Anion-induced ferroelectric polarization in a luminescent metal-organic cage compound

Metal-organic crystalline solids with ferroelectric properties have attracted significant attention recently as materials for high-tech applications. Here, we describe two crystalline assemblies that contain cationic metal-organic cages {[Zn-6(H2O)(12)][TPTA](8)}(NO3)(12)center dot 26H(2)O (1) and {[Zn-6(H2O)(12)][TPTA](8)}(ClO4)(12)center dot 18.75H(2)O (2) featuring the tripodal ligand [PS(NH3Py)(3)] (TPTA). Ferroelectric measurement on a single crystal of 1 gave a remnant (Pr) polarization of 1.2 mu C cm(-2) at room temperature. The ferroelectric response originates from the toggling of nitrate anions and solvate molecules found in pockets between the cages. The temperature-dependent permittivity of 1 shows an anomalous dielectric peak at 20 degrees C. This is attributed to the desolvation-assisted dielectric relaxation behaviour and signifies the role of solvate molecules in the ferroelectric behaviour of 1. In addition, this material is highly luminescent exhibiting a bright blue emission under UV light. This multifunctional behaviour is unique among metalorganic cage frameworks.