PHOTOREDUCTTON OF SYNTTHETIC DYES 51 



and photoreactivation) where reversals take place on using light of 

 two different wavelengths. 



A particularly interesting electron donor is the chelating agent, 

 ethylene diamine tetracetic acid (EDTA). Nickerson and Merkel 

 (3) have shown that certain dyes in the presence of this substance 

 are photoreduced by visible light. We have examined the kinetics of 

 reduction of methylene blue in the presence of EDTA and found that 

 the quantum yield varies with pH in the same way as does its degree 

 of ionization and its chelating power (4). The leuco dye, in this case, 

 is not fluorescent and does not revert to the normal form on irradia- 

 tion with ultraviolet Ught. As with all leuco dyes, however, the nor- 

 mal dye is obtained by flushing the solution with oxygen. EDTA is by 

 no means a reducing agent in the ordinary sense. For example, we 

 found that we could effect the photo-reduction of methylene blue in 

 the presence of EDTA even when a ten thousand-fold excess of hy- 

 drogen peroxide is present in the solution. We are now engaged in a 

 more extensive study of the nature of transfer of electrons in photo- 

 chemical reactions invohdng EDTA. 



The leuco dye is itself a reducing agent and can reduce other sub- 

 stances. For example, leuco methylene blue will reduce tetrazolium 

 salts to their corresponding insoluble formazans (compare refs. 3a 

 and 5). The reduction potential of leuco methylene blue is about zero 

 and it is therefore not a powerful reducing agent. Leuco acriflavine 

 (produced, for example, by photoreduction of acriflavine in the 

 presence of EDTA) is, on the other hand, a very powerful reducing 

 agent (reduction potential about - 1 volt) and, if properly utilized, it 

 should be more effective than acidified zinc as a reducing agent. 

 For example, nitrobenzene is reduced by the leuco dye, the latter then 

 reverting to its normal form. Thus, the dye serves as a sensitizer for 

 the reduction of nitrobenzene and is not consumed in the overall 

 process. If the leuco dye has a suflSciently low reduction potential 

 (about -0.1 volt or less) it will produce free radicals (probably hy- 

 droxyl radicals) on reacting with oxygen. Thus, if some vinyl mono- 

 mer is present the chain polymerization of the monomer will ensue, 

 resulting in a high polymeric product (6). Under conditions where 

 chain termination is suppressed the overall quantum yield for mono- 

 mer conversion may run as high as one billion (7). 



