Photoinduced Self-Epitaxial Crystal Growth of a Diarylethene Derivative with Antireflection Moth-Eye and Superhydrophobic Lotus Effects

Abstract

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.

Photochromic Crystalline Systems Mimicking Bio‐Functions

“…Small needle-shapedc rystals of 3c covered the surface, whereas CA reached 1738 and SA was less than 18,i ndicating very excellent super water-repellency. [12,13] These resultsi ndicate that the crystal growth proceeds above the compound's T g . The importance of T g was already reported by Ts ujioka et al for metal vapor deposition on diarylethene crystalline surfaces.…”

“…Another surface of 3 o after UV irradiation was stored for nine days at 30 °C, which is above the glass transition temperature ( T g ) of 3 o , was observed. Small needle‐shaped crystals of 3 c covered the surface, whereas CA reached 173° and SA was less than 1°, indicating very excellent super water‐repellency . These results indicate that the crystal growth proceeds above the compound's T g .…”

“…This agrees with the low activation energy of formation of 3 c The 3 o crystals below the 3 c crystals act as the seeds of formation of 3 c crystals. In the above results, the nano crystals of 3 c were always formed by UV irradiation standing at 60°, regardless of which substrate was applied …”

“…In the above results, the nano crystalso f3c were always formed by UV irradiation standing at 608,r egardless of which substrate was applied. [13] Due to the highly ordered structure, the rough surfaces are expected to show the moth-eye anti-reflection effect in the infrared region. The closed-ring isomer 3c formed by UV irradiation has an absorbance whose band tail extends to around 800 nm.…”

Selective Metal-vapor Deposition on Organic Surfaces

“…One organic surface that enables light‐controlled selective metal‐vapor deposition is the photochromic diarylethene (DAE) surface . Photochromism is a reversible change in molecular structure upon light irradiation, and various properties of the molecule or bulk, which include color, refractive index, crystal shape, fluorescence, hydrophobicity, electrical conductance, and so on, change, based on the isomerization reaction. Photochromic diarylethenes undergo electrical isomerization and can be applied to organic semiconductor memory devices .…”