Figure 2.

Triplet excited state dynamics in PDT: The diagram captures the photodynamic action of a PS which, upon light absorption, transitions from a ground state to a singlet excited state. It then undergoes intersystem crossing to achieve a triplet excited state. In Type I reactions, the triplet PS engages with cellular substrates to simultaneously generate radical ions or free radicals. These entities can inflict damage on cellular substrates and, through further reactions with molecular oxygen, give rise to diverse ROS that cause enhanced damage. Concurrently, the PS can directly interact with oxygen to initiate a reaction cascade, leading to potent damage from the hydroxyl radical, resulting in cell death. In Type II reactions, the triplet PS directly transfers energy to molecular oxygen, forming singlet oxygen (¹O₂) that damages cellular components. Both Type I and Type II pathways can proceed in tandem, with repeated cycles of ROS generation culminating in extensive cell damage, cell death, and the degradation of the PS.