Computational Analysis of Electron Transfer Kinetics
for CO2 Reduction with Organic Photoredox Catalysts
Posted on 2020-06-18 - 13:06
We present a fundamental
description of the electron transfer (ET)
step from substituted oligo(p-phenylene) (OPP) radical
anions to CO2, with the larger goal of assessing the viability
of underexplored, organic photoredox routes for utilization of anthropogenic
CO2. This work varies the electrophilicity of para-substituents
to OPP and probes the dependence of rate coefficients and interfragment
interactions on the substituent Hammett parameter, σp, using constrained density functional theory (CDFT)
and energy decomposition analysis (EDA). Large electronic couplings
across substituents indicates an adiabatic electron transfer process
for reactants at contact. As one might intuitively expect, free energy
changes dominate trends in ET rate coefficients in most cases, and
rates increase with substituent electron-donating ability. However,
we observe an unexpected dip in rate coefficients for the most electron-donating
groups, due to the combined impact of flattening free energies and
a steep increase in reorganization energies. Our analysis shows that,
with decreasing σp, flattening OPP
LUMO levels lower the marginal increase in free energy. EDA reveals
trends in electrostatics and charge transfer interactions between
the catalyst and substrate fragments that influence free energy changes
across substituents. Reorganization energies do not exhibit a direct
dependence on σp and are largely
similar across systems, with the exception of substituents containing
lone pairs of electrons that exhibit significant deformation upon
electron transfer. Our study therefore suggests that while a wide
range of ET rates are observed, there is an upper limit to rate enhancements
achievable by only tuning the substituent electrophilicity.
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Kron, Kareesa
J.; Gomez, Samantha J.; Mao, Yuezhi; Cave, Robert J.; Mallikarjun Sharada, Shaama (2020). Computational Analysis of Electron Transfer Kinetics
for CO2 Reduction with Organic Photoredox Catalysts. ACS Publications. Collection. https://doi.org/10.1021/acs.jpca.0c03065
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AUTHORS (5)
KK
Kareesa
J. Kron
SG
Samantha J. Gomez
YM
Yuezhi Mao
RC
Robert J. Cave
SM
Shaama Mallikarjun Sharada
KEYWORDS
OPP LUMO levelsOrganic Photoredox Catalystsanthropogenic CO 2CDFTσ penergy decomposition analysisrate coefficientsElectron Transfer Kineticssubstituent electron-donating abilityelectron transfercharge transfer interactionsadiabatic electron transfer processsubstituent Hammett parameterCO 2 Reductionreorganization energiesEDAenergy changesET rate coefficients