Enantiomers chiral nematics

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

The first phase of this type was found in esters of cholesterol and became known as the cholesteric phase (Ch). However, this is the chiral analog of the nematic phase and is not confined to derivatives of cholesterol. It is therefore more correctly named the chiral nematic phase N. The structure of this phase is shown in Figure 2.2. Opposite optical enantiomers give helices of opposite handedness. 

Lyotropic polymeric LC, formed by dissolving two aromatic polyamides in concentrated sulphuric acid, have been studied using variable-director 13C NMR experiments.324 The experimental line shapes at different angles w.r.t the external field were used to extract macromolecular order and dynamic in these ordered fluids. An interesting application of lyotropic LC is for the chiral discrimination of R- and S-enantiomers, and has recently been demonstrated by Courtieu and co-workers.325 The idea was to include a chiral compound 1-deutero-l-phenylethanol in a chiral cage (e.g., /1-cyclodextrin) which was dissolved and oriented by the nematic mean field in a cromolyn-water system. Proton-decoupled 2H NMR spectrum clearly showed the quad-rupolar splittings of the R- and S-enantiomers. The technique is applicable to water-soluble solutes. 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

Cholestogenic compounds (i.e. such which produce cholesteric phases) are optically active, i.e. their molecules are chiral under equivalent conditions the enantiomers form countercurrent, but otherwise identical helical structures. The close relationship of cholesteric and nematic phases is emphasized by the fact that a racemic mixture of cholestogenic compounds does not form a mesophase but a nematic one (Leclercq et al., 1969). A nematic phase can also be formed by mixtures of non-enantiomeric cholesto-gens which tend to form oppositely coiled structures on their own. However, the ratio... 

Such twisted nematic phases are called induced cholesteric solutions and - as schematically outlined in Fig. 4.6-9 - enantiomers cause countercurrently twisted structures. As discussed by Korte and Schrader (1981) this effect offers the potential of sensitively characterizing the chirality of small amounts of optically active compounds. There are no restrictions as to the type of chirality, and the experiments can advantageously be based on infrared spectroscopy. The application of induced cholesteric solutions was later reviewed again by Solladie and Zimmermann (1984). The host phase is the more twisted the more of the optically active guest compound is dissolved. Quantifying the twist by the inverse pitch z and the concentration by the molar fraction x, the ability of a chiral. solute to twist a given nematic host phase is characterized by the helical twisting power (HTP Baessler and Labes, 1970). For small values of a this quantity P is defined by the relation... 

Figure 4.6-13 Optical rotation q recorded as outlined in Fig. 4.6-12 Spectra of two differently concentrated solutions of S-tyrosine-methylester in the nematic mixture EBBA/MBBA (equimolar mixture of N-(p-ethoxybenzylidene)-p - -butylaniline and its methoxy analogue 2 of Table 4.6-1 Riedel-de Haen), left RCE (molar fraction x fa 0.024) related to the selective reflection band indicating pitch and handedness of the. structure, thus characterizing the chirality of the solute molecules by the helical twisting power right Sequence of ACE (,v se 0.0024, therefore the RCE should occur around 200 cm ) each of which indicates the induced handedness and therefore, discriminates enantiomers (Koite, 1978).

Two series of polycatenar complexes containing the same metallic fragments were also reported [115]. As expected for polycatenar complexes, nematic (54 n=6, 10 and 11), Sc (54 n-lO and 11) and Colh (54 n=12-16) phases were obtained. Resolution of the pure enantiomers gave rise to the equivalent chiral phases for the same n (N and Sc ), and Colh phases (chiral ) for n> 12, with slight differences in the transition temperatures as compared to the racemic mixtures. Small spontaneous polarisations were measured. 

This section pertains to reports on oriented molecules in which phases other than the usual thermotropic nematics have been used. Studies in chiral, smectic, columnar, lyotropic and polymeric liquid crystals as well as other unusual phases have been presented. The use of carbon-proton heteronuclear selective refocusing 2D NMR experiment designed for the spectral analysis of enantiomers dissolved in weakly ordering chiral liquid crystal solvents has been proposed." The method permits the extraction of carbon-proton residual dipolar couplings for each enantiomer from a complex or unresolved proton-coupled... 

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