Fluorescence and photoinduced proton transfer in the protolytic forms of fluorescein: Experimental and computational study

In contrast to the well-studied absorption spectra of different protolytic forms of fluorescein, the complex structure of the fluorescence spectra in a wide pH range is not completely understood because of the interplay between emission and photoinduced proton transfer in the electronic excited states. We provide insight into this interplay through a combined analysis of the experimental data, obtained by absorption and steady-state fluorescence spectroscopy at pH 0.3-10.5, and the time-dependent density functional theory (TD-DFT). The TD-DFT based computational model is validated on dianion and used to model the spectra of other protolytic forms. The protolytic/tautomeric forms of fluorescein are classified according to the partial charges on the triple chromophore ring, and electronic transitions are analyzed in terms of changes in molecular geometries and orbitals. A linear regression analysis between the calculated and experimental results based on both absorption and well-understood dianionic and cationic fluorescence peaks is used to assign the monoanionic (496 nm), neutral quinoid (550 nm) and neutral zwitterionic (483 nm) fluorescence peaks, whose positions were not clear prior to this work. The values of the excited-state dissociation microconstants pk(a)*for different forms of fluorescein are calculated by means of the Forster cycle in conjunction with the spectroscopic measurements and computational data.