Search for nematic biaxiality in bent-core mesogens: an X-ray diffraction perspective

ABSTRACT

Since its theoretical prediction in 1970, the search for the biaxial nematic phase in thermotropic systems has challenged generations of liquid crystal scientists. Over the last 10 years, bent-core mesogens have drawn much interest as promising candidates for nematic biaxiality. However, despite a number of disputed claims, conclusive evidence of proper (spontaneous and macroscopic) biaxial order in these materials is still missing. By contrast, it is now widely recognised that biaxiality exists on a local scale, in the form of nano-sized clusters of molecules (cybotactic groups) possessing smectic-like positional order and biaxial orientational order. This article provides a review of X-ray diffraction studies on biaxiality and cybotaxis in bent-core nematics, discussing the most relevant issues related to this research field.

GRAPHICAL ABSTRACT

KEYWORDS:

• Biaxial nematic
• bent-core mesogens
• cybotactic nematic
• X-ray diffraction

Acknowledgements

I gratefully acknowledge my debt to O. Francescangeli (Università Politecnica delle Marche) and E. T. Samulski (University of North Carolina at Chapel Hill) for introducing me to this fascinating topic and to E. Scharrer (University of Puget Sound) for the inspiring discussions.

Disclosure statement

No potential conflict of interest was reported by the author.

Notes

1. The values in the original Leadbetter et al.’s paper are slightly different from those reported here as d was calculated as 1.2 2π/q₀, assuming cylindrical symmetry in the transverse molecular packing.