Photochromic compounds have attracted much interest because of their potential to modulate physical and chemical properties by light irradiation. Although UV light has been frequently used to induce photochromic reactions of conventional artificial photochromic molecules, visible light has been utilized in photoreceptors in nature, such as the cis–trans isomerization of retinal in rhodopsin proteins. Visible light has several advantages over UV light, reduction of degradation in devices and mutagenesis of cells and the ability to penetrate into the inside of the sample rather than UV light. Therefore, the development of visible light sensitive photochromic compounds has been recently received much attention. In this review, various molecular designs to develop visible-light-responsive photochromic molecules are overviewed.

Reaction mechanisms of photochromic crystal based on X-ray crystal structure analysis are reviewed. Although various organic photochromic crystals have been presented so far, reaction mechanism was not always clarified because of the lack of structural information. In order to obtain three dimensional structure of molecules in crystals before and after photochromic reactions, X-ray crystal structure analysis should be made very carefully. Thanks to recent development of XRD apparatus and analysis software, molecular structure of small amount of unstable photoproducts in crystals can be analyzed. In this review, recent progress of the relationship between the reactivity of photochromic crystal and molecular packing structure in the crystals are reviewed. In addition, the novel method in controlling photochromism, i.e., in situ control of photochromism by modified crystal structure with visible light is presented.

The most significant feature of heterogeneous photocatalyses is possible multielectron-transfer processes to drive significant reactions, such as water oxidation to oxygen or its backward reaction, which cannot be driven by homogeneous-phase photoreactions induced by molecules or metal complexes. It is easily expected that the multielectron transfer in heterogeneous photocatalyses requires absorption of multiple photons and accumulation of photoexcited electrons or positive holes in each photocatalyst particle and that photocatalyses including those multielectron transfer processes may show light-intensity dependence. In this review, fundamental physical chemistry for photoinduced multielectron transfer, e.g., quantum efficiency and standard electrode potential, is interpreted first, and then the results, predominantly reported by the present authors' group, on light intensity dependence analysis on oxidative decomposition of organic compounds and oxygen evolution are discussed to propose new concepts of effective particle size and digital kinetics.

Luminescence behavior of crystalline diaroylmethanatoboron difluoride (1BF₂ ) were studied. Derivatives of 1b-gBF₂ with various alkyl substituents in size and geometry show remarkably different fluorescence in the crystalline state, as a result of different intermolecular interactions. The behavior is explained by intermolecular interaction taking place in multiple molecules, referred to as “excited multimer”. It is also revealed that isopropyl derivative (1fBF₂ ) and iodine-substituted derivative 1iBF₂ show white-light emission and intense room temperature phosphorescence, respectively. The luminescent characteristics of crystals of 1BF₂ are controlled by the crystal-phase composite interaction.

Chirality is ubiquitous in biology and chemistry. Indeed, molecular chirality plays crucial roles in biological activity. Recently, chiral materials are extensively explored for the use in 3D displays, medical endoscopes, information storage/communication, as well as studying excited-state chirality and enantioselective sensing. Among various types of chirality, propeller chirality is unique for its cooperative dynamism. In this report, we will demonstrate how the propeller chirality of hexaarylbenzenes and related molecules offers exceptional chiroptical responses derived from its dynamics.

Stimuli-responsive soft materials that can respond to external stimuli such as temperature, light, pH, and chemicals have been intensively investigated in recent decades. Among various stimuli-responsive systems reported to date, light-responsive materials have attracted significant attention because of their beneficial properties, including high spatial and time resolutions, contactless manipulation, and non-invasiveness. In this topic, we briefly review our recent development of photoresponsive polymer gels based on photodimerization reactions. By introducing photo-dimerizable chromophores into polymer architectures, novel dynamic functions can be imparted to polymer gels such as photo-induced dynamic viscoelastic changes and photo-healing ability.