Study of orientational order of some nematogenic compounds by spectroscopy methods using linearly polarized light

The orientational behaviour of liquid crystals from homologous series of 4-(trans−4′-n-alkylcyclohexyl)-isothiocyanatobenzene (nCHBT) and trans-4-n-alkyl(4′-cyanophenyl)-cyclohexane (PCHn) was studied by means of classical methods of optical spectroscopy: absorption of the polarized light in the visible region, fluorescence depolarization and Raman scattering depolarization. The dichroic dye dimethylamino-nitrostilbene was added to liquid crystals and acted as guest probe reflecting the host orientation in absorption and fluorescence measurements. The absorption, emission and Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the nematic phase. On the basis of these spectra the order parameters 〈P2〉 and 〈P4〉 as well as the molecular distribution function were determined. It was found that the orientational order of liquid crystals molecules depends on the terminal group as well as on the alkyl chain length. The odd-even effect for the order parameters, explained as the alternation of interaction between alkyl chains, was observed. Moreover, the effect of the dye presence on the orienting properties of liquid crystalline hosts was discussed.