Li+ and K+ ionic conductivity in ionic nematic liquid crystals based on 18-diaza-crown ether substituted with six decylalkoxy-p-cyanobiphenyl chains

Abstract

Two macrocycles containing six decylalkoxy or decylalkoxy cyanobiphenyl chains symmetrically distributed around an 18-diaza-crown ether core have been prepared. The one with decylalkoxy cyanobiphenyl chains and its adducts with KI, K₂[PtCl₄], K[AuCl₄], 1/2 K[AuCl₄], K₄[Fe(CN)₆] or LiI for the second macrocycle are enantiotropic liquid crystals that display a nematic mesophase with melting points in the range of 16.7–30.0 °C and clearing points in the range of 86.4–98.6 °C. The dielectric permittivity of the complexes has been determined via impedance spectroscopy in the solid, in the mesophase and in the isotropic liquid state, under a variable temperature between 160 K and 390 K. The equivalent ionic conductivity of these mesogens in their mesophases and as isotropic liquids is in the range of 4.01 × 10⁻⁷ to 9.67 × 10⁻¹⁰ S cm⁻¹ at 350 K, and in the range of 2.02 × 10⁻⁶ to 1.64 × 10⁻⁸ S cm⁻¹ at 390 K. The maximum conductivity values are obtained for the LiI derivative (4.01 × 10⁻⁷ S cm⁻¹ at 350 K and 2.02 × 10⁻⁶ S cm⁻¹ at 390 K) with an activation energy of 0.74 eV. Interestingly, the conductivity in the mesophase for the compound with 1/2 K[AuCl₄] (diaza-crown-ether : AuK = 2 : 1) is about 20 times higher than for K[AuCl₄] (diaza-crown-ether : AuK = 1 : 1), which supports the supposition that the presence of vacant diaza-crown-ether sites in the mesogen improves the conductivity for K⁺. A model of ionic conductivity by K⁺ or Li⁺ charge carriers explaining the experimental results as well as the effect of the counteranions is proposed, which considers as an important contribution the existence of momentary spontaneous alignment induced by the molecular shape of the compounds.