New organic ionic plastic crystals based on pyrrolidinium dication for a solid-phase electrolyte

A series of 1,4-bis(N-alkylpyrrolidinium)butane salts combined with Br-, I-, PF6-, and Tf2N- anions were newly designed and synthesized. In nonpolar CDCl3 solution, the axial and equatorial proton peaks of the bis-pyrrolidinium cation ring are distinguished in H-1 NMR. Also, the strengths of ion paring could be estimated from the H-1 NMR chemical shift changes of the bridge and pyrrolidinium proton peaks. 1,4-Bis(N-hepthylpyrrolidinium)butane I- is an organic ionic plastic crystal showing the lowest fusion entropy value (delta S-f = 3.4 J Kmiddotmol(-1)). Softer crystalline morphologies were found by polarized optical microscope images at plastic crystal phase of each sample and the results were corresponding to differential scanning calorimetry results. 1,4-Bis(N-ethylpyrrolidinium)butane Tf2N- showed the softest crystal phase. Ionic conductivity changes and electrochemical stabilities of the bis-pyrrolidinium PF6- and Tf2N- salts were also measured, and they were stable up to 4.3 V (vs. Li/Li+) and expected to be applied as an electrolyte component.