Blue cholesterics

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

The B4 phase is complex a phase seemingly dominated by defects, similar to the cholesteric blue phase. While the detailed structure is not known, some facts are clear. From the viewpoint of this discussion, the key observation is the chirality of domains of the B4 phase in 4-. im LC cells. This... 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

By addition of each of several diesters of isosorbide, isomannide, and isoidide to a nematic phase, cholesteric phases can be produced. All compounds exhibit a large twisting power. In the cholesteric phase, helix inversion, large or small temperature-dependencies of the pitch, and broad blue phases were achieved.183... 

A different phenomenon is the appearance of the so-called blue phases in case of chiral LC s, first observed at cholesteric LC s. Sometimes, one or more blue phases occur close below the clearing point with a very small phase width and bright blue or green colours. Figure 15 shows some typical representatives containing one or more X=Y groups in... 

Experimental evidence was reported for the existence of various additional phases a pre-cholesteric order in the form of a network of double-twisted cylinders, analogous to the thermotropic blue phases [27], a hexatic phase that replaces the hexagonal columnar in very long DNA fragments [31], and a structure with orthorhombic symmetry appearing in the transition to crystalline order [27]. 

It was really only a matter of time until researchers in the field started doping blue phases with quasi-spherical nanoparticles. This area is very much in its infancy, but the few recent reports already show some promising results. Yoshida et al., for example, reported on an expansion of the temperature range of cholesteric blue phases from 0.5 to 5°C by doping blue phases with gold nanoparticles (average diameter of 3.7nm) as well as a decrease in the clearing point of approximately 13°C [427]. A similar effect was also observed by Kutnjak et al. for CdSe quantum dots simultaneously capped with oleyl amine and TOP (diameter of the core 3.5 nm) in CE8 (Merck) and CE6 (BDH). The authors found that particularly blue phase III was stabilized in these mixtures, blue phase II destabilized, and... 

Cladis, P. E., Pieranski, P., Joanicot, M., Elasticity of blue phase-I of cholesteric liquid-crystals. Phys. Rev. Lett. 1984, 52, 542-545. 

The valency of the hydrogen bond donors and acceptors can be varied to produce chiral mesophases [85]. Mixtures of the divalent 4,4 -bipyridine with 4-[(S)-2-methylbutoxy]benzoic acid in ratios between 1 9 and 4 6 show LC behaviour, but the chiral mesophases are exhibited for only a small range of compositions [86]. Here, the cholesteric and a blue LC phase were observed. The association of the acid to the bipyridine was confirmed by a crystal structure of the 1 2 complex. 

Hgure 4.33 Representation of the local structure of some chiral mesophases on heating. The matches describe the relative orientations of chiral molecules in space. As the temperature is raised, the system transforms from a crystalline phase (left) to a cholesteric phase (centre) characterised by a single twist, to a double-twist blue" phase (right). 

The structure of the blue phase is of some importance. Among the lipoproteins carrying lipids in the blood, low-density lipoproteins (LDL) have attracted much attention. They are the factors mainly responsible for plaque formation, which ultimately leads to atheriosclerotic changes and heart disease. The major components of the LDL-particles are cholesterol fatty acid esters. A remarlmble property is the constant size of LDL particles [28], which indicates that the interior must possess some degree of order. It seems probable that the structure proposed above for cholesterol esters in the cholesteric liquid-crystalline structure should occur also in the LDL-particle. In that case the LDL particle can be viewed as a dispersed blue phase, whose size is related to the periodicity of the liquid-crystalline phase, and the protein coat at the surface is oriented parallel to adjacent specific crystallographic planes of the blue phase. These amphiphilic proteins will expose lipophilic segments inwards emd expose hydrophilic groups towards tiie enviroiunent. 

L-glucose enantiomer (Figure 5.38). Correspondingly, the green cholesteric liquid crystalline phase becomes red upon addition of D-glucose and blue upon addition of L-glucose. Diasteromeric hexoses gave similar effects. ... 

Solid Cholesteric. . Blue phase I Blue phase U... 

Some cholesteric materials show the blue phase as the temperature increases from that of the cholesteric phase and before it reaches that of the isotropic phase. The blue phase is a cubic phase. There have been three blue phases found so far BP I, BP II and BP III phases. It is now understood that the BP I phase is a body-centered cubic, the BP II phase is a primitive cubic and the BP III phase is a fog phase with no structural symmetry. Generally the temperature range of the blue phase is quite narrow, less than 1 degree... 

Both the mononuclear palladium complexes, with a chiral alkyl chain in the ether substituent of one of the aromatic rings of the Schiff base, and the dinuclear complexes, where the bridge joining the two palladium atoms is itself chiral, exhibit a blue phase in a glassy state.Pure, they form a cholesteric phase, right-handed for the (5)-Pd (5)-(6.24) derivative and left-handed for the (/f)-Pd2 (R)-(6.25) derivative (Figure 6.15). 

The strange blue phase liquid-crystalline condition of certain cholesteryl esters (nonanoate and myristate) exists over a very narrow temperature range between the cholesteric and isotropic phases. Its structure has now been probed by study of deuterium-labelled materials. The H n.m.r. spectra have been inter-... 

The blue phases occur in cholesteric systems of sufficiently low pitch, less than about 5000 A. They exist over a narrow temperature range, usually 1 C, between the cholesteric liquid crystal phase and the isotropic liquid phase (see (1.3.5)). The first observation of a blue phase was described by Reinitzer himself in his historic letter to Lehmann as follows On cooling (the liquid phase of cholesteryl benzoate) a violet and blue phenomenon appears, which then quickly disappears leaving the substance cloudy but still liquid. Although Lehmann recognized it as a stable phase, not until the 1970s was it generally accepted that the blue phases are thermodynamically distinct phases. The nature of these phases has now become a subject of considerable interest to condensed matter physicists. 

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