Donor-acceptor pairs charge-transfer process

A third type of semiconductor junction uses a conducting liquid to contact the semiconductor. Generally, the solvent and electrolyte do not participate in the charge-transfer process. Therefore, to control the Fermi level of the liquid phase, a donor/acceptor pair (a redox couple ) must be added to the solution. The electrochemical potential, or Fermi level, of the solution phase is then given by the Nemst equation (equation 8) ... 

The charge transfer process between donor and acceptor molecules influences the mode of the addition reaction. This charge transfer process is closely related to the free energy change AG of the radical ion pair formation and can be calculated from the Rehm-Weller equation [9] using the oxidation potential of the donor (D), the reduction potential of the acceptor (A), and the excitation energy of the excited species and coulombic interaction energy by the radical ion pair at the encounter distance for their interaction. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

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