Electrogenerated chemiluminescence (ECL) coreactants are crucial towards their roles in forming radical intermediates for chemical oxidation or reduction of the ECL emitters, such as rubrene (RUB) or 9,10-diphenylanthracene (DPA), to form excited state species that emit light. During the cathodic potential scanning, ECL coreactants such as benzoyl peroxide (BPO) and ammonium persulfate were believed to generate benzyloxy free radical (C₆H₅CO₂^• or BPO^•) and sulfate anion radical (SO₄^-•), respectively, resulting in the chemical oxidation of the ECL emitter and eventually the ECL production. The direct electron paramagnetic resonance (EPR) evidence of the above radical intermediates, generated via chemical reduction of the ECL coreactant benzoyl peroxide in MeCN and ammonium persulfate in water using hydrazine as the reducing agent and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent, was obtained. Additionally, EPR spectra of C₆H₅CO₂^• (without DMPO trapping) generated from solid state using UV irradiation were consistent with the spectra obtained from solution phase. The EPR spectra of SO₄^-•(with DMPO trapping) were also obtained. Simulations of the experimentally acquired EPR spectra were conducted with EPR simulating software, such as PEST winsim.

Financial support from the NSF CAREER Award (CHE-0955878) is gratefully acknowledged.