Determining Bimolecular Rate Constants for Annihilation Reactions of Electrogenerated Ru(bpy)₃²⁺ Radical Ions by Means of Electrochemiluminescence Transients at a Conventional Electrode and an Ultramicroelectrode: Toward Single

This work presents a thorough analytical procedure for measuring ion-annihilation electrochemiluminescence (ECL) reaction kinetics exemplified with Ru(bpy)₃²⁺. Using a 2 mm diameter platinum electrode and a 25 μm diameter platinum ultramicroelectrode (UME), we captured the rapid kinetic profiles of ECL transients during potential-stepping experiments through the use of advanced time-resolved instrumentation. The associated bimolecular annihilation rate constant, <i>k</i>_ann, was modeled using two-dimensional (2D) finite element analysis in COMSOL Multiphysics software. The <i>k</i>_ann was estimated to be (9 ± 1) × 10⁹ M^-1 s^-1 by fitting simulated ECL transients to experimental data at frequencies of 0.5 Hz, 5 Hz, and higher frequencies. In fact, our 2D model simulating diffusion, reaction, and decay of electroactive species in the vicinity of both conventional macroelectrodes (2 mm) and UMEs has, for the first time, demonstrated effectiveness in extracting crucial chemical information (e.g., reaction mechanisms, intermediate species) and kinetic information (e.g., rate constants, reaction kinetics, mass transport dynamics) from electrochemiluminescence systems. It was discovered that ECL transients at the 25 μm UME represent single events, whereas those at the 2 mm electrode display ensemble ECL kinetic behavior.