Photoreduction of Synthetic Dyes 



GERALD OSTER, Polytechnic Institute of Brooklyn, Brooklyn, New Y'ork 



The work in our laboratory has been concerned mainly with 

 studies of the kinetics of the photoreduction of synthetic dyes in 

 aqueous media using visible light as the source of energy. The ener- 

 gies associated with visible light vary between 2 and 3 electron volts. 

 It would be useful if this energy could be utilized to bring about the 

 reduction of noncolored molecules as well, the dye serving as the re- 

 ceptor of light. In some cases, we have been able to utilize a large 

 fraction of the absorbed energy for this purpose. I wish to summarize 

 for you the results obtained so far in the hope that they may be of 

 help in understanding the photochemical steps in photosynthesis. 



PHOTOREDUGTIONS 



It has long been appreciated that some reactions which involve 

 colored substances and which are thermodynamically impossible 

 will proceed, however, in the presence of visible light. For example, 

 Rabino witch (1) has shown that thionine in the presence of acidified 

 ferrous sulfate is photoreduced to give the leuco (colorless) dye, al- 

 though in the absence of light this reaction is thermodynamically im- 

 possible. In this reaction, the ferric ion which is produced oxidizes 

 the leuco dye back to the colored form and a steady state is reached. 

 We have found that a wide variety of other electron donors such as 

 ascorbic acid, ghitathione, cysteine, thiourea, allyl thiourea, and 

 stannous chloride under pH conditions where thionine (or methylene 

 blue) is not reduced in the dark will give a stable leuco dye on irradia- 

 tion with visible light if oxygen is rigorously excluded (2). The leuco 

 dye exhibits a strong yellow fluorescence (phosphorescence in highly 

 viscous media) and has the property of being oxidized to the normal 

 dye even in the absence of oxygen if irradiated with near ultraviolet 

 light. The fading with visible hght, recovery with ultraviolet hght, 

 fading with visible light, and so on can be repeated several times and 

 is reminiscent of certain biological processes (e.g., photoperiodism 



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