Polymorphism and Substitutional Effect on the Thermoresponsive and Luminescence Properties of Organic Molecular Crystals

Smart molecular crystals that macroscopically respond to various external stimuli such as light, thermal, pressure, or humidity have emerged as potential candidates for their applications as actuators, sensors, optoelectronics, and other machinery devices. These materials have the unique capability to transform stimuli-induced molecular response to bulk-scale property change. Compared to single-stimuli-responsive crystals, developing multistimuli-responsive crystals that can respond differently to various stimuli is a challenging task. In this manuscript, we report a new class of multistimuli-responsive acylhydrazone derivatives in which thermosalient (TS) actuation and solid-state luminescence can be tuned using polymorphism and substitutional variation. The crystals of methyl-substituted compound (N′-(4-(dimethyamino)benzylidene)-4-hydroxybenzohydrazide), NMe₂OH, display prominent TS actuation and bright cyan luminescence in the solid state. On the contrary, two polymorphs of the ethyl-substituted compound (N′-(4-(diethyamino)benzylidene)-4-hydroxybenzohydrazide), NEt₂OH-FL (Fluorescent) and NEt₂OH-NFL (Nonfluorescent), exhibit drastically different thermal and optical responses. While NEt₂OH-FL crystals display fascinating TS actuation and luminescence behavior, the NEt₂OH-NFL crystals are nonthermosalient and nonluminescent. With the aid of the single-crystal, powder X-ray diffraction study, we have established that distinct stimuli-responsive properties of the crystals are rooted in their molecular conformations and crystal packing. Our work provides an effective approach to tune the stimuli-responsive behavior of molecular crystals for various technological applications.