Donor acceptor solvent reorganization energy

Depending on the solvent polarity and redox potentials of a donor and an acceptor, the ions resulting from electron transfer may remain associated either as a contact IRP or as a solvent-separated IRP. In the contact pair, back electron transfer can take place. For such electron back-transfers, the solvent reorganization energy is less than 5% of the total reorganization energy (Serpa and Arnaut 2000). 

Rather full calculations of /. (r) vs. r for various p values must be compared to the experimental results to determine p. Equation (6) gives a widely used expression for solvent reorganization energy that can be substituted into k expressions. It was derived by Marcus over 40 years ago and is both simple and useful [61]. It models the donor and acceptor as two conducting spheres imbedded in a dielectric continuum. 

In a second paper, Sanders and coworkers presented a simple model mediated by pyridine ligation to zinc porphyrin. " In the donor-spacer-acceptor system, photoinduced ET occurs with a rate con.stant of 2.13 x I0 s for zinc mesoporphyrin II dimethyl ester 141 and 53.3 X lO s" for zinc tetraphenylporphyrin, while the recombination rate constants were 6.35 x 10 s and 3.81 X 10 s , respectively. Although the two porphyrins have similar redox potentials, the forward ET rates were unambiguously different. The authors speculated that the two photodonor porphyrins have different solvent reorganization energies. 

On the electrolyte side. Figure 1.12 shows the probability density distributions W for finding a single R or O species in the electrolyte with energy e of its donor or acceptor state. The maxima of these distributions are shifted by 2X, where X is the solvent reorganization energy. Probability density distributions of finite width arise because of thermal fluctuations of the solvation state of R and O in solution. They... 

The rate constant (kst) of the electron transfer reaction from an electron donor (D) and an electron acceptor (A) is related to the electron coupling matrix element between D and A, ( V ), standard Gibbs free energy change of the electron transfer step (AG°), and the sum of molecular and solvent reorganization energies (1) by [40]... 

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