A New Insight into the Singlet Oxygen Mechanism for Photodynamic Therapy.

Modern photodynamic therapy has been built on the mechanism of the interaction between the photosensitizer (porphyrin derivatives) and oxygen to produce singlet oxygen, which relies on energy transfer from the triplet excited state (T 1 ) of porphyrin to the excited state of oxygen. In this process, the energy transfer from the singlet excited state (S 1 ) of porphyrin to oxygen is believed to be not pronounced as the rapid decay of S 1 and the large energy mismatch. Here, we have evidenced the existence of an energy transfer between S 1 and oxygen, which can contribute to the production of singlet oxygen. For hematoporphyrin monomethyl ether (HMME), the Stern-Volmer constant of S 1 ( K SV ') is 0.023 kPa -1 , according to the oxygen concentration-dependent steady fluorescence intensities. In addition, fluorescence dynamic curves of S 1 under various oxygen concentrations have also been measured through ultrafast pump probe experiments to further verify our results.