Fast photoactuation of elastic crystals based on 3-(naphthalen-1-yl)-2-phenylacrylonitriles triggered by subtle photoisomerization

A series of ( Z )-3-(naphthalen-1-yl)-2-phenylacrylonitrile derivatives have been synthesized via the Knoevenagel condensation reaction. The photoisomerization takes place in the molecular crystals, which triggers the crystals to bend backward from the light source rapidly. In particular, significant photomechanical motions are observed in the initial several seconds of UV irradiation, during which the conversion ratio from Z - to E -isomer is less than ca. 0.4%. Such fast photoactuation induced by subtle photoisomerization might originate from the relatively large change in the molecular width after the photochemical reaction and strong intermolecular interactions to amplify the deformation at the molecular level into the macroscopic mechanical motion of the crystals. Therefore, the crystals performed reversible photo-induced bending and unbending by turning around the irradiation directions more than 100 times on account of the retained crystal integrity during photoactuation and the good elasticity of the crystals. The needle-like crystals also exhibit photomechanical effects in water. Such photochemical reaction-powered photomechanical molecular crystals may be employed as actuators in many practical application scenarios. The subtle photoisomerization of diarylacrylonitrile crystals can trigger rapid and significant mechanical motion, during which the crystal integrity is retained, favoring reversible bending and unbending for more than 100 times without fatigue.