Electron transfer, activation control mediated

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Conversely, controlled immobilization of enzymes at surfaces to enable high-rate direct electron transfer would eliminate the need for the mediator component and possibly lead to enhanced stability. Novel surface chemistries are required that allow protein immobilization with controlled orientation, such that a majority of active centers are within electrontunneling distance of the surface. Additionally, spreading of enzymes on the surfaces must be minimized to prevent deactivation due to irreversible changes in secondary structure. Finally, structures of controlled nanoporosity must be developed to achieve such surface immobilization at high volumetric enzyme loadings. 

In fact, the surface may mediate the requisite chemistry of the initially formed radical cation so that different products can be observed from the same intermediate when generated photoelectrochemically or by other means. The radical cation of diphenyl-ethylene, for example, gives completely different products upon photoelectrochemical activation 2 than upon electrochemical oxidation at a metal electrode or by single electron transfer in homogeneous solution, Eq. (31) . Surface control of... 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

Poly(amidoamine) dendrimers of the fourth generation with -OH terminal groups were used as templates to produce stable metal nanoparticles [170-172], The dendrimers (in aqueous solution) are first loaded with a predetermined amount of Cu + or Pt + metal ions following chemical reduction, metal nanoparticles are formed inside the dendrimer structure, where they are protected from agglomeration. This procedure permits both particle stability and control over particle size. Dendrimers containing Pt metal clusters were also attached to gold electrodes, and were found to be active as electrocatalysts for O2 reduction [172]. This demonstrates that the nanoparticles inside the dendrimer can mediate electron transfer processes between the electrode surface and reactants in solution. 

The redox potentials of organic cofactors are directly responsible for controlling the equilibrium behavior of the corresponding cofactor-mediated electron transfer processes. The relative redox potentials of the cofactor and its redox partner are also intimately related to the rate of adiabatic electron transfer Ret through the classical Marcus equation [13, 14], and nonadiabatic electron transfer through the semi-classical Marcus equation [15, 16]. The direct dependence of both the kinetics and thermodynamics of electron transfer processes on the cofactor redox potential makes the control of these potentials a key determinant of the activity of redox proteins. 

It also was found that the direction of the photobiocatalytic switch of the nitrospiropyran-FAD-reconstituted enzyme is controlled by the electrical properties of the electron transfer mediator. With ferrocene dicarboxylic acid as a diffusional electron transfer mediator, the enzyme in the nitrospiropyran-FAD state (10a) was found to correspond to the OFF state bio-catalyst, while the protonated nitromerocyanine state of the enzyme (10b) exhibits ON behavior. In the presence of the protonated 1-[1-(dimethyl-amino )ethyl]ferrocene, the direction of the photobioelectrocatalytic switch is reversed. The nitrospiropyran-enzyme state (10a) is activated toward the electrocatalyzed ox idation of glucose, while the protonated nitromerocyanine enzyme state (10b) is switched OFF, and is inactive for the electrochemical oxidation of glucose. This control of the photoswitch direction of the photoisomerizable reconstituted enzyme was attributed to electrostatic interactions between the diffusional electron mediator and the photoisomefizable unit... 

Perhaps the most exciting recent advances in the organometallic chemistry of fluorocarbons have been the contemporaneous and complementary discoveries of examples of catalytic activation and functionalization of perfluorocarbons in laboratories led by Crabtree and Richmond [23]. Kiplinger and Richmond [64] showed that Group 4 metallocenes function as catalysts in the synthesis of per-fluoronapthalene from perfluorodecalin using activated Mg or Al as the terminal reductant. Low valent zirconocene or titanocene species were postulated as intermediates in the catalytic cycle and control experiments showed the central role played by the metallocene in mediating electron transfer in these systems. Turnover numbers up to 12 (net removal of 120 fluorines/metallocene) were noted [64]. 

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