Two Hydrogen-Bonded Organic Frameworks with Imidazole Encapsulation: Synthesis and Proton Conductivity

On the basis of the carboxylic structure of trimesic acid (TMA) and 5-borono-1,3-benzenedicarboxylic acid (B-BDC), here two hydrogen-bonded organic frameworks (HOFs), HOF 1 (MA-TMA) and HOF 2 (MA-B-BDC), were prepared by reacting melamine (MA) with trimesic acid and 5-borono-1,3-benzenedicarboxylic acid, respectively. The as-prepared HOF 1 and HOF 2 with 3D supramolecular structure exhibit excellent performance in proton transport. AC impedance test shows that the proton conductivities of HOF 1 and HOF 2 reach 3.11 × 10–4 S·cm–1 (343 K, 98% RH) and 4.32 × 10–4 S·cm–1 (323 K, 98% RH), respectively. To increase the proton conductivity, Im@HOF 1 and Im@HOF 2 were obtained by introducing imidazole (Im), which acts as a jumping site for proton transfer. The proton conductivities of Im@HOF 1 and Im@HOF 2 reached 4.12 × 10–4 S·cm–1 (333 K, 98% RH) and 1.20 × 10–3 S·cm–1 (353 K, 98% RH), respectively. This is the first time that guest molecules were introduced into HOFs. The stability and proton conductivity of the HOFs are improved due to the interaction of imidazole with carboxyl groups and water molecules.