101 



proceeds to a high level of the ground singlet state. 



KASHA: Oh, the ground singlet. Then you cannot get over to the triplet in 

 general. 



BURTON: You cannot get over unless there happens to be an interaction. 



KASHA: That is right. 



BURTON: Suppose there is an interaction. In these particular cases you 

 happen to have atoms around which will promote the transition. That is all I 

 was saying. 



That is a mechanism for the radiomimetic effect mentioned by Dr. Kamen. 

 What can be done with the idea I don't know. 



KASHA: Incidentally, there is only one electronic system which I think the 

 spectroscopists know in which most often the ground state does intersect with 

 all of the excited states and that is the system which includes anethylenic double 

 bond. At least you can say that no one has ever observed any emission. No 

 matter what electronic state is excited in ethylene you finally get hot ethylene in 

 the ground state and the photo processes are almost identical with the thermal 

 processes. 



LINSCHITZ: There is a possibility that proteins may act to increase the 

 lifetime of triplet states, by decreasing the chance of thermal quenching. In a 

 highly organized protein there may be enough internal interaction and cross- 

 linking to establish a rigid structure and impede vibrations. Triplet states ex- 

 cited in molecules within this framework, or closely associated with it, would 

 be stabilized since internal conversion to the ground state would be impeded, as 

 in the case of molecules which show fluorescence or phosphorescence only in 

 rigid solvents or when they are absorbed on suitable materials. By this means 

 the excitation energy may be preserved intact, as electronic energy, possibly 

 even as long as the natural radiative lifetime of the triplet, and the chance of 

 chemical reaction is thus enhanced. 



KASHA: In that special case a pseudo potential is introduced into the mole- 

 cule by the environment which prevents the normal crossing of the excited po- 

 tential surfaces and the ground state. 



PLATZMAN: If I understand the situation correctly, Dr. Kasha suggests 

 that in certain kinds of molecules there occurs internal conversion of all types 

 of excited states to long-lived states and, therefore, states in which the energy 

 is chemically available. Just exactly what kinds of molecule? Dr. Kamen sug- 

 gests all cellular proteins. This seems likely. On the other hand, we all agree 

 that simple systems like pure H^O do not qualify. Well, in general can we all 

 agree upon the cases where this long-lived chemical availability will pertain be- 

 cause that, of course, as Dr. Kamen says, is extremely important. 



KAMEN: Sterols and pigments should be added to the protein. 



KASHA: I would say as a general rule any molecule which has a double bond 

 in it and which does not contain an ethylenic link between that double bond and 

 the rest of the molecule, will have stable triplet states. You see, there is the 

 complication in the case of complete conjugation through an ethylenic bond which 

 leads to total degradation of the electronic energy as heat. 



