A theoretical study of dimerization equilibria for fluorescent dyes in aqueous solutions is described. The work is based on the comparison of calculated data provided by absorbance, fluorescence and thermal lens measurements with respect to the analytical concentration of the dye. Deviations from linear response due to dimerization are computed for rhodamine 6G and the diamide of the zinc-tetracarboxyphthalocyanine with known values of the fluorescence quantum yield of the monomer, of the dimerization constant and of molar absorptivities. It is demonstrated that the photothermal method gives valuable information and can be more sensitive than absorbance and fluorescence to detect dimer formation provided the monitoring wavelength has been well chosen.
