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, 1 alpha and 1 beta, and investigated their different photomechanical behaviors. Under ultraviolet (UV) light irradiation, the thin plate-like 1 alpha crystal bent away gradually and strongly from the light source, with some twist caused by enol-keto photoisomerization. In contrast, the thin, needle-like 1 beta crystal did not bend by photoisomerization; however, the thick 1 beta 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 1 alpha 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.

Automated summary

Mechanically responsive materials have been extensively studied in the past two decades. In this research, two polymorphic crystals of a salicylideneaniline derivative were investigated for their different photomechanical behaviors. The results showed that these crystals could achieve distinct bending motions through photoisomerization and photothermal effects, offering a novel approach to diversify the mechanical motions of molecular crystals and expand their potential and versatility as actuation materials.