Energy Transfer via Exchange Interaction

The electron exchange interaction [see Eq. (6.16)] requires overlap of the electronic wave functions of the donor and acceptor. Dexteri evaluated the exchange expectation value equation and obtained the following result  

Inasmuch as K and L are constants not readily available from experimental data, only the form of Eq. (6.22) is of interest. For example, the rate constant should decrease exponentially with increasing separation R between the donor and acceptor. Also, because donor and acceptor multiplicities can change during the transfer, the overlap integral is calculated with both the donor emission and acceptor absorption normalized to unity. 

When energy transfer takes place via an exchange mechanism, the system obeys Wigner s spin rule. If xi and X2 are the initial spin quantum numbers 

The triplet-triplet energy transfer, which is doubly forbidden by a resonance mechanism, is allowed by an exchange mechanism  

Energy transfer by the following route is forbidden both by exchange (spin) and resonance transfer  

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