Studies of spin relaxation and molecular dynamics in liquid crystals by two-dimensional Fourier transform electron spin resonance. II. Perdeuterated-tempone in butoxy benzylidene octylaniline and dynamic cage effects

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Abstract
Two-dimensional Fourier transform (2D-FT)-electron spin resonance (ESR) studies on the small globular spin probe perdeuterated tempone (PDT) in the liquid crystal solvent 4O,8 (butoxy benzylidene octylaniline) are reported. These experiments, over the temperature range of 95 degrees C to 24 degrees C, cover the isotropic (I), nematic (N), smectic A (S-A), smectic B (S-B), and crystal (C) phases. The 2D-ELDOR (two-dimensional electron-electron double resonance) spectra confirm the anomalously rapid reorientation of PDT, especially in the lower temperature phases. The model of a slowly relaxing local structure (SRLS) leads to generally very good non-linear least squares (NLLS) global fits to the sets of 2D-ELDOR spectra obtained at each temperature. These fits are significantly better than those achieved by the standard model of Brownian reorientation in a macroscopic orienting potential. The SRLS model is able to account for anomalies first observed in an earlier 2D-ELDOR study on PDT in a different liquid crystal in its smectic phases. Although it is instructional to extract the various spectral densities from the COSY (correlation spectroscopy) and 2D-ELDOR spectra, the use of NLLS global fitting to a full set of 2D-ELDOR spectra is shown to be more reliable and convenient for obtaining optimum model parameters, especially in view of possible (incipient) slow motional effects from the SRLS or dynamic cage. The cage potential is found to remain fairly constant at about k(B)T over the various phases (with only a small drop in the S-B phase), but its asymmetry increases with decreasing temperature T. This value is significantly larger than the weak macroscopic orienting potential which increases from 0.1 to 0.3k(B)T with decreasing T. The cage relaxation rate, given by R(c) is about 3x10(7) s(-1) the I phase, but increases to about 10(8) s(-1) in the S-A, S-B, and C phases. The rotational diffusion tensor for PDT shows only a small T-independent asymmetry, and its mean rotational diffusion coefficient is of order 10(10) s(-1), with however, a small increase in the S-B phase. These results are consistent with a model previously proposed for PDT in benzylidene liquid crystal solvents, that as T is reduced the PDT molecules are partially expelled from the hard core (dipolar) region of the liquid crystalline molecules toward the more flexible aliphatic chain region as a result of increased core packing from smectic layer formation, and it thus experiences a more fluid (for a given temperature) local cage structure

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