Photosensitization is a reaction to light that is mediated by a light-absorbing molecule, which is not the ultimate target. There are many substances which do not normally react when exposed to light. lf, however, another substance is added to it, photochemical reaction readily proceeds. The substance thus added does not undergo any chemical change. It merely absorbs light energy and then passes it on to the reactant substance. The added substance is called a photosensitizer, since it sensitizes the reaction. The process is termed as photo-sensitization. The photosensitizer acts as a carrier of energy from the excited molecule to the reactant molecule. Photosensitization can involve reactions within living cells or tissues, or they can occur in pure chemical systems. In photobiology, we are concerned with the reactions in living systems.
If only hydrogen gas is irradiated by ultraviolet light of λ = 253.70 nm the molecules do not dissociate to the atoms. But if the same radiation acts on hydrogen in presence of Hg-vapour, the hydrogen molecules undergo dissociation to the hydrogen atoms.
Hg + hv → Hg–
Hg+ + H2 → Hg + 2H
The radiation excites the mercury atoms and the excited mercury atoms transfer the energy to the hydrogen molecules by collision. The reaction occurs as a result of transference of energy from pholochemically excited molecules by collision- in this case from Hg to H2 molecules.
Another photosensitized reaction is the decomposition of oxalic acid in presence of uranyl sulphate.
UO22+ + hv → UO22+*
UO22+* + (COOH)2 → UO22+ + CO2 + CO + H2O
The uranyl ion acts as a photosensitizer.
The decomposition of ozone in ordinary light by a trace of chlorine is an example of photosensitized reaction. In ordinary photographic films or plates silver bromide is the active ingredient which is decomposed by visible light of the shorter wavelength region. Thus red and orange bodies appear equally dark in the print. However, if suitable photosensitizes are mixed with the silver bromide the latter becomes sensitive to the entire part of the visible spectrum. Such films are known as panchromatic films. These give a more natural and livelier picture than the orthochromatic films.