Figure 3.

Photodynamic therapy-customized Perrin–Jablonski energy diagram. After cellular uptake of the photosensitizer (PS) and irradiation with light emitting a specific wavelength, this radiation is absorbed by the PS in the ground state (PS S₀), which transitions to the first or second excited singlet state (PS S₁ and PS S₂, respectively). Variations between PS S₁ and PS S₂ or between the various energy levels of the same state occur by internal conversion. Radiative (fluorescence and consequent return to S₀) or nonradiative (intersystem crossing to the PS T₁) processes can occur in the PS S₁. The PS T₁ can interact with biological substrates, forming reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), hydroxyl radical (^•OH), or superoxide anion (^•O₂⁻), by type I reactions, or singlet oxygen (¹O₂) by type II reactions. In high concentrations, these ROS led to cell damage in essential structures for normal cell functioning, culminating in cell death.