Reordering Frustration in Enantiomer-Enriched Solid Solutions of Ionic Plastic Crystals for Proton-Conducting Materials Design

A series of triflate (TFO^-) salts with various globular cations, namely, the achiral 3-quinuclidonium [QHco]⁺, the racemic (3)-hydroxyquinuclidinium S/R-[QH]⁺, and the enantiopure (3)-hydroxyquinuclidinium R-[QH]⁺, have been synthesized and characterized via a combination of X-ray diffraction and microcalorimetric techniques. Despite the similarity among their components, the three salts displayed different behaviors. At room temperature, the achiral [QHco]-TFO crystallized in an ordered phase, and no transition to the plastic phase was detected. In contrast, the racemic and enantiopure salts, namely, S/R-[QH]-TFO and R-[QH]-TFO at room temperature crystallized as disordered phases; only the latter underwent a transition from a plastic phase to an ordered one upon cooling, whereas the former remained permanently disordered, even at the lowest accessible temperature. The formation of solid solutions was investigated for the pair S/R-[QH]-TFO/R-[QH]-TFO, S/R-[QH]-TFO/[QHco]-TFO and R-[QH]-TFO/[QHco]-TFO and found to be successful only in the first case, whereas it led to physical mixtures in the other two. Notably, the S/R-[QH]-TFO/R-[QH]-TFO solid solution retained its disordered phase over a wide temperature range, a behavior attributed to a phenomenon we describe as "reordering frustration". Raman spectroscopy complemented and confirmed the structural analysis, and electrochemical impedance spectroscopy was successfully applied to study proton conductivity associated with temperature variations in the pure salts and S/R-[QH]-TFO/R-[QH]-TFO solid-solutions under inert and dry conditions.