Highly Proton-Conducting Mixed Proton-Transferred [(H2PO4–)(H3PO4)]∞ Networks Supported by 2,2′-Diaminobithiazolium in Crystals

Hydrogen-bonding organic acid–base salts are promising candidates for the chemical design of high-performance anhydrous proton conductors. The simple molecular crystals between the π-planar molecules of 2,2′-diaminobithiazolium (DABT) derivative and hydrogen-bonding H3PO4 formed the proton-transferred salts with proton conductivities above ∼10–4 S cm–1 and anisotropic behavior. Controlling the crystallization condition facilitated the formation of binary salts between di-cationic H2DABT2+ and (H3PO4–)2 or mixed proton-transferred (H2PO4–)2(H3PO4)2 with different hydrogen-bonding networks, including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) networks. The structural isomers of 2,2′-diamino-4,4′-bithiazolium (2,4-DABT) and 2,2′-diamino-5,5′-bithiazolium (2,5-DABT) formed a different type of packing structure even with the same crystal stoichiometry of (H2DABT2+)­(H2PO4–)2 and/or (H2DABT2+)­(H2PO4–)2(H3PO4)2 where the latter salt had different protonated species of H2PO4– and H3PO4 in the hydrogen-bonding network. Four and 10 protons per H2DABT2+ molecule (H+: carrier concentration) were present in the (H2DABT2+)­(H2PO4–)2 and (H2DABT2+)­(H2PO4–)2(H3PO4)2 salts, respectively, which accounted for the highly proton-conducting behavior in the latter mixed protonated crystal. To design anhydrous intrinsic H+ conductors, both the mixed proton transfer state and uniform O–H···O hydrogen-bonding interaction are essential factors that must be considered.