Back electron transfer from encounter

In the photochemical conversion model (Fig. 3), the most serious problem is the undesired and energy-consuming back electron transfer (shown as dotted arrows) as well as side electron transfer, e.g., the electron transfer from (Q) to (T2)ox. It is almost impossible to prevent these undesired electron transfers, if the reactions are carried out in a homogeneous solution where all the components encounter with each other freely. In order to overcome this problem, the use of heterogeneous conversion systems such as molecular assemblies or polymers has attracted many researchers. The arrangement of the components on a carrier, or the separation of the Tj—Q sites from the T2—C2 ones in a heterogeneous phase must prevent the side reactions of electron transfer. 

However, our concern is with the cationic surface which promotes a rapid exchange of an electron from dimethylaniline to pyrene, and thereafter maintains a long-lived ion which can react with further solutes added to the system. Hie concept of the experiment is, that dimethylaniline transfers the electron rapidly to pyrene via a diffusion controlled reaction, which occurs by movement of the reactants on the surface of the micelle until they encounter each other. Electron transfer then occurs, and the back reaction of the two ions is prevented by the surface of the micelle, which holds the reactants in an unsuitable configuration for back reaction to occur. However, the repulsive positive force of the micelle on the dimethylaniline cation rapidly drives it away from the micelle, and effective and efficient charge separation is achieved, with a quantum yield Q of unity for the process of charge separation. 

This charge separation effect induced by the micelles is explained schematically in Fig. 4.10. Ci4MV2+ due to its hydrophylic character is mainly present in the aqueous phase and does not associate with the CTAC aggregates. The forward electron transfer will therefore occur in water. In the reduced state the viologen relay acquires hydrophobic properties. This leads to rapid solubilisation in the CTAC assemblies. The oxidized porphyrin on the other hand is prevented from approaching the micelles by the positive surface charge. As the -potential of CTAC is at least +100 mV the probability of encounter with ZnTMPyP5+ is smaller than 2 x 10 9. This explains the effective micellar inhibition of the back reaction. 

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