The primary mechanism of ROS generation in PDT involves the excitation of the photosensitizer to its singlet excited state upon light absorption. This excited state can undergo intersystem crossing to form a triplet state, which can then transfer energy to ground state molecular oxygen to produce singlet oxygen, a highly reactive form of oxygen. Additionally, the triplet state photosensitizer can engage in electron transfer reactions to generate other ROS like superoxide anions and hydroxyl radicals. These reactions are examples of photocatalytic processes.
