Bent-core mesogens, achirality

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.

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. 

Weissfiog, W. Dunemann, U. Schroder, M. W. Diele, S. Pelzl, G. Kresse, H. Grande, S. Field-induced inversion of chirality in SmCPA phases of new achiral bent-core mesogens. J. Mater. Chem. 2005,15, 939-946. 

Keith, C. Reddy, R. A. Tschierske, C. The first example of a hquid crystalline side-chain polymer with bent-core mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem. Commun. 2005, 7, 871-873. 

The main features of the two experimental interference patterns in Figure 5-23 - two pairs of diffuse peaks with an azimuthal angular separation of 80° in the absence and one pair of reflections in the presence of the electric field - were found in the calculated patterns as well. This unambiguously proves the biaxial nature of the investigated nematic phase. Latest polarization microscopy and X-ray experiments conducted on azo-substituted achiral bent-core mesogens revealed the existence of a uniaxial and a biaxial nematic phase as well as three smectic phases [36]. 

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. 

Spontaneous formation of chiral LC phases from achiral mesogens—an equivalent to spontaneous resolution of achiral compounds in crystals but in a fluid phase—is a relatively recent phenomenon [9,130-132]. The first compounds discovered with this extraordinary property were the bent-core or banana-shaped mesogens [133,134],... 

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... 

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