2.1 Silole-based AIEgens

In 2001, Tang's group first observed AIE phenomenon in “1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS)”,⁹ which is nonemissive in the solution state, but highly emissive in the aggregated (solid) state. Silole derivatives have high electron affinity and fast electron mobility because of the unique σ*–π* conjugation effect from the σ* orbital of the two exocyclic Si-C bonds and the π* orbital of the butadiene unit. MPPS can show an EQE of up to 8%, which exceeds the theoretical limit.²² In general, silole derivatives shown strong emission in green and yellow regions, but seldom in blue region. In 2016, Li's group reported green to blue AIEgens without the σ*–π* conjugation effect, by replacing the Si atom with a C atom, thereby, achieving a limited degree of intramolecular conjugation. In these AIEgens, a new core, tetraphenylcyclopentadiene, was connected with TPE or TriPE as peripheral moieties.²³ Furthermore, a blue or deep-blue AIEgens were achieved by controlling the conjugative effect via the adjustment of the linkage mode. Nondoped devices based on these resulting new luminogens could show good performance with blue emission (λ_EL = 440 nm) rather than the green or yellow-green emission of siloles. In order to construct blue/deep-blue AIEgens based on silole derivatives, recently Tang's research group synthesized a new building block, tetraphenylbenzosilole (TPBS), by replacing the two phenyl groups of hexaphenylsiole (HPS) at the 2,3-positions with one phenyl group consist with a silole core via a silyl radical cascade process with intermolecular radical cyclization.²⁴ TPBS core can effectively increase the skeleton rigidity and reduce the number of excited-state conformations, which is beneficial for obtaining practical deep-blue emission. As expected, the resulting four new TPBS derivatives not only inherited the AIE feature from HPS but also exhibited high-efficiency blue/deep-blue emission in the aggregated state. The remarkable performance of these derivatives was attributed to their tunable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), increased skeleton rigidity, and reduced the vibrational–rotational motions of peripheral phenyl groups. Thus, nonradiative channel of excited state was gradually blocked, and radiative channels were activated. Thanks to the hydrophobic nature of resulting luminogens, which are gathered into aggregated states when adding water (f_w ≥ 80%) to THF solution; therefore, the resulting spatial limitation reduced the motion of rotational units and the nonradiative channels of excited state were gradually blocked. Consequently, as shown in Figure 2B, the TPBS-based derivatives exhibited drastically enhanced PL intensity and increased their PLQYs (Φ_F) in the aggregated state, which reveals a typical AIE feature. As a result, the resultant blue nondoped OLEDs with the configuration of ITO/PEDOT:PSS/TFB (40 nm)/emitting layer (about 15 nm)/TmPyPB (30 nm)/LiF (1.5 nm)/Al (120 nm) displayed excellent performance with high EQEs (3.1–3.6%) at CIE coordinates of (0.15, 0.10) (Figure 2C). These results demonstrate the effectiveness of a fused benzo-based strategy to obtain building blocks for deep-blue AIEgens that could overcome the deficiency of HPS derivatives for constructing blue/deep-blue OLED emitters.

As known, TPSs are excellent solid-state light emitters with AIE characteristics, but the bifunctional materials capable of both light emitting and electron transporting in OLEDs are much rare. To create bifunctional materials, Tang's group reported three derivatives of 2,3,4,5-TPS, connected with dimesitylboryl groups.²⁵ Owing to the synergistic effect between the silole ring and dimesitylboryl moieties, the new derivatives exhibited lower LUMO energy levels than the corresponding parent moiety 2,3,4,5-TPSs and improved electron-transporting ability, which offered them to serve as dual functioning of light-emitting layers and electron-transporting layers (ETL) in a single device. In addition, the branched conformation of dimesitylboryl unit greatly suppresses close π–π stacking, which helps to exhibit AIE characteristics for these silole derivatives with high PLQY values of 56–62% in solid film state. The nondoped double-layered configuration of these devices revealed excellent performances with high electroluminescence efficiencies up to 4.35%, 13.9 cd A⁻¹, and 11.6 lm W⁻¹ relative to the corresponding triple-layer configured devices. Therefore, these are behaving as excellent bifunctional materials with both light-emitting and electron-transporting properties. Furthermore, to improve the electron-transporting ability of light emitters, Tang's group synthesized another set of four AIE active n-type emitters based on silole derivatives, (PBI)2DMTPS, (PBI)₂MPPS, (PPI)2DMTPS, and (PPI)₂MPPS, by introducing 1-phenyl-1H-benzo[d]imidazole (PBI) and 1-phenyl-1H-phenanthro[9,10-d]imidazole (PPI) groups to 2,3,4,5-TPS core.²⁶ The resulting four luminogens revealing AIE feature by enhancing PL emission when f_w exceeded 50% in THF–water system because the intramolecular rotations were restricted by the steric constraint, and gradually the nonradiative internal conversion (IC) channels were deactivated, it helps the strong emission of the molecules in aggregated state with a high PLQY values of 49.5–62.1%. Thanks to the synergistic effect of the silole and PBI units of new silole derivatives, which showed better electron-transporting abilities compared with 2,2′,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). Impressively, a double-layer OLED based on (PBI)₂DMTPS without an additional ETL exhibited a maximum EQE (EQE_max) of 4.25%, which is a comparable result with the EQE_max (4.22%) of its corresponding trilayer OLED. Nondoped trilayer-configured light-emitting device based on (PPI)₂DMTPS displayed a high EQE_max of 4.84%, which is close to the theoretical limit of fluorescent OLEDs. This work provided an effective strategy to achieve bifunctional materials (i.e., n-type light emitters) for highly efficient, simplified, and low-cost OLEDs. The relevant data from the aforementioned representative reports of silole-based AIEgens are summarized in Table 1, and their corresponding chemical structures are shown in Figure 2.

2.2 TPE-based AIEgens

Amongst the various AIEgens, TPE and TriPE are the most common and versatile AIEgens, which can be incorporated into several chromophores. As is well known, the highly twisted conformation, restriction of intramolecular motion (RIM), and weak intermolecular interactions of AIEgens containing a TPE core efficiently suppress ACQ in solid films of OLEDs. As a result, AIEgens exhibit great potential for fabricating nondoped OLEDs with high efficiency and small efficiency roll-off. AIEgens containing TPE/TriPE cores have been widely used to develop high-performance nondoped OLED materials so far.

In 2017, Tang et al.²⁷ synthesized thiophene/furan-cored AIEgens via the cascaded cyclization of diyne, abbreviated as TPE-T and TPE-F. In dilute THF solution, both luminogens were weakly emissive due to their active intramolecular rotations that acted as the fast IC channels and efficiently deactivated the excited state via nonradiative relaxation. The addition of water into THF solution (aggregated state) enhanced their PL emission; the maximum emission was achieved at a f_w of 99%, due to the greater restriction in the intramolecular rotations and the nonradiative IC channels were blocked. Owing to its higher restriction of RIM in TPE-F, stronger PL emission and higher PLQY (50%) than TPE-T (18%) were shown. As well, OLED devices of TPE-F displayed high EL efficiencies comparable with the TPE-T-based device. The nondoped OLED device using TPE-F as the emitter exhibited low turn-on voltage (V_on), the maximum luminescence (L_max), current efficiency (CE_max), power efficiency (PE_max), and EQE_max values of 3.3 V, 24,298 cd m⁻², 9.98 cd A⁻¹, 7.02 lm W⁻¹, and 3.67%, respectively. These results are better than those of other similar hetero aromatic AIEgens containing benzo-2,1,3-thiadiazole and silole with the same device structure reported earlier by the same group.²⁸ The authors also reported a new series of AIE-active sulfur/sulfone-bridged TPE derivatives connected with carbazole (Cz) and diphenylamine (DPA) units that exhibited a wide energy band gap and pure deep-blue color narrow emission with full width at half maximum (FWHM) of 55–65 nm at 434–473 nm in the solid state. A nondoped device based on the sulfur-bridged TPE–DPA derivative exhibited excellent performances (L_max: 2918 cd m⁻², CE_max: 3.94 cd A⁻¹, PE_max: 4.13 lm W⁻¹ and EQE_max: 2.15% at λ_EL: 468 nm) comparable with that of doped OLEDs.²⁹

However, the fluorophores using TPE as AIEgens with emission peaks below 470 nm are still rare. The first report of such fluorophores was by Tang et al.³⁰ based on TPE substituted on pyrene (TTPEPy). These fluorophores exhibited excellent EL performance with an EQE of 4.95% and sky-blue EL emission at 488 nm; the same group reported two sky-blue AIEgens, by substituting TPE or TriPE at 2,7-positions of pyrene, named Py–TPE and Py–TriPE, respectively.³¹ Both luminogens exhibiting AIE feature in THF/water mixture, for example, Py–TPE has weak fluorescence with a maximum peak at 450 nm in THF solution, while the emission intensity was enhanced by ∼60-fold when increasing water fraction until 60%, then PL emission intensity gradually increased to ∼200-fold and a large red-shifted emission peak at 491 nm when the f_w reached 99%. A nondoped device with Py–TPE as emitter showed a EQE_max of 3.19%, but its emission extended beyond the blue region. Later on, some reports were devoted based on TPE–pyrene, exhibiting red-shifted emission due to extended π-conjugation with pyrene. In order to achieve blue emission, a few strategies have been developed to control the degree of π-conjugation in AIEgen by changing the linkage pattern or break up the π-conjugation or to make the twisted structure.³² Based on these strategies, Hu et al.³³ reported a twisted TPE-substituted pyrene-based blue AIEgen, abbreviated as TTPE(1,3,5,9)Py by modifying TTPEPy. Similar to TTPEPy, the TTPE(1,3,5,9)Py showed robust AIE features in f_w from 40% to 90% and enhanced its PLQY value by about 10-fold in f_w > 40%, this value further increased to 65% when the f_w reached 90%. A nondoped device based on TTPE(1,3,5,9)Py showed pure-blue emission (λ_EL: 468 nm) with EQE_max and CE_max values of 4.10% and 7.38 cd A⁻¹, respectively, as well as exhibited negligible efficiency roll-off and good color purity. In addition, the same research group integrated two TPE or TriPE units into 5,9-positions of pyrene moiety in the shape of butterfly, namely, Py(5,9)BTPE and Py(5,9)BTriPE.³⁴ Py(5,9)BTPE displayed significant AIE characteristics upon aggregation in a THF/water system, as well as high PLQY of 57.7% in the thin film state. A nondoped OLED based on Py(5,9)BTPE exhibited a sky-blue emission with an EQE_max of 3.35%, whereas a nondoped device with Py(5,9)BTriPE displayed a pure-blue emission (CIE: 0.16, 0.17) with a moderate EQE_max of 1.27% and a small efficiency roll-off. On the other hand, Xu et al.³⁵ synthesized two sky-blue AIEgens, namely, 9CzTPE and 3CzTPE, by tuning the linkage modes between TPE and the Cz unit. Both compounds exhibited remarkable AIE behavior with PLQY values of 63.4 and 65.5%, respectively, in a solid state. A nondoped OLED based on 3CzTPE displayed an EQE_max of 2.81%, high luminous efficiency of 4.35 cd A⁻¹ and with sky-blue emission at 479 nm.

In 2018, Xue and coworkers,³⁶ reported TPEPPI as a deep-blue AIEgen, using phenothiazine (PTZ) as a donor and TPE as the AIEgen. TPEPPI is hardly luminescent in THF solution, but its PL intensity increased by 5.5-fold when f_w reached 95%. A nondoped device with TPEPPI as the emitter showed EQE_max, CE_max, PE_max, and λ_EL values 2.36%, 4.25 cd A⁻¹, 3.35 lm W⁻¹, and 467 nm, respectively, as well, the device showed a negligible efficiency roll-off of 3.3% with increasing luminance. As known, phenanthroimidazole (PI) core is one of the most widely used π-conjugated rigid structure that can be used to explore deep-blue to green electroluminescence when combined with AIE blocks. For instance, in 2019, Tang's group³⁷ synthesized six TPE-PI-based derivatives with different conjugation patterns at the C2 and N1 substituent positions of the PI moiety according to HLCT strategy, which is another strategy to harvest 75% of nonemissive triplet excitons for light emission. In which, hot excitons (excitons in second or higher excited state) are injected to the high-lying triplet state and are transferred to the singlet state via RISC process. The resulting TPE-PI-based derivatives revealed AIE characteristics by enhancing PL emission as water fraction (f_w) increased, due to the restriction of rotational motions by the spatial constraint in the aggregated state and leads to suppression of nonradiative decay of the excited state. A nondoped OLED employing ppCTPI as the emitter exhibited excellent performance with a L_max, CE_max, and EQE_max values of up to 31,070 cd m⁻², 18.46 cd A⁻¹, and 7.16%, respectively, as well this device exhibited a very small efficiency roll-off of 4.0% at 1000 cd m⁻², which is the best reported thus far for nondoped OLEDs based on TPE-substituted AIE molecules. However, the EL peak wavelength of blue fluorophores based on TPE-substituted AIE cannot be controlled in the blue-light-emission region, thus, fabricating efficient AIE-active organic blue light-emitting materials is fairly difficult. Encouragingly, Xue et al.³⁸ synthesized TPE-substituted efficient AIE-active blue-fluorescent D–A molecule, TPETPAPI, composed with TPA and PI as the donor and acceptor units, respectively. TPETPAPI revealed AIE feature with the emission intensity increased by up to ∼100 folds upon aggregation, as well, showed higher PLQY of 73% in film state than in the THF solution (1.3%). A nondoped device bearing TPETPAPI as the emitter exhibited excellent EL performance with EQE_max, L_max, CE_max, and PE_max values of 6.05% 10,780 cd m⁻², 12.2 cd A⁻¹, and 11.9 lm W⁻¹, respectively; these values are among the best reported to date for small AIEgens used in TPE-substituted nondoped blue OLEDs in λ_EL of 480 nm.

Jayabharathi and coworkers³⁹ reported two D–A-based positional isomers of TPE-substituted blue AIEgens, namely, TPE–NPPB and TPE–APPB, containing PI and TPA cores. A nondoped OLEDs using them as emitters exhibited high EQE of 3.2 and 5.3%, CE of 4.32 and 5.28 cd A⁻¹, and PE of 4.01 and 4.92 lm W⁻¹, respectively. These materials also showed reversible mechanochromism between the colors blue and green. In very recent work, the same research group synthesized TPE-based AIE active PIs, namely, NSPI–DVP and CNSPI–DVP.⁴⁰ These materials not only exhibited HLCT states, but also showed reversible mechanochromism between the colors blue and green. The nondoped devices based on NSPI–DVP and CNSPI–DVP as the emitters displayed EQE_max of 5.09 and 5.23%, CE_max of 5.61 and 5.03 cd A⁻¹, and PE_max of 4.99 and 4.72 lm W⁻¹, respectively (Figures 3B and 3C), as well exhibited negligible efficiency roll-off [NSPI–DVP 1.76% and CNSPI–DVP 0.96%] due to the effective h⁺–e⁻ recombination and reduced exciton quenching. In comparison, CNSPI–DVP shows higher EQE than NSPI–DVP under the same device configuration because the special excited state of CNSPI–DVP can trigger the fast k_RISC with a hot exciton process and harvesting more triplet excitons, which leads to a higher exciton utilization efficiency of 64.0% than NSPI–DVP (36.0%). In 2020, Tang's research group also explored PI-based three blue-emissive AIEgens, CNNPI, 2TriPECNNPI, and 2CzPh–CNNPI, with HLCT characteristics.⁴¹ A nondoped OLED based on 2CzPh–CNNPI exhibited pure blue emission with an impressive EQE_max of 5.09%. Moreover, the material could simultaneously be employed as an emitter, emissive dopant, and sensitizing host. The nondoped/doped deep-blue OLEDs and HLCT-sensitized green fluorescent OLEDs prepared from this material are among the most efficient OLEDs prepared via the “hot exciton” approach reported thus far. 2TriPE-BPI-MCN was developed as a blue AIEgen by inserting para-cyano and ortho-methyl groups to the phenyl unit at the C2 position of PI moiety from its matrix 2TriPE–BPI by adopting HLCT approach.⁴² Interestingly, the excited state of 2TriPE–BPI–MCN explored RISC channel to obtain high exciton utilization efficiency during the EL process. A nondoped blue OLED based on 2TriPE–BPI–MCN exhibited excellent EL performance with a high EQE of 4.60% and high exciton utilization efficiency of 50.2%; these values are better than those of its matrix part 2TriPE–BPI (3.74 and 29.9%). Lu et al.⁴³ developed three blue aggregation-induced emission enhancement (AIEE) type luminogens, namely, PIAnTPE, TPAAnTPE, and CzAnTPE, based on TriPE core substituted PI/TPA/Cz moieties at 9,10-positions of anthracene, respectively. The resultant materials displayed AIE characteristics upon aggregation and significant PLQY values of 65, 70, and 46%, respectively, in the film state. The nondoped OLEDs using them as emitters showed the EQE_max values of 4.46, 4.13, and 4.04%, respectively, as well, realized negligible efficiency roll-offs. Huo et al.⁴⁴ reported three TPE–PI derivatives with different alkoxy chain lengths, namely, TPEO1–PPI, TPEO4–PPI, and TPEO6–PPI, observed high solid-state PL efficiencies along with AIE and mechanochromic luminescence (MCL) characteristics in these materials. A nondoped OLED based on TPEO4–PPI showed excellent EL performance with a EQE_max and L_max values of 4.07% and 15995 cd m⁻², respectively, as well as the device showed negligible efficiency roll-off.

Owing to the poor charge carrier mobility of TPE core, issue still exists to make high performance of TPE-based devices; some researchers believe that this issue could be overcome by balancing the charge carriers in the device rather than the addition of carrier transporting layer to the OLED device. Previous reports noted the favorable ability of bipolar materials based on the donor–acceptor (D–A) model inject and transport both holes and electrons.⁴⁵ These bipolar materials have the ability of balance charge carrier transportation in the device, as a result can reduce production costs and simplify the manufacturing process. For instance, Ge et al.⁴⁶ reported three bipolar AIEgens with a D–A configuration by combining TPE with Cz and benzimidazole, named TPE-1, TPE-2, and TPE-3. These three derivatives exhibited weak PL emission in THF solution because of nonradiative decay through strong intramolecular motion of TPE units, which is gradually reduced and enhanced PL emission intensity in the aggregated state; upon addition of water to its THF solutions, it indicates typical AIE feature of resulting luminogens. As well as PLQY values were high as 98.7, 86.2, and 84.0% respectively, in the solid state. A nondoped OLED using TPE-2 as the emitter exhibited good performances with CE_max, PE_max, and EQE_max values of 15.1 cd A⁻¹, 14.8 lm W⁻¹, and 5.34%, respectively. Tong and coworkers⁴⁷ also synthesized two bipolar blue molecules, PPI–PIM–TPE and 2PPI–TPE, the resultant materials showed AIE and mechanochromic characteristics, as well as prominent PLQY values of as 61.9 and 73.4%, respectively, in the solid state. A nondoped blue OLED using 2PPI–TPE as the emitter exhibited an EQE, CE, and PE values of 2.48%, 6.46 cd A⁻¹, and 4.72 lm W⁻¹, respectively. In addition, TriPE substituted with Cz or acridan units featuring fluorinated or nonfluorinated fragments (TCz, TCz-F, TAc, and TAc-F) have been reported as bipolar charge transporting AIEE emitters for blue nondoped OLEDs.⁴⁸ TPE–triazole hybrids with AIE characteristics could simultaneously function as intrinsic blue-light emission and electron-transport layer in OLEDs.⁴⁹ TPETAZ exhibited PLQYs of 75.7% in the film state. A nondoped device with ETL 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB) showed EQE_max and L_max of 2.4% and 1020 cd m⁻², respectively, with an EL emission peak of 468 nm. Whereas, nondoped device without the TmPyPB layer exhibited EQE_max values of up to 2.0%, which is only 18% lower than that of the device with TmPyPB as ETL, and L_max of 1171 cd m⁻². These findings could be attributed to the ability of TPETAZ to function as both light-emitting layers and ETLs in OLEDs. The relevant data from the aforementioned representative reports of tetra/TPE-based AIEgens are summarized in Table 2, and their corresponding chemical structures are shown in Figure 3.

2.3 TPP-based AIEgens

TPP is a new AIE-active unit introduced by Tang's group in 2015 with the benefits of facile synthesis, easy modification, and excellent thermal stability.⁵⁰ In addition, TPP unit exhibits narrow emission at 390 nm, which is more beneficial for fabricating deep-blue AIE emitters for OLED applications. However, the previous report based on TPA–TPP derivatives are limited to the sky-blue region.⁵¹ Owing to the strong interactions between the TPA donor and TPP acceptor, as well as the increased rotational freedom of the TPA moiety in the excited state, red-shifted emission with widened spectra, which is unfavorable for the construction of efficient deep-blue OLEDs, was observed. To address this issue, the group developed TPP–Cz, TPP–PhCz, TPP–2Cz, and TPP–2PhCz, by employing the Cz unit instead of TPA unit; Cz features a good donor capacity and can restrict rotation via a locked single bond.⁵² The four TPP-based luminogens revealed AIE phenomenon, by increasing their PL emission intensities when formation of aggregates with increasing water fraction to THF solutions, due to the RIM of TPP unit in the molecules. Thanks to the formation of stable quinone conformation in the excited state of these derivatives that led to great enhancements on their radiative transition decay rate, which contrasts the behavior of traditional AIEgens. A nondoped OLED based on TPP-Cz exhibited the optimal EL performance with an EQE of 1.49% and ideal roll-off properties with a retaining CIE coordinate of (0.16, 0.11).

In general, large torsion has been observed between the pyrazine center and peripheral phenyl groups in the ground state, when forms quinone conformation in the excited state, its excited conjugation is expanded for planarization, which is referred to as excited-state quinone-conformation-induced planarization. Taking advantage of the restricted intramolecular rotation mechanism of TPP and its ability to form a stable quinone conformation in the excited state, Tang's group tailored two AIEgens, using the strategy of planarized ICT (PLICT), namely, TPP–PPI and TPP–PI.⁵³ Both the materials exhibited blue emission with the peak at 452 and 463 nm in the dilute THF solution, along with fluorescence quantum yields (Φ_F) of 10.0 and 13.8%, respectively. Their PL intensities remain low up to the water fraction (f_w) is less than 60% in the THF/water mixture, but it increases gradually when the f_w is more than 80%, which indicates its AIE feature. Due to the formation of nanoaggregates in the THF/water mixture, which restricts intramolecular motions and blocks the nonradiative decay channel, the radiative decay rate (K_r) increase and surely contributes to the enhanced emission (Figure 4B). Nondoped devices bearing TPP–PPI and TPP–PI with the configuration of ITO/HATCN (5 nm)/NPB (40 nm)/TCTA (5 nm)/EML (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al exhibited stable sky-blue emission with the peaks at 474 and 484 nm, respectively, close to their PL emission in the neat film. These nondoped devices displayed significant EQE_max values of 4.85 and 4.36%, respectively, and exhibited small efficiency roll-off of 5% (EQEs remain to 4.51 and 4.12%, respectively, at the luminescence of 1000 cd cm⁻²) (Figure 4C). On the other hand, tetra/triphenylpyrazine–3Cz derivatives, namely, TPP–3C and TrPP–3C, were synthesized by replacing the phenyl group with a methyl group at the 3-position of pyrazine to obtain deep-blue emitters; meanwhile, 3-Cz was melded instead of 9-phenyl-Cz in TPP–PhCz to improve the luminescence efficiency of the resultant devices.⁵⁴ Compared with an OLED bearing TPP–3C, a nondoped OLED employing TrPP–3C as the emitter showed better EL performance with a significant EQE_max of 2.89%.

Furthermore, the same group synthesized three blue fluorophores by inserting a phenyl group at the ortho-, meta-, and para-positions of a pyrazine unit containing 3-Cz, to investigate the role of each phenyl group in the TPP core.⁵⁵ AIE characteristics were observed in the derivative with a phenyl group at ortho-position (23-B3C) but not in those with phenyl groups at the meta- and para-positions (i.e., 26-B3C, 25-B3C) because of the large steric hindrance between the ortho-phenyl group and pyrazine core, which distorts the planarity of pyrazine. Among the nondoped OLED devices, 26-B3C exhibited the best EL performance with an EQE_max of 3.32% and deep-blue emission at CIE coordinates of (0.16, 0.07). Moreover, to investigate AIE features inherited from TPE and TPP, the same group tailored three AIE isomers by connecting TriPE at the para, meta, and ortho-position to TPP unit, respectively, namely, TPP-p-TPE, TPP-m-TPE, and TPP-o-TPE.⁵⁶ The isomerism effect of the AIEgens shortened their conjugation length, and their emission gradually weakened from the para to the ortho isomer, owing to decrease in RIM due to the their loose intermolecular packing. Meanwhile, the molecular porosity increases, which showed low emission efficiency but improved sensitivity as fluorescent probes. Based on distinct properties, the resultant isomers investigated not only in nondoped blue OLEDs, but also in nanofluorescent probes, and molecule-capturing porous crystals. Nondoped devices using TPP–p–TPE and TPP–m–TPE as EMLs exhibited EQE_max values of 2.74 and 1.14%, respectively. Moreover, the device prepared with TPP–p–TPE showed sky-blue emission at 488 nm, whereas TPP–m–TPE revealed deep-blue emission at 454 nm. The relevant data from the aforementioned representative reports of TPP-based AIEgens are summarized in Table 3, and their corresponding chemical structures are shown in Figure 4.

2.4 TPB-based AIEgens

Qin and Tang et al. introduced another AIE core, TPB, and significant research has been conducted on it in the past few years. TPB is similar to TPE and TPP, but the nitrogen atom is replaced with a carbon atom in TPP. TPB emits at 380 nm in the aggregated state with good thermal stability and is more suitable for constructing stable deep-blue emitters in OLEDs. The same group has reported many deep-blue AIEgens based on the TPB core by extending the conjugation or introduction of donors and acceptors. For example, TPB–AC was synthesized by melting a DPA moiety and a cyano group on the TPB core.^[57] The TPA core in TPB–AC enhances the hole-transport properties of the device, whereas the cyano group reduces the LUMO energy level, diminishes the electron injection barrier, and improves the intermolecular interactions. It emits intensely at 479 nm in a THF solution. With an increase in the water fraction (f_w) from 0 to 70%, the emission intensity gradually decreased, along with a red shift in the emission peak, owing to a TICT process. With a further increase in the water fraction (f_w > 70%), the emission intensity gradually increased, with a blue shift in the peak wavelength, owing to the formation of aggregates and restriction of the intramolecular rotation of the phenyl rings in TPB–AC. In addition, it exhibited a high PLQY of 69.4% in the aggregated state. A nondoped OLED based on TPB–AC exhibited deep-blue electroluminescence with its V_on as low as 3.2 V and η_ext,_max up to 2.92% with the device configuration of ITO/NPB (40 nm)/TPB–AC (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al. The low performance of the device based on TPB–AC may be due to the unbalanced electron–hole transport system provided by the device structure. After optimization of the device structure as ITO/HAT–CN(5 nm)/TAPC(50 nm)/TCTA(5 nm)/TPB–AC(20 nm)/BmPyPB(40 nm)/LiF(1 nm)/Al(120 nm), high-efficiency deep-blue EL emission with a η_ext,_max of 7.0% was obtained, which exceeded the theoretical limit and was ascribed to the high horizontal dipole orientation of TPB–AC. Furthermore, the EQEs remained at 6.3% at a luminance of 1000 cd m⁻², exhibiting a very low roll-off. In addition, TPB–AC can also be used as a good host for highly efficient green/orange/red phosphorescent OLEDs and hybrid WOLEDs because of its high triplet energy level.^[57] Moreover, the same research group studied the luminescence mechanisms in TPB–AC-based OLEDs using magnetic field effect measurements to determine the reason for the efficiency of nondoped OLEDs exceeding the theoretical limitation.^[59] They found that the theoretical efficiency was exceeded owing to the energy conversion process of T₂→S₁ in TPB–AC. Low-field rise and high-field reduction line shapes of magneto-electroluminescence were realized, which cannot be ascribed to the TTA process. In addition, time-dependent density functional theory (TD–DFT) studies proved the presence of an extra T₂→S₁ conversion in TPB–AC, which is directly related to the high EQE and characteristic line shapes of magneto-electroluminescence. Furthermore, the efficiency and efficiency roll-off of TPB–AC AIEgen-based blue OLEDs were investigated by introducing a phosphorescent (FIrpic)-doped layer adjacent to the TPB–AC layer separated by 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCZPPY). The maximum EQE reached 7.93% and remained at 7.57% at a luminance of 1000 cd m⁻². Thus, this study provided physical insight for designing high-performance blue OLEDs using AIE materials. Inspired by the high horizontal orientation of TPB–AC molecules and to improve the device performance, the same group attempted to increase the molecular length and developed two deep-blue AIEgens, CN–TPB–AD and CN–TPB–TPA, by attaching TPA or DMAC as donors in the direction of the long molecular axis of TPB–AC.^[58] The two AIEgens exhibited deep-blue emission with PLQY values of 55.3 and 93.2% in the film state, respectively, and CN–TPB–TPA showed a good horizontal dipole orientation of 83.6%; thus, a device with η_out > 30% was obtained. Notably, a nondoped OLED based on CN–TPB–TPA exhibited a maximum external quantum efficiency of 7.27%, with a low efficiency roll-off and CIE coordinates of (0.15, 0.08). Additionally, CN–TPB–TPA also acts as a good host for hybrid warm WOLEDs, with the maximum current, power, and external quantum efficiency of 58.0 cd A⁻¹, 60.7 lm W⁻¹, and 19.1%, respectively. To further blue-shift the emission of TPB-based AIEgens, Qin et al.^[58] developed three TPB-based violet-blue AIE emitters, namely, TPBCzC1, TPBCzC2, and TPBCzC3, by altering the donor group from DPA or TPA to Cz with weak electron-donating ability, and the cyano group was used as an acceptor. In these derivatives, TPB acts as a π-conjugate bridge and helps to reduce the intramolecular interference between the donor and acceptor groups; further, it decreases the intermolecular charge transfer (ICT) to enhance the emission efficiency in the solid state, which is a desirable feature for emitters in the aggregate state. These three AIEgens exhibited excellent PL quantum yields (Φ_F) of over 98% in their film states. They also showed the AIE feature by gradually enhancing the PL intensity along with blue-shifted emission in THF:water mixtures, because of the restriction of intermolecular π–π stacking interactions and inhibition of intramolecular motions (vibration and rotation) in the aggregated state. Consequently, violet–blue nondoped OLEDs were constructed using these three AIEgens as emitters. Among them, the TPBCzC1-based device exhibited a maximum EQE of 4.34% with CIE coordinates of (0.160, 0.035), which is the first example of a nondoped violet–blue AIEgen-based OLED with a CIE_y smaller than 0.046. The relevant data from the aforementioned representative reports of TPB-based AIEgens are summarized in Table 4, and their corresponding chemical structures are shown in Figure 5.

2.5 Chiral binaphthyl-based AIEgens

A few reports have been devoted to circularly polarized OLEDs (CP–OLEDs) based on small chiral organic molecules with TADF and AIE characteristics. The first report of such materials were published in 2018; Ma, Tang and coworkers⁵⁹ explored four pairs of chiral AIEgens with DF, namely, R/S–BN–CF, R/S–BN–CCB, R/S–BN–DCB, and R/S–BN-F, for CP–OLEDs, by using a binaphthalene unit as the chiral source, cyano group as the acceptor, and Cz or tert-butyl Cz (tCz) and dimethylacridine (DMAC) as donor units. The resulting chiral luminogens revealed AIE features along with TICT behavior, for example, the emission intensity of S-BN–CF gradually increased by as much as 48-fold as f_w increased from 70 to 99%, and the material showed a large blue-shift of as high as 46 nm with an emission peak centered at 506 nm. Nondoped and doped CP–OLEDs fabricated using these materials as emitters exhibited EQE_max values of up to 3.5 and 9.3%, respectively, with multicolor EL emissions ranging from 537 to 597 nm and from 496 to 571 nm, respectively. More importantly, the roll-off of current efficiency for nondoped OLED (11.7%) is much smaller than that of the roll-off efficiency for doped OLED (53.3%). Cheng's research group⁶⁰ synthesized two pairs of chiral AIEgens based on binaphthalene-TADF emitters with xanthenone or benzophenone units as acceptor and phenoxazine (PXZ) as the donor; these luminogens were named, R/S-1 and R/S-2, respectively. The PL emission values of R-1 at 590 nm and R-2 at 542 nm were enhanced by 59 and 40-fold, respectively, when f_w was increased to 99%, which is indicative of AIE characteristics. However, owing to the fixed conjugation structures of R/S-1, the AIEgens revealed CP–EL signals; R/S-2 revealed no CP–EL signals because of the rotatable nature of its benzophenone structure, which can limit the chirality transfer process. Thus, doped and nondoped CP-OLEDs using R/S-1 as the emitter exhibited EQE_max of 4.1% and 0.22% respectively, with orange-red emission.

Besides, the same group developed a pair of sky-blue chiral D–A type emitters(S-/R-BN-tCz) by using tCz, instead of PXZ as the donor in R/S-1.⁶¹ The resultant nondoped blue CP–OLEDs exhibited an EQE_max of 1.0%; furthermore, solution-processed white CP–OLEDs are fabricated by combining the aforementioned blue and orange CPL AIEgens. The properties of the resultant OLEDs were optimized by adjusting the doping ratios of the orange emitters. The device exhibited a low V_on of 4.3 V, L_max of 10,200 cd m⁻², and CE_max of 2.0 cd A⁻¹ with intense CP–EL signals in the range of 450–650 nm. Two pairs of blue chiral AIEgens, namely, R-/S-5 and R-/S-6, developed by introducing pyrene groups on BINOL at 3,3ʹ or 6,6ʹ-positions. Nondoped CP–OLEDs using them as emitters exhibited EQE_max of 2.79 and 3.09%, respectively, with blue and sky-blue emission peaks at 460 and 480 nm, respectively.⁶²

In addition, a few reports have been devoted about CP–EL of AIE-active chiral polymer emitters based on chiral binaphthyl core and AIE-active diphenylethane linker or TPE unit. For instance, Cheng's group⁶³ reported a pair of AIE-active chiral binaphthyl enantiomers (S-/R-6) containing a TPE core. The resulting chiral luminogens showed excellent AIE characteristics upon aggregation in a THF–water system, for example, S-6 showed almost no fluorescence emission in pure THF solution and no change was observed until f_w reached 80%. When f_w increased up to 99%, the emission intensity of S-6 gradually increased by as much as 262-fold, the luminogen exhibited bright yellow fluorescence emission appears at 532 nm and a PLQY of 32.1%, these features indicate that S-6 is an excellent AIE-active emitter in the aggregated state. Nondoped CP–OLEDs using S-/R-6 exhibited bright yellow CP–EL emission at 534 nm with EQE_max values of 0.48 and 0.45%, respectively, as well as CE_max values of 1.32 and 1.26 cd A⁻¹, respectively. Another pair of chiral polymer enantiomers (S-/R-P) were also reported by Cheng et al.,⁶⁴ these materials showed typical AIE features with up to ∼16-fold enhancements in emission intensity upon aggregation at f_w of 90%; such enhancement is due to the restricted vibration of phenyl rings in the trans-1,2-diphenylethane linker. A nondoped device using S-/R-P luminogens emitted green CP–EL at 512 nm with CE_max values of 0.92/0.83 cd A⁻¹ and the PE_max values of 0.39/0.42 lm W⁻¹, respectively.

However, the development of CP-OLEDs with high EQE and small efficiency roll-off remains challenging. To overcome this issue, a few attempts were made by Zheng et al.,⁶⁵ who reported a pair of octahydro-binaphthyl (OBN)-based chiral AIEgens, namely, (R/S)–OBN–DPA, by merging chiral (R/S)–OBN units with the TADF skeleton of cyan CN–DPA derivatives. Owing to the twisted structure of (R/S)-OBN, there was a small ΔE_ST of 0.09 eV, which suggested the fast RISC leading to improved radiative efficiency. Both doped and nondoped CP–OLEDs fabricated by employing them as emitters displayed high EL performances with EQE_max values of 12.4 and 6.6%, respectively, as well observed low efficiency roll-offs with an EQE of 11.5% at 1000 cd m⁻² for doped one. Owing to the steric hindrance of 16 peripheral hydrogen atoms in the cyclohexane unit, exciton annihilation was effectively suppressed by reducing the intermolecular interactions of the molecules, resulting in small efficiency roll-off at high current density in the OLEDs. Soon afterward, the same group successfully constructed doped and nondoped CP–OLEDs with EQE_max values of 32.6 and 14.0%, respectively, by using Cz, instead of DPA, as the donor.⁶⁶ The resultant isomers, (R/S)–OBN–Cz, displayed a small ΔE_ST of 0.037 eV, and a high PLQY of 92%. Furthermore, in particular, the efficiency roll-off of doped CP–OLEDs were extremely low (i.e., 2.5, 3.0, and 5.9% at 1000, 3000, and 5000 cd m⁻², respectively), and their EQE was maintained at 30.6% even at a high luminance of 5000 cd m⁻². As well as nondoped OLEDs exhibited milder efficiency roll-off with an EQE of 13.8% maintained at 3000 cd m⁻² are almost the same as the maximum values. The roll-off ratios of the nondoped OLEDs is 1.0%, which is smaller than that of the doped OLEDs (i.e., 3.0%), owing to their shorter delayed lifetime and the AIE effect (Figures 6B and 6C). The relevant data from the aforementioned representative reports of chiral binaphthyl-based AIEgens are summarized in Table 5, and their corresponding chemical structures are shown in Figure 6.

3.1 Blue AIDF emitters

In general, blue luminogens are very needful for constructing commercialized OLEDs. However, the development of efficient blue luminogens is limited by the wide band gap and low PL efficiency of these materials in the solid state. The features of AIDF emitters offer rich opportunities for developing efficient blue-emitting luminogens for OLED applications. The development of blue AIDF emitters with stable and high color purity for OLEDs is a challenging endeavor because of the difficulty of injecting charge carriers from the adjacent layers of a device due to the wide bandgap (>3.0 eV) of blue-emitting materials. Thus, the design and synthesis of blue-emitting systems with AIDF materials must be investigated further.

Tang, Cheng, and coworkers⁶⁷ developed two pairs of AIE active efficient blue TADF isomers and named them 2Cz2tCzBn, 2tCz2CzBn, 2PhCz2tCzBn, and 2tCz2PhCzBn, respectively, these materials featured a twisted acceptor–hetero–donor configuration with cyanobenzene as the acceptor unit and Cz or 3,6-diphenylcarbazole as one donor, and 3,6-di-tert-butylcarbazole as an another donor. Owing to the large twisted dihedral angle between the donor and acceptor units of 55.4° to 83.9°, the materials showed increased molecular rigidity and reduced nonradiative decay, resulting in high RISC rate constant (k_RISC) for the DF. The ΔE_ST values for these emitters are calculated to be 0.13–0.15 eV, which are small enough for efficient RISC process and TADF characteristics. The incorporation of tert-butyl groups in the molecules could effectively increase the solubility and reduce the ACQ effect by inhibiting the intramolecular vibrational relaxation and the intermolecular π–π stacking. Nondoped OLEDs bearing 2Cz2tCzBn, 2tCz2CzBn, 2PhCz2tCzBn, and 2tCz2PhCzBn, exhibited record-high EL performances with EQEs of 25.8, 24.5, 19.5, and 19.1%, respectively. High-performance white OLEDs were obtained by employing a single EML with this blue TADF emitter as a host to orange-red 3DMAC-BP as a TADF dopant, which exhibited EQE_max and CE values of 27.3% and 67.0 cd A⁻¹, respectively.

Our group recently developed two ultra-deep-blue AIDF emitters, namely, TB–tCz and TB–tPCz, based on the D–A configuration using 3,6-substituted Czs as the donor and an oxygen-bridged boron core (TB) as the acceptor.⁶⁸ The hetero atom (O or N) bridged aryl boron core structure restricted the bathochromic shift of these materials. Both materials revealed significant TADF characteristics with AIE features. Compared with TB–tCz, TB–tPCz was found to be a superior AIDF material because of its stronger AIE behavior (≈24-fold greater), lower ΔE_ST, larger SOC constant, and faster RISC rate. The corresponding nondoped devices exhibited ultra-deep-blue EL emissions at 416–428 nm and high color purity with a narrow FWHM of 42.2–44.4 nm. The TB–tCz-based nondoped device showed excellent EL performance with an EQE_max of 15.8% and excellent color purity close to the National Television System Committee (NTSC) deep-blue standard of (0.14, 0.08). Moreover, both AIDF emitters exhibited outstanding device performances with EQE_max of 14.1–15.9% in doped OLEDs. We also prepared a series of deep-blue to sky-blue OLEDs based on D–A-type AIDF emitters, namely TB–3Cz, TB–P3Cz, and TB–DACz, using the same TB acceptor and Cz or DPA substituted on the Cz derivatives as different donor entities.⁶⁹ The AIDF emitters showed high PLQYs of 90–99%, and high k_RISC values in the neat-film state, as well as AIE characteristics with 11–25-fold enhancements in PL emission intensity when f_w reached 90%; such enhancements could help suppress the nonradiative decay process. Nondoped device based on TB–3Cz, TB–P3Cz, and TB–DACz exhibited a EQE_max values of 9.90, 6.13, and 6.04%, respectively. Interestingly, among these emitters, TB–3Cz and TB–P3Cz exhibited deep-blue emission at CIE coordinates of (0.17, 0.07) and (0.15, 0.08), which are close to the NTSC blue standards.

In 2018, Yang et al.⁷⁰ explored two blue AIDF isomers, namely o-ACSO2 and m-ACSO2, which were based on diphenyl sulfone (DPS) as the acceptor and DMAC as the donor. The engineering design of these isomers was inspired by the DMAC–DPS emitter reported by Adachi et al.^[5] Both luminogens exhibit good TADF and AIE characteristics owing to the large twisted angles between donors and acceptor unit. The meta-isomer, m-ACSO2, displayed small ΔE_ST of 0.07 eV, high PLQY of 76%, and excellent morphology of neat films. As a result, a nondoped solution-processed sky-blue OLED employing m-ACSO2 as the emitter achieved high EL performance with a EQE_max, CE_max, and PE_max values of 17.2%, 37.9 cd A⁻¹, and 23.8 lm W⁻¹, respectively. The same group developed another pair of blue chiral emitters, namely, (S)-NPE–AcDPS and (R)-NPE–AcDPS, with CPL, TADF, and AIEE characteristics by fusing chiral amines at the ortho-positions of the DPS unit in DMAC–DPS emitter.⁷¹ A doped deep-blue OLED based on (S)-NPE–AcDPS exhibited a EQE of up to 18.5%. Indeed, Guo's group initially reported m-ACSO2 as bis(3-(9,9-dimethyl-9,10-dihydroacridine)phenyl)sulfone (mSOAD) based on DMAC and DPS units with a highly twisted zig-zag configuration, and compared with its linear isomer, DMAC–DPS.⁷² The twisted zig-zag structure of mSOAD led to a very low ΔE_ST of 0.01 eV, compared to para- substituted DMAC–DPS (ΔE_ST of 0.08 eV), as well as the resulting luminogen concurrently exhibits both AIE and TADF characteristics. The EL performance of a nondoped blue OLED employing mSOAD was higher than that of an OLED bearing DMAC–DPS, with EQE_max, CE_max, and PE_max values of 14.0%, 31.7 cd A⁻¹, and 28.4 lm W⁻¹, respectively.

Grazulevicius and coworkers⁷³ developed three multifunctional luminescent materials composed of pyrimidine-5-carbonitrile with different substituted Czs, and named the materials CzPCN, tCzPCN, and MeOCzPN. The resulting molecules simultaneously exhibit both AIE and TADF properties. These materials could be used not only as blue emitters for OLEDs but also as oxygen probes for optical sensors. Nondoped and doped devices based on CzPCN, exhibited EQE_max values of 12.8 and 14%, respectively. You and coworkers⁷⁴ employed a series of imidazole derivatives with TADF and AIE characteristics for the first time to prepare AIEgens namely, DMAC–CNIM, DMAC–CNIB, and DMAC–CNBIM. Owing to attachment of two cyano groups with electron-withdrawing properties to the imidazole moiety, TADF property was induced with small ΔE_ST values (0.06–0.10 eV); while, the insertion of methyl/phenyl at N1 position of imidazole moiety can twist the geometry between imidazole and phenyl bridge, thus the imidazole derivatives transformed from ACQ emitter into AIEgens. To obtaining insight into the AIE mechanism, DMAC–CNIH (hydrogen at N1 position of imidazole moiety) is also synthesized for comparison, which gets a planar geometry with strong N-H···N intermolecular hydrogen bonding interaction, thus leads to severe ACQ effect. Meanwhile, the presence of a steric hindrance group at the N1 position of imidazole moiety induced AIE feature in THF/water system. As a result, nondoped OLED based on DMAC–CNBIM AIEgen as emitter exhibited a high EQE of 20.0% and low efficiency roll-off (EQE at 1000 cd m⁻², 16.1%). These results represented the state-of-the-art performance for all imidazole-based OLEDs. Qi's research group⁷⁵ explored a series of AIDF luminogens based on D–A–Dʹ configuration featuring phenyl(pyridyl)methanone as the acceptor and DMAC or PXZ as the donor. Interestingly, the molecules became rigid on account of the formation of intramolecular hydrogen bonds with the nitrogen atom in the acceptor. Thus, the resultant AIEgens demonstrated small ΔE_ST (0.03–0.10 eV), suppressed nonradiative decay and increased luminescence efficiency in the solid state. The PLQYs were realized to be 46.7, 66.8, and 53.3% for CBM–DMAC, 3CPyM–DMAC, and 2CPyM–DMAC, respectively, these enhanced emission values can ascribe to be the suppressed nonradiative transition by virtue of the formation of intramolecular hydrogen-bonding interactions and the intramolecular rotation can be locked. Nondoped devices using these emitters exhibited EL emission from sky-blue to orange, and the device employing CBM–DMAC as the emitter revealed an EQE_max of 6.7 %.

The relevant data from the aforementioned representative reports of blue AIDF emitters are summarized in Table 6, and their corresponding chemical structures are shown in Figure 7.

3.2 Green AIDF emitters

Many green AIDF materials have been reported as EML in OLEDs, and some of these materials show very high EL performance. For example, in 2019 Chi's group reported a green AIDF emitter 2Cz–DPS, which was obtained by inserting DPS between Cz and PTZ donors with an asymmetric D–A–Dʹ configuration.⁷⁶ The resultant material showed dual-charge transfer pathways, that is, intramolecular electrostatic attraction and through-space charge transfer (TSCT) because the Cz donor and DPS acceptor groups were connected at the ortho-position to achieve the spatially close D–A interaction. The dual charge transfer process significantly increased the oscillator strength (f = 0.0207) for faster radiative decay, in comparison with the linearly linked conformation of 4Cz–DPS (f = 0.0074), exhibiting a high PLQY of 91.9% in solid state. In addition, strong intramolecular interactions created in 2Cz–DPS molecule, which assist the increase in molecular rigidity, suppress nonradiative decay channels, and increase the radiative transition rate led to effective AIE and TADF characteristics. A nondoped OLED constructed based on 2Cz–DPS with the configuration of ITO/PEDOT:PSS (30 nm)/mCP (20 nm)/2Cz–DPS (30 nm)/TPBI (40 nm)/LiF (1 nm)/Al exhibited a record-high EL performances with EQE_max, CE_max, and PE_max values of 28.7%, 82.3 cd A⁻¹, and 51.8 lm W⁻¹, respectively. Two D–A–Dʹ-configured AIDF luminogens based on DPS derivatives, namely, 3CP–DPS–PXZ and 3CP–DPS–DMAC, were explored by Wang et al.⁷⁷ The materials were prepared with an asymmetric configuration to understand the effect of donor on their luminescence properties with different donors Dʹ, namely, PXZ and DMAC, herein, 9-phenylcarbazole was as the fixed donor D. Both emitters exhibited clear AIE behavior, high PLQY in the neat film, as well, observed a very small ΔE_ST of 0.03–0.07 eV. They also demonstrated TADF features with microsecond-scale lifetimes of 0.96–2.36 μs. The molecule containing PXZ featured stronger electron-donating capability compared with 9,9-DMAC, thus 3CP–DPS–PXZ exhibited a red-shift of emission and enhanced AIE effect and higher RISC (k_RISC) rate constant in comparison with 3CP–DPS–DMAC. Nondoped devices using 3CP–DPS–PXZ and 3CP–DPS–DMAC as EMLs exhibited EQE_max values of 17.9 and 9.1%, respectively. As well, 3CP–DPS–PXZ showed low efficiency roll-off that EQE remained 14.5% at the luminance of 1000 cd m⁻².

In 2015, Adachi's group constructed nondoped OLEDs with a symmetrical D–A–D configuration and obtained excellent EL performances (EQE_max = 18.9%) and negligible efficiency roll-off by using benzophenone, instead of DPS as the acceptor.^[5] In 2017, the same group developed three AIDF emitters with an asymmetric D–A–Dʹ configuration by using dibenzothiophene (DBT) as the donor, benzophenone as the acceptor, and PXZ, PTZ, and DMAC as another donor; these emitters were named DBT−BZ−PXZ, DBT−BZ−PTZ, and DBT–BZ–DMAC.⁷⁸ Indeed, they initially reported DBT–BZ–DMAC^[78] revealed AIE feature by gradually enhancing its PL intensity along with blue-shifted emission in THF:water mixtures because intramolecular rotational and vibrational motions are inactive in aggregated state, resulting in suppressed nonradiative decay of the excited state, leading to enhanced radiative decay in the form of luminance. DBT–BZ–DMAC showed its PL maximum at 505 nm in the neat film, as well realized PLQY of 80.2% and smallest ΔE_ST of 0.08 eV in the neat film. A nondoped OLED fabricated using DBT–BZ–DMAC exhibited excellent EL performances with CE_max and EQE_max values of 25.6 cd A⁻¹ and 14.2%, respectively, and high value of small CE roll-off of 0.46%. By comparison, nondoped devices based on DBT−BZ−PTZ and DBT–BZ–DMAC as emitters displayed green/yellow EL emissions with EQE_max values of 9.2 and 9.7%, respectively, as well as small efficiency roll-off.^[78] The corresponding, doped devices also exhibited good EL performances, with EQE_max values up to 19.2%. Soon afterward, the same group tailored a green AIDF emitter (DCPDAPM) with a twisted D–A–Dʹ configuration by replacing the DBT with two Czs as D in the aforementioned report, benzophenone as the acceptor, and DMAC as another donor Dʹ.⁷⁹ Owing to its twisted conformation, the ΔE_ST of DCPDAPM in the solid state was calculated to be 0.10 eV and showed a high solid-state PLQY of 76.1%. The resulting green emitter simultaneously exhibits AIE and TADF characteristics. A nondoped device fabricated using DCPDAPM as the emitter exhibited EQE_max and CE_max values of 8.15% and 26.8 cd A⁻¹, respectively. By comparison, a doped OLED with 20 wt% DCPDAPM in CBP showed EQE_max and CE_max values of 19.6% and 61.8 cd A⁻¹, respectively.

To develop more twisted D–A–D-based AIDF emitters, Wang's group introduced a triazatruxene (TAT) unit as strong donor on both sides of benzophenone and named the resultant materials TATC–BP and TATP–BP.⁸⁰ Owing to the different substituents on TAT unit, the material showed different AIE and MCL characteristics. TATC–BP with flexible hexyl substituents showed color-changing AIE and clear MCL behaviors with a large red-shift of 59 nm. By contrast, TATP-BP, which features rigid phenyl substituents, exhibited no color-changing AIE and MCL behavior with a smaller red-shift of 36 nm. Nondoped OLEDs prepared with TATP–BP and TATC–BP showed good EL performance with EQE_max values of 5.9 and 6.0%, respectively, and exhibited remarkable small efficiency roll-off. In particular, TATP–BP showed the roll-off ratio of 3.3% in EQE at 1000 cd m⁻² was much lower than that of TATC–BP (18.6%). To further develop TAT-based AIDF luminogens, the same group explored two green AIDF emitters based on TAT as the donor and benzophenone as the acceptor with D–A configuration, namely, TAT–BP and TAT–2BP.⁸¹ Owing to their highly twisted rigid structures with large dihedral angles, the emitters showed ΔE_ST smaller than 0.1 eV, thus both emitters displayed not only typical AIE behavior, but also a distinct TADF nature along with high PLQY and a relatively short DF lifetimes (<1 μs). Nondoped devices bearing TAT–2BP and TAT–BP showed EQE_max values of 9.8 and 6.4%, respectively. In addition, these devices exhibited small efficiency roll-off characteristics, in particular, the TAT–2BP-based devices showed a roll-off ratio of 1.0% in EQE at 1000 cd m⁻².

Unfortunately, AIDF emitters are insufficient to obtain ideal nondoped OLED emitters. Indeed, previous reports indicated that AIDF emitters suffer from Dexter energy transfer (DET), which is mainly involved in the complete quenching process of TADF in neat films. Triplet exciton-related quenching may also explain the occurrence of efficiency roll-off in practical brightness at high current densities, which is a major issue in EL process. To address this problem, Zhang and coworkers⁸² tailored two green AIDF emitters by reshaping the D–A-based TADF emitter BP–DPAC and named the resultant materials SPBP–DPAC and SPBP–SPAC.⁸³ These emitters were uniquely designed by locking the phenyl rings of benzophenone unit via a rigid fluorine moiety. Thus, the materials showed not only enhanced molecular rigidity but also reduced the intermolecular interaction, resulting suppressed ACQ and showed potential AIE characteristics by enhancing their PLQYs. The ΔE_ST of both luminogens was calculated to be less than 0.1 eV and PLQYs of SPBP–DPAC and SPBP–SPAC in neat films, at 93 and 98%, respectively, were twice that of BP–DPAC of 47%. Thus, the materials demonstrated TADF with very short delayed lifetimes of 2.5 and 1.3 μs, respectively, such a short lifetime are favorable to suppress the device efficiency roll-off by accelerating triplet conversion to DF. Nondoped OLEDs using SPBP–DPAC and SPBP–SPAC exhibited excellent EQE_max values of 22.8 and 21.3%, respectively, and a very negligible efficiency roll-offs owing to the short delayed lifetimes of 2.5 and 1.3 μs for both emitters, respectively.

Tang's group prepared the same type of twisted D–A–Dʹ-based green AIDF emitters by using carbonyl group as the acceptor and DMAC as an electron donor. The authors melded 9,9-dimethylfluorene (DMF) and 9,9-diphenylfluorene (DPF) to the carbonyl group to decrease intermolecular π–π stacking and increase the steric hindrance and named the resultant products DMF–BP–DMAC and DPF–BP–DMAC, respectively.⁸⁴ The ΔE_ST values of these materials were calculated to be to be 0.14 and 0.09 eV, respectively, which are sufficiently low to achieve effective TADF feature. In particular, DPF–BP–DMAC revealed a high PLQY of 62.3%, and its nondoped device exhibited good EL performances, with an EQE_max of up to 14.4%, and retained an EQE of 12.6% at 1000 cd m⁻², indicating a very small efficiency roll-off. Another series of D–A–Dʹ-based AIDF emitters were synthesized by Zhao and coworkers,⁸⁵ by introduction of 4-(phenoxazin-10-yl) benzoyl group to the common chromophores of Cz-substituted fluorene derivatives, namely, DCDMF–BP–PXZ, DCDPF–BPPXZ, and DCSBF–BP–PXZ. DCDMF–BP–PXZ and DCDPF–BP–PXZ showed very high PLQY values of 88.5 and 89.0%, respectively, films than DCSBF–BP–PXZ of 39.6% because of poor molecular π-conjugation as well as strong intermolecular π–π interaction of DCSBF–BP–PXZ. Consequently, nondoped OLEDs using DCDMF–BP–PXZ and DCDPF–BP–PXZ exhibited high EL performances with high EQE_max values of 19.0 and 18.5%, respectively. A nondoped OLED using DCSBF–BP–PXZ displayed a low EQE_max value of 3.3%, owing to its low PLQY and unbalanced charge-carrier transport ability. Doped OLEDs based on DCDMF–BP–PXZ and DCDPF–BP–PXZ and DCSBF–BP–PXZ with a CBP host exhibited high EQE_max values of 21.7, 24.4, and 22.3%, respectively.