ИСТИНА

EPR spectroscopy of nitroxide radicals in liquid crystals is used at present mainly for determination of rotation molecular mobility and estimation of mean field potential. The additional important information can be obtained by means of consideration of dipole-dipole and spin exchange of radicals in liquid crystalline media. The dipole-dipole interaction is known to be anisotropic. Present work demonstrates that experimental recording of angular dependence of EPR spectra for oriented liquid crystals is appropriate for measurement of the anisotropic dipole-dipole broadening of EPR spectra. The quantitative description of the broadening makes possible the precise determination of the mutual positions and orientations of molecules in liquid crystalline medium. This possibility is illustrated in the cases of liquid crystals, which care the nitroxide moiety, as well as in the cases of probe nitroxides specially induced in the medium. The pair location of the radicals in the structure of smectic liquid crystals is revealed and discussed (Fig.1). The dynamic spin exchange can be characterized by measurements of concentration dependence of spectra. The characteristics of molecular diffusion are extracted from value of the exchange frequency. The results of such measurements for liquid crystals are presented. The methods of quantitative determination of dipole-dipole and exchange broadening in the case of the ordered liquid crystals are described and discussed.