Developments of Anhydrous Super-protonic Conductors Based on the Molecular Internal Degrees of Freedom

Although the development of anhydrous super-protonic conductors are highly demanded, theory of protonics has not been established yet. This account describes our recent fundamental research on anhydrous proton conduction using acid–base-type molecular single crystals, which enable the precise elucidation of intrinsic structure–property relationships. Especially, we have focused on the molecular internal degrees of freedom such as molecular motions and proton tautomerism, which are related to intra-molecular proton transfer in the Grotthuss-type conduction mechanism, to realize anhydrous super-protonic conductors. We have achieved super-protonic conduction with significant three-dimensional molecular rotational motions in a crystal. Furthermore, we demonstrated control over the molecular motion and conductivity via chirality. We have also proposed a novel, low-barrier conduction mechanism based on proton tautomerism, which circumvents the need for molecular motions, providing new design principles for advanced protonic materials.