Recent ultrafast spectroscopic researches on semiconductor nanoparticles (NPs) are summarized in this review. Taking a research result as an example, the initial exciton dynamics of semiconductive NPs studied by femtosecond transient absorption spectroscopy and picosecond time-resolved luminescence spectroscopy is explained. The ultrafast exciton quenching with a time constant of 200 fs was observed, which may be related with LO phonon in the excited state. In addition, higher electronic state and biexciton with short lifetime were clearly detected by the excitation intensity dependence of time-resolved luminescence and transient absorption spectra. These relaxation processes of semiconductive NPs are very important not only for understanding the generation of charge carrier but also for constructing the high-efficiency charge separation and electric luminescence systems.

In this review, several aspects of visible-to-ultraviolet (UV) photon upconversion (UC) based on triplet–triplet annihilation (TTA) are introduced and discussed. Because UV-UCs use higher energy triplet states, they are generally more prone to suffer from photodegradation compared to visible-to-visible UCs. First, the background, utility, and research situation of UV-UCs are explained and then some example of low photostability in previously reported UV-UC sample system is presented contrasted with the stability of visible-to-visible UC carried out in ionic liquid. Our recent findings regarding UV-UCs for the wavelength shorter than 340 nm are presented and discussed. Based on them, some general guideline for improving sample photostability is proposed.

Recently, photoindued decarboxylative radical reactions of aliphatic carboxylic acids using a photoredox catalyst have been established as an effective and environmentally-friendly method for the generation of alkyl radicals. This is because carboxylic acids occur widely in nature and are easy to handle; moreover, decarboxylation releases the nonflammable and nontoxic product CO₂ under mild conditions (e.g., at room temperature), and light is a traceless reagent. The process is initiated by oxidation of the carboxylate ions via photoinduced electron transfer to form carboxy radicals, which are subsequently decarboxylated to form radicals. In this review, I describe four typical processes of photoinduced decarboxylation of carboxylic acids using a photoredox catalyst and the following representative radical reactions.

Development of photocatalysts working under visible light is an important topic from a practical point of view. In previous studies, Au nanoparticles supported on metal oxide have been applied as a new type of photocatalysts responding to visible light. In this article, we introduce two methods for control of surface plasmon resonance (SPR) of supported metal nanoparticles by 1) post-calcination of Au/TiO₂ and 2) modification of Au nanoparticles with Ag and Cu. Elevation of calcination temperature induced a red shift of SPR. Modification with Ag and Cu induced blue and red shifts, respectively. These plasmonic photocatalysts were active for mineralization of formic acid and selective oxidation of alcohols to carbonyl compounds under visible-light irradiation and exhibited the best performance when irradiated by light overlapping with their SPR.

The shape and symmetry of molecular orbitals determine the photophysical properties of molecules. Helicenes are promising candidates for chiral optoelectronic materials because of their helically twisted molecular orbitals. One of major drawbacks of helicenes is low emission intensity (Φ_f < 0.05 for unsubstituted carbohelicenes) due to a small oscillator strength for the S₁→S₀ transition. In this article, logical guidelines for improving photophysical properties of [5] and [7] helicenes are introduded in terms of “photostability”, “emission intensity”, and “dissymmetry factor for circularly polarized luminescence (CPL)”. By introducing electron-withdrawing and -donating groups into appropriate substitution positions, a [5]helicene derivative showing high emission quantum yield (Φ_f) and short lifetime (Φ_f = 0.23, <τ_f> = 1.5 ns) as well as a [7]helicene derivative showing a high Φ_f value and a large dissymmetry factor for CPL (Φ_f = 0.17, |g_CPL| = 0.013) were synthesized. The experimentally observed photophysical properties were well reproduced by TD-DFT calculations.

Luminescent coordination compounds have been attracting wide range of interest because of their characteristic photophysical and photochemical properties. Usage of bridging ligand for coordination compounds leads to formation of luminescent coordination polymers, where multiple luminescent units are compiled by coordination bonds. Introduction of hetero units to such compounds causes not only the addition of the emission from the units but also the modification of luminescent properties by the interaction among the homo and hetero units. We have synthesized the mixed coordination polymers as solid solutions by utilizing the isomorphous relation among the copper(I) and silver(I) coordination polymers having the formula of [M₂X₂(PPh₃)₂(μ-L)]_n. Halogenido-mixed and bridging-ligand-mixed coordination polymers have been also prepared to show the rapid energy migrations between units, and in several systems, directional energy migrations occurred to make them antenna systems.

The web system "touche (TOtal User-Centered Html-generating Engine)", open for non-profitable meeting organizing committees free of charge, for organizing a meeting (hopefully) entirely on the Internet is introduced. Use of "touche" is highly recommended to save time for management of meetings and to re-think the meaning of meetings making the meeting high quality and to enjoy meetings as organizers as well as participants.

Mr. Tadataka KATAOKA has passed away on March 2020. He was an owner of one glass company supplying quartz cells, mercury lamps, and other photochemistry-related instruments with high quality. He supported personally foreign students and young Japanese researchers as well as formally many activities of Japanese Photochemistry Association. Some memories on his kind and warm understanding on photochemistry and photochemists in Japan are described and acknowledged.