Diiridium(III) Complexes with Fluorenylpyridyl Cyclometalating and μ₂-Oxamidato Bridging Ligands and their High Efficiency Phosphorescent Solution-Processed OLEDs

Three neutral diiridium(III) complexes with 2-fluorenylpyridyl (flpy) or 5-fluoro-2-fluorenylpyridyl (flpyF) as C^N cyclometalating ligands and a μ₂-oxamidato bridge have been synthesized. NMR spectroscopy shows that the complexes are inseparable mixtures of diastereomers (rac, ΔΔ/ΛΛ and meso, ΔΛ) with bridges in anti and syn configurations. Each isomer was determined using ¹⁹F NMR data on the flpyF complex. Single crystal diffraction studies of two complexes revealed meso diastereomers with anti configuration of the bis-(t-butylphenyl)oxamidato bridge but an uncertain configuration of the unsubstituted μ₂-oxamidato bridge. The complexes are highly emissive (Φ_PL 57–82 % in solution) with excited state lifetimes of τ_p ca. 40 μs. The vibronic emissions with maxima at 553–561 nm and at 587–595 nm in solution are attributed to mixed metal-ligand to ligand charge transfer (³MLLCT). Density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations establish the involvement of the fluorenyl groups and the vibronic structures in the emissions. The bridge mediates intramolecular interactions between iridium centers based on electrochemical measurements Phosphorescent organic light-emitting diodes (PhOLEDs) using these complexes as the emissive dopants with a solution-processed active layer have bright greenish-yellow emission with λ_max^EL ca. 560 nm, luminous efficiency up to 26 cd/A and high external quantum efficiency (maximum η_ext ca. 20 %).