Orientational order of tetra n-hexylesters of perylene and tetrachloroperylene tetracarboxylic acids in low-molar-mass liquid crystal investigated using absorption and fluorescence methods

The orientational order parameters <P₂> and <P₄> of perylene-3,4,9,10-tetra-(n-hexylester) (PTHE) and 1,6,7,12-tetrachloroperylene-3,4,9,10-tetra-(n-hexylester) (4Cl-PTHE) dye molecules dissolved in low-molar-mass liquid crystal, 4-(trans-4-n-hexylylcyclohexyl)-isothiocyanobenzene, are calculated from the experimental values of polarised absorption and fluorescence anisotropies. The experimental data show that the substitution of chlorine atoms instead of hydrogen in the perylene core reduces the value of <P₂> parameter. Moreover, the <P₂> and <P₄> values for molecules of the perylene derivatives, playing the role of the guest in the liquid crystal matrix, are much less than these values for molecules of the liquid crystal host. The extremely low negative values of <P₄> for the perylene dyes are explained as the result of the preferable oblique orientation of the main molecular symmetry axis of the probe with respect to the direction of the macroscopic orientation of the liquid crystal host molecules.