Liquid crystal chiral, bent-core molecules

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. 

The free energy density terms introduced so far are all used in the description of the smectic phases made by rod-like molecules, the electrostatic term (6) being characteristic for the ferroelectric liquid crystals made of chiral rod-like molecules. To describe phases made by bent-core molecules one has to add symmetry allowed terms which include the divergence of the polar director (polarization splay) and coupling of the polar director to the nematic director and the smectic layer normal ... 

It is important to note that also nonchiral molecules are capable of forming chiral mesophases. In particular, molecules with a bent core ( bananashaped molecules) can build polar, and even chiral liquid crystal structures [75]-[78]. Bent-core molecules form a variety of new phases (B1-B7, Table 1.3) which differ from the usual smectic and columnar phases (see also Chapter 8). As a consequence of the polar arrangement, antiferroelectric-like switching was observed in the B2 phase formed by bent-core molecules, and second harmonic generation was found in both the B2 phase and the B4 phase. The latter phase is probably a solid crystal. It consists of two domains showing selective reflection with opposite handedness. In the liquid crystalline B2 phase, the effective nonlinear susceptibility can be modulated by an external dc field [79] (Figure 1.15). 

A special example of chiral liquid crystals is the tilted smectic phases of bent-core molecules, which can be chiral even though the molecules do not contain any stereo-center carbons. The reason is that a bent-core tilted layer structure with polar order has a three-dimensional structure defined by the polar vector and the tilt direction, which can be illustrated either by our left or right hands of Figure 1.12. We note that the latest investigations indicate other sources of chirality of bent-core molecules, which are termed conformational chirality, due to a propeller type configuration of the two molecular legs. ... 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanabe J (2000) Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit. J Am Chem Soc 122 7441-7448 Tschierske C, Dantlgraber G (2003) From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana J Phys 61 455-481 Ungar G, Percec V, Zuber M (1992) Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on l-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R=lluoro, chloro and methyl and flexible spacers containing an odd number of methylene units. Macromolecules 25 75-80 Urayama K (2007) Issues in liquid crystal elastomers and gels. Macromolecules 40 2277-2288 Vorlander D (1908) About transparently clear, crystalline liquids. Rep Ger Chem Soc 41 2033-2052... 

First it was argued to be due to a chiral molecular configuration characteristic of the particular type of bent-shape molecules, such as twisted or propeller shape (conformational chirality). The concept of conformational chirality was supported by simulations by Earl et al. [61], and was demonstrated by the observation that doping calamitic cholesteric liquid crystal by achiral bent-core molecules can lead to a decrease of the helical pitch, indicating an enhanced rotatory power of the mixture [62]. Unfortunately there is no proof that the decrease of the pitch is not due to a decrease of the twist elastic constant caused by the addition of bent-core units. Although the conformational chirality is usually not questioned in the solid B4 phase [20], its role has been questioned by Walba et al. [20] by arguing that these chiral conformations have very short lifetime, therefore they average out in fluid smectic, such as SmCP or SmCo phases. 

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... 

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... 

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. ... 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

The ferroelectric properties of the chiral smectic-C phase and the electroclinic effect of the smectic- phase appeared as a result of the symmetry breaking caused by the presence of chiral molecules. One can think of smectic phases in which nonchiral molecules arrange themselves in a polar order [86], and it seems that such phases were recently observed, indeed experimentally. The molecules which establish these phases are not chiral but possess a bent core resembling a bow- or banana-like shape [87] a second class of nonchiral liquid crystals showing polar ordering consists of certain polymer-monomer mixtures [88],... 

Study of free-standing membranes is also extremely important to map the polar nature of ferroelectric, ferrielectric and antiferroelectric liquid crystals of chiral and bent-core achiral molecules. An excellent overview of smectic membranes with detailed results and list of literature has been published recently by de Jeu et al. ... 

We see from Figure 8.13 that a number of liquid crystal phases, SmC of chiral rod-shape and the tilted columnar phase of chiral disc shape molecules, as well as the SmCP of achiral bent-core, and the tilted bowl-shape molecules all have C2 symmetry with eight independent piezoelectric coefficients. The direct and converse i- piezoelectric effects have been mostly studied in the fluid SmC liquid crystals. 

In achiral rod-shape molecules one does not expect any optical activity, but as we have seen achiral bent-core liquid crystals can have chiral layer structures. An interesting question therefore if we see optical activity in those materials. Observing planar textures of bent-core liquid crystals between slightly uncrossed polarizers, it is indeed often found [40, 50-59] that the texture splits into darker... 

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