DISCUSSION 75 



thing must be done il wc arc cmt to uiulcrstand thf pliotoi luinistry ol this 

 kind of system at all. A distiiulion must be made between the singlet and 

 triplet. One must accumulate data and try to correlate this with whether 

 the n ^ -77* or the tt -* -tt* are the lowest state and this sort of thing. 



Dr. Rabinowitch: Since you brought up the reaction between a thiazine 

 dye and ferrous ions, I thought it would be proper to mention some results 

 recently obtained by Ainsworth in our laboratory. He concluded that, with 

 thionine as oxidant, the reaction with ferrous ions goes partly through 

 thionine molecules in the triplet state encountering ferrous ions, and partly 

 through excitation and electron transfer in a complex of a ferrous ion and 

 a dye molecule. Even in the latter case, however, there seems to be a delay 

 between excitation and reaction. It looks as if in the complex, too, thionine, 

 excited to the singlet state, can go down to the triplet state and stay there 

 for at least 10"^ seconds, before accepting an electron from the ferrous ion. 

 It seems a little pecidiar that one can preser\e an excited dye molecule next 

 to an electron donor in a complex for this length of time. 



Dr. Porter: Are you saying that even in the complex the singlet does 

 not react? 



Dr. Rabinowitch: No, I didn't mean that. There must be a competition 

 between singlet and triplet reaction but quenching observations suggest that 

 the average time between excitation and the electron transfer is considerable, 

 and that means that even within a complex, much of the reaction goes 

 through the intermediary of a long-lived triplet state. 



Dr. Porter: Yes. 



Dr. Kasha: I was very impressed to see the progress in the photochemistry 

 of the triplet states. I am bothered by a few items here which I would like 

 to bring up. One is the very high energy at which the triplet-triplet absorp- 

 tion comes. For example, in anthracene I calculate from your slide there are 

 38,000 wave numbers above the ground state in the first excited triplet that 

 you find. You start at 14,000, and the first triplet -> triplet is at 24,000 

 wave numbers, so you go 38,000 wave numbers up. That means you miss 

 two triplets, which obviously require infra-red flash spectroscopy. 



Dr. Porter: I missed two triplets, but the transitions to them are not 

 allowed. As a matter of fact, the triplets which we do observe correlate very 

 beautifully with Pariseo's calculations. If one only takes his fully allowed 

 transitions and plots those for tlie linear polyacenes they agree very nicely 

 with yours for the ground state, with ours for the two principal excited states 



Dr. Kasha: But the forbidden ones would simply be weaker in intensity 

 and should still be observable. 



Dr. Platt: They should be down by a factor of 10 to 1,000 just the same 

 as the weak bands in the aromatics. 



Dr. Porter: I thought it was even more than that, the calculated / values 

 were I think a good thousand down. 



Dr. Platt: Yes, but they don't make a count of vibrations. Vibrations are 

 equally allowed in the aromatics. There is only a factor of ten or so. 



