Solvent and Temperature Effects on Photoinduced Proton-Coupled Electron Transfer in the Marcus Inverted Region.

Laura F Cotter,James M Mayer,Leif Hammarström,Belinda Pettersson Rimgard,Giovanny A Parada

doi:10.1021/acs.jpca.1c05764

Laura F Cotter, James M Mayer + Show 3 more

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https://doi.org/10.1021/acs.jpca.1c05764

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Abstract

Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science2019, 364, 471–475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene–phenol–pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (−ΔG°) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (ΔG*CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in ΔG° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B1999, 103, 9130–9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both ΔG°CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

Highlights

The thermochemistry and kinetics of electron transfer (ET) and proton transfer (PT) are often intimately correlated in processes known as proton-coupled electron transfer (PCET) reactions.[1−10] These associated electron−proton transfer reactions are critical to numerous fundamental energy conversion processes, from photosynthesis and respiration to combustion and fuel cells
Stepwise proton transfer followed by electron transfer (PTET) and electron transfer followed by proton transfer (ETPT) were excluded by considering that the PT step of PTET should be significantly uphill (ΔpKa > 10, for phenol and pyridinium in MeCN2), which would not allow for the observed rate constant of ∼1011 s−1
Satisfactory fits to the data could be obtained using a simplified model with a single vibronic transition that represents a weighted average of the contributing vibronic transitions and using molecular solvent models (MSMs) to correct for the temperature dependence of the activation barrier

Summary

Introduction

The thermochemistry and kinetics of electron transfer (ET) and proton transfer (PT) are often intimately correlated in processes known as proton-coupled electron transfer (PCET) reactions.[1−10] These associated electron−proton transfer reactions are critical to numerous fundamental energy conversion processes, from photosynthesis and respiration to combustion and fuel cells. Such processes may become even more favorable when high-energy intermediates can be bypassed via a concerted mechanism, where PT and ET occur in a single kinetic step (CPET).

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