Electron spin resonance studies of anisotropic ordering, spin relaxation, and slow tumbling in liquid crystalline solvents. 4. Cholestane motions and surface anchoring in smectics

ESR line shape studies of cholestane (CSL) in monodomain liquid crystals of HOAB, 40,6, and 40,8 oriented between glass surfaces are reported. The line shapes are analyzed in terms of the Polnaszek-Bruno-Freed theory for spectra from slow tumbling probes in ordered fluids. Preparation of monodomain smectic crystals, free from previously observed effects of static director distribution due to magnetic torques, and therefore adequate for accurate studies of dynamic line shapes, both as a function of temperature and orientation, is described. CSL exhibits fast motion in HOAB but slow motion in 40,8 and 40,6. In the nematic phases the anisotropy in the diffusion tensor (N ≈ 5) is consistent with the geometry of the probe molecule, as expected for simple Brownian type of diffusion in a mean potential field. In the smectic phases, an "apparent" anisotropy was found by simulations to be substantially greater than predicted by the molecular configuration and appears to be increasing dramatically as the temperature is decreased. It is argued that this is untenable in the context of the model. Instead, it is shown that an approximate model of fluctuating torques could be successfully used to interpret the apparent anomalies. These observations are interpreted in terms of the effect of the local solvent structure on the nature of the overall dynamic mode of the probe, implying considerable local cooperativity in these smectic phases. It is also shown by preliminary studies with the PD-Tempone probe that the very well-aligned samples may experience substantial elastic distortions with magnetic fields of 3 kG when the field is tilted relative to the normal to the plates. Since this is contrary to predictions of previous theory, a new theoretical analysis is given which suggests that (1) the conditions of weak vs. strong anchoring are different in the nematic vs. smectic phases and (2) it is quite possible to have weak anchoring in the smectic phase of a sample which exhibits rather strong anchoring in the nematic phase.