Photoactuators Based on Plastically Flexible α‐Cyanostilbene Molecular Crystals Driven by the Solid‐State [2+2] Cycloaddition Reaction

Abstract

Organic photomechanical molecular crystals are promising candidates for photoactuators, which have potential applications as smart materials in various fields. However, it is still challenging to fabricate photomechanical molecular crystals with flexibility because most of the molecular crystals are brittle and the mechanism of flexible crystals remains controversial. Here, a plastically flexible α‐cyanostilbene crystal has been synthesized that can undergo solid‐state [2+2] cycloaddition reaction under violet or UV irradiation and exhibits excellent photomechanical bending properties. A hook‐shaped crystal can lift 0.7 mg object upward by 1.5 cm, which proves its potential for application as photoactuators. When complex with the agarose polymer, the molecules will be in the form of macroscopic crystals, which can drive the composite films to exhibit excellent photomechanical bending performance. Upon irradiation with UV light, the composite film can quickly lift 18.0 mg object upward by 0.3 cm. The results of this work may facilitate the application of macroscale crystals as photoactuators.

Superelastic and Photomechanical Behavior of Acylhydrazone Derivative Crystals

“…Later, in 1964, Schmidt et al developed the concept of “topochemistry” by studying the solid-state photochemical reactions of cinnamic acid derivatives . Since then, the photomechanical effects of organic crystals have been extensively studied. − Many studies have described [2 + 2] cycloaddition reactions involving olefins, − isomerization processes such as trans ( E )– cis ( Z ) isomerization in diarylethenes or azobenzenes, − [4 + 4] photodimerization reactions occurring in anthracene, − and ring opening and closing of diarylethenes, − leading to photoinduced mechanical responses in organic molecular crystals.…”

“…Remarkably, due to the low light reaction efficiency, these crystals maintained their elastic bending properties even after exposure to light irradiation . Hao’s group reported a plastically flexible α-cyanostilbene crystal with excellent photomechanical bending properties . Meanwhile, other photomechanical movements of elastic crystals have been reported in recent years. − Based on these aforementioned studies, it is clear that the combination of flexibility and photomechanical properties in crystals is undeniably crucial for improving their survivability as photosensitive materials.…”