Thermo-Induced Single-Crystal-to-Single-Crystal Transformations and Photo-Induced [2+2] Cycloaddition Reactions in Polymorphs of Chalcone-Based Molecular Crystals: Multi-Stimuli Responsive Actuators.

The polymorphs of 2ClChMe-4 in Form I (ribbon-like crystal) and Form II (block-like crystal) were prepared, and they exhibited curling/flipping and expansion upon heating on account of single-crystal-to-single-crystal transformations. The irreversible phase transformations occurred separately at 53.2 °C and 57.8 °C for the crystals in Form I and Form II, during which the molecular conformation of 2ClChMe-4 changed and the molecules slipped along the (100) plane. Movement at the molecular level resulted in changes of cell parameters, which in turn led to macroscopic motions of the crystals upon heating. Additionally, the ribbon-like crystals of 2ClChMe-4 showed photo-induced bending driven by [2+2] cycloaddition. Accordingly, an actuator showing reversible bending behavior was fabricated triggered by light and heat successively. Like biomimetic self-actuators, such multi-stimuli mechanical responsive molecular crystals might have potential applications in soft robots, artificial muscles and microfluidic systems.