Key Takeaways & Executive Findings
- •• • SCZ-4AnCN achieves a record EQE_max of 10.02% with CIE coordinates (0.154, 0.052), closely matching the BT.2020 blue standard, demonstrating the viability of anthracene-based emitters for high-efficiency deep-blue OLEDs. • • Nondoped SCZ-4AnCN devices exhibit an EQE_max of 7.51% and maintain 6.74% EQE at 1000 cd m−2, corresponding to a mere 10% efficiency roll-off, indicating superior operational stability for practical applications. • • The spirofluorene bridge introduces sp3-hybridized bridgehead carbons that restrict π-conjugation extension and fine-tune donor–acceptor interactions, stabilizing the S1 state with dominant LE character and suppressing intermolecular π–π stacking. • • Sensitization experiments with 1-indanone (ET1 = 3.29 eV) and 2-acetonaphthone (ET1 = 2.58 eV) provide conclusive evidence for hRISC channels in both emitters, confirming the presence of high-lying triplet states (Tn, n≥2) above 2.58 eV that enable efficient triplet harvesting.
Abstract
Deep-blue organic light-emitting diodes (OLEDs) remain the most challenging primary-color emitters due to stringent exciton energy requirements. We strategically designed two innovative deep-blue emitters, SCZ-4AnCN and STPA-4AnCN, via systematic functionalization of an anthracene core with arylamino-decorated spirofluorene donors and cyano-substituted phenyl acceptors. Comprehensive theoretical and experimental analyses demonstrate that these spirofluorene-anthracene hybrids adopt precisely engineered distorted configurations, effectively suppressing detrimental intermolecular π–π stacking in condensed phases. The sp3-hybridized bridgehead carbons in spirofluorene units play a pivotal role by simultaneously restricting π-conjugation extension and fine-tuning donor–acceptor interactions, thereby stabilizing the lowest excited singlet (S1) state with dominant local excitation (LE) character. This molecular engineering yields exceptional deep-blue emission with remarkable efficiency. Notably, the materials exhibit unique high-lying reverse intersystem crossing (hRISC) behavior, enabling efficient triplet harvesting. Optimized doped devices incorporating SCZ-4AnCN achieve outstanding performance, including a maximum external quantum efficiency (EQE_max) exceeding 10% and CIE coordinates (0.154, 0.052) approaching the BT.2020 blue standard. Nondoped devices maintain impressive performance with an EQE_max of 7.51% and superior operational stability, demonstrating less than 10% efficiency roll-off at 1000 cd m−2. This work validates anthracene-based molecular architectures for deep-blue electroluminescence and establishes a transformative design paradigm for next-generation OLED emitters.
1. Introduction
Deep-blue OLEDs remain the most formidable challenge among primary-color emitters due to stringent exciton energy requirements and the inherent spin statistics that limit electroluminescence efficiency to 25% singlet excitons. Conventional fluorophores suffer from low external quantum efficiency (EQE) because they cannot harvest the 75% triplet excitons. Heavy-metal phosphorescent emitters achieve 100% exciton utilization via strong spin–orbit coupling but are hindered by high cost and instability. All-organic alternatives like triplet-triplet annihilation (TTA) systems are limited to a theoretical EQE of 62.5%, while thermally activated delayed fluorescence (TADF) emitters suffer from severe efficiency roll-off due to microsecond-scale triplet lifetimes. These bottlenecks underscore the critical need for novel molecular designs that can achieve high efficiency and stability simultaneously.
This work addresses these limitations by engineering anthracene-based deep-blue emitters with spirofluorene bridges that mediate electronic structure modulation. The strategic incorporation of spirofluorene donors and cyano-substituted phenyl acceptors induces distorted configurations that suppress π–π stacking, while the sp3 bridgehead carbons restrict conjugation extension and stabilize the S1 state with dominant local excitation character. More importantly, the materials exhibit high-lying reverse intersystem crossing (hRISC), enabling efficient triplet harvesting without the drawbacks of TADF. The resulting devices achieve record EQE values and operational stability, offering a transformative design paradigm for next-generation OLED emitters.
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Ruicheng Wang, Zhichao Mao, Zhiju Chen, Jieying Lin, Zhaoxi Liu, Caishen Huang, Dehua Hu, Jia-Xiong Chen, Yanping Huo, Shaomin Ji, Yuguang Ma (2026). Engineering High-Efficiency Anthracene-Based Deep-Blue Emitters via Spirofluorene Bridge-Mediated Electronic Structure Modulation. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4197-4
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Frequently Asked Questions
What is the mechanism behind the high-lying reverse intersystem crossing (hRISC) in SCZ-4AnCN and STPA-4AnCN, and how does it contribute to the high EQE?
The hRISC mechanism involves the population of high-lying triplet states (Tn, n≥2) via Dexter energy transfer from a sensitizer, followed by reverse intersystem crossing to the S1 state. Sensitization experiments with 1-indanone (ET1 = 3.29 eV) and 2-acetonaphthone (ET1 = 2.58 eV) confirmed that the Tn states of both emitters exceed 2.58 eV, enabling efficient hRISC. This pathway allows harvesting of triplet excitons without the long-lived triplet states typical of TADF, thus minimizing efficiency roll-off and achieving high EQE_max values (10.02% for doped SCZ-4AnCN devices).
How do the spirofluorene bridges influence the molecular conformation and photophysical properties of the emitters?
The spirofluorene bridges introduce sp3-hybridized bridgehead carbons that create a rigid, distorted three-dimensional structure. This distortion suppresses intermolecular π–π stacking in the solid state, reducing aggregation-caused quenching. Additionally, the spiro bridges restrict π-conjugation extension, which helps maintain a dominant local excitation (LE) character in the S1 state, leading to high photoluminescence quantum yields and deep-blue emission with CIE coordinates (0.154, 0.052).
What are the operational stability metrics of the nondoped devices, and how do they compare to doped devices?
Nondoped SCZ-4AnCN devices achieve an EQE_max of 7.51% and maintain 6.74% EQE at 1000 cd m−2, corresponding to only a 10% efficiency roll-off. This indicates superior operational stability compared to many TADF-based devices that suffer from severe roll-off. The doped devices achieve a higher EQE_max of 10.02% but may have different stability characteristics; the nondoped configuration offers a simpler fabrication process and potentially longer operational lifetime.
What is the significance of the CIE coordinates (0.154, 0.052) in relation to the BT.2020 standard?
The BT.2020 standard for deep-blue emission requires CIE coordinates very close to (0.131, 0.046). The achieved coordinates (0.154, 0.052) are among the closest reported for anthracene-based emitters, indicating a high color purity that is essential for wide-color-gamut displays. This level of color accuracy is critical for commercial OLED displays to meet next-generation broadcasting standards.
How does the donor-acceptor design in SCZ-4AnCN and STPA-4AnCN affect charge injection and transport barriers?
The strategic electronic modulation through fluorene bridge engineering effectively decouples the HOMO and LUMO distributions, reducing charge injection and transport barriers. This is achieved by placing arylamino-decorated spirofluorene donors and cyano-substituted phenyl acceptors, which create a donor-acceptor architecture that facilitates balanced charge transport. The reduced barriers contribute to the high EQE and low efficiency roll-off observed in the devices.
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