Achiral dyes

Circularly Polarized Luminescence from Achiral Dyes Doped... 

Some of the very early applications of CPL spectroscopy involved chiral polymeric systems. In particular, the CPL from achiral dye molecules dissolved in cholesteric liquid crystals has been used to probe chirality changes. CPL from chromophores attached to a chiral poly-amino acid may also be used to study exciton coupling between aromatic chromophores. In these polymeric systems, it is often observed that gijjjjj is quite large. In some cases it is so large, in fact, that detection using static quarter-wave plates is possible. 

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). 

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... 

Kirstein, S., von Berlepsch, H., Bottecher, C., et al. (2000). Chiral J-aggregates formed by achiral cyanine dyes. Chem. Phys. Chem., 1, 146-50. 

Recognition of amino acids has been attempted more frequently with receptors that are likewise zwitterionic in nature (compare 35). Some early studies include the transport of phenylalanine by a merocyanine dye through a liposomal bilayer [53]. Rebek s dicarboxylate-complexing cleft 9 (see Sect. 2) turned out to be a selective (though achiral) binder of trypthophane, phenylalanine, and tyrosine methyl ether [54]. A reasonable structure for a phenylalanine complex of 2 1 stoichiometry, as deduced from NMR studies, is schematically represented in formula 39 [55]. 

Complexes between chiral polymers having ionizable groups, and achiral small molecules become, under certain conditions, optically active for the absorption regions of the achiral small molecules. Dyes such as acridine orange and methyl orange have been used as achiral species, since they are in rapport with biopolymers through ionic coupling. This phenomenon has been applied to the detection of the helix chirality in poly-a-amino acids, polynucleotides, or polysaccharides when instrumental limitations prevent direct detection of the helices. 

Association of molecules changes their As as has been proven for metal complexes, charge transfer complexes, Lewis acids, etc. In any case, complex formation can be followed by the variation of the CD. Molecular association is the first step to an aggregation and, furthermore, is the presupposition for supramolecular structures. Compounds with inter-molecular exciton interaction, e.g., carotenoids or cyanine dyes, have been successfully analyzed. The spontaneous association to chiral associates from achiral compounds, e.g., cyanines, have been proven and analyzed with the help of ECD spectroscopy. In this context, the successful CD analyses with Langmuir-Blodgett films as well as the analyses of membranes have to be mentioned where suprastruc-tural chirality can also emerge from achiral monomers. 

The first fundamental studies on this system were published by Styrer and Blout [28] in 1959. They took poly-a-L or poly-a-D glutamic acid as the chiral polyelectrolyte, and a cationic dye such as acridine orange or pseudoisocyanine as the achiral counterion. If the... 

Miyagawa, T. Yamamoto, M. Muraki, R. Onouchi, H. Yashima, E. Supramolecular helical assembly of an achiral cyanine dye in an induced helical amphiphilic poly(phenylacetylene) interior in water. J. Am. Chem. Soc. 2007, 129, 3676-3682. 

In the case of the DNA solutions, neither the temperature nor the dye concentration was changed, indicating that the DNA was promoting aggregation of the dye. This was verified using circular dichroism (CD) spectropolarimetry, which takes advantage of the fact that achiral molecules exhibit no CD in solution, but when bound to a chiral molecule such as DNA can exhibit induced CD [19]. A strong, exciton coupled CD... 

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