Photochemistry, Volume 37 (Specialist Periodical Reports)

Photochemistry Volume 37 A Specialist Periodical Report Photochemistry Volume 37 A Review of the Literature Publishe...

Author: Angelo Albini

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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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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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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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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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98 Q.-H. Song, H.-B. Wang, X.-B. Li, X.-M. Hei, Q.-X. Guo and S.-Q. Yu, J. Photochem. Photobiol. A, 2006, 183, 198. 99 X.-M. Hei, Q.-H. Song, X.-B. Li, W.-J. Tang, H.-B. Wang and Q.-X. Guo, J. Org. Chem., 2005, 70, 2522. 100 V. Lhiaubet-Vallet, S. Encinas and M. A. Miranda, J. Am. Chem. Soc., 2005, 127, 12774. 101 N. Belmadoui, S. Encinas, M. J. Climent, S. Gil and M. A. Miranda, Chem. Eur. J., 2006, 12, 553. 102 B. Lohmeyer and P. Margaretha, Photochem. Photobiol. Sci., 2005, 4, 637. 103 B. Lohmeyer, K. Schmidt and P. Margaretha, Helv. Chim. Acta., 2006, 89, 854. 104 K. Schmidt, J. Kopf and P. Margaretha, Helv. Chim. Acta, 2005, 88, 1922. 105 P. Margaretha, K. Schmidt, J. Kopf and V. Sinnwell, Synthesis, 2007, 1426. 106 L. Meyer and P. Margaretha, Photochem. Photobiol. Sci., 2004, 3, 684. 107 J. Rosenthal, D. I. Schuster, R. J. Cross and A. M. Khong, J. Org. Chem., 2006, 71, 1191. 108 E. Karpuk, D. Schollmeyer and H. Meier, Eur. J. Org. Chem., 2007, 1983. 109 R. Shen and E. J. Corey, Org. Lett, 2007, 9, 1057. 110 I. Akritopoulou-Zanze, A. Whitehead, J. E. Waters, R. F. Henry and S. W. Djuric, Org. Lett., 2007, 9, 1299. 111 M. Kemmler, E. Herdtweck and T. Bach, Eur. J. Org. Chem., 2004, 4582. 112 D. E. Minter and C. D. Winslow, J. Org. Chem., 2004, 69, 1603. 113 M. G. Organ and D. Mallik, Can. J. Chem., 2006, 84, 1259. 114 W. D. Shipe and E. J. Sorensen, J. Am. Chem. Soc., 2006, 128, 7025. 115 M. Ichikawa, S. Aoyagi and C. Kibayashi, Tetrahedron Lett., 2005, 46, 2327. 116 V. Singh, S. Pal, D. K. Tosh and S. M. Mobin, Tetrahedron, 2007, 63, 2446. 117 V. Singh, P. Vedantham and P. K. Sahu, Tetrahedron, 2004, 60, 8161. 118 J. R. Ragains and J. D. Winkler, Org. Lett., 2006, 8, 4437. 119 S. M. Ng, S. J. Bader and M. L. Snapper, J. Am. Chem. Soc., 2006, 128, 7315. 120 D. Dondi, A. M. Cardarelli, M. Fagnoni and A. Albini, Tetrahedron, 2006, 62, 5527. 121 J. Moran, T. Suen and A. M. Beauchemin, J. Org. Chem., 2006, 71, 676. 122 J. D. Winkler and E. C. Lee, J. Am. Chem. Soc., 2006, 128, 9040. 123 V. Singh, S. Pal and S. M. Mobin, J. Org. Chem., 2006, 71, 3014. 124 V. Singh, G. D. Praveena, K. Karki and S. M. Mobin, J. Org. Chem., 2007, 72, 2058. 125 D. Armesto, M. J. Ortiz, A. R. Agarrabeitia and N. El-Bouﬃ, Angew. Chem. Int. Ed., 2005, 44, 7739. 126 V. Singh, P. K. Sahu and S. M. Mobin, Tetrahedron, 2004, 60, 9925. 127 S.-Y. Chang, S.-L. Huang, Shih-Lin, N. R. Villarante and C.-C. Liao, Eur. J. Org. Chem., 2006, 4648. 128 D. Armesto, M. J. Ortiz, A. R. Agarrabeitia and M. Martin-Fontecha, Org. Lett., 2004, 6, 2261. 129 N. T. Tzvetkov, B. Neumann, H.-G. Stammler and J. Mattay, Eur. J. Org. Chem., 2006, 351. 130 Y. Zhang, L. Wang, M. Zhang, H.-K. Fun and J.-H. Xu, Org. Lett., 2004, 6, 4893. 131 T. Koizumi, K. Harada, E. Mochizuki, K. Kokubo and T. Oshima, Org. Lett., 2004, 6, 4081. 132 K. Wakamatsu, Tetrahedron Lett., 2004, 45, 4627. 133 F. W. Lichtenthaler, T. Weimer and S. Immel, Tetrahedron Asymm., 2004, 15, 2703. 134 P. A. Waske, J. Mattay and M. Oelgemoeller, Tetrahedron Lett., 2006, 47, 1329. Photochemistry, 2009, 37, 149–174 | 171 This journal is

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135 R. G. Brinson, S. C. Hubbard, D. R. Zuidema and P. B. Jones, J. Photochem. Photobiol. A, 2005, 175, 118. 136 M. Shi, W.-G. Yang and S. Wu, J. Photochem. Photobiol. A, 2007, 185, 140. 137 Y. Chen and M. G. Steinmetz, Org. Lett., 2005, 7, 3729. 138 Y. Chen and M. G. Steinmetz, J. Org. Chem., 2006, 71, 6053. 139 F. Bergeron, K. Klarskov, D. J. Hunting and J. R. Wagner, Chem. Res. Toxicol., 2007, 20, 745. 140 A. E. Alegria, N. Cruz-Martinez, S. K. Ghosh, C. Garcia and R. Arce, J. Photochem. Photobiol. A, 2007, 185, 206. 141 H. Goerner, Photochem. Photobiol. Sci., 2006, 5, 1052. 142 H. Goerner, Photochem. Photobiol. Sci., 2004, 3, 933. 143 H. Goerner, Photochem. Photobiol., 2006, 82, 71. 144 D. E. Nicodem, R. S. Silva, D. M. Togashi and M. F. V. da Cunha, J. Photochem. Phototobiol., A, 2005, 175, 154. 145 Y. Harada, S. Watanabe, T. Suzuki and T. Ichimura, J. Photochem. Photobiol A, 2005, 170, 161. 146 R. Das and B. Venkataraman, Res. Chem. Intermed., 2005, 31, 167. 147 Y. Pan, Y. Fu, S. Liu, H. Yu, Y. Gao, Q. Guo and S. Yu, J. Phys. Chem. A, 2006, 110, 7316. 148 R. C. White, V. Gorelik, E. G. Bagryanskaya and M. D. E. Forbes, Langmuir, 2007, 23, 4183. 149 A. Bose, D. Dey and S. Basu, J. Photochem. Photobiol. A, 2007, 186, 130. 150 Y. Harada, S. Watanabe, T. Suzuki and T. Ichimura, J. Photochem. Photobiol. A, 2005, 170, 161. 151 H. Yamada, Y. Yamashita, M. Kikuchi, H. Watanabe, T. Okujima, H. Uno, T. Ogawa, K. Ohara and N. Ono, Chem. Eur. J., 2005, 11, 6212. 152 R. Mondal, R. M. Adhikari, B. K. Shah and D. C. Neckers, Org. Lett., 2007, 9, 2505. 153 R. Mondal, B. K. Shah and D. C. Neckers, J. Am. Chem. Soc., 2006, 128, 9612. 154 N. K. Urdabayev, A. Poloukhtine and V. V. Popik, Chem. Commun., 2006, 454. 155 R. Ruzicka, L. Barakova and P. Klan, J. Phys. Chem. B, 2005, 109, 9346. 156 S. Lopes, A. Gomez-Zavaglia, L. Lapinski and R. Fausto, J. Phys. Chem. A, 2005, 109, 5560. 157 C. J. Mortko and M. A. Garcia-Garibay, J. Am. Chem. Soc., 2005, 127, 7994. 158 M. J. E. Resendiz and M. A. Garcia-Garibay, Org. Lett., 2005, 7, 371. 159 J. Zhang, M. Gembicky, M. Messerschmidt and P. Coppens, Chem. Commun., 2007, 2399. 160 T. Choe, S. I. Khan and M. A. Garcia-Garibay, Photochem. Photobiol. Sci., 2006, 5, 449. 161 M. I. Guzman, A. J. Colussi and M. R. Hoﬀmann, J. Phys. Chem. A, 2006, 110, 3619. 162 M. I. Guzman, A. J. Colussi and M. R. Hoﬀmann, J. Phys. Chem. A, 2006, 110, 931. 163 T. Vencel, K. Gaplovska, A. Gaplovsky, S. Toma and F. Sersen, J. Photochem. Photobiol. A, 2004, 162, 53. 164 L. A. Huck, M. Xu, K. Forest and P. Wan, Can. J. Chem., 2004, 82, 1760. 165 L. Llauger, M. A. Miranda, G. Cosa and J. C. Scaiano, J. Org. Chem., 2004, 69, 7066. 166 V. Lhiaubet-Vallet, N. Belmadoui, M. J. Climent and M. A. Miranda, J. Phys. Chem. B, 2007, 111, 8277. 167 H. Suzuki, T. Suzuki, T. Ichimura, K. Ikesue and M. Sakai, J. Phys. Chem. B, 2007, 111, 3062. 172 | Photochemistry, 2009, 37, 149–174 This journal is

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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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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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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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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 kcal/mol) with a rate constant of 1 107 s1, while at ambient temperature, singlet nitrene cyclizes to either of the two azirines (28 or unstable 29) and/or to didehydroazepines (30–32) faster than it relaxes to the triplet.

In order that the ensuing reaction has synthetic interest, nucleophilic trapping must be eﬀective and selective, avoiding that polymeric tars (typically resulting by polymerization of the didehydroazepine) or at any rate mixtures of products are formed. Apart from synthesis, important is that azides have found many technological applications in various ﬁelds, such as photopolymerization, photoresists in lithography, photoaﬃnity labeling and others. This has fueled the interest of photochemists for whom azides have long been a favorite ﬁeld of investigation, due to the intriguing mechanistic issues raised and the attempt to rationalize (and then direct) the chemistry occurring through the role of the intermediate involved. Thus, advancement has essentially relied upon the application of spectroscopic techniques that in turn became available for the detection of intermediates and the determination of the related kinetics. Studies thus involve ﬂash photolysis with various detection techniques and matrix isolation, besides obviously computations. The situation heavily depends on the conditions of the experiment. Photochemistry, 2009, 37, 213–239 | 219 This journal is

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Thus, much has been learned in recent years about the structure of intermediates by photolysis in argon matrix with UV, IR and EPR detection. Work in this direction continues and is quite important since it reveales which paths may be followed, even if not how these compete in solution. Recent matrix studies have documented essentially the same general pattern for 4-aminophenylazide,49 2,6-diﬂuorophenylazide and pentaﬂuorophenylazide, although earlier studies had suggested that ring enlargement did not occur in the last case. Thus, recent studies demonstrated that the lack of detection of cyclized intermediates depended on their further photoreaction under non monochromatic irradiation.50 Fragmentation processes are observed in some cases. Thus, 2-quinazolylnitrene (33) undergoes both ring expansion and ring cleavage to give either a diradical (34) or a diazo compound (35) that have been both characterized.51

With 3-iodo-2,4,5,6-tetraﬂuorophenyl azide cleavage of the CI bond also occurs and is followed by ring opening to yield a d-alkynyl-a,b-unsaturated nitrile (36) via intermediate (37).52

Besides Argon matrix, another convenient situation for individuating the intermediates is the use of guest molecules of the appropriate shape. Thus, photolysis of an azide prepared in such a way that the N3 group inserts within the cavity of a cryptand produces an unusually long-lived singlet nitrene.53 In a similar vein, hemicarcerands have been used for the study of the nitrene reactions. Under this condition dimerization or reaction with the medium are prevented and highly reactive species can be directly observed, as is the case for the highly strained didehydroazepine (38), for products from further rearrangement (39, 40) and for triplet nitrene.54,55

Technical advancements, however, now make possible to obtain a detailed picture of the early events in solution at room temperature by 220 | Photochemistry, 2009, 37, 213–239 This journal is

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ultrafast spectroscopy, so that the characterization of the intermediates is directly relevant for the rationalization of the photochemistry in solution. Thus, decomposition of p-methoxyphenylazide gave the singlet nitrene that relaxed to the triplet with a time constant of 108 ps.56 After ﬂashing naphthylazide the S2 state of the molecule was revealed, with a lifetime of tenths of ps, while the S1 state was not detected because its electronic structure was more prone to cleavage and gave the singlet nitrene over a 2 kcal/mol barrier. The latter intermediate was formed in a vibrationally excited state and relaxed, possibly with concurrent cyclization to the azirine in 15 ps.57 IR detection has been greatly helpful because some of the intermediates do not absorb in the accessible UV-Vis window. In this way, it has been distinguished whether ring enlargement to a didehydroazepine occurs directly from vibrationally excited (singlet) phenyl- or biphenyl nitrene or after relaxation via the azirine.58 As seen above, in most cases the end products arise from the trapping of the didehydroazepine by an external nucleophile, not directly from short lived singlet nitrene. A recurring question has thus been whether the latter type of reaction could occur, at least through an intramolecular path. This has been studied with molecules where an accessible nucleophilic group, such as a phenyl or a pyrazole, was adjacent to the nitrene. It resulted that in the singlet manifold (41a) ring expansion (42) competes even with such seemingly facile intramolecular attack, such as that leading to (43), a fact that limits the likelihood of forming carbazole analogues via the singlet path. Furthermore, having adjacent to the nitrene a trap for radicals such as a dimethylpyrazole does not lead to an eﬃcient capture of triplet nitrene (41b). This rather dimerizes to the azo compound, conﬁrming the high stabilization of this intermediate (the often commented ‘‘lazy’’ character of this species). However, the excited triplet is a good hydrogen abstractor and gives biradical (44) and the ﬁnal product from it.59

A case of exclusive intramolecular electrophilic attack by singlet nitrene has however been documented with o-nitrophenylazide, where it has been demonstrated that the lifetime for cleavage of the azide to produce singlet nitrene (45) is 0.5 ps and that for cyclization to benzofuroxan is 8.3 ps Photochemistry, 2009, 37, 213–239 | 221 This journal is

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(in MeCN).60 In fact, in a number of cases photolysis gives preparatively interesting results, particularly among heteroaromatics, a recent example being the preparation of 5H-1,3,5-triazepin-2,4-dione by ring expansion from 4-azidouracil.61 Likewise, there are examples where neither singlet nitrene nor its isomers react, and only the triplet path is productive. A recent instance is that of 4-azidopyridine N-oxide, where the photoreaction involves the azide, not the likewise quite photoreactive N-oxide function, and leads to the azo compound, the typical product from the triplet nitrene dimerization. This is due to the stabilization of both nitrene spin states by biradical structures, a fact that hinders the customary ring enlargement.62 A more complex case is in the photolysis of various pyridinediazides that leads to quintet state dinitrenes (46) that were characterized by EPR, where the 2,6-dinitrenes were clearly distinguished by the larger zero ﬁeld splitting in comparison to the 2,4-dinitrenes.63 Yet another process, interconversion between phenylnitrene and ortho-pyridylcarbene has been studied by the same techniques.64

Protonation of nitrenes gives nitrenium ions. These species have been the objects of the attention by several laboratories in recent years because these species seem to have an important role in biochemistry, since these attack selectively guanine bases in DNA. Time resolved resonance Raman spectroscopy has demonstrated to be a well suited technique for studying this phenomenon and indeed the protonation of 4-methoxyphenyl- and 2-ﬂuorenylnitrene (47) has been documented in this way (the protonation of the ﬁrst species is much faster).65 In turn, 2-ﬂuorenylnitrenium (48) reacts with ﬂuorenylazide on the microsecond time scale to give an intermediate tentatively identiﬁed as the tetraazadiene cation (49) resulting from the addition onto the terminal nitrogen that then evolves into the azo compound.66 Notice that protonation obviously gives only monosubstituted nitrenium ions RN+H; the disubstituted derivatives RN+R 0 are obtained through a fragmentation process from 1-(N,N-dialkylamino)pyridinium or, as recently demonstrated, from diarylhydrazinium salts (50).67 Increased understanding of the azide photochemistry fosters progress in applications, in particular for biological experiments, such as photochemical labeling of aminoacids or of nucleic acids. Since the low-energy triplet nitrene will not cause a cross-link with biomolecules, the key factor is whether singlet nitrene or the didehydroazepine arising from it are trapped by a nucleophile. As an example, photolysis 4-acetylphenylazide produces the singlet nitrene (and in hydrophobic medium also the didehydroazepine). The ﬁrst intermediate is too short-lived (1 ps in water) for entering in a 222 | Photochemistry, 2009, 37, 213–239 This journal is

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bimolecular reaction. However, in 4-acetyltetraﬂuorophenylnitrene, the lifetime increases by a factor of 4104 (43 ns in water, arrives at 172 ns in benzene), so that in this case the singlet becomes a convenient reagent for labeling.68 Apart from these exceptions, singlet nitrene generally is too short-lived. A valid alternative may be the didehydroazepine, which often is suﬃciently stable in water that reaction with various N, O, S-nucleophiles can be observed (rate Z 1 107 s1). A nitro group enhances the reactivity that is on the contrary depressed by a hydroxy group, since the didehydroazepine will then rapidly tautomerize from the enol to the unreactive keto form.69 The electrophile may be yet another intermediate. Thus, 8-azidoadenosine undergoes photoinduced cleavage in water to give the singlet nitrene (51) that undergoes a rapid (o0.4 ps) tautomerization to a new adenosine intermediate with a diazaquinodimethane structure (52). This is a long-lived species (lifetime ca. 1 min in water) that however eﬃciently adds to good nucleophiles (1 to 6 107 mol1 s1 with S, N nucleophiles) and is again well suited for photoaﬃnity labeling.70

This report further evicences the variety and versatility of reagents that can be generated photochemically from azides under mild conditions. 1.4

Amides

a-Ketoamides are known to be photolabile and hydrogen transfer from the CH a to the amino group to the keto group has been proposed as the primary process. The proposal has been now supported by the study of Photochemistry, 2009, 37, 213–239 | 223 This journal is

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derivatives bearing an acyloxy group in b that is eliminated during the reaction. 5-Methylene-4-oxazolidinones (53) are thus formed and the intermediate is best envisioned as a zwitterion (54).71

As one may expect, the sensitized oxidation of proteins involves attack to sulfur-containing functions or to aromatic side chain, certainly not the amide backbone.72–74 (N-Acylarylyden)aminoacid esters were found to undergo photocylization of the acyl group onto the CQC double bond and form oxazolines (55, with a low enantiomeric eccess in the presence of chiral amines).75

Napthalenediimide, just as perilenediimide, is largely used as electron acceptor and this has fostered the study of the generation of, and reaction with, singlet oxygen and superoxide.76 Long chain N,N-bis-(alkyl) derivatives of perilenediimide has been used as oxygenation photosensitizers of alkenes in solution and, better, in immobilized sol–gel phase.77 1.5

CQN bonded functions

Lehn has observed that compounds containing the CQN group, such as imines and their derivatives undergo syn–anti isomerization of the CQN bond by two diﬀerent mechanisms, viz by out-of-plane rotation via an orthogonal state (photochemically) and by in-plane nitrogen inversion via a linear transition state (thermally). He has pointed out that the sequencial occurring of the two processes regenerates the initial molecule (a closed process) but with a diﬀerent course of each step. Thus, in a chiral imine photoinduced rotation is expected to occur in one sense in preference to the opposite one and the sum of photochemical and thermal isomerization results in unidirectional molecular motion. Compounds with these characteristics can be envisioned as unidirectional molecular photomotors able to convert light energy into mechanical motion.78 Conjugated imines may be useful as materials for optics, as is the case with a-cyano-4 0 -nitrobenzal-4-dimethylaminoaniline that, just as the corresponding stilbene, shows large photoinduced optical anisotropy, as a consequence of the increased rate of isomerization brought about by the introduction of the cyano group.79 Imines have been also found to give photoreactions of some preparative interest, such as the reduction of aldimines (both aliphatic and aromatic, N-substituted or not) to amines by irradiation in alcohol solution, a versatile reaction that satisﬁed the requirements of ‘green’ chemistry.80 224 | Photochemistry, 2009, 37, 213–239 This journal is

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Under semiconductor photocatalysis conditions, N-benzoylimines were allylated in the presence of cycloalkenes via coupling of the corresponding radicals, provided that CdS/ZnS was used as the photocatalyst, since with silica supported CdS a thermal side reaction hindered the photoreaction.81 1,4-Diaryl-1-azabutadienes underwent [4 + 2] cycloaddition with 2,3-dihydrofuran under ET photocatalysis by 2,4,6-triphenylpyrylium tetraﬂuoroborate to produce the corresponding styrylfuroquinolines in preparatively interesting yields.82 Hydrazones, tautomers of azo compounds, are often photoreactive. This is the case for some arylaldehyde N-methyl-N-phenylhydrazones that are cleanly cleaved at the CQN double bond by singlet oxygen, apparently via ET and rate determining C–O bond formation in the caged radical ion pair.83 The reaction is more eﬃcient at low temperature (78 1C) than at room temperature, an eﬀect that has been explained by the large negative entropic term relative to the key step. Oxidation of oximes via photosensitized electron transfer (PET) gives the corresponding ketones. Intermediates are the radical cation of the starting materials and iminoxyl radicals.84 The (photo)decomposition of diazo compounds is a favorite path for the generation of carbenes. A study at the fs level of the photolysis of p-biphenylyldiazomethane has now allowed to detect an excited state of the starting compound with lmax = 490 nm and lifetimes below 300 fs (in cyclohexane, MeCN, MeOH). Decay of this transient is concomitant with the growth of singlet carbene (1BpCH:) absorption at 360 nm. Photolysis of p-biphenyldiazoethane (56) likewise produces the excited state in the same yield, but the yield of the corresponding carbene (57, 1BpC:Me) is 30–40% lower in all three solvents. This is due to a competing rearrangement in the excited state of (156) to form p-vinylbiphenyl (58, BpCHQCH2) in parallel with nitrogen extrusion. This study has further revealed that the decay of (57) is biexponential, while that of 1BpCH: is monoexponential, probably due to the additional low-frequency vibrational modes introduced by the methyl group. As for reactions, addition of (57) to methanol is much faster than spin equilibration, but the reverse holds true for the solvent trapping in cyclohexane.85

Another synthetically important application of diazo decomposition is that of a-diazoketones. A computational study has shown that transition states for the concerted Wolﬀ rearrangement (leading to ketene 59) and for the formation of carbonyl carbenes (60) have a very similar geometry. However, in rigid systems such as small- to medium-sized cycles, the Photochemistry, 2009, 37, 213–239 | 225 This journal is

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orthogonal conformation required for stabilizing the carbene is hindered and the former process predominates.86

This does not apply to large rings. Thus, although calculations and NMR measurements show that in crown-6 (61) diazo and carbonyl groups are in a Z,Z arrangement, as is the potassium complex (62), while the sodium analogue (63) has a Z,E conﬁguration, all of the three species show the same chemistry via the dicarbonyl carbene, as it happens in open chain analogues.87

1.6

Heteroatom-N bond

Interest in the photolysis of nitrosoamines continues, in view of pharmaceutical application of compounds suitable for the controlled generation of NO under mild chemical (including of course photochemical) or enzimatic conditions. Recent additions to the ﬁeld include some photoactivated molecules, such as a new N-nitrosamine of cyclam,88 as well as substituted benzyl N–nitrosocarbamates89 and Se-nitrososelenols.90 The reaction between phosphines and azides leads to phosphazenes under elimination of nitrogen (Staudinger reaction). The intermediacy of an adduct (phosphazide) has been demonstrated in a number of cases. Recently it has been found that when an o-phosphineborane is used, the adduct is stabilized to the point that the photochemistry can be investigated. Interestingly, it has been found that it involves ring enlargement with shift from B–Na (64) to B–Nb (65) interaction.91

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2 2.1

Functions containing diﬀerent heteroatoms Silicon

The eﬀect of silicon-based substituents on the photochemistry of aromatics, heterocyclics and ketones is discussed in the corresponding sections. Furthermore, these compounds have found a wealth of technical applications. Only a couple of examples of the photochemistry of silicon containing derivatives is presented here. A computational study of the 1,3-silyl group sigmatropic shift in allylsilanes supported that it occurred with retention of stereochemistry (in accord with the Woodward-Hoﬀmann rules, but not with a previous study), although it was accompanied by homolytic dissociation, the importance of which increased in derivatives that have a longer Si–C bond.92 As shown below, aromatic compounds of formula p-X-C6H4CPh2–SiMe3 underwent photochemical cleavage and gave geminate radical pairs, either in the singlet (66) or in the triplet state (67) according to the nature of the reactive excited state as determined by the choice of substituent X. In the ﬁrst case [XQH, PhC(OCH2CH2O)] the singlet fragmented within 0.5 ps and the radicals combined either to give back the starting material or to give the Fries products, in the latter one (XQPhCO, NMe2) freely diﬀusing radicals were formed.93

Stable disilenes are a class of compounds which have received considerable attention since their discovery. The irradiation of the mixture of cyclic polysilanes obtained by reductive coupling of neat dimesithyldichlorosilane with potassium/ graphite yields tetramesityldisilene (68).94 Tetramethyldistannene (69) has been directly detected by ﬂash photolysis in solution and in the gas phase.95 Silylenes are divalent derivatives of silicon similar to carbenes that can be accessed photochemically. The ground state is a singlet with two electrons in a non bonding (sp2) orbital, then there are excited singlet and triplet states Photochemistry, 2009, 37, 213–239 | 227 This journal is

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with n1p1 structure. Heavily substituted silylenes are stable, which makes possible to study their photochemistry. The triplet is the lowest state (by 25.1 kcal/mol for Me2Si:), but ISC is relatively slow. As a consequence, singlet reactions are observed with alkenes to yield silacyclopropanes (kad ca. 106 mol1 s1, a stepwise process, but with simmetry conservation since ring closure ensues with a rate constant 4109 sec1) and to benzene to yield a silepine (at a lower rate).96 Finally it is useful to recall that ultraﬁne particles of organosilicon compounds were obtained by irradiation at 313 nm of a gaseous mixture of CS2 and trimethyl-(propynyloxy)silane.97 As for directing the course of the reaction by inserting a silyl group, one should mention at least the review by Yoon and Mariano on the SET photochemistry of silicon-substituted polydonor-linked phthalimides98 and the formation of a ﬁve- (70), along with the expected four-, membered ring (71) in the photocyclization of enone derivatives.99

2.2

Phosphorous

The photooxygenation of phophines, long neglected in comparison to that of sulﬁdes, has been studied in recent years, at least for the case of triaryl derivatives that are suﬃciently stable in the presence of O2 for considering the photoprocess. The reaction of singlet oxygen with p-substituted triarylphophines yield the phosphine oxides as the only product. The rate of formation of these products depends on the nucleophilicity of the phosphine and is consistently twice as much than that of singlet oxygen removal, supporting that electrophilic attack by 1O2 leads only to chemical reaction, not to physical quenching. o-Substituted derivatives, however, give aryl phosphinates (72), where both oxygen atoms are incorporated in the same product, along with phosphine oxides (73) in a yield that increases with decreasing starting material concentration. This and other pieces of evidence suggest that with phosphines the ﬁrst-formed intermediate is a phosphadioxirane (74) rather than an open chain peroxide as for the sulﬁdes.100

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Phosphine oxides are also smoothly obtained through a SET path by using DCA/biphenyl as the photosensitizer.101 Among P derivatives at a higher oxidation level, one may mention 1,2-diarylphosphaalkenes (protected by a m-terphenyl group in order to obtain a suﬃcient stability) that have been shown to undergo E/Z isomerization reactions. The thermodynamic parameters for rotation around the P–C bond are DH# 13.8 kcal/mol and 1.3 DS# eu in the ground state and the lowest excited state has pp* character.102 Irradiation of CBr4/PPh3 induced the smooth oxidation of alcohols to acids (an example is shown below).103

Acylphosphine oxides are well known photoinitiators of radical polymerization, but irradiation of alkenyl acyldiphenylphosphine oxides (75) at 300 nm led eﬀectively to cyclization.104

2.3

Sulfur

The mechanistic issues linked to the photo-oxygenation of sulﬁdes have long been a favorite ﬁeld of research and certainly this did not decline in recent years (see ref. 105 for a review). The reaction with singlet oxygen gives the sulfoxide via a persulfoxide (76) as the primary intermediate. In aprotic, non nucleophilic solvents a further intermediate is involved, viz a S-hydroperoxysulfonium ylide (77, evidence for the generality of this mechanism is accumulating),106 while in alcohols a single intermediate, an alkoxysulfurane (78), is involved. The persulfoxide lacks stabilization and cleavage to the components limits the chemical yield, unless protonation of the outer oxygen or nucleophilic attack to the sulfur prevents unproductive decay. Appropriately located heteroatoms likewise may oﬀer the required stabilization, as demonstrated for a series of thiacyclooctanes having a S, O, NH in position 5 through an electrostatic eﬀect.107 The role of reversible intermediates had been initially determined by Foote mainly through trapping by diphenylsulﬁde and diphenylsulfoxide, themselves stable under these conditions, but co-oxidized in the presence of dialkyl sulﬁdes. It has recently been found that the corresponding selenium derivatives behave as similar quenchers and likewise are co-oxidized.108 The oxygenation of a-ethylbenzyl phenyl sulﬁde (79) has been obtained with up to a 43% Photochemistry, 2009, 37, 213–239 | 229 This journal is

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enantiomeric excess when using a chiral ruthenium phenanthroline complex immobilized on montmorillonite as the sensitizer. Calculations on the system have been carried out and it has been suggested that chiral discrimination is achieved because of the sterical constraints imposed during the approach of the reagent to the immobilized sensitizer.109

The known eﬀect of steric hindrance and of phenyl conjugation on the interaction of singlet oxygen with sulﬁdes, with regard both to physical quenching and to chemical reaction has been further documented.110 Extending the comparison to benzyl, a-methylbenzyl and cumyl sulﬁdes evidenced again the importance of sterical hindering as well as the determining role of a proton in a (see above the ylide intermediate).111 The solventdriven partitioning between ene reaction, dioxetane formation and sulfur oxidation has been studied in phenyl vinyl sulﬁdes.112 The relatively low oxidation potential of sulﬁdes makes them accessible also to ET sensitization. It is not trivial to distinguish the two mechanisms, because many of the commonly used sensitizers are quenched both by sulﬁdes and by oxygen at a comparable rate, as clearly shown by Baciocchi in 2003, although the key intermediate may be diﬀerent (thiadioxirane).113 Activity in this ﬁeld has continued on the part of several groups. A comparison of various sensitizers in the oxygenation of di-n-butyl sulﬁde has shown that these can be classed in three groups, viz Bengal Rose, chloranil and dicyano anthracene that give the sulfoxide, 4-benzoyl benzoic acid and anthraquinone that give mainly the thiosulfonate and butanesulfonic acid, and ﬁnally triphenylpyrylium and TiO2 that give both types of products.114 Further studies show that in the DCA/R2S system, the sulﬁde increases the eﬃciency of the sensitizer ISC, both when the sulﬁde reacts with 1 O2 (Et2S) and when it does not (Ph2S). The dependence on the solvent nature and the eﬀect of some quenchers support that the sulﬁde radical cation + superoxide mechanism is followed with N-methylquinolinium tetraﬂuoborate (NMQ+, 80), indeed recognized as a typical ET sensitizer. However, the results are similar with triphenylpyrylium tetraﬂuoborate (81) as the sensitizer, despite that this is known not to produce superoxide. This has led to the hypothesis that combination of sulﬁde radical cation with molecular oxygen may be yet another viable path for sulfoxidation.115

The ET photooxidation often involves a cleavage of the initial skeleton. It has been explored whether fragmentation may occur independently from the oxidation. However, NMQ+ sensitization of some tert-alkyl phenyl 230 | Photochemistry, 2009, 37, 213–239 This journal is

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sulﬁdes showed that no cleavage occurred under nitrogen and supported the key role of a sulfadioxirane intermediate in the fragmentation.116 Obviously, cleavage processes occur more easily when favored, as is the case for sulfanyl-1,2-diphenylethanol derivatives that fragment to give benzaldehyde and the dimers of the sulfanylmethyl radical under NMQ+ sensitization. 4-Methylthiophenylacetic acid (82) undergoes two diﬀerent homolytic processes, decarboxylation from the singlet and cleavage of the phenyl-S bond from the triplet. The actually isolated products arise from further reactions of the primarily formed radicals.117 Benzophenone sensitization results in ET. In water the radical ions separate and are solvated independently, and under these conditions decarboxylation occurs. In MeCN, on the other hand, proton transfer from the a position generates a diﬀerent radical.118 With phenylthioacetic acid the main process is homolysis of the S–CH2 and with S-benzylthioglycolic acid of the S–CH2Ph bond.119 Particularly important is the oxidation of methionine, in view of the relation this reaction has with the Alzheimer’s disease. A recent review highlights that of the two reactions that this aminoacid undergoes with ROS, single electron transfer to give the radical cation and overall two electron oxidation to give the sulfoxide, the most important is the ﬁrst one. In fact this leads to radicals and irreversible degradation, while sulfoxides are enzymatically reduced back.120 OH radical initiated oxidation in the gas phase involves both hydrogen abstraction and addition to the S atom.121 TiO2 deposited on quartz bars by the sol–gel method eﬃciently degrades gas phase dimethyl sulﬁde: a short residence time is required and the only intermediate present in the eﬄuent is dimethyldisulﬁde. On the other hand, with DCA encapsulated in a silica gel network, no intermediate is present in the gaseous eﬄuent, since the main product is the sulfoxide that is totally absorbed on silica gel.122 Indeed, a complete degradation of dimethyl sulﬁde in the gas phase is obtained in a ﬂow reactor and under visible light irradiation thanks to photocatalysts such as DCA and anthraquinone adsorbed on commercial silica beads or (for DCA) incorporated in sol–gel monoliths. Partially oxidized products are trapped by these material and no toxic compound or foul odor is present in the outlet.123 The oxidation of dimethyl sulﬁde in the atmosphere and in the oceans is important because this process generates sulfate aerosols and cloud condensation nuclei. These phenomena seem to have a role in climate control, probably counteracting the warming eﬀect of greenhouse gases. The oxidation of ‘reduced sulfur’ (Me2S, H2S, CS2 etc) contributes in average for ca. 15% to the amount of SO2 present in the atmosphere, with large diﬀerences according to the season.124 As for the aqueous phase, recent studies suggest that the photolysis of nitrate has a key role in the oxidation of dimethyl sulﬁde in nutrient-rich waters125 (as NOx have in the DMS oxidation in the atmosphere).126 Unhindered thioaldeydes are generally instable and oligomerize readily. A way for the in situ generation of such compounds is the photolysis of phenacyl sulﬁdes (83). Thioaldehydes (84) are then trapped through a cycloaddition reaction. This method has been applied to the multigram scale synthesis of a chiral sulﬁde, both in a batch reactor (starting from 18 g Photochemistry, 2009, 37, 213–239 | 231 This journal is

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of reagent) and in a continuous ﬂow reactor (38 g). This sulﬁde is used as a catalyst for asymmetric transformations.127

The radical addition of compounds of formula RS–H across C–C multiple bond (thiol-ene coupling) is a convenient method for the synthesis of various sulfur containing products. Thus, thioacetic acid and thiobenzoic acid add eﬃciently to 2,3-diallyltetraazathiapentalene derivatives under acetone or benzophenone sensitization.128 The addition of 2-aminoethane thiol hydrochloride onto o-alkenylglycosides (85) occurred quantitatively with no competition by the amino group.129

A similar addition onto an O-allyl mannose polysaccharide has been developed for the multigram scale synthesis of C. albicans (1 - 2)-a-Dmannopyran epitopes.130 ET sensitization by NMQ+ causes the racemization of enantiomerically pure thioanisole sulfoxides.131 A convenient access to sulfones is the photo-Fries rearrangement of sulfonates (via the singlet excited state). As an example, 2-carbazolyl sulfonates are converted into a mixture of 1-(86, the main one) and 3-sulfonyl-2-hydroxycarbazoles (87), together with a small amount of 2-hydroxycarbazole (88).132

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Homolytic cleavage of S–O bond in sulfonates has been also studied in view of possible applications of these derivatives as non ionic photoacid generators (PAG).133 Very recently, imino sulfonates and N-hydroxyimide sulfonates have been tested as i-line sensitive PAG for UV-curing.134,135 Notice however that aryl sulfonates (and phosphates as well), in particular those prepared from an electron-donating substituted phenols (89), undergo cleavage of the aryl–oxygen bond (from the triplet) and give phenyl cations (90), highly useful synthetic electrophilic intermediates.136

As regard sulfones, one may recall the particular role of propargylic sulfones as DNA intercalating agents.137 A derivative containing the anthraquinone chromophore (91) has been synthetized and it has been conﬁrmed that it binds to DNA duplex by intercalation and thus causes DNA cleavage.138

2.4

Halogen

The photochemistry of organic halogenated derivatives has long occupied a minor place and seemed to be limited to homolytic cleavage, particularly of weakly bonded iodides. The most important reaction certainly is the SRN1 process that applies not only to aromatic compounds, but also to activated aliphatic derivatives, as recently shown for the reaction of 1-iodoadamantane with the anion of nitroalkanes.139 The panorama is becoming more varied, however. One of the grounds for that is the increasing concern about persistant pollutants, ranging from some herbicidals (including some the use of which has been discontinued like aldrin) to some antibiotics, particularly those involved in a large amount for the therapeutic use for livestock, like ﬂuoroquinolones, accumulate e.g. in polar ice or in soil. Most of these are halogenated compounds and photochemistry oﬀers one of the few, or the only, degradation path in ice. Important is a laboratory study on the photochemistry of some pollutants under environmentally relevant conditions of concentration, temperature (25 1C) and air exchange. This showed that the main path for some PCB contaminants was reductive dehalogenation, probably involving traces of volatile organic compounds as the hydrogen donors.140 The other ‘new’ process is the photoheterolysis of aryl chlorides (and ﬂuorides, including the above mentioned ﬂuoroquinolones), of which until Photochemistry, 2009, 37, 213–239 | 233 This journal is

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recently very little had been reported, while in the last years has been rather extensively investigated, mainly in view of the remarkable synthetic potential it has revealed (see below). This has fostered mechanistic studies, in particular ﬂash photolysis experiments demonstrating the initial formation of a triplet phenyl cation (92) from 4-chlorophenol and derivatives.141,142 This deprotonates to form a carbene (93), identiﬁed by the diagnostic addition of oxygen to form a carbonyl oxide (94). By contrast the cation is not aﬀected by molecular oxygen, nor adds methanol, by which is rather reduced (compare above). The characteristic reaction of this species is addition to a p nucleophile, e.g. an alkene to give a phenonium ion (95).

This behavior is reminescent more of that of triplet carbenes than of that of usual carbocations. As hinted above, the method has shown considerable synthetic potential, and under several aspects can be considered a metal-free alternative of popular transition-metal catalyzed procedures. Recent applications include substitution of a chloro by an allyl (see compound 96),143 an alkynyl (97)144 and a cyano group (98),145 besides the preparation of benzyl (or phenyl) g- and d-lactones (see formula 99) by reaction with 3, 4, or 5-alkenoic acids as illustrated below.146

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131 C. Aurisicchio, E. Baciocchi, M. F. Gerini and O. Lanzalunga, Org. Lett., 2007, 9, 1939. 132 L. K. Crevatı´ n, S. M. Bonesi and R. Erra-Balsells, Helv. Chim. Acta, 2006, 89, 1147. 133 J. Andraos, G. G. Barclay, D. R. Medeiros, M. V. Baldovi, J. C. Scaiano and R. Sinta, Chem. Mater., 1998, 10, 1694. 134 M. Shirai and H. Okamura, Prog. Org. Coat., 2009, 64, 175. 135 P. A. Arnold, L. E. Fratesi, E. Bejan, J. Cameron, G. Pohlers, H. Liu and J. C. Scaiano, Photochem. Photobiol. Sci., 2004, 3, 864. 136 M. De Carolis, S. Protti, M. Fagnoni and A. Albini, Angew. Chem. Int. Ed., 2005, 44, 1232. 137 M. Kar and A. Basak, Chem. Commun., 2006, 3818. 138 K. Haruna, H. Kanezaki, K. Tanabe, W.-M. Dai and S. Nishimoto, Bioorg. Med. Chem., 2006, 14, 4427. 139 A. N. Santiago, S. M. Basso, C. A. Toledo and R. A. Rossi, New J. Chem., 2005, 29, 875. 140 N. Matykiewiczova, J. Klanova and P. Klan, Environm. Sci. Tech., 2007, 41, 8308. 141 I. Manet, S. Monti, M. Fagnoni, S. Protti and A. Albini, Chem.-Eur. J., 2005, 11, 140. 142 I. Manet, S. Monti, P. Bortolus, M. Fagnoni and A. Albini, Chem.-Eur. J., 2005, 11, 4274. 143 S. Protti, M. Fagnoni and A. Albini, Org. Biomol. Chem., 2005, 3, 2868. 144 S. Protti, M. Fagnoni and A. Albini, Angew. Chem. Int. Ed., 2005, 44, 5675. 145 V. Dichiarante, M. Fagnoni and A. Albini, Chem. Commun., 2006, 3001. 146 S. Protti, M. Fagnoni and A. Albini, J. Am. Chem. Soc., 2006, 128, 10670.

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Photochemistry and photophysics of transition-metal complexes Andrea Maldotti* DOI: 10.1039/b812719k This chapter aims to cover important developments on the photophysical and photochemical properties of transition-metal complexes for the period between June 2004 and July 2007. Because of the enormous number of systems investigated, I was compelled to leave articles with strongly applied character out of this overview, which is mainly focused on fundamental aspects of metal complexes also as units of supramolecular assemblies. For the same reason, I could report only on good impact factor articles. A great number of the considered works concern photoinduced electron (PET) and energy (PEnT) transfer studies in mononuclear compounds and multicomponent arrays. An important purpose of these articles is the possibility to obtain intensely phosphorescent systems and long-lived charge-separated states, in view of their possible interest in the development of luminescent materials, light emitting diodes, sensors, solar cells, artiﬁcial photosynthetic systems. A non-negligible number of articles (about 20%) deal with the excited-state redox activity of transition-metal compounds. In this ﬁeld, particular attention is devoted to the photocatalytic functionalization of organic substrates, to environment decontamination and to CO2 reduction.

1.

Introduction

Some reviews were published covering the following subjects: tridentate ligands for Ru(II) complexes with long luminescence lifetimes;1 elucidation of intercomponent interactions and molecular motion in supramolecular systems;2–4 synthetic tailoring of the excited state properties of Ir(III) complexes;5 PEnT in covalently linked cyclic metalloporphyrin arrays;6 photocatalysis with polyoxometalate-containing composite systems;7 lanthanide photophysics in polynuclear complexes,8 in emissive systems based on Ln(III),9 in the presence of organic chromophores,10 for switching and bioassays;11 A number of contributions at a ‘‘Forum on Solar and Renewable Energy’’ and at the ‘‘15th and 16th International Symposia on the Photochemistry and Photophysics of Coordination Compounds’’ have been published on two special issues of Inorganic Chemistry (2005, 44, issue 20) and Coordination Chemistry Reviews (2005, 249, issues 13–14) respectively. Many aspects of the electronic spectroscopy of inorganic compounds have been covered in an entire issue of Coordination Chemistry Reviews (2007, 251, issue 257). Three annual periodical reviews give an exhaustive summary of the photophysical properties of metal complexes.12–14 V. Balzani and S. Campagna are the Editors of two books on the photochemistry and photophysics of coordination compounds published in 2007.15,16 Universita di Ferrara, Dip Chimica, Via L Borsari 46, Ferrara 44100, Italy

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2. 2.1

Chromium, molybdenum, tungsten Chromium

Some investigations in the ﬁeld of Cr-complexes concern ultra fast dynamics. Femtosecond time-resolved absorption spectroscopy has been used to elucidate the dynamics associated with formation of the 2E excited state in the complex Cr(acac)3 (acac: deprotonated monoanion of acetylacetone).17 Photophysical diﬀerences among trans-dicyano-Cr(III) complexes of topologically constrained tetraazamacrocycles were explained on the basis of steric and symmetry reasons.18 Other literature data on Cr-complexes regard intra- and inter-molecular redox processes, PEnT and photosubstitution reactions. [Cr(cyclam)Cl2]+ with a covalently-bound anthracene group shows phosphorescence from the doublet d–d excited state of Cr(III) at low temperature as a consequence of antracene excitation.19 When the axial anionic ligands are nitrites, sensitisation of the Cr based doublet state results in ejection of NO, suggesting a potential interest of these complexes for medicinal applications.20 Ejection and substitution of one molecule of CO in the complexes M(CO)6 (M = Cr, Mo, W) can be induced upon irradiation in the presence of Et3SiH,21 2-methyl-2,3-dihydrofuran or 2,3-dihydropyran.22 A photodissociation process involving transient Cr-macrocyclic N-bond cleavage explains the thermally activated 2Eg relaxation.23 Photoexcited azido-Cr(III) Schiﬀ-base complexes induce the cleavage of bovine serum albumin at multiple sites.24 Cr-complexes such as [Cr(Z6-arene)(CO)3] photoinitiate acrylate polymerization.25 2.2

Molybdenum

The luminescence properties of [Mo2(O2C-9-anthracene)4] have been investigated; its 3MLCT excited-state has been found to have a long lifetime (t = 76 ms).26 Photosubstitution reaction in Mo-carbonyls may lead to the formation of reactive intermediates. Photolysis of Mo(CO)6 in n-heptane creates a coordinatively unsaturated d6 species, which can react with Et2SiH2 to give Mo-H and Mo-Si bonds.27 Photochemical reaction of Cp*Mo(CO)3Mes (Mes = 2,4,6-Me3C6H2) with the hydrodisilane HSiMe2SiMeMes2 gives silyl-silylene Mo half-sandwich [Cp*Mo(CO)2(SiMes2)(SiMe3)].28 2.3

Tungsten

A number of reactions of interest in synthesis and catalysis may be induced by photoexcited W-complexes and poly-oxo-tungstates. Photochemical reaction of W(CO)6 with GeCl4 is a source of germyl and germylene compounds acting as initiators for ring-opening metathesis polymerization of norbornene.29 Photoinduced b-hydrogen elimination from the complex CpW(CO)3C2H6 is followed by the formation of a a-H stabilized complex; in a side reaction, CpW(CO)3 is formed via ethyl ligand dissociation.30 Photochemical excitation of transition metal substituted heteropoly tungstates in nonpolar media causes their multielectron reduction, yielding promising photocatalysts of interest also for CO2 reduction.31 It has been reported that luminescence from the Photochemistry, 2009, 37, 240–299 | 241 This journal is

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complex trans-(N2)2W(R2PCH2CH2PR2)2 (R = 4-CF3-Ph,) is composed of simultaneous emission from near-degenerate 3MLCT and 3(d–d) terms.32 There are several articles describing the use of the decatungstate W10O324 for inducing ‘‘green’’ photocatalytic oxidation processes by O2. Cyclohexane can be converted to cyclohexanone when the decatungstate is heterogenized on an ion-exchange resin.33 Photocatalytic oxidation of tetrasubstituted alkenes34 and aromatic alcohols35 leads to the formation of allylic hydroperoxides and carbonylic compounds respectively. Solvent-free, heterogeneous photooxygenation of hydrocarbons occurs by Hyﬂon membranes embedding the ﬂuorous-tagged W10O324.36 Photoexcitation of (nBu4N)4W10O32 hydrogenised with Amberlite IRA-900 and in the presence of Br induces the bromination of arenes and alkenes.37 Hydrogen is eﬃciently abstracted from aliphatic compounds by excitation of (nBu4N)4W10O32; trapping by a,b-unsaturated nitriles, esters and ketones is a convenient ‘‘green’’ method for their alkylation.38 It was shown that, in addition to acetonitrile and water, acetone can be used as solvent in decatungstate photocatalysis even if its reactivity towards the photoexcited decatungstate is 3–4 times lower than that of acetonitrile.39 The photocatalytic multielectron-transfer-based H2-formation through the dehydrogenative oxidation of methanol (CH3OH - H2 + HCHO) can be achieved with the Keggin-Ti/W-mixed polyanion [(A-b-GeTi3W9O37)2O3]14.40 3. 3.1

Manganese, rhenium Photophysics of mononuclear complexes

A lot of articles on rhenium have been published in the area of diimine complexes with CO as co-ligand (a prototype of this class of complexes is shown in Fig. 1). Part of this work regards the photophysical properties of mononuclear complexes of Re(I). The extent of metal-to-ligand charge-transfer has been found to be greater with [ReL(CO)3(5-NO2-phen)]+ compared to [ReL(CO)3(phen)]+ (L = Cl, 4-ethylpyridine, imidazole).41 In [Re(5-NO2-phen)(CO)3Cl] the 3 MLCT state decays very fast (10 ps) to a lower-lying ligand-centered state, which is short-lived (30 ps).42 The growth of the MLCT state of [Re(bpy)(CO)3Cl] in the solid state has been found to be slower using X-ray excitation than UV light, because the cation-electron recombination generates the MLCT excited state only in the ﬁrst case.43 Two isomers of [Re(CO)3Cl(phen-O)]

Fig. 1

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(phen-O = phenanthroline-5,6-epoxide), which diﬀer in the relative orientation of epoxide and Cl, present weaker luminescence than [Re(CO)3Cl(phen)].44 [Re(CO)3(NN)L]+ (NN = a bpy or phen derivative, L = 2,6- Ph2–C6H3NC) show stronger luminescence than [Ru(bpy)3]2+.45 The character of the lowest excited state of [Re(NCS)(CO)3(NN)] is a combination of Re NN MLCT and thiocyanate-NN LLCT.46 In [Re(CO)3Cl(phen)] bearing alkynyl substituents at the C5 position of phen the lowest excited state is 3MLCT at room temperature and 3LC phen-centred at 77 K.47 Re(I) compounds of formula [(bpy)(CO)3Re(PCA)]+, [(bpy)(CO)3Re(PCA)Re(CO)3(bpy)]2+ and [(bpy)(CO)3Re(PCA)Ru(NH3)5]3+ (PCA = 4-pyridinecarboxaldehydeazine) in acidic aqueous solutions show luminescence from MLCT excited states.48 Luminescent Re(I)-iminopyridine or Re(I)-phenanthroline complexes were prepared with pendant crown or thiacrown ethers attached to the diimine ligand.49,50 Several mixed ligand bpy and phen Re(I) carbonyl complexes, [Re(CO)2(PP)(NN)]+ (PP = chelating diphosphene) display very diﬀerent excited state lifetimes (25–1147 ns) and quantum yields (0.002–0.11).51 The HOMO of the amido and phosphido complexes fac-[Re(ER2)(CO)3(bpy)] (ER2 = NHPh, NTol2, PPh2, Tol = 4-methylphenyl) is localized at the amido/phosphido ligand; the lowest excited state is the corresponding triplet 3LLCT, which relaxation occurs with complex dynamics ranging from units to tens of picoseconds.52 A number of Re(CO)3(L) complexes, where L is a tridentate ligand derived from quinoline, benzimidazole, or tryptophan, show emission from MLCT or LC excited states.53 In [Re(CO)3Cl(bzpy)2] (bzpy = 4-benzoylpyridine) the 3MLCT excited state is localised on one bzpy ligand, whereas this excited state in [Re(CO)3(bzpy)(bpy)]+ is localised on the bpy ligand.54 The photophysical properties of [Re(dmb)(CO)2(PR3)(PR 0 3)]+ are aﬀected by intramolecular interactions between ligands.55 The ﬂuorescence of Re(III) metallocarborane [7,8,9-ReC2B7H9] containing a {Re(CO)(PPh3)2} fragment was ascribed to metal-perturbed 1LC states.56 The clusters [Re6(m3-S)8X6]4 (X = halide) show temperature dependent luminescence due to the presence of four emissive triplet excited state.57 3.2

Multicomponent systems

Tetrametallic, molecular rectangles that have the form ([Re(CO)3]2BiBzIm)2m,m 0 -(LL)2, where BiBzIm is 2,2 0 -bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand have been prepared; their singly reduced forms are members of a category of mixed-valence compounds in which the ligands themselves are the redox centers and interligand electronic communication is controlled by direct ligand orbital overlap rather than by superexchange through the metal ions.58 The 3MLCT excited states of Re(I) compounds, [{Re(CO)3(m-4,40-bpy)Br}{Re(CO)3-(m-L)Br}] (L = 4,40-dipyridylalkynes) are quenched by electron transfer reactions from aromatic amines more quickly than monomeric Re(I) complexes.59 A derivative of [Re(CO)3(bpy)(py)]+ with a catecholamide group pendant acts as a luminescent sensor for oxometallates, which quench the Re-based luminescence.60 A number of Re-diimine luminophores have been prepared with pendant groups able to bind proteins.61–63 A photoactive supramolecular assembly consisting of a Photochemistry, 2009, 37, 240–299 | 243 This journal is

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methyl viologen-functionalized barbiturate host and a [Re(Br)(CO)3(barbi-bpy)] (barbi-bpy = 5-[4-(4 0 -methyl)-2,2 0 -bipyridyl]methyl-2,4,6(1H,3H,5H)-pyrimidinetrione) complex has been investigated; upon excitation of the rhenium compound, an ultrafast electron-transfer process occurs from the metal-based component to the acceptor unit.64 Tricarbonyl Re(I) phenanthroline estradiol compounds exhibit long-lived 3MLCT emission that is increased upon binding of estrogen receptor a.65 Re(I) tricarbonyl amidodipyridoquinoxaline biotin complexes display a signiﬁcant enhancement of the 3MLCT emission intensity upon binding to the avidin protein.66 Aggregation in nanobundles and the eﬀect of diverse environments on the solution-phase photochemistry and photophysics of the polymers {(vpy-[Re(CO)3(2,2 0 -bpy)])m(vpy-[Re(CO)3(phen)]n(vpy)p(CF3SO3)m+n} (vpy = 4-vinylpyridine, m = 131, n = 131 or m = 200, n = 150, and m + n + p = 600) have been investigated.67 The heterodinuclear complex [{fac-(CO)3Re(dppz)}(m-dpp[5]){Ru(tpm)(dppz)}]3+ (dpp[5] = 4,4-dipyridylpentane; tpm = tris(1-pyrazoyl)methane] shows PEnT from Re to Ru. Its luminescence is enhanced after DNA binding, and photoirradiation leads to DNA cleavage.68 3.3

Photoinduced reactivity and photocatalysis

There are several articles showing that photoexcitation of Re(I)-complexes may lead to the formation of radical species. A covalently-linked [Re(CO)3(bpy)CN]/tyrosine adduct undergoes tyrosine Re PET upon excitation, yielding a phenoxy radical.69 Time-resolved spectroscopies were used to monitor the kinetics involved in the photogeneration of ﬂuorotyrosyl radicals from a Re(I) tricarbonyl polypyridyl complex.70 Irradiation of [Re(R)(CO)3(dmb)] (R = methyl or ethyl) in MeCN yields [Re(MeCN)(CO)3(dmb)]d and Rd radicals together with a 3MLCT excited state, which, in CH2Cl2 gives the radical anion [Re(Cl)(CO)3(dmb)]d.71 Other Re-complexes have been investigated as photocatalysts for CO2 reduction. Ru(II)–Re(I) binuclear complexes linked by the bridging ligands 1,3-bis(4 0 -methyl-[2,2 0 ]bipyridinyl-4-yl)propan-2-ol and 4-methyl-4 0 -[1,10]phenanthroline-[5,6-d]imidazol-2-yl)bipyridine have been investigated for constructing supramolecular photocatalysts for CO2 reduction.72 Two series of complexes [MX(diimine)(CO)] (M = Tc, Re) (X = anionic ligands) work as photocatalysts for the reduction of CO2 to CO.73 Intramolecular interactions between ligands have been successfully applied as a tool for controlling various properties of a series of cis, trans-[Re(dmb)(CO)2(PR3)(PR 0 3)]+type complexes, including photocatalysis for CO2 reduction.74 Some investigations have been also reported on the use of Re- and Mn-complexes for generating reactive intermediates of interest in synthesis. Studies of disubstituted dichloroarenes and Cp*Re(CO)3 under UV irradiation show C-C bond activation.75 Photoexcitation of fac-[Re(bpy)(CO)3Cl and derivatives induces CO dissociation and ligand substitution of some interest for the synthesis of dicarbonyl compounds.76 A Re(I) carbonyl complex of triethylphosphine undergoes competitive photosubstitution of both triethylphosphine and CO.77 The complexes [Re(CO)4(phen)]+ and [Re(CO)3CN(phen)] and analogous Ir(III) compounds act as good sensitisers 244 | Photochemistry, 2009, 37, 240–299 This journal is

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of singlet oxygen.78 Photoexcitation of a charge-transfer band of the manganeseoxo ‘‘cubane’’ complexes, models of the photosynthetic water oxidation site, causes dissociation of one phosphinate and two core oxygen atoms.79 4. 4.1

Iron Multicomponent systems

Several examples of eﬃcient PET systems based on C60 with ferrocene units on their periphery have been reported (see for example the system reported in Fig. 2, ref. 83). Fluorescence quenching of a C60 acceptor with linked ferrocene and N,N-dimethylaniline donors indicates that charge separation takes place through singlet excited state of the C60 moiety.80 The photoinduced chargeseparated state lifetimes in fulleropyrrolidine–oligothienylenevinylene dyads increase when a donor ferrocene is bound at the end of the thienylenevinylene chain.81 In fullerodendrimers with two and four ferrocene units on their periphery the lifetimes of the [(C60)d-(dendron)d+] state vary signiﬁcantly depending on the solvent polarity.82 Photophysical studies of bucky ferrocene and ruthenocene have been reported; photoexcitation of fullerene results in a rapid charge separation, with radical ion pair lifetimes of about 30 ps; no charge separation has been found in the corresponding ruthenocene.83 Very eﬃcient stabilization of the photoinduced charge-transfer state has been achieved upon photoexcitation of simple subphthalocyaninesferrocene dyads, reaching radical pair lifetimes of about 0.2 ms.84 4.2

Photoinduced reactivity and photocatalysis

The photoinduced ligand (L) release from Fe-complexes (step a in Fig. 3) has been examined in a number of publications. The photodetachment of NO from [M(III)(CN)5NO]2 (M = Fe, Ru, Os) is accompanied by the formation of [M(II) (CN)5H2O]3; the most active complex is the iron derivative.85 For the dye-derivatized clusters [Fe2(m-SR)2(NO)4] (R = organic dye molecule) eﬃcient PEnT from the dye to the metal centre occurs with the concomitant release of NO.86,87 Photochemistry of [S5Fe(NO)2] leads to the reversible formation of [S5Fe(m-S)2FeS5]2; this is relevant to the repair in vitro of nitric oxide-modiﬁed [2Fe-2S] ferredoxin by cysteine desulfurase and L-cysteine.88 The iron nitrosyl complex [(PaPy(2)Q)Fe(NO)](ClO4), where PaPy(2)QH is

Fig. 2

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Fig. 3

(N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide, undergoes NO release under photoexcitation.89 Fe-complexes may also undergo CO release upon irradiation. Allyl Fe-complexes of the type (Z3-2-R-C3H4)Fe(CO)(NO)(X) (R = H or Cl; X = CO or PPh3) undergo photoinduced loss of a CO ligand at 20 K.90 Photolysis of Fe2(CO)6(m-S2) in a Nujol matrix gives the butterﬂy diradical Fe2(CO)6S2 species; photolysis at higher energies results in CO release from this diradical.91 The photochemistry of Fe(CO)5 has been investigated in several solvents; Fe(CO)4 and Fe(CO)3(solvent) are formed as primary photoproducts.92 Matrix photolysis of a mixture of endo- and exo-Fe(Z3-C3H5)(CO)3Br gives the C1-mer isomer with visible light and CO-loss species with UV photolysis. It is proposed that the C1-mer isomer arises from a Bailar twist.93 Other works concern intra- and inter-molecular PET in Fe-complexes (Step b in Fig. 3) of interest in synthesis and in bioinorganic chemistry. Irradiation of [Fe(4-R-benacen)(CH3OH)(NCS)] where 4-R-benacen2 = tetradentate N,N 0 -ethyle-ligands (R = H, Cl, Br, OCH3) induces the reduction of Fe(III) and the oxidation of the 4-R-benacen-ligands.94 Photoexcitation of the phenolate-to-Fe(III) charge-transfer band in a Fe-dipyridoquinoxaline leads to cleavage of supercoiled pUC19 DNA to form its nicked circular form.95 The LMCT excited state of [Fe(edta)(H2O)] or [Fe(edta)(OH)]2 undergoes self-quenching to give the [(H2O)(edtad)-FeII(m-OHx)FeIII(edta)]x4 species.96 Intermolecular PET was observed from [Ru(bpy)3]+ to a model of the iron-only hydrogenase active site, [(CO)6Fe2{(m-SCH2)2X}] (X = CH2, NCH2C6H5).97 Photocontrolled ring-opening polymerisation of Sila[1]ferrocenophanes with multiple alkyne substituents occurs using NaCp as initiator.98 Ferrocene-based organic radicals exhibit broad absorption bands in the near-IR that correspond to the excitation of a neutral donor–acceptor (DA) ground state to a short lived charge-separated D+A state.99 A transitionmetal-catalyzed cycloaromatization of conjugated enediynes under photochemical conditions has been achieved, for the ﬁrst time, utilizing [(Z5-C5Me5)Fe(NCMe)3]PF6 as precursor.100 Photoexcitation of FeCl4 hydrogenised with Amberlite causes the conversion of several cycloalkanes to the corresponding monochlorinated products with a selectivity higher than 95%.101 Photochemical reactions of Fe(CO)5 with monometal alkynyls and free alkynes lead to the formation of [(Z5-C5Me5)Fe2Mo(CO)7{m-Z1:Z4:Z2-C(H)C(Ph)C(Ph)C}] and ferrocenylquinones.102 Catalytic C-C bond cleavage of MeCN and C–Si bond formation were attained in the photoreaction of MeCN with Et3SiH in the presence of a Cp(CO)2FeMe.103 246 | Photochemistry, 2009, 37, 240–299 This journal is

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5.

Ruthenium

A lot of Ru-complexes have been investigated from the photophysical and photochemical points of view. In particular, a great number of articles have been published on polypiridine complexes, since they are able to absorb visible light, they present intense and long-lived luminescence, they undergo reversible redox processes. A prototype of this class of complexes is [Ru(bpy)3]2+ (see Fig. 4). Ru-complexes are also the subunits of a great number of photoactive multicomponent structures.

5.1

Mononuclear complexes

The photophysical properties of mononuclear complexes of Ru(II) have been extensively investigated. A Ru(II)bis(2,2 0 :6 0 ,200 -terpyridine) complex bearing a single ethydylene substituent shows luminescence in ﬂuid solution at room temperature only from the lowest energy MLCT triplet state.104 A novel series of Ru-terpyridine complexes with bichromophoric units separated by more than one nm displays higher luminescence lifetimes than the parent Ru(tpy)22+ chromophore.105 Hetero- and homoleptic Ru- complexes of tridendate ligands based on 2-aryl-4,6-di(2-pyridyl)-s-triazine show 3MLCT luminescence in solution; the triazine-based ligands result in longer luminescence than that observed for [Ru(tpy)2]2+ by lowering the LUMO.106 The complex [Ru(bpy)2(L)](PF6)2 (L = tetra-peri-(tert-butylbenzo)-di-peri-(pyrimidino)-coronene) shows highly red-shifted 1MLCT absorption and 3MLCT luminescence.107 A complex based on N-heterocyclic carbenes exhibits interesting luminescence properties with a lifetime of 820 ns in acetonitrile and of 3100 ns in water at room temperature.108 [Ru(bpy)2(bpy-crown)]2+ complexes, where one of the bpy ligand is appended with a crown ether, display luminescence that is responsive to complexation with metal ions.109 Addition of Ba2+ ions to [Ru(phen)3]2+ complexes, with azacrown ethers attached to the phenantrolines, causes shifts of up 87 nm in the emission spectra, and shifts of up to 370 mV in the redox potential.110 The bidentate phosphine-ether ligands (POR) in [Ru(bpy)2(POR)]2+ [POR = 1,2-C6H4(OR)(PPh2)] causes signiﬁcant changes in the emission properties with respect to [Ru(bpy)3]2+.111 [Ru(tpy-4 0 -CN)2]2+ displays luminescence in ﬂuid solution because the electron-withdrawing eﬀect of the cyano groups lowers the energy of the emissive 3MLCT state and makes the non-emissive 3d–d state less

Fig. 4

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accessible.112 The luminescence from Ru(II) terdentate chelates can be improved by bipyridyl-pyridyl-methane ligands, which have a stronger ligand ﬁeld than tpy because of a more octahedral coordination.113 The luminescence of [Ru(bpy)2(phen-5-COOH)]2+ has been investigated in view of possible applications for pH sensors.114 A series of eilatin complexes [Ru(bpy)x(eil)y]2+ show a red-shift of MLCT absorption as a consequence of replacement of bpy by eilatin with concomitant low-energy luminescence.115 A similar eﬀect occurs in substitution of bpy ligands by diazaperylene.116 Several polypyridinic heteroleptic complexes of the type Ru(NN)m(L)n (NN = bpy, dcb) and Ru(terpy)(L)n present peculiar photophysical properties depending on the nature of L.117–120 Variations in the ligands of [Ru(L)(bpy)] (L = four mono-, two bi- or one tetra-dentate amine ligands) result in large changes in the shape of emission band, due to diﬀerent vibronic contributions and excited state distortions.121 A Ru-complex based on the 2,6-bis(8 0 -quinolinyl)pyridine ligand shows 3MLCT excellent photophysical properties at room temperature: emission with a lifetime of 0.3 ms and quantum yield of 0.02.122 DFT calculations on [Ru(NN)]32+ (NN = bpy, bpz) complexes indicate that emission originates from two quasi-degenerate 3MLCT states.123 A signiﬁcant improvement of the photophysical properties was obtained with a series of Ru(II) complexes of general formulas [(R-pm-tpy)Ru(tpy)]2+ and [Ru(tpy-pm-R)2]2+ (tpy = 2,2 0 :6 0 ,200 terpyridine; R-pm-tpy = 4 0 -(2-pyrimidinyl)-2,2 0 :6 0 ,200 -terpyridine with R = H, methyl, phenyl, perﬂuorophenyl, chloride, and cyanide) because of the enhanced separation between the MLCT and MC excited states.124 The luminescence in the solid state of [Ru(L)(L 0 )(CN)2], where L and L 0 are bpy derivatives, depends strongly on hydration.125 Only the closed form of (1,2-bis(2-methylbenzo[b]thiophen-3-yl) hexaﬂuorocyclopentene works as a photochromic energy transfer quencher of the MLCT based luminescence of [Ru(4,7-Ph2phen)]2+ which allows the luminescence lifetime of the Ru–phen complex to be used as a readout for the open/closed state of the photochromic unit.126 The 3MLCT excited state of Ru(II) complexes with 1,4,5,8-tetraazaphenanthrene ligands in the presence of tryptophan yields an electron transfer process from the aminoacid, with subsequent formation of a covalent adduct.127 Excitation of a [Ru(bpy)3]2+/amino-acid adduct (amino- acid = tryptophan or tyrosine) generates a Ru(III) centre, which oxidises the amino acid via a proton-coupled electron-transfer process.128 X-ray absorption spectra of aqueous [Ru(II)(bpy)3] have been recorded in its ground and excited states. A Ru–N bond contraction by about 0.03 A˚ in the excited state is observed.129 Femtosecond ﬂuorescence-upconversion studies of aqueous [Ru(bpy)3]2+ show a short-lived vibrationally hot emission of the singlet MLCT state, with an ultrafast intersystem crossing to the triplet state.130 Substitution of L with L 0 in the complex [Ru(bpy)2(L)]2+ (L = diacetyl dihydrazone, L 0 = an adduct of L with acetone) induces a change of the lowest excited state from Ru(II)–L to Ru(II)—bpy charge transfer.131 The complex [Ru(bqp)2]2+ [bqp = 2,6-bis(80-quinolinyl)pyridine] has a remarkably long-lived 3MLCT excited-state lifetime at room temperature, due to the octahedral geometry of the Ru(II) core that raises the energy of the short-lived triplet metal-centred state.132 Ru(II) polypyridyl complexes containing bpy and terpy subunits with pyrromethene-BF2 248 | Photochemistry, 2009, 37, 240–299 This journal is

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bodipy chromophores present long-lived (ms) non-emissive excited states at room temperature; however, at 77 K all the complexes exhibit bodipy based ﬂuorescence and, for tpy complexes, also phosphorescence.133 Excited-state absorption and circular dichroism of Ru(phen)32+ have been investigated in the ultraviolet region.134 For the pyridine-1,2,4-triazolato based complex of two series of Ru(II) polypyridyl compounds [Ru(bipy)2(phpytr)]+ and [Ru(bipy)2(phpztr)]+ (where Hphpytr = 2-(5-phenyl1H-[1,2,4]triazol-3-yl)-pyridine and Hphpztr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyrazine) the lowest emissive excited state is exclusively bipy based; however, for the pyrazine based complexes, localisation of the excitation on particular ligands depends markedly on solvent nature and pH.135 Excited state energies and light-harvesting ability of the [Ru(dcb)2(NCS)2] and [Ru(dcb)2(L)2] (L = CN) dyes were compared by DFT methods.136 Mechanism and kinetics of quenching by oxygen of the excited states of [Ru(NN)(CN)4]2 [NN = bpy, phen, dmb, 1-ethyl-2(2-pyridyl)benzimidazole] complexes in aqueous media was investigated.137 The photophysical properties of Ru complexes can be tuned using pyrroleand pyrrolidine-containing polypyridine ligands.138 The kinetics and mechanism for quenching of the MLCT excited-state of [Ru(bpy)2(bpz)]2+ by proton-coupled electron transfer from 1,4-hydroquinone was elucidated.139 Extending the nitrogen-heterosuperbenzene family, a new system has been prepared and characterized, where the low energy of the p* orbitals causes low-energy MLCT absorption and emission.140 The complex [Ru(phen)2dpq-n][PF6]2 (phen = phenanthroline, dpq-n = dipyridoquinoxaline-norbornene)+is the ﬁrst example of a Ru(II) diimine complex capable of undergoing ring-opening metathesis polymerisation with a quantum eﬃciency increasing in polar solvents.141 The use of heterogeneous or microheterogeneous systems is a suitable means for controlling the luminescence properties of Ru-complexes. A one-dimensional polymeric rotaxane was prepared based on a {Ru(bpy)2(m-4,4 0 -bpy)}n chain with a cyclodextrin unit surrounding each 4,4 0 -bpy ligand. The presence of the cyclodextrin causes a strong Ru-based luminescence.142 Time-resolved luminescence and transient absorption studies of [Ru(bpy)3]2+ constrained in various alkali-metal exchanged zeolites show ﬂuorescence quenching of the 3MLCT by PET to the alkali metal cations.143 The luminescence of [Ru(TAP)(dpphen)2]2+ (TAP = tetraazaphenanthrene) is sensitive to the local environment of the complex, so allowing to probe oligonucleotides.144 5.2

Multicomponent systems

5.2.1 With other Ru-complexes. Polynuclear complexes containing more than one Ru centres have been extensively investigated. A dinuclear ruthenium complex with a large, planar bis-tridentate bridging ligand has been prepared; this species has an unusually long-lived excited lifetime and high emission quantum yield, ascribed to the delocalisation of the promoted electron on the plane of the bridge.145 An analogous multichromophore approach with dinuclear Ru(II) complexes, containing anthryl-terpy derivatives as ancillary ligands and pyrimidine-terpy derivatives as bridging Photochemistry, 2009, 37, 240–299 | 249 This journal is

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ligands, aﬀorded long-lived luminescence at room temperature.146 The photophysical properties have been reported for a series of binuclear Ru(II) bis(2,2 0 :6 0 ,200 -terpyridine) complexes containing a geometrically constrained, biphenyl-based bridge; the luminescence quantum yield increases with decreasing temperature and the nonradiative decay from the 3MLCT state depends on the dihedral angle of the central C–C bond.147 Possible eﬀects of excited-state localization on photophysical behaviour are observed for two isomeric dinuclear Ru(II) polypyridine species.148 Sequential or concerted proton-coupled, multi-electron reduction of [(phen)2Ru(tatpp)Ru(phen)2]Cl4 (phen = 1,10-phenanthroline; tatpp = 9,11,20,22-tetraaza tetrapyrido[3,2-a:2 0 3 0 -c:300 ,200 -l:2000 ,3000 ]-pentacene) occurs under both photochemical and electrochemical conditions in aqueous solution as a function of pH.149 The analogous complex [(phen)2Ru(tatpp)Ru(phen)2][PF6]4 accepts up to four electrons and two protons on the central tatpp bridging ligand via a combination of stoichiometric chemical reductions and protonations.150 Helical assemblies containing several proline residues and ordered arrays of a tris-Ru(II)-bipyridyl type chromophore and a phenothiazine electrontransfer donor have been synthesized.151 A system based on [Ru(bpy)3]2+ and [Ru(py)(NH3)5]3+ connected by an oligo-proline chain undergoes PET through the chain from Ru(II) to Ru(III) centers, with a mechanism of superexchange or electron-hopping depending on the metal centres distance.152 A long-lived radical ion pair can be also formed by photoexcitation of a dinuclear assembly of Ru(II) phthalocyanines bridged by an electron acceptor perylenediimide derivative.153 A series of weakly luminescent or no luminescent 2,2 0 -bipyrimidine-based Ru complexes have been studied to probe their electronic structure and the dynamic behaviour of their excited states on the picoseconds and nanoseconds time scales; the lifetime of the lowest energy 3MLCT excited states of dinuclear complexes is shorter than that of the analogous mononuclear species.154 Red-light emitting devices of di- and tri-nuclear Ru(II) complexes based on linear and star shaped 2-(2 0 -pyridyl)benzimidazolyl derivative ligands have been achieved.155 Functionalisation of the naphthyl group of naphthalenebisimide (NBI) with two [Ru(bpy)3]2+ units causes an improvement of electronic coupling and an increase in the rate of Ru NBI PET.156 PEnT in the complex cis-[{(phen)2Ru(PHEHAT)}2RuCl2]4+ (PHEHAT = 1,10-phenanthroline[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene) occurs from the periphery Ru(II) ions to the central Ru atom; on the contrary, PEnT takes place in the other direction for the complexes [{(phen)2Ru(PHEHAT)}2Ru(NN)]6+ (NN = phen, 1,4,5,8-tetrazaphenanthrene).157 In mononuclear and binuclear wire-like Ru(II) complexes with oligo-diethynyl-thiophene bridged back-to-back terpyridine ligands, 3MLCT-based luminescence arises from the Ru–tpy units; weak coupling between the metal and the chain gives reason of the fact that the 3MLCT luminescence is not quenched by low-lying 3p–p* states of the bridging ligand.158 A number of metal cations (Li+, Na+, K+, Cs+, Ba2+) may control the 3MLCT luminescence energy of [Ru(bpy)(CN)4]2; as a consequence, it is possible to control the direction of PEnT in a bis-bipyridyl bridged [Ru(bpy)3]2+/[Ru(bpy)(CN)4]2 dyad.159 Conformational factors aﬀect the photophysical properties of dyads based on triphenylpyridinium-functionalized bipyridyl complexes of Ru(II).160 250 | Photochemistry, 2009, 37, 240–299 This journal is

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5.2.2 With other metal complexes. Polynuclear complexes combining Ru(II) centers with Fe(II) centers bridged by terpyridyle type ligands have been investigated. The trimetallic mixed Ru(II)/Fe(II) bis-terpyridyl complexes displays emission a long lived emission despite the presence of the Fe(II) terpyridyl center.161 The model of the Fe-hydrogenase active site [(ADT)Fe2(CO)6] (ADT = azadithiolate (S-CH2-NR-CH2-S), (R = 4-bromophenyl) has been connected to a [Ru(terpy)2]2+ photosensitizer. This trinuclear complex represents one synthetic step toward the realization of light-driven proton reduction.162 The complex [Ru(terpy- COOH)(terpy-NH2)] linked to a ferrocene unit at the N-terminal group, is signiﬁcantly more luminescent than the analogous complexes with C-terminal attached ferrocene.163 Ru(II)-based luminescence in hydrogen bonded assemblies of the complex [Ru(NN)(H2biim)]2+ (NN = bpy, 4,4 0 -tBu-bpy; H2biim = biimidazole) with [M(CN)6]3 enhances when M = Co and decreases when M = Fe.164 Spectroscopic and photochemical properties of [Ru(bpy)3]2+ are aﬀected by interaction with polyoxotungstates. In fact, the UV-vis spectrum of the adduct between [Ru(bpy)3]2+ and [S2W18O62]4–shows new optical transitions due to charge-transfer transitions in the ion-pair165 and the excited state of [Ru(bpy)3]2+ is quenched by the heteropolyanions [P2W17O61]10 with the involvement of a low-energy MMCT excited state.166 PEnT Ru Pd in [(bpy)2Ru(m-L)Pd(Me)(X)] (L = bpym, 5,5-Me2-bpym, 5,5-Br2-bpym; X = Cl or solvent) is most eﬃcient when the bridging ligand is the electron-withdrawing species 5,5-Br2-bpym.167 Binding of {Cu(dien)S}2+ units (S = solvent) to two of the externallydirected cyanide of [Ru(4,4 0 -tBu2bpy)(CN)4]2– quenches the Ru-based 3 MLCT luminescence; addition of cyanide results in detachment of the Cu(II) fragments, restoring the luminescence.168 [{CrIII(cyclam)}(m-NC){RuII(NH3)5}]5+ displays luminescence from an MMCT state at about 830 nm.169 A bpy ligand with two pendant Zn-cyclen units was used to prepare a complex containing a [Ru(bpy)3]2+ core; strong binding of polyphosphates to the Zn centres causes a strong enhancement of the Ru-based luminescence.170 Eﬃcient PEnT Ir((III) Ru(II) occurs in heteroleptic bimetallic systems having an aryl linker between one metal centre coordinated to two terpy ligands and a second metal centre coordinated to three bpy ligands.171 In a naphthalenediimide/[Ru(bpy)3]2+/Mn2 triad the phenolate-bridged Mn2 complex acts as electron donor, generating a MnIIMnIII unit and a naphthalenediimide radical anion; the charge-separated state has a long lifetime due to high reorganisation energy for back electron-transfer.172 Complexes containing [Ru(tpy)2]2+–[Co(tpy)2]3+ or [Ru(bpy)2]2+–[Co(bpy)2]3+ units connected by a bis-tpy or tpphz (tetrapyridophenazine) bridging ligand respectively undergo Ru Co PET to give a Ru(III)–Co(II) charge-separated state.173 Excitation of the Ru centre in a trinuclear species obtained through attachment of HgCl2 to the cyanide lone pairs of [Ru(bpy)2(CN)2] results in a cleavage of the N–Hg bond.174 Excitation transfer in a novel tetranuclear complex based on Ru(II) and Pd(II) chromophores occurs upon photoexcitation; ultrafast dynamic processes ranging from 2 to 220 ps were monitored by using coherent four-wave-mixing spectroscopy.175 Photochemistry, 2009, 37, 240–299 | 251 This journal is

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5.2.3 With organic molecules. There have been several examples of PEnT or PET between Ru-chromophores and organic molecules: in [Ru(bpy)2L]2+ (L = a bpy derivative with anthracene covalently linked);176 in a mixture of [Ru(dmb)3]2+ and either anthracene or 9,10-diphenylanthracene;177 in a [Ru(bpy)3]2+-copolymer array of acenaphthyl and coumarin.178 PEnT were studied in a covalently-linked free-base-porphyrin[Ru(tpy)2]2+ dyad; PEnT from the Ru-based 3MLCT state to the porphyrin triplet occurs; on the other hand, the S1 state of the porphyrin transfers energy to the 3MLCT state by the Dexter mechanism.179 Photoexcitation of Ru with three bipyridyl ligands, one of which is modiﬁed by attaching one or two hydroxamic acids groups, converts 3O2 to 1O2, which, in turn, oxidises the hydroxamic acid group to a nitroxide.180 The adduct of [Ru(bpy)(CN)4]2 with 1,4,8,11-tetrakis(naphthylmethyl)cyclam and a dendrimer consisting of a cyclam core appended with 12 dimethoxybenzene and 16 naphthyl units has been investigated. As shown by the sensitised Ru-based luminescence, Napthyl (Ru-bpy) PEnT occurs upon excitation.181 The triplet emitting state of an indacene (of 50 ms lifetime) was observed for the ﬁrst time in Ru(II)-complexes based on bipartite ligands carrying one or two indacene subunits linked via phenylethynyl connectors to terpyridine fragments.182 Preparations and characterizations of bichromophoric systems composed of a Ruthenium polypyridine complex connected to a diﬂuoroborazaindacene or a Zn-phthalocyanine chromophore have been carried out.183 A dual signal responding ﬂuoride sensor formed of a Ru(II) bpy ﬂuorophore and a 2,4-dinitrophenylhydrazone chromophore displayed a new intense absorption band at 580 nm upon addition of F.184 In [Ru(tpy- 4 0 -CCPh)(tpy-4 0 -H2Q)]2+ (H2Q = covalently linked hydroquinone) the oxidation of hydroquinone to quinone results in fast Ru quinone PET.185 A salen-type binding unit pendant from a [Ru(bpy)3]2+ core undergoes phenolate [*Ru(bpy)3]2+ PET to give a long-lived phenoxy radical.186 A rotaxane in which both the macrocycle and the thread contain bpy units coordinated to Ru(II) is of potential interest for systems displaying light-switchable molecular motion. In fact, it undergoes expulsion of the macrocyclic bpy containing fragment on irradiation of the Ru(II) chromophore.187 Irradiation of a [Ru(bpy3)]2+/ methylviologen (MV2+) adduct included within cucurbit[8]uril causes a PET process and the formation of a long-lived charge-separated state (Ru3+- MV+d—cucurbit[8]uril.188 A signiﬁcant improvement of the photophysical properties of [Ru(terpy)2]2+ was obtained with [Ru(NNN)2]2+ [NNN = anthracene and bromo-anthracene derivatives of bis(pyridyl)triazine ligands].189 Long wavelength excitation of [Ru(dmb)2(bpy-An)]2+(bpy-An is 4-methyl-40 -(9-anthrylethyl)-2,20 -bipyridine) in CH3CN solution produces upconverted delayed singlet anthracene ﬂuorescence via bimolecular triplet– triplet annihilation.190 Derivatives of [Ru(bpy)3]2+ containing ethynyl– pyrene substituents show that the lowest excited states are pyrene-based.191

5.2.4 With biomolecules. Ru polypyridyl complexes are of interest as luminescent marker or probes of biomolecules such as DNA. The complex [Ru(bpy)3]2+ with attached oligonucleotides form duplex DNA sequences 252 | Photochemistry, 2009, 37, 240–299 This journal is

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with other molecules giving cyclic polygonal arrays containing double stranded (ds)-DNA edges and photoactive [Ru(bpy)3]2+ units as vertices.192 Other derivatives of [Ru(bpy)3]2+ with pendant oligonucleotide strands hybridise with complementary oligonucleotide strands to give (ds)-DNA units pendant from the metal core.193 Calculations on the complex [Ru(phen)2(dppz)]2+ when intercalated into DNA reveal a strong eﬀect of the adjacent base-pairs on the electronic properties of the complex and the nature of its excited state.194 Addition of methyl substituents to a Ru-phen complex diminishes luminescence in organic solvents but improves it in water; binding of the complex to DNA increases the luminescence intensity.195 Excitation of [Ru(HAT)2(phen)]2+ (HAT = hexaaza-triphenylene) intercalated in DNA results in covalent attachment of the complex to DNA at a guanine residue.196 The emission intensity of the complex rac-[Ru(dmp)3]2+ (dmp = 5,6-dimethylphen) increase strongly as a consequence of binding with DNA.197 Ru(II) complexes containing the tris(1-pyrazolyl)methane (tpm) ligand have been prepared: [Ru(tpm)(L)(dppn)]n+ (where n = 1, 2; L = Cl, L = MeCN and pyridine; dppn = benzo[i]dipyrido[3,2-a:2 0 ,3 0 -c]phenazine) have lower emission quantum yields (=104) and lifetimes (=50 ns) than the analogous dppz-based complexes, but similar binding aﬃnities with CT-DNA.198 DFT studies were used to elucidate photoredox mechanism of DNA photocleavage by [Ru(phen)2(6-OH-dppz)]2+199 and the photophysical properties of [Ru(NN)2(dppz)]2+ (NN = phen or 1,4,5,8tetraazaphenanthrene).200 Photoactivation of [{(Z6-arene)RuCl}2(m-dpp)]2+ [arene = indan, benzene; dpp = 2,3-bis(2-pyridyl)pyrazine] in aqueous solutions containing DNA results in the formation of a Ru(II) complex that can bind to DNA.201 The MLCT luminescence intensity of a heteronuclear Ru(II)–Pt(II) dimer, containing a [Ru(bpy)3]2+ derivative attached to a cis-PtCl2 unit, is enhanced by complexation with DNA; importantly, visible-light-induced strand scission of DNA is found to be eﬃciently enhanced in the presence of this dimer.202 The binding of [Ru(bpy)3]2+ complexes containing a biotin moiety to avidin has been studied by luminescent titrations and competitive assays.203 Luminescent Ru(II)- and Re(I)-diimine wires that bind to the oxygenase domain of nitric oxide synthase (NOS) has been studied as inhibitors of this protein.204 Photochemical reduction of a Ru(II) complex containing a nicotinamide adenine dinucleotide (NAD+) model ligand in the presence of proton sources results in the formation of an organic hydride donor mimicking the enzymatic NAD+/NADH system.205 Upon visible excitation of the tyrosine-containing Ru(II)-complexes PET from the Ru(II) chromophore to methyl viologen occurs followed by intramolecular electron transfer from the tyrosine moieties to Ru(III).206 PET from [Ru(terpy)2]2+ to the covalently linked cytochrome c was observed upon continuous irradiation at 480 nm.207

5.2.5 With fullerene. Photophysical experiments show that in the complex [Ru(bpy)2(bpy-C60)]2+ a rapid intramolecular quenching of the Ru(II) 3MLCT excited state occurs by transudation of the triplet excited state energy to the fullerene.208 Quantitative intramolecular triplet energy Photochemistry, 2009, 37, 240–299 | 253 This journal is

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transfer occurs also between a [Ru(bpy)3]2+ based donor and C60 acceptor bridged by a short ethylene spacer; the energy-transfer mechanism is dominated by super-exchange interactions at low temperature and a through-space mechanism at higher temperatures as a result of a linker distortion.209 The rate of PEnT from a [Ru (bpy)3]2+ unit to a covalentlylinked C60 unit is independent of the length of a bridging group, which consists of an alternating phenylene/alkynyl sequence.210 5.3

Photoinduced reactivity and photocatalysis

The photoinduced release of NO or CO from Ru-complexes has been examined in several investigations. Irradiation of trans-[RuCl(tetraazamacrocycle)NO]2+ with near UV light results in the release of NO.211 Ruthenium nitrosyls with planar dicarboxamide tetradentate N4 ligands release NO upon illumination giving solvated low-spin Ru(III) species; the extent of NO release and its wavelength dependence can be modulated by changes of either the in-plane or the axial ligand ﬁeld strength.212 Visible light photoexcitation of [Ru(NO)(Me2bpb)(resoruﬁn)] [H2Me2bpb = 1,2bis(pyridine-2-carboxamido)-4,5-dimethylbenzene] results in NO release.213 Ultra fast dissociation of equatorial CO ligand occurs upon irradiation of trans(X,X)-[Ru(X)2(CO)2(bpy)] (X = Cl, Br, I).214 Dissociation of CO from oxo-centered trinuclear ruthenium clusters [Ru3(m3-O)(m-OOCCH3)6(CO)(pyridine)2] has been observed in organic solvents upon photolysis at l 4 290 nm.215 Finally, the photochemistry of Ru3(CO)12 was investigated on the ultrafast time scale using UV-vis pump, infrared probe spectroscopy in order to study the transient intermediates formed upon photoexcitation.216 Ru-complexes have also been used for generating reactive intermediates of interest in synthesis and photocatalysis. Photoexcitation of [Ru(dmb)3]2+ in the visible region induces eﬃcient bimolecular [4+4] cycloaddition between two anthracene molecules.217 A synthetic cycle for conversion of phosphaalkynes to 1H-phosphindoles with photoexcited Ru-dppe complexes has been developed.218 Steady-state irradiation under CO, C2H4 and Et3SiH of Ru(etp)(CO)H2 (etp = PhP(CH2CH2PPh2)2) yields Ru(etp)(CO)2, Ru(etp)(CO)(C2H4), and Ru(etp)(CO)(SiEt3)H, respectively.219 A photocatalytic system based on [Ru(bpy)3]2+, methyl-viologen and oxygen is able to convert aqueous ammonia to N2.220 Photocatalytic decomposition of NH3 to N2 has been also carried out with systems based on [Ru(bpy)3]2+/K2S2O8 or [Ru(bpy)3]2+/methyl viologen dichloride/O2.221 [RuL3]2+ (L = bpy, dmb) induces photocatalytic oxidation of various manganese complexes using an aryl diazonium salt as electron acceptor.222 Two photon excitation of [Ru(bpy)2(apy)2]2+ (apy = 4-aminopyridine) generates an excited state that ejects the 4-aminopyridine.223 Visible-light irradiation of [(bpy)2Ru(m-bpym)PdMeCl]+ induces photocatalytic dimerisation of a-methylstyrene at the Pd centre.224 The photo-reduction of the Dawson polyoxometalate anions [S2M18O62]4 in the visible region was sensitised by [Ru(bpy)3]2+ cations.225 Compounds of the type [Ru(tpy)(bpy)(dmso)]2+ undergo photoinduced isomerization through a mechanism that does not involve the ligand ﬁeld states.226 The complex [Ru(NN)2(MeOH)2]2+ (NN = 2,7-diphenyl-phen) undergoes a cis-trans 254 | Photochemistry, 2009, 37, 240–299 This journal is

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photoisomerisation process.227 Photolysis of [Ru(NH3)5(pz)]2+ (pz = pyrazine) gives the Ru(III) analogue together with singlet oxygen and superoxide.228 Derivatives of [Ru(bpy)3]2+ in which one ligand has highly sterically hindering substituents were investigated in the search for reversible photoinduced decomplexation reactions which might be of interest in light-driven molecular machines.229 Photoexcitation of cis-[RuCl2(dmso)4] in dmso leads to geometric isomerization to give the photostable trans complex, whereas in acetonitrile and aqueous solution, both isomerization and substitution processes occur.230 6. 6.1

Osmium Mononuclear complexes

A number of Os-complexes containing imine ligands have been prepared and characterized. Two geometrical isomers of the [Os(CO)2(bptz)2] (bptz = 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazolate)present markedly diﬀerent photophysical properties.231 The photophysical properties of the complexes [Os(CO)2(N–NH)2] (N–NH = 3-CF3-5-{2-pyridyl}pyrazole or 3-CF3-5{2-pyridyl} triazole) and the isomeric forms of [Os(CO)(H)(PPh2-Me)2(N-NH)] have been investigated; some photophysical diﬀerences among these species were attributed to the p-accepting character of CO.232 [Os(NN)2(CO)L] [HNN = 5-(triﬂuoromethyl)-3-(2-pyridyl)- pyrazole; L = a monodentate pyridine derivative] gives ILCT and LLCT emissions depending on the nature of L.233 The lowest excited triplet state of a spiropyran substituted [Os(bpy)3]2+ in its open merocyanine form is higher in energy than the 3 MLCT state, resulting in spiropyran-to-metal PEnT.234 For the [Os(tpy)2]2+ complex, high-energy triplet states above the lowest 3MLCT have been detected.235 A [Os(terpy)2]2+ derivative attached to a cyclam receptor unit acts as a ﬂuorogenic sensor, with quenching of emission from the Os(II)-complex in the presence of Cu(II) and Ni(II) ions.236 Os(II)-complexes of 8-hydroxyquinolinate display dual emission due to incomplete inter-system crossing from the excited singlet state.237 6.2

Multicomponent systems

The photochemical behaviour of [Os3(CO)10(a-diimine-MV)]2+ (donor– acceptor) dyad can be controlled by an externally applied electronic bias.238 Photoexcitation of supramolecular triads consisting of a central trisbipyridine Ru(II) chromophore with one or more bound phenothiazine electron donors and a diquat-type electron acceptor yields long-lived charge separated states with unusually high quantum eﬃciency.239 A series of dinuclear complexes incorporating Ru(II)- and Os(II)-tris(2,2 0 -bipyridine) units have been prepared; in these compounds the metal-containing moieties are separated by 3,4-dibutyl-2,5-diethenylthiophene spacers and end-capped by 3,4dibutyl-2-ethenylthiophene subunits; RuOs and OsOs derivatives exibit triplet emission of 3Os LCT character.240 Some dyads based on Os(II) bis-tpy complexes functionalized by 2,4,6-triarylpyridinium groups show conformational gating of photoinduced processes, with competing energy transfer or electron-transfer to a charge-separated state responsible of the Photochemistry, 2009, 37, 240–299 | 255 This journal is

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quenching of the lowest-lying triplet excited states.241 DFT calculations on these systems conﬁrm the existence of a triplet state that involves the nitro substituents.242 [Ru(bpy)3]2+ and [Os(bpy)3]2+ were bound to oligonucleotides; quenching of the Ru-based emission by Ru Os PEnT was observed.243 The complexes [(phen)2M(tatpp)M(phen)2]4+, where M = Ru or Os, tatpp = 9,11,20,22-tetraazatetrapyrido[3,2-a:2 0 3 0 -c:300 ,200 -l:2000 ,3000 ]pentacene have diﬀerent types of lowest excited states depending on the nature of the metal; the excited-state kinetics is strongly solvent-dependent.244 A series of heteronuclear Os and Ru polypyridyl complexes with the bridging ligands 1,3-bis(5-(2-pyridyl)-1H-1,2,4-triazol-3-yl)benzene and 1,4-bis(5-(2-pyridyl)-1H-1,2,4-triazol-3-yl) shows dual emission, suggesting that energy transfer predominantly takes place via a dipole-dipole, Fo¨rster type, mechanism.245 Os(II) and Ru(II) complexes of [4 0 -(p-phenyl)]terpyridyl ligand derivatized with an electron acceptor of triphenylpyridinium show intramolecular PET to form a charge separated state.246 Several thiophenecontaining hybrids based on the bent conjugated backbone of a rigid ditopic ligand, the dimeric moiety 3,4-dibutyl-2,5-bis{50 -[(3,4-dibutylthien-2-ylethynyl)2,2 0 -bipyridin-5-yl]ethynyl}thiophene, have been investigated; in the heterodinuclear complexes containing Os and Ru, eﬃcient Ru Os energy-transfer occurs via a double electron exchange mechanism mediated by the oligomeric bridge.247 An assembly consisting of a Ru(II) bpy-cyclodextrin based junction with anthracene and Os(II) terpy guest molecules has been prepared; excitation of the anthracene moiety induces energy transfer processes leading to the formation of a Os-based triplet excited state.248 7. 7.1

Cobalt and rhodium Photophysical properties

Cyanide-bridged dinuclear complexes [(NC)5Ru(II)–CN–Co(III)(mac)] or [(NC)5Fe(II)–CN–Co(III)(mac)] (mac = a pentadentate N-donor macrocycle) form, after excitation, short-lived MMCT states.249 The binuclear cyclopentadienylcobalt carbonyls Cp2Co2(CO)n (n = 3, 2, 1; Cp = Z5-C5H5) were studied by DFT and compared with binuclear iron carbonyls.250 Coordination of acid-functionalized {Ru(tpy)2}2+ units (tpy = 2,2 0 :6 0 ,200 terpyridine) to a tetracarboxylato Rh-dimer leads to multinuclear polypyridyl-Ru(II)-complexes; eﬃcient PEnT from the Ru-based MLCT levels to the lower-lying level involving the Rh dimer takes place at room temperature.251 Homobinuclear systems, which bring together the complexities of ligand-bridged species and multiply bridged metal-metal Rh dimers present long-lived state due to their MLCT excited state.252 7.2

Photoinduced reactivity and photocatalysis

A Rh-supramolecular assembly opens up various possibilities for increasing the photocatalytic eﬃciency for H2 production and selective hydrogenation processes; for example, the HOMO/LUMO diﬀerence in the bridgingligand/metal fragment can be adjusted by functionalization of the bridge, by introduction of other metals or by variation of the coordinated anions.253 Two-electron photoprocesses of Rh-Rh species all possessing 256 | Photochemistry, 2009, 37, 240–299 This journal is

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excited states of dp* ds* type provide the basis for the photocatalytic hydrogen production.254 Rhodium-carbonyl complexes with a quinolyl functionalized Cp-ligand have been prepared and characterized from the photochemical point of view.255 Photolysis of one isomeric form of Rh2II,II(tfepma)3H2Cl2 (tfepma = MeN(P[OCH2CF3]2)2) results in a stoichiometric quantity of H2 formed through reductive elimination of H2.256 Irradiation in CHCl3 or CH2Cl2 of dicarbonyl [Z5-2,3,4,5-tetramethyl-1(8-quinolyl)cyclopentadienyl]Rh(I) gives the dichloro derivative; photoexcitation in the presence of a silane leads to the corresponding Si-H oxidative addition products.257 Evidence for Si–C and C–H activation pathways is observed upon photochemical reactions of [CH2(Z5-C5H4)2][Rh(C2H4)2]2 with silane.258 Irradiation of Rh(CO)2 on the surface of dealuminated zeolite Y leads to the production of reactive = Rh(CO) surface species of interest in heterogeneous photocatalysis.259 The Rh centered mixed-metal supramolecular complex [{(bpy)2Ru(dpp)}2RhCl2]5+ undergoes photoreduction in the presence of an electron donor to generate the two-electron reduced form.260 Low temperature irradiation of [(Z5-C5H5)Co(C2H4)2] in the presence of silanes enables the characterization of unstable ﬂuxional Co(III) silyl hydride complexes.261 8. 8.1

Iridium Mononuclear complexes

Mononuclear Ir(III)-complexes have been deeply investigated since their long-lived and intense emission in the visible region makes them of interest as phosphorescent dopants in light-emitting diodes. From this point of view, several tris-chelate Ir(III)-complexes containing cyclometallating N,C and/or N,N-donor ligands have been prepared and characterized. The eﬀect of ﬂuorination and of the pyridine/pyrazole ratio on the emission properties of mixed (diﬂuoro)phenylpyridine/(diﬂuoro)phenylpyrazole tris-cyclometalated Ir-complexes have been studied; increasing ﬂuorination and replacement of pyridine by pyrazole lead to a widening of the HOMO–LUMO gap and, generally, to a blue shift in emission.262 A series of heteroleptic Ir(III)-complexes bearing two N-phenyl-substituted pyrazoles and one 2-pyridyl pyrazole (or triazole) ligands were synthesized for attaining highly eﬃcient, room-temperature blue phosphorescence.263 Ir(III)-complexes with orthometalated quinoxaline ligands have been prepared for subtle tuning of emission to the red colour.264 The ancillary ligand structure enables facile and eﬃcient colour tuning over the whole visible range in cyclometalated heteroleptic Ir-complex.265 A series of diiminoiridium(III) complexes exhibit moderately intense and long-lived emission dependent on the substituents on the diimine ligands; the origin of the emission is assigned to a triplet metal-to-ligand charge-transfer excited state.266 The properties of the lowest excited state of in cyclometalated Ir(III)-complexes can be tune by employing diﬀerent ancillary ligands.267 The use of a negatively charged ligand such as a triazolyl pyridine allows a facile preparation, maintaining the high energy emission in the blue region, of heteroleptic complexes, with quantum yield values depending on the position of F or CF3 substituents on the phenylpyridine ligand.268 Complexes with a N3C3 donor set arising from one Photochemistry, 2009, 37, 240–299 | 257 This journal is

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C,N,C-donor and one N,C,N-donor cyclometallating ligand show luminescence at 585 nm in solution.269 The emission wavelength of mononuclear Ir(III)-complexes has been also tuned using other N,N-donor ligands, such as quinoline-based compounds,270 quinoline-based N,C-donor compounds,271 and benzimidazoyl substituted terpy-derivatives having N,C,N and N,N,N-donors.272 The 3LC luminescence of [Ir(tpy)2]3+ changes to 3MLCT when benzamide groups are added to the C4 0 positions of the terpyridyl ligands.273 [Ir(tpy)(CNC)]+ containing a bis-cyclometallating 2,6-diphenylpyridine derivative as a terdentate ligand gives luminescence at about 680 nm, varying with the substituents on the CNC species.274 Several Ir(III)-complexes containing the N,N,N-donor 2,6-bis(benzimidazolyl)pyridine or the N,C,N-donor 1,3-bis(benzimidazolyl) benzene, display quantum yield values up to 19% for the 3MLCT luminescence.275 Some bis-terdentate Ir(III)-complexes, which contain a cyclometalated N,C,N-coordinating 1,3-di(2-pyridyl)benzene derivative, are luminescent in ﬂuid solution; DFT calculations support the assignment of the emission to charge-transfer excited states with signiﬁcant MLCT character.276 Simultaneous two-photon excited ﬂuorescence and one-photon excited phosphorescence was observed from thin ﬁlms of [Ir(ppy)3] dispersed in poly(methylmethacrylate).277 X-ray excited optical luminescence techniques indicate that the excited state lifetimes of [Ir(ppy)3] ﬁlms are shorter than those observed in solution.278 Triplet energy transfer from [Ir(4-tBu-ppy)3] to [Ir(CN)2(acac)] (CN = 2-(2-benzothienyl)pyridinato, acac = acetylacetonate) occurs via Fo¨rster mechanism when molecular diﬀusion is inhibited by polymer matrix.279 Photoexcitation of [Ir(ppy)3] causes a singlet ﬂuorescence from pyrene or 3,8-di-tert-butylpyrene by triplet-triplet annihilation.280 A rapid combinatorial synthesis and screening methodology using resin bound show that Ir(III)-complexes display red emission in electroluminescence devices.281 Photophysical studies have been carried out for functionalised biscyclometalated complexes [Ir(CN)2(X2)]0/+, where X2 is acetylacetonate,282 or isocyanides.283,284 Photoexcited Ir-complexes have been also investigated for photocatalytic purposes. The Ir(III) complexes [Ir(NC)2(NN)]+ (NN = bpy or phen derivative; NC = 2-phenylpyridine derivative) are able to assist the photochemical production of H2 by water, with a quantum yield also 37 times higher than that of [Ru(bpy)3]2+.285 The photochemical C-H activation of cycloalkanes by aminoethyl-functionalized cyclopentadienylIr-complexes proceeds like that of their unfunctionalized analogues; in the presence of CO, carbonylation to form the carboxaldehyde occurs with both systems.286 8.2

Multicomponent systems

There are a number of multicomponent arrays containing Ir-complexes as photoactive units for inducing PEnT or PET processes. In an assembly made of a Ir(terpy)23+ unit connected through an amidophenyl spacer to a naphtalene bisimide, energy transfer from its 3MLCT level to the triplet localized on the naphtalene bisimide occurs upon excitation of the Ir(III) 258 | Photochemistry, 2009, 37, 240–299 This journal is

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moiety.287 Excitation of a series of para-phenylen bridged heterodinuclear Ir/Ru complexes with increasing bridge length leads to emission from the lower energy excited Ru(II)-unit; this suggests a very fast and nearly length independent Ir Ru energy transfer.288 Topological linkages have been used for the preparation of multi-component systems. A tetranuclear complex containing the sequence of Ir(III)–Ru(II)–Ru(II)–Ir(III) units shows independent Ru-based and Ir-based luminescence at room temperature because a meta-phenylene spacer in the bis-terpyridyl bridging ligands inhibits the PEnT process.289 Two equivalents of an [Ir(NN)(NC)2]+ unit bind to Eu(III) via a carboxylate, giving an Ir2Eu assembly; this gives red emission from Eu(III) and some blue emission from Ir(III).290 The Ir-tpy luminescence is quenched by the Cu-phen unit in a dinuclear pseudorotaxane.291 Chiral, facial tris-cyclometalated Ir(III)-complexes, fac-D-Ir(pppy)3, fac-D-Ir(pppy)3, fac-D-IrL (where pppy is (8R,10R)-2-(2 0 -phenyl)-4,5pinenopyridine and L is a tripodal ligand comprising three pppy moieties connected through a mesityl spacer) have been prepared; their luminescence properties and their sensitivity to dioxygen were evaluated.292 Ir Ru PEnT in a heterometallic [(ppy)2Ir(bpy)(phenyl)2(bpy)Ru(bpy)2]3+ complex and in a related tetranuclear (Ir)2–Ir–Ru assembly occur by a through-space Fo¨rster mechanism.293 The emission intensity of Ir(III) diimine-bis(biotin) complexes [Ir(CN)(NN)]+ (CN = biotin-functionalised ppy ligands; NN = 4,7-diphenyl-phen or 3,4,7,8-tetramethyl-phen) enhances as a consequence of binding to the avidin protein.294 9.

Nickel, palladium, platinum

9.1

Mononuclear complexes

The photochemical and photophysical properties of d8 platinum complexes strongly depend on their square planar geometry (Fig. 5). Pt(II)-complexes with polypyridine ligands have been deeply explored because of their interesting luminescent properties. Luminescence of [Pt(4,4 0 -tBu2-bpy)(CCR)2] (R = SiMe3, CC-SiMe3 or tBu) may be controlled by the nature of R, which aﬀects the sigma-donor strength of the acetylide.295 [Pt(4,4 0 -tBu2-bpy)(CC-pyrenyl)2] shows very fast migration of excitation energy through the alkyne linker.296 The nature of R also inﬂuences luminescence of [Pt(ttpy)(CC-R)]+ (ttpy = 4 0 -p-tolyl-terpyridine; R = H, Br, NO2, NMe2, OMe, naphthyl, quinolinyl), which may occur from either 3MLCT or 3LC states.297 Pt(II)-terpyridyl-alkynyl complexes display strong colour change and enhancement of luminescence intensity as a consequence of amine protonation.298 A luminescent diarylethenefunctionalized Pt(II) diimine bis(alkynyl) complex displays interesting photochromic behavior; photosensitization by excitation into the 3 MLCT/LLCT excited state of this complex induces photocyclization.299 Luminescence studies at 77 K show that the excited state of [Pt(bpy)(ttcn)]2+ (ttcn = trithia-cyclononane) is stabilised by the axial Pt–S link.300 [Pt(q-phen)Cl]+ (q-phen = 2-(8-quinolinyl)-1,10-phenanthroline) shows strong luminescence from a 3p–p* excited state.301 The photophysical properties of structurally similar diimine Pt(II)-complexes can be controlled Photochemistry, 2009, 37, 240–299 | 259 This journal is

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Fig. 5

using bulky coordinated thiolate and phenolate ligands.302 Steric hindrance of the cyclic diacetylide in [Pt(dbbpy)] yields unexpected high photophysical properties.303 [Pt(thpy)(aa)] (where ‘aa’ is a N,O-chelating amino acid monoanion such as phenylalanine) binds strongly to HAS causing an increase in luminescence intensity.304 The yellow luminescence from Pt(II)-complexes of the bis(pyrrolyl)-diimines is red-shifted because of the excimer or oligomer emission.305 [Pt(tpy-4 0 -NMe2)(CN)]+ is strongly luminescent due to a ligand-centred excited state based on the tpy-4 0 -NMe2 ligand.306 Coordinating perylene-diimide units bearing pendant pyrene chromophores can bind Pt(II); PEnT and PET among the components is accelerated in comparison to the free ligands.307 The excited state absorption properties of Pt(II) terpyridyl acetylide complexes have been investigated.308 The Pt(II) complexes [Pt(fppz)(m-pz)]2 [fppzH = 3-CF3-5-(2-pyridyl)pyrazole; Hpz = pyrazole or 3,5-Me2-pyrazole] display strong phosphorescence at low temperature from thin ﬁlms or CH2Cl2 solutions.309 Emission in [Pt(4-phenyl-terpy)(CN)]+ is from an excited state with 3MLCT character, while emission from [Pt(4-R-terpy)(CN)]+ complexes (R = o-CH3C6H4, o-ClC6H4 or o-CF3C6H4) is intraligand 3p–p*.310 The inﬂuence of cyclometallation on the excited states properties of Pt-complexes has been investigated. A number of luminescent cyclometallated Pt(II)-complexes of formula [Pt(ppy-4-styryl-R)(OCCR 0 CHCR 0 CO)] (R 0 = CH3, tert-butyl; R = H, OMe, NEt2, NO2) have been found to be weakly emissive in solution.311 Pt(II)-complexes containing 7,8-benzoquinolinate and various phosphine ligands are strongly phosphorescent as solids, depending on the nature of acetylide and phosphine co-ligands.312 In [Pt(thpy)(CO)(S-C6H4CO2Me)] (thpy = the cyclometallated monoanion of 2-thienyl-pyridine) the thpy 3p–p* luminescence is short-lived (o1 ns) because of a close-lying CT state involving the thiolate; replacement of the thiolate by chloride causes a much longer-lived luminescence.313 Protonation of the amine causes a reversible switch from a 3p–p* to an ILCT state in [Pt(NCN-C6H4NMe2)Cl] [NCN-C6H4NMe2 = the cyclometallating anion of 1,3-bis(2-pyridyl)benzene, with a substituent at the C5 position of the phenyl ring].314 Cyclometalation of trans-[PtCl2(Ph2C = NH)(RR 0 SO)] [R, R 0 = Me, Me; n-Pr, n-Pr; Me, Ph; Me, p-MeC6H4] yields a family of complexes that are emissive at room temperature and in the solid state.315 Some phosphorescent Pt(II)-complexes containing alkyl substituted 2-thienylpyridine cyclometallating ligands act as turn-on optical chemosensors.316 260 | Photochemistry, 2009, 37, 240–299 This journal is

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Short ﬂuorescence lifetimes and low quantum yields were observed for [Pt(PnBu3)2(CRCR)2] (R = thiophenealkynyl or phenylalkynyl) complexes due to a fast intersystem crossing to a triplet excited state.317 Dithiolate Pt-complexes show luminescence in the solid state or in solution, which is similarly to the behaviour of analogous 1,2-dithiolene complexes.318 Acetylene- and (buta-1,3-diyne)-Pt(0) complexes exhibit emissions at 380–550 nm due to the 3[P(dp*) Pt] transition.319 [Pt(dppe)(CCR)2] (R = Me, CCH, C6H4–CCH) show ﬂuorescence from a singlet excited state.320 The gas-phase photofragmentation of transbis(triﬂuoroacetato)bis(N,N 0 -dimethylethylenediamine)Ni( II) leads to mono- and di-imine species that remain coordinated to nickel.321 Some organic dye s-bonded metal complexes of Pd(0) display strong ﬂuorescence due to weak electronic interactions between the metal center and the dye-system.322 9.2

Multicomponent systems

Pt(II)-multicomponent systems also present interesting luminescent properties. The photophysical properties of trinuclear cyclometalated Pt(II)-complexes assembled by bis(diphenylphosphinomethyl)phenylphosphine depend on the nature of the oligophosphine; these complexes show strong red or near-infrared phosphorescence with a lifetime of microseconds.323 Luminescent dinuclear Pt(II)-terpyridyl complexes display a high tendency towards self-association, in which the two Pt terpy units are connected due to metal–metal and p–p interactions.324 Heteronuclear PtM2 (M = Cu, Ag, Au) clusters containing dithiolene, bipyridyl and diphosphine ligands show luminescence from an LMCT (S Pt) state.325 Phosphorescence from SCS and NCN pincer Pt(II)-complexes, which derive from 3,5-bis(anilinothiocarbonyl)toluene, originates from 3MLCT or 3(p*p) excited states.326 Luminescence from the trimetallic complex [{Mn(NCS)(bpy)2(m-SCN)}2Pt(SCN)2] originates from a d-d transition of the square-planar [Pt(SCN)4]2 moiety.327 Covalent linkage of [Pt(pybim)(Ph)2] groups [pybim = a 2-(2-pyridyl)benzimidazole N,N-chelate] to a central aromatic spacer produces polynuclear complexes displaying luminescence at low temperature.328 A new Pt-terpyridyl-based triad with covalently linked donor and acceptor has been prepared and characterized; its Pt-based luminescence is quenched by electron donation from a trimethoxybenzene group.329 The nature of the emissive state of polynuclear complexes containing multiple trans-{Pt(PR3)2(CCR)2} depends on the substituent; the direction of PEnT processes can be controlled.330 The ferrocenyl group of Pt(phen 0 )(CC-R)2] (phen 0 = a poly-substituted phenanthroline) (R = Fc or C6H4–CC–Fc) quenches the luminescence.331 Several Pt complexes formed by reaction of [Pt(CRCR)4]2 (R = alkyl or aryl) and [M2(m-dppm)2]2+ (M = Cu, Ag, Au) display decreasing emission energies with increasing donor ability of R, indicating that the emissive states originate from ligand-to-cluster [RCRC-PtM] transitions.332 Metallopolyenes of formula [M(C,RCRCRC)M 0 (CRCRCRC)]n (M = Hg, M 0 = {Pt(PBu3)2}, R = 9,90-dioctylﬂuorene) show opticalpower limiting responses with good transparency in the visible region.333 Photochemistry, 2009, 37, 240–299 | 261 This journal is

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Dinuclear [{PtTl2(CRCR)4}2] (R = 4-CH3–C6H4, 1-naphthyl) and polymeric [{PtTl2(CRC-C6H4-4-CF3)4}2] complexes show visible emission in solution due to Tl-p(alkynyl) interactions.334 The changes in the photophysical properties of a new series of m-pyrazolate-bridged cyclometalated platinum complexes can be related to the Pt–Pt distance.335 The luminescent species [{Pt(NC)(C6X5)2}Ag(PPh3)] (NC = 7,8-benzoquinolate) exhibit two structured emissions ascribed to 3MLCT and 3LC states.336 The luminescent cyclometalated complexes [Pt(L)Cl], [Pt2(L)2] and [Pt(L)(PPh3)]+ [HL = 2-phenyl-6-(1H-pyrazol-3-yl)pyridine] display diﬀerent luminescent quantum yield at room temperature.337 One-dimensional luminescent micro- and nano- wires formed from [Pt(CNtBu)2(CN)2] molecules with weak Pt–Pt interactions display intense green emission.338 Similar excited-states are responsible for the luminescence observed in organometallic polymers with the fragments [M2(dppm)2(CN–R–NC)] (M = Pt, Pd; R = aryl or alkyl).339 Increasing the chain length of oligomeric [(HCRCRCRC){Pt(dppe)(CRCRCRC)}nH] (R = phenyl, diphenyl; n = 1, 2, 3) causes a decrease of the optical band gap and an increase of the triplet-state emission intensity.340 The onedimensional polymer [{Pt(C6F5)4}2Tl]n shows photophysical properties that are strongly dependent on concentration, solvent and excitation wavelength.341 Deprotonation of polycarboxylic acids induces a self-assembly of terpyridyl-Pt(II)-complexes and triggers remarkable changes in their UV/Vis spectra and emission intensities as a consequence of the aggregation of the complexes.342 The electronic absorption and the emission properties derived from aggregate formation of Pt(II)-terpyridyl-alkynyl complexes were found to depend on the nature of the counter anions, suggesting their potential as colorimetric anion probes.343 The complex [Pt(L)(NHCOtBu)2] (L = 1,2-diaminocyclohexane) yields luminescent polymers upon reaction with Tl(I) ions.344 Pt(II) complexes containing derivatives of 8-hydroxyquinoline with pendant arms form organogels in various solvents; strong red phosphorescence from the Pt(II) units is favoured because the diﬀusional quenching by O2 is inhibited in the gels.345 New phosphorescent materials were obtained by coupling of trialkylgallate-functionalized alkynes to a planar Pt(II)-terpyridine unit; Pt–Pt interactions in these complexes lead to the formation of colored organogels and liquid crystals.346 The red-shift of the emission bands upon aggregation of [Pt(CNMe)2(CN)2]n (n = 1–4) is due to shortening of the Pt–Pt distance in the T1 state, which results in promotion of an electron from the s*[Pt(dz2)] to s[Pt(pz)] orbital.347 External binding of a Pt(II)-complex induces luminescence enhancement, whereas intercalation leads to luminescence quenching.348 Luminescence of [Pt(NNN)Cl]+ shifts towards blue and increases in intensity on binding to serum albumin, likely due to an interaction with hydrophobic pockets of the protein.349

9.3

Photoinduced reactivity and photocatalysis

Photoexcitation of [Pt(CRCPh)(4 0 -p-tolyl-terpy)]+ in the presence of a donor (triethanolamine), an acceptor (methyl viologen) and colloidal Pt leads to the generation of molecular hydrogen from water.350 The 262 | Photochemistry, 2009, 37, 240–299 This journal is

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photoinduced evolution of hydrogen with [(bpy)2Ru(m-bridge)PtCl2]2+ (bridge = tetraimine ligand) is connected to the extent of intramolecular quenching of the Ru-based 3MLCT excited state by Pt(II) and by steric eﬀects.351 Photoexcited Cp 0 Pt(CH3)3 (Cp 0 = Z5-C5H4CH3) reacts with Et3SiH with a high photochemical eﬃciency.352 Femtosecond pump-probe spectroscopy indicates that excitation of PtBr62 to excited 1T1g state is followed by a decay indicating that the photoaquation quantum yield is independent on the excitation wavelength.353 The luminescent complex Pt(dpphen)bis(arylacetylide) (dpphen = 4,7-diphenylphenanthroline and arylacetylide = 4-ethynylbenzaldehyde) has been employed in the synthesis of donor–chromophore imine-linked and amine-linked dyads; photolysis experiments reveal that both the imine and amine linkages are photochemically unstable, resulting in regeneration of the aldehyde-containing chromophore.354

10. 10.1

Copper Mononuclear complexes

Several emissive Cu(I)-bisphenanthrolines and Cu(I)-complexes containing N- and P-coordinating ligands have been investigated. Fig. 6 shows the tetrahedral coordination environment typical of Cu(I)-complexes. Distortion from ideal geometry of [Cu(I)(bfp)2]+ (bfp = 2,9-bis(triﬂuoromethyl)1,10-phenanthroline) aﬀects signiﬁcantly the luminescence, leading to longer emission wavelengths and shorter emission lifetimes.355 [Cu(pqx)(PPh3)2]+ [pqx = 2-(2 0 -pyridyl)quinoxaline] undergoes a large structural reorganisation in its MLCT excited state, as demonstrated by Raman spectroscopy and DFT calculations.356 A Cu-complex containing phenanthroline ligands with 3,5-di-tert-butyl-4-methoxyphenyl and 2,4,6trimethylphenyl substituents shows unusually strong emission at 77 K, because the sterically protected coordination environment prevents the formation of nonemissive pentacoordinated excited species.357 The 3 MLCT lifetime and the emission intensity of [Cu(2,9-Me-phen)(PP)]+ (PP = diphosphine ligand) complexes increase as the bulkiness of the

Fig. 6

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diphosphine ligand increases.358 The complexes [Cu(PPP)X] [PPP = the tripodal triphosphine (Ph2PCH2)3CMe; X = Br, I, PhS, phenylacetylide] show strong blue phosphorescence in the solid state; the greater ﬂexibility in solution allows more eﬃcient vibrational quenching with consequent weaker luminescence.359 The [Cu(NN)2]+ (NN = 2,9-dimethyl- or 2,9diphenyl-1,10-phenanthroline) complexes undergo slow (10–15 ps) ISC from the 1MLCT state to the 3MLCT state and fast (about 80 fs) structural rearrangement.360 Doping of mononuclear Cu(II)-complexes into polymer ﬁlms causes MLCT emissions with long decay lifetimes at 77 K.361 The analysis of the Cu(I)- and Re(I)-complexes [Cu(PPh3)2(dppz-11-COOEt)]BF4, [Cu(PPh3)2(dppz-11-Br)]BF4, [Re(CO)3Cl(dppz-11-COOEt)] and [Re(CO)3Cl(dppz-11-Br)] (dppz-11-COOEt = dipyrido-[3,2a:2 0 ,3 0 c]phenazine-11-carboxylic ethyl ester, dppz-11-Br = 11-bromo-dipyrido[3,2a:2 0 ,3 0 c]-phenazine) reveals that the dominant chromophore for the complexes measured at 356 nm is ligand-centered, except for [Re(CO)3Cl(dppz-11-Br)], which appears to have additional chromophores at this wavelength.362 Reduction of the complex [Re(L)(CO)3Cl] (L = ethyl dipyrido[3,2-a:2 0 ,3 0 -c]phenazine-11-carboxylate or 11-bromodipyrido[3,2-a:2 0 ,3 0 -c]phenazine) causes structural changes across the entire dppz ligand; the unusually long-lived excited states of the copper complexes result from metal-to-ligand charge transfer transitions.363 Cu(II)-complexes have been also investigated as ﬂuorescent sensors for NO. Reduction by NO causes ﬂuorescence increase of Cu(II) with anthrancenyl and dansyl based ﬂuorophore ligands.364 Cu(II) coordination by p-conjugated polymers containing bipyridyl or terpyridyl substituents induces ﬂuorescence quenching of the polymer emission; however, a strong increasing of the metallopolymer emission is observed as a consequence of Cu(II) reduction by NO.365,366 10.2

Multicomponent systems

A variety of supramolecular arrays and metal-clusters based on the use of Copper exhibit interesting luminescent properties. Complexes containing [Cu(pbim)(PPh3)2]+ units [pbim = 2-(2-benzimidazolyl)pyridine] bound to a central spacer show long lived phosphorescence at 77 K.367 Cu(I)–Cu(I) and Cu(I)–Ru(II) dinuclear complexes bridged by the 2,5-bppz (2,5-bis(2-pyridyl)pyrazine) ligand show photoluminescence in the solid state, which should arise from MLCT states.368 The halide-bridged dimers [Cu2(m-X)2(PPh3)L] (L = a monodentate or bidentate N-donor heterocycle) show MLCT luminescence, whose energy depends on the reduction potential of L.369 Addition of Cu(I) to the random-coil peptide, C16C19-GGY, produces a self-organized, metal-bridged 4-helix bundle; it displays an intense room-temperature luminescence at 600 nm, which is likely due to Cu4S4 cluster.370 In [{Cu(AsPh3)2}(m-2,2 0 -bipyrimidine)]2+ the intramolecular aromatic stacking between phenyl rings from the AsPh3 ligands and the central bpym ligand may inﬂuence the luminescence properties.371 A Cu-complex with a diamond-shaped Cu2N2 core displays very intense blue luminescence due to the steric protection of the metal by the ligand substituents.372 Self-assembly of nanometer scale [Cu24I10L12]14+ (L = triazole 264 | Photochemistry, 2009, 37, 240–299 This journal is

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based N-heterocyclic ligand) cages with Keggin anions, [PMoV2MoVI10O40]5 exhibits photoluminescence at room temperature attributed to a mixture of I–Cu charge transfer and d–s transitions due to Cu–Cu interactions.373 Multicomponent systems based on the use of Cu-complexes may also undergo PET processes. An array made of a bis-Cu(I) helicate core and two peripheral fullerene subunits has been prepared; it undergoes an electron transfer from the photoexcited Cu(I)-complexed unit to C60.374 PET from the central metal-complexed unit to the external fullerenes of two fullerenesubstituted m-phenylene-bis-phenanthroline ligands occurs by excitation of both moieties.375 Ternary Cu(II)-complexes containing a diimine ligand and an additional ligand [bidentate lysine,376 tridentate PhCH2CH2N (CH2CH2-2-pyridyl)2,377 or a tris(pyrazolyl)borate378] bind to DNA and cause its photocleavage and 1O2 generation. The Cu-doped tungsten bronze type potassium niobate K2Nb4O11 shows high photocatalytic activity for the degradation of acid red G.379

11. 11.1

Silver, gold Silver

The halogeno(cyano)argentites [Ag5(CN)Br6]2+ and [Ag2(CN)2Cl]+ show diﬀerent photophysical properties; the ﬁrst one exhibits a strong blue ﬂuorescent emission at 450 nm, the second one is characterized by a weak band at 390 nm.380 Strong emission is observed in the solid-state from Ag cyano coordination polymers; this emission originates from an MLCT transition (Ag-bpy) or from a coupled MLCT (Ag CN) and metal centered transition.381 The complex [Ag2(L2)(ClO4)2] (L = 4,5-diazospirobiﬂuorene) displays luminescence properties in a single crystal.382 Ag(I) and Au(I) complexes of a chelating N,C-donor (pyridyl/carbene) ligand have metal–metal contacts showing strong blue luminescence.383 Schiﬀ-base ligands are characterized by intense luminescence when coordinated to Ag(I) centers, due to their conformational rigidity that reduces nonradiative decay of the intraligand 1(p–p*) excited state.384

11.2

Gold

A great part of articles on Au-compounds deals with their luminescent properties. Au–Au interactions among Au(I) centers arranged in a planar rhomboidal array in tetranuclear Au(I) alkynylcalix[4]crown-6 complexes give rise to long-lived excited states and high luminescence quantum yields.385 Au(I) and a calixarene containing ethynyl groups form tetranuclear clusters; they show strong luminescence at lower energy than that of previously investigated dinuclear Au(I) systems.386 An Au(I) square metallamacrocyclic complex with 4,4 0 -bipyridyl and anthracenyl-9,10(PPh2)2 bridging ligands shows anthracene-based luminescence; this can be quenched by a variety of organic substrates.387 The heterometallic clusters [Au5Ag8(m-dppm)4(CRC–C6H4R)7], containing phosphine and aryl-acetylide ligands, display intense red luminescence from an excited Photochemistry, 2009, 37, 240–299 | 265 This journal is

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state with MLCT and cluster-centered character.388 AgAu3 clusters [Ag3(m3-E)Au(Ph2Ppy)3]3+ (Ph2Ppy = a substituted 2-diphenylphosphinopyridine; E = O, S, Se) show very strong blue (E = O), yellow (E = S) or orange (E = Se) luminescence with lifetimes of many microseconds.389 [Au(PPh3)An] and [{Au(An)2}(m-PP)] complexes (PP = a bridging diphosphine dppm; An = 9-anthracenyl) show anthracene-based luminescence inﬂuenced by coordination to Au(I).390 [AuCl(PPh2phen)] (PPh2phen = 9-diphenylphosphinophenanthrene) shows blue-green phosphorescence from the phenanthrene triplet because of the heavy-atom eﬀect of Au.391 Stable mesomorphic tetraﬂuorophenyl-Au(I) biphenylisocyanide complexes display strong luminescence in the smectic C mesophase, in the solid state, and in solution; in the solid state these complexes show good mesophase stabilization without Au–Au interactions and short intermolecular ﬂuorine–ﬂuorine interactions.392 The dinuclear complex {[Tl(Z6-toluene)][Au(C6Cl5)2]} shows intense blue phosphorescence (3MMCT) in the solid state.393 Dinuclear Au(I)-phosphine complexes exhibit rich luminescence properties associated with their alkynyl triplet states.394,395 [Au2{(Ph2Sb)2O}3]2+ shows a large Stokes shift for its phosphorescence because of a structural distortion of the excited state.396 A chiral Au16 ring displays intense green phosphorescence.397 Cyclic Au trimers are characterized by solid-state luminescence, which is strongly related to the Au–Au interactions.398 However, these interactions in a series of Au(I) thiolates do not aﬀect signiﬁcantly emission energy and excitation maxima.399 Cyclic Au(I) triazolate trimers form aurophilically bonded dimmer of trimers that give phosphorescence bands both in the solid state and in solution.400 Luminescence from the di-Au(I)-complexes (R3P)Au-CC-(th)n-CC-Au(PR3) (th = thiophene2,5-diyl; n = 1, 2, 3) likely originates from a singlet excited state of the bridging group.401 A series of luminescent cyclometalated alkynyl-Au(III)complexes shows an absorption band at about 400 nm mainly originated from a LC transition of the cyclometalated ligand.402 The pentanuclear Au(I)–Cu(I) and Au(I)–Ag(I) complexes [nBu4N][Au3M2(CRC–C6H4-p-R)6] (M = Cu, Ag; R = alkoxy group) display emission from [(CRC–C6H4-p-R) Au3M2] or [(AuCRC–C6H4-p-R) Au3M2] charge-transfer excited states.403 The emission energy of the clusters [E(AuL)3M]2+ [M = Cu, Ag; E = O, S, Se; L = PPh2-py, PPh2(CH2)2-py] depend on the nature of E and is inﬂuenced by the chalcogenide.404 Some Au-complexes have been investigated for the reactivity of their excited states. It has been demonstrated that photolysis of LAuICl (L = RNC or CO) leads to formation of free L, AuIII, and Au0.405 Activation of the Pt(III)–Au(II) bond is achieved by irradiation of [Pt(III)Au(II)(dppm)2PhCl3]PF6 with visible light.406

12.

Zinc, cadmium, mercury

Four-coordinate ZnL2 (HL = N,O-chelating 2-(2-hydroxyphenyl)benzimidazole and substituted derivatives) show strong blue ligand-centered luminescence.407 The weak luminescence of macrocyclic Robson-type 266 | Photochemistry, 2009, 37, 240–299 This journal is

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Schiﬀ-base ligands is blue-shifted and increased in intensity as a consequence of coordination to Zn(II).408 Zn(II)-complexes of 4,4 0 -diphenyl-6,6 0 dimethyl-2,2 0 -bipyrimidine (pmbp) show blue luminescence both in the solid state and in solution, which is originated from a ligand-centered excited state.409 Complexes of Zn(II) with a phenanthroline-based N-donor macrocycle containing an anthracenyl group are characterized by an exciplex emission at 550 nm.410 The number of phenylene vinylene units in the pyrene-oligo(phenylene vinylene)-2,2-bipyridine molecular rods controls the ﬂuorescence emission of the complexes with Zn(II).411 Zn(II) diimine bis(thiolate) complexes with photochromic diarylethene-containing phenanthroline ligands display strong phosphorescence in the solid state.412 Supramolecular Cd(II) dicarboxylate networks show strong emission, depending on the solid-state coordination mode.413 The emission energy of a porous metal–organic framework based on the cluster [Cd3L12] [L = 2,6-di(4-triazolyl)pyridine] can be tuned in the UV-Vis range by controlling the number of guest molecules.414 Tetranuclear Cd(II) clusters containing a salen-type N,N,O,O-donor Schiﬀ-base ligand show strong blue ligand-centered phosphorescence at low temperature.415 Signal ratio ampliﬁcation occurs through modulation of resonance energy transfer of a dithiazacrown-substituted benzaldehyde cation receptor connected to a boradiazaindacene dyad; it displays a 35-fold change in emission ratio upon addition of Hg(II) ions.416 Fluorescein based ﬂuorescent sensors with pyridyl-amine-thiol groups selectively bind Hg(II) ions and are able to detect traces of Hg(II) in aqueous solution.417 13. 13.1

Lanthanides Luminescent lanthanide complexes

The development of luminescent systems based on the use of lanthanide complexes has attracted a great deal of interest, since these compounds are characterized by peculiar long-lived luminescence in the visible and near infrared spectral regions. The complex [Ln(trp)4] (trp = tropolonate anion, an O,O-donor chelate) shows near-infrared luminescence when Ln = Nd, Er, Yb, Ho, Tm, because the tropolonate ligand acts as energy-donor for low-lying f–f states.418 Ln(idp)3 complexes [idp = the imidi-diphosphinate ligand Ph2P(O)-(N)-PPh2(O)], in which several phenyl substituents avoid solvent interaction with the metal, give unusually long near-IR luminescence lifetimes.419 Perﬂuorination of the phenyl rings to give Ln(Fidp)3 [Fidp = (C6F5)2P(O)-(N)-P(C6F5)2(O)] increases signiﬁcantly the luminescence lifetimes, due to the removal of CH oscillators from the proximity of the metal ion.420 The triplet energies of pyrazolone ligands in Eu complexes can be tuned by changing the substituents to optimise the ligand-to-metal PEnT processes.421 The green luminescence of Tb(III) can be sensitized by guanidinates [RN = C(NR2)-NR].422 Isoxazolone ligands are eﬀective sensitisers of Eu(III) luminescence.423 An Er-quinolinolate trimeric cluster displays single-exponential luminescence from Er(III).424 Eight- and nine-coordinated Eu(III)- and Tb(III)-complexes with carbonyl group have been prepared using the monodentate ligand Photochemistry, 2009, 37, 240–299 | 267 This journal is

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2,4-diamino-6-hydroxy pyrimidine; they are water-soluble and exhibit luminescence in the solid state as well as in aqueous and methanolic solutions.425 Circular dichroism has been detected using luminescence spectroscopy for the chiral complex [Eu(dipicolinate)3]3 with a D3 symmetry.426 An optically-pure helical Eu complex shows circularlypolarised luminescence from the Eu(III).427 The eﬀect of the inductive properties of the ligand substituents on the emission quantum yields was investigated in a series of complexes [Ln(acac)3(NN)] (NN = a substituted bpy or phen derivative; Ln = Eu, Tb, Er, Yb),428 It was found that some Pr(III) complexes of poly(pyrazolyl)borate ligands show emission from two diﬀerent f–f levels.429 Two-photon excitation using red or IR laser pulses generates luminescence from Tb(III) and Eu(III) complexes.430,431 An Eu(III)-chelate-based phosphorescence probe speciﬁc for 1O2432 and a rigid assembly of four bidentate chromophores in water-stable highly luminescent lanthanide complexes have been synthesized and characterized.433 The room-temperature luminescence of tris(heterocyclic b-diketonato)Eu(III)-complexes of general formula Eu(PBI)3 L, where HPBI = 3-phenyl4-benzoyl-5-isoxazolone and L = H2O, bpy, 4,40 -dimethoxy-2,20 -bipyridine, phen, 4,7-diphenyl-1,10-phenanthroline, are composed of the typical Eu3+ red emission; the results demonstrate that the substitution of solvent molecules with bidentate nitrogen ligands enhances the quantum yield and lifetime values.434 In three Eu-complexes, [Eu(PBI)3 3H2O], [Eu(PBI)3 2TOPO], and [Eu(PBI)3 2TPPO H2O] (TOPO, and TPPO are tri-n-octylphosphine oxide and triphenylphosphine oxide, respectively), with diﬀerent neutral ligands, the substitution of water molecules by TOPO leads to a greatly enhanced quantum eﬃciency.435 Some Nd(III)- and Er(III)-complexes with 1-(9-anthryl)-4,4,4-triﬂuoro-1,3-butandione produces sensitised near-infrared luminescence as a consequence of anthracene excitation.436 The relationship between the energy of ligand triplet state and the photophysical properties of a series of NIR emitting Nd(III) complexes containing b-diketonate ligands has been discussed.437 A stable and highly luminescent Eu(III)-complex has a very intense 5D0 7F2 transition and, consequently, a high quantum yield (f = 0.215).438 The cocrystalline complex [Yb0.5Er0.5(OO)3(O = PPh3)2] (OO = hexaﬂuoroacetylacetonate) shows sensitised NIR Er-emission with intermolecular energy transfer from the Yb-complex.439 Clusters containing nine lanthanide and six sodium ions are crystallized with the help of a tridentate ligand bearing a phosphonic acid group; the structure of the Eu(III) compound reveals a host–guest assembly, which exhibits luminescence with a good quantum yield.440 Polyaryl dendrimers with a 4-phenylacetyl-5-pyrazolone-based Tb(III)-complex as core have overall quantum yields that increases as the dendritic generation increases.441 Binuclear frameworks [Er2(BDC)(DMF)n] DMFn (BDC = terephthalate, n = 0; BDC = perﬂuoroterephthalate, n = 1) exhibit Er(III) emission that is signiﬁcantly less intense for the non-ﬂuorinated analogue, due to the quenching eﬀect of the vibrational C–H bond.442 There have been a number of studies on sensitised Ln(III) ion emission induced byvarious sensitisers based on carbazole,443 ferrocene,444 tartaric acid,445 tetradentate 1-hydroxypyridin-2-one derivatives,446 268 | Photochemistry, 2009, 37, 240–299 This journal is

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8-hydroxy-5-nitroquinoline.447 Near-IR emission in lanthanide tris 2-carboxamide-8-hydroxyquinoline complexes is signiﬁcantly enhanced by ligand disubstitution with bromine.448 Emission from 8-coordinate Ln(III) complexes (Ln = Nd, Gd, Er, Yb) of a 2-(2-pyridyl)benzimidazole unit containing an anthacenyl group can be sensitised by anthracene as a consequence of the fact that anthracene ﬂuorescence is quenched by intramolecular PET to the benzimidazole unit.449 Emission from some luminescent lanthanide cations [Sm(III), Eu(III), Tb(III), Dy(III)] can be sensitised by octadentate 2-hydroxyisophthalamide ligands.450 Two-photon absorption of pyridine dicarboxamide ligands sensitises emission from Eu(III).451 The lanthanide-organic array [LnLn(L)3(H2O)2](NN) (H2L = adipic acid; Ln = Tb; Ln = Tb, Eu; NN = 4,4-bpy) show Ln-emission sensitised by 4,4-bpy.452 Emission eﬃciency of self-assembled hexametallic rings of [Eu(6-CO2-terpy)2]+ incorporating a central lanthanide are similar to that of the monomeric derivative, indicating the absence of intramolecular quenching eﬀects.453 NIR luminescence of Er(III) in [Er1.4Yb0.6(benzoate)6(phen)2] is enhanced compared to the homodinuclear Er2 complex due to a Yb Er energy transfer process.454 Heterodimetallic bisporphyrins complexes (YbZn, YbPd and YbPt) display sensitised NIR emission with lifetimes and intensities increasing in the order YbZn o YbPd o YbPt.455

13.2

With multi-armed ligands

Multi-armed ligands have been often employed for optimising luminescence properties. A tetrapodal ligand with four 8-hydroxyquinolinate units forms stable 8-coordinate Ln(III)-complexes; they enable to induce NIR luminescence from Yb, Nd and Er in aqueous solutions.456 Yb(III)- and Er(III)- complexes of high-denticity ligands show sensitised luminescence on excitation of the aromatic donors.457 Ln(III)-complexes of a 9-dentate phen ligand show sensitised luminescence in water from all potentially luminescent members of the lanthanide series.458 Addition of [15]crown-5 to SmI2 in acetonitrile produces a sterically encumbered complex characterized by a reduction of its solvent induced luminescence quenching and by an increase of its excited state lifetime.459 The p-tert-butylsulfonylcalix[4]arene ligand acts as a good antenna chromophore for UV light; the sensitised emission from Eu(III)- and Tb(III)-complexes depends on the energy of the ligand excited states, which, in turn, can be controlled by conformational changes of the calixarene.460 Also calix[4]azacrowns capped with aminopolyamide bridges act as antenna chromophores for energy-transfer sensitisation of luminescent excited states of Ln(III).461 Ln-complexes based on the macrocycle ‘cyclen’ with three carboxylate arms and one additional functional group attached to the fourth site (DO3A ligands) have been extensively investigated. One of this complex containing a triazolonaphthalazine chromophore is able to sensitise luminescence from Nd, Er, Yb and Eu.462 In another complex of this type, deprotonation of a pendant carboxylic residue results in a reversible coordination of the carboxylate to another metal centre; this causes water expulsion from Eu or Tb so increasing the luminescence intensity.463 Photochemistry, 2009, 37, 240–299 | 269 This journal is

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Emission intensity of Tb/DO3A derivatives with a pendant azacrown macrocycle can be controlled by binding Na+ or K+ in the macrocycle.464 Eu-based luminescence of a Eu/DO3A complex with a binaphthyl group as chromophore is enhanced when the hydrophobic environment around the binaphthyl group lowers its 3p–p* energy close to the f–f state of Eu(III).465 The interaction between Ln-DO3A complexes and chromospheres bearing phosphonates or carboxylates was investigate in order to clarify the eﬀect of sensitising chromospheres on NIR emission.466 The dinuclear Tb(III)-complex of a p-xylene-bridged DO3A derivative is characterized by an enhancement of the Tb(III) emission after titration of the mono- and di-carboxylate anions.467 Nd(III) and Yb(III) complexes of asymmetrically substituted DO3A derivatives having an antenna (rhodamine B) and a quencher (nucleoside) attached to the cyclen scaﬀold have been prepared; chemical switching of the Ln-based emission is observed upon addition of adenosine or uridine to the nucleoside.468 A Eu(III)/DO3A complex functionalised with a tris-pyridyl-amine acts as a sensor for Zn(II).469 13.3

Luminescent lanthanide complexes in d-block transition-metal systems

Excitation of a lanthanide ion may be achieved with a sensitising transition metal chromophores (antenna) through an intramolecular PEnT process (Fig. 7). Simple derivatives of [M(bpy)3]2+ (M = Ru, Os) and [Re(bpy)(CO)3Cl] form ternary complexes with seven-coordinated lanthanide centers; the Nd-, Yb-, and Er-containing derivatives show Ln-centered emissions sensitized by the MLCT states of the d-block components.470 Attachment of {Ln(diketonate)3} units to [ReCl(CO)3(bpym)] and [Pt(CC–C6H4–CF3)2(bpym)] results in NIR luminescence from Yb, Nd or Er.471 In a molecular square where alternating {Ru(bpy)2}2+ and {Nd(tta)3} corners (Htta = thenoyltriﬂuoro-acetylacetone) are connected by 4,4 0 -bipyridyl bridges, the 3MLCT emission of Ru(II) is quenched by energy-transfer to the lanthanide that, in turn, presents near-infrared luminescence.472 Covalent binding of [Re(bpy)(CO)3Cl] or [M(bpy)3]2+ units (M = Ru, Os) to Ln(III)/dtpa complexes (Ln = Nd, Er, Yb) results in PEnT processes and NIR lanthanide luminescence; this depends on the metal–metal distances in the dyads.473 Mono- and C3-symmetric, tris(ligand) complexes of Sm(III), Y(III) and Eu(III) with sulfur-bridged binaphtholate ligands have been synthesised and characterised from the photophysical point of view.474 [Ru(bpy)3]2+ derivatives with pendant calixarene groups bind Ln(III); the Ru–bpy emission is quenched, so resulting in sensitised lanthanide luminescence.475 Yb-based luminescence was investigated from the tetranuclear cluster [Zn2Yb2L2(m-OH)2Cl2] (H2L = a compartmental Schiﬀ-base ‘salen-type’

Fig. 7

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dinucleating ligand).476 Cyanometallate luminophores such as [M(CN)2] (M = Ag, Au)477 or [Ru(bpy)(CN)4]2–478 have been used to form cyanidebridged coordination networks with Ln(III); in the second case Ru Ln PEnT (Ln = Nd, Pr, Er, Yb) leads to the quenching of the Ru-based emission and to the sensitisation of the Ln-luminescence. The long-lived excited states of Ru(II) and Cr(III) generate sensitized near-IR lanthanide emission with lifetimes between ms and ms in a series of heterodinuclear helicates containing the d-block Cr(III) or Ru(II) ions and Ln is Yb, Nd or Er.479 Supramolecular self-assemblies made of Nd- and Yb-based tetraamidefunctionalized cyclen complexes bearing a single 1,10-phenanthroline moiety coordinating to a RuII(bipy)2 unit have been prepared; excitation of the Ru(II) MLCT band in water gave rise to long-wavelength sensitized emission from the Yb(III) or Nd(III) centers.480 In an analogous manner, other mixed f–d self-assemblies have been developed and characterized from the photophysical point of view.481,482 Luminescent [Pt2(m-dppm)2(CRC–NN)4] (NN = bpy, phen) were used for preparing a series of Pt2Ln2 and Pt2Ln4 arrays, in which MLCT excitation induces sensitisation of Ln-luminescence.483 Interpenetration of a Nd(III) b-diketonates imidazole thread with a macrocycle containing a Re(I)-bpy to form a pseudorotaxane complex, is demonstrated by sensitised NIR lanthanide emission.484 Heterotrimetallic helicates [M-Ln-M] (M = Cr, Zn; Ln = Eu, Tb) were prepared using segmental bidentate-tridentate-bidentate benzimidazoyl pyridine ligands; for M = Zn, only Ln-centred emission was observed; Cr(III)-complexes displays Ln Cr PEnT.485 The complexes 2n [{Ru(CN)4}n(mn-HAT)] (n = 1, 2, 3; HAT = hexaaza-triphenylene) show strongly solvatochromic and intense MLCT absorptions.486 Covalently linked polynuclear systems containing Pt(II) chromophores, with [Pt(bpy)(diacetylide)] cores and 4-pyridyl or phen units attached to {Ln(b-diketonate)3} fragments, show lanthanide based sensitised emission as a consequence of Pt Ln PEnT.487 The diimine binding site of the luminescent complexes [Re(CO)3Cl(bppz)] and [Pt(CRC–C6H4CF3)2(bppz)] [bppz = 2,3-bis(2-pyridyl)pyrazine] were used to bind {Ln(diketonate)3} fragments and sensitise near-IR emission.488 14.

Miscellaneous transition metal compounds

Photosensitive uranyl ions anchored onto MCM-41 mesoporous molecular sieves serve as remarkable photocatalysts in the degradation of alcohols, under ambient conditions.489 Photolysis of UO2(tBu4-salphen)(THF) in the presence of cobaltocene in THF yields [Cp2Co][UO2(tBu4-salphen)(OH)] in high yield; no photoreaction occurs in toluene.490 Photoexcitation of a mixture of 1,1,2,2-tetraphenylethane and di-n-butylzirconium diethoxide (1:2 ratio) in THF at room temperature leads quantitatively to the formation of Ph2CH–Zr(OEt)2–CHPh2, which with H2O (D2O) provides Ph2CH2 (Ph2CHD).491 Reactive decay of the excited states of TpTiCl3 (Tp = hydrotris(pyrazol-1-yl)borate) and Tp*TiCl3 (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) produces metal center Ti(III) radicals via homolytic cleavage of the Tp–Ti (Tp*–Ti) bond.492 Photochemistry, 2009, 37, 240–299 | 271 This journal is

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Fig. 8

15.

Metalloporphyrin and analogous complexes

A great number of metalloporphyrins have been investigated from the photophysical and photochemical points of view (see in Fig. 8 the structure of the porphyrin ring). In fact these compounds are characterized by several peculiar features such as rigid planar geometry, good stability, tuneable optical and redox properties. Metalloporphyrins are often employed as subunits of photoactive multicomponent arrays, where they can play a major role as active chromophores. 15.1

Mononuclear complexes

The photophysical properties of metalloporphyrin complexes have been reported in several articles. The presence of a non-emissive state has been observed in Au(III)-porphyrins, whose energy lies between the 1p–p* and the 3 p–p* states; this dark state may control the formation of the lowest triplet state.493 A TiIV(4-sulfonatophenyl)porphyrin shows ﬂuorescence from diﬀerent excited states both in water and methanol.494 The S2- and S1-state properties of Zn(II)-porphyrins incorporated inside polypeptides depend on the polymerisation, which can aﬀect the distance between the complexes.495 Photoexcitation of the dimethylaminophenyl substituents in the Zn(II)tetrakis-(p-Me2N–C6H4)-porphyrin results in PEnT to the Zn-porphyrin core and sensitised luminescence.496 The photophysical properties of excited states of Zn(II) porphyrins have been investigated using steady-state absorption, ﬂuorescence spectrometry and time-dependent DFT calculations.497 Zn(II)-tetrakis{4-(N-methyl)pyridinium}porphyrin acts as a chiroptical probe to distinguish between the B and Z forms of DNA.498 Binding of halide ions to Sn(IV)-complexes of N-confused porphyrins and oxoporphyrins results in a signiﬁcant ﬂuorescence enhancement, so indicating that these complexes are of interest as ﬂuorescent halide sensors.499 An azulene fused Ni(II)-porphyrin presents a very eﬀective p-electron conjugation pathway throughout the porphyrin moiety, with intense near-IR absorption and large two-photon absorption cross-section.500 Benzoannulation and octabutoxy substitution modify the deactivation mechanism of the S1(p,p*) state of Ni(II)-phthalocyanines, inducing signiﬁcant changes in the relative energies of excited states.501 Spectroscopic studies of a series of tetrasubstituted thiol oxo-Ti-phthalocyanines show low ﬂuorescence yields and relatively long triplet lifetimes.502 The radical anion and radical 272 | Photochemistry, 2009, 37, 240–299 This journal is

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cation of 2,7,12,17-tetraphenylporphycene and of its Pd(II)-complex have been produced by photoirradiation; both these radicals absorb at longer wavelengths than the triplet states.503 Adsorption of Pt–octaethylporphyrin in a polystyrene ﬁlm onto Ag surfaces gives a strong enhancement of the phosphorescence.504

15.2

Multicomponent systems

15.2.1 With other porphyrins. Energy-transfer between ZnTPP and H2TPP units has been found to occur in the time-scale of tens of picoseconds depending on the covalent linker.505 Singlet–singlet ZnP H2P PEnT through H-bonded bridges occurs in oligomers with pendant carboxylate and amidinium units; this process occurs faster than predicted by the Fo¨rster mechanism.506 The conformation of an array of four Zn-porphyrins connected to a central H2-porphyrin unit by nucleoside-containing linkers aﬀects the PEnT process.507 In other dyads containing both Zn-porphyrins and free-base porphyrin units, PEnT is controlled by both the distance between the components and the nature of the linking groups.508–510 Experimental and theoretical methods show that singlet energy transfer in a system based on the use of a Zn(II)-porphyrin as donor, p-phenyleneethynylene as bridge and a free porphyrin as acceptor occurs by both Forster and Dexter mechanisms.511 The 3MLCT excited state of [Ru(terpy)(bpy)]+ fragments attached to a tetra(pyridyl)porphyrin is quenched by electron transfer to the porphyrin at room temperature; this process does not occur at low temperature, so allowing emission from the Ru-unit.512 Singlet–singlet energy transfer rates have been evaluated for a series of cofacial dyads having a Zn(II)-porphyrin acceptor and corrole or porphyrin free base donor with a relatively ﬂexible spacers.513 Optical separation, excitation coupling and eﬃcient excitation energy migration have been investigated in porphyrin boxes constructed by homochiral self-sorting assembly.514 Photoinduced processes have been investigated in a [2]-catenane containing a Zn(II)-porphyrin and an Au(III)-porphyrin; upon excitation of the Zn-porphyrin, a PEnT process from the Zn-unit to the Cu-complex is followed by an electron transfer process to the Au–porphyrin, producing a long-lived (10 ms) charge-separated state.515 In a Zn-porphyrin-anthraceneAu–porphyrin triad, PET can be either direct or through a multistep mechanism depending on the solvent polarity.516 A dyad containing Zn(II)-phthalocyanine and boron sub-phthalocyanine units can show either ZnPc subPc PET or subPc ZnPc PEnT, depending on the nature of substituents on the macrocycle cores.517 Singlet–singlet and triplet–triplet PEnT was observed in a nonameric porphyrin assembly from eight peripheral Zn-porphyrins to a central free base, which is connected by nucleosidic linkers.518 Macrocyclic arrays of m-bis(ethylene)phenylenelinked Zn(II)-imidazolyl-porphyrins are characerized by shorter excitation energy hopping times for the hexagonal array in comparison with the pentagonal arrangement, due to a better orientation of transition dipoles.519 The photophysical properties of ethyne-bridged Zn(II)-porphyrin/ Fe(III)-porphyrin complexes have been studied in detail; unusually long Photochemistry, 2009, 37, 240–299 | 273 This journal is

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excited state lifetimes were observed for Fe-porphyrin pentacoordinate complexes.520 PET and PEnT processes were investigated by ultrafast absorption spectroscopy in cofacial Zn(II)-porphyrin/Zn(II)-phthalocyanine dimers; long-lived charge-separated states were obtained with a high eﬃciency.521 Giant porphyrin wheels comprising of six meso-linked tetra-Zn(II)porphyrins shows an eﬃcient excitation energy hopping rate around the wheel.522 A triad based on the use of two free-base porphyrins linked by a [Ru(terpy)2]2+ bridge displays PEnT from the Ru-3MLCT state to the lowest energy p–p* triplet of the porphyrins.523 [Ru(bpy)2(1,2-benzosemiquinone radical anion)]+ units quench the luminescence of the covalently attached ZnTPP by providing additional vibrational modes for nonradiative quenching.524 Ultrafast charge separation in a system containing Mg- and Zn-porphyrins, covalently linked via an amide bond to [Re(bpy)(CO)3] units occurs upon excitation of the porphyrin, with the formation of chargeseparated state.525 A Zn-chlorophyll tetramer exhibits ultrafast energy transfer between porphyrins signiﬁcantly faster than that of the selfassembled porphyrin tetramers.526 The kinetics of charge recombination and charge separation was investigated in a series of arrays containing a Zn(II)-porphyrin donor, a phenyl bridge and Au(I) or Fe(III) acceptors.527 Fluorescence from a Zn(TPP) is quenched by intramolecular PET to Eu(III)-bis(phthalocyaninato) complexes; this process is aﬀected by the connecting position and the number of porphyrin moieties.528 The cavity present in cofacial M-porphyrin dimers (M = Zn, Cu, Pd), covalently bound via their meso positions is able to trap O2, so improving the eﬃciency of phosphorescence quenching by this species.529 A Zn(II)-phthalocyanine with 4 pendant perylene-diimide units forms columnar heptameric stacks in solution; PEnT from these units to Zn(II)-phthalocyanine is followed by exciton hopping along the stack.530 PET was observed in single-wall carbon nanotubes bearing covalently linked Zn(II)-phthalocyanines.531 Timeresolved transient absorption and ﬂuorescence decays indicate that the eﬃciency of PEnT processes to give eﬀective light-harvesting systems of various hexaarylbenzene anchored polyester Zn(II)-porphyrin dendrimers is high for densely packed dendrimers.532

15.2.2 With inorganic units. A system based on a Zn-porphyrin and [M(tpy)2]2+ units (M = Ru, Os) linked by an alkynyl group shows broad near-infrared absorptions giving a highly delocalised long-lived excited state with some charge-separated character.533 A (m-alkoxo)bis(m-carboxylato)diruthenium complex quenches luminescence from covalently attached ZnTPP units, likely by PET.534 The electronic coupling between ferrocene and porphyrin controls the ﬂuorescence emission of Zn-imidazolyl-porphyrin dimers, functionalised with ferrocene.535 In nanomechanical butterﬂy-type devices, composed of a terpy scaﬀod and of two appended porphyrin moieties, the photophysical properties are reversibly modulated by cation coordination to the terpy unit that switches the geometry from an extended open W to a compact closed U form.536 A series of dyads consisting of a Zn- or Mg-porphyrin appended to a platinum terpyridine acetylide complex 274 | Photochemistry, 2009, 37, 240–299 This journal is

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via a para-phenylene bisacetylene spacer have been described;537 excitation of the porphyrin leads to a very rapid PET to the platinum complex followed by an ultra fast charge recombination; the results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor–acceptor distance. PET from ZnTPP to a tri-ruthenium cluster is mediated by the di-hexyl-viologen dication.538 PET processes that occur upon Soret excitation of Zn(II)-porphyrins meso-linked directly to ferrocene were studied by femtosecond up-conversion and pump-probe techniques.539 A perylene-diimide group binds to the axial positions of Ru(TPP)(CO) fragments to give an assembly whose photophysical properties depend on excitation wavelength.540 15.2.3 With organic units. Anthracene-Zn(TPP) PEnT is more eﬀective when the anthracene unit is stacked over the Zn(TPP) unit.541 In a Zn-porphyrin-calixarene system, addition of benzoquinone results in quenching of the Zn-porphyrin luminescence through direct p-stacking between the Zn-porphyrin and the quinone or by insertion of the benzoquinone into the calixarene cavity.542 The luminescence of the Zn-porphyrin units may provide information on the conformation of the attached calixarene.543 In a ﬂexible Zn-porphyrin/(methyl-viologen)2+ dyad, PET gives a {ZnP}d+/(MV)d+ pair, in which the positively charged units move apart, so inhibiting the back electron-transfer process.544 Photoexcitation of ZnTPP surrounded by four perylene-3,4:9,10-bis(dicarboximide) chromophores (PDI) causes the quantitative formation of [ZnTPP+–PDI], which recombines to produce the lowest triplet state of PDI.545 A bis Zn-porphyrin tweezer, with anthracene units as apex and side-arms, works as a host for a series of dipyridylporphyrin guests; PEnT from the host to the guest occurs with high eﬃciency.546 15.2.4 With fullerene. Artiﬁcial systems mimicking natural photoinduced processes are the subject of continuing research activity. In this framework, PET between Zn-porphyrins and C60 continues to be a topic of great interest. A ‘‘highlight’’ article reports that, in addition to the conventional approach of utilizing the cascade eﬀect, charge separation can be stabilized in artiﬁcial photosynthetic systems simply by the geometry.547 A meso,meso-linked porphyrin trimer, (ZnP)3 has been incorporated into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C60) as an electron acceptor; irradiation of this molecular array (Fc–(ZnP)3–C60) results in a PET from the porphyrin trimer to C60 to produce the intermediate Fc–(ZnP)3+–C60 which, ﬁnally, produces the charge-separated state Fc+–(ZnP)3–C60; the lifetime of this species is in the timescale of seconds.548 In a Zn-porphyrin-C60 dyad, the charge-separated Zn-porphyrin+-C60 state is generated quickly (about 10 ps), but the back electron-transfer is two orders of magnitude slower and depends on the molecular conformation.549. The separated-charge in the previous dyad may collapse either to the ground state or to a C60-based triplet excited state, depending on the nature of the solvent.550 The reaction medium also aﬀects Photochemistry, 2009, 37, 240–299 | 275 This journal is

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the nature of the oxidation process (metal vs. porphyrin centered) in an analogous Co(II)-porphyrin-C60 dyad.551 A triad assembled by ligation of an imidazole-appended C60 derivative to a ZnTPP-(boron-dipyrrin) dyad acts as a model for the antenna group of the photosynthetic reaction centre.552 Photoexcitation of other triads containing ZnTPP-Cu(phen)2+-C60 components connected in a rotaxane leads to (ZnTPP)d+/C60d chargeseparated states with lifetimes up to 30 ms.553 In analogous Zn(TPP)/C60 dyads, in which the components are connected via hydrogen bonding554 or covalent linkages,555 very long lifetimes of the charge-separated Zn(TPP)+/C60 states are obtained. Zn(TPP)/C60 dyads, in which the C60 has a pyridyl group for binding the metal centre, have well-deﬁned distances and orientations between the components; addition of pyridine to separate the components results in longer charge-separated lifetimes.556 An amphiphilic Zn-phthalocyanine-C60 conjugate can self-assemble into 1-D nanotubules, which show very long charge-separated states.557 Eﬃcient photoinduced charge separation (f = 0.97) in a dyad consisting of a Zn(II)–N-confused porphyrin donor, and fullerene acceptor, has been obtained.558 Excitation of the Mg-porphyrin unit in a Mg-porphyrin-C60 dyad leads to the MgP+/C60d state, whose lifetime can be up to 520 ns because of a relatively slow back electron-transfer process.559 In a fullerene with two pendant pyridyl substituents bound to a bis-Zn(porphyrin) receptor, the Zn-porphyrin luminescence is quenched by PEnT to generate *1C60.560 Modelling studies indicate that the rate of PET in a conformationally ﬂexible metalloporphyrin/C60 dyad can be accounted on the basis of a folded conformation.561 The intra-rotaxane PET between the spatially positioned C60 and ZnP in rotaxanes has been investigated.562 An array formed by C60 derivative with two pendant 4-pyridyl groups bound to a ﬂexible Zn-porphyrin undergoes fast Zn-porphyrin C60 PET.563 Photophysical studies on a fullerene derivative containing a Zn-porphyrin and liquid-crystalline dendrimer show quenching of the Zn-porphyrin ﬂuorescence.564 A Zn(II)-phthalocyanine-C60 dyad undergoes PET to generate a CS state with a lifetime (3.0 ns) shorter than that of the analogous porphyrin dyad (2.02 ms).565 Electron transfer between ferrocene and C60 was investigated using butadiyne-linked Zn(II)-porphyrin oligomers.566 The on–oﬀ communication between fullerene substituents in Ni-tetraazachlorin-C60 system depends on the nature of the ligands.567

15.3

Photoinduced reactivity and photocatalysis

‘‘Non emitting’’ metalloporphyrins are characterized by intramolecular photoreactivity of interest in biomimetic catalysis. Carbon monoxide migration can be induced through photoexcitation of a synthetic heme-Cu-complex; the results obtained are analogous to those observed in the active site of cytochrome c oxidase.568 Two-photon absorption process of a Ru(II) carbonyl octaethylporphyrin complex has been shown to occur upon nanosecond visible-light irradiation to yield a decarbonylation 276 | Photochemistry, 2009, 37, 240–299 This journal is

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process.569 A Roussin red-salt ester with a pendant porphyrin chromophore was prepared and investigated as a precursor for the photochemical generation of nitric oxide.570 Sn(IV)- and Zn(II)-complexes of tetrakis(2,6dichloro-3-sulfonato-phenyl)porphyrin were employed for photodegradation of phenols in aqueous solution through the formation of 1O2.571 Photoexcitation of the Cr(III) octaethylpoprhyrin complex, [Cr(OEP)(Cl)(L)] (L = H2O, Py, OPPh3), in dichloromethane causes the generation of a coordinately unsaturated intermediate [Cr(OEP)(Cl)], which reacts with ligands in solution to give the parent complex, [Cr(OEP)(Cl)(L)], or a transient species, [Cr(OEP)(Cl)(H2O)], when L = Py or OPPh3.572 Other non-emitting metalloporphyrins have been employed for the biomimetic photooxidation of organic substrates. The C-H bonds of hydrocarbons are oxidized catalytically by the electron-deﬁcient bis-Fe(III)-m-oxo Pacman porphyrin using visible light and O2 as oxygen atom source.573 Photoexcitation of chloro [meso-tetrakis(2,6-dichlorophenyl)-porphyrin]Fe(III) induces the conversion of the coordinated N-(4-chlorophenyl)-N 0 -hydroxyguanidine to N-(4-chlorophenyl)urea and NO; this process presents important similarities with the mechanism of NO-synthase enzymes.574 The excited state reactivity of an Fe(II)porphyrinogen that has extremely short-lived excited states was studied.575 Monooxygenating species such as Mn(V)- and Mn(IV)-oxo-porphyrins can be produced in organic solvents by laser ﬂash photolysis of the corresponding Mn(III)-porphyrin perchlorate and chlorate complexes.576 Photoexcitation of 5,10,15-tris(pentaﬂuorophenyl)corrole-Fe(IV) chlorate or nitrate gives a highly reactive Fe-oxo transient identiﬁed as an Fe(V)-oxo species, which acts as an oxo transfer agent for the oxygenation of cyclooctene.577 Chemical oxidation of an Fe(III)-porphyrin complex followed by irradiation with 355 nm laser light gives a highly reactive porphyrin-Fe-oxo transient able to induce biomimetic oxidations of alkenes and arylalkanes.578

Abbreviations bpy bpz cyclam dcb DFT Dmb Dppe dppm dppz PEnT PET Phen Terpy TPP

2,2 0 -bipyridine 2,2 0 -bipyrazine 1,4,7,11-tetraazacyclotetradecane 2,2 0 -bipyridine-4,4 0 -dicarboxylic acid Density Functional Theory 4,4 0 -dimethyl-2,2 0 -bipyridine 1,2-bis(diphenylphosphino)ethane 1,2-bis(diphenylphosphino)methane dipyrido[3,2-a:2 0 ,3 0 -c]phenazine photoinduced energy transfer photoinduced electron-transfer 1,10-phenanthroline 2,2 0 :6 0 ,200 -terpyridine meso-tetraphenylporphyrinate dianion Photochemistry, 2009, 37, 240–299 | 277 This journal is

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Photochemistry, 2009, 37, 240–299 | 299 This journal is

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Photocatalysis and solar energy conversion (chemical aspects) Nick Serpone,w*a Alexei V. Emelineb and Satoshi Horikoshic DOI: 10.1039/b812720b This review presents some of the studies published in the years 2004–2007 focusing in large part on developments in three major areas: (1) photocatalysis with metal oxides, (2) generation of solar hydrogen through photoinduced (real) water splitting, and (3) progress in dye-sensitized solar cells.

1.

Metal-oxide photocatalysis

First generation metal-oxide photocatalysts based mostly on pristine TiO2 have been the object of much debate in the last three decades relative to (i) the nature of the oxidative agent (dOH radicals vs. holes h+); (ii) the site where reaction takes place (surface vs. bulk solution); (iii) is TiO2 indeed a photocatalyst since turnover numbers are diﬃcult to determine?; and (iv) how can process eﬃciency be ascertained? To the extent that the absorption edge of anatase TiO2 is at 387 nm (EBG ca. 3.2 eV), how can sunlight visible radiation be used eﬀectively to drive surface photoredox reactions? One successful strategy gaining momentum is to dope TiO2 with suitable dopants (e.g. metal ions and/or non-metals) to shift the absorption edge to longer wavelengths. Several review articles have addressed some of the above issues along with progress in degradation and mineralization of organic contaminants.1 Herein we address second generation TiO2 specimens that display visible-light photoactivity. The current debate occupying doped-TiO2 materials (i.e. 2nd generation or visible-light-active, VLA, photocatalysts) regards the root causes of the red-shift of the absorption edge of doped titanias. One school suggests that these red-shifts involve mostly oxygen vacancies in the metal-oxide lattice (surface and bulk) acting as electron traps to yield Ti3+ and/or F-type color centers. The ﬁrst signiﬁcant reports of anion-doped TiO2 began to appear in the early 1990s, although not until the 2001 study by Asahi et al.,2 who doped TiO2 with various anions (e.g. A = N, C, and S) to produce VLA-TiO2s, did these materials attract so much attention. In particular, the suggestion by these authors2 that doping of TiO2 shifts the absorption edge of TiO2xAx systems to lower energies and increased photoactivity owing to a narrowing of the TiO2 band gap by the dopants is currently a hotly debated issue in photocatalysis. In this regard, Kuznetsov and a

Gruppo Fotochimico, Dipartimento di Chimica Organica, Universita di Pavia, via Taramelli 10, Pavia 27100, Italia. E-mail: [email protected]; Fax: (+39) 0382-987316; Tel: (+39) 0382-987323 b V.A. Fock Institute of Physics, St. Petersburg State University, St. Petersburg 198504, Russia. E-mail: [email protected] c Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. E-mail: [email protected] { Professor Emeritus, Department of Chemistry and Biochemistry, Concordia University, Montreal (QC), Canada. Email: [email protected]

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Serpone3 have argued that the photoactivity of VLA-TiO2s originates mostly, if not completely, from the existence of oxygen vacancies and color centers and that there is no narrowing of the intrinsic band gap of TiO2 other than involvement of dopant mid-gap energy levels to account for (in some cases) the photoactivity of the doped materials to degrade and mineralize organic contaminants. 1.1

Undoped TiO2 specimens

The temperature of the pretreatment stage can inﬂuence signiﬁcantly the photoactivity of TiO2.4 Photoreduction of O2 on electron surface centers occurs on trapping photogenerated free electrons by surface defects, whereas photooxidation of H2 takes place at photogenerated hole surface centers (trapped holes). Formation of color centers is initiated by photoholes and assisted by organic molecules, whereas destruction of the same centers is initiated by photoelectrons and assisted by oxygen.5 SIMS measurements of VLA-TiO2 thin ﬁlms fabricated by a radio-frequency magnetron sputtering (RF-MS) method6 revealed that the O/Ti ratio decreases from the top surface (2.00 0.01) to the inner bulk (1.93 0.01). Commercially available undoped TiO2 photocatalyst specimens, including Degussa P25 TiO2, show appreciable activity in the photodegradation of various molecules even under visible-light irradiation.7,8 Hydrolysis of Ti(i-PrO)4 in aqueous isopropanol in dilute H2SO4 followed by calcination at ca. 700 1C yields a sulfated anatase TiO2, which displays weak absorption in the 400–600 nm region. XPS spectra show loss of surface acidic hydroxyl groups below 500 1C that is followed by decomposition of the sulfate species. This leads to formation of oxygen vacancies (VOs) within the anatase lattice, some of which are stable to calcination above 600 1C for low O/Ti ratios.9 Common methods used to create defect sites (e.g. VO and/or Ti3+) on the TiO2 surface are typically UV-light irradiation, annealing the sample in vacuum, ion sputtering and plasma treatment. These methods follow a previous step, which involves preparing crystalline TiO2 powders/ﬁlms. Suriye et al.10 generated surface defects on TiO2 by a sol–gel method by varying the quantities of oxygen during the calcination process. Trapped electrons in oxygen vacancies (F centers) determine the VLA activity of TiO2 because nanoparticles with F centers provide unique energy levels that correspond to visible-light excitation. Sun et al.11 examined F centers quantitatively by EPR spectroscopy in high-surface-area TiO2 anatase nanoparticles prepared by metal-organic chemical vapor deposition (MOC-VD) techniques; surface rather than bulk processes dominate the temperature- and time-dependent concentration of F centers through a process summarized by eqns (1–3). OOx # VOdd (F++) + 2 e + 1/2 O2(g) VOdd (F++) + e # VOd (F+) OOx # VOd (F+) + e + 1/2 O2(g)

(1) (2) (3)

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Hydrolysis of Ti(n-BuO)4 in ethanol/water acidiﬁed with HNO3 yields TiO2, which when calcined at 150–300 1C is particularly visible-light-active in the degradation of NOx pollutants.12 With a slightly modiﬁed but similar procedure, Dong et al.13 placed Ti(n-BuO)4 in absolute alcohol acidiﬁed with HNO3 to produce a yellowish anatase TiO2 specimen co-doped with both C and N (XPS spectra). 1.2

TiO2 specimens doped with transition metal ions

An interesting innovative method of using visible-light radiation to photoactivate pristine TiO2 and to photogenerate electrons and holes involves doping TiO2 with the upconversion luminescence agent 40CdF2d60BaF2d1.0Er2O3 that converts visible light at 488 nm into ﬁve UV wavelengths between 200 and 450 nm.14 Unique visible-light-active TiO2 materials have also been developed by implantation of Vn+ ions into TiO2 thin ﬁlms supported on a quartz substrate; the ﬁlms were photoactive in degrading HCOOH to CO2 and H2O under visible light irradiation (l 4 450 nm).15 Electrochemical anodization of Ti yields layers of TiO2 nanotubes which, subsequent to Cr doping by metal-ion implantation, produces anatase TiO2 with a certain degree of amorphization.16 Visible-light-active Cr-doped TiO2s can also be obtained by a combined sol–gel/hydrothermal treatment method in which the quantity of Cr decreases from the surface into the lattice (XPS, AAS); EPR analysis showed Cr3+ occupies Ti4+ positions in the anatase lattice (XRD) along with Cr4+ ions to maintain electroneutrality.17 Visible-light-active Fe-doped TiO2 has been prepared by a one-step ﬂame spray pyrolysis technique for which an increase of the Fe/Ti ratio from 0.005 to 0.30 gradually shifts the absorption threshold from 396 to 564 nm.18 Donor levels of the Fe dopants are located within the TiO2 band gap close to the valence band. These levels allow for extrinsic excitation with visible light. Doping Fe3+ into TiO2 during the hydrothermal crystallization to optimize the distribution of iron in the TiO2 anatase lattice (XRD) was achieved19 by doping TiO2 with diﬀerent amounts of Fe3+ through a combined sol–gel/ hydrothermal treatment method in a manner otherwise similar to that used for Cr-doped TiO2.17 EPR spectra conﬁrmed incorporation of Fe3+ into the anatase lattice. Doping with Fe3+ introduces additional VOs on the surface and in the bulk of TiO2, thus favoring adsorption of H2O, formation of surface OH group, and promoting photoactivity. Eﬃcient separation of photogenerated charge carriers (eqn (4)) leads to increased activity as the Fe3+ species trap photogenerated holes (eqn (5)) at the Fe3+/Fe4+ energy levels located above the valence band edge of anatase TiO2; trapped holes as Fe4+ migrate to the surface and oxidize OH groups to dOH radicals (eqn (6)). TiO2 + hn - ecb + hvb+ Fe 4+

Fe

3+

+

- Fe

(ads)

- Fe

+ hvb

+ OH

4+

3+

(4) (5)

+ dOH(ads)

(6)

Visible-light-active zinc-ferrite doped titania (TiO2/ZnFe2O4), prepared by a sol–gel method and calcined at diﬀerent temperatures (400 to 600 1C), shows a stable TiO2 anatase phase (XRD).20 302 | Photochemistry, 2009, 37, 300–361 This journal is

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Thin ﬁlms of VLA gold-doped anatase TiO2 on various supports display a red-shift of the absorption edge (band at 550 nm) with XRD results indicating no rutile forms until 600 1C; evidently, the anatase-to-rutile phase change is inhibited, or at best slowed down, because of the presence of gold.21 VLA platinum-loaded TiO2 thin ﬁlms decompose H2O under visible light irradiation at l Z 420 nm in the presence of CH3OH (H2 evolved); O2 is evolved in the presence of AgNO3. The latter process also occurs at l 4 550 nm on thin ﬁlms fabricated at 600 1C with the RF-MS method.6,22,23 Ti3+ centers conﬁrmed by EPR spectroscopy lead to low visible-light-activity toward O2 evolution because they act as charge carrier recombination sites. Cerium(III)-doped TiO2, prepared by a sol–gel process with Ti(n-BuO)4 hydrolyzed in the presence of Ce(NO3)3, absolute ethanol and acetic acid, contains Ti3+, Ce3+ and Ce4+ ions in the TiO2 lattice (XPS analysis); signiﬁcant absorption between 400 and 500 nm was ascribed to excitation of valence band electrons to the Ce 4f levels.24

1.3

Anion-doped TiO2 specimens

Thermal treatment of TiO2 photoelectrodes in a hexane-rich environment leads to incorporation of C into the TiO2 lattice after annealing in a n-hexane/argon atmosphere at 500 1C and to creation of oxygen vacancies. The blackish-color of TiO2 originates from surface C deposits that fail to enhance photoactivity in the visible spectral range but does prolong the anatase-to-rutile phase conversion to temperatures greater than 800 1C.25 Using a diﬀerent approach, Xu et al.26 prepared C-doped TiO2 by hydrolyzing TiCl4 in the presence of (C4H9)4NOH as the C source and in the presence of glucose/NaOH. Visible light absorption was extended to 800 nm. An alternative synthesis subjected TiO2 to ethanol vapors at 150 to 400 1C to yield a VLA carbon-doped TiO2 that could photodegrade phenol.27 The use of a hydrothermal method at temperatures as low as 160 1C also produced visible-light-active TiO2xCx in two stages:8 (1) prepare amorphous TiO2 by controlled hydrolysis of Ti(i-PrO)4, and (2) autoclave the aqueous TiO2/glucose (C source) suspension at 160 1C. Tachikawa et al.28 examined the photoactivities of photogenerated h+ during UV and visible laser ﬂash photolyses of pure anatase TiO2, and S- and C-doped TiO2 powders by time-resolved diﬀuse reﬂectance (TDR) spectroscopy. One-electron oxidation of methanol and 4-(methylthio)phenylmethanol occurs under 355-nm laser irradiation but no oxidation occurs during the 430-nm laser photolysis of S- and C-doped TiO2 powders. Three types of carbonaceous species were detected in TiO2 samples prepared by sol–gel methods using Ti(i-PrO)4 in i-propanol.9 The principal species were: (i) organic residues from the alkoxide precursor, (ii) slightly oxidized organics, and (iii) further oxidized carboxylates. The organic-like C could be removed after calcination in air at 697 1C, although some graphitic carbon and some CO32-like species remained in TiO2 at C levels greater than 4.5 at.%. Photochemistry, 2009, 37, 300–361 | 303 This journal is

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Mixing Ti(i-PrO)4 and thiourea in ethanol followed by evaporation of the solvent and calcination of the white residue at 400 1C in air yields S-doped anatase TiO2 with a S content of 1.6 at.%.29 Except for S4+ species, XPS analyses revealed no peaks for either C or N atoms or S2 ions after calcination. This has led to some controversy about the nature of S dopants in TiO2xSx because of the likelihood that S2 anions in O2 sites may have been too few to be detected and that SO32 ions may have been produced by the partial oxidation of thiourea when mixed with TiO2 particles.30 The red-shift of the absorption edge of S-doped TiO2 was attributed to an increased valence band width and to a narrowing of the band gap.28 Adsorption of Fe3+ ions (0.90 wt%) on S-doped TiO2 increases the photoactivity of TiO2xSx but decreases it at loadings of 1.0 wt% or greater.31 Calcination of a mixture of TiCl3 and NH4SCN at 400 1C yields a yellowish S-doped anatase TiO2 whose XPS spectra shows a S 2p signal at 170 eV, consistent with an S6+ species doped on the TiO2 surface.32 The 170-eV peak vanishes after Ar+-ion etching inferring the presence of SO42 species on the TiO2 surface. Degradation of methylene blue was signiﬁcantly greater for the VLA sulfur-doped specimens than for the ST-01 TiO2 at l 4 400 nm. Subjecting a mixture of TiO2 and TiS2 powdered samples of diﬀerent ratios to mechanical grinding produces S-doped anatase TiO2 whose S doping limit is around 1%.33 Spray pyrolysis of aqueous H2TiF6 at temperatures 800, 900, 1000 1C produces F-doped TiO2 powders with a single anatase architecture at T o 900 1C (XRD analysis); it exhibits signiﬁcant visible-light activity even though F-doping has no eﬀect on the fundamental absorption edge.34 Evidently, the visible-light-driven photoactivity is achieved by excitation into the absorption bands of the VOs, i.e. by excitation of the F (465 nm) and F+ (525 nm) centers. The existence of Ti3+ centers is not precluded. Several methods have been used to synthesize visible-light-active N-doped TiO2 speci-mens. Yates et al.35 have divided these methods into three classes: (i) modiﬁcation of an existing TiO2 by ion bombardment, (ii) modiﬁcation of an existing TiO2 in powdered form, ﬁlm, and single crystal, or else modify TiN by gas phase chemical impregnation, and (iii) growth of TiO2xNx crystals from either liquid or gaseous precursors. Yang and Gao36 obtained N-doped TiO2 by hydrolysis of tetrabutyl titanate in the presence of thiourea in ethanolic media. Yang et al.37 obtained crystallized anatase TiO2xNx ﬁlms with a considerable amount of substitutional N atoms (1.8 at.%) and chemisorbed molecular N2 by ion-assisted electron-beam evaporation using rutile TiO2 and molecular nitrogen. The N 1s peaks in the XPS spectra at ca.400–402 eV and 396 eV are due to molecularly surface-chemisorbed g-N2 and to atomic b-N (O substituted by N in the TiO2 lattice), respectively. No peaks attributable to S were observed. Using a simple nanoscale synthetic route, Gole et al.38 produced TiO2xNx samples in short time at room temperature by direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. XPS analysis with Ar+-ion sputtering revealed the presence of N dopants not only at the surface but also incorporated into the sublayers of the TiO2xNx agglomerates 304 | Photochemistry, 2009, 37, 300–361 This journal is

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(N content: 3.6–5.1 at.%). Little if any XPS evidence of atomic b-N binding at 396 eV was found that might be associated with TiO2xNx or Pd-treated oxynitride samples. Rather, the XPS data were consistent with nonstoichiometric surface-based Ti–O–N bonding. From a mechanistic study into the photooxidation of water (evolution of O2) involving measurements of anodic photocurrents at N-doped TiO2 ﬁlm electrodes, Nakamura et al.39 suggested that the visible-light responses of N-doped TiO2s originate from a N-induced mid-gap level that forms slightly above the top of the O 2p valence band. They also concluded that photooxidation of organic compounds under visible-light illumination proceeds mainly by reactions with dOH radicals or superoxide radical anions (O2d), and not by direct reactions with holes that may have been trapped at the N-induced mid-gap level. Ion implantation of atomically clean TiO2 (110) surfaces with mixtures of N2+ and Ar+ ions followed by subsequent annealing under ultrahighvacuum conditions leads to incorporation of N into the TiO2 lattice;40 XPS spectra reveal only the N 1s feature at 396.6 eV attributed to substitutionally bound nitride nitrogen (O2 ions substituted by N2 anions). N-doped TiO2 (110) rutile single crystals pretreated in the presence of an NH3/Ar gas mixture at ca. 600 1C exhibits photoactivity at the lower photon energy of 2.4 eV, i.e. 0.6 eV below the band gap energy of rutile TiO2 (3.0 eV).41 The active dopant state of the interstitial N responsible for this eﬀect shows a N 1s binding energy at 399.6 eV attributed to a form of nitrogen likely bound to H, diﬀerent from the substitutional nitride state which displayed a N 1s binding energy at 396.7 eV. Ar+-ion sputtering led to extensive depletion of the former signal. Inferences made by the Yates group contrast earlier suggestions that nitridic N species that substitute O2 ions into the TiO2 lattice are the dopant species for TiO2 photoactivity in the visible-light region. Thomson and Yates42 re-emphasized later that the exclusive XPS N 1s signal at 396.7 eV, typically attributed to substitutional b-N in ion-implanted N-doped TiO2, in and by itself alone cannot account for the decrease in the photothreshold of TiO2(110) as observed for interstitially located N–H bound species. Although there is clear XPS evidence for the incorporation of bsubstitutional N in N-doped TiO2, there is no ﬁrm evidence of any appreciable photoactivity when these doped systems are irradiated with visible light,35 a point also raised by Frach et al.43 who noted no improvement in visible-light activity on N-doping TiO2, and by Li et al.44 who reported that the nature and level of visible-light activity depend on the nitriding compound employed. Although TiO2xNx ﬁlms display absorption features in the visible spectral region, so do nominally undoped TiO2 ﬁlms indicating that N incorporation cannot be assumed on the basis of red-shifts of the absorption edge, a point that cannot be overemphasized enough. XPS evidence is required. Moreover, even though b-N incorporation and absorption spectral features in the visible region are seen in N-doped TiO2 specimens, no visible-light-induced photoactivity has been observed, while the more conventional UV photoactivity is considerably reduced compared to ﬁlms grown in the absence of NH3. Evidently, the presence of b-N alone cannot be claimed to induce visible-light activity in Photochemistry, 2009, 37, 300–361 | 305 This journal is

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N-doped TiO2 ﬁlms, a point also raised by Mrowetz and coworkers45 who failed to oxidize HCOO to CO2d, or NH3–OH+ to NO3 under visiblelight illumination. These observations contrast those of Aita et al.46 who reported signiﬁcant visible-light activity at l 4 510 nm for N-doped TiO2 nanoparticles prepared by a solvothermal process. Similar N-doped nanocrystalline TiO2 (yellow) powders were synthesized by heating commerciallyavailable ST-01 TiO2 under a dry N2 gas ﬂow in the presence of a small quantity of carbon.47 The RF-MS deposition method has also been used to prepare photoactive N-substituted TiO2 thin ﬁlms with a calcined TiO2 plate under various N2/Ar mixtures as the sputtering gas.48 XPS and XRD measurements show signiﬁcant substitution of lattice O atoms of TiO2 by N atoms, which the authors suggested may play a crucial role in band gap narrowing of the TiO2 thin ﬁlms (from 2.58 to 2.25 eV relative to 3.2 eV for anatase) thereby enabling the visible-light photoresponse. Specimens with 6.0% N exhibited the highest visible-light activity in the photooxidation of isopropanol in aqueous media at l Z 450 nm, and the photooxidation of H2O at wavelengths up to 550 nm. Factors that govern the relationship between photoactivity and preparative conditions of visible-light-active N-doped TiO2 materials were reported in a study by Joung et al.49 Visible-light photoactivity of N-doped TiO2 materials is sensitive to the preparative routes because even though such systems absorb visible light, they are nonetheless frequently inactive in photooxidations. In this regard, In et al.50 prepared a series of TiO2xNx systems with nominal N loadings from 0.2 to 1.0 wt% involving the sequential reaction of H2O with a small excess of TiCl4 in toluene under dry O2-free argon. They found that (i) calcination at 400 1C yields a solid with pronounced absorption in the visible spectral region but no visible-light photoactivity, (ii) 500 1C calcination produces an eﬀective (yellow) visible-light-active sample, and (iii) heat treatment at 600 1C results in an inactive white material. In addition to ﬁlms and powdered specimens, self-organized N-doped TiO2 nanotubes can be fabricated by electrochemical anodization of titanium in HF/H2SO4 electrolyte, followed by calcination at diﬀerent temperatures (range 300–600 1C) in pure NH3.51 In two extensive reports, Belver and coworkers52,53 prepared and characterized a series of nanosized N-doped TiO2-based materials by a reverse micelle microemulsion method using Ti(i-PrO)4 and three N sources (2-methoxyethylamine, N,N,N 0 ,N 0 -tetramethylethylenediamine and 1,2phenylenediamine). XANES evidence conﬁrmed anatase TiO2xNx, and revealed no correlation between the number of oxygen vacancies (VOs) and N content. A joint XANES/EXAFS investigation showed that the defect distribution was not simply related to VOs but conﬁrmed point defects to be VOs. No interstitial defects were seen and the O/Ti atom ratio was less than 2. DRIFTS spectra, however, indicated the presence of several anion-related impurities of a substitutional (Nn) and interstitial (NO+) nature. Although impurities contributed to the absorption features (around 500 nm), no clear correlation exists between these species and photoactivity that best correlated with an optimal number of oxygen vacancies. 306 | Photochemistry, 2009, 37, 300–361 This journal is

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Fig. 1 Schematic illustrations of the photocatalytic reaction processes of S adsorbed on the surfaces of pure, N-, S-, and C-doped TiO2 nanoparticles. Reproduced with permission from Tachikawa et al., J. Phys. Chem. B, 2007, 111, 5259. Copyright 2007 by the American Chemical Society.

N-doped or N-/S-co-doped titania TiO2xAy (A = N, S) systems that can be excited by visible light (400 to 550 nm) have been prepared by mixing aqueous TiCl3 with various nitrogen sources, e.g. NH2OH, HMT, (NH2)2CO and (NH2)2CS followed by hydrothermal treatment at 190 1C in an autoclave.54 These specimens displayed good visible-light photoactivity in the oxidative destruction of NO, except for the powder prepared from TiCl3/NH2OH solutions. XPS data showed only the N 1s peak around 400 eV but not the peak at 396 eV. Highly visible-light-active N-doped TiO2 specimens have been synthesized using a layered titania/isostearate nanocomposite prepared by a sol–gel technique with N-doping achieved by treating the composite with aqueous NH3 followed by calcination either in an O2/N2 mixture or in pure N2 at various temperatures. Visible-light photoactivity failed to correlate with N content.55 Visible-light-active TiO2 systems doped with C, S or N possess, in most cases, good attributes toward the photooxidation of organic and inorganic (e.g. NOx) substrates, particularly N-doped TiO2 materials prepared by various but otherwise simple methods. Methods have been varied but otherwise simple in a large number of cases. Although they all displayed absorption features and red-shifted absorption edges (at least to 550 nm), their photoactivity under visible light has not always correlated with such absorption features. Tachikawa et al.56 addressed these issues and described mechanisms of the photoactivity of VLA TiO2 specimens. Photooxidations of organic compounds by VLA TiO2xNx systems proceed by O2d and/or dOH radical oxidation and not by direct reaction with h+ trapped at the N-induced mid-gap level as illustrated in Fig. 1. 1.4

N,F- and N,S-codoped titanium dioxides

Vivid yellow N,F-co-doped TiO2 powders have been prepared by spray pyrolysis at various temperatures from a mixed aqueous solution containing TiCl4 and NH4F to introduce new active sites by F-doping, while visiblelight absorption was due to N-doping.57,58 The band at 465 nm was attributed to an oxygen vacancy with two trapped electrons (F center), whereas the 525-nm band is due to an F+ center, whereas the band at 627 nm was likely a consequence of the Franck-Condon principle and the Photochemistry, 2009, 37, 300–361 | 307 This journal is

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polarizability of the lattice ions surrounding the vacancy. The 700-nm band originates from doped N atoms and assigned to transfer of excited electrons between the F+ center and some impurity energy state. The band at 905 nm was assigned to transfer of excited electrons between the F center and some unidentiﬁed energy state. The photoactivity of the N,F-co-doped TiO2 powder prepared at 900 1C proved superior to that of P25 TiO2 under both UV- and visible-light irradiation in the oxidation of CH3CHO. Enhancement of photoactivity in N,F-co-doped systems from a contribution of N-doping to visible-light photoactivity through VOs may not be the sole factor since N,F-codoped TiO2 powders displayed greater photoactivity than N-doped TiO2 alone. UV-visible spectra unequivocally indicates that N-doping causes no narrowing of the band gap of TiO2 as there was no red-shift of the fundamental absorption edge of TiO2. F-doping leads to several beneﬁcial eﬀects on photoactivity: (1) F-doping causes formation of new active sites such F and F+ centers, (2) F-doping results in the formation of surface acid sites that lead to increased adsorptive ability of the N,F-TiO2 powders for a substrate and in acting as electron acceptors, and (3) F-doping increases photogenerated electron mobility in TiO2 and diﬀusion from the inner lattice to the particle surface. An overall comparative study carried out on N-doped, F-doped, and N,F-co-doped TiO2 powders (denoted NTO, FTO, NFTO) synthesized by spray pyrolysis conﬁrmed the origins of the visible-light-driven photoactivity.58 Anatase N,F-codoped TiO2 powders could also be prepared by a sol–gel/ solvothermal method using tetrabutyl titanate as a precursor, triethylamine as the N source and ammonium ﬂuoride as the F source.59 F atoms cause no signiﬁcant shift in the fundamental absorption edge of TiO2. N,F-co-doped TiO2 samples display good visible-light activity in the photoassisted degradation of p-chlorophenol and rhodamine-B under visible-light irradiation (400–500 nm). N 1s XPS spectra of N,F-codoped TiO2 and N-doped TiO2 displayed a peak at 400.0 eV ascribed to N atoms from adventitious N–N, N–H, O–N, or N-containing organic compounds adsorbed on the surface,59 and a small peak at 396.0 eV taken as evidence for Ti–N bonds formed when N atoms replace the oxygen in the TiO2 lattice. No evidence of TiF4 and TiOF2 bonds were seen in the XRD spectra. A simple method for preparing highly photoactive nanocrystalline mesoporous N,S-co-doped TiO2 powders involves hydrolysis of Ti(SO4)2 in aqueous NH3 at room temperature yielding various xerogels after calcination at diﬀerent temperatures.60 An XPS survey spectrum of N,S-co-doped TiO2 powders calcined at 500 1C show powders contained Ti and O elements and a small amount of N, S and C. In S 2p3/2 XPS spectra, two isolated peaks at binding energies 168.7 and 162.3 eV are due to S6+ and S2 species, respectively, with the former associated with SO42 ions on the surface of TiO2, whereas the latter peak is due to Ti–S bonds from substituting O2 in TiO2 by S2 species. The red-shift of the absorption edge of N,S-co-doped TiO2 was taken to be due to band gap narrowing by the presence of dopants.60 These specimens also showed high visible-light photoactivities toward the photooxidation of acetone and formaldehyde. 308 | Photochemistry, 2009, 37, 300–361 This journal is

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1.5

DFT-calculated band gap energies of doped TiO2

A key question that keeps coming up in the literature is the chemical nature and the location of species that lead the absorption edge of TiO2 to be red-shifted and consequently to visible-light activity of TiO2. Species such as NOx, NHx, and N2 have been proposed, as well as hyponitrite, nitrite and nitrate species that were conﬁrmed experimentally. Another key question regards the electronic structure(s) of (anion)-doped materials and their fate when subjected to UV and/or visible light irradiation. S-doped TiO2 displays signiﬁcant visible-light activity attributed to the presence of S4+ species substituting Ti4+ in the lattice.29 Matsushima et al.30 re-examined the electronic structure through ﬁrst-principles DFT band calculations. They concluded that (i) the S atom is located at either Ti or O sites in the anatase structure depending on the preparative conditions, and that (ii) S atoms located at Ti sites in S-doped TiO2 lead to lower visible-light activity. These predictions contrast earlier results of Umebayashi et al.61 who noted that as-prepared S-doped TiO2 was twofold more visible-light-active than as-prepared N-doped specimens62 under visible light irradiation. The eﬀects of S-doping have been further examined by Tian and Liu63 using plane-wave-based pseudo-potential DFT to characterize the electronic structure when S atoms substitute O atoms in anatase TiO2. Evidently, S-doped anatase TiO2 is converted into a direct band gap semiconductor at the G position in line with results for S-doped rutile TiO2 caused by S 3p states localized above the upper edge of the valence band.64 The DFT analysis63 also showed that the band gap energies are concentration-dependent and that the width of the valence band increases as the doping level increases. The exact cause of the absorption edge red-shift of TiO2 of various N-doped TiO2 (anatase) powders became confused with the report from Yates’ group40 that the absorption edge of a N-doped TiO2 rutile single crystal shift to higher energy. Spin-polarized DFT calculations by Di Valentin et al.65 reveal that in anatase the localized N 2p states are located just above the O 2p states of the valence band and red-shift the absorption edge to lower energy, whereas in rutile the tendency to red-shift the absorption edge is oﬀset by the concomitant contraction of the O 2p band, resulting in an overall increase in the optical transition energy by 0.08 eV (experimental blue-shift, 0.20 eV). DFT-calculated band gaps for pure undoped anatase and rutile TiO2 were 2.19 eV and 1.81 eV versus the experimental values of 3.2 eV and 3.0 eV, respectively. These demonstrate serious shortcomings of the DFT method. Regardless, the simultaneous presence of N dopants and VOs may lead to charge transfer states that also contribute to the visible-light photoactivity (eqn (7)). Subsequent DFT VO (F) + N - VOd (F+) + N

(7)

calculations performed using the plane-wave-pseudopotential approach together with the Perdew-Burke-Ernzerhof exchange correlation functional and ultrasoft pseudopotentials deduced, among others, that N-doping leads to a substantive reduction of the energy of formation of VOs (4.3 eV to 0.6 eV for anatase) with important consequences in the generation of F-type Photochemistry, 2009, 37, 300–361 | 309 This journal is

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and Ti3+ color centers.66 Substitutional N-doping is stabilized by the presence of oxygen vacancies (NSO + VO) under oxygen-poor experimental conditions, whereas under oxygen-rich conditions interstitial N species (NI) are favored. A consequence of the large number of VOs under oxygen-poor conditions in N-doped TiO2 is the partial quenching of Nbd paramagnetic species that are transformed into Nb through reduction by Ti3+ color centers (eqn (8)) as evidenced by EPR spectroscopy. Nbd + Ti3+ - Nb + Ti4+

(8)

Removal of an O atom from the TiO2 lattice leaves behind two electrons that form either neutral F centers (VO in the Kroger-Vink notation) or are trapped by neighbouring Ti4+ species to give two Ti3+ color centers, which Henderson et al.67 earlier positioned at 0.8 eV below the bottom of the conduction band. Other studies indicate otherwise. Although there are electron traps around this energy no Ti3+ color centers have been evidenced by EPR measurements.68 The question of the blue-shift of the absorption edge of single crystals of N-doped TiO2 rutile that contrasts the red-shifts in N-doped TiO2 powders was taken up in a DFT study by Yang et al.69 using the plane-wave method. Results conﬁrmed those of Di Valentin et al.65 in that some N 2p states lie above the O 2p valence band when N substitutes O in the TiO2 lattice and when N is located at interstitial positions. No band gap narrowing was predicted by these calculations.69 DFT calculations of C-doped TiO2 at low C concentrations under oxygen-poor conditions indicate that substitutional (to oxygen) carbon and oxygen vacancies are favored, contrary to oxygen-rich conditions in which both interstitial and substitutional (to Ti) C dopings are preferred.70 Though DFT calculations also indicate that C-doping favors formation of oxygen vacancies in bulk TiO2, again they underestimate the band gaps of anatase and rutile TiO2. Additional calculations along with experiments have been reported that aimed at determining the fate of doped specimens when irradiated at diﬀerent wavelengths in the presence of adsorbates.71 Wang and Lewis72 have explored the electronic properties of C-, N-, and S-doped TiO2 materials in a comprehensive theoretical investigation of substitutional anion doping in TiO2 using an ab initio tight-binding method (FIREBALL) based on density functional theory and a nonlocal pseudopotential scheme. 1.6

The question of band gap narrowing in doped TiO2

Taken literally, band gap narrowing in doped TiO2 materials means that the band gap energy of TiO2 decreases in the presence of dopants. What in fact does change is the energy photothreshold for activating doped titania specimens to carry out surface photoinduced redox processes. A better term in referring to the long-wavelength absorption edge might be (i) the red-limit of TiO2 photocatalysis used in the past to refer to photooxidations and photoreductions that occur on excitation in the visible spectral region, or as (ii) the extrinsic band gap(s) of doped TiO2 versus the term intrinsic band gap (anatase, 3.2 eV; rutile, 3.0 eV) that is used for pristine undoped titania. 310 | Photochemistry, 2009, 37, 300–361 This journal is

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An extensive examination of the experimental and theoretical literature reveals a lack of consensus on whether or not there is band gap narrowing in doped TiO2 materials based on DFT calculations. Some studies deduced that there is a rigid shift of the valence band edge to higher energies, thus narrowing the intrinsic band gap of TiO2 as a consequence of doping, while others found otherwise. The discrepancies are not a semantic problem. Anion-doping (or for that matter any type of doping) of TiO2 does shift the absorption edge of the doped metal oxide to longer wavelengths, thus aﬀording potentially visible-light photoactive materials in several important applications of processes that take place on the TiO2 surface. In a comprehensive examination of published diﬀuse reﬂectance spectra of metal- and anion-doped TiO2 specimens, Kuznetsov and Serpone5 concluded that light absorption in the visible spectral region originates from the existence of color centers, rather than from a narrowing of the intrinsic band gap of TiO2 (Ebg = 3.2 eV; anatase), as originally espoused by some workers,2,73 through mixing of oxygen and dopant states. True narrowing of the original (i.e. intrinsic) band gap of the metal-oxide TiO2 semiconductor would necessitate heavy anion or cation doping that would require high concentrations of the dopants. In the latter case, however, one must ask whether the metal oxide retains its original integrity. Serpone74 tentatively attributed the convoluted absorption spectra of doped TiO2s at wavelengths greater than 400 nm to Jahn-Teller split 2T2 - 2E transitions of Ti3+ centers (428–413 nm and 517–477 nm) whose existence was conﬁrmed by EPR examination of N-doped TiO2 specimens calcined at diﬀerent temperatures,10,49 and to a transition from the ground state of the F+ center to its corresponding excited F+* state (729–590 nm), though transition to the conduction band of TiO2 was not precluded. If as suggested by some workers the absorption features seen in the visible spectral region are truly due to this rigid shift, then irradiation into these bands should cause no bleaching of the absorption bands. However, if the absorption features are due to the existence of color centers (F-type and/or Ti3+), then bleaching of the spectral features should be observed if destruction of photoinduced color centers occurs, as recent work has demonstrated for both TiO2/polymer compositions5 and for an N-doped TiO2 system.75 2.

Solar energy conversion (water splitting)

The Sun is the largest energy source that keeps Earth’s engine moving forward and is the principal resource that can potentially meet all the global energy demand, even though the power density of the sunlight reaching Earth is relatively low. A 2005 report76 from the Basic Energy Sciences Workshop on Solar Energy Utilization sponsored by the US Department of Energy identiﬁed key scientiﬁc challenges and research directions that will enable eﬃcient and economic use of the solar resource to provide a signiﬁcant fraction of global primary energy by the mid-21st century. In the present context, two of the three challenges are: (a) production of solar fuels (e.g. H2 from water splitting) to overcome the inherent day–night and sunny–cloudy cycles of solar radiation; and (b) conversion of sunlight to solar electricity by photovoltaic solar cells and dye-sensitized solar cells. Photochemistry, 2009, 37, 300–361 | 311 This journal is

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Current use of sunlight represents but less than 1% of total electricity produced from renewable sources, even though deployment of photovoltaics has increased, albeit slowly, during the last two decades owing (i) to a poor competitive edge in a market dominated by fossil fuels, (ii) to an initial high capital cost, and (iii) to modest conversion eﬃciencies and intermittency.77 Two types of photoelectrochemical cells are worth noting: photoelectric cells that convert light into electricity, and photochemical cells that use light to drive chemical reactions to produce fuels.78,79 Hydrogen production technologies involving sunlight have been a signiﬁcant area of research since the 1970s.79 Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored.80 Another approach uses heat from solar concentrators to drive steam reformation of natural gas to increase overall H2 yields.81 It is relevant to emphasize that the water splitting reaction involves the direct breakup of the water molecule by some process that lilely entails the use of some (photo)catalyst to yield hydrogen and oxygen in the proper ratio as per reaction (9). Other reactions that require sacriﬁcial donors or acceptors to produce either hydrogen or oxygen cannot be considered a water splitting process, since such processes produce either hydrogen or oxygen, but not both. A few review articles on water splitting have appeared in the 2004–2007 period.82 2H2O + Cat + hn - 2H2 + O2 + Cat

(9)

Overall water splitting under visible light irradiation has been achieved by construction of a Z-scheme photocatalytic system that employs visiblelight-driven photocatalysts for H2 and O2 evolution and the Fe3+/Fe2+ redox couple as the electron relay (Fig. 2).83 A NiO (0.2 wt%)/NaTaO3:La (2 wt%) photocatalyst with a 4.1-eV band gap shows high activity for water splitting to H2 and O2 with an apparent quantum yield of ca. 56% at 270 nm. Examination of the eﬀects of metal-ion doping or replacement on the performance of d10 and d0 metal-oxide and d10 metal-nitride photocatalysts {e.g. (1) a-Ga22xIn2xO3 and ZnGa22xIn2xO4 with the In3+ ion

Fig. 2 Water splitting system by a two-photon process with visible light irradiation. Adapted from Kudo et al. Chem. Letters 2004, 33, 1534.

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added to Ga2O3 and ZnGa2O4, respectively, (2) YxIn2xO3 from a solid solution of In2O3 and Y2O3, (3) metal-ion doped CeO2, and (4) metal-ion doped GaN} in water splitting showed that the photocatalytic activity of 1 wt% RuO2-loaded a-Ga22xIn2xO3 increased sharply with increasing x to a maximum at ca. x = 0.02, and then decreased considerably with further increase in x as occurred for ZnGa22xIn2xO4.84 A series of R3TaO7 and R3NbO7 systems (R = Y, Yb, Gd, or La) have been prepared by a polymerization complex technique and investigated for their photocatalytic activity in the splitting of pure water into H2 and O2.85 The crystal structures of R3TaO7 and R3NbO7 change with increasing ionic radius of the R3+ ion from a ﬂuorite-type cubic structure to a pyrochlore-type cubic structure, and ﬁnally to a weberite-type orthorhombic structure. Water splitting to H2 and O2 in the appropriate stoichiometric ratio proceeds over NiOx-loaded La3TaO7 and La3NbO7 photocatalysts that possess a distorted orthorhombic structure. For both La3TaO7 and La3NbO7, the phase transition from cubic to orthorhombic occurs around 1000–1050 1C, incurring a drastically increased photocatalytic activity from this phase change. Clearly, there is a signiﬁcant inﬂuence of the crystal structure of the photocatalysts on the photoactivity of these metal-oxide semiconductor materials. Fig. 3 displays the reaction time course of H2 and O2 evolution from distilled water (400 mL) over 1 wt% NiOx-La3TaO7 and 1 wt% NiOx-La3NbO7 photocatalysts under UV light irradiation. Rates of formation of the two gases are rather small (kH2 B 79 mmol hr1 and kO2 B 35 mmol hr1 for the weberite-orthorhombic La3TaO7 catalyst); the niobates proved very disappointing photocatalysts. In a subsequent study, Abe et al.86 prepared a series of R3MO7 and R2Ti2O7 (R = Y, Gd, La; M = Nb, Ta) by the same polymerized complex method for the splitting of pure water. Whereas the phase transition of La3TaO7 and La3NbO7 from cubic pyrochlore to orthorhombic weberite occurs around 1050 and 1000 1C, respectively, the crystal structures of R2Ti2O7 change from cubic pyrochlore to monoclinic perovskite with increasing ionic radius of R3+. The overall photocatalyzed

Fig. 3 Reaction time courses of gas evolution from distilled water (400 mL) over 1 wt% NiOx-La3TaO7 and 1 wt% NiOx-La3NbO7 photocatalysts under UV light irradiation: (a) La3TaO7 photocatalyst prepared by calcination at 1000 1C (cubic) and at 1100 1C (orthorhombic); (b) La3NbO7 photocatalyst prepared by calcination at 900 1C (cubic) and 1100 1C (orthorhombic). Reproduced with permission from Abe et al. J. Phys. Chem. B, 2004, 108, 811. Copyright 2004 by the American Chemical Society.

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water splitting proceeds over La3TaO7 and La3NbO7 (orthorhombic weberite structure), Y2Ti2O7 and Gd2Ti2O7 (cubic pyrochlore structure), and La2Ti2O7 (monoclinic perovskite structure). All these metal-oxide photocatalysts possess a network of corner-shared octahedral units of metal cations (TaO6, NbO6, or TiO6). Materials without such a network, e.g. Y3TaO7 and Y3NbO7 (ﬂuorite-cubic structure), Gd3TaO7 and Gd3NbO7 (cubic pyrochlore structure) and La3TaO7 and La3NbO7 (cubic pyrochlore structure), tend to be inactive toward the photocatalytic water-splitting reaction. These results again emphasize the signiﬁcant inﬂuence of crystal structure, particularly the network of octahedral units of metal cations, on the photocatalytic activity of these materials. Ikeda and coworkers87 prepared the analogous tantalate and niobate pyrochlores Ca2M2O7 (M = Nb, Ta) and A2Ta2O6 (A = Na, K) by a hydrothermal method. These oxides were crystallized directly from tantalum or niobium alkoxides in the presence of Na+, K+ or Ca2+ cation in alkaline solutions at temperatures between 373 and 448 1K. Their crystallinity improved by calcination in air. Stoichiometric formation of H2 and O2 from the water splitting process occurs over tantalate pyrochlores loaded with a small amount of nickel oxide (NiO) as a catalyst for H2 production. By contrast, the NiO-loaded niobate pyrochlore shows no such photocatalytic activity. The photocatalytic functions of the tantalate pyrochlores are attributed to the energy levels at the bottom of conduction bands being much more negative than those of the niobate pyrochlores. Rates were in the 100s of mmol hr1. The oxide semiconductor K3Ta3B2O12 with a pillared structure and a band gap of 4.2 eV is photocatalytically active for splitting water into H2 and O2 without the assistance of co-catalysts.88 The electron dynamics in the K3Ta3B2O12 photocatalyst for water splitting were traced using timeresolved infrared absorption spectroscopy.89 A structureless absorption appears at 3000–1500 cm1 under 266 nm light pulses, assigned to infrared absorption by UV-excited electrons that are then trapped in mid-gap states. Electron-hole recombination observed in an Ar atmosphere is sensitive to the boron content in the starting material in the catalyst preparation. Decay of the electrons is accelerated by exposing the catalyst to water vapor. The rate of decay qualitatively correlates with the rate of H2 production under steady-state UV irradiation. Defect pyrochlore-type oxides, AMWO6 (A = Rb, Cs; M = Nb, Ta), loaded with nickel oxide show photocatalytic activity for overall water splitting under UV light irradiation.90 The conduction bands of the materials are thought to be composed of hybrid orbitals between W 5d and Nb 5d or between W 5d and Ta 4d orbitals. Fig. 4 depicts possible band structures along with the band gap energies compared with WO3. The layered perovskite tantalates A2SrTa2O7 (A = H, Li, K, and Rb) systems have been shown to be photocatalytically active in producing H2 and O2 from the water splitting process occurring under UV irradiation.91 Tantalates containing a hydrated interlayer space {e.g. A2SrTa2O7 nH2O where A = H, K, and Rb} show greater rates of formation of H2 than does the anhydrous layered tantalate Li2SrTa2O7. By comparison, H2SrTa2O7 nH2O, K2Sr-Ta2O7 nH2O and the anhydrous perovskite 314 | Photochemistry, 2009, 37, 300–361 This journal is

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Fig. 4 Possible band structures of Rb(Cs)TaWO6, Rb(Cs)NbWO6 and WO3 for comparison (eV vs. NHE). Adapted from Ikeda et al. Catal. Letters, 2004, 98, 229.

tantalite KTaO3 show high activity for overall water splitting without loaded co-catalysts. The reaction over H2SrTa2O7 nH2O occurs steadily for more than 70 hr, demonstrating the high durability of this metal-oxide catalyst. Results also suggest that the availability of an interlayer space in layered tantalites as reaction sites is an important factor in improving the photocatalytic activity of Ta-based semiconductor materials. Indeed, results from photoluminescence spectroscopy and H2 evolution from an aqueous n-butylamine solution support the notion that the high activity of A2Sr-Ta2O7 nH2O systems results from their hydrated layered structure within which the photogenerated electrons and holes are eﬀectively transferred to the interlayer water. In the presence of hole scavengers (e.g. n-butylamine), the rate of H2 formation increases substantively by a factor of ca. 4. For example, for the H2SrTa2O7 nH2O photocatalyst the rate of H2 evolution without the hole scavenger is 385 mmol hr1 whereas in the presence of n-butylamine the rate is 1490 mmol hr1.91 Layered perovskite materials display high photocatalytic activity in water splitting reactions under UV irradiation. The electronic structure of the perovskite slab forming the layered structure (total cation valency) is a most crucial factor.92 Both the excessive cation valency and the layered structure are required for active photocatalysts, whereas the slab thickness of the layered perovskites has little if any eﬀect on the water-splitting activity. Doping the La2Ti2O7 system with an alkaline-earth element (e.g. Ba, Sr, and Ca) enhances markedly the photocatalytic activity and introduction of an alkaline hydroxide into the reaction system as an external additive further enhances the activity: apparent quantum yields around 50%. Matsumoto and coworkers93 examined the photoelectrochemical properties of such photocatalysts as TiO2, NaTaO3, along with TiO2 and SrTiO3 doped with either Cr or Sb to evaluate the reverse reactions to water splitting, namely the photooxidation of H2 and the photoreduction of O2 in N2-, H2- or O2-saturated electrolytes. Photoreduction of O2 and photooxidation of H2 reactions occur preferentially in O2- and H2-saturated Photochemistry, 2009, 37, 300–361 | 315 This journal is

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electrolytes, respectively, with cathodic and anodic photocurrents larger than those from N2-saturated electrolytes. Results from both reverse photoreactions explained the rather poor activity of TiO2 because of the high anodic photocurrent of H2 and high cathodic photocurrent of O2. The increased photocurrents in La-doped NaTaO3 accord with the La-dopant induced increase in photocatalytic activity. A relatively high O2 photoreduction current was observed, but not for the H2 photooxidation at NaTaO3 without the presence of the NiO cocatalyst. That is, water photolysis on these catalysts is suppressed by O2 photoreduction. By contrast, NiO loading suppresses the O2 photoreduction and/or increases the photooxidation of water since hardly any O2 photoreduction current was observed for the NiO-loaded NaTaO3 samples. Consequently, the high photocatalytic activity of the NiO-loaded and La-doped NaTaO3 is brought about by the negligible reverse reactions of H2 and O2. Oxygen photoreduction and hydrogen photooxidation currents were observed under visible light irradiation for SrTiO3 and TiO2 doped with either Cr or Sb, respectively. Perovskite-type NaTaO3 derived from a sol–gel synthesis (SG) exhibits a larger surface area and a remarkable greater photocatalytic activity in water splitting than does the solid-state (SS) prepared NaTaO3.94 The SG and SS NaTaO3 possess diﬀerent crystalline structures (monoclinic P2/m and orthorhombic Pcmn, respectively), diﬀerent band gap energies (SG 4 SS) and diﬀerent electronic band structures. The sol–gel NaTaO3 has an indirect band gap, whereas the solid-state NaTaO3 displays a direct band gap structure, which together with diﬀerences in surface area explains the greater photocatalytic activity of the sol–gel NaTaO3. The recombination rate for electron–hole pairs should be lower in systems with an indirect band gap, thereby causing the lifetimes of free electrons and holes to be longer and thus to an increased probability of their participations in the water splitting process. The photodeposition of gold particles on several niobate and tantalate photocatalysts (e.g. K4Nb6O17, Sr2Nb2O7, KTaO3, NaTaO3, and NaTaO3 doped with La) signiﬁcantly improves the photocatalytic activities of these metal oxides for water splitting.95 The nano-sized gold particles function as eﬃcient co-catalysts for the photocatalytic water splitting by assisting the H2 evolution. Au-loaded Sr2Nb2O7 and KTaO3 produce H2 and O2, but are not active without the Au co-catalyst. By contrast, K4Nb6O17, NaTaO3, and La-doped NaTaO3 are active even without a co-catalyst (for the latter kH2 = 404 mmol hr1 and kO2 = 187 mmol hr1); however, their activities are improved fourfold by the Au loading (kH2 = 1950 mmol hr1 and kO2 = 880 mmol hr1). The back-reaction between H2 and O2 to produce water on the Au cocatalyst is negligible, as was the case with the NiO co-catalyst, in comparison with that on a Pt cocatalyst. Apparently, the ﬁne Au particles play an important role in the creation of active sites for H2 evolution and in the enhancement of charge separation. Jang and coworkers96 noted that the presence of nanosized Fe2O3 particles in the interlayer space of the layered compounds HTiNbO5 and HTiTaO5 induces absorption of visible light that is absent for the host-layered systems. The iron oxide so intercalated into the interlayer 316 | Photochemistry, 2009, 37, 300–361 This journal is

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space has electronic and crystal structures similar to those of Fe2O3. However, an interesting feature of these intercalated nanosized Fe2O3 particles is their high electron deﬁciency relative to bulk Fe2O3, which suggests a strong electronic interaction between the nanoparticles and the host layers. Such interaction can work favorably in photocatalysis because of an eﬃcient electron–hole separation. The HTiNb(Ta)O5 compounds have proper band positions for both water reduction and oxidation, but absorb only UV light owing to the large band gap energy. The Fe2O3–HTiNb(Ta)O5 systems absorb visible light, but the conduction band position of Fe2O3 is not negative enough to reduce water. Unlike an earlier report, Fe2O3-intercalated layered compounds do not produce H2 from an aqueous CH3OH solution in a photocatalytic manner under visible light, but do produce a stoichiometric mixture of H2 and O2 photocatalytically under UV irradiation and produce O2 from an aqueous AgNO3 solution photocatalytically under visible light illumination. TaON and MTaO2N (M = Ca, Sr, Ba) have been prepared by nitriding Ta2O5 and M2Ta2O7, respectively.97 They have small band gap energies (TaON, 2.5 eV; MTaO2N, 2.5–2.0 eV) and absorb visible light at 500–630 nm via the N 2p orbitals at the top of the valence bands. Under visible light irradiation, TaON reduces H+ to H2 and oxidizes water to O2 in the presence of sacriﬁcial electron donors and acceptors, respectively, without signiﬁcant degradation of the materials. This indicates stable visible light-driven photocatalysts with good redox potentials. TaON oxidizes water to O2 eﬃciently (maximum quantum yield, 34%) and MTaO2N reduces H+ into H2 in the presence of sacriﬁcial electron donors under visible light irradiation. The morphology of nanostructured a-Fe2O3 ﬁlms is strongly inﬂuenced by silicon doping, which causes a decrease of the size of the nanocrystallites. In a tandem cell conﬁguration with two series-connected dye-sensitized solar cells that provide a bias voltage of 1.4 V and a current density of 4 mA cm2 at 0.5 Sun, the best performing Fe2O3 photoanode yielded a solar-to-chemical conversion eﬃciency of 2.1% based on the heat of hydrogen combustion (upper value, 280 kJ mol1, 1.45 eV electron1).98 The photophysical and photocatalytic properties of the photocatalyst Ca2NiWO6 synthesized by a solid-state method have been investigated; it has a double-perovskite layered structure with corner-shared WO6 and NiO6 octahedra.99 Although Ca2NiWO6 responds to visible light, photocatalytic H2 evolution occurs only under UV light irradiation, whereas O2 evolution occurs under visible light due to a special band structure. A band structure based on DFT calculations has been proposed in which photoexcitation of electrons from the hybridized O 2p/Ni t2g valence band to the W 5d level is responsible for H2 evolution under UV light. The presence of oxygen vacancies that act as electron–hole recombination centers causes the photoactivity of the photocatalyst to be rather low. Certain oxynitrides such as TaON, LaTiO2N, and the (Ga1xZnx)(N1xOx) solid solution (band gap, 2.4–2.8 eV) are promising stable photocatalysts for overall water splitting (OWS) under visible-light irradiation.100 Although these oxynitrides exhibit high photocatalytic activity for water oxidation in the presence of an appropriate electron acceptor, their activity for water reduction Photochemistry, 2009, 37, 300–361 | 317 This journal is

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is nearly an order of magnitude lower than that for water oxidation. Hence some of these oxynitrides were modiﬁed to promote water reduction and improve their overall eﬃciency. Rh-loaded GaN:ZnO exhibits little photocatalytic activity for OWS even after extended periods of irradiation because of rapid water formation on the Rh nanoparticulates (NPs). However, GaN:ZnO loaded with a Rh/Cr2O3 core/shell co-catalyst yields stoichiometric quantities of H2 and O2 from pure water, which occurs steadily and as the reaction proceeds indicating migration of electrons photogenerated in GaN:ZnO to the surface of the Cr2O3 shell to reduce H+ to H2. Addition of the Rh-loaded catalyst to the reactant suspension containing the Rh/Cr2O3loaded catalyst results in a marked decrease in the rates of both H2 and O2 evolution owing to water formation from H2 and O2 on unmodiﬁed Rh nanoparticles, which is rather signiﬁcant in the OWS process. Clearly, in such cases suppression of water formation is imperative to achieve eﬃcient evolution of H2 and O2 gases. If silver nitrate is used as the sacriﬁcial electron acceptor, the quantum eﬃciency for O2 evolution rises to 51% at 420–440 nm, a nearly 20-fold greater eﬃciency than for the OWS process.101 The photocatalytic activity of the modiﬁed (Ga1xZnx)(N1xOx) solid solution can be increased further by improving the H2-evolution site and by modifying its composition to extend the absorption edge to longer wavelengths. The photocatalytic activity of (Ga1xZnx) (N1xOx) for OWS is strongly dependent on both the co-catalyst and the crystallinity and composition of the material. The quantum eﬃciency of (Ga1xZnx)(N1xOx) with Rh and Cr mixed-oxide nanoparticles is 2–3% at 420–440 nm, the highest reported eﬃciency for overall water splitting in the visible-light region.102 Fig. 5 illustrates the time course of the OWS process using (Ga1xZnx)(N1xOx) with various cocatalysts under visible light irradiation (l4 400 nm).

Fig. 5 Time course of the OWS process with (Ga1xZnx)(N1xOx) with various co-catalysts under visible light irradiation (l 4 400 nm). Circles denote Rh–Cr mixed-oxide co-catalyst, triangles denote RuO2, solid symbols denote H2 production, open symbols denote O2 evolution: catalyst (0.3 g); aqueous H2SO4 solution adjusted at pH 3.0 for RuO2-loaded sample and at pH 4.5 for Rh2–yCryO3-loaded sample (370 mL); light source, high-pressure Hg lamp (450 W); inner irradiation-type reaction vessel made of Pyrex with an aqueous NaNO2 solution (2 M) ﬁlter. Reproduced with permission from Maeda et al. Pure Appl. Chem., 2006, 78, 2267. Copyright 2006 by the International Union of Pure and Applied Chemistry.

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Fig. 6 Schematic illustration of the mechanism of the overall H2O splitting on Rh2yCryO3/ (Ga1xZnx)(N1xOx) photocatalyst. Reproduced with permission from Maeda, J. Phys. Chem. B, 2006, 110, 13107. Copyright 2006 by the American Chemical Society.

Fig. 6 displays the mechanism of the OWS process on the Rh2yCryO3/ (Ga1xZnx)-(N1xOx) photocatalyst. The photocatalytic performance of the latter is strongly pH-dependent but largely independent of gas pressure.103 It exhibits stable and high photocatalytic activity at pH 4.5 for 72 hrs but much lower at pH 3.0 and pH 6.2, attributed to corrosion of the co-catalyst and hydrolysis of the catalyst. The Rh2yCryO3 dispersion as the co-catalyst on the (Ga1xZnx)(N1xOx) surface promotes H2 evolution, taken to be the rate-determining step for overall water splitting. Among the range of co-catalysts examined, the Rh2yCryO3 mixed oxide is the most eﬀective for enhancing the photocatalytic activity of (Zn1+xGe)(N2Ox), consisting of a solid solution of ZnO and ZnGeN2 (band gap, 2.7 eV), primarily through the production of highly eﬀective H2-evolution sites. The optimally nitrided catalyst with composition (Zn1.44Ge)(N2.08O0.38) loaded with Rh2yCryO3 (3.0 wt% Rh, 0.2 wt% Cr) provides photocatalytic overall water decomposition under visible light irradiation with a quantum eﬃciency ca. 0.20% at 420 nm.104 Loading the zinc germanium oxynitride (Zn1.44Ge)-(N2.08O0.38) catalyst (band gap 2.7 eV) with RuO2 as the co-catalyst results in an eﬀective photocatalyst for overall water decomposition, achieving stoichiometric and stable H2 and O2 production under both UV and visible light irradiation.105 A ﬁrst successful example of a non-oxide photocatalyst (b-Ge3N4) for overall water splitting was reported by Sato and coworkers.106 Although b-Ge3N4 displays negligible activity for overall water splitting, it becomes photocatalytically active under UV irradiation (l 4 200 nm) when loaded with RuO2 nanoparticles as the H2 evolution sites. The photocatalytic activity of RuO2-loaded b-Ge3N4 for OWS depends strongly on reaction conditions, with the highest activity obtained in 1 M H2SO4 aqueous solution.107 Rates of both H2 and O2 evolution decrease as reaction progresses due to the photoreduction of O2, to the collapse of the catalyst surface by elusion of Ge cations, and to a loosened interfacial contact between the b-Ge3N4 and the loaded RuO2 nanoparticles. However, up to 80% of the initial activity can be recovered by calcination of the used catalyst at 673 1K in air for 5 hrs followed by reloading with RuO2. Photochemistry, 2009, 37, 300–361 | 319 This journal is

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Solar water splitting with a composite polycrystalline-Si/doped TiO2 thin-ﬁlm electrode is a promising new approach to high-eﬃciency and low-cost solar-to-chemical energy conversion.108 This composite thin-ﬁlm electrode has a strong advantage over thin-ﬁlm solar cells in that it avoids the use of expensive transparent conductive oxides such as indium tin oxide. Visible-light-responsive TiO2 thin ﬁlms (vis-TiO2) exhibit unique declined O/Ti composition from the surface (2.00 0.01) to deep inside the bulk (1.93 0.01). They were successfully developed at 873 1K by applying a radio-frequency (rf) magnetron sputtering deposition method.109,110 Pt-loaded vis-TiO2 decomposes water in the presence of methanol for H2 production or in the presence of a 0.05 M AgNO3 solution for O2 production under visible light (l 4 420 nm) irradiation. The stoichiometric and separate evolution of H2 and O2 from H2O was achieved successfully using an H-type glass container even under visible light. Selli and coworkers111 also proposed a two-compartment Plexiglas cell to produce H2 and O2 separately from photocatalyzed water splitting on a thin TiO2 layer deposited on a ﬂat Ti electrode. Of the many metal oxides (e.g. TiO2, WO3, and Fe2O3) that can be used as photoanodes in thin-ﬁlm form, TiO2 remains one of the most promising because of its low cost, chemical inertness, and photostability. As such, novel C-doped TiO2 (TiO2xCx) nanotube arrays with high aspect ratios for maximizing the photocleavage of water under white light irradiation have been developed by Park et al.112 Fig. 7 illustrates schematically the structure of the TiO2 photoanode to optimize its photocatalytic activity for water splitting: (1) TiO2 absorbs a considerable fraction of the solar spectrum; (2) all e-h pairs on excitation are located within the space charge layer (5–200 nm) at the electrolyte/ semiconductor interface; and (3) a thicker TiO2 photoanode is desirable to maximize absorption of solar photons. The synthesized TiO2xCx nanotube arrays show much greater photocurrent densities and a more eﬃcient water splitting process under visible light illumination

Fig. 7 Schematic structure of the TiO2 photoanode to optimize its photocatalytic activity for water splitting. Reproduced with permission from Park et al. Nano Letters 2006, 6, 24. Copyright 2006 by the American Chemical Society.

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(l 4 420 nm) than do pure TiO2 nanotube arrays. The total photocurrent was more than 20-fold greater than with a P-25 TiO2 nanoparticulate ﬁlm under white light illumination.

3.

Solar energy conversion (solar cells)

Electrical power generation through photovoltaic devices and dye-sensitized solar cells (e.g. the Graetzel DSSC cells) inherently implicate TiO2 owing to low cost and stability features of this metal oxide. Proof of concept of these DSSCs was established in the many studies of the 1990s.113 Current studies try to enhance the eﬃcacy of DSSCs through increased eﬃciency of energy conversion and use of dyes and processes that are stable and durable. A recent review article114 discussed three major ways to utilize nanostructures for the design of solar energy conversion devices: (i) mimicking photosynthesis with donor–acceptor molecular assemblies or clusters, (ii) semiconductor-assisted photocatalysis to produce fuels (e.g. H2), and (iii) nanostructure semiconductor-based solar cells. The article further highlighted recent developments in these areas pointing out factors that limit optimization eﬃciency. Also discussed were strategies to employ semiconductor and metal nanoparticles ordered assemblies, inorganic– organic hybrid assemblies, and carbon nanostructures in energy conversion schemes. One such nanostructured assembly involves size-quantized CdTe nanoparticle as the donor and a water-soluble C60 derivative as the acceptor that have led photovoltaic thin ﬁlms to display photon-to-current conversion eﬃciencies of 5.4%.115 Dye-sensitized solar cells with a nanostructured TiO2 ﬁlm continue to be investigated extensively ever since the ﬁrst report by O’Regan and Graetzel in 1991113 that attracted considerable academic and industrial attention as an eﬃcient alternative to conventional inorganic photovoltaic devices because of potential low costs, a large surface area of the nano-porous ﬁlms to anchor photosensitizers at near-monolayer levels for eﬃcient light harvesting and electron injection; the incident photon-to-electric current conversion eﬃciency from the UV to the near IR region was ca. 10.6% (AM 1.5). Several excellent review articles have appeared.116 In this regard, recent progress on processes taking place in dye-sensitized nano-crystalline solar cells has been reviewed by Peter;117 areas characterized by controversy or a poor understanding have been highlighted. 3.1

Dye-sensitized solar cells (DSSCs)

The operating principle of a dye sensitized solar cell or photochemical solar cell is similar to the photosynthetic conversion system in that a RuII-based dye anchored on nanocrystalline TiO2 particles harvests natural or simulated sunlight.118,119 Photoexcitation of the sensitizer (S) is followed by injection of an electron into the conduction band of the metal-oxide semiconductor ﬁlm, subsequent to which the dye is regenerated by the redox system, typically the I/I3 couple, itself being regenerated at the counter-electrode by electrons that have passed through the load. Photochemistry, 2009, 37, 300–361 | 321 This journal is

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For practical applications that necessarily require large scale modules other than the 1 cm2 laboratory cells, sealing volatile organic-solvent based electrolytes has been an issue with the thermostability of the DSSCs. Overcoming such drawback has required usage of p-type inorganic semiconductors, organic hole-transport materials, and solvent-free polymer electrolytes incorporating the redox couple triiodide/iodide.119 Usage of (non-volatile) solvent free room-temperature ionic liquid electrolytes (e.g. 1-ethyl-3-methylimidazolium di-cyanamide) yields an eﬃciency of 6.6% under simulated full sunlight (AM 1.5; 100 mW cm2) with the iodide/triiodide couple, although this led to some instability under visible light.119 Despite such shortcomings, however, recent demonstration of relatively long-term thermostability with an amphiphilic polypyridyl RuII(NN)3 sensitizer has injected some new momentum in the research of DSSCs with TiO2 and related metal oxides. Use of 1-propyl-3-methylimidazolium iodide (PMII) and 1-ethyl-3methylimidazolium thiocyanate yields a 6.4% eﬃciency under simulated AM 1.5 full sunlight. Co-grafting the Z-907 amphiphilic RuII(NN)3 sensitizer and 3-phenylpropionic acid (PPA) as the co-adsorbed spacer leads to increased eﬃciencies under similar conditions.120 Eﬃciencies of 10.2% were obtained with a newly developed heteroleptic Ru(II) sensitizer (Z-910).121 The oxidized states of two amphiphilic polypyridyl Ru(II) sensitizers [Ru(dcbpy)(L)(NCS)2, {dcbpy = 4,40 -dicarboxylic acid-2,2 0 -bipyridine and L = N,N-di(2-pyridyl)dodecylamine or N,N-di(2-pyridyl)tetradecylamine} proved more stable than other RuII(NN)3 sensitizers with the SCN ligand. Anchoring these sensitizers on dye-sensitized TiO2 solar cells gave eﬃciencies of 8.2% (100 mW cm2; AM 1.5) and eﬃciencies Z 8.7% at lower irradiances.122 With other amphiphilic ligands, the performance of the heteroleptic Ru(II) complexes, when anchored onto thick TiO2 nanocrystalline ﬁlms via two carboxylic acid groups in TiO2-based solar cells, gave an overall conversion eﬃciency of ca. 9% under AM 1.5 sunlight.123 Coordinative interactions between diﬀerent components in a dye-sensitized solar cell and especially with the 1-methylbenzimidazole (MBI) additive have been examined by resonance Raman scattering.124 In a complete solar cell with I2 and LiI in the electrolyte, I3 exchanges with the SCN ligand of the dye bis(tetrabutylammonium)-cis-bis-(thiocyanato)-bis(2,20 -bipyridine-4-carboxylicacid-4 0 -carboxylate)Ru(II). The choice of cation in the salt inﬂuences the ligand stability of the dye. In a complete solar cell the concentrations of Li+ and MBI have to be balanced to avoid SCN loss from the dye. Quinoline additives inﬂuence the I–V characteristics in an I/I3 redox electrolyte solution on the performance of a bis(tetrabutylammonium)-cisbis(thiocyanato)-bis(2,2 0 -bipyridine-4-carboxylic acid-4 0 -carboxylate)Ru(II) sensitized TiO2 solar cell125 and enhance the open-circuit photovoltage (Voc) and ﬁll factor (ﬀ), but reduce the short circuit photocurrent density (Jsc). Aminotriazole additives also inﬂuence the current-voltage characteristics of this dye-sensitized TiO2 solar cell under AM 1.5 (100 mW cm2),126 most of which enhance the Voc, the ﬀ and the solar energy conversion eﬃciency (Z), but reduce the short-circuit photocurrent density (Jsc). Addition of 3-amino-1H-1,2,4-triazole gave the highest Z (7.6%) comparable to that of 4-t-butylpyridine. 322 | Photochemistry, 2009, 37, 300–361 This journal is

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A solar-to-electric conversion eﬃciency of 5.1% was achieved with a highly redox-stable indoline dye, compared to 5.8% for the [cis-RuII(2,2 0 bipyridyl-4,4 0 -dicarboxylate)2-(NCS)2] dye under otherwise identical experimental conditions.127 The expected lower costs and the ease of preparation of the indoline dyes make them especially attractive for mass production. Overlayer thin ﬁlms of Ru(II) and Zn(II) vinyl bipyridine complexes have been formed on top of Ru(II) dye complexes adsorbed on TiO2 by reductive electropolymerization to create an eﬃcient, water-stable photoelectrode (or electrodes).128 The thermodynamics and kinetics of binding onto nanocrystalline TiO2 of ﬁve structurally diﬀerent Ru(II) complexes in the number of possible Photochemistry, 2009, 37, 300–361 | 323 This journal is

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anchoring carboxy groups (1, 2, 4, or 6) attached to coordinated bipyridyl ligands and in the number of auxiliary ligands (bipyridine, CN, or SCN) have been reported.129 The dyes predominantly chemisorb on TiO2 in a bridging mode in which carboxy group oxygens bond to separate Ti atoms on the TiO2 surface. The more weakly bound monocarboxy dye displays the lowest short-circuit current density and open-circuit voltage under simulated solar illumination in a photoelectrochemical cell. The most weakly bound species (viz. the monocarboxy dye) yields inferior photoelectrode properties, whereas diﬀerences between dyes that contain at least one dicarboxy ligand result primarily from diﬀerences in the light absorption and energetic properties of the metal complexes. Such observations suggest an important role for the linkage to the TiO2 surface in achieving temporal stability and in tuning both the steady-state quantum yield and the magnitude of the predominant back-reaction rate in dye-sensitized TiO2-based solar cells. Given the role that porphyrins play as light harvesters in photosynthesis and the relative ease with which a variety of covalent or noncovalent porphyrin arrays can be constructed, some metalloporphyrins were tested on TiO2 semiconductors and reasonable eﬃciencies measured.130 Some of these show relatively high incident monochromatic photon-to-current conversion eﬃciencies compared to those observed for the Cu-containing paramagnetic metalloporphyrins. The photovoltaic performance of several porphyrin-derivatized TiO2 ﬁlms in regenerative photoelectrochemical cells has been reported and compared to that in cells sensitized with the Ru(2,2 0 -bipyridyl-4,4 0 dicarboxylate)2(NCS)2 dye.131 Diﬀerences in eﬃciencies of the porphyrin sensitizers tetrakis-(3 0 ,5 0 -di-tert-butylphenyl)porphyrin, tetrakis(3 0 ,5 0 -ditert-butylphenyl)porphyrinzinc(II), and tetrakis(4 0 -carboxyphenyl)porphyrin are due to poor electronic coupling of the latter two sensitizers to the TiO2 conduction band. Anchoring the unsymmetrical phthalocyaninato-metal complexes, ZnL and Ru(4-picoline)2L gave the highest monochromatic incident photon-to-current conversion eﬃciency (IPCE) of 1.6% at 690 nm for a solar cell based on the Pc-Zn sensitized nanostructured TiO2 electrode; an IPCE of 23% at 630 nm was obtained for the Pc-Ru sensitized electrode.132 Overall conversion eﬃciencies (Z) at a simulated AM 1.5 (100 mW cm2) of 0.03% and 0.40% for the Zn and Ru complexes were achieved, respectively. A large sized DSSC (100 mm 100 mm) was fabricated to examine the inﬂuence of electrode distance, TiO2 nanoparticle size, thickness of TiO2 nanoporous layer in the 1-ethyl-3-methylimidazolium bis(triﬂuoromethanesulfonyl)amide electrolyte system.133 The short-circuit current (Jsc) is greatly inﬂuenced by these factors compared with a conventional volatile electrolyte system; Z = 4.5% for a 9 mm 5 mm sized cell. For a larger cell, Z = 2.7% in the ionic liquid system and 2.4% in an ion–gel system, based on the active area (2.3 and 2.0%, respectively, based on total area). Fabrication of a DSSC in an ionic liquid environmentally friendly electrolyte LiI(C2H5OH)4–I2 gives Z = 4.9% under AM 1.5 irradiation (100 mW cm2).134 Dye-sensitized solar cells constructed with nanocomposite organic– inorganic sol–gel electrolytes and a TiO2 nanocrystalline ﬁlm, also based 324 | Photochemistry, 2009, 37, 300–361 This journal is

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on a sol–gel nanocomposite material, have, among other advantages, connected with nanocomposite electrolytes the balance between hydrophilic and hydrophobic domains. These allow reducing the polarity-connected repulsive forces developed between the titania-dye system and the electrolyte. The overall eﬃciencies were 5 to 6% under 100 mW cm2 illumination (1 Sun, AM 1.5).135 A DSSC constructed by spray pyrolysis deposition incorporating a porous TiO2/dense TiO2/SnO2:F junction drastically shortened the fabrication period of the working electrodes.136 Optimizing the porosity of the TiO2 ﬁlm gave a conversion eﬃciency of 5.1% under quasi-sunlight illumination (AM 1.5; irradiance 100 mW cm2). Several room-temperature ionic liquids based on 1-alkyl-3-methylimidazolium cations, have been used to dissolve the 1-ethyl-3-methylimidazolium iodide/I2 redox couple in electrolytes to examine performance and characteristics of the DSSCs;137 the conversion eﬃciency under 100 mW cm2 was ca. 5.5%. Rigid and ﬂexible dye-sensitized solar cells employing mechanically stable porous thick ﬁlm electrodes of nanocrystalline TiO2, prepared at low temperature from mixed pastes of nanocrystalline TiO2 particles and TiCl4, TiOSO4, or Ti-tetraisopropoxide, achieved conversion eﬃciencies as high as 4.2% (rigid DSSC) and 2.5% (ﬂexible DSSC) under AM 1.5 simulated sunlight (1 Sun).138 The DSSC eﬃciency improved signiﬁcantly by post-heat treatment of the ﬁlms at 450 1C. A new method for fabricating plastic ﬁlm-based mesoporous TiO2 electrodes for eﬃcient DSSCs has been described in which TiO2 nanocrystalline layers electrophoretically deposited on indium-tin-oxide (ITO)-coated polyethylene terephthalate (PET) ﬁlm were post-treated with chemical vapor deposition (CVD) of Ti alkoxide, followed by UV light irradiation at temperatures below 110 1C.139 A ﬁlm electrode bearing a Ru(II) complex dye-sensitized TiO2 on ITO–PET yielded a solar energy conversion eﬃciency of 3.8%. Cost-eﬃcient and ﬂexible dye-sensitized solar cells have been reported for Pt counter-electrodes prepared by direct and pulse current electrodeposition.140 Application of pulse-electrodeposited Pt which has a 1.86 times greater surface area compared to direct current electrodeposited Pt increases the short-circuit current and the conversion eﬃciency from 10.34 to 14.11 mA cm2 and from 3.68% to 5.03%, respectively. A ﬂexible solar cell fabricated with a pulse current electrodeposited Pt counterelectrode displayed a conversion eﬃciency of 0.86%. Commonly used transparent conducting oxides (TCOs) in electrodes of dye-sensitized solar cells can be replaced by conducting meshes to provide a TCO-less ﬂexible working electrode.141 Preliminary results indicate that the electrode can be sintered; an eﬃciency of 1.49% at an irradiance of 100 mW cm2 was obtained in a liquid-state DSSC. Thin ﬁlms of nanoporous TiO2 semiconductor prepared on a conducting F-SnO2/glass substrate by sol–gel and by a paste-squeegee method showed that an increase in ﬁlm thickness increases both the amount of dye absorbed and the energy conversion eﬃciency.142 A multimode 28-GHz microwave heating system was used to produce a TiO2 nanocrystalline material to fabricate a well-sintered thin ﬁlm electrode on transparent conductive Photochemistry, 2009, 37, 300–361 | 325 This journal is

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PET-ITO electrode for use in DSSCs;143 the energy conversion eﬃciency was 2.16%. Nanocrystalline titania containing a nanotube structure (TiNT) has been synthesized by a surfactant-assisted templating mechanism using tetraisopropyl orthotitanate (TIPT) modiﬁed with acetylacetone (ACA)/laurylamine hydrochloride (LAHC);144 the solar energy conversion eﬃciency was greater (Z = 8.43%; ﬁlm thickness, 8.2 mm). A mesoporous nanocrystalline TiO2 layer prepared on a conductive ITO-coated poly-(ethylene terephthalate) (PET) ﬁlm by a low-temperature process was used to fabricate a solar cell from a dye-sensitized TiO2-coated ITO-PET ﬁlm with a Ru(II)-bipyridyl complex used as the photoanode in a methoxyacetonitrile-based iodide/triiodide redox electrolyte; conversion eﬃciencies of 4.1% and 4.3% were obtained, respectively, for incident solar energies of 100 and 23 mW cm2.145 Kim and coworkers146 have reported a solar cell in the conﬁguration ITO glass/Ru)II)-red dye-adsorbed TiO2/iodine electrolyte/sputtered Pt/ITO glass, and used a Eu-doped LiGdF4 (LGF) luminescent material for high-energy-photon dividing; the conversion eﬃciency of the cell at l = 600 nm was 4.8%. A tandem structure has been introduced to improve the spectral response of DSSCs without loosing their high external quantum yield.147 The tandem-structured cell employing two Ru(II) complexes exhibited a high photocurrent (15.3–15.9 mA cm2) and a greater conversion eﬃciency (7.3–7.6%) under AM 1.5 (100 mW cm2) irradiation than either of the single DSSCs (5.2–6.1%). Amao et al.148 have developed a DSSC using visible-light sensitization of chlorophyll-a derivative (Chl-e6) immobilized on a TiO2 ﬁlm, which leads to an eﬀective electron injection from the excited singlet state of Chl-e6 to the conduction band of TiO2. The IPCEs of the DSSCs were 11.0% at 400 nm, 4.7% at 541 nm and 7.9% at 661 nm. New lignin derivatives (lignophenols) containing phenol, p-cresol, catechol, resorcinol and pyrogallol have been examined as possible sensitizers for DSSCs of porous TiO2; the conversion eﬃciencies, however, were rather low (Z ca. 0.11%; AM 1.5 irradiation; 100 mW cm2).149 Multi-colored organic dye-sensitized solar cells (dyes 1 to 4) have been developed by optimizing electrolytes and counter-electrodes to enhance the transparence of both by reducing the concentration of I2 dissolved in imidazolium-based electrolytes and by employing a 0.5-nm thick Pt layer.150 When connected in series, the DSSCs exhibited a 2.1% conversion eﬃciency under AM 1.5 irradiation (1 Sun, 100 mW cm2); the total semiconductor area was 25 cm2. Using a poly(vinylideneﬂuoride-co-hexaﬂuoropropylene) (PVDF-HFP)based polymeric solid electrolyte (PSE) ﬁlm for DSSCs is almost always accompanied with a decrease in short-circuit current density (Jsc).151 The behaviors of the I/I3 redox reaction on chemically polymerized p-toluenesulfonate-doped poly(3,4-ethylenedioxythiophene) (PEDOT-TsO) and sputtered-Pt electrode have been characterized to compare performance as counter-electrodes in DSSCs. The photovoltaic performance of the DSSCs with the PEDOT-TsO counter-electrode improved with the thickness of PEDOT-TsO in an ionic liquid electrolyte.152 The thickness of the Pt ﬁlm 326 | Photochemistry, 2009, 37, 300–361 This journal is

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in a platinized counter-electrode had signiﬁcant consequences on the performance of DSSCs. When the Pt ﬁlm thickness exceeded 100 nm, further Pt deposition had no signiﬁcant eﬀect on conductivity improvement, and in the range 10–415 nm, the Pt ﬁlm thickness had no signiﬁcant inﬂuence on the performance of the DSSC.153 A DSSC composed of a 10-nm Pt ﬁlm counterelectrode showed a conversion eﬃciency of ca. 5%. Three novel perylene polyimides (PPIs) with band gap energies of 2.16, 2.19 and 2.25 eV containing p-n diblock units were designed and synthesized for use in dye-sensitized mesoporous TiO2 solar cells; introduction of 4,4 0 -bisaminetriphenylamine enhanced the optoelectronic conversion eﬃciency as well as the solubility that is favorable in the fabrication of DSSCs.154 New large size (100 100 mm2) transparent conductive oxide (TCO) ﬁlms, i.e. F-doped tin oxide (FTO) ﬁlms coated on indium-tin oxide (ITO) ﬁlms, have been developed for dye-sensitized solar cells using spray pyrolysis deposition at a substrate temperature of 350 1C in ITO and 400 1C in FTO. DSSCs made by the conventional method displayed an energy conversion eﬃciency of 3.7%.155 However, the conversion eﬃciency of large Photochemistry, 2009, 37, 300–361 | 327 This journal is

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cells on scale-up of DSSCs can be reduced drastically owing to high internal resistance when DSSCs are fabricated with ordinary materials and processes of mini-size cells.156 Hence, the need to reduce sheet resistance of the transparent conductive substrates. Highly conductive transparent substrates fabricated with nickel grids formed by an electroplating process and F-doped SnO2/ITO double layered TCOs achieved low sheet resistance (0.28 Om) and 66% light transmittance, along with a 4.3% conversion eﬃciency based on total area and 5.1% based on the active area (100 mm 100 mm size cell). A titania nano-network structure composed of single-crystal-like anatase nanowires has been successfully synthesized by a surfactant-assisted ‘‘oriented attachment’’ mechanism at low temperature (353 1K).157 Application of these titania anatase nanowires as the TiO2 thin ﬁlm of dye-sensitized solar cells achieved a light-to-electricity conversion yield of 9.3%. Naphthyridine and acridinedione coordinated ruthenium(II) complexes have been employed as photosensitizers in dye-sensitized solar cells.158 A maximum current conversion eﬃciency of ca. 7.7% was obtained for the 5-amino-4-phenyl-2-(4-methylphenyl)-7-(pyrrolidin-1-yl)-1,6-naphthyridine8-carbonitrile coordinated Ru(II) complex. Time constants measured by kinetic techniques have been interpreted,159 and the chemical diﬀusion coeﬃcient of electrons in nanostructured semiconductor electrodes and dye-sensitized solar cells has been determined.160 Core–shell type nanoparticles with SnO2 and TiO2 cores and zinc oxide shells have been prepared and characterized by surface-sensitive techniques to probe their morphology for use in DSSCs.161 For the SnO2 core, addition of ZnO improves overall cell eﬃciency owing to improved light scattering, dye uptake and decreased recombination. By contrast, dye-sensitized solar cells based on Zn-modiﬁed TiO2 ﬁlms show decreasing eﬃciency upon addition of Zn, primarily because of a drop in photocurrent due to slow electron transport associated with the formation of the ZnO coating around each particle. A photoelectrode using ZnO-coated TiO2 nanoparticles was prepared for use in ﬂexible DSSC devices to provide an inherent energy barrier between the electrode and the electrolyte interface.162 The overall conversion eﬃciency increased from 0.71 to 1.21% under a light irradiance of 20 mW cm2 (=0.2 Sun) due to the reduced recombination of photoinjected electrons without any post-treatment. A ZnO-covered TiO2 ﬁlm prepared by incorporating a small quantity of particulate ZnO in a TiO2 matrix using the thermal chemical vapor deposition has been used in a dye-sensitized solar cell; a 23% increase in the solar conversion eﬃciency was obtained relative to a DSSC containing a bare TiO2 ﬁlm electrode.163 Formation of a potential barrier by ZnO at the TiO2/electrolyte interface and the blocking of the TiO2 surface states by deposited ZnO reduces back-electron transfer from the conduction band of TiO2 to I3 in the electrolyte and increases both the open-circuit voltage by 17% and the photocurrent density by 12%. Use of pure anatase TiO2 nanocrystallites prepared by hydrolysis of TiCl4 aqueous solution, and TiO2 mesoporous ﬁlms with precisely controlled morphology prepared using TiO2 nanocrystallites for dye-sensitized solar cells shows that the amount of adsorbed dye on the ﬁlm and photocurrent of 328 | Photochemistry, 2009, 37, 300–361 This journal is

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the cell are proportional to the roughness factor of the ﬁlms, but depend neither on the particle size nor on pore size of the ﬁlms under a roughness factor of 1400.164 The highest photon conversion eﬃciency (5.65%) was obtained for ﬁlm morphologies with a roughness factor of 1341. Polyanilines used as hole conductors on the photovoltaic behaviors of dye-sensitized solar cells show that the conductivity of the polyanilines greatly aﬀects the performance of the devices, with an intermediate conductivity of 3.5 S cm1 being optimal.165 A conducting polymeric material with a large band gap, a suﬃcient conductivity, a homogeneous surface, and a small cluster size seems desirable as a hole conductor in DSSCs. TiO2 nanocrystalline electrodes for use in dye-sensitized solar cells have been fabricated on transparent conductive FTO glass electrode by 28-GHz microwave irradiation at 0.7 kW for 5 min for rapid processing; the photoelectron energy conversion eﬃciency achieved was 5.51%.166 The room-temperature ionic liquid N,N-diethyl-N 0 ,N 0 -dipropyl-N00 -hexylN00 -methylguanidinium iodide (SGI) has been used successfully as an iodide source for a nanocrystalline DSSC sensitized with the Ru(II) dye Z-907.167 Devices with solvent-free, ionic liquid electrolytes yield a power conversion eﬃciency of 5.9% under AM 1.5 radiation (9.47 mW cm2). The slow diﬀusion of triiodide in solvent-free SGI-based electrolyte appears to be the limiting step for high power application. Moreover, the organic solventbased electrolyte containing SGI as the iodide source gives as good a photovoltaic performance as that obtained with an imidazolium salt. Evolution of device eﬃciencies using the N-719, Z-907, and K-19 Ru(II) sensitizers during the thermal aging of DSSCs at 80 1C shows that the N-719 dye is rather unstable, even though it exhibits an 11% photon-to-energy conversion eﬃciency. The poor stability is likely caused by sensitizer desorption at the high temperature.168 In contrast, the two amphiphilic Z-907 and K-19 sensitizers retain over 92% of their initial performance for 1000 hrs under this thermal stress. Cells based on the K-19 sensitizer show excellent photochemical stability when submitted to accelerated testing in a solar simulator at an irradiance of 100 mW cm2. Under otherwise identical conditions, the conversion eﬃciencies are 6.7% (N-719 sensitizer), 6.0% (Z-907) and 7.0% for the K-19 sensitizer. Design and performance of a ZnO nanowire-based dye-sensitized solar cell (Fig. 8) with the ZnO nanowires possessing a branched structure have been described. Dye-sensitized solar cells constructed with this wide band gap semiconductor exhibit energy conversion eﬃciencies of 0.5% with internal quantum eﬃciencies of 70%.169 The nanowires provide a direct conduction path for electrons between the point of photogeneration and the conducting substrate; they further oﬀer improved electron transport compared to ﬁlms fabricated with sintered nanoparticles. The devices have light harvesting eﬃciencies under 10%, indicating that current densities and eﬃciencies can be improved by an order of magnitude on increasing the nanowire surface area. Single-crystal (100–300 nm long) anatase TiO2 nanorods (diameter, ca. 20–30 nm) have been synthesized successfully by a surfactant-assisted hydrothermal method. A high light-to-electricity conversion yield of 7.29% was achieved by applying the titania as a thin ﬁlm in dye-sensitized Photochemistry, 2009, 37, 300–361 | 329 This journal is

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solar cells.170 Incorporating anatase TiO2 nanorods at a ca. 10 wt% level in a P-25 TiO2 ﬁlm electrode of a DSSC device enhances the solar energy conversion eﬃciency by about 42% as a result of a higher short-circuit photocurrent compared with that of a cell fabricated with bare P-25.171 Reasons for the invariance of sizes of nanostructures and for the enhancement of the solar energy conversion eﬃciency were discussed. Dye-sensitized solar cells made from TiO2 nanoribbon electrodes have a better photoelectrical energy conversion eﬃciency (Z = 2.2%) than those made from TiO2 nanorod electrodes (Z = 1.8%) under irradiation with a xenon lamp emitting an irradiance of 50 mW cm2.172 The rise and decay times of the photovoltage transients are 0.37 and of 9.32 ms for the nanoribbon solar cells, respectively, whereas for the nanorod cells they are 1.39 and 6.11 ms. The reason is that only saturated Ti(IV) species exist in the TiO2 nanoribbons surface, whereas there are unsaturated coordination Ti(III) species in the TiO2 nanorods surface that can trap the photogenerated 330 | Photochemistry, 2009, 37, 300–361 This journal is

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Fig. 8 Schematic of the nanowire-based dye sensitized solar cell. Reproduced with permission from Baxter and Aydil, Appl. Phys. Letters 2005, 86, 053114. Copyright 2005 by the American Institute of Physics.

electrons. Accordingly, the nanoribbon TiO2 solar cells have a better charge collection eﬃciency and lower interfacial charge recombination. The transfer of electrons from the conducting glass substrate to I3 ions in solution in dye-sensitized nanocrystalline solar cells is an important loss mechanism that can be suppressed using thin compact blocking layers of TiO2.173 Model simulations show that reliable information about DSSC properties, such as trapping distributions, can only be derived from transient or periodic photovoltage responses if the back-reaction is suppressed by the use of suitable blocking layers. A series of new benzothiazolium hemicyanine dyes (HC-1, HC-2, HC-3, HC-4, HC-5) have been synthesized for sensitization of nanocrystalline TiO2 electrodes by introducing either carboxyl, hydroxyl, or sulfonate anchoring groups onto the dyes skeletons.174 Results show that (i) ﬂuorescence quenching eﬃciencies of the dyes by colloidal TiO2, (ii) monochromatic incident photon-to-current conversion eﬃciencies (IPCEs) for dye-sensitized TiO2 electrodes, and (iii) the overall photoelectric conversion eﬃciencies (Z) for the dye-sensitized solar cells based on these hemicyanines depend strongly on the anchoring group types and decrease in the order: (carboxyl + hydroxyl) 4 carboxyl 4 (sulfonate + hydroxyl). This indicates the importance of the dyes’ adsorbing groups for their sensitization eﬀects in DSSCs. The combination (carboxyl + hydroxyl) as the anchoring groups lead to highly eﬃcient IPCEs over a wide spectral region with the maximum IPCE of 73.6% and a Z of 5.2% under AM 1.5 global simulated light (80 mW cm2) for the HC-1 based DSSC, resulting from the complex formation between HC-1 and TiO2 and the cathodic shift of the excited state oxidation potential. The inﬂuence of 1-methyl-3-propylimidazolium iodide on the I3/I redox behavior in 3-methoxypropionitrile has been examined; the photovoltaic performance of dye-sensitized solar cells reveals a photoelectric conversion eﬃciency of 7.17% under one Sun conditions (AM 1.5).175 Photochemistry, 2009, 37, 300–361 | 331 This journal is

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Diﬀerent solar cell photosensitizers such as [Pt(II)(diimine)(dithiolate)] complexes of general formula [Pt{X,X 0 (CO2R)2-2,2 0 -bipyridyl}(maleonitrile dithiolate)] (X = 3, 4, or 5 and R = H or Et) have been attached to a TiO2 substrate.176 When tested as solar cell sensitizers (R = H), the 3,3 0 -disubstituted bipyridyl complex gave an intermediate dye loading but superior photovoltaic performance relative to those of the other two. Compared to the cell eﬃciency of the well-known Ru polypyridyl N-719 sensitizer {ditetrabutylammonium salt of [RuL2-(NCS)2] (L = 2,2 0 bipyridyl-4,4 0 -dicarboxylato)} the device eﬃciency was relatively low (0.13% to 0.64%) because of a relatively low optical absorbance of these Pt sensitizers relative to the N-719 system. Novel conjugated organic dyes that have N,N-dimethylamine (DMA) moieties as electron donors and cyanoacetic acid (CAA) as electron acceptors have been used in dye-sensitized nanocrystalline TiO2 solar cells attaining maximal solar energy-to-electricity conversion eﬃciency of 6.8% under AM 1.5 irradiation (100 mW cm2). The DSSC was based on 2-cyano-7,7-bis(4-dimethylaminophenyl)hepta-2,4,6-trienoic acid (NKX-2569), approaching the 7.5–7.8% eﬃciency displayed by solar cells based on the N-719 dye sensitizer under otherwise identical conditions.177 The high 332 | Photochemistry, 2009, 37, 300–361 This journal is

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performance of the solar cells shows that highly eﬃcient electron injection from excited dyes to the conduction band of TiO2 occurs. High-eﬃciency organic-dye-sensitized solar cells can be controlled by the thickness of the nanocrystalline-TiO2 electrode.178 For instance, a DSSC system comprising a mesoscopic bilayer TiO2 ﬁlm as an electron collector and the D149 dye (see below) in conjunction with an acetonitrile- or ionic-liquid-based electrolyte exhibits a remarkable performance: conversion eﬃciencies were 6.67% for a 6.3-mm layer and 9.03% for a 12.6-mm layer. Fig. 9 illustrates the conﬁguration of the solar cell.

The synthesis and photophysical/electrochemical properties of a series of novel triphenylamine (TPA)-based organic dyes (TPAR1, TPAR2, TPAR4, and TPAR5), together with their application in nanocrystalline TiO2 DSSC devices have been reported by Liang et al.179 The TPA group and the rhodanine-3-acetic acid act as the basic electron donor unit and the electron acceptor, respectively. Introduction of a CH2QCH– group into the TPA unit exhibited better photovoltaic performance due to the increase of the electron-density donor moiety. The introduction of a methine (–CHQCH–) unit to the p bridge resulted in a red-shift and in a broadening of the absorption spectrum due to the expansion of the p-conjugation system. The TPAR4-sensitized DSSC showed an overall conversion eﬃciency of 5.84% under AM 1.5 irradiation (100 mW cm2). The two organic dyes with the donor-conjugated chain-acceptor (D-p-A) general structure involving the electron donor (pyrrolidine) and acceptor (cyano acrylic acid) groups, 2-cyano-3-{5-[2-(4-pyrrolidin-1-ylphenyl)vinyl]thiophen-2-yl}acrylic acid (denoted PT) and 2-cyano-3-{5-[2-(5-pyrrolidin-1ylthiophen-2-yl)vinyl]-thiophen-2-yl}acrylic acid (TT), have been investigated as sensitizers for nanocrystalline TiO2 solar cells.180 Solar-to-electrical energy conversion eﬃciencies are 2.3% for solar cells based on PT but less Photochemistry, 2009, 37, 300–361 | 333 This journal is

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Fig. 9 Molecular structure of the dye D149 and the conﬁguration of the DSSC. Reproduced with permission from Ito et al. Adv. Mater. 2006, 18, 1202. Copyright 2006 by Wiley-VCH Verlag GmBH & Co.

than 0.05% for those based on TT under simulated AM 1.5 radiation (100 mW cm2). Photoinduced absorption measurements show that the TT dye is not regenerated properly by the redox electrolyte after electron injection because of the 0.3 eV less positive HOMO level for the TT dye compared to PT. This results in a lower driving force for the regeneration of the oxidized dye by iodide in the electrolyte. Moreover, regeneration of the oxidized TT dye and electron injection from the excited TT dye is likely poor because of formation of dye aggregates and/or surface complexes. Such results highlight the notion that a small structural change in the dyes can result in signiﬁcant changes in the redox energies and binding features that dramatically aﬀect the performance of these dyes in DSSCs. Spherical voids acting as light scattering centers in nanocrystalline TiO2 ﬁlms obtained with polystyrene particles (diameter, 400 nm) enhance the photovoltaic performance by 25% on large areas, and provide an indication that these ﬁlms can be used with electrolytes of higher viscosity.181 Fast charge transport based on the exchange reaction of the I/I3 redox couple, observed in ionic liquids but not in molecular liquids, contributes to the high performance of dye-sensitized solar cells using ionic liquids in spite of their high viscosity.182 Particle size distribution of nanocrystalline TiO2 plays an important role on the performance of DSSCs. A narrow particle size distribution of nanocrystalline TiO2 increases the eﬃciency of DSSCs, whereas a wide distribution decreases it. Results reported by Pan et al.183 show that dye-sensitized solar cells with a double TiO2 layer display conversion eﬃciencies ranging from 2.91% to 4.21%. A dye-sensitized heterojunction with the conﬁguration n-TiO2/PD-CuPC-MV/ p-CuSCN {PD = 3,4-pyridinedicarboxylic acid anchored to TiO2, CuPC = copper(II) phthalocyanine tetrasulfonic acid ionically linked to PD, and MV is Methyl Violet complexed to CuPC} has been developed to demonstrate the applicability of molecular rectiﬁcation in dye-sensitized 334 | Photochemistry, 2009, 37, 300–361 This journal is

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solar cells as a strategy of suppressing recombination.184 Short-circuit photocurrent, open-circuit voltage, energy conversion eﬃciency, and incident photon-to-photocurrent conversion of this system are higher than those of the heterojunctions of conﬁgurations n-TiO2/PD-MV/p-CuSCN, n-TiO2/CuPC-MV/p-CuSCN, and n-TiO2/MV/p-CuSCN. The impressively high rectiﬁcation ratio and the mode of anchoring CuPC to TiO2 may be the cause of superior photovoltaic action of the cell TiO2/PD-CuPC-MV/ p-CuSCN.

The I3/I redox behavior in 3-methoxypropionitrile containing alkalimetal iodide complexes with crown ether and cryptand macrocycles has been examined by Shi and co-workers185 together with their application to Photochemistry, 2009, 37, 300–361 | 335 This journal is

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dye-sensitized solar cells. The photoelectric conversion eﬃciency of the various DSSCs ranged from 6.00% to 6.85%. Use of the new ionic liquid crystal 1-dodecyl-3-methylimidazolium iodide and iodine as the electrolyte in dye-sensitized solar cells led to a high shortcircuit photocurrent density and to a high light-to-electricity conversion eﬃciency, due to a self-assembled structure of the imidazolium cations resulting in high conductivity of the electrolyte.186 A series of allyl-functionalized imidazolium salts reported by Fei et al.187 (1-allyl-3-ethylimidazolium iodide and 1-allyl-3-propylimidazolium iodide) and used as electrolytes provide excellent eﬃciencies and good stability in dye-sensitized solar cells when subjected to an accelerated-light soaking test at 60 1C. Devices based on a binary ionic liquid electrolyte (B) and an organic solvent electrolyte (C; contains 3-methoxypropionitrile) exhibit an overall eﬃciency of 6.8% and 7.8%, respectively. The photoelectric conversion eﬃciency reached 8.0% under an irradiance of 30 mW cm2. Usage of a thin TiO2 semiconductor layer in dye-sensitized solar cells reduces the amount of dye coverage and thus fewer electrons are generated on illumination, so that it becomes necessary to include a light scattering layer to compensate.188 Scattering layers considered included TiO2-Rutile, ZrO2, and layers consisting of these two in various proporions. With a 4-mm thin titanium dioxide semiconductor layer as the photoelectrode and an additional light scattering layer (TiO2-Rutile and ZrO2 in a ratio of 1:3), eﬃciencies of 6.8% were achieved. Linking an N3 dye to another TiO2-attached N3 dye through trans-1,2bis(4-pyridyl)-ethylene (Fig. 10) enhanced the short-circuit photocurrent and yielded a higher conversion eﬃciency for the dye-sensitized solar cell with the pertinent TiO2 ﬁlm electrode.189 The overall energy conversion eﬃciency of the N3 dye-linked cell increased from 4.6 to 5.3% relative to a single N3 dye cell. Interfacial charge recombination in low-temperature processed dyesensitized solar cells can be retarded by addition of quaternary ammonium

Fig. 10 Schematic representation of the N3 dye linked to another N3 dye which is immobilized on a TiO2 particle. Adapted from Jang et al. Chem. Commun. 2006, 103.

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cations. This provides an alternative method to increase the eﬃciency of DSSCs by designing electrolytes that control the interfacial charge transfer of photogenerated electrons in the solar cells.190 In this regard, the electron lifetime (t) in DSSCs that use electrolytes containing the I/I3 redox couple in systems containing quaternary ammonium cations with various alkyl chain lengths increases in the order tTPA E tTBA . . . tTHA E tTHpA, where TPA, TBA, THA and THpA represent the tetra-propyl, -butyl, -hexyl and -heptyl ammonium cations, respectively. Under 1 Sun conditions (100 mW cm2), newly prepared electrolytes showed at most a 40% increase in energy conversion eﬃciency (Z) in comparison to conventional electrolytes, with the highest Z being 4.7% among reported Z of low-temperature processed DSSCs. The increase of Z was achieved by higher short-circuit current induced by longer electron diﬀusion length and higher open-circuit voltage as a result of higher electron density. Binary TiO2-GeO2 oxide electrodes enhance the eﬃciency of dyesensitized solar cells; they also yield signiﬁcantly higher short-circuit photocurrents Jsc when compared to cells fabricated with a TiO2 electrode.191 The ruthenium(II) complex [Ru(II)L2(NCS)2] (K8) with the novel ligand 4,4 0 -bis(carboxyvinyl)-2,2 0 -bipyridine (L) has been used as a charge transfer photosensitizer when anchored to a nanocrystalline TiO2 ﬁlm in TiO2-based solar cells fabricated with an electrolyte consisting of 0.6 M methyl-N-butylimidiazolium iodide, 0.05 M iodine, 0.05 M LiI, and 0.5 M 4-tertbutylpyridine in a 50/50 (v/v) mixture of valeronitrile and acetonitrile; the overall conversion eﬃciency was ca. 8.64% under standard AM 1.5 sunlight irradiation.192 Fig. 11 illustrates the UV-vis absorption spectrum of the Ru(II) K8 and N3 photosensitizers measured in DMF. The greater UV-vis absorption by K8 opens the way to designing more eﬃcient panchromatic sensitizers that absorb all the visible light, including the near-IR radiation by further modiﬁcation of the ligand architecture so as to improve power conversion eﬃciencies of dye-sensitized solar cells.

Fig. 11 UV-vis absorption spectrum of K8 (ı´max 555 nm, e 18 000 M1 cm1) and N3 (ı´max 535 nm, e 13 600 M1 cm1) measured in DMF. Reproduced with permission from Klein et al. Inorg. Chem. 2005, 44, 178. Copyright 2005 by the American Chemical Society.

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The dynamics of charge separation and recombination in liquid-electrolyte and solid-state solar cells have been examined employing a series of amphiphilic ruthenium dyes with varying hydrocarbon chain lengths that act as an insulating barrier for electron–hole recombination.193 Increasing the dye alkyl chain length results in slower charge recombination dynamics to both the dye cation and the redox electrolyte or solid-state hole conductor (spiro-OMeTAD); they are paralleled by reduced rates of both electron injection into the TiO2 electrode and dye regeneration by the I/I3 redox couple or by the spiro-OMeTAD system. Kinetic competition between electron recombination with dye cations and dye ground state regeneration by the iodide electrolyte is a key factor for liquid electrolyte cells, with the optimum device performance being obtained when the dye regeneration is just fast enough to compete with electron–hole recombination. Depending on chain length (C1, C6, C9, C13 and C18), conversion eﬃciencies from 1.3 to 3.2% for liquid-electrolyte DSSCs and from 2.3 to 3.1% for solid-state solar cells have been obtained.

The two novel heteroleptic sensitizers Ru((4,4-dicarboxylic acid-2,2 0 -bipyridine)(4,4 0 -bis-(p-hexyloxystyryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2 0 -bipyridine)-(4,4 0 -bis(p-methoxystyryl)-2,2 0 bipyridine)(NCS)2, coded K-19 and K-73, respectively, with high extinction coeﬃcients have enabled the fabrication of a new generation of thin ﬁlm dye-sensitized solar cells that yield high conversion eﬃciencies at full sunlight, even when viscous electrolytes based on ionic liquids or nonvolatile solvents are used.194 Electron transfer from excited K-19 to the conduction band of TiO2 is complete within 20 fs while charge recombination has a half-lifetime of 800 ms. An eﬃciency greater than 9% was obtained under standard AM 1.5 simulated sunlight conditions (100 mW cm2) when the K-73 sensitizer was combined with a nonvolatile ‘‘robust’’ electrolyte. A cell comprised of the K-19 dye and a binary ionic-liquid electrolyte gave a conversion eﬃciency of 7.1%. The devices exhibited good stability under light soaking at 60 1C for 1000 hrs. An energy conversion eﬃciency of ca. 7.0% was also obtained at full AM 1.5 sunlight for a DSSC that employed a 6.8 mm thick transparent TiO2 layer (20 nm sized particles), a scattering layer 338 | Photochemistry, 2009, 37, 300–361 This journal is

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(TiO2, 400 nm sized particles) of 4 mm thickness, a Z-907Na dye solution in a mixture of acetonitrile and tert-butyl alcohol (ratio, 1:1 v/v), and 3-phenylpropionic acid as co-adsorbent.195 At lower light irradiances of 53 and 30 mW cm2 the eﬃciencies were 7.4% and 7.7%, respectively. Ionic liquids containing nitrile and vinyl functional groups attached to imidazolium cations combined with various anions, e.g., iodide, bis[(triﬂuoromethyl)sulfonyl]imide ([TFSI]), or dicyanamide ([N(CN)2]), have been used successfully as electrolytes in dye-sensitized solar cells based on nanocrystalline TiO2 with the amphiphilic Ru(II) sensitizer K-19.196 The iodide salt was used in 3-methoxypropionitrile-based electrolytes. The performances of both types of devices demonstrate that the functional groups do not exert a detrimental eﬀect on the performance of the DSSCs. Good energy conversion eﬃciencies were seen (ca. 8%) at low light irradiances using a low-vapor-pressure organic solvent. The hole conducting materials thiophene-based homopolymer (P3HT) and the copolymers (poly(3TA-co-3HT)-1 and poly(3TA-co-3HT)-2) have been used in SnO2:F/TiO2/dye/polymer/Pt dye-sensitized solar cells.197 Overall power conversion eﬃciencies with the (poly(3TA-co-3HT)-1 and poly(3TA-co-3HT)-2) copolymers were greater (Z = 2.47% and 2.74%, respectively) than the eﬃciency of a similar device that used the P3HT alone (Z = 2.50%). This shows that the eﬀective interface adsorption between hole conducting polymers and the TiO2 layer enhances the overall photovoltaic performance. The devices further showed the usefulness of hole conducting polymers in lieu of liquid electrolytes in DSSCs. Application of an as-prepared transparent luminescent ﬁlm of uniform LaVO4:Dy nanocrystals which absorb UV light through backside illumination and down-convert it to visible light in dye-sensitized solar cells not only enhanced the lifetime of the solar cell but also improved the energy conversion eﬃciency by 23.3% relative to a similar solar cell coated with an undoped LaVO4 ﬁlm, since the down-converted light could be re-absorbed by the solar cell and generate current.198 Backside illuminated solar cells based on 6 mm long highly-ordered nanotube-array ﬁlms sensitized by a self-assembled monolayer of bis(tetrabutylammonium)-cis-(dithiocyan-ato)-N,N 0 -bis(4-carboxylato-4 0 -carboxylic acid-2,2 0 -bipyridine)ruthenium(II) (N719) show a power conversion eﬃciency of 4.24% under AM 1.5 sunlight.199 A superior photoresponse is obtained with acetonitrile as the dye solvent, attributed to the improved wetting characteristics of the dye solution in acetonitrile enabling self-assembled monolayers with higher surface coverage to be formed inside the nanotubes. Possible improvements in the eﬃciency of dye-sensitized photovoltaic cells using dyes capable of singlet ﬁssion into two triplets that produce two electron–hole pairs from a single photon absorbed have been examined by Paci and coworkers;200 the semi-empirical Pariser-Parr-Pople method and the time-dependent DFT method were used. Roh and coworkers201 report the use of a chemically deposited ZnO recombination barrier layer to improve the eﬃciency of TiO2-based dye-sensitized solar cells. With a 30-nm ZnO thick layer, the solar cells displayed a conversion eﬃciency of 4.51% due to the formation of an eﬃcient recombination barrier at the electrode/electrolyte interface. Further Photochemistry, 2009, 37, 300–361 | 339 This journal is

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increase in ZnO barrier thickness, however, leaks the electrons injected from the dye, owing to the low electron eﬀective mass of 0.2me, and decreases the eﬃciency. Highly porous and high surface area anatase TiO2 ﬁlms have been fabricated using a new spongy replica prepared by a layer-by-layer selfassembly technique involving a silver acetate solution; use of these porous TiO2 ﬁlms as the photoelectrode in DSSCs impoves the photocurrent/voltage characteristics of the solar cell resulting in a conversion eﬃciency of 2.66%.202 The characteristics of dye-sensitized nanostructured TiO2 solar cells with high eﬃciencies of light-to-electricity conversion of 11.1% and 10.2% have been investigated using impedance spectroscopy. It appears that such eﬃciencies result from the excellent transport and low recombination rate of electrons in TiO2.203 The high conduction band position maintains excellent electron injection from the excited dye molecules, a key reason for such highly eﬃcient DSSCs, as well as the high quality of the mesoscopic TiO2 ﬁlm and the optimized TiO2-electrolyte interface. In addition, the low series resistance and optimal distribution of the surface states in TiO2 result in an excellent ﬁll factor. Charge recombination processes at interfaces between F-doped tin oxide (FTO), TiO2, dye, and electrolyte play an important role in limiting the photon-to-electron conversion eﬃciency in dye-sensitized TiO2 solar cells. Xia et al.204 examined a high work function material such as titanium deposited by sputtering on FTO as an eﬀective blocking layer for preventing electron leakage from FTO without inﬂuencing electron injection. The XPS analysis indicated that Ti4+, Ti3+, and Ti2+ species as well as a small amount of Ti0 exist on FTO. Electrochemical and photoelectrochemical measurements also revealed that TiOx thin ﬁlms work as a compact blocking layer between FTO and the TiO2 nanocrystaline ﬁlm, thereby improving Voc and the ﬁll factor, and yielding a 15–20% better conversion eﬃciency for dye-sensitized TiO2 solar cells with ionic liquid electrolytes (Z = 3.5 to 4.4%). Thin Nb2O5 layers function as remarkable blocking layers when deposited by the rf magnetron sputtering method between FTO and a mesoporous TiO2 layer, thereby improving the open-circuit photovoltage and ﬁll factor; a power conversion eﬃciency over 5.5% was obtained at 1 Sun irradiation of TiO2 solar cells sensitized with the Z-907 dye in ionic liquid electrolytes.205 The two ruthenium(II) complexes [Ru(dcbpy)L(NCS)2] with dcbpy being 4,4 0 -dicarboxylic acid-2,2 0 bipyridine and L being either 3.8-bis(4-octylthiophen-2-yl)-1,10-phenanthroline (denoted CYC-P1) or 3,8-bis(4-octyl5-(4-octylthiophen-2-yl)thiophen-2-yl)-1,10-phenanthroline (CYC-P2) have been used as photosensitizers in nanocrystalline TiO2 dye-sensitized solar cells.206 The diﬀerence in light-harvesting ability between CYC-P1 and CYC-P2 has been associated mostly with the location of the frontier orbitals, in particular the HOMOs as described by semi-empirical computations (Fig. 12). An increase in conjugation length of the ancillary ligand decreases the energy of the metal-to-ligand charge transfer (MLCT) transition and reduces the molar extinction coeﬃcient because the HOMO is partially located on the ancillary ligand of the Ru(II) complex. Overall conversion eﬃciencies of the CYC-P1- and CYC-P2-sensitized solar cells are 6.01% and 3.42%, respectively, compared to the eﬃciency of 7.70% for a solar cell 340 | Photochemistry, 2009, 37, 300–361 This journal is

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Fig. 12 Energy level diagrams for the CYC-P1, CYC-P2 and the N3 ruthenium(ii) complexes, together with the bands of the TiO2 semiconductor and the I/I3 redox couple. Adapted from Chen et al. Adv. Funct. Mater. 2007, 17, 29.

fabricated with the same process and measured under otherwise identical conditions for the cis-di(thiocyanato)-bis-(2,2 0 -bipyridyl)-4,4 0 -dicarboxylateruthenium(II) sensitizer (N3).

A new high molar extinction coeﬃcient ion-coordinating ruthenium(II) sensitizer, [Ru-(4,4 0 -dicarboxylicacid-2,2 0 -bipyridine)(4,4 0 -bis(2-(4-(1,4,5,10tetraoxyundecyl)phenyl)ethenyl)-2,2 0 -biprydine)(NCS)2] (K60), in combination with a nonvolatile organic solvent-based electrolyte, has been used to fabricate a DSSC device that displays a photovoltaic eﬃciency of 8.4% under standard AM 1.5 sunlight (100 mW cm2).207 The device is reasonably stable under a continuous thermal stress at 80 1C or under light soaking at 60 1C for 100 hrs. Stability is maintained by stabilizing the TiO2/dye/ electrolyte and the Pt/electrolyte interface during the aging process.

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Succinonitrile, a molecular plastic crystal, has been modiﬁed to become a thermostable gel over a wide temperature range by introducing the hydrogen bond (O–H F) network on addition of silica nanoparticles and 1-butyl-3-methylimidazolium tetraﬂuoroborate (BMI-BF4).208 The gel electrolyte containing the I/I3 redox couple is also thermostable and highly conductive in the temperature region 20–80 1C. Addition of silica nanoparticles and BMI-BF4 had no adverse eﬀects on the mobility of I and I3 ions, whereas the absence of succinonitrile deteriorated this mobility. DSSC devices with this gel electrolyte showed solar-to-electric energy conversion eﬃciencies of 5.0–5.3% over the temperature range 20–80 1C. Aging tests showed that the cell maintains 93% of its initial conversion eﬃciency after being stored at 60 1C for 1000 hrs. Dye-sensitized solar cells with zinc stannate (Zn2SnO4) as the electron conductor and the N719 sensitizer yielded higher open-circuit potentials and lower IPCEs compared with similar standard TiO2-based solar cells; the overall eﬃciency was lower for zinc stannate even though the electrontransport properties of the latter oxide are as good as those of TiO2.209 The main diﬀerence in the behavior of both oxides is the injection eﬃciency, which in zinc stannate is caused by a higher conduction band position. Nonetheless, Zn2SnO4 shows promising properties for practical use in DSSC if dyes with higher LUMOs than those of the N719 dye sensitizer could be found.

The inﬂuence of the alkyl chain spacer length of perylene monoimide (PMI) dyes on device performance in dye-sensitized solar cells was reported by Zafera et al.210 Dyes with longer and branched alkyl chains exhibit higher eﬃciencies in DSSCs, with the highest (Z = 1.61%) obtained under standard conditions for the perylene imide derivative PMI-DA1. 342 | Photochemistry, 2009, 37, 300–361 This journal is

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The DSSC with the cyclohexyl derivative of PMI-CH yielded a conversion eﬃciency of 0.37%, whereas another derivative with a geometrically similar spacer (i.e. the 2,6-diisopropylphenyl derivative, PMI-AR) gave a twofold greater eﬃciency (Z = 0.60%). The diﬀerence is not due to mobility diﬀerences, but to diﬀerences in electrochemical potentials. The aromatic ring in PMI-AR is a stronger electron donor in comparison to cyclohexyl. Also, the higher electron density of the PMI-AR molecule, with the LUMO level higher in energy than in PMI-CH, results in a better electron injection to the TiO2 conduction band from the excited PMI-AR molecule.

DSSC devices based on nanocrystalline TiO2 have been fabricated with the amphiphilic ruthenium(II) sensitizer Na+[Ru(2,2 0 -bipyridine-4carboxylic acid-4 0 -carboxylate)-(4,4 0 -di-nonyl-2,2 0 -bipyridine)(NCS) 2] (Z-907), and a series of o-guanidinoalkyl acids as co-adsorbents.211 The guanidinoalkyl acids co-adsorbents increase the open-circuit voltage due either to suppression of the recombination process or to an upward shift of the TiO2 band-edge to negative potentials, but seem to have no adverse eﬀect on the photocurrent if an appropriate amount is used. Up to a 15% increase in total power conversion eﬃciency was achieved for the DSSC device using the GBA co-adsorbent (see below); the 8.3% eﬃciency under AM 1.5 irradiation is unprecedented for DSSCs that use the Z-907 sensitizer. Photochemistry, 2009, 37, 300–361 | 343 This journal is

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The amphiphilic polypyridyl Ru(II) complex cis-di(isothiocyanato)(4,4 0 -di-tert-butyl-2,2 0 -bipyridyl)(4,4 0 -dicarboxy-2,2 0 -bipyridyl)ruthenium(II) (K005) sensitizes TiO2 over a notably broad spectral range due to its intense MLCT bands at 537 and 418 nm; solar cells sensitized with the K005 dye display an eﬃciency of 3.72% (100 mW cm2, AM 1.5 simulated sunlight) without optimization of the TiO2 ﬁlms and the electrolyte.212 Zhu et al.213 examined the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells that incorporate oriented TiO2 nanotube (NT) arrays consisting of closely packed NTs of several mm length, with typical wall thicknesses and intertube spacings of 8–10 nm, and pore diameters of ca. 30 nm. The calcined material was fully crystalline with individual NTs consisting of B30 nm sized crystallites. Charge recombination is much slower in the NT ﬁlms, indicating that the NT-based DSSCs have signiﬁcantly higher charge-collection eﬃciencies than their nanoparticle (NP)-based counterparts. Dye-sensitized nanotube- and nanoparticle-based solar cells gave conversion eﬃciencies in the range 1.7–3.0% and 2.1–3.2%, respectively (AM 1.5 simulated sunlight). 344 | Photochemistry, 2009, 37, 300–361 This journal is

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3.2

Quasi-solid-state and solid-state solar cells

Dye-sensitized nanocrystalline TiO2 solar cells provide a promising alternative to conventional p–n junction photovoltaic devices. Unfortunately, liquid-state DSSCs tend to show low stability since a volatile liquid electrolyte is typically utilized. An eﬀective approach to resolve this problem is replacing the volatile liquid electrolyte with a solid-state or a quasi-solidstate hole conductor such as p-type semiconductors, ionic liquid electrolytes and polymer electrolytes. Mechanisms of operation of solid-state DSSCs have been summarized in a recent review214 that also discusses (i) the hole transfer process at dye/hole conductor interfaces, (ii) the ionic transport inside the hole conductor media and (iii) the factors that depress the eﬃciency of solid-state solar cells. Also, in a mini review article Robertson215 discusses dye design in the context of novel alternatives to the standard liquid electrolyte and describes the rapid progress made in improving the eﬃciencies of solid-DSSCs and quasi-solid-DSSCs that promise cheap, eﬃcient, and robust photovoltaic systems. Dye-sensitized solid-state solar cells typically have low energy and quantum conversion eﬃciencies because of high recombination rates at the n-type semiconductor/dye/p-type semiconductor interfaces. Deposition of an ultrathin layer of MgO on the TiO2 crystallites can signiﬁcantly enhance the eﬃciency of DSSCs with nanocrystalline TiO2 as the n-type material and CuI as the p-type material.216 Transparent (480% in the 400–900 nm range) semiconducting CuI ﬁlms fabricated by pulsed laser deposition and examined for their structural and optoelectronic properties in power output for TiO2/ Dye/CuI cells show eﬃcient charge generation on illumination of the TiO2 layer of the cells and display a power conversion eﬃciency of 2.8%.217 Volatile solvent-free quasi-solid solar cells fabricated using acid-doped polyaniline (PANI) covalently grafted on surface-modiﬁed nanocrystalline TiO2 substrates via self-assembled monolayer (SAM) of a silane-bearing aniline compound {C6H5NHC3H6Si-(OMe)3} have been examined for their photovoltaic performances with diﬀerent additives; eﬃciency around 0.12%.218 Solid-state dye-sensitized TiO2 solar cells fabricated with 4-dodecylbenzenesulfonic acid-doped polyaniline (PANI-DBSA) blended with LiI and 4-tert-butylpyridine (tBP) as the hole conductors show a signiﬁcantly improved photovoltaic behavior and a solar-to-energy conversion eﬃciency of 1.15%; an appropriate hole conductor matrix should further improve performance.219 A clay-like conductive material that comprises polyaniline-loaded carbon black particles and an ethylene oxide-substituted imidazolium iodide sandwiched between dye-coated porous TiO2 and a counter-electrode form a solid-state DSSC device that works with overall conversion eﬃciencies of 3.48% and 4.07% at an irradiance of 100 mW cm2 and 23 mW cm2 irradiation, respectively (AM 1.5 simulated sunlight).220 A signiﬁcant merit of the fabrication method of this solid-state DSSC device is the relative ease in the making and in the handling of the device, a method that can be applied to the fabrication of a plastic ﬁlm-type ﬂexible photocell by way of screen-printing the composite conductive paste on the TiO2 layer formed at low temperature. Photochemistry, 2009, 37, 300–361 | 345 This journal is

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Zhang and coworkers221 have developed a novel room-temperature method to prepare high performing porous TiO2 ﬁlms in dye-sensitized solar cells by addition of a small quantity of TiIV-isopropoxide to an ethanolic paste of TiO2 nanoparticles, on which the isopropoxide hydrolyzes in situ and connects the TiO2 particles to produce a homogeneous and mechanically stable ﬁlm of up to 10 mm thickness leading to a remarkable improvement of the cell eﬃciency. Solar-to-electrical energy conversion eﬃciencies of 4.00% and 3.27% were achieved for DSSCs with conductive glass and plastic ﬁlm substrates, respectively, under irradiation with AM 1.5 simulated sunlight (100 mW cm2). Quasi-solid-state DSSCs have also been fabricated using an oligomer having three polymerizable reactive groups; a 7% polymer concentration in the electrolyte yielded a stable quasi-solid, three-dimensional polymer network structure. An overall conversion eﬃciency of 8.1% under AM 1.5 irradiation (100 mW cm2) was observed when the quasi-solid-state DSSCs were fabricated with a high conducting polymer electrolyte.222 Electrolytes containing a novel polymer quaternary ammonium iodide {i.e. a polysiloxane with quaternary ammonium side groups (PSQAS)} exhibit maximal ambient conductivity at room temperature. A quasi-solid-state dye-sensitized solar cell fabricated with a polymer gel electrolyte based on polyacrylonitrile (PAN) and PSQAS displayed an overall conversion eﬃciency of 2.67% on irradiation (100 mW cm2).223 Combination of the amphiphilic Ru(II) dye Z-907 with the quasi-solid-state gel electrolyte 1,3:2,4-di-O-dimethylbenzylidene-D-sorbitol/3-methoxypropionitrile yields an overall solar energy conversion eﬃciency of 6.1% at AM 1.5 sunlight illumination (99.8 mW cm2); the DSSC was thermally stable at 80 1C over a month period.224 An eﬃcient solid-state solar cell involving the amorphous organic hole transport material 2,2 0 ,7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)-9,9 0 -spirobiﬂuorene (spiro-OMeTAD) as the hole conductor and an organic metal-free indoline dye as the sensitizer has been fabricated with a conversion eﬃciency well over 4% (Z = 4.1%), a value greater than a similar cell fabricated with the (Bu4N+)2[Ru(dcbpyH)2(NCS)2]2 (N719) dye sensitizer (Z = 3.2%).225 The N719 dye attached on diﬀerent TiO2 samples in acetonitrile was easily degraded when subjected to 532-nm laser light irradiation.226

A solid-state dye-sensitized solar cell comprised of TiO2 and CuI together with the dye mixture of [Ru(2,2 0 -bpy-4,4 0 -dicarboxylicacid)(NCS)2] and [Ru(4,4 0 ,400 -tricarboxy-2,2;6,200 - terpy)(NCS)3] on a TiO2 thin ﬁlm showed 346 | Photochemistry, 2009, 37, 300–361 This journal is

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a cell eﬃciency of 2.8%, whereas a cell fabricated with the single dyes displayed a cell eﬃciency of 1.7% and 1.2%, respectively.227 A new imidazole polymer synthesized by co-polymerization of alkylbis-(imidazole) and diiodoalkyls has been used to develop an ionic polymer electrolyte for quasi-solid-state DSSCs, which show a photon-to-electron conversion eﬃciency of 1.3% under AM 1.5 irradiation (100 mW cm2).228 Dye-sensitized solar cells solidiﬁed with the chemically cross-linked gelators polyvinylpyridine and 1,2,4,5-tetra(bromomethyl)benzene, together with nanoporous TiO2 electrolytes in 1-methyl-3-propylimidazolium iodide brought about suﬃcient physical contacts between gel electrolytes and the TiO2 nanocrystals that led to a signiﬁcant increase in performance.229 Solid-state dye-sensitized solar cells fabricated using a polymer matrix in polymer electrolyte to improve the durability of DSSCs consist of I2, LiI, an ionic liquid, ethylene carbonate/propylene carbonate (EC/PC) and a polymer matrix cast onto the TiO2 electrode impregnated with a Ru(II) dye photosensitizer; solar energy conversion eﬃciencies, however, were rather low (o0.2%).230 A straightforward synthetic route for the preparation of a new heteroleptic Ru(II) polypyridine sensitizer functionalized with a tert-thiophene unit attached through a non-conjugated ethanyl spacer has been developed.231 The electronic independence of the Ru(II) complex and the pendant oligothiophene unit is crucial in maintaining the intrinsic sensitizing eﬃciency of the complex. The complex (3) shown below was tested in dye-sensitized solar cells using the liquid electrolyte (I2/LiI/ tert-butylpyridine/propylene carbonate) or the solid poly(3-octylthiophene) as the hole conductor. The overall photon-to-energy conversion eﬃciency of the sandwich-type DSSC with the Ru(II) sensitizer under simulated solar irradiation (AM 1.5, 100 mW cm2) was tenfold greater (0.46%) in a liquid electrolyte than in the solid-state device (0.047%).

A supramolecular electrolyte designed for use in DSSCs on modiﬁcation of a low molecular weight PEG (1000 g mol1) at both ends with functional groups having quadruple H-bonding sites resulted in improved interfacial contact between the dye-adsorbed TiO2 nanoparticles and the electrolyte.232 The relatively high overall conversion eﬃciencies of 3.34% at 100 mW cm2 and 4.59% at 42.9 mW cm2 irradiation are among the highest eﬃciencies reported for a solid-state DSSC device due mostly to the improved interfacial contact produced by the supramolecular electrolyte. A novel eﬃcient absorbent (absorbency index ca. 3.65) for a liquid electrolyte based on poly(acrylic acid)–poly(ethylene glycol) (PAA–PEG) Photochemistry, 2009, 37, 300–361 | 347 This journal is

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hybrid has been prepared to produce a quasi-solid-state DSSC device with an overall energy conversion eﬃciency of 3.19% under irradiation of 100 mW cm2.233 The poor ion transport ability of the polymer gel electrolyte compared with that of a liquid electrolyte does not aﬀect cell performance at low light intensities, but does so at high light intensities resulting in lower energy conversion eﬃciencies (Z = 3.19% at 100 mW cm2 and 3.95% at 20 mW cm2) compared to the liquid electrolyte (Z = 4.6% at 100 mW cm2). Dye-sensitized solar cells using the solid-state hole conductor poly(3,4ethylenedioxythiophene) (PEDOT) have been fabricated using in situ photoelectrochemical polymerization giving a device with on average an overall conversion eﬃciency of 1.25% under AM 1.5 conditions.234 The performance of the PEDOT DSSCs was improved when Xe lamp illumination was performed from the cathode side during the in situ photoelectrochemical polymerization of PEDOT, which resulted in a conversion eﬃciency Z = 2.1%, also under AM 1.5 conditions. Polydiacetylenes used as hole transport materials in solid-state dyesensitized solar cells were incorporated into the nanoporous TiO2 ﬁlm by solution casting of an amphiphilic di-acetylene and subsequent in situ polymerization. The material exhibits high charge carrier mobility and the overall power conversion eﬃciency of the DSSC was a meagre 0.62% under AM 1.5 illumination (100 mW cm2).235 A new type of hybrid gel electrolyte matrix based on tetraethyl orthosilicate (TEOS) and poly(ethyleneglycol) (MW 200, PEG200) has been developed for quasi-solid-state dye-sensitized solar cells, which showed an overall solar energy conversion eﬃciency of 4.1% for the hybrid gel containing TEOS:PEG200 at a ratio of 10:1 by volume.236 A range of diﬀerent hole conductor materials and a number of key parameters that aﬀect the performance of solid-state dye-sensitized solar cells have been examined by Schmidte-Mende and Graetzel.237 Wetting and pore-ﬁlling of the nanoporous TiO2 layer by the hole transporter play a critical role in determining the ﬁnal eﬃciency of the cell. Comparison of results showed the importance of complete ﬁlling in contrast to just wetting of the nanoporous TiO2 layer, a point generally underestimated. Micro-/nano-composite TiO2 porous ﬁlms have been prepared by an electrohydrodynamic (EHD) method applied successfully to fabricate dye-sensitized solar cells.238 Considering that micro-/nano-composite structures based on the EHD technique are better for the ﬁlling of ionic liquid electrolytes and quasi-solid-state electrolytes than liquid-state electrolytes, total photoelectric conversion eﬃciencies (Z) were 6.4% for the ionic liquid electrolyte and 5.3% for the quasi-solid-state electrolyte. The composite hierarchical structure beneﬁts light collection because of strong light scattering. The EHD technique can also produce large area DSSC devices with continuous fabrication that can be extended to make ﬁlms of other oxides (ZnO, SnO, Al2O3, ZnO–TiO2, among others). TiO2 electrodes electrospun directly onto a conducting glass substrate from a mixture of titanium(IV) isopropoxide and poly(vinyl acetate) (PVAc) in dimethyl formamide provide a new alternative to conventional electrodes in DSSCs consisting of nanocrystalline TiO2 particles because of the 348 | Photochemistry, 2009, 37, 300–361 This journal is

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possible enhanced penetration of the polymer gel electrolyte.239 The performance of the gel electrolyte system with an electrospun electrode is greater than 90% of an equivalent liquid electrolyte system. Electrospun TiO2 nanoﬁbers have been employed in quasi-solid state DSSCs with porous electrodes, which enhances the penetration of viscous polymer gel electrolytes.240 The TiO2 ﬁbers electrospun from the poly(vinyl acetate) matrix formed a one-dimensionally aligned ﬁbrillar morphology as an islands-in-a-sea structure. Photocurrent generation in the DSSC with polymer gel electrolytes and the new TiO2 electrodes was over 90% of the performance in a dye-sensitized solar cell with liquid electrolytes (Z = 3.80% versus 4.01%, respectively). The feature of the electrospun TiO2 electrode in the DSSC can be judged by the degree of penetration of the highly viscous gel electrolyte into the porous space of the TiO2 layer. The ionic conductivity of polymer electrolytes and their interfacial contact with dye-attached TiO2 particles are enhanced markedly by addition of an amorphous oligomer into polymer electrolytes that result in a relatively high overall energy conversion eﬃciency (Z = 3.84% at 100 mW cm2).241 The new compound LiI(HPN)2 possesses 3-D transporting paths for iodine and a mono-ion transport feature that make LiI(HPN)x (2 r x r 4) systems suitable candidates as solid electrolytes for DSSCs.242 The performance of the DSSC with the electrolyte proved unsatisfactory, however, owing to the relative low conductivity, poor ﬁlling and poor contact of the electrolyte with the porous TiO2 electrode. However, addition of mm-sized and nm-sized SiO2 particles into the solid electrolyte enhances the conductivity of the electrolyte and greatly improves the interfacial contact between electrode and electrolyte; using an optimized composite solid electrolyte system to fabricate the DSSC achieved a light-to-electricity conversion eﬃciency Z of 5.4% under AM 1.5 simulated solar light illumination (note that Z decreased to 70% after a month. A new polymer electrolyte consisting of heteropolyacid (HPA)-impregnated polyvinylidene ﬂuoride (PVDF) with I2/I has been prepared that biomimicks natural photosynthesis.243 A dye-sensitized solar cell fabricated with the N3 dye-adsorbed over TiO2 nanoparticles (photoanode) and conducting carbon cement coated on conducting glass (photocathode) showed overall energy conversion eﬃciency up to 2.77% with this new polymer electrolyte. The variation of electrolyte components on I3 ion diﬀusion properties and charge transfer resistance at a Pt electrode have been examined in quasi-solid-state polymer electrolytes prepared from poly(vinylideneﬂuorideco-hexaﬂuoropropylene) (PVDF-HFP) as the gelator for 1-ethyl-3-methylimidazolium based ionic liquids {triﬂuoromethanesulfonate [EMIM]-[TfO] and bis(triﬂuoromethanesulfonyl)imide [EM-IM][Tf2N] anions}, polyacrylonitrile (PAN) for gelation of 1-ethyl-3-methylimidazolium dicyanamide [EMIM]-[DCA] and the I/I3 redox couple.244 Iodine addition on the solid-state electrolyte LiI/3-hydroxypropionitrile (1:4) for dye-sensitized solar cells causes a dramatic decrease of the ionic conductivity that diﬀers from the conduction behavior of the Grotthuss transport mechanism observed in liquid or gel electrolytes.245 A 1.5% of light-to-electricity conversion eﬃciency of the solid-state DSSC was Photochemistry, 2009, 37, 300–361 | 349 This journal is

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obtained at the optimum ratio of LiI/HPN/I2 = 1:4:0.05 under the radiation of 100 mW cm2 (AM 1.5). Quaternary ammonium polyiodides as ionic liquid/soft solid electrolytes in DSSCs have been examined by Santa-Nokki and coworkers.246 Under illumination from a halogen lamp source at an irradiance of 10 mW cm2, the highest power conversion eﬃciency was 2.4%, obtained with the (Me2Hex2N)I:I2 (10:1) liquid electrolyte containing tert-butylpyridine (TBP), compared to an eﬃciency of 5.4% obtained from similar cells with traditional 3-methoxypropionitrile-based electrolyte under otherwise identical conditions. The best eﬃciency with the soft solid electrolyte (Et2Hex2N)I:I2 (10:1) with TBP was 2.3%. 3.3

Long term stability of DSSCs

A major issue that has hampered wide acceptance of dye-sensitized solar cells has been their stability and photostability. Much of the work on DSSCs has involved laboratory-scale studies with cells of dimensions less than 1 cm2. Experience from laboratory work, however, has formed the basis for market expectations concerning performance and stability of DSSCs in commercial applications. Clearly, a new set of variables need to be addressed when taking the technology from a laboratory-scale DSSC to a module at product level. Stability and performance are two critical issues that must be considered when scaling up DSSCs. Tulloch247 has described background cell technology and module design considerations for ﬁrst production of DSSC modules for outdoor applications that may be commercially viable in the next several years. A light-soaking test and aging tests carried out on dye-sensitized solar cells of diﬀerent electrolyte compositions by examining thermal stress under both illuminated and dark conditions showed that at constant artiﬁcial 1-Sun illumination at ambient temperature imposed no signiﬁcant stress on the solar cell, whereas constant heating in the dark at 85 1C led to severe decrease in performance, albeit restored by continuous illumination (ca. 1 Sun) at moderate temperatures.248 The thermal stress at 85 1C for all electrolyte compositions at constant illumination led to a most severe decrease in cell performance, if no recovery in the dark was allowed. Cell composition, purity of starting materials, processing conditions, and humidity all played an important role in the stability of manufactured solar cells. Sommeling et al.248 emphasized that their results were not valid for extrapolating to outdoor lifetimes of the DSSC devices. Rather, their tests qualiﬁed the mechanical integrity of the complete module in relation to thermal stress and humidity. A ﬁrst long-term outdoor performance test of large scale DSSC modules was performed over a 6-month period by examining 64 DSSC cells connected in series to elucidate possible challenges for outdoor practical use of full-ﬂedged DSSC modules; the size of each solar cell in a module was 10 cm 10 cm.249 Although the modules needed a larger area than conventional Si solar cell modules to attain the same rated output because of lower rated energy conversion eﬃciency, the data indicated that DSSC modules generated about 10–20% more electricity annually than 350 | Photochemistry, 2009, 37, 300–361 This journal is

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conventional crystalline Si modules of the same rated output power. Results also indicated that the energy conversion eﬃciency under 1 Sun conditions did not always coincide with the electricity generated annually outdoors, and as such is not a crucial measure in evaluating solar cell performance. The outputs of four modules showed a monotonic slow and steady decrease, clearly indicating that signiﬁcant challenges remain for potential outdoor use of the DSSC devices in attaining high performances while retaining long-term stability. The performance of dye-sensitized solar cells assembled with a plasticized polymer electrolyte based on a poly(ethylene oxide) derivative has been evaluated as a function of active area size.250 With area enlargement, the performance of the cell decreased from 3% (0.25 cm2) to 0.8% (4.5 cm2) as a consequence of the loss of current ﬂow through the device, caused mainly by an increase in the internal resistance of the device. While the performance of solar cells assembled with 4.5 cm2 active areas is low, 13 such cells connected in series to compose the ﬁrst solar module built with solid-state DSSCs showed a promising performance generating 8 V under outdoor conditions. After 30 days of solar irradiation (100 mW cm2) no changes in cell eﬃciency occurred. The stability of the TiO2/Ru(II) dye/CuI solid-state solar cell subjected to continuous full-spectrum simulated sunlight illumination degraded rapidly; however, when the UV portion of the illumination spectrum was removed the cell showed rather good stability.251 XPS measurements indicate that TiO2 can oxidize CuI in UV light. Long-term stability of the solid-state DSSC improved under simulated sunlight by coating the TiO2 porous electrode with an ultra-thin MgO layer that blocks the photooxidative activity of TiO2. Addendum In case it was missed in an earlier Report, we wish to draw the readers’ attention to a comprehensive review article published in 2003 following a DOE-sponsored workshop on the current status of ‘‘Charge Transfer on the Nanoscale’’.252 It should be of particular interest to aﬁcionados of the two themes of the present article. References 1 Selected recent reviews on photocatalysis: M. Addamo, M. Del Arco, M. Bellardita, D. Carriazo, A. Di Paola, E. Garcia-Lopez, G. Marci, C. Martin, L. Palmisano and V. Rives, Res. Chem. Intermed., 2007, 33, 465; G. Mele, R. Del Sole, G. Vasapollo, E. Garcia-Lopez, L. Palmisano, L. Jun, R. Slota and G. Dyrda, Res. Chem. Intermed., 2007, 33, 433; I. Oller, P. Fernandez-Ibanez, M. I. Maldonado, L. Perez-Estrada, W. Gernjak, C. Pulgarin, P. C. Passarinho and S. Malato, Res. Chem. Intermed., 2007, 33, 407; H. C. Pehlivanugullari, E. Sumer and H. Kisch, Res. Chem. Intermed., 2007, 33, 297; C. McCullagh, J. M. C. Robertson, D. W. Bahnemann and P. K. J. Robertson, Res. Chem. Intermed., 2007, 33, 359; M. D. Hernandez-Alonso, J. M. Coronado, J. Soria, J. C. Conesa, V. Loddo, M. Addamo and V. Augugliaro, Res. Chem. Intermed., 2007, 33, 205; R. I. Bickley and L. T. Hogg, Res. Chem. Intermed., 2007, 33, 333; M. Anpo, S. Dohshi, M. Kitano and Y. Hu, in Metal Oxides: Chemistry and Photochemistry, 2009, 37, 300–361 | 351 This journal is

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209 T. Lana-Villarreal, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2007, 111, 5549. 210 C. Zafera, M. Kusa, G. Turkmena, H. Dincalpc, S. Demica, B. Kuband, Y. Teomand and S. Icli, Solar Energy Mater. Solar Cells, 2007, 91, 427. 211 Z. Zhang, N. Evans, S. M. Zakeeruddin, R. Humphry-Baker and M. Graetzel, J. Phys. Chem. C, 2007, 111, 398. 212 F.-T. Kong, S.-Y. Dai and K.-J. Wang, Chin. J. Chem., 2007, 25, 169. 213 K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Letters, 2007, 7, 69. 214 B. Li, L. Wang, B. Kang, P. Wang and Y. Qiu, Solar Energy Mater. Solar Cells, 2006, 90, 549. 215 N. Robertson, Angew. Chem. Int. Ed., 2006, 45, 2338. 216 G. R. A. Kumara, M. Okuya, K. Murakami, S. Kaneko, V. V. Jayaweera and K. Tennakone, J. Photochem. Photobiol. A: Chem., 2004, 164, 183. 217 M. Rusop, T. Siga, T. Jimbo and M. Ameno, Surf. Rev. Letters, 2004, 11, 577. 218 G. K. R. Senadeera, T. Kitamura, Y. Wada and S. Yanagida, J. Photochem. Photobiol. A: Chem., 2004, 164, 61. 219 S. Tan, J. Zhai, M. Wan, Q. Meng, Y. Li, L. Jiang and D. Zhu, J. Phys. Chem. B, 2004, 108, 18693. 220 N. Ikeda, K. Teshima and T. Miyasaka, Chem. Commun., 2006, 1733. 221 D. Zhang, T. Yoshida, T. Oekermann, K. Furata and H. Minoura, Adv. Funct. Mater., 2006, 16, 1228. 222 R. Komiya, L. Han, R. Yamanaka, A. Islam and T. Mitate, J. Photochem. Photobiol. A: Chem., 2004, 164, 123. 223 W. Li, J. Kang, X. Li, S. Fang, Y. Lin, G. Wang and X. Xiao, J. Photochem. Photobiol. A: Chem., 2005, 170, 1. 224 N. Mohmeyer, P. Wang, H.-W. Schmidt, S. M. Zakeeruddin and M. Graetzel, J. Mater. Chem., 2004, 14, 1905. 225 L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi, H. Miura, S. Ito, S. Uchida and M. Graetzel, Adv. Mater., 2005, 17, 813. 226 V. A. Quan, H.-T. Cam-Hoai and T. Lund, ‘‘Degradation Rate of the Graetzel Solar Cell Sensitizer N719 on Diﬀerent TIO2 Samples’’, North American Catalysis Society (NACS)-20th North American Meeting (NAM), Houston, TX, June 17–22, 2007 (Poster P-S14-25A). 227 J. Bandara and H. Weerasinghe, Solar Energy Mater. Solar Cells, 2006, 90, 864. 228 K. Suzuki, M. Yamaguchi, S. Hotta, N. Tanabe and S. Yanagida, J. Photochem. Photobiol. A: Chem., 2004, 164, 81. 229 S. Sakaguchi, H. Ueki, T. Kato, T. Kado, R. Shiratuchi, W. Takashima, K. Kaneto and S. Hayase, J. Photochem. Photobiol. A: Chem., 2004, 164, 117. 230 M.-R. Kim, S.-H. Jin, S.-H. Park, H.-J. Lee, E.-H. Kang and J.-K. Lee, Mol. Cryst. Liq. Cryst., 2006, 444, 233. 231 C. Houarner-Rassin, E. Blart, P. Buvat and F. Odobel, J. Photochem. Photobiol. A: Chem., 2007, 186, 135. 232 Y. J. Kim, J. H. Kim, M.-S. Kang, M. J. Lee, J. Won, J. C. Lee and Y. S. Kang, Adv. Mater., 2004, 16, 1753. 233 Z. Lan, J. Wu, J. Lin and M. Huang, J. Power Sources, 2007, 164, 921. 234 N. Fukuri, N. Masaki, T. Kitamura, Y. Wada and S. Yanagida, J. Phys. Chem. B, 2006, 110, 25251. 235 Y. Wang, K. Yang, X. Wang, R. Nagarajan, L. A. Samuelson and J. Kumar, Org. Electron., 2006, 7, 546. 236 J. Joseph, K. M. Son, R. Vittal, W. Lee and K.-J. Kim, Semicond. Sci. Technol., 2006, 21, 697. 237 L. Schmidt-Mende and M. Graetzel, Thin Solid Films, 2006, 500, 296. 360 | Photochemistry, 2009, 37, 300–361 This journal is

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238 Y. Zhao, J. Zhai, S. Tan, L. Wang, L. Jiang and D. Zhu, Nanotechnol., 2006, 17, 2090. 239 M. Y. Song, D. K. Kim, K. J. Ihn, S. M. Jo and D. Y. Kim, Nanotechnol., 2004, 15, 1861. 240 S. M. Jo, M. Y. Song, Y. R. Ahn, C. R. Park and D. Y. Kim, J. Macromol. Sci., Part A: Pure Appl. Chem., 2005, 42, 1529. 241 M.-S. Kang, J. H. Kim, Y. J. Kim, J. Won, N.-G. Park and Y. S. Kang, Chem. Commun., 2005, 889. 242 H. Wang, H. Li, B. Xue, Z. Wang, Q. Meng and L. Chen, J. Am. Chem. Soc., 2005, 127, 6394. 243 S. Anandana, S. Pitchumania, B. Muthuraamana and P. Maruthamuthu, Solar Energy Mater. Solar Cells, 2006, 90, 1715. 244 G. Nazmutdinova, S. Sensfuss, M. Schro¨dner, A. Hinsch, R. Sastrawan, D. Gerhard, S. Himmler and P. Wasserscheid, Solid State Ionics, 2006, 177, 3141. 245 H. Wang, X. Liu, Z. Wang, H. Li, D. Li, Q. Meng and L. Chen, J. Phys. Chem. B, 2006, 110, 5970. 246 H. Santa-Nokki, S. Busi, J. Kallioinen, M. Lahtinen and J. Korppi-Tommola, J. Photochem. Photobiol. A: Chem., 2007, 186, 29. 247 G. E. Tulloch, J. Photochem. Photobiol. A: Chem., 2004, 164, 209. 248 P. M. Sommeling, M. Spa¨th, H. J. P. Smit, N. J. Bakker and J. M. Kroon, J. Photochem. Photobiol. A: Chem., 2004, 164, 137. 249 T. Toyoda, T. Sano, J. Nakajima, S. Doi, S. Fukumoto, A. Ito, T. Tohyama, M. Yoshida, T. Kanagawa, T. Motohiro, T. Shiga, K. Higuchi, H. Tanaka, Y. Takeda, T. Fukano, N. Katoh, A. Takeichi, K. Takechi and M. Shiozawa, J. Photochem. Photobiol. A: Chem., 2004, 164, 203. 250 J. N. de Freitas, V. C. Nogueira, B. I. Ito, M. A. Soto-Oviedo, C. Longo, M.-A. De Paoli and A. F. Nogueira, Int. J. Photoenergy, 2006, 75483. 251 X.-T. Zhang, T. Taguchi, H.-B. Wang, Q.-B. Meng, O. Sato and A. Fujishima, Res. Chem. Intermed., 2007, 33, 5. 252 D. M. Adams, L. Brus, C. E. D. Chidsey, S. Creager, C. Creutz, C. R. Kagan, P. V. Kamat, M. Lieberman, S. Lindsay, R. A. Marcus, R. M. Metzger, M. E. Michel-Beyerle, J. R. Miller, M. D. Newton, D. R. Rolison, O. Sankey, K. S. Schanze, J. Yardley and X. Zhu, J. Phys. Chem. B, 2003, 107, 6668.

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Multi-component arrays for interfacial electronic processes on the surface of nanostructured metal oxide semiconductors Andrew Kopecky and Elena Galoppini* DOI: 10.1039/b813865f The growing interest in the sensitization of nanostructured metal oxide semiconductors for the development of dye sensitized solar cells and photocatalytic systems has led to the development of multi-component dyes, able to perform well-deﬁned, stepwise electronic processes on the semiconductor surface. Important applications include the development of DSSCs with improved eﬃciencies and the control of intermolecular and interfacial electronic processes at the dye/semiconductor interface. This review summarizes the progress in this area through the description of model dyes prepared from polymetallic complexes and organic molecules, and ends with a perspective on light harvesting arrays.

1.

Background

In the past decade the quest to ﬁnd renewable energy sources to replace fossil fuels has been broadly recognized as one of the most urgent needs and one of the greatest technological challenges the world will face in the next 50 years.1,2 Lewis,w3 Hoﬀert,4,5 and others6,7 have presented welldocumented perspectives of the energy demand for the next decades. The current and projected atmospheric levels of CO2, the impact on the global climate change and on the environment, and the scale of the projected power demands (an estimated additional 30 TW by 2050 to the current B14 TW)z2,5 have motivated an unprecedented research eﬀort among scientists worldwide to develop renewable energy sources. There is some consensus that the problem can be alleviated (if not solved) by a combination of alternative energy sources, i.e. wind, biomass, geothermal, solar, and others. Solar energy, however, is the only source that, alone, can oﬀer a solution to the energy problem. Although only a small portion of the Sun’s energy reaches the Earth, the Earth’s surface receives each day more energy from the Sun than humans use in a year.2,5 Unfortunately, photovoltaics remain one of the least exploited renewable energy sources (in 2005, solar accounted only for 0.6% of all energy sources in the US).y8 Hence, the need to ﬁnd more eﬃcient materials and develop the technology that is necessary to eﬃciently harvest and store energy from sunlight. Rutgers University–Newark, 73 Warren St., Newark, NJ, USA. E-mail: [email protected]; Tel: 973-353-5317 { For presentations and articles about the technical, political, and economic challenges involved with the development and use of renewable energy technologies see Lewis, Nathan S., ‘‘Powering the Planet—Global Energy Perspective’’ http://nsl.caltech.edu/energy.html. { 1 Terawatt (TW) = 1012 watts. } In 2005 solar was only 1% of renewable energy sources, which are B6% of the total energy sources.

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One of the most promising developments in this quest is the progress towards materials based on photoactive or redox-active molecules bound to nanostructured metal oxide semiconductor surfaces. In 1972, Fujishima and Honda discovered the photocatalytic splitting of water into H2 and O2 by TiO2.9 This important discovery marked the beginning of the ﬁeld of heterogeneous photocatalysis involving metal oxide semiconductors for the production of ‘‘solar fuel’’. Since the original experiment, many laboratories worldwide have studied this process and tried to improve its eﬃciency.10–15 Molecule-semiconductor interfaces are also the basis for new types of solar cells. By extending the absorption range of a wide band-gap semiconductor (typically TiO2, ZnO or SnO2) with a dye that absorbs in the visible, it is possible to perform useful photoelectrochemical processes by irradiating the dye-semiconductor material with sunlight. The 1991 seminal paper by Gra¨tzel and O’Regan16 was based on this principle. Currently, scientists are studying heterogeneous charge transfer to prepare dye-sensitized solar cells (DSSCs) to generate electricity from sunlight. New materials used to develop DSSCs6 and the factors that inﬂuence the metal oxide semiconductor sensitization process have been extensively reviewed.17–20 In a Gra¨tzel-type DSSC (Fig. 1), which is a ‘‘sandwich’’ solar cell, a photo excited dye (eqn (1)) injects an electron into the conduction band of the semiconductor to which is attached. This process is called sensitization, and the dye is often termed sensitizer (eqn (2)). The injection process is typically ultrafast21–25 (femto- to picoseconds). The recombination process (eqn (3)) is frequently orders of magnitude slower. TiO2–S + hn - TiO–S*

(1)

TiO2–S* - TiO2(e )–S

+

TiO2(e)–S+ - TiO2–S

(2) (3)

The semiconductor is cast as a thin, optically transparent, nanoparticulate ﬁlm on conductive glass, which is the photoanode. The dye is attached to the ﬁlm covalently, usually by reaction of carboxylate or phosphonate substituents. Following injection, the oxidized dye oxidizes, in turn, an electron mediator in a redox couple that is dissolved in the electrolyte (typically I3/I, Red Ox in Fig. 1). The photoelectrochemical cycle is completed by the reduction of the mediator at the cathode. Fig. 1 illustrates, in a simpliﬁed manner, the key processes occurring in this type of device. The most eﬃcient dyes to date remain the dyes employed in the ﬁrst DSSCs, i.e. bipyridine (bpy) complexes of Ru(II).17 Because of the urgent need for green energy, much eﬀort is devoted to the technical optimization of the devices (higher eﬃciencies, higher photovoltages, improved stability, etc.), and to the quest for an ‘‘ideal’’ dye. However, the study of heterogeneous charge transfer between a chromophore and a semiconductor, and the controlled functionalization of a semiconductor interface are interesting problems from the fundamental view-point, and are relevant to many applications of semiconductor nanomaterials. The past decade has seen enormous progress in this ﬁeld, partly thanks to the development of increasingly sophisticated molecular model dyes. Photochemistry, 2009, 37, 362–392 | 363 This journal is

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Fig. 1 Top: Key electronic processes in a solar cell sensitized with a dye bound to the surface of the nanoparticles: hv = photo excitation of the dye; kinj = electron injection; kreg = regeneration; krec = competitive recombination processes. This simpliﬁed ﬁgure does not illustrate intersystem crossing, vibronic relaxation or other photophysical processes of the dye. Bottom: Schematic representation of a dye/TiO2 nanoparticle photoanode. The mesoporous ﬁlm is typically B10 mm thick and is made of nanoparticles 10 to 20 nm in diameter. On the right, an ITO electrode with the dye/TiO2 ﬁlm (active area B1 cm2).

Overall the main goal of the ‘‘supersensitizer’’26 design described in this review is to control the charge injection and recombination processes at the dye/semiconductor interface, rather than providing better dyes for eﬃcient DSSCs.17,26,27 In fact, most of the dye molecules reported here are too complex to ﬁnd applicability for viable DSSCs or photocatalysts. However, multi-component dyes, through clever molecular design, have proven to be important models to study the complex injection and recombination dynamics, improve light harvesting, and perform other important functions. This review is limited to multi-component dyes made of structurally and functionally distinct units (donor, acceptor, spacer, chromophoric unit, anchor group) each designed to perform a speciﬁc function or a separate electronic step at the surface of wide band-gap metal oxide semiconductor. We have not included model dye-linker-anchor unit, such as the model sensitizers developed by us,28–32 because they are not multichromophoric and do not contain donor–acceptor dyads. We also have limited the review to supramolecular systems bound to metal oxide surfaces. For this reason, we have excluded many elegant systems (dyads, triads etc.) studied in solutions and inspired by natural photosynthesis, such as, for instance, the bimetallic complexes developed by Hammarstro¨m and coworkers for photocatalytic processes.33–35 We have included a description of porphyrin 364 | Photochemistry, 2009, 37, 362–392 This journal is

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arrays on TiO2 for DSSCs in the last section, to describe the eﬀort of constructing antennas for eﬃcient light harvesting and energy conversion. This area has experienced a fast growth in the past few years, thanks to the availability of porphyrin arrays developed for artiﬁcial photosynthesis studies. 2. Design of supramolecular dyes for stepwise electron transfer on the surface of nanostructured MOn The organic or inorganic molecules described here consist of multiple components, each diﬀering in their respective redox or photo physical properties. Compared to the huge number of semiconductor/dye systems studied to date, the examples of multi-component dye/semiconductor (sometimes referred to as ‘‘heterosupramolecular’’) systems remain limited. In addition, the synthetic challenge of constructing the assemblies often comes with even more challenging heterogeneous ultrafast charge transfer studies involving multiple donor and acceptor units. While most dyes are designed to achieve eﬃcient sensitization, multi-component dyes of varying complexity have been developed to study and control one or more of the following: longer-lived charge separated states, inhibited recombination (eqn (3)), extension of absorptive range, enhanced extinction coeﬃcient, tuning of HOMO–LUMO level, and inhibition of recombination processes involving the Red Ox couple in the electrolyte. The rationale for designing multi-component (supramolecular) synthesizers is that well designed assemblies can perform relatively elaborate processes and can ultimately improve the control of charge transfer events at the interface. It is the same fundamental concept that is central to the ﬁeld of supramolecular chemistry and was inspired by the ‘‘antenna’’ systems found in nature. In the scheme below we have summarized some of the ‘‘stepwise’’ processes that have been studied through the diﬀerent multi-component dye systems described in this review. Various concepts of intramolecular and interfacial electron transfer processes involving supramolecular dyes are shown in Scheme 1. These consist of a sensitizer (S), an electron donor (D) or an acceptor (A), connected in diﬀerent-order, and bound to the semiconductor. The various components are linked to each other directly or through a bridging unit. In Schemes 1A and 1B, following photo excitation, a series of stepwise intramolecular electron transfer processes result in a remote charge separated state, with an electron injected from the dye into the semiconductor. In the case of a p-type semiconductor system (Scheme 1C) the semiconductor is the electron donor and the hole is transferred to a remote acceptor unit on the supramolecular dye. Scheme 1D illustrates an ‘‘antenna’’ system able to transfer vectorially energy through the array and ultimately forming a charge separated state. The antenna systems are only brieﬂy mentioned in this review, as the focus remained that of stepwise charge transfer processes. The various recombination processes have been omitted from Scheme 1 for clarity, but these are discussed case by case in the following sections. Photochemistry, 2009, 37, 362–392 | 365 This journal is

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Scheme 1

3.

Dyads containing transition metal coordination complexes

Polymetallic coordination compounds based on polypyridine complexes of transition metals have been studied for over three decades to design and construct multi-component (supramolecular) systems capable of performing useful light-induced and/or redox-induced functions.36–39 Linear, branched, and dendritic arrays have been developed, and their photochemical and electrochemical properties used to design artiﬁcial photosynthetic arrays, multi-component light harvesting systems, or for optoelectronic applications.40–44 More recently, they have been bound to semiconductor thin ﬁlms to perform some of the stepwise processes shown in Scheme 2. In most cases, they led to sensitizers that exhibited eﬃcient photoinduced electron injection and slow recombination.27

3.1

Ru(bpy)-phenothiazine dyad45,46

The ﬁrst dyad for stepwise sensitization processes on the surface of semiconductors performing the function illustrated in Scheme 1B was realized by Bignozzi and Meyer.27,45 The ultimate goal of such design was to form, upon excitation with visible light, an interfacial charge separated pair with an electron in the semiconductor and a hole localized on a molecular unit away from the semiconductor surface, resulting in a long-lived charge separated state and inhibited recombination from the electron injected into the semiconductor to the oxidized dye. 366 | Photochemistry, 2009, 37, 362–392 This journal is

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The dyad, [Ru(dcbH2)2(4-CH3,40 -CH2-PTZ-2,20 -bipyridine)]2+, abbreviated here as Ru-PTZ, was made of a Ruthenium bpy complex with a phenothiazine (PTZ) unit attached to one of the ligands. PTZ had been used as an electron donor in an electrolyte solution to regenerate the Ru(II) following injection in TiO2.47 In solution, photoexcitation of the Ru complex resulted in the formation of the MLCT excited state, which was rapidly quenched by the PTZ donor with a rate constant 2.5 108 s1, so that only residual emission from the Ruthenium excited complex was observed. When the dyad was bound to TiO2, excitation was followed by ultrafast (fs)21 injection in the semiconductor (step 1 in Fig. 2). This ﬁrst step was followed by the intramolecular charge transfer process from the phenothiazine unit to the oxidized complex, Ru(III). The rapid formation of the charge separated state TiO2(e)–Ru–PTZ+ was monitored by nanosecond transient absorption spectroscopy. Interestingly, the rate of recombination of the electron with the PTZ+ (process 4 in Fig. 2) was three orders of magnitude slower (3.1 103 s1) than that observed in a model Ru complex without the PTZ unit. In conclusion, the strategy of removing the hole away from the surface of the semiconductor resulted in considerably slower recombination rates.

Fig. 2 Interfacial and intramolecular electron transfer processes in [Ru(dcbH2)2(4-CH3,4 0 CH2-PTZ-2,2 0 -bipyridine)]2+ anchored to TiO2. In the dyad, the Ru complex is the sensitizer and the phenothiazine unit is the donor. Top ﬁgure adapted from Fig. 2 of ref. 27.

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3.2

Ru-ligand-Os dyad

A bimetallic coordination compound behaving like a molecular photodiode on TiO2, was realized with a bimetallic complex of Ru(II) (the sensitizer) and Os(II) (the donor) attached together by a 1,2-bis(4-pyridyl)ethane bridge, [Ru(dcb)2(Cl)-bpa-Os(bpy)2(Cl)]-(PF6)2, abbreviated here as Ru-bpa-Os, Fig. 3. The multistep processes follow the same scheme of the Ru-PTZ dyad (Scheme 1B), but in this case, the donor is also a chromophore that can be selectively photoexcited. Conceptually this experiment was, on a molecular level, the replica of the bi-layer polymeric ﬁlms of Ru(II) and Os(II) obtained by electro polymerization, where charge was trapped for hours in the Os layer.48 The Py coordinating bridge provided only weak electronic coupling between the metal centers, which behaved spectroscopically as separate units. The absorption spectrum of Ru-bpa-Os was close to the sum of the spectra of the Ruthenium and Osmium individual dyes, with Ru and Os ligand charge transfer bands overlapping at B450 nm and the forbidden Os - bpy 3MLCT charge transfer band centered at about 720 nm. Upon excitation, very fast injection into TiO2 (kinj 4 108 s1) was observed, and fast (ket Ru–Os 4 108 s1) hole transfer from Ruthenium to Osmium led to TiO2(e)/Ru(II)-bpa-Os(III) as the end species, observed in absorption diﬀerence spectra. Ruthenium III species, the product of the ﬁrst charge transfer step (kinj) was not observed. The transfer rates were independent of whether the Os (lexc = 417 nm) or Ru (lexc = 417 nm) units were directly excited. In conclusion the bimetallic dyad acted as a charge rectiﬁer: electron injection rates were at least 5 orders of magnitude faster than electron recombination. The contribution of a population where direct charge transfer occurs from the Osmium unit directly into the semiconductor (and vice versa) could not be excluded because the bridge in the Ru-bpa-Os system is ﬂexible. 3.3

Ru-ligand-Rh dyad49

The concept of bimetallic dyads was further developed in Ru(II)–Rh(III) dyads connected by bridging bipyridyl ligands (BL): RhIII(dcb)2-(BL)-RuII(dmp) 2 and RhIII(dcb)2-(BL)-RuII(bpy)2, Fig. 4. A Ruthenium complex without Rhodium RuII(dcb)2(dmb) was studied for comparison. Each dyad contained a Rh(III) complex of bipyridine bridged to a Ru(II) complex of phenanthroline or bipyridine, respectively. The attachment to the surface of the semiconductor occurred through the dcb ligands of the Rh complex. The ligand-centered transitions of the Rh(III) component were in the region l o 350 nm, which is obscured by the absorption of the semiconductor when the dyads are bound, and the Ru(II) complex exhibited the characteristic MLCT band at 450 nm. In solution, selective photo excitation of Ru(II) resulted in fast and eﬃcient (490%, B109 s1) electron transfer to the Rhodium center, which acts as the electron acceptor in the dyad (eqn (4)). The dmp-based dyad, with a decrease in Ru(III)/(II) potential, was expected to show a corresponding increase in driving force for photo induced electron transfer, and faster excited-state decay rate. Ru(II) polypyridine MLCT emissions, with lmax at 650 nm for the model complex and 368 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 3 Idealized representation of the binding and observed intramolecular charge transfer and injection processes of the Ru-bpa-Os model compound. See Scheme 1A. Top ﬁgure taken From Scheme 2 of ref. 48.

Fig. 4 Ru–Rh Dyads and observed intramolecular charge transfer and injection processes. Top ﬁgure taken from Fig. 2 of ref. 49.

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Fig. 5 Energy level diagram and photo physical processes for the TiO2-RhIII(dcb)2-(BL)-RuII(bpy)2 heterotriad. Taken from Fig. 12 of ref. 49.

620 nm and 615 nm for the dyads, both blue shifted because of the presence of the BL bridge. Because of the presence of the Rh quencher, the emission intensity of the dyads was low. In solution, fast back electron transfer occurred (eqn (5)) so that the Rh(II)–Ru(III) species was not observed in the transient absorption spectra. In conclusion, in solution the two dyads behaved as expected on the basis of molecular design. Rh(III)–Ru(II)* - Rh(II)–Ru(III)

(4)

Rh(II)–Ru(III) - Rh(III)–Ru(II)

(5)

On TiO2 the dyads were expected to perform heterogeneous stepwise electron transfer upon selective excitation of the Ru(II) chromophore, as illustrated in Scheme 1A and Fig. 4, with a long-lived charge separated state as the end product. The model RuII(dcb)2(dmb) showed fast quenching of the MLCT state upon binding to TiO2, with complex kinetics. Both dyads injected electrons into TiO2 following light absorption, but the emission decays of both surface-bound dyads were similar to those observed in solution. The proposed mechanism for injection is shown in Fig. 5. In summary, both non-ﬂuorescent dyads were, as expected, poor sensitizers. The photocurrent eﬃciency was rather low, mainly because of low charge injection yields, and the injection mechanism (stepwise vs. direct injection or both) was not clear, presumably because of non-ideal orientation of the dyads at the surface and accidental contacts in the pores of the ﬁlm. Nevertheless, the recombination between injected electron and oxidized sensitizer was a few milliseconds in both dyads/TiO2, i.e. 3 orders of magnitude slower compared to that observed for RhIII(dcb)2-(dmb)/TiO2 the time scale for recombination was milliseconds. 3.4

Re–Ru dyads50

Most bimetallic dyads described earlier did exhibit some conformational mobility at the bridging unit connecting the two metal complexes. An approach aimed at retaining control over the orientation of the component units on the semiconductor surfaces involved the preparation 370 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 6 Structure and intramolecular energy transfer followed by remote charge injection in the binuclear complex [Re(dcbH2)(CO)3(CN)Ru(bpy)2(CN)]+. Top ﬁgure adapted from Fig. 5 of ref. 27.

of dyad [Re(dcbH2)(CO)3(CN)Ru(bpy)2(CN)]+, abbreviated here as Re–Ru. The fac-[ReI(dcbH2)(CO)3] unit anchors to the surface, orienting the Ru(bpy) complex proximate to the surface of the semiconductor, as shown in Fig. 6. Solar cells prepared from the Re–Ru dyad were eﬃcient, and the rapid (k 4 5 108) formation of the charge separated state TiO2(e)–Re(I)–Ru(III) was observed by transient absorption spectroscopy suggesting the possibility of intraligand (bpy - dcb) charge transfer from Ru(III)(bpy–) to Re(I)(dcb). This result demonstrated fast and eﬃcient injection into the semiconductor occurring from a chromophoric group (the Ru complex) which is not directly bound, and not electronically coupled, to the semiconductor.

3.5

Ru-Tyrosine-dipicolinamide dyads51

Primary charge recombination is a competing process in dyads. To solve this problem, Sundstro¨m et al. devised a dyad with an electron donor (a tyrosine unit) that regenerates the oxidized dye without participating in the electron injection, performing the process indicated in Scheme 1B, in a strategy similar to that used for the Ru-PTZ dyad.51 The model dyad (Ru-TP) consisted of a Ru(II) tris-bpy complex unit with ester (COOEt) anchor groups, attached through an alanine bridging unit to a tyrosine substituted with dipicolinamide ligands (the electron donor), Chart 1. The Photochemistry, 2009, 37, 362–392 | 371 This journal is

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Chart 1

model was designed to mimic processes occurring in photosystem II (PS II) between tyrosine and P680.52 Compared to an earlier Ru(II) complex-tyrosine model compound, which had a simple tyrosine unit and exhibited low (15%) intramolecular ET eﬃciencies,53 the Ru-TP model had added a H-bonding interaction between the phenol of tyrosine and dipicolinamide units, as shown in Chart 1. This interaction was added to mimic a recently discovered H-bonding interaction in PS II that seems to play a crucial role in promoting eﬃcient charge separation in PS II. The solution emission spectra of Ru-TP and the reference Ru-Ala (which lacks the tyrosine-picoline unit) followed single exponential kinetics and had similar emission lifetimes; 1150 ns and 1120 ns, respectively. As expected, the tyrosine moiety did not quench the excited states of the Ruthenium chromophore, as in the Ru–Rh dyad described earlier. Time-resolved transient absorption spectra of both Ru-TP/TiO2 and Ru-Ala/TiO2 showed bleaching of the ruthenium II chromophore at 460 nm as a result of formation of ruthenium III following electron injection into TiO2, Fig. 7. The features at 390 and 550 nm were assigned to emission from 3MLCT from unquenched Ru(II). The time evolution for the Ru-TP dyad was very diﬀerent: the bleaching signal decayed almost completely after 2 microseconds; indicating that ruthenium III formed during the injection process is quickly regenerated (reduced) by the tyrosine-dpa unit. Hence, the intramolecular ET process (Tyrosine-dpa to Ru(III)) was successfully competing with the charge recombination. The intramolecular ET reaction from the tyrosine was fast (550 ns) and almost quantitative (F over 90%), forming a charge separated state on TiO2, in which holes are removed the surface and in which the internal donor unit (tyrosine-dpa) regenerates the dye but does not compete in the electron injection process. In the case of tyrosine alone52 the intramolecular ET and the charge recombination processes competed with each other, resulting in low quantum yields. The presence of the H-bonding interaction in the new dyad was crucial for the eﬃciency of the injection. 372 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 7 Time-resolved absorption diﬀerence spectra recorded after pulsed light excitation at 450 nm of the dye-sensitized ﬁlms in 0.1 M LiClO4 acetonitrile. (A) Reference compound RuAla/TiO2 data recorded at 50 ns (&), 500 ns (J), 2 ms (n), and 20 ms (,). (B) dyad RuTP/TiO2 data recorded at 50 ns (&), 200 ns (O), and 2 ms (n). Insets: recovery kinetics at 470 nm. Taken from Fig. 3 of ref. 51.

3.6 Dyads made of Ru(terpyridine) with tyrosine-dipicolinamide or carotenoid units54 More recently the same concept employed for the Ru-TP dyad was used by Sundstro¨m and Sun to develop a series of Ru-terpyridine-based model compounds. The use of the terpyridine (tpy) ligand, allowed the substitution of two 4 0 -positions of the Ru complex thus maintaining the axial symmetry and a rod-like shape. In the series of bis(terpyridine)Ru(II) models, one of the terpyridines was functionalized in the 4 0 -position with the anchoring group (phosphonic or carboxylic acid group) for attachment to TiO2. The other tpy was substituted with an electron donor in the 4 0 -position with tyrosine or hydrogen bonded tyrosine, or a carotenoic amide, Fig. 8. Here the systems have been abbreviated as Ru(tpy)-TP for the compounds carrying tyrosine, or hydrogen bonded tyrosine, and Ru(tpy)-car for the compounds substituted with carotenoid amide. Although the systems were termed triads by the authors because are made of donor-dye-anchor units, eﬀectively the number of components is the same as the earlier Ru-TP models. The triads in Fig. 8 were studied in solution, bound and in solar cells devices. The choice of the carotenoid unit as the donor allowed exploitation of the excellent electron donor properties of carotenoids. In addition, oxidized carotenoids have a strong absorption band in the 800–1000 nm region, which is a useful spectroscopic probe to monitor the intramolecular electron transfer from the donor unit to the oxidized Ru(III) complex (Step 2 in Fig. 8). It was anticipated that, in all triads, selective light excitation of the Ru complex would result in a stepwise process similar to that observed for Photochemistry, 2009, 37, 362–392 | 373 This journal is

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Fig. 8 Structure of the Ru(tpy) based triads and stepwise electronic processes. Structures are adapted from Chart 1 of ref. 54.

Ru-TP models, according to Scheme 1B, producing a long lived charge separation. Selective excitation of the Ru center, however, could not be achieved as carotenoids also strongly absorb in the 400–500 nm region. In solution, excitation of 2a produced transient absorption spectra consistent with formation of the carotenoid triplet state, and on TiO2 2a showed a band at 840 nm which was assigned to an all-trans carotenoid radical, Fig. 9. The charge recombination between the carotenoid radical and TiO2 occurred on the time scale of a few ms, Fig. 9. Surprisingly, the charge separated state could only be observed for 2a. DFT calculations, showing in all cases the LUMO favorably localized on the tpy connected to the TiO2 could not rationalize the observed behavior. 3.7

Ru(tpy)triphenylamine dyads

Gra¨tzel and coworkers proved that Ru(tpy)-based supramolecular dyads could also be employed to develop eﬃcient photochromic devices that perform the type of stepwise processes described in Scheme 1B.26,55,56 The compounds were studied in a single semiconductor layer (bound to a TiO2 374 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 9 Transient absorption spectra at 0.1 ms (full symbols) and 2 ms (open symbols) obtained after 520 nm excitation of 2a/TiO2. The inset shows kinetics measured at 840 nm for 2a/TiO2 (lexc = 520 nm). Taken from Fig. 2 of ref. 54.

thin ﬁlm Fig. 10A–C) as well as in a conﬁguration that involved a second semiconductor layer (SnO2), to produce a photochromic device, Fig. 10D. The dyads were anchored to TiO2 nanoparticle ﬁlms through a phosphonate group, and two types of non-chromophoric p-methoxy-triphenylamine electron donors were used to regenerate the dye unit though intramolecular charge transfer as in Scheme 1B and Fig. 10A–B. A comparison was made by co-binding the individual components of the dyads to the surface of the semiconductor (Fig. 10C) to determine the importance of synthesizing a covalent dyad system and to assess the inﬂuence of lateral charge transfer as a competing process. Several important observations were made by studying the systems bound to TiO2: (1) Photoexcitation of the Ru complexes in the dyads resulted in the formation of long lived charge-separated states (t1/2 for 2 = 300 ms). The recombination kinetics were very diﬀerent for the system studied, with half lives ranging from 3 ms for 1/TiO2 to 300 ms for 2/TiO2. (2) The charge separation process was more eﬃcient in the heterotriad 2/TiO2, with the LUMO localized on the anchoring ligand. (3) The hole transfer process (D–S+/TiO2(e) - D+–S/TiO2(e), step 2 in Scheme 1B) was much faster than the charge recombination, and biphasic kinetics were observed indicating multiple conformation of the molecule on the semiconductor or intramolecular D - S+ transfer within the monolayer. (4) The 3+5/TiO2 ﬁlms behaved as 2/TiO2. (5) It is necessary to control the distance and geometry (orientation) of such triads on the surface of the semiconductor and to separate the dyes to avoid lateral electronic processes that short–circuit the vectorial intramolecular and interfacial processes as planned in Scheme 2B. (6) Finally, the observed photoelectrochromism (Fig. 10D and Fig. 11) paves the way to novel types of devices involving multi-component dyes and semiconductors.

3.8

N845

The series of Ru-complexes linked to a secondary organic donor moiety was further developed by Hirata, Durrant and coworkers26 using a N719 dye modiﬁed with a N,N-(di-p-anisylamino)phenoxymethyl group as the Photochemistry, 2009, 37, 362–392 | 375 This journal is

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Fig. 10 A, B, C. Idealized distances, energy levels, and electron-transfer half-lives in the heterotriads 1/TiO2 (A), 2/TiO2 (B), and 3+5/TiO2 (C). Adapted from Fig. 5 of ref. 55. D. Proposed electron ﬂuxes in the illuminated photochromic systems 2/TiO2 on SnO2. Photoexcitation of the sensitizer S is followed by electron injection (a) into the TiO2 and oxidation (b) of the donor D. Lateral conduction (c) inside the monolayer allows electrons to ﬂow from the SnO2 (d) to the oxidized donors. Adapted from Fig. 7 of ref. 55.

internal donor, abbreviated as N845, Fig. 12. Photoexcitation of the dye bound to TiO2 nanoparticle ﬁlms resulted in the formation of a remarkably long-lived charge separated state (t1/2 = 0.7 s, about 1000 times slower than N719). The HOMO for the studied dye was shifted to the secondary electron donor moiety, displacing the hole in the oxidized form of the dye from the thiocyanate ligands closer to the triphenyl amine group. The increased

Fig. 11 Absorption spectra of the heterotriads D-SjTiO2 under positive polarization (0.5 V), in the dark (dotted lines) and after 10 min. illumination (full lines). Adapted from Fig. 6 of ref. 55.

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Fig. 12 Schematic representation of the molecular structure and electronic processes of the N845 dye (structure at right). Taken from cover artwork of ref. 26.

separation of the HOMO orbital from the TiO2 surface is an essential property to achieve slower charge-recombination dynamics. 4. 4.1

Organic multicomponent systems Peryleneimide–naphthalenediimide dyad57

Perylene imides (PI) and diimides are widely used to develop solid state organic thin ﬁlms, organic photovoltaics, and organic light emitting diodes.58–60 Perylene diimides are chemically and photo chemically stable and have been used commercially for decades as red pigments in paints. In addition, they have excellent photophysical properties. They absorb strongly in the visible, and the spectrum of the reduced dye is well separated from the absorption spectrum of the excited singlet state. Morandeira and coworkers studied a peryleneimide-naphthalenediimide (PI-NDI) dyad covalently bound to nanoporous nickel oxide (NiO, Ebg = 3.55 eV) electrodes in the presence of the redox couple I3/I.57 The dyad was designed to solve a serious drawback in the development of eﬃcient NiO-based DSSCs, i.e. the fast recombination between the hole in the semiconductor and the reduced sensitizer. The NDI unit acted as a secondary electron acceptor that regenerates the dye and places the electron further away from the interface, resulting in a longer-lived charge separated state, according to Scheme 1C. A comparison was made with perylene diimide (PI) which lacks the NDI acceptor unit, Chart 2. The phenoxy (OAr) substituents decreased the acceptor properties of the PI unit, and the t-Bu units were added to avoid dye aggregation. DFT calculations showed that the LUMO in PI is evenly distributed with little or no charge transfer character. However, the LUMO for PI-NDI is located on the NDI unit, Fig. 13. The localized nature of the frontier orbitals in the PI-NDI dyad was explained by the minimized geometry of the molecule (the PI and NDI rings are not in plane in the ground state minimized geometry) Photochemistry, 2009, 37, 362–392 | 377 This journal is

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Chart 2

Fig. 13 Frontier orbitals of the PI dye (left) and the dyad (right). Taken from Fig. 2 of ref. 57.

resulting in weak electronic coupling between the two units. In summary, the dyad behaved like the sum of two electronically separated units. Upon selective photo excitation of the PI unit an electron is donated from the valence band of NiO to the dye: the metal oxide electrode functioned as the electron donor and the dye as the acceptor, Fig. 14. The dye/NiO electrode is eﬀectively a photo cathode, Fig. 14. The charge transfer process was monitored by femtosecond transient absorption spectroscopy. The charge transfer was ultrafast (t = 0.5 ps) in both PI and PI-NDI. In the case of the dyad, however, a new charge separated state was observed onto the naphtalenediimide unit (NDId). 378 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 14 Energy diagram describing the charge separation events in PI-NDI/NiO and stepwise processes according to Scheme 1C. Top ﬁgure taken from Scheme 2 of ref. 57.

The use of the secondary electron acceptor (the NDI unit) in the dyad resulted in a signiﬁcantly longer lived charge separated state. The timescale of the recombination process is a problem in the application of NiO DSSCs as it competes with injection. In the p-semiconductor NiO, recombination timescales for most dyes are 100 ps to 10 ns, much faster than on n-type systems (TiO2) that often exhibit recombination times on the order of microseconds. In the case of PI-NDI/NiO, however, the NDId was stable on a ns time scale, contrasting the rapid decay previously observed from PI on NiO. Although there was no evidence that the charge separation was a stepwise process in the dyad (neither a PId nor a PId+ intermediate was observed), the remote attachment of an acceptor on the surface of the semiconductor was a successful approach. The two key results are that (1) in the dyad, NDId still formed very rapidly but recombination with the hole (NiO(+)) was substantially slower than in the case of PI/NiO. (2) The long lived charge separation also resulted in higher IPCE values: a threefold improved absorbed photon to current conversion eﬃciency (APCE) was observed in the DSSC made of PI-NDI/NiO compared to PI/NiO, Fig. 15. 4.2

BODIPY-based dyads

Boron dipyrromethenes (BODIPYs) are useful ﬂuorophores for a variety of application including ﬂuorescent probes in biological studies, ﬂuorescence imaging, and BODIPY-porphyrins donor acceptor dyads for artiﬁcial photosynthesis models.61 BODIPYs have attractive photophysical properties, including a strong visible absorption at B500 nm, large quantum yields, long excited state lifetimes (few nanoseconds), and the possibility of Photochemistry, 2009, 37, 362–392 | 379 This journal is

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Fig. 15 IPCE for a DSSC with a 800 nm thick, nanoporous NiO ﬁlm as photocathode, sensitized with PI (dashed), PI-NDI (solid), or bare NiO (dotted). Inset: the visible absorption spectrum of the ﬁlm sensitized with PI-NDI. Taken from Fig. 5 of ref. 57.

functionalization on the pyrrole rings. The BODIPY radical anion can inject an electron in the conduction band of TiO2.62–64 A BODIPY-based dyad, 8-(2,4,5-trimethoxyphenyl)-4,4-diﬂuoro-1,3,5,7-tetramethyl-4-bora-3a,4adiaza-s-indacene (MEOPHBDP in Chart 3), where the BODIPY unit was modiﬁed with an electron donor (2,4,5-trimethoxybenzene, MEOPH), was recently developed by the team of Fukuzumi, Nagano and Lemmetyinen.65 The dyad was designed to follow a stepwise injection strategy according to Scheme 1B. DFT calculations revealed that in the dyad MEOPHBDP, the HOMO-1 is localized on the MEOPH moiety (the donor) and the LUMO is localized on the BDP moiety (the acceptor), as illustrated in Fig. 16. Photoexcitation of the BODIPY moiety to the singlet excited state and intramolecular charge transfer from the donor unit results in formation of the charge

Chart 3

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Fig. 16 (a) HOMO-1 and (b) LUMO orbitals of MEOPHBDP. The geometry optimization was calculated by a DFT method with Gaussian 98 (B3LYP/6-31G* basis set). Taken from Fig. 4 of ref. 65.

separated state MEOPHd+–BDPd on the surface of TiO2. BDPd can inject an electron into TiO2, as the redox potential of BDP/BDPd, 1.11 V vs. NHE, is lower than that of the TiO2 conduction band, 0.5 V vs. NHE. In solution, the ﬂuorescence emission of MEOPHBD, following excitation at 420 nm of the BODIPY moiety (BDP) to the singlet excited state, was signiﬁcantly quenched by intramolecular electron transfer from the donor MEOPH. Because of intramolecular quenching, the emission quantum yield was very low (the dye was virtually non-ﬂuorescent) and the ﬂuorescence lifetime was considerably shorter in the dyad (1.2 ps) than in the reference PHBDP (3.2 ns). The dyad with carboxylic acid anchoring groups, MEOPHBD-COOH was bound to TiO2 ﬁlms as well as ZnO–SnO2 composite ﬁlms, and the injection process and spectroelectrochemical properties of solar cells prepared from them was compared with a reference compound lacking the donor unit, PHBD-COOH. The rate of formation of the charge-separated state (MEOPHd+–BDPd) in solutions of MEOPHBD-COOH was very fast (8.5 1011 s1), and the decay of the transient absorption of BDPd at 580 nm was monitored by femtosecond laser ﬂash photolysis. The decay of the absorbance at 580 nm obeyed ﬁrst-order kinetics with the rate constant of 1.7 1010 s1 and was due to the back-electron transfer in the radical ion pair to regenerate the original ground state. This result indicated that the MEOPHd+–BDPd state was very short lived (B59 ps). Interestingly, appreciable photocurrent generation, following illumination with white light, was observed in DSSCs prepared from the non-ﬂuorescent dyad MEOPHBD-COOH, indicating that the short lived-charge separated state can inject an electron into the semiconductor, Fig. 17. The eﬃciencies were, however, lower than for the DSSCs prepared from PHBD-COOH. This was expected because of the shorter lifetime and low quantum yield of EOPHBD-COOH. 4.3

Dyads for tuning HOMO–LUMO energy levels

Multicomponent organic sensitizers consisting of an electron donor unit (triphenylamines), a p-conjugated linker that acts as the chromophore (oligovinylthiophene), and an acceptor group that anchors to the surface Photochemistry, 2009, 37, 362–392 | 381 This journal is

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Fig. 17 (A) Comparison of photocurrent action spectra (IPCE values) of (a) OTE/TiO2/ MEOPHBDP-COOH with (b) OTE/TiO2/PHBDPCOOH electrodes. (B) Photocurrent– photovoltage curves of (a) OTE/TiO2/MEOPHBDP-COOH and (b) OTE/TiO2/PHBDP-COOH electrodes under white light illumination. For details see ref. 65. Taken from Fig. 8 of ref. 65.

Chart 4

(cyanoacrylic acid) have been developed by Sun and coworkers to have a pronounced push-pull eﬀect upon photoexcitation, i.e. an intramolecular charge transfer from a remote donor moiety to an acceptor that is bound, and strongly electronically coupled to, the semiconductor, Chart 4.66,67 The ultimate goal of engineering the three individual components in the series of dyes (particularly the structure of the donor66 and the length of the conjugated bridge67) was to tune the HOMO and LUMO energy levels in organic dyes to prepare more eﬃcient solar cells with increased photovoltages (Voc in Fig. 1). In a sensitizer, the LUMO should be suﬃciently negative to inject an electron into the conduction band of the semiconductor, and the HOMO suﬃciently positive to allow eﬃcient dye regeneration by the redox couple in the electrolyte. The Voc of a solar cell is determined by the diﬀerence between the conduction band in the TiO2 under illumination and the Fermi level of the electrolyte, as shown in Fig. 1. It is proposed that higher Voc could be obtained either by slower recombination (eqn (3)), or by adjusting the band edge with respect to the redox potential of the redox mediator. Ultimately, careful tuning of the HOMO–LUMO levels can lead to both larger Voc, and enhanced spectral absorption in the visible. The methoxy 382 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 18 Molecular orbital energy diagram and isodensity surface plots of the HOMO and LUMO of D9 and D11. Adapted from Fig. 3 of ref. 67.

moieties were added in the more recent generation of dyes67 (see Chart 4) to increase the electron donor properties and inhibit recombination between injected electrons and the triiodide in the electrolyte, a problem that was observed in the earlier work.66 TDDFT calculations showed that in these types of dyes, the lowest excitation is a HOMO–LUMO charge transfer transition, with a LUMO delocalized on the bridge-anchor unit, and with negligible LUMO contribution in the donor groups (see Fig. 18). The absorption spectra of the dyes in the series exhibited a strong (e B 33,000 M1cm1) p–p* transition band centered at 450 nm. Longer p linker lengths resulted in moderate red shifts in the visible spectrum. However, increasing the electron donor character of the phenylamine units by functionalization with p-OMe groups produced almost a 20 nm red shift, HOMO destabilization, and lower oxidation potentials. Current–voltage characteristics of the dyes were studied in DSSCs with TiO2 layers of varying thickness, and higher Voc were observed with D11 (for the structure see Fig. 18). This dye exhibited overall conversion eﬃciency of 7.20% and an open-circuit voltage of 740 mV, demonstrating that dye engineering at a molecular level can tune (and improve) the properties of the devices. The eﬀect of modiﬁcation of the linker/chromophore unit and of the anchor/electron acceptor group was studied in a separate series of compounds (see Chart 5).68 The addition of electron withdrawing substituents to the p-conjugated linker resulted in a pronounced bathochromic shift in

Chart 5

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Fig. 19 (a) J–V curves and (b) IPCE of DSSCs. Taken from Fig. of ref. 68.

the absorption spectra, increased light-harvesting range in the visible region, and shifted the HOMO levels of the dyes to a more positive potential. These dyes also showed considerable diﬀerences in solar cell eﬃciencies based upon the electron accepting group, Fig. 19. The dyes with the cyanoacrylic anchor group showed faster and more eﬃcient electron injection into the semiconductor.

4.4

Electrochromic materials

Research into supramolecular systems attached to metal oxide surfaces for devices has also been extended to the development of new types of electrochromic windows. These are devices based on a redox active molecule (typically methyl-viologen, MV) anchored to TiO2.69 The conﬁguration of the window is similar to a DSSC cell, except that the semiconductor acts as the electron donor to the bound molecule. When a voltage is applied to the ITO/TiO2/Ox electrode, an electron is donated to the acceptor (Ox), which changes color upon reduction. MV, for instance, turns from pale yellow into a deep purple-blue. The color change of the window is reversible, as the redox cycle is repeated. Fitzmaurice and coworkers have studied supramolecular methyl-viologen systems attached to TiO2 for use in electrochromic devices.70 A heterosupramolecular system consisting of a tripodal viologen, adsorbed to TiO2, threaded a crown ether to form a pseudorotaxane. In the case of the heterosupramolecular system in Fig. 20, the crown ether shifts the reduction and oxidation potentials of the viologen complex. The tripodal linker has a dual function: it attaches the MV to the semiconductor surface and maintains it at a distance, so that the crown ether can thread on it (see Fig. 20). 5.

Light harvesting strategies

One the attractive properties of polynuclear complexes and multicomponent dyes is the possibility to use them to prepare molecular devices based on ‘‘antennae’’ sensitizers. This concept is schematically illustrated in Scheme 1D. An array or multilayer of sensitizer units or light harvesters is deposited on the electrode. Photoexcitation results in vectorial energy or electron transfer that produce a charge separate state. To perform this function, there are three key requirements: (1) the antenna must be eﬃcient in channeling the absorbed energy towards the surface, (2) the molecular unit coupled to the semiconductor must inject electrons into the conduction band, and (3) the surface orientation of the array must be controlled. 384 | Photochemistry, 2009, 37, 362–392 This journal is

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Fig. 20 Heterosupramolecular methyl viologen systems for electrochromic windows. Top structure adapted from Scheme 1 of ref. 70.

In artiﬁcial photosynthesis71 light harvesting arrays are developed to collect sunlight and produce high energy chemicals, and there are numerous examples of elegant multilayer assemblies on surfaces72,73 and light harvesting arrays studied in solution designed to perform this function.17,74–83 We have limited this section to a few representative examples of multicomponent molecules that were anchored and studied on the surface of metal oxide semiconductors. One of the main goals of the multichromophoric arrays described here is to increase the light harvesting eﬃciency (LHE) of the devices by acting as antennae systems. This property is expected to be useful to sensitize planar semiconductor ﬁlms or electrodes for the production of thin ﬁlm cells. One of the reasons why the Gra¨tzel-type cells work so well is that the mesoporous nanoparticle ﬁlms have a huge surface area for the physisorption of the dyes (about three orders of magnitude higher compared to planar crystal surfaces). Eﬃcient sensitization cannot be achieved for planar surfaces, unless a light harvesting array is used. 5.1

Multimetallic arrays

Meyer and coworkers have recently proposed multimetallic arrays, prepared from cis- or trans-polynuclear Ru(II) complexes [Ru(bpy)2(ina)(pz)[Ru(bpy)2(pz)]38+ units on the surface of semiconductor, that could act as antenna sensitizer.84 This concept is illustrated in Fig. 21. One of the most interesting aspects of this work is the study of the diﬀerence in binding between stereoisomers. Meyer noted that the footprints’ sizes of the cis- and trans-isomers of Ru complexes on the surface of the semiconductor would be very diﬀerent, Fig. 21. This is important because the LHE of a solar cell is related to the fraction of light Photochemistry, 2009, 37, 362–392 | 385 This journal is

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Fig. 21 Proposed models of multimetallic cis- and trans-polynuclear Ru(II) complexes on the (100) surface of anatase TiO2 as possible light harvesting arrays. The cis-form shown on the left hand side occupies 202 A˚2, while the trans-form has a footprint of only 70 A˚2. Taken from Fig. 1 of ref. 84.

Fig. 22 Proposed injection processes for trans Ru complexes. Taken from Scheme 2 in ref. 84.

absorbed, a (absorptance) of a monolayer on a ﬂat surface, which is related to the area occupied on the surface according to eqn (6)84 a = 3.82 105 e/footprint

(6)

The smaller footprint of the trans-isomer indicates that this geometry is preferable for sensitization of planar semiconductor electrodes as it would form a well-ordered monolayer with close packing of the antennae units, and the vertical orientation would increase the light harvesting eﬃciency. To probe this concept, trans-[Ru(phen-NH-phen)(ina)2](PF6)2 was studied on the surface of nanoparticle TiO2 thin ﬁlms. Ultrafast and eﬃcient (F = 0.8) injection was observed on the surface of nanocrystalline TiO2 ﬁlms. Interestingly, compared to an N3-type dye, injection from the trans complex with isonicotinic acid (ina) and phenanthroline (phen) ligands occurs remotely, as illustrated in Fig. 22. 5.2

Porphyrin arrays

Porphyrins are eﬃcient and robust dyes for DSSCs and, after the polypyridine complexes of Ruthenium, they are probably the most widely studied sensitizers for semiconductors.85 A wide variety of large porphyrin arrays can now be prepared, using a building-block approach or by self assembly, and the binding of such arrays to the surface of semiconductors (including planar surfaces) has the potential of increasing greatly the dye coverage in the cell. Most interestingly, it could lead to conﬁgurations with great control of energy and electron transfer processes. 386 | Photochemistry, 2009, 37, 362–392 This journal is

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Oﬃcer and colleagues, in a comprehensive review on the use of porphyrins as light-harvesters for DSSCs, describe results obtained in their own laboratory with smaller models for porphyrin antennae bound to the surface of semiconductors, and oﬀer a perspective of this ﬁeld.86 Two of the proposed designs for porphyrin-based antennae are the branched, and the linear one, Fig. 23. The branch design, with parallel energy processes is expected to be more energy eﬃcient than the linear arrangement, but the wider and bulkier antenna would lead to lower surface coverages. Model compounds Zn2-2 and Zn2-3 synthesized to test the two arrays gave similar, and low, Isc and Voc values, and poor surface binding, compared with a model compound containing a single porphyrin. An earlier example of the linear design was realized with two dyads by Koehorst et al., each made of a Zn(II) porphyrin attached to a free base porphyrin and diﬀering in the order of attachment to the TiO2 surface, Fig. 24.87 Cells were constructed from the two dyads, anticipating very diﬀerent results, since in dimer A the electron transfer processes are consecutive, while in B are competitive. The fact that similarly poor results were obtained for both dyads in terms of eﬃciencies (IPCE B3%) and photocurrents was ascribed to an ‘‘antenna eﬀect’’. It is important to remember however, that excellent control of surface orientation and strong binding are crucial, as shown by the example with Zn2-2 and Zn2-3, and it is not clear whether aggregation or non ideal binding modes (ﬂat on the surface, for instance) could be, at least in part, the reason for the observed behavior. Precisely to control the binding, ‘‘sticky’’ porphyrin array with numerous COOH anchor groups were prepared and studied by Oﬃcer et al.86 For each pair, the arrays with more acid groups gave the lower Isc values. Since

Fig. 23 Branched and linear array design as proposed by Oﬃcer et al.86 for antennae made of porphyrins, and dyads prepared to test the concept.

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Fig. 24 Linear array of free base/Zn(II) porphyrin dyads and proposed electronic processes. Figure taken from Fig. 8 of ref. 86.

the arrays with the free isopthalic acid (Phm(COOH)2) moieties gave the highest outputs this suggests that the porphyrin array is a better photosensitizer when it is not held too close to the surface. Overall, single porphyrin dyes were generally more eﬃcient than a variety of porphyrin arrays. This result reﬂects more on the large size and poor surface adsorption of the arrays than their ability to act as light-harvesting antennae. Lindsey, Holten and Bocian, who have developed and studied numerous conﬁgurations of porphyrin arrays, have recently reported a theoretical prediction of light harvesting and energy conversion properties of rod-like arrays consisting of porphyrins oligomers linked together with diphenylethyne groups.88 Such linear arrays could orient perpendicularly to the TiO2 surfaces, as proposed in Fig. 25. The visible and electrochemical properties of these arrays were modeled as a sum of the properties of the individual porphyrins, based on the properties of multiporphyrin arrays developed by the authors.89–91 The phenylethynyl linker groups provide weak electronic coupling, allowing fast singlet excited state intramolecular energy transfer (ps timescale) and ground state electron/ hole transfer (submicrosecond time scale). The gradient of redox potential in the array was designed so that, upon photoexcitation, the hole moves away from the metal oxide surface, and an electron is injected in the semiconductor by the porphyrin unit anchored to the surface, as illustrated in Fig. 25. The

Fig. 25 Proposed multi-porphyrin arrays on the surface of the semiconductor, replicating the linear array of Scheme 1D. Top ﬁgure taken from Fig. 1 of ref. 88.

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theoretical work was also extended to model arrays where porphyrins are substituted with perylene imide dyes to increase light absorption in the spectral region between the Soret and Q-bands. The results of this work, which combines theoretical studies with data obtained from porphyrin arrays in solution and bound, underscore the importance of parameters such as surface orientation, ﬁne-tuned energetics and electrochemical properties, for the design of light harvesting arrays with optimized solar conversion eﬃciencies. 6.

Conclusions

Well-developed theoretical strategies and synthetic methodologies are now available to design and prepare ﬁne-tuned multicomponent model compounds and arrays (organic and inorganic) for the sensitization of semiconductors through stepwise electronic processes. Important applications include the development of DSSCs with improved eﬃciencies, increased light harvesting on planar surfaces and, more importantly, the capability to tune and control electronic processes at the interface between a molecule and nanostructured semiconductor surfaces. This important interface is crucial to the development not only of DSSCs but for a variety of electronic devices. An important limiting factor in this ﬁeld remains the control over the dye/interface contact at the molecular level: orientation, distance, electronic coupling and control over the adsorption mode are likely to dominate the photophysical and photoelectrochmical behavior of otherwise well designed arrays and, ultimately, of the devices.

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