ESR OF SPIN-LABELED NEMATOGENLIKE PROBES DISSOLVED IN NEMATIC LIQUID CRYSTALS

We show that qualitative information on molecular ordering in nematic mesophases is conveniently obtained from the ESR spectra of trace quantities of spin-labeled rodlike probe molecules dissolved in the nematogen. The synthesis of the probe [2,2,6,6-tetramethyl-4-(p-octyloxy)-benzoylamino-piperidine-1-oxyl] is described and ESR spectra of the probe taken in nine solvents at various temperatures are reported. Large contractions of the 14N hyperfine patterns, as well as positive g shifts, have been observed. This indicates that the alignment of the host molecules along the magnetic field direction constrains the motion of the probe molecules so that they too are aligned parallel to the field. The rotation of the radical group about the bond connecting it to the rest of the probe molecule is partly hindered. In general, the probe molecules described here are less ordered than the host molecules.