Photosensitizers in the ground state are excited to a high-energy singlet state, which could subsequently be converted into a triplet state via ISC. This triplet state may further undergo photochemical reactions via either a type I or a type II process to generate ROS. (Abs, absorption; IC, internal conversion; ISC, intersystem crossing; SET, single electron transfer; ET, energy transfer; SOD, superoxide dismutase)