Grafting Free Carboxylic Acid Groups onto the Pore Surface of 3D Porous Coordination Polymers for High Proton Conductivity

Development of proton conduction materials with a high performance via functionalization of porous coordination polymers (PCPs) is challenging yet promising. Herein, a facile and efficient method to synthesize multicarboxylate-group-functionalized rigid ligands has been developed. Interestingly, when utilizing these ligands to prepare PCPs, the peripheral carboxylate groups of the ligands selectively coordinate to the metal ions for constructing the PCP frameworks while ortho-dicarboxylic acid groups on the central benzene ring remain free in the pores. Single-crystal X-ray diffractions and various gas sorption analyses reveal the characteristics of high-density free carboxylic acid groups in the pores of these PCPs, not only forming hydrogen-bond clusters and three-dimensional hydrogen-bond networks potentially, but also stimulating the flexibility of these very robust PCP frameworks. Based on this unique structure, the PCP functionalized with four free carboxylic acids on every ligand (TJU-102) exhibits a high proton conductivity of 5.26 × 10^-2 S cm^-1 at 90 °C and 98% relative humidity. The activation energy for proton conduction of TJU-102 is ca. 0.59 eV, suggesting that the conduction mechanism is predominantly vehicular.