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 CH2Cl2. 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 cdA−1), with 1931 Commission Internationale de L'Eclairage (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.