Water splitting using semiconductor photocatalysts under solar light has attracted much attention as one of the promising methods for clean hydrogen production by harvesting solar light energy. The effective utilization of visible light is one of the most important challenges to achieve the desired efficiency in practical application. In this review, we overview recent progress in development of semiconductor photocatalysts for water splitting under visible light. We firstly introduce early studies on photocatalytic water splitting using oxide semiconductors under UV light, along with the importance and difficulty of achieving visible-light-driven water splitting. Then, we introduce some successful examples of visible-light-driven water splitting using a single photocatalyst (i.e. a one-step photoexcitation system). Finally, some examples of visible-light-driven water splitting via two-step photoexcitation systems, so-called Z-scheme systems inspired by natural photosynthesis, will be introduced, especially focusing on such systems with visible-light-responsive non-oxide and mixed-anion photocatalysts.

For Sustainable Development Goals (SDGs) adopted by all United Nations Member States in 2015, the development of new renewable and sustainable energy sources is an issue of global concern to be addressed. Among sustainable energy sources, dye-sensitized solar cells (DSSCs) have received considerable attention from the viewpoint of their interesting construction and operational principles, decorative natures, high power conversion efficiency, and low cost of production, since Grätzel and O’Regan reported high performance DSSCs based on ruthenium(II) pyridyl complex dyes adsorbed on a nanocrystalline n-type semiconductor TiO₂ electrode in 1991. To improve photovoltaic performances of DSSCs, much effort has been made towards the development of various types of organic dye sensitizers as well as Ru complex dyes and there has been a gradual accumulation of information about the relationship between the chemical structures and photovoltaic performances of DSSCs. In this review, progress of dye sensitizers for DSSCs are overviewed.

Despite importance of elucidating photoinduced charge-generation mechanisms for development of the efficient photon-to-energy conversion systems, it has been quite difficult to characterize molecular geometries, electronic couplings and charge-mobilities in the photoinduced initial charge-separated (CS) states for heterogeneous molecular environments in protein complexes and in bulk-heterojunction interfaces between electron donor-acceptor domains in the photoactive layers of the organic solar cells. The time-resolved electron paramagnetic resonance (TREPR) methods have been employed to characterize several electron spin polarizations (ESP) of the photoinduced CS states as different geometries, electronic couplings of spin-correlated radical pairs by determinations and extractions of the anisotropic magnetic interactions. A novel tool of mapping the ESPs to space directions is herein demonstrated to be powerful for understanding how several nuclear motions contribute to CS-state geometries and their mobilities for light-induced primary processes.

Efficient production of triplet excited states without relying on heavy atom effect is a key to practical realization of the long-lifetime phosphorescence for time-resolved imaging, the upconversion luminescence by triplet-triplet annihilation, and the ¹O₂-producing photosensitizers for photodynamic therapy. Herewith, we focus on spin-orbit charge transfer (SOCT) as an intersystem crossing (ISC) mechanism without heavy atoms. Whereas the most of studies regarding SOCT-ISC have employed orthogonally fixed donor (D)-acceptor (A) diads, we used free rotatable biphenyl derivatives as the D-A diads. The free rotation allows us to directly excite it to CT state in planar conformation, which is immediately twisted to give a twist CT state with orthogonal D-A dihedral angle permitting SOCT-ISC. The new free rotatable D-A diad can be applied as a photosensitizer for photodynamic therapy, which can pass through glucose transporters overexpressed on cancer cells.

To investigate excited states is important for elucidating mechanisms of photo-functional materials. Here, we have focused on the third-generation organic light emitting diodes (OLED) materials, namely, thermally activated delayed fluorescence (TADF) molecules, from a viewpoint of molecular geometry in the electronically excited states. We investigated four carbazole-benzonitrile (Cz-BN) derivatives that possess identical energy difference between S₁ and T₁ but show distinct TADF activities. We systematically compared their geometrical changes upon photoexcitation using time-resolved infrared (TR-IR) vibrational spectroscopy. We found that the most TADF-active molecule, 4CzBN, shows little structural change after photoexcitation, while the TADF-inactive molecules show relatively large deformation upon S₁−T₁ conversion. This implies that the suppression of structural deformation is critical for minimizing the activation energy barrier for reverse intersystem crossing in cases of the Cz-BN derivatives.

Scanning near-field optical microscope (SNOM) is a powerful tool for nanoscale optical imaging of elementary excitations in mesoscopic materials. Recently, we have explored the spatial and temporal characteristics of the plasmons excited in two-dimensional gold mesoplates by combining the SNOM with the various spectroscopic methods. From the theoretical calculations based on the Schrödinger equation, we found that the spatial patterns observed in the near-field images are qualitatively reproduced by the eigen functions of a free particle confined inside the boundary of the mesoplates. We also found that not only the in-plane polarized modes but also out-of-plane polarized modes are excited in the mesoplates. From the three-dimensional and time-resolved near-field optical measurements, we revealed that the out-of-plane modes confine the light fields more tightly compared with the in-plane modes. Our results provide the fundamental and profound knowledge on the near-field characteristics of the mesoscopic materials.