Optical rotation, cholesteric

From the diameter of the disclinations, the pitch can be measured30 furthermore, clockwise rotation of the lens generates a shrinking or an expansion of the circles, according to the cholesteric handedness from this, the helical handedness can be deduced.31 It should be stressed that this is not a spectroscopic method and gives results completely independent from those obtained with CD or optical rotation. 

From the stereochemical point of view, (3 characterizes a chiral compound similar to the classical rotatory power [a]0. However, the two quantities have a different origin The optical rotation is a consequence of the chiral interaction between light and matter, while the cholesteric induction originates from a... 

Solutions of cellulose in NH3/NH4SCN (27 73 w/w) are liquid crystalline at concentrations from 10-16% (w/w) depending on the cellulose molecular weight (64). Optical rotations of the solutions indicate the mesophase is cholesteric with a left-handed twist. The solvent does not react with cellulose. Recently, Yang (60) foimd that cellulose (D.P. 210) formed a mesophase at 3.5% (w/w) concentration at a NH3/NH4SCN of 30 70 (w/w). 

Organic materials with large optical rotations include cholesteric liquid crystals, molecules and polymers with chiral jt-conjugated systems, especially [n]helicenes [21, 31, 139]. The most important factor contributing to their large optical rotations is anomalous optical rotatory dispersion (ORD), which is associated with the presence of absorption (or reflection) with large rotational strength (Fig. 15.30). 

A solution of PBLG in certain solvents such as methylene chloride and dioxane can form the cholesteric structure and exhibits stroi form optical rotation. A theoretical description for this in terms of observable quantities is in the form (4S) ... 

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.

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).

As early as in 1951 de Vries showed that a twisted stack of birefringent layers is an adequate model for a cholesteric structure in order to reproduce a principally correct spectral dependence of the optical rotation also around the selective reflection band as it was recorded in a different way for Fig. 4.6-8. Even if the layer thickness is formally reduced to zero the optical rotation and its spectral dependence is preserved. Several other approaches were reported to describe particular effects of the cholesteric structure such as the selective reflectance or the rotatory anomaly (e.g. Chandrasekhar and Prasad, 1971 Chandrasekhar and Ranganath, 1974 SchSnhofer et al., 1983 Eidner et al., 1989). 

Cholesteric lyotropic mesophases of cellulose in dimethylacetamide-LiCl solutions have been observed by Ciferri and coworkers (9-11). While cellulose/TFA-CH2Q2 mesophases have positive optical rotations, the cellulose/ LiCl/DMAC mesophases have negative rotations. 

The sense of the cholesteric helix must be positive in the case of CELLOH and negative for CTA. The reason for the hi er optical rotations for the CELLOH solutions compared to the CTA solutions is not obvious based on the number of chiral centers per structural unit, but may be related to the tightness of the pitch. While some data (17) exists on the pitch of CTA in TFA-CH2CI2. none is available for CELIXIH in TFA-CH2Q2 due to the small domain size of the mesophase, that is, a fingerprint pattern is not observed in the case of CELLOH. 

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

Other equally remarkable optical properties are associated with the selective reflection. At normal incidence, the reflected light is circularly polarized one circular component is totally reflected, while the other passes through unchanged. Also, quite contrary to what is found in normal substances, the reflected wave has the same sense of circular polarization as that of the incident wave. This is an important difference between the nature of the optical rotation of normal substances and of cholesteric liquid crystals. While the more familiar cases of optical rotation have their origin in the selective absorption of one circularly polarized component of the light, the form optical rotation of the twisted structure in cholesteric liquid crystals originates in the selective reflection of one circularly polarized component of the light. 

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

Unfortunately, there is no report on the detailed physical characterization of these polymers. Such information as unidirectional twist angle and form optical rotation, as well as their dependence on chemical structures and temperature, can be very useful in further understanding the molecular orientations of the polymers in the cholesteric phase. In contrast, a number of studies have been made on the physical-chemical properties of cholesteric lyotropic polymer systems, especially polypeptides. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

Cholesteric liquid crystals have more optical activity than conventional crystals do. The optical rotation power is a function of wavelength, cholesteric pitch, and birefringence, i.e.,... 

The pitch P is the most important parameter of cholesteric liquid crystals. The physical properties of cholesteric liquid crystals are associated with P, such as selective reflection, optical rotation dispersion, circular dichroism, etc. The helical pitch is sensitive to the temperature and external field, for example, electric and magnetic field, chemical environment, pressure or radiation, etc. 

The cholesteric-like structure has been found in many biological bodies. The report on these discoveries is scattered, mainly based on optical rotation observations and chiral structure. For example, the wings and shells of some insects can rotate incident light and selectively reflect one circular polarized light (Shimamura et al., 1981). Very few experiments on these materials are available because of the difficulty in extracting and separating the effective materials from biological bodies. 

No racemization was observed when the electrode potential was scanned only to a value where the dianion is formed. Upon formation of the tetraanion, subsequent chemical reactions were found. With a slightly different electrolyte salt (Mc4NBF4 instead of BU4NF6), reversibility without racemization was found even up to the tetraanion formation. Further examples include the spectroelectrochemistry of vitamin D2 [139], which has been studied with a long pathlength cell similar to the design described by Zak et al. [44]. Optical rotary dispersion and CD of optically active polybithiophene that has been electropolymerized in a cholesteric electrolyte have been studied [140]. The optical rotation of this chiral polymer could be controlled via the electrode potential. 

Fig. 4.7.1. Anomalous optical rotation in the isotropic phase of a cholesteric liquid crystal. Open and closed circles are measurements on two different samples appropriately normalized. Cholesteric-isotropic transition temperature 60.57 C. X = 0.6328 /mi. (After Cheng and Meyer. )...

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