Degradation of various pollutants by photocatalysis has been widely studied over the last 30 years. Titanium dioxide is still the widely used photocatalyst for environmental remediation because there is no satisfactory alternative. Sol-gel method is efficient for tailoring nanostructure materials with high specific surface area without the need for using expensive equipment. We have developed a synthetic method to prepare porous TiO₂ with BET surface area higher than 200 m² g^-1 by using water as solvent and conducting dialysis in the sol-gel method. Recently, many attempts have been devoted to enhance the photocatalytic activity of TiO₂ by extending its spectral response to visible light and/or suppressing the recombination of the photogenerated hole and electron. In this review, the preparation of metal-ion-doped TiO₂ working under visible light by conducting dialysis in the sol-gel method and enhancement of the photocatalytic activity obtained by mixing TiO₂ and WO₃ sol are presented.

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A review of the magnetic field effects (MFEs) in the photochemical reactions of Flavin and its derivatives is presented. Starting from the MFE observation of intramolecular electron transfer reaction of Flavin adenine dinucleotide (FAD) in low pH condition, the various radical pairs such as in a micelle, protein was investigated. The experience of the study on these systems helps the discovery of MFE in cryptochromes and photolyase, which is the most feasible candidate of so called “chemical compass” mechanism for the navigation of the animals. In parallel to the investigation of the blue light receptor flavoproteins, the novel techniques, cavity ringdown and enhanced spectroscopy, optical cavity based transient absorption (OCTAS), and prompt fluorescence detected MARY (PF-MARY) has been developed and applied to the study on the magneto reception of more precious flavin related samples.

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For efficient utilization of light energies, the control of the exciton dynamics is most important. Particularly, exciton annihilation which causes the decrease in the number of the generated excitons has to be suppressed for the efficient utilization of the excitons. In the present article, we introduce a new method to control the relaxation dynamics of multiexciton, i.e., photon emission statistics from the multiexciton state, in a single colloidal quantum dot (QD) using plasmonic nanostructures. A single QD can emit single-photon and multiphoton emission depending on the occurrence of the annihilation process which is called the Auger recombination. We demonstrate that the photon emission statistics can be controlled on demand by applying the enhancement effect of the radiative decay by the plasmonic nanostructures to a single QD. The effect of the plasmonic nanostructures shown in this article can open new plasmonic routes for manipulating multiexcitonic process to utilize the excitons efficiently.

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The reversible crystal growth of open- and closed-ring isomers of photochromic diarylethenes were discovered upon alternate irradiation with UV and visible light, accompanied with the topographical changes on the microcrystalline film surface. The photogenerated crystals form needle-shaped crystals for the closed-ring isomer, while cubic-shaped crystals for open-ring isomer for the initially discovered derivative. The former surfaces showed a superhydrophobic wetting property, which is classified as a lotus or petal effect of wetting depending on the size of the needle-shaped crystals. The needle-shaped crystals were randomly oriented. We tried epitaxial crystal growth to orient the needle shaped crystals. Then we found a self-epitaxial crystal growth of the needle-shaped crystals which grow on the lattice of the open-ring isomer, and form a well-ordered surface. Due to the orderings and submicron-sized crystals, the surface shows not only super-water-repellence but also antireflection moth-eye effect in near infrared region.

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Nitric oxide (NO) is now known as an endogenous signaling molecule and involved in the various physiological signaling such as control of blood pressure. NO has gaseous property and short half-life due to susceptibility to oxidation under ambient conditions, so that it is difficult to handle it in biological experiments directly. NO donor compounds which release NO under physiological conditions have been instead developed, and photocontrollable NO releaser, that is caged NOs, are also developed which are advantageous for treatment with NO in a spatiotemporally controlled manner. We recently developed caged NOs and adopted them to cellular, tissue, and in vivo experiments. One of our caged NO, Flu-DNB, released NO by NIR fs-pulse laser irradiation, and induced blood vessel response in a living mouse brain, and another one, NOBL-1, released NO by blue light, and succeeded to induce relaxation responses in a vessel tissue strip. These compounds would be useful for the investigation of NO biology and also for the NO-related therapeutic research.

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Supramolecular sensing with polymers is particularly important not only for basic science but also for many applications as detection of pollutants in the environment, of explosives for homeland security, bioimaging, and practical diagnostic technique, since a number of polymers can greatly enhance signal gains upon interactions with analytes. Of many approaches to polymer-based sensing examined so far, "Supramolecular Allosteric Signal-amplification Sensing (SASS)", proposed by us very recently, provides us with unique, versatile methodologies for detecting biologically important analytes such as amino acids, peptides, and saccharides, which are alternatives to the conventional molecular chemosensors. In this Topic, the author highlights the SASS with recent examples of chirally modified polythiophenes (synthetic conjugated polymer) and curdlans (one of the simplest β-1,3-glucans).

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