Self-assembly, phase behaviour and ion-conducting property of pyridinium-based ionic liquid-crystalline oligomers

Series of oligomeric ionic liquid crystals (OILCs) based on various anions including bromide (Br−), tetrafluoroborate (BF4−), dodecylbenzenesulphonate (A-SO3−) and (+)-10-camphorsulphonate (D-SO3−) are synthesised by use of pyridine-containing monomers, terminal bromo-substituted alkanes and anion replacement reagents. The effect of anions on the chemical structure, liquid-crystalline behaviour, and ionic conductivity is investigated. The OILCs show various mesophases including smectic, hexagonal columnar and bicontinuous cubic phase due to the change in anions. These mesophase types are determined by two incompatible segments derived from ionic mesogens and flexible alkyl chains in the microstructure. For the OILCs based on A-SO3− and D-SO3− anions, a curvature of the interfaces between the layers arises due to the increased size of one segment, resulting in columnar and cubic mesophases. The ionic conductivity is investigated in the way that a lithium ion solution passing through the oriented OILC films at room temperature. For the OILC films with various anions, ionic conductivities of the measuring system increase a little in the order A-SO3− > D-SO3− > BF4−> Br−. The values based on A-SO3− anion films show nearly twofold increase compared with those based on Br- anions, which can be attributed to the effects of both ion migration and ionic channels in the microstructures.