Ultrahigh proton conductivity of four ionic hydrogen-bonded organic frameworks based on functionalized terephthalates

Recently, the utilization of hydrogen-bonded organic frameworks (HOFs) with high crystallinity and inherent well-defined H-bonding networks in the field of proton conduction has received increasing attention, but obtaining HOFs with excellent water stability and prominent proton conductivity (σ) remains challenging. Herein, by employing functionalized terephthalic acids, 2,5-dihydroxyterephthalic acid, 2-hydroxyterephthalic acid, 2-nitro terephthalic acid, and terephthalic acid, respectively, four highly water-stable ionic HOFs (iHOFs), [(C₈H₅O₆)(Me₂NH₂)]∙2H₂O (iHOF 1), [(C₈H₅O₅)(Me₂NH₂)] (iHOF 2), [(C₈H₄NO₆)(Me₂NH₂)] (iHOF 3) and [(C₈H₅O₄)(Me₂NH₂)] (iHOF 4) were efficiently prepared by a straightforward synthesis approach in DMF and H₂O solutions. The alternating-current (AC) impedance testing in humid conditions revealed that all four iHOFs were temperature- and humidity-dependent σ, with the greatest value reaching 10^-2 S·cm^-1. As expected, the high density of free carboxylic acid groups, crystallization water, and protonated [Me₂NH₂]⁺ units offer adequate protons and hydrophilic environments for effective proton transport. Furthermore, the σ values of these iHOFs with different functional groups were compared. It was discovered that it dropped in the following order under 100 °C and 98 % relative humidity (RH): σ iHOF 1 (1.72 × 10^-2 S·cm^-1) > σ iHOF 2 (4.03 × 10^-3 S·cm^-1) > σ iHOF 3 (1.46 × 10^-3 S·cm^-1) > σ iHOF 4 (4.86 × 10^-4 S·cm^-1). Finally, we investigated the causes of the above differences and the proton transport mechanism inside the framework using crystal structure data, water contact angle tests, and activation energy values. This study provides new motivation to develop highly proton-conductive materials.