Spectroscopic study of singlet oxygen photogeneration by lipophilic photosensitiser in liposomes

The time- and spectrally-resolved phosphorescence measurements of protoporphyrin IX (PpIX), haematoporphyrin (HpD) and singlet oxygen in liposomal samples under different oxygen concentrations were performed. We observed two different phosphorescence lifetimes of two distinct groups of photosensitisers (PSs). The group with shorter lifetime is located deep inside the nonpolar lipid bilayer, whereas the group with longer lifetime is exposed to H2O due to its localisation near the bilayer surface. When the oxygen concentration in H2O is increased about five times, a significant change in the slower decay component of the group of PS near the surface was observed from 3.8 to 1.3 μs (HpD) and from 3.3 to 1.2 μs (PpIX). On the other hand, the shorter phosphorescence components exhibit less-pronounced changes in lifetimes from 0.42 to 0.39 μs (HpD) and from 0.28 to 0.25 μs (PpIX). The singlet oxygen decay time decreases from 7.3 to 3.5 μs (PpIX) and from 8.0 to 3.5 μs (HpD) in H2O. The results are discussed in the frame of a model, where an increase of the oxygen concentration in the aqueous medium is accompanied by only a slight increase of the oxygen concentration inside the lipid bilayer.