Volume 77, Issue 6

Photochemistry is a mature science. A characteristic hallmark of a consolidated scientific discipline is that it increasingly broadens its scope of interests from an initial central core toward the periphery where it interacts with other areas. Most of the current scientific research is characterized by an enriching multidisciplinarity, focusing on topics that combine backgrounds from different fields. In this way, the largest advances are taking place at the interphase between areas where different fields meet. This multidisciplinarity is, I believe, also a characteristic feature of the current situation for photochemistry. Thus, photochemistry was initially focused on the understanding and rationalization at a molecular level of the events occurring after light absorption by simple organic compounds. Molecular organic photochemistry constituted the core of this discipline, and it largely benefited from advances in the understanding of the electronic states provided by quantum mechanics. Later, photochemistry started to grow toward areas such as photobiology, photoinduced electron transfer, supramolecular photochemistry, and photochemistry in heterogeneous media, always expanding its sphere of interest. This context of increasing diversity in topics and specialization is reflected in this issue of Pure and Applied Chemistry . The contributors correspond to some of the plenary plus two invited lectures of the XXth IUPAC Symposium that was held 17ñ22 July in Granada, Spain. The program included plenary and invited lectures and oral contributions grouped in 13 sections covering femtochemistry, photochemistry of biomacromolecules, single-molecule photochemistry, and computational methods in photochemistry to nanotechnology, among others. These workshop titles give an idea of the breadth of themes that were included in this symposium. While it is obvious that the list of contributions correspond to different subdisciplines in photochemistry, all of them have a common scientific framework to rationalize the facts. The purpose of the symposium was to present an overview of the current status of some research fronts in photochemistry. This issue begins with the 2004 Porter Medal Lecture awarded jointly by the Asian, European, and Interamerican Photochemical Societies that was given to Prof. Graham Fleming (University of California, Berkeley) for his continued advances in photosynthesis. Prof. Flemingís studies have constituted a significant contribution to the understanding of the interplay between the structure of photosynthetic centers of green plants and the mechanism of energy migration toward the photosynthetic centers. These events take place in a very short time scale and are governed by the spatial arrangement of the constituents. Continuing with photobiology, the second article by Prof. Jean Cadet (Grenoble University) describes the type of photochemical damage and photoproducts arising from DNA UV irradiation. Knowledge of these processes is important for a better understanding of skin cancer and the possibilities for DNA repair. Closely related with DNA damage occurring upon irradiation, the article by Prof. Tetsuro Majima (Osaka University) provides an account of his excellent work on photosensitized oneelectron oxidation of DNA. The concept of "conical intersection", developed initially by Robb and Bernardi to rationalize the relaxation of excited states, led to the foundation of computational photochemistry, which has proved to be of general application to photochemical reactions. In this issue, Prof. Massimo Olivucci (University of Siena) shows that quantum chemical calculations can also be applied to photochemical reactions occurring in photobiology and, in particular, to the problem of vision. These calculations are characterized by the large number of atoms that are included and the fact that they have to estimate at a high calculation level and with high accuracy the energy of states differring in a few kcal mol -1 . The next article corresponds to one of the two invited lectures included in this issue. The one given by Dr. Virginie Lhiaubet-Vallet (Technical University of Valencia) in the workshop Photophysical and Photochemical Approaches in the Control of Toxic and Therapeutic Activity of Drugs describes the enantioselective quenching of chiral drug excited states by biomolecules. Moving from photobiology to free radical polymerization with application in microlithography, the article by Prof. Tito Scaiano (University of Ottawa) reports among other probes an extremely elegant approach to detect the intermediacy of radicals in photochemical reactions based on a silent fluorescent molecular probe containing a free nitroxyl radical. Solar energy storage is a recurrent topic and a long-desired application of photochemistry. In her comprehensive contribution, Prof. Ana Moore (Arizona State University) summarizes the continued seminal contribution of her group to the achievement of an efficient solar energy storage system based on the photochemical generation of long-lived charge-separated states. Another possibility of solar energy storage consists of water splitting. In his article, Prof. Haruo Inoue (Tokyo Metropolitan University) deals with artificial photosynthetic methods based on the use of ruthenium porphyrins as photosensitizers for the two-electron oxidation of water with formation of dioxygen. Also in applied photochemistry, Prof. Luisa De Cola (University of Amsterdam) reports on intramolecular energy transfer in dinuclear metal complexes having a meta-phenylene linker. The systems described by Prof. De Cola have potential application in the field of light-emitting diodes, since most of the complexes described exhibit electroluminescence. The second invited lecture is by Dr. Alberto Credi (University of Bologna), one of Europeís most promising young photochemists. In his interesting article, the operation upon light excitation of a rotaxane molecular machine is described. A macro-ring acting as electron donor moiety in a charge-transfer complex is threaded in a dumbbell-shaped component having two viologen units with different redox potential. Light absorption produces the cyclic movement of the macro-ring from one viologen station to the other. The last two contributions fall within the more classic organic photochemistry realm. Prof. Axel Griesbeck (University of Cologne) describes the multigram synthesis of antimalarial peroxides using singlet-oxygen photosensitizers adsorbed or bonded to polymer matrices. The last contribution comes from Prof. Heinz Roth (University of Rutgers), who has worked during his entire career in the fields of organic photochemistry and radical ion chemistry. Prof. Roth has summarized his vast knowledge in radical ion chemistry, reviewing the mechanism of triplet formation arising from radical ion pair recombination. This mechanism for triplet formation is currently gaining a renewed interest owing to the potential applicability to the development of phosphors. I hope that the present selection will be appealing and attractive for a broad audience of readers interested in photochemistry and will give readers an idea of the state of the art of some current topics in this area. Hermenegildo García Conference Editor

The fundamental interactions between naturally occurring pigments in light-harvesting systems are responsible for the high efficiency of the photosynthetic apparatus. We describe the role of carotenoids (Cars) in light-harvesting systems, including our work elucidating the mechanism of energy transfer from the optically dark Car singlet excited state (S 1 ) to chlorophyll (Chl) and calculations on the electronic structure of Cars by means of time-dependent density functional theory (TDDFT). We highlight new studies on the charge-transfer state of the Car, peridinin (Per), which enhances the light-harvesting efficiency of the Car by increasing the electronic coupling to Chl. The role of another Car, zeaxanthin (Zea), is discussed with respect to its role in the mechanism of the feedback deexcitation quenching in green plants, a vital regulation process under light conditions which exceed photosynthetic capacity. Lastly, we provide insight on how the 96 Chls in Photosystem I are optimized to generate a pigment-protein complex which utilizes solar energy with near unit efficiency.

The effects of solar light on cellular DNA are mostly explained by both direct excitation of nucleobases and photosensitized reactions that are mediated by UVB and UVA radiation, respectively. A large body of information is now available on the main photodynamic reactions to DNA, which involve guanine as the preferential target of both one-electron oxidation and singlet oxygen oxidation, as the result of type I and type II mechanisms. Most of the final products of the photosensitized reactions of guanine base in isolated DNA have been characterized, and comprehensive mechanisms for their formation have been proposed. Further insights into the mechanisms of solar radiation-induced modifications within cellular DNA have been gained from accurate measurement of the main photoproducts using recently designed sensitive chromatographic and biochemical methods. Thus, the distribution pattern of the 12 possible bipyrimidine photoproducts has been shown to be similar in the DNA of UVB-irradiated rodent and human cells. Cyclobutyl pyrimidine dimers are also generated at di-thymine and thymine-cytosine sites within nuclear DNA upon exposure to UVA radiation as the likely result of triplet energy transfer. In addition, oxidative reactions that involved mostly singlet oxygen, and to a lesser extent, . OH radicals are also implicated, although less efficiently.

Photosensitized one-electron oxidation of DNA has attracted much interest because it causes oxidative damage which leads to mutation, and because it is involved in the basic mechanism of photodynamic therapy. In the present article, we describe the mechanistic study of photosensitized DNA damage, especially addressing the kinetics of hole transfer by adenine(A)-hopping and its effect on the DNA damage. The combination of the transient absorption measurement and DNA damage quantification by high-performance liquid chromatography clearly demonstrate that the yield of the DNA damage correlates well with the lifetime of the charge-separated state caused by A-hopping, showing that hole transfer helps DNA damage. These findings led us to propose a new method to accomplish the efficient DNA damage using a combination of two-color, two-laser irradiation.

In this paper, we discuss the results of our recent studies on the molecular mechanism, which stand at the basis of the photochemical processes occurring in photobiological systems. These results are obtained using modern, robust, and fairly accurate high-level quantum chemical methods.

Photoreactivity of chiral carprofen (CP) and ofloxacin in the presence of two biomolecules, namely, DNA and human serum albumin (HSA), has been reported. Analysis of the photosensitization of 2'-deoxyguanosine and thymidine (Thd) by high-performance liquid chromatograpy has shown that racemic ofloxacin and levofloxacin [its ( S )-stereoisomer] acts by a mixed type I/type II mechanism, while CP does not lead to significant degradation of the nucleosides. Studies on DNA relaxation have revealed formation of single-strand breaks and specific alterations of DNA bases. Ofloxacin and levofloxacin photoinduce direct single-strand breaks and formation of purine and pyrimidine oxidative photoproducts; no Thd dimer has been detected. CP produces only photosensitized single-strand breaks. Moreover, DNA photosensitization has shown a weak enantiodifferentiation in favor of levofloxacin and ( S )-CP. In the case of HSA, a remarkable stereodifferentiation has been found in the interaction between the excited triplet state of CP and protein. Time-resolved laser flash photolysis measurements revealed the presence of two components with different lifetimes that have been assigned to complexation of CP to the two binding sites of albumin. Moreover, photobinding of the drug to protein and formation of the dehalogenated photoproduct of CP proceed via stereodifferentiating photoprocesses.

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S 2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (<100 fs) between the chromophores. In this assembly, the energy transduction process can be initiated by light absorbed by the polyene.

Fluorescence sensors have been developed to monitor radical reactions, DNA damage, acid-catalyzed processes, detection of electron-deficient molecules, and zeolite applications. The examples presented emphasize applications in polymer and supramolecular systems.

Recent progress in the field of artificial photosynthesis is reviewed. Among various approaches to oxidizing the water molecule, attention has been focused on the two-electron chemical conversion processes mediated by metallo-porphyrins upon visible light irradiation. Photochemical oxygenation reactions such as the epoxidation of alkenes sensitized by ruthenium(II) porphyrins with water as both electron and oxygen atom donor have been found. Ru porphyrin induces highly efficient two-electron oxidation of water with a quantum yield of 60 % to form an epoxide from the alkene with high selectivity. The water molecule serves as both an efficient electron and oxygen atom donor in the reaction, in which the oxygen atom of water is incorporated in a useful product, i.e., epoxide.

The synthesis and photophysical properties of heterometallic dinuclear complexes based on ruthenium and osmium trisbipyridine units, Ru- m Ph 3 -Os and Ru- m Ph 5 -Os, in which the metal complexes are linked via an oligophenylene bridge centrally connected in the meta position, are described. Electronic energy transfer from the excited ruthenium-based component (donor) to the osmium moiety (acceptor) has been investigated using steady-state and time-resolved spectroscopy. The results obtained for the meta -substituted compounds are compared with the analogous systems in which the phenylene spacers are substituted in the para position. The mechanism of energy transfer is discussed.

A molecular machine is an assembly of a definite number of molecular components designed to perform mechanical motions as a result of an appropriate external stimulation. Like their macroscopic counterparts, nanoscale machines need energy to operate. Energy can be supplied through (i) chemical reactions, (ii) electrochemical processes, and (iii) photoinduced processes. Although most molecular motors of the biological world are fueled by chemical reactions, for several reasons light is a very good choice to operate artificial molecular machines. Rotaxanes, owing to their peculiar architecture, are attractive candidates for the construction of artificial nanoscale machines. By adopting an incrementally staged design strategy, photoinduced electron-transfer processes have been engineered within rotaxane-type structures with the purpose of obtaining light-powered molecular machines. Such an approach is illustrated by describing the behavior of prototypes investigated in our laboratories.

Photooxygenation involving the first excited singlet state of molecular oxygen is a versatile method for the generation of a multitude of oxy-functionalized target molecules often with high regio- and stereoselectivities. The efficiency of singlet-oxygen reactions is largely dependent on the nonradiative deactivation paths, mainly induced by the solvent and the substrate intrinsically. The intrinsic (physical) quenching properties as well as the selectivity-determining factors of the (chemical) quenching can be modified by adjusting the microenvironment of the reactive substrate. Tetraarylporphyrins or protoporphyrin IX were embedded in polystyrene (PS) beads and in polymer films or covalently linked into PS during emulsion polymerization. These polymer matrices are suitable for a broad variety of (solvent-free) photooxygenation reactions. One specific example discussed in detail is the ene reaction of singlet oxygen with chiral allylic alcohols yielding unsaturated β-hydroperoxy alcohols in ( threo ) diastereoselectivities, which depended on the polarity and hydrogen-bonding capacity of the polymer matrix. These products were applied for the synthesis of mono- and spirobicyclic 1,2,4-trioxanes, molecules that showed moderate to high antimalarial properties. Subsequent structure optimization resulted in in vitro activities that surpassed that of the naturally occurring sesquiterpene-peroxide artemisinin .

Return electron transfer (RET) in radical ion pairs may populate the reagent ground states or, in the case of triplet pairs, one reagent triplet state. The efficiency of triplet RET is governed by the free energies of singlet and triplet RET and by the topologies of the potential surfaces of parent molecules, radical ions, and accessible triplet states or biradicals. RET in triplet radical ion pairs is exemplified for two distinct relationships between the three potential surfaces.