74 LIGHT AND LIFE 



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DISCUSSION 



Dr. Platt: Would you draw us an energy level diagram of duroquinone 

 and benzophenone showing us the differences between the energy levels and 

 which one is abstracting hydrogen? 



Dr. Porter: I can't because I don't know the energy level of the triplet 

 state of duroquinone. I don't mean that I just haven't read the literature, 

 I just don't think it is known. It doesn't phosphoresce as far as I know. The 

 triplet Slate for benzophenone is well known at 24,000 cm ' and the singlet 

 absorption for both compounds is about the same, that is, the longest wave- 

 lengh peak is at about 3500 angstroms. I don't know the triplet level of 

 duroquinone. It hasn't been measured. We have looked for the phos- 

 phorescence but never observed it. 



Dr. Platt: Would you summarize these results in about two .sentences? 



Dr. Porter: Yes. In duroquinone the production of the semiquinone 

 occurs from the singlet state, while in benzophenone that of ketyl radical 

 occurs from the triplet state. 



Dr. Franck: Do you regard it probable, that a reaction proceeding at the 

 cost of the measurable triplet energy will not be promoted by the higher 

 energy of the first excited singlet? 



Dr. Porter: In the case of benzophenone it could well be that the singlet 

 was highly reactive, but the transition was very efficient to the triplet. But 

 in duroquinone the converse is not true. The triplet is there but does not 

 react with anything like the efficiency of that of ben/opiienone. 



Dr. Franck: If the energy stored in the triplet is sufficient, tiien the reac- 

 tion by the triplet is preferred because of its long lifetime but I am sure 

 that the yield of the reaction per impact should be at least as higli for the 

 first excited state. 



Dr. Porter: This is very reasonable but energy is not the only considera- 

 tion—electron density distril)ution in tlie two states is also important. I am 

 rather loath to put forth explanations in terms of energy levels and lifetimes 

 on so few examples. I did want to point out, however, that this sort of 



