Protic Ionic Liquids Can Be Both Free Proton Conductors and Benign Superacids

Abstract

Superacids have been the source of much spectacular chemistry but very little interesting physics despite the fact that the states of cations formed by transfer of the superacid proton to molecular bases can approach that of the cations in free space. Indeed, some of the very strongest acids, such as HPF6 and HAlCl4, have no independent existence due to lack of screening of the bare proton self-energy: their acidities can only be assessed by study of the conjugate bases. Here we show that, by allowing the protons of transient HAlCl4 and HAlBr4 to relocate on pentafluoropyridine, PFP (a very weak base that is stable to superacids), we can create glass forming protic ionic liquids (PILs) that are themselves superacids but, being free of superacid vapors, are of benign character. At Tg, conductivities exceed "good"ionic liquid values by 9 decades, so must be superprotonic. Anomalous Walden plots confirm superprotonicity.