Abstract
Herein, we report the thermodynamics, kinetics, and mechanism for electrochemical proton-coupled electron transfer involving the anthracene-based azo dye azo-OMe. The peak reduction potential of azo-OMe with organic acids spanning the pKa range of 2.6–23.51 shows a dependence upon the pKa of the acid when the acid pKa falls between 8 and 20 (in acetonitrile). A potential-pKa diagram is constructed and used to estimate the pKa of the azo-OMe species. Heterogeneous electron-transfer rate constants are obtained using rotating disk electrode voltammetry in combination with Koutecký–Levich and Tafel analysis. Electrochemical analysis shows that the reactions are diffusion limited and are kinetically controlled by the electron-transfer step. Kinetic isotope studies indicate a concerted proton, electron transfer event occurs in the pKa-dependent range when using trifluoroacetic acid.
Highlights
Nature has evolved a mechanistic advantage in carrying out electron-transfer steps, as well as associated chemical reactions, by coupling the movement of electrons along reaction pathways to proton motions
Kinetic isotope studies indicate a concerted proton, electron transfer event occurs in the pKa-dependent range when using trifluoroacetic acid
The coupling of proton and electron movements play an important role in energy conversion reactions, energy storage, and small molecule activation reactions.[1,2,3,4,5]
Summary
Electrochemical proton-coupled electron transfer of an anthracene-based azo dye†. We report the thermodynamics, kinetics, and mechanism for electrochemical proton-coupled electron transfer involving the anthracene-based azo dye azo-OMe. The peak reduction potential of azo-OMe with organic acids spanning the pKa range of 2.6–23.51 shows a dependence upon the pKa of the acid when the acid pKa falls between 8 and 20 (in acetonitrile). A potential-pKa diagram is constructed and used to estimate the pKa of the azo-OMe species. Heterogeneous electron-transfer rate constants are obtained using rotating disk electrode voltammetry in combination with Koutecky–Levich and Tafel analysis. Electrochemical analysis shows that the reactions are diffusion limited and are kinetically controlled by the electron-transfer step. Kinetic isotope studies indicate a concerted proton, electron transfer event occurs in the pKa-dependent range when using trifluoroacetic acid
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