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Orientational ordering of small molecules in nematic liquid crystals Danilovič, Zorana


Orientational ordering in nematic liquid crystal phases arises from the presence of anisotropic intermolecular forces. To date NMR experiments, theory and Monte Carlo simulations indicate the importance of two main contributions to orientational ordering of small solutes in various liquid crystals and liquid crystal mixtures. The first contribution is well defined and involves short-range interactions that depend on the size and the shape of the solute. The second contribution, which accounts for long-range (electrostatic) interactions, is believed to have lesser impact on the molecular ordering. Which of the electrostatic interactions (induction, electric quadrupole or polarization) are most important is still debated. In order to investigate the impact of electrostatic interactions on molecular ordering, small symmetric molecules with the same size and shape, and therefore the same short-range interactions, but different electrostatic properties were dissolved in various liquid crystals and mixtures of liquid crystals. Second rank orientational order parameters of solutes in various liquid crystal phases are obtained from analysis of high-resolution NMR spectra. For high-spin systems, initial spectral parameters needed to solve very complicated high-resolution spectra are estimated from selective multiple-quantum NMR spectra, collected using a 3D selective MQ-NMR technique. Structural: parameters of the solutes are calculated -using non-vibrationally corrected nuclear dipolar coupling constants accurately obtained from analysis of highresolution NMR spectra. The contribution of the electrostatic interactions to the orientational ordering of small solutes in liquid crystal phases is discussed in terms of different solutes and different types of liquid crystals by comparing experiment with theoretically determined order parameters. Those comparisons seem to suggest that dipoles have the least impact on orientational ordering of small molecules in nematic liquid crystals. Quadrupole contribution results predict opposite signs of the electric field gradient to the one obtained in similar previous studies. Experiments with zero-electric-field-gradient mixtures ('magic mixture') show no significant contributions of the electrostatic long-range interaction to the orientational mechanism in the special mixture. The polarizability effect appears strongly dependent on molecular geometry and in this study appears to be an important electrostatic mechanism of orientation.

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