An ESR study of conformation, dynamics, and local ordering of doxyl and azethoxyl nitroxides dissolved in smectic S2

We report on a careful line-shape study of ESR spectra from the smectic A_d phase of S2 doped with the common doxyl and the "minimum-perturbation" azethoxyl nitroxides, with the NO group attached at the midchain region. The primary goal of this investigation is to address unresolved questions related to chain conformation, dynamics, and local ordering in anisotropic solvents, as reflected in the spin relaxation of the above-mentioned paramagnetic hydrocarbons. We find that the NO groups of both doxyl and azethoxyl nitroxides experience similar, relatively low, and only slightly temperature-dependent local ordering. On the other hand, both motional and conformational properties of the two types of spin probes are at variance. Thus, the azethoxyl nitroxides reorient at average rates of roughly 2.0 × 10⁷ s^-1 at the lower end of the smectic A_d phase, which are about twice higher than the reorientational rates of the doxyl analogues at similar temperatures. Azethoxyl nitroxides reorient basically as individual particles whereas with doxyl nitroxides, we detect substantial dynamic coupling to collective modes of the liquid-crystalline solvent. The doxyl chains were found to be fully extended whereas the azethoxyl chains bent. We discuss the unique value of slow-motional ESR in complementing information obtained previously with both NMR and ESR in the motional-narrowing regime.