Long Description for Photodynamic Therapy

This image illustrates the process of Photodynamic Therapy (PDT) and the photophysical pathways of a photosensitizer (PS) upon laser irradiation. Here’s what’s happening:

1. Laser Absorption:
   • The ground-state photosensitizer (PS, S₀) absorbs laser energy and transitions to the singlet excited state (S₁).
2. Pathways from Singlet Excited State (S₁):
   • Photothermal Therapy: The excited PS releases energy as heat (red wavy arrow).
   • Fluorescence Imaging: Some of the excited PS molecules emit fluorescence, which can be used for imaging (green arrow).
   • Internal Conversion: Some energy is lost non-radiatively, leading back to the ground state.
   • Intersystem Crossing: Some molecules undergo a transition to the triplet excited state (T₁) (dashed red arrow).
3. Pathways from Triplet Excited State (T₁):
   • Phosphorescence: Some molecules return to the ground state via emission of lower-energy light.
   • Type I Reaction (Electron Transfer): The excited triplet PS can transfer electrons to surrounding biomolecules, leading to the formation of reactive oxygen species (ROS) such as hydroxyl radicals (HO•).
   • Type II Reaction (Energy Transfer): The triplet PS transfers energy to molecular oxygen (³O₂), converting it into singlet oxygen (¹O₂), a highly reactive form of oxygen.
4. Photodynamic Therapy (PDT) Effect:
   • The generated ROS (¹O₂, HO•) can damage cancerous or diseased cells, making this process effective for cancer treatment and other medical applications.

This diagram effectively summarizes how light-activated photosensitizers can be used in PDT for cancer therapy and related biomedical applications.