Photochemistry of Fe(III) and sulfosalicylic acid aqueous solutions

Femtosecond and nanosecond laser flash photolysis was used to determine the photophysical and photochemical processes in aqueous solutions of Fe(III) ion and 5-sulfosalicylic acid (SSA) containing the FeSSA complex and the free ligand. Excitation of the FeSSA complex in the charge transfer band (λmax = 505 nm) is followed by an ultrafast relaxation to the ground electronic state with two characteristic times of 0.26 and 1.8 ps. The shorter time constant is ascribed to internal conversion to the vibrationally hot electronic ground state of FeSSA and the 1.8 ps time constant is assigned to the vibrational cooling of the ground state. The UV irradiation of the solution (308 nm) leads to the excitation of both the free ligand and the FeSSA complex. The latter relaxes rapidly and the free ligand undergoes intersystem crossing to the triplet state. This system undergoes an irreversible photochemical reaction originating from an electron transfer (k = (9 ± 2) × 108 M−1 s−1) from the free ligand in the triplet state to the FeSSA complex. This electron transfer is accompanied by an energy transfer between these species (k = (1.3 ± 0.2) × 109 M−1 s−1).