The performance of electrochemically induced chemiluminescence (ECL) assays strongly depends on the stability of electrogenerated coreactant radicals, which governs the spatial extension of the ECL emitting layer. This dependence is even more pronounced for heterogeneous bead-based ECL, where the emission efficiency is widely assumed to scale proportionally with the thickness of the ECL emitting layer. To date, however, establishing a consistent relationship between ECL intensity and radical stability has remained elusive: apparent rate constants fail to isolate intrinsic deprotonation kinetics, and the rational design of amines with long-lived radical cations remains experimentally challenging. In this work, using ECL microscopy, we address both issues by correlating the ECL intensity from individual Ru(bpy)32+-labeled beads with the intrinsic deprotonation kinetics of model electrogenerated radical cations, including a novel long-lived intermediate arising from an aniline derivative. Kinetic constants derived from density functional theory (DFT) calculations, in the framework of a hybrid cluster-continuum approach, disentangled intrinsic radical decomposition rates from experimental artifacts such as concentration, pH, and buffer effects. This combined electrochemical imaging and computational approach reveals a non-monotonic, volcano-type dependence between the deprotonation rate of radical cations and ECL intensity, defining an optimal stability window for efficient ECL emission.
Fracassa et al. (Mon,) studied this question.