Cholesteric liquid crystalline

Partial photochemical decomposition of racemic alkyl aryl sulphoxides in the presence of chiral amines as sensitizers gave non-decomposed sulphoxides in optically active form with optical purity of about 3%339. The report340 on the use of cholesteric liquid crystalline reaction media to change the enantiomeric composition of racemic sulphoxides at high temperatures could not be reproduced341. 

In Chapter 7, Gottarelli and Spada extend further the theme of liquid crystalline materials. They note that although much is understood, important mysteries remain about the relationships between the handedness of cholesteric liquid crystalline phases and the chirality of the constituting building blocks. This may seen surprising given the intense interest in these materials, including their... 

Finkelmann, F., Ringsdorf, H., Siol, W. and Wendorff, H. Synthesis of cholesteric liquid crystalline polymers. Makromol. Chem. 179, 829 (1978)... 

In an attempt to discern the factor(s) most responsible for ordered solvent induced alterations of reaction rates and specificities, we have investigated the influence of cholesteric liquid-crystalline and other optically active media upon the induction or loss of optical activity in the atropisomers of 1,1 -blnaphthyl (BN, equation 1). We find that optical induction is negligible from thermal (ground-state) lsomerizations (usually <0.1%) but is larger for excited-state lsomerizations conducted in cholesteric mesophases (up to 1.1%). The factors responsible appear to be the geometry and polarizability of the 15N triplet state and rather specific solvent-solute interactions in ordered... 

Figure 4. Hypothetical potential energy curves of BN along the coordinate for atroplsomerIc Interconversion In a cholesteric liquid-crystalline phase (---) and an Isotropic...

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Tunable Cholesteric Liquid Crystallinity of Helical Polysilane. 172... 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Fig. 31 a Changes in the cholesteric pitch and ICD intensity of 32-HC1 versus the enantiomeric excess (% ee) of 80 (S rich) in concentrated (20wt%) and dilute (inset, lmgmlr1) water solutions, b Polarized optical micrographs of cholesteric liquid crystalline phases of 32-HC1 (20 wt%) in the presence of 0.001 equivalent of (S)-80 and 5% ee (S rich) of 80 (0.1 equivalent) in water. (Reprinted with permission from [151]. Copyright 2004 American Chemical Society)... 

The cholesteric mesophase formed by cholesteryl p-nitrobenzoate at 200 °C has been used as the solvent to effect an asymmetric synthesis lrans-but-2-enyl p-tolyl ether gave the product of an ortho-Claisen rearrangement, 2-(but-1 -en-3 -yl)-4-methylphenol. This material exhibited circular dichroism, although neither the optical yield nor the configuration of the product is yet known.262 Decarboxylation of ethylphenylmalonic acid in cholesteryl benzoate at 160 °C (cholesteric liquid-crystalline phase) also proceeded with asymmetric induction to give (R)-(—)-2-phenylbutyric acid, with 18% optical yield.263 Electric dipole moments are reported for some esters of 5a-cholest-8(14)-en-3j8-ol there is some slight correlation with melting points.264... 

Similar results have been observed in the photodimerization reaction of acenaphthylene [732, 733] cf. Eq. (5-161) in Section 5.5.8. A considerable increase in the production of the traro-adduct was reported in cholesteric liquid-crystalline media compared to the isotropic solvent benzene, in which the cu-adduct is the dominant product... 

Cholesteric liquid crystals are optically active nematic phases as a result of their gradual twist in orientational alignment. Therefore, cholesteric liquid-crystalline solvents are expected to induce enantioselectivity in chemical reactions see reference [713] for a review on photoasymmetric induction by chiral mesophases. The existing results are not very promising. So far, the maximum photoasymmetric induction reported has... 

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. 

Using appropriate melting point apparatus, characterize the melting behavior of the purified solid. Cholesteryl nonanoate exhibits a chiral nematic, more commonly known as a cholesteric, liquid crystalline phase around 85°C and melts to the isotropic liquid around 93 C. The cholesteryl nonanoate first melts to form a smectic phase around 75°C and with further heating transforms to the cholesteric phase around 85°C. 

Nucleation studies on supercooled cholesteric liquid crystalline materials (cholesteryl caproate and nonanoate) are reported. The presence of sodium deoxycholate induces optical activity in bilirubin, with significant Cotton effects near 410 and 460 nm. ... 

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast fdms and thermotropic mesomorphic state in some copolypeptides are also discussed. 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

Fig. lla and b. Cholesteric liquid crystalline structure of PBLG in m-cresol a, striation patterns observed under a polarizing microscope b, optical diffraction pattern with a beam from the He—Ne gas laser. Concentration is 17% volume fraction of polymer, and cell thickness is 2 mm... 

Petn, A., Kummer, S., Anneser, H., Eeiner, E, and Btauchle, C. Photoinduced reorientation of cholesteric liquid crystalline polysiloxanes and applications in optical information storge and second harmonic generation. Ber. Bunsenges. Phys. Chem. 97, 1281 (1993). 

Bowen, J.M. Crone, T.A. Hermann, A.O. Purdie, N. Circular dichroism spectra of opium alkaloids in a cholesteric liquid crystalline solvent system. Anal. Chem. 1980, 52, 2436-2440. 

Figure 4.31 (Left) Schematic view of the relative arrangement of chiral molecules (extended lozenges) in the cholesteric liquid crystalline mesophase (after [59]. The twist between layers is greatly exaggerated. In reality approximately 10 layers lie between equally inclined layers. (Right ) Helical arrangement of molecules, with a relative twist between molecules along one direction only the axis of the helical ribbon.

The most remarkable feature of the cholesteric liquid crystalline phases is their ability to rotate the plane of transmitted plane-polarised light. Other liquid crystalline phases (for example those formed by certain polypeptides) with such properties have also been classified as cholesteric liquid crystals. 

The structure of the blue phase is of some importance. Among the lipoproteins carrying lipids in the blood, low-density lipoproteins (LDL) have attracted much attention. They are the factors mainly responsible for plaque formation, which ultimately leads to atheriosclerotic changes and heart disease. The major components of the LDL-particles are cholesterol fatty acid esters. A remarlmble property is the constant size of LDL particles [28], which indicates that the interior must possess some degree of order. It seems probable that the structure proposed above for cholesterol esters in the cholesteric liquid-crystalline structure should occur also in the LDL-particle. In that case the LDL particle can be viewed as a dispersed blue phase, whose size is related to the periodicity of the liquid-crystalline phase, and the protein coat at the surface is oriented parallel to adjacent specific crystallographic planes of the blue phase. These amphiphilic proteins will expose lipophilic segments inwards emd expose hydrophilic groups towards tiie enviroiunent. 

L-glucose enantiomer (Figure 5.38). Correspondingly, the green cholesteric liquid crystalline phase becomes red upon addition of D-glucose and blue upon addition of L-glucose. Diasteromeric hexoses gave similar effects. ... 

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