Resonant superexchange

The photo-induced charge conduction mechanism in double-strand guanine-rich DNA-based molecules, and AT pairs attenuating the conductivity by off-resonance superexchange steps is broadly supported. Mechanistic mapping has been based on both direct charge transfer kinetics in the nano-, pico- and femtosecond ranges... 

Fig. 2 (a) Schematic representation of the energy levels diagrams for a DBA system and a MBM junction in which the electron transfer process is dominated (b) by superexchange or non-resonant tunnelling, (c) by resonant tunnelling or (d) by hopping ... 

The role of mediator molecules in donor-acceptor electron transfer processes is an item of considerable recent interest [73 — 81]. A lot of research has been done on intermediate acceptors in the electron transfer in photosynthesis and theoretical studies bases on the superexchange interaction have been carried out [76 — 82]. In Refs. [83,84], electron transfer in the presence of ordered mediator molecules with arbitrary energy levels in one-dimensional case [83] as well as electron transfer in the presence of one resonant mediator [84] were considered. 

Superexchange occurs when the bridge and donor energy levels are off-resonance, typically when the bridge levels lie in excess of 2 eV higher than the donor levels. 

Figure 2.7 Schematic illustrations of the proposed mechanisms for electron transfer through a bridge (a) superexchange, in which electron transfer is a concerted process mediated through a bridge whose energy levels lie well outside of resonance with the donor orbitals (b) electron hopping, where the electron is transferred sequentially through a bridge whose energy levels lie in close resonance with the donor orbitals...

Figure 15. (a) Resonance field positions Hres of 5 and 5 and (b) normalized transverse superexchange interaction Jx/Jz as a function of digging field Hdlg... 

Fig. 1.6) defined the physical positions of each of these components more precisely [38]. For the past decade or more, there has been some dispute as to whether the accessory BChl serves as an actual intermediate, or as an off-resonant site, for the ET from (BChl)J to BPh. At present, the data favor the physical intermediate, BChl", over the superexchange mechanism [39]. 

The same series of Ru /Ru systems that were used to compare T,y values obtained from direct use of the MH model (Eq. 82) with the 7)/ values obtained from a superexchange model (Eq. 94), as discussed in Section 1.5.1, were also employed for a comparison of optical and thermal J)y values [135]. In this case the thermal T/y values were inferred indirectly from the estimated resonance-exchange component (AG() of the free energy change (AG, ) associated with the measured com-proportionation equilibrium between the 3-(-/3-(-, 2+/2+, and 2+/3+ oxidation states. According to the perturbative model adopted for the Robin/Day class-ll system [136], one obtains... 

This form illustrates the superexchange coupling via off-resonance intermediate electronic states. is the energy gap between the... 

Equations 7-3 and 7-6 represent the two limits of molecular conductivity in a simplified useful form directly useful in real data analysis. One limit is superexchange with electron tunneling through strongly off-resonance electronic levels. The other one is hopping with successive electronic population and depopulation. By their physical nature and the form of the equations, the two modes are conceptually and physically quite different. Other systems and formal aspects are discussed in Chapter 8. 

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