Electron-transfer models

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

The dissociative electron transfer model has been improved since then by a more accurate estimation of the activation entropy, taking into account that R and X are formed within a solvent cage from which they successively diffuse out.49 The free energy and entropy of activation are thus obtained from equations (14)... 

To summarize, in this article we have discussed some aspects of a semiclassical electron-transfer model (13) in which quantum-mechanical effects associated with the inner-sphere are allowed for through a nuclear tunneling factor, and electronic factors are incorporated through an electronic transmission coefficient or adiabaticity factor. We focussed on the various time scales that characterize the electron transfer process and we presented one example to indicate how considerations of the time scales can be used in understanding nonequilibrium phenomena. 

Experimental Test of Bridge-assisted Electron Transfer Models... 

The calculation of the transmission coefficient for adiabatic electron transfer modeled by the classical Hamiltonian Hajis based on a similar procedure developed for simulations of general chemical reactions in solution. The basic idea is to start the dynamic trajectory from an equilibrium ensemble constrained to the transition state. By following each trajectory until its fate is determined (reactive or nonreactive), it is possible to determine k. A large number of trajectories are needed to sample the ensemble and to provide an accurate value of k. More details... 

Figure 22 shows the same quantities for the intramolecular electron-transfer Model IVb. Similar to what occurs in the pyrazine model, the classical level density obtained with y = 1 overestimates the total and state-specific level density while for y = 0 the classical level densities are too small. Employing a ZPE correction of y = 0.8 results in a very good agreement with the total quantum mechanical level density, while the criterion to reproduce the state-specific level density results in a ZPE correction of y = 0.6. 

Figure 29. Comparison of quantum path-integral results (thick tines) and ZPE-corrected mapping results (thin lines) for the diabatic electronic populations of a three-state electron transfer model describing (a) sequential and (b) superexchange electron transfer.

The POs identified above can also be used for the analysis of other observables of our simple electron-transfer model. For example, it has been shown that a calculation employing two POs qualitatively reproduce the short-time evolution of the probability distribution P(x, t) = (f) /2) x) (x ( i2 l I (O)... 

The semiclassical mapping approach outlined above, as well as the equivalent formulation that is obtained by requantizing the classical electron-analog model of Meyer and Miller [112], has been successfully applied to various examples of nonadiabatic dynamics including bound-state dynamics of several spin-boson-type electron-transfer models with up to three vibrational modes [99, 100], a series of scattering-type test problems [112, 118, 120], a model for laser-driven... 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

Ultimately an understanding of electron transfer processes in dye-sensitized solar cells must be expressed in terms of a model which takes the specific nature of metal oxide surfaces into account [97]. Moreover, the nanostructured devices often involve oxide nanoparticles which approach the limit where quantum-size effects become important. It would be a great step forward if this could be incorporated into an electron-transfer model. 

The implications of the obtained structural and electronic information on the binding and the surface electron transfer models in dye-sensitized solar cells have been discussed. Calculated strong binding, and strong electronic surface-adsorbate interactions, is consistent with experimentally observed ultrafast photoinjection processes in stable dye-sensitized electrochemical devices. It will be important, however, to combine results from explicit calculations of... 

Figure 25. Two types of electron-transfer models, (a) A chromophore-linked Mb. (b) A protein assembly using an interface-attached Mb.

Figure 26. Electron-transfer models using Mb. (a) A functionalized prosthetic group linked by ruthenium complex, (b) Electron transfer occurs from the photoexcited ruthenium complex to the hemin and then Ru(III) is reductively quenched by EDTA. (c) Electron transfer occurs from the photoexcited ruthenium complex to Co(III) complex and then one electron is abstracted from the hemin.

The Arrhenius plot shows an apparent overall activation energy of about 8 kcal/mol, well below the initiation by a hydrogen abstraction [(Eq. (7)] and more consistent with an electron transfer model for initiation reaction [(Eq. (8)]. 

Tanaka, S. and Marcus, R.A. (1997) Electron transfer model for electric field effect on quantum yield of charge separation in bacterial photosynthetisc reaction centers, J. Phys. Chem. 101, 5031-5045. 

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