Chirality macroscopic crystals

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... 

In the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

A young Louis Pasteur observed that many salts of tartaric acid formed chiral crystals (which he knew was related to their ability to rotate the plane of polarization of plane-polarized light). He succeeded in solving the mystery of racemic acid when he found that the sodium ammonium salt of racemic acid could be crystallized to produce a crystal conglomerate. After physical separation of the macroscopic enantiomers with a dissecting needle, Pasteur... 

Addadi, L., A Link Between Macroscopic Phenomena and Molecular Chirality, Crystals as Probes for the Direct Assignment of Absolute Configuration of Chiral Molecules, 16, 1. 

Recent experimental and theoretical studies on crystal growth, especially in the presence of tailor-made inhibitors, provide a link between macroscopic and microscopic chirality. We shall discuss these principles in some detail for chiral molecules. Furthermore, we shall examine whether it is indeed feasible today to establish the absolute configuration of a chiral crystal from an analysis of solvent-surface interactions. Since these analyses are based on understanding the interactions between a growing crystal and inhibitors present in solution, we shall first illustrate the general mechanism of this effect in various chiral and nonchiral systems. 

A LINK BETWEEN MACROSCOPIC PHENOMENA AND MOLECULAR CHIRALITY CRYSTALS AS PROBES FOR THE DIRECT ASSIGNMENT OF ABSOLUTE CONFIGURATION OF CHIRAL MOLECULES... 

Jakli A, Nair GG, Lee CK, Su R, Chen LC (2001) Macroscopic chirality of a liquid crystal formed nonchiral molecules. Phys Rev E 63 061710-1-4. 

L. A link between macroscopic phenomena and molecular chirality crystals cis prohes for the direct assignment of absolute configuration of chiral molecules. Topics in Stereochemistry 16, 1-85 (1986). 

Rikken proposed that the EMCA effect could also result from the simultaneous application of a magnetic field and a current to a crystal with an enantiomorphous space group, and that it is a universal property. He showed the existence of this effect in the case of chiral single-walled carbon nanotubes.For most of the investigated tubes, a dependence of the resistance is observed that is odd in both the magnetic field and the current. These observations confirm the existence of EMCA not only for a macroscopic chiral conductor but also for a molecular conductor with chirality on the microscopic level. 

Materials that are composed of chiral molecules are without doubt somewhat special, particularly those with biological activity. However, ehiral molecules that generate liquid crystal phases are also very speeial and imnsnal. The aim of this chapter is to discuss the special nature of hquid crystal phases that are composed of chiral molecttles and look at their srritabihty for technological apphcations. In certain fltrid, ordered liquid crystalline phases, the chirality of the molectrles (molectrlar chirahty) causes chirality in the macroscopic liquid crystal phase stractrrre (form chirality). The chiral liquid crystalhne molectrles oiganise into an asytrrmetric, chiral stractrrre that takes the form of a helix. The... 

Chirality is a fascinating subject for liquid crystal research [1]. Molecular chirality is transferred to macroscopic chirality and the resulting effects are ... 

Sugars are a commonly used source for amphiphilic liquid crystals [10]. These materials show lamellar, columnar, and cubic phases, but chiral phases are very rarely observed. Thermotropic cholesteric phases are never observed and lyotropic cholesteric phases based on asymmetric micelles only in a few cases [11]. The bicontinuous cubic phase of these glycolipids may have macroscopic chiral ordering, but this has not been resolved hitherto [12], [13]. Thus, alkylated sugars are chiral compounds, but not effective... 

Liquid crystal induced circular dichroism (LCICD) examines the differential absorption of circularly polarized radiation by an achiral solute oriented in a cholesteric liquid crystal. The circular dichroism results from an induced Cotton effect in the achiral solute due to the macroscopic chirality of its ordering in the helical solvent matrix. The effect has been shown theoretically to arise... 

The CSM and CCM approaches can, of course, be used for the analysis of macroscopic structures. An example is the treatment of the chirality of the macroscopic shape of crystals as a continuous structural property, rather than as an either/or property. This was demonstrated on the classical chiral crystal of ammonium sodium tartrate. ... 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

Along with the prediction and discovery of a macroscopic dipole in the SmC phase and the invention of ferroelectric liquid crystals in the SSFLC system, the discovery of antiferroelectric liquid crystals stands as a key milestone in chiral smectic LC science. Antiferroelectric switching (see below) was first reported for unichiral 4-[(l-methylheptyloxy)carbonyl]phenyl-4/-octyloxy-4-biphenyl carboxylate [MHPOBC, (3)],16 with structure and phase sequence... 

The purpose of this review is two pronged. We have discussed the far-reaching manifestations of molecular chirality in crystals on the macroscopic scale, and have described the various direct methods for the assignment of absolute configuration of chiral molecules in solids. Our analysis has been confined to the solid state, because of the difficulties associated with the direct assignment of absolute configuration in the dispersed phase (12). 

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... 

Xu Z, Gao C. Graphene chiral liquid crystals and macroscopic assembled fibers. Nat Commun. 

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