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

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 [Ln^III = Tb^III (Tb-MOF), Eu^III (Eu-MOF), Sm^III (Sm-MOF), and Dy^III (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.