Photochemical reactions are driven by light or near-visible electromagnetic radiation. In general, incoming units of energy, known as photons, excite effected molecules, raising their energy to a point where they can undergo reactions that would normally be exceedingly difficult. The process is distinguished from thermal reactions, which take place with molecules in their normal energy states. Under sunlit conditions, photochemical processes can generate small amounts of extremely reactive molecules which initiate important chemical reaction sequences.
To initiate a photochemical reaction, two requirements need to be met. First, the photon must have enough energy to initiate the photochemical reaction in the molecule. Second, the compound must be colored, in order to be able to react with visible or near-visible photon radiation.
In environmental chemistry , photochemical reactions are of considerable importance to the trace chemistry of the atmosphere . The Los Angeles photochemical smog is an example of a system of photochemical reactions. Perhaps one of the most important reactions in this system is the photochemically driven conversion of nitrogen dioxide, a major component of automobile exhaust, to nitric oxide and an atom of oxygen, which usually occurs in pairs in the air. The oxygen atom subsequently attaches to an oxygen molecule to form the secondary pollutant, ozone . Photochemically initiated reactions are also responsible for generating the all important hydroxyl radical, a key intermediate in the trace chemistry of the lower atmosphere.
Photochemical reactions are also important in natural waters. Here they may be responsible for enhancing the reaction rate of organic compounds or changing the oxidation state of metallic ions in solution. However, some of these compounds are not colored, and are unable to absorb light. In such cases reactions may be mediated by photosensitizers, compounds capable of being energized by absorbed light which then pass the energy onto other molecules. Seawater appears to contain natural photosensitizers, perhaps in the form of fulvic acid or chlorophyll derivatives. Photosensitizers activated by sunlight can react with organic compounds or dissolved oxygen to produce the reactive singlet-oxygen, which can react rapidly with many seawater compounds.
Photochemical reactions also occur in solids. Since solids often lack the transparency required for the light to penetrate the surface, reactions are usually limited to the surface, where incoming photons initiate reactions in the top-most molecules. There is also some evidence that, under extremely bright desert sunlight, traces of nitrogen and water absorbed on titanium and zinc oxides can be photochemically converted to ammonia. Such processes are no doubt also important on planetary surfaces.
[Peter Brimblecombe ]
Findlayson-Pitts, B. J., and J. N. Pitts. Atmospheric Chemistry. New York: Wiley, 1986.