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

Journal Article · · Proceedings of the National Academy of Sciences of the United States of America

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. 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 HDA> 1,000 cm–1; and a xylyl–thiophene bridge (x) that prevents planarization and decreases HDA< 100 cm–1without 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.

Research Organization:
University of North Carolina, Chapel Hill, NC (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
SC0013461
OSTI ID:
1457096
Alternate ID(s):
OSTI ID: 1540281
Journal Information:
Proceedings of the National Academy of Sciences of the United States of America, Vol. 115, Issue 28; ISSN 0027-8424
Publisher:
National Academy of SciencesCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 24 works
Citation information provided by
Web of Science

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Remote control of electronic coupling – modification of excited-state electron-transfer rates in Ru(tpy) 2 -based donor–acceptor systems by remote ligand design journal January 2019
Barriers for interfacial back-electron transfer: A comparison between TiO 2 and SnO 2 /TiO 2 core/shell structures journal January 2019
Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes journal January 2020