1 Introduction  1 引言

Recently, the pioneering discovery of emitting materials based on BN-containing multiple-resonance (MR) characteristics has laid a solid foundation for the development of organic electroluminescent (EL) materials with narrowband emission and provided considerable opportunities for the manufacture of organic light-emitting diodes (OLEDs) with ultrahigh color purity.^[1] The emission color purity of red (R), green (G), and blue (B), three primary color, emitters is dominate for the color gamut of organic EL displays. Therefore, organic R, G, and B emitters with superhigh color purity are critical for achieving wide color gamut OLED displays. Indeed, for a display, the color gamut defines the palette of available colors that can be represented realistically.^[2] The OLED industry is pursuing high-performance and ultrahigh-definition display technology, which can accurately reproduce the authentic colors of the images.^[3] Currently, for active-matrix OLED (AMOLED) displays, the desired color purity can be obtained by cutting off the margin region of original broad EL band with optical apparatus (e.g., color filter or optical microcavity).^[4] However, the drawback is that these optical processing approaches remarkably lead to decrease in luminous efficiency and increase in power consumption. In addition, it also notably promotes the manufacturing cost of AMOLED displays. Fortunately, the employment of organic MR emitters with thermally activated delayed fluorescence (TADF) property opens an avenue to construct high color purity and wide color gamut AMOLED displays without additional optical treatments.^[5
近年来，基于含硼氮多重共振（MR）特性的发光材料的开创性发现，为窄谱带发射有机电致发光（EL）材料的发展奠定了坚实基础，并为超高色纯度有机发光二极管（OLED）的制造提供了广阔前景。红（R）、绿（G）、蓝（B）三基色发光体的发射色纯度主导着有机 EL 显示器的色域表现。因此，具有超高色纯度的有机 RGB 发光体是实现广色域 OLED 显示的关键所在。对于显示器而言，色域决定了能够真实再现的色彩范围。 ² OLED 产业正致力于发展高性能超高清显示技术，以实现图像真实色彩的精准还原。 ³ 当前在有源矩阵 OLED（AMOLED）显示器中，可通过光学器件（如滤色片或光学微腔）裁切原始宽谱电致发光谱带的边缘区域，从而获得理想色纯度。 （注：根据学术翻译规范，特殊符号""已规范处理为"氮"；完整保留原文中的文献标注 ¹ 、 ²、 ³；技术术语如"multiple-resonance"译为"多重共振"、"color gamut"译为"色域"、"active-matrix OLED"译为"有源矩阵 OLED"等均采用领域标准译法；复合句式在保持专业性的前提下进行了符合中文表达习惯的重组。） 然而，这些光学处理技术会显著降低发光效率并增加功耗，同时大幅提升 AMOLED 显示屏的制造成本。幸而，具有热活化延迟荧光（TADF）特性的有机多重共振发光材料的应用，为构建无需额外光学处理即能实现高色纯度与宽色域的 AMOLED 显示屏开辟了新路径。^]

For MR-type emitters, the fundamental molecular design paradigm is to para-align boron and nitrogen atoms into a six-membered ring (p-BNR) and then embed them into a polycyclic aromatic hydrocarbon (PAH) skeleton. In the p-BNR-embedded aromatic system, the electron densities of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) reside on the atoms of conjugated framework and adopt an alternating distribution mode. This so-called complementary MR effect can minimize the spatial overlap of HOMO and LUMO, and lead to small singlet–triplet energy splitting (ΔE_ST) between the lowest singlet (S₁) state and the lowest triplet (T₁) state that enables the strongly endothermic-assisted reverse intersystem crossing (RISC).^[6] Therefore, the intramolecular charge-transfer (ICT) excited state of MR molecules shows a typical TADF characteristic. Moreover, the significant molecular rigid geometry and distinctive electronic structure can endow the transitions from the ground (S₀) state to S₁ state, and S₁ to S₀ with short-range reorganization of electron density.^[7] Consequently, the unique excitonic characteristics are bestowed in the Franck–Condon transition, including narrow full-width at half-maximum (FWHM) characteristics, large extinction coefficient, high oscillator strength (f), and near-unity photoluminescence (PL) quantum yield (Φ_PL). As a result, these class emitters provide unprecedented opportunity for the fabrication of high-performance OLEDs with perfect color purity.
对于多共振型发光体，其基本分子设计范式是将硼、氮原子对位排列构成六元环（p-BNR）并嵌入多环芳烃（PAH）骨架。在 p-BNR 嵌入的芳香体系中，最高占据分子轨道（HOMO）与最低未占据分子轨道（LUMO）的电子密度定域于共轭框架原子，并呈现交替分布模式。这种所谓的互补多共振效应可最小化 HOMO 与 LUMO 的空间重叠，导致最低单重态（S ₁ ）与最低三重态（T ₁ ）间产生微小单重态-三重态能隙（ΔE _ST ），从而实现强吸热辅助的反向系间窜越（RISC） ⁶ 。因此，多共振分子的分子内电荷转移（ICT）激发态展现出典型的热激活延迟荧光（TADF）特性。此外，显著的分子刚性几何结构和独特电子结构可赋予从基态（S ₀ ）向 S ₁ 态、以及 S ₁ 向 S ₀ 态的跃迁以短程电子密度重组特性。 因此，这类发光材料在 Franck-Condon 跃迁中展现出独特的激子特性：包括窄半峰全宽（FWHM）特性、大消光系数、高振子强度（f）以及接近 100%的光致发光量子产率（Φ _PL ）。这些特性为制造具有完美色纯度的高性能 OLED 器件提供了前所未有的机遇。

Recently, significant progress has been made in EL materials and devices based on BN-containing MR framework.^[8] However, almost all synthetic methods of MR molecules are based on the preborylation procedure (PBP). In other words, some MR molecules with complex aromatic amine ligand structures must also be synthesized previously, and borylation was laid out in the last step. It has been demonstrated that PBP suffers a number of serious deficiencies: i) for the synthesis of MR molecules based on bulky and irregular oligoaromatic amine ligands, the separation and purification are very hard or impossible, because the resulting reaction mixture includes target product, ligand, borylation isomers, and by-products with the same or very near molecular weight as well as very similar polarity; ii) PBP is not suitable for the synthesis of MR molecules composed of ligands with strong electron-withdrawing groups (EWGs)—especially, the EWG is located on the para-carbon position of B-substituted phenyl ring, because the borylation reaction of the CH bond is highly electrophilic; iii) the alkyl-lithium-assisted borylation may lead to unpredictable side chemical reactions of the aromatic amine ligands with active groups, such as cyano, carbonyl, and so on.^[9
近期，含硼氮多共振框架的 EL 材料与器件研究取得重大突破。 ⁸ 然而几乎所有多共振分子的合成方法均基于预硼化过程（PBP）。换言之，某些具有复杂芳香胺配体结构的多共振分子仍需预先合成，最终一步进行硼化反应。 研究表明，PBP 方法存在若干严重缺陷：其一，基于大位阻不规则寡聚芳香胺配体的多重共振分子合成过程中，由于反应混合物包含目标产物、配体、硼化异构体及分子量相同或接近、极性高度相似的副产物，分离纯化极为困难甚至无法实现；其二，PBP 不适用于含强吸电子基团（EWG）配体的多重共振分子合成——尤其当吸电子基团位于硼取代苯环对位碳时，因 CH 键硼化反应具有高度亲电性；其三，烷基锂辅助硼化可能导致含氰基、羰基等活性基团的芳香胺配体发生不可预测的副化学反应。 ^{9 ]}

In this context, it is still an extremely urgent and challenging topic to develop universal and outstanding synthetic methodologies for expanding the diversity of MR molecular architectonics, which is critical for the academic research and commercial application of organic electronics. In this contribution, a synthetic methodology for precisely functionalizing BN-containing an MR framework has been proposed. The introduction of the Br group makes the postfunctionalization of MR compounds possible, facile, and diversified. The brominated parent MR framework can be hybridized with universal functional groups, such as donors, acceptors, or moieties without obvious push–pull electron properties. Herein, the parent molecule BNCz was selected as the representative skeleton (Scheme 1),^[8] which could be easily transformed into m-Br–BNCz via N-bromosuccinimide (NBS). It can be inferred that the bromination of BNCz can be extended to its carbazole-based analogs. Four representative molecules were carefully investigated to demonstrate the superiority and applicability of our synthetic method. It is necessary to pursue a balance between ICT strength and FWHM considering their antagonistic relationship. Therefore, the phenyl bridge was inserted between the donor and MR framework to guarantee the redshift emission and also avoid strong ICT caused by the direct CN bond coupling between them that can broaden the FWHM.^[10] Finally, the m-DPAcP–BNCz-based OLED exhibits green emission with an FWHM of 28 nm and high a maximum external quantum efficiency (EQE) of 40.6%.
在此背景下，开发通用且卓越的合成方法学以拓展多共振分子架构的多样性，仍是当前有机电子学学术研究与商业应用领域中极具紧迫性和挑战性的课题。本研究提出了一种对含硼氮（BN）多共振框架进行精确功能化的合成策略。溴基团的引入使得多共振化合物的后功能化处理具备了可操作性、简便性与多样性。溴化母体多共振框架可与各类通用官能团（如给电子基团、吸电子基团或不具有明显推拉电子特性的结构单元）实现杂化。本文选取母体分子 BNCz 作为代表性骨架（图式 1）， ⁸ 通过 N-溴代丁二酰亚胺（NBS）可将其便捷转化为间位溴代产物 m-Br-BNCz。由此可推断，BNCz 的溴化反应可拓展至其咔唑类类似物。通过对四种代表性分子的深入研究，充分验证了本合成方法的优越性与普适性。 考虑到分子内电荷转移(ICT)强度与半峰全宽(FWHM)的拮抗关系，需在二者间寻求平衡。为此，我们在给体与多重共振(MR)骨架间引入苯基桥连结构，既保障了发射光谱红移，又避免了直接 CN 键耦合引发的强 ICT 效应——该效应会导致 FWHM 展宽。 ¹⁰ 最终，基于 m-DPAcP–BNCz 的有机电致发光器件(OLED)实现了 28 nm 窄带绿光发射，同时获得高达 40.6%的最大外量子效率(EQE)。

2 Results and Discussion
结果与讨论

Four compounds were successfully synthesized through the palladium-mediated Suzuki cross-coupling reaction with m-Br–BNCz as a precursor. They were purified by column chromatography and then further purified by gradient sublimation under high vacuum to afford highly pure samples. The prerequisite of obtaining the target compounds was the synthesis of the precursor, which could be prepared by one-step easy handling bromination reaction with NBS (Scheme S1, Supporting Information). Bromination is an electrophilic reaction, which needs to be closed to the position where the electron cloud density is concentrated. Therefore, the position of HOMO population induced by the MR framework should be selected preferentially. In addition, combined with the substituent localization effect of boron and nitrogen atoms, the bromination reaction can only occur on the meta-carbon position of the B-substituted phenyl ring. The molecular structure of m-Br–BNCz was unambiguously confirmed by mass spectrum, single crystal (Scheme S2, Supporting Information), and ¹H NMR spectrum (Figure S1, Supporting Information). The preparation processes of the key intermediate and target compounds are robust and thus have the potential to manufacture the target compounds with commercial scale. The NMR, mass spectra, and high-performance liquid chromatogram (HPLC) of the target compounds are shown in Figures S3–S16 (Supporting Information).
以间溴-BNCz 为前体，通过钯催化的 Suzuki 交叉偶联反应成功合成了四种化合物。经柱色谱纯化后，在高真空条件下进行梯度升华进一步提纯，获得高纯度样品。合成目标化合物的关键在于前体制备，该前体可通过 NBS 一步溴化反应便捷制得（支撑信息 Scheme S1）。溴化作为亲电反应，需作用于电子云密度富集区域，因此优先选择多重共振框架诱导的 HOMO 布居位点。此外，结合硼氮原子的取代基定位效应，溴化反应仅能发生在硼取代苯环的间位碳原子上。通过质谱、单晶衍射（支撑信息 Scheme S2）及 ¹ 氢核磁共振谱（支撑信息 Figure S1）确证了间溴-BNCz 的分子结构。 关键中间体与目标化合物的制备工艺可靠，具备实现目标化合物商业规模生产的潜力。目标化合物的核磁共振谱、质谱及高效液相色谱图（HPLC）详见支持信息中的图 S3-S16。

To elucidate the implication and discrepancy of different push–pull electron groups on the geometric and electronic properties, the S₀ state geometries were preliminarily optimized by employing density functional theory (DFT), and the geometric configurations of S₁ state geometries were simulated using time-dependent DFT (TDDFT). The HOMO/LUMO distributions, HOMO–LUMO energy level gaps (E_gap), oscillator strengths, electrostatic potential (ESP) distribution, and spin–orbit coupling matrix elements (<S|Ĥ_SOC|T>s) are depicted in Figure 1. The LUMO distribution in m-PCz–BNCz, m-DPAcP–BNCz, m-BN–BNCz, and m-SF–BNCz is approximately identical to that of the parent molecule BNCz and distributes on boron and carbon atoms at their ortho/para-positions in BNCz moiety, comparatively, but their HOMO distribution is somewhat different. For m-PCz–BNCz and m-DPAcP–BNCz, the HOMO distribution is predominantly localized on nitrogen and carbon atoms at the ortho/para-positions in the BNCz moiety, and partially extends to the attached phenyl-carbazole and triphenyl-acridine moieties. In particular, the presence of a sp³-hybrid spiro-carbon atom in m-DPAcP–BNCz truncates the conjugation, and the two benzene rings at the peripheral terminal do not participate in the frontier molecular orbital (FMO) distribution. For m-BN–BNCz and m-SF–BNCz, their HOMO distribution is also principally resided on nitrogen and carbon atoms at the ortho/para-positions in the BNCz moiety. Intriguingly, only a small part spreads to the adjoining benzene ring of the attached benzonitrile and spirofluorene moieties, and does not reach the electron-withdrawing group of cyano unit and neutral spiro-carbon atom. This is because the benzene ring is directly connected to the HOMO of the parent molecule, BNCz, and still maintains a certain degree of conjugation, and the FMOs can show transition directly. As a result, compared with the parent molecule BNCz (HOMO: −5.06 eV, LUMO: −1.71 eV, f: 0.4098), the HOMO energy levels of m-PCz–BNCz, m-DPAcP–BNCz, and m-SF–BNCz increase significantly, while their LUMO energy levels change slightly, thereby leading to enhanced ICT state and should result in redshift emission. In contrast, the HOMO and LUMO energy levels of m-BN–BNCz decreased slightly, but the E_gap remains basically unchanged relative to that of the parent molecule BNCz; thus, the similar emission should occur. Because the investigated molecules have a rigid π-conjugated MR framework, the oscillator strengths of their HOMO–LUMO transition remain large, which contributes to the realization of high Φ_PL. According to the FMO distribution and oscillator strengths, it is rational that the small FWHM values of four molecules can be consummately maintained due to the restricted impact of these substituents on the MR effect of the BNCz skeleton. Therefore, the four molecules should have the MR gene and inherit the original “fingerprint” features of BNCz.
为阐明不同推拉电子基团对几何与电子性质的影响机制及差异，研究首先采用密度泛函理论（DFT）优化了 S₀态几何构型，并通过含时密度泛函理论（TDDFT）模拟了 S₁态几何构型。图 1 展示了 HOMO/LUMO 分布、HOMO-LUMO 能级差（E[2]）、振荡强度、静电势（ESP）分布及自旋轨道耦合矩阵元素（s）。m-PCz–BNCz、m-DPAcP–BNCz、m-BN–BNCz 和 m-SF–BNCz 的 LUMO 分布与母体分子 BNCz 基本一致，均位于 BNCz 部分中硼原子及其邻/对位碳原子上；而其 HOMO 分布存在差异：m-PCz–BNCz 与 m-DPAcP–BNCz 的 HOMO 主要定域于 BNCz 部分中氮原子及其邻/对位碳原子，并部分延伸至连接的苯基咔唑和三苯基吖啶部分。 尤其，m-DPAcP–BNCz 分子中 sp ³ 杂化的螺碳原子截断了共轭链，外围末端的两个苯环并未参与前线分子轨道（FMO）分布。对于 m-BN–BNCz 和 m-SF–BNCz，其最高占据分子轨道（HOMO）分布同样主要位于 BNCz 结构单元的邻/对位氮原子与碳原子上。值得注意的是，仅小部分电子云延伸至相邻的苯甲腈与螺芴基团的苯环，且未触及氰基单元的吸电子基团及中性螺碳原子。这是因为苯环直接连接母体分子 BNCz 的 HOMO 轨道，仍保持一定程度的共轭性，使得前线分子轨道能直接呈现电子跃迁。因此相较于母体分子 BNCz（HOMO: -5.06 eV, LUMO: -1.71 eV, f: 0.4098），m-PCz–BNCz、m-DPAcP–BNCz 和 m-SF–BNCz 的 HOMO 能级显著升高，而 LUMO 能级变化微弱，从而增强分子内电荷转移态，预期将导致发射光谱红移。 相比之下，m-BN–BNCz 的 HOMO 和 LUMO 能级略有下降，但其 E _gap 值相对于母体分子 BNCz 基本保持不变，因此应产生相似的发射特性。由于研究对象具有刚性π共轭多重共振骨架，其 HOMO-LUMO 跃迁的振荡强度仍然较大，这为实现高Φ _PL 提供了基础。根据前线分子轨道分布和振荡强度分析，四种分子之所以能完美保持较小的半峰宽值，是因为这些取代基对 BNCz 骨架多重共振效应的限制性影响。由此可见，四种分子应具备多重共振基因，并继承了 BNCz 原始的"指纹"特征。

The introduction of substituents with different push–pull electronic properties expands the ESP distribution to varying degrees, especially the introduction of donor makes the ESP of parent core BNCz more negative, and the introduction of benzonitrile acceptor makes them more positive.^[11] This is the origin that the HOMO energy levels of m-PCz–BNCz and m-DPAcP–BNCz increase significantly, while their LUMO energy levels change relatively little, and the HOMO/LUMO energy levels of m-BN–BNCz decrease in varying degrees. It is found that there are appreciable <S|Ĥ_SOC|T>s with nonadiabatic vibronic couplings between S₁ state and two T_n states (closest to the energy of S₁ state) of four molecules, especially for m-PCz–BNCz and m-DPAcP–BNCz; their upperlying T₂–T₃ states offer exceptionally intense <S|Ĥ_SOC|T>s with S₁ states, which may provide multiple cooperative spin-flipping channels for the upconversion of excitons from T_n state to S₁ state.^[12] The strong spin−orbit couplings (SOCs) are reminiscent of the probable mixed short-range charge-transfer (SRCT) state (from BNCz) and long-range charge-transfer (LRCT) state (from donor → BNCz) of their S₁ and T_n states.^[13] In addition, this is inseparable from the intensified intramolecular noncovalent interactions (NCIs) between the attached donor and the parent molecule BNCz,^[14] which can be clearly revealed by the reduced density gradient (RDG) analysis (Figure 2).^[15] The brown regions of the RDG isosurfaces show the congested stereohindrance effect, resulting in highly folded and twisted geometrical structures, and the green regions of the RDG isosurfaces corresponding to several spikes in the range from −0.015 to 0.015 a.u. of sign (λ₂)ρ function in the scattering diagrams depict significant intramolecular noncovalent van der Waals interactions. It is precisely because such folded geometries can trigger specific NCIs and thus increase additional charge-transfer pathways, which is also expected to enhance the important S₀ → S₁ transition dipole moment (TDM).^[6] The mixed SRCT/LRCT nature of these states can potentially offer a desirable balance between the small S–T gap, lead to large SOCs between S and T states, and thereby ultimately facilitate the efficient RISC process.^[10] In addition, considering the main role of short-range Dexter energy transfer in their molecular aggregates, it is reasonable to conjecture that under high doping concentration, the highly twisted structures can suppress the electron exchange between adjacent molecules and inhibit the intermolecular collisional quenching.
引入具有不同推拉电子性质的取代基会不同程度地扩展电子表面电势（ESP）分布，特别是供体基团的引入使母核 BNCz 的 ESP 更负，而苯甲腈受体的引入则使其更正。 ¹¹ 这正是 m-PCz-BNCz 和 m-DPAcP-BNCz 分子最高占据分子轨道（HOMO）能级显著升高、最低未占分子轨道（LUMO）能级变化较小，而 m-BN-BNCz 的 HOMO/LUMO 能级均不同程度下降的根源。研究发现，四种分子中单重激发态（S ₁ ）与两个能量最接近的三重激发态（T _n 、T ₁ ）之间存在显著的非绝热振动耦合，尤其对于 m-PCz-BNCz 和 m-DPAcP-BNCz；其高位 T ₂ -T ₃ 态与 S ₁ 态间产生异常强烈的耦合，这可为激子从 T _n 态至 S ₁ 态的上转换提供多重协同自旋翻转通道。 强自旋轨道耦合（SOCs）暗示了其单重态（S）与三重态（T）可能同时存在短程电荷转移态（SRCT，源于 BNCz）和长程电荷转移态（LRCT，源于给体→BNCz）。此外，这与引入的给体和母体分子 BNCz 之间增强的分子内非共价相互作用（NCIs）密不可分，这一点可通过约化密度梯度（RDG）分析清晰呈现（图 2）。RDG 等值面图中的棕色区域显示出拥挤的空间位阻效应，导致形成高度折叠扭曲的几何构型；而绿色区域对应的散射图中 sign(λ)ρ函数在-0.015 至 0.015 原子单位区间内的多个尖峰，则描绘了显著的分子内非共价范德华相互作用。正是这类折叠构型可触发特定 NCIs，从而增加额外电荷转移路径，预期也将增强关键的 S1→S0 跃迁偶极矩（TDM）。 ⁶ 这些态兼具短程/长程电荷转移混合特性，可能在维持较小单重态-三重态能隙的同时，诱导产生显著的自旋轨道耦合效应，从而最终促进高效的反向系间窜越过程。 ¹⁰ 此外，考虑到短程德克斯特能量转移在其分子聚集体中的主导作用，有理由推测：在高掺杂浓度下，高度扭曲的分子结构可有效抑制相邻分子间的电子交换，进而阻碍分子间碰撞淬灭现象的发生。

The preliminary photophysical properties of four compounds in dilute toluene solution (1 × 10^-5 m) were recorded (Figure 3 and Table 1), including ultraviolet–visible (UV–vis) absorption and PL spectra. For m-PCz–BNCz, m-DPAcP–BNCz, m-BN–BNCz, and m-SF–BNCz, the electronic absorption spectra have intense absorption peaks at 474, 472, 472, and 474 nm, respectively, which are attributed to the ICT absorption transition. They all show strong fluorescence, with sharp emission peaks at 494, 491, 488, and 491 nm; small Stokes shifts of 20, 19, 16, and 17 nm; and narrow FWHM values of 25, 26, 25, and 25 nm, respectively. The maximum emission wavelengths of m-PCz–BNCz and m-DPAcP–BNCz have redshift values of 13 and 10 nm, respectively, compared with that of BNCz (481 nm). The redshift amplitude is not as large as that of m-Cz–BNCz.^[8] This is because the inserted phenyl bridge between the donor and MR framework disperses the electron cloud density of the donor, resulting in the very weak ICT state. It is observed that the UV–vis and PL spectral profiles show good mirror symmetry, because the excited state configurations of four molecules have less structural deformation compared with the ground state configurations (Figure S17, Supporting Information), which coincide with the very rigid MR framework. The corresponding root-mean-square deviations (RMSDs) of m-PCz–BNCz, m-DPAcP–BNCz, m-BN–BNCz, and m-SF–BNCz are calculated to be 0.26567, 0.44896, 0.14625, and 0.65935, respectively, which visually quantify the small change in the overall structure during the S₀ and S₁ state switching. Noteworthy, though the RMSDs of m-DPAcP–BNCz and m-SF–BNCz are much larger than those of m-PCz–BNCz and m-BN–BNCz, but they basically have no influence on their FWHM values. This is because RMSD measures the change of the overall structure of molecule. For m-DPAcP–BNCz and m-SF–BNCz, many of the increased RMSD values come from the huge fluctuation of the two benzene rings at the peripheral terminal. Fortunately, they are not involved in the FMO distribution, that is, they are not in the chromophore, so they will not have a substantial impact on the FWHM values.
在稀甲苯溶液(1×10 ^-5 m)中记录了四种化合物的初步光物理性质(图 3 与表 1)，包括紫外-可见吸收光谱与光致发光光谱。m-PCz—BNCz、m-DPAcP—BNCz、m-BN—BNCz 及 m-SF—BNCz 的电子吸收光谱分别在 474、472、472 和 474 纳米处呈现强吸收峰，归属于分子内电荷转移吸收跃迁。四者均表现出强荧光特性：发射峰尖锐(494、491、488 及 491 纳米)，斯托克斯位移微小(20、19、16 及 17 纳米)，半峰宽值狭窄(分别为 25、26、25 及 25 纳米)。相较于 BNCz(481 纳米)，m-PCz—BNCz 与 m-DPAcP—BNCz 的最大发射波长分别产生 13 纳米与 10 纳米红移，但其红移幅度小于 m-Cz—BNCz。 ⁸ 这是因为供体与多共振框架间插入的苯基桥分散了供体的电子云密度，导致分子内电荷转移态强度显著减弱。 观察到紫外-可见吸收光谱（UV-vis）与光致发光光谱（PL）谱图呈现良好镜像对称性，这是因为四种分子的激发态构型相较于基态构型结构形变较小（图 S17，支持信息），这与高度刚性的多共振（MR）框架特性相符。经计算，m-PCz-BNCz、m-DPAcP-BNCz、m-BN-BNCz 和 m-SF-BNCz 的均方根偏差（RMSD）值分别为 0.26567、0.44896、0.14625 和 0.65935，直观量化了 S@2 态与 S@3 态转换过程中整体结构的微小变化。值得注意的是，尽管 m-DPAcP-BNCz 和 m-SF-BNCz 的 RMSD 值远高于其他两种分子，但对其半峰全宽（FWHM）值基本无影响。这是因为 RMSD 衡量的是分子整体结构变化，而 m-DPAcP-BNCz 和 m-SF-BNCz 的 RMSD 增量主要源于外围末端两个苯环的大幅振动。值得庆幸的是，这些振动区域未参与前线分子轨道（FMO）分布，即不属于发色团组成部分，因此不会对 FWHM 值产生实质性影响。

The absolute Φ_PL values of m-PCz–BNCz, m-DPAcP–BNCz, m-BN–BNCz, and m-SF–BNCz in oxygen-free toluene solution are as high as 97%, 97%, 93%, and 96%, respectively. According to the onsets of fluorescence and phosphorescence spectra at 77 K (Figure S18, Supporting Information), the ΔE_ST values are estimated to be 0.15, 0.14, 0.16, and 0.16 eV, respectively. They are sufficient to support the exciton upconversion from T₁ state to S₁ state, resulting in TADF characteristics. The photophysical properties of these compounds in doped thin films were further systematically investigated. 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-3,9′-bicarbazole (PhCzBCz) was selected as the host, and 3 wt% doping concentration of the guest was adopted to ensure sufficient energy transfer and prevent bimolecular processes in the rigid matrix. Compared with the emission in low-polar toluene solution, the doped films show vividly green emission, with a certain degree of redshifting and slightly broadening FWHM (Table 2; Figure S19, Supporting Information), which may be due to the enhancement of molecular interaction in the films and the increase of matrix PhCzBCz polarity. The transient PL decay curves of the doped films at room temperature exhibit bi-exponential decays, that is, a prompt fluorescence component with lifetime in the nanosecond regime and a delayed fluorescence component with lifetime in the microsecond regime (Figure S20, Supporting Information), which are the manifestations of TADF characteristics. When the temperature gradient increases from 120 to 320 K, the proportion of delayed fluorescence components gradually increase (Figure S21, Supporting Information), undoubtedly confirming the TADF characteristic. Furthermore, the photophysical rate constants of prompt fluorescence (k_F), internal conversion (k_IC), and reverse intersystem crossing (k_RISC) were analyzed by using the transient PL spectroscopic data and Φ_PL values (Table S1, Supporting Information). Thanks to the significant MR electronic effect, the k_F values of these compounds are as high as 10⁸ orders of magnitude, which are much higher than their k_IC values (10⁶ orders of magnitude), indicating that the nonradiative decay energy loss is minimal and negligible. However, their k_RISC values are only about 10⁴ orders of magnitude. Generally, k_RISC is positively correlated with <S|Ĥ_SOC|T> and negatively correlated with ΔE_ST (k_RISC ∝ <S|Ĥ_SOC|T>²/ΔE_ST).^[16] Therefore, in order to obtain large k_RISC, both must be taken into account, that is, not only to enhance SOC, but also to minimize ΔE_ST. In the newly constructed TADF systems, although they have considerable <S|Ĥ_SOC|T>s, their ΔE_ST values (dominant S₁ state and T₁ state) are relatively large. This is because there is a relatively large degree of HOMO–LUMO overlap in the MR framework, and the introduced ICT state is also very weak due to the inserted phenyl bridge between substituents and MR framework.
在无氧甲苯溶液中，m-PCz–BNCz、m-DPAcP–BNCz、m-BN–BNCz 及 m-SF–BNCz 的绝对Φ值分别高达 97%、97%、93%和 96%。根据 77K 条件下的荧光与磷光谱起始点（图 S18，支持信息），估算其ΔE 值分别为 0.15、0.14、0.16 和 0.16 eV。该能隙值足以支撑三重态（T）至单重态（S）的激子上转换过程，从而产生热活化延迟荧光特性。我们进一步系统研究了这些化合物在掺杂薄膜中的光物理性质。选用 9-(2-(9-苯基-9H-咔唑-3-基)苯基)-9H-3,9′-联咔唑（PhCzBCz）作为主体材料，并采用 3wt%的客体掺杂浓度，以确保刚性基质中能量充分转移且抑制双分子过程。相较于低极性甲苯溶液中的发射光谱，掺杂薄膜呈现出鲜艳的绿色发光，同时伴随一定程度的红移现象及半峰全宽轻微展宽（表 2；图 S19，支持信息），这可能是由于薄膜内分子相互作用增强及基质 PhCzBCz 极性提升所致。 室温下掺杂薄膜的瞬态 PL 衰减曲线呈现双指数衰减特性，即纳秒级的瞬时荧光组分与微秒级的延迟荧光组分（图 S20，支持信息），这正是 TADF 特性的体现。当温度梯度从 120 K 升至 320 K 时，延迟荧光组分的占比逐渐增加（图 S21，支持信息），确凿地证实了 TADF 特征。此外，通过瞬态 PL 光谱数据和Φ _PL 值（表 S1，支持信息）分析了瞬时荧光(k _F )、内转换(k _IC )及反向系间窜越(k _RISC )的光物理速率常数。得益于显著的多重共振电子效应，这些化合物的 k _F 值高达 10^9 数量级，远高于其 k _IC 值（10^11 数量级），表明非辐射衰变能量损耗极低可忽略。然而其 k _RISC 值仅约为 10^13 数量级。 通常，k _RISC 与呈正相关，与ΔE _ST 呈负相关（k _RISC ∝ ² /ΔE _ST ）。 ¹⁶ 因此，为获得较大的 k _RISC ，需同时兼顾二者：既要增强自旋轨道耦合（SOC），也要最小化ΔE _ST 。在新构建的热活化延迟荧光（TADF）体系中，尽管存在显著的值，但其主导单重态（S ₁ ）与三重态（T ₁ ）间的ΔE _ST 值仍然较大。这是由于多共振框架中最高占据分子轨道-最低未占分子轨道（HOMO-LUMO）重叠度较高，且取代基与多共振框架间插入的苯基桥导致引入的分子内电荷转移（ICT）态强度极弱。

According to the oxidation and reduction onset potentials measured by cyclic voltammetry (CV; Figure S22, Supporting Information), the HOMO energy levels are estimated to be −5.17, −5.22, −5.27, and −5.23 eV for m-PCz–BNCz, m-DPAcP–BNCz, m-BN–BNCz, and m-SF–BNCz, respectively, while the LUMO energy levels are estimated to be −2.47, −2.50, −2.53, and −2.47 eV, respectively. Compared with BNCz (HOMO: −5.40 eV, LUMO: −2.61 eV), the HOMO energy levels of m-PCz–BNCz and m-DPAcP–BNCz increase significantly, and the LUMO energy levels are almost homologous, suggesting that their emission should be bathochromic shift. Thermogravimetric analysis (TGA) measurements show that these compounds have outstanding thermal stability, with decomposition and/or sublimation temperatures (T_d/s, corresponding to 5% weight loss) as high as 474, 488, 441, and 477 °C (Figure S23, Supporting Information), respectively, supporting the vacuum thermal deposition process for OLED manufacturing. From differential scanning calorimetric (DSC) measurements, the glass transition temperatures (T_g) of m-PCz–BNCz, m-DPAcP–BNCz, and m-SF–BNCz are high enough to be 240, 249, and 269 °C, respectively, and the T_g of m-BN–BNCz is not observed (Figure S24, Supporting Information). The high T_g values make the compounds not easy to crystallize in organic films, and can fabricate OLED devices with good quality and excellent stability.
根据循环伏安法（CV；图 S22，支持信息）测得的氧化还原起始电位，m-PCz–BNCz、m-DPAcP–BNCz、m-BN–BNCz 和 m-SF–BNCz 的 HOMO 能级估值分别为-5.17、-5.22、-5.27 和-5.23 eV，LUMO 能级估值分别为-2.47、-2.50、-2.53 和-2.47 eV。相较于 BNCz（HOMO：-5.40 eV，LUMO：-2.61 eV），m-PCz–BNCz 与 m-DPAcP–BNCz 的 HOMO 能级显著提高，LUMO 能级基本同源，预示其发射光谱将发生红移。热重分析（TGA）表明这些化合物具有优异的热稳定性，分解/升华温度（T _d/s ，对应 5%失重）分别高达 474、488、441 和 477°C（图 S23，支持信息），完全满足 OLED 制造所需的真空热沉积工艺要求。 通过差示扫描量热（DSC）测试表明，m-PCz–BNCz、m-DPAcP–BNCz 和 m-SF–BNCz 的玻璃化转变温度（T _g ）分别高达 240°C、249°C 和 269°C，而 m-BN–BNCz 的 T _g 未检测到（图 S24，支持信息）。这些高 T _g 值使化合物在有机薄膜中不易结晶，可制备出高质量且稳定性优异的 OLED 器件。

The EL performances of four compounds as emitters were evaluated by multilayer OLEDs with the configuration of indium tin oxide (ITO)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (50 nm)/tris(4-carbazolyl-9-ylphenyl)amine (TCTA) (5 nm)/PhCzBCz:x wt% of the investigated compounds (30 nm)/3,3″-[5″-[3-(3-pridinyl)phenyl][1,1″:3″,1″″-terphenyl]-3,3″″-diyl]bispyridine (TmPyPB) (30 nm)/LiF (1 nm)/Al (100 nm)] (x = 3, 5, 10, and 20). Due to the high triplet energy (E_T1 = 2.96 eV) and balanced bipolar carrier-transport ability, PhCzBCz was designated as the host of the emitting layer (EML), which can well confine the excitons inside the EML. TAPC and TmPyPB were utilized as the hole-transporting layer (HTL) and electron-transporting layer (ETL), respectively. To confine the excitons to the EML, TCTA was employed as the electron-blocking layer (EBL). The minimum doping concentration of the investigated compounds was 3 wt% to ensure efficient energy transfer from the host to the emitters. Furthermore, the doping concentrations of 5, 10, and 20 wt% of the investigated compounds were used for modulating and optimizing the device performances. The energy level diagram of devices and molecular structures of used materials are illustrated in Figure S25 (Supporting Information). The device performances consisting of EL spectra and EQEs are shown in Figure 4; the current density (J), voltage (V), and luminance (L) are depicted in Figure S26 (Supporting Information); the current efficiency (CE) and power efficiency (PE) are displayed in Figure S27 (Supporting Information); and the detailed device parameters are compiled in Table 3 and Table S2 (Supporting Information), respectively.
通过多层 OLED 器件评估了四种化合物作为发光体的电致发光性能，器件结构为[氧化铟锡(ITO)/1,1-双[(二-4-甲苯氨基)苯基]环己烷(TAPC)(50 nm)/三(4-咔唑-9-基苯基)胺(TCTA)(5 nm)/PhCzBCz：x wt%目标化合物(30 nm)/3,3″-[5″-[3-(3-吡啶基)苯基][1,1″:3″,1″″-三联苯]-3,3″″-二基]二吡啶(TmPyPB)(30 nm)/氟化锂(LiF)(1 nm)/铝(Al)(100 nm)]（x=3、5、10 和 20）。基于 PhCzBCz 的高三重态能量（E _T1 =2.96 eV）和平衡的双极性载流子传输能力，将其选定为发光层主体材料，可有效将激子限制在发光层内。TAPC 与 TmPyPB 分别作为空穴传输层和电子传输层，TCTA 则用作电子阻挡层以实现激子限域。目标化合物的最低掺杂浓度为 3 wt%以确保主体材料向发光体的高效能量转移，同时采用 5、10 及 20 wt%的掺杂浓度梯度调控优化器件性能。 器件能级图及所用材料的分子结构如图 S25（支持信息）所示。图 4 展示了包含电致发光光谱和外量子效率的器件性能；电流密度（J）、电压（V）和亮度（L）数据描绘于图 S26（支持信息）；电流效率（CE）与功率效率（PE）呈现于图 S27（支持信息）；详细器件参数则分别汇总于表 3 及表 S2（支持信息）。

All devices have low turn-on voltages (V_on at 1 cd m⁻²) of about 3.0 eV, indicating efficient carrier injection and transport. With the increase of doping concentration, the turn-on voltage decreases gradually, suggesting that at higher doping concentration, carriers tend to be injected directly into the investigated emitters rather than through matrix PhCzBCz. For m-PCz–BNCz-, m-DPAcP–BNCz-, and m-SF–BNCz-based emitters, the devices all achieve the best EL performances at 10 wt% doping concentration. The devices achieve the optimized maximum EQE values of 36.8%, 42.0%, and 41.1%; maximum CE values of 94.9, 93.9, and 91.1 cd A⁻¹; maximum PE values of 99.2, 96.2, and 90.8 lm W⁻¹ for m-PCz–BNCz, m-DPAcP–BNCz, and m-SF–BNCz, respectively. They emit vivid green light with sharp peaks at 504, 496, and 496 nm; narrow FWHM values of 29, 28, and 28 nm, and Commission Internationale de I'Éclairage (CIE) coordinates of (0.11, 0.61), (0.09, 0.54), and (0.09, 0.53), respectively. In comparison, for the m-BN–BNCz-based emitter, the device with 3 wt% doping concentration exhibits the optimized maximum EQE, CE, and PE values up to 35.0%, 72.5 cd A⁻¹, and 71.7 lm W⁻¹, respectively, and the device displays bluish green emission with a sharp peak at 492 nm, a narrow FWHM of 28 nm, and CIE coordinates of (0.09, 0.48). With the increase of doping concentration of m-PCz–BNCz-, m-DPAcP–BNCz-, m-BN–BNCz-, and m-SF–BNCz-based emitters, the emission maxima are basically unchanged or slight redshifted, and the FWHM values remain basically stationary. Obviously, the emission of matrix PhCzBCz in their EL spectra disappears, and all emissions of devices originate from pure MR-TADF emitters, indicating that the radiative transition excitons are well limited on the emitters. During the measurement of EL data in air, all un-encapsulated devices with the best EL performances based on the investigated emitters exhibit very good spectrum stability over a wide range of operating voltages from 3 to 8 V (Figures S28–S31, Supporting Information), indicating that the energy transfer is very complete from the host to the emitters, and there is only one stationary recombination zone in the devices. In fact, beyond the emitting layer, the device stability is also strongly controlled by other functional layers, such as HTLs, ETLs, EBLs, and buffer layers at the anode and cathode. To fabricate long lifetime devices, the corresponding configuration and fabrication process should be carefully optimized in the future. Remarkably, the extremely high efficiency is achieved in 10 wt% doped devices for m-DPAcP–BNCz and m-SF–BNCz without any light outcoupling enhancement, providing the maximum EQE value of over 40.0% for single junction OLED so far, and notably, they can concurrently achieve narrowband emission.^[17] To confirm the reproducibility and reliability of maximum EQE of the device based on these compounds, six groups of devices based on four emitters were fabricated in parallel (doping concentration selects the corresponding concentration of the best EL performances of each emitter). The results show that the EQE–L curves of these compounds are nearly consistent, demonstrating that the device efficiencies of these four emitters are completely reproducible and not accidental (Figure S32 and Table S3, Supporting Information).
所有器件均表现出约 3.0 eV 的低启亮电压（1 cd m⁻²下的 V[ _on ]），表明载流子注入与传输效率优异。随着掺杂浓度提升，启亮电压逐步降低，说明在高浓度掺杂时载流子倾向于直接注入研究对象发光体而非通过基质 PhCzBCz 传输。基于 m-PCz–BNCz、m-DPAcP–BNCz 和 m-SF–BNCz 的发光器件均在 10 wt%掺杂浓度下实现最佳电致发光性能：其最大外量子效率（EQE）分别优化至 36.8%、42.0%和 41.1%；最大电流效率（CE）达 94.9、93.9 和 91.1 cd A⁻¹[ ⁻¹ ]；最大功率效率（PE）分别为 99.2、96.2 和 90.8 lm W⁻¹[ ⁻¹ ]。这些器件发射出鲜绿色光，在 504、496 和 496 nm 处呈现尖峰；半峰全宽（FWHM）窄至 29、28 和 28 nm；国际照明委员会（CIE）色坐标分别为(0.11, 0.61)、(0.09, 0.54)和(0.09, 0.53)。 相比之下，基于 m-BN–BNCz 的发射体制备的器件在 3 wt%掺杂浓度下表现出最优性能：最大外量子效率（EQE）达 35.0%，电流效率（CE）为 72.5 cd A ⁻¹ ，功率效率（PE）达 71.7 lm W ⁻¹ 。该器件呈现蓝绿光发射，其发射光谱在 492 nm 处具有尖锐峰，28 nm 的窄半峰宽，CIE 坐标为（0.09, 0.48）。随着 m-PCz–BNCz、m-DPAcP–BNCz、m-BN–BNCz 及 m-SF–BNCz 基发射体掺杂浓度增加，其发射峰位置基本不变或发生轻微红移，半峰宽值保持稳定。值得注意的是，电致发光光谱中基质 PhCzBCz 的发射完全消失，所有器件的发光均源于纯净的多重共振热激活延迟荧光（MR-TADF）发射体，这表明辐射跃迁激子被很好地限制在发射体上。 在空气中测量电致发光数据时，基于所研究发光体的所有未封装最佳性能器件，在 3 至 8 伏的宽工作电压范围内均展现出优异的光谱稳定性（图 S28-S31，支持信息），表明主体材料向发光体的能量传递极为充分，且器件内仅存在单一稳态复合区。实际上，除发光层外，器件稳定性还受到空穴传输层（HTLs）、电子传输层（ETLs）、电子阻挡层（EBLs）以及阴阳极缓冲层等功能层的显著调控。为制备长寿命器件，未来需对其相应结构及制备工艺进行精细优化。值得注意的是，m-DPAcP-BNCz 与 m-SF-BNCz 在 10 wt%掺杂浓度下无需任何光取出增强即实现了超高效率，创造了单结 OLED 迄今超过 40.0%的最大外量子效率值，并同步实现了窄带发射特性。 ¹⁷ 为验证基于这些化合物的器件最大外量子效率（EQE）的可重现性与可靠性，我们平行制备了六组基于四种发光材料的器件（掺杂浓度选取各发光材料最佳电致发光性能对应的浓度）。结果表明，这些化合物的 EQE-亮度曲线高度一致，证明四种发光材料的器件效率具备完全可重现性，并非偶然现象（支撑材料图 S32 及表 S3）。

It is observed that the efficiency roll-off of the devices based on four investigated emitters exhibits low-efficiency roll-off at 100 cd m⁻², but relatively large self-emission quenching at 1000 cd m⁻². Considering these four compounds have peripheral auxiliary donors or bulky groups with electronic inertness, the efficiency roll-off can be mainly attributed to the intrinsically long triplet exciton lifetime/small k_RISC of the emitters. This type of efficiency attenuation can be alleviated by device optimization, that is, modulating host or material types of other functional layers, such as using exciplex as the host or selecting an appropriate TADF or phosphorescence sensitizer. For all that, m-PCz–BNCz-, m-DPAcP–BNCz-, and m-SF–BNCz-based devices are much smaller than that of the compounds with similar scale k_RISC values, and are at a medium level. More importantly, the m-PCz–BNCz-, m-DPAcP–BNCz-, and m-SF–BNCz-based devices show high maximum EQE values of over 32.8%, 36.3%, and 36.3%, respectively, in the wide range of doping concentration from 3 to 20 wt%. In addition, other concentration-independent EL performances of these devices also include the emission peaks, FWHMs, and CIE coordinates. The main reason for the above phenomena is that the molecules have highly folded and distorted geometrical structures, which are due to the crowded stereohindrance effect caused by their linking mode between the auxiliary electron moiety and the parent molecule BNCz. This has been clearly elucidated by RDG analysis (vide supra). The suppressed efficiency roll-off can be largely attributed to the introduction of peripheral steric hindrance groups, which enlarge the intermolecular distance and inhibit the interchromophore aggregation and interaction, thus reducing the occurrence of collision and annihilation events related to triplet excitons, such as singlet–triplet annihilation (STA) and triplet–triplet annihilation (TTA).^[18] In addition, the highly twisted structure can effectively suppress the aggregation-induced emission quenching at high doping concentration, so as to maintain high EQE, narrow FWHM, and curb unfavorable redshift emission. It is worth noting that the concentration independence of m-SF–BNCz-based devices is particularly prominent. Even if the doping concentration is 20 wt%, the maximum EQE remains at 40.9%, and the emission spectrum maintains unaltered. Therefore, the investigated compounds also solve the problems of spectrum broadening and luminescence quenching of traditional MR-TADF molecules due to the aggregation of their large planar structure under high doping concentration. This work also provides a feasible approach to construct highly efficient and slight concentration-dependent narrowband emitting materials. In fact, the devices with high doping concentration will typically avoid insufficient energy transfer and serious phase segregation.^[19] Meanwhile, maintaining accurate and consistent doping percentage will also simplify the mass production processes and reduce the production cost from a practical viewpoint. In contrast, the efficiency roll-off of m-BN–BNCz-based device is the most serious, which can be ascribed to the most sluggish k_RISC and the smallest stereohindrance effect among four investigated emitters. Moreover, the latter factor directly leads to its obvious concentration-dependent EL performances. The above results also convincingly demonstrate the rationality and universality of our molecular structure strategy for constructing high-efficiency narrowband organic electroluminescent materials in the long-wavelength region. Compared with the emission of parent molecule BNCz, although the wavelength redshift amplitudes of m-PCz–BNCz and m-DPAcP–BNCz are relatively limited, the ingenious synthetic method and the extremely excellent device performances given in this study are very enlightening for the development of emissive materials with desirable color purity in the long-wavelength region. For example, the parent core can be flexibly adjusted. Assuming the green emissive compound DtBuPhCzB is selected as the parent core for bromination,^[8] and then can be used to construct narrowband emitting materials in the perfect green region and fabricate high-performance OLEDs through solution processing. As we know, solution-processed OLED is a more economical method for mass production of displays than vacuum-deposited OLED, and it is also the focus of current academic and industrial research.^[20
研究发现，基于四种目标发光体的器件在 100 cd m⁻² ⁻² 亮度下效率滚降较低，但在 1000 cd m⁻² ⁻² 亮度下自发光淬灭效应显著增强。鉴于这些化合物均含有外围辅助供电子基团或具有电子惰性的空间位阻基团，效率滚降主要源于发光体固有的长三重态激子寿命/小速率常数 k _RISC 。此类效率衰减可通过器件优化策略缓解：如调控主体材料或其他功能层材料类型——采用激基复合物作为主体材料，或选用合适的 TADF/磷光敏化剂。值得注意的是，m-PCz–BNCz、m-DPAcP–BNCz 及 m-SF–BNCz 基器件的效率滚降程度远低于具有相近 k _RISC 值的化合物，处于中等水平。更重要的是，在 3-20 wt%的宽掺杂浓度范围内，上述三种器件分别实现了 32.8%、36.3%和 36.3%的最大外量子效率峰值。 此外，这些器件其他与浓度无关的电致发光性能还包括发射峰、半峰全宽和 CIE 坐标。上述现象主要源于分子具有高度折叠且扭曲的几何结构，这归因于辅助电子单元与母体分子 BNCz 连接模式产生的空间位阻效应。通过 RDG 分析已明确阐明此点（见前文）。效率滚降的抑制主要得益于外围位阻基团的引入——其增大了分子间距，抑制了发色团间的聚集与相互作用，从而减少了与三重态激子相关的碰撞湮灭事件，例如单重态-三重态湮灭（STA）和三重态-三重态湮灭（TTA）。 ¹⁸ 同时，高度扭曲的结构能有效抑制高掺杂浓度下的聚集诱导发光淬灭，从而维持高外量子效率、窄半峰全宽，并遏制不利的红移发射现象。 值得注意的是，基于 m-SF-BNCz 的器件展现出尤为显著的浓度不敏感性。即使掺杂浓度高达 20 wt%，其最大外量子效率仍保持在 40.9%，且发射光谱维持不变。因此，所研究的化合物同时解决了传统 MR-TADF 分子因大平面结构在高浓度下聚集导致的光谱展宽和发光淬灭问题。本工作为构建高效且浓度依赖性弱的窄带发光材料提供了可行路径。实际上，高掺杂浓度器件通常能规避能量转移不足和严重相分离问题 ¹⁹ 。从实用角度看，保持精准一致的掺杂比例还能简化量产工艺并降低生产成本。相比之下，基于 m-BN-BNCz 的器件效率滚降最为严重，这归因于其在四种研究材料中最缓慢的 k _RISC 速率及最弱的立体位阻效应。 此外，后一因素直接导致其电致发光性能呈现显著的浓度依赖性。上述结果亦有力证实了我们在长波长区域构建高效窄带有机电致发光材料的分子结构策略具有合理性与普适性。相较于母体分子 BNCz 的发射波长，尽管 m-PCz-BNCz 和 m-DPAcP-BNCz 的红移幅度相对有限，但本研究提出的精妙合成方法及展现的卓越器件性能，对开发长波长区域高色纯度发光材料具有重要启示意义。例如，母核结构可进行灵活调控——若选用绿色发光化合物 DtBuPhCzB 作为溴化母核 ⁸ ，则能用于构建完美绿色区域的窄带发光材料，并通过溶液加工法制备高性能 OLED 器件。 众所周知，溶液加工 OLED 相较于真空蒸镀 OLED 是实现显示面板大规模量产更经济的方案，也是当前学界与工业界共同的研究焦点。 ^{20 ]}

3 Conclusion  3 结论

We have proposed and performed a synthetic methodology for functionalizing BN-containing MR framework with multifarious functional groups, such as donors, acceptors, and moieties without obvious push–pull electron properties. For this synthetic method, the key is the successful bromination of parent molecule BNCz, which opens a broad space for the facile introduction of various functional groups. The m-DPAcP–BNCz-based OLED exhibits green emission with an FWHM of 28 nm and a high EQE of 40.6%. The outstanding results reported here are ascribed to the perfect combination of the inherent advantages of MR framework and donor–acceptor configuration, which can simultaneously achieve bathochromic shift and narrowband emission, high Φ_PL and Θ_//. This study demonstrates that our molecular design strategy is of significance for the development of high-efficiency narrowband organic electroluminescent materials that can be utilized to fabricate OLED display with wide high color purity. Importantly, the approach is beneficial to get rid of the predicament situation that the synthetic methodology of MR framework is very deficient currently. The approach represents a powerful tool for quick generation of novel and efficient narrowband emitting materials and may establish a more controllable and accurate method to construct organic luminescent materials. On the other hand, this contribution is highly attractive for elucidating the structure–property relationship for MR molecules. Indeed, the bromination reaction of BN-containing MR framework can be employed as a “platform reaction” to extremely enrich the MR molecular library.
我们提出并实践了一种合成方法学，用于将含 B-N 键的多重共振骨架功能化，引入多样化官能团——包括给电子体、受电子体以及无明显推拉电子特性的基团。该方法的关键在于母体分子 BNCz 的成功溴化，这为便捷引入各类官能团开辟了广阔空间。基于 m-DPAcP-BNCz 的有机发光二极管(OLED)展现出半峰全宽 28 纳米的绿光发射，同时实现 40.6%的高外量子效率。这些卓越性能源于多重共振骨架固有优势与给-受体结构的完美结合，既能实现红移发射又能维持窄谱带特性，同时达成高Φ _PL 与Θ _// 。本研究证明，该分子设计策略对开发高效窄谱带有机电致发光材料具有重要意义，此类材料可应用于制备广色域高色纯度的 OLED 显示器。尤为关键的是，该方法有助于摆脱当前多重共振骨架合成方法学极为匮乏的困境。 该方法为快速开发新型高效窄带发射材料提供了有力工具，并可能建立更可控、更精准的有机发光材料构筑策略。同时，该研究对阐明 MR 分子的构效关系极具吸引力。事实上，含 BN 基团的多共振框架溴化反应可作为"平台反应"，极大丰富 MR 分子库资源。

Conflict of Interest  利益冲突

The authors declare no conflict of interest.
作者声明无利益冲突。