Electrogenerated chemiluminescence with amine and benzoyl peroxide coreactants: reactivity and reaction mechanism studies
The electrogenerated chemiluminescence (ECL) of tris(2,2′- bipyridine)ruthenium(II), Ru(bpy)3 2+, was investigated using amine coreactants. When amine coreactants were oxidized, they produced both sufficiently strong reducing (R2NC• HR′) and oxidizing (R2N•+ CH2R′) agents and these agents were relatively stable to diffuse and react with Ru(bpy)3 2+ or electrochemically generated Ru(bpy)3 3+ to generate Ru(bpy)3 2+*. Most amine coreactants produced two ECL waves in a plot of ECL vs electrode potentials: the first occurred before the oxidation of Ru(bpy)3 2+ and the second occurred immediately after the oxidation of Ru(bpy)3 2+, implying two different ECL mechanisms participate in the first and second ECL, except for N, N, N′, N′-tetra-n-propylmethane diamine and N, N, N′, N′-tetra-n-propylethylene diamine. ECL produced by monoamine coreactants gave stronger emissions than diamines, and acyclic amines produced stronger ECL than heteroalicyclic amines. The ECL intensity increased as the number of carbon atoms increased in the same group of amines except for those with the piperazine group. Tri-n-propylamine produced the most intense first and second ECL. Benzoyl peroxide (BPO) was studied as a new coreactant for neutral red (NR), ter(9,9-diarylfluorene)s (TDAFs) and Ru(bpy)3 2+. When BPO was reduced, it produced the strong oxidizing agent, C6H5CO2 • via an ECE process. Then electrochemically reduced NR•−, TDAF•−, or Ru(bpy)3 + reacted with C6H5CO2 • to produce ECL. At a high concentration of BPO (e.g. > 10 mM compared to 0.5 mM NR), ECL quenching by BPO was significant, as confirmed by fluorescence quenching experiments. NR could be used as both a pH indicator and ECL emitter in acetonitrile. A yellow-to-red color change was observed when a NR solution was titrated with acids. The protonated form of NR (NRH+ ) did not produce ECL due to two-electron reduction of NRH+ and lack of sufficient energy for the ECL reaction. However, NR generated ECL emission of 610 nm. TDAFs emitted at ∼ 400 nm (blue ECL). In annihilation ECL, significant emission of TDAF excimers, mainly formed by coulombic interactions when radical cations were annihilated by radical anions, interfered with their blue emission (φECL ≤ 0.05 %) Excimers were not observed in fluorescence. No significant excimer emission was found when BPO coreactant was used.