Study of orientational order of some nematogenic compounds by spectroscopy methods using linearly polarized light D. Bauman * , E. Chrzumnicka, E. Mykowska, M. Szybowicz, N. Grzelczak Faculty of Technical Physics, Poznan ´ University of Technology, Nieszawska 13a, Poznan ´ 60-965, Poland Received 9 September 2004; accepted 4 November 2004 Available online 18 January 2005 Abstract The orientational behaviour of liquid crystals from homologous series of 4-(transK4 0 -n-alkylcyclohexyl)-isothiocyanatobenzene (nCHBT) and trans-4-n-alkyl(4 0 -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 hP 2 i and hP 4 i 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. q 2004 Elsevier B.V. All rights reserved. Keywords: Liquid crystal; Dichroic dye; Order parameter; Absorption and emission of light; Raman scattering 1. Introduction Among various possible display technologies, liquid crystal displays technology has been one growing most rapidly and fruitfully. The most physical properties of liquid crystals are strongly dependent on the molecular orientation [1]. The knowledge of the degree of order is thus of great value not only in understanding various aspects of physics of these substances, but also in most their application. Although the utilization of various kinds of liquid crystals are being investigated in many laboratories, in most information display devices still nematogenic substances are used. In order to study anisotropic systems the classical methods of molecular spectroscopy, i.e. magnetic reson- ances and optical spectroscopy with polarized light, have been widely applied [2–7]. Linearly polarized light is of special interest because it represents a powerful tool for the investigation of oriented assemblies of molecules, such as crystals, thermotropic and lyotropic liquid crystals as well as lipid bilayers [6]. In the paper, we present the results of the study of the orientational order of some liquid crystals from homologous series of 4-(transK4 0 -n-alkylcyclohexyl)-isothiocyanato- benzene (nCHBT) and trans-4-n-alkyl(4 0 -cyanophenyl)- cyclohexane (PCHn) by using following optical spectroscopy methods: absorption in visible spectral region, fluorescence depolarization and depolarization of Raman scattering. In two former methods liquid crystals were doped with small amount of the dichroic dye, dimethyla- mino-nitrostilbene (DANS), which acted as the probe reflecting the matrices orientation (‘guest–host’ effect [8]). 2. Experimental The liquid crystal materials used in the study, namely the series of 4-(trans-4 0 -n-alkylcyclohexyl)-isothiocyanato- benzene (nCHBT) for nZ3–9 and of trans-4-n-alkyl(4 0 - cyanophenyl)-cyclohexane (PCHn) for nZ5–8, were 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.11.035 Journal of Molecular Structure 744–747 (2005) 307–313 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: C48 61 665 3164; fax: C48 61 665 3201. E-mail address: bauman@phys.put.poznan.pl (D. Bauman).