Enhancement of Ionic Conductivity in Organic Ionic Plastic Crystals by Introducing Racemic Ammonium Ions

An organic ionic plastic crystal (OIPC) consisting of racemic N1233'Tf2N (1) (N-1233' = NR1R2R3R4, R-1 = Me, R-2 = Et, R-3 = n-propyl, R-4 = isopropyl, Tf2N = bis(trifluoromethanesulfonyl)imide) is developed. 1 displays three solid phases in addition to the high-temperature liquid phase, and two intermediate phases are assigned to OIPC by DSC, powder XRD and solid-state NMR measurements. Ionic conductivity of 1 showed increase from the low-temperature region, and diffusion of Tf2N anions was confirmed even at -27 degrees C by NMR spectroscopy. This behavior is distinct from that of symmetric N1223'Tf2N (2), which showed much lower ionic conductivity at low temperatures. The introduction of racemic ions in OIPCs weakens the net intermolecular interactions and it enhances molecular motion and accordingly the ionic conductivity.