Creating Concentration‐Insensitive TADF Luminogens With Spiro‐Fused Xanthone Acceptors for Highly Efficient OLEDs

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 , 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 , 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 , 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 , 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 , 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 , 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 , 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 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 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 | PL of (a) 2MCz-XT and (b) 2TBCz-XT in doped Download figure Download PowerPoint TADF molecules are in highly twisted D–A structures, which to efficient blue emissions because of a strong ICT effect. To this we a and effective strategy of modulating torsion angles of D–A groups for the of efficient blue delayed-fluorescence materials, based on a of deep-blue and pure-blue luminescent molecules consisting of Cz donor and XT By gradually decreasing the torsion the PL peak of Cz-XT is apparently blueshifted relative to those of MCz-XT and 2MCz-XT, accompanied by increased ΔEST and elongated By the electron-donating ability of Cz via the introduction of tert-butyl 2TBCz-XT shows a moderately redshifted PL and its ΔEST and τdelayed smaller and respectively, in comparison with Cz-XT. Although Cz-XT and 2TBCz-XT have more planar structures and larger they strong deep-blue and pure-blue delayed fluorescence in doped films with excellent ΦPLs of and respectively. The EL emissions of these molecules the as the PL emissions, thus achieving blue high-performance The device using Cz-XT as radiates deep-blue light with an EL peak at 442 nm (CIEx,y = 0.15, 0.08) and a high ηext,max of 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 and 32.1%, respectively. These OLEDs are among the current state-of-the-art TADF OLEDs with similar which may the of efficient blue organic luminescent materials by of D–A torsion Supporting Information Supporting Information is and and fabrication and crystal data of Cz-XT and 2MCz-XT, analysis and differential scanning calorimetry cyclic transient PL decay fluorescence and phosphorescence and device performance of The of study is by the Science of China the Science of Guangdong and the State Key of Luminescent Materials and Devices, South China University of TADF Emitter and Efficient and Doped OLEDs with Wu Chen Ma Zhao Tang OLEDs with and by a of Efficient Materials for Blue Organic Efficient in Organic Yang Efficient by for Chen Wu of Blue and and Their OLEDs with Efficient Blue Based on and from and Their to Organic for Blue Organic Efficient Blue Based on Single and Organic The Key of Chen Wu for Efficient

This work demonstrates successful large area inkjet printing of a thermally activated delayed fluorescence (TADF) material as the emitting layer of organic light-emitting diodes (OLEDs). TADF materials enable efficient light emission without relying on heavy metals such as platinum or iridium. However, low-cost manufacturing of large-scale TADF OLEDs has been restricted due to their incompatibility with solution processing techniques. In this study, we develop ink formulation for a TADF material and show successful ink jet printing of intricate patterns over a large area (6400 mm2) without the use of any lithography. The stable ink is successfully achieved using a non-chlorinated binary solvent mixture for a solution processable TADF material, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one dispersed in 4,4’-bis-(N-carbazolyl)-1,1’-biphenyl host. Using this ink, large area ink jet printed OLEDs with performance comparable to the control spin coated OLEDs are successfully achieved. In this work, we also show the impact of ink viscosity, density, and surface tension on the droplet formation and film quality as well as its potential for large-area roll-to-roll printing on a flexible substrate. The results represent a major step towards the use of TADF materials for large-area OLEDs without employing any lithography.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Recently, the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs) have been dramatically upgraded due to development of organic materials and device structure. In particular, an intramolecular or intermolecular complex based on thermally activated delayed fluorescent (TADF) materials has greatly contributed to improving OLED device performance. Although high external quantum efficiency has been the main objective of the development of TADF materials as hosts and emitters, recent interest has been directed towards the lifetime of TADF-material-based OLEDs. For the past several years, remarkable advances in the lifetime of phosphorescent and TADF OLEDs have been made using TADF materials as hosts or emitters in the emitting layer. Therefore, since TADF materials are useful as both hosts and emitters for a long lifetime, this work discusses the recent progress made in developing TADF materials for long-lifetime OLEDs.

Organic light emitting diodes (OLEDs) have been attracting much attention and have been studied extensively by scientists and engineers in both academic and industrial communities, due to their great advantages of self-emitting, fast response, wide view-angle, full color capacity, high efficiency, low power consumption, ultra-thinness, lightness and flexibility for the applications of new generation flat panel displays and solid-state lighting sources. With the full use of active-matrix OLED (AMOLED) screen in iPhone X, OLED technology has become the mainstream information display technology and has gained more than 40% of market share. In the era of information, liquid crystal display technology will be gradually replaced by OLED technology. However, OLEDs technology still faces many challenges, including low yield, high cost, short service life, environmentally unfriendly and so on, which urgently requires more in-depth and detailed research in material development and device design. Since the first OLED reported by CW Tang, OLED technology has developed from the first generation of fluorescence (FL) technology, the second generation of phosphorescence (PH) technology to the third generation of thermally activated delayed fluorescence (TADF) technology. In general, PH materials are heavy metal complexes, in which triplet excited state is the main radiative excited state. Since singlet exciton can translate into triplet exciton through intersystem crossing (ISC) process, therefore, in theory, 100% electrogenerated excitons can be used in phosphorescence materials. Compared with the first generation of fluorescence materials, which can only use 25% single state electrogenerated excitons, the electro-optic conversion efficiency is greatly improved in phosphorescence materials. However, heavy metal ions in PH materials not only increase the cost, but also bring some potential environmental pollution. TADF technology can realize the transformation of non-radiative triplet excitons into radiative singlet excitons through reverse ISC (RISC) process, and thus achieving 100% exciton utilization. TADF material is usually pure organic molecule based on a push-pull electron system, thus making up for the defects of phosphorescent material. Therefore, as a kind of molecular electroluminescent devices, innovation of material system is the foundation for the development of OLED technology. With the insulating effect, moderate electronic effect, large steric hindrance, multi-functionalizability and coordination effect, aryl phosphine (oxide) group is one of the few groups with multiple functions. Through comprehensively utilizing these functions, aryl phosphine (oxide) group reveal the unique advantage in selectively optimizing molecular optoelectronic properties, further realizing high-efficiency OLED, and providing a platform for selective investigation of the influence of single property variation on optoelectronic performance. Recently, phosphine optoelectronic materials have been gradually developed as one of the focuses in OLED field, whose excellent performance and clear structure-property relationship reveal the significant theoretical and application values regarding to enriching OLED material, suggesting the methods in material design and optimization and promoting the OLED technology innovation. Based on material design strategy, we developed phosphine host materials, emitters and electron transporting materials and deeply investigated the photophysical properties, excited-state characteristics, electrical properties and OLED performance. In this paper, the main works are summarized, with the expectation to provide reference for the subsequent study on phosphine optoelectronic materials.

Thermally activated delayed fluorescent (TADF) materials are those that have a small energy gap between the lowest singlet and triplet excited states, enabling singlet excitons through thermal upconversion of triplet excitons even at room temperature. Thus, they can achieve external electroluminescence (EL) quantum efficiency (EQE) equal to that of phosphorescence based organic light-emitting diodes (OLEDs) [1]. Considering fluorescence and phosphorescence based OLEDs as the first and second generations, respectively, TADF materials can be positioned as the third generation realizing highly efficient OLEDs. Recently, the best EQE of 31.2% in green TADF based OLEDs was demonstrated [2]. Further, EQE can be greatly improved by transition dipoles horizontally oriented to a substrate surface; such a preferential dipole orientation is achievable by controlling molecular orientation.

Macrocyclic compounds have garnered considerable attention in the realms of supramolecular chemistry and optoelectronic materials owing to their distinctive geometries, exceptional stability, and potential chiral properties. As third‐generation organic light‐emitting diodes (OLEDs) emitters, thermally activated delayed fluorescence (TADF) materials present themselves as promising contenders due to their ability to achieve complete exciton utilization through the harnessing of triplet excitons. The incorporation of macrocyclic structures with TADF characteristics holds the potential to yield TADF emitters endowed with unique attributes, thereby synergistically enhancing the performance and longevity of TADF materials for diverse device applications, ultimately propelling their commercialization. Within this comprehensive review, a thorough exploration of the molecular structures, synthesis methodologies, structure‐property relationships, and applications of the reported macrocyclic TADF systems in OLEDs are delved. Additionally, the prevailing challenges and prospective research directions are highlighted that demand attention in the realm of macrocyclic TADF materials. It is hoped that this review will provide researchers engaged in optoelectronic materials research with a lucid comprehension and inspiration concerning the intricacies of these macrocyclic TADF systems.

9 - Thermally activated delayed fluorescent materials for other applications

High‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness.

Dual-band amplified spontaneous emission (DB-ASE) can serve as an effective spectral analysis technique for the detection of specific chemical or biological molecules. In this study, thermally activated delayed fluorescence (TADF) materials were employed to improve the DB-ASE performance of the PFO: BUBD-1 system and the optoelectronic properties of the corresponding organic light-emitting diodes (OLEDs). Due to the reverse intersystem crossing (RISC) effect and Förster energy transfer (FRET) in the TADF material DPAC-PPM, the ASE threshold of blend films decreased from 12.29 ± 0.49 to 3.78 ± 0.28μJ/pulse (460 nm) and from 12.69 ± 0.51 to 3.83 ± 0.28 μJ/pulse (490 nm). Moreover, OLEDs incorporating TADF materials in the emission layer exhibited improved performance. The energy transfer analysis revealed that the singlet (both prompt and delayed) energy of TADF material can be transferred to both PFO and BUBD-1, promoting fluorescence and thereby enhancing both DB-ASE and OLED performance. The incorporation of TADF materials in the emission layer offers a promising pathway toward the development of organic semiconductor lasers.

Thermally activated delayed fluorescence (TADF) is a temperature-dependent luminescence mechanism that allows the harvesting of both singlet and triplet excitons for fluorescence emission. Consequently, TADF-active compounds exhibit excellent photophysical properties, such as high luminescence efficiencies and long excited-state lifetimes. In recent times, TADF materials have found widespread applications in material and life science, most prominently in organic light-emitting diodes (OLEDs). Many design strategies for TADF emitters have emerged, amongst which the use of transition metal complexes constitutes an appealing approach. In this chapter, the theoretical background of the TADF phenomenon is discussed, different types of metal-based TADF emitters are introduced, selected case studies are highlighted, and an overview is given of the many applications of TADF materials in light-driven systems.

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 (Adv. Mater.2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017–2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

To achieve highly‐efficient organic light‐emitting diodes (OLEDs), great efforts have been devoted into constructing thermally activated delayed fluorescence (TADF) with high horizontal dipole ratios (Θ//). Here, we proposed a design strategy by integrating a rigid electron‐accepting oxygen‐bridged boron core with triple electron‐donating groups, which exhibited a “shamrock‐shape”, namely BO‐3DMAC and BO‐3DPAC. Benefiting from the rigid and large‐planar skeletons brought by shamrock‐shaped design, BO‐3DMAC and BO‐3DPAC exhibit high Θ// of 84%/70% and 93%/94% in neat/doped films, respectively, and finally furnish excellent external quantum efficiencies (EQEs) of up to 28.3% and 38.7% in 20 wt% doped OLEDs with sky‐blue emission, as well as adequate EQEs of up to 21.0% and 16.7% in nondoped OLEDs. This work unveils a promising strategy to establish high‐Θ// TADF emitters by constructing large‐planar molecular structures using shamrock‐shaped design.

Fabricating solution-processible host material for thermally activated delayed fluorescence (TADF) emitter remains a formidable challenge for organic light-emitting diodes (OLEDs). In this work, two new host materials, namely 3CzAcPy and 9CzAcPy, are found to exhibit high triplet energy levels, high thermal stability, and excellent film morphology from a solution process. An in-depth analysis on the photophysical data and device performance reveals the isomeric effect of the host materials has a significant impact not only on the host properties, but also on the host-dopant interactions and thus the performance of the resulting solution-processed TADF OLEDs. Impressively, the new hosts are proven to be suitable for both donor-acceptor type and multi-resonance TADF emitters, achieving state-of-the-art device performance. By using the new host 9CzAcPy, solution-processed OLED based on a donor-acceptor TADF emitter of DPAC-PCN, a maximum external quantum efficiency (EQE) of 29.5% is achieved, and solution-processed narrowband OLED based on a multiple-resonance TADF emitter of BN-CP1 acquires a maximum EQE of 26.6%. These efficiencies represent the highest values among the solution-processed TADF OLEDs. This study highlights the significance of host-dopant interactions in modulating the electroluminescence performance of TADF emitters, and provides an effective design principle for solution-processible host materials.

Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔEST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (kRISC)5–20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.