Light-induced processes in aqueous solutions of porphyrins

An analysis of the spectral changes of porphyrins in aqueous solutions detected upon irradiation with visible light shows that a few photoprocesses take place. The decrease in absorption and emission intensity throughout the whole spectra reflects: (1) a photoinduced destruction of the porphyrin macrocycle and (2) a photoinduced chemical modification leaving the porphyrin macrocycle intact. The photodestruction of the porphyrin macrocycle is related to: (1) the formation of di- and mono-pyrrolic photoproducts absorbing in the UV and (2) the formation of negligible amounts of bilirubin-like photoproducts with the emission maximum at 545 nm. Another photoprocess is related to the formation of chlorin and bacteriochlorin type photoproducts with absorption maxima at about 640 - 660 nm, maintaining the porphyrin macrocycle intact. A scheme of the photomodifications of porphyrins is presented. The phototransformations of sensitizers should be taken into account in clinical photochemotherapy.