Electron transfer charge-shift reactions

In a general description of intramolecular electron-transfer (ET) processes one has to differentiate between charge separation in donor/acceptor (D/A) systems via the formation of photoexcited states and a charge-transfer or charge-shift reaction that is thermally activated (Cannon, 1980 Fox and Chanon, 1988 Meyer, 1978). 

If a third component (M), which can specifically stabilize one of the products of electron transfer, is introduced into the D -A system, the free energy change of photoinduced electron transfer is shifted to the negative direction, when the activation barrier of electron transfer is reduced to accelerate the rates of electron transfer, as shown in Figure 3, where M forms a complex with A ". It should be emphasized that there is no need to have an interaction of M with A and that the interaction with the reduced state (A ") is sufficient to accelerate the rate of photoinduced electron transfer. This contrasts sharply with the catalysis on conventional ionic or concerted reactions, in which the catalyst interacts with a reactant to accelerate the reactions. The initial interaction between M and A in the complex A-M, where charge is partially transferred from A to M, would also result in acceleration of the photoinduced electron transfer, since the reduction potential of A-M is shifted to the negative direction as compared to that of A. 

D and A are represented as neutral species for simplicity, although often one or both is charged (and electrostatic effects can influence ET rates). When both D and Areally are neutral, the forward electron transfer is sometimes referred to as charge separation (CS) and the backreaction as charge recombination (CR). When D or A bears a charge, forward ET is sometimes referred to as a charge-shift reaction. 

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