Chiral dopants

The fimetionahty of chiral dopants is to induce twist in blue phases. Blue phases only appear as the chirahty ( o = ntP) exceeds a certain value. To increase chirality, we can either increase the chiral dopant concentration or employ a chiral dopant with a high helical twisting power (HTP). Since the solubility of a chiral dopant limits its maximum concentration, it is preferred to use a high HTP chiral dopant. For example, Merck developed a chiral dopant with HTP 170/pm so that the required concentration is less than 5% [27]. 

Besides material optimizations, photo-initiator and UV irradiation conditions also affect the performance of a polymer-stabihzed BPLC composite, such as photo-initiator concentration. 

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If the molecules are chiral or if a chiral dopant is added to a discotic Hquid crystal, a chiral nematic discotic phase can form. The director configuration ia this phase is just like the director configuration ia the chiral nematic phase formed by elongated molecules (12). Recendy, discotic blue phases have been observed. 

Tbe purpose of tbe bydroxyl group is to acbieve some hydrogen bonding with the nearby carbonyl group and therefore hinder the motion of the chiral center. Another way to achieve the chiral smectic Cphase is to add a chiral dopant to a smectic Chquid crystal. In order to achieve a material with fast switching times, a chiral compound with high spontaneous polarization is sometimes added to a mixture of low viscosity achiral smectic C compounds. These dopants sometimes possess Hquid crystal phases in pure form and sometimes do not. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

In Section 1.2.1, we discussed the phenomenon of adsorbate-induced chirality whereby the adsorption of achiral species (e.g., glycine) results in the formation of two mirror equivalent domains on the surface. It has recently been shown that the presence of relatively small mole fractions of chiral dopants can result in the exclusive... 

Supramolecular chirality can be introduced in columns of discotic molecules by using specific interactions, apart from the arene-arene interactions. To achieve this, a study with chiral dopants has been performed.73 Hexa-n-hexyloxytriphenylenes were mixed in dodecane solutions with a variety of chiral electron-deficient dopants and the resulting charge transfer complexes... 

Several natural10 and synthetic (e.g., polyisocyanates11) polymers form lyotropic cholesterics with the appropriate solvent also micellar systems formed by amphiphilic molecules and water, if chirality is introduced by either using a chiral amphiphile or adding a chiral dopant, can give cholesteric phases.12... 

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 ... 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

The classical cholesteric phase materials show only a weak anisotropic interaction with electric fields and hence are of limited use in electro-optical response applications. Cholesteric phases for these outlets are consequently produced by adding chiral dopants to nematic liquid crystals. 

From Table 8 it is obvious that the resolution always increases with an increase of the number of benzene rings and that riboflavine is a more powerful selector than the nucleotides, but not as good as TAPA. An interesting experiment shows that it is not always necessary to have the selector coated or bound to the solid phase but that it can sometimes be used as well, dissolved in the mobile phase. The n-dodecyl ester of N-(2,4-dinitrophenyl)-L-alanine is able to discriminate between the enantiomers of l-aza-[6]-helicene, when used as a chiral dopant in the mobile phase in HPLC on a reversed phase column 93) (see Table 9). The usefulness of this dopant must be due to the known ability of a dinitrophenyl moiety to form CT-complexes with polycyclic aromatic hydrocarbons the presence of a chiral site near this group causes resolution of helicenes, because the steric interactions in diastereomeric complexes will be quite different. 

Table 9. Resolution of l-Aza-6-Helicene by the n-Dodecyl Ester of N-(2,4-Dinitrophenyl)-L-Alanine as a Chiral Dopant in the Mobile Phase3 931...

Research Focus Preparation of helical polyaniline using chiral dopant acids. 

Enhanced chirality has also been observed in the induction and stabilization of BPs. The first related experiment was made by Nakata et al. [26]. It is known that BPs emerge between the isotropic and N phases only in highly chiral systems. For instance, we cannot have any BPs in a commercial NLC ZLI-2293 with chiral dopant MLC6248 at least up to 25%, but instead only the N phase emerges. By doping such N phases with BSMs, BP emerges and expands with increasing... 

Stabilization of BP consisting of bent-core N with chiral dopant was also investigated [29-31]. One of the most dramatic results is that BPIII is easily induced by adding a very small amount of chiral dopant such as 1% [29]. The BPIII temperature range was more than 20°. If BSMs show the N phase at room temperature, the BPIII phase over 20° including room temperature is easily realized [30]. Since BSMs have low compared with Kn [25, 32], the effect of elastic constant on the BP stabilization is confirmed [28]. 

Spontaneous resolved two chiral domains are formed in equal probability. In other words, enantiomeric excess (ee) is zero. We now ask whether ee can be controlled or not. The answer is yes. Several methods used in bent-shaped mesogenic phases will be introduced. The direct method is of course an addition of chiral dopant. Actually this has been shown to be a viable method [6, 61]. Use of chiral surface is also effective [62], By using polyimide with chiral side chains at both substrate surfaces, imbalance of two chiral domains (10% ee) has been achieved. Another method using macroscopic helical structure was demonstrated by Jakli et al. [63]. They used a nonchiral polymer network, which was formed in the N phase. After the polymerization, N compounds were washed out, then bent-core mesogens were introduced. Because of the polymer helical fibers, bent-core mesogen shows a chiral domain. 

Thisayukta J, Niwano H, Takezoe H, Watanabe J (2001) Effect of chiral dopant on a helical Sml phase of banana-shaped N-n-O-PIMB molecules. J Mater Chem 11 2717-2721... 

Fig. 11 Mixed monolayer alkylthiol-capped gold nanoparticle end-functionalized with (S)-naproxen chiral dopant moieties [349, 365, 366]...

Yokoyama et al. showed that the binaphthol derivatives of indolylfulgides 11 and 12 functioned as chiral dopants to generate cholesteric phases on addition to nematic liquid crystal 26. Photoirradiation induced dramatic changes in cholesteric... 

Scheme 25 Photochemical switching processes of LC-phase 41 and chiral dopants M-c/s-17a and P-trans-17b, representing a three position switch.

One of the most recent observations in supramolecular surface chirality is the induction of homochirality on surfaces via cooperatively amplified interactions in molecular monolayers. As discussed in Sect. 2, adsorption-induced chirality leads to both mirror motifs. However, in the presence of additional chiral bias, lattice homo chirality can be installed in the entire molecular layer. Such bias comes from a chiral dopant, small ee or physical fields in combination with symmetry breaking of the surface. 

The coadsorption of chiral molecules into racemic layers is an efficient way to induce further asymmetrization towards single handedness. While in heterogeneous chiral catalysis the stationary ratio of modifier and reactant at the surface is assumed to be one, a small amount of chiral dopant can be sufficient for induction of homochirality on the entire surface SU on Cu( 110), for example, forms two enantiomorphous domains in its bisuccinate phase [27]. 

Fig. 7 Induction of chiral mesophases in achiral LCs by addition of chiral dopants...

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