Accepted 00th January 20xx
接受日期 20xx 年 1 月 00日

DOI: 10.1039/x0xx00000x
DOI：10.1039/x0xx00000x

Recent progress of multi-resonance thermally activated delayed fluorescence emitters with efficient reverse intersystem crossing process
高效逆系间窜越多共振热激活延迟荧光发射体研究进展​​​​​​​​​

Xu-Feng Luo,^a,b Xunwen Xiao*^,a and You-Xuan Zheng*^,b
Xu-Feng Luo, ^{a b}肖迅文* ^a和You-Xuan Cheng* ^b

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have become an active research topic in the forefront of organic light-emitting diodes (OLEDs) owing to their excellent photophysical properties such as high efficiency and narrow emission characteristics. However, MR-TADF materials always exhibit slow reverse intersystem crossing rates (k_RISC) due to the large energy gap and small spin orbit coupling values between singlet and triplet excited states. In order to optimize RISC process, strategies such as heavy-atom-integration, metal perturbation, π-conjugation extension and peripheral decoration of donor/acceptor units have been proposed to construct the efficient MR-TADF materials for high-performance OLEDs. This article provides an overview of the recent progress in MR-TADF emitters with efficient RISC process, focusing on the structure-activity relationship between molecular structure, optoelectronic feature, and OLED performance. Finally, the potential challenges and future prospects of MR-TADF materials are discussed to gain a more comprehensive understanding of the opportunities for efficient narrowband OLEDs.
多共振热激活延迟荧光（MR-TADF）发射器由于其高效率和窄发射特性等优异的光物理特性，已成为有机发光二极管（OLED）前沿的活跃研究课题。然而，由于单重态和三重态激发态之间的能隙大和自旋轨道耦合值小， MR-TADF材料总是表现出缓慢的反向系间窜越率s （ k _RISC ）。为了优化RISC工艺，提出了重原子集成、金属微扰、π共轭延伸和给体/受体单元外围装饰等策略来构建用于高性能OLED的高效MR - TADF材料。本文概述了采用高效 RISC 工艺的 MR-TADF 发射器的最新进展，重点关注分子结构、光电特性和 OLED 性能之间的构效关系。 最后，重新讨论了MR-TADF材料的潜在挑战和未来前景，以更全面地了解高效窄带OLED的机会。

Since the groundbreaking work of Tang and Vanslyke in the 1980s,¹ the development of organic light-emitting diodes (OLEDs) has been thriving for decades due to the immense potential in flat-panel displays and solid-state lighting.^2-4 Generally, extensive efforts have been made to improve the OLED performance by focusing on efficient organic emitters, which exhibits a high exciton utilization rate to achieve high electroluminescence (EL) efficiency and possess a narrowband emission profile for achieving high color purity.⁵ According to the spin statistics under the electrical excitation, the lowest singlet (S₁) and triplet (T₁) excitons are generated in a ratio of 1:3.⁶ Consequently, the strategy to utilize the non-emissive T₁ excitons has become a hot topic in the research of OLEDs.^7-9 By employing the metal atoms such as iridium (Ir(III)) or platinum (Pt(II)) to significantly enhance spin-orbit coupling (SOC), both S₁ and T₁ excitons can be effectively used for phosphorescence emission, theoretically achieving 100% internal quantum efficiency (IQE).^10-16 However, considering the expensive price disadvantage faced by these phosphors in production applications, new high-performance emitters should be explored.
自 20 世纪 80 年代 Tang 和 Vanslyke 的开创性工作以来¹ ，由于平板显示器和固态照明的巨大潜力，有机发光二极管 (OLED) 的发展几十年来一直蓬勃发展。 ^2-4一般来说，人们通过关注高效的有机发射器来努力提高 OLED 性能，这种发射器具有高激子利用率，可实现高电致发光 (EL) 效率，并具有窄带发射曲线，可实现高色纯度。 ⁵根据电激发下的自旋统计，最低单线态(S ₁ )和三线态(T ₁ )激子以1:3的比例产生。 ⁶因此，利用非发射性T ₁激子的策略已成为OLED研究的热点。^7-9通过使用铱（ Ir （III） ）或铂（Pt （II） ）等金属原子来显着增强自旋轨道耦合（SOC），S ₁和T ₁激子都可以有效地用于磷光发射，理论上实现100%的内量子效率（IQE）。 ^10-16然而，考虑到这些荧光粉在生产应用中面临昂贵的价格劣势，应探索新的高性能发射器。

To overcome these drawbacks, the adoption of noble metal-free thermal activated delayed fluorescence (TADF) emitters has emerged as a promising solution in OLED applications.^5,7,17 In general, spatially twisted donor (D) and acceptor (A) structures facilitates the reduction of exchange integral (J) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thus resulting in a small energy gap (∆E_ST) between S₁ and T₁ excited states.¹⁸ For traditional TADF materials, the structural relaxation and vibrational coupling between the excited and ground states are severe, rendering them with large Stokes shifts and wide emission spectra (> 70 nm) (Scheme 1). Even though optical techniques such as filters or microcavities^19,20 can be relied on to filter out the edge regions of the original wide EL spectra to improve color purity, these treatments will increase product costs and power consumption. Therefore, the development of TADF materials that can balance color purity, emission efficiency, cost, and power consumption is urgent.
为了克服这些缺点，采用不含贵金属的热激活延迟荧光 (TADF) 发射器已成为 OLED 应用中一种有前途的解决方案。 ^5,7,17​​ 一般来说，空间扭曲的供体（D）和受体（A）结构有利于降低最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）之间的交换积分（ J ），从而产生在S ₁和 T ₁激发态之间的小能隙 (Δ E _ST )中。 ¹⁸对于传统的 TADF 材料，激发态和基态之间的结构弛豫和振动耦合很严重，使其具有较大的斯托克斯位移和较宽的发射光谱 (> 70 nm ) （方案 1） 。尽管可以依靠诸如滤光片或微腔^19,20之类的光学技术滤除原始宽EL光谱的边缘区域以提高色纯度，但这些处理将增加产品成本和功耗。因此，开发能够平衡色纯度、发射效率、成本和功耗的TADF材料刻不容缓。

In 2016, Hatakeyama et al. proposed an innovative molecular construction strategy for designing polycyclic aromatic hydrocarbons (PAHs, DABNA-1 and DABNA-2) with embedded heteroatoms, also known as multiple resonance TADF (MR-TADF) materials.²¹ The electron-withdrawing (B atom) and electron-donating (N atom) are arranged at the relative positions of the benzene ring. Through the localization effect, HOMO and LUMO are induced to alternately distribute within the entire resonance skeleton. Benefitting from the short-range charge transfer (SR-CT) effect (Scheme 1), high rate of radiation transition and narrow emission spectra can be achieved simultaneously. Correspondingly, OLED based on DABNA-1 exhibited an ultra-pure EL emission with a full width at half-maximum (FWHM) of only 28 nm, which is significantly lower than that of current commercial blue OLEDs (40-60 nm), and is almost a symbol of epoch-making. Moreover, the maximum external quantum efficiency (EQE_max) can reach 13.5%. By modifying the peripheral benzene ring and diphenylamine, the EL emission of DABNA-2 red-
2016 年，Hatakeyama 等人。提出了一种创新的分子构建策略，用于设计具有嵌入杂原子的多环芳烃（PAHs 、 DABNA-1和DABNA- 2 ），也称为多重共振TADF （MR-TADF）材料。 ²¹吸电子（B原子）和给电子（N原子）排列在苯环的相对位置上。通过局域化效应，HOMO和LUMO被诱导在整个共振骨架内交替分布。受益于短程电荷转移（SR-CT）效应（方案1） ，可以同时实现高辐射跃迁率和窄发射光谱。相应地，基于DABNA-1的OLED表现出超纯EL发射，半峰全宽（FWHM）仅为28 nm，明显低于目前商业化的蓝色OLED（40-60 nm） ，并且几乎是划时代的象征。此外，最大外量子效率（ EQE _max ）可达13.5%。通过修饰外围苯环和二苯胺， DABNA-2的EL发射红光

Scheme 1. Emission principles corroborating with D-A type TADF and MR-TADF materials
方案1. E任务原理与DA型TADF和MR-TADF材料的佐证.

Figure 1. strategies for accelerating k_RISC of MR-TADF materials.
图 1.加速 MR-TADF 材料的k _RISC的策略。

In this article, we systematically summarize the recent progress of MR-TADF materials with efficient RISC processes, and their OLED performances. The main purpose of this article is to reveal a comprehensive understanding of the structure-activity relationship between molecular structure design and optoelectronic properties. Besides, the challenges in this field were also considered to provide assistance in optimizing the performance of narrowband OLEDs.
在本文中，我们系统地总结了高效RISC工艺的MR-TADF材料的最新进展及其OLED性能。本文的主要目的是揭示对分子结构设计与光电性能之间的构效关系的全面理解。此外，该领域的挑战也被认为可以为优化窄带OLED的性能提供帮助。

1. The strategies for accelerating k_RISC
1. k _RISC加速策略

As is well known, in the EL process of TADF materials, the ratio of S₁ and T₁ excitons is 1:3 according to the spin-statistics. The dissipation of S₁ excitons can be achieved through three pathways: fluorescence radiative transition,²² intersystem crossing (ISC) behavior,²³ and non-radiative transition.²⁴ Interestingly, the T₁ excitons can up-convert to the emissive S₁ state by a RISC process for a delayed fluorescence (DF),²⁵ with a clear microsecond delayed lifetime. Therefore, in the process of implementing TADF progress, the behaviour of T₁ excitons is an important factor that restricts EL performance of TADF materials.
众所周知，在TADF材料的EL过程中，根据自旋统计， S ₁和T ₁激子的比例为1:3。 S ₁激子的耗散可以通过三种途径实现：荧光辐射跃迁、 ²²系间窜越（ISC）行为、 ²³和非辐射跃迁。 ²⁴有趣的是， T ₁激子可以通过 RISC 过程上转换为发射S ₁态，从而产生延迟荧光(DF)， ²⁵具有明显的微秒延迟寿命。因此，在TADF进展的实施过程中， T ₁激子的行为是制约TADF材料EL性能的重要因素。

To maximize the utilization of S₁ and T₁ excitons, enhancing efficient RISC process is a key to avoid the triplet-triplet annihilation (TTA)²⁶ or singlet-triplet annihilation (STA).²⁷ According to the Fermi's Golden Rule, k_RISC in the TADF process can be quantified as a ratio of SOC and ∆E_ST between S₁ and T₁ states, as shown in the equation: k_RISC ≈ |<S₁|Ĥ_SOC|T₁>/ΔE_ST|².^28,29 Therefore, the most intuitive strategy for accelerating k_RISC of MR-TADF emitters is to increase SOC between S₁ and T₁, as well as, reduce ΔE_ST, to improve the performance of MR-TADF-based OLEDs.
为了最大限度地利用S 1_和T ₁激子，提高高效RISC工艺是避免三重态-三重态湮灭(TTA) ²⁶或单重态-三重态湮没(STA)的关键。²⁷根据费米黄金法则， TADF 过程中的k _RISC可以量化为S ₁和 T ₁状态之间SOC和 Δ E _ST的比值，如公式所示： k _RISC ≈ |<S ₁ Ĥ _SOC | T ₁ >/Δ E _ST ^{2 28,29}因此，加速 MR-TADF 发射器k _RISC最直观的策略是增加 S ₁和 T ₁之间的 SOC ，同时减小 Δ E _ST ，以提高基于 MR-TADF 的 OLED 的性能。

2. Increasing SOC value
2.增加SOC值

SOC is the dominant force driving spin conversion,³⁰ and typically, the value of SOC is proportional to Z⁴, where Z is the nuclear charge.^31,32 Therefore, the introduction of heavy atoms such as sulfur (S), and selenium (Se) can easily induce large SOC in MR-TADF materials, resulting in a high k_RISC (>10⁵ s^-1).
SOC 是驱动自旋转换的主导力， ³⁰ ，通常，SOC 的值与Z ⁴成正比，其中Z是核电荷。 ^31,32因此，硫（S）和硒（Se）等重原子的引入很容易在MR-TADF材料中诱发大的SOC，从而产生高k _RISC （>10 ⁵ s ^-1 ）。

2.1 Heavy-atom-integration MR-TADF molecules
2.1 重原子整合MR - TADF分子

The heavy atom effect can accelerate the SOC between S₁ and T₁ states, and the organic integration of heavy atom effect and MR-TADF material design has been proven to be a strategy to induce efficient TADF performance. Several MR-TADF materials incorporating heavy elements, such as S and Se, have recently been produced. These materials can serve as breakthrough emitters that enable fast spin interconversion between the excited S₁ and T₁ states. In this section, a heavy-atom family of multi-heteroatom MR-TADF materials is presented.
重原子效应可以加速S ₁和T ₁态之间的SOC ，并且重原子效应与MR - TADF材料设计的有机结合已被证明是诱导高效TADF性能的策略。最近已经生产出了几种含有S和Se等重元素的 MR-TADF 材料。这些材料可以作为突破性的发射体，实现激发的S ₁​​ 和 T ₁态之间的快速自旋相互转换。在本节中，介绍了多杂原子 MR-TADF材料的重原子族。

2.1.1 Sulfur-containing MR-TADF emitters
2.1.1含硫MR - TADF发射体

Because the incorporation of S atom facilitates a spin-flip RISC progress via the heavy-atom effect, MR-TADF materials containing S atom have received particular attention (Figure 2). In 2021, Yang et. al. developed a S-containing MR-TADF material, 2PTZBN,³³ by replacing the diphenylamine fragments of DABNA-1 with S-bridged phenothiazine units. Due to the enhanced SOC, a high k_RISC of 2.76 × 10⁵ s^-1 was obtained, and the 2PTZBN-based OLED exhibited an EQE_max of 25.5% with a low efficiency roll-off. Similarly, Lu et. al. reported two green S-containing MR-TADF materials, Cz-PTZ-BN and 2Cz-PTZ-BN,³⁴ whose emissions peaking at 510 and 505 nm with small FWHMs of 37 and 39 nm in toluene, respectively. Benefitting from the asymmetrically peripheral protection and enhanced SOC values, high k_RISC values of 0.81 × 10⁵ and 1.05 × 10⁵ s^-1 and high photoluminescence quantum efficiencies (PLQYs) were obtained. As a result, OLEDs based on Cz-PTZ-BN and 2Cz-PTZ-BN displayed pure-green emissions with EQEs of 26.6% and 32.8%. Notably, even at 1000 cd m^-2, two OLEDs still maintained EQEs of 17.3% and 23.5%. Interestingly, Zhu et. al. synergistically disrupted the symmetry of the peripheral region by combining phenol thiazide and acridine substituents (BN5),³⁵ which increases the racemic barrier, and induces a circularly polarized luminescence (CPL). Efficient RISC processes with a high k_RISC of 1.6 × 10⁵ s^-1 and a short delayed lifetime of 8.3 μs for BN5 were achieved due to the heavy S atom effect. Consequently, chiral enantiomers of BN5-based OLED showed EQEs of 20.6% and 19.0%, respectively. Very recently, our group introduced the S-substituted spirofluorene structure featuring asymmetric chiral C-center into the multiple resonance skeleton and obtained a pure-green emitter, Spiro-BNCz,³⁶ with CPL feature. The introduction of S atom also endowed Spiro-BNCz with a high <S₁|Ĥ_SOC|T₁> of 0.36 cm^-1 and a high k_RISC of 2.2 × 10⁵ s^-1. The Spiro-BNCz-based OLED exhibited an EQE_max of 34.2% with a low efficiency roll-off. These works provide theoretical and experimental basis for the efficient combination of S-containing motif and MR-TADF skeleton.
由于S原子的掺入通过重原子效应促进了自旋翻转RISC的进展，因此含有S原子的MR-TADF材料受到了特别关注（图2）。 2021年，杨等人。等人。通过用 S-桥联吩噻嗪单元替换DABNA-1的二苯胺片段，开发了一种含 S 的 MR-TADF 材料2PTZBN ³³ 。由于SOC的增强，获得了2.76 × 10 ⁵ s ^-1的高k _RISC ，并且基于2PTZBN的OLED表现出25.5%的n EQE _max和低效率滚降。同样， Lu等人。等人。报道了两种绿色含硫 MR-TADF 材料， Cz -PTZ-BN和2Cz-PTZ-BN ^34，其发射峰在 510 和 505 nm，在甲苯中的小半高宽分别为 37 和 39 nm。受益于不对称的外围保护和增强的 SOC 值，高k _RISC值为 0。获得了81×10 ⁵和1.05×10 ⁵ s ^-1以及高光致发光量子效率( PLQYs ) 。结果，基于Cz -PTZ-BN和2Cz-PTZ-BN的OLED显示出纯绿光发射， EQE分别为26.6%和32.8%。值得注意的是，即使在1000 cd m ^-2下，两种OLED仍然保持17.3%和23.5%的EQE 。有趣的是，朱等人。等人。通过组合苯酚噻嗪和吖啶取代基（ BN 5 ）， ³⁵协同破坏外围区域的对称性，从而增加外消旋势垒，并诱导圆偏振发光（ CPL ）。高效RISC 工艺，具有1.6 × 10 ⁵ s ^-1的高k _RISC和8 的短延迟寿命。由于重 S 原子效应， BN 5达到了 3 μs 。因此， BN 5基OLED的手性对映体的EQE分别为20.6% 和19.0%。最近，我们课题组将具有不对称手性C中心的S取代螺芴结构引入到多重共振骨架中，得到了具有CPL特征的纯绿光发射体Spiro-BNCz ³⁶ 。 S原子的引入还赋予Spiro- BNCz 0.36 cm ^-1的高<S ₁ Ĥ _SOC |T ₁ >和2.2 × 10 ⁵ s ^-1的高k _RISC 。基于 Spiro-BNCz 的 OLED的n EQE最大值为₃₄ 。2%，效率滚降较低。这些工作为含S基序与MR-TADF骨架的有效结合提供了理论和实验基础。

Unlike introducing S atom at the peripheral N/B/N resonance skeleton mentioned above, Yasuda et al. reported a N/B/S-based MR-TADF emitter, CzBS.³⁷ Based on the resonance mode of N/B/S, CzBS still exhibited blue emission with a FWHM of 28 nm in toluene solution. Notably, a high k_RISC of 4.0 × 10⁵ s^-1 was achieved because of the direct resonance participation of S atom in N/B/S system. The OLED based on CzBS displayed a sky-blue emission peaking at 473 nm with a FWHM of 31 nm, an EQE_max of 23.1%, accompanied by an alleviated efficiency roll-off. To enrich the MR-TADF molecular library based on N/B/S resonance mode, Yasuda et. al. proposed a nanographitic fused-nonacyclic π-system (BSBS-N1).³⁸ BSBS-N1 showed a narrowband sky-blue emission, peaking at 478 nm, with a FWHM of 24 nm. Additionally, the SOC value was enhanced by incorporating two S atoms, thereby offering a high SOC value of 0.31 cm^-1 between S₁ and T₁. The BSBS-N1 based OLED achieved an
与上述在外围 N/B/N 共振骨架引入 S 原子不同，Yasuda 等人。报道了一种基于 N/B/S 的 MR-TADF 发射体CzBS ³⁷基于 N/B/S 的共振模式， CzBS在甲苯溶液中仍然表现出半高宽为 28 nm 的蓝色发射。值得注意的是，由于N/B/S体系中S原子的直接共振参与，实现了4.0×10 ⁵ s ^-1的高k _{R ISC} 。基于CzBS的 OLED显示出天蓝色发射峰，峰值波长为 473 nm，半高宽为 31 nm ， n EQE_最大值为 23.1%，同时效率滚降得到缓解。为了丰富基于NB S共振模式的MR-TADF分子库，Yasuda等人。等人。提出了纳米石墨稠合非无环π-系统（ BSBS-N1 ）。 ³⁸ BSBS-N1显示窄带天蓝色发射，峰值为 478 nm，半高宽为 24 nm。此外，通过加入两个S原子提高了SOC值，从而提供了高达0的SOC值。S ₁和T ₁之间为31cm ^-1 。基于BSBS-N1的OLED实现了n

Figure 2. Molecular structures of S-containing MR-TADF emitters.
图2.含硫 MR-TADF 发射体的分子结构。

EQE_max of 21.0%, with a suppressed efficiency roll-off. The characterization of related photophysical properties revealed that embedding various boron, nitrogen, oxygen, and sulfur heteroatoms in the fused polycyclic π-system can finely regulate the narrowband pure-blue and deep-blue emissions of the emitters, and achieved efficient RISC processes.³⁹ Consequently, OLEDs based on BOBS-Z, and BSBS-Z demonstrated narrowband and ultrapure-blue EL emissions, peaking at 456/463 nm and FWHMs of 22/23 nm, leading to CIEy coordinates of 0.06/0.08. Particularly, the EQE_maxs of two OLEDs reached 26.9% and 26.8%, respectively, and small efficiency roll-offs at a practical luminance were achieved.
EQE_最大值为 21.0%，效率滚降受到抑制。相关光物理性质的表征表明，在稠合多环π体系中嵌入各种硼、氮、氧和硫杂原子可以精细调控发射器的窄带纯蓝色和深蓝色发射，并实现高效的RISC过程。 ³⁹因此，基于BOBS-Z和BSBS-Z的 OLED表现出窄带和超纯蓝色 EL 发射，峰值为 456 463 nm，半高宽为 22 23 nm，导致CI​​E y坐标为 0.06 0.08。特别是，两种OLED的EQE _max s分别达到26.9%和26.8%，并且在实际亮度下实现了较小的效率滚降。

In 2022, our group developed two ternary N/B/S-based polycyclic heteroaromatic emitters (SBSN and DBSN) from bluish-green (489 nm) to yellow (553 nm) via tuning the coordination between B/N and S atom, aiming to increase CT delocalization of the emitters and improve photophysical properties.⁴⁰ This strategy endows the two emitters with small FWHMs of 27 and 28 nm, respectively. Additionally, considerable k_RISC values were achieved owing to the small ∆E_ST values of 0.10 - 0.13 eV and large SOC values. Consequently, the OLEDs emitting bluish-green and yellow emissions based on these two emitters showed the EQE_maxs of 17.6% and 21.8%, respectively, with low efficiency roll-offs. Subsequently, by the ternary combination of para-arrayed B-π-B, N-π-N, and S-π-S, Wang et. al. developed two red narrowband emitters, DBNS and DBNS-tBu,⁴¹ whose emissions peaking at 631 and 641 nm with FWHMs of 40 and 39 nm in toluene. Benefiting from the heavy-atom effect of S atom, high k_RISC values of 2.1 × 10⁵ s^-1 and 2.2 × 10⁵ s^-1 and short τ_ds of 11.2 and 10.2 μs were obtained. These results indicated that introducing S atoms into the resonance skeleton can effectively improve the SOC values and k_RISC for MR-TADF emitters, thereby achieving efficient OLEDs with low efficiency roll-offs
2022年，我们课题组通过调节B/N和S原子之间的配位，开发了两种基于三元N/B/S的多环杂芳族发射体（SBSN和DBSN），从蓝绿色（489 nm）到黄色（553 nm）增加发射器的 CT 离域并改善光物理性质。40 这种策略使两个发射器分别具有 27 和 28 nm 的小 FWHM。此外，由于 0.10 - 0.13 eV 的小 ΔEST 值和大的 SOC 值，获得了相当大的 kRISC 值。因此，基于这两种发射器发出蓝绿光和黄光的 OLED 的 EQEmax 分别为 17.6% 和 21.8%，效率滚降较低。随后，Wang等人通过对位排列的B-π-B、N-π-N和S-π-S的三元组合。等人。开发了两种红色窄带发射器 DBNS 和 DBNS-Bu41，其在甲苯中的发射峰值为 631 和 641 nm，半高宽为 40 和 39 nm。受益于S原子的重原子效应，获得了2.1×105 s-1和2.2×105 s-1的高kRISC值以及11.2和10.2 μs的短τds。这些结果表明，将S原子引入到共振骨架中可以有效提高MR-TADF发射器的SOC值和kRISC，从而实现具有低效率滚降的高效OLED.

2.1.2 Selenium-containing MR-TADF emitters
2.1.2 含硒MR -TADF发射体

Considering the main group effect, Se atom has a stronger heavy atom effect compared to the S atom, so organic integration of Se and MR-TADF materials may endows it with a more efficient RISC processes (Figure 3). Yasuda et. al. firstly doped Se atom into a MR-TADF emitter (CzBSe),³⁷ yielding a record k_RISC exceeding 10⁸ s^-1, which is three orders of magnitude higher than that of common MR-TADF emitters. The theoretical calculations indicated that CzBSe possesses a quite large SOC (5 cm^-1 for <S₁|Ĥ_SOC|T₂> and 10 cm^-1 for <S₁|Ĥ_SOC|T₃>) value, which higher than that of CzBS and CzBO. Benefitting from an ultrafast triplet-exciton up-conversion of CzBSe, corresponding OLED offered an EQE_max of 23.9%, with a narrow blue emission (λ_EL = 481 nm, FWHM = 33 nm) and a significantly alleviated efficiency roll-off. Based on this framework, Wang et al introduced additional tert-butyl and peripheral 3,6-di-tert-butylcarbazole moieties to two Se-integrated MR-TADF emitters, Cz-BSeN and DCz-BSeN.⁴² Cz-BSeN and DCz-BSeN showed sky-blue emissions peaking at 479 and 472 nm with FWHMs of 30 and 28 nm, respectively. Notably, these two emitters showed large SOC values (2.51 and 3.96 cm^-1) and high k_RISC of 7.5 × 10⁶ and 8.8 × 10⁶ s^-1 for Cz-BSeN and DCz-BSeN, respectively. The OLEDs based on Cz-BSeN and DCz-BSeN displayed narrowband EL emissions with peaks at 490 and 481 nm, as well as small FWHMs of 36 and 32 nm, accompanied by EQE_maxs of 20.3% and 22.3%, respectively. Impressively, these two OLEDs achieved suppressed efficiency roll-offs with EQE values of 16.9%/19.6% at 100 cd m^-2 and 13.7%/15.6% at 500 cd m^-2, respectively. This work revealed the N/B/Se-based MR-TADF molecules for the development of highly efficient narrowband blue OLEDs with low-efficiency roll-offs
考虑到主族效应，Se原子比S原子具有更强的重原子效应，因此Se和MR-TADF材料的有机整合可能赋予其更高效的RISC工艺（图3）。安田等。等人。首先将 Se 原子掺杂到 MR-TADF 发射器 (CzBSe) 中，37 产生了超过 108 s-1 的创纪录的 kRISC，比常见的 MR-TADF 发射器高出三个数量级。理论计算表明CzBSe具有相当大的SOC（5 cm-1和 10 cm-1 为）值，高于CzBS和CzBO。受益于 CzBSe 的超快三线态激子上转换，相应的 OLED 的 EQEmax 为 23.9%，具有窄蓝色发射（λEL = 481 nm，FWHM = 33 nm），并且效率滚降显着减轻。基于该框架，Wang等人向两个Se集成的MR-TADF发射体Cz-BSeN和DCz-BSeN42Cz-BSeN和DCz-BSeN引入了额外的叔丁基和外围3,6-二叔丁基咔唑部分。 -蓝色发射峰值在 479 和 472 nm，半高宽分别为 30 和 28 nm。值得注意的是，这两个发射器表现出较大的 SOC 值（2.51 和 3.96 cm-1），Cz-BSeN 和 DCz-BSeN 的 kRISC 分别为 7.5 × 106 和 8.8 × 106 s-1。基于Cz-BSeN和DCz-BSeN的OLED显示出窄带EL发射，峰值为490和481 nm，以及36和32 nm的小FWHM，EQEmax分别为20.3%和22.3%。令人印象深刻的是，这两种 OLED 实现了抑制效率滚降，100 cd m-2 时的 EQE 值分别为 16.9%/19.6%，500 cd m-2 时的 EQE 值分别为 13.7%/15.6%。这项工作揭示了基于 N/B/Se 的 MR-TADF 分子，用于开发具有低效率滚降的高效窄带蓝色 OLED.

Furthermore, Yang et al reported a heavy Se atom incorporating emitter, BNSeSe,⁴³ showing a PLQY~100% and the k_RISC of 2.0 × 10⁶ s^-1, which is prior to most MR-TADF materials. The corresponding OLED exhibited an excellent EQE_max of 36.8% with a low efficiency roll-off (efficiency roll-off values of 2.8% and 14.9% at 1000 cd m^-2 and 10000 cd m^-2, respectively). Besides, BNSeSe was firstly used as an assist dopant to sensitize the low-energy MR-TADF emitter (BN3),⁴⁴ the hyperfluorescence OLED exhibited a significant EQE_max beyond 40% with a low efficiency roll-off. This work sheds a new light on MR-TADF emitters or sensitizers towards high-performance OLEDs. Later, Yang et. al. developed a green narrowband MR-TADF emitter, BN-STO,⁴⁵ through peripherally introducing a Se-embedded selenoxanthone unit to a MR-TADF skeleton. BN-STO exhibited a green narrowband emission, peaking at 506 nm with a small FWHM of 29 nm. And a high k_RISC value of 1.2 × 10⁵ s^-1 was achieved owing to the strong SOC caused by the peripheral Se atom integration. The BN-STO-based OLED showed a high EQE_max of 40.1% with a suppressed efficiency roll-off. These results demonstrate that the peripheral modification of heavy-atom and direct integration of heavy atom into the MR-TADF framework is helpful for improving RISC processes of MR-TADF emitters.
此外，Yang等人报道了结合发射极BNSeSe ^{43 的}重Se原子，其PLQY ~100%和k _RISC为2.0×10 ⁶ s ^-1 ，这优于大多数MR-TADF材料。相应的OLED表现出优异的EQE max 36.8%和低效率滚降（在1000 cd m ^-2和10000 cd m ^-2时效率滚_降值分别为2.8%和14.9%）。此外， BNSeSe首先被用作辅助掺杂剂来敏化低能 MR-TADF 发射体 ( BN3 )， ⁴⁴超荧光OLED表现出超过 40%_{的显着 EQE 最大值}，且效率滚降较低。这项工作为高性能 OLED 的 MR-TADF 发射器或敏化剂提供了新的思路。后来，杨等人。等人。通过在 MR-TADF 骨架外围引入 Se 嵌入的硒氧蒽酮单元，开发了一种绿色窄带 MR-TADF 发射器BN-STO ⁴⁵ 。 BN-STO表现出绿色窄带发射，峰值为 506 nm，半高宽为 29 nm。由于外围Se原子集成引起的强SOC，实现了1.2×10 ⁵ s ^-1的高k _RISC值。基于BN-STO的OLED表现出_高达40.1% 的高EQE ，并且效率滚降受到抑制。这些结果表明，重原子的外围修饰以及重原子直接集成到MR-TADF框架中有助于改进MR-TADF发射器的RISC工艺。

Figure 3. Molecular structures of Se-containing MR-TADF emitters.
图3 .含硒 MR-TADF 发射体的分子结构。

2.2 Metal-perturbed MR-TADF materials
2.2 金属扰动MR- TADF材料

The aforementioned publications have shown that heavy atoms can enhance the SOC values of MR-TADF materials, thereby improving k_RISC. In addition to the usual heavy atoms (S, Se), some metal atoms with high atomic numbers can also improve k_RISC. Of course, the selection of metal ions requires careful consideration, otherwise the strong SOC effect will cause molecules to exhibit phosphorescence rather than TADF. Considering the actual difficulty of synthesis and the selectivity of metal ions with weak SOC, there are only a few successful examples have been reported regarding metal perturbations, such as gold (Au(I)) and Pt(II) (Figure 4). By introducing metal centers to fundamentally modify the excited state dynamics, a feasible solution for improving MR-TADF emission without sacrificing color purity has been demonstrated. In 2022, Yang et al proposed a MR-TADF emitter, DCzBN-Au,⁴⁶ through coordination of Au with a B/N-embedded polycyclic ligand. Due to the Au(I)-complex perturbation, the k_RISC value was dramatically accelerated to 2.3 × 10⁷ s^-1, leading to a τ_d as short as 4.3 µs. Meanwhile, a PLQY of 95% and a FWHM of 39 nm were essentially unchanged after metal Au(I) coordination. Consequently, the OLED based on DCzBN-Au exhibited an EQE_max of 35.8% without TADF sensitization. And the EQE was still maintained as high as 32.3% at 10000 cd m^-2. At the same time, Chi et al also developed a series of similar Au-based MR-TADF material, (BzIPr)AuBN, (PzIPr)AuBN and (PylPr)AuBN.⁴⁷ All the MR-TADF emitters displayed green emissions with the maximum peaks at around 510 nm with small FWHMs as 30 nm. Benefiting from the influence of Au(I) coordination disturbance, the high rates k_RISC ≈ 10⁶ s^-1 were afforded, which were significantly enlarged about two orders of magnitude compared to those metal-free B/N-based MR-TADF emitters, and consequently, their τ_ds were notably reduced to 5.5-27.0 μs with a mono-exponential function. Consequently, the OLEDs utilizing these Au(I)-based MR-TADF emitters showed green narrowband EL emissions, peaking at 508-517 nm with narrow FWHMs of 34-40 nm. Notably, Au(I)-based OLEDs displayed high EQEs of up to 30.3% and suppressed efficiency roll-offs of 0.8%−6.0%, as well as a long operational lifetime (LT₆₀) of 1210 h at 1000 cd m^-2. These metal-perturbed MR-TADF molecular design can greatly promote T₁ harvesting, thereby achieving high-performance MR-TADF-OLEDs and opening up a new dimension for the design of practical MR-TADF emitters.
上述出版物表明，重原子可以提高MR-TADF材料的SOC值，从而提高k _RISC 。除了通常的重原子（S、Se）之外，一些原子序数较高的金属原子也可以提高k _RISC 。当然，金属离子的选择需要仔细考虑，否则强SOC效应会导致分子表现出磷光而不是TADF。考虑到合成的实际难度以及弱SOC金属离子的选择性，成功的例子屈指可数。有关金属扰动的报道已有报道，例如金 (Au(I)) 和Pt( II) （图 4） 。通过引入金属中心从根本上改变激发态动力学，已经证明了一种在不牺牲色纯度的情况下改善 MR-TADF 发射的可行解决方案。 2022年，Yang等人通过Au与B/N嵌入的多环配体的配位提出了一种MR-TADF发射体DCzBN-Au ⁴⁶ 。 由于Au (I)复合物扰动， k _RISC值急剧加速至2.3 × 10 ⁷ s ^-1 ，导致τ _d短至4.3 µs。同时，金属Au (I)配位后， PLQY为95%， FWHM为39 nm，基本没有变化。因此，基于DCzBN -Au的 OLED在没有 TADF 敏化的情况下表现出35.8% 的n EQE _max 。并且在10000 cd m ^-2时EQE仍保持高达32.3% 。同时，Chi等人还开发了一系列类似的Au基MR-TADF材料， ( BzIPr ) AuBN ( PzIPr ) AuBN和( PylPr )AuBN ⁴⁷所有MR-TADF发射器都显示出绿光发射，最大峰值为约 510 nm ，半高宽小为 30 nm。 受益于Au(I)配位扰动的影响，获得了高速率k _RISC ≈ 10 ⁶ s ^-1 ，与无金属B/N基MR-TADF发射器相比，显着提高了约两个数量级，因此它们的τ _d s通过单指数函数显着降低至 5.5-27.0 μs 。因此，利用这些基于 Au (I)的 MR-TADF 发射器的 OLED显示出绿光窄带 EL 发射峰值为 508-517 nm，半峰宽为 34-40 nm。值得注意的是，基于Au (I)的OLED 表现出高达30.3% 的高 EQE，并抑制了0.8%−6 的效率滚降。0%，以及在1000 cd m ^-2下1210 h的长工作寿命（LT 60 ）这些金属_扰动的MR-TADF分子设计可以极大地促进T ₁收集，从而实现高性能MR-TADF-OLED和为实用 MR-TADF 发射器的设计开辟了新的维度。

Figure 4. Molecular structures of metal-containing MR-TADF emitters.
图4 .含金属 MR-TADF 发射体的分子结构。

Recently, Liu et. al. designed and synthesized a Pt(II)-based MR-TADF emitter (BNCPPt) by integrating a reported MR-TADF unit into the classical heavy metal Pt(II) complex. As expected, BNCPPt exhibited larger <S₁|Ĥ_SOC|T₁>s from S₁→T₁ and T₁→S₀ (<S₁|Ĥ_SOC|T₁> = 2.82 cm^-1, <T₁|Ĥ_SOC|S₀> = 14.19 cm^-1) due to the strong SOC effect induced by the Pt(II) participation. The solution-processed OLED employing BNCPPt displayed a green emission, peaking at 507 nm, with a narrow FWHM of 35 nm, and realized an EQE_max of 13.5% with a very low efficiency roll-off of 4.4% at 1000 cd m^-2.⁴⁸ This design concept provides an ingenious combination of multi-resonance motif and metal complex, proposing a novel strategy to construct MR-TADF emitters toward high-performance OLEDs.
最近，刘等人。等人。通过将报道的 MR-TADF 单元集成到经典的重金属Pt ( II)配合物中，设计并合成了Pt ( II)基 MR-TADF 发射器 ( BNCPPt ) 。正如预期的那样， BNCPPt从 S ₁ → T ₁和 T ₁ → S ₀表现出更大的 <S ₁ Ĥ _SOC | T 1 % _3E （<S ₁ Ĥ _SOC |T ₁ > = 2.82 cm ^-1 , <T ₁ Ĥ _SOC |S ₀ > = 14.19 cm -1 ⁾由于强 SOC效应铂（ II）参与。 采用BNCPPt的溶液处理 OLED显示出绿光发射，峰值为 507 nm，半峰宽为 35 nm，并实现了13.5%的n EQE _max ，在 1000 cd m 时效率滚降为 4.4% ^{。 2 48}这种设计理念提供了多共振基序和金属配合物的巧妙结合，提出了一种构建 MR-TADF 发射器的新策略高性能 OLED。

2.3 π-Conjugation extended MR-TADF emitters
2.3 π-共轭扩展MR- TADF发射器

In addition to enhance the SOC of MR-TADF emitters to promote the RISC process of T₁ excitons, reducing ∆E_ST is also an effective strategy to improve k_RISC (Figure 5). In 2019, Zysman-Colman and Olivier calculated and simulated a series of B/N based emitters. Impressively, as the conjugated backbones of the molecule extends, the intramolecular charge transfer delocalization becomes more pronounced. As a result, ∆E_ST will decrease and f _OSC will increase.⁴⁹ At the same time, Hatakeyama et. al. developed a narrowband deep-blue emitting MR-TADF material ν-DABNA, which consisting of five connected benzene rings, two boron and four nitrogen atoms, as well as two diphenylamino substituents.⁵⁰ Compared to the DABNA-1 and DABNA-2, the CT delocalization of ν-DABNA is more pronounced, providing a smaller ∆E_ST of 0.017 eV and a larger k_RISC of 2.0 × 10⁵ s^-1. The deep-blue OLED based on ν-DABNA showed an excellent EQE_max of 34.4%. Notably, the efficiency roll-off (1.6% and 8.6% at 100 and 1000 cd m^-2, respectively) is smaller than most deep-blue OLEDs.
除了提高MR-TADF发射极的SOC来促进T ₁激子的RISC过程外，降低Δ E _ST也是提高k _RISC的有效策略（图5 ）。 2019 年，Zysman-Colman 和 Olivier 计算并模拟了一系列基于 B/N 的发射器。令人印象深刻的是，随着分子的共轭主链延伸，分子内电荷转移离域变得更加明显。结果，Δ E _ST将减少， _OSC将增加。 ⁴⁹同时，Hatakeyama等人。等人。开发了一种窄带深蓝色发射MR-TADF材料ν -DABNA ，它由五个相连的苯环、两个硼和四个氮原子以及两个二苯氨基取代基组成。 ⁵⁰与DABNA-1和DABNA-2相比， ν -DABNA的 CT 离域更加明显，提供更小的 Δ E _ST为 0。017 eV 和更大的k _RISC 2.0 × 10 ⁵ s ^-1 。基于ν -DABNA的深蓝色OLED表现出优异的EQE_最大值为34.4%。值得注意的是，效率滚降（在 100 cd m -2 和 1000 cd m ^-2下分别为 1.6% 和 8.6%）小于大多数深蓝色 OLED。

The π-conjugation extension of MR-TADF emitters is diverse. In 2019, Wang et al. constructed a bluish-green narrowband emitter, DtBuCzB, by replacing tert-butyl carbazole with diphenylamine.⁵¹ Furthermore, by expanding the π-conjugated skeleton of the parent bis(di[t-butyl]carbazol)phenylene, an efficient narrowband green emissive molecule, DtBuPhCzB, was successfully developed. DtBuPhCzB exhibited a smaller ∆E_ST and better TADF performances than that of DtBuCzB. Based on DtBuPhCzB dopant, a green narrowband OLED was prepared, with an EQE_max of 25.5% and a FWHM of 33 nm. In 2022, Wang et al. used DtCzB as a synthon and utilized the Scholl oxidative coupling reaction to construct a large π-conjugated MR-TADF material, BN-TP,⁵² which exhibited a vivid green emission, peaking at 523 nm, and a narrow FWHM of 34 nm. Benefitting from efficient exciton up-conversion from T_n state to S₁ state, BN-TP exhibited good TADF performances with a k_RISC of 10⁴ orders of magnitude, and OLED based on BN-TP showed an ultrapure green emission with a peak of 528 nm, and CIE coordinates of (0.26, 0.70), as well as an EQE_max of 35.1%.
MR-TADF 发射体的 π 共轭延伸是多种多样的。 2019 年，王等人。通过用二苯胺取代叔丁基咔唑，构建了蓝绿色窄带发射器DtBuCzB 。 ⁵¹此外，通过扩展母体双(二[ -丁基]咔唑)亚苯基的π-共轭骨架，成功开发了一种高效的窄带绿光发射分子DtBuPhCzB 。与DtBuCzB相比， DtBuPhCzB表现出更小的 Δ E _ST和更好的 TADF性能。基于DtBuPhCzB掺杂剂，制备了绿色窄带OLED，其n EQE _max为25.5%，半高宽为33 nm。 2022 年，Wang 等人。使用DtCzB作为合成子，并利用Scholl氧化偶联反应构建了大型π共轭MR-TADF材料BN-TP ^52，该材料表现出鲜艳的绿光发射，峰值波长为523 nm，半高宽为34 nm。 受益于从T _n态到S ₁态的高效激子上转换， BN-TP表现出良好的TADF性能， k _RISC达到10 ⁴数量级，并且基于BN-TP的OLED表现出超纯绿光发射，峰值为 528 nm，CIE坐标为 (0.26, 0.70)， n EQE_最大值为 35.1%。

In 2019, Duan and co-workers successfully synthesized a novel hybridized MR-TADF material, AZA-BN,⁵³ whose emission peaking at 522 nm with a FWHM of 28 nm in toluene solution. Due to the reasonable orbital hybridization and CT delocalization, AZA-BN exhibited a small ∆E_ST of 0.18 eV and a high PLQY of 94%. And the corresponding pure-green OLED displayed an EQE_max of 28.2% with a FWHM of merely 30 nm and a CIEy of 0.69. Furthermore, through the precise synthesis of multiple resonance fragments, the extended π-conjugation length, increased molecular rigidity, and reduced vibration frequency can be simultaneously reflected on BN-ICz molecule, which showing a pure green emission peaking at 521 nm and a PLQY of 99%. The OLED based on BN-ICz exhibited a record-high CIEy of 0.74 and an EQE_max of 30.5%.⁵⁴
2019年，段和同事成功合成了一种新型杂化MR-TADF材料AZA-BN ^{5 3，}其在甲苯溶液中的发射峰为522 nm，半高宽为28 nm。由于合理的轨道杂化和CT离域， AZA-BN表现出0.18 eV的小ΔE ST和_高达94 %的PLQY 。相应的纯绿色 OLED 显示n EQE _max为 28.2%，FWHM 仅 30 nm ， CIE y为 0.69。此外，通过多个共振片段的精确合成，延长的π共轭长度、增加的分子刚性和降低的振动频率可以同时反映在BN - ICz分子上，在521 nm处呈现出纯绿光发射峰，PLQY为99%。基于BN- ICz的OLED表现出创纪录的0.74的CIE y和30.5%的n EQE _max 。 ^{5 4}

Later, Yasuda group proposed two para B-π-N and B-π-B strategies to change donor and acceptor strengths, which aiming to adjust molecular ∆E_ST.⁵⁵ And narrowband deep-blue and red emitters, BBCz-DB and BBCz-R, exhibited moderate k_RISC values of 1.9 and 1.2 × 10⁴ s^-1, which are better than the parent molecule BBCz-SB. OLEDs based on BBCz-DB and BBCz-R displayed the EQE_max values of 29.3% and 22.0%, respectively.
随后，Yasuda课题组提出了两种para B - π - N和B - π - B策略来改变供体和受体强度，旨在调整分子 Δ E _ST ^{5 5}和窄带深蓝色和红色发射体， BBCz -DB和BBCz -R ，表现出中等的k _RISC值，分别为 1.9 和 1.2 × 10 4 ^s -1 ^，优于母体分子BBCz -SB 。基于BBCz -DB和BBCz-R的OLED显示EQE最大值分别为29.3 %和_22.0 %。

In 2020, Hatakeyama et al reported a series of carbazole-based DABNA analogs (CzDABNAs)⁵⁶ from triarylamine by regioselective one-shot single and double borylation. This facile and scalable synthesized strategy achieved the comprehensive construction of CzDABNAs by selectively boronation at the ortho-position of the carbazole group, exhibiting a narrowband emission. Similar to the results of previous calculated simulations by Oliver et al., the introduction of double borylation further increases the CT delocalization of molecules, while achieving smaller ∆E_ST and higher k_RISC. For example, the ∆E_STs are 0.18 and 0.11 eV for single borylation CzDABNA-NP-M/TB and double borylation Cz2B2-M/TB, respectively, with k_RISCs of 1.1 × 10⁵ s^-1 and 3.1 × 10⁵ s^-1, respectively. This work indicated that π-conjugation extension can effectively increase CT delocalization, thereby reducing ∆E_ST and accelerating k_RISC. Later, Hatakeyama reported an ultra-pure blue MR-TADF material (ν-DABNA-O-Me)⁵⁷ by oxygen atom incorporation. Because of restricted π-conjugation of the HOMO rather than the LUMO induced by oxygen atom incorporation, ν-DABNA-O-Me showed a hypsochromic shift compared to the parent ν-DABNA. And OLED based on ν-DABNA-O-Me exhibited an EL emission at 465 nm with a small FWHM of 23 nm and a high EQE_max of 29.5%. Recently, our group proposed two π-extended MR-TADF emitters, NBO and NBNP,⁵⁸ whose emissions peaking at 487 and 500 nm by fusing conjugated high-triplet-energy units (carbazole, dibenzofuran)
2020年，Hatakeyama等人通过区域选择性单次和双硼化从三芳胺中报道了一系列基于咔唑的DABNA类似物（CzDABNAs）56。这种简便且可扩展的合成策略通过在咔唑基团的邻位选择性硼化实现了 CzDABNA 的全面构建，并表现出窄带发射。与 Oliver 等人之前计算模拟的结果类似，双硼化的引入进一步增加了分子的 CT 离域，同时实现更小的 ΔEST 和更高的 kRISC。例如，单硼化 CzDABNA-NP-M/TB 和双硼化 Cz2B2-M/TB 的 ΔEST 分别为 0.18 和 0.11 eV，kRISC 分别为 1.1 × 105 s-1 和 3.1 × 105 s-1 。这项工作表明π共轭延伸可以有效增加CT离域，从而减少ΔEST并加速kRISC。后来，Hatakeyama通过氧原子掺入报道了一种超纯蓝色MR-TADF材料（ν-DABNA-O-Me）57。由于氧原子掺入引起的HOMO而不是LUMO的受限π共轭，ν-DABNA-O-Me与母体ν-DABNA相比表现出低色位移。基于ν-DABNA-O-Me的OLED在465 nm处表现出EL发射，FWHM小为23 nm，EQEmax高达29.5%。最近，我们小组提出了两种π扩展的MR-TADF发射器，NBO和NBNP58，通过融合共轭高三线态能量单元（咔唑、二苯并呋喃），其发射峰值在487和500 nm

Figure 5. Molecular structures of π-conjugation extended MR-TADF emitters.
图5 . π-共轭离子扩展 MR-TADF 发射器的分子结构。

into a B/N skeleton, aiming to increase CT delocalization of the B/N skeleton and minimize ∆E_ST. As a result, considerable k_RISC values of 9.3 × 10⁵ s^-1 and 3.1 × 10⁵ s^-1 for NBO and NBNP, respectively, were obtained due to the small ∆E_ST values (0.12 and 0.09 eV) and large spin-orbital coupling values. Consequently, the OLEDs based on NBO and NBNP exhibited EQE_max values of 26.1% and 28.0%, respectively. Even at the high brightness of 1000 cd m^-2, the EQEs of devices still reached 25.6% and 20.0%, respectively, which are superior to the parent BBCz-SB. In order to further increase CT delocalization of MR-TADF materials, our group designed and synthesized two green narrowband emitters, VTCzBN and TCz-VTCzBN, containing indolo[3,2,1-jk]carbazole unit and boron-nitrogen skeletons. Particularly, TCz-VTCzBN exhibited a green emission, peaking at 521 nm, with a FWHM of 29 nm.⁵⁹ Meanwhile, high k_RISC values of above 10⁶ s^-1 are obtained due to their small ∆E_ST values (0.06 eV and <0.01 eV) and large spin-orbital coupling data. Notably, planar molecular frameworks along the transition dipole moment direction endow them with high horizontal emitting dipole ratios of up to 94%. Consequently, the corresponding OLEDs showed EQE_maxs of 31.7% and 32.2%, respectively. Particularly, OLED based on TCz-VTCzBN displayed an ultra-pure green emission with CIE coordinates of (0.22, 0.71). Besides, our group introduced tricoordinate B atom into the indolo[3,2,1-jk]carbazole (ICz) fused MR skeleton to achieve a MR fluorophore to a MR-TADF emitter conversion, realizing a rigidly green B/N framework based MR-TADF emitter (LTCz-BN),⁶⁰ showing a small FWHM of 27 nm and a high PLQY of 93%, as well as a high k_RISC of 8.3×10⁵ s^-1. Consequently, the LTCz-BN-based OLED simultaneously exhibited an EQE_max of 27.2% with a small FWHM of 34 nm, and a much suppressed efficiency roll-off, whose value still reached 25.4% at 3000 cd m^-2
进入B/N骨架，旨在增加B/N 骨架的 CT 离域并最小化 Δ E _ST 。结果，由于较小的 Δ E _ST值（0.12 和 0.09 eV）和较大的自旋， NBO和NBNP的k _RISC值分别为9.3 × 10 ⁵ s ^-1和 3.1 × 10 ⁵ s ^-1 -轨道耦合值。因此，基于NBO和NBNP 的OLED的E QE_最大值分别为26.1% 和 28.0%。即使在1000 cd m ^-2的高亮度下，器件的EQE仍分别达到25.6%和20.0%，优于母体BBCz -SB 。 为了进一步提高MR-TADF材料的CT离域性，我们课题组设计合成了两种含有吲哚并[3,2,1- jk ]咔唑单元和硼氮骨架的绿光窄带发射体VTCzBN和TCz-VTCzBN 。特别是， TCz-VTCzBN表现出绿色发射，峰值位于 521 nm ，半高宽为 29 nm。 ⁵⁹同时，由于较小的 Δ E _ST值（0.06 eV 和 <0.01 eV ）和较大的自旋轨道耦合数据，获得了高于 10 ⁶ s ^-1的高k _RISC值。值得注意的是，沿过渡偶极矩方向的平面分子框架赋予它们高达94%的水平发射偶极比。因此，相应的OLED 的EQE最大值_分别为31.7% 和 32.2%。 特别是，基于TCz-VTCzBN的OLED显示出超纯绿色发射， CIE坐标为(0.22，0.71)。此外，我们课题组将d三配位B原子引入吲哚[3,2,1- jk ]咔唑（ICz）稠合MR骨架中，实现了MR荧光团到MR - TADF发射体的转换，实现了基于刚性绿色B/N骨架的绿色B/N骨架。 MR-TADF发射器( LTCz -BN )， ⁶⁰显示出27 nm的小FWHM和93%的高PLQY，以及8.3×10 ⁵ s ^-1的高k _RISC 。因此，基于LTCz -BN的OLED同时表现出27.2%的n EQE _max和34 nm的小FWHM，以及大大抑制的效率滚降，其值仍然达到ed 25。3000 cd m ^-2时为 4%.

To reveal the π-extended effect on the MR-TADF performance, Yang et al. developed three narrowband deep-blue MR-TADF materials, BN1-BN3, with gradually extended π-conjugation skeleton.⁶¹ Three MR-TADF emitters were prepared from the same precursor by lithium-free borylation reactions in high yields. And BN1-BN3 displayed extremely narrow deep-blue emission spectra with FWHMs below 18 nm and small ΔE_ST values (≤ 0.20 eV) in toluene due to the rigid and extended molecular skeleton. Because of 13 fused rings within entire molecule, BN3 showed the highest PLQY of 98% and k_RISC of 2.55 × 10⁵ s^-1 among them. As a result, BN3-based deep-blue OLED offered a high EQE_max of 37.6% with a small efficiency roll-off, implying that the π-extended effect is beneficial to improve the RISC process and device performance of MR-TADF materials.
为了揭示 π 扩展对 MR-TADF 性能的影响，Yang 等人。开发了三种窄带深蓝色MR-TADF材料BN1 - BN3 ，具有逐渐延伸的π-共轭离子骨架。 ⁶¹由相同的前体通过无锂硼化反应以高产率制备了三种 MR-TADF 发射体。由于其刚性且延伸的分子骨架， BN1-BN3在甲苯中显示出极窄的深蓝色发射光谱，半峰宽低于 18 nm，且 Δ E _ST值较小（≤ 0.20 eV） 。由于整个分子内有13个稠合环， BN3在其中表现出最高的PLQY（98%）和k _RISC为2.55×10 ⁵ s ^-1 。结果， BN3基深蓝色OLED提供了高达37.6%的EQE_最大值和较小的效率滚降，这意味着π延伸效应有利于改善MR-TADF材料的RISC工艺和器件性能。

Recently, You et. al. proposed a medium-ring molecular design strategy for a blue MR-TADF emitter, DTBA-B2N3. By reasonably introducing two highly twisted heptagonal tribenzo[b,d,f]azepine as donors, the distance between multiple resonance units was expanded, and π-π interaction was suppressed. Importantly, DTBA-B2N3 exhibited a high k_RISC of 1.6 × 10⁶ s^-1. And DTBA-B2N3-based OLED demonstrated an EQE_max of 30.9% and an EQE value of 20.5% at 1000 cd m^-2, respectively.⁶² Later, Wang et. al. developed a MR-TADF emitter, m-DBCz,⁶³ by assembling MR-building blocks (MR-BBs) to form a more π-extended and rigid molecular configuration. Based on optimized MR-BBs’ geometric arrangement, m-DBCz displayed outstanding photoelectrical characteristics with a small FWHM of 32 nm, and a high PLQY of 95%, as well as a k_RISC of 1.5 × 10⁵ s^-1. And the green emissive OLED based on m-DBCz showed an EQE_max of 34.9% and a small FWHM of 35 nm.
最近，你等人。等人。提出了蓝色 MR-TADF 发射体DTBA-B2N3的中环分子设计策略。通过合理引入两个高度扭曲的七方三苯并[ b d ]氮杂卓作为供体，扩大了多个共振单元之间的距离，抑制了π-π相互作用。重要的是， DTBA-B2N 3表现出1.6 × 10 ⁶ s ^-1的高k _RISC 。基于DTBA-B2N3的OLED在1000cd m ^-2下分别表现出30.9%_的EQE最大值和20.5%的EQE值。 ⁶²后来，Wang等人。等人。通过组装MR构建块（MR-BB）以形成更加π延伸和刚性的分子构型，开发了一种MR-TADF发射器m -DBCz ^63。基于优化的MR-BBs几何排列， m - DBCz表现出出色的光电特性，半高宽仅为32 nm，PLQY高达95%， k _RISC为1.5 × 10 ⁵ s ^-1 。基于m - DBCz 的绿色发射 OLED显示出34.9% 的n EQE_最大值和 35 nm 的小 FWHM 。

Impressively, molecular π-conjugated skeleton can be extended through lithium-free boronation reactions. In 2022, Hatakeyama et. al. proposed an expanded heterohelicene consisting of three BN₂-embedded[4]helicene subunits (V-DABNA-Mes) by one-shot triple borylation.⁶⁴ Due to the multiple resonance effect of three boron and providing new synthetically accessible chemical space. ω-DABNA,⁶⁵ the proof-of-concept material, exhibited narrowband green TADF characteristics with a FWHM of 22 nm and a small ∆E_ST of 0.013 eV. The ω-DABNA-based OLED exhibited an ultra-pure green emission
令人印象深刻的是，分子π共轭骨架可以通过无锂硼化反应延伸。 2022 年，Hatakeyama 等人。等人。通过一次性三硼化，提出了一种由三个 BN2 嵌入的[4]螺烯亚基 (V-DABNA-Mes) 组成的扩展杂螺烯。 64 由于三个硼的多重共振效应，并提供了新的可合成的化学空间。 ω-DABNA65 是概念验证材料，具有窄带绿光 TADF 特性，半高宽为 22 nm，ΔEST 为 0.013 eV。基于ω-DABNA的OLED表现出超纯绿色发射

Figure 6. Molecular structures of MR-TADF emitters with peripheral decoration D/A units.
图6 .具有外围装饰 D/A 单元的 MR-TADF 发射器的分子结构。

peaking at 512 nm, with CIE coordinates of (0.13, 0.73) and a high EQE_max of 31.1%, accompanied by an alleviated efficiency roll-off. Very recently, Hatakeyama et. al. reported a series MR-core structures, consisting of B/N-embedded higher-order acene frameworks.⁶⁶ Notably, these compounds are the largest MR-TADF frameworks reported so far. The effective embedding of multiple B/N types endows the materials with ultra-narrowband FWHMs (12 - 16 nm) and high PLQYs (92% - 94%). Interestingly, the π-conjugation expansion of the MR-TADF emitters leads to improvements in ΔE_ST and k_RISC, reaching 3.9 meV and 6.5×10⁵ s^-1, respectively, for CzB8. The OLED employing CzB4-oPh exhibited a small FWHM of 21 nm and an EQE_max of 28.7% with a small efficiency roll-off (2.9% at 1000 cd m^-2). These publications indicate that the π-conjugation extension strategy is an active and effective means of enhancing the RISC process of MR-TADF materials.
峰值位于 512 nm，CIE 坐标为 (0.13, 0.73)， EQE_最大值高达31.1%，同时效率滚降有所缓解。最近，畠山等人。等人。报道了一系列 MR 核心结构，由 B N 嵌入的高阶并苯框架组成。 ⁶⁶值得注意的是，这些化合物是迄今为止报道的最大的 MR-TADF 框架。多种B/N类型的有效嵌入赋予材料超窄带FWHM（12 - 16 nm）和高PLQY（92% - 94%）。有趣的是，MR-TADF发射器的π共轭离子扩展导致了Δ E _ST和k _RISC的改进，对于CzB8分别达到3.9 meV和6.5×10 ⁵ s ^-1 。采用CzB4-oPh的OLED表现出21 nm的小FWHM和28.7%的n EQE _max ，并且效率滚降小（1000 cd m ^-2时为2.9% ）。 这些出版物表明π共轭延伸策略是增强MR-TADF材料RISC工艺的积极有效的手段。

2.4 Peripheral decoration D/A units
2.4 外围装饰D/A单元

The decoration of D/A units on the periphery of B/N resonance core has been proven to enable more complete separation of HOMO and LUMO within entire molecule, offering a smaller ∆E_ST (Figure 6). In 2020, Wang et al. attached the auxiliary donor (3,6-di-tert-butylcarbazole) to the parent MR molecule (BNCz), resulting in a novel molecule, m-Cz-BNCz,⁶⁷ with twisted D-A and MR structural features. Due to the meta-substitution of DtBuCz, HOMO of m-Cz-BNCz has been well extended. As a result, m-Cz-BNCz exhibited a green emission peaking at 519 nm, with a narrow FWHM of 38 nm, as well as a high k_RISC of 1.0×10⁶ s^-1. Additionally, m-Cz-BNCz displayed a negligible aggregation-induced shifting of the emission and broadening behaviors, which indicates that the intermolecular interactions of the emitter is efficiently inhibited owing to the highly twisted D/A structure. The OLED based on m-Cz-BNCz exhibited a pure green EL emission, peaking at 520 nm, with a small FWHM of 43 nm. The CIE coordinates of (0.23, 0.69) and a high EQE_max of 27.0% were recorded at a 3 wt% doping concentration.
B/N 共振核心外围的 D/A 单元的装饰已被证明可以使整个分子内的 HOMO 和 LUMO 更完全分离，从而提供更小的 Δ E _ST （图 6）。 2020 年，王等人。将辅助供体（ 3,6-二叔丁基咔唑）连接到母体 MR 分子（ BNCz ）上，产生具有扭曲 DA 和MR结构特征的新型分子m -Cz-BNCz ⁶⁷ 。由于DtBuCz的间位取代， m - Cz - BNCz的HOMO得到了很好的扩展。结果， m - Cz - BNCz表现出绿光发射峰值在519nm，具有38nm的窄FWHM，以及1.0×10 ⁶ s ^-1的高k _{R ISC} 。 此外， m - Cz - BNCz显示出可忽略不计的聚集引起的发射偏移和展宽行为，这表明由于高度扭曲的 D/A 结构，发射体的分子间相互作用被有效抑制。基于m - Cz - BNCz的OLED表现出纯绿色EL发射，峰值在520 nm，半高宽较小，为43 nm。在3 wt % 掺杂浓度下记录的CIE坐标为 (0.23, 0.69) 和 27.0% 的高EQE _max 。

In 2019, Duan et. al. designed and synthesized three green emissive MR-TADF materials, 2F-BN, 3F-BN and 4F-BN.⁶⁸ Benefitting from amplifying the influence of skeleton and peripheral units, three MR-TADF materials showed shorter τ_d and smaller ∆E_ST values compared to the parent molecule BCz-BN, resulting in accelerated k_RISC values (0.22 × 10⁵ s^-1, 0.39 × 10⁵ s^-1, 0.44 × 10⁵ s^-1 for 2F-BN, 3F-BN and 4F-BN, respectively). EQE_maxs of 22.0%, 22.7% and 20.9% were recorded for OLEDs based on 2F-BN, 3F-BN, and 4F-BN, respectively. These results demonstrated that efficient MR-TADF emitters with significantly bathochromic shift emission could be achieved by amplifying the influence of skeleton and the peripheral units. To achieve full-color, narrowband and high-efficient emitters, Yasuda et. al. introduced electron-withdrawing and electron-donating units at different sites of the parent molecule BBCz-SB,⁵⁵ reasonably extending the HOMO and LUMO of the compound to generate Eg with different energies. These MR-TADF emitters showed full-color narrowband emissions from blue (466 nm) to red (615 nm) and small ∆E_ST values below 0.2 eV. Owing to their ideal emission features, full-color and narrowband OLEDs were achieved with notably high EQE_max values of up to 31.8% based on this family of MR-TADF emitters, which further guide the design of full-color, efficient, narrowband emissive OLEDs. Subsequently, they developed wide-range color tuning of narrowband emissions in MR-TADF materials via the rational imine/amine functionalization. Through the reasonable introduction of peripheral carbazole and methyl-substituted diphenylamine, molecular HOMOs were well extended. As a result, TCz-B and DACz-B⁶⁹ exhibited red-shifted narrowband emissions and smaller ∆E_STs, as well as higher k_RISC values compared to parent Cz-B. Consequently, the corresponding narrowband deep-blue to yellow OLEDs achieved high EQE_max values of 19.0%-29.2% with desirable EL color purity. In 2023, Yang et al. proposed a delicate manipulation of excited state energy levels and successfully constructed a narrowband emitting MR-TADF material, CzBN3. Owing to the RISC enhancement by ³SRCT, CzBN3 showed a 23-fold increase of k_RISC than that of parent BCzBN. And OLED based on CzBN3 exhibited a high EQE_max of 37.1% and a relatively slow efficiency roll-off (30.4% at a luminance of 1000 cd cm^-2).⁷⁰
2019年，Duan等人。等人。设计并合成了三种绿色发射 MR-TADF 材料， 2F-BN 、 3F-BN和4F-BN ⁶⁸受益于放大骨架和外围单元的影响，三种 MR-TADF 材料表现出更短的τ _d和更小的 Δ E _ST值s与母体分子BCz -BN相比，导致加速的k _RISC值（ 2F-BN 、 3F-BN和4F-为0.22 × 10 ⁵ s ^-1 、0.39 × 10 ⁵ s ^-1 、0.44 × 10 ⁵ s ^-1分别为BN ）。基于2F-BN 、 3F-BN和4F-BN的 OLED 的EQE_最大值分别为22.0%、22.7% 和 20.9% 。 这些结果表明，通过放大骨架和外围单元的影响，可以实现具有显着红移发射的高效 MR-TADF 发射器。为了实现全彩、窄带和高效发射器，Yasuda等人。等人。 Lu等人在母体分子BBCz-SB ⁵⁵的不同位点引入吸电子和给电子单元，合理扩展了化合物的HOMO和LUMO，产生不同能量的E g 。这些 MR-TADF 发射器显示出从蓝色 (466 nm) 到红色 (615 nm) 的全色窄带发射，并且 Δ E _ST值低于 0.2 eV。由于其理想的发射特性，基于该系列 MR-TADF 发射器实现了全彩和窄带 OLED，其E QE_最大值高达31.8%，这进一步指导了全彩、高效、窄带的设计发射型 OLED。随后，他们通过合理的亚胺/胺官能化开发了 MR-TADF 材料中窄带发射的宽范围颜色调谐。通过合理引入外围咔唑和甲基取代二苯胺，分子HOMO得到了很好的延伸。 结果，与母体Cz -B相比， TCz -B和DACz-B ⁶⁹表现出红移窄带发射s和更小的 Δ E _ST ，以及更高的k _RISC值。因此，相应的窄带深蓝色至黄色 OLED 实现了19.0% - 29.2%的高EQE_最大值以及理想的 EL 色纯度。 2023年，杨等人。提出了对激发态能级的精细操纵，并成功构建了窄带发射 MR-TADF 材料CzBN3 。由于³ SRCT对 RISC 的增强， CzBN3 的k _RISC比亲本BCzBN增加了 23 倍。基于CzBN3的OLED表现出_高达37.1%的高EQE和相对较慢的效率滚降（在1000 cd cm ^-2亮度下为30.4% ）。⁷⁰

Recently, You et. al. represented a cyano (CN)-functionalization strategy to render the red-shifted emission while maintaining good color purity in MR-TADF skeleton. Based on the peripheral decoration D/A units engineering, the electron-withdrawing (-CN) group located on the LUMO distribution could decrease the LUMO energy level, leading to red-shifted emission. Meanwhile, molecular structure relaxation was restricted via adopting a co-planar conformation with the MR-skeleton, which is helpful to maintain a narrowband FWHM. As a result, a CN functionality was introduced on a blue-emitting MR-TADF skeleton (BCz-BN), leading to a 15 nm red-shifted emission with a small FWHM of 21 nm for CN-BCz-BN, while a CN functionality was introduced on a yellow-emitting MR-TADF skeleton (BBCz-Y), giving a more significant red-shifted emission of 32 nm with a small FWHM of 42 nm for CNCz-BNCz.⁷¹ Owing to the effective separation of HOMO and LUMO, CNCz-BNCz showed a small ΔE_ST value of 0.18 eV and a high k_RISC value of 4.2×10⁵ s^-1. Consequently, The CNCz-BNCz-based OLED, utilizing TADF sensitized mechanism, exhibited an outstanding EQE_max as high as 33.7%, representing the state-of-heart performance for orange-red OLEDs. Very recently, based on their previous work, You et. al. introduced larger steric hindrance donors (phenylcarbazole, triphenyl) and receptors (triazine) into the MR-TADF emitters and synthesized two green MR-TADF materials, TRZCzPh-BNCz and TRZTPh-BNCz. Benefitting from space-confined donor-acceptor engineering, the introduction of space-confined D/A units onto the MR-skeleton induces intermediate triplet states, which leads to a multichannel RISC process and thus increases the k_RISC. And TRZCzPh-BNCz and TRZTPh-BNCz showed high k_RISC values of 2.13×10⁶ s^-1 and 1.55×10⁶ s^-1, respectively.⁷² Notably, their OLEDs exhibited high EQE_maxs of 32.5% and 31.4%, respectively. Even at the luminance of 1000 cd m^-2, the EQEs still maintained 22.9% and 23.1% for TRZCzPh-BNCz- and TRZTPh-BNCz-based OLEDs, respectively. These works demonstrated the superiority of modifying D/A units on the periphery of MR-TADF materials.
最近，你等人。等人。代表了氰基（CN）功能化策略，以呈现红移发射，同时在 MR-TADF 骨架中保持良好的颜色纯度。基于外围装饰D/A单元工程，位于LUMO分布上的吸电子(-CN)基团可以降低LUMO能级，导致发射红移。同时，通过采用与MR骨架的共面构象来限制分子结构弛豫，这有助于维持窄带FWHM。因此，在发蓝光的 MR-TADF 骨架（ BCz -BN ）上引入了 CN 功能，导致CN- BCz -BN产生 15 nm 的红移发射，半高宽为 21 nm ，而 CN在发黄光的 MR-TADF 骨架 ( BBCz -Y ) 上引入了功能性，由于 HOMO 和 LUMO 的有效分离， CNCz-BNCz ⁷¹具有更显着的 32 nm 红移发射和 42 nm 的小 FWHM ， CNCz-BNCz表现出0.18 eV的小ΔE ST_值和4.2×10 ⁵ s ^-1的高