Abstract
New asymmetrical/symmetrical styryl cyanines (<(3a)under bar>-(d) under bar,<(4a)under bar>-(d) under bar), monomethine cyanine dyes (<(5a)under bar>-(d) under bar,<(6a)under bar>-(d) under bar) and trimethine cyanine dyes (<(11a)under bar>-(g) under bar,<(12a)under bar>-(g) under bar) incorporating bis pyrazolo-[2,3-b;2',3'-b'] oxazine and/or pyrazolo [2,3-b]-oxazolo [2',3'-b] oxazine were prepared. The new synthesised cyanines were identified by elemental and spectral analyses. The uv-visible absorption spectra of some selected dyes were investigated in pure and mixed solvents as well as in aqueous buffer solutions. Molecular complex formation with ethanol was verified by mixed solvent studies. Electronic transitions were attributed to either locally excited or predominantly charge transfer states. The spectral shifts were discussed in relation to molecular structure and in terms of medium effects. The variation of absorbance with pH was utilized for the determination of the pKa values for some selected compounds.