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 (H _DA ) 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 _DA > 1,000 cm ^-1 ; and a xylyl-thiophene bridge (x) that prevents planarization and decreases H _DA < 100 cm ^-1 without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, K _eq In agreement with theory, K _eq (p) were closer to unity compared to K _eq (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.
Competing Interests: The authors declare no conflict of interest.