Photoinduced topographical surface changes and photoresponse of the crystals of 7-methoxycoumarin

Photoinduced topographical changes, bending, and photosalient effect due to the dimerization reaction were observed on a single crystal of 7-methoxycoumarin, upon deep UV (254 nm) light irradiation.

Crystal Growth Technique for Formation of Double Roughness Structures Mimicking Lotus Leaf

Bio-inspired functional materials have received much attention for their potential to provide sustainable and advanced materials. The lotus effect has proven to be one of the most remarkable biomimetic effects since it was discovered by Barthlott. A superhydrophobic surface with the ability to bounce water droplets is the origin of the self-cleaning mechanism that keeps the surface clean by removing dust using water droplets moving with momentum. We have developed a crystal growth technique (CGT) of photochromic diarylethenes over the past decade, and from this, we fabricated a surface structure that closely resembles the natural lotus leaf's characteristic of controlling the Laplace pressure and clarified the importance of the double roughness structure of the surface. The bouncing ability is also discussed in terms of the characteristic size of the double roughness structure theoretically. Moreover, this work clarifies the exquisiteness of the double roughness structure of the leaf. We also show that the CGT is a versatile technique with the potential to fabricate desired structured surfaces.

Photochromic Crystalline Systems Mimicking Bio-Functions

Photoresponsive crystalline systems mimicking bio-functions are prepared using photochromic diarylethenes. Upon UV irradiation of the diarylethene crystal, the photogenerated closed-ring isomers self-aggregate to form needle-shaped crystals on the surface. The rough surface shows the superhydrophobic lotus effect. In addition, the rose-petal effects of wetting, the anti-reflective moth-eye effect, and a double-roughness structure mimicking the surface of a lotus leaf are observed by controlling the heating procedures, UV irradiation processes, and molecular structural modification. By changing the molecular structure, a superhydrophilic surface mimicking a snail shell can be generated. We also find the crystal of a diarylethene derivative that shows a photosalient effect. The effect is observed partly due to the hollow structure of the crystal. It is demonstrated that a photo-response similar to the response of impatiens plant to stimulation is observed by packing small beads in the hollow. These photoresponsive functions are unique, and they demonstrate a macroscopic response by means of microscopic molecular movement induced by light. In the future, such a molecular assembly system will be a promising candidate for fabricating photoresponsive architectures and soft robots.