Electron-spin relaxation and ordering in smectic and supercooled nematic liquid crystals

We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid-crystal-like spin probe dissolved in a benzilidene-derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the S_B phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co-workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed.