Two‐dimensional electron–electron double resonance and electron spin–echo study of solute dynamics in smectics

Electron spin–echo (ESE) and two‐dimensional electron–electron double resonance (2D ELDOR) experiments have been performed as a function of director orientation and temperature in the smectic A phase of the liquid crystal S2 for the spin–probe PD‐tempone(2×10⁻³ M). Over the entire temperature range studied (288–323 K) we observe significant 2D ELDOR cross peaks only for ΔM_I =±1 indicative of ¹⁴N spin–relaxation and negligible Heisenberg exchange. From the angular dependent ¹⁴N spin–relaxation rates we obtain the dipolar spectral densities at the hyperfine (hf) frequency, whereas from a combination of ESE and 2D ELDOR we obtain the dipolar and Zeeman‐dipolar spectral densities at zero frequency. The angular dependent spectral densities were successfully decomposed into their basic components in accordance with theory. The angular dependent spectral densities at the hf frequency are not predicted by a model of anisotropic rotational diffusion in a nematic orienting potential, but are consistent with predictions of a model due to Moro and Nordio of solute rototranslational diffusion in a McMillan‐type potential. The angular dependence also indicates that order director fluctuations in the smectic phase are suppressed at frequencies on the order of 10 MHz. An additional contribution to solute reorientation due to cooperative hydrocarbon chain fluctuations is suggested to account for the behavior of the observed spectral densities at zero frequency. An evaluation of the relevance of several other dynamical models to this experimental work is also presented.