Abstract
<jats:p>Mechanical flexibility in molecular crystals is widely attributed to the cooperative action of weak intermolecular interactions, with dispersion-driven π⋯π stacking. Here, we report a mechanically flexible zwitterionic polymorph of the drug clonixin (CLXZ) that challenges this paradigm. The flexible zwitterionic polymorph CLXZ is significantly more stable than the neutral and brittle polymorphs of this drug, with a lattice cohesive energy difference as high as ~100 kJ mol-1. This unusual stability difference originates from strong electrostatic interactions. Despite the strong electrostatic stabilization of the crystal, mechanical flexibility in CLXZ is rendered by anisotropic dispersion interactions. This contrasts with the interaction topologies observed in zwitterionic piezoelectric amino acid crystals. An unusually high spontaneous polarization of 11.6 µC cm-2 in the zwitterionic polymorph was revealed by X-ray quantum crystallography (QCr). The coexistence of elastic flexibility and intrinsic molecular charge separation produces pronounced piezoelectric (d33 ≈ 10.4 pm V-1), pyroelectric, and flexoelectric responses under bending-induced strain gradients in CLXZ. An energyharvesting device fabricated from CLXZ crystals generates output voltages of up to ~75 V with a power density of 255 µW cm-2. Together, these multifunctional responses establish zwitterionic pharmaceutical solids as a promising class of materials for self-powered implantable bioelectronic devices.</jats:p>