PHOTOCHEMISTRY (Gr. 4&n,

See also:

• LIGHT

light, and "

See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

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 of y 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 (q.v.) . 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 ill 1857; and in 1887 Pringsheim suggested that the reaction proceeded in two stages: 1120+ Cl2 = 0120+ 112, 2H2+C120=H2O+2HC1 . 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 8o% 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: 2HgC12+ (NH4)2 C204 = 2HgCl + 2NH4C1 + 2CO2, the extent of the decomposition being determined by the amount: of mercurous chloride or carbon dioxide liberated . The article PHOTOGRAPHY (q.v.) 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, COC12, 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, C6H5CH2C1, in sunlight, and chlortoluene, C5H4(CH)3C1, in the dark; with benzene it gives an addition product, C6H6C16, 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 a hydrocarbon and an acid . Among aromatic compounds, benzaldehyde gives a trimeric and tetrameric benzaldehyde, benzoic acid and hydrobenzoin (G . L . Ciamician and P . Silber, Atli . R . Accad . Lincei, 1909); in alcoholic solution it gives hydrobenzoin; whilst with nitro-benzene it is oxidized to benzoic acid, the nitrobenzene suffering reduction to nitrosobenzene and phenyl-#-hydroxylamine; the latter isomerizes to ortho- and 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•C6H4•CH(OC2H6)2 (E . Bamberger and F .

Elgar, Ann . 191o, 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., 19oi, 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 i3-naphthylhydrazones (the a-compounds are not phototropic), and of A . Senier and F . G . Shepheard (Journ .

Chem . Soc., 1909, 95, p . 1943) on the arylidene- and 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 a form of benzyl-0-aminocrotonic ester into the $ form, whereas light reverses this; similarly heat and light have reverse actions with as-diphenyl ethylene, CH2: C(C6H6)2 (R . Stoermer, Ber., Igoo, 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 photo-chemical 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: (I) 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 igo8 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) .