Iridium Complexes of N‑Heterocyclic Carbene Ligands: Investigation into the Energetic Requirements for Efficient Electrogenerated Chemiluminescence

A series of five heteroleptic Ir­(III) complexes of the general form Ir­(ppy)2(C∧C:) have been prepared (C∧C represents a bidentate cyclometalated phenyl-substituted imidazolylidene ligand). The five complexes arise from the cyclometalated phenyl ring of the NHC ligand being unsubstituted or having electron-donating (OMe and Me) or electron-withdrawing (Cl and F) groups at the 2- and 4-positions of the ring. The synthesized phenyl-substituted imidazole precursors, imidazolium salts, and Ir­(III) complexes have been characterized by elemental analysis, NMR spectroscopy, cyclic voltammetry, and electronic absorption and emission spectroscopy. The molecular structures for two imidazolium salts and two Ir­(III) complexes were determined by single-crystal X-ray diffraction. Each of the Ir­(III) complexes exhibited intense photoluminescence in acetonitrile solution at room temperature with quantum yields (ϕp) ranging from 42% to 68% and excited-state lifetimes on the order of 2 μs. Voltammetric experiments revealed one formal metal-based oxidation process and two ligand-based reductions for each complex. All complexes gave moderate to intense annihilation electrochemiluminescence (ECL); however, only the fluorinated complex produced significant coreactant ECL. The combined electrochemical, spectroscopic, and theoretical investigations offer insights into the reasons for this behavior and suggest useful strategies for the design of ECL emitters. A plot of oxidation potential versus emission color is proposed as a convenient reference guide to aid in the prediction of energy sufficiency in ECL reactions.