Kinetics teach that electronic coupling lowers the free-energy change that accompanies electron transfer.
basic_science · Level V
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- Record sourced from PubMed, PMID 29941573.
- Also identified by DOI 10.1073/pnas.1722401115 and PMC identifier 6048547.
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Abstract
Electron-transfer theories predict that an increase in the quantum-mechanical mixing (H<sub>DA</sub>) 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 H<sub>DA</sub> > 1,000 cm<sup>-1</sup>; and a xylyl-thiophene bridge (x) that prevents planarization and decreases H<sub>DA</sub> < 100 cm<sup>-1</sup> without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, K<sub>eq</sub> In agreement with theory, K<sub>eq</sub>(p) were closer to unity compared to K<sub>eq</sub>(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.