Thermotropic ionic liquid crystals by molecular assembly and ion pairing of 4,4′-bipyridinium derivatives and tris(dodecyloxy)benzenesulfonates in a non-polar solvent

Abstract

The present work reports a simple and versatile pathway to incorporate redox-active 4,4′-bipyridinium (“viologen”) units with two, three, or six charges into dynamically ordered liquid crystalline structures by ionic interactions. A detailed analysis of the structure–property relationship was supported by molecular modelling experiments. Three redox-active 4,4′-bipyridinium (“viologen”) cores, 1,1′-dimethyl (4-pyridin-4-ylpyridinium) di- (MV2+2+2+2+), 1,1′,1′′-[benzene-1,3,5-triyltris(methylene)]tris(4-pyridin-4-ylpyridinium) tri- (V3+3+3+3+3+), and 1,1′,1′′-[benzene-1,3,5-triyltris(methylene)]tris[(methyl) 4-pyridin-4-ylpyridinium] hexa- (MV6+6+6+) cationic halides, were combined with an anionic caesium 3,4,5-tris(dodecyloxy)benzene sulfonate (DOBS) under phase transfer conditions to yield complexes of constant stoichiometry. The formed complexes, MV2+2+2+(DOBS)222, V3+3+3+(DOBS)333 and MV6+6+6+(DOBS)666, were characterized by NMR spectroscopy. Depending on the molecular structure, significantly different characteristics in electrochemical behaviour of ionic liquid crystals (ILCs) with two DOBS anions and a rod-like core (MV2+2+2+2+) were observed compared with ILs with three and six DOBS anions and y-shaped cores (V3+3+3+3+3+), and (MV6+6+6+) cations respectively. We report for the first time a new electroactive thermotropic liquid crystal, MV2+2+2+(DOBS)222, which changes colour from yellow to blue-green above 75 °C typically for redox units in the reduced state. The reduced states of the MV2+2+2+(DOBS)222 complex at E = −2 V exhibit reversible redox waves and have influence on molecular arrangement in the crystal structure as seen by polarisation microscopy. Our ionic liquid crystals have potential applications, due to their low melting temperature, wide temperature range, ionic conductivity, and thermal- and electro-chromism, for instance, for the design of electrochromic displays.