Determination of relative positions and localizations of paramagnetic probe molecules in liquid crystal by analysis of concentration broadening of EPR spectra

Highlights: • Concentration broadening of EPR spectra of probe reflects the molecular arrangement. • Experimental angular dependence proves the magnetic dipolar molecular interaction. • Relative positions of molecules for smectic liquid crystals are determined. • Spin probes were found to form pairs in liquid crystalline media. • Weak molecular bonding is discussed. The angular dependences of concentration broadening of EPR spectra for nitroxide spin probes in liquid crystals were experimentally measured. The obvious angular dependence of the broadening found for oriented smectic liquid crystal HOPDOB proves the paired localization of the probe molecules. The numerical calculation of the angular dependence taking into account the magnetic dipolar and spin exchange interactions have been used for quantitative determination of position of probes in the pairs. The probable localization of the probes in the smectic layer is discussed.