Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes

Abstract

The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C^∧N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C^∧N₂Ir(LX). The C^∧N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C^∧N₂Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C^∧N₂Ir(acac) complexes is a mixture of ³MLCT and ³(π−π*) states. By choosing the appropriate C^∧N ligand, C^∧N₂Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C^∧N ligands, which lead to bathochromic shifts of the free ligands, lead to similar bathochromic shifts in the Ir complexes of the same ligands, consistent with “C^∧N₂Ir”-centered emission. Three of the C^∧N₂Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs). The three Ir complexes, i.e., bis(2-phenylpyridinato-N,C^2‘)iridium(acetylacetonate) [ppy₂Ir(acac)], bis(2-phenyl benzothiozolato-N,C^2‘)iridium(acetylacetonate) [bt₂Ir(acac)], and bis(2-(2‘-benzothienyl)pyridinato-N,C^3‘)iridium(acetylacetonate) [btp₂Ir(acac)], were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy₂Ir(acac)-, bt₂Ir(acac)-, and btp₂Ir(acac)-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current−voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%, with the ppy₂Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, >50 Cd/A). The btp₂Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These C^∧N₂Ir(acac)-doped OLEDs show some of the highest efficiencies reported for organic light emitting diodes. The high efficiencies result from efficient trapping and radiative relaxation of the singlet and triplet excitons formed in the electroluminescent process.