An Overview of Molecular Design Strategies for Blue Thermally Activated Delayed Fluorescence
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摘要: 热激活延迟荧光（thermally activated delayed fluorescence，TADF）分子由于三重态上的激子可以通过反向系间窜越到单重态并辐射发出荧光，因此在有机发光二极管（organic light-emitting diode, OLED）中理论上可以实现100%的激子利用率。具有TADF特性的发光材料融合了第一代荧光材料和第二代磷光材料的优点，不仅可以实现100%的内量子效率，还有助于降低器件的材料成本，被誉为第三代OLED发光材料，并成为突破高效稳定蓝光OLED瓶颈的潜在解决方案。本文从发光机理出发，系统阐述了高效稳定蓝光TADF分子的设计策略，包括高荧光量子产率、短延迟荧光寿命、窄发射光谱半峰宽、显著的水平分子取向和良好的光电稳定性等。本文旨在为高性能蓝光TADF分子的开发提供理论支持。最后，总结了当前蓝光TADF材料存在的问题，并对其未来的发展前景进行了展望。Abstract: Recent developments have shown that purely organic molecules exploiting thermally activated delayed fluorescence (TADF) in organic light-emitting diodes (OLEDs) can utilize non-emissive triplet excitons for light emission through reverse intersystem crossing (RISC) processes. As a result, the exciton utilization efficiencies of TADF-based OLEDs can approach 100%. The electroluminescent molecule with TADF characteristics combines the advantages of the first-generation fluorescent emitters and the second-generation phosphorescent emitters by realizing 100% internal quantum efficiencies while reducing the material cost for commercialization. TADF materials now represent the third-generation of OLED emitters and offer an effective approach to breaking through the bottleneck of blue OLEDs. In this review, we focus on the development of high-performance blue TADF molecules, systematically elaborating on the design strategy of efficient and stable blue TADF molecules. Our discussion includes the achievement of high photoluminescence quantum yield, short excition lifetime, narrow full width half maximum, good molecular horizontal orientation and excellent stability. We aim to provide a scientific basis for the development of high-efficiency and stable blue TADF molecules. Additionally, we include an overview of outstanding issues and the research prospects of blue TADF molecules. We hope that this review contributes to the advancement of the field and provides insight into the design of high-performance blue TADF materials.
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图 1 有机电致发光材料的发展历程：第一代传统荧光OLED（a）、第二代磷光OLED（b）和第三代热活化延迟荧光OLED（c）电致发光机制的示意图（F和P分别对应荧光和磷光）
Figure 1. Development of organic electroluminescent materials: Schematic diagram of electroluminescence (EL) mechanism of the first-generation of traditional fluorescent OLED (a), the second-generation phosphorescent OLED (b) and the third-generation of TADF OLED (c) (F and P represent fluorescence and phosphorescence)
图 2 高性能蓝光TADF分子设计策略的示意图
Figure 2. Schematic diagram of design strategy for high-performance blue TADF molecules
图 3 蓝光TADF分子内电子给体与电子受体的连接方式
Figure 3. Different linkages between donor and acceptor units for design of blue TADF molecules
图 4 构建蓝光TADF分子常用的电子给体和电子受体
Figure 4. Common donor and acceptor units for design of blue TADF molecules
图 5 不同光色TADF分子能级分布的示意图，从左到右分别代表深蓝、天蓝、绿光TADF分子的典型发光机制（ISC，IC，RIC，
$ {E}_{{}_{}{}^{3}\mathrm{C}\mathrm{T}} $ 及$ {E}_{{}_{}{}^{3}\mathrm{L}\mathrm{E}} $ 分别代表系间窜越，内转换，反向内转换，3CT的能级及3LE的能级）[41]Figure 5. Schematic diagram of molecular energy level distribution of TADF materials with different light colors . From left to right, each picture represents typical emission mechanism of deep blue, sky blue and green TADF molecules (ISC, IC, RIC,
$ {E}_{{}_{}{}^{3}\mathrm{C}\mathrm{T}} $ and$ {E}_{{}_{}{}^{3}\mathrm{L}\mathrm{E}} $ represent intersystem crossing, internal conversion, reverse internal conversion, energy level of 3CT and energy level of 3LE)[41]
图 13 商业化应用中使用滤光片提高OLED发光色纯度的示意图[72]：器件结构（a）和使用滤光片与没有使用滤光片的对照光谱的示意图（b）
Figure 13. Schematic diagram of the use of color filters (CF) strategy to improve color purity of OLED in commercial applications[72]: Schematic diagram of device structure (a) and the comparison of emission spectra with and without CF (b)
图 14 Franck-Condon原理与分子光谱关系的示意图：发光分子辐射跃迁（a）和荧光光谱的示意图（b）
Figure 14. Schematic diagram of the relationship between Franck-Condon principle and molecular spectrum: Schematic diagram of radiative transitions of luminescent molecules (a) and fluorescence spectrum (b)
图 15 减小发光分子光谱FWHM的策略示意图：通过减少分子振动能级减小光谱展宽的示意图（a）；普通OLED分子的光谱示意图（b）和通过减小分子S1和S0构型差异减小光谱展宽示意图（c）
Figure 15. Schematic diagram of the strategy for narrowing FWHM of luminescent molecules: Schematic diagram of suppressing the spectral broadening by suppressing molecular vibration energy level (a); Schematic diagram of the emission spectrum of OLED molecules (b) and schematic diagram of suppressing the spectral broadening by suppressing the configuration difference between S1 and S0 (c)
图 17 一般TADF分子中化学键的键解离能（a）；蓝光TADF发射体的降解机制示意图（b）；蓝光TADF材料电氧化和光氧化协同过程的降解机制示意图（c）
Figure 17. Bond dissociation energy of the chemical bonds in common TADF molecules (a); Schematic diagram of the degradation mechanism of blue TADF emitters (b); Schematic diagram of the degradation mechanism of blue TADF materials by means of synergistic processes of electro-oxidation and photo-oxidation (c)
表 1 示例分子及其器件的常用参数列表
Table 1. List of common parameters for the reported molecule and their device
Molecule ΔEST/eV kRISC/s−1 PLQY/% EQEmax/% CIE T50/h Cz-TRZ3 0.17 1.53×104 60.0 19.20 0.15, 0.10 − TDBA-PAS 0.15 1.53×106 92.3 22.35 0.16, 0.04 − P-Ac95-TRZ05 0.02 − 51.0 12.10 0.18, 0.27 − P1-05 0.04 3.3×106 29.0 7.10 0.17, 0.17 − 4CzIPN 0.04 2.03×106 94.0 − − − 3Cz2DPhCzBN 0.15 9.90×105 80.0 20.90 0.21, 0.44 − 5CzTRZ 0.02 1.50×107 92.0 29.30 − − 3CTF 0.04 2.57×106 − 22.60 0.29, 0.57 − MCz-TXT − 1.10×108 79.0 25.80 0.21, 0.46 − CzBSe 0.15 1.80×108 98.0 23.90 0.10, 0.24 − BCzT − − 95.6 21.70 − − 2a 0.09 − 74.0 20.80 0.19, 0.35 − 2b 0.28 − 57.0 16.80 0.17, 0.27 − 2c 0.32 − 72.0 14.60 0.18, 0.28 − PXZN-B 0.28 4.54×104 93.0 12.70 0.13, 0.15 − DMACN-B 0.27 2.45×104 90.0 10.00 0.15, 0.05 − DM-B 0.17 1.80×105 96.0 27.40 0.20, 0.44 − mCz-Xo-TRZ 0.16 3.00×105 90.0 21.00 0.15, 0.20 − PNBTAc-TRZ5 − 4.40×106 81.0 18.80 0.20, 0.31 − SBA-2DPS 0.07 5.38×105 60.0 25.50 0.15, 0.20 − CzBPCN 0.27 − − 14.00 0.14, 0.12 11.01) v-DABNA 0.02 2.00×105 82.0 34.40 0.12, 0.11 − BN3 0.15 2.55×105 98.0 37.60 0.14, 0.08 − DABNA-2 0.14 1.48×104 89.7 20.20 0.12, 0.13 − BOBS-Z 0.12 8.60×105 93.0 26.90 0.14, 0.06 − mDBIC 0.31 5.0×103 68.0 13.50 0.16, 0.05 − BCZ-TRZ 0.31 − 92.0 20.50 0.18, 0.34 32.02) 1) L0 = 200 cd/m2；2) L0 = 500 cd/cm2；T50 represents the time to reach 50% of inital lunminance 
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