Structural design and functional characterization of dicationic pyrazolium salts as organic ionic plastic crystals

Abstract

A series of dicationic pyrazolium hexafluorophosphate (PF₆⁻) salts connected by a butylene bridge were synthesized with side linear alkyl chains ranging from C₁ to C₁₂. Thermal analysis revealed that most compounds exhibit one or more solid–solid phase transitions (T_ss), and several compounds displayed clear characteristics of organic ionic plastic crystals (OIPCs), with entropy of fusion (ΔS_f) values below 40 J mol⁻¹ K⁻¹. Among them, 1,4-bis[N-(N′-octylpyrazolium)]butane PF₆⁻ exhibited the softest plastic crystal morphology, as confirmed by polarized optical microscopy (POM) and wide-angle X-ray scattering (WAXS), and achieved a high ionic conductivity of 1.31 × 10⁻³ S cm⁻¹ at 70 °C upon incorporation of 30 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTf₂N). These findings demonstrate that the structural tunability of dicationic pyrazolium salts plays a key role in modulating solid-state ionic conductivity, providing insights into the design of next-generation electrochemical materials.