Electrochemical proton-coupled electron transfer of an anthracene-based azo dye

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 pK(a) range of 2.6–23.51 shows a dependence upon the pK(a) of the acid when the acid pK(a) falls between 8 and 20 (in acetonitrile). A potential-pK(a) diagram is constructed and used to estimate the pK(a) 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 pK(a)-dependent range when using trifluoroacetic acid.