oxadiazole derivatives

oxadiazole derivatives

Organic Electronics 69 (2019) 77–84 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel N...

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Organic Electronics 69 (2019) 77–84

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Nondoped deep-blue fluorescent organic electroluminescent device with CIEy = 0.06 and low efficiency roll-off based on carbazole/oxadiazole derivatives


Yu Tan, Zhende Wang, Chen Wei, Zhiwei Liu∗, Zuqiang Bian, Chunhui Huang Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China



Keywords: Deep-blue OLED Nondoped EBU Carbazole/oxadiazole derivatives

Deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de L'e'clairage (CIE) coordinates matching the European Broadcasting Union (EBU) standard blue CIE (x, y) coordinates of (0.15, 0.06) are very limited and in enormous demands. Here, we designed and synthesized four blue-emitting compounds with D-A structures based on carbazole and oxadiazole units. After the systematic studies of their photophysical, thermal and electrochemical properties, it is found that their photoluminescent properties were tuned successfully via changing the electronic structures of Ar group connecting with the oxadiazole and they all have high photoluminescent quantum yields (PLQYs). Then we selected the outstanding one as the emitter to prepare nondoped deep-blue OLEDs. The devices have maximum brightness up to 4406 cd m−2 and high maximum external quantum efficiency (EQE) up to 4.0% along with very small efficiency roll-off. Furthermore, the CIE coordinates of (0.16, 0.06) almost reaches the EBU standard blue. The comprehensive performance of the device is competitive among the best reported nondoped deep-blue fluorescent OLEDs with EBU standard blue CIE coordinates.

1. Introduction Organic light-emitting diodes (OLEDs) have achieved tremendous development in the last decades and even been called as the new generation of display and lighting technologies [1–9]. But some issues still need to be resolved, among which the significant one is the lack of deep-blue emitting materials [10] especially with the Commission Internationale de L'e'clairage (CIE) coordinates matching the National Television System Committee (NTSC) standard blue with CIE coordinates of (0.14, 0.08) or further the European Broadcasting Union (EBU) one of (0.15, 0.06) [11–19]. As a matter of fact, deep-blue emitters play very important roles in displays and lightings. They can not only cut down power consumptions and extend color gamut, but also manage the color temperature and generate other visible emissions [13,17,19–22]. Recently, some deep-blue emitters have been developed with special luminous mechanisms such as thermally activated delayed fluorescence (TADF) [5,23–25], hybridized local and charge transfer (HLCT) [12,19,26,27] and triplet-triplet annihilation (TTA) [28–30] etc. Although many of the OLEDs based on them achieved high efficiency, but the number of ones with CIEy below 0.10 is still very

limited, letting alone that close to 0.08 or 0.06. Moreover, most of these OLEDs adopted doped device structures which needs hosts with a wider energy gap. Although this doped method can indeed improve the performance of OLEDs, it not only increases the difficulties in the rational match of host-dopant systems, but also raises the costs of device fabrication and easily leads to the phase separation problem [10,31–34]. In comparison, nondoped devices can meet the requirements of simplifying device structures and reducing preparation costs. But as the pure emitting layer, an emitting material had better be capable of balancing the charge injection and transport in the nondoped device. To achieve this, a “D-A” type structure is usually designed for an emitting molecule, in which “D” is electronic donor while “A” is electronic acceptor [19,20,27,35–37]. But another factor need to be considered, that is the D and A groups must be selected deliberately to realize an energy gap as wide as possible for a satisfying low CIEy [19,24]. In addition, a certain degree of conjugacy and twist of the molecular structure also need to be equipped to ensure high photoluminescent quantum yields (PLQYs) in neat film state [13,20,28,37–39]. Carbazole unit is widely used to construct blue or deep-blue emission compounds because of its deep highest occupied molecular orbital (HOMO) energy level, high

Corresponding author. E-mail address: [email protected] (Z. Liu).

https://doi.org/10.1016/j.orgel.2019.02.028 Received 25 November 2018; Received in revised form 26 January 2019; Accepted 24 February 2019 Available online 04 March 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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lowest unoccupied molecular orbitals (LUMOs) values, we can find that the LUMOs energy levels gradually become deeper for Cz-Oxa-Ph, CzOxa-Py and Cz-Oxa-Pz, respectively. Analyzing the reason, it should be responsible for this tendency that the more introduction of “N” heteroatom reduces the electron cloud density of Ar group [46]. The eventual presentation is the lower energy gaps and red-shifted PL spectra for these three compounds, which is consistent with the experimental results. As for Cz-Oxa-BPh and Cz-Oxa-Ph, their LUMOs values are close to each other, which could explain their close energy gaps and emission spectra as mentioned above. In their optimized geometry structures, all of them have a certain degree of twist between carbazole and the adjacent phenyl with similar dihedral angles (about 54 °C), which suggests that they are potentially suitable to serve as efficient emitters in neat film states. We also measured their PLQYs in dilute solution, as Table 1 shows. They all have very high PLQYs in solution. Next, we investigated their photoluminescent properties in neat films. Fig. 1b shows the PL spectra of these films. In comparison, the emissions of Cz-Oxa-Ph and Cz-OxaBPh films locate in the bluest region with PL peaks below 420 nm. Moreover, they both have PLQYs above 80%, indicating that they may act well as nondoped deep-blue emitters in OLEDs. Furthermore, to explore whether these compounds have TADF characteristics, we measured the transient PL decay spectra of the CzOxa series films in short and long time range, as Fig. S1 (see Supporting Information) shows. Obviously, four compounds all decay without longlifetime components with microsecond magnitude, but show short lifetime with nanosecond one (Table S1). Further, in order to evaluate their energy level differences (ΔEST) between singlet state and triplet state, we measured the fluorescence spectra at room temperature and phosphorescence spectra at 77 K in 2-MeTHF (∼10−5 mol L−1), as Fig. S2 shows, in which the phosphorescence spectra were achieved after being delayed by 5 ms. From the highest energy peaks of phosphorescence spectra, the energy levels of triplet (ET1) can be estimated to be 2.66 eV, 2.66 eV, 2.61 eV and 2.52 eV for Cz-Oxa-Ph, Cz-Oxa-Py, CzOxa-Pz and Cz-Oxa-BPh respectively. Then, their ΔEST can be calculated, which are 0.62 eV, 0.56 eV, 0.50 eV and 0.73 eV for Cz-Oxa-Ph, Cz-Oxa-Py, Cz-Oxa-Pz and Cz-Oxa-BPh respectively. Their ΔEST are large that the reverse intersystem crossing (RISC) process could not be realized effectively. These evidences indicate that the Cz-Oxa series compounds are probably not TADF emitters. These experimental data are summarized in Table S1.

Scheme 1. Synthetic routes and molecular structures of Cz-Oxa-Ph, Cz-OxaPy, Cz-Oxa-Pz and Cz-Oxa-BPh.

luminous efficiency, good hole transporting property and stability [40,41]. Oxadiazole unit possesses outstanding electron transporting property and is frequently utilized to build electron transporting layer materials or bipolar fluorescent compounds [23,42–44]. Hence, we used a simple molecular constructing strategy to design a series of D-A structure blue-emitting compounds, which contain carbazole as electronic donors while oxadiazole as acceptors. Their molecular structures and synthetic routes are showed in Scheme 1. They all have high PLQYs. Based on one emitter, we prepared nondoped deepblue OLEDs. The device shows high maximum external quantum efficiency (EQE) up to 4.0% along with very small efficiency roll-off. Moreover, the CIE coordinates of (0.16, 0.06) are very close to the EBU standard blue. 2. Results and discussion 2.1. Photophysical properties Firstly, the photophysical properties in solution (CH2Cl2) of the four compounds were studied, as Fig. 1a shows. They demonstrated similar absorption bands shapes, where the absorption peaks at about 340 nm should be assigned to the charge transfer transitions between carbazole and central electron-withdrawing oxadiazole [18,45,46]. From the onset of absorption spectra, the energy gaps (Eg) were estimated to be 3.28 eV, 3.22 eV, 3.11 eV and 3.25eV for Cz-Oxa-Ph, Cz-Oxa-Py, CzOxa-Pz and Cz-Oxa-BPh respectively. They all showed blue emission in solution with the maximum photoluminescent (PL) peaks at 420 nm, 434 nm, 498 nm and 422 nm respectively in the above order (Fig. 1a). The key photophysical data is listed in Table 1. It is found that for the three compounds (Cz-Oxa-Ph, Cz-Oxa-Py and Cz-Oxa-Pz), the energy gaps were decreasing and the emission spectra were red-shifted with the “N” heteroatom sequentially introduced into the Ar group. Moreover, the Eg and emission of Cz-Oxa-BPh are close to that of Cz-OxaPh, indicating that a more phenyl didn't have an obvious influence on these properties as the “N” heteroatom worked. Hence, we used density functional theory (DFT) calculations to understand these behaviors in depth, as Fig. 2 and Table 1 shows. Their HOMOs distributions are almost the same, concentrating on carbazole and the adjacent phenyl, so the differences between their HOMOs values are small. Comparing their

2.2. Thermal properties Good thermal stability plays a positive role in fabricating device with vacuum evaporation method and enhancing the long-term operation of device, including high sublimation/decomposition temperature (Ts/Td) and glass transition temperature (Tg). The thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the thermal properties of the four compounds, the results showed in Fig. 3 and Table 1. They have good thermal stability

Fig. 1. (a) UV-vis absorption spectra in CH2Cl2 and (b) PL spectra in neat films of the Cz-Oxa series compounds. 78

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Table 1 The key photophysical, thermal and electrochemical properties of Cz-Oxa series compounds. Compd.

λabsa [nm]

λPLb [nm]

λPLc [nm]

PLQYd [%]

Tse/Tg [°C]

Egf [eV]


Cz-Oxa-Ph Cz-Oxa-Py Cz-Oxa-Pz Cz-Oxa-BPh

283, 341 286/341 290, 341 293, 341

420 434 498 422

413 432 461 417

87/98 96/69 89/64 98/84

341/63 336/73 338/81 394/88

3.28 3.22 3.11 3.25

−5.73/-2.03 −5.76/-2.19 −5.79/-2.48 −5.72/-2.10

a b c d e f g

The maximum absorption wavelength, measured in CH2Cl2 (1 × 10−5 mol L−1). The fluorescent emission peak in CH2Cl2 (1 × 10−5 mol L−1) at room temperature. The fluorescent emission peak in neat film. Photoluminescence quantum yields in CH2Cl2 and neat film respectively. Corresponding to 5% weight loss. Optical gaps calculated from the absorption onset. Theoretical values, based on DFT calculations at the B3LYP/6-311G** levels.

(Fig. 3a), while the Tg are less than 100 °C (Fig. 3b), among which CzOxa-BPh has the highest Tg (88 °C) likely due to its heaviest molecular weight. Considering the joule heat generated in an operational device, a high Tg is necessary to stabilize film morphology. Hence, Cz-Oxa-BPh is more suitable to be employed as an emitter in OLEDs in terms of thermal properties.

blocking layer because of its low-lying HOMO level. Comparing two devices, the hole transporting layers adopted different configurations. In device 1, NPB (N, N′-di-1-naphthyl-N, N′-diphenylbenzidine) was used as hole transporting layer and mCP (1, 3-bis (N-carbazolyl) benzene) as exciton-blocking layer. In device 2, the hole transporting layer was changed into a 20 nm-thickness mCP doped with MoO3 (20 wt%) followed by a pure mCP layer, which is intended further to enhance hole injection [49]. The device performance is showed in Fig. 5 and Table 2. The two devices showed deep-blue emission with EL peaks at 423 nm (Fig. 5b). The EL spectra are similar to the PL ones, indicating that the emissions were from only Cz-Oxa-BPh rather than other layers or any exciplex formed. From Fig. 5c, we see that both devices have high maximum brightness with 2683 cd m−2 and 4406 cd m−2 respectively. Moreover, both devices have good EQE with 3.1% and 4.0% for device 1 and device 2 respectively, as Fig. 5d shows. But in comparison, the EQE roll-off of device 1 is severer than that of device 2. For device 2, the EQE can still retain 3.8% and 3.1% at 100 cd m−2 and 1000 cd m−2 respectively. Therefore, the performance of device 2 is more excellent than that of device 1, which can be attributed to the balanced charge injection and transport in the device after the optimization of the hole transporting layers. In addition, device 2 has the CIE coordinates of (0.16, 0.06) at a high brightness (3000 cd m−2), which is actually very close to the EBU standard blue. Comparing with the reported nodoped deep-blue fluorescent OLEDs with EBU emission (see Table 3), the comprehensive performance of our device is competitive among the best ones regardless of the EQE or the efficiency rolloff. The brilliant performance of the nondoped device is inseparable from the emitting material itself, which indicates that our molecular design strategy is successful in obtaining a good deep-blue emitter. Moreover, we adopted two hosts (DPPOC [50] and CBP, their molecular structures as Fig. S6 shows) to prepare doped devices with the same structure of device 2. The comparison of device performance for nondoped and doped OLEDs is shown in Fig. S7 and Table S3. We can

2.3. Electrochemical properties We used cyclic voltammetry to study their electrochemical properties, the results as Fig. 4 shows. Their oxidation processes present considerable reversibility, indicating they have good electrochemical properties and are capable of accomplishing well the functions of charge carriers in the operation of devices. From the oxidation potentials, the HOMOs values can be evaluated [47] and then the LUMOs values obtained according to the calculated HOMOs and the energy gaps deduced from the absorption onsets. 2.4. Electroluminescent properties After the study of aforementioned properties of the four compounds, Cz-Oxa-BPh stands out as a deep-blue emitter on the whole. So, we used Cz-Oxa-BPh as pure emitting material to fabricate nondoped OLEDs with three-layers structures below: ITO/MoO3 (2 nm)/NPB (30 nm)/mCP(10 nm)/Cz-Oxa-BPh (20 nm)/TPBi(40 nm)/LiF (1 nm)/ Al (100 nm) (named as device 1) and ITO/MoO3 (2 nm)/mCP: MoO3 (20 wt%, 20 nm)/mCP(20 nm)/Cz-Oxa-BPh (20 nm)/TPBi(40 nm)/LiF (1 nm)/Al (100 nm) (named as device 2). The energy-level diagram of these materials used in nondoped OLEDs is showed in Fig. 5a, in which ITO (indium tin oxide) and Al were used as anode and cathode respectively. 2 nm-thickness MoO3 was used to modify the work function of ITO to enhance hole injection [20,48] and LiF was used as the electron injecting layer. TPBi (1, 3, 5-tris (1-phenyl-1H-benzimidazol-2yl) benzene) was used as electron transporting layer as well as hole

Fig. 2. DFT-calculated results of the Cz-Oxa series compounds, including the optimized geometry structures and HOMOs-LUMOs distributions. 79

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Fig. 3. (a) TGA and (b) DSC curves of the Cz-Oxa series compounds.

confinement in the doped devices. Moreover, the choice of host materials also has great influence on the device performance. So there are still substantial room to further improve the performance of doped devices from some respects such as the optimization of functional transporting layer or host materials.

3. Conclusion In this work, we designed and synthesized four blue-emitting compounds with D-A structures based on carbazole and oxadiazole. Through changing the electronic structures of Ar group connecting with the oxadiazole, the photophysical properties including the emission spectra and PLQYs of these materials were successfully tuned. Nondoped deep-blue OLEDs were fabricated based on one emitter. The device has a maximum brightness up to 4406 cd m−2 and high maximum EQE up to 4.0% along with very small efficiency roll-off. Moreover, the CIE coordinates of (0.16, 0.06) are extremely close to the EBU standard blue. These results are competitive with the best nondoped deep-blue fluorescent OLEDs with EBU CIE coordinates. We think this work will have a positive effect on exploring new deep-blue emitters which can be applied to nondoped OLEDs.

Fig. 4. Cyclic voltammetry curves of the Cz-Oxa series compounds.

see that the performance parameters of doped devices are inferior to that of nondoped one. Analyzing the reason, the device structure is optimal for nondoped device rather than for doped ones, which led to imbalanced injection and transporting of carriers and poor exciton

Fig. 5. (a) Energy-level diagram of the materials used in nondoped OLEDs, (b) EL spectra at 1000 cd m−2, (c) luminance-voltage-current density (L-V-J) curves of the devices and (d) power efficiency-luminance-external quantum efficiency (PE-L-EQE) curves of the devices. 80

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Table 2 Key device performance of nondoped deep-blue OLEDs based on Cz-Oxa-BPh. Device

Vona [V]

λELb [nm]

Lmaxc [cd m−2]

CEmaxd [cd A−1]

PEmaxe [lm W−1]

EQEf [%]

device 1 device 2

4.0 3.3

423 423

2683 4406

1.3 1.4

1.0 1.3

3.1/2.0/1.4 4.0/3.8/3.1

a b c d e f

At 1 cd m−2 At 1000 cd m−2 Maximum brightness. Maximum current efficiency. Maximum power efficiency. Maximum EQE, the values at 100 cd m−2 and 1000 cd m−2respectively.

4. Experimental section

temperature, the reaction mixture was poured into 1 L ice water and then acidified with dilute hydrochloric acid to get white precipitate. After filtration, the filter cake was washed with water for 2–3 times and then dried in vacuum chamber. After recrystallization (absolute ethanol), the colorless needle-like crystals were obtained as the final product (10.6 g, yield: 85%). 1H NMR (400 MHz, DMSO‑d6, δ): 8.358.33 (d, 1H), 8.29-8.27 (d, 2H), 7.93-7.91 (d, 2H), 7.53-7.46 (q, 4H),7.35-7.31 (t, 2H), 4.36 (m, 1H). MS: m/z: [MH+] = 312.1.

4.1. General information All chemicals were received from commercial resources. 1H NMR spectra were recorded on a Bruker-400 M NMR spectrometer using CDCl3 or DMSO as the solvent. 13C NMR spectra were recorded on a Bruker-500 M NMR spectrometer using CDCl3 as the solvent. Mass spectra were measured on a Bruker Apex IV FTMS. Elemental analyses were performed on a VARIO EL analyzer (GmbH, Hanau, Germany).

4.2.3. 2-(4-(9H-carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (CzOxa-Ph) 2 (2.49 g, 8 mmol) was firstly dissolved in super-dry pyridine (20 mL). Then benzoyl chloride (1.23 g, 8.8 mmol) was added dropwise into the above solution under nitrogen protection. The mixture was refluxed for 2 h. After cooling to room temperature, the reaction mixture was poured into 200 mL water and then extracted with dichloromethane to get an orange solution. The solution was dried by anhydrous sodium sulfate and then the solvent was removed in reduced pressure. A pale yellow solid was obtained (2.61 g) and then further purified by thermal gradient sublimation (200 °C-150 °C-100 °C) at ultralow pressure to get colorless crystals (1.60 g, yield: 51%). 1H NMR (400 MHz, CDCl3, δ): 8.38 (dd, 2H), 8.24-8.11 (m, 4H), 7.78 (dd, 2H), 7.61-7.40 (m, 7H), 7.33 (t, 2H). 13C NMR (126 MHz, CDCl3, δ): 164.82, 164.04, 140.97, 140.35, 131.87, 129.17, 128.60, 127.28, 127.03, 126.25, 123.92, 123.86, 122.56, 120.63, 120.50, 109.74. MS: m/z: [MH+] = 388.1. EA: C:80.67%, H:4.53%, N:10.75% (theoretical value: C, 80.60%; H, 4.42%; N, 10.85%).

4.2. Synthesis 4.2.1. 4-(9H-carbazol-9-yl)benzonitrile (1) Carbazole (8.37 g, 50 mmol), 4-fluorobenzonitrile (6.06 g, 50 mmol) and anhydrous potassium carbonate (20.7 g, 150 mmol) were added into in DMSO (50 mL). The mixture was stirred at 140 °C for 12 h under a nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into 1 L water and a lot of gray solids were obtained and then followed by vacuum drying. The crude product was purified by silica gel chromatography (dichloromethane: petroleum ether = 1:3) and then recrystallization (ethanol: dichloromethane = 1:1). The colorless needle-like crystals were obtained as the final product (5.36 g, yield: 40%). 1H NMR (400 MHz, CDCl3, δ): 8.15-8.13 (d, 2H), 7.91-7.88 (d, 2H), 7.75-7.72 (q, 2H), 7.47-7.41 (m, 4H), 7.35-7.31 (m, 2H). MS: m/z: [MH+] = 269.1. 4.2.2. 9-(4-(1H-tetrazol-5-yl)phenyl)-9H-carbazole (2) 1 (10.7 g, 40 mmol), sodium azide (2.86 g, 44 mmol) and ammonium chloride (2.35 g, 44 mmol) were dissolved in DMF (100 mL). The mixture was stirred at 100 °C for 24 h. After cooling to room

4.2.4. 2-(4-(9H-carbazol-9-yl)phenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (Cz-Oxa-Py) Picolinic acid (1.49 g, 12 mmol) and N, N′- dicyclohexyl

Table 3 Key performance parameters of the recently reported nondoped deep-blue OLEDs with CIE coordinate close to EBU standard. Compound

Von [V]

EQEa [%]

λEL [nm]

CIE (x, y)



3.3 4.8 3.5 3.2 3.2 4.1 3.6 3.0 3.0 3.5 4.0 4.75 3.6 3.6 2.0 – 5.8

4.0/3.8/3.1 4.56b 1.97/−/− 3.63/3.61/3.36 4.2/-/2.96 5.80/−/− 6.11/-/1.30 5.4/−/− 4.95/−/− 4.21/4.20/3.19 3.69/−/− – 3.03/−/− 2.31/−/− 0.72/−/− 3.02/−/− –

423 434 – 428 424 – 440 – 436 426 436 420 424 428 424 428 428

0.16, 0.06 0.15, 0.06 0.16, 0.06 0.16, 0.06 0.16, 0.06 0.153, 0.067 0.155, 0.058 0.15, 0.06 0.16, 0.06 0.157, 0.055 0.16, 0.06 0.166, 0.064 0.162, 0.057 0.165, 0.068 0.15, 0.06 0.16, 0.056 0.16, 0.06

This work [51] [52] [53] [54] [18] [55] [56] [15] [57] [58] [59] [26] [26] [60] [61] [62]

a b

EQE corresponding to the maximum value, the value at brightness of 100 cd m−2 and 1000 cd m−2, respectively. At 300 cd m−2. 81

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instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 10 °C min−1 from 25 to 560 °C. Differential scanning calorimetry was performed on a Q100DSC instrument unit at a heating rate of 10 °C min−1 from 20 to 250 °C under nitrogen. The glass transition temperature was determined from the second heating scan.

carbodiimide (2.49 g, 12 mmol) were dissolved in toluene (25 mL) and the mixture was stirred for 0.5 h. Then 2 (2.49 g, 8 mmol) was added rapidly into the solution and the mixture reacted at 90 °C for 24 h under nitrogen protection. After cooling to room temperature, the reaction mixture was further cooled in the freezing chamber until white precipitate was obtained. Filtered, the filtrate was retained and then the residual toluene was removed by distillation to afford black oily matter, which was subsequently purified by silica gel chromatography (ethyl acetate: petroleum ether = 3:2) to afford light brown solids (0.93 g, yield: 20%). The product further underwent recrystallization (absolute ethanol) and thermal gradient sublimation (200 °C-150 °C-100 °C) at ultralow pressure and colorless crystals were obtained finally. 1H NMR (400 MHz, CDCl3, δ): 8.86-8.85(d, 1H), 8.49-8.46(d, 2H), 8.39-8.36 (d, 1H), 8.17-8.15(d, 2H), 7.97-7.92(t, 1H), 7.81-7.79(d, 2H), 7.53-7.50(t, 3H), 7.47-7.43(t, 2H), 7.35-7.31(t, 2H). 13C NMR (126 MHz, CDCl3, δ): 165.02, 164.08, 150.39, 143.65, 141.26, 140.30, 137.24, 128.98, 127.19, 126.26, 125.89, 123.87, 123.35, 122.19, 120.64, 120.49, 109.74. MS: m/z: [MH+] = 389.1. EA: C: 77.52%, H: 4.27%, N: 14.51% (theoretical value: C, 77.30%; H, 4.15%; N, 14.42%).

4.5. Cyclic voltammetry measurements Cyclic voltammetry was carried out in degassed CH2Cl2 solution (10−3 M) at room temperature with a CHI600C voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was as the supporting electrolyte and ferrocene as an external standard. The conventional three-electrode configuration included a platinum working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl wire pseudo-reference electrode. The scan rate is = 100 mV s−1. 4.6. Theoretical calculations The optimized molecular structures of ground state (S0) and HOMOs-LUMOs distributions were calculated through DFT at the B3LYP/6-311G** levels using a Gaussian suite of programs (Gaussian 09, Revision A.01).

4.2.5. 2-(4-(9H-carbazol-9-yl)phenyl)-5-(pyrazin-2-yl)-1,3,4-oxadiazole (Cz-Oxa-Pz) The synthetic process of Cz-Oxa-Pz was similar to that of Cz-OxaPy, the difference was just replacing picolinic acid with pyrazine-2carboxylic acid. Finally, yellow solids were obtained (2.34 g, yield: 50%). The product was also further purified through recrystallization and thermal gradient sublimation (200 °C-150 °C-100 °C). 1H NMR (400 MHz, CDCl3, δ): 9.59(m, 1H), 8.80(m, 2H), 8.48-8.46(d, 2H), 8.178.15(d, 2H), 7.83-7.81(d, 2H), 7.53-7.51(d, 2H), 7.48-7.43(t, 2H), 7.367.32(t, 2H). 13C NMR (126 MHz, CDCl3, δ): 165.41, 162.24, 146.50, 144.63, 144.35, 141.57, 140.21, 139.69, 129.05, 127.25, 126.27, 123.89, 121.73, 120.71, 120.50, 109.70. MS: m/z: [MH+] = 390.1. EA: C: 73.62%, H: 3.80%, N: 17.93% (theoretical value: C, 74.02%; H, 3.88%; N, 17.98%).

4.7. OLED fabrication and measurements The ITO-coated glass, MoO3, electron transporting material TPBi, hole transporting materials NPB and mCP, electron injection material LiF and cathode Al were available commercially. ITO anode has a sheet resistance of 17–18 Ω square−1 and 80-nm thickness. The ITO substrate was cleaned with detergent, deionized water, acetone and ethanol before loading into a deposition chamber. The organic and metal layers materials were evaporated in different vacuum chambers with a base pressure lower than 8 × 10−5 Pa. The devices were capsulated in a glove box under a nitrogen atmosphere. All electrical testing and optical measurements were performed under ambient conditions. The EL spectra, current density-voltage-luminance (J-V-L) and EQE characteristics were measured by computer controlled Keithley 2400 source meter and absolute EQE measurement system (C9920-12, Hamamatsu Photonics) with the photonic multichannel analyzer (PMA-12, Hamamatsu Photonics).

4.2.6. 2-(4-(9H-carbazol-9-yl)phenyl)-5-([1,1′-biphenyl]-4-yl)-1,3,4oxadiazole (Cz-Oxa-BPh) 2 (1.86 g, 6 mmol) was firstly dissolved in super-dry pyridine (60 mL). Then 4-biphenylcarbonyl chloride (1.32 g, 6.06 mmol) was added dropwise into the above solution under nitrogen protection. The mixture was refluxed for 2 h. After cooling to room temperature, the reaction mixture was poured into 200 mL water and white precipitates then were separated out. After filtration, the filter cake was washed with water for 2–3 times and then dried in vacuum chamber. The crude product was purified by silica gel chromatography (dichloromethane) to afford colorless solids (2.23 g, yield: 80%), which also finally underwent thermal gradient sublimation (245 °C-195 °C-100 °C) at ultralow pressure. 1H NMR (400 MHz, CDCl3 δ): 8.41 (d, 2H), 8.25 (d, 2H), 8.16 (d, 2H), 7.79 (d, 4H), 7.67 (d, 2H), 7.56-7.39 (m, 7H), 7.33 (t, 2H). 13C NMR (126 MHz, CDCl3, δ): 164.73, 164.04, 144.66, 140.97, 140.35, 139.82, 129.04, 128.61, 128.26, 127.79, 127.47, 127.28, 127.19, 126.26, 123.87, 122.63, 122.57, 120.63, 120.51, 109.75. MS: m/z: [MH+] = 464.2. EA: C: 83.07%, H: 4.50%, N: 9.12% (theoretical value: C, 82.92%; H, 4.57%; N, 9.07%).

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