Measuring Electron-Transfer Reaction Kinetics in Single Electrochemiluminescence Events at an Ultramicroelectrode for the Ru(bpy) ₃ ²⁺ /Tri- n- propylamine Coreactant System

Electrochemiluminescence (ECL) provides a unique analytical platform for probing electron-transfer kinetics via time-resolved photon detection. In coreactant ECL systems, however, quantitative determination of individual homogeneous rate constants has remained challenging due to the simultaneous operation of multiple reaction pathways and limited access to fast dynamics. Herein, we report time-resolved ECL methodologies that enable experimental determination of charge-transfer rate constants governing all three excited-state generation mechanisms in the Ru(bpy)₃²⁺/tri-n-propylamine (TPrA) coreactant system. By combining ultramicroelectrode-based potential pulsing, alongside nanosecond-resolved photon counting with two-dimensional finite-element simulations, we selectively suppress or activate coreactant pathways and extract electron-transfer rate constants from fast ECL transients in single events. The rapid regime (<0.5 ms) was discovered to be highly sensitive to reaction kinetics and reliably accessed by minimizing capacitive and solution-resistance effects with ultramicroelectrodes. Rate constants for reactions of TPrA radical cation (TPrA+•) with Ru(bpy)₃+• (k₁) and of TPrA radical (TPrA•) with Ru(bpy)₃3+• (k₃) were determined to be (1.7 ± 0.6) × 10⁷ M⁻¹ s⁻¹ and (9.3 ± 2.1) × 10⁹ M⁻¹ s⁻¹, respectively. Together with prior annihilation-ECL studies, this work establishes a general, time-resolved ECL framework for quantitatively probing electron-transfer kinetics that is readily extendable beyond the Ru(bpy)₃²⁺/TPrA system to other luminophores and coreactants.