Volume 78, Issue 12

The XXIst IUPAC Symposium on Photochemistry was chaired jointly by Profs. Masahiro Irie (Kyushu University, International Committee) and Atsuhiro Osuka (Kyoto University, Local Committee) on 2-7 April 2006 in Kyoto, the beautiful and ancient capital of Japan in the season of cherry blossoms. This series of Symposia was initiated by Prof. George S. Hammond, and the first was held in Strasbourg in 1964. Since then it has been held almost every two years, and this is the first time that it has been held outside Europe. The opening presentation of the Symposium was dedicated to the founding pioneer, Prof. Hammond, who passed away in 2005 at the age of 84. In a eulogy, Prof. David G. Whitten (University of New Mexico) paid a tribute to his memory with an account of his personality and his scientific works in photochemistry and physical organic chemistry. The scientific program of the Symposium featured 25 plenary and invited lectures, and three workshops on the topics "Organic light-emitting diodes: Present and future", "Photocatalysis: Water splitting and environmental purification", and "Photoinduced electron transfer: Present and future". In addition to 462 posters on display, 131 were selected for short oral presentations. The poster sessions were spirited, with the younger participants engaged in stimulating discussions with their more senior colleagues, and the contributions of three young authors were additionally recognized by the award of prizes. The numerous presentations reflected the broad scope and interdisciplinarity of modern photochemistry. The spectrum of topics ranged from the classical fields of inorganic, organic, physical, and theoretical chemistry, to physics, biological chemistry, and materials science. In summary, the exciting scientific program maintained the proud tradition of this series, and demonstrated that photochemistry continues to be a vivid, active, and evolving discipline, some details of which have been introduced by Prof. Jochen Mattay [ Angew. Chem., Int. Ed. 45 , 3570 (2006)]. In addition to the scientific program, the participants and their accompanying guests enjoyed a variety of social and cultural programs provided by the Local Organizing Committee, which took full advantage of the spring season in Kyoto. A highlight of the meeting was the presentation of the Porter Medals, named for 1967 NobelLaureate, George Porter. On this occasion, two medals were awarded, to recognize the outstandingachievements of Prof. Howard E. Zimmerman (University of Wisconsin, Madison, USA), one of thefounders of modern organic photochemistry whose lifetime's work has contributed to our understandingof fundamental processes in excited states, and Prof. Hiroshi Masuhara (Osaka University, Japan)for his pioneering work on the integration of time and spatially resolved measurements on the emergingfields of nanosciences. This issue of Pure and Applied Chemistry offers a representative collection of papers, based upon plenary and invited lectures by the two medallists and a number of internationally recognized scientists, who covered traditional as well as modern photochemistry, and discussed the science as well as related technologies such as materials for photonics, nanotechnology, and photobiology. This demonstrates the broadening scope of photochemistry from an initial central core toward a periphery that is increasingly interactive and interdisciplinary. As editors, we appreciate all the important contributions made by our authors and hope that this collection enables us to convey to readers some of the excitement of photochemistry and the status of recent achievements in the field. Hiroshi Miyasaka Masahiro Irie Conference Editors

This exposition summarizes a personal perspective on past and current developments in organic photochemistry. It comprises three portions. The first section describes the beginnings of our photochemical studies and how it was possible to relate photochemical reactivity to excited-state electronic structures. The second selectively relates some of the reactions and concepts developed in the intermediate years. Finally, the third portion describes our recent research.

Various time- and space-resolved spectroscopies have been developed and applied to thin films and nanoparticles. Dynamic reflection spectroscopies of total internal, diffuse, and regular reflection modes analyze photophysical and photochemical processes at the interface/surface layers with thicknesses of a few tens to a few hundreds of nm and of optically scattering materials. The excited singlet, triplet, and ionic states are identified, and intersystem crossing, isomerization, electron transfer and recombination, and photothermal conversion due to excited-state annihilation are analyzed, just as by transmittance mode spectroscopy. Fluorescence and Rayleigh light-scattering spectroscopies are developed for elucidating excited-state dynamics of single nanoparticles. The optical properties are related to their size, shape, internal structure, and environmental conditions. We prove that organic molecular materials show novel nanometer-size effects due to structural confinement. The high-intensity laser excitation induces ablation whose dynamics and mechanism are considered on the basis of time-resolved spectroscopy and imaging. For nanosecond and femtosecond ablation, we propose cyclic multiphotonic absorption and photomechanical mechanisms, respectively, while purely photochemical ablation was confirmed. Ablation studies have opened a new research approach toward expansion and contraction dynamics of polymer films, nanoparticle preparation, crystal growth control, crystallization in saturated solution, and others.

An efficient photoinduced radical addition of tertiary amine, mainly cyclic derivatives, to electron-deficient alkenes was developed. The reaction was applied to the asymmetric synthesis of the pyrrolizidine alkaloids laburnine and isoretronecanol. The method was then optimized for the addition of a larger variety of tertiary amines, in particular acyclic ones. Radical tandem addition cyclization reactions with unsaturated tertiary amines have also been investigated. A detailed mechanistic study using isotopic labeling enabled the optimization of a corresponding reaction with N , N -dialkylaniline derivatives. The origin of the high reaction stereoselectivity achieved with menthyloxyfuranone was elucidated. The radical addition of tertiary amines was also performed with heterogeneous photocatalysis using inorganic semiconductors as sensitizers.

Two examples of artificial supramolecular host-guest systems derived from resorc[4]arenes (calix[n]arenes based on resorcinol) and ammonium ions as guests have been studied by atomic force microscopy (AFM). For the first time, real single-molecule events have been determined for this type of supramolecular complexes and off-rates as well as molecular parameters of single-molecule aggregates such as the depths of the binding pocket (molecular length parameter) could be measured by applying the methods of dynamic force spectroscopy. In addition, this technique was also applied to differentiate between the two states (open and closed) of a photoswitchable resorc[4]arene-anthracene tweezer. An investigation of the exchange rates of various complexes in the gas phase by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry confirmed the results of the AFM study.

Evidence for intramolecular photoinduced electron transfer (ET) in synthetic systems consisting of a triphenylamine-perylenediimide donor-acceptor dendrimer or a triphenylamine-peryleneimide dendrimer at the ensemble and single-molecule (SM) level is presented. Moreover, for the first time a direct observation of the forward as well as the backward ET step is made in a single emitting entity. Fluctuations in the values of the rate constants for forward and backward ET were observed, induced by the local environment as well as by conformational changes of the dendrimer itself. The results obtained in a weakly coupled system can also be extended to a strongly coupled donor-acceptor system based on peryleneimide and penta-phenylene.

Oxynitride photocatalysts with d 10 electronic configuration are presented as effective non-oxide catalysts for overall water splitting. Germanium nitride (β-Ge 3 N 4 ) having a band gap of 3.8-3.9 eV modified with RuO 2 nanoparticles as a cocatalyst is shown to achieve stoichiometric decomposition of H 2 O into H 2 and O 2 under UV irradiation (λ > 200 nm). A novel solid solution of GaN and ZnO, (Ga 1- x Zn x )(N 1- x O x ), with a band gap of 2.4-2.8 eV (depending on composition) achieves overall water splitting under visible light (λ > 400 nm) when loaded with an appropriate cocatalyst. The narrower band gap of the solid solution is attributed to the bonding between Zn and N atoms at the top of the valence band. The photocatalytic activity of (Ga 1- x Zn x )(N 1- x O x ) for overall water splitting is strongly dependent on both the cocatalyst and the crystallinity and composition of the material. The quantum efficiency of (Ga 1- x Zn x )(N 1- x O x ) with Rh and Cr mixed-oxide nanoparticles is 2-3 % at 420-440 nm, which is the highest reported efficiency for overall water splitting in the visible-light region.

Drug-biomolecule interactions in the excited state are relevant from a photobiological point of view as they can be correlated with a number of photosensitization disorders such as photocarcinogenicity, photoallergy, phototoxicity, etc. Nonsteroidal anti-inflammatory 2-arylpropionic acids and antibacterial fluoroquinolones have been selected as typical examples of photoactive drugs. Protein photosensitization has revealed photoadduct formation; the major amino acids involved are Tyr, Trp, and His. Generation of specific antibodies has allowed us to identify relevant structures of the drug epitopes. Then, drugs have been submitted to systematic steady-state and time-resolved studies on their photophysical properties, alone and in the presence of biomolecules: proteins, DNA, and their simple building blocks. The results are discussed in the framework of the chemical mechanisms underlying photosensitization by drugs and also in connection with the potential of drug excited states as (chiral) reporters for the binding sites of biomolecules.

Short DNA duplexes can be stabilized by the presence of organic chromophores, which serve as hairpin linkers or end-capping groups. Capped hairpins possessing one or more base pairs form stable folded structures in aqueous solution. Increasing the number of base pairs separating the two chromophores increases both the distance between the two chromophores and the dihedral angle between their electronic transition dipoles. Thus, duplex DNA can serve as a helical scaffold for the study of electronic interactions between two chromophores. Three types of electronic interaction have been investigated: (a) exciton coupling (EC) between two identical chromophores, as probed by exciton-coupled circular dichroism (EC-CD); (b) fluorescence resonance energy transfer (FRET) between a fluorescent donor and acceptor; and (c) photoinduced electron transfer (PET) between an electron donor and acceptor. EC and the efficiency of fluorescence energy transfer are dependent upon both the distance and dihedral angle separating the two chromophores. Electron transfer occurs via both single-step superexchange and bridge-mediated hopping mechanisms, neither of which displays angular dependence. The competition between these mechanisms is dependent upon both the energetics of hole injection into the base-pair bridge and the distance between the donor and acceptor chromophores, superexchange dominating at short distance and hole hopping at longer distances.

One-electron oxidation of DNA results in chemical damage to nucleobases, particularly guanine in multiple G sequences. Oxidation may be triggered by numerous events, including photosensitization. We describe studies of photoinduced oxidations of DNA triggered by irradiation of covalently linked anthraquinone derivatives under various conditions that affect the global structure of the DNA. These structural changes have subtle effects on the result of the one-electron oxidation.

There is increasing interest in single nucleotide polymorphism (SNP) typing since they can be used as markers to identify the genes that underlie complex diseases and to realize the full potential of pharmacogenomics in analyzing variable response to drugs. Among the different methodologies for SNP genotyping, the homogenous assay is more amenable than the heterogeneous one. In this article, we will describe some of our most recently developed novel base-discriminating fluorescent (BDF) nucleosides useful for homogenous SNP typing. Our novel concept led to the investigation of a new type of pyrene-labeled BDF nucleosides Py U, Py C, 8py A, and MePy dA, which emitted strong fluorescence only when the bases opposite the BDF bases are A, G, T, and C, respectively. The DNA probes containing four different BDF bases enabled us to distinguish single-base alterations by simply mixing with a sample solution of target DNA. An example of SNP typing of c-Ha-ras SNP sequence has also been demonstrated. Detection of base insertion in insertion/deletion (indel) polymorphisms using pyrene excimer fluorescent probe has also been explored.

We recently found that certain cyanines form tight complexes with carboxymethylamylose (CMA) in aqueous solutions and that in these complexes the cyanine exists as a strongly fluorescent and stable J-aggregate. Cyanine dyes are characterized by their ability to form J-aggregates showing very narrow absorption and fluorescence spectra relative to the monomer. Although they have found uses in sensing applications, the practicability has been limited in many cases due to the low quantum efficiencies for J-aggregate fluorescence. The CMA-cyanine complex is formed by a cooperative self-assembly in which both components undergo conformational changes during the association. The CMA exists as a random coil in solution prior to complex formation; helix formation is prevented due to repulsion of the charges on the carboxymethylated glucose units. The cyanine exists as a nonfluorescent monomer in the same solutions. A helical atomic force microscopy image and large induced circular dichroism (CD) spectra of the cyanine J-aggregate indicate that the self-assembly is a superhelix scaffold of CMA decorated with J-aggregates of the cyanine. Similar behavior was also observed with carboxymethylated cellulose (CMC). Enzymatic disruption of the helical structures (e.g., by the use of amylase to disrupt the structure of CMA helix) leads to the disappearance of the J-aggregate-associated fluorescence. The photophysical behavior and applications of this complex for sensing are discussed.

The emission properties of selected benzo[ b ]quinolizinium (acridizinium) derivatives in the presence of double-stranded DNA and proteins are presented. Spectrophotometric studies and linear dichroism (LD) spectroscopic experiments reveal that benzo[ b ]quinolizinium derivatives bind to DNA, mainly by intercalation. In contrast to the 9-aminobenzo[ b ]quinolizinium, which exhibits a moderate emission quantum yield in water, the 6-aminobenzo[ b ]quinolizinium ion as well as N -phenyl-9-aminobenzo[ b ]quinolizinium derivatives are almost nonfluorescent. The low intrinsic fluorescence quantum yields of the latter compounds are caused by conformational changes in the excited state, as shown by a linear double-logarithmic plot of the emission quantum yield vs. the solvent viscosity. Most notably, the fluorescence intensities of these dyes increase significantly by a factor of 10 to 50 upon addition of double-stranded DNA or proteins such as human serum albumin (HSA) or chicken egg albumin (CEA). Thus, these compounds exhibit ideal properties to be used as DNA- or protein-sensitive light-up probes.

The self-assembling and photophysical properties of soluble fluorescent linear condensed aromatic gelators are reported. The gels formed by 2,3-alkoxy derivatives are constituted of nanofibers weaving a 3-dimensional supramolecular network and imprisoning the solvent. These nanofibers are efficient light-harvesting systems in which complete energy transfer toward a fluorescent acceptor can be achieved at doping levels below 1 mol % of acceptor.

Perylene bisimides bearing four aryloxy substituents in the 1,6,7,12 positions (so-called bay area) and two receptor units at the imide nitrogens are most useful fluorescent building blocks for the realization of a broad variety of supramolecular architectures. As discussed in this review, this versatility is not only due to the outstanding fluorescence properties of these dyes with quantum yields close to unity, but also a consequence of pronounced twisting of the π-system with an inversion barrier of around 60 kJ/mol (leading to atropisomerism) and conformational flexibility of aryloxy units. The latter two properties endow these dyes with high solubility even in least polar aliphatic solvents and enable the dyes to maintain a distance of >4 Å in their supramolecular assemblies. This feature particularly reduces the, otherwise often observed, fluorescence quenching in dye aggregates. Thus, bright luminescent cyclic and polymeric supramolecular architectures have become available from these dyes by hydrogen-bond and metal-ion directed self-assembly. In those cases where additional π-π-stacking interactions come into play, luminescent organogels and liquid crystals are obtained.

The use of photoresponsive hexatriene architectures to regulate chemical reactivity is described. The major focus of this report is how the two isomers of dithienylethene derivatives exhibit different steric and electronic properties. The ring-open form is structurally flexible, and the thiophene rings are electronically insulated from each other, while the ring-closed counterpart has a rigid structure, and there is a linear π-conjugated pathway along the molecular backbone. Representative examples that demonstrate how these photoswitches can be used to influence metal coordination, catalysis, and nucleophilicity are highlighted in this overview.