ИСТИНА

The most detailed characteristic of molecular orientation is orientation distribution function (ODF) that gives a quantity of molecules oriented in an given angular interval. ODF can be obtained by numerical analysis of EPR spectra recorded at various orientation of anisotropic sample in magnetic field [Vorobiev A. Kh., Chumakova N. A., J. Magn. Reson. 175 (1) (2005) 146-157.]. In the present work this method was applied for study of structure and dynamic properties of aligned liquid crystals. Radicals R1 [Naohiko Ikuma, Rui Tamura, at all., Adv. Mater.18 (2006) 477-480.], that have rigid molecules with anisometric shape, and radicals R2 [Shin`ichi Nakatsuji at all., J.Org.Chem 72 (2007) 2021-2029], that combine photoactive azobenzene and paramagnetic nitroxide moieties, were used as a paramagnetic probes. The orientation distribution functions of the spin probes in liquid crystals 5CB and MBBA oriented by magnetic field and by surface of pores in porous polyethylene have been obtained. It was found that radicals R1 more correctly reflect the alignment of liquid crystals in comparison with well known spin probe TEMPOL. ODF of R2 changes dramatically as a result of cis-trans isomerization of azobenzene fragment. The ODF of biaxial sample is discussed.