How much do coulombic interactions stabilize a mesophase? Ion pair and non-ionic binary isosteric derivatives of monocarbaborates and carboranes

Abstract

Replacement of the B⁻ atom in the monocarbaborate anion, 1[10] or 1[12], and the N⁺ atom in the pyridinium cation [Pyr] of a liquid crystalline ion pair with C atoms leads to an isoelectronic and isosteric non-ionic binary liquid crystalline mixture of carborane (2[10] or 2[12]) and benzene ([Ph]) derivatives lacking coulombic interactions. A comparison of mesogenic properties of ion pairs, 1[10]c–[Pyr]c and 1[12]c–[Pyr]c, with their analogous non-ionic mixtures, 2[10]c–[Ph]c and 2[12]c–[Ph]c, shows a 181 K higher clearing temperature, T_c, for the ion pair. This corresponds to a DFT-calculated difference in association energy ΔΔH_a = 24.5 kcal mol⁻¹ in a typical dielectric medium (ε = 2.5). Pure compounds and binary mixtures were characterized using thermal, optical, and XRD methods.