New ionic mesogens with high ionic conductivity based on a cyanoborate building block and lipophilic or hydrophilic counter ions

Abstract

New ionic mesogens have been prepared based on a cyanoborate head group, which is tethered to a lipophilic periphery via a short flexible spacer. As counter ions, either tetra-n-butyl ammonium cations or sodium and potassium cations stabilized by 18-crown-6 have been used. Although all compounds exhibit SmA enantiotropic mesomorphism, X-ray diffraction, modelling and solid-state NMR uncover distinct self-assembly of the LC materials in double layers. While the ammonium ions are located between the borate anions and are in contact with spacers and aromatic units, the crown ethers with the coordinated alkali metal cations are completely nanosegregated between two borate anion layers. Homogenous and homeotropic alignment has been achieved. Electrical impedance spectroscopy reveals ionic conductivity values in the single-ion conductor that are two to three orders of magnitude higher in the correctly-aligned samples. Values of ca. 10⁻⁴ S cm⁻¹ in the SmA phase at 85 °C and 10⁻⁵ S cm⁻¹ at 40 °C have been obtained.