Synthesis and characterisation of imidazolium-based ionic liquid crystals bearing a cholesteryl mesogenic group

A series of cholesteryl-containing imidazolium chlorides and imidazolium tetrachloroaluminates were synthesised, and the chemical structure, liquid crystalline behaviour and ionic conductivity were characterised by several technical methods. Whereas the imidazolium chlorides show chiral smectic A (SmA*) phase on heating and cooling cycles, the imidazolium tetrachloroaluminates display chiral nematic (N*) phase, which is uncommon for ionic liquid crystals (ILCs). The imidazolium chlorides display similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than the different alkyl substituent groups. The imidazolium tetrachloroaluminates show lower melting point temperatures and lower clear point temperature than the imidazolium chlorides. The mesophases exist at rather moderate temperatures. Non-mesomorphic imidazolium tetrachloroaluminate(III) salts with short alkyl substituents have been known for a long time, and the synthesised imidazolium tetrachloroaluminates are the first examples of tetrahalogenoaluminate(III)-containing ILCs. For the imidazolium tetrachloroaluminates, imidazolium cations combine loosely with AlCl₄^? ions because AlCl₄^? ions are large and occupy more space in spite of the hydrogen bond and electrostatic attraction interaction, indicating that the layer structure can be destroyed easily to form N* phase on heating.