A comparative spectroscopic study of thiourea effect on the photophysical and molecular association behavior of various phenothiazine dyes.

This paper describes the role of a structure breaking additive (thiourea) on the photophysical and molecular association of a series of phenothiazine dyes in aqueous media using the absorption and fluorescence spectroscopic methods for the first time. The studied dyes were thionine, azure A, azure B, toluidine blue, and methylene blue. The spectral data were analyzed using DECOM program. Relevant spectral parameters in the dye solutions were estimated and discussed based on the chemical structure of the additive and excitonic treatment. The observation of spectral changes in the spectral data of the (water-additive-dye) system indicates the possible structure formation between the dye and additive molecules. However, it is found that in the high concentrations of thiourea the dimer geometries (H- and J-type dimers) are influenced by the dye-additive interactions. As a result, a competition between the dye-additive and dye-dye interactions was also observed.