54 G. OSTER 



Oster : Much less than 1 %. 



Wassink : What happens if you leave out the reducing agent? 



Oster : Nothing. 



Wassink : Do you observe any long-Hved species of the dye? 



Oster : My only indications are the fading of the dye or the initiation of poly- 

 merization. Purely chemical criteria. Gaffrou many years ago sensitized the oxi- 

 dation of allylthiourea in methanol by ethyl chlorophyllide; he was following the 

 oxidation of the reducing agent, while I am following the photoreduction of the 

 dye, by the disappearance of its color. 



Gaffron: At that time we could say only that there must be a long-Hved state; 

 afterwards it was interpreted as the triplet state of the dye. You need only traces 

 of oxygen for sensitized autoxidation to go with a high yield. This can be under- 

 stood only if either the sensitizing dye, or all the substrate undergoing oxidation 

 lives long enough in its excited state. 



Wassink: In the sensitized reaction, does the sensitizer, chlorophyll itself, 

 change appreciably? 



Oster: I think, at least for ethyl chlorophyllide (I cannot speak for chloro- 

 phyll), that it is reduced, although I did not see a loss of color. As with the other 

 dyes, the reduced form reduces other substances present. Ethyl chlorophyllide 

 is the only dye which I have studied which does not exhibit a readily discernible 

 loss of color. 



Rosenberg: How long is the reduced form of your dyes stable in aqueous 

 systems if you keep out oxidizing agents? 



Oster: Leuco methylene blue in solution where oxygen has been rigorouslj' 

 excluded (and no ultraviolet light is present) seems to remain stable forever. 



Becker : I would like to point out one important thing. Oxygen in its ground 

 state is paramagnetic, and if oxygen is in intimate contact, so to speak, with the 

 dye molecule, this paramagnetism will allow the triplet state to be occupied to a 

 much higher degree than is allowed ordinarily. 



A second point is quenching. Collisional quenching you cannot avoid. You 

 cannot effectively remove electronic energy in small portions unless you have a 

 tremendous number of collisions; so, if you want to remove large amounts of 

 electronic energy, you must have a receiving molecule with an electronic fre- 

 quency somewhere near the one that is in the donor. 



Oster: As a matter of fact, we feel, with no evidence other than what I have 

 indicated here, the reason why certain dyes will inhil)it the reaction and others 

 won't, may be that there is a properly situated long-lived (say triplet) state, 

 which we should be able to pick up spectroscopically. That is something we 

 have to look for. 



Lumry : Did you try any single-electron reducing agent? 



Oster : Chromous chloride is in this category. 



Lumry: The organic substances which you have listed as oxidants all require 

 two electrons for their reduction. Some must pass through rather diffi cult-to-form, 

 one-electron intermediates. Do you believe that two electrons are migrating simul- 

 taneously? 



Oster : Yes. 



Limvry • Glutathione in the absence of oxygen or high hydroxyl concentration 



