Phosphorescent Cationic Heterodinuclear IrIII/MI Complexes (M=CuI, AuI) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge

A novel class of phosphorescent cationic heterobimetallic Ir-III/M(I)complexes, where M-I=Cu-I(4) and Au-I(5), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an Ir(III)and a M(I)center, respectively. Complexes 4and5have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M-I(IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4and5display intense red emission arising from a low-energy excited state that is located onto the Ir(C<<^>>N) moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer ((IL)-I-3/(MLCT)-M-1) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating Ir(III)complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4and5due to the smaller energy gap between the(3)IL and(1)MLCT manifolds, which mix via spin-orbit coupling.