Contribution of C−H⋯π Interactions to Polymorphic Transitions of a Molecular Crystal with Disordered Fragments

Polymorphic transition is important for the functionality of crystalline materials. However, the underlying mechanism remains unclear, especially when the crystal structure contains disordered fragments. We report that C−H⋯π interactions play an important role in polymorphic transitions in a molecular crystal with disordered fragments. The crystal has three phases, namely the a (< -80°C), β (-80-40°C), and γ (< 40°C) phases, which are reversible through single-crystal-to-single-crystal transformation in association with temperature change. Disorder of bulky tert-butyl substituents appears at high-temperature in the β and γ phases. Intermolecular interaction analysis based on Hirshfeld surfaces and related fingerprint plots revealed that the proportion of π⋯π interactions decreased, while that of C−H⋯π interactions increased, at the transition from a to β phase. The proportion of C−H⋯π interactions also increased at the transition from β to γ phase, but continuously decreased in the β phase due to elevated temperature. Intermolecular interaction energies clarified the contribution of C−H⋯π interactions to the stability of high-temperature crystal β and γ phases via polymorphic transitions. Our results potentially lead to design molecular crystals with polymorphic transitions.