Bent-core

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

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. 

Figure 8.19 Structures of bent-core double-Schiff bases of Matsunaga are given. Mesogenicity of isomers of type 8 was found superior to that of isomers of type 9.

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.

Stimulated by the Tokyo Tech results, several groups began studying similar bent-core mesogens. At the European Conference on Liquid Crystals, held in Zakopane, Poland, in March of 1997, Heppke et al. confirmed the Tokyo Tech results,46 and Weissflog et al. did as well, with the important exception that the Halle group found antiferroelectric behavior in the B2 phase.47... 

Figure 8.21 Symbolic bow-and-arrow-based representations of bent-core mesogen structures used in this discussion are given.

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

In order to test this very simple idea, the MHOC tail was incorporated into the prototypical bent-core double-Schiff-base structure, providing as target the mesogen (Rac)-MHOBOW (9),53 whose structure and phase sequence is shown in Figure 8.30. The racemate was prepared in order to avoid complications in the interpretation of the results caused by molecular enantiomeric... 

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.

Benidipine, 5 130, molecular formula and structure, 5 125t Bentazon, 13 323 Bent-core liquid crystals, 15 98 Bentonite, 6 664, 686, 696 9 11, 17. 

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

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. 

Nowadays, rods and discs are considered as model structures because some other unconventional shapes have been recently described such as bananas (bent-core), cones and rings. 

Also mesogens with more complex shapes, such as, for example, those with bent aromatic cores (bent-core mesogens [47]), star mesogens [48], or cone-like... 

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]...

Bent-Core Mesogens with Perfluorinated Segments... 

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... 

Special effects on mesogenic self-assembly have also been observed for highly core fluorinated bent-core mesogens [420],... 

Polar and Apolar Columnar Phases Made of Bent-Core Mesogens... 

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