Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction.

Abstract

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.