Symmetries and alignment of biaxial nematic liquid crystals

TL;DR

This paper investigates the symmetry properties of biaxial nematic liquid crystals, proposing monoclinic symmetry as more likely than orthorhombic, and discusses methods to differentiate symmetries using NMR and IR spectroscopy.

Contribution

It introduces a detailed methodology for identifying the symmetry of biaxial nematic phases and challenges the common assumption of orthorhombic symmetry.

Findings

Monoclinic ($C_{2h}$) symmetry is more probable for biaxial nematics.

Differentiation methods using NMR and IR spectroscopy are detailed.

Implications for measurement, quantification, and applications of biaxial nematics are discussed.

Abstract

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic ($C_{2h}$) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitc tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.