Dinuclear iridium(iii) complexes of cyclometalated fluorenylpyridine ligands as phosphorescent dopants for efficient solution-processed OLEDs

Abstract

Phosphorescent cyclometalated dinuclear iridium(III) complexes of fluorenylpyridine ligands bridged by bis(μ-Cl) and bis(μ-NCO) units have been synthesised and characterised by NMR, UV-vis absorption and emission spectroscopy, cyclic voltammetry, spectroelectrochemistry and X-ray diffraction. The complexes display green luminescence in solution from mixed ³π–π*/³MLCT (metal-to-ligand-charge transfer) states in the range of λ_max 542–552 nm. The solution PL quantum yields of 21–41% are remarkably high for diiridium complexes. Cyclic voltammetric and UV-visible spectroelectrochemical studies establish that the mixed valence cations are stable with the two Ir³⁺/Ir⁴⁺ centres interacting strongly. DFT/TD-DFT calculations provide further insights into the electronic and photophysical properties of the complexes, with good agreement between computed and experimental data. Solution-processed organic light emitting devices (OLEDs) using the diiridium complexes as the emissive dopants in poly(9-vinylcarbazole) (PVK) host polymer exhibit performances of up to 12 cd A⁻¹ and external quantum efficiencies of up to 4%. The device architecture is: ITO/PEDOT:PSS/PVK–complex (5 wt%)–PBD (40 wt%)/Ba/Al. These electroluminescence efficiency values are unusually high for dinuclear metal complexes and confirm that dinuclear iridium complexes are suitable phosphors for OLEDs using simple device architectures. Analogous mononuclear fac-Ir(III) complexes of fluorenylpyridine ligands are reported for comparison. They have PL quantum yields of 30–43%, device performances of up to 26 cd A⁻¹ and external quantum efficiencies of up to 8.6%.