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.