Electron transfer process rate attenuation

In addition photoexcitation can also result in the transfer of an excited state electron to a distant acceptor group resulting in charge separation. This process can be understood within the framework of Marcus theory and subsequent more sophisticated theoretical treatments.2,5 The rate of electron transfer (ke]) drops with distance according to an attenuation factor / el ke °c exp(—/ el /yB) where /Xb is the distance between donor and acceptor components A and B. When the donor and acceptor components are separated by a vacuum J3el is estimated to be ca. 2-5 A-1. However when some kind of material substance is involved such as a bridge L the electron transfer process can be... 

The dependence of the logarithm of the rate constants of electron transfer (log kEr) between the donor (D) and acceptor (A) centers on the D-A distances is similar to the correspondent dependence for the superexchange attenuation coefficient (log Yet) (Fig.2.6). Therefore, we can conclude that, similar to primary events in RCs from bacteria, the primary fast electron transfers take place as nonadiabatic and conformationally nonequilibrium processes. 

Recent studies of the dynamics of electron-transfer reaction between Et and Z conducted with femtosecond resolution [15] provided further evidence that rates of DNA-mediated electron transfers are not greatly attenuated by distance. Biphasic kinetics were detected with time constants of 5 ps and 75 ps. Only the absolute amplitudes of the electron-transfer rates measured in this high-resolution study changed as a function of distance, supporting the idea that the distance dependencies of some DNA-mediated processes include contributions from events (e.g., base dynamics) occurring on a slower timescale. 

Fig. 2.9 Analysis of the exchange energy transfer mechanism in terms of electron-and hole transfer processes. The relationships between the rate constants and the attenuation factors of the three processes are also shown...

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