Vibrational spectra of liquid crystals. XI. Correlation functions obtained from 4-octyloxy, 4′-cyanobiphenyl CN stretching in smectic, nematic, isotropic, and solution phases

Abstract

Correlation functions from the Raman scattering associated with the C=N stretching mode of the mesogen 4‐octyloxy, 4′‐cyanobiphenyl (80 CB) in aligned smectic, aligned nematic, and isotropic phases have been calculated. These functions, calculated from I_VH spectra, show that the band contour is dominated by vibrational relaxation processes. Correlation functions calculated from spectra of the C=N band of the mesogen dissolved in benzene, carbon tetrachloride, chloroform, and methyl thiocyanate suggest that ’’vibrational dephasing’’ is primarily responsible for the bandwidth and shape in liquid crystal phases, in the isotropic phase, and in solutions. The solution studies indicate the presence of a wide distribution of local environments in the mesomorphic and isotropic phases of the mesogen as well as in CHCl₃ and CH₃SCN. These environments average out to a more narrow distribution, i.e., there is ’’motional narrowing,’’ in C₆H₆ and CCl₄. The results lead to an understanding of why motional narrowing occurs for certain bands and certain solvents.