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 ¹H NMR. Also, the strengths of ion paring could be estimated from the ¹H 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 (ΔS_f = 3.4 J K·mol⁻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.