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

of 3.6% and CIE coordinates of (0.15, 0.10) as the best spin‐coating type of AIEgens of nondoped OLEDs reported so far [56].

      Tetraphenylpyrazinyl (TPP) was a novel blue‐emissive AIE emitter first found in 2015 [57, 58]. Based on this moiety as electron donor and imidazole as electron acceptor, two deep‐blue AIE emitters of TPP‐PPI and TPP‐PI were prepared, with the property of planarized intramolecular charge transfer (PLICT). The nondoped blue OLEDs based on these emitters (TPP‐PPI and TPP‐PI) were prepared showing good performance, with maximum EQE of 4.85 and 4.36% [59]. Also nonpolar blue emitter of TPP was further modified with carbazole, and its nondoped OLEDs exhibited optimal performance, with CIE coordinates of (0.16, 0.11) and maximum EQE of 1.49% [60].

      1.2.2 Green Aggregation‐induced Emissive Emitters

In comparison to their blue and red counterparts, green AIEgens are quite facile to design, with suitable conjugated length and polarity. In this section, we exemplify the green AIE emitters sorted by nonpolar and polar emitter. Tanyeli et al. attached three peripheral moieties of indole, tert‐butylcarbazole, and tetrahydrocarbazole to the TPE core, to obtain TIPE, TTBCPE, and TDCPE (Figure 1.3), respectively, through two facile steps. Based on these emitters, nondoped green OLED devices with different constructions could be prepared with a maximum brightness, CE, and EQE of 18 000 cd/m², 7.7 cd/A, and 3.2%, respectively [61]. By the same method, they further took advantage of another four N‐heteroaromatic rings to attach the TPE cores, and the AIE emitters were therefore prepared, based on which green OLED devices could be prepared and exhibited relatively lower EL performance, with brightness, CE, and EQE of 2600 cd/m², 3.6 cd/A, and 1.5%, respectively [62]. Similarly, Tang et al. used the well‐known and hole‐transporting materials of NPB to covalently integrate to the TPE core, with the resulted TPE‐NPB as a green AIEgen (Figure 1.3). Using TPE‐NPB as emitters, nondoped green OLED devices were fabricated, with maximum luminance and CE of 11 981 cd/m² and 11.9 cd/A, respectively [63]. Still some other nonpolar green emitters were reported. Lu and Wang et al. tethered 2‐aryl‐3‐cyanobenzofuran fluorophore with TPE core via 5‐, 6‐, and 7‐position of 2‐aryl‐3‐cyanobenzofuran, and its nondoped OLED devices were prepared with the structure of ITO/PEDOT : PSS (40 nm)/EML (60 nm)/TBPI (25 nm)/Ca (10 nm)/Ag (80 nm), where 6‐position substituted emitter‐based OLEDs exhibited the best performance with brightness, CE, PE, and EQE of 620 cd/m², 1.62 cd/A, 0.73 lm/W, and 0.63%, respectively [64]. Silole moiety was also adopted as effective building units to construct the green AIE‐active emitters, simply by increasing the conjugating length. For example, Zhao et al. designed and prepared three novel green‐emissive AIE‐active silole derivatives 2,2′‐MTPS‐CaP, 3,3′‐MTPS‐CaP, and 9,9′‐MTPS‐CaP (Figure 1.3) by connecting silole and electron‐donating groups of 9‐phenyl‐9H‐carbazole through different linkage. And the OLEDs based on the emitter 9,9′‐MTPS‐CaP had the best EL performance, with the EQE of 5.63%, above the limit of 5% in traditional fluorescent OLEDs [65].

Figure 1.3 Molecular structures of green conventional AIE‐active emitters.

      Green AIEgens of donor– π –acceptor (D– π –A) systems, with intramolecular charge transfer (ICT) from donor end to acceptor, also attracted great attention in molecule design and found broad applications in construction of OLEDs [66]. Tang et al. took diphenylamino and dimesitylboryl groups as donor and acceptor, respectively, to combine with the TPE core, and a series of AIE‐based star‐shaped bipolar TPE derivatives TPE‐2PN2PB, TPE‐PN3PB, and TPE‐4PB (Figure 1.3) were prepared with PLQY of up to 95% in the solid state. Based on these emitters, nondoped OLEDs through solution process were successfully fabricated, with the structure of ITO (130 nm)/PEDOT : PSS (40 nm)/EML (70 nm)/TPBi (30 nm)/Ba (4 nm)/Al (120 nm). These OLEDs showed peak luminance and EQE of 11 665 cd/m² and 2.6%, respectively [67]. In another research, they introduced carbazole moiety with hole‐transporting property and dimesityl boron groups with electron‐transporting property as peripheries, into TPE core, to obtain two green AIE emitters of PDPBCE and BDPBCE (Figure 1.3). Based on these two emitters, nondoped OLEDs were fabricated with the structure of ITO/HATCN (10 nm)/NPB (50 nm)/EML (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (150 nm). Both the devices showed green‐light emission, with a maximum brightness of 67 500 cd/m² and a maximum CE of 11.2 cd/A [68]. Furthermore, they applied N‐ethyl‐carbazole group, and dimesitylboron or (dimesitylboranyl)phenyl groups were attached TPE core and linked to the para‐ or meta‐position of TPE to obtain four AIE luminogens of p‐DPDECZ, p‐DBPDECZ, m‐DPDECZ, and m‐DBPDECZ (Figure 1.3). Compared to the meta‐linked, compound‐based blue OLEDs, the nondoped OLED devices with para‐linked compound exhibited better EL performance, with a maximum brightness of 30 210 cd/m² and a maximum CE of 9.96 cd/A, due to the increased conjugated length [69].

      1.2.3 Red Aggregation‐induced Emissive Emitters

Red emitters, as one indispensable part of the three‐primary‐color system, play an important role in the full‐color displaying and white‐light emitting systems. Therefore, various AIE emitters with different structures and high efficiency have been prepared and applied in the OLED area [70]. In addition, red AIEgens also found broad applications in biological areas, especially for in vitro and in vivo bioimaging, due to being less detrimental to tissues, deeper penetrating length into tissues, and lower overlapping with background fluorescence, compared with other chromatic emitters [71, 72]. However, compared to AIE luminogens with blue or green emission, the number of red and near infrared (NIR) emissive AIEgens are relatively smaller, and they also share the lower efficiency, due to two reasons: (i) low energy gap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the red emitters resulting in high intersystem conversion (IC) and (ii) large dipole moments of donor and acceptor with a high tendency to self‐quench [70]. The red AIE emitters should have lower energy gap, and the main strategies to decrease the energy gap by expanding the π ‐conjugating length and/or strengthening the donor–acceptor interactions. Since most AIE emitters shared much more twisted molecular structure with shorter conjugating length, it could be observed from the red AIE emitters already published that most contained strong donor, such as triphenylamine, and acceptor, such as benzothiadiazole and cyano [73].

The moiety of heterocyclic benzo‐2,1,3‐thiadiazole (TD) was a well‐known building block for red emitters due to its strong electron‐withdrawing properties. Zhao et al. took advantage of TPE, TD, and thiophene building blocks to construct two red emitters BTPETTD and BTPEBTTD (Figure 1.4), with strong built‐in push–pull interactions. OLED devices [ITO/NPB (60 nm)/EML (20 nm)/TPBi (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm)] were fabricated based on these red emitters BTPETTD and BTPEBTTD that exhibited emission at 592 and 668 nm, respectively, with maximum CE, PE, and EQE of 8330 cd/m², 6.4 cd/A, and 3.1% [74]. Furthermore, they modified BTPEBTTD by attaching an additional TPE unit to the thiophene moiety to obtain the AIE emitter of TTPEBTTD (