photolysis

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The chemical decomposition of matter due to absorption of incident light. For example, illumination of microcrystals of silver bromide embedded in gelatin results in formation of metallic silver, and is the basis of the photographic process.

Numerous metal complexes, azides, nitrides, and sulfides, and most organometallic compounds undergo decomposition upon illumination, often with concomitant evolution of a gaseous product. In the presence of water and oxygen, illumination of many semiconductors results in their corrosion; for example, an aqueous suspension of cadmium sulfide (CdS) undergoes rapid decomposition upon irradiation with sunlight. The outcome of these photochemical processes can often be controlled by addition of adventitious materials. For example, illumination of cadmium sulfide in aqueous solution containing hydrogen sulfide (H₂S) and colloidal platinum results in evolution of hydrogen gas. Here, the semiconductor photosensitizes (or photocatalyzes) decomposition of hydrogen sulfide into its elements, and the reaction can be used to remove sulfides from industrial waste. See also Coordination complexes.

Many ketones, for example, acetone, abstract hydrogen atoms from adjacent organic matter under illumination. According to the circumstances, the resultant free radicals may be used to initiate polymerization of a monomer or cause decomposition of a plastic film. Both reactions have important commercial applications, and photoinitiators are commonly used for emulsion paints, inks, polymers, explosives, fillings for teeth and for development of photodegradable plastics. Photolysis of carbonyl compounds, released into the atmosphere by combustion of fossil fuels, is responsible for the onset of photochemical smog. Many other types of photochemical transformation of organic molecules are known, including isomerization of unsaturated bonds, cleavage of carbon-halogen bonds, olefin addition reactions, halogenation of aromatic species, hydroxylation, and oxygenation processes. Indeed, photochemistry is often used to produce novel pharmaceutical products that are difficult to synthesize by conventional methods. See also Free radical; Photodegradation.

The most important photochemical reaction is green plant photosynthesis. Here, chlorophyll that is present in the leaves absorbs incident sunlight and catalyzes reduction of carbon dioxide to carbohydrate.

Excitation of an organic molecule results in spontaneous generation of the singlet excited state. In most molecules, this highly unstable excited singlet state may undergo an intersystem-crossing process that results in population of the corresponding (less energetic) excited triplet state, in competition to fluorescence. The excited triplet state, because of spin restriction rules, retains a significantly longer lifetime than is found for the corresponding excited singlet state, and may be formed in high yield.

Almost without exception, these triplet states react quantitatively with molecular oxygen (O₂) present in the system via a triplet energy-transfer process. The resulting product is singlet molecular oxygen. This species is a potent and promiscuous reactant, and it is responsible for widespread damage to both synthetic and natural environments. Indeed, plants and photosynthetic bacteria contain carotenoids to protect the organism against attack by singlet oxygen. The same species is known to be responsible, at least in part, for photodegradation of paint, plastic, fabric, colored paper, and dyed wool. Secondary reactions follow from attack on a substrate by singlet oxygen, resulting in initiation of chain reactions involving free radicals. However, modern technological processes have evolved in which singlet molecular oxygen is used to destroy unwanted organic matter, such as tumors, viruses, and bacteria, in a controlled and specific manner. In photodynamic therapy a dye is injected into a tumor and selectively illuminated with laser light. The resultant singlet oxygen destroys the tumor. Similar methodology can be used to produce photoactive soap powders, bleaches, bactericides, and pest-control reagents. See also Chain reaction (chemistry); Triplet state.

Chemical decomposition by light.