90 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



and when formaldehyde was found (as in the case of malachite green), it could be 

 detected also in absence of carbon dioxide, and thus must have originated in the decom- 

 position of the dyestuff. 



In later papers, Baur was not satisfied with the provision of a sensitizer, but at- 

 tempted also the substitution of a stronger redudant (in place of water) or of a less 

 reluctant oxidant (in place of carbonic acid). He was guided in these experiments by 

 a concept of photochemistry as "molecular electrochemistry." He considered a 

 molecule excited by light absorption as "polarized," with a positive and a negative pole, 

 and treated all photochemical reactions as "depolarizations" brought about by the 

 transfer of charges from the " hght-polarized " molecule to appropriate acceptors. 

 Although this picture has little in common with well-founded concepts of molecular 

 excitation, it can be used without much harm as a description of certain facts of sen- 

 sitization. An excited molecule has no "plus" and "minus" pole, but it can have 

 both an increased affinity for an electron (i. c, the properties of an oxidant), and the 

 tendency to lose an electron {i. e., the properties of a reductant). When the excited 

 molecule meets a reductant, it may oxidize it, by taking away an electron (Baur's 

 "cathodic depolarization"). If it meets an oxidant, it can reduce it by donating an 

 electron (Baur's "anodic depolarization"). 



Baur's conception proved useful in practice in that it induced him to pay attention 

 to the nature of the "depolarizers," that is, to provide complete oxidation-reduction 

 systems, and not to be satisfied with the reduction of carbon dioxide without asking 

 whether the part of the reductant was played by water, by the sensitizer itself, or by 

 some accidental component of the system. 



In a series of experiments, Baur has attempted to achieve the photochemical 

 reduction of carbon dioxide by providing, in addition to sensitizers, reductants ("anodic 

 depolarizers") which can be expected to donate their electrons more wilhngly than the 

 water molecules. He tried (1928) urea, cyanamide, cyanide, benzidine and sodium 

 sulfite (in benzene), with eosin, resinate dyes or chlorophyll as sensitizers. He also 

 attempted the fixation of the sensitizer by adsorption on carbonates (magnesia alba) 

 and the combination in one molecule of the oxidant (carbonate ion) and sensitizer 

 (uranyl ions, ferrous ions). He also used iron-substituted permutites (for a still stronger 

 fixation of the sensitizers) and colored lacquers, in which tannin was supposed to create 

 a molecular link between carbonate and sensitizer. All these experiments gave negative 

 results; no formaldehyde was produced, and no oxygen was hberated. Similarly 

 negative results were obtained also by Reggiani (1932), who used eosin, quinine sulfate, 

 methylene blue, rhodamine, thionine, and methyl orange as sensitizers, in both artificial 

 light and sunlight, and sodium sulfide, hydrogen, zinc, Dewarda alloy, pyrogallol and 

 hydroquinone as reductants. 



In another series of experiments, Baur substituted carboxyl groups for carbonate 

 ions as substrates of reduction (c/. page 80). At first (1928), he used /3-resorcyhc acid 

 and other polyphenolcarboxylic acids. Then he tried carboxyl-containing dyestuffs, 

 gallocyanin and pseudopurpurin, \vith different reductants ("anodic depolarizers"), in 

 the hope that uniting sensitizer and oxidant (carboxyl) in one molecule might yield 

 some success. However, no oxygen or formaldehyde were obtained in these experiments 

 as well. 



In subsequent experiments, (1935), Baur arrived at the conclusion 

 that chlorophyll is capable of producing formaldehyde by the reduction 

 of its two carboxyl groups (c/. Formula 16.III), and, what is more, that 

 this oxidation can be carried out at the cost of water, by the intermediary 

 of an "auxiliary" reversible oxidation-reduction system, e. g., methylene 



