Unusually Flexible Indium(III) Metal-Organic Polyhedra Materials for Detecting Trace Amounts of Water in Organic Solvents and High Proton Conductivity.

Humidity-induced single-crystal transformation was observed in the indium metal-organic polyhedra [In2(TCPB)2]·2H2O (In1), where H3TCPB is 1,3,5-tri(4-carboxyphenoxy)benzene. When the humidity is above 58% relative humidity (RH) at room temperature, the neutral compound In1 could be successfully converted into the positively charged compound In1-H along with the color change from yellow to deep red, which also undergoes a reversible transformation into In1 driven by thermal dehydration. Notably, the color of In1 takes only 5 min to change under 58% RH at room temperature, which is much quicker than common desiccant bluestone. As the water content is increased from 0.0% to 0.2% in acetonitrile solvent, compound In1 exhibits rapid detection of trace amounts of water through turn-off luminescence sensing mechanism with a low detection limit of 2.95 × 10-4%. Because of the formation of extensive hydrogen-bonding network between the metal-organic polyhedra (MOPs) and surrounding guest OH- ions, compound In1-H, along with isostructural Ga1-H, displays excellent proton conductivity up to 2.84 × 10-4 and 2.26 × 10-4 S cm-1 at 298 K and 98% RH, respectively. Furthermore, the activation energies are found to be 0.28 eV for In1-H and 0.34 eV for Ga1-H. This method of incorporation of OH- ions to obtain high proton conductivity MOPs with low activation energy demonstrates the advantage of OH- ion conduction in the solid-state materials.