Chirality, dyes

Colourants are perhaps the oldest recognised synthetic molecular materials, and they present a wide range of properties, both optical and other, and a varied supramolecular chemistry [216,217]. Several chiral dyes are known and their aggregates have been studied [218,219]. Here we shall concentrate on the more recent advances in the area. 

Dyes, and their absorption changes, can be used to sense chirality and differentiate between two enantiomers with the naked eye , i.e. without the use of a polarimeter or any other device that measures optical activity. In principle every diastereomeric interaction between a chiral medium and the two enantiomers of a chiral dye must lead to different spectral characteristics. In practice, however, the effects are... 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

Circularly Polarized Luminescence from Achiral Dyes Doped in Chiral Matrices... 

M Voigt, M Chambers, and M Grell, Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal, Liq. Cryst., 29 653-656, 2002. 

Phthalazinone, 19 332 o-Phthaldialdehyde-2-mercaptoethanol, chiral derivatizing reagent, 6 76t Phthalein dyes, 19 304-305 Phthalic acid, 19 332... 

Tris(tribromoneopentyl) phosphate, 11 492 Tris(tribromophenyl) triazine, 11 467 Tris- 1-phenylethylcarbamate chiral stationary phase, 6 88t Tris-3,5-dichlorophenylcarbamate chiral stationary phase, 6 88t Tris-3,5-dimethylphenylcarbamate chiral stationary phase, 6 88t Trisaccahrides, 4 697 Trisanyl, 4 358t Trisaturates, 10 813 Trisazo dyes, 9 363... 

Chowdhury A, Wachsmann-Hogiu S, Bangal PR, Raheem I, Peteanu LA (2001) Characterization of chiral H and J aggregates of cyanine dyes formed by DNA templating using stark and fluorescence spectroscopies. J Phys Chem B 105 12196—12201... 

Zhang Y, Xiang J, Tang Y, Xu G, Yan W (2007) Chiral transformation of achiral J-aggregates of a cyanine dye templated by human serum albumin. ChemPhysChem 8 224-226... 

Another method for assaying the activity and stereoselectivity of enzymes at in vitro concentrations is based on surface-enhanced resonance Raman scattering (SERRS) using silver nanoparticles (116). Turnover of a substrate leads to the release of a surface targeting dye, which is detected by SERRS. In a model study, lipase-catalyzed kinetic resolution of a dye-labeled chiral ester was investigated. It is currently unclear how precise the method is when identifying mutants which lead to E values higher than 10. The assay appears to be well suited as a pre-test for activity. 

Figure 5.10 Schematic of a guest-host (GH) LCD using dichroic dyes and a chiral nematic...

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