino-nitro compound.43 The authors interpret this change in regioselectivity in terms of a mechanistic shift from a SN2Ar* process in the latter case, in which the nitro group is meta-directing, to a route involving electron transfer from the amino nucleophile to the excited state nitroanisole [SN(ET)Ar*] in the presence of the cyclodextrins. The size of the H-D isotope effect observed in the 1,4-dicyanonaphthalene photosensitized cyanation of biphenyls (see Scheme 4) is reported to be dependent on the concentrations of both the substrate and the cyanide ion as well as the ionic strength of the medium.44 These experimental observations are considered to originate from intermolecular electron exchange between the biphenyl and its radical cation, in competition with attack of the cyanide ion. From an investigation of the photoreactions of 4-chlorophenol and 4-chloroanisole in a variety of solvents, it has been shown that reductive dehalogenation is the principal process, which proceeds by a homolytic pathway in cyclohexane and a heterolytic route in methanol or trifluoroethanol.45 The

100


Scheme 4

latter path has been exploited as a convenient route to triplet 4-hydroxy- and 4methoxyphenyl cations, which in the presence of p-nucleophiles (e.g. 2,3dimethylbut-2-ene or benzene) can provide a medium-to-good yield access to arylated compounds: these reactions are summarized in Scheme 5. The formation of products such as (45) from irradiation of cyanobenzenes and 2,3dimethylbut-2-ene in methanol solution is a commonly observed process (photo-NOCAS), but as noted this year this reaction can be suppressed by acrylonitrile, and then (46) predominates.46 The irradiation of 1-bromonaphthalene and sodium sulfide in DMSO solution followed by quenching with methyl iodide is reported to yield 1-methylthionaphthalene (16%), bis (1naphthyl) sulfide (17.5%) and naphthalene (42%) by a radical chain mechanism.47 A similar reaction is also observed with the thiourea anion, and this gives the 1-methylthionaphthalene in a 50% yield after quenching. Many accounts have described the attack of photochemically generated radicals on aromatic compounds, but the recent report of generating such species with electron withdrawing groups by Se–CF2 bond cleavage in (47) and their subsequent reaction with arenes and heteroarenes to give, for example (48) in reasonable yield, is worthy of note in this section.48

101


Scheme 5

5

Cyclization Reactions

Photoinduced 6p-electrocyclization occurs for a wide variety of aromatic systems and attracts considerable research activity, which, over the years, has resulted in a number of commercial applications particularly in the areas of photochromism and molecular switches. Investigations into stilbene derivatives have a long history in this area. trans 3,3 0 ,5,5 0 -Tetramethoxystilbene yields the cis isomer on irradiation, which then undergoes photocyclization and oxidation to 2,4,5,7tetramethoxyphenanthrene (49), and no reversal of the geometric isomerization is observed.49 Similarly, irradiation of (50) obtained by Hofmann degradation of 1,2,3,4-tetrahydro-2,2-dimethylisoquinolinium iodides (51) with methanolic KOH, affords (52),50 and 6p-photoelectrocyclization of (53) gives good yields

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of (54), which provides access to the 2-azachrysene (55) in three further steps.51 This type of process with the dihydroisoquinolin-1(2H)-one derivative (56) is a key step in the first total synthesis of the tetrasubstituted 1,2,3,4-tetrahydronaphtho[2,1-f]isoquinoline (57).52 The photoconversion of cyclohexane or acetonitrile solutions of trans (58) directly into 1-hydroxychrysene in good yields (one-photon absorption) has provided evidence for the involvement of an adiabatic trans* to cis* isomerization,53 but in aqueous solvents (MeCN:H2O, 1:1) trans 3-hydroxystilbene affords solely the hydration products (59) and (60), in marked contrast to the 2- and 4-hydroxystilbenes, which undergo trans-cis photoisomerization.54 Further examples of the use of photocyclizations of mono- and distyryl substituted polynuclear arenes in the synthesis of helicenes have been published.55 In these examples, irradiation of toluene solutions of (61) and (62) with iodine in the presence of propylene oxide as an HI scavenger, gives 62–96% and 70–90% of the [5]- and [7]carbohelicenes (63) and (64), respectively, depending on the substituents. Mechanistic studies into the photochromism of arene [e]-annelated dimethyldihydropyrenes (65) have been described, and appreciably larger quantum yields for the ring opening reaction than for simpler dihydropyrenes have been observed, which, the authors suggest, makes such compounds as (65) more useful as photoswitchable units.56 Stilbenes with functionality on the ethene moiety are not commonly used in this 6p-photoelectrocyclization process. It is, however, now reported that irradiation of 1, 2-diaryl-1-tosylethenes (66) in the presence of base gives the corresponding phenanthrenes and heterocyclic analogues in yields which vary with the base employed (DBU, Et3N, or CaCO3) and the solvent (THF, benzene, CH2Cl2, or methanol).57 For example, 3,6-dimethylphenanthrene is formed in 95% yield from irradiation of (67) in THF in the presence of five equivalents of DBU; the mechanism is considered to involve base-induced elimination of p-toluenesulfinic acid from the intermediate 9-tosyl-4a,4b-dihydrophenanthrene.


103

104



105

Again this year considerable activity has been focused on the photochromic properties of 1,2-diheteroarylethenes, and two groups have reported the results of theoretical studies on these systems. The role of conical intersections in these processes58 and whether the photochromism can be explained by the reaction path alone59 have both received attention. At present, the heteroarylethenes favoured for application in molecular devices would seem to be those based on 1,2-bis(methylthienyl) and 1,2-bis(methylbenzothienyl)perfluorocyclopentenes, and much effort has been devoted to assessing the influence of structural modification and physical state on their photochromic properties and characteristics. For example, the asymmetric compounds (68) and (69), having different sites of attachment to the two heteroarenes, have been investigated and found to be sensitive to wavelengths in the 400–500 nm region and to have good fatigue resistance,60 and for (70) the change of the commonly-used methyl substituents at the 2- and 2 0 -positions by isopropyl groups reduces the efficiency of the cyclization and requires higher temperatures for the thermal reversal.61 The introduction of cyano groups in place of the 2,2 0 -methyl substituents in (71), however, increases the quantum yield of cycloreversion by approximately 30-fold.62 Russian workers have developed methods to introduce functional groups (e.g. halogen, –CHO, –CO2H) at the 4- and 4 0 -positions of the thienyl moieties with the purpose of producing photochromic condensation polymers,63 and a further report into the exceptionally high thermal stability of the cyclized isomer (72) of (73) also notes that the efficiency of the photochemical cycloreversion is comparable to that of the corresponding dithienyl compound.64 The influence of linking photochromic dithienylperfluorocyclopentenes has been investigated with such diverse systems as (74), (75), (76) and (77). Multicolour photochromism has

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been is observed in (74) using 313, 365 and 436 nm irradiation, and by varying the exposure time colour changes in solution through yellow, green and blue have been accomplished, although only one of the two units is cyclized at any one time.65 Again for (75) only one of the open forms cyclizes, and this results in a change from colourless to dark blue,66 and (76) is reported to exhibit reversible photochromism both in solution and in the bulk amorphous phase.67 In contrast to most other systems, the blue colouration induced on 313 nm irradiation of hexane solutions of (77) arises from the presence of both the mono- and dicyclized isomers, and furthermore (77) also exhibits photochromism as a single crystal.68 Interestingly, these dithienyl chromophores are aligned almost perpendicular to each other in the crystal and, using linearly polarized light, one of the photoactive units in (77) can be selectively ring closed. Indeed, a number of reports have been published in the year outlining the photochemistry of dithienylperfluorocyclopentenes in the bulk amorphous phase, crystals, co-crystals, nanoparticles and films. For example, irradiation (334 nm) of (78) in the bulk amorphous phase produced a new absorption band at 600 nm and analysis showed that the closed isomer was formed with a diastereoisomeric excess of 25%.69 The optically active photochromic dithienylethene (79) forms two crystalline phases (a- and b-) both of which undergo 6p-photoelectrocyclization, and, while the a-phase showed no diastereoselectivity in the closure, only the (S,R,R) diastereomer (80) was formed on irradiation of the b-phase.70 The achiral compound (81) is reported to undergo absolute asymmetric photocyclization in chiral crystals, and the two enantiomers of both the open and the closed forms are observed not to be interconverted.71 The three different types of dithienylethenes (82), (83) and (84) form novel two- and three-component multicolour photochromic crystals and in the latter, by using radiation of appropriate wavelengths, the crystals are turned yellow, orange, red, purple, blue, green or black.72 The authors note that such multicoloured photochromic crystals may have application in full-colour displays and other optoelectronic devices. Other workers have also investigated the potential of three-component systems as multi-wavelength photochromic storage devices.73 The derivative (85) having two pentafluorophenyl groups is reported to afford stoichiometric 2:1 and 1:1 co-crystals with benzene and naphthalene, respectively, by using intermolecular non-covalent sandwich aryl-perfluoroaryl interactions.74 Both these species display the photochromism of the dithienylethene unit, but the absorptions of the photoisomers in the co-crystals differ and depend on the conformations of the photoactive molecules. Nevertheless, although the dithienyl ethene pair (85) and (86) also has the possibility of aryl-perfluoroaryl interactions and do form alternate nanolayers of molecular sheets in the 1:1 stoichiometric co-crystal, there is no evidence for the p–p stacking of the molecules.75 Both (85) and (86) undergo photochromism in one-component crystals but in the co-crystals only (86) photoreacts and then only to a conversion of 20%, indicating energy transfer from (85) to (86) in the crystal. Nanoparticles of the dithienylethene (87) are formed by a reprecipitation method, but the aggregation resulted in a decrease in the efficiency of the cyclization process and in a blue shift in the absorption of the closed isomer.76 A self-assembled monolayer technique has been used to prepare


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gold nanoparticles capped with the photochrome (88), which is reported to undergo the expected photochromic activity regardless of the environment.77 Incorporation of dithienylperfluorocyclopentenes in DNA-quarternary ammonium complex transparent self-standing films does, however, have varying effects on the photocyclization process and although derivatives without hydroxy groups behave similarly to their reactions in solution, the presence of such functionality as in (89) markedly slows the reaction in the film.78 Further reports have appeared on the electrochemical ring closure and ring opening reactions of photochromic bis(terthiophene)perfluorocyclopentene,79 and other workers have shown that (90) also undergoes cyclization when its excited state is accessed by electron-carrier injection.80

108



109

The influence on the photochromism in dithienylethene systems of replacing the favoured perfluorocylopentene moiety by other cyclic ethene units has attracted the attention of a number of research groups. Dutch workers have shown that the use of the perhydrocyclopentene unit as in (91) has little effect on the photochromic behaviour of (92) but note that in situations where fatigue resistance and thermal stability are critical, the fluorinated moiety would be preferable.81 The cyclization quantum yield of (93) is reported to increase in the presence of metal ions,82 and more detailed accounts have been published of the interesting application of the dithienylcyclopentene-tethered b-cyclodextrins (94) as photoswitchable hosts for meso tetrakis(4-sulfonatophenyl)porphyrin.83,84

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Several groups have described the photochromic activities of diheteroarylmaleimides. Of the dithienyl compounds (95), that with R1 ¼ CN and R2 ¼ NO2 displayed photochromism even in polar solvents (MeCN, EtOH), but the derivatives with R1 ¼ Ph or OMe and R2 ¼ H were inactive,85 whereas the dithieno[3,2-b]pyrroles (96) with an N-substituted imide moiety are reported to undergo cyclization in acetonitrile solution.86 Interest in the closure reaction of the bis indolylmaleimides (97) arises from the need to access biologically active, naturally occurring indolocarbazole alkaloids such as rebeccamycin (98).87 Indeed, irradiation of (97) in the presence of iodine gave the required indolo[2,3-a]pyrrolo[3,4-c]carbazoles (99) in yields of 84–90%. By using a related photocyclization, French workers have successfully synthesized a new family of isogranulatimide analogues in which the 7-azaindole moiety replaces the indole unit.88 Thus irradiation of (100) in acetonitrile solution affords the cyclized products (101) and (102) in respective yields of 21 and 19%, with a further 24% of (101), without the BOC protection. Oxidation and deprotection of the isolated products gave the two isomeric analogues of isogranulatimides A (103) and B (104). The novel dithienylethene (105) has been shown to act both as a photochromic and as an acidichromic yellow dye, as outlined in Scheme 6,89 and the diheteroaryl derivatives (106), having a 2, 5-dihydrothiophene bridge, have been synthesized although as yet there is no account of their photochemical properties.90 In contrast to other dithienyl ethenes, the dithienyl[1,3]dithiol-2-ones (107) and (108) undergo photodecarbonylation rather than photocyclization and this leads to the 1,4-dithine (109) from the former isomer, while (108) produces the thieno[3,4-c]dithiine (110) in 76% yield by way of a unique ring cleavage.91 The photochemical reactivity of the two dithienylethene systems (111) and (112), having fixed conformations but varying molecular rigidity, has been examined in acetonitrile solution, and although (111) undergoes reversible photochromism the more constrained compound (112) is reported to be unreactive.92 Excellent fatigue resistance in repeat cycle photochromism and high thermal stability of the coloured isomer are reported for the novel 1-(isopropylidene)-2-(thien-3-yl) perfluorocyclopentene (113), which has a quantum yield of cyclization to (114) with 313 nm radiation of 0.43 and of cycloreversion with 405 nm radiation of


111

0.16.93 Such 6p-electrophotocyclization is commonly observed for a variety of 1-arylbuta-1,3-dienes and is also the basic process for the photochromism of fulgides and fulgimides. The mechanism of these reactions has been subjected to a theoretical investigation using PM3 semiempirical quantum chemistry and MECI CI methods, and the results are reported to support the mechanism

112


Scheme 6

evolved from experimental observations.94 The molecular conformational changes in relation to the crystal structure of the photochromic system based on the 3-furylfulgide (115) have been studied.95 Single crystals of (115) turn from yellow to red on irradiation, but during the cyclization process giving (116) the crystallinity is lost. In addition, x-ray diffraction analysis of crystallized (116) has been carried out and is reported to provide the first determination of the molecular conformation and crystal structure of such cyclized compounds. Fulgides of type (117) in PMMA matrices exhibit photochromism which is reported to display no fatigue in over 450 writingerasing cycles and hence is suggested to have the characteristics required for halographic applications.96 A novel class of photochromic (Z)-4-oxazolylfulgimides (118) has been synthesized and, not surprisingly, the absorption characteristics of the ring opened and ring closed isomers are found to be dependent on the nature of the aryl substituent.97 6p-Photocyclization of the 1-arylbuta-1,3-diene moiety in curcumin (119) in non-degassed solution produces 2 0 -hydroxy-5 0 ,6 0 -benzochalcone (120) along with the secondary photoisomer (121) as well as the degradation products benzaldehyde and cinnamaldehyde.98 The authors note that the cyclization of (120) to give (121) is the unique example of the photoconversion of a diarylheptanoid isomerizing to a flavanoid. Interestingly, non-conjugated phenylpolyalkenes of appropriate structure as in (122), for example, undergo intramolecular photocyclization in aqueous acetonitrile and in the presence of an electronacceptor couple such as 1,4-dicyanotetramethylbenzene and biphenyl, to give the polycyclic compound (123) in 22–25% yield.99 This procedure has the virtue of providing a convenient pathway to polycyclic compounds in a single operation.


113

114



115

Further details have been published concerning the efficient photocyclization of the syn rotomer of 2-vinylbiphenyls to give 8a,9-dihydrophenanthrene intermediates (124), which undergo a rapid sigmatropic shift to yield the 9,10dihydrophenanthrenes (125).100 The photochemistry of stilbenylpyrroles (126) in benzene or acetonitrile solution is reported to give a new approach to indole (127) and isoindole (128) derivatives (respective yields 33 and 5%), and, as the authors note, the reaction provides the first example of intramolecular addition of a pyrrole molecule to the ethene moiety of a stilbene.101 The treatment of bis indole and the keto lactam (129) with BF3 Et2O affords a 57% yield of (130), which undergoes efficient 6p-photocyclization and thereby gives a simple direct access to the indolo[2,3-a]carbazole alkaloid staurosporinone (131).102 The indolinylphenylethenes (132) undergo electrocyclization of the 1-aza-1,3,5-hexatriene moiety, giving (133), which give good yields of (134) following oxidation.103 Both 2-vinylstyryl-furans (135) and pyrroles (136) undergo photoinduced cyclization processes. The intermediate (137) from (135) with R2 ¼ H has been previously trapped with oxygen to give the hydroperoxide (138),104 but the derivatives with R ¼ Me, Br, or –CH ¼ CH2 are now reported to give both exo and endo isomers of the benzobicyclo[2.1.1]hexenes (139) and the phenanthrenes (140).105 The former arise by (2pþ2p) photocycloaddition while the latter are explained by a mechanism involving a (4þ2) process followed by dehydration, as outlined in Scheme 7. The photobehaviour of (136), not surprisingly, basically follows that described104 for (135; R2 ¼ H) and gives the tetrahydromethanobenzo[4,5]cyclohepta[1,2-b]pyrroles (141), albeit in low yields.106

116


The photocyclizations of 1-allyloxy- and 1-allylamino-2-halogenoarenes giving dihydrofurans and indoles respectively has been observed to be remarkably enhanced by the presence of enolate ions as an entrainment reagent.107 For example, (142) is obtained in 55% yield from irradiation of 1-allyloxy-2-chlorobenzene but this is increased to 91% when the CH3COCH2 ion is present, and similarly under the latter conditions the yield of (143) from (144) is greater than 96%. Three reports have appeared within the year describing various features of the photocyclization reactions of aryl-substituted N-acyl-a-dehydroalanine derivatives (145), which yield both isoquinolines (146) and 1-azetines (147). The influence of meta substituents on the process has been reported, and while both the 6- and 8-substituted isomers of (146) are formed for R ¼ Me, Cl and CF3, the preference for the former is explained in terms of steric effects, whereas a meta methoxy group exerts both steric and electronic influences, giving 6-substituted2-quinolinone and isoquinolinone derivatives.108 This reaction has been extended to other substituted derivatives of (145), and the mechanism for formation of (146) and (147) has been outlined and is depicted in Scheme 8.109 Both processes occur with 1-naphthyl analogues of (145), and for the conversion of (148), having a chiral auxiliary, into the 3,4-dihydrobenzo[f]quinolinones isomers in the presence of a

117


Scheme 7

tertiary amine, the magnitude of the diastereomeric excess for (S,S)-(149) varied from 0 to 55% dependent on the characteristics of the amine and the solvent.110

118


Scheme 8

The antimicrobial additive Triclosan (150) is commonly detected in surface waters and worryingly has now been shown to undergo a process of dehydrohalogenation-cyclization to yield 2,8-dichlorodibenzo-p-dioxin (151) in both buffered and natural water.111 The photocyclization of phenyl-1,4-benzoquinone giving 2-hydroxybenzofuran has been subjected to time-resolved photoacoustic calorimetric and flash photolytic investigations, and a long-lived intermediate, suggested to be a diradical, has been confirmed.112 The wellknown photoconversion of diphenylamine derivatives into carbazoles has been studied theoretically for Ph2NCH3nPhn (n ¼ 0–3) using semiempirical PM3


119

methods, and while the cyclization is predicted for Ph2NMe, N–C bond cleavage is considered to be favoured for Ph2NCPh3, which is in agreement with experimental findings.113 The photocyclization of N-methylated enaminones (152) has been investigated as a key step in a synthetic approach to analogues of indoloquinoline and pyridocarbazole alkaloids.114 The formation of (153) by this route allowed the preparation in three further steps of the pyridopyrroloquinolines (154) for in vitro testing.114

6

Dimerization Processes

The (4pþ4p) photodimerization reactions of anthracene and its derivatives continue to attract research interest for a variety of reasons. The process with 9-methyl- and 9-chloroanthracenes has been used to monitor structural transformations in crystals. In the former derivative, the cell volume increases at the start of the dimerization process and decreases subsequently, and the product molecules are not fixed during the reaction but are reported to move smoothly in a manner that includes a rotational component.115 It is noted for 9-chloroanthracene that the symmetry of the dimer is defined by the orientation of the monomer, but that the molecules of the monomer in the crystal pack differently from those in the recrystallized dimer.116 Anthracenes 2,6-disubstituted with decyloxy or (Z)-dec-4-enyloxy chains are liquid crystal materials exhibiting smectic A or C phases, in which the photodimerization yield is low compared to the solution phase reaction, mainly as a result of the instability of the dimer at the high temperatures of the mesophases.117 The photodimerization of 1-(anthracen-9-ylmethyl)-3-ethylimidazolium iodide (155), giving the headto-tail dimer (156), has been the subject of two reports within the year,118,119 and the process with 2-anthracenecarboxylate in an aqueous buffer (pH 7) has been shown to be subject to enantiodifferentiation using bovine serum albumin.120 In this case, the regioselectivity in the formation of the four (4pþ4p) dimers (157)–(160) is changed from head-to-tail to head-to-head by adding the

120


albumin, and this also yields the achiral dimers (158) and (159) with enantiomeric excesses of 29 and 41%, respectively, at 5% conversion. The hydrogenbonded molecular assembly (161) has been generated by using the head-to-tail photodimerization of the 9-substituted anthracene (162),121 while intramolecular dimerization of the two anthracene moieties in the resorcin[4]arenes (163) with 350 nm radiation can yield thermally stable (4pþ4p) adducts, and thereby produce a system with potential as a photoswitch.122 Irradiation of an argondegassed cyclohexane solution of the soluble tetracene (164) is reported to give the head-to-tail and head-to-head dimers (165) and (166), respectively, each in yields of 20%.123 Differentiation between the structures of the two dimer isomers is achieved by the excimer fluorescence from (166) and the naphthalene monomer fluorescence from (165), and the authors note that this is the first account of photodimerization for substituted tetracenes. Not surprisingly, irradiation of 9-(phenyloxymethyl)anthracene does not yield an intramolecular cycloaddition product, and instead bond cleavage occurs.124


7

121

Lateral Nuclear Shifts

The photo-Fries reaction of 2-benzoyl-4-benzoyloxyphenol (167) affords 2, 3- and 2,5-dibenzoyl-1,4-dihydroxybenzenes (168) and (169) in respective yields of 48 and 19% on 254 nm irradiation,125 and similarly both 2- and

122


4-rearrangement products are formed from the 5,8-dihydro-1-naphthyl esters (170) in relative and absolute yields which are very dependent on the nature of the R group.126 Two examples have been reported within the year which well illustrate the potential for encapsulation mediation of the photo-Fries rearrangement. Thus, while irradiation of 1-naphthyl phenyl acetates (171) in hexane solution gives eight products from this b-cleavage process, excitation of the solid inclusion complex of (171) in g-cyclodextrin yields essentially only the 2-substituted naphthols (172), which is attributed to the cyclodextrin cavity inducing conformational and translational restrictions on both the naphthyl ester and the reaction intermediates.127 The same research group has reported similar effects by mediating the photoreactions of the same esters in the zeolite NaY.128 In this case, the high selectivity in product formation is considered to arise from the alkali metal ions in the zeolite restricting the mobility of the primary radical pair. Furthermore, these workers also report that the dibenzyl ketones (173), which yield solely decarbonylation products in solution, also afford the rearrangement isomers (174) on irradiation in heavy atom (Cs, Tl) exchanged zeolites. The 313 nm irradiation at 401C of substituent blocked naphthyl esters has allowed Japanese workers to isolate, for the first time, labile cyclohexadienones which are the type of intermediates postulated in the photo Fries rearrangement.129 Thus the 4-substituted rearrangement product (175) was obtained in 22% yield from (176), and, although the 2-isomer (177) was not isolated, it was detected by HPLC at low reactant conversion. Irradiation of (178) under similar conditions, however, did induce rearrangement to give (179) with the acyl group at the 2-position. The two isomeric acylcyclohexadienones have been shown to have quite different photochemical reactivities and, while (175) gives both the starting ester (176) and the 2-rearranged product (177) as well as the naphthol on 313 nm irradiation, (179) under the same experimental conditions proved to be photostable.130 The photo-Fries rearrangement of dibenzofuran-2-yl ethanoate (180) has been investigated under both Lewis acid catalysis and on irradiation.131 Formation of the rearrangement products (181) and (182) is reported to be more efficient under the latter conditions and with CH2Cl2 as the solvent. Similarly, both the 1-acyl- and 3-acyl-2-hydroxycarbazoles (183) and (184), respectively, are formed on either 254 or 313 nm irradiation of the 2-acyloxycarbazoles (185) in various solvents, and the authors suggest this photo Fries rearrangement as a convenient step in the synthesis of carbazole alkaloids.132 Phenylurea herbicides in aqueous solution are reported to display photochemical behaviour which is markedly dependent on the nature and position of substituents on the arene, and, although photosolvolysis dominates for halogenated derivatives, the photo-Fries reaction is observed in other cases.133 Such rearrangement of the p-toluenesulfonanilides (186), giving the 2- and 4-amino-substituted diaryl sulfones (187) and (188), respectively, appears to be a general process, the yields of which (38–72%) are increased by N-alkylation of the sulfonanilides but are not drastically reduced by electron withdrawing substituents on the N-phenyl ring.134 Since 2- and 4-aminoaryl sulfones are not easily accessible, this route would seem to have synthetic merit.


123

124


The photochemistry of diphenylether in methanol solution has been reported to be affected by ultrasound and, while the rate of rearrangement to 2hydroxybiphenyl remains unchanged by sonication, that of the 4-hydroxy isomer is decreased and phenol production is increased.135

8

Miscellaneous Photochemistry of Aromatic Systems

The photochemistry of many aromatic compounds which have a methyl or substituted methyl group in a 2-position to a carbonyl or nitro functionality (H-abstraction) or a hydroxyl group (formal loss of water) is frequently dominated by the formation of quinone methides as reaction intermediates. Many examples of trapping these structural moieties by Diels-Alder reactions have been used in synthetic pathways. The reaction of such intermediates from substituted benzaldehydes with some 17 dienophiles illustrates the considerable potential of the process, added to which, yields of the adducts are generally good.136 For example, trapping the hydroxy-o-quinodimethane species (189) from the aldehyde (190) with methyl methacrylate in toluene solution gives an 81% yield of a 2:1 mixture of diastereoisomers (191).137 The formation of the adduct (192) by irradiation of the tetrahydro-2-oxoquinoline-5-aldehyde (193) with acrylonitrile illustrates the molecular complexity that can readily be achieved from this photoenolization and, furthermore, irradiation of these particular systems at 601C in the presence of the chiral complexing agent (194) produced adducts with excellent enantioselectivity (91–94% ee).138 The photoactive [4-(11-mercaptoundecyl)-phenyl](2-methylphenyl)methanone (195) has been co-adsorbed with other thiols to the surface of monolayer-protected

125


clusters.139 The dienol moieties from (195) are reported to be formed efficiently on irradiation and readily trapped with dimethyl acetylene dicarboxylate. The photoremoval of the 1-[2-(2-hydroxyalkyl)phenyl]ethanone protecting group for carboxylic acids is considered to arise by intramolecular hydrogen abstraction in (196) to form the quinone methide (197),140 and a similar photodeprotection mechanism has also been reported for irradiation of 2,5-dimethylphenacyl esters (198).141 Evidence has been presented that the mechanism of the photorelease of methanol from 2-nitrobenzyl methyl ether arises by initial formation of the quinone methide analogue (199) in a pathway outlined in Scheme 9.142 The quinone methide (200) is proposed to be involved in the selective phototransformation of hydroxy-substituted aromatic nitrones (201) into N,N-diaryformamide derivatives (202), but in this case the intermediate is formed by ring opening of the primary photoproduct, the oxaziridine (203), as shown in Scheme 10.143 Chromene photochromism is known to proceed by formation of quinone methides, and an interesting example of the process described this year is that observed for the novel materials (204) bearing a monoazacrown ether moiety, which allows interaction with metal ions.144

Scheme 9

126


Scheme 10


127

Irradiation of hydroxybiphenyl alkenes and alcohols is reported to yield novel biphenyl quinone methides such as (205) from (206), and in aqueous acetonitrile solution it is these intermediates that undergo attack by water to yield the observed photohydration products.145 Similar photoinduced proton tautomerization in 2-hydroxyphenazine (207) yields (208), which is the origin of the observed photochromism at 77 K in this and related systems.146 Irradiation of 1,4,4,4-tetraphenylbut-2-en-1-one (209) in benzene or acetonitrile solution leads to rearrangement and the formation of trans 1-benzoyl-2,2,3-triphenylcyclopropane (210).147 This reaction is a new example of the concerted mechanism in which an aryl group migrates synchronously with formation of a cyclopropane ring, as much earlier described by Zimmermann and Hancock.148 Dibenzobarrelenes are known to undergo photorearrangement to dibenzocyclooctadienes, and such a reaction of (211) in THF solution to give a 95% yield of (212) has been used in a synthetic route to a tetraphenylene derivative.149 The photoelimination of indan from (213) has provided access to the highly strained dibenzotetrakisdehydro[12]annulene (214), which was trapped by its Diels Alder adduct with furan.150

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Photo-oxidation and Photo-reduction BY NIALL W.A. GERAGHTY Chemistry Department, National University of Ireland, Galway, Ireland

1

Introduction

Photo-oxidation and photo-reduction remain among the most active areas of photochemical research, with interest in general centring on systems involving photo-induced electron transfer (PET), heterogeneous photo-oxidation and photooxygenation. The interest in PET results from the ongoing efforts directed towards producing systems capable of artificial photosynthesis, whereas that in heterogeneous oxidation systems, particularly TiO2, is occasioned by their value in photodegrading potential pollutants. Although there continues to be interest in photooxygenation as a clean or green oxidation process, much of the work in this area remains linked to issues relating to photodynamic therapy (PDT). A number of major new photochemistry books have been published in the period covered by this review1,2 and a new edition of an older work has also appeared.3 Although these books deal with a variety of topics in photochemistry, photobiology and photophysics, many of their chapters constitute useful and up-to-date reviews of areas relating to photooxidation and photo-reduction. A number of these reviews, and of those published elsewhere, are of general interest in the context of the sub-divisions used in this review. Thus reviews dealing with PET reactions in organic chemistry in general,4 with the control elements in PET reactions,5 and with appropriate and readily accessible computational methods for such reactions,6 have been published. A discussion of synthetic organic chemistry in mesoporous silicas,7 which deals with the photodecarboxylation of a-hydroxy carboxylic acids, the oxidation of arylmethyl halides and alcohols and the oxidative cleavage of styrene, and a review which deals with the application of PET reactions in heterocyclic chemistry,8 have also appeared. Reflecting the continuing interest in using zeolites as a medium for photochemical reactions, photo-oxidations,9,10 PET,11 and the potential for achieving selectivity, including enantioselectivity, in this constrained environment12 have been reviewed. A comparison of PET reactions of organic substances with oxygen in zeolite nanocavities and in solution has been provided13 as has a review of some of the most recent achievements in the field of photochemical molecular devices, Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 133

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including a discussion of those involving PET.14 Light-induced charge transfer (CT) at phthalocyanine surfaces and its technological applications,15 and the reactivities of various Ti-based photocatalysts in the context of a range of different reactions have also been reviewed.16

2

Reduction of the Carbonyl Group

Many of the carbonyl group reductions reported involve PET from an amine. Thus the mechanism of the photo-reduction of benzophenone by N,N-dimethylaniline has been discussed with particular reference to experimental results and theoretical models relating to the dynamics of proton transfer in the radical ion pair formed by PET.17 The photo-reduction of o-benzoquinones, such as (1), in the presence of p-bromo-N,N-dimethylaniline is reported18 to give pyrocatechols and hydroxyphenyl ethers (Scheme 1), the former being produced as a result of the disproportionation of two semi-quinone radicals. The hydroxyphenylethers subsequently form pyrocatechols as a result of a thermal decomposition. Quinones feature in a number of other recent papers relating to the photo-reduction of the carbonyl group. The effect, for example, of alcohols and amines on the intermediates involved in the photo-reduction of p-benzoquinones has been studied.19 The reduction step leading to the formation of hydroquinones involves electron transfer (ET) in the presence of amines and hydrogen transfer in the case of alcohols. Photo-reduction processes involving 1,4-naphthoquinone in water have also been investigated using nanosecond flash photolysis and product analysis (Scheme 2).20 A number of transients were observed, to which structures (2) and (3) were assigned. An acid catalysed hydrogen-atom migration subsequently gives 1,4,5- and 1,4,7trihydroxynaphthalene, which in turn form the observed products, 1,4-dihydroxynaphthalene and 5- and 7-hydroxy-1,4-naphthoquinone, following a fast oxido-reduction with 1,4-naphthoquinone. PET between the S1 state of the dihdroxyquinones (4) and (5) and a range of aromatic and aliphatic amines has been studied.21 A significant difference was observed in the diffusion controlled fluorescence-quenching constants obtained for the aromatic and the aliphatic amines, the latter being about 46% lower. The results were rationalized on the basis that there is an orientational restriction on the encounter complexes involving the aliphatic amines, arising from the shape of their HOMOs. Finally, photo-induced hydrogen abstraction by the carbonyl group of homoquinones But

O But

C 6H 6

O (1)

OH

λ > 500nm

+

OH

But

NMe2

But

+

But

But

O N

OH Me

Br

Scheme 1

Br

135


OH

O

OH

O HO hv H 2O

H OH (2) + O

OH +

OH

O

NQ O

OH

HO O

OH

H+

+

+

HO

HO

OH

H

NQ

(3)

OH

O

OH

Scheme 2

O

O

OH

O

OH

Me Ph

(4)

O

OH

O

(5)

Ph Me (6)

O

OH

Me N N Me

X

R3

O R1 R

2

O

R4

(7) or (8) hv, THF BDMAP

O

HO

R1

R3 R4

R2

(7) X = 2-OH (8) X = 4-OH Scheme 3

such as (6) and their participation in PET with amine and arene donors have been reviewed.22 In relation to other carbonyl systems, it is reported that the amines 2-(2 0 hydroxyphenyl)-1,3-dimethylbenzimidazoline (7) and 2-(4 0 -hydroxyphenyl)1,3-dimethylbenzimidazoline (8) promote the PET reactions of a,b-epoxy ketones forming b-hydroxy ketones (Scheme 3).23 The reaction involves an initial single electron transfer (SET) to the carbonyl group, and overall the amines act as the formal donors of two hydrogen atoms. The use of 1,6bis(dimethylamino)pyrene (BDMAP) as a sensitizer allows the reaction to be extended to epoxy ketones that could not be sufficiently activated by direct irradiation using pyrex filtered light. A consideration of the conversion of a,bepoxy ketones to b-hydroxy ketones forms part of a review which focuses on the PET generation of ketyl radicals and their subsequent rearrangement.24 The review also considers the PET reactions of a-bromomethyl ketones and

136

Photochemistry, 36, 2007, 133–204 R1

O 1

R

sunlight R

2

2-propanol

Yield (isolated) = 70-96%

R

R1 R2

2

HO OH R1 = Ph, R2 = H R1 = 4-ClC6H4, R2 = CH3 R1 = R2 = Ph R1 = Ph, R2 = CH3

Scheme 4

4-tribromomethylcyclohexa-2,5-dienones. In general the photo-reduction of ketones and aldehydes can result in the formation of the corresponding alcohol or of 1,2-diols as a result of radical coupling. The amine mediated photoreduction of aryl ketones in N-heterocyclic ionic liquids is reported to give preferentially benzhydrols rather than the expected benzpinacols.25 The benzydrol:benzpinacol ratio is a function of the reduction potential of the intermediate ketyl radical, the results obtained being consistent with a reaction mechanism which involves a dark ET between this radical and an a-amino radical. On the other hand it has been reported that the reductive coupling of aromatic aldehydes and ketones giving 1,2-diols can be carried out in 2propanol using sunlight as the photochemical driving force (Scheme 4).26 Negligible amounts of simple photo-reduction products are formed in this case. Although the reaction proceeds in high yield in many cases, it is however less successful for nitrobenzaldehydes, hydroxybenzaldehydes, furfural and cyclohexanone. In the presence of electron-donor sensitizers such as N,Ndimethylaniline (DMA) or 1,4-dimethoxynaphthalenes (DMN), b,g-unsaturated aldehydes such as (9) undergo a range of photochemical transformations which involve oxa-di-p-methane rearrangement (forming (10)), decarbonylation, and proton transfer (Scheme 5).27 The outcome of the reaction depends on the electron donor used and the fact that competitive ketyl- and alkene-centred radical anion formation is possible. The ferrocene-anthraquinone dyad (11), which has a rigid amide spacer, is typical of structures designed to mimic natural photosynthetic systems.28 A remarkable 7 106-fold extension in the lifetime of its charge separated (CS) state (Fcd1– AQd) was obtained in benzonitrile at 298 K by the addition of Y(OTf)3, which binds strongly to the AQd unit and in so doing decelerates back electron transfer (BET).

3

Reduction of Nitrogen-containing Compounds

A review of the phototochemistry of N-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic N-oxides.29 A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested.30

137

Photochemistry, 36, 2007, 133–204 Me

Me

Me

Me Me

Me CHO

+ (10) 1% 16%, inseparable mixture Me Me CHO

8%

Me Me

hv DMN

CHO

Me Me

HO CHO hv DMA Me Me

(10) 25%

(9)

13% OH

Me

Me OH

O

16%

4%

Scheme 5

A number of papers have appeared which deal with mechanistic issues relating to the reduction of nitrogen-containing compounds. Thus an EPR study has been carried out on the photo-reduction of nitroso compounds in suspensions of TiO2 in alcohol.31 Nitrosobenzene, for example, is efficiently reduced under these conditions, forming exclusively the stable radical (12). This observation is consistent with the generally accepted mechanism for the photoreduction. The photo-reduction by amines of a series of nitrobenzenes with electron-donating and electron-withdrawing substituents in the 4-position has also been studied.32 EPR spin-trapping experiments showed that photo-reduction in the presence of triethylamine (TEA) gave only the a-aminoethyl radical when an electron-donating group was present. This behaviour is reflected in the fact that these compounds in the presence of an amine are efficient photoinitiators of methyl methacrylate polymerization. On the other hand, polymerization in the presence of nitrobenzenes with electron-withdrawing groups was extremely inefficient, reflecting the fact that a-aminoethyl radicals are not formed in these cases. The nature of the transients observed in a time-resolved UV-visible spectroscopic study of the amine-mediated photo-reduction of dinitroarenes, the ultimate product of which is a nitrosoarene, depends on whether TEA or a N,N-dialkylaniline is used.33 In the case of the former, the observed transient is believed to be the radical (13), whereas the use of the latter involves the formation of (14). The photo-reduction of p-nitroaniline has also been the subject of an EPR study.34 The pulse radiolysis of 8-bromoadenosine (15) (Scheme 6) has been studied with a view to providing a molecular basis for the behaviour of RNA radicals and of biologically important adenosine derivatives.35 It was found that the reaction of hydrated electrons with (15) resulted in the formation of a C-8 radical which, following rapid hydrogen abstraction, gave the sugar-based radicals (16) and (17). The C-5 0 radical (16) subsequently

138


O

N H

Fe

O Y(OTf)3

(11)

R

N

R1 O2NC6H4

N H O

HO

R = Me, R1= H R = Et, R1 = Me

(12)

N

O

(14)

(13)

N HO

N O

N

N

Br

HO

N

N

O

N

OH

e-(aq)

N

N O HO

OH

N

HO

OH

NH2 N

N HO

N O

40% HO

OH (17)

N

OH

NH2

N

N

O

(16)

NH2 N

HO

HO

60%

N

HO

N

(15) HO

NH2

NH2

NH2

N

N N N

HO

HO

N + O OH

Scheme 6

adds to the adenine unit whereas bond cleavage in the C-2 0 radical (17) releases adenine. Many papers are concerned with systems and issues relating specifically to the phenomenon of PET. There is ongoing interest in phthalimides as electron acceptors in PET processes, and their photochemistry in this regard has been reviewed.36 A series of fluorescent dimethoxyphthalimides (18) has been used to explore the detail of the ET process itself.37 Although strong intramolecular fluorescence quenching was observed for the thioether and the tertiary amine derivatives, the glycine derivative was the only carboxylate to demonstrate strong activity. The nature of PET in the 1,8-naphthalimide-spermine conjugate (19) and related systems has also been explored.38 Ground-state aggregation was observed for (19) and the naphthalimide radical anion, and triplet species were identified from its transient absorption data. The intramolecular dimer is believed to play an important role in the intramolecular PET processes

139


in this system. The importance of short range supramolecular interactions emerges from a study of PET with carboxylated Ru(II) complexes.39 Thus the use of the carboxylated ligands (20) or (21) facilitates a supramolecular interaction with methyl viologen (MV21), leading to the formation of methyl viologen radical cation (MVd). The peripheral carboxylate is considered to define a ‘bubble’ of electrostatically attractive space that allows pre-association with MV21, providing PET with an entropic advantage. Two suitably disposed carboxylate groups provide an even more favourable electrostatic environment. Ruthenium complexes, specifically dipolar ruthenium(III/II) pentaamine-Nmethyl-(4,4 0 -bipyridinium) complexes, in a two-phase (water-benzene) system in which they are soluble only in the water, constitute the first example of a light-driven molecular switch which is activated by selective irradiation of the water and benzene layers at 254 and 528 nm, respectively.40 The easily synthesized 5-(2-pyridyl) pyrazolate boron complexes (22) may have potential uses in organic light emitting devices as they exhibit dual fluorescence in solution as a result of a PET from the phenyl group to the pyrazolate ligand.41 The photophysical properties of a molecular conjugate formed by including a highly fluorescent acceptor guest (23) in an electron-donating macrocyclic host (24) has been investigated.42 The crown ether sits loosely at the end of the anthradiisoquinolinium polycycle, and there is evidence that a 3-pseudorotaxane is formed, based on an interaction with two crown ethers, as the mole fraction of the former is increased. Photochemically the interesting feature of this system is that the triplet excited state localized on the polycyclic guest has an energy which is much lower than the CT state formed by intra-complex PET. This means that charge recombination populates the triplet state of the guest with much higher probability than in the absence of complexation. O MeO

X N

MeO (18)

O

O

n

O X = SMe, n = 2 X = NEt2, n = 1 X = CO2H, n = 1-3, 5

N

H N

N H

N O

O

(19)

COO

(21)

N N

N

N

(20) CO2

N

N

140


R R1 N

N N B

Ph

(22)

Ph

R1 = CF3, CF3CF2, Ph, But; R = H R1 = CF3; R = Me

O

O

O

O Me

N

O (24)

N Me

PF6-

PF6O

(23)

O O

O

O

A number of new sensor systems based on PET processes in which nitrogencontaining units are the acceptors have been described. The 1,3-bisisothiouronium-naphthalene conjugate (25) operates on the basis of PET from the naphthalene to the electron deficient isothiouronium unit.43 The most noteworthy feature of this anion sensing system is that, although it forms a 2:1 host-guest complex with HPO42, binding with acetate involves biphasic 1:1 and 1:2 hostguest stoichiometries. This suggests that the binding modes of (25) vary according to the shape of the oxoanions involved, a feature that could be exploited in developing fluorescent sensors. Donor-acceptor interactions in the naphthalenediiminopyridine ligands (26) are promoted44 by complexation with Zn21. Thus, although the free ligands do not generate donor-acceptor adducts, the formation of light-emitting CT species is promoted by metal ion coordination. This effect is due not only to the fact that complexation results in the creation of an electron deficient pyridine ring, but also to the geometry imposed on the ligand in the complex, which favours intracomplex donor-acceptor interactions. Such an effect could again be of importance in the design of sensor systems. Me Ph

S

NH R NH

N H N

H N

R

N N

Me

S

(25)

(26) R = H, Me Ph

141

Photochemistry, 36, 2007, 133–204 O R2

R3

O

MeCN, aq NaOH NH

R1

R3 N R1

hv

R2

O

O R 1 = R 2 = R 3= H R1 = Me, R2 = R3= H R1 = R2 = Ph, R3= H R1 = R2 = H, R3= OMe R1 = Me, R2 = H, R3= OMe

Yield: 59-90% (based on 73-92% alkene conversion)

Scheme 7

There continues to be an interest in the development of novel reaction environments for photochemical reactions. The potential of ‘quasi-solids’, which are obtained from polysaccharides and contain a lot of water, has been explored45 using the ET quenching of photoexcited Ru(bpy)321 by MV21. In agarose quasi-solids, the PET occurs by a dynamic mechanism identical to that operating in water. PET quenching in anionic k-carrageenan quasi-solid, however, involved not only a dynamic mechanism but also a static mechanism that does not require the reaction components to move at all. A microreactor, based on SiO2 capillaries (length, 5 cm; ID, 530 and 200 mm), to whose inner wall TiO2 coated colloidal SiO2 particles are bound, has been described.46 The reduction of methylene blue was carried out by injecting a solution of the reactant through the microcapillary using a syringe pump, while irradiating at 254 nm. A 150-fold increase in the reduction rate was obtained relative to a comparable batch system. The photo-reduction by silver nanoparticles of 4-nitrobenzenethiol in self-assembled monolayers on a gold surface has been reported.47 The reduction is initiated by visible light (514.5 nm, Ar1 laser) and involves the emission of photoelectrons by silver nanoparticles in contact with the organic monolayer. There have been two papers relating to the synthetic application of the photo-reduction of nitrogen-containing compounds. One of these deals with the photoaddition of phthalimides to unactivated doubles bonds such as those in cyclohexene or styrenes.48 The reaction is regioselective in the case of the styrenes (Scheme 7), giving N-phenylethylphthalimides that are effectively protected phenethylamines. The reaction involves phthalimide in equilibrium with its conjugate base, and it is proposed that it proceeds via the nucleophilic attack of phthalimide anion on the alkene radical cation produced by SET to excited phthalimide. The N-phenethylphthalimides (27) and (28) were prepared by the reaction of phthalimide with isosafrole and indene, respectively. The electron-acceptor properties of the phthalimide group is the key element in a novel synthesis of cyclic peptides (Scheme 8).49 The acyclic substrates contain N-terminal phthalimides as the light absorbing electron acceptor and C-terminal a-amidosilane or a-amidocarboxylate groups. Following initial SET to the excited phthalimide, a series of intra-chain SETs generates a set of equilibrating zwitterionic biradicals. The conversion of one of these intermediates to a neutral biradical is the product determining step, with the loss of an

142


O

O O

N

N Me

O

O

(28) 90%, 75% conversion

(27) 51%, 71% conversion

O O

R O

N

R

R N

N R

O

O

O

HO N

R

E

R

R

N N

hv

O n

SET

n

R

N O

O

N

R

R

Scheme 8

electrofugal group from a carbon centre next to a radical cation site being an ideal control process. Recombination of the resulting biradical gives the cyclic peptide in modest to good yield.

4

Miscellaneous Reductions

A small number of papers dealing with matters of general mechanistic interest in the context of photo-reduction have appeared. Benzyne radical cations have been produced using the photo-induced reaction of complexes derived from fluorine substituted benzenes and Mgd1 (Scheme 9).50 The complexes considered divide themselves into two groups: those that involve o-difluoro substituted benzenes and bidentate coordination, such as (29), and those in which the Mgd1 can only coordinate to one fluorine atom, for example (30). On photolysis, the former were found to generate benzyne radical cations efficiently (Scheme 9), whereas yields of the radical cation from the latter were low. It is believed that the formation of oxetanes by nucleotide bases constitutes an important route for photo-induced DNA damage and that the photosensitized cleavage of oxetanes is a correspondingly important repair mechanism. In this context, the dimethyluracil adducts (31) were synthesized and a study of the photosensitized cycloreversion carried out (Scheme 10).51 It was shown that the F Me

Me

F Mg

Me

hv

+ MgF2

F

F (30)

(29) Scheme 9

Mg

143


O Me O

R

N

O +

N

Ph

hv, λ > 290nm R1

hv, photosensitizer

Me

N

O

R = H, R1 = Ph R1 R = H, R1 = H R = Me, R1 = Ph H Ph O

N Me (31)

Me

R

Scheme 10

cleavage process was consistent with the involvement of oxetane radical anions, providing further support for the ET repair mechanism. Much of the other work in this area also relates to PET processes. The PET reactions of organosilicon compounds, particularly monosilanes, have been reviewed.52 Particular consideration is given to the mechanistic aspects of these reactions in which the organosilanes act as electron-donors. The first definitive evidence that PET occurs in fluorescein derivatives has been obtained for systems such as (32) in which the electron-donor aromatic unit is directly attached to the acceptor xanthene unit.53 Laser flash photolysis experiments produced transient absorption spectra showing bands due to the radical cation of the aromatic unit and the radical anion of xanthene. The fluorescence properties of the fluorescein probes considered were shown to be dependent on the rate of PET from the aromatic donor to the singlet excited state of the xanthene. It is suggested that this work provides a quantitative basis for the rational design of fluorescence probes. Intramolecular PET has been demonstrated for a donor-acceptor system based on ferrocene attached to a single-wall carbon nanotube (SWNT).54 The photoexcitation of the SWNT-ferrocene combination with visible light results in PET and the formation of a long-lived (SWNTd)(ferrocened1) species, as indicated by steady-state electrolysis and time-resolved pulse radiolysis. The general behaviour of these nanohybrids suggests that they have a promising future in terms of solar energy conversion.

(32)

MeO

CO2Me

O

O

A number of light promoted reductions are of interest from the synthetic perspective. The irradiation with a xenon laser of a DMF solution of electrogenerated nickel(I) salen, water and a primary alkyl halide such as 1-bromohexane, followed by a brief exposure to air, results in the formation a product mixture containing a significant amount of aldehyde (Scheme 11).55 A more

144


Me

Br

n-C16H34

Ni(I) salen, H2O, O2 DMF, hv

H

+

+

n-C8H18

O

33%

42% +

Me 1%

5%

Scheme 11

Me

O

Me

Me

O

Me O

O

X

2

X

O

X

O Me O

(33), X = Cl; Yield (X = H, 88%) (34), X = SePh; Yield (X = H, 84%)

Me (35), X = OTs; Yield (X = H, 77%)

Cl

O CO (50 atm), SmI2 68%

THF, >400 nm

Scheme 12

Me LiAlH4 Me O Me Me (36)

Et2O hv

+ Me Me

Me

Me

Me

Me

+ Me Me

Me

Me

Me 21%

Me OH

OH

OH

68%

7%

Scheme 13

complicated mixture, which contains some 2-octanone, is obtained from 2bromooctane. Although the details of the mechanism are unclear, the product composition is clearly typical of a process involving alkyl radicals. Irradiation with visible light also has a profound effect on the ability of samarium diiodide to reduce organic halides and chalcogenides, for example (33)–(35), to hydrocarbons.56 Carbonylation occurs if the photo-reduction of alkyl chlorides (RCl) is carried out under an atmosphere of CO (Scheme 12). It is suggested that the reaction involves the formation of acylsamarium intermediates (RC(O)SmI2) which dimerize and give the observed products following reduction with further SmI2. A combination of UV irradiation and LiAlH4 was used to carry out the reductive cleavage of the allylic ether unit in (36) (Scheme 13), a key intermediate in the synthesis of jungianols.57 The reaction involves the photochemical electrocyclic ring opening of the oxacyclohexa-1,3-diene and reduction with

145

Photochemistry, 36, 2007, 133–204 H

Me

H

Br

H

t

liq. NH3, BuOH

+

+

hv

X

X

CO2Et (37)

X = O, NR

O 3-7%

91-100%

Scheme 14

Me (37)

ET hv

X (38)

+ X

X

CO2Et

Scheme 15

O

O

O TEA, LiClO4 H

+

H

MeCN, hv (254 nm)

Me

H

3%

(39) 40% Scheme 16

O O TEA, LiClO 4 MeCN, hv (254 nm) (40) 74% Scheme 17

LiAlH4 of the resulting dienone. A photochemical method for the tin-free reductive cyclization of suitably substituted aryl halides and the hydrodehalogenation of aryl and alkyl halides has also been reported (Scheme 14).58 The key feature of the method is that the hydrogen donor is the monoanion (37) obtained from ethyl benzoate. PET generates the radical anion of the substrate that fragments to give the radical (38); cyclization and hydrogen transfer from the benzoate anion completes the formation of the product (Scheme 15). Under the same conditions, bromoadamantane gives adamantane in 70% yield. Finally, reductive PET-induced fragmentation of bicyclo[n.3.0]alkanones, followed by cyclization, has been used to prepare a new triquinane (39) (Scheme 16) and a new propellane (40) (Scheme 17).59

146

5


Singlet Oxygen

5.1 Singlet Oxygen. – Most of the recently published work dealing with the generation of singlet oxygen (1O2) in general, or the design and synthesis of new photosensitizers in particular, is related to the topic of photodynamic therapy (PDT). Transient grating measurements have been proposed as a method for evaluating Type II PDT agents on the grounds that no additional chemical agent has to be added to the dye solution and that it is sensitive enough to be used on a micromolar scale.60 The method was applied to a series of sensitizers, including the recently introduced phthalocyanine (41), which share a common aromatic macrocycle and metal centre and differ only in terms of peripherally attached groups. The importance of fluorination on the properties of dyes is underlined by results obtained for the polyfluorinated cyanine dye (42), which has been synthesized as a potential fluorescent label.61 The dye exhibited significantly reduced aggregation in aqueous media, an enhanced fluorescence quantum yield and a greater resistance to photobleaching through direct irradiation or reaction with 1O2. Quantum yields for 1O2 generation by the squarylium cyanine dyes (43) have been reported.62 The values obtained for a number of the dyes, coupled with the fact that they absorb in the so-called ‘phototherapeutic window’ (600–1000 nm), has led to the suggestion that they may be of value as PDT sensitizers. The direct timeresolved spectroscopic observation of 1O2 phosphorescence has been used to provide time constants for its population and depopulation, and a measure of photosensitizer phosphorescence lifetime.63 The behaviour of meso-tetraphenylporphine was considered and it was shown that the lifetime of 1O2 decreases with increasing photosensitizer concentration, confirming that it can also act as a quencher of 1O2. The potential of using ceramic-based nanoparticles as carriers of photosensitizers for PDT has been described.64 Thus the anticancer drug 2-devinyl-2-(1-hexyloxyethyl)pyropheophorbide (44) has been encapsulated in the non-polar core of micelles by hydrolysis of triethoxyvinylsilane. The resulting particles are uniform in size, having an average diameter of 30 nm, and are stable in an aqueous medium. The uptake of the nanoparticles by tumour cells, and significant cell death following 1O2 generating irradiation at 650 nm, has been demonstrated in vitro. A number of new photosensitizers have been developed specifically with PDT in mind. A series of octaalkynyl tetra-6,7-quinoxalinoporphyrazines have been synthesized and their efficiency in generating 1O2 has been assessed using a 1,3diphenylisobenzofuran assay.65,66 The compound (45) was found to be particularly efficient, generating 1O2 with a quantum yield (0.7) that exceeds those of some sensitizers currently in use in PDT. The number of carboxylic acid groups on the water-soluble core-modified porphyrins (46; X ¼ S) had little effect on the quantum yields for 1O2 generation (F ¼ 0.74–0.80).67 The corresponding Se analogues were less efficient (F ¼ 0.30). An in vitro study of the dark and phototoxicities, cellular uptake and localization of these

147


photosensitizers was also carried out. The production of 1O2 by the sugarpendant C60 derivatives (47), obtained together with the corresponding bis-sugar derivatives from carbohydrate-linked azides, has been demonstrated by the direct observation of 1O2 emission at 1270 nm.68 The photocytotoxicity of the mono-sugar derivatives is a function of the pendant sugar unit. A 1:1 complex of lanthanum ion with hypocrellin A (48) combines high efficiency (F ¼ 0.9) for 1O2 generation with a large absorption in the therapeutic window and high water solubility, and is thus another promising sensitizer in terms of PDT.69 Finally, the efficient nuclease activity of a ternary copper(II) complex, [Cu(phen)(met)(MeOH][ClO4], has been attributed to the intercalating properties of the 1,10-phenanthroline, coupled to the ability of the L-methionine to generate 1O2 on irradiation with UV (312 nm) or visible light (436, 532 nm).70 It is suggested that the complex will be of value as a PDT agent as it possesses the bio-essential components copper and L-methionine.

(F3C)2FC

CF(CF3)2

F

F

F

N

(F3C)2FC

CF(CF3)2

N

N

N

F

CF(CF3)2

N

Zn

N

F

N

N

(F3C)2FC

F

F

F

(F3C)2FC

CF(CF3)2 (41)

O F

F

F

F S F

EtSO4

F

Et

Et

F

N

N R

N

X

X

S

N (42)

F

O

R

(43) X = S; R = Me, Et, n-C6H13 X = Se; R = Me, Et, n-C6H13 X = HC=CH; R = Me, Et, n-C6H13

148


R

R

N

N

O(CH2)5CH3 Me

Me Me

NH

N

N

Me R

N

N

N

HN

R

Me

N

N

N

N

N

R t

Bu

O C O2H

R

N

Zn

N

Me

N

N

(44)

(45) R = N

N

R

R

t

Bu

R4

R1

C60

(47)

X Sugar:

N

N

sugar

N

OH

OH O

HO HO

X

OH

HO

HO O

HO HO

O

O

OH O

HO

R3

R2

OH

(46) 1

2

O

HO HO

O

O

OH

OH

3

4

HO HO

R = OCH2CO2H, R = R = R = H; X = S R1 = R4 = OCH2CO2H, R2 = R4 = H; X = S R1 = R3 = R4= OCH2CO2H, R2 = H; X = S R1 = R2 = R3 = R4 = OCH2CO2H; X = S R1 = R4 = H, R2 = R3 = OCH2CO2H; X = Se

O

OH HO

O

O HO

OH OMe Me

MeO MeO

OH COMe OMe O

O

OH (48)

OH

O


149

A number of papers relating to the generation of 1O2 in a more general context have also appeared. The incorporation of C60 into a Y zeolite or an MCM-41 silicate framework, for example, results in the significant lengthening of its triplet-state lifetime such that it persists for several minutes.71 The system is also capable of generating 1O2 efficiently, as indicated by the detection of a near-IR emission at 1270 nm. The generation of 1O2 via the triplet metal-ligand CT (MLCT) states of Ru(II) and Os(II) bipyridyl complexes has been reported, the efficiencies varying between 0.10 and 0.72.72 The factors affecting these values include the oxidation potential of the complex, the energy of its lowest excited state and the spin-orbit coupling constant of the central metal. The potential of another complex, Pt(binap)2, as a photosensitizer has also been explored, again using the established 1O2 scavenger, 1,3-diphenylisobenzofufan.73 Finally, there have been a number of mechanistic contributions relating to the generation and behaviour of 1O2 in a variety of situations. A detailed study on the effect of solvent on the quantum yield for 1O2 generation by fluoren-9one has been reported.74 The quantum yield for intersystem crossing (ISC) decreases as the polarity and protic character of the solvent increases, resulting in a corresponding reduction in the quantum yield for 1O2 generation. Quantum yields of unity were observed in alkanes. Although the main agents in TiO2 photocatalysis are hydroxyl and superoxide radicals (O2d), there have been reports suggesting that 1O2 may also be involved. The generation of 1O2 in an aqueous photocatalytic TiO2 system has now been demonstrated by the observation of near-IR phosphorescence at 1270 nm.75 It is suggested that the most plausible route for its formation is the photocatalytic oxidation of O2d at the TiO2 surface. The possible role of 1O2 in photocatalysis is discussed. 5.2 Oxidation of Aliphatic Compounds. – A review of the photooxygenation of 1,3-dienes that includes a discussion of its mechanism and applications, and of the diastereoselectivity of the [4þ2] cycloaddition reaction with 1O2, has appeared.76 A number of papers have provided additional information about the mechanism of these oxidation reactions. A kinetic comparison of the reaction of 1O2 with unfunctionalized alkenes with prenol-type allylic alcohols, ethers and acetates provides evidence that a combination of hydrogen-bonding interactions and physical quenching controls the reactivity, regioselectivity and diastereoselectivity of the ene reaction (Scheme 18).77 The photochemistry of the retinoid A1E (49) reflects a competition between cyclization, giving pyridinium terpenoids (50), and peroxide formation, with the outcome being determined by the concentration of A1E and oxygen, and the lifetime of 1O2, in the solvent used (Scheme 19).78 A series of physical measurements supports the involvement of 1O2 in the formation of the cyclic peroxide (51) with, it is proposed, the A1E acting as a sensitizer. There continues to be considerable interest in the effect of the reaction medium on the outcome of 1O2 oxidations. The photo-oxidation of hydrocarbons in cation-exchanged zeolites has been the subject of a review in which the authors focus on their work in this area and assess the industrial potential of these reactions.79 The examples provided in another review emphasize that the

150

Photochemistry, 36, 2007, 133–204 OH

Me

Me

1

OH

OH

O2 Me

Me

Me

+

Me

Me OOH

OOH d.r. CCl4 93 : 7 CH3OH 73 : 27 CD3OD 68 : 32 HO

> 95

Me

80

MeO Me

Me

d.r. 93 : 7 krrel = 1.0

82

AcO Me

Me

Me d.r. 40:60 krrel = 0.09

d.r. 72 : 28 krrel = 0.25 Scheme 18

Me Me

Me

Me

OH

N X (49)

Me hv (λ > 400 nm) MeOH

hv (λ > 400 nm) CCl4, O2

H Me

Me

N

Me

Me

Me OH

N

Me

X

Me

OH Me

(50)

O

O

(51)

Scheme 19

OOH Me

Me Me

O2, hv

Me

HOO Me

Me

Me

+

C60, C60-Al2O3 or C60-SiO2

Me 1:1

Scheme 20

reaction cavities of zeolites provide a confined reaction environment that is well-defined and capable of delivering a high level of selectivity.80 The regioselectivity obtained for the 1O2 oxidation of alkenes in zeolite Y due to alkali metal cation-olefin interactions is one of the examples discussed. Very active photocatalysts for the liquid phase oxidation of alkenes have been prepared by depositing C60 on silica and alumina, the use of the latter resulting in a somewhat more efficient catalyst.81 The oxidation of 2-methyl-2-heptene was used as a model reaction (Scheme 20), and an approximately one hundred-fold

151


increase in photocatalytic activity was observed for the supported photosensitizer relative to unsupported fullerene. The same reaction selectivity was obtained with the supported and unsupported form of the photocatalyst. The platinum(II) terpyridyl acetylide complex (52) was incorporated into Nafion membranes and was used as a photosensitizer to promote the photo-oxidation of 7-dehydrocholesterol, a-pinene and cyclopentadiene.82 It was found initially that reaction in deuteriomethanol was much more efficient than in methanol, owing to the much shorter lifetime of 1O2 in the deuteriated solvent. Using a small amount of D2O to swell the complex-incorporated Nafion membrane prior to its use in standard undeuteriated solvents produced a highly efficient photosensitizer that could be recovered and reused 20 times without a significant loss in efficiency.

N MeO

N

ClO4 Pt

N (52)

Finally, there have been a number of papers that deal with the synthetic potential of the photo-oxidation of aliphatic substrates. The photooxygenation of trans-8-(acetyloxy)bicyclo[4.2.0]octa-2,4-dien-7-yl acetate (53) (Scheme 21), for example, is a key step in the stereospecific synthesis of a new inositol analogue (54).83 The same group has also used photooxygenation in two different routes to the cyclohexenetriol (55), which was subsequently converted to another highly oxygenated system, ()-proto-quercitol (56).84 One route involves the conversion of cyclohexa-1,4-diene to a mixture of bicyclic endoperoxides (Scheme 22) and the other the formation of the epoxy-hydroperoxide (57a) from cyclohexene epoxide. The 1O2 cleavage of a vinyl ether giving an aldehyde has been used in the synthesis of tetrazoylacroleins (Scheme 23).85 An interesting difference in the reactions of cyclopropylidenecyclobutenes (58a) and cyclopropylidenecyclobutanes (59a) with 1O2 has been reported (Scheme 24).86 The former were found to react slowly, via a photoinitiated, radical based, autoxidative epoxidation process, forming spiroketones. The cyclopropylidenecyclobutanes on the other hand react rapidly with 1O2, OH H

H

OAc OAc

H 1

O2, TPP, hv

CCl4, rt, 70%

OAc

HO

OAc

HO

H

OH

O O H

(53)

HO

H (54)

Scheme 21

OH

152

Photochemistry, 36, 2007, 133–204 OH

OH HO HO

OH OH

OH OH

(55)

(56)

OOH

OOH

TPP, O2

O

hv

O

OOH

+

O

O

12 : 88 Scheme 22

OOH TPP, O2

O

90%

hv (57a)

O

OHC N

N N MeO

N

N

N

N

N

O2, TPP

70%

hv, 4-7 days CH2Cl2 Cl

Cl Scheme 23

O R1

R1

O

R1

sens, O2 hv R

R

R

(58a) O

HOO sens, O2

OH

hv R

R (59a) Scheme 24

R

153


generating a cyclopropyl hydroperoxide which then undergoes a Hock cleavage. It is suggested that the low reactivity of the cyclobutenes may be due to the fact that reaction would lead to the formation of a cyclobutadiene. 5.3 Oxidation of Aromatic Compounds. – Most contributions relating to the 1 O2 oxidation of aromatic compounds in the period covered by this review are mechanistic in nature. In work directed towards providing a better understanding of the genotoxic, carcinogenic and mutagenic effects of 1O2, the reaction of 2 0 -deoxyguanosine in the presence of methylene blue under aerobic conditions has been investigated (Scheme 25).87 The formation of the novel diimino-imidazole (57) is proposed on the basis of spectroscopic and chromatographic data. It was also shown that (57) is produced directly and that 8-oxo7,8-dihydro-2 0 -deoxyguanosine is not involved in its formation. The reaction of 1 O2 with indolizines (58) has been shown to follow a general pattern that involves initial attack at C-3, forming a peroxidic zwitterionic intermediate (59) (Scheme 26).88 In methanol this intermediate is trapped giving 3-(2pyridinyl)propenoates (60), whereas in acetonitrile the dioxetane (61) in equilibrium with (59) undergoes homolysis forming a 3-(2-pyridinyl)-2-oxiranecarboxaldehyde (62). A study of the effect of substituents on the conversion of

O N HO

N O

H N

HN

NH

NH N

NH2

methylene blue

HO

N

HN O

+

hv, O2 OH

OH

other products

(57)

Scheme 25

Ar COAr

COAr Ph

N

(58)

R 1

R

MeOH

Ph

N

HO

COAr

N O OH

R

Ph

+

N

CO2Me R

(60)

O2 COAr

N

(59)

Ph

Ph

ArOC O

MeCN

O

N O O

R

COAr

N

O R (61)

Scheme 26

R

Ph CHO (62)

O O Ph

154


Ar Me

S

Me O H

O O (63) Me HOO Me Me Me

Me

N

Me

TPP, O2 OH

Me

OH N

HOO

N

Me

CH2Cl2

O

N

Me Me major product +

Me

O

Me

Me

O

O

HOO

N

O

Me Me 50%, 5:1

Scheme 27

4-substituted thioanisoles to the corresponding sulfoxides provides evidence for an electrophilic mechanism as the rate correlates with s (r ¼ 1.97) and Eox.89 In methanol, sulfoxides are formed via a persulfoxide intermediate, which is stabilized by the alcohol through hydrogen bonding, nucleophilic addition or a combination of both hydrogen bonding and nucleophile stabilization (63). The reaction is slow in aprotic solvent but quickens significantly if carboxylic acids are added, the protonated persulfoxide being formed in this case. A few examples of the use of 1O2 as a selective reagent in synthesis have been reported. Chiral bicyclic lactams have been prepared using a route in which the diastereoselective oxidative cyclization of 2-methylpyrrole derivatives using 1O2 is the key step (Scheme 27).90 The trans isomers are the major products in all cases. Considerable selectivity is also displayed in the use of 1O2 as part of a sequence that discriminates between the two indole units in (64) and completes the first enantioselective total synthesis of the heptacyclic bisindole alkaloid okaramine N (65).91 Stable endoperoxides are formed on photooxygenation of 3-alkylidenedihydrofuran-2,4-diones (66) in the presence of CuSO4, with the reaction involving an initial [4þ2] cycloaddition of 1O2 to the enolic form of the dione (Scheme 28).92 In contrast, photooxygenation using O2 and tert-butyl hydroperoxide leads to the formation of hydroperoxides, which slowly decompose to diols. Finally, the use of a photoxidation-reduction sequence has been used to convert prenylated dihydroxycoumarins and trihydroxyxanthones to the corresponding o-(2-hydroxy-3-methylbut-3-enyl)phenol derivatives in yields of 8-65% (Scheme 29).93 The distribution of the photooxygenation products is controlled by the so-called ‘large group’ effect and a stabilizing effect due to the phenol groups.

155


Me N Me H N

O H

i) MTAD (ii) O2, sunlamp methylene blue

H

H

Me

H

O

O

N

N Me H Me

H

N

O O

O

OH

O

Me O2, hv CuSO4 p-TsOH CH2Cl2

N

O

Me

Me Me

H

N Me H Me (65)

Me Me

(64)

Me

N

N

N H

Me

OH

N H

N

O O

N (iii) Me2S O

Me

OH

N H

Me

O

N

Me

Me OOH

Me

O

O O

O2, hv TBHP

O

CH2Cl2

n

n (66)

n 60 - 67%

OH

O

60 -72%

Scheme 28

Me

O HO

Me

R (i) 1O2

Me HO

O

O

O HO

R

Me

(ii) PPh3

HO HO

O

O

Me

Me Me O

O

Me

OH

Me O

OH

Me

OH

(i) 1O2 OH

(ii) PPh3

O

OH

OH

OH Scheme 29

6

Other Oxidation Methods

6.1 Oxidation of Aliphatic Compounds. – In volume terms, the use of TiO2 for the photodegradation, and ultimately photomineralization, of a wide variety of substances continues to dominate this area of photochemistry. Although many of the publications resulting from this work involve the standard application of

156


the technology to particular chemical systems, the following examples are of more general interest. The use of photoelectrochemical methods to study photocatalytic oxidation processes at surfaces such as TiO2 enjoys a number of advantages, including the fact that the half-reactions occur in different places rather than on the same particle, as would be the case with colloidal suspensions. A photoelectrochemical study of the photo-oxidation of glucose, an effective photo-hole scavenger, has been carried out using nanoporous TiO2 electrodes with a view to studying the effect of potential bias, light intensity, and the concentration of the hole scavenger on the kinetics of the photocatalytic oxidation process.94 A model for the overall photocatalytic oxidation process was proposed on the basis of the results obtained. Extending the excitation wavelength into the visible is a common theme in relation to TiO2 systems. One way this can be achieved is through surface sensitization which involves excitation of the sensitizer followed by ET to the semiconductor surface, the primary oxidizing agent under these conditions being O2d. The ESR spectrum of O2d on the surface of TiO2 nanoclusters sensitized by the anisyltritolylporphyrin (67) has now been detected directly for the first time at room temperature.95 An alternative approach to producing visible light active TiO2 based catalysts involves their nitridation96. The TiO2xNx photocatalysts are formed by the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts, and can be tuned to absorb across the visible region. It is reported that the activity of the catalysts in promoting the photodegradation of methylene blue is better than that of commercially available TiO2 catalysts or those produced by other doping procedures. It was also found that although nitridation at the nanoscale was facile, little or none occurred with larger TiO2 particles. The diffusion of oxidizing species from illuminated areas of a TiO2 surface to ‘dark’ areas, resulting in the degradation of molecules anchored in these areas, has been confirmed.97 The authors suggest that their findings have implications for how the contribution of dark pores in TiO2 particles is viewed and make the suggestion that relatively large, but highly porous particles may have photocatalytic potential. A number of papers have considered the relationship between TiO2 and other photocatalytic systems. In general it is difficult to prevent complete photodegradation of hydrocarbons on TiO2, and it is in this context that the use of V2O5-Al2O3 is proposed as an alternative.98 The system is selective in terms of the liquid-phase oxidation of cyclohexane to cyclohexanol and cyclohexanone, and a high ketone:alcohol ratio (3.8:1) is also observed. In this context it has also been reported that the TiO2 photocatalysed reaction of adamantane in the presence of O2 leads to the formation of 1- and 2-adamantanol, and adamantanone, with limited degradation but with modest conversion (Scheme 30).99 The process involves the formation of an adamantyl radical that can be trapped not only by oxygen but also by electrophilic alkenes. The reaction of simpler monocyclic hydrocarbons is less successful. A comparative study of the photocatalytic behaviour of heterogeneous TiO2 and homogeneous polyoxometalates (POMs) has been carried out.100 Although the two systems are similar in that they both involve photo-induced CT and feature OH radicals

157


Me

N H N

N

(67)

H N

CO2Me

Me Me

OH

CN

OH

CN Me

CN

Me

CN +

75%

NH2

O

Me

TiO2, Ag 15% conversion

TiO2, hv O2, H2O 36% conversion

38%

23%

O

O

O

8%

7%

Scheme 30

as the main photo-oxidant, the study identifies a number of crucial differences. Thus, although the initial photo-oxidation occurs at comparable rates, photomineralization is significantly slower with POMs. Photo-oxidation with TiO2 involves both OH radicals and direct hole transfers, whereas in the case of POMs the former is the sole effective oxidant. Significant differences also emerge with reducible substrates where the rate of ET from POM is significantly slower than the rate of conduction band ET on TiO2. Finally, the study indicates that surface reactions between substrates and intermediates play a key role in TiO2 mediated processes, but are obviously not relevant in the context of a homogeneous system. The potential of POMs as photocatalysts continues to excite interest. The fact that POM promoted reactions involve the formation of radicals has led one group to suggest the decatungstate anion, W10O324, as a reference sensitizer for 1O2 free radical based oxygenations of organic molecules.101 Thus time-resolved and steady-state methods have been used to compare the photooxygenation of simple compounds, for example 2-methyl2-pentene and 2,3-dimethylbutene, using W10O324 and standard 1O2 sensitizers, such as methylene blue and ruthenium complexes. These studies confirm that W10O324 sensitized oxygenation occurs exclusively by a radical pathway that differs clearly from both Type I and Type II oxygenations. The W10O324 photosensitized oxidation of p-substituted 1-aryl-1-alkanols has been investigated (Scheme 31).102 The possibility that the rate-determining step could be either an ET or a hydrogen-atom transfer between the substrate and the photocatalyst was considered, but it is concluded on the basis of product

158


R

hv, W10O324O2, MeCN

X

O

O

OH

R

H + X

X 97-100%

X = NO2, CF3, F, H, Me, MeO R = Me, tBu

0-3%

30-47% conversion Scheme 31

Cl

OH

O

OOH

OH

SiO2/W10O324-

Product Distribution (%)

MeCN CH2Cl2

+

O +

+

+

+

hv

Cl 82.5 24.5

10 16

6 8.5

1.5 1.5

0 42.5

0 1.5

Scheme 32

analysis and kinetic data that the latter is in fact involved. The same group has shown that this photooxygenation can also be carried out using DCA as a photosensitizer.103 In this case product analysis and Hammett correlations support an ET mechanism. The importance of radicals in POM promoted reactions is underlined by the use of tetrabutylammonium decatungstate to generate cycloalkyl radicals from cycloalkanes.104 The radicals react in a Michael fashion with electron deficient alkenes, giving addition products in 60–65% yield with complete conversion of the alkene. The presence of CH2Cl2 has a significant effect on the photocatalytic oxidation of cyclohexene and cyclooctene using (Bu4N)4W10O32 supported on silica (Scheme 32).105 The CH2Cl2 undergoes oxidation, giving radical intermediates that are involved in the subsequent epoxidation of the alkenes. Although cyclooctene epoxide is stable under the reaction conditions, cyclohexane epoxide undergoes ring opening to form 2-chlorocyclohexanol. The photodegradation to CO2 and F of nonafluoropentanoic acid, a model perfluorinated acid, by H3PW12O40 has been reported.106 The result is significant because the TiO2 mediated photomineralization of trifluoroacetic acid is not very efficient. The effect of O2 on the photoisomerization of all-trans-1,6-diphenyl-1,3,5-hexatriene, a model for long-chain polyenes related to retinal and vitamin A, has been reported.107 The presence of air enhances the photoisomerization of the terminal alkene unit but reduces that in the centre. Evidence is provided that excludes the involvement of 1O2 and also the radical cations of the all-trans material in this new isomerization pathway to the cis,trans,trans isomer. A mechanism is proposed which involves CT from the S1 state of all-trans material to O2 followed by collapse of the exciplex to either a zwitterionic or a biradicaloid species. The process is completed by rotation about the new single bond in this intermediate followed by reversion to the triene and O2.

159


The synthetic applications of the photo-oxidation of aliphatic systems include the formation of 1,4-monoprotected dialdehydes, 1,4-ketoaldehydes, g-lactols and g-lactones by means of the radical alkylation of a,b-unsaturated aldehydes in organic and organic-aqueous media (Scheme 33).108 The key step in the process is the generation of radicals from 1,3-dioxolanes or alcohols by photomediated hydrogen abstraction using benzophenone in an organic solvent, or benzophenone disodium disulfonate in water. These radicals then add to the a,b-unsaturated aldehyde. The outcome of the oxidative fragmentation of the norcholestanyl acetate (68) using Pb(OAc)4 depends on whether the reaction is carried out thermally or photochemically (Scheme 34).109 The latter results in a mixture of products, effectively formed via the radical intermediate (69), in which nor-secosteroidal ketones predominate. The reaction with Pb(OAc)4 under thermal conditions results in the stereoselective formation of O

R1 O

CHO

H

O

CHO O

R

R2

Ar2CO, hv R1 water or MeOH

R

R2

R3

OH

R4

H

R3

Ar2CO, hv water or MeOH

O

OH

R4 R1

R2

Scheme 33

H C8H17

Me

AcO

H AcO

Pb(OAc)4

H

Me

H H

H

Me

Me

H

+

H

AcO

O 11%

O 16%

hv Me

H

OH (68)

+

H AcO

Pb(OAc)4, ∆

Me

H H

O

+ AcO

OAc

O

(A) 8%

22% (A) 65%

H

H H

+

H

O

+

AcO 17%

O AcO 22%

Me

H H

AcO

O (69)

Scheme 34

H

160


5,10-ethers with a b-oriented epoxy bridge. It is suggested that cationic intermediates may be involved in this case. The potential of alkyl aryl ketones as delivery systems for the controlled release of fragrances has also been considered.110 The required precursors can be readily synthesized and on irradiation in undegassed solvent with a xenon lamp or using sunlight undergo a standard Norrish Type II reaction, releasing the perfumery alkenes. GC-MS analysis of the irradiated solutions allowed a number of by-products, which had not been previously detected in such reactions, to be characterized. The majority of these are formed via the intermediate (70) that results from the reaction of the standard 1,4-biradical with O2. The 7,9-dimethylcyclohepta[b]pyrimido[5,4–d]furan-8(7H),10(9H)-dionylium ion (71) and its S-analogue (72) are isoelectronic with the 5-ethyl-3-methyllumiflavinium ion (73). This relationship to flavin systems provides the rationale for mechanistic studies designed to provide further insight into the role played by these cofactors in biological redox reactions.111,112 These ions have been prepared as their tetrafluoroborates and it has been shown that they participate on irradiation in the autorecycling oxidation of alcohols under aerobic conditions (Scheme 35). Evidence is provided that the process involves ET from excited (71) or (72) to the alcohols. ET is also involved in a range of other processes of synthetic interest, a number of which centre on ring-opening reactions of small strained ring systems. In addition to its synthetic potential the oxidative cycloreversion of oxetanes is of biological importance as it is involved in the enzymatic repair of

HO

X

N N O

R

Me O

+

O

(70)

Me X

O

Me

Me

N

Me

+ N Et (73)

(71) X = O (72) X = S

N

O N

Me

O

O

OH (71)

12%, based on alcohol consumed 311%, based on (71)

hv, MeCN Scheme 35

161


UV damaged DNA. The ET-mediated cycloreversion of trans,trans-2,3-diphenyl-4-methyloxetane has been found to be photosensitizer dependent (Scheme 36).113 The reaction involves cleavage of the C2–C3 bond in the oxetane radical cation and the formation of the trans-b-methylstyrene radical cation when chloranil is used as a sensitizer. In the case of pyrylium salts, the trans-stilbene radical cation, formed via O–C2 bond cleavage in the oxetane radical cation, is involved. Radical cation and diradical intermediates have been simultaneously observed for the first time, using a variety of spectroscopic techniques, in the PET-initiated degenerate rearrangement of the methylenecyclopropane (73) (Scheme 37), for which a radical-cation cleavage, diradical cyclization, mechanism has been proposed.114 Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of (73) with, for example, 9,10-dicyanoanthracene (DCA) identifies two transients, which have been assigned to the trimethylenemethane radical cation (74) and the corresponding diradical (75). The same group has obtained evidence that the related rearrangement of the 2-methylenecyclobutanone (76) involves the intermediate (77), a novel oxa analogue of the tetramethyleneethane radical cation (Scheme 38).115 The formation of the 2-silacyclobuta[2.3]cyclophane (78) and its cycloreversion take

Me O +

pyrylium salts Me

Ph

hv

O

Ph

+

MeCHO +

Ph

Ph

Ph

Ph

chloranil Ph

Me

Ph

O

hv

PhCHO

+ Ph

Ph

Ph

Ph

+ + Me

Me

Scheme 36

Ar Ar

Ar

Ar

Ar D

D (74)

D DCA hv Ar

Ar

Ar

Ar D D

D BET

DCA hv

Ar

Ar

Ar D

D ( (73)

D

D ((75) D Scheme 37

D

162


Me

Me Me

Me Me

chloranil

Ar

Ar

hv

O

Ar

Ar

O

(76)

O

Ar

Ar (77) Scheme 38

Me Si Me

sens

sens hv

hv Me

Me

(78)

Si

Si Me

Me (78)

Me Si Me

(79) sensitizer = p-dicyanobenzene/phenanthrene

Scheme 39

t

Bu Me

Ar

t

.

.

Ar

Bu

DCA, hv +

ROH

Me

Ar = Ph, p-ClC6H4

MeCN, 12 h

R = Me, Et, Pr

Ar Ar

OR

. H 26 - 60%

(80) Scheme 40

place under PET conditions (Scheme 39).116 The quantum yield for the cycloreversion is greater than one, leading to the suggestion that a chain mechanism, in which the key step is ET between the open-form radical cation (79) and (78), is in operation. Finally, the photoaddition of alcohols to a 1,2,3-butatriene type cumulene (80) in the presence of a catalytic amount of DCA has been reported (Scheme 40).117 The reaction is regioselective and involves the formation of the cumulene radical cation as an intermediate. The design and construction of PET systems designed to mimic photosynthesis continues to be a very active research area. An example of such a system is the carotene (C)-porphyrin (P)fullerene (C60) triad (81) for which a Cd1–P–C60d CS state with a lifetime of 340 ns and a dipole moment of greater than 150 D has been reported.118

163

Photochemistry, 36, 2007, 133–204 C60

N n

N H

n

Bu

Me N

N Me

(81) Me

Me Me

H N

Me

Me

Me

Bu

H N

Me

n

Bu

n

Bu

O Me

Me

Me

6.2 Oxidation of Aromatic Compounds. – As was the case for aliphatic compounds, the use of TiO2 based systems and other so-called ‘advanced oxidation technologies’ (AOTs) dominate this area of photochemistry. The examples that follow are some of the more interesting recent contributions in this research field. The effect of ozone on the photocatalytic oxidation of toluene, chosen as a model VOC, was considered in a comparative study that included the use of O3-UV, TiO2-UV and O3-TiO2-UV in a flow reactor.119 It was concluded that the O3-TiO2-UV system was the most efficient, as photocatalyst deactivation was not a problem and residual O3 concentration was low. As the use of UV light is required with standard TiO2 systems, many contributions in this area are concerned with the development of photo-oxidation systems that are activated by visible light. Thus it is reported that S-doped TiO2 photocatalysts absorb visible light (l 4 440 nm) strongly and display high activity for methylene blue degradation.120 A range of metal-ion based photocatalysts have also been developed for the degradation of organic pollutants. These include copper phthalocyaninesulfonate, which was found to be effective in aerated aqueous solution for the visible light (l 4 450 nm) degradation of 4chlorophenol, 2,4-dichlorophenol, 2,4,6-tri-chlorphenol and methyl orange.121 The novel b-cyclodextrin-hemin complex (82) has been used at neutral pH and at l 4 450 nm to activate hydrogen peroxide for the oxidation and mineralization of 2,4-dichlorophenol and rhodamine B.122 The photocatalyst combined high activity with high turnover numbers and excellent stability. Another group has looked for an explanation for the effect of visible light on the rate of oxidation of chlorinated phenols, including 2,4,6-trichlorophenols, by hydrogen peroxide in the presence or absence of iron catalysts.123 They conclude that the photo-acceleration is due to the formation of the light-sensitive 2,6-dichloro-1,4-dibenzoquinone (83), whose photochemical conversion to 2,6-dichlorohydroquinone and 3,5-dichloro-1,2,4-benzenetriol completes the early phase of the oxidation process (Scheme 41). The ability to utilize visible or near-UV light is one of the attractions of POMs in terms of photo-oxidation. The sulfo-polyoxometalate anion clusters S2W18O624, S2Mo18O624, and SMo12O402, for example, have been used together with a 7 W lamp

164


Me

N

Me

N Fe Cl

N

N Me

Me

O

O

O

O

(82)

Cl

Cl

Cl

Cl

Cl

H2O2

Cl

Cl

+ HO OH

O

OH Cl

H 2O hv

catalyst Cl

OH

O

OH

OH

(83) Scheme 41

(312-700 nm) to photo-oxidize benzyl alcohol, ethanol, and (–)-menthol.124 Although oxidation of the reduced form of the tungsten POM occurs in the presence of H1 or O2, this method of closing the catalytic cycle is not feasible for molybdenum clusters. The oxidation of the reduced forms of all three POMs, however, may be achieved, or accelerated, by working in the presence of an electrode held at a potential more positive than the first redox potential of the POM anion. A sunlight promoted procedure based on these POMs, and involving particularly simple reaction conditions, has been developed by the same group for the selective oxidation of benzyl alcohol.125 The use of a variety of surface-modified TiO2 photocatalysts has been described. The effect of fluorination has been explored in terms of the behaviour of four model compounds – phenol, acid orange 7, and sodium di- and trichloroacetate – chosen because their photodegradation mechanisms are quite different.126 It was found that the fluorinated TiO2 was more effective than pure TiO2 for the photocatalytic oxidation of phenol and acid orange 7, but less so for dichloroacetate. It is concluded that OH radical mediated oxidation pathways are enhanced on the fluorinated surface, whereas hole-transfer mediated oxidations become less effective owing to the hindered adsorption of substrates. The reductive dechlorination of trichloroacetate is less efficient on the fluorinated surface, a fact that is attributed to the high electronegativity of the

165

Photochemistry, 36, 2007, 133–204 h+

h+ hv O

Ar

Ar

Ar

Ar Ar

TiO2 O

Ar

Ar

Ar Ar Ar

Scheme 42

fluorine in surface Ti–F species, which reduces interfacial ET rates. The TiO2-WO3 system has generated a lot of interest in terms of its photo-electrochemistry. It has been reported that the ability of TiO2 to promote the photooxidation of acetaldehyde in the gas phase and of 2-naphthol in the liquid phase is considerably reduced when it is coupled with WO3, an effect attributed to a decrease in the rate of ET from the semiconductor to O2.127 When the electrons are removed electrochemically from a WO3 film with a SnO2 underlayer, however, the photocatalytic activity for the oxidation of 2-napththol exceeds that of TiO2. A number of other papers have appeared which underline the importance of TiO2 based photocatalysis. The possibility of a cascade hole transfer from TiO2 to a free alkene via adsorbed p-phenylbenzoic acid has been demonstrated using time-resolved diffuse reflectance spectroscopy (Scheme 42).128 It is suggested that a cascade process such as this has synthetic potential. A key element in the operation of a dye-sensitized solar cell is that BET to the radical cation of the dye should be as slow as possible. It has now been shown that the modification of the surface of TiO2 nanoparticles with sodium dodecyl benzyl sulfonate significantly reduces the rate of BET.129 The results relate to a femtosecond transient absorption spectroscopy study of alizarin-sensitized surface-modified and unmodified TiO2 nanoparticles. It is suggested that the energy of the Fermi level of the nanoparticles increases on surface modification, thus raising the overall free energy of reaction for the BET process. The selective photo-oxidation of aromatic substrates has been the focus of attention for a number of groups. Although photocatalysis using TiO2 generally results in the significant photodegradation of organic compounds, it can result in more selective oxidation in certain cases. Thus the TiO2 catalysed photooxygenation of electron-rich 1,1-diarylethenes and 1,1,8,8-tetraaryl-1,7octadienes gives 1,2-dioxanes in high yield (Scheme 43).130,131 It is also reported that the PET process is significantly accelerated by the addition of MgClO4. Evidence has been obtained for the involvement of epoxide and aldehyde intermediates in the zeolite-promoted oxidation of 1,1-diarylethenes, and this has led to the formulation of a detailed mechanism for these reactions (Scheme 44).132 Evidence was also obtained that the aldehyde intermediates undergo a novel photooxygenation in competition with a-cleavage. Zeolites also feature in another selective photo-oxidation system which involves iron(II) pipyridine

166

Photochemistry, 36, 2007, 133–204 Ar

hv ( λ > 280 nm) O2

Ar

Ar

TiO2, Mg(ClO4)2 MeCN

Ar

Ar

H

H Ar

Ar

TiO2, Mg(ClO4)2 MeCN

Ar

Ar

O O

hv ( λ > 280 nm) O2

Ar Ar

Ar

Ar

O O

Ar

Ar2CH

+

Scheme 43

O2

Ar

O

O

hv

Ar

hv

Ar2CH

Ar

CHO

H

Ar hv O O

Ar

O O

Ar

Ar2CH

Ar

Ar

Ar2CH2 +

O2 H ∆, hv

- H 2O

O

O Ar2CHOOH

CO

Ar2CH

Ar2CH

OOH

O2 H

Scheme 44

Me2N

NMe2 Cl O

O Fe(bipyr), O2 hv (visible) H 2O Me2N

+ Me2N major

minor

NMe2

Scheme 45

supported on NaY zeolite and demonstrates excellent reactivity without leading to extensive photodegradation (Scheme 45).133 The oxidation of aromatic groups with O2 in the presence of N-bromosuccinimide (NBS) has been reported (Scheme 46).134 It is suggested that the reaction involves hydrogen abstraction from the aromatic methyl by a photochemically generated bromine atom and the subsequent reaction of the benzylic radical with oxygen. The photocatalysed oxidation of an aromatic methyl to the aldehyde level, in systems containing both electron-withdrawing (CN) and electron-donating

167


CO2H

NBS (1eq), hv EtOAC X

X

32-85%

t

X = H, p- Bu, p-OMe p-CN, p-Ph, p-NO2 m-OMe Scheme 46

CN

Me

F

CHO

F

Ph

Me

O2, F

F

N Me

CN

hv, λ > 300nm

CN

CHO

O2,

CN

hv, visible light

Me

Me

Scheme 47

N

N

Sc(OTf)3

Sc(OTf)3

N

N

Me

Me

Me

Me

Me

Me

Me

hv

Me

Me

Me

Me

Me

Me

O2

CH2OH

Me

radical chain Me

Me Me

Scheme 48

(Me) groups, has been described.135 The appropriate choice of photocatalyst allows a high level of selectivity to be achieved in these reactions (Scheme 47). The key step in this reaction is PET from the aromatic system to the singlet excited state of the photocatalyst. The visible light driven conversion of a-methylstyrene to acetophenone has been carried out in the same way using 10-acridinium perchlorate as photosensitizer.136 The same group has also discovered that Sc(OTf)3 has a significant promotional effect on PET in photooxygenation reactions of this type (Scheme 48).137 The effect is not only apparent with acridine, which can form a complex with Sc(OTf)3 in the ground state, but also with pyrene, for which no such interaction occurs. No photooxidation is observed in the absence of Sc(OTf)3. The mechanism of the photohydroxylation of 1,4-benzoquinone in water, which results in the formation of equal amounts of 2-hydroxy-1,4-benzoquinone and hydroquinone, has been re-examined.138 The quantum yield and laser

168


O

OH

H 2O

O

O

O

OH

OH

O

+ O

O

(84) O

H 2O

OH O H OH

O OH OH

OH

OH Scheme 49

flash photolysis data obtained are explained in terms of a rapid equilibrium between monomeric excited benzoquinone and an exciplex (84) (Scheme 49). Product formation can occur through addition of water to the monomeric benzoquinone to give an adduct which is immediately oxidized by benzoquinone or which prior to this oxidation undergoes rearrangement to a trihydroxy benzene. Alternatively, the exciplex (84) can undergo ET and water addition, again ultimately forming 2-hydroxy-1,4-benzoquinone and hydroquinone. The mechanism thus discounts any suggestion that the photohydroxylation involves hydroxyl radicals. The visible light initiated photochemistry of riboflavin in the presence of 3- or 7-hydroxyflavone has been investigated, in part because the processes involved may operate in nature.139 The triplet state of riboflavin is generated in solutions containing mM or sub-mM concentrations of the flavones, which quench the excited state through an ET process. The reaction of the semireduced riboflavin thus formed with dissolved O2 produces O2d, which in turn reacts with riboflavin or 3-hydroxyflavone. No evidence was found for the involvement of 1O2 or the reaction of 7-hydroxyflavone with O2d. The photochemistry of flavothione (85), and a number of hydroxyflavothiones, in ethanol has been described.140 The primary photochemical process for flavothione is hydrogen abstraction from the solvent to give a radical that in the absence of O2 forms dimers; in the presence of O2 the major product is flavone (Scheme 50). This reaction involves ground-state O2 rather than 1O2. The behaviour of the hydroxyflavothiones depends on the substitution pattern and ultimately on the ordering of the excited states. Both 6- and 7-hydroxyflavothione, and 7,8-dihydroxyflavothione, which like flavothione itself have lowest (n,p*)3 states, produce the parent flavone and dimers. The hydroxyflavothiones with lowest (p,p*)3 states – 3,6- and 3,7-dihydroxy-, and

169


H

(85)

O

Ph

EtOH

O

Ph

hv

H

Ph

+

Ph

O

O

O2 O

hv

SH

S (85)

Ph

O

Ph

Ph

O

Scheme 50

3-hydroxyflavothione – act as photosensitizers, producing 1O2, which then reacts with the ground state of the flavothione. Pterins, which also occur in a wide variety of biological systems, are also light sensitive. The photo-oxidation of 6-(hydroxymethyl)pterin (86) in alkaline aqueous solutions, the only product of which is 6-formylpterin, has been investigated.141 The data obtained are consistent with the formation of an intermediate (87) in an O2-independent, photochemical process and its rapid conversion to 6-formylpterin in a reaction that also produces one equivalent of hydrogen peroxide. Although 6-(hydroxymethyl)pterin (86) is a relatively good sensitizer for 1O2 formation (F ¼ 0.21), this active oxygen species is not involved in the conversion of 6-(hydroxymethyl)pterin (86) to 6-formylpterin.

N

N H 2N

N (86)

O

O

O

N

CH2OH hv

H N

N H 2N

N H

N

CH

O2

N

N H 2N

O

O

H2O2

N

CH

N

(87)

PET is the key feature of many other photo-oxidations involving aromatic substrates. It has been shown, for example, that pyrene and anthracene which are covalently attached to silica, gold or indium-doped tin oxide (ITO), undergo a photo-oxidation forming dihydroxy/dione derivatives.142 The reaction involves O2d, formed by ET between excited pyrene, or anthracene, and O2, and it is suggested that the implications of such a photodegradation need to be considered when polycyclic aromatic hydrocarbons (PAHs) are used as spectroscopic probes in surface adlayers. The redox photosensitized amination of 1,2-benzo-1,3-cycloalkadienes, arylcyclopropanes, and quadricyclane with ammonia and primary amines, using 1,2,4-triphenylbenzene (1,2,4-TPB) or 2,2 0 -methylenedioxy-1,1 0 -binaphthalene in the presence of m- or p-dicyanobenzene (DCB), has been described (Scheme 51).143 The process involves the formation of the radical cation of 1,2,4-TPB, for example, by PET to the DCB, followed by hole transfer from the radical cation to the substrate, the latter

170


R

1

1,2,4-TPB m-DCB

2

+

R NH2

NHR2

MeCN-H2O R1 hv R 1 = R2 = H 79% R1 = Me, R2 = H 83% R1 = H, R 2 = iPr 52%

1

R = H, Me

+ Ph

NH3

1,2,4-TPB m-DCB

NH2 71% Ph

MeCN-H2O hv, Et4NBF4

Ph

Ph

Scheme 51

Ph Ph Ph

Me

R

R

Ph

R

Ph hv

Ph

Ph Me

DCA

DCA Ph

Ph

Me

hv Ph

Ph Ph

(88) R = H, Me Scheme 52

being the key step. The PET-induced rearrangement of the bicyclopropenyls (88) gives both benzene and Dewar benzene derivatives in product ratios that depend on the irradiation time (Scheme 52).144 These experimental results indicate that the PET-promoted bicyclopropenyl-benzene rearrangement occurs via a Dewar benzene intermediate, and as similar mechanisms have been proposed for the Ag1 catalysed and thermolytic rearrangement of bicyclopropenyls, they suggest that all three routes share a common structural pathway. The PET promoted isomerization of hexamethyl Dewar benzene to hexamethylbenzene using cyanonaphthalenes as electron acceptors has been the subject of a kinetic study which addresses the importance of the reorganization energy involved in the BET in the radical cation-radical anion exciplexes.145 A different diastereoselectivity is obtained for the [2þ2] cycloaddition of stillbene to a chiral fumarate if selective excitation of the CT complex is employed rather than direct excitation of the substrate (Scheme 53).146 Reflecting the fact that the structures of the excited CT complex and the normal exciplex are different, the diastereomeric excess (de) of the product and its temperature profile depend on the manner of excitation. The product de increases with increasing temperature, and the chirality of the product is also temperature dependent. Overall, the approach provides another method of controlling the stereoselectivity of photochemical reactions. Quantum yield data have been used to show that the photochemical Diels-Alder reaction of anthracenes, such as 9,10-dimethylanthracene (DMA), with fumaronitrile and p-benzoquinone proceeds via

171

Photochemistry, 36, 2007, 133–204 CO2R

Ph +

hv, 313 nm direct

RO2C

Ph

Ph exciplex Ph + Ph

hv, 366 nm

CT complex

CO2R

CO2R

excited CT complex

CT complex excitation

CO2R

CO2R Ph

Scheme 53

O O

Me

O

O H H Me

Me

O

DMA1

O O

Me

O

Me Me

Scheme 54

Me

Me

Me

Me

1,4-dicyanotetramethylbenzene

Me

Me

Me

Me

Me

Me

Me

- e, -H+ or - H

HO

H2O, - H+ Me

Me Me

Me

Me

PET

Me

Me

HO Me

Me

Scheme 55

ET from the singlet excited state of the anthracene to the dieneophiles.147 Subsequent cation-anion coupling gives a diradical which closes to give the cycloadduct (Scheme 54). Spectroscopic evidence – a transient absorption spectrum at 298K for the radical ion pair and ESR data at 77 K for the diradical – is provided in support of the proposed mechanism. A biomimetic photochemical cascade cyclization process has been developed for the synthesis of aromatic polycyclic terpenoids.148 The regioselective and stereoselective reaction involves a PET produced radical cation, which is trapped by water, leaving a radical which initiates a cascade cyclization leading to an all-chair polycyclic structure (Scheme 55).

172


Finally, a series of donor-acceptor PET systems have been reported in which the donor is a aromatic system. The anthracene-fluorescein-C60 triad (89) and a related structure have been synthesized.149 The anthracene, acting as an ‘energy antenna’, collects light in the 320–400 nm range and transfers it to the ‘reaction centre’, the fluorescein. The fluorescein, following absorption of light in the visible region, is involved in the generation of a CS state through PET to C60. Coordination of a Ru(II) terpyridyl metallo-fragment to the terpyridine unit in (90) results in the appearance of a weak phosphorescence at 710 nm, which is attributed to MLCT, or to an intra-ligand CT from pyrene to the coordinated terpyridine unit.150 It is suggested that the system, because of the position of the luminescence maximum, the long emission lifetime, and the possibility of using the thiophene as an anchor through appropriate functionalization, may have potential as a bio-marker. Both PET (k E 6.2 109 s1) and energy transfer (k E 6.2 109 s1) processes have been identified in a pyrene-perylene bisimide dyad (91) using time-resolved emission and femtosecond transient absorption spectroscopy.151 Temperature-dependent time-resolved emission spectroscopy provides evidence that the dyad can exist in two conformations – folded and stretched – that have different barriers to ET. The potential of this system in constructing metallo-supramolecular structures through the coordination of metal ions to the pyridine units has been highlighted.

PET COO(CH2)2OOC C60 COO(CH2)2O

O

O

N energy transfer

Me (89)

N N S N Et (90)

173


O O

O

O

O

N

N O

O

O O

O (91)

O

The spectroscopy and dynamics of a dyad in which the donor and acceptor are linked by a spacer – the heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecane system – that is characterized by high symmetry, rigidity and linearity have been reported.152 The importance of the alignment of the p-orbitals of the donor and acceptor is underlined by the data obtained for (92) and (93), which show that the rate of PET in the former is significantly greater (3.9 1010 s1) than in the latter (7.0 108 s1). Remarkably, the ET state of the 9-mesityl-10methylacridinium ion (94) has a lifetime (2 h at 203 K) which is much longer than that of the CS state of the natural photosynthetic centre which can extend to several seconds.153 Its energy (2.37 eV) is also significantly larger than that that of the natural system (0.50 eV). These features result from the fact that the ET state of (94) is formed through a single-step PET and because of the minimal reorganization energy of the system. The dyad (95), based on eosin and tris(2,2 0 -bipyridine)Ru(II), acts as an efficient photosensitizer for reactions such as the photooxygenation of anthracene.154 The active oxygen species is O2d, generated by ET from the CS state of (95), which is generated by PET. The photosensitization ability of (95) and its broad absorption range in the visible region suggest that it should have applications in many situations involving ET. The photophysics and photochemistry of benzyl derivatives of isoquinoline N-oxides (96) have been studied.155 The only products formed on irradiation were the parent amine and a product resulting from an intramolecular hydroxylation (Scheme 56). A two-step rather than a concerted mechanism is preferred for the reaction. The reaction involves hydroxyl radicals, formed by N–O bond scission in the radical cation produced through PET from the benzene ring to the isoquinoline acceptor. The regioselectivity of

174


subsequent C–O bond formation is controlled by the electron distribution of the radical cation. CN

CN

S

S CN

CN S

S (92)

(93)

Me

Me

Me

Me

Me

Me

hv N

N

Me

Me

(94)

2 O O Br

N Br

N

N

2ClO4-

Ru O

NaO Br

O

N

N

Br

N

(95)

O N

N

R R

N

R

hv, λ > 300 nm

R

R +

R

CH2Cl2, TFA R

R

R

R R

R

(96)

Scheme 56

R

HO

R R

R

R

R

175


7

Oxidation of Nitrogen-containing Systems

Arising from the facility with which many nitrogen-containing systems can donate an electron, there have been many reports relating to the photooxidation of nitrogen-containing systems. Much of the work in this area focuses on donor-acceptor systems which are of importance in the context of artificial photosynthesis or as photosensors. A number of contributions which are not specifically related to PET are broadly synthetic in scope. A number of papers have appeared which describe the synthesis of indolocarbazoles, substances which possess a wide range of biological activities, and use a photochemical cyclization as the key step. Thus the preparation of indolo[2,3-a]pyrrolo[3,4-c]carbazoles via the I2 promoted oxidative photocyclization of bisindolylmaleimides has been reported (Scheme 57).156 A second group has prepared indolocarbazoles using the same method, or using acetone as the effective oxidizing agent (Scheme 58).157 In the case of the 2-naphthyl derivative (97), the regioselectivity of the two methods is quite different. This group has also developed a route to the same indolocarbazoles which is based on a Heck cyclization. A comparison of the photochemical and the palladium-catalysed routes indicates that, although lower yields are obtained with the latter, it does not require high dilution conditions and involves shorter reaction times. Another oxidative photocyclization, in this case involving the formation of a C–N bond, is involved in the preparation of the stable heteroaromatic cation (99) (Scheme 59).158 The reaction proceeds faster when I2, or a mixture of I2 and AgClO4, is included in the reaction mixture. The

H N

O

R

R R

N H

H N

O

O

R

N H

hv, I2 THF-MeCN

O

R

R

3-24 h, 84-90% R

N H

R

N H

Scheme 57

H N

O

H N

O

O

O

H N

O

hv + N H

N H

N H

(97) benzene, I2 : 74%, 100 : 25 acetone: 73%, 100 : 0

Scheme 58

O

176

Photochemistry, 36, 2007, 133–204 Me Me

Me Me

Me Me R

N

hv, O2

H

OH-

R

R

R

N

R

N

R

(99)

(98) O

O R = R = OMe

R=R=

O O

O

Scheme 59

O

O NOH

NaOMe, MeOH

OH

hv, rose Bengal

Me

Me

Me

O

1

N O

R

O O O

R

N O

R1

MeOH

O

O R

OMe

+

Me 1: 2.8, 92%

O O2

R1

esters acids

O

NO R1

O O

O R1

O O

R

R

O (100)

R1

Scheme 60

crown ether derivative (98) forms a complex with Mg21, but in this form cyclizes more slowly as a result, it is suggested, of hydride removal being less efficient as a result of complexation. The photoxygenation of the C ¼ N in a-iminocarbonyl compounds results in C–C cleavage and the formation of mixtures of acids and esters (Scheme 60).159 The key intermediate in the proposed mechanism is the trioxa[2.1.0]pentane (100). An autorecyling process is suggested to account for the greater than 100% yields obtained for the oxidation of some amines by 6,9-disubstituted cyclohepta[b]pyrimido[5,4-d]pyrrole-8(6H),10(9H)-dione derivatives such as (101).160 A possible mechanism involves an initial PET between (101) and the amine, followed by the generation of O2d and the regeneration of (101) (Scheme 61). The actual product isolated is the result of a secondary reaction between the initially formed imine and excess amine. The related

177

Photochemistry, 36, 2007, 133–204 O2

O2

Bn

Bn

NH2

N

N

R1

O

N

hv

N

NH2

O + R1

N

Me

O

N

R2

Me

O

(101)

R2 O2

R1

NH2 R2

N

R1 NH3

R2

R1

R2 R1

NH R2

Scheme 61

O

Me

O

Me N

N

N Me

O

N

O N

(102)

N Bn

(103)

R H

R NH HN

+

C60

N

N

hv degassed toluene

C60

Scheme 62

1,6-methanocycloundeca[b]pyrimido[5,4-d]pyrrole-12,14-dione derivatives (102) and (103) also participate in the same type of autocycling photo-oxidation of certain amines and alcohols.161 A photochemical method involving oxidative dehydrogenation has been used to carry out the mono-addition of substituted piperazines carrying alkyl, ether, amide and hydroxyl functional groups to C60 (Scheme 62).162 The reactions, which are believed to proceed via radical-ion pairs formed by ET from the amine to the fullerene, are selective in that they involve monoaddition and the substituents on the piperazine ring adopt an exo configuration relative to the surface.

178


A number of general reviews have appeared that deal with aspects of PET in systems in which the donor is a nitrogen-containing unit, among them being a review of the role of metal ions in controlling photochemical and thermal ET through complexation with radical anions.163 Another review focuses on the aheterolytic reactions of aminium radicals which are formed by one-electron oxidation of amines.164 PET and triplet sensitized di-p-methane rearrangements have been reviewed with particular reference to the aza-di-p-methane variation of the reaction.165 There have been a number of reports relating largely to the mechanistic aspects of PET in nitrogen-containing systems. Details of the first examples of 2aza-di-p-methane rearrangements have been provided.166 Specifically it has been shown that 2-aza-1,4-dienes undergo a triplet sensitized rearrangement, with (104), for example, giving a vinylaziridine (105) and a cyclopropylamine (106) which results from the SiO2 promoted hydrolysis of an initially formed imine (Scheme 63). Of greater importance is the finding that these rearrangements also take place under SET photosensitized conditions, with (104) giving again the vinylaziridine (105) and in addition a new cyclopropylamine (107) (Scheme 64). Ph

Ph

Ph sensitizer

N Ph

Ph (104)

Ph

+

(104)

hv

Ph

Ph

Ph

N

Ph Ph

Ph

SiO2

SiO2

H

+ N

Ph

Ph

Ph

(105) Ph

Ph

Ph

NH2 Ph

3%

Ph

Ph NH2

Ph

Ph

(106) 11%

83%

Scheme 63

Ph

Ph N

Ph

Ph (104)

Ph

Ph

Ph

Ph

Ph

DCA, biphenyl

Ph

N

MeCN, hv Ph

Ph (108)

Ph

Ph

Ph Ph

H

N

N

Ph

Ph Ph

Ph

Ph

Ph

Ph Ph (109)

Ph H

Ph Ph

Ph

Ph Ph NH2

N

+

N

Ph

Ph

(110)

Ph SiO2

Ph

Ph

H Ph

Ph Ph N Ph

(105) 11%

(107) 19%

Scheme 64

179


In this case, the SET generated radical cation (108) can produce (105) via an aziridinyl radical cation (109), or (107), via the radical cation (110) formed by phenyl migration. Fluorescence quenching in the fluorescein derivative (111) is possible because of a conformational equilibrium which involves the linking methylene group between the electron-donor and fluorescein adopting an axial position, thus allowing the aniline nitrogen to approach the fluorescein unit.167 The fluorescence quantum yield for (111) is thus only 20% of that for 2 0 ,7 0 dichlorofluorescein. The quantum yield for (112) is even lower, at 3.6% of the fluorescein value, providing evidence of cooperative quenching by the two aniline nitrogens. This conclusion is reinforced by the increase in fluorescence intensity that occurs when b-cyclodextrin is added to a solution of (112), a fact that is attributed to one of the aniline nitrogens being unable to participate in quenching due to complexation with the cyclodextrin.

CO2H

CO2H Cl

Cl

Cl O

Cl O

O

N

N

N N

(111)

O

C6H4pOMe pMeOC6H4

N

N

(112)

C6H4pOMe

A number of synthetic applications of PET reactions of N-donor systems have been reported. A change in the regioselectivity of photoamination of veratrole (113) with ammonia and amines occurs when the reaction is carried out in the presence of cyclodextrins.168 The effect is attributed to a change from an SN2Ar* mechanism in which the nitro group is meta-directing to a mechanism involving ET which leads to para-substitution. It is suggested that enhanced ET occurs in the presence of cyclodextrin due to the formation of a veratrole-amine complex in its cavity. A modest diastereomeric excess was observed for the PET-promoted cyclization of N-acyl-a-dehydro(1-naphthyl)alanines (Scheme 65).169 The H

O

NHR

NHCOR1 O N

NHCOR

R

H

NHCOR1 O N

+

R

1

O

MeOH/MeCN (1:9)

NHCOR1 NHR

TEA, hv R = (S)- CH(Me)CO2Me; R1 = Me Conversion: 63%; de = 55%

R N

+

Scheme 65

R1

O +

180

Photochemistry, 36, 2007, 133–204 NO2

NO2

NO2

MeNH2 OMe

+ NHMe

hv

OMe

OMe

OMe

(113)

NHMe

THF/H2O (1 : 9) γ -CD, MeOH/H O (1 : 1) 2

9.5 : 1 1 : 15

Me

+ Me

33%

Me

HN

Me

N hv

+ 5%

C6H6 or MeCN HN

10%

(114)

HN

Scheme 66

R

N

R Nu

HN hv

R

NuH HN

HN R = H, Me

H N

HN

R

R

NuH = MeOH, Et2NH

R

Scheme 67

diasteoselectivity is attributed to hydrogen-bonding control of the enolization step that completes the formation of the 3,4-dihydrobenzo[f]quinolinone. The conversion of the stilbenylpyrrole (114) to a mixture of an indole and an isoindole is initiated by an intramolecular PET from the pyrrole to the alkene (Scheme 66).170 Subsequent transfer of the pyrrole N–H to the radical anion generates two biradicals, closure of which gives the observed products. This is believed to be the first intramolecular addition of a pyrrole to a stilbene double bond. The same group has also used this PET, pyrrole N–H transfer and radical recombination sequence, to convert styryl dipyrroles to unstable 2-[[1-(2Hpyrrol-2-ylidene)methyl]-2-indanyl]pyrroles which can be trapped by nucleophiles giving functionalized indanylpyrroles (Scheme 67).171

181


An enormous amount of resources and imagination continues to be invested in the synthesis and study of molecular and supramolecular donor-acceptor systems designed to mimic photosynthesis. Many of these systems involve nitrogen-containing molecules, particularly porphyrins, as electron-donors. Photo-induced energy-transfer processes in fullerenes, currently the most commonly used acceptors in these systems, has been reviewed.172 The issues affecting the competition between energy transfer and ET are given particular attention. Fullerene-porphyrin systems, the most widely used donor-acceptor combination, are the subject of a second review which considers a range of issues including the self-assembly of porphyrin-fullerene monolayers and the significance of the fact that fullerenes have small reorganization energies.173 The CS state of the zinc imidazoporphyrin-C60 dyad (115) is reported to have a particularly long lifetime (310 ms) for a donor-acceptor system that involves a single-step ET.174 The extended lifetime is attributed to the nature of the linkage, the energy of the CS formed and solvent effects. An exceptionally longlived CS state (120 s in frozen benzonitrile at 120 K) is obtained following PET from the zinc chlorin unit to C60 in the dyad (116).175 The very short donoracceptor separation is the key contributing factor in determining the lifetime of the CS state. The single-step ET process that is involved, again avoids the loss of energy that is associated with multi-step ET. Up to 15 electrons can be accommodated in the triply fused diporphyrin-C60 conjugate (117).176 Because of its low-lying and very short-lived singlet state, however, the photo-induced process which occurs in (117) is energy transfer from the C60 units rather than the ET to C60 that occurs in the bisporphyrin analogue. But

Ar

Ar

Me N

N N

Zn

N

N

N H

N Ar

Ar = But

C60

Ar (115)

C60 N

Me Et

N

N

Me

Me

Zn N

(116) N

Me

Me

O

N C6H13

O

CO2Me

182

Photochemistry, 36, 2007, 133–204 Ar

Ar

N

N

N

Zn

Zn N

N

N

N

N

O

O

O

O Ar

Ar O

C60

O

C60

(117)

O

O

Me

Me

The ET dynamics in zinc porphyrin-arene diimide systems with aryl(ethynyl) spacers, such as (118), have been explored using pump-probe transient absorption spectroscopy.177 The conjugated nature of the (aryl)ethynyl bridge plays a role in the charge-recombination reactions of these species by increasing the extent to which the porphyrin radical cation is delocalized. A series of dyads in which an imide is connected directly to a zinc porphyrin in the meso postion has been used to explore the factors affecting the energy gap involved in PET charge separation and thermal charge recombination.178 The effect of Sc31 and Lu31 complexation on PET in the zinc porphyrin-naphthalenediimide dyad (119) and a related zinc porphyrin-pyromellitdiimide-naphthalenediimide triad has been reported.179 The metal ions bind to the radical anion of the diimide unit and thus have no effect on the rate of charge separation, as complexation occurs only after PET. The driving force for the charge-recombination process, however, decreases as the concentration of the metal ion is increased, suggesting that metal complexation can be used as a method of controlling BET in appropriate cases. The use of light to control PET has been achieved by covalently linking a porphyrin to a photochromic dihydroindolizine unit.180 Irradiation with UV light generates the ring-opened form of the photochromic unit and initiates rapid PET to give the CS state (Scheme 68). Charge recombination occurs in 2.9 ps, generating the ground state. As the ring opening process can be reversed photochemically or thermally, light can be used to switch PET on and off. The characteristics of a dyad consisting of two directly tethered quinoxaline-fused porphyrin systems, one containing Zn(II) and the other Au(III), have been reported.181 The CS state generated by PET in this dyad, ZnPQd1-AuPQ, has an exceptionally long lifetime (10 ms) in non-polar solvents, sufficiently long enough to allow its ESR spectrum to be measured at 143 K. Ph

H17C8

O

O

N

N C6H4

O

O

N

N Zn

N

C6H4 N

Ph (118)

O

O

N

N C8H17

O

O

183

Photochemistry, 36, 2007, 133–204 O Tol N N

O

N Zn

Tol N

O

C6H13 O

N

N

O

N (119)

Tol

PET NC O

O CN

P

hv, 366 nm P

N H

N N

CN

hv, 590 nm ∆

N H

CN

N N

Scheme 68

A number of non-porphyrin donor-acceptor systems have also been described. Thus a comparison of azulene-C60 and naphthalene-C60 dyads, on the basis of flash photolysis experiments, shows that azulene is a better donor, even though both it and naphthalene have a 10p-electron system.182 The synthesis and properties of a series of semi-rigid donor-(sigma spacer)-acceptor systems (120) and (121) has been reported.183 PET generates a CS state in all cases, and with the dicyanovinyl acceptors this undergoes conformational ‘folding’ in nonpolar solvents. The results show that changes can be made in the donor and the acceptor without changing the basic photophysical properties of the systems. This has led to the suggestion that these materials are suitable for incorporation into organized arrays or polymers. PET in the donor-(steroid spacer)-acceptor dyad (122) occurs184 with FSET ¼ 0.14 and kSET ¼ 1.6 107 s1. The observed isomerization of the norbornadiene unit to quadricyclane is shown to occur as a result of triplet-energy transfer and ET, both of which, it is suggested, occur by a through-bond mechanism. The dynamics involved in PET systems consisting of an imide acceptor and multiple donor sites has been explored (Scheme 69).185 On the basis of the results obtained it is concluded that if the lowest energy zwitterionic biradical contains the most reactive radical cation then an efficient and regioselective reaction will ensue. However if the lowest energy zwitterionic biradical is the least reactive then the reaction will have a low quantum efficiency and its outcome will depend on the relative energies of the transition states for the competing reactions. In other words the Curtin-Hammett principle applies in these cases.

184


MeO2 R

Me

CN

1

N

N

CN

R2

Me

MeO2

(120)

(121)

R1 = R2 = CN R1 = CO2Me, R2 = CN R1 = CO2Me, R2 = CO2Me R1 = CO2(CH2)11Me, R2 = CO2(CH2)11Me

O Me

OCCH2 N

Me O CO (122) CO2Me

O

O N

Y

X

E

PET hv

N

X

Y

E

Y

E

O

O Y

HO

O X N

N

X

O O E = SiMe3, CO2NBu4; X =NMe, O, S, NMs; Y = NMs, NAc Scheme 69

Sub-picosecond photo-induced charge injection from ‘molecular tripods’ into mesoporous TiO2 has been achieved over the distance of 24 A˚ (Scheme 70).186 The three-point attachment to the TiO2 surface and the rigidity of the spacer in molecules of such remarkable architecture as (123) facilitate control of the distance and orientation of the sensitizer with respect to the surface. Details of PET in two dendrimer system have been reported. The first contains a perylenediimide as an acceptor core, a rigid second generation polyphenylene

185

Photochemistry, 36, 2007, 133–204 o

24 A TiO2

O

O

O

TiO2

N O

2

Ru(bpy)2 N

TiO2 (123)

O TiO2

O PET, 240 fs

Scheme 70

skeleton and triphenylamine donors on the outside.187 Dendritic molecules of this type have been used to demonstrate forward and BET at a single-molecule level. The second involves dendritic multiporphyrin arrays with a C60 core.188 It is considered that the actual PET involves the porphyrin directly connected to the C60 core, but that the energy required is harvested by the porphyrins in the dendritic antenna. It is also proposed that the PZnd1 generated by the PET at the core should migrate towards the outside of the dendrimer by means of a hole-hopping mechanism. Thus overall the porphyrin array not only harvests light but also retards the BET process. In natural photosynthesis, photosystems I and II act in concert to achieve significant charge separation. The tetrad (124), which contains two donor-acceptor pairs, has been designed to explore this aspect of the natural system.189 The zinc porphyrin and the perylenedicarboximide can be excited selectively by irradiation at 420 nm and 540 nm, respectively. In both cases, however, the CS states that result undergo rapid charge recombination, thus corresponding to a zero output by the system. Nevertheless, the sequential formation of both CS states on a 50 ps time scale leads to the central ion pair undergoing charge recombination, leaving the distal ion pair. This CS state has a lifetime of 650 s. In this way the tetrad (124) demonstrates AND gate behaviour in analogy to photosystems I and II. C5H11 R O

N

N

Me N

Zn

N

N

Me

N O

R = 3,5-di-t-butylphenoxy

N

N N

R

(124)

O

C5H11

O

O N C8H17 O

186


PET also been studied in a number of supramolecular systems. Adamantanamine which is known to bind strongly to b-CD has been covalently linked to a porphyrin via a flexible hydrocarbon chain.190 In aqueous solution, PET between this porphyrin and mono-6-p-nitrobenzoyl-b-cyclodextrin was found to involve both free donor and free acceptor, and donor and acceptor bound in a supramolecular complex (125). The latter was found to be remarkably fast (kSET E 1 109 s1), being very close to that of a covalently linked porphyrin-nitrobenzene dyad. Intrarotaxane PET has been reported by two groups. Sauvage type rotaxanes (126) have been synthesized by a convergent route.191 The topology of these systems requires that electronic interaction between the porphyrin and the C60 must occur through the [Cu(phen)2]1 complex. Spectroscopic evidence was obtained for the (ZnPd1)2-[Cu(phen)2]1-C60d CS state, which had a lifetime of the order of ms, significantly longer than the value (180 ns) obtained previously for a structurally related porphyrin-C60 rotaxane held together by hydrogen bonds.192 A 3-rotaxane consisting of an anionic phenylene ethynylene dumbbell (127) threaded through two cationic cyclophanes has been synthesized, and its ability to participate in PET with the acceptors methyl viologen, dipropyl-4,4 0 bipyridinium disulfonate, and anthraquinone-2,6-disulfonate has been assessed.193 A Stern-Volmer analysis shows that quenching of the fluorescence of the dumbbell by PET to the quencher is inhibited by the presence of the threaded cyclophane. This effect, which is both steric and electrostatic in the case of cationic quenchers, has led to this rotaxane being described as an ‘insulated molecular wire’. If a rotaxane is to operate as a molecular machine in which the threaded unit moves in response to PET mediated changes in the dumbbell, then it is essential that the lifetime of ET states should be long enough to allow this movement. It has been established that the lifetime of such states is increased by several orders of magnitude in the pore of zeolites and related aluminosilicates. Although attempts to incorporate a rotaxane based on the dumbbell molecule (128) into the mesopores of Al/MCM-41 were unsuccessful, the dumbbell molecule (128) was incorporated and the photochemistry of the resulting combination was explored.194 It was found that the 4,4 0 -bipyridinium radical cation formed by PET was very long lived and that PET occurred between the aluminosilicate framework and excited units in (128).

NO2

PET

O

Ph

O N N H

N Zn

(CH2)n N

Ph N

(125) Ph

187


ZnP

N

PF6-

N Cu+

N

C60

N

ZnP (126) SO3-

-

O3S

(127) SO3-

-

O3S

Me

Me

Me Me N N Me

N 2+ N Ru N N

Me

N

Me

Me

Me

N N

N t

Me

(128)

Bu

O O

Me Et t

Bu

PET involving nitrogen-containing units as donors is the key process underpinning the operation of many fluorescence detectors. The application of PETmodulated fluorescence and luminescence in the context of chemosensors, and of biosensors which depend on conformationally induced alterations in PET efficiency, has been reviewed.195 The potential of these systems to deliver singlemolecule sensitivity in relation to monitoring individual binding events in solution, on surfaces or in polymers is addressed. The boronic acid derivatives (129) have been evaluated as fluorescent disaccharide sensors,196 their action depending on a covalent interaction between sensor and substrate. The selectivity of the sensor was a function of the length of the linking methylene chain, and in general considerable specificity was displayed for disacchadides for which pyranose-furanose isomerization is possible. A number of new cation sensors have been described. A series of sensors based on p-phenylenediamine

188


and dansyl groups as the donor and acceptor units, respectively, have been synthesized and evaluated in terms of their response to Group I and Group II cations.197 The sensors (130) and (131) displayed the greatest selectivity between Mg21/Ca21 and Na1/K1 owing to size considerations and the charge-density sensitivity of the phenylenediamine moiety. A series of N-arylN 0 -(9-methylanthryl)diaza-18-crown-6 derivatives has also been prepared by the same group and assessed as photosensors, with (132) showing particular selectivity for Ca21 in comparison to Mg21, Na1 and K1.198 A 1H NMR study indicates that the Ca21 induces large downfield shifts of the crown-ether protons, whereas the other cations have a lesser effect. There is thus a good correlation between the fluorescence response and the NMR data. The sensor (133), consisting of 4-amino-1,8-naphthalimide as fluorophore and a structurally simple receptor, demonstrates a remarkable selectivity for Zn21.199 Other physiologically important divalent cations, such as Ca21 and Mg21, and ions such as Cu21, Co21, Ni21, Hg21 and Cd21, produce either no fluorescence enhancement, or only a minor enhancement at high concentration. OH B

OH

OH

HO

N

B

N n

Ph

(129) n = 3 - 8

Me O

N

O

Me N

O

N

N SO2

O (130)

Me N

O

O

Me SO2

N

N

N O

O (131)

189


O

N

N O

O

Et O

O

N

(132)

CO2Na CO2Na HN

(133)

The 4-amino-1,8-naphthalimide group has been linked via the same spacer to an anion receptor, a thiourea, giving (134), systems which function as combined fluorescent and colorimetric chemosensors.200 On interaction with anions such as F or AcO the fluorescence of (134) is quenched as a result of PET, whereas in the presence of large concentrations of F, there is a pronounced change in the absorption spectrum, where the lmax shifts from 455 to 550 nm. It is suggested that this is the first example of a combined fluorescent-colorimetric sensor. Two new sensors based on boradiazaindacene (BODIPY) have been reported. The paramagnetic BODIPY derivatives (135) are designed to provide information through ESR and fluorescence measurements, as the nitroxide is sensitive to the redox status of the system, and the fluorescence intensity is decreased if acid or a cation is added due to protonation/chelation of the pyrroline nitrogen atom.201 The bis-BODIPY substituted bipyridyl zinc complex (136) displays a 25-fold enhancement in fluorescence intensity in the presence of HPO42.202 A similar enhancement is displayed by other anions with the exception of sulfate and fluoride. The effect is due to a simple electrostatic interaction between the complexed Zn21, and the anion which stops the PET from the BODIPY unit and restores its bright green fluorescence. The fluorescent chemosensor (137), bearing two imidazolium groups at the 1,8-position of anthracene, selectively recognizes the biologically important H2PO4 ion.203 Ab initio calculations suggest that (137) should bind selectively to H2PO4 ion, in preference to Cl, Br or I ions, through a ‘tweezer-like’ binding action. In practice, the selectivity shown by (137) for H2PO4 ion was approximately 200 times that for ions such as Br or I. Evidence for strong binding of anions to the sensor was also obtained using 1H NMR spectroscopy. Finally, the use of the resorufin derivative (138) as a PET sensor involves an entirely different approach.204 Although the natural fluorescence of the phenoxazinone system in (138) is quenched by PET from the p-nitrophenyl group, the addition of Pb21 ions leads to the hydrolysis of the phosphate ester bonds and a fluorescence enhancement which is directly proportional to the concentration of the Pb21 ions. The system showed good selectivity for Pb21 ions in preference to a range of alkaline earth and transition metal ions.

190


R Et O

N

O S

NH NH (134)

HN

R = H, CF3

Me

N

F B

Me

Me

Me

Me Me

Me

Me Me N

N

N

N

N

Me

Me n

Me R

Me

F

O

B

N

F

Me F F n = 0, R = H, Me, Br n = 1, R = H (135)

N

B F

N

Zn2+ (136)

Me Me

Me

Me

O2N

N

N N

N

O O

2PF6

P

O

O

O

O N

-

(138)

(137)

8

Miscellaneous Oxidations

There have been a number of contributions relating to photo-oxidations in which silicon plays a role. The photochemistry of silicon substituted phthalimides has been the subject of a review which discusses how organosilanes can participate efficiently as donors in PET, with the resulting radical cations undergoing rapid desilylation, producing radicals that form carbon-carbon

191


bonds through recombination or addition to unsaturated systems.205 The PETpromoted formation of the radical cations of silyl enol ethers and their use in intramolecular reactions has also been reviewed.206 The domino cyclization of such silyl enol ether radical cations has been used for the construction of steroid backbones.207 Thus the irradiation of the silyl enol ether (139) (Scheme 71) in the presence of a catalytic amount of sensitizer results in a domino cyclization generating five additional stereogenic centres in a single reaction. A further example of the use of this methodology involves the PET-mediated conversion of cyclopropyl(vinyl) silyl ethers carrying an olefinic or acetylenic side chain into bi-, tri-and tetracyclic ring systems via radical and radical cation intermediates (Scheme 72).208 Not surprisingly, all the cyclization products feature cis ring fusions, reflecting what would be the highly strained nature of trans ring junctions in the polycyclic products. Me Me H Me H Me

H

O

Me

H

Me DCA, hv (420 nm)

Me

Me

+

Me

Me H H

OTMS

O

H

Me (139)

H

Me H +

Me H

H

O 26% (47 : 41 : 12)

H

Scheme 71

O

OTMS

OTMS DCN

nucleophile

hv (350 nm) MeCN

O

O H H

Scheme 72

H

192


R2S

+

O2

hv sens

R 2S

R

+

+

O

O2

O

R

R 2S

2 R2SO

(140) Scheme 73

R2S +

O2

hv sens

R 2S

+

1

+

O2

R 2S O O

R2SO

(141) Scheme 74

A number of groups have reported findings in relation to the photooxidation of sulfides and sulfoxides. The oxidation of aryl methyl sulfoxides using Ru(III)-polypyridyl complexes, generated photochemically using O2 as oxidant, has been the subject of a flash-photolysis study.209 The observation of the absorption spectrum of a transient that could be assigned to a sulfoxide radical cation confirms ET as the rate determining step in these oxidations. The relative importance of ET and 1O2 mechanisms in the photooxygenation of dibutyl sulfide and thioanisole using N-methylquinolinium tetrafluoroborate (NMQ) and DCA has been assessed.210 It has been shown that oxidation of both sulfides with NMQ involves ET (Scheme 73). Although ET is again involved in the DCA oxidation of thioanisole, the principal route for oxidation of dibutyl sulfide involves 1O2 (Scheme 74). Evidence is presented which suggests that a thiadioxirane (140) rather than a persufoxide (141) is involved in the ET photo-oxidations of sulfides. It has also been reported that aryl selenides and aryl selenoxides are efficient trapping agents for the intermediates involved in the reaction of 1O2 with sulfides, the latter trapping the intermediate persulfoxide (141) and former trapping the S-hydroperoxysulfonium ylide (142).211 The process has synthetic potential in the context of the oxidation of selenides and selenoxides to selenones, as the photooxygenation of selenides in the presence of 4-5 equivalents of dimethyl sulfide, or of selenoxides in the presence of 1.5 equivalents of dimethyl sulfide, leads to the formation of the selenone in greater than 90% yield. This dye-sensitized photo-oxygenation has also been successfully applied to the conversion of phosphites to phosphates.212 A PET involving the tetrabutylammonium salt of (phenylthio)acetic acid plays a role in the formation of what is believed to be the first example of an ion pair consisting of a benzophenone radical anion and an organic cation, and of a photo-induced Hofmann elimination in quaternary ammonium ions (Scheme 75).213 Two synthetically useful functional group transformations involving sulfur compounds have been described. A photocatalytic system for the oxidation of sulfides to sulfoxides, which is based on a heterogeneous nafion membrane containing a lead ruthenate pyrochlore catalyst and a [Ru(bpy)3]21sensitizer, has been developed.214 Over-oxidation to the sulfone level is not a problem with this system, separation of the product is straightforward and the

193

Photochemistry, 36, 2007, 133–204 O R

OH R1

S (142)

Me

Me Me

Me

N

N

O

Ph O

O

Me

O

Me

O

Ph

Me

Me

Ph

S

S

Ph

Ph

O

Ph

Me Me

Me + Ph

Ph N

N

O

Ph

+

S

+ CO2

Ph OH

Ph

Me

Me

Me

Me Me

Scheme 75

multi-component membrane can be recycled. A yield of 96% was obtained for the oxidation of p-acetylphenyl methyl sufide. The regeneration of carbonyl compounds from 1,3-dithianes by means of a photochemical oxidation has also been described.215 The experimentally simple procedure involves 30% hydrogen peroxide and proceeds in relatively high yield. Tetrathiafulvalene (TTF) is an excellent donor in PET processes, and so it has been incorporated into a number of donor-acceptor systems that have been constructed in the context of artificial photosynthesis. A series of supramolecular dyads based on TTF and C60, of which (143) is typical, have been constructed, the interesting feature being the fact that the donor and acceptor units are held together through biomimetic hydrogen bonding.216 Through-space ET was confirmed in these systems using transient absorption spectroscopy, with the lifetime of the CS state being of the order of hundreds of nanoseconds to microseconds. Picosecond transient absorption spectroscopy has been used217 to show that PET in the dumbbell triad (144), on which the ester substituents confer solubility in organic solvents, takes place from the singlet excited state of C60. The C60-TTFd1-C60d state decays with a lifetime of 20 ns in benzonitrile. The synthesis of a donor-acceptor dyad (145),

194

Photochemistry, 36, 2007, 133–204 H17C8 N

O NH

C60

NH

O

NH

O

ET S

N S

S S (143)

TBDPSO

CO2(CH2CH2O)3CH3 S

CO2(CH2CH2O)3CH3

S C60

C60 S

H3C(OCH2CH2)3CO2 H3C(OCH2CH2)3CO2

S

(144)

S

S CH2CO2C(CO2Et)2 C60F15

S

S 3

(145)

C8H17

Me N S

S S MeS

S H17C8

S

S

(146) S

S

C60

195


MeS

S

MeS

S

S

O

O

S(CH2)12 N

N O

O

S

(CH2)12S

S S

S S

SMe SMe

(147)

S

S

S

S

SCH2CH2O

OCH2CH2S

(148)

H13C6

S

S

H13C6

S

S

S(CH2)12O

(149)

Me Me

N visible light or heat

uv

O

NO2

Me

featuring an all-trans 18p annulenic fluorofullerene (tranulene) and three pendant tetrathiafulvalene units, has been described.218 Tranulenes are particularly attractive light-harvesting units in donor-acceptor assemblies as they have a strong absorption at 665 nm that allows the CS state to be produced using visible light. In this case the CS state is energetically low-lying and relatively long-lived (870 ns). The donor-acceptor dyad (146) involves a C60 unit that is connected to a tetrathienylethylene (TTE) via a quaterthiophene (4T), both of which could donate electrons to the C60 through PET.219 Nanosecond transient absorption spectra in benzonitrile provides evidence for the formation of (TTE-4T)d1-C60d, in which the radical cation is delocalized over both the TTE and the 4T sub-units. The lifetime of the CS state in benzonitrile is only 18 ns. A series of triads, including (147), based on TTF and naphthalenediimide (NIm) building blocks, have been constructed and the photophysical consequences of varying the spacer units and the endgroups have been assessed.220 The efficiency of PET, was lower for (147) than for its analogue in which the flexible methylene chain was replaced by a more rigid cyclohexyl spacer. Absorption bands at 480 and 760 nm in the nanosecond transient spectra in benzonitrile were attributed to the TTFd1-NImdTTF CS state. Similar triads based on a perylenediimide acceptor have also been synthesized and investigated.221 The reduced fluorescence and

196

Photochemistry, 36, 2007, 133–204 H13C6

S

S

S

H13C6

NO2

S

Me Me

S(CH2)12O

(150) N

O

Me Scheme 76

Me

Me

Me PR2 H

R 2P

Me B H

Ru H

R 2P

PR2 H

R 2P Ru

B

R 2P

Me Me

PR2

PR2

Me

Me

(152)

(151) Scheme 77

fluorescence lifetimes observed for these triads relative to N,N 0 -dihexylperylenediimide was attributed to PET. The triad (148) has potential as a redox fluorescence switch, as, although it is only weakly fluorescent as a result of PET, fluorescence enhancement occurs in the oxidized radical cationic form.222 Thus sequential electrochemical oxidation and reduction results in the reversible modulation of its fluorescence. The possibility of using light to control a redox process – effectively the inverse of what happens with (148) – has also been demonstrated by the same group.223 The spiropyran unit in the dyad (149) can be photochemically switched to an open form (150) (Scheme 76). The addition of Fe31 to a solution of (149) results in ET, the conversion of the TTF unit to TTFd1 and the formation of Fe21. Subsequent irradiation produces the open form of the spiropyran to which the Fe31 coordinates, resulting in a significant reduction in its oxidation potential. This results in ET back to the TTFd1 and so the presence of the spiropyran unit in (149) facilitates the photochemical control of ET between Fe31 and TTF. Finally, although ruthenium phosphine hydrides such as (151) do not react with pinacol borane thermally, they undergo photochemical H–B oxidative addition to give metal boryl hydride complexes224 (Scheme 77). The major product (152) has a cis stereochemistry although the trans isomer is also formed as a minor product. References 1. H.S. Nalwa (ed.), Handbook of Photochemistry and Photobiology, American Scientific Publishers, California, 2003.


197

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190. Y.-H. Wang, M.-Z. Zhu, X.-Y. Ding, J.-P. Ye, L. Liu and Q.-X. Guo, J. Phys. Chem. B, 2003, 107, 14087. 191. K. Li, D.I. Schuster, D.M. Guldi, M.A. Herranz and L. Echegoyen, J. Am. Chem. Soc., 2004, 126, 3388. 192. N. Watanabe, N. Kihara, Y. Furusho, T. Takata, Y. Araki and O. Ito, Angew. Chem., Int. Ed., 2003, 42, 681. 193. P.N. Taylor, A.J. Hagan and H.L. Anderson, Org. Biomol. Chem., 2003, 1, 3851. 194. M. Alvaro, B. Ferrer, H. Garcia, E.J. Palomares, V. Balzani, A. Credi, M. Venturi, J.F. Stoddart and S. Wenger, J. Phys. Chem. B, 2003, 107, 14319. 195. M. Sauer, Angew. Chem. Int. Ed., 2003, 42, 1790. 196. S. Arimori, M.D. Phillips and T.D. James, Tetrahedron Lett., 2004, 45, 1539. 197. A.J. Pearson and W. Xiao, J. Org. Chem., 2003, 68, 5361. 198. A.J. Pearson and W. Xiao, J. Org. Chem., 2003, 68, 5369. 199. T. Gunnlaugsson, T.C. Lee and R. Parkesh, Org. Biomol. Chem., 2003, 1, 3265. 200. T. Gunnlaugsson, P.E. Kruger, T.C. Lee, R. Parkesh, F.M. Pfeffer and G.M. Hussey, Tetrahedron Lett., 2003, 44, 6575. 201. T. Kalai, E. Hideg, J. Jeko and K. Hideg, Tetrahedron Lett., 2003, 44, 8497. 202. A. Coskun, B.T. Baytekin and E.U. Akkaya, Tetrahedron Lett., 2003, 44, 5649. 203. S.K. Kim, N.J. Singh, S.J. Kim, H.G. Kim, J.K. Kim, J.W. Lee, K.S. Kim and J. Yoon, Org. Lett., 2003, 5, 2083. 204. M. Sun, Ming, D.H. Shang Guan, H.M. Ma, L.H. Nie, X.H. Li and S.X. Xiong, Chin. Chem Lett., 2003, 14, 1024. 205. U.C. Yoon and P.S. Mariano, in ref. 3, p. 85/1. 206. J.O. Bunte and J. Mattay, in ref. 3, p. 10/1. 207. J.O. Bunte, S. Rinne and J. Mattay, Synthesis, 2004, 619. 208. H. Rinderhagen and J. Mattay, Chem.-Eur. J., 2004, 10, 851. 209. M. Ganesan, V.K. Sivasubramanian, S. Rajagopal and R. Ramaraj, Tetrahedron, 2004, 60, 1921. 210. E. Baciocchi, T. Del Giacco, F. Elisei, M.F. Gerini, M. Guerra, A. Lapi and P. Liberali, J. Am. Chem. Soc., 2003, 125, 16444. 211. N. Sofikiti, C. Rabalakos and M. Stratakis, Tetrahedron Lett., 2004, 45, 1335. 212. N. Sofikiti and M. Stratakis, ARKIVOC, 2003(iv), 30; (URL: http://www. arkat-usa.org/ark/journal/2003/I06_Varvoglis/AV-672A/672A.asp). 213. A. Wrzyszczynski, M. Pietrzak, J. Bartoszewicz, H. Kozubek, G.L. Hug, B. Marciniak and J. Paczkowski, J. Am. Chem. Soc., 2003, 125, 11182. 214. J.-M. Zen, S.-L. Liou, A.S. Kumar and M.-S. Hsia, Angew. Chem. Int. Ed., 2003, 42, 577. 215. M.H. Habibi, S. Tangestaninejad, I. Mohammadpoor-Baltork and M. Montazerozohori, Phosphorus, Sulfur Silicon Relat. Elem., 2004, 179, 597. 216. M. Segura, L. Sanchez, J. De Mendoza, N. Martin and D.M. Guldi, J. Am. Chem. Soc., 2003, 125, 15093. 217. D. Kreher, P. Hudhomme, A. Gorgues, H. Luo, Y. Araki and O. Ito, Phys. Chem. Chem. Phys., 2003, 5, 4583. 218. G.A. Burley, A.G. Avent, O.V. Boltalina, I.V. Gol’dt, D.M. Guldi, M. Marcaccio, F. Paolucci, D. Paolucci and R. Taylor, Chem. Commun., 2003, 148. 219. K. Yamanaka, M. Fujitsuka, Y. Araki, O. Ito, T. Aoshima, T. Fukushima and T. Miyashi, J. Phys. Chem. A, 2004, 108, 250. 220. X. Guo, Z. Gan, H. Luo, Y. Araki, D. Zhang, D. Zhu and O. Ito, J. Phys. Chem. A, 2003, 107, 9747.

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221. X. Guo, D. Zhang, H. Zhang, Q. Fan, W. Xu, X. Ai, L. Fan and D. Zhu, Tetrahedron, 2003, 59, 4843. 222. G. Zhang, D. Zhang, X. Guo and D. Zhu, Org. Lett., 2004, 6, 1209. 223. X. Guo, D. Zhang and D. Zhu, J. Phys. Chem. B, 2004, 108, 212. 224. P.L. Callaghan, R. Fernandez-Pacheco, N. Jasim, S. Lachaize, T.B. Marder, R.N. Perutz, E. Rivalta and S. Sabo-Etienne, Chem. Commun., 2004, 242.

Photoelimination BY IAN R. DUNKIN Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, G1 1XL, Glasgow, UK

1

Introduction

This chapter deals with photoinduced fragmentations of organic and selected organometallic compounds, and especially reactions accompanied by loss of small molecules such as nitrogen, carbon monoxide or carbon dioxide. Photodecompositions which produce two or more larger fragments and other miscellaneous photoeliminations are reviewed in the final section. Photofragmentations of carbonyl compounds taking place, for example, by Norrish Type I and II processes, are discussed systematically in Chapter 1, although some eliminations from carbonyl compounds are also included here, e.g. certain decarbonylations and decarboxylations. A major publication event in the field of photochemistry during the review period was the appearance of the second, and much enlarged, edition of the CRC Handbook of Organic Photochemistry and Photobiology.1 This comprehensive work comprises reviews of many topics, including several chapters on various specific types of photoelimination, references to which are made in the appropriate sections below. Of more general relevance to this chapter are reviews of the matrix-isolation technique in photochemical studies,2 the matrix photochemistry of small ring compounds,3 both of which include examples of photoeliminations, and a chapter on photoremovable protecting groups.4 Two short historical reviews of the development of flash photolysis in George Porter’s laboratory at Cambridge have been published, one dealing with applications to gas-phase systems,5 the other studies of triplet states and free radicals in solution.6 A comparison of theories of reversible dissociation of molecules following photoexcitation has been made on the basis of integral kinetic equations.7 A number of different theories were brought to the same integral form.

2

Elimination of Nitrogen from Azo Compounds and Analogues

For quite a number of years now, the study of photo-induced nitrogen elimination from azoalkanes and analogues has appeared to be the exclusive Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 205

206


domain of Waldemar Adam’s Wu¨rzburg group, who have published a long series of detailed studies, especially on the mechanism of N2 elimination from 2,3-diazabicyclo[2.2.1]hept-2-enes. This and related work has been reviewed in the CRC Handbook.8 In addition, an extensive theoretical study of the denitrogenation of diazabicycloheptene (1) (Scheme 1) has recently been reported.9 This made use of CASPT2//CASSCF computations to investigate reaction pathways, and has indicated that the exo-axial conformer of the intermediate diazenyl diradical (2) is the primary photoproduct. Furthermore, it appears that (2) is located in a shallow region of the ground-state potential energy surface, which provides access to five different reaction pathways, and that production of the experimentally observed inverted housane (3inv) is inconsistent with thermal equilibration of (2), but rather requires an impulsive population of an axialto-equatorial pathway. Similarly, the decrease of inversion stereoselectivity or even the retention (the so-called stereochemical memory effect) observed for certain substituted derivatives of (1) can be explained by dynamic effects, in which the axial-to-equatorial motion is restrained by either inertia or steric hindrance of the substituents. On the back of a large body of previously published experimental results, this theoretical study must be regarded as providing significantly enhanced understanding. Hitherto it was not well understood how a photoreaction of this type, i.e., an elimination in which a large amount of energy is deposited by the initial photon absorption, could show such high degrees of stereoselectivity.

(3ret)

−N2 retention (stereochemical memory)

N

N N

hν

inversion

N (1)

−N2

(2)

−N2

(3inv)

stereochemical memory loss + (3ret)

(3inv)

Scheme 1

Further experimental corroboration of the intermediacy of diazenyl diradicals, such as (2), has been provided by an investigation of the viscosity

207


dependence of both stereoselectivity and product selectivity in the photoelimination of N2 from a 7-spirocyclopropane-substituted derivative of (1).10

3

Elimination of Nitrogen from Diazo Compounds and Diazirines

3.1 Generation of Alkyl and Aryl Carbenes. – A review of recent experimental and theoretical progress in the photochemistry of diazirines has been published.11 These compounds, unlike their valence isomers, diazo compounds, are relatively stable to most organic reagents and are reasonably stable in dilute solutions, and are therefore useful photochemical and thermal precursors to carbenes. A number of diaryloxydiazirines (4: X ¼ Y ¼ H, MeO, Me, Cl; X ¼ H, Y ¼ MeO, Cl; X ¼ MeO, Y ¼ Cl) (Scheme 2) have been synthesized and their thermolyses and photolyses investigated in various media.12 Although the thermolysis of (4) afforded products only from the expected carbenes (6), photolysis gave both carbenes (6) and aryloxy radicals by a-scission. The fragmentation of diaryloxycarbenes (6) to aryloxy radicals appears to be unprecedented and, moreover, thermodynamically improbable, as indicated by computations. The authors suggest, therefore, that the a-scission takes place from the excited state of the diazirine, which they depict as the ring-opened diradical (5).

X

Ar1O

O N N

Y

O (4)

hν

N

−N2

N

Ar 2O

Ar1O

Ar2O

(5)

(6) −N2

α-scission Ar1O + Ar2OC Scheme 2

The brominated diaryldiazomethane (7) was found to be stable enough to survive Suzuki coupling, thus allowing the synthesis of mono-, bis- and tris(diazo) compounds, e.g. (8).13 The photoproducts of these diazo compounds were characterized by EPR and SQUID measurements, which indicated that triplet, quintet and septet ground-state species were generated from the mono-, bis- and tris(diazo) precursors, respectively. Temperature-dependent EPR and UV-visible measurements and also laser flash photolysis studies showed that all three species are stable up to 160 K, and have half-lives of a few seconds in solution at room temperature.

208


N2

N2

N2 Br

(7) (8) N2

N N

Cl

N

Cl (11)

hν N

N

N

N2

N

N

N Cl

N Cl

(9)

Cl (10)

N

(12)

N

hν

(11)

N

Cl

N

−HCl

Cl

N

N

N

N

N Cl

(15)

Cl (14)

(13)

Scheme 3

In earlier studies of the thermolysis and photolysis of m-phenylene-bis(chlorodiazirine) (9) in the presence of 2-vinylpyridine (11) (Scheme 3), it was found that the reaction proceeded in a stepwise fashion, with generation of a monocarbene

209


(10), followed by a number of steps, as shown in Scheme 3, leading to the indolizine derivative (14), and then by similar reaction of the second diazirine to the bis(indolizine) (15). In semi-preparative conditions, however, the reaction was reported to stop at (14), after formation of the first indolizine moiety. This has potential synthetic utility, since routes to unsymmetrical products could then be developed, but raises the question of why the second diazirine ring is seemingly inert photochemically. Several possible answers to this question were suggested, including screening of the second diazirine by the intense absorption of the first indolizine, or localization of the excitation energy from the second photon in the indolizine system of (14). In a new study of the photolysis of (9) and (11),14 making use of laser flash photolysis and both absorption and fluorescence monitoring, coupled with computations, it has been established that, although the S1 state of (14) has the excitation localized largely in the indolizine system as previously suggested, there is a higher excited state of (14) that can be readily thermally populated from S1, in which the excitation energy is localized on the diazirine. Indeed, the quantum yield for decomposition of the diazirine ring of (14) was estimated as B0.5, and the conclusion is that the indolizine ring does not effectively inhibit this photodecomposition. It seems instead that the best explanation for the reluctance of the second diazirine ring to react in more concentrated solutions is the formation and precipitation of the hydrochoride of (11), which gives rise to turbidity and thus light scattering. The photolysis of 3-pyridyldiazomethane (16) (Scheme 4) in an argon matrix at 7–10 K gives the corresponding carbene (17).15 Further photolysis of (17) generates the nitrile ylide (19) as the major product, providing a new example of this recently discovered type of ring opening. A minor amount of the ring expanded didehydroazepine (20) was also formed.

CHN2

CH

hν

N

N

(16)

(17)

N

C N

CH (19)

(18)

CH N (20)

Scheme 4

Derivatives of C60 for photoaffinity labelling studies have been synthesized, including the cis and trans diazirine compounds (21a).16 The photochemical properties of these are currently being studied in various applications. An

210


analogue of tautomycin containing an aryl diazirine moiety has also been synthesized for use as a photoaffinity probe.17 R a: R =

N

Me

N CF3

b: R = N3

N O Me (21)

3.2 Photolysis of a-Diazo Carbonyl and Related Compounds. – The photochemical reactivity of a-diazo carbonyl compounds has been reviewed, especially with respect to recent experimental and theoretical contributions to furthering the understanding of the reaction mechanisms.18 The question of oxygen migration and the involvement of oxirene intermediates in the Wolff rearrangement has been addressed in two papers by Haiss and Zeller.19,20 In the first of these, crossover experiments between isotopomers of 2-diazo-1-oxo-1-phenyethane (22) (18O, 13C and D substituted) have established that the partial oxygen migration observed in the Wolff rearrangement products following photoelimination of N2, see for example (24) in Scheme 5, is not the result of an intermolecular process.19 This is consistent with a carbenecarbene rearrangement via an intermediate oxirene (23). The second study was concerned with the Wolff rearrangement of 3-diazo-1,1,1-trifluoro-2-oxo-propane (25b) (Scheme 6).20 The trifluoromethyl group had been considered as having a low tendency to migrate to a carbene centre, and this served to explain why photolysis of the diazoester (25a) results in hydrogen abstraction, to give (26), rather than rearrangement. On this basis, the observed Wolff rearrangement of (25b) was assumed to involve equilibration of the initially formed oxocarbene (28b) with the isomeric (30b) via oxirene (29b), followed by the H Ph

H hν

13

C

N2

Ph

O 1-[13C]-(22)

H

13

C

13

C

−N2

Ph

Ph 13C

H

O O

O (23)

Ph O

13

Ph 13

C

C C

H 1-[13C]-(24) Scheme 5

O

H 2-[13C]-(24)

211


O

OR2

F3C

F3C

OEt

O

(26)

(27) (25a) hν,

(25b) O

R2OH

1 * R

F3C

hν, R2OH

N2 (25) a: R1 = CO2Et b: R1 = H c: R1 = H, * = 13C (25b,c) hν O

O

O H

*

F3C

F3C

*

H

(29)

(28)

H O

F3C

(30)

F3C

* C CF3

* H

* C

H2O HO

O

H H2 O O

* CF3

F3C

O (31c)

* OH (32c)

Scheme 6

favourable migration of an H atom in (30b). An investigation of the photolysis of the 13C-labelled isotopomer (25c) in water-saturated ether, however, has shown that rearrangement of the oxocarbene (28c) occurs to only a minor extent: from the ratio of the yields of the isolated isotopomeric acids (31c) and (32c), it was estimated that 93.5% of the rearranged product was produced by trifluoromethyl migration. As a consequence, it is proposed that the difference in photochemical behaviour between (25a) and (25b) is likely to be due to a larger singlet-triplet splitting in the oxocarbene (28b), which allows the intramolecular Wolff rearrangement to compete efficiently with singlet-triplet interconversion. The photochemistry of diazo Meldrum’s acid (33) (Scheme 7) has been investigated experimentally and theoretically.21 Irradiation of (33) in MeOH with 254 nm light results in efficient loss of N2 and Wolff rearrangement, affording the ester (36) via the intermediate ketene (35). There was no product derived from trapping of a singlet carbene, suggesting that the Wolff

212

Photochemistry, 36, 2007, 205–231 254 nm O O O

O N

355 nm

N

∆

O N2 O

O

254 nm −N2

O

O

O

C O

O

(34)

(33)

MeOH triplet sens.

(35)

MeOH triplet sens.

MeOH

O O O O (37)

O

O

O

CO2Me

(36) Scheme 7

rearrangement is concerted with nitrogen loss. Irradiation of (33) in MeOH at 355 nm led almost exclusively to the isomerized diazirine (34). This isomerization and the loss of N2 leading to Wolff rearrangement clearly occur from two different electronic excited states. The diazirine (34) reverts to the diazo precursor (33) on mild heating, while UV-irradiation of (34) yields a mixture of the Wolff rearrangement product and the diazo isomer (33). From the productevolution profiles, it seems that the Wolff rearrangement, at least initially, occurs directly from (34) and not via the diazo compound (33). Finally, triplet sensitization of both (33) and (34) gives Meldrum’s acid (37), presumably via an intermediate triplet carbene. A laser flash-photolysis study has been made of the bis(arylsulfonyl)diazomethanes (38: X ¼ H, Me).22 Analysis of the results indicated the involvement of two transient species, and a partitioning of the reaction pathway between the sulfonyl analogue of the Wolff rearrangement and what seems to be intramolecular carbene oxidation by a sulfonyl group. N2 X

S O2

S O2

X

(38)

4

Elimination of Nitrogen from Azides and Related Compounds

The photochemistry of organic azides has been reviewed.23 In addition to the photolysis of alkyl, vinyl, acyl, aryl and heteroaryl azides, this review also

213


covers the chemistry of singlet and triplet nitrenes and the photogeneration of nitrenium ions. Photolysis of 2-, 3- and 4-fluorophenylazide (39a,b,c) in aniline gives the products (40), (41) and (42) in the yields shown in Scheme 8.24 With 2,6difluoro- and 2,3,4,5,6-pentafluorophenyl azide, however, ring expansion to azepine products analogous to (42) is suppressed, and the corresponding unsymmetrical azobenzenes (cf. 41) are formed in 50% and 55% yield, respectively. The azobenzenes are presumed to arise from insertion of a singlet nitrene into an N–H bond of aniline, followed by oxidation of the resulting hydrazine. NH2

N3

NH

hν X

NHPh

+ X

X (40) a: 8% b: 12% c: –

(39) a: X = 2-F b: X = 3-F c: X = 4-F

N

NPh

+

PhN3 X

N

(41) a: 10% b: – c: –

(42) a: 15% b: 15% c: 30%

Scheme 8

When o-azidobenzoic acid and its potassium salt were photolysed in H2O, EtOH, THF, EtOH-H2O or THF-H2O, the major product was 2,1-benzisoxazolone, from intramolecular cyclization.25 Varying amounts of azepine products were observed, depending on the solvent. Intramolecular nitrene cyclization was also observed in the photolysis of the azido-functionalized cryptand (43), which afforded the benzopyrazole (44) after an aerobic workup.26 The paper which reported the photo-induced ring opening of 3-pyridylcarbene (17) (see Section 3.1) also described a study of the matrix photolysis of 3azidopyridine (45) (Scheme 9).15 Irradiation of (45) at 222 nm in Ar at 7 K gave a product with IR and UV absorptions which indicated that it was the ringopened ylide (47); two rotamers were identified. The ylide also underwent a 1,7H shift to give the ketenimine (48) in two rotameric forms.

N3

N

N

N

N

N

O O

O (43)

O

O

O (44)

214

Photochemistry, 36, 2007, 205–231 N3

N

hν N2

N

C

N

(45)

C

N

NH

N

N

CH (47)

(46)

C (48)

Scheme 9

A matrix-isolation investigation of the photolysis of azido- and diazidophenylethynes has been carried out.27 Simple ethynyl or phenylethenyl substitution of phenyl azide has little effect on the reaction pathway, no matter which regioisomer is considered. Thus irradiation (l 4 350 nm) of the metaethynyl azides (49a) and (49b) in Ar matrices at 13 K affords the corresponding triplet carbenes, which can be photochemically interconverted with the ‘normal’ ring-expanded dehydroazepine products (51a,b). Exactly analogous results were obtained for the para isomers (50a) and (50b). On the other hand, with the diazides (52), the regiochemistry of the azido substitution substantially affects the products obtained. Whenever the two azido groups are placed in conjugating positions with respect to each other – i.e., p,p 0 - (52a), p,o 0 - (52b) and o,o 0 - (52c) – a quinoidal diimine diradical is obtained, e.g. (53) from (52b), or the stereoisomers (54) and (55) from (52c). The m,m 0 - and p,m 0 -isomers (52d,e), however, both gave photoproducts with EPR spectra in good agreement with what was expected for a dinitrene quintet state. In addition, (52d), but not (52e), gave a dehydroazepine product, as indicated by the matrix IR spectrum. CR C N N3

N3 (49)

C

CR

C

(50)

CR

(51)

a: R = H b: R = Ph X

Y

(52) a: X = 4-N3; Y = 4′ -N3 b: X = 4-N3; Y = 2′ -N3 c: X = 2-N3; Y = 2 ′-N3 d: X = 3-N3; Y = 3′ -N3 e: X = 4-N3; Y = 3′ -N3 N N

N

C C

(53)

N

N

N

C C

C C

(54)

(55)

215


Quintet m-phenylenedinitrenes have been generated in low-temperature matrices by photolysis of the corresponding diazide precursors, and their secondary photochemical transformations studied.28 Two competing ring-opening pathways were identified, and it was also concluded that quintet dinitrenes are much more photochemically reactive than triplet nitrenes. An EPR study of radical intermediates formed in the photooxidation of 4,4 0 -diazidodiphenyl in benzene and toluene has also been published.29 The radicals apparently arise by abstraction of H atoms from the solvent by a triplet nitrene–O2 complexes. Two related reports have been made on the generation of 2,4,6-trinitreno1,3,5-triazine, a septet trinitrene, by photolysis of the triazido precursor (56).30,31 Stepwise generation of the mono-, di- and trinitrene was observed in lowtemperature matrices by IR and EPR spectroscopy. Comparison of experimental IR spectra with those calculated by DFT aided the assignments. The trinitrene readily decomposed into three NCN molecules upon further irradiation. N3 N N3

N N

N3

(56)

As well as the aziridine group in (21a) (see Section 3.1), an azide functionality has also been incorporated into a C60 derivative (21b) for use as a photoaffinity label.16

5

Photoelimination of Carbon Monoxide and Carbon Dioxide

A new statistical quantum mechanical approach – the statistical adiabatic product distribution method – has been applied to the photodissociation of ketene in parallel theoretical32 and experimental33 studies. The main focus of the work, however, was H-atom production via C–H cleavage rather than the elimination of CO. The photodecomposition of formohydroxamic acid (HCONHOH) has been investigated in matrix-isolation FTIR and DFT studies.34 Irradiation of the acid in Ar or Xe matrices with the full output of a xenon arc lamp generated H-bonded HNCO H2O and NH2OH CO complexes. In the latter case, the IR spectra also suggest the existence of a structure with the NH2 group interacting with the carbon atom. The photochemistry of a-pyrone (57a) (Scheme 10) was the subject of some of the earliest matrix-isolation studies of organic species. This system still has some interest for further study, however, especially on account of the success that DFT computations have had in assisting with the assignments of matrix IR spectra. Recently, the matrix photolyses of both a-pyrone35 and its 4,6-dimethyl derivative (57b)36 have been investigated with the aid of DFT calculations. When (57a) is irradiated (l 4 285 nm) in Ar or Kr matrices, rapid formation of rotamers, (58a) and (59a), of the Z isomer of the ring-opened

216

Photochemistry, 36, 2007, 205–231 R

R

C O

O

R O + O

C R

(58)

R hν O

O

hν R

(59)

R

R

(57)

hν C

O

hν

O

O

C

O

(62) hν

a: R = H b: R = Me R

O

R

+

R

R

R

+ CO2

O

(60)

R

(61)

(63) Scheme 10

aldehyde-ketene results. Further irradiation generates rotamers of the E isomer: (60a) and probably (61a). Upon subsequent irradiation at l 4 337 nm, the Z forms revert to the ring-closed starting material (57a), but the E forms do not react. At l 4 285 nm, the ring-opening reaction of (57a) is accompanied by a much slower isomerization to the bicyclic lactone (62a), which in turn is converted into cyclobutadiene (63a) and CO2 at shorter wavelengths (l 4 235 nm). These findings are in accord with those from previous investigations, and the main novelty lies in the positive identification of particular isomers and rotamers of the ring-opened product, which has now been made possible for the first time. The matrix photochemistry of 4,6-dimethyl-a-pyrone (57b) is something of a contrast to that of the parent compound. Although both ring opening and valence isomerization to the bicyclic lactone (62b) are observed on UV irradiation (l 4 315 nm), the latter reaction is by far the more efficient. 1,3-Dimethylcyclobutadiene (63b) is generated from the lactone at shorter wavelengths (l 4 235 nm). The photochemistry of ketoprofen (64) has been studied because of the known phototoxicity of this compound; the photoproduct is 3-ethylbenzophenone, formed via a short-lived carbanion (65). In a new study of this reaction,37 the lifetime of the carbanion has been extended to several hours by generating it in carefully controlled conditions in THF. Intermolecular photoinduced decarboxylative additions of alkylcarboxylates to N-substituted phthalimides gave the corresponding hydroxyphthalimidines in yields of 39–89%.38 Timeresolved monitoring on the fs-scale of transient intermediates in the photodecarboxylation of organic peroxides has been carried out in conjunction with theoretical modelling.39,40 O

Me

O CO2−

(64)

(65)

Me

217


5.1 Photoelimination of CO from Organometallic Compounds. – A procedure for the sunlight-induced synthesis of (Z2-C60)M(CO)5 complexes from fullerene and M(CO)6 (M ¼ Mo, W) has been suggested as an instructive laboratory experiment.41 Sequential loss of CO ligands has been observed in the formation of W(CO)4(Z4-2,5-dimethylfuran) during laser photolysis (308 nm) of cyclohexane solutions of W(CO)6 containing excess 2,5-dimethylfuran.42 An initially generated unstable complex, W(CO)5(Z1-2,5-dimethylfuran) rapidly eliminated a second CO molecule to give the final product in a first-order process. NearUV irradiation of Ru(dcbpy)I2(CO)2 in solution induces loss of one of the CO groups, generating a vacant coordination site, which is occupied by a solvent molecule.43 The kinetics of this ligand exchange in EtOH were studied with femtosecond resolution, by probing the CO stretching vibrations of the reactant and product molecules. It was found that photoelimination of CO occurs on a sub-picosecond time scale, with an overall quantum yield well below unity (0.3). Recovery of the parent molecule also occurs on a sub-picosecond time scale. Five new complexes, [M(CO)5(DTTT)] (M ¼ Cr, Mo, W), [Re (CO)4Br(DTTT)] and [Mn(CO)2Cp(DTTT)], have been prepared by the photochemical reaction of the metal carbonyls M(CO)6 (M ¼ Cr, Mo, W), Re(CO)5Br and Mn(CO)3Cp with 3,5-dimethyltetrahydro-2H-1,3,5-thiadiazine-2-thione (DTTT) (66).44 Spectroscopic and x-ray diffraction studies showed that DTTT behaves as a monodentate ligand, coordinating to the metal centre via the sulfur (C ¼ S) donor atom. Similar sulfur (P ¼ S) donation has been observed in the novel complexes (68) and (69), obtained by photolysis of Re(CO)5Br in the presence of tetraalkyldiphosphine disulfides (67: R ¼ Me, Et, Pr, Bu, Ph) (Scheme 11).45 Analogous complexes have also been synthesized in the photochemical reactions of Re(CO)5Br with diphosphines, Ph2P(CH2)nPPh2 (n ¼ 1–3).46 Spectroscopic characterization of the products suggests cis-chelate bidentate coordination of the ligand in fac-[Re (CO)3Br{Ph2P(CH2)nPPh2}] (n ¼ 1–3), and cis-bridging bidentate coordination between two metals in [Re2(CO)8Br2{cis-m-Ph2P(CH2)PPh2}] (n ¼ 1–3). Photoinduced substitution of CO in iron selenocarboxylate complexes CpFe(CO)2SeCOR with triphenylphosphine, -arsine or -antimony (EPh3) gave exclusively and in high yields the monosubstituted products CpFe(CO)(EPh3)SeCOR (R ¼ Ph, 2-Me-C6H4, 4-NO2-C6H4, 3,5-(NO2)2C6H3; E ¼ P, As, Sb).47 Me

N

N S

Me

S

(66) 5

Photochemical reaction of (m -C5Me5)Re(CO)3 with 2,4,5-trichloroanisole and 3,4,5-trichlorotrifluoromethylbenzene yields (Z5-C5Me5)Re 5 (CO)2(C6H2Cl2(MeO))Cl and (Z -C5Me5)Re(CO)2(C6H2Cl2(CF3))Cl, respectively, by insertion of the intermediate (Z5-C5Me5)Re(CO)2 into a C–Cl bond

218

Photochemistry, 36, 2007, 205–231 R

Br OC CO Re OC CO CO

S +

R

R

P

P

R

R

hν −CO

S

Br S OC Re OC CO CO

R P R

P

S R

(67) −CO R R

Br S P R OC Re OC P R CO S R (68)

R

R

Br S OC Re CO OC CO

R

S OC

Br Re CO

CO CO

(69)

Scheme 11

of the chloroarene.48 In the trichloroanisole, C–Cl bond activation occurs ortho to the methoxy group, while the reaction with C6H2Cl3(CF3) occurs at the chlorine meta to the CF3 group.

6

Photoelimination of NO and NO2

The past few years have seen a flurry of activity in research into the photoelimination of NOx species, particularly on account of the biological activity of NO as well as interest in the environmental impact of nitrogen oxides. In the period under review at present, however, only three reports concerning photoelimination of NO or NO2 have appeared. A high-level theoretical study of low-lying singlet and triplet potential energy surfaces in the photolysis of nitromethane has been carried out, which reinterprets available experimental results in a consistent manner.49 Two reaction paths were found in the photolysis of CH3NO2 with 193 nm light, which generates initially the 2A00 excited state. In the major channel, intersystem crossing gives the 2A 0 state, which dissociates into CH3(1A 0 1) þ NO2(1 2B1); the photoeliminated NO2 then undergoes intersystem crossing to the 1 2A1 state, which dissociates into NO(A 2S1) and a mixture of O(3P) and O(1D). In the minor channel, the initial 2A00 state of CH3NO2 dissociates directly into CH3(1A 0 1) and NO2(1 2A2), which in turn gives NO(X 2P) and a mixture of O(3P) and O(1D). No ionic species were implicated in any dissociation pathway. The velocity and angular distributions of NO produced by UV photolysis of nitrosobenzene have been determined by velocity-map ion imaging.50 With light of l ¼ 290.5 (S2 state) and 226 nm (Sn (n Z 3) states), completely isotropic velocity distributions were observed, leading to the conclusion that photodissociation occurs on a timescale much slower than rotation of the parent molecule, and after redistribution of the excess energy into the vibrational modes.

219

Photochemistry, 36, 2007, 205–231 O Et2N

N

O N

O

hν Et2N

(70)

N

+ NO

(71) H+

e− transfer Et2NNO + 3NO−

Et2NH + 2 NO Scheme 12

The aqueous photochemistry of the sodium salt of 1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (70) (Scheme 12) has been examined experimentally and theoretically.51 Photolysis of (70) results in formation of the radical anion (71) and NO, via intersystem crossing and a triplet excited state. The product pair either undergoes electron transfer, before escape of NO from the cage, to form triplet NO and nitrosamine (minor pathway), or the radical anion (71) dissociates to give a second NO molecule and (ultimately) diethylamine (major pathway).

7

Miscellaneous Photoeliminations and Photofragmentations

7.1 Photoelimination from Hydrocarbons. – The visible fluorescence of excited CH fragments in the A2D and B2S states has been observed following photodissociation of ethene in the 11.7–1.4 eV energy range.52 Two channels were identified: CH* þ CH3 and CH* þ H2, and it is proposed that both pass via an ethylidene intermediate (H3CCH:). The photofragmentation dynamics of ethyne have been studied via vibrationally mediated photodissociation.53 Near-IR excitation prepared rovibrational states in the region of three C–H stretch quanta (B9640 cm1 ), and subsequent irradiation at 243.1 nm dissociated these pre-excited C2H2 molecules into C2HþH; the resulting H atoms were probed by UV resonantly enhanced multiphoton ionization. An organic molecule containing krypton, HKrCCH, has been generated in low-temperature matrices by 193-nm irradiation of solidified mixtures of C2H2 and Kr at 8 K, and subsequent thermal mobilization of the resulting H atoms by warming to Z 30 K.54 The novel organokrypton species was characterized by IR absorption spectroscopy and identified with the aid of ab initio calculations. Photodissociation dynamics of propene at 157.6 nm have been investigated in a molecular beam apparatus by photofragment translational spectroscopy.55 The eleven photofragments identified were assigned to eight dissociation channels: C3H5 þ H, C3H4 þ H þ H, C3H4 þ H2, C3H3 þ H2 þ H, C2H4 þ CH2, C2H3 þ CH3, C2H2 þ CH4 and C2H2 þ CH3 þ H, with branching ratios of 1%, 7%, o0.2%, 17%, 6%, 4%, 5% and 60%, respectively. This somewhat complicated process seemingly has a preference for triple dissociations, notably to C2H2, CH3 and H. The photodissociation dynamics of CD3CH¼CH2 at the same wavelength was also studied.56 Labelling of the methyl group with

220


deuterium allowed discrimination between elimination of atomic and molecular hydrogen from the vinyl and methyl moieties. Hydrogen migration before photodissociation was also revealed by this means. The dynamics of allene and propyne photofragmentation at 193.3 and 121.6 nm have been investigated by H(D) Rydberg atom translational spectroscopy.48 The total kinetic energyrelease spectra for H2C¼C¼CH2, H3CCRCH and D3CCRCH photolysis at 193.3 nm were essentially identical – contradicting several previous studies of propyne photochemistry at this wavelength. The results were most easily rationalized by assuming that the fragmentations are preceded by internal conversion to the S0 ground state, and that the isomerization rate of the resulting highly vibrationally excited molecules exceeds that of the unimolecular decay. The analogous spectra from 121.6 nm photolysis, however, are significantly different. This observation can be accounted for by assuming two competing pathways for propyne: one involving selective cleavage of the H3CCC–H bond, the other arising via radiationless transfer to a lower excited state, isomerization and then unimolecular decay. The benzyl radical and several deuteriated isotopomers have been generated in methylcyclohexane at 4.2 K by UV-photolysis of toluene or deuteriated toluene, and a detailed vibrational analysis of the fluorescence spectra carried out.57 The photodissociation dynamics of azulene at 193 nm have been studied in a molecular beam using multimass ion-imaging techniques.58 Most of the excited molecules produced at this wavelength relax rapidly to the electronic ground state through internal conversion, then isomerize to naphthalene, and eventually dissociate by H-atom elimination. 7.2 Photoeliminations from Organohalogen Compounds. – A review has been published of the photobehaviour of alkyl, aryl, benzylic and homobenzylic halides in solution and of alkyl halides in the gas phase.59 UV-induced dissociation of CD3Cl trapped inside thin amorphous solid water layers on Ru(001) has been examined.60 Stable products, such as C2D6, CHD3, CD3CD2Cl and CD3OH, were detected via post-irradiation desorption. An investigation of the UV-induced photodecomposition of CHBr3 multilayers on Cu/Ru(001) has been carried out.61 Methyl radicals accumulated on the surface were the precursors for the formation of methane and ethene. A photofragment translational spectroscopic study of the 248-nm photolysis of methyl hypochlorite (CH3OCl) – a molecule that acts as an atmospheric chlorine reservoir – has shown that the primary photodissociation channel is cleavage of the O–Cl bond to produce Cl atoms and CH3Od radicals.62 The results are in accord with previous theoretical and experimental findings, but also suggest that CH3OCl could serve as a precursor of CH3Od radicals with high and well-defined rotational and translational energies. An experimental and theoretical study of the 355-nm photodissociation of CH2Cl2 and CH2Br2 has shown that both molecules follow similar ionization patterns, with H1, CH21, X1, CX1 and CH2X1 (X ¼ Cl, Br) as the major ions.63 The proposed mechanism involves dissociation of CH2X2 into CH2X, which then dissociates further by several competitive channels, the products of


221

which are subsequently ionized. Dichloromethane molecules vibrationally excited to the second C–H stretch overtone have been photodissociated with light of l ¼ B235 nm, and the Cl atoms produced were studied by resonantly enhanced multiphoton ionization.64 The results suggest fast dissociation and the involvement of several upper states of different symmetries, and also higher non-adiabaticity for vibrationally excited than for vibrationless ground-state molecules. The photochemistry of CH2I2 at 266 nm in acetonitrile solution has been investigated by femtosecond and nanosecond laser photolysis.65 The CH2I–I isomer was produced with a quantum yield of B70%. High isomer quantum yields (470%) were also measured for n-hexane, dichloromethane, methanol and ethanol solutions. The isomer is formed within B15 ps after excitation, and survives for nanoseconds or microseconds before decaying by a mixture of first- and second-order processes. The measured rate constants for nhexane solution correspond to intramolecular decomposition of CH2I–I to the dCH I radical and an I atom, with the additional formation of I in MeCN. 2 The observation of H1 and I following UV photolysis of CH2I2 in water has been reported.66 Since the concentrations of the two ions are about the same, this suggests a reaction which produces HI as a leaving group. An ab initio investigation was also carried out for the reaction between isodiiodomethane, CH2I–I, and H2O, which showed how the CH2I–I could react with an O–H bond by insertion and HI elimination, and that this process would be catalysed by a second water molecule. The rotational spectrum of HCBr produced by 193-nm photolysis of bromoform has been studied by kinetic microwave spectroscopy (420–472 GHz).67 Pulsed multiphoton photolysis of CHBr3 at 248 nm in the presence of excess O2 has led to the first observation of CO UV-visible chemiluminescence.68 The luminescence is best accounted for by postulating reactions between O2 and vibrationally excited methylidene radicals in two different electronic states: CH(X2P) and CH(a4S). The photodissociation of CF3I at 248 nm has been studied in a high-resolution photofragment translational spectrometer.69 Dichlorocarbene, CCl2, has been generated by 213-nm photolysis of CCl4, and the kinetics of its reactions with C2H4, NO, N2O and halomethanes determined.70 The photodissociation dynamics of n-alkyl bromides, RBr (R ¼ Et, n-Pr, nBu, n-pentyl), at B234 and 267 nm have been investigated using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry.71 Bromine atoms were produced by direct dissociation of the R–Br bond, and in this wavelength range ground-state bromine was the major product. A concerted three-body dissociation of CH2XCH2Y (X, Y ¼ Br, Cl) into X þ Y þ C2H4 has been discovered by product translational spectroscopy following absorption of 193-nm photons.72 No stable CH2BrCH2 or CH2ClCH2 was detected. Two theoretical studies of the of HCl elimination from 193-nm photodissociation of vinyl chloride have been published, the first using AM1 trajectory calculations with specific reaction parameters,73 and the second a direct MP2/6-31G(d,p) classical trajectory approach.74 Broadly speaking, the results were in good agreement, and support an earlier proposal that three-centre elimination of HCl and isomerization of vinylidene to ethyne

222


take place in a concerted, non-synchronous manner. In addition, the results indicate that the four-centre channel produces rotationally hot HCl molecules, while low rotational states of HCl derive from the three-centre channel, which is at odds with a previous conclusion. Photodissociation of allyl-d2 iodide, H2C¼CDCH2I at 193 nm and the dynamics of the resulting allyl-d2 radical have been investigated by photofragment translational spectroscopy.75 A previous study had found the allyl radical stable at internal energies up to about 60 kJ mol1 higher than the barrier to allene þ H formation as the result of a centrifugal barrier, and this new study shows that the allyl-d2 radical is stable to allene þ D formation at energies of about 10 kJ mol1 higher than that of the non-deuteriated analogue. This is thought to be the result of the difference in zero-point energies and a reduction in the impact parameter of the fragments due to the decrease in frequency of the C–D bending modes. A minor fraction of the allyl-d2 radicals isomerize to the 2-propenyl radical, but the dominance of the direct alleneþH(D) formation is in accord with previous conclusions. Photolysis (300 nm) of the tricyclic molecule (72) (Scheme 13) generates carbomethoxyfluorocarbene (73) and indan.76 Laser flash photolysis studies of this reaction in solution, with UV-visible and IR detection, demonstrated that, contrary to expectations, (73) is more reactive than its chloro analogue. F

CO2Me hν +

F O

(72)

(73)

Scheme 13

A theoretical examination of the excited state properties of fluoro-, chloro-, bromo- and iodobenzene using CASSCF and CASPT2 methods has been carried out with the aim, amongst others, of understanding the experimental observation that chloro- and bromobenzene each have one fast dissociation channel, whereas iodobenzene has two.77 The computations indicate that the dissociation channels for chloro- and bromobenzene and the slower iodobenzene dissociation channel are due to intersystem crossing from a bound (p,p*) singlet excited state to a repulsive (n,s*) triplet excited state. The faster iodobenzene dissociation channel involves direct dissociation of an antibonding (n,s*) singlet excited state. Another theoretical study was concerned with the ground states and several low-lying excited states of bromobenzene, o-, m- and p-dibromobenzene and 1,3,5-tribromobenzene.78 Experimentally, it was known that o- and m-dibromobenzene each have two ultrafast predissociation channels, while bromobenzene, p-dibromobenzene and 1,3,5-tribromobenzene each have only one such channel. The calculated potential energy curves provide an explanation, showing that the reduction of symmetry from C2v to Cs along the main reaction coordinate, which occurs only in o- and m-dibromobenzene, opens up a second predissociation channel.


223

In an investigation of the photodissociation dynamics of fluorobenzene and d5-fluorobenzene at 193 nm, it has been found that four-centre elimination of HF or DF was the major dissociation channel, although small amounts of Hor D-atom elimination were also observed.79 The photodissociation of bromobenzene at 267 and 234 nm has also been examined experimentally.80 The branching ratio, Br(2P1/2)/Br(2P3/2), was found to be smaller than that for the dissociation of 1-bromoheptane, indicating a different photodissociation mechanism for aryl and alkyl halides. In addition, the branching ratio for bromobenzene decreases with a decrease in excitation wavelength. The photodissociation dynamics of o-, m- and p-bromotoluene at 270 nm have been investigated by femtosecond pump-probe spectroscopy.81 The initially excited S1 (p,p*) state is predissociative via either the repulsive 3(n,s*) state or high vibrational levels of the S0 state. In the photodissociation of o-, m- and pchlorotoluene at 193 nm, photoreactions corresponding to ClC6H4CH3 ClC6H4CH2 þ H and ClC6H4CH3 - ClC6H4 þ CH3 are observed as well as Cl-atom elimination.82 The photodecomposition of iodobenzene adsorbed on the Mo2C/Mo(100) surface has been studied in the 100–1200 K temperature range.83 7.3 Photofragmentations of Organosilicon Compounds. – There have been several studies of the infrared multiphoton decomposition of silanes. Isotopeselective IR multiphoton dissociation of difluorosilane was achieved with 977.2 cm1 light from a pulsed CO2 laser, the dissociation rate for 28SiH2F2 being approximately twice as fast as for the other isotopomers (29SiH2F2 and 30 SiH2F2).84 It was estimated, however, that only a small proportion of the molecules interact with the IR radiation, owing to a ‘rotational bottleneck’ effect; the dissociation is therefore very inefficient. IR multiphoton dissociation of trichlorosilane molecules, irradiated by pulses from CO2 and NH3 lasers, gives mainly HCl and a solid product, presumed to be (SiCl2)n, in yields which depend on the laser frequency and the pressure of SiHCl3.85 In a related study, the IR multiphoton dissociation of SiHCl3 was carried out in the presence of scavenger gases, such as O2, CO2, OCS, halomethanes, BCl3 and TiCl4.86 In some cases, stable volatile products were detected. At relatively high pressures of SiHCl3 ( Z 400–800 Pa), its photolysis in mixtures with F-, Cl- or Brcontaining scavengers gave products of the type SiX4nCln (X ¼ F, Br; n ¼ 1–4); a conversion into SiCl4 higher than 70% could be achieved. At low SiHCl3 pressures, however, a stable volatile product was observed only with BCl3, this resulting from insertion of SiCl2 into the B–Cl bond. In the IR multiphoton dissociation of diethylsilane unders collisionless conditions, dissociation of both C–Si and C–C bonds were found as the main unimolecular processes.87 Photolysis of the silacyclopropenes (74: R ¼ Me, Et) at 254 nm in methylcyclohexane glasses at 9–80 K led to the observation of a characteristic broad EPR signal due to bis(tri-tert-butylsilyl)silylene, (tBu3Si)2Si:, the temperature dependency of which established that the silylene has a triplet ground state.88 The identified products, which included the disilacyclobutane derivative (75) and (tBu3Si)2SiH2, support this conclusion. The effects of ortho-methyl

224


substitution on the reactivity of 1,1-diarylsilenes has been investigated by photolytically generating five silenes (76: Ar1 ¼ Ar2 ¼ 2-MeC6H4; Ar1 ¼ Ph, Ar2 ¼ 2,6-Me2C6H3; Ar1 ¼ Ph, Ar2 ¼ 2,4,6-Me3C6H2; Ar1 ¼ 2-MeC6H4, 2,6Me2C6H3; Ar1 ¼ Ar2 ¼ 2,6-Me2C6H3) from the corresponding 1,1-diarylsilacylobutanes (77).89 UV absorption spectra and lifetimes of the silenes (76) were determined in MeCN and hexane solution, along with the absolute rate constants for their reactions with MeOH and AcOH; rates were found to vary over 3–4 orders of magnitude. Photolysis of the silacyclobutane (78) gave three isomeric products, derived from intramolecular trapping of a biradical intermediate, as well as the expected ether-stabilized silene (79).90 Recent advances in the silylation of fullerenes by active species – silylenenes and silyl radicals – generated by photolysis of polysilane have been summarized.91 H But3Si

SiBut3 Si

But3Si

Si

But2Si

R

Me

R

Me

(74)

(75)

Ar1

Ar1 Si Ar2

CH2

(76)

Ar2

Si (77)

OMe

CH2OMe Ph

Si

Si

CH2

Ph (78)

(79)

7.4 Photofragmentations of Organosulfur Compounds. – The gas-phase Raman spectra of cyanogen isothiocyanate (NCNCS) and its photolysis product, isocyanogen (CNCN), have been detected in a study of the 266 nm photodissociation of NCNCS.92 Direct C–S bond cleavage appears to be the most likely pathway for the formation of CNCN. UV-irradiation of the 4-(2-halobenzyl)-1,2,4-triazole-3-thiones (80) under basic conditions gave ring-closed benzothiazines (81) and desulfurized triazoles (82) (Scheme 14).93 Typical yields are given in the scheme. Apart from the two o-chlorophenyl derivatives (80f,g), ring closure predominated. On the other hand, benzophenone photosensitization gave exclusively desulfurized triazoles. The results suggest that ring closure occurs via a singlet excited state and desulfurization via a triplet excited state.

225

Photochemistry, 36, 2007, 205–231 R N

R N N X

NH

254 nm

N +

MeCN, H2O, 2M NaOH

N

S

N

X

S (80)

a b c d e f g

R

N

N

(81)

(82)

R

X

%

%

Ph 4-MeC6H4 4-MeOC6H4 2-MeC6H4 PhCH2 2-ClC6H4 2-ClC6H4

Cl Cl Cl Br Cl Cl Br

41 47 38 50 45 0 0

10 8 15 0 0 47 48

Scheme 14

7.5 Photolysis of o-Nitrobenzyl Derivatives and Related Compounds. – Timeresolved FTIR monitoring of the flash photolysis of several 1-(2-nitrophenyl)ethyl ethers (83) (Scheme 15) have shown that two parallel pathways are involved, but that release of the alcohol (89) is rate-limited by the decomposition of a common hemiacetal intermediate (88).94 The two pathways are (i) direct formation of the hemiacetal (88) from the initially generated aci-nitro species (84) (the curly arrows shown in (84) refer to this process) and (ii) formation of the bicyclic intermediate (87) via proton transfer. One of the compounds (83d) was studied in greater detail by time-resolved FTIR and UV-visible spectrosocopy, and by product studies. The latter confirmed clean photolytic decomposition to the alcohol (89d) as well as the by-product (90). Computations suggest that the pKa of (84) is likely to determine the partition between the two pathways. Although this reaction scheme seems to be general for 1-(2-nitrophenyl)ethyl ethers, it contrasts with results for the related 2-nitrobenzyl ether (91), which photolysed without the involvement of a long-lived hemiacetal. Wavelength dependent differential release of compounds from a solid-phase resin has been demonstrated, using beads bifurcated with nitroveratryl (92) and pivaloyl glycol (93) photo-linkers.95 A narrow bandwidth tuneable pulsed laser was used for photolysis, and it was found that the nitroveratryl linker undergoes cleavage over a wide range of wavelengths, with maximum cleavage rates at 320 and 340 nm. On the other hand, the pivaloyl glycol linker is photo-stable at wavelengths above 340 nm. 6-Nitroveratryloxycarbonyl chloride (94) has been used to protect amino groups, allowing selective labelling of peptides in solution.96 The photolabile protecting groups are easily removed after the labelling reaction. This technique is claimed to offer the advantages of solidphase synthesis combined with the flexibility of labelling reactions in solution,

226


hν

O

R

Me

R

NO2

N

O

−H

OH

+H+

N

−

R

Me +

O

+H

O−

−H+

O O−

N

O− (85)

O (84)

(83)

R

Me +

OH (86)

−OH

Me

Me O

O

H

O

+ RCH2OH

NO

Me

O

R

R

O N

NO

OH (90)

(88)

(89)

a: R = CO2− b: R = CH2N+Me3 c: R = CH2OPO32− d: R = CH2OP(OMe)O2−

(87) O

CO2−

NO2 (91)

Scheme 15

thus permitting selective labelling of nanomole amounts. This work has also been the subject of a patent.97

O

O HO

OTMS

NO2

O

OH

MeO

O OTBDMS

O

HO

(92)

(93)

The 2-(2-nitrophenyl)propyloxycarbonyl group has been applied as a photolabile amino protecting group for amino acids to be used as building blocks in photolithographic solid-phase peptide synthesis.98 The protected amino acids (95) are cleaved by UV radiation about twice as fast as the corresponding 2nitroveratryloxycarbonyl-protected analogues. A 2-mercapto-1-(2-nitrophenyl)ethylamine moiety has been used as photoremovable ligation auxiliary in polypeptide synthesis.99 MeO

O NO2

O MeO

O

CO2H

Cl O NO2 (94)

(95)

N H

R

227


7.6 Other Photofragmentations. – The 193.3 nm photodynamics of propan-1ol and propan-2-ol and their partially deuteriated isotopomers have been investigated using the high-n Rydberg-atom time-of-flight technique.100 The results show that O–H bond fission is the primary H-atom production channel in both cases. Indeed, the photodissociations of these two molecules are nearly identical and similar to those of methanol and ethanol, suggesting a common RO–H dissociation mechanism: after the nO - s*(O–H) excitation, localized on the H–O–C moiety, the H atom is ejected promptly in the H–O–C plane in a time scale shorter than a rotational period of the parent molecule. In the light of the fact that dimethyl ether has been identified in the interstellar gas in cold molecular clouds, the vacuum-UV photodissociation of this compound has been studied at 10 K in Ar and N2 matrices, in the solid phase and in amorphous water ice.101 The primary photoproducts were identified by FTIR spectroscopy as formaldehyde and methane, which are both produced via two channels without subsequent reactions with water ice. The 193.3 nm photodissociation dynamics of ethyl ethynyl ether have been examined by photofragment translational spectroscopy and laser-induced fluorescence.102 Only cleavage of the ethynyl–O bond to form HCRCOd and an ethyl radical occurred. Neither cleavage of the other C–O bond nor molecular elimination to give C2H4 þ CH2CO was observed. As was briefly mentioned previously in Section 5, the H-atom production channel in the 193–215 nm photodissociation of ketene has been studied both experimentally and theoretically.32,33 The observed product energy disposal was interpreted on the basis of one-photon absorption to the 1B1 electronic excited state, followed by internal conversion to high vibrational levels of the electronic ground state and subsequent unimolecular decay to H þ HCCO. Bhc-diol (6-Bromo-4-(1,2-dihydroxyethyl)-7-hydroxycoumarin) (97) can be used under simulated physiological conditions as a photolabile protecting group for aldehydes and ketones.103 The photo-induced deprotection reaction is shown in Scheme 16, where the protected carbonyl compound, R1COR2, is released from the caged precursor (96). An advantage of this method of protection is that the Bhc-diol can be removed either with UV light (350 nm), by one-photon absorption, or with 740 nm light, by two-photon absorption. The latter procedure has the obvious advantage of not exposing R2 R

1

O

OH

O

HO 1- or 2-photon excitation

Br −O

O

Br

O +

−O

O

(96)

O (97)

Scheme 16

O

R1

R2

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potentially delicate molecules to UV radiation, and has been demonstrated for the photo-release of benzaldehyde, piperonal, acetophenone and cyclohexanone. Classical trajectory studies have been carried out for the photodissociation of sym-triazine for energies equivalent to 193, 248, 266, 285 and 295 nm excitation.104 The calculated average translational and rotational energies of the HCN molecules produced are in good agreement with some experimental results. Moreover, the calculated shapes of the translational energy distributions are also in good agreement with experiment, at least for the lower energies, though somewhat narrower than the experimental distributions at higher energies.

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Polymer Photochemistry BY NORMAN S. ALLEN Department of Chemistry, Manchester Metrapolitan University, Chester Street, Manchester, M1 5GD

1

Introduction

The field of polymer photochemistry continues to be recognized as an active area in applied photochemistry, with many topics growing in industrial development. Photopolymerization and photocuring science and technologies are being developed particularly with regard toward designing novel and specific initiators and materials for specialist applications. Interest in active ionic initiators and radical-ionic processes remains a high priority, while the photocrosslinking of polymers is attractive in terms of enhancing the physical and mechanical properties of electronic materials and the development of liquid crystalline materials. The optical properties of polymers remains an active area of strong development with a continued growth in photochromic and liquid crystalline materials. Last year saw the first ever literature explosion in LEDs (light emitting diodes). In this year’s review it represents one of the largest specialized topics in photochemistry and photophysics. The photooxidation of polymers on the other hand continues to remain at a low profile. Bio- and photodegradable plastics continue to be useful for agricultural usage, although interest here is again minimal. The same applies to polymer photostabilization, where commercial applications dominate very much, with emphasis on the practical use of stabilizers. For dyes and pigments, stability continues to be a major issue in so far as this section is concerned.

2

Photopolymerization

A number of reviews have appeared on the function of different types of photoinitiators and their future development and applications.1–3 A number of articles have targeted interest in photosensitive polyelectrolyte diazo systems,4 pressure-sensitive adhesives and coatings,5,6 bonding of epoxy resins,7 electrodeposition materials,8 heat transfer in thick films,9 ring-opening metathesis10 and curing for microelectronics.11 Photochemistry, Volume 36 r The Royal Society of Chemistry, 2007 232


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2.1 Photoinitiated Addition Polymerization. – Many new photoinitiator systems continue to be developed for photopolymerization. Novel macroinitiators based on thioxanthone have been developed and found to be highly efficient when bound to a polymer chain and used in conjunction with a free amine cosynergist.12 Here the polymer chain was found to influence the reactivity of the alkylamino radicals and not the excited state reactivities of the initiator molecule. In a study on a series of 2-substituted thioxanthones in the presence of diethyldiethanolamine, it has been shown that photoinitiation efficiency is dependent upon the electron-withdrawing efficiency of the substituent.13 Here the singlet state of the initiator was found to be deactivated by the amine, with a rate constant near the diffusion controlled limit, and evidence of strong ketone-amine complexation. In another study on a thioxanthone-amine interaction with acrylic acid polymerization, the propagation rate was found to be linear with monomer concentration.14 In another study using 2-chlorothioxanthone as the initiator, the structure of the amine cosynergist has been found to be important in the photopolymerization of methyl methacrylate (MMA).15 So hydroxyl substituted alkylamines were found to be more reactive than simple alkylamines, while dimethylanilines with electronacceptor groups in the 4-position were more efficient than those with electrondonor groups. Ketone-amine complexation was again found to be highly important in the overall reaction. Block copolymers of MMA and styrene with hydroxyl terminal groups have been made using an alkanolamine as a co-synergist with benzophenone as initiator.16 These end groups were then subsequently modified into bromine groups via esterification with 2-bromopropionyl bromide. These block copolymers were also found to exhibit living radical characteristics. Similar work on MMA polymerization using benzoquinones as initiator found that the steric hindrance of substituents on the ring played a crucial role in initiation activity.17 Propagation rate constants have been found not to be available in the 355 nm laser pulsed irradiation of N-vinylcarbazole.18 It was found that, even in the presence of a free radical acetophenone-based initiator, radicals or cations produced directly from the monomer were also responsible for the polymerization. Using a mixture of cerous salts and an aminoethylene phosphonic acid as an initiator, the photopolymerization of various acrylic and styrenic monomers gave polymers of decreasing molecular weight with increasing initiator concentration.19 Solvent type has been found to influence the complexation and triplet-state activity in the photopolymerization of 2-oxyethylmethacrylate and polyvinylpyrrolidone,20 while novel organic azides have been synthesized and found to be effective photoinitiators of the polymerization of MMA.21 Diazides were found to be more photoreactive than monoazides, both giving a dark reaction. Also, the azide group was found to be reactive only when substituted onto an aromatic carbonyl chromophore. Photocopolymer formation with L-lysine hydrochloride has been found to be optimal in dioxane,22 while bis(trichloromethyl)-1,3,5-triazine has been found to synergize effectively in the dye-amine photopolymerization of acrylic monomers.23 This enhanced effect was found to be due to the radical inhibitor scavenging ability of the triazine molecule. Phenylsilanes have also been found to enhance the photopolymerization of MMA, giving rise to silyl terminated chains,24 whereas ethylene has been successfully photopolymerized over a chromium doped

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sieve.25 Here mesoporous sieves were found to be superior to those of silicalite zeolites. Photoinitiator end-labelled polystyrenes have been prepared and investigated as photoinitiators,26,27 as have novel terephthaloylbis(phosphonates)28 and poly(caprolactones).29 In the latter case, hydroxyl functionalized benzoin ethers were grafted to the polymer chains. Benzoyl species have also been found attached to MMA chains,30 while N-alkoxypyridinium salts have been found to give alkoxy radicals via cleavage of N–O bonds on irradiation.31 Using dyes as sensitizers, their reduction potential has been found to be related to initiation efficiency. Rates of electron-transfer processes have been measured between dyes and borate anions using picosecond photolysis.32 Here the data indicated that the rate of electron transfer and the free energy for reaction in polymerization displayed typical ‘normal Marcus region’ kinetic behaviour, indicating that the rate is a function of the free energy change of electron transfer. Macroazo initiators have been bound to polyethylene glycol chains and used to polymerize 2-hydroxyethyl methacrylate,33 while a well defined polystyrene has been prepared through the photoinduced properties of a 2-phenylacetophenone-ferric tri(N,N-diethyldithiocarbamate).34 A novel ethereal monomer N-(p-phenoxyphenyl)methacrylamide has been found to undergo electron transfer with other electron deficient acrylic monomers inducing their polymerization,35 whereas poly(vinyl alcohol) has been made as a product of the photoinduced emulsion polymerization of vinyl acetate.36 The photolysis of tetrasubstituted bi(phosphine sulfides) has been found to give disubstituted thiophosphinyl radicals capable of initiating the photopolymerization of MMA.37 Regiospecific polymers were obtained by this method with phosphorus containing chain ends. Tetraalkylammonium salts undergo an intramolecular electron-transfer process upon irradiation, to give a radical ion pair that will initiate polymerization of vinyl monomers.38 Monomer concentration was found to be crucial in this regard, with higher concentrations giving rise to an anion generated process, while at low concentrations a radical process is operative. The photoinduced polymerization of acrylic monomers using dyes in a protein restricted medium has been reported, where it was found that the dye photophysics is altered when bound to the photopolymer.39 Here the dye-protein combination was found to be crucial. Photo-Fenton reagents have been used to polymerize Nvinylpyrrolidone,40 and thin films of fullerene have been photopolymerized with 254 nm light.41 Cationic processes remain high on the list. Diaryliodonium salts continue to be attractive in this regard. In one study, the encapsulation of diaryliodonium salts into montmorillonite allowed the formation of nanobubbles upon irradiation in solutions containing tert-butyl methacrylate.42 Here butene and acrylic acid products were formed due to photocleavage processes, the former being trapped in the matrix causing expansion. Under visible light, oxiranes have been found to undergo ring opening reactions via the same initiator,43 as have dioxolanes.44 The initiators are also photosensitized by the presence of electron-transfer polynuclear aromatics with hydroymethyl groups,45 other heterocyclics46 and titanocene.47 Propagation rate constants have been measured in the cationic polymerization of cyclic acetals,48 while in the dual radical and cationic initiation of the polymerization of THF the rate curve displayed both


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independent processes.49 Using a thioxanthone and a triphenylsulfonium salt, electron transfer was observed from the former to the latter. Similar photosensitization processes have also been observed using a benzoin ether,50 but in this case the hydroxyl benzyl radicals were oxidized by the onium salts to give cationic species. Triarylamine sulfonium salts with near-UV sensitivity have been prepared where photoacid generation was found to be independent of the counterion,51 while the living nature of THF polymerization by diphenyliodonium hexafluorophosphate has been associated with the stability of the cationic species due to ion-pair formation with a less nucleophilic complex metal halide anion.52 The radical and cationic polymerizations of 2-(N,Ndimethylthiocarbamoyl)ethyl vinyl ether have been found to give low and high molecular weight polymers respectively53 in good yields, whereas novel bicontinuous microemulsions for cationic polymerization of MMA have been developed utilizing ruthenium tris(4,7-diphenyl-1,10-pheanthroline)dichloride with a surfactant acryloyloxyundecyltrimethylammonium chloride.54 For other ruthenium complexes based on tris(2,2 0 -bipyridine), the most effective amine co-synergists for acrylamide polymerization were found to be those that quenched the complex fluorescence the most effectively.55 Here electron transfer from the complex to the amine gives rise to a radical anion. Other studies of interest include polymerizations via onium borates,56 cyclohexene oxide by Nphenacyl-N,N-dimethylanilinium hexafluoroantimonate,57,58 oxiranes via allyl ammonium salts with benzophenone moieties,59 the novel iodonium tetrakis(pentafluorophenyl)gallate,60 carbazolyl and phenothiazinyl thiiranes via iodonium fluoroborates,61 propenyl ether and epoxy functionalized siloxanes as hydrogen donors,62 reactivity of 1-propenyl vinyl ether systems63 and aliphatic diepoxides via FeBF4 complexes.64 The inclusion of poly(3,4-epoxybut-1-ene) into photopolymerizing systems has been found to exhibit a number of beneficial effects.65 It behaves as an effective hydrogen donor and chain-transfer agent, as well as undergoing oxidation thus inhibiting oxygen quenching effects. Vinyl ethers have also been found to enhance the polymerization of dicycloaliphatic epoxides.66 It is also shown that maleimide-vinyl ether combinations will photopolymerize, with the need for an initiator. A number of workers have investigated the photoinduced polymerization of C60. In C60 crystals under high pressure, tunneling processes have been observed,67 whereas a simultaneous deposition and irradiation process has also been found to be successful.68 The adiabatic and energy-transfer processes in photogenerated excitons in C60 crystals have also been investigated.69 In single crystals, structural defects were found to exist even at low temperatures. On a different subject, nanowires of polydiacetylene have been prepared by the chain polymerization of the monomers onto graphite surfaces.70 Morphological changes in this type of polymer have also shown the formation of nodules in addition to rod-like polymer crystallites with smooth surfaces.71 The photoinduced polymerization of 3-alkylated-2-pyrrolidones has been found to increase with substitution on the pyrrolidone ring system,72 while vinylidene fluoride (VDF) in the presence of hydrogen peroxide gives telechelic PVDF through the formation of hydroxyl radicals, which can react with the

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fluorine sites to give carboxyl end groups.73 The photoinduced polymerization of vinyl acetate has been found to depend upon the type of initiator and temperature,74 as does furfuryl methacrylate,75 while rod-like molecules of bis[4-(1,3-octadynyl)phenyl]terephthalate have been vapour deposited and then UV cured.76 On the other hnad, monolithic macroporous polymers have been prepared from functionalized methacrylate monomers.77 These monoliths were prepared in spatially defined positions on fused slica capillary and microfluidic chips. Deuterated MMA has been emulsion photopolymerized without an initiator,78 whereas living amphiphilic poly(ethylene oxide) macromonomers with varying p-vinylphenylheptyl end-groups have been made in micellar media.79 High yields of a 1:1 alternating copolymer of styrene and MMA have been prepared using BCI380 and dibenzyl trithiocarbonate81 as initiators. Through both methods narrow polydispersed well-defined structures were made. Allylic sulfide monomers have also copolymerized with MMA82 via a pulsed laser system, while self-written waveguides have been prepared in a photopolymerizable system,83 and 3D polymer structures made by Q-switched laser irradiation of methacrylate monomers.84 Alternating copolymers have been prepared from vinyl ethers and chlorotrifluoroethylene on account of their marked differential polarities,85 while visible light has been found to photopolymerize MMA via azobisisobutyronitrile.86 Temporal variations in the homopolymerization of acrylamide have been monitored for holographic media,87 as have molecular dynamics in glassy polymers.88 Polysilane-acrylamide copolymers have been prepared for application on silica hybrid thin films,89 whereas thin films of vapour phase poly((silyl)methyl methacrylate) polymers have been grown continuously by irradiation.90 A benzilmonooxime methacrylate polymer has been found to copolymerize and sensitize the photopolymerization of MMA via benzyl and benzoyl radicals,91 poly[2-(10-unedecenoyloxy)ethyl methacrylate] has been prepared92 and peptide functionalized amphiphiles have been used to make liposomes.93 2.2 Photocrosslinking. – Various photoinitiator systems continue to be developed and investigated for photocuring and crosslinking or resins and polymer materials. A series of novel oligomeric amine activators have been developed for curing multifunctional acrylates,94 as have oligomeric benzoin ethers95,96 and acylphosphonates.97–99 With regard to the development of the latter, novel acyl phosphine oxides have been bound to tetramethylene oxide side-chains and their effectiveness as initiators found to be unaffected compred with free initiator types.100 Similar conclusions were reached for novel camphorquinone based polymeric initiators,101,102 while siloxane based photoinitiators have been found to be less reactive.103 Novel vinyl-functionalized initiators have also been developed with high efficiency,104 as have those based on cyclodextrin.105 Quaternary ammonium salts with aromatic ketone structures have been found to exhibit photobase generating activities,106 whereas xanthene dyes have been found to enhance the curing of acrylated resins via a benzophenone-amine complex.107 Intermolecular transfer from the amine to the benzophenone is enhanced by the dye, with sulfur based amines such as methionine being highly


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effective. Quaternary ammonium thiocyanates are also highly effective initiators through photobase generation,108 as are zirconocenes.109 The latter were found to be highly stable with an efficiency dependent upon the electron-donor ability of the complexes and any steric effects. Several cationic systems have also been developed. A number of commercial systems include phenyl glycidyl ether,110 epoxy acrylates,111 prototype epoxies for injection moulding,112 carbon fibre reinforced systems,113 diazonaphthoquinones in Novolaks for deep UV curing,114 epoxy norbornenes for packaging applications115 and epoxy glass reinforced resins.116 Several accelerators have been investigated for cationic curing. Benzyl alcohols have been found to be effective, with efficiency being dependent upon the substitution pattern on the aromatic ring.117 Here benzyl groups attach themselves to the ends of the polymer chains. Pyridinium salts have been found to be accelerated by the addition of benzophenone,118 as have diarlyiodonium salts via iron-arene complexes.119 Macroinitiators based on phenothiazine grafted vinyl ether groups are also highly effective sensitizers120 especially for onium salts, as are telechelic polydimethylsiloxanes modified with vinyl ether groups.121 Electron donating substiuents on the aromatic ring of trarylcyclopropenium tetrakis(pentafluorophenyl)gallates have been found to slow down their activity by reducing their ability to release acid,122 whilst the presence of N-vinylcarbazole is an accelerator for the iodonium salt induced photocuring of glycol diglycidyl ether.123 Nitro substituted amines are effective photobase generators in the curing of epoxide-thiol systems,124 whereas bissubstituted diaryl iodonium hexafluorophosphate initiators work better through a sensitization mechanism.125 Blends of poly(vinyl phenol)-diepoxides undergo thermal cleavage processes,126 and the photocuring of butyl methacrylate in the presence of dry triethylamine occurs through an anionic mechanism.127 The 2 þ 2 cycloaddition of polymer materials maintains some interest. The photodimerization of styrylpyridinium groups bound to poly(vinyl alcohol) has been found to be constant at 0.7 mol-% in solid films.128 The optimum degree of polymerization for the chain was found to be 2500, with pH of the medium also having an important influence on the rate of cure. Water soluble phenolic resins with cinnamate groups have been prepared by a ring opening reaction of epoxy phenolic and cinnamate groups.129 Similar polymers have also been prepared based on polyesters.130 Polymers with p-phenylenediacryloyl moieties also undergo a 2 þ 2 cycloaddition reaction.131 Temperature and solvent effects were both found to be crucial. A new technique of solid-state crystalline photopolymerization, called induction filming solid state photopolymerization, has been developed, with the first polymer, crystalline poly(acrylamide), being developed successfully.132 The photocrosslinking of polymer materials continues to be attractive in many applications for electronic, electrical, insulation and property enhancements. NIsopropylacrylamide with di-methylmaleinimidoacrylamide has been crosslinked to produce thermally sensitive nano-gels.133 These gels made in micellar media exhibited major changes in hydrodynamic diameter in the vicinity of the phase transition temperature of the polymer, with increasing temperature, micellar concentration and chromophore content all decreasing particle size. Similar

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studies have been undertaken using di-methylmaleinimido-N-ethylacrylamide as the chromophore.134 Here surface-plasmon resonance spectroscopy was used to measure the degree of swelling and phase-transition temperature range of the polymer. Film thickness reduced with increasing chromophore content. Various vinyl benzene glycidyl ether based styreneic monomers have been cured via azobisisobutyronitrile in the absence and presence of silane copolymers.135 In this case, polymers with pendant 2-methylglycidyl structures were found to be more reactive than those that were non-substituted, while an ether link between the epoxy group and the aromatic ring lowered reactivity. Benzophenone induced photocrosslinking of polyethylene has been discussed in detail,136 while a novel di(4-hydroxybenzophenone)sebacate has been found to be effective for improved compatibility in the same matrix.137 Phenylindene has been used as a crosslinking agent for polystyrene via complexation with iron.138 Intramolecular crosslinking was found to be the major process, with no oxidation of the iron complex owing to protection by the polymer matrix. On the other hand, sodium benzoate has been found to photocrosslink poly(vinyl alcohol) (PVA)139 through hydrogenatom abstraction from the polymer backbone followed by reaction with the OH groups, giving rise to ether links between the chains. Similar photocrosslinking studies were undertaken on PVA using furan and thiophene chromophores140 and PVC using sodium diethyldithiocarbamate as sensitizer.141 Fire retardant polypropylene has been made by grafting with maleic anhydride and then photocrosslinking with 2,3-dimethyl-2,3-diphenylbutane,142 and polyethylene has been crosslinked via various quinones.143 Under 146 nm light, poly(vinylphenol) has been silylated and coupled with crosslinking,144 while photosensitive fluorescein polyurethane ionomers have been prepared145 and photocrosslinked via benzoin. Novel polymer materials with 4-benzyloxy and 4-acryloyloxy functionalities have been photocrosslinked,146 as have two-stage systems based on benzaldehyde containing copolymers and quinones.147 Benzophenone has again been used to photoinduce the crosslinking of acrylamide copolymers,148 and acylphosphine oxides used to crosslink polybutadiene copolymers by doping with multifunctional acrylates.149 The vulcanization of siloxane rubbers has been undertaken via built-in oligodimethylsiloxane initiators,150 and hydrogels have been made from N-cinnamoyloxymethyl acrylamide.151 Waxes have been found to influence cure rates of wood finishes,152 and new prepolymers have been developed from epoxy and methacrylic acid resin.153 Trapped radicals in the photopolyerization of MMA polymers have been characterized via EPR154 as well as their viability for 2D and 3D polymerizations.155 A new method for the ultra prototyping of microfluidic systems using liquid-phase polymerizations has been developed156 via plastic and glass cartridges. This process is shown to be superior to that of conventional SU-8 50 photoresist technology. Phenylazide end-capped copolymers photocrosslink effectively with increasing end group concentration157 and are found to be effective for immobilizing cells. The relationship between Tg and cure temperature has been monitored for a variety of multifunctional acrylates.158 Here the differences were found to be significantly greater for the more heterogeneous resins, thus facilitating a higher Tg as a function of cure than is possible in a conventional homogeneous system.


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Polyamic acids are useful resists especially when containing 2,2 0 -dinitrodiphenylmethane segments,159 while a Ti:sapphire laser has been found to be effective for 3D curing and microfabrication.160 On a theoretical note, a direct correlation has been found between the calculated Boltzmann-averaged dipole moment and the measured maximum rate of photoinitiated radical polymerization of acrylic monomers.161 Liquid crystalline systems have important applications in optoelectronics. The cholesteric structure of an ethylcyanoethyl cellulose/acrylic acid formulation is somewhat changed upon photocuring.162 The wavelength for selective reflection was shifted to higher energies coupled with a decrease in intensity, although the variation ratio of cholesteric pitch after polymerization was not linear with polymerization volume, the shrinkage ratio of the solvent and the polymer concentration, though it varied with them. The discotic nematic phase has been examined on a triphenylene based liquid crystalline material from the view point of the effects of molecular orientation change due to polymerization.163 In this case, there is a homeotropic alignment in the nematic phase when initially coated, but following polymerization the order of the aromatic core in the nematic phase markedly decreases during the early stages to a steady 80% of the total. When using polarized light for curing, a series of methacrylate systems with substituted biphenyl chromophores have been found to exhibit a small negative optical anisotropy.164 Here 3D molecular reorientation and inclined out-of-plane reorientation was controlled by adjusting the exposure angle and nature of the side-group. Similar functional monomers have also been developed165 where it was found that the crosslinked polymer exhibited higher thermal stability that that of the side-chain liquid crystalline material. The crosslinked material also exhibited pyroelectric activity up to 1701C, whereas the side-chain polymers lost their activity at 381C. Electrooptic and birefringence measurements showed that crosslinking in the unwound smectic phase prevented the re-occurrence of the helical superstructure. Highly ordered nanostructured lyotropic liquid crystalline materials have been developed based on a cationic amphiphile.166 Both the type and degree of phases formed depended upon the composition and temperature. For example, the incorporation of a non-polymerizable surfactant gave hexagonal, bicontinuous cubic and lamellar morphologies. The last exhibited the fastest polymerization rate due to diffusion restrictions associated with the highly ordered state, with the slowest occurring in the less ordered hexagonal state. The lyotropic liquid crystalline phase was, however, retained even above the thermal phase transitions. For a group of 4-substituted acryloyloxy-cyanobiphenyl monomers, orientation of the liquid crystalline textures was found to be retained on photopolymerization.167 Only under isotropic phase conditions, were polygonal or continuous phase-separated LC structures observed for dissimilar mesogenic blends of monomers. A series of diglycidyl ether-bisphenol-A-aniline based polymers have been prepared containing azo chromophores168 and found to be useful for surface relief gratings, while the reflective wavelength range for LC polymers can be modified by the chiral agent and temperature.169 Mono and diacrylate dielectric networks have been prepared by in-situ photocuring of the mesogenic reactive phase,170 as have switchable gratings from photopolyerized

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nematic diglycidyl ether diacrylate blends with polyether urethane acrylates.171 Fractal dimensional analysis has also been employed to obtain a quantitative measure of the morphology of polymer networks produced by UV irradiation of reactive mesogenic monomers in an LC host medium.172 Director gradient controlled polymerizations have been found useful for assembling micron scale polymer architectures,173 while a dichroic photoinitiator enables control over polymerization kinetics through the state of polarization of the UV light used in the initiation.174 In the latter case, when a chiral-nematic monomer mixture is used in combination with polarized UV light, the polymerization kinetics are modulated over the length scale of half the cholesteric pitch. This consequently induces diffusion processes where the most reactive components of the mixture will diffuse toward the volume area with the highest polymerization rate. Under these conditions the helixes of the cholesteric networks formed were found to be periodically deformed. When the deformed helix is combined with a pitch gradient over the film thickness, the built-in retardation can be used for wideband cholesteric polarizers that directly generate linearly polarized light without an additional quarter-wave foil. A photopolymerizable LC reactive mesogen with high electron mobility has also been synthesized175 for CT media, and the morphology of anisotropic networks formed in-situ in anisotropic solvents has been extended to 3D by changing the polymerization direction during cure.176 Here the increasing homeotropic character of the anisotropic network resulted in response times and threshold voltages that were lower than those of conventional anisotropic gels. Methods for monitoring properties of resins during photocuring continue to attract interest. Pigmented177 and stabilized178 systems have been monitored, as have acrylated systems through gellation and cure depth,179–181 EPR182 and anti-solvent precipitation methods.183 Dilatometric184 and refractive index185 changes have also been measured. Differential scanning calorimetry is a commercially viable method and able to differentiate rates and processes, although volatile systems can be problematic.186,187 Real-time monitoring of infrared changes in the curing of resins also continues to attract some interest. This includes the use of the method for measuring temperature effects on photocuring,188,189 depth profiling190,191 and initiator concentration effects.192 Fluorescence on the other hand is by far the most widely utilized technique, although it must be said that the method is purely academic and is of little or no interest to industry. Its use is primarily as a molecular probe to examine the properties and nature of photocuring kinetics. In the photocuring of acrylamide, the molecular probe, 1-anilinonaphthalene-8-sulfonate, has been found to give enhanced fluorescence at the surface of the resin layers owing to the hydrophobic nature of this region.193 Gel-phase transitions have also been monitored in photocuring of MMA and glycol dimethacrylate,194,195 wherein pyrene is used as the molecular probe. Here pyrene fluorescence lifetimes decreased as swelling increased, no doubt owing to quenching and excimer formation. In diglycidyl ether bisphenol-A196 and polyurethanes197 using naphthalene and nitro-stilbene as fluorescence probes, changes in chemical transformations, such as primary reactions of secondary amino groups, gel


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point and glassy state were all seen, with the method being developed further to devise new mathematical models.198 The method is also viable for thermal curing and has been demonstrated for phenolic resole resins.199 For acrylic adhesives, double bond conversion rate was influenced by the actual molecular size of the fluorescent probe,200 while dependence upon the microscopic environment is also considered to be important.201 Fluorescent probes have also been localized on the surface of epoxidized layers of resins using silane coupling agents.202 Shifts in fluorescence emission maxima were related to depth and degree of cure. The use of nitrostibene derivatives has also been shown to be independent of temperature during cure203 as well as being useful for monitoring nanoscale mixing of formulations. Other curing processes exhibiting success are cyclotrimerization of dicyanate esters,204 copolymerization of bismaleimides205 and swelling in networks of polyethylene-poly(styrene-cobutyl methacrylate).206 Using 7-nitro-2-oxa-1,3-diazol as a molecular probe, fluorescence and DSC techniques have also been correlated.207 Reports of new materials and formulations and resin properties are prolific. Articles of a topical or applied interest include probes for in-situ hardness measurements on adhesives,208 photobase generators for image recording devices,209 oxygen inhibition in packaging applications,210 resins for sign boards,211 potentiometric sensors,212 new photodefinable polyimides,213 visible curable resists,214 clay composites,215 putties,216 silica fillers,217 curable paints,218,219 soluble photocurable systems,220 fluorinated coatings221 and inhibition processes on wood surfaces.222 In the last case, it is claimed that phenolics from the wood surface inhibit the photocuring. A number of hyperbranched systems have been prepared. These include polyimides with methacryloyl groups at the chain ends,223 methacrylated polyamine esters,224 polyurethanes acrylates,225 polyester acrylates226,227 and polyisophthalate esters.228 All materials exhibit 3D sphere like structures with excellent properties. Sol-gel acrylated systems have been investigated and include organosilanes,229 poly(L-lactide)-block-poly(ethylene oxide),230 bismethacrylates,231 methacryloxypropyltrimethoxysilanes,232 poly(N-isopropylacrylamide)233 and N-cinnamic oxygenmethyl acrylamide.234 Apparently, in the Mir space station, gel formation during photocuring is far more homogeneous than when carried out on Earth conditions.235 Photosensitive polyimides are also high on the list of useful materials. Novel interlayer dielectric polyimides have been prepared in N-methyl-2-pyrrolidone solvent, giving values of 2.4–3.0 in the 1 to 20 GHz region,236 while a series of polyamic acids have been made which were subsequently subjected to thermal imidization to polyimides237 for image generation. The degree of photoreactivity of a series of polyimides has been found to increase with constant UV exposure time238 owing to the presence of partially crosslinked precursors. Polyimides with an indan structure have also been produced239 with high thermal stability and transparency. Polysiloxane based systems also have useful properties. Naturally occurring phenols such as eugenol have been reacted with siloxanes to give excellent photocrosslinkable systems,240 while polysiloxanes with oxaalkylene styrenyl

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groups form excellent release properties with good thermal resistance.241 The photocrosslinking of vinyl polysiloxanes has been found to be more effective in the presence of arlynitrenes compared to sulfonylnitrenes,242 whereas in another study aromatic bisazides have been found to exhibit their highest value at a 3% w/w concentration.243 Mesomorphic poly(di-n-alkylsiloxanes) have been photocrosslinked in an iso-butyl acrylate matrix to give materials with properties varying from waxes through to plastics, depending upon the nature of the alkyl substituent.244 The microscopic properties of these materials were found by AFM to be dependent upon the degree of microseparation in the domains. Phase separation has been measured in blends of (2,2-bis(4-(acryloxy diethoxy)phenyl)propane and polysulfone.245 Bi-continuous phases were observed at high cure temperatures while at low temperatures semi-interpenetrating networks are formed. In epoxy modified acrylic copolymers, network heterogeneities were found to be strongly influenced by structure.246 An acid-resistant water-based photosensitive MMA resin has been developed,247 as have diacrylate resins with quaternary ammonium salts with excellent antistatic properties.248 Polyurethane acrylate adhesives have been found to exhibit excellent optical and mechanical properties,249 while bifunctional sensitizers with both initiation and grafting capabilities have been shown to exhibit high activity.250 Rather interestingly, humidity has been found to reduce the cationic photopolymerization of epoxy resins.251 Polymers with endocyclic epoxy groups, however, show enhanced activity with humidity. Here there is a reaction between the chain ends and water via proton transfer, and the more positive the end groups the greater the degree of interaction. In the photoreaction of a series of vinyl ether-oxirane based resins, induction times to curing are reduced by ethyl4-diethylaminobenzoate co-synergist,252 whereas a reaction between glycidyl methacrylate and butane-1,4-diol in the presence of triethylamine gives glycidol253 via a transesterification process. In terms of rates of cure, ITX has been found to be the best of five intiators in phenolic resins containing quaternary ammonium salts,254 concentration of photoinitiators in oligo(methacrylates)255 and polyurethane acrylates,256 methacryloyl content in polyether-polyurethanes,257 unsaturated methacrylic side groups in polyester-urethanes,258,259 free radical inhibitors in methacrylic-anhydrides,260 calixarene derivatives in a variety of methacrylic and vinyl ether resins261 and calcium phosphate cements in acrylamide polymers.262 High gel formation has been observed in the photocrosslinking of poly(ethylene glycols), but only at the expense of high levels of benzophenone.263 On the other hand, in the case of oligo(oxyethylene) with methacrylate groups, crosslinking decreased with increasing LiCIO4 concentration,264 while the amount of dissolution of the aggregated phase increased. Termination models have been developed for the post-curing effects of a dimethacrylate monomer,265 whereas a new oligomer based on the chalcone structure end-capped with methacrylic acid has good thermal properties after cure.266 Light-sensitive naphthoquinone-(1,2)diazide-5-sulfonic acid esters have been developed as good positive photoresists267 with high solubility. A norbornyl epoxidized linseed oil has been developed and found to be poorer than that of an aliphatic epoxidized oil.268


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Triethylene glycol divinyl ether had to be added to give a good cure rate. Other newly synthesized polymers include unsaturated polyesters with polyepoxy groups,269 poly(3-methacryloylstyryl-2-naphthyl ketone),270 4-cinnamoyl-phenylacrylate,271 digylcidyl ether diacrylate with polyurethane diacrylates for relief plates272 and alkoxysilane methacrylates.273 Finally, the curing of 1,6-hexanediol dimethacrylate has been investigated in a PMMA matrix,274 where interphase attractive forces were important in controlling the film properties and the postcured photostability of commercial epoxy coatings has been studied.275 2.3 Photografting. – Surface modification continues to be utilized for property modifications of materials. Nanostructred graft block copolymer micelles have been developed,276 as have LC biphenyl monomers onto polyisoprene.277 Benzophenone has been used to initiate the grafting of acrylic acid onto LDPE,278 as has benzoin for styrene onto polypropylene.279 In the latter case, the surface was then sulfonated with chlorosulfonic acid followed by complexation with metal ions, and utilized for ammonia adsorption. Polypropylene has been light-stabilized through grafting of 2-hydroxy-4-(acryloyloxyethoxy)benzophenone,280 and composite membranes of polyimides developed through photografting of methyl acrylate via benzophenone.281 Novel particles with designed hairs have been prepared by photografting acrylic acid onto core particles of poly(N-isopropylacrylamide),282 while, from a scientific point-ofview EPR, has confirmed cage effects in the grafting of acrylamide onto poly(methacrylic acid).283 Blue leather surfaces have also been photografted with dyed acrylated monomers,284 while styrene mono-layers have been grafted onto gold surfaces.285 In reverse, polystyrene has been photografted with 2-hydroxyethyl methacrylate to give a material with an enhanced contact angle.286 Membranes of polysulfones have been modified with different hydrophilic monomers, such as N-vinyl-2-pyrrolidinone, to control protein permeability.287 New LC materials have been made from an allyl(p-fluorocinnamate) polysiloxane grafted copolymer288 with excellent thermal and alignment properties on ITO plates. Finally, the wettability of PTFE has also been modified through photografting with N-isopropylacrylamide.289 3

Luminescence and Optical Properties

A number of reviews have appeared of topical interest. These include photoisomerization of azo dyes,290 photofunctional polysilanes,291 photochromic pigments,292 rare earth complexes,293 pressure sensitive paints,294 electron-transfer processes,295 electroactive dentrimers,296 chiral polyisocyanates,297 photodefinable benzocyclobutene,298 photoconductive polymers,299 excited states in conjugated polyenes,300 photosensitive materials301 and polyazomethanes.302 Urea linkages have been monitored during the extrusion production of a polyurethane-urea through doping with a fluorescent aromatic diamine chain extender,303 whereas soluble poly(arylene ether)s, thioethers and sulfones have been synthesized, which emit strongly in blue and red regions.304 Brownian

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dynamics have been used to determine the distribution of fluorescent resonance energy-transfer between the two ends of a stiff polymer chain.305 Collapsed or ordered conformations were easily distinguishable by this technique. An ABA triblock conjugated copolymer with strong two-dimensional quantum confinement of excitons has also been synthesized,306 while on a more commercial front 3-pyridinecarbonitrile compounds have been examined in paper formulations,307 and fluorescent products have been removed from paper by thermal treatment.308 Isothermal luminescence has been investigated from poly(ethyleneterephthalate)309 and composite polyethylene.310 Luminescence from crosslinked by-products311 and additives312 has also been monitored in polyethylene. Luminescence has also been monitored in degraded isotactic polypropylene tape,313 whereas diacetylene monomer has been polymerized into high molecular weight polyethylene to produce a material with optical anisotropy for security applications.314 Chemiluminescence of polymer materials maintains an active interest as an analytical probe. The effectiveness of various commercial antioxidants in polyethylene has been determined through their chemiluminescence activity and related well to their ability to inhibit hydroperoxide formation.315 The antioxidants themselves were also found to exhibit weak emission due to partial oxidation of their structures during manufacture and storage. With temperature ramping, the chemiluminescence was significant and related well to the melting points of the additives in the polymer. The same group of workers have also shown that the chemiluminescence and activation energies for the thermal decomposition of a series of polyolefins depended upon their rates of oxidation under heat and light.316 On thermal oxidation the activation energies were found to decrease with time and followed the order high density polyethylene 4 metallocene polyethylene 4 linear low density polyethylene, and this correlated with the chemiluminescence. Similar results were obtained for light-stability, with the overall effect being controlled by branching. Other workers have indicated that the source of the chemiluminescence from polyolefins is a consequence of excitation energy remote from that of carbonylic groups.317 Early prediction of the degradation of polymers is also possible by this method318 as well as monitoring the photooxidation of cured resins.319,320 Localized chemiluminescence spreading in polypropylene has been confirmed to be due to the generation of gaseous products such as aldehydes and acids,321 and charge couple devices have been useful for monitoring such effects.322,323 The kinetics of non-isothermal chemiluminescence of cellulose has also been established as first-order,324 while the Fe catalysed decomposition of chitosan is due to cleavage of the glycosidic bond via a six-membered intermediate giving hydroperoxide radicals and a ketone group.325 Excimer properties in polymers have special interest in various contexts. Polystyrene prepared by an anionic end-to-end coupling reaction has been found to give anomalous emission between 300 and 320 nm, attributable to styrenic chain end impurities.326 Other workers also associate polystyrene emission with chemical defects in the main chain and optical impurity centres.327 In the same region (300–350 nm), excimer emission is observed, while


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longer wavelength emission is due to phenylhexatriene groups. Two different time regions of excimer formation, on the other hand, were observed from poly(methylphenylsiloxane).328 These were associated with a fast relaxation, which is connected to unrestricted skeletal motions at the dyad level and a slower relaxation, attributed to segemental chain rearrangements required to release temporary isolated hindered monomer units; both being unaffected by molecular weight. In the same polymer system, monomer fluorescence has been associated with the diphenylsiloxane units, while the excimer emission does not increase with Si–O content.329 The fluorescence of perylene-doped phenanthrene in crystalline polymer media revealed the characteristic emission of perylene and partial quenching of phenanthrene emission.330 Excimer formation due to perylene was not observed under these conditions, while in polyethylene 1,3-di(1-pyrenyl)propane gives excimer emission which is temperature dependent.331 In phenanthroline based polymers, the enforcement of a cisoid geometry in the bipyridine unit of the main chain via an ethenylene bridge does not significantly change the spectral and deactivation behaviour of the emission processes.332 In the case of model systems, 2,2 0 -bipyridine analogues show an intense bathochromically shifted emission due to excimer or aggregate states. Poly(2,5-pyridinedyl), on the other hand, has been found to give blue fluorescence in solution and green emission in the solid state.333 The latter is associated with phosphorescence, and delayed fluorescence is enhanced when associated with oxygen. The electron-hole transporting properties of poly(aryl ethers) has been enhanced through doping with carbazole and 1,3,4-oxadiazole molecules.334 Dye-doping systems of this type have useful optoelectronic and photonic applications.335 The photoelectric properties of N,N 0 -bis(4 0 aminophenyl)-1,4-quinonenediimine are influenced by the addition of a heteropolyacid material,336 while rubrene doped in PMMA with N,N 0 -diphenylN,N 0 -di(m-tolyl)benzidine appears to exist in two chemical environments.337 Some novel benzofuranonaphthoquinol type clathrate hosts have been observed to give rise to blue shifted emission on exposure to amine vapours,338 while, on the other hand, the application of an external electric field reduces the emission from thiacarbocyanine dyes doped in poly(N-epoxypropylcarbazole).339 Rotor probes have also been developed for studying mobility in nanoscopically confined polymers,340 whereas phenolphthalein doped into hydrotalcites can act as useful probes for determing the physical properties of the material.341 Thus, the fluorescence of the dye gave emission spectra dependent upon whether the dye was surface adsorbed or intercalated. Photogenerated triflic acid in polyaniline has been used to alter its conductivity.342 Here delocalization of electrons in the proton-doped polymer lowered the work function of the polymer. Highly luminescent polymer zinc complexes have been prepared from 8-hydroxyquinoline,343 while solvatochromic probes have been used to measure solvent-water ternary mixtures where the emission of the probe depends upon its partitioning.344 Polyimidazopyrrolone foils have been made into 3D structures,345 as have optical sensors based on luminescence quenching of platinum porphyrins.346 Protoporphyrins on the other hand have been used to measure interpolymer adduct formation between self-organized mixtures of

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polycarboxlyic acids and poly(N-vinylpyrrolidone).347 Here the fluorescence increases with degree of complexation due to the expulsion of water from the phase. The kappa number of single fibres has also been measured using Acridine Orange as a molecular probe,348 whereas the phototransformations of naphthalene in cellulose have been shown to depend upon zone of location.349 Fullerene dispersed in PMMA has been used as an optical molecular thermometer,350 while the fluorescence from fullerene copolymers has been shown to depend upon the aromaticity of the solvent.351 The ability of fullerene to bind into a polymer is measured by the extent of quenching of doped anthracene fluorescence.352 Fluorescence may also measure the extent of nitroxyl radical termination in polystyrene chains353,354 as well as the nature of different polystyrene composites.355,356 The layer-by-layer deposition of polystyrenesulfonate and a water-soluble ionene permitted the formation of single layer emitting devices.357 Real-time monitoring of fluorescence anisotropy has been undertaken in biaxially stretched polypropylene film358 and has also been used to measure the Tg of polymers.359 Fluorescent probes have been useful for measuring polymer processing,360 water uptake in epoxy resins,361 polymerization of N-(1-pyrene)methacrylamide362 and micro-sphere formation.363 Fluorescence measurements on gels are numerous. These include gel formation and film thickness,364 photochemical switching365 and bilayer memebranes366 in photocrossslinakble N-isopropylacrylamide, degradation in kappa-carrageenan,367 molecular weight effects on PMMA gels,368 morphology changes in styrene-divinyl benzene gels,369 self-assembly of gels based on 4,4 0 -bis(a-methylacryloyloxy-1,3-ethyeneoxycarbonylpropionamido)diphenylmethane,370 swelling in crosslinked PMMA,371,372 core-hair type microgels,373 photoreversibility in poly(ethyleneglycol) hydrogels374 and electroactivity in acrylamide gel actuators.375 A highly polarized luminant polypyridine has been prepared through complexation with camphorsulfonic acid followed by reaction with 4-hexylresorcinol,376 and for polyaniline the first hyperpolarizability and multiphoton induced fluorescence has been observed.377 Polarized luminescence has also been reported from poly(2,5-pyridinediyl)378 wherein side-groups may also be cleaved to give solid films which retain their optical anisotropy. PMMA films have also been prepared with a non-linear directional coupler379 where the coupler length changes with the intensity of the light input. Poly(p-phenylene vinylene) (PPV) has been modified through embedding it with wide-gap nanocrystals (antidots),380 thus altering the time distribution of the luminescence. The exposure of a photosensitive poly(hydroxyaminoether)-based polymer to light leads to the formation of an image with non-linear optical and photoelectric properties, hence, photorefractive behaviour.381 A series of bifunctional acrylate monomers have also been used to produce self-organized striped patterns of refractive indexes.382 Using an electron probe, the striped patters were associated with crosslinked density modulations in the cured resins. The photoluminescence quantum yield of poly(9,9-dioctylfluorene) has been found to depend upon its morphological state, with temperature, in turn, having an additional effect.383 Generally, temperature reduces the quantum yield as expected, except in the


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b-phase helix conformation, where a reduction in temperature reduces the quantum yield owing to enhanced polaron formation, which can act as quenching sites and traps for the singlet excitons. Production conditions for polystyrene composites also affect their luminescence and optical clarity,384 while a copolymer with terminal nitroxyl groups can be tagged with luminescent stilbene groups.385 The recording, relaxation and erasing cycles in a poly(malonic ester) with p-cyanoazobenzene groups has been associated with reorientation of the polymer backbone,386 while residual stresses in carbon black filled PVC doped with 9-methylanthracene may be monitored via surface fluorescence analysis.387 The use of melamine formaldehyde particles has allowed the development of luminescent CdTe nanocrystal spheres,388 while a conductive polyaniline has been prepared via doping and desposition with viologen.389 The optical properties of a series of water soluble perylene tetracarboxylic diimide derivatives have been determined.390 A number of articles have appeared on polyacetylenes and their derivatives. Amplified spontaneous emission has been observed from poly(diphenylacetylenes) with Si and Ge substituents.391 The nature of this emission was found to be dependent upon the nature of the substitution and the atoms. With the same class of polyacetylenes, other workers show that electron-electron interactions can cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain to the phenyl groups.392 On the other hand, in urethane substituted polyacetylenes, irreversible thermochromic behaviour has been observed.393 This type of conversion shows that there is a threshold of light intensity which depends upon photon energy, indicating that the photoinduced phase transitions in this polymer are mediated by the photogenerated electronhole pairs. Bis(biarylacetylenes) have been modified by oxidative cyclization to produce effective narrow-band luminescent dibenzo(g,p)chrysenes,394 while polyacetylenes with trans-4-n-alkylcyclohexanoate groups exhibit smectic LC behaviour, which apparently is enhanced on blending with PMMA.395 Photoluminescent silicate sticks containing nanodomains of conjugated polymers segregated by ordered silicate channels have been prepared from diacetylene surfactants.396 The sol-gel route for these materials produced excellent nanostructured optoelectronic and electroconductive materials. Carbazole side groups in polydiacetylenes have been found to be responsible for a peculiar supramolecular organization seen in the powdered phase of the red form of the material,397 while the fluorescence of mono-layers of polydiacetylenes has been measured and compared with those from single crystals.398 There are a number of studies on pyrene labelled or doped polymer materials. Dendritic size and electrostatic forces in poly(amido) dendrimers with pyrene residues have been determined through the use of a variety of quenchers.399 As the dendritic size increases so the Stern-Volmer quenching constant decreases, owing to blocking of the pyrene chromophore by the growing dendritic network. The photophysical properties of poly(acrylic acids) tagged with pyrene has been measured in micellar media at various pHs.400 The ratio of monomer to excimer gave valuable information on the micropolarity sensed by the pyrene label, as well as the influence of external stimuli. The surfactant

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was found to interact with the polymer at a pH 3 to form a complex with a critical aggregation constant lower than that of the CMC of the surfactant. Similar studies have been undertaken on poly(sulfone-amine) labelled with pyrene401 where the excimer emission increased with increasing hydrophobicity of chain branches. The hydrophobicity of nanodomains in heat-induced precipitation of poly(N-isopropylacrylamide) has also been monitored through the excimer emission of pyrene tags.402 Poly(N-propargylamide) tagged with pyrene has also been shown to respond rapidly to changes in temperature,403 while chiral interactions in poly(N-1-(1-pyrenyl)ethyl-1-glutamines have been found to affect excimer formation.404 Aggregation and precipitation effects have been determined for pyrene tagged polystyrene in solvent-water mixtures,405 and thermal relaxations monitored in pyrene probed nylon406 and EVA407 polymers. In the last case, no simple relationship was found between the relaxation processes and the vinyl acetate content, which could be explained by the morphology of the copolymers. In a similar way pyrene probed PMMA has measured the extent of desorption of polymer chains from disks.408 Pyrene has also been shown to exhibit negative desorption from crosslinked polystyrene nanoparticles.409 The interesting feature in this work was the observed marked 40-fold enhancement in the pyrene emission compared with solution media. Self-assembly of hydrous metal oxide states has also been monitored for zirconia media using 1-pyrenecarboxylic acid as a probe.410 Radical-ion recombination processes in pyrene doped PMMA have also been monitored,411 as have phase changes in hydrogen-bonded gels of poly(methacrylic acid-codimethyl acrylamide).412 Light emitting diode polymers (LEDs) are the subject of the most prolific area in this field at this point in time, followed closely by photochromic materials. Triple bonds have been incorporated into the PPV backbone to give excellent electron transporting materials413 for LEDs. Although the basic unsubstituted polymer had a high fluorescence quantum yield, significant reductions were observed on alkylation to give more processable materials. In another study, electron attracting, octafluoro biphenyl and hole transporting, carbazole and dialkoxyphenyl groups have been incorporated into PPVs wherein conjugation length had restricted the emission to the blue region giving poor device performance.414 Rod-coil block copolymers of PPV have been made with styrene and ethyl acrylate,415 while other workers have prepared a whole range of PPVs and categorized them in terms of LED efficiency.416 A red coloured PPV with C1–4 chains has been prepared, but is restricted by solvent solubility,417 as are PPVs treated with N-chlorosuccinimide.418 Model compounds of PPVs have been examined in micellar media,419 and novel solvent- and water-soluble materials have also been prepared.420,421 Poly(thiophene-phenylenevinylene) has been characterized to give efficient intramolecular energy-transfer and self-assembly ability,422 while biphenyl and terphenylene derivative of PPV have high thermal stability.423 Two main electron traps have been identified in poly[2-methoxy-5-(2 0 -ethylhexyloxy)-1,4-phenylene vinylene] wherein the activation energies and level filling was enhanced by exposure to air.424 The deeper traps were located near the surface, in the contact region,


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while the shallower traps were located across the depth of the material. In this material, the re-application of a charge was found to give rise to long-lived polarization due to some possible chain restructuring. In fact, electron-hole capture processes in PPVs have been modelled by the dissipative dynamics of a multi-level electronic system coupled to a phonon bath.425 In this model, the authors have considered the recombination kinetics of an initially separated CT pair and address the issue of electroluminescent enhancement in LEDs. Overall, a remarkable linear relationship has been found between chain length and singlet-triplet branching ratio, which was explained in terms of the binding energies of the final excitonic states. PPVs with 1-4 styryl units have also been characterized in terms of their vibronic coupling.426 PPVs with various alkoxy groups have been prepared and characterized.427 Thermotropic and lyotropic behaviour was observed, coupled with a high degree of electron affinity and good photoconductivity. Although the nature of the substituent affects the emission maxima and quantum yields, the octyloxy group gave the highest yield, with narrow spectral emissions. Long chain hydrocarbons induced high solvent solubility and intense red colouration, but strong interchain interactions caused significant quenching and excimer formation. PPVs with p-terphenyl groups in the main chain give violet-blue emissions due to well-defined chromophoric interactions,428 while PPVs with a different trans-olefin configuration give green emissions.429 Shifts in emission maxima and concentration effects have also been undertaken on the octyloxy PPV,430,431 while a new dibromo PPV gives green emission at 492 nm with a very high yield of 72%.432 Thermally stimulated emission from PPV has been found to contain two peaks, an upper due to intrinsic localized states and a lower one due to aggregates.433 Carbazole containing PPVs are high on the list as effective LEDs. A number of similar studies relate to the formation of novel systems with carbazole groups with high solvent solubility and greenish-blue photoluminescence with high efficiency.434–437 The incorporation of carbazole groups with octyl groups give good solubility and spin coatability, displaying two reversible redox processes,438 while the incorporation of PVK directly into PPV gave a 2.5-fold enhancement in emission intensity.439 PPVs with pendant carbazole groups give green emissions, the lower wavelengths providing more effective LEDs.440 Doping or reacting with metal complexes or salts is also proving attractive for LED devices. Vinylene linked PPVs doped with antimony pentachloride have been found to give new red-shifted optical absorption bands due to CT species,441 whereas those doped with poly(pyridinium salts) exhibited thermotropic LC properties.442 New azo-conjugated ferrocene oligomers have been shown to give unique redox behaviour443 as well as green light isomerism due to strong electrostatic interactions. Other PPVs with thienylenethynylene groups show sensitivity to doping with transition metal ions,444 whereas PPVs substituted with crown ethers exhibit worm like images when complexed with potassium ions.445 These images apparently grow slowly on account of self assembly and complexation with the K1 ions. The photosensitivity of PPVs has been shown to be enhanced upon doping with Ru and Rh diimine complexes.446 Here the CT

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process is facilitated beyond 500 nm, with carrier mobility being dependent upon the metal content in the polymer. Other studies include coating PPVs onto gold substrates447 and reaction with iridium derivatives.448 PPVs based on triphenylamine have been studied. Pure poly(triphenylamines) have been made and found to exhibit high blue emission quantum efficiencies of 0.78,449 while PPVs with triphenylamine in-chain functionality have good hole-injection ability and high mobility450–452 and strong yellow emissions. PPVs with 1,2-diphenylmaleimide groups in the backbone, on the other hand, exhibited a curing process in the temperature range 165–2501C owing to crosslinking between acetylenic groups.453 These polymers also give yellowish-green emissions. Fullerene containing systems are also of interest and have been covered in some detail in terms of applications and properties.454 Strong photoenergy transfer has been observed between pendant fullerene groups in PPV copolymers and main chain chromophores, especially at low temperatures.455 In PPVfullerene composite films, an increase in the substituted side-chain length has been found to lower the energy of the emission bands, while the fullerene dopant also quenched the emissions. In this way the electronic affinity of the PPVs can be controlled.456 Conjugated polyelectrolytes also have applications as fluorescent sensors.457 Several types of siloxane based PPVs have also been developed with good processability and LED efficiency. Highly soluble cyclohexylsilyl and phenylsilyl PPVs have been prepared with high molecular weight and electroluminescence performance (0.82) with emission maxima at around 512 nm.458 Poly[(formylphenyl)methylsilanediyl] has also been prepared and tagged with pyrene and biphenyl molecules, to give highly electroluminescent materials in double layers of up to 0.06%.459 Ladder-like poly(phenylsilsesquioxane) has been prepared and found from excimer-monomer emission ratios to give a linear Arrhenius plot with one break point, typical for polyorganosiloxanes.460 Thermotropic LC behaviour has been observed in poly[2,5-bis(dimethyloctylsilyl)-1,4-phenylenevinylene], coupled with a high LED output in multilayer systems.461 Oligophenylenevinylenes have also been directly attached to a Si atom to give blue-green emitting products,462 while other blue emitting products include poly(2,5-dialkoxy-1,4-phenylene/1,3-divinyl-1,1,3,3tetramethyldisiloxane)s,463 polymers coupled to 2-trialkylsilyloxy-6-methylphenol464 and various oligocarbosilanes.465 Fluorene containing polymers exhibit interesting LED properties. These are based on poly[9,9-bis(20 -ethylhexyl)fluorine-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanovinyl)-1(2 0 -ethylhexyloxy)-4-methoxybenzene-500 ,5000 -diyl] with strong absorption at 380 nm, increasing with thiophene addition,466 poly(quinoxaline-fluorene) with blue emission at 447 nm and absorption at 407 nm,467 polyfluorene derivatives with emission at 420 nm,468 highly soluble poly(9,9-di-n-hexyl-2,7-fluorene-diylethynylene-alt-9-trimethylsilyl-2,7-fluorene-ethynylene) with emission maxima at 490 and 540 nm,469 blue emitting copolymers of fluorene and oxadiazole,470 green emitting polymers based on 9,9-dialkyl-fluorene-2,7-diboronate esters.471 A red emitting copolymer at 675 nm has also been made based on diocylfluorene and


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4,7-dithien-2-yl-2,1,3-benzothiadiazole472 and blue light emitting poly(2,7-fluorene) networks.473 A hypsochromic shift of the new fluorescence spectra in PPV doped trinitrofluorenone has been interpreted in terms of increasingly frustrated spectral relaxation of singlet excitons within a homogeneously broadened distribution of hopping states.474 PPVs with oxadiazole groups are also useful LEDs. Two new luminescent copolyethers with isolated 2,5-distyrylthiophene emitting groups and electron transporting 2,5-diphenyl-1,3,4-oxadiazole chromophores have been prepared.475,476 These highly soluble materials have good LED characteristics with yellow-green light. PPVs with oxadiazole chromophores also give green emission with excellent LED properties as a triple layer device.477,478 Highly thermally stable poly(oxadiazole-amides) with good solvent solubility give blue emission and form excellent devices on Si wafer on the nanoscale.479 Poly(pphenylenebenzobisthiazole) is a rod-like polymer with similar properties and is voltage tunable.480 A benzothiophene PPV copolymer has been found to be photochromic, turning bright red upon irradiation owing to the formation of a ring closed dimer481 with a high quantum yield and, as such, could have considerable potential in optical reading systems. Thienyl building blocks in PPVs also enabled photoluminescence tuning,482,483 while those bearing coumarin,484 Nalkylcarbazole,485 iodine,486 imides,487 paracyclophane,488,489 anthracene490,491 and dinaphthylanthracene492 give blue emissions with good LED properties. Soluble and thermally stable diyne containing PPVs have been synthesized.493 Here the energetically arduous migration of electrons through the diyne units required a higher threshold voltage for the detection of photoconduction, although the electron withdrawing effect of the triple bonds conferred good electron accepting properties suitable for LEDs. Chemiluminescence has also been generated from PPVs electrochemically, with anodic polarization giving a higher efficiency.494 A series of arylamine crosslinked polymers have been tuned for LEDs,495 while LC polyfluorenes have been successfully aligned on photoaddressable polymers such as polyacrylates with mesogen azobenzene side groups.496 Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Conductivity in poly(1-hexyl-3,4-dimethyl-2,5-pyrrolylene) has been correlated to chain structure and conformation as well as the torsion angle of the pyrrole bonds,497 whereas nanocrystal formation in pyridine-doped poly(4-vinylpyridine) is undertaken by ring-opening followed by photoproduct aggregation.498 PPVs have been made with ionically conductive triethylene oxide units that are soluble and excellent LEDs,499–501 while several blue-orange polythienylenes have been made with LC properties giving while luminescence.502 Biphenyl poly(1-alkynes) have been found to developed enantiotropic SmA and SmB mesophases when the spacer length was increased to 4.503 LED properties are also enhanced in a similar fashion. A new fluorescent instrument has also been made using an array of LEDs,504 while poly(vinyl toluene) becomes luminescent upon irradiating with ionizing radiation.505 LED devices have been made from norbornene and Al 8-hydroxyquinoline,506 polyamic acids with quaternary ammonium

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salts,507 polyphenylene dendrimers,508 2,6-linked quinquepyridine derivatives,509 polyquinolines,510 polypyridinium salts,511 tritolylamine in polycarbonate,512 PPVs with aldehydic and aminooxy groups513 and carbon nanotubes.514 PPVs with pyrazine groups gave suitable red-shifted emissions,515 and in pyridine based PPVs the chain rigidity could be controlled.516 Femtosecond spectroscopy has been used to measure electronic excitation oscillations in PPVs,517 whereas electric field induced fluorescence quenching and optical charge generation in poly(p-phenylenes) is associated with exciton dissociation into geminate electron-hole pairs rather than their full dissociation into free charge carriers.518 Delayed fluorescence in the same polymers has been associated with the same process,519 whereas intersystem crossing rates have been found to be the highest in polythiophenes.520 PPVs with fluorine groups showed a stable enantiotropic Sc phase, characteristic of ferroelectricity as well as fluorescence,521 while the geometries of PPVs and their ramifications have been optimized by using a quantum chemical method.522 Time-dependent excitation studies on PPVs shows that self-trapping of excitations occurs on about 6 repeat units during the course of photoexcitation relaxation, as well as specific slow torsion and fast bond-stretching nuclear motions, all strongly coupled to the electronic degrees of freedom.523 At low temperatures PPVs occupy a more planar structure,524 whereas the photostability of PPVs can be enhanced by doping into silica nanoparticles,525 silicate composites526 and treatment with solvents like THF.527 The optical properties of polythiophenes have been actively investigated. A series of 3,4-disubstituted poly(thiophenes) have been made, with the nitro derivative exhibiting a large Stokes shift of 218 nm associated with the planar geometry of the group lowering the excited state energy level.528 Phenyl substituted polythiophenes exhibit two well defined ordered-disordered phases,529 the conductivity being enhanced via the addition of fullerenes. The blue phase has an enhanced spectral absorption range, with strong bimolecular recombination when compared to that for the orange phase, which is more appropriate for LEDs. Regular bithiophene-alt-thiophene-S,S-dioxides have been made by aniodic coupling and exhibit a finite window of conductivity.530,531 Here the dioxide substitution tends to increase the torsion angle between the rings, thus influencing the states at the optical band edges. Polythiophenes with pendant fulleropyrrolidine moieties operate as double cable polymers for LED applications,532,533 while other workers534 have shown that the LED properties of polythiophenes decrease upon continuous irradiation owing to the growth in photooxidation products. Both phosphorescence and delayed fluorescence have been observed from poly(3methyl-4-octyl-thiophene)535 owing to geminate pair decay, while the potential of such materials for LED applications have been determined through a structural versus efficiency evaluation.536 Carbazole groups, on the other hand, have been shown to reduce the photoluminescence efficiency of polythiophenes,537 whereas in blends of poly(vinyl alcohol) with poly(3-thiopheneacetic acid) emission intensities are enhanced through laser irradiation.538 Rigid cyclic polythiophenes are also highly emissive,539 as are m-linked polythiophenes.540 A new poly(3-phenylgalvinoxylthiophene) has been made and shown to undergo a reversible blue-red


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colour change due to radical and anion formation,541 whereas poly(2-ethynylthiophene) undergoes cis-trans isomerization.542 Other systems of interest include conformational changes in poly{3-[2-(N-dodecylcarbamoyloxy)ethyl]thiophene},543 poly(arylene ethynylenes) doped with polythiophenes,544 crystalline oligothiophenes,545 ring closure in a dithiophene containing polymer,546 nanomolecules from oligo(octithienylene-diethynylenes)547 and chiral assemblies of amphiphilic polythiophenes in aqueous media.548 Electron- and energy-migration processes cut across numerous systems. Triplet-triplet annihilation due to intra-chain diffusion exhibits bimolecular reaction kinetics associated with a turnover from dispersive to non-dispersive regimes, as borne out by the delayed fluorescence decay.549 Millisecond photoexcitations in poly(p-phenylene sulfoxide) are due to triplet excitons and not ring-torsional polarons found in previous polyanilines.550 Intersystem crossing processes have been manipulated in 4-thienyl polymer systems,551 and a new series of donor-acceptor trivalent boron systems developed with two-photon excited up-conversion fluorescence.552 An analytical expression has been developed to account for luminescence from a polymer chain in the static regions where rotational motion of monomers giving excimer formation is frozen.553 The conditions are apparently found when the inhomogeneous broadening of the chromophoric spectra and conformational motion of the polymer chain should be taken into account. A mathematical model has also been presented to show that the up-gap state of a polaron possesses negative polarizability,554 whereas, for polymers with randomly labelled chromophores, analytical equations have been obtained for donor-decay curves for both pseudo-ideal and self-avoiding walk polymer chain models.555 Photocarrier transfer theory in a photorefractive polymer has also been developed556 to give analytical solutions and numerical data for both temporal and steady-state processes of spacecharge fields in materials under different conditions. The results of a Brownian dynamics simulation of the time-dependent survival probability of a donoracceptor pair embedded at the two ends of a Rouse chain have been compared with the well-known Wilemski-Fixman (WF) theory.557 While the WF theory has been found to be satisfactory for small reaction rates, the agreement was found to be progressively poorer as the rate was increased. In this theory an approximate reduced propagator technique was introduced for a 3D system and then solved by combining a Green’s function solution with a discretized sink method. Poly(Schiff’s bases), in which non-radiational energy transfer occurs, have been made,558 whereas trifluoromethyl-norbornadiene donoracceptor moieties exhibit repeated cycles of interconversion.559 Intramolecular energy transfer has been confirmed via femto-second laser photolysis experiments,560 as has photoinduced electron-transfer acid generation due to intramolecular conjugated CT in triazine compounds.561 Only triplet excitons have been observed in alkyl methacrylate polymers with halogenated carbazolyl pendant groups,562 while CT processes have been observed in poly(acrylonitrile)-C60 copolymers.563 Ultrafast electron-transfer reactions have been initiated by excited CT states of push-pull perylene systems,564 whereas in CT complexes of copolymers of naphthyl methacrylate with vinyl carbazole exhibit

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only naphthalene fluorescence at low concentrations.565 Conjugated polymers with longer excited state lifetimes than those of phenyl analogs have been designed based on triphenylene poly(arylene ethynylenes),566 while electron transporting copolymers have been developed containing 1,3,4-oxadiazole groups.567 Polyfunctional carbazole containg methacrylate copolymers also exhibit highly efficient energy migration,568 as do novel discotic triphenylenes with a comblike polymethacrylate wrapping.569 Dendrimers continue to display interesting photooptical properties. Charge transfer and applications of dendrimers have been well covered.570–573 Polyarylether dendrimers have been found to be highly effective at shielding phosphorescent Pd porphyrins from oxygen,574 while the analysis of dendrimer-embedded polymers indicated that, upon hole burning, the structural relaxation of polymer chains outside the dendrimer does not have an influence on the resonant frequency of the porphyrin core.575 Hyperbranched PPVs exhibit significant red-shifted emissions,572 whereas poly(3,4,5-trihydroxybenzoate) dendrimers give strong chemiluminescence.576 Exclusive Forster energy transfer is observed from dendrimers with four donors and one acceptor chromophore.577 Donor emission is observed only following bleaching of the acceptor molecule, which in this case is terrylenediimide. A perylene diimide dye has been used to build-up a polyphenylene dendrimer to give a pentaphenyl building block.578 In this case, however, although aggregation is prevented, the fluorescence quantum yield decreases with increasing generation. Dendrimers with all-azobenzene repeat units have been prepared for the first time,579 as have photochromic dendrimers with six azo functionalities.580 In poly(propylene amine) dendrimers with naphthalene and trans-azobenzene units, electron transfer from the amine groups quenches the naphthalene fluorescence.581 In this process, the trans-azobenzene units are converted to the cis form. Phosphorus dendrimers with azobenzene groups have also been made wherein isomerization from the E to the Z form is the primary reaction on irradiation,582 with little change in reaction rate versus generation number. Azobenzene groups deep in the framework, however, showed different kinetics. The kinetics of hyperbranched polystyrenes have been investigated via a photofunctional thio inimer583 and perylene initiator.584 In the latter, the polystyrene arms were found to suppress aggregation of the perylene chromophores and, at low degrees of polymerization, the rigid-core moiety significantly affected the segmental motion of the arms. Spin casting gave films with a high fluorescence quantum yield and additional properties for migration fastness. Self-association has been observed in hyperbranched poly(sulfone-amines)585 where the excimer emission from a perylene probe was found to give an intersection at 35%. Phenyleneacetylene dendrimers have also been labelled with perylene, with potential applications for fluorescence based thermometry.586,587 Poly(propyleneimine) dendrimers, on the other hand, functionalized with oligo(p-phenylenevinylene) end-groups based on distyrylbenzene show fluorescence which is dependent upon the generation number.588 Rapid migration of exciton energy and chain coupling caused a red-shift in the emission spectrum, implying that interchain interactions are very strong, causing delocalization


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over more than one group. Delayed emission was also observed from the dendrimer but not the isolated units. Highly luminescent distyrylstilbenes bearing oxadiazole surface functionalized poly(benzyl ether) type dendritic wedges at one end have been prepared.589 This novel system shows for the fist time that an emissive core bearing asymmetric substituted functionalized dendrons exhibits more favourable luminescence than that of a symmetrical core. Energy-transfer efficiency was found to be extremely high with excellent LED properties. A six-generation methyl orange functionalized poly(propyleneamine) dendrimer has been made with both pH and light sensitivity.590 The isomerization process showed no dependence upon generation, while pH did. In a similar fashion, phenol blue has been bound to poly(propyleneimine) dendrimers.591 In solution two discrete dye populations were observed in the inner and outer cores. Carboxylic diads have also been bound onto the same dendritic structure,592 where in this case intermolecular ionic interactions were observed to form templates. A quinoline tautomer bound to a poly(aryl ether) dendrimer has been found to influence and decouple the excited state intramolecular proton-transfer process as well as the core planarity.593 Although the rate was found to be slowed with increasing generation number, the luminescence still increased owing to the formation of isolated surface units. Dendrimers based on 5-aminovulvinic acid have been used for photodynamic therapy,594 while chiral dendrimers have been made based on bi-2-naphthyl cores.595 Photocrosslinkable dendrimers have also been made based on poly(1,5-dioxepan-2-one).596 Various tagged polymer systems have been investigated as optical probes. Fluorene based and oxadiazole substituted polymers have been made597,598 in order to facilitate electron-migration processes, as have poly(aryl ethers) with carbazole and fluorine groups599 and poly(methyl acrylate) grafted with poly(thiophene).600 In the solid state, the acrylate traps the thiophene polymer into a solution-like state. For dansyl sulfonate tagged groups to an E-type glass, it has been found that the solvent dipolar coupling relaxation mechanism is dominated by thermodynamic interactions of the polymer with the solvent in aprotic media, while in protic media this mechanism is dominated by specific interactions between the solvent molecules and the excited state chromophores.601 Poly(styrene-co-maleic anhydride) has been tagged with a 1,8-naphthalimide dye602 and aminobenzothiazoles603 to give fluorescent sensitive systems, as have cationic poly(acrylamides) with various anthracene, pyrene and dansyl groups604 as quantitative probes. Using acenaphthalene as a probe, poly(vinyl pyrrolidone) has been shown to adopt a loose coiled conformation over a wide pH range.605 This polymer unusually binds to molecules with active hydrogens and can be used, therefore, to modify the conformational behaviour of some water-soluble polymers with hydrophobic micro-structures. The fluorescence properties of Schiff’s bases are also affected by the cationic or anionic form of the amine groups,606 and the fluorescence of 2,2 0 :6 0 ,600 -terpyridines is significantly influenced by the solvent and the number of pyridine rings.607 The question of intermixing of hydrocarbon systems has been investigated through the use of pyrene labelled poly(N-isopropylacrylamides).608 In micellar media

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excimer formation indicated that polymeric chains can exchange among micelles of hydrocarbon-modified polymers, but not hydrocarbon and fluorocarbon modified polymers. The latter were found to segregate into nanodomains within a hydrophobic core. The same study and polymer system has been used to measure the mixing of hexane and methanol, where the rotational motion of chromophoric groups is slowed down at the critical mixing point.609 Molecular orientation has been investigated through the use of polyethers with quinquephenyl and anthracene segments.610 Using CD measurements, the polymer chains have been found to orientate parallel to the direction of the draw, giving polarized blue and yellow light emissions. Superquenching has been observed on silica spheres containing cyanine dye bound polylysine,611 cogelation monitored in trimethoxysilane-derived oligarylene-vinylene fluorophores,612 thermal properties of PPMA copolymers with benzazolylvinylene chromophores determined,613 tunable electrochemical interactions observed in polystyrenes with anthracenyl and tetrathiafulvalenyl groups,614 crosslinking achieved in fluorene containing polymers615, energy transfer examined in N-allyl-2-aminobenzoic acid and benzamides,616 processing effects on pyrene tagged acrylate and vinyl pyridine polymers studied617 and polymerizations with 9-acrydyl derivatives of aromatic amines monitored.618 Fluoroalkyl end-capped cooligomers containing benzoylbenzyl segments have been found to have excellent oil repellency on polymer surfaces such as PMMA619 as well as antibacterial and anti-HIV activity in-vitro. Water-soluble conjugated ionic poly(ethynylpyridinium bromide) exhibited good film properties,620 oxypyridinium functionalized methacrylates and styrene undergo intramolecular cyclization reactions,621 cyanine dyes in PMA undergo photoisomerization,622 and 1,8naphthalimide dyes have been bound to styrene.623 The optical properties of polymers in micellar media are widespread in interest. Only the protonated base of Acridine Orange dimerizes in waterheptane media.624 At pH 4 10 only the basic form is detected in heptane, while at a pH 4 above the critical micelle concentration the basic form disappears giving the protonated dimer. Overall, three processes were identified: (1) the distribution of the dye between the organic phase and the micellar interface followed by protonation to give the mono form, (2) dimerization of the dye at the interface and (3) the conversion of the dimer form to the mono form by the micelle at high dye concentrations. The important parameters that are responsible for the solid-matrix phosphorescence quenching of phosphors adsorbed onto filter paper have been determined for phenanthrene and benzo[e]pyrene.625 Here the changes in phosphorescence intensities and lifetimes followed a simple exponential function with moisture content and were due to changes in the modulus of the paper. Ultrasonic methods have also been used and compared with fluorescence analysis to determine critical micelle concentrations of liposomes and surfactants.626 Here the CMCs were found to increase dramatically with an increase in the polymer chain length. Amphiphilic combshaped polymers based on a hydrophobic poly(p-alkylstyrene) with a hydrophilic poly(ethylene oxide) graft have been examined in water using a pyrene probe.627 Direct observation of a single macromolecule revealed a high


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molecular weight cylindrical brush-like conformation of 12.8 nm thickness. Fine particles were also observed, suggesting the formation of a unimolecular micelle in water. In polyelectrolyte samples of poly((2-acrylamido)-2-methylpropanesulfonic acid)), energy transfer between pyrene and naphthalene probes showed that the charged polymer chains interpenetrated each other rather than reduce their coil volume.628 By binding with surfactant, the polyelectrolyte chain became more coiled, with intramolecular energy transfer occurring below 4 105 M and intermolecular energy transfer occurring above it, eventually giving hydrophobic aggregation between the micellar tails bound on the polyelectrolyte chains. On the other hand, polyion complex formation between polylysine and chondroitin sulfate has been measured in the presence of pararosaniline leucohydroxide.629 Between pH 6 and 12, the chondroitin was fully dissociated, while the polylysine is reduced with increasing pH, resulting in a random coil-to-a-helix transition around a pH 9.4. Complex formation was in fact controlled by irradiation. The aggregation behaviour of a biodegradable amphiphilic poly(aspartic acid) with long alkyl chains has been characterized by size, interfacial properties and aggregate formation.630 These polymers formed aggregates on ultrasonication, with higher amounts of alkyl chain grafts inducing higher aqueous stability of the self-aggregates. In aqueous media the incorporation of a C18 chain did not produce surface activity owing to the physical crosslinking nature of the octadecyl chains. Shorter chains were surface active. Aggregated assemblies of hexadecyltrimethylammonium bromide (HTAB) and phospholipids have been monitored at a water-air interface and in bulk media.631 Two kinds of aggregation processes were observed, identified by three breaks at low lipid concentrations. The first process involved the incorporation of the HTAB into the lipid vesicles and the subsequent disruption of the vesicular structure leading to the formation of mixed micelles. The second aggregation process indicated the completion of the mixed micelles and the initiation of independent micelle formation process which was completed at the third stage. At higher concentrations of HTAB, no first stage was observed owing to the presence of large vesicles. Mixed polyaromatic hydrocarbon-surfactant solutions have been photochemically treated for surfactant recovery,632 while the aggregation of a non-ionic surfactant Triton X has been measured as a function of pressure using energy transfer between pyrene and coumarin probes.633 Similar studies have been undertaken on water-soluble oligomers, such as 1,4-bis[9 0 ,9 0 -bis(N,N,N-trimethylammonium)hexyl]-2 0 -fluorenyl]benzene tetraiodide,634 while a series of amphiphilic polyanions containing 9-carbazolylalkyl methacrylamide have been prepared and their fluorescence properties examined in organic and aqueous media.635 Thus, whilst in organic media only monomer emission was observed with no self-quenching, in water fluorescence quenching was observed. In polyethylene oxide-polystyrene-polyethylene oxide triblock amphiphilic copolymers, N,N-diethyldithiocarbamate pendant sites have been incorporated that can react with benzyl radicals upon irradiation to give locked polystyrene cores.636 Chemiluminescence from luminol has been reduced by surfactants,637 while the colour change of triphenylmethane dyes induced by surfactants at concentrations much greater

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than their CMCs are found to be accompanied by enhanced fluorescence.638 This is associated with the hydrophobic environment in the micelles preventing non-radiative relaxation processes in the dyes. Thus, the more hydrophobic the micelle the more fluorescence is observed. The low anisotropy values for a poly(trifluoroethoxyphosphazine) and poly(ethyleneoxide) amphiphile has been associated with the flexibility of the trifluoroethoxy group, allowing enhanced rotational diffusion of the probe.639 Water-soluble nanospheres have been made from dicarboxylic acids of poly(tert-butyl acrylate)-block-poly(2hydroxyethyl methacrylate),640 and a blob model has been used for studying the dynamics of polymer segmental encounters as a function of time in near and good solvents.641 Fluorescence depolarization kinetics have been used to measure microviscosity changes in poly(ethylene oxide) and poly(propylene oxide) triblock copolymers.642 At a 20% w/w concentration in solution, although the viscoelastic modulus increased by an order of magnitude when the sol-gel transition was crossed, the microviscosity of the hydrophilic medium exhibited only minor changes. Also, faster and slower components of the fluorescence depolarization were tentatively assigned to the dye in free solution and associated in the micelles. CTAB micelles in the presence of hydroxypropyl-cyclodextrins have shown the presence of two types of micellar arrangements.643 One was a pure surfactant micelle while the other was a surfactant monomer complex with the cyclodextrin. Hydrogen bonding has been observed between a photochromic multiplayer of assemblies of poly(4-vinyl pyridine) and poly(acrylic acid),644 while luminescent polyelectrolytes have been prepared by growing ZnO nanoparticles in a polyethylene matrix.645 Other studies of interest include random ionomers of polystyrene,646 viscosity probes in micelles using 4-hydroxycoumarin derivatives,647 microviscosity of polypropylene oxide melts,648 precipitation of polyethylene oxides in toluene649 and development of a new PVC membrane for phosphates based on fluorescence analysis.650 Optical studies on rare earth complexes as probes, and metal ions in general in polymeric media, have been significant in the last year. Polystyrene containing mixed rare earth triisopropoxides undergoes complexation to yield a highly fluorescent material with improved impact strength,651 whilst improved cation selectivity has been obtained through the use of multilayer polyelectrolyte membranes based on poly(acrylic acid)/poly(allylamine hydrochloride).652 Fluorescent composites have also been made based on poly(urethanes) with samarium benzoate,653 Eu(III) and Tb(III) complexes in a sodium styrenesulfonate-ethylene glycol-sodium styrenesulfonate triblock copolymer654 and Eu(III) complexes in acrylate-styrene655 and acrylamide-acrylic acid656 systems. Hybrid macroporous silica materials have also been developed based on monovacant Keggin-type polyoxometalates.657 Europium complexes with polyester macroligands have been prepared to form heteroarm stars and nanoscale assemblies,658 with a lamellar structure able to be thermally modified. Eu(III) with polymaleic acid forms a 1:1 complex659 with two different environments, while with epoxy resins energy transfer has been observed from the polymer to the complex.660 High quantum efficiencies for the Eu(III)


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complex were observed and have been discussed in many other applications.661 Lanthanide(III) methacrylate monomers have also been prepared,662 as have Eu(III) polymers of styrene-acrylic acid.663 A ruthenium(II) tris(2,2 0 -bipyridine) complex has been anchored to a polysiloxane matrix664 where one of the ligands gave rise to a mobile metal centre. Quenching studies on anthracene showed that mobilities and accessibilities were higher than for conventional polysiloxanes. Fast energy transfer has been observed from carbazole block copolymers to Ru(III) complexes.665 Nanosized micelles have also been made from Ru(III) complexes with poly(styrene)-block-poly(vinyl pyridine) copolymers666 wherein solution media disk and rod-like micelles could be formed. Based on the amplified fluorescence quenching of polyquinolines, Ag(I) and Hg(II) ions have been used as chemosensors,667 while polyurethane-CdS cured complexes are useful membranes.668 Hexacyclane fluorophores undergo energy transfer to rare earths,669,670 while phototunable systems have been made based on 4,4 0 -conjugated[2,2 0 ]-bypridines complexed with various Zn(II), Hg(II) and Re(VII) ions.671 A highly fluorescent and thermally stable complex has also been prepared from Tb(III) ions with thienyltrifluoroacetone-electropolyrushthiol,672 while the same ions also complex well with poly(vinyl sulfonate),673 displacing some six water molecules in the process. Here complexation results in energy transfer, which is found to be inhibited in alcoholic media but enhanced by the addition of ions such as salt. Here quantification of the energy-transfer data allowed one to determine with a high degree of accuracy the binding distance between the metal ions and the polymer chains. Similar studies have been undertaken on Eu(III) with 4-vinylpyridine674 and Ru(II) ions with a viologen containing poly(1-vinylimidazole).675 LC polymer systems continue to be developed for optical data recording media. Polyurethane layers with good LC properties have been made,676 as have template lyotropic LCs based on hydroxyethyl acrylate with dodecyltrimethyl ammonium bromide.677,678 In the polymerization of the latter, lamellar aggregates were found to form faster than either cubic or isotropic morphologies owing to diffusional limitations on the growing chain. A statistical theory has been developed to account for the compensating effect when anchoring with two orthogonally photoaligned polymers.679 The data include the angular dependence on the two aligned directions as well as a suitable procedure for getting controlled anchoring strength in-situ. A theory has also been developed for the fluorescence depolarization of a re-orientating molecular probe in a mesophase with local uniaxial symmetry and a random distribution of directors tilted with respect to the axis of the structure.680 Expressions were evaluated for the polarized fluorescence intensities as a function of the cone opening as well as their dependency on the orientational correlation functions and order parameters for both a uni- and a bi-axial probe. Photoinduced molecular re-orientation and relaxation processes in a liquid crystalline polymer have also been investigated in a series of pump-probe experiments.681 Here both the frustration effect of the trans-cis photoisomerization transition on the nematic phase, and hence the photoinduced isothermal transition from a perturbed nematic to a totally frustrated

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(isotropic) phase, and the approach of the glass transition were studied. The elaboration and spectral properties of a passive broad-band reflector have also been investigated.682 Photocrosslinkable chalcone polyimides undergo photodimerization,683–685 while copolyester-imides emit blue light and operate as effective LEDs.686,687 A case for a randon-field Ising model has been made studying photoactive guest-non-photoactive host LC systems,688 while the LC properties of diazadibenzoperylene dyes have been controlled by covalent and hydrogen bonded attachment of mesogens.689 Polyoxetanes with active 4-cinnamoylbiphenyl mesogens show enantiotropic LC properties690 where the spacer length influenced the thermal transitions and the nature of the mesophase. The isotropization temperatures showed an even-odd effect as a function of the spacer length. Main chain viologen polymers with organic counterions have been found to exhibit either a high or low order smectic phase.691 The LC temperature range was also influenced by the nature of the counter ion, as were the fluorescence spectra. Thermotropic LC polyesters of 4,4 0 -biphenol and phenyl-substituted 4,4 0 -biphenols have been found to exhibit very high thermotropic crystal-nematic transitions at 4501C when reacted with terephthalic or naphthalenedicarboxylic acids,692 whereas the annealing of films of polyacrylates with mesogen side-chains above their Tg gave significant amplification of their photoinduced anisotropy.693 Triptycene groups have been found to impart a high degree of solubility to PPVs, to form effective nematic LCs.694 Here the conjugated backbones were found to align with the direction of the nematic LC and can be re-orientated by the application of an electric field. An intensity grating has been formed in an LC polymer with azobenzene side groups through a photoinduced alignment of the mesogens,695 while smectic LC poly(methacrylates) with p-methoxyazobenzene groups gave rise to photoinduced optical anisotropy upon irradiation to polarized light.696 On annealing, the polymers showed either out-of-plane or in-plane orientation, depending upon the structure of the material. The concentraion of a photoinitiator has been found to influence the LC properties of a phenyletheracrylate,697 whereas the encapsulation of monomer molecules in LC droplets dispersed in a thermoplastic matirx can provide a convenient method to control the orientation of LC directors.698 LC poly(1-alkynes) with biphenyl bridges are also highly fluorescent in the near-UV.699 Continuing on from LC systems are photochromic materials, many displaying LC properties. Polymers of a special interest include thionines,700 multifunctional acrylics,701 phthalocyanine hybrids of bisthienylene,702 diarylethenes,703 fluorinated naphthopyrans,704 cyclobutene-1,2-dione705 and azo-polymers.706,707 Poly[oxy[trans-4-(2-phenylethenyl)pyridiniomethyl]-1,2ethanediyl chloride] undergoes trans-cis isomerization on irradiation, with prolonged irradiation causing cycloaddition and then conversion to the original linear oligomer.708 A novel polyamidoamine side-chain polyester with azobenzene motifs in the polymeric core has been found to display cis-trans isomerization on light exposure,709 whereas polesters with 4-cyano-4 0 alkoxyazobenzene groups displays LC properties as well as a strong tendency to aggregate.710 In this case the photoorientation effect is co-operative whereby


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the orientation of the chromic side-group induces alignment of the ester unit and in-chain methylenes. Very high linear dichroism and birefringence are observed and are unaffected by J-aggregation effects. In polynorbornenes with azo groups, isomerization was found to depend upon the azo groups only and not the polymer chain itself,711 whereas 2H-chromenes with pyranofluorenone groups did not exhibit photochromism, whilst those with pyranofluorenols did.712 Polyazophenols, on the other hand, have been found to isomerize very slowly,713 while poly(vinyl alcohol) with azobenzene groups gives two monolayers, each exhibiting an alignment transition due to isomerization, with the top layer have perpendicular rod-like structures.714 For polystyrene-b-poly(1,2isoprene-ran-3,4-isoprene) block copolymers with azobenzene side groups, typical microphase separated morphologies such as sphere, cylinder and lamellar structures were observed.715 while dewar benzene groups on polyacrylics did not hinder photoisomerization,716 whereas PMMA doped with zinc tetrabenzoporphyrin produced a long-lived reversible triplet state.717 Hybrids incorporating triethoxysilane end-capped polyethylene glycols with tungsten oxide have been prepared by a sol-gel process,718 while actuators have been made from polydiacetylene with alkylurethanes.719 The latter undergoes a bluered phase shift on irradiation. Conjugated polymers with di-hetarylethene and benzylidene-anthrone groups in the chain exhibit photoreversible magnetic properties,720 whereas azobenzene-modified copolyaramides undergo reversible light and heat induced trans-cis isomerization.721 In the latter case isomerization-induced perturbations to the local dihedral angles residing in the atropisometric binaphthylene main chain units serve as the genesis for the photo and thermal regulated behaviour in these materials. Cationomers based on polyurethanes with nitroazobenzene groups have been shown to undergo photoisomerization with an irreversible photobleaching effect,722 whereas blends of poly(pyridinium salts) give enhanced colour changes depending on the electron withdrawing nature of groups on the polymer backbone.723 Polyanilines with azobenzene sulfonic acid dopant form nanotubes which can undergo cis-trans isomerization,724,725 while several polyurethanes with various azobenzene groups exhibt birefringence properties,726–728 each with different growth patterns depending upon the nature of the azobenzene group. Spirooxazines have been looked at as usual. These include mercocyanine dyes729,730 where binding to polysiloxanes only gave stability to the coloured form of the dye in solid media and not in solution. Molecules with long alkyl groups731 and allyl groups732 have also been made, with the latter exhibiting photochromism between the two isomers of the coloured open form. Using quantum chemical analysis, the pathways for ring opening show that the most stable mercocyanine forms,733 TTC and CTC, can be obtained after C–O bond cleavage of the R and S enantiomeric closed forms. The less stable mercocyanine forms, CTT and TTT, revert to the corresponding closed form through an inversion mechanism at the N1 0 nitrogen atom, and this explains the fast component of the kinetics of the thermal fading reaction of the dye. Enantiomerization was achieved after ring opening by cis-trans isomerization, either between two mercocyanines (i.e., TTC and CTC) or two s-cis intermediates. Apparently, the high energy barrier

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of the bond rotation necessary for the interconversion between TTC and CTC isomers makes this process less favourable than the direct interconversion between the s-cis intermediates (CCC and TCC). Thermal back reactions of spirooxazines have also been calculated and measured in both solution and poly(methacrylate) media.734 Azobenzene groups have been bound to numerous acrylic polymers, with 29 mol-% of photochromic groups being required to give a smectic phase.735 Changes in refractive index, restricted motions and crosslinking effects have all been investigated on the photoisomerization.736–740 Aggregation in micelles and its effect on isomerization of azo groups has also been probed.741–745 This includes sensitivity of absorption spectra, formation of nanotubes, fabrication of multilayers through self-assembly processes, weak anisotropy and reversibility. Spirobenzopyrans have also been investigated in polymeric media.746,747 When copolymerized to N-isopropylacrylamide the isomerization was found to be very slow, enabling the isomers to be separated by water precipitation,746 while chelation gives some novel properties.747 Azobenzene groups have also been attached to polysiloxanes.748–751 These include orientational control of alkyl chain lengths,748 microphase separation of cisisomeric molecules,749 packing and tail lengths in monolayers750 and broad thermotropic properties.751 Photochromism in crown ethers is also possible.752– 755 Spironaphthoxazines when bound on the 6 0 -position of the naphthalene ring exhibit bathochromic shifts in absorption spectra depending upon the metal ion.752 Isomerization reduces the binding ability of metal ions. A new calixacrown ring has been synthesized753 where the kinetics of thermal decay are biexponential, whereas dithiacrown ether styryl dyes form dimer complexes capable of undergoing 2þ2 cycloaddition reactions.754 Reversible Z-E isomerization also occurs, with an amphiphilic form giving relatively stable monolayers on water surfaces. Several other novel crown-ether systems have also been discussed.755 Photochromism has also been reported for polymethine dyes,756 Nile Red dye in dipalmitoylphosphatydilcholine surfactants,757 Eriochrome dyes758 and annulated coumarins.759 A number of articles have also appeared on poly(acrylamides)760–762 where the isomerization reversibility is very effective and useful for probing the effects of sol-gels.

4

Photodegradation and Photooxidation Processes in Polymers

The photodegradation of polymer materials continues to attract limited interest although, as in the last few years, activity continues to decline. Thermal degradation and stabilization processes rank much more highly in this particular field. Laser ablation processes continue to grow in interest with applications in electronics. A number of useful reviews on various aspects of the subject have appeared, giving extensive coverage of techniques, applications and mechanisms, and also numerous other topical articles of interest. Amongst these are degradation processes in polyurethane adhesives,763 chemiluminescence and degradation processes,764 bond cleavages,765 activation spectra,766 polystyrenes,767 environmental issues,768 wavelength sensitivity,769


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biodegradability,770 the current status today771 and a general overview.772 Some specialist articles include photoradical ageing,773 identification and quantification of processes,774–776 acceleration of processes,777 nature of light sources,778 active intermediates,779 chemiluminescence for screening,780 weathering of many polymers,781 product failure,782 electrooptical materials,783 melt viscoelasticity,784 shopping bag problems in the environment785 and the value and use of integrating spheres.786 4.1 Polyolefins. – Polyolefins continue to be widely investigated, with novel submicrometric methods for thermal and infrared analysis being developed for examining surface effects.787 A number of photodegradants continue to be pursued, including ceric stearate,788 ferric stearate789,790 and ferric diethyldithiocarbamate.791 Similar toughened polypropylene compounds were found to behave differently with photoirradiation time,792 while photosensitized degradation was induced by grafting with epoxidized lignosulfates,793 and a high degree of stability achieved using sericite-tridymite-cristobalite blends.794 Polyethylene improves the photostability of natural rubber in blends,795 whereas under vacuum polypropylene degrades about 8 faster than polyethylene under similar conditions of exposure.796,797 Oxygen accelerates the photooxidation of polyethylene,798 while the application of an electric fied improves its mechanical properties.799 4.2 Polystyrenes. – Through quantum yield measurements, the photooxidation of polystyrene has been attributed to a photochain process, in the main involving the dissociation of excited peroxides, whereas hydroperoxides are shown to contribute to only 10% of the free radicals formed.800 In another study on polystyrene hydroperoxides, benzaldehyde and hydroxyacetophenone were identified as major products as well as some phenyl glycol.801 Mass changes and crosslinking reactions have also been pursued,802,803 as has degradation in tropical climates.804 Photodegradable blends of polystyrene and carbon monoxide copolymers have also been found valuable for environmental issues.805 On a similar note, recycled polystyrene degrades faster than virgin material,806 and benzophenone, as expected, is also a powerful photosensitizer.807 Photoproducts formed under natural and accelerated weathering conditions have also been measured and compared.808 4.3 Poly(Acrylates) and -(Alkyl Acrylates). – Structured nanopore films of poly(styrene-block-methyl methacrylate) copolymers have been made with controlled spectral sensitivity, such that each block is sensitive to a specific degradation wavelength.809 In copolymers of 2,2,2-trifluoroethyl methacrylate with vinyl ethers, the photosensitivity is controlled by the vinyl ether units.810 Photodegradation occurs at the tertiary positions of the ether units followed by lactone formation and chain scission processes. Furthermore, the fluorinated side chains have been found to inhibit cyclization reactions.

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4.4 Poly(Vinyl Halides). – Poly(vinyl acetate) has been found to improve the inherent light stability of PVC.811 The effect is apparently associated with an efficient quenching of macro-radicals by low radicals formed in primary photochemical stages in both phases. The carbonyl groups in the PVAc also protect the polymer. Rare earth carboxylates photosensitize the degradation of PVC but inhibit crosslinking reactions,812 while the addition of small amounts of ferric and cobaltous chloride initially inhibit the photodegradation of PVC, thereafter accelerating the process, but again the presence of the metal ions prevents crosslinking reactions.813 4.5 Polyamides and Polyimides. – Rate constants for the decrease in the intensity of the amide groups in a model nylon 6,6 compound have been determined through pulsed laser Raman spectroscopy,814 while in side-chain polyimides photobleaching processes have been measured.815 Simple models were developed here, involving four different molecular states. Preferential degradation of polyimide molecules has been found to occur under polarized light exposure parallel to the direction of the UV light.816 4.6 Poly(Aromatics). – Poly(p-xylylene) undergoes photooxidation at the methylene group, initially via hydrogen atom abstraction817 followed by attack on the ring structure. Such instability prevents the long term use of this material in outdoor applications. The backbone structures in PPVs influence the rates of their photooxidation.818 4.7 Silicone Polymers. – The photolysis of an alternating copolymer of 2,6diethylene-acetophenone-disiloxane in the absence of oxygen resulted in complete polymer recovery with no change in molecular weight, whereas in the presence of oxygen significant chain scission was observed.819 In this case hexamethyldisiloxane and 1,2-diacetylbenzene were formed as products. 4.8 Polyurethanes and Rubbers. – AFM analysis on photodegraded surfaces of a polyurethane coating has identified blisters while under continuous exposure, whereas under dark-light cycles the blisters retained a constant size and shape.820 The former, as may be expected, was more damaging. In a polyoctenamer evidence has been provided for the presence of low concentrations of hydroperoxides being responsible for the photoinduced crosslinking.821 The nature of the crosslinks, however, is disputed as being solely due to C–C or C– O–C bonds. Using ATR-FTIR, the photodegradation of ABS has been found to be heterogeneous down to a layer of 50 microns only.822 Carbonyl and hydroxyl groups were readily observed, along with the disappearance of the 1,4-butadiene units. The butadiene units were the most severely degraded, with the styrene-acrylic units remaining intact. 4.9 Polyesters. – Positron lifetime techniques have been used to measure structural relaxation processes in irradiated poly(ethylene terephthalate) (PET).823 Three relaxation processes with different kinetics were observed in


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irradiated material, compared with only two for the unirradiated polymer. Photohydroxylation has been found to be a primary procees in the photooxidation of PET fibres,824 while DSC analysis of photooxidized PET films showed the reformation of crystalline regions associated with the re-alignment of amorphous material.825 Contaminated water in PET bottles has been found to be destroyed only after some 5-6 hours of sunlight exposure.826 The bio- and photodegradabilities of aliphatic and aromatic polyesters have also been investigated in terms of physical mechanical property changes.827 4.10 Photoablation of Polymers. – Structural relaxation and electron-phonon interactions have been investigated during the photochemical hole-buring of several polymer systems.828 In porphyrin containing DNA-lipid complexes, marked hole broadening was observed but no hole filling at 40 K, owing to intercalation of the porphyrin molecules into the double helix of the DNA. Dendrimer porphyrin neat films showed sharp holes at 20 K, with a higher generation exhibiting good thermal stability due to effective surface mobility. Doped PMMA and polystyrene films have been optimized for laser ablation work.829 Here the importance of employing relatively strongly absorbed wavelengths in laser processing relates, besides the efficient etching and good surface morphology, to the minimization of the chemical modifications. In contrast, it is claimed that pulsed irradiation work is highly disadvantageous with regard to the chemical integrity of the substrate. PTFE surfaces have enhanced adhesion to copper plates following ArF excimer laser ablation under a hydrazine and ammonia atmosphere.830 Other adhesion promoters include the use of acrylated photoinitiators.831 Synchrotron radiation has been used to crosslink PTFE,832 and the real-time evolution of luminescence properties of ion-irradiated polymer films has been investigated by means of ion-beam induced luminescence measurements.833 Marked narrowing of spectral holes in dyed polymer films has been observed through the use of spectral hole burning under high pressures.834 Under Xe2 excimer laser ablation, polycarbonate material undergoes severe surface roughening followed by the evolution of chemical functional groups due to the severing of aromatic C–C and carboxylic groups.835 Polymers have been categorized into those with low, medium and high fluency for laser ablation.836,837 Polymers with triazine groups were found to have the lowest threshold for ablation. 4.11 Natural Polymers. – Camphorquinone is a sensitizer in cellulose fibres,838 while wood flour is a good filler and stabilizer for PVC.839 Pine wood lignin gives primarily cis-sinapyl alcohol and sinapyl aldehyde on irradiation, associated with oxidation of the guaiacyl units.840 Wood pulp treated with hydrogen periodide has been examined by laser flash photolysis,841 while acetylated and bleached aspen chemithermomechanical pulp have been examined through the use of UV-visible diffuse reflectance spectroscopy.842,843 It was concluded that light-induced yellowing was not excusively an oxygen aided process, and in fact in an oxygen saturated atmosphere it impaired the yellowing process. In a similar fashion acid anhydrides also inhibited the yellowing of fir sapwoods in

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the order phthalic 4 maleic 4 succinic 4 acetic 4 butyric anhydride.844 Hydrogenolysis of lignin also eliminated yellowing.845 Hydroxyl radicals have been found to be the prime cause of initiating the degradation of glucosidic units,846 and again guaiacyl units are found to be more degradable than syringyl units.847 4.12 Miscellaneous Polymers. – UV photolysis of methoxylated melamine formaldehyde gives methyl radicals followed by more stable methane radicals, brought about by side-chain reactions,848 while acrylic-melamine coatings undergo primarily hydrolysis under irradiation conditions with high humidity.849 The degree of degradation of cellulose triacetate decreases with enzymatic degradation,850 while polycarbonate undergoes loss of optical properties on irradiation owing to a photo-Fries rearrangement and oxidation processes.851 High humidity also caused photobleaching of the yellowing in polycarbonates. Vinyl ester composites undergo severe degradation when exposed to UV light, with changes in surface chemistry, morphology and the bulk matrix,852 whereas quinine-methides formed from phenolic antioxidants in polymer materials are photobleached with UV light.853 Thermosetting acrylic clearcoats undergo loss of functional groups with the concommitant formation of carboxyl and hydroxyl groups,854 as do acrylic-urethane clearcoats.855,856 Surface weathering has been studied in glycol acrylate composites,857 epoxy matrices858 and poly(vinyl methyl ether).859 Other studies of interest include photodegradation of PTFE,860 shopping bags,861 poly(pyridinium salts),862 calcium carbonate doped polymers,863 recyclable materials864 and polyimides.865 A new method has been developed for measuring chain scissions in polymer degradation,866 and a model developed for linking field and laboratory trials.867

5

Photostabilization of Polymers

Photostabilization processes have not received much attention in the last year other than a multitude of review and specialist industrial articles. These include stabilization of polypropylene fibres,868 new stabilizer developments,869–882 sorting of stabilizers for polyethylene,883 trends in UV absorbers,884 hindered piperidines (HALS) for polyacrylics885 and urethanes,886 new stabilizers for polyolefins,887 enhanced service-life applications,888 stabilizer masterbatches for polyester,889 stabilizers for UV-cured acrylic overcoats for PVC890 and stabilization of clearcoats and various commercial packages.891 Apparently, pre-irradiating polyethylene containing a HALS with a low pressure Hg lamp imparts some surface stabilization toward sunlight exposure.892 Anthracene-HAS stabilizer molecules are found to be more effective stabilizers than the separate molecular moieties,893 while the nature of the polymer composition affects the distribution of nitroxyl radicals in stabilized materials.894 Polyethylene with terminal HAS groups has been found to be very light stable,895 as were reactive acrylic coatings with acrylated HAS systems.896 A new epoxide amine based HAS has been found to be quite effective in


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stabilizing PP,897 as was a new rubber grafted maleic anhydride HAS system.898 Cyclic bridged amines are also effective co-synergists with HAS,899 as are lactones.900 The polymer matrix has been found to influence the excited state properties of UV absorbers,901 while novel effective adducts of HAS and 2-hydroxyphenylbenzotriazoles have been made.902 UV absorbers have been shown to undergo photolysis reactions in polymer materials, the loss being quite critical, depending upon the nature and form of the material.903 Spectroscopic methods have been developed to design more effective UV absorbers which function via intramolecular proton-transfer in the excited state.904 Other effective stabilizers for polymers includes thioureas for PMMA,905 aryl nitrenes for PP,906 diphenylnitrone for polyethylene,907 zinc glycerolate in PVC908 and hexazoclanes for cellulosics and nylons.909

6

Photochemistry of Dyed and Pigmented Polymers

Dye and pigment fading and sensitization continue to be highly active areas of interest, especially with regard to photocatalytic chemistry. The sensitized production of singlet oxygen is known to play an important role in polymer oxidation reactions with dyes. Zinc phthalocyanine sulfonate is widely used to chemically treat dyed fibres as a sterilizing treatment in washing powders.910 It was found to be detrimental to the photostability of a range of dyestuffs on different fibres after a washing treatment. Singlet oxygen production and hydroperoxide formation were key steps in the enhanced photofading of the dyes. The addition of a hydroxyl KSCN radical scavenger was found to impair the photofading process. The photooxidation of three new bicycle-boron dipyrromethane difluoride dyes depended upon the nature of the solvent,911 while for a series of vat dyes in cellulose photofading was found to occur mainly by oxidation in dry fabric, whereas in wet fabric under anaerobic conditions fading occurred through a photoreductive process to the leuco form of the dye.912 Intramolecular triplet energy transfer is an important process in controlling the photostability of many dyes,913 which may also be induced through the use of absorbers.914 Singlet oxygen is a common species in dye photofading chemistry, with basic dyes such as Crystal Violet being a noteable example. Here the use of absorbers is found to be ineffective against fading, whilst nickel quenchers improve against the fading significantly.915 Azo dyes have also been co-reacted with stabilizer moieties for enhanced stabilization,916 while Rhodamine 6G dye has been copolymerized with 1,8-naphthalimide derivatives.917 Photo-Fenton reagent was used to sensitize the photofading of Malachite Green dye in the presence of various aromatic compounds.918 The compounds did not influence the type of degradation pathways, just the rate process, which primarily involved OH and N-demethylation cleavage. Using molecular orbital analysis and fading reactions, cyanine dyes have also been found to photofade via a singlet oxygen reaction mechanism.919 Acidic pH conditions have been found to enhance the photofading of an Acid Blue 25 anthraquninone dye in nylon 6,6 film.920 This process is already well-known and associated usually

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with protonation of the dye and enhanced potential for hydrogen-atom abstraction. Photo and molecular transformation processes of methyl red in PVA film on laser irradiation have been investigated,921 while in another article dye fading has been considered based on singlet oxygen and aggregation effects.922 Carthamin dye in cellulose triacetate film also fades via a singlet oxygen mechanism and is inhibited by nickel quenchers.923 Factors influencing the photofading of vat dyes have been determined, especially those related to the dye bath operation,924,925 while certain phthalocyanine dyes have been found to emit intense singlet oxygen sensitized delayed fluorescence upon laser irradiation in oxygen.926 The photofading of azo dyes varies with the type of added heterocycle, such as furan or thiophene,927 whereas a disperse yellow dye was found to fade faster when in a less aggregated form.928 Multifilament nylon 6 yarns are found to photodegrade faster than single filaments,929 and the Blue Wool ISO standard scale of lightfastness measurement has been found to depend significantly upon the nature of the light source used for weathering.930 Numerous articles of a general interest have appeared on photocatalytic chemistry. These include nanoparticles,931 protection of buildings,932–934 antibacterial properties,935 ageing of pigment blends,936 modified titanium dioxides,937 photocatalytic paints,938 stabilization of pigmented systems,939 sensitization of acid dyes940 and clay masterbatches.941 Anatase has been found to have a detrimental effect on the light stability of inks,942 while similar effects have been prevented by encapsulating dyes in polymers.943 In PVC, titanium dioxide has a catalytic effect due to its semiconductor properties,944 whereas in paper systems titania supported in an inorganic fibre matrix gives good stabilization.945 Titanium dioxide also photosensitizes the degradation of plasticizers in PVC, with carbon dioxide being the major breakdown product.946 The latter has also been used as a monitor for the photooxidation rates of titania filled alkyd paint films.947 Here degradation rates were found to increase with increasing humidity. In polyethylene film, nanoparticle uncoated titanium dioxide pigments have been shown to be more photoactive than pigmentary grades and operate as powerful prooxidants during prior processing generating hydroperoxides and carbonylic groups.948 The same behaviour is operative in alkyd based paint films, with SEM analysis showing deep pits being formed around the nanoparticles. Pigment activity was also related to charge-carrier generation properties as measured by dielectric microwave spectroscopy. In acrylic and isocyanate based paint films, coated nanoparticles of titanium dioxide have been shown to be powerful UV absorbers above 350 nm, and during weathering can outperform commercial HAS and UV absorber stabilizers.949 The nanoparticles were also found to antagonize the stabilizing effect of HAS, while in wood coatings the nanoparticles were found to be excellent inhibitors of colour formation, owing to their sensitizing effect in photobleaching the lignin by-products. References 1. J.P. Fouassier (ed.), Research Trends, Trivandrum, India, 2001, 190 pp. 2. J.P. Fouassier, Recent Res. Dev. Photochem. Photobiol., 2000, 4, 51.

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