Chiral nematics helical 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 ... 

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... 

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... 

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-... 

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. 

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 4.6-13 Optical rotation q recorded as outlined in Fig. 4.6-12 Spectra of two differently concentrated solutions of S-tyrosine-methylester in the nematic mixture EBBA/MBBA (equimolar mixture of N-(p-ethoxybenzylidene)-p - -butylaniline and its methoxy analogue 2 of Table 4.6-1 Riedel-de Haen), left RCE (molar fraction x fa 0.024) related to the selective reflection band indicating pitch and handedness of the. structure, thus characterizing the chirality of the solute molecules by the helical twisting power right Sequence of ACE (,v se 0.0024, therefore the RCE should occur around 200 cm ) each of which indicates the induced handedness and therefore, discriminates enantiomers (Koite, 1978).

For the CB0n0.fSj2MB series with n - 7 and 9 a blue phase was observed but not for n = 6 and 8 thus, the chiral properties of these materials do indeed exhibit an odd-even effect as expected. This was rationalised in terms of the smaller pitch for the odd relative to the even membered dimers which arises from the smaller twist elastic constant of odd dimers and is related to their lower orientational order. Surprisingly, the helical twisting power of the dimers in a common monomeric nematic solvent appears to depend solely on the nature of the chiral group, the 2-methylbutyl chiral centre, and not on its environment. Thus similar helical twisting powers are observed for both odd and even membered dimers. We will return to the nature of the phases exhibited by some of these chiral dimers in Sect. 4.4. 

Another important parameter of Ch LC systems is the so-called helical twisting power (HTF), defined as the ability of a chiral group to induce Ch mesomorphism in a nematic host. HTF depends on the dipole-quadruple... 

R. Thomas, Y. Yoshida, T. Akasaka, N. Tamaoki, Influence of a change in helical twisting power of photoresponsive chiral dopants on rotational manipulation of micro-objects on the surface of chiral nematic liquid crystalline films. Chem. Eur. J. 18, 12337-12348 (2012)... 

T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie, M. Irie, Photoswitching of helical twisting power of a chiral diarylethene dopant pitch change in a chiral nematic liquid crystal. Chem. Mater. 12, 869-871 (2000)... 

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) ... 

The pitch of the helix depends on concentration c of a dopant for small c Po ac and a is called helical twisting power of the dopant [15]. However, with increasing c the dependence becomes nonlinear and the heUx handedness can even change sign (the case of cholesteryl chloride dopant in p-butoxybenzyli-dene-p -butylaniline, BBBA, see Fig. 4.24). The same chiral, locally nematic phase with a short pitch in the range of 0.1-1 pm is traditionally called cholesteric phase because, at first, it has been found in cholesteryl esters. Such short-pitch phases manifest some properties of layered (smectic) phases. 

It is weU known that E Z photoisomerization of azobenzene-containing LC molecules can lead to a nematic-to-isotropic transition as well as photochromism [167]. It was reported that UV irradiation of a nematic mixture doped by chiral azobenzene bent-core LCs leads to the N -I transition and shift in selective reflection band position of the N phase [177]. These chiral bent-core molecules can induce the N helix upon doping them into a nematic base mixture, and their helical twisting power (HTP) is given by P = UPC where P is the helical pitch length, and C is the concentration of a chiral dopant. The P value for the exclusively E isomer is maximum and decreases with the increase in the ratio of the Z isomer. UV irradiation causes E Z conversion, therefore increasing the helical pitch and shifting the selective reflection band of the N phase. 

Typical examples of chiral azobenzenes are (/f)-4-(2-methylbutyl)-4 -substituted azobenzenes (9-13) which appeared with the emergence of synthetic methods which allowed the preparation of chiral precursors, such as (-i-)-2-methylbutylbenzene [51-54]. In the case of compound 12, the presence of two chiral terminal moieties was found to increase the helical twisting power ()8) by a factor of two [51]. Both the compounds 9 and 10 show enantiotropic chiral nematic phases which have relatively low clearing points, unlike compounds 11 and 12 which are not liquid crystalline. 

Equation (11) of Nordio et al [20] can be generalised to develop a theory with a non-traceless pseudotensor of second rank Wy, the chirality interaction tensor. Because the contribution of the interaction energy from the chiral/chiral potentials of the molecules is small in comparison to the chiral/achiral contribution, the helical twisting power is then given for a guest/host system as well as for a single component chiral nematic phase by ... 

The N -LC is used as an asymmetric liquid reaction field and it is prepared by adding a small amount of a chiral dopant to a nematic liquid crystal (N-LC). The formation of the N -LC can be confirmed under the polarizing microscope (POM) by the change of the characteristic nematic Schlieren texture to a fingerprint-like or striated Schlieren texture of the chiral nematic phase. The distance between the fingerprint lines of the optical pattern is equivalent to half of the helical pitch of the N -LC phase. Therefore, the stronger twisting power of the dopant can be observed by the shorter helical pitch of the optical pattern under the POM (Fig. 9.33). 

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