Prediction and Validation of Mechanical Flexibility in Molecular Crystals: Dispersion Interactions Dictate Bending

Mechanically flexible crystals are a rapidly growing class of functional molecular materials. Typically, such flexible crystals are discovered by serendipity. Herein, we have predicted mechanical flexibility in a series of molecular crystals based on a structure screening approach that combines interaction topology and the presence of nitrile···nitrile interactions-a supramolecular motif hitherto not associated with bending property. Further, we have experimentally validated plastic/elastic bending properties in a series of crystal structures thus predicted. However, four out of five of these crystals showed the bending direction along π···π stacking despite the fact that the direction of strongest interaction anisotropy was rendered by the nitrile···nitrile interaction motifs. This is contrary to the commonly perceived anisotropy model and underscores the dominant role of dispersion forces over the electrostatically stabilized motifs in dictating the bending phenomena in molecular crystals. The interaction energies of these motifs have been evaluated using accurate structures from X-ray quantum crystallography. Analyses combining elastic tensors, interaction anisotropy indices, thermal expansion studies, and high-pressure simulations quantify the relative roles of the nitrile···nitrile motif and π···π stacking in mechanical flexibility. Our results point to the possibility of expanding the realm of flexible molecular materials to novel structural types aided by predictive computational models.