Modulating Stimuli-Responsive and Fluorescence Properties in Organic Single Crystal via Solvent Strategy

Abstract

Multiple stimuli-responsive crystalline materials have attracted huge attention in versatile fields. Solvent strategy for regulating structures and properties is effective but remains underexplored. This work achieved the integration of multiple stimuli responses including photomechanical, thermomechanical, fluorescent, and chemical-chromic properties using solvent strategy. We synthesized [(5-cyanothiophene-2-yl) methylene]-4-(trifluoromethyl) benzohydrazide (CTB) and obtained its neat crystal (cryA) and two methanol solvates (cryB and cryB1). The two forms of CTB exhibit different photomechanical response, including curling (cryA), bending (cryB), and breaking (cryB) due to the distinctive molecular conformations and packing patterns. Although these photomechanical responses are all assigned to the E→Z isomerization, they display altered fluorescence properties. Especially for cryA and cryB, their photochromism can be restored by acid-induced fumigation, also via E→Z isomerization. Furthermore, the solvates exhibit unique thermomechanical behaviors: cryB jumps triggered by methanol desorption at higher temperature, whereas cryB1 jumps induced by anisotropic expansion at lower temperature. Finally, according to the properties of cryB, reversible color-switching demonstrations highlight its potential for applications in dynamic anticounterfeiting. This work demonstrates the ability of solvent strategy for diversifying smart crystalline materials.