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 (1a) was found to undergo a thermodynamic phase transition at 180 degrees C to form a needle-like crystal, designated as 1a-gamma. The phase transition involves melting of the initial a-crystal and growth of the gamma-crystal. The phase transition temperature decreased with the presence of the closed-ring isomer (1b) in the crystal because of the decrease in the melting temperature. Upon irradiation with ultraviolet (UV) light, compound 1a in the alpha-crystal was converted into 1b to an extent of 20%. Consequently, the alpha-crystal containing 20% of 1b underwent the phase transition accompanied by melting of the crystal and growth of the gamma-crystal even at 170 degrees C. Photomicropatterning by the phase transition upon irradiation with UV light using a photomask, followed by heating at 170 degrees C, was successfully accomplished with a resolution in the microcrystalline pattern of about 20 mu m. The contact angle with water on the gamma-microcrystalline phase on a glass substrate was larger than that on the alpha-microcrystalline phase by 20 degrees. This can be ascribed to a difference in the roughness of the surface. Furthermore, the gamma-microcrystal was also found to be formed upon heating an amorphous film of 1a in poly(methyl methacrylate) for 2 min at 130 degrees 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 1b in 1a. Photomicropatterning by the crystallization in the polymer showed a pattern with a higher resolution of about 4 mu m, 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.