Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]⁻) and various organic cations. In particular, the OIPC composed of [TCB]⁻ and spiro-(1,1′)-bipyrrolidinium ([spiropyr]⁺) cations can transform into its plastic phase at ultralow temperature (T_p = −55 °C) while maintaining a high melting point (T_m = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high T_m was attributed to the relatively small interionic distance between [spiropyr]⁺ and [TCB]⁻. At the same time, the low T_p was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.