Symmetries and alignment of biaxial nematic liquid crystals

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 calamitic-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.

Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

“…These two tensor order parameters Q and B, however, are generally insufficient for the description of a biaxial phase with the orthorhombic symmetry (D 2h ) as shown by Gorkunov et al 5 In principle a low symmetry biaxial nematic phase with C 2h symmetry may also exist. 12 The absorbance profile vs. the angle of polarization of the IR beam can in principle give a direct evidence of departure from the orthorhombic symmetry. Lower symmetry shall break the overall symmetry of the absorbance profile as shown.…”

Molecular field theory for biaxial nematic liquid crystals composed of molecules withC2hpoint group symmetry

“…For example, it has long been appreciated that there should be more than one type of biaxial nematic each with a different point group symmetry. Recently, however, it has been shown how NMR spectroscopy might be used to distinguish between biaxial nematic phases with different point group symmetries; in particular those with D 2h and C 2h symmetries [1].…”

“…In view of the subtleties in the differences between non-polar biaxial nematics with C 2h and D 2h symmetries as well as C i the identification of these new biaxial nematics promises to present even greater problems [1]. To help in this challenging task we have developed a molecular field theory for the phases, isotropic, uniaxial nematic, biaxial nematic both D 2h and C 2h formed by molecules with C 2h…”

Biaxial nematics: symmetries, order domains and field-induced phase transitions†

“…The situation is, however, different for the presently available observations of biaxial orientational order in thermotropic nematics. As detailed in ref (20), the experimental results involving measurements on aligned samples, although routinely analysed assuming a D 2h symmetry, do not particularly support this symmetry and in some cases appear to contradict it. Thus, the NMR measurements on bent-core nematics (9) using deutriated probe solutes indicate the presence of biaxial order but are not detailed enough to single out a particular symmetry for the observed phase biaxiality.…”

“…Thus, the NMR measurements on bent-core nematics (9) using deutriated probe solutes indicate the presence of biaxial order but are not detailed enough to single out a particular symmetry for the observed phase biaxiality. On the other hand, the biaxial order in calamitic tetramer nematics, reported first from IR spectroscopy studies on aligned samples (10) and subsequently confirmed by NMR measurements using deuteriated probe solutes (21), when analyzed on the assumption of D 2h symmetry leads to inconsistently large values of biaxial order parameters and to mutually conflicting inferences regarding the molecular attributes underlying phase biaxiality (20). In addition to these difficulties, X-ray diffraction studies in the nematic phase of bentcore compounds (22)(23), as well as in the nematic phase of the side-on monomers which form the calamitic tetramer compounds (24)(25), show the presence of local biaxial order that is characteristic of tilted-layer domain structures, thus favouring a monoclinic rather than orthorhombic symmetry for the biaxial phase that is formed when these domain structures become macroscopically ordered in the transverse direction (17).…”