Molecule banana-shaped

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. 

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. 

Note 1 At one time it was thought that a non-amphiphilic molecule had to be long and rod-like for mesophase formation, but it has now been established that molecules of other types and shapes, for example, disc-like and banana-shaped molecules, may also form mesophases. (See ref. 6). 

Mesogen constituted of bent or so-called banana-shaped molecules in which two mesogenic groups are linked through a semi-rigid group in such a way as not to be co-linear. 

Fig. 32. Schematic representation of the flexo-electric effect, (a) The structure of an undeformed nematic liquid crystal with pear- and banana-shaped molecules (b) the same liquid crystal subjected to splay and bend deformations, respectively.

Likewise for the N()2 ion, the two 0 atoms must be treated the same way as participating in double-bond formation. All three atoms must use sp- orbitals, with end-overlap between each 0 and N to form cr bonds. And of course the three p orbitals that rise perpendicularly from their common plane will side-overlap and merge to give a delocalized split-banana-shaped n molecular orbital, with one half-banana above the plane of the molecule and the other half below (Figure 9-28). Four of the 18 valence electrons are held in the two cr bonds, 10 are held as five lone pairs in the sp- orbitals (two on each O, and one on the N), and the remaining four are delocalized over the length of the molecule in the n bond (two above the plane and two below). 

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... 

Thisayukta J, Niwano H, Takezoe H, Watanahe J (2002) Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules. J Am Chem Soc 124 3354-3358... 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanahe 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... 

Heppke G, Parghi DD, Sawade H (2000) Novel sulphur-containing banana-shaped liquid crystal molecules. Liq Cryst 27 313-320... 

Pelzl G, Diele S, Jakli A, LischkaCh, Wirth I, Weissflog W (1999) Helical superstructures in a novel smectic mesophase formed by achiral banana-shaped molecules. Liq Cryst 26 135-139... 

Kurosu H, Kawasaki M, Hirose M, Yamada M, Kang S, Thisayukta J, Sone M, Takezoe H (2004) Solid-state C-13 NMR study of chiral twisted conformation attributable to chirality in smectic phases of achiral banana-shaped molecules. J Phys Chem A 108 4674 -678... 

Thisayukta J, Takezoe H, Watanabe J (2001) Study on helical structure of the B4 phase formed from achiral banana-shaped molecule. Jpn J Appl Phys 40 3277-3287... 

Thisayukta J, Niwano H, Takezoe H, Watanabe J (2001) Effect of chiral dopant on a helical Sml phase of banana-shaped N-n-O-PIMB molecules. J Mater Chem 11 2717-2721... 

Diborane (B2H6) is a dimer composed of two molecules of borane (BH3). The bonding in diborane is unconventional, using three-centered (banana-shaped) bonds with protons in the middle of them. Diborane is in equilibrium with a small amount of borane (BH3), a strong Lewis acid with only six valence electrons. 

Bent-core LC molecules are one fascinating LC class featured by a banana-shaped or bent-core molecular shape which have attracted a great deal of interest for then-exciting electro-optical behavior [65,66]. When the bent-core molecule 13 was used (Scheme 4.5), a columnar phase of hybrid GNPs were obtained (Fig. 4.10) [50]. 

It is clear that the two polar vectors respect the apolar nature of n. There is also an obvious analogy of the above mechanism with the orientation polarization of a liquid dielectric, which was used by Helfrich to relate the two flexocoefficients with molecular properties. The intrinsic splay or bend can be related to an appropriate angle and molecular dimensions. The relevant component of the electric dipole moment and the curvature elastic constant, viz., and the splay constant Ki or /ux and the bend constant It s figure in the estimation of the flexocoefficients. Nematic liquid crystals made of banana-shaped molecules have been studied only recently, and a comparison of the experimental measurements with the Helfrich formula leads to interesting inferences, as will be mentioned later in this chapter, and covered more thoroughly in the companion Chapter 3 by Jakli et al. ... 

As described above, the kink in the molecular shape and the requirement to fill the space as effectively as possible are not compatible with a three-dimensional fluid order. In other words, when translating a bent-core molecule in the melt of the neighbouring bent-core molecules, it experiences a periodic potential with its periodicity determined by the length I of the molecules. To allow for fluidity at the macroscopic level, one needs to frustrate the bent-core structure so that they do not lock into smectic layers easily. Such a frustration can be introduced by some steric or electrostatic disturbance of the bare bent-core (or peeled banana ) shape, which has been seen in some modulated smectic phases.As discussed by Bailey and Jdkli,a steric or electrostatic inclusion in the core of the molecules leads to layer modulation, an SmCc structure and broken smectic layers that effectively correspond to a columnar phase, as the inclusions increase. Following this picture, here we postulate that bent-core nematics are probably more frustrated than the electrically unswitchable B7 (columnar) phases, in which the broken smectic ribbons are separated by melted fluid nematic regions. Such over-frustrated B7 materials are characterized... 

In addition to the pear-shaped molecules, bent-shaped molecules were used to illustrate the dipolar origin of the flexoelectric effects in nematic liquid crystals. It was assumed that the constituent molecules of the nematic liquid crystals are free to rotate around their axes, and in the absence of electric fields, their dipole moments average out so the net polarization of the material is zero. However, when liquid crystals made from polar pear- or banana-shaped molecules are subjected to splay or bend deformations, respectively, they can become macroscopically polar, because the polar structures correspond to a more efficient packing of the molecules. It follows from symmetry considerations that the deformation-induced fiexo-electric polarization Pa can be written as ... 

The dipolar mechanism is sensitive to the molecular shape. By dimensional considerations one can estimate the flexocoefficients due to dipolar mechanism as ei, 63 < /Xe/a, where /Xg 1-5 debye (1 D = 3.3x 10 Cm) is the molecular dipole moment and a 2-4 nm is the typical molecular dimension for a low molecular weight liquid crystal. This means that e and 63 are expected to be of the order of pCm. Assuming a random three-dimensional distribution of the centre of masses of the constituent bent-core (banana-shaped) molecules, Helfrich and Derzhanski and Petrov derived a more precise expression for the macroscopically testable bend fiexo-electric coefficient ... 

G. Heppke, A. Jakli, D. Kriierke, C. LShning, D. LStzsch, S. Pans, R. Rauch and K. Sharma, Polymorphism and electro-optical properties of banana shaped molecules. In European Conference on Liquid Crystals (ECLC 97), Zakopane, March 1997, p. 34. 

Let us look at Fig. 11.24. In the upper two sketches, we can see undistorted nematic liquid crystals with pear- and banana-shape molecules. Such nematics in the bulk... 

Fig. 11.24 Dipolar flexoelectric polarization. Pear-shape and banana-shape molecules in undistorted nematic liquid crystals without any polar axes (a) and appearance of polar axes and flexoelectric polarization along the z-direction in the same nematics due, correspondingly, splay and bend distortion (b)...

As has been shown, the splay and bend distortions of a nematic create electric polarization. There is also a converse effect the external electric field causes a distortion due to the flexoelectric mechanism. For example, if the banana-shape molecules with transverse dipoles are placed in the electric field, the dipoles are partially aligned along the field and their banana shape induces some bend. This effect takes place even in nematics with zero dielectric anisotropy. 

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