Control of surface wettability and photomicropatterning with a polymorphic diarylethene crystal upon photoirradiation.

A photochromic diarylethene crystal of 1,2-bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene (1 a) was found to undergo a thermodynamic phase transition at 180 °C to form a needle-like crystal, designated as 1 a-γ. The phase transition involves melting of the initial α-crystal and growth of the γ-crystal. The phase transition temperature decreased with the presence of the closed-ring isomer (1 b) in the crystal because of the decrease in the melting temperature. Upon irradiation with ultraviolet (UV) light, compound 1 a in the α-crystal was converted into 1 b to an extent of 20 %. Consequently, the α-crystal containing 20 % of 1 b underwent the phase transition accompanied by melting of the crystal and growth of the γ-crystal even at 170 °C. Photomicropatterning by the phase transition upon irradiation with UV light using a photomask, followed by heating at 170 °C, was successfully accomplished with a resolution in the microcrystalline pattern of about 20 μm. The contact angle with water on the γ-microcrystalline phase on a glass substrate was larger than that on the α-microcrystalline phase by 20°. This can be ascribed to a difference in the roughness of the surface. Furthermore, the γ-microcrystal was also found to be formed upon heating an amorphous film of 1 a in poly(methyl methacrylate) for 2 min at 130 °C. The crystallized area exhibited a higher water contact angle than the amorphous area. Upon irradiation of the amorphous film with UV light, such crystallization did not take place because of the impurity effect of 1 b in 1 a. Photomicropatterning by the crystallization in the polymer showed a pattern with a higher resolution of about 4 μm, which was much better than that of the neat crystal. This photopatterning process represents a useful tool for controlling the surface wettability in relevant applications.