Compounds biaxial nematics

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...

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,... 

Hexacatenar compounds derived from cinnamic acid exist as dimeric molecules (Table 41) [213]. The compounds represent the seldom occurring biaxial nematic phase. Some compounds with four rings in the core are also nematic however, their biaxiality has not been proven [213],... 

In order to investigate the gap between dis-cotic and calamitic liquid crystals and in an attempt to obtain biaxial nematics, Fletcher and Luckhurst [235] synthesized hybrids of rod-like and disk-like mesogens, by linking two units to dimers. Among the several compounds of this type is one example that shows nematic behaviour (21) however, the... 

Figure 8. Interference patterns of the biaxial nematic phase of compound 9 (/j=12) at 137°C. (a) The trace of the optic axial plane lies parallel to the polarizer or analyzer (extinction position) (b) and (c) represent the trace of the optic axial plane enclosing an angle of +45° or -45° with the polarizer position (from [24]).

Heilmeier displays 281 polyamides, biaxial nematics 940 polycatenar compounds, thermotropic cubic... 

The first thermotropic compound for which a biaxial nematic phase was claimed is the laterally broadened mesogen shown in Figure 5-1. The assignment of the phases was based on the observation of the optical textures and the X-ray diffraction pattern [2]. 

Several low molecular weight compounds have been synthesized incorporating the features described above in an attempt to observe biaxiality in the nematic phase. Many such copper(II) complexes were synthesized [21] exhibiting the nematic phase in a metastable state. The overall shape of the molecules of these complexes resemble that of a pipet. One of these complexes, bis [ 1 -(p-n-decylbiphenyl)-3-(p-ethoxyphe-nyl)propane-l,3-dionato] copper(II) (compound 2) showed evidence of biaxiality in... 

Figure 3. Plot of the transition temperatures (y-ax-is) versus the number of methylene units (j -axis) in the alkyl bridge of the twin ethers (compound 9). C crystal Nu t, discotic nematic biaxial I isotropic x melting points clearing points (from [18]).

Prior to discussing the structural features of discotic mesogens in Section C, some other distinguishing features of these fascinating, discotic - for many still exotic - materials will be discussed here induction of nematic phases and the possibility of thermotropic phase biaxiality envisaged for them. Along with thermotropic Nd phases of pure compounds, further fundamental modes of formation of nematic phases of disc-like molecules have become known. 

Subclass 02, biparallel structures. This class, with nematic structures, was obtained by Hessel and Finkelmann and called biaxial. While the word biaxial is quite appropriate, it is also used in the liquid crystal field in connection with structures of smectic C phases, hence a new name was proposed. A low-molecular-mass 02 compound was synthesized by Malthete and his colleagues. Properties of a polysiloxane with phenyl benzoate mesogens obtained by Finkelmann were studied by Hotz and Strobl. " ... 

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