Analysis of slow-motional electron spin resonance spectra in smectic phases in terms of molecular configuration, intermolecular interactions, and dynamics

ESR spectra from two oxazolidine derivative spin probes (CSL and 1,14-stearic acid) dissolved in the smectic phase of S2 were carefully analyzed and the results interpreted in terms of ordering characteristics, molecular conformation, and dynamics by using powerful and comprehensive spectral simulation techniques. The rigid CSL is highly ordered in the smectic A phase as expected for strong interactions with the rigid aromatic cores of the liquid-crystal molecules and shows reorientational motion well approximated as Brownian in the mean potential of the solvent molecules. The principal axes of ordering of CSL are found to be tilted with respect to the principal axes of the magnetic tensor, and the Euler angles specifying this tilt could be determined, because of the sensitivity of the slow-motional spectra to these parameters. The considerable sensitivity of these spectra to the shape of the orienting potential is also demonstrated, and values for coefficients in the expansion of the potential in the spherical harmonics through L = 4 are estimated. The flexible 1,14-stearic acid probe is only weakly ordered in the smectic phase. It shows an anisotropy in its rotational diffusion tensor that is smaller than predicted for an extended all-trans conformation, suggesting an average configuration with a decreased length-to-width ratio. However, the end segment containing the N-O group appears to be in an all-trans configuration, judging by the observed collinearity between the principal symmetry axis of diffusion and ordering with that of the hyperfine tensor. Its low ordering and reduced activation energy for reorientation in the smectic phase suggest primary coupling to the alkyl chain region of the solvent molecules. The spectral simulations are improved by introducing asymmetry in the viscosity, but related anomalies suggest the likelihood of some other mechanism such as a fluctuating torque model. A comparison of the results of this and other studies is made to show how, by the use of different spin probes, one can obtain insights into the intermolecular interactions operating in anisotropic fluids.