Phototautomeric equilibrium in the lowest excited singlet state of 3-hydroxyacridone

The pH dependencies of the absorption and fluorescence emission of 3-hydroxyacridone (3-HA) were studied. Three species (the cation, the neutral molecule, and the anion) were identified in absorption. The same species, albeit at different pH ranges, were identified in fluorescence. Their decay times are highly different. In addition, a new species, having an unusually long wavelength emission that has no equivalent in the ground state was identified in the pH 4 to H-0 -4 range. This species is assigned to an excited-state tautomer formed predominately by adiabatic double-proton transfer during the lifetime of the excited state. Two mechanisms are operative: At pH 4 to H-0 0, the neutral (uncharged) species is photoprotonated at the carbonyl group and quickly undergoes photodissociation at the hydroxy group. In more acidic solutions (H-0 -3 to H-0 -2), the cation is exclusively excited, followed by photodissociation from the 3-hydroxy group to form the phototautomer. In the region H-0 -2 to H-0 0, both mechanisms are operative, and therefore, the continuity of the fluorimetric titration curve of the cation suggests that the photoexcited cation and the phototautomer are in equilibrium, a photochemical and spectroscopic rarity.