Figure 1.

Basic mechanism of photosensitizer excitation in PDT: The photosensitizer (PS) is represented as a single atom for conceptual ease, highlighting the electron transitions between energy levels. The diagram illustrates the initial stage of PS activation, where the PS, in its ground state, absorbs a photon within the therapeutic window (600–800 m), elevating an electron to the singlet excited state. The inset clarifies the electron’s energy jump and subsequent fluorescence emission as it returns to the ground state, a precursor event before intersystem crossing and reactive oxygen species generation in PDT.