Figure 2.

Simplified Jablonski diagram showing the photosensitized generation of singlet oxygen. Following excitation to the singlet state (S₁) by absorption of a photon, the photosensitizer undergoes intersystem crossing, thus transitioning to its triplet state (T₁). One of the non-radiative relaxation pathways involves transfer of the photosensitizer excitation energy to ground state oxygen (³O₂), which results in the formation of singlet oxygen (¹O₂) and the relaxation of the photosensitizer back to its ground state (S₀). Radiative relaxation pathways include the emission of fluorescent (hν_f) or phosphorescent (hν_p) photons.