Photomechanical macroscopic movements driven by photoinduced topochemical [2 + 2] cycloaddition reaction in tridentate cyanostilbene

Abstract

Photomechanical crystals are capable of converting light energy into mechanical energy, making them as potential candidates for a wide range of applications in areas such as actuators, artificial muscles, and optical switches. In this work, we successfully synthesized a tridentate cyano styrene derivative molecule, (2Z,2′Z,2′Z)-3,3′,3′-(benzene-1,3,5-triyl)tris-(2-(4-(trifluoromethoxyl)ph-enyl)acrylonitrile) (C3-CF₃), along with its three different polymorphs. Both blocky Form I and sheet-like Form II, exhibit photosalient effects (ballistic splitting, moving, and jumping) under both ultraviolet and visible lasers. More importantly, they could even respond to sunlight and light-emitting diode white light with a subdued photomechanical response. ¹H nuclear magnetic resonance indicates that Form I and Form II undergo photo-induced [2 + 2] cycloaddition reactions under various light sources. The kinetic analysis of the photoreactions reveals that both polymorphs demonstrate rapid reaction kinetics and high photodimerization conversion when exposed to 420 nm visible laser, with conversions reaching 97.6% and 95.26% after 300 s of irradiation, respectively. Additionally, although the needle-shaped Form III of C3-CF₃ lacks photomechanical activity, it possesses notable mechanical flexibility. Finally, films composed of powders from Form I or Form II with poly(vinyl alcohol) were created, which were capable of controllable photo-induced bending.