Frustrated chiral phases

Frustrated Phases. Chiral molecules normally form chiral phases, hui in some cases this is dune in an interesting way. For example, it is not... 

Finally, we have attempted to evaluate the possible impact of an intermediate liquid crystalline phase and the possibility of transfer of helical hand information from the melt to the crystal throughout this process. Assuming that the melt is structured, the melt of chiral but racemic polyolefins would be made of stretches of helical stems that are equally partitioned between left- and right-handed helices. Formation of antichiral structures (such as in a iPP) could be interpreted as indicating a possible transfer of information (but the problem of the sequence of helical hands would still remain). This analysis is, however, ruined by the observation that many of these polymers also form chiral structures (frustrated p phase of iPP, Form III of iPBul). For the achiral poly(5-methyl-pentene-l), the chiral, frustrated phase is actually the more stable one, and can be obtained by melting and recrystallization of a less stable antichiral phase. 

Another class of frustrated phase results from the frustration between bend or twist deformations in smectic phases (Section 5.6) and the tendency to form a layered structure. Twisted grain boundary phases are frustrated smectic phases and both SmA and SmC versions have been observed. The phases are denoted TGBA and TGBC respectively and are formed by chiral mesogens. The phases are macroscopically chiral and result from arrays of screw dislocations (i.e. defects in lattice order) which lead to a twist in the director between grains of layers, i.e. to a helical rotation of layers. 

The third interesting characteristic of banana-shaped liquid crystals, besides polarity and chirality, is the frustrated (dissipative) phases Bl, B5, B6, and B7. It is understood that all those phases have been produced by some kind of dissipation of the spontaneous polarization [142]. 

Besides molecular shapes and amphiphilicity, chirality also has a large influence on LC self assembly, leading to series of LC phases with helical superstructures, reduced symmetry, and chirality induced frustration [43-46]. 

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

Recent progress in material science, notably with the development of new materials exhibiting blue phases, has generated a renewed interest in the incorporation of the functional properties with the unique structure of frustrated phases. Synthesis of a monosubstituted ferrocene-based chiral Schiff s base derivative which exhibits TGBA and blue phases has been reported [17] (Fig. 9). Other metallomesogens leading to blue phases have been found for palladium complexes [18] (Fig. 10). Optically active materials incorporating... 

Keywords Liquid crystals, self-organizing systems. Chirality, Frustrated phases. Superconductivity, Abrikosov flux phases... 

Molecular shape anisotropy plays a very important role in determining whether or not liquid crystal phases will be formed, and indeed, when they are formed, which particular modification will be generated in preference to other mesophases [3, 4]. Molecules that form liquid crystals are usually carbon based, but they need not be. They often have preferred shapes, based on rods, bananas, boards or discs. In the following account, the properties of some unusual rod-like organic systems which have asymmetric structures thereby making them optically active and chiral will be described. In particular, the unusual self-organizing properties and the structures of the frustrated phases of the (R), (S), and racemic forms of 1-methylheptyl 4 -(4-n-... 

Keith, C. Reddy, R. A. Prehm, M. Baumeister, U. Kresse, C. H. Chao, J.L. Hahn, H. Lang, H. Tschierske, C. Layer frustration, polar order and chirality in liquid crystalline phases of silyl-terminated achiral bent-core molecules. Chem. Eur. J. 2007, 13, 2556-2577. 

In the cholesteric phase, the helical configuration of the director is the consequence of the molecular chirality. From the microscopic point of view, unlike in the nematic phase where the molecules have a tendency to be locally parallel to each other, in cholesteric materials, the adjacent molecules have a tendency to form a small angle as shown in Figure 2.6. As we will point out in Section 2.6, such a local tendency cannot be satisfied globally and the system is frustrated. 

For a detailed discussion of the frustrated phases, such as blue, TGB, and chiral columnar phases, see the chapters by Bock, Crooker, Kitzerow, and Pieranski. 

At a normal chiral nematic to smectic A transition, the helical ordering of the chiral nematic phase collapses to give the layered structure of the smectic A phase. However, for a transition mediated by a TGB phase, there is a competition between the need for the molecules to form a helical structure due to their chiral packing requirements and the need for the phase to form a layered structure. Consequently, the molecules relieve this frustration by trying to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the chiral nematic phase), yet at the same time they also try to form a lamellar structure, as shown in Fig. 21. These two... 

The results reached until now in the transformation of chiral or prochiral substrates under the usual phase transfer conditions are rather frustrating as far as the optical yield is concerned. 

Liquid crystal molecules usually tilt in the same direction over the smectic layers (synclinic [212]) in the smectic C (SmC) phase. However, in one of the smectic A (SmA) phases, called de-Vries phase [213,214], molecules tilt but the tile direction is random so that the overall molecular tilt cannot be recognized optically. Frustration can be produced between aligning and random orders [215]. There is another style of tilt, in which the tilting direction is aligning in one direction in each smectic layer however, tilting direction alternates between the adjacent layers (anticlinic [212]). It has been well known that the introduction of chirality into the synclinic and anticlinic stmctures produces the ferroelectric and antiferroelectric properties, respectively. Frustration between the ferroelectric and antiferroelectric properties produces the ferroelectric structure in which the spontaneous polarization is partially canceled by the different magnitude between plus and minus polarization directions [216, 217]. The anticlinic order, NOT the antiferroelectric order, has been reported to be created by achiral systems [218, 219], indicating that the frustration between synclinic and anticlinic structures occurs, without any polar effects. The clinicity is determined by the style of the molecular order between the adjacent smectic layers, and therefore, the molecular structures at the peripheral... 

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