High and Tunable Proton Conduction in Six 3D-Substituted Imidazole Dicarboxylate-Based Lanthanide-Organic Frameworks.

Six isostructural three-dimensional (3D) Ln(III)-organic frameworks, {[Ln2(HMIDC)2(μ4-C2O4)(H2O)3]·4H2O}n [LnIII = GdIII (1), EuIII (2), SmIII (3), NdIII (4), PrIII (5), and CeIII (6)], have been fabricated by using a multifunctional ligand of 2-methyl-1H-imidazole-4,5-dicarboxylic acid (H3MIDC). Ln-metal-organic frameworks (MOFs) 1-6 present 3D structures and possess abundant H-bonded networks between imidazole-N atoms and coordinated and free water molecules. All the six Ln-MOFs demonstrate humidity- and temperature-dependent proton conductivity (σ) having the optimal values of 2.01 × 10-3, 1.40 × 10-3, 0.93 × 10-3, 2.25 × 10-4, 1.11 × 10-4, and 0.96 × 10-4 S·cm-1 for 1-6, respectively, at 100 °C/98% relative humidity, in the order of CeIII (6) < PrIII (5) < NdIII (4) < SmIII (3) < EuIII (2) < GdIII (1). In particular, the σ for 1 is 1 order of magnitude higher than that for 6, and it enhances systematically according to the decreasing order of the ionic radius, indicating that the lanthanide-contraction tactics can effectively regulate the proton conductivity while retaining the proton conduction routes. This will offer valuable guidance for the acquisition of new proton-conducting materials. In addition, the outstanding water stability and electrochemical stability of such Ln-MOFs will afford a solid material basis for future applications.