Highly phosphorescent bis-cyclometalated iridium complexes: Synthesis, photophysical characterization, and use in organic light emitting diodes

The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated ((CN)-N-boolean AND) ligands and a single monoanionic, bidentate ancillary ligand (LX). i.e., (CN2Ir)-N-boolean AND(LX). The (CN)-N-boolean AND ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all beta -diketonates, with the major emphasis placed On acetylacetonate (acac) complexes. The majority of the (CN2Ir)-N-boolean AND(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1-0.6), and microsecond lifetimes (e.g., 1-14 mus). 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 (CN2Ir)-N-boolean AND(acac) complexes is a mixture of (MLCT)-M-3 and (3)(pi-pi*) states. By choosing the appropriate (CN)-N-boolean AND ligand, (CN2Ir)-N-boolean AND(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the (CN)-N-boolean AND 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 "(CN2)-N-boolean AND-Ir"-centered emission. Three of the (CN2Ir)-N-boolean AND(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) [ppy(2)Ir(acac)], bis(2-phenyl benzothiozolato-N,C-2')iridium(acetylacetonate) [bt(2)Ir(acac)], and bis(2-(2'-benzothienyl)-pyridinato-N,C-3')iridium(acetylacetonate) [btp(2)Ir(acac)], were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy(2)Ir(acac)-, bt(2)Ir(acac)-, and btp(2)Ir(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 ppy(2)Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, > 50 Cd/A). The btp(2)Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These (CN2Ir)-N-boolean AND(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.