I'HOTOREDUCTIOX OF SYNTHETIC DYKS 55 



has to go through a radical unless the process involves an improbable activated 

 complex containing two SH-containing molecules. 



Hendricks: This discussion is only on the photochemistry of a particular 

 system, but you might be interested to know that the system is a model for the 

 photoreaction of photoperiodism, which is a photoreduction. 



Frenkel: What is the maximum energy j-ield in some of these reactions? I 

 mean, actually is there any conversion of light energy into chemical energy? 



Oster: About 10% in the case of methylene blue. My interest is not so much 

 the energy yield as it is the very strong reduction potential of the leuco dye pro- 

 duced. For example, polarographic studies indicate that leuco acriflavine at high 

 pH values has a reduction potential of —1.33 volt. 



Weigl : \Miat is the nature of this photoreduced species with its fabulous reduc- 

 ing potential? If it is an excited state of the photoreduced dye, can you get to this 

 state by exciting the photoreduced dye? 



Oster : The photoreduced species is the normal leuco dye. 



It is a colorless substance. Some of these dyes you cannot reduce in water by 

 any known agent. 



Rabinowitch : The difficulty of reducing a substance is not in itself an indica- 

 tion of a lower potential. It may be just an inactive system. It may be a problem 

 of kinetics and not a problem of low potential. 



Oster : In the case of acriflavine the indications are that it has a very low re- 

 duction potential. 



Rabinowitch : Your leuco compound does not reduce everything in the world 

 to carbon. 



Kamen : You emphasize this is only the dye-absorbed polymer, not the free dye. 



Oster: Not necessarily. For example, acriflavine will not oxidize allylthioiirea 

 unless it is bound to the substrate. 



Kamen : But you get the same result in the end. 



Oster : You get a reduced dye, yes. 



Gaflfron : Apparently it is still more complicated because you cannot expect 

 that the two hydrogens go over at once. So you must have some radical intermediate. 



Linschitz : Under what conditions do your dyes photosensitize polymerization? 



Oster : The photoreduced form of the dye in the presence of vinyl monomer is 

 stable as long as oxygen is rigorously excluded. On the introduction of oxygen, 

 however, the monomer is rapidly polymerized. 



Linschitz : Hence the photoreduced dye is not a free radical. 



Oster: Yes. Free radicals which initiate polymerization are formed during the 

 reaction between the leuco dye and oxygen. The radicals may be OH radicals or 

 the semiquinone form of the dye. 



Holt : With ethyl chlorophyllide I tried a system using versene and there is a re- 

 versible color change. 



Oster: That is very interesting. I want to point out that versene is not what 

 we call a standard reducing agent. As I said before, you can bubble oxygen 

 through it and you don't decompose the material. But it is a reducing agent to 

 light-excited molecules. 



Lumry: I want to get this point about Krasnovskii's work clarified if I can. 



