Experimental confirmation of proton conductivity predicted from intermolecular hydrogen-bonding in spatially-confined novel histamine derivatives

The new compounds with one (N-(2-(1H-imidazole-4-yl)ethyl)benzamide; A1), two (N,N′-bis(2-(1H-imidazole-5-yl)ethyl)terephthalamide; A2) and three (N,N′,N″-tris(2-(1H-imidazole-5-yl)ethyl)benzene-1,3,5-tricarboxamide; A3) histamine units were synthesized with an expectation of the realization of high proton conductivity in a high temperature region in the presence of imidazole groups. The X-ray crystal structure analysis revealed the network structures of the strong intra- and intermolecular hydrogen bonds. The thermal degradation temperatures were much higher than those for the original histamine. The proton conductivities of the single crystals were measured in a wide temperature range. The results of these three compounds were shown at different temperatures. Referring to the crystal structure, the water molecules are confined in a local position and the protons cannot spread freely over the whole crystal lattice. In this way, the information from X-ray analyzed crystal structure allowed us to predict the proton transfer behaviors of these compounds, which were confirmed by performing the proton conductivity experiment.