Photoinduced reversible topographical changes on photochromic microcrystalline surfaces

Abstract

Diarylethenes are well known as thermally irreversible photochromic compounds, and some of their derivatives undergo the photochromic reactions even in the crystalline state. The reversible crystal growth of open- and Closed-Ring isomers of diarylethenes was discovered upon alternate irradiation with UV and visible light, accompanied with the topographical changes on the microcrystalline film surface. The photogenerated crystals form Needle-Shaped crystals for the closed-ring isomer, while Cubic-Shaped crystals for Open-Ring isomer for the initially discovered derivative. The former surfaces show a superhydrophobic wetting property, which is classified as a lotus or petal effect of wetting depending on the size of the needleshaped crystals, while the latter does not show such superhydrophobicity . The needle crystals were randomly oriented. We tried epitaxial crystal growth to orient the Needle-Shaped crystals. The first epitaxial crystal growth was performed on a 110 surface of a strontium titanate single crystal whose lattice constant is similar to that of a diarylethene crystal. Then we found a Self-Epitaxial crystal growth of the Needle-Shaped crystals which grow on the lattice of the Open-Ring isomer and form well-ordered surface. Due to the orderings and Submicron-Sized crystals, the surface shows not only super water repellence but also Antireflectionmoth-Eye effect in Near-Infrared region.