Enantiomeric mesophase

The simplest mesophase is the nematic phase. It is very fluid and involves highly disordered molecules having only short-range positional order, but with the molecules preferentially aligned on average in a particular direction (the director). If the constituent compound is racemic then it is possible to form a phase from the enantiomerically pure compound which is a chiral nematic phase. 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

The experiments of Table 6 indicate that a cholesteric mesophase with a right-handed helix gives an excess of (+)-[6]-helicene, which is known to have also a right-handed helix. The optical yield is small but significant. Nakazaki et al. 72) did not find induction in an isotropic mesophase (Table 6, c), while Hibert73) got still an enantiomeric excess in an isotropic and in several compensated nematic mesophases (Table 6, f, i, j, k, 1). Hibert explained these findings by discerning two effects ... 

Hie first helical SmC mesophase for a gold(I) mesogen has been prepared by means of complexation to an enantiomerically pure chiral isocyanide (Figure 7.32) [30]. This... 

Research utilizing the thiepine skeleton in material science has been reported. For example, liquid crystals with chiral dihydrodibenzo[c,i ]thiepines were studied. The enantiomerically pure (-)-(i )-3,9-bis[4-(dodecyloxy)benzoyl-oxy]-5,7-dihydro-l,l l-dimethyldibenzoR,f]thiepine 80 and (—)-(R)-3,9-bis[4-(dodecyloxy)benzoyloxy]-5,7-dihydro-1,1 l-dimethyldibenzoR,f]thiepine dioxide 82 display smectic C mesophases by a rigid twisted biphenyl core and axial chirality <1998JOC3895>. 

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. 

A variety of chiral alignment media for organic solvents is known with the most widely used and best characterized being the poly(amino acids) PBLG and PCBLL. Both polymers form lyotropic mesophases and possess ot-helical structures for which many examples of enantiomeric differentiation have been shown. In addition to chiral poly(amino acids), it was demonstrated that achiral media with chiral cages like cyclodextrines serve as alignment media with the potential of chiral discrimination.121... 

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... 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Pn-3m AMS-10 Bicontinuous cubic mesophase composed of an interwoven enantiomeric pair of 3D networks Effective pore diameter 40 -50 A... 

When a chiral substance is dissolved in a nematic liquid crystal, a cholesteric mesophase is obtained(8). The cholesteric structure is characterized by its handedness (P-or M-helix) and pitch. Equal amounts of enantiomeric solutes of equal optical purity induce helical structures with identical pitch and opposite handedness... 

Lecithins and glycolipids are naturally occurring chiral compounds, which can create a big variety of mesophases (lamellar, hexagonal, cubic, ripple, gel phases, etc.). Classical chiral phases like cholesteric or ferroelectric smectic phases are hitherto not reported and, if they were found, they would not be very typical for this class of compounds. Some of the bicontinuous cubic phases may have enantiomeric pure chiral space groups. Further, lyotropic cholesteric and blue phases might be formed (see below). Ordered tilted lamellar phases have been reported for lecithin and other amphiphilic compounds, but the special effects of chirality, like ferroelectric properties or helical order, are unknown. 

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