Chirality layer

Figure 1.13 Schematic drawing of possible lateral cut of TiCl t structural layer. Chirality of Ti atoms on termination of layer is indicated. ElectroneutraUty conditions impose that metal atoms on this lateral cut are bonded to four Cl atoms bridge bonded to further metal atoms and one Cl atom not bridge bonded and that coordination position is left free. Two nonequivalent coordination positions, which will become available to monomer and growing polymer chain, inward and outward, are indicated by i and o, respectively (see text).

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

Chiral supramolecular architectures are sometimes formed by molecules that stay achiral as a single entity. Hence, chirality arises just because of close-packed self-assembly on the surface. The single pentane molecule in its linear configuration remains achiral. For the close-packed monolayer, a rectangular unit cell has been identified by neutron diffraction. In addition, a tilt of the molecular axis with respect to the adlattice vectors would make the whole layer chiral [33]. For a particular mirror domain, the tilt angle i// can be either turned clockwise or counterclockwise (Fig. 12). 

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. 3.2. Illustration of layer chirality of bent-core liquid crystals (an example of a typical molecular structure is shown in the upper picture) using the left and right hands.

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). 

Let us take a closer look at the nature of these interactions. The achiral bilinear interactions have rather simple desires . If they are negative, they favour a parallel orientation in the interacting layers. If they are positive, they favour antiparallel orientations in the interacting layers. Chiral interactions are different. They have opposite signs in oppositely handed enantiomers. Interactions favour, in this basic linear approximation, perpendicular tilts in interacting layers. The favourable sense, i.e. the 90°... 

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. 

The tilting of molecules in the B2 phase is clearly confirmed from the observation that the spherulites emerging from the isotropic phase show an electric field dependence of the position of the optical extinction lines (Fig. 9.26). Because of the tilting of banana molecules to the layer, chirality is spontaneously generated in addition to the polarity this fact sounds shocking but is so simple to be understood [132, 133]. If the molecule is rotated around their polar axis (the orientation of the bent in the molecules), which is akin to tilting the molecules in the layer, the rotation operation cannot be achieved by a simple translation (see Fig. 9.27). That is, these two states are in a mirror relation with left-handed and right-handed chirality. This is called the layer chirality. When the chirality couples with the polarity of the molecules, one would consider various smectic liquid crystal structures. There are two homochiral phases in which either (—) or (-I-) chiral molecules stack in the layers and a racemic phase in which layers are alternately stacked with layers of (—) and (-I-) chiral molecules. Each of those phases can be either ferroelectric or antiferroelectric, so that in total six different phases are present... 

Importantly it was also realized by the Boulder group [6], that the combination of polar packing and the tilt of the molecular planes gives the smectic layers a chiral structure, which is usually referred as layer chirality. Since the molecules (at least those studied first) do not contain any chiral carbons, smectic layers can form two different structures that are non-superposable mirror images of each other. To distinguish between these two structures, we can define the chiral order parameter as [42]... 

Bottleneck 2. From the results discussed in this section it clearly comes out that densitometric detection applied to the thin layer chiral separations has to be performed in a very cautious and also very conscious manner, that is, anticipating the possibility of quite significant deviations of the chiral analytes migration tracks from the straight-line direction. In this sense, the present section contains a troubleshooting advice addressed to all those, who use TLC with densitometric detection for chiral separations and — what is even more important — for quantification of the analyzed mixtures composition. The recommended troubleshooting solution is to scan relatively broad bands on the respective chromatograms instead... 

---