Influence of nature, concentration and pH of buffer acid-base system on rate determining step of the electrochemiluminescence of Ru(bpy)(3)(2+) with tertiary aliphatic amines

The electrogenerated chemiluminescence (ECL) of Ru(bpy)3 2+ (bpy = 2,2-bipyridyl) with tertiary aliphatic amines as co-reactants, was theoretically and experimentally studied as a function of the pre-equilibria involved in the ammonium proton lost and in relation to the nature of the rate determining step. Transient potential steps were used with a 3-mm glassy carbon disk electrode or carbon fiber ultramicroelectrodes array to investigate emission behavior in a variety of aqueous solution types, containing phosphate, tartrate and phthalate acid–base systems at differing pH values. The emission of Ru(bpy)3 2+ resulting from the reaction with n-tripropylamine (TPrA), tri-isobutylamine (TisoBuA), n-tributylamine (TBuA), methyl-di-n-propylamine (MeDPrA) and triethylamine (TEtA) in varying acid–base media was interpreted on the basis of the quoted pre-equilibria, ammonium pKa being known. The nature of the rate determining steps changes depending on pH. Above pH≈5 the amine neutral radical formation is the rate determining step and, is independent of pH with rate constant close to 103 s−1; below pH≈5 the rate determining step becomes the deprotonation of the ammonium ion, operated by different bases present in solution. Different amines in the same acid–base system showed analogous ECL behavior, conditioned by the chosen acid base system. A single amine in different acid–base systems showed different kinetic behaviors, due to the dissociation constants of the chosen buffers. The concentration of the acid–base system also played an important role and influenced emission intensity and shape. ECL emission were simulated by finite difference methods, implementing a previously proposed mechanism by including the relevant pre-equilibria. Simulation may also give estimates of the pKa values of the ammonium ions. An ion pair formation between R3N•+ and the mostly charged species present in solution is hypothesized to explain the contradictory experimental results concerning the reaction mechanism of the proton lost of the radical cation.