Figure 1. Jablonski diagram illustrating the principles of PDT.

A ground-state photosensitizer (⁰PS) absorbs a photon, transitions to the short-lived (nsec) excited singlet state (¹PS) that can lose energy by fluorescence, internal conversion to heat, or else can undergo intersystem crossing to the long-lived (μsec) excited triplet state (³PS). The³PS can relax to ground state by emitting phosphorescence, but can also undergo energy transfer with ground state triplet oxygen (³O₂) to form reactive singlet oxygen (¹O₂, Type 2) or else can undergo an electron transfer reaction to form hydroxyl radicals (HO•, Type 1). Both these ROS (¹O₂ and HO•) can efficiently kill cancer cells (see Figure 2).