Metal clusters composed of well-defined numbers of metal atoms and ligands exhibit unique structural and physicochemical properties different from those of bulk materials and nanoparticles. Some metal clusters display photoluminescence (PL) property. While the detailed mechanism of PL in clusters has not been fully elucidated, a number of approaches including heteroatom doping in the core, ligand engineering, and aggregation of clusters have been demonstrated to improve their PL property. The origin of PL in clusters has been discussed to be assigned roughly to two mechanisms including an inner core (superatom)-localized excited state and charge transfer states between the core and ligand-layer. The PL from the core-localized excited state has been modified through the geometrical control of superatom core and heteroatom doping. The excited state geometrical relaxation is possible to associate with the excited state transitions such as S₁-T₁ intersystem crossing. As for the ligand-involved charge transfer PL, the ligand engineering through rigidification is effective to enhance the PL property. The stabilization of CT state through ionic interactions has been demonstrated to lead to intense PL in the NIR-I region. Furthermore, tailoring of surface ligands as well as core through metal species (Au or Ag) has controlled the radiative and nonradiative pathways, leading to highly bright NIR-II PL with a quantum yield as high as 0.35.

Singlet fission and triplet-triplet annihilation have attracted much attention in recent years due to their potential application to organic photo devices. In both processes, triplet pairs are generated as an important intermediate which determines the efficiency of these processes. Since magnetic field affects spin states in triplet pairs, efficiency of singlet fission and triplet-triplet annihilation are often show remarkable magnetic field effects. Measurements of magnetic field effects on fluorescence have been the powerful tools to investigate singlet fission and triplet-triplet annihilation in organic crystals. In this review, first, the mechanism of the magnetic field effects on fluorescence in organic crystals is described with spin states mixing in triplet pairs. Then the magnetic field effects on fluorescence observed under the magnetic field lower than 2 T are introduced. Finally, several recent reports with magnetic fields up to 10 T are introduced with the discussion on exchange interactions in triplet pairs. Brief explanations are also given for words that are sometimes difficult to understand.

Piezofluorochromism (PFC) is a phenomenon that fluorescence color changes reversibly in response to isotropic pressure. Under extremely high pressure at the order of GPa, organic molecular crystals may undergo unusual changes of electronic structures due to extraordinal condition of the molecular and packing structures. Namely, PFC is one of the fundamental methods to explore extraordinal electronic structures that cannot be achieved under ambient conditions via changes of visible properties, and thus has attracted much attention in terms of some applications. In this review, we introduce the recent progress in PFC of organic molecular crystals.

We have developed a negative photochromic 1-(1-naphthyl)pyrenyl-bridged imidazole dimer (NPy-ImD) that has three different isomers. The colored isomers 5MR-R and 5MR-B are photochemically isomerized to the colorless isomer 6MR via the short-lived biradical (BR) upon irradiation with blue light and red light, respectively. 6MR is formed from the short-lived BR through a kinetically controlled reaction. 6MR and 5MR-B can then be converted to the more stable 5MR-R through a thermodynamically controlled reaction. Notably, 5MR-R photoisomerizes to 6MR upon exposure to CW-UV light, whereas it photoisomerizes to 5MR-B by a two-photon process when exposed to nanosecond UV laser pulses. NPy-ImD is a unique type of negative photochromic molecule, which has a nonlinear response to excitation light intensity.

Photochemical reactions can be a powerful tool in organic synthesis, as they can facilitate reactions that are often challenging under conventional thermal conditions. This paper introduces our recent efforts in pursuing the realization of photoinduced reduction reactions, focusing on non-transition metals, high reduction power, and visible light excitation as key factors. Our approach primarily involves the development of carbazole-based molecules as photosensitizers. These developed photochemical reactions hold significant potential in addressing global issues such as efficient biomass utilization and carbon dioxide fixation.

In this topic, I will provide an idea of ultrafast structural dynamics measurements on the organic molecules using a table-top ultrafast time-resolved electron diffraction setup for the field of photochemistry. The former part of this topic is focused on the instrumentation itself and the sample preparation for ultrafast time-resolved electron diffraction measurements, which may be more interesting for the researchers working on material synthesis. The latter part of this topic shows an example of ultrafast structural dynamics of representative organic molecules, azobenzene liquid-crystalline molecules. The measurements revealed that the ultraviolet photoexcitation induced trans-to-cis isomerization of azobenzene molecules, which subsequently generated the collective motions in the liquid-crystalline phase through the free space changes. Lastly, one of the further challenges of ultrafast time-resolved electron diffraction for photochemistry is also given.