Molecular twisting power

The (5)-(6.24) compound shows good solubility in a nematic host (ZLl-1275 Merck) and is an effective doping with a molecular twisting power (mtp) of between 1500 and 2400m mol , for a doping agent concentration of 17.43 wt% and at a temperature of... 

From a cholesteric induction experiment, one can obtain chiral information on the induced cholesteric (namely, pitch and handedness) and therefore the helical twisting power of the dopant in that solvent (at a certain temperature). If a model or molecular theory relating molecular chirality to mesophase chirality is available, one can infer stereochemical information about the dopant (absolute configuration, preferred conformation). 

Several stereochemical applications of cholesteric induction have been described.52 55 Reliable results are obtained especially when the dopant has a high twisting power. One of the molecular fragments associated to a high value of twisting power is, as anticipated above, the biaryl unit therefore many experiments have been done on compounds containing this unit. 

In contrast to binaphthyls, chiral biphenyl derivatives are challenging systems because their twist ability shows a strong dependence on the molecular structure, which does not conform to the empirical correlation rule described above. In fact, homochiral biphenyls 33-40 are characterized by P helicity along the biphenyl axis. In spite of this common feature, the twisting power spans from a highly positive value for 33-45 to a relatively negative value... 

Although biaryl-based chiral molecules are the most extensively studied, also other molecular frameworks are associated to high twisting powers and thus suitable for stereochemical studies. One of the first chiral structures used in... 

A theoretical study of numerous chiral molecules including bridged biaryls 5 and 6 has been undertaken using a molecular Monte Carlo simulation approach coupled with calculations of molecular chirality based on a chirality order parameter. The method successfully predicts the helical twisting powers <2003JCP10280>. 

The induction of chirality in liquid crystals (LCs) has a long history [100-104]. The supramolecular induction can be used to assign absolute configurations [105-108], conformations of molecules [109,110] and the interplay between inter- and intra-molecular interactions [111], and models can be developed to justify the sense of the inductions that are observed. Twisting powers of dopants—the twist per mole—can be pushed to extraordinary values [112]. Given the history and vast body of work, we will focus here on the more contemporary aspects of work in this area. 

Fig. 14. Theoretical line with experimental points of the dependence of the twisting power of PBLG on the molecular weight in EDC. (c x 0.20)...

Szamiecka and Samulski have attributed the inversion of the twisting power occuring with the changes of temperature and of solvent to the transformation in the sense of coiling of the polymer. According to the theory by Kimura et al., the inversion can occur thermally, with no such essential transformation in molecular conformation. 

Numerous investigators have proposed theories for the temperature dependence of pitch. " However, none completely explain the experimental results obtained for the various cellulosic systems. Of those proposed, Osipov s approach, which is based on a molecular statistical theory,takes into account steric and chiral interactions in solution to predict the influence of temperature and solvent on the pitch and twist sense of cellulose deiivatives. Assuming the cellulosic chain adopts a twisted belt as opposed to a helix, and the persistence length, /, of the cellulose chains is much smaller than that of a rigid chain, the twisting power could be expressed by... 

Goossens, W.J. Molecular theory of cholesteric phase and of twisting power of optically active molecules in a nematic liquid crystal. Molec. Crystals Liquid Crystals 1971, 12 (3), 237. 

To get more insight into the polymer-solvent interaction, especially to investigate the correlation between chirality on a molecular level (chiral centers, chiral conformation) and the helical twisting power. 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

Alam MZ, Yoshioka T, Ogata T, Nonaka T, Kurihara S. 2007. The influence of molecular structure on helical twisting power of chiral azobenzene compounds. Liq Cryst 470 63 70. Bahr C, Escher C, Fliegner D, Heppke G, Molsen H. 1991. Behavior of helical pitch in cholesteric and chiral smectic C phases. Ber Bunsen Ges 95(10) 1233 1237. 

R non-mesogenic molecules showing a molecular rotatory power and able to twist a nematic liquid. 

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

Finally, the other property which is also strongly dependent on molecular symmetry associated with the chiral centre is the twisting power, )8, which is usually expressed... 

Investigations of the helical twisting power in mixtures of laterally aryl substituted mesogens each other or with rod-like mesogens (one of the components of which is chiral) are concerned with relationships between the molecular constitution and the values ofthe helical twisting power [69,72]. 

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