Crystal actuation switching by crystal thickness and light wavelength

Photomechanical molecular crystals have been developed over the past two decades, and actuation switching is an important requirement for their practical application. In this study, we developed a method for actuation switching of the isomorphic crystals of two salicylideneaniline derivatives, where bending direction and speed are controlled by crystal thickness and light wavelength. Upon ultraviolet (UV) light irradiation of its top face, a thin crystal bends toward the light source through photoisomerisation. In contrast, a thick crystal bends away rapidly through the photothermal effect. Slightly thick crystals exhibit two-step bending that involves a combination of these two mechanisms, first bending away quickly through the photothermal effect and then bending toward the light source through photoisomerisation. The bending motions of thin and slightly thick crystals switch to two-step bending and bending away, respectively, upon UV light irradiation of the side face. Visible light irradiation alters the bending motions of thin and slightly thick crystals to no significant bending and bending away, respectively. These results provide a novel, convenient, and useful approach for switching the crystal bending direction and speed, thereby expanding the versatility of molecular crystals as actuation materials.