Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer
(2018) Proceedings of the National Academy of Sciences — Vol. 115, n° 28, p. 7248-7253 (2018)
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Sampaio, Renato N. 
- Abstract
- Electron-transfer theories predict that an increase in the quantum-mechanical mixing (HDA) of electron donor and acceptor wavefunctions at the instant of electron transfer drives equilibrium constants toward unity. Kinetic and equilibrium studies of four acceptor–bridge–donor (A-B-D) compounds reported herein provide experimental validation of this prediction. The compounds have two redox-active groups that differ only by the orientation of the aromatic bridge: a phenyl–thiophene bridge (p) that supports strong electronic coupling of HDA > 1,000 cm−1; and a xylyl–thiophene bridge (x) that prevents planarization and decreases HDA < 100 cm−1 without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, Keq. In agreement with theory, Keq(p) were closer to unity compared to Keq(x). A van’t Hoff analysis provided clear evidence of an adiabatic electron-transfer pathway for p-series and a nonadiabatic pathway for x-series. Collectively, the data show that the absolute magnitude of the thermodynamic driving force for electron transfers are decreased when adiabatic pathways are operative, a finding that should be taken into account in the design of hybrid materials for solar energy conversion.
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Sampaio, R. N., Piechota, E. J., Troian-Gautier, L., Maurer, A. B., Hu, K., Schauer, P. A., Blair, A. D., Berlinguette, C. P., & Meyer, G. J. (2018). Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer. Proceedings of the National Academy of Sciences, 115(28), 7248-7253. https://doi.org/10.1073/pnas.1722401115 (Original work published 2018)