Solute alignment in liquid crystal solvents The Saupe ordering matrix for perylene and pyrene

The complete second rank ordering matrix for perylene-d₁₂ and pyrene-d₁₀ in four different thermotropic liquid crystals has been determined over large temperature ranges, by interpreting the quadrupolar splittings observed from the deuterium N.M.R. spectra. The dipolar couplings also observed in the spectra were determined by computer simulation for both probes, allowing the commonly made assumptions of rigidity and planarity of these molecules to be tested. It is found that the perylene results are consistent with a non-rigid and non-planar structure with an average twist angle between the napthtalene units of 11.6°, whereas pyrene is rigid and planar within experimental error. It is also observed that both molecules are highly biaxial in their orientational behaviour and therefore another assumption often made especially in fluorescence measurements, that of disc-like cylindrical symmetry, is invalid. The results are interpreted in terms of a molecular field theory which predicts the variation of (S_xx - S_yy) with S_zz by calculating these quantities from the potential of mean torque, U(β, γ), of rigid solutes, expressed solely in terms of one adjustable parameter, the biaxiality parameter, λ. This parameter is predicted by some theories to be both temperature and solvent independent, but our results show that there is a weak temperature dependence and a stronger solvent dependence.