98 



BURTON: You say the singlet lifetime would be longer than we think? 



KASHA: Yes, the observed lifetime will be shorter than that calculated from 

 the integrated absorption, as a result of radiationless transition to the triplet 

 state. 



BURTON: With regard to the long lives of the singlet states, it is pertinent 

 to mention the work of Duncan at Rochester. He worked on things like acetalde- 

 hyde, acetone, etc. and, as I recall, it turned out that the half-lives for these 

 excited states before fluorescence are greater than 10" 5 seconds. 



KASHA: They are 10~ 3 but you correct them for a quantum efficiency of 10 

 per cent. But they have not been shown to be singlet- singlet, and vibrational 

 analysis of the vapor absorption indicates that there are two electron 

 transitions, remember. There is a school of thought, which includes me, which 

 thinks that the lowest emitting level is actually a triplet and tt antibonding. 



BURTON: The point is, however, that the molecule must get into that state 

 in some way or other. 



KASHA: In fact, pyridine and acetone are rather analogous in their electron- 

 ic systems. 



BURTON: Is it the vapor state which you are talking about in the case of 

 pyridine? 



KASHA: The transition takes place in any state. The emission is observed 

 only in rigid glasses or solid material. 



BURTON: I was talking about gases here. 



KASHA: There is a trivial detail. The intrinsic lifetime is 3.0 seconds. 

 So you cannot observe this in the gas, but in the case of ketones you can. It 

 happens to be 10"^ seconds and it can be 10" 5 , but the quantum efficiency comes 

 in. 



BURTON: If you have such a long lifetime for your emitting state then, 

 since the singlet state was presumably formed in the absorption act, the singlet 

 emission must be a slow process compared to the other processes in which that 

 state can be involved. 



KASHA: The singlet-singlet emission would be about 10 -5 or 10-6 seconds. 

 The emission is not observed, but the decay constant is calculated from the ob- 

 served singlet-singlet absorption curve. 



LINSCHITZ: I didn't make myself clear a little while ago when I asked about 

 the lifetime of the excited singlet. The point was not that the lifetime is short- 

 ened by the conversion to triplet but that in so many cases the maximum singlet 

 lifetime is already so short that no appreciable conversion to the triplet can oc- 

 cur. If you calculate the singlet- singlet lifetime just from the absorption you 

 get a radiative lifetime which would be an upper limit for the actual lifetime. 

 Then you come out certainly with something much shorter than 10" sec. , but 

 you need this much time for the singlet- triplet conversion. 



KASHA: In what case do you mean? 



LINSCHITZ: Take a dye. 



