Liquid crystals bent-core mesogens

Keywords Liquid crystals Bent-core mesogens Polar propeties X-ray structure studies... 

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... 

A survey of recent studies of biaxial liquid crystals, in the context of the reported biaxial nematic phase in low molecular mass bent-core mesogens, with emphasize on the use of NMR spectroscopy is available. A discussion of orientational order parameters particularly in smectic-C and biaxial nematic phases and their determination by various techniques including NMR has been presented. The relation between molecular dynamics and biaxiality of nematic polymers and elastomers has been studied by H NMR spectroscopy. "" It was observed that the side-on attachment of the mesogens hinder the rotation of the molecules around their long axes and that in nematic polymers and elastomers phase biaxiality... 

Boron clusters are effective and unusual structural elements for designing calamitic and bent-core mesogens. Their structural and electronic diversity combined with judicious choice of substituents makes it possible to address fundamental aspects of liquid crystal research, to design LCs with unusual properties, and to prepare technologically important materials. Properties of these formally inorganic-organic hybrid mesogens, such as photophysical behavior, polarity, and phase structure, can be tuned by judicious choice of the cluster and substituents. 

Within the last few years, two different experimental techniques could successfully prove the existence of a biaxial nematic phase in liquid crystals based on bent-core mesogens. Madsen et al. [32] found indications for a phase biaxiaUty with polarization microscopy and conoscopy and also obtained additional quantitative evidence by means of deuterium NMR. Acharya and co-workers [33] used X-ray... 

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]. 

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],... 

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. 

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

Although more than 60 years ago Vorlander had already reported the synthesis of bent-shaped liquid crystals, ° they did not attract much interest until Mat-sunaga et al. synthesized new mesogenic compoimds with "banana-shaped" molecular stmctures. Only recently it was realized that not only rod-shape (calamitic) or discotic molecules can form liquid crystals, but bent-core (bowlike or banana-shape) molecules can, too. ... 

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. 

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