Thermosalient Effect in an Organic Salt Crystal Induced by Moiety Twisting

Molecular flexibility offers a powerful means to generate stimuli-responsive dynamic crystals. This study reports temperature-induced reversible dynamic properties of an organic salt crystal containing a conformationally flexible homopiperizine (HPZ) molecule. The salt crystals exhibit thermosalient properties, such as jumping, hopping, and fragmentation, followed by a phase transformation. Generally, the mechanism of thermosalient properties has been rationalized by anisotropic thermal expansion and phase transformation. In this study, the mechanism of the dynamic properties of the crystal has been elucidated by variable-temperature single-crystal x-ray diffraction (VT-SCXRD), which reveals twisting of HPZ resulting in generating strain in the crystal. Further, the dynamic behavior of HPZ has been observed by variable temperature solid state ¹³C CP-MAS NMR studies. The NMR spectrum revealed that in the absence of dynamicity of the crystals at room temperature and below to it, all peaks corresponding to carbon atoms were well resolved. However, merging of some of the peaks corresponding to HPZ moiety confirmed the dynamic effect manifested by salient behavior.