Electron transfer excited organic species

Equation (18) has been used in a further recent paper on electron transfer reactions of excited organic species, " and the leveling of log k at highly negative AG is found also in electron transfers from neutral molecules to acceptors such as trapped holes in glasses. " " " [Plots of log k rather than AG against AG are now generally preferred, in view of the nonapplicability of equation (2) to nonadiabatic reactions.]... 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

Because of this difference in electron-donorand electron-acceptor properties, excited states have very different redox properties from those of related ground states. The effect is so marked that many photochemical processes begin with a complete transfer of an electron from (or to) an excited state (1.2), and the subsequent chemistry is that of radical cations and radical anions, species that are regarded as unusual in ground-state organic reactions. The importance of photochemical electron transfer is underlined by its extensive involvement in photobiological processes such as photosynthesis. 

This article will illustrate several methods by which back electron transfer can be obviated and hence by which organic transformations can be accomplished. Because this field has been so active, a comprehensive review of all work accomplished toward these objectives would be impossible. The coverage of this article is therefore restricted to recent, rather arbitrarily chosen, experiments which exemplify the basic principles governing both electron exchange between excited organic molecules and appropriate redox partners and the subsequent chemical reactivity of the reduced and oxidized species formed in the photochemical step. 

A mechanism that accounts for the oxidative addition of halocarbons has been proposed for the two d8-d8 dimers (Figure 4) (23). The mechanism involves the oxidative quenching of the triplet excited state of the metal dimer as the primary photoprocess. This gives a radical anion species that dissociates a halide, thereby producing an organic radical. The dissociated halide adds to the partially oxidized metal dimer to form a mixed valence Ir -Ir -X or Pt 1-Pt -X intermediate. This intermediate reacts further with the remaining organic radical (presumably in a second, thermal electron transfer step) to form the final d2-d2 dihalide dimer. 

Electromagnetic radiation, besides being a probe of surface structure, can excite electrons in the species in solution (especially in organic compounds) or in the electrode itself (especially in semiconductor electrodes). This photon excitation can lead to electron transfer between electrode and solution. The study of these phenomena is photoelectrochemistry and can be very important in conversion of solar energy into electricity in order to convert substances (photoelectrolysis). 

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