Excitation transfer, very weak coupling

Resonance energy transfer between the aromatic amino acids proceeds by very weak coupling between the donor and acceptor.151,52) Very weak coupling implies that the interaction between the donor and acceptor wave functions is small enough so as not to perturb measurably the individual molecular spectra. This transfer process, which is distinct from the trivial process of absorption of an emitted photon, involves radiationless deexcitation of an excited-state donor molecule with concomitant excitation of a ground-... 

The finite width of an individual vibronic transition may be interpreted as resulting from life-time broadening. This is due to the redistribution of vibrational energies within the separate molecules, under assistance of external degrees of freedom of the molecules involved in transfer and of others. It is nothing else than thermal relaxation which in condensed systems restricts the lifetimes of individual vibronic states to 10 sec or less at room temperature. Under very weak coupling conditions an excited molecule not only performs several vibrations but also occupies different vibronic levels... 

In the author s opinion, the conceptions of delocalized excitation and of excitation transfer are complementary but the second one is more appropriate to very weak coupling conditions. Amorepenetratinginvestigation has led Bay andPearlstein to the result that even in very weakly coupled systems theexcitationshould better be described as delocalized. This does not, however, lead to different predictions for experimental results. 

As mentioned earlier, vibrational relaxation of electronically excited states in solution is very rapid and thus this situation often prevails for solute-solute transfer. In spite of the fact that the coupling is very weak in this case, quantitative theoretical treatments lead to predictions of resonance transfer between appropriate molecules over large distances, i.e., 50-100 A.81... 

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