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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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9,9'-Bianthracene-Based Blue Fluorescent Emitters for High-Performance Organic Light-Emitting Diodes

  • Aowei Zhu 1 ,
  • Zhanfeng Li , 1 ,
  • Kunping Guo , 2 ,
  • Yanqin Miao , 3 ,
  • Baoyou Liu 4 ,
  • Gang Yue 4
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  • 1. Key Laboratory of Advanced Transducers and Intelligent Control System,Ministry of Education and Shanxi Province,Taiyuan University of Technology,Shanxi 030024,China
  • 2. Shaanxi Engineering Research Center of Flat Panel Display Technology,Shaanxi University of Science and Technology,Xi'an,710021,China
  • 3. Key laboratory of Interface Science and Engineering in Advanced Materials,Ministry of Education,Taiyuan University of Technology,Taiyuan 030024,China
  • 4. Ningxia Hui Autonomous Region Screen Display Organic Materials Engineering Technology Research Center,Ningxia Sinostar Display Material Co. ,Ltd. Yinchuan 750003,China
* (Zhanfeng Li);
(Kunping Guo);
(Yanqin Miao)

Received date: 2024-05-22

  Revised date: 2024-08-07

  Online published: 2024-09-18

Supported by

National Nature Science Foundation of China(22379105)

National Nature Science Foundation of China(62305202)

Natural Science Foundation of Shanxi Province(20210302123110)

Key Research and Development Program of Shaanxi(2023-YBGY-198)

Ningxia Sinostar Display Material Co., Ltd. Open Fund Project, Science and Technology Innovation Talent Team Project of Shanxi Province(202204051001013)

Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province(20230007)

Key Research and Development Program of Xianyang(L2023-ZDYF-QYCX-061)

Abstract

Organic light emitting diodes (OLEDs) have attracted extensive attention and research interest in advanced display and solid-state lighting due to their self-luminescence, low drive voltage, wide color gamut, surface luminescence, flexibility and rapid response. One of the primary colors of OLED, the development of blue emitter is still lagging far behind. Interestingly, 9,9'-bianthracene as a promising blue-emitter for high-performance fluorescent OLEDs exhibits excellent optoelectronic performance in recent years. Here, we review the progress with the development of 9,9'-anthracene-based blue fluorescent materials and gain insight into their contribution towards enhanced OLED performance. Different approaches to achieve blue emission from molecular design including isomerization, fluorine substitution, asymmetrical structuring, and steric hindrance effects are discussed, with particular focus on device efficiency and stability. Furthermore, an outlook for future challenges and opportunities of OLEDs from the development of new molecular structures, understanding of luminescence mechanisms as well as innovation in flexible and large-scale panels is provided.

Contents

1 Introduction

1.1 OLED structure and principle

1.2 OLED emissive materials

2 9,9'-Bianthracene-based blue light-emitting materials and device performance

2. 1 Basic structure of bianthracene

2. 2 9,9'-Bianthracene-based blue light-emitting materials and devices

2. 3 Structures and chemical properties of other bianthracene derivatives

2. 4 BT. 2020 blue light

Correlation between the structure and performance of 9,9'-Bianthracene-based blue light-emitting materials

3. 1 Isomerization effects in bianthracene

3. 2 Halogen substitution effects

3. 3 Asymmetric effects

3. 4 Steric hindrance effects

3. 5 Blue-emitting device design

4 Conclusion and prospects

4. 1 Summary

4. 2 Prospects

Cite this article

Aowei Zhu , Zhanfeng Li , Kunping Guo , Yanqin Miao , Baoyou Liu , Gang Yue . 9,9'-Bianthracene-Based Blue Fluorescent Emitters for High-Performance Organic Light-Emitting Diodes[J]. Progress in Chemistry, 2025 , 37(3) : 317 -331 . DOI: 10.7536/PC240520

1 Introduction

With the rapid growth of the global economy and the swift development of technology, display technology has become an indispensable part of modern society, widely applied in various occasions from small personal devices to large public display screens, gradually becoming a significant pillar in the information technology field1-4. As an emerging representative of display technology, organic light-emitting diodes (OLED), with their unique advantages such as self-emission, fast response speed, lightweight, flexibility, high contrast, and full-color display, have dominated the downstream small-size display market in recent years. BOE, the domestic display leader, through cooperation with local smartphone brands like Huawei and entering Apple's supply chain, is gradually becoming the main supplier of OLED mobile phone panels. As OLED technology becomes increasingly dazzling, Sony, Samsung, and LG, the three giants in the international market, stand neck and neck. Their medium and large-sized OLED panels are progressively expanding into automotive and TV products, clearly forming the top competitive landscape in this field5-10. Moreover, due to its outstanding chromatic quality, dimming ability, and environmental friendliness, OLED also indicates potential applications in solid-state lighting, optical communication, and the Internet of Things market11-16. Today, OLED is not only an important development direction of display technology but also a driving force for industry progress and innovation.

1.1 Structure and Principle of OLED

Organic light-emitting diodes are a revolutionary technology in the contemporary fields of display and lighting, and their working mechanism based on current-driven organic material luminescence marks unique self-luminescent characteristics, which stand in sharp contrast to traditional display technologies that rely on external backlight sources10. The basic structure of OLED is shown in Fig. 1 (a), typically consisting of a substrate, transparent conductive anode, hole transport layer (HTL), emitting layer (EML), electron transport layer (ETL), and metal cathode. In addition, to further enhance performance, OLED structures usually incorporate a hole injection layer (HIL), electron injection layer (EIL), hole blocking layer (HBL), and electron blocking layer (EBL)17-25. Transparent conductive glass substrates generally use indium tin oxides (ITO) as the anode material for bottom-emission OLED structures. Typical top electrodes of bottom-emission OLEDs commonly use highly conductive metals such as silver and aluminum, which also possess high reflectivity.
图 1 Schematic Diagram of OLED Structure (a) and Its Luminescence Principle (b)

Fig. 1 (a) Structures diagram of OLED and (b) schematic diagram of luminescence mechanism

The typical OLED luminescence process is shown in Fig. 1(b), mainly including the following four steps: (1) Carrier injection: under the action of an externally applied driving voltage, electrons and holes from the cathode and anode will move towards the organic luminescent layer of the device; (2) Carrier transport: electrons and holes respectively pass through the corresponding transport layers to reach the organic luminescent layer, where they accumulate in large quantities at the interface of this layer; (3) Carrier recombination: when a certain number of holes and electrons accumulate at the interface of the organic luminescent layer, they will recombine to form excitons; (4) Exciton luminescence: excitons produced by recombination at the interface of the luminescent layer will activate the organic luminescent material, causing its outermost electrons to transition from the ground state to the excited state. Due to the instability of the high - energy excited state, the electrons will return to the ground state and release energy in the form of light, thus completing the entire device's luminescence process26.
The core of OLED display technology lies in the luminescent materials, which determine the final performance of the device, including color saturation, brightness, and lifespan. If high-quality OLED displays and solid-state lighting are to be achieved, it is essential to develop energy-saving and long-lasting luminescent materials that can be widely used. Based on the trichromatic theory, the effective combination of red, green, and blue light is the key to achieving full-color gamut display27. Although red and green OLED materials have been widely used in commercial products, the development of highly efficient and long-lasting blue light OLED materials remains a significant challenge in the field.

1.2 OLED Luminescent Materials

According to the classification based on the luminescence mechanism and the development of OLED luminescent materials, blue OLED materials can be divided into three categories: first-generation traditional fluorescent materials, second-generation phosphorescent materials, and third-generation thermally activated delayed fluorescence (TADF) materials28. The luminescence mechanisms of the three generations of materials are shown in Figure 3. In the process of organic electroluminescence, both singlet and triplet excitons are generated simultaneously, with 25% being singlet excitons and 75% being triplet excitons. Excitons at high energy states are usually unstable and lose energy through different pathways to return to the ground state. These pathways include radiative transitions and non-radiative transitions. Radiative transitions refer to light emission through fluorescence or phosphorescence processes, while non-radiative transitions include vibrational relaxation, internal conversion, external conversion, and intersystem crossing (ISC)29. Singlet excitons emit fluorescence through radiative transitions to the ground state, and triplet excitons emit phosphorescence through radiative transitions to the ground state. Due to the forbidden effect of radiative transitions, triplet excitons mainly undergo non-radiative decay, contributing little to luminescence and relying only on singlet excitons for radiative transition light emission30. Therefore, for traditional organic fluorescent materials, the internal quantum efficiency (IQE) is at most 25%31. By utilizing heavy metal complexes to excite 75% triplet excitons, the second generation of OLED luminescent materials (i.e., phosphorescent materials) was developed. People utilize the enhanced spin-orbit coupling effect induced by heavy metal atoms, resulting in heavy metal complexes that can accelerate radiation deactivation from the lowest triplet state T1 to the ground state S0 through phosphorescent emission, promoting ISC from the lowest singlet state S1 to T132. This strategy and relaxation pathway using triplets can achieve an internal quantum efficiency of 100% for phosphorescent heavy metal emitters. Thermally activated delayed fluorescence occurs because of the small energy gap between the lowest singlet state and the lowest triplet state. When the lifetime of T1 excitons is long enough, the normally spin-forbidden reverse intersystem crossing (RISC) process is thermally activated, allowing triplet excitons to up-convert to the S1 state, from which they radiatively transition to the ground state, achieving fluorescence emission involving triplet excitons33. For TADF materials, theoretically, IQE can also reach 100%, which is key to obtaining high external quantum efficiency (EQE) OLED devices34.
图2 Schematic Diagram of Classification and Luminescence Mechanism of Blue OLED Materials

Fig. 2 Schematic diagram of the emission mechanism of different types of blue OLED materials

图3 Molecular Structure Diagram of Representative Materials for Blue OLEDs

Fig. 3 Molecular diagram of representative blue OLED materials

In the field of OLED technology, by precisely controlling the material composition and energy level structure of the red, green, and blue emission layers, it is possible to achieve a continuous spectrum covering the visible light spectrum range from a single device. This spectral synthesis effect enables OLED devices to exhibit highly efficient and high-quality white light illumination. Among these three primary colors, red and green luminescent materials have reached a highly mature level of industrial production due to long-term research and technological innovation, demonstrating excellent performance stability and color purity. However, although blue OLED materials have made significant progress in recent years, their performance optimization and lifespan improvement still lag behind those of red and green materials. This is mainly due to the technical challenges faced by blue materials in terms of charge injection efficiency, energy conversion efficiency, and long-term operational stability. Therefore, breakthroughs in blue light technology are particularly crucial for achieving high-efficiency, energy-saving white light and the development of the entire lighting industry. The chromaticity range of blue light materials can be subdivided into four main categories through CIEy values: ultra-deep blue (CIEy < 0.05), deep blue (CIEy < 0.010), blue (CIEy < 0.25), and sky blue (CIEy < 0.35)[35]. Developing deep blue emitters is a fundamental requirement for achieving high-quality displays and solid-state lighting. Additionally, promising deep blue emitters must also comply with the National Television System Committee (NTSC) standards and the International Commission on Illumination (CIE) coordinates (0.14, 0.08) to meet the requirements of full-color display applications[36].
The classic building blocks of traditional blue fluorescent materials mainly include anthracene, pyrene, fluorene, carbazole, phenanthroimidazole, etc. Most blue fluorescent materials are derivatives of anthracene. For example, MADN and TMADN (structures as shown in Figure 3) and other commercialized blue light materials are designed and synthesized based on anthracene[37]. Anthracene-derived blue light materials not only have good stability but also high fluorescence quantum yield. This is because they possess a wide bandgap, rigid chemical structure, and excellent charge transport properties. Additionally, anthracene-based materials exhibit good thermal stability and film-forming properties, which are beneficial for OLED fabrication. However, due to the planar structure of anthracene, it is prone to aggregation-induced crystallization issues, thus typically requiring chemical modification to avoid this problem. To prevent fluorescence quenching and aggregation-induced crystallization, bulky substituents are usually introduced at the 9 and 10 positions of anthracene, adjusting the emission from blue to deep blue and enhancing its thermal stability[38].
Blue phosphorescent materials are typically complexes of iridium or platinum39-40. Blue phosphorescent materials can be traced back to 2001, when Forrest et al. first used Firpic to prepare a blue phosphorescent OLED (CIE = 0.16, 0.29) with an EQE of 5.7% and an emission wavelength of 475 nm41. Firpic (as shown in Figure 3) is also the most reported blue phosphorescent complex. When preparing phosphorescent devices, because the energy gap of blue light materials is relatively large, selecting suitable wide-energy-gap blue light host materials presents a considerable challenge. Extensive research has shown that the most important factor in improving the efficiency of blue phosphorescent devices is choosing the appropriate host material. In recent years, researchers have successfully developed numerous blue phosphorescent materials, which not only aim to further enhance the optoelectronic performance of OLED devices but also focus on extending their service life and improving color purity. The progress in this research direction provides new momentum and possibilities for the future development of OLED technology.
The earliest research on blue TADF materials began in 2012, pioneered by Adachi et al. The DTC-DPS (Fig. 3) they synthesized exhibited excellent device performance, marking the starting point of TADF research[42]. In the early studies of blue TADF materials, the properties of TADF molecules were finely tuned by designing donor-acceptor structures and adjusting different combinations of donors and acceptors. The key was to ensure effective separation between the donor and acceptor, thereby achieving effective separation of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), thus reducing the energy gap (ΔEST) between the lowest singlet excited state (S1) and the lowest triplet excited state (T1)[42], which is crucial for accelerating the reverse intersystem crossing (RISC) process. Recently, the introduction of B-N resonance structures has brought new directions to TADF research, known as Multi-Resonance Thermally Activated Delayed Fluorescence (MR-TADF)[43]. MR-TADF materials utilize the resonance effect produced by electron-deficient boron groups/carbonyl groups and electron-rich oxygen/nitrogen atoms, minimizing the bonding/antibonding of frontier molecular orbitals to reduce vibrational relaxation in polycyclic structures, thereby achieving a narrow Full Width at Half Maximum (FWHM) and high luminescence efficiency[44-45].
Although blue phosphorescent and TADF materials can theoretically achieve 100% internal quantum efficiency (IQE) and OLED devices with higher external quantum efficiency have been fabricated, they still face many challenges in practical applications. For instance, phosphorescent materials are costly and pose environmental pollution problems due to the necessity of using heavy metal atoms, while TADF materials, although reducing costs and improving EQE, still have issues with color purity and lifespan that need to be addressed, and the phenomenon of efficiency roll-off is relatively severe. Currently, traditional fluorescent materials based on anthracene derivatives still dominate among commercial blue materials, demonstrating the importance of stability and cost-effectiveness in commercial applications.

2 9,9'-Bianthracene-Based Blue Light Materials and Device Performance

As an anthracene derivative, triphenylene has attracted much attention because it possesses the advantages of wide bandgap, rigid chemical structure, good thermal stability, and excellent charge transport properties of anthracene materials46-47. Moreover, its more twisted molecular structure can effectively avoid fluorescence quenching and exhibits unique luminescent properties, demonstrating its great potential in the development of high-performance blue organic light-emitting diodes.

2.1 Basic Structure of Anthracene and Its Low-Cost Synthesis

The molecule of bianthracene is composed of two anthracene units connected by a single carbon-carbon bond, and its molecular structure is diverse with a total of five isomers: 1,1'-bianthracene, 2,2'-bianthracene, 1,9'-bianthracene, 2,9'-bianthracene, and 9,9'-bianthracene (the molecular structures are shown in Figure 4). The structural characteristics of each isomer influence their photoelectric properties such as light absorption, luminescence efficiency, and stability, which also determine their application potential in OLED devices to varying degrees. As a key core material for OLEDs, the low-cost synthesis of organic materials is a complex and diversified field that encompasses various chemical methods and processes. During the synthesis process, optimizing reaction conditions and catalysts to reduce by-product formation and improve the quality and production efficiency of the target product is crucial for cost reduction. The synthetic route for bianthracene-based blue light materials is relatively simple, especially since many isomeric bianthracene monomers can be synthesized in a simple one-step reaction. In 2019, Li Zhanfeng's team reported a series of 9,9'-bianthracene derivatives with high-efficiency blue-light-emitting hosts and deep-blue luminescent materials synthesized at low cost. By simplifying the synthesis process without affecting the luminescence performance of the materials, the manufacturing cost of blue-light-emitting materials was minimized to the greatest extent48. In fact, the synthesis process of 9,9'-bianthracene derivatives is generally simple. This characteristic not only ensures the safety and controllability of industrial-scale production but also greatly promotes the low-cost and highly efficient preparation of large quantities of blue-light-emitting materials on a gram scale or even larger, driving the low-cost development and application of OLED panels.
图4 Molecular Structures, Optimized Configurations, Frontier Molecular Orbitals, and Electrostatic Potentials of Five Anthracene Isomers

Fig. 4 Molecular structure, optimized geometric structure, frontier molecular orbital surface and ESP of the five bianthracene isomers

2.2 9,9'-Bianthracene-Based Blue Light Materials and Devices

Among numerous anthracene isomers, 9,9'-bianthracene materials have significantly improved the luminescence efficiency and service life of OLED devices due to their outstanding high fluorescence quantum efficiency and excellent thermal stability. Their unique wide bandgap structure ensures high color purity blue light emission, meeting the stringent requirements of international standards. Meanwhile, their characteristics of low turn-on voltage, high current efficiency, and high energy efficiency effectively reduce the power consumption of the device and enhance the overall device efficiency. Moreover, the bipolar transport properties of 9,9'-bianthracene materials simplify the device structure design, making them highly favored and extensively explored in the research and application of OLED blue-emitting materials. The following will delve into the application progress of 9,9'-bianthracene and its derivatives in the OLED field and comprehensively summarize their device performance.

2.2.1 Symmetric Structure 9,9'-Bianthracene-Based Blue Light Materials

In the research of organic light-emitting diode (OLED) technology, symmetrically structured 9,9'-bianthracene-based blue light materials have garnered significant attention due to their excellent electronic properties and optimized molecular symmetry. These materials leverage their symmetrical molecular structure to achieve efficient charge balance and superior photoelectric conversion efficiency, thereby significantly enhancing the performance of OLED devices. Figure 5 and Table 1 summarize 9,9'-bianthracene-based molecules with symmetric structures and their performance in OLEDs.
图5 Schematic Diagram of Molecular Structure of Symmetric 9,9'-Bianthracene-Based Blue Light Materials

Fig. 5 Schematic diagram of the molecular structure of symmetric 9,9'-bianthracene blue light-emitting materials

表1 对称9,9'-联蒽类蓝光材料的OLED器件性能总结

Table 1 Performance summary of OLED devices using symmetrical 9,9'-bianthracene blue light materials

Compound EL λ (nm) CE (cd·A-1 PE (lm·W-1 L max (cd·m-2 EQEmax (%) CIE (xy Ref
1a 472 3.95 3.87 6470 2.47 (0.17, 0.22) 48
1b 432 1.36 0.85 1860 3.46 (0.16, 0.05) 48
1c 472 12.06 5.59 19 200 5.72 (0.15, 0.33) 48
2a 446 2.11 1.74 788 1.9 (0.16, 0.12) 54
2c 459 5.18 3.72 2059 3.52 (0.15, 0.19) 54
5a 444 1.87 0.92 1383 1.1 (0.17, 0.16) 56
5c 468 6 2.88 8726 3.19 (0.16, 0.27) 56
6a 450 2.21 1.51 1881 2.02 (0.16, 0.12) 57
7a 460 3.58 2.58 2367 1.94 (0.26, 0.26) 57
8a 450 4.27 2 4.26 (0.15, 0.12) 58
9a 499 2.72 1.14 1.5 (0.21, 0.29) 58
10b 438 1.66 1.02 1647 2.67 (0.15, 0.08) 60
10c 472 8.8 5.79 25 065 4.14 (0.17, 0.33) 60
11b 440 1.28 0.89 1669 1.99 (0.15, 0.08) 60
11c 468 8.67 4.93 25 832 4.86 (0.16, 0.27) 60
12b 437 1.22 0.87 2766 1.86 (0.15, 0.08) 60
12c 468 9.64 6.77 28 667 5.43 (0.15, 0.27) 60
13b 435 1.47 1.1 2016 2.75 (0.15, 0.07) 60
13c 468 7.05 4.46 23 887 3.35 (0.18, 0.37) 60
14b 437 2.11 1.45 838 3.56 (0.15, 0.08) 60
14c 478 7.98 5.65 17 452 4.02 (0.17, 0.31) 60
15a 444 1.11 0.46 2136 0.93 (0.15, 0.13) 62
15c 468 5.88 2.69 12 690 2.98 (0.15, 0.30) 62
16a 444 0.94 0.36 1919 0.84 (0.16, 0.17) 61
16c 472 4.38 0.36 14 200 2.29 (0.16, 0.32) 61
17b 435 3.05 2.62 3588 5.02 (0.15, 0.08) 59
18a 0.63 0.42 2290 0.31 (0.21, 0.29) 63
19a 444 2.94 1.9 3112 2.22 (0.18, 0.17) 57
20a 2.24 1.78 15 600 0.97 (0.28, 0.33) 63
21a 448 3.7 3.9 6137 3.9 (0.15, 0.10) 64

a Undoped device.b Devices prepared by doping as guest materials.c Devices prepared by doping as host materials.

In 2013, Natarajan et al. conducted a systematic study on the optoelectronic properties of 9,9'-bianthryl, revealing the regulation potential of the 9,9'-bianthryl unit in luminescence performance by in-depth analysis of its photoluminescence (PL), redox characteristics, and electrochemiluminescence49. Therefore, it has broad application prospects in the fields of OLEDs, organic field-effect transistors50, organic photovoltaic devices51, and fluorescent probes52. In 2019, Li et al. further applied 9,9'-bianthryl (referred to as BA) in the field of OLEDs, evaluating the electroluminescent performance of BA by constructing three different types of devices (non-doped devices, guest-doped devices, and host-doped devices)48. The results showed that the OLED with BA (1) as the host-doped material exhibited the best device performance, with a maximum external quantum efficiency of 7.16%, a maximum brightness value of 19,200 cd·m-2, and a maximum current efficiency of 12.06 cd·A-1, indicating that BA has broad application potential in the field of high-efficiency blue OLEDs53.
Early research focused on symmetrically substituting the 10 and 10' positions of 9,9'-bianthracene to obtain its derivatives and explore their performance in OLEDs. These studies covered the performance evaluation of simple substitution, halogen substitution, and heteroatom substitution of BA molecules in terms of thermal stability, photoelectric properties, and film-forming properties. Simple substituted BA molecules can be traced back to four anthracene derivatives synthesized by Jang et al. in 2012, where doped and non-doped devices were prepared to study their electroluminescent properties. Among them, the non-doped device prepared with 2 achieved a maximum EQE of 3.52%, with CIE color coordinates of (0.15, 0.19)54. In the same year, Jiang et al. introduced larger triphenylmethyl and triphenyl tert-butyl units at the 10 and 10' positions of B to obtain 3 and 4. The results showed that these two materials not only have blue light emission and high fluorescence quantum efficiency but also good thermal stability and excellent thin-film morphology, with the potential to improve the efficiency and lifespan of OLED devices55. In 2016, Li et al. reported a bianthracene material 5 with high thermal stability and high luminescence efficiency. The doped device using 5 as the main material achieved a maximum current efficiency and maximum external quantum efficiency of 6.00 cd·A-1 and 3.22%, respectively, with very weak efficiency roll-off observed56. In 2017, Lee et al. synthesized four different materials by changing the types of substituents at the 10 and 10' positions of the BA molecule: 2, 6, 7, and 19. Among them, 6 had the best EL performance, with a maximum EQE reaching 2.22% and a maximum brightness reaching 3112 cd·m-2[57. In 2017, Lee et al. systematically changed the types of side substituents to study the relationship between the core and side substituents, synthesizing 8 and 9. Among them, the device performance of 8 was the most excellent, with a maximum EQE reaching 4.26% and color coordinates of (0.15, 0.12)58.
The earliest application of halogen substitution in blue light OLED molecular design was by Yu et al. in 2013 and 2014, who introduced fluorine into the BA core to synthesize six materials 10, 11, 12, 13, 14, and 17. It was found that the introduction of F and CF3 electron-withdrawing groups had a significant impact on the photophysical properties and OLED performance of the materials. Among them, the color coordinates of the doped OLED device using 17 as the dopant were (0.156, 0.083), and the highest external quantum efficiency also reached 5.02%59-60. In 2015, Li and Gao et al. reported the influence of trifluoromethyl substituents at different positions, preparing two materials 15 and 16 and applying them to OLEDs. The devices using 15 as the host material exhibited excellent performance, with a maximum current efficiency of 5.88 cd·A-1, a maximum external quantum efficiency of 3.15%, and CIE color coordinates of (0.15, 0.30)56, 61-62. In 2018, Jhulki et al. introduced methoxy and dibenzofuran into the anthracene core, developing two new electroluminescent materials 18 and 2063. Among them, 20 could lead to white light emission (CIE ≈ 0.28, 0.33) in the prepared multilayer non-doped OLED devices, with a power efficiency of 2.24 lm·W-1 and a maximum brightness of up to 15,600 cd·m-2. In 2013, Zhang et al. combined alkyl chain-substituted carbazole with anthracene to prepare 21. This material not only had the advantages of very stable and highly efficient deep blue emission but also exhibited characteristics that significantly improved the efficiency of radiative exciton generation; however, it was not applied to OLEDs64.

2.2.2 Asymmetric Structure 9,9'-Bianthracene-Based Blue Light-Emitting Materials

Asymmetrically structured 9,9'-bianthracene-based blue light-emitting materials have become a research hotspot due to their unique optoelectronic properties. By introducing asymmetric functional groups onto the basic skeleton of 9,9'-bianthracene, these materials achieve precise control over electron distribution, thereby optimizing their luminescence efficiency and device performance in OLEDs. Figure 6 and Table 2 summarize the molecular structures and performance characteristics of a series of asymmetric 9,9'-bianthracene derivatives in OLED applications. In 2017, Lee et al. explored the relationship between the central core and side groups by synthesizing a series of materials including 22, 23, 24, and 25[58]. Using these materials as the emissive layer, they fabricated OLED devices without doping. Among them, material 25 exhibited outstanding electroluminescent performance, with a current efficiency of 8.97 cd·A-1, power efficiency of 4.43 lm·W-1, and an external quantum efficiency reaching 6.37%. In the same year, Kim et al. designed and synthesized two 9,9'-bianthracene derivatives, 26 and 27, which aimed to enhance the electroluminescent performance of OLEDs by reducing molecular self-aggregation. Material 27 demonstrated a current efficiency of 2.09 cd·A-1, power efficiency of 0.92 lm·W-1, current density of 20 mA·cm-2, external quantum efficiency of 1.72%, and CIE chromaticity coordinates of (0.16, 0.22)[65]. In 2018, Yu et al. introduced two organic molecules, 28 and 29, adopting a highly twisted donor-bianthracene-acceptor (D-BA-A) structure. These molecules were successfully utilized to fabricate OLED devices with high luminescence efficiency and extremely low efficiency roll-off through the design of a twisted intramolecular charge transfer (TICT) intermediate excited state[66].
图6 Molecular Structure Diagram of Asymmetric 9,9'-Bianthracene Blue Light Materials

Fig. 6 Schematic diagram of the molecular structure of asymmetric 9,9'-bianthracene blue light-emitting materials

表2 非对称9,9'-联蒽类蓝光材料的OLED器件性能总结

Table 2 Performance summary of OLED devices using asymmetrical 9,9 '-bianthracene blue light materials

Compound EL λ(nm) CE (cd·A-1 PE (lm·W-1 L max (cd·m-2 EQEma (%) CIE (xy Ref
22a 455 5.1 2.33 4.02 (0.17, 0.15) 58
23a 501 7.79 3.79 3.17 (0.19, 0.52) 58
24a 466 6.78 3.55 4.71 (0.15, 0.20) 58
25a 466 8.97 4.43 6.37 (0.14, 0.19) 58
26a 469 2.1 1.16 959 1.74 (0.16, 0.22) 65
27a 481 2.7 1.89 935 1.55 (0.18, 0.24) 65
28a 464 4.85 3.83 7175 2.72 (0.17, 0.22) 67
28c 512 21.39 15.9 74 922 6.69 (0.28, 0.63) 67
29a 452 5.93 4.91 10 608 4.09 (0.16, 0.16) 67
29c 508 23.83 19.19 82 235 7.13 (0.27, 0.61) 67

a Undoped device.b Devices prepared by doping as guest materials.c Devices prepared by doping as host materials.

2.2.3 Alkyl-Substituted 9,9'-Bianthracene-Based Blue Light Materials

In the OLED field, alkyl-substituted 9,9'-bianthryl derivatives have attracted widespread attention due to the significant improvement in their optoelectronic properties after alkyl substitution on the 9,9'-bianthryl core. Through systematic research on these materials, researchers have found that alkyl substitution plays a crucial role in enhancing molecular spacing, suppressing π-π stacking, and optimizing luminescence performance, as well as positively impacting the luminous efficiency and stability of devices. Figure 7 and Table 3 summarize the molecular structures and OLED device performance of alkyl-substituted 9,9'-bianthryl derivatives, highlighting the potential and application prospects of this material system in the deep blue light emission field.
图7 Molecular Structure Diagram of Alkyl-Substituted 9,9'-Bianthracene Blue Light Materials

Fig. 7 Schematic diagram of the molecular structure of alkyl- substituted 9,9'-bianthracene blue light-emitting materials

表3 烷基取代9,9'-联蒽类蓝光材料的OLED器件性能总结

Table 3 Summary of OLED devices using alkyl-substituted 9,9 '-bianthracene blue light-emitting materials

Compound EL λ (nm) CE (cd·A-1 PE (lm·W-1 L max (cd·m-2 EQEmax (%) CIE (xy Ref
30a 456 2.26 1.92 3546 2.51 (0.15, 0.10) 48
30b 436 2.76 2.4 3742 3.6 (0.16, 0.08) 48
30c 468 16.54 16.57 13 800 9.47 (0.15, 0.26) 48
31a 440 2.52 2.73 2013 3.2 (0.15, 0.06) 48
31b 436 4.16 3.84 2310 4.56 (0.15, 0.07) 48
31c 460 11.33 10.45 10 900 7.16 (0.15, 0.20) 48
32a 444 2.74 2.26 965 3.94 (0.16, 0.07) 57
32c 460 9.97 8.73 6822 6.96 (0.14, 0.18) 57
33a 436 2.07 1 2284 2.43 (0.17, 0.09) 68
33b 440 2.46 1.45 3860 4.57 (0.15, 0.05) 68
33c 480 20.7 46.24 23 670 8.69 (0.17, 0.40) 68
34a 444 2.35 1.73 3450 1.81 (0.18, 0.15) 68
34b 444 2.93 1.96 5489 4.11 (0.15, 0.07) 68
34c 480 19.24 12.42 20 920 8.42 (0.16, 0.38) 68
35a 440 4.63 4.15 1110 6.11 (0.15, 0.05) 68
35b 444 4.09 3.67 3570 6.67 (0.15, 0.05) 68
35c 476 23.01 23.99 21 190 11.52 (0.16, 0.38) 68
36a 444 1.18 0.57 1423 1.66 (0.15, 0.07) 68
36b 440 4.43 4.21 3316 6.25 (0.15, 0.05) 68
36c 476 18.63 12.9 22 010 7.9 (0.17, 0.40) 68
37a 448 2.64 1.9 2584 3.61 (0.14, 0.09) 67
37b 444 2.58 2.23 2769 4.66 (0.14,0.06) 67
38a 444 2.45 1.62 2364 3.03 (0.15, 0.10) 67
38b 448 2.34 1.77 2978 3.85 (0.14, 0.07) 67
39a 452 2.44 1.08 1444 2.54 (0.14, 0.15) 67
39b 444 2.22 1.74 2775 4.16 (0.14, 0.06) 67
40a 448 2.78 1.94 248 3.72 (0.14, 0.09) 67
40b 440 2.85 2.49 663 5.25 (0.14, 0.06) 67
41a 448 4.77 3.37 217 3.07 (0.15,0.21) 67
41b 440 2.66 2.14 386 5.03 (0.14, 0.05) 67
42a 456 2.03 1.58 1164 2.66 (0.14, 0.09) 70
42b 444 0.95 0.66 3370 1.72 (0.16, 0.07) 70
42c 468 11.2 8.72 21 250 6.89 (0.15, 0.25) 70
43a 452 1.95 1.46 1458 3.16 (0.15, 0.07) 70
43b 448 0.92 0.73 3168 1.51 (0.15, 0.07) 70
43c 464 6.11 3.91 9256 4.11 (0.15, 0.22) 70
44a 448 1.04 0.45 1261 1.57 (0.15, 0.08) 70
44b 444 0.8 0.61 2744 1.1 (0.16, 0.08 70
44c 464 7.35 4.2 9563 4.91 (0.15, 0.22) 70
45a 452 1.75 1.14 1138 2.78 (0.15, 0.07) 70
45b 452 1.07 0.83 2363 1.38 (0.15, 0.09) 70
45c 464 8.64 6.76 9913 5.88 (0.15, 0.21) 70
46a 444 0.63 0.3 963 1.04 (0.15, 0.07) 70
46b 452 1.04 0.77 2569 1.16 (0.15, 0.11) 70
46c 460 4.24 3.11 11 120 2.86 (0.15, 0.21) 70

a Undoped device.b Devices prepared by doping as guest materials.c Devices prepared by doping as host materials.

In 2017, Li et al. successfully synthesized 32 by introducing tert-butylbenzene at the 10 and 10' positions of 3,3'-dimethyl-bianthracene (abbreviated as MBAn) (30). In 32, the bulky tert-butyl effectively prevents π coupling, reduces intermolecular stacking, and improves material performance[57]. When 32 is used as the luminescent host with two different guest dopants to prepare devices, both achieved excellent performance, with a maximum EQE of 6.96% and CIE chromaticity coordinates of (0.16, 0.07). In 2018, Si et al. synthesized a series of 3,3'-dimethyl-bianthracene derivatives 33, 34, 35, and 36 by adjusting the position and number of fluorine substituents. The OLED device based on 35 achieved maximum external quantum efficiencies of 6.65% and 6.15% in non-doped and doped device structures, respectively, with CIE chromaticity coordinates of (0.155, 0.058) and (0.153, 0.055). Particularly, when 35 was used as the host material, the device current efficiency reached 23.01 cd·A-1, and the maximum EQE was 11.52%[68]. Additionally, in 2018, Wang et al. incorporated 35 into the novel luminescent material BBPA, achieving a maximum EQE of 10.27% and CIE chromaticity coordinates of (0.15, 0.05) for the prepared OLED device, validating the effectiveness of the alkyl substitution strategy in optimizing material performance[69]. In 2018, Lv reported five new fluorinated tert-butyl bianthracene derivatives (42, 43, 44, 45, and 46), which demonstrated excellent thermal stability and outstanding optoelectronic efficiency by adjusting the number and distribution of fluorine atoms and trifluoromethyl groups on the benzene ring to modulate the optoelectronic properties of the materials[67]. In 2019, Li et al. found that introducing methyl or tert-butyl groups at the 3- and 3'-positions of the twisted 9,9'-bianthracene structure helps reduce molecular aggregation, providing good thermal properties, film-forming ability, and EL performance[48]. In 2024, Zhu et al. further explored the impact of halogen substitution on MBAn by varying the type, position, and number of halogen substituents on the benzene rings at the 10 and 10' positions of the MBAn core, synthesizing five new MBAn derivatives. When 40 was used as the guest dopant material for the OLED device, the device exhibited excellent electroluminescence performance with an external quantum efficiency as high as 5.25%, and the CIE chromaticity coordinates of the device were (0.14, 0.06)[70].

2.3 Structures and Properties of Other Anthracene-Based Materials

Beyond the exploration of 9,9'-bianthryl and its applications in organic light-emitting diodes, the bianthryl family also encompasses other structurally diverse isomers such as 1,1'-bianthryl, 2,2'-bianthryl, 1,9'-bianthryl, and 2,9'-bianthryl (molecular structures shown in Figure 8). These different structural variants not only broaden the research horizon of bianthryl materials but also provide new avenues for application development in the optoelectronic field. Although derivatives of 1,1'-bianthryl, such as DMBA (47)[71] and (48)[72], have not yet been applied in the optoelectronic field, 2,2'-bianthryl-derived TPBA (49) is often used as a green-light emitting host material or hole transport layer material in OLEDs and organic field-effect transistors[49, 73-77]. Meanwhile, although the derivative development of 1,9'-bianthryl and 2,9'-bianthryl is still in its early stages, they have already demonstrated potential application value in specific fields such as photon upconversion luminescence (TTA)[78-79].
图8 Schematic Diagram of Molecular Structures of Other Anthracene-Based Materials

Fig. 8 Molecular structure schematic diagrams of other 9,9'-bianthracene-based materials.

2.4 BT.2020 Blu-ray

With the development of ultra-high-definition 4K and 8K resolution displays, their unprecedented fine image quality and immersive visual impact are driving the rapid innovation of display light sources. In this context, the BT.2020 standard has emerged as a color benchmark for ultra-high-definition video content, complementing 4K/8K technology and jointly defining the standards for a new generation of visual feasts80. At the same time, the BT.2020 standard broadens the color performance capabilities of these OLED display devices, enabling them to present a wider, more accurate, and vivid color range, providing users with a near real-world visual experience.
Compared with the NTSC standard, the BT.2020 standard allows a 1.5-fold increase in color gamut, covering approximately 75.8% of the CIE 1931 color space chromaticity diagram. Such a large expansion of the color gamut is due to the redefinition of the RGB primary colors' CIE coordinates: red light as (0.708, 0.292), green light as (0.170, 0.797), and blue light as (0.131, 0.046)81. To achieve this leading standard, the development of high color purity blue OLED emitting materials is the core critical issue. Traditional luminescent materials require extremely deep blue emission to meet the BT.2020 blue light standard because their wide spectrum can easily cover adjacent green regions, thereby significantly affecting the CIEy value. Currently, reported luminescent materials with color coordinates close to the BT.2020 blue light standard mainly rely on dynamic combinations of electron-donating/electron-accepting pairs, such as Cz-derived donor units modulating acceptor units or acridine-derived donor units paired with weak acceptor units. Among 9,9'-bianthryl materials, only a few exhibit electroluminescence close to the BT.2020 standard blue light, such as BA (component 1) with an emission peak wavelength of 432 nm and a color coordinate CIEy of 0.0548, and MBAn-(3,4,5)-F (component 35) with an emission peak wavelength of 440 nm and a color coordinate CIEy reaching 0.05368. Therefore, future design of 9,9'-bianthryl blue luminescent materials may achieve appropriate blue shifts while maintaining electroluminescent conversion efficiency by introducing strong electron acceptor units, modulating intramolecular charge transfer, and increasing molecular rigidity, thereby aligning their CIE coordinates with the BT.2020 blue standard.

3 Structure-Property Relationship of 9,9'-Bianthryl Blue Light Materials

3.1 Anthracene Isomerization Effect

In the in-depth exploration of the optoelectronic properties of bianthracene materials, the structural differences between isomers are particularly crucial to their performance. We applied density functional theory (DFT) using the Gaussian 09 software with the B3LYP/6-31G(d) basis set to model and analyze the ground state (S0) structures and frontier molecular orbital energy levels of five different molecular structures of bianthracene (as shown in Figure 4). Through theoretical calculations, we discovered significant effects of bianthracene isomerization in three main aspects. First, different molecular structures result in different spatial configurations, directly affecting the packing modes and interactions between molecules. For example, as can be seen from Figure 4, the dihedral angles between anthracene units in 1,1'-bianthracene, 2,2'-bianthracene, 1,9'-bianthracene, 2,9'-bianthracene, and 9,9'-bianthracene exhibit clear differences, being 76.7°, 90.3°, 38.2°, 75.8°, and 90.0° respectively. These structural twists not only enhance the thermal stability of the material but also promote fluorescence quantum yield by increasing molecular rigidity and reducing the possibility of non-radiative transitions. In particular, the reduction in intermolecular interactions in orthogonal structures helps mitigate fluorescence quenching caused by molecular aggregation, thereby further enhancing luminescence efficiency. Second, variations in molecular structure also affect electronic structure, influencing electron injection and transport properties in devices. For instance, 1,9'-bianthracene exhibits HOMO and LUMO distributions on different anthracene units, unlike other structures centered on bianthracene, suggesting that 1,9'-bianthracene may have stronger intramolecular charge transfer, potentially causing a redshift in the spectrum, which is detrimental to luminescent performance. Finally, different molecular structures lead to variations in photophysical properties. In 2021, Zhao et al. found that 9,9'-bianthracene with an orthogonal geometry showed more pronounced solvent polarity dependence, longer fluorescence lifetimes, and higher spin-orbit charge transfer efficiency compared to the more coplanar 2,9'-bianthracene[78]. In 2023, Kang et al.'s research further explored the differences in symmetry-breaking charge transfer (SB-CT) between 9,9'-bianthracene and TPBA due to different anthracene unit connection methods, revealing significant short-range CT coupling differences between geometrically planarized TPBA and 9,9'-bianthracene[82]. Through these comprehensive analyses, we not only deepen our understanding of the effects of bianthracene isomerization on material optoelectronic properties but also provide important scientific guidance for the future design and development of high-performance blue OLED materials.

3.2 Halogen Substitution Effect

The substitution of halogen has a significant impact on the performance of bianthracene-based blue light organic electroluminescent diode materials. Research has found that derivatives introducing halogen atoms onto the bianthracene molecular skeleton exhibit superior electroluminescent performance in OLED devices. This enhancement in performance is mainly attributed to the unique electrochemical properties of halogen atoms, including their high electronegativity, which enables them to effectively attract electron clouds within the molecule, thereby lowering the molecular energy levels. Additionally, halogen substitution can significantly reduce the injection barriers for electrons and holes, promoting efficient transmission of electrons and holes, thus enhancing the luminescence efficiency of OLED devices. The introduction of halogen atoms not only affects electronic properties but also significantly improves the overall stability of the material. The carbon-halogen bonds formed between halogen and carbon atoms possess high polarity and bond energy, greatly enhancing the chemical stability of halogen-substituted bianthracene derivatives under light or current excitation. Moreover, the presence of halogen atoms reduces π-π stacking between molecules, diminishing fluorescence quenching caused by molecular aggregation, thereby further optimizing luminescent performance. Halogen substitution not only alters the electronic structure and optical properties of molecules but also plays an important role in their spatial arrangement and intermolecular interactions. For instance, halogen substitution can improve intermolecular interactions and packing modes by influencing the three-dimensional configuration of molecules, which is crucial for controlling exciton formation and transport in OLED devices. Furthermore, halogen substitution can optimize the photophysical properties of molecules, such as adjusting emission wavelengths and increasing quantum yields, providing new strategies for designing and developing high-performance blue light OLED materials. In summary, the halogen substitution effect plays a key role in enhancing the performance of bianthracene-based blue light OLED materials. By finely tuning the position and quantity of halogen substitutions, the electronic structure and optoelectronic properties of materials can be further optimized, paving new pathways for developing highly efficient and stable OLED luminescent materials.

3.3 Asymmetric Effect

In the technology of organic light-emitting diodes, the asymmetric effect of bianthracene molecules plays a crucial role. This effect is mainly reflected in two aspects: first, the asymmetrically substituted 9,9'-bianthracene molecules affect the properties of their excited states, including local excited states and charge transfer states, by altering their electronic structure. Such structural changes can not only cause shifts in absorption and emission spectra but also alter fluorescence quantum yield. Additionally, the typical symmetry-breaking charge transfer characteristics of 9,9'-bianthracene enable directional charge separation under low driving force, providing a new approach for optimizing the performance of OLED materials. By applying asymmetric substitution to 9,9'-bianthracene, researchers are able to modulate the carrier migration rate, thereby optimizing the overall performance of OLED devices. This asymmetric substitution strategy not only helps achieve higher luminescence efficiency and a broader range of emission spectrum modulation but also effectively enhances the thermal stability and operational lifetime of the materials.
With the deepening understanding of the influence mechanism of asymmetric effects, researchers have begun to explore the use of this effect to design and synthesize novel bianthracene derivatives. For instance, by introducing various functional side chains at different positions of 9,9'-bianthracene, the electronic properties and spatial configuration of the molecules can be further adjusted, thereby achieving precise control over OLED performance. These studies not only broaden our understanding of the application of asymmetric effects in optoelectronic materials but also provide important design principles and synthetic strategies for developing next-generation high-efficiency blue-light OLED materials.

3.4 Steric Hindrance Effect

Appropriate steric hindrance can not only prevent over-dense packing between molecules, thereby reducing exciton quenching, but also significantly improve charge transport efficiency. By introducing bulky side chains or functional groups, intermolecular π-π interactions can be effectively reduced, thus decreasing aggregation-induced fluorescence quenching and improving the thermal stability of the material.
Recent studies have shown that steric hindrance not only improves the electronic and photophysical properties of molecules but also directly affects the photoelectric conversion efficiency and lifetime of OLED devices by regulating molecular packing modes and charge transport characteristics. In particular, designing molecules with steric hindrance can promote more efficient charge injection and balance charge transport, thereby improving the operational lifetime of the device. For example, deep blue OLEDs based on TBBA anthracene derivative 30 have a half-life of only 42 minutes at an initial brightness of 500 cd·m-2 when the device is placed in an inert gas glove box48. However, the deep blue OLED device using the DITBDAP emitter, developed through effective steric hindrance effects, has been reported by researchers to achieve a T95 lifetime of up to 250 hours under high brightness conditions of 3000 cd·m-2. In fact, compared to red and green OLED devices, blue OLED devices generally exhibit shorter operational lifetimes. Therefore, adopting an effective steric hindrance strategy enables the development of high-performance 9,9'-bianthracene derivative blue light materials that combine efficiency and stability83.
Moreover, specific spatial steric configurations can lead to large Stokes shifts and multiple emission phenomena, providing new possibilities for achieving wide color gamut and high color purity in OLED display technology. Additionally, the spatial steric effect also helps to break the conversion barrier between triplet and singlet states, improving exciton utilization. This is particularly important for enhancing the luminescence efficiency of OLEDs, as the formation and utilization of triplet excitons is one of the key strategies for improving luminescence efficiency in many OLED materials.

3.5 Design of Blue Light Device Structure

By intricately designing and synthesizing bianthracene-based luminescent materials with effective steric hindrance, the performance of OLED devices can be significantly enhanced, including improvements in luminous efficiency, thermal stability, and extended device lifespan. However, compared to red and green luminescent materials, wide-bandgap 9,9'-bianthracene blue luminescent materials exhibit lower exciton utilization and are prone to efficiency roll-off at high current densities. Particularly for blue phosphorescent and TADF luminescent materials, high emission energy (>2.7 eV) and long triplet exciton lifetimes can easily lead to hot exciton or hot polaron losses through exciton annihilation processes, thereby accelerating material and OLED device aging. An effective approach is to disperse 9,9'-bianthracene blue luminescent materials in a high-energy host to reduce exciton concentration and mitigate exciton annihilation. Typically, the host exciton energy in the emissive layer of OLED devices does not generate photons directly through the host but is transferred to 9,9'-bianthracene blue dopant guests via Förster and Dexter energy transfer mechanisms. Ultimately, the dopant radiatively emits light with the assistance of the host suppressing intermolecular interactions and non-radiative quenching. Based on this, by optimizing factors such as the thickness, energy level matching, and doping concentration of adjacent transport layer materials, it ensures that carriers can be efficiently balanced and transported across functional layers and recombine to emit light efficiently, thereby enhancing the electroluminescent blue light performance of the device. Currently, the performance of wide-bandgap host materials such as exciplex hosts, mixed hosts, and bipolar hosts surpasses that of unipolar hosts and single TADF hosts84. Among mixed hosts, exciplexes outperform traditional mixed hosts in terms of device lifetime. In singlet hosts, bipolar and TADF hosts exhibit better device longevity than unipolar hosts. Therefore, different device structures need to be designed based on the performance of various hosts to maximize the regulation of 9,9'-bianthracene blue OLED performance.

4 Conclusions and Prospects

4.1 Conclusion

This paper delves into the application of 9,9'-bianthracene-based blue light-emitting materials in organic light-emitting diodes (OLEDs), providing a detailed analysis of their molecular structure, performance relationship, and impact on the performance of OLED devices. By systematically reviewing research progress on symmetric structures, asymmetric structures, and alkyl-substituted 9,9'-bianthracene-based blue light-emitting materials, it highlights the importance of these materials in improving the performance of blue light OLED devices. In addition to conventional bianthracene derivatives, this review also extends to other members of the bianthracene family, such as 1,1'-bianthracene and 2,2'-bianthracene, which, although still in the exploratory stage for OLED applications, have already shown potential in the optoelectronic field. The research on these materials not only broadens the application scope of bianthracene-based materials but also provides new ideas and directions for future optoelectronic device design and development. Finally, special emphasis is placed on the important role of bianthracene isomerization effects, fluorine substitution effects, asymmetric effects, and steric hindrance effects in modulating the luminescence performance of blue light OLED materials. These molecular design strategies are not only significant for understanding the fundamental science of blue light OLED materials but also have practical application value for designing and developing novel high-efficiency blue light-emitting materials.

4.2 Prospects

To further develop high-performance bianthracene-based blue light OLED materials and devices, the future research work on 9,9'-bianthracene will mainly progress from the following aspects:

(1) Novel Anthracene-Based Molecular Structural Engineering

New anthracene-based molecular designs can achieve breakthroughs in four aspects: First, by introducing conjugated aromatic substitutions parallel and perpendicular to the minor axis of the molecular backbone, it is possible to control the ICT process in 9,9'-bianthryl compounds as well as the proportion of LE state emission. In 2019, Komskis et al. found that aromatic substitution along the ICT reaction path led to an extension of the π-electron conjugation, which enhanced the oscillator strength and also increased LE state emission, favoring blue light emission85. Second, expand the application of halogens; it is known that fluoro-substituted bianthryl exhibits excellent EL performance, and chlorine or bromine substitution may theoretically improve device performance. Bromine and chlorine atoms have a heavy atom effect, which enhances spin-orbit coupling and increases exciton utilization efficiency. Third, enrich asymmetric structures by finding suitable substituents, which can stabilize the charge transport properties of materials and further enhance device performance. Fourth, full deuterium substitution; in 2024, Huang et al. found that full deuteration effectively suppresses high-frequency molecular vibrations, thereby reducing vibrational coupling, lowering non-radiative transition rates, and significantly improving external quantum efficiency86. Some commercial blue fluorescent materials also adopt deuteration strategies, indicating that full deuteration is a wise choice.

(2) In-Depth Study of the Luminescence Mechanism

In future research, an in-depth exploration of the luminescence mechanism of 9,9'-bianthracene materials will be crucial for enhancing OLED performance. This review has already touched upon the importance of improving material properties through molecular design strategies such as isomerization, fluorine substitution, asymmetric structure design, and steric hindrance effects. Moving forward, research should focus more on understanding how these molecular designs affect the electronic structure, exciton formation, and migration mechanisms of materials, as well as how they work together to improve the photoelectric conversion efficiency and stability of OLEDs. Advanced characterization techniques, such as time-resolved fluorescence spectroscopy, electron spin resonance, and ultra-high-resolution microscopy, can reveal finer intermolecular interactions and exciton dynamics. Additionally, establishing more accurate theoretical models and computational methods to predict the luminescent performance of new materials will greatly accelerate the discovery and optimization of high-performance OLED materials. With a deeper understanding of the luminescence mechanism, we anticipate developing OLED materials with higher efficiency, longer lifespan, and broader color gamut, thereby promoting the application prospects of OLED technology in display and lighting fields.

(3) Anthracene-Based Phosphorescent/TADF Molecules

With the deepening research on blue light materials, significant breakthroughs have been achieved in highly efficient and stable blue phosphorescent materials. For instance, in 2024, Forrest et al. from the University of Michigan utilized polarization-enhanced Purcell effects, increasing the lifetime of blue phosphorescent OLEDs by 5.3 times compared to traditional devices, and extending the lifetime by up to 14 times when compared with similar deep-blue phosphorescent OLEDs87. In the market aspect, the leading OLED material company, UDC Corporation of the United States, previously announced that "preparations for mass production of blue phosphorescent OLED components will be completed by the end of 2024," indicating that the cost bottleneck of blue phosphorescence may no longer be the main issue hindering its industrialization. Recently, Flask Co., Ltd. of Japan stated that it has successfully developed highly efficient blue TADF materials with initial industrialization capabilities using existing patented technologies. Therefore, developing phosphorescent or TADF molecular derivatives with 9,9'-bianthryl groups will bring new development opportunities for such materials and promote the industrialization of blue OLED materials.

(4) Challenges of Large Size and Flexible OLED

With the increasing demand for display quality, high-resolution, wide color gamut, and high color rendering index OLED screens have been continuously introduced to the market, becoming the top choice for smartphones and wearable devices in pursuit of the ultimate visual experience. With the development of small-sized OLED panels, medium and large-sized and flexible OLED panels can be widely used in tablets, automotive displays, large-sized TVs, etc., and are gradually expanding into diversified application scenarios. At present, manufacturers at home and abroad are also constantly exploring new design forms such as curved screens, full-screen displays, and foldable screens, all of which rely on the support of OLED technology. Especially for foldable screens, their unique folding mechanism highly depends on flexible OLED displays, thereby promoting the rapid development and application of flexible OLED technology. Although 9,9'-bianthracene blue light materials have shown excellent optoelectronic performance under laboratory conditions, achieving their application in flexible and large-sized OLEDs still requires overcoming a series of challenges. As flexible OLED panels use bendable plastic substrates instead of traditional glass substrates, this places higher demands on the flexibility, heat resistance, and corrosion resistance of 9,9'-bianthracene materials. Meanwhile, the high stability of 9,9'-bianthracene blue light materials is another challenge, requiring that material performance remains unaffected during bending and folding processes. To achieve good bending performance, flexible OLEDs need to adopt special packaging structures and multi-layer designs. These designs must ensure no delamination or cracking between layers when bent, while 9,9'-bianthracene luminescent materials maintain stable electrical and optical performance under these special conditions.

(5) New Technology for Crystallized Thin-Film OLED

The disordered molecular arrangement in non-crystalline materials leads to lower carrier mobility, limiting the carrier transport capability and photon output capability, thereby affecting device efficiency[88]. Crystalline thin-film organic light-emitting diodes (C-OLED) can achieve high light emission and low Joule heat loss at low driving voltages[89].
Future commercial applications require new materials with low synthetic complexity, low cost, green solvent processing, and high stability, as well as advanced device fabrication processes and assembly techniques. Moreover, achieving a high color gamut index and high external quantum efficiency for blue OLEDs is crucial for promoting the development and large-scale application of white lighting and display technologies. In summary, through in-depth research and development of 9,9'-bianthracene-based blue OLED materials, it is expected to overcome the limitations of existing technologies and pave the way for next-generation high-performance and high-efficiency OLED lighting and display technologies.
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