Heterogeneous dynamics of diffusive motion in organic ionic plastic crystal studied using spin-spin relaxation time: N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide

The temperature dependences of the spin-spin relaxation times (T2) of 1H and 19F nuclei were measured for N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide with a plastic crystal phase. In the plastic crystal phase, 2 types of T2 were observed in both 1H and 19F experiments, which were considered to be the appearance of heterogeneous dynamics of diffusive motion. By examining temperature dependences of the T2 values and the existence ratios, the following conclusions were reached. (i) The prepared plastic crystal sample was in a polycrystalline state, and each crystallite comprised 2 phases: the core phase (plastic crystal phase) and the surface phase formed to relieve surface stress. (ii) The 1H-T2 (19F-T2) values of the 2 phases differed, and ions in the surface phase were more mobile. The 1H-T2 (19F-T2) values for the 2 phases increased with temperature rise. In particular, the 1H-T2 (19F-T2) values of the surface phase were smoothly connected to the liquid T2 values. (iii) The cations and anions exhibited a cooperative diffusive motion. (iv) When the temperature was considerably lower than the melting point, the ratio of the surface phase did not significantly differ from when it first formed. However, it rapidly increased near the melting point and became liquid. Graphical Abstract Free induction decay signal of 1H-NMR for N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide, [C2epyr][FSA], at 390 K. The chart shows that there are 2 types of diffusive motion of the [C2epyr]+ cations, namely, a hard component (green curve, ratio 0.844) and a soft component (orange curve, ratio 0.156).