Solvation sites in mesophases of discotic liquid crystals

The deuterium quadrupole splitting, νQ, of C6D 6 dissolved in a number of discotic liquid crystals has been measured over the temperature range of the mesophase stability. Three different types of mesophases were used : (i) Dho, which is a columnar hexatic phase with ordered molecular stacking; (ii) Drd, which is a rectangular biaxial columnar phase with disordered molecular stacking ; (iii) a nematic discotic, ND. The three types of mesophases exhibit different temperature dependence of ν Q : In the Dho phase the splittings are relatively small and strongly temperature dependent. This behaviour is interpreted in terms of a fast dynamic equilibrium between two solvation sites consisting of (1) molecules intercalated within the discotic columns, and (II) molecules dissolved in the aliphatic side chain region between the columns. In the Drd phase the splittings are considerably larger and fairly constant with temperature indicating that solvation in this phase is predominantly in the intercalation site. Finally in the nematic phase the splittings are large and follow a Maier-Saupe type temperature dependence. This is consistent with the structure of the nematic phase where no different sites are expected