Molecular
Orientational Order of Nitroxide Radicals
in Liquid Crystalline Media
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Abstract
The
orientational distribution of a set of stable nitroxide radicals
in aligned liquid crystals 5CB (nematic) and 8CB (smectic A) was studied
in detail by numerical simulation of EPR spectra. The order parameters
up to the 10th rank were measured. The directions of the principal
orientation axes of the radicals were determined. It was shown that
the ordering of the probe molecules is controlled by their interaction
with the matrix molecules more than the inherent geometry of the probes
themselves. The rigid fused phenanthrene-based (A5) and 2-azaphenalene
(A4) nitroxides as well as the rigid core elongated C11 and 5α-cholestane
(CLS) nitroxides were found to be most sensitive to the orientation
of the liquid crystal matrixes