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 ligan ds can be any of a wide variety of organometallic ligands. The LX ligands u sed for this study were all beta -diketonates, with the major emphasis plac ed On acetylacetonate (acac) complexes. The majority of the (CN2Ir)-N-boole an AND(acac) complexes phosphoresce with high quantum efficiencies (solutio n 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 signi ficant spin-orbit coupling of the Ir center, The lowest energy (emissive) e xcited 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-bool ean 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 liga nds, lead to similar bathochromic shifts in the Ir complexes of the same li gands, 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-phenylp yridinato-N,C-2')iridium(acetylacetonate) [ppy(2)Ir(acac)], bis(2-phenyl be nzothiozolato-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 g reen, yellow, and red electroluminescence, respectively, with very similar current-voltage characteristics. The OLEDs give high external quantum effic iencies, ranging from 6 to 12.3%, with the ppy(2)Ir(acac) giving the highes t efficiency (12.3%, 38 lm/W, > 50 Cd/A). The btp(2)Ir(acac)-based device g ives saturated red emission with a quantum efficiency of 6.5% and a luminan ce efficiency of 2.2 lm/W. These (CN2Ir)-N-boolean AND(acac)-doped OLEDs sh ow some of the highest efficiencies reported for organic light emitting dio des. The high efficiencies result from efficient trapping and radiative rel axation of the singlet and triplet excitons formed in the electroluminescen t process.