Highly Emissive Red Heterobimetallic Ir^III/M^I (M^I = Cu^I and Au^I) Complexes for Efficient Light-Emitting Electrochemical Cells

A class of four bright heterobimetallic emitters is presented, which features both a chromophoric [Ir(C^N)₂] center and a positively charged, linear bis-NHC M(I) ancillary moiety bridged through a Janus-type ligand. An in-depth investigation of their optical and electrochemical properties is also reported, which are further elucidated by means of time-dependent density functional theory including spin-orbital coupling effects. All complexes display efficient, vibrant red phosphorescence with a higher quantum yield and a faster radiative rate constant compared to the mononuclear parental complexes. This effect was elucidated in terms of better S-T excited-state mixing as well as increased rigidity favored by the multimetallic architecture. Finally, their electroluminescence performances are investigated by using these bimetallic complexes as electroactive materials in light-emitting electrochemical cells, achieving external quantum efficiency values up to 6% and resulting in among the most efficient ones for red emitters. This result is also attributable to the charge-neutral nature of an emitting Ir complex bearing a charged, wider energy gap, metal complex as an ancillary moiety.