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.