81 



BURTON: Do you mean for this case or a comparable one? Dr. Linschitz 

 knows about the work that we did on the benzene system. 



LINSCHITZ: Yes, I want to come to that. 



CURTTS: May I ask how you know you produce the excitation of this one 

 bond? 



LINSCHITZ: Simply by the absorption spectrum and the wavelength of the 

 exciting light. It is known that the excitation appears originally in the aromatic 

 part of the complex molecule. I might say that the various possible, almost 

 circumstantial, shapes of energy curves which happen to be involved in the 

 transition -- just those special modes of vibration that happen to be involved 

 will result in subsequent photochemistry or lack of photochemistry because this 

 will set up specific preferred channels for the vibrational energy to follow 

 through the molecule, after internal conversion. As a general rule, one can 

 say that the more complex the system the more will be the tendency to heat up 

 rather than to split, but if there are weak bonds adjacent to the chromophore 

 these may break also. 



To go on to the bimolecular processes which might be involved, I think the 

 most important one from the standpoint of radiobiology is oxidation or reduction 

 of the excited molecule. 



In the simplest case, an excited molecule MH2 can lose a hydrogen atom 

 (or electron) to a suitable acceptor A to form a pair of radicals: 



MH* 2 + A — > MH- + AH- 



If A is oxygen, this leads to the powerful oxidizing radical H0 2 . Several reac- 

 tions may now ensue. The two radicals may react further to give M + AH2, or 

 MH- can react with another molecule RH 2 : 



MH- + RH 2 — > MH 2 + RH- 



regenerating MH and permitting 



RH- + AH- V R + AH 2 - 



In this case MH 2 acts as a redox catalyst. This is probably the simplest mech- 

 anism one can write for photo-sensitization by MH 2 . 



BURTON: Give us an example of that, please. 



LINSCHITZ: The photo oxidation of amines by oxygen, sensitized by chloro- 

 phyll (5) essentially runs through such a sequence. 



Another possibility is for MH 2 to react as an acceptor rather than donor, to 

 give again a pair of radicals 



MH* + RH 2 ► MH 3 - + RH- 



and subsequent reactions analogous to these given previously. 



As evidence for this, one can cite experiments on fluorescence quenching of 

 various dyes by oxidants or reductants and the observed over-all photochemical 

 effect, sensitized oxidation-reduction (6K 



