Liquid crystals achiral molecules

Finally here, it is important to note that the supramolecular nature of liquid crystal mesophases, in conjunction with polarity, can also lead to the induction of chirality in nonchiral materials. Current interest dates back to 2006 when Niori et a/. reported the observation of ferroelectric switching in some achiral, bent-core liquid crystals. The molecules are shown in Figure 6 and are unusual inasmuch as convention suggests that liquid-crystalline molecules should be highly anisotropic. Matsnnaga eta/. had prepared these materials in the 1990s and had noted that they did indeed form a liquid crystal phase. However, what Niori et al. showed was that the symmetry of the liquid crystal phases must be broken in order to observe a ferroelectric response and further that chiral domains could be observed. ... 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

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... 

Chiral Liquid Crystals from Achiral Molecules Banana Phases... 

CHIRAL LIQUID CRYSTALS FROM ACHIRAL MOLECULES ... 

Niori, T. Sekine, T. Watanabe, J. Furukawa, T. Takezoe, H. Distinct Ferroelectric Smectic Liquid Crystals Consisting of Achiral Molecules with Banana Shape, Abstracts of the 16th International Liquid Crystal Conference, Kent State University, Kent, OH, 1996, p. 126. 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

Niori T, Sekine T, Watanabe J, Takezoe H (1996) Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules. J Mater Chem 6 1231-1233... 

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. 

Takanishi Y, Shin GJ, Jung JC, Choi S-W, Ishikawa K, Watanabe J, Takezoe H, Toledano P (2005) Observation of very large chiral domains in a liquid crystal phase formed by mixtures of achiral bent-core and rod molecules. J Mater Chem 15 4020-4024... 

The terminus of chirality induction is used for processes, in which the structural information of a chiral molecule is transferred to an initially achiral collective which then will form a superstructural chiral phase. One of the most prominent examples can be found in the field of liquid crystals The doping of a nematic LC phase with chiral mesogenes (dopants) can lead to a twisted, helical cholesteric phase. Noteworthy is the fact that the length scales of the chiral information that characterizes the involved species can differ by several orders of magnitude a few Angstrpms in a single chiral molecule, but the pitch of a helical cholesteric phase amounts typically a few microns. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

The focus of this chapter is on isotropic chiroptical properties. Therefore, we briefly mention here some of the few available studies of measurements of ECD and OR tensors, and relevant TDDFT computations. Experimental techniques for measurements of OR tensors have been reported in [66-68]. For an overview see Clabom et al. [69]. Measurements of anisotropic CD in liquid crystals have been reported and reviewed in [70-72]. A measurement of the OR tensor at X = 670 nm has been reported for pentaerythritol which crystallizes in the achiral point group S4 [73]. TDDFT computations were also performed on a single pentaerythritol molecule (B3LYP/aug-cc-pVDZ). The computations yielded good agreement with experiment regarding the orientation of the OR tensor. TDDFT computations of... 

In the following, a description of the melting processes of liquid crystals will be given, and then the general structures of the nematic, smectic A and smectic C achiral and chiral phases that are involved in TGB phenomena will be discussed. When mesophases are formed by molecules (such as 1) that have asymmetric or dissymmetric structures, a reduction in the environmental space symmetry occurs, which in some cases can induce the creation of helical... 

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]. 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

Chiral molecules, only left or only right, form chiral phases, left and right chiral molecules in equal amount form achiral (enantiomorphic) phases [6]. Consider a chiral molecule of a popular compound DOBAMBC (D(or L)-p-decyloxybenzyli-dene-p -amino-2methylbutyl cinnamate). It has an asymmetric carbon in its tail and form a chiral SmC phase in the range of 95-117°C, Fig. 3.5a. A molecule with a chiral tail looks like an ice-hockey stick and forms a helical liquid crystal phase. Feft and right forms of a chiral tail result in the left and right handedness of a molecule Fig. 3.6. On the other hand, chirality of cholesterol esters is exclusively due to a curvature of the molecular skeleton Fig. 3.5b. 

---