Induced cholesteric solution

Figure 4.6-9 Induced cholesteric solutions Schematic outline of experiment and evaluation of the optical rotation p(A) related to the selective reflection band (reflection Cotton effect, RCE, centred at the wavelength A/ ) in order to characterize the chirality of the solute molecules by the helical twisting power.

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 4.6-10 Infrared spectra of transmittance T, reflectance R, and optical rotation p of an induced cholesteric solution at 22 °C (Ch) and as isotropic liquid at 63 °C (Is). Solvent eutectic mixture of the isomeric N-oxides of p-methoxy-p -n-butylazobenzene, see 3 of Table 4.6-1 (Nematic Phase IV Licristal E. Merck) solute 17-3-acetoxy-5-/3-androst-l-en-3-one molar fraction x = 0.05.5 (Korte and Schrader, 1981).

The phenomenon of induced cholesteric solutions can be considered as an amplification of chirality by transforming it from the intramolecular level into macroscopic order. Induced cholesteric solutions proved to be an often very sensitive tool to reveal and to characterize molecular chirality, the experimental data can be achieved from simple infrared spectra. For a detailed interpretation of the cholesteric optical activity, the optical model used must account for the often masked anisotropy. 

Korte EH (1985) The characterization of entiomers by induced cholesteric solutions principles and experimental techniques. In Proceedings of the Internationa) Conference on Circular Dichroism, vol 6. Bulgarian Academy of Sciences, Sofia, p 145 Korte EH (1988) Appl Spectrosc 42 428 Korte EH (1990a) Vibrat Spectrosc 1 179... 

Korte EH, Schrader B (1981) A new chiroptical method Infrared rotatory dispersion of induced cholesteric solutions. In Clark RJH, Hester RE (eds) Advances in infrared and Raman spectroscopy, vol 8. Heyden, London, p 226 Korte EH, Schrader B, Bualek S (1978) J Chem Res M 1978 3001 Korte EH, Staat H (1989) SPIE 1145 296 Korte EH, Staat H (1993) Fresenius J Anal Chem 347 454 Koriiim G (1969) Refiexionsspektroskopie, Springer, Berlin Kortiim G, Delfs H (1964) Spectrochim Acta 20 405 Kosloff R (1988) J Chem Phys 92 2087 Kostyk E, Welsh HL (1980) Can J Phys 58 534... 

Korte, E.H., andB. Schrader. 1981. New chiroptical method infrared rotatory dispersion of induced cholesteric solutions, in Advances in Infrared and Raman Spectroscopy, H. Clark and C. Hester (Eds.). Heyden, London. 

Jordanov, B., and D. Tsankov. 1984. A method for measuring circular dichroism of induced cholesteric solutions in the infrared region. J. Mol. Struct. 117 261-264. 

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

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. 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

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... 

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... 

Liquid crystalline solutions of KB LG (or of RBDG) and films prepared from such srdutions also ow significant CD in the wavelength range of the aromatic absorption bands 46,47) the CD for PBDG in methylene chloride is positive (46). CD bands are also induced when dye mdecules are introduced in liquid crystal films of polypeptide (PMDG). These induced CD bands are interpreted as arising from the dissymmetric field of the cholesteric structure 48). 

As a first step towards the measurement of single molecule effects, Schrader and Korte (1972) reported the measurement of the infrared rotatory dispersion of carvone in liquid crystalline solution. They used a modified commercial spectrometer. They observed a huge effect which is not the result of the carvone itself but of the liquid crystal in which a helical arrangement (cholesteric state) is induced by the chiral solute (Sec. 4.6.4). In this case the liquid crystal acts as a kind of molecular amplifier which allows the absolute configuration of tiny amounts of solutes to be determined reliably. At about the same time Dudley et al., (1972) measured the infrared circular dichroism of (-)-menthol in a liquid crystal. Their equipment consisted of a normal infrared spectrometer supplemented by a Fresnel rhomb made from sodium chloride. 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

The thermally induced inversion of the cholesteric sense was considered to be related to the intermolecular hydrogen-bond formation of the side chain ester groups of PBLG with m-cresol, since it was first observed in m-cresol solutions. However, our subsequent investigations revealed that the sense inversion also occurs in 1,2,3-trichloropropane(TCP) which does not have any particular functional groups. A similar finding was recently reported in 1,1,2,2-tetrachloroethane. ... 

They found that substrate 1 and the eomplex catalyst [RhCl(PPh3)3] in the presenee of the cholesteric liquid crystal (CTD) in butanol solution should induce eireular dichroism with maximal value in the temperature range of the... 

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