Liquid-crystal-induced circular dichroism

Liquid Crystal-Induced Circular Dichroism (LCICD)... 

We arrive at this conclusion from the lack of more than a slight atropisomeric excess (ca. 0.1% in all but one anomalous experiment) after equilibration of racemic BN in the cholesteric phases at several temperatures (TablelV). The lack of change in the ratio of atropisomers in the cholesteric phases is consistent with our observation that liquid-crystal induced circular dichroism spectra (67) of ISN in cholesteric mixture D are due to a macroscopic property of the solvent the LCICD spectra disappear when mixture D is heated to an isotropic temperature. 

LCICD liquid-crystal-induced circular dichroism... 

The u.v. and c.d. spectra of (364 R = Me, Et, Pr, Bu , or PhCH2) in EtOH and in iso-octane have been reported.The chiroptical properties of the oxaziridine chromophore were generally characterized by a positive pattern at 190—350 nm. The notable exception to this was a clear negative Cotton effect for (364 R = Pr ). The seemingly anomalous behaviour of this compound was interpreted in terms of the possible existence of solvation equilibria and conformational rotamers about the N—CHMe2 bond. The relatively new technique of Liquid-Crystal-Induced Circular Dichroism (LCICD) has been used in the... 

Liquid crystal induced circular dichroism (LCICD) examines the differential absorption of circularly polarized radiation by an achiral solute oriented in a cholesteric liquid crystal. The circular dichroism results from an induced Cotton effect in the achiral solute due to the macroscopic chirality of its ordering in the helical solvent matrix. The effect has been shown theoretically to arise... 

On the other hand, it was reported that either several reactions could not be reproduced or no asymmetric effect was found Negative results in several works were explained by the large steps of the helix in the structure of cholesteric liquid crystals (300-400 nm) and by the absence of chirodiastaltic interaetion with the small molecules of the substrates. In the case of large molecules of complicated stereostmctures it was possible to wait for the appearance of such interactions as revealed by the appearance of induced circular dichroism (ICD) in the absorption bands of chromophors of components and products of the reaction. 

H. Qi, J. O Neil, T. Hegmann, Chirality transfer in nematic liquid crystals doped with (S)-naproxen-functionalized gold nanoclusters an induced circular dichroism study. J. Mater. Chem. 18, 374-380 (2008)... 

Guo J-X and Gray DG (1995) Induced circular dichroism as a probe of handedness in chiral nematic polymer solutions. Liquid Crystals 18 571-580. 

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. 

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. 

A sufficient amount of oriented chiral molecules can be obtained in an induced cholesteric liquid crystal phase if the induced helical structure has been untwisted by an electric field. In the following description tensors are needed for the sake of simplicity (At least there are three tensors required the transition moment tensor (absorption tensor ,y), the rotational strength tensor (circular dichroism tensor A ,y), and the order tensor g,y33 (i,j= 1,2,3). If the molecules do not possess any symmetry, the principal axes of all of these tensors are differently oriented with respect to the molecular frame (the coordinate system in which only the three diagonal elements of a tensor are different from zero).) The only tensorial property, needed here explicitly, is the existence of three coordinates (components) of a tensor with respect to three specially chosen mutually perpendicular axes. This means that three information instead of one information about a molecule are needed instead of one CD value, namely Ae, three CD values, namely As, (i=l, 2, 3), have to be introduced. Ac is then one-third of a sum of the three so-called tensor coordinates of the CD tensor ... 

In addition, it was also reported that circular dichroism and circular birefiingence can be induced in thin films of achiral polymer liquid crystals containing 7 to 15 mol % azobenzene chromophores by irradiation with circularly polarized light The polymer can be illustrated as follows ... 

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