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

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(ΔS_ss)) 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⁺).