Guest-induced proton conductivity of two-dimensional layered hydrogen-bonded organic frameworks

Abstract

It is a widely held view that the continuous hydrogen bonding network plays a crucial role in proton transport. Herein, we report the cyclic single-crystal-to-single-crystal transformation of three hydrogen-bonded organic frameworks (HOFs), BPPA, BPPA-azo, and BPPA-gua, induced by the change of auxiliary ligands, which explicitly demonstrates that the hydrophilic interlayers of BPPA possess good self-adaptability to guest molecules. Their proton conducting performance was studied under different humidities at varying temperatures, indicating that the proton conductivity of BPPA with continuous hydrogen bonding networks formed by water molecules reaches 5.14 × 10⁻² S cm⁻¹ under 80 °C and 95% relative humidity (RH), which is four orders of magnitude higher than those of BPPA-azo and BPPA-gua with their continuous hydrogen bonding networks formed by 1,2,4-triazole and guanidine molecules, respectively, under the same conditions. The evidence from this study suggests that the rotational degrees of freedom of hydrogen bonding donors and acceptors play the most important role in the construction of efficient proton conduction pathways.