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

Abstract

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. Here, 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^–1without 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.

Authors:

Sampaio, Renato N. ;

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Piechota, Eric J.; Troian-Gautier, Ludovic; Maurer, Andrew B.; Hu, Ke; Schauer, Phil A.; Blair, Amber D.; Berlinguette, Curtis P.; Meyer, Gerald J.

Publication Date:

2018-06-25

Research Org.:

Univ. of North Carolina, Chapel Hill, NC (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1457096

Alternate Identifier(s):

OSTI ID: 1540281

Grant/Contract Number:  

SC0013461

Resource Type:

Published Article

Journal Name:

Proceedings of the National Academy of Sciences of the United States of America

Additional Journal Information:

Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 115 Journal Issue: 28; Journal ID: ISSN 0027-8424

Publisher:

Proceedings of the National Academy of Sciences

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Science & Technology; Other Topics; electron transfer; electronic coupling; Gibbs free energy; equilibrium; solar energy