68 LIGHT AND LIFE 



Dr. Abrahamson: If I understand you correctly, you say that in chlorophyll- 

 chlorophyll b to be precise— one can obtain phosphorescence from the ?z — ^ 77* 

 as well as the it ^> -* triplet. 



Dr. Kasha: Both, depending on your solvent. Fernandez and Becker have 

 found it from the n, 'tt* triplet in very dry solvents and Becker and Kasha 

 have found phosphorescence from the lower ir, -tt* triplet. So you can get 

 either one, curiously enough! 



Dr. Abrahamson: It is generally true that tt -> tt* triplets lie at lower 

 energies than n —^ tt* triplets? 



Dr. Kasha: Not always. This is a very important thing. The n, 'tt* 

 singlet-triplet split is small in some types oi. n -^ tt* promotion, but not 

 in heterocyclics. In heterocyclics this order can be inverted. You can have 

 a small n — » vr* singlet-triplet split or a large -tt — ^ "tt* singlet-triplet split. 

 So we have every arrangement of singlet and triplet states of the two orbital 

 types that you like. If you name it, we probably can give you an example, 

 because the singlet-triplet splitting is variable and can be very large in 

 n — > 77*, as much as 7,000 wave numbers. This is something which had not 

 been considered possible before so that the order can interchange. 



Dr. Franck: Flash experiments by Livingston, Linschitz, and others on 

 chlorophyll dissolved in different organic liquids at room temperature showed 

 that under these conditions only one mctastable triplet was detectable by 

 its absorption spectrum, supposed to be the lowest tt, tt* metastable triplet. 

 It therefore seems that radiationless transition beween the triplet levels of 

 n, TT* and tt, tt* exist. 



Dr. Kasha: I am not certain of that and I see Professor Porter is on the 

 program, and perhaps he can tell us. The flash experiments which people 

 do in liquid solutions would be, perhaps, the only way that one could dis- 

 tinguish between which lowest triplet is available. If there is a restriction 

 on the conversion from the n, tt* triplet to tt, tt* triplet, then one should 

 get a different triplet-triplet absorption in the dry solvent compared with 

 the wet. 



Dr. LiNsciirr/: It has been foinid that one gets the same type of spectrum 

 whether one works in polar or non-polar solvents. 



Dr. Kasha: Well then, that is the answer. Dr. Franck. That establishes 

 the radiationless transition in the presence of the liquid solvent. 



