85 



ALLEN: Do you consider the difference between 1 per cent and 2 per cent 

 is outside the range of error? 



LINSCHITZ: Probably not, but the important thing is the order of magni- 

 tude. The fraction of energy which is transferred from xylene to anthracene 

 could be 100% or maybe 10%, but is certainly very high, because you are al- 

 ways dealing with a solute-solvent ratio of 1:1000. 



The mechanism of this transfer presumably is the same as that in sensi- 

 tized fluorescence, except that in these systems vibrational deactivation oper- 

 ates to trap the energy in the molecule whose absorption lies furthest to the 

 red (13). Suppose we assume that a molecule of solvent is excited into some 

 arbitrary state. Any excess vibrational energy will be lost quickly. If another 

 molecule is now adjacent, energy transfer may occur to that vibrational level 

 of the second molecule which offers the best energy match or resonance with 

 the first. Any excess vibrational energy of the second molecule will then also 

 be quickly dissipated, so that the resonance is destroyed and the energy cannot 

 get back to the original or similar molecule. In this way the energy may drift 

 to molecules having still lower lying excited states, as demonstrated by Kall- 

 mann, until ultimately, fluorescence or radiationless transition to the ground 

 state occurs. 



CURTIS: Did I understand you to say that the maximum energy efficiency 

 is 50% when the ratio is 1 to 1000? 



LINSCHITZ: That is correct. 



CURTIS: If this picture is correct, why would it be a maximum? 



LINSCHITZ: Well, it turns out that if you plot the fluorescence efficiency 

 of anthracene, for example, as a function of its concentration, then you find a 

 curve with a maximum. A phenomenon called self- quenching appears, so that 

 as you go to higher anthracene concentration, although you expedite the resona- 

 tive transfer from xylene to anthracene, you run into the self- quenching effect 

 and that cuts down the total light yield. This ratio, which is roughly 1:1000, 

 represents the optimum product of the probability of energy transfer into the 

 anthracene and the lack of the quenching of the anthracene by itself. 



FANO: Quenching means that the light is emitted but it is captured again 

 by some other anthracene and it does not get out? 



LINSCHITZ: No, this is not quenching due to absorption of fluorescent 

 light. 



FANO: What sort of quenching is this? 



LINSCHITZ: The actual mechanism of self- quenching is really a problem. 

 Of the many theories probably the one which makes the most sense is that of 

 Forster (6). Coupling between two dipolar oscillators leads to two states, one 

 symmetrical and one anti-symmetrical, of which the anti-symmetric will be the 

 state of lowest energy and longest radiative lifetime. The increase in the ra- 

 diative lifetime due to this coupling then permits more of the excited molecules 

 to lose their energy by competing radiationless processes. This is one possible 

 mechanism of self-quenching. 



KAMEN: Does Pringsheim have anything to say about this? 



