In the field of synthetic organic chemistry, development of novel electron injection methods has led to a new phase of synthetic techniques. Over the past decade, photoredox catalysis (PRC) has emerged as a powerful synthetic strategy. In particular, use of organic photoredox catalysts (OPCs) has rapidly spread among synthetic chemists as a game-changing strategy because of its sustainability and versatility. This review focuses on modern electron injection strategies by highly reducing OPCs: (i) oxidative quenching by *OPC and (ii) systems involving the photoexcitation of electron-primed catalytic species (OPC·^–), which can be engaged in photochemically (conPET: consecutive photoinduced electron transfer) or electrochemically (e-PRC: electrochemically mediated photoredox catalysis). I believe the use of sophisticated OPC systems will be promising for achieving elusive molecular transformations.

This review describes how the 5-fs pulse laser can be applied to develop and study new organic chemical reactions which could not be performed by traditional methods. Stability of the pulse laser systems was extremely improved because of the requirement in the measurement and application using these pulses. We have developed stable ultrashort pulse laser systems for visible and ultraviolet using non-collinear optical parametric amplifier (NOPA) and self-phase modulation, respectively. These broadband ultrashort pulse laser has pulse duration shorter than the molecular vibrational period of organic molecule, thus these pulses excite various molecular vibrational levels coherently to produce wavepacket. Analysis of the signal modulation caused by the wavepacket motion calculates instantaneous frequency of molecular vibrational modes which can visualize the dynamics of molecular structure after photo irradiation. When the photon energy of the ultrashort pulse is not enough for one photon excitation of the sample, thermal reaction was triggered in the electronic ground state whose reaction dynamics was also observed in demonstration of pericyclic reaction. These pulses have ability to produce novel compounds which cannot be produced by general thermal reaction or photoreaction, which was demonstrated using a thioglucoside.

In past several decades, molecular metal complexes and semiconductors have been developed as promising candidates of photocatalysts for molecular conversion such as CO₂ reduction and water splitting. Although these two materials have been individually studied in most cases for a long time, hybridization strategies to maximize the strengths and overcome weaknesses of metal complexes and semiconductors have recently been emerged and growth at an accelerating rate. This review overviews the molecule–semiconductor hybrid photocatalysts with classification by their driving principles. The molecule-semiconductor hybrids can be mainly classified into three groups: (1) molecular photosensitizer–semiconductor catalyst (dye-sensitized photocatalyst), (2) molecular catalyst–semiconductor photocatalyst, and (3) molecular photocatalyst–semiconductor photocatalyst (artificial Z-scheme). In addition, the design principles of hybridization are described, including our recent study on the hybrid photocatalysts constructed from molecular building blocks.

We have developed a method of quantitatively predicting radiative and nonradiative decay rate constants. The method is based on the excited-state computation and rate-constant calculation with the Fermi golden rule. First, we briefly review the rate constant formulae. Second, we present the application of our method to the excited-state decay mechanism of photoexcited benzophenone. We successfully reproduce all the experimentally obtained rate constants, including those for fluorescence, phosphorescence, and intersystem crossing. Calculated results show that the dominant decay channel is the indirect S₁→T₂→T₁ process. Finally, we highlight the application to multiple-resonance thermally activated delayed fluorescence (MR-TADF) mechanism of DABNA-1. We successfully reproduce experimental photophysical properties, including electronic-transition rate constants, lifetimes, and PLQYs relevant to TADF. We show that TADF in DABNA-1 occurs via a higher triplet state (T₂).

The phenomenon of crystal melting by light irradiation, coined as photoinduced crystal-to-liquid transition (PCLT), can dramatically alter material properties with high spatiotemporal resolution. However, the molecular framework of PCLT-active compounds is severely limited to a few photochromic frameworks. Here, we report on heteroaromatic 1,2-diketone as the new class of PCLT-active compound. Along PCLT, the crystal exhibits dynamic multistep changes in the luminescence color and intensity. This luminescence evolution can be ascribed to the sequential PCLT processes of crystal loosening and conformational isomerization before macroscopic melting. Our results demonstrate the integration of photofunction with PCLT, provide fundamental insights into the melting process of molecular crystals, and will diversify the molecular design of PCLT-active materials beyond classical photochromic scaffolds such as azobenzenes.

A novel fabrication method of a homogeneous liquid crystal (LC) alignment is introduced by exposure of polarized ultraviolet (UV) to a cell including a mixture of the LC material and the monomer carrying an azobenzene group. The azobenzene group is known to align the LC molecules uniaxially by the polarized UV exposure. First, the mixture was injected into the empty cell without forming a conventional alignment layer on a pair of substrates. The polarized UV was then exposed to the cell from a normal direction, and finally the homogeneous alignment was obtained. The homogeneous alignment was confirmed by polarized optical microscopic images, and transmittance ratio between bright and black states shows over 900, which is enough ratio for display application. The important point of this method is conducting both the alignment control and polymerization by the exposure of the polarized UV.