Electron-spin relaxation and ordering in smectic and supercooled nematic liquid crystals

Abstract

We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid-crystal-like spin probe dissolved in a benzilidene-derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the SB phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co-workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed.

Nonlinear-Least-Squares Analysis of Slow-Motion EPR Spectra in One and Two Dimensions Using a Modified Levenberg–Marquardt Algorithm

“…cally require the solution of large matrix equations (11)(12)(13) for the calculation of a single spectrum.…”

Modeling the Effects of Structure and Dynamics of the Nitroxide Side Chain on the ESR Spectra of Spin-Labeled Proteins

“…Extension of the slow-motional ESR theory 42 to include fast internal motions has been performed in the following way.…”

Cu(ii)–porphyrin molecular dynamics as seen in a novel EPR/Stochastic Liouville equation study

“…For higher temperatures an anisotropic diffusion model may be needed as is seen from nitroxide spin probe studies. 18 Such a model is already implemented, however, it adds an unnecessary complexity to the task of finding the best candidate spin Hamiltonian parameters from low temperature fluid state EPR experiments.…”

“…Extending EPRLL and including a complete Liouville spin space is done in a program EPRLF and valid for transition metal complexes, also developed by Freed et al 18 More recently gadolinium complexes have been studied within the frequency domain approach. 22 However, the main hurdle comes with the large Liouville space when electron spin couples up to five nuclear spins as is the case for Cu(II) heme-protein.…”

“…Little work has been done on solving the Stochastic Liouville equation explicitly including ligand nuclear spin for transition metal complexes and thus arriving at a microscopic interpretation of the fluid-phase EPR experiment. For nitroxide spin probes an efficient numerical program for solving the SLE in the frequency domain (EPRLL) was developed by Freed et al 18,19 A first complication in extending the work for nitroxide spin probes to transition metals is the large anisotropy of the spin Hamiltonian parameters where the dimension reductions introduced in EPRLL are not valid. The validity problem is seen in work on vanadyl complexes.…”