Giant thermal expansion associated with a macroscopic polarization change in a single crystal of a Zn(ii) complex

In this study, we prepared a polar flexible single crystal of [Zn(4-ethylpyridine)2(NCS)2]. This material shows unusually large anisotropic linear thermal expansion as the tetrahedral coordination geometry of the complex can be modified in response to the thermal dynamic state of the ethyl group in 4-ethylpyridine ligands. Correspondingly, a giant thermally controlled reversible shape change of 10% elongation/contraction is observed in the rod-like single crystal along its long axis. Such a large thermal mechanical deformation is among the largest values in the reported thermal dynamic single crystals. Because the magnitude of dipole moment of uniaxially aligned molecules is varied by the large molecular structural change, the polarization of the single crystal is switched and manifests as a continuous pyroelectric current, particularly in the vicinity of phase transition points where molecular geometries are largely tuned. This study demonstrates that the large thermal expansion of flexible single crystals can be used to control the polarization of materials, providing a promising strategy to prepare superior pyroelectric materials.