Concentration effect on the absorption spectra of oxazine1 and methylene blue in aqueous and alcoholic solutions

The visible absorption spectra of oxazine-1 (OX) in aqueous and alcoholic solutions were studied as a function of the dye concentration. Due to the structural similarity of OX and MB, the optical characterization of methylene blue, MB, was also investigated for comparison. OX in aqueous solutions appears to exist almost in its monomeric form at concentrations below about 1 × 10− 4 M, while MB exhibit strong tendency to aggregate even at the low concentrations. The dimerization behavior of OX and MB in water was analyzed in terms of monomer-dimer equilibria. The nature of the interacting pairs in these dyes was discussed using the exciton theory.