Bent-core molecules

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. 

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. 

Fig. 60 Examples of anchor-shaped (bent-core) bolaamphiphiles and comparison of the hexagonal honeycombs (a) 6-molecule hexagons (p6mm symmetry) as formed by 186b) and (b) 3-molecule hexagons (p iml symmetry) as formed by 186a (dark gray = fluorinated cores, small light gray dots = glycerols) [344]...

Fig. 66 (a) Examples of bent-core mesogens with semiperfluorinated terminal chains [348-351] and (b) models of the organization of the molecules in polar smectic phases... 

Fig. 1 Due to their strongly bent shape and the excluded volume effects bent-core molecules tend to form polar layers, v is the layer normal and p the electric polarization...

It seems that, in contrast to the rod-like molecules, in the case of the bent-core molecules there are no special molecular prerequisites for the formation of 2D structures they are frequent and found for symmetric as well as asymmetric molecules, for molecules with strong and weak dipole moments - multiple terminal chains are also not required (see for example [6]). 

So far we have discussed 2D density modulated phases that are formed by deformation or breaking of the layers. However, there are also 2D phases with more subtle electron density modulations. In some cases additional peaks observed in the XRD pattern (Fig. 10) are related to a double layer periodicity in the structure. As double layer periodicity was observed in the bent-core liquid crystals formed by the asymmetric as well as symmetric molecules [22-25] it should be assumed that the mechanism leading to bilayers must be different from that of the pairing of longitudinal dipole moments of molecules from the neighboring layers, which is valid for smectic antiphases made by asymmetric rod-like molecules. 

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 ... 

The first term in (7) describes the coupling between the polarization splay and tilt of the molecules with respect to the smectic layer normal. This coupling is responsible for the chiral symmetry breakdown in phases where bent-core molecules are tilted with respect to the smectic layer normal [32, 36]. The second term in (7) stabilizes a finite polarization splay. The third term with positive parameter Knp describes the preferred orientation of the molecular tips in the direction perpendicular to the tilt plane (the plane defined by the nematic director and the smectic layer normal). However, if Knp is negative, this term prefers the molecular tips to lie in the tilt plane. The last term in (7) stabilizes some general orientation (a) of the polar director (see Fig. 7) which leads to a general tilt (SmCo) structure. 

Coleman DA, Jones CD, Nakata M, Clark NA, Walba DM, Weissflog W, Fodor-Csorba K, Watanabe J, Novotna V, Hamplova V (2008) Polarization splay as the origin of modulation in the B1 and B7 smectic phases of bent-core molecules. Phys Rev E 77 021703... 

Gorecka E, Vaupotic N, Pociecha D, Cepic M, Mieczkowski J (2005) Switching mechanisms in polar columnar mesophases made of bent-core molecules. Chemphyschem 6 1087-1093... 

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... 

Gorecka E, Pociecha D, Matraszek J, Mieczkowski J, Shimbo Y, Takanishi Y, Takezoe H (2006) Polar order in columnar phase made of polycatenar bent-core molecules. Phys Rev E 73 031704... 

Keywords Axial chirality Bent-core molecule Chirality Chirality control Deracemization... 

Stabilization of Blue Phase by Bent-Core Molecules. 309... 

Let us consider an N phase doped with BSMs, where BSMs interact with chiral host molecules. The interaction energy between left-handed bent-core conformation and the chiral host molecule f/LH is different from that between right-handed bent-core conformation and the same chiral host molecule t/RH- The nonzero difference AU = f/LH Crh induces finite ee in BSMs, resulting in increased chiral molecules in the system. At the same time, we have to consider the dilution effect. 

Finally, the difference of chirality enhancement in the N and SmC phases should be mentioned. As shown in Sect. 2.1, enhancement rate in SmC is about one order of magnitude larger than that in N. In the SmC chirality enhancement is attributed to two effects (1) the interaction between bent-core and chiral host molecules and (2) the coupling between ee, tilt, and spontaneous polarization. The latter effect is absent in the N phase and is an additional effect in SmC. Moreover, the chiral discrimination parameter AU is expected to be larger in SmC than in N because of a confined geometry, i.e., smectic layer. 

Fig. 6 Reciprocal optical pitch as a function of dopant content. Host material is a cholesterol derivative used in the enhanced chirality in N with bent-core molecules [5]...

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... 

Earl DJ, Osipov MA, Takezoe H, Takanishi Y, Wilson MR (2005) Induced and spontaneous deracemization in bent-core liquid crystal phases and in other phases doped with bent-core molecules. Phys Rev E 71 021706-1-11... 

Lee M, Hur S-T, Higuchi H, Song K, Choi S-W, Kikuchi H (2010) Liquid crystalline blue phase I observed for a bent-core molecule and its electro-optical performance. J Mater Chem... 

Kundu B, PratibhaR, MadhusudanaNV (2007) Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rodlike and bent-core molecules. Phys Rev Lett 99 247802-1-4... 

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... 

Guo L, Dhara S, Sadashiba BK, Radhika S, Pratibha R, Shimbo Y, Araoka F, Ishikawa K, Takezoe H (2010) Polar switching in the smectic-A PA phase composed of asymmetric bent-core molecules. Phys Rev E 81 011703-1-6... 

Fig. 17 The stereochemical possibilities for packing in bent-core molecule phases...

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. 

Liquid crystals are interesting supramolecular systems which can show second harmonic generation when they are aligned appropriately. Ferroelectric LCs [250] as well as bent-core molecules have been used to this purpose, and show reasonable second harmonic generation [251]. These materials combine non-linear optical effects with simple processing procedures on account of their liquid crystalline flow characteristics and the possibility of organising them with electric and magnetic fields. 

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