Structural and optical investigations of TGB_A and TGB_C mesophases exhibiting cylindrical and cone-like domain textures

Abstract:

The characterization by optical microscopy, X-ray diffraction and differential scanning calorimetry of two new liquid crystalline compounds is presented. The compounds under consideration incorporate in their molecules two chiral centres (one asymmetric carbon and one asymmetric sulphur of a sulphinate group). While the carbon chirality is fixed in the R configuration the sulphur chirality may be either in the S or in the R configuration. A TGB_A mesophase in a large temperature domain is evidenced for one of the two diastereomers, while both TGB_A and TGB_C mesophases are observed in a similar temperature domain for the other one. For both diastereomers and in both mesophases it is possible to observe the coexistence of two different types of optical textures, namely planar cholesteric textures and developable domains. The latter are coiled in a different way than proposed in an earlier publication. More precisely, they form cylindrical or cone-like domains with double-twist properties as observed in the blue phases. The pitch of the helix in the TGB mesophases is evaluated by means of optical reflection observations in samples exhibiting the planar cholesteric textures. For both diastereomers it is also possible to evaluate structural parameters related to the organization within the TGB_A mesophases. Finally, the induced phase transition TGB_C-S_C* is studied by the application of an AC electric field.