Ferroelasticity versus superelasticity in molecular crystals: the role of weak switchable interaction motifs and low shear moduli

Molecular crystals that mimic metal alloys in mechanical properties, such as ferroelasticity and superelasticity, form an intriguing class of functional materials. The structural factors that lead to twinning deformation in ferroelastic crystals are not well understood. Here, we report ferroelasticity in crystals of 4-fluorobenzonitrile, in which an audible crackling sound, indicative of a collective and abrupt structural change, accompanies the twin domain transformation. The roles of specific interaction motifs, molecular rotational energy barriers, and shear moduli linked to this ferroelastic twin domain transformation have been examined. The crystal structure shows supramolecular motifs involving weak C–H⋯F and C–H⋯NC dipolar interactions, which are key structural features of this class of ferroelastic and superelastic crystals. Electronic features of these motifs have been characterized by X-ray quantum crystallography, which provides quantitative insights into these motifs that facilitate domain-switching/twinning. While elastic tensors and Young's moduli do not capture molecular orientational changes during the ferroelastic transition, our detailed analysis of stiffness constants, along with simulations of molecular rotational pathways and energy barriers, provides insight into the possible mechanism of ferroelasticity.