Three Hydrogen-Bonded Organic Frameworks with Water-Induced Single-Crystal-to-Single-Crystal Transformation and High Proton Conductivity

In the development of proton conductors, it is very important to increase the density of proton carriers to adjust proton conduction pathways. In this work, two novel isomeric hydrogen-bonded organic frameworks, [(HDATA)2(H2BPYBPA)­(H2O)2] (UPC-H1 and UPC-H2), together with their dehydrated counterpart, [(HDATA)2(H2BPYBPA)] (UPC-H3), were successfully assembled from acidic H4BPYBPA and alkaline DATA. Single crystal X-ray diffraction analysis clearly reveals the three-dimensional lattice water molecules-involved hydrogen-bonded networks for UPC-H1 and UPC-H2 and no lattice water molecule-involved hydrogen-bonded network for UPC-H3. Detailed structural analysis with the aid of water adsorption tests discloses their easy single-crystal-to-single-crystal (SCSC) transformation and good self-adaptability to water molecules through hydrogen-bonded reorganization under the help of electrostatic interactions. Owing to the lack of restriction from the lattice water molecules in UPC-H3, the adsorbed water molecules more easily reorganize hydrogen bonds to form smooth proton conduction pathways, endowing it good proton conductivity over a wide temperature range from 30 to 80 °C at 95% relative humidity (RH), with the highest value of 9.0 × 10–2 S cm–1 at 80 °C and 99% RH, despite the lack of channels and permanent voids in its crystal structure.