Ligand design strategy to construct metal-organic frameworks with high-density carboxylic groups and high protonic conduction

Nowadays, the synthesis of crystalline solid materials with high proton conductivity, and excellent stability has become an important research topic. Bearing this in mind, we select a multifunctional organic ligand, 4-(2,4,6-tricarboxyphenyl)-2,2′,6,2″-terpyridine (H3tcp) that holds three carboxylate and three pyridine units to solvothermal prepare two one-dimensional MOFs, {[Cu(H1.5tcp)Cl0.5(H2O)0.5]·0.2H2O}n (1) and {[Cu(H2tcp) (H2O)]·1/2SO4·2H2O}n (2). Note that both the two MOFs contain a large amount of free carboxylate units and complicated H-bonded systems benefiting the proton transmission. AC impedance analyses demonstrated that their highest H2O-assisted proton conductivities (σ) could be 4.45 × 10−4 and 4.23 × 10−4 S cm−1 at 90 °C and 98% RH, respectively. Apparently, the high σ values are due to the efficient proton hopping across the H-bonding network built by the adsorbed H2O molecules and chemical composition such as, free –COOH and crystalline solvent units inside the frameworks. Furthermore, the lower Ea values of 1 (0.22 eV) and 2 (0.13 eV) under 98% RH confirm that the protons can effectively hop along the H-bonded networks. This research offers a new inspiration for the preparation of high proton conducting MOFs with free carboxylic groups.