Photochemistry

Photochemistry (Gr. φῶς, light, and "chemistry"), in the widest sense, the branch of chemical science which deals with the optical properties of substances and their relations to chemical constitution and reactions; in the narrower sense it is concerned with the action of light on chemical change. The first definition includes such subjects as refractive and dispersive power, colour, fluorescence, phosphorescence, optical isomerism, spectroscopy, &c. - subjects which are treated under other headings; here we only discuss the subject matter of the narrower definition.

Probably the earliest photochemical investigations were associated with the darkening of certain silver salts under the action of light, processes which were subsequently utilized in photography. At the same time, however, it had been observed that other chemical changes were regulated by the access of light; and the first complete study of such a problem was made by J. W. Draper in 1843, who investigated the combination of hydrogen and chlorine to form hydrochloric acid, a reaction which had been previously studied by Gay-Lussac and Thenard. Draper concluded that the first action of sunlight consisted in producing an allotrope of chlorine, which subsequently combined with the hydrogen. This was denied by Bunsen and Roscoe in 1857; and in 1887 Pringsheim suggested that the reaction proceeded in two stages: H₂O + Cl₂ = Cl₂O+H₂, 2H₂ + Cl₂O = H₂O + 2HCl. This view demands the presence of water vapour (H. B. Baker showed that the perfectly dry gases would not combine), and also explains the period which elapses before the reaction commenced (the "photochemical induction" of Bunsen and Roscoe) as taken up by the formation of the chlorine monoxide necessary to the second part of the reaction. The decomposition of hydriodic acid into hydrogen and iodine was studied by Lemoine in 1877, who found that 80% decomposed after a month's exposure; he also observed that the reaction proceeded quicker in blue vessels than in red. A broader investigation was published by P. L. Chastaing in 1878, who found that the red rays generally oxidized inorganic compounds, whilst the violet reduces them, and that with organic compounds the action was entirely oxidizing. These and other reactions suggested the making of actinometers, or instruments for measuring the actinic effect of light waves. The most important employ silver salts; Eder developed a form based on the reaction between mercuric chloride and ammonium oxalate: 2HgCl₂ + (NH₄)₂ C₂O₄ = 2HgCl + 2NH₄Cl + 2CO₂, the extent of the decomposition being determined by the amounts of mercurous chloride or carbon dioxide liberated.

The article Photography deals with early investigations on the chemical action of light, and we may proceed here to modern work on organic compounds. That sunlight accelerates the action of the halogens, chlorine and bromine, on such compounds, is well known. John Davy obtained phosgene, COCl₂, by the direct combination of chlorine and carbon monoxide in sunlight (see Weigert, Ann. d. Phys., 1907 (iv.), 24, p. 55); chlorine combines with half its volume of methane explosively in sunlight, whilst in diffused light it substitutes; with toluene it gives benzyl chloride, C₆H₅CH₂Cl, in sunlight, and chlortoluene, C₆H₄(CH)₃Cl, in the dark; with benzene it gives an addition product, C₆H₆Cl₆, in sunlight, and substitutes in the dark. Bromine deports itself similarly, substituting and forming addition products with unsaturated compounds more readily in sunlight. Sometimes isomerization may occur; for instance, Wislicenus found that angelic acid gave dibromangelic acid in the dark, and dibromtiglic acid in sunlight. Many substances decompose when exposed to sunlight; for example, alkyl iodides darken, owing to the liberation of iodine; aliphatic acids (especially dibasic) in the presence of uranic oxide lose carbon dioxide; polyhydric alcohols give products identical with those produced by fermentation; whilst aliphatic ketones give hydrocarbon and an acid.

Among aromatic compounds, benzaldehyde gives a trimeric and tetrameric benzaldehyde, benzoic acid, and hydrobenzoin (G. L. Ciamician and P. Silber, Atti. R. Accad. Lincei, 1909); in alcoholic solution it gives hydrobenzoin; whilst with nitrobenzene it is oxidized to benzoic acid, the nitrobenzene suffering reduction to nitrosobenzene and phenyl-β-hydroxylamine; the latter isomerizes to orthoand para-aminophenol, which, in turn, combine with the previously formed benzoic acid. Similarly acetophenone and benzophenone in alcoholic solution give dimethylhydrobenzoin and benzopinacone. With nitro compounds Sach and Hilbert concluded that those containing a ·CH· side group in the ortho position to the ·NO₂ group were decomposed by light. For example, ortho-nitrobenzaldehyde in alcoholic solution gives nitrosobenzoic ester and 22' azoxybenzoic acid, with the intermediate formation of nitrobenzaldehydediethylacetal, NO₂·C₅H₄·CH(OC₂H₅)₂ (E. Bamberger and F. Elgar, Ann. 1910, 371, p. 319). Bamberger also investigated nitrosobenzene, obtaining azoxybenzene as chief product, together with various azo compounds, nitrobenzene, aniline, hydroquinone and a resin.

For the photochemistry of diazo derivatives see Ruff and Stein, Ber., 1901, 34, p. 1668, and of the terpenes see G. L. Ciamician and P. Silber, Ber., 1907 and 1908.

Light is also powerful in producing isomerization and polymerization. Isomerization chiefly appears in the formation of stable stereo-isomers from the labile forms, and more rarely in inducing real isomerization or phototropy (Marckwald, 1899). As examples we may notice the observation of Chattaway (Journ. Chem. Soc. 1906, 89, p. 462) that many phenylhydrazones (yellow) change into azo compounds (red), of M. Padoa and F. Graziani (Atti. R. Accad. Lincei, 1909) on the β-naphthylhydrazones (the α-compounds are not phototropic), and of A. Senier and F. G. Shepheard (Journ. Chem. Soc., 1909, 95, p. 1 943) on the arylideneand naphthylidene-amines, which change from yellow to orange on exposure to sunlight. Light need not act in the same direction as heat (changes due to heat may be termed thermotropic). For example, heat changes the α form of benzyl-β-aminocrotonic ester into the β form, whereas light reverses this; similarly heat and light have reverse actions with as-diphenyl ethylene, CH₂:C(C₆H₅)₂ (R. Stoermer, Ber., 1909, 42, p. 4865); the change, however, is in the same direction with Senier and Shepheard's compounds. With regard to polymerization we may notice the production of benzene derivatives from acetylene and its homologues, and of tetramethylenes from the olefines.

Theory of Photochemical Action. - Although much work has been done in the qualitative and quantitative study of photochemical reactions relatively little attention has been given to the theoretical explanation of these phenomena. That the solution was to be found in an analogy to electrolysis was suggested by Grotthuss in 1818, who laid down: (1) only those rays which are absorbed can produce chemical change, (2) the action of the light is analogous to that of a voltaic cell; and he regarded light as made up of positive and negative electricity. The first principle received early acceptance; but the development of the second is due to W. D. Bancroft who, in a series of papers in the Journal of Physical Chemistry for 1908 and 1909, has applied it generally to the reactions under consideration. Any electrolytic action demands a certain minimum electromotive force; this, however, can be diminished by suitable depolarizers, which generally act by combining with a product of the decomposition. Similarly, in some photochemical reactions the low electromotive force of the light is sufficient to induce decomposition, but in other cases a depolarizer must be present. For example, ferric chloride in aqueous solution is unchanged by light, but in alcoholic solution reduction to ferrous chloride occurs, the liberated chlorine combining with the alcohol. In the same way Bancroft showed that the solvent media employed in photographic plates act as depolarizers. The same theory explains the action of sensitizers, which may act optically or chemically. In the first case they are substances having selective absorption, and hence alter the sensitivity of the system to certain rays. In the second case there are no strong absorption bands, and the substances act by combining with the decomposition products. Bancroft applied his theory to the explanation of photochemical oxidation, and also to the chlorination and bromination of hydrocarbons. In the latter case it is supposed that the halogen produces ions; if the positive ions are in excess side chains are substituted, if the negative the nucleus.

Standard treatises are: J. M. Eder, Handbuch der Photographie, vol. i. pt. 2 (1906); H. W. Vogel, Photochemie (1906). An account of the action of light on organic compounds is given in A. W. Stewart, Recent Advances in Organic Chemistry (1908).