Biaxial nematic phase

Disk-shaped molecules based on a metal atom possess discotic Hquid crystal phases. An example is octasubstituted metaHophthalocyanine. FiaaHy, metallomesogens which combine both rod-like and disk-like features iato a single molecule adopt the biaxial nematic phase. In addition to there being a preferred direction for orientation of the longest molecular axis as is tme for the nematic phase, perpendicular to this direction is another preferred direction for orientation of the shortest molecular axis (12). NonmetaHomesogens which combine both rod- and disk-like features iato a single molecule also adopt a biaxial nematic phase, but at least ia one case the amount of biaxiaHty is very small (15). 

Fig. 3 Hekates with flexible spacers. Compounds 1, 2 Thermotropic properties as a function of spacer length. Compounds 3, 4 LC properties with unchanging spacer length and variable peripheral chains. Compounds 5, 6 Nematogens claimed to form biaxial nematic phases. Cr crystal, g glass, N nematic, N cholesteric, SmX unidentified smectic phase, Colh columnar hexagonal phase. All temperature are given in °C...

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

NMR experiments carried out on one of these co-dendrimers have proved that the symmetry of the nematic phase is imiaxial, in contrast to the biaxial nematic phase observed for some side-on-polymers [219]. 

However, in 1986, Chandrasekhar [70] published a derivative (Figure 35) which he claimed to show a biaxial nematic phase. This report was interesting because the biaxial nematic phase (Nb), demonstrated in lyotropic systems [71], had been long sought after in thermotropic materials. Further, the molecules were described as bridging the gap between rod- and disc-like materials (a reference perhaps better reserved for polycatenar liquid crystals—vide infra)... 

The existence of the biaxial SmA phase (SmAj,), also known as the McMillan phase, has been demonstrated in some charge-transfer complexes (CT complexes) formed with the metallomesogens 126 and TNF by textural observations and X-ray investigations. (Like the biaxial nematic phase mentioned earlier, the SmAt phase has additional long-range order in a direction perpendicular to the principal director, n (Figure 7)). Two novel... 

Conoscopic studies suggest that this compound in fact exhibits a biaxial nematic phase although these experiments indicated only a small opticd biaxiality. Indeed, more recently, NMR spectroscopy using selectively deute-rated materials has shown that for other materials the assignment of a biaxial nematic phase on the basis of conoscopy alone is not necessarily reliable [118], This is discussed further in Sect, 3,4,... 

The existence of the thermotropic biaxial nematic phase was theoretically predicted almost thirty years ago [119] but such a phase has yet to be unambiguously identified. Indeed, as we saw in the previous section, a biaxial nematic phase has been claimed for a class of discotic dimers [111], Theory predicts that mixtures of rod-like and disc-like molecules should exhibit the biaxial nematic phase [120] but experimentally such systems phase separate... 

This experimental difficulty was overcome by simulating the behaviour of a model system and a biaxial nematic phase was indeed obtained [122], One way by which the problem of phase separation could be solved experimentally was suggested by Fletcher and Luckhurst and involved covalently linking a rod-like and disc-like unit via a flexible spacer yielding a non-symmetric discotic-calamitic liquid crystal dimer [123] ... 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

Fig. 6.6.1. Two examples of molecules which exhibit the biaxial nematic phase (a) (bis-l-(p-n-decylbiphenyl)-3-(p-ethoxyphenyl) propane-1,3-dionatocopper (II))(8i.82) (hereafter referred to as complex A) (b) I,12- >w(pentakis((4-pentyl-phenyl)ethy nyl)pheny loxy)dodecane. ...

Fig. 6.6.3. Raw microdensitometer scans of the X-ray intensity plotted against diffracting angle (20) for magnetically aligned nematic samples (a) the uniaxial nematic phase of 80CB at 77 °C (b) the biaxial nematic phase of complex A at 168.5 C, (i) meridional scan (parallel to H), (ii) equatorial scan (perpendicular to H). M represents the diffraction peaks from the mylar film which covered the windows of the sample holder and heater assembly. ...

Fig. 6.6.4. Schematic diagram of the molecular order in (a) the uniaxial nematic phase and (b) the biaxial nematic phase.

Combined LC polymers have been investigated intensively in a search for biaxial nematic phases [21]. However, it has never been possible to prove the existenee of such phases. 

In 1970 the existence of a biaxial nematic phase for thermotropic LCs was theoretically predicted by Freiser [73] the first-order transformation from the isotropic state to... 

Figure 11. The three possible half integer disclination lines in the biaxial nematic phase of Vectra B 950 . (a) Rotation around the X axis, (b) rotation about the Y axis and (c) rotation around the Z axis [53], It is worth noting that the Ey line is perpendicular to the main director Y.

In certain cases, nematic (orientational but not positional) order may be observed in phases of (relatively small) micellar rods (canonic) or disks (discotic) [177], which are associated with lower temperature hexagonal and lamellar phases, respectively [178]. (See Fig. 17.) The nematic isotropic phases were thought to be built up of discrete aggregates of different shapes, but the presence of continuous aggregates has also been recently suggested in this case [178]. Some systems (e.g., potassium laurate-decanol-water) form biaxial nematic phases. In this case, the micelles are believed to be neither rods nor disks but rather to... 

For the ideal nematic with sim = 0 and %- 1 there is no difference between cases b and c. The locally (microscopically) biaxial nematic phase should not be confused with macroscopically biaxial phases to be discussed in the next section. 

Now the biaxial nematic phase has two order parameters Q and and, in general, three different phases can be distinguished, namely, isotropic (Q = Qi = 0), uniaxial nematic Q, Q i = 0) and biaxial nematic (01,02) phases. Note that biaxial molecules may form both biaxial and uniaxial phases the latter appear due, for instance, to free rotation of biaxial molecules around their long molecular axes. As to the uniaxial molecules, they may also form either uniaxial (as a rule) or biaxial phases the latter may be formed by biaxial dimers or other building blocks formed by uniaxial molecules. 

Yu, L.J., Saupe, A. Observation of a biaxial nematic phase in potassium-laurate -1-decanol-water mixtures. Phys. Rev. Lett. 45, 1(X)0 (1980)... 

Acharya, B.R., Primak, A., Kumar, S. Biaxial nematic phase in bent-core thermotropic mesogens. Phys. Rev. Lett. 92(14), 145506 (2004)... 

Nb Dzh X T(3) Biaxial nematic phase possessing long range orientational order and no translational order... 

A survey of recent studies of biaxial liquid crystals, in the context of the reported biaxial nematic phase in low molecular mass bent-core mesogens, with emphasize on the use of NMR spectroscopy is available. A discussion of orientational order parameters particularly in smectic-C and biaxial nematic phases and their determination by various techniques including NMR has been presented. The relation between molecular dynamics and biaxiality of nematic polymers and elastomers has been studied by H NMR spectroscopy. "" It was observed that the side-on attachment of the mesogens hinder the rotation of the molecules around their long axes and that in nematic polymers and elastomers phase biaxiality... 

The chemical shifts allowed the local order parameters to be computed which indicated the uniaxial to biaxial nematic phase transition. The nematic phase of a deuterated fiuorenone nematogen has been studied by NMR and X-ray and evidence for biaxial order close to its glass transition temperature has been inferred. The possible symmetries of the biaxial nematic phase have been examined based on experimental results and it is concluded that a monoclinic symmetry rather than an orthorhombic symmetry that is more likely to be the cause for the observed phase biaxiality in thermotropic bent-core and calamitic-tetrapode nematic systems. Density functional theory has been employed in a detailed conformational study of a bent-core mesogen The chemical shielding... 

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