A New Insight into the Singlet Oxygen Mechanism for Photodynamic Therapy

Modernphotodynamic therapy has been built on the mechanism ofthe interaction between the photosensitizer (porphyrin derivatives)and oxygen to produce singlet oxygen, which relies on energy transferfrom the triplet excited state (T-1) of porphyrin to theexcited state of oxygen. In this process, the energy transfer fromthe singlet excited state (S-1) of porphyrin to oxygen isbelieved to be not pronounced as the rapid decay of S-1 andthe large energy mismatch. Here, we have evidenced the existence ofan energy transfer between S-1 and oxygen, which can contributeto the production of singlet oxygen. For hematoporphyrin monomethylether (HMME), the Stern-Volmer constant of S-1 (K (SV)') is 0.023 kPa(-1), according to the oxygen concentration-dependent steady fluorescenceintensities. In addition, fluorescence dynamic curves of S-1 under various oxygen concentrations have also been measured throughultrafast pump probe experiments to further verify our results.

Automated summary

Modern photodynamic therapy relies on the interaction between photosensitizer and oxygen to produce singlet oxygen. Energy transfer from the triplet excited state of photosensitizer to the excited state of oxygen is well-established, while the transfer from the singlet excited state is believed to be insignificant. However, our study provides evidence of energy transfer between the singlet excited state and oxygen in the production of singlet oxygen. This finding has been verified through fluorescence intensity measurements and ultrafast pump probe experiments.