Polymorphism and\nSubstitutional Effect on the Thermoresponsive\nand Luminescence Properties of Organic Molecular Crystals

Smart molecular crystals that macroscopically respond\nto various\nexternal stimuli such as light, thermal, pressure, or humidity have\nemerged as potential candidates for their applications as actuators,\nsensors, optoelectronics, and other machinery devices. These materials\nhave the unique capability to transform stimuli-induced molecular\nresponse to bulk-scale property change. Compared to single-stimuli-responsive\ncrystals, developing multistimuli-responsive crystals that can respond\ndifferently to various stimuli is a challenging task. In this manuscript,\nwe report a new class of multistimuli-responsive acylhydrazone derivatives\nin which thermosalient (TS) actuation and solid-state luminescence\ncan be tuned using polymorphism and substitutional variation. The\ncrystals of methyl-substituted compound (<i>N</i>′-(4-(dimethyamino)benzylidene)-4-hydroxybenzohydrazide), <b>NMe</b><sub><b>2</b></sub><b>OH</b>, display prominent\nTS actuation and bright cyan luminescence in the solid state. On the\ncontrary, two polymorphs of the ethyl-substituted compound (<i>N</i>′-(4-(diethyamino)benzylidene)-4-hydroxybenzohydrazide), <b>NEt</b><sub><b>2</b></sub><b>OH-FL (</b>Fluorescent<b>)</b> and <b>NEt</b><sub><b>2</b></sub><b>OH-NFL</b> (Nonfluorescent), exhibit drastically different thermal and optical\nresponses. While <b>NEt</b><sub><b>2</b></sub><b>OH-FL</b> crystals display fascinating TS actuation and luminescence behavior,\nthe <b>NEt</b><sub><b>2</b></sub><b>OH-NFL</b> crystals\nare nonthermosalient and nonluminescent. With the aid of the single-crystal,\npowder X-ray diffraction study, we have established that distinct\nstimuli-responsive properties of the crystals are rooted in their\nmolecular conformations and crystal packing. Our work provides an\neffective approach to tune the stimuli-responsive behavior of molecular\ncrystals for various technological applications.