Photolysis of sulfosalicylic acid in aqueous solutions over a wide pH range

Nanosecond laser flash photolysis, absorption and fluorescent spectroscopy were used to study the influence of pH on the photophysical and photochemical processes of 5-sulfosalicylic acid (SSA) in aqueous solutions. Information on the excited singlet state intramolecular proton transfer (ESIPT) of the SSA ions could be deduced from the dependence of the quantum yield and the spectral maximum of SSA fluorescence on the pH of the medium. The main photochemical active form of SSA at pH < 10 is the dianion (HSSA2−). Excitation of this species gives rise to the HSSA2− triplet state, to the SSA2− radical anion and to the hydrated electron. In a neutral medium, the main decay channels of these intermediates are T–T annihilation, recombination and capture by the HSSA2− dianion, respectively. A decrease of pH leads to an increase of the second-order rate constants of disappearance of both HSSA2− triplet state and SSA2− radical anion due to their protonation.