Vibrational Spectra of Liquid Crystals. XII

Recent papers by Gray et al.1 have discussed the synthesis, structure, and properties of the mesomorphic 4-cyano, 4′-alkyl (or alkoxy) biphenyls. The reports on these materials have made mention of the existence of solid state dimorphism for certain homologs. This paper discusses this dimorphism as it manifests itself in the Raman spectrum of the octyloxy homolog (hereinafter referred to as 80CB) in the CN stretching region. The approach used involves direct measurement of the Raman spectrum as a function of temperature, as well as computation of a correlation function obtained from the Raman band envelope by means of Fourier transformation. The application of this correlation function technique to this system has been discussed by us recently in detail.2,3 Gray and Mosley,4 in a recent study of the Raman spectrum of 4-cyano, 4′-pentylbiphenyl note that the CN stretch of this compound is a doublet in the solid phase (2224 and 2233 cm−1), but only the 2224 cm−1 band appears in the nematic phase. They further report that only a single band appears in the CN stretching spectrum of solid 4'-heptyl-, -hexyl-, and -butyl-derivative. They speculate that the doublet might appear in the spectrum of the metastable solid phase of the heptyl homolog. In this work, we show that for 80CB this is not the case. Rather, a doublet is observed for the stable solid but not for the metastable. The origin of this doublet is discussed, and all of the CN stretches for stable solid, metastable solid, and smectic liquid crystal are analyzed from the viewpoint of the vibrational relaxation correlation function.