Photosensitization
Energy Transfer in Photochemical Reactions
Photosensitization & Quenching Reactions
Photosensitization was discovered by Frank and Cario in 1922. Some chemical reactions take place not by the absorption of light by one of the reactants but by a third substance which transfers the absorbed energy to the reactants. The third substance which itself does not undergo any change is called the photosensitizer and the process is known as photosensitization.
Among the photosensitizers commonly used are mercury, cadmium and zinc as atomic sensitizers and the molecular photosensitizers such as benzophenone and SO2.
Consider a general donor-acceptor system in which only the donor D i.e. the sensitizer, absorbs the incident quantum of energy and the triplet state of the donor is higher in energy than the triplet state of the acceptor A i.e. the reactant. Absorption of the photon produces the singlet excited state of the donor, 1D which via inter-system crossing (ISC) gives the triplet excited state of the donor 3D.
This triplet excited state of the donor then collides with the acceptor producing the triplet excited state of the acceptor 3A and the ground sate of the donor. If 3A gives the desired products, the mechanism is called photosensitization. If, however, the products of interest result from 3D, then A is called the quencher and the process is known as quenching.
The reactions depicting photosensitization and quenching may be represented as-
D + h𝜈 → 1D
1D ---ISC→ 3D
3D + A → D + 3A
3A → products (Photosensitization)
3D Products (quenching)
It may be noted that the triplet excited state of the sensitizer must be higher in energy than the triplet excited state of the acceptor (Reactant) so that the energy available is enough to raise the reactant molecule to its triplet excited state.
Example-
The most important example of Photosensitization is the photosynthesis of carbohydrates in plants from CO2 and H2O in which chlorophyll, the green colouring matter of plants, acts as a photosensitizer. Neither CO2 nor H2O absorbs any radiation in the visible range but chlorophyll does absorb in the visible range and then transfers to CO2 + H2O molecules which then react to form carbohydrate.
