Achiral Bent-Core Molecules

As described in Chapter 1, smectic liquid crystal phases of bent-core molecules can form polar strcutures. 

The orthogonal arrangement, where both the molecular plane and the long axis (the virtual line connecting the end points) are parallel to the layer normal. It is a transversely polarized achiral SmA-type structure with C21, symmetry. This corresponds to the structure proposed originally by Niori et al. but observed only later.  

The single-tilted arrangement when only the molecular plane is tilted. 

In this case the smectic layers have monoclinic chiral symmetry C2 It is denoted as SmCP phase (P stands for polar ). Most of the experimental results concern this phase, and will be summarized below. 

The other single-tilted structure, when only the long axis is tilted with respect to the smectic layers, has an achiral monoclinic symmetry Cg. To our knowledge, so far there is no experimental evidence of this phase. 

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Zhang Y, Baumeister U, Tschierske C, O Callaghan MJ, Walker C (2010) Achiral bent-core molecules with a series of linear or branched carbosilane termini dark conglomerate phases, supramolecular chirality and macroscopic polar order. Chem Mater 22 2869-2884... 

Otani T, Araoka F, Ishikawa K, Takezoe H (2009) Enhanced optical activity by achiral rodlike molecules nano-segregated in the B4 structure of achiral bent-core molecules. J Am Chem Soc 131 12368-12372... 

K. Ishikawa and H. Takezoe, Alternating twist structures formed by electroconvection in the nematic phase of an achiral bent-core molecule, Phys. Rev. E 77(4), 041708/1-5, (2008/. doi 10.1103/PhysRevE.77.041708... 

Usually, it is possible to observe chiral mesophases exhibiting ferroelec-tricity in mesogens made from optically active molecules, but in some particular cases, achiral bent-core molecules can form polar ordered smectic phases... 

Fig. 4.25 Hypothetical chiral trimers formed by rod like molecules due to specific Van der Waals interaction (a) and achiral bent-core molecules capable of formation chiral domains (b)...

Keith, C. Reddy, R. A. Prehm, M. Baumeister, U. Kresse, C. H. Chao, J.L. Hahn, H. Lang, H. Tschierske, C. Layer frustration, polar order and chirality in liquid crystalline phases of silyl-terminated achiral bent-core molecules. Chem. Eur. J. 2007, 13, 2556-2577. 

Keith C, Reddy RA, Prehm M, Baumeister U, Kresse H, Chao JL, Hahn H, Lang H, Tschierske C (2007) Layer finstration, polar order and chirality in liquid laystalline phases of silyl-terminated achiral bent-core molecules. Chemistry 13(9) 2556-2577. doi 10.1002/ chem.200600876... 

Figure 1.15. Intensity of a second harmonic generation as a function of applied electric field strength in the phase B2 which is formed by achiral bent-core molecules. Courtesy by F. Kentischer and R. Macdonald (from [79]).

Illustration of the formation of chiral structures in achiral bent-core molecules if they form polar tilted smectic phase. The molecules below the hands illustrate the similarity to the typical chiral molecules s-ahne and r-aUne, respectively. 

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. 

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. 

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 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. 

Deb, R. Nath, R. K. Paul, M. K. Rao, N. V. S. Tuluri, F. Shen, Y Shao, R. Chen, D. Zhu, C. Smalyukh, I. I. Clark, N. A. Four-ring achiral unsymmetrical bent core molecules forming strongly fluorescent smectic hquid crystals with spontaneous polar and chiral ordered B7 and B1 phases. J. Mater. Chem. 2010, 20, 7332-7336. 

Dantlgraber, G. Eremin, A. Diele, S. Hauser, A. Kresse, H. Pelzl, G. Tschierske, C. Chirality and macroscopic polar order in a ferroelectric smectic liquid-crystalline phase formed by achiral polyphilic bent-core molecules. Angew. Chem. Int. Ed. 2002, 41, 2408-2412. 

Keith, C. Reddy, R. A. Hauser, A. Baumeister, U. Tschierske, C. SUicon-containing polyphUic bent-core molecules the importance of nanosegregation for the development of chirality and polar order in hquid crystalline phases formed by achiral molecules. J. Am. Chem. Soc. 2006,128,3051-3066. 

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 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. 

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... 

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... 

Experiments by Binet et al. [72], carried out on chiral bent core materials containing biphenyl (BP) cores with S or R hydrocarbon chains, and on achiral biphenyl core molecules with chiral dopants, reveal the effect of the molecular chirality on the polarization stmcture. In addition the polarization Pb due to the closed packing of the bent-shape molecules, another polarization, P is introduced due to the chiral and tilted molecular structure. It was found [72] that in the antiferroelectric racemic domains at low fields, of the synclinic - racemic... 

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... 

This situation changed dramatically in 1996 with the discovery of strong electro-optic (EO) activity in smectics composed of bent-core, bowshaped, or banana-shaped achiral molecules.4 Since then, the banana-phases exhibited by such compounds have been shown to possess a rich supermolecular stereochemistry, with examples of both macroscopic racemates and conglomerates represented. Indeed, the chiral banana phases formed from achiral or racemic compounds represent the first known bulk fluid conglomerates, identified 150 years after the discovery of their organic crystalline counterparts by Pasteur. A brief introduction to LCs as supermolecular self-assemblies, and in particular SmC ferroelectric and SmCA antiferroelectric LCs, followed by a snapshot of the rapidly evolving banana-phase stereochemistry story, is presented here. 

The smectic C phase formed by chiral molecules (SmC phase) has also a helical superstructure having a pitch incommensurate with the smectic layer thickness. Theoretically chiral phases can also be formed by achiral molecules due to very specific packing [16]. For instance, three achiral rod-Uke molecules of dijfer-ent length may form a chiral trimer or a tripod due to Van der Waals interactions between their fragments, see Fig. 4.25a, and such trimers, in their turn, may form a kind of helical structure. Another example is bent-core or banana like-molecules [17]... 

It has been shown [34] that molecules containing a bent rigid core, so-caUed banana molecules, exhibit spontaneous polarization an chirality in their smectic LC phases in spite of the fact that the molecule itself is achiral. The banana molecules in a 1.3-benzene bis [4- (4- -alkoxyphenyliminomethyl) benzoate] (P -0-PIMB) homologous series, as illustrated in Fig. 1C, have a various characteristic physical properties in the LC phases, which are classified and designated as B-phases [35]. It has been reported by... 

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. ... 

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