Electron transfer in donor-bridge-acceptor supermolecules

6 Electron transfer in donor-bridge-acceptor supermolecules 

This equation, which is a particular case of eq. 4.50, has several interesting imphcations. It predicts a strictly exponential fall-off of (and hence with n (and hence Tda). Noting from eq. 4.52 that = i/A, where and are the values 

The effect of solvent can be investigated by adding discrete solvent molecules into the DBA ensemble (as well as a peripheral dielectric continuum). Computational results indicate that, where a covalent link connects D and A and fills the space between them, through-space and peripheral routes involving solvent molecules are unimportant. However, where solvent rather than a covalently linked bridge fills the space between D and A, the as-the-crow-flies pathway it provides will dominate D-A coupling and the ET reaction channel. 

Both the coupling element (eq. 4.43) and the transfer integral (eq. 4.51) fall off essentially exponentially with the distance between D and A. Substituting either of these equations into eq. 4.43 gives 

Exponential fall-off of with r A is to be expected only over a hmited range of distances. As r A becomes very short (with only one or two bridge units between D and A, or close contact between them), the D-A couphng is likely to become so strong that the reaction becomes adiabatic. The rate equation then takes the form cf. eq. 4.29) 

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