Oxetanes, four-membered cyclic ethers, possess a high ring-strain and polar CO bonds. Thus, the ring-opening reaction of oxetanes, accompanying bond-formation reactions, is very useful for synthetic purposes. Since the oxetane ring is an important structural component of biologically active compounds, efficient and selective methods to synthesize the strained structure are currently active areas of research. The photochemical [2+2] cycloaddition reaction of carbonyl compounds with alkenes, so-called Paternò-Büchi (PB) reaction, was developed in 1909, and is currently one of the most widely used methods for oxetane synthesis. In this review article, recent developments of the selective formation of oxetane are summarized. Methods that allow us to control the regioselectivity and stereoselectivity in the formation of synthetically important oxetanes will be described.

Two types of electron transfer reactions (exciplex formation and Marcus electron transfer) that occur in the excited state have been discussed in detail. Femtosecond fluorescence upconversion experiment suggests that exciplex is formed by ultrafast short-distance electron transfer (ET). A mechanism was proposed to account for this type of ET. Calculation of the free energies G of donor (D) and acceptor (A) systems shows that the change in D-A distance r can induce ET reactions even in nonpolar solvents where solvent polarization x is absent. The intermolecular distance can be regarded as another important reaction coordinate of ET reactions. The calculation was extended to polar solvents where G is a function of both x and r . This enabled us to discuss how exciplex formation and Marcus ET are related to each other. The two-dimensional free energy surfaces (2D-FES) with appropriate parameter values were used to explain several experimental results already reported.

The target size of laser trapping have recently decreased to nanometer scale and, molecular systems have arisen as a promising application target. The review article overviews the recent applications of radiation pressure in particular to molecular systems, which include i) optical trapping of polymer chains, molecular clusters, and nanometer-sized micelles in solutions, ii) inducement of phase transitions of molecular systems, such as the crystallization of amino acids and the volume-phase transition of a polymer gel, and iii) confinement of the photopolymerization and solidification of a liquid resin to nanometer-sized spaces. For rational understanding of the trapping mechanism, the effect of radiation pressure on the lateral diffusion and concentration of nanoparticles is quantitatively evaluated by combining a computational simulation method and fluorescence correlation spectroscopy (FCS). Local temperature rise induced via the absorption of trapping laser light by solvents is also quantitatively evaluated by using FCS.

The unique properties of polymer materials result from the large degree of freedom for the conformation and dynamics of the polymer chains. In order to understand the fundamental properties of the polymer materials, the direct observation of individual molecules would provide valuable information. Fluorescence imaging is the most effective method to detect a single molecule embedded in a bulk medium. The recent progress in the near-field optics and super-resolution techniques enable us to obtain an optical image with spatial resolution on the order of 10 nm. This article deals with the principle of the optical imaging methods with the resolution beyond the diffraction barrier and its application to study the structure of polymer materials at the single molecule level.

This short review gives an overview of the emissive metal complexes bearing H-heterocyclic carbene ligands. At first, dinuclear copper(I) complexes having bridging N-Heterocyclic carbene ligands are described. The complexes emit strong light in the solid state. The second part shows heteroleptic copper(I) complexes bearing a monodentate N-heterocyclic carbene ligand. The complexes have bipyridine or diphosphine ligand as a second ligand. The luminescence data of the complexes are shown briefly. The structures and photophysics of the platinum(II) complexes are described in the third part. The complexes have an N-heterocyclic carbene chelate and a cyclometallated ligand. Some of these complexes show strong emission in solution at room temperature. The intensity of the emission strongly depends on the length of the methylene chain which connects two carbene parts in the N-heterocyclic ligand. The photophysical properties of the platinum(II) complexes are discussed with the results of the TDDFT calculation.

Human serum albumin (HSA) has been exploited as a biomolecular host for mediating chiral photoreactions. Thus, the enantiodifferentiating [4+4] photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by HSA affords chiral AC-dimers in high enantiomeric excesses of up to 90%. Nevertheless, this bio-supramolecular photochirogenesis has not been expanded to a catalytic system or mechanistically elucidated in detail due to the complexity of the complexation and photochemical behaviors. In this account, we briefly describe our latest studies on (i) the catalytic use of HSA in the enantiodifferentiating photocyclodimerization of AC and (ii) the mechanistic elucidation of the origin of the high enantioselectivity by measuring the time-resolved fluorescence anisotropy decay.