Polymorph-Derived Diversification of Crystal Actuation by Photoisomerization and the Photothermal Effect

Mechanically responsive materials have been investigated extensively over the past two decades. Diversification of actuation modes is essential for the practical application of mechanical materials. Polymorphic crystals with different crystal structures composed of the same compound can exhibit distinct mechanical motions. Here, we focused on two polymorphs of a salicylideneaniline derivative with a 4-fluoro substituent in enol form, 1α and 1β, and investigated their different photomechanical behaviors. Under ultraviolet (UV) light irradiation, the thin plate-like 1α crystal bent away gradually and strongly from the light source, with some twist caused by enol-keto photoisomerization. In contrast, the thin, needle-like 1β crystal did not bend by photoisomerization; however, the thick 1β crystal bent away quickly from the light source because of the photothermal effect, ultimately achieving 500 Hz high-speed bending under pulsed UV laser irradiation. Moreover, the thick plate-like 1α crystal exhibited two-step motion: fast bending forward by the photothermal effect and then slow bending away by photoisomerization. We succeeded in creating four motions using two polymorphic crystals and two distinct mechanisms, thereby providing a novel approach to diversify the mechanical motions of molecular crystals and expanding the potential and versatility of molecular crystals as actuation materials.