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Search for biaxiality in a shape-persistent bent-core nematic liquid crystal

Oleg D. Lavrentovich ,Young-Ki Kim ,Madhabi Majumdar 2,Bohdan I. Senyuk ,Luana Tortora ,Jens Seltmann 3,Matthias Lehmann 4,Antal Jakli ,Jim T. Gleeson 2,Samuel Sprunt 2

Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH, 44242, USA
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA.
2Department of Physics, Kent State University, Kent, OH 44242, USA.
3Institute of Chemistry, Chemnitz University of Technology, 09107 Chemnitz, Germany
4Institute of Organic Chemistry, Julius-Maximillians-Universit˘ĉat W˘ĉurzburg, 97074 W˘ĉurzburg, Germany

Using a range of optical techniques, we have probed the nature of orientational order in a thermotropic bent-core liquid crystal, which features a shape-persistent molecular architecture designed to promote a biaxial nematic phase. In the upper range of the nematic phase (enantiotropic regime), dynamic light scattering reveals strong fluctuations attributable to the biaxial order parameter, in addition to the usual uniaxial director modes. Assuming a Landau-type expansion of the orientational free energy, we estimate the correlation length associated with these fluctuations to be ~100 nm. At lower temperatures, and mainly in the monotropic regime of the nematic, we observe by optical conoscopy an apparently biaxial texture, which develops when the sample temperature is changed but then relaxes back to a uniaxial state over time scales much longer than observed in the light scattering measurements. A combination of fluorescence confocal polarizing microscopy and coherent anti-Stokes Raman scattering confirms that the conoscopic texture arises from a flow-induced reorientation of the molecules, associated with a large thermal expansion coefficient of the material, rather than from the spontaneous development of a macroscopic secondary optical axis. We discuss a model to account for the observed behavior at both high and low temperatures based on the temperature-dependent formation of nanoscale, biaxially ordered complexes among the bent-core molecules within a macroscopically uniaxial phase.

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