Multielectron Charge Storage Enables Cascade Electrochemiluminescence in an Atomically Precise Silver Nanocluster

Atomically precise metal nanoclusters combine discrete electronic states with access to multiple redox levels, enabling multielectron charge storage and electrochemical light generation. Herein, cascade electrochemiluminescence (ECL) is demonstrated for the silver nanocluster Ag₆PL₆ (PL = 4-phenylthiazolidine-2-thione) with benzoyl peroxide (BPO) as a coreactant. Three sequential reductions of Ag₆PL₆ enable multielectron charge storage, while BPO reduction generates the oxidizing benzoate radical, which successively oxidizes the three reduced cluster states, enabling cascade ECL. The cascade ECL regime is 460 times more efficient than the first regime, indicating substantial enhancement through the cascade mechanism. The cascade ECL peak wavelength is red-shifted by 60 nm relative to the neutral-state photoluminescence (665 versus 605 nm), and photoluminescence under reducing conditions also exhibited a red-shift, supporting the assignment of cascade ECL to reduced nanocluster states. Finite-element simulations of ECL transients were fitted to experimental transients to deconvolute reaction kinetics from species diffusion, yielding rate constants of (2.5 ± 0.6) × 10³, (3.0 ± 1.2) × 10⁴, and (3.0 ± 1.2) × 10⁴ M^-1 s^-1 for reactions of Ag₆PL₆ ^•-, Ag₆PL₆ ^2-, and Ag₆PL₆ ^•3- with the benzoate radical, respectively. These results quantitatively connect multielectron charge storage, charge-state-dependent emission, and cascade ECL kinetics in an atomically precise nanocluster.