High And Tunable Proton Conduction In Six 3d-Substituted Imidazole Dicarboxylate-Based Lanthanide-Organic Frameworks

Six isostructural three-dimensional (3D) Ln(III)-organic frameworks, {[Ln(2)(HMIDC)(2)( mu(4)-C2O4)(H2O)(3)]center dot 4H(2)O} n [Ln(III) = Gd-III (1), Eu-III (2), Sm-III (3), Nd-III (4), Pr-III (5), and Ce-III (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 (sigma) having the optimal values of 2.01 x 10(-3), 1.40 x 10(-3), 0.93 x 10(-3), 2.25 x 10(-4), 1.11 x 10(-4), and 0.96 x 10(-4) S.cm(-1) for 1-6, respectively, at 100 degrees C/98% relative humidity, in the order of Ce-III (6) < Pr-III (5) < Nd-III (4) < Sm-III (3) < Eu-III (2) < Gd-III (1). In particular, the s 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 lanthanidecontraction 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.