Blue Electrophosphorescence from Iridium(III) Phosphors Bearing Asymmetric Di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene Chelates

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximum EQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Ir(III) Metal Emitters with Cyano-Modified Imidazo[4,5-b]pyridin-2-ylidene Chelates for Deep-Blue Organic Light-Emitting Diodes

Ir(III) carbene complexes have been explored as one of the best blue phosphors for their high performance. Herein, the authors designed and synthesized a series of blue-emitting Ir(III) phosphors (f-ct9a-c), featuring fac-coordinated cyano-imidazo[4,5-b]pyridin-2-ylidene cyclometalates. These Ir(III) complexes exhibit true-blue emission with a peak maximum spanning 448-467 nm, with high photoluminescence quantum yields of 81-88% recorded in degassed toluene. Moreover, OLED devices bearing phosphors f-ct9a and f-ct9b deliver maximum external quantum efficiencies (EQEmax) of 25.9% and 30.3%, together with Commission Internationale de L'Eclairage (CIEx,y) coordinates of (0.157, 0.225) and (0.142, 0.169), respectively. Remarkably, the f-ct9b-based device displays an incredible EQE of 29.0% at 5000 cd·m-2. The hyper-OLED device based on f-ct9b and ν-DABNA exhibits an EQEmax of 34.7% and CIEx,y coordinates of (0.122, 0.131), affirming high potentials in achieving efficient blue electroluminescence.

C-H, N-H, and O-H Bond Activations to Prepare Phosphorescent Hydride-Iridium(III)-Phosphine Emitters with Photocatalytic Achievement in C-C Coupling Reactions

Complex IrH5(PiPr3)2 (1) activates two different σ-bonds of 3-phenoxy-1-phenylisoquinoline, 2-(1H-benzimidazol-2-yl)-6-phenylpyridine, 2-(1H-indol-2-yl)-6-phenylpyridine, 2-(2-hydroxyphenyl)-6-phenylpyridine, N-(2-hydroxyphenyl)-N'-phenylimidazolylidene, and 1,3-di(2-pyridyl)-4,6-dimethylbenzene to give IrH{κ3-C,N,C-[C6H4-isoqui-O-C6H4]}(PiPr3)2 (2), IrH{κ3-N,N,C-[NBzim-py-C6H4]}(PiPr3)2 (3), IrH{κ3-N,N,C-[Ind-py-C6H4]}(PiPr3)2 (4), IrH{κ3-C,N,O-[C6H4-py-C6H4O]}(PiPr3)2 (5), IrH{κ3-C,C,O-[C6H4-Im-C6H4O]}(PiPr3)2 (6), and IrH{κ3-N,C,C-[py-C6HMe2-C5H3N]}(PiPr3)2 (7), respectively. The activations are sequential, with the second generally being the slowest. Accordingly, dihydride intermediates IrH2{κ2-C,N-[C6H4-isoqui-O-C6H5]}(PiPr3)2 (2d), IrH2{κ2-N,N-[NBzim-py-C6H5]}(PiPr3)2 (3d), IrH2{κ2-N,N-[Ind-py-C6H5]}(PiPr3)2 (4d), and IrH2{κ2-N,C-[py-C6HMe2-py]}(PiPr3)2 (7d) were characterized spectroscopically. Complexes 3 and 5 are green phosphorescent emitters upon photoexcitation, exhibiting good absorption over a wide range of wavelengths, emission quantum yields about 0.70 in solution, long enough lifetimes (10-17 μs), and reversible electrochemical behavior. In agreement with these features, complex 3 promotes the photocatalytic α-amino C(sp3)-H arylation of N,N-dimethylaniline and N-phenylpiperidine with 1,4-dicyanobenzene and 4-cyanopyridine under blue LED light irradiation. The C-C coupling products are isolated in high yields with only 2 mol % of photocatalyst after 24 h.

Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

Narrowband blue emitters are indispensable in achieving ultrahigh-definition OLED displays that satisfy the stringent BT 2020 standard. Hereby, a series of bis-tridentate Ir(III) complexes bearing electron-deficient imidazo[4,5-b]pyridin-2-ylidene carbene coordination fragments and 2,6-diaryloxy pyridine ancillary groups were designed and synthesized. They exhibited deep blue emission with quantum yields of up to 89% and a radiative lifetime of 0.71 μs in the DPEPO host matrix, indicating both the high efficiency and excellent energy transfer process from the host to dopant. The OLED based on Irtb1 showed an emission at 468 nm with a maximum external quantum efficiency (EQE) of 22.7%. Moreover, the hyper-OLED with Irtb1 as a sensitizer for transferring energy to terminal emitter v-DABNA exhibited a narrowband blue emission at 472 nm and full width at half-maximum (FWHM) of 24 nm, a maximum EQE of 23.5%, and EQEs of 19.7, 16.1, and 12.9% at a practical brightness of 100, 1000, and 5000 cd/m2, respectively.