Electrogenerated Chemiluminescence of the Tris(2,2′-bipyridine)ruthenium(II)/Tri-n-propylamine (TPrA) System: Crucial Role of the Long Lifetime of TPrA^•+ Cation Radicals Suggested by Electrode Surface Effects

We describe the effects of electrochemical pretreatment of glassy carbon electrodes (GCEs) on the electrogenerated chemiluminescence (ECL) of the Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine)/tri-n-propylamine (TPrA) system. It has been found that the oxidation of TPrA at the anodically pretreated GCEs became more facile, while the intensity of ECL could be greatly suppressed. The contravention between the electrochemical reaction kinetics and the light emission intensity of the coreactant ECL system was reported for the first time and has been attributed to the rapid deprotonation of TPrA^•+ cation radicals by the aid of oxygen-containing surface species formed on the GCE during the pretreatment. Because the lifetime of TPrA^•+ was reduced, the products of the deprotonation reaction, TPrA^• free radicals, would be more subject to oxidative consumption on the electrode subsequently, leading to weaker ECL signals. When the ECL was produced mainly following a catalytic route, however, the electrode surface effect was much less significant. This study suggests that the long lifetime of TPrA^•+ cation radicals may be crucial for the intense light emission, which allows a sufficient amount of highly reducing intermediate radicals, TPrA^•, to participate in the ECL process within a relatively thick reaction layer before being destroyed by the electrode.