Photophysical properties of 8-hydroxyquinoline-5-sulfonic acid as a function of the pH: a TD-DFT investigation.

Time dependent density functional theory (TD-DFT) in conjunction with a hybrid exchange correlation functional (PBE0) were applied to characterize the photophysical behavior of the 8-hydroxyquinoline-5-sulfonic acid (8-HQS) in solution as a function of the pH. In particular, absorption and emission spectra of each species as well as their relative stability in the first excited state were computed. From these calculations it is possible to directly derive quantities otherwise hardly experimentally accessible such as excited state acidic dissociation constants (pK(a)*) and corresponding distribution diagrams at the excited state. These two latter quantities were determined by first principles from the relative stabilities of the species at the excited state computed at the TD-DFT level. Consequently, the evolution of the absorption and emission spectral properties of 8-HQS as a function of the pH could be fully simulated from first principles. Finally, insights on energetics and the mechanism of the phototautomerization reaction supposed to be responsible for the absence of fluorescence of the 8-HQS molecule were derived from the calculations.