Interpretation of cw-ESR spectra of p-methyl-thio-phenyl-nitronyl nitroxide in a nematic liquid crystalline phase

Abstract

In this paper we report on the characterization by continuous wave electron spin resonance spectroscopy (cw-ESR) of a nitronyl nitroxide radical in a nematic phase. A detailed analysis is performed by exploiting an innovative modeling strategy alternative to the usual spectral simulation approach: most of the molecular parameters needed to calculate the spectrum are evaluated a priori and the ESR spectrum is obtained by direct application of the stochastic Liouville equation. Allowing a limited set of fitting parameters it is possible to reproduce satisfactorily ESR spectra in the temperature range 260 K-340 K including the nematic-to-isotropic phase transition (325.1 K). Our results open the way to a more quantitative understanding of the ordering and mobility of nitronyl nitroxide radicals in nanostructured environments.

First determination of the spin relaxation properties of a nitronyl nitroxide in solution by electron spin echoes at X-band: A comparison with Tempone

“…In PTIO the unpaired electron is coupled with two nitrogen nuclei whose isotropic hyperfine coupling constants are equal. As regards the dipolar hyperfine interaction tensors, even though the principal values are equal, the principal axes are differently oriented [32,33], hence the two nitrogen nuclei are not completely equivalent. In this case, for any simple choice of representation the relaxation matrix is not diagonal; besides, the off-diagonal elements are expected to be comparable with the diagonal ones [53].…”

“…Nitronyl nitroxide radicals [14,28], often used in different fields of application, have rarely been used as spin probes [29][30][31][32][33], and their relaxation properties have not been systematically explored so far.…”

“…Recently a new computational tool has been developed, the so-called Integrated Computational Approach, for the calculation of spectral profiles in any régime and anisotropy of reorientational motions [36]. This method, which uses as input parameters merely the chemical structure of the radical and the bulk properties of the solvent, has been recently implemented to calculate the spectral profiles of radicals with hyperfine couplings up to four nuclei [33]. Taking advantage from this evolution, this type of calculation has been recently successfully applied to analyse the EPR spectra of a nitronyl nitroxide radical in toluene in a wide temperature range [32], or performing anisotropic slow motions in a liquid crystalline phase [33].…”

“…This method, which uses as input parameters merely the chemical structure of the radical and the bulk properties of the solvent, has been recently implemented to calculate the spectral profiles of radicals with hyperfine couplings up to four nuclei [33]. Taking advantage from this evolution, this type of calculation has been recently successfully applied to analyse the EPR spectra of a nitronyl nitroxide radical in toluene in a wide temperature range [32], or performing anisotropic slow motions in a liquid crystalline phase [33]. These studies aroused the interest towards a systematic determination of the relaxation properties of nitronyl nitroxides [37].…”

Chapter 2. Computational approaches for simulating motional EPR spectra

“…More recently, Polimeno and co-workers have studied nematic liquid crystalline environments by performing SLE simulations of the EPR spectra of the prototypical nitroxide probes 4-hexadecanoyloxy-2,2,6,6-tetramethylpiperidine-1-oxy and pmethylthiophenylnitronyl nitroxide in isotropic and nematic phases of nematic liquid crystals. 54,56 These authors reported a software tool based on the numerical solution of SLE that evaluates EPR spectra for rigid, or flexible, single or multi-labelled paramagnetic molecules in both isotropic and ordered phases. In particular, simulation of EPR spectra of macromolecules labelled with two dipolar-coupled nitroxide spin labels have been reported.…”