Deuterium NMR of biaxial discotic liquid crystals

Deuterium NMR spectra of deuterated probe molecules dissolved in several discotic liquid crystals possessing biaxial mesophases are reported. The spectra exhibit typical features characteristic of biaxial phases. This is particularly manifested through the asymmetry parameter of the average quadrupole tensor. The effect is demonstrated on two types of biaxial discotic phases: (i) a rectangular Drd mesophase which appears in homologs of triphenylenehexa-n-alkanoates; and (ii) a tilted Dt mesophase which occurs in homologs of benzenehexa-n-alkanoates. The theory relating to the motional constants in biaxial phases and the asymmetry parameter of the quadrupole tensor (and other tensorial properties) is reviewed, and applied to some specific examples.