99 



KASHA: Possibly I should mention an additional point. Many molecules in 

 which the fluorescent lifetime is of the order of 10"8 or 10 _ 9 seconds success- 

 fully emit with the smaller conversion to the triplet states. Most of the aromat- 

 ic hydro-carbons fall into that category. 



For example, in anthracene most of the emission is from the singlet, and it 

 is extremely difficult to observe the triplet. 



LINSCHITZ: How about Gilmore, McClure and Gibson? 



KASHA: The cases where very high phosphorescence yields are obtained 

 are those usually involving n ■> pi transitions. 



KAMEN: What you said was that undoubtedly in a solid peptide where you 

 have transition from the triplet to the ground state you might get a concentration 

 of this extra energy in some sensitive spot. Would this happen with a peptide in 

 solution? 



KASHA: I would put it this way: Suppose we take something for which the 

 lifetime is known. Let's say, pyridine -- with a lifetime of three seconds. 

 Under what conditions can we observe the pyridine three second phosphores- 

 cence? I know of two. A crystal of pyridine or pyridine in a rigid glass. 



There are two things which really complete the picture. In the fluid system 

 or in an absorbed system, deactivation collision and reaction could very well 

 have a higher yield or a greater probability, and so that could occur. 



BURTON: Isn't there a possibility, for example, of a straight forward re- 

 action of an excited polypeptide molecule with water? 



KAMEN: Most proteins in the cellular systems are insoluble; that is, the 

 active proteins. They are in the interfaces mostly, and water, which is supposed 

 to be competing or giving reaction one way or another, may not be there at all 

 in the case we are talking about. We keep harping about protein and water, but 

 actually the cellular systems involved are insoluble systems. They are extract- 

 ed as such. I doubt whether you should really worry about losing this mechanism 

 because of the presence of water. 



I think maybe you have something when you say that an inactivation could 

 come about from migration of energy to this long-lived triplet state. 



KASHA: There is one other point that I forgot to mention. It is perhaps the 

 most interesting. What causes singlet-triplet transition? Well, there is a basic 

 process, spin-orbital coupling, which is involved and by which such transitions 

 occur. This can be enhanced by external electric or magnetic fields. One way 

 in which this can be done is to replace the atoms in a molecule with heavier ones. 



KAMEN: Can you give a specific example of this? 



KASHA: Yes, for example if you take benzene, one way of increasing the 

 atomic number of a critical part of the system is to put in a sulfur. There you 

 have gone from an atomic number of - - well, that of oxygen to sulphur, from 8 

 to 16. The Z ratio increase of two causes an enormous increase in spin-orbit 

 coupling. 



In addition to that effect, which is internal, you can simply replace a hydrogen 

 by some heavier atom and here you have a wide range of substitutions that are 



