Twisting power

A quantitative study of the cholesteric induction and of the chiral transfer from dopant to phase requires the definition of the helical twisting power 3. This quantity expresses the ability of a chiral dopant to twist a nematic phase and can be numerically expressed in Eq. (2) where p is the cholesteric pitch, c the dopant molar fraction, and r its enantiomeric excess its sign is taken to be positive or negative for right-handed (P), or left-handed (M) cholesterics, respectively. This relation holds for molar fractions <0.01-0.05 ... 

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... 

Helical Twisting Powers of Compounds 15-24 Determined at Room Temperature in... 

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 last example of a dopant whose chirality has been investigated by the LC technique is represented by helicenes and related molecules. Once again, compounds very different spectroscopically, such as 46-49, and hence hardly comparable with chiroptical techniques, are very similar in shape and give helical twisting powers of the same sign and of comparable intensity80 the twisting powers of helicenes have been successfully calculated by the shape model of Ferrarini et al.73... 

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. 

Kricheldorf [17] studied liquid-crystalline cholesteric copoly(ester-imide)s based on 1 or 2. The comonomers to obtain these chiral thermotropic polymers were N-(4-carboxyphenyl)trimellitimide, 4-aminobenzoic trimellitimide, 4-aminocinnamic acid trimellitimide, adipic acid, 1,6-hexanediol, and 1,6-bis(4-carboxyphenoxyl) hexane. Apparently the poly (ester imide) chains are so stiff that the twisting power of the sugar diol has little effect. 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

By addition of each of several diesters of isosorbide, isomannide, and isoidide to a nematic phase, cholesteric phases can be produced. All compounds exhibit a large twisting power. In the cholesteric phase, helix inversion, large or small temperature-dependencies of the pitch, and broad blue phases were achieved.183... 

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>. 

Crown ether binaphthyl derivatives 128-131 (Scheme 71) were synthesized and investigated by Akagi [139], Compounds 128-131 were used to induce chiral nematic phases (N ) in liquid crystals. It was found that the helical twisting power increased with decreasing ring size. Helical polyacetylenes were synthesized in the N phases. It was found that the interdistance between the fibril bundles of the helical polyacetylene was equal to the half-helical pitch of the N liquid... 

Enhanced Twisting Power by Doping B4 Phase with Rod-Like Molecules. 319... 

Fig. 3 Inverse structural pitch (1/nP) as a function of dopant ratio. Bent-shaped molecule P12-0-PIMB enhances the twisting power, which is proportional to 1/nP, whereas rod-shaped molecule TBBA gives just a dilution effect the helical pitch increases...

This result indicates that the pitch becomes smaller with increasing content of BSMs if lb tanh (p0AU/2kT) /-(, namely the dilution effect is much smaller than the helical twisting power of BSMs. In the above discussion, we have neglected the effect of BSMs on the elastic constant K22. The effect on K22 has not been reported, whereas it has been experimentally shown that K V in bent-core nematogens is remarkably small compared with that in rod-like nematogens [25]. 

Thisayukta J, Niwano H, Takezoe H, Watanahe J (2002) Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules. J Am Chem Soc 124 3354-3358... 

Choi S-W, Fukuda K, Nakahara S, Kishikawa K, Takanishi Y, Ishikawa K, Watanahe J, Takezoe H (2006) Amplification of twisting power in chiral mesophase by introducing achiral rod-like compound with ester group. Chem Lett 35 896—897... 

T-type chromophores 37 (j-tum structure 418 twisting power 55 two-level system (TLS) pentacene 3 perylene 3 terrylene 3... 

When the twisting power of the closed-ring isomer is larger than that of the openring isomer, it is expected that UV irradiation should induce the phase change from the nematic to chiral nematic phases.1481 A diarylethene 31a, with two diarylethene units in a chiral cyclohexane, was incorporated into K15 and the phase change concomitant with photoisomerization was measured. 

Optically active bis-imine-functionalized diarylethene (2-4 %) (Scheme 13) was used as a chiral, photoresponsive dopant in the nematic LC materials K15 and ZLI-389, resulting in stable cholesteric phases. For the open form of 26a, [5m values of 11 [tm-1 (K15) and 13 xm 1 (ZLI-389) were measured, while the closed form 26b did not show any helical twisting power. Irradiation at 300 nm (30-50 s) resulted in the closed form and disappearance of the cholesteric phase. Irradiation with visible light restored the cholesteric phase. The gradual decrease in pitch, representing a multi-... 

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. 

Recently, the importance of the structure of chiral metal complexes on the handedness of the mesophases induced in a nematic LC was exemplified [114]. The chiral metal complexes 10 and 11—in which the alkyl substituents are aligned almost perpendicularly to the C2 axis in the former and parallel in the latter—show very different induction phenomena. Not only are the induced helicities in the nematic LC of opposite sense for the two compounds, but the helical twisting power of 10 is much higher than that of 11. The reason for these differences is the way in which the molecules are incorporated into the host nematic phase and exert their force upon it to create the twist between the layers. 

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.

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