A systematic study on alkyl urocanates related to the proton conductivity performances to clarify the role of molecular mobility and hydrogen bond in proton transfer is carried out. Depending on the methylene units, the melting (T-m) and degradation temperatures (T-d) change remarkably. When methylene unit is four, C4U shows the lowest melting point (as low as 46 degrees C) and this suggests the favorable molecular mobility in the molten state. The short hydrogen bond distance and the short T-1 relaxation time lead to a scheme of proton conductivity of C4U to be under a regular imidazole arrangement with highly active alkyl chain molecular motion. When C4U is in molten state, the proton transfer is under vehicle mechanism clarified by Volgel-Tammann-Fulcher (VTF) equation. By applying C4U as a proton conductive additive in a sulfonated poly(ether ether ketone) (SPEEK) membrane without any acid dopants, the proton conductivity in the heating process up to 170 degrees C continuously increases to be similar to 10(4) times higher than that of the neat SPEEK. The present work not only demonstrates the thermal mobility as a key factor to govern the proton conductivity but also proposes the effective proton transfer of heterocyclic compounds based on the molten state. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.