Resolution of absorption spectra of rhodamine 6G aggregates in aqueous solution using the law of mass action

There is striking dissimilarity among the absorption spectra of rhodamine dimers reported by different authors. To address this issue, we measured the absorption spectra of aqueous solutions of rhodamine 6G over a wide range of concentrations. The lack of an isosbestic point near 508 nm indicates the presence of at least three species. Data analysis using the program spectrabind described in [D. Toptygin, L. Brand, Anal. Biochem. 222 (1995) 330] confirms the presence of trimers and tetramers in addition to dimers and monomers. The absorption peaks of the monomer, dimer, trimer, and tetramer are at 526, 498, 485, and 475 nm, respectively.