Double electron exchange process

For this double electron exchange process to operate, there should be a molecular orbital overlap between the excited donor and the acceptor molecular orbital. For a bimolecular process, intermolecular collisions are required as well. This mechanism involves short-range interactions ( 6—20 A and shorter). Because it relies on tunneling, it is attenuated exponentially with the intermolecular distance between the donor and the acceptor.17 The rate constant can be expressed by the following equation. 

The Dexter mechanism is a nonradiative energy transfer process that involves a double electron exchange between the donor and the acceptor (Fig. 12).16 Although the double electron exchange is involved in this mechanism, no charge separated-state is formed. 

Depending on the electromagnetic nature of Ti , a double-electron exchange (Dexter) mechanism or an electrostatic multipolar (Forster) mechanism have been proposed and theoretically modeled. They are sketched on Fig. 8 for the simple S - T -Ln path. Their specific dependences on the distance d separating the donor D from the acceptor A, i.e., for double-electron exchange and for dipole-dipolar processes, respectively, often limit Dexter mechanism to operate at short distance (typically 30-50 pm) at which orbital overlap is significant, while Forster mechanism may extend over much longer distances (up to 1,000 pm). 

The exchange interaction can be regarded (Fig. 2.8) as a double electron transfer process, one-electron moving from the LUMO of the excited donor to the LUMO of the acceptor, and the other from the acceptor HOMO to the donor HOMO. This important insight is illustrated in Fig. 2.9, from which it is clear that the attenuation factor for exchange energy transfer should be approximately equal to the sum of the attenuation factors for two separated electron transfer processes, i.e. for electron transfer between the LUMO of the donor and acceptor (2.29), and j " for the electron transfer between the HOMO (superscript ht denotes for hole transfer from the donor to the acceptor). 

Figure 1 shows the double-well potential which is often used to represent the electron transfer act in oxidation-reduction. The special case of a self-exchange process was chosen for simplicity. (For a full discussion of the issues being discussed in relation to the potential energy diagram, see Ref. 11. Ref. 13 gives a more compact treatment.) The implications of the diagram may not be immediately obvious, and the... 

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