highlight 4 2021

The driving force, ΔG, of photoinduced electron transfer between the donor and acceptor species is a key parameter for describing observed. This parameter is typically calculated from the Weller equation which expresses ΔG as the sum of the donor and acceptor reduction potentials and a term C, which uses Coulomb’s law to account for the free energy difference between the neutral reactants and charged products. The magnitude of C in polar solvents is debated, but cannot be measured directly. To address this issue, we used umbrella sampling molecular dynamics simulations  to determine C by taking the difference, Δw(r), of the potentials of mean force between a number of neutral and ionic donor/acceptor pairs in various solvents (J. Phys. Chem. B, 124, 9945−9950). The simulations demonstrate that C is typically 2 times more negative than calculated using Coulomb’s law, which can be a large effect due to the exponential dependence of kET on ΔG. One can recover the correct value of C by using the π-stack contact distance in the Weller equation instead of the van der Waals radius, but this is due to a fortunate cancellation of the compensating effects of overestimating the dielectric screening at contact and neglecting charge dilution and desolvation.

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Eric Vauthey

Physical Chemistry Department - Sciences II - University of Geneva
30, Quai Ernest Ansermet - CH-1211 Geneva 4 (Switzerland)


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