By alternate UV and visible light irradiation, reversible topographical changes were observed on a newly synthesized diarylethene microcrystalline surface between the rough crystalline surface of an open-ring isomer and flat eutectic surfaces. The contact angle changes of a water droplet between 80° and 150° and peak intensities changes of the open-ring isomer in XRD patterns within 2 h of repeating cycle were observed. The results indicated that reversibly photogenerated rod-shaped crystals on the surface were produced based on the lattice of the open-ring isomer crystals in the subphase.

Reversible topographical changes were observed on a photochromic diarylethene microcrystalline film surface by alternate irradiation with UV and visible light. Two types of surfaces were prepared from this film: 1) Storage of the film at 30 °C for 24 hours in the dark after UV irradiation afforded a surface that was covered with needle-shaped crystals, whose diameter and length were approximately 1 μm and 10 μm, respectively, and showed a superhydrophobic lotus effect. 2) Storage of the film at 70 °C for 3 hours in the dark caused the needle-shaped crystals to be converted into larger rod-like crystals (5~8 μm wide and 20~30 μm long) by Ostwald ripening and a disappearance of the lotus effect. The obtained activation energy of the formation of the needle- and rod-shaped crystals was 143 and 162 kJ mol(-1), respectively. Subsequent UV irradiation to the surface, which was followed by storage at 50 °C for 1 hour in the dark, gave a doubly rough structure; small needle-shaped crystals were formed between the larger rod-shaped crystals. The surface showed both superhydrophobic properties and the pinned effect of the water droplet: the petal effect. Fractal analysis of both surfaces were carried out using a box-counting method, and the lotus effect was observed in the presence of smaller-sized crystals, whilst the petal effect was observed with larger sized crystals (ca. 100 μm). We demonstrated that the hydrophobic property was controlled by the distribution in crystal size of the closed-ring isomer of the diarylethene. Visible-light irradiation of both rough surfaces afforded surfaces with cubic-shaped micro-crystals of the open-ring isomer.

Diarylethene photochromic switches use light to drive structural changes through reversible electrocyclization reactions. High efficiency in dynamic photoswitching is a prerequisite for applications, as is thermal stability and the selective addressability of both isomers ring-opened and -closed diarylethenes. These properties can be optimized readily through rational variation in molecular structure. The efficiency with regard to switching as a function of structural variation is much less understood, with the exception of geometric requirements placed on the reacting atoms. Ultimately, increasing the quantum efficiency of photochemical switching in diarylethenes requires a detailed understanding of the excited-state potential energy surface(s) and the mechanisms involved in switching. Through studies of the temperature dependence, photoswitching and theoretical studies demonstrate the occurrence or absence of thermal activation barriers in three constitutional isomers that bear distinct π-conjugated systems. We found that a decrease in the thermal barriers correlates with an increase in switching efficiency. The origin of the barriers is assigned to the decrease in π-conjugation that is concomitant with the progress of the photoreaction. Furthermore, we show that balanced molecular design can minimize the change in the extent of π-conjugation during switching and lead to optimal bidirectional switching efficiencies. Our findings hold implications for future structural design of diarylethene photochromic switches.

Three types of diarylethenes having one phenol moiety as an aromatic unit were synthesized in order to control pH change upon irradiation with UV and visible light. Upon irradiation with 313 nm light, these open-ring isomers were transformed to thermally unstable colored isomers in methanol solution. During the storage of the photoirradiated diarylethene solutions in the dark for 10 min, the thermally unstable closed-ring isomers were converted to thermally stable keto isomers. The structures of these isomers were characterized by 1H NMR and X-ray crystallography. The keto isomers were transformed to the initial phenol isomer upon irradiation with visible light. Additionally, we also found the formation of a novel photochromic diarylethene derivative having one keto group on the cyclopentane ring during UV irradiation. A pH change between the two isomers was observed in a mixture of ethanol and water.

Diarylethene 1 shows reversible transformation between the open-ring isomer (1o) and the closed-ring isomer (1c) by alternate UV and visible light irradiation, accompanied with reversible melting and crystallization of the microcrystalline film of 1 at 67 °C, which is the eutectic temperature of 1o and 1c. The reversible epitaxial crystal growth of an 1o of a diarylethene derivative was observed on a 110 surface of a strontium titanate (SrTiO3) single crystal by maintaining a constant temperature and monitoring six reversible intensity changes (within 2 h of repeating cycles) of the reflection of 004 of 1o on XRD measurements.