Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

      1.3.3 Hybridized Local and Charge Transfer Materials Aggregation‐induced Emissive Emitters

      Different from Kasha’s rule, “hot exciton” process can happen between higher, excited triplet states to emissive singlet T_m → S_n (m ≥ 2), which was discovered in 1962 [123]. This process with 100% IQE in theory was often accompanied by HLCT excited state, where LE state decays fluorescently, while CT state upconverted from ³CT to ¹CT and then to emissive ¹LE state, as a result of close LE and CT states [28–31]. Ma et al. first reported several HLCT emitters of donor–acceptor (D–A) structures, and the related OLEDs’ excellent EL performance [28]. As the AIE characteristics also grafted into the HLCT materials, it enabled these emitters to have high efficiency in aggregated state, with spectrum from blue to green and red.

Our group utilized the most common AIE‐active group of TPE to combine with imidazole, and three blue novel emitters TPEI, TPEMeOPhI, and 3TPEI were prepared (Figure 1.8), showing both HLCT and AIE behaviors. Their nondoped OLEDs can be fabricated through facile process, with medium maximum EQE of 2.41, 2.16, and 3.13%, and slight efficiency roll‐off [124]. Tang et al. also took advantage of TPE moiety to prepare two green AIE‐active emitters TPE‐NB and TPE‐PNPB (Figure 1.8), both of which exhibited double slops of stokes shift versus different solvent parameters, suggesting the existence of both LE and CT state. The OLED based on the emitter of TPE‐PNPB showed better EL performance, with maximum EQE of up to 5.35% as a result of weaker D–A interaction [125]. With regard to the red HLCT‐AIE emitters, the TPE’s CN derivatives were usually applied as a high‐efficient luminescent building block, based on which Lu et al. prepared TPATCN (Figure 1.8) with another building of 2,3‐bis(4‐bromophenyl)fumaronitrile. The density functional theory (DFT) calculation showed larger energy gap between T₂ and T₁ and smaller energy splitting between T₂ and S₁, which prohibited the intersystem conversion and activated the “hot exciton” process. The nondoped OLED based on TPATCN exhibited strong NIR emission with maximum EQE and luminance of 2.58% and 7025 cd/m², respectively [126]. The contribution of AIE property to the highly efficient HLCT mechanisms were further confirmed by Wang et al. through the QM/MM method [127]. Similarly, based on the same AIE‐active moiety, Yuan and Zhang et al. prepared AIE‐HLCT emitter of BDPACS (Figure 1.8). Its nondoped OLED could only reach the maximum EQE of 0.41%, as a result of exciplexes or excimers formed, while the doped OLED with BDPACS embedded into host of 2‐methyl‐9,10‐di(2‐naphthyl)anthracene (MADN) exhibited significantly enhanced maximum EQE of 6.8% [128]. Through substitution of the vinylene groups with CN groups at different positions, two isomers of α‐CN‐APV and β‐CN‐APV (Figure 1.8) were prepared by Ma et al., and the isomer with closer intermolecular stacking showed higher maximum EQE ( β‐CN‐APV : 5.3%) [129].

Figure 1.8 The structures of hybridized local and charge transfer materials aggregation‐induced emissive emitters.

1.4 Conclusion and Outlook

OLED technology has experienced a rapid progress in last few decades, and numerous emitters of various structures with different photophysical mechanisms have been applied to this area. Due to OLEDs’ nature of solid‐state lightening, the emitters’ aggregated states also play a vital role in the performance of OLEDs, especially for the efficiency and luminance. The ACQ properties have the drawbacks of quenching and lowering the EQE of OLEDs; therefore, in order to overcome ACQ behaviors, the doping technology was often resorted to, causing complex process and unstable performance for the devices. In contrast to ACQ, AIE property means that emitters have increased emission in aggregated states, enabling them to prepare nondoped OLED devices, still with high efficiency and luminance. Till now, numerous AIE emitters have been applied in OLED devices; therefore, in this review, we mainly demonstrated the emitters based on AIE integrated with different photophysical mechanisms and their applications in OLED devices. So far, most of AIE emitters are traditional fluorescent. Due to the facile molecule design, the traditional fluorescent AIE emitters have covered wide range of spectrum, and blue to green from red. And even a lot of white OLEDs based on these emitters have been fabricated. However, due to the theoretic EQE limitation of 5% for traditional AIE emitter‐based OLED, AIE property has been combined with other high EUE photophysical mechanisms to prepare highly efficient OLED emitters. Among these emitters, the AIPE emitters with both AIE property and phosphorescent emission were developed first, but with only little progress regarding the emitters reported and the related EQE achieved. The AIDF and HLCT‐AIE materials started quite recently, but with both numerous emitters and high EQE efficiency reported in comparison to their nonAIE‐active counterparts.

      Increasing the efficiency of OLED devices is a complex and integrated challenge in the long term. As for the EMLs, they might have multiple photophysical mechanisms coexisting in the emitters both at the molecular levels and in the aggregated state, therefore the strategic design for the emitters was highly required. Apart from the influence of emitters, many other factors also play significant roles in determining the efficiency of OLED devices such as processing methods, doping, and lighting‐extraction. When it comes to the application of emitters from the laboratory to commercial, these factors, including cost, stability, and lifetime of the device cannot be ignored. From these points, AIE emitters have already been applied in nondoped OLEDs with evident advantages of high efficiency, low cost, and high stability. In spite of these achievement, there is still enough room for developing AIE emitters in OLEDs, such as improving the efficiency of OLEDs or designing novel AIE emitters with other high EUE mechanism of triplet–triplet annihilation (TTA) [130, 131], triplet exciton‐polaron annihilation (TPA) [33], or singlet fission [132]. Finally, we hope that OLED devices will profit more from AIE emitters both academically and commercially in the future.

Acknowledgments

      This work is financially supported by the National Key R&D Program of China (2016YFB0401000, 2017YFE0106000), National Natural Science Foundation of China (21805296, 21876158, 51773212, 21574144 and 21674123), Zhejiang Provincial Natural Science Foundation of China (LR16b040002), Natural Science Foundation of Ningbo City (2018A610134), Ningbo Municipal Science and Technology Innovative Research Team (2015B11002 and 2016b10005), CAS Key Project of Frontier Science Research (QYZDB‐SSW‐SYS030), CAS Key Project of International Cooperation (174433KYSB20160065), and Zhejiang Provincial Natural Science Foundation of China (Grant No. LY20B040002).

References

      1 1 Tang, C. W., VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987; 51(12):913–5.

      2 2 Jeong, H., Shin, H., Lee, J., Kim, B., Park, Y. I., Yook, K. S., et al. Recent progress in the use of fluorescent and phosphorescent organic compounds for organic light‐emitting diode lighting. J. Photon. Energy. 2015; 5:23.

      3 3 Wei, Q., Fei, N., Islam, A., Lei, T., Hong, L., Peng, R., et al. Small‐molecule emitters with high quantum efficiency: mechanisms, structures, and applications in OLED devices. Adv. Opt. Mater. 2018; 6(20):1800512.

      4 4 Zou, S. J., Shen, Y., Xie, F. M., Chen,