Electron spin resonance studies of anisotropic ordering, spin relaxation, and slow tumbling in liquid crystalline solvents. 2

Abstract

In this work the nonaxial ordering and spin relaxation of PD-Tempone spin probe in several liquid crystalline solvents exhibiting smectic A and B phases were studied utilizing methods previously employed by Polnaszek and Freed for the study of nematic liquid crystals. The results rleported here for the isotropic and nematic phases are generally in accord with those obtained previously. An analysis of isotropic hyperfine shifts, changes in the ordering tensor, and anomalous relaxation behavior in the smectic phases suggest a model in which the PD-tempone probes are partially expelled from the dipolar region of the liquid crystalline molecules toward the more flexible hydrocarbon end chains as a result of the packing of the smetic layers, and concomitantly the probes increasingly experience a slowly relaxing local structure (SRLS) in a cavity-like location. Differences in observations from different types of smectic liquid crystals are interpreted in terms of their differing structures based on X-ray studies. It is shown that the angular dependent line widths in the smectic phases are significantly affected by the size and shape of the sample. These inhomogeneous broadening effects are discussed in detail in terms of static distortions of the smectic layering induced by wall effects and magnetic-field induced torques, and are in reasonable agreement with predictions of a simple model. The residual homogeneous widths are discussed in terms of combined models of anisotropic rotation and anisotropic viscosity as well as associated SRLS models. For the former case, the problem of defining the rotational diffusion tensor, which must be time dependent in any axis system, is discussed in some detail.