Photoinduced reversible topographical changes on diarylethene microcrystalline surfaces with biomimetic wetting properties.

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 (58 μm wide and 2030 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.