Blue Phosphorescence with High Quantum Efficiency Engaging the Trifluoromethylsulfonyl Group to Iridium Phenylpyridine Complexes

Incorporation of an electron-withdrawing −SO2CF3 substituent to cyclometalating C^N-phenylpyridine (ppy) ligand resulted in an expected blue-shifted phosphorescence in the corresponding homoleptic Ir­(ppySCF3)3 complex, showing the emission of λem = 464 nm at 300 K. One of its heteroleptic derivatives, modified by a pyrazolyl borate LX ligand, Ir­(ppySCF3)2(bor), exhibited further blue-shifted phosphorescence of λem = 460 nm at 300 K. Cyclic voltammograms (CVs) and density-functional theory (DFT) calculations supported the efficacy of the electron-withdrawing capability of the SO2CF3 substituent lowering HOMO energy and obtained widened bandgaps and resumed blue emissions for all of the iridium complexes studied. The homoleptic complexes of both substituents, Ir­(ppySCF3)3 and Ir­(ppySF)3, reached the higher quantum yields (ΦPL) of (0.89 and 0.72), respectively. Similarly, emission quantum yields (ΦPL) of the heteroleptic derivatives were reported to be (0.75, 0.83, and 0.87) for Ir­(ppySCF3)2(acac), Ir­(ppySCF3)2(bor), and Ir­(ppySCF3)2(pic), respectively. Emission kinetics support the enhanced quantum efficiency when kr and knr values are compared between Ir­(ppySCF3)3 and Ir­(ppySF)3, and both values favorably contribute to attaining a higher quantum efficiency for Ir­(ppySCF3)3. Among solution-processed multilayered devices having an ITO/PEDOT:PSS/TCTA:Ir dopant (10:1, w/w)/TmPyPB/Liq/Al structure, a heteroleptic dopant, Ir­(ppySCF3)2(bor), exhibited better device performance, reporting an external quantum efficiency (EQE) of 1.14%, current efficiency (CE) of 2.31 cd A–1, and power efficiency (PE) of 1.21 lm W–1, together with blue chromaticity of CIEx,y = (0.16, 0.32)

Triplet Energy Transfer between a Sacrificial PMP and Blue TPF2 Iridium Dopants Leading to Enhancement of OLED Device Performance

In this study, we prepared phenylimidazole-based C^N-cyclometalated Ir­(III) complexes (DMP, TPF2) and a C^C-cyclometalated Ir­(III) complex (PMP), and investigated the energy transfer process by examining the intermolecular interactions between the two cyclometalated Ir­(III) complexes. In films doped with 3% Ir­(C^C)3 complex (PMP) and 15% Ir­(C^N)3 complex (DMP or TPF2), the PMP effectively induced energy transfer to the DMP or TPF2. This intermolecular energy transfer process was investigated using a picosecond time-resolved emission spectroscopic method. In the case of mixing PMP with DMP, where two types of luminescence were observed at 470 and 580 nm, the emission at 470 nm was due to DMP, while the emission at 580 nm can be assigned as the intermolecular exciplex emission. By contrast, in the case of mixing PMP with TPF2, the emission at 465 nm corresponding to the PMP emission region decreased for 18.5 ns, while the emission at 530 nm corresponding to TPF2 increased. This emission can be attributed to the energy transfer from PMP to TPF2. In addition, no change was observed in the longer wavelength region than the TPF2 emission region for 10 μs. We analyzed the energy transfer process when PMP was added to the dopant (DMP and TPF2) and found that TPF2 was more efficient than DMP in the device without PMP doping, but it showed performance deterioration in high current density (>1 mA/cm2) owing to activation of fluorinated ligands. Finally, it was confirmed that the operation lifetime and efficiency of the device were improved by doping 3% of PMP in emissive layer (EML)

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir­(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir­(III) complexes is designed and prepared: Ir­(dfppy)₂­(pic-OH) (<b>1a</b>), Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>), Ir­(ppy)₂­(pic-OH) (<b>2a</b>), and Ir­(ppy)₂­(pic-OMe) (<b>2b</b>). The emission lifetime of Ir­(dfppy)₂­(pic-OH) (<b>1a</b>) (τ_em = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>) (τ_em = 0.94 μs), because of a substantial difference in the nonradiative decay rate (<i>k</i>_nr = 0.28 × 10⁵ s^–1 for (<b>1a</b>) vs 2.99 × 10⁵ s^–1 for (<b>1b</b>)). These results were attributed to the intramolecular OH···OC hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when <b>1a</b> was used as the dopant compared to <b>FIrpic</b>, a well-known blue dopant. Device <b>III</b> (<b>1a</b> as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device <b>IV</b> (<b>FIrpic</b> as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir­(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir­(III) complexes is designed and prepared: Ir­(dfppy)₂­(pic-OH) (<b>1a</b>), Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>), Ir­(ppy)₂­(pic-OH) (<b>2a</b>), and Ir­(ppy)₂­(pic-OMe) (<b>2b</b>). The emission lifetime of Ir­(dfppy)₂­(pic-OH) (<b>1a</b>) (τ_em = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>) (τ_em = 0.94 μs), because of a substantial difference in the nonradiative decay rate (<i>k</i>_nr = 0.28 × 10⁵ s^–1 for (<b>1a</b>) vs 2.99 × 10⁵ s^–1 for (<b>1b</b>)). These results were attributed to the intramolecular OH···OC hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when <b>1a</b> was used as the dopant compared to <b>FIrpic</b>, a well-known blue dopant. Device <b>III</b> (<b>1a</b> as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device <b>IV</b> (<b>FIrpic</b> as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability