Porosity regulation of metal-organic frameworks for high proton conductivity by rational ligand design: mono- versus disulfonyl-4,4′-biphenyldicarboxylic acid

Porous crystalline metal-organic frameworks (MOFs) bearing sulfonic groups (-SO3H) are receiving increasing attention as solid-state proton conductors because the -SO3H group can not only enhance the proton concentration, but also form hydrogen bonding networks for high proton conductivity. A large number of 1,4-phenyldicarboxylic acids or biphenyldicarboxylic acids bearing two -SO3H groups have been applied for the synthesis of proton-conducting MOFs. Surprisingly, 4,4'-biphenyldicarboxylic acid bearing one -SO3H group has never been explored for the construction of proton-conducting materials. Herein, we first designed and synthesized 2-sulfonyl-4,4'-biphenyldicarboxylic acid (H3L). By applying this ligand to react with lanthanide salts, a series of three-dimensional MOFs, (Me2NH2)(2)(H3O)[LnL(2)]center dot 8H(2)O (Ln = Eu (1), Gd (2), Tb (3)) have been prepared. Due to the presence of the uncoordinated -SO3H group and the encapsulation of high concentrations of dimethylammonium and hydronium cations in the cavity, the MOFs 1-3 show a high proton conductivity (8.83 x 10(-3) S cm(-1)) at 95 degrees C and 60% relative humidity (RH). More importantly, this high proton conductivity can be maintained over 72 hours without any significant decrease at low RH.