Paramagnetic Liquid Crystals

The advent of the FFC instruments has opened a number of important application areas (molecular dynamics of liquid crystals, paramagnetic contrast MRI agents, proteins, polymers, etc.) and has thus provided a powerful impulse for further development of variable-field NMR relaxometry. Since 1996, Stelar entered the field and, building on the Noack-Schweikert technology (67), started producing the first commercial FFC NMR relaxometers. The availability of such instruments has further enhanced the drive towards new applications, apart from confirming the enormous potential of the technique as a primary tool for the study of molecular dynamics of even quite complex systems. 

C. Benzi, M. Cossi and V. Barone, Accurate prediction of electron-paramagnetic-resonance tensors for spin probes dissolved in liquid crystals, J. Chem. Phys. 123 (2005) 194909. 

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

Deuteron NMR was recently treated by Mantsch et al. (3) and the halogens, with particular emphasis on chlorine, by Lindman and Forsen. (1) Some of the more specialized applications of alkali metal NMR, such as those that involve large biomolecules, paramagnetic systems, and liquid crystals, will be ignored. Besides the already cited review on alkali metal and alkaline earth NMR, (2) a brief but comprehensive summary on sodium NMR has recently appeared. (4) Nitrogen-14 NMR has been thoroughly discussed in a text edited by Witanowski and Webb. (5) Oxygen-17 NMR and its chemical applications has been reviewed by Klemperer (6) and will also be the subject of a chapter in volume 11 of this series. 

In this subsection we show the effect of the orienting potential on the ESR lineshape of paramagnetic species of particular practical interest, namely, the nitroxide stearic spin probe and Cu ion complex dissolved in a nematic liquid crystal. 

Thus when Q < the director makes a constant angle

electron spin resonance spectrum of a paramagnetic probe dissolved in a nematic liquid crystal which is spun in a magnetic field. The spacing between the hyperfine lines was found to be in quantitative accord with (3.6.2). 

A series of Schilf-base Cu(II)-complexes have been synthesized by a transesterification reaction of random liquid crystal terpolymers with a functionalized tetradentate low molecular weight Cu(II)-complex [8], Between 5 and 20 mol% of the organometallic unit was incorporated without disrupting the liquid crystallinity. The aim of the research was to obtain new magnetoactive organic systems which combine the anisotropic paramagnetic susceptibility of metal entities and the cooperative reorientation of liquid crystals in external fields (for a review see [9]). 

Information on the spatial distribution of paramagnetic molecules deduced from ESRI experiments has been used successfiilly for measurement of the translational diffusion. Diffusion coefficients of paramagnetic diffusants can be deduced from an analysis of the time dependence of the concentration profiles along a selected axis of the sample. The determination of diffusion coefficients for spin probes in liquid crystals and model membranes, and the effect of polymer and probe poly-dispersity, have been described in a series of papers by Freed and co-workers (44). These papers represent an effort to move beyond phantoms, and to extract quantitative information from ESRI experiments. 

Organometallic complexes are of particular interest in the study of magnetooptical phenomena in liquid crystals when they contain paramagnetic atoms. 

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