Handedness chiral nematics

AEC) in chloroform, with the increase of degree of acetylation, the rigidity of the cellulose chain increases monotonically. However, the pitch of the mesophase first increases to infinity and then decreases with an inversion of the handedness of the chiral nematic phase, from left-handed to right-handed. 

Several examples of textures of the rod-like, disk-like, and biaxial chiral nematics are shown and discussed in a paper by Melnik and Saupe [14]. Contact preparations can be used to determine the handedness of the twist if a sample of known handedness is available. The observation of a nonchiral (compensated nematic) region between two twisted parts of the sample is a strong hint to opposite handedness. 

Fig. 14, then it is found that the number of atoms (n) that the chiral center is removed from the rigid central core determines the handedness of the helical structure of the chiral nematic phase. As the atom count by which the chiral center is removed from the core (n) switches from odd (o) to even (e) (parity), so the handedness of the helix alternates from left to right or vice versa. Sim-... 

F re 3. Schematic diagram of selective reflection from a right-handed chiral nematic planar texture for A= inside the liquid ciystal. The linearly polarized input light may be considered as counter-propagating right-handed (RH) and left-handed (LH) circular components. The RH component in which the E field matches the sense of the helicoidal structure is back reflected due to director fluctuations, whilst the LH component is almost totally transmitted. By convention, the handedness is delined in terms of the progression of the E field vector in time relative to the observer. The RH rotating wave therefore has the same spatial structure as the chiral nematic at any time. 

The selectively polarized light (for Aj -p) is circularly polarized with the same handedness as the chiral nematic helix. The other circular polarization is almost totally transmitted. 

The first phase of this type was found in esters of cholesterol and became known as the cholesteric phase (Ch). However, this is the chiral analog of the nematic phase and is not confined to derivatives of cholesterol. It is therefore more correctly named the chiral nematic phase N. The structure of this phase is shown in Figure 2.2. Opposite optical enantiomers give helices of opposite handedness. 

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 mentioned in the introduction, the first empirical correlation between the absolute configuration of dopants and the handedness of induced cholesterics was proposed in 1975.20 The first attempt to find a general correlation was a few years later Krabbe et al.58 related the sense of the cholesteric to a stereochemical descriptor of the dopant based on the effective volume of the substituents and listed many compounds following this rule. However, exceptions were described at that time,59 and, furthermore, this approach neglects the role of the structure of the nematic solvent in determining the sense of the cholesteric. It is well known that chiral compounds may induce cholesterics of opposite handedness in different nematics.60,61... 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

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. 

Twisting a nematic structure around an axis perpendicular to the average orientation of the preferred molecular axes, one arrives at the molecular arrangement commonly called cholesteric (Kelker and Hatz, 1980). The twisted nematic phase is optically uniaxial, however with the axis perpendicular to the (rotating) director. Such a mesophase combines the basic properties of nematics with the implications of chirality The structure itself is chiral and as a consequence, a non-identical mirror image exists as it is shown schematically in Fig. 4.6-7. Besides the order parameters mentioned before, the essential characteristics of a cholesteric mesophase are the pitch, i.e., the period of the helical structure as measured along the twist axis, and its handedness, i.e., whether the phase is twisted clockwise or anticlockwise. 

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).

The theory predicts that the handedness of cellulosic liquid crystalline solutions, designated by the sign of the pitch, depends not only on temperature (T) and on steric repulsion of the chain X), but also on an attractive interaction parameter, %, which depends on the nature of the solvent. The chiral forces are balanced when (x XkT) = 0. In this compensated condition, the pitch of the mesophase should become infinite, and the mesophase resembles a normal nematic phase. 

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