First examples of de Vries‐like smectic A to smectic C phase transitions in ionic liquid crystals

In ionic liquid crystals the orthogonal smectic A phase is the most common phase while the tilted smectic C phase is rather rare. We now present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core while the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature dependent layer spacing as well as their translational and orientational order parameters were studied by X-ray diffraction. The X-ray tilt angles derived from the X-ray layer contraction and the optical tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries-like behavior with a layer shrinkage that is far less than what has to be expected for conventional materials. These materials can thus be considered as the first de Vries-type materials among ionic liquid crystals.