Key Takeaways & Executive Findings
- •• • mer-Ir(Bpmb)3 achieves a maximum external quantum efficiency (EQE) of 20.0% in OLEDs, with current efficiency (CE) of 70.8 cd A−1 and power efficiency (PE) of 92.6 lm W−1, demonstrating superior device performance for green phosphorescent emitters. • • The rigid BO-fused ring design elevates the MLCT character to 21.69% (mer-Ir(Bpmi)3) and 17.30% (mer-Ir(Bpmb)3), compared to 13.01% and 15.99% for reference complexes, enhancing spin-orbit coupling and radiative transitions. • • Both complexes exhibit high thermal stability with decomposition temperatures of 491°C (mer-Ir(Bpmi)3) and 540°C (mer-Ir(Bpmb)3), ensuring robustness for vacuum thermal evaporation in OLED manufacturing. • • Short phosphorescence lifetimes (846 ns for mer-Ir(Bpmi)3; 1083 ns for mer-Ir(Bpmb)3) and high radiative rate constants (10.04 × 10^5 s−1 and 7.29 × 10^5 s−1) reduce efficiency roll-off at high brightness, critical for display applications.
Abstract
Two novel N-heterocyclic carbene (NHC)-based ligands featuring rigid boron-oxygen (BO) fused-ring units, named Bpmi and Bpmb, and the two corresponding homoleptic meridianal iridium complexes, namely mer-Ir(Bpmi)3 and mer-Ir(Bpmb)3, were designed and synthesized. Single-crystal structures revealed a meridional coordination geometry for both complexes. Shorter Ir–C carbene bond lengths and rigid planar BO-fused ring units contribute to enhanced stability. Both complexes exhibit efficient green phosphorescence (λem = 536/521 nm in toluene, ΦPL > 78%) with short lifetimes (τ = 846/1083 ns), leading to high radiative rate constants (Kr = 10.04 × 10^5 and 7.29 × 10^5 s−1, respectively). Theoretical calculations indicate significantly increased metal-to-ligand charge transfer (MLCT) character (21.69% for mer-Ir(Bpmi)3; 17.30% for mer-Ir(Bpmb)3) compared to reference complexes (13.01% for mer-Ir(pmi)3; 15.99% for mer-Ir(pmb)3). Both complexes exhibit exceptional thermal stability with decomposition temperatures of 491°C (mer-Ir(Bpmi)3) and 540°C (mer-Ir(Bpmb)3). OLED devices using mer-Ir(Bpmb)3 and mer-Ir(Bpmi)3 as emitters demonstrate maximum external quantum efficiencies of 20.0% and 15.6%, respectively. This research pioneers boron-fused ring-containing NHCs and their phosphorescent iridium(III) complexes, establishing a novel design strategy for high-performance NHC-based OLED phosphorescent emitters.
1. Introduction
Conventional NHC-based iridium complexes, such as mer-Ir(pmi)3 and mer-Ir(pmb)3, emit in the ultraviolet region with low photoluminescence quantum yields, limiting their utility in OLED displays. The wide bandgap and weak MLCT character result in inefficient triplet harvesting and poor device stability. To overcome these bottlenecks, the introduction of rigid, electron-withdrawing boron-oxygen fused rings into the NHC framework is proposed to narrow the bandgap, shift emission to the visible spectrum, and enhance MLCT contributions.
This work synthesizes two novel BO-fused NHC ligands (Bpmi and Bpmb) and their homoleptic iridium complexes. The rigid BO units enforce a planar geometry, shorten Ir–C bonds, and increase MLCT character, leading to efficient green phosphorescence with high quantum yields and short lifetimes. The complexes also exhibit exceptional thermal stability, making them suitable for vacuum-deposited OLEDs. This design strategy addresses the long-standing trade-off between emission color, efficiency, and stability in NHC-based phosphors.
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Fuzheng Zhang, Zhenghao Zhang, Jingsheng Miao, Feiyang Li, Jing Zhang, Qiuxia Li, Wenqing Yu, Aihua Yuan, Chao Shi, Chuluo Yang (2026). Rigid Oxygen-Bridged Boron NHC-Based Homoleptic Phosphorescent Iridium Complexes: Structures, Photophysics and OLED Application. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3820-5
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Frequently Asked Questions
What is the impact of the BO-fused ring on the MLCT character and how does it affect the radiative decay rate?
The BO-fused ring increases the MLCT character from 13.01% (mer-Ir(pmi)3) to 21.69% (mer-Ir(Bpmi)3) and from 15.99% (mer-Ir(pmb)3) to 17.30% (mer-Ir(Bpmb)3). This enhanced MLCT character strengthens spin-orbit coupling, leading to higher radiative rate constants (Kr = 10.04 × 10^5 s−1 for mer-Ir(Bpmi)3 and 7.29 × 10^5 s−1 for mer-Ir(Bpmb)3) and short lifetimes (846 ns and 1083 ns, respectively).
How do the synthesized complexes compare to the reference complexes in terms of photoluminescence quantum yield and emission wavelength?
The synthesized complexes exhibit green phosphorescence with emission peaks at 536 nm (mer-Ir(Bpmi)3) and 521 nm (mer-Ir(Bpmb)3) in toluene, with photoluminescence quantum yields exceeding 78%. In contrast, the reference complexes mer-Ir(pmi)3 and mer-Ir(pmb)3 emit in the UV region with lower efficiency, as noted in the introduction.
What are the thermal stability parameters and why are they critical for OLED fabrication?
The decomposition temperatures are 491°C for mer-Ir(Bpmi)3 and 540°C for mer-Ir(Bpmb)3. These high thermal stabilities are essential for withstanding the high temperatures during vacuum thermal evaporation, a common method for depositing organic layers in OLED manufacturing, ensuring material integrity and device longevity.
What are the key device performance metrics for the OLEDs using these emitters?
The OLEDs achieve a low turn-on voltage of 2.4 V. For mer-Ir(Bpmb)3, the maximum EQE is 20.0%, CE is 70.8 cd A−1, and PE is 92.6 lm W−1. For mer-Ir(Bpmi)3, the maximum EQE is 15.6%, CE is 51.8 cd A−1, and PE is 58.1 lm W−1. These metrics indicate high efficiency and low power consumption, suitable for display applications.
How does the rigid BO-fused ring influence the structural stability of the complexes?
The rigid planar BO-fused ring units contribute to shorter Ir–C carbene bond lengths, as revealed by single-crystal X-ray diffraction. This structural rigidity reduces non-radiative decay pathways and enhances the overall stability of the complexes, as evidenced by high decomposition temperatures and efficient phosphorescence.
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