Photochemistry Volume 40

A Review of the Literature Published Between May 2011 and April 2012

By Angelo Albini

The Royal Society of Chemistry

Copyright © 2012 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-437-0

Contents

Preface Angelo Albini, v,
Periodical reports: Organic and computational aspects,
Introduction and review of the years 2010–2011 Angelo Albini, 3,
Computational Photochemistry and Photophysics: the state of the art Ya-Jun Liu, Daniel Roca-Sanjuán and Roland Lindh, 42,
Alkenes, alkynes, dienes, polyenes Takashi Tsuno, 73,
Photochemistry of aromatic compounds Kazuhiko Mizuno, 106,
Organic aspects. Oxygen-containing functions M. Consuelo Jiménez and Miguel A. Miranda, 146,
Functions containing a heteroatom different from oxygen Angelo Albini and Elisa Fasani, 174,
Highlights in photochemistry,
The history of the European Photochemistry Association Ugo Mazzucato, 197,
History of the Asian and Oceanian Photochemistry Association (APA) Haruo Inoue, 230,
Photoprotection of human skin Bernd Herzog, 245,
Photo-induced water oxidation: New photocatalytic processes and materials Serena Berardi, Giuseppina La Ganga, Fausto Puntoriero, Andrea Sartorel, Sebastiano Campagna and Marcella Bonchio, 274,
Any colour you like. Excited state and ground state proton transfer in flavonols and applications Stefano Protti and Alberto Mezzetti, 295,

CHAPTER 1

Periodical reports: Organic and computational aspects

Introduction and review of the years 2010–2011

Angelo Albini

DOI: 10.1039/9781849734882-00001

After a short introduction on the changes adopted in the format of this series, some representative findings on photochemistry and applications published in 2010–11 are reviewed.

1 Introduction

The present volume, no. 40 in the series ‘Photochemistry’ of the Specialist Reports published by the Royal Society of Chemistry makes a further step forward in the direction indicated in volumes 37–39. This choice arises from the idea that the role of photochemistry has changed by a large degree in the more than 40 years intervening since then the series was planned (volume 1 was published in 1970). In the Sixties, photochemistry was a young science (see below, however) that had been just established as a consistent discipline and the advancement in the rationalization of key issues was pointed out year after year by each volume. This fact, along with the much greater work then required for literature search, made these series a much welcome opportunity for the many scientists then entering the field and for anybody wishing to keep abreast with the advancement of this discipline in a time-effective way. Nowadays, literature search is done in a much faster, although not necessarily dependable, way, while photochemistry has become a pervasive science with a variety of remarkably diverse applications.

Thus, the problem is not so much that of making available new notions to the photochemical comunity, but rather that of offering the information to various communities of scientists, some of which do not consider themselves full-time photochemists, and facilitate the exchange between them. Indeed, differently for example from some spectroscopic methods, where having a crytically compiled list of the data published each year remains useful, offering inventories of the new publications in photochemistry is probably not sufficient. Thus, after that with the previous two volumes the delay accumulated had been eliminated, it was felt that a structure change was advisable. Thus, next (yearly) volumes will be prepared in the following way.

The periodical on the different photochemical disciplines will be published every other year. The biennal coverage should help in clarifying the development of specific studies and their significance for photochemistry in general, while it is hoped that the delay in reporting part of the data has a limited effect, because appropriate literature surveys are generally available. Of course, the short review of the last two years that is done in this chapter refers to the whole field of photochemistry, indeed is meant to give a flavour of the large field of applications.

The specific reports mentioned above will correspond to about a half of each volume, the other half being occupied by highlights, prepared by well known specialists. For the reasons mentioned above, these will be mainly devoted to applicative aspects of photochemistry.

It is hoped that this dual structure may contribute to maintaining some connection among the various fields of photochemistry, whether these refer to the core discipline or to a practical application.

As a result, volume 39 contains reports on spectroscopic and physicochemical aspects (coverage: year 2010), as well on inorganic aspects and solar energy conversion (among the Authors, F. Punturiero and K. Kalyanasundaran contribute for the first time to this series), while organic and theoretical aspects are reviewed in volume 40 (two years coverage, 2010 and 2011, Y. Liu contributing for the first time).

As for the highlights, these had been introduced in volumes 37 and 38 in the number of three and five respectively and should remain in that range, as it is the case for the present volume. Next to scientific reports, a historic account on two of the main photochemical societies, the Asian and Oceanian and the European, are presented.

Two further topics should be rapidly mentioned. The first one as to do with history. In July 2010 a minisymposium for celebrating the 100th birthday of photochemistry was organized. The choice of the date may be discussed. This originated from the recognizment that, although the action of solar light on a variety of chemicals had been long known and some photochemical reactions had been well described in the 19th century and earlier, it is only through the work by Giacomo Ciamician, Emanuele Paternò, Hans Stobbe and a few others that a sufficient number of reactions was thoroughly studied, so that generalizations could be made. The work by these scientists was for the main part published by 1910 and by that year many – if not most of the – photochemical processes that today are applied in the lab and taught in the classes were known.

Apart from some historic note, the meeting attempted to re-create the spirit of a hundred years ago, when photochemistry seemed to be the science of the future. This was done through seven lectures figuring out what may be the contribution of photochemistry to the development of chemistry (solar light conversion, organic synthesis, molecular machines, single crystal photochromism, computation and photochemistry, new chemistry and biology via singlet oxygen, photomedicine), as well as by asking every participant which he/she felt the most important contribution photochemistry may give in the future.

Finally, one may ask the question, which is the place of photochemistry at present? Perhaps not the science of the future, as it was in the first decade of the 20th century, nor it is expanding as it did in the 1950 and 1960. Certainly, it pervades chemistry, physics, biology and allows advancement that would not be possible without the insight we now have of photochemical processes.

One way for assessing how important is deemed this discipline is looking for the most often red papers. As an example, the American Chemical Society publishes a list of the ten most accessed papers for each of its journals in 2010. It is remarkable that six out of the ten most accessed articles in Accounts of Chemical Research is directly concerned with photochemistry (this is in part due to the printing of a special issue on the theme, but this is not sufficient to explain the great success of the topic). As it appears from the titles below, five of these have to do with the various aspects of solar energy conversion. It would appear that investigations on this problem are again experiencing a lively development.

– Recent Advances in Sensitized Mesoscopic Solar Cells.

– “Plastic” Solar Cells: Self-Assembly of Bulk Heterojunction Nanomaterials by Spontaneous Phase Separation.

– Molecular Understanding of Organic Solar Cells: The Challenges.

– Solar Fuels via Artificial Photosynthesis.

– Visible Light Water Splitting Using Dye-Sensitized Oxide Semiconductors.

– Using Singlet Oxygen to Synthesize Polyoxygenated Natural Products from Furans.

This impression is confirmed by the fact that the other ACS journal where photochemical articles are among the top ten is Inorganic Chemistry with four.

– Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells.

– Chemistry of Personalized Solar Energy.

– Catalytic Water Oxidation by Single-Site Ruthenium Catalysts.

– Photoelectrochemical Behavior of Sensitized TiO2 Photoanodes in an Aqueous Environment: Application to Hydrogen Production.

However, organic chemistry and material science are not cut down. An indication is the presence in the list of an account on the synthetic utility of singlet oxygen (see above) and of a paper on click chemistry for surface immobilization in the specific JACS list for surface patterning.

– High Density Orthogonal Surface Immobilization via Photoactivated Copper-Free Click Chemistry.

2 Review of the years 2010–2011

2.1 Books, reviews

After the two textbooks published in 2009, two further photochemical books of general interest became available in 2010. One of them is a handbook of synthetic photochemistry, addressing the practical issues of how to carry out a photochemical preparation and reviewing in ten chapters the main photoreactions. These are classed according to the chemical transformation occurring (type of bond formed, linear or cyclic product etc.) in order to facilitate the inclusion of photochemical steps in synthetic planning. The second one is a two volume set (40 chapters, 1200 pages) on hydrogen transfer in excited states. Then, in 2011 Ramamurthy and Inoue edited a conspicuous (640 pages) book devoted to supramolecular photochemistry and Wypych published a Handbook of UV degradation and stabilization. Further major multi-authors books or special issues in scientific journals has concerned photochromism, solar chemistry and photocatalysis, the plenary lectures at the XXIII IUPAC Symposium in Photochemistry.

A number of excellent reviews have been published in various journals. Besides those mentioned above and some more that will be indicated when discussing the specific reactions below in this section, a few of the topics considered are listed below. This serves at least to have a taste of how varied is the scope of the applications of photochemistry.

– Using perfluoroazides for the modification of surfaces and the synthesis of nanomaterials.

– Advances in patterning materials for 193 nm immersion lithography.

– Fluorescent analogs of biomolecular building blocks: design, properties, and applications.

– Imaging and photodynamic therapy: mechanisms, monitoring, and optimization.

– Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar cells.

– Engineering Metal Organic Frameworks for Heterogeneous Catalysis.

– Recent Studies of Laser Science in Paintings.

– Conservation and Research Role of the πσ* State in Molecular Photophysics.

– Ultrafast Interfacial Proton-Coupled Electron Transfer.

– Reviews photoinitiated polymerization: advances, challenges, and opportunities.

2.2 Organic synthesis

A special mention deserves the overview Nick Turro has published of his work in physical organic, organic supramolecular and spin chemistry during his five decades carrier at Columbia. A tutorial review has been published on the utility of photolabile protecting groups in chemical synthesis and in biology. A wide scope crytical review has been devoted to the 2 + 2 cycloaddition reaction involving allenes and includes several photochemical examples.

A preparatively interesting synthesis of some Z-cynnamic acid derivatives has been reported, based on the fact that the salts of these acids with amines crystallize out of an acetonitrile solution (Scheme 1).

The first examples of enyne [4 + 4] adducts have been isolated from the photocycloaddition to a 2-pyridone as a mixture of regio and stereochemical isomers. These are too strained to allow isolation, but the products of further 2 + 2 dimerizations have been identified. Further products formed arise from 2 + 2 cycloaddition (or from Cope rearrangement of the above 4 + 4 adduct, see Scheme 2).

2-Napthoquinone-3-methides are conveniently generated via the efficient photodehydration (Φ = 0.2) of 3-(hydroxymethyl)-2-naphthols. These intermediates undergo facile hetero-Diels-Alder addition (k ≈ 4 × 104 M-1 s-1) to electron-rich olefins in an aqueous solution. In this way, photostable benzo[g]chromans are formed in excellent yield. The fraction of the quinone methide that is not trapped is rapidly hydrated (k ≈ 125 s-1) and regenerates the starting naphthol. The fact that hydration competes with cycloaddition makes the former process selective; actually only vinyl ethers and enamines are sufficiently nucleophilic for adding. The photochemistry of 1,2-bis(butadienyl)benzene is affected by the introduction of methyl groups on the chain. These limit planarity and thus alter absorption and photophysical parameters and affect the competition between di-π-methane and 6 π e cyclization.

Photoremovable protecting groups are becoming important in organic synthesis and this increases the interest in the mechanism of fragmentation. A study of two phenacyl phosphates showed that the reactive triplet has a mixed nπ*/ππ* character and its chemistry depends on the structure (diphenyl or diethyl phosphate) and solvation (in accord with the prediction from DFT calculations). Thus, in MeCN the triplet is long-lived (100 ns) and essentially unreactive, while in more solvating media, such as fluorinated alcohols or mixed aqueous solvents, the triplet lifetime is shortened to ca. 5 ns and rearrangement and cleavage occur (at least with a good nucleofugal group, e.g. diphenyl rather than diethyl phosphate). Hydrogen abstraction competes when the above conditions are not met (see Scheme 3).

A related investigation on 2-methylphenacyl epoxides has been carried out for exploring the viability of preparing pharmaceutically active hydroxyalkylindanones via hydrogen abstraction from the methyl group. Some positive results were obtained, although the situation was complex, in particular because of different cleavages competitively occurring, e.g. that of the epoxide ring (see Scheme 4).

A stereoselective Wol? rearrangement of α-diazo-N-methoxy-N-methyl-β-ketoamides (formed from enantiopure aminoacids) leads to enantiopure β–lactams (see Scheme 5). The reaction is conveniently carried out in a continuous-flow photochemical reactor made from inexpensive laboratory equipment and is amenable to scale-up.

Chiral photochemistry is an intrinsically difficult task, because this implies controlling the shortlived, weakly interacting and highly reactive species such as an electronically excited state. A welcome example, that has been tagged “dual-chiral, dual-supramolecular” photochirogenesis approach, has been applied to the [4 + 4] photocyclodimerization of 2-anthracenecarboxylate tethered to an R-cyclodextrin scaffold. The reaction was accelerated by a γ-cyclodextrin or cucurbit[8]uril host and gave a single enantiomeric cyclodimer (out of four) in up to 98% chemical and 99% optical yield.

Higher members of the acene series have demonstrated to be a very difficult synthetic target. However, photochemical reactions can be carried out at a low temperature and are thus well suited for arriving to products of limited stability. A remarkable success has been the synthesis of octacene and nonacene. Thus, doubly bridged derivatives prepared by Diels Alder and elimination reactions have been oxidized to α-diketo derivatives. These compounds have been irradiated at 30K, where long-wavelength irradiation causes partial decarbonylation, but prolonged irradiation in the UV (several hours) eliminated also the second bridge (see Scheme 6). High acenes have been predicted to have antiferromagnetic properties and thus organic materials of interest as semiconductors may be prepared by this path, provided that one can devise a pattern of substituents that impart a sufficient kinetic stability to these compounds.

(Continues…)Excerpted from Photochemistry Volume 40 by Angelo Albini. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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