Properties of molecules are strongly dependent on their electronic states. Accordingly, we can expect different reactions of the molecule by selecting the electronic state through the change of the wavelength in the photoexcitation. Actually, reaction depending on the excitation wavelength has been reported for many molecules consisting of several atoms in the gas phase. Large molecules composed of ca.10 or more atoms, however, undergo the ultrafast intramolecular relaxation into the lowest excited state and the longitudinal relaxation of the vibrational excess energy follows this rapid conversion in the condensed phase. These fast relaxation processes have confined most of the photochemical processes to the reaction starting from the thermally equilibrated S₁ states. Reactions in higher excited states, however, have been recently reported for several molecules in the condensed phase. In the present article, we introduce these examples relating to isomerization and electron transfer reactions and discuss the prospect for future.

Photocatalytic CO₂ reduction is indispensable for solving environmental and energy issues toward establishing a sustainable society. Extensive efforts have been devoted to developing photocatalytic systems for CO₂ reduction using metal complexes as catalysts to produce CO as well as formic acid (HCOOH) as 2-electron reduced products. In this review, we survey the progress in photocatalytic CO₂ reduction using molecular Ni complexes as catalysts, inspired by the active site of a Ni-containing CO₂-fixing enzyme. Photocatalytic CO₂ reduction using Ni complexes requires photosensitizers such as Ru(II)-diimine complexes and appropriate reductants such as BIH; the reaction shows a remarkable feature in high CO selectivity, in contrast to Ru complexes as catalysts.

In an aging society, the enhancement of the preventive healthcare system based on Point-of-Care Testing (POCT) and Digital Health technologies is required. Smart and portable / wearable biosensors are indispensable to conduct daily health check based on quantitative analysis of molecules in body fluid, which gives evidence for medical diagnosis. Development of biosensors aiming for POCT and digital health applications including photochemical processes and techniques are introduced in this review. By combining photooxidative polymerization and enzyme reaction, protein biomarkers in a small volume sample are measured just by focusing a visible laser beam. Localized surface plasmon resonance (LSPR) biosensor fabricated by a one-step nanoimprint lithography demonstrates the tunable, high throughput, reproducible and cost-efficient merits. A simple and portable electrochemiluminescence platform shows excellent performance in various biosensing applications based on the reactive oxygen species (ROS) detection.

Ligand-protected metal clusters are well-defined, ultrasmall particles with a diameter of less than 3 nm (less than 1000 constituent atoms) whose structure and properties vary dramatically at the single-atom level. In this review, we first discuss the fundamental properties of metal clusters, and then introduce the geometrical and electronic structures, photophysical processes, and photoluminescence properties of thiolate-protected noble metal clusters, which can be precisely synthesized in solution. Focusing on the fact that these clusters possess wide absorption bands extending into the near-infrared region and long-lived excited states on the order of microseconds, we also present recent studies on the application of thiolate-protected noble metal clusters as photosensitizers for sensitized solar cells, photocatalysis, ¹O₂ generation, and triplet-triplet annihilation photon upconversion.

The potential application of luminescent techniques to various systems, such as organic, organometallic, and inorganic electroluminescence devices and optoelectronic devices, has attracted considerable attention. Analogous to the chirality associated with molecules, there exists chirality of light, which is referred to as circularly polarized luminescence (CPL). In general, CPL is generated from a chiral luminophore. That is, chiral structure is essential for organic and organometallic luminophores with CPL property. In contrast, it is difficult to introduce a chiral structure into inorganic and metal-cluster luminophores. In this topic, magnetic circularly polarized luminescence (MCPL) systems are reported as non-classical CPL (NC-CPL) systems. Under an external magnetic field, achiral and racemic luminophores are able to emit MCPL. This MCPL system is beneficial in developing versatile CPL systems from various luminophores, covering a wide range of UV–visible–near infrared region in the future.

Solution-processed organic photovoltaic (OPV) devices have attracted extensive attention owing to their advantageous features such as light weight, low cost, and large-area fabrication. Herein, we report a novel nonfullerene acceptor (NFA), TACIC, having an electron-rich, self-assembling nanographene unit, thienoazacoronene, at the center with electron-withdrawing groups at both ends. The TACIC film exhibited not only a narrow band gap (1.59 eV) but also an extremely long singlet exciton lifetime (1.59 ns), which is highly advantageous for exciton diffusion in OPV devices. Consistently, when combined with a conjugated polymer donor, PBDB-T, exciton diffusion efficiency was sufficiently high (>96%) even in large domains of TACIC. This unique property of TACIC is significantly advantageous for large-scale OPV production with high efficiency, stability, and reproducibility.

How a newly developed transient absorption technique, the Randomly Interleaved Pulse Train (RIPT) method, made practical is described. Its additional function to measure fluorescence lifetime using an oscilloscope-based time correlated single photon counting technique is also introduced.

Since July 1994, Bioorganic Chemistry Kusatsu Seminars have been held in Biwako Kusatsu Campus (BKC) of Ritsumeikan University, where we had 158 nice lectures. Additionally, International Workshops on Supramolecular Nanoscience of Chemically Programmed Pigments (SNCPP) have been annually organized in BKC since June 2004. These meetings inspired my laboratory members to grow up in scientific filed.