Bimetallic N-Heterocyclic Carbene−Iridium Complexes: Investigating Metal−Metal and Metal−Ligand Communication via Electrochemistry and Phosphorescence Spectroscopy

Bimetallic [Ir(COD)Cl] and [Ir(ppy)2] (COD = 1,5-cyclooctadiene; ppy = 2-phenylpyridyl) complexes bridged by 1,7-dimethyl-3,5-diphenylbenzobis(imidazolylidene) (1), in addition to their monometallic analogues supported by 1-methyl-3-phenylbenzimidazolylidene (2), were synthesized and studied. Electrochemical analyses indicated that 1 facilitated moderate electronic coupling between [Ir(COD)Cl] units (ΔE = ∼60 mV), but not [Ir(ppy)2]. The metal-based oxidation potentials for the bimetallic complexes were within 20 mV of those for their monometallic analogues. Furthermore, spectroscopic analyses of the [Ir(ppy)2] bimetallic and monometallic complexes revealed nearly identical phosphorescence profiles, indicating that carbene coordination does not affect the energy of the emissive states. Collectively, these results suggest that N-heterocyclic carbenes (NHCs) such as 1 could link together two emissive fragments without altering their fundamental phosphorescence profiles. Ultimately, employing multitopic NHCs as non-interfering molecular connectors could facilitate the rational design of new phosphorescent materials as well as second-generation phosphor dopants.