Proton conduction in two Cu/Zn dimer-based hydrogen-bonded supramolecular frameworks from imidazole multi-carboxylate

Abstract

To date, the research on hydrogen-bonded supramolecular frameworks (HSFs) has been extremely limited. Herein, by hydrothermal reactions of CuCl₂·2H₂O or Zn(Ac)₂·2H₂O with 2-(o-carboxylic)phenyl-4,5-imidazole dicarboxylic acid (o-CPH₄IDC), two dimer-based HSFs, [Cu₂(o-CPH₂IDC)₂(phen)₂]·3H₂O (phen = 1,10-phenanthroline) (1) and [Zn₂(o-CPH₂IDC)₂(2,2-bipy)₂]·0.5H₂O (2,2′-bipy = 2,2′-bipyridine) (2) were assembled and characterized. The two HSFs demonstrated temperature- and humidity-dependent proton conductive properties. Moreover, at 98% RH and 100 °C, the highest σ values of 3.13 × 10⁻⁴ S cm⁻¹ for 1 and 0.55 × 10⁻⁴ S cm⁻¹ for 2 can be attained, which can be compared to the values of reported HSFs. Moreover, the proton transfer mechanisms of the two HSFs were highlighted. Obviously, the existence of free –COOH units and H-bonds in the frameworks is responsible for the proton transport. Our research will provide useful guidance for the design of proton conductive H-bonded materials.