Macroscopic chirality

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. 

From these examples, it becomes clear that molecular symmetry can spontaneously break into chiral domains in the absence of any external force or seed. However, in every known case, the net symmetry remains intact, and the overall chirality sums to zero in the environment. Nevertheless, the spontaneous formation of macroscopic chiral regions in systems of associating achiral molecules is of interest to those who contemplate induction of molecular chirality in the context of prebiotic reaction chemistry. 

Link DR, Natale G, Shao R, Maclennan JE, Clark NA, Korblova E, Walba DM (1997) Spontaneous formation of macroscopic chiral domains in a fluid smectic phase of achiral molecules. Science 278 1924—1927... 

The above observation suggests a unique feature of ester molecules, which tend to induce chirality in LC systems. Actually it is know that molecules are strongly bent at an ester linkage to form P- and M-conformers. Moreover, molecules are bent at the ester linkage. Yan et al. concluded that the system with bent molecules and chiral molecules results in macroscopic chiral aggregation using simulation [34]. 

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. 

The principal goal is to define those factors which lead to the macroscopic chirality of the dendrimer. Despite numerous studies on this topic, the relation between the molecular chirality of the dendritic building blocks and the macroscopic chirality of molecules has still not been completely elucidated [12]. Yet an understanding of this relation is important for the development of new materials, including polymers, whose properties and function depend upon their macroscopic chirality [13]. 

The local chirality of a core unit is not necessarily manifested in a macroscopic chirality of the dendrimer. This is clearly apparent from the example of the dendrimers constructed by Meijer et al. from a glycerol unit as chiral core unit and four Frechet-type dendrons of different generations (Fig. 4.66). No optical activity could be measured with available instrumentation for the two enantiomeric forms (S)-l and (i )-l [2 a]. 

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. 

Ruslim C, Ichimura K. 2000. Conformational effect on macroscopic chirality modification of cholesteric mesophases by photochromic azobenzene dopants. J Phys Chem B 104(28) 6529 6535. 

Even though the first SmC materials were synthesized at the beginning of the 20th century [10], it took decades until the macroscopic chirality of the SmC phase was discovered. The existence of a hypothetical twisted smectic phase was first discussed by Saupe in 1969 [11]. Two years later, in 1971, Helfrich and Oh [12] detected the SmC phase as such for the first time due to its ability to selectively reflect light. The ferroelectricity of the SmC phase was then theoretically predicted, explained and experimentally proved by Meyer et al. [13] in 1975 for the first time. Five years later, Clark and Lagerwall published their groundbreaking work [14], which demonstrated the ferroelectric switching of the SmC phase if surface-stabilized. 

Fig. 5.23 a Helical pitch versus temperature for the three macroscopically chiral phases and corresponding texture images of b the phase at 65 °C, c the TGBL phase at 68 °C and d the N phase at 71 °C... 

The investigated surfactant/solvent mixtures of the diol C50 and water or for-mamide, respectively, are the first lyotropic systems to form a lamellar, fluid and tilted liquid crystalline phase which contains chiral surfactant molecules. The main issue of the present chapter is thus to demonstrate whether or not the lyotropic SmC analog phase exhibits similar chirality effects as known from its thermotropic counterpart. The most outstanding manifestations of chirality in the thermotropic SmC phase are helicity, due to a chirality-induced precession of the director, and ferroelectricity, due to its polar C2-point group symmetry. Thus, the focus of this chapter is on the detection and analysis of those two macroscopic chirality effects. 

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... 

The above-described considerations have been confirmed by the different behaviors of films consisting of either an even or an odd number of smectic layers, and by electro-optic switching studies which show the spontaneous formation of macroscopic chiral domains possessing opposite handedness. An important point is that the inversion of the spontaneous polarization, induced by the inversion of an applied field, occurs preferably by the inversion of the tilt direction and not by an inversion of the chiral handedness (inversion of b). 

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. 

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... 

This preferential handedness is also present in the nano-scopic world of molecules in fact, it is the presence of chiral molecular building blocks that often imparts macroscopic chirality. Thus, many chiral organic compounds exist in nature as only one enantiomer, although some are present as both. Specific handedness is associated with a specific biological function, dictated by the presence of chiral receptor... 

Before discussing macroscopic chirality, we should first discuss some aspects of the evolution of chirality from the very primitive self-replicating chemical system to the organized living cell and then more recognizable forms of life. 

Should we be surprised to find chiral structures in nature Surely we should not, because aU living things are made up of chiral amino acids, sugars, and other components. One major questions is whether the fact that the building blocks of life are chiral controls the overall chirality of the macroscopic system, or whether this macroscopic chirality has a more macroscopic origin. We have already discussed the many... 

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