Thiazine dyes: Evaluation of monomeric and aggregate forms

• The aggregation of thiazine dyes was evaluated in organic and aqueous media.
• The monomeric species presented an absorption maximum around 500 nm.
• Different J-aggregates were observed in aqueous medium.

The aggregation phenomenon of Azure B, monobrominated Azure B, Thionine and Methylene Blue was studied by UV–Visible spectrophotometry in different media as a function of dye concentration and temperature variations. The tests carried out in organic solvents allowed the identification of monomeric species of these compounds, which have not been reported in literature and have been wrongly assigned for years. The results obtained in water allowed demonstrating that different kinds of aggregates are present in this medium. In addition, the aggregation tendency of these dyes in organic solvent and aqueous media was established. Several parameters such as lipophilicity, effect of bulky substituents and interactions with media were considered to interpret the aggregation behavior of thiazine dyes.

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