Abstract
New asymmetrical dimethine cyanine dyes were prepared by the condensation of 4-benzylideno-oxazol-/imidazol-5-one-2-carboxaldehyde derivatives with 2-methylpyridinium(quinolinium)-2-yl salts. Cyclocondensation reaction of these dyes with urea gave new asymmetrical oxazolo(imidazo)-[4,5-d]-pyrimidine-2(2)-dimethine cyanines. The ultraviolet and visible absorption spectra of some selected dyes were investigated in single and mixed solvents, and also in aqueous buffer solutions. Molecular complex formation with ethanol was verified through mixed-solvent studies. Electronic transitions have been attributed to either locally excited or predominantly charge- transfer states. The spectral shifts are discussed in relation to molecular structure and in terms of medium effects. The variation of absorbance with pH is utilised for the determination of pKa. values for some selected compounds.
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