The photo-CE mechanism: the oxidation of tris-(p-tolyl)amine in acetonitrile at an illuminated platinum electrode

The oxidation of tris-(p-tolyl)amine (A) has been studied in acetonitrile solution using a channel electrode flow cell. In the dark a simple reversible one-electron oxidation to the corresponding (stable) radical cation is observed. However on irradiation with light of wavelength 300 nm a ‘pre-wave’ is additionally seen ca. 200 mV cathodic of the simple ‘dark’ process. Action spectrum measurements, preparative experiments and photocurrent/solution flow rate data are used to show that this process has the kinetic characteristics of a ‘photo-CE’ reaction in which the product of a photochemical pre-equilibrium involving A undergoes a disproportionation reaction forming an electroactive species. The nature of the species involved is deduced and a detailed mechanistic scheme presented. Rate and equilibrium constants are estimated. The ‘photo-CE’ reaction is shown to have the unusual property for an electrode reaction at a channel electrode that the photocurrents decrease with solution flow rate.