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Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence

Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed... hv photonics Communication Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence 1 1 1 1 2 1 , Jie Li , Jincheng Zhang , Heqi Gong , Li Tao , Yanqing Wang and Qiang Guo * College of Optoelectronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China; lijie@cuit.edu.cn (J.L.); 3201105004@stu.cuit.edu.cn (J.Z.); 3201105022@stu.cuit.edu.cn (H.G.); taoli@cuit.edu.cn (L.T.) College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China; yanqingwang@scu.edu.cn * Correspondence: qiangguo@cuit.edu.cn Abstract: We report an efficient deep-blue organic light-emitting diode (OLED) based on a heptazine- based thermally activated delayed fluorescent (TADF) emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA). The deep-blue-emitting compound, HAP-3DPA, was designed and synthesized by combining the relatively rigid electron-accepting heptazine core with three electron-donating dipheny- lamine units. Due to the rigid molecular structure and intramolecular charge transfer characteristics, HAP-3DPA in solid state presented a high photoluminescence quantum yield of 67.0% and obvious TADF nature with a short delayed fluorescent lifetime of 1.1 s. Most importantly, an OLED incor- porating HAP-3DPA exhibited deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13), a peak luminance of 10,523 cd/m , and a rather high external quantum efficiency of 12.5% without any light out-coupling enhancement. This finding not only reports an efficient deep-blue TADF molecule, but also presents a feasible pathway to construct high-performance deep-blue emitters and devices based on the heptazine skeleton. Citation: Li, J.; Zhang, J.; Gong, H.; Keywords: organic light-emitting diode; thermally activated delayed fluorescence; heptazine; deep Tao, L.; Wang, Y.; Guo, Q. Efficient Deep-Blue Electroluminescence blue emitter; electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence. Photonics 2021, 8, 293. https://doi.org/10.3390/ 1. Introduction photonics8080293 Considerable progress in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) has triggered intensive effort to develop high- Received: 30 June 2021 performance pure organic electroluminescent (EL) materials over the past decade [1–5]. Accepted: 21 July 2021 As the third generation of organic light-emitting materials in comparison with traditional Published: 22 July 2021 fluorescent and phosphorescent materials, TADF emitters can harvest both singlet and triplet excitons without the use of noble metals, which are consequently considered as Publisher’s Note: MDPI stays neutral the promising option for next-generation OLEDs with numerous features, such as high with regard to jurisdictional claims in efficiency, metal-free, diverse molecular design, and low cost [6–8]. To get full-color displays published maps and institutional affil- or white-light OLEDs, the utilization of blue emitters is indispensable, and many kinds of iations. molecular skeletons (e.g., boron-containing, diphenylsulfone-based, triazine-pyrimidine- based) have been developed [9]. To date, a large number of blue TADF emitters have been developed, whereas most of them belong to the sky-blue region [10–17]. Hence, highly efficient deep-blue TADF emitters are still urgently required. Copyright: © 2021 by the authors. An effective separation of electron densities of the highest occupied molecular orbital Licensee MDPI, Basel, Switzerland. (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a single molecule is This article is an open access article essential for the realization of a small energy gap (DE ) between the lowest excited singlet ST distributed under the terms and state (S ) and the lowest excited triplet state (T ) [18]. In order to obtain a small DE and 1 1 ST conditions of the Creative Commons efficient TADF emitters, molecules featuring electron donor-acceptor strcutures are very Attribution (CC BY) license (https:// popular and effective. Thereinto, the heptazine core, which has a considerably planar and creativecommons.org/licenses/by/ rigid heterocyclic system of six C = N bonds surrounding a central sp -hybridised N-atom, 4.0/). Photonics 2021, 8, 293. https://doi.org/10.3390/photonics8080293 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 293 2 of 8 is an ideal strong electron acceptor [19–22]. To the best of our knowledge, several highly efficient heptazine-based red and green TADF emitters have been reported [22–25], while there are no published heptazine-based blue or deep-blue TADF emitters. In this study, we designed and synthesized an efficient heptazine-based deep-blue TADF emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA), by integrating three electron-donating diphenylamine units into the strong electron-accepting heptazine core. The photoluminescence (PL) and electroluminescence (EL) properties of HAP-3DPA were systematically investigated. On account of the pretty rigid molecular geometry and charge transfer (CT) characteristics, HAP-3DPA in solid film exhibited high thermally stability, a high photoluminescence quantum yield (PLQY) of 67%, and apparent TADF nature along with a short delay emission lifetime of 1.1 s. More importantly, an HAP-3DPA-based OLED showed deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13) and a reasonably high external quantum efficiency (EQE) of 12.5%, together with a peak luminance of 10,523 cd/m without any light out-coupling en- hancement. 2. Materials and Methods 2.1. Synthesis of 2,5,8-Tris(Diphenylamine)-Tri-s-Triazine (HAP-3DPA) Diphenylamine and extra dry solvents (xylene stored with molecular sieves) were obtained from commercial suppliers and used without further purification. A flame-dried Schlenk tube with a magnetic stir bar was charged with mixture of cyameluric chloride (1.09 mmol, 300 mg), diphenylamine (17.7 mmol, 3.3 g), and dry xylene (20 mL) under a N atmosphere. The resulting mixture was heated at 180 C for 24 h. After cooling to room temperature, the solvent was removed by vacuum distillation. The residue was purified by column chromatography on silica gel and recrystallized from ethyl acetate/petroleum ether mixtures to provide the desired product as white solid (577 mg, 78%). H NMR (400 MHz, DMSO-d ): d = 7.32 (td, J = 8.0 Hz, J = 2.0 Hz, 12H), 7.17–7.24 (m, 18H) ppm. C NMR 6 1 2 (100 MHz, DMSO-d ): d = 164.2, 155.9, 143.1, 129.2, 128.1, 126.9 ppm. High-resolution mass + + spectrometry (HRMS) (ESI ): calcd. for C H N [M + H] 675.2733, found 675.2732. 42 30 10 Elemental anal. calcd. for C H N (%): C, 74.76; H, 4.48; N, 20.76; found: C 74.72, 4.47, 42 30 10 N 20.80. 2.2. OLED Fabrication and Measurement The OLED was fabricated by vacuum thermal evaporation under a pressure lower than 5  10 Pa. An 150-nm-thick indium-tin-oxide (ITO) precoated glass substrate was used as the anode. Prior to the deposition of the organic layers and cathode, the substrate was firstly cleaned with ultra-purified water, acetone, and isopropyl alcohol (IPA) in sequence, then treated with UV-ozone for 15 min, and finally transferred to a vacuum thermal deposition system. The intersection of ITO and the metal electrodes gave an active device area of 4 mm . The OLED device was characterized under atmospheric conditions without any encapsulation or light out-coupling enhancement. The EL spetrum, EQE, and current density-voltage-luminance (J-V-L) characteristics of the OLED were recorded and measured with an Agilent E5273A semiconductor parameter analyzer and a Newport 1930C optical power meter. EL spectra were recorded using an Ocean Optics USB2000 multi-channel spectrometer. 3. Results and Discussion As depicted in Scheme 1, HAP-3DPA was synthesized by cyameluric chloride and diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key inter- mediate and was prepared according to the literature [25]. The target compound was 1 13 characterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supplementary Materials), and HRMS. Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 3. Results and Discussion As depicted in Scheme 1, HAP-3DPA was synthesized by cyameluric chloride and diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key interme- 3. Results and Discussion diate and was prepared according to the literature [25]. The target compound was char- As depicted in Scheme 11, HAP13 -3DPA was synthesized by cyameluric chloride and acterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supple- diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key interme- mentary Materials), and HRMS. diate and was prepared according to the literature [25]. The target compound was char- Photonics 2021, 8, 293 3 of 8 1 13 acterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supple- mentary Materials), and HRMS. Scheme 1. Synthetic route of HAP-3DPA. Quantum chemical calculations of HAP-3DPA were carried out based on density Scheme 1. Synthetic route of HAP-3DPA. Scheme function 1.al Synthetic theory (DFT) route of an HAP-3DP d time-depen A. dent DFT (TD-DFT) to predict the molecular con- figuration, electron cloud density distribution, and energy levels. As depicted in Figure 1 Quantum chemical calculations of HAP-3DPA were carried out based on density Quantum chemical calculations of HAP-3DPA were carried out based on density and Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine functional theory (DFT) and time-dependent DFT (TD-DFT) to predict the molecular config- functional theory (DFT) and time-dependent DFT (TD-DFT) to predict the molecular con- units and the heptazine core, respectively, which is in accordance with the electron-do- uration, electron cloud density distribution, and energy levels. As depicted in Figure 1 and figuration, electron cloud density distribution, and energy levels. As depicted in Figure 1 nating feature of DPA and electron-accepting character of heptazine core. Accordingly, Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine units and and Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine the small overlap between the HOMO and LUMO leads to a small ΔEST of 0.23 eV, which the heptazine core, respectively, which is in accordance with the electron-donating feature units and the heptazine core, respectively, which is in accordance with the electron-do- is beneficial to the realization of the TADF process. Interestingly, as compared to the pre- of DPA and electron-accepting character of heptazine core. Accordingly, the small overlap nating feature of DPA and electron-accepting character of heptazine core. Accordingly, viously reported high-performance red-emitting heptazine derivative, 4,4,4″- between the HOMO and LUMO leads to a small DE of 0.23 eV, which is beneficial to the ST the sma 1 ll overlap between the HOMO and LUMO leads to a small ΔEST of 0.23 eV, which (1,3,3a ,4,6,7,9-heptaazaphenalene-2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) realization of the TADF process. Interestingly, as compared to the previously reported high- is beneficial to the realization of the TADF process. Interestingly, as compared to the pre- (HAP-3TPA), which has a similar ΔEST of 0.27 eV and a small energy band gap between performance red-emitting heptazine derivative, 4,4,4”-(1,3,3a ,4,6,7,9-heptaazaphenalene- viously reported high-performance red-emitting heptazine derivative, 4,4,4″- HOMO and LUMO (Eg) of 2.76 eV (Figures S4 and S5), HAP-3DPA possesses a much 2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) (HAP-3TPA), which has a similar DE 1 ST (1,3,3a ,4,6,7,9-heptaazaphenalene-2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) larger Eg of 4.29 eV and a rather higher S1 energy level of 3.53 eV, indicating the significant of 0.27 eV and a small energy band gap between HOMO and LUMO (E ) of 2.76 eV (Fig- (HAP-3TPA), which has a similar ΔEST of 0.27 eV and a small energy band gap between importance of subtle structural change of electron-donating moieties on photophysical ures S4 and S5), HAP-3DPA possesses a much larger E of 4.29 eV and a rather higher S g 1 HOMO and LUMO (Eg) of 2.76 eV (Figures S4 and S5), HAP-3DPA possesses a much properties. Additionally, we found that the natural transition orbitals (NTOs) for T1 of energy level of 3.53 eV, indicating the significant importance of subtle structural change larger Eg of 4.29 eV and a rather higher S1 energy level of 3.53 eV, indicating the significant HAP-3DPA have the intramolecular charge transfer (CT) character and are related to the of electron-donating moieties on photophysical properties. Additionally, we found that importance of subtle structural change of electron-donating moieties on photophysical π→π* transitions from HOMO to LUMO (Figure S6), while the NTOs for S1 of HAP-3DPA the natural transition orbitals (NTOs) for T of HAP-3DPA have the intramolecular charge properties. Additionally, we found that the natural transition orbitals (NTOs) for T1 of (HOMO-3 to LUMO) are deriving from more localized n→π* transitions involving lone- transfer (CT) character and are related to the !* transitions from HOMO to LUMO HAP-3DPA have the intramolecular charge transfer (CT) character and are related to the pair electrons of N heteroatoms and π antibonding molecular orbitals (Figure S7). There- (Figure S6), while the NTOs for S of HAP-3DPA (HOMO-3 to LUMO) are deriving from π→π* transitions from HOMO to LUMO (Figure S6), while the NTOs for S1 of HAP-3DPA fore, it could be anticipated that the different NTOs nature of S1 and T1 can facilitate the more localized n!* transitions involving lone-pair electrons of N heteroatoms and (HOMO-3 to LUMO) are deriving from more locali 3 zed n→ 1 π* transitions involving lone- reverse intersystem crossing (RISC) process from ππ* to nπ* state according to the El- antibonding molecular orbitals (Figure S7). Therefore, it could be anticipated that the pair electrons of N heteroatoms and π antibonding molecular orbitals (Figure S7). There- Sayed rule [26]. different NTOs nature of S and T can facilitate the reverse intersystem crossing (RISC) 1 1 fore, it could be anticipated that the different NTOs nature of S1 and T1 can facilitate the 3 1 process from * to n* state according to the El-Sayed rule [26]. 3 1 reverse intersystem crossing (RISC) process from ππ* to nπ* state according to the El- Sayed rule [26]. Figure 1. Frontier molecular orbital distributions and energy levels of the lowest excited singlet and triplet states of HAP-3DPA by theoretical calculations. The thermal stability of HAP-3DPA was measured by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure 2a and Table 1, HAP- 3DPA showed a fairly high initial decomposition temperature (T with a 5 wt% loss) of 448 C and a high glass-transition temperature (T ) of 112 C, implying its excellent thermal stability and suitable for vacuum thermal evaporation process, which should be ascribed to the relatively rigid and planar molecular structure of HAP-3DPA. Furthermore, the HOMO energy level of HAP-3DPA was determined to be 6.2 eV by atmospheric ultraviolet photoelectron spectroscopy (Figure 2b). Moreover, HAP-3DPA in a neat film showed Photonics 2021, 8, x FOR PEER REVIEW 4 of 9 Figure 1. Frontier molecular orbital distributions and energy levels of the lowest excited singlet and triplet states of HAP-3DPA by theoretical calculations. The thermal stability of HAP-3DPA was measured by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure 2a and Table 1, HAP-3DPA showed a fairly high initial decomposition temperature (Td with a 5 wt% loss) of 448 °C and a high glass-transition temperature (Tg) of 112 °C, implying its excellent thermal stability and suitable for vacuum thermal evaporation process, which should be Photonics 2021, 8, 293 4 of 8 ascribed to the relatively rigid and planar molecular structure of HAP-3DPA. Further- more, the HOMO energy level of HAP-3DPA was determined to be −6.2 eV by atmospheric ultraviolet photoelectron spectroscopy (Figure 2b). Moreover, HAP-3DPA in a neat film showed deep-blue emission with a peak wavelength (λem) of 420 nm (Figure deep-blue emission with a peak wavelength (l ) of 420 nm (Figure 2c). Consequently, the em 2c). Consequently, the LUMO energy level of HAP-3DPA can be calculated to be −2.9 eV LUMO energy level of HAP-3DPA can be calculated to be 2.9 eV from the differences from the differences between the HOMO and optical Eg. Meanwhile, the PLQY and between the HOMO and optical E . Meanwhile, the PLQY and transient PL decay of transient PL decay of HAP-3DPA in a neat film were carried out. HAP-3DPA exhibited HAP-3DPA in a neat film were carried out. HAP-3DPA exhibited comparably high PLQYs comparably high PLQYs of 34.7% and 39.2% in air and N2 atomosphere, respectively, of 34.7% and 39.2% in air and N atomosphere, respectively, which should be associated which should be associated with the rather rigid molecular structure of HAP-3DPA in with the rather rigid molecular structure of HAP-3DPA in solid state. The relatively small solid state. The relatively small difference (4.5%) induced by oxygen should be ascribed difference (4.5%) induced by oxygen should be ascribed to the TADF process considering to the TADF process considering the weak delayed emission component in Figure 2d. the weak delayed emission component in Figure 2d. Figure 2. (a) The thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) Figure 2. (a) The thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of HAP-3DPA. (b) The HOMO energy level of HAP-3DPA determined by atmospheric curves of HAP-3DPA. (b) The HOMO energy level of HAP-3DPA determined by atmospheric ultraviolet photoelectron spectroscopy. (c) The PL spectrum of HAP-3DPA in a neat film. (d) The ultraviolet photoelectron spectroscopy. (c) The PL spectrum of HAP-3DPA in a neat film. (d) The transient PL decay of HAP-3DPA in a neat film. transient PL decay of HAP-3DPA in a neat film. Table 1. Thermal and photophysical properties of HAP-3DPA. Table 1. Thermal and photophysical properties of HAP-3DPA. HOMO/ λem (nm) τp (ns) τd (μs) PLQY Compound Td/Tg (° C) t (ns) l (nm) p t (s) PLQY HOMO/ em d a b a b a b a b LUMO (eV) Sol /Film Sol /Film Sol /Film Sol /Film Compound T /T ( C) d g a b a b a b a b LUMO (eV) Sol /Film Sol /Film Sol /Film Sol /Film HAP-3DPA 448/112 −6.2/−2.9 521/442 4.0/3.0 1.2/1.1 16%/67% HAP-3DPA 448/112 6.2/2.9 521/442 4.0/3.0 1.2/1.1 16%/67% a b sol: measured in oxygen-free toluene. film: measured in a 6 wt% HAP-3DPA:DPEPO doped film. The ultraviolet-visble (UV-vis) absorption and PL spectra of HAP-3DPA in diluted 4 1 toluene at a concentration of 1  10 mol L are shown in Figure 3a. The strong absorption band centered at 313 nm can be assigned to !* electronic transition with regard to the  conjugated molecular system. Meanwhile, HAP-3DPA displayed green emissions in diluted toluene with l = 521 nm and a quite low PLQY of 16% (oxygen- em free condition), indicating a large molecular geometry change of HAP-3DPA induced by toluene molecules in comparison to that in the neat film. To confirm the TADF nature, the transient PL decay of HAP-3DPA both in air-saturated and oxygen-free toluene were measured (Figure 3b). Delayed emission components could be clearly observed both in air-saturated and oxygen-free toluene although the PL intensities are weak. Obviously, as compared to the lifetime of delayed component (t = 256 ns) in air-saturated condition, d Photonics 2021, 8, 293 5 of 8 the t in oxygen-free condition was greatly enhanced to be 1.2 s. Therefore, this oxygen- sensitive delayed component should be attributed to the TADF. Meanwhile, the lifetimes of prompt emission (t ) are 4.0 ns both in air-saturated and oxygen-free toluene, showing no oxygen-dependence. To verify that the TADF occurs in HAP-3DPA in solid state, the photophysical properites of HAP-3DPA in a doped film were performed. As a famous host material for blue emitters, bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) possesses a high T level over 3.0 eV and was chosen as the host for HAP-3DPA [27], and a 6 wt% HAP-3DPA:DPEPO-doped film was fabricated and characterized. Herein, the concentration of 6 wt% was chosen based on the optimization of luminescence efficiencies at various concentrations (Table S1). The doped film showed deep-blue emission with l = 442 nm (Figure 3a), which is significantly blue shifted compared with that in toluene. em Transient PL decay of the doped film was shown in Figure 3c and Figure S8. Similar to that in toluene, the doped film exhibited strong prompt and weak delayed components, with t = 3.0 ns and t = 1.1 s. Such a short delayed fluorescence lifetime in solid p d state demonstrated that efficient RISC from T to S could occurr and efficient harvest of 1 1 triplet exctons could be expected in EL performance. Excitingly, the doped film displayed a relativley high PLQY of 67%, which is much higher than that of HAP-3DPA in a neat film or diluted toluene, suggesting efficient radiative transition of singlet excitons from S to the ground state (S ). The photophysical properties of HAP-3DPA in toluene and doped film are summarized in Table 1. To better elucidate the delayed emission, the prompt and delayed emission spectra of the 6 wt% HAP-3DPA:DPEPO doped film were characterized Photonics 2021, 8, x FOR PEER REVIEW 6 of 9 (Figure 3d). The well-overlapped PL spectra confirm that all photons were generated from the same excited state. Figure 3. (a) The UV-vis and PL spectra of HAP-3DPA in toluene (Tol) and doped film (6 wt% HAP-3DPA:DPEPO). (b) Figure 3. (a) The UV-vis and PL spectra of HAP-3DPA in toluene (Tol) and doped film (6 wt% HAP- Transient PL decay of HAP-3DPA in air-saturated (Air-satd.) and oxygen-free toluene. Inset: Transient PL decay of HAP- 3DPA:DPEPO). (b) Transient PL decay of HAP-3DPA in air-saturated (Air-satd.) and oxygen-free 3DPA in oxygen-free toluene with a time range of 60 ns. (c) Transient PL decay of the 6 wt% HAP-3DPA:DPEPO-doped toluene. Inset: Transient PL decay of HAP-3DPA in oxygen-free toluene with a time range of 60 ns. film. Inset: Transient PL decay in a time range of 60 ns. (d) Prompt and delayed PL spectra of the 6 wt% HAP- (c) Transient PL decay of the 6 wt% HAP-3DPA:DPEPO-doped film. Inset: Transient PL decay in 3DPA:DPEPO-doped film. a time range of 60 ns. (d) Prompt and delayed PL spectra of the 6 wt% HAP-3DPA:DPEPO-doped film. To evaluate EL performance of HAP-3DPA, an OLED device incorporating an To evaluate EL performance of HAP-3DPA, an OLED device incorporating an emit- emitting layer of 6 wt% HAP-3DPA:DPEPO was fabricated. The solubility of HAP-3DPA ting layer of 6 wt% HAP-3DPA:DPEPO was fabricated. The solubility of HAP-3DPA is is not enough to be applied into solution-processed OLED fabrication on the basis of high insolubility of heptazine core. Therefore, the OLED was prepared by vacuum thermal evaporation with a structure of ITO/α-NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/6 wt% HAP-3DPA:DPEPO (20 nm)/DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al (Figure 4a), where α-NPD, TCTA, CzSi and TPBI represent N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10- biphenyl-4,4′-diamine, 4,4′,4′′-tris(N-carbazolyl)triphenylamine, 9-(4-tert-butylphenyl)- 3,6-bis(triphenylsilyl)-9H-carbazole, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene, respectively. The molecular structures of organic compounds employed in the device are shown in Figure S9. Here, the OLED architecture was optimized and chosen as compared to the EL performance with a simple three-layered structure (Figure S10 and Table S2). −2 The EL spectra of this device measured at 1, 10, and 100 mA cm are well-overlapped with a maximum EL peak (λEL) of 440 nm, and in good agreement with PL spectrum of the emitting layer (Figure 4b). Meanwhile, no detectable host emission was observed, sug- gesting excellent exciton confinement in the OLED. More importantly, the OLED showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quan- tum efficiency (EQE) of 12.5%, a turn-on voltage (Von) of 4.1 V, and a peak luminance (Lmax) −2 of 10,523 cd/m without any light out-coupling enhancement (Figure 4c,d, Figure S11, and Table 2). Moreover, comparably stable CIE coordinates in the deep-blue range were ob- served with applied voltage increasing (Figure S12). The excellent EL performance of this OLED is partly attributed to the well-balanced electron and hole fluxes into the emitting zone. Meanwhile, it should be ascribed to efficient up-conversion of triplet excitons from T1 to S1 through TADF process. Photonics 2021, 8, 293 6 of 8 not enough to be applied into solution-processed OLED fabrication on the basis of high insolubility of heptazine core. Therefore, the OLED was prepared by vacuum thermal evaporation with a structure of ITO/ -NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/6 wt% HAP-3DPA:DPEPO (20 nm)/DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al (Figure 4a), 0 0 where -NPD, TCTA, CzSi and TPBI represent N,N -diphenyl-N,N -bis(1-naphthyl)-1,10- 0 0 00 biphenyl-4,4 -diamine, 4,4 ,4 -tris(N-carbazolyl)triphenylamine, 9-(4-tert-butylphenyl)- 3,6-bis(triphenylsilyl)-9H-carbazole, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene, respectively. The molecular structures of organic compounds employed in the device are shown in Figure S9. Here, the OLED architecture was optimized and chosen as compared to the EL performance with a simple three-layered structure (Figure S10 and Table S2). The EL spectra of this device measured at 1, 10, and 100 mA cm are well-overlapped with Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 a maximum EL peak (l ) of 440 nm, and in good agreement with PL spectrum of the EL emitting layer (Figure 4b). Meanwhile, no detectable host emission was observed, sug- gesting excellent exciton confinement in the OLED. More importantly, the OLED showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quantum Furthermore, it should be noted that there is a dramatic efficiency roll-off at high efficiency (EQE) of 12.5%, a turn-on voltage (V ) of 4.1 V, and a peak luminance (L ) −2 −2 on max current densities for the OLED, 12.5% at 0.01 mA cm , 12.3% at 1 mA cm , 10.0% at 10 of 10,523 cd/m without any light out-coupling enhancement (Figure 4c,d, Figure S11, −2 −2 −2 −2 mA cm , 8.0% at 100 mA cm , 5.8% at 200 mA cm , and 4.5% at 300 mA cm (Figure 4d). and Table 2). Moreover, comparably stable CIE coordinates in the deep-blue range were As shown in Figure S13, this effect can be predominantly ascribed to triplet-triplet anni- observed with applied voltage increasing (Figure S12). The excellent EL performance of this hilation (TTA) on the basis of theoretical TTA fitting [28]. In view of the short delayed OLED is partly attributed to the well-balanced electron and hole fluxes into the emitting fluorescence lifetime (1.1 μs) of HAP-3DPA, a long device operational lifetime could be zone. Meanwhile, it should be ascribed to efficient up-conversion of triplet excitons from expected and will be systematically evaluated later [29–32]. T to S through TADF process. 1 1 Figure 4. (a) The OLED structure. (b) The EL spectra at various current densities. (c) The current efficiency-voltage- Figure 4. (a) The OLED structure. (b) The EL spectra at various current densities. (c) The current luminance (J-V-L) characteristics. (d) EQE as a function of current density. efficiency-voltage-luminance (J-V-L) characteristics. (d) EQE as a function of current density. Table 2. Summary of the OLED performance based on HAP-3DPA. Table 2. Summary of the OLED performance based on HAP-3DPA. a −2 Emitter Von (V) λEL (nm) Lmax (cd/m ) EQE (%) CIE (x, y) a 2 Emitter V (V) l (nm) L (cd/m ) EQE (%) CIE (x, y) on EL max HAP-3DPA 4.1 440 10,523 12.5 0.16, 0.13 HAP-3DPA 4.1 440 10,523 12.5 0.16, 0.13 a −2 Turn-on voltage at 1 cd/m . a 2 Turn-on voltage at 1 cd/m . 4. Conclusions In summary, we designed, synthesized, and characterized an efficient heptazine- based deep-blue TADF emitter, HAP-3DPA, which has an electron-accepting heptazine core and three electron-donating diphenylamine units. Deep-blue-emitting HAP-3DPA in solid state presented good thermal stability, a high PLQY, and obvious TADF nature with a short delayed emission lifetime. Most importantly, an OLED containing HAP-3DPA showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external −2 quantum efficiency (EQE) of 12.5% and a peak luminance of 10,523 cd/m without any light out-coupling enhancement. This study does not merely provide a highly efficient deep-blue TADF emitter, but rather offers a feasible pathway to construct high-perfor- mance deep-blue light-emitting materials and devices based on heptazine derivatives. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, instru- 1 13 mentation; copies of H and C NMR spectra of HAP-3DPA (Figures S1 and S2); quantum chemical calculations (Figures S3–S7); photophysical characteristics (Tables S1 and S2, Figures S8-S13). Photonics 2021, 8, 293 7 of 8 Furthermore, it should be noted that there is a dramatic efficiency roll-off at high 2 2 current densities for the OLED, 12.5% at 0.01 mA cm , 12.3% at 1 mA cm , 10.0% at 2 2 2 2 10 mA cm , 8.0% at 100 mA cm , 5.8% at 200 mA cm , and 4.5% at 300 mA cm (Figure 4d). As shown in Figure S13, this effect can be predominantly ascribed to triplet- triplet annihilation (TTA) on the basis of theoretical TTA fitting [28]. In view of the short delayed fluorescence lifetime (1.1 s) of HAP-3DPA, a long device operational lifetime could be expected and will be systematically evaluated later [29–32]. 4. Conclusions In summary, we designed, synthesized, and characterized an efficient heptazine-based deep-blue TADF emitter, HAP-3DPA, which has an electron-accepting heptazine core and three electron-donating diphenylamine units. Deep-blue-emitting HAP-3DPA in solid state presented good thermal stability, a high PLQY, and obvious TADF nature with a short delayed emission lifetime. Most importantly, an OLED containing HAP-3DPA showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quantum efficiency (EQE) of 12.5% and a peak luminance of 10,523 cd/m without any light out- coupling enhancement. This study does not merely provide a highly efficient deep-blue TADF emitter, but rather offers a feasible pathway to construct high-performance deep-blue light-emitting materials and devices based on heptazine derivatives. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 1 13 10.3390/photonics8080293/s1, instrumentation; copies of H and C NMR spectra of HAP-3DPA (Figures S1 and S2); quantum chemical calculations (Figures S3–S7); photophysical characteristics (Tables S1 and S2, Figures S8–S13). Author Contributions: Conceptualization, J.L. and Q.G.; methodology, J.Z. and H.G.; software, J.Z. and H.G.; validation, J.L. and Q.G.; formal analysis, J.L. and Q.G.; investigation, J.L. and Q.G.; resources, L.T. and Y.W.; data curation, L.T. and Y.W.; writing—original draft preparation, J.L.; writing—review and editing, Q.G.; visualization, J.L. and Q.G.; supervision, L.T. and Y.W.; project administration, J.L. and Q.G.; funding acquisition, J.L. and Q.G. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (grant numbers: 61505015, 21801028, 11704050), Department of Science and Technology of Sichuan Province (grant numbers: 2019YJ0358, 2017FZ0085, 2020YFG0038, 2020YFH0104), Fundamental Research Funds for the Central Universities (grant number: YJ201952) and Department of Human Resources and Social Security of Sichuan Province (grant number: 2019Z226). 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Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence

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hv photonics Communication Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence 1 1 1 1 2 1 , Jie Li , Jincheng Zhang , Heqi Gong , Li Tao , Yanqing Wang and Qiang Guo * College of Optoelectronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China; lijie@cuit.edu.cn (J.L.); 3201105004@stu.cuit.edu.cn (J.Z.); 3201105022@stu.cuit.edu.cn (H.G.); taoli@cuit.edu.cn (L.T.) College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China; yanqingwang@scu.edu.cn * Correspondence: qiangguo@cuit.edu.cn Abstract: We report an efficient deep-blue organic light-emitting diode (OLED) based on a heptazine- based thermally activated delayed fluorescent (TADF) emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA). The deep-blue-emitting compound, HAP-3DPA, was designed and synthesized by combining the relatively rigid electron-accepting heptazine core with three electron-donating dipheny- lamine units. Due to the rigid molecular structure and intramolecular charge transfer characteristics, HAP-3DPA in solid state presented a high photoluminescence quantum yield of 67.0% and obvious TADF nature with a short delayed fluorescent lifetime of 1.1 s. Most importantly, an OLED incor- porating HAP-3DPA exhibited deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13), a peak luminance of 10,523 cd/m , and a rather high external quantum efficiency of 12.5% without any light out-coupling enhancement. This finding not only reports an efficient deep-blue TADF molecule, but also presents a feasible pathway to construct high-performance deep-blue emitters and devices based on the heptazine skeleton. Citation: Li, J.; Zhang, J.; Gong, H.; Keywords: organic light-emitting diode; thermally activated delayed fluorescence; heptazine; deep Tao, L.; Wang, Y.; Guo, Q. Efficient Deep-Blue Electroluminescence blue emitter; electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence. Photonics 2021, 8, 293. https://doi.org/10.3390/ 1. Introduction photonics8080293 Considerable progress in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) has triggered intensive effort to develop high- Received: 30 June 2021 performance pure organic electroluminescent (EL) materials over the past decade [1–5]. Accepted: 21 July 2021 As the third generation of organic light-emitting materials in comparison with traditional Published: 22 July 2021 fluorescent and phosphorescent materials, TADF emitters can harvest both singlet and triplet excitons without the use of noble metals, which are consequently considered as Publisher’s Note: MDPI stays neutral the promising option for next-generation OLEDs with numerous features, such as high with regard to jurisdictional claims in efficiency, metal-free, diverse molecular design, and low cost [6–8]. To get full-color displays published maps and institutional affil- or white-light OLEDs, the utilization of blue emitters is indispensable, and many kinds of iations. molecular skeletons (e.g., boron-containing, diphenylsulfone-based, triazine-pyrimidine- based) have been developed [9]. To date, a large number of blue TADF emitters have been developed, whereas most of them belong to the sky-blue region [10–17]. Hence, highly efficient deep-blue TADF emitters are still urgently required. Copyright: © 2021 by the authors. An effective separation of electron densities of the highest occupied molecular orbital Licensee MDPI, Basel, Switzerland. (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a single molecule is This article is an open access article essential for the realization of a small energy gap (DE ) between the lowest excited singlet ST distributed under the terms and state (S ) and the lowest excited triplet state (T ) [18]. In order to obtain a small DE and 1 1 ST conditions of the Creative Commons efficient TADF emitters, molecules featuring electron donor-acceptor strcutures are very Attribution (CC BY) license (https:// popular and effective. Thereinto, the heptazine core, which has a considerably planar and creativecommons.org/licenses/by/ rigid heterocyclic system of six C = N bonds surrounding a central sp -hybridised N-atom, 4.0/). Photonics 2021, 8, 293. https://doi.org/10.3390/photonics8080293 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 293 2 of 8 is an ideal strong electron acceptor [19–22]. To the best of our knowledge, several highly efficient heptazine-based red and green TADF emitters have been reported [22–25], while there are no published heptazine-based blue or deep-blue TADF emitters. In this study, we designed and synthesized an efficient heptazine-based deep-blue TADF emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA), by integrating three electron-donating diphenylamine units into the strong electron-accepting heptazine core. The photoluminescence (PL) and electroluminescence (EL) properties of HAP-3DPA were systematically investigated. On account of the pretty rigid molecular geometry and charge transfer (CT) characteristics, HAP-3DPA in solid film exhibited high thermally stability, a high photoluminescence quantum yield (PLQY) of 67%, and apparent TADF nature along with a short delay emission lifetime of 1.1 s. More importantly, an HAP-3DPA-based OLED showed deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13) and a reasonably high external quantum efficiency (EQE) of 12.5%, together with a peak luminance of 10,523 cd/m without any light out-coupling en- hancement. 2. Materials and Methods 2.1. Synthesis of 2,5,8-Tris(Diphenylamine)-Tri-s-Triazine (HAP-3DPA) Diphenylamine and extra dry solvents (xylene stored with molecular sieves) were obtained from commercial suppliers and used without further purification. A flame-dried Schlenk tube with a magnetic stir bar was charged with mixture of cyameluric chloride (1.09 mmol, 300 mg), diphenylamine (17.7 mmol, 3.3 g), and dry xylene (20 mL) under a N atmosphere. The resulting mixture was heated at 180 C for 24 h. After cooling to room temperature, the solvent was removed by vacuum distillation. The residue was purified by column chromatography on silica gel and recrystallized from ethyl acetate/petroleum ether mixtures to provide the desired product as white solid (577 mg, 78%). H NMR (400 MHz, DMSO-d ): d = 7.32 (td, J = 8.0 Hz, J = 2.0 Hz, 12H), 7.17–7.24 (m, 18H) ppm. C NMR 6 1 2 (100 MHz, DMSO-d ): d = 164.2, 155.9, 143.1, 129.2, 128.1, 126.9 ppm. High-resolution mass + + spectrometry (HRMS) (ESI ): calcd. for C H N [M + H] 675.2733, found 675.2732. 42 30 10 Elemental anal. calcd. for C H N (%): C, 74.76; H, 4.48; N, 20.76; found: C 74.72, 4.47, 42 30 10 N 20.80. 2.2. OLED Fabrication and Measurement The OLED was fabricated by vacuum thermal evaporation under a pressure lower than 5  10 Pa. An 150-nm-thick indium-tin-oxide (ITO) precoated glass substrate was used as the anode. Prior to the deposition of the organic layers and cathode, the substrate was firstly cleaned with ultra-purified water, acetone, and isopropyl alcohol (IPA) in sequence, then treated with UV-ozone for 15 min, and finally transferred to a vacuum thermal deposition system. The intersection of ITO and the metal electrodes gave an active device area of 4 mm . The OLED device was characterized under atmospheric conditions without any encapsulation or light out-coupling enhancement. The EL spetrum, EQE, and current density-voltage-luminance (J-V-L) characteristics of the OLED were recorded and measured with an Agilent E5273A semiconductor parameter analyzer and a Newport 1930C optical power meter. EL spectra were recorded using an Ocean Optics USB2000 multi-channel spectrometer. 3. Results and Discussion As depicted in Scheme 1, HAP-3DPA was synthesized by cyameluric chloride and diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key inter- mediate and was prepared according to the literature [25]. The target compound was 1 13 characterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supplementary Materials), and HRMS. Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 3. Results and Discussion As depicted in Scheme 1, HAP-3DPA was synthesized by cyameluric chloride and diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key interme- 3. Results and Discussion diate and was prepared according to the literature [25]. The target compound was char- As depicted in Scheme 11, HAP13 -3DPA was synthesized by cyameluric chloride and acterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supple- diphenylamine in a good yield of 78%. Thereinto, cyameluric chloride is the key interme- mentary Materials), and HRMS. diate and was prepared according to the literature [25]. The target compound was char- Photonics 2021, 8, 293 3 of 8 1 13 acterized and confirmed via H and C NMR spectroscopy (Figures S1 and S2 in Supple- mentary Materials), and HRMS. Scheme 1. Synthetic route of HAP-3DPA. Quantum chemical calculations of HAP-3DPA were carried out based on density Scheme 1. Synthetic route of HAP-3DPA. Scheme function 1.al Synthetic theory (DFT) route of an HAP-3DP d time-depen A. dent DFT (TD-DFT) to predict the molecular con- figuration, electron cloud density distribution, and energy levels. As depicted in Figure 1 Quantum chemical calculations of HAP-3DPA were carried out based on density Quantum chemical calculations of HAP-3DPA were carried out based on density and Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine functional theory (DFT) and time-dependent DFT (TD-DFT) to predict the molecular config- functional theory (DFT) and time-dependent DFT (TD-DFT) to predict the molecular con- units and the heptazine core, respectively, which is in accordance with the electron-do- uration, electron cloud density distribution, and energy levels. As depicted in Figure 1 and figuration, electron cloud density distribution, and energy levels. As depicted in Figure 1 nating feature of DPA and electron-accepting character of heptazine core. Accordingly, Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine units and and Figure S3, the HOMO and LUMO are mainly distributed over the diphenylamine the small overlap between the HOMO and LUMO leads to a small ΔEST of 0.23 eV, which the heptazine core, respectively, which is in accordance with the electron-donating feature units and the heptazine core, respectively, which is in accordance with the electron-do- is beneficial to the realization of the TADF process. Interestingly, as compared to the pre- of DPA and electron-accepting character of heptazine core. Accordingly, the small overlap nating feature of DPA and electron-accepting character of heptazine core. Accordingly, viously reported high-performance red-emitting heptazine derivative, 4,4,4″- between the HOMO and LUMO leads to a small DE of 0.23 eV, which is beneficial to the ST the sma 1 ll overlap between the HOMO and LUMO leads to a small ΔEST of 0.23 eV, which (1,3,3a ,4,6,7,9-heptaazaphenalene-2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) realization of the TADF process. Interestingly, as compared to the previously reported high- is beneficial to the realization of the TADF process. Interestingly, as compared to the pre- (HAP-3TPA), which has a similar ΔEST of 0.27 eV and a small energy band gap between performance red-emitting heptazine derivative, 4,4,4”-(1,3,3a ,4,6,7,9-heptaazaphenalene- viously reported high-performance red-emitting heptazine derivative, 4,4,4″- HOMO and LUMO (Eg) of 2.76 eV (Figures S4 and S5), HAP-3DPA possesses a much 2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) (HAP-3TPA), which has a similar DE 1 ST (1,3,3a ,4,6,7,9-heptaazaphenalene-2,5,8-triyl)tris(N,N-bis(4-(tert-butyl)phenyl)aniline) larger Eg of 4.29 eV and a rather higher S1 energy level of 3.53 eV, indicating the significant of 0.27 eV and a small energy band gap between HOMO and LUMO (E ) of 2.76 eV (Fig- (HAP-3TPA), which has a similar ΔEST of 0.27 eV and a small energy band gap between importance of subtle structural change of electron-donating moieties on photophysical ures S4 and S5), HAP-3DPA possesses a much larger E of 4.29 eV and a rather higher S g 1 HOMO and LUMO (Eg) of 2.76 eV (Figures S4 and S5), HAP-3DPA possesses a much properties. Additionally, we found that the natural transition orbitals (NTOs) for T1 of energy level of 3.53 eV, indicating the significant importance of subtle structural change larger Eg of 4.29 eV and a rather higher S1 energy level of 3.53 eV, indicating the significant HAP-3DPA have the intramolecular charge transfer (CT) character and are related to the of electron-donating moieties on photophysical properties. Additionally, we found that importance of subtle structural change of electron-donating moieties on photophysical π→π* transitions from HOMO to LUMO (Figure S6), while the NTOs for S1 of HAP-3DPA the natural transition orbitals (NTOs) for T of HAP-3DPA have the intramolecular charge properties. Additionally, we found that the natural transition orbitals (NTOs) for T1 of (HOMO-3 to LUMO) are deriving from more localized n→π* transitions involving lone- transfer (CT) character and are related to the !* transitions from HOMO to LUMO HAP-3DPA have the intramolecular charge transfer (CT) character and are related to the pair electrons of N heteroatoms and π antibonding molecular orbitals (Figure S7). There- (Figure S6), while the NTOs for S of HAP-3DPA (HOMO-3 to LUMO) are deriving from π→π* transitions from HOMO to LUMO (Figure S6), while the NTOs for S1 of HAP-3DPA fore, it could be anticipated that the different NTOs nature of S1 and T1 can facilitate the more localized n!* transitions involving lone-pair electrons of N heteroatoms and (HOMO-3 to LUMO) are deriving from more locali 3 zed n→ 1 π* transitions involving lone- reverse intersystem crossing (RISC) process from ππ* to nπ* state according to the El- antibonding molecular orbitals (Figure S7). Therefore, it could be anticipated that the pair electrons of N heteroatoms and π antibonding molecular orbitals (Figure S7). There- Sayed rule [26]. different NTOs nature of S and T can facilitate the reverse intersystem crossing (RISC) 1 1 fore, it could be anticipated that the different NTOs nature of S1 and T1 can facilitate the 3 1 process from * to n* state according to the El-Sayed rule [26]. 3 1 reverse intersystem crossing (RISC) process from ππ* to nπ* state according to the El- Sayed rule [26]. Figure 1. Frontier molecular orbital distributions and energy levels of the lowest excited singlet and triplet states of HAP-3DPA by theoretical calculations. The thermal stability of HAP-3DPA was measured by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure 2a and Table 1, HAP- 3DPA showed a fairly high initial decomposition temperature (T with a 5 wt% loss) of 448 C and a high glass-transition temperature (T ) of 112 C, implying its excellent thermal stability and suitable for vacuum thermal evaporation process, which should be ascribed to the relatively rigid and planar molecular structure of HAP-3DPA. Furthermore, the HOMO energy level of HAP-3DPA was determined to be 6.2 eV by atmospheric ultraviolet photoelectron spectroscopy (Figure 2b). Moreover, HAP-3DPA in a neat film showed Photonics 2021, 8, x FOR PEER REVIEW 4 of 9 Figure 1. Frontier molecular orbital distributions and energy levels of the lowest excited singlet and triplet states of HAP-3DPA by theoretical calculations. The thermal stability of HAP-3DPA was measured by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure 2a and Table 1, HAP-3DPA showed a fairly high initial decomposition temperature (Td with a 5 wt% loss) of 448 °C and a high glass-transition temperature (Tg) of 112 °C, implying its excellent thermal stability and suitable for vacuum thermal evaporation process, which should be Photonics 2021, 8, 293 4 of 8 ascribed to the relatively rigid and planar molecular structure of HAP-3DPA. Further- more, the HOMO energy level of HAP-3DPA was determined to be −6.2 eV by atmospheric ultraviolet photoelectron spectroscopy (Figure 2b). Moreover, HAP-3DPA in a neat film showed deep-blue emission with a peak wavelength (λem) of 420 nm (Figure deep-blue emission with a peak wavelength (l ) of 420 nm (Figure 2c). Consequently, the em 2c). Consequently, the LUMO energy level of HAP-3DPA can be calculated to be −2.9 eV LUMO energy level of HAP-3DPA can be calculated to be 2.9 eV from the differences from the differences between the HOMO and optical Eg. Meanwhile, the PLQY and between the HOMO and optical E . Meanwhile, the PLQY and transient PL decay of transient PL decay of HAP-3DPA in a neat film were carried out. HAP-3DPA exhibited HAP-3DPA in a neat film were carried out. HAP-3DPA exhibited comparably high PLQYs comparably high PLQYs of 34.7% and 39.2% in air and N2 atomosphere, respectively, of 34.7% and 39.2% in air and N atomosphere, respectively, which should be associated which should be associated with the rather rigid molecular structure of HAP-3DPA in with the rather rigid molecular structure of HAP-3DPA in solid state. The relatively small solid state. The relatively small difference (4.5%) induced by oxygen should be ascribed difference (4.5%) induced by oxygen should be ascribed to the TADF process considering to the TADF process considering the weak delayed emission component in Figure 2d. the weak delayed emission component in Figure 2d. Figure 2. (a) The thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) Figure 2. (a) The thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of HAP-3DPA. (b) The HOMO energy level of HAP-3DPA determined by atmospheric curves of HAP-3DPA. (b) The HOMO energy level of HAP-3DPA determined by atmospheric ultraviolet photoelectron spectroscopy. (c) The PL spectrum of HAP-3DPA in a neat film. (d) The ultraviolet photoelectron spectroscopy. (c) The PL spectrum of HAP-3DPA in a neat film. (d) The transient PL decay of HAP-3DPA in a neat film. transient PL decay of HAP-3DPA in a neat film. Table 1. Thermal and photophysical properties of HAP-3DPA. Table 1. Thermal and photophysical properties of HAP-3DPA. HOMO/ λem (nm) τp (ns) τd (μs) PLQY Compound Td/Tg (° C) t (ns) l (nm) p t (s) PLQY HOMO/ em d a b a b a b a b LUMO (eV) Sol /Film Sol /Film Sol /Film Sol /Film Compound T /T ( C) d g a b a b a b a b LUMO (eV) Sol /Film Sol /Film Sol /Film Sol /Film HAP-3DPA 448/112 −6.2/−2.9 521/442 4.0/3.0 1.2/1.1 16%/67% HAP-3DPA 448/112 6.2/2.9 521/442 4.0/3.0 1.2/1.1 16%/67% a b sol: measured in oxygen-free toluene. film: measured in a 6 wt% HAP-3DPA:DPEPO doped film. The ultraviolet-visble (UV-vis) absorption and PL spectra of HAP-3DPA in diluted 4 1 toluene at a concentration of 1  10 mol L are shown in Figure 3a. The strong absorption band centered at 313 nm can be assigned to !* electronic transition with regard to the  conjugated molecular system. Meanwhile, HAP-3DPA displayed green emissions in diluted toluene with l = 521 nm and a quite low PLQY of 16% (oxygen- em free condition), indicating a large molecular geometry change of HAP-3DPA induced by toluene molecules in comparison to that in the neat film. To confirm the TADF nature, the transient PL decay of HAP-3DPA both in air-saturated and oxygen-free toluene were measured (Figure 3b). Delayed emission components could be clearly observed both in air-saturated and oxygen-free toluene although the PL intensities are weak. Obviously, as compared to the lifetime of delayed component (t = 256 ns) in air-saturated condition, d Photonics 2021, 8, 293 5 of 8 the t in oxygen-free condition was greatly enhanced to be 1.2 s. Therefore, this oxygen- sensitive delayed component should be attributed to the TADF. Meanwhile, the lifetimes of prompt emission (t ) are 4.0 ns both in air-saturated and oxygen-free toluene, showing no oxygen-dependence. To verify that the TADF occurs in HAP-3DPA in solid state, the photophysical properites of HAP-3DPA in a doped film were performed. As a famous host material for blue emitters, bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) possesses a high T level over 3.0 eV and was chosen as the host for HAP-3DPA [27], and a 6 wt% HAP-3DPA:DPEPO-doped film was fabricated and characterized. Herein, the concentration of 6 wt% was chosen based on the optimization of luminescence efficiencies at various concentrations (Table S1). The doped film showed deep-blue emission with l = 442 nm (Figure 3a), which is significantly blue shifted compared with that in toluene. em Transient PL decay of the doped film was shown in Figure 3c and Figure S8. Similar to that in toluene, the doped film exhibited strong prompt and weak delayed components, with t = 3.0 ns and t = 1.1 s. Such a short delayed fluorescence lifetime in solid p d state demonstrated that efficient RISC from T to S could occurr and efficient harvest of 1 1 triplet exctons could be expected in EL performance. Excitingly, the doped film displayed a relativley high PLQY of 67%, which is much higher than that of HAP-3DPA in a neat film or diluted toluene, suggesting efficient radiative transition of singlet excitons from S to the ground state (S ). The photophysical properties of HAP-3DPA in toluene and doped film are summarized in Table 1. To better elucidate the delayed emission, the prompt and delayed emission spectra of the 6 wt% HAP-3DPA:DPEPO doped film were characterized Photonics 2021, 8, x FOR PEER REVIEW 6 of 9 (Figure 3d). The well-overlapped PL spectra confirm that all photons were generated from the same excited state. Figure 3. (a) The UV-vis and PL spectra of HAP-3DPA in toluene (Tol) and doped film (6 wt% HAP-3DPA:DPEPO). (b) Figure 3. (a) The UV-vis and PL spectra of HAP-3DPA in toluene (Tol) and doped film (6 wt% HAP- Transient PL decay of HAP-3DPA in air-saturated (Air-satd.) and oxygen-free toluene. Inset: Transient PL decay of HAP- 3DPA:DPEPO). (b) Transient PL decay of HAP-3DPA in air-saturated (Air-satd.) and oxygen-free 3DPA in oxygen-free toluene with a time range of 60 ns. (c) Transient PL decay of the 6 wt% HAP-3DPA:DPEPO-doped toluene. Inset: Transient PL decay of HAP-3DPA in oxygen-free toluene with a time range of 60 ns. film. Inset: Transient PL decay in a time range of 60 ns. (d) Prompt and delayed PL spectra of the 6 wt% HAP- (c) Transient PL decay of the 6 wt% HAP-3DPA:DPEPO-doped film. Inset: Transient PL decay in 3DPA:DPEPO-doped film. a time range of 60 ns. (d) Prompt and delayed PL spectra of the 6 wt% HAP-3DPA:DPEPO-doped film. To evaluate EL performance of HAP-3DPA, an OLED device incorporating an To evaluate EL performance of HAP-3DPA, an OLED device incorporating an emit- emitting layer of 6 wt% HAP-3DPA:DPEPO was fabricated. The solubility of HAP-3DPA ting layer of 6 wt% HAP-3DPA:DPEPO was fabricated. The solubility of HAP-3DPA is is not enough to be applied into solution-processed OLED fabrication on the basis of high insolubility of heptazine core. Therefore, the OLED was prepared by vacuum thermal evaporation with a structure of ITO/α-NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/6 wt% HAP-3DPA:DPEPO (20 nm)/DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al (Figure 4a), where α-NPD, TCTA, CzSi and TPBI represent N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10- biphenyl-4,4′-diamine, 4,4′,4′′-tris(N-carbazolyl)triphenylamine, 9-(4-tert-butylphenyl)- 3,6-bis(triphenylsilyl)-9H-carbazole, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene, respectively. The molecular structures of organic compounds employed in the device are shown in Figure S9. Here, the OLED architecture was optimized and chosen as compared to the EL performance with a simple three-layered structure (Figure S10 and Table S2). −2 The EL spectra of this device measured at 1, 10, and 100 mA cm are well-overlapped with a maximum EL peak (λEL) of 440 nm, and in good agreement with PL spectrum of the emitting layer (Figure 4b). Meanwhile, no detectable host emission was observed, sug- gesting excellent exciton confinement in the OLED. More importantly, the OLED showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quan- tum efficiency (EQE) of 12.5%, a turn-on voltage (Von) of 4.1 V, and a peak luminance (Lmax) −2 of 10,523 cd/m without any light out-coupling enhancement (Figure 4c,d, Figure S11, and Table 2). Moreover, comparably stable CIE coordinates in the deep-blue range were ob- served with applied voltage increasing (Figure S12). The excellent EL performance of this OLED is partly attributed to the well-balanced electron and hole fluxes into the emitting zone. Meanwhile, it should be ascribed to efficient up-conversion of triplet excitons from T1 to S1 through TADF process. Photonics 2021, 8, 293 6 of 8 not enough to be applied into solution-processed OLED fabrication on the basis of high insolubility of heptazine core. Therefore, the OLED was prepared by vacuum thermal evaporation with a structure of ITO/ -NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/6 wt% HAP-3DPA:DPEPO (20 nm)/DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al (Figure 4a), 0 0 where -NPD, TCTA, CzSi and TPBI represent N,N -diphenyl-N,N -bis(1-naphthyl)-1,10- 0 0 00 biphenyl-4,4 -diamine, 4,4 ,4 -tris(N-carbazolyl)triphenylamine, 9-(4-tert-butylphenyl)- 3,6-bis(triphenylsilyl)-9H-carbazole, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene, respectively. The molecular structures of organic compounds employed in the device are shown in Figure S9. Here, the OLED architecture was optimized and chosen as compared to the EL performance with a simple three-layered structure (Figure S10 and Table S2). The EL spectra of this device measured at 1, 10, and 100 mA cm are well-overlapped with Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 a maximum EL peak (l ) of 440 nm, and in good agreement with PL spectrum of the EL emitting layer (Figure 4b). Meanwhile, no detectable host emission was observed, sug- gesting excellent exciton confinement in the OLED. More importantly, the OLED showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quantum Furthermore, it should be noted that there is a dramatic efficiency roll-off at high efficiency (EQE) of 12.5%, a turn-on voltage (V ) of 4.1 V, and a peak luminance (L ) −2 −2 on max current densities for the OLED, 12.5% at 0.01 mA cm , 12.3% at 1 mA cm , 10.0% at 10 of 10,523 cd/m without any light out-coupling enhancement (Figure 4c,d, Figure S11, −2 −2 −2 −2 mA cm , 8.0% at 100 mA cm , 5.8% at 200 mA cm , and 4.5% at 300 mA cm (Figure 4d). and Table 2). Moreover, comparably stable CIE coordinates in the deep-blue range were As shown in Figure S13, this effect can be predominantly ascribed to triplet-triplet anni- observed with applied voltage increasing (Figure S12). The excellent EL performance of this hilation (TTA) on the basis of theoretical TTA fitting [28]. In view of the short delayed OLED is partly attributed to the well-balanced electron and hole fluxes into the emitting fluorescence lifetime (1.1 μs) of HAP-3DPA, a long device operational lifetime could be zone. Meanwhile, it should be ascribed to efficient up-conversion of triplet excitons from expected and will be systematically evaluated later [29–32]. T to S through TADF process. 1 1 Figure 4. (a) The OLED structure. (b) The EL spectra at various current densities. (c) The current efficiency-voltage- Figure 4. (a) The OLED structure. (b) The EL spectra at various current densities. (c) The current luminance (J-V-L) characteristics. (d) EQE as a function of current density. efficiency-voltage-luminance (J-V-L) characteristics. (d) EQE as a function of current density. Table 2. Summary of the OLED performance based on HAP-3DPA. Table 2. Summary of the OLED performance based on HAP-3DPA. a −2 Emitter Von (V) λEL (nm) Lmax (cd/m ) EQE (%) CIE (x, y) a 2 Emitter V (V) l (nm) L (cd/m ) EQE (%) CIE (x, y) on EL max HAP-3DPA 4.1 440 10,523 12.5 0.16, 0.13 HAP-3DPA 4.1 440 10,523 12.5 0.16, 0.13 a −2 Turn-on voltage at 1 cd/m . a 2 Turn-on voltage at 1 cd/m . 4. Conclusions In summary, we designed, synthesized, and characterized an efficient heptazine- based deep-blue TADF emitter, HAP-3DPA, which has an electron-accepting heptazine core and three electron-donating diphenylamine units. Deep-blue-emitting HAP-3DPA in solid state presented good thermal stability, a high PLQY, and obvious TADF nature with a short delayed emission lifetime. Most importantly, an OLED containing HAP-3DPA showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external −2 quantum efficiency (EQE) of 12.5% and a peak luminance of 10,523 cd/m without any light out-coupling enhancement. This study does not merely provide a highly efficient deep-blue TADF emitter, but rather offers a feasible pathway to construct high-perfor- mance deep-blue light-emitting materials and devices based on heptazine derivatives. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, instru- 1 13 mentation; copies of H and C NMR spectra of HAP-3DPA (Figures S1 and S2); quantum chemical calculations (Figures S3–S7); photophysical characteristics (Tables S1 and S2, Figures S8-S13). Photonics 2021, 8, 293 7 of 8 Furthermore, it should be noted that there is a dramatic efficiency roll-off at high 2 2 current densities for the OLED, 12.5% at 0.01 mA cm , 12.3% at 1 mA cm , 10.0% at 2 2 2 2 10 mA cm , 8.0% at 100 mA cm , 5.8% at 200 mA cm , and 4.5% at 300 mA cm (Figure 4d). As shown in Figure S13, this effect can be predominantly ascribed to triplet- triplet annihilation (TTA) on the basis of theoretical TTA fitting [28]. In view of the short delayed fluorescence lifetime (1.1 s) of HAP-3DPA, a long device operational lifetime could be expected and will be systematically evaluated later [29–32]. 4. Conclusions In summary, we designed, synthesized, and characterized an efficient heptazine-based deep-blue TADF emitter, HAP-3DPA, which has an electron-accepting heptazine core and three electron-donating diphenylamine units. Deep-blue-emitting HAP-3DPA in solid state presented good thermal stability, a high PLQY, and obvious TADF nature with a short delayed emission lifetime. Most importantly, an OLED containing HAP-3DPA showed deep-blue emission with CIE coordinates of (0.16, 0.13) and a rather high external quantum efficiency (EQE) of 12.5% and a peak luminance of 10,523 cd/m without any light out- coupling enhancement. This study does not merely provide a highly efficient deep-blue TADF emitter, but rather offers a feasible pathway to construct high-performance deep-blue light-emitting materials and devices based on heptazine derivatives. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 1 13 10.3390/photonics8080293/s1, instrumentation; copies of H and C NMR spectra of HAP-3DPA (Figures S1 and S2); quantum chemical calculations (Figures S3–S7); photophysical characteristics (Tables S1 and S2, Figures S8–S13). Author Contributions: Conceptualization, J.L. and Q.G.; methodology, J.Z. and H.G.; software, J.Z. and H.G.; validation, J.L. and Q.G.; formal analysis, J.L. and Q.G.; investigation, J.L. and Q.G.; resources, L.T. and Y.W.; data curation, L.T. and Y.W.; writing—original draft preparation, J.L.; writing—review and editing, Q.G.; visualization, J.L. and Q.G.; supervision, L.T. and Y.W.; project administration, J.L. and Q.G.; funding acquisition, J.L. and Q.G. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (grant numbers: 61505015, 21801028, 11704050), Department of Science and Technology of Sichuan Province (grant numbers: 2019YJ0358, 2017FZ0085, 2020YFG0038, 2020YFH0104), Fundamental Research Funds for the Central Universities (grant number: YJ201952) and Department of Human Resources and Social Security of Sichuan Province (grant number: 2019Z226). 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Jul 22, 2021

Keywords: organic light-emitting diode; thermally activated delayed fluorescence; heptazine; deep blue emitter; electroluminescence

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