Energy transfer processes, donor-acceptor interaction

Nonradiative excitation energy transfer from a donor D to an acceptor A summarized in Figure 13.9 requires interactions between them. These interactions can be of the Coulombic type, or due to an orbital overlap between the two species, or be a combination of both types. Two mechanisms can explain nonradiative energy transfer in donor-acceptor systems. As is the case of electron transfer, energy transfer can also happen in cascade processes. For clarity reasons, however, we will only describe the basic theoretical background for energy dyads comprising one donor D and one acceptor A. 

In host-guest systems based on electron donor/ acceptor interactions, association/dissociation can be driven by redox processes so that it is possible to design electrochemical switches than can be used to control energy- and electron-transfer processes. 

Donor-Acceptor Interactions in Energy Transfer Processes... 

According to this sequence, formation of cis- and tnms-stilbenes is preceded by formation of a magnetosensitive ion radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and fram-stilbenes, and the isomerization rate must depend on the intensity of the magnetic field. These predictions were confirmed experimentally (Lyoshina et al. 1980). Hence, the ion radical route for trans/cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account donor-acceptor interaction. This interaction makes the process energetically more favorable. 

Eq. (127) indicates that it is possible to estimate the energy transfer rate if the absorption spectrum of the acceptor and the fluorescence spectrum of the donor are known. However, the optical properties of the acceptor and donor are not always available especially for the case in which ultrafast energy transfer takes place. In this case, it is necessary to solve the coupled GMEs with appropriate interactions that account for the energy transfer process. 

In the Forster mechanism, energy transfer occurs through dipolar interactions. This process is not coupled through bond interactions, and therefore orbital overlap and inter-component electronic coupling are unimportant. Dipole-dipole interactions may occur efficiently in systems where the donor and acceptor species are over 100 A apart, whereas Dexter energy transfer is typically efficient only up to distances of approximately 10 A. 

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