Effect of asymmetric substitution on the mesomorphic behaviour of low-melting viologen salts of bis(trifluoromethanesulfonyl)amide

We report on the synthesis and characterisation of novel ionic liquid crystals and ionic liquids based on asymmetric viologen salts of bis(trifluoromethanesulfonyl)amide (Tf₂N⁻). The thermotropic behaviour of the salts has been thoroughly characterized by DSC, X-ray diffraction and polarized light optical microscopy. The occurrence of smectic mesophases and the temperature range over which they are stable has been discussed as a result of the asymmetry of the viologen dications. Strongly asymmetric systems, not showing mesomorphism, have a rather low melting point (just beyond 40 °C) while slightly asymmetric samples exhibit a smectic phase with a wide range of stability, from about 0 °C up to above 140 °C, thus increasing the range of stability of the mesophase observed for the symmetric compounds. Finally, we report on the lyotropic behaviour in non-polar solvents, such as benzene, where liquid clathrates have been found.