Banana phases bent-core mesogens

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.

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... 

Xu JD, Dong RY, Domenici V, Fodor-Csorba K, Veracini CA (2006) C-13 and H-2 NMR study of structure and dynamics in banana B-2 phase of a bent-core mesogen. J Phys Chem B 110 9434-9441... 

Nadasi, H. Weissflog, W. Eremin, A. Pelzl, G. Diele, S. Das, S. Grande, S. Ferroelectric and antiferroelectric banana phases of new fluorinated five-ring bent-core mesogens. J. Mater. Chem. 2002,12, 1316-1324. 

An accepted experimental proof for the existence of a biaxial nematic phase in a thermotropic liquid crystal remained missing for a very long time. However, in recent years, biaxial nematic phases have been found in liquid crystalline polymers as well as in liquid crystals made of rod-disc mesogens, banana-shaped (bent-core) molecules, and organo-siloxane tetrapodes. Here, some characteristics of these systems and the corresponding experimental procedure for the investigation of phase biaxiality will be introduced. Further details for the individual systems can be found in the cited literature. 

Recent optical experiments have probed the effect of flexible polymeric solutes in both lyotropic [36] and thermotropic liquid-crystal [37, 38] environments. Long-chain solutes feel an average effective anisotropy, and are found to exhibit much stronger ordering than smaller solutes. The synthesis of the bent-core mesogens ( banana molecules) has resulted in a novel biaxial smectic-A (both pure phase [39] and solute-induced [40]) as well as nematic phases [41,42]. 

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],... 

Another class of mesogen, which has proven to be a successful candidate for the formation of a biaxial nematic phase, is made up of the so-called bent-core, or banana mesogens. In this kind of mesogen, two rod-like molecules are chemically linked together via a central unit. The molecular structure of such a banana mesogen is schematically given in Figure 5-19. 

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