A Mechanism for Reversible Solid-State Transitions Involving Nitro Torsion

Reversible solid-state transformations are important in stimuli-responsive materials. The current understanding is limited on what kind of structure enables reversible transitions. We report that for molecular solids containing nitro groups, reversible phase transitions can occur if the nitro group is a hydrogen bond (HB) acceptor and has torsional freedom. We use the polymorphs of nifedipine (NIF) to illustrate this phenomenon taking advantage of their different molecular packing but identical chemical structure. NIF has six known polymorphs with four being kinetically stable at 100 K. Upon heating, two polymorphs undergo reversible phase transitions with large volume change, while the others do not. In the transforming structures, the nitro group is an HB acceptor and rotates to optimize HBs to offset loss of close packing, while in the inactive structures, the nitro group has similar torsional freedom but is not engaged in HBs. We test the generality of this phenomenon using all available systems in the Cambridge Structural Database, including NIF's derivative nisoldipine, and suggest possible applications in designing materials with controlled mechanical response.