Light‐Triggered, Non‐Centrosymmetric Self‐Assembly of Aqueous Arylazopyrazoles at the Air–Water Interface and Switching of Second‐Harmonic Generation
Yuki Nagai, Keita Ishiba, Ryosuke Yamamoto, Teppei Yamada, Masa‐aki Morikawa, Nobuo Kimizuka
Abstract
Trans-p-methoxy arylazopyrazole spontaneously forms non-centrosymmetric polar crystals, which reversibly undergo liquefaction upon photoisomerization to the cis-isomer. This liquid cis-isomer has a large electric dipole moment and is highly soluble in water (solubility up to ≈58 mM), which is remarkably higher than that of the trans-isomer (690 μM). Vis-light illumination of the aqueous cis-isomer generates macroscopically oriented, non-centrosymmetric crystals at the air-water interface. Polar crystals are also formed in sandwich glass cells (spacing, 20 μm) upon photo-induced crystallization of the liquid cis-isomer. The trans-crystals thus formed showed second harmonic generation (SHG) whose intensity is switched on/off in response to the photo-induced phase transition.