Molecular dynamics and EPR spectroscopic studies of 8CB liquid crystal

Abstract

We report successful simulation of motional EPR spectra of the liquid crystal 8CB doped with a cholestane nitroxide spin probe from fully atomistic molecular dynamics (MD) simulations. The spectra are calculated directly and completely from MD trajectories using our novel MD-EPR methodology. Predicted changes in molecular order, dynamics and EPR spectra across the N–I phase transitions show excellent agreement with experimental results. A nanosecond exchange dynamics between disordered and partially ordered meta-stable states is revealed at the N–I transition point and is confirmed by EPR measurements. This study demonstrates that a unique combination of state-of-the-art molecular modelling at the atomistic level and EPR spectroscopy, with introduced paramagnetic probes, allows accurate estimation of the local order and motional parameters of the mesogens. In particular, it is shown that an accurate estimation of the rotation correlation times for different molecular axes in liquid crystals can be achieved and correlated directly with the motions of the spin probe. We also demonstrate the successful simulation of a low temperature smectic-A liquid crystal phase in 8CB. Here, the simulations correctly predict the experimental layer spacing in 8CB and show directly the presence of a strong local preference for anti-parallel arrangements of molecules. The latter leads to a layer-spacing of D ≈ 1.4 molecular lengths.