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, SLzz and SLxx–SLyy, 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 C2,0 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.