Forward Electron Transfer

FIGURE 10. Possible origin of dispersive kinetics of primary electron transfer. Forward electron transfer is indicated by the solid arrows, thermal repopulation of the P state (a minor process) by the dotted arrows. (A) Static heterogeneityoelectron transfer takes place from the P state to P Ha states with distribution of free energies The reaction therefore occurs with a distribution of driving forces, and hence a distribution of rates. Most thermal repopulation of the P state occurs from the P Ha" states that are highest in energy. A similar model can be constructed based upon a P —> P a" with two or more values for the... 

As tire reaction leading to tire complex involves electron transfer it is clear that tire activation energy AG" for complex fonnation can be lowered or raised by an applied potential (A). Of course, botlr tire forward (oxidation) and well as tire reverse (reduction) reaction are influenced by A4>. If one expresses tire reaction rate as a current flow (/ ), tire above equation C2.8.11 can be expressed in tenns of tire Butler-Volmer equation (for a more detailed... 

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Forward Electron Transfer as Studied by Fluorescence Quenching 69... 

Since the electrostatic potential sharply decreases with increasing distance from the polyelectrolyte cylinder, the degree of reactivity modification by functional groups fixed to the polyion is strongly dependent on the distance from the cylinder surface. Considerable electrostatic potential effects on the photoinduced forward and thermal back electron transfer reactions, which will be discussed in the following chapters, can be attributed to the functional chromophore groups directly attached to the polyelectrolyte back-bone through covalent bonds. 

The forward ET rate for the poly(A/St/Phen)-MV2 + system was extremely fast i.e., the rate constant (kEX) was at least of the order of 1011 s It is reasonable to consider that the fast primary electron transfer occurs from D to the... 

Consider again the electron-transfer reaction O + ne = R the actual electron transfer step involves transfer of the electron between the conduction band of the electrode and a molecular orbital of O or R (e.g., for a reduction, from the conduction band into an unoccupied orbital in O). The rate of the forward (reduction) reaction, Vf, is first order in O ... 

FIGURE 3-9 Square-wave voltammograms for reversible electron transfer. Curve A forward current. Curve B reverse current. Curve C net current. (Reproduced with permission from reference 9.)... 

A rapid, nonphotochemical, transfer of the alkyl group has been observed at 44° C between [MeCo(BAE)H20)] and [Co"(tfen)] in both the forward and reverse directions, and from the ethyl, but not the pentafluoroethyl, analog and it was suggested that the reaction involved electron transfer accompanied by the transfer of the alkyl group (174a). Whatever the best description of the mechanism, it presumably involves direct transfer... 

In the model presented above the forward dark current corresponds to an electron transfer via the conduction band. Using, however, a redox couple of a relatively positive standard potential the empty states of the redox system occur rather close to the valence band and the cathodic current could be due to an electron transfer via the valence band as illustrated in Fig. 3 b. In this case one still obtains the same i — U characteristic but the saturation current is now given by... 

In an EC mechanism the ratio of the forward and backward reaction rates is decisive for k/ d in , the chemical follow-up reaction has no influence here, so that for a sufficiently rapid electron transfer step the limiting current remains diffusion controlled.)... 

Three-pulse ESEEM spectrum of perdeuterated P-carotene imbedded in Cu-MCM-41 exhibits an echo decay with an echo modulation due to deuterons. The three-pulse ESEEM is plotted as a function of time, and curves are drawn through the maximum and minima. From ratio analysis of these curves, a best nonlinear least-squares lit determines the number of interacting deuterons, the distance (3.3 0.2A), and the isotopic coupling (0.06 0.2MHz). This analysis made it possible to explain the observed reversible forward and backward electron transfer between the carotenoid and Cu2+ as the temperature was cycled (77-300 K). 

The extent to which the ion-radical pair suffers a subsequent (irreversible) transformation (with rate constant k characteristic of highly reactive cation radicals and anion radicals) that is faster than the reverse or back electron transfer (/cBET) then represents the basis for the electron-transfer paradigm that drives the coupled EDA/CT equilibria forward onto products (P)20 (equation 8). 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Much fundamental work yet remains in the study of intramolecular donor-acceptor molecules to find out what structural parameters of the donor, acceptor and particularly the linkage enhance the efficiency of forward electron transfer while at the same time inhibiting the rate of reverse electron transfer. Progress so far is very promising. 

Denote the forward and backward rate constants of this reaction by ka and kb- When the reaction proceeds under stationary conditions, the rates of the chemical and of the electron-transfer reaction are equal. Derive the current-potential relationship for this case. Assume that the concentrations of A and of the oxidized species are constant. 

Here, i is the faradaic current, n is the number of electrons transferred per molecule, F is the Faraday constant, A is the electrode surface area, k is the rate constant, and Cr is the bulk concentration of the reactant in units of mol cm-3. In general, the rate constant depends on the applied potential, and an important parameter is ke, the standard rate constant (more typically designated as k°), which is the forward rate constant when the applied potential equals the formal potential. Since there is zero driving force at the formal potential, the standard rate constant is analogous to the self-exchange rate constant of a homogeneous electron-transfer reaction. 

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