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

The pH dependencies of the absorption and fluorescence emission of 3-hydrox yacridone (3-HA) were studied. Three species (the cation, the neutral molec ule, 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 identif ied in the pH 4 to H-0 -4 range. This species is assigned to an excited-sta te tautomer formed predominately by adiabatic double-proton transfer during the lifetime of the excited state. Two mechanisms are operative: At pH 4 t o H-0 0, the neutral (uncharged) species is photoprotonated at the carbonyl group and quickly undergoes photodissociation at the hydroxy group. In mor e acidic solutions (H-0 -3 to H-0 -2), the cation is exclusively excited, f ollowed by photodissociation from the 3-hydroxy group to form the phototaut omer. In the region H-0 -2 to H-0 0, both mechanisms are operative, and the refore, the continuity of the fluorimetric titration curve of the cation su ggests that the photoexcited cation and the phototautomer are in equilibriu m, a photochemical and spectroscopic rarity.