Rate-Determining Step in the Electrogenerated Chemiluminescence from Tertiary Amines with Tris(2,2'-bipyridyl)ruthenium(II)

The rates and mechanism of coreactant electrogenerated chemiluminescence (ECL) from tris(2,2-bipyridyl)- ruthenium(II) (Ru(bpy)3 2+) and the tertiary amines, tripropylamine (TPrA) and trimethylamine (TMeA), in aqueous solution were investigated. Transient (0.5 ms) potential steps were used with microelectrodes to investigate the emission time course under a variety of solution conditions. With amine concentrations that are low with respect to Ru(bpy)3 2+, the emission rises continually during the transient potential step and decays slowly after its termination. In contrast, the emission approaches a plateau during the potential step and is rapidly extinguished afterward with concentrations of Ru(bpy)3 2+ that are much lower than the amine concentration. At intermediate pH values, the emission intensity increases approximately linearly with pH. The emission after the potential step is unaffected by the rest potential. To simulate these temporal characteristics by finite difference methods, a mechanism employing 15 discrete chemical and electrochemical steps was employed, using literature-based thermodynamic values and electron-transfer rate constants evaluated from Marcus theory. The rate-limiting step was found to be the deprotonation of the amine radical cation. In addition, the simulations required a rate constant for the homogeneous oxidation of the tertiary amine by electrogenerated Ru(bpy)3 3+ value much below its Marcusian-calculated value to match the experimental data.