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.