Fig. 3.

Generation of reactive oxygen species and excited states of the photosensitizer. Light promotes the excitation of an electron from a low-energy singlet state (S_o) to high-energy singlet states (S_1,2). Such states can lose their energy via fluorescence (radiative emission, light) or internal conversion (non-radiative emission, heat). The spin flipping of the high-energy electron takes places via intersystem crossing, which leads to a long-lived excited triplet state (T₁). Type I and II reactions favor the formation of free radicals and singlet oxygen (¹O₂), respectively, in the presence of ³O₂. Significantly modified from Ref. [11]