Layers chiral phases

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

The mixture was agitated at 200 rpm at 30 °C (to monitor the reaction, aliquots were extracted with ethyl acetate and the organic layer analyzed by chiral-phase GC). 

Downstream extraction. The culmre broth was diluted with ethyl acetate and the aqueous phase separated using a separation funnel. The organic layer was collected and dried over anhydrous sodium sulfate. Removal of the solvent by rotary evaporator gave (5)-7-methyl-2-oxepanone as a light yellow oil (6.5 g, 38 % yield). Chiral-phase GC showed 99 % ee and >97 % purity. EI-MS and NMR confirmed the product. Note the unconverted (R)-2-methyl cyclohexanone evaporated completely under the aeration conditions used during the overnight incubation. 

Fig. 8 Definition of layer chirality in the bent-core tilted smectic phase...

Few chiral phases are used in TLC one of the main reasons for this is that stationary phases with a very high ultraviolet (UV) background can be used only with fluorescent or colored solutes. For example, amino-modified ready-to-use layers bonded or coated with Pirkle-type selectors [3], such as A-(3,5-dini-trobenzoyl)-L-leucine or i (—)-a-phenylglydne, are pale yellow and strongly adsorb UV radiation. 

Almost all the smectic phases, in which the molecules are arranged in layers and are tilted with respect to the layers, have counterpart chiral phases. The most important one of this class is the chiral smectic C phase — Sc phase. In these chiral liquid crystal phases, the molecules are tilted at a constant angle with respect to the layer normal but the tilt azimuthal rotates uniformly along the chiral axis and forms a helical structure. 

In this analysis it must be emphasized that the TGB phase, is not simply a layered chiral nematic phase, and should not be confused with this concept. A layered chiral nematic phase simply cannot exist on a macroscopic scale, and it is a requirement that defects must be formed. 

Fig. 30. The normal layered structure of the smectic A phase of a side chain liquid crystal polymer, and the proposed twisted structure of the layered smectic phase of a chiral side...

M.W. SchrSder, S. Diele, G. Pelzl and W. Weissflog, Field-induced switching of the layer chirality in SmCP phases of novel achiral bent-core hquid crystals and their unusual large increase in clearing temperature under electric field application, Ghem. Phys. Chem. 5(1), 99-103, (2004). 

The flexoelectric effect is a phenomenon where a space variation of the order parameter induces polarization. Chiral polar smectics are liquid crystals formed of chiral molecules and organized in layers. All phases in tilted chiral polar smectic liquid crystals have modulated structures and they are therefore good candidates for exhibiting the flexoelectric effect. The flexoelectric effect is less pronounced in the ferroelectric SmC phase and in the antiferroelectric SmC. The flexoelectric effect is more pronounced in more complex phases the three-layer SmCpu phase, the four-layer SmCFi2 phase and the six-layer SmCe a phase. 

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. 

The smectic C phase formed by chiral molecules (SmC phase) has also a helical superstructure having a pitch incommensurate with the smectic layer thickness. Theoretically chiral phases can also be formed by achiral molecules due to very specific packing [16]. For instance, three achiral rod-Uke molecules of dijfer-ent length may form a chiral trimer or a tripod due to Van der Waals interactions between their fragments, see Fig. 4.25a, and such trimers, in their turn, may form a kind of helical structure. Another example is bent-core or banana like-molecules [17]... 

The most popular thin layer chromatography (TLC) techniques for separation of enantiomers are described here 1) use of non-chiral phases for indirect resolution of optical isomers after derivatization to obtain the corresponding diastereoisomers and 2) direct resolution of enantiomers using chiral stationary phases or chiral mobile phases. Advantages and limits of all reported techniques are discussed. 

S. Jaradat, N. W. Roberts, Y. Wang, L. S. Hirst and H. F. Gleeson, Remarkably wide four-layer smectic phases in mixtures of liquid crystals and highly chiral dopants, J. Mater. Chem., 16, 3753-3761 (2006). 

FIGURE 6.9 (a) AF and FE switching in SmCP phases formed by bent-core molecules (b) two basic switching mechanisms switching around the molecular long axes (A) reverses layer chirality, and switching on a smectic cone (B) retains layer chirality. The polar direction is not illustrated in (b) since it can be either parallel or antiparallel to the bend direction depending on the molecular structures. 

CHIRAL PHASE. A layer used for separation of optically active compounds (enantiomers). The most widely used chiral phase is prepared from C-18 (octa-decyl) chemically bonded silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative and operates with a ligand-exchange mechanism. 

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