Spectral properties of the prototropic forms of fluorescein in aqueous solution

The commonly used fluorescent probe, fluorescein, can exist in seven prototropic forms. We have used global analysis procedures to reanalyze the absorption data of Diehl and Horchak-Morris (Talanta 34, 739-741, 1987) in terms of five alternative ionization models. We identify the forms of fluorescein present in aqueous solution and the pK a of each ionisation transition. The pKa values of the neutral xanthene, carboxylic acid, and cationic xanthene groups are 6.3, 3.1-3.4, and 3.1-3.4, respectively, and the pKa value of lactonization is 2.4. As a consequence, the neutral form of fluorescein is a mixture of the lactone (70%), zwitterionic (15%), and quinoid (15%) forms. A knowledge of the forms present in solution permits the characterization of their spectral properties. It is shown that the quinoid and monoanion forms have similar absorption spectra, while the zwitterion spectrum is similar to that of the cation but blue-shifted by 3 nm. The emission spectra of the monoanion and quinoid forms are also identified and shown to be similar but not identical. A model for the excited-state reactions of fluorescein is presented.