Synthesis chiral nematics

During the first week of this experiment, you will synthesize cholesteryl nonanoate, which is an ester derivative of cholesterol, and perform some simple characterizations to test your successful synthesis. Cholesteryl nonanoate (ChNon) is known to exhibit a chiral nematic, i.e., a cholesteric, liquid crystalline phase. 

As this compound was one of the higher homologues in the series, and because we knew that the earlier homologues did not exhibit a chiral nematic phase, it was clear that the new phase also could not be a chiral nematic phase. In addition, it was clear from the formation of the defect structures seen in the microscope that the phase first formed from the isotropic liquid possessed a helix, see Plate 1, which had its heli-axis at right angles to the heli-axis in the lower temperature chiral ferroelectric smectic phase. This simple observation meant that if the phase was a lamellar smectic phase then the helix would have to be formed, inconceivably, in a direction parallel to the layers. Synthesis of the achiral variant confirmed that the phase formed first on cooling from the isotropic liquid was indeed a smectic A phase, and thus we immediately knew that we had found a smectic A phase where the helical macro structure formed in the planes of the layers, and thus the helix must... 

Recently Ribeiro et al. [65] reported on the synthesis and characterization of a variety of tolanes that had optically active sulfinate groups. Some of these compounds, see 23, were found to possess a phase that exhibited oily-streak textures typical of chiral nematic phases and also defect pattern associated with columnar phases (the earlier photomicrograph Plate 6 for 14P1M7 is similar). 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

In comparison to the synthesis of separate chiral and nematic precursors, the idea of a hybrid chiral-nematic precursor, exemplified as Ch2 in Reaction Scheme 6.4, is quite appealing. Moreover, the synthesis of a cholesteric GLC can be completed in a single step at a 66 % yield [36], which compares favorably with the preceding statistical and deterministic approaches. Of particular significance is that (26) with (5 )-3-bromo-2-methylpropanol incorporated as the chiral linkage produced a left-handed cholesteric GLC film with Xr located at 413 nm. 

Scheme 6.4 Deterministic synthesis of a cholesteric GLCs consisting of hybrid chiral-nematic pendant groups...

In view of the unique roles of cholesteric GLCs in material synthesis and device application, the commonly practiced statistical synthesis and the continuing effort in deterministic synthesis are highlighted. All factors considered, a hybrid chiral-nematic precursor is much preferred over separate chiral and nematic precursors in terms of synthesis and subsequent product separation and purification. 

H.M.P. Chen, D. Katsis, S.H. Chen, Deterministic synthesis and optical properties of glassy chiral-nematic liquid crystals. Chem. Mater. 15, 2534—2542 (2003)... 

The synthesis of conjugated polymers in chiral nematic liquid crystal fields has been reviewed [9]. 

Piao, G., et al. 2001. Synthesis of well-controlled helical polyacetylene films using chiral nematic liquid crystals. Curr Appl Phys 1 121. 

Synthesis of Nematic Liquid Crystals and Chiral Dopants... 

Materials that generate the chiral nematic phase are very common and in many respects parallel those that give the nematic phase except that a chiral unit is present. Accordingly, it is the chiral unit that is of interest in this chapter. The chiral rmit is almost always found in the terminal chain because of the relative ease of synthesis (see Chapter 8). The inclusion of a chiral centre in the terminal chain invariably produces a sizeable steric effect (usually from a branched chain) which inevitably depresses the clearing point. So if... 

A necessary condition for the formation of the cholesteric mesophase, or chiral nematic phase, is the presence of a chiral center chemically bound to the molecules or an optically active additive. Such a centre determines the helical twisting meso-gens in both low-molecular-mass and polymeric liquid crystals. Wide opportunities for the synthesis of cholesteric polymers are provided by the copolymetization of nematogenic and chiral monomers (Figure 19). 

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