How does photosensitization transfer light energy from one substance to another?

Photosensitization is a process used to initiate a chemical reaction by employing a substance—a sensitizer—that absorbs light and transfers that energy to the intended reactants. This technique is particularly valuable for driving reactions that require specific wavelengths of light that may not be readily available from a direct source.1

The transfer of energy typically occurs through the following mechanism:

• Absorption: The sensitizer (such as mercury vapor, zinc, or certain organic dyes) absorbs incident light energy, which promotes the sensitizer to an excited electronic state.1
• Collision: The excited sensitizer molecule then collides with the target reactant molecules. During these collisions, the sensitizer transfers its excess energy to the reactants.1
• Deactivation: After transferring the energy, the sensitizer returns to its initial, lower-energy ground state, while the reactant molecules—now in an excited state—undergo a chemical reaction, such as decomposition.1

In some instances, the sensitizer may also participate directly in the chemical reaction rather than acting solely as an energy intermediary.1