From flexible crystals to piezoelectrics: The advent of a new class of flexible functional molecular materials

The recent discoveries of mechanically flexible molecular crystals have fuelled a resurgence of research interest in molecular piezoelectrics. This has raised the quest to explore structure-property relations in molecular piezoelectric crystals, which remain largely obscure. In this article, we discuss the fundamental structural features associated with organic molecular piezoelectric crystals in relation to their mechanical and supramolecular flexibility. Along with the electrostatic properties such as molecular dipole moments and spontaneous crystal polarization, we have explored possible correlations of piezoelectric coefficients with intermolecular interaction topologies and their anisotropy – features often pointing towards mechanical flexibility in molecular crystals. In addition, the possible roles of crystal packing efficiency, lattice cohesive energies, Young’s moduli, and its anisotropy from elastic tensors have been examined. Our quantitative overview suggests that piezoelectric response in molecular materials is a complex interplay of several structural and electrostatic factors. Based on these analyses and the fundamental aspects of electromechanical coupling, it becomes apparent that combining mechanical flexibility and supramolecular chirality/polarity can be a promising approach to discovering soft molecular piezoelectrics for novel actuators and energy-harvesting materials.