Determining the annihilation electrochemiluminescence rate constant of protoporphyrin IX dimethyl Ester

Electrochemiluminescence (ECL) turns out to be a powerful tool to probe the bimolecular electron-transfer dynamics of the reactive species in various reaction systems. Herein, we employed fast photon counting data acquisition for ECL transients in the annihilation pathway under controlled potential pulsing experiments at a 2 mm diameter platinum electrode, and a one-dimensional COMSOL-based model developed for the system to investigate the bimolecular annihilation rate constant (k ann ) of an electrochemically active porphyrin, protoporphyrin IX dimethyl ester (Pro-IX). Simulated ECL transients of the Pro-IX system with various defined rate constants were superimposed on experimentally recorded ones for extracting the intrinsic annihilation kinetics. The k ann for the annihilation reaction between oxidized and reduced Pro-IX radical species was estimated to be (6.0 ± 0.1) × 10 5 M −1 ·s −1 . This value provides an insight into the interplay between molecular electronic structure, redox potential alignment, and excited-state formation efficiency. This study presents the general applicability of transient ECL measured along modelling as a quantitative tool for characterizing annihilation dynamics beyond the classical Ru(bpy) 3 2+ system, empowering the design of new luminophores and applications in advanced ECL sensing and imaging. • Experimental ECL transients of Pro-IX reveal annihilation ECL reaction rates. • CV fitting yields radical generation/decay kinetics for Pro-IX. • COMSOL 1D simulations extract k ann from ECL transients at macroelectrodes.