Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Abstract

Open AccessCCS ChemistryCOMMUNICATIONS2 Sep 2022Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor Jinke Chen, Xing Wu, Hao Liu, Nuoling Qiu, Zhangshan Liu, Dezhi Yang, Dongge Ma, Ben Zhong Tang and Zujin Zhao Jinke Chen State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Xing Wu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Hao Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Nuoling Qiu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Zhangshan Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Dezhi Yang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Dongge Ma State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author , Ben Zhong Tang School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 AIE Institute, Guangzhou Development District, Huangpu, Guangzhou 510530 Google Scholar More articles by this author and Zujin Zhao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202196 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Constructing blue thermally activated delayed-fluorescence materials for high-performance organic light-emitting diodes (OLEDs) remains challenging due to the intrinsically strong intramolecular charge transfer nature of the nearly orthogonal connection of electron donor (D) and acceptor (A), which results in long-wavelength emission. Herein, an effective delayed-fluorescence design strategy of modulating D–A torsion angles is proposed and efficient sky-blue, pure-blue, and deep-blue delayed-fluorescence molecules consisting of a xanthenone acceptor and carbazole-based donors are created by decreasing the torsion angles. They exhibit strong delayed fluorescence with high photoluminescence quantum yields of 85–94% in doped films, and their delayed-fluorescence lifetimes are elongated from 1.0 to 27.6 μs as the torsion angles decrease. These molecules can function as excellent emitters in OLEDs, providing efficient electroluminescence peaking at 442 nm (CIEx,y = 0.15, 0.08), 462 nm (CIEx,y = 0.15, 0.18), and 482 nm (CIEx,y = 0.17, 0.30) with state-of-the-art external quantum efficiencies of up to 22.2%, 33.7%, and 32.1%, respectively, demonstrating the proposed molecular design for efficient blue delayed-fluorescence molecules is successful and promising. Download figure Download PowerPoint Introduction Efficient blue organic luminescent materials are highly desired because they are one of the fundamental elements of the three primary colors in organic light-emitting diodes (OLEDs).1–7 Organic fluorescence molecules with blue emission, which are employed as the first-generation luminescent materials in OLEDs, can be readily designed, but only 25% of the electro-generated excitons under electrical excitation are used, leading to low external quantum efficiency with an upper limit of 5–7.5%.8–10 Several strategies, such as triplet–triplet fusion11–13 and hybridized local and charge-transfer excited states,14–17 have been proposed to enhance triplet exciton utilization of fluorescence molecules, but full exciton harvesting remains difficult. Second-generation noble-metal-containing phosphorescence materials have been developed, which can reach unity exciton utilization by converting singlet excitons to triplet excitons via intersystem crossing based on heavy-atom induced large spin–orbit coupling (SOC). But, because of the intrinsic metal-to-ligand charge-transfer (CT) characteristics, pure blue emissions are hardly achieved in most phosphorescence materials, and the long lifetimes of triplet excitons result in poor stability of these materials in OLEDs.18–21 After decades of continuous research, purely organic thermally activated delayed-fluorescence (TADF) molecules have been invented and are currently emerging as the third-generation luminescent materials for the fabrication of high-performance OLEDs, thanks to the advantages of easy molecular design, high exciton utilization, noble metal-free structures, and so on.1,22–28 The reported TADF molecules generally have a highly twisted conformation, consisting of an electron donor (D) and acceptor (A) connected in a nearly perpendicular manner, to minimize the exchange integral between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, the energy split (ΔEST) between the lowest singlet excited (S1) state and the lowest triplet excited (T1) state can be reduced to allow fast reverse intersystem crossing (RISC), which results in the occurrence of delayed fluorescence.3,29–31 Although numerous efficient sky-blue to red TADF molecules have been successfully explored based on this design method,24,31–37 deep-blue to blue TADF molecules are still challenging because these nearly orthogonal D–A systems are inevitably accompanied by a strong intramolecular charge-transfer (ICT) effect that causes redshifted emissions.38–41 Weakening the ICT effect by choosing weak D and A groups3,42–44 or designing a through-space CT framework45,46 can to some extent shift the emission peaks to the short-wavelength region, but the corresponding electroluminescence (EL) efficiencies are often unsatisfactory. Therefore, modulating the torsion angles of proper D and A groups could be a promising strategy to explore high-efficiency deep-blue and pure-blue TADF molecules. As a proof of concept, we wish to report an effective design of blue luminescent molecules based on a xanthenone (XT) acceptor and two carbazole (Cz) donors (Figure 1a–c). XT is selected as electron acceptor because of its relatively weak electron-withdrawing nature, high structural rigidity, and ability to promote RISC by enlarging SOC stemming from the n−π* transition of the carbonyl group.40 A previous study demonstrated the ability to tune the color of blue TADF emitters by the introduction of methyl substituents.47 Here, the torsion angles between XT and Cz are tuned progressively by introducing methyl groups at the 1 and 8 positions of Cz, and the strength of the D–A interaction is further optimized by modification at the 3 and 6 positions of Cz with electron-donating tert-butyl groups. We found that the ICT effect is weakened sequentially as the torsion angles between XT and Cz diminishes, gradually blueshifting the emissions from 2MCz-XT to MCz-XT and then to Cz-XT. The introduction of tert-butyl groups can enhance the ICT effect, leading to moderately redshifted emission of 2TBCz-XT relative to Cz-XT. Meanwhile, all these molecules exhibit apparent delayed fluorescence, while the lifetimes of the delayed fluorescence are closely associated with the torsion angles and the strength of the ICT effect. By adopting these new molecules as emitters, highly efficient deep-blue, pure-blue, and sky-blue OLEDs with EL peaks at 442, 462, and 482 nm and outstanding maximum external quantum efficiencies (ηext,maxs) of 22.2%, 33.7%, and 32.1%, respectively, are obtained. These impressive EL performances demonstrate the significance of modulating torsion angles in the design of blue TADF emitters. Figure 1 | (a) Molecular design strategy. (b) Chemical structures of the new molecules with calculated torsion angles and (c) crystal structures of Cz-XT and 2MCz-XT with observed torsion angles. (d) Distributions of HOMOs and LUMOs and the calculated energy splits (ΔESTs) of the new molecules. Download figure Download PowerPoint Results and Discussion The target molecules Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT were facilely synthesized in good yields by palladium-catalyzed Buchwald–Hartwig C–N coupling reactions of 3,6-dibromoxanthen-9-one with Cz and Cz derivatives ( Supporting Information Scheme S1). The molecular structures were characterized by 1H NMR and 13C NMR ( Supporting Information Figures S1–S4) and high-resolution mass spectrometry with satisfactory results. They are thermally and morphologically stable with high decomposition temperatures of 397–459 °C and high glass-transition temperatures over 210 °C, as determined by thermogravimetry analysis and differential scanning calorimetry measurements, respectively ( Supporting Information Figure S5). Their electrochemical properties were measured by cyclic voltammetry using ferrocene as the calibration compound ( Supporting Information Figure S6). The experimental HOMO and LUMO energy levels of Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT are calculated to be −5.69 and −2.92; −5.66 and−2.93; −5.56 and −2.94; and −5.60 and −2.92 eV, respectively. Single crystals of Cz-XT and 2MCz-XT were obtained from a mixture of n-hexane and dichloromethane via slow solvent evaporation. Single-crystal X-ray crystallography analysis reveals that 2MCz-XT adopts a highly twisted D–A connection with large torsion angles of 83° and 91°, due to the severe steric hindrance imposed by the two methyl groups at the 1 and 8 positions of Cz. In contrast, Cz-XT shows a more planar molecular conformation, in which the torsion angles are decreased to 37° and 40°, indicating Cz-XT has a better π-conjugation between Cz and XT than 2MCz-XT. The optimized structures and molecular orbitals of these new molecules were calculated employing a density functional theory (DFT) method.48 As depicted in Figure 1d and Supporting Information Figure S7, the optimized geometry of 2MCz-XT has a similar highly twisted conformation to its crystal structure, with large torsion angles of 83°–84° between XT and Cz. However, MCz-XT and Cz-XT show gradually decreased torsion angles of 70° and 51°–52° due to the reduced steric hindrance. Similar molecular geometry is simulated for 2TBCz-XT compared with Cz-XT. The electron clouds of the HOMOs and LUMOs of these molecules are primarily distributed on Cz and XT, respectively. Due to the highly twisted molecular geometry, 2MCz-XT has the highest degree of separation between the HOMO and LUMO, which leads to the smallest ΔEST of 0.01 eV. The ΔEST of MCz-XT is increased to 0.12 eV due to decreased torsion angles. Cz-XT and 2TBCz-XT have overlapping HOMOs and LUMOs because of the relatively planar conformation. Thus, they have much larger ΔESTs of 0.23 and 0.21 eV than 2MCz-XT and MCz-XT. As displayed in Figure 2a, Cz-XT and 2TBCz-XT have strong absorption maxima at 365 and 384 nm in tetrahydrofuran (THF) solution, which are mainly comprised of the π–π* transitions. MCz-XT and 2MCz-XT have relatively weak absorption maxima at 362 and 368 nm, associated with the ICT states. 2MCz-XT exhibits a green photoluminescence (PL) peak located at 501 nm in THF solution, whereas the PL peaks are blueshifted progressively to 481 nm for MCz-XT and 459 nm for Cz-XT (Figure 2b) due to the weakened ICT effect. The PL peak of 2TBCz-XT is redshifted to 481 nm, which is ascribed to the strengthened ICT effect due to the presence of the tert-butyl groups. To evaluate the ICT effect, the PL spectra of the four molecules in various solvents are tested ( Supporting Information Figure S8). The spectral displacements gradually increase from Cz-XT (60 nm) to MCz-XT (63 nm) and then to 2MCz-XT (71 nm), in good agreement with the increased dihedral angles and strengthened ICT effect. 2TBCz-XT exhibits a larger spectral displacement of 70 nm than Cz-XT (60 nm) because of a stronger ICT effect. These solvation effects further validate that both enlarging D–A dihedral angles and introducing electron-donating groups strengthen the ICT effect of the molecules. When doped in (diphenylphosphoryl)-dibenzo[b,d]-furan (PPF) host at a concentration of 15 wt %, Cz-XT shows a PL peak at 459 nm, similar to that in THF solution, whereas MCz-XT and 2MCz-XT have redshifted PL peaks at 466 and 483 nm, respectively. The PL peak of 2TBCz-XT is located at 468 nm, which is redshifted by 9 nm compared with that of Cz-XT. The photoluminescence quantum yields (ΦPLs) of these molecules in doped films are in the range of 85–94%, higher than those in THF solution (Table 1). The molecular motions are active in solution, largely dissipating the excited-state energy and thus leading to low ΦPL values. But in the doped films, the intramolecular motions of the molecules are greatly suppressed so that the nonradiative dissipation pathways are blocked, accounting for the significantly improved ΦPL values.16,35 Figure 2 | (a) Absorption and (b) photoluminescence (PL) spectra of the new luminogens in THF solutions (10−5 M) and in doped films with a doping concentration of 15 wt % in PPF. Temperature-dependent transient PL decay spectra of (c) Cz-XT, (d) MCz-XT, (e) 2MCz-XT, and (f) 2TBCz-XT doped in PPF host with a doping concentration of 15 wt %, measured under nitrogen. Download figure Download PowerPoint Table 1 | Photophysical Properties of the New Molecules Solutiona Doped Filmb λabs (nm) λem (nm) ΦPLc (%) λem (nm) ΦPLc (%) τdelayedd (μs) Rdelayede (%) kFf (×107 s−1) kICg (×107 s−1) kRISCh (×105 s−1) ΔESTi (eV) Cz-XT 365 459 47 459 85 27.6 64 9.9 1.7 1.0 0.15 MCz-XT 362 481 52 466 86 11.3 51 7.0 1.1 1.8 0.04 2MCz-XT 368 501 47 483 91 1.0 60 1.1 0.1 25.0 0.01 2TBCz-XT 384 481 71 468 94 17.0 50 12.4 0.8 1.2 0.10 aMeasured in THF solution (10−5 M) at room temperature. bVacuum-deposited on a quartz substrate with a doping concentration of 15 wt % in PPF. cPhotoluminescence quantum yield (ΦPL) determined by a calibrated integrating sphere under nitrogen at room temperature. dDelayed fluorescence lifetime (τdelayed) evaluated at 300 K under nitrogen. eRatio of delayed component. fFluorescence decay rate. g Internal conversion decay rate from S1 to S0. hRate constant of RISC process. iEstimated from the high-energy onsets of fluorescence and phosphorescence spectra at 77 K. From the onsets of fluorescence and phosphorescence spectra of doped films ( Supporting Information Figure S9), the experimental ΔESTs of these molecules are calculated to be 0.01–0.15 eV, which are small enough to ensure the occurrence of RISC and thus delayed fluorescence (Figure 2c–f). By progressively reducing the torsion angles between Cz and XT, the ΔEST increases from 0.01 eV of 2MCz-XT to 0.04 eV of MCz-XT and to 0.15 eV of Cz-XT. The ΔEST of 2TBCz-XT is 0.10 eV, smaller than that of Cz-XT, although both molecules adopt nearly identical molecular conformations. These results demonstrate that enlarging the torsion angles and strengthening the ICT effect between D–A groups are conducive to achieving a small ΔEST. Because of the smaller ΔEST, 2MCz-XT exhibits a shorter delayed-fluorescence lifetime (τdelayed) of 1.0 μs and faster RISC, rate constant (kRISC) of 2.5 × 106 s−1, than Cz-XT (27.6 μs, 1.0 × 105 s−1) and MCz-XT (11.3 μs, 1.8 × 105 s−1). Compared with Cz-XT, 2TBCz-XT displays faster RISC, corresponding to a shorter τdelayed of 17.0 μs and a larger kRISC of 1.2 × 105 s−1 (Table 1). The temperature-dependent transient PL decay spectra indicate that Cz-XT and 2TBCz-XT have greatly promoted RISC with apparently enhanced delayed components (Rdelayeds) at high temperatures ( Supporting Information Table S1). However, the change in delayed fluorescence of 2MCz-XT by increasing temperature is obviously diminished, and its τdelayed and Rdelayed vary slightly from 77 to 300 K. These results manifest that the very small ΔEST allows 2MCz-XT to enjoy fast RISC even at low temperatures, while the large ΔESTs make Cz-XT and 2TBCz-XT more dependent on the thermal activation for sufficient RISC. The energy levels of Cz-based donors, XT acceptor, and the new molecules are measured from the phosphorescence spectra and shown in Supporting Information Figure S10. Generally, the locally excited triplet (3LE) energy levels of the donors are close to the 1CT states of MCz-XT, 2MCz-XT, and 2TBCz-XT, whereas the 3LE energy level of XT is close to the 1CT state of Cz-XT. For 2MCz-XT, the 3LE energy level of the donor is close to both 1CT and 3CT states, which may contribute to the fastest RISC and most efficient delayed fluorescence.49 Furthermore, the time-dependent DFT method is employed to gain insights into the RISC in these blue molecules. The natural transition orbital analysis reveals that the S1 and T1 states of the four molecules are dominated by CT transition, whereas the second triplet excited (T2) states are energetically close to the T1 states with LE transition characteristics ( Supporting Information Figures S11 and S12). The different transition natures of S1 and T2 are favorable for RISC.50,51 Furthermore, the calculated SOC matrix elements are also considerable between S1 and T2. These results suggest T2 is involved in RISC and facilitates the occurrence of delayed fluorescence in these molecules, which could be important for Cz-XT and 2TBCz-XT, who have small torsion angles and relatively large ΔESTs. To evaluate the EL performances of these blue molecules, doped OLEDs are fabricated with the configuration of indium tin oxide (ITO)/hexaazatriphenylenehexacabonitrile (HATCN) (5 nm)/1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (50 nm)/tris[4-(carbazol-9-yl)phenyl]amine (TcTa) (5 nm)/1,3-di(carbazol-9-yl)benzene (mCP) (5 nm)/emitting layer (EML) (20 nm)/PPF or diphenyl-4-triphenylsilylphenyl-phosphine oxide (DPEPO) (5 nm)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1″-terphenyl]-3,3″-diyl]bispyridine (TmPyPB) (30 nm)/lithium fluoride (LiF) (1 nm)/Al (Figure 3a–d), where the doped films of these molecules in PPF or DPEPO hosts with varied doping concentrations of 10, 15, and 20 wt % work as EMLs, HATCN and LiF serve as hole- and electron-injection layers, respectively, TAPC and TmPyPB perform as hole- and electron-transporting layers, respectively, TcTa serves as electron-blocking layer, PPF and DPEPO work as hole-blocking layers, and mCP functions as exciton-blocking layer. The key performance data of all the devices with corresponding configurations are summarized in Supporting Information Figures S13–S20 and Tables S2–S5. In general, these devices turn on at low voltages of 2.7–3.6 V and radiate strong light in deep-blue to sky-blue regions. In comparison with Cz-XT, MCz-XT and 2MCz-XT exhibit apparently redshifted EL emissions, and 2TBCz-XT shows redder EL emission than Cz-XT (Table 2). These EL behaviors are consistent with their PL behaviors. Cz-XT and MCz-XT have better EL efficiencies in the DPEPO host, whereas 2MCz-XT and 2TBCz-XT give better EL efficiencies in the PPF host. Whether in PPF or DPEPO, the EL spectra remain stable with minor redshifts less than 8 nm, but the maximum luminance (Lmax) is enhanced greatly by increasing doping concentrations from 10 to 20 wt %. Figure 3 | (a) Energy level diagram and chemical structures of the functional layers. (b) EL spectra at 4 V. (c) Plots of luminance–voltage–current density and (d) external quantum efficiency–luminance of the OLEDs based on the new luminogens. EML, doped films of the new molecules in PPF or DPEPO hosts. Download figure Download PowerPoint Table 2 | EL Performances of the Doped OLEDs Based on the New Molecules Emitter Von (V) ηC (cd A−1) ηP (lm W−1) ηext (%) Lmax (cd m−2) CIE (x, y) λEL (nm) Maximum Value/at 100/at 500 cd m−2 Cz-XT 3.5 15.9/10.3/5.8 13.9/7.5/3.5 22.2/14.4/8.0 1989 (0.15, 0.08) 442 MCz-XT 3.5 29.7/26.4/21.6 25.9/19.3/13.8 24.0/21.3/17.5 5751 (0.15, 0.15) 460 2MCz-XT 3.0 64.4/57.6/53.3 65.2/47.6/38.9 32.1/28.7/26.6 42250 (0.17, 0.30) 482 2TBCz-XT 2.9 47.9/38.6/31.9 50.2/33.7/24.5 33.7/27.1/22.4 20370 (0.15, 0.18) 462 Abbreviations:Von, turn-on voltage at 1 cd m−2; ηC, current efficiency; ηP, power efficiency; ηext, external quantum efficiency; Lmax, maximum luminance; CIE, Commission Internationale de I’Eclairage coordinates; λEL,EL peak. Cz-XT radiates deep blue-light with an EL peak at 442 nm, Commission Internationale de l’Eclairage (CIE) color coordinates of (0.15, 0.08), and an Lmax of 1989 cd m−2 in DPEPO host at a doping concentration of 10 wt %. The full width at half maxima value of the EL spectrum is 63 nm, similar to that of the PL spectrum (64 nm). The maximum current efficiency (ηC,max), maximum power efficiency (ηP,max), and ηext,max are 15.9 cd A−1, 13.9 lm W−1, and 22.2%, respectively. More importantly, 2TBCz-XT shows pure-blue light with an EL peak at 462 nm (CIEx,y = 0.15, 0.18) and a Lmax of 20370 cd m−2 in PPF host at a doping concentration of 20 wt %. The ηC,max, ηP,max, and ηext,max are 47.9 cd A−1, 50.2 lm W−1, and 33.7%, respectively. The device comprised of 2MCz-XT in PPF host at a doping concentration of 10 wt % displays sky-blue light with an EL peak at 482 nm (CIEx,y = 0.17, 0.30) and provides an ηext,max of 32.1% similar to that of 2TBCz-XT ( Supporting Information Tables S2–S5). In addition to the efficient RISC that ensures nearly full exciton utilization and excellent ΦPLs of 91% (2MCz-XT) and 94% (2TBCz-XT), the high horizontal orientation ratios of 78.0% and 84.0% of 2MCz-XT and 2TBCz-XT (Figure 4a,b), respectively, account for the outstanding ηext,maxs exceeding 30%.7,24,25,34,35,52,53 To the best of our knowledge, these impressive ηext,maxs demonstrate Cz-XT and 2TBCz-XT are among the currently reported state-of-the-art deep-blue and pure-blue TADF materials ( Supporting Information Table S6). Figure 4 | Measured p-polarized PL intensity of (a) 2MCz-XT and (b) 2TBCz-XT in doped films. Download figure Download PowerPoint Conclusion Common TADF molecules are designed in highly twisted D–A structures, which makes it difficult to acquire efficient blue emissions because of a strong ICT effect. To address this issue, we propose a facile and effective strategy of modulating torsion angles of D–A groups for the creation of efficient blue delayed-fluorescence materials, based on a series of eminent deep-blue and pure-blue luminescent molecules consisting of Cz donor and XT acceptor. By gradually decreasing the torsion angle, the PL peak of Cz-XT is apparently blueshifted relative to those of MCz-XT and 2MCz-XT, accompanied by increased ΔEST and elongated τdelayed. By enhancing the electron-donating ability of Cz via the introduction of tert-butyl groups, 2TBCz-XT shows a moderately redshifted PL peak, and its ΔEST and τdelayed become smaller and shorter, respectively, in comparison with Cz-XT. Although Cz-XT and 2TBCz-XT have more planar structures and larger ΔESTs, they possess strong deep-blue and pure-blue delayed fluorescence in doped films with excellent ΦPLs of 85% and 94%, respectively. The EL emissions of these molecules follows the same trend as the PL emissions, thus achieving blue high-performance OLEDs. The device using Cz-XT as emitter radiates deep-blue light with an EL peak at 442 nm (CIEx,y = 0.15, 0.08) and a high ηext,max of 22.2%. Even more efficient OLEDs are achieved by adopting 2TBCz-XT and 2MCz-XT as emitters, providing pure-blue and sky-blue light peaking at 462 nm (CIEx,y = 0.15, 0.18) and 482 nm (CIEx,y = 0.17, 0.30) with outstanding ηext,maxs of 33.7% and 32.1%, respectively. These OLEDs are among the current state-of-the-art TADF OLEDs with similar colors, which may inspire the exploration of efficient blue organic luminescent materials by means of D–A torsion angle modulation. Supporting Information Supporting Information is available and includes general information, synthesis and characterization, OLED fabrication and characterization, crystal data of Cz-XT and 2MCz-XT, thermogravimetric analysis and differential scanning calorimetry curves, cyclic voltammograms, electronic distributions, transient PL decay spectra, fluorescence and phosphorescence spectra, photophysical data, and device performance comparisons. Conflict of Interest The authors declare no conflicts of interest. Acknowledgments This study is financially supported by the National Natural Science Foundation of China (grant no. 21788102), the Natural Science Foundation of Guangdong Province (grant no. 2019B030301003), and the State Key Lab of Luminescent Materials and Devices, South China University of Technology.