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the ability of a substance to undergo a reversible transformation, upon exposure to optical radiation, from one state to any other state in which an optical absorption spectrum appears or changes markedly in the substance. A reversible transformation is a transformation in which a substance later reverts to its initial state. Many substances undergo such reversible transformations when exposed to, for example, X rays or microwaves. Nevertheless, such substances are photochromic in the strict sense only if they undergo reversible transformations upon exposure to ultraviolet, visible, or infrared radiation.
A general description of the photochromic process follows. In the initial state A, a substance, having absorbed optical radiation of a specific spectral composition, undergoes a transformation to a light-induced state B, which is characterized by a different absorption spectrum and a certain lifetime that is specific for a given state. The reverse transformation B → A occurs spontaneously as a result of heat energy and may be accelerated to an extremely high degree when the substance is heated or absorbs light in state B.
Photochromism is characteristic of a very large number of organic and inorganic substances. The photochromism of organic substances is based on several photophysical processes or numerous photochemical reactions. If photochromism is based on photochemical reactions, the reactions are accompanied either by a rearrangement of valence bonds or by a change in the configuration of the atoms in the molecules, which is known as cis-trans isomerism (seeISOMERISM). Valence bonds are rearranged during, for example, dissociation, dimerization, the rearrangement of atoms in a molecule, oxidation-reduction reactions, or tautomeric transformations (seeTAUTOMERISM). The photochromism of inorganic substances is caused by various processes, such as reversible light-induced electron transfer (which results in the appearance of various types of color centers and in a change in the valence of metal ions) or reversible photodissociation reactions.
Photochromic materials have been developed on the basis of organic and inorganic photochromic substances. The use of photochromic materials in science and technology depends on the photosensitivity of the materials, the reversibility of the photophysical and photochemical processes that occur in the materials, the appearance or change of color or absorption spectra immediately upon exposure to light, and differences in the thermal, chemical, and physical properties of the initial and light-induced states of the photochromic substances.
REFERENCESTerenin, A. N. Fotonika molekul krasitelei i rodstvennykh organicheskikh soedinenii. Leningrad, 1967.
Barachevskii, V. A. “Fotokhromizm.” Zhurnal Vsesoiuznogo Khimicheskogo ob-va im. D. I. Mendeleeva, 1974, vol. 19, no. 4, pp. 423–33.
Barachevskii, V. A., G. I. Lashkov, and V. A. Tsekhomskii. Fotokhromizm i ego primenenie. Moscow, 1977.
Photochromism. New York .
V. A. BARACHEVSKII