Intermolecular Guest-Host Interactions and the Optical Order Parameter of Pleochroic Dyes
The optical order parameters Sop of anisotropic dye molecules in various nematic hosts have been studied as a function of temperature, in order to compare different guest-host systems over the same range of reduced temperature. Two anthraquinone dyes, one of which has hydrogen bonding and the other not, have been studied in cyano and non-cyano two-ring nematic host mixtures, with and without cyano and non-cyano high clearing point additives. Extrapolation of Sop to absolute zero is attempted in order to determine experimentally the angle between the transition moment and the “long axis” of the molecule. It is found that the temperature dependence of Sop has a different shape to that of the host order parameter, the reason perhaps being the rather strong deviations from cylindrical symmetry of the anthraquinone molecule. The comparison of reduced temperature curves of Sop reveals two effects: (i) the hydrogen-bonding dye is more sensitive to the chemical nature of the host, presumably due to varying competition between intramolecular and intermolecular interactions; (ii) although the host order parameter was unchanged by addition of the high clearing point additives studied, the presence of such materials in a mixture lowers Sop significantly, indicating some kind of local structure effect, possibly prefential solvation of the dye by the larger molecule. The shift in clearing point on adding dye broadly correlates with direct evidence on the degree to which each host orders the dye. We speculate on a possible role for dipole/induced-dipole interactions in guest-host interactions.
KeywordsGuest Molecule Transition Moment Absolute Zero Preferential Solvation Nematic Director
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