Electrochemiluminescence of Heteroleptic Iridium(III) Complexes Featuring Substituted 2,4-diphenylpyridinato Ligands

Five heteroleptic iridium(III) complexes bearing cyclometalated 2,4-diphenylpyridinato ligands and a 4,4'-dimethyl-2,2'-bipyridine ancillary ligand were synthesized. Modulation of substituents at the 4'-position of the cyclometalated phenyl ring provided systematic variation in oxidation potentials, emission wavelengths, and luminescence quantum yields, used herein to interrogate the relationship between electronic structure and electrochemiluminescence (ECL) response. With tri-n-propylamine as co-reactant, the relative ECL intensities of the complexes depended strongly on the excitation pathway, for which distinct optima in co-reactant concentration were also identified. The complexes bearing more electron-withdrawing substituents exhibited the greatest ECL intensities via the reductive excitation (direct) pathway, but rendered the oxidative excitation (indirect) pathway energetically inaccessible. Under cathodic potentials with benzoyl peroxide as co-reactant, the unsubstituted complex exhibited the highest ECL intensities within the series, exceeding those of [Ru(bpy)₃]²⁺ under selected conditions. These findings highlight that ECL performance is governed by luminescence efficiency, the excitation pathway and energetic alignment with the co-reactant system, and that even subtle changes in luminophore properties can have multiple, sometimes competing, effects on ECL intensity.