Ion transport in triphenylene metal–organic columnar mesophases

Abstract

Ionic liquid crystals (iLCs) constitute an emerging materials type that combines the properties of an ionic liquid, such as ionic mobility, with the supramolecular organization of a liquid crystal. This makes them suitable candidates for applications such as anisotropic ion conductors and next-generation energy storage systems. In this work, we report ion conducting columnar mesophases based on bis(triphenylene-isocyanide) metal complexes of the type [M(CNR)₂]Y (CNR = 2-(6-(4-isocyanophenoxy)hexyloxy)-3,6,7,10,11-pentakisdodecyloxy-triphenylene; M = Ag(I), Au(I); Y = NO₃⁻, BF₄⁻). All the gold and silver complexes display enantiotropic mesomorphism in the temperature range 35–80 °C. They self-assemble into rectangular columnar mesophases, whose X-ray diffraction profiles reveal the simultaneous stacking of triphenylene disks into one-dimensional columns and the aggregation of metallic fragments assembled into filaments that are arranged parallel between the triphenylene columns. The ionic conductivity of these mesophases is based on the counterion mobility and increases regularly as the molecular slice thickness increases along the columns, ranging from 1.70 × 10⁻⁹ to 1.64 × 10⁻⁸ S cm⁻¹. Although the conductivity values obtained so far are lower than those required for commercial displays, our results demonstrate the key role of packing efficiency in ion transport in soft materials, and the potential of organic/inorganic nanosegregated mesophases to develop tailored and advanced ion-conducting materials. The gold and silver complexes display fluorescence behavior related to the triphenylene core in solution.