What is "photosensitizer"

n. (context chemistry English) A molecular or atomic species that initiates a photochemical reaction

A photosensitizer is a molecule that produces a chemical change in another molecule in a photochemical process. Photosensitizers are commonly used in polymer chemistry in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers generally act by absorbing ultraviolet or visible region of electromagnetic radiation and transferring it to adjacent molecules. Photosensitizers usually have large de-located π systems, which lower the energy of HOMO orbitals and its absorption of light might be able to ionize the molecule.

Chlorophyll act as a photosensitizer in photosynthesis of carbohydrates in plants:

6CO + 6HO → CHO + 6O