A High-Frequency Electrochemiluminescent Investigation of the Reaction Pathway Between Tris(2,2'-bipyridyl)ruthenium(II) and Tripropylamine Using Carbon Fiber Microelectrodes

The electrochemiluminescent (ECL) reaction mechanism between tris(2,2 ' -bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) and tripropylamine (TPrA) in aqueous solution at pH 7.4 was examined using fast potential pulses at carbon fiber microelectrodes. High-stability ECL emission was found with 0.5 ms pulses and a 25% duty cycle. In addition, stability was increased with negative rest potentials. Direct evidence for the strongly reducing free radical intermediate was obtained when the light was quenched upon addition of nitrobenzene derivatives. The formation of this free radical becomes rate-limiting at high concentrations of Ru(bpy))(3)(2+) and TPrA, as its production can be limited when there is an insufficient supply of one or both of its precursors (TPrA and TPrA(.+)) relative to Ru(bpy)(3)(2+) When TPrA is insufficient excess of Ru(bpy)(3)(2+), the ECL efficiency (photons emitted/Ru(bpy)(3)(3+) generated) was determined to be very high (similar to 90%) by comparison to a Ru(bpy)(3)(2+)/Ru(bpy)(3)(+) standard in acetonitrile. Rapid potential pulses also generated ECL from Ru(bpy)(3)(2+) when other tertiary amines, trimethylamine, diisopropylethylamine, and histamine, were used as co-reactants. The secondary amine epinephrine also produced light, but not norepinephrine, a primary amine.