Water-Induced SCSC Transformation in Ionic Hydrogen-Bonded Organic Frameworks Enhances Single Crystal Proton Conductivity

The rich hydrogen bonding network of hydrogen-bonded organic frameworks (HOFs) provides a large number of effective sites for proton conduction. In this study, two examples of ionic HOF (iHOF) in ammonium arylphosphonate systems are designed via a charge-assisted strategy. Among these, iHOF-58 is formed through the alternating linkage of -PO 3 H – and -NH 3 + groups, resulting in a 1D chain hydrogen bonding network. Upon exposure to water, the hydrogen bonding network undergoes a transformation to form iHOF-59, which is composed of alternating links of phosphonate anions, amine cations, and water molecules, forming a more dense hydrogen bonding network. This confers upon it higher stability (electrostatic attraction between acid–base pairs) and more favorable sites and pathways for proton transport. Under conditions of 98% relative humidity (RH) and 80 °C, the a -axis of iHOF-59 single crystal exhibits an ultrahigh proton conductivity of 0.18 S cm –1 . This value is 2 orders of magnitude higher than both the conductivity perpendicular to the a -axis (⊥ a -axis) and the powder conductivity. It exhibits high anisotropy and confirms that the dense hydrogen bonding network along the a -axis serves as an efficient pathway for proton transport. This approach offers new insights into the design and development of high proton-conductivity HOFs.