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.