Abstract
This paper comparatively reports the role of a molecular additive (caffeine) on the absorption and emission spectral characteristics and molecular association of a series of phenothiazine dyes in aqueous solutions. The studied dyes were five phenothiazine dyes (thionine, azure A, azure B, toluidine blue O, and methylene blue) with strong close molecular structures. The spectral data of the water-caffeine-dye systems were systematically analyzed and discussed based on the chemical structure of the constituent molecules. The aggregate structure and the dimer characteristics were analysed based on the exciton model. The additive was to act as strong water structure-breaker over the range of studied concentration. The main driving force for breaking the dye aggregation is water-caffeine or dye-caffeine interactions at low and moderate caffeine contain. At the high additive concentrations, the caffeine-caffeine, dye-dimer caffeine, and caffeine-water-caffeine might have crucial role in the spectral and aggregation behavior of the dyes. Caffeine induces markedly fluorescence quenching in emission spectra of the studied dyes.
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