Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer

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 repor...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2018-07, Vol.115 (28), p.7248-7253
Main Authors: Sampaio, Renato N., Piechota, Eric J., Troian-Gautier, Ludovic, Maurer, Andrew B., Hu, Ke, Schauer, Phil A., Blair, Amber D., Berlinguette, Curtis P., Meyer, Gerald J.
Format: Article
Language:English
Subjects:
Adiabatic
Adiabatic flow
Chemical equilibrium
Chemical-mechanical polishing
Coupling
Electron transfer
electronic coupling
Electrons
Energy
Energy conversion
equilibrium
Experiments
Free energy
Gibbs free energy
INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Kinetics
Other Topics
Physical Sciences
Quantum physics
Science & Technology
Solar energy
Solar energy conversion
Unity
Wave functions
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Summary: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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1722401115