Phosphorescent iridium(III) complexes with nonconjugated cyclometalated ligands

A series of blue phosphorescent iridium(III) complexes 1-4 with nonconjugated N-benzylpyrazole ligands were synthesized and their structural, electrochemical, and photophysical properties were investigated. Complexes 1-4 exhibit phosphorescence with yields of 5-45% in degassed CH2Cl 2. Of the compounds, 1 showed emission that was nearly true blue at 460 nm with a lack of vibronic progression. These photophysical data clearly demonstrate that the methylene spacer of the cyclometalated N-benzylpyrazole chelate effectively interrupts the π conjugation upon reacting with a third L∧X chelating chromophore. This gives a feasible synthesis for the blue phosphorescent complexes with a sufficiently large energy gap. In another approach, these complexes were investigated for their suitability for the host material in phosphorescent OLEDs. The device was synthesized by using 1 as the host for the green-emitting [Ir-(PPy)3] dopant, which exhibits an external quantum conversion efficiency (EQE) of up to 11.4% photons per electron (and 36.6 cd A-1), with 1931 Commission Internationale de L'E-clairage (CIE) coordinates of (0.30, 0.59), a peak power efficiency of 21.7 lmW-1, and a maximum brightness of 32000 cdm-2 at 14.5 V. At the practical brightness of 100 cdm-2, the efficiency remains above 11% and 18 lmW-1, demonstrating its great potential as the host material for phosphorescent organic light-emitting diodes. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.