Photomechanical motion of three polymorphic salicylideneaniline crystals with distinct bending directions, speeds, and repeatability

Abstract Photomechanical crystals are fascinating candidates for their applications to light-fueled actuators. Over the past 2 decades diverse examples have been reported. However, research that revealed the structure–photomechanical behavior relationships is rare. Here, we focused on 3 polymorphs of a salicylideneaniline derivative with a 3-carboxy group in the enol form, 1α, 1β, and 1γ. All polymorphic crystals were stable in air at room temperature and underwent reversible photoisomerization in the solid state. Absorption spectrum measurements revealed that the γ form, previously considered as nonphotochromic, is weakly photochromic. Upon ultraviolet light irradiation, thin 1α and 1β crystals bent away from the light source quickly and slowly, respectively, through photoisomerization while a thin 1γ crystal bent toward the illumination direction by the same mechanism. The 1α and 1β crystals exhibited high repeatability for photomechanical bending, maintaining their original shape over 100 cycles. In contrast, the 1γ crystal gradually deformed during repeated bending because of its plasticity due to the slip planes. Besides the 1α crystal underwent an irreversible thermal phase transition to the 1β and 1γ crystals upon heating. This work is expected to highlight the relationship between structures and actuation performance, especially bending repeatability, of photomechanical crystals.