Various types of photomechanical motion have drawn much attention because there is a potential to create photomechanical actuators from molecular-scale to macro-scale. To construct photoactuators, it is necessary to utilize a molecular assembly with a small free volume. Photochromic compounds undergo photoreversible isomerization between the original colorless isomer and the photogenerated colored isomer upon alternating irradiation with UV and visible light. Among many known photochromic compounds, diarylethenes　undergo photochromic reactions even in the crystalline phase. The present review introduces recent development in study of photomechanical crystals including crystal shape changes, bending velocity, dependence of the bending behavior on irradiation wavelength, the behavior in mixed crystal, new types of photomechanical motion, and applications. These photomechanical behaviors are based on geometrical structure changes in the crystalline phase, and can be applied to macro-sized light-driven actuators.

Liquid crystals (LCs) are promising materials not only for the display purpose, but also for active matters, such as biological membrane. They are soft and flexible in nature, caused by the long molecular interaction. However, the dynamic change of liquid crystals has been difficult to clarify because of their nonlinear responses, collective behavior, orientation dependence, even though various time-resolved techniques have been utilized to study the dynamics by monitoring the change of LCs after a photo-induced trigger. Here, the history of such studies is reviewed, and it is described that the recent development of new techniques gradually have clarified the complicated processes. Especially the transient grating method is a powerful tool to study the photo-induced dynamics of LCs, by applying a simple optical setup and large-area (homogeneous) excitation. The studies on the host-guest interaction, photochemical phase transition, molecular orientation propagation, are introduced.

Layered inorganic materials such as clay minerals and layered metal oxide semiconductors (LMOSs) offer a two-dimensionally wide and expandable layered structure. Their expandability enables the incorporation of various types of chemical species into their interlayers with layer distance can vary. Moreover, the exfoliation of layered materials as “nanosheet” are promising for new types of inorganic layered materials. In many cases, the adsorbed chemical species on layered materials form a regularly aligned structure by the interaction with layer surface. These two-dimensionally wide spaces allow inter-molecular interactions between identical or different adsorbed chemical species. Thus, such two-dimensionally oriented systems are very interesting as photochemical reaction fields. In fact, the photochemical behaviors of adsorbed molecules are completely different from those in homogeneous solutions. Unique and characteristic photochemical properties of dyes in the interlayer spaces or on the surfaces of inorganic nanosheets are reviewed.

Photofunctionalities (e.g., absorption/emission characteristics and photoinduced reaction behaviors) of the MLCT-type transition metal complexes are sensitive to the microenvironment around the molecule as well as the chemical structure of the molecule itself. Therefore, understanding how the complexes behave under the rigid environment is of primary importance since the solid-state environments are often employed as the media for the metal complexes in photochemical applications/devices such as organic light-emitting diodes (OLEDs) and dye-sensitized solar cells (DSSCs). In this article, the spectroscopic, photophysical and photochemical properties of the MLCT-type transition metal complexes introduced into the polymerizable material or metal–organic framework systems are described as typical examples of the rigid medium effects on the photofunctionalities of the metal complexes.

Research on mechanochromic luminescence has rapidly grown over the past decade owing to their potential utility such as force sensing. However, there are very few studies on the luminescent mechanochromic materials with emissions in the long wavelength regions, including deep- to infrared (IR). Here, we describe 9-anthryl gold(I) isocyanide complex 3, which exhibits a bathochromic shift of its emission color from the visible to the IR region upon mechanical stimulation. Prior to exposure to the mechanical stimulus, as-prepared 3a exhibit emission wavelength maxima (λ_em,max) at 448 nm. Upon grinding, the λ_em,max of the resulting ground powder 3b shows red-shift to 900 nm with ∆λ_em,max 452 nm or 1.39 eV. Complex 3 thus represents the first examples of mechanochromic luminescent materials with λ_em,max in the IR region. The origin of the unprecedentedly large shift of the emission wavelength of 3 was investigated via X-ray diffraction analyses, theoretical calculations, and spectroscopic analyses.

A considerable attention has been focused on the development of near-infrared (NIR) absorption and emission dyes used for bio-imaging and recordable disks. In order to make NIR dyes, expansion of π-conjugated system is one of the most straightforward strategies. However, large π-conjugated molecules such as higher acenes are often chemically unstable and difficult to handle because of their intrinsic high reactivity and insolubility. Thus, developing NIR dyes with smaller π-system (molecular size) is highly desired. Xanthene dyes, e.g. rhodamines and fluoresceins, display long-wavelength absorption and emission despite their relatively small molecular size. These are achieved by small bond length alternation originated from the contributions of resonance structures in the ionic form. Here, we introduce the photophysical properties of structurally well-defined π-expanded fluorones. We find that new dibenzofluorones FBX display the sharp and the longest NIR absorption and emission among the naphthofluorones so far reported upon the addition of base. Hence, we demonstrates the utility of dibenzo[b,i]-fluorone dye as an efficiently π-conjugated material.