Chiral thermotropic liquid crystal

In Part I we present a survey of the work done in the preparation and characterization of synthetic and semi-synthetic chiral thermotropic liquid crystal polymers. For convenience, we have grouped the polymeric materials (until now reported) according to the nature of the repeat unit and relevant position of the mesogen, side chain and main chain polymers. In Part II we report on the results obtained in our laboratories on optically active thermotropic polyesters containing mesogenic aromatic dyads or triads based on / -oxybenzoic acid. 

Side Chain Chiral Thermotropic Liquid Crystal Polymers... 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

An analogous enhancement in the optical rotatory power of the isotropic phase of a thermotropic liquid crystal has been observed near the isotropic-cholesteric phase transition. Patel and DuPre concluded that it is attributable to short-range chiral ordering of the long axes of the macromolecules. 

In thermotropic liquid crystals, the cholesteric sense is determined by the chirality of the constituent molecule the cholesteric sense of the liquid crystals of an optical isomer must be opposite to that of its mirror image isomer. When equal moles of both isomers are mixed (racemic mixture), the twisting power falls to zero and the... 

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

In the case of thermotropic liquid crystals, a surface description is mathematically useful, but physically misleading, since these surfaces are fictional they serve only to describe the three-dimensional variation of molecular orientation. An alternative description of blue phases in tem s of close-packing of chiral rods can be found in die next Chapter (section 5.1.8). 

Sage, I. Materials Requirements for Nematic and Chiral Nematic Electrooptical Displays. In Thermotropic Liquid Crystals-, Gray, G. W., Ed. Wiley Chichester, 1987 Chapter 3, pp 64-98. 

To our knowledge, this is the first example of the coexistence of both twisted smectic and cholesteric phases in thermotropic liquid crystal polymers. Previous preparations of thermotropic polymers by the use of chiral derivatives both incorporated in the macromolecular backbone and pendant to it as side chain substituents (comb-like polymers) resulted in either cholesteric or smectic " polymeric products. 

Only a few examples of optically active ferrocene-containing thermotropic liquid crystals have been reported. Compounds (304) and (305) have been prepared by functionalizing ferrocene with a chiral fragment bearing an asymmetric carbon atom. Ferrocene derivative (304) showed an SmC phase, while for (305), SmC, SmA, TGBA, N, and blue phases (blue phases are referred to in Section 7.9.4.3.3) were obtained. " Ferrocene has also been substituted with a cholesterol derivative but the mesophases were not identified. ... 

Samulski et al2D suggested that an asymetric helical molecular conformation may provide an explanation for the cholesteric structure, and very small oscillations of helices relative to their neighbors in the liquid crystal would be biased to one side of the parallel position because of the chirality of the van der Waals surface of the polypeptide molecules. Further, they found a linear relationship between the pitch and the reciprocal of temperature. This relation holds quite well for a number of thermotropic liquid crystal systems and was explained theoretically by Keating ), who treated the macroscopic twist as a rotational analogue of thermal expansion with the dominant anharmonic forces coming from nearest neigh-... 

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