A superhydrophobic surface on which the contact angle of a water droplet exceeds 170° was reversibly produced by alternate irradiation with UV and visible light. Superhydrophobicity is due to the formation of densely generated submicrometer sized needle-shaped crystals (less than 0.2-0.3 μm diameter and 2.2-2.5 μm long) at 30 °C, which is much lower than the eutectic temperature of either isomers of the diarylethene. Below the eutectic temperature, the generated crystals were much smaller than those generated above the eutectic temperature. These smaller crystals more effectively enhanced the superhydrophobicity.

We identified the mechanism of the formation of needle-shaped microcrystals on which the contact angle of a water droplet exceeds 170° [Nishikawa, N. et al. Langmuir, 2012, 28, 17817-17824]. The standing needle-shaped crystal of the closed-ring isomer of a diarylethene 3c grew at a much lower temperature than the eutectic temperature by irradiation of UV light on the thin films of the open-ring isomer 3o, due to the epitaxial growth of the 013 plane of 3c over the 110 plane of the crystal lattice of 3o in the subphase. Therefore, the new crystal-growth mechanism triggered by the photoisomerization does not require special inorganic single-crystal substrates and may be called self-epitaxial crystal growth. The needle-shaped crystals appeared well-ordered and stood inclined at an angle of about 60° to the surface. Consequently, the photo-induced rough surface shows not only the superhydrophobic lotus effect, but also the antireflection moth-eye effect, and these effects were switchable by alternate irradiation with UV and visible light.

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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.