Pressure and Temperature Induced Dual Responsive Molecular Crystals: Effect of Polymorphism

The rapid conversion of energy into mechanical motion is a crucial requirement in the design of advanced flexible sensors and optoelectronic devices. Responsive materials that show mechanical movement when exposed to different external stimuli such as heat, pressure, humidity, or light can serve as technomimetics in smart actuator and microrobotic devices. However, it is quite challenging to design molecular crystals which can respond to two or more external stimuli. Herein, we report dimorphic forms (1a and 1b) of 2-hydroxy-3,5-dibromobenzylidine-4-fluoro-3-nitroaniline (Crystal 1) where form 1a is brittle and form 1b is elastically bendable. Both polymorphs exhibit progressive thermosalient responses which are seemingly different in nature. One polymorph (1a) shows jumping, while the other (1b) shows bending. The property difference is solely attributed to the inherent crystal packing and linked to the differences in the molecular conformations, structural packing, and intermolecular interactions. 2-Hydroxy-3-bromo-5-chlorobenzylidine-4-fluoro-3-nitroaniline (Crystal 2) was nonthermosalient but was mechanically flexible. Variable temperature powder X-ray diffractometry revealed anisotropic crystallographic face expansion-a possible explanation for the different thermosalient behaviors of the dimorphs.