Lanthanide–Potassium Biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate Frameworks: Gas Sorption, Proton Conductivity, and Luminescent Sensing of Metal Ions

A novel sulfonate–carboxylate ligand of biphenyl-3,3′-disulfonyl-4,4′-dicarboxylic acid (H4–BPDSDC) and its lanthanide–organic frameworks {[LnK­(BPDSDC)­(DMF)­(H2O)]·x(solvent)}n (JXNU-2, where JXNU denotes Jiangxi Normal University, DMF indicates dimethylformamide, and Ln = Sm3+, Eu3+, and Pr3+) were synthesized and structurally characterized. The three isomorphous lanthanide compounds feature three-dimensional frameworks constructed from one-dimensional (1D) rod-shaped heterometallic Ln–K secondary building units and are an illustration of a Kagome-like lattice with large 1D hexagonal channels and small 1D trigonal channels. The porous material of the representive JXNU-2(Sm) has an affinity to quadrupolar molecules such as CO2 and C2H2. In addition, the JXNU-2(Sm) compound exhibits humidity- and temperature-dependent proton conductivity with a large value of 1.11 × 10–3 S cm–1 at 80 °C and 98% relative humidity. The hydrophilic sulfonate group on the surface of channels facilitates enrichment of the solvate water molecules in the channels, which enhances the proton conductivity of this material. Moreover, the JXNU-2(Eu) material with the characteristic bright red color shows the potential for recognition of K+ and Fe3+ ions. The enhancing Eu3+ luminescence with the K+ ion and quenching Eu3+ luminescence with the Fe3+ ion can be associated with the functional groups of the organic ligand.