76 LIGHT AND LIFE 



Dr. Portkr: 11 its only a factor of ten, I would think we might see them, 

 but I would not say that we do not see them. We do, in fact, if you look, at 

 the tracing of anthracene, give a longer wavelength transition than the one 

 you see, but you have to make the principal one a complete blackout to see 

 it. There are some weaker long wavelength transitions. 



Dr. Kasha: Secondly, on the phenanthrene transfer to naphthalene, I 

 believe that the Foerster condition for resonance transfer is perfectly satisfied 

 because the phenanthrene emission comes at about 4700 Angstroms which is 

 well within the naphthalene singlet-triplet absorption region. I think a test 

 of your contention would be to use phenanthrene-anthracene which would 

 have a blank in the region of phenanthrene emission. 



Dr. Porter: We have done that actually and a number of others as well. 

 Transfer from phenanthrene to anthracene is very efficient. There is no 

 correlation with the overlap calculated by the Foerster theory. Furthermore, 

 you say that conditions are perfect, but they are not, because the Foerster 

 mechanism predicts a very low probability for triplet-singlet, singlet-triplet 

 transfer. 



Dr. Kasha: The Foerster spectral overlap conditions are satisfied. As to 

 the excited state coupling mechanism, one might anticipate a different 

 r-dependence. 



Dr. Porter: Well, except that you have a condition of spin transfer. This 

 makes the probability so low that the transfer distance becomes very small 

 and I think exchange transfer will predominate. 



Dr. Kasha: I want to raise one more question. 1 his is about the viscosity 

 dependence of the radiationless process. I would tend to agree with Whiffen 

 just because it would imply conservation of angular momentum as an im- 

 portant process in singlet-triplet intramolecular conversion. On the other 

 hand, your ideas would suggest that the spectral features should show a 

 viscosity dependence as well, I think. Or don't you think so? In other 

 words, either the triplet-triplet or the triplet-singlet absorption emission 

 spectra should then become strongly viscosity-dependent if you are saying 

 that the potential function actually changes with viscosity. 



Dr. Porter: I agree, but what I am not sure about is how much. There 

 is a viscosity dependence in our triplet-triplet absorption spectra. In the 

 first spectra of Windsor wc pointed out that as one goes to paraffins and 

 alcohols and so on, irresjjcc live of the chemical constitution of these things, 

 there is a gradual shift wlii(h correlates with viscosity rather than with any- 

 thing else of the triplet-triplet absorption spectrum. 



Dr. Kasha: It is not so much the frequency which should change, or the 

 spectral envelope. 



Dr. Porter: Well, of course, there is not a great deal of detail in the 

 absorption. It depends upon how much change is expected. There is a shift 

 in the thing as a whole. I don't think the spectral data are precise enougii 

 to rule out a change in the potential energy diagram. 



Dr. Linsc;hit/: In the (pienching of the triplet state by \arious agents, I 



