Photophysical studies of pheophorbide a and pheophytin a. Phosphorescence and photosensitized singlet oxygen luminescence

The triplet states of pheophorbide a and pheophytin a were studied in several environments by direct measurement of the phosphorescence of the pigments and photosensitized singlet oxygen (1O2) luminescence. The spectra, lifetimes and quantum yields of phosphorescence and the quantum yields of 1O2 generation were determined. These parameters are similar for monomeric molecules of both pigments in all the environments studied. Aggregation of the pigment molecules leads to a strong decrease in the phosphorescence and 1O2 luminescence intensities, which is probably due to a large decrease in the triplet lifetime and triplet quantum yield in the aggregates. The results obtained for pheophorbide a and pheophytin a are compared with those previously reported for chlorophyll alpha. The data suggest that the photodynamic activity of the pigments in living tissues is probably determined by the monomeric pigment molecules formed in hydrophobic cellular structures. Aggregated molecules seem to have a much lower activity.