Different types of organic doublet radicals have been investigated in isotropic, nematic, and smectic phases of liquid crystals by performing electron nuclear double resonance (ENDOR), general TRIPLE and special TRIPLE experiments. For this purpose phenalenyls (perinaphthenyls), galvinoxyl, tetra-tert.-butylcarbazolyl, and bis(biphenylenyl)propenyl radicals have been chosen including some partially deuterated and 13C-labeled compounds. Anisotropic hyperfine interactions and deuterium quadrupole couplings could be determined. Smectic A phases have proved to be advantageous as compared to nematic phases in these studies. Detailed information about the orientation of the radicals within the liquid-crystalline solvent and about the conformations of the radicals could be obtained.
B. Kirste and H. Kurreck, "Electron Nuclear Double Resonance (ENDOR) Investigations of Organic Doublet Radicals in the Nematic and Smectic Phase of Liquid Crystals," Appl. Spectrosc. 34, 305-310 (1980)