Photochemistry Volume 38

A Review of the Literature Published Between July 2007 and December 2009

By Angelo Albini

The Royal Society of Chemistry

Copyright © 2011 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-054-5

Contents

Preface Angelo Albini, v,
Reports,
Review of the period July 2007–December 2009 Angelo Albini, 1,
Physical and theoretical aspects,
Recent trends in computational photochemistry Luis Serrano-Andrés, Daniel Roca-Sanjuán and Gloria Olaso-González, 10,
Light induced reactions in cryogenic matrices Rui Fausto and Andrea Gómez-Zavaglia, 37,
Dynamics and photophysics of oligomers and polymers João Pina, Telma Costa and J. Sérgio Seixas de Melo, 67,
Organic aspects,
Alkenes, alkynes, dienes, polyenes Takashi Tsuno, 110,
Oxygen-containing functions M. Consuelo Jiménez and Miguel A. Miranda, 143,
Photochemistry of aromatic compounds Kazuhiko Mizuno, 168,
Functions containing a heteroatom different from oxygen Angelo Albini and Elisa Fasani, 210,
Inorganic aspects and solar energy conversion,
Photophysics of transition metal complexes Francesco Barigelletti, 234,
Photochemical and photocatalytic properties of transition-metal compounds Andrea Maldotti, 275,
Highlights,
New materials for sensitized photo-oxygenation Sylvie Lacombe and Thierry Pigot, 307,
Prebiotic photochemistry Daniele Dondi, Daniele Merli and Luca Pretali, 330,
Industrial applications of photochemistry: automotive coatings and beyond Kurt Dietliker, Adalbert Braig and Andrea Ricci, 344,
Trends in Photolithography Materials Will Conley and Cesar Garza, 369,

CHAPTER 1

Review of the period July 2007–December 2009

Angelo Albini

DOI: 10.1039/9781849730860-00001

1 A bit of history

Anniversaries have come up in these years. The present reporter has remarked that a century has elapsed since photochemistry came of age. The chemical effects that light produced had of course been known since the beginning of chemistry itself and the interest had much grown in the 19th Century due to the development of photography. However, photochemical experiments had remained sparse and conclusive evidence about the exact nature of that effect had been very limited until the beginning of the following century, when things changed mainly thanks to the contribution by Ciamician and Silber and by Paterno` in Italy and by Stobbe in Germany.

All of the three groups published their view of the state of the art in 1909 and recognized the great advancement that had taken place. It is indeed remarkable that most of the key reactions of (unsaturated) carbonyl derivatives, nitro compounds and alkenes, oxygenations reactions and photochromism were then discovered and rationalized in a way that has resisted time.

In the hundred years since, photochemistry has been first neglected, then has taken a considerable time in rediscovering what had been in meantime forgotten. When this happened, however, (in the 1950s) the understanding of molecular structure and bonding had much grown and the new ‘molecular’ photochemistry, as indicated in the title of Turro’s book became an essential part of ‘mechanistic’ chemistry in research and in university courses. The first volume of the present Royal Chemical Society series edited by D. Bryce-Smith was printed 40 years ago at the high mark of this process and represented photochemistry as a consistent and articulated theory, growing at a lively pace in different (applicative) directions.

After four decades, what most impresses an observer is how far the applications of photochemistry has become detached from the core of the discipline. Indeed, photochemistry has pervaded fields so far one from another that they are not only independent one from another, but are even forgetful that there is a single core discipline.

2 Photochemical literature: the present state

Examining the photochemical literature in the 2 1/2 years period considered, one first of all notices that this discipline has an important role and certainly advances at no slackened pace, with regard to both research papers and patents. The yearly number of photochemical papers is since some time essentially unchanged. A more detailed consideration evidences some characteristics of the photochemical literature that had been highlighted in the Introduction to the previous volume.

Thus, if one takes into consideration the journals that have most often hosted photochemical papers, as an example referring to year 2009, and lists the journals according to the number of papers on this subject published that year, one find that 32 journals contained about 35% of the total number of the papers of that year. The type of journal is an indication of the audience that photochemistry practitioners think to address. What comes out is that the percentage of papers is distributed according to the key topic of the journal as follows.

– General chemistry, 6.6% of the total (JACS, the single journal most often chosen makes 3.2 %, the others are Chem. Commun., Angew. Chem., Chem. Eur. J., Proc. Natl. Acad. Sci.)

– Physical chemistry, 7% (J. Phis. Chem. A, B, C, Phys. Chem. Chem. Phys., Chem. Phys. Lett.)

– Organic chemistry 2.6% (J. Org. Chem, Org. Lett., Org. Biom. Chem.)

– Inorganic chemistry 2.1% (Inorg. Chem., Dalton Trans.)

– Materials and surfaces 5.1% (J. Hazard. Mat., J. Mater. Chem., Langmuir, J. Coll. Inter. Sci.)

– Environment 3.7% (Env. Sci. Technol., Atm. Chem., Atm. Environ., Chemosph.)

Further topics among the most used specialist journals containing photochemical papers are catalysis (Appl. Cat. B), applied physics (Proc. SPIEE, Opt. Express), polymer science (J. App. Pol. Sci.), biochemistry (Biochem.).

As remarked in the introduction to Vol. 37 in this series, a noticeable fact is the relatively small amount of papers published in journals specifically devoted to photochemistry. The three journals in the field (J. Photochem. Photobiol., Photochem. Photobiol.,Photochem. Photobiol Sci.) make together 3.5% of the total (see Fig. 1), a half of the papers in the general chemistry category and a much smaller number than in other fields.

In the opinion of the present reporter, this fact does not necessarily imply a negative connotation. It simply indicates that photochemistry is important in many fields and plays a role in each of them that is felt more important than that in photochemistry itself. In particular, remarkable is the high fraction of papers in general chemistry journals, the largest part of them appearing as fast communication in prestigious journals, an indication of the recognized position that this discipline has maintained. The use of devoted journals is much more extensive in other chemical disciplines, e.g. in electrochemistry, but this has little to do with the importance and the role that each discipline has.

The determining fact is that dedicated journals are available so that any scientist can refer to them for good science, if a further portion of good science is found elsewhere, no problem. In this sense, if a concern must be expressed, this is rather that photochemistry, while remaining in the first line, has lost some position with respect to other advancing fields. As an example, if one considers JACS, inevitably the reference journal, the papers in photochemistry certainly remain a high fractions of the articles published, but clearly the highest point has been reached two-three decades ago and such levels are no more to be reached (see Fig. 2). This corresponds to the feeling one has when browsing other chemistry journals or attending meetings.

As to where photochemistry is done, there are a considerable number of laboratories where photochemistry is the main businness. In 2009 the most prolific author has been Prof. Shunichi Fukuzumi from the University of Osake, but there are many other scientists following with a slightly lower production, almost equally distributed between Japan, USA, Europe and China. Fortunately, there is also an important production from laboratories where photochemistry is only one of the research theme and, importantly, patents maintain a large share in the photochemical literature.

3 Review

Some years have elapsed from the last publication of a textbook in photochemistry and in 2009 we had the much wellcome opportunity of having two in a few months. One of these is the new edition of what indoubtely has been the reference text for over 40 years, Turro’s book now titled ‘Modern Molecular Photochemistry of Organic Molecules’, with V. Ramamurthy and J. C. Scaiano as co-Authors, grown to over 1000 pages, but maintaining the same, quite captivating approach due to the origin from courses and lectures (the first part of the text, exluding the chapters on the chromophore photochemistry, is separately available).

True to its title, the second one, Photochemistry of Organic Compounds. From Principles to Practice by Kla` n and Wirz5 presents a substantial course of photochemistry (5 chapters) followed by a long and very informative chapter (250 pages) on the chemistry of excited states, presented by chromophore. The discussion is enlivened by the frequent introduction of ‘Case Studies’ and ‘Special Topics’ that greatly help both in understanding the mechanism involved and in appreciating the application in diverse fields.

Another important event is the publication of the two volumes Photochemistry and Photophysics of Coordination Compounds, edited by V. Balzani and S. Campagna (part of the Top. Curr. Chem. Series, two volumes, 273 + 627 pages). This gives a complete account of the really varied photochemistry of the complexes of block d and block f ions.

As mentioned, research reports in the field continue to appear at a steady pace. Here, the reporter avows that he is unable to distinguish the main lines of the development among many thousand papers. Browsing through the literature causes panic, first of all because of the rapid advancement of experimental techniques and computational methods that allow to arrive at an in-depth understanding of the mechanism in cases that were not even taken into consideration only a few years ago.

The advancement of computational chemistry is particularly apparent in photochemistry. ‘Old’ problems have been confronted in a new way. Thus, a multiconfiguration complete active space self-consistent field (CASSCF) method has been applied to the determination of intermediates involved in radiationless processes for acetophenone and derivatives.

An excellent agreement has been obtained between experimental and computed coulombic coupling matrix elements for donor-spacer-acceptor systems, which consist of a boron dipyrromethane donor and acceptor in various stages of protonation. Noteworthy, this correlation holds, despite the fact that the validity of Förster theory applied to intramolecular electronic energy transfer (ET) over short (e.g. 20 A? ) distances is disputed.

New approaches are used for fundamental processes such as proton transfer (e.g. from the dimethylaniline radical cation to benzophenone radical anion), where a new theory suggest that the transition state occurs within the solvent coordinate, not the proton transfer coordinate, and proton transfer may occur either adiabatically or nonadiabatically. Quite interesting a computational study rationalized the mechanism of intramolecular oxo-hydroxy phototautomerism in pyridones and analogues that has been obtained by IR irradiation in matrix. The tautomerism involves πσ* states that are repulsive toward the stretching of N-H or O-H bond. How one of the key photoreactions, C = C isomerization, is confronted computationally and experimentally can be appreciated e.g. in a study on fumaric amide.

On the other hand, it has been shown that orbital-energy correlation diagrams (by using an artificially high-spin ROHF method) and state-energy correlation diagrams (by using a state-averaged CASSCF method) can be computed ab initio, as shown for the electrocyclic ring opening of cyclobutene and the addition between photoexcited oxygen and nitrogen.

The computational approach extends to materials, as shown e.g. for the application of a hybrid molecular dynamics–Monte Carlo technique to simulate laser ablation in poly(methyl methacrylate).

Photochemistry remains one of the best techniques for the generation of intermediates under controlled conditions. A typical example is that of benzylic carbanions that are smoothly generated by photolysis of the corresponding phenylpropionates. Under these conditions the lifetime of the carbanion is a remarkable 200 ns in water and up to several minutes in a rigorously anhydrous solvent (see Scheme 1).

New intermediates are often attainable in matrix. Among the many noticeable advancements is the synthesis of the first molecule containing two noble gas atoms, HXeOXeH, by UV photolysis of water in solid xenon and subsequent annealing of the matrix at 40-45 K. This may be considered the first step towards the preparation of linear (Xe-O)n chains (and thus contribute to the debate on the ‘missing xenon’ question). Apropos matrices, it has been noticed that irradiation may give different results in different matrices. This depends not on the chemical composition but on the different rigidity of the organic glass at the temperature of the experiment.

Inorganic photochemistry is enjoying a period of hefty development. Metal carbonyls, as an example, are a favourite topic due both to the efficient and varied photochemistry these undergo and to the complex vibrational spectroscopy that allows a prompt identification. Two-dimensional infrared (2DIR) spectroscopy is an excellent tool for testing the accuracy of ab initio quantum chemical calculations.17 Another interesting topics is the chirality conservation, as observed e.g. in the photochemical mer -> fac geometrical isomerization of Tris(1phenylpyrazolato,N,C2′)iridium(III), a complex pertaining to a class of highly fluorescent complexes used in organic LEDs (OLEDs).

The photochemistry of metal complexes and that of materials finds ample application in solar energy conversion. A noticeable progress is taking place with regard to water oxidation to dioxygen. This is a key feature in fundamental processes, such as both water splitting into hydrogen and oxygen

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and reduction of CO2 to methanol or hydrocarbons

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This function is carried out by a cubic manganese oxide moiety in Photo-System II (see below) which continues to be studied and modeled.

Organic molecules are no less valuable substrates for photochemical reaction and have been exploited as key steps in complex synthetic sequences (e.g. 2 + 2 cycloaddition in the total synthesis of tetracyclic sesquiterpene (±)-Punctaporonin C (see Scheme 2) as well as in the enantioselective total synthesis of the Melodinus alkaloid (+)-Meloscine (see part of the synthesis in Scheme 3).

Further examples of solid-state asymmetric photochemical studies using the ionic chiral auxiliary approach have been reported. Contrary to previous conclusions, a recent study on the dimerization of 2-anthracenecarboxylic acid derivatives demonstrated that remarkable regio-, diastereo-, and even enantioselectivities can be induced by liquid crystals in a photochemical reaction. Indeed, the selectivity is conserved also upon changing the shape of the molecule, much more than in the solid state.

Although external factors such as crystal packaging may be determining in many cases, intramolecular interactions may dominate in other ones. Thus, although dibenzylketones usually fragment, the bis-(thienylmethyl) ketone S,S-dioxide in Scheme 4 undergoes cycloaddition both in solution and in the solid state (see Scheme 4).

An interesting case of different products depending on the impinging ?ux has been observed in the photochemistry of 2-ethylindandione.

In this case the use of a 312 nm lamp leads to Norrish Type II fragmentation or Yang cyclization via the triplet, whereas irradiation by a 355 nm laser causes Norrish Type I fragmentation via the singlet. Apparently, in the latter case T1 is re-excited and reforms S1 before it reacts (see Scheme 5).

The peculiar mildness of photochemical reactions is advantageous in many cases, as exemplified for the generation of carbenes from diazo compounds or diazirines. An example is the addition of a carbene unto a fullerene. Encapsulation of a metal ions in the cavity of the molecule imparts a reactivity not present in the metal free fullerene, e.g. by favoring addition onto the positions around fused pentagons (see Scheme 6).

Likewise, the photolabilization of a ligand is an excellent method for the synthesis of 2-azetidinone incorporating carbene chromium units that come useful for the synthesis of peptides containing penicillin or cephalosporin moieties.

(Continues…)Excerpted from Photochemistry Volume 38 by Angelo Albini. Copyright © 2011 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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