A Well-Established POM-based Single-Crystal Proton-Conducting Model Incorporating Multiple Weak Interactions

Three new proton conductors with simple structures based on isolated olyoxometalate anions as well as protonated imidazole and benzimidazole, namely, NNU-6–8, have been successfully prepared by hydrothermal reaction. We could control the number of proton sources by selecting different types and changing the charges of POM anions. The single crystal sample of NNU-6 along a-axis shows a highest proton conductivity of 1.91×10⁻² S cm⁻¹, which is two and three orders of magnitude higher than that of 2.42×10⁻⁴ and 8.90×10⁻⁵ S cm⁻¹ along b- and c-axes, respectively, due to the more unobstructed H-bonding network and stronger π–π stacking between benzimidazole rings as proton-transferring pathway along a-axis than that along b and c axes. It is a straightforward model to understand the metaphysical proton-conducting process, and this is the first time to put forward the idea that π–π stacking could assist proton transfer and be in favor of proton conduction, which has been demonstrated by calculating potential energy surfaces of proton transfer between benzimidazole molecules.