Many efforts have been devoted to synthesis of photocatalysts that respond to visible light. This article reviews recent studies on new application of surface plasmon resonance (SPR) of gold (Au) nanoparticles as plasmonic photocatalysts working under irradiation of visible light. The plasmonic photocatalysts exhibited unique photoabsorption properties and photocatalytic performances in the minerarization of organic acid, selective oxidation, H₂ formation, and reduction of organic compounds under irradiation of visible light. The functionalized SPR photocatalysts with platinum, silver and palladium co-catalysts worked effectively in H₂ formation, reduction of nitrobenzene and dechlorination of chlorobenzene, respectively. The results introduced in this review can be widely applied for design and development of new plasmonic photocatalysts exhibiting further excellent activities and selectivities.

Design principle to enhance two-photon absorption (TPA) of organic molecules and some leading strategies are reviewed. Relation between one- and two-photon absorption spectra is discussed in view of molecular symmetry with examples of centro- and noncentrosymmetric molecules. The lowest energy TPA band of noncentrosymmetric molecules can be described by the two-state model, resulting in the correspondence to the one-photon absorption peak, while the higher TPA bands and the TPA bands of centrosymmetric molecules originate from more complicated processes. Transition dipole moment is focused as a controlling factor to enhance TPA cross section by electron donor/acceptor substitution. Extension of π-conjugation system across the core of even-numbered branched systems and molecules having the partial open-shell electronic structures are also discussed as strategies to enhance TPA cross section. Moreover, measurement techniques and quantum chemical calculation are mentioned briefly.

In natural photosynthetic systems such as in purple bacteria, a light-harvesting system (LHS), which is composed of regularly arranged molecules collects the sunlight efficiently and carries the excitation energy smoothly to the reaction center. One of the strategies to realize an efficient artificial LHS is a construction of such regularly arranged structure of functional dyes. A fair amount of research has been carried out to realize an artificial LHS. Supramolecular assemblies, covalently linked systems and dendrimer systems have been examined for the same purpose. Recently, we have achieved the quantitative excited energy transfer reaction between cationic porphyrins on an anionic clay surface. The efficiency reached up to ca. 100% owing to the “Size-Matching Effect” that is a novel methodology to arrange molecules on the inorganic surfaces using electrostatic interactions. It was revealed that the important factors for the efficient energy transfer reaction are (i) suppression of aggregation that causes the decrease of excited lifetime, (ii) suppression of the self-quenching between adjacent dyes, and (iii) suppression of the segregated adsorption structure of two kinds of dyes on the clay surface.

Laser irradiation for colloidal nanoparticles has attracted much attention as a conventional size control method of nanoparticles. In general, focused laser beam at high intensity has been used in this method to induce the fragmentation of nanoparticles. Recently, it was found that laser irradiation using non-focused laser beam at moderate intensity induces the fusion of nanoparticles, resulting in the formation of spherical submicron-sized particles (SSMPs). In this study, SSMPs of gold were prepared by using this method. It was found that control of the agglomeration of the source nanoparticles is necessary to generate SSMPs. In addition, when the source gold nanoparticles stabilized by ligands are used, the agglomeration of the source nanoparticles is induced by laser irradiation.

Upon laser flash photolysis of 2-(4-benzoylphenyl)-2-phenyl-1-methylenecyclopropane (3), fluorescence from the corresponding excited biradical ³4^••* is observed. Mechanistic studies by using laser flash photolysis (LFP) reveal that ³4^••* is directly generated from ³ 3* via an “excited state C-C bond cleavage”. In this topic, we describe intriguing photochemical and photophysical properties of 3 obtained by employing a two-color two-LFP technique.