Energy transfer very weak coupling

Fig. 4.12. Energy level scheme of donor and acceptor molecules showing the coupled transitions in the case where vibrational relaxation is faster than energy transfer (very weak coupling) and illustration of the integral overlap between the emission spectrum of the donor and the absorption of the acceptor.

Dexter s formulation of exchange energy transfer (very weak coupling) In contrast to the inverse sixth power dependence on distance for the dipole-dipole mechanism, an exponential dependence is to be expected from the exchange mechanism. The rate constant for transfer can be written as... 

In many instances tire adiabatic ET rate expression overestimates tire rate by a considerable amount. In some circumstances simply fonning tire tire activated state geometry in tire encounter complex does not lead to ET. This situation arises when tire donor and acceptor groups are very weakly coupled electronically, and tire reaction is said to be nonadiabatic. As tire geometry of tire system fluctuates, tire species do not move on tire lowest potential energy surface from reactants to products. That is, fluctuations into activated complex geometries can occur millions of times prior to a productive electron transfer event. 

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-... 

Very Weak Coupling. When AE /3 there is another energy parameter, a, to be considered. The size of a determines the efficiency of redistribution of vibrational energy.79 If AE 13 a, vibrational relaxation determines the overall electronic relaxation, and oscillation between the initial and final states takes place. When AE a /3, the electronic transition is slow compared with the vibrational relaxation time therefore, as soon as the energy passes to the acceptor, vibrational relaxation occurs and there is little chance of back transfer. 

According to Eqn. 3, the very weak coupling transfer rate is proportional to the square of the vibronic interaction energy. For dipole-dipole interaction this leads to an inverse 6th power distance dependence of the transfer rate . The transfer is then often said to occur by inductive resonance . 

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... 

Singlet excitonic energy transfer between chls is most commonly discussed in terms of the two limiting cases of very strong and very weak electronic coupling (J) between donor (D) and acceptor (A) transition dipoles [159-161]. J(cm" ) may be calculated by the expression given by Pearlstein [162]... 

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