Synthesis and ferroelectric behaviour of an axially symmetric octahedral [Cu6L8]12+ cage

Abstract

Metal-organic hybrid supramolecular architectures based on lighter transition metal ions are an emerging class of ferroelectric materials due to their highly crystalline nature and less toxicity. However, synthesizing polar metal-ligand assemblies is still challenging as well-defined design strategies are unknown for obtaining these crystalline solids in non-centrosymmetric structures. Herein, we report a new discrete octahedral metal-organic cage [Cu6(TPPA)8(H2O)12]·(NO3)12·32H2O] (1) by employing a tripodal phosphoramide ligand, [PO(NH3Py)3] (TPPA). Ferroelectric measurements on 1 showed improved polarization vs. the electric field (P-E) hysteresis loop characteristics than those observed for similar cages, with a sizable remnant polarization (Pr) value of 39.2 µC/cm2. The non-centrosymmetric structure of 1 can be tracked to the uniform rotation of the octahedra around the metal centre. At the same time, the polarization in the framework stems from the toggling of the disordered nitrate anions. The temperature-dependent dielectric constant measurements on 1 showed a desolvation-assisted dielectric relaxation behaviour, indicating the involvement of solvate molecules in establishing the long-range polar order.