Amphiphiles, i.e. molecules containing hydrophilic and hydrophobic moieties, spontaneously form supramolecular structures such as micelles, mono- and multilayers or spherical bilayer membranes, called liposomes or vesicles in a medium. If an amphiphile contains a suitable chromophore, light can be used to trigger changes in surfactant activity, aggregation structure, viscosity, microemulsion separation and solubilization. Irradiation can cause cis-trans isomerizations, cyclizations, polymerizations, fragmentations, changes in head-group polarity, or oxidative reactions in these amphiphiles. Because light offers an attractive method to couple temporal and spatial control, these photochemical reactions have created considerable interest in the control of colloidal materials. In this review article, molecular assemblies of amphiphiles regulated by photochemical reactions are overviewed with representative papers.

Photochromic behaviors of transition-metal complexes have attracted considerable attention due to interests in controlling the optical and magnetic functionalities and the high potentials for switchable reaction catalysts. A conventional method to give photochromic properties to transition-metal complexes is introduction of photochromic functional groups such as azobenzenes and diaryethenes to the ligands. However, this kind of photochromic complexes frequently shows low reaction rates and efficiencies for the photochromic processes, due to the fact that the lowest excited states of the complexes are derived from the metal-centered state such as metal-to-ligand charge transfer state. In recent years, several examples for photochromic complexes showing the structural changes around the central metals have been reported. This kind of photochromic behaviors is more useful to directly control the properties of metal complexes such as catalytic activity. Majority of the photochromic complexes reported so far affords a photostationary state under light illumination, hampering complete structural changes. Very recently, we have reported two Ru^II-complexes, which exhibit complete photochromic structural changes around the central metals based on two different reaction mechanisms.

Ryleneimide-containing compounds have attracted much interest due to their photophysical properties and electron-accepting characteristics, which can be promising candidates for the active electron-transporting component of organic photovoltaics (OPVs). In many cases, the active organic layers of OPVs are composed by blends of electron-donating and electron-accepting materials, so-called bulk heterojunction (BHJ) type. To construct the charge-transporting pathways in the BHJ devices, the structural modification of ryleneimide-containing organic semiconductors at the molecular level is increasingly important. In this review, electronic properties and photovoltaic characteristics of ryleneimide compounds bearing multidimensional structures are overviewed.

The mechanisms of water photooxidation reaction and sub-reactions (PL emission by recombination with electrons, non-radiative recombination and surface roughening reaction) were investigated using single crystal n-TiO₂ (rutile) electrodes. It was revealed that the reaction efficiency of those competitive processes strongly depends on the atomic level surface local structures, such as Miller index and step-terrace structures. We also revealed that those 4 kinds of reaction processes are highly correlated with each other, indicating that the controlling of the sub reactions is key factor to control the photocatalytic efficiency. The present work has thus opened a new way to investigate the photocatalytic reactivity and stability of these materials on an atomic level, which is important to search for new active materials.

The oxygen concentration of the internal living cells and tissues is one of the key parameters in various physiological, pathological, and therapeutic processes. Phosphorescence imaging techniques are useful for quantifying the oxygen concentrations in cells and tissues, because the intensity and lifetime of phosphorescence are significantly dependent on the concentration (partial pressure) of the quenching oxygen. Here we report that phosphorescence of an iridium(III) complex (BTP) is quenched in the presence of oxygen in solution and lipid membrane. The application is demonstrated for the phosphorescence imaging of the oxygen concentrations in living cells using BTP and BTP derivatives which function as a mitochondria-specific oxygen probe and a ratiometric molecular sensor. We show hypoxic tumor imaging of tumor-bearing nude mice using BTP and a modified BTP which provide near-infrared emission.

Concentration-dependent variations in the fluorescence spectra in polymer matrices were investigated. The fluorescence spectroscopy of films formed from varying concentrations of a dye within a polymer matrix can therefore be applied as a means to find out the state of molecular assembly or crystal nucleation and growth of the dye. We have proposed that concentration-dependent fluorescence spectral changes are due to matrix isolation and/or freezing of crystal nuclei growth by the polymer chains, which we term “polymer matrix isolation.” In this article, the concentration-dependent fluorescence change of perylene derivatives both static and dynamic trials is reported. Studying concentration-dependent fluorescence spectral changes during solvent evaporation not only provides insight into the molecular dynamics of the casting process and the compatibility between the dispersed material and the polymer matrix, but also provides information concerning molecular assembly and the nucleation and growth of crystals of the fluorescent organic molecules.