Photophysical processes of perylene have been studied in various aggregate forms including isolated molecule, excimer, two crystal polymorphs and molten liquid. We observed different fluorescence color, blue, green and red-orange, among these aggregates, which is induced by intermolecular interaction caused by specific molecular configuration of the aggregates. For crystalline phase, excited state can migrate as an exciton and the diffusion coefficient is evaluated based on the annihilation reaction of two singlet excitons. After the exciton annihilation, a highly excited state is populated and subsequently relaxes through various channels including photoconductivity, photoelectron emission and singlet fission. Transient absorption measurements have also been carried out to study electronic state formed by aggregation. We observed the characteristic charge transfer absorption band in near-IR wavelength range due to the formation of sandwich excimer. We discuss the energy position of the charge transfer state as a precursor state for charge separation.

Luminescent lanthanide (Ln) compounds are already applied for fingerprint recognition for security codes and bioprobes because the luminescence colors remain steady because of their quite stable electronic transitions related to the inner-core of Ln. However, the ff emission of rare earth ions is originally weak because it is parity-forbidden. The light emitting efficiency is remarkably increased by the antenna effect of the π-electron organic ligand because the ff emission of the Ln ion appears as narrow bands because of the inner-core transitions being shielded from the outer-sphere. Also, the introduction of helicity into the molecular structure breaks the symmetry of the molecule and allows electronic transitions. Introduced here is a luminescent rare-earth complex that introduces chirality into its helical molecular structure. We compared circularly polarized luminescence properties of two kinds of rare earth complexes stably existing in solution, and clarified the correlation between their molecular structures and luminescence properties.

In this review, studies on the dynamics of excited radical ions are summarized. The enhanced reactivities of excited radical ions have been recognized by many researchers based on product analysis studies around 1980. Nanosecond laser flash photolysis studies provided evidence for the reactions of excited radical ions, and analyses of their kinetics revealed properties such as their excited-state lifetime despite an indirect manner. Recent femtosecond laser flash photolysis studies provided detailed pictures of excited radical ions, though such studies are rather limited. Furthermore, studies on dyad or triad electron donor-acceptor systems, including excited radical ions, showed electron transfer dynamics and disclosed characteristics of excited radical ions that differ from those in neutral states. Larger electronic coupling and smaller dumping factor were reported as important characteristics of electron transfer systems of excited radical ions. These systematic studies on excited radical ions demonstrate their suitability for applications.

Photocatalytic water splitting is one of the promising strategies to generate solar H₂. For large-scale solar H₂ production, it is desirable for the photocatalyst to harness a wide spectral part of the solar light. Mg-doped La₅(Ti_0.99Mg_0.01)₂CuS₅O₇ (Mg-LTC) is an oxysulfide-based H₂ evolution photocatalyst with a bandgap of 1.84 eV. Despite intensive research, LTC-based photocatalysts are yet to demonstrate high photoconversion efficiency. In this article, transient absorption spectroscopy was employed to unveil the key loss processes reducing the efficiency of H₂ evolution in Mg-LTC. At least 50% of the electrons decayed by trapping to defects in tens of picosecond. As a result, the key electron-transfer process from Mg-LTC to the Pt cocatalyst is found to occur from the microsecond long-lived electrons with a time constant of 0.26 μs and 30.5% efficiency. The results provide insight into the perspective of material-design and photocathode fabrication procedure to further advance the H₂-generation efficiency in Mg-LTC.

The solar light in the infrared (IR) region is a potential source of untapped energy. Plasmonic energy conversion holds the key to conversion of IR light through materials that efficiently absorb photons of a desired wavelength. To convert IR-light to energy, we investigated plasmon induced carrier transfer using plasmonic heavily doped semiconductor nanocrystals. We discovered that efficient hot carrier transfer proceeds from a heavily doped semiconductor nanocrystal to a wide band gap semiconductor through the hetero junction. In addition, we achieved photocatalytic H₂ evolution based on excitation at long wavelengths of the solar spectrum (i.e., 2500 nm). The apparent quantum yield of photocatalytic H₂ evolution based on a catalyst with a plasmonic heavily doped semiconductor as a light harvesting material represents highly efficient conversion of IR energy to fuel. The plasmon induced charge separation using heavily doped semiconductor nanocrystal paves the way for use of the undeveloped low-energy light as a solar energy resource.

In in vivo environments, DNA coexists with enormous biomolecules. A high concentration of biomolecules definitely changes both the environment around DNA and DNA conformations. The behavior of DNA-mediated electron transfer (ET) in vivo is expected to be different from that in dilute aqueous solution, but no studies have thus far been conducted on DNA-mediated ET in a crowded molecular medium. We reported that the efficiency of guanine decomposition induced by DNA-mediated ET under crowded environments formed by various cosolutes obviously differed from the efficiency in dilute aqueous solution. DNA is also often packed into a condensed phase under crowded environment. We investigated the efficiency of guanine oxidation in DNA condenses using pyrene-modified oligonucleotides. The results showed that guanine decomposition via photoinduced electron transfer in DNA condensate was dramatically promoted.