90 



cule long enough to see the long-lived triplet excited states in phosphorescence, 

 or even in absorption. For these triplets radiative lifetimes lie in the range of 

 possibly 10~4 to one second, which are very long lifetimes, indeed. In addition, 

 these molecules are interesting chemically because they are diradicals. Thus 

 ethylene in the triplet state would be written by a chemist with two unpaired 

 electrons, available for bond formation. These two properties make such states 

 very interesting as possible intermediates in the photo-chemical processes that 

 can occur. 



Another feature of the triplet state is that although it is reached generally 

 with low efficiency optically, it may be excited with high efficiency by ionizing 

 radiation, in particular, via recombination of ions and electrons. This was 

 seen, for example, in the rigid solvent experiments cited earlier, in which the 

 recombination luminescence corresponded to the triplet-singlet emission. Opti- 

 cally, the transition will occur between the singlet ground state and other ex- 

 cited singlets, and the probability of getting into the triplet state is relatively 

 small because of the restriction on the radiationless transition between the 

 singlet and triplet. 



KASHA: Could you state that again, please, because I don't think it sounded 

 right. 



LINSCHITZ: The probability for getting into the triplet state from the ex- 

 cited singlet is relatively small. 



KASHA: You mean from the ground state? 



LINSCHITZ: From the excited singlet. I mean optically getting into the ex- 

 cited singlet followed by radiationless transition into the triplet. 



KASHA: If you have not seen the paper by Gilmore, McClure and Gibson 

 (18), they have shown that at least for the half dozen cases they studied the 

 quantum efficiency of the triplet-singlet emission is close to unity for several 

 molecules none of which contain any heavy atoms which would enhance spin-orbit 

 coupling. 



In other words, the probability in many complex molecules of going from the 

 singlet to the triplet is very high. 



LINSCHITZ: Are not these molecules studied in rigid glasses in which the 

 deactivation of the excited singlet is relatively slow and the competing triplet 

 conversion from that state can then occur with high yield? If you take, on the 

 other hand, the average aromatic molecule in a liquid solvent I wonder what 

 would be the yield. 



KASHA: The yield is zero, but you cannot say that it does not get into the 

 triplet state just because you cannot see any emission. 



LINSCHITZ: That is true. 



KASHA: In fact, that has been demonstrated by others. 



LINSCHITZ: Has anybody measured triplet yield in cases where you do not 

 have such short lifetimes for the excited states? 



KASHA: Those were the ones covered partially by McClure, Gilmore and 

 Gibson. The singlet- singlet probably would give a lifetime of about 10 -y sec - 



