The efficiency of a photosensitizer in converting light energy to a reactive species is quantified by its quantum yield . Quantum yield is the ratio of the number of reactive species generated to the number of photons absorbed by the photosensitizer. The value of quantum yield can range from 0 to 1, with 1 being the highest efficiency.The photochemical mechanism involved in the conversion of light energy to a reactive species by a photosensitizer can be briefly explained in the following steps:1. Absorption of light: The photosensitizer absorbs light energy usually in the visible or near-infrared region and gets excited from its ground state S0 to a higher-energy excited state S1 or S2 .2. Internal conversion: The photosensitizer undergoes internal conversion, where it relaxes from higher excited states S2 to the lowest excited state S1 by losing energy in the form of heat or vibrational energy.3. Intersystem crossing: The photosensitizer undergoes intersystem crossing, a process in which it transitions from the singlet excited state S1 to the triplet excited state T1 by changing its electron spin. The triplet state is longer-lived and more reactive than the singlet state.4. Energy transfer: In this step, the photosensitizer in the triplet excited state T1 can interact with a target molecule or a substrate, transferring energy and generating a reactive species. There are two primary pathways for this energy transfer: a Type I mechanism: The photosensitizer in the triplet state undergoes electron transfer with the substrate, generating free radicals or radical ions. These reactive species can further react with other molecules, leading to a chemical reaction or a cascade of reactions. b Type II mechanism: The photosensitizer in the triplet state transfers energy directly to molecular oxygen O2 , generating highly reactive singlet oxygen 1O2 . Singlet oxygen can then react with the substrate or other molecules, causing oxidation reactions.5. Deactivation: Finally, the photosensitizer returns to its ground state S0 and can be reused in the process.The overall efficiency of a photosensitizer in generating reactive species depends on various factors, including the absorption properties of the photosensitizer, its ability to undergo intersystem crossing, and the efficiency of energy transfer to the target molecule or substrate.