Orientational properties of perylene tetracarboxylic diimide molecules in liquid-crystalline matrices

Molecular orientational order of perylene tetracarboxylic diimide, N, N'-di(n-hexyl)-1,6,7,12-tetrachloroperylene-3, 4,9,10-carboxylic acid imide (Cl4-PTCDI-6C), dissolved in homologous series of liquid crystals, i. e. 4-(trans-4'-n-alkylcyclohexyl)-isothiocyanato-benzene (nCHBT) and 4-n-alkyl-4'-cyanobiphenyl (nCB), is investigated by using UV-V is absorption and fluorescence spectroscopies. The orientational order parameter P2 , determined experimentally for Cl4-PTCDI-6C molecules in nCHBT and nCB, is much lower than that for some other derivatives of perylene dissolved in the same liquid crystal hosts. The order parameter'P2 , obtained for Cl4-PTCDI-6C in nCHBT and nCB, is also smaller than'P2'obtained previously from Raman scattering in the liquid crystal hosts. The orientational order parameter'P4'for the Cl4-PTCDI-6C probe takes extremely low negative values, which indicates the importance of higher rank order parameters in the description of the dye distribution in the liquid-crystalline matrices. The bulky chlorine atoms, substituted in the perylene tetracarboxylic diimide molecular core, seem to be primarily responsible for the small orientational order parameters of the dye molecules. Finally, the odd-even effect, manifesting itself in an alternating increase and decrease of'P2'and'P4 for Cl4-PTCDI-6C along with increasing number of carbon atoms in the alkyl chains in nCHBT homologous series, is discussed.