Anion-driven mesogenicity: a comparative study of ionic liquid crystals based on the [closo-1-CB9H10]− and [closo-1-CB11H12]− clusters

Abstract

A series of structurally analogous esters derived from monocarbaborates, [closo-1-CB9H10]− (A) and [closo-1-CB11H12]− (B), containing a maximum of 4 rings in the rigid core were prepared and investigated by thermal, optical, and XRD methods. Ion pairs containing a total of 4 rings with either N-butyl-4-heptyloxypyridinium (Pyr) or cetyltrimethylammonium (Cetyl) cation form a SmA phase ranging from 90 °C to 229 °C. Ion pairs 1e[Pyr] and 2e[Pyr] containing an azo group in the anisometric anion exhibit an enantiotropic nematic phase above the SmA phase. The cesium salt 1h[Cs] did not exhibit mesogenic behavior. Partial binary phase diagrams were investigated for 1d[Pyr] and 2d[Pyr] in 2e[Pyr], and for 1f[Pyr] in non-ionic mesogens. Variable temperature powder XRD analysis of 5 ionic liquid crystals, 1e[Pyr], 1f[Pyr], 2f[Pyr], 1g[Pyr], and 2g[Pyr], demonstrated about 20% interdigitation in the SmA phase and provided thermal expansion coefficients.