Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well as smectic A phases. Their phase sequences and tilt angles were studied by polarizing microscopy, and their temperature-dependent layer spacings and orientational order parameters were investigated by X-ray diffraction. We present 1 new amidinium azobenzene mesogen that forms enantiotropic smectic C and A phases and another amidinium as well as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic C and enantiotropic smectic A phases. With this study, we show that azobenzene is indeed an SmC-promoting group in ILCs. Comparing these results with our earlier results on azobenzenes with an N-methylimidazolium head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116-4123), we show that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases.