Effects of electron-rich ancillary ligands on green and yellow-emitting bis-cyclometalated iridium complexes

Six new green to yellow-emitting heteroleptic bis-cyclometalated iridium(III) complexes of the type Ir(C⁁N)2(L⁁X) (C⁁N = cyclometalating ligand, L⁁X = monoanionic chelating ancillary ligand) bearing two widely used cyclometalating ligands (C⁁N = 2-(2-thienyl)pyridine (thpy) and 2-phenylbenzoxazole (bo)) and six different ancillary ligands were prepared. In this study, the complexes include structurally diverse ancillary ligands that allow us to investigate several aspects of structure-property relationships. Ancillary ligands used in this study are small-bite-angle N-phenylacetamidate (paa), N-isopropylbenzamidate (ipba) and N,N′-diisopropylbenzamidinate (dipba), and larger bite-angle β-ketoiminate (acNac), β-diketiminate (NacNac), and β-thioketoiminate (SacNac). The emission color is governed by the choice of the cyclometalating ligand, but the ancillary ligands influence the electrochemical and photophysical properties. Electrochemical analysis shows that the energy of the HOMO varies substantially as the L⁁X structure is altered, whereas the energy of LUMO remains nearly constant. The emission maxima range from 537 nm to 590 nm, with solution quantum yields between 0.0094 and 0.60 and microsecond lifetimes. The results here reveal the ancillary ligands provide a channel to control redox properties and excited-state dynamics in cyclometalated iridium complexes that luminesce in the middle regions of the visible spectrum.