photochemistry, study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible light or ultraviolet radiation. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photon, and go to a higher-energy state, or excited state (see quantum theory). Such a molecule is then much more reactive than a ground-state molecule and can undergo entirely different reactions than the more stable molecule, following several different reaction pathways. One possibility is that it can simply emit the absorbed light and fall back to the ground state. This process, called chemiluminescence, is illustrated by various glow-in-the-dark objects. Another possibility is for the molecule to take part in a photo-induced chemical reaction; it may break apart (photodissociate), rearrange, isomerize, dimerize, eliminate or add small molecules, or even transfer its energy to another molecule. Photochromic compounds—compounds that change color reversibly in going from the dark to the light—are generally compounds that are capable of reversible isomerization, or rearrangement. In the absence of light, the compound exists in its most stable form, which exhibits a particular color; in the presence of light, the compound goes to a less stable form, which exhibits a different color. After removal of the light, the compound will revert back to its original state. The best-known and most important photochemical reaction is photosynthesis, the complex, chlorophyll-catalyzed synthesis of sugars from carbon dioxide and water in the presence of light. Other extremely important and complex photochemical reactions take place in the eye. Photochemistry is indispensible to industries involved with dyes, photography, television, and many other applications of light and color.
Photochemistry, a sub-discipline of chemistry, is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation).

Scientific background

Like most scientific disciplines, photochemistry utilizes the SI or metric measurement system. Important units and constants that show up regularly include the meter (and variants such as centimeter, millimeter, micrometer, nanometer, etc.), seconds, hertz, joules, moles, the gas constant R, and the Boltzmann constant. These units and constants are also integral to the field of physical chemistry.

The first law of photochemistry, known as the Grotthuss-Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.

The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction. This is also known as the photoequivalence law and was derived by Albert Einstein at the time when the quantum (photon) theory of light was being developed.

Photochemistry may also be introduced to laymen as a reaction that proceeds with the absorption of light. Normally a reaction (not just a photochemical reaction) occurs when a molecule gains the necessary activation energy to undergo change. A simple example can be the combustion of gasoline (a hydrocarbon) into carbon dioxide and water. This is a chemical reaction where one or more molecules/chemical species are converted into others. For this reaction to take place activation energy should be supplied. The activation energy is provided in the form of heat or a spark. In case of photochemical reactions light provides the activation energy.

The absorption of a photon of light by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise inaccessible reaction path, as described by the Woodward-Hoffmann selection rules. A 2+2 cycloaddition reaction is one example of a pericyclic reaction that can be analyzed using these rules or by the related frontier molecular orbital theory.

Spectral regions

Photochemists typically work in only a few sections of the electromagnetic spectrum. Some of the most widely used sections, and their wavelengths, are the following:

  • Ultraviolet: 100–400 nm
  • Visible Light: 400–700 nm
  • Near infrared: 700–2500 nm
  • Mid infrared: 2500 - 25000 nm
  • Far infrared: 25–1000 µm


There are important processes based in the photochemistry principles. One case is photosynthesis, which some plants use light to create glucose in their chloroplasts to contribute to cell metabolism. The glucose is used by the plant's mitochondria to produce adenosine triphosphate. Medicine bottles are made with darkened glass to prevent the medicine itself from reacting chemically with light. In fireflies, an enzyme in the abdomen works to produce bioluminescence. The mercaptans or thiols produced by Chevron Phillips Chemical Company are produced by photochemical addition of hydrogen sulfide (H2S) to alfa olefins. Among their many uses as a chemical reagent these mercaptans are used to provide a distinctive odor (an odorant) to otherwise odorless natural gas. Many polymerizations are started by photoinitiatiors which decompose upon absorbing light to produce the necessary free radicals for Radical polymerization.

See also


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