Dicationic pyridinium salts as new organic ionics: Changes in solid-state phases and thermal/electrochemical properties

Abstract

To explore a new family of organic ionic materials, a series of α,ω-bis[N,N′-(4-alkylpyridinium)]alkane salts combined with iodide (I−), tetrafluoroborate (BF4−), and bis(trifluoromethanesulfonyl)imide (Tf2N−) anions and alkylene bridges of different lengths are synthesized. From 1H NMR chemical shift changes, the proton acidity of at the bis-pyridinium cations can be estimated for different anions. All the synthesized I− and BF4− salts have one or multiple solid–solid phase transitions, which are typical features of plastic and liquid crystals. The I− or BF4− salts with large values (>90 J mol−1 K−1) of total entropy changes in solid–solid phase transitions (Σ(ΔSss)) would have a very soft crystalline phase. Combined with polarized microscope images and wide-angle X-ray diffraction spectra, 1,2-bis[N,N′-(4-n-dodecylpyridinium)]ethane 2BF4– may be a true plastic crystal at temperatures above 70 °C. The thermal properties of the bis-pyridinium Tf2N− salts are quite different; they show only a melting transition and are thermally more stable than those of I− and BF4−. The electrochemical window of the bis-pyridinium BF4– and Tf2N− salts is stable up to 4.3 V (vs. Li/Li+).