Achiral systems liquid crystals

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

Enantiopure thiahelicenes can transfer their molecular chirahty to the whole phase of an achiral liquid crystal phase thus acting as dopant systems. Coupling the analysis of CD spectra with the study of cholesteric meso-phases induced in nematic liquid crystals (LC), a model has been proposed for the transfer of chirality from thiahehcene (Al)-114 (Figure 13) to the whole liquid crystal phase (cholesteric induction) (1996JO2013). The dopant thiahehcene presents a twisted chiral surface, which is homochiral with the induced cholesterics as a consequence of the interaction of its... 

Li), the lamellar Ld), and the nematic one (No), which in this case consists of disk-like micelles. One outstanding feature of the system is the extremely wide range of stability of the lamellar as well as of the nematic phase. Moreover, the capability to perform phase transitions lamellar/nematic/ isotropic by temperature variation makes the handling of the samples easy. The short, perfluorinated chains of the surfactant are responsible for the unusual properties of this lyotropic liquid crystal which in some aspects behaves very similar to a thermotropic phase. Both of the briefly introduced achiral lyotropics have been used extensively as host phases for the induction of phase chirality by means of chiral dopants. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

Some of the very early applications of CPL spectroscopy involved chiral polymeric systems. In particular, the CPL from achiral dye molecules dissolved in cholesteric liquid crystals has been used to probe chirality changes. CPL from chromophores attached to a chiral poly-amino acid may also be used to study exciton coupling between aromatic chromophores. In these polymeric systems, it is often observed that gijjjjj is quite large. In some cases it is so large, in fact, that detection using static quarter-wave plates is possible. 

Bearing in mind the odd-even effect, I tried to synthesize new polymer types in order to express the ferroelectric phase in an achiral system. The opportunity to study chiral ferroelectric liquid crystals was ripe at that time, as I already mentioned in the opening sentence. Their discoverer Mayer [118] argued that by introducing a chiral molecule into a Sc phase, the twofold axis of the Sc phase becomes the polar axis because of the extinction of a vertical mirror symmetry (Fig. 9.20). The reduction of the symmetry by the introduction of chirality into the system is in a sense conventional, simple, and straightforward. 

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... 

Another example is the swollen B4 phase of banana-type (bend-type) liquid crystals. The banana-type molecules form smectic hquid crystals in which the molecules are tilted with respect to the layer normal. This tilt and the orientation of the bow-shaped molecules have been of interest, because even achiral molecules can express a chiral structure. In addition, in the mixed system of pentylcyanobiphenyl (5CB), a typical nematic rodlike mesogen, and (P-8-0-PIMB), one of the banana-type liquid crystals, the specific B4 phase is formed in a wide mixing range of up to 107 wt% 5CB [63]. In addition, there is a new Bx phase in the low-temperature... 

Tables 4 and 5 show the general structures and mesomorphic characteristics of the homo- and co-polycondensate liquid crystal systems, respectively. Chiral thermotropic polyesters have been studied mainly by Blumstein and Krigbaum, almost simultaneously with, or immediately following, the first report by Strzelecki and co-workers.The latter report described also the preparation and properties of a cholesteryl end-capped polyester based on achiral components 4 -hydroxyphenyl 4-hydroxy benzoate and pimelic acid (Table 5).

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