English: Illustrative scheme of photodynamic reactions. The photosensitizer absorbs light and is promoted from its ground singlet state (1PS) to an excited singlet state (1PS*). Alternatively, the photosensitizer can convert to an excited triplet state (3PS*) by inter-system crossing. This is a longer-living state that allow sufficient time for chemical reactions to occur. A photosensitizer in 3PS* state can return to ground state (1PS) either by emitting phosphorescence, or by photochemical reactions that occur through donation of charges or energy. These photochemical reactions can generate cytotoxic reactive oxygen species (ROS) via the Type I or II photodynamic reactions. In a cellular microenvironment, these ROS have a short lifespan, and react with and destroy biomolecules, such as proteins, nucleotides, and lipids, very close to the illumination source. Type I: Charges, such as electrons, are transferred or donated to surrounding substrates (R), forming radicals (R•) due to the presence of the unpaired electron that was received. Molecular oxygen (O2) participates directly or indirectly in this reaction pathway forming the radical anion known as superoxide (O2•–). The superoxide radical can be further reduced to form hydrogen peroxide (H2O2), which can also be reduced to form highly reactive free hydroxyl radicals (HO•) via Fenton-like reactions. Type II: Energy is transferred to ground state triplet molecular oxygen (3O2), creating singlet oxygen (1O2*), an excited form of oxygen that is much more reactive than its ground state triplet counterpart. 1PS = Ground Singlet State of Photosensitizer; 1PS* = First Excited Singlet State of Photosensitizer; 3PS* = First Excited Triplet State of Photosensitizer; ISC = Inter System Crossing; 3O2 = Ground State Triplet Oxygen; 1O2 = Excited State Singlet Oxygen.