506 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



On the other hand, Timiriazev (1869) asserted that chlorophyll 

 solutions in ethanol are bleached by light, even in absence of oxygen, 

 and that they smell of aldehyde after exposure, thus showing that the 

 bleaching must have been caused by a reduction of chlorophyll and 

 oxidation of alcohol to aldehyde. The observations of Osterhout (1918) 

 on the formation of aldehydes in illuminated chlorophyll solutions (c/. 

 page 496) also may be explained by a photoxidation of alcohols (present 

 as solvents or impurities) , rather than by a photochemical decomposition 

 of chlorophyll itself. The observations of Knorr and Albers (1935) and 

 Albers and Knorr (1935) on the photodecomposition of chlorophyll 

 solutions in acetone in the absence of oxygen, which were attributed on 

 page 501 to a photoxidation of the pigment at the cost of the solvent, 

 may equally well be explained by a photoreduction of the pigment, and 

 oxidation of acetone. 



As mentioned above, reductive bleaching is a characteristic property 

 of many dyestuffs. Some of them are reduced reversibly (e. g., the 

 thiazines, the oxazines, and the triphenylmethane derivatives), others 

 irreversibly (e. g., many azo dyes). The reductive bleaching of "vat 

 dyes" was investigated, for instance, by Kogel and Steigmann (1925), 

 Kogel (1926), Mudrov^ic (1929), and Weber (1931), who used diethyl 

 thiourea, piperonal, and other organic compounds as reductants, as well 

 as ferrous salts. Later (1936), Weber reported that the bleaching of 

 chlorophyll exhibits a similarity to that of the reducible dyes in that it, 

 too, is accelerated by the presence of small quantities of diethyl thiourea, 

 diallyl thiourea, and other substances which are efficient reductants of 

 vat dyes. However, Weber found that larger quantities of the same 

 reductants act as inhibitors, probably by retarding the oxidative bleaching 

 of chlorophyll (as discussed on pp. 501-502). We may thus assume that 

 chlorophyll can be bleached either by oxidation or by reduction; small 

 quantities of diethyl thiourea and similar reductants cause an increase in 

 bleaching by accelerating the reductive bleaching; while larger quantities 

 of the same reductants have the opposite effect, by preventing the 

 oxidative bleaching. This is merely a tentative explanation, and new 

 experiments on the interaction of illuminated, oxygen-free chlorophyll 

 solutions with organic reducing agents are desirable. 



It must be mentioned that, according to Weber (1931), certain 

 reducing agents (hydroquinone, pyrogallol, phenol, cyanide, iodide) 

 inhibit the reductive bleaching of many dyestuffs as well. The mechanism 

 of this paradoxical effect is as yet not clear. 



The photoreduction of many dyestuffs is accelerated by neutral salts (sodium 

 chloride, potassium chloride, etc.; see Weber 1931), whereas the photoxidation often 

 appears retarded by them (c/. Noack 1925, 1926). This result can be compared with 

 the observation of Rabinowitch and Weiss (1937) that the reversible reaction of chloro- 

 phyll with ferric chloride (both in the dark and in light) is inhibited by neutral salts. 



