Bridging states

In studies of photochemically induced CT, the energy of the donor and acceptor orbitals are close or higher in energy than the bridge states, and occupation of the bridge is understandable. In contrast, our electrochemical measurements employ redox active intercalators with reduction potentials... 

Figure 15. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the bridge states lie above the D level consequently the electron is transferred in one coherent jump and is never localized within the bridge. The distance dependence behavior is exponential decay.

Molecular wire behavior The bridge states are energetically comparable to the donor level the electron may be thermally injected into the bridge, whereupon, it moves from donor to acceptor incoherently, as a defect such as a polaron. The distance dependence behavior is Ohmic (varies inversely with distance). 

Hamiltonian term describing the interaction between the bridge state B ) and the acceptor state A)... 

Electron transfer through a molecular bridge can occur by single or multiple-step mechanisms [5-7]. The multiple steps may involve real (hopping) or virtual (superexchange) bridging states. Single electron transfer steps... 

Fig.6 The distance dependence of electron-transfer rates in DNA hairpins [51]. The acceptor is a photoexcited derivatized stilbene (SA) or phenanthrene (PA) the electron donor is guanine (G), deazaguanine (Z), or inosine (I). The decay is much more rapid in the Z-PA couple compared to the G-SA couple because the tunneling energy is further from the bridge states in the case of Z-PA...

The experimental data of Figs. 6 and 7 provide strong support for the two-site model of adsorption of NO. The dissociation products of NO occupy the same sites on the Ru(001) surface as the bridged state of molecular NO. When the initial coverage of dissociation products is sufficiently high, as in Fig. 7, the surface is poisoned with respect to subsequent adsorption of bridged NO. 

NO coverage, with the bridged state being populated exclusively at coverages below approximately a third of saturation. 

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