91 



onds. 



LINSCHITZ: From the absorption spectrum ? 



KASHA: Yes. 



LINSCHITZ: I see. These figures are very relevant. What kind of mole- 

 cules were they? 



KASHA: Among them were aromatic ketones which are electronically pecul- 

 iar in a certain sense and that really is why they do this. 



I think there is one missing link in what Dr. Linschitz presented, and this 

 has to do with what is the longest time an internal conversion takes. If it is 

 10-13 for a singlet-singlet radiationless transition, and there is a prohibition 

 factor as great as 1 million for an inter-combination at the same time that an 

 internal conversion takes place (in other words, an internal conversion between 

 a singlet and triplet state) then you multiply the latter by 10" 13 and you still get 

 10~7. If the singlet-singlet fluorescence lifetime is comparable, then, so that 

 10"? can compete with it very well, then between those processes there is about 

 equal competition (19). There is really bi-partition of the electronic energy, but 

 in some molecules that takes place overwhelmingly in favor of intercombination, 

 so that triplet-singlet emission is the main one observed and that includes aro- 

 matic ketones, aromatic nitro compounds and aromatic N-heterocyclics, which 

 are not only fluorescent but they are also phosphorescent if observed in any 

 rigid system. 



LINSCHITZ: In any case there is another process by which one can get to the 

 triplet state, with certainly indisputably high yield, and that is recombination of 

 the radical ions, R + , and electrons that have been set free. You would excite 

 the triplet state with a yield of 75 per cent for this process. 



FANO: When you have ionizing radiations all the slow secondaries can give 

 you a singlet- triplet transition easily. 



BURTON: I want to ask just one question that I want to be clear about. When 

 you say a singlet- triplet transition is an internal conversion process do you 

 mean to rule out the possibility of these processes occurring between two mole- 

 cules? As you know, if two molecules are involved, the multiplicity selection 

 rule may be much less restrictive. 



KASHA: It could work except there is one thing that rules against it. Don't 

 you think that those experiments of McClure (20), and of Yuster and Weissman 

 (21), in which they substituted heavy atoms right in the same molecule, showed 

 that the transition possibilities were influenced within the single molecule? 



BURTON: I was not arguing. I just wanted to ask the question. 



KASHA: I think that might rule it out. 



LINSCHITZ: I don't know if everybody is acquainted with those experiments 

 Dr. Kasha mentioned. What was done was to substitute progressively into a 

 given molecule various heavy atom constituents which would enhance the spin- 

 orbit coupling. It was found by Weissman and Yuster, and by Dr. Kasha that the 

 yield of phosphorescence increased along with the atomic number of the substitu- 

 ent, and the lifetime of phosphorescence decreased, indicating that what you were 

 dealing with was transition involving a change in multiplicity within a single mole- 



