Modeling of Electronic Spectra of Ionic Forms of Eosin and Erythrosin

Multistage dissociation of fluoroscein dyes, widely used in biological labeling, yields a variety of ionic and tautomeric forms in a wide range of pH values. In contrast to well-studied absorption spectra, the emission spectra are not quite readily interpreted due to their strong overlapping and proton transfer in electronically excited states. The least studied are the fluorescent properties of eosin and erythrosin dyes containing heavy atoms (Br, I), in which the characteristics of the dianionic form only are reliably determined. In the framework of the density functional theory using the B3LYP-functional including nonequilibrium solvation, the geometries of the series of ionic forms of eosin and erythrosin in the ground and excited states are found, and the electronic spectra are calculated. Based on the identified linear regression of the calculated and experimental data for the earlier resolved electronic spectra, for the first time, the emission spectrum maxima of the monoanionic, neutral quinoid, and cationic forms of the dyes are determined. The spectral peculiarities (Stokes shifts) are discussed in terms of variation of the molecule and ion geometries in the ground and excited states.