The orientational order of solutions of a dye molecule in liquid-crystalline solvents

Abstract

Deuterium n.m.r. has been used to study both the major and the biaxial local order parameters, S^L_zz and S^L_xx–S^L_yy, respectively, for a dye molecule dissolved in several liquid-crystalline solvents. The dye molecule, 4-[4-(N,N-dimethylamino)phenylazo]azobenzene, designated as D2 by BDH Chemicals, was selectively deuteriated in the phenyl ring. The liquid-crystalline solvents used were 4,4′-di-n-heptylazoxybenzene and 4-n-pentyl-4′-cyanobiphenyl (5CB), together with two commercial mixtures, Merck Phase 5 and BDH E63. The solvent 5CB contained some material with a single deuteron at the first position in the alkyl chain, which enabled the order parameters of the dye to be compared directly with C_2,0 the major conformationally averaged order parameter of the solvent. The biaxiality in solute ordering is interpreted in terms of λ, a measure of the departure from cylindrical symmetry of the assumed second-rank potential of mean torque. The parameter λ is observed to depend strongly on the nature of the solvent, and possible explanations for this are considered.