Synthesis and thermal behaviour of liquid crystalline pyridinium bromides containing a biphenyl core

A range of new pyridinium bromides was synthesized by the quaternization of different substituted pyridines with a group containing a biphenyl core and alkyl chains of differing lengths. The phase behaviour of the pyridinium bromides was studied by differential scanning calorimetry, polarizing optical microscopy and powder X-ray diffraction. It is shown that pyridinium moieties, linked to a rod-like biphenyl core via an alkyl spacer, can form ionic liquid crystals. Unsubstituted pyridinium groups promote mesomorphism. Liquid crystalline phases are formed only from 2- and 4-ethyl substituted pyridinium groups with sufficiently long alkyl terminal chains and spacers; i.e. decyl chains on both sides of the biphenyl core. Both the substitution pattern at the pyridinium group and the alkyl chain length have an influence on the polymorphism of the smectic phases. 3,5-Dimethyl substitution hinders mesophase formation.