PHOTOCHEMISTRY

From Big Medical Encyclopedia

FOTOHYMIYa — the section of chemistry devoted to studying of chemical transformations to-rye proceed under the influence of light radiation.

Subject of studying F. the photochemical reactions (see) including having the major obshchebiol serve. and medical value, napr, photosynthesis (see), biosynthesis of vitamins (see), transformation of light energy during the functioning of rhodopsins (see) in the course of sight (see), etc.

The big actual material about photochemical transformations of organic and inorganic matters which is saved up in 18 — the beginning of 19 centuries gave the chance to formulate the first ideas of the nature of photochemical processes. In 1818 Grot-gus (Ch. J. D. Grothus) rejected the hypothesis of the thermal mechanism of chemical effect of light occurring until then (see) and formulated situation, according to Krom only such light can be the cause of chemical action, to-ry it is absorbed by a body (Grotgus's law). In 1839 Dagerr (L. J. The m of Daguerre) invented the photo based on photochemical decomposition of silver halides.

According to the quantum theory (see) a radiant energy extends in the form of the discrete portions called by quanta, or photons.

In 20 century the law of quantum equivalence was formulated (Einstein's law — Stark), software Krom each absorbed quantum (photon) in primary act is capable to activate only one molecule. For F. this law is fair only in the sense that at absorption of one photon one excited molecule is formed (see the Molecule, excited states of molecules), however only their some part can be exposed to chemical transformations, other molecules are exposed to deactivation or back reaction. In case of chain reactions (see) one photon can cause transformation of a large number of molecules. In 1862 the correlation between quantity of a product of photochemical reaction, intensity of light and a radiation time (quantity of a product of photochemical reaction is proportional to the work of intensity of incident light for the period of its influence) which received the name of the law of Bunsen — Rosko was established { a hedgehog. Bunsen — Rosko the law) * in the middle of 20 century thanks to achievements of spectroscopy, quantum chemistry and kinetics became possible to study photochemical, including, fotobiol. reactions, using the theory of the elementary chemical act.

From all range of electromagnetic radiation for F. only the narrow area including the near Uv-radiation and visible radiation (wavelength of 200 — 760 nanometers) matters. This results from the fact that in the field energy of photons is commensurable with energy of chemical bonds and, therefore, light absorption can have photochemical effect.

At absorption of a photon the electron from an external orbit passes to higher orbit, and atom (molecule) from a ground state — in excited. Absorption of a photon happens only in that case when its energy is precisely equal to a difference of energy excited and the basic energetiches them at more exactly molecule absorbing light. Except power conditions, the probability of absorption of a photon depends as well on a resolution of transition to excited state. The molecule excited by the absorbed photon is exposed then deactivations, edges can be realized fotofizichesk by radiation — fluorescence (see), a phosphorescence (see), by nonradiative conversion (return of a molecule to a reference state and calorification) or photochemically — with formation of new chemical substances.

Researches on photochemistry are also abroad conducted to the USSR in high fur boots at departments of physical chemistry, biophysics, biochemistry and phytophysiology, etc., and also in chemical in-ta and laboratories. Bibliography: Baraby V. A. Biolo

gichesky action of ultra-violet lighting, At the joint venture. sovr. biol., t. 94, century 2 (5), page 269, 1982; B and r l of t r about p D. and

Coyle D. Excited states in organic chemistry, the lane with English, M., 1978; Vlad and m and r about in Yu. A. Fotokhimiya and a luminescence of proteins, M., 1965, bibliogr.; Konev S. V. and Bolotov to and y I. D. Introduction to molecular photobiology, Minsk, 1971, bibliogr.; About to and e X. Photochemistry of small molecules, the lane with English, M., 1981, bibliogr.; Smith K. and X e N e at about l of t F. Molecular photobiology, the lane with English, M., 1972; T e p e N and A. N. Fotonik's N of molecules of dyes and related organic compounds, L., 1967; T at r -

r about N. J. Molecular photochemistry, the lane with English, M., 1967, bibliogr.

V. I. rozengart.

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