Photochemistry, Volume 37 (Specialist Periodical Reports)

Photochemistry Volume 37

A Specialist Periodical Report

Photochemistry Volume 37 A Review of the Literature Published between July 2004 and June 2007 Editor Angelo Albini, University of Pavia, Pavia, Italy Authors Paolo Coppo, Brunel University, Uxbridge, UK Rui Fausto, University of Coimbra, Coimbra, Portugal Elena Galoppini, Rutgers University, Newark, USA Andrea Maldotti, Universita` degli Studi di Ferrara, Ferrara, Italy Miguel A. Miranda, Universidad Politecnica de Valencia, Valencia, Spain Kazuhiko Mizuno, Osaka Prefecture University, Osaka, Japan J. Se´rgio Seixas de Melo, University of Coimbra, Coimbra, Portugal Nick Serpone, University of Pavia, Pavia, Italy Takashi Tsuno, Nihon University, Narashino, Japan

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ISBN: 978-0-85404-455-9 ISSN: 0556-3860 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2009 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org Typeset by Macmillan India Ltd, Bangalore, India Printed and bound by Henry Ling Ltd, Dorchester, UK

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

Introduction and review of the period July 2004–June 2007

11

Angelo Albini Introduction Review of the period July 2004–June 2007

11 15

Photophysical processes in polymers and oligomers Telma Costa, Joa˜o Pina and J. Se´rgio Seixas de Melo Processes occurring upon electronic excitation: general considerations Hydrophobically modified polymers The nature of excited states in conjugated polymers Conclusions

44

Light induced reactions in cryogenic matrices

72

Rui Fausto and Andrea Go´mez-Zavaglia Introduction UV/Visible induced reactions in cryomatrices IR induced reactions in cryomatrices

72 74 99

44 45 58 66

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Alkenes, alkynes, dienes, polyenes

110

Takashi Tsuno Introduction Photochemistry of alkenes Photochemistry of dienes Photochemistry of polyenes Photochemistry of alkynes Oxidation of alkenes, dienes, and polyenes Photochemistry of haloalkenes

110 110 126 131 133 136 140

Oxygen-containing functions

149

M. Consuelo Jime´nez and Miguel A. Miranda Norrish Type I reactions Hydrogen abstraction Paterno´-Bu¨chi photocycloadditions Photoreactions of enones and quinones Photodecarbonylation Photodecarboxylation Photo-Fries and photo-Claisen rearrangements Photocleavage of cyclic ethers

149 151 156 157 162 162 164 166

Photochemistry of aromatic compounds Kazuhiko Mizuno Introduction Isomerization reactions Addition reactions Substitution reactions Intramolecular cyclization reactions Dimerization reactions Lateral-nuclear rearrangements Heterocycles

175

Functions containing a heteroatom different from oxygen Angelo Albini and Elisa Fasani Nitrogen-containing functions Functions containing different heteroatoms

213

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175 175 181 184 187 195 199 203

213 227

Photochemistry and photophysics of transition-metal complexes

240

Andrea Maldotti Introduction Chromium, molybdenum, tungsten Manganese, rhenium Iron Ruthenium Osmium Cobalt and rhodium Iridium Nickel, palladium, platinum Copper Silver, gold Zinc, cadmium, mercury Lanthanides Miscellaneous transition metal compounds Metalloporphyrin and analogous complexes

240 241 242 245 247 255 256 257 259 263 265 266 267 271 272

Photocatalysis and solar energy conversion (chemical aspects) Nick Serpone, Alexei V. Emeline and Satoshi Horikoshi Metal-oxide photocatalysis Solar energy conversion (water splitting) Solar energy conversion (solar cells) Addendum

300

Multi-component arrays for interfacial electronic processes on the surface of nanostructured metal oxide semiconductors

362

Andrew Kopecky and Elena Galoppini Background Design of supramolecular dyes for stepwise electron transfer on the surface of nanostructured MOn Dyads containing transition metal coordination complexes Organic multicomponent systems Light harvesting strategies Conclusions

300 311 321 351

362 365 366 377 384 389

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The day lighting became organic

393

Paolo Coppo Introduction: white light OLEDs: basic operating principles Multilayer WOLEDs Stacked devices Single layer WOLEDs Excimer white light emission White polymer LEDs White light emitting electrochemical cells The light outcoupling issue Conclusions

393 394 395 398 399 399 400 403 404 404

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Introduction and review of the period July 2004–June 2007 Angelo Albini* DOI: 10.1039/b812704m

1.

Introduction

In his Introduction to Volume 36 of the Specialistic Report in Photochemistry (covering the literature from July 2003 to June 2004, published in 2007), Professor Dunkin stated that important changes would intervene in the series, as required in the era of easily available on-line literature searching. He thought that ‘from Volume 37 onwards, Photochemistry will aim at providing a critical analysis of recently published research in photochemistry, with less of an attempt at comprehensiveness. It also is intended to emphasize applications of photochemistry in, for example, synthesis, fabrication of new devices and materials, medicine, pollution control, etc’. He could not pursue this task, but RSC decided to continue the series and Volume 37 appears now with a three years coverage (July 2004–June 2007) and only a partial move towards the same target. The point is, it is not only the way of keep abreast with literature that has changed. The series on Photochemistry began in 1970 at a time of rapid development and consolidation of this discipline. Professor Bryce-Smith remained the Senior Reporter up to Volume 18 and again for Volumes 19–25 in association with Professor Gilbert, who took over the charge for Volumes 25–33 before passing it to Professor Dunkin. The team of the reporters formed in the first years was to prove a strength of the series, with many of them contributing to many, many volumes, indeed in some cases to all of them. The presentation of the state of the art in all of the aspects of photochemistry was a novelty. In the introduction to Volume 1 (covering the literature from July 1968 to June 1969), Professor Bryce-Smith stated that, while up to about 1960 photochemistry had been ‘largely a branch of physical chemistry’, things had changed and the idea now was that of producing a book, ‘which as a whole will be of value to both physical and organic chemists, and . . . to inorganic and organometallic chemists’. Indeed, a unitary perspective is clearly perceived in this and in the following volumes, while photochemistry progressed at a good pace and took the shape of a compact discipline. ‘The year under review’ he observed ‘has seen solid progress in many areas, and a lot of enthusiastic ‘wildcatting’ (to borrow a term from the petroleum industry) in others. Quite often one finds that apparently arbitrarily chosen complicated molecules are studied long before their structural parents have been looked at. Such approaches are probably relatively inefficient in promoting a greater understanding of the subject as a whole’. Dept. Organic Chemistry, via Taramelli 10, 27100 Pavia, Italy. E-mail: [email protected]; Fax: 39 0382987323; Tel: 39 0382987316

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After 40 years the situation is changed in the sense that photochemical work continues at an even increasing pace in various field, and certainly there is a lot of ‘wildcatting’, but perhaps photochemistry as a unitary science does not advance any more, or at least not as much as when the foundations of photochemistry were actually laid. Some would say that the divergent development towards different sub-disciplines makes a common perspective insignificant, others that this very fact makes exchanging ideas between different fields the more important and potentially fruitful. Thus, the question is, is a periodical presentation of the literature still useful? In a new form? But actually how different? The answer to such a question depends in part on the general evolution of scientific literature, in part on the specific characteristics of photochemical literature. The latter point can be appreciated with reference to some figures. As mentioned, there is no doubt that this is a lively topic nowadays. The yearly output of contributions on photochemistry has attained and overcome the target of 10 000. The trend involves a slight increase over the years 2000 to 2008 (see Fig. 1) after the more lively expansion of the previous decades. In these years, the ratio of patents vs. open articles has been ca. 1/7th, a positive indication of the sound place this discipline has in the industry. It is also important to consider in which ways is photochemical science published. Always limiting the analysis to years 2000–2008, it is noticed that photochemical information is quite diffuse among a variety of sources. 41 bibliographic sources have at least 250 items in this period, which amounts to 39% of all the information (see Fig. 2). Examining in detail the role of the 41 sources with respect to all the publications, one notice that 15.4% are patents (more than 4% Japanese!). As for the open literature, there are well established journals devoted to photochemistry that collect an important fraction of photochemical science, viz: – Journal of Photochemistry and Photobiology. A: Chemistry, B: Biology, together 1.66% – Photochemistry and Photobiology, 1.28% – Photochemical Photobiological Sciences (established 2002), 1.01%

Fig. 1 Yearly output of reports on photochemistry, electrochemistry and nanomaterials.

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Fig. 2 Distribution of the reports on the considered topic journal dedicated to that specific topic vs. those in broad chemical disciplines.

Thus, even taking into account the late establishment of PPS, dedicated journals intercept a relatively small part of the contributions in this discipline, overall around 4%. The reason for this is that photochemistry is innervated in all of the parts of chemistry, much more than are other disciplines (see below). Symptomatically, the single largest contribution comes from JACS with 3.0%, along with a further 1.95% in other journals of general chemistry (always limiting the analysis to the 41 sources above). Practically equivalent is physical chemistry, with the Journal of Physical Chemistry A at 1.66% and B at 1.44%, and others in the field 1.74%. Then follow: – Journal of Organic Chemistry 1.18%, other organic 1.48% – Environmental 3.53%, an obvious stronghold – Polymers 1.71% – Physics 2.19% – Biochemistry 0.62% – Inorganic chemistry 0.58% The distribution will vary somewhat if also less frequent sources are considered, but the main characteristics of present day photochemistry are Photochemistry, 2009, 37, 11–43 | 13 This journal is

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well expressed by these data and are even more apparent when compared to other disciplines, both long established such as electrochemistry and more recent such as nanomaterials. Electrochemistry may in a sense be considered the traditional antagonist of photochemistry as an ‘alternative’ or ‘mild’ method for carrying out chemical reactions avoiding the use of chemical activators. Scientific activity has alway been at least as intense in this field, indeed in the last decade it has doubled that in photochemistry, likewise with a good proportion of patents (see Fig. 2). A close look shows important differences in the journal chosen, however. Thus, individual journal contribute by a lower amount each. The largest single share is that of Electrochemica Acta (3.17%), then five other journals specifically devoted to this discipline follow before a journal of physical chemistry, J. Phys. Chem. A (1.40%), and one of general chemistry, J. Am. Chem. Soc. (1.31%), are encountered (see Fig. 2). After these two, one finds again several electrochemical journals. Completing the examination until one arrives at 32% evidences that in electrochemistry the proportion of publication in dedicated journals is much larger than that observed in photochemistry (almost four times as much, see the histogram in Fig. 2), while that in journals of general chemistry is much lower. This difference is not limited to ‘old’ disciplines. Let us take as an example for a ‘new’ topic that of nanomaterials, a recent addition and a lively developing topic (see Fig. 1). Here a substantial contribution comes from abstracts of presentations at meetings, certainly an indication that this is a timely subject. Apart from this, the single main contribution comes from J. Am. Chem. Soc., but then examination of the top bibliographic sources (contributing 30.7%) evidences again a large proportion of articles is in dedicated journals (10.0%) created ad hoc in the last years. The impression that one gets from the data above is that the attitude of photochemistry practitioners differs somewhat from those of electrochemistry. The former scientists are in a way camouflaged within the traditional areas of chemistry and thus form different groups, each of which well inserted within that area, sometimes with a less strong relation to the other photochemical groups than to their area: an organic photochemist is a recognized member of the community of organic chemists, perhaps with a closer relation to it than to that of photochemists. On the other hand, photochemical methods and even more photochemical expertise are not ubiquitary. The largest part of the research in this field is done by dedicated photochemists, so that a large part of this science is developed in a relatively small number of laboratories (a tenth of groups published more than 100 papers in photochemistry over the years considered, most of whom, as one may expect, Japanese). In some areas, this is due to the use of sophisticated and expensive instruments, such as those required for fast kinetics investigations or matrix studies in physical chemistry. In other ones, such as organic or environmental chemistry, photochemical methods require no specific instrument and the relatively limited diffusion simply results from a partial knowledge of the topic. The situation in electrochemistry is largely different: there the experimental method is quite specific and it appears that this peculiarity has brought about a more clear cut separation of the discipline itself. In fact, in many 14 | Photochemistry, 2009, 37, 11–43 This journal is

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contexts electrochemistry is considered an independent area of chemistry, while this is not the case with photochemistry. Yet another situation is that of a novel topic, such as nanomaterial. To some degree at least, here there is a rush towards building a new independent area rather than having a recognized role in the traditional ones, despite the fact that these studies do not always require peculiar instruments or technique. These figures may, perhaps, help everyone when forming his/her opinion. Is it more important to offer an interface for a science sparse over different disciplines or to review a more unitary discipline? Personally, I would say that the former is (may be) more useful if the information is organized in a way that makes it easily accessible and perhaps facilitates each photochemist in finding something interesting outside the paths he or she usually follows. Thus, the present volume is an attempt of favoring the scientific exchange between different part of photochemistry, in the belief that observing the situation over a wider angle may help in obtaining a better rationalization of specific cases and contribute to a more extensive advancement of the whole discipline. If this has to be done, then it must involve all of the main areas of photochemistry. The literature has to be examined in some depth, although certainly not aiming at a complete coverage. Taking from the original structure of these Reports, the physical, organic and inorganic aspects are covered in this volume, although within each of these only a fraction (of various breadth) of the possible topics has been considered. Once again, the idea is not so much that of offering a balanced account, but rather one of the many possible view, attempting to make it sufficiently polyphonic that some cross fertilization may be hoped for. A section on photocatalysis has been added, recognizing the role that the photocatalytic/environmental aspects have reached. A section on polymers is missing, although it may be present in the future and at any rate some aspects are discussed in parts 1, 2. Then there is the new part, the one that should be more characteristic, with the highlights. These are personal accounts of various length about a topical subject that address questions of some applicative significance. The interrelation between the two parts will become one of the key issue in the future. In view of the above, it appeared unappropriate—and indeed impossible-to try and build again a team of reporters strictly interconnected and able to accept ‘cabinet responsibility’ for the entire volume. Rather, diversity has been priviledged in every aspect, from geographic location to scientific background. Any comment or suggestion would be highly wellcome. 2.

Review of the period July 2004–June 2007

For the reasons stated above, it is probably less obvious to identify the key advancements of such a multiform and well established discipline such as contemporary photochemistry than it was in the years of more heftly development, such as the 60s and 70s. What can certainly be said is that the advancement has not slackened off in the three years considered and important achievements can be easily listed in all of the areas of photochemistry. Furthermore, the yearly number of reports has increased by a Photochemistry, 2009, 37, 11–43 | 15 This journal is

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factor of 5 since 1970 and the papers considered are those published over a three years time span, not a single one. As a consequence what follows is not even a personal choice but rather a quick ride grabbing at some of the many stimulating ideas from the enormous amount available. In view of the changes in the preparation of this report, references are supplied in this section, rather than referring, as in previous issues, to the appropriate chapter. First of all, several books have been published. Photochemistry has a role also in teaching and the production of introductory textbooks, handbooks and advanced or specialized books has been above the average of chemistry in the time span considered here. The second edition of the CRC Handbook of Organic Photochemistry and Photobiology,1 now expanded to 145 chapters for 2900 printed pages has appeared, perhaps with some superimposition and a pair of weak points, but unquestionably the reference work on the photochemical reactivity of molecules. Further, several volumes in the series Molecular and Supramolecular Photochemistry (series ed. Ramamurthy and Schanze) were published in these years, namely Vol. 11 to 14 concerning chiral photochemistry,2 preparative organic photochemistry,3 computational photochemistry,4 and organic photochemistry and photophysics5, respectively. Further collective books include a volume of Advances in Photochemistry, the eldest of the series in the field,6 another text of computational photochemistry,7 two volumes in ‘Topics of Current Chemistry’ on the photochemistry and photophysics of coordination compounds,8 books on UV curing9 and on environmental photochemistry,10 as well as a highly wellcome experimental Handbook,11 as a much enlarged version of the old Murov popular among photochemists. The variety of topics involving photochemistry is briefly illustrated in the following. 2.1

Computational photochemistry

The high level of calculations nowadays available has allowed to confront successfully a number of photophysical and photochemical processes. Spectacular advancement have been made in explaining the photochemistry of complex molecules, such as retinal, unapproachable until recently12–14 as well as the intricate photophysics of Green Fluorescent Protein, where various proton transfer steps occur after photoexcitation.15 Important achievements involve also old problems on small molecules, such as the isomerization of butadiene.16 The anomalous fluorescence of 4-dimethylaminobenzonitrile (a red shifted emission in polar solvents, besides the expected short wavelength emission in apolar solvent) and related molecules has been known since 1961 and has been attributed to an internal charge transfer state (ICT), in contrast to the locally excited (LE) state. As for the structure of the ICT state, different models have been suggested (twisted, planar). A recent CASSCF study in a way reconciled previous hypotheses and evidenced a conical intersection between S2 (ICT) and S1 (LE). It resulted that both a torsional motion and the formation of a non planar quinoid structure are important factors in stabilizing the ICT structure in polar solvents.17 16 | Photochemistry, 2009, 37, 11–43 This journal is

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Among reactions, recent CASSCF calculations gave evidence that in the pyridine (1)—Dewar pyridine conversion the preferred path is that leading to compound (2) rather than to the isomer (3), in accord with the experiment. On the other hand, calculations suggested that phosphine should isomerize the other way around, and predicted that arsine should not isomerize at all.18

Related to this topic is the computational and experimental report on the rearrangement of Dewar benzene to benzene via the radical cation, an orbital symmetry forbidden process that has been found to involve potential surfaces of different symmetry and electronic nature. The radical cation has the odd electron in a p orbital and the reaction starts by stretching the central C–C bond, but then reaches an excited state of benzene radical cation through an avoided crossings and the ground state through a further avoided crossing.19 2.2

Mechanistic studies

2.2.1 In matrix. Both simple and complex molecules have been examined. In most studies, the identification of the intermediates is based on the calculation of the IR spectra and the experimental observations are supported by calculations for determining the best reaction path. As an example, the photodecomposition of acetyl cyanide (4) at 12 K has been shown to involve equilibration with the isocyanide (5), fragmentation a to the carbonyl group, loss of CO from intermediate complexes (6), (7), and further steps before arriving at the end products, acetonitrile (8) and methyl isocyanide (9).20

Many other intermediates have been generated in solution: e.g. thiopyran2-one (10) opened up to the thioaldehydeketene (11) that then underwent Photochemistry, 2009, 37, 11–43 | 17 This journal is

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various geometrical isomerizations. The aldehydethioketene (13) correspondingly formed from pyran-2-thione (12) also underwent intramolecular hydrogen transfer to give (11).21 In another case of homolysis leading to a ring opening process, ethynylthioketene (15), not the likewise conceivable diethynyl sulfide, was obtained from the irradiation of matrix-isolated 2,5-diiodothiophene (14).22 The long investigated photochemistry of azides has been clarified in many aspects though matrix studies. A recent example is that of pentafluorophenylazide (16) that at 10 K gave the ground state nitrene, that is the triplet (17). However, isomerization to the benzoazirine (18) and the didehydroazepine (19) were also observed. With tetrafluoroiodophenylazides loss of iodine also took place, as shown for the case of the meta derivative (20) that underwent ring opening to allene (22), reasonably via homolytic fragmentation to the 3-dehydronitrene (21) and recombination.23

The initial product from the photolysis of 1,3-diphenyldiazomethane (23) was triplet carbene (24) that conserved a conformation similar to that of the starting compound. Annealing, however, led to a highly symmetric species that is best envisaged as an allenic biradical (25). Further irradiation at short wavelength led to cyclization (see formula 26).24

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Another way of arriving at carbenes is from diazirines. As suggested by flash photolysis data, halogenated phenyldiazirines (27) underwent fast (fs) cleavage of a C–N bond to form a diradical/zwitterion (28) with excess vibrational energy. After relaxation, competing path from intermediate (28) were loss of nitrogen to form the carbene and thermalization. In turn the relaxed open-chain intermediate closed back to the starting diazirine or cleaved in picoseconds.25 To the many intermediates generated in matrix, phenylborylene (30), formed by photolysis of diazidophenylborane (29), can be added. On further photolysis at l 4 350 this underwent cyclization to (31), analogously to what observed with phenylnitrene.26 The persistent p radicals delocalized over sulfur and nitrogen atoms are studied for possible application as building blocks for molecular magnets. The analogue 1,2,3-benzodithiazolyl radicals (33) are formed by photolysis of 1,3,2,4-benzodithiadiazine (32) at room temperature. The mechanism of formation has been investigated in matrix, where various isomers were detected that upon selective irradiation could be intercoverted.27

2.2.2 In solution. The relatively large diffusion of flash photolysis with fs detection allowed considerable advancement over a some mechanistic issue that have long been under scrutiny. Key photophysical parameters have been experimentally determined. Important examples involve the mode of conversion of upper excited state. Thus, after excitation in the S2 state (pp*) the E-azobenzene molecule rotated out of planarity and rapidly dropped down to the S1 (np*) surface (time constant 0.13 ps). The latter state was reached with a large excess energy, a fact that was determining in the ensuing evolution of the system.28 Conversion between excited states has become a favorite topic of investigation and new studies document ultrafast phenomena in molecules that are quite familiar to photochemists. As an example the rate of the S2–S1 IC in benzophenone has been measured (0.53 ps). It has been found that inserting a iodine atom in 4 has no effect either on IC or on ISC.29 The rate of ISC has been measured also in some nitro polycyclic aromatic hydrocarbons, and turned out to be the fastest known ISC in solution; as an example the fluorescence lifetime of 9-nitroanthracene (34) was 50 fs. Shorter lifetimes have been observed only for metal complexes. This was the case for Ru(II) tris(bipyridyl) (35), also in that istance due to fast ISC, so that the triplet state (in this case emitting) decayed in 15 fs.30 Photochemistry, 2009, 37, 11–43 | 19 This journal is

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The result is a nice confirmation of El Sayed rule, since in these compounds both S1 and T1 states have a pp* nature, but an upper triplet state of np* nature is available and is involved in the above transition.31

Chemical reactions have been studied by the same technique and details of the mechanism have been revealed. As an example, it has been possible to detect the appearance of the emission from the excited tautomer of some 3-hydroxyflavones and thus to determine that the excited state intramolecular proton transfer (ESIP) process characteristic of these molecules occurred with a rate 3  1010 to 3  1011 s1.32 The photoisomerization of o-nitrobenzaldehyde (36) to o-nitrosobenzoic acid (39), a long known reaction, is a model photochemical process because it occurs cleanly despite the considerable molecular motion it involves. The detailed mechanism has been recently investigated at the fs time scale. It has been shown that the first step was hydrogen transfer and led to ketene (37) in 400 fs. This step was followed by intramolecular nucleophilic addition (90 ps) to give a ring closed intermediate (38) that then evolved to the end product (3 ns).33

Apropos hydrogen transfer, a basic question that arises when a good donor is involved is whether formal hydrogen transfer to a triplet acceptor (e.g. a carbonyl, a nitro group) occurs as a single step or via sequential electron and proton transfer, a distinction for which it may be difficult to find experimental support. A flash photolysis study by using 3,6-diphenyl-1,2,4,5-tetrazine (40) as the acceptor showed that the H transfer path was followed with N-methyldihydroacridine (41), where a kinetic deuterium effect was observed. However, using increasingly better acceptors such as the bis(2-chlorophenyl)tetrazine and the 20 | Photochemistry, 2009, 37, 11–43 This journal is

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bis-(2-pyridyl)tetrazine led, respectively to competition of the two mechanisms and to operation of the sequential e/H+ path only. Likewise, the last mechanism was followed with isopropylmethyldihydroacridine, since proton transfer was slowed down by the bulky substituent. In this case both intermediates, the radical cation (42) and the neutral radical (43) were actually detected.34

The course of the photochemistry of azides is primarily determined by the fast intramolecular reaction of singlet nitrene that isomerizes to benzoazirines and/or didehydroazepines. The elusive first formed intermediate has however now been directly observed in solution (besides than in matrix, see above) and the key kinetic parameters have been determined. Thus, the lifetimes of singlet ortho- and para-biphenylnitrene and of 1-naphthylnitrene were, respectively of 11, 16 and 12 ps in acetonitrile.35 Dimethylstannylene Me2Sn: (45, M = Sn) was generated in hexane solution and in the gas phase by cycloreversion from 2,2-dimethylstannacyclopentene (44, t 193 ns). In solution this intermediate decays to tetramethylstannene (46, t ca. 10 ms).36 Likewise, diphenylgermylene has been generated and shown to dimerize to the digermene.37

Phenyl cations may be added to the range of useful intermediates that are generated photochemically. Their significance is due both to the easy formation (not only from diazonium salts, but also from aryl chlorides, fluorides, mesylates and triflates (47) at least as long as they bear an electron donating substituent) and to the selective reaction of the triplet phenyl cation (348), actually the ground state in this case. These intermediates react with p nucleophiles such as alkenes (to give products 49), alkynes (50), aromatics (51), heteroaromatics as well as with anions (52), but not with n Photochemistry, 2009, 37, 11–43 | 21 This journal is

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donors. Thus, the reaction was conveniently carried out in alcohols or acetonitrile and resulted in a mild arylation method.38

The research on enediynes cyclization and in particular on the photochemical version is quite active in view of the possible application for photodynamic therapy. As an example, enediyneallenes (53) reacted exclusively to form products containing a five-membered ring (55), (56), via the analogue of the thermal Myers-Saito process, rather than a six-membered ring. Flash photolysis allowed the detection of the intermediate singlet biradical (54). Apparently, two slowly interconverting rotamers of this intermediate were present and this determined the formation of the two final products.39

2.3

Selectivity

2.3.1 In the solid state. The most appealing aspect of photochemistry for synthetically-minded chemists is the selectivity often obtained in complex transformations. Furthermore, the selectivity can be directed by carrying out the reaction in a restricted environment, which turns out to be 22 | Photochemistry, 2009, 37, 11–43 This journal is

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particularly convenient with such ‘cold’ reactions. The concept and the applications of templated photochemistry have been reviewed.40 The simplest instance of a template is the crystal and photochemistry in the crystal state, perhaps considered in the past as an exotic and limited field, is developing at fast pace and continues to reveal new applications. The longest-known of these reactions—indeed the first photochemical reactions to be accurately reported—is the dimerization of anthracene derivatives, of which there are many examples involving the 9,10 positions and some involving the 1,4 positions. Recently, the scope of this reaction has been enlarged by the first finding of an asymmetric 1,4–9,10 dimer (58) from some crystalline salts of anthracenylmethylamine (57). The unusual course of the reaction has been rationalized, just as the more common paths, on the basis of the distances between the reacting carbons in the crystals.41

The 2-methoxynaphthamide (59) crystallizes in a chiral space group and crystals composed of one enantiomer were formed. Large batches of a single enantiomer were obtained by crystallization from the melt by using such crystals as seed. When the crystals were dissolved, racemization by rotation around a single bond was rather slow (lifetime of minutes at 15 1C in aprotic solvents and of hours in alcohols). One could thus take advantage of the efficiency of photochemical reactions in the cold and exploit the ‘frozen’ chirality in the naphthamide by carrying out a cycloaddition to 9-cyanoanthracene and induce chirality in the newly formed bonds. Irradiation at 20 1C in THF gave a single product (60), 100% yield, 95% e.e. Overall, the process corresponds to an asymmetric synthesis in solution in the absence of any chiral agent.42

2.3.2 In solution. Most of (photo)chemical reactions are obviously carried out in solution. Processes that involve in-depth structure changes, with formation and cleavage of several bonds in a single step, and Photochemistry, 2009, 37, 11–43 | 23 This journal is

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occur under mild conditions, while maintaining an elevate selectivity, are typical of photochemistry and their potential is far from being exhausted. The most impressive instance is that of adducts from aromatic compounds, where a simple, flat molecole is transformed into a stereochemically defined, structurally complex product containing several sp3 carbons. As an example the well-established meta-cycloaddition continues to find new applications, in particular intramolecularly starting from o-alkenylbenzenes, most often anisoles. An example is the recent transformation of (61) into stereoisomers (62) and (63), a key step towards the synthesis of aphidicolin, a tetracyclic diterpene with antiviral and antimitotical properties.43 Remarkable is also the result of the irradiation of the alkenylcyanonaphthalene (64) where a single product with a tetraquinane skeleton (65) accumulated.44 Another way for trasforming an inexpensive aromatic starting material in a valuable synthetic building block is isomerization. This has been successfully applied to pyridinium salts. A recent example is the photocyclization of 1-methoxyethoxymethyl-3-pivaloxymethylpyridinium perchlorate (66) to generate bicyclic aziridine intermediates (67) and (68). Opening of the aziridine ring then gave access to trehazolamine, the key structural component of the trehalase inhibitor trehazolin, through a remarkable short path. Furthermore, a stereoselective synthesis was obtained when a chiral auxiliary was used.45 The intermediates prepared by the cyclization of pyridinium salts have been used also for the synthesis of polyhydroxylated indolizidines.46

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Ketoprofen, an antiinflammatory non steroidal drug is known for its phototoxic effect. The presence of the benzophenone chromophore caused DNA cleavage through three mechansims, viz. dimerization of the thymine upon energy transfer, oxidation of guanine upon electron transfer and addition of the drug to thymine forming an oxetane. Interestingly, it has been found that the two enantiomers of the drug exhibited a different rate for the quenching by nucleosides. As an example, the triplet of the (R) isomer (69) was quenched by thymidine (71) at a rate 40% larger than the enantiomer (70).47

This example of stereodifferentiation may be significant for the design of chiral drugs. An intramolecular version of this system has also been investigated by studying the photochemistry of diastereomeric molecules where the ketoprofen moiety was linked through a spacer at a hydrogen donating moiety. A clear cut differentiation was found at both levels, viz. the initial hydrogen transfer step (as deduced from the different triplet lifetime) and the intramolecular recombination of the formed biradicals to give macrocyclic photoproducts (as shown by the diastereoselectivity in C–C coupling. An example is ketone (72), for which attack at both the benzylic and the a-amino positions took place giving biradicals (73) and (74).48

An example of chiral discrimination has been found in a helicene based on the benzothiophene skeleton and bearing chiral moieties at the end of the two arms (75). This was a reversible photochromic system and the photostationary state with the highest content of the closed form (76) was obtained by irradiation at 400 nm and had only 40% of it, but this was a single Photochemistry, 2009, 37, 11–43 | 25 This journal is

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stereoisomer. This result was attributed to the asymmmetric orientation of the transition moments in some of the conformers but not in others.49

In the examples above, selectivity was induced by the structure of the reagents involved. However, also in solution the mild conditions make photochemical reactions well suited for a medium-directed behavior. Thus, supramolecular complexation can affect the photophysical properties. As an example, cucurbit[n]urils, synthetic pumpkin-shaped molecules that act as cation complexing agents, have shown excellent properties for the thermal and photochemical stabilization of fluorescent dyes.These serve as probes for biological systems as well as, in favorable cases, as promising drugs for photodynamic therapy and as antimicrobial agents.50 As for photochemical reactions, attention for their control in solution by using cyclodextrins or dendrimers is increasing. As an example, dendrimers terminating by 5-alkoxy-1,3-dicarboxylic acid moieties were dissolved in basic solutions and the presence of nonpolar locations was established. Some photochemical reactions were carried out under these conditions (a-cleavage of benzoin ethyl ester, dimerization of acenaphthylene) and it resulted that the larger effect took place in third generation dendrimers rather than in first or second generation.51,52 Photochirogenesis, or asymmetric synthesis through an electronically excited state, provides a unique access to optically active compounds, which is alternative to the well-established thermal and enzymatic methods. The photocyclodimerization of 2-anthracenecarboxylic acid salts in the presence of cyclodextrins has been studied in depth and it has been found that the ee of the products obtained was proportional to the relative stability of the complexes formed by the ground state acid in the suitable pre-orientation. Such ‘supramolecular photochirogenesis’ based on pre-orientation is a versatile method. Thus, a dicationic group attached to the cyclodextrin can switch the products distribution in favour of the head to head dimers rather than of the head to tail dimers otherwise preferred and at the same time enhance the complex stability so that the ee of the dimer increased from 3 to 41%.53 Shifting to Human Serum Albumin as the photochirogenic host seems to be advantageous and led to a high enantiomeric excess (ee 4 80).54 Permethylated 6-O-modified b-cyclodextrins were synthetized as novel photosensitizing hosts. These differ from the non methylated host by having a more flexible skeleton. Sensitization by permethylated benzoyl b-cyclodextrins of Z-cyclooctene gave the E isomer in moderate ee. The enantiomeric excess strongly depended on temperature, and the products ratio was reversed in the extreme case. This behavior was taken as indication of a key role of complexation entropy with this flexible host.52 26 | Photochemistry, 2009, 37, 11–43 This journal is

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Metal complexes have likewise been used as a template for selective synthesis. Thus, an octahedral Pd nanocage has been found to act as a ‘reaction vessel’. Under these conditions, a group of coumarins were found to give selectively the syn head–head dimer, whereas in water they yielded either a mixture of dimers or a different isomer.55 Another field that is gaining an increasing role is the photochemistry occurring at the molecule–metal interfaces (as well as at the molecule– semiconductor surface, in particular with reference to TiO2).56 This situation deeply changes the characteristics both of the ground state and of the excited state. Various aspects of the field, including experiments at the fs time scale,57 have been reviewed.58,59 The short lifetime of photochemical intermediates offers another dimension for selectivity that can be explored by using different excitation wavelength. Thus, a ‘two colors’ experiment has been carried out with naphthylmethyl phenyl ethers. Laser irradiation at 355 nm served for reaching the triplet state of ether (77) via benzophenone sensitization. The energy of the T1 state (377) was not sufficient for cleavage of the arylmethyl-oxygen bond, but absorption of a second photon at 430 nm led to radical (78). In the case of 1,8-di-(phenoxymethyl) derivate (79), a ‘three colors’ experiment was reported. First, the S1 state was reached, from which ISC crossing led to T1 (379). The latter was promoted to an excited triplet Tn that underwent cleavage to the mono naphthyl radical (80). Finally, an excited state of the radical was reached that cleaved to diradical (81), not detected because of the fast ring closure to acenaphthylene (82).60

2.4

Modification of bulk properties by photochemistry

Equally interesting is the modification of the properties of the medium through the photoreaction of a component. The photoproduction of acidity is an important issue for its catalytic action in a variety of systems. Photochemistry, 2009, 37, 11–43 | 27 This journal is

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Chloranil irradiated in methanol produced acidity that caused the acetalization of ketones, and was regenerated through a disproportionation reaction.61 In this regard, one may mention that while 6-hydroxyquinoline underwent tautomerization in the excited state, a change in the nitrogen basicity by passing to the N-oxide made the compound a strong photoacid (pKa -3.1 in MeOH–H2O 1–1).62 Fast proton transfer is involved in various biological processes, such as transport through ion channels, photosynthesis and protein folding. An approach for the rapid imposition of pH change is flash photolysis, as illustrated in the case of nitrobenzyl photochemistry. For this application benzyl sulfate (83) has been chosen and the process has been monitored by photoacustic spectroscopy. The aci-nitro form (84) was formed in 7 ns at pH 7 and deprotonation occurred at a rate depending on the initial pH of the solution. The liberation of sulfuric acid had only a little buffering effect, so that strong pH jumps could be obtained that would cause protein denaturation.63

A reversible light controlled switch for alkaline cations has been devised. This is a renium bipyridine tricarbonyl complex with a ligand containing an aza-crown ether (85). Irradiation into the metal-to-ligand band caused expulsion of the cation in the ns time scale, but decay to the ground state and thermal equilibration of the complex resulted in recapture in microseconds.64

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A hydrocarbon containing three dihydropyrene units has been synthetized as a multistate switch. The most stable form (86) had the central ring open. Irradiation in the visible led to the fully closed isomer, whereas UV irradiation led to the fully opened product.65 Photochromism can be used for inducing macroscopic changes. As an example, dithienylethylene (87) bearing amphiphilic side-chains forms aggregates in water. Heating loosened the association and caused clouding of the liquid within a narrow temperature range (1.5 1C). The phenomenon was reversed upon cooling. The ring closed isomer (88) had a clouding temperature lower by some degrees, and thus turbidity could be induced by a photochemical change.66 Photoactive surfactans are useful for changing colloidal properties. As an example, photoinduced phase separation was obtained by using sodium 4-hexylphenylazosulfonate as an anionic surfactant. This is photolabile and when a salting out electrolyte was present, the initial micelle system broke down upon irradiation and phases were separated. The change was spectacular when a dye insoluble in water was present. This system may be used for recovering the dye, or more generally, for separating hydrophobic (and/or hydrophilic) components from a multicomponent mixture.67

2.5

Preparative photochemistry

Bringing to reaction inactive molecules under mild conditions is another typical application of photochemistry. An illustrative case is the mild and selective activation of alkanes. As an example, it has been found that adamantane is accommodated within the self-assembled cage formed by a multinuclear palladium complex (six ions bridged by four 2,4,6-tris(4-pyridyl)-1,3,5-triazine units). Irradiation of this complex in aqueous solution led to 1-adamantanol and the corresponding hydroperoxide. The process involved electron transfer from the hydrocarbon to the excited Photochemistry, 2009, 37, 11–43 | 29 This journal is

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triazine, at close contact in the complex, followed by deprotonation and trapping by oxygen of the resulting radical.68 Oxidation at a benzylic position by using molecular oxygen can be conveniently achieved via diiron(III)-m-oxo bisporphyrin complexes. The Fe–O bond was thermally inactive in these complexes, but irradiation generated geminal Fe(II)/Fe(IV) pairs in which the ferryl moiety [Fe(IV)] was active in the oxidation.69 The above reactions fulfil the conditions for being considered examples of ‘green chemistry’, among which photochemical reactions have a particular place since the reactive that causes the reaction, the photon, is adsorbed and leaves no residue behind to be taken care of, contrary to what occurs with chemical reagents. This quality is one of the reasons that should favor the development of photochemical procedures for synthesis in an industrially significant scale. Although activity in this field is certainly not overwhelming, some significant results deserve mention. A continuous flow arrangement is the best solution for scaling up photoreactions and compact reactors have been constructed and optimized to perform continuous organic photochemistry on a large scale. These were built from commercially available or customized immersion well equipment around which a UV-transparent, solvent-resistant fluoropolymer tubing was coiled. The usefulness of such reactors was assessed by carrying out the intra and intermolecular [2 + 2] photocycloaddition of malemides to form compounds (89) and (90) at the hectogram scale.70

This demonstrates that scaling up a photochemical reaction by using an inexpensive set up is possible, although the use of the light has not been optimized at this stage. The other line of advancement involves the use of solar light. An account of the results obtained at the PROPHIS (parabolic trough-facility for organic photochemical syntheses in sunlight) facility has been reported71 that uses moderately concentrated sunlight and has been demonstrated to be an efficient tool for the synthesis of chemicals on a semi-technical scale, including oxygenations, isomerizations, acylations and others. It may be mentioned that photochemical reactions, in particular when solar driven, are considered among the items of interest to process R&D chemists and engineers.72 30 | Photochemistry, 2009, 37, 11–43 This journal is

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Demonstrating that the scaling up of photochemical reaction is not more difficult than that of a thermal alternative is important for making more synthetic-minded chemists interested in photochemistry and use this method more largely. In most cases, however, new synthetic methods are explored on a small scale and here there is no reason to exclude the photochemical alternative. A topic that is attracting an increasing interest is that of photoremovable groups, where the peculiar mildness of the method finds an ideal application. Protecting groups that are removed via a reductive sensitization, such as phenacyl esters (91) and N-methylpyridinium esters (92) have been reviewed.73 New groups have been devised that apply also to poor leaving groups such as alcohols and amines. A promising candidate for this role is the 2,5-dimethylphenacyl group. In this case the cleavage was obtained by direct irradiation and was initiated by photoenolization, not requiring further reagents. The corresponding carbamates (93) have been found to be a viable solution for amines and aminoacids that were photoliberated in good chemical yield, although inefficiently (quantum yield 0.04 to 0.09).74 The same principle has a biological application for the release of a biologically active molecule from a inactive precursor, for the study of the biological response or for the release of a drug. 7-Diethylaminocoumarin is one of the photoremovable groups used for this purpose. Glycine ‘caged’ as ester (94) was liberated by visible irradiation in a buffered solution. The quantum yield was 0.12 and flash photolysis experiments indicated a cleavage rate of ca. 4  105 s1.75

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2.6

Energy conversion

2.6.1 Solar energy exploitation. The exploitation of solar light for a clean and inexhaustible source of energy obviously continues to be a favorite field of investigation, in view of the importance of the energy issue in the coming decades,76,77 although it has been observed that the level of funding in the field is well below that for research of comparable or lower importance for mankind.78 Many new systems for solar light harvesting have been proposed. Crucial is unidirectional energy transfer. A wealth of supramolecular systems have been developed for this function, in many cases based on metal cations, although also the combination metal ion—semiconducor as well as purely organic systems have been investigated. Elaborated molecular assemblies have been built starting from separate components for the different functions, light absorption and unidirectional energy transfer devised taking into account the free-energy gradients and incorporating the appropriate oxidative and reductive catalysts, e.g. for the production of hydrogen as a fuel.79 Studies of compounds with covalent bonds between the donor and acceptor have provided considerable insights into how spin, distance, and thermodynamic driving force influence electron-transfer rate constants. It must been taken into account that in such rigid structures (molecular wires)80 the connection by ligands ensues the position of the metal ions, but has also an effect on the electronic structure and the energy and elecron transfer rates. As an example, in bimetallic compounds of structure (95), the extensive electron delocalization over the fully aromatic fused-ring bridging ligand stabilized not only the MLCT, but also, and indeed to a greater extent, the LC states. Thus, while with most ligands the Os(II) and the Ru(II) exhibit a similar behavior, with the lowest excited state of MLCT nature and triplet character, in this case there was an inversion with Ru(II) that now had a LC lowest state.81

Efficient systems for capturing solar energy are obtained also by self-assembling of flexible structures. This is for example the case of complex (96), that is based on a Ru(II) tris-bipyridyl complex with three cyclodextrins pending group that are able to bind hydrophobic chains. Assembling results from the inclusion of the chain of 8-anthracenyloxyoctanoic acid into the two a-cyclodextrin moieties and of the adamantyl 32 | Photochemistry, 2009, 37, 11–43 This journal is

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group, in turn tethered to an Os(II) tris-bipyridyl complex, into the b-cyclodextrins moiety. In this way, initial absorption by the anthracene chromophore is followed by unidirectional energy transfer to the Ru ion and then to the Os ion.82

The assembling of different components can be obtained also by using organic molecules. As an example, a system composed of a central pyrrolylperilenediimide flanked by four benzyloxyperilenediimides spontaneously formed dimers (97). Light absorption led to energy transfer from the outer to the inner perilenediimide moieties and electron transfer between the two central perilenediimide units, due to their close proximity in the self-assembled dimer. Therefore, this molecule mimiks the functions of the photosynthetic centre, both in the very extended absorption and in the capability of light harvesting, directing energy transfer and separating charges.83

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A highly promising approach for solar energy conversion is having a (dp)6 metal complex such as a Ru(II) tris-bipyridyl derivative anchored though a carboxyl group or a phosphonate to a nanocrystalline wide-band-gap semiconductor, such as TiO2, see (98).84 The high surface area of this material ensured a good coverage, so that light absorption by the complex led directly to electron injection into the conduction band of the semiconductor. Thus, charge-separated states were formed at the surface, a favorable situation for efficient solar energy conversion into electrical power. Rigid rod linkers85 (99) and linkers of tripodal structure86 (100) that ensure a predictable distance beween the metal centre and the TiO2 surface have been synthesized and tested for assessing the efficient conversion of light into electrical energy. The reductive power accumulated by photocatalysis is used for producing hydrogen or for the fixation of CO2. An interesting application is the reduction of fumarate to succinate and of oxaloglutarate to oxalosuccinate that occurs by irradiation in the presence of colloidal ZnS. This corresponds to carrying out some steps of the reverse Krebs cycle, an important process in prebiotic chemistry for the reduction of CO2.87

2.6.2 Molecular motors. Conceptually, the conversion of light into a mechanical motion can be obtained in simple systems, as observed by Lehn, who pointed out that the syn-anti isomerization of imines occurs by different mechanisms under thermal and photochemical conditions and thus in a chiral molecule the sum of the two processes results in an unidirectional movements.88 Molecules that are able to rotate repeatedly in the same sense along an axis, while consuming energy, have been devised and indicated as molecular motors. These generally are hindered alkenes where the barrier for E/Z isomerization can be modulated by the choice 34 | Photochemistry, 2009, 37, 11–43 This journal is

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of substituents. Thus, low temperature (60 1C) irradiation of alkene (101, R = Me, Et) caused isomerization of the CQC bond to form the Z isomer as the a high energy conformer. Allowing the temperature to reach 20 1C induced helical inversion of the two rings to give the more stable conformer. A second photochemical step led back to the E isomer, first as the less stable conformer, then, upon helix inversion induced by heating at 60 1C, in the stable starting conformation.89,90

In alkenes (102) an increase of the bulk of substituent R ‘accelerated’ the rate of rotation around the CQC bond because this was weakened.90 Comments on the theoretical treatment of the excited state dynamics associated with the isomerizations91 on which such devices are based and on the strategy to design crystals capable of supporting structurally programmed molecular motions92 have been published. A new system capable of acting as an extension cable has been prepared by self assembling. This was able of transferring electrons from a light-powered source to a drain through two distinct, reversible, plug/socket junctions, operated independently by orthogonal chemical inputs.93

2.7

Polymers

The well known lability of benzylic derivatives has been exploited in new photoinitiators. Thus, naphthylmethyl anthracenyl ethers (103) underwent cleavage of the CH2–O bond, both in the homolytic and in the heterolytic way, and initiated the polymerization both of styrene and of cyclohexene oxide.94

A photochemical variation of radical living polymerization has been reported. This was based on the photofragmentation of disulfide (104) and the initiation of the polymerization by the tetrazolylsulfide radical (105). The recombination of the propagating radical (107) with the Photochemistry, 2009, 37, 11–43 | 35 This journal is

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dithiocarbamate radical (106) was photochemically reversible and this determined the living character of the propagation.95,96

A drawback of photoinitiation is that decomposition of residual initiator, for practical reasons always added in excess, continues during the lifetime of the polymer and causes yellowing of the manufactured article. Monomers capable of photopolymerization in the absence of an added initiator overcome this limitation. It is known that maleiimide is capable of self-initiation, but this monomer is too expensive. Recently, however, monomers that are quite efficient for this aim have been found in enol derivatives. Vinyl acrylate did polymerize, but was too volatile for practical use, but divinyl fumarate and maleate had a higher boiling point and a reasonable absorption above 300 nm (e313 4  102). These derivatives were tested for their ability to induce the copolymerization of 1,6-hexadienol diacrylate. This was indeed obtained under conditions where the diacrylate alone was stable. The rate of polymerization was much lower than in the presence of an equimolecular amount of a conventional photoinitiator (a part of which remained non reacted, however), but the rate could be increased by increasing the amount of the photoactive monomer.97 The versatility of photochemical method makes it ideal for applications where particular requirements must be met. Among polymer transformations, a recent application involves in situ photochemical cross-linking during electrospinning. In this technique, ultra thin fibers of peculiar properties are formed when the polymer is ejected upwards against gravity from a nozzle kept at 0.75 kV towards a target maintained at ground potential. A jet of methyl methacrylate—hydroxyethyl methacrylate copolymer functionalized by cinnamate groups was irradiated and 2 + 2 photodimerization was obtained ‘in flight’. Noteworthy, this cross-linking involved chains within a single electrospun fiber and not in different fibers.98 The photodimerization of cinnamates or coumarins is also used for the photoalignment of liquid crystals. When irradiating with linearly polarized light of low fluence a thin film where coumarin moieties were randomly oriented, only those with absorption dipoles falling along the polarization axis reacted. This resulted in a non random localization of coumarin dimers that caused a re-orientation along the light polarization axis of the overlying nematic liquid crystals. On the other hand, irradiation at high fluence led to coumarin dimers almost randomly, while the remaining unreacted coumarin moieties mainly had their absorption dipoles perpendicular. This oriented 36 | Photochemistry, 2009, 37, 11–43 This journal is

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the overlying nematic liquid crystal perpendicular to the polarization axis. This fact has been known since some time, but only recently a satisfactory kinetic model has been proposed and predictive criteria have been formulated.99 A photoactive molecule incorporated in a polymer serves either for the characterization of the polymer structure or for changing in some way one of the properties of the polymer. Largely used probes are azobenzene derivatives due to the thermally and photochemically reversible isomerization and some examples are reported in sec. 2D. Photosensitive materials based on the azobenzene isomerization are of interest and thin films are easily prepared by self-assembling. A limitation is that when incorporated in an organized assembly (and in water), these compounds tend to form stable aggregates via p–p interactions. However, a dimethylaminopropylmethacrylamide based copolymer containing a portion of the analogue monomer bearing an azobenzene moiety was prepared. This did not form aggregates and exhibited reversible E/Z isomerization at a rate remarkably similar to that observed in solution.100 Another application regarded the fact that thin and ultra thin polystyrene films behave as they had a glass transition temperature different from that of the bulk material. This point has been investigated by incorporating a 1% amount of an azobenzene derivative. The extent of the E/Z isomerization occurring for a given irradiation time was monitored and was found to depend on the film thickness. The observed dependence could be rationalized through the previously presented hypothesis that at the polymer surface the isomerization occurs in the same way as in a liquid.101 2.8

Biological applications

The photochemical and photophysical behavior of chemical compounds in a biological environment may have an important diagnostic role about the biological functions. As an example, the mode of binding of ruthenium(II) and rhenium(I) diimine ‘wires’ to nitric oxides synthase has been investigated by fluorescence quenching rate measurements, from which complexing constants and distance from the heme-Fe have been determined.102 More generally, the conducting properties of DNA are a topic of continuous interest, and electron transfer from a photoexcited donor is a suitable technique for injecting excess electrons that then migrate through reduction of the proximal nucleobase. This has been documented by using N,N,N 0 ,N 0 -tetramethyl-1,5diaminonaphthalene (TMDN) and 1,5-diaminonaphthalene (DAN) as the donors. The use of DNA containing bromodeoxyuridine allowed to explore the distance to which the electron could migrate from the initial injection point to this heterocycle by measuring the fraction of it reduced.103 A peculiar application of the properties of singlet oxygen in a biological system exploits the time resolved phosphorescence of singlet oxygen to obtain images of single cells. A light pulse has been concentrated on a small volume (o1 fL). The relatively long lifetime of singlet oxygen gives the possibility of following events for several ms. This appears a formidable tool for following the evolution of cell components, e.g. the complex phenomenon of apoptosis.104,105 Photochemistry, 2009, 37, 11–43 | 37 This journal is

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Singlet oxygen and other reactive forms of oxygen (ROS) have, as it is well known, an important role in photoactivity of certain molecules against tumoral cell and viruses (photodynamic therapy, PDT). The effect is based on oxygen photosensitization by some compounds, mostly heteroaromatic derivatives (typically, psoralens), but several cases of metal complexes with photoinduced biological activity have been recently reported. An example is a rhenium complex that attacks the genome of the sindbis virus, avoiding the risk of indiscriminate damage to the blood components, a typical side effect in photodrugs based on singlet oxygen.106 In a recent study, a dirhenium complex has been found that underwent photoaquation accompanied by a marked increase in cytotoxycity (by a factor of 34!). This finding may open the way for a new photodynamic therapy, where the metal complex would act in a way similar to cisplatin, but with the advantage of local activation.107 The liberation of NO from photolabile metal nitrosyls has an important role for PDT, cardiovascular diseases and other therapeutic uses because no additional biological activation is required.108 Recently a ruthenium and a manganese complexes have been reported that are notable for the low level or irradiation required for NO release. On the other hand, psoralens of more elaborated structure have been introduced that are devised for a localized action.109 The problem of localized activation is a central theme of research in this field (and an intrinsic advantage of photochemical activation). Among the problems that remain, one of the most important is the wavelength used. This must be in the visible for having sufficient energy to activate oxygen, but penetration of visible light in the skin is limited. A way out of this is changing to two photons, rather than single photon, excitation. Photons of lower energy would then be required and this may allow to exploit the transparent window of the skin between 700 and 1000 nm. An approach that has been recently devised involves the use of a bis-[(aminostyryl)styryl]anthracene (108) that in view of its structure was characterized by increased two-photons absorption and strong fluorescence. Near infrared photons were absorbed by the nanoaggregates of the dye that re-emitted in the visible. The dye and the oxygen sensitizer, a pyropheophorbide known to be effective in this role and already in phase I/II clinical trial were co-encapsulated in nanoparticles. Thus, the sensitizer was indirectly excited through fluorescence resonance energy transfer (FRET) from the nanoaggregates of the dye.110

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The unique luminescence properties of certain lanthanides are valuable features in aiming at more sensitive label technologies for clinical diagnostic use. A series of water soluble complexes (109) with a new ligand where a phenanthroline ring was connected to a diethylenetriaminetetracarboxylic moiety through a flexible methylene group has been reported [with Tb(III), Eu(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), Er(III)].

Spectroscopic studies demonstrated that both moieties of the ligand participated in the complexation of the metal (see the folded arrangement in structure) and that there was no water molecule in the first coordination sphere of the La(III) cation. These complexes were characterized by an intense luminescence in the visible and near-infrared.111 2.9

Depollution

As mentioned in the introduction, this is one of the most active field of research in photochemistry, mainly based on the use of titanium dioxide as the photocatalyst, and it is practically impossible to summarize the advancement of the last years. There has been some controversy about the actual effectiveness of this method for the mineralization of pollutants, which is always determining for large scale application. A recent assessment has evidenced the large variations in efficiency on the pollutants structure and conditions, e.g. low irradiation rates are more effective and there is an optimal catalyst loading.112 One may mention that attention is progressively extended to new classes of pollutants, e.g. drugs from human or veterinary use that often are quite persistent in water.113,114 Important in this connection is to note that the rate of drugs photodegradation has been found to greatly vary with water hardness.115 A couple of points that may have a general bearing on this question may be mentioned. One has to do with mechanism. Thus, the photocatalyzed degradation is mainly based on the generation of OH radicals. But these are generated also in natural waters by effect of dissolved organic matters and may have a considerable role.116 Second, more attention is given now to the combination of different activation methods, e.g. sonochemistry and photochemistry, and this trend might acquire more weight in the future.117

Acknowledgements Special thanks are due to Professor Elisa Fasani and to Drs Stefano Protti and Valentina Dichiarante. Photochemistry, 2009, 37, 11–43 | 39 This journal is

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References 1 W. H. Hoorspool and F. Lenci, CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, 2nd edn, 2004. 2 Y. Inoue and V. Ramamurthy, Chiral Photochemistry, Marcel Dekker, New York, 2004. 3 A. Griesbeck and J. Mattay, Preparative Organic Photochemistry, Marcel Dekker, New York, 2005. 4 Computational Photochemistry, ed. A. Kutateladze, CRC Press, Boca Raton, 2004. 5 V. Ramamurthy and K. S. Schanze, Organic Photochemistry and Photophysics, Marcel Dekker, New York, 2005. 6 Advances in Photochemistry, ed. D. Neckers and T. Wolff, Wiley, Hoboken, NJ, vol. 28, 2005. 7 Computational Photochemistry in Theor. Comput. Chem., ed. M. Olivucci, Elsevier, Amsterdam, 2005, p. 16. 8 Photochemistry and Photophysics of Coordination Compounds I and II in Top Curr. Chem., ed. V. Balzani and S. Campagna, 2007, pp. 280 and 281. 9 Photochemistry and UV Curing New Trends, ed. J. P. Fouassier, Research Signpost, Trivandrum, India, 2006. 10 Photo/Electrochemistry & Photobiology in the Environment, ed. S. Kaneko, Energy and Fuel, Research Signpost, Trivandrum, India, 2006. 11 Handbook of Photochemistry, ed. M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi, CRC Press, Boca Raton, Fla, 3rd edn, 2006. 12 M. Olivucci, A. Lami and F. Santoro, Angew. Chem. Int. Ed., 2005, 44, 5118. 13 J. A. Gascon, E. M. Sproviero and V. S. Batista, Acc. Chem. Res., 2006, 39, 184. 14 A. Cembran, F. Bernardi, M. Olivucci and M. Garavelli, J. Am. Chem. Soc., 2004, 126, 16018. 15 O. Vendrell, R. Gelabert, M. Moreno and J. M. Lluch, J. Am. Chem. Soc., 2006, 128, 3564. 16 F. Sicilia, L. Blancafort, M. J. Bearpark and M. A. Robb, J. Phys. Chem. A, 2007, 111, 2182. 17 Y. Amatatsu, J. Phys. Chem. A, 2005, 109, 7225. 18 M.-D. Su, J. Phys. Chem. A, 2007, 111, 971. 19 T. Bally, S. Matzinger and P. Bednarek, J. Am. Chem. Soc., 2006, 128, 7828. 20 Z. Guennoun, I. Couturier-Tamburelli, S. Combes, J. P. Aycard and N. Pitri, J. Phys. Chem. A, 2005, 109, 11733. 21 S. Breda, I. Reva, L. Lapinski, M. L. S. Cristiano, L. Frija and R. Fausto, J. Phys. Chem. A, 2006, 110, 6415. 22 Y. S. Kim, H. Inui and R. J. McMahon, J. Org. Chem., 2006, 71, 9602. 23 W. Sander, M. Winkler, B. Cakir, D. Grote and H. F. Bettinger, J. Org. Chem., 2007, 72, 715. 24 J. T. DePinto, W. A. deProphetis, J. L. Menke and R. J. McMahon, J. Am. Chem. Soc., 2007, 129, 2308. 25 J. Wang, G. Burdzinski, J. Kubicki, M. S. Platz, R. A. Moss, X. Fu, P. Piotrowiak and M. Myahkostupov, J. Am. Chem. Soc., 2006, 128, 16446. 26 H. F. Bettinger, J. Am. Chem. Soc., 2006, 128, 2534. 27 N. P. Gritsan, E. A. Pritchina, T. Bally, A. Y. Makarov and A. V. Zibarev, J. Phys. Chem. A, 2007, 111, 817. 28 H. Satzger, C. Root and M. Braun, J. Phys. Chem. A, 2004, 108, 6265. 40 | Photochemistry, 2009, 37, 11–43 This journal is

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Photophysical processes in polymers and oligomers Telma Costa, Joa˜o Pina and J. Se´rgio Seixas de Melo* DOI: 10.1039/b908480k

Processes occurring upon electronic excitation: general considerations The initial state of molecules prior to excitation is usually the ground electronic state. Upon electronic excitation the molecule returns to the ground electronic state decaying through several deactivation, radiative (fluorescence and phosphorescence) and radiationless (internal conversion and intersystem crossing), channels, see Scheme 1. Upon electronic excitation to any state above the first excited singlet state (S1), de-excitation occurs (through vibrational relaxation) commonly to the S1 state. From this state, possible deactivation processes include fluorescence emission, continued de-excitation to the ground-state, with no emission, crossing to the triplet state or any combination of these (Scheme 1). If the triplet becomes populated, phosphorescence and/or de-excitation to the ground-state, with no emission, can occur. It is the overall balance between radiative and non-radiative deactivation pathways that gives rise to the characteristic properties of molecules. The lifetime of an excited state of a molecule is one of its four main characteristics; the others are the energy, quantum yield (f) and polarization (P or anisotropy, r). Following pulsed excitation, fluorescence decays exponentially with time, according to I = I0et/t, where I0 is the initial intensity settled at an arbitrary time zero, I is the intensity at some latter time, t, and t is a constant. When the time t is equal to t, the intensity has fallen to 1/e of its initial value. The value of t is therefore defined as the mean decay time for the emission process or the mean life of the excited state. When fluorescence is the only deactivation process, the value of t is commonly designated as t0 with the meaning of intrinsic or natural lifetime.

Scheme 1 Jablonski-type diagram schematizing the overall set of deactivation processes occurring upon excitation. vr—vibrational relaxation; IC—internal conversion; ISC—intersystem crossing.

a

Department of Chemistry, University of Coimbra, P3004-535 Coimbra, Portugal. E-mail: [email protected]

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The observed or measured lifetime t will only be equal to t0 in the absence of any deactivating process, either internally or externally induced. The observed lifetime, tF, is determined by all the deactivation processes and is expressed as: tF ¼

1 ; kF þ kIC þ kISC þ kq ½Q

where kF (fF/tF), kIC (fIC/tF), kISC (fISC/tF) and kq are the rate constants for, respectively the fluorescence, internal conversion, intersystem crossing and quenching processes, and Q is a quencher. Additionally, several other factors can influence and add new pathways for dissipation of energy. Among these, is the formation of new species (for example excimer formation or acid–base equilibria) and/or quenching. Oxygen, present in all solvents in equilibrium with air, acts as a very efficient quencher. This frequently is due to energy transfer to the ground-state of oxygen (a triplet) to generate singlet molecular oxygen (1270 nm, E1 eV). Obviously, the efficiency of oxygen quenching depends on the lifetime of the probe being quenched and, particularly, on the state undergoing quenching. In the case of triplet states, due to their longer lifetimes, rigid matrices (frozen solutions or glasses for example) can be used to prevent diffusional collision between molecular oxygen and the molecular probe, thus avoiding quenching. A comprehensive study of the excited states of a molecule involves the determination of the above mentioned characteristics (summarized in Scheme 1) for the singlet and triplet excited states. 1.

Hydrophobically modified polymers

Fluorescence is an important and useful tool to investigate physicochemical, biochemical and biological systems. Several techniques, based on the detection of fluorescence, have been developed in terms of spectrometry, microscopy, flow cytometry, and of photophysical methods. Included in the photophysical methods are steady-state and time-resolved fluorescence, anisotropy, fluorescence resonance energy transfer (FRET), two-photon absorption. Fluorescence techniques offer a huge variety of advantages due to its high sensitivity and selectivity to a high number of analyte targets. Intrinsic or natural probes are frequently found in proteins, where the aromatic amino acids, such as tryptophan (trp), tyrosine (tyr), and phenylalanine (phe), exhibit fluorescence emission.1–3 However, the majority of the systems under investigation (polymers, micelles, lipids, DNA, . . . ) are non-fluorescent and the use of an extrinsic or intrinsic fluorescence probe must be therefore considered in order to investigate these systems at a molecular level. The choice of the fluorescence probe should be made taking into consideration its sensitivity to the property of the system to be measured.4 There is a wide range of fluorophores, or fluorescence probes, that can be used: coumarins, fluoresceins, eosin, rhodamines, cyanines, etc. However, the design of new fluorescent molecular sensors, that are selective to a given analyte is, nowadays, an area of growing research. Photochemistry, 2009, 37, 44–71 | 45 This journal is

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1.1 The use of fluorescence probes in the investigation of polymer dynamics and self-assembly phenomena 1.1.1 Fluorescent probes in polymers and models of analysis. The grafting of a fluorescent probe onto the polymer backbone provides information at a molecular level. By employing photophysically active groups, either randomly distributed or at both ends of the polymer, a direct molecular-level study of the association and, consequently, adopted molecular conformation can be made. From photophysical studies it is possible to follow events occurring on a very short time scale and, in ideal situations, of ‘‘zero intramolecular concentration’’. Probes with long lifetimes are required in order to probe large scale motions of the polymer chains. Steady-state and time-resolved fluorescence measurements, based on excimer formation/decay kinetics, are also often used as tools to follow the self-association of these polymers in aqueous solution. Excimer (excited dimers) imply the diffusive encounter between a molecule in its ground state with one in its excited state. Due to its long lifetime (100 to 600 ns depending on the solvent and substitution) pyrene is the most popular probe known to present excimer formation. There are however situations where an ‘‘excimer-like’’ emission can be observed, resulting from pre-associated (or preformed) dimers in the ground state, GSD. This is commonly seen when the chromophores are linked by a polymer chain,4–6 when they are bound or absorbed on silicas,5,6 aluminas,6 clays or zeolites7–9 or when they form complexes with cyclodextrins,5,10 calixarenes11 or metal ions.12 The presence of ground-state dimers has a strong influence on the excimer-to-monomer steady-state emission ratio (IE/IM) and in the obtained excimer association and dissociation rate constants.13 Ground-state dimers are known to have a lower fluorescence quantum yield (fFGSD) compared to the excimers formed through a dynamic mechanism. Duhamel et al.14 reported the first quantitative information about the fluorescence quantum yield of pyrene dimers (fFGSD), for the interaction between Hydrophobically modified Alkali Swellable Emulsion (HASE) copolymer labelled with pyrene (Fig. 1a) and surfactant sodium dodecyl sulfate (SDS),15,16 where a decrease of the

Fig. 1 Polymer structures: (a) Hydrophobically modified Alkali Swellable Emulsion (HASE) copolymer, (b) poly(acrylic acid), and (c) poly(styrene) labelled with pyrene.

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ground state association was observed by increasing the SDS concentration. It was found that the fFGSD is 4.5 times smaller than that of dynamic excimers. In addition to GSD, conformationally different excimers,10,17 monomers that give rise to excimer (MAGRE) with different association rate constants18 and monomers that do not give rise to excimer, i.e., free monomers,13,17 can co-exist in randomly labelled polymers as a result of the different interpolymeric distances between adjacent chromophores. Efforts and different methods of analysis have been developed over the past years in order to solve the observed complex time dependent luminescence behavior. The more widespread methods involve fittings to multiexponential decay laws; however, these differ in methodology. The compartmental analysis, sum of exponentials, and the blob model are nowadays the main methods of analysis of fluorescence decays. The compartmental analysis has been extensively used by De Schryver, Boens, Amellot et al. for different systems,19–22 in which these are divided into ground- and excited-state subsystems composed of distinct species which act kinetically in a unique way. The compartmental analysis was the topic of a recent work by Boens and Amellot19 where the concepts of this modelling were revised. The exponential analysis is the classical model of analysis where experimental fluorescence decays are fitted to a sum of exponentials. The obtained decay times and preexponential factors are later on used to determine the value of the excimer association (ka) and dissociation (kd) rate constants.10,13,18 This is probably the most direct and clean method to identify species in a given system. However, in systems where more than two species exist, for example in polymers randomly labelled with fluorescent probes, the analysis of such a system becomes much more complex and sometimes impossible through the derivatization of analytical expressions. Poly(acrylic acid) (PAA) randomly labelled with naphthalene (Np)13,18 and pyrene (Py)10,17 (Fig. 1b) have showed to present different excited state behavior depending on external conditions such as the pH, temperature and solvent. In aqueous solution, its behavior is ruled out by the balance between the hydrophobic and electrostatic interactions. In view of these competing processes, the polymer chain can undergo severe conformational changes. The presence of excited-state (as well as ground-state) dimers in addition to monomer emission, due to locally excited probe, gives evidence for hydrophobic association between chromophores. This association was found to become much less important at higher pH due to the electrostatic repulsion between different chain segments. However, it was noted that even at high pH there is a significant self-association. For the naphthalene labelled polymer, from time-resolved fluorescence data, three kinetically coupled species were found and attributed to the MAGRE monomers, free monomers and one excimer (formed either by a dynamic and/or static mechanism). For the pyrene labelled-PAA instead of one excimer, two excimers, with two different conformations (twisted and parallel), were observed. For the Np labelled-PAA the apparent and the intrinsic activation energies (for excimer formation) were determined from Stevens-Ban plots and from time-resolved fluorescence data, respectively.13 A comparison between the two values revealed that the intrinsic is always smaller than the Photochemistry, 2009, 37, 44–71 | 47 This journal is

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apparent activation energy, which shows the importance of the contribution of ground-state association to the activation energy for excimer formation. When a good solvent (for the chromophore groups), such as dioxane or methanol, is gradually introduced in the system, a decrease of the chromophore–chromophore interactions was observed, which was shown to be consistent with the presence of two types of monomers and one excimer. Both monomers were found to be able to form excimers in the excited state (two MAGRE monomers): one involving the movement of long distance chromophores and the other involving a local reorientation of nearby chromophores.18 In pure organic solvents (methanol and dioxane), one MAGRE, one excimer (more stable) and free monomers were observed and with this somehow less complex kinetic scheme, the determination of all the rate constants in these polymers was achieved.10,18 The effect of the size of the polymer backbone was investigated and showed to influence the photophysics of PAA polymers.17 For low molecular weight polymers (Mn = 2000 g/mol), a number of peculiar effects were observed which were attributed to the adoption of ‘‘micelle-like’’ conformation, at intermediate pH values, where the PAA chain surrounds the hydrophobic core formed by the pyrene groups to shield it from water; these structures promote the occurrence of GSD. At higher pH values, the higher electrostatic repulsion expands the polymer chain, as happens with the long PAA chain polymers.17 Duhamel and co-workers15,16,23–27 have developed an alternative model to interpret excimer formation kinetics with different kinds of pyrenelabelled polymers, based on the fact that excimer formation can be described by a distribution of rate constants. This distribution is due to the different distances between the interacting chromophores, attached to the polymer backbone, and since cyclization rate constants depend on the chain length (between the chromophores), different rate constants could be obtained. The model divides the polymer coil into several blobs and is known as the ‘‘fluorescent blob model’’ (FBM).16,24,25 The FBM was applied to four different randomly labelled polystyrene polymers with different pyrene derivatives (see for example Fig. 1c), and it was concluded that the more efficient excimer formation was achieved with longer and more flexible linkers.25 Additionally, it was also found that different grafting procedures lead to different distributions of pyrene groups along the polymer chain. The FBM was also applied to two different poly(2-vinylpyridine) polymers, i.e., mid- and end-tagged anthracene-labelled poly(2-vinylpyridine),26 which was previously investigated by Clements et al.28 and whose behaviour showed to be dependent on the degree of ionization (a). By increasing a, the intrapolymer fluorescence quenching becomes efficient due to the electron transfer from the excited anthracene to neighbouring pyridinium units. Similar results were obtained with the FBM, in which the quenchers randomly distribute themselves within the blobs.26 This method has been successfully applied to the study of pyrene-labelled polystyrene,25,29 and poly(dimethylacrylamide)27,30 polymers. A study on a series of double pyrene labelled end-capped polystyrene [PS(X)Py2, where X is the polymer molecular weight] and pyrene randomly labelled polystyrene 48 | Photochemistry, 2009, 37, 44–71 This journal is

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Fig. 2 Molecular structure of a new pyrene-based fluorescence probe.37

using steady-state and time-resolved techniques, where the nature of the labelling and of the linker connecting the pyrene group to the polystyrene chain has been taken into account, was reported.29 It was observed that the random labelling of the polymer backbone generates excimer formation with higher efficiency and with a higher value for the association rate constant, than in the case of end-capped polymers, due to the formation of pyrene rich-domains in the former case. On the other hand, the flexibility and stiffness of the linker also showed to be of relevance; short and stiff linkers are associated with slower rate constants than longer and flexible ones. However, in both cases, from the normalization of the obtained IE/IM ratio and of the cyclization rate constants, taking into account the different pyrene content in the polymer, it was found that, independently of the labelling degree, nature of the linker and of the labelling, they merge into a single value and have the same tendencies of variation with the solvent viscosity.29 These observations lead to an important conclusion: the long range polymer chain dynamics depend only on the polymer backbone. The use of randomly labelled polymers seems therefore an advantage for the study of long-range interactions, since they are easier to prepare (than the monodisperse end-capped polymers) and the encounters between the chromophores occur more frequently.29 The FBM was also used to the determination of the radius of the polymer chain ratio through the number of ground-state pyrene groups per chain, the mean number of ground-state pyrene groups in each blob and the excimer association rate constant within a blob.27 The obtained values were further compared with the hydrodynamic radius obtained through dynamic light scattering experiments, where a fairly good agreement between the values was found.27 Moreover, the exponential and the blob models were used to investigate the effect of the addition of SDS on the intramolecular interactions within a HASE copolymer labelled with pyrene.16 Both the methodologies of analysis yielded identical results, leading to the conclusion that the obtained rate constants seem independent of the model of choice.16

1.1.2 Hydrophobically modified polymers with fluorescent probes: trends and developments. Hydrophobically modified polymers (HMP) have important applications for surface modification, structuring and, in particular, rheology control. The rheological behavior of hydrophobically modified water soluble polymers is usually followed for polymers in which Photochemistry, 2009, 37, 44–71 | 49 This journal is

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Fig. 3 Tetrakis(1-pyrenyl)-substituted rigid hinge-like molecule (2); pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer (3)–(7); 1,4-di(1-pyrenyl)butadiyne (8); para-alkynylpyrene oligomers (n = 1–4) (9).

alkyl chains are incorporated into the water soluble backbone. In order to validate and correlate the information obtained at the molecular level with the macroscopic viscoelastic properties found for these kind of polymers, pyrene groups were incorporated onto the polymer chain instead of the alkyl chains.31 It was observed that pyrene confers the necessary hydrophobic properties for associative thickeners, i.e., a large shear thinning effect was observed. Complementary information about HMP in solution in different concentration regimes can be found using fluorescence and rheological techniques. Polymer-surfactant systems constitute the basis of several technological formulations, such as detergents and cosmetics. The understanding of the mechanisms involved in the polymer-surfactant complexation mechanism is still an important area of research. Haldar et al. have studied the interaction between pyrene-end capped poly (ethylene oxide) and surfactants using the IE/IM and I1/I3 ratios as monitoring tools.32,33 Different behaviors were found depending on the charge of the surfactant head group (non-ionic, cationic and anionic).32 In the case of the non-ionic (TX-100) and cationic (CTAC) surfactants a cooperative interaction was observed. However, in the former case the complex formation is accompanied by a decrease of the excimer contribution and of a decrease of the polarity felt by the pyrene groups. In the latter, a different behavior was found; initially the excimer formation is promoted (accompanied by an increase of the I1/I3 ratio) and up to a given concentration it decreases. The formation of charged aggregates in the vicinity of the polymer leads to an expansion of the polymer chain and the end-to-end cyclization is precluded. In the case of the anionic surfactant (SDS) the same IE/IM vs. surfactant concentration was attained. Nevertheless, specific interactions between the SDS and the micelle 50 | Photochemistry, 2009, 37, 44–71 This journal is

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core have been found, i.e., some parts of the PEO chain are incorporated into the micelle. This was further confirmed through an increase of the polarity felt by the pyrene groups and thought isothermal titration calorimety.33 The micellization process of sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) and sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied in the presence of hydrophobically modified poly(acrylamide), and it was concluded that a stronger binding is attained with the SBPBS surfactant rather than it is with AOT.34 The interaction between poly(maleic acid/octyl vinyl ether) (PMAOVE) with SDS, i.e., the interaction occurring between a negatively charged polymer and a surfactant, was studied by Deo et al.35 using viscosimetry, pyrene solubilisation, light scattering and analytical ultracentrifugation. The results obtained with the different techniques were all in favour of the formation of mixed micelles involving SDS hydrophobic domains, along the polymer chain, and with the disruption of the intramolecular hydrophobic microdomains, formed by the n-octyl chains of the PMAOVE polymer. The amphiphilic properties of poly(ethylene oxide) labelled at one end with pyrene have also been investigated by Siu et al.36 Micelle-like structures with a hydrodynamic radius of 7.2  3.1 nm and an aggregation number of 20  2 were obtained. The pyrene hydrophobic core is surrounded by collapsed poly(ethylene oxide) chains. According to the authors this constitutes the first attempt to characterize aggregates formed by PEO

Fig. 4 Fluorescence sensors 25,27-Bis[N-(1-pyrenylmethyl)aminocarbonyl methoxy]calyx[4]crown-5 in the 1,3-alternate conformation (10); 25,27-Bis[N-(1-pyrenylmethyl)aminocarbonylmethoxy]-26,28-dipropyloxycalyx[4]-arene in the 1,3-alternate conformation (11); 16-(1-Pyrenylmethyl)-1,11-dimethyl-3,6,9,13,19,21,24,27-octaoxa-16-azabicyclo[9.9.7]heptacosane (12); 6-(1-Pyrenylmethyl)-1,11-dimethyl-3,9,12,15,18,20,23-heptaoxa-6-azabicyclo[9.7.6]tetracosane (13); binaphthyl-crown ether bis(pyrene) compound (14), N-pyrenylacetamide-substituted sugar-aza-crown ethers (15); dioxocyclam bearing two N-anthracen-2-yl-2-acetamide units (16).

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chains labelled at one end with pyrene and where fluorescent properties of the micelle (pyrene core) were used to determine the aggregation number. 1.2

Advances in excimer-forming fluorescence sensors

The photophysical properties that make a fluorophore suitable as an effective probe are essentially two: to have a high fluorescence quantum efficiency (large FF) and a negligible overlap between the absorption and fluorescence spectra (large Stokes shift). However, additional characteristics should also be considered. In the case of probing a polymer conformation, long decay times, efficient ability of excimer formation and low spectral superposition between monomer and excimer emissions, are amongst these important features. Pyrene itself, pyrene-3-carboxaldehyde (PCA), sodium pyrene-3-sulfonate, and 1-anilinonaphthalene-sulfonate (ANS) are well established fluorescence probes for hydrophobic environments. However, although these fluorophores showed to be highly sensitive, the synthesis of new sensors with, selective sensing ability, is growing due to its importance in basic and applied chemistry. As mentioned above fluorescence is one of the most powerful methods to probe the influence of external factors, such as solvent, pH and temperature, on a system and to detect small amounts of a given compound, such as metal cations or anions. An example of a new pyrene-based fluorescent probe is compound (1) (Fig. 2) that showed to be successfully employed in the determination of critical micellar concentration (cmc) values, where it was tested with two ionic and four nonionic surfactants.37 The determination of the cmc with this particular probe is based on the IE/IM ratio and not on the I1/I3 ratio, since in aqueous solution it only presents excimer emission (ground state association), with no monomer emission. The gradual addition of surfactant leads to the appearance of monomer emission, indicating that the probe is being solubilised inside the hydrophobic core of the micelle.37 Above the cmc, the IM remains constant. An important class of molecular sensors is based on p–p interactions due to the importance of p-stacking interactions in the DNA base pairs for the stabilization of its double-helices, in proteins, in the stabilization of host–guest complexes, in crystalline structures, etc. Several theoretical studies have been performed in order to understand the strength and the

Fig. 5 Fluorescence chemosensors: 1,4-bis(1-pyrenyl)-2,3-diaza-1,3-butadiene (17), 2,4-diaminoxylopyranoside (18); pyren-1-ylmethyl-[2-(6-{2-[(pyren-1-ylmethylimino)-methyl]phenoxymethyl}pyridin-2-ylmethoxy)-benzylidene]-amine (19); pyrene end-capped polyamine (20).

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relative geometry of the molecules involved.38,39 Aromatic compounds, such as pyrene and naphthalene, exhibit excellent p–p stacking ability which is reflected in their fluorescence emission properties through the excimer emission band. Excimer emission can therefore be employed as a sensor, in which the chain connecting the two pyrene groups and the effect of the substitution are factors of major importance.40 Additionally, sensors based on the excimer-to-monomer ratio are based on a ratiometric measurement which constitutes a further advantage. Fig. 3 shows the chemical structures of compounds exhibiting pyrene-pyrene p-stacking interactions. The tetrakis(1-pyrenyl)-substituted rigid hinge-like molecule (2) (Fig. 3) showed a strong contribution from pre-associated pyrenes which was evidenced by the broad absorption spectra and the large Stokes shift (only excimer-like emission is observed).41 A hindered rotation around the major axis was proposed to be promoted by increasing the temperature. The effect of substitution on pyrene-pyrene interactions was considered by Benniston et al., for pyrene derivatives covalently linked through a 1,3-disubstituted phenylene spacer (3)–(7) (Fig. 3).42 The volume of the blocking groups was found to have a high effect on the fluorescence properties: by increasing the volume, the fluorescence quantum yield increases, the IE/IM ratio decreases, the excimer association rate constant decreases and the free energy of stabilization decreases [except for (4), in Fig. 3].42 These results show that substitution of a hydrogen in (3) leads to a higher intramolecular distance between the pyrene groups and a perturbation of the excimer state. The compound 1,4-di(1-pyrenyl)butadiyne [(8) in Fig. 3], in which the pyrene groups are connected through a stiff and short chain, showed quite different properties from pyrene itself and from pyrene connected through short and flexible chains.43 Its fluorescence emission spectrum shows a small Stokes shift, insensitivity to the polarity of the solvent, and absence of excimer emission (intramolecular excimer formation is precluded). Due to the high extension of the p-conjugation, the two pyrene groups in the molecule behave as a unique entity. Additionally, it possesses a very short decay time (B1.2 ns) when compared to pyrene itself, greatly decreasing the quenching by oxygen. Similar results were obtained for other alkynylpyrene derivatives which were also tested as probes in peptides, proteins, and DNA, an example of which is compound (9) (Fig. 3).44,45 A high degree of p-conjugation was observed for the para- when compared with the meta-linked oligomers.44 As expected, the increase of chain length (n) led to a more flexible structure and, consequently, to an increase in the competition between the radiative (fluorescence) and the radiationless processes, with, nevertheless, high fluorescence quantum yields for all the compounds. The design and development of new fluorescence probes, highly selective and sensitive to metal cations, based on excimer emission, has been a subject of high interest. These sensors mainly consist of bichromophoric compounds connected by a chain. Crown ethers46–52 and polyamines chains53–55 functionalized with pyrene have showed to be promising candidates to be used as ‘‘on-off’’ molecular sensors. Kim et al.46 synthesized a new Photochemistry, 2009, 37, 44–71 | 53 This journal is

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chemosensor with two possible sites of cation binding on the lower rims of a 1,3-alternate calyx[4]crown (10) (see Fig. 4). Hence depending on the metal cation present the excimer formation in acetonitrile can be either promoted or precluded. The two amine oxygens constitute preferential sites for complexation of Pb2+ which results in a quenching of the excimer. On the other hand, with K+ the complexation preferentially occurs with the polyether units, which has a minor effect on the excimer formation ability of (10). When the polyether units are replaced with propyl groups (Fig. 4), compound (11), this becomes selective to Pb2+. A similar pyrene-labelled calix[4]arene compound, sensitive to Fe3+ metal ion in acetonitrile:water [1:4 (v:v)] mixture at pH = 6.1, was also synthesized.52 Although crown ether probes are highly selective, their selectivity showed to be effective only in organic solvents; the strong hydration of these compounds lead to a decrease of their complexation ability.47,56 This disadvantage was overcame by the addition of surfactant to the aqueous solutions of (12) (Fig. 4).47,56 In aqueous solution the monomer fluorescence emission (no excimer is observed) of (12) is highly reduced (due to the quenching of the nitrogen lone pairs) and no complex formation could be observed with Ba2+. The addition of non-ionic surfactant, at concentrations above the cmc, provides the less polar environment required for the complexation, which was detected by a gradual increase of the emission fluorescence intensity with increasing Ba2+ concentration.47 This method showed to be successfully employed in other systems, and its selectivity was found to be dependent on the size of the crown ether, as well as the surfactant used. For instance, compound (13), in aqueous solutions, showed to be highly selective for Ca2+ and Mg2+ in the presence of sodium n-dodecylbenzenesulfonate and tetramethylammonium dodecyl sulfate, respectively.56 Alternatively, Pallavicini et al.48 have used surfactant micelles as containers in aqueous media for assembling of supramolecular sensors, in which both fluorophores and quenchers are kept inside the hydrophobic core of the micelle. They showed its efficiency for the fluorescence sensing of Hg2+, where the on-off sensor was transformed into an off-on fluorescence sensor. A new binaphthyl-crown ether-bis(pyrene), compound49 (14), was synthesised and showed to be highly selective to Cu2+, relatively to the other metal ions tested, leading to a 1:1 (14):Cu2+ complex formation. In crown ether of (14) complexes, the Cu2+ cation promotes the py–py ground-state association, leading to a (rare) blue shifted pyrene excimer emission.

Fig. 6 Functionalized deoxynucleoside with terphenyl (21) and a pyrene derivative (22).

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The single labelled pyrene derivative also showed selectivity to the Cu2+ metal ion being, however, the effects on the fluorescence emission spectrum much weaker. A Cu2+-sensitive fluorescence quenching chemosensor based on sugar-aza-crowns, compound (15), was synthesised by Xie et al.50 This molecular sensor exhibits high stability constants and low detection limits. It is also based on IE/IM ratio and, additionally, on the fluorescence quenching upon complexation with Cu2+. The dioxocyclam chemosensor (16), bearing two N-anthracen-2-yl-2acetamide units, showed to recognize and complex the Cu2+ and Hg2+ metal ions, with detection limits of 1.5  106 and 7.8 106 M, respectively. Comparing the detection limit, to the copper metal cation obtained with (15) (40 nM), it can be observed that this is more sensitive than with (16). Fig. 5 shows other very efficient chemosensors not based on crown ether compounds. Similarly to the chemosensor (16), compound (17) showed to be selective to both Cu2+ and Hg2+ metal cations.57 The 2,3-diaza-1,3butadiene unit is the binding site of these cations. The complex formation is easily visualised from the enhancement of the excimer emission band intensity. Furthermore, the complexes formed between (17) and the metal cations can be easily distinguished through colour change: green and orange fluorescence are obtained for the Hg2+ and Cu2+ complexes, respectively. Yuasa et al.58 designed chemosensor (18) (Fig. 5). They demonstrated the application of a hinge sugar in an excimer fluorescence sensor selective to Zn2+and Cd2+ metal ions. When this compound forms complexes with the metal ion, it leads to the rearrangement of the diequatorial orientation of the arms into a diaxial orientation. More recently, Lodeiro et al.59 synthesized a new probe derived from the attachment of two methylaminopyrene units to the carbonyl precursor 2,6-bis(2-formylphenoxymethyl)-pyridine [(19) in Fig. 5]. Its emission properties were investigated in water:acetonitrile solutions [99.5:0.5 (v:v)] as a function of pH and in the presence of Zn2+, Cu2+ and barbituric acids. The dependence of the IE/IM ratio with the pH showed an off-on-off behavior, due to photoinduced electron transfer. The addition of barbituric acids was shown to strongly reduce excimer formation, whereas, and in contrast to this, the addition of metal ions does not have a significant effect on the emission of this molecular probe. The fluorescence sensor (20) showed to be sensitive to pH due to bending pHdependent behavior of the polyamine chain connecting the two pyrene units.53–55 Besides the ‘‘on-off’’ behavior found in a great majority of the fluorescent pH indicators, these chemosensors showed to be more complex when acetonitrile is present in solution. The pyrene (monomer) emission is independent of the presence of the organic solvent, displaying a typical ‘‘on-off’’ profile; the increase in pH promotes the deprotonation of the nitrogen atoms (of the polyamine chain), leading to electron transfer from the unprotonated nitrogen atoms to the photoexcited pyrene groups.59 However, these sensors are able to form excimer and its emission band displays a much more rich behavior: ‘‘off-on’’, ‘‘off-on-off-on’’ and ‘‘off-on-off’’ profiles are observed depending on the content of acetonitrile in solution. Therefore the excimer emission band can be modulated by slight changes in the solvent, acting as a ‘‘multiply configurable’’ fluorescent pH indicator.53 Photochemistry, 2009, 37, 44–71 | 55 This journal is

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1.3

Biosensors

Fluorescent DNA base analogues are of rising interest which comes from several developments in organic chemistry, physics, engineering, biochemistry, and biology.60 These fluorescent sensors have been used in structural studies of DNA and RNA, and in enzymatic processes involving DNA.60 The fluorescent DNA base replacements show some advantages relatively to other types of labelling; they are less disruptive to local structure and to interactions with other biomolecules, the fluorescent DNA base analogues can be stacked directly, within the double helix, and their orientation and location is known. This last feature is important in terms of DNA repair and replication, and makes these probes very important in the rapid detection of infections and genetic diseases.61 Okamoto et al. reported the synthesis of a novel base-discriminating fluorescence nucleobases and their application to single nucleotide polymorphism typing, using 1-pyrenecarboxymide.61 This chromophore is known to show a strong polarity dependence, due to the change in the properties of the first singlet excited state with increasing solvent polarity. This property makes it a suitable probe to monitor the changes in DNA duplexes. Therefore, the synthesis of new fluorophore-labelled nucleobases, in which a natural DNA base is replaced by a different structure with recognition, structuring and self-assembling properties, in addition to fluorescent properties, constitutes an important and growing field of interest and intensive research. Terphenyl [(21) in Fig. 6], pyrene and terthiophene had been used as fluorophores for these studies.62 Comparison between these fluorescent probes and other commercially available probes, such as fluorescein, showed that the formers have a larger Stokes shift and a higher fluorescence quantum yield, which is a major advantage in terms of detection sensitivity. There are several examples on the literature, on the synthesis of new fluorophore-labelled nucleobases, such as pyren-1-yl-2 0 -deoxyuridine,63 8-(Pyren-1-yl)-20-deoxyguanosine,64 5-(10-methyl-phenothiazin-3-yl)-2 0 65 deoxyuridine, perylene bound to oligo-2 0 -deoxyribonucleotides,66 phenanthridinium-containing DNA,67–691-ethynylpyrene,70 5-(pyren-1-yl)20-deoxyuridine71 and 5-(10-methyl-phenothiazin-3-yl)-20-deoxyuridine.71 Pyrene covalently bound to deoxyadenosine bases showed to induce a self duplex of the oligodeoxyadenylate, associated with a change of its fluorescence properties depending on the relative location of the pyrene groups.72 It can display three different colours: reddish-orange, green, and blue. Dual fluorescence, originating from the equilibrium between locally excited (LE) and intramolecular charge transfer (ICT) states, was used to monitor DNA hybridization,73 by means of the incorporation of a functionalized deoxynucleoside with a pyrene derivative [(22) in Fig. 6] into DNA. This fluorescent nucleoside allowed Okamoto et al.73 to distinguish between single stranded (ss) and double stranded (ds) DNA, by a colour change. In the case of a perfect match between the base pairs, pyrene is located outside the double helix (hydrophilic environment) giving rise to strong fluorescence emission, and the opposite is seen when a mismatched is observed.61 Gao et al.74 reported a library of over 14 000 DNA-analogue sensors of colour-changing which can be used for biochemical and biophysical 56 | Photochemistry, 2009, 37, 44–71 This journal is

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applications. In these compounds the aromatic compounds are stacked in a face-to-face oligonucleotide-like arrangement, on the DNA phosphodiester backbone, which leads to a large Stokes shift. These compounds are light sensitive. An obvious improvement of this library would be the synthesis of other sensors which respond to properties such as temperature and pH, as was suggested by the authors. Fluorescent molecular beacons (MB) are also very useful in the in vitro RNA and DNA monitoring, gene monitoring (in living systems) and in the study of proteins.75 MBs consist of single-stranded oligonucleotides labelled with a fluorophore and a quencher; this means that these sensors are based on an ‘‘on-off’’ behavior. If two pyrene groups are bound at the extremities of these, the ‘‘on-off’’ behavior is measured in terms of the IE/IM ratio. In the absence of the target high excimer emission is attained, whereas in the presence of the target, which can be for instance DNA76 or mRMN,77 a significant decrease of the IE/IM ratio is observed. Chen et al.78 proposed a new strategy to convert MerR-type proteins into fluorescence biosensors. This approach joins the high sensitivity of fluorescence techniques with the high selectivity of the MerR proteins. MerR proteins are known to be highly selective to metal cations in the picomolar– femtomolar concentration range. Furthermore, they are stable dimers in solution which bind to specific sequences of DNA without distorting its double-helix in the absence of the metal ion. In the presence of the metal, the MerR proteins lead to the disruption of the base pairs. This disruption can be probed by fluorescence. In the work by Chen et al.,78 pyrrolo-C was used as a fluorescence probe and placed into the middle of the protein-binding sequence. Its emission is quenched by the DNA double-helix and is highly enhanced when complexation of the protein with the metal ion occurs. Different selectivity towards metal ions was found for different MerR family proteins: MerR for Hg2+, CueR for Cu+ and PbrR for Pb2+. The same authors also reported the use of pyrene excimer emission as a way for studying protein–DNA interactions.79 In this case the two pyrene groups are placed in the DNA sequence recognized by the MerR protein. In the presence of the metal ion the excimer emission is replaced by a blue-shifted vibronically resolved monomer band. This biosensor showed a high selectivity to Hg2+, against Pb2+, Cd2+, Co2+ and Zn2+, where no signal was obtained. Biosensors, based on excimer emission, can work as ratiometric biosensors, with the advantage of using an internal correction promoted by slightly different experimental conditions, e.g., concentration, pH, temperature, etc. Excimer emission has also been used, for instance, in the study of the protein dynamics, such as to distinguish between the bovine serum albumin and human serum albumin using pyrene-end capped poly(ethylene oxide),80 to study the conformation of actin labelled with N-(1-pyrene)iodoacetamide,81 and to determine the platelet-derived growth factor.82 Two final examples of developments in sensors for relevant properties of biological interest, temperature and water, based on luminescence, will be shortly described. Pinheiro et al. reported a ruthenium complex, [Ru(bpy)(CN)42], deriving from the ability of the nitrile goup to participate in H-bond interactions, that was explored to develop a sensor for low Photochemistry, 2009, 37, 44–71 | 57 This journal is

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concentrations of water on common organic aprotic solvents and found to respond linearly to water concentrations in the range 5–70 ppm.83 The sensitivity of this sensor opens new possibilities to investigate particular environments where water is found in very low concentrations. The report of a sensitive and broad-ranged optical thermometer based on the thermally activated delayed fluorescence of fullerene C70, with a working range from 80 to at least 140 degrees Celsius, was observed for C70 molecularly dispersed in a polymer film.84

2.

The nature of excited states in conjugated polymers

Conjugated organic polymers have, in the past recent years, been devoted to extensive interest namely on understanding the nature of its excited states in solution and in the solid state. This knowledge is of great interest, especially for development and fabrication of electroluminescent and optical devices. Many of the applications involve electroluminescence whereby electrons and holes are introduced into films of the polymers by charge injection from appropriate electrodes. Charge recombination then leads to excited state formation and light emission. In the molecular exciton picture85 the ground state is normally a singlet state and the excited singlet and triplet states are formed by electron–hole recombination. From this point of view the electron and hole have strong interaction and for charge separation additional energy is needed. The exciton binding energy can be correlated to the energy difference between the optical transition and the energy of independent electron and hole in the polymer.86 The latter can be defined as the difference between the ionization potential and the electron affinity which can be evaluated by the HOMO and LUMO energies. The binding energy of an optical excitation is a key parameter for the understanding of the opto-electronic properties of organic solids in general and in conjugated polymers in particular. It determines the energetics of the dissociation of an excitation of either singlet or triplet character into a pair of free charges and the reverse process, that is, the recombination of an electron–hole pair yielding an excitation which can decay radiatively or nonradiatively. If the optical transition energy is large, photogeneration of charge carriers is an endothermic, that is, inefficient process.85 Obviously, in a photovoltaic device, the optical transition energy should be as small as possible, whereas in a light emitting diode it is the opposite. Knowledge of the S0–T1 energy gap [and triplet energy (ET)] of p-conjugated oligomers and polymers is also of relevance because ET in combination with the S1 ’ S0 transition provides the triplet exciton binding energy (an upper limit to the singlet exciton binding energy). In general ET can be calculated from the emission from the triplet state, that is from its phosphorescence spectra. In the absence of phosphorescence the triplet state energy can be estimated by triplet quenching studies. It is generally held that triplet state formation represents one of the major factors for energy loss in polymer-based light emitting devices (LEDs). Long lived triplet states may also act as traps, reducing the concentration of emitting species and in nonencapsulated systems they may also sensitize formation of singlet 58 | Photochemistry, 2009, 37, 44–71 This journal is

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oxygen, which can react with conjugated polymer chains and ultimately degrade them and limit the performance and lifetime of the device. In contrast to these negative aspects of triplet state formation, interest has been focused on the possibility of using this triplet energy in electrophosphorescent devices (PHOLEDs).87,88 PHOLEDs are based on phosphorescence from organometallic complexes (often iridium or platinum complexes)89,90 in which spin-orbit coupling allows efficient radiative decay of the triplet. In some of these devices the emission can come from both the singlet and triplet states, significantly increasing the efficiency.91 In addition to excited-state formation in the charge recombination process, direct photoexcitation of p-conjugated polymers produces the singlet excited state as the primary species. From the excited state a variety of radiative and non-radiative photophysical processes may emerge such as emission, intersystem crossing and oxidation or reduction. For optical and electrical excitation the intersystem crossing process, between singlet and triplet manifolds, can be a significant non-radiative decay mechanism limiting the overall fluorescence efficiency. Therefore, a detailed understanding of the photophysical processes involved in the excited-state deactivation of p-conjugated polymers and oligomers is necessary for the development of devices based on these materials. In the past recent years (2004–2007) several works, focused on the determination of photophysical parameters of several organic conjugated polymers and oligomers, were carried on. As emerging field it is, a comprehensive description of these works is out of the scope of this contribution. Instead we will report some of the relevant findings with respect to photophysical characteristics of oligomers and polymers, applications of these and the relevance of investigating oligomers in the understanding of a polymer behavior. Photophysics of conjugated polymers and oligomers. The nature and photophysics of triplet states in poly(3-dodecylthiophene) were characterized using, among other techniques, photothermal beam deflection, photoacoustic calorimetry and photoluminescence spectroscopy and the results suggested that internal conversion plays a major role in the excited state deactivation.92 The photophysical and spectroscopic properties of cruciform oligothiophenes, were investigated in solution (293 K and 77 K) and in the solid state.93 The study focused on absorption, emission, and triplet–triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allowed the determination of the rate constants for all decay processes. It was found that, in solution, the main deactivation channels for the compounds are the radiationless processes (internal conversion and intersystem crossing). In the solid state it was seen that, in general, the fluorescence quantum yields decreased relative to solution. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (SD E 1) provided support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds. Photochemistry, 2009, 37, 44–71 | 59 This journal is

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King et al.94 reported the measurement of the quantum yield of triplet formation for the prototypical conjugated polymer polyspirobifluorene (in solution and solid state) using an updated method based on femtosecond time-resolved ground state recovery following photoexcitation of the polymer. The fast decay of photoexcited singlet excitons results in a rapid recovery of the ground state absorption and the considerably longer lifetime of the triplet excited state results in an additional slow recovery of the absorption. When viewed in a double logarithmic presentation, these manifest themselves in a peak and plateau in the ground state recovery kinetics; the peak represents the total number of excited states formed and the plateau, after the fast recovery from the decay of the singlet, represents the number of triplet excited states formed. The ratio of the two was used to calculate the triplet yield of the polyspirobifluorene (fT = 0.05  0.01) in solution. In polymer films a high triplet yield value (fT = 0.12  0.02) was obtained and found to be temperature independent; the higher fT value, relative to solution, was attributed to triplet recombination from charged states.94 The photophysical and electroluminescent properties of the 4,7-di-2thienyl-2,1,3-benzothiadiazole (DBT) dye blended into blue-emitting polyfluorene as a host were compared with that from polyfluorene-co-4,7di-2-thienyl-2,1,3-benzothiadiazole (PFO-DBT) copolymer of the same molar composition, with DBT incorporated covalently into the polyfluorene backbone.95 The role of intra- and interpolymer chain interaction and energy transfer in the polymer light-emitting devices were discussed. On the basis of the direct comparison of blend and copolymer from the same molecular composition, it was suggested that, in conjugated polymers, intrachain energy transfer, along the polymer backbone, is more efficient than that via interchain interaction and plays a more important role in polymer light-emitting devices. Deactivation of the first singlet excited state in conjugated polymers and oligomers: energy transfer and conformational relaxation. Sundstro¨m et al.86 focused on light-polymer interactions and discussed the processes initiated by light absorption in a conjugated polymeric material, from the formation of the initially excited state to conversion of the light energy into more longlived excitations. They considered that light absorption is best described within a molecular picture than the semiconductor band picture and that the major primary photoexcitation, in conjugated polymers, is a neutral strongly bound exciton. Following the formation of an exciton, by light absorption, this can migrate along the polymer chain on which it was formed. If many polymer chains are in close contact, like in a solid state film of a neat conjugated polymer, the exciton may as well jump to a neighbouring chain. Both intra- and interchain transfer are efficient and proceed on a sub-picosecond to tens (and hundreds) of picoseconds time scale. However, recent reports on PPV oligomers have lead to the conclusion that fast components on the fluorescence decays should account for the occurrence of conformational relaxation, concurrently with energy transfer.96,97 A comprehensive photophysical and spectroscopic study of p-phenylenevinylene oligomers (PPV-trimers) possessing different alkyl and alkyloxy side chain substituents and different end groups (aldehyde, CQC, 60 | Photochemistry, 2009, 37, 44–71 This journal is

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phenylene and anthracene units) was described.97 With one of the alkyloxy derivatives, a more detailed study at the onset of the fluorescence decay was made, and it was found that upon excitation a fast conformational relaxation process of the initially excited oligomer occurs, leading to a more planar conjugation segment. A more detailed study was performed by Di Paolo et al.96 showing that for PPV derivatives, the fast decay component should be assigned/interpreted to account for the occurrence of conformational relaxation, concurrently with energy transfer. 2.1

Applications

Organic semiconducting polymers are currently of broad interest as potential low-cost materials for photovoltaic and light-emitting display applications.98–103 Below a brief overview of the developments in this area is given. 2.1.1 Organic light emitting devices (OLEDs). When an electric current passes through some conducting polymers light is emitted. In its simple form, an OLED consists of a layer of the luminescent material placed between two electrodes (Fig. 7). When an electric current is applied, light is emitted with a colour that depends on the particular material used. At the anode holes are created while at the cathode, electrons are added to the material (charge injection). The conducting properties of the polymer allow the holes and electrons to migrate together (charge transport) and when they meet a localized excited state, of the polymer, is created (charge recombination), analogous to the state created upon absorption of a photon. The excited state energy is released in the form of fluorescence (electroluminescence), thereby producing light. Exciton formation due to electrical excitation is governed by spin statistics which leads to a ratio of 25:75 for singlet/triplet excitons. Although the issue of singlet/triplet formation ratio is still a matter under debate, since for example 50% singlet generation fractions have been reported.89,104,105 Both experimental and theoretical work showed that, as the chain length increases (monomers vs. polymers), a significant difference in the singlet generation fraction appears.104 It was found that a different spin-dependent

Fig. 7 Schematic drawing of a single-layer electroluminescence device.

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mechanism appear to be at work in polymers, which favours singlet formation so that the 25% limit does not apply, whereas for small molecules the singlet fraction remains limited to 25%.89 In most conjugated organic systems the lack of heavy atoms in the molecular structure dictates that only singlet excitons can decay radiatively (fluorescence) while phosphorescence from longer-lived triplet states is highly improbable. Displays based on organic polymers are already in use and also prototypes of displays that are highly flexible, able to bend and twist to a considerable degree have also been prepared.104 Recent developments in the field of light emitting devices. Jenekhe et al.106 reported the photophysics and highly efficient blue electroluminescence of a series of four new n-type conjugated oligomers, 6,60 -bis(2,4-diphenylquinoline) (BIPPQ), 6,6 0 -bis(2-(4-tert-butylphenyl)-4-phenylquinoline) (BtBPQ), 6,6 0 bis(2-p-biphenyl)-4-phenylquinoline) (B2PPQ), and 6,6 0 -bis((3,5-diphenylbenzene)-4-phenylquinoline) (BDBPQ). They emit blue photoluminescence with high (0.73–0.94) fluorescence quantum yields and 1.06–1.42 ns lifetimes in chloroform solutions. A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene–fluorenone– fluorene trimer model compound have been described.107 The photophysics, and origin of the green emission, and electroluminescence of this class of light-emitting materials was investigated. The new oligofluorene trimer, with a central fluorenone moiety, was found to be an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, the controversial 535 nm green emission band was suggested to originate from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers.107 In other works, in which copolymers including fluorene and fluorenone defects were included, the green emission was clearly proved to be due to fluorenone emission.108,109 Bright green electroluminescence (EL) centred at 535 nm was achieved from single-layer copolymer light-emitting diodes (LEDs), ITO/PEDOT/copolymer/Al, with luminances of 1600–3340 cd m2 that varied with fluorenone content.107 The EL data suggest that the fluorene– fluorenone copolymers are very promising materials for green LEDs. Two kinds of fluorene-acceptor copolymers containing three emitting segments were investigated as the emissive layer in white-light-emitting devices (WLEDs).110 The light-emitting copolymers PFQTP and PFBTTP either contain blue-emitting (9,9-dihexyl-fluorene, F), green-emitting (quinoxaline, Q) or yellow-emitting (2,1,3-benzothiadiazole, BT), and red-emitting (thieno[3,4-b]-pyrazine, TP) units in the backbone. The experimental results suggest that only a relatively small fraction of the acceptor moiety incorporated into the fluorene can achieve white light emission through energy transfer. The energy transfer from BT to TP is more efficient than that of Q to TP because of the degree of spectroscopic overlap between absorption and luminescence. The electroluminescent device with PFBTTP1 (0.1% of BT and 0.25% of TP) as an emissive layer exhibits a luminance of 1870 cd m2 under the condition of maximum 62 | Photochemistry, 2009, 37, 44–71 This journal is

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luminance yield of 1.92 cd A1 and without significant variation with driving voltages. The study suggests that the single polymers based on three fluorene-acceptor segments could be potentially used for pure-white-lightemitting diodes. Jung et al.111 reported electroluminescent polymers (poly(9,9-dioctylfluorene-co-thieno[3,2-b]thiophene-co-benzo[2,3,5]thiadiazole) (P1) and poly(9,9 0 -dioctylfluorene-co-thieno[3, 2-b]thiophene -co -benzo[2,3,5]thiadiazole-co-[4-(2-ethylhexyloxyl)phenyl] diphenylamine (P2)) possessing holetransporting or electron-transporting units or both in the main chains. Electron-deficient benzothiadiazole and electron-rich triphenylamine moieties were incorporated into the polymer backbone to improve the electron-transporting and hole-transporting characteristics, respectively. Owing to the incorporation of the electron-deficient benzothiadiazole unit, P1 and P2 exhibit remarkably lower LUMO levels than PFTT and thus, it should facilitate the electron injection into the polymer layer from the cathode electrode. Consequently, because of the balance of charge mobility, LED devices based on P1 and P2 exhibit greater brightness and efficiency (up to 3000 cd m2 and 1.35 cd A1) than devices that use the pristine PFTT.

2.1.2 Organic photovoltaic devices (OPV). The demand for alternative energy resources stimulated the investigation of new photovoltaic devices with increased energy harvesting and conversion efficiencies has potential sources of renewable electrical energy.112 Solar cells based in solution processable organic conjugated polymers have the advantage of low-cost fabrication in large areas and low weight on flexible substrates.99,101,113 Furthermore, organic semiconductors may display high absorption coefficients exceeding 105 cm1, which makes them good chromophores for optolectronic applications.101 The overall power conversion efficiency of organic solar cells depends on many factors which, among others, are photon absorption, charge carrier photogeneration, separation and transport, that are intrinsic properties of the active material. Recently, power conversion efficiencies of E5% were reported for solar cells based on bulk heterojunctions consisting of poly(3-hexylthiophene) (P3HT) as donor material and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor.114–116 Nevertheless, for commercial purposes solar cells with an efficiency exceeding 10% are required.113,117 Blending conjugated polymers with high-electron-affinity molecules like fullerene (C60) (as in the bulk-heterojunction solar cell) has proven to be an efficient way for rapid exciton dissociation. Conjugated polymer-C60 interpenetrating networks exhibit ultrafast charge transfer (o40 fs).118 As there is no competing decay process for the optically excited electron–hole pair located on the polymer in this time regime, an optimized mixture with C60 was found to convert absorbed photons to electrons with an efficiency close to 100%.119 Efforts have been made for designing better materials to improve coverage of the solar spectrum and optimizing charge transport properties of the polymeric materials.98,114,120 The use of low-bandgap polymers improves Photochemistry, 2009, 37, 44–71 | 63 This journal is

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the spectral overlap between the polymer absorption and the solar irradiance spectrum by extending the absorption to longer wavelengths, and is therefore a promising route toward increased light harvesting and higher power conversion efficiency of polymer photovoltaics.98,99,121 However, although several organic polymers with lower band gaps than P3HT have been reported, the achieved power conversion efficiency was typically lower when compared to the best achieved results for P3HT based cells. Typically, the efficiency of low band gap polymer based solar cells is in the range B2.2–3.2%,122–125 while efficiencies exceeding 4–5% have been only recently reported for a poly(3-hexylthiophene)/fullerene derivative.126,127 It should be noted that most of the efforts in developing new materials are focused on novel donor materials and in establishing what properties the donor material should have in order to result in efficient solar cells based on a bulk heterojunction with PCBM.118 An alternative method for improving device performance could be placed on changing the acceptor material, since it has been proposed that lowering of the LUMO level of the acceptor would be more beneficial to the cell performance than lowering the band gap of the polymer.117 However, although efficient cells with other acceptor molecules have been demonstrated,113,128–130 the majority of polymer solar cells are based on a PCBM acceptor. Nevertheless, polyfluorene based polymer yielding similar efficiency in bulk heterojunctions with P3HT and PCBM were reported128 showing that significant improvements in the device performance of all-polymer solar cells can be expected through improved device architecture design and by tuning the energy levels and absorption profile of conjugated polymers.113 Extensive studies of polymer based photovoltaic cells have been largely focused on blends or nanocomposites of donor polymer with acceptor materials including fullerenes, CdSe nanocrystals, TiO2 nanoparticles, carbon nanotubes, acceptor polymers and acceptor small molecules.113,129 Similar architectures to LEDs are used for fabrication of photovoltaic devices, although opposite underlying operation mechanisms are observed for these devices. While in LEDs the main focus is charge recombination, in PV devices the final research target is charge separation. Here the goal is to convert the optically created exciton into free charge carriers. The simplest device structure is a layer of organic material sandwiched between two different conducting contacts, typically indium tin oxide (ITO) and a low work function metal such as Al, Ca or Mg. However, very low conversion efficiencies were observed for this type of devices113 due to the strong tendency of photogenerated electrons and holes to recombine in conjugated polymers. In order to improve device performance, conjugated polymers should be combined with electron donor–acceptor groups.113 The observed high performance of that bilayer heterojunction is attributed to the efficient charge generation at the interface between the two organic materials with different electron affinity. In bulk heterojunction devices, two organic conjugated materials with electron–acceptor (A) and electron–donor (D) properties are sandwiched between the transparent cathode and the anode (Fig. 8). On light absorption electron–hole pairs (excitons) are generated in the donor layer (1). Within the exciton lifetime and finite diffusion length the excitons can reach the donor–acceptor interface (2) where charge 64 | Photochemistry, 2009, 37, 44–71 This journal is

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Fig. 8 (a) Schematic representation of a bulk heterojunction photovoltaic device. To ensure that all photogenerated excitons reach a donor–acceptor interface the heterojunction formed between the two materials has to be scaled down to the nanometer level to form an archichecture that is referred to as bulk heterojunction. The advantage of the bulk heterojunction is that it can be formed by simply mixing the donor–acceptor materials in a common solvent and cast with well-known solution deposition techniques. The deposited donor– acceptor layer consists of a bicontinous interpenetrating network of the two materials that enable both efficient charge generation and extraction of the charges to the electrodes. The bulk heterojunction can be regarded as an ensemble of nanoscale heterojunctions distributed all over the volume forming a bicontinuous network; (b) diagram of the photoinduced electron transfer (ET) occurring in the donor–acceptor interface.

separation occurs (3). There, exciton dissociation results in electron transfer to the acceptor layer and the hole remaining on the donor (Fig. 8b). Finally, the separated charges, which overcome their mutual Coulomb field, move away from the interface and are collected selectively at the respective electrodes (4).

2.2

What can oligomers tell us about the behavior of conjugated polymers?

Well-defined oligomers work as model compounds in studying the mechanisms involved in photoexcitation of conjugated polymers and in determining the nature and the decay of their excited states. The main advantage of investigating the nature, and temporal evolution of photoexcitations, of conjugated oligomers is their well-defined molecular structure and unique chain length that allow structure-property relationships to be determined with great detail. Moreover, high-level quantum chemical calculations, which cannot be performed in polymeric systems, are often amenable with oligomers, providing additional insights that cannot be obtained from experiment alone. As a material, conjugated oligomers form molecular solids or crystals. Upon going from an isolated molecule to a macroscopic material, optical properties will change as a result of the intermolecular interactions that appear in the solid. In this view, the ultimate goal is to understand, and control, the route from a molecule to a material. Such an insight is likely to yield the intrinsic limits of conjugated materials and will be of importance in designing new conjugated polymers. By using oligomers, with varying number of monomer units, it is possible to evaluate the effect of increasing conjugation length and to estimate properties for the more complex polymer. This was shown by Skene et al.131,132 where the effect of the number of thiophene and azomethines bonds on the photophysics of novel conjugated azomethines (consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared Photochemistry, 2009, 37, 44–71 | 65 This journal is

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from a stable diaminothiophene) was studied. A high degree of conjugation was obtained and doping with methanesulphonic acid resulted in lowering of the HOMO–LUMO energy gaps to 1.3 eV. Increasing the degree of conjugation was found to shift the nonradiative mode of singlet excited state energy deactivation from internal conversion to intersystem crossing.131,132 Wegner et al.133 studied oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center (OFnK: n = 3, 5, or 7) as models for understanding the origin of the low-energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. The X-ray analysis of the OF5K structure revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit; the packing pattern precludes formation of excimers. The photophysics of OFnK in solution and thin films suggested efficient tunnelling of excitation energy from the photoexcited fluorene segments to the low-energy fluorenone sites by both intra- and intermolecular hopping events, whereby they give rise to green emission.

3

Conclusions

In this contribution the recent developments of two kinds of polymers and oligomers were briefly reviewed: organic conjugated and water soluble polymers. With the water soluble polymers and oligomers most of its potential applications are related with its use as fluorescence sensors for cations, DNA, etc. The incorporation of pyrene into water soluble polymers allows following the dynamics of these in a molecular scale. The complex excited state kinetics that is found with polymers leads to different approaches and methodologies of analysis. After the 1980’s and mid 1990’s when Birks’ kinetics was found to be largely inapplicable to polymers with fluorescent excimer-forming probes (pyrene, naphthalene, carbazole, etc.) and other approaches had to be considered, little attention was paid to the development of alternative models for interpretation of complex decays. In recent years the exponential method (with analytical solution for systems with 3–4 species), the fluorescence blob model, the stretched exponential method, etc., have been used to model and interpret the kinetics of these HMP but also of organic conjugated polymers. It is anticipated that in the next years new polymers and new findings, as a result of the application of these and other models, will be presented. A great deal of work has been carried out in recent years related to the investigation of photophysical processes in organic conjugated polymers and oligomers. The development of new materials for lightning or energy harvesting applications requires the intrinsic knowledge of the photophysical properties shown by these materials and optimization of those properties regarding the desired future application. However, the design of efficient devices requires highly interdisciplinary research between macromolecular chemistry, supramolecular chemistry, physical-chemistry, colloid chemistry, photophysics/photochemistry, device physics, nanostructural analysis, and thin film technology. In this field there are great challenges 66 | Photochemistry, 2009, 37, 44–71 This journal is

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91 H. Christian-Pandya, S. Vaidyanathan and M. Galvin, in Handbook of Conducting Polymers, ed. T. A. Skotheim and J. R. Reynolds, CRC Press Taylor & Francis Group, Boca Raton, editon edn, 2007. 92 Y. F. Huang, H. L. Chen, J. W. Ting, C. S. Liao, R. W. Larsen and W. Fann, J. Phys. Chem. B, 2004, 108, 9619. 93 J. Pina, J. Seixas de Melo, H. D. Burrows, A. Bilge, T. Farrell, M. Forster and U. Scherf, J. Phys. Chem. B, 2006, 110, 15100. 94 S. M. King, C. Rothe, D. Dai and A. P. Monkman, J. Chem. Phys., 2006, 124, 234903. 95 S. W. Fan, M. L. Sun, Z. Chen, J. Luo, Q. Hou, J. B. Peng, H. Yang, D. Q. Zhang, F. Y. Li and Y. Cao, J. Phys. Chem. B, 2007, 111, 6113. 96 R. E. Di Paolo, J. Seixas de Melo, J. Pina, H. D. Burrows, J. Morgado and A. L. Mac¸anita, ChemPhysChem, 2007, 8, 2657. 97 J. Seixas de Melo, J. Pina, H. D. Burrows, R. E. Di Paolo and A. L. Mac¸anita, Chem. Phys., 2006, 330, 449. 98 E. Bundgaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2007, 91, 954. 99 R. Koeppe, O. Bossart, G. Calzaferri and N. S. Sariciftci, Sol. Energy Mater. Sol. Cells, 2007, 91, 986. 100 F. So, B. Krummacher, M. K. Mathai, D. Poplavskyy, S. A. Choulis and V. E. Choong, J. Appl. Phys., 2007, 102, 091101. 101 S. Gunes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107, 1324. 102 S. P. Price, J. Henzie and T. W. Odom, Small, 2007, 3, 372. 103 W. Y. Wong, G. J. Zhou, X. M. Yu, H. S. Kwok and Z. Y. Lin, Adv. Funct. Mater., 2007, 17, 315. 104 J. L. Bredas, D. Beljonne, V. Coropceanu and J. Cornil, Chem. Rev., 2004, 104, 4971. 105 N. Koch, ChemPhysChem, 2007, 8, 1438. 106 C. J. Tonzola, A. P. Kulkarni, A. P. Gifford, W. Kaminsky and S. A. Jenekhe, Adv. Funct. Mater., 2007, 17, 863. 107 A. P. Kulkarni, X. X. Kong and S. A. Jenekhe, J. Phys. Chem. B, 2004, 108, 8689. 108 F. B. Dias, M. Knaapila, A. P. Monkman and H. D. Burrows, Macromolecules, 2006, 39, 1598. 109 F. B. Dias, M. Maiti, S. I. Hintschich and A. P. Monkman, J. Chem. Phys., 2005, 122, 054904. 110 W. C. Wu, W. Y. Lee and W. C. Chen, Macromol. Chem. Phys., 2006, 207, 1131. 111 E. Lim, B. J. Jung and H. K. Shim, J. Polym. Sci. Pol. Chem., 2006, 44, 243. 112 K. W. J. Barnham, M. Mazzer and B. Clive, Nat. Mater., 2006, 5, 161. 113 K. M. Coakley and M. D. McGehee, Chem. Mat., 2004, 16, 4533. 114 J. Y. Kim, S. H. Kim, H. H. Lee, K. Lee, W. L. Ma, X. Gong and A. J. Heeger, Adv. Mater., 2006, 18, 572. 115 W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617. 116 M. Reyes-Reyes, K. Kim and D. L. Carroll, Appl. Phys. Lett., 2005, 87, 083506. 117 L. J. A. Koster, V. D. Mihailetchi and P. W. M. Blom, Appl. Phys. Lett., 2006, 88, 093511. 118 M. C. Scharber, D. Wuhlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger and C. L. Brabec, Adv. Mater., 2006, 18, 789. 119 P. Schilinsky, C. Waldauf and C. J. Brabec, Appl. Phys. Lett., 2002, 81, 3885. 120 V. D. Mihailetchi, H. X. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P. W. M. Blom and L. J. A. Koster, Appl. Phys. Lett., 2006, 89, 012107. 70 | Photochemistry, 2009, 37, 44–71 This journal is

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121 C. Soci, I. W. Hwang, C. Yang, D. Moses, Z. G. Zhu, D. Waller, R. Gaudiana, C. J. Brabec and A. J. Heeger, in Organic Photovoltaics VII, ed. Z. H. Kafafi and P. A. Lane, 2006, pp. U52–U62. 122 Q. M. Zhou, Q. Hou, L. P. Zheng, X. Y. Deng, G. Yu and Y. Cao, Appl. Phys. Lett., 2004, 84, 1653. 123 F. L. Zhang, W. Mammo, L. M. Andersson, S. Admassie, M. R. Andersson, L. Inganas and O. Ingands, Adv. Mater., 2006, 18, 2169. 124 D. Muhlbacher, M. Scharber, M. Morana, Z. G. Zhu, D. Waller, R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884. 125 J. H. Hou, Z. A. Tan, Y. Yan, Y. J. He, C. H. Yang and Y. F. Li, J. Am. Chem. Soc., 2006, 128, 4911. 126 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497. 127 W. Y. Wong, X. Z. Wang, Z. He, A. B. Djurisic, C. T. Yip, K. Y. Cheung, H. Wang, C. S. K. Mak and W. K. Chan, Nat. Mater., 2007, 6, 521. 128 C. R. McNeill, A. Abrusci, J. Zaumseil, R. Wilson, M. J. McKiernan, J. H. Burroughes, J. J. M. Halls, N. C. Greenham and R. H. Friend, Appl. Phys. Lett., 2007, 90, 193506. 129 M. M. Alam and S. A. Jenekhe, Chem. Mater., 2004, 16, 4647. 130 Y. Yao, C. J. Shi, G. Li, V. Shrotriya, Q. B. Pei and Y. Yang, Appl. Phys. Lett., 2006, 89, 153507. 131 M. Bourgeaux and W. G. Skene, J. Org. Chem., 2007, 72, 8882. 132 S. A. P. Guarin and W. G. Skene, Mater. Lett., 2007, 61, 5102. 133 C. Y. Chi, C. Im, V. Enkelmann, A. Ziegler, G. Lieser and G. Wegner, Chem.-Eur. J., 2005, 11, 6833.

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Light induced reactions in cryogenic matricesw Rui Fausto* and Andrea Go´mez-Zavaglia DOI: 10.1039/b812717b This chapter deals with light induced reactions in cryogenic matrices. Photochemical reactions induced by UV/visible irradiation of matrix isolated organic compounds, ranging from conformational isomerizations to complex bond-breaking/bond-forming processes, will be addressed. The effects of the media on the chemical processes, including direct participation of the matrix noble gas as a reactant to form novel covalently bound noble gas containing molecules, will also be discussed. Finally, (N)IR induced reactions in cryomatrices will be considered, as well as intramolecular vibrational energy relaxation/redistribution processes involved in these reactions.

1.

Introduction

During the period July 2004–June 2007 a substantial amount of reports on light induced reactions in cryogenic matrices appeared in the specialized journals. This fact testifies the great power of the matrix-isolation method, coupled with spectroscopic techniques, in studying photochemical reactions, involving electronic excited states, and also hot vibrational chemistry processes, in which vibrationally excited molecules in their ground electronic state undergo chemical transformations upon infrared excitation. This approach revealed itself to be specially powerful in the identification of reaction intermediates and establishment of reaction mechanisms. Of fundamental importance to the success of this approach was the concomitant development of computational chemistry, supported by an enormous quality jump regarding both hardware and software capabilities, which provided sound theoretical foundations for the interpretation of the experimental data, and the availability at relatively low cost of tunneable lasers that could be used as adequate irradiation sources to investigate specific processes in an elegant and powerful way. Nevertheless, the physical conditions typical for a matrix isolated chemical species are still the key for the unique capabilities of the matrix isolation method. In particular, (i) the simplification they introduce in the spectra due to the quenching of the rotational transitions, as well as hot vibrations and subtractive-combination vibrational transitions, leading to an increased intrinsic spectral resolution, (ii) the possibility they open of studying Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal. E-mail: [email protected]; Fax: +351 239 27703; Tel: +351 239 852063 { Copyright and Licenses Note: The following figures were based on or copied from the original articles, cited in the corresponding captions, with permission of their publishers: Fig. 2 and 9, Royal Society of Chemistry; Fig. 6, 7, 11 and 12, Elsevier; Fig. 16 and 19, American Physical Society; Fig. 3–5, 10, 13, 14, 15, 17, 18, 20 and 21, American Chemical Society.

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otherwise short life intermediates, which can be produced in the gas phase during deposition of the matrix or, after deposition, for example by photolysis of a suitable precursor, (iii) the general simplification of the photochemistry they introduce by avoiding recombination of photoproduced fragments originally belonging to different molecules, once molecular diffusion in cryogenic matrices can be controlled and the processes are then cage confined, and, finally, (iv) the easy and direct comparison of the experimental spectroscopic results obtained for matrix isolated species with theoretical data for a single molecule in vacuum, which in the case of vibrational spectroscopy is particularly useful since the vibrational spectrum of a molecule isolated in a matrix is, in general, practically identical to the pure vibrational spectra of a molecule in vacuum (host-guest interactions are minimized and, as mentioned above, rotations are inhibited for almost all molecules but the very small ones). The present chapter is organized in two main sections, the first one dealing with UV/visible induced photochemical processes and the second one with IR induced ground state hot vibrational chemistry. This last topic has emerged in the period covered by this review as an important topic of research, in particular as applied to the study of the mechanisms of energy redistribution within a molecule, though its fundamentals were already known for several years. But this renaissance was stimulated by the finding that IR excitation is matrix site selective, i.e., no energy is transmitted efficiently to other molecules through the matrix media. Such property opened a wide range of possibilities in relation with the investigation of the effects of the local environment on the intramolecular energy relaxation processes. On the other hand, UV/visible induced chemistry in cryogenic matrices was extensively applied during the period covered by this review both to organic, organometallic and inorganic species and to the study of simple conformational isomerization processes as well as to complex fragmentation bond-breaking/bond-forming reactions. Whenever possible, these different types of processes are covered in this chapter in different sections. A separate section is dedicated to those processes in which the matrix noble gas takes an active role as reactant in a photochemical reaction giving rise to novel covalently bound noble gas containing molecules. For the readers less familiarized with the matrix isolation method, we would like to refer to the classical books by Meyer,1 Andrews and Moskovits,2 Barnes et al.,3 Dunkin4 or Fausto,5 or to more specific and more recent reviews on this technique,6–11 a few of them also published during the period covered by this review. The book on reactive intermediate chemistry edited by Moss, Platz and Jones12 also contains a full chapter dedicated to the matrix isolation method where, in relation to the chemistry of reactive species, Thomas Bally describes the fundamentals of the technique and provides some important practical information for those who wish to implement this technique in their own laboratories. A review by Haas and Schweke,13 on simulation of matrix trapped molecules, which includes photochemistry and spectroscopy as important topics, was also recently published. Photochemistry, 2009, 37, 72–109 | 73 This journal is

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2.

UV/Visible induced reactions in cryomatrices

The main body of the research reported during the period covered by this review which is relevant to the subject of this chapter refers to UV/visible light induced reactions. In the two first parts of this section, we present the most relevant studies dealing with organic compounds, considering in separate the photoinduced conformational isomerization processes and the bond-breaking/bond-forming reactions. The photochemistry of matrix-isolated inorganic and organometalic compounds will not be covered in this Chapter, being briefly mentioned in the chapters of this book dealing with these types of compounds. The third part of this section addresses the subject of noble gas chemistry, where the matrix noble gas atoms take an active role as reactants in a photochemical reaction giving rise to novel covalently bound noble gas containing molecules. 2.1

Organic Compounds

2.1.1 Conformational isomerism. A combined matrix isolation FTIR and molecular orbital study on the photochemistry of a-pyrone was reported by Breda et al.14 a-Pyrone general photochemistry had been known for many years,15–17 but it had never been described in detail before, in particular in what concerns to the relative importance in mechanistic terms of the different isomeric forms of the open-ring photoproduct. The most efficient of the observed photoprocesses (Fig. 1) was found to be the Norrish type I, ring opening reaction leading to a conjugated

Fig. 1 Summary of the observed14 a-pyrone (1) photochemical reaction pathways: fast photoequilibration with the conjugated aldehyde–ketene (3) and slow ring-closure to the Dewar isomer (2). In the first process, several conformers are produced, with some of them further reacting to yield 4-formyl-2-cyclobutene-1-one (4). The Dewar isomer of a-pyrone subsequently expels CO2, to produce cyclobutadiene. The two latter species are confined in the matrix in the same cage and form a complex adopting a stacked structure with intermolecular distance of ca. 3.45 A˚.

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aldehyde–ketene ((3), in Fig. 1). Upon broadband irradiation with UV light (l 4 285 nm), rapid formation of Z isomers of the aldehyde–ketene was observed (Fig. 2). After a few minutes of irradiation, the IR bands assigned to these photoproducts stopped growing. Further irradiation generated, by excited state internal rotation around the central CQC bond, new aldehyde–ketene forms, which were identified as conformers of its E isomer. Upon subsequent UV irradiation (l 4 337 nm), the Z aldehyde–ketene

Fig. 2 Ketene antisymmetric stretching infrared spectral region.14 E and Z indicate bands assigned to E and Z ketene isomers, respectively: (A) Progress of UV irradiation of a-pyrone isolated in Xe matrix (T = 25 K) using a Xenon arc lamp with a cut-off filter l 4 285 nm. The traces correspond to recorded spectra after 0, 1, 6, 11, 17, 32, 42 and 62 min of irradiation. (B) Progress of subsequent UV irradiation with a l 4 337 nm cut-off filter. Spectra recorded after 5, 10, 20, 30 and 55 min of irradiation. The arrows indicate the general direction of changes.

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isomers reverted back to a-pyrone, while the E forms did not react further. For both Z and E ketene isomers, only the most stable conformers (depicted in Fig. 1) were observed experimentally. This result was interpreted as indicating the occurrence of thermal deactivation in the excited state. The observed ring opening reaction was found to be accompanied by the very slow valence photo-isomerization of a-pyrone to the corresponding Dewar form ((2), in Fig. 1). Observation of cyclobutadiene, produced by shorter wavelength UV irradiation (l 4 235 nm) of (2), was also described. The formed matrix-cage-confined complex of cyclobutadiene with CO2 was shown to exhibit a stacked (parallel) geometry, with an equilibrium distance between the two molecules (cyclobutadiene and CO2) of ca. 3.45 A˚, the stabilization energy of the complex being 3.45 kJ mol1. Another very interesting fact was pointed out in connection with the absence in the photolysed matrix of specific conformers of the aldehyde–ketene photoproduct. In the excited state, the aldehyde–ketene shall adopt preferentially conformations where the C–CQC–C dihedral angle is ca. 901, as is usually the case for a,b-unsaturated carbonyl compounds. Starting from such excited state geometry the system can adopt, during dissipation of energy excess, either Z or E orientation at the CQC double bond. Because the barriers for rotations around single C–C bonds are much lower than the electronic excitation energy, the rotations around such bonds should be possible for a molecule carrying large excess of energy gained by absorption of a UV photon. Hence, in principle, all the possible isomers of the conjugated aldehyde–ketene should be accessible on the photochemical way. However, no spectral signatures of conformers having the ketene group and the central CQC bond in the s-cis geometry could be found in the experimental spectra. This result was interpreted as an indication that these conformers, once formed, undergo a fast secondary reaction upon irradiation. Indeed, the presence in the spectra of the irradiated matrices of features ascribable to the cyclobutenone formyl derivative (4), suggests the occurrence of a pericyclic ring closure reaction in which the missing conformers of the aldehyde–ketene act as reactant species. The ground state of (4) was predicted to be higher by ca. 50 kJ mol1 with respect to those of the s-cis aldehyde–ketenes, but once (4) is photochemically created it should be stable, because the barrier for the ground state opening of its ring is high (calculated value equal to 55 kJ mol1). Another interesting study where excited state internal rotations about CQC bonds play a fundamental mechanistic role was reported by Minoura et al.18 In that study, the mechanism of UV-induced conformational changes among chelated and non-chelated enol isomers of (trifluoroacetyl)acetone in solid argon was investigated by infrared spectroscopy and quantum chemical calculations. In the isolated molecule situation, the keto forms of (trifuoroacetyl)acetone are known to be less stable than the enol ones, which are strongly stabilized by intramolecular hydrogen bonding of the CQO  H–O–C type. The compound has 16 possible enol isomers, shown in Fig. 3: F-xxx and xxx-F denote the isomers of 1,1,1-trifluoro-4-hydroxy-3-penten-2-one and those of 5,5,5-trifluoro-2-hydroxy-3-penten-4-one, respectively. 76 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 3 The sixteen possible isomers of enol-type (trifluoroacetyl)acetone. Isomers of 1,1,1-trifluoro-4-hydroxy-3-penten-2-one and 5,5,5-trifluoro-4-hydroxy-3-penten-2-one are denoted as F-xxx and xxx-F, respectively. Broken lines represent hydrogen bonds.18

It was found that the infrared spectrum of the as-deposited matrices of the compound contains only features due to the F-ccc chelated enol isomer. The absence of the ccc-F species in the matrix was explained assuming that this form (which according to the Boltzmann distribution law should exist at the evaporation temperature used as 15% of the total population) is promptly converted to the most stable F-ccc form in the matrix by proton tunneling. Such effect has been observed, for example, in the case of the s-cis-s-trans conversion of matrix-isolated formic acid.19–21 When the matrix sample was exposed to the UV light from a superhigh-pressure mercury lamp through UV28 short-cut and water filters (l 4 280 nm), it was found that the F-ccc conformer converts to non-chelated enol isomers by rotational isomerization after destruction of the intramolecular hydrogen bond. Comparison of the infrared spectra measured after UV irradiation with the spectral patterns obtained by density functional theory calculations allowed for identification of the photoproduced non-chelated species as being F-ctc, ctt-F and tct-F. In addition, a transient species that could only be detected when the IR spectra were obtained simultaneously with the UV irradiation could also be identified, corresponding to the F-ctt isomer. Kinetic studies were also performed along the irradiation time and from these studies a mechanism for the observed photoreactions was established (Fig. 4). This analysis was helped by a complete set of calculations on the relative energies of the different conformational species. The proposed mechanism was also able to explain the photochemical behaviour previously observed for the analogous molecules, acetylacetone and hexafluoroacetylacetone.22,23 Photochemistry, 2009, 37, 72–109 | 77 This journal is

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Fig. 4 Mechanism of the one-photon photoreactions of matrix-isolated (trifluoroacetyl)acetone upon UV irradiation.18

In the case of (trifluoroacetyl)acetone, the first photochemical events correspond to photoisomerization of the most stable F-ccc isomer to F-ctt (a transient species produced during the UV irradiation), F-ctc (a precursor of the observed tct-F) and ctt-F (a final product). All these products commonly have the cis conformation around the C2–C3 bond, i.e., the first italic letter of the adopted name scheme for the conformers is c. To explain this fact, it was assumed a perpendicular structure in the pp* transition state of the C3QC4 bond, caused by the UV irradiation: in this state, the C3QC4 bond has single-bond character, while the C2–C3 bond has double-bond character. Therefore, the conformational changes around the C3QC4 bond occur keeping the cis conformation around the C2–C3 bond and produce the F-cct, F-ctc, and F-ctt isomers. The F-cct isomer, whose energy is 61.9 kJ mol1 higher than that of F-ccc, is immediately isomerized to F-ccc by internal rotation around C4–O5 in thermal relaxation. Similarly, F-ctt, which has an infrared spectrum only observable during UV irradiation, is isomerized to F-ctc by the same conformational change. Furthermore, if the UV-induced conformational changes occur with the hydrogen atom moving from O–H to CQO, the cct-F, ctc-F, and ctt-F isomers might be produced from ccc-F, in the xxx-F group. Similarly to the F-xxx group, cct-F and ctc-F thermally relax to ccc-F and ctt-F, respectively, by internal rotation around C4–O5. This scheme then explains why the infrared spectrum of ctt-F was observed, but not that of ccc-F, which changes to F-ccc by hydrogen atom tunneling. A second photon reaction was also observed,18 in which F-ctc, that is the most stable isomer among the non-chelated F-xxx isomers, changed to tct-F, which is the most stable isomer among the non-chelated xxx-F isomers. To produce xxx-F from F-xxx, the hydrogen atom in the OH group must move to the CQO group. This transfer readily occurs between the chelated enol isomers, F-ccc and ccc-F, in the electronic ground (S0) state but is difficult between the non-chelated isomers in the S0 state because the hydrogen atom in the OH group is located far from the CQO group. In contrast, the hydrogen atom may approach the CQO group in the electronically excited state of the F-ctc isomer, where the O-H group is perpendicular to the CQO group. Then, the F-ctc isomer converts to non-chelated xxx-F isomers upon UV irradiation and, finally, to the most 78 | Photochemistry, 2009, 37, 72–109 This journal is

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stable non-chelated enol isomer in the xxx-F group, i.e., tct-F. Interestingly, it was found that this conformational change never occurs in the first photon isomerization, although the structure of the electronically excited state of this process is identical to those involved in the second photon reaction. To explain this fact, Minoura et al.18 assumed that the photoexcitation energy from F-ccc is insufficient to surmount the barrier to yield tct-F in the electronically excited state, while it is sufficient to surmount that for F-ctc, whose relative energy was predicted to be 42.4 kJ mol1 higher than that of F-ccc. Changes in the average conformation of matrix-isolated highly flexible molecules promoted by UV excitation was demonstrated for benzil by Lopes et al.24 It was shown that the low-frequency (ca. 25 cm1), large-amplitude torsional vibration around the C–C central bond in this molecule strongly affects the structural and spectroscopic properties exhibited by the compound and the population distribution in solid argon. The authors used a classical treatment of this C–C torsion to estimate the equilibrium conformational distribution of molecules with different OQC–CQO dihedral angles existing at room temperature in the gas phase and trapped in a low-temperature (T = 9 K) inert matrix, assuming a fast matrix deposition. They then showed that this population distribution can be changed either by in situ irradiation with UV light (l 4 235 nm) or by annealing the matrix to higher temperatures (T = 34 K). In the first case, the observed increase of the average OQC–CQO angle, as estimated from the observed frequency shifts in the IR spectra of the matrix-isolated compound (Fig. 5), was shown to result from conformational relaxation in the excited electronic states (S1 and T1), whose lowest-energy conformations correspond to a nearly planar molecular configuration, with the OQC–CQO dihedral angle equal to 1801. In the second case, the observed decrease of the average value of the OQC–CQO dihedral angle was shown to be a consequence of the change in the S0 C–C torsional potential, resulting from interactions with the matrix media, that favors the stability of the more polar structures, which correspond to structures with smaller OQC–CQO dihedral angles. Photochemical cis–trans isomerizations about CQN bonds were reported by Lapinski et al.25,26 for a series of N4-methoxy- and N4-hydroxycytosines isolated in argon and nitrogen matrices. Upon UV (l 4 295 nm) irradiation, photoconversion of the cis isomers into the corresponding trans forms was observed (Fig. 6). Very interestingly, these results demonstrated that for the N4-methoxycytosines, somewhat unexpectedly, although the cis–trans photoisomerization concerns rotation of a large methoxyl group, steric interactions with the matrix cage do not preclude it. It was also shown that, for all N4-methoxycytosines studied,25 the final stage of the reaction was a photostationary state and that subsequent UV (l 4 335 nm) irradiation led to photoisomerization in the opposite direction, resulting in partial recovery of the cis isomers, whereas for the hydroxy analogues the reactivity was found to be very much dependent of the substituents.26 Another interesting observation was the fact that replacement of the argon matrix environment by solid nitrogen results in a systematic shift of the position of the photostationary states in favor of a higher population of Photochemistry, 2009, 37, 72–109 | 79 This journal is

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Fig. 5 Selected spectral regions of the IR spectra of benzil in an argon matrix showing the results of irradiation (l 4 235 nm) experiments [doted line, as deposited (substrate temperature: 9 K); dashed and plain lines, after 20 min and after 417 min of irradiation, respectively] and calculated spectra for different values of the OQC–CQO dihedral (doted line, 901; dashed line, minimum-energy conformation; plain line, 1501).24

the trans photoproduct.25 As for the cases discussed before in relation with photochemically induced conformational changes about CQC bonds, photoisomerization reactions involving rotation around a CQN double bond can be rationalized in terms of the shapes of the ground state and excited state potential energy surfaces. In compounds with an imino group, the ground state minima corresponding to cis and trans isomers are separated by a high (150/200 kJ mol1) energy barrier. However, in the first excited singlet state there is only one minimum on the potential energy surface and the molecule in this minimum has a geometry similar to that corresponding to the transition point in the ground state. For both N4-methoxycytosine and 1-methyl-N4-methoxycytosine in the excited state minimum energy conformation, the methoxyl group is no longer coplanar with the heterocyclic ring but located nearly in the plane perpendicular to it. Small shifts of the position of the excited state minimum with respect to the top of the ground state barrier due to changes in the chemical environment can then significantly influence the relative rates of the cis-trans and trans-cis phototransformations.25 In consonance with this hypothesis, for 80 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 6 Comparison of the experimental spectra of: (B) N4-methoxycytosine isolated in Ar matrix; (C) the photoproduct generated upon UV (l 4 295 nm) irradiation; with the calculated spectra of cis (A) and trans (D) isomers of the oxo-imino tautomer of the compound.25

N4-hydroxycytosine a conical intersection between S0 and S1 was located for a structure with perpendicular orientation of the hydroxylamino group with respect to the heterocyclic ring.26 Internal rotation in matrix-isolated substituted aromatic molecules were also reported during the period covered by this review. Ohno et al.27 investigated the three structural isomers of pyridinecarboxaldehydes (2-, 3- and 4-pyridine-carboxaldehyde). Two rotamers (anti and syn) were identified upon photoexcitation for 2- and 3-pyridinecarboxaldehyde and one rotamer for 4-pyridinecarboxaldehyde (Fig. 7). Both infrared spectroscopy and DFT calculations revealed that for 2- and 3-pyridinecarboxaldehydes the syn rotamer is a less stable isomer, in the first case with a non-significant population in the gas phase at room temperature and only observed upon UV-excitation of the most stable conformer. The intramolecular N–H  N hydrogen bond existing in the anti rotamer of 2-pyridinecarboxaldehyde was shown to result in a shortening of the aldehyde C–H bond length and led to a blue shift of the aldehyde C–H stretching mode. The presence of the intramolecular hydrogen bond in 2-pyridinecarboxaldehyde seems also to facilitate the photolysis of the compound to CO and pyridine, since among the 3 structural isomers only this one was found to undergo photolysis upon excitation at 370 4 l 4 300 nm. The same laboratory also reported a study on the UV-induced conformational isomerization of the matrix-isolated structural isomers of Photochemistry, 2009, 37, 72–109 | 81 This journal is

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Fig. 7 Optimized geometries (bond lengths in A˚) for 2-, 3- and 4-pyridinecarboxaldehyde. The relative energies (kJ mol1) of the rotamers are given in parentheses.27

benzenedicarboxaldehyde (1,2-, 1,3-, and 1,4-derivatives).28 Two rotamers were identified for 1,2- and 1,4- and three rotamers for 1,3-benzenedicarboxaldehyde in infrared spectra upon UV-irradiation (Fig. 8). In a similar way as for 2-pyridinecarboxaldehyde,27 it was shown that both the intramolecular C–H  H–C interaction in the H-syn rotamer and the C–H  OQC hydrogen bonding in the anti rotamer of 1,2-benzenedicar-boxaldehyde result in the blue-shift of the aldehyde C–H stretching band and the shortening of the aldehyde C–H bond length. Also as found for the structural isomers of pyridinecarboxaldehyde,27 only the benzenedicarboxaldehyde isomer bearing an intramolecular hydrogen bond (the 1,2- form) was found to undergo simultaneous photoinduced rotational isomerization and rearrangement. Together with theoretical results for conformational energy differences and barriers for conformational isomerization, the spectroscopic results indicated that upon irradiation at l 4 370 nm the most stable anti form converts into the H-syn conformer whereas the O-syn rotamer is produced upon irradiation at higher energy (370 4 l 4 300). Besides the conformational isomerization reactions, rearrangement reactions, i.e., enolization and cyclization to phthalide, were also observed to take place upon irradiation of the matrix-isolated 1,2-benzenedicarboxaldehyde (see Fig. 8).28 For irradiation at l o 300 nm, formation of benzaldehyde and CO was also observed. Isozaki et al.29 presented evidence for the existence of a non-planar conformer of o-fluoroanisole, which could be obtained from the lowest energy planar trans conformer by UV irradiation (410 4 l 4 240 nm) of the matrix-isolated compound. After ca. half an hour of irradiation, a photostationary state was reached. It was shown that the S1 states of both conformers were involved in the photoequilibrium. The non-planar conformer was also shown to gradually decay by a unimolecular process 82 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 8 Conformers of 1,2-benzenedicarboxaldehyde (1-3) and observed reactions and photoproducts. The photoproduct identified as being the enol (5) was proposed to be obtained from a different conformer (4) which should correspond to the primary photoproduct.28

under dark conditions at 16 K, by tunneling. The back-reaction rate was estimated as 3  104 min1, and the reaction dynamics between the trans and non-planar conformers was discussed. Intramolecular hydrogen atom tunneling in 2-chlorobenzoic acid has been investigated by Nishino and Nakata.30 The vibrational signatures of two relatively stable syn isomers, SC and ST, were observed in the infrared spectra of the compound in argon and xenon matrices. Upon UV irradiation of the matrices, two less stable anti isomers, AT (which has an OH  Cl intramolecular hydrogen bond) and AC, were produced from the most stable syn isomers. Furthermore, when the matrix samples were kept in the dark after UV irradiation, AT and AC changed to ST and SC, respectively, by spontaneous isomerization around the C–O axis in the carboxyl group. The rate constants for the AT-ST isomerization in a Xe matrix were estimated from the absorbance changes at various matrix temperatures. They showed a drastic decrease in deuteration of the hydrogen atom of the carboxyl group and were found not to follow the Arrhenius law. These findings led to the conclusion that the AT-ST and AC-SC isomerizations in the low-temperature rare-gas matrices proceed through intramolecular hydrogen atom tunneling. 1,2-Diaminobenzene isolated in solid argon was found to yield 3,5-cyclohexadiene-1,2-diimine upon UV irradiation at l o 270 nm.31 Photochemistry, 2009, 37, 72–109 | 83 This journal is

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Subsequent irradiation of the obtained initial photoproduct at different wavelengths led to observation of photoinduced conformational changes: upon irradiation at l 4 410 nm the cis–cis conformer converted into the cis–trans and trans–trans conformers while irradiation in the 410 4 l 4 350 nm range resulted in photoisomerization from cis–trans to cis–cis, in addition to photocyclization to yield 7,8-diazabicyclo[4.2.0]octa-1,3,5-triene. Similar photoisomerization reactions among the isomers of 4-methyl-3,5-cyclohexadiene-1,2-diimine and 4,5-dimethyl-3,5-cyclohexadiene-1,2-diimine were also observed, but not the photocyclization.31 The wavelength dependence for the photoisomerization and the methyl-substitution effect for the photocyclization were elucidated in terms of the p-p* and n-p* vertical transition energies and oscillator strengths obtained by time-dependent density functional theory calculations.31 Other organic molecules exhibiting photoinduced conformational isomerism in cryogenic matrices studied during the period covered by this review were linear and isopropyl nitrites, propiolic acid and the dimethyl ester of squaric acid.32–34 Matyus, Magyarfalvi and Tarczay32 measured the infrared spectra of both constitutional isomers (n and i) of propyl nitrite in an argon matrix and were able to identify spectral lines ascribable to different conformers by utilizing their different rate of photodecomposition as well as by employing conformational cooling using a supersonic jet as the inlet source for matrix deposition. The rate of photo-decomposition was found to be primarily determined by the steric alignment of the nitrite group, whereas jet cooling affects mainly the conformation of the alkyl tail. On the basis of these observations and computational predictions 2 to 3 conformers of isopropyl nitrite and 8 conformers of n-propyl nitrite were identified. After broadband ultraviolet-visible (UV/vis) photolysis of isopropyl nitrite in the matrix, HNO, acetone, HNO:acetone complex, acetaldehyde and nitrosomethane were identified as the main products.32 Furthermore, in a small amount, NO and possibly the isopropoxy radical were also observed in the photolysed matrix. Photolysis of n-propyl nitrite was found to yield HNO, propanal and their 1:1 complex as the main products, together with a small amount of NO and cis-1-nitrosopropanol. Photolysis (l = 193 nm) of propiolic acid (HCCCOOH) was studied in different noble gas matrices by Isoniemi et al.33 The higher-energy s-trans conformer of the compound was efficiently formed upon irradiation, and found to decay back to the ground-state conformer on a time scale of B10 min, by tunneling of the hydrogen atom through the torsional energy barrier. In addition, the photolysis produced a number of 1:1 molecular complexes such as HCCH:CO2, HCCOH:CO and H2O:C3O, whose structures and relative stabilities were elucidated.33 The UV-induced (l 4 337 nm) conformational isomerization of the dimethyl ester of squaric acid in solid argon was studied by Breda et al.,34 who observed conversion of the most stable trans–trans conformer (C2v) of the compound into the cis–trans form (Cs). In addition to the conformational isomerization, the compound was observed to undergo a ring-opening reaction to the bisketene, 2,3-dimethoxy-buta-1,3-diene-1,4-dione. Upon higher energy irradiation (l 4 235 nm), the main observed photoproducts were found to be CO and deltic acid dimethyl ester, the latter being obtained 84 | Photochemistry, 2009, 37, 72–109 This journal is

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in two different conformations (trans–trans and cis–trans). According to the experimental data, deltic acid dimethyl ester was produced by decarbonylation of the bisketene and not by direct CO extrusion from the initial reactant.34 2.1.2 Bond-breaking/bond-forming photoprocesses. A considerable number of papers on the photochemistry of different types of carbonyl containing molecules in cryogenic matrices were published during the period covered by this review. Photofragmentation of salicylhydroxamic acid in argon matrix and methanol-water solution upon irradiation with visible and UV (l 4 320) light was investigated by Kaczor et al.35 o-Hydroxyphenylisocyanate was found to be the main photolysis product in the matrix, while salicylamide was identified as the primary photolysis product in methanol-water solution. The intermediate species (B, in Fig. 9), whose structure lies between a nitrene and an oxazirene, was also identified in the IR spectra of the irradiated matrix. The reaction in solution was found to occur via the same intermediate as in the matrix. Matrix-isolated aminoacids were observed to easily decarboxylate when submitted to UV irradiation (l 4 200 nm). Hence, phenylalanine releases CO2 and transforms to phenethylamine,36 whereas serine gives rise to ethanolamine.37 In the case of serine, a less important photochemical reaction channel was also observed, where the compound undergoes decarbonylation, with formation of CO, H2O and acetamide. Very interestingly, the two observed photoprocesses observed for serine were found to be dependent on the conformation assumed by the reactant molecule, with conformers bearing intramolecular hydrogen bonds where the alcohol group acts as donnor undergoing decarboxylation and those where the intramolecular hydrogen bond is of the type OH(carboxylic)  N undergoing decarbonylation.37

Fig. 9 Mechanism of photolysis of salicylhydroxamic acid in matrix and solution.35

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The unimolecular UV isomerization of formamide (HCONH2) trapped in xenon, nitrogen, argon, and neon cryogenic matrices was studied by Duvernay et al.38 When irradiation was carried out at 240 nm, n - p* excitation led to formation of several isomers of formimidic acid, H(OH)CQNH, the amount of each form produced depending on the polarizability of the matrix. Subsequent vacuum UV photodecomposition (l 4 160 nm) of this latter molecule was found to proceed by two main channels, dehydrogenation and dehydration. In the first case, the HNCO:H2 complex was formed, whereas in the second case, HNC and HCN in interaction with water were yielded. The dehydration reaction concerning amides or their tautomers was reported for the first time in that study. These results were followed by a similar study by the same research group on acetamide,39 where HNCO:CH4, CO:CH3NH2 and CH3CN:H2O molecular complexes were observed upon photolysis of the matrix-isolated compound, together with production of acetimidic acid. Duvernay et al.,40,41 also investigated the photolysis of urea upon vacuum ultraviolet irradiation (l 4 160 nm) of the compound isolated in argon and xenon matrices. Several primary photoproducts, such as HNCO:NH3 and CO:N2H4 molecular complexes, and isourea, H2N(OH)CQNH, which was reported for the first time, were characterized. Isourea is found in the argon matrix in its most stable configuration. It was found to be an intermediate in the photodecomposition process, its dehydration leading to the NH2CN:H2O complex.40 In the xenon matrix, the photochemistry of urea was found to yield the HNCO:NH3 complex as major product, whereas the CO:N2H4 complex was only observed in trace amounts. The observed differences between the argon and xenon matrices suggested the crossing between S, and T, potential surfaces of urea to be responsible for the formation of the HNCO:NH3 complex. Lopes et al.,42 studied the photochemistry of 1-phenyl-1,2-propanedione in solid xenon resulting fom irradiation with UV light (l 4 235 nm). They found that decarbonylation of the compound took place, with generation of acetophenone and carbon monoxide, with an almost complete consumption of the reagent after 1100 min of irradiation (k = 2.8  102 min1). A mechanism for the decarbonylation reaction implying intersystem crossing to the T1 triplet state, where the homolytic cleavage of the intercarbonyl C–C bond takes place, was proposed. The primary photoproducts are then the benzoyl and acetyl radicals. The acetyl radical is known to be considerably less stable than the benzoyl radical43 and is known to decompose easily into dCH3 and CO upon irradiation at l 4 235 nm.44 Then, before the benzoyl radical has time to decompose to CO+phenyl radical, the acetyl radical decomposition shall take place and recombination of the methyl radical resulting from this process with the benzoyl radical occurs, leading to production of acetophenone. UV photolysis of CCl3COCl in argon and O2 matrices was studied by Tamezane et al.45 In the Ar matrix, CCl4 and CO were found to be dominant products over Cl2CQCQO. The CCl3 and COCl species were observed as intermediates and indicated occurrence of the C–C bond cleavage. The C–C bond cleavage was also evidenced by the formation of ClC(O)OO in the O2 matrix. The marked difference in the photochemistry 86 | Photochemistry, 2009, 37, 72–109 This journal is

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of matrix-isolated CCl3COCl from that of CH3COCl, where CH2QCQO is the predominant product, was attributed to the triplet surface reaction of CCl3COCl, as supported by DFT calculations.45 The photochemistries of other very interesting simple carbonyl and analogous thiocarbonyl compounds in cryogenic matrices were also reported by Della Ve´dova and co-workers, which were used in the preparation of novel sulphenyl-halides such as, for example, syn-iodocarbonylsulfenyl bromide46 or methanesulfenyl fluoride.47 Regarding the photochemisty of carbonyl (or thiocarbonyl) cyclic compounds, considerable effort was put on the study of pyrone and pyran-thione derivatives. Besides the fundamental work on the photochemistry of unsubstituted a-pyrone already referred to in this review,14 several studies were reported by Fausto and co-workers which considered other molecules of this family, like 4,6-dimethyl-a-pyrone,48 methyl 2-pyrone-5-carboxylate (5-methyl coumalate),49 coumarin,50 thio-pyrone and pyran-2-thione.51,52 From these studies, a quite complete understanding of the photochemistry of this type of molecules emerged. In particular, it could be understood why the Dewar isomer formation is much more effective in the case of the substituted a-pyrone derivatives, while the ring-opening photoreaction, leading to the aldehyde–ketene, proceeds easier for the unsubstituted molecule. The ring-opening reaction leading to the ketene species is believed53,54 to originate from excited states with np* character, whereas the process leading to formation of the Dewar isomers should start from pp* states. In a-pyrone, the lowest excited singlet state has np* character.55 Hence, the np*-type photochemistry is favored for such compounds when they are free of any substituents. On the other hand, conjugative type substitution with groups such as –OH, –CH3 or –OCH3, blue-shifts the S1 ’ S0 (np*) transition and red-shifts the S2 ’ S0 (pp*) transition,55 reducing the gap between the S1 and S2 states and favoring the process leading to the Dewar formation. The authors call the attention for the fact that the ring-opening reaction in S1 is not a barrierless process and some excess of excitation energy is needed to promote the a-bond cleavage. Hence, this process does not necessarily need to dominate, even though the lowest of the excited states is of np* character. If the energy gap between the np* and pp* states is not too large, the processes typical for pp* photochemistry can compete with the a-bond cleavage or even dominate. Isomerization to the Dewar form observed as dominating process for 4,6-dimethyl-a-pyrone,52 for example, is a good example of such behavior. Another interesting result regarding the photochemistry of matrixisolated derivatives of a-pyrone was the observation that, when the substituent is a volumous group at the position 3 of the a-pyrone ring, the reaction leading to the formation of the Dewar species becomes less easy and at least in some cases it does not occur at all.49 The possible reason precluding transformation of these 3-substituted compounds into their Dewar forms was considered to be related with the steric properties of these molecules, whose Dewar forms are strongly non-planar in oposition to the reactant species. Then, a substantial rearrangement of the matrix cage would be required to accommodate the newly formed species. In practice, Photochemistry, 2009, 37, 72–109 | 87 This journal is

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this energetically demanding process hinders the transformation for example of 3-methyl coumalate into its Dewar isomer.49 The ring-opening reaction was also found to be easier in a-pyrone than in coumarin, and factors explaining this observation were presented,50 which include the existence for coumarin of a third active photoreaction channel, decarbonylation to benzofuran, that is identical to that found previously to be the preferred one for coumarin in the gas phase.56 In the case of the sulphur containing analogues of a-pyrone, it was shown that UV irradiation (l 4 337 nm) of the studied compounds isolated in low-temperature matrices results mainly in the ring-opening reaction by means of the cleavage of the a-bond. Other photoprocesses, not involving the a-bond-cleavage step (such as generation of the Dewar isomer) were found to correspond to minor reaction channels both in pyran-2-thione and 2-thiopyrone.51,52 The observed ring-opening photoreaction in pyran-2-thione represented the first reported case of an a-bond cleavage in a compound with a CQS group attached to a sixmembered ring, in which internal strain practically does not exist, whereas the corresponding reaction in 2-thiopyrone (a cleavage of a C–S bond in the a position with respect to a carbonyl group) was also reported for the first time.51,52 Following the ring-opening reactions, isomerization processes and intramolecular hydrogen shift reactions were observed, enabling production of 2-thiopyrone from pyran-2-thione and vice versa. In the papers by the Fausto’s group,51,52 a detailed analysis of such processes was reported, and kinetical and mechanistical data were presented and discussed (Fig. 10). The photochemistry of tetrazoles and tetrazolones was also the subject of extensive investigation during the period covered by this review, enabling the observation of several novel molecular species, like for example antiaromatic diazirines, or reactive isocyanates and azides (e.g., 3-methoxy1-phenyl-1H-diazirine, 3-ethoxy-1-phenyl-1H-diazirine, 3-allyloxy-1-phenyl1H-diazirine, 1-phenyl-diaziridin-3-one, 1-allyl-2-phenyldiaziridin-3-one, allylisocyanate, allyl-azide).57–61 The fundamental photochemistry of matrix-isolated tetrazoles involves fragmentation of the tetrazole ring

Fig. 10 Proposed general scheme of reactions resulting from UV-irradiation of pyran-2-thione (PT) and 2-thiopyrone (TP) isolated in a cryogenic matrix. TKn and TAn represent specific conformers of the open-ring aldehyde-thioketene and thioaldehyde–ketene isomeric forms of TP and PT, respectively.51 Bold lines connect main observed photoproducts. All reactions are reversible.

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Fig. 11 Photochemical reactions observed for 1-phenyl-tetrazolone isolated in solid argon upon irradiation with UV (l 4 235 nm) light. Pathway (d) could not been unequivocally established.57

(Fig. 11). The specific substituents present in the ring, however, may determine in large amount the preferences for the different available primary photochemical reaction channels and strongly determine secondary processes in which the initial photoproducts participate. As a general rule, it was shown57–61 that the tetrazole ring can undergo photolytic cleavage in three different ways: (a) through the (N-1)–(N-2) and (N-3)–(N-4) bonds, releasing molecular nitrogen and forming a diazirine derivative; (b) through the (N-1)–(C-5) and (N-3)–(N-4) bonds and (c) through (N-1)–(N-2) and (N-4)–(C-5) bonds. In the two latter cases, the precise nature of the photoproducts varies depending on the substituents present in the tetrazole ring,57–61 but one of the products is always an azide, which then can undergo subsequent reactions, most of times eliminating N2 to form the nitrene that then can further react to form the final product (e.g., 1-aza-1,2,4,6-cycloheptatetraene, produced by ring expansion of phenyl nitrene). The number of available reaction channels was shown to correlate with the number of formally single bonds in the tetrazole ring. When four single bonds are present (like in tetrazolones or tetrazole-thiones), the 3 photochemical fundamental types of reactions can take place, a fourth reaction path being sometimes also activated, corresponding to cleavage through the (N-1)–(C-5) and (N-4)–(C-5) bonds. In general, for molecules exhibiting only three formally single bonds in the tetrazole ring, only two reaction channels are active, with that corresponding to direct N2 elimination always playing the most important role.57–61 The presence in the ring or in the substituents of labile hydrogen atoms was always found to increase the complexity of the photochemistry, mainly Photochemistry, 2009, 37, 72–109 | 89 This journal is

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because of the occurrence of different tautomeric forms,58 whereas the conformational flexibility of the substituent may have effects difficult to predict a priori. These effects were shown to be strongly determined by the number and relative energies of the possible conformers and conformational interconversion barriers.60,61 On the other hand, the presence of a phenyl substituent at N-1 (or N-4) in alkyloxy tetrazoles was found to result in strong activation of the channel leading to production of phenylazide and the corresponding alkyl-cyanate (that undergoes major isomerization to the isocyanate).59,61 In tetrazolones,57,60 a phenyl or alkyl substituent at N-1 (or N-4) favours the pathway leading to direct production of the corresponding isocyanate, which is in general inhibited in the alkyloxy and allyloxy tetrazoles.59,61 It is interesting to point out that that most of the prevalent photoproducts of the photochemistry of tetrazole derivatives can be easily identified by IR spectroscopy, a fact that was on the basis of the great progress achieved in the study of this type of compounds by this technique coupled with matrixisolation. In fact, three groups of bands, occurring in the usually clean spectral regions allow an easy identification of the main products resulting from the three fundamental reaction paths in tetrazoles’ photochemistry: diazirines and diaziridinones give rise to characteristic bands in the 1800–1900 cm1 spectral region, azides have their most intense band around 2100 cm1 (nNQN+QN antisymmetric stretching) and isocyanates strongly absorb in the 2290–2300 cm1 range (nNQCQO antisymmetric stretching); Fig. 12. Aziridines by themselves have been shown to have a very interesting photochemistry. As it could be expected, the reactivity of 2H-azirines is strongly related with the appreciable trend of the three-membered ring to open. It has been shown that the thermally activated cleavage of the C–N bond, leading to formation of a ketene imine, is generally favored, whereas photolysis results most of times in ring-opening through heterolytic cleavage of the C–C bond, yielding nitrile ylides. The effect of electron withdrawing substituents at the azirine ring carbon atoms on the photochemistry of these type of compounds was evaluated by Kaczor, Go´mez-Zavaglia et al.,62,63 by comparison of the photochemistry of two methoxycarbonyl substituted 2H-azirines (methyl 3-methyl-2H-azirine-2carboxylate and its chlorosubstituted analogue, methyl 2-chloro-3-methyl2H-azirine-2-carboxylate) as isolated species in cryogenic noble gas matrices. These studies constituted the first evidence for C–N bond cleavage preference in non-aromatic azirines and enabled to shed light on the relative importance of the chlorine and methoxycarbonyl substituents in determining the preferred pathways for the photochemical reactions. In particular, it was shown that the unusual photochemical C–N bond cleavage observed for both molecules should be attributed to the presence in the molecule of the methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T1 triplet state and, in this way, favors the C–N bond cleavage.63 Another very interesting topic of research on heterocyclic molecules which was the subject of several reports appearing during the period covered by this review is photoinduced H-migration and tautomerism. The groups leaded by Nowak, Aida and Nakata have been particularly active in this field.64–74 90 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 12 2350–1800 cm1 spectral region of the irradiated (l 4 235 nm) matrix of 1-phenyltetrazolone at different times of irradiation [2 (black line), 10 (blue), 30 (green) and 60 (red) minutes] and calculated spectra, in this spectral range, for the observed photoproducts.57

Unimolecular phototautomerization reactions in several sulphur analogues of nucleic acid bases (e.g., 2-thiouracil, 6-aza-2-thiothymine, 1-methyl-2-thiouracil, 3-methyl-2-thiouracil, 2,4-dithiouracil and 6-aza-2,4-dithiouracil),64,66 pyrimidinones and hypoxanthynes,65,69 and trithiocyanuric acid67 isolated in low-temperature matrices were investigated by Nowak and coworkers. All the thiouracils studied adopted exclusively oxo-thione tautomeric forms before UV irradiation. Upon UV (l 4 320 nm) irradiation, excited state proton transfer took place and several oxo-thiol tautomers were generated. For some molecules, hydroxy-thiol tautomers were also observed as minor photoproducts.64 In the case of dithiouracils, the initially deposited dithione tautomeric forms were found to be converted to the corresponding dithiol forms upon irradiation.66 Trithiocyanuric acid (C3H3N3S3) corresponds to a particularly interesting molecule regarding phototautomerism, since this compound exists exclusively in its trithione tautomeric form in gas phase and in the freshly deposited matrices prepared from the vapour of the compound, but converts to the trithiol tautomeric form (trimercaptotriazine) upon UV irradiation (l 4 270 nm).67 This was the first observation of an intramolecular triple trithione - trithiol photoinduced proton transfer. Photochemistry, 2009, 37, 72–109 | 91 This journal is

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Like the thiouracils and dithiouracils, which exist in the freshly deposited matrices exclusively in their thione forms,64,66 the studied pyrimidinones and hypoxanthynes were found to exist in the as-deposited matrices predominantely in their oxo tautomeric forms.65,69 Irradiation of the matrices (l 4 230 nm or l 4 270 nm) led to proton transfer, converting the oxo forms into the corresponding hydroxy tautomers.65,69 Generation of conjugated ketenes as minor photoproducts was also observed.65 Both Aida’s and Nakata’s research groups focused their attention on substituted pyridines or resembling compounds.70–74 Intramolecular H-migrations in 2-hydroxy-3-nitropyridine was found to take place in the initially deposited in argon matrix anti-enol isomer, upon UV and visible light (l 4 350 nm) irradiation, to yield as final product the keto tautomer (Fig. 13).70 During the irradiation, the presence of the syn-enol species in the matrix was observed. The bands of syn-enol disappeared completely when the irradiation was stopped, while those of the original isomer, anti-enol, reappeared (Fig. 14). No reverse isomerization was observable in the corresponding deuterated species. This led to the conclusion that the isomerization from syn to anti-enol occurs through hydrogen-atom tunneling. The dynamics of the hydrogen-atom migrations between anti- and syn-enols, and between the syn-enol and keto form were discussed in terms of the potential energy surfaces obtained theoretically.70 2-(Methylamino)pyridine was found to exist in a freshly prepared matrix as a mixture of two nearly isoenergetic amino tautomers and to be transformed into methyl-imino tautomers by intramolecular H-atom (or proton) transfer upon UV irradiation (320 4 l Z 300 nm).72 Aida and co-workers also found that the reverse tautomerism took place by longer-wavelength light irradiation (370 4 l Z 340 nm), but only one of the methyl-imino tautomers was found to be photoreactive.72 The same research group also investigated the photochemistry of matrix-isolated 2-amino-5-methylpyridine.72 Photoinduced reversible amino (NQC–NH2)/imino (NH–CQNH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changed to the imino tautomer by UV irradiation (340 4 l Z 300 nm) and the reverse

Fig. 13 (a) Observed difference spectrum of a photolysed matrix of 2-hydroxy-3-nitropyridine (spectrum measured after 10 min UV and visible light (l 4 350 nm) irradiation minus that before irradiation) and (b) calculated spectral pattern of the compound keto form.70

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Fig. 14 Observed and calculated IR spectra of syn-enol: (a) difference spectrum between the spectra measured during irradiation minus that before UV and visible-light irradiation (l 4 350 nm), (b) difference spectrum between the spectra measured after irradiation minus that during the irradiation, and (c) calculated spectral pattern of syn-enol. Bands marked with ’ represent keto.70

change occurred by longer-wavelength light irradiation (420 4 l Z 340 nm). Results from CASSCF calculations73 revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from the electronic excited state to the ground state. Tautomerization by H-transfer in selenourea isolated in low temperature argon matrix was also studied by Nowak’s group.68 The results obtained indicated that monomers of this compound adopt exclusively the selenone tautomeric form in the gas phase and freshly deposited matrix. UV irradiation (l 4 345 nm) of the matrix isolated selenone led to generation of the selenol tautomer (isoselenourea). Two conformers of the selenol tautomer with imino N–H bond oriented anti or syn with respect to the Se–H group were photoproduced. For the matrix kept at 10 K and in darkness, a proton tunneling transforming the photoproduced selenol anti form back into the initial selenone tautomer was observed. The time constant of this process was found to be 16 h, this being the second example known of very slow ground state proton transfer through a high potential energy barrier (the asymmetric barrier for the selenol-selenone proton tunneling was estimated theoretically to be 95 kJ mol1),68 only comparable to that observed for thiourea.75 Another important area of research where photochemistry, spectroscopy and matrix isolation have been used altogether very fruitfully is the investigation of reactive intermediates, such as nitrenes, carbenes, radicals and carbocations. During the period covered by this review several studies were reported on this subject. In the in situ production of nitrenes, azido compounds were used many times as adequated precursors. Wang et al.76 started from p-biphenylyl, Photochemistry, 2009, 37, 72–109 | 93 This journal is

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o-biphenylyl and 1-naphthyl azides deposited in argon and generated in situ triplet nitrenes, azirines, and didehydroazepines upon UV excitation. In the presence of HCl, photolysis of these azides led to production of the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion was found very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation were also provided. The photochemistry of p-azidoaniline was studied in argon matrices in the absence and presence of oxygen by Pritchina, Gritsan and Bally.77 With the help of quantum chemical calculations these authors were able to characterize the triplet p-aminophenylnitrene as well as the cis- and trans-p-aminophenylnitroso oxides. It was found that the latter two isomers can be interconverted by selective irradiation and that they were ultimately converted into p-nitroaniline. Sander et al.78 investigated the photochemistry of 3-iodo-2,4,5,6-tetrafluoro-phenyl azide and 3,5-diiodo-2,4,6-trifluorophenyl azide by IR and EPR spectroscopy in cryogenic argon and neon matrices. Both compounds were found to give rise to the corresponding nitrenes as primary photoproducts in photostationary equilibria with their azirine and ketenimine isomers. In contrast to fluorinated phenylnitrenes, ring-opened products were obtained upon short-wavelength irradiation of the iodine-containing systems, what was interpreted as indication of C–I bond cleavage in the nitrenes or didehydroazepines under that conditions. Mechanistic aspects of the rearrangements leading to the observed products were discussed on the basis of DFT as well as high-level ab initio calculations. The computations indicated strong through-bond coupling of the exocyclic orbital in the meta position with the singly occupied in-plane nitrene orbital in the monoradical nitrenes. In contrast to the ortho or para isomers, this interaction was found to result in low-spin ground states for meta nitrene radicals and a weakening of the C1–C2 bond, causing the kinetic instability of these species even under low-temperature conditions.78 Bettinger and Bornemann79 produced the donor atom stabilized borylnitrene, 2-nitreno1,3,2-benzodioxaborole from the corresponding boronazide by UV irradiation (l = 254 nm) of the matrix-isolated precursor and characterized the obtained borylnitrene by IR, UV, and ESR spectroscopy as well as multiconfiguration SCF and CI computations. Under the conditions of matrix isolation, the triplet borylnitrene was found to be photochemically and thermally stable toward rearrangement to the corresponding cyclic iminoborane.79 Photochemical irradiation (l 4 550 nm) causes an efficient reaction with molecular nitrogen, lying in matrix sites nearby, to reform the original boronazide. Similarly, photochemical, but not thermal, trapping of the borylnitrene with CO could be achieved and resulted in the corresponding isocycanate. Bettinger and Bornemann79 also shown that the thermal reaction of 2-nitreno-1,3,2-benzodioxaborole with O2 in doped argon matrices at 35 K could be observed by IR spectroscopy to result in borylnitroso-O-oxide. This diradical was found to be very photolabile and quickly rearranges to the corresponding nitritoborane upon irradiation. 94 | Photochemistry, 2009, 37, 72–109 This journal is

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A quite extensive review, by Gritsan,80 was published in the Russian Chemical Reviews during 2007, where the general subject of photochemical transformations of organic azides as studied by matrix isolation techniques was addressed. Among the radicals studied, vinyl radical was that deserving more attention. Yang et al.,81 studied the reaction of vinyl radical, produced through high frequency discharge of ethylene, with molecular oxygen in solid argon. The vinyl radical reacted with oxygen spontaneously on annealing to form the vinylperoxy radical C2H3OO with the O–O bond in a trans position relative to the C–C bond. The vinylperoxy radical was then found to undergo visible photon-induced dissociation to the CH2OH(CO) complex (or CH2OH + CO), which was considered as a possible photoproduct for the first time in this study. The CH2OH(CO) product was postulated to be formed by hydrogen atom transfer from the initially obtained H2CO–HCO pair and was calculated to be thermodynamically more accessible than the previously reported major HCO + H2CO channel.81 Tanska¨nen et al.,82 were able to obtain the vinyl radical by annealing-induced reaction of mobilized hydrogen atoms with acetylene molecules in solid argon, krypton and xenon. The hydrogen atoms were generated from acetylene by UV photolysis or fast electron irradiation. The methyl radical was studied in relation with its rotation in cryogenic matrices by Popov et al.,83 using ESR. The radical was produced by dissociating methane by plasma bursts generated either by a focused 193 nm laser radiation or a radio frequency discharge device during the gas condensation on the substrate. The ESR spectrum exhibited axial symmetry at the lowest temperature (14 K) and was ascribed to ground state molecules with symmetric total nuclear spin function I = 3/2. The hyperfine anisotropy was found to be 0.01 mT, whereas that of the g value was 2.5  105. The anisotropy was observed for the first time in argon.83 Elevation of temperature led reversibly to the appearance of excited state contribution having antisymmetric I = 1/2. The experimental observations were interpreted in terms of a free rotation about the C3 axis and a thermal activation of the C2-type rotations above 15 K. The ground and excited rotational state energy levels were found to be separated by 11.2 cm1 and to exhibit significantly different spin-lattice coupling. A crystal field model was applied to evaluate the energy levels of the hindered rotor in the matrix, and a 60 cm1 effective potential barrier for rotation of the C3 axis was obtained. Other interesting and unusual radicals studied during the period covered by this review were the FC60 radical,84 1,2,3-tridehydrobenzene,85 the first representative of the tridehydrobenzenes to be spectroscopically characterized and isolated in cryogenic matrices, and the Berson-type diradical, 2-isopropylidenecyclopentane-1,3-diyl.86 The latter was produced from photodenitrogenation of 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene in an argon matrix at 10 K and was found to have a planar structure. Oxygen-trapping reaction of the diradical produced regioselectively a fused peroxide, 3,3-dimethyl-3,5,6,6a-tetrahydrocyclopenta [1,2]dioxole, in an oxygen-doped argon matrix at 10 K. UV irradiation of the maytrix-isolated diradical afforded 6-methylhept-5-en-1-yne (Fig. 15). Photochemistry, 2009, 37, 72–109 | 95 This journal is

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Fig. 15 Reactions of 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene in argon matrices.86

Contrarily to nitrenes and radicals, carbenes were not much investigated by photochemical methods in conection with matrix-isolation during the period covered by this review. To the best of our knowledge, the sole studied published on this matter considered the photochemistry of the a-diazo sulfoxide, cis-3-diazo-5,6-dimethyl-1,4-oxathian-2-one S-oxide isolated in solid argon.87 In that study, O’Sullivan et al. were able to indirectly detect the sulfinyl carbene derivative of this precursor, which underwent photochemically induced hetero-Wolff rearrangement to the a oxo-sulfine intermediate, detected and characterized following irradiation at 248 nm. During the reviewed period, several other studies on different types of compounds were reported and deserved to be mentioned. These studies will be briefly referred to now. Miyazaki and Yamada88 studied the photochemistry of matrix-isolated cyclopentadiene, describing the production of bicyclo[2.1.0]pent-2-ene upon irradiation with a super-high-pressure mercury lamp. When the compound was irradiated with shorter wavelength using a low-pressure mercury lamp, further reactions of bicyclo[2.1.0]pent-2-ene were found to produce allylacetylene and vinylallene. While the photochemistry of cyclopentadiene to form bicyclo[2.1.0] pent-2-ene was already known in solution, the observed production of allylacetylene and vinylallene had never been previously reported. Cyanocompounds were studied by Guennoun et al.,89 Akal, Kudoh and Nakata,90 and Coupeaud et al.91,92 These studies enabled identification and characterization of some novel compounds with relevance in astrophysics, like, for example, cyanobutadiyne,92 which is potentially important for chemical models of the Titan’s atmosphere. Haynie, Morgan and Baumann93 photolysed matrix-isolated trifluoronitrosomethane with 633 and 670 nm light and observed formation of bis(trifluoromethyl)dioxodiazine, a previously uncharacterized species. Breda et al.,94 reported UV-induced photoisomerization of matrix-isolated pyrazine to pyrimidine, which was proposed to take place through an intermediate similar to benzvalene. Bariseviciute, Ceponkus and Sablinskas95 investigated the photochemical behaviour of ethene secondary ozonide in argon matrix in the context of developing a new method for the separation of this species from the other products of ethene ozonization reaction. Clark and Foley96 studied the photolyses of matrices of either BrCHQCHBr/NO2/Ar or ClCHQCHCl/NO2/Ar using quartz-filtered radiation (l 4 240 nm) and noticed that infrared bands attributable to carbonyl, carbon monoxide, and ketene species were observed in the irradiated matrix, discussing the possible reaction mechanism. Finally, Bucher et al.97 studied the cleavage of several phosphiranes and shown that triplet mesityl-phosphinidene is the principal product of the studied photolytic reaction. 96 | Photochemistry, 2009, 37, 72–109 This journal is

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2.2

Noble gas chemistry

This section focus on reports appearing during the period considered in this review on chemical processes in which the matrix noble gas takes an active role as reactant in a photochemical reaction, giving rise to novel covalently bound noble gas containing molecules. Most of the studies on this subject resulted from the efforts of Markku Ra¨sa¨nen and Leonid Kriachtchev’s laboratory in Helsinki, who were stimulated by their success in preparing the first covalently bound argon compound.98 An interesting review on the topic of formation of novel rare-gas molecules in low temperature matrices was published by Gerber.99 The focus of that review was on rare-gas molecules of the form HNgY, where Ng is a noble-gas atom and Y is an electronegative group, prepared by photolysis of HY in the rare-gas matrix, though other related types of new molecules of noble-gas atoms were discussed as well, including molecules resulting from photochemically induced insertion of Ng into hydrocarbons, such as HXeCCH. The main topics discussed in that review included: (a) the nature of bonding and the energetic stability of the compounds; (b) the vibrational spectroscopy of the molecules, and its role in identification of the species; (c) the mechanism and dynamics of photochemical formation of HNgY in the matrices and the pathways for thermal and infrared-induced decomposition; (d) molecules of the lighter rare gases Ar, Ne, and He, focusing on the experimentally prepared HArF98 and on theoretical predictions suggesting the existence of other molecules; and (e) clusters of HNgY with other molecules. The novel Ng containing molecules have been prepared in noble-gas matrices using UV photolysis of HY precursors and thermal mobilization of H atoms to promote their reactions with neutral Ng–Y centers. HNgY molecules have a strong (HNg)+Y ion-pair character and consequently large dipole moments. Because of this, these molecules have a very strong H–Ng stretching absorption in the infrared and can be easily detected by IR spectroscopy. Their lifetimes have been shown to be practically unlimited at low-temperatures (B10 K). Tanska¨nen et al.,100 investigated the formation mechanism of HXeCCXeH in a Xe matrix (Fig. 16), showing that the HXeCCXeH

Fig. 16 Annealing-induced IR absorption spectra of Xe- and Kr-containing noble-gas compounds in the H-Ng stretching region. The acetylene/Ng samples (12C compounds) were first irradiated at 193 nm and then annealed at 45 K for the Xe matrix (upper trace) and at 30 K for the Kr matrix (lower trace). The bands marked with an asterisk probably belong to the vinyl radical.100

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molecules are formed in secondary reactions involving HXeCC radicals formed upon photolysis of acetylene in a xenon matrix. The experimental data on the formation of HXeCCXeH was explained based on a model involving the HXeCC + Xe + H - HXeCCXeH reaction. This reaction was the first case reported when a noble-gas hydride molecule is formed from another noble-gas molecule. Kriachtchev, Tanska¨nen and Ra¨sa¨nen studied the light-induced H + XeCC 2 HXeCC reaction in solid xenon, and demonstrated the possibility of full optical control of this reaction.101 By narrowband excitation in the IR spectral region, HXeCC radicals can be decomposed to a local metastable configuration and then selectively recovered by resonant excitation of the XeCC vibrations. The novel recovery process was explained by short-range mobility of the reagents promoted by vibrational energy redistribution near the absorbing XeCC. This fact demonstrated that a chemical reaction can be selectively promoted in a desired place where the chosen absorber is located.101 Preparation of the HXeCCH molecule in Ar and Kr matrices and its characterization by IR spectroscopy was also reported, demonstrating that this molecule could be produced in another host than the polarizable Xe environment.102 The HXeCC radical and HXeCCH were also found to be formed from a xenon–acetylene low temperature matrix subjected to irradiation with fast electrons.103 The infrared and EPR spectra of these species were studied by Feldman et al.,103 who also reported results from relativistic density functional calculations with inclusion of anharmonic corrections on these systems, achieving a quantitative description of their vibrational spectra. These authors demonstrated that HXeCCH is formed in the H + Xe + CCH reaction and HXeCC in the H + XeCC reaction. The production of several other new Ng compounds by photolysis of an adequate precursor in a Ng matrix, such as HKrCCH, HXeCCH, HXeCC, HXeCCCN and HKrCCCN) was also reported.102,104 The HNgCCCN compounds with Kr and Xe were found to have similar stabilities to the previously reported HKrCN and HXeCN species and to absorb also at quite similar IR frequencies as these latter compounds, in agreement with the theoretical predictions.104 No strong candidates for an Ar compound were observed in the IR absorption spectra of the photolysed HCCCN/Ar matrix, what was also found to be in consonance with the theoretical predictions104 that the HArCCCN molecule should have a weaker H–Ar bond than the previously identified HArF molecule.98 The HXeCCH:CO2 complex was also found to be obtained upon UV photolysis of propiolic acid (HCCCOOH) in a xenon matrix, followed by thermal mobilization of H atoms at 45 K.105 Photolysis of propiolic acid led to formation of the HCCH:CO2 complex as one of the primary photoproducts, which was subsequently photolyzed to the HCC:CO2 complex. Thermal annealing of the matrix then allowed for the formation of HXeCCH:CO2. The details of the IR spectra of the simplest Ng-hydrides with general formula HNgY in cryogenic matrix media were studied by Kriachtchev et al. and Bochenkova et al., including the spectroscopic signature of their hindered rotation.106–109 The close doublet observed in the H–Ng stretching 98 | Photochemistry, 2009, 37, 72–109 This journal is

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region as the main spectral feature was explained as being originated in molecules occupying two different matrix sites and being subjected to different specific interactions with noble-gas matrix atoms,106,107 whereas the observed weaker satellite at higher frequency, was attributed to the hindered rotation (libration) of the embedded molecules in the lattice.106,108 The IR spectra of the (NgHNg)+ cations (Ng = Ar and Kr), also produced via photolysis of HF/Ar, HF/Kr, and HBr/Kr solid mixtures, were also studied in order to investigate the mechanism of decay of these species.109 It was proposed that the decay of (NgHNg)+ in noble gas matrices results from the neutralization of the solvated protons by electrons involving tunneling of an electron from an electronegative fragment or another trap to the (NgHNg)+ cation. The proposed electron-tunneling mechanism was presented110 as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids. The interaction of the (NgHNg)+ cation with a nitrogen molecule was investigated by Lignell et al.,111 both experimentally and theoretically. The computations reveal two stable structures, linear and T-shaped congurations, with interaction energies of the order of 1000 cm1. However, none of the predicted structures was found to accurately agree with the matrix-isolation experiments, the differences originating possibly from the influence of the surrounding matrix. Based on the obtained data, the mechanism of cation decay in noble-gas matrices was discussed. The observed similar decay of (NgHNg)+ and its N2 complex110,111 indicated that the solvated proton is unable to tunnel and stays immobile in noble-gas matrices. The observations for the cation decay111 were then consistent with the electron neutralization mechanism above mentioned.110 3.

IR induced reactions in cryomatrices

As mentioned in the Introduction of this chapter, controlled use of infrared radiation to promote chemical reactions in cryomatrices has emerged in the period covered by this review as an important topic of research. An interesting short review on this field was published by Mac¸oˆas et al.,112 which focused specifically the authors’ studies on narrowband infraredinduced conformational isomerization processes of small carboxylic and dicarboxylic acids (formic, acetic, propionic, malonic, fumaric, oxalic and maleic acids). The smaller members of the carboxylic acid family were indeed the molecules most studied by this approach. The initial studies of the trans-cis IR-induced conformational isomerization in formic acid in Ar, Kr and Xe matrices19,113 were extended to studies where the compound was embedded in neon and hydrogen matrices.114,115 In neon matrix, besides the selective IR induced conversion of trans to cis monomeric formic acid, that had been previously observed for the compound in the heavier noble gases matrices,19,113 conversion of the trans/trans to the trans/cis dimer of the compound was observed.114 The proton tunneling decay of the cis monomer was found to be surprisingly fast in solid neon (two orders of magnitude faster than in solid argon). It was also noticed that the stability of the trans/cis dimer against proton tunneling is enormously enhanced in solid Photochemistry, 2009, 37, 72–109 | 99 This journal is

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neon compared to the monomer (by a factor of B300), and these results were discussed in terms of matrix solvation and hydrogen bonding.114 In solid hydrogen, the quantum yield of the trans to cis conformational process was measured to be about two orders of magnitude smaller than in solid argon.115 This result was explained considering the efficient coupling of the vibrationally excited trans conformer with the host vibrations, which deactivates the conformational change. Another important result was the observation that the trans to cis conformational process could be efficiently promoted by excitation of the hydrogen-matrix rovibrational transitions (host excitation), which confirmed the strong coupling between vibrations of the host and embedded molecule in this case.115 These results demonstrate a unique process of conformational reorganization mediated by vibrational excitation of the host. The tunneling decay of the cis formic acid monomer in solid hydrogen was found to be 4 times faster than in solid argon but 30 times slower than in solid neon, what was interpreted based on the different solvation properties of the hosts.115 Marushkevich et al.116 measured the vibrational spectrum of the trans/cis dimer of formic acid in argon matrix formed by selective vibrational excitation of the trans/trans dimer of the compound and concluded that the stability of the cis/trans dimer against proton tunneling was also strongly improved compared to that of the monomer in this matrix media, especially at elevated temperatures (430 K). This phenomenon was explained by differences in dynamical, energetic, and vibrational properties of the dimer and monomer.116 The obtained results showed that the proton tunneling reactions can be strongly modified in the hydrogen-bonded solid network compared to the monomeric species and stabilize less stable conformers. This general effect occurs when the energy difference between conformers is smaller than the hydrogen bond interaction energy, which opens perspectives in chemistry on intrinsically unstable conformers. The same authors also studied hydrogen bonding between formic acid and water in solid argon and identified the first water complex with the higher-energy cis form of formic acid.117 In sharp contrast to the isolated cis monomer but as found for the trans/cis dimer of formic acid,116 cis formic acid complex with water was concluded to be very stable at low temperatures.117 This stability was explained by the strong O–H  O hydrogen bonding existing in the cis-formic acid/water dimer. Infrared-induced isomerization was also described to take place in water polymers in cryogenic matrices as well as in other matrix-isolated complexes of water with small organic molecules, by Coussan, Roubin and Perchard.118,119 The structure of ROH:R–OH heterodimers (R, R 0 = H, CH3, C2H5) trapped in argon and nitrogen matrices was examined using selective irradiations in the nOH region for the CH3OH:H2O and C2H5OH:H2O dimers. Type-I-Type-II interconversion (where Type-I and Type-II designate complexes whether its smaller component acts as proton acceptor or proton donor, respectively) was observed in N2 matrix (Fig. 17), while no effect was detected in Ar.119 Acetic acid trans-cis IR-induced conformational isomerization was also studied in detail by Mac¸oˆas et al.120–122 in the continuation of the first production of the less stable cis conformer by nOH fundamental and 1st 100 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 17 Effect irradiation at 3527 cm1 on a C2H5OH/H2O/N2) 1/3/600 annealed matrix. Recording temperature: 4 K. a: before irradiation. b: difference spectrum after 35 min irradiation. W2: (H2O)2; I, II: Type-I, Type-II C2H5OH:H2O dimer (water acting as proton acceptor or proton donnor, respectively; see text).119

Fig. 18 Calculated and experimental vibrational spectra of cis and trans acetic acid. The lower trace is the experimental difference spectrum showing the result of vibrational excitation of the trans conformer in an Ar matrix at 8 K.19

overtone selective IR irradiation of the most stable trans conformer (Fig. 18).123 Emphasis was put on the effects of medium and isotopic substitution on the observed quantum yields. Several isotopologues of acetic acid were studied in different matrices (Ar, Kr, Xe) and it was found that, for excitation of acetic acid at energies above the predicted isomerization energy barrier, the measured quantum yields were in average 2%–3%, i.e., one order of magnitude smaller that the corresponding values for formic acid (Fig. 19). Such reduction of the reaction quantum yields was explained as been due to a competition between the isomerization reaction coordinate (torsion arround the C–O bond) and other low energy dissipative channels which are active in acetic acid but absent in formic acid Photochemistry, 2009, 37, 72–109 | 101 This journal is

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Fig. 19 Quantum yields for the IR-induced trans-cis isomerization of acetic acid [CH3COOH in solid Ar (’), Kr (m); and Xe ( ); and CD3COOH (J), CH3COOD (R) and CD3COOD (K) in solid Ar]. The results for the trans-cis isomerization of formic acid in solid Ar are shown for comparison (*). The error bar represents B50% of the estimated quantum yields.120

(e.g., the CH3 torsion).120,121 The cis-trans back-reaction tunneling rates were also measured and the possible conformational dependence of UV-induced photodissociation of acetic acid investigated. However, contrarily to what was found previously for formic acid,124 no evidence of conformer specific photodissociation was found.121 Very interestingly, the UV photolysis of the matrix-isolated acetic acid revealed very different products from the gas phase. Methanol complexed with carbon monoxide was found to be the major product of photolysis of acetic acid isolated in argon matrices whereas it had never been observed as a photolysis product in the gas phase. Narrowband IR-induced conformational isomerization in several isotopologues of propionic acid isolated in cryogenic matrices was also investigated.125 Propionic acid is a four conformational states system (Tt, Tg, Ct and Cg; Fig. 20), the ground-state Tt conformer being the only form which was found to be present in the as-deposited matrices. Narrowband excitation of the first hydroxyl stretching overtone of the conformational ground state promoted the Ca–C and C–O internal rotations, producing the Tg and Ct conformers, respectively. Subsequent vibrational excitation of the initially produced Tg form induced its conversion into the Cg conformer, by rotation around the C–O bond (Fig. 21). On the whole, all the higher energy conformers of propionic acid could be produced and characterized spectroscopically for the first time.125 In the dark, the newly obtained conformers were found to decay to the conformational ground state at different rates. Duvernay et al.38 used also narrowband IR irradiation to characterize, for the first time, all the isomers of formimidic acid, H(OH)CQNH. Selective excitation was performed by pumping at the nOH stretching mode wavelength. On the other hand, nNH stretching excitation was used by Dian et al.126 to study the dynamics of conformational isomerization in melatonin and 5-methoxy N-acetyltryptophan methyl amide. The IR-UV 102 | Photochemistry, 2009, 37, 72–109 This journal is

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Fig. 20 Spectral changes upon excitation of the 2nOH mode of Tt and Tg conformers (traces a and c, respectively) and dark decay of the Ct (’) and Cg ( ) conformers (traces b and d, respectively). Difference spectra a and c are obtained by subtracting the spectra recorded before irradiation from those recorded under irradiation; difference spectra b and d by subtracting the spectra recorded under irradiation from those recorded after a few minutes in the dark.125

Fig. 21 Carbonyl stretching region in the IR spectra of matrix-isolated glycolic acid. Dashed line—spectrum of glycolic acid deposited in Kr matrix at 16 K. Spectra after shorter and longer irradiations are presented with thin and thick solid lines, respectively. SSC corresponds to the most stable conformer; AAT and GAC to the two higher energy forms produced upon IR irradiation.127

hole-filling spectroscopy and IR-induced population transfer spectroscopy were used in the study. An IR pump laser was used for the selective Photochemistry, 2009, 37, 72–109 | 103 This journal is

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excitation of each of the amide NH stretch fundamentals. Some conformation speciality was observed in the isomerization quantum yields for melatonin, but not hint of vibrational mode speciality. It was also found that for the 5-methoxy derivative, there was no isomerization out of the single conformational well populated in the expansion in the absence of the infrared excitation.126 Broadband IR irradiation was used by Reva et al.127 to produce high-energy conformers of glycolic acid (HOCH2COOH) isolated in lowtemperature argon and krypton matrices from the initially deposited conformational ground state (Fig. 21). Systematic monitoring of the progress of the IR-induced conformational isomerizations (over a time scale of 20–140 min) allowed to distinguish between the primary and secondary photoproducts, implying that the conformational isomerization processes occured in a stepwise way. The identification of the observed conformers and the interpretation of the transformation dynamics were assisted by high-level theoretical calculations of the possible pathways for intramolecular rotation. Lack of conformers other than the 3 experimentally observed in the IR-irradiated matrices was explained in terms of energy barriers separating different forms of the compound.127 Finally, an interesting study by Raston and Anderson128 was also reported during the period covered by this review in which irradiation of Cl atom doping parahydrogen solid with broadband infrared radiation was found to induce reaction of atomic Cl with the parahydrogen matrix, to form HCl. The infrared-induced chemistry was attributed to solid parahydrogen absorptions that lead to the creation of vibrationally excited H2 (v =1), which supply the necessary energy to induce reaction. The kinetics of this low temperature infrared-induced reaction was studied using IR spectroscopy of the HCl reaction product. The HCl formation kinetics was found to be first-order and the magnitude of the effective rate constant for the infrared-induced reaction to be dependent on the properties of the near infrared radiation.

Acknowledgements The authors thank the Portuguese Science Foundation (FCT, Lisbon, Project PTDC//QUI/71203/2006) and CYTED (108RT0362) for financial support. References 1 B. Meyer, Low Temperature Spectroscopy, American Elsevier Publishers Company, New York, 1971. 2 Chemistry and Physics of Matrix Isolated Species, ed. L. Andrews and M. Moskovits, Elsevier, Amsterdam, 1989. 3 Matrix Isolation Spectroscopy, ed. A. Barnes, W. J. Orville-Thomas, R. Gaufrhs and A. Muller, Springer, 1981. 4 I. R. Dunkin, Matrix Isolation Techniques: A Practical Approach, Oxford University Press, 1998. 5 Low Temperature Molecular Spectroscopy 1996, ed. R. Fausto, NATO-ASI Series C483, Kluwer, Amsterdam, 1996. 6 V. A. Apkarian and N. Schwentner, Chem. Rev., 1999, 99, 1481. 104 | Photochemistry, 2009, 37, 72–109 This journal is

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39 F. Duvernay, P. Chatron-Michaud, F. Borget, D. M. Birney and T. Chiavassa, Phys. Chem. Chem. Phys., 2007, 9, 1099. 40 F. Duvernay, T. Chiavassa, F. Borget and J. P. Aycard, J. Phys. Chem. A, 2005, 109, 6008. 41 F. Duvernay, T. Chiavassa, F. Borget and J. P. Aycard, Chem. Phys., 2004, 298, 241. 42 S. Lopes, A. Go´mez-Zavaglia, L. Lapinski and R. Fausto, J. Phys. Chem. A, 2005, 109, 5560. 43 R. K. Solly and S. W. Benson, J. Am. Chem. Soc., 1971, 93, 2117. 44 S. North, D. A. Blank and Y. T. Lee, Chem. Phys. Lett., 1994, 224, 38. 45 T. Tamezane, N. Tanaka, H. Nishikiori and T. Fujii, Chem. Phys. Lett., 2006, 423, 434. 46 Y. A. Tobon, L. I. Nieto, R. M. Romano, C. O. Della Ve´dova and A. J. Downs, J. Phys. Chem. A, 2006, 110, 2674. 47 R. M. Romano, C. O. Della Ve´dova and A. J. Downs, Chem. Eur. J., 2007, 13, 8185. 48 S. Breda, L. Lapinski, I. Reva and R. Fausto, J. Photochem. Photobiol. A—Chemistry, 2004, 162, 139. 49 I. D. Reva, M. J. Nowak, L. Lapinski and R. Fausto, Chem. Phys. Lett., 2006, 429, 382. 50 N. Kus, S. Breda, I. Reva, E. Tasal, C. Ogretir and R. Fausto, Photochem. Photobiol., 2007, 83, 1237. 51 S. Breda, I. Reva, L. Lapinski, M. L. S. Cristiano, L. Frija and R. Fausto, J. Phys. Chem. A, 2006, 110, 6415. 52 S. Breda, I. D. Reva, L. Lapinski and R. Fausto, ChemPhysChem, 2005, 6, 602. 53 N. J. Turro, Modern Molecular Photochemistry, University Science Books, Sausalito, California, 1991. 54 M. Klessinger and J. Michl, Excited States and Photochemistry of Organic Molecules, VCH, New York, 1995. 55 J. S. Seixas de Melo, G. Quinteiro, J. Pina, S. Breda and R. Fausto, J. Mol. Struct., 2001, 565/566, 59. 56 T. Yatsuhashi and N. Nakashima, J. Phys. Chem. A, 2000, 104, 1095. 57 A. Go´mez-Zavaglia, I. D. Reva, L. Frija, M. L. Cristiano and R. Fausto, J. Photochem. Photobiol. A—Chemistry, 2006, 179, 243. 58 A. Go´mez-Zavaglia, I. D. Reva, L. Frija, M. L. Cristiano and R. Fausto, J. Phys. Chem. A, 2005, 109, 7967. 59 A. Go´mez-Zavaglia, I. D. Reva, L. Frija, M. L. S. Cristiano and R. Fausto, J. Photochem. Photobiol. A—Chemistry, 2006, 180, 175. 60 L. M. T. Frija, I. D. Reva, A. Go´mez-Zavaglia, M. L. S. Cristiano and R. Fausto, Photochem. Photobiol. Sci., 2007, 6, 1170. 61 L. M. T. Frija, I. D. Reva, A. Go´mez-Zavaglia, M. L. S. Cristiano and R. Fausto, J. Phys. Chem. A, 2007, 111, 2879. 62 A. Kaczor, A. Go´mez-Zavaglia, A. L. Cardoso, T. M. V. D. Pinho e Melo and R. Fausto, J. Phys. Chem. A, 2006, 110, 10742. 63 A. Go´mez-Zavaglia, A. Kaczor, A. L. Cardoso, T. M. V. D. Pinho e Melo and R. Fausto, J. Mol. Struct., 2007, 834/836, 262. 64 A. Khvorostov, L. Lapinski, H. Rostkowska and M. J. Nowak, J. Phys. Chem. A, 2005, 109, 7700. 65 A. Gerega, L. Lapinski, I. Reva, H. Rostkowska and M. J. Nowak, Biophys. Chem., 2006, 122, 123. 66 A. Khvorostov, L. Lapinski, H. Rostkowska and M. J. Nowak, J. Photochem. Photobiol. A—Chemistry, 2006, 184, 322. 106 | Photochemistry, 2009, 37, 72–109 This journal is

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67 H. Rostkowska, L. Lapinski, A. Khvorostov and M. J. Nowak, J. Phys. Chem. A, 2005, 109, 2160. 68 H. Rostkowska, L. Lapinski, A. Khvorostov and M. J. Nowak, Chem. Phys., 2004, 298, 223. 69 A. Gerega, L. Lapinski, M. J. Nowak and H. Rostkowska, J. Phys. Chem. A, 2006, 110, 10236. 70 M. Nagaya and M. Nakata, J. Phys. Chem. A, 2007, 111, 6256. 71 K. Ujike, S. Kudoh and M. Nakata, Chem. Phys. Lett., 2005, 409, 52. 72 N. Akai, K. Ohno and M. Aida, J. Photochem. Photobiol. A—Chemistry, 2007, 187, 113. 73 N. Akai, T. Harada, K. Shin-Ya, K. Ohno and M. Aida, J. Phys. Chem. A, 2006, 110, 6016. 74 N. Akai, H. Yoshida, K. Ohno and M. Aida, Chem. Phys. Lett., 2005, 403, 390. 75 L. Lapinski, H. Rostkowska, A. Khvorostov and M. J. Nowak, Phys. Chem. Chem. Phys., 2003, 5, 1524. 76 J. Wang, G. Burdzinski, Z. D. Zhu, M. S. Platz, C. Carra and T. Bally, J. Am. Chem. Soc., 2007, 129, 8380. 77 E. A. Pritchina, N. B. Gritsan and T. Bally, Phys. Chem. Chem. Phys., 2006, 8, 719. 78 W. Sander, M. Winkler, B. Cakir, D. Grote and H. F. Bettinger, J. Org. Chem., 2007, 72, 715. 79 H. F. Bettinger and H. Bornemann, J. Am. Chem. Soc., 2006, 128, 11128. 80 N. P. Gritsan, Russ. Chem. Rev., 2007, 76, 1139. 81 R. J. Yang, L. Yu, X. Jin, M. F. Zhou and B. K. Carpenter, J. Chem. Phys., 2005, 122, Mns. 014511. 82 H. Tanska¨nen, L. Khriachtchev, M. Ra¨sa¨nen, V. I. Feldman, F. F. Sukhov, A. Y. Orlov and D. A. Tyurin, J. Chem. Phys., 2005, 123, Mns. 064318. 83 E. Popov, T. Kiljunen, H. Kunttu and J. Eloranta, J. Chem. Phys., 2007, 126, Mns. 134504. 84 E. Y. Misochko, A. V. Akimov, V. A. Belov and D. A. Tyurin, Russ. Chem. Bull., 2007, 56, 438. 85 S. Venkataramani, M. Winkler and W. Sander, Ang. Chem.—Itnl. Ed., 2006, 44, 6307. 86 M. Abe, S. Kawanami, A. Masuyama and T. Hayashi, J. Org. Chem., 2006, 71, 6607. 87 O. C. M. O’Sullivan, S. G. Collins, A. R. Maguire, M. Bohm and W. Sander, Eur. J. Org. Chem., 2006, 13, 2918. 88 J. Miyazaki and Yamada, J. Mol. Struct., 2004, 692, 145. 89 Z. Guennoun, N. Pietri, I. Couturier-Tamburelli and J. P. Aycard, Chem. Phys., 2004, 300, 23. 90 N. Akal, S. Kudoh and M. Nakata, Chem. Phys. Lett., 2004, 392, 480. 91 A. Coupeaud, M. Turowski, M. Gronowski, N. Pietri, I. Couturier-Tamburelli, R. Kolos and J. P. Aycard, J. Chem. Phys., 2007, 126, Mns. 164301. 92 A. Coupeaud, R. Kolos, I. Couturier-Tamburelli, J. P. Aycard and N. Pietri, J. Phys. Chem. A, 2006, 110, 2371. 93 B. C. Haynie, M. J. Morgan and C. A. Baumann, J. Phys. Chem. A, 2005, 109, 5307. 94 S. Breda, I. D. Reva, L. Lapinski, M. J. Nowak and R. Fausto, J. Mol. Struct., 2006, 786, 193. 95 R. Bariseviciute, J. Ceponkus and V. Sablinskas, Centr. Eur. J. Chem., 2007, 5, 71. Photochemistry, 2009, 37, 72–109 | 107 This journal is

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96 R. J. H. Clark and L. J. Foley, Spectrochim. Acta A—Molecular and Biomolecular Spectroscopy, 2005, 61, 1389. 97 G. Bucher, M. L. G. Borst, A. W. Ehlers, K. Lammertsma, S. Ceola, M. Huber, D. Grote and W. Sander, Ang. Chem.—Itnl. Ed., 2005, 44, 3289. 98 L. Khriachtchev, M. Pettersson, N. Runeberg, J. Lundell and M. Ra¨sa¨nen, Nature, 2000, 406, 874. 99 R. B. Gerber, Ann. Rev. Phys. Chem., 2004, 55, 55. 100 H. Tanska¨nen, L. Khriachtchev, J. Lundell and M. Ra¨sa¨nen, J. Chem. Phys., 2004, 121, 8291. 101 L. Khriachtchev, H. Tanska¨nen and M. Ra¨sa¨nen, J. Chem. Phys., 2006, 124, Mns. 2201445. 102 H. Tanska¨nen, L. Khriachtchev, J. Lundell and M. Ra¨sa¨nen, J. Chem. Phys., 2006, 125, Mns. 074501. 103 V. Feldman, F. Sukhov, A. Orlov, I. Tyul’pina, E. Logacheva and D. Tyurin, Russ. Chem. Bull., 2005, 54, 1458. 104 L. Khriachtchev, A. Lignell, H. Tanska¨nen, J. Lundell, H. Kiljunen and M. Ra¨sa¨nen, J. Phys. Chem. A, 2006, 110, 11876. 105 H. Tanska¨nen, S. Johansson, A. Lignell, L. Khriachtchev and M. Ra¨sa¨nen, J. Chem. Phys., 2007, 127, Mns. 154313. 106 L. Khriachtchev, A. Lignell, J. Juselius, M. Ra¨sa¨nen and E. Savchenko, J. Chem. Phys., 2005, 122, Mns. 014510. 107 L. Khriachtchev, A. Lignell and M. Ra¨sa¨nen, J. Chem. Phys., 2004, 120, 3353. 108 A. V. Bochenkova, L. Khriachtchev, A. Lignell, M. Ra¨sa¨nen, H. Lignell, A. A. Granovsky and A. V. Nemukhin, Phys. Rev. B, 2008, 77, Mns. 094301. 109 A. V. Bochenkova, D. A. Firsov and A. Nemukhin, Chem. Phys. Lett., 2005, 1–3, 165. 110 L. Khriachtchev, A. Lignell and M. Ra¨sa¨nen, J. Chem. Phys., 2005, 123, Mns. 064507. 111 A. Lignell, L. Khriachtchev, H. Lignell and M. Ra¨sa¨nen, Phys. Chem. Chem. Phys., 2006, 8, 2457. 112 E. M. S. Mac¸oˆas, L. Khriachtchev, M. Pettersson, J. Lundell, R. Fausto and M. Ra¨sa¨nen, Vibrat. Spectrosc., 2004, 34, 73. 113 E. M. S. Mac¸oˆas, J. Lundell, M. Pettersson, L. Khriachtchev, R. Fausto and M. Ra¨sa¨nen, J. Mol. Spectrosc., 2003, 219, 70. 114 K. Marushkevich, L. Khriachtchev and M. Ra¨sa¨nen, J. Chem. Phys., 2007, 126, Mns. 241102. 115 K. Marushkevich, L. Khriachtchev and M. Ra¨sa¨nen, Phys. Chem. Chem. Phys., 2007, 9, 5748. 116 K. Marushkevich, L. Khriachtchev, J. Lundell and M. Ra¨sa¨nen, J. Am. Chem. Soc., 2006, 128, 12060. 117 K. Marushkevich, L. Khriachtchev and M. Ra¨sa¨nen, J. Phys. Chem. A, 2007, 111, 2040. 118 S. Coussan, P. Roubin and J. P. Perchard, Chem. Phys., 2006, 324, 527. 119 S. Coussan, P. Roubin and J. P. Perchard, J. Phys. Chem. A, 2004, 108, 7331. 120 E. M. S. Mac¸oˆas, L. Khriachtchev, M. Pettersson, R. Fausto and M. Ra¨sa¨nen, J. Chem. Phys., 2004, 121, 1331. 121 E. M. S. Mac¸oˆas, L. Khriachtchev, R. Fausto and M. Ra¨sa¨nen, J. Phys. Chem. A, 2004, 108, 3380. 122 E. M. S. Mac¸oˆas, L. Khriachtchev, M. Pettersson, R. Fausto and M. Ra¨sa¨nen, Phys. Chem. Chem. Phys., 2005, 7, 743. 123 E. M. S. Mac¸oˆas, L. Khriachtchev, M. Pettersson, R. Fausto and M. Ra¨sa¨nen, J. Am. Chem Soc., 2003, 125, 16188. 124 J. Lundell and M. Ra¨sa¨nen, J. Mol. Struct., 1997, 436/437, 349. 108 | Photochemistry, 2009, 37, 72–109 This journal is

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125 E. M. S. Mac¸oˆas, L. Khriachtchev, M. Pettersson, R. Fausto and M. Ra¨sa¨nen, J. Phys. Chem. A, 2005, 109, 16. 126 B. C. Dian, G. M. Florio, J. R. Clarkson, A. Longarte and T. S. Zwier, J. Chem. Phys., 2004, 120, 9033. 127 I. D. Reva, S. Jarmelo, L. Lapinski and R. Fausto, J. Phys. Chem. A, 2004, 108, 6982. 128 P. L. Raston and D. T. Anderson, Phys. Chem. Chem. Phys., 2004, 8, 3124.

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Alkenes, alkynes, dienes, polyenes Takashi Tsuno* DOI: 10.1039/b812707g This review briefly covers the literature concerning photochemistry of alkenes, alkynes, dienes, and polyenes published during July 2004 and June 2007.

1.

Introduction

This review briefly covers the literature concerning photochemistry of alkenes, alkynes, dienes, and polyenes published during July 2004 and June 2007.

2.

Photochemistry of alkenes

A review of CQC photoinduced isomerization reactions, focused on synthetic organic photochemistry, has been published by Mori and Inoue.1 Ro¨hrig et al. have reviewed the E–Z isomerization in organic and biological systems using computational dynamic studies.2 Kato et al. have found that zigzag relationships exist between the number of p-electrons of the monocyclic annulenyl substituent and the profile of the T1 potential energy surface for the E–Z isomerization of an alkene through quantum chemical calculation.3 This result could likely be helpful for the design of alkene-containing molecular switch and memories. Time-resolved CIDNP gives detailed information about the complex reaction mechanism of the isomerization via radical-ion pairs in the triplet state of several alkenes.4 Inoue et al. have reported the enatiodifferentiating E–Z photoisomerization of (Z)-cyclooctene (1) sensitized by using permethylated 6-O-benzoylb-cyclodextrin5 and homochiral mesoporous POST-1.6 The conformational flexibility of the permethylated cyclodextrin host enables the entropy-driven cyclooctene complexation in the ground state and the entropy-controlled enatiodifferentiation in the excited state.5 The photochemistry of optically active (E)-cyclooctene upon irradiation at 254 nm by the use of a mercury lamp and at 266 nm with an Nd-YAG laser has been studied. The racemization is slightly faster than the E - Z isomerization.7 Photochemical E–Z isomerization of encapsulated (Z)-enecarbamates (2) within the hydrophobic chiral cavities of g-cyclodextrin shows higher diastereoselectivities in the products.8 Further, the E–Z isomerization upon direct and triplet sensitized irradiation with chiral/achiral sensitizers has

Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Narashino, Chiba 275-8575, Japan. E-mail: [email protected]; Fax: +81-(0)47-474-2579; Tel: +81-(0)47-474-2569

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been investigated. The enhanced product diastereoselectivity is found to depend on the solvent and temperature.9

2.1

Stilbene and derivatives

The light driven E–Z isomerization of stilbenes as ligands in rhenium complexes has been reviewed.10 The (E)-isomer of 3-(N-phenylamino)stilbene and its analogues have been studied by electronic spectroscopy and photochemistry.11 The N-phenyl derivatives have lower fluorescence quantum yields and higher photoisomerization quantum yields by the m-amino conjugation effect. Yan et al. have studied the one-photon fluorescence and two-photon fluorescence of compound (3) as a photopolymerization initiator.12 Yang et al. have investigated the substituentdependent photoinduced intramolecular charge transfer in N-aryl-substituted (E)-aminostilbenes (4) and reported that their photochemical behavior in solvents more polar than THF strongly depends on the substituent in the N-aryl group.13 Stilbene (4a) leads to the formation of a planer intramolecular charge transfer state, while (4b) affords a twisted charge transfer state. The isomerization of (E)-stilbene to the (Z)-isomer has been investigated in sodium dodecyl sulfate/benzyl alcohol/water.14 The yield of the (Z)-isomer increased with an increase in water content or with the decrease in benzyl alcohol content.

Debnarova and Techert have applied the ab initio treatment of time-resolved X-ray scattering to the photoisomerization of (E)-stilbene.15 This technique will be applied to the investigation of bioorganic systems. A time-resolved absorption study on (Z)-stilbene in solution has also been performed.16 An oscillatory signal at 220 cm1 suggests a clear wavepacket motion for the reaction coordinate of the isomerization of (Z)-stilbene. Fuß et al. have reported a hula-twist pathway for the photochemical E–Z isomerization of stilbene and concluded that the hula-twist can take place in free molecules without any constraint.17 Other quantum calculations concerning the photoisomerization of stilbene have also been carried out.18,19 In addition, the laser UV photoionization of (E)-stilbene in a channel of acidic20 or nonacidic21 ZSM-zeolite has been examined. The Photochemistry, 2009, 37, 110–148 | 111 This journal is

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photochemical reactivity of model organic systems using stilbene and 1,4-diphenyl-1,3-butadiene in organic glasses has been investigated and compared with their reactivity in solution and in organized media. The close interactions between host molecules and the alkenes in amorphous organic glasses have made volume-conserving hula-twist process a preferred of E–Z isomerization in organized media.22 Cascade hole transfer from surface bound the radical cation of 4-phenylbenzoic acid to (E)-stilbene at the surface of TiO2 powder slurried in acetonitrile have been observed.23 The observation of the formation of stilbene+ suggests a hula-twist reaction between the radical cation of 4-phenylbenzoic acid and the ground state of stilbene at the TiO2 surface. The photophysical properties of perdeuterated (E)-stilbene grafted on polystyrene have been investigated.24 In polar solvents, the polymer shows a red-shifted emission. The photoexcitation of the stilbene chromophore is affected by the photoisomerization process. The role of adiabatic pathways has been discussed about the E–Z isomerization of some stilbene-like derivatives on the basis of the results of combined fluorimetric and qphotochemical measurements and statistical analysis.25

Because the intersystem-crossing quantum yields of pyrene-benzoylthiophene bichromophores (5) and (6) have been determined to be 0.91 and 0.79, respectively, (5) and (6) act as good triplet sensitizers for the E - Z isomerization of stilbene.26 Natarajan et al. have synthesized (Z)-diaryl alkenes (7)–(11) and investigated their photochemistry in the solid state.27 The alkenes (7)–(10) undergo the one-way Z - E isomerization in the solid state, but no (11). Because the geometrical isomerization requires large motion and the molecules of crystalline (11) are tightly packed, neither the one-bond flip nor the hula-twist is able to produce the trans-isomer. Bridged phenylthienylethenes and dithienylethenes (12)–(16) have been prepared by Pd-catalyzed doublecyclization reaction of diarylhexadienylenes and their photochemical E–Z isomerization examined.28 Only phenylthienylethene (12) undergoes the one-way Z - E isomerization. The effect of the thienyl groups on the E–Z isomerization of five thienylethene compounds (17)–(21) has been investigated.29 The presence of one or two thienyl groups and their positional isomerism affect the spectral behavior, the relaxation properties, the isomerization mechanism, and the ground state rotamerism. Basaric´ et al. have studied photochemistry of o-pyrrolylstilbenes (22).30 Stilbenes (22) show two photochemical processes: E–Z isomerization and hydrogen transfer of NH to the double bond moiety. In the presence of 9,10-dicyanoanthracene as a sensitizer, 1,1-diaryl-2-t-butylethene (23) undergoes one-way isomerization upon irradiation.31 The (E)-4-dodeoxystilbene (24) bearing an oxalyl 112 | Photochemistry, 2009, 37, 110–148 This journal is

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amide group acts as a versatile gelator of various organic solvents, whereas its (Z)-form shows poor gelation ability.32 Gogoll et al. have prepared the photoswitchable stilbene-type b-hairpin mimetic (25) capable of light triggered conformational changes.33 Arai et al. have stereoselectively synthesized (E)- and (Z)-oligo(phenylenevinylene)s (26)–(29) and studied their photoisomerization concerning effect of p-conjunction chain length.34 Monomers, (E)- and (Z)-(26), undergo two-way isomerization, while (27)–(29) undergo one-way isomerization from all-(Z)-form to all-(E)-form. The singlet excited state of (E)-1-(1 0 -naphthyl)-2-(3-hydroxyphenyl)ethane (30) gives the (Z)-isomer via adiabatic E - Z photoisomerization. Then the resulting singlet excited state of the (Z)-isomer undergoes photocyclization to afford hydroxychrysene.35 Irradiation of (Z,Z)-1,4-bis(2 0 -quinoylethenyl)benzene (31) has been found to cause direct adiabatic isomerization to a product having the same life-time as the (E,E)-isomer.36 Irradiation of (32) in the presence of iodine and propylene oxide in cyclohexene affords heptahelicen-2-yl(diphenyl)phosphine oxide. Reduction of the phosphine oxide with trichlorosilane gives Photochemistry, 2009, 37, 110–148 | 113 This journal is

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2-(diphenylphsphino)heptahericene (33), which coordinates metals such as ruthenium and palladium as a monodentate ligand.37 A dithienylethene (34) with chiral pinene moieties gives an M-[7]-dithiahelicene upon irradiation with 400 nm.38 It is expected that the photochromism can be utilized as a chiroptical photoswitch. Moreover, Yokoyama et al. have been reported chiral helicenoid diarylethenes (35)39 and (36)40 with a large change in the specific optical rotation by photochromism. Dithienylethenes (37) readily 114 | Photochemistry, 2009, 37, 110–148 This journal is

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undergo photocyclization in the absence of iodine as an oxidant to give thiahelicenes in 70% yield.41

Two isomeric bichromophoric species (38), each containing [Ru(bpy)3]2+ and 1,2-bis(biphenyl)ethene units covalently linked together by an ether tether, have been synthesized and characterized.42 The (Z)-isomer (38) isomerizes 250 times faster than the non-metalated alkene + [Ru(bpy)3]2+. This result would be applied to the development of precipitons as energydriven metal scavengers. Meier et al. have prepared the dendrimers (39) with (E)-stilbene chromophores in the core and studied the photochemistry in solution and in neat films.43 The irradiation of (39) leads to E–Z geometrical isomerization and intra-/intermolecular C–C bond formations. The C–C bond formations predominate in concentrated solution (4 103 M) when energy-rich UV light is used. Meier et al. have also investigated novel two dendrimers (40) and (41) with (E)-stilbene chromophores in peripheral positions of the dendrons.44 Arai et al. have reported both singlet and triplet energy transfer in stilbene-cored benzophenone dendrimers (42).45 The efficiency of singlet energy transfer from stilbene to benzophenone is estimated to be 99.7%, while the energy transfer from benzophenone triplet to stilbene is estimated to be 97%. The authors have also investigated the E–Z isomerization and the fluorescence properties of p-substituted benzyl type stilbene dendrimers (43)–(45).46 The E–Z isomerization occurs in the subnano-second time scale within the interior of the dendrimers. Furthermore, the photoinduced electron-transfer reactions and two-photon ionization process of a series of stilbene bearing 3,5-dibenzyl ether-type dendrons have been reported.47 The methacrylic monomer (46) containing a stilbene unit has been prepared and radically copolymerized with methyl methacrylate to investigate the photochemical and photophysical properties. The photoreactivity of (46) upon irradiation is higher than that of the copolymer (47).48 The photochemical E–Z isomerization of the stilbene moiety has been applied to photo-driven molecular devices such as a molecular motor,49,50 a Photochemistry, 2009, 37, 110–148 | 115 This journal is

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molecular shuttle,50 mechanical switching,49,50 etc.49,50 Novel light-driven molecular motors (48),51 (49),52 (50),53 and (51)54 have been prepared and studied by Feringa et al. Compound (48a) is effective in inducing helical organization in a liquid-crystal film51a,b and acts as a reversible full-range control of a cholesteric liquid-crystalline film.51c Feringa et al. have also applied it to control the twist sense of a helical polymer55 and the rotary motion on a quartz monolayer56 or on a gold surface.57 Harada et al. have also reported a new model of light-powered chiral molecular motors (52).58 Interestingly, Tour et al.59 have developed a motorized nanocar60 using a photo-driven molecular motor. Axially chiral BINOL-appended stiff-stilbenes (53) have been utilized as novel photoswitchable chiral hosts.61 The stilbene derivatives (54) with an alkoxy chain length larger than 6 exhibit a nematic phase.62 Upon irradiation at 340 nm, compounds (54) 116 | Photochemistry, 2009, 37, 110–148 This journal is

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clearly undergo E–Z isomerization to the Z-isomers in an almost complete manner. Photochemistry, 2009, 37, 110–148 | 117 This journal is

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The diarylethene (55a) in solution shows photochromism like dithienylethene derivatives.63 Compound (55b) exhibits a strong green emission both in solution and as a thin film at 540 nm.64

Branda et al. have presented a new approach to deliver chemical species on the basis of combining reactivity-gate photochemistry and photogate reactivity.65,66 Cyclohexadiene (56) in solvents does not show any photochromic response. However, the [4 + 2] cycloadduct (57) derived by the Diels-Alder reaction of (56) with maleic anhydride shows photochromic response.65 Dithienylfulvenes (58) also does not show photochromic response and readily undergo reversible Diels-Alder reaction between (58) and the bicyclic compounds (59) when treated dicycanofumarate.66 118 | Photochemistry, 2009, 37, 110–148 This journal is

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The photochromic properties derived from the isomerization of (59) and (60) has been found.

Diarylethene derivatives involving dithienylethenes and fulgides have been well known as photochromic compounds67 and many papers have been published for three years. The other chapters will fully cover their photochemistry implying photocyclization, photochromic properties, etc. 2.2

Photochemical [2 + 2] cycloaddition

General reviews of photocycloaddition of alkenes to alkenes68 and to dienes69 have been published. Furthermore, absolute asymmetric photochemical transformations including [2 + 2] cycloaddition of alkenes have been reviewed by Sakamoto.70 Frisˇ cˇicˇ and MacGillivary have published a review on single-crystal-to-single-crystal [2 + 2] photodimerizations.71 2.2.1 Intermolecular [2 + 2] cycloaddition and dimerization. Ramamurthy et al. have studied the templating photodimerization of stilbazoles with water-soluble calixarenes, cucurbit[6]uril (CA[6]) and cucurbit[8]uril (CA[8]). CA[6] and CA[8] help to localize the stilbazoles and them in a specific geometry to afford anti-head-to-tail [2 + 2] cycloadducts.72,73 For example, irradiation of (61) included in CB[8] in water gave anti-head-to-tail [2 + 2] cycloadducts (62) in 84% over yields.72 The solid state photochemistry of two styrylquinoline compounds (63) and (64) and their N-ethylquinolinium salts has been examined.74 The compounds (64) and the N-ethylquinolinium salts of (63) and (64) upon irradiation afford syn-head-to-tail [2 + 2] cycloadduct only, while (63) lead to the formation of syn-head-to-tail [2 + 2] cycloadduct, along E–Z geometrical isomerization of the initial product.

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(Benzothiazoyl)ethenylbenzocrown ether (65) in the presence of Ba2+ forms a stable sandwich complex whose photolysis with visible light results in a stereoselective [2 + 2] cycloaddition to give two-head-to-head dimers.75,76 On the other hand, bis(benzothiazoyl)ethenylbenzocrown ether (66) upon irradiation at 365 nm undergoes E–Z isomerization and intermolecular [2 + 2] cycloaddition, while the complex with Ba2+ under the same condition affords intermolecular [2 + 2] cycloadducts.76,77 N,N 0 -[(1,3-Diphenylene)methylene]bis(stilbazolium) tetraperchlorate (67), having two 18-crown-6-ether fragments, undergoes intramolecular [2 + 2] cycloaddition in the presence of 1,3-diammoniumpropane diperchlorate to give a syn-head-to-head cyclobutane only.78 In addition, this reaction was applied to the intermolecular [2 + 2] cycloaddition of the 18-crown-6-ether styryl compound (68) as a host molecular and N-aminostyryl compound as a guest molecule.79 Photodimerization of acenaphthylene (69) within a self-assemble host (70) takes place and in situ crystallographic observation reveals that this reaction occurs smoothly without preorganization of reaction centers at a preferred geometry.80 Stilbene (71) can form intramolecular N–I bonding in the solid state to give a 1D heterodiopic self-complementary tecton. Upon UV irradiation in the solid state the tecton dimerizes to form a 2D tecton (72).81 Furthermore, a solution of (73) and tecton (74) affords a supramolecular architecture (2:1 = (73):(74)) as co-crystals.82 When the 120 | Photochemistry, 2009, 37, 110–148 This journal is

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powdered crystalline material is irradiated at 300 nm, [2 + 2] cycloaddition takes place under topochemical control. In addition, (73) reacts with a dinuclear Zn Schiff-base complex (75) to give a ladder-like coordination polymer.83 Upon irradiation with UV light this polymer undergoes [2 + 2] cycloaddition in the solid state. Wu and Shimizu have investigated synthesis of molecularly imprinted polymers as catalysts in the selective solid-state [2 + 2] photocycloaddition of (73).84 The reaction of (76) with MnCl2  2H2O, NaSCN gave two supramolecular co-crystals [Mn(NCS)2(OH2)4]  4(76).85 Upon irradiation with UV light in the solid state both co-crystals afforded a rett-dimer in ca. 55–60% yield. Jeannin and Fourmigue´ have reported the solid-state intermolecular [2 + 2] photocycloaddition of trifluoromethyl-substituted tetrathiafulvalene (77).86 Irradiation of (E,E)-1,4-bis(o-trifluoromethylphenyl)-1,3-butadiene (78) in solution results in the conversion to the E,Z-isomer in 95% yield, while photolysis in the powdered crystalline state affords the [2 + 2] cycloadduct (79) in a quantitative yield.87 Furthermore, photoreaction of m- and p-trifluoromethyl-substituted analogues has been studied.87 Photochemical intermolecular [2 + 2] cycloaddition can be applied to the synthesis of ladderanes.88 The intermolecular [2 + 2] cycloaddition of tetrahydrodianthracene (80) with syn-tricyclooctadiene (81) gives the ladderane (82) and a mixture of stereoisomers (83).89 When the isolated Photochemistry, 2009, 37, 110–148 | 121 This journal is

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(83) was irradiated at 254 nm, two Mo¨bius-type annulenes (84) were produced for the first time and one Hu¨ckel-type annulene was also obtained.

Irradiation of b,b 0 -di(2-fulyl)-substituted o-divinylbenzenes (85)–(87) results in intramolecular and intermolecular [2 + 2] cycloadditon to give bicyclo[3.2.1]octadiene and cyclophane derivatives.90,91 On the other hand, the primary process of 2,3-divinylfuran derivatives (88) is the E–Z geometrical isomerization to the (E,E)-isomers followed by photorearrangement of the furan ring giving transformation products.91,92 122 | Photochemistry, 2009, 37, 110–148 This journal is

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The pyrene-benzoylthiophene system (5) led to the formation of an exciplex, which interacts with 1,3-cyclohexadiene (89) or styrene (90) to yield a reactive excited triplex.93 The triplex reacts with other (90) to give the [2 + 2] cycloadduct (91).

2.2.2 Intramolecular [2 + 2] photocycloaddition. Irradiation of dienes (92) in pentane gave intramolecular [2 + 2] cycloadducts.94

It is known that the singlet reactivity of myrcene (93), norbornadiene (94), and dicyclopentadiene (95) is different from the triplet state.95 It is found that heavy cations such as Tl+ and Cs+ effectively promote intersystem crossing to the triplet excited state within zeolites. Ghandi et al. have reported that the dienes (93)–(95) within Y-zeolite exchanged Tl+ and Cs+ afford triplet photoproducts in good yields.95 Also 5,5-diphenyl-1,3cyclohexadiene (96) in the same condition undergoes the di-p-methane rearrangement via the triplet excited state.95

It is well known that the dienes readily undergo intramolecular Cu-catalyzed [2 + 2] cycloaddition. Bach et al. have studied the [2 + 2] cycloaddition of 1,3-divinylcyclopentanes (97) by the use of the more stable copper(II) trifluoromethanesulfonate [Cu(OTf)2] instead of CuOTf.96 The reaction proceeds smoothly and the [2 + 2] cycloadduct is obtained in good yield and with endo diastereoselectivity. It has been reported that intra- and intermolecular copper(I)-catalyzed [2 + 2] photocycloaddition reactions of nonconjugated alkenes can be achieved efficiently at room temperature in ionic liquids such as [tmba][NTf2].97 Sarker and Gosh succeeded in preparing enenatiomerically pure cis-1,2-disubstituted Photochemistry, 2009, 37, 110–148 | 123 This journal is

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cyclobutanes such as (-)-grandisol via copper(I)-catalyzed [2 + 2] photocycloaddition reaction.98

Nishimura et al. synthesized dimeric 3-vinylpyridine (98) and its homologues (99) and (100) and studied their intramolecular [2 + 2] photocycloaddition in stereoselectivity.99 [2.n](2,6)Pyridinecrownnophanes can be readily prepared by intramolecular [2 + 2] cycloaddition of (101).100

Okada et al. have synthesized novel calixarenes having a tweezer-type structure via the intramolecular [2 + 2] photocycloaddition of (102).101

In the solid state bis(stilbazole)Ag complexes (103) undergo [2 + 2] cycloaddition to give a dinuclear complex (104) in 100% yield.102

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Mizuno et al. synthesized novel stilbenophanes (105)–(108) tethered by silyl chains and studied their photochemistry.103 On direct irradiation of (105)–(107) intramolecular [2 + 2] cycloaddition takes place stereoselectively, while on benzophenone- or benzyl-triplet sensitization E–Z geometrical isomerization occurs. Direct photolyses of o-, m-, and p-stilbenophanes (108) afford phnenanthrenophanes. Mizuno et al. also reported that the intramolecular [2 + 2] cycloaddition of b-stilbazoles (109) tethered by silyl chains took place with efficiency. Complexation of (109) with dicarboxylic acid or catechol enhanced both the efficiency and stereoselectivity.104

Bondarenko et al. synthesized a series of pseudo-geminally substituted [2.2]paracyclophanes (110)–(112) and investigated their photochemical reactions.105 Divinyl-substituted and vinyl-ethynyl-substituted [2.2]paracyclophanes (110) and (111), upon irradiation with the sunlight, formed the intramolecular [2 + 2] cycloadducts (113) and (114). Cycloadduct (114) undergoes the Diels-Alder reaction with cyclopentadiene to give (115) in 60% yield. Direct irradiation of the diethnyl-substituted [2.2]paracyclophane (112) (R = H) gives polymers, whereas bis(tetramethylsilylethynyl)substituted [2.2]cyclophane (112) (R = TMS) in the presence of hydrogen chloride affords (116) in 62% yield. 2.3

Miscellaneous

The fluorescence of trans-methoxy-substituted stilbene, styrene, and 1-arypropene derivatives is quenched by 2,2,2-trifluoroethanol.106 Irradiation of these alkenes in 2,2,2-trifluoroethanol gave solvent adducts. The irradiation of (E)-cinnamyl acetate (117) in methanol leads to E–Z isomerization, 1,3-migration and dissociation of the acetoxy group.107 The dissociation of the acetoxy group in the 4-methoxy substituted compound readily gave a cationic intermediate which is trapped with methanol. Irradiation of 9-methylene-9,10-dihydrophenanthrene (118) in a nonpolar solvent such as benzene or cyclohexane undergoes photoinduced ene-reaction to give an ene-adduct (119) as a main product and 9-methylphenanthrene Photochemistry, 2009, 37, 110–148 | 125 This journal is

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(120) as a by-product.108 Also a photoinduced ene-reaction of (117) with some alkenes was found to yield (121).

Methylenecyclopropane (122) in acetonitrile undergoes photorearrangement to (123) in 29% yield.109 Furthermore, (124) and (125) are obtained in much lower yields. In the solid-state photolysis of (122), a dimer (126) is obtained in 3% yield. The intervention of 2,2-diphenylcyclobutylidene carbene as an intermediate has been proposed.

3 3.1

Photochemistry of dienes Photochemistry of conjugated dienes

Fuß et al. discussed the forward and backward pericyclic photochemical reactions of 1,3-butadiene.110 The probabilities of transitions between the states of allene and allylene in the process of the photochemical reaction have been calculated.111

When the s-cis-(E,E)-diene (127) was irradiated at 300 nm for 1 h in degassed benzene-d6, the s-cis-(E,Z)-diene (128) was quantitatively 126 | Photochemistry, 2009, 37, 110–148 This journal is

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obtained. The resulting (128) was further irradiated to give s-cis-(Z,Z)-diene (129) slowly.112

Naphthalene-modified cyclodextrins act as supramolecular chiral photosensitizing hosts for the enantiodifferentiating photoisomerization of (130) to (131).113 3.2

Di-p-methane rearrangement

Some reviews of the di-p-methane rearrangement have been published.114 Zimmerman has reviewed theoretical applications of organic photochemistry involving the di-p-methane rearrangement.115 The photochemistry of 2-allylphenol derivatives including the di-p-methane rearrangement has been reviewed by Miranda et al.116

The di-p-methane rearrangement of (132) (R = Me) in the presence of a soluble ionic liquid sensitizer in the ionic liquid [bmin]BF4 gives (133) in 87% yield.117 Chiral ionic liquids have been evaluated as chiral induction solvents for the di-p-methane rearrangement of (132) (R = H) to (134).118

A chiral mesoporous silica has been prepared and the stereoselectively of the di-p-methane rearrangement of 11-formyl-12-methyldibenzobarrelene (135) within the silica has been investigated.119 An enantiomeric excess of 24% for the resulting photoproduct at 11% conversion of benzobarrelene has been found. Photoreactions of the o-substituted allylbenzene (136) in cyclodextrin cavities have been reported.120 Upon irradiation the complexes of allylbenzenes (136b) or (136c) with b-cyclodextrin in water upon irradiation undergo di-p-methane rearrangement and hydroxylation, (136a) affords benzofuran. Photochemistry, 2009, 37, 110–148 | 127 This journal is

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Acetone-sensitization in the di-p-methane rearrangement of (137) gave tetracyclic sultams (138) in moderate yields.121

4,4-Dialkyl-2,6-diphenyl 4H-thiopyran-1,1-dioxides (139) underwent the di-p-methane rearrangement to give bicyclic compounds (140) in high yields.122 On the other hand, 4H-thiopyrane derivative (141) upon UV irradiation afforded the two 2H-isomers (142) and (143). Isomer (143) is derived from the intermediate of the di-p-methane rearrangement, while isomer (142) is produced via a four-electron suprafacial 1,3-sigmatropic rearrangement.123

The photorearrangement of (144) to (146) involves a triplet-di-p-methane rearrangement mechanism of (145).124

Bicyclo[2.2.2]oct-5,7-dien-2-ones (147a–c) which are endowed with the structural features required for both oxa-di-p-methane and di-p-methane rearrangement, undergo acetophenone-sensitized di-p-methane rearrangement to afford (148a–c) in good yields.125 128 | Photochemistry, 2009, 37, 110–148 This journal is

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The photochemical intermolecular [2 + 2] cycloadduct (149) undergoes the di-p-methane rearrangement upon irradiation at 254 nm to give (150) in 43% yield.126

3.3

Miscellaneous

Photochemical reactions of vinylidenecyclopropane derivatives have been reviewed by Maeda and Mizuno.127 Frutos et al. have calculated the photochemical reaction pathway from tricyclo[3.3.0.02,6]octa-3,7-diene (151) to cyclooctatetraene (152) and semibullvalene (153) from CASSCF and CASPT2/6-31G(d).128

Panquette et al. reported the photochemistry of two sultams (154) and (155).129,130 The direct photolysis of (154) in hexane afforded an endotricyclic compound (156) in 5% yield,129 whereas (155) under the same conditions gave (157) and (158) in 50% and 5% yields, respectively.130 The photoinduced electron transfer from the diene group of (159) to C60 gives a radical cation which undergoes ring opening toward the methoxy group. The reaction of the resulting dienyl radical cation with the C60 radical anion affords cycloadduct (160).131 In addition, the photoinduced electron-transfer reaction of 2-(1,3-pentadienyl)-2-phenylcyclopropane Photochemistry, 2009, 37, 110–148 | 129 This journal is

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(161) with C60 affords five-, seven-, and nine-membered fused adducts (162)–(164).132 Inoue et al. reported the photosensitized diastereodifferentiating addition of methanol to (R)-(+)-(Z)-limonene in a microreactor.133 The quantity of photoproducts increases with increasing time of irradiation with UV light and the diastereomeric excess of the products is slightly larger than that obtained in a batch reaction system.

Nanosecond time-resolved absorption spectroscopy has shown clearly that there is an o-quinodimethane radical cation (165) which intervenes in the photoinduced electron-transfer electrocyclization of 1,2-bis(a-styryl)benzene (166).134

Diene (167) upon irradiation with two wavelengths (254 and 350 nm) affords a precursor (168) of 1a-hydroxyvitamine D5 (169) in good yield.135 Its homologue, 17-epi-calcitriol has also been synthesized with the photochemical method.136 Perfluoroalkyl iodides undergo the photoinduced iodoperfluoroalkylation with several alkenes, alkynes, dienes, diynes, and allenes in the presence of benzotrifluoride.137 130 | Photochemistry, 2009, 37, 110–148 This journal is

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4.

Photochemistry of polyenes

Frank et al. examined in detail the effect of isomer geometry on the spectroscopic properties and photophysics of the excited states of some carotenoid derivatives.138 Spectral properties of amphiphilic lipids with linear conjugated polyene fluorescent groups have been reported.139 The linear polyene 2,4,6,8,10,12,14-hexadecaheptaene (170) undergoes the E–Z geometrical isomerization at the 4-position. Emissions at 77 K indicate that the S1 states of the linear polyene have local energy minima separated by relatively low barriers (2–4 kcal). Catala´n et al. reported the photophysics of all-(E)-polyenes.140 Three geometrical isomers of 1-(1 0 -naphthyl)-6-phenylhexa-1,3,5-triene (171) have been characterized by spectrophotometry and NMR spectroscopy.141 The properties of their lowest excited states of singlet and triplet multiplicity have been reported. Irradiation of (Z,E,Z)-1,6bis(4-alkoxycarbonylphenyl)hexa-1,3,5-trienes (172) in the solid state gives one-way isomerization to the (E,E,E)-isomers.142

It has been expected that the (E)-triene (173) is a precursor of betulinic acid anologues.143 The triene (173) readily undergoes the E–Z geometrical isomerization to give the (Z)-isomer (174), but the electrocyclization of the resulting isomer unfortunately has not been found under photochemical and thermal conditions. Triplet-sensitization photochemistry of cyercene A (175) and its homologues (176) and (177) have been discussed. When (175) was irradiated in the presence of oxygen, singlet oxygen was produced at a significantly higher rate than corresponding to other pyrone homologues.144 The photoisomerization of (175) was quenched with piperylene. In contrast, the isomerization of (176) and (177) was not quenched by piperylene. It is expected that this photochemical behavior is effective in the chemical defense of mollusks. Photochemistry, 2009, 37, 110–148 | 131 This journal is

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Spectinabilin (178) represents an unusual nitroaryl-substituted polyene metabolite produced by Streptomyces.145 Immunosuppressive SNF4435C/D (179), intriguing constitutional isomers from S. spectabilis, have been derived from a 8p–6p electrocyclic rearrangement cascade of Spectinabilin initiated by a photoinduced E–Z isomerization in vivo and in vitro. The related tetraene (180), bearing a methoxycarbonyl group, behaves differently and its photolysis affords (181) via a pathway similar to the photoreaction of (144).146

Racemic and the (1R,2R)-, (1S,2S)-enantiomers of N,N 0 -bis[5-(diethylaminophenyl)-penta-2,4-dienylidene]cyclohexane-1,2-diamine (182) have been synthesized and their photoinduced E–Z transitions have been simulated.147

Singh and Hota have reviewed recent developments in bacteriorhodopsin analogues and studies of charge separated excited states in the photoprocesses of linear polyenes.148 Metarhodopsin III is known to be an inactive intermediate thermally formed following light activation of the visual pigment rhodopsin, whose photoreactions have been reported by Vogel et al.149 The photoisomerization of retinal chromophore model-compounds has been calculated at CASPT2/CASSCF/6-31G*150 and CASSCF/AMBER levels.151 132 | Photochemistry, 2009, 37, 110–148 This journal is

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Giacalone et al. have reported the synthesis, structural, and electronic studies of a series of donor–acceptor arrays, incorporating p-conjugated oligo(phenylenevinylene) wires of different length between p-extended tetrathiafulvalenes as the electron donor and C60 as the electron acceptor.152 Photoinduced electron-transfer processes over distances of up to 50 A˚ afford highly stabilized radical ion pairs, whose life times are B500 ns in benzonitrile. A different life time (4350 ns) was observed for the molecular wire (183). 5.

Photochemistry of alkynes

Wessig et al. have reviewed the photodehydro-Diels-Alder reaction.153 They have also synthesized the alkyne derivatives (184) which undergo the photodehydro-Diels-Alder reaction to give 1H-benzo[g]isochromen-4(3H)-ones (185) and (186) in good yields.154

Imidazol-fused enediynes (187) and (188) give the cycloaromatized products (189) and (190) in a photoinduced Bergman cycloaddition155 in good yields.156

Enediynes (191) which are prodrugs, undergoes the photochemical E–Z isomerization followed by an allylic rearrangement to the putative epoxy enediyne, resulting in efficient DNA cleavage.157 Enyne-allenes (192) and (193) undergo Myers-Saito reaction (C2–C7 bonding) and C2–C6 cyclization in the presence of 1,4-cyclohexadiene upon irradiation with UV light.158 Photochemistry, 2009, 37, 110–148 | 133 This journal is

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Enediyne derivatives (194) undergo the Bergman cycloaddition. The correlation between affinity and photoinduced protein degradation activity has been investigated.159 Alabugin et al. have studied the photoinduced damage of oligonucleotides by (195a,b) bearing a basic lysine moiety.160 O’Connor et al. have reported transition-metal catalyzed enediyne cycloaromatization.161 When the (E)-enediyne (196) was irradiated in the presence of a catalytic amount of the Fe(II)-complex (197a) and an excess of 1,4-cyclohexadiene, (198) was obtained in 83% yield, whereas a similar reaction with the Ru(II)-complex (197b) failed owing to arene ligand photodissociation of (197b). However, when (Z)-isomer of (196) was used, the catalytic cycle was found.

Alca´zar et al. have reported that alkynyldiols (199) underwent endo-selective cyclization in the presence of W(CO)6 as a catalyst to give the seven-membered cyclic enol ethers (200) in good yields.162

The complex [ReCl(CO)5] acts as an effective photocatalyst for the tandem cyclization of o-acetylenic dienol silyl ethers (201).163 In the presence of 10% catalyst upon irradiation the dienol silyl ether (201) gives the bicyclic and tricyclic compounds (202) and (203) in 98% yield in total. 134 | Photochemistry, 2009, 37, 110–148 This journal is

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Narrow-rim alkynyl-substituted calix[4]arenes (204) undergo cyclization/ rearrangement reactions to afford (205) and (206).164

Zeidan reported an efficient method to prepare 1,5-diaryl-substituted tetracyclo[3.3.0.02,8.02,6]octanes (homoquadricyclanes) (207) by means of the photocycloaddition of diarylethyne, in which at least one of aryl groups is a pyridine.165

Fahr and Laufer studied the photochemistry of allene, propyne, and 2-butyne at 193 nm through time-resolved UV-absorption and timeresolved GC/MS.166 The clear difference between the time-resolved absorption trace and wavelength-dependent spectra derived following 193 nm photolysis of allene with those of propyne and 2-butyne suggest that observed absorption features following photolysis of their alkynes are likely to be composite with contribution from propagyl radical and other transition species. Maeda et al. prepared various alkynylpyrene derivatives and investigated them in detail with a view to develop a new class of pyrenebased biomolecular probes.167

Ogawa et al. reported that diphenyldiselenide and tetraphenyldiphosphine upon irradiation gave the corresponding heteroradicals which were readily trapped with alkynes (208) to yield (209)168 and (210).169 They also found a novel domino reaction of diphenyl diselenide with (208) and two kinds of alkenes to provide the corresponding cyclic four-component coupling products in good yields.168 Photochemistry, 2009, 37, 110–148 | 135 This journal is

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Geragthy et al. have found the photomediated reaction of alkynes with cycloalkanes or 1,3-dioxolanes which occurs via a chain mechanism.170 This reaction promoted under the sunlight.171 6.

Oxidation of alkenes, dienes, and polyenes

It is well known that singlet oxygen is a good oxidant for alkenes. Some reviews have been published: advances in singlet oxygen chemistry,172 photooxygenations of the [4 + 2] and [2 + 2] type,173 and photooxygenations of the ene-type.174 Alberti and Orfanopoulos reported in detail stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen.175 Majima et al. studied the effect of oxygen on the formation and decay of the stilbene radical cation during the resonant two-photon ionization.176 Pace and his group found long-live singlet oxygen (12 ms) in NaY zeolite doped fluoro-organic cations such as 1-methyl-4-trifuoromethylpyridinium hexafluorophosphate.177 Some alkenes were readily oxidized in this zeolite. Natarajan et al. reported that regioselective oxidation of (211) to (212) (490%) with singlet oxygen in a water-soluble and deep-cavity cavitant.178

Chiral alkenes were oxygenated diastereoselectively with singlet oxygenNa+ within thionin-supported zeolite NaY.179 Such dye-sensitized intrazeolite photooxygenations has been reviewed by Stratakis.180 In addition, a short review of Orfanopoulos et al. has dealt with a number of factors, such as solvents, electronic effects, and non-bonded interactions, concerning the stereoselectivity of the sensitized photooxygenation of alkenes with singlet oxygen.181

Photooxygenation of pregnane steroids with singlet oxygen has been investigated.182 For the ene-reaction involving C(17)QC(20) bond in the steroids, high regioselectivity has been found when the allylic OH group is located C(16) on the b-side. The stereoselectivitity is dependent not only steric factors, but also the activating or deactivating effect of the allylic OH groups at C(16) and C(20). 1-Methyl-1,4-cyclohexadiene (213) undergoes initially an ene-reaction with singlet oxygen, derived from tetraphenylporphyrin (TPP) sensitization. Then the resulting ene-adducts (214) and 136 | Photochemistry, 2009, 37, 110–148 This journal is

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(215) react with singlet oxygen to yield the bicyclic endoperoxides (216) and (217).183 Chiral-auxiliary functionalized E/Z-enecarbametes (2) oxidize in highly diastereoselectivity to give the enatiomeric (R)- or (S)-methyldeoxybenzoin in 97%ee over.184 The formation of the preferred methyldeoxybenzoin enantiomer depends for the E-isomer on the chosen solvent and temperature, but not for the corresponding Z-isomer. The enantioselective photooxidation reaction has also been studied in zeolite Y supercages.185 In addition, it has compared this photooxidation reaction with the oxidation reaction with ozone.186

Alkyl- and aryl-substituted 2,3-dihydrofurans (218) undergo the photooxygenation with TPP sensitization to give hydroperoxide derivatives (219).187 The resulting (219) is treated with a ferrous sulfate solution to afford a,b-unsaturated g-lactones (220) in 60% yield. Ricco et al. found a regioselective entry to bromo-g-hydroxybutenolides (222) and (223) by photooxygenation of 3-bromofuran (221) with singlet oxygen in the presence of a suitable base.188 The photooxygenation in the presence of phosphazene afforded the 3-bromo-compound (222) as the main product, while DBU yielded the 4-bromo isomer (223) only. A 1,2-difuryl alkene (224) undergoes a double photooxygenation in the presence of Rose Bengal (RB) under oxygen to give the spirocyclic compounds (225) which are the bis-spiroketal core of the prunolide natural products.189 Bis(hydroxyalkyl)substituted furan derivatives (226) undergo photooxygenation with singlet oxygen. The treatment of the hydroperoxide intermediates with dimethylsulfide yields [5,5,5]- or [6,5,6]-bis-spiroketals (227) in a good yield.190 In addition, the photooxygenation of 2-alkenyl-substituted 3-methylfurans (228) has been applied to the synthesis of first generation litseaverticillols (229).191

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Balci et al. reported the TPP-sensitized photooxygenation of the cycloheptatriene derivatives (230) and (231).192 These compounds readily undergo the photooxygenation with singlet oxygen to give endo-peroxides (232) and (233) in moderate yields. a-Santonin (234) also undergoes the photooxygenation by TPP-sensitization to give endo-peroxide (235) in 20–30% yield.193

It has been found that acetonitrile assisted the highly selective photocatalytic epoxidation of alkenes on titanium-containing silica with oxygen.194 In this reaction, acetonitrile suppresses selectively side reactions. Diiron bisporphyrin (236) which has a Fe(III)–O–Fe(III) bond, is also a good aerobic photooxidizing catalyst for alkenes.195 The cyclometalated platinum(II) 4,6-diphenyl-2,2 0 -bipyridine complex (237) supported by the cationic ion-exchange resin amberlite IRA-200 is a good photooxidation sensitizer for alkenes.196 Furthermore, the complex has also been incorporated into the (3-aminopropyl)triethoxysilanemodified channels of ordered mesoporous molecular sieves and the 138 | Photochemistry, 2009, 37, 110–148 This journal is

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photooxidation of alkenes under oxygen using this complex has been examined.197 Porphyrin-functionalized dendrimers act as recyclable photooxidizing catalysts in a nanofiltration membrane reactor.198 For the model photooxygenation of 1-methyl-1-cyclohexene, a turnover number between 3000 and 5000 is reached for all catalysts. A novel photooxygenation catalyst with solid-supported seco-porphyrazine (238) has been developed.199 To investigate the reproducibility of the reaction, the photooxygenation of a-terpinene was repeated several times by the use of the same batch of catalyst and no loss in activity was observed.

Formation of 1,2-dioxetane (240) from tetraphenylethylene (239) occurs via formation of the electron transfer state of 9-mesityl-10-methylacridinium ion (241) under visible light.200 It is known that the decatungstate anion W10O324 acts as a good photocatalyst in the oxygenation and alkylation of alkenes. Orfanopoulos et al. studied the photocatalyzed oxygenation of tetrasubstituted alkenes with W10O324.201 Furthermore, Albini et al. reported the W10O324 photosensitized alkylation of electrophilic alkenes.202 On the other hand, Maldotti et al. reported the bromide-assisted bromination of alkenes in the presence of oxygen through heterogenized W10O324.203 Chromium-containing silica gives photocatalytic oxidation of alkenes by visible light irradiation.204 A redox-photosensitized reaction of indene using a photosensitive surfactant in an oil/water emulsion has been found. In this reaction which is strongly influenced by the oil droplet size and surfactant charge, 2-hydroxyindan was obtained.205 Photochemistry, 2009, 37, 110–148 | 139 This journal is

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The photooxygenation of cyclopentadiene with singlet oxygen has been investigated in a flowing-film microreactor.206 Nakanishi and co-worker investigated the superoxidation of retinoide derivatives and b-carotene with FAB-, ESI-, or APCI-MS techniques.207 The MS spectra of the photoproducts of retinoide derivatives show a maximum mass an increase of +32 Da, corresponding to the parent ion of the retinoide plus two oxygen atoms. The photooxydation of bisretinoides and b-carotene results in the addition of up to 4 or 10 oxygen atoms.207b 7.

Photochemistry of haloalkenes

Gonza´lez et al. investigated the isomerization energy profiles and conical interaction of a chiral (4-methylcyclohexylidene)fluoromethane by ab initio calculation.208 Upon irradiation halostyrenes (242) and (243) with a methyl or trifluoromethyl substituent at the a- or b-position in methanol lead to the formation of highly destabilized vinyl cations.209

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139 E. Quesada, J. Delgado, V. Hornillos, A. U. Acuna and F. Amat-Guerri, Eur. J. Org. Chem., 2007, 2285. 140 J. Catala´n and J. L. G. De Paz, J. Chem. Phys., 2006, 124, 034306/1; J. Catala´n, H. Hopf, C. Mlynek, D. Klein and P. Kilickiran, Chem. Eur. J., 2005, 11, 3915. 141 E. Marri, G. Galiazzo, F. Masetti, U. Mazzucato, C. Zuccaccia and A. Spalletti, J. Photochem. Photobiol. A: Chem., 2005, 174, 181. 142 Y. Sonoda, Y. Kawanishi, S. Tsuzuki and M. Goto, J. Org. Chem., 2005, 70, 9755. 143 M. E. Jung and B. A. Duclos, Tetrahedron, 2006, 62, 9321. 144 D. R. Zuidema and P. B. Jones, J. Photochem. Photobiol., B: Biol., 2006, 83, 137; D. R. Zuidema and P. B. Jones, J. Nat. Prod., 2005, 68, 481. 145 M. Mueller, B. Kusebauch, G. Liang, C. M. Beaudry, D. Trauner and C. Hertweck, Angew. Chem., Int. Ed., 2006, 45, 7835. 146 M. F. Jacobsen, J. E. Moses, R. M. Adlington and J. E. Baldwin, Tetrahedron, 2006, 62, 1675. 147 G. Lemercier, M. Alexandre, C. Andraud and I. V. Kityk, Chem. Phys., 2004, 298, 299. 148 A. K. Singh and P. K. Hota, Photochem. Photobiol., 2007, 83, 50. 149 R. Vogel, S. Lu¨deke, I. Radu, F. Siebert and M. Sheves, Biochemistry, 2004, 43, 10255. 150 L. De Vico, M. Garavelli, F. Bernardi and M. Olivucci, J. Am. Chem. Soc., 2005, 127, 2433; A. Cembran, F. Bernardi, M. Olivucci and M. Garavelli, J. Am. Chem. Soc., 2004, 126, 16018. 151 A. Migani, A. Sinicropi, N. Ferre´, A. Cembran, M. Garavelli and M. Olivucci, Faraday Discuss., 2004, 127, 179. 152 F. Giacalone, J. L. Segura, N. Martı´ n, J. Ramey and D. M. Guldi, Chem. Eur. J., 2005, 11, 4819. 153 P. Wessig, G. Mu¨ller, C. Pick and A. Matthes, Synthesis, 2007, 464. 154 P. Wessig, G. Mu¨ller, R. Herre and A. Kuehn, Helv. Chim. Acta, 2006, 89, 2694; P. Wessig and G. Mu¨ller, Chem. Commun., 2006, 4524. 155 M. Kar and A. Basak, Chem. Rev., 2007, 107, 2861. 156 Z. Zhao, J. G. Peacock, D. A. Gubler and M. A. Peterson, Tetrahedron Lett., 2005, 46, 1373. 157 Y. Tachi, W.-M. Dai, K. Tanabe and S. Nishimoto, Bioorg. Med. Chem., 2006, 14, 3199. 158 M. Schmittel, A. A. Mahajan and G. Bucher, J. Am. Chem. Soc., 2005, 127, 5324. 159 F. S. Fouad, J. M. Wright, G. Plourde II, A. D. Purohit, J. K. Wyatt, A. El-Shafey, G. Hynd, C. F. Crasto, Y. Lin and G. B. Jones, J. Org. Chem., 2005, 70, 9789. 160 B. Breiner, J. C. Schlatterer, S. V. Kovalenko, N. L. Greenbaum and I. V. Alabugin, Angew. Chem., Int. Ed., 2006, 45, 3666. 161 J. M. O’Cnnnor, S. J. Friese and B. L. Rodgers, J. Am. Chem. Soc., 2005, 127, 16342. 162 E. Alc´azar, J. M. Pletcher and F. E. McDonald, Org. Lett., 2004, 6, 3877. 163 H. Kusama, H. Yamabe, Y. Onizawa, T. Hoshino and N. Iwasawa, Angew. Chem., Int. Ed., 2005, 44, 468. 164 H. Al-Saraierh, D. O. Miller and P. E. Georghiou, J. Org. Chem., 2007, 72, 4532. 165 T. A. Zeidan, R. J. Clark, I. Ghiviriga, S. V. Kovalenko and I. V. Alabugin, Chem. Eur. J., 2005, 11, 4953; T. A. Zeidan, S. V. Kovalenko, M. Manoharan, R. J. Clark, I. Ghiviriga and I. V. Alabugin, J. Am. Chem. Soc., 2005, 127, 4270. 146 | Photochemistry, 2009, 37, 110–148 This journal is

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166 A. Fahr and A. H. Laufer, J. Phys. Chem. A, 2005, 109, 2534. 167 H. Maeda, T. Maeda, K. Mizuno, K. Fujimoto, H. Shimizu and M. Inoue, Chem. Eur. J., 2006, 12, 824. 168 K. Tsuchii, M. Doi, I. Ogawa, Y. Einaga and A. Ogawa, Bull. Chem. Soc. Jpn., 2005, 78, 1534. 169 S. Kawaguchi, S. Nagata, T. Shirai, K. Tsuchii, A. Nomoto and A. Ogawa, Tetrahedron Lett., 2006, 47, 3919. 170 N. W. A. Geraghty and A. Lally, Chem. Commun., 2006, 43; R. A. Doohan, J. J. Hannan and N. W. A. Geraghty, Org. Biomol. Chem., 2006, 4, 942. 171 R. A. Doohan and N. W. A. Geraghty, Green Chem., 2005, 7, 91. 172 E. L. Clennan and A. Pace, Tetrahedron, 2005, 61, 6665. 173 M. R. Iesce, in Molecular Supramolecular Photochemistry, Synthetic Organic Photochemistry, ed. A. G. Griesbeck and J. Mattay, Marcel Dekker, New York, 2005, ch. 11, vol. 12, p. 299. 174 E. L. Clennan, in Molecular Supramolecular Photochemistry Synthetic Organic Photochemistry, ed. A. G. Griesbeck and J. Mattay, Marcel Dekker, New York, 2005, ch. 12, vol. 12, p. 365; A. G. Griesbeck, A. Bartoschek, J. Neudo¨rfl and C. Miara, Photochem. Photobiol., 2006, 82, 1233. 175 M. N. Alberti and M. Orfanopoulos, Tetrahedron, 2006, 62, 10660. 176 M. Hara, S. Samori, C. Xichen, M. Fujitsuka and T. Majima, J. Org. Chem., 2005, 70, 4370. 177 A. Pace, P. Pierro, S. Buscemi, N. Vivona and E. L. Clennan, J. Org. Chem., 2007, 72, 2644. 178 A. Natarajan, L. S. Kaanumalle, S. Jockusch, C. L. D. Gibb, B. C. Gibb, N. J. Turro and V. Ramamurthy, J. Am. Chem. Soc., 2007, 129, 4132. 179 M. Stratkis, C. Raptis, N. Sofikiti, C. Tsangarakis, G. Kosmas, I.-P. Zaravinos, D. Kalaitzakis, D. Stavoulakis, C. Baskakis and A. Stathoulopolou, Tetarhedron, 2006, 62, 10623. 180 M. Stratakis, Curr. Org. Synth., 2005, 2, 281. 181 M. N. Alberti and M. Orfanopoulos, Tetrahedron, 2006, 62, 10660. 182 M. A. Ponce, R. Erra-Balsells, A. C. Bruttomesso and E. G. Gros, Helv. Chim. Acta, 2004, 87, 2987. 183 R. D. Yardımcı, N. Kaya and M. Balci, Tetrahedron, 2006, 62, 10633. 184 J. Sivaguru, M. R. Solomon, H. Saito, T. Poon, S. Jockusch, W. Adam, Y. Inoue and N. J. Turro, Tetrahedron, 2006, 62, 6707; T. Poon, J. Sivaguru, R. Franz, S. Jockusch, C. Martinez, I. Washington, W. Adam, Y. Inoue and N. J. Turro, J. Am. Chem. Soc., 2004, 126, 10498. 185 J. Sivaguru, H. Saito, M. R. Solomon, L. S. Kaanumalle, T. Poon, S. Jockusch, W. Adam, V. Ramamurthy, Y. Inoue and N. J. Turro, Photochem. Photobiol., 2006, 82, 123; J. Sivaguru, T. Poon, R. Franz, S. Jockusch, W. Adam and N. J. Turro, J. Am. Chem. Soc., 2004, 126, 10816. 186 J. Sivaguru, T. Poon, C. Hooper, H. Saito, M. R. Solomon, S. Jockusch, W. Adam, Y. Inoue and N. J. Turro, Tetrahedron, 2006, 62, 10647; J. Sivaguru, H. Saito, T. Poon, T. Omonuwa, R. Franz, S. Jockusch, C. Hooper, Y. Inoue, W. Adam and N. J. Turro, Org. Lett., 2005, 7, 2089. 187 Y.-Z. Chen, L.-Z. Wu, M.-L. Peng, D. Zhang, L.-P. Zhang and C.-H. Tung, Tetrahedron, 2006, 62, 10688. 188 M. Aquino, I. Bruno, R. Riccio and L. Gomez-Paloma, Org. Lett., 2006, 8, 4831. 189 N. Sofikiti, M. Tofi, T. Montagnon, G. Vassilikogiannakis and M. Stratakis, Org. Lett., 2005, 7, 2357. 190 T. Georgiou, M. Tofi, T. Montagnon and G. Vassilikogiannakis, Org. Lett., 2006, 8, 1945. Photochemistry, 2009, 37, 110–148 | 147 This journal is

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191 G. Vassilikogiannakis, I. Margaros, T. Montagnon and M. Stratakis, Chem. Eur. J., 2005, 11, 5899. 192 C¸elik and M. Balci, ARKIVOC, 2007, 150; A. DaStan and M. Balci, Tetrahedron, 2006, 62, 4003; M. Gu¨ney, Z. C¸. Ceylan, A. DaStan and M. Balci, Can. J. Chem., 2005, 83, 227. 193 C. W. van der Westhuyzen and C. J. Parkinson, S. Afr. J. Chem., 2005, 58, 41. 194 M. Morishita, Y. Shiraishi and T. Hirai, J. Phys. Chem. B, 2006, 110, 17898; Y. Shiraishi, M. Morishita and T. Hirai, Chem. Commun., 2005, 5977. 195 J. Rosenthal, B. J. Pistorio, L. L. Chng and D. G. Nocera, J. Org. Chem., 2005, 70, 1885. 196 K. Feng, L.-Z. Wu, L.-P. Zhang and C.-H. Tung, Tetrahedron, 2007, 63, 4907. 197 K. Feng, R.-Y. Zhang, L.-Z. Wu, B. Tu, M.-L. Peng, L.-P. Zhang, D. Zhao and C.-H. Tung, J. Am. Chem. Soc., 2006, 128, 14685. 198 S. A. Chavan, W. Maes, L. E. M. Gevers, J. Wahlen, I. F. J. Vankelecom, P. A. Jacobs, W. Dehaen and D. E. De Vos, Chem. Eur. J., 2005, 11, 6754. 199 M. J. Fuchter, B. M. Hoffman and A. G. M. Barrett, J. Org. Chem., 2006, 71, 724. 200 K. Ohkubo, T. Nanjo and S. Fukuzumi, Catal. Today, 2006, 117, 356; K. Ohkubo, T. Nanjo and S. Fukuzumi, Org. Lett., 2005, 7, 4265. 201 I. N. Lykakis, G. C. Vougioukalakis and M. Orfanopoulos, J. Org. Chem., 2006, 71, 8740. 202 D. Dondi, M. Fagnoni and A. Albini, Chem. Eur. J., 2006, 12, 4153. 203 A. Molinari, G. Varani, E. Polo, S. Vaccari and A. Maldotti, J. Mol. Catal. A: Chem., 2007, 262, 156. 204 Y. Shiraishi, Y. Teshima and T. Hirai, J. Phys. Chem. B, 2006, 110, 6257. 205 Y. Yoshimi, T. Itou and M. Hatanaka, Tetrahedron Lett., 2006, 47, 3257. 206 K. Ja¨hnisch and U. Dingerdissen, Chem. Eng. Technol., 2005, 28, 426. 207 (a) I. Washington, N. J. Turro and K. Nakanishi, Photochem. Photobiol., 2006, 82, 1394; (b) I. Washington, S. Jockusch, Y. Itagaki, N. J. Turro and K. Nakanishi, Angew. Chem., Int. Ed., 2005, 44, 7097. 208 M. Schreiber, M. Barbatti, S. Zilberg, H. Lischka and L. Gonza´lez, J. Phys. Chem. A, 2007, 111, 238. 209 K. van Alem, G. Belder, G. Lodder and H. Zuilhof, J. Org. Chem., 2005, 70, 179.

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Oxygen-containing functions M. Consuelo Jime´nez and Miguel A. Miranda* DOI: 10.1039/b812715h This chapter is mainly devoted to the photochemistry of carbonyl compounds, although it also contains some photoreactions of other oxygen containing functions, for instance photochemical ring opening of strained cyclic ethers. Due to space limitations and to the broad time coverage (mid 2004 through mid 2007), only those articles containing original experimental results are quoted. In general, reviews and theoretical calculations are not included. The material is organised according to the well-known types of reactions (e.g., Norrish, Paterno`-Bu¨chi, photo-Fries, etc.). Within each section, the basic organic photochemical findings are reported first. They are followed by more specific aspects, such as stereoselectivity or applications to the synthesis of natural products or structurally interesting molecules. Special attention is paid to photochemical reactions in heterogeneous and constrained media, including zeolites, polymers or organic crystals. Finally, the mechanistic advances are highlighted, especially when they are based on direct detection of reaction intermediates by means of laser flash photolysis or related time-resolved techniques.

1.

Norrish Type I reactions

Atmospheric photolysis of (Z)-3-hexenal produces CO and 2-pentenal via a Norrish Type I process.1 With 2-phenylcyclododecanone2 and 9-acylfluorenes,3 a-photocleavage has also been observed to take place. The Norrish Type I reaction of dibenzyl ketone and 4-methyldibenzyl ketone, producing benzyl radicals and their recombination products, has been reported in frozen aqueous solutions over a broad temperature range.4 Isomeric (E/Z)-3-alkylidene-3H-isobenzofuranones (1) are obtained by photoisomerisation of 2-aroyl-2-methylbenzylindan-1,3-diones (2) in high yields, by a-cleavage and subsequent intramolecular radical coupling.5 In b,g-unsaturated ketones with electron-withdrawing substituents at the g-position, oxa-di-p-methane competes with a-photocleavage in the S2 (p,p*) excited state.6 The Norrish Type I photoreaction has found some interesting synthetic applications. Thus, the photochemical reactivity of isoeriocephalin (3) and teucrolivin B (4) has been evaluated;7 the former leads to the 6-b-epimer and the e-lactone (5), whereas the latter affords a spiro-g-lactone (6). The total synthesis of iridoid monoterpene ether (7) has been described, with photofragmentation of intermediate (8) as the key step.8 Likewise, one of the key stereodefining steps in the total synthesis of nitiol (9) is construction of the A-ring fragment through a Norrish Type I photocleavage.9 Stereocontrolled photocyclisation of 1,2-diketones has been applied to carbohydrate models Departamento de Quimica/Instituto de Tecnologia Quimica UPV-CSIC, Universidad Politecnica de Valencia, Av. Naranjos s/n, E-46022, Valencia, Spain. E-mail: [email protected]

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as a new entry to C-ketosides.10 The reaction has also been used for the preparation of (+)-juvabione (10), a natural sesquiterpene exhibiting insect juvenile hormone activity, from 4-(2-formylethyl)cyclohexanone.11

Several studies on the Norrish Type I photoreaction in heterogeneous media have been published during this period. Styrene-based water-soluble polymers12 and methanol-swollen nafion beads13 have been explored for their use as hosts of lipophilic substrates in aqueous medium. Other unimolecular reactions (photo-Fries rearrangement of naphthyl esters and Norrish Type II reactions of benzoin alkyl ethers) have also been examined in the same media. Irradiation of a-alkyl dibenzyl ketones yields Norrish Type I products in organic solvents, while in aqueous media, in the presence of b-cyclodextrin, Norrish Type II products are formed.14 As regards the mechanistic aspects of the process, photolysis of alkyl phenylglyoxalates (11) has been studied by time-resolved FT-IR spectroscopy. The observed transients have been assigned to benzoyl and alkyl mandelate ester radicals, respectively.15 Further radicals, such as acyl, hydrated acyl, alkyl and ketyl radicals (generated by photolysis of aliphatic aldehydes) have been detected by NO spin trapping and EPR techniques.16 In the case of 2,2 0 -thenil, in addition to a-cleavage, the presence of triplet state-oxygen adducts (12) (a type of species previously proposed but never observed in a direct manner) has been readily observed following laser flash photolysis studies.17 150 | Photochemistry, 2009, 37, 149–174 This journal is

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2. 2.1

Hydrogen abstraction Norrish Type II reactions

Photoexcited heptanal undergoes rapid intersystem crossing to the triplet manifold, followed by intramolecular H-abstraction, to form biradical intermediates.18 The Norrish Type II reaction of valerophenone in aqueous media has been found to be temperature-sensitive.19 Valuable chiral drimanic dienic synthons have been prepared from the corresponding 14,15-bisnorlabdene-13-ones (13)–(14).20 As a practical application, the Norrish Type II reaction has been used for elimination of photolabile protecting groups. Thus, photolysis of a new UV absorber derived from 4-tert-butyl-4 0 -methoxydibenzoylmethane results in elimination of the alkyl side chain, with generation of the active sunscreen ingredient.21 Likewise, PEG-supported 2-methylphenacyl ester has been found to be a new photocleavable linker. Here, photocleavage is also based on an efficient intramolecular hydrogen abstraction and gives the corresponding carboxylic acid in high yield and purity.22

The chemistries of a monoradical (15), of the ultrafast ‘‘radical-clock’’ type, and a structurally related singlet biradical (16), generated by Norrish type II photochemistry have been compared.23 Only the former undergoes cyclopropane ring opening at room temperature. The diastereomers of ketones (17) and (18) exhibit distinct photochemical reactivities due to conformational preferences; while the anti isomers undergo efficient Yang cyclisation in 75–90% yields with a remarkable diastereoselectivity (490%), the syn isomers predominantly undergo Norrish Type II elimination.24 The biradicals arising from propiophenone and g-methylvalerophenone undergo photoionisation more efficiently than their precursor triplet states; this highlights that a spontaneous intramolecular transformation accompanied by small structural and electronic changes-the conversion of a triplet moiety into a radical moiety by a Norrish II reaction-can have a pronounced effect on the electron yield.25

Stereoselective synthesis of 3-alkylated cis-1,2-cyclobutanediols and derivatives by Norrish-Yang photocyclisation has been achieved.26 The Norrish Type II photoreaction has also found some applications in the field of carbohydrates. Thus, highly functionalized glyco-conjugated Photochemistry, 2009, 37, 149–174 | 151 This journal is

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hexahydroazepinediones have been obtained from saccharide imides via the Norrish type II reaction.27 An interesting development in bioorganic chemistry has been the first antibody-catalyzed Norrish-Yang photocyclisation, which can be achieved with antibodies elicited against cis- and trans-2,3-diaryloxetanes.28 The influence of anisotropic media on the outcome of the Norrish Type II photoreaction has attracted considerable attention in the last years. Benzoin alkyl ethers encapsulated in a cisoid conformation within water-soluble p-sulfonatocalixarene derivatives preferentially yield the Norrish Type II reaction product deoxybenzoin.29 Application of the solid state ionic chiral auxiliary method for asymmetric synthesis to 2-ethyl-1-arylbutan-1-one derivatives affords the corresponding cyclobutanols in enantiomeric excesses as high as 92%, at 91% conversion.30 Chiral inductor and chiral auxiliary approaches have also been examined within zeolites with the aim of achieving asymmetric induction during the photocyclisation of cyclohexadienone, naphthalenone and pyridone derivatives.31 Monolayerprotected nanoparticles (MPNs) with different core have been prepared and functionalised with a variety of aryl ketone substrates. Upon irradiation in benzene solution, the aryl ketone-modified MPNs undergo the Norrish Type II photoreaction and yield alkene- or acetophenone-modified MPNs exclusively, with no evidence for the generation of cyclobutanol.32 A number of studies have been devoted to the Norrish Type II photoreaction in the crystalline state. g-Hydrogen abstraction has been found to be the primary photoprocess in the crystalline state of a-oxoamides, through photochemical and X-ray structural studies. A remarkable asymmetric induction has been observed during Yang cyclisation of the resulting biradicals.33 2-Benzoylcyclohexanone undergoes an efficient Type II photoreaction in solution, but not in the crystal.34 Cleavage of the enantiotopic C2–C3 or C2–C30 bonds in bicyclic 1-hydroxy-1,4-biradicals leads to enantiomeric products. Absolute configuration correlations demonstrate that, in the crystalline state, cleavage occurs mainly in one direction and is controlled by the orientation of the cleaving bonds with respect to the p orbital at the C1 position.35 Absolute asymmetric photocyclisation of isopropylbenzophenone derivatives using a cocrystal approach involving single-crystal-to-single-crystal transformation has been observed.36 The behaviour of 1,4-hydroxybiradicals has been revealed through crystal structure-solid-state reactivity correlations.37 Crystallisationinduced asymmetric transformations in solid state organic photochemistry have been used for the enantioselective Yang photocyclisation of endo-bicyclo[2.1.1]hexyl aryl ketones.38 Irradiation of cis-bicyclo[4.3.0]non-8-ylacetophenone derivatives in solution and in the solid state yields cis-3a,4,5,6,7,7a-hexahydro-1H-indene (19).39 The photochemistry of 1-isopropylcycloalkyl aryl ketones has also been investigated in this framework, paying particular attention to ring size or medium effects, and asymmetric induction.40 Elimination is suppressed in the photoreaction of valerophenone in the solid-state, in the presence of various cyclodextrins, giving the Norrish-Yang cyclisation with a remarkable diastereoselectivity.41 152 | Photochemistry, 2009, 37, 149–174 This journal is

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Time-resolved spectroscopy has been widely used for mechanistic elucidation. Laser flash photolysis of dialkylphenacylsulfonium salts reveals transient species showing absorption at 480 nm, assigned to phenacyl radical, as a result of Norrish Type II process.42 A diastereomeric discrimination in the lifetimes of triplet 1,4-biradicals and stereocontrolled partitioning of their reactivity (Yang cyclisation versus Type II fragmentation) has been observed.43 Triplet 1,4-biradicals generated by g-hydrogen transfer from a-heteroatom-substituted b-branched butyrophenones have also been detected by transient absorption measurements.44 Likewise, detection of the involved transient species has been achieved in the photochemical g-hydrogen abstraction of ethyl 4-formyl-1,3-dimethylpyrazole5-carboxylate45 (20) and in the photoenolisation of methyl 2-(2-methylbenzoyl)benzoate.46 Compared to laser flash photolysis, time-resolved FT-IR has been less frequently used. An interesting example is the observation of two triplet state conformations of alkyl phenylglyoxylates.47

When the carbonyl group and the g-hydrogen are separated by an aromatic ring, photoenolisation occurs instead of fragmentation or cyclobutanol formation; the resulting photoenols can be trapped by Diels-Alder cycloaddition. In the case of dicarbonyl substituted benzenes, benzocyclobutenols have also been obtained.48 A combination of photoenolisation and Diels-Alder trapping has been used for the synthesis of N,N 0 -dioctyl-3,9-diphenylperylene1,2,7,8-tetracarboxylic bisimide (21), the first derivative of a new class of perylene bisimides with extended, (Z)-shaped structure.49 A highly diastereoselective tandem photoenolisation-hetero-Diels-Alder cycloaddition reactions of o-tolualdehydes has been observed in the solid state.50 Irradiation of 2-(alkoxymethyl)-5-methyl-a-chloroacetophenones and 2-(methoxymethyl)5-methylphenacyl benzoate in dry, non-nucleophilic solvents affords 3-alkoxy-6-methylindan-1-ones in very high chemical yields. However, 3-methylisobenzofuran-1(3H)-one is isolated as the major photoproduct in the presence of trace amounts of water.51 Irradiation of 3-benzoyl-2-benzyl6,7-difluoro-1-propyl-1H-quinolin-4-one leads to formation of a mixture of photoproducts through two, thermally reversible, photoenols.52

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Photoenolisation of a dibenzoyl-p-xylene derivative has been used to synthesise new, highly substituted anthracene-based molecules (22).53 In the field of photoremovable protecting groups, photoenolisation of 2,5-dimethylphenacyl carbamates54 or carbonates55 has been used for the release of amines, amino acids, alcohols or phenols in high chemical yields, albeit with quantum yields F lower than 0.1. The photoenolisation mechanism has been investigated by means of timeresolved spectroscopy in a number of cases. This includes 2- and 4-(2-methylbenzoyl)benzoate,56 as well as some photochromic compounds (3-benzoyl-2-benzyl-1-methyl-1H-quinolin-4-one and 3-benzoyl-1,2-dibenzyl1H-1,8-naphthyridin-4-one) and the nonphotochromic 3-benzoyl-1-benzyl2-methyl-1H-1,8-naphthyridin-4-one.57 Likewise, a systematic investigation on a broad set of aldehydes has revealed that the lifetimes of (Z)-photoenols can be modulated by variation of the substituents.58 The rate of deuterium transfer in the photoenolisation of triplet 1,4-dimethyl-10H-anthracene-9one with varying degrees of deuterium label in the methyl groups has been investigated as a function of temperature between 5 and 77 K.59 2.2

Other intramolecular hydrogen abstractions

g-Hydrogen abstraction has been observed upon irradiation of 2-mesyloxyphenyl ketones in 1,1,2-trichloro-1,2,2-trifluoroethane, to give products resulting from subsequent elimination and cyclisation of the generated 1,3-biradicals.60 Photocyclisation of 2-(N-benzoyl-N-benzylamino)ethyl benzoylacetate occurs via remote hydrogen abstraction, to give 8-membered azalactones in high yields. Biradical cyclisation is enhanced by intramolecular hydrogen bonding.61 Competition between photocyclisation and long-range hydrogen abstraction has been observed in the photochemistry of N-(2-acylphenyl)-2methylprop-2-enamides.62 In a related work, irradiation of 2-acylphenyl methacrylates gives the tricyclic lactones (23) in good yields, together with small amounts of 2-acylphenols (the O–CQO bond cleavage products). The mechanism is thought to involve formation of a 1,7-diradical through z-H abstraction (1,8-H transfer) by the excited carbonyl group.63 Two dyads (24) synthesised by esterification of a-cholesterol with (S)- or (R)-ketoprofen are efficient photogenerators of the 7-allyl radicals by intramolecular H abstraction.64 Triplet state reactivity and regio-/stereoselectivity in the macrocyclisation of diastereomeric ketoprofen–hydrogen donor systems have been studied by means of steady-state and time-resolved photolysis.65

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2.3

Intermolecular hydrogen abstraction reactions

A series of mechanistic studies based on time-resolved spectroscopy have appeared during this period. Absolute rate constants for hydrogen abstraction from 4-methylphenol by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile.66 Laser flash photolysis of a series of bichromophoric compounds containing the 2-benzoylthiophene and phenol or indole moieties has been used to determine possible geometrical effects in the intramolecular quenching of the triplet excited ketones, resulting in formal hydrogen abstraction.67 The technique has also allowed to detect three neutral radicals (ketyl, indolyl and skatolyl radicals) resulting from formal hydrogen abstraction from N-BOC-tryptophan methyl ester by excited benzoylthiophenes.68 The reaction dynamics of ketoprofen (KP) has been investigated in methanol, with and without triethylamine. After excitation, triplet KP abstracts a hydrogen atom to form the ketyl radical.69 Excited phenacyl and 3-pyridacyl esters of benzoic acid react with an excess of aliphatic alcohols in a chain reaction process involving hydrogen transfer, leading to benzoic acid in addition to acetophenone and 3-acetylpyridine, respectively.70 This principle has found application in the development of photoremovable protecting groups. In connection with antioxidant activity, solvent effects on the kinetics of hydrogen abstraction from a lactone have been determined for alkoxyl and nitroxyl radicals; their reactivity differs by about 7 orders of magnitude.71 In the same field, a study has appeared dealing with the influence of substitution at the benzylic position on the behaviour of stereoisomeric phosphorus compounds as precursors of stabilized carbon-centered radicals.72 The behaviour of a highly reactive lactone-derived acrylate monomer (25) has been compared with that of isodecyl acrylate. Time resolved absorption spectroscopy allows direct detection of the lactone-derived radical, formed through a hydrogen abstraction reaction between (25) and the benzophenone triplet state.73 Some attention has been devoted to the course of the reaction in heterogenous media. Thus, benzophenone has been used as a probe for the study of silica and reversed-phase silica surfaces.74 Likewise, the photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined in the presence of a chiral amine. Most of them are photoreduced to the corresponding alcohols; however, in some cases, intramolecular hydrogen abstraction has been observed.75 Diketones can also participate in intermolecular hydrogen abstraction. This has been shown for the reaction of triplet excited 1,2-diketopyracene with phenols by means of laser flash photolysis and DFT-theoretical calculations.76 Rate constants for hydrogen abstraction, electron transfer and energy transfer processes have also been measured for ninhydrin and its 5-methoxy derivative.77 Likewise, the photophysics and photochemistry of five aromatic a-diketones exhibiting different geometries have been reported. The ground state geometry has been correlated with the absorption spectra and redox properties.78 A more sophisticated approach makes use of the so-called two-colour two-laser flash photolysis. In this way, the absorption spectrum of an Photochemistry, 2009, 37, 149–174 | 155 This journal is

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excited benzophenone ketyl radical has been obtained.79 When this species is generated in a model bichromophoric compound (26), it reduces the N-methylphtalimide moiety to form a diphenylmethanol cation and a phthalimide radical anion. The cation quickly deprotonates, regenerating the ketone. In the presence of an appropriate electron acceptor, oxidation of the radical anion regenerates the initial state. As a result, the circular reaction is completed.80 In a few cases, alternative physical techniques have been employed for mechanistic studies of this process. Thus, nanosecond time-resolved resonance Raman spectroscopy has been applied to the reaction of the triplet state benzophenone with 2-propanol.81 In addition, the anomalous net absorptive CIDEP spectra observed in the photoinduced hydrogen abstraction reaction by chromone and chromone-2-carboxylic acid from 2-propanol, in the presence of added hydrochloric acid, has been investigated by FT-EPR.82 3.

Paterno`-Bu¨chi photocycloadditions

The photoreaction of several carbonyl compounds with vinylene carbonate has been used to prepare oxetanodioxolanones.83 The photocycloaddition of aromatic ketones with furans gives the corresponding Paterno`-Bu¨chi adducts in a diastereoselective fashion.84,85 Concerning stereoselectivity, a hydroxy directing effect has been noticed in the reaction between tertiary 2-furylmethanols and aromatic carbonyl compounds.86 Notable temperature effects on the stereoselectivity of the reaction with 2,3-dihydrofuran-3-ol derivatives have also been reported.87 With benzo[b]furan derivatives as the alkene component, 3-benzofurylmethanols are obtained.88 Photocycloaddition of aldehydes and a-keto esters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high to moderate (exo) diastereoselectivity.89 If alkynes are used instead of alkenes, oxetenes are formed. This has been shown for 1-acetylisatin, where oxetene formation is regioselective and provides an easy access to 3-alkylideneoxindoles and dispiro[oxindole[3,2 0 ]furan[3 0 ,300 ]oxindole]s (27).90

As regards synthetic applications, a more complex oxetane (28) is obtained from a 2,11-diaza[3,3](9,10)anthracenoparacyclophane and benzophenone.91 Pseudo-geminally substituted [2.2]paracyclophanes are also known to undergo the reaction, although in the intramolecular version.92 In a further example of the intramolecular photocycloaddition, 5-aryl-4,4dimethyl-hex-5-enals have been used to assemble the hindered cyclopentane skeleton of the cuparene (29) and herbertane (30) sesquiterpenes.93 156 | Photochemistry, 2009, 37, 149–174 This journal is

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Merrilactone A (31) constitutes another application in the field of natural products synthesis.94 The Paterno`-Bu¨chi photoreaction has attracted considerable attention in connection with its biological implications in DNA damage and repair. This process has been found to compete with energy and electron transfer in the photosensitisation of thymine nucleobase by benzophenone derivatives.95 The photochemical [2 + 2] cycloaddition of 1,3-dimethylthymine or 1,3-dimethyluracil with benzophenone and its 4,4 0 -disubstituted derivatives generates two series of oxetanes, the head-to-head and the head-to-tail isomers.96 Moreover, the reaction exhibits a remarkable solvent-,97 substituent-98 and temperature-dependent regioselectivity.99 Excited state enantiodifferentiating interactions have been observed between ketoprofen, a chiral benzophenone derivative, and thymidine. Photoproduct studies indicate that the enantiodifferentiation is associated with a Paterno`-Bu¨chi reaction, leading to the formation of oxetanes.100 Analogous intramolecular processes have been observed in benzophenone-thymine dyads, where highly strained macrocyclic oxetanes (32) and (33) are formed.101

4. 4.1

Photoreactions of enones and quinones Enones

One of the most frequently reported photoreactions of enones is [2 + 2] cycloaddition to alkenes, such as the reaction between cyclohex-2-enones and alkylidenemalononitriles102 or penta-1,2,4-triene,103 or between 2,3-dihydro-2,2-dimethyl-4H-thiopyran-4-one and alkenes or dienes,104 including the regio- and/or stereospecific processes with 2-chloroacrylonitrile and ethylidene-malononitrile.105 In the case of 4-pent-4-enylcyclohex-2-enones, an unexpected 1,3-H atom transfer in the primary biradical intermediates has been reported.106 Further examples are the [2 + 2] photocycloaddition of 3-methyl-2-cyclohexenone and C70107 or the photodimerisation of tricinnamoylbenzenes to give cyclophanes.108 Stereochemical studies on Photochemistry, 2009, 37, 149–174 | 157 This journal is

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the photoreactions between chiral 2-cyclohexenones and olefins have led to a working hypothesis for the geometry of the enone triplet p–p* excited state.109 The reaction has found wide application in organic synthesis. For example, a new methodology for the construction of 3-azabicyclo[4.2.0]octan-4-ones (e.g. (34)) has been developed by combining the Ugi multicomponent reaction of unsaturated cyclic carboxylic acids, isonitriles, carbonyl compounds, and unsaturated amines with subsequent [2 + 2] enone-olefin photocycloaddition.110 The enol forms of 1,3-dicarbonyl compounds may undergo a two-step [2 + 2] photocycloaddition-ring expansion. This process is known as the de Mayo reaction and has been widely applied to the synthesis of larger rings. Intramolecular [2 + 2] photocycloaddition of various tetronates leads to the tri- and tetracyclic products (e.g. (35)) in good yields and with excellent diastereoselectivities. The reaction tolerates a high degree of substitution at both the tetronate and at the alkene double bond.111 A five-step, atomefficient synthesis of the galanthan tetracyclic skeleton (36) has been developed, where the key step is an unusual intramolecular de Mayo reaction, using a isocarbostyril substrate (e.g. (37)).112 A one-step synthesis of cyclopentanoid (38) (along with an equal amount of (39)) has been achieved based on the photoreaction between an enone and an allylsilane. The ability of the silyl moiety to stabilise the intermediate biradical is believed to be responsible for this unusual transformation, where the [2 + 2] cyclobutane adduct is the normally expected product.113

A convergent strategy leading to the enantioselective syntheses of guanacastepenes, including the natural (+)-guanacastepene E (40), has made use of an efficient p-allyl Stille cross-coupling reaction, followed by an intramolecular enone-olefin [2 + 2] photocycloaddition and a stereoelectronically controlled, reductive fragmentation of the resulting cyclobutyl ketone.114 An enantioselective synthesis of ()-incarvilline (41) has been reported employing a three-component coupling reaction and an intramolecular enone-olefin [2 + 2] photocycloaddition of the N-allyl enone (42), followed by a SmI2-induced cyclobutane ring-opening.115 A new, 158 | Photochemistry, 2009, 37, 149–174 This journal is

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stereoselective formal synthesis of hirsutic acid (43) and medium ring carbocyclic systems from salicyl alcohol has been reported. Cycloaddition between electron deficient partners, such as cyclohexa-2,4-dienone and methyl methacrylate, triplet sensitized 1,2-acyl shift and ring-closing metathesis are the key features of this approach.116 A total synthesis of hirsutene (44) is based on the photochemical sigmatropic 1,2-acyl shift in 3-hydroxy-2-methyl-endo-tricyclo[5.2.2.02,6]undeca-10-en-8-one, followed by radical induced cleavage of the peripheral cyclopropane bond, olefination and Simmon-Smith reaction.117 Intramolecular photocycloaddition of diketone (45) leads to formation of a photoadduct, where selective bond fragmentation provides a new entry for the stereoselective construction of the bicyclic core of peduncularine (46).118 The regio- and stereoselectivity of intramolecular [2 + 2] photocycloadditions of 2 0 -hydroxyenones are solvent-dependent. In aprotic solvents, the results are consistent with the involvement of an intramolecular hydrogen bond, whereas in protic solvents, intermolecular interactions appear to disrupt the hydrogen bond, providing products with complementary diastereoselectivity. If the facial accessibility of the a-tethered olefin is limited, formation of the head-to-tail or head-to-head regioisomers depends on the nature of the employed solvent.119

In addition to the well-known [2 + 2] photocycloaddition to alkenes, conjugated enones undergo radical addition reactions. For instance, Photochemistry, 2009, 37, 149–174 | 159 This journal is

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cycloalkylketones are prepared through a photomediated addition of cycloalkyl radicals (generated from the corresponding cycloalkanes via hydrogen abstraction) onto the b-enone position.120 Likewise, photoinduced 1,4-addition of indoles to cyclic and some acyclic enones has been reported to occur with modest to excellent yields.121 Making use of a radical-like addition to enones, thiolane 1-oxides ((47), X = O, NH) have been prepared by photorearrangement of sulfoxides (48) in high yields; further manipulation of the photoproducts leads to formation of diverse heterocyclic structures.122 The behaviour of b,g-enones follows different pathways. Thus, upon triplet photosensitisation, the bicyclic enone (49) undergoes oxa-di-pmethane rearrangement; by contrast, direct irradiation results in 1,3-acyl migration.123 Related processes have been reported for tricyclo[5.2.2.01,5]undecanes.124 The triplet energies of the sensitisers play an important role in this type of photoreactions.125 Oxa-di-p-methane rearrangement of bis-annulated bicyclo[2.2.2]octenones has provided a new entry into angular tetraquinane and other polycyclic systems.126 Direct photolysis of 1,3,3-trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones leads to aromatisation through photoelimination of dimethylketene; by contrast, triplet sensitisation affords the oxa-di-p–methane rearranged product.127 Electron-donor sensitisers have a marked influence on the photoreactions of b,g-unsaturated aldehydes. With 1,4-dimethoxynaphthalene, oxa-di-pmethane and decarbonylation are observed; by contrast, using N,N-dimethylaniline, other photoproducts resulting from proton abstraction by ketyl- and alkene-centered radical-anions are also obtained.128 Polyquinanes (e.g. (50)) and analogous ring systems have also been synthesised from enones where the alkene moieties are more distant from the carbonyl group.129

4.2

Quinones

New examples of photocycloadditions with quinones have been reported. Thus, 9,10-phenanthraquinone reacts with oxazoles, to give [4 + 4] products. A similar photoreaction is observed for 1-acetylisatin, but the initial product undergoes a further [2 + 2] cycloaddition with another excited isatin unit (51).130 Likewise, diarylhomobenzoquinones give [2 + 2] photocycloadducts with diphenylacetylene.131 In the case of chloranil, its photocycloaddition to isocyanates affords quinone imine dyes (52) as final products.132 Photocycloaddition of phenanthrenequinone and acenaphthenequinone to 3,4,6-tri-O-acetyl-D-glucal proceeds with distinctly different regioselectivities: the former preferentially adds with both carbonyl oxygens 160 | Photochemistry, 2009, 37, 149–174 This journal is

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to give the [4 + 2] cycloadduct (53) while the latter reacts with only one carbonyl group to exclusively yield the [2 + 2] addition product (54).133

Photochemical acylations of 1,4-naphthoquinones with various aldehydes furnishes C-acylated hydroquinones and quinones in moderate to good combined yields.134 An intriguing photochemical rearrangement of 1-allyloxy-9,10-anthraquinones, to give (55) has been reported. The mechanism is thought to involve initial photoinduced hydrogen abstraction and subsequent intramolecular electron transfer.135 Photolysis of 2,5-dichloro-3,6-bis(dialkylamino)-[1,4]benzoquinone at 4 500 nm affords the elimination products (56) in good yields via d-hydrogen abstraction.136 This type of photoreactivity has been exploited to develop photoremovable protecting groups. Thus, amino-substituted 1,4-benzoquinones (57) can be photoactivated using visible light for release of free carboxylate and phenolate leaving groups.137,138

Some of the observed photoreactions of quinones present interesting biological implications. For instance, near-UV photolysis of 2-methyl-1,4naphthoquinone-DNA duplexes leads to reversible and stable interstrand Photochemistry, 2009, 37, 149–174 | 161 This journal is

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cross-links (58) between quinone and adenine moieties.139 Aziridinylquinones also give covalent photoadducts to DNA.140 Finally, laser flash photolysis studies have been performed to gain further insight into the mechanistic aspects involved in fundamental photoprocesses of quinones. In this context, H-atom or electron transfer from different donors to triplet excited quinones has attracted considerable attention during the last years.141–150 5.

Photodecarbonylation

Photodecarbonylation of bridged diketones (59), (60) and (61) has been used to prepare pentacene,151 hexacene152 and heptacene.153 Irradiation of cyclopropenones with 800 nm pulses of ultrafast laser results in a photodecarbonylation reaction via nonresonant two-photon absorption of light.154

In frozen aqueous solution, radical intermediates produced by photodecarbonylation of dibenzyl ketones are trapped by copper(II) chloride.155 Irradiation of 1-phenyl-1,2-propanedione in low temperature xenon matrixes leads to decarbonylation, with generation of acetophenone.156 Using a crystal to crystal photoreaction, stereoselective decarbonylation of trans-a,a 0 -dialkenoylcyclohexanones has been observed.157 Photodecarbonylation of crystalline 1,3-diarylacetones proceeds with high chemoselectivity and chemical yield. Substituent effects are explained by means of Hammett analysis.158 In 4-hydroxy substituted derivatives, photodecarbonylation in the solid state is quenched by intermolecular hydrogen bonding; the reactivity is recovered by co-crystallization with 4,4 0 -bicyclohexanone.159 While 1,6-biradicals produced by photodecarbonylation of dimethyl 11-oxodibenzo[c,h]bicyclo[4.4.1]undeca-3,8-diene-1,6-dicarboxylate (62) react exclusively by disproportionation in benzene solution, the reaction in crystals leads to radical–radical combination in almost quantitative yield.160

6.

Photodecarboxylation

Photodecarboxylation of pyruvic acid has been investigated in aqueous solution and in the glass form.161,162 This compound is a representative of the a-oxocarboxylic acids widely found in the atmospheric aerosol, and its 162 | Photochemistry, 2009, 37, 149–174 This journal is

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photolysis yields 2,3-dimethyltartaric and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic acids, rather than 3-hydroxy-2-oxobutanone as previously reported. Ultrasound enhanced photodecarboxylation of phenylglyoxylic acid has been reported in acetonitrile containing low amounts of water.163 Irradiation of 3- and 4-acetylphenylacetic acids in aqueous solution leads to the efficient formation of benzyl carbanions, which can be detected by laser flash photolysis.164 Likewise, benzyl-substituted carbanions are produced by photodecarboxylation of ketoprofen (63) or its derivatives in basic aqueous medium.165 This is a very fast process, which occurs in the subnanosecond timescale; however, when the benzophenone moiety and the carboxylic acid are separated by a longer spacer (for instance, in the glycine derivative of ketoprofen), photodecarboxylation is markedly slower.166 Triethylamine and histidine have a significant influence on the reaction dynamics of excited ketoprofen.167,168 The photobehaviour of S(+)- and R()-ketoprofen in the bovine serum albumin has been studied by steady-state photolysis and transient absorption spectroscopy. Photodecarboxylation is slower than in buffered aqueous solution, and the triplet excited states are longer lived. Some stereodifferentiation is observed in these processes.169 Photodecarboxylation of p-benzoylphenylacetic acid in aqueous solution produces the elongated enol (64), whose strength as an oxygen acid makes it more acidic than simple enol analogues by several orders of magnitude.170 Irradiation of 2- and 4-xanthoneacetic acid in aqueous buffer (pH 7.4) leads to efficient (F = 0.67 and 0.64, respectively) intermediate benzylic carbanions through C–C bond heterolysis from the singlet excited state.171 Steady state photolysis of flurbiprofen (65) in aqueous medium leads to (S)-2-hydroxy-a-methyl-4-biphenylacetic acid via photonucleophilic aromatic substitution, in addition to photodecarboxylation products.172 The photochemistry of zomepirac (66) also involves decarboxylation as one of the major reaction pathways.173

Solid-state photodecarboxylation occurs in a chiral cocrystal of acridine and diphenylacetic acid to afford a chiral condensation product in modest enantiomeric excess. A chiral cocrystal of 9-methylbenz[c]acridine and diphenylacetic acid undergoes similar photodecarboxylation but gives an almost racemic product.174 Mesoporous silica has also been found to catalyse oxidative photo-decarboxylation of a-hydroxy carboxylic acids, phenylacetic acid derivatives, and N-acyl-protected a-amino acids.175 Direct photolysis of 4-(methylthio)phenylacetic acid in acetonitrile leads to 4-(methylthio)benzaldehyde, 4-(methylthio)benzyl alcohol, methyl p-tolyl sulfide, dimethyl disulfide, phenylacetic acid, benzaldehyde, benzyl alcohol, toluene, and 1,2-diphenylethane.176 Similar studies have been performed for phenylthioacetic acid and (S)-benzylthioglycolic acid.177 Using light sensitive HgO or mercuric fluoride, decarboxylative Photochemistry, 2009, 37, 149–174 | 163 This journal is

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photooxygenation of arylacetic acids affords aldehydes and ketones in high yield, through trapping of benzylic radical intermediates by oxygen.178,179 The oxidative photodecarboxylation of naproxen can be achieved using aromatic ketones as photocatalysts; the latter have been shown to play a dual role, acting as triplet-triplet photosensitisers and subsequently as ground state electron acceptors.180 Photodecarboxylative benzylation of N-alkyl, N-arylalkyl, and N-arylphthalimides with arylacetic acids in aqueous solution proceeds via electron transfer from the arylalkanoate to the excited triplet state of the phthalimide, either formed directly or upon sensitisation with acetone.181 This reaction serves as a convenient route to methoxy-substituted precursors of phenanthrene lactam alkaloids.182 Phthalimidophenoxyalkanoates also undergo photodecarboxylative cyclisations.183 A similar reaction has been observed for o-phthalimidoalkynoates, and the influence of the geometrically restricted acetylene linker on the photocyclisation efficiency has been investigated. This process has been exploited for the synthesis of cycloalkynes.184 Based on phthalimide photochemistry, a new class of photoremovable protecting groups, involving a photoinduced decarboxylation reaction coupled with the elimination of the caged molecule, has been described for 2-phthalimido-3-hydroxypropionate derivatives (67).185 Likewise, photolysis of amino acids bearing a 2-nitrobenzyl protecting group on the a-amino nitrogen involves a normal 2-nitrobenzyl-type photocleavage, to release the amino acid, but also a second mechanistic pathway initiated by single electron transfer from the amino group to the excited state of the nitroaromatic. The final result of the latter pathway is photodecarboxylation.186

In addition to free carboxylic acids or their carboxylate forms, a number of esters have also been found to photodecarboxylate. Thus, 2,4,6-trimethylphenyl (S)-(+)-2-methylbutyrate, whose ester group has a chiral center alpha to the carbonyl carbon, undergoes facile photodecarboxylation under a variety of conditions and with complete retention of configuration. In this case, photo-Fries rearrangement is blocked by the methyl substituents.187 Photochemical transformations of methyl coumalate have been studied by matrix-isolation technique. Upon UV (l 4 295 nm) irradiation, one of the preferred pathways is isomerisation to the Dewar form, followed by decarboxylation.188 7.

Photo-Fries and photo-Claisen rearrangements

The photochemical behaviour of phenylurea herbicides at wavelengths o300 nm in aqueous solution has been found to be dependent on the nature and position of ring substituents. The main reaction is an 164 | Photochemistry, 2009, 37, 149–174 This journal is

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intramolecular rearrangement, similar to photo-Fries rearrangement, whereas photohydrolysis becomes the main pathway with halogenated derivatives.189 In an independent study, the main photoproducts of UVC (l = 193 nm) photodegradation of phenylureas have also been found to correspond to photo-Fries rearrangement.190 The photo-Fries process has found many synthetic applications. Thus, o-hydroxybenzoylarenes can be easily obtained from aryl benzoates.191 A synthetic route has been reported for 2,6-dialkyl-3,7-diphenylbenzo[1,2-b:4,5-b0 ]difurans (68) from p-dimethoxybenzene, utilising photocyclisation and photorearrangement reactions.192 Another example is the expedient preparation of 3,5-disubstituted 1,4-benzodiazepines starting from anilides.193 Photolysis of 2-acyloxycarbazoles has proven to be a potential entry to carbazole alkaloids.194 Likewise, photo-Fries rearrangement of carbazol-2-yl sulfonates is an efficient tool for the introduction of sulfonyl groups into polycyclic aromatic compounds.195 A series of O-heteroaryl phenols have been synthesised and structurally characterised. Photo-Fries rearrangement of these compounds represents a useful way to access the corresponding C-heteroaryl derivatives.196 Asymmetric synthesis of dihydrofurans has been achieved via a formal retro-Claisen photorearrangement of syn-7benzoylnorbornene derivatives (69).197 Selective dealkylation of calix[4]arene crown ethers (e.g. (70)) using laser irradiation leads to free phenols, whose formation is explained by partial photo-Claisen rearrangement.198 From the mechanistic point of view, the photo-Fries rearrangement has attracted considerable attention as the archetypal model for generation of caged radical pairs. Using two-colour femtosecond pump probe spectroscopy, it has been found that the primary carbonyl-oxygen cleavage of 4-tert-butylphenyl acetate (dissolved in cyclohexane) occurs within 2 picoseconds, while radical recombination within the solvent cage takes 13 picoseconds.199 In-cage singlet radical-pair motions from irradiations of 1-naphthyl (R)-2-phenylpropanoate (71) have been analysed in n-alkanes of different viscosities, at different temperature, and in polyethylene films of different crystallinities.200,201 In addition to formation of the photo-Fries products, loss of carbon monoxide also occurs to some extent. The regioand sterochemistries of the decarbonylated radical recombination products depend on the viscosity of the medium and on the employed temperature. These results are compared with those obtained by photo-Claisen

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rearrangement of 1-naphthyl (R)-1-phenylethyl ether. Photo-Fries rearrangements of 4-dodecylphenyl phenylacetate and two structurally related esters in hexane and polyethylene cages have been compared.202 Similar studies have been performed on 1-naphthyl (R)-2-phenylpropanoate and 1-naphthyl (R)-1-phenylethyl ether, both in the liquid and solid phases of n-nonadecane.203 A number of reports have appeared on the use of anisotropic media to perform this type of reaction. Thus, the behaviour of singlet radical pairs resulting from photocleavage of 1-naphthyl esters has been investigated in aqueous medium, inside dendritic reaction cavities.204 The dendritic microenvironment restricts the mobility of radicals and encapsulates the substrates, intermediates and products; this effect is higher than in micelles. Likewise, the water soluble capsule formed by a deep cavity cavitand with eight carboxylic acid groups controls product distribution during photoFries rearrangement of naphthyl esters in water, by restricting the mobility of primary singlet radical pair.205 8. 8.1

Photocleavage of cyclic ethers Oxiranes

Photochemistry of alkylidenefluorene oxides in a constrained system results in free radical 1,2-alkyl migrations.3 Photolysis of 1,2-dihydronaphthalene oxide leads to indan as a primary photoproduct through a triplet reaction pathway.206

Photosolvolysis of diastereomeric limonene and carvomenthene oxides in methanol, in the presence of Lewis acids, affords kinetic resolution of the (+)-trans isomers (72) and (73) in excellent yields.207 Highly stereoselective formation of 1,3-dioxolanes (e.g. (74)), has been observed on photoinduced electron transfer ring opening of a-epoxy ketones (e.g. (75)), catalyzed by 1-benzyl-2,4,6-triphenylpyridinium tetrafluoroborate in acetone solution.208 Both 2-(20 -hydroxyphenyl)-1,3-dimethylbenzimidazoline and 2-(40 -hydroxyphenyl)-1,3-dimethylbenzimidazoline have been found to act as formal two hydrogen atom-donors for photoinduced electron transfer reactions of epoxy ketones.209 166 | Photochemistry, 2009, 37, 149–174 This journal is

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8.2

Oxetanes

The photosensitised cycloreversion of oxetanes has attracted considerable attention, due to the biological importance of (6-4) photoproducts repair in DNA by photolyases. Actually, thymine oxetanes have been used as charge traps for chemical monitoring of nucleic acid mediated transfer of excess electrons.210 A variety of photosensitisers have been found to be suitable for this process, including carbazole derivatives.211 Oxetanes tethered to flavin units (76) have been used as model compounds for mechanistic investigations.212 Model studies have also been performed with thymine oxetanes covalently linked to tryptophan213 or to b-cyclodextrins;214 in the latter case efficient photocycloreversion is observed by non-covalent binding of N,N-dimethylaniline or indole as electron donor units (77). Intramolecularly sensitised photocleavage of diastereomeric dyads (78) has been achieved in acetonitrile and chloroform as solvents. Interestingly, a higher photoreactivity has been found in acetonitrile, whereas a significant stereodifferentiation has been found in chloroform.215 An efficient carbonylalkene metathesis of bicyclic oxetanes occurs in the photoinduced electron transfer reduction of the Paterno-Buchi adducts from 2,3-dihydrofuran and aromatic aldehydes.216 Theoretical calculations have been performed on the cycloreversion of oxetane radical cations,217 including examples related to the electrontransfer induced repair of (6-4) photoproducts in DNA.218

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168 T. Suzuki, T. Okita and T. Ichimura, Photomed. Photobiol., 2006, 28, 17. 169 S. Monti, I. Manet, F. Manoli, R. Morrone, G. Nicolosi and S. Sortino, Photochem. Photobiol., 2006, 82, 13. 170 Y. Chiang, A. J. Kresge, I. Onyido, J. P. Richard, P. Wan and M. Xu, Chem. Commun., 2005, 4231. 171 J. A. Blake, E. Gagnon, M. Lukeman and J. C. Scaiano, Org. Lett., 2006, 8, 1057. 172 M. C. Jime´nez, M. A. Miranda, R. Tormos and I. Vaya, Photochem. Photobiol. Sci., 2004, 3, 1038. 173 C.-C. Wang, F.-A. Chen, C.-J. Chen, S.-H. Chao and A.-B. Wu, Biomed. Chromatogr., 2004, 18, 820. 174 H. Koshima, Mol. Cryst. Liq. Cryst., 2005, 440, 207. 175 A. Itoh, T. Kodama, Y. Masaki and S. Inagaki, Chem. Pharm. Bull., 2006, 54, 1571. 176 P. Filipiak, G. L. Hug, K. Bobrowski and B. Marciniak, J. Photochem. Photobiol., A, 2005, 172, 322. 177 P. Filipiak, G. L. Hug and B. Marciniak, J. Photochem. Photobiol. A, 2006, 177, 295. 178 M. H. Habibi and S. J. Farhadi, J. Chem. Res., 2004, 296. 179 S. Farhadi, P. Zaringhadam and R. Z. Sahamieh, Tetrahedron Lett., 2006, 47, 1965. 180 J. Perez-Prieto, R. E. Galian and M. C. Morant-Minana, ChemPhysChem, 2006, 7, 2077. 181 K.-D. Warzecha, H. Goerner and A. G. Griesbeck, J. Phys. Chem. A, 2006, 110, 3356. 182 A. G Griesbeck, K.-D. Warzecha, J. J. Neudoerfl and H. Goerner, Synlett, 2004, 2347. 183 A. R. Kim, K.-S. Lee, C.-W. Lee, D. J. Yoo, F. Hatoum and M. Oelgemoeller, Tetrahedron Lett., 2005, 46, 3395. 184 D. J. Yoo, E. Y. Kim, Y. Eun, M. Oelgemoeller and S. C. Shim, Photochem. Photobiol. Sci., 2004, 3, 311. 185 A. Soldevilla and A. G. Griesbeck, J. Am. Chem. Soc., 2006, 128, 16472. 186 A. Barth, S. R. Martin and J. E. T. Corrie, Photochem. Photobiol. Sci., 2006, 5, 107. 187 T. Mori, R. G. Weiss and Y. Inoue, J. Am. Chem. Soc., 2004, 126, 8961. 188 I. D. Reva, M. J. Nowak, L. Lapinski and R. Fausto, Chem. Phys. Lett., 2006, 429, 382. 189 A. Amine-Khodja, A. Boulkamh and P. Boule, Photochem. Photobiol. Sci., 2004, 3, 145. 190 M. Canle Lopez, M. I. Fernandez, S. Rodriguez, J. A. Santaballa, S. Steenken and E. Vulliet, ChemPhysChem, 2005, 6, 2064. 191 K. K. Park and J. Jeong, Tetrahedron, 2005, 61, 545. 192 K. K. Park, S.-H. Kim and J. W. Park, J. Photochem. Photobiol. A, 2004, 163, 241. 193 S. Ferrini, F. Ponticelli and M. Taddei, J. Org. Chem., 2006, 71, 9217. 194 S. M. Bonesi, L. K. Crevatin and R. Erra-Balsells, Photochem. Photobiol. Sci., 2004, 3, 381. 195 L. K. Crevatin, S. M. Bonesi and R. Erra-Balsells, Helv. Chim. Acta, 2006, 89, 1147. 196 S. Ferrini, S. Fusi, F. Ponticelli and M. Valoti, J. Pharm. Pharmacol., 2007, 59, 829. 197 W. Xia, C. Yang, B. O. Patrick, J. R. Scheffer and C. Scott, J. Am. Chem. Soc., 2005, 127, 2725. Photochemistry, 2009, 37, 149–174 | 173 This journal is

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198 C. K. Jankowski, C. Hocquelet, S. Arseneau, C. Moulin and L. Mauclaire, J. Photochem. Photobiol. A, 2006, 184, 216. 199 S. Lochbrunner, M. Zissler, J. Piel, E. Riedle, A. Spiegel and T. Bach, J. Chem. Phys., 2004, 120, 11634. 200 J. Xu and R. G. Weiss, J. Org. Chem., 2005, 70, 1243. 201 J. Xu and R. G. Weiss, Photochem. Photobiol. Sci., 2005, 4, 348. 202 C. Luo, P. Passin and R. G. Weiss, Photochem. Photobiol., 2006, 82, 163. 203 J. Xu and R. G. Weiss, Photochem. Photobiol. Sci., 2005, 4, 210. 204 L. S. Kaanumalle, J. Nithyanandhan, M. Pattabiraman, N. Jayaraman and V. Ramamurthy, J. Am. Chem. Soc., 2004, 126, 8999. 205 L. S. Kaanumalle, L. D. Gibb, B. C. Gibb and V. Ramamurthy, Org. Biomol. Chem., 2007, 5, 236. 206 R. C. White, B. E. Arney Jr. and K. M. White, J. Org. Chem., 2006, 71, 8173. 207 B. K. Bettadaiah and P. Srinivas, J. Photochem. Photobiol. A, 2004, 167, 137. 208 H. M. Memarian, A. Saffar-Teluri and M. K. Amini, Heterocycles, 2006, 68, 1861. 209 E. Hasegawa, N. Chiba, T. Takahashi, S. Takizawa, T. Kitayama and T. Suzuki, Chem. Lett., 2004, 33, 18. 210 T. Stafforst and U. Diederichsen, Angew. Chem. Int. Ed., 2006, 45, 5376. 211 J. Trzcionka, V. Lhiaubet-Vallet, C. Paris, N. Belmadoui, M. J. Climent and M. A. Miranda, ChemBioChem, 2007, 8, 402. 212 M. G. Friedel, M. K. Cichon and T. Carell, Org. Biomol. Chem., 2005, 3, 1937. 213 Q.-H. Song, H.-B. Wang, W.-J. Tang, Q.-X. Guo and S.-Q. Yu, Org. Biomol. Chem., 2006, 4, 291. 214 W.-J. Tang, Q.-H. Song, H.-B. Wang, J.-Y. Yu and Q.-X. Guo, Org. Biomol. Chem., 2006, 4, 2575. 215 R. Perez-Ruiz, S. Gil and M. A. Miranda, J. Org. Chem., 2005, 70, 1376. 216 R. Perez-Ruiz, M. A. Miranda, R. Alle, K. Meerholz and A. G. Griesbeck, Photochem. Photobiol. Sci, 2006, 5, 51. 217 M. A. Izquierdo, L. R. Domingo and M. A. Miranda, J. Phys. Chem. A, 2005, 109, 2602. 218 O. A. Borg, L. A. Eriksson and B. Durbeej, J. Phys. Chem. A, 2007, 111, 2351.

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Photochemistry of aromatic compounds Kazuhiko Mizuno* DOI: 10.1039/b812712n This chapter deals with the photoisomerization, photoaddition, photo-substitution, intramolecular photocyclization, photodimerization, photo-rearrangement of aromatic compounds and related photoreactions.

1.

Introduction

This chapter on the photochemistry of aromatic compounds maintains the classification into the same categories adopted in the previous reviews in the series. The subject of aromatic photochemistry has again been developed in a variety of photoreactions. In particular, a large number of papers appeared in the area of intramolecular photocyclization. During the years considered, a handbook on organic photochemistry was edited in by Horspool and Lenci.1 Many reactions of aromatic compounds appeared in this book. A series of ‘‘Molecular and Supramolecular Photochemistry’’ including several reviews on aromatic photochemistry also appeared in the period considered.2–4 2.

Isomerization reactions

The cis-trans photoisomerization of stilbene (E-1 and Z-1) and analogues has been widely investigated from 1960’s. Although some studies are considered in a separated chapter (see section 2A), the most characteristic photoreactions are described here. Arai has reported the trans-cis photoisomerization of cross-linked 1,3,5-tristyrylbenzene dendrimer (E,E,E-2;12-cross-links) to isomer Z,E,E-2 through the hula-twist mechanism.5 N-Aryl-substituted trans-4-aminostilbenes (3) have been photoisomerized via a twisted intramolecular charge-transfer (TICT) state depending on substituents. The nature of the TICT states has been discussed on the basis of the ring-bridged model compounds (4–7).6 The photoirradiation of b-(4-sydnoyl)-o-divinylbenzene (8) caused the cis-trans isomerization and decarboxylation. In the presence of acrolein, pyrazoline derivatives (9) were obtained via dipolar intermediates.7 Ramamurthy has reported some volume demanding one-way photoisomerization of cis-styrylpyrenes (10) in solid state.8 In this photoreaction, the presence of an empty space near the reaction centre is determining. Thus, 1,2-diazulenylethene (11) did not isomerize. The occurrence of a conformer specific photoisomerization via an adiabatic pathway has been reported for the case of cis-styryl-2-anthracene (Z-12).9 Styrylstilbenes and related compounds (13 and 14) showed one-way photoisomerization to give all trans-isomers in solution and in the solid states.10,11 With the dimethyl derivative of 1,2-bis(4-pyridinium)ethylene 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. E-mail: [email protected]; Fax: +81-72-254-9289; Tel: +81-72-254-9289

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(15) cis-trans photoisomerization took place efficiently, but with the dibenzyl analogue (16) it occurred in a low quantum yield.12

Irradiation of E-styrylfurans (E-17) led to the Z-isomers (Z-17), then gave the cyclized product (19) under aerated conditions. However, in the absence 176 | Photochemistry, 2009, 37, 175–212 This journal is

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of oxygen rearomatization occurred to give benzofuran derivatives (20). Dihydrophenanthrene-type intermediates (18) followed by radical intermediates have been observed.13 The photoisomerization of azobenzenes has been largely developed for photochromic systems. Very many papers appeared in this field, ranging from basic chemisty to applications. Some of the most typical examples are mentioned Photochemistry, 2009, 37, 175–212 | 177 This journal is

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below. A photoswitching behaviour was characterized by the use of chiral azobenzenes (21–23).14 An optical switching character was observed in sol–gel hybrid TiO2/ormosil organic–inorganic matrices.15 Peptidomimetic boronate esters containing an azobenzene unit (24 and 25) were photoisomerized by UV and visible light to give mixtures enriched in the E- and Z-isomers.16 Azobenzenes having suitable substituents (26) were used as photoinducible turning elements to investigate and control the folding and stability of b-sheets.17 The photoinduced E–Z isomerization of azobenzene units in polymers has been widely investigated. Cycloaliphatic-aromatic polyimides bearing azobenzene groups (27) were characterized and the E–Z photoisomerization was examined.18

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The E–Z photoisomerization of azobenzene units containing ferrocenophanes (28 and 29) has been reported to give a photostationary state.19 The femtosecond fluorescence and absorption dynamics have been examined in the case of 4-nitro-4 0 -(dimethylamino)azobenzene (30) as a typical push-pull substituted azobenzene.20 The photoisomerization of chalcones in flavylium photochromic systems was found to depend on substituents and solvents.21 The diastereoselective cis-trans photoisomerization of 2,3-diphenylcyclopropane1-carboxylic acid (31) and its derivatives (32–34) in zeolites has been reported by Ramamurthy. Some chiral auxiliaries placed at a remote location from the isomerization site functioned much better within a zeolite than in solution.22

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Yang and Li have reported norbornadiene- and carbazole-labeled poly(aryl ether)dendrimers (35), which were photochemically isomerized to quadricyclanes (36).23

Photochemical transposition reactions such as that of pyrazine (37) to pyrimidine (38), or that of aniline (39) to 4-methylpyridine (40) and vice versa have been reported by Su and Ni, independently.24,25 Ni proposed the intermediacy of seven-membered ring isomers (41) by 193 nm irradiation of aniline and 4-methylpyridine. The intramolecular photocyclization of 2-vinyldiphenylacetylenes (42 and 43) afforded 1-phenylnaphthalene (45 and 48) and 1-methylene-4-phenylnaphthalene derivatives (46, 47 and 49) via unstable cyclic allene (isonaphthalenes) intermediates (44).26

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3.

Addition reactions

Review articles on photochemistry in natural products synthesis and comments on current topics on intramolecular meta-photocycloaddition have been published by Hoffmann, Gilbert, Russel and Mattay, respectively.27–30 The photocycloaddition of alkenes to naphthalenes commonly occurs at both 1,2- and 1,4-positions of the naphthalene rings. The intramolecular photocycloaddition of 2-alkenyl-1-cyanonaphthalenes (50) has been first reported by McCullough.31 Mizuno obtained a highly efficient and regioselective [2 + 2] photocycloaddition at the 1,2-position using a microreactor.32,33 These [2 + 2] photocycloadducts (51) underwent cycloreversion to the starting compounds. Then, 1,3-cycloadducts (53), which were characterized by X-ray crystallographic analyses, were obtained in good to high yields.34

This is a rare example of the 1,3-photocycloaddition of alkenes to naphthalenes giving polycyclic compounds in a single step. Intramolecular ortho- and meta-photocycloaddition of 5-phenyl-1-pentene (54) in the gas phase using 254 nm light was reported by Morrison. In solution, meta cycloadducts (55), (56) and (57) were obtained. In the gas phase, however, Photochemistry, 2009, 37, 175–212 | 181 This journal is

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compounds (55)–(59) were produced. Tricyclic derivative (59) was assigned as a two photons product in this photoreaction.35

Miranda has reported the triplet-photosensitized Diels-Alder reaction between indoles (60) and cyclohexadienes to give [4 + 2] cycloadducts (61) in a mechanistic study.36 Benzoylthiophene used as a triplet photosensitizer was found to interact with the NH group of indoles. Pyrene-benzoylthiophene sensitized photoreaction between cyclohexadiene and styrenes gave endo- and exo-[2 + 2] photocycloadducts (62) and (63).37

The intramolecular [4 + 4] photocycloaddition between anthracene and naphthalene derivatives with bulky N substituents has been investigated as chiral photochromic system.38 The two enantiomers (64) and (65) gave cycloadduct (66), which caused a slow racemization. The foldamer (67) having an anthracene moiety in the center connected with two naphthalene moieties below and above it was found to be dichroism (CD) active in the [4 + 4] photocycloaddition and afforded cycloadduct (68). 182 | Photochemistry, 2009, 37, 175–212 This journal is

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Cases of intermolecular photocycloaddition between unsaturated compounds and aromatic compounds have also been reported. Cage compounds (70) were formed by photocycloaddition of furan to electron-deficient naphthalenes (69) in the excited singlet state. It has been suggested that a dipole–dipole interaction between the excited singlet state of naphthalene and furan is important for this photocycloaddition.39 The diastereoselective [2 + 2] photocycloaddition of methyl 9-phenanthrenecarboxylate (71) with cholesteryl cinnamate (72) afforded [2 + 2] cycloadduct (73) in liquid crystalline phase.40

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The 1,4-addition of benzene to a dihydrocyclopenta[a]indene diradical photochemically generated from compound (74) where two ethynylanthracene chromophores were present was reported to give (75) that further evolved with the formation of (76).41 Irradiation of 2-allylphenol (77) in hexane afforded dihydrobenzofuran (78) as the sole product. The photoproducts distribution was strongly depended on the reaction conditions, such as using inclusion in cyclodextrin (CD) cavities. Cyclopropylphenol (79) and hydrated product (80) were obtained in a- and b-CD.42 The oxo-hydroxy tautomerism of heterocyclic compounds such as 2-quinolinone, 1-isoquinolinone, 3-hydroxyisoquinolinone, 2-quinoxalinone, and 4-quinazolinone was discussed systematically.43 Padwa has reported the efficient photodesulfonylation of N-sulfonyl indoles (81) initiated by electron transfer from triethylamine.44 The photoinduced ene-type reaction of 9-methylene-9,10-dihydrophenanthrene (82) with styrene, 1,1-diphenylethene, and 1,3-pentadiene has been reported.45 In the absence of alkenes, the dimerized product was obtained. Yasuda reported a review article of photoaddition of amines and ammonia via photoinduced electron transfer.46

4.

Substitution reactions

The photo-NOCAS reaction (photochemical nucleophile-olefin combination, aromatic substitution) has been developed and reviewed by Arnold.47 Some excellent reviews of aromatic photosubstitution reactions have appeared in this period.48–50 184 | Photochemistry, 2009, 37, 175–212 This journal is

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The photoreaction of 4-nitroanisol (83) with NaCN afforded 4-cyanoanisole (85) and the stable nitronate ion adduct (86). However, photosubstitution of 2-halo-4-nitroanisoles with NaCN depended on halogen, alternatively giving 2-cyano-4-nitroanisole (84) via SN2Ar* mechanism.51 Photosubstitution of 4-nitroanisole (83) by amines para to nitro group occurred via formation of a meta s complex, followed by an unprecedented sigmatropic rearrangement to give (87) and (88).52 The photosubstitution reaction of 4-nitroanisole (83) with hydroxide ion occurred selectively to give 4-nitrophenol (89) in high-temperature water using microwave (MW) at 170 1C. However, both (89) and 4-methoxyphenol (90) were obtained in a 1:1 ratio at 100 1C.53

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The photoreaction of 2-fluoro-4-nitroanisole (91) with N-acetyllysinamide gave the substitution product (92).54 The excited triplet state of 6-bromopicolinic acid (93) in the presence of Cl afforded 6-chloropicolinic

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acid (95) via the 6-bromo-2-carboxypyridinyl radical (94) in ca. 90% yield. This radical was characterized by laser flash photolysis (lmax = 318 nm).55 Miranda has reported the nucleophilic aromatic photosubstitution reaction of 6-fluoroquinolones (96) in basic media, giving the corresponding 6-hydroxy derivatives (97).56 Aryl methyl sulfides (100) and diaryl sulfides (101) have been prepared by photoreactions of potassium thioacetate (98) with aryl halides (99) such as 1-bromonaphthalene and 4-bromobiphenyl. Without isolation, the reaction with methyl iodide gave aryl methyl sulfides accompanied by diaryl sulfides.57 The photoreactions of 1,2,4,5-tetracyanobenzene (102) with styrene derivatives (103) gave (2,4,5-tricyanophenyl)tetralin derivatives (104) as the main products via photoinduced electron-transfer.58 The photoalkylation of 2,3-dicyanonaphthalene (105) by 1,2-diarylcyclopropanes (106) in benzene has been reported to give 1-(1,3-diarylpropyl)-2,3dicyanonaphthalenes via exciplex (107). The typical emission from a intramolecular exciplex was observed between 2,3-dicyanonaphthyl and 4-methoxyphenyl groups.59 Irradiation of 2 0 -halobenzanilide (108) gave 2-phenylbenzoxazole (109) and phenanthridone (110) as substitution products accompanying some rearranged products.60

5.

Intramolecular cyclization reactions

Seven-membered oxacyclic compounds (112) were produced via a photochemically unprecedented 7-exo-dig cyclization from a 1,3-bis(phenylethynyl)-p-tert-butylcalix[4]arene (111).61 The photocyclization of quinazolin-1-oxides (113) afforded 1aH-[1,2]oxazireno[2,3-a]quinazolines (114) depending on substituents. The irradiation of (114) gave (115) in THF and toluene, and (116) and (117) in CH2Cl2 in good yields, respectively.62 Photochemistry, 2009, 37, 175–212 | 187 This journal is

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Efficient photolabile protecting group for carboxylic acid were reported. As an example, 2-hydroxy-1,2,2-triphenylethanone (118) was protected as the corresponding ester (119). Upon photodeprotection, however, intramolecular cyclization occurred and afforded 2,3-diphenylbenzofuran (120). In turn, this compound underwent cyclization and oxidation to give benzo[b]phenanthro[9,10-d]furan (121).63

The irradiation of 1-dienyl-2-phenylcyclopropanes (122) with C60 gave five-, seven-, and nine-membered [60]fullerene adducts (123).

It was suggested that the reactions involved a photoinduced electron transfer mechanism from the cyclopropanes (122) to the excited triplet state of C60.64 Six-membered aza-heterocyclic compounds were obtained via iminyl radical (125) generated by irradiation of acyloximes (124), (127) 188 | Photochemistry, 2009, 37, 175–212 This journal is

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and (129). These cyclized to phenanthridines (126) and isoquinolines (128).65 The photochemical radical cyclization of g, d-unsaturated ketone oximes (130) gave 3,4-dihydro-2H-pyrroles (132) via radical anion (131) resulting from the photoinduced electron transfer from 1,5-dimethoxynaphthalene (DMN) to (130).66

Irradiation of 4-chlorophenol (133) in the presence of 3-butenoic acid, 4-pentenoic acid, and 5-hexenoic acid gave benzyl or phenyl lactones (136) and (137) via photoinduced tandem Ar–C, C–O bond formation. Albini postulated phenonium ions (134) as reactive intermediates. These rearranged to aryl d-lactones (135).67

Intramolecular aromatic photocyclization of 2-iodo-N-(2-arylethyl)imidazoles (138) afforded 5,6-dihydroimidazo[2.1-a]isoquinolines (139) via imidazol-2-yl radicals. Also the related nitrogen-containing heteroaromaric Photochemistry, 2009, 37, 175–212 | 189 This journal is

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compounds have been prepared.68 Substituted dihydrothieno[20 ,30 :4,5]thieno[2,3c]-quinolin-6-ones (140) and tetra-hydrodithieno[2,3-b:20 ,30 -d]thieno[200 ,300 c:200 ,300 c0 ]-diquinolin-6,14-dione (141) have been prepared by photoinduced intramolecular cyclization of the corresponding anilides and dianilides.69 Several angularly fused polyaromatic compounds such as 9,14-dimethoxynaphtho[1,2a]-anthracene (142) were synthesized by photoirradiation in the presence of potassium tert-butoxide.70 The doubly annulated tropylium ion 2,4-dimethylfuro[2,3-d]pyrimidine-1(2H),3(4H)-dione (144) has been prepared by irradiation of compound (143).71

The photoinduced-Bergman cyclization of porphyrin endiyne chimeras (145) was reported to give (147) via 1,4-diyls (146).72

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Second generation poly(propyleneamine) dendrimer (148) functionalized with E-stilbene underwent E–Z photoisomerization and photocylization of Z-isomer.73 Some Z-stilbenophanes (149) afforded phenanthrenophanes (150) in good yields. However, (149) did not isomerize to E-isomers.74

Irradiation of cis-stilbene and its analogues frequently results in electrocyclic closure of the excited singlet state to give dihydrophenanthrenes, which are readily oxidized to phenanthrenes.75 However, 1,2-diheteroarylethenes bearing substituents at 2 and 2 0 positions were cyclized, but not oxidized. The photochemical cyclization of a variety of 1,2-diheteroarylethenes has been developed in view of the photochromic properties of these molecules. Several reviews appeared in this field.76–81 Irie and his coworkers reported many papers in this field.77–107 A variety of 1,2-diheteroarylethene compounds have been prepared and again were of interest as typical photochromic systems. In some cases, the quantum yield of such reactions were quite high. A theoretical study of the quantum yield of the photochromic cyclization and cycloreversion reactions was carried out on both normal-type 1,2-dithienylethenes [151(o) and 151(c)] and inverse-type 1,2-dithienyl ethenes [152(o) and 152(c)].80 Irie has noticed that upon irradiation by UV light the anti-parallel conformer [153(a)] does cyclize to a closed form [153(c)], but the parallel conformer [153(p)] cannot cyclize at all. Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethene units having two nitroxides radicals (154) and (155).100 Photochemistry, 2009, 37, 175–212 | 191 This journal is

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Full color photochromism was accomplished by preparing fused dithienylethene trimer (156)–(160). Although (156) is colorless, the closed-ring isomer of the central one (157), the terminal one (157), and the terminal one (158) showed blue, red, and yellow colors, respectively. The double closed-ring isomer (160) showed orange.103 Robust fluorescence molecules, having perylene bisimide as the fluorescent unit and diarylethene as the switching unit, were prepared and the photochromic reactions were measured at the single-molecule level in various polymer matrics.83 Single crystals of the closed-isomer of (162(c)) underwent a ringopening reaction with high conversion while keeping their crystalline shape and transparency.86 The digital photoswitching of fluorescence based on the photochromism of diarylethenes at single-molecule level has been studied by the use of bis(phenylethynyl) anthracene units and an adamantyl spacer (163(o)).106 192 | Photochemistry, 2009, 37, 175–212 This journal is

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Feringa developed the photochromism of 1,2-diheteroarylethenes.108–118 Dynamic chiral selection and amplification by using photoresponsive organogelators and liquid crystalline phases (164)–(166) have been reported.116–118

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Molecular memory devices based on 1,2-dithienylethene switch (167) modified ITO electrodes undergo reversible ring opening/closing under both photo- and electro-chemical conditions, offering an example of nondestructive electrochemical readout.110 The atropisomers of photochromic 1,2-dithienylethenes (168) were isolated and their photochemical properties were examined. The parallel rotamer did not underwent intramolecular photocyclization, but the anti-parallel rotamer gave the cyclized product upon 313 nm irradiation.108 Pu and Chen prepared a variety of 1,2-diheteroarylethenes having photochromic properties.119–134 Some highly colored cyclized compounds (169)–(171) and some reversible photochromic systems (172)–(174) are shown below.113,119,125,134

The photoreversible cyclization of 3-(2-benzylbenzoyl)quinolinone (175) as highly efficient photochromic compound was reported.135 Irradiation of (175) afforded two thermally stable, but photochemically reversible, diastereomeric benzoacridones (176). Mariano reported the intramolecular photocyclization of pyridinium salt (177) and (180) to give cyclopentylamine derivatives (179) and (181) as precursors of natural products.136–139 Yoon and Mariano developed the intramolecular photocycloaddition of phthalimides and naphthalimides for building macrocyclic poly-ethers, thioethers, sulfonamides and related compounds.140–147 Naphthalimide derivatives (182) afforded cyclized poly-ethers (183) in good yields.143 194 | Photochemistry, 2009, 37, 175–212 This journal is

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The enantioselective intramolecular photocycloaddition of 4-(2 0 -aminoethyl)-quinolones (184) gave tetracyclic tetrahydro-1aH-pyrido[4 0 ,3 0 :2,3]cyclobuta[1,2-c] quinoline-2,11(3H,8H)-diones (185) by the use of chiral complexing agent (186).148 Lewis reported the intramolecular photocyclization of conformationally constrained 2-vinylbiphenyl (187), 2-ethynylbiphenyl (188), 2,6-diarylstyrenes (189), and 2,6-diheteroarylstyrenes (190).149–151

6.

Dimerization reactions

Review articles on the [2 + 2] photocyclodimerization of cinnamic acid derivatives, of acenaphthylenes and of related arylalkenes appeared in this period.152–155 Irradiation of some cinnamophane vinylogs (191) afforded Photochemistry, 2009, 37, 175–212 | 195 This journal is

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cyclobutane derivatives (192) by intramolecular photocyclodimerization.156 The direct photolysis of stilbenophanes (193) and (195) afforded the intramolecular [2 + 2] cycloadducts (194) and (196) in good yields. Only cis-trans photoisomerization took place in the benzophenone-sensitized photoreaction of (193). However, the triplet-sensitized photoreaction of (195) also gave (196) quantitatively.74 [4.4.4](1,3,5)Cyclophane (198) and (198 0 ) were synthesized by photodimerization of (E,E,E)-1,3,5-tricinnamoylbenzene (197) and (197 0 ) in solution. In the crystalline state, (197) yielded the syn-head-to-tail dimer (199) topochemically.157 (E)-3 0 - and 4 0 -Nitro-3-azachalcones (200) photodimerized to give syn-dimers (201) in a stereoselective manner. However, (E)-2 0 -nitro-3-azachalcone did not give a cyclobutane dimer.158 3,3 0 -Dimethoxy-4,2 0 -dihydroxybiphenyl (202) has been photochemically prepared from 2-methoxyphenol.159 Octahedrane (204) was synthesized 196 | Photochemistry, 2009, 37, 175–212 This journal is

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by intramolecular photocyclodimerization of 2,11-diaza[3.3]-paracyclophanes (203) and was accompanied by (205). However, the carbon bridged analogue (206) was almost photochemically inert.160 Amphiphilic hexabenzocoronene (207) having two coumarin pendant groups underwent reversible photodimerization to give head-to-head and head-to-tail cyclobutanes (208) and (209). These dimers underwent cycloreversion to give back (207).161 Ramamurthy reported the photodimerization of cinnamic acids (210) in cucurbit[8]uril (CB[8]) and ~ g-cyclodextrin (~g-CD) as templates to give syn-head-to-head dimers (211).162 He also reported the (CB[8])-mediated photodimerization of protonated azastilbenes (212) to give the syncyclodimer (215) in a highly selective manner along with a small amount of the anti-dimer (216).163 In an aqueous solution, the isomerized product (213) and the cyclized product (214) were obtained. The photodimerization of coumarin was rationalized on the basis of simple ab initio calculations.164 Irradiation of (E)-8-styrylxanthine derivatives (217) in solid states afforded syn-head-to-tail cyclobutanes (218). Photochemistry, 2009, 37, 175–212 | 197 This journal is

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However, E–Z photoisomerization occurred in solution.165 Some solid-state photodimerization of chalcone, 9-acetyl- and 9-methoxycarbonylanthracenes has been reported in inclusion compounds such as 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol.166 Irradiation of 6,13-bis((triisopropylsilyl)ethynyl)pentacene (219) afforded two types of [4 + 4] dimerized products (220) and (221).167 The intermolecular dimerization of stilbene derivatives occurs inefficiently. However, the intramolecular photocycloaddition of b-stilbazoles (222) tethered by silyl chains led to smooth dimerization to give cyclobutanes (223) in the presence of catechol or 1,2-cyclohexane dicarboxylic acid.168 The inter- and intra-molecular [4 + 4] photocycloaddition of anthracenes has been developed for molecular recognition by using templates.169–173 198 | Photochemistry, 2009, 37, 175–212 This journal is

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Nakamura reported the template effects of metal cations (225) and (226) on the photodimerisation of N,N 0 -linked bis(anthracenecarboxamides)dimerisation of anthracene derivatives (227) has been developed by Ihara.170 Enantioselective [4 + 4] photocyclodimerisations of 2-anthracenecaroboxylate (228) within g-cyclodextrin modified by pyridinium and ammonium cations have been reported by Ikeda and Inoue, independently.171,172 Two cyclodimers (229) and (230) are chiral, but the other ones, (231) and (232), are achiral.

7.

Lateral-nuclear rearrangements

Several reviews on this type of photorearrangements have been published.174–181 The di-p-methane rearrangement of dibenzobicyclo[2.2.2]octatrienes (233) was explored when using of chiral ionic liquids, but the Photochemistry, 2009, 37, 175–212 | 199 This journal is

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enantiomer excess of the rearranged products (234) and (235) was low.182 The effect of substituents on the photo-Fries reaction of triphenylmethylsilane derivatives (236)–(239) was examined in the excited singlet and triplet states by means of 248 and 308 nm nanosecond laser flash photolysis (ns-LFP), femtosecond LFP, and so on.183 In the case of (237) and (238), photo-Fries products such as (240) were obtained from the singlet radical pair (241) in solvent cage. On the other hand, (236) and (239) efficiently generated the triplet radical pair and gave free radicals. The Fries rearrangement of sulfonamide and sulfonate crystals (242) induced by elecron-beam was much more extensive than that of the corresponding carboxylic acid and amide derivatives (243).184 The photo-Fries rearrangement of carbazol-2-yl sulfonates (244) afforded two kinds of rearranged products (245) and (246).185 200 | Photochemistry, 2009, 37, 175–212 This journal is

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The photorearrangements of 1-methyl-8-nitronaphthalene (247) in the excited triplet state were studied by means of DFT-calculations.186 The nitrite type isomer (248) was produced from intermediate (249), and (250)

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was formed from the triplet biradical (251). The photochemical rearrangement of dibenzo[1,4]dioxins (252) proceeded through reactive spirocyclohexadienone (253) and biphenylquinone (254) intermediates to give 2,2 0 -dihydroxybiphenyls (255).187 Both photoinduced para-nitro Smiles rearrangement and para-nitro Meisenheimer complex formation took place from nitrophenoxyethylamines (256) and (257) in aqueous solution with a high concentration of hydroxide ion. The reactions occurred from the excited triplet states of the nitrophenyl ethers (256) and (258) to give (257) and (259).188 Some selective photodecarboxylations of chiral and cyclic aryl esters (260)–(262) have been reported by Mori and Inoue.189 In this photoreaction, the photo-Fries rearrangement was blocked by methyl substitutents. The photochemical rearrangements of imidazoles (263) and 1,2,4-oxadiazoles (264) were discussed on the basis of theoretical calculations.190,191

8.

Heterocycles

Several preparation of heterocyclic compounds have been reported, implying electrocyclic rearrangements and other photocylisation reactions. These are discussed here, although some of them have been mentioned in the previous sections. The electron transfer initiated asymmetric photocyclisation of chiral N-acyl-a-dehydro(1-naphthyl)alaninamides (265) led to the 3,4-dihydrobenzo[f]-quinolinone derivatives (266) and (267).192 Similar intramolecular photocyclisation reactions were described, such as the conversion of (268) to related N-heterocyclic compounds like quinoline derivatives (269).193–195 The intramolecular photocyclization of N-(2-acyl-phenyl)-2-methylprop-2-enamides (270) gave cyclic amides (271) via dipolar intermediates (272) followed by migration of acyl group (273).196–198 Some other intramolecular photocyclisation reactions were reported. Thus, the light-induced generation of iminyl radical (275) from acyloximes (274) was followed by cyclization to give N-heteroaromatic compounds such as phenanthrolines Photochemistry, 2009, 37, 175–212 | 203 This journal is

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(276).199 Similar intramolecular photocyclizsation afforded heterocyclic and N-hereroaromatic compounds.200–203

Apart from the dithienylethenes discussed above,212–225 a variety of other heterocyclic systems are involved in photochromic systems.204–209 Some naphthopyrans and related compounds (277)–(279) were

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characterized as photochromic molecules. Hybrid biphotochromic system (280) and strained dibenzo-acridinium cation (281) were also reported.210,211 References 1 Review: CRC Handbook of Organic Photochemistry and Photobiology, ed. W. Horspool and F. Lenci, CRC Press, second edn, 2004. 2 Review: Chiral Photochemistry, ed. Y. Inoue and V. Ramamurthy, Marcel Dekker, 2004. 3 Review: Synthetic Organic Photochemistry, ed. A. G. Griesbeck and J. Mattay, Marcel Dekker, 2005. 4 Review: Organic Photochemistry and Photophysics, ed. V. Ramamurthy and K. S. Schanze, Taylor & Francis, 2006. 5 M. Uda, A. Momotake and T. Arai, Tetrahedron Lett., 2005, 46, 3021–3024. 6 J.-S. Yang, K.-L. Liau, C.-M. Wang and C.-Y. Hwang, J. Am. Chem. Soc., 2004, 126, 12325–12335. 7 K. Butkovic, N. Basaric, K. Lovrekovic, Z. Marinic, A. Visnjevac, B. Kojic-Prodic and M. Sindler-Kulyk, Tetrahedron Lett., 2004, 45, 9057–9060. 8 A. Natarajan, J. T. Mague, K. Venkatesan, T. Arai and V. Ramamurthy, J. Org. Chem., 2006, 71, 1055–1059. 9 G. Bartocci, G. Galiazzo, E. Marri, U. Mazzucato and A. Spalletti, Inorganica Chimica Acta, 2007, 360, 961–969. 10 Z. Fengqlang, J. Motoyoshiya, J. Nakamura, Y. Nishii and H. Aoyama, Photochem. Photobiol., 2006, 82, 1645–1650. 11 H. Katayama, M. Nagao, F. Ozawa, M. Ikegami and T. Arai, J. Org. Chem., 2006, 71, 2699–2705. 12 R. Ballardini, A. Credi, M. T. Gandolfi, C. Giansante, G. Marconi, S. Silvi and M. Venturi, Inorganica Chimica Acta, 2007, 360, 1072–1082. 13 S. Samori, M. Hara, T.-I. Ho, S. Tojo, K. Kawai, M. Endo, M. Fujitsuka and T. Majima, J. Org. Chem., 2005, 70, 2708–2712. 14 M. Z. Alam, T. Yoshioka, T. Ogata, T. Nonaka and S. Kurihara, Chem. Eur. J., 2007, 13, 2641–2647. 15 W. Que, X. Hu, X. L. Xia and L. Zhao, Optics Express, 2007, 15, 480–485. 16 D. Pearson and A. D. Abell, Org. Biomol. Chem., 2006, 4, 3618–3625. 17 A. Aemissegger, V. Krautler, W. F. van Gunsteren and D. Hilvert, J. Am. Chem. Soc., 2005, 127, 2929–2936. 18 E. Schab-Balcerzak, L. Grobelny, A. Sobolewska and A. Miniewicz, Eur. Polym. J., 2006, 42, 2859–2871. 19 T. Sakano, M. Horie, K. Osakada and H. Nakao, Eur. J. Inorg. Chem., 2005, 644–652. 20 B. Schmidt, C. Sobotta, S. Malkmus, S. Laimgruber, M. Braun, W. Zinth and P. Gilch, J. Phys. Chem. A, 2004, 108, 4399–4404. 21 A. Roque, J. C. Lima, A. J. Parola and F. Pina, Photochem. Photobiol. Sci., 2007, 6, 381–385. 22 J. Sivaguru, R. B. Sunoj, T. Wada, Y. Origane, Y. Inoue and V. Ramamurthy, J. Org. Chem., 2004, 69, 6533–6547. 23 J. Chen, J. Chen, S. Li, L. Zhang, G. Yang and Y. Li, J. Phys. Chem. B, 2006, 110, 4663–4670. 24 M.-D. Su, J. Phys. Chem. A, 2006, 110, 9420–9428. Photochemistry, 2009, 37, 175–212 | 205 This journal is

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118 J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch and B. L. Feringa, Science, 2004, 304, 278–281. 119 S.-Z. Pu, G. Liu, L. Shen and J.-K. Xu, Org. Lett., 2007, 9, 2139–2142. 120 S.-Z. Pu, G. Liu, G. Li, R. Wang and T. Yang, J. Mol. Structure, 2007, 833, 23–29. 121 T. Yang, S.-Z. Pu, B. Chen and J.-K. Xu, Can. J. Chem., 2007, 85, 12–20. 122 S.-Z. Pu, T. Yang, R. Wang, F. Zhang and J. K. Xu, Spectrochimica Acta, A, Mol. Biomol. Spectroscopy, 2007, 66A, 335–340. 123 S.-Z. Pu, H. Tang, B. Chen, J. K. Xu and W. Huang, Mater. Lett., 2006, 60, 3553–3557. 124 S.-Z. Pu, F. Zhang, J.-K. Xu, L. Shen, Q. Xiao and B. Chen, Mater. Lett., 2006, 60, 485–489. 125 S.-Z. Pu, T. Yang, J.-K. Xu, L. Shen, G. Li, Q. Xiao and B. Chen, Tetrahedron, 2005, 61, 6623–6629. 126 N. Xie, D.-X. Zeng and Y. Chen, J. Electroanal. Chem., 2007, 609, 27–30. 127 N. Xie and Y. Chen, J. Mater. Chem., 2007, 17, 861–865. 128 N. Xie and Y. Chen, New J. Chem., 2006, 30, 1595–1598. 129 Y. Chen and N. Xie, J. Photochem. Photobiol. A, Chemistry, 2006, 179, 320–323. 130 N. Xie and Y. Chen, J. Mater. Chem., 2006, 16, 982–985. 131 D.-X. Zeng and Y. Chen, Chinese J. Chem., 2006, 24, 264–268. 132 S. Luo, K. Chen, L. Cao, G. Liu, Q. He, G. Jin, D.-X. Zeng and Y. Chen, Optics Express, 2005, 13, 3123–3128. 133 Y. Chen, D.-X. Zeng, N. Xie and Y. Z. Dang, J. Org. Chem., 2005, 70, 5001–5005. 134 Y. Chen and D.-X. Zeng, J. Org. Chem., 2004, 69, 5037–5040. 135 J. Berthet, J.-C. Micheau, V. Lokshin, M. Vales, A. Samat, G. Vermeersch and S. Delbaere, J. Photochem. Photobiol. A, Chemistry, 2007, 187, 269–274. 136 Z. Zhao, E. Duesler, C. Wang, H. Guo and P. S. Mariano, J. Org. Chem., 2005, 70, 8508–8512. 137 Z. Zhao, L. Song and P. S. Mariano, Tetrahedron, 2005, 61, 8888–8894. 138 X. Feng, E. N. Duesler and P. S. Mariano, J. Org. Chem., 2005, 70, 5618–5623. 139 Z. Zhao and P. S. Mariano, Tetrahedron, 2006, 62, 7266–7273. 140 Review: U.-C. Yoon and P. S. Mariano, J. Photosci., 2005, 12, 155–162. 141 Review: U.-C. Yoon and P. S. Mariano, Bull. Kor. Chem. Soc., 2006, 27, 1099–1114. 142 Review: see ref. 3, U.-C. Yoon and P. S. Mariano, pp. 179–206. 143 R. Wang, Z. Zhao, P. S. Mariano, K.-H. Choi, S.-H. Kim and U.-C. Yoon, J. Photochem. Photobiol. A, Chemistry, 2005, 175, 232–241. 144 D.-W. Cho, M. Fujitsuka, U.-C. Yoon and T. Majima, J. Photochem. Photobiol. A, Chemistry, 2007, 190, 101–109. 145 S.-W. Oh, J.-Y. Kim, D.-W. Cho, J.-H. Choi and U.-C. Yoon, Bull. Kor. Chem. Soc., 2007, 28, 629–634. 146 D.-W. Cho, M. Fujitsuka, A. Sugimoto, U.-C. Yoon, P. S. Mariano and T. Majima, J. Phys. Chem. B, 2006, 110, 11062–11068. 147 D.-W. Cho, M. Fujitsuka, K.-H. Choi, J. Man, U.-C. Yoon and T. Majima, J. Phys. Chem. B, 2006, 110, 4576–4582. 148 P. Selig and T. Bach, J. Org. Chem., 2006, 71, 5662–5673. 149 F. D. Lewis, P. C. Karagiannis, M. C. Sajimon, K. S. Lovejoy, X.-B. Zuo, M. Rubin and V. Gevorgyan, Photochem. Photobiol. Sci., 2006, 5, 369–375. Photochemistry, 2009, 37, 175–212 | 209 This journal is

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150 D. Lewis, E. M. Crompton, M. C. Sajimon, V. Gevorgyan and M. Rubin, Photochem. Photobiol., 2006, 82, 119–122. 151 M. Sajimon and F. D. Lewis, Photochem. Photobiol. Sci., 2005, 4, 789–791. 152 Ref. 1, D. M. Bassani, 20/1–20. 153 Ref. 1, N. Haga and K. Tokumaru, 21/1–21. 154 L. MacGillivray, G. S. Papaefstathiou, T. Friscic, D. B. Varshney and T. D. Hamilton, Topics Curr. Chem., 2005, 248, 201–221. 155 T. D. Hamilton, G. S. Papaefstathiou and L. MacGillivray, J. Solid State Chem., 2005, 178, 2409–2413. 156 H. Greiving, H. Hopf, P. G. Jones, P. Bubenitschek, J.-P. Desvergne and H. Bouas-Laurent, Eur. J. Org. Chem., 2005, 558–566. 157 E. Kalpuk, D. Schollmeyer and H. Meier, Eur. J. Org. Chem., 2007, 1983–1990. 158 N. Yayh, M. Kucuk, O. Ucuncu, A. Yasar, N. Yayh and S. A. Karaoglu, J. Photochem. Photobiol. A, Chemistry, 2007, 188, 161–168. 159 D. Braga, C. Christophis, S. Noll and N. Hampp, J. Photochem. Photobiol. A, Chemistry, 2005, 172, 115–120. 160 H. Okamoto, K. Satake, H. Ishida and M. Kimura, J. Am. Chem. Soc., 2006, 128, 16508–16509. 161 J. Motoyanagi, T. Fukushima, N. Ishii and T. Aida, J. Am. Chem. Soc., 2006, 128, 4220–4221. 162 M. Pattabiraman, A. Natarajan, L. S. Kaanumalle and V. Ramamurthy, Org. Lett., 2005, 7, 529–532. 163 M. V. S. N. Maddipatla, L. S. Kaanumalle, A. Natarajan, M. Pattabiraman and V. Ramamurthy, Langmuir, 2007, 23, 7545–7554. 164 M. D’Auria and R. Racioppi, J. Photochem. Photobiol. A, Chemistry, 2004, 163, 557–559. 165 J. Hockemeyer, J. C. Burbiel and C. E. Muller, J. Org. Chem., 2004, 69, 3308–3318. 166 I. Zouev, T. Lavy and M. Kaftory, Euro. J. Org. Chem., 2006, 4164–4169. 167 P. Coppo and S. G. Yeates, Adv. Mater., 2005, 17, 3001–3005. 168 H. Maeda, R. Hiranabe and K. Mizuno, Tetrahedron Lett., 2006, 47, 7865–7869. 169 H. Hiraga, T. Morozumi and H. Nakamura, Eur. J. Org. Chem., 2004, 4680–4687. 170 T. Ihara, T. Fujii, M. Mukae, Y. Kitamura and A. Jyo, J. Am. Chem. Soc., 2004, 126, 8880–8881. 171 A. Nakamura and Y. Inoue, J. Am. Chem. Soc., 2005, 127, 5338–5339. 172 C. Yang, G. Fukuhara, A. Nakamura, Y. Origane, K. Fujita, D.-Q. Yuan, T. Mori, T. Wada and Y. Inoue, J. Photochem. Photobiol. A, Chemistry, 2005, 173, 375–383. 173 H. Ikeda, T. Nihei and A. Ueda, J. Org. Chem., 2005, 70, 1237–1242. 174 Review: F. Galindo, J. Photochem. Photobiol. C, Photochem. Rev., 2005, 6, 123–138. 175 Review: see ref. 3, D. Armesto, M. J. Ortiz and A. R. Agarrabeitia, pp. 161–187. 176 Review: see ref. 1, K. Seki and K. Ohkura, 105/1–16. 177 Review: see ref. 1, A. Albini and M. Fagnoni, 99/1–21. 178 Review: see ref. 1, D. Amesto, M. J. Ortiz and A. R. Agarrabeitia, 95/1–16. 179 Review: see ref. 1, H. E. Zimmerman, 75/1–11. 180 Review: see ref. 1, M. A. Miranda and F. Galindo, 42/1–11. 181 Review: see ref. 1, K. Mizuno and H. Maeda, 31/1–12. 210 | Photochemistry, 2009, 37, 175–212 This journal is

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182 J. Ding, V. Desikan, X. Han, T. L. Xiao, R. Ding, W. S. Jenks and D. W. Armstrong, Org. Lett., 2005, 7, 335–337. 183 A. K. Zarkadis, V. Georgakilas, G. P. Perdikomatis, A. Trifonov, G. G. Gurzadyan, S. Skoulika and M. G. Siskos, Photochem. Photobiol. Sci., 2005, 4, 469–480. 184 J. Kato, Y. Maekawa and M. Yoshida, Chem. Lett., 2005, 34, 266–267. 185 L. K. Crevatin, S. M. Bonesi and R. Erra-Balsells, Helv. Chim. Acta, 2006, 89, 1147–1157. 186 S. V. Kombarova and Y. V. Il’ichev, J. Org. Chem., 2005, 70, 6074–6084. 187 S. Rayne, R. Sasaki and P. Wan, Photochem. Photobiol. Sci., 2005, 4, 876–886. 188 G. G. Wubbels, N. Ohta and M. L. Crosier, Org. Lett., 2005, 7, 4741–4744. 189 T. Mori, R. G. Weiss and Y. Inoue, J. Am. Chem. Soc., 2004, 126, 8961–8975. 190 M.-D. Su, J. Phys. Chem. A, 2007, 111, 1567–1574. 191 A. Pace, S. Buscemi, N. Vivona, A. Silvestri and G. Barone, J. Org. Chem., 2006, 71, 2740–2749. 192 K. Maekawa, A. Shinozuka, M. Naito, T. Igarashi and T. Sakurai, Tetrahedron, 2004, 60, 10293–10304. 193 K. Maekawa, K. Kubo, T. Igarashi and T. Sakurai, Tetrahedron, 2005, 61, 11211–11224. 194 K. Maekawa, K. Fujita, K. Iizuka, T. Igarashi and T. Sakurai, Heterocycl., 2005, 65, 117–131. 195 H. Hoshina, K. Maekawa, K. Kobayashi, T. Igarashi and T. Sakurai, Heterocycl., 2006, 68, 993–1006. 196 T. Nishio, M. Tabata, H. Koyama and M. Sakamoto, Helv. Chim. Acta, 2005, 88, 78–86. 197 T. Nishio, H. Koyama, D. Sasaki and M. Sakamoto, Helv. Chm. Acta, 2005, 88, 996–1003. 198 T. Nishio, N. Sakurai, K. Iba, Y. Hamano and M. Sakamoto, Helv. Chim. Acta, 2005, 88, 2603–2609. 199 R. Alonso, P. J. Campos, B. Garcia and M. A. Rodriguez, Org. Lett., 2006, 8, 3521–3523. 200 M. M. V. Ramana, R. H. Sharma and J. A. Parihar, Tetrahedron Lett., 2005, 46, 4385–4386. 201 T. Dhanabal, R. Sangeetha and P. S. Mohan, Tetrahedron, 2006, 62, 6258–6263. 202 A. Pietrangelo, M. J. MacLacholan, M. O. Wolf and B. O. Patrick, Org. Lett., 2007, 9, 3571–3573. 203 J. F. Guastavino, S. M. Barolo and R. A. Rossi, Euro. J. Org. Chem., 2006, 3898–3902. 204 X. Sallenave, S. Delbaere, G. Vermeesch, A. Saleh and J.-L. Pozzo, Tetrahedron Lett., 2005, 46, 3257–3259. 205 F. Pina, A. J. Palora, M. J. Melo, C. A. T. Laia and C. A. M. Afonso, Chem. Commun., 2007, 1608–1610. 206 M. Suzuki, T. Asai and H. Masuhara, J. Photochem. Photobiol. A, Chemistry, 2006, 178, 170–176. 207 P. J. Coelho, M. A. Salvador, M. M. Oliveira and L. M. Carvalho, J. Photochem. Photobiol. A, Chemistry, 2005, 172, 300–307. 208 A. Domenech, H. Garcia, I. Casades and M. Espla, J. Phys. Chem. B, 2004, 108, 20064–20075. 209 J. Berthet, J.-C. Micheau, A. Matelistsa, G. Vermeersch and S. Delbaere, J. Phys. Chem. A, 2004, 108, 10934–10940. 210 S. Delbaere, G. Vermeersch, M. Frigoli and G. H. Mehl, Org. Lett., 2006, 8, 4931–4934. Photochemistry, 2009, 37, 175–212 | 211 This journal is

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211 A. C. Benniston and D. B. Rewinska, Org. Biomol. Chem., 2006, 4, 3886–3888. 212 S.-J. Lim, B.-K. An, S. D. Jung, M.-A. Chung and S. Y. Park, Angew. Chem., Int. Ed., 2004, 43, 6346–6350. 213 T. A. Golovkova, D. V. Kozlov and D. C. Neckers, J. Org. Chem., 2005, 70, 5545–5549. 214 S. Kawai, T. Nakashima, K. Atsumi, T. Sakai, M. Harigai, Y. Imamoto, H. Kamikubo, M. Kataoka and T. Kawai, Chem. Mater., 2007, 19, 3479–3483. 215 H. Okada, N. Nakajima, T. Tanaka and M. Iwamorto, Angew. Chem., Int. Ed., 2005, 44, 7233–7236. 216 M. Frigoli and G. H. Mehl, Angew. Chem., Int. Ed., 2005, 44, 5048–5052. 217 V. Lemieux, S. Gauthier and N. R. Branda, Angew. Chem., Int. Ed., 2006, 45, 6820–6824. 218 V. Lemieux and N. R. Branda, Org. Lett., 2005, 7, 2969–2972. 219 S. Wang, W. Shen, Y. Feng and H. Tian, Chem. Commun., 2006, 1497–1499. 220 S. Kobatake and Y. Terakawa, Chem. Commun., 2007, 1698–1700. 221 M. Frigoli, C. Welch and G. H. Mehl, J. Amer. Chem. Soc., 2004, 126, 15382–15383. 222 Z. Zhou, S. Xiao, J. Xu, Z. Liu, M. Shi, F. Li, T. Yi and C. Huang, Org. Lett., 2006, 8, 3911–3914. 223 J. Kuehni and P. Belser, Org. Lett., 2007, 9, 1915–1918. 224 G. Jiang, S. Wang, W. Yuan, Z. Zhao, A. Duan, C. Xu, L. Jiang, Y. Song and D. Zhu, Eur. J. Org. Chem., 2007, 2064–2067. 225 M. K. Hossain, M. Takeshita and T. Yamato, Eur. J. Org. Chem., 2005, 2771–2776.

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Functions containing a heteroatom different from oxygen Angelo Albini* and Elisa Fasani DOI: 10.1039/b812716f The main photochemical reactions of chromophores containing nitrogen, phosphorus, silicon, sulfur and halogen atoms are briefly reviewed.

1

Nitrogen-containing functions

The reactions of nitrogen-containing organic compounds are discussed starting from those bearing a sigma-bonded C–N function with a single and with more nitrogen atoms in decreasing oxidation level order, followed by double-bonded CQN functions. 1.1

C–N, a single nitrogen atom

The photoreactivity of nitroaromatics has been known from the earliest stage of this discipline and continues to be studied. The intermolecular reduction by alcohols proceeds via initial H-abstraction to give a (N-hydroxy)arylnitroxide radical PhN(Od)OH or via electron transfer to give the corresponding radical anion PhNO2d. These mechanisms have been further supported by EPR and laser flash photolysis studies for nitrobenzene and 2-nitroresorcinol, respectively.1 The reduction by amines (ET path) has been likewise mechanistically investigated. Trapping experiments show that proton exchange to form a-aminoalkyl radical ensues when the nitrobenzene bears an electron-donating substituent.2 The nitrobenzene/amine system is an effective photoinitiator for promoting acrylate polymerization2 and for the ionic curing of epoxy resins.3 Among intramolecular processes, the rearrangement to nitrite ester has been studied for some nitro polycyclic aromatic hydrocarbons (NPAH). Loss of NO with formation of an aryloxy radical has been demonstrated from the singlet excited state (pp*) in competition to ISC via the T2 (np*) state.4 This supports earlier contentions about the intermediacy of such derivatives in the reaction of nitro polycyclic aromatic hydrocarbons (NPAH) bearing the group perpendicular to the ring. It has been observed that the dependence of the photobehavior from the geometry of the nitro group is a rule that has exceptions, however. At any rate, when NPAHs are adsorbed onto a surface it is the nature of the adsorbent that directs the reaction, not geometry.5 Intramolecular H transfer has been extensively studied. In o-nitrobenzaldehyde, the first intermediate is ketene (1), detected by IR 1 ps after excitation. In ethanol, this decays in 90 ps via OH nucleophilic addition, both intermolecularly by the solvent and intramolecularly by the N-hydroxy University of Pavia, Department of Organic Chemistry, via Taramelli 10, Pavia 27100, Italy

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group to give intermediates (2) and (3), the spectrum of which corresponds to calculations. These evolve to the final products (4) and (5) on a ns scale.6 An aci-nitro intermediate (6) is formed in the related rearrangement of o-nitrobenzyl alcohol. H transfer and dehydration to the final nitrosobenzaldehyde then occurs through either of the two mechanisms shown, over the nitroso hydrate (7) or over the benzoxazoline (8) and the aldehyde hydrate.

The competition between the two mechanisms depends on conditions, the former predominating in dry solvents or under strongly acidic or basic conditions, the latter one under neutral conditions.7

The kinetics of the process and the effect of ring substituents have been studied in detail.8 A similar mechanism operates in other systems likewise bearing an activated o-alkyl group, such as the o-nitrobenzylpyridines, the photochemistry of which and the attending applications have been recently reviewed.9 Another group is that of o-nitrobenzylamines. The last chromophore is the reacting moiety in photocleavable linkers for oligonucleotides containing combinatorial libraries based upon derivatives of 4-aminomethyl-3-nitrobenzoic acid such as (9).10

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Again similarly, 4-(20 -nitrophenyl)-1,4-dihydropyridinedicarboxylates (10) are converted to the corresponding nitrosopyridines. Attention to this reaction arose from the use of these molecules as cardiac drugs and their absorption in the visible, which suggested the possibility of a phototoxic effect. In these derivatives two separate chromophores are present (DHP–PhNO2) and the long wavelength absorption corresponds to excitation of the DHP moiety (1DHP–PhNO2). The reaction is thought to involve intramolecular charge/ proton transfer to yield intermediate (11) and the end product (12) from it.11

In the 3-nitro analogue only a slow reduction takes places via the lowest lying triplet (DHP–3PhNO2). This is reached via intramolecular ET from the easily oxidized DHP chromophores to the nitrobenzene moiety and back electron transfer.12 Light has an important role in the equilibrium between the monomeric (colored) and the dimeric (colorless) forms of nitroso derivatives. Thus, in a criogenic matrix at 12K UV irradiation converts dimers to monomers, a reaction that is reverted by irradiation in the visible or by warming above 170 K. This photochromic system has been considered a photothermal ‘‘chemical switch’’ with potential use in supramolecular self-assembly systems.13 As for amines, calculations on the excited states of glycine have been published, but the photochemistry of the amine group has a limited importance, while anilines and other aromatic amines are easily oxidized.14 The oxidation of anilines has i.a. a medical importance for the role of kynurenine (13), an aminoacid arising from the degradation of tryptophan that binds to the lens proteins and makes them liable to oxidation (involving singlet oxygen). This phenomenon increases with increasing age and is one of the causes of cataracta.15

Synthetically useful reactions are those involving electron transfer followed by proton exchange, a mild method for carrying out radical processes. Thus, benzylamines are dehydrogenated to imines upon ET photosensitization.16 With tertiary amines, radical alkylation of electronpoor alkenes has been obtained under these conditions.17 1.2

C–N, two nitrogen atoms

The mechanism of isomerization of azobenzene has been a favorit topic in mechanistic studies and work aiming to clarify the mechanism continues. As it is well known, two mechanisms (rotation and inversion) are possible for the isomerization. A computational study on substituted azobenzenes has Photochemistry, 2009, 37, 213–239 | 215 This journal is

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individuated a conical intersection between S1 and S0 for each of them on the rotation path. Furthermore, it has been evidenced that the S2 surface is very close in energy to S1 at some points, making relaxation to S1 facile.18 Recent contributions suggest that rather than having two mechanisms from two states (S1 and S2), the deactivation path has a more subtle dependence on the dynamics of the process. The isomerization implies movements of large amplitude on the S1 surface and the charateristics of the medium, in particular viscosity, have an important role.19–21 Apart from the mechanistic intricacies, the smooth E–Z photoisomerization of azobenzene has continued to find a wealth of applications, making this chromophore the most reliable choice for a reversibly isomerizable moiety. As an example, both enantiomers of a chiral bicyclic compound containing two azobenzene chromophores have been synthetized and it has been found that their racemization rates could be controlled. Indeed, the alternate exposure to right and left circularly polarized light repeatedly led to the partial enrichment of either enantiomer. Possibly, this is a model for a rewritable recording medium.22 The smooth E/Z isomerization of azo compounds leads to considerable changes in the molecular shape and dipole moments offers a convenient way for changing the properties of materials and in particular of polymers. A continuously increasing number of applications is published. Thus, in polyethylene imines bearing as side chains 4-butylazobenzene groups linked through methylene spacers of various length the molecular orientation could be photochemically controlled.23 On the other hand, photoactivated supramolecular crosslinked linear poly(trimethylene iminium trifluorosulfonimide) polymer gels have been prepared. In these materials, the sol–gel transition was induced by light thanks to the isomerization of an azo group present in the cross-linker.24 The aminophenylazothiophene moiety has been likewise incorporated into polymers used for obtaining crosslinked thin films of superior stability.25 Fibers of cinnamate functionalized poly(methyl methacrylate-co-2-hydroxyethyl acrylate) were cross-linked in situ by UV irradiation during electrospinning.26 The isomerization is likewise useful for polymer characterization and has been used for determining whether the glass transition temperature of polystyrene changes in thin films.27 Photomechanical effects have been observed with thin films, where the morphology is changed upon irradiation due to azo E/Z isomerization.28,29 A similar photomechanical effect probably explains the spontaneous surface patterning that azobenzene thin films udergo when exposed to light intensity and/or polarization gradients.30 In the biological field, the azobenzene chromophore has been incorporated in bispropargyl sulfone derivatives. The thermally stable E-form (14) has been prepared and shown to convert into the Z isomer (15), a more efficient DNA-cleaving agent under irradiation.31

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The 1,2-diazadiene (16) underwent electrocyclic rearrangement (both photochemical and thermal) to give a dihydrocinnoline.32

The light-fastness of azo dyes under applicative conditions is a problem of obvious importance. In an in-depth investigation, Hihara and associates measured the rate of bleaching under illumination of cellulose films dyed with a variety of azo dyes immersed in an aqueous solution of Rose Bengal. The calculation of the standard enthalpy of formation of these dyes by semiempirical molecular orbital calculations (PM 5) allowed to determine whether these were present as the azo or hydrazone tautomer. The propensity of reacting with singlet oxygen either to form dioxetanes (17) or to form hydroperoxides (18) and the site of preferred reaction were then estimated on the basis of Fukui’s frontier electron densities for electrophilic reactions fr(E) (the weighted sum of the squares of the coefficients of each LCAO MO).33

The overall reactivity with singlet oxygen estimated as the sum of parameters f (E) for all of the double bonds involved correlated nicely with r the experimental rates. This sum could be considered a descriptor of the sensitivity to singlet oxygen for each dye.33–35 Likewise important is azo dyes degradation for water depollution. Every year 4700 000 tons of dyestuff is produced, 50% of which is azo and therefore a large number of studies have been devoted to this problem, of which a small fracion is quoted here. UV irradiation (by Hg arcs, but also using microwave activated electrodeless lamps)36 has been demonstrated to offer one of the best solutions for decolorizing waste water and for the mineralization of the dyes, at least in the presence of H2O2 (the active species are hydroxyl radicals).37 At the moment, however, the photochemical treatment is considerably more energy consuming than other oxidative methods. Thus, it was found that UV/H2O2 used the most energy, 5–11 times than that of the UV/O3 process and 265–520 times more than that of the ozonization process.38 Given the radical nature of the degradation process, quenchers of Photochemistry, 2009, 37, 213–239 | 217 This journal is

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radicals decrease decoloration.39 On the other hand, effective degradation is obtained by using radical initiators, such as a-hydroxyketones.40 Largely used for the same target is photocatalysis. It has been shown that UV irradiation is more effective at low dye concentration (5 mg/L), photocatalysis at high (50 mg/L).41 Photolysis of aryldiazonium salts is the most convenient way for obtaining a clean heterolytic fragmentation to give the phenyl cation. The process has been studied with various 4-substituted phenyldiazonium salts (19a–g) and it has been found that the parent compound and electrondonating substituted derivatives (substituent = H, tert-butyl, NMe2) cleave from the singlet (119) and give the singlet phenyl cation (120), an unselective electrophile that adds to the solvent. Electron-withdrawing substituents (NO2, COMe), on the contrary, cleave from the triplet (319) and give (320), which attacks p nucleophiles selectively. With the 4-cyano and the 4-bromo derivatives the situation is intermediate.42

1.3

Azides

Acylazides are known to undergo thermally or photochemically induced (Curtius) rearrangement to isocyanates. In the first case a concerted rearrangement takes place, while in the latter one an acylnitrene may be an intermediate. A study in matrix at 12K supplemented by calculations showed that the two processes actually compete and that the nitrene (21, in the singlet state, with a geometry intermediate with that of an oxazirene, O–C–N angle, 901) further isomerizes to both cyanate and isocyanate.43

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Insertion of a methylene group separates the two functions and a-azidoacetophenone has almost degenerate np* and pp* states as lowlying triplets, both localized on the carbonyl functions. The np* state undergoes a-cleavage, while reaction via the pp* ends up in azide fragmentation after intramolecular energy transfer. The main product is N-(benzoyl)-benzoylmethylamine (22), from the coupling of benzoyl radical and nitrene.44 A study of the same and related molecules in the crystal state has supplied further evidence of the a-cleavage in that the main product under these conditions is N-benzoylformimine (23), resulting from rearrangement of the first formed azidomethyl radical (24) to an iminyl radical (25) that couples with the benzoyl radical (a crystal to crystal reaction). Insertion of a phenyl group in a (RQPh) lends additional stabilization to the radical (24, PhCH  N3) so that the formimine is produced also in solution.45 By far more important than aliphatic derivatives are aryl and heteroaryl azides, the smooth photodecomposition of which continues to be studied.46,47 As summarized in a review by Gritsan and Platz, apart from the radicalic triplet nitrene, the intermediates are electrophilic species, viz singlet nitrene and rearranged benzoazirine and didehydroazepine.48 Taking 1-naphthylazide as an example, photolysis in 3-methylpentane glass at 77K yields the singlet nitrene (26) that intersystem crosses to the triplet (27, stabilized by 13.9 k