The Electron Transfer Process

This process is particularly important in such photonic applications as solar cells. Simple migration of energy is a thermodynamically neutral process. It allows the excitation energy deposited at a site in a solid or in a concentrated solution to move to another position by transferring the excitation energy in the absence of an intermediate quencher. Electron transfer, however, is a process by which an electron is passed from an electron-rich donor to an electron-deficient acceptor [95]. This reaction is substantially accelerated when the donor or acceptor is excited. Electron transfer from an excited state donor molecule D* to a ground state acceptor A generates a radical cation D+• and a radical anion A •. The resulting radical ion pair exists as a charge-separated pair of ions:

The oxidized and reduced species are usually highly energetic, storing a substantial fraction of the energy absorbed from the photon. The charge separation that occurs in such a photo-induced electron transfer provides a way to convert the excitation energy of the excited molecule to a chemical potential in the form of a radical ion pair. Electron migration can also be a movement of an electron either to a neutral electron donor from an oxidized one) D + D→ D+D) or from a reduced acceptor to a neutral one) A+A → A+A ([93]. These thermoneutral processes that are called hole and electron migrations, respectively, permit further spatial charge separation between an excited donor, D*, and a reduced acceptor, A•. This separation is beyond one that is initially produced in an ion pair by photo-induced electron transfer. After the absorption of light by a to a sensitizer, the energy migrations or the energy transfer move the excited state site where the excitation energy is converted to a radical ion pair by photo induced electron transfer. A kinetic competition then takes place between the rates of several possible next steps. These steps can be chemical reaction of the radical ions, or they can be further charge migrations by sequential electron or hole transfers, or actually nonproductive charge recombination, called back-electron transfer. The back-electron transfer regenerates the ground states of both the donor and the acceptor.

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مواضيع ذات صلة

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts

Immobilized Catalysts

Special Gels for Drug Release

Support Materials Other Than Polystyrene

Materials Based on Polystyrene

Polymer Supports for Reagents, Catalysts, and Drug Release

Degradation of Polymeric Materials by Ionizing Radiation