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A Specialist Periodical Report
Photochemistry Volume 33 A Review of the Literature Published between July 2000 and June 2001 Senior Reporter A. Gilbert, Department of Chemistry, University of Reading, UK Reporters N.S. Allen, Manchester Metropolitan University, UK A. Cox, University of Warwick, UK I. Dunkin, University of Strathclyde, Glasgow, UK A. Harriman, University of Newcastle upon Tyne, UK W.M. Horspool, University of Dundee, UK A.C. Pratt, Dublin City University, Ireland
~~
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ISBN 0-85404-435 - 3 ISSN 0556-3860 A catalogue record for this book is available from British Library 0 The Royal Society of Chemistry 2002
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Contents
Part I
Introduction and Review of the Year By Andrew Gilbert
1
Physical Aspects of Photochemistry
11
PhotophysicalProcesses in Condensed Phases By Anthony Harriman
13
1 Introduction
13
2 General Aspects of Photophysical Processes
13
3 Theoretical and Kinetic Considerations
16
4 Photophysical Processes in Liquid or Solid Media 4.1 Detection of Single Molecules 4.2 Radiative and Non-radiative Decay Processes 4.3 Amplitude or Torsional Motion 4.4 Light-induced Proton-transfer Reactions 4.5 Quenching of Excited States 4.5.1 Energy-transfer Reactions 4.5.2 Electron- t r ansfer Reactions 4.6 Photophysics of Fullerenes
18 18 19 21 23 24 25 25 27
5 Applications of Photophysics
29
6 Advances in Instrument Design and Utilisation 6.1 Data Analysis 6.2 Instrumentation
29 29 30
7 References
31
Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002
vi
Part I1
Contents
Organic Aspects of Photochemistry
51
Chapter 1 Photolysis of Carbonyl Compounds By William M . Horspool
53
1 Norrish Type I Reactions
53
2 Norrish Type I1 Reactions 2.1 1,SHydrogen Transfer 2.2 Other Hydrogen Transfers
55 55 58
3 Oxetane Formation
60
4 Miscellaneous Reactions 4.1 Decarbonylation and Decarboxylation 4.2 Reactions of Miscellaneous Haloketones and Acid Chlorides 4.3 Other Processes
61 61
5
References
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactionsof Dienones and Quinones By William M . Horspool
66 66 69 74
1 Cycloaddition Reactions 1.1 Intermolecular Cycloaddition 1.1.1 Open-chain Systems 1.1.2 Additions to Cyclopentenones and Related Systems 1.1.3 Additions to Cyclohexenones and Related Systems 1.2 Intramolecular Additions 1.2.1 Intramolecular Additions to Cyclopentenones 1.2.2 Additions to Cyclohexenones and Related Systems
74 74 74
2 Rearrangement Reactions 2.1 a$-Unsaturated Systems 2.1.1 Isomerisation 2.1.2 Hydrogen Abstraction Reactions 2.1.3 Rearrangement Reactions 2.2 P,y-Unsaturated Systems 2.2.1 The Oxa Di-n-methane Reaction and Related Processes 2.2.2 Other Rearrangements
84 84 84 86 87 88
75 77 80 81 81
88 89
vii
Contents
3 Photoreactions of Thymines and Related Compounds 3.1 Photoreactions of Pyridones 3.2 Photoreactions of Thymines etc. 3.3 Miscellaneous Processes
91 91 91 94
4 Photochemistry of Dienones 4,l Cross-conjugated Dienones 4.2 Linearly Conjugated Dienones
95 95 96
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 5.3.1 Phthalimides and Related Compounds 5.3.2 Fulgides and Fulgimides
97 97 98 100 102 106
6 Quinones 6.1 o-Quinones 6.2 p-Quinones
108 108 108
7 References
110
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M . Horspool
1 Reactions of Alkenes 1.1 cis,trans-Isomerisation 1.1.1 Stilbenes and Related Compounds 1.1.2 The Dithienylethene System and Related Compounds 1.2 Miscellaneous Reactions 1.2.1 Addition Reactions 1.2.2 Electron-transfer Processes 1.2.3 Other Processes 2 Reactions Involving Cyclopropane Rings 2.1 The Di-n-methane Rearrangement and Related Processes 2.1.1 The Aza-di-n-methane Rearrangement and Related Processes 2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds 3 Reactions of Dienes and Trienes 3.1 Vitamin D Analogues
119
119 119 119 122 127 128 128 128 129 129 132 132 135 139
V l... ll
Contents
+ 2.n)-IntramolecularAdditions
139
5 Dimerisation and Intermolecular Additions 5.1 Dimerisation
140 141
6 Miscellaneous Reactions 6.1 Reactions Involving Cations and Radicals 6.2 Miscellaneous Rearrangements and Bond Fission Processes
141 141
7 References
146
4 (2.n
Chapter 4 Photochemistry of Aromatic Compounds B y Andrew Gilbert
144
155
Introduction
155
Isomerisation Reactions
155
Addition Reactions
156
Substitution Reactions
164
Cyclisation Reactions
168
Dimerisation Processes
178
Lateral Nuclear Shifts
181
Miscellaneous Photochemistry of Aromatic Systems
183
References
188
Chapter 5 Photo-reduction and -oxidation By Alan Cox
194
1 Introduction
194
2 Reduction of the Carbonyl Group
194
3 Reduction of Nitrogen-containing Compounds
201
4
205
Miscellaneous Reductions
5 Singlet Oxygen
212
ix
Contents
6 Oxidation of Aliphatic Compounds
213
7 Oxidation of Aromatic Compounds
217
8 Oxidation of Nitrogen-containing Compounds
223
9 Miscellaneous Oxidations
232
10 References
232
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By Albert C. Pratt
1 Introduction
242
2 Nitrogen-containing Compounds 2.1 E,Z-Tsomerisations 2.2 Photocyclisations 2.3 Photoadditions 2.4 Other Processes
242 242 244 254 26 1
3 Sulfur-containing Compounds
275
4 Compounds Containing Other Heteroatoms 4.1 Silicon and Germanium 4.2 Phosphorus 4.3 Other Elements
287 287 290 292
5 References
294
Chapter 7 Photoelimination By Ian R. Dunkin
307
1 Introduction
307
2 Elimination of Nitrogen from Azo Compounds and Analogues
307
3 Elimination of Nitrogen from Diazo Compounds and Diazirines 3.1 Generation of Alkyl, Alicyclic and Heterocyclic Car benes 3.2 Generation of Aryl and Heteroaryl Carbenes 3.3 Photolysis of Diazo Carbonyl Compounds and Sulfur Analogues
308 308 311
313
Contents
X
4 Elimination of Nitrogen from Azides 5
Photoelimination of Carbon Monoxide and Carbon Dioxide 5.1 Photoelimination of CO from Organometallic Compounds
6 Photoelimination of NO and NOz
3 14 316 318
321
7 Miscellaneous Photoeliminations and 322 Pho tofragmentations 322 7.1 Photoelimination from Hydrocarbons 7.2 Photoelimination from Organohalogen Compounds 322 7.3 Photofragmentations of Organosilicon and 324 Organogermanium Compounds 7.4 Photofragmentations of Organosulfur, Organoselenium and Organotellurium Compounds 327 7.5 Photolysis of o-Nitrobenzyl Derivatives and Related 327 Compounds 329 7.6 0t her Phot ofragment ations 8
Part I11
References
330
Polymer Photochemistry B y Norman S. Allen
337
1 Introduction
339
2 Photopolymerization 2.1 Photoinitiated Addition Polymerization 2.2 Photocrosslinking 2.3 Photografting
339 340 345 351
3 Luminescence and Optical Properties
352
4 Photodegradation and Photooxidation Processes in Polymers 4.1 Polyolefins 4.2 Polystyrenes 4.3 Poly(acry1ates)and Poly(alky1 acrylates) 4.4 Polyesters 4.5 Polyamides and Polyimides 4.6 Poly(alky1 and aromatic ethers) 4.7 Silicone Polymers 4.8 Polyurethanes and Rubbers
369 369 370 370 37 1 37 1 37 1 371 372
x1
Contents
4.9 4.10 4.1 1 4.12
Part IV
Poly(viny1 halides) Photoablation of Polymers Natural Polymers Miscellaneous Polymers
372 372 373 373
5 Photostabilization of Polymers
374
6 Photochemistry of Dyed and Pigmented Polymers
375
7 References
375
Photochemical Aspects of Solar Energy Conversion B y Alan Cox
405
1 Introduction
407
2 Homogeneous Photosystems
407
3 Heterogeneous Photosystems
408
Photoelectrochemical Cells
4 10
4
5 Biological Systems
411
6 References
412
Author Index
415
Introduction and Review of the Year BY ANDREW GILBERT
The chapter and reference numbers of the reports cited in this Introduction and Review can be found by using the Author Index. As usual, this subjective reflection on the literature published within the review period follows the order of the chapters in this volume and so begins with the more physical aspects of the subject. Phenyl-substituted polyacetylenes are important materials for light-emitting polymeric devices and a significant conclusion from their detailed theoretical study is that polyacetylenes have a smaller optical band gap than polyenes of the same chain length (Shukla et al.). The enormous increase in interest in the photochemistry of dendrimers, which have multiple chromophores, arises principally because such systems can be used as models for the natural light-harvesting complexes. Thus several groups have reported the fluorescence properties of organic-based dendrimers (see references 80-85 in Part I) and Balzani et al. have described a dendrimer which hosts 32 dansyl groups. In other studies, optically active dendrimers have been synthesised which are capable of enantioselective fluorescence sensing at modest levels (Gong et ul.), and picosecond laser flash photolysis has been used to monitor twisted intramolecular charge-transfer states in dendrimers (Drobizhev et al.). A further area of increasing interest is the construction of molecular-scale wires for use in molecular opto-electronic devices, and indeed ultrafast energy transfer has been observed in such systems fashioned from zinc porphyrins (Kim et ul.). The spectroscopic investigation of single or isolated molecules has been the subject of considerable attention for some time and Enderlein and Sauer have described a new algorithm for singlemolecule identification by time correlated single-photon counting techniques, while Bereshkovski et al. have developed an analytical approach using singlemolecule fluorescence spectroscopy to evaluate rate constants for slow conformational exchange. A new approach for measuring the rate constant of oxygen quenching of longlived triplet excited states is based on the time-resolved measurement of resultant singlet molecular oxygen (Kruk and Korotkii), and a new technique, the socalled ‘piston source method’ has been used to measure the absolute concentrations of singlet molecular oxygen in solution (Dun et ul,). New treatments have also been presented for the analysis of kinetic data, particularly those of nonexponential decay processes (Wen and McCormick, inter alia).
Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002 1
2
Photochemistry
Publications on the more organic aspects of photochemistry are now considered. The photochemistry of acetone continues to attract interest and cornputational procedures have been used to probe its photodissociation (Liu et aE.), and Aloisio and Francisco have reported its photochemical behaviour in the presence and absence of water. Irradiation of the ketone (1) provides an efficient route to the cyclopropyl ketone (2) and this method, which is initiated by a Norrish Type I1 hydrogen abstraction, can be used to synthesise such bicyclic compounds as (3) (Wessig and Muhling). 0
OH
OMS
i
OMS
(1)
n
V (2)
(3)
The formation of the oxetanes (4) and ( 5 ) from the photoaddition of benzophenone to cis- and trans-cyclo-octene is subject to a remarkable temperature effect (Adam et al.), and the specific addition of benzaldehyde to ethenes (6) has been used in a synthetic approach to preussin (7) (Bach et d.).Barton esters such as (8) have been used in new photochemistry of boronic esters [e.g. (9)] to give (10)as the major product isomer (Cadot et al.) and Horton et al. have described a new photolabile linker (11)which, in the presence of tributyltin hydride, liberates indole. The photoaddition of cyclic amines to 5-(R)-(1)-menthyloxy-2(5H)furanone has been developed into a new method for the synthesis of chiral cycloaminobutyrolactones (Wang et aE.) and dienamides (12) are reported by Bois et al. to undergo efficient photocyclisation in the presence of sodium borohydride to give (13) which is considered to be a convenient intermediate in the synthesis of (S)-( +)-pipecoline.
(5)
(6) R = alkyl or CH2Ph, n = 1 or 2
(7)
3
Introduction and Review of the Year
A
A
Me Ph (12) R = alkyl
Me
Ph
(13)
As in previous years, the photochemistry of molecules as guests in host systems continues to attract a high level of interest. Examples in the review period include the formation of the dimer (14) with 100% ee from irradiation of the inclusion complex of the enone (15) with (16) as the host (Tanaka et al.)?and the report of the host-dependent outcome of the photoinduced rearrangement of (17) (Zimmerman et al.). Macrocyclic systems can be formed from phthalimide derivatives both by photoaddition of 2-phenylpropene to appropriately N-substituted compounds followed by cyclisation [e.g. (1S)] (Zhu et al.)?and by photocyclisation of such molecules as (19) giving (20) (Yoon et al.). Novel indol-2-ylfulgimides (21) have been synthesised by Heller et al. and their cyclisation with 336 nm radiation yields the stable photochromes (22). A useful source of the cyclopentanone (23) for conversion into the natural product P-necrodol can be obtained from the product (24) of the copper triflate assisted intramolecular (2n + 2n) cycloaddition in the diene (25) (Samajdar et al.) and irradiation of racemic norbornadiene (26) with r-circular polarised 290 nm radiation is reported to excite the (-)-enantiomer selectively and yield the (+)-quadricyclane (27)(Nishino et al.)
$$$) \
/
/
Ph2COH
0
(18) R’ = Me, R 2 = Ph, 16% R’ = Ph, R2 = Me, 14%
__t
0
0 (19) n=2-6
(20)
CN
4
Photochemistry
(21) X = O o r S , R ’ = M e o r P h , R 2 = M e o r -
(22)
Wakita et aE. have synthesised the stable silabenzene (28) and report that this is converted with 320 nm radiation into the silabenzvalene (29). The first example of using P-cyclodextrin for asymmetric induction in the intramolecular meta photocycloaddition of arene-ethene bichromophores has been described by Vizvardi et al., and Morley and Pincock have reported an unprecedented intramolecular photoaddition of a carbonyl group to a naphthalene moiety from their study of the ester (30)in methanol solution. Tbt
Tbt
6n-Photocyclisation in stilbene derivatives continues to provide a convenient access to a variety of polyarenes (inter alia Martinez et a!. and Sat0 et al.), and Irie and co-workers have reported on the photochromism of a number of derivatives of 1,2-bis(methylthienyl)-perfluorocyclopenteneand related systems. The formation of (31) from irradiation of the linked bi-naphthyl compounds (32) under oxygen provides the first example of trapping a triplet biradical intermediate in aromatic cycloadditions (Kohmoto et al.), and the previously unknown species 4-iminocyclohexa-2,5-dienylidene (33) has been observed by nanosecond transient absorption spectroscopy from irradiation of 4-halogeno-anilines (Othmen et al.). Saito et al. report that the sequential irradiation of 1,2:5,6-naphthalene tetracarboxylic dianhydride (34) in an argon matrix provides the first preparation of dec-5-ene-1,3,7,9-tetrayne (35).
5
Introduction and Review of the Year
%
R I
-
R'
'0
H
d
,c?
0 L
c
.c?
O
Diastereo- and enantio-meric excesses of the order of 96% are observed in the product (36) from irradiation of chiral esters such as (37) in the solid state (Cheung et al.), and evidence has been provided from a study into the magnetic field effect on the photoinduced electron transfer between benzophenone and starburst dendrimers that those of higher generations act as both an electron donor and as a supercage in the photoprocess (Akimoto et d.). Using a-benzoylpropionic acid derivatives as substrates, Wessig et al. have obtained results which give an insight into the factors which determine the stereochemistry of the Norrish-Yang reaction, and a new route to vicinal diamines by photoreductive coupling of pyridine- arene- and alkyne-carboxalddimines has been described (Campos et al.). From studies into the excited state dynamics of methylviologen, it has been shown for the first time that a hydrogen bonding solvent can act as an electron donor in ultrafast intermolecular electron transfer (Peon et al.), and control over competitive photochemical and photophysical pathways to allow maximisation of electron and proton pathways can be gained by manipulation of the species in the novel complex of 8-hydroxy-1,3,6-pyrenetrisulfonate anion (38) and methylviologen using ionic micellar aggregates (De Borba et al.). X
hv
&x
Me Y
(37) x = -0
Me
Me
H
6
Photochemistry
-03sms03Interest in the photophysics and photochemical reactions of fullerenes continues unabated at a high level and covers a wide variety of topics. Furthermore, there appear to have been numerous attempts to incorporate such compounds into essentially every type of photosystem. Indeed the molecular dyads comprising fullerene and porphyrin terminals seem to attract the most interest and have been the most intensely studied (see references 495-505 in Part I). Reports have appeared describing the one-step multiple addition process of secondary amines to c 6 0 to give the corresponding tetra(amin0)fullerene epoxide in moderate to excellent yields (Isobe et d), and the photoinduced properties of C1200, which is a dimer of c 6 0 linked through a saturated furan ring (Fujitsuka et d.). Electron transfer to give the c 6 0 * - and C70--species has also been a subject which has attracted considerable attention (Part 11, Chapter 5, references 89-1 13), and the photophysical properties of self-assembled supramolecular ensembles from fullerene derivatives have been recorded (Deviprasad et d.). A new method for the production of singlet oxygen consisting of passing molecular oxygen over an irradiated sensitiser (e.g.Methylene Blue) formed from an impregnated pigment on a support such as silica, alumina or titania has been reported by Matsuura and Suzuki. p-Xylene has been converted into p-tolualdehyde with 100% selectivity using the 10-methyl-9-phenylacridinium ion as the electron acceptor sensitiser with visible light (Ohkubo and Fukuzumi) and a-amino radicals produced photochemically from tertiary amines (e.g. N , N dialkylanilines) undergo diastereoselective addition to (39) which can then lead to tetrahydroquinolines such as (40) in a tandem process (Bertrand et d.).
hv
0
==Q;,
PhNRZ2
0
g;
(39) ‘R’
Maier and Endres have identified the products from irradiation of the carbene (41) in matrices at 313 nm as the (s-E)-(E)-conformer(42) of triplet pent-2-en-4yn-1-ylidene which is converted into 3-ethynylcyclopropene (43) with 436 nm radiation, and novel triplet anthryl(ary1)carbenes (44) have been generated from the corresponding diazo compound in rigid matrices and characterised by ESR
7
Introduction and Review of the Year
H
V
C
, cH ,
(41)
(42)
F C = C - H (43)
Ph (44) Ar = Ph or mesityi
spectroscopy (Itakura and Tomioka). The first report of the direct observation of a carbine-ether ylide has been made from studies into the laser flash photolysis of methoxycarbonyl-2-naphthyldiazomethanewhich yields the ether ylide (45)as a transient (Wang et ul.), and novel surface modifications of platinum by the photoysis of 3- and 4-pyridyl a-diazoketones (46) are considered to have potential as a basis for general surface alterations (Pitters et aL). A synthesis of novel mesoionic amides such as (47) by irradiation of the azidotetrazolium salts has been reported by Araki et al., and studies into the photolysis of M(C0)6 in supercritical fluids have provided the first observation of organometallic noble gas complexes [e.g. M(C0)5(Kr)](George et al.). The first experimental measurements of triplet ethene near its equilibrium geometry have been made (Qi et d.) and the cinnamyl radical (PhCH = CHCH20*),generated from the corresponding 4-nitrobenzenesulfonate, is reported to undergo unprecedented ring closure to give the oxiranyl benzyl radical (Amaudrut and Wiest). Papageorgiou and Corrie suggest that the photoylic release of carboxylic acids (RC02H) from 1-acyl-7-nitroindolines (48) would provide a convenient method to generate neuroactive amino acids, and in a remarkable process that involves a ring contraction and loss of a nitrogen atom, irradiation of the benzodithiadiazine (49) gives the radical (50)in almost quantitative yield (Vlasyuk et ul.). Me
hN2 0-
(46)
(45)
(47)
In marked contrast to stilbene, the photostationary E/Z-ratio of azobenzene in zeolite cavities is closely similar to that in cyclohexane (Kojima et al.). The key intermediate (51) in an enantioselective synthesis of the antitumor alkaloids (+)-narciclasine and (+)-pancratistatin has been obtained by a stereo- and
8
Photochemistry
regio-controlled photocyclisation of the arylenamide (52) (Rigby et al.), and a similar photoreaction of the dienamide (53) and its enantiomer in the presence of sodium borohydride and methanol has been used in the synthesis of (S)-(+)pipecoline and of (S)-( -)- and (R)-(+)-coniine (Bois et al.). The P-ketoamides (54) undergo &hydrogen abstraction and loss of methanesulfonic acid on irradiation to give the enolate diradical(55) which cyclises regioselectively to form (56): this is the first example of C-0 bond formation in the Norrish-Yang reaction (Wessig et al.). The (27c + 2.n) photodimerisation of cinnamic acid and its derivatives is very well documented in the literature and has now been reported for the cinnamoyldopamines (57) and (58) but, in contrast, (59) photodimerises by ethene addition to the benzene giving (60), which is the first example of this type of process in the solid state (It0 et al.).
(54) X = -(CH2In-
(55) (56) (n = 2-5), -CH20CH2CH2-, -CHMeCHMeCH2-
q& 7 &
HO, HO
,O OH H
I
H N H
0
-
L
O
L
O
H
O/
C” I 0 C”
I
I
\W
N
H
R’0 (57) R’ = Me, R2-R3 = -OCH20(58) R’ = Me, R2 = CI, R3 = H (59) R’ = Me, R2-R3 = -OCH2-
The considerable number of publications reviewed in Part I11 of this report reflects the enormous and continuing activity in ‘polymer photochemistry’. The
Introduction and Review of the Year
9
volume of reports concerning light emitting diode systems has increased yet again and from this measure the topic has become one of the largest specialised areas in photochemistry and pho t oph ysics, with poly(phenyleneviny1ene)s (PPVs) creating the greatest academic and technical interest and attention. Much of the effort in this area is to devise polymer systems with highly efficient luminescence in specific wavelength region. Lipson et al. have shown that preparation and encapsulation of these polymers under argon greatly enhances (70%) their luminescence intensity, and Chen et al. report that PPVs with dentritic side chains self-organiseinto highly ordered structures in the solid state. Complexes of metal ions with PPVs exhibit an ionchromic effect with potential applications in optical switching devices (Huang et al.) and oligo-PPVs-fullerene dyads are reported to undergo rapid electron-transfer steps (Peeters). New initiator systems for photopolymerisation continue to be developed for general and specialised purposes. For example, poly(ethy1 methacrylate) with high acetone insolubility is produced using bis(cyclopentadieny1)titaniumdichloride as the initiator (Sato et al.), and a star-shaped polymer of tetrahydrofuran is formed by photoinduced cationic polymerisation in the presence of pentaerythritol tetrakis(3,4-epoxybutanoate)(Mah et al.). A new method has been developed by Lavrov et al. for the synthesis of Cso-polyfullerenes, and Cataldo has reported that such a polymer can be converted into a piezopolymer that is as hard as diamond. A novel photosensitive polyimide/silica hybrid has been prepared by a sol-gel route which yields material with high tensile and thermal stability (Cao et al.), and Wurtz et al. have described a new method on the sub-micrometre scale for curing nanometric polymer dots, A series of novel polyfluorinated epoxides have been synthesised which, following a cationic cure, give a segregated surface with low free energy (Matuszczak and Feast), and polyesters doped with 1,4-phenylenebis(acrylicacid) undergo (2.n + 2.n) cycloaddition to give a photochemical set (Vargas et aZ.). Sykora et al. have synthesised a functionalised polystyrene which allows the attachment of transition metal complexes such as ruthenium-polypyridine for use as a system for light harvesting energy through electron transfer, and a new photobioreactor incorporating diluted whey as the substrate has been evaluated: it is reported that on sunny days the yield of hydrogen production corresponds to a conversion efficiency of approximately 25% (Modigell and Holle). Sadly, this volume of Photochemistry is the last that will benefit from Alan Cox’s considerable talents as a reporter. Alan has had a long and somewhat varied history of contributions to this series. He joined the team for Volume 10 reporting on the Photochemistry of Transition Metal Complexes to which he added chapters on the Photochemistry of Transition Metal Organometallic Compounds, the Photochemistry of Main Group Elements and Photo-reduction and -oxidation for Volume 14. Alan continued reporting in these four areas up to Volume 22 when the inorganic aspects of photochemistry were dropped from the series. However, in that Volume he also took on the reporting of the Photochemical Aspects of Solar Energy Conversion. Alan continued contributing his two chapters up to and including the present volume but also added the Photochemistry of Aromatic Compounds to his portfolio for Volumes 29-32
10
Photochemistry
inclusive. The depth and breadth of Alan’s reporting over the years reflect his considerable insight across the various areas of science into which Photochemistry has spread over the years. All of us involved in the reporting and production of the volumes of Photochemistry wish Alan well in his ‘retirement’ and I extend my gratitude to him for his precise and concise reporting and for always meeting the deadlines!
Part I Physical Aspects of Photochemistry By Anthony Harriman
Photophysical Processes in Condensed Phases BY ANTHONY HARRIMAN
1
Introduction
This review follows the format adopted in recent years, with minor modification according to the volume of work presented in particular areas. It appears that interest in single-molecule photophysics is less than in previous years but that there has been an increase in the number of publications concerning fullerenes. Several research groups are making serious efforts to design molecular-scale photochemical devices and there has been a tremendous upsurge in interest in the synthesis of dendrimers containing multiple chromophores. No attempt has been made to cover all the literature pertaining to the application of luminescent dyes for the detection of solutes in solution and only a few such highlights are given. There has been a progressive increase in the use of quantum chemistry to gain an improved understanding of photophysical processes and it is clear that such approaches, especially quantum dynamics and molecular dynamics simulations, will make major contributions to photophysics research in the near future. Increased interest has also been shown in intramolecular proton-transfer reactions, since the ultrafast instrumentation often needed to follow such processes is now available. The chapter is organised to cover all important processes leading to the deactivation of an excited state in a condensed phase. Special attention has been given to the various fullerenes because of the exceptionally high interest paid to these compounds over the past few years. Other sections consider theoretical concepts, instrumental methods for monitoring photophysical processes and applications. The huge number of journals now in the market place precludes complete coverage of the subject. 2
General Aspects of Photophysical Processes
Various aspects of excited state behaviour have been reviewed or highlighted during the relevant period. Thus, several general reviews of organic photochemistry have appeared'.*and the importance of luminescencespectroscopy has been ~tressed.~ The photophysics, photochemistry and optical properties of polyimides have been discussed in terms of charge-transfer effects." Related work has illustrated the importance of ultrafast transient spectroscopy for elucidating the Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002 13
14
Photochemistry
primary photophysical processes inherent to tailor-made organic chromophores in s ~ l u t i o nThe . ~ special effects exerted by intense laser pulses have been highlighted6 while the use of the phase of the incident light to establish coupling mechanisms has been r e ~ i e w e dSpecifically, .~ this latter work has examined how the phase of a transition dipole matrix element can be measured by the interefence between competing quantum mechanical paths. The effect of organised media on the photophysical properties of organic molecules has been considered with particular reference to relating dynamics of the probe molecule to the microscopic properties of the host medium.* Light harvesting for solar energy conversion, especially with regard to semiconductorbased solar cells, has been reviewed in a comprehensive fashion? An interesting account'o has been given of n,n* photochemistry for compounds other than aromatic ketones while the effect of ultrasound on the photopinacolisation of benzophenone has been reported." It appears that ultrasound can modify the course of bimolecular processes originating from triplet excited states. Various aspects of photochemical isomerisation have been reviwed, with special attention given to the so-called 'hula-twist' mechanism12and to isomerisation from the triplet excited state.13 The more common singlet state induced photoisomerisation has also been re~iewed.'~ The photophysics of phenyl-substituted polyacetylenes, these being important materials for light-emitting polymeric devices, have been subjected to detailed theoretical e~amination.'~ An important conclusion to emerge from this work is that polyacetylenes display a smaller optical band gap than found for polyenes of the same chain length. Fluorescence excitation spectra have been reported for some organic radicals16 and a new technique, the so-called 'piston source method', has been introduced to measure absolute concentrations of singlet molecular oxygen in s ~ l u t i o n .A '~ review has appeared'* that covers the basic principles involved in the solvation dynamics of triplet excited states in viscous liquids or glassy solids. It appears that there are many cases where the phosphorescence signals are strongly influenced by local dipolar reorientation dynamics and the mechanisms for such effects have been discussed in detail. The photophysical properties of tetrapyrrolic pigments continue to attract attention" and increased interest has been given to deactivation of the upperlying excited singlet states.20-22 The underlying mechanisms whereby light-emitting polymeric devices operate have been and the role of electrontransfer reactions in photoinitiation of polymers has been examined.26Considerable attention has been given to the photophysics of transition metal complexes, especially with respect to metal-to-ligand, charge-transfer excited s t a t e ~ . A ~~-~~ direct o b ~ e v a t i o nhas ~ ~ been made of the charge-transfer-to-solvent reactive mode in the photoexcited alkali metal anion Na-. A theoretical evaluation has been made of photoluminescence from ~ e m i c ~ n d ~ c t o r ~ . ~ ~ The intramolecular magnetic interactions between two nitrosyl nitroxide radicals separated by a thiophene residue have been probed and compared with the corresponding phenylene-linked compound.36Closely-related systems have also considered the photoswitching of intramolecular magnetic interactions in radical-substituted c h r o m o p h ~ r e s Photoinduced .~~~~~ spin states have been re-
I : Photophysical Processes in Condensed Phases
15
ported4' for compounds known to undergo a light-induced phase change and the mechanisms for such magnetic interactions have been considered?l The potential for generating molecular-scale magnets has been highlighted?' has received The use of confined environments, such as zeolites and considerable attention while many aspects of bimolecular photochemistry occurring in crystals have been r e ~ i e w e d ?A~general kinetic model has been proposed to account for the optically and thermally stimulated luminescence observed with samples of pure quartz?' There is continued interest in using luminescent compounds to detect analytes in ~ o l u t i o n . 4Similar ~ - ~ ~ attention has been given to the analytical applications of chemil~minescence.5~ The design of liquid membranes bearing light switches has been highlighted.58In such systems, a liquid membrane is used to separate two different solutes, usually dissolved in aqueous solution. Selective transport across the membrane is facilitated by doping the membrane with a light-activated carrier molecule. The general technique of sonoluminescence has been reviewed, especially with regard to single-bubble sono1uminescence.59-65 A light-drived moleular rotor, capable of unidirectional rotation, has been described.66Other interesting molecular-scale photochemical devices have been constructed from catenanes and r o t a x a n e ~while ~ ~ a fluorescent probe has been reported to mimic the functions of a simple logic gate.68Ways to control the helix content of short peptides by photochemical means have been reviewed69 whilst the design of 'off-on' luminescent systems has received much a t t e n t i ~ n . ~ ' - ~ ~ A reversible molecular shuttle has been produced73where translational motion is controlled by hydrogen bonds. Related molecular switching events have been A review has covered the application of near-field fluorescence imaging to the detection of single pigment molecules using an open-ended Recent years have seen a major initiative made into placing a large number of chromophores in close proximity, primarily to build models for the natural light-harvesting complexes. A variety of approaches have been advocated and the effects of spatial crowding on the photophysical properties of the chromophores have been documented. Thus, the fluorescence properties have been described for nano-sized star-like m01ecules,7~organic-based dendrirner~~'-'~ and doughnut-like assemblies.86A dendrimer has been described that hosts 32 dansyl groupsg7and optically active dendrimers have been synthesised that are capable of modest levels of entioselective fluorescence sensing." Other dendrimers have been reported to display 'off-on' luminescence switching effects in the presence of certain solute^.'^-^^ Photophysical probes for organised assemblies have been described93while artifical light-harvesting arrays have been assembled by way of non-covalent association^.^^-^^ The photophysical properties of large aggregates of tetrapyrrolic pigments have been and the fluorescence behaviour of other nano-sized aggregates has been r e ~ o r d e d . ' ~ - * ' ~ Parallel to the studies devoted to the preparation of photoactive dendrimers, there has been a concerted effort to construct linear molecular-scale wires for future use in molecular opto-electronic devices. Thus, ultrafast energy transfer has been observed to take place in long molecular wires formed from zinc
16
Photochemistry
porphyrins.'@' Related meso,meso-linked porphyrin arrays have been de~ c r i b e d ' and ~ ~ ~the ' ~ use ~ of ethynylene bridges to couple together porphyrinbased chromophores has been highlighted.'07 Related porphyrin-based arrays have been formed"' by fusing adjacent porphyrins at the pyrrole positions. Many different dendrimers have been reported to contain photoactive transition metal-based chromophores.'09-113The emission properties of a molecular rectangle have been de~cribed."~ Great interest has also been shown in the design of novel light-emitting polymeric devices and the photophysical properties of appropriate model compounds and oligomers have been measured. In particular, the importance of interchain exchange effects has been s t r e s ~ e d "while ~ the significance of triplet excited states has been considered.''6 The luminescence properties of highlyconjugated oligomers have been reported with a view to better establishing the mechanism for light emission from the corresponding polymeric d e ~ i c e s . l l ~ - l ~ ~
3
Theoretical and Kinetic Considerations
Theory has always been an integral part of photophysical investigations and the current availability of cheap but powerful computers has greatly aided the detailed examination of experimental data. There is a growing use of quantum chemical calculations to interpret decay kinetics and to explore how the solvent enters into photophysical processes. Experimental verification has been provided for the theoretical prediction of a kinetic transition in a reversible binding reaction driven by the difference in effective lifetimes of bound and unbound species.I2' A hopping model has been proposed to account for thermally stimulated luminescence in disordered organic molecules.129 The model is based on the premise that such emission arises from radiative recombination of long-lived geminate pairs of charge carriers. A theoretical model has been presented that allows determination of the donor-acceptor distribution functions in Forstertype energy fran~fer.'~' Unlike previous approaches to this problem, the new model makes no a priori assumptions about the nature of the distribution and it is reported that the method has particular application to measuring the acceptor distribution in luminescence sensing protocols. The possible role of inversion symmetry in intramolecular vibrational relaxation has been ~onsidered'~' and the dynamics of vibrational motion in electron donor-acceptor complexes has been addressed by ultrafast transient s p e c t r o s ~ o p y . ' ~ ~ The photodynamics of ethylene have been explored by ab initio quantum chemical calculations of the conical interes~tion.'~~ It is reported that the twisted geometry of ethylene corresponds to a saddle point, rather than being a local minimum. Other reports have shown the value of the conical intersection while a theoretical analysis has been made of the absorption spectra and dynamics of photosynthetic reaction centres.136This latter work is based on a microscopic exciton-vibrational model that includes temperature effects and that takes into account the inherent inhomogeneity of the reaction centre complex. An approximate analytical solution has been provided for photochromic
I: Photophysical Processes in Condensed Phases
17
and photorefractive gratings observed with certain mafe~ia1s.l~~ A configuration-interaction description has been given for intersystem crossing and spin-orbit coupling in conjugated polymers.'38An analytical routine has been described for non-linear least-squares fitting of fluorescence quenching data139and a quantum dynamics approach has been applied14' to analyse fractional wave packet behaviour in random phase fluorescence interferometry. Theoretical studies have been used to probe the photophysics and structure of adenine,14' various aromatic amino and 7-a~aindole.l~~ Semi-empirical AM 1 calculations have been used to calculate potential energy surfaces relating to isomerisation of unsymmetrical carbocyanine dyes.144It was found that the isomerisation potential surface was highly dependent on chain length and on the nature and position of the terminal groups. The results of this study also indicated the importance of steric hindrance around the isomerising bond. Related studies have addressed the triplet potential energy surface for hexatriene.I4' Twisting around the C-7-C-6 and C-4-C-7 bonds in coumaric acid has been studied by ab initio M O c a l ~ u l a t i o n swhile ' ~ ~ related calculations have been applied to the problem of photochromiticity in substituted dithienylethene~'~~ and to the photoreactivity of f~1gides.l~~ A theoretical study has considered the mechanism of energy transfer in metal cation-containing crypt ate^'^^ and separate work has focussed on the nature of the Kekule vibration in styrene for the S1 state.'" An ab initio study has considered the mechanism for photoisomerisation of acrylic acid'51 and has shown the importance of the triplet state as a reactive intermediate. Theoretical investigations have explored the spectroscopic properties of charge-transfer complexes'52and have described anharmonic effects in electron-transfer proc e ~ s e s . 'A~ ~computational study has considered the factors that govern the triplet state reactivity of 1 , 4 - ~ e n t a n o n ewhile ' ~ ~ other studies have examined how the fluorescence properties of highly conjugated organic molecules are affected by changes in molecular g e ~ m e t r y . 'The ~ ~ reaction pathway for electrocyclic reactions has been studied by ab initio multistate, second-order perturbation t heory.156 The ground- and excited-state structures of intramolecular donor-acceptor complexes have been examined by DFT calculation^'^^ while large-scale conformational exchange has been studied by molecular dynamics ~ i m u l a t i o n s .The '~~ role of molecular symmetry in intersystem crossing processes perturbed by an external magnetic field has been c o n ~ i d e r e dand ' ~ ~ a dynamical theory has been proposedI6' to account for time-resolved fluorescence spectroscopy. Propagator calculations have been described for the electronic spectra of photochromic spiro-oxazines.161 The ultrafast energy- and electron-transfer reactions occurring in bacterial photosynthetic systems have been explained in terms of a microscopic and contributions of short-distance donor-quencher pairs in intermolecular fluorescence quenching have been ~ 0 n s i d e r e d . lIncorporating ~~ such effects into conventional Rehm-Weller quenching expressions is reported to explain the discrepancies between theory and experiment. The special case of reversible intramolecular energy transfer has been treated in terms of integral encounter theory.164
Photochemistry
18
Certain aspects of photochemical ring-opening reactions have been subjected to theoretical e ~ a m i n a t i o n .The ' ~ ~ question of non-Arrhenius temperature dependencies in electron-transfer processes has received further study.166 Theoretical studies have also addressed the low-lying excited singlet states in styrene,167 reversible intermolecular photochemical processes,168Franck-Condon factors in polyar~matics,'~~ and proton-transfer rea~ti0ns.l~' This latter study used a semiclassical molecular dynamics simulation to construct the relevant potential energy surfaces. Proton transfer was found to be greatly affected by isotopic substitution and to be coupled to internal vibrational modes. Potential energy surfaces have also been constructed for light-induced metal-ligand bond weakening.171 4
Photophysical Processes in Liquid or Solid Media
4.1 Detection of Single Molecules. - The most elegant photophysical processes are undoubtedly those attributed to single or isolated molecules and this type of spectroscopic investigation has been popular for a number of years. The review period has seen little progress in this area, however, and most research has been devoted to looking at isolated molecules on inert surfaces. A technique has been introduced, based on near-field fluorescence imaging, that allows detection of single molecules with a spatial resolution of about 10 n111.l~~ This high resolution is attributed to the onset of non-radiative energy transfer from the fluorescent molecule to the coated metal of the probe. A theoretical investigation has been made for single-molecule fluorescence detection on thin metallic layers using a classical electrodynamics A correlation has been made*74between the fluorescence intermittency and spectral diffusion for single semiconductor quantum dots. A new algorithm has been described for single-molecule identification by time-correlated, single-photon counting techniques.17' Fluorescence correlation spectroscopy has been used to investigate singlemolecule dynamics in thin polymer An analytical approach has been developed to evaluate rate constants for slow conformational exchange using single-moleculefluorescence s p e c t r o s ~ o p y .The ' ~ ~statistics ~ ~ ~ ~ of photobleaching of single dye molecules have been monitored using renewal The method uses a five-state model where bleaching occurs exclusively from the triplet excited state. An exact formulism allows calculation of the distribution of bleaching number and accounts for photostable dye molecules. Enhancement of single-molecule fluorescence under metallic and dielectric tips has been explained'" and linewidth measurements have been made for single molecules dispersed in disordered media.Ig1A study of the dynamics of single latex beads in polyvinyl alcohol films has been made by confocal microscopy's2 and the multistep deactivation of single luminescent conjugated polymers has been des~ribed.''~ A description has been given that accounts for the effects of solutes on single-bubble sonoluminecence.'84The concept of single atom lasers and masers has been introd~ced.''~
I : Photophysical Processes in Condensed Phases
19
4.2 Radiative and Non-radiativeDecay Processes. - A comprehensive correlation has been attempted between the fluorescence properties, the photochemical stability and the lasing properties of aromatic compounds with their molecular symmetry.'86It is reported that the molecular symmetry has a profound effect on the ability of the compound to function as a laser dye, especially with respect to the threshold of laser action. A somewhat related study has compared the optical properties, and the fluorescence quantum yield in particular, to the molecular packing of the compound in a crystal or sublimed film.lB7Other correlations have been made between the molecular vibrational structure and the fluorescence quantum yield for a range of organic The latter results have particular application to light-emitting polymeric devices. The photophysics of certain esters of 4-cyanobenzoic acid have been interpreted in terms of Norrish Type I1 hydrogen atom abstraction reaction^.'^^ The effect of an applied magnetic field on the molecular photophysics of s-triazine has been c~nsidered"~ in terms of promoted intersystem crossing. A statistical approach to the study of singlet-triplet interactions in small polyatomic molecules has been advo~ated.'~' This study made use of surface electron ejection by laser excited metastable species and laser-induced fluorescence spectroscopic techniques. The fluorescence properties of conjugated polyenes in non-polar solvents have been described.192The extraordinary hypercojugation of the methyl group in the S1excited state of 8-methylquinoline has been reported on the basis of red-shifted emission and polariation Fluorescence from porphyrin aggregates present at extremely low concentration has been observed in certain mixed solvents.194The structure and reactivity of 4,4'-bipyridine in the S1 excited state have been addressed by picosecond Raman spectro~copy.'~~ Very fast formation of radical species in methanol was observed to follow laser excitation. Related Raman studies have focussed on models for conducting p01ymers.l~~ The photophysical properties of tryptophan in water,197phenyl-substituted polyacetyl e n e ~and ' ~ ~pyrromethene-BF3dyes'99have been described. The fluorescence radiative lifetime of Rhodamine 6G in a polymeric matrix has been evaluated200and the mechanism for photodegradation of this dye has been considered.201The photophysical properties of poly(4'-ethoxyacrylophenone) have been measured202and the role of intersystem crossing in the deactivation of the singlet excited state of aminofluorenones has been examined.*03While the rate of intersystem crossing in such molecules remains slow and insensitive to the nature of the solvent, it is recognised that internal conversion is both rapid and solvent dependent. Phenyl-substituted terpyridine shows evidence for intramolecular charge transfer under illumination2" while the fluorescence spectra of ketocyanine dyes depend on the nature of the ~olvent."~ Photophysical studies have been reported for some acridine derivatives,206anthracene-based carbonyl compounds207and some asymmetrically substituted ethenes in solution.208An unusual temperature dependence has been reported209 for anthracene in ethanol. The photophysical properties of 4-aminobenzophenone have been revisited:" as has the photochemistry of triacylmethene dyes:' ' Michler's ket one2' and certain benzo thiazoles.2 l 3 Considerable attention has been given to characterising the S2excited states of
20
Photochemistry
large polyatomic molecules, especially metalloporphyrins. Thus, fluorescence from the S2 state of tetraphenylporphyrins has been measured by ultrafast spectroscopy in various The energy gap between the S2 and S1states is somewhat dependent on the nature of the central metal cation and this can influence the lifetime of the upper-lying excited state. Related measurements and pyrene.216 This latter have monitored S2 emission from 1,4-anthraquinone~~'~ study indicates that the lifetime of the S2 state in pyrene is only 150 fs. Pyrene derivatives are also known to form intramolecular excimers217while anthracene undergoes facile photodimerisation?l' The anomalous fluorescence properties of Terylene in a frozen neon matrix have been reported.219Curcumin displays solvent-dependent photophysical properties, possibly due to formation of an intramolecular charge-transfer state?20Rapid decay of the S1state of trans-stilbene, and the effects of vibrational cooling by solvent molecules, has been monitored by picosecond Raman spectroscopy.221The solvation dynamics of Nile Blue in ethanol confined in porous sol-gel glasses have been measured222and the effect of solvent polarisability on the dual fluorescence of 1-phenyl-4-(1-pyrene)-l,3-butadiene has been deIt is seen that emission from a thermally populated upper-lying state disappears at low temperature and in highly polarisable solvents such as carbon disulfide. The general effect of solvent exchange on excited state relaxation processes has been considered224and solvation of acridone has been reported in terms of a microscopic solvation The mechanism for the photoionisation of Methyl Viologen has been addressed on the basis of transient abiorption spectroscopic studies?26The effects of microheterogeneous media on the photophysics of certain dyes continues to be a source of considerable a ~ t i v i t y . Phosphorescence ~~~-~~~ from large aromatic ketones has been described in terms of mixing between nearby n,n* and n,n* excited triplet ~ t a t e s . 2The ~ ~ low-lying excited states of pyridine have been assigned from high-resolution singlet-to-triplet absorption spectroscopy and phosphorescence spectral rnea~urements.~~' Phosphorescence has also been recorded for the chlorotoluenes at low temperat~re.'~~ Photophysical properties have been described for some peripherally metallated porphyra~ines,2~~ vitamin B2:41 tetraki~(4-N-methylpyridinium)porphyrin,2~~ and phe0phorbide-a.2~~ Triplet-triplet annihilation has been observed in some soluble conjugated polymers244and in certain fluoranthrene derivative^.^^' Triplet-triplet annihilation has also been measured for some tetraphenylporphyrins in liquid A description has been given for the triplet states of a series of Pt-containing e t h y n y l e n e ~The . ~ ~ ~photophysical properties of zinc porphyrins in microemulsions have been Hole burning spectroscopy has been applied to the electronic states of coordination compounds in order to probe local The luminescence properties of several new ruthenium(I1) and osmium(I1) polypyridine complexes have been d e s ~ r i b e d ? ~and ~ ? ~the ~ ~efficiency of electrogenerated chemiluminescencefrom such compounds has been related to the corresponding free energy change.2s2
I : Photophysical Processes in Condensed Phases
21
4.3 Amplitude or Torsional Motion. - Light-induced conformational changes can provide a facile way by which to promote internal conversion. These geometry changes may be small, such as slight twisting around a connecting bond, or large-scale, leading to formation of a geometrical isomer. Such processes can be extremely fast and highly efficient means for deactivating the excited state. Because of frictional forces with surrounding solvent molecules, light-induced torsional motion provides a unique opportunity by which to study the structure of the host medium. The photophysical properties of some 2,6distyrylpyridines, and the corresponding hetero-analogues, have been investigated in order to monitor conformational exchange processes.253It has been reported that photoisomerisation of certain chiral azobenzenes leads to enhanced helical twisting capabilities.254The ultrafast barrierless rotation of Fast Auramine 0 has been studied using femtosecond laser relaxation within the excited state manifold occurs by way of torsional diffusion of the phenyl rings. It is believed that this twisting motion involves chargetransfer intera~tions.2~~ A report has been made of the light-induced reorientation of triacylpyrylium cations in solution resulting from excited state twisting and reverse The unusual fluorescence properties noted for 3,4,6-triphenyl-a-pyrones have been attributed to internal rotation of the aromatic rings.258A synthetic strategy has been devised to control the dihedral angle between porphyrin rings in covalently-linked b i s - p ~ r p h y r i n sThe . ~ ~ ~mechanism and reaction dynamics for conformational exchange in non-planar porphyrins have been examined by ultrafast transient spectroscopy.260,261 The role of a transient dipole moment in stabilising intramolecular chargetransfer states in solution has been examined262for Coumarin 440 in solvents of differing polarity. A considerable enhancement of x-electron delocalisation, and a concomitant increase in fluorescence intensity, can be achieved for 1,6diphenyl- 1,3,5-hexatriene by covalent r i g i d i f i ~ a t i o nThe .~~~ importance of internal rotation and intramolecular charge transfer has been stressed for some donor-acceptor carbobazole and for some substituted carbostyr i l ~A. dual-mode ~~~ molecular switch based on a chiral binaphthol compound has been reported.266Several other compounds are believed to undergo internal rotation following promotion to the excited singlet267or triplet268states in solution. A series of donor-acceptor substituted biphenylenes has been proposed as highly selective and sensitive fluorescence probes for monitoring changes in pH.269It is argued that protonation shows some degree of selectivity for the twisted rotamer. Charge recombination within some planar donor-acceptor systems leads to weak emission that can be explained in terms of a simple model based on the magnitude of the electronic coupling matrix elementF7' The importance of internal rotation within the intramolecular charge-transfer state is highlighted. Intramolecular charge transfer, coupled to structural modification, has been described for donor-acceptor substituted b u t a d i e n e ~ ~and ~ l for N-phenylphenanthridinones in solution.272Light-induced intramolecular charge transfer
22
Photochemistry
has been observed for the sodium salt of 4-(N,N-dimethylamino)benzenesulfonate in water but not in organic solvents.273 Considerable attention continues to be given to the so-called twisted intramolecular charge-transfer (TICT) states where internal motion accompanies intramolecular electron transfer, often leading to the appearance of two or more fluorescence bands. Picosecond laser flash photolysis studies have monitored TICT state formation in d e n d r i m e r ~and ~ ~ ~highly-flexible long-chain mole c u l e ~In . ~the ~ ~latter case, coordination of cations to the polyether linker can affect the photophysical properties of the TICT state. Transient infrared spectroscopic studies have provided additional insight into the mechanism leading to TICT state formation in 4-dimethylamin0-4’-nitrostilbene.2~~ The photophysics of 1-dimethylaminonaphthalene in binary solvents have been interpreted in terms of TICT state formation277while the archetypal TICT molecule, 4dimethylaminobenzonitrile, has been shown to undergo light-induced electron transfer with carbon tetra~hloride.2~~ Using fluorescence anisotropy measurements, the mechanism for TICT state formation in this prototypic molecule has been studied It is reported that the TICT mechanism gives a better representation of the experimental data that obtained with alternative models based on planar ICT states or having the cyano group undergo bending modes. A comprehensive study has addressed the concept of internal conversion of 3,5-dimethyl-4-(methylamino)benzonitrilein alkane solvents.280Intramolecular charge transfer has been reported for a set of pyrene-2,2’-bipyridine-based dyads281and for various substituted 1,2-diar~lethenes.2~~ Time-resolved fluorescence studies have been used to follow TICT state formation in aminostyryl pyridinium dyes in both homogeneous solution and microheterogeneous media.283Incorporating the molecule into the cavity of P-cyclodextrins perturbs TICT state formation284while the effects of added polymers on TICT-forming molecules have been reviewed.28s The excited-state Raman spectrum has been recorded for trans-stilbene and used to discuss vibrational relaxation within the S1state.286Similar studies have been applied to the corresponding cis-isomer, where it is seen that there is an unusually high intensity of low-frequency bands for the S1 The significance of the rneta-effect in controlling the photophysical properties of donoracceptor substituted trans-stilbenes has been c ~ n s i d e r e d .The ~ ~ ~dynamics , ~ ~ ~ for photoisomerisation of 4-(methano1)stilbene have been compared with the Kramers-Hubbard and it is concluded that the barrier to isomerisation decreases with increasing solvent polarity. A range of novel stilbenes displaying relatively high fluorescence yields has been ~ynthesised~~l and light-induced isomerisation has been described in some multiply substituted a l k e n e ~ . ~ ~ ~ The effect of macromolecular isomerisation on the photomodulation of dendrimer properties has been reported for some azobenzene-subsituted photosysIntramolecular hydrogen bonding can affect the rate of light-induced i s ~ m e r i s a t i o nwhile ~ ~ ~the photoisomerisation of certain cyclic olefins occurs via both singlet and triplet excited with the possible involvement of chargetransfer effects.296The photophysical properties have been recorded for linear p0lyenes,2~~ substituted b ~ t a d i e n e sand ~ ~ ~h e x a t r i e n e ~ and ~ ~ ~simple , ~ ~ poly-
I : Photophysical Processes in Condensed Phases
23
e n e ~ . ~The ” importance of light-induced isomerisation has been stressed in each case. Evidence has been presented for the involvement of neutral soliton pairs in the relaxation pathways of photoexcited p ~ l y e n e s . ~ ’ ~ Photoisomerisation of azobenzene derivatives has been shown to take place at the air/water interface303while a solvent isotope effect has been reported for the rate of isomerisation of an azo dye.304It is known that azobenzene undergoes light-induced isomerisation when included inside the cavity of a zeolite305whereas related studies have reported on the photochemical contol of the microstructure of cholesteric liquid crystals functionalised with azobenzene residues.306
4.4 Light-induced Proton-transfer Reactions. - Light-induced proton transfer reactions, which are often extremely fast, have been known for a long time but it is only recently, with the availability of ultrafast laser spectroscopic tools, that the dynamics of such processes have been resolved. Excited-state proton-transfer reactions have been reported for some substituted naphthols in lipo~omes,~’~ l,l’-binaphthyL4,4’-diol in various solvents and as a function of pH308 and 2,2’-bipyridyl-3,3’-diol in polymeric media.309The latter system is intended as a UV stabiliser for plastics. A report has appeared of the ground- and excited-state reactivity of 2,2’and 4,4’-biphenyldiols with a range of proton acceptor^.^" A theoretical evaluation has been made of light-induced proton transfer and internal motion in l-hydro~y-2-acetophenone?~~ The photochemistry and photophysics of other a-hydroxy ketones has been reviewed3I2and the excitedstate proton transfer reactions of 2,5-diphenyl-1,3,4-oxadiazole have been reported.313Photochemical proton-transfer reactions have been recorded for a variety of other organic c o m p ~ u n d s . ~All ~ ~ the - ~ ~above-mentioned * photosystems involve proton transfer from the S1state of the chromophore but it has been reported3I9 that hypericin in a micellar dispersion undergoes proton transfer from the lowest-energy triplet state. The mechanism for rapid photoacid-base reactions has been assessed by ultrafast transient spectroscopy.320 The results have been interpreted in terms of a diffusion model that allows for electrostatic interactions and distinguishes between H20 and D20. Competition between inter- and intra-molecular proton transfer has been observed for some 2-hydroxy derivatives of 2,5-diphenyl-1,2oxazole in media of varying acidity.321The rapid proton transfer from pyranine to water has been followed, with several transient species being identified before proton transfer occurs.322 Photoinduced tautomerism has been monitored in several molecule^.^^^-^^^ Such processes are necessaily fast and difficult to resolve from other inherent deactivation pathways. Intramolecular proton transfer has been described for Light-induced S-H the anionic form of 2-(2’-acetamidophenyl)benzimidazole.328 bond cleavage has been detected by laser flash photolysis techniques for some substituted thiones in non-polar Light-induced proton transfer has been observed for a variety of hydrogen-bonded c ~ m p l e x e s . ~ ~while ’ - ~ ~the ~ nature of the solvent exerts a strong effect on the rate of intramolecular proton Ab initio calculations have been transfer in hydroxy-substituted flavothione~.~~’ used to augment laser spectroscopic detection of proton transfer in 3-hydroxy-2-
24
Photochemistry
naphthoic Excited-state double proton transfer has been observed for certain pyrimidines.337 4.5 Quenching of Excited States. - Various aspects of the interaction between an excited state and molecular oxygen have been considered. In particular, the mechanism for photodegradation of certain cyanine and merocyanine dyes has been explored.338A new method has been proposed to determine the rate constant for quenching of a long-lived triplet excited state by oxygen, based on the time-resolved measurement of the resultant singlet molecular oxygen.339 The factors that combine to control the efficiency of singlet molecular oxygen sensitisation by expanded porphyrins have been explored340and the involvement of charge-transfer interactions in oxygen quenching of triplet naphthalene derivatives has been appraised.341A separate study has considered the fluorescence quenching by oxygen of 9,lO-dimethylanthracene in liquid solution and in supercritical The effect of a heavy atom on the photophysical properties of various classes of compound has been studied with a view to promoting triplet state f 0 r m a t i o n . 3 ~ ~ - ~ ~ ~ There have been several reports of how covalently-attached stable organic radicals interact with excited ~ t a t e ~The . ~ most ~ ~ popular - ~ ~ ~ radical has been TEMPO and it has been shown that the lifetime of the excited state depends markedly on the number of attached radicals. The lifetimes of both singlet and triplet excited states are perturbed by the radical centres and it appears that the magnetic properties of the overall molecule is influenced by excitation.348 Quenching of room-temperature phosphorescence of polycyclic aromatic compounds has been observed350while hydrogen atom abstraction by triplet excited states is a common phenomenon.351A comprehensive study of the exciplex formation mechanism, often known as the Rehm-Weller model, has been made.352The mechanism for enhanced intersystem crossing in certain gable-type bis-porphyrins has been considered in terms of a through-bond, spin-orbit coupling interaction.353It is reported that the lifetime of the free-base porphyrin subunit is markedly dependent on the geometry and nature of the connecting spacer residue. It has been reported354that the rate of energy transfer can be controlled by selective protonation of one of the reactants. Ultrafast electronic energy transfer has been reported to take place in linear and crossed porphyrin arrays355while energy migration and subsequent trapping have been detected in a polymer Related work has examined the photophysics and energy-transfer reactions of 9,10-diphenylanthracene in solution.357 Investigations have been carried out to probe the conformations of tethered poly(ethy1eneglycol) chains anchored on polystyrene latex particles using fluorescence energy transfer to establish the distance between donor and acceptor species.358 A newly developed semi-empirical method has been applied to the fast energy-transfer steps occurring in photosynthetic purple bacteria.359It is reported that the method gives a good representation of exciton interactions. The concept of photoswitching of intramolecular charge and energy transfer has been discussed in terms of donor-spacer-acceptor tripartite s y ~ y e m sThe . ~ ~switching ~ function was achieved by incorporating optically bistable photochromic units
I : Photophysical Processes in Condensed Phases
25
into the spacer. Efficient energy- and electron-transfer processes have been reported for certain bis-p~rphyrins~~' and for a range of naphthalene d i i m i d e ~ . ~ ~ ~ The development of stable dye-injection solar cells requires the identification of appropriate sensitisers and much work has centred on the use of transition metal polypyridine complexes as the photoactive electron donor. However, ultrafast charge injection into the semiconductor particles has been monitored with coumarin s e n s i t i ~ e r sRapid . ~ ~ ~ interfacial electron transfer has been reported to occur from both singlet and triplet excited states of certain ruthenium(I1) complexes.364The latter report is a rare example of photoreactivity from the singlet excited state of a transition metal complex. 4.5.1 Energy-transfer Reactions. Intramolecular singlet energy transfer has been reported for a series of coumarin-based molecular dyads included into p-cyclod e ~ t r i n sAn . ~ ~experimental ~ protocol has been devised to ensure the onset of room-temperature phosphorescence generated during triplet-triplet energy transfer between dyes and polycyclic aromatic hydrocarbons solubilised in anionic m i ~ e l l e s Intermolecular .~~~ energy transfer has been detected between selected laser dyes and Rhodamine l10.367Triplet energy transfer has been observed between hydrogen-bonded reactants,368although the inherent flexibility of the tethers prevents a detailed mechanistic study. Energy transfer has also been detected within the geminate radical pair formed by light-induced charge ~ lsolids374 - ~ ~ ~ and in transfer:69 within mono layer^:^^ in polymeric m a t r i ~ e s , ~in certain pure Triplet-triplet energy transfer in various transition metal polypyridine complexes has been reviewed.376A particularly efficient conduit for Dexter-type electron exchange seems to be acetylene-based bridges and long-range triplet energy transfer has been achieved with such linkers.377Here, electron exchange has been detected over distances in excess of 50 A. Much less efficient throughbond electron exchange occurs across spiro-based bridges but through-space triplet energy transfer has been detected in such molecular dyads.37sThe synthesis of putative porphyrin-based dyads has been reported379and ways to achieve structural control over the direction and dynamics of energy transfer in porphyrinic arrays have been discussed.380A possible two-step triplet-energy transfer process has been described.381Porphyrin-based models for the natural light-harvesting antenna continue to attact attention382and artificial arrays capable of establishing a cascade of energy-transfer steps are now available.383 4.5.2 Electron-transfer Reactions. Research into light-induced electron-transfer
processes continues to be highly popular and there have been numerous attempts to employ such reactions for the engineering of photochemical molecular-scale devices. Most work has been carried out in fluid solution. By measuring the changes in enthalpy, entropy and volume that accompany electron transfer in fluid solution it has been concluded that the size of the reactants has only a modest effect on the efficiency of the process.384This finding is in apparent contradiction to earlier work carried out with charge-transfer complexes but the discrepancy might relate to the nature of the reactants used in the various
26
Photochemistry
experimental studies. A separate approach has considered the importance of stereochemical factors in bimolecular electron-transfer reactions.385Charge transfer between strongly-coupled redox partners is often accompanied by exciplex formation, with the reaction following non-Marcus b e h a ~ i o u rA. ~detailed ~~ analysis of the kinetic factors associated with diffusional electron-transfer quenching has been made387with a view to evaluating the size of the electronic coupling matrix element for such reactions. Several experimental studies have examined the role of aliphatic and aromatic amines as qunchers of various excited states in polar solvent^.^^^-^^^ In most cases, evidence for light-induced charge transfer has been obtained by laser flash photolysis and kinetic parameters for both forward and reverse steps have been elucidated. The primary intermediates formed by electron transfer from 1methylcytosine to anthraq~inone-2~6-disulfonate in water have been monitored by FT-EPR spectroscopy.393Novel photosensitisers have been tested394>395 for their ability to operate as electron-transfer promoters in biological systems and it has been noted that triplet 1-nitronaphthalene is able to oxidise trans-stilbene in polar solvents.396The same system undergoes light-induced excitation energy transfer in non-polar solvents. Ultrafast anisotropy measurements indicate a complicated mechanism for light-induced electron transfer with ruthenium(I1) tris(2,2'-bipyridine) in nitrile solvents due to diffusive solvation dynamics.397 The regioselectivity of photoinduced electron-transfer reactions involving unsymmetrical phthalimides is controlled by the spin density distribution of the intermediate radical anions.398Electron transfer to the triplet excited state of 10-methylphenothiazines is influenced by an applied magnetic field.399The involvement of fluorescent Lewis acid-base exciplexes and triplexes has been demonstrated for numerous types of redox pairs in solution.400Light-induced electron transfer across the interface between two immiscible liquids has been reported4" and related to ion transport across the interface. It has been reported that bond cleavage triggered by electron transfer may follow either a stepwise or a concerted mechanism:'* It is well documented that the nature of the solvent can exert a powerful effect on the outcome of electrontransfer especially when the solvent plays a direct role in charge transfer.404In fact, a microscopic model involving translational and rotational motion of the solvent molecules has been developed to account for rapid electron transfer from N,N-dimethylaniline to oxazine when the latter is dissolved in the former. The quantum yield of radical ion pairs formed by light-induced electron transfer has been measured by transient photoconductivity studies4'' and related work has addressed several aspects of charge recombination within geminate radical ion pairs.406The effect of solution viscosity on the efficiency of bimolecular electron-transfer reactions has been considered407and ways to control electron transfer using hydrogen bonds have been c o n ~ i d e r e d . 4 Charge-transfer '~~~~~ processes have been monitored for ion pairs4" and in conjugated polymer^.^"-^^^ Directed electron transfer has been demonstrated in elaborate catenanes4I4 and rotaxanes,415intended as models for the photosynthetic bacterial reaction centre complex. The catalytic effect of molecular oxygen on the rate of intramolecular electron transfer has been shown for a porphyrin-fullerene molecular
I : Photophysical Processes in Condensed Phases
27
dyad?16 Several studies have indicated that the selective coordination of cations can influence the dynamics of electron transfer between remote redox partners.417-419 Other studies have monitored light-induced electron transfer in porphyrin-based molecular dyads and triads.420Various aspects of intramolecular charge transfer have been explored by reference to specific, flexibly-linked It is often difficult to resolve the intimate reaction donor-acceptor dyads.421-424 mechanism in such systems, because of competing diffusion, but strong indications for through-space electron transfer have been observed with carefully designed U-shaped dyads having varying bite angles."25 In order to monitor the effects of through-bond electron transfer is is usually necessary to study rigidly-linked donor-acceptor systems and this has proved to be a rich research field.426-429 Charge recombination within such dyads can result in formation of the corresponding triplet state with an uncommon spin polarisat i ~ n . ~Orientational ~' effects have been reported to be important for lightinduced charge transfer between closely-spaced while other work has reported a novel double-electron transfer in certain donor-bridge-acceptor dyads.432Attaching the chromophores to a polymeric support might lead to the isolation of model compounds able to mimic some of the essential features of the photosynthetic a p p a r a t ~ s . 4 ~ ~ Although most work in this field has centred around the use of porphyrinbased chromophores there has been a parallel effort to design molecular dyads and triads around ruthenium(I1)p0lypyridines.4~~ Indeed, such complexes can be used to drive a wide variety of electron-exchange reactions leading to longdistance triplet energy transfer along rigid spacer^.^^^,^^^ A ruthenium(I1bmanganese(I1) mixed-metal binuclear complex has been proposed437as a model for the oxygen-evolving catalyst present in green plant photosynthetic organisms. Several systems have been designed to undergo the photoswitching of electron t r a n ~ f e r ? ~Usually, ~ - ~ such systems are designed such that a conformational exchange can be promoted by selective excitation or coordination and where the two conformers display markedly disparate rates of electron transfer. This is a rapidly expanding area of electron-transfer research, driven by the need to identify appropriate components for use in molecular-scale opto-electronics. Photophysics of Fullerenes. - Research into the photochemistry and photophysics of the various fullerenes continues unabated, aided by the proliferation of specialised journals, and there have been numerous attempts to include such materials in virtually every kind of photosystem. Although the photophysical properties of the basic clusters are now well established, fullerenes have been functionalised in such a way as to make them attractive components in LEDs and in artificial photosynthetic devices, where their unusually low reorganisation energy provides important benefits. Recent advances in the photophysics of fullerenes have been highlighted,441i442 and high-resolution fluorescence spectra have been recorded for c 6 0 in toluene at 5 K.443Thermally-activated processes contributing to the overall excited-state properties of fullerenes have been rev i e ~ e d ,and ~ . laser ~ ~ flash photolysis studies have been reported for fine particles of c 6 0 prepared by re-precipitation techniquesM6Separate reports deal 4.6
28
Photochemistry
with the photochemistry of the higher fullerenesM7and with the photophysics of ring-opened c 6 0 derivati~es.4~~ Using argon ion laser excitation, quantum yields for photodegradation and singlet molecular oxygen production have been meas~ ~ photoured for solutions of c60, c70, c76 and Cg4at room t e m p e r a t ~ r e . 4The physical and photochemical properties of C1200,a dimer of c60 linked through a saturated furan ring, have been rep0rted.4~' Singlet oxygen generation via fullerene-based sensitisers has been described in The temperature-dependent fluorescence properties of phenylated and chlorinated c 7 0 have been described454and the photophysical properties of multiphenylated derivatives of c 7 0 have been rec0rded.4~~ Related studies, including laser flash photolysis, fluorescence and phosphorescence spectroscopy and kinetic measurements, have concentrated on a mono-ben~yne-C~~ a d d ~ c tSome .~~~ unusual luminescence properties have been reported457for hexapyrrolidine derivatives of c60 and the photophysical properties of several methano derivatives of c 6 0 have been Other work has addressed the photophysics of cisand trans-stilbenomethano f~llerenes,"~~ carborane-functionalised fullerenes,460 and a c 7 0 derivative equipped with a crown ether linkage.461Several reports have been directed towards exploring the photophysics of fullerenes covalently linked to unsaturated c o m p o ~ n d s . 4Because ~ ~ ~ ~of ~ ~on-going interest in using fullerenes in conjunction with conducting polymers, or light-emitting diodes, much research has focussed on attaching fullerenes to 0ligo-thiophenes,4~-~~~ oligop h e n y l e n e ~ i n y l e n e and s ~ ~tetra-thiaf~lvalenes.4~~ ~~~~~ The role of c 6 0 adducts in light-induced electron-transfer reactions has been reviewed473and numerous energy- and electron-transfer processes driven by triplet c60 have been described in ~ o 1 ~ t i and o n organised ~ ~ ~ ~ media.476 ~ ~ ~ Lightinduced reduction of fullerene derivatives by amines has been c o n ~ i d e r e d ~ ~ ~ - ~ ~ ' and other electron-transfer processes have been reported between c60 and various redox-active reagent^?^^-^^^ Intramolecular light-induced energy and/or electron transfer has been described for a wide variety of fullerene-based dyads!86-494 In most cases the course of reaction has been followed by laser flash photolysis techniques and rates of forward and reverse transfer steps have been evaluated. The most interesting, and most intensely studied, molecular dyads are those comprising fullerene and porphyrin terminals and several such systems have been reported during the current review ~ e r i o d . 4 The ~ ~ ~low ~ ' reorganisation ~ energy associated with oneelectron reduction of c 6 0 means that fast rates of charge separation can be realised at modest thermodynamic driving forces while charge recombination falls within the Marcus inverted region, and is therefore relatively slow. Attaching additional redox-active subunits has allowed extension to form molecular triads displaying long-lived charge-separated state^.^'^.^'^ Again, tetrapyrrolic pigments are the most popular chromophores for use in such systems and the fullerene residue serves as the primary electron acceptor. Interest is growing in the use of fullerene derivatives to form self-assembed supramolecular ensembles. Several such assemblies have been formed recently and their photophysical properties recorded.s08 Likewise, functionalised fullerenes have been incorporated into films.509~510 These latter systems have
I : Photophysical Processes in Condensed Phases
29
genuine opportunities to be built into photochemical devices where some kind of cooperative orientation is essential.
5
Applications of Photophysics
The study of photophysics, especially time-resolved fluorescence spectroscopy, provides unique opportunities to explore complex molecular systems, to selectively transfer information at the molecular level, to label biological materials, and to design new analytical protocols. Perhaps the most popular applications of photophysics concerns the selective detection of analytes and measurement of the fluidity, polarity, electric surface, effective dielectric constant or composition of microheterogeneous media. Here, we mention only a few such applications. Thus, the analytical applications offered by using cyclodextrins to induce roomtemperature phosphorescence have been reviewed.511Fluorescence anisotropy can be used as a measure of chiral r e ~ o g n i t i o nand ~ ' ~ novel fluorescence anisotropy tools have been developed to monitor liquid crystals513and to estimate microvis~osity.~'~ Luminescence techniques have also been developed to follow the entire range of surfactant aggregation in aqueous Fluorescence microscopy has been applied to the problem of monitoring the concentration of oxygen dissolved in polymer r n a t r i c e ~ .Fluorescence ~'~ quenching techniques are also available to follow radical-induced cross-linking of monomers.517A protocol based on the re-absorption of laser-induced fluorescence has been adapted to measure film Finally, a method has to measure the size distribution of colloidal particles by using been the well-known photofading and subsequent recovery stategy.
6
Advances in Instrument Design and Utilisation
Photophysics research depends critically on the availability of appropriate instrumentation and adequate computational protocols. To a large degree, progress in the field is limited by new developments in the type and scope of instrumentation, but the importance of a steady supply of pure and tailor-made molecules must never be underestimated. Improvements in the precision with which conventional measurements can be made and the opportunities to undertake new types of photophysical investigation continue to be reported; not all are expensive or subject to the simultaneous use of several sophisticated lasers.
6.1 Data Analysis. - Improved methods have been proposed for the analysis of fluorescence a n i ~ o t r o p y , ~fluorescence ~' decay kinetics,521solvation dynam i c and ~ ~ fluorescence ~ ~ quenching in the presence of high concentrations of q ~ e n c h e r .New ' ~ ~ treatments have also been given for the analysis of kinetic data, especially non-exponential decay p r o c e s s e ~ . A ~ ~direct ~ - ~ observation ~~ has been made of non-RRKM behaviour in femtosecond laser spectroscopic and improved modelling of ultrafast photophysical processes has been re-
30
Photochemistry
ported?28An experimental protocol has been established that allows estimation of fluorescence quantum yields for heterogeneous samples529while ways to determine rate constants from time-gated fluorescence correlation spectroscopy have been highlighted?30Related studies have proposed improved routines for monitoring energy transfer by luminescence t e c h n i q ~ e s ~and ~ l . ~correcting ~~ background signals in Raman spectra533and in 2D fluorescence measurement s.534 A kinetic treatment has been given to account for triplet-triplet annihilation in disordered media.535The importance of a detailed analysis of fluorescence excitation spectra has been stressed536and an absolute calibration of laser-induced fluorescence can be obtained from optical depth analysis.537Extraction of parameters from time-resolved fluorescence specroscopy has been considered538and new applications have been found for fluorescence polarisation techniques.539 Two-photon fluorescence excitation spectra have been r e c ~ r d e dand ~ ~a ~ ~ ~ ~ ~ robust local regression procedure has been introduced for baseline subtract i ~ nAnalytical . ~ ~ ~ models have been presented for fluorescence correlation spect r o ~ c o p yscanning-fit ,~~~ analysis of fs spectroscopic data,544to account for noise on fluorescence correlation data545and for detailed analysis of single-photon counting results.546 6.2 Instrumentation. - Several aspects of the instrumentation used in photophysics research have been reviewed during the relevant period. Thus, the types of instrumentation used for direct observation of transient species have been described,547the technique of time-resolved infrared spectroscopy has been rethe applications of ultrafast transient grating spectroscopy have been highlighted549and the multifarious applications of time-resolved EPR spectroscopy to supramolecular chemistry have been described in Several reports have concentrated on the development and use of the optical Kerr-gate effect for femtosecond time-resolved luminescence s p e c t r ~ s c o p y .Other ~ ~ ~ -ap~~~ proaches have been used to record femtosecond l u m i n e ~ c e n c and e ~ ~infrared ~~~~~ spectra557while a description has been given of up-conversion spectroscopy using square-wave excitation A near-field fluorescence microscope with a spatial resolution of about 100 nm has been described559and a set-up having somewhat improved spatial resolution, achieved using the technique of near-field shadowing, has been reported.560The design of a rapid-scanning, spectrally-resolved fluorescence microscope has been provided561while other studies have led to the development of a detector for time-resolved emission working at wavelengths greater than 1500 nm.562,563 The advantages of two-colour excitation fluorescence microscopy have been highlighted.564It is clear that fluorescence correlation spectroscopy is gaining popularity and recent advances in this area have been r e p ~ r t e d . ~ ~ ~ , ~ ~ ~ fields to perturb The application of strong e l e ~ t r i c a lor ~~~?~~~ photophysical properties continues to provide valuable information about the processes under investigation. The technique of rotational coherence spectroscopy has been described572and a critical comparison has been made of the optical geometries needed for combined flash photolysis and total internal
I : Photophysical Processes in Condensed Phases
31
reflectance fluorescence microscopy.573Techniques for use in nanometre-resolved 2D photochemistry have been discussed574and a review has reported on tunable picosecond optical parametric amplifiers for time-resolved Raman spect r o ~ c o p yThe . ~ ~ application ~ of photoacoustic calorimetry for following photoisomerisation has been highlighted576while the construction of a 40 ns timeresolved, step-scan FTIR instrument has been
7 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
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474, D. M. Martino and H. Van Willigen, J . Phys. Chem. A, 2000,104,10701. 475. 0.Ito, T. Konishi and M. Fujitsuka, Proc. - Electrochem. SOC.,2000,8 (Fullerenes: Electrochemistry and Photochemistry), 140.. 476. H. Imahori, S. Fukuzumi, H. Yamada, H. Norieda, Y. Sakata, Y. Nishimura, I. Yamazaki, M. Fujitsuka and 0. Ito, Proc. - Electrochem. SOC.,2000,8 (Fullerenes: Electrochemistry and Photochemistry), 79. 477. Q. J. Li, Q. J. Gong, L. M. Du and W. J. Jin, Spectrochim. Acta, Part A, 2001,57, 17. 478. J. Sun, Y. Liu, D. Chen and Q. Zhang, J . Phys. Chem. Solids, 2000,61,1149. 479. S. Komamine, M. Fujitsuka, 0.Ito, K. Moriwaki, T. Miyata and T. Ohno, J . Phys. Chem. A, 2000,104,11497. 480. N. Martin, L. Sanchez, B. Illeescas, S. Gonzalez, M. Angeles Herranz and D. M. Guldi, Carbon, 2000,38, 1577. 481. V. Brezova, D. Dvoranova, P. Rapta and A. Stasko, Spectrochim. Acta, Part A , 2000,56,2729. 482. T. Konishi, M. Fujitsuka, 0.Ito, Y. Toba and Y. Usui, Bull. Chem. SOC.Jpn., 2001, 74, 39. 483. A. Masuhara, M. Fujitsuka and 0.Ito, Bull. Chem. SOC.Jpn., 2000,73,2199. 484. B. Komamine, M. Fujitsuka, 0.Ito and A. Itaya, J. Photochem. Photobiol. A, 2000, 135, 111. 485. V. Brezova, M. Gembicka and A. Stasko, Fullerene Sci. Technol., 2000,8,225. 486. H. Imahori, K. Tamaki, H. Yamada, K. Yamada, Y. Sakata, Y. Nishimura, I. Yamazaki, M. Fujitsuka and 0.Ito, Carbon, 2000,38,1599. 487. J.-F. Nierengarten, J.-F. Eckert, D. Felder, J.-F.Nicoud, N. Armaroli, G. Marconi, V. Vicinelli, C. Boudon, J.-P. Gisselbrecht and M. Gross, Carbon, 2000,38,1587. 488. B. Jing, D. Zhang and D. Zhu, Tetrahedron Lett., 2000,41,8559. 489. D. M. Guldi, M. Maggini, N. Martin and M. Prato, Carbon, 2000,38,1615. 490. M. Diekers, A. Hirsch, C. Luo, D. M. Guldi, K. Bauer and U. Nickel, Org. Lett., 2000,2,2741. 491. G. Torres-Garcia, D. M. Guldi and J. Mattay, J . Ink Rec., 2000,25,273. 492. I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl and B. J. Schwartz, Chem. Phys. Lett., 2000,327,253. 493. S. Nath, D. K. Palit and A. V. Sapre, Chem. Phys. Lett., 2000,330,255. 494. 0. Ito, M. Yamazaki and M. Fujitsuka, Proc. - Electrochem. Soc., 2000, 8 (Fullerenes:Electrochemistry and Photochemistry), 306. 495. D. I. Schuster, Carbon, 2000,38,1607. 496. H. Imahori, S. Fukuzumi, K. Tamaki, K. Yamada and Y. Sakata, Proc. - Electrochem. SOC.,2000,9 (Fullerenes:Functionalised Fullerenes), 60. 497. 0.Kutski, M. Wedel, F. P. Montforte, S. Smirnov, S. Cosnier and A. Walter, Proc. Electrochem. SOC.,2000,8 (Fullerenes:Electrochemistry and Photochemistry), 172. 498. N. Armaroli, G. Marconi, L. Eschegoyen, J.-P. Bourgeois and F. Diederich, Proc. Electrochem. SOC.,2000,9 (Fullerenes:Functionalised Fullerenes), 92. 499. T. Da Ros, M. Prato, D. M. Guldi, M. Rizzi and L. Pasimeni, Chem.: Eur. J., 2001,7, 816. 500. S. MacMahon, S. R. Wilson and D. I. Schuster, Proc. - Electrochem. Soc., 2000,S (Fullerenes: Electrochemistry and Photochemistry), 155. 501. F. P. Montforte and 0.Kutski, Angew. Chem., Int. Ed., 2000,39,599. 502. N. V. Tkachenko, A. Y. Tauber, V. Vehmanen, A. A. Alekseev, P. H. Hynninen and H. Lemmetyinen, Proc. - Electrochem. SOC., 2000, 8 (Fullerenes: Electrochemistry and Photochemistry), 161.
I: Photophysical Processes in Condensed Phases
47
503. A. Ikeda, M. Kawaguchi, Y. Suzuki, T. Hatano, M. Numato, S. Shinkai, A. Ohta and M. Aratono, J . Inclusion Phenom. Macrocycl. Chem., 2000,38,163. 504. N. Armaroli, G. Marconi, L. Eschegoyen, J.-P. Bourgeois and F. Diederich, Chem.: Eur. J., 2000,6, 1629. 505. H. Imahori, M. E. El-Khouly, M. Fujitsuka, 0. Ito, Y. Sakata and S. Fukuzumi, J . Phys. Chem. A, 2001,105,325. 506. H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, 0.Ito, Y. Sakata and S. Fukuzumi, J . Am. Chem. Soc., 2001,123,2607. 507. J. L. Bahr, D. Kuciauskas, P. A. Liddel, A. L. Moore, T. A. Moore and D. Gust, Photochem. Photobiol., 2000,72, 598. 508. G. Deviprasad, M. E. Zandler and F. D’Souza, Proc. - Electrochem. SOC.,2000,8 (Fullerenes:Electrochemistry and Photochemistry), 182. 509. K. Noworyta, E. P. Krinichnaya, W. Kutner, P. M. Smith, G. Deviprasad and F. D’Souza, Proc. - Electrochem. Soc., 2000,S (Fullerenes:Electrochemistry and Photochemistry), 54. 510. D. M. Guldi, C. Luo, M. Maggini, M. Enzo, S. Mondini, N. A. Kotov and M. Prato, Proc. - Electrochem. SOC.,2000,s (Fullerenes: Electrochemistry and Photochemistry), 202. 511. A. M. de la Pena, M. C. Mahedero and A. B. Sanchez, Analusis, 2000,28,670. 512. M. E. McCarroll, F. H. Billiot and I. M. Warner, J . Am. Chem. SOC.,2001,123,8173. 513. W. J. Joo, H. D. Shin, C. H. Oh, S. H. Song, P. S. Kim, B. S. KO and Y. K. Han, J . Chem. Phys., 2000,113,8848. 514. R. Pramanik, P. Jumar Das and S. Bagchi, Phys. Chem. Chem. Phys., 2000,2,4307. 515. M. M. da Garca, Adv. Collid. Interface Sci., 2001,89, 1. 516. K. A. Kneas, J. N. Demas, B. A. DeGraff and A. Periasamy, Microsc. Microanal., 2000,6, 551. 517. 0.Pekcan, D. Kaya and M. Erdogan, J . Appl. Polym. Sci., 2001,80,1907, 518. C. R. Hidrovo and D. P. Hart, Meas. Sci. Technol., 2001,12,467. 519. B. Fong, W. Stryjewski and P. S. Russo, J . Collid. Interface Sci., 2001,239,374, 520. H. J. Egelhaaf, L. Luer, A. Tompert, P. Bauerle, K. Mullen and D. Oelkrug, Synth. Met., 2000,155,63. 521. V. V. Apanasovich, E. G. Novikov and N. N. Yatskov, J . Appl. Spectrosc., 2000,67, 842. 522. R. Argaman, T. Molotsky and D. Huppert, J . Phys. Chem. A, 2000,104,7934. 523. D. T. Cramb and S. C. Beck, J . Photochem. Photobiol. A , 2000,134,87. 524. V. Capek, Czeck J . Phys., 2001,51,513. 525. M. Wen and A. V. McCormick, Macromolecules, 2000,33,9247. 526. A. J. Garcia-Adeva and D. L. Huber, J . Lumin., 2000,92, 65. 527. I. R. Lee, W. K. Chen, Y. C. Chung and F. Y. Cheng, J . Phys. Chem. A, 2000,104, 10595. 528. T. Palszegi, V. Szoca, M. Breza and V. Lukes, NATO Sci. Ser., 2000,79,139. 529. M. Corboz, I. Alxneit, G. Stoll and H. R. Tschudi, J . Phys. Chem. B, 2000, 104, 10569. 530. D. C. Lamb, A. Schenk, C. Rocker and G. U. Nienhaus, J . Phys. Org. Chem., 2000, 13,654. 531. T. Heyduk and E. Heyduk, Anal. Biochem., 2001,289,60. 532. W. P. Partridge and N. M. Laurendeau, Appl. Phys. B: Laser Opt., 2000,71,237. 533. M. J. Pelletier and R. Altkorn, Appl. Spectrosc., 2000,54,1837. 534. H. Malm, G. Sparr, J. Holt and C. F. Kaminski, J . Opt. SOC.Am. A , 2000,17,2148. 535. S. A. Bagnich and A. V. Ronash, Chem. Phys., 2001,263,101.
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Photochemistry
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I : Photophysical Processes in Condensed Phases
49
571. T. C. Yang, D. J. Sloop, S. I. Weissman and T. S. Lin, Chem. Phys. Lett., 2000,331, 489. 572. A. Weichert, C. Riehn and B. Brutschy, J . Chem. Phys., 2000,113,7830. 573. P. B. Conibear and C. R. Bagshaw, J . Microsc., 2000,200,218. 574. S. De Feyter, J. Hofkens, M. Van der Auweraer, R. J. M. Nolte, K. Mullen and F. C. De Schryver, Chem. Commun., 2001,585. 575. M. Towrie, G. Gaborel, P. Matousek, A. W. Parker, W. Shaikh and R. H. Bisby, Laser Chem., 1999,19,153. 576. K. Takenshita, N. Hirota and M. Terizima, J . Photochem. Photobiol. A, 2000, 134, 103. 577. X. Ho and T. G. Spiro, Laser Chem., 1999,19,141.
Part II Organic Aspects of Photochemistry
Photolysis of Carbonyl Compounds BY WILLIAM M. HORSPOOL
Several reviews have been published during the past year that are pertinent to this section. Among these is a review highlighting the photochemistry in mixed crystals, co-crystals and the solid state of mixtures.' Others have detailed some aspects of bimolecular reactivity in single crystals.2 There is growing interest in photochemistry carried out under the constraints of the crystalline phase or in zeolites. R a m a m ~ r t h yis~active ? ~ in this area and he has published extensive reviews of photochemistry carried out under both condition^.^ Yamashita and Anpo6 have discussed pore effects in ZSM-5 zeolites in relation to the photochemical reactions of pentan-2-one under such constraints. Others have reported both theoretical and experimental studies of the reactivity of the same ketone in zeolites. The ratio of products (Norrish Type 1/11) is dependent to a large extent upon the cation within the cage. Reviews have been published on photochemical processes controlled by electron-transfer processess and asymmetric photochemical reactions in sol~ t i o nA. ~short review has described the nn*-excited state reactivity of ketones." Specific studies on the behaviour of ketones, such as the detailed report of photophysical properties of p-aminobenzophenone, are also worthy of mention." The irradiation of acetophenone in aerated solutions using wavelengths >200 nm is also of interest and results in the formation of 2-hydroxy- and 3-hydroxy-acetophenone as the principal products.12 Asymmetric recognition has been demonstrated using chiral fluorenone derivatives such as 1[( 1S,2R,5S)( +)-menthyloxycarbonyloxy]fluoren-9-one.'3A study of the photophysical behaviour of the sunscreen menthyl anthranilate in a variety of solvent systems has been reported.14While the triplet state is readily quenched by oxygen it can be observed in low-temperature glasses.
1
Norrish Type I Reactions
Several studies dealing with the photochemistry of acetone under a variety of conditions have been reported. Thus, irradiation of the ketone in air affords acetyl radicals by a conventional Norrish Type I pro~ess.'~ The influences of pressure and of wavelength on the efficiency of the reaction were determined. Acetyl radical and methyl radicals are also formed on infrared multiphoton Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002 53
54
Photochemistry
irradiation of acetone.16The fragments generated by the irradiative decomposition of acetone have been studied using time-of-flight mass ~pectrometry'~ and computative procedures such as CASSCF methods have been used to probe the photodissociation of acetone.'* A study of the photochemical (at 248 and 308 nm) behaviour of acetone in the presence and absence of water has been carried 0 ~ t . I ~ The photochemical reaction of acetone and HBr has been investigated.20Excitation at 266 or 309 nm results in the reaction of HBr/acetone complexes. Other simple carbonyl compounds such as CO(CN)2also undergo Norrish Type I processes on irradiation at 193 nm.21This treatment has shown the similarity of behaviour to acetone and two different CN fragments are produced. One of these is formed by the conventional a-fission process while the other arises by cleavage of the resultant COCN radical. The photochemistry and the photophysics of the arylpropanones (1)have been investigated?2These compounds are models for the photoinitiation of free radical polymerisation. a-Cleavage of the compounds (1) is favoured with p-fluoro and p-chloro substituents where the nn*lowest triplet state is active. The character of the lowest excited state is changed with p-dimethylamino and p-thio ether substituents. The spectra of the persistent radicals formed on irradiation of a series of 4,4'-dialkylsubstituted benzophenones in MFI zeolites have been recorded.23Further studies of the photochemical behaviour of dibenzyl ketone A review has highlighted the derivatives in MFI zeolites have been carried processes involved in electron spin polarisation in supramolecular systems such as zeolites.25Others have also reported on phenacetyl radicals that are formed by irradiation of dibenzylketones.26The decarbonylation of the radicals was studied and the influence of the p-substituents (MeO, Me, C1, CF3) was assessed. Cyclobutanones also undergo Norrish Type I processes and calculations relating to the photochemical activity of this cyclic ketone have been carried A study of the cis,trans-isomerism of some 2-azetidinones has been reported.28Theproducts obtained from the process are dependent both on the substitution on the nitrogen atom and the ability of substituents at C-3 and C-4 to stabilise radical centres.
Norrish Type I fission occurs on the irradiation of the a-alkylphenylindanones (2).29Thefinal products from this reaction mode have been identified as oformylstilbenes as a result of disproportionation within the 1,5-biradical. Photodissociation of cyclopentanone and cyclohexanone has been studied using irradiation at 800 nm with a pulsed-laser system.30
1111: Photolysis of Carbonyl Compounds
2
55
Norrish Type I1 Reactions
2.1 1,s-Hydrogen Transfer. - The Norrish Type I1 photoreactivity of alkyl aryl ketones on silica gel surfaces and in solution has been investigated?' The authors report that the amount of acetophenone produced on irradiation of valerophenone is dependent upon the surface loading.32The yield increases linearly up to a maximum loading of 60%. Valerophenone is photochemically active to some extent in frozen solvents such as benzene, cyclohexane, t-butanol, hexadecane and water. This activity is observed even although there is restricted C-C bond rotation. It appears that a fraction of the valerophenone molecules are oriented so that a 1,5-hydrogen abstraction reaction can occur even in this constrained environment. There is also conformational restriction of butyrophenone and valerophenone P-cyclodextrin complexes and as a result the photochemistry observed is different from that in the solution phase.33The Norrish Type I1 behaviour of the aryl ketone (3)in a monolayer on gold has been studied for the first time.34The photo-deconjugation of (4) to give (5) occurs on 254 nm irradiation in methylene chloride at - 10 "C and this step, a Norrish Type I1 process, has been used as part of a synthesis of (R)-sesquilavandulol.35
The irradiation of the ketone (6) at h> 300 nm in methylene chloride provides an efficient method for the synthesis of the cyclopropyl ketone (7). The process involved is a standard Norrish Type I1 hydrogen abstraction with the formation of a 1,4-biradical (8). This subsequently eliminates the leaving group, OMS, to reform the carbonyl group and afford a 1,3-biradical which cyclises to give (7). The scope of the reaction was investigated using the derivatives (9). As can be seen in the results, affording (lo),(11)and (12),there is a preference for cyclisation within the 1,3-biradical to yield the trans-cyclopropane (10) but the cis-cyclopropane (1 1) and the alkene (12) can also be formed. The reaction can also be used for the synthesis of bicyclic molecules such as (13) obtained by irradiation of (14).36 The asymmetric induction encountered in the formation of the cyclobutanol resulting from the irradiation of the ketone (15a) with different chiral auxiliaries
56
Photochemistry
(9)
R'
R2
Me H Ph H OBn
H Me H C02Me H
Yield (%)
R3
Me Me Ph C02Me OBn
63 90
26 59
46
-
16
18 10
-
-
-
has been evaluated.37The photoreaction of (15a) yields the corresponding cyclobutanol with only 14% de in solution. A more dramatic effect is observed in the crystalline phase when a de of 96% is obtained. The results are less encouraging with (15b) where only 18% de is obtained in the crystal and 26% de in solution. Ketone (16)is converted photochemically into the three products (17,47%), (18, 47%) and (19,6%) on irradiation in a~etonitrile.~' The reaction pathway changes dramatically in single crystals of (16) irradiated through Pyrex at - 20 "C when (20) is formed exclusively. The outcome of the reaction in whatever medium is used is controlled by the ability of the biradicals to cyclise. The authors suggest that the biradical formed by abstraction of the hydrogen at C-10 in (16)is slow to cyclise in solution but becomes the dominant process in the crystal. Decomposition of a-keto esters of primary alcohols occurs by irradiation at 350 nm from the triplet The predominant reaction encountered is yhydrogen abstraction and fragmentation of the resultant biradical. y-Hydrogen abstraction is also the outcome of irradiation of g-benzoylpropionic acid derivatives!' Pincock and his co-workers4' have studied the photochemical reactivity of trans-2-phenylcyclohexyl4-cyanobenzoate(21). Irradiation brings about cleavage to give 1-phenylcyclohexeneand 4-cyanobenzoic acid. A Norrish Type I1 process is thought to be involved from the usually inefficient xn* excited state of the ester moiety. Interestingly the naphthyl benzoates (22) do not undergo this reaction and instead afford products of intramolecular cycloaddition!2 13Hydrogen transfer reactions are also brought about on irradiation of the (S)ketone (23).43This process yields the 1,4-biradical which cyclises to give the azetidinols (24). The reaction was developed further and used the photocyclisation of (S)-3,4-diacetoxy-C6H3COCH2N(COCH3)Me (from adrenaline) as a syn-
57
I111 :Photolysis of Carbonyl Compounds
s$l-'&-
(16) A r = e C O 2 M e
(15) a; X =
b; X =
& 0-
(18 )
(20)
(19)
thetic path to 3-hydroxy-3-azetidinecarboxylic acid. Nishino et aE.44have examined the pH-dependent photochemistry of amino acids such as leucine (25) using circularly polarised light. Under these conditions racemic leucine undergoes enantiomeric enrichment to afford (26,1.3% ee). The photochemical reactivity of musk ketone (27) in cyclohexane and methanol has been studied.45
-bN
0 II
0-C-Ar
p h h N $ CH~R'
Ph R'
'R2
The reaction dynamics for the triplet-state-induced hydrogen transfer in 2methylbenzophenone have been The photochemical process in such systems results in the formation of enols by 1,5-hydrogen transfer. The photochromism exhibited by (28) in the solid state has been studied in detail, and apparently the process is the result of an intramolecular hydrogen abstraction to afford the enol (29).47 The photochemically induced hydrogen transfer reactivity in the salicylate derivatives (30) has been reported,"' as has the photoinduced proton transfer within 3-hydroxy-2-naphthoic acid (3l).49In the latter case a large Stokes-shifted
58
Photochemistry
emission is observed which is dependent upon pH, solvent, temperature and excitation wavelength. The large Stokes shift is the result of intramolecular hydrogen transfer. A detailed study of the photoinduced proton transfer within the acetonaphthol (32) has been carried out in order to investigate the internal twisting processes within the molecule.5oPhotoenolisation of the hydroxyquinoline derivative (33) occurs on irradaiti~n.~’
2.2 Other Hydrogen Transfers. - The photochemical rearrangements encountered in the unsaturated ketones (34) have been examined.52The reactions involve either hydrogen abstraction by the excited carbonyl group from the P-position, which in this instance is situated between the two vinyl substituents, or a di-n-methane process. This latter reaction affords the cyclopropane products (35). The free radical path involving the intermediate (36) formed by hydrogen abstraction has two possible reaction paths. The usual mode is bonding at ‘a’ in the biradical intermediate (36) which affords the cyclopropyl ketone (37) that rings opens to afford the furan derivatives (38). Alternatively, within this vinylogous system, bonding can occur at ‘b’ or ‘c’ to yield the cyclopentenes (39) and (40). The reaction outcome as may be expected, is to some extent dependent upon the substitution on the starting material and the influence these substituents have upon the stability of the biradical intermediates. The influence of environment on the photochemical cyclisation of the acetophenones (41) into the indanols (42), brought about by a 1,6-hydrogen transfer, has been assessed.53This detailed investigation has shown that there is a correlation between the reactivity and the crystal structure. A &hydrogen abstraction is also involved in the conversion of the derivatives (43) into the benzofurans (44).54Thi~ cyclisation mode has been used as a path to the synthesis
59
1111: Photolysis of Carbonyl Compounds
(34)
R' Ph Me Et Me
R2 H H H Ph
(38)
-
-
12 21
7 11
OH
-
63 26 30 17
-
27
OMe
R'
Ph
R*
(41)
R'
R2 or vice versa
F CN C02H C02Me
H H H H
(43) a; R = H b: R = M e 0
(42)
(44)
of coumestrol. The photochemical cyclisation of suitably ortho-substituted arylketones has been re~iewed.'~ These reactions arise from the nn* excited state and a 1,6-hydrogen transfer. The resultant 1,5-biradicals can readily cyclise to yield benzofuran derivatives. A study of the photochemical dynamics of the irradiation of (45a) and (45b) has been reported.56The irradiation of (45a) affords only a single product identified as (46), but the oxirane (45b),while following the same reaction path to yield (47), also undergoes ring-opening to yield (48). The influence of the oxygen in the three-membered ring on the outcome of the reaction is discussed.
60
Photochemistry
(45) a; X = CH2
(46)
b;X=O
3
Oxetane Formation
Adam and c o - w ~ r k e r shave ~ ~ examined the photochemical addition of benzophenone to both cis- and trans-cyclooctene and have uncovered a remarkable temperature effect on the formation of the oxetane products (49) and (50). The results show that the formation of the trans-oxetane (50)from the cis-cyclooctene is favoured at higher temperatures. Thus the ratio of (50):(49)changes from 98:2 at - 95 "Cto 20230 at 110 "C. Addition of benzophenone to trans-cyclooctene also favours the formation of the trans-oxetane (50). There is also a temperature effect in this addition and at - 80 "C the ratio is 3565 (c:t).Even at 40 "C there is still a slight preference for the trans product (ratio 4951;c:t). At 110°C the reaction favours the formation of the cis-oxetane (ratio 70:30; c:t). Among a variety of factors that control the outcome of this reaction the authors suggest that conformational factors are important.
The diastereoselectivity of the photo-addition of aldehydes to the alkenes ( 51) has been demonstrated to be excited state dependent.s*Thus, a low ds is obtained in the product oxetanes (52)from the singlet state while a higher ds is returned from the triplet excited states of the carbonyl compounds. The photochemical addition of benzophenone to 5-methyl-2-furylphenylmethanolyields two oxetane derivatives in a ratio of l:l.59 The influence of substituents upon the outcome of the Paterno-Buchi cycloadditions between the furans (53) and the aldehydes and ketones (54) has been assessed.6' These high-yielding additions affording the oxetanes (55) and (56) are brought about in degassed acetonitrile using wavelengths >290 nm. The path followed within the system is to a large extent dependent upon both the type of carbonyl function and the substituents on the furan. The multiplicity of the carbonyl excited state is also important. The aldehydes, for example, add quite randomly independent of the excited state multiplicity. The ketones (54d, e), on the other hand, react from the triplet state giving regioselective formation of the oxetanes (56). Acetone, which apparently reacts from its singlet state, is once more stereo-random in the addition mode. Further studies by Bach and his co-workers61 have given details of the Paterno-Buchi addition of benzaldehyde to alkenes such as (57). The addition of
61
II/1: Photolysis of Carbonyl Compounds
(52) R = Me, Et or Bn
(51) n = 1 or 2
~1
(53) a; H b; H
~2
H H c; H Me d; Me H
SiR3 TIPS TBDMS TBDMS TBDMS
n
R3 R4 (54) a; Me b; Pr"
H
c; Ph
H
H
d; Ph Me e; Ph Ph f; Me Me
(55) a + a 61 : 39 c + a 5 4 : 46 d + a 69 : 31 b + b 53 : 47 a + c 50 : 50 c + c 40 ; 60 d + c 5 6 : 44
(56) a + d 5 : 95 c + d 7 : 93 d + d 42 : 58 b+e7:93 c + e 5 : 95 d + e 15 : 85 c + f50:50
the aldehyde takes place in a syn-manner and gives products where the groups are cis to each other. This specific addition mode has been made use of in a synthetic approach to preussin (58). Bach62has reviewed this class of photochemical addition aldehydes to N-acyl enamines. Unstable oxetanes are obtained on the irradiation of benzil derivatives in the presence of (S)-2-(2metho~yrnethylpiperidiny1)propenenitrile~~
(KR I
Ph I
C02Me
Me
(57) R = Me, Et, CH2Ph, C9HI9 or C4H9,n = 1
R = H. n = 2
4
Miscellaneous Reactions
4.1 Decarbonylation and Decarboxylation.- Laser irradiation at 370 nm has been used to detect formaldehyde in the primary flame front of a Bunsen flame.64 The photochemical dynamics for the fragmentation of methanal has been studied t he~retically.~' Morokuma and his co-workers66have suggested that the photodissociation of ketene in its TI excited state is highly non-statistical. Irradiation of ketene at 193 nm has shown that there are four different decomposition paths.67Two of these afford CO and either triplet or singlet methylene. The spectrum of singlet methylene generated by the photochemical decomposition of ketene has been recorded.68 The acetone-sensitised photochemical decarbonylation of cyclobutanones to cyclopropane derivatives has been d e ~ c r i b e dLaser . ~ ~ irradiation of the ketone (59)brings about decarbonylation and the formation of the biradical (60).70This same radical can be produced by irradiation of the dichloro-compound (61). The
62
Photochemistry
biradical (60) does not cleave to yield p-xylylene but either ring-closes to the paracyclophane (62) or dimerises to yield (63) in low yield. The dianhydride (64) undergoes interesting photochemistry when irradiated in a low temperature Irradiation at 308 nm affords the naphthyne monoanhydride (65) and prolonged irradiation at this wavelength converts (64) into the anhydride (66). Irradiation of naphthyne anhydride (65) at 248 nm brings about the second decomposition step and the formation of the naphth-175-dyne(67). This intermediate undergoes ring opening to yield the polyyne (68).
Both experimental and theoretical methods have been used to explore the photodissociation of formic A comparison of the decomposition of acetic acid and benzamide on different types of T i 0 2 catalysts has been Apparently the specific area of the catalyst does not affect the decomposition of benzamide. The decomposition of butanoic acid on a Ti02 catalyst, giving acetic and succinic acids, has also been examined and the influence of changes in pH have been q ~ a n t i f i e dAcrylic . ~ ~ acid undergoes photochemical dissociation from Thus the loss of a hydroxy radical occurs on the T2-S1 several excited interface while formation of a vinyl radical arises on the TI surface. Aliphatic amino acids undergo rapid decarboxylation when irradiated in the presence of 4-~arboxybenzophenone.7~ Under these conditions the triplet state of the benzophenone is the active species. The photophysical properties of phenylalanine have been The decomposition of phenylglycine has been investigated under pyrene sensitisati~n.~~ The reaction can be accelerated by the addition of diethyl isophthalate or terephthalonitrile as electron-accepting sensitisers. Photo-oxidative decarboxylation of amino acids in mesoporous silica has been investigated with the protected amino acids (69).79Irradiation of such compounds with a 400 W lamp for 36 hours in hexane as the solvent provided the imides (70).
63
I I / l : Photolysis of Carbonyl Compounds 0
0
K,K,
R1
H (69)
(70)
I
H R'
R2 Yield (%)
Ph
Me
70
Ph
H
45
Ph
Bu'
54
Ph
Ph
74
BnO Me
51
BnO
66
H
The photochemical reactivity of aryl-substituted acetic acids in acetonitrile with HgF2 has been described and the corresponding benzyl radicals that are formed dimerise to afford 1,2-diar~lethanes.~' The photochemical decomposition of 4-chlorophenoxy- and 2,4-dichlorophenoxyacetic acid has been studied in air saturated mixtures and in the presence of traces Fe3+.81Irradiation in the 245-250 nm range of the acids (71-73) in water/acetonitrile mixtures set at pH 7 controlled by the addition of sodium hydroxide brings about efficient decarboxylation.82 Dark controls demonstrate that the reactions are truly photochemical. The decarboxylation occurs with high quantum yields (71, @ =0.66; 72, = 0.62; 73, = 0.22). The reactions are thought to arise from the excited singlet state and result in the formation of the corresponding anion following decarboxylation. The leaving group need not be carbon dioxide, a fact demonstrated using the alcohol (74) when a 70% yield of 3-methylbenzophenone is obtained, formally a loss of formaldehyde. Photodecarboxylation of the anion of ketoprofen has been studied by laser-induced optoacoustic spectros~opy.~~ There is a marked enhancement of decarboxylation when phenyl and 1-naphthyl esters are irradiated in polyethylene films at sub-ambient temperature^.^^ The photochemical reactivity of the naphthyl esters (75) in stretched and unstretched polymer films has been ~tudied.'~ Decarboxylation of (76) results on irradiation at 254 nm in acetonitrile solution and this yields cyclohexylmesitylene (77).86When the irradiation is repeated in solutions containing a trace of acid and ethanol the reaction follows a different path and yields ethyl cyclohexanecarboxylate and the 2,4,6-trimethylphenol. The decarboxylation of (78) to afford (79) can be carried out efficiently by irradiation in benzene in the presence of Bu'SH/quinoline to give (78), necrodol, in 8 1% yield.87The photochemical decarboxylation of chromone-2-carboxylic acid in ethanol affords 4-hydroxycoumarin and 2-(1'-hydroxyethyl)chromone.88 The photochemical equilibria exhibited by anthracene-9-carboxylic acid in a A laser flash study of the activity of the variety of media have been in~estigated.~~ fluoroquinolone antibiotic flumequine has been rep~rted.~' A detailed study of the photofragmentation undergone by other fluoroquinolone antibiotics has been carried out." Medium effects were also investigated?2 The results of a study of the photochemical reactions undergone by some furocoumarins have been p~blished?~
+
+
64
Photochemistry
0 (73) (72)
(74)
(75) R = CH2Ph or CHPh
(76)
I
(77)
Me
(78)
(79)
The Barton ester (80) cleaves on laser irradiation at 355 nm and the resultant radical (81)undergoes bond fission with the formation of the radical cation (82).94 Other studies have focused on the Barton esters (83) which on irradiation yield the radicals (84):' Irradiation of Barton ester (85) provides a good route to the radical (86) within which the rate of ring ppening of the cyclobutane ring was studied.96Irradiation of Barton ester (87) in acetonitrile or a sodium phosphate buffer at pH 7.4 leads to 0-C bond fission and the production of the ubiquinol radical (88).97
(80) R = Ph or Et
(84)
(82)
(85)
(83) X = Br or OP(0)(OPh)2
(86)
Use of the Barton ester (89) has been made in new photochemistry of the alkyl boronic esters (90).98Irradiation of (89) with a 300 W halogen lamp in the presence of (90) affords the mixture of adducts (91) and (92) in a ratio of 6:l. Several boronic esters were examined and the best yields (68%) were obtained with the catechol derivative of (90).The study also included reactions of (93) with
65
I I I l : Photolysis of Carbonyl Compounds
Meo*Me M e0
OH
R
Me0
OH (87)
the Barton ester and a variety of alkenes (94).This reaction provided moderate to excellent yields of the adducts (95) as a mixture of trans and cis in ratios greater than 80:20. Reactions were also carried out with indene derivatives of (93).
I
(92) (93) n = 1 or 2
S
3
(94) R = CN, P(O)(OEt),,SO,Ph
or C0,Me
(Q -
P
R
Yield (%)
(95) n = 1, R = CN: 54 n = 1, R = P(O)(OEt)2: 79 n = 1, R = SOzPh: 70 n = 1, R = C0,Me: 55 n = 2, R = C02Me: 82
The radical (96) can be formed by the irradiation of (97) in a~etonitrile.~~ Cleavage of the 0-N bond is solvent dependent and is not as efficient when non-polar solvents are used. The products formed from the reaction were identified as the dimer (98) and anthracene-9-carboxylic acid. Anthracene-9-carboxy radical is also formed by irradiation at 308 nm of (99).lo0The polymer supported thiazole thione (100) has been developed as a means of producing free alkoxy radicals."' Irradiation liberates the R radical from the substrate.
66
Photochemistry
0 ' OR
U
Ph (89)
(90) OR=Me, O
~
O
M
~
o
. oro%
/
o
/
4-R (94) R = CN, P(0)(OEt)2,S02Phor C02Me
Yield (YO) (95) n = 1, R = CN: 54 n = 1, R = P(O)(OEt)2: 79 n = 1, R = S02Ph: 70 n = 1, R = C02Me: 55 n = 2, R = C02Me: 82
4.2 Reactions of Miscellaneous Haloketones and Acid Chlorides. - The photochemical reactivity of a series of 2-substituted N-(2-halogenoalkanoyl) anilines and cyclic amines has been reported.lo2 Fission of a C-C bond occurs on irradiation of the iodocycloalkanones (101). This is an extension of earlier worklo3and the present report details the bond fission processes in alcohol (R3 OH) The principal products are the esters (102) that are formed in yields of 65-88%. The authors suggest that the photochemical reaction brought about by initial C-I bond fission using wavelengths >300 nm involves an electron transfer with the resultant formation of the ions (103), for example. The reactions are carried out in a trace of water and it is at this stage that water adds to the cation to afford the 2-hydroxycyclohexanone. Although there is no experimental support for the next step of the sequence the authors'04propose the formation of the alkoxy radical (104) which then undergoes the necessary C-C bond fission that ultimately yields the products. 4.3 Other Processes. - The fragment HCO is formed on irradiation at 193 nm of p r ~ p e n a l . "The ~ photochemical reaction between the dianions of phenylacetic acid (105) and aryl halides has been studied.lo6The reaction is dependent on the nature of the counterion and with K+ only the biphenylacetic acid (106) is formed. Mixtures of (106) and (107) are obtained using the Na+ salt while with Li+ only a-arylation is observed affording (107). A study of the alkylation of glycine derivatives (108) has been reported.Io7The process involves the irradi-
67
I l l l : Photolysis of Carbonyl Compounds
(99)
0 R'
R2
0
II
I
I
R30CCH2CH(CH2),CHCH(OR3)2 R2
1
H H M e H Me Me Me H Me H Me 2 H 3 H H Me H Me, Et, P P o r Pr' 7 H
1 1
10 H
0 ; k O 2 (105)
M'
M = K, Na or Li
H
Me
Ar
0
CH2C02H
(106)
& H (2 C -0 2 -H A r Ar (107)
ation of the glycine in solution containing di-t-butyl peroxide, benzophenone and toluene. Several products are formed as shown in the Scheme. The conditions for the formation of the principal product (109) have been optimised. Irradiation of mixtures of the pyran (110) with a variety of ketones has been reported.lo8The excited-state ketones abstract hydrogen from the 4-position of the pyran and combination between the resultant radicals affords the substituted derivatives (111). A new photolabile linker (112)has been described which on irradiation at 350 nm in THF with tributyltin hydride results in liberation of the indole in 55%
68
Photochemistry
yield."' The Norrish Type I cleavage in (113) affords a radical that allows for the release of the immobilised alcohols.11oA copolymer containing the t-butyl-4vinylphenyl carbonate (114) moiety undergoes photochemical decomposition and this has been used as a means of producing a photopattern."' A new photoremovable protecting group containing a 2,5-dimethylphenacyl chromophore"' and a photolabile linker based on 3'-methoxybenzoin have been #-C02Me (108)
R
a; Bz b; Boc c; Cbz
light
7
C02Me
R - N X R-N C0,Me
Ph
Me
+
R-NACo2Me+
+ Hofp
R-NLC02Me
I
I
H
I
H
Bz-N
H
H (109)
Yield 11 18
2
(YO)
29 30 37
6 21 11
C02Me
I N
-
1 3
3 4
-
Scheme
R'$OH
TMS
0
TMS
pJsq
R'
R2
Yield (%) 12
Ph
H
Me
COMe
42
Me
C02Et
62
But
C02Et
38
Ph
C02Me
52
4-MeoC~H4 C02Me
49
I-naphthyl
44
C02Me
Me I
0
/
But
I
'e
R' I
QG
Me
O
0
X
H OR
69
I I I I : Photolysis of Carbonyl Compounds
NMe2
0-
+9
de~cribed."~ Further developments in the study of photoremovable protecting groups have extended the range of absorption of such ~pecies.''~ A detailed investigation of photoremovable protecting groups based on (115) and (116) has been carried out."' The reactions involve single electron transfer with the generation of zwitterionic biradicals such as (117) formed from the irradiation of (115). The collapse of the intermediate (117) liberates acetic acid. Peptide synthesis based on t-Boc chemistry has been described.l16
References
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70
Photochemistry
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I I I l : Photolysis of Carbonyl Compounds
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71
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72
Photochemistry
69. J. Ramnauth and E. Lee-Ruff, Can. J . Chem., 2001,79,114. 70. M. A. Miranda, E. Font-Sanchis, J. Perez-Prieto and J. C. Scaiano, J. Org. Chem., 2001,66,2717. 71. T. Sato, H. Niino and A. Yabe, Chem. Commun., 2000,1205. 72. H. Su, Y. He, F. Kong, W. Fang and R. Liu, J . Chem. Phys., 2000,113,1891. 73. 0.Heintz, D. Robert and J. V. Weber, J. Photochem. Photobiol. A : Chem., 2000,135, 77. 74. C . Guillard, J. Photochem. Photobiol. A: Chem., 2000,135,65. 75. W. H. Fang and R. 2. Liu, J . Am. Chem. SOC.,2000,122,10886. 76. G. L. Hug, M. Bonifacic, K.-D. Asmus and D. A. Armstrong, J. Phys. Chem. B, 2000,104,6674. 77. A. Rzeska, J. Malicka, K. Stachowiak, A. Szymanska, L. Lankiewicz and W. Wiczk, J. Photochem. Photobiol. A - Chem., 2001,140,21. 78. S . Ikeda, S. Murata, K. Ishii and H. Hamaguchi, Bull. Chem. SOC.Jpn., 2000, 73, 2783. 79. A. Itoh, T. Kodama, S. Inagaki and Y. Masaki, Chem. Lett., 2000,542. 80. M. H. Habibi and S. Farhadi, Asian Chem. Lett., 1998,2, 111 (Chem. Abstr., 2001, 134, 115697). 81. S. Klementova and J. Matouskova, Res. J . Chem. Environ., 2000, 4, 25 (Chem. Abstr., 2001, 1556). 82. M. S. Xu and P. Wan, Chem. Commun., 2000,2147. 83. C. D. Borsarelli, S. E. Braslavsky, S. Sortino, G. Marconi and S. Monti, Photochem. Photobiol., 2000,72, 163. 84. W. Q. Gu, D. J. Abdallah and R. G. Weiss, J . Photochem. Photobiol. A- Chem.,2001, 139,79. 85. W. Q. Gu and R. G. Weiss, Tetrahedron, 2000,56,6913. 86. T. Mori, T. Wada and Y. Inoue, Org. Lett., 2000,2,3401. 87. S. Samajadar, A. Ghatak, S. Banerjee and S. Ghosh, Tetrahedron, 2001,57,2011. 88. H. Kawata, T. Kumagai, T. Morita and S. Niizuma, J. Photochem. Photobiol. A: Chem., 2001,138,281. 89. M. S. A. Abdel-Mottaleb, H. R. Galal, A. F. M. Dessouky, M. El-Naggar, D. Mekkawi, S. S. Ali and G. M. Attya, Int. J . Photoenergy, 2000,2,47 (Chem. Abstr., 2000,809159). 90. M, Bazin, F. Bosca, M. L. Marin, M. A. Miranda, L. K. Patterson and R. Santus, Photochem. Photobiol., 2000,72,45 1. 91. E. Fasani, A. Albini, M. Mella, M. Rampi and F. B. Negra, J . Photoenergy, 1999,1, 7 (Chem.Abstr., 2000,629163). 92. Z. Liu, Z. Huang and R. Cai, Spectrochim. Acta, Part A , 2000, 56, 1787 (Chem. Abstr., 2000,133, 157079). 93. F. Bordin, J . Photoenergy, 1999,1, 1 (Chem. Abstr., 2000,629162). 94. M. Newcomb, N. Miranda, X. H. Huang and D. Crich, J . Am. Chem. SOC.,2000,122, 6128. 95. B. C. Bales, J. H. Horner, X. H. Huang, M. Newcomb, D. Crich and M. M. Greenberg, J. Am. Chem. SOC.,2001,123,3623. 96. S. Y. Choi, J. H. Horner and M. Newcomb, J. Org. Chem., 2000,65,4447. 97. B. E. Schultz, K. C. Hansen, C. C . Lin and S. I. Chan, J. Org. Chem., 2000,65,3244. 98. C. Cadot, J. Cossy and P. I. Dalko, Chem. Commun., 2000,1017. 99. Y. Saitoh, K. Segawa, H. Itoh and H. Sakuragi, Tetrahedron Lett., 2000,41,8353. 100. Y. Saitoh, M. Kaneko, K. Segawa, H. Itoh and H. Sakuragi, Chem. Lett., 2001,82. 101. L. De Luca, G. Giacomelli, G. Porcu and M. Taddei, Org. Lett., 2001,3, 855.
I I / l : Photolysis of Carbonyl Compounds
73
102. T. Nishio, H. Asai and T. Miyazaki, Helu. Chim. Acta, 2000,83, 1475. 103. S . J. Ji, E. Takahashi, T. T. Takahashi and C. A. Horiuchi, Tetrahedron Lett., 1999, 40,9263 104. S . J. Ji and C. A. Horiuchi, Bull. Chem. SOC.Jpn., 2000,73, 1645. 105. Y.-T. Kao, W.-C. Chen, C.-H. Yu and I-C. Chen, J . Chem. Phys., 2001,114,8964. 106. G. C. Nwokogu, J. W. Wong, T. D. Greenwood and J. F. Wolfe, Org. Lett., 2000,2, 2643. 107. H. S. Knowles, K. Hunt and A. F. Parsons, Tetrahedron Lett., 2000,41,7121. 108. D. Saleur, J. P. Bouillon, C. Portella and N. Hoffmann, Tetrahedron Lett., 2000,41, 5199. 109. J. R. Horton, L. M. Stamp and A. Routledge, Tetrahedron Lett., 2000,41,9181. 110. R. Glatthar and B., Org. Lett., 2000,2,2315. 111. T. S . Li, M. Mitsuishi and T. Miyashita, Chem. Lett., 2000,608. 112. P. Klan, M. Zabadal and D. Heger, Org. Lett., 2000,2, 1569. 113. E. R. Felder, P. Petriella and P. Schneider, Proc. ECSOC-1: First Int. Electron. C o n , Synth. Org. Chem., 1997-1998,563 (Chem. Abstr., 2001,134,193007). 114. P. G. Conrad, R. S . Givens, J. F. W. Weber and K. Kandler, Org. Lett., 2000,2,1545. 115. K. Lee and D. E. Falvey, J. Am. Chem. SOC.,2000,122,9361. 116. J. P. Pellois, W. Wang and X. Gao, J . Comb. Chem., 2000,2,355 (Chem.Abstr., 2000, 133,150874).
2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones BY WILLIAM M. HORSPOOL
1
Cycloaddition Reactions
1.1 Intermolecular Cycloaddition. - I .I .I Open-chain Systems. The results of detailed calculations on the photochemical addition of alkenes to a,p-unsaturated enones such as acrolein have been published.' Mixtures of (2.n+ 2.n)photodimers are formed on irradiation of the furan derivatives (1). Similar behaviour is reported for the corresponding thiophene derivatives.2
(2) R = Me, H or CI
(1)
Panja et aE. have reported details of a study into the charge transfer exhibited by 4-N,N-dimethylaminocinnamaldehydeencapsulated in P-cy~lodextrin.~ A further study of this system has examined the photodynamics of 4-N,Ndimethylaminocinnamaldehyde and the authors have suggested that the anomalous fluorescence in polar aprotic solvents can be attributable to a twisted intramolecular charge transfer! The crystal structures of the trans-cinnamides (2) have been determined and irradiation of the crystals results in (2.n+2.n)photodimerisation without destruction of the crystalline form.' A study of the dimerisation of derivatised cinnamates (3) has been reported and interestingly, while cinnamate esters have often been shown to be reluctant to dimerise unlike free cinnamic acid, in the present investigation intermolecular complexation and irradiation of (3) affords the three cyclobutane derivatives (4-6).6 Similar quantum yields are observed when (3) is complexed with 0.5 mole of (7), but the major change occurs when (8) is used as the complexing agent, when better quantum yields for the dimerisation were observed. The cinnamides (9) also undergo (2.n + 2n)-cycloaddition to afford the dimers (10) and (11)in variable yields with cinnamide (9a)itself giving only 18% of the dimer.7Control over the dimerisation of these cinnamides (9) can be exercised using hydrogen bonding within coPhotochemistry, Volume 33
0The Royal Society of Chemistry, 2002 74
75
1I12: Enone Cycloadditions and Rearrangements
MeoYNYNH
NYN NH2 (3)
x x
X
I
#
Ar Ar
Ar A
Ar @X Ar
(4)
(6)
(5)
alone I$ = 0.7 x with 0.5 of (7) 0.7 x 0.5 of (8) 2.3 x
r
0.1
10”
'^ suggest that the reaction involves cyclisation to the biradical(l49) which rearranges to the ketene (150) and this is the intermediate for the products. Chang and Parks6report that the irradiation of the bicyclohexenones (151) at 365 nm converts them efficiently (high chemical and quantum yields) into the naphthols (152).The reaction arises from the triplet state and both sensitisation and quenching experiments have been used to substantiate this claim. The ketene (153) is considered to be the intermediate but attempts to trap this were unsuccessful.
91
II/2: Enone Cycloadditions and Rearrangements
w
a OMe
0
9142) R = M O M
0
OMe
(144)
(143)
0 OMe
(147)
(145)
&p
* 0
\
\
R (151) R = H or Me
3
(149)
Me
+% R
/
yAr
(152) R = H; I$ = 0.92 R = Me; I$ = 0.95
R (1 53)
Photoreactionsof Thymines and Related Compounds
3.1 Photoreactionsof Pyridones. - The chiral host (154) has been employed in the reactions of the pyridones shown in Scheme 6.87Thus, irradiation affords the Dewar pyridones when ether derivatives are used. With pyridones (X = H) high enantioselective addition to cyclopentadiene affords the derivative (155). Gauvry and Huetg8 have used the method originally described by DillingS9for the synthesis of (156) from 2-hydroxypyridine (157) and have established that the reaction needs to be carried out in dilute solution M) and with long irradiation periods to ensure a high yield of product. The photochemical cyclisation of (158) to afford (159) has been reported previously and this has now been used as a key intermediate in the synthesis of tax01.~'Tautomerism results on irradiation of 1,3,6,8,10-pentamethyl- and 1,3,5,7,9-pentamethylcyclooctapyrimidine-2,4-dionesP1 3.2 Photoreactionsof Thymines etc. - Uracil and cytosine undergo photooxidation on irradiation in the presence of pero~ydiphosphate.~~ Titanium dioxide
92
Photochemistry
(1 55)
Scheme 6 OMe
I
HN q
0 Me
Me (158)
(159)
mediated oxidation of uracil, thymine and 6-methyluracil is retarded by the presence of C U ~and ~ uracil , ~ ~undergoes photochemical addition when irradiated in phosphate buffered saline to give 6-phosphoryloxyuraci1.94 Irradiation of 6-chloro-1,3-dimethyluracil in TFA at low temperatures and in the presence of mesitylene affords 1,3,5,7,9-pentamethylcyclooctapyrimidine2,4-dione which is also photochemically reactive.95Irradiation of 6-chloro- 1,3dimethyluracil in mesitylene with added TFA for one hour affords a 1,3,6,8,10pentamethylcyclooctapyrimidine derivative as well as diazapentacyclo-
[6.4.0.0'~3.02~5.04~8]dodecane?6 The cyclobutane dimer formed from 1,3-dimethyluracil undergoes photochemical cleavage to the monomer when irradiated in the presence of (1,lOphenanthroline)tricarbonylrhenium(I)chloride as the s e n s i t i ~ e rChanges .~~ in the mass spectral intensity shown after multiphoton ionisation of thymine and uracil clusters have been interpreted as evidence for photodimerisation?8 1 -Alkylthymine dimerisation has been studied in the crystalline phase and the length of the chain is found to affect the crystal structure and the dimerisation process.99 Thus, 1-pentyl, 1-nonyl and 1-decylthymines give a trans,syn-photodimer while 1-0ctylthymine affords a trans,anti-dimer. The photocrosslinking of PVA containing uracil and thymine units results from (2n+ 2n)-cycloaddition between the
II/2: Enone Cycloadditions and Rearrangements
93
uracil and thymine groups and this has been discussed in a short review."' A study of the photochemical reactivity of thymine in solid layers on a quartz surface using h = 280 nm has revealed that dimerisation occurs in the crystalline phase of the layers."' The outcome of the irradiation of polymer films containing thymine derivatives with long alkyl chains has been reported.lo2The thin polymer films were irradiated at 280 nm and dimerisation of the thymine units was observed in a rate of dimerisation which was directly related to the length of the alkyl chains (i.e. increasing length required longer irradiation times). Annealing the films diminished the photodimerisation and this was thought to be due to the production of inactive micro crystals. The oxetanes (160) are formed by the addition of aryl ketones or aldehydes to thymine. A laser flash study of their decomposition has shown that they decompose adiabatically to yield ground-state thymine and triplet-state ketone or aldehyde. Results of this type are considered to have implications in the general area of DNA photo-damage and photo-enzymatic repair.Io3Radical ion intermediates have been demonstrated to be involved in the sensitised repair of thymine 0~etane.l'~ The cyanobenzophenone derivative (161) undergoes photoThe template-directed addiinduced SET reactions in duplex DNA ~ystems.''~ tion reactions between (162) and the vinyldeoxyuridine (163) have been investigated using 366 nm radiation which gives the cycloadduct ( 164).'063'07A study of the formation of cyclobutane dimers on irradiation of skin cells has been reported.Io8A detailed investigation of the photosensitised bond fission processes encountered in the isomeric dihydrothymine dimers (165) and (167) in aqueous solution has been described.''' The influence of conformational effects on the photophysical characteristics of some C5-C5' dihydrothymine dimers has been assessed.'lo
(160)
Ar Ph 4-MeC6H4 4-MeOC6H,
R' Ph H H
OH
The photochemical isomerisation of the oxime derivative of cytosine has been observed."' Photo-decomposition of (168) has been used as a route to the corresponding thyminylmethyl radical (169) and, likewise, the ketone (170) affords the 2'-deoxyuridin-l'-yl radical (171) in a Norrish Type I process."2 The t-butyl ketone (172, R = But) is inert to irradiation at 350 nm and 300 nm but the benzyl ketone (172, R = PhCH2) is photochemically reactive and provides a route to the radical (173) again by a Norrish Type I fission p r o ~ e s s . " ~
94
Photochemistry
0
I
R1
Me Me
0
0
0
I
R'
0
OH
OH (173)
3.3 Miscellaneous Processes. - Good cross-linking ability has been shown for l,4-bis[n'-(8-psoralenoxyalkyl]piperazine~14 The photochemical activity of some psoralen derivatives linked to triplet helix forming oligonucleotides has been examined.'15 Laser photoionisation has been used to generate the radical cations of a variety of psoralens such as the 8-methoxy derivative.'16The reactions encountered between these species and biological substrates such as nucleotides and amino acids have been studied and the results demonstrate that electron-transfer processes are important in the use of psoralens as photoactivated drugs. Studies have revealed that tiaprofenic acid sensitises cellular DNA to subsequent i1-radiati0n.l'~A patent application has been lodged dealing with photodeprotection of immobilised nucleoside derivatives and the method can be used in the production of DNA chips."* Further details relating to the photoenzymatic repair of the so-called (6-4)-photoproducts of DNA has examined the
95
II/2: Enone Cycloadditions and Rearrangements
involvement of oxetane and azetidine intermediates and the role by electrontransfer proces~es."~ The driving force dependence of photoinduced electron-transfer dynamics in duplex DNA has been investigated for 16 synthetic DNA hairpins which have an acceptor chromophore serving as a linker between two complementary oligonucleotide arms containing a single donor nucleobase located either adjacent to the linker or separated from the linker by two unreactive base pairs.12'The rate constants for both charge separation and charge recombination processes have been determined by means of subpicosecond time-resolved transient absorption spectroscopy and the results analysed using quantum mechanical Marcus theory. This analysis provides intimate details about electron-transfer processes in DNA including the distance dependence of the electronic coupling between the acceptor and nucleobase donor and the solvent and nuclear reorganisation energies. Electron-transfer processes within a series of synthetic DNA hairpins have been studied,l*l and the results of a laser flash photolytic study of N-acetylhistidine with 2,2'-dipyridyl have been reported.12*
4
Photochemistry of Dienones
4.1 Cross-conjugatedDienones. - Details of the photochemical rearrangement of the cyclohexadienones (174) into the bicyclopentenones (175) have been r e ~ 0 r t e d . This l ~ ~ work was the subject of some earlier publications by the same a ~ t h 0 r s .The I ~ ~cyclohexadienones (176) aromatise on irradiation at 300 nm and the resulting phenols (177) are all formed via an alkyl group migration from C-4 to C-3 within the cyclohexadienone moiety.12' 0
MejSi
.B
K
O
A
C
RIA
C02Me
OAc
(174) R = Me, allyl, PhCH2, CI(CH2),
CH2CH2=CHz or CH,CH2CHz-&)
OH
n
(175) YieldrSO%
(176) R' = H, R2 = alkyl
C02Me (177)
R' = MeO, = alkyl
0
The santonin derivative (178a) undergoes photochemical conversion into the enone (179) on irradiation in acetic acid.'26The product is typical for this type of rearrangement of a cross-conjugated cyclohexadienone and is a key intermediate in an approach to the synthesis of 4a-hydroxy-8,12-guaianolides. Further use has been made of the rearrangement of (178b) into (180) in the presence of acetic acid.127The product (180) of this photochemical reaction has been used in the stereoselective synthesis of 7,l l-guaien-8,12-olides. Pedro and co-workers12* have made use of the photo-rearrangement of the dienone (181) under acidic conditions to yield (182) which is a key intermediate in a synthetic approach to oxaguaianolides.
96
Photochemistry
The photochromism exhibited by pyrimidinespirocyclohexadienones has been reviewed.129
- -0Ac I
--Me
C02Me Me02C
(178) a; R = Ac b;R=H
(179)
0
4.2 Linearly Conjugated Dienones. - The cyclohexadienone (183) undergoes ring opening on irradiation in H20/THF or MeOH/THF to give the expected ketene (184) which can be trapped by water or methanol and is also photochemically labile undergoing cyclisation to afford the biradical(1 85).I3OThis representation is preferred to the alternative, a highly strained allene. 1,3-Silyl migration occurs in (185) to give the final products (186).When irradiation is carried out in di-isopropylamine/THF the product isolated is (187) where a second addition of amine has occurred. Irradiation of cyclohexa-2,4-diene-1-ones is well known to bring about ring opening with the formation of a ketene, and when the reaction is carried out in the presence of diamines bis-amides are formed.131Ring-opened amides have been synthesised following the irradiation of some cyclohexa-2,4dienone derivatives in the presence of a m i n e ~ .An ' ~ argument ~ has been presented
CONPr', MeO@ M :s TMS 0
(186) a; R = H b; R = M e
0
97
IIf2: Enone Cycloadditions and Rearrangements H
O M Ar 0
(188) n
Ar
R a b
Cyclodextrin
Host:guest
ee(%)
a
C; R = PhCH2
C
P
d; R = PhCH2CH2
d
P
1:l 1:l 2:1 2:1
28 20 20 25
a; R = Me
b; R = Et
a
Scheme 7
that casts doubt on the involvement of the colchicine triplet state in its isomerism into p- and y-lumic~lchicine.~~~ The cycloadditions undergone by the pyrones (188) with maleimide in the crystalline phase afford the adducts (189).134 Irradiation of the tropolone ethers (190) results in conversion to the isomer (191) which on prolonged irradiation is converted into (192) by a process that was reported many years ago. The rearrangement of these systems has been studied in the confines of cyclodextrins to examine the possibility of enantioselective cyclisations and modest ee percentages were obtained as shown in Scheme 7.'35
5
1,2-, 1,3- and 1,4-Diketones
5.1 Reactions of 1,2-Diketones and Other 1,2-Dicarbonyl Compounds. - The photofragmentation of glyoxal involves the singlet-excited state and decay from this to the ground state is accompanied by fission into hydrogen and C0.'36An ab initio study on the unimolecular dissociation of glyoxal has been carried OUt.137,138 A further patent dealing with the chlorocarbonylation using oxalyl chloride of 1,4-dinitrocubane has been filed.139A study of the irradiation of a series of
98
Photochemistry
1,2-diketones in an outdoor reaction chamber has been rep~rted,'~'and photolysis frequencies for some dicarbonyl compounds (biacetyl, methyl glyoxal and glyoxal) in the atmosphere have been determined.14' The photodissociation of methylglyoxal in the range 290-440 nm has been and the excitation spectra of biacetyl have been r e ~ 0 r d e d . l ~ ~ Further interest has been shown in the control that compounds such as (193) can exercise on photochemical cyclisations in solid-state crystals.14 In this example the hydrogen abstraction process within the amide (194) to afford (195) and (196) has been studied. The outcome is different dependent upon the inclusion compound used. Thus with (193, n = 1) the product formed is (-)-(195) in 21% yield and with an ee of 99%. When (193, n=2) is used the product is (+)-(196) obtained in 48% with an ee of 98%. P Ho h - HHMe (
Ph>i
Me PhZCOH
\
0
0
Me
0
Me
The photochemical disappearance of 4,4'-dihalosubstituted benzils is enhanced when ultrasound irradiation is used.1452-0xabicyclo[3.2.0] heptane-2,3dione derivatives are formed on irradiation of 5-phenylfuran-2,3-dione in the presence of styrene.'46 1-Phenylpropane-1,2-dione and butane-2,3-dione have been used as sensitisers using visible light for polymerising dental resins.'47The photodecomposition of derivatives of 1-phenyl-3-sulfonyloxypropane1,2diones has been rep~rted.'~' A complex mixture of products is formed on irradiation of indane-1,2,3-trione in methylene chloride solution with 2,3-dimethylbut-2-ene as the addend with hydrogen abstraction, oxetane and dioxene formation being among the reaction modes 0 b s e r ~ e d .The l ~ ~ irradiation of N-acetylisatin (197) with phenylacetylene, for example, affords the 2:l cycloadduct (198). Other acetylenes also undergo this addition reaction.lS0The isatin derivative (197) also undergoes oxetane formation when it is irradiated in the presence of alkenes. For example, the addition of the styrene derivatives (199) yields the oxetanes (200).151The triones (201) are photochemically reactive when irradiated in the presence of the alkynes (202).'52 With diphenylacetylene the polycyclic dione (203, X = H ) is obtained as the principal product with the minor product (204) resulting from ring opening of an oxetene. The major product from the photoreaction arises by dehydrogenative cyclisation. 5.2 Reactions of 1,3-Diketones. - Irradiation (m*excitation) in a supersonic in its enol form brings about jet of 1,1,1,5,5,5-hexafluoropentane-2,4-dione C-OH bond fission and the formation of hydroxy radi~a1s.l~~ The solid-state photochemical reactivity of dialkyl 1,3-acetonedicarboxylates(205) has been examined and decarbonylation and rebonding between the resultant radical pair
99
IIf2: Enone Cycloadditions and Rearrangements 0
I
COMe
go
(199) R = H, Ph or Me
(200)
X
I
0
X e - P h 0 (201) R = Me, H or Ph
(202) X = H, CI or OMe
\
N,
/
R
0
(203)
to give dialkyl succinates (206)has been de~cribed.'~~ A study of the formation of the radicals (207) using laser flash or pulse radiolysis has been r e ~ 0 r t e d .The l~~ radical (207) is formed initially but is rapidly transformed into the phenoxyl radical, and the importance of such species in cancer chemoprevention was discussed. The influence of solvent on the photophysical properties of the naturally occurring dione (207a) has been assessed.'56
0
0
0 OMe OR2
JXo
A photophysical study of the excitation of 4-cyclopentene-1,3-dione has been carried A review of the photochemical behaviour of compounds such as the Meldrum's acid derivative (208) has been publi~hed.'~~ Photochemical reactivity of vinyl or allenyl methane derivatives such as methylene Meldrum's acid has been reviewed.159Both (2.n + 2.n) and (4.n + 2.n)-photocycloadditionsoccur on irradiation of substituted 1-acetonaphthones and 2-morpholinoacrylonitrile.160
100
Photochemistry
5.3 Reactions of 1,4-Diketones. - Pyrex-filtered irradiation of powdered samples of benzoylbenzamides (209) results in their transformation into asymmetric phthalimides (210).I6lSeveral mechanistic paths were considered for this process but the results indicate that the preferred route involves a radical pair thought to be (211), and cyclisation within this species and rebonding to give the final products. The irradiation of the diketones (212) as suspensions in water using Pyrex-filtered light has been examined.’62The product from the irradiation of (212a) is the thermally unstable ketene (213b) that is formed by phenyl migration within the cis-diketone moiety. The stability of the ketene produced by this rearrangement can be enhanced by a methyl group at the bridgehead as in (212b) and this then yields the ketene (213b) which is thermally stable to around 40°C but at which point readily adds methanol to give the ester (214). The migration of the phenyl group always occurs in these solid-state reactions from the more crowded benzoyl group. This tendency was also demonstrated in the ketones (212c) and (212d). The yields of the ketenes obtained are in the 25-40% range.
a;
R’
R2
R3
Me Me
Ph Ph
H Me
co
b; CO C; CH2 d; CO
CH;! CH2 CO CH2
Me0 Me Me Me
Yield (Yo) 0°C 15 “C
H H H Me
>99 99
ee (%) 80 42
Yield (%) -50°C -50°C
99 99
ee (%) 87 87
H H H Me
The homoquinones (215 ) undergo addition reactions with ethyl vinyl ether from irradiation with wavelengths > 300 nm in benzene solution under an argon a t m 0 ~ p h e r e .The l ~ ~reactions exhibit some substitution dependence and addition of the vinyl ether to the homoquinone derivatives (215a-e) yields conventional (2n:+ 2n:)-cycloadducts identified as (216) in good to excellent yields. Only with the dibromo derivative (215f) does a different reaction occur and this yields the adduct (217, 57%). All of the processes are thought to involve a biradical
101
I I / 2 : Enone Cycloadditions and Rearrangements
presumed to be (218) and C-C bond formation will afford the cyclobutanes. The authors suggest that the homoquinone (215f) reacts via the cation (219) which cyclises by C-0 bond formation. A similar series of products is formed using the positional isomers (220) from which (221) and (222) are formed. Exclusive oxetane formation occurs on irradiation of 2,6,6-trimethylcyclohex-2-en-1,4dione in the presence of a l k e n e ~ . ' ~ ~
4YPh 0
0
a; b; c;
Ph
R'
R2
Yield (Yo)
Me Me Me
Me CI Br
69 72 80 89 82 mixture of adducts
d;
CI
CI
e; f;
Me0 Br
Me Br
EtO + (219)
a; b; c; d;
R'
R2
Me CI Me0 Br
Me CI Me Br
Yield (%) 89 95 88 (mixture)
-
-
-
Maleimide is reported to exhibit rapid tautomerism in the triplet-excited state. This apparently has prevented the authorP5 from measuring the triplet-state quantum yield. N-Alkylation, however, prevents this tautomerism and the quantum yield for triplet-excited state formation has been measured as 0.03 for the N-methyl, 0.07 for the N-ethyl and 0.05 for the N-propyl derivatives. Irradiation of the P-hexopyranosyl imide (223) in acetonitrile for 2.5 h brings about its conversion to the lactam (224).'66The reaction is quite selective and involves a hydrogen abstraction process to give the 1,4-biradical(225)which is the key to the transformation: cyclisation and rearrangement eventually affords (224) in 69% yield. The a-isomer of (223)is also photochemically reactive. The hydrogen abstraction path and cyclisation selectively affords the final product (226, 83%).
102
Photochemistry
Manno derivatives (e.g. 227) were also examined and in this instance two products (228, 20%) and (229, 62%) were obtained. A low regioselectivity is observed in the photochemical addition of (230) to alkyl-substituted naphthalenes and the principal products are the adducts (231).'67 The photochemical properties of some derivatives of p-phenylenediacrylic acids have been studied.16' A detailed examination of the photoconversion of a mixture of stereoisomers of (232) into the carboxylic acid (233) has been reported.' 69
OR
OR
10
OR
(223)R = TBDMS
/
OR
RO% OR OR
Ro
Ho&o
W
O
OR
A'% N
'
R
OH OR
0
0
NMe
0 R
(231)
(229)
n
C02H C02H
(232)
(233)
1
5.3.1 Phthalimides and Related Compounds. The Pyrex-filtered irradiation of (234) in acetone/acetonitrile has provided a route to the cyclobutane derivatives (235).I7OThe outcome of the reactions is variable and with (234, R = Me) the product (235, R=Me) is obtained in 65% yield while only 8% of (235, R,R = [CH&) is obtained from (234, R = [CH&). A series of intramolecular cycloaddition reactions of the maleimide derivatives (236) and (237) have been described.171 Products (238) and (239) were obtained in moderate to excellent yields. Such formation follows the normal (2.n+ 2n)-addition mode to the maleimide C=N group in (240) and ring opening of the resultant adduct affords the final products, The addition via (240) explains the observed regio- and stereo-chemistries.
1112: Enone Cycloadditions and Rearrangements
(234) R = Me or R-R
= -(CH2)4-
103
(235) R = Me, 65% R-R = (CH2)4, 8%
R2
n
H H Me Me -(cH2)4Me Me
1 1 1 2
R' R2 H Q R :!'
N
0
M$Nj;;; .Q
Yield (YO) 50 83 89 90
Me
90% 1 : l n = 2
0
0
(237)
H
1"
0
(239)
(240)
*N-Me 0
f
R (241) a; H
b; c; d; e; f;
0
Pr' CH(Me)Et But CH2CHMe2 Me
Yield %
a b C
d e f
52 86 73 84
-
Scheme 8
104
Photochemistry
(243)
Rq
Z
H
(244)
(YO)
R'
R2
Me
H
54
H
Me
75
H
Yield
Pr'
81
But
83
0 (245)
+o Rq \
0o X
0
0 H'.
- -C02K
(246) R = H or CI
(247) X = H (65%) X = CI (48%)
Further examples of the acetone-sensitised reactivity of N-methylphthalimide with a-ketocarboxylates (241) have been published and a variety of products are reported as shown in Scheme 8.172Griesbeck and his c o - ~ o r k e r s have ' ~ ~ also described intramolecular examples of the utility of the decarboxylative cyclisations. They have shown that subtle changes in the structure can influence the results greatly. Thus, the derivative (242) does not cyclise but merely decarboxylates. However, the derivatives (243) are synthetically useful and irradiation affords the products (244). More complex structures can be synthesised such as (245)from the irradiation of (246).These cyclisations afford products with 86 and 79% ee. Other cyclisations affording (247) have also been described. A further example of decarboxylative cyclisation occurs on irradiation of (248) in acetone/water to give (249).174Associated with other work on such systems, Griesbeck and c o - ~ o r k e r s have l ~ ~ examined the influence of deactivation processes such as hydrogen bonding upon the photodecarboxylation of mphthalimido potassium carboxylates. The same workers have also published a review dealing with photocyclisations of this type and others that occur stereo~electively.'~~ The macrocyclic cyclisations of a series of phthalimide derivatives (250) has
105
II/2: Enone Cycloadditions and Rearrangements
s-O &
N
P
o
0
@N/””
Me
0
0 (255) R1 = Me, R 2 = Ph
R’ = Ph, R2 = Me
(256) R’ = Me, R2 = Ph, 16% R1 = Ph, R2 = Me, 14%
been studied.17’Irradiation of (250, n = 2 or 3) in acetone results in the quantitative formation of the derivatives (251).Irradiation of (250)in acetonitrile results in loss of the side chain while the derivatives (250, n = 5 or 10) behave in the same manner when irradiated in acetone. The influence of side chain substitution was also investigated and (252) is converted into (253) in 61% yield on irradiation in acetone. Other researchers have described interesting additions of 2-phenylpropene to the phthalimide derivative (254)which give the cyclised product (255) in good yield as a mixture of isomers.’78The reaction was extended to the synthesis of large ring compounds such as (256). Again the reaction involves a suitably substituted phthalimide and the propene and results in the formation of the two isomers (256) in 16 and 14%. Earlier studies by Mariano and his co-workers demonstrated that efficient cyclisation of (257) could be carried out. The formation of the spiro product (258) is the result of a SET process. Such cyclisations have been extended to the phthalimide derivatives (259) and (260).179
106
Photochemistry
The products (261) formed from (259) are obtained in good yield. Larger ring compounds can also be synthesised. Thus, irradiation of (260) affords the products (262, n = 1) and (262, n = 2) in 80% and 72%, respectively. The photochemical ring expansion of cycloalkanones has provided a route to allylic N-phenylimides."' Electron transfer within the naphthalimide derivatives (263) has been studied.'" The fluorescent behaviour and the pH dependence have been evaluated for a series of tetracarboxydiimides.lS2The addition reactions encountered with the carboxamide (264)have been reported.lg3A study of the photophysics of N,N-ditridecyl-3,4:9,1O-perylenetetracarboxylic diimide has been reported.lg4
b,
N-SiMe,
&N
@-
Jph
I
0
CH2Ph (257)
x -SiMe,
(259) X
(258)
= NMe, NAc, 0 or S
(261) X = NMe, NAc, 0 or S
(260)
(262)
R'
\
/
\
\
I
R2 (263) n = 2-6
R'
R2
Pi Pi Bn Bn
H Me H Me
(264)
5.3.2 Fulgides and Fulgimides. Considerable improvement on the reversibility in photochromism of fulgides has been reported from studies in films,lg5and progress continues to be shown in the development of new photochromic systems. Thus, the photochromic fulgides (265,266) have been synthesised and patented.lg6The effect of pressure on the photochromicity of the fury1 fulgide { 2 4 1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]-3-isopropylidenesuccinicanhydride} has been e~a1uated.l~~ Others have also investigated the photochromism of some anhydrides and fulgides.'s8 Ab initio calculations have been carried out on some thienylf~lgides,'~~ and the photochromic properties of benzofurylfulgidescondensed with binaphthol have been investigated.'" Details of the synthesis and the photochromic properties of the fulgides (267,268) have been reported.lgl Cyclisation in toluene solution by irradiation at 366 nm of the thiofulgides (269) affords the thermally stable photochromes (270).192The
11/2: Enone Cycloadditions and Rearrangements
107
heliochromism of some benzothienylf~lgides'~~ and other photochromic fulgides has been r e ~ 0 r t e d . The l ~ ~ fatigue resistance of some fulgides, in so-called naked spin-coated polymer, can be increased by careful exclusion of air.195A convenient synthetic approach to some photochromic fulgides has been described using carbonylation of but-2-yne-1,4-diols.lg6Y ~ k o y a m a has ' ~ ~ reviewed some of the photochemistry undergone by fulgides and this article in particular has focused upon the use of such molecules for photochemical switches. A review of the photochromism exhibited by fulgides has been pub1i~hed.l~~ Calculations have been carried out to assess modelling for the design of new photochromic systems.lg9 The photochromic properties of some novel indol-2-ylfulgimides have been studied.200Irradiation of (271) at 366 nm brings about cyclisation to yield the stable 'photochromes' (272).201
Ry!+$* R3
Me
Me Me
Me
Me
(265) R' = R2 = alkyl
X
R Ph
0
Ph Me Me
C(CN)2 0 C(CN)2
(266) R' = ally1 or vinylbenzyl R2 = R3 = alkyl, aryl or arylalkyl
X 0
Ar 4-FCeH4
4-FCGH4 C(CN)2 Ph OorC(CN),
Y 0
(269) R = Me or Ph
0 S
r? H2
R'
(271)
0
X
R'
R2
0 0 S S
Me Me Me Ph
Me Cyclopropyl
Me Cyclopropyl
272
(270)
108
6
Photochemistry
Quinones
6.1 o-Quinones. - A study of the photoreduction of benzoquinones by N,Ndimethylaniline derivatives has been reported.202The irradiations were carried out in the 400 nm region (n,n* excitation) and irradiation at 600 nm brings about an nn* excitation. The authors suggest that a reversible triplet exciplex is involved in the photoreductions. The biradicals produced on irradiation of quinones with norbornadiene or quadricyclane have been studied by CIDNP.203 Photochemical allylation of 1,2-naphthoquinones has been reported. The reaction involves irradiation of the quinones with allylsilanes and a triplet exciplex is implicated. This reaction produces a [3 + 21-cycloadduct that is converted into the final product (Scheme 9).2a4Irradiation at 450 nm of a series of phenanthraquinone derivatives in the presence of alkenes yields dioxenes usually with reasonable stereochemical integrity.205The reactions are efficient and phenanthraquinone itself undergoes addition with unit quantum efficiency. Studies of the factors that control photoinduced electron transfer within a porphyrin-p henoxynaph thacenequinone pho t ochromic system have been evaluated.206 OH
0
R’
R2
H H H H Br
H
R3 = H or Me
CN Me0 AC H
Scheme 9
6.2 p-Quinones. - The addition of benzaldehyde to benzoquinone can be carried out efficiently by irradiation in benzene in the presence of benzophenone. A recent study has demonstrated that the process is more efficient in supercritical carbon dioxide and under these conditions, as shown in Scheme 10, yields as high as 8 1Yo can be a~hieved.~” Electron transfer from 1,3,5-trimethoxybenzene to a series of quinones (benzoquinone, 2-methylnaphthoquinone and anthraquinone) has been reported.208The low-lying electronic states of p-benzoqwinone radical anion have been studied from a theoretical standpoint.209Both laser flash photolysis and continuous irradiation have been used to establish the mechanism whereby 3-arylbenzoquinones undergo cyclisation to yield 2-hydroxybenzofuran derivatives. The triplet nn* excited state of the quinone is involved.210 A series of products is formed when halogenated 1,4-benzoquinones (e.g. p chloranil) are irradiated in the presence of 2,3-dimethylbut-2-ene or 3,4dimethylpent-2-ene?” The products were identified as monoallyl ethers (273) and (274) of the corresponding hydroquinones. Calculations have been used to
109
II/2: Enone Cycloadditions and Rearrangements 0
II
OH
0
56%
scicop+ 5%
B~OH 5735 psi
81%
Scheme I 0
OH
OH
optimise the structures of 2,3-dicyano-5,6-dichloro-p-benzoquinone and its radical anion.212 Both hydrogen abstraction and electron transfer reaction paths have been reported in a study of the laser irradiation at 248 nm of 1,2- and 1,4-naphthoq~inones.2'~ The topochemical photo-polymerisation of the bisquinone derivatives (275) has been The m*triplet excited state of 1,4-anthraquinone has been examined using flash and steady-state p h o t o l y s i ~ .Dimerisation ~'~ and hydrogen abstraction reactions were reported and no (2n + 2n)-cycloadducts were detected when the quinone was irradiated in the presence of alkenes. The photochemical oneelectron reduction involving radical ions of 1,4-dihydroxyanthraquinone has been studied in the presence of l-benzyl-l,4-dihydronicotinamideand 5,5dimethyl- 1-pyrroline N-oxide.2161-Hydroxyanthraquinone (276a) undergoes 0
II
0
R'
R2
R3
H Me Me Me
H
H H
F CI Br
H Me Me
H Me
H
H
H H
H
H
Photochemistry
110
photochemical amination on irradiation in the presence of n-butylamine to give the two products (276b) and (276c) in a ratio that is dependent on the reaction In acetonitrile under an atmosphere of air the ratio of (276b):(276c) is 51. This changes to 0.3:l when the reaction is run under nitrogen. Interestingly the corresponding 1-aminoanthraquinone does not undergo amination. The quinones (277) also undergo amination with the same amine to yield the 4butylaminoquinone (277, R =NHBu"). Rapid proton transfer within the quinone (278) is the result of formation of the lowest excited singlet state.218A series of oligomers based on the system shown in (279) has examined one-electron transfer to the anthraquinone moiety.219The possibility that one-electron transfer occurred from thymine dimers to anthraquinone resulting in the repair of the DNA was investigated. The results showed that there was little or no repair at such sites. A review has highlighted the photo- and radiation chemistry of quinones that are of value in
n
O
R
0 (276) a; R' = R2 = H b; R' = H, R2 = Bu"NH c; R2 = H, R' = Bu"NH
(277) R = H or Br
(278) n = 1 or 2
A stable radical ion pair is formed when 3,4-di-O-benzylhypericin is irradiated The photochemiin the presence of bi~-1,8-N,N-dirnethylaminonaphthalene~~~ cal rearrangement of 3-0-benzylhypericin has been and a synthesis of racemic methylenomycins A and B has been reported making use of the photochemical rearrangement of quinones as the key step.223A review of the general area of photochromism in quinones has been
7 1.
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II/2: Enone Cycloadditions and Rearrangements 2. 3. 4. 5.
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111
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III2: Enone Cycloadditions and Rearrangements
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114
Photochemistry
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3 Photochemistry of Alkenes, Alkynes and Related Compounds BY WILLIAM M. HORSPOOL
1
Reactions of Alkenes
1.1 cis,trans-Isomerisation.- A review has detailed recent work carried out on photochemical cispans processes in the singlet state.' A photochemical step has been utilised in the synthesis of irones. One of the key steps was the photochemical isomerisation of the a-double bond.2 The enantiodifferentiating photochemistry of cyclooctene has been described using the nucleoside (1) as the chiral ~ensitiser.~ Benzoate substituted cyclodextrins have also been used to photoisomerise 2-cyclooctene.4The yields obtained are better than those found using alkyl benzoates as the sensitisers. Irradiation of trans-P-methyl-P-nitrostyrene in acetonitrile brings about isomerisation to the corresponding cis-isomer with a quantum yield of 0.8.5The photochemical isomerisation of the derivative (2) into (3) is a key step in the synthesis of locked side chain analogues of calcitriol.6The spectral properties of some 6-styryl-2,4-disubstituted pyrylium salts have been m e a ~ u r e dThe . ~ trans@-isomerism within the naphthalenophane (4) has been studied.* A detailed account of photochemical reactions of alcohol protecting groups (5)has been presented.' The deprotection of the alcohol is dependent on a primary trans,cis-isomerisation path on irradiation at 254 nm which is followed by a photochemical 1,5-hydrogen migration to give intermediate ( 6 )and then by a 1,5-silicon migration to yield (7). Collapse of this intermediate affords the free alcohol. Some of the classes of alcohol and the percentage yields are shown.
I .I .I Stilbenes and Related Compounds. Long-wavelength trans,cis photochemistry of stilbene and some derivatives has been described." The cooling kinetics of Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002 119
120
Photochemistry
OH
(5)
R' = Me or Pr'
0,
Me0
T
87%
photoexcited trans-stilbene have been studied using time-resolved Raman spectroscopy." A resonance Raman spectrum has been observed following irradiation of cis-stilbene at 267 nm.I2 Irradiation of trans-stilbene at a silica gel/air interface affords considerable amounts of two d i m e r ~ . 'Some ~ isomerisation to the cis-stilbene also takes place as well as oxidation to give benzaldehyde. The isomerism kinetics of the stilbene (8) have been measured.14Stilbenes are well known to undergo photochemical cyclisation to phenanthrene derivatives on irradiation. A recent study of this cyclisation reaction has examined the potential for the synthesis of ketones following the cy~lisation.'~ Many examples were reported but the irradiation of (9) in acetonitrile with 0.5 M HCl to yield the expected intermediate (10) is typical. 1,9-Hydrogen migration occurs in (10) to afford the enol ether (11) which on hydrolysis under the acid conditions is converted into (12) in 96% yield. A further study of this reaction has revealed that the products obtained from (9) in acetonitrile solution are dependent on the concentration of acid used.16Thus the enone (1 3) is formed with 5 x lop3M HC1 while (12) is formed, as reported originally, with 0.5 M HC1. An examination of the excited-state properties of the stilbene derivatives (14) has sought further information on the rneta-amino effect.17A detailed study of the photophysics of a series of 1,2-diarylethenes (15) has been carried out." Various computative methods have been used to investigate the isomerisation of simple stilbene derivatives and the stilbene super molecule ( 16).19The photophysical properties of several triazine/stilbene fluorescent brighteners have been studied in aqueous and alcoholic The photoisomerism of 4,4'-bis(benzoxazoly1)stilbene in a variety of solvents has been studied and the activation energy for the trans-cis-isomerisation was measured.*l The photoisomerisation of 4,4'-diaminostilbene-2,2'-disulfonateat a variety of wavelengths has been studied and the reaction was shown to be pH dependent.22
121
I I / 3 : Photochemistry of Alkenes, Alkynes and Related Compounds CH20H /
OMe
I
a!?
PhT OMe
\
(9) 0
OMe
\
\
\
flRi3 flR4 /
R1
/
R1
R2 (15)
(14)
R’
R2
R3
R4
R’
R2
R3
H H H CN
H H CN H
NMe2 H NMe2 H
NMe2 H NMe2 H
H H H CN H CN
H Me0 CN Me0 Me0 Me0
H Me0 H H H Me0
CN C N- l>
NC-C,
/ NH2
\ \
(16)
A study of the cis,trans-photoisomerism of the suberanes (17-19) has examined the suitability of such molecules as optical switches.23Irradiation of some cis-stilbenomethanofullerenes converts them quantitatively into the corresponding trans-i~omers.~~ Reviews have detailed both one-way and two-way isomerisation in a variety of stilbene derivative^^^ as well as other aspects of the photo-
122
Photochemistry
(17) R' = R2 = R3 = R4 = H (18) R' = R2 = H, R3-Re4 = -(-CH=CHf2 (19) R1-Re2
= +CH=CHf,,
R3 = R4 = H
chemistry of stilbene.26The conformational equilibrium of some derivatives of (E,E)-2,6-di(arylvinyl)pyridinehas been in~estigated.2~ The 2,E-isomerism of (20)has been studied in The photoluminescence of E-l-(9-anthrylethenyl)-4-chloromethyl-2,5-dimethoxybenzene (21) has been inve~tigated?~ and the results of a study into the photoisomerisation of 1-(9anthryl)-2-(N-quinolinyl)ethene derivatives have been p~blished.~' The efficient trans-cis-isomerisation of 1-(9-anthry1)-2-pyridylethanehas been de~cribed,~' and the photophysical properties of l-pyraziny1-2-(3-quinolinyl)ethenehave been measured.32 Experiments have shown that it is possible to control the alignment of polymethacrylates using the photochemically induced E,Z-isomerism of styrylpyridine ~ i d e - c h a i n sAnthracene .~~ has been shown to be the most efficient catalyst to effect cis-trans-isomerisation in 1-(3,5-di-t-butylstyryl)pyrene which occurs with a quantum yield 11.5 times higher than for the uncatalysed process.34
mPh \
CH&I (21)
1.1.2 The Dithienylethene System and Related Compounds. In recent years a considerable number of detailed studies have been reported into the photochromism of (22) and related systems both in solution and in constrained environments. Irie35has published a review detailing the photochromism of such species in constrained environments. A further review has dealt with their behaviour in the crystalline phase36and a general review has surveyed the recent advances in the field.37A new read-out system has been suggested as a method for determining degradation in optical memory systems using photochromic diar~1ethene.s.~~ The photochromism of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopent ene (22) and 1,2-bis(2-meth yl-6-ni t ro- 1- benzo t hiophen- 3-yl) per-
123
I I / 3 : Photochemistry of Alkenes, Alkynes and Related Compounds
fluorocyclopentene in the solid state has been A further study has demonstrated that irradiation of the chiral cyclohexane (23) provides a large pitch change in the chiral nematic phase!' The photochromism of the dithienyl alkene (24) in clays has been s t ~ d i e d ; ~while , ~ ~ the photochemical colourisation of (22) and some of its derivatives has been studied in polymer The radiation sensitivity of such cyclopentene derivatives has also been examined.44 An X-ray crystallographic study of the photochromism exhibited by (22) in the The unsubstituted perfluorocyclopentene crystalline state has been carried derivative (25) has also been shown to undergo the usual photochemical cyclisation but degradation by dehydrogenation to afford (26) is a competing Interestingly compound (22) also undergoes photo-decomposition to give (27) which is thought to arise by S-C bond fission of the ring-closed form to yield biradicals such as (28).47Rearrangement within (28) affords (27). F2
F-
t--.(
Y
Me'
(24)
(26)
(25)
A
Me
S Me I Me S
(27)
I
Me
Me
MeMe
Me
(28)
Many variations on the substituents surrounding the basic skeleton of (22) have been carried out over the years. Aryl substitution of the thiophene rings has The compounds been applied and a pulse-laser study of (29) has been generally show photochromism, as do all the derivatives (30),when irradiated at 366 nm, which induces their conversion to the blue cyclised form. Irradiation of the cyclised isomers at wavelengths >480 nm reverses the process in a rate dependent upon the substituents on the aromatic ring. The fastest reversal is achieved with the 4-tolyl substituent.4' Photochromism is exhibited on irradiation of crystals of 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (31), and changes to the surface morphology of the crystal were observed
124
Photochemistry
Ph
Ar
Me Me
(30) Ar = Ph, 4-MeC6H4or 4-Bu‘C6H4
(29)
following the isomeri~ation.~~ Other changes to the substitution around the rings, such as increasing the size of the substituents at the 2-position, do not appear to influence adversely the photochromicity of the systems. Thus (32) has been shown to cyclise to the closed form even in the single-crystalline phase.51The influence of 2,2’-isopropyl substituents (e.g. 33) on the photoactivity of the photochromic bisbenzothienylethenes has been and refractive index changes have been studied in photoisomerism of some diarylethene derivatives with 2,2’-t-butyl s u b ~ t i t u e n t sThe . ~ ~ effects of other substituents have also been investigated as in the diastereoselective cyclisation of the photochromic diarylethene (34).5’356Other workers have described perfluorocyclopentene derivatives with an optically active group at the 2-position of the thiophene ring.57 F2
-
Me Ph
F2
I \
Me
/ \ Me Me
Ph
Ph
/ \
Pr‘ PIJ
Ph
F2
F7
R’
Et Et
I \
R’
Ph
Me
Ph
(331 R’ = Ph. R2 = H
The alkene (35) undergoes cyclisation to afford a blue compound on irradiation at h > 366 nm.” The reverse reaction occurs on irradiation at h > 408 nm. Irradiation of amorphous films of the substance shows the same coloration and the same wavelength dependence. Studies into the photochromic behaviour of (36)have revealed that the quantum yield of cyclisation is solvent de~endent.’~ In the crystalline phase the alkene (37) undergoes photochemical cyclisation on irradiation at 366 nm to give a blue form.6oIrradiation at longer wavelengths (578 nm) reforms the starting material. Photochromism is also observed when the alkene is complexed with Zn(hfa~)~.2H~O. The dye (38) shows enhanced fluorescence with quantum yields of the emission rising to 0.83 from 0.001 when the thienyl unit specifically is irradiated at 313 nm which also brings about the usual ring closure of the alkene moiety.61
125
IIj3: Photochemistry of Alkenes, Alkynes and Related Compounds
Me Me
4-toly1,
4-tolyl
4-tolyl
(35)
The alkene (39) undergoes ring closure on irradiation at 313 nm in solution.62 These compounds form extended aggregates in solution and provide a new self-assembly system for photochromic switches. The ring closure is photoreversible and irradiation at h > 520 nm reforms the starting material. A new series of phot ochromic compounds based on 1,2-bis(2-et h yl t hio- 3-t hieny1)perfluorocyclopentene has been synthesised and studied.63Another example of the versatility of the dithienylethene photochromic system has been reported and this involves the first example of incorporation of a porphorynic moiety.64Thus compound (40)undergoes ring closure on irradiation at 313 nm and ring opening of the closed form can be brought about using h > 480 nm. Phot oreversible phot ochromism of 1,2-bis(2-methyl- 1-benzo thiophen- 3y1)perfluorocyclopentene derivatives (41)65and of (42) and (43) in the amorphous state has been observed.66A full account of the photochromism exhibited by (44) has been published67and a study of photoswitching within such molecules has been described.68Thediarylethene (45) has been studied as a dopant in liquid crystals and on UV-irradiation this causes a disruption of the cholesteric phase, a process which is reversed by irradiation with visible light.69 The aryl groups in such systems have also been substituted with amino functions as in (46) and the control on cyclisation of these alkenes that can be exercised by cyclodextrins has also been asses~ed.~' The study has also been extended to examine the behaviour of the tetramethylammonium salt derivatives
Photochemistry
126
A H I
R/
ROC
Me Me
Me Me
COR
0
0
= C12H25
(39)
R = But R=
'0(40) Ar = 4-MeC5H4
Me Me '0'
\
0'
(44)
F2 ( 3 F 2
R 0
&
5
g
&
0
.R
R = -0
127
I I / 3 : Photochemistry of Alkenes, Alkynes and Related Compounds
(47).71There are two arrangements possible for the triene, the one illustrated as (47), the antiparallel form, where cyclisation can occur and the one shown schematically as (48). This latter is referred to as the parallel arrangement and this does not photocyclise. In P-cyclodextrin the amount of the antiparallel form is enhanced and the quantum yield for the formation of the cyclic form is also enhanced. In y-cyclodextrin, however, the reverse is true and the parallel form is preferred leading to a reduction in the quantum yield for cyclisation. Some photochromic compounds based on 1-(3-methylbenzo[b]thiophen-2-y1)-2-(2methylindol-3-y1)cycloalkeneshave been synthesised and, as these compounds have absorption bands around the 440 nm range, they can be excited using InGaN blue lasers.72
(46) R = H (47) R = Me
Other studies by Irie and his c o - w ~ r k e r sreport ~ ~ three-dimensional erasable optical recording using the photochromism of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene. Studies on the photochromic properties of diarylethenes with terthiophene components have been and the photocyclisation of the 1,2-dicyanodiarylethene (49) has been studied.75The efficiency of the cyclisation is wavelength dependent as shown by the quantum yield data for the reaction ($365 = 1.1, $405 = 0.32). The ring opening reaction is induced by irradiation at longer wavelength ($532 = 0.16). The behaviour of such systems in amorphous films, cast polymer films and colloidal solutions has also been examined.76 A examOther workers have also reported photochromism in these m01ecules.~~ ination of the photochromicity of compounds (50) and (51) has been reported.'8 Me Me
Ph
cN -Me CN
CN
(49)
1.2 Miscellaneous Reactions. - Ab initio studies on the photodynamics of ethene have been reported.79Pulsed laser photolysis has been used to measure the absolute rate constant for the reaction of ethynyl radicals with ethyne.80 Irradiation of phenylacetylene in a molecular beam at 193 nm results in the
128
Photochemistry
formation of ethyne and a C6H4 fragment. This fragment breaks down into he~a-l,3,5-triene.~’ Complex hydrogen bonding has been detected in the crystals of the diyne (52).82 1.2.1 Addition Reactions. A review has highlighted the applications of photoinduced alkylation of electrophilic a l k e n e ~Further . ~ ~ studies on the methoxycarbonylation of alkenes have been reported.84The addition products (53) and (54) and the reduction product (55)are formed when (56) is irradiated in the presence of allyltrimethyl~ilane.~~ The reaction is dependent on the present of an ‘additive’ such as phenanthrene and the ratio of the products is dependent on the particular additive used. Cyclic alkanes undergo abstraction of hydrogen to afford the corresponding cycloalkyl radical when they are irradiated in the presence of benzophenone (or a polymer tethered derivative) as the hydrogen abstracting reagent.g6The resultant radicals add moderately efficiently to alkynes and selected results are shown in Scheme 1.
-
w
I: C-OEt
+ Me02C*C02Me
c
-
o
2
E
ITCo2Me C02Me
t
;
Yier3(%)
EIZ
70 3 5
46 47
1.33 1.53 1.oo 1.14
1 2
50 15
100% Z 0.29
Scheme 1
2 2 . 2 Electron-transfer Processes. Arnold and his co-workersg7have reported the photochemical deconjugation of the arylcyclohexenes (57) to yield (58). The reactions are brought about using single electron-transfer photochemistry. Calculations have been carried out to examine the electron-transfer behaviour of the tet rac yanoe thene/tet ramet hylethene system.88A single electron-t ransfer react ion is involved in the conversion of (59) into (60).89The reactions are carried out using DCA as the electron-accepting sensitiser and the radical cation of the styryl moiety cyclises to give the intermediate (61). A study of photochemically induced intramolecular charge separation in the derivatives (62) and (63) has been carried
1.2.3 Other Processes. The photodissociation of several substituted alkenes has been reported over the past year. Thus the fission processes encountered on the
129
II/3: Photochemistry of Alkenes, Alkynes and Related Compounds R
R
(57)
R = OMe, Me F, H, CF, or CN
P
h
-
N
X
X
P
h
(62) X = C=C(CN)* or CH2
-
N
m
(63)
irradiation at 193 nm of vinyl chloride have been investigated:' and similarly and chlorotrithe photochemical dissociations of l-chloro-l-fluoroethene92 f l ~ o r o e t h e n ehave ~ ~ been studied at the same wavelength. Several dissociation channels were identified for the former compound, while three principal fragmentation paths were detected for the latter molecule. Other workers have also studied the primary photofragmentation of 2-chloropropene induced by irradiation at 193 nm.94395 Three processes were elucidated including two that involve C-Cl bond fission producing so called fast Cl and slow Cl species. Elimination of HCl is also a recognised decomposition pathway. A spectroscopic study of allyl radicals generated by irradiation of either allyl iodide or hexa-1,5-diene has been reported?6 Interestingly, the use of longer wavelength irradiation (254 nm) of halo compounds does not always bring about C-halogen bond rupture.97For example, the irradiation of (64) in hexane with a low-pressure Hg lamp brings about complete conversion to the isomer (65). A photochemical desilylation of silyl enol ethers has been described?* Me0
Me0 Br
Br (64)
2
(65)
Reactions Involving Cyclopropane Rings
2.1 The Di-n-methane Rearrangement and Related Processes. - A re-investigation of the photochemical behaviour of 1,3-diphenylpropene has shown that irradiation in cyclohexane affords the products shown in Scheme 2.99 This outcome is independent of wavelength and either 254 nm or 300 nm is effective,
130
Photochemistry
but the former also induces isomerisation of the cis-cyclopropane to the transisomer (66). Product formation is solvent dependent and there is a marked difference when acetonitrile is used. Thus irradiation of the 1,3-diphenylpropene at 254 nm in acetonitrile affords the trans-isomer (66) as the major product (50% chemical yield), and the quantum yield for the di-n-methane rearrangement is an order of magnitude greater than that observed in cyclohexane. The authors suggest that the change in behaviour in changing from cyclohexane to acetonitrile is the result of excitation of the alkene to a higher singlet state, Direct irradiation at 1 ~ 3 0 nm 0 of the trienes (67) results in efficient di-n-methane rearrangement to afford the homobarrelene derivatives (68).'O0 The barrellene (69) is also reactive in this mode and irradiation (Pyrex filter) in acetonitrile solution for 20 min brings about the formation of the cyclooctatetraene (70, 27%) and the semibullvalene derivative (71, trace)."' Irradiation of (69) in toluene gives the same products but the yield of (71) is enhanced to 43%. The irradiation of the cyclooctatetraene derivative (70) in deuteriochloroform induces ring closure in the octatetraene followed by bond fission to give (72) in 68% yield. The influence of substituents on the mode of cyclisation of the barrellenes (73) has been studied in detail.lo2The derivatives (73a and b) cyclise by the 3,ll-bonding pathway to yield the semibullvalenes (74) quantitatively, whereas derivatives (73c and d) on irradiation exhibit both 3,11 and 2,12 bonding to afford mixtures of (74) and (75). Derivative (73e) exclusively follows the 2,12-
-phyP.. A +
Ph-Ph
Ph
Scheme 2
Ar
(67)
R
Ar
C02Et C02Et COMe COMe
Ph 4-CICeH4 Ph 4-CIC6H4
CN
CN
(68) Yield (Yo) 70 78 80 94
Ph
Ph
+
Ph
Ph
131
IIf3: Photochemistry of Alkenes, Alkynes and Related Compounds
bonding path to give (75e). Interestingly, the dibenzodihydropentalenofurans (76) are photochemically reactive and irradiation converts them back into the dibenzosemibullvalene derivatives (77) by way of the triplet radicals (78) which absorb around 410 nrn.Io3Further studies on the photochemical rearrangement of bridgehead-substituted dibenzobarrelenes have been reported.lo4 COPh COPh
COPh
& &
2
\
/
R2 (73)
\
Ph
I
/
R2 (75)
R2
(74)
R'
3, 1I-bonding
R2
a; PhCO b; PhCO
c; MeCO d; PhCO e; PhCO
Me Me0 Me C& Ph
a;
2, 12-bonding 10 100
b;
,!
c; d; e;
Ph
I
47 35
-
,I
,
-
qq
53
it
65
"
100
Ph
(76) R' H H AC
COPh 4-MeOC6H4
R2 But
(77)
(78)
AC
Me Me H
A full account of the tri-n-methane photochemical reactions of, for example, (79) which results in the conversion to the two principal products (80) and (81) has been published.1o6. The intermediate biradical (82) is the key to the rearrangement.
Ph
\
Ph (79) R = Me or PhCH2
Ph
(80)
Ph
Ph Ph
Ph
(81
(82)
Calculations regarding the photochemical cyclisation from the S1 state of cyclo-octatetraene into semibullvalene have been r e p ~ r t e d "and ~ Wilseylo8has examined theoretically the rearrangement processes open to the non-conjugated
132
Photochemistry
1,4-dienes.The various photochemical reactions of non-conjugated dienes have been reviewed.log 2.1 .I The Aza-di-n-methane Rearrangement and Related Processes. Further examples of the aza-di-n-methane reaction within some pyrazino- and quinoxalino-fused naphthobarrelenes have been reported."'
2.2 Miscellaneous Reactions Involving Three-membered Ring Compounds. The photoisomerism of the cyclopropane carboxylic esters (83) has been investigated in a variety of organised media. The best results for the conversion to (84) were obtained by irradiation either in the pure crystalline state or in zeolites."' Irradiation of cyclopropyl iodide affords ally1radicals following the fission of the C-I bond and ring opening of the cyclopropyl radical.l12 The photochemical y-Irradireactivity of [1.1.I]-propellane with methylene has been de~cribed."~ ation of (85) in a matrix at 77 K results in the conversion to the radical cation (86) which is photolabile and irradiation with visible light brings about its rearrangement to the phenalene radical ati ion."^ The tricyclic hydrocarbon (87) undergoes photochemical (254 nm) extrusion of dimethylcarbene and the formation of indane.'15 The nature of the solvent used influences the efficiency of the reaction and the best yields (15-20%) are obtained in cyclohexane. The cleavage reaction is less efficient in other solvents (benzene 13-15%, cyclohexylamine 6%, propan2-01 10% and isobutylene 7%). Ph H
Ph
Y
\
(85) R = H or Me
/
(86)
(87)
A study of the influence of medium on the outcome of the irradiation of 2-methylcyclopropene has been reported.ll6 In an argon matrix no reaction is observed, but when the matrix is xenon or bromine-doped xenon irradiation at 254 or 313 nm brings about ring opening and the formation of buta-1,3-diene and methylallene by way of carbenes. Cyclopropene has also been demonstrated to undergo ring opening to give propyne and allene. The photochemical reactivity of the cyclopropenes (88) in both the singlet and the triplet excited states has been e ~ a m i n e d . "The ~ cyclopropenes (89, 90) are photochemically reactive and irradiation at 254 nm brings about their conversion into the corresponding allenes (91,92)."*
133
1113: Photochemistry of Alkenes, Alkynes and Related Compounds
SiMe3
I
Me3Si.../SiMe,
*re3
Fe Me3Si-SiMe,
(88)
R = Me or P i Me3Si)
Me3Si
(90) II = 1-3
(89)
.
Me3Si
{SiMe, SiMe,
(91)
Me3Si Me3Si (92)
The photochemical isomerisation of the vinylidenecyclopropanes (93) has been studied in some detail."' The outcome of the reaction, in terms of the photostationary states attained, is dependent on the excited state involved. On sensitisation a cis:trans ratio of 30:70 is achieved while direct irradiation gives a 5050 mixture. Only the 2-naphthyl derivative behaves differently from this general rule and both direct and sensitised irradiation give the same isomer ratio. In another study the cis-trans-isomerisation of (94) into (95) has been shown to be brought about by a SET process using DCA as the sensitiser.12'Irradiation using h > 400 nm in aerated acetonitrile brings about the isomerisation with a quantum yield of 0.67. The isomerism involves the ring opening of the radical cation of (94) into the open isomeric radical cation (96). When additives such as LiC104 or Mg(C104)2are added, the quantum yield rises and can be as high as 13.7. The authors suggest that an electron-transfer chain process has to be involved. Irradiation of the tetramethyl substituted derivatives (97) in benzene brings about the isomerisation into the butatrienes (98) by way of radical cations.12' Mizuno and co-workers have shown that the vinylidenecyclopropanes (99) undergo ring opening on irradiation to afford the biradicals (100) which can be trapped efficiently by suitably substituted alkenes such as (101).'22The products are the adducts (102) which are formed in moderate to good yields. The recent photochemistry of cyclopropanes, methylenecyclopropanes and vinylidenecyclopropanes has been reviewed and a variety of processes were discussed such as cis-trans isomerisation, polar additions and photo-~xygenation.'~~ A description of the formation of a 1,3-dipole by irradiation of 2,3-diphenyl2H-azirine and additions to the isomers of 3-(tosyloxymethylene)tetrahydrofuran-2-one has been p~b1ished.l~~ Photoionisation of chloropropylene oxide has been studied and the photoionisation efficiency spectra for the ions were re~ 0 r d e d . The I ~ ~photoionisation and photodissociation of epichlorohydrin have been studied.'26 Phenanthrene type products are produced on irradiation of cis-stilbene oxide (103) and the quantum yield for the consumption of starting material is 1.1 x 10-2.'27Quantum chemical studies on 2-oxabicyclobutane have sought to explain its unusual chemical reactivity.'**
134
Photochemistry
Ar)=.-a’Me
/IMe Ar,
R
FMe
“Me
(93) R
(9
Ar
Ar+4Me
Ar+cqMe
Ar
Me Me
Ar
But
Ar (99)
Ar
R’
a; Ph b; Ph C; 4-CIC&
Me H Me
+R2 R3
a‘; H b‘; Me c‘; H d’; Me
CN CN C02Et C02Me
vinylidene
added
Yield (YO)
a a a a b b
a’ b’
44
C’
32
d‘
32 71 88 32
C
a’ b’ b’
85
135
II/3: Photochemistry of Alkenes, Alkynes and Related Compounds
3
Reactions of Dienes and Trienes
A short review has highlighted the photochemical reactivity of a l l e n e ~ .Irradi'~~ ation (h> 300 nm, Xe-lamp, Pyrex filter) of allenes (104)in the presence of CIOFZII can be an efficient method for the formation of the adducts (105).I3OThe yields and the E / Z ratios are shown below the products (105). A study of the photochemical reaction of sensitisers such as (106) with allenes has been carried out and a typical result, a photo-NOCAS process, is shown in Scheme 3.13' The selenium-substituted allenes are both thermally and photochemically reactive and the products shown in Scheme 4.132 R q n - c 7 0 F 2 , (105)
E:Z
Yield (%)
R
But Bun
88 75 58 65
n-Hex Cyclohexyl
63 28 27 27
: 37 : 72 : 73 : 73
MeCNlMeOH (3:l)
*
NC NC
Scheme 3
8; phup Ph
Ph
Ph
s e w s e n =3or4
n =3 n =4
Ph
8 Ph
Se
'\h
23% 7%
29% 8%
8% (n = 1) 51% (n = 2)
Scheme 4
The photochemical isomerisation of germacrene D has been studied.'33The disrotatory ring opening of photochemically excited cyclobutene has been studied using ab initio molecular dynamics.134Other studies have been carried out using the ab initio multiple spawning (AIMS) method that permits the solution of nuclear dynamics and electronic structure problems simultaneously. Application of this method to the ring opening of cyclobutene has shown that the disrotatory motion is established within the first 50 fs following e ~ c i t a t i 0 n . l ~ ~ The triplet-state isomerisation of (107) involves allylmethylene intermedia t e ~The . ~ wavelength ~ ~ dependence for the isomerism exhibited by the dienes (108) has been established: the results are summarised in the Table.'37 The authors suggest that these results implicate a twisted dipolar state depicted as (109). Patents have been filed for the synthesis of photochromic chromene
derivative^.'^*-'^
136
Photochemistry
.CO2Me (108) a; R = C 0 2 b;R=CN c; R = COMe d; R = CO~OAC e; R = C 0 2 0 H
Table Isomerisation of diene (1 08) Composition Compound
h (nm)
Time (min)
all-trans
4-cis
a
> 400
b
> 400 > 400 > 300
5 10 15 15
58 20 8 8 9 43
42 80 92 92 91 57
~~
C
15 45
A theoretical study of the ring opening of cyclohexa-1,3-diene has been reported14' and its electrocyclic ring opening has been studied using ultrafast diffraction imaging.142The irradiation of the dihydropyridine derivatives (110) results in the formation of the 2-azabicyclo[2.2.0]hex-5-enes (111) in low to moderate ~ i e 1 d s . I ~ ~ The photophysical properties of the dienes (112) 1-(p-cyanophenyl)-4-phenylbuta-lE,3E-diene, l-(p-methoxyphenyl)-4-phenylbuta-lE,3E-diene and 1-(p-cyanophenyl)-4-(p-methoxyphenyl)-lE,3E-diene have been r e ~ 0 r t e d .The l ~ ~photoisomerisation of l-(n-pyridyl)-4-phenylbutal,3-diene ( n= 2, 3, or 4) has been studied and shown to arise from the singlet manifold in low quantum ~ i e 1 d s . l ~ ~ The photophysical properties of E,E- 1-phenyl-4-(1'-pyreny1)buta-1,3-dienehave been described,146 and a theoretical study of the wave functions of buta-1,3-diene has been carried Photochemically excited diacetylene is reactive with some a r e n e ~ .Photoad'~~ dition of the diyne (113) to dimethyl fumarate affords the three products shown in Scheme 5.'49A detailed mechanistic study of the process has provided evidence that the cyclobutene (114)is the primary product and that further photochemical addition occurs to give the other two products. Computational studies have been carried out on the photochemical Bergman cyclisation of enediynes such as (115).150The photo-Bergman cyclisation of (116) affords (117) in moderate ~ i e 1 d s . lJones ~ ~ and c o - w ~ r k e r s have ' ~ ~ reported the photochemical Bergman cyclisation of the diyne (118) which yields (119) in 44% yield. The carbodiimide derivatives (120) undergo inefficient reaction in ~un1ight.l~~
137
1113: Photochemistry of Alkenes, Alkynes and Related Compounds
&
-C02Et
R'
R'
R2
R3
Yield (%)
H H
H Me H Me
H H Me Me Ph H
50
H
R3
H H Me
16
21 13 15 25
"2 (111)
\
H H
R2
CN R'
H R2
Me0 CN
H Me0
(11 4
Ph
Ph
- P
h
g
Me02C
P
h
'XR+ 'Yph
+
Ph
Ph
o
'C02Me
\
R
R
(114) Scheme 5
300 nm and on direct irradiation (120a-c) undergo cyclisation to give (121) in a process considered to involve the triplet state of the carbodiimide moiety and its cyclisation to the biradical (122). The proof of the triplet nature of the reaction was demonstrated using sensitised irradiation at 254 nm in toluene or acetone which induces cyclisation of the derivatives (12Od-g) in yields ranging from 66-96%. Acetophenone was shown to be the best of the triplet sensitisers studied and cyclisation also occurred using the derivative (123). The synthesis of a fully reversible optical switch based on a tetraethynylethene-1,l'-binaphthalenehas
138
Photochemistry
R'
Mes (123) R = TMS orBu'
been r e ~ 0 r t e d . Photochemically I~~ induced intramolecular cycloadditions within 1-substituted-2-pentamethyldisilanylethynes (substituents are o-hydroxymethyl, acetoxymethyl or allyloxymethylphenyl) have been reported,155and the photodegradation of ethylnylestradiol has been studied using a variety of automated techniques.156The photodissociation dynamics of propyne on irradiation at 157 nm has been studied in detail and the process has been shown to involve elimination of hydrogen atoms from both the methyl group and the alkyne moiety.157Photodissociation of propargyl bromide has been in~estigated.'~' Photochemical SET triggered cyclisation of (124) affords (125) which can be converted into racemic stypoldione ( 126).159 A review has highlighted the pericyclic reactions of conjugated dienes and trienes,16' and quantum yields for the 1,3,5-triene have been measured.161 isomerisation of all-trans-l,6-diphenylhexaThe photophysics of the trienes (127) have been studied and their photoisomerisation has been investigated and shown to be solvent polarity dependent.162An examination of the So to S1 spectra of the tetraene (128) and the transient has been dependence upon laser power has been r e ~ 0 r t e d . IA~novel ~ observed following subpicosecond time-resolved absorption spectroscopy of all-trans-p-~arotene.~~~ Liu and H a m m ~ n d have ' ~ ~ reviewed the examples in the literature of photochemical cis-trans-isomerisation with special attention being paid to medium effects and conformational changes. The photoisomerisation of all-E,3S,5R,6R,3'R)-3,6,-epoxy-5,6-dihydro-~,~-carotene-5,3'-diol has been investigated.166
139
II/3: Photochemistry of Alkenes, Alkynes and Related Compounds
(124) R = CH,OAc or C0,Et
(125)
(127) R = NO2 or CN
(128)
3.1 Vitamin D Analogues. - The results from the study of the UV irradiation of pro-vitamin D3 (7-dehydrocholesterol) in human keratinocytes show that vitamin D3 is produced.'67Calculations have been carried out to ascertain conformational abundances in a 3-desoxy-previtamin D model compound.'68 A new light source for the synthesis of vitamin D2 has been described and the best transformations are achieved using a 283 nm UV source.169
4
(2n: + 2n:)-IntramolecularAdditions
The hydrocarbon (129) has been synthesised in two steps from ~arvone.'~' Nominally the product (129) can be obtained by a (2n+2n)-cycloaddition in limonene (130) and calculations have shown that the enthalpy to achieve this cyclisation is 7.2 kcal mol- I, an endothermic reaction. Bach and c o - w ~ r k e r s ' ~ ~ , ~ ~ ~ have reviewed the synthetic potential of cyclisations of non-conjugated dienes such as (131).Irradiation with acetophenone as the sensitiser in acetone solutions gives an 80% yield of (132) and (133) in a ratio of 6633 respectively. Copper triflate assisted cycloaddition of (134) gives the adduct (135) which is a useful source of the cyclopentenone (136) for conversion into the natural product P-necrod01.l~~ CUT-controlled(2n + 2n)-photoadditions of some tethered alkenes have also been Typically the irradiation of (137) affords a 1:l mixture of the adducts (138) and (139). A (2n + 2n)-intramolecular cycloaddition is encountered in the photochemical transformations of l,n-bis[trans-4-{2-(5-phenyl1,3,4-oxadiazolyl)}ethenyl]alkaneswhere n = 3,4 or 6.17' ,Ph
tPh
z-N+,
A' (129)
(130)
(131)
H
(132)
Me02C
(133)
Z = benzyloxycarbonyl
Inoue and c o - w ~ r k e r shave ' ~ ~ described a method in a patent application for synthesising optically active compounds using circularly polarised light. The
140
Photochemistry
example cited involves irradiation of the racemic carboxylate (140)in acetonitrile with r-circular polarised light at 290 nm which results in the selective excitation of (-)-(140) and its cyclisation into (+)-(141). Paramagnetic species are formed on irradiation of 7,7-dimethyl-1,4,5,6-tetraphenyl-2,3-benzo-7-silanorbornadiene.177 A detailed study of the synthesis of pagodanes by (2n + 2n)-photocycloaddition has been published.17*Examples are the acetone-sensitised cycloaddition of (142) to afford (143) and (144).Phenanthrene anellated polycyclic hydrocarbons can be obtained by irradiation of [n.2]metacy~lophanes.'~~ Further studies on the photochemical addition reactions encountered on the irradiation of the cyclophane (145) have been carried out.'80
(142)
5
(142) X = CHz; 70% X = C(OCH2); 70%
(143) X = CHZ; 5% X = C(0CH-J; 10%
(145)
Dimerisation and Intermolecular Additions
The photochemical cycloaddition of the aryl alkene (146) to pyrene derivatives (147) yields the two 1:l cycloadducts (148) and (149) and one 2:l adduct (150).18' The involvement of singlet exciplexes in the cycloaddition of alkenes to 9cyanophenanthrene has been investigated,'82 and the photoaddition of benzofuran to the pyridine derivatives (151) occurs in benzene solution using excita-
II/3: Photochemistry of Alkenes, Alkynes and Related Compounds R
141
R
R (147)
tion at h > 300 n111.l~~ The four products are illustrated in Scheme 6. The likely path to these products involves (2n + 2n)-cycloaddition as the first step to yield (154) which on ring opening affords the aza-cyclooctatriene (155) and it is this that is transformed into the products (152) and (153). DopplB4has reviewed the photochemical addition reactions of captodative alkenes.
5.1 Dimerisation. - Heterocyclic substituted alkenes undergo photochemical dimerisation through both the singlet and the triplet excited states and the adducts are formed with good regio- and stereo-~electivity.~~~ A review has highlighted the solid-state photodimerisation of 1,4-dihydr0pyridines.l~~ The incorporation of the stilbene derivative (156)into a SAM on gold affords a gold cluster that undergoes trans-cis-isomerisation on irradiation but no (27c + 2n)-photoaddition processes were detected.187Irradiation at 313 nm of films of liquid crystalline polymer containing the trans-4,4'-stilbene biscarboxylate chromophoric systems leads to the disappearance of the stilbene system which is attributed to the formation of (2n:+ 2n)-cycloaddition products.'88 The trans-cis-isomerisation of 2-styrylpyridine in faujasite zeolites has been examined and at low concentrations is the only photochemical reaction, but at higher loading levels (2n 2n)-cycloaddition also takes ~ 1 a c e . l ' ~
+
6
MiscellaneousReactions
6.1 Reactions Involving Cations and Radicals. - There are many reports dealing with the photochemical activity of alkyl halides using a variety of excitation wavelengths. Thus, photodissociation of methyl chloride, bromide and iodide can be brought about by excitation at 121.6 nm.190 The photodissociation of
142
Photochemistry
h 5 300 nm
Me
benzene
*
+
R2
R2
OR~ (152) R*
+
R'
R2
R3
Yield (%) (155) 32 25 34 27 31 11
(154)
H H Me
H H H
Me Et Me
Scheme 6
""2
HS(CH2)e-0
(156)
methyl iodide has been studied from a theoretical standpoint ,191,192 and the angular distribution of photo fragments produced by irradiation of methyl iodide at 304 nm has been studied.'93 Chlorobromomethane undergoes photodissociation in the 193-242 nm range194and also by irradiation specifically at 254 nm.'95 The irradiation of dibromomethane is reported to yield detectable amounts of isodibromomethane.196The same authors have also examined the photoisomerisation of bromoi~domethane.'~~ Further evidence for the formation of an isomer has been obtained following the study of the photodissociation of chloroiodomethane'98 and of d i i o d ~ m e t h a n e . ' ~In~ ,the ~ ~ latter case, the species formed has been identified as isodiiodomethane (CH2-I-I).'99~200 The authors suggest that this species is probably involved in the cyclopropanation of alkenes. The primary photodissociation path for tribromomethane on irradiation at 193 nm is loss of a bromine atom.201The photochemical reactivity between
IIf3: Photochemistry of Alkenes, Alkynes and Related Compounds
143
tribromomethane and diphenylamine has been used to develop a new detection method based on fluorimetry.202 The photodissociation of dichlorofluoromethane has been studied,203and irradiation at 193 nm of benzotrifluoride brings about C-F fission as the primary process.204 The phototransformations undergone by some halomethanes (CC13Br,CBr3F, CHC12Br and CH2BrC1) under aerobic and anaerobic conditions have been in~estigated.2'~ Irradiation at 185 and 254 nm of some chlorinated hydrocarbons has been carried out in the absence of an oxidant.206Evidence has been gathered from the photodissociation (at 235 nm) of dichlorofluoromethane that a threebody decay path is operative.207 The photofragmentation of difluorodiiodomethane at 193 nm has been reported:" and irradiation of CF31has been A study of the studied using the fourth harmonic of the YAG f~ndarnenta1.2'~ photochemical dissociation of CBrClF2 at 157.6 nm has shown that several fragmentation paths occur with the formation of bromine and chlorine atoms as well as CF2.210,211 The irradiation at 355 nm of ethyl bromide brings about the formation of the corresponding cation and a study of the fragments produced in the decay of this species has been made using TOF-MS.212Perdeuterated ethyl iodide has been photolysed in solid parahydrogen at 4.4 K and the perdeuterated compounds ethylene, ethane and ethyl radicals were detected as was DI.213 Calculations have been carried out to ascertain the paths for the elimination of a hydrogen atom from excited ethyl The competition between ionic and radical paths in the photochemical reactions encountered with the dihalo-1,2-diphenylethanes has been studied.215The photodissociation dynamics of 1-chloro-2-iodoethane have been measured.216The ethanes 1-chloro-1-fluoroethane?l7 1,l-dichloro-lfluoroethane218and l,l-dichloro-l-fluoroethan~19 all undergo photodissociation. The photochemical decomposition of several chlorinated hydrocarbons has been investigated using 185 and 254 nm irradiation.220The study was of environmental interest since many of the chlorohydrocarbons (tetrachloroethene, trichloroethene, 1,2-dichloroethene, chloroform and carbon tetrachloride) are contaminants in ground water. Photodissociation dynamics have been established for a series of partially fluorinated alkyl iodides such as CF3CH21,221and fluorinated alkyl iodides are reported to undergo photodissociation on irradiation at 266, 280 and 305 nm.222The trifluoromethyl anion has been obtained by electron bombardment of h e x a f l ~ o r o e t h a n e . ~ ~ ~ The peracetylated pyranosyl bromo chlorides (157) are photochemically reactive in the presence of allyltributyltin and yield (158) and (159).224 The irradiation of (160, R = Br) in T H F solution in the presence of tri-t-butyltin hydride results in the formation of the corresponding radical from C-Br bond fission.225This radical can add to acrylonitrile, for example, to give the adduct (161) which is accompanied by the reduced compound (160, R = H). The cation (162) can be prepared by irradiation of the alcohol (163) at low temperature in strong acid?26The cation apparently undergoes a 1,2-hydrogen shift to afford (164) at ambient temperatures. The cations (165) can be formed by photochemically induced heterolysis of the fluorenols (166) in zeolites.227Other workers have demonstrated the formation of the cation (165, R = H ) from
144
Photochemistry
%
AcO
R3
Br
R'
R2
R3
R4
H
OAc H OAc
OAc H OAc
H OAc H
AcO H
D-glUC0 D-galacto D-manno
(157)
R3
R3
AcO
AcO
CI (158)
OH (159) a; 7.5% b; 23% C; 8%
a; 86% b; 51% c; 31%
OAc
phYph
OAc
Ph-Ph
Ph+Ph
\
R = H, Me,Et or Pr'
irradiation of fluorenol in both polar and non-polar solvents.228Wan and coworkers have reviewed the photochemical reactivity of hydroxyaromatic compound~.*~~
6.2 Miscellaneous Rearrangements and Bond Fission Processes. - Methane is photochemically decomposed into hydrogen, methylene and methyl radicals under 6.4 eV photon irradiation on a Cu(111) surface.230 Decomposition has also been studied using 121.6 nm i r r a d i a t i ~ n ~and ~ ' a study of the photochemical decomposition of methane at 10.2 eV has been reported.232The photofragmentation of several straight chain alkanes such as propane, butane, pentane hexane, heptane, octane and decane by excitation at 157 nm has been reported and the work has been extended to examine the behaviour of some branched alkanes (2-methylpropane, 2-methylbutane and 2-methyl~entane).~~~> 234 Laser irradiation of (167) in the cavities of NaY zeolites induces a single
IIf3: Photochemistry of Alkenes, Alkynes and Related Compounds
145
electron transfer to yield the corresponding radical cation of (167) which then cleaves to give the cumyl cation and the cumyl The involvement of an electron-transfer process in this reaction is supported by the reactions of (167) carried out in the presence of electron donors such as p-chloranil when the same fission process occurs. The photochemical isomerisation of (168) into (169) has been reported and the reactions of the triene (169) were investigated.236A further report by Albini and co-workers has focused attention on the SET induced ring opening of a c e t o n i d e ~ .Thus, ' ~ ~ the simplest acetonide (170) is irradiated in the presence of TCNB and yields the acid (171). The TCNB undergoes alkylation with the formation of the tricyanobenzene (172).This reaction type was extended to include a study of (173) whereby (174) and (175) are formed in a 1:l ratio and total yield of 43%. In more complex systems such as (176) intramolecular trapping yields (177, 33%). Irradiation of (178) at 254 nm brings about the formation of o-quinone methide.238
A review has given details of the photochemical cleavage reactions involving benzyl-heteroatom single bonds.239Benzyl radicals produced by irradiation of benzyl chloride in a glassy medium have been and Leigh and Owens have reported on the one- and two-photon photochemistry undergone by some benzylsilacyclobutanes?41The photochemical SRNlreactivity of the iodopropane (179)with some anions (Scheme 7 ) has been studied in detail and photochemical yields and chain propagation steps were quantified.242An electron-transfer mechanism is suggested to account for the reaction between 2-naphthoxide and haloadamantane~?~~ and a laser flash study has examined the formation of the alkoxides (180) from the corresponding The photodissociation of ethoxy radicals has been investigated,245and hydroxymethyl radicals undergo photochemical loss of hydrogen to yield methana1.246Irradiation of acrylonitrile adsorbed on a copper surface brings about expulsion of cyanide anion?47
146
Photochemistry
Scheme 7
cko(180)
7 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
n = 1 or2
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150
Photochemistry
111. E. Cheung, K. C. W. Chong, V. Ramamurthy, J. R. Scheffer and J. Trotter, Org. Lett., 2000,2,2801. 112. P. A. Arnold, B. R. Cosofret, S. M. Dylewski, P. L. Houston and B. K. Carpenter, J. Phys. Chem. A , 2001,105,1693. 113. W. B. Lee, D. Oh, P. Won, S. Mi, D. H. Hwang, C. J. Cheong and H. Y. Moon, J. Photosci., 1999,6,57 (Chem. Abstr., 2000, 588182). 114. T. Bally, Z. D. Zhu, J. Wirz, M. Fulscher and J. Y. Hasegawa, J. Chem. SOC.,Perkin Trans. 2,2000,23 11. 115. I. R. Likhotvorik, E. Tipmann and M. S. Platz, Tetrahedron Lett., 2001,42,3049. 116. G. Maier, C. Lautz and S. Senger, Chem.-Eur. J., 2000,6, 1467. 117. E. I. Klimova, M. M. Garcia, T. Klimova, C. A. Toledano, R. A. Toscano and L. R. Ramirez, J . Organomet. Chem., 2000,205,89 (Chem. Abstr., 2000,586632). 118. A. de Meijere, D. Faber, U. Heinecke, R. Walsh, T. Muller and Y. Apeloig, Eur. J . Org. Chem., 2001,663. 119. K. Mizuno, H. Sugita, T. Hirai and H. Maeda, Chem. Lett., 2000, 1144. 120. K. Mizuno, K. Nire, H. Sugita and H. Maeda, Tetrahedron Lett., 2001,42,2689. 121. K. Mizumo, H. Maeda, H. Sugita, S. Nishioka, T. Hirai and A. Sugimoto, Org. Lett., 2001,3, 581. 122. K. Mizuno, H. Sugita, T. Hirai, H. Maeda, Y. Otsuji, M. Yasuda and K. Shima, Tetrahedron Lett., 2001,42,3363. 123. K. Mizuno, N. Ichinose and Y. Yoshimi, J . Photochem. Photobiol., C, 2000, 1, 167 (Chem.Abstr., 2001,134,245080). 124. J. Castulik, J. Jonas and C. Mazal, Collect. Czech. Chem. Commun., 2000, 65, 708 (Chem.Abstr., 2000,378684). 125. F. Y. Liu, C. X. Li, G. H. Wu, H. Gao, F. Qi, L. S. Sheng, Y. W. Zhang, S. Q. Yu, S. H. Chien and W. K. Li, J . Phys. Chem. A , 2001,105,2973. 126. F. Liu, C. Li, H. Gao, L. Sheng and Y. Zhang, Wuli Huaxue Xuebao, 2000,16,758 (Chem. Abstr., 2000,133,266459). 127. S. Hayashi, M. Nose and Y. Yamashita, Kenkyu Kiyo-Nihon Daigaku Bunrigakubu Shizen Kagaku Kenkyushu, 2001,36,127 (Chem. Abstr., 2001,306960). 128. S. Okovytyy, L. Gorb and J. Leszczynski, Tetrahedron, 2001,57,1509. 129. K. Yamamoto. Kenkyu Kiyo - Konan Joshi Daigaku, 1998,35,143 (Chem. Ahstr., 2001,134,162764). 130. A. Ogawa, M. Imura, N. Kamada and T. Hirao, Tetrahedron Lett., 2001,42,2489. 131. D. Mangion, D. R. Arnold, T. S. Cameron and K. N. Robertson, J . Chem. SOC., Perkin Trans. 2,2001,48. 132. T. Shimizu, D. Miyasaka and N. Kamigata, J . Org. Chem., 2001,66,1787. 133. N. Bulow and W. A. Konig, Phytochemistry, 2000,55, 141 (Chem.Abstr., 2001,134, 86387). 134. M. Ben-Nun and T. J. Martinez, J . Am. Chem. SOC.,2000,122,6299. 135. M. Ben-Nun, J. Quenneville and T. J. Martinez, J . Phys. Chem. A, 2000,104,5161. 136. J. Saltiel, 0. Dmitrenko, W. Reischl and R. D. Bach, J. Phys. Chem. A , 2001, 105, 3934. 137. M. J. R. Reddy, C. V. Rao, K. M. Bushan, M. J. R. Reddy, V. R. Gopal and V. J. Rao, Chem. Lett., 2001, 186. 138. J. Hyakuta and K. Komuro, Jpn. Kokaz Tokkyo Koho Jp., 2000,219,685 (Chem. Abstr., 2000, 133, 170276); Jpn. Kokai Tokkyo Koho Jp., 2000, 219,687 (Chem. Abstr., 2000, 133, 170277); Jpn. Kokai Tokkyo Koho Jp., 2000, 219,686 (Chem. Abstr., 2000, 133, 170278).
I I / 3 : Photochemistry of Alkenes, Alkynes and Related Compounds
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4 Photochemistry of Aromatic Compounds BY ANDREW GILBERT
1
Introduction
During the year, a number of reviews have been published which provide an overview for a variety of aspects of the photochemistry of aromatic compounds. Topics which have formed the subject of these reviews include the photochemistry of hydroxy aromatic compounds,' photoinduced ortho (2n 2n) cycloaddition of ethenes to triplet benzenes: perspectives of photoinduced electron transfer in organic ~ynthesis,~the synthetic potential of phthalimide SET photochemistry,4 the synthesis of benzofurans using photocyclisation reactions of aromatic carbonyl compound^,^ and the photocarbo-functionalisation of fullerenes.6
+
2
Isomerisation Reactions
cis-trans Photoinduced interconversions of stilbenes and related systems are reviewed in Part 11, Chapter 3 of this Volume. The results of an extensive study into the phototranposition reactions of 25 ortho, meta and para disubstituted benzenes in acetonitrile solution have been reported by Pincock et aL7 Photostationary states are reached by methylbenzonitriles and trifluorobenzonitriles while some other derivatives, such as methylanisoles and methoxybenzonitriles for example, are unreactive. In the presence of 2,2,2-trifluoroethanol, all the photoreactive benzenes yield the ether derivatives of bicyclo[3.1 .O]hex-3-en-2-01, which in the case of benzene and alkylbenzenes arise from addition of the alcohol to the corresponding benzvalene. However, for other reactive benzenes, carbenes or bicyclic diradicals are considered to be the probable intermediates. The latter feature has been examined in detail with the six isomers of dimethylbenzonitrile.' All of these alcohol addition products are rationalised in terms of nucleophilic attack on the bicyclo[3.l.0]hex-3-en-l-yl cation intermediates [e.g. (1) from 3,4-dimethylbenzonitrile] formed by protonation of the photoisomer (2). It is concluded from these and earlier studies that the essential feature which controls the formation of the phototransposition isomers and the alcohol addition products is the position Photochemistry, Volume 33
0The Royal Society of Chemistry, 2002 155
156
Photochemistry
of the cyano group relative to the other substituents. It has been earlier reported that, in a argon matrix, parent silabenzene underwent photoisomerisation to give Dewar silabenzene? Further studies into the isomerisation of these systems have, apparently, been inhibited by the lack of stable silabenzenes, but Japanese workers have recently synthesised the derivative (3) in which the reactive silicon centre is protected by the bulky 2,4,6tris[bis(trimethylsilyl)methyl]phenyl group.” Irradiation (320 nm) of (3) in C6D6 is reported to yield the silabenzvalene (4)which, in moist air, is converted into the single stereoisomer (5), the structure of which was determined by X-ray crystallography. Tbt
Tbt
I
__t
*
3
Tbt
Addition Reactions
Photoinduced (2n + 2n) cycloadditions involving arylethene moieties are reviewed in Sections 1.1 and 1.2 of Part 11, Chapter 2. The present section is concerned with reactions which involve aromatic rings directly in the photoprocess. Intramolecular meta photocycloaddition of 5-phenylprop-1-enes has been used as the key step in a wide variety of synthetic sequences and interest in the photochemistry of the corresponding bichromophores with heteroatoms in the tether between the benzene and ethene units continues. It is known that the photoreactivity of 4-phenoxybut- 1-enes is markedly influenced by the nature and position of the substituents on the benzene ring.” Further work in this area has been undertaken with the arene bearing carbomethoxy (6), carbomethoxymethyl (7) and carbomethoxyethyl(8) substituents.12As expected, for 2’-(6) only (9), derived from the initial ortho cycloadduct (lo), was formed, the 3’- isomer produced minor amounts of the substituted 1,6-bridged dihydrosemibullvalene compound (1l),and intractable polymer resulted from 4’-(6). In marked contrast, irradiation (300 nm) of the isomers of (7) and (8) all gave substituted derivatives of the intramolecular 2’,6’- (meta) cycloaddition product with the highest yields resulting from 2’-(7) and 2’-(8). Both these bichromophores and the 4’-(7) and 4’-(8) isomers also yielded adducts having structures corresponding to (9) from the intramolecular ortho cycloadditon. The isomers of the two series (7) and (8) of the 4-phenoxybut- 1-enes have been complexed with P-cyclodextrin and
157
IIf4: Photochemistry of Aromatic Compounds
C02Me .(6)R = C02Me (7) R = CH2C02Me (8) R = CH2CH2C02Me
(11)
(9)
(12) R’ = H, R2 = CH2C02Me (13) R’ = CH,CO2Me, R2 = H
irradiated in cellophane envelopes with a low-pressure mercury lamp.13 This work provides the first example of using P-cyclodextrin for asymmetric induction in the intramolecular meta photocycloaddition of benzene-ethene bichromophores. For example, ee values of 11.2 and 17?4 are observed for the two adducts (12)and (13)(ratio 1 : 3) respectively from irradiation of complexed 3’-(7). It was reported last year that short irradiation (1 h) of 6-chloro-1,3dimethyluracil in mesitylene in the presence of trifluoroacetic acid (TFA) gave the two adduct isomers (14) and ( 15).14The same researchers have now reported that prolonged irradiation of this addend pair produced a complex mixture of adducts of pentalenopyrimidine derivatives (see Scheme 1) including the novel diazapentacyclo [6.4.0133.02~6.04~8] dodecane ( 16)” Hydrogen bonding between the addends has been used in the well-known (2.n + 2.n) photocycloadditions of ethenes to 1-cyanonaphthalene in order to control the regio- and stereo-selectivities of the process.16Thus the cyanoarenes (17) and the ethenes (18) give the endo adducts (19) selectively: this selectivity control is increased from an endo : exo ratio of 3.3 : 1.0 to 13 : 1 respectively by lowing the temperature from ambient to -60°C. Further studies into the photocycloaddition of captodative ethenes to arenes having electron withdrawing substituents have been published and the type of product obtained from the 1-acetonaphthones (20)is shown to be dependent upon the nature of the second s~bstituent.’~ For example, while only the photosubstitution product (21) results from irradiation of (20a) with 2-morpholinoacrylonitrile, for (20b)both substitution and addition processes occur whereas (20c) gives solely (22) and (23) is formed exclusively from (20d).It is noted that these adducts are thermally labile and readily revert to the starting materials on heating and that the present photocycloaddition reactions support the intermediacy of exciplexes as proposed earlier.ls Irradiation at wavelengths longer than 355 nm of 9-cyanophenanthrene in the presence of cyclopentadiene or cyclohexa-1,3-diene is reported to yield both the arene dimer (24) and (2.n+2.n) c y c l o a d d ~ c t s .The ~~ mechanism of the addition is suggested to be influenced by the ionisation potential of the diene. The endo adduct isomers (25) and (26) from cyclopen-
158
Photochemistry
I
Me
18 hrs
'
I
Me
Me
(16) 1.9%
5.8%
2.2%
Me
I Me
: Me
2.2%
3.7% +(14) 3.3%
2.3%
Scheme I
x - - - -- _ - - Y\ hv
@
* \
MeJ2
Me (18) Y = OH or OMe
CN (17) X = CHZOH or CH20Me
Q + & ;
CN
Me
Me
(19)
tadiene and the dimerisation of the phenanthrene are deduced to arise from the excited singlet state of the arene but in the presence of Michler's ketone this system also gives the exo isomer (27) and the (2n + 4n) cycloadduct (28). Electron-deficient arylethenes such as methyl cinnamate are reported to undergo stereoselective (2n 2n;) photocycloaddition to the 5,6-positions of chrysene in benzene solvent to give (29) by way of an excited singlet sandwich exciplex.20Similarly, pyrene gives the two adducts (30) and (31) stereospecifically and with endo selectivity in high yield. The same workers have described an intramolecular version of the addition to pyrene from the bichromophores (32).22 In this case, the efficiency of the reaction is very dependent upon the ethene
+
c6H6 159
l I / # : Photochemistry of Aromatic Compounds
Me,
Me, @R1 \
+O
+c2TcN >280 nm
/
R2 (20) a; R’ b; R’ c; R’ d; R’
+O
CN
= H, R2 = CI = H, R2 = F = H, R2 = Me = Me, R2 = H (21)
+%I2 (24) 54%
(25) 24%
(26) 7%
202M 3 H Ph
Ph
/
I
+
l
hv
-
C02Me
\
/
\
(29)
substituents. Thus while both (32a) and (32b) gave the intramolecular (27t + 27c) photoadducts (33a) and (33b) in 83 and 81% yields respectively, (32c) afforded only a 22% yield of the adducts and (32d) was unreactive, although the corre-
160
Photochemistry
sponding intermolecular process of pyrene and the ethene (34) resulted in the formation of (35) exclusively and in high yield. The regiospecific photocycloaddition of (32) is explained by a sandwich-type singlet exciplex involving the pyrene and benzene rings. The (2n 2n) photocycloadducts (36) and (37) from irradiation of 2-cyanonaphthalene in the presence of the cyclohexa-l,3-diene (38) have been used to synthesise the ‘cyclodimers’ (39) and (40) of the cyanoarene and benzene in 64 and 30% yields re~pectively.2~
+
+ ph\
A!...+ C02Me
(30) R1 = R4 = H, R2 = Ph, R3 = C02Me (31) R2 = R3 = H, R’ = Ph, R4 = C02Me (35) R’ = R4 = Ph, R2 = CN, R3 = H
g 0 Y R2 R3 ~ 1
/
\
I
(32) a; R’ = R2 = H , R3 = C02Me b; R‘ = R2 = H, R3 = CN c; R’ =CN, R2 = H, R3 = Ph d; R’ = H, R2 = CN, R3 = Ph
(33) a;R’ = H, R2 = C02Me b; R’ = H, R2 = CN c; R’ =CN, R2 = Ph
(34)
Further studies into the application of the intramolecular photocycloaddition of ethenes to the furan ring to the synthesis of ginkgolide B (41) have been described.24Indeed the synthesis of the complex molecular architecture of this potent PAF antagonist has now been achieved by using the stereoselective intramolecular photocycloaddition of (42) to construct the core skeleton of (41). Irradiation of (42) in hexane-benzene solution with wavelengths longer than 350 nm gave an 85% yield of the adduct isomers (43) and (44) in a ratio of >25 : 1 respectively. Regioselective cleavage of the cyclobutane ring in (43) and further elaboration provided the key pentacyclic intermediate for (41). Benzofuran has been reported to undergo (2n+ 2n) photocycloaddition to 2-alkoxy-3-cyanopyridine in benzene solution to give a respective 32 : 25 ratio of the endo and exo isomers of (45) by the usual sequence of ortho cycloaddition yielding (46), ring opening to the cyclo-octatriene (47) and photochemical 4.n-ring closure.25
161
I I / 4 : Photochemistry of Aromatic Compounds OH
?H
HoY)
dCN \
/
CN
I
I
~350 nm OTMS CMe,
OTMS (42)
(43)
CMe3
OTMS
CMe,
(44)
Photochemical positional isomerisation in the pyridine and its dimerisation to give (48)26also occur but neither the N-methylindole nor benzothiophene undergoes the addition process. The same series of pyridines give novel adducts with 1-cyan~naphthalene?~ For example, 3-cyano-2-methoxypyridinegives a 32% yield of the pyridine dimer and 24% of the adduct (49) which is reasonably proposed to arise from initial (2n+2n) cycloaddition between the C-2 and C-3 positions of the pyridine and the C-3 and C-4 positions of the naphthalene. The cyclo-octatriene (50),formed from ring opening of the primary photoproduct, now unexpectedly undergoes an intramolecular (2n + 27c) photoaddition to yield (49) rather than a 4n-closure to give adducts of type (45). The adducts (49) are stable under ambient conditions but at 130°C yield the isomer (51) quantitatively. Interesting photochemistry of 24 1-naphthy1)ethyl benzoates (52) has been described by Morley and Pincock.28In both compounds, the naphthalene fluorescence is quenched, and in the case of (52a) this is accompanied by a solvent-
162
Photochemistry R’
R’
+
Me
CN
2290 nm Me
R’ = H or Me R2 = Me or Et
0R2 (46)
(47)
R’@
Me
OR2
(45)
R’
CN
R’
IVICU
R1 = H or Me R2 = H,Me or OMe
CN (48)
(49)
6
R’
CN
A
N-
/
R2
CN
&Me
CN
(51)
dependent emission from the exciplex, but (52b) exhibited no such fluorescence. On the other hand, while (52a) is remarkably unreactive photochemically, (52b) undergoes an unprecedented intramolecular addition of the ester carbonyl to the naphthalene ring followed by the reaction pathway outlined in Scheme 2 to give the isomers (53) and (54). Novel intermolecular cycloaddition is reported from studies into the photoreactivity of 1,4-dicyanobenzene in the presence of 1 , l di~henylethenes.2~ For these systems, it is suggested that the exciplex and/or contact radical ion pair (55) collapses to the dihydroisoquinoline (56) which is oxidised in air to give (57), The reaction is controlled by the ability of the 2-substituents on the ethene to stabilise the radical cation species and, furthermore, while these additions occur in benzene, the reaction of the addend (58) is diverted in acetonitrile and the isomer (59) results. The photochemistry of fullerenes continues to attract considerable attention. Interestingly, photoinduced one-step multiple addition of secondary amines occurs to Cs0under aerobic conditions to give tetra(amin0)fullerene epoxide, the relevant section of which is shown in (60), in moderate to excellent yields dependent on the amine structure.30 Other workers have reported that in a spectroscopic study on amine-Cm systems a slow addition reaction was
Ill#: Photochemistry of Aromatic Compounds
163
d -- , hv
0
R
(52)a; R = CN b:R=H
Scheme 2 Ph
CN
N-
phl-p Q CN R = H, Me, Et or n-C5H1,
CN
(55)
Ph) - - n / v y M e
4
Ph
M e -
hv \
Me
Me
MeCN Me Me
(58)
(59)
(60) 98% for R1-R2
= -NMe
L f 42% for R' = R2 = Me 0% for R' = R2 = Et
Ph
164
Photochemistry
observed that was 'dramatically catalysed' by UV r a d i a t i ~ n .These ~ ~ adducts have strong fluorescence emission in the 519 nm region and from this feature the dynamic properties of the aminofullerenes have been explored. (2n + 2n) Photocycloaddition of 2- and E-4-propenylanisole to C60is reported to occur stereospecifically to give the trans-substituted cyclobutane and hence it is concluded that a stepwise mechanism operates in the reaction: results from studies with deuteriated phenylethenes are consistent with the intermediacy of an open species in the rate-determining step.32Photocycloaddition of 3-methyl-2-cyclohexen-1-one to C70is reported to give an appreciably more complex mixture than that observed from the corresponding reaction with c60, but NMR spectral studies suggest that the reaction occurs preferentially at the two 6,6-bonds nearest to the pole of the c 7 0 molecule.33
4
Substitution Reactions
In aerated aqueous solution (pH = 6), acetophenone is reported to undergo substitution with wavelengths 2 200 nm to give the 2- and 3-hydroxy isomers.34 The reaction is greatly inhibited by nitrogen degassing which is suggested to show that the substitution process arises by attack of a 'reactive' oxygen species formed from acetophenone sensitisation. The mechanism of photoinduced chlorination of pyridine has been assessed by density functional theory and, of the transition states of the three possible pathways, that to form the 2-chlorosubstituted product has the lowest activation energy (114.6 kJ mol-') which is in agreement with the experimental result.35Visible light irradiation of aromatic hydrocarbons in the presence of N-bromosuccinimide is reported to yield monobromides and dehydrogenated products from methyl-substituted and hydrogenated arenes respectively whereas anthracene yields solely the 9,lO-dibromo deri~ative.~~ In recent years, the photoreactions of arene-tetranitromethane systems have been subjected to detailed studies.37 Such processes with phenols, 1,2dimethoxybenzene, and anisoles have been investigated by 15NNMR spectroscopic examination of the photoreaction with "N-enriched C(N02)4.38 In the formation of nitrophenols and 1,2-dimethoxy-4-nitrobenzene, the reactions are deduced to arise from triplet excited states of the arenes or by free radical encounters to give radical pairs from radical cations or phenoxy radicals and -NO2.The NMR signals of these nitro products appear in emission whereas, in contrast, the ''N NMR signals from the nitration products of anisole and the 3,5-dimethylanisole appear in enhanced absorption which suggests the involvement of singlet radical pairs such as [anisole+*, -NO21 for example: these are proposed to arise from decomposition of an unstable nitro-trinitromethyl adduct intermediate. The irradiation of chlorobenzene in water is known to yield phenol derivat i v e ~ ; but ~ it has now been reported that unusual behaviour occurs from the chloroarene in ice+ .' Under these conditions biphenyl, terphenyl and their chlorinated derivatives are formed as a result, it is suggested, of a free radical process
165
I I / 4 : Photochemistry of Aromatic Compounds
in aggregates of the chlorobenzene even in very dilute solid solution. The formation of triphenylene from this reaction is reasonably proposed to involve photodehydrochlorination of the intermediate 2-chloroterphenyl (61). The photoformation of propylbenzene from 2-chloropropylbenzene in trifluoroethanol can readily be accounted for by radical intermediates but the production of indane and trifluoroethoxypropylbenzene suggests the involvement of the 2-propylphenyl cation?l Reaction pathways in this photosolvolysis process have been assessed by density function calculations and, indeed, the results strongly implicate the cation as an intermediate. Good yields of stannanes such as (62) are formed by an SRNlmechanism from irradiation of chlorobenzenes in liquid ammonia in the presence of NaSnMe3and the reaction has been developed into a one-pot process to give phenylated compounds again in excellent yield.422-(4N,N-Dimethylaminophenyl) heteroarenes (63) can be readily obtained by irradiation of 4-chloro-N,N-dimethylaniline in acetonitrile solution in the presence of furans, pyrroles or t h i ~ p h e n e s When . ~ ~ the a-positions of the heteroarenes are blocked, the reaction occurs at the P-position with equal efficiency and the observed high selectivity of the process is accounted for by the intermediacy of the N,N-dimethylamino cation (64) formed by heterolytic cleavage of the C-CI bond in the triplet excited aniline. Aryl bromides and iodides and phenylacetate dianions (65) in liquid ammonia solution react photochemically by an SRNl process to give arylated phenyl acetic acids (66) and (67),44and irradiation of 2,3-di-iodo-5-nitrothiophene in the presence of arenes or heteroarenes is reported to give the 2-aryl derivatives (68) in good to excellent yield.45The low efficiency of the latter reaction with 2,4-di-iodo-5-nitrothiophene is rationalised
SnMe, I
CI I
Ph I
hv/liq. NH3 Me,SnNa CI
CI
SnMe,
NMe,
NMe,
I
I
NMe, I
+
I CI
Ph
Ph
X = NH, S or 0 R’, R2 = H or Me
R*x
(64)
R’ (63)
166
Photochemistry
CH-C02M
pArn Q
ArXNH, hvlliq.
CHpCOpH .+
M
Ar
(65) M = Li', Na' or K'
CH-COZH /
(66)
d,
(67)
hvlMeCN
O2N
ArH
*O2N
Ar
(68) Ar = Ph or
/Q
in terms of the homolytic cleavage of the C-I bond occurring from the no* triplet state which is the lowest state of the 2,3-di-iodo isomer, but the 2,4-di-iodo thiophene has a m*lowest state. Irradiation of aryl iodides in the presence of azulene is reported to be a convenient method for arylating the electron rich I-position of a ~ u l e n e . ~ ~ The photoinduced substitution reactions of cyanoarenes continues to be a topic of much interest. Thus ortho and para dicyanobenzenes and 4-cyanopyridine undergo photosubstitution with formamides and 1-alkyl-2-pyrrolidone to yield, for example (69) and (70) re~pectively.4~ Allylbenzene derivatives such as (71) are formed as 1:1:l adducts from irradiation of 172,4,5-tetracyanobenzene and tetramethylallene in methanol solution by a mechanism involving photoinduced electron transfer and described as a photo-NOCAS (photochemical nucleophile-olefin combination, aromatic substitution) pro~ess.~' 1,4Dicyanobenzene behaves similarly but 1,4-dicyanonaphthalene undergoes reactions which are initially similar to the photo-NOCAS process but which yield addition products (72)-(75) with 1,l-dimethylallene, rather than the ally1 compounds corresponding to (71). The efficiency of all these electron-transfer initiated processes is enhanced by the presence of biphenyl. Indeed, in the absence of this co-donor, the photoinduced electron transfer in the dicyanonaphthalene-1,l -dimethylallene system is markedly reduced and instead an exciplexmediated reaction to give the (471:+27c)cycloadduct (76) becomes the major pathway. 0
CHO
$-.
M
e
0
N\Me
NC
CN
9 Me
GMe 167
IIf4: Photochemistry of Aromatic Compounds CN
NC
OMe
e; ;a OMe \
Me
OMe
\
Me Me h e
NC
NC N C-
a -6, (73)20%
(72)22%
Me
q
M
:
I \
e
\
OMe
I
CN
cN OMe (74) 18%
CN
(75) 8%
(76)
-n* “nCN+ ENc C=NH .Ph I CH + CN
PhCH2CN
NC
CN
NC
NC
CN
I
N NC
C \
d
p /NNH2
h
N NC
C
d
-
PNH h
\
syn-isomer-
Ethanol NC
CN
NH2 Scheme 3
Irradiation of the co-crystals of 1,2,4,5-tetracyanobenzeneand benzyl cyanide (respective ratio 1:2) leads to substitution at the aryl nitrile group giving the stilbene derivative (77) by the pathway outlined in Scheme 3.49 In ethanol/acetone solution, (77) is converted into the syn isomer which cyclises to give the isoindole derivative (78). The cyano group in 6-phenanthridinecarbonitrile is substituted on 254 nm irradiation of its propan-2-ol/water solutions to afford phenanthridine, (79) and (80) from the common intermediate, the 6phenanthridinyl radical.50Further studies by the same group have shown that the reaction proceeds exclusively from the m*singlet of the c~anoheteroarene.~’ The lowest triplet state of diacetylene has been laser-generated and the reactivity of this species with benzene and toluene has been investigated by time of flight mass ~pectrometry.’~ The products are identified as phenylacetylene and isomers of phenyldiacetylene, and these reactions have been discussed in relation to hydrocarbon growth in sooting flames. Finally in this section, it is interesting to note that phenylalanines (81) can be obtained in yields up to 50%, based on recovered starting material, from irradiation of the protected glycines (82) in the presence of di-t-butyl peroxide, benzophenone and substituted toluenes.53
168
Photochemistry Me,
+O C I
C 0 2 E ,tN H ,
hv
PhMe (82)
5
-
Me, I
+O
C
H,N 420 nm) of a mixture of acenaphthylene and p - ~ h l o r a n i lThese . ~ ~ products arise from oxetane formation followed by its hydration, and are stereoselectively converted into an identical naphthopyranylacetal(10;R = Me, Et) in primary alcohols. Quinones have been photoreduced to the corresponding hydroquinones by 5,6-0-isopropylidene-L-ascorbic acid rather than undergoing the analogous PaternoBuchi rea~tion.~'A study has been made of the photoreduction of
197
I I / 5 : Photo-reduction and -oxidation
(8)
(9)
(1 0)
o-benzoquinones at wavelengths corresponding to the S(n +n*) and S(n+.n*) transitions, (Amax -400 and 600 nm), in the presence of dimethylaniline and derivatives, and the apparent rate constants for the transformations determined by the free energy of electron transfer from the amine molecule to a photoexcited o-quinone molecule.41A mechanism for the transformation has been proposed in which the rate-determining step is reversible formation of a triplet exciplex, and in which hydrogen transfer proceeds in parallel with electron transfer within the exciplex. A time-resolved FT-EPR spectroscopic study of the photoreduction of duroquinone by triethylamine in methanol has shown that the spin polarised (CIDEP) duroquinone triplet undergoes deactivation by electron transfer from triethylamine to generate the duroquinone radical anion and amine radical Hydrogen transfer from the solvent to produce durosemiquinone radical and hydroxymethyl radical also occurs. The durosemiquinone radical is reported to be transformed into duroquinone radical anion in the presence of triethylamine in solution. Photoinduced one-electron reduction of 1,4-dihydroxyanthraquinonein the presence of l-benzyl-l,4-dihydroxynicotinamideor 5,5-dimethyl-l-pyrroline N-oxide has been shown to occur by a radical-ion mechani~m?~ and the control of photoinduced electron transfer within a hydrogen-bonded porphyrin-phenoxynaphthacenequinone photochromic system by reversibly changing the electronic properties of the quinone electron acceptor has been de~cribed.4~ The carbonyl oxygen atoms of benzopyrones such as chromones and flavones in their lowest excited triplet states with mixed nn*-n,n* character are capable of abstracting H atoms from s0lvents.4~Ketyl radicals are formed, and even though these are indistinguishable from p-enol type radicals they undergo different reaction types. The photophysical properties of the porphyrinic phenoxynaphthacenequinones (11; M = H2, Zn) and (12)have been assessed with a view to determining their potential use as gated photoinduced electron-transfer systems."6This has revealed that the photochemical isomerisation of the naphthacenequinone moiety is prevented by its close association with the porphyrin ring system. Photochemical redox reactions of the trimethyl ester of coenzyme PQQ (PQQTME) with benzyl alcohol derivatives, THF, and cyclohexa-1,4diene have been observed to give PQQTMEH2, the reduced PQQTME in the quinol form.47Flash photolysis spectroscopy has enabled the lifetimes of the triplet states of the o-quinones to be determined, and deuterium isotopic studies indicate that the photoreduction occurs by electron transfer from the substrate to the triplet excited state of the o-quinone followed by proton and hydrogen
198
Photochemistry
0
\
NH
atom transfer to yield the quinol and the corresponding oxidation products. The orientation of donor and acceptor molecules in the intermolecular electron transfer between coumarin and dimethylaniline has been ascertained using ultrafast visible and IR polarisation spectroscopy."' An examination of the use of keto esters as delivery systems for the controlled release of some aldehydes and ketones in sunlight has shown that the dominant process is a Norrish Type I1 fragmentation of the ester side chain.49In addition, important subsidiary reactions include y-H abstraction from an alkyl side chain and intramolecular Paterno-Buchi reaction or epoxidation of the alkene. The observations have been rationalised using ab initio and density functional calculations; the results of these investigations may find practical application in the perfumery industry. A silica gel surface has been shown able to provide a polar medium capable of reducing the energy separation between the lowest 3(n,n*) and the upper 3(n,713*)states to a small value, and in some circumstances this can cause inversion of nearby 3(n,n*)and 3(n,n*)states." In valerophenone-p-methyl-
199
I1/5:Photo-reduction and -oxidation
valerophenone and valerophenone-p-methoxyvalerophenonesystems, internal filter effects can be sufficiently strong that direct observation of energy-transfer processes in solution is inhibited. Photoreactivity studies of valerophenone in frozen solution have shown that physical restraints present in the solid solvent cavity are able to prevent reaction proceeding in parts of the molecule.51Larger conformational changes are unable to occur, although some H abstraction processes are still apparent irrespective of the solvent used. The Norrish Type I1 reaction was studied as a function of temperature, and semi-empirical PM3 and molecular mechanics MM3 force field calculations have been performed to evaluate the stabilities of ground-state valerophenone conformations. A study has been made of the Norrish-Yang reaction of some a-benzoylpropionic acid derivatives as a function of substituent and reaction condition^.'^ Both cyclic and open-chain products such as cyclobutanes, pyrrolidines, tetrahydrofurans, 6lactones, and pinacols are formed, and these may be mostly obtained with high regio- and diastereo-selectivity. The results provide an insight into the factors determining the stereochemistry of the Norrish-Yang reaction. Irradiation of trans-2-phenylcyclohexyl4-cyanobenzoatein methanol is reported to induce a Norrish Type I1 type reaction with formation of phenylcyclohex-1-ene and 4-cyanobenzoic The transformation is thought to occur by a singlet-state intramolecular electron transfer which is followed by intramolecular proton transfer and finally cleavage of the 174-biradical.The corresponding stereoisomer cis-2-phenylcyclohexyl 4-cyanobenzoate probably undergoes cis to trans isomerisation before fragmentation. 1-(0-Toly1)-1-benzoylcyclopropane (13) yields a single photoproduct (14) resulting from intramolecular hydrogen transfer from the methyl group to the carbonyl group to give a biradical intermediate which c y ~ l i s e s By . ~ ~contrast, irradiation of 2H2-substituted 2-(o-tolyl)-2-benzoyloxirane induces hydrogen atom abstraction from the oxiranyl ring to give a biradical which undergoes transformation into a second oxiranyl ring-opened intermediate that subsequently rearranges. These differences in behaviour have been ascribed to increases in acidity and instability of the oxirane moiety.
(13)
(14)
A study of the excited state reactions of short-lived 2-methylbenzophenone enols using a stepwise two-colour excitation time-resolved thermal lensing technique has a~peared.’~ This reveals that with a 532 nm laser at which wavelength only the E-enol is excited, the increase of the transient absorption is apparent at wavelengths less than 420 nm, with no spectral changes corresponding to the 2-enol. These observations suggest that following excitation, it is only the E-enol which goes on to produce dihydroanthrone. Photolysis of o-tolualdehydes leads to the formation of o-quinodimethanes, and these have been found to react efficiently with [60]fullerene to form stable ad duct^.'^ Such products possess a hydroxyl group which is available for further functionalisation.
200
Photochemistry
An examination of the regioselective and threo-diastereoselective [2 + 21 photocycloaddition of benzophenone to chiral allylic alcohols of the form Me$=CHCHR(OH) (R = Me, Et, CHMe2, CMe3)has revealed that the process is directed by the hydroxyl g r o ~ p . The ~ ’ product oxetanes (15) are obtained with both high stereo- and regio-selectivity, and this is rationalised in terms of hydrogen bonding which promotes regioselective cycloaddition, and a combination of hydrogen bonding with 1,3-allylic strain which produces high n-facial differentiation. An unusual temperature dependence has been observed in the diastereoselectivity of the [2 + 21 photocycloaddition of benzophenone to cis- and trans-cyclooctene through conformational control.58In this reaction the lower energy substrate diastereomer, cis-cyclooctene (cis-16), affords trans-oxetane (trans-17),the higher energy product with increasing temperature, but (trans-16), the more strained diastereomer, retains its configuration in the cycloadduct (17) over a wide temperature range. These observations have been accounted for in terms of the thermodynamic preference of the trans triplet diradical conformer, along with the kinetically controlled conversion of the cis into the trans triplet diradical conformer. Irradiation of benzophenone in the presence of 5-methyl-2furylphenylmethanol leads to the formation of two [2 + 21 adducts in a ratio of H
Ph
oH P Ph
Hh
m
Ph Me (15)
(17)
1:1.59 Similar reactions with 4,4‘-dimethoxybenzophenone, benzaldehyde or 4methoxybenzaldehyde form adducts on the side of the furan proximate to the methyl group, but reactions involving 4,4’-dichlorobenzophenone lead to adducts on the other side. These observations are rationalised in terms of a single electron-transfer process followed by a radical coupling reaction in which the regioselectivity is explained in terms of the stabilities of the intermediates. A mechanism has been reported which accounts for the regioselectivity and diastereoselectivity of the photoinduced cycloaddition reactions of l-acetylisatin (18) with alkenes (19; R = H , Ph, Me) and (20) to give spiroxetanes such as (21), and with the related alkenes (22) and (23).60For electron-rich alkenes, single electron processes involving (3 18*) and ion-radical pair formation operate, and the regioselectivity of the cycloaddition depends upon charge and spin-density distribution in the ion-radicals; diastereoselectivity is also decided by ion-pair collapse. By contrast, with alkenes of high oxidation potential where single electron-transfer processes are not involved, regioselectivity is rationalised by frontier molecular orbital considerations. The photocycloaddition reactions of quinones with norbornadiene have been followed by the CIDNP method in which the relevant signals arise both from the 1,Sbiradicals and from their related radical ion pairs from which they are derived.6l The routes by which the biradicals form and decay can be traced by using polarisations as labels. A study has shown that the Paterno-Buchi reactions of the silyl 0,s-ketene acetals
II/5: Photo-reduction and -oxidation
20 1
(SKA), P,P-dimethyl-0,s-SKA (Me2C=C(SR1)OSiR3,SiR3/R' = TBDMS/Me, TMS/Me, TIPS/Me, TBDMS/Bu', TMS/Buc; (E)- and (2)-EtCH= C(Sbut)OTBDMS)and aromatic aldehydes (ArC(0)H; Ar = Ph, p-NCC6H4,p MeOC6H4,mesityl) give regio- and stereo-selectively trans-3-siloxyoxetanes independent of the aldehyde, the substituents SR' and SiR3,as well as the reaction medium!* The regioselectivity has been accounted for in terms of the relative stability of the 1,4-diradicalsand the relative nucleophilicity of the sp2-carbonsin 0,s-SKA, and the S atom in 0,s-SKA effects control of the trans selectivity. [2 + 21 Photocycloaddition of R3COR4(R3= Me, Pr', Ph; R4= H, Me, Ph) with 2-silyloxyfuransproceeds with stereoselectiveformation of the exo-oxetanes and occurs in high yields.63The regioselectivity for adducts (24; R', R2=H, Me; R3= Pr', tert-BuMe2)and (25; same R', R2, R3) is dependent upon the carbonyls, the substituents on the furan ring, and the excited state of the carbonyls. However, reactions with aldehydes are regiorandom and independent of the excited state. It is suggested that an important factor in the approach direction of the electrophilic oxygen of the excited carbonyls is significant for exo-stereoselection, and the Griesbeck model is successful in rationalising the regio- and exo-selective formation of oxetanes in the triplet-state photoreaction. The main product of irradiating benzene solutions of 2-alkynylcyclohex-2-en1-ones in the presence of an excess of 2-methylbut-1-en-3-yne at 350 nm is l(2H)-naphthalenone which arises by a 1,6cis-fused 3,4,4a,5,6,8a-hexahydrocyclisation of the common biradical intermediate, together with some bicycl0[4,2,O]octan-2-one.6~
3
Reduction of Nitrogen-containing Compounds
An investigation of the excited-state dynamics of methylviologen has been de~cribed.~' In particular, the photophysical and photochemical deactivation pathways have been studied in several polar solvents at room temperature, and the results clearly show the strong electron-accepting character of the lowest singlet excited state. This work also demonstrates for the first time that a hydrogen bonding solvent can function as the electron donor in an ultrafast
202
Photochemistry
intermolecular electron-transfer reaction, and in addition is the first report of an efficient radiative decay pathway for methylviologen in fluid solution. A study has appeared of the photoreduction of methylviologen by eosin-Y (EY2-)in the presence of triethanolamine in water-methanol mixtures.66Both steady-state and time-resolved investigations have been undertaken, and evidence is presented which confirms that contributions are made by both the oxidative and reductive routes of 3(EY2-)*to the formation of methylviologen radical cation. Steady-state and time-resolved examinations of the photoreduction of methylviologen by 10-methylacridine orange in aqueous ethanol mixtures containing triethanolamine have been reported.67Rate constants have been measured for the various processes, and the effects of added salts also determined. A report has appeared of the Cm-photosensitised reduction of methylviologen mediated by molecular oxygen in organic solvents.68Thus on irradiation of a system consisting of c60, electron donors such as triethanolamine and tetraphenylborate, c60'is formed. Subsequent introduction of molecular oxygen followed by further irradiation causes the c60'- to disappear with simultaneous appearance of superoxide anion (Oy-). Addition of MV2+leads to electron transfer from 02*to MV2+in aprotic solvents, and by irradiating a system consisting of Cso/electron donor/Oz/MV2+, MV-+ was also observed to be generated. The action spectrum for the photoreduction of methylviologen in a three-component system consisting of triethanolamine, (sulfonatophthalocyaninato)zincate(II) and methylviologen has been compared with the absorption and excitation spectra of the zinc complex alone, and this has enabled a quenching process for the system to be determined.69The distribution of the complexant species in the novel complex between pyranine, 8-hydroxy- 1,3,6-pyrenetrisulfonateanion (26), and methylviologen can be manipulated using ionic micellar aggregates, and this permits control over competitive photochemical and photophysical pathways allowing maximisation of electron- and proton-transfer routes.70Along with other observations, this may have implications for the development of a photocatalyst whose properties can be adjusted by suitable disposition of the partners in supramolecular aggregates. Comparison of the quenching of the electronically excited singlet state of a series of simple N-alkylated pyridiniumyl- 1,8-naphthalimides and a series of polymethylene-linked 1,8-naphthalimide/viologen dyads (27) has shown that attachment of the viologen promotes quenching?l From flash photolysis and other studies, the conclusions have been drawn that the quenching can be ascribed to both intra- and inter-molecular processes and that these arise by electron transfer from the excited state of 1,8-naphthalimide to methylviologen, It has been reported that visible light excitation of [Ru(bpy)3I2+-
II/5: Photo-reduction and -oxidation
203
tethered titania will induce electron transfer to methylviologen to form the cation radical in an electron migration process which occurs on the titania surface.72 A new route to vicinal diamines by the photoreductive coupling of pyridine-, arene- and alkynecarboxaldimines has been de~cribed.7~ Irradiation of 10methylacridinium ion in acetonitrile containing allylic silanes and stannanes leads to allylated dihydroacridines (28),but with unsymmetrical allylsilanes ally1 groups are introduced at the a position.74Photoreduction of lo-methylacridinium by tributyltin hydride and tris(trimethylsilyl)silane, however, gives the corresponding 1,4-dihydroquinolines exclusively. These differences are accounted for in terms of nucleophilic uersus electron-transfer pathways.
/ \
\
N Me
/
R’
R’
I
R’
A study has compared the photosensitised reductive splitting of stereoisomeric CS-CS’-linked dihydrothymine dimers [meso compound of (5R,5’S)- and (5S75’R)-bi-5,6-dihydrothymines (29; R’ = Me, R2= H, Me); racemic compound of (5R75’R)-and (5S75’S)-bi-5,6-dihydrothymines (30 and 31; same R’, R2)] in aqueous solution with the one-electron oxidative splitting mechanism and photorepair reaction of cyclobutane pyrimidine p h ~ t o d i m e r s Reaction .~~ with photochemically generated hydrated electrons converts the C5-C5’-linked dihydrothymine dimers to the corresponding 5,6-dihydrothymine derivatives, and time-resolved studies indicate that one-electron adducts of the C5-C5’linked dimers undergo C5-C 5’-bond cleavage to produce 5,6-dihydrothymin-5yl radicals and the 5,6-dihydrothymine C5-anions leading to formation of 5,6dihydrothymine derivatives by protonation at C5. Photoinduced reductive ring contractions have been verified for l74-dihydro-6-methy1-2,4-diphenylpyrimidine, 1,4-dihydro-2,4,6-triphenylpyrimidine, 1,2-dihydro-3,6-diphenyl172,4,5-tetrazine, 1,2-dihydro-2,4,6-triphenyl1,3,5-triazine and 1,2-dihydro-1methyl-2,4,6-triphenyl-1,3,5-triazineto give the fully unsaturated heterocycle^.^^ Dihydropyrazines such as (32; R’,R2=H, C1, Me, F3C) are also reported to undergo photoreductive ring contraction to give 1,2,5-triarylpyrrolesof the type (33). (34) and 3-phenylquinoxalin-2-one Both l-methyl-3-phenylquinoxalin-2-one (35) have been efficiently photoreduced in the presence of amines to give the sernireducedquinoxalin-2-ones (34-H)- and (35-H)- in unit quantum yield by an electron-proton-electron transfer process.77This is followed by an almost quantitative reversion to the parent substrate in a dark reaction. A study of the photochemistry of 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrinin dimethylformamide using he,= 347 nm induces photoreduction of the porphyrin
204
Photochemistry
(32)
(33)
and subsequently formation of protonated The solvent acts as reducing agent, and in air-saturated solutions chlorin molecules are formed, whereas in deoxygenated solution the transformation sequence is porphyrin -+ phlorin + porphomethene ---* porphyrinogen. Gramicidin S analogues containing a pair of D-1-pyrenylalanine and L- or D-p-nitrophenylalanine residues have been synthesised, and following photoexcitation electron transfer from the excited pyrenyl group to the nitrophenyl group was observed to occur.79Comparisons have been made between the rates of electron transfers in these examples and those observed in a-helix model polypeptides. Rates of photoinduced electron transfer from the excited pyrenyl group to the nitrophenyl group in a-helical polypeptides containing L- 1-pyrenylalanine and L-4-nitrophenylalanine separated by 0-8 amino acid residues have been measured.80The rate constants show a complex dependence on the number of spacer amino acids, but a simple exponential dependence on the edge-to-edge distance between the two chromophores. The photolabile sugars 2,6-di-O-onitro benzyl- and 3,6-di-0+nit ro benzyl-methylmannoside have been depr otected by irradiating at 350 nm to afford methylglycosides.*’An examination of the effects of modifying the surface of nanocrystalline titanium dioxide on the photocatalytic degradation of nitrobenzene has been reported.82Arginine, lauryl sulfate, and salicylic acid have been found to bind T i 0 2 through their oxygencontaining functional groups, and arginine will facilitate the transfer of photogenerated electrons from the T i 0 2 conduction band to the adsorbed nitrobenzene. This study reveals that such a modification is an effective route to enhanced photodecomposition of nitroromatic compounds. Azobenzene has been photocatalytically reduced to hydrazobenzene in a 2e- process by irradiating at he, > 300 nm in the presence of Ti02.83Irradiation in the presence of T i 0 2 loaded with nanometre-sized particles of Pt, however, leads to N=N bond cleavage by a 4e- reduction. The photoreduction of the triplet states of the electron-deficient 3-phenyl-1,2,4-benzotriazine, 3-phenylazaarenes 3,5,6-triphenyl-1,2,4-triazine, 1,2,4-phenanthro[9,10-e]triazine, and tetraphenylpyrimidine have been investigated.84In the presence of 1,4-diazabicyclo[2,2,2]octane(DABCO), the secondary transient is ascribed to the radical anion, but in the presence of TEA or diethylamine H-adduct radicals having maxima around 400 nm are observed. A separate group of workers also reports the photoreduction of 3,5,6-triphenyl1,2,4-triazine in neat triethylamine to form 2,5-dihydro-3,5,6-triphenyl-1,2,4triazine together with the products of reductive ring contraction, 3,5-diphenyl1,2,4-triazole and 2,3-di-(3,5-diphenyl-1,2,4-triazole-1-yl)butane, the latter being produced as a mixture of racemic and rneso-diastereoisomers.85Fluorescence
I I / 5 : Photo-reduction and -oxidation
205
quenching studies of naphthalene diimides have revealed their electron acceptor capabilities.86Although naphthalenediimides do not seem to produce 02('Ag),in the presence of these compounds styrene has been photooxidised to benzaldehyde, and it has been speculated that this may occur by radical chain reactions involving the superoxide anion radical. Investigations of the photochemical reduction of a series of aromatic imines to the corresponding amines by 2phenyl-N,N-dimethylbenzimidazolinein the presence of magnesium perchlorate has shown that the reaction proceeds by a Mg2+-mediatedphotoinduced electron-transfer me~hanism.'~ The quantum yields of triplet state and radical ion formation of various maleimides have been determined; these parameters are of particular importance in the use of such substrates as electron-transfer photoinitiators.88
4
Miscellaneous Reductions
A study of the photoinduced electron donor-acceptor interactions between c 6 0 and aliphatic amines of various chain lengths, including diethylamine, triethylamine, tri-n-amylamine, propylethylamine, n-butylamine, n-heptylamine, dodecylamine and ethylenediamine, has established a correlation between structure and the length of the alkyl chain in both the ground and excited Factors influencing dynamic properties of the C60/(aliphaticamine) such as AHet and AS,, have been investigated. ESR investigations of photoinduced electron transfer between some water-soluble amine donors and the C6o-y-cyclodextrin inclusion complex have shown the presence of both the monoanion, c60-, and the dianion, C62-.90This study also suggests that one of the most important factors affecting the half-life of the dianion radical is the stability of the corresponding donor cations. In some related work an examination has been made of the radical ions generated by photoinduced electron transfer between amines and 3C60*/~-CD and C70*/y-CD.91In both instances studied, rates were found to be slower than in corresponding cases in solution, and for reversible systems involving stable radical cations of amines, both C60*-/Y-CD and C70--/y-CD decayed slowly by back electron transfer. In the presence of methylviologen, persistent MV*+was generated in equilibrium with C60--/y-CD, and this suggests that C,o/y-CD can act as an efficient photosensitiser and an electron mediator to produce MV-+ for which nitrilotriethanol is used as sacrificial donor. Irradiation of three-, four- and five-membered cyclic silicon compounds in the presence of fullerene in benzonitrile as solvent leads to formation of the fullerene radical anion, c60*-, in tandem with rapid decay of the fullerene triplet state suggesting that electron transfer occurs via 3C60*.92Increases in the number of silicon units are matched by decreases in the rate constant and quantum yield for electron transfer. FT-EPR has been used to study the energy and electron transfer from porphyrins in their triplet excited state to c 6 0 in toluene and in ben~onitrile.9~ The primary route of electron transfer is shown to be oxidative quenching of magnesium tetraphenylporphyrin triplets. Dioxygen has been reported to accelerate back electron-transfer processes between a fullerene rad-
Photochemistry
206
ical anion and a radical cation of zinc porphyrin (ZnP) in photolytically generated ZnP*+-Cm--and ZnP,+-H2P-Cso--radical ion pairs.94In these systems, partial coordination of 0 2 to ZnP'+ occurs and this facilitates an intermolecular electron transfer from c 6 0 * - to 02. Consequently, molecular oxygen can act as a novel catalyst in the acceleration of back electron transfer in Cm---ZnP-+radical ion pairs. A model system incorporating c 6 0 has been described which shows that c 6 0 adducts can serve as visible-light harvesters which are capable of triggering electron-transfer processes between partners that do not absorb visible light.9s Introduction of a cyclopropyl grouping on the c 6 0 chromophore renders it suitable for participating in both triplet energy-transfer processes and in electron-transfer processes. A study of photoelectron-transfer processes involving Cso or c 7 0 and zinc octaethylporphyrin (ZnOEP) in polar media has shown that, following selective excitation of ZnOEP, transient absorption bands attributable to the fullerenes can be observed?6 Analogously, following excitation of the fullerenes, decays of 3C60*and 3C70*can also be detected. Electron-transfer rate constants and quantum yields of c60 and c70 formation via 3ZnOEP* and 3C60* or 3C70*have been determined, and were found to increase with solvent polarity. In benzonitrile solution, C70 forms a ground state charge-transfer complex with 3,3',5,5'-tetramethylbenzidine, and on selective excitation of C70 EPR singlets ascribable to C70 mono- and di-anion are In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals have been obtained. A study of the photochemistry of water-soluble isomeric bis(pyrro1idinium) salts with C60(C4HloN+)2 as cationic moiety [36=(36a-36d) in which one pyrrolidinium ring is fixed at the top 6-6 fusion as shown, the second is located at the dotted bond labelled eq =equatorial (36a), the dotted bond t4 = trans-4 (36b), t3 = trans-3 (36c), or t2 = trans-2 (36d)l has been compared to bis(carboxy1ates) c6O[c(co2-)2]2and to y-CD-encapsulated C60.98The electron-withdrawing character of the pyrrolidinium groups confers enhanced electron-acceptor properties on the bis(pyrro1idinium) salts, and photolysis of (36a-36d) gives singlet state absorptions that closely resemble observations on the pyrrolidine precursor. Intramolecular electron transfer and singlet-singlet energy transfer have been observed to occur competitively in the C60-oiigo(naphthylenevinylene) dyad (37).99Photoinduced charge separation and recombination in a tetrat h i 0 ~ h e n e - Cdyad ~ ~ has been investigated in solvents of various polarities, and Me, + ,Me
CN>
IIf5: Photo-reduction and -oxidation
207
has been found to occur with almost unit quantum yield and at about 10IOs-' in polar solvents, and to be totally absent in solvents such as toluene.'00 The observation of a second charge-separated state of unprecedentedly long lifetime was made in benzonitrile, and interpreted in terms of an equilibrium between the charge-separated state and the triplet excited state. A comparative study has been made of the photoinduced energy and electron-transfer processes in some fullerene-oligothiophene-fullerene triads ( C ~ O - ~ T -T C= ~O thiophene, ; n = 3, 6, 9) to those of mixtures of oligothiophenes (nT) with N-methylfulleropyrrolidine (MP-C60).101 Preferences have been observed for intra- and intermolecular energy- and electron-transfer reactions as a function of conjugation length and solvent permittivity, and these are found to be consistent with predictions made using the Weller equation for the change in free energy upon charge separation. In a study of photoinduced electron-transfer processes from oligothiophenes (nT)/polythiophene (poly-T) to fullerenes (c60/c70), it has been shown that selective photoexcitation of the fullerene in polar solvents promotes electron transfer from nT to the excited triplet state of the fullerene.'O* The efficiency of electron transfer is a maximum at n = 4 and falls to smaller values at higher figures suggesting that energy transfer may be occurring. In non-polar solvents, energy transfer is the dominant deactivation process. A series of novel donor-bridge-acceptor dyads has been synthesised in which the pyrrolidine[3',4': 1,2] C60lfullerene is covalently attached to the electron donor tetrathiafulvalene either directly at the 2' position or through one or two vinyl groups.'o3Observations suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate the (C60°-)-(TTF*+) pair. Excitation of fulleropyrrolidines and fullerotriazolines covalently attached to tetrathiafulvalene as electron donor leads to the formation of the fullerene excited singlet state which then undergoes intramolecular electron transfer to the charge separated state.lo4Back electron transfer occurs following formation of the fullerene excited triplet state. c 6 0 has been used as a subunit for the construction of molecules which exhibit light-induced electron transfer from a porphyrin
208
Photochemistry
donor to a fullerene ac~eptor.'~'A significant advantage of fullerenes over quinones, the preferred choice of nature, is the ability of fullerenes to accept up to six electrons and the lower reorganisation energy of c60 compared to quinones according to Marcus theory. Time-resolved optical and transient EPR spectroscopies have been used to investigate photoprocesses associated with the complexation of a pyridine-functionalised c 6 0 fullerene derivative to ruthenium- and zinc-tetraphenylporphyrins.'06 The study has shown that following excitation in polar solvents electron transfer from porphyrin to the fullerene occurs. Intramolecular charge separation and charge recombination processes have been observed in a dyad comprising covalently linked c60 and N,N-di(6-tert-butylbiphenyl)benzenamine, as well as in intermolecular electron transfer to methan~fullerene.'~' In moderately polar solvents, ion-pair recombination was found to occur within a few nanoseconds giving the ground state and the triplet excited state of the C ~ moiety, O whereas in polar solvents decay occurred through two steps. The existence of an equilibrium between the charge-transfer and triplet states has been proposed. The solvent dependence of charge separation and charge recombination rates in zinc p ~ r p h y r i n - C dyads ~ ~ have been examined in a range of different solvents."* It has been shown that, irrespective of solvent polarity, the chargeseparated state ZnP*+-C60--is formed, but that it decays to different energy states depending upon its energy level with respect to those of the singlet and triplet excited states of the c 6 0 fragment. In non-polar solvents, charge recombination occurs to give first the c 6 0 singlet state and subsequently, following intersystem crossing, the C60 triplet state. In more polar solvents, the chargeseparated state is lower than the c 6 0 singlet excited state so that the c 6 0 triplet state is formed directly, whereas in benzonitrile the charge-separated state decays directly to the ground state. An examination of electron-transfer processes in a variety of porphyrin-linked c 6 0 dyads and triads has shown that, compared with c 6 0 or naphthalenediimide with similar reduction potentials, accelerated photoinduced charge separation can be observed in the former.lo9This has been accounted for by the small reorganisation energy in c60. Studies on porphyrin-pyr0mel1itimide-C~~ triads suggest that the c 6 0 moiety accelerates the electron transfer via a through-bond process, or enhances the direct throughspace electron transfer from the excited singlet state of the porphyrin. These studies may have implications for the construction of a solar energy conversion system. Time-resolved transient absorption spectroscopy and fluorescence lifetime measurements have been used to investigate photoinduced charge separation and charge recombination processes in a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strengths.11oThe series comprises the free base porphyrin-Ca dyad (H~P-C~O), zinc porphyrin-C60 dyad (ZnP-Cbo), ferrocene-zinc porphyrin-Cm triad (Fc-Z~P-C~O), ferrocene-free base porphyrin-C6o triad (ZnPH2P-Ca),and zinc porphyrin-free base p ~ r p h y r i n - Ctriad ~ ~ (ZnP-HZP-C,). The cyclophane-type molecular dyads (38; M = 2H, Zn) in which a doubly bridged porphyrin donor adopts a close, tangential orientation relative to the surface of a fullerene acceptor have been prepared along with the porphyrin derivatives (39;
209
iiJ5: Photo-reduction and -oxidation
M = 2H, Zn)." ' Structural investigations indicate that the preferred conformations of the latter compounds are such that one of the carbon spheres nests on the porphyrin surface resulting in an orientation analogous to that of the fullerene moiety in the doubly bridged systems. Time-resolved luminescence studies have shown (38; M = Zn) and (39; M =Zn) to have similar photophysical behaviour Ph
suggesting that tight donor-acceptor distances can be present in singly bridged dyads as a consequence of favourable fullerene-porphyrin ground-state interactions. A report has appeared of the synthesis and properties of novel porphinfullerene dyads as well as their use in an investigation of light-induced energy and electron transfer.'12 It is suggested that the advantages of fullerenes over quinones is their ability to accept up to six electrons and the lower reorganisation energy compared to quinones according to Marcus theory. Novel donoracceptor compounds formed by phytochlorin and fullerene residuals have
210
Photochemistry
been examined in solution and in solid Langmuir-Blodgett films.113The chargetransfer state has a relatively short lifetime in solution (< 100ps), but by contrast the lifetime in films is found to be extraordinarily long for a dyad (- 30 ns). The conclusion is drawn that in the films the dyads have uniform orientation and perform a vectorial CT on photoexcitation, so that alternating the DA layer with the layer composed of secondary donor molecules permits the CT distance to be increased with concomitant increase in lifetime of the CT state. A study has shown that photoinduced intermolecular electron transfer in mixtures of oligo(pphenyleneviny1ene)s (OPVns, with n = 2-7, the number of phenyl rings) and N-methylfulleropyrrolidine in o-dichlorobenzene occurs to the triplet state of the fullerene from the OPVn for n > 2.'14 This observation is in full agreement with the calculated free energy change for charge separation. Irradiation (h> 350 nm) of deaerated mixtures of nitrobenzene and cyclohexene leads to the formation of C6H5N(0)==NC6H5,C ~ H S N = N C ~ H ~ , C6H5NH2, and C6H5N(H)C6H5,and these products are also obtained when the irradiations are carried out in the presence of dispersions of Ti02, W03, or CdS."' It has been observed, however, that the relative product distribution depends upon both the competitive adsorption-desorption equilibrium of the reagents used and the intermediates on the solid surfaces, as well as upon the differing reducing powers of the photoexcited semiconductors. A study of the reactions of triplet 1-nitronaphthalene with trans-stilbene in both non-polar and polar solvents has shown that in polar solvents the substituted naphthalene acts as an electron acceptor, but that in non-polar solvents only energy transfer to trans-stilbene is observed.'16The change from energy to electron transfer in line with solvent polarity has been rationalised in terms of Marcus-Hush theory. Aryl-substituted tropylium ions have been photoreduced in deaerated acetonitrile at room temperature using 9,10-dihydro-10-methylacridine( A C ~ H ) ~ , 2,4,6-triphenyl-4H-pyran, 10,10'-dimethy1-9,9'-bisacridane,or 2,2',4,4',6,6'-hexamethyl-4,4'-bi-(4H-pyran) to 4-methoxyphenyltropylium perchlorate and 4dimethylaminophenyltropylium perchlorate in an electron-transfer pro~ess."~ Following both steady-state and laser flash spectroscopic studies, a mechanism has been proposed involving photoionisation of ( A c ~ H ) ~ . The reaction between hydrogen and photoexcited carbon dioxide over Zr02 has been studied using kinetic isotope measurements, reaction temperature dependence, and EPR."' The results suggested that the hydrogen is activated in the dark to react with the photoexcited C02--. An IR study has shown that the surface species arising during the photoreduction of carbon dioxide with methane over zirconium oxide are probably surface acetate and surface formate."' Evidence from EPR studies suggests that photoexcitation of adsorbed carbon dioxide gives CO2*-, which then reacts with methane in the dark; from these observations a mechanism to been proposed. Magnesium oxide has been reported to be a catalyst for the photoreduction of carbon dioxide to carbon monoxide, and surface formate has been shown to be a reaction intermediate in the process.120Surface formate is also a reductant for the conversion of a second molecule of C 0 2 to CO. A study of the photocatalytic reduction of carbon dioxide by cobalt and iron phthalocyanines indicates that although their
I I / S : Photo-reduction and -oxidation
21 1
tetrasulfonated derivatives in aqueous solutions are readily reduced to [Co(I)Pcl- and [Fe(I)Pc]-, they do not react with C02.I2lHowever, further reduction of [Co(I)Pc]- gives [CO(I)PC*-]~-,a species which reacts rapidly with carbon dioxide to produce CO and formate in a process whose photochemical yields are greatly enhanced by addition of p-terphenyl. Photolysis of the rhenium complex [Re(bpy)(C03)P(OC6H13)3][BArF](2BArF-) (BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in compressed carbon dioxide and in the presence of triethylamine leads to reduction of the medium C02 to C0,122and carbon dioxide has also been photocatalytically and selectively reduced to formic acid using macrocyclic Ni(I1) and tris(2,2'-bipyridine)ruthenium(II) complexes immobilised into a Nafion membrane.'23 The conditions necessary for the abiotic photoreduction of chloropropionic acid in solutions containing Na2Sand quinones have appeared.'24Kinetic studies have been reported for the photoinduced electron-transfer reduction from carbazoles and anthracenes to various halomethanes in a~etonitrile.'~~ The fundamental parameters were determined by application of the Rehm-Weller Gibbs energy relationship for one electron reduction, and good linear correlations were obtained when these parameters were related to a range of thermodynamic parameters. Examination of the free energy dependence of electron transfer in some donor-acceptor systems having hydrogen bonding appendages has shown that two types of electron transfer can operate.'26 A unimolecular process occurs between hydrogen bonded species and this obeys the Marcus equation, but where there is free diffusion electron transfer is bimolecular and Rehm-Weller behaviour is observed. The absence of the inverted region in bimolecular chargeseparation reactions has been attributed to diffusion in the region of large driving force. An investigation of the time resolved fluorescence quenching of a pyrylium salt by toluene in acetonitrile solution gives rise to a non-exponential decay as a consequence of operation of the transient effect at higher concentrat i o n ~ Following .~~~ deconvolution, use of the Smoluchowski-Collins-Kimball model yields the intrinsic rate constant of the bimolecular electron-transfer reaction; the Marcus electron transfer/diffusion model was also used. A study has been reported of solvent and substituent effects on the efficiencies of photoinduced intramolecular electron-transfer processes in esters of 9-anthracenemethano1.12*Estimated rates of electron transfer were found to show a linear correlation with G values, and values of pwere calculated for methanol, sodium dodecyl sulfate and Triton X. Observed variations were accounted for in terms of the microviscosity and micropolarity of the interior of the micelle systems. Heptacyclo[6.6.0.6~6.03~13.04~1'.05~9.010~'4] tetradecane has been used as a spacer group for regulating photoinduced electron-transfer Typical derivativesare(40; R'=H, R2=OH,X=O;R1,R2=0,X=O;R1=H,R2=OH,X=S; R', R 2 = 0 , X=S), (41; X = O , S), and (42; R = 0 , C(CN)?, same X), and high efficiency is observed if the donor and acceptor groups are coplanar. It has been reported that the product distribution from photolysis of methyl (p-nitropheny1)diazoacetate in an acetonitrile/methanol solvent system is altered by addition of an electron-donating amine.I3OCarbene-derived products are com-
212
Photochemistry
pletely suppressed, and evidence suggests that single electron transfer to give the carbene radical anion is the most likely pathway. Irradiation of some 2-0 and 3-0 thiobenzoate derivatives such as (43) in dichloromethane solution containing triethylamine induces solvent incorporation followed by cyclisation to the tricyclic product (44) via an electron-transfer process.'31
'SJ (43)
5
(44)
Singlet Oxygen
A new method for the manufacture of O#A,) has been described and consists in passing molecular oxygen over a sensitiser fabricated from an impregnated pigment on a carrier under i r r a d i a t i ~ n .The ' ~ ~ support may be one of a range including silica, alumina, or titania, and the photosensitiser can be selected from Methylene Blue, Rose Bengal, a phthalocyanine, or a tetraphenylporphin. The ESR technique has been used to monitor the generation efficiency of O*('A,) using C60 and C70 by following the signal intensity of TEMPO, the stable nitroxide arising from attack of 02(lA,) on TMP (2,2,4,4-tetramethyl~iperidine).'~~ Porphyrin-fullerene hybrids have been synthesised, and photophysical properties such as quantum yields for formation of 02(lAg)and fluorescence quenching determined.134Efficiencies of 02('A,) generation using some vinyl-linked benzoaza- 15-crown-5-bipyridine ruthenium(I1) complexes as sensitisers have been found to lie in the range 0.26-0.69.'35 Lower values are characteristic of those compounds having lower potentials for oxidation of the conjugated ligands. The photophysical properties and 02('Ag) generation efficiency of tetrathiarubyrin have been investigated to elucidate the possibility of its use as a photodynamic therapy photo~ensitiser.'~~ The results show that the efficiency of O2(lA,)generation during the oxygen quenching of the triplet state is close to unity, an observation which may be accounted for in terms of the hydrogen
I I / 5 : Photo-reduction and -oxidation
213
bonding of ethanol impeding the deactivation pathway of the charge-transfer complex with oxygen to the ground state, and the reduced probability of aggregate formation. Photoexcited T i 0 2 and ZnO are reported to be a convenient source of 02(1Ag).137 Their use in this respect has been demonstrated by analysis of the oxidation products of methyl oleate and 2,2,6,6-tetramethyl-4-piperidone. Rate constants have been determined for the quenching by O2of triplet states T1for a series of naphthalene sensitisers of very different oxidation potential E,,, but of almost constant ET.'38 An analysis of these and other results suggests that quenching of these oxygen triplet states leads to 0 2 ( ' & + ) , O2(lAg),and 02(3Zg-) with varying efficiencies by two different channels, each of which is capable of producing all three product states. Measurements of the phosphorescence lifetime of 02('Ag)in supercritical and liquid carbon dioxide have shown that raising the pressure leads to corresponding reductions in the lifetime.139Phosphorescence quenching constants have been obtained, and bimolecular quenching constants and activation volumes derived. A new method has appeared for determining the rate constant of quenching of the excited electronic states of molecules by O2from measurements of the kinetics of photosensitised luminesThis has been used in the case of quenching by molecular cence of 02(1Ag).140 oxygen of the excited triplet states associated with the biopolymers of tetrapyrrole in aqueous media. Photooxidations of alkenes by the O2(lAg) ene reaction, and which occur within Methylene Blue doped Nay, have been observed to proceed with novel regi~chemistry.'~'This selectivity has been rationalised in terms of cationic complexation with the alkenes, and electrostatic interaction between the cation and the pendant oxygen atom on the developing perepoxide.
6
Oxidation of Aliphatic Compounds
The photocatalytic oxidation of methane to methanol by molecular oxygen on water-preabsorbed porous TiOz-based catalysts has been reported, and Mocontaining porous T i 0 2 catalysts have been found to exhibit higher catalytic activity than pure Ti02.142,143 Photocatalytic oxidation of methane to formaldehyde on a W 0 3 surface has been achieved with greater than 90% selectivity from 0.01-0.05 % conversion using visible ~ a d i a t i 0 n .This I ~ ~ high selectivity is a result partly of the powerful electron-accepting capability of its short-lived photoinduced 0 -centres which strongly polarise the adsorbed methane, as well as the stability of the W-0-W moiety during the photocatalytic process. Several materials have been investigated as possible catalysts for the photooxidation of propane in a fixed bed flow r e a ~ t 0 r . lThe ~ ~ highest activity and selectivity for propanone formation was achieved by alkali-ion-modified silica-supported vanadium oxide, and this has been ascribed to the resistance of the catalyst to structural changes and its ability to withstand being poisoned by water. Photocatalytic oxidation of n-butane has been observed to occur at a steady state over a silica-supported vanadium oxide catalyst modified with Rb to form methyl ethyl ketone.'46Hydroxylation of cyclohexane has been catalysed by 5,10,15,20-
214
Photochemistry
tetraphenylporphyrinatoiron(II1) chloride (TPPFeCl) using O2 in a range of solvents.147 The rate of the reaction was found to be a function of the solvent and increased in the order acetone < benzene < acetonitrile. These results may be of importance in the development of a model of cytochrome P450. Cyclohexane has been photooxidised to cyclohexanone using alumina-supported vanadium oxide as specific catalyst.'48Evidence is advanced to suggest that the active species are stable isolated V 0 4 units on alumina. Titanium dioxide nanoparticles have been derivatised with a Fe(II1)-porphyrin by a procedure which leaves the aminopropylsilane function contained by the complex, and this has been characterised using various technique~.'~~ These show that the nature of the solvent is highly significant in determining the redox characteristics of the grafted polymer. Assessment of the photocatalytic activity of this grafted polymer has been accomplished by studying the monooxygenation of cyclohexane, and the results show that increases in efficiency and selectivity are achieved. 8-Methyl-8-(1methylethyl)bicyclo[5.1.O]oct- l(7)-ene (45), 8-ethyl-8-methylbicyclo[ 5.1.O]octl(7)-ene (46), and 9,9-dimethylbicyclo[6.l.O]non-1(8)-ene(47) have been subjected to photooxygenation using polymer Rose Bengal as sen~itiser."~ Both (45) and (46) yield dienes and enones, whereas (47) gives enones exclusively. Experimental data indicate that a photosensitiser-initiated free radical autoxidative process is involved with likely intermediates being epoxides for (45) and (46) and hydroperoxides for (47). The absence of O2('A,)-derived products may be attributed either to the relatively long Ca-Hallylic distance in alkylcyclopropanes or to their relatively high IP. Alkenes are reported to undergo reaction with molecular oxygen using the heterofullerenes C59HNand (C59N)2as sen~itisers.'~~ In particular, 2-methylbut-2-ene and a-terpinene undergo both ene and DielsAlder photooxygenation reactions respectively to produce the corresponding peroxides. The seco-porphyrazine (48) is reported to induce [4 + 21 cycloaddition of 02('Ag) to a variety of 1,3-dienes in chlorinated solvents to give the corresponding endoperoxides under mild ~ 0 n d i t i o n s . l ~ ~ NMe,
The kinetics of the chemiluminescence in the oxidation of cyclo-octene by molecular oxygen have been studied.'53Cyclopentadiene has been photooxidised by visible light using 2,9,16,23-tetrasulfophthalocyaninesalong with various central metal ions as photosen~itisers.'~~ The use of heterogeneous photosensitisers immobilised on the cationic exchange resin Amberlite IRA-400 was
215
I I / 5 : Photo-reduction and -oxidation
also examined. 9,lO-Dicyanoanthracene and lumiflavin have been reported to act as sensitisers for the photo-oxygenation of 3-substituted cholesterols and 7-substituted cholesterols to give oppositely-positioned enol derivative^.'^^ An ene reaction involving 02(lAg)has been proposed, followed by subsequent rearrangement of the initially formed 5a-hydroperoxides. Photo-oxidation of the tetranortriterpenoid cedrelone gives (49)whose structure has been established by both NMR measurements and by X-ray ~rystallography.'~~ Addition of Rose Bengal has been found to increase the rate of photo-oxidation. The photooxidation of a-pinene and trans,trans- 1,4-diphenylbuta-1,3-diene using 9,lOdicyanoanthracene as sensitiser in mixed surfactant vesicles has been selectively directed towards products derived from either the 02('A,) or superoxide radical anion routes.157This has been achieved by the appropriate choice of vesicles. In one case studied, the sensitiser was incorporated within the bilayer membrane of the vesicles and the substrate solubilised in another set of vesicles, or by having both sensitiser and substrate incorporated in the bilayers of the same set.
&Me
0
\
Me Me OH
Irradiation (h> 300 nm) of deoxygenated solutions of c60 in liquid diphenylmethane results in the formation of Ph2CH radicals which react with electrondeficient c 6 0 to give Ph2CHC60H,(n = 1, 3, 5).15*It is suggested that excitation of the fullerene leads to an increase in its solvation, and hence to an increase in the acidity of the methylene hydrogens in PHCH2. c 6 0 has been alkylated using visible radiation in benzonitrile solutions containing the alkylcobalt(II1) complexes, [ R C O ( D H ) ~ (R ~ ~=] Me and PhCH2; (DHh = bis(dimethylg1yoximato); py =pyridine) to give R2C60.159 The transformation, which proceeds through the excited state of the cobalt complex, is retarded by trapping agents such as the 2,2,6,6-tetramethyl-1-piperidinyloxylradical, and this observation suggests that the transformation proceeds by photocleavage of the cobalt-carbon bond of [RCo(DH)2py]. Photooxygenative partial ring cleavage of the bis(ful1eroid)derivative (50;R' = R2= C02Me;C02CH2CF3,C02Bu') has been investigated, and found to constitute a useful high yield route to novel diketone derivatives (51; same R', R2)having 12-memberedrings on the surface of the fullerene; these arise via (52; same R', R2).160 Both experimental and theoretical approaches have been used to investigate the Norrish Type I and Norrish Type I1 reactions of pentan-2-one included
216
Photochemistry
within an alkali metal cation-exchanged ZSM-5 zeolite.'61Exchanging the cations affects both the absorption state as well as the photochemical reactions of the included ketones, and molecular orbital calculations indicate that the zeolite framework promotes delocalisation of the charge density of the alkali metal cations, resulting in significant changes in the photolysis of the ketones. Using visible radiation, ethanol has been photo-oxidised to a mixture consisting largely of carbon dioxide together with small amounts of acetaldehyde, formic acid, and carbon monoxide.162The vanadium-doped, supported T i 0 2 photocatalyst has a comparable activity and generates a similar product distribution to analogously prepared Ti02 thin-film monolayer catalysts. An examination of the photo-oxidation of aqueous solutions of isopropanol containing Fe(II1) on the surface of semiconductor electrodes has refuted the possibility that the Fe(II1) ions act as electron acceptors from the oxidation intermediates of the s ~ b s t r a t e .A '~~ study has been made of the photocatalytic dehydrogenation of propan-2-01 on the (110) and (100) planes of Ti02, and both thermal and photochemical pathways have been It is found that in the presence of light and with hv < 3.2 eV, the reaction proceeds readily and is not thermally activated, but on the (100) surface both thermally activated and photocatalytic pathways are observed. Differences are accounted for in terms of the site geometry on the different surfaces, and it has been concluded that the photocatalytic pathway is dominant on the (110)surface because hydrogen abstraction occurs faster from the cation resulting from hole trapping than through proton transfer from the neutral molecule. The photocatalytic oxidation of propan-2-01 on Ti02 powder and on a Ti02 monolayer catalyst anchored on porous Vycor glass (Ti02/PVG)has been studied by solid state NMR.'65Two adsorbed propan-2-01 species were identified on the TiOz powder, a hydrogen bonded species and a 2-propoxide species. Two parallel routes seem to be followed in the oxidation process, the first of which proceeds from the H-bonded propan-2-01 species and which is followed by a condensation to give mesityl oxide, and a second route which occurs through the relatively rapid and complete oxidation of 2-propoxide to carbon dioxide. Irradiation of diethyl ether-oxygen chargetransfer complexes in the presence of Sn(I1) or Cu(I1) salts is reported to give higher yields of oxidation products such as ethyl acetate, acetaldehyde, ethanol, ethyl formate, and methanol than in their absence.'66 Photolysis of oxygensaturated tetrahydrofuran or dibutyl ether gives y-butyrolactone or butanol and butyl butyrate. The mechanism of the photo-oxidation of decanethiol, self-assembled on
217
I I / 5 : Photo-reduction and -oxidation
roughened silver, has been examined by surface-enhanced Raman spectroscopy (SERS), and in combination with an examination of the oxidation kinetics the results show that, under the experimental conditions chosen, the oxidation mechanism is dominated by O3 and not by light.167The slow rate of photooxygenation of diethyl sulfide in aprotic solvents is enhanced by addition of alcohols, and an investigation has shown that this can be rationalised by the interaction of the protic additives on the persulfoxide intermediate in competition with cleavage processes.'68A kinetic analysis has rationalised this effect as a general acid catalysis. Studies of the quantum yields of the photocatalytic oxidation of formate in aqueous T i 0 2 suspensions under periodic illumination have shown them to be always smaller than, but at sufficiently high intermittence to approach values obtained under continuous i l l ~ m i n a t i o n .The ' ~ ~ conclusion is drawn that photocatalytic oxidation of formate in 10 nm T i 0 2nanoparticle suspensions under periodic illumination behaves kinetically as a homogeneous photochemical system. Photolysis of matrix isolated cycloalkyl nitrites leads to the formation of the corresponding cycloalkyl ketones as complexes with HN0.17' However, cyclobu t yl nit rite results in 4-nit rosobu t anal formation.
-
7
Oxidation of Aromatic Compounds
A study of the catalytic performance of Mo complexes with Mo1-Mo4 nuclearities grafted on mesoporous silica FSM-16 in the hydroxylation of benzene to phenol has appeared.I7' The highest catalytic activity using hydrogen peroxide as oxidant is exhibited by a trinuclear Mo 0x0 complex grafted on FSM-16, and at 300 K turnover numbers for phenol exceed 700. Studies of photoinduced intramolecular electron transfer in the two donor-bridge-acceptor systems (53) and
Ph
OMe /
(53)
Ph
(54)
218
Photochemistry
(54) have been reported, and in all solvents examined fast electron transfer was 0 b ~ e r v e d . IInvestigations ~~ exclude a solvent-mediated electron-transfer pathway. From gas phase (U)HF ab initio MO calculations on (59,a less computationally demanding case, the centre-to-centre distance between the two chromophores was evaluated. Irradiation of 'naphtho-o-carborane' (56) in the presence of donors such as cyclohexa-1,4-diene induces a quantitative double hydrogen abstraction to give (57); supercoiled DNA is reported to behave ~imi1arly.l~~ A diradical intermediate (58) has been proposed for these transformations. The same authors have also shown that the quinone, 5,s-diketonaphtho-0-carborane H,
t;'
has been produced by irradiating 'naphtho-o-carborane' under 0 ~ y g e n .In l ~the ~ presence of the hydrogen donors acetonitrile or cyclohexa-1,4-diene,a mixture of this same quinone and 5-ketodihydronaphthocarboraneis formed. However, under similar conditions, photolysis of 'benzocarborane' leads to a highly stereoand regiospecific dimerisation only . Excitation of bis[4,5-di(methylsulfanyl)-1,3dithiol-2-ylidene]-9,lO-dihydroanthracene (59) in chloroform solution produces (59'+),which in degassed conditions disproportionates to (59'2+),but which in aerated solutions gives 10-[4,5-di(methylsulfany1)-1,3-dithiol-2-ylidene]anthracene-9-(10)-0ne.'~~ The crystal structure of dication (59'2+)has been determined and this indicates that the planar anthracene and 1,3-dithiolium rings form a dihedral angle of 77.2" in contrast to the saddle shaped structure of (59). A study of the photoinduced electron-transfer quenching of singlet state excited pyrene and 1,2,5,6-dibenzanthraceneby 3-cyanopyridine and o-dicyanobenzene in protic and aprotic solvents has rationalised the charge separation efficiencies, &s, in singlet-state photoelectron transfer using aprotic solvents and a model based upon the macroscopic properties of the Investigations of the electron-transfer quenching of pyrene by the diphenyliodonium cation in a series of straight chain carboxylic acid solvents suggests that specific solvation of the pyrene by the polar head groups of the acids may be i m ~ 0 r t a n t . lHydrogen ~~ bonding between the carboxyl groups and the Tc-cloud of the pyrene may occur, leading to the electron-transfer quenching process not being diffusion controlled. It is suggested that the head groups of the carboxylic acids are involved in the solvent relaxation, Some anthracene derivatives of [60]fullerene have been found to react with photochemically produced 0 2 ( l A g ) at the anthryl group to give 9,lO-epidi~xides.'~~ An investigation of the photocatalytic oxidation of gaseous toluene on polycrystalline Ti02 has found that use of Merck T i 0 2 leads to benzaldehyde as the main product, and the study shows that, in the absence of water vapour, the benzaldehyde is held on the catalyst surface.'79Where Ti02 Degussa P25 is used
II/5: Photo-reduction and -oxidation
219
as an alternative catalyst, no gas-phase products are detected, and the main materials formed are benzoate-like species which are strongly absorbed onto the catalyst. An exploration of the liquid-phase photo-oxidation of ethylbenzene in air in the presence of Rose Bengal supported on a polymer has centred on the effects of temperature and amount of sensitiser on the ethylbenzene conversion and the ethylbenzene hydroperoxide product selectivity, and on the kinetics.I8' The active species in this process is thought to be 02('Ag). p-Xylene has been photo-oxygenated to p-tolualdehyde with 100% selectivity in a photoinduced electron-transfer process by irradiating with visible light in the presence of l0-methyl-9-phenylacridinium ion as excited electron acceptor.181A study has been reported of the photooxidation of toluene and p-xylene with molecular oxygen using visible light in the cation-exchanged zeolites X, Y, ZSM-5, and Beta.182Large electric fields are thought to promote the photooxidation reaction by stabilising the internal charge-transfer state ( R e + 0 2 - ) formed following excitation by visible light, and a correlation was found to exist with measured electric field and product yield. This was highest for divalent cation-exchanged zeolites with high Si/Al ratios. Photosensitised electron transfer has been used to deconjugate some arylhex1-enes to the corresponding aryl~yclohex-3-enes.'~~ Studies of substituent effects in the aryl ring have provided useful insights into the mechanism of the reaction, and to its scope and limitations. Radical cations of 1,l-diarylethylenes, generated by direct excitation using short-wavelength radiation within zeolites, have been observed to react with solvent hexane, whereas those generated by longwavelength excitation of the diarylethylene/oxygen complex react with superoxide anion as counter anion r a d i ~ a 1 . These l ~ ~ observations show that within zeolites the 1,l-diarylethylene radical cations undergo abstraction of a hydrogen atom followed by reaction with superoxide anion, and further indicate that reactive organic radical cations can be generated within zeolites in the absence of a sensitiser. A mixture of o-(2-hydroxy-3-methylbut-3-enyl)phenols and 4 3 hydroxy-3-methylbut- 1-eny1)phenolshas been produced by the photoxygenation [O,( 'Ag)] of o-prenylphenols followed by reduction by triphenylphosphine at low t e m p e r a t ~ r e ,and ' ~ ~ irradiation of oxygenated mixtures of perfluoroalkyl iodides and a-chlorostyrenes in the presence of hexabutylditin has been reported to lead to fluoroalkylated a$-unsaturated ketones of the form RCOCH=CF(CF2),CF3 (R = Ph, 4-C1C6H4, 4-MeC6H4,1-naphthyl, 2-naphthyl; n = 3, 5, 9).18(j An investigation of the photo-oxidation of trans- 1,2-dimethoxystilbene,transstilbene, and trans,trans-l,4-diphenylbuta-1,3-diene as well as 2,2,6,6-tetramethylpiperidine in mixed surfactant vesicles has been carried out by using either tetraphenylporphyrin or Methylene Blue as sensitiser incorporated in the bilayers or aqueous inner compartments of one set of vesicles, with the substrates in the bilayer membranes of a second set of ~esic1es.l~~ Observations suggest that 02('Ag), generated in either the bilayer or inner water pool of one vesicle, is capable of diffusing out and may enter the bilayer of a second vesicle. Under these conditions, trans-stilbene and trans,trans-1,4-diphenylbuta-1,3-diene are observed to undergo 1,2-cycloaddition with the O$Ag). In some related work by
220
Photochemistry
the same authors, an investigation of the 9,lO-dicyanoanthracene-sensitised photooxidation of a-pinene, trans-stilbene, and trans,trans-l,4-diphenylbuta1,3-dienein mixed surfactant vesicles indicates that oxidation within the vesicles selectively yields either the 0 2 ( *Ag)mediated or the superoxide radical anion mediated products according to the locations of the substrate and sensitiser in the reaction medium.”’ Following photoinduced cis-trans isomerisation, cis-3styrylthiophene has been converted to dihydronaphtho[ 1,2-b]thiophene which can be oxidised to naphthor 1,241thiophene.’” Photoirradiation of 3-styrylthiophene in the presence of oxygen gives (60) along with benzaldehyde and 3-thiophenecarboxaldehyde as well as dimerisation to bis(naphtho[ 1,241 thiophene) (61). It has been suggested that the latter two reactions occur by charge-transfer complex formation between oxygen and the substrate.
A rhodamine substituted with two 4-( 1-pyreny1)butyl moieties has been observed to show a biexponential fluorescence decay, and this has been interpreted as a reversible intramolecular photoinduced electron transfer.’” Fluorescence decay measurements permitted the determination of different rate constants of the excited state equilibrium. Photocyclisation of 3-chloro-N-(3-phenanthryl)naphtho[ 1,241thiophene-2-carboxamide is reported to give naphtho[2’,1’:4,5]thieno[2,3-~]naphtho[ 1,2-flquinolin-6(5H)-one as the only one of two possible isomer^.'^' This has been further converted to (62) and the corresponding triazole and tetrazole.
Secondary alcohols such as benzhydrol have been irradiated with visible light in the presence of molecular oxygen within a titanium-substituted mesoporous molecular sieve, Ti-MCM-41, to give the a-hydroperoxoalcohol, and subsequently hydrogen peroxide.’” These peroxide species have been found to react with alkenes and sulfides with selective formation of epoxides and sulfoxides respectively. This procedure may represent a new method for activation of oxygen in the presence of alcohols. Quantum yields ( + o ~ s )of the colloidal TiOzsensitised photooxidation of ring methoxy-substituted benzylic alcohols have been determined, and the true quantum yields (+o) thence obtained.’93Inter- and
I I / 5 : Photo-reduction and -oxidation
22 1
intramolecular deuterium isotope effects were found to be consistent with a kinetically significant C,-H bond-breaking process following the electron-transfer step. An examination of the dependence of the oxidation of phenol by 02('Ag) photosensitised using [Ru(bpy)J2+ on such quantities as quantum yield of formation of the benzoquinone product as a function of [O,], [PhOH], temperature, pH, and composition of the solvent has been r e ~ 0 r t e d . IA~ mechanism ~ consistent with these data has been proposed and this involves formation of an endoperoxide intermediate from the reaction of 02(lAg) with phenol. A heterogeneous copper catalyst employing the mesoporous molecular sieve MCM-41 as support has been developed in which different loadings of copper are impregnated onto the s ~ p p o r t . ' ~Evaluation ' of the performance showed that the catalyst is able to increase the oxidation rate significantly, and studies are also reported of the effects of copper loading and catalyst dosage. Kinetic analysis of the photo-oxidation of phenol on naked T i 0 2 has indicated that 98% of the transformation occurs by reaction with surface bound hydroxyl radicals, and On that the remaining 10% proceeds through direct reaction with h01es.l~~ Ti02/F the reaction takes place almost exclusively by homogeneous hydroxyl radicals. These observations may have implications for the use of alcohols as a diagnostic tool for analysing photocatalytic mechanisms. A study has been made of the kinetics of the oxidation of the three isomeric trihydroxybenzenes by O#AJ as a function of pH and ionic strength in water, as well as benzene and a~etonitrile.'~~ These results show that in aqueous media trihydroxybenzenes undergo spontaneous and fast photo-oxidation, and that they may have relevance to solar-promoted photo-oxidation under field conditions. Some porphyrins and a chlorin possessing an aromatic group at the meso position have been synthesised and used as sensitisers to photooxidise various phenols and naphthols to q u i n ~ n e s .The ' ~ ~ reactions involve formation of 02('Ag) which adds to the substrate. In all of the cases studied generation of 02('Ag) was found to be highly efficient, and this is particularly so for $1 0,15,20-tetrakis(2,6-dichlorophenyl)porphyrin, presumably because of its high 02('A,) yield and its high phot ostabilit y. Aromatic aldehydes have been oxidised with molecular oxygen in the presence of photocatalysts such as meso-tetra-[4-(p-toluenesulfonyloxy)phenyl]porphyrin and the corresponding Co(I1) and Mn(I1) c ~ m p l e x e s .Measurements '~~ of the photooxidation of p-chlorobenzaldehyde showed that the kinetics are first order in the disappearance of substrate. The main intermediates in the direct photolysis (A2200 nm) of acetophenone in aerated aqueous solution are 2Evidence is cited which indicates that hydroxy- and 3-hydroxya~etophenone?~~ the photodegradation occurs through attack on the aromatic ring by reactive oxygen species which themselves originate from reaction of dissolved molecular oxygen with the excited organic substrate. Further hydroxylation to give dihydroxyacetophenone is observed. It has been reported that cleavage of p-methoxybenzyl2-cyclohexylethylether by 9,lO-dicyanoanthracene in the presence of air to give a mixture of anisaldehyde and 2-cyclohexylethanol may be accelerated by co-sensitisation with
222
Photochemistry
biphenyl and driven in favour of cleavage products by replacing oxygen with bromotrichloromethane as sacrificial electron donor.201Doubly heterogeneous conditions in which both sensitiser and ether are fixed on different silica beads were used. The effects of oxygenated substituents on the [4 + 2) cycloaddition of 0 2 ( l A g ) in the photo-oxygenation of water-soluble naphthyl ethers have been investigated.2o2 In cases such as (63),mesomeric interactions between oxygen and the naphthalene ring lead to extreme reactivity. However, when a methylene linker separates the oxygen atom from the aromatic ring as in (64), a mixture of 1,4- (65; R = CH2CH(OH)CH20H)and 5,8-endoperoxides (66) results. A subpicosecond study has been reported of the electron-transfer kinetics of some rigid dyads and triads containing N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as donors and the dicyclovinyl group (CV) as acceptor.2o3The rate of charge separation decreases exponentially with the number of o-bonds in the bridge for dyads such as DMN[n]DCV, and in triads such as DMA[4] DMNC8lDCV primary electron transfer occurs within 10 ps in solvents of low and medium polarity. The rates of secondary electron transfer and the ensuing deactivation processes were observed to depend upon the conformation. ?-POH
O
A
O
H
‘ O A O H
RO
(65)
Solutions of 3-acetyl-5-aryl-2-methylfurans in acetonitrile have been photooxygenated to 2,2-diacetyl-3-aroyloxiranes in the presence of Rose Bengal as sensitiser through the corresponding endo-peroxide intermediate.204Destruction However, of the product with water gives 3-acetyl-l-arylpent-2-ene-1,4-diones. direct irradiation of the same substrate in a stream of dry air affords 3-acetyl-laryl-2-hydroxypent-2-ene-1,4-diones. An investigation of the photosensitised oxidation of furfural in butanol at 60°C has been undertaken both in the presence and in the absence of added water, and gives 2,5-dihydro-5-hydroxyfuran-2-one and 2,5-dihydr0-5-butoxyfuran-2-one?~~ The same authors have presented evidence to support the view that formic acid, an important product in the photosensitised oxidation of furfural in butanol, is oxidised under the conditions of the reaction and constitutes a source of water.206Such an
I I / 5 : Photo-reduction and -oxidation
223
observation could explain the formation of 2,5-dihydro-5-hydroxyfuran-2-one in those cases where no water is added. Reaction of 4,7-dimethylbenzofurazan (67) in chloroform solution with 02('Ag) at 0 "C,generated by irradiation of C60,leads to 4,7-dimethylbenzofurazan 4,7-endoperoxide (68), and is the first direct observation of endoperoxide formation from a benz~furazan.~'~ Evidence for the intermediacy of the hydroperoxide has been obtained, and the rate constant for the oxidation step measured. Irradiation of the neem triterpenoid nimbin (69) promotes oxidation of the furan ring with formation of two isomeric products containing a hydroxybutenolide; salannin (70; Tig = COCHMe=CHMe-(E)) undergoes an analogous The mechanism of the polyoxometalate-mediated photocatalytic oxidation of chlorinated organic compounds using 1,2-dichlorobenzene as a model has been studied and the possible role of hydroxyl radicals has been elucidated.209
Me
8
Me
AcO'
Oxidation of Nitrogen-containingCompounds
Photolysis of (pheny1amino)piperidine(71) in aqueous acetonitrile containing Methylene Blue and in the presence of oxygen has been reported to give the (pheny1azo)pentanal(72).210An examination of the photocatalytic oxidation of Z(CONHNH2)2 (Z =bond, NHNH, CO) and Z'(C02Et), (Z' =NHNH, NHNHCOCH2CONHNH) in Ti02 dispersions has shown that photomineralisation of the N and C atoms occurs along with formation of N2, NH4+, NO3-, and C02; carboxylic acids are also produced.211These experimental observations along with the results of MO simulation of frontier orbital calculations implicate a mechanism in which cleavage of the bonds between the carbonyl group C atoms and the N atoms of the adjacent hydrazo groups occurs in the initial photooxidation. a-Aminoalkyl radicals produced by photochemical induced electron transfer from tertiary amines such as N,N-dialkylanilines and
pyrrolidine derivatives have been added diastereoselectively to (5R)-5-menthyloxy-2[ 5H]-furanone (73) and subsequently used to produce polycyclic molecules and tetrahydroquinolines (74) in a tandem reaction.212Facial diastereoselectivities in excess of 90% have been observed. Photo-oxidation of
224
Photochemistry
thebaine (75)leads to the formation of hydrodibenzofuran (76)in a process which occurs by a [4+2] addition of 02('Ag) to the diene function followed by oxidation at the nitrogen atom, together with the benzofuran (77).213 A similar photooxidation of thebaine ammonium salt gives good yields of thebaine endoperoxide (78). Structures have been synthesised in which a N,N-dimethylaniline chromophore is linked to a phenacyl ester of acetic acid, and it has been found that on irradiation these cleave to release acetic Flash photolysis investigations suggest that an intramolecular charge-transfer state is formed which partitions between bond scission with formation of acetic acid, and a chargerecombination pathway which returns to the ground state. Such covalently linked electron donor-acceptor systems may form a useful photochemically removable protecting group. Photoinduced intramolecular charge separation has been observed in structures composed of either a bicyclohexylidene (79) or a bicyclohexyl(80) substituted with an aniline donor and a dicyanoethylene electron acceptor.215Folding has been shown to occur on the nanosecond time-scale for (SO), and for (79) charge separation proceeds from either a fully folded conformation or on a sub-nanosecond time-scale. The presence of the exocyclic double bond leads to efficient quenching as well as to an increased charge recombination rate. Photoinduced charge separation has been studied in a series of rod-like donor-bridge-acceptor molecules in order to gain some insight into the role of bridge energy levels on electron transfer rates.216Structures of the type ANI-diMe-NI and ANI-diMeO-NI (AN1= 4-aminonaphthalene- 1,s-imide; NI = 1,8:4,5-naphthalenediimide; diMe and diMeO =phenyl bridge substituted at the 2 and 5 positions with methyl or methoxy groups respectively) were examined, and relative energies of the ion pair states suggest that '*D-B-A+DB -A- occurs by a double electron-transfer process. Evidence seems to indicate that electron transfer from the naphthalene 1,s-imide ring of '*AN1to NI occurs +
D
/
Me0
Me0
I g + < N , M e CHO
0
I
/
(76)
CHO (77)
Me
IIl.5: Photo-reduction and -oxidation
225
concomitantly with electron transfer from the p-dimethoxybenzene bridge to the electron-deficient amine nitrogen atom in ‘*ANI.Some secondary amines have been reported to undergo multiple addition to [GOlfullerene under photochemical aerobic conditions to produce a tetra(amin0)fullerene e p ~ x i d e . ~ ~ ~ Quenching of 4-carboxybenzophenone triplets (3CB*)by amino acids in basic solution occurs by electron transfer to 3CB* to give CB-- and the zwitterion aminium radical anion R2N*+CR2C02-.218 Values of the primary quenching constants have been obtained, and transfer of protons from aminium radicals within the solvent cage gives aminyl radicals, RNCR2C02- -, which undergo p-elimination of CO2*-. The rate constant for transfer of the proton from R2N*+CR2C02-to CB*- within the solvent cage was also determined. The mechanism of the pyrene-sensitised photodecomposition of N-phenylglycine has been established as proceeding by electron transfer from the N-phenylglycine to the excited singlet state of the pyrene by emissive exciplex formation.219 PhNHCHy also participates as a reactive intermediate, and electron acceptors such as terephthalonitrile and diethyl isophthalate are reported to enhance the efficiency of the photodecomposition. The regioselectivitiesof photoinduced electron-transfer reactions of quinolinic and trimellitic acid imides have been studied in the cases of potassium butyrate and hexanoate (81; n = 3,5 respectively)and the cysteine derivative (83), and give photocyclisation products (82) and (84) with moderate selectivities for ortho cyclisation.220However, photoreaction of potassium propionate with the methyl ester of N-methyltrimellitic acid imide gives only the para addition product. These regioselectivities are rationalised in terms of donor-acceptor interactions prior to electron transfer, and spin density magnitudes in the corresponding imide radical anions.
Irradiation of a 1:l mixture of the E and 2 isomers of N-methoxy-4methoxyphenyl-4’-methylphenylmethanimineat birr > 360 nm using 9,lOdicyanoanthracene as photosensitiser in acetonitrile leads to an isomer ratio of 4/96,221and a study of the photooxidation of acetone semicarbazone in the presence of Ti02 indicates that increases in solvent polarity enhance the yield of product, and that the reaction rate increases with increase in the band gap of the A structure-activity relationship has been proposed and a reaction mechanism suggested. An examination of the photocatalytic oxidation of methylviologen in airsaturated aqueous suspension has found that the initial photonic efficiency increases as the light intensity decreases.223The consequences of increasing the surface methylviologen concentration have also been elucidated. Studies of the
Photochemistry
226
transient absorption of the 1:1 and 1:2 charge-transfer complexes of methylviologen and iodide following ultrafast excitation in their charge-transfer band has shown that excitation of the 1:l complex results in formation of the MV-+/Iradical pair, while excitation of the 1:2 complex gives the MV*+/I*and MV*+/12 radical pairs.224 8-Hydroxyquinolines (85; R', R2, R3=H, halo, CI-CS alkyl, CHO, OH, OR, C02H, CN, C02R4, CONHR4, CONR4Rs, CH2N(CH2C02R5)C02R4, R4 and RS= c1-c6 alkyl and C6-CI4aryl) have been irradiated in methylene dichloride solution containing TPP with oxygen purging, followed by agitation with sodium sulfate to yield the quinoline-5,8-diones (86; same R', R2,R3, R4, and R5).225 It has been suggested that the reaction may proceed via the peroxide (87; same R', R2, R3, R4, and Rs) which undergoes a sodium sulfate-mediated decomposition. Photo-oxygenation of substituted 8-hydroxyquinolines gives substituted q~inoline-5,8-quinones?~~
I
RZ*
I
;;qo
R3
(85)
0
0
OH
(86)
(87)
The products arising from photolysis of acridine- 1,8-dione dyes have been shown to be the result of a substituent-dependent process in which the cation radical and the solvated electron are the primary photo product^.^^^ The anion radical arises from reaction of the ground-state molecule with a solvated electron, and the anion radical and enolic form of the cation radical are apparent at 480 and 550 nm respectively. Transients such as radical cations and radicals, produced sequentially in the oxidation of 3,6-diamino-10-methylacridan (88), an uncharged precursor of acriflavine, have been characterised using pulse radiolysis and laser flash photolysis.228 An examination of the photophysics of coupled Cd(OH)z-coated Q-CdS with colloidal Ti02 has appeared.229The photoactivity of this coupled semiconductor may be enhanced by interaction between Cd(OH)2-coated Q-CdS and TiO2, leading to the possible formation of [CdS-Ti02(0H)2] and a photogenerated hole CdS(h+)which gives an emissive complex with the indole from which indigo is produced. Irradiated in the presence of cyanide ion and a sensitiser capable producing 02('Ag), ( +)-catharanthine (89) and ( -)-16-O-acetylvindoline (90) have been reported to give ( +)-3P-cyanocatharanthine and ( -)-16-O-acetyl-3acyanovindoline re~pectively.~~' Photochemical oxidation of 3-methylcarbazole in methanol leads to the formation of murrayaquinone-A, an alkaloid which has been isolated from Murraya euchrestif~Eia.~~~ This transformation has also been successfully applied to the similar photooxidation of 3,6-dimethylcarbazole to 3,6-dimethylcarbazole1,4-quinone. In a study of the photophysics and the mechanisms of the photochemical aromatisation of 1,2,3,4-tetrahydro-7H-pyrido[3,4-b)indolein 40% methanol/water, the rate of disappearance of substrate is linearly dependent
227
II1.5: Photo-reduction and -oxidation
Me
Me0
OCOMe
(90)
upon both the concentration of acid and the intensity of the exciting radiation.232 A two-step mechanism appears to be involved, in the first of which the excited substrate reacts with ground state molecular oxygen to form an indolenine, followed subsequently by an acid-catalysed rearrangement to the corresponding dehydro derivative. A study of the kinetics of the dye-sensitised photo-oxidation of 2-amino-4hydroxy-6-methylpyrimidine has shown that rates are enhanced in alkaline media.233The presence of the 4-hydroxyl group greatly influences the reaction rate and the experimental evidence suggests the involvement of a charge-transfer mediated mechanism and participation of an initial excited encounter complex. Fourier-transform EPR investigations have been reported of radicals formed by electron transfer from cytosine and 1-methylcytosine to the laser-induced triplet state of anthraquinone-2,6-disulfonic On the nanosecond timescale, the main products are derived from the base radical cations. The photo-oxidation of uracil and cytosine has been carried out in the presence of peroxydiphosphate in aqueous solution and at neutral pH.235Analysis of the results indicates that the transformation probably occurs by production of the phosphate radical anions which add to the C-5 or C-6 position of the pyrimidine ring with formation of the pyrimidine radical. Following reaction with peroxydiphosphate both 5,6-dihydroxypyrimidine and isobarbituric acid are formed. It has been reported that the rates of the Ti02-mediated photo-oxidation of uracil, thymine, and 6-methyluracil are retarded by the presence of Cu2+,and this has been accounted for in terms of a short-circuiting role for C U * + .Decreases ~~~ in the photocatalytic activity of T i 0 2brought about by increasing the calcination temperature of the T i 0 2 have been explained by decreases in the extent of surface-bound peroxospecies. Methylene Blue-mediated photooxidation of guanosine has been reported to give spiroiminodihydantoin as the major and 2’deoxyguanosine 5’-monophosphate (dGMP)has been oxidised by flavin adenine dinucleotide as sensitiser in a process for which direct evidence has been obtained for electron transfer from dGMP to either the triplet state of FAD or oxidised FAD Ab initio calculations have been performed on the ground and lowest excited
228
Photochemistry
state of pyrrole and pyrrole-water clusters and full geometry optimisation of the Ins* state implies that there is formation of a charge-transfer-to-solvent state.239 These studies indicate that such clusters form good models for studying the mechanistic details of electron solvation processes occurring on excitation of organic chromophores in water. The antiaromatic isophlorin, N21,N22-( 1,2diphenylethen~)-N~~,N~~-(carboet hoxymethano)-5,10,15,20-mesotet raphenylisophlorin (91) has been dioxygenated at the Cmeso-Cpyrrole-a double bond of the dipyrrylmethene group having the N21,N22-( 1,2-diphenyletheno) bridge, and leads to the 19-benzoylisobilirubin (92) which has been ~haracterised.~~'
Ph
Ph
N-(Ary1amino)piperidines and N-(ary1amino)pyrrolidines have been converted into the corresponding N-(arylamino) lac tarn^.^^^ For example, photocyanation of the (nitropheny1amino)piperidine (93) has been achieved by irradiating in oxygenated aqueous solution in the presence of trimethylsilyl cyanide and gives the cyanopiperidine (94). Photooxidation of (94) occurs in aqueous acetonitrile containing Methylene Blue, to produce the piperidinone (95). OZNyJ
'3 02Ny& 3
\
NI I N H
N" I
H
CN
N' I
H
"3 O
The products of p hot olysis of 4,5-diphenyl-3-(4-methylphenyl)-4-oxazoline-2thione in hydrocarbon solvents in the presence of 04'8,)at hlrr 450 nm are benzil, N-(4-methylphenyl)benzamide, and N,N-dibenzoyl-4-methylaniline, whereas in protic solvents benzil, N-(4-methylphenyl)benzamide, N,N-dibenzoyl-4-methylaniline, and benzoic acid are formed.242The suggestion is made that a dioxetane is generated, which after cleavage gives two radicals which subsequently lead to the above products. Chiral oxazolidine auxiliaries have been shown to be effective in steering the diastereoselectivity and regioselectivity of the ene mode of 02('Ag) reaction by means of a hydrogen bonding process.243For example, (96; X = OCMe3, Ph, NHPh, NHC6H4N02-4,NMePh) will react with molecular oxygen under photochemical conditions, and following treatment with PPh3the alcohol (97; same X) is obtained.
229
II1.5: Photo-reduction and -oxidation Me
Me
An investigation of electron-transfer pathways from the lowest triplet excited states of (2-substituted)-lO-methylphenothiazineshas shown that quenching occurs by various electron acceptors in polar solvents such as propan-2-01 and a~etonitrile.2~~ Two types of intermediate may be involved, a triplet contact radical ion pair (3CRIP) or a triplet exciplex (3E~*),and a triplet solventseparated radical ion pair (3SSRIP).Effects of magnetic fields and heavy atoms on the efficiency of free ion formation are described. Multistep non-covalent and covalent electron transfer have been successfully achieved in a catenane triad of the type [98.Q]C16 and consisting of phenothiazine as donor, [Ru(bipy)J, and cyclobis(paraquat-p-phenylene)(Q).245 Me
h
Me
Electron transfer rates have been measured for the charge separation process of the porphyrin-spacer-benzoquinones (99, X = Br, C1, H) in which the spacers are trans-decalin and dihalosubstituted tricyclo[4.4.l.O]undecane including a three-membered ring.246These show that rates for compounds having the threemembered rings are about 50-60 times larger than those with a trans-decalin spacer in THF. This acceleration has been attributed to an increase of the electronic coupling and a decrease of the reorganisation energy. Ab initio calculations suggest that this may arise from the bent geometry of the spacer or from the mixing pathway induced by a very low lying antibonding orbital in the dihalosubstituted cyclopropane. A study has been reported of the photoinduced electron transfer from the S1state of ZnTTP to a covalently linked Ru(bpy), unit in the dyad T T P - C H ~ N H C O R U (composed ~ ~ ~ ) ~ , of 5,10,15,20-tetra(p-tolyl)porphyrin (TTP) and ruthenium tris(2,2’-bipyridyl)subunits functionalised for con-
230
Photochemistry Me
Me
0
nection by an amide linkage.247Strong fluorescence quenching has been observed in systems consisting of two porphyrins bridged by a biquinoxalinyl spacer, and this has been interpreted as arising from long range (> 8 A) through-biquinoxaline bridge-mediated electron transfer from free base to porphyrin to the gold(II1) of the caroteno-porphyrin-fullerene triad (C-P-c60) p ~ r p h y r i n .Excitation ~~~ triggers electron transfer from the porphyrin first excited state or to the fullerene first excited state to yield C'P'+-c60*-, and this is followed by further electron Investigations transfer to give C-+'P-c60*- as the final charge-separated suggest that the energies of the charge-separated states of these fullerene-based systems are much less sensitive to changes in solvent dielectric constant than are those of similar molecules possessing quinone electron acceptors. (100;Ar =p-nitrophenyl) Irradiation of 3-methyl-2-(4-nitrophenyl)-2H-azirine in the presence of molecular oxygen in fluid solution and in low-temperature matrices induces cleavage of the C-N bond of the azirine ring to give biradical (101), which in turn leads to acetonitrile oxide.250A new synthetic protocol has been described in which azidyl radicals and molecular oxygen can be added to electron donor and electron acceptor substituted acyclic and cyclic alkenes to produce 1,2-azidohydroperoxides, which themselves can be easily reduced.25' This reaction can be thought of as a complex sequence of photoinduced electron transfer, addition, oxygen-trapping and subsequent electron-transfer processes. The MO LCAO quantum mechanical method in the AM1 approximation has been used to consider hypothetical models of the interaction of deoxypeganine (102) with a solvent.252Excited triplet states of deoxypeganine and some analogues have been calculated enabling a free-radical mechanism of photochemical oxidation to be proposed. Helimeric mixtures of ( -)-(M,7S)-isocolchicine (103) and ( -)-(P,7S)-isocolchicine (104) have been photooxygenated using 02('$), and it has been found that cycloaddition occurs with high regioselectivity at the 7a,ll-positions and predominantly at the diene face to the amidic substituent at the stereogenic centre, C-7.253The two products are the syn endoperoxide (105)
231
l I J 5 : Photo-reduction and -oxidation OMe
OMe
Po
Me
Me
MeO,
OMe o
w
O
M
e
MeCO
Po
Me
HLfO
Me
and the anti endoperoxide (106).Monometallic colloidal dispersions such as Au, Pt, Pd, Rh, and Ru, and bimetallic nanoclusters such as Au/Pt, Au/Pd, Au/Rh, and Pt/Ru have been used as catalysts for visible light induced hydrogen generati0n.2~~ It has been observed that the rate of electron transfer from methylviologen cation radical to the metal nanoclusters is proportional to the hydrogen generation rate. Free-radical intermediates are reported to be produced during the visible irradiation of aqueous Sulforhodamine B in the presence of T i 0 2and air, and have been detected using the spin-trapping technique with 5,5-dimethyl1-pyrroline N-oxide and N-tert-butyl-~t-phenyl-nitrone?~~ These are the hydroxyl and hydroperoxyl radicals, together with the hydrated electron. Their mechanism of generation has been discussed, but the main oxidising agent has been suggested to be oxygen molecules. The photodegradation of methanolic Methyl Orange has been examined in the presence of ferric ions and hydrogen peroxide using the spin-trapping EPR Intermediates produced were detected by IR and GC-MS methods, and a mechanism suggested for the transformation under both UV and visible light excitation. It has been reported that trifunctional electron donor-donor-acceptor molecules are capable of undergoing the photoinduced charge separation D2-D1-A*-+D2-DI +-Ap. followed by a charge migration step D2-D1+-A-(CSl)-+D2+-D1-A-(CS2) to give a charge-separated state which is relatively long lived.257Increases in charge migration rate occur in solvents of increased polarity within a series of alkyl ethers or alkyl acetates.
232
9
Photochemistry
Miscellaneous Oxidations
Ketyl radicals and ketyl radical anions along with various sulfur radical cations have been identified as transients following quenching of triplet 4-carboxybenzophenone by methionine-containing peptides in aqueous solution, and the quantum yields of formation of these transients have been determined.258The presence of such transients suggests that the triplet-triplet quenching process occurs by electron transfer. Competitive donation of protons to the 4-carboxybenzophenone radical anion takes place within the charge-transfer complex, and this competition occurs between protons on carbons adjacent to the sulfur radical centre and protons on the protonated amino groups of the radical cation. A competition also exists between the two intramolecular two-centred, three-electron bonded species (SS)+ and (SN)+ which appear in the secondary kinetics. Visible light photooxidation of dilute aqueous and aqueous-ethanolic solutions of sulfathiazole and succinylsulfathiazole have been studied kinetically in the presence of riboflavin and Rose Benga1.259The results may have microbiological implications. An analysis of the kinetics of the photooxidation of 3-(2-benzothiazolyl)-7-diethylaminocoumarinat 254 nm in halomethane solvents has shown that H-bond donation of the solvents is important in controlling the rate of product formation.”’ The observations imply that the process of activation is controlled by diffusion of dye into the solvent cage. In a study of the photocatalytic decomposition of water to oxygen over pure W03,Ce02,and Ti02,it has been demonstrated that the yield of oxygen depends upon both the type and surface of the cation present in the electron acceptor, and upon the salt counter ion. Highest long-term yields of molecular oxygen are given by Fe,; .261 Dimethyl methylphosphonate has been photooxidised on powdered Ti02 using UV radiation to give CO, C02 and formate ions together with water along with concurrent destruction of the PCH3and POCH3groupings.262Studies were mainly carried out at 200 K under which conditions a hydrolytic pathway is suppressed. An examination of the SET-initiated photorearrangements of the cis- and trans-2-phenylallyl phosphites (107) to the corresponding phosphonates (108) has shown the process to occur with retention of configuration at the phosphorus atom.263 +
t-BuX0;P, >Ph 0 0
10 1.
t-BU
References T. Slonka, Pr. Nauk, Inst. Technol. Nieorg. Nawozow Miner. Politech. Wroclaw, 2000,48,220.
11/5:Photo-reduction and -oxidation
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
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6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen BY ALBERT C.PRATT
1
Introduction
Reviews have been published on the photochemistry of 1,2-dithiins,' fluoroquinolone antibiotics: fulleropyrrolidines? sultams? boron c o m p ~ u n d s , ~ ~ ~ photocleavage processes of benzyl-heteroatom o - b ~ n d sphotoamination ,~ by electron transfer,' the synthetic potential of phthalimide single electron transfer (SET) photochemistry:." photo- and thermal-degradation mechanisms of photomerocyanines,ll molecular tailoring of photochromics,12 synthesis of dihetarylethene~'~ and their photochromism in confined reaction spaces,14SET reactions of cyclic organosilane~,'~ photophysics of fullerene-porphyrin dyads,16 solid state reactions of N-heterocy~les,'~,~~ dimerisation of heterocycle-substituted alkenes," photoisomerisation of pentaatomic n,n* excited-state chemistry of azoalkanes,*' bimolecular photoreactions in solution,22 oxetanes from thiophenes and ~ e l e n o p h e n eand s ~ ~asymmetric photoreactions in and in crystalline ammonium carboxylate
2
Nitrogen-containing Compounds
2.1 E,Z Isomerisations. - Of the quinolyl-9-anthrylethenes (1),the 3-quinolyl isomer is strongly fluorescent but does not undergo E,Z photoisomerisation. In contrast 2- and 4-quinolyl-9-anthrylethenes fluoresce relatively strongly and undergo inefficient E,Z photoisomerisation in non-polar solvents but fluoresce weakly and undergo efficient photoisomerisation in polar solvents.262-Pyridyl-, 4-pyridyl- and 2-pyrazinyl-anthrylethenes (2) exhibit solvent-dependent fluorescence and efficient E,Z photoisomerisation, accompanied in non-polar solvents by efficient oxidative cyclisation. In polar solvents photocyclisation was not 0bserved.2~The role of rotamers and intramolecular H-bonding between the nitrogen atom and the vinylic hydrogens in influencing the photophysical and photochemical properties of the E,E-2,6-di(arylvinyl)pyridines (3) have been discussed.28Pulsed and stationary fluorimetric techniques and laser flash photolysis have been used to investigate the excited states of the E,E- and Z,E-isomers of the three 1-pyridyl-4-phenyl-1,3-butadienes. Intramolecular hydrogen bond~~
Photochemistry, Volume 33 0The Royal Society of Chemistry, 2002 242
243
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
ing influences the excited-state properties of the 2,E-isomer of the 2’-pyridyl deri~ative.2~ The fluorescence and E,Z photoisomerisation of a series of triazinestilbene fluorescent brighteners have been reported.3G32Potential energy surfaces have been calculated (AMl) for ground and first singlet excited-state isomerisation about the carbonxarbon bonds of symmetrical carbocyanines and correlated with polymethine chain length.33In non-polar and low-polarity solvents the cationic 3,3’-diethyl-2,2’-oxadicarbocyanines (4) form ion pairs. The counter ions affect the barrier height for isomerisation and AM1 calculations have been used to rationalise the influence of these on the lifetimes of the photois~mers.~~
& \
\
’
(1) Ar = 2-, 3-, 4-quinolyl
(2) Ar = 2-, 4-pyridy1, 2-pyrazin yl
N
A & rAr\ (3) Ar = 2-pyridy1, 2-fury1, 2-thieny1, 2-thiazolyl
Q ’-:
OJf N
Lt I Et (4) X - = CI-, Br-, I-, BF4-, C104-
An unusual oxygen effect has been reported for the 9,lO-dicyanoanthracene (DCA) SET sensitisation of oxime ether ( 5 ) [E:Z ratio = 1:1] under oxygen, resulting in geometrical isomerisation with high 2-selectivity [photostationary E:Z ratio =4:96]. Addition of superoxide anion, from DCA radical anion and oxygen, to the radical cation of (5) to give an open N,O-1,4 biradical was proposed, with free rotation and subsequent loss of oxygen resulting in geometrical isomeri~ation.~~ For a series of O-acyl-a-oxo oximes (6) geometrical isomerisation occurred from the triplet excited state whereas radical formation via N-0 bond homolysis was a singlet excited-state process.36Efficient E,Z photoisomerisation ($I = 0.2-0.8) was the main process observed for aroylhydrazones (A~CONHN=CAI-~).~~ Geometrical photoisomerisation was also observed for iminochromones (7) and (8), accompanied by irreversible formation of products consistent with competing abstraction of hydrogen from the CH=N group by the imide carbonyl group and a - c l e a ~ a g eand , ~ ~ for Z-l-methyl-N4hydroxycytosine (9) in low-temperature matrices.39The ONIOM method has been used to investigate the first singlet excited-state photoisomerisation pathways in protonated Schiff bases and extended to the isomerisation energy profile of the entire retinal protonated Schiff base.4O
(5) X = OMe, R’ = 4-MeOC6H4, R2 = 4-MeC6H4 (6) X = OCOAlkyl, OCOPh, R’ = Me, R2 = COMe, COPh
z (7) X = CI, Me, NO2; Y = 2 = H (8) X = H, Y-Z = benzo
Me
(9)
244
Photochemistry
The enthalpy change and volume contraction accompanying E,Z photoisomerisation of azobenzene and p-coumaric acid have been determined by a hybrid transient grating and photoacoustic method.4l In marked contrast to that observed for stilbene, the photostationary E,Z ratio for azobenzene in zeolite cavities is very similar to that in cyclohexane.42The geometrical photoisomerisation of an azobenzene linker has been used to perturb the periphery of a polyamidoamine starburst dendrime1-,4~ to regulate the 1:1 us. 1:2 stoichiometry of the ferric complex of a trihydroxamate siderophore," to control the catalytic activity of a P-cyclodextrin bearing a histidine to photoswitch the peptidomimetic inhibition of a-chymotrypsin,'!6 to photocontrol the formation of triple helices between modified oligothymidine species and oligothymidine/oligodeoxyadenosine double helices, 47 to photomodulate the conformations of pep tide^^"^^ and to modulate the fluorescence from the basal chromoE,Z Photoisomerisation has been rephore in a phosphorus(V) p~rphyrin.~' ported for amorphous films and solutions of triphenylamine derivatives containing azobenzene branche~,'~ for substituted azobenzene amphiphiles in reversible optically-induced switching processes, 52 for self-assembled monolayers of azobenzene and stilbene derivatives capped on colloidal gold and for azobenzene derivatives containing a positively charged head group at the air/water i n t e r f a ~ e . ~ ~ ? ~ ~ 2.2 Photocyclisations. - 2-Azabicyclo[2.2.0] hex-5-enes (13)< 15) were obtained by acetone sensitised stereoselective photocyclisation of the 1,2-dihydropyridines (10)-(12) ~ e s p e c t i v e l y ,(14) ~ ~ . and ~ ~ (15) being intermediates in a synthesis of nicotinic acetylcholine receptor agonist ABT-594 analogues. Electrocyclic 4n ring-closure of 2-pyridone, 4-methoxy-2-pyridone and 4-benzyloxy-2pyridone to 3-oxo-2-azabicyclo[2.2.O]hex-5-enesoccurred with 20-23% enantiomeric excess at - 20 "C in the presence of chiral lactam host (16)? The singlet excited state of the alkaloid (-)-colchicine undergoes tropolone ring 4n electrocyclisation to give P- and y-lumicolchicines. Solvent-solute interactions involving the amide group determine the partitioning between the isomeric prodUCts.59
&
Me
Me
---(N Me/
(10) R',R2,R3 = H, Me, Ph; R4 = Et (13) (11) R' = R2 = H, R3 = CH20H, R4 = Me (14) (12) R' = CH20H, R2 = R3 = H, R4 = Me (15)
(16) R =
0
(17) R = C02Menthyl
Stilbene-type 6n: electrocyclisation of (18) and (19)provided the phenanthrene 1,2-fJisoquinolines(20).60 ring in a synthesis of l-methyl-1,2,3,4-dihydronaphtho[ 3-Styrylpyridine (21)underwent regioselectivephotoconversion to 2-azaphenanthrene (24) under anaerobic conditions, a rapid thermal 1,7-hydrogen shift
245
I I / 6 : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
R'
R2
(18) R' = H, MeO; R2 = H, OMe; R3 = H, R4 = CH2CH2NHCOMe (19) R' = H, MeO; R2 = OMe; R3-R4 = CHMeN(C02Et)CH2CH2
Me
(20)
converting primary photoproduct (23) to the relatively stable 1,4-dihydropyridine (27) which yielded (24). Anaerobic cyclisation of 3-styrylpyridine rotamer (28) yielded primary photoproduct (29) which reverted to starting material rather than tautomerise to the less stable 1,2-dihydropyridine (26). However, in the presence of oxygen both regioisomeric dihydroazaphenanthrenes (27) and (29) were oxidised, giving a mixture of 2- and 4-azaphenanthrenes, (24) and (30). 3-Aminostyrylpyridine (22)exhibited analogous anaerobic regioselectivity to yield (25)whereas, in the presence of oxygen, both (25) and (31) were formed.61
(21) R = H (22) R = NH2
R=H
\
anaerobic 1,7-H shift
/
(24) R = H (25) R = NH2
anaerobic or O2
HN R* (26) R = H
(28) R = H
k (29) R = H
(27) R = H
(30) R = H (31) R = NH2
Photochromic materials have been intensively studied in recent years due to the prospects for application in photonic memory, switching or display devices. The photochromic 1,2-diarylcycloalkenes (32) underwent conrotatory 6n electrocyclisation on irradiation with UV light to give the closed forms (33) whose absorption, centred at 440 nm, is within the wavelength range of the InGaN blue and -cyclolaser?* The analogous 1,2-bis(1,3-dimethylindo1-2-yl)-cyclopentenes hexenes similarly yielded coloured cyclised forms. These have greater thermal stability than those of the 172-bis(1-ethyl-2-methylindol-3-yl)cyclopentenes and cycl~hexenes.~' Fulgides (2,3-dialkylidenesuccinicanhydrides) also continue to attract interest as photochromic materials for technological applications. Z-Fulgide (34) is
246
Photochemistry
& /
254 nm
7
440 nm
(32)n = 3,4; R = Me, Et, CH2Ph,n-C16H33 (33)
Z-Open form
(34)x = 0 (35)X = C(CN)2
€-Open form
Closed form
(36) X = 0 (37) X = C(CN)2
(38) X = 0 (39) X = C(CN)2
non-photochromic whereas E-fulgide (36) gave closed form (38) which reverted to colourless E-open form (36) on irradiation with visible light. The introduction of a 5-dicyanomethylene group resulted in Z-isomer (35) being highly photochromic, undergoing facile conversion to E-isomer (37) which in turn underwent reversible closure to (39).64Photochromic properties (UV/visible absorption, coloration, bleaching and fluorescence quantum yields, and photochromic cycle fatigue resistance) have been reported for a series of N-substituted indol-2ylfulgimides (2,3-dialkylidenesuccinimides).65The helical chirality of the hexatriene portion of an open-form E-fulgide results in formation of a stereogenic quaternary carbon on the cyclohexadiene portion of the closed form because of the required photoinduced conrotatory ring closure. When an enantiomer of resolved indolylfulgide (40), or either of the (R)-binaphthol derived indolylfulgides (41) or (42), was mixed with a nematic liquid crystal (4-cyano-4'-pentylbiphenyl) the cholesteric phase was induced and the cholesteric pitches were reversibly changed by photoirradiation.66The photochromic behaviour of three indolylfulgenates containing crown-ether moieties has been investigated in the presence and absence of Li+, Na+ and K+. The association constants for the Eand Z-forms of (44) and (45) are greater than for the closed forms and for Na+-(44) and K+-(45) no photocoloration was observed. No effects of alkali metal cations were observed for the photochromism of (43).67Changes in the UV/visible absorption spectra resulting from 254 nm irradiation of various fulgides and monoalkylidenesuccinic anhydrides have been reported?* Ab initio MO studies at the HF/6-31G and HF/6-31G* levels have been reported for the open and closed forms of 3-furyl, 3-pyrryl and 3-thienyl f~lgides.~' Interest continues in the photochromism of benzopyrans involving photoinduced conversion of the colourless closed form to the coloured open form and
I I / 6 : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(41)X = NMe, R = P r ' (42)X = NMe, R = Pr"
247
(43) n = 2 (44) n = 3 (45)n = 4
thermal reversion to the original closed form. Carbazole 6,7-annelated 2,2diphenyl-2H- 1-benzopyrans exhibited enhanced colourability and kinetics of thermal blea~hing.~' Nanosecond laser flash photolysis studies of 3-phenyl-3[1,2-dimethylindol-3-yl]-3H-naphtho[2,l-b]pyran, 2-phenyl-2-(2-thieny1)-2Hbenzo [b]furano [3,2--benzopyran and 2,2-diphenyl-2H-benzo[b]furano [3 , 2 - benzopyran have shown that, for each, both the singlet and triplet excited states are involved in the photocoloration p r o c e ~ s . ~ Photocoloration ~,'~,~~ of the nitrospirobenzopyranindolines (46) and (47) is a triplet-state process and open form E- and 2-merocyanines (59) and (59a) were observed, the quantum yield for coloration with 308 nm pulses ranging from 0.3 to 0.8 in low-polarity solvents and decreasing to 88%) to give oxetanes (145), which are useful intermediates in an efficient route to 2-arylmethyl-3-piperidinols. The ability of D H P to bind to chiral lactam host (16) through two hydrogen bonds may be used to differentiate the enantiotopic faces of its double bond.'25The thymine oxetanes (146)underwent highly efficient photocycloreversion to the triplet excited states of the aromatic ketones.'26 1-Acetylisatin underwent efficient [2 + 21-photocycloaddition to a
ZZf6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
MeN
k
257
o
AJ-
0
H Hb R1 , R2 O
o
(145) R',R2= H, Ph; Ph, Ph; H, 3-Me3CC02C6H4; Me, Ph; C02Me, Ph
I
R2 (146) R1,R2= H, Me; H, OMe; Ph, H
\
C02Me (147) R' = Ph, R 2 = Me (148) R' = Me, R2 = Ph
variety of styrene derivatives: for example, (147) and (148) are formed in 96% yield (ratio 0.61:l respectively)from a-methylstyrene in benzene. For high oxidation potential alkenes the regioselectivity was rationalised by consideration of frontier MO interactions and the diastereoselectivity by the Salem-Rowland rules for diradical intersystem crossing. For more electron-rich alkenes SET is involved, regioselectivity being rationalised by consideration of charge and spin density distributions in the ion-radical pairs and diastereoselectivity by ionradical pair collapse consideration^.'^^ Diphenylacetylene gave oxetene (155) and both regioisomeric oxetenes, (156)/(157), and (158)/(159), were formed from 1-acetylisatin and 4-methoxy- and 4-chloro-substituted diphenylacetylenes, respectively. The oxetenes underwent spontaneous ring opening to the corresponding E- and 2-2-indolones (149)-(153).The a,P-unsaturated aldehyde (154) from phenylacetylene via oxetene (160) was not isolated; secondary intermolecular hydrogen abstraction from the aldehydic C-H bond yielding the radical pair (165)/(166) occurred. Coupling, intramolecular nucleophilic attack of the hydroxyl group on the ketene and ketonisation of the resulting enol(l67) furnished both diastereomers of (168).12*Cycloaddition of triplet 1,3,4-(2H)-isoquinolinetrione to diphenylacetylenes also yielded unstable oxetenes which rearranged to E- and Z(161) and E- and 2-(162). Photoisomerisation of the E- to the Zisomers, accompanied by oxidative cyclisation, resulted in formation of (163)and (164) in a high-yield one-pot Anthracene, benz[a]anthracene and dibenz[a,c]anthracene gave [4 + 21-adducts when irradiated in the presence of 2-morpholinopropenenitrile.'30Direct irradiation of Z-P-(N-benzylaziridin-2y1)acrylonitrile yielded two head-to-head dimers (170) by 1,3-dipolar cycloaddition of photogenerated azomethine ylide (169) to the precursor aziridinylacrylonitrile. Analogous cycloadducts were obtained with acrylonitrile, methyl acrylate, t-butyl acrylate, pent-2-enone, N-phenylmaleimide and methyl propiolate, the regiochemistry of the additions being contrary to that expected from MO theory.13' Hydrogen bonding of the inert chiral lactam host (16) to 2-pyridone in toluene resulted in [I4 41-photocycloaddition to cyclopentadiene to give a 2:3 mixture of endo and exo adducts in 87% and 84% ee re~pective1y.l~~ The amino acridizinium salt (171) gave the syn and anti head-to-tail [4+4]photodimers in equal amounts in solution whereas (172)yielded the anti head-totail [4 41-photodimer along with both labile head-to-head dimers. Irradiation of (171) or (172) in the presence of supercoiled DNA led to pronounced strand
+
+
258
Photochemistry 0
A?
I
COMe X = NCOMe, CONR (0and (3(149) Ar' = Ar2 = Ph (155) (150) Ar' = 4-MeOC&, A$ = Ph (156) (151) Ar' = Ph, Ar2 = 4-MeOC& (157) (152) Arl = 4-CICcH4, A? = Ph (158) (153) Ar' = Ph, Ar2 = 4-CIC6H4 (159) (154) Ar' = Ph, A r 2 = H
+
%NR I
'Isatin
R'
(154)
w t
R'k-l 0 .R1
.R1+ +
,,;..
/
NR
\ *NR
0 (163)
8
C
O 0 0M
R1 = H, OMe, CI
0 (164)
e-
HO \ I
COMe
Whereas the N-methylated tethered pyrindinone-pyridone (173) yielded exclusively intramolecular [4 + 4]-trans-cycloadduct (175), the nitrogenunsubstituted system (174) yielded both trans- and cis-cycloadducts (176) and (177) respectively. Formation of the cis-adduct (177) involves formation of a self-assembled hydrogen-bonded dimer of (174). In toluene at 0°C (174) was quantitatively converted to cis-isomer (177), which is a key intermediate in a synthesis of the central features of the fusicoccins.'34The magnetic properties of anthracene derivatives, and their [4 + 41-photodimers, containing stable radical substituents such as TEMPO and verdazyl, have been i n ~ e s t i g a t e d . ' ~ ~ , ' ~ ~ Diastereoselective intramolecular [4 + 4]-photocycloadditions of chiral acyclic imides (178) and (179) were compared in the solid state and in solution. For (178) reversal of diastereoselectivity occurred on changing the reaction phase.
259
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
pcN CH2Ph I
CH2Ph I
4
y
CN
q
N,CH2Ph
CN (170)
0
(171) R1 = NH2, R 2 = H (172) R’ = H, R2 = NH2
0 +NR
&p
OTBS
(173) R = Me (174) R = H
(175) R = Me (176) R = H
J2? OTBS
(177)
Solid state photodimerisation of (179)resulted in almost 100% diastereoselectivit^.'^^ Molecular oxygen trapped the triplet syn- and anti-1,s-biradicals generated on irradiation of 1-naphthyl-N-(1-naphthylcarbony1)carboxamides(180) and (18 1) and gives evidence of stepwise aromatic cycloaddition. In the absence of oxygen the biradicals were converted to [2+2]- and syn- and anti-[4+4]cycloadducts. The anthryl derivatives (182) and (183) afforded the [4 +41-cycloadduct in quantitative yield even under an oxygen a t m o ~ p h e r e . The ’ ~ ~ 1:l crystalline furoic acid salt of 9-(N,N-dimethylaminomethyl)anthraceneyielded the head-to-tail photodimer ( ~ 4 ) .There ’ ~ ~has been much interest in the photocycloaddition of tertiary amines R’N(CHR& to [60]-fullerene. New alkaloidfullerene systems have been reported from the photoreaction of tazettine, gramine, scandine and 10-hydroxyscandine to C6,,.All gave the expected [6,6]monoadduct of type (185). The scandine and 10-hydroxyscandine [6,6]-monoadducts underwent a secondary reaction involving [2 + 21-cycloaddition of a
260
Photochemistry
free vinyl group with a proximate C60double bond.'40Photoinduced inter- and intra-molecular reactions of N-(o-hydroxyalky1)tetrachlorophthalimideswith alkenes resulted in the formation of medium to large sized heterocyclic rings, initiated by SET from the alkene. Capture of the radical cation (187) by the side chain hydroxyl group of the radical anion (186) followed by radical pair recombination gave the products, for example (188) and (189) from a-methylstyrene. Rings of different structure and size can be readily constructed, for example the 2-(2-hydroxyethoxy)ethyl ester of N-tetrachlorophthaloylglycine with a-methylstyrene gave 13-membered lactones (190) and (191). Irradiation of (192) gave spirooxetane isomers (193) and ( 194).I4lIrradiation of 5-(R)-menthyloxy-2[5H] furanone in the presence of benzophenone and tertiary cyclic amines resulted in regio- and stereo-specific addition to the less hindered face of the enone to yield adducts (195). With secondary cyclic amines, chiral adducts (196) ( > 98% de) were obtained.'42Analogous reactions occurred with other unsaturated lactones using semiconductors (Sic, T i 0 2 or ZnS) as photosensitisers, rather than benzophenone, though with little selectivity at the asymmetric carbon a-to the nitrogen. SET from the amines to either benzophenone or the semiconductor generated the amine radical cation which, on deprotonation, resulted in C- or N-centred radicals which added to the enone in a radical chain process.143
(188) R' = Me, R2 = Ph (189) R' = Ph, R2 = Me
0
(193) R' = H, R2 = Ph (194)R' = Ph, R2= H
(190)R' = Me, R2 = Ph (191)R' = Ph, R2 = Me
MeN
X
*X
7
0 N
4
0
HOMenthyl
O~o~.'OMenthyl
(197)R'=a-H, (198)R' = P-H, (199)R' = a - H , (200)R' = P-H,
R2=H, n = l R2 = H, n = 1 R2 = Me, n = 0 R 2 = Me, n = O
1116: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
261
Greater that 90% facial diastereoselectivity has been obtained in the photoinitiated tandem addition-cyclisation reactions of N-alkenyl and N-propargyl a-aminoalkyl radicals to (5R)-5-menthyloxy-2[5H]-furanone, the radicals being formed following SET from a tertiary amine and loss of an a-proton. For example adducts (197H200) are obtained on irradiation of the furanone in the presence of N-( l,l-dimethyl-2-propenyl)pyrrolidine with 4,4'-dimethoxybenzophenone as SET sensitiser. Analogous reactions with N,N-dialkylanilines also proceed with high facial diastereo~electivity.'~~ The topochemical polymerisation on UV irradiation of the 1-naphthylmethylammonium salts of monomethyl 2,Zand E,E-muconic acids (Me02CCH=CHCH=CHC02H)and E,E-sorbic acid (MeCH=CHCH=CHC02H) in the crystalline state has been investigated from the viewpoint of polymer crystal engineering.'45 2.4 Other Processes. - Interest continues in excited-state intramolecular proton transfer (ESIPT) processes. Spectroscopic studies on the photochromism of the Schiff bases N-salicylidene-l-de~ylamine'~~ and 7-ethylsalicylidenebenzyla m i r ~ e and ' ~ ~ on the photochromic (and liquid crystalline) properties of some N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene] derivatives of met hoxy- or eth~xy-anilines'~~ have been reported. Spectroscopic and quantum-chemical methods have been applied to salicylidene alkylimines and to more rigid structures (201).'49 Salicylidene-N-methylimine and salicylidene-N-(a-methylbenzy1)imine have been the subject of theoretical s t u d i e ~ . ' ~ ~ Investigations ~'~' of photoinduced proton transfers in 2-hydroxy-l-(N-morpholinomethyl)naphthalene and 7-hydroxy-8-(N-morpholinomethyl)quinoline~52 in the anion of 2-(2'- acetamidophenyl)benzimidazole 53 in 2-(3,4,5,6-tetr afluoro-2-hydroxyphenyl)benzo~azole,'~~ in o-hydroxy derivatives of 2,5-diphenyl-1,3,4-0~0d i a ~ o l e , ' ~ in ~ - '10-hydroxybenzo[h]quinoline"8 ~~ and in ring-substituted [2,2'bipyridy1]-3,3'-diol~'~~ have been reported. In the singlet excited state of 9acetoxy-2,7,12,17-tetra-n-propylporphycene, one tautomer is strongly favoured while both exist in equilibrium in the ground state.'60Studies on the excited-state H-bonding interactions of p-methoxy-2-styrylquinolines,'61 l-methyl-9H-p~rido[3,4-b]indole and 9-methy1-9H-pyrid0[3,4-b]indole,'~~and 2-(2'-hydroxyphenyl)benzimidazole163have been reported. Steady-state and picosecond time-resolved spectroscopies have been applied to the investigation of the azo-enol and hydrazone-quinone tautomeric forms of a series of bisazo comp o u n d ~ . ' ~Nanosecond ~.'~~ laser photolysis has been utilised to investigate the ESIPT pathways and their rate constants for 2-(2,4-dinitroben~yl)pyridine.'~~ Intermolecular proton transfer occurred in the solid state photochromism of pyrazolones (202) and (203), involving conversion of the enolic forms to keto forms. Compounds (204H206) are non-photo~hromic.'~~ Further investigations of the photophysics of 7-azaindole, the doubly H-bonded dimer of which undergoes intermolecular phototautomerism and has been used as a model for Hbonded base pairs in DNA, have been r e p ~ r t e d . ' ~Earlier ~ ? ' ~ conclusions ~ that the tautomerism occurs via concerted excited-state double proton transfer (ESDPT) have been strongly challenged and a stepwise mechanism has been prop o ~ e d . ' ~ESDPT ~ ~ ' ~ ' studies on 2-amino-4,6-dimethylpyrimidineand 2-amino-4-
262
(201) R = Me, (CH2)17Me,CH2Ph
Photochemistry
(202) (203) (204) (205) (206)
Ph R’ = Ph, R’= Ph, R’ = Ph, R’ = Me, R’ = Me,
Ph
R 2 = NH2, X = S R2=SMe, X = S R2 = 3-pyridy1, X = 0 R 2 = NH2, X = S R2 = SMe, X = S
methoxy-6-methylpyrimidineH-bonded dimers and acetic acid complexes have also been reported,*72as have the kinetics of excited-state proton transfer of doubly protonated 2-amin0a~ridine.l~~ Time resolved laser spectroscopy has shown that in 0-acyloxime triplets the energy is localised on the imino portion of the molecule. In sufficiently flexible oxime esters in non-vertical energy transfer, involving a change in geometry between ground and excited states, occurs.’740-Alkyl aryl aldoxime ethers give alkoxy and aryliminyl radicals in very low yields on p h o t o l y ~ i s , ’whereas ~~ aldoxime esters undergo N-0 bond cleavage and are convenient radical precursors. Oxime esters containing unsaturated alkyl groups yielded cyclised products in good yield providing a ‘green’ alternative to organotin-mediated radical processes. For example sensitised irradiation of (207) yielded methylenecyclopentane in 77% yield.176Cyclohexyl, cycloheptyl and cyclooctyl nitrites in argon matrices underwent 0-NO bond photocleavage and disproportionation to yield a complex of the ketone and HNO. Cyclobutyl nitrite and, to a lesser extent, cyclopentyl nitrite formed cycloalkyl radicals which underwent ring opening with the formation of nitrosoaldehydes. The major differences in outcome from those which occurred in fluid solution were due to the influence of the rigid matrix environment on the potential reaction pathways.’77Alkoxy radicals produced on photolysis of N-alkoxythiazolethiones (208H211) underwent stereoselective 5-em-trig cyclisation and were trapped by water-soluble thiols to afford disubstituted tetrahydrofurans with satisfactory to excellent diastere~selectivity.~~~ Photoconversion of N-cyclopentoxy- and carbohydrate-derived N-alkoxythiazolethiones in the presence of hydrogen atom donors (R3SnH) yielded substituted aldehydes or formyl esters via regioselectivefragmentation of alkoxy radi~a1s.I~~ Polymer supported reagents (212)and (213)can be used for the generation of alkyl or alkoxy radicals respectively under very mild conditions. Following irradiation of a dispersion of the reagent in the reaction medium with a tungsten lamp, the products are simply isolated by filtration and removal of solvent. With (212) N-0 bond cleavage and decarboxylation yielded the corresponding alkyl radicals which were trapped by BrCC13to give the corresponding alkyl bromides. With (213) the alkoxy radicals underwent 5-em-trig cyclisation Photocleavage to yield the corresponding phenyl 2-methyltetrahydrof~rans.~~~ of the N-0 bond of N-(9-anthroyloxy)-9-fluorenylideneamineand 1-(9-anthroyloxy)-2-pyridone yielded 9-anthroyloxy radicals with much lower reactivity in decarboxylation, alkene additions, and hydrogen abstraction than ben-
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(207)
(208) (209) (210) (211)
R' = Ph, R2 = R3 = H, R4 = Me R' = R3 = R4 = H, R2 = Ph R' = R3 = R4 = H, R2 = Bu' R'=R2=R4=H, R3=Bd
263
@ = Wang resin (212) R = COCMe3, COCH2Ph, COCHMeCH2Ph (213) R = CHPhCH*CH2CH=CH2, CH2CHPhCH2CH=CH2
'hfio\b zphp Me OMe
Me OMe
!PhJie.]'
X (215)
(214) X = Br, OP(0)(OPh)2
N, Me
Me
\
H (216 )
H Ar
zoyloxy and 1-naphthoyloxy radicals.181Product and laser flash photolysis studies on the radicals generated by photolysis of pyridine-2-thione esters (214) show that the initially formed radicals (215) undergo heterolytic fragmentation of the P-substituent to generate the olefin radical cation (216). Increased radical reactivity and decreased cationic reactivity of (216) are important features of radical cations that may possibly be capable of synthetic exploitation.lg2Diazeniumdiolates R2N-N(0) =NOR' (R = Et; R' = Me, CH2Ph or 2-NOZC6H4) are photosensitive and two primary pathways operate. Extrusion of nitrous oxide (N20)with simultaneous radical generation (R2N*and R'O.), which then formed amines, aldehydes and alcohols, comprised the minor pathway. Cleavage of the N=N bond formed a carcinogenic nitrosamine (R2NN-0) and an alkoxy nitrene (R'ON) which rearranged to a C-nitroso compound (R'N=O) and subsequently tautomerised to the oxime.lg3Photolysis (and thermolysis) of 1,3,2,4-benzodithiadiazines(217) yielded stable 1,2,3-benzodithiazolyl radicals (218), possibly involving loss of a nitrogen from a ring-contracted 1,2,3-benzodithiaz01-2-ylnitrene.~~~ The initial step following excitation of 1,4-dihydro1,4-dihydropyridine, may be pyridines, for example l-methy1-2,4,4,6-tetraphenylradical formation rather that the purely intramolecular processes previously assumed responsible for their photochromic behaviour.lg5Irradiation of azirine (219) yielded biradical (220) by C-N bond cleavage in solution or in low-temperature matrices where ketenimine (221) was concluded to be the product.186 The quantum yields for intersystem crossing of N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide and maleimide have been determined to be 0.03, 0.07,0.05 and unity respe~tive1y.l~~ The photoinduced SRNlreaction of potassium
264
Photochemistry
phthalimide with l-iodoadamantane in DMSO in the presence of 18-crown-6 yielded ring-substitution products 2-(l-adamanty1)phthalimide (12%) and 3-(1adamanty1)phthalimide (45%) rather than N-substituted products.ls8 Photodecarboxylative addition of glyoxylate and secondary or tertiary a-ketocarboxylates (RCOC02Na)to N-methylphthalimide resulted in the formation of phthalimidines (222). In contrast primary a-ketocarboxylates gave solely acylation products, for example (223) and/or (225). The primary product (223) undergoes ring expansion by ring-chain tautomerisation and subsequent further reaction with a-ketocarboxylate to give (225).ls9Decarboxylative cyclisation of phthalimido dipeptides (227) yielded cyclodipeptides (228)l9’ and the process has also been used to prepare macrocycloalkyne (226) from the corresponding o-phthalimidoalkynoic acid in 2 1% yield,’” and trans-pyrrolo[ 1,4]benzodiazepines (229)from N-phthalolylanthranilic acid derivatives (230).The proline derivative (230b) yielded pure trans-(229b) in 86 % enantiomeric excess. High
M e: * X
\
0 (222) R = H, CHMe2, CHMeEt, CMe3; X = H (223) R = COCH2CHMe2,COMe (224) R = Et, X = C02Me
Q\ $R /
&
N Me
\
0 (225) R = CH2CHMe2,Me
0 (226) n = 9
R3
0 (227) (a) XR’ = CH2; R2 = H, YR3 = (CH2)11 (b) XR’ = (CH2)Z; R2 = H, YR3 = (CH2)11 (c) XR’ = (CH2)2; R2 = H, YR3 = (CH2)3 (d) XR’ = (CH2)3; R2 = H, YR3 = (CH2)11 (e) XR’ = (CH2)”; R2 = H, YR3 = (CH2)2 (f) XR’ = (CH2)11; R2 = H, YR3 = (CH2)11 (9) XR’ = CHMe; R2-R3 = (CH2)3, Y = CH (h) XR’ = CHBU”; R2-R3 = (CH2)3, Y = CH (i) XR’ = CHBu’; R2-R3 = (CH2)3, Y = CH (j) XR’=CH2; R2=Me, YR3=CH2
(229) (a) R‘ = H, Me; R2 = H, Me, Pr‘, Bu’ (b) R1-R2 = (CH2)3
p’
0 (228) (a) 26% (b) 57% ( C ) 42% (d) 68% (e) 71% (f) 80% (9) 23% (h) 21% (i) 21% (j) 12%
(230) (a) R’ = H, Me; R2 = H, Me, Pr‘, Bu’ (b) R’-R2 = (CH2)3
I I / 6 : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
265
activation barriers to rotation about the central C-N bond in the intermediate (atropisomeric) 1,7-triplet biradicals result in preservation of their absolute axial chirality during reaction and diradical combination which proceeds with complete inversion of configuration at the stereogenic ~ t - c e n t r e . With ' ~ ~ ~the ~ ~more ~ flexible ethylene linked precursor N-phthaloyl-2-(~-alanyl)-2-azabicyclo[3.3.0] octanoate, approximately equal amounts of two diastereomers, (231)and (232), were is01ated.l~~ Unsymmetrical imides (233) and (234) underwent photodecarboxylation to yield 0- and rn-cyclised products (235) and (236), and (237) and (238) respectively, with modest regioselectivity favouring the o-isomer in each case. Photocyclisation of the imide (239) gave rn- and o-isomers (241) and (243) respectively with preference for o-cyclisation. Unsymmetrical trimellitic acid imide (240) yielded p- and rn-photoproducts (242) and (244) with preference for p-cyclisation. In contrast irradiation of N-methyltrimellitic acid imide in the presence of potassium propionate gave solely the p-product (224). DFT and ab initio calculations for the imide radical anions were consistent with the observed regioselectivities being determined by differences in spin densities in the corresponding imide radical anions rather then donor-acceptor interactions prior to SET from either the carboxylate or sulfur donors to the imide.195Photo-Friesrearrangement of 12- and 14-membered N-phenylimides occurred readily to give 0-and p-cyclophanes as primary
(231) H,OHa(232) H,OH p-
(239) X = N, Y = H (240) X = C H , Y =C02Me
(233) n = 3 (234) n = 5
(235) (236) (237) (238)
n = 3, n = 3, n = 5, n = 5,
X = CH, Y = N X 1N, Y = CH X = CH, Y = N X = N, Y = CH
(68%) (32%) (57%) (43%)
(241) X = N, Y = H (81%) (243) X = N, Y = H (19%) (242) X = CH, Y = C02Me (75%) (244) X = CH, Y = C02Me (25%)
In the presence of a phosphate buffer, antibacterial 7-amino-6-fluoroquinolones underwent reductive defluorination and piperazine side chain oxidation, which are photoprocesses not observed in neat water. Norfloxacin (245) underwent quantitative defluorination to an unstable major product, though not to (248), the main product in neat water. A minor product (249) was characterised. Enoxacin (246) yielded (251) and (252) whereas lomefloxacin (247) yielded (250) and (253)-(255).The reactions are initiated by SET quenching by phosphate anion, an unexpectedly efficient reducing agent for excited states. This, coupled with the radical reactivity of the phosphate radical anion, led to the
266
Photochemistry
y&co2H q
H
I
Et I
R
(245) (246) (247) (248)
yQ4Jco2H
Y&C02H R
I Et
X = CH, Y = F, R = H X = N, Y = F, R = H X=CF, Y =F, R = M e X=CH, Y =OH,R = H
(249) Y = H, R = Me (250) Y = F, R = COMe
R3Y-Y R'NH
R*
Et
(251) X = N, Y = H, R' = CHO, R2=Me, R 3 = H (252) X = N, Y = H, R' = Me, R2=CH0, R 3 = H (253) X=CH, Y=F, R ' = R 2 = R 3 = H (254) X = CH, Y = F, R' = R2 = H, R3 = Me (255) X = CH, Y = F, R1-R2 = CO, R3 = Me
unprecedented photoconversion of these 7-amino-6-fluoroquinolones.197~198 Incorporation of the cationic form of lomefloxacin (247)in anionic sodium dodecyl sulfate micelles results in much increased ph~tostability.'~~ Photolysis of clinafloxacin (256)yielded eight new degradation products from the two processes of dechlorination followed by further reactions of the quinolone ring, and pyrrolidine side-chain degradation.*@' The bichromophoric sulfonylurea, chlorsulfuron (257), followed different reaction pathways depending on whether the benzene or the triazine chromophore was excited. Chlorine substitution by hydrogen or hydroxyl in water occurred in the former case, whereas the most efficient process in the latter case was S-N bond cleavage in the sulfonylurea bridge.201Photodegradation of the human pharmaceutical dichlofenac, 2-(2',6'dichloropheny1)aminophenylacetic acid, occurs in lake water, the initial photoproduct, 8-chlorocarbazole-1-acetic acid, photodegrading more rapidly than the parent compound.202 0
NMe2 tio.@
HO
CH20H
Further modifications of the triacetyl derivative of 4-aminocyclopentenetrans,trans-3,5-diol, obtained by photohydration/acid-catalysed ring opening of pyridine in aqueous perchloric acid, has led to a convenient synthesis of (-)allosamizoline (258).*03cisltrans Interconversion generally occurred readily for N-alkyl-2-azetidinones, probably involving C-3-C-4 bond cleavage, whereas N-phenyl-2-azetidinones were unreactive, and N-CO bond cleavage/reclosure did not lead to isomerisation of the p-lactam ring.2o4Irradiation of furazans (259) in the presence of amines resulted in extrusion of benzonitrile, capture of the ring-cleaved intermediate by the amine, cyclisation of the resulting N acylaminoamidoxime and formation of the 3-amino-5-perfluoroalkyl-1,2,4-
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(259) R1 = C3F7, CTF,,
(260) R1 = C3F7, C7F15; R2 = H, Me, C3H7, C&7
267
(261) R’
(262)X = Si, Sn
NPh Me (263) R = H, CH2C02Me
(264)
(265)
PY PhCH2 Py = 2-pyridyt (266) n = 1; R = H, C02Me (267) n = 0; R = H, C02Me
J
oxadiazoles (260).*05Irradiation of 10-methylacridinium perchlorate (261) with allylic silanes and stannanes (262) led to the corresponding 5-allylated dihydroacridines (265). With unsymmetrical allylsilanes and allylstannanes, the allylic groups were introduced selectively at the a-position though, in the case of allylstannanes, y-adducts were also obtained. SET from the organosilanes or organostannanes to singlet excited 10-methylacridinium ion followed by radical coupling yielded the products.206The 3-pyrazolines (263) underwent ring contraction to (264) on irradiation.207Irradiation of the triazolines (266) gave cyclobutane cleavage products, in addition to the anticipated aziridines (267). These included pyridazinonorbornadiene (268),its isomer (269)and triazoles (272). The unusual cleavage of (266),leading to (268) and (269),has been attributed to extra stabilisation, via (271), of diradical(270) provided by the pyridazine and pyridyl nitrogens. Breakdown of (271) by route a yielded triazole (272) and norbor-
268
Photochemistry
nadiene (268) whereas route b yielded (272) and rearranged product (269). Separate irradiation of (268) yielded (269) via a di-n-methane rearrangement, though this does not compete with formation of (269) during irradiation of (266)?08Irradiation of pyrazino- and quinoxalino-fused naphthobarrelenes resulted in triplet excited-state di-n-methane rearrangement via heteroaromaticvinyl bridging, and not naphtho-vinyl bridging, to yield the corresponding semibullvalenes?09Photolysis of isoxazolone (273) in acetone at 300 nm gave pyrrole (276) via carbene (275).In contrast isoxazolone (274) yielded the acetone cycloaddition product (280), consistent with an electron-withdrawing group close to the nitrogen of the iminocarbene endowing it with 1,3-dipole (277) reactivity. In acetonitrile isoxazolone (274) yielded pyrrole (278), via cyclisation of carbene (275),and cycloaddition product (279).210>211 yH=CHC02Et
(273) R’ = H, R 2 = Ph (274) R’ = Me, R2 = C02Et
C02Et
. .. (276)
(275) R’ = Me R2 = C02Et
CH=CHC02Et I
C02Et
‘Me (277)
C02Et tN __t
Ph
Ph (286)
Irradiation of acridine and carbazole, either as a polycrystalline mixture or in solution, yielded the condensation product 9-carbazol-9-yl-acridine, possibly involving 2,7-Dihydroazepine (281) photorearranged to 2,3-dihydroazepine (282) which on further irradiation underwent ring contraction to give (283).213Irradiation of 3,5,6-triphenyl-1,2,4-triazine in neat triethylamine gave 2,5dihydrotriazine (284), 3,5-diphenyl-1,2,4-triazole (285) and dimeric triazole (286).214Photophysical studies of this and related electron-deficient azaarenes
I I / 6 : Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
269
have shown that the triplet state is involved in efficient SET from amines. Subsequent rapid proton transfer within the collision complex forms the hydroAqueous gen-adduct radicals which react slowly to yield subsequent solutions of the sodium salt of N-bromo-4-methylbenzenesulfonamide (bromamine) yielded 4-methylbenzenesulfonamide, N,N-dibromo-4-methylbenzenesulfonamide and bromine as photoproducts.216Irradiation of aldimines (R1CH=NR2) in propan-2-ol/acetone through Pyrex resulted in reductive dimerisation to the corresponding vicinal diamines [R'CH(NHR2CHR1NHR2] in good to excellent yields, the meso-diamine normally being in excess.217Aromatic imines (Ar1CH=NAr2)were efficiently photoreduced to the corresponding amines (Ar1CH2NHA?)in the presence of 2-phenyl-N,N-dimethylbenzimidazole as electron donor and magnesium cation as SET mediator.218lH-Azepine-2,7dione, on irradiation in aqueous acetonitrile containing either morpholine or potassium hydroxide, underwent carbonyl photoreduction to give 7-hydroxy1H-azepine-2-0ne.2'~The use of catalytic amounts of P-lapachone, a triplet SET photosensitiser, resulted in C-16-C-21 bond cleavage of the catharanthine radical cation (287)and reaction with trimethylsilyl cyanide to yield 2la-cyano-16a(methoxycarbony1)cleavamine(288) in 88% yield.220
Light-controlled synthesis of peptides, employing photogenerated acids for deprotection of N-t-Boc groups, has potential for parallel synthesis of addressable, combinatorial molecular microarrays, with photolysis of triarylsulfonium or diaryliodonium hexafluoroantimonates in dichloromethane a source of the photogenerated acid.221Laser flash photolysis has been used to study photoacid generation from N-oxysuccinimidoarylsulfonates and 1,2-di(arylsulfonyl)hydrazines. Sulfonic acids were generated following reaction of arylsulfonyl radical with molecular o ~ y g e n .Decahydroacridone ~~~,~~~ dyes are efficient SET sensitisers for decomposition of diaryliodonium and triarylsulfonium salts. The resulting acridone radical cations release a proton. The dye singlet excited state is involved in the photosensitisation of triarylsulfonium salts whereas both singlet and triplet excited states are involved in photoacid generation with diaryliodonium salt s.224Quaternary ammonium dithiocarbamates quantitatively release a tertiary amine, for example diazabicyclo[2.2.2]octane from 1phenacyl-( 1-azonia-4-azabicyclo[2.2.2]octane)-~,N-dimethyldithiocarbamate, and are excellent photobase generators for use in polymer photocrosslinking.225-227 Preferential excitation-decomposition, using circularly polarised light, of one of the enantiomers of a racemic a-amino acid by the Norrish Type I1 mechanism (leucine, valine or isoleucine, all of which contain the necessary y-H
270
Photochemistry
atom) yielded an enantiomerically enriched sample.228Fluorescence quenching, laser flash photolysis and product characterisation have confirmed SET from N-phenylglycine to singlet excited pyrene. The anilinomethyl radical is an intermediate in formation of the decomposition products N-methylaniline, aniline and f ~ r m a n i l i d eIn . ~neutral ~~ argon-saturated aqueous solution no dependence of the dipeptide decomposition quantum yield on the sequence of amino acids exists on 193 nm laser i r r a d i a t i ~ n . ~The ~ ’ photocleavage of proteins, bovine serum albumin or lysozyme, by a series of 1-pyrenyl peptide probes Py(CH2)3CONHCH2COX(where X = Trp, Tyr, Phe or His) in the presence of an electron acceptor has been investigated. Both BSA and lysozyme were photocleaved by the phenylalanine and histidine analogues while the tyrosine and tryptophan analogues did not cause fragmentation of either compound. Flash photolysis of the probe-protein mixtures indicate that the initially produced pyrene cation radical is strongly quenched by the tyrosine and tryptophan residues.231It is of interest to biomedical processes related to cataract induction, photoageing, photodynamic therapy and stabilisation of biomaterials such as porcine or bovine pericardial tissues, that the FMN-sensitised intermolecular cross-linking of N-acetyl-L-tyrosine results in formation of three tyrosine-tyrosine products: C6,C6-linkeddi-(N-acetyltyrosine), C6,07-linked di(N-acetyltyrosine) and C6,C4-linked di-(N-a~etyltyrosine)?~* Thymidine and uridine, also calf thymus DNA, sensitise the geometrical photoisomerisation of 2-cyclooctene, producing the chiral E-isomer in enantiomeric excesses of up to 15%.233 Cytosine and 1-methylcytosine radical cations, generated by SET to triplet excited anthaquinone-2,6-disulfonic acid, underwent deprotonation on the nanosecond timescale. Cytosine radical cation deprotonated at N- 1yielding cytosin-1-yl radical whereas 1-methylcytosineradical cation deprotonated at the side-chain amino group t.0 yield an aminyl radical. Each parent compound yielded an additional long-lived radical of unknown structure on the nanosecond to microsecond t i m e ~ c a l eFlavin . ~ ~ ~ adenine dinucleotide (FAD), a photonuclease model, has been used as a sensitiser of dGMP, which is a DNA model. Direct evidence of SET from dGMP was obtained. Sensitiser reactivity was not markedly influenced by the nucleotide environment as shown by a comparison of nucleotide-free and -bound ribofla~in.2~~ Irradiation of the 7-nitroindole nucleosides (289) yielded the 2’-deoxyribonolactones (290) and 7-nitrosoindole. The process provides a general route to the efficient preparation of oligonucleotides containing the labile deoxyribonolactone moiety at a preselected position.236Nitropiperonyl2-deoxyriboside has been investigated as a universal photocleavable DNA base analogue. Thus when it is incorporated into pentacosanucleotides (29l), irradiation followed by piperidine treatment caused specifically located strand cleavage to give the corresponding 3’- and 5’-phosphate~?~~ The 2-(3,4-methylenedioxy-6-nitrophenyl)propoxycarbony1 group is an effective photoremovable protecting group for the 5’-hydroxyl protection of n u ~ l e o s i d e s . ~ ~ ~ Protected peptides, peptideamides and peptide N-alkylamides (PeptideCOXH) may be photolytically released from the peptidyl resin (292) on which they have been assembled. In this case, the 2-nitrobenzyl unit serves the
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
3
I: -0P I -0
OR (289) R = H, 4-MeC6H4C0
27 1
8
02N
OR (290)
dual function of an anchoring linkage between the supporting resin and the growing peptide chain, and of latent reagent for release of the assembled peptide.239The normal photocleavage of the a-methyl-6-nitroveratryl linker used in peptide synthesis has been found to be accompanied by side product formation arising from competing reactions with the amino and thiol groups of other molecules present in the reaction l-Acyl-7-nitroindolines have been investigated as photolabile precursors of carboxylic acids, particularly neuroactive amino acids. 4-Methoxy substitution improved the photolysis efficiency whereas the 4-NJV-dimethylamino analogue was ~ h o t o i n e r t . AM1 ~ ~ ' calculations on the mechanism of 2-nitrobenzyl photochemistry suggest that a new mechanism, consistent with results from time resolved spectroscopy and acid catalysis, must be The recently introduced 2-nitrofluoren-9-ylmethoxycarbonyl peptide-protecting group underwent solvent-dependent photocleavage. Although lacking an o-benzylic proton; a mechanism involving solvent-mediated proton transfer from the 9-position to the rn-nitro group has been proposed. The resulting intermediate (293) then breaks down as shown to release the peptide, with simultaneous formation of 2-nitrodiben~ofulvene.2~~
8= cross-linked polystyrene resin (292) X = 0, NH, NMe, NEt
I
Peptide
(293)
Irradiation of the a-D-glucopyranosylpyridinium chloride (294) in aqueous potassium carbonate yielded a 1:1 mixture of photohydration products (295a) and (296), with aziridine (295a) being readily separable on a gram scale as the pentaacetate (295b).244Irradiation of N-substituted (2-bromoacy1)anilides(297) resulted in competing cyclisation to oxindoles (299) and dehydrobromination to alkene (300), accompanied by secondary six-electron photocyclisation of (300) to dihydrocarbostyrils (302). In contrast N-unsubstituted (2-bromoacy1)anilides (298) yielded only dehydrobromination products (301), cyclisation being prevented by the almost exclusive trans geometry around the amide carbon-nitro-
272 R20
(294) R ’ =
\
Photochemistry
c+ \
R2=H
,N-,
(b) R‘=Ac OR*
OR2
OR2
,
R2=H
aB;fL so axMe Me Me \
Y
I
R
0
Y
R
+
0
Y
I
R
(297) R = Me, Et, Ph, CH2Ph (299) (300) Y = 4-Me, 4-CI, 4-Me0 (298) R = H, Y = H, 4-CL 4-C02Et, 4-Me0, 2,4-(Me0)2 (301)
gen bond.245Chlorine radical n-complexes have been identified as intermediates in the photolysis of 4-(2-chlorobenzoylamino)pyridineswhich result in intramolecular cyclisation to (303) via aryl radical attack on the complexed pyridinyl ring.246Intramolecular photosubstitution to yield 2-phenyl-1,3-benzoxazolewas the major process on irradiation of 2’-bromobenzanilide in acetonitrile accompanied by photoreduction and photo-Fries type products. For 2’-chlorobenzanilide, benzoxazole formation is not the major Photolysis of the allal azidoformate (304) in the presence of an alcohol provided a convenient route to P-2-amido allopyranosides (305), presumably via a transient aziridine intermediate.248 When 2’,3’,5’-tri-O-acetylbredinin(306) or 2’,3’-0-ipropylidenebredinin (307) were irradiated in dilute acetic acid, the 2aminomalonamides (308) and (309) were obtained respectively. Appropriate modifications of the 5’-position of (309) may be made and condensation with triethyl orthoformate permits reconstruction of the imidazole base moiety, pron . ~ ~ ~ of viding convenient access to 5’-modified analogues of b r e d i ~ ~ iIrradiation cyanoaromatics (ArCN) in the presence of formamides or pyrrolidones resulted in the formation of a-aryl amides (310) 550 nm). Valleys were formed simultaneously on the (010)surface on 366 nm irradiation and disappeared on subsequent exposure to visible light. These surface changes arise from molecular structural changes occurring within the diarylethenes packed in the single crystal.315Polystyrene films containing compounds (346) or (349) turned blue or red respectively on exposure to y-radiation, colour intensities increasing linearly with the dose
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
279
absorbed, the colour change permitting an estimation of radiation dose. Excitation energy transfer from polymers to dithienylethenes may play a role in the coloration process since different polymers display quite different coloration e f f i c i e n c i e ~ . ~Irradiation '~?~~~ of (366), (367) or (368), containing two 1,2dithienylethene photochromes, resulted in cyclisation of only one of these moieties. Prolonged irradiation of (367) yielded the rearranged product (369) quantit a t i ~ e l y . ~A~ 'system containing two dithienylperfluorocyclopentene moieties, linked to a fluorescent bis(phenylethyny1)anthracene reside underwent cyclisation of only one of these moieties on irradiation at 313 nm, accompanied by a reduction in fluorescence quantum yield from 0.83 to 0.001. A similar decrease in laser emission intensity was observed on exposure to UV light. Reversal of both observations occurred by irradiation with visible light (h > 500 nm).319The blue fluorescence of the photochromic compound (357) was also suppressed by ring closure on irradiation. The spectroscopic properties and reaction dynamics have been investigated and analysed, taking into account the presence of reacting and non-reacting conformers. The presence of different conformers is argued to be a requirement for applications relying on efficient switching of the Similar photocontrol of fluorescence has been reported for the non-fluorinated analogue of porphyrinic dithienylethene (358), the intense emission from the open form being eliminated by conversion to the non-fluorescent closed form on irradiation at 313 nm and restored on irradiation with wavelengths greater than 480 nm, accompanying regeneration of the open form (358). The fluorescence intensity may be conveniently regulated by toggling between open and closed forms by alternate UV (313 nm) and visible light irradiation (h>480 nm), demonstrating the potential of (358) to act as a reversible data processing system using fluorescence detection.321
R
(366) R = X = H (367) R = CI, X = H (368) R = H, X = F
x2
cis- 1,2-Dicyano-1,2-dithienylethene underwent photochromic cyclisation as efficiently in colloidal solution as in amorphous films or in hexane solution. The photocyclisation efficiency in a polymer matrix was essentially independent of the nature of the polymer. In contrast only amorphous films of 2,3-bis(2,4,5trimethyl-3-thieny1)maleicanhydride coloured slightly on UV irradiation whereas colloidal solutions and polycrystalline samples showed no photochromism. In polymer matrices it showed significant dependence on the glass transition temperature and polarity of the The closure reactions for two terthiophene-substituted perfluorocyclopentenes occurred within about 2.7 ps in
280
Photochemistry
hexane and within 1 ps in more polar acetonitrile, suggesting that charge transfer in the excited singlet state is important in the photochromic process.323The quantum yields for the photoinduced closure and opening reactions of a series of 1,2-dithienylperfluorocyclopentenesexhibited clear threshold behaviour as a function of the So-S1 excitation energy but were otherwise insensitive to the nature of s u b ~ t i t u t e n t s .Both ~ ~ ~ the closure and opening processes for the bis(nitrony1 nitroxide) (340, n = 0) occurred with almost 100% efficiency, and magnetic susceptibility measurements showed that they are accompanied by change of magnetic interaction between the two radical centres, associated with changes in the planarity and aromaticity of the system and providing the basis for molecular switching devices for logic c i r c ~ i t s .For ~ ~the ~ ,bis(nitrony1 ~~~ nitroxides) (340, n = 1,2) the p-phenylene spacers regulate the strength of the exchange interaction and highly efficient switching was observed by ESR spectroscopy.327-330 For the diarylethene dimer (370) there are three, rather than two, photochromic states: open-open (00),closed-open (CO) and closed-closed (CC). Bond alternation is disconnected at the open form moieties of the 00-and CO-forms so that two spins cannot interact. In contrast the IT system of the CC-form is fully delocalised and the exchange interaction between the radical centres is facilitated. On irradiation of the 00-form (370) with 313 nm light sequential conversion to the CO- and CC-forms occurred. Cycloreversion occurred using 576 nm light. ESR spectroscopy confirmed that the magnetic interaction was much greater in the CC-form than in either the 00- or C O - f ~ r m s . ~ ~ ~ Semiempirical M O calculations (AM1 and PM3) have been applied to the optimisation of the conformers of photochromic dithienylethenes (cis-1,2dicyano-1,2-dithienylethene,2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride and 1,2-bis(2,4,5-t rimethyl- 3-thien y1)perfluorocyclopentene in the ground and first excited singlet states. Charge distributions, energies and dipole moments have been calculated, in addition to energy barriers between the open and closed forms.332
0
(370)
0'
Efficient reversible photochromism requires very high reproducibility of the open/ring-closed/open cycle. Some diarylethenes with thiophene rings cease their photochromic cycles in less than several hundred cycles. An understanding of the various fatigue mechanisms is essential if highly fatigue-resistant materials are to be developed for use in optoelectronic devices. Prolonged irradiation of non-substituted 1,2-bis(3-thienyl)perfluorocyclopentenewith 313 nm radiation yielded (375), resonance structure (371) rationalising cyclisation to (373), with subsequent dehyrogenation yielding (375). Blocking the dehydrogenation step by incorporating 2-and 2'-methyls, for example as in (372), did not produce a product analogous to (375) due to the difficulty of eliminating a methyl group.
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
28 1
Rather, elimination of hydrogen fluoride occurred from (374) to yield (376).With methyls in the 4- and 4'-positions of the thiophene rings no by-product analogous to (375) was obtained. In the presence of oxygen, compound (371) yielded minor by-product (377).333 The stable by-product (378), from the closed isomer (359), was obtained from 1,2-bis(2,5-dimet hyl-3-thien y1)perfluorocyclopentene (349).334The dihydroazulene-dithienylethene conjugate (380) underwent photoconversion to both isomeric dihydrothienobenzothiophene (379) and vinylheptafulvene (382). In contrast the dihydroazulene-diphenylethene conjugate (381)yielded only the vinylheptafulvene (383) on irradiation. Vinylheptafulvenes (382) and (383) reverted thermally to the dihydroazulene forms (380) and (381) respectively.335
(371) R = H (372) R = Me
(373) R = H (374) R = M e
(375)
Me Me
-
Me
Me
hv
NC
(379)
NC
A
(380) Ar = 2,5-dimethyC3-thienyI (382) (381) A r = Ph (383)
Thiofulgides (384) cyclised to thermally stable coloured isomers (387) which showed large bathochromic shifts of 40-60 nm of their long-wavelength absorption bands relative to the oxygen analogues (388) obtained from (385). White light resulted in cycloreversion of (387) to (384).336 The alkyl-substituted fulgides (386) were irradiated with 366 nm light, The resulting closed forms (389) underwent a thermal 1,5-sigmatropic hydrogen shift to yield heliofulgides (390) which yielded the open-form (391) on irradiation with 366 nm light. Reversion to (390) occurred on standing.337
282
Photochemistry
2R
R5%y
Vis.
7
s
w
y
‘‘r
Hw
S
1,5-shift
R2
R2
0
R’ R2 R2
(384) X = 0;Y = S; R’ = R3 = Me; R2 = cyclo-C3H5; (387) R 4 = H; R 5 = Me, Ph (385) X = Y = 0 ; R’ = R3 = Me; R2 = cyclo-C3H5; (388) R4 = H; R5 = Me, Ph (386) X = S; Y = 0 ; R’ = H; R2-R2 = adarnantylidene; (389) R3 = Me, Pr’, Ph, C6D5, 4-FC6H4; R4-R5 = benzo
H
0
O
(390)
0
Structure-property relationships have been proposed based on a combination of molecular modelling and experimental determination of photochromic parameters (absorption wavelength of open and closed forms, rate constants of thermal bleaching and coloration ability) for photochromic thiophene-substituted [3H]-naphtho[2,1-b]pyrans. The calculations were used to make qualitative predictions on the variation of the absorption wavelength of the open form.338For the crowned spirobenzothiopyran (58) the stability of the photoinduced open coloured isomer was enhanced by the metal ion complexing ability of the crown ether moiety, especially Li+, and by metal ion affinity, especially by Ag+, for the thiophenolate anion.339Spectrokinetic parameters have been reported for a series of thiophene-fused 2H-chromenes which includes 2,3-dimethyl-8,8-diphenyl[8~]chromene[7,8-d]thiophene, 2,3-dimethyl-7,72,3-dimethyl-6,6-diphenyl[6H] diphenylC7H-jchromene[ 6,5-d] t hiophene, chromene[ 5,6-d] thiophene and 2,3-dimethyl-5,5-diphenyl[5H]chromene[8,7-d] thi~phene?~’and also for a range of 5-methoxycarbonyl-8,8-diaryl[8H] chromenec7,8-d] t hiophenes and 8-methoxycarbonyl-5,5-diaryl[ 5H] chromene[ 8,741thi0phenes.3~’ Fluorescence was observed for several oligothiophene-substituted chromenes, the absorbed light inducing oligothiophene fluorescence rather than ring opening of the chromene. The photochromism/fluorescence ratio depended on the polythiophene chain length and on the chromene substitution site.342-344 Oxidative photocyclisation of the 9-(arylvinyl)thieno[3,2-~]quinolizinium perchlorates (392) and (393) in methanol containing iodine yielded the corresponding thiaazonia[ 51helicenes, for example (394) from (392). The isomeric 9-arylvinylthieno[2,3-a] quinolizinium perchlorates underwent analogous cycl i ~ a t i o nIrradiation .~~~ of N,N-dibenzyl-a,P-unsaturated thioamides (395)-(398) in benzene yielded a-thiolactams (400)-(403) respectively. From (395) and (396), for which two diastereomeric P-thiolactams are possible, only the 2-isomer was obtained. The unsaturated thioamides (396) and (397), although achiral, crystal-
283
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
(392) Ar = 2-thienyl (393) Ar = 3-thieny1, Ph
(394)
lised in chiral form and solid-state photocyclisation resulted in the formation of the corresponding optically active P-thiolactams (401) and (402) in high enantiomeric excess. Consideration of the absolute configurations of crystalline ( + )(396) and its cyclisation product ( +)-(2)-(401) were consistent with the involvement of a zwitterionic intermediate (399) which undergoes conrotatory cyclisation to yield the Z - P - t h i ~ l a c t a m . ~ ~ ~
(395) (396) (397) (398)
R’ = R3 = Me, R2 = H R’ = H, R2 = R3 = Me R’-R2 = (CH2)4, R3 = H R‘-R2 = (CH2)5, R3 = H
(400) (401) (402) (403)
(399)
R’ = R3 = Me, R2 = H R’ = H, R2 = R3 = Me R’-R2 = (CH2)4, R3 = H R1-R2 = (CH2)5, R3 = H
The 4-aryl-4-methyl-2,6-diphenyl-4H-thiopyrans (404) photorearranged by selective migration of the aryl groups to form the corresponding 2H-thiopyrans (408) quantitatively via the 6-aryl-5-methyl-1,3-diphenyl-2-thiabicyclo[3.1.0] hex-3-enes (406) as intermediate^.^^^ For the 3,5-substituted 4H-thiopyrans (405), direct conversion to hexasubstituted 2H-thiopyrans (409) occurred without intermediates (407) being observable by NMR spectroscopy.3484,4-Diphenyl-2,6di(4-methoxyphenyl)-4H-thiopyran-l,l-dioxide photoconverted to 3-(4methoxyphenyl)-6,6-diphenyl-2-thiabicyclo[3.l.0]hex-3-ene-2,2-dioxideby a thia-di-n-methane rearrangement involving initial vinyl-vinyl bridging.349Similar formation of syn- (major) and anti-thiabicyclo[ 3.1.O] hex-3-ene-2,2-dioxides occurred for 4-meth yl-2,4,6- t riphen yl-4H-t hiopyran- 1,l-dioxide?50Phot olysis of N-phenyl-0-benzylthiocarbamate (PhNHCSOCHZPh), N-phenyl-O-phenylthiocarbamate (PhNHCSOPh) and N-phenyl-S-phenylthiocarbamate (PhNHCOSPh) in acetone containing traces of benzophenone resulted in homolysis of the N-CS, 0-CS and S-CO bonds and the resulting free radicals yielded the products by hydrogen-abstraction, dimerisation, disproportionation and/or fragmentation R
R1&Ar Ph
Ph Ph (404) R’ = H; R2 = Me, Ph; Ar = Ph, 4-BrC6H4, 4-FsCC6H4, 4-MeCsH4, 4-MeOC& 4-Me2NC& (405) R1 = Me, Ph; R2 = Me, Ph; Ar = Ph
S
Ph
Ph
’
h
;
S
Ar
(406)
(408)
(407)
(409)
284
Photochemistry
Details of the photochemical reactions of N-acylbenoxazole-2-thiones with alkenes have been reported.3s2Initial [2 21-cycloaddition of the alkene to the carbon-sulfur double bond yielded the unstable aminothietane (410), the regiochemistry of which was determined by formation of the more stable diradical intermediate. Carbon-sulfur bond cleavage, followed by intramolecular acyl transfer from nitrogen to sulfur, resulted in formation of 2-substituted benzoxazoles (411) whereas the analogous steps initiated by carbon-oxygen bond cleavage led to iminothietanes (412). Irradiation of a series of N-3- or N-4alkenylthioglutarimides (413) gave thietanes (414) as primary photoproducts. For thio- or dithio-glutarimides (413; X = 0, S; n = 2; R1= R2= R3= Me), thietanes (411) were the major products isolated whereas in all other cases their fission products were obtained. Products from Norrish Type I1 y-hydrogen abstraction were very minor.3s3 Irradiation of 2-trimethylsilyl-2-phenyl- 1,3-
+
(410) R’ = Me, Ph, CN, OEt; R2 = Me, H, CN, Ph, C02Me, CMe=CH2; R3 = H, Me; R4 = H, Me, CH=CMe2
(41 1)
R’
6
In
N-(CH2)nCR’= CR2R3
Ph
X
(413)n = 2,3; X = 0 , S; R’ = H, Me; R2 = H, Me; R3 = H, Me
h
@
(41X4)
(415)
dithiane or 2-pentamethyldisilanyl-2-phenyl1,3-dithiane in propan-2-01 yielded benzyltrimethylsilane or benzylpentamethyldisilane in 34% and 28 YOyield respectively, the outcome of initial carbon-sulfur bond cleavage.354Laser flash photolysis studies with a series of dithiane-carbonyl adducts (415) support a mechanism for deprotection involving SET from the dithiane moiety to excited benzophenone, followed by benzophenone radical anion facilitated O-deprotonation, coupled to carbon-carbon bond scission and release of the carbony1 The method has been applied to the deprotection of calixarene and dibenzocrown ether derivative^.^'^ Irradiation of the 2-0-thiobenzoin ate derivative (418) of methyl 4,6-benzylidene-a-~-glucopyranoside dichloromethane containing triethylamine resulted in solvent incorporation and cyclisation to diastereoisomers (416) and (417). The 3-0-thiobenzoate derivative (419) similarly yielded (421) and (422). SET from triethylamine to the triplet excited thiobenzoyl group of (419) yielded a radical anion which abstracted a hydrogen atom from the triethylamine radical cation. The resulting thiolate anion reacted with dichloromethane, generating chloromethyl sulfide (420) with cyclisation to the adjacent hydroxyl group completing conversion to (421) and
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
285
(422).357Irradiation of the N-t-butyl-benzothiazole-2-sulfenamide (423) in carbon tetrachloride in the presence of water yielded benzothiazole, 2-mercaptobenzothiazole, 2-chlorobenzothiazole and benzothiazol-2-0ne.~~~ The resinbound thiohydroxamic acid linker (424) has potential as an efficient traceless linker, revealing an aliphatic C-H bond on photolysis at 350 nm. Thus irradiation of resin bound N-methylindole-3-acetic acid (425) in the presence of a variety of hydrogen donors (Me3CSH,Bu3SnH,(Me3Si)3SiH)resulted in decarboxylative release of 1,3-dimeth~lindole.~~~
‘OMe (416) R’ = H, R2 = Ph (417) R’ = Ph, R2 = H ]
-
OR2 (418) R’ = H, R2 = CSPh (419) R’ = CSPh, R2 = H (420) R’ = CHPhSCH&I, R2 = H
-[
’OMe
(421) R’ = H, R2 = Ph (422) R’ = Ph, R2 = H
2,3-Diiodo-5-nitrothiophene (426), on irradiation in the presence of aromatic compounds (benzene, thiophene, 2-bromothiophene, 2-chlorothiophene), gave high yields of the corresponding 2-aryl derivatives (427) and (428). 2-Iodo-5nitrothiophene (429)underwent an analogous conversion to (430)in the presence of m-xylene, and 2-iodo-5-nitroimidazole behaved similarly. Homolytic cleavage of the carbon-iodine bond was proposed to occur from the lowest triplet n,n* state. In contrast the di-iodo compound (426) converted rn-xylene to a mixture of 3-methylbenzaldehyde and m-tolualdehyde by an SET process. For the isomeric 2,4-diiodo-S-nitrothiophene (431)substitutions occurred in very low yields, PM3 calculations showing that the lowest triplet is 7c,n*. With thiophene, (432) was obtained whereas with benzene a mixture of the anticipated product (433) and the transposed product (427) was formed, transposition occurring from the initially formed radical. 2-Bromo-5-nitrothiazole has a lowest n,n* excited state with insufficient energy to cleave the carbon-bromine bond and, in the presence of benzene or indene, replacement of the nitro group occurred to yield 2-bromo5-phenylthiazole or 2-bromo-5-( 1H-inden-2-y1)thiazole respectively.360Steadystate and pulsed techniques, and also semi-empirical quantum-mechanical calculations, have been applied to an investigation of the photosubstitution reactions of 2-iodo-S-nitrothiophene, 2-iodo-S-cyanothiophene, 2-bromo-5-cyanothiop hene and 4-iodonit ro benzene.361 In water, direct photolysis of 4-ClC6H4CH2SCOEt yielded 4-chlorobenzaldehyde and 4-chlorbenzyl alcohol, and 2-MeNHCOOC6H4CH2SEtwas converted to a mixture of the corresponding sulfoxide, 2-MeNHCOOC6H4Meand two unidentified oxidation products.362The solid-state photoreactions of twocomponent molecular crystals of 2-thienylacetic acid with acridine yielded 9-(2thienyl)methyl-9,lO-dihydroacridineand biacridane, also obtained from solution-phase irradiation. In the solid state bis(2-thieny1)aceticacid-acridine and
286
Photochemistry Me I
a 7 F SN N H U v l e 3 (423)
e
DsK"oR
o
S
@ = Polymer resin (424) R = H (425) R = COCH2-3-(Nmethylindolyl)
02N
R3
(426) R' = H, R2 = R3 = I (427) R' = H, R2 = I, R3 = Ph (428) R' = H, R2 = I, R3 = 5-thienyl, 5-brorno-2-thienyl, 5-chloro-2-thienyl = R~ = H, R~ = 1 (429) RI (430) R' = R2 = H, R3 = 2,4-Me2C6H, (431) R ' = R ~ = I , R ~ = H (432) R' = I , R2 = H, R3 = 2-thienyl (433) R' = I , R2 = H, R3 = Ph
bis(2-t hieny1)acetic acid-p henanthridine yielded bis(2-t hieny1)methane and 9,9bis(2-thienyl)methyl-9,1O-dihydroacridineor 6-(di-2-thienylmethyl)-5,6-dihydrophenanthridine respectively. In solution, the bis(2-thieny1)methylradical from The bis(2-thieny1)aceticacid dimerised to give 1,1,2,2-tetraki~(2-thienyl)ethane.~~~ lack of racemisation on triplet sensitisation and of quenching of racemisation by dienes, and the much greater impact on singlet photophysics by the sulfinyl substituent than on triplet behavour, led to the conclusion that photoracemisation of a series of aryl methylsulfoxides is intimately tied to non-radiative singlet decay. Activation energies for sulfoxide photoracemisation are CIDNP measurements have been used to study the SET quenching of singlet sensitisers naphthalene and 9,lO-dimethylanthracene by triphenylsulfonium hexafluoroantimonate. Formation of phenyl radicals and the 9,lO-dimethylanthracenyl cation respectively were observed.365CIDNP has also been used to investigate the equilibrium between the open-chain protonated, open-chain deprotonated and cyclic (two-centre, three-electron bond between sulfur and nitrogen) forms of the methionine radical cation, generated by photoinduced SET to 4-~arboxybenzophenone?~~ Quenching of the fluorescence of 3-carboxyethyl-7-methylthioxanthen-9-oneby di- and tri-methoxybenzenes displayed Rehm-Weller behaviour, whereas with methyl-substituted benzenes the behaviour followed a sigmoidal curve arising from exciplex quenching.367Rapid solvent-dependent intramolecular SET quenching occurred on excitation of the fullerene moiety of .n-extended tetrathiafulvalene-containing fulleropyrrolidine dyads.368 Bis[4,5-di(methylthio)-1,3-dithiol-2-ylidene]-9,10-dihydroanthracene formed a transient radical cation, with a half life of approximately 80 ps on flash photolysis in chloroform. Disproportionation to the dication occurred in degassed solutions whereas in aerated solutions l0-[4,5-di(methylthio)-1,3-dithiol2-ylidene]anthracene-9( 10H)one was Time-resolved visible and near-IR absorption and EPR have been used to investigate charge separation in photoexcited polythiophene-fulleropyrrolidine dyads. Photoexcitation of the oligothiophene moiety in some fullerene-oligothiophene-fullerene triads, with three, six or nine thiophene units, s-') intramolecular SET to the fullerene moiety, resulted in very fast (1012-1013
II/6: Photoreactions of Compounds Containing Heteroatoms Other than Oxygen
287
whereas in oligothiophene/fullerene mixtures intermolecular triplet energy transfer occurred.372 Excitation of the phenothiazine moiety in phenothiazine-bridge-pyromellitdiimide and phenothiazine-bridge-pyrromellitdiimide-nitroxide radical systems, where the bridge is a semi-rigid biphenyl4,4'-bis(methy1ene) unit, resulted in efficient singlet initiated SET from the phenothiazine to the pyromellitdiimide unit to give charge-separated states whose decay kinetics were determined by the interplay between spin conversion and back electron
4
Compounds Containing Other Heteroatoms
4.1 Silicon and Germanium. - The stable silabenzene (434) yielded silabenzvalene (435) on irradiation.374The disilanylethynylbiphenyl(436)was converted to reactive silacyclopropene (437) on irradiation. In the presence of methanol, (437) was converted to the E-adduct (438) and to two dimers, a 1,2- and a 1,4disilacyclohexadiene. Irradiation of (438) yielded only the 2-isomer (439). In the presence of acetone, photoadducts (437) yielded two acetone adducts, in addition to some 4,4'-bis(trimethylsilylethynyl)biphenyl,the latter consistent with liberation of dimethylsilene from either (437) or its adduct with A series of trimet h ylsilyl-substituted cyclopropenes has been investigated to distinguish between the possible occurrence of cyclopropylidene intermediates (445), produced by a fast 1,2-silyl shift in a 1-silyl-substituted cyclopropene (440), or vinylcarbene intermediates (442) in the formation of allenes (443) from cyclopropenes (440). For example 254 nm irradiation converted tetrakis(trimethy1sily1)cyclopropene(441)quantitatively to allene (444).The alkenyl cyclopropenes (446), offering an intramolecular trap for a cyclopropylidene intermediate, yielded only allenes (447), without bridged spiropentanes (448). Alkenyl cyclopropenes (449) and (450)similarly yielded only allenes, (451)and (452)respectively, without bridged spiropentanes analogous to (448). The experimental results, supported by computational considerations, therefore rule out the involvement of cyclopropylidene intermediates in these rearrangement^.^^^ The efficient photodeprotection of t-butyldimethylsilyl enol ethers occurred in the presence of dichloronaphthoquinone or chloranil as sensitiser and propan-2-01 as solvent. Under the same conditions silyl alkyl ethers were inert. For example (453) was converted to 4-t-butyldimethylsilyloxycyclohexanonein 98 % yield.377Direct photolysis of benzylsilanes (454) and (455) in solution resulted predominantly in the formation of the corresponding 6-silylisotoluene derivatives (456) and (457), secondary photolysis of which accounted for most of the subsequently isolated products. Thus (458) was produced quantitatively from l-benzyl-l-methylsilacyclobutane (454) in methanolic hexane whereas irradiation of 1-benzyl-1phenylsilacyclobutane (455) yielded 1-benzyl-1-phenylsilene and a complex product mixture consistent with competing formation of benzyl and l-phenylsilacyclobutyl radicals from isotoluene (457). Benzyldimethylphenylsilane also yielded the corresponding isotoluene derivative whereas benzyltrimethylsilane was essentially p h o t o ~ t a b l e . ~ ~ ~
288
Photochemistry C S i R
C S i R
(434) R = 2,4,6-[CH(SiMe3)2]3C6H2
(435)
Razy
(436) R = Me,SiSiMe,
Me3Si R&siMe20Me (438) €-isomer (439) Z-isomer
(437)
(440) (441) R' = R2 = R3 = R4 = SiMe3
e-Q-
(442)
(443) (444) R' = R2 = R3 = R4 = SiMe3
(445)
SiMe,
3:Me3
Me3Si
Me3Si
Me3Si
(446)
n = 1, 2, 3
Me3Si
Me3si>*
570 nm induces a partial back reaction to diazocyclopentadiene. The same authors have also reported a matrix study of 4H-imidazol-4-ylidene (25), generated in solid argon by 3 13 nm photolysis of 4-diaz0-4H-imidazole.~~ Photoexcitation of (25) at > 570 nm yields the ringopened singlet carbene (26), which can be converted into (27) by irradiation at > 310nm. Carbene (26) could not be observed in N2matrices, probably owing to efficient back reaction with nitrogen to regenerate the diazo precursor. It was also shown to react with CO, yielding the corresponding ketene.
Cyclopent [alacenaphthylenylidene (28) has been generated from the corresponding diazo precursor compound, and has been shown to give spirocyclopropanes with alkenes.16 With trans-P-methylstyrene, the carbene addition was stereospecific, and with 2,3-dimethylbutadiene both 1,2- and 1,4-addition were observed. Competition experiments for the reaction of (28) with styrenes gave a linear Hammett correlation with p=O.38. Accordingly, (28) is regarded as a nucleophilic singlet carbene in these reactions. Nevertheless, some triplet products (e.g. the dimer and H-abstraction products) were also obtained from reactions of (28). Photolysis of benzotriazole in Ar and N2matrices has been studied by both IR and UV-visible absorption spectroscopy (Scheme 4).'' The reactions are complicated by the existence of the 1H (29) and 2H (30) tautomers of the triazole. At 254 nm, the 1H form (29) photolyses more quickly than the 2H form, yielding the
I I f 7 : Photoelimination
311
diazoimine (31) by N-NH bond scission. Irradiation of (31) at 420nm resulted predominantly in cycloreversion to (29), but a minor pathway led to ketenimine (33), presumably via iminocarbene (32), and ultimately to cyanocyclopentadiene (34). Although the 2H tautomer (30) was also photolysed slowly in these experiments, it was likely that it reacted by initial photoinduced tautomerization to (29).
aN2 aN\\N [.Q Q ]] 420 nm 420nm
____c
*
N:
254 420 nm 420 nm
NH NH
NH NH
H
(29)
(34)
(33)
Scheme 4
3.2 Generation of Aryl and Heteroaryl Carbenes. - Three novel triplet anthryl(ary1)carbenes (35) were generated by irradiating the corresponding diazo precursors in rigid matrices at low temperatures and were characterized by EPR spectroscopy." The anthryl and aryl groups appear to act as good reservoirs for the unpaired electrons and also confer kinetic stability. Thus, in comparison with the EPR signals of most triplet carbenes, which disappear below 90 K, the EPR signals of (35a) in 2-methyltetrahydrofuran glasses persisted to 120 K, those of (35b) to 130 K, and those of the very bulky (35c) to 175 K, where the sample was fluid. The triplet benzanthracenylidene (36) has been obtained in n-hexane matrices at 1.7 K by in situ photolysis of the corresponding diazo compound, and ESR and hole burning studies were performed to determine its zero-field splitting parameters." a; A r =
* Me
Ph
b: A r = (35)
.Me
Me
c; A r =
312
Photochemistry
Sulfur ylides (37) formed from arylchlorocarbenes and trimethylene sulfide have been studied by flash-photolysis of the diazirine precursors.2oIn preparative experiments, these intermediates gave thioacetal products when the ylides underwent ring opening induced by HCl. Absolute rate constants have been determined for the reactions of chlorophenylcarbene, bromophenylcarbene and chloro-p-nitrophenylcarbene with tetramethylethylene in a range of solvents.21 The carbenes were generated from diazirine precursors and showed similar reactivity in pentane, Freon- 113, benzene, anisole, THF, ethyl acetate and acetonitrile. Thus, solvation of the carbenes has little influence on their reactivity towards tetramethylet hylene. 2-Benzofurylchlorocarbene (38) has been isolated in low-temperature matrices by photolysis of the corresponding diazirine and has been found to possess an interesting photochemistry of its own.22By choice of wavelength, (38) can be selectively interconverted with the ring-opened quinone methide (39) and the strained allene (40); and there is evidence that the latter undergoes a photoinduced 1,3-aryl shift to give the benzocyclobutadiene (41).
(38)
(39)
(40)
(41)
*
Carbonyl oxides (R’R’COO), derived from the photolysis of diphenyld i a ~ o m e t h a n e ~and ~ , ’ ~phenylmethyldia~omethane~~ in the presence of 0 2 , have been subjected to kinetic studies at 295 K. Reaction of benzophenone 0-oxide with sulfoxides afforded predominantly sulfones, but in the case of Ph2S0 some diphenyl sulfide was also formed.23The latter product led to the postulate of parallel reaction pathways: (i) nucleophilic attack of the carbonyl oxide at the sulfur atom of the sulfoxide and (ii) formation of a cyclic intermediate by 1,3-dipolar addition of Ph2COO to the S=O bond. In the formation of the carbonyl oxides, contributions by quantum chain processes involving triplet ketones and singlet oxygen have been identified.24 The influence of the reaction environment has been investigated for the photolysis of four racemic 1,2-diaryldiazopropanes (42; Ar’, Ar2= phenyl, bi~henyl).~’ In solution, mixtures of products were obtained, containing E and 2 stereoisomers of products from both aryl and hydrogen migration to the carbene centre generated by loss of N2. Photolysis of the crystalline compounds, however, gave the H-shifted products as their Z isomers (43) with >99% selectivity in most cases. Photolysis of the compounds in amorphous solids did not reproduce this selectivity, demonstrating that rigidity alone is not sufficient. The observed products can be rationalized by considering the conformations of the diazo compounds in the crystal and modelling reaction trajectories. The ability to control reaction pathways, including stereochemistry, in this way, even for such high energy reactions as diazo photolysis, suggests that there could be many worthwhile applications of crystal photochemistry in synthesis. Several photoaffinity probes containing aryl(trifluoromethy1)diazirine moieties (44) have been One of these reports describes a complex
313
I I j 7 : Photoelimination
trifunctional probe, containing the photolabel, a biotin tag and a moenomycin ligand,27and another a simple approach for the preparation of biotinyl photoprobes from unprotected carbohydrates,**which should facilitate research into carbohydrate receptors. 3.3 Photolysis of Diazo Carbonyl Compounds and Sulfur Analogues. - Laser flash photolysis of methoxycarbonyl-2-naphthyldiazomethane in Freon-1 13 solution containing T H F led to the observation of a transient absorption (La, = 330 nm), which was assigned to the ether ylide (45).29The rate of formation of the transient had a first-order dependence on T H F concentration. This appears to be the first reported direct observation of a carbene-ether ylide. The mechanism of the reaction of ketocarbenes with methanol has been investigated in kinetic studies of the photolysis of a series of p-substituted phenyl-2-diazopropiophenones (46; X = MeO, Me, H, F).30A wide difference in the activation enthalpies for electron-donating and electron-withdrawing substituents was noted, the former being consistent with diffusion-controlled processes. Qualitative energy surfaces for the singlet and triplet carbenes were proposed to account for the kinetic results and observed products.
0 O+
&OMe
(45)
0
.flM (46)
Ketene ylides have been detected following flash-photolysis of 2-diazo- 1,3dipheny1propane-1,3-dione7 (PhC0)2CN2, in the presence of amines (although not including ~ y r i d i n e )It . ~was ~ also found that the triplet state of the starting material had a lifetime of several microseconds - unusual for a diazo compound - but that it was not a precursor of the ketene, which must therefore have resulted from singlet excited state fragmentation. Product distributions from photolysis of methyl (pnitropheny1)diazoacetate in MeCN-MeOH were greatly altered by the addition of the electron-donating amine N,N,N’,N’-tetramethyl-p~henylenediamine.~~ In particular, the a-methoxy product from trapping of the corresponding carbene by MeOH was completely suppressed. Other amines had similar but less dramatic effects. To account for this observation, it was proposed that single-electron transfer to the carbene generated the carbene radical anion as the key intermediate. Some confirmation of this proposal was derived from the observation of the radical cation of the amine in flash-photolysis experiments. The hydration of the carbene formed by flash-photolysis of 4-diazochroman3-one (47) has been A short-lived species, identified as the enol
3 14
Photochemistry
tautomer of the lactone, 4-hydroxyisochroman-3-one, was detected. 2-Ethoxycarbonyl-1-silacyclobutanes(49; X = Cl, N3, NCO, NCS) have been synthesized photochemically from the silyl-a-diazoacetates (48) by intramolecular C-H insertion of the intermediate c a r b e n e ~The . ~ ~silacyclobutanes undergo facile thermal ring expansion by a 1,3(C40) shift, yielding the oxasilacyclohexenes (50).
(47)
(48)
(50)
(49)
Novel surface modifications of platinum by the 3-pyridyl a-diazoketone ( 5 1) and its 4-pyridyl isomer, together with the ketenes formed by photolysis of the diazoketones, have been studied by means of ultra high vacuum reflectionabsorption IR spectro~copy.~~ This approach is claimed to have potential as the basis for a large range of surface modifications. In a laser flash-photolysis study of diphenylsulfonyldiazomethane (52), the sulfene (53) and the ylide formed by trapping of this sulfene by pyridine were both The reactions of (53) with other nucleophiles (e.g. acetate, azide, cyanide and MeOH) were also examined. Photolysis of the stereoisomeric adiazo sulfoxides (54) in argon matrices gave the sulfine (55) by hetero-Wolff rearrangement of the sulfinyl carbene, which was not itself detected.37On further UV irradiation, (55) rearranged to (56) or lost COS to give (57).Novel diazosulfonyldiazomethanes have been patented as photoacid generators for chemically amplified resist^.^'
moc&.7 s=o
0
4
H
&Lo H
0
Elimination of Nitrogen from Azides
A critical review of the complex literature on the photo-oxidation of organic azides has been published and a reaction scheme proposed which appears to explain most of the available experimental and theoretical results.39A study of the photolysis of 4,4'-diazidostilbenes in polymer matrices has revealed that cis-trans isomerization competes with degradation of the azido
I I / 7 : Photoelimination
315
Although the majority of photochemical studies of azides in the review period have been concerned with aryl or heteroaryl azides, an interesting exception was provided by the report of a photoinduced amidoglycosylation. Photolysis of the allal azidoformate (58) in the presence of alcohols yielded the 2-amidoallopyranosides (59),presumably by nitrene addition to the C-C bond followed by ring opening of the resulting aziridine!' Yields were modest but potentially useful (35-40%) for simple alcohols (MeOH, EtOH, Pr'OH), but significantly lower for a number of more complex alcohols. Me
Me
I
"xNH
0 y 3 0
0
(59)
(58)
The effects of substitution on the yield of high-spin nitrenes from the photolysis of 2,6-diazidopyridine~~~ and the part played by orbital control in the selective have been photolysis of azido groups in 2,4,6-triazid0-3,5-dichloropyridine~~ investigated. In the former study, it was found that the progressive introduction of cyano groups disfavours the formation of high-spin products, probably owing to enhancement of pyridine-ring fragmentation. Azidopyridine (60), upon irradiation, has the possibility of N2 elimination from the azido group and fragmentation of either or both triazole rings, yielding potentially nitrene, carbene, carbenonitrene and dicarbenonitrene species with triplet, quintet or septet spin states. Photolysis of (60) at 77 K in 2-methyltetrahydrofuran led to the detection of several EPR signals.44Besides triplet signals belonging to isolated carbene and nitrene centres, a quintet signal was also observed, which was attributed to the carbenonitrene (61). A photochemical synthesis of novel mesoionic amides [ e . g . (62)] starting from azidotetrazolium salts has been d e ~ c r i b e d . ~ ~ Me
(60)
(61)
(62)
Photoaffinity labels containing aryl azide groups have been developed by several research groups. Efficient syntheses of 4-azidotetrafluoroaniline have been reported, and the potential of this compound as a heterobifunctional photoaffinity label was tested in model photolyses in cyclohexane!6 A tyrosine derivative containing a 5-azido-2-nitrobenzoyl moiety has been prepared, and the structures of its photo-cross-linking products have been investigated?' Testosterone derivatives with various azidoaryl groups have been synthesized to
316
Photochemistry
provide reagents with different linker lengths for the photoaffinity labelling of sex-hormone binding globulins and androgen receptors!* Arylazide- 1,3-disubstituted cyclohexanes have been prepared as leukotriene Bq photoaffinity pr0bes.4~8-Aminohydrocinchonidine was coupled with 4-azidosalicylic acid and labelled with 125 I to provide a photoaffinity label for proteins:' and 12-[(4azidosalicyl)amino]dodecanoic acid was used to prepare acetylated gangliosides, which were then radioiodinated to give photolabels for erythrocyte membrane proteins.51
5
Photoeliminationof Carbon Monoxide and Carbon Dioxide
Rotational and vibrational distributions have been determined for the C O fragments produced by vacuum-UV photodissociation of OCS in the 150-155 nm region, which takes place through the 2 'C+state of 0CS.52As one of the few small molecules with competing chemically distinct reaction channels at similar energies, HNCO continues to be the subject of both experimental and theoretical studies. In the review period, three computational studies of the photodissociation of HNCO have been p ~ b l i s h e d . The ~ ~ - pathway ~~ for S1+So internal conversion has received particular a t t e n t i ~ n . ~ ~ , ~ ~ Photolysis of ketene at 193nm has been studied by measuring the yields of atomic hydrogen formed when very dilute mixtures of ketene and argon or ketene and Hz were subjected to single pulses from an ArF laser.56Quantum yields for four reaction channels were determined: H 2 C C 0+ hv giving (i) CH2(3B1) C O (0.628),(ii) CH2(lAI)+ C O (0.193),(iii) HCCO + H (0.107) and (iv) C20(b'C+)+ H2(0.072).The [HI profile was found to depend mainly on the rate of the reaction H + HCCO+CH2 + CO. A theoretical study has been made of the photodissociation of formaldehyde to give HZ+ CO, including classical trajectory calculations by MP2 and density functional theory methods.57The predicted translational energy distributions of the products were in better agreement with experiment than for previous Hartree-Fock calculations, and good representations of product rotational distributions and the C O vibrational state populations were also obtained. The photodissociation of formic acid has been investigated both experimentally and the~retically.~' Ab initio calculations were performed for five reaction channels on the So, S1 and TI potential energy surfaces; and the vibrationally excited products were detected using time-resolved FTIR after laser photolysis at 248 or 193 nm. At 248 nm, the HCOOH molecule is first excited to the S1state but the dissociation takes place on the So surface, giving vibrationally excited CO, C 0 2 and H2. At 193nm, an additional dissociation pathway which produces OH and HCO radicals was identified. The sequential photolysis (308 and 248 nm) of 1,2;5,6-naphthalenetetracarboxylic dianhydride (63) (Scheme 5 ) has been investigated in Ar matrices.59 Dec-5-ene-1,3,7,9-tetrayne (67) was tentatively identified as the final product, and naphthyne intermediate (64) and ketene (65)were detected directly by IR spectroscopy. It seems unclear, however, whether the ketene (65) lies on the pathway to (67) or whether it decomposes to other, unidentified products. The
+
317
II/7: Photoelimination
likely immediate precursor of the final product, the naphthadiyne (66), was not detected. All IR identifications were made on the basis of comparisons with spectra predicted by density functional theory.
Scheme 5
A series of aroyl-substituted phenylacetic acids and p-acetylphenylacetic acid have been shown to undergo photodecarboxylation with quantum yields in the range 0.2-0.7, when irradiated with 254-350nm light in aqueous solutions at pH > PK,.~OQuantum yields decreased when the pH was lowered. In most cases the product arising from protonation of the corresponding arylmethyl carbanion was obtained in high yield. Mesityl cyclohexanecarboxylate lost C02 to give cyclohexylmesitylene in good yield upon excitation at 254 nm in neutral acetonitrile solutions.61In the presence of ethanol and acid, however, the same ester underwent transesterification upon irradiation. Quenching of the 4-carboxybenzophenone triplet by amino acid anions in basic aqueous solution has been investigated in a nanosecond laser flash-photolysis study.62Rapid decarboxylation was observed with rate constants estimated at 8.7 x 10" s-', which is at least an order of magnitude faster than the decarboxylation of aliphatic acyloxy radicals in aqueous media. The pyrene-sensitized photodecomposition of N-phenylglycine has been shown to be accelerated by the addition of an electron acceptor, such as tere~hthalonitrile.6~ A mechanism involving electron transfer from the amino acid to singlet excited pyrene through exciplex formation and the intermediacy of the radical PhNHCH2- was proposed. The photodecarboxylative addition of a-keto carboxylates (RCOC02Na)to N-methyl- phthalimide gave alkylation products (68; R = Pr', Bus, But) in yields
318
Photochemistry
of 73-86%, while glyoxylate (HCOC02Na) gave the reduction product (68; R = H) in 52% yield.64In contrast, pyruvate (MeCOC02Na)gave a 53% yield of the ring-expansion product (69). An intramolecular application of this photochemistry was provided by stereoselective syntheses of pyrrolo[ 1,4Jbenzodiazepines (70; R’=Me, H; R 2 = H , Me, Pr’, Bu’) from the precursors (71);65 yields were in the range 54-83%. Quantum yields for the 350nm photodecarboxylation of 6-carboxypterin (72) in aqueous solutions were shown to be dependent on both pH and oxygen concentration.66A study has been made of the photodecarboxylation of chromone-2-carboxylic acid in both aerated and deaerated ethanol; and ketohydroperoxide intermediates in the aerated reaction were detected by chemiluminescence.67 0
OKMe I
O% q 0
R’,N
Y R2
N\
--R2
R’
C02K
(71)
Dimethylvinylidene (73), together with the a-lactone (74), dimethylpropadienone (75) and dimethylketene, were observed as products of the photolysis of the bis-peroxy ester (76) in argon matrices.68Expected intramolecular rearrangement products of the carbene (73), but-2-yne and l-methylcyclopropene, were not, however, detected. Identifications were made by comparison of experimental and computed IR spectra.
5.1 Photoelimination of CO from Organometallic Compounds. - A review of the quantitative photochemistry of organometallic complexes and the mechanisms of their photoreactions has been published.69This contains sections on the photoelimination of CO from selected Fe, W and Rh carbonyl complexes.
I I / 7 : Photoelimination
3 19
Reviews of the photochemistry of Group 8 and Group 6 cyclopentadienyl metal carbonyl compounds have also a~peared.~O*~l A detailed experimental and theoretical study has been made of the ultrafast 267 nm photodissociation of Group 6 metal hexacarbonyls M(CO)6 (M = Cr, Mo, W) in the gas phase.72Five consecutive processes were identified following excitation: the first two correspond to relaxation along a Jahn-Teller active coordinate and internal conversion, while in the third the molecules change over to a repulsive ligand-field surface and dissociate, giving M(CO)5in the S1 state. Thereafter, the excited M(CO)5molecules relax in an ultrashort time through a Jahn-Teller induced conical intersection to the So state, a pathway which corresponds to a pseudorotation. The total times taken to reach the So state of M(C0)5 were found to be 110, 165 and 195fs for M = Cr, Mo and W, respectively. In So, M(CO)5eliminates a second C O molecule in about 1ps, owing to excess vibrational energy, but this step can be suppressed in solution by cooling. Nanosecond time-resolved infrared spectroscopy has been used to study the photolysis of M(CO)6(M = Cr, Mo, W) in supercritical fluids (C02, Kr and Xe).73 The sensitivity of the v(C0) IR bands to the molecular structure assists greatly in identifying reactive species. For the first time organometallic noble gas complexes [e.g. M(CO)5(Kr)] have been observed in solution. Moreover, evidence was obtained for q l - 0 bound C 0 2 in the complexes M(CO)5(C02) formed in supercritical COz.Time-resolved infrared spectroscopy on the nanosecond timescale has also been used to investigate the photoelimination of C O from [(C5Me5)Cr(C0)2]2.74 In this case, loss of terminal CO bands was accompanied by the appearance of a single band in the bridging CO region, consistent with the formation of the triply bridged species (C5Me5)Cr(p-C0)3Cr(C5Me5). Room-temperature photolysis of M(CO)6 (M = Cr, Mo, W) in the presence of tetracyanoethylene (TCNE) or fumaronitrile (FN) yielded trans-(q2TCNE)2M(CO)4or tran~-(q~-FN)~M(Co),, respectively, with no evidence for the UV irradiation of Cr(C0)6in heptane containing corresponding cis an excess of AsPh3 gave only tr~ns-Cr(CO)~(AsPh~)~, in contrast to previous studies of the reaction in the presence of y-alumina, which gave mixtures of the cis and trans complexes.76 Propene complexes, Cr(C0)5(q2-C3H6)and ( C S R ~ ) M ~ ( C ~ ) ~ (R ( ~=~H,- C Me), ~ Hhave ~ ) been synthesized by UV photolysis of Cr(C0)6 and (C5R5)Mn(C0)3,respectively, in liquid propene under high pre~sure.'~ Similarly, (CSH5)Mn(C0)2(N20) has been prepared in near-critical N20 at room temperature and identified by its v(C0) and v(N20)IR bands.78 This complex has a lifetime of about 5 minutes at room temperature and appears to decay by two pathways, one of which may involve 0-atom transfer, to give (C5H5)Mn(CO),(N2). Aldol-type condensations of cyclic ketones can be initiated by UV irradiation of W(C0)b and CCb; a mechanism involving intermediate carbene complexes of tungsten has been A metal-carbene active species has also been proposed for the polymerization of alkynes and strained cyclic alkenes by the w(co)6/cc~)/hv system, with NMR evidence to support this suggestion.80The 'photocatalytic' role of Fe(CO)5in the silylation of olefins with vinylsilanes and hydrosilanes has also been investigated." Benzenetricarbonylchromium(O),when photolysed in CHCl3, gives CrC13with a quantum yield
320
Photochemistry
of 1.4, consistent with a radical mechanism.82 The photochemistry of some chromium aminocarbene complexes, (C0)5Cr[C(NR12)R2](R' = H, Me, Bz; R2= H, Me, Ph), has been investigated in solution by flash photolysis and in low-temperature matrices.83Some of these complexes were known to undergo efficient photoreactions with imines to form p-lactams, supposedly via metal-ketene complexes. The only photochemical process observed in this study, however, was C O loss, and the major product in all cases was apparently C~S-(CO)~C~[C(NR'~)R~]. The photochemical reactions of the niobium and tantalum complexes (77; M = Nb, Ta) and their indenyl analogues with CO, H2and N2have been studied in solution at room temperaturex4 and at low temperatures in polyethylene matrices and liquid xenon.85The reactions occur via initial C O loss. A noteworthy observation with H2 was that either classical or non-classical dihydrides could result. Thus, in n-heptane saturated with H2, the tantalum complex (77; M = Ta) gave the classical dihydride (78; M = Ta), while the niobium complex (77; M = Nb) gave both the classical dihydride (77; M = Nb) and the non-classical isomer (79). The 267 nm photolysis of (C5H5)Ir(C0)2 in cyclohexane solution at room temperature has been examined by picosecond time-resolved IR spectros c ~ p yAlkane . ~ ~ C-H bond activation was observed directly, with formation of (C5H5)Ir(CO)(cyclohexyl)(H)following C O loss, although about 80% of the excited molecules relaxed without dissociation. The rate of activation of the cyclohexane was fast enough (2 ps) to suggest a negligible activation barrier. Comparison of the photochemistry of a series of rhodium dicarbonyl complexes, XRh(C0)2 (X=C5H5, C5H4Me,C5HMe4,C5Me5,q5-C9H7 and acac), under a variety of experimental conditions has shown that the photoefficiency of C O elimination is substantially dependent on the unique ligand X and the excitation wavelength:' quantum yields varied over three orders of magnitude in the order C5H5> C5H4Me> C5Me5>> acac > q5-C9H7.
Photolysis of C02(CO)~(alkyne)complexes in frozen Nujol at about 90 K resulted in CO loss, to give Co*(CO)s(alkyne)complexes in which CO loss appeared to be from an axial position.88Conversion to a second isomer, presumably with an equatorial vacancy, was observed on annealing at 140K. Photolyses of some phosphine substituted derivatives, ~ X ~ ~ ~ - C O ~ ( C O ) ~ ( P R ' ~ ) ( C (R' = Bu, Ph, OPh; R2= H, Ph), gave two isomeric CO-loss products in each case. UV irradiation of the iron complex (80)in the presence of P(OR)3(R = Ph, Bu, Pr, Et, Me) led to regioselective substitution of two C O ligands on different iron centres, to give mixtures of cis and trans complexes (81).89 Studies of the photoinduced replacement of C O in cyclopentadienyl(dicarbony1)iron thiocarboxylate complexes9oand in a heterometallic osmium-manganese complexg1have also been reported. UV irradiation of Fe(C0)3[P(OPh)3]2
II/7: Photoelimination
32 1
co
co
Me2 / / o ,Si -Fe,
,'Fe -S i Me2 OC (80)
oc" 1
P(oP~),
oc (81)
(82)
(83)
gave the orthometallated iron hydride (82).92The reactions of this hydride with a series of alkynes (R'C = CR2)were studied, and in some cases double carbonylation was observed, yielding complexes of structure (83).
6
Photoeliminationof NO and NO2
Photochemical reactions of cycloalkyl nitrites c-CnH2,- 1 0 N O ( n = 4-8) have been studied in argon matrices.93The larger ring compounds, cyclohexyl, cycloheptyl and cyclooctyl nitrites, gave complexes of the corresponding cycloalkyl ketones with HNO, presumably via initial 0 - N cleavage followed by disproportionation of the resulting cycloalkyloxyl radicals and NO. In the case of cyclobutyl nitrite, ring opening occurred, to give 4-nitrosobutanal as the major product, while cyclopentyl nitrite gave a mixture of 5-nitrosopentanal and the cyclopent anone-HNO complex. Excitation at 400 nm of the charge-transfer complex between tetranitromethane and naphthalene in acetonitrile and dichloromethane has been investigated on the femtosecond t i m e ~ c a l eThe . ~ ~ excitation produces a radical ion pair, comprising the naphthalene radical cation and the tetranitromethanide radical anion. The latter eliminates NO2 to give tetranitromethanide within 200 fs. In the search for versatile methods of NO generation for biomedical applications, a possibility for achieving controlled photochemical release of N O from a solid substrate has been dem~nstrated?~ In this work, a gold substrate was first derivatized with monolayers of dithiothreitol (DTT-SH), in which a thiol group is exposed. Attempts to nitrosate these layers in situ with N a N 0 2were, however, unsuccessful. Nitrosation of the surface was subsequently accomplished by deriva tiza t ion with previously prepared S-nit r osodit hio t hreit ol (DTT-SN 0). The DTT-SNO layer was found to be thermally stable, and N O was released by irradiation with visible light. The dynamics of N O ejected in the photodissociation of methyl nitrite on Ag(ll1) surfaces have been studied at 248 and 351 nm, with and without thick The photoejecspacer layers of hexane, and at various coverages of MeON0.96,97 tion of N O appears to be dominated by direct excitation of MeONO; there was no evidence for N O ejection as a result of substrate excitation.
322
7
Photochemistry
Miscellaneous Photoeliminationsand Photofragmentations
7.1 Photoelimination from Hydrocarbons. - Vacuum-UV photolysis of jetcooled methane has been investigated in detail by photofragment translational s p e c t r o ~ c o p yand, ~ ' ~ ~in~one study, the results compared with similar photolyses of silane and ge~rnane.~' Products arise via cleavage of one C-H bond, to give H + CH3,or by cleavage of two C-H bonds, to give H + H + CH2.A photochemical model has been applied to the role of methane photochemistry in the atmosphere of Titan@ .' ' Three significant dissociation channels for propane at 157nm have been identified: elimination of atomic hydrogen, molecular hydrogen and methyl radical.'O' Interestingly, elimination of H2 from the central carbon of propane (2,2-H2elimination) was found to be more favourable than H2 elimination from vicinal carbons ( 1,2-H2elimination). A similar study of H-atom dynamics in the photodissociation of jet-cooled ethyl radical has also been published.'02 A theoretical study of the photodynamics of ethylene has identified eight conical intersections involving the optically accessible V state, which are likely to be relevant to the photochemistry of eth~1ene.I'~ Experimentally, H-atom and H2 elimination from ethylene excited at 157 nm have been investigated, and three different molecular elimination processes observed: l,l, 1,2-cis and 1,2-tran~.'~ HCC radicals have been generated by laser photodissociation of acetylene at 193nm, and their reactions with acetylene studied both experimentally and the~retically.'~~ The 193nm photodissociation of HCC radicals, to give C2 mainly in the B 'Ag state, has also been investigated.lo6Photofragments from the 157nm photodissociation of propyne and its isotopomer MeCCD have been examined in similar experiments.lo7H-atom elimination from both the methyl group and terminal alkyne carbon was observed; elimination of H2 also occurred but with much smaller yields. The 243 nm photodissociation of vibrationally pre-excited CD3CCH resulted in both methyl C-D and acetylenic C-H bond rupture, with the former process predominating."' The photofragmentation of phenylacetylene at 193nm gave acetylene and C6H4as the only detected primary products.'09 Some of the C6H4molecules subsequently decomposed to 1,3,5hexatriyne and H2. There was no evidence for the formation of phenyl and ethynyl radicals, even though these had been observed in the pyrolytic decomposition of phenylacetylene. Photodissociation of 4-ethyltoluene at 266 nm in n-heptane solution proceeds by C-H bond fission of the CH2 group to give the corresponding benzyl radical at a relatively slow rate (4.0 x lo7s-')."' Since the S1 [nn*(benzene)] state populated by the initial excitation does not correlate adiabatically with the dissociative oo*(C-H) state, it was proposed that the photodissociation takes place via intersystem crossing to the Tl [nn*(benzene)] state, which in turn crosses to the oo*(C-H) state. 7.2 Photoelimination from Organohalogen Compounds. - The photolysis of simple organohalogen compounds continues to attract a large amount of very sophisticated experimentation, and our knowledge of ultrafast fundamental
IIf7: Photoelimination
323
processes has increased as a consequence. In the review period, reports on this type of research have included studies of the photodissociation of CH3C1,"' CH3Br,"' CH31,111-113 CH2BrC1,"4CH2BrI,"5~"6 CH2C11,'17CH2Br2,Il8CH212,119,120 CF212,1l 6 m I 22 CHFC12123 and CBrC1F2.124*125 In the photodissociation of CH212, formation of isodiiodomethane (H2C-1-1) from the reaction of the initial fragments, CH21and I, has been found to be favoured by s o l ~ a t i o n , " while ~ ~ ' ~ related ~ isodihalomethanes have also been observed in other systems: H2C-C1-I from CH2C1I,ll7H2C-Br-Br from CH2Br2lI8and H2C-I-Br from CH2BrI.'16The effect of the surface on the formation of CH3fragments has been investigated for the photodissociation of CH3Br adsorbed on CaF2(111) surfaces modified either by electron impact or by H-atoms.126CF2Br and Br were the major products, and C2F4Br2 and Br2 the minor products, when CF2Br2was adsorbed on highly ordered pyrolytic graphite and irradiated at 225-350 nm.127 Besides halogenated methanes, there have also been studies of the fragment photodynamics of a number of halogenated C2 and C3 molecules: CH3CFC12,128 CH2ClCH21,129 CF3CH21,13' CF3CF21,'12 CH2=CHC1,l3I CH2=CFC1,132,133 CF3CF2CF21,'l2(CF3)2CFI,"2and CH2=CC1CH3.'34To take two examples, five primary dissociation channels were found for CH2=CFCl at 193nm [elimination of Cl (by a fast process), HCl, HF, C1 (by a slow process) and F],'32 and three primary channels for CFZ-CFCl at the same wavelength (to give CFCl CF2, C2F2Cl F and C2F3 C1).'33 The elimination of HCl from CH2=CHCl has been found to proceed by competing three- and four-centre ~hanne1s.I~' IR spectroscopy was employed to study the photolysis of CD3CD2I in solid parahydrogen at 4.4 K.135Under these conditions, the iodide precursor existed in both monomeric and dimeric units. The monomers underwent competing reactions to give CD3CD2*+Ior CD,=CD2+12; the dimers gave CD2=CD2 + C2D6 + 12, either directly or via CD3CD2- radicals, followed by a slow disproportionation proceeding by quantum mechanical tunnelling of a D atom. In a comparative study of the A-band photodissociation of partially fluorinated alkyl iodides, CF3CH21, C6F13CH2CH21and C8FI7CH2CH2I, eliminated I atoms in both ground (2P3/2)and excited (2P1,2)states were detected by twophoton laser induced fluores~ence.'~~ It was found that F atoms at the P-position increase the quantum yield of excited I atoms but at the y-position have far less effect, and that excited I atoms tend to be the major product over the entire A-band (222-305 nm). A theoretical study has been made of competing C-Cl and C-Br bond fission in '[nn*(CO)] photoexcited bromoacetyl ch10ride.l~~ Although computed absolute rate constants were smaller than those measured experimentally, calculated branching ratios were close to experimental values. The triplet a-ketocarbene (84) has been detected in flash-photolysis experiments as the HBr-loss primary product in the photolysis of 2-bromophen01.l~~ The triplet ketocarbene had UV absorptions with A, = 360, 375 and 388 nm, and its identity was confirmed by product studies. An alternative pathway gave the ring-contracted ketene (85), possibly via the singlet ketocarbene. The principal photoproducts of 2-chloro-, 2-bromo- and 2-fluoroaniline in aqueous sol-
+
+
+
3 24
Photochemistry
utions were found to be 2-aminophenol, aniline and 1-cyanocyclopenta-1,3diene.'38In the formation of 2-aminophenol, the fluoro derivative reacted significantly more efficiently than the chloro and bromo congeners; so it was concluded that the reaction was a heterolytic process, probably the substitution of halide by water. Although a definitive conclusion was not reached, it seemed likely that the ring-contracted product, cyanocyclopentadiene, was formed via initial elimination of the corresponding hydrogen halide to give iminocarbene (86). Photochemical reactions of a series of N-(2-bromoalkanoyl)anilines (87) gave HBr-loss products (88), (89) and (90) in yields and proportions depending on the substituents R and X.13' Some cyclic analogues (91; n = 1-2; X = Cl, Br) underwent similar photoreactions.
X&o
R
cx:x
X
R
Chlorobenzene in ice exhibits unusual photochemistry when irradiated at 254 nm.140 Thus, biphenyl and terphenyl, together with chlorinated derivatives, and triphenylene were formed in ice, possibly via free radicals, in contrast to liquid water, in which phenolic products are formed almost exclusively. The photolysis of C6&16 in the presence of a spin trap has been examined in an EPR study.141A C-C1 bond underwent homolytic cleavage to give a stable fullerene radical of the cyclopentadienyl type. 7.3 Photofragmentations of Organosilicon and Organogermanium Compounds. - The literature on the gas-phase laser photolysis of organosilicon compounds for chemical vapour deposition has been reviewed.'42 Time-resolved photoionization mass spectrometry has been used to study the kinetics of the No reaction of Si2H2with formation of Si2H2by 193nm photolysis of di~i1ane.l~~ H2, CH4, SiH4 or Si2H6was observed, but decay rates for Si2H2reacting with 02, NO and HC1 were measured and exhibited negative dependence on the total pressure. 1-Benzyl-1-methylsilacyclobutane (92; R = Me) undergoes a rearrangement to (93) in quantitative yield when photolysed in methanolic hexane solution, by a sequential two-photon process involving intermediate (94; R = Me); whereas the 1-phenyl analogue (92; R = Ph), under similar conditions, gives a complex mixture of products consistent with competing formation of 1-benzyl-1-phenylsilene
I I / 7 : Photoelimination
325
and benzyl- and phenylsilacyclobutyl 1adica1s.l~~ The silene intermediate was detected directly in flash-photolysis experiments with A, at 315 nm; while the radical intermediates were shown to arise by secondary photolysis of the other primary photoproduct (94; R = Ph). Photolysis of the dihydro-9-silaanthracene (95; R = Ph) produced 9-phenyl-9-silaanthracene (96; R = Ph); the parent dihydro compound (95; R = H) gave (96; R = H), but in very low yield.145These reactions were also studied at 77 K in 3-methylpentane glasses, and this showed that the unsubstituted silaanthracene (95; R = H) decomposed to carbon-centred radicals and silenes, while the phenyl derivative (95; R = Ph) gave radicals as well as (96; R = Ph).146
The stereospecific formation of 1,3-disilacyclobutanes has been observed to occur in the photolysis of organometallic precursors, as exemplified by the conversion of the meso compound (97) exclusively into the trans product (98) (Scheme 6).14' The racemic mixture diastereoisomeric with (97) gave only the cis stereoisomer of (98). Paramagnetic intermediates formed by photolysis of the silanorbornadiene derivative (99) in the presence of electron-density donors (PPh3and 0 2 ) have been investigated by spin-chemistry methods (CIDNP and magnetic field effects).I4*The results have provided the second example of a reaction of triply excited dimethylsilylene. Cyclotetrasilenes (100; R' = Pr', R2= Pr', Bur) have been generated photolytically from ladder oligosilanes (101) and trapped as Diels-Alder adducts with 2,3-dimethylbuta-l,3-dieneor a n t h r a ~ e n e . ' ~ ~Both photolysis and thermolysis of betaines have been (R13P+-CR2R3-SiR4R5-S-) containing the fragment + P-C-Si-Sshown to follow two main pathways: (i) elimination of Ph3P (R'=Ph) and formation of a silathiirane and (ii) a elimination of R3P=CR2R3and generation of a silanethione R4R5Si=S.150 oc co 1,
Me, ph ,CH,-Fe ,ii-Fi Fe-cH2 Me 'Ph
&
oc' 'co
hv C6D6
*
(97)
Ph' A 'M'Ph e Me' *siVSi'
+
[CpFe(C0)2]2
(98) Scheme 6
In an investigation of the formation of aerosol particles in the gaseous photolysis of mixtures of allyltrimethylsilane and acrolein, it was shown that the silane underwent a retroene elimination of propene to give 2-methyl-2-silapropene, as well as C-Si homolysis to give ally1 and trimethylsilyl radi~a1s.I~~ The decomposition of 1,3-dimethyldi~iloxane'~~ and its diethyl analogue'53induced by IR lasers or UV photolysis has been examined as a means of producing nano-structured
326
Photochemistry
hydridoalkylsilicones. With the diethyldisiloxane, IR laser-induced thermolysis is dominated by 1,l-H2and ethylene elimination, while UV photolysis results mainly in l,l-Hz and ethane elimination; this difference influences the composition of the resulting nanotextured films.
(99)
(100)
(101)
The photodissociation of jet-cooled silane and germane by Lyman-a light (121.6nm) yields H atoms with low kinetic energies, consistent with a three-body fragmentation to, primarily, H H + SiH2 (or GeHz)." The IR multiphoton excitation of propyltrimethylgermane in the collisionless regime yields propene, with a threshold energy above the dissociation energy of the starting compound, in agreement with a recombination mechanism involving H atoms.'54 EPR signals belonging to triphenylsilyl and triphenylgermyl radicals were observed following photolysis of hexaphenyldisilane and hexaphenyldigermane in solution, but the triphenylstannyl radical could not be similarly detected within the available time resolution (ca. 80 n ~ ) . The ' ~ ~photochemistry of trimethylsilyltriphenylgermane (Ph3GeSiMe3), triphenylsilyltrimethylgermane(Ph3SiGeMe3) (Ph3GeGeMe3) in hydrocarbon and 1,1,l-trimethyl-2,2,2-triphenyldigermane solvents has been studied by steady state and flash photoly~is.'~~ In each case, the major products were derived from either homolysis of the Ge-Si or Ge-Ge bond or from extrusion of dimethyl- or diphenylgermylene. The 350 nm photolysis of the siladigermirane (102; Mes = 2,4,6-trimethylphenyl) in toluene in the presence
+
Bu:
But Sil
/ \ .Mes
Mes,
?-Ye
Mes
Mes
( 102)
of MeMgI gave a complex product mixture (after work-up with NH4Cl), from which five products were isolated and identified: MeszGeHMe, MesGeHMe2, (Bur2MeSi)MesGeHMe, rn-( di-t-but ylmet hylsilyl)t oluene and p-(di-t-but ylmethy1silyl)tol~ene.'~~ The first of these products was thought to arise by addition of MeMgI to photoextruded dimesitylgermylene, the second by formation of dimesitylmethylgermylmagnesiumiodide from the first product, followed by elimination of MesMgI to give mesitylmethylgermylene and then addition of MeMgI. The third product was supposed to derive from addition of MeMgI to Mes2Ge=SiBur2,followed by elimination of MesMgI and a second addition of MeMgI, while the most reasonable explanation for the two aromatic products was the addition of di-t-butylmethylsilyl radicals to the solvent, toluene, followed by H-atom abstraction.
I I f 7 : Photoelimination
327
7.4 Photofragmentations of Organosulfur, Organoselenium and Organotellurium Compounds. - The photodissociations of dimethyl ~ u l f i d e ' ~and * ~ ' ~its~ fully deuteriated is~topomer"~ have been investigated by velocity map imaging. Electronic excitation of dimethyl sulfide in the first absorption band produced MeS. and Me- radicals with substantial translational but little vibrational energy. The dissociation dynamics of ethylene sulfide have been studied by means of tunable synchrotron radiation, and the results have suggested the existence of a reaction channel giving S(3P)in conjunction with triplet ethylene, CzH4(3B1,), and have allowed the first experimental measurement of the energy of the latter species near its equilibrium geometry, in which the two methylene groups lie in perpendicular planes.160 A number of alkoxyl and cycloalkoxyl radicals (ROO),along with the 4have been generated by flash photolynitrobenzenethiyl radical (4-N02C6H4S*), sis of 4-nitrobenzenesulfenate esters (103), and rate constants determined for their p-scission or 1,5-H abstraction reactions.161 The cinnamyloxy radical (PhCH=CHCH20-)has also been generated from the corresponding 4-nitrobenzenesulfenate, and was found to undergo an unprecedented epoxide ring closure, to give the oxiranyl benzyl radical.'62 Calculations (B3LYP/6-31G*) suggested that the closed form of the radical is about 20 kJ mol-' more stable than the open form; so the ring closure appears to be thermodynamically driven. The 2,2-diphenylcyclobutylcarbinyl radical, along with the 2-pyridylthiyl radical, was generated from (104), by photo-induced 0-N cleavage followed by
nNo2 pht-r--goLb Ph
R/o,
S
decarboxylation, and the kinetics of its ring opening were in~estigated.'~~ As a result, the 2,2-diphenylcyclobutylcarbinylradical was proposed as a useful calibrated radical clock, which is somewhat faster than the cyclopropylcarbinyl radical. The sulfine (55), generated in argon matrices by photoelimination of N2 from (54), gave (57) by photoelimination of COS, as well as the rearrangement product (56), as mentioned earlier in this chapter (Section 3.3).37Note also the photoelimination reactions of betaines containing the fragment P-C-Si-Sdescribed above (Section 7.3).I5O Laser-induced photolysis of gaseous selenophene and tellurophene affords but-1-en-3-yne and ethyne as major products, with very minor amounts of butadiyne, and results in chemical vapour deposition of selenium and tellurium films.la +
7.5 Photolysis of o-Nitrobenzyl Derivatives and Related Compounds. - The photoinduced transfer of hydrogen from methylene groups to nitro in o-nitrobenzyl compounds has been examined by time-resolved resonance Raman and absorption spectro~copy.'~~ Although these processes are not themselves photo-
328
Photochemistry
eliminations, the data acquired in this study are relevant to an understanding of the more usual photocleavages of o-nitrobenzyl ethers or esters. EPR spectra have been obtained for radical species generated by photolysis of a series of o-nitrobenzyl compounds, including the caged ATP (105) and the caged monomethyl phosphate ( 106).166 The photolysis of caged ATP has been investigated previously and the accepted mechanism for its photofragmentation leads to the generation of ATP- with no role postulated for free radicals. This latest study shows, however, that the photolysis of (105) is quite complicated: the resulting reactions include photoisomerization, photofragmentation, electron transfer, intramolecular addition and spin-trapping reactions of the nitroso group produced via molecular rearrangement. Unwanted complexity was also uncovered in the photochemical cleavage of an a-methyl-6-nitroveratryl-based photolabile linker for peptide ~ynthesis.’~’ Fortunately it was found that the undesired effects could be largely reduced by the choice of appropriate reaction conditions.
(105)
(106)
Photolysis of 1-acyl-7-nitroindolines (107) in aqueous solution releases the carboxylic acid (RC02H)and a 7-nitrosoindole, and it was suggested that this reaction would provide a convenient source of photochemically generated carboxylic acids, particularly neuroactive amino acids.’68The effect of electrondonor substituents (X) in the 4-position was investigated as part of this research, and it was found that a methoxy group improved the photolysis efficiency by more than two-fold, but a 4-dimethylamino group suppressed the reaction completely. Indirect phosphorylation of hydroxylic solvents has been accomplished by UV photolysis of the nitrobenzyl ester (108), by a mechanism which seems to involve photo-induced de-esterification followed by dissociation of the resulting species, (H0)2P(O)C(NOH)C02H.169 X
(107)
(108)
An examination of wavelength selectivity in the removal of photolabile protecting groups, including o-nitrobenzyl derivatives, has been p~blished.”~ For some groups, the order of reactivity at 254nm was found to be reversed at 49 1 nm. The syntheses of photolabile phosphotriester derivatives of dinucleoside phosphates, containing o-nitrobenzyl or o-nitroveratryl moieties, have been
329
I I / 7 : Photoelimination
r e p ~ r t e d . ' ~Photolabile ' protecting groups for nucleosides, based on o-nitrophenylethylcarbonate groups, have been the subject of a patent,'72 and a novel photoscissile poly(ethy1ene glycol)-based hydrogel has been developed, which exploits nitrocinnamate pendant groups. 173
7.6 Other Photofragmentations.- The photodynamics of the Lyman-a photodissociation of HCN'74,'75and DCN'75have been investigated both theoretically and experimentally, and an improved value of the dissociation energy Do(H-CN) obtained. At this wavelength, HCN fragments to H + CN, with CN in both A and B states, whereas for DCN no significant branching to CN(B) occurs. Time-offlight mass spectrometry has been utilized to study the irradiation of CH3CN by soft X-rays, which results in the formation of CF3+, CF2+ and C N + fragrnent~.'~~ Photolysis of p-(a-hydroxyethy1)toluene at 266 nm in n-heptane results in C-OH bond fission."' The dissociation rate was found to be > 1.0 x 109s-', which is much greater than that of the C-H bond fission observed under the same conditions for p-ethyltoluene (see Section 7.1). In contrast to p-ethyltoluene, the initially populated S1 [m*(benzene)] state of p-(a-hydroxyethy1)toluene crosses adiabatically to the dissociative n p ( 0 ) o*(C-0) state, thus allowing rapid C-OH bond fission. A series of ten substituted aryl t-butyl ethers gave as the major products the corresponding phenols, as well as t-butyl substituted phenols, when irradiated at 254 nm.177Quenching studies with 2,3dimethylbutadiene indicated that the photoreactions took place from the singlet excited state; quantum yields and singlet lifetimes were found to correlate reasonably well with oh"values, with p = -0.77, consistent with polarity of bond breaking in the transition state of the type 0 ( 6 -). C ( 6 +). The photochemical dissociation of alkylperoxy radicals on activated silica surfaces has been examined, and parallel pathways involving rupture of both 0-0 and C-0 bonds were found.'78 The photorelease of diethyl phosphate from the p-hydroxyphenacyl derivative (109), a potentially useful means of achieving fast release for monitoring physiological responses, has been shown to proceed via the triplet excited Photolyses of aldoxime esters (ArCH=NOCOR), containing a range of alkyl and cycloalkyl groups, resulted in N-0 bond cleavage and the formation of aryliminyl (ArCH=N*)and alkyl (Re) radicals.lgOThe process was favoured by 4-methoxyacetphenone, added as a photosensitizer, and by methoxy substituents on the aryl ring; 4-nitro and pentafluoro substitution, on the other hand, were deleterious. The analogous photolyses of aldoxime ethers (ArCH=NOR) gave alkoxy and aryliminyl radicals, but only in very low yields.'" o-Quinone methide (110) has been generated in water by thermolysis and photolysis of (2-hydroxybenzy1)trimethyl iodide (11l ) . l g 2 Alkylations of various amines and sulfides, including amino acids and glutathione, were accomplished in good yields by Michael additions to (110) generated in this way. Photodissociation dynamics of s-triazine at 193 and 248 nm have been studied by probing the HCN fragments using coherent anti-Stokes Raman spectroscopy (CARS).lp3Room-temperature photolysis of the benzodithiadiazine (112) affords radical (1 13) in nearly quantitative yield, in an extraordinary reaction that *
330
Photochemistry 0
involves a ring contraction and loss of a nitrogen atom.'84Radical (113) and a series of substituted analogues were also produced by thermolysis of the corresponding benzodithiadiazine precursors, and EPR spectra of these species were obtained. Carbazolyl nitrenium cations, which can be in either the singlet or triplet state, are generated by photolysis and thermolysis of l-(carbazol-9-yl)2,4,6-triphenylpyridinium tetrafl~oroborate.'~~
Photorelease of the cyclopentadienyl radical is among the topics covered in a review of the electronic spectra and photoreactivity of cyclopentadienyl complexes.186UV irradiation of C P R ~ ( C ~ (Cp H ~= )~ q5-C5H5)gives products arising from the initial photoelimination of C P R ~ .The ' ~ ~synthesis of nanoparticles by the laser-induced photodissociation of ferrocene has been in~estigated.'~~.''~ The monomeric carbenoid complex [LiCH2SPh(pmdta)] (pmdta = N,N,N',N",N''-pentamethyldiethylenetriamine)undergoes extrusion of the methylene group when photolysed in toluene, with formation of CH4 and LiSPh(pmdta).'" In refluxing toluene, the thermal reaction follows a dimerizing a-elimination pathway, to give LiSPh(pmdta) and ethylene. The 157nm photodissociation of p01yamides'~land the photodegradation of poly~xymethylene'~~ at 122, 147 and 193nm have been examined. The latter study was aimed at understanding the photochemical evolution of organic molecules in comets, and the main products identified were H 2 C 0 ,CO, HC02H, C 0 2 ,MeOH, MeOCHO, MeOCH20Meand C3H603(trioxane).
8 1. 2. 3.
4. 5. 6.
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Photochemistry
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1117: Photoelimination
73. 74. 75. 76.
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Photochemistry
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Part Ill Polymer Photochemistry By Norman S. Allen
Polymer Photochemistry BY NORMAN S. ALLEN
1
Introduction
The field of polymer photochemistry remains an active area in applied photochemistry with many topics growing in industrial development. Photopolymerization and photocuring science and technologies as always continue to be developed particularly with regard toward designing novel and specific initiators and materials for specialist applications. Interest in active ionic initiators and radical/ionic processes continues 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 a major 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 are important for agricultural usage although interest here is again minimal. The same applies again to polymer stabilization where commercial applications dominate with emphasis on the practical use of stabilizers. For dyes and pigments stability continues to be a major issue.
2
Photopolymerization
Over twenty review articles or papers of topical interest have appeared in the last year on all aspects of this major subject area. A number of reviews have appeared on the function of different types of cationic photoinitiators and their future development and application^'-^. A number of articles have targeted interest in photosensitive systems5v6,pressure sensitive adhesives and coatings7-'', modelling and general photo initiator^"-^^, photoinitiators for visible light14,maleirnide~'~, nitro-aromatic compounds16, urethane resins1', drying and curing photocurable paint^'^, polymeric photobased systems2', problems in photocuring acrylics2', wood coatings22,dye based initiators and electron-transfer proc e s ~ e and s ~ ~the current state of the art and problem development^^^. Photochemistry, Volume 33 0The Royal Society of Chemistry, 2002 339
340
Photochemistry
2.1 Photoinitiated Addition Polymerization. - Many new photoinitiator systems continue to be developed for photopolymerization. A number of watersoluble methacrylate copolymers with pendant benzil groups have been synthesized and ~haracterised~~. These initiators were found to be much less sensitive to self-quenching reactions and exhibited greater efficiency than non-polymeric derivatives. The benzil ketyl radical was determined to be the active initiating entity in radical polymerization. Using ESR other photofragmenting initiators based on [Z]-sulfonyl-2-oximinoketonesgive a-ketoiminyl radicals associated with N-0 cleavage reactions26.These systems have the advantage of inducing the acid-catalysed polymerization of melamine resins. Although many studies tend to concentrate on the primary initiator other workers have developed novel ~ ~ . esters were co-synergistic N,N-dimethylaminobenzoates and b e n ~ a m i d e sThe found to be more effective cosynergists than the corresponding amides. The activity of the triplet-exciplex formed with the ketone initiator was found to be highly dependent on many factors including structure, electron donation, absorption and photolytic stability. A number of water-soluble anthraquinone copolymers have been synthesized based on sulfonic acid and trimethylammonium salts2' and found to exhibit activities equivalent to the model systems. Photoreduction quantum yields were found to correlate well with photopolymerization rates. Polystyrene macroinitiators with acylphosphine oxide end groups have also been found to be effective29as have acrylated photofragmenting initiators based on acet~phenone~'. In the case of poly(styrene peroxide) effective photopolymerization was observed for various vinyl monomers3'. In the case of acrylamide and acrylonitrile the styrene peroxide was found to be catalytic and not attach itself to the polymer chain. However, in the case of methyl methacrylate polystyrene blocks were found in the final polymer. Acrylic polymers with pyrimidinyl moieties have been found to be effective water-soluble initiator^^^. They will also enter the final polymer product in the chain giving products with enhanced molecular weights. For a range of thioxanthone initiators photoactivity has been found to be highly dependent upon the ketone structure with competition between monomer and amine cosynergist quenching playing a major role33.Methyl substitution has been found to influence significantly the photochemistry of 2-(2-hydroxy-3[bis(2-hydroxyethyl)amino]propoxy)thioxanth0n~~ while a range of novel water-soluble thioxanthones have been prepared and the activities found to correlate well with structure/photochemical a ~ t i v i t i e s ~Trithianes ~ - ~ ~ . have been reported to accelerate the benzophenone-initiated photopolymerization of vinyl However, the rates did not correlate with derived radical formation from laser flash photolysis. Quenching effects may account for this as well as the formation of derived radicals from the trithianes. Astramol polypropyleneimine dendrimers are highly effective co-synergists when compared with simple aliphatic tertiary a m i n e ~Such ~ ~ . molecules have low volatility and graft into the polymer network. From a series of novel benzoin ether initiators, photoinitiation activity has been related to their ability to generate free radicals via cleavage rather than being related to their absorption properties in the near UV-visible regions4*.In the case of p-nitroaniline, photoinitiation occurs through the forma-
I I I : Polymer Photochemistry
34 1
tion of nitro and amino radicals41formed by proton transfer from the amine to the nitro group. Photoinitiation was found to be more active in non-polar media. Similar reactions have been found where N-acetyl-4-nitro- 1-naphthylamine is a powerful sensitizer coupled with the use of N,N-dimethylaniline4*,Other organic non-ketonic type initiators include poly(dialky1 or alkylphenyl~ilanes)~~ which have been found to exhibit high quantum yields of conversion for vinyl and acrylated monomers. Sulfur-containing carboxylic acids synergize effectively in the photoinduced polymerization of acrylamide using 4-carboxybenzophenone as an initiator4. The yield of secondary processes is considered important here following the initial step of electron transfer. Decarboxylation is important especially in the case of aromatic carboxylic acids where this process is facile. Molecules that do not undergo facile decarboxylation were found to be poor initiators. Exciplex formation between 3-amino-9-ethylcarbazole and acrylonitrile gives rise to polymerization4’ while for 1-phenyl-3-sulfonyloxy-1,2-propanediones sulfonic acid is formed46.Various chromophoric groups attached to TEMPO have been found to operate as effective systems for forming living polymers47.Intramolecular quenching within the TEMPO structure was considered to be important in controlling the initiator triplet lifetime and efficiency. Monochloroacetic acid with dimethylaniline is also claimed to be an effective initiator complex48.Non-ideal kinetics on the polymerization rate indicated initiator termination steps and/or degradative initiator transfer with solvent playing a major role. Halomethyl aromatic compounds operate in a similar way in accelerating the efficiency of benzophenone or anthraquinone initiator^^^. In this case oxygen remains an effective inhibitor with photolysis of the initiator complex palying a role in controlling the escape of radicals from the ‘complexcage’. Dye complexes continue to show high efficiency in specialised applications. Cyanine-butyltriphenylborate salts have been found to undergo efficient electron transfer to give sec-butyl radicalsso.Dye bleaching during polymerization did not influence the overall rate of reaction. Coumarin dyes in conjunction with iodonium salts effectively initiate the polymerization of acrylated monomers51. This reaction is found to proceed via the singlet state of the coumarin complex due to its oxygen insensitivity. The only problem with this complex was the potential role of coumarin radicals in acting as chain terminators. Similar complexes have been found between Methylene Blue dye and diethanolamine with iodonium The iodonium salt operates as an effective oxidizing agent to convert the dye back to its original state allowing it to re-enter the primary process. Eventually, the iodonium salt will photolyse to give initiating phenyl radicals. Squarylium dyes with iodonium salts are likewise effective electron-transfer initiators giving rise to effective radical formation and dye bleaching54.Coumarin or ketocoumarin have been found to interact with other sensitizers such as bi~imidazole~~. Here the coumarin derivatives form radicals via electron transfer while ketocoumarin undergoes energy transfer. Such systems had applicability in laser imaging. The presence of 1-naphthol with iodonium salts also undergo electron transfeP. Other highly active compositions include benzothiazoles and aminostyryl dyes with 3,3’4,4’-tetrakis(tert-butyl-
342
Photochemistry
peroxycarb~nyl)benzophenone~~~~~, quinoxalines with aminesS9,dyes with hexaarylbiimidazole60and xanthene dyes with61,62 and without a m i n e ~ ~ ~ . Metal-based initiators continue to have specialized applications. Poly(ethy1 methacrylate) has been prepared using bis(cyclopentadieny1)titanium dichloride as initiator to give a polymer that has high acetone in~olubility~~. Pentacarbonylrhenium(1)halides have been found to effectively ring open cyclohexene oxide65. Removal of the carbonyl ligand deactivates the initiator. Tungsten hexacarbonyl has been found to polymerize alkynes and strained cyclic olefin!P as does dirhenium deca~arbonyl~~. Zinc chloride on the other hand polymerizes acrylonitrile to give an unusually insoluble polymer, which decomposes at only 160"C to give the cyclic tetrahydronaphthyridine ring chains68.Ruthenium bipyridyl will polymerize dimercapto- 1,3,4-thiadiazole on irradiatiod9. The polymer is also able to be removed upon electrochemical reduction. A radical mechanism operates in the polymerization of styrene with triphenylbismuthonium ylide7' while a series of trialkyl derivatives of Si, Ge and Sn have been found suitable for producing 'living polymer systems'71.Benzoyl-substituted ferrocenes have been shown to be effective anionic initiator^^^'^^. Irradiation gives rise to ring-metal cleavage to give a benzoyl-substituted cyclopentadienide carbanion species. This process occurs with apparently high efficiency for the dibenzoyl derivatives. Platinum(I1) diketonates have been found to ring-open 1,1,3,3-tetramethyl-1,3disilacyclobu tane74. Biorenewable monomers have been discussed which can easily be photopolymerized uia cationic initiator^^^. Epoxide-vinyl ether mixtures apparently undergo photoinduced cationic polymerizations without cop~lymerization~~. Although interactions take place this is highly dependent upon the nature of the epoxide monomer. In general, the vinyl ether polymerization is slower and usually completes only after the epoxide polymerization has ceased. A mechanism involving an equilibrium between alkoxy-carbenium and oxonium ions was proposed to account for the rates. The presence of N-vinylcarbazole proved to be a sensiti~er~~. A series of novel sulfonium salts have been prepared where the presence of an indanonyl group proved to be highly effective7s.Apparently, sulfonium salts possessing polycyclic aromatic structures were the least effective. A series of triphenylphosphonium salts on the other hand only operate as effective initiators for oxiranes when co-radical generating initiators are present79.It is claimed that the addition of free radicals to a double bond causes fragmentation of an adduct giving rise to reactive onium radical cations. The presence of polyols has been found to enhance the rate of photoinduced polymerization of cyclohexene oxide using sulfonium salts" while onium borates can be sensitized by anthracene'l. The rate of polymerization was found to increase with a corresponding decrease in the free energy change from the excited singlet state of anthracene to the onium cation of the onium borates. Diphenyliodonium salts are also effective with dimethylaminobenzophenones2. The enhancement of photoinduced cationic initiators with free radical types has also been demonThe free radicals are oxidized by the strated by the use of pyridinium type pyridinium ions while at the same time the free radicals induce decomposition of the pyridinium ions. Novel block copolymers can be made in this way. The
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343
cationic polymerization of 2,3-dihydrofuran by sulfonium salts is very rapid giving a good film forming Cobalt(II1) salts of azidopentaamine are effective in inducing the polymerization of 2-hydroxyethyl methacrylate". In water the reaction is autoaccelerating giving a gel at high monomer concentrations. A star shaped polymer of THF has been made by the photoinduced cationic polymerization of THF in the presence of pentaerythritol tetrakis(3,4epoxybutanoate)s6.Epichlorohydrin gave the same effect and appeared to operate by stabilization of the growing cationic chain through ion-pair formation. The molar ratio of the two systems controlled the arm length of the star. Diaryiodonium salts have been shown to be effective near-IR initiators87and triarylsulfonium salts induce the ring opening of 1,3-dioxepane8'. A number of novel polymer materials have been prepared. Bis(si1anes)have been found to photocopolymerize with methyl methacrylate (MMA)89.The presence of the silane functionality apparently accelerated the reaction. A diblock copolymer has been made from styrene and vinyl acetate, which was then hydrolysed to give an amphiphilic diblock copolymer9'. Nanostructured polymers have also been prepared through hydrogen bonding with nano-liquid crystalline materialsg1.Polyacrylamide nanocomposites have been made with various metal ions such as Cd, Zn and Pb92.Polyesters containing conjugated diacetylenes have been photopolymerized and monitored by DSC93while a photopolymerizable hydrogel based on an acrylated poly(viny1alcohol) has been developed for skin implants94.Long chain oligomeric amines have been found to play an important role in the photosensitized polymerization of oligocarbonate met ha cry late^^^ via complexation while an alternating copolymer of alkyl sorbates with peroxides has been prepared by photopolymerization of the monomer with oxygen96.The polymeric tributylstannyl ester of silicic acid has been found to be a useful intermediate in the preparation of a polysiloxane derivative, which possesses methacryloyloxypropyl groups97. When MMA is photopolymerized with 2,3-diphenylbutadiene in the presence of a template polymer, poly(N-isopropylacrylamide), using benzoin ether initiators, globular nanoparticles are obtained9*.The template polymer was not significantly adsorbed into the particles. A new method for the synthesis of C6o-polyfullerenes has been developed in aqueous media99 while other workers have photopolymerized aerosol particles of mixtures of benzyl chloride with acrolein'OO. Monomers of 1,1,2,2-tetrahydroperfluorodecyl acrylate have been photopolymerized in lowpressure microwave plasmas'o' whereas the presence of polyethylene oxide as a macromonomer has been found to decrease the emulsion polymerization of alkyl methacrylateslo2.The particle size distribution was found to increase with increasing size of the alkyl group on the monomer while polymer molecular weights were inversely proportional to the particle size. A few studies have been undertaken on the photopolymerization of maleate systems. The triplet excited state of maleimides is known to initiate the polymerization of acrylate monomers through their ability to abstract hydrogen atoms. By use of laser flash photolysis studies, the rate of quenching of triplet maleimides by vinyl ethers has been shown to be independent of the presence of labile Using hydrogens on the C-atom attached to the central 0 of the vinyl ether'03J04.
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Photochemistry
RTIR the monomer feed composition has been shown to play a decisive role on the polymerization kinetics of such mixture^"^. Thus, when the vinyl ether is in excess, the two monomers disappear at equivalent rates to yield an alternating copolymer. When the maleimide (MI) is in excess, the copolymerization and homopolymerization of MI occur simultaneously to give copolymers with isolated vinyl ether units. Apparently vinyl ether radicals act as the main propagating species. Computer simulation methods have been developed to monitor such polymerizations using Quantum Monte Carlo theory'06. Using fluorescence analysis electron-donor complexes have been identified between vinyl ether-maleimide r n i x t u r e ~ ~Here ~ ~ ~polymerization '~~. rates were also found to depend upon several factors such as light intensity, initiator concentration and oxygen while temperature did not play a significant role. Hybrid monomers of isopropenyl ether and epoxy-cycloalkane functionalities have been synthesized that are capable of cationic polymeri~ation'~'. A few studies have appeared on the use of photoiniferters. N,N-Diethyldithiocarbamate has been actively studied. The polydispersity of styrene and methyl methacrylate polymers was found to be very wide using this carbamate as iniferter indicating the formation of a living polymer"'^' ll. Polytetrahydrofurans on the other hand were found to be narrow in their molecular weight distribution' 12. Copolymers could be made with methyl methacrylate. A macro iniferter has been made by copolymerizing styrene with an acrylated 2-N,N-diethyldithiocarbamyl acetate Hypervalent iodine iniferters have been investigated based on 10-1-3-iodane~"~. They were found to regulate effectively the polymerization of styrene and acrylate monomers especially when used in conjunction with Cu(1) salts or complex forming agents such as dipyridyl. The interesting feature of the results in this work was the observation that the iodanes behaved as iniferters only under visible light irradiation. UV light initiation simply initiated radical polymerization. Propagation rate coeficients have been measured using pulsed laser polymeri z a t i o n ~ " ~ ~The " ~ .activation energy for the polymerization of 3-Ctris(trimethy1silyloxy)silyl]propy1 methacrylate has been found to be significantly less than that for lower alkyl methacrylates but similar to dodecyl metha~rylate"~. A similar study has been undertaken on N-vinylcarbazole' 16. Quenching rate constants have also been measured for a number of monomers on various tripletexcited ketones using laser flash phot~lysis"~. Using a semi-empirical calculation the reactivity of excited ketones could be predicted. From this work it was found that the bimolecular quenching depended on the free enthalpy of formation of the regioselective favoured 1,4-biradical between the ketone/monomer. In another study stilbene probes have been utilized to measure the polymerization stages for methyl methacrylate' 18. It was found that coreactive probes were more fundamental to the rate changes rather than free probes. Butyl acrylate polymerization has been assessed by the spinning disk method"' while the release of TEMPO from a benzoyloxystyrene initiator has been measured by laser flash spectroscopy'20. The gelation processes in the polymerization of acrylic acid have been measured by interferometryI2l. This was found to be a valuable non-destructive
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345
method. The UV-induced decomposition of azo groups attached to the surface of silica has been monitored in the photopolymerization of acrylate monomers.'22 The effect of monomer hydrophobicity has been measured on the rate of emulsion polyrnerizati~n'~~. For 2-ethylhexyl methacrylate, initiation was associated with radicals generated within the microaggregates. One third of the radicals were actually found not to recombine. c 6 0 fullerene photopolymer has been prepared that can be converted to a piezopolymer that is as hard as diamond'24. c 6 0 has also been used as a supported i ~ ~ i f e r t ewhile r ' ~ ~2,5-dimethoxyphenyl and quinone substituted octa-3,Sdiynes have been photopolymerized to give blue and then red products'26.The insertion of an ester or sulfonyl group into the hydrophobic part of a diyne molecule apparently increases its activity for topochemical p~lymerization'~~. Pulsed irradiation of diyne crystals led to the formation of dimer, trimer and tetramer radicals that have been implicated in the topochemical polymerization'28.
2.2 Photocrosslinking. - The role of various initiators for photocuring has been discussed in a number of articles. Camphorquinone/amine systems have been used to photocure mixtures of acrylic acid with triethyleneglycol dimetha ~ r y l a t e ' ~The ~ . formation of a product with two cure rates was observed. General mechanistic processes of this system have also been ~onsidered'~'. Benzoin ethers have been grafted to urethane resins via a reactive hydroxyl f~nctionalityl~l as have silicon containing oligomers onto 2-hydroxyethylacrylate'32.High temperature curing of resins has been accelerated by the use of UV photo inti at or^'^^ while quinolyl sulfides have been successfully photocured in the presence of vinyl ethers using visible light'34.This process was proposed as being valuable for the development of UV curable powder coatings. Poly(furfury1 alcohol) with varying amounts of oxymethylene bridges has been synthesized using trifluoroacetic acid and p-toluenesulfonic acid'35.These polymers had a tendency to retain acids and became insoluble upon storage. With maleic anhydride they were useful for the preparation of negative photoresists. The Michael addition reaction of a secondary amine with an acrylate resin is an established patented technology in the field. However, a recent publication has incorrectly claimed novelty in this regard'36.Microwave dielectrometry grafted benzoin methyl ethers have been found to be less effective in UV curing than the model non-grafted A new bis(methylethylamin0) derivative of benzophenone has been synthesized and claimed to be non-carcinogenic with equivalent photoactivity to that of Michler's ketone'38.Aminoketones have been found to be effective for photocuring epoxy-based resins'39in base-catalysed imaging systems. On irradiation the benzoyl moiety is cleaved and an active tertiary amine base is liberated. A novel photosensitive polyimide/silica hybrid has been prepared by a sol-gel route to give a product with high tensile and thermal stability'40. Also, a new method for sol-gel analysis data treatment has been verified using experimental data on the photocrosslinking of p~lyolefins'~'. The dynamics of photofabrication processes during surface relief gratings (SRG) have been monitored with azobenzene functionalized p01yrners'~~. The writing behaviour of photofabricated SRGs was found to depend upon the irradiated light
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Photochemistry
intensity and not the spacing of the interference light pattern. Failure modes have been assessed in adhesion joints using a combination of IR-ATR and fluorescence reflection technique^'^^ while gelation in free radical copolymerization has been measured by a transient fluorescence methodI4. Excimer fluorescence has been found valuable for on-line monitoring of the cure of silicone release 1ine1-s'~~ whereas two photon visible chromophores generate light capable of activating photoinitiators for laser induced polymerization^'^^. Charge recombination luminescence (CRL) from epoxy resins produced during cure has been correlated with the extent of reaction of epoxy It was found that for CRL to be observed the resin must contain enough OH groups to stabilize the electron traps by solvation and be sufficiently vitreous to prevent immediate recombination of electron611 while in other work relaxation effects depend highly on temperature effects612.Intra- and intermolecular hydrogen bonding effects are also important between chain side-chains613while thermal decay reactions depend upon the alkyl chain lengths in the cop01ymer~'~. In ester methacrylates fluctuations in local free volume observed in cis-trans isomerism of azobenzene chromophores have been related to local structural relaxations615. Macroconformations in polystyrene have been controlled through the use of azobenzene Side-chainP while a series of azobenzene modified poly(amides)fitted with spirobi-indane and chiral binaphthyl chromophores have been found suitable for high performance applications617.trans-cis Isomerization was induced through UV irradiation and reversed using thermal or visible light. Irradiation of the polymer samples to drive the trans-cis isomerization reaction resulted in an immediate chiroptical response, with CD band intensities and optical rotations significantly reduced. These effects were fully reversible and were attributed to the presence of putative one-handed helical conformations in the trans-azobenzene-modified polymers that were severely disrupted following trans-cis isomerization. Poly(azoaromatic viologens) have been prepared618that undergo a reversible photoinduced supramolecular assembly and also form complexes with the dye Eosin that have enhanced conductivity on irradiation. Such complexes are considered to be photoswitchable. Photodynamic properties of azo-modified polymers are found to depend upon the degree of functionali~ation~~~ while photochromism in tungstophosphate acid acrylamide polymer is related to the presence and diffusion of oxygen620.Transi-
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111: Polymer Photochemistry
ent diradical species are observed in polyene sequences621while photochromic dyes have been developed that can photoswitch to bind and release metal ions622. Here spiropyrans can be used to control environmental pollution. A novel mechanism has been proposed for the observed fast relaxation of photoinduced anisotropy in a poly(ma1onic ester) with p-cyanoazobenzene These workers observed an increase in the induced birefringence in the decay process just after the pumping beam polarized perpendicularly to the recording beam was illuminated in the relaxation process. They considered a new model based on an elastic force between the side-chain and the backbone of the polymeric film. Here the backbone could be reorientated together with the side-chain with fast relaxation being associated with the elastic force. Photochromic dihetarylethenes have been made and shown to undergo photocyclization which decreases in the order of substituents COOH > COOMe > CONHAf24.Photochromic organic-inorganic hybrids have been made through a sol-gel reaction of tetramethoxysilane in the presence of spiropyran modified poly(N,Ndimethyla~rylamide)~~~. Exposure to UV light gave a new absorption at 557 nm that was little affected by the presence of the silica. The photochemical and photodynamic behaviour of E/Z isomerization of an azobenzene side-chain polymer have been related through UV and dielectric spectroscopy626.Poly(3,4ethylenedioxypyrrole) undergoes electrochromic switching from red to blue forms627while a new photochromic spirothiopyranobenzenopyrylium dye undergoes very rapid reversible ring opening on irradiation-dark reaction628.An improvement has been made on the theory of all-optical poling629whereas monolayers of PVA with azobenzene spacers undergo reversible expansion and contraction processes on irradiatiod3'. An increase in the length of the spacers gave a non-linear response in the reversible process whereas short chain spacers gave a linear response. The non-linear behaviour is associated with co-operativity stemming from the self-assembling nature of the trans-azobenzene sidechains. Photochromic poly(tetramethy1ene oxide)/tungsten trioxide hybrid materials have been developed that undergo yellow-blue reversible changes63'. Long chain organomercury(I1) dithionate complexes undergo similar changes632. Ion-selective amphiphilic crown ether dyes in monolayer form collapse in the presence of Na(I), Ca(I1) and Mg(I1) ions but this is reversed in the presence of K(1) ions633.Here dye-cation complexes are formed in the monolayers, which influences its expansion. With K ions photodimerization takes place. Local free volume has been investigated for poly(methylsi1sesquioxane) probed with azobenzene c h r o m ~ p h o r e swhere ~ ~ ~ the final cis fraction in uncured PMSQ decreased markedly below 250 K, which is claimed to be unusual for linear polysiloxanes. Polyurethane cationomers undergo reduced isomerism with increasing azo group while multilayers have been developed of azopolyelectrolytes636~637 that also exhibit a photochromic dependence on the azo group concentration. Photochromic liquid crystalline copolymers containing a photochromic liquid crystalline monomeric unit showed only a smectic phase while those with non-LC monomeric units show a chiral nematic phase638. while poly(4Photochromic inks have been made based on bacteriorh~dopsin~~~ polyphenylazophenol) has been synthesized using an enzymatic method640.This +
364
Photochemistry
polymer has a long relaxation time and behaves as a glassy macromolecular dye. An LC polymer has been made based on 4,4‘-dialkoxystilbene tagged metha ~ r y l a t e as ~ ~has ’ a 6-[4-(4-ethylphenyl)diazenylphenyloxy]hexyl methacrylate copolymer642.Azobenzene multilayer assemblies of poly(viny1 sulfate) undergo rapid isomerism without fatigue643as do cationic and anionic bolaamphiphiles based on azobenzene derivatives644.In the latter case photoswitching can be controlled by the nature of the counterion. Other photochromic systems of interest include polymers with 2-phenyl- 1,3-indandione moieties64s,imidazolone a z o p o l y ~ y a n u r a t e s ~2-~[~[4-( , 4-cyanophenylcarbamoyl)phenyl]ethyl] eth yl m e t h a ~ r y l a t e ~ ~ ~dimethyl-6-aryl-2,2-dimethyl-2H-chromene-7,8-dicarboxy, late^^^*, azo-tagged p ~ l y a c e t y l e n e s composites650, ~~~, ~ p i r o p y r a n s ~azo-tagged ~’, cyclodextrins6s2,benz~furans~’~, azo-tagged p~lystyrene~’~, dithien~lethene~’~, poly(dihe~ylsi1ane)~~~ and articles on general topic^^'^-^^^. Over twenty articles have been published relevant to LC polymer materials. Photo-orientation effects of poly(methacry1ates) with azobenzene side-chains have been investigated660.The polymer exhibits excellent thermal stability and high optical anisotropy with a well-ordered domain. In-plane orientation was generated in the glassy state as well as above the Tgand irradiation at 90 “Cgave rise to a distinct transformation from in-plane orientation at the early stage to successive out-of-plane orientation, which was also accompanied by H-aggregation. Such systems are claimed to be valuable for recording optical images on the basis of the differential in birefringences between the two orientational modes. Two fluorescence bands are observed from LC polyesters with naphthoate mesogen units at 410 and 430 nm661.The former band is due to partially overlapping naphthoate units while the latter is due to ground-state fully overlapping complexes. The latter assignment is confirmed by the observation that increasing viscosity increases the 430 nm peak intensity. Non-LC and LC based polymer blends have been mixed to give systems in which the former reduces the emission at 420 nm but generates a new band at 480 nm662.Prior thermal treatment of the blends and casting onto rubbed polyimide film increased the new emission by over an order of magnitude. O n the other hand blends composed of cholesteric copolymer and a chiral monomer give rise to a thermodynamically incompatible matrix giving rise to a separate amorphous phase663. This causes a decrease in the concentration of the chiral component in cholesteric phase and, as a result, leads to untwisting of the helix bringing about a shift in the maximum of the selective light reflection to longer wavelengths. Such materials may also be used for optical data storage. The thermal quenching and optical bleaching of the luminescence from polyacrylates with cyanoterphenyl side groups is dependent upon the composition of the matrix664.However, there is no dependence upon the degree of crosslinking of the polymer. The chain packing in polyesters with biphenyl side groups influences their luminescence properties66s.Here the rotational diffusion coefficient was found to increase gradually with temperature showing a sudden jump at the liquid crystal-isotropic transition temperature. In another area a modified stereolithographic process was developed using a magnetic field to align the liquid crystal monomers in each layer of a multiplayer part666.Thus, multilayered photopolymerized
I I I: Polymer Photochemistry
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parts with various layer alignments were obtained. In this way it was possible to achieve any desired in-plane coefficient of thermal expansion values between 0 and 90 "C for unidirectional alignments. A new chiral menthone-based acrylic polymer has been made that is capable of cis-trans isomerization coupled with a nematogenic monomer667.On irradiation the step of the helix is changed with a planar orientation. A new crosslinked LC network has also been made containing a fluorescent probe668,From birefringence measurements no appreciable loss in alignment was found up to 200 "C. However, it was found that the fluorescent chromophore was less well ordered than the liquid crystal. The thermoreversible gelation of rod-like poly(y-stearyl-a-1-glutamate) has been investigated and it was found that its long-range features such as a cholesteric twist remain frozen by g e l a t i ~ n The ~ ~ ~nano. and meso-scale morphologies of polymer films of varying LC concentration have been examined670.These polymers, photocrosslinked and based primarily on a penta-acrylate matrix with an LC dopant, reveal increasing nanocale heterogeneity with increasing LC content. In this type of matrix an increase in LC content causes a coalescent regime of aggregated beads of LC polymer the size and uniformity of which increase with increasing LC concentration. Phase separation has also been investigated theoretically using a time-coupled dependent Ginzburg-Landau equatiod7'. Here the simulations on the spatio-temporal evolution of the coupled LC concentration and orientation order parameter initially lag behind those of the concentration order parameter. The polymer-induced phase separation is characterized only by the late stage of phase separation. Also, the growth behaviour and simulated morphology consisting of LC droplets dispersed in a matrix of polymer appears the same for all compositions, the only exception being that the size increases with increasing LC concentration. Of particular interest with this model is that the simulation captures the observed domain topologies. LC systems containing azobenzene side-groups become translucent on irradiation due to the isomerism672. The light scattering is associated with a biphasic morphology produced transiently due to a partial photochemical transition. The topochemical polymerization of 1,3diene muconic and sorbic acids with naphthylammonium salt counterions forms st ereoregular meso- or erythro-diisot actic-trans-2,5-polymers irrespective of their starting c o n f i g ~ r a t i o nMicroscopic ~~~. mechanisms of LC photoalignment have been considered for poly(methylphenylsilanes)674and polarization in methacrylate polymers with azobenzene s i d e - g r o ~ p s ~ Novel ~ ~ . photocrosslinkable polymers with biphenyl or naphthalene groups have been prepared with anchoring and tilt angles676whereas new photochromic spiropyrane acrylic monomers with varying methylene spacers give ternary cholesteric copolymers when reacted with hematogenic and chiral monomers677.The latter exhibit selective light reflection in the visible region of the spectrum while under UV light irradiation they form a merocyanine form of dye, the absorption maximum of which corresponds to the maximum of light reflection. Other studies of interest include discotic materials coated onto p ~ l y i m i d e s bismaleimides ~~~, with divinyl comp o u n d ~ poly(3,7-di-tert-b~tylnaphthyleneethynylenef~~, ~~~, chiral diacrylates681, PMMA with azo-side discotic epoxy systems682,polymers with chiral and polythiophenes with ionic viologen mesogenic ~ i d e - c h a i n sIn ~~~.
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one study using Langmuir-Blodgett films it was shown that intermolecular interactions decrease the efficiency of photochromic transformations686. Several articles have appeared dealing specifically with excimer formation in polymers. Novel photoactive heterocyclic polyimides containing naphthalene groups have been synthesized and found to give rise to excimer emission687,as do copolymers of methacrylates with carbazole and naphthalene groups688.Electronic energy migration and excimer dissociation control the fluorescence kinetics in copolymers of 1-vinylnaphthalene and MMA689.A similar study has also shown that excimer formation between naphthalene goups in related copolymers cannot be inhibited690.The emission spectra of diblock and triblock spherical vesicles of polyquinoline-polystyrene have features that are characteristic of their supramolecular m ~ r p h o l o g y ~Strong ~ ' . excimer emission was observed due to J-like aggregation with the potential of developing photonic materials. Sulfonated polystyrene gives692excimer emission in concentrated solutions of above 2.0 g L-' while macrocyclic polystyrene shows significant enhancement of excimer emission with decreasing molecular Copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonateand N-oleylmethacrylamide have been tagged with naphthalene groups in order to monitor the formation of hydrophobic domains and their self-association b e h a v i o ~ rcis-Ethene ~~~. bonds in the oleyl group were found to form clusters in the hydrophobic domains coupled with a strong tendency to undergo intramolecular association. Orientation of PMMA chains tagged with naphthalene groups results in an enhancement of excimer formation due to chain alignment and close intermolecular packing695and indene-PMMA also gives excimer Naphthalene groups undergo energy tranfer in polyrotaxane molecules697while in poly(dioctylfluorene), excimers are formed with only a small change in intermolecular separation698.In this polymer the exciton diffusion constant was found to be two orders of magnitude larger than the excimer diffusion constant thus accounting for strong quenching effect of excitonic luminescence by quenching sites. Here the excitonic emission exhibits a significant polarization anisotropy, which is consistent with the migration of excitons between regions of different orientation of the polymer chains. This is in contrast to excimer formation between such domains, which will be inhibited by the fact that excimers experience the domain borders as potential barriers. Excimer formation in poly(ethy1enenaphthalate) is also enhanced by elongation due to chain chromophore alignment699while front face geometry is claimed to be better for observing excimer emission than a right-angled arrangement as it corrects for r e - a b s o r p t i ~ n Excimer ~~. formation of methylphenylsiloxanes has been associated with diad conformations in the chains where distances between the C atoms were 4.2 A with an angle of rotation of 1lo" in DS to angles between phenyl rings of 15" coupled with a staggering of the aromatic groups and angles between the Si-C bonds between neighbouring rings of below 45" 701.It is claimed that the conformation of excimer forming sites in these polymers is significantly different from those in normal hydrocarbon analogues with large movements being necessary to form the excimer sites. Energy migration continues to be investigated in a variety of polymer systems. Vinyloxy monomers with l,%naphthalimide groups have been synthesized and
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fluoresce with intensities which are dependent upon the spacer lengths between the naphthalimide groups7o2.Self-quenching occurs by intramolecular chargetransfer interactions between the electron-donating vinyloxy and electron accepting naphthalimide groups. Molecular dynamics of polystyrene with anthryl end groups have been studied by fluorescence d e p o l a r i ~ a t i o nwhile ~ ~ ~ the luminescence from cation-doped poly(fluorene) is observed to depend upon the cation layer thicknesses704.Highly fluorescent poly(cyc1odiboranes) have been synthesized705and photoinduced electron transfer observed between pyrene and a xanthene dye706.Long-range energy transfer has been observed between layers of PVK on poly(9,9'-di-n-hexyl-2,7-fluorenylvinylene)707 while poly(fluorene) copolymers exhibit exciton migration and trapping to the copolymer cyanostilbene The copolymer exhibits red-shifted emission and enhanced colour stability. Stern-Volmer quenching analysis has been undertaken on polysilanes using different halogenated solvents709and steady-state triplet exciton densities have been measured in poly(2,5-diheptyl-l,4-phenylene-alt-2,5-thienylene~1o. Polymers of 3-methyl-acrylamide-9-carbazole have been found to exhibit higher fluorescence intensities than the corresponding monomer analogues7" and electron-hole generation has been examined for poly(N-epoxypropylcarabzole)712. In carbazole-methacrylate copolymers there is little interaction between the ethene bonds of the methacrylate groups and the carbazole c h r o m ~ p h o r e s ~ ' ~ . Self-quenching was found to be highly dependent upon the polarity of the solvent, high polarity inducing intramolecular association and hence quenching. Meso-linked zinc porphyrins doped in PMMA exhibit photoinduced charge transfer across the arrays714while other studies of interest have concentrated on dichromophoric copolymers with l,%naphthalimide groups715,polycoxy- 1,4phenylenecarbonyl- 1,4-phenyleneoxy-1,4-phenylene[(2-carboxyphenyl)methylidynel-1,4-phenylene) doped with TCNQ716and aggregation effects in water soluble polymers717. Polymers in micellar media continue to attract significant interest. Pyrene solubilised in poly(N-isopropylacrylamide) shows unusual fluorescence anisotropy behaviour7". Rather than decay to zero, as might be expected for a freely rotating species in solution, the emission attains a minimum finite value. After 100s of nanoseconds the anisotropy increases and becomes more polarized with time. This behaviour reflects the heterogeneous nature of the medium in which the probe is dispersed; that is to say whether it is free or occluded in the polymer host. The latter will evidently give rise to the unusually longer-lived growth in anisotropy. The Gemini surfactant 1,4-bis-(2'-(N-dodecylpyridinio-4"-yl)etheny1)benzene dibromide exhibits a large fluorescence enhancement and shift in maxima in alcoholic solvents719.This polymer effectively solubilizes pyrene and totally quenches its emission. Pyrene has also been used as a probe to study Here the effect of pH on viscosity of acrylamide copolymers in viscosity increases with pH due to the polymer changing from a compact (less soluble state) to an expanded state (more soluble). The critical solution temperature of poly(N,N-dimethylmethacrylamidophenylammoniumpropane sultone)has been found to be less than 0 0C721. Pyrenylacrylic acid has been used as a probe and found to give varying emission spectra depending upon the solvent
368
Photochemistry
polarity722.The probe also shows two stages of protonation (i.e. of the negative carboxylic group and carbonyl oxygen) and this is claimed to be useful for identifying vicinal versus geminal OH groups on silica particles. Polyurethane ionomers tagged with fluorescent dyes form aggregates and give rise to UV The intermolecular interactions between the hydrophilic groups of the fluorescent dye become stronger with increasing concentration causing an increase in average particle size. For polystyrene-graft-polyoxyethylenes the higher the temperature and shorter the side-chain lengths the lower is the CMC value724.Double diene lipids with spacer lengths greater than six show enhanced stability toward surfactants while solubility in organic solvents is Thus, heterobifunctional amphiphiles with long spacer lengths tend to favour crosslinking. Amphiphilic block copolymers of poly(2-methyl-2-oxazoline) show good solubility in aqueous media with an aggregation dependent on molecular while cationic fluorophores on methacrylate copolymers are quenched by halide ions727.For a series of acrylic terpolymers increasing the hydrophobic chain lengths increases their viscosity in solution728.Low levels of baterial growth have been detected in polymeric detergents729while the fluorescence quenching behaviour of an anionic conjugated polymer towards small amounts of quenchers can be modulated by complexation with a countercharged detergent730. For example, upon adding dodecyltrimethylammonium bromide to a conjugated polymer, cationic quenchers such as methylviologen become less effective while the quenching by neutral agents, most notably nitoraromatics, is enhanced. Such polymer-detergent complexes provide a new method for sensing chemical agents. Crosslinked gelatin gives strong inherent blue fluorescence due to dimeric t y r ~ s i n e ~and ~ l this is enhanced in non-solvents due to increased intramolecular associations. Complexa tion bet ween pol y(acrylamide) and poly(methacry1ic acid) is influenced by pH732and the emission from poly(2,5methoxypropyloxysulfonate) is enhanced by a cationic ~ u r f a c t a n tOther ~ ~ ~ . studies of interest include those investigating phosphor containing acrylic quinoline-styrene-quinoline triblock pyrene in mixed surfact a n t ~ aminocoumarin ~~~, dyes in restricted media737,diazo resins with sodium dodecyl ~ulfonate~~', pyrene with amphiphilic polymer aggregates739,bis(2-ethylhexy1)sodium sulfosuccinate) with polymer s ~ r f a c t a n t sketocyanine ~~~, dyes in micellar media741,hydrophobically modified p o l y a ~ i d s perylene ~~~, and effects of urea and thiourea on Safraninine T dye emission in r n i ~ e l l e s ~ ~ ~ . Fluorescence has been used to measure the degree of bonding in adhesive joints for polyester/polyethylene material^^^^,^^^ while fluorescence from recycled paper has been removed using chlorine Thermally stimulated luminescence from a poly(ester urethane) has been found to vary with film thickness748which is assumed to be due to the diffusion effect of oxygen. Poly(4vinylpyridine) exhibits variable emissions depending upon the degree and wavelength of irradiation749.This effect is due to a photoinduced directional ordering of the polymer chains in a special quasi-crystal formation and originates from protonation of the side-chain pyridine groups after solvation. This sol-gel transformation process is reversible. PPMA beads with coumarin dyes have been synthesized as photonic Poly(cyc1ophane) and its poly([2]-
I I I : Polymer Photochemistry
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catenane) have been made and the latter is found to require a higher oxidation potential to reach its conductive state than that of the former75'.The emission from cellulose was found to depend upon heating and cooling At temperatures above 145 "C degradation sets in and yields different products. Polyerythrosin has been made electrochemically753 and the thermoluminescence of polystyrene sulfonic acid examined754.Polycarbonate luminescence has been observed following irradiation with uranium ions755and a theoretical investigation has been undertaken on the redox behaviour of p-n diblock conjugated polymers756.The fluorescence form LDPE has been found to depend upon radiation dosage757while the permeability of oxygen in alkylaminothionylphosphazine films has been monitored by phosphorescence analysis75*.Depolarization of the fluorescence of labelled macromolecules has been determined by second rank time orientational correlation functions (OCFs) of the absorption and emission components759.The OCFs under certain conditions exhibit an oscillatory behaviour, which is irreproducible within the studied diffusion equation. Other studies of interest include those of aromatic failure of adhesive joints761,relaxation in density gratings762,spectroscopic behaviour of novel p0ly(pyridine-2,5-diyl)~~~, fluorescence of Schiff-base polymers764,aromatic and aliphatic c ~ p o l y a m i d e s ~ p ~ ~l y, s i l y l e n e sand ~ ~ metal ~ ~ ~ ~containing ~ polymer~~~*.
4
Photodegradation and PhotooxidationProcesses in Polymers
As discussed in the last report interest in polymer photodegradation/oxidation has remained at a steady but low level of activity in the published journals compared with what was at one time a highly prolific field of interest. Reviews of this topic include a comparative assessment of weathering devices769, degradable polymer materials770,photooxidation of bio- and photodegradability772and dehydr~halogenation~~~.
4.1 Polyolefins. - This class of polymers tends to be one of the most widely studied. Photooxidation of polypropylene has been described in terms of three kinetic This involves a typical induction period and autoacceleration build-up of hydroperoxides, an intermediate slower hydroperoxide growth and finally a very slow hydroperoxide growth. The early stages also appear to oscillate strongly, possibly due to heterogeneous oxidation sites. Polyethylene has been studied in great detail. Ethylene carbon monoxide copolymers disintegrate rapidly on irradiation and give substantial amounts of ether Norrish Type I is the primary reaction with the formation of acyl and alkyl terminal together with water and carbon dioxide as the major volatile products. Irradiation of isotactic polypropylene in a mixture of 3202 and 3602 generates 3 4 0 2 due to a pseudo-termination reaction through the recombination of peroxy radicals to give t e t r o x i d e ~ ~The ~ ~ .latter then decompose to give molecular oxygen and alkoxy radicals. Several agents can speed up the photooxidation of polyethylene such as grafted methylenebutandioic iron diethyl
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d i t h i o ~ a r b a m a t eand ~~~ ferric , ~ ~ salts7g1. ~ In the latter case the presence of starch acts as a synergist for photosensitization. Polyethylene and polyethylene waxes are biodegradable when the molecular weight is less than 5000782and the activation energy of thermal oxidation of polyethylene is sensitive to all processes resulting from the exposure to UV light7g3.Surface cracking in polyethylene has been modelled by per~olation~'~. Here the geometric characteristics of the generated Voroni decompositions and simulated clusters do not affect the dependence of the characteristic size of clusters on their capacity, At the same time the rate of surface cracking is characterized by the presence of a strong size effect. Surprisingly, the use of compatibilizers such as anthraquinone enhances the mechanical performance of polyolefin blends after irradiation7g5.Differences in natural and artifical ageing of LDPE have been measured and shed some doubt on the issue of hydroperoxides as key intermediates in the photooxidation For recyclability in pipe applications it has been formulated that only up to 50% by weight of recycled material should be used with fresh virgin polymer7g7and metallocene polyethylene has been In the latter case for two grades of polymer metallocene material has been found to be more photostable that conventional Ziegler-Natta polymer. 4.2 Polystyrenes. - Poly(styrene peroxides) undergo a chain unzipping mechanism with stability being dependent upon the bialkoxy radicals7g9.Polystyrene copolymers with benzil and benzoyl peroxide undergo the same rates of degradat i ~ n ~UV ~ ' .exposure dose enhances the surface wettability of p ~ l y s t y r e n e ~This ~'. is due to the formation of low molecular mass products at the surface of the polymer material increasing hydrophilization. Poly(methy1 vinyl ether) and polystyrene blends exhibit strong interactions on p h o t o ~ x i d a t i o nwhereas ~~~ polystyrene irradiated with fluorescent tubes undergoes random scission processes with crosslinking playing a more important role on the surface rather than the The latter is associated with the usual oxygen starvation effect in the bulk of the material. The presence of sensitizers such as benzophenone hampers the crosslinking reaction in polystyrene794.Diketonic groups have been co-reacted into polystyrene to enhance reactivity795while post-reactions have been observed in the photooxidation of butadiene-styrene copolymers doped with diphenylethanedi~ne~~~. For SIS triblock copolymers the isoprene units only undergo oxidation resulting eventually in phase d e m i ~ i n gMultivariate ~~~. analysis procedures have been used to measure the photooxidation of ABS798.
4.3 Poly(acry1ates)and Poly(alky1acrylates).- Of a range of poly(alky1methacrylates), PMMA has been found to be the slowest for photooxidation but fastest for p h ~ t o d e g r a d a t i o n ~Flexibility ~~. and mobility of the free radicals account for this differential in reactivity. For another range of acrylate and methacrylate polymers the former were found to be more reactivegm.Thus, with shorter alkyl side groups, chain scissions prevailed over crosslinking reactions in both acrylate and methacrylate samples. Only the butyl methacrylate undergoes rapid crosslinking and fragmentation. With 248 nm radiation, side-chain scission predominates coupled with some main chain scissongol.There is a strong
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stereochemical influence on the methacrylate spectra, which manifests itself through changes in time-resolved EPR analysis as a function of tacticity and temperature. Similar studies have been undertaken on a range of acrylate coatings and the side-chain length again appears to be important in controlling the reaction rates802.Polymers with long ester groups undergo fast and extensive crosslinking reactions while methyl and ethyl groups are relatively stable.
4.4 Polyesters. - Polyethylene terephthalate and its copolymer with 1,4-cyclohexanedimethanol have been photolysed under different condition^"^. Under vacuum pure photoytic processes occur involving Norrish Type I and I1 reactions with hydroxylated and carbonyl products being enhanced in the case of the copolymer. This effect is associated with the labile hydrogen atom on the tertiary carbon atom of the cyclohexane units. Under oxidation conditions the aliphatic portions of the molecular chains also undergo attack forming hydroperoxides that eventually result in formic and acetic acid production. Hydroxylation of the aromatic rings also occurs as well as the formation of terephthalic acid. In the photooxidation of poly(buty1enes terephthalate) anhydride formation has been confirmed through reaction with tetramethylammonium hydroxide to give methyl-4-methoxybutyrate8~. 4.5 Polyamides and Polyimides. - Very little published work has appeared on these materials. Nylon 6,6 has been shown to exhibit an increase in crystallinity upon irradiation although this fact is already ~ell-established'~~. What is new is the observation that there is no change in X-ray diffraction, indicating a new type of crystalline morphology. Crack formation was observed at the centre of spherulitic structures, which increased with irradiation time. 4.6 Poly(alky1 and aromatic ethers). - Poly(oxyhexylenoxy-4,4'-benzilylene) undergoes rapid degradation on irradiation with crosslinking and fragmentation depending upon the source intensity806.Crosslinked epoxy resins undergo rapid surface photooxidation developing a growth in hydroxy absorptionso7while laminates have also been found to exhibit growths in ester and carboxylic acid'O'. Photoageing has also been found to influence the mechanical properties of epoxy systemsgo9. Multifunctional polymers with phenacyl ester and vinyl ether groups undergo direct cleavage of the ester groups upon irradiation to give pendant carboxylic acid groups'". The latter, in turn, react with the vinyl ether groups to give acetal linkages. Degradation products of poly(ethy1ene oxide) have been analysed by pyrolysislGC'". Poly(ether ether ketone) has been investigated and found to undergo pinacolization, photo-Claisen and direct chain scission processes8I2.Intra- and intermolecular phenylation reactions also occur coupled with discoloration. 4.7 Silicone Polymers. - Irradiation of c60 and a polysilane in solution afforded an adduct of the two molecules with unique electronic propertie~''~.A variety of linear and crosslinked polysilanes have been converted into silicon
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oxides by ozone/UV treatment proces~es~'~. Under this type of treatment all of the organics are removed as volatiles. 4.8 Polyurethanes and Rubbers. - The physical properties of EPDM rubber seals for automobiles have been examined under weathering conditionss'' while impurities in raw natural rubber can sensitize its p h o t o o x i d a t i ~ nThe ~ ~ ~erosion . resistance of protective elastomeric coatings during UV exposure has been examined8I7as has the degradation of polyurethanes based on 4,4'-dibenzyl diisocyanate818 and 4,4'-methylene bis(4-phenyli~ocyanate)~l~. The extent of photolysis is greater for soft segments resulting eventually in phase separation from the hard-aromatic segments.
4.9 Poly(viny1halides). - PVC has been examined by depth profiling on irradiations2' and conjugated double bonds generated by UV irradiations21.Dehydrochlorination is associated with C-Cl bond cleavage and using a KrF laser conjugation sequences of up to 30 C atoms can be made822.Iron pigments have been used in PVC laminates and, as might be expected, this affects the degradation rate823. 4.10 Photoablation of Polymers. - Excimer lasers (UV) have been used to induce the reaction between phenylsilane and methylphenyl~ilane~~~ while the same lasers used for ablating polypropylene show that photochemical and thermal effects are co-operatives2'. Water repellant fabrics have been made through plasma irradiation of vinylidene fluoride and 1,1,1,2-tetrafluorethylene826while self-assembled organo-silane layers undergo direct C-C and C-Si bond scissions from 172 nm radiation827.Using a nitrogen laser polyimide films have been shown to generate surface carbonyl groups82s.PTFE has been ablated with pulsed nano- and femto-second laserss29and experimental uncertainties in the processes have been discussed. The products of degradation under 157 and 248 nm irradiation from typical lithographic materials have been examineds3'. Hydroxystyrene polymers were found to undergo crosslinking while acrylates and methacrylates undergo chain scission. The latter show film loss on irradiation while hydroxystyrenes do not. Consideration of these processes was felt to be important during laser curing reactions at 157 nm. Halonaphthalene dopants in PMMA undergo significant degradation on 248 nm excimer laser ablations3* while for a triazeno polyether only neutral products were identified using the same irradiation at fluences below 1.3 J cm-2 832,833. The irreversible increase in hole width and decrease in hole area for polysiloxane matrixes has been investigated and found to be due to local relaxation of the siloxane chains834.Surface interactions of radical species have been examined during plasma irradiation of polymer surfaces by fluorocarbon plasmas835.Three types of surface interactions were seen, namely generation of CF2 (S> l), surface loss of CF2(S
