photochemistry

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photochemistry,

study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
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 or ultraviolet radiationultraviolet radiation,
invisible electromagnetic radiation between visible violet light and X rays; it ranges in wavelength from about 400 to 4 nanometers and in frequency from about 1015 to 1017 hertz.
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. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photonphoton
, the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent.
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, and go to a higher-energy state, or excited state (see quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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). 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 photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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, 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

 

the branch of chemistry concerned with chemical reactions that occur through the effect of light. Photochemistry is closely related to optics and optical radiation. The first photochemical laws were established in the 19th century, but photochemistry came into its own as an independent branch of science only in the first third of the 20th century with the Einstein photochemical equivalence law, which became the fundamental law of photochemistry.

With the absorption of a quantum of light, a molecule moves from the ground state to an excited state, and in this latter state it takes part in a chemical reaction. The products of this primary (strictly photochemical) process often enter into various secondary processes (“dark” reactions), which yield the final products. Photochemistry may thus be defined as the chemistry of the excited molecules formed on the absorption of light quanta. Often, the more or less significant portion of excited molecules does not enter into a photochemical reaction but rather is returned to the ground state as a result of various types of light-induced deactivation processes. In many cases, these processes may be accompanied by the emission of a quantum of light (fluorescence or phosphorescence). The ratio of the number of molecules taking part in a photochemical reaction to the number of absorbed light quanta is called the quantum yield of the reaction. The quantum yield of the primary process cannot be greater than one, and usually it is considerably smaller than one owing to the considerable deactivation. As a result of the dark reactions, however, the overall quantum yield may be considerably greater than one.

The most typical photochemical reaction in the gaseous phase is that for the dissociation of molecules into atoms and radicals. Thus, ultraviolet radiation of short wavelength creates excited molecules of oxygen (O2*), which dissociate into atoms:

O2 + hv → O2*, O2* → O + O

These atoms then take part in a secondary process with O2, forming ozone:

O + O2 → O3

Such processes occur in, for example, the upper atmospheric layers under the effect of solar radiation.

When a mixture of chlorine and saturated hydrocarbons RH (R being an alkyl) is exposed to light, the hydrocarbons undergo chlorination. The primary process is the dissociation of chlorine molecules into atoms; this reaction is followed by a chain reaction forming chlorohydrocarbons:

CI2 + hv → CI2* → CI + CI

CI + RH → HCI + R

R + CI2 → RCI + CI(and so forth)

The overall quantum yield of the chain reaction is considerably greater than one.

When a mercury-vapor lamp is used to illuminate a mixture of mercury vapor and hydrogen, light is absorbed only by the mercury atoms. Upon excitation, these atoms bring about the dissociation of hydrogen molecules:

Hg* + H2 → Hg + H + H

This is an example of a photosensitized reaction. Under the effect of a quantum of light possessing sufficient energy, molecules are converted into ions. This process, called photo-ionization, may be readily observed with the aid of a mass spectrometer.

The simplest photochemical process in the liquid phase is that of electron transfer, that is, an oxidation-reduction reaction induced by light. For example, when an aqueous solution containing Fe2+, Cr2+, V2+, and other ions is exposed to ultraviolet light, an electron passes from an excited ion to a water molecule. This transfer can be seen in the reaction

(Fe2+)* + H2O → Fe3+ + OH + H+

Secondary processes then produce a molecule of hydrogen. Electron transfer, which can proceed upon the absorption of visible light, is characteristic of many dyes. Light-induced electron transfer with the participation of a chlorophyll molecule is the first step in photosynthesis, a complex photochemical process for living systems that occurs in green leaves on exposure to sunlight.

In the liquid phase, molecules of organic compounds having multiple bonds and benzene rings can take part in a variety of dark reactions. Aside from bond splitting, which leads to the formation of radicals and biradicals—for example, carbenes—and from heterolytic substitution reactions, numerous photochemical processes of isomerization, rearrangement, and ring formation are known. There are organic compounds which, on exposure to ultraviolet light, isomerize and take on a color; on illumination with visible light, these compounds again become colorless. Known as photochromism, this phenomenon is an example of a reversible photochemical conversion.

Research on the mechanism of photochemical reactions is extremely difficult. The absorption of a quantum of light and the formation of an excited molecule occur within a period of approximately 10–15 sec. In the case of organic molecules having multiple bonds and benzene rings, which photochemically are the most interesting compounds, there are two types of excited states. The states differ in the value of the total spin angular momentum quantum number, which may be either zero (in the ground state) or one. They are referred to as, respectively, singlet and triplet states. A molecule passes directly into the singlet excited state upon the absorption of a quantum of light. The transition from the singlet to the triplet state occurs as a result of a light-induced physical process. The lifetime of a molecule in the excited singlet state is ~10–8 sec; in the triplet state it varies between 10–5–10–4 sec (liquid media) and 20 sec (rigid media, such as solid polymers). Therefore, it is in the triplet state that many organic molecules enter into chemical reactions. As a result, the concentration of molecules in the triplet state may become so great that the molecules begin to absorb light and pass into a highly excited state, in which they take part in reactions involving the absorption of two light quanta. The excited molecule A* often forms a complex with a nonexcited molecule A or with a molecule B. Such complexes, existing only in the excited state, are called eximers (AA)* and exiplexes (AB)*, respectively. Exiplexes are often the precursors of primary chemical reactions. The primary products of photochemical reactions—radicals, ions, ion-radicals, and electrons—rapidly enter into subsequent dark reactions within a period usually not exceeding 10 –3 sec.

One of the most effective methods of studying the mechanism of photochemical reactions is flash photolysis, which in essence involves creating a high concentration of excited molecules by illuminating a reaction mixture with short but powerful bursts of light. The resulting short-lived particles (more precisely, the excited states and the primary products of the photochemical reactions mentioned above) are detected by their absorption of the “probing” beam. This absorption, together with the change of the absorption with time, is recorded by means of a photomultiplier and oscillograph. In this way, it is possible to determine both the absorption spectrum of an intermediate particle (at the same time identifying the particle) and the kinetics of the particle’s formation and disappearance. Here, laser pulses of 10–8 sec and even 10–11–10–12 sec duration are used, permitting an investigation of the earliest stages of the photochemical process.

Photochemistry has practical applications in many areas. Methods of chemical synthesis based on photochemical reactions are currently under development. Photochromic compounds have found application, particularly for recording information. Photochemical processes have made it possible to obtain relief images in microelectronics and to prepare printing plates. Photochemical chlorination, mainly of saturated hydrocarbons, is of practical significance. The most important field of application of photochemistry is photography. Apart from the photographic process based on the photochemical decomposition of silver halides, primarily AgBr, various methods of silver-free photography are becoming increasingly important; the photochemical decomposition of diazo compounds, for example, forms the basis of the diazo process.

REFERENCES

Turro, N. J. Molekuliarnaia fotokhimiia. Moscow, 1967. (Translated from English.)
Terenin, A. N. Fotonika molekul krasitelei i rodstvennykh organicheskikh soedinenii. Leningrad, 1967.
Calvert, J. G., and J. N. Pitts. Fotokhimiia. Moscow, 1968. (Translated from English.)
Bagdasar’ian, Kh. S. Dvukhkvantovaia fotokhimiia. Moscow, 1976.

KH. S. BAGDASARIAN

photochemistry

[¦fōd·ō′kem·ə·strē]
(physical chemistry)
The study of the effects of light on chemical reactions.