Banana smectics

As briefly mentioned in the Introduction, the extensive flow of chirality-related topics has been initiated by the discovery of polar switching in bent-core mesogens [11, 45]. Among the first banana smectic phases B1-B8, at least the B2 [10], B4 [14], and B7 [15] phases can be chiral. The chirality in the B2 phase was... 

The Bj textures show characteristic helical filamentary growth in cooling from isotropic melt. i- 2 However they denote at least two distinct phases. X-ray measurements clarified that the original By materials, which form freestanding strands, - just like columnar liquid crystals of disc-shape molecules (see Chapter 2), indeed have a columnar phase (hereafter we will label them as Bj j). Other By-type materials (we call them Byu) were foimd to have modulated layer structures. We note that the By and B2 type banana-smectics also form strands of fibers. ... 

In explaining the fiber formation of "banana smectics," we have to recognize the analogy between the cross section of tire banana smectic and the columnar fibers. This means that the compression of the smectic layers will have the same effect as the compression of the columns in colimmar materials. Accordingly, we can use the same arguments and equation that was used to describe the stability of colimmar fibers. Even the typical surface tension and compression modulus data are similar (7=2 x 10 /m and B 1(F J/tn ) so we get D, 0.15 fim. Again, similar to colmnnar fibers, this is an order of magnitude smaller than the experimentally observed smallest... 

Proposed "jelly-roll" structure of the banana smectic fibers. 

Presently it is not clear why smectic phases of rod-shape molecules do not form fibers, but banana smectics do. It is believed that the fiber forming ability of tire smectics is related to the asymmetry of the surface tension. 

Figure 2-8. Textures of 4- j,m cells of a banana-smectic in chiral state viewed without polarizers. The pictures represent lOO-jim x 70-p,m areas. Permission for Reprint, courtesy Society for Information Display...

Chirality can also be introduced when one or more chiral carbons are incorporated in the molecules, for example in the hydrocarbon terminal chains [67, 68], within the bent-core [69], or by addition of chiral dopants [6, 70], It was noted during the early research that the handedness of the homochiral structures is very sensitive to chiral dopants [6], or even on chiral surfaces [71]. On the other hand, it was observed that banana-smectics made of enantiomeric chiral molecules form synclinic - antiferroelectric [44] and anticlinic ferroelectric [67] domains. This combination of tilt and polar order implies that the phase is racemic, with a rigid alternation of right- and left-handed chiral layers. This shows that the molecular chirality has no or minor effect on deciding about anticlinic or synclinic packing (which is mainly determined by entropic reasons), but it can bias the otherwise degenerate tilt directions. 

In the effort to make free-standing films of the first B1 material it was found that instead of films they form free-standing strands [25] just like columnar liquid crystals of disc-shape molecules [95], Such observation was confirmed with other B7 materials [98], and later it was found that the B7 and B2 type banana-smectics also form strands of fibers [99]. Although the most stable fibers are the B7 materials with slenderness ratio as large as 5000, the B7 and B2 fibers also have aspect ratios over 1000 and 100, respectively. The values are orders of magnitudes larger than of the Rayleigh-Plateau limit [100] of Newtonian isotropic fluids and of nematic and smectic liquid crystals of rod-shape molecules [96]. 

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. 

Figure 8.18 Smectic dimer of Watanabe, possessing an odd number of methylene units in linking group. This material self-assembles into intercalated smectic structure very similar to B6 banana phase. As for B6 phase, this achiral phase is also neither ferroelectric nor antiferroelectric.

Figure 8.20 Structure and phase sequence of prototypical bent-core mesogen NOBOW (8) are given, along with space-filling model showing one of many conformational minima obtained using MOPAC with AMI force field. With observation by Tokyo Tech group of polar EO switching for B2 smectic phases formed by mesogens of this type, banana LC field was bom. Achiral, polar C2v layer structure, with formation of macroscopic spontaneous helix in polarization field (and concomitant chiral symmetry breaking), was proposed to account for observed EO behavior.

Figure 8.23 Illustration of chiral layer structure proposed for NOBOW high-temperature smectic banana phase is given. On left layer with (+) configuration is shown, while on right enantiomeric (—) configuration is illustrated. These mirror image fluid smectic layer configurations are not superposable.

As can be easily seen by inspection of these illustrations of the SuiCsPa and ShiCsPf phases, while the director tilt in the tilt plane is synclinic for both, the layer interfaces have a different character when observed in projection in the bow plane. The antiferroelectric diastereomer has synclinic character at the layer interfaces, while the ferroelectric diastereomer is anticlinic in the bow plane. This suggests a very simple reason for the tendency toward antiferroelectric bananas, this being basically the same as the tendency toward ferroelectric calamitic smectics preference for synclinic layer interfaces. 

Figure 8.29 Illustration showing how all-anticlinic bilayer smectic should be ferroelectric is given. In case of covalent dimers (bent-core mesogens), equilibrium tilt of director combined with anti clinic layer interfaces in bow plane provides SmCsPF ferroelectric banana structure.

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. 

This volume of Topics in Stereochemistry could not be complete without hearing about ferroelectric liquid crystals, where chirality is the essential element behind the wide interest in this mesogenic state. In Chapter 8, Walba, a pioneering contributor to this area, provides a historical overview of the earlier key developments in this field and leads us to the discovery of the unique banana phases. This discussion is followed by a view of the most recent results, which involve, among others, the directed design of chiral ferroelectric banana phases, which display spontaneous polar symmetry breaking in a smectic liquid crystal. 

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... 

Watanabe J, Niori T, Sekine T, Takezoe H (1998) Fmstrated structure induced on ferroelectric smectic phases in banana-shaped molecular system. Jpn J Appl Lett 37 L139-L142... 

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... 

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... 

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