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 ' -bi pyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) and tripropylamine (TPrA) in aqueous solution at pH 7.4 was examined using fast potential pulses at carbon fibe r microelectrodes. High-stability ECL emission was found with 0.5 ms pulses and a 25% duty cycle. In addition, stability was increased with negative r est potentials. Direct evidence for the strongly reducing free radical inte rmediate was obtained when the light was quenched upon addition of nitroben zene derivatives. The formation of this free radical becomes rate-limiting at high concentrations of Ru(bpy))(3)(2+) and TPrA, as its production can b e limited when there is an insufficient supply of one or both of its precur sors (TPrA and TPrA(.+)) relative to Ru(bpy)(3)(2+) When TPrA is insufficie nt 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 comparis on to a Ru(bpy)(3)(2+)/Ru(bpy)(3)(+) standard in acetonitrile. Rapid potent ial pulses also generated ECL from Ru(bpy)(3)(2+) when other tertiary amine s, trimethylamine, diisopropylethylamine, and histamine, were used as co-re actants. The secondary amine epinephrine also produced light, but not norep inephrine, a primary amine.