Figure 1.

Schematic of photodynamic therapy. When light of a particular wavelength interacts with the PS, the PS goes from a ground singlet state to an excited singlet state. Thereafter, intersystem crossing occurs, and the PS enters an excited triplet state. It then interacts with (1) O₂ to form ¹O₂ and (2) biomolecules to form ROS. Some excited singlet state PS molecules will return to their ground state, releasing fluorescence; others will return from the excited triplet state to the ground state, releasing phosphorescence.