Singlet oxygen production by lipophilic photosensitizers in liposomes studied by time and spectral resolved phosphorescence

In this contribution, excitation energy transfer from lipophilic photosensitizers incorporated into the liposomes to singlet oxygen was investigated. Measurements were carried out on photosensitizers used in photodynamic therapy: haematoporphyrin derivatives and protoporphyrin IX. Solutions of l-α-phosphatidylcholine large unilamellar vesicles of 100 nm diameter prepared by extrusion were used. Kinetics of photosensitizer triplet states and singlet oxygen in the liposomes together with processes of excitation energy transfer were examined by direct detection of their weak near-infrared phosphorescence (894 and 1278 nm, respectively) with time and spectral resolution. The life-times of photosensitizer triplets and singlet oxygen in H2O and D2O solutions of liposomes were compared to those in acetone and in lipids droplets. The higher complexity of the liposomal systems is reflected in the fact that the phosphorescence kinetics of the photosensitizers do not follow monoexponential decay.