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
A R T I C LE I N FO
A B S T R A C T
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  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]
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.
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
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 . 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
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
Table 1 The key photophysical, thermal and electrochemical properties of Cz-Oxa series compounds. Compd.
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 . 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  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  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
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
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
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
Table 2 Key device performance of nondoped deep-blue OLEDs based on Cz-Oxa-BPh. Device
Lmaxc [cd m−2]
CEmaxd [cd A−1]
PEmaxe [lm W−1]
device 1 device 2
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
CIE (x, y)
Cz-Oxa-BPh AFpTPI PTPA m-BBTPI An-1 pBFCz-26DPPM MBAn-(4)-F POA 3nPI-BP-4PI CzS1 BDPP CzB-PIM DPACFPPI DPACTPI PATPA M2 TPA-(3)-F
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                
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
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
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%).
Acknowledgments We gratefully acknowledge ﬁnancial support from the National Key R&D Program of China (No. 2016YFB0401001). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.02.028. References  C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913–915.  Y. Im, S.Y. Byun, J.H. Kim, D.R. Lee, C.S. Oh, K.S. Yook, J.Y. Lee, Recent progress in high-eﬃciency blue-light-emitting materials for organic light-emitting diodes, Adv. Funct. Mater. 27 (2017) 1603007.  J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Light-emitting diodes based on conjugated polymers, Nature 347 (1990) 539–541.  C. Adachi, Third-generation organic electroluminescence materials, Jpn. J. Appl. Phys. 53 (2014) 060101.  M.Y. Wong, E. Zysman-Colman, Purely organic thermally activated delayed ﬂuorescence materials for organic light-emitting diodes, Adv. Mater. 29 (2017) 1605444.  M.A. Baldo, D.F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Highly eﬃcient phosphorescent emission from organic
4.3. Photophysical measurements UV-vis absorption spectra were measured on a Shimadzu UV-3100 spectrometer. The ﬂuorescence spectra were measured on an Edinburgh Analytical Instruments FLS980 spectrophotometer. PLQYs were measured on C9920-02 absolute quantum yield measurement system (Hamamatsu Company). 4.4. Thermal property measurements Thermogravimetric analysis was measured with a Q600SDT 82
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
(2015) 3892–3901.  G.Y. Zhong, Z. Xu, J. He, S.T. Zhang, Y.Q. Zhan, X.J. Wang, Z.H. Xiong, H.Z. Shi, X.M. Ding, W. Huang, Aggregation and permeation of 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq, Appl. Phys. Lett. 81 (2002) 1122–1124.  X. Yang, X. Xu, G. Zhou, Recent advances of the emitters for high performance deepblue organic light-emitting diodes, J. Mater. Chem. C 3 (2015) 913–944.  A.L. Fisher, K.E. Linton, K.T. Kamtekar, C. Pearson, M.R. Bryce, M.C. Petty, Eﬃcient deep-blue electroluminescence from an ambipolar ﬂuorescent emitter in a singleactive-layer device, Chem. Mater. 23 (2011) 1640–1642.  S.T. Zhang, L. Yao, Q.M. Peng, W.J. Li, Y.Y. Pan, R. Xiao, Y. Gao, C. Gu, Z.M. Wang, P. Lu, F. Li, S.J. Su, B. Yang, Y.G. Ma, Achieving a signiﬁcantly increased eﬃciency in nondoped pure blue ﬂuorescent OLED: a quasi-equivalent hybridized excited state, Adv. Funct. Mater. 25 (2015) 1755–1762.  S.J. Woo, Y. Kim, M.J. Kim, J.Y. Baek, S.K. Kwon, Y.H. Kim, J.J. Kim, Strategies for the molecular design of donor-acceptor-type ﬂuorescent emitters for eﬃcient deep blue organic light emitting diodes, Chem. Mater. 30 (2018) 857–863.  X.Y. Tang, Q. Bai, T. Shan, J.Y. Li, Y. Gao, F.T. Liu, H. Liu, Q.M. Peng, B. Yang, F. Li, P. Lu, Eﬃcient nondoped blue ﬂuorescent organic light-emitting diodes (OLEDs) with a high external quantum eﬃciency of 9.4% @ 1000 cd m(-2) based on phenanthroimidazole-anthracene derivative, Adv. Funct. Mater. 28 (2018) 1705813.  S.F. Xue, X. Qiu, S. Ying, Y.S. Lu, Y.Y. Pan, Q.K. Sun, C. Gu, W.J. Yang, Highly eﬃcient nondoped near-ultraviolet electroluminescence with an external quantum eﬃciency greater than 6.5% based on a carbazole-triazole hybrid molecule with high and balanced charge mobility, Adv. Opt. Mater. 5 (2017) 1700747.  K. Wang, W. Liu, C.J. Zheng, Y.Z. Shi, K. Liang, M. Zhang, X.M. Ou, X.H. Zhang, A comparative study of carbazole-based thermally activated delayed ﬂuorescence emitters with diﬀerent steric hindrance, J. Mater. Chem. C 5 (2017) 4797–4803.  B. Wex, B.R. Kaafarani, Perspective on carbazole-based organic compounds as emitters and hosts in TADF applications, J. Mater. Chem. C 5 (2017) 8622–8653.  N. Tamoto, C. Adachi, K. Nagai, Electroluminescence of 1,3,4-oxadiazole and triphenylamine-containing molecules as an emitter in organic multilayer light emitting diodes, Chem. Mater. 9 (1997) 1077–1085.  A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Electron transport materials for organic light-emitting diodes, Chem. Mater. 16 (2004) 4556–4573.  Y. Tao, W. Qiang, C. Yang, Z. Cheng, Z. Kai, J. Qin, D. Ma, Tuning the optoelectronic properties of carbazole/oxadiazole hybrids through linkage modes: hosts for highly eﬃcient green electrophosphorescence, Adv. Funct. Mater. 20 (2010) 304–311.  P. Rajamalli, N. Senthilkumar, P. Gandeepan, P.Y. Huang, M.J. Huang, C.Z. RenWu, C.Y. Yang, M.J. Chiu, L.K. Chu, H.W. Lin, C.H. Cheng, A new molecular design based on thermally activated delayed ﬂuorescence for highly eﬃcient organic light emitting diodes, J. Am. Chem. Soc. 138 (2016) 628–634.  F. Wei, J. Qiu, X.C. Liu, J.Q. Wang, H.B. Wei, Z.B. Wang, Z.W. Liu, Z.Q. Bian, Z.H. Lu, Y.L. Zhao, C.H. Huang, Eﬃcient orange-red phosphorescent organic lightemitting diodes using an in situ synthesized copper(I) complex as the emitter, J. Mater. Chem. C 2 (2014) 6333–6341.  B.W. D'Andrade, S. Datta, S.R. Forrest, P. Djurovich, E. Polikarpov, M.E. Thompson, Relationship between the ionization and oxidation potentials of molecular organic semiconductors, Org. Electron. 6 (2005) 11–20.  J. Lee, H.-F. Chen, T. Batagoda, C. Coburn, P.I. Djurovich, M.E. Thompson, S.R. Forrest, Deep blue phosphorescent organic light-emitting diodes with very high brightness and eﬃciency, Nat. Mater. 15 (2016) 92–98.  X. Li, J. Zhang, Z. Zhao, L. Wang, H. Yang, Q. Chang, N. Jiang, Z. Liu, Z. Bian, W. Liu, Z. Lu, C. Huang, Deep Blue Phosphorescent Organic Light-Emitting Diodes with CIEy Value of 0.11 and External Quantum Eﬃciency up to 22.5%, Adv. Mater., 2018 1705005.  Z.Q. Chen, F. Ding, F. Hao, Z.Q. Bian, B. Ding, Y.Z. Zhu, F.F. Chen, C.H. Huang, A highly eﬃcient OLED based on terbium complexes, Org. Electron. 10 (2009) 939–947.  S.S. Reddy, V.G. Sree, W. Cho, S.H. Jin, Achieving pure deep-blue electroluminescence with CIE y < = 0.06 via a rational design approach for highly eﬃcient non-doped solution-processed organic light-emitting diodes, Chem. Asian J. 11 (2016) 3275–3282.  S. Jhulki, A.K. Mishra, A. Ghosh, T.J. Chow, J.N. Moorthy, Deep blue-emissive bifunctional (hole-transporting plus emissive) materials with CIEy similar to 0.06 based on a 'U'-shaped phenanthrene scaﬀold for application in organic light-emitting diodes, J. Mater. Chem. C 4 (2016) 9310–9315.  W.-C. Chen, G.-F. Wu, Y. Yuan, H.-X. Wei, F.-L. Wong, Q.-X. Tong, C.-S. Lee, A metamolecular tailoring strategy towards an eﬃcient violet-blue organic electroluminescent material, RSC Adv. 5 (2015) 18067–18074.  Z.Q. Wang, W. Liu, C. Xu, B.M. Ji, C.J. Zheng, X.H. Zhang, Excellent deep-blue emitting materials based on anthracene derivatives for non-doped organic lightemitting diodes, Opt. Mater. 58 (2016) 260–267.  C.F. Si, Z.F. Li, K.P. Guo, X. Lv, S.H. Pan, G. Chen, Y.Y. Hao, B. Wei, Functional versatile bipolar 3,3 '-dimethy1-9,9 '-bianthracene derivatives as an eﬃcient host and deep-blue emitter, Dyes Pigments 148 (2018) 329–340.  L. Xiao-Ke, Z. Cai-Jun, L. Ming-Fai, X. Jing, L. Chun-Sing, F. Man-Keung, Z. XiaoHong, A multifunctional phosphine oxide-diphenylamine hybrid compound as a high performance deep-blue ﬂuorescent emitter and green phosphorescent host, Chem. Commun. 50 (2014) 2027–2029.  J. Ye, Z. Chen, M.K. Fung, C. Zheng, X. Ou, X. Zhang, Y. Yuan, C.S. Lee, Carbazole/ sulfone hybrid D-π-A-structured bipolar ﬂuorophores for high-eﬃciency blue-violet electroluminescence, Chem. Mater. 25 (2013) 2630–2637.  S.B. Lee, K.H. Park, C.W. Joo, J.I. Lee, J. Lee, Y.H. Kim, Highly twisted pyrene derivatives for non-doped blue OLEDs, Dyes Pigments 128 (2016) 19–25.
electroluminescent devices, Nature 395 (1998) 151–154.  Y.G. Ma, H.Y. Zhang, J.C. Shen, C.M. Che, Electroluminescence from triplet metalligand charge-transfer excited state of transition metal complexes, Synth. Met. 94 (1998) 245–248.  H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly eﬃcient organic light-emitting diodes from delayed ﬂuorescence, Nature 492 (2012) 234–238.  T. Yu, L. Liu, Z. Xie, Y. Ma, Progress in small-molecule luminescent materials for organic light-emitting diodes, Sci. China Chem. 58 (2015) 907–915.  M. Zhu, C. Yang, Blue ﬂuorescent emitters: design tactics and applications in organic light-emitting diodes, Chem. Soc. Rev. 44 (2013) 4963–4976.  C.Y. He, H.Q. Guo, Q.M. Peng, S.Z. Dong, F. Li, Asymmetrically twisted anthracene derivatives as highly eﬃcient deep-blue emitters for organic light-emitting diodes, J. Mater. Chem. C 3 (2015) 9942–9947.  T. Shan, Y. Liu, X.Y. Tang, Q. Bai, Y. Gao, Z. Gao, J.Y. Li, J. Deng, B. Yang, P. Lu, Y.G. Ma, Highly eﬃcient deep blue organic light-emitting diodes based on imidazole: signiﬁcantly enhanced performance by eﬀective energy transfer with negligible eﬃciency roll-oﬀ, ACS Appl. Mater. Interfaces 8 (2016) 28771–28779.  W.C. Chen, Y. Yuan, Z.L. Zhu, Z.Q. Jiang, L.S. Liao, C.S. Lee, Polyphenylnaphthalene as a novel building block for high-performance deep-blue organic light-emitting devices, Adv. Opt. Mater. 6 (2018) 1700855.  I.S. Park, H. Komiyama, T. Yasuda, Pyrimidine-based twisted donor-acceptor delayed ﬂuorescence molecules: a new universal platform for highly eﬃcient blue electroluminescence, Chem. Sci. 8 (2017) 953–960.  M. Chen, Y. Yuan, J. Zheng, W.C. Chen, L.J. Shi, Z.L. Zhu, F. Lu, Q.X. Tong, Q.D. Yang, J. Ye, M.Y. Chan, C.S. Lee, Novel bipolar phenanthroimidazole derivative design for a nondoped deep-blue emitter with high singlet exciton yields, Adv. Opt. Mater. 3 (2015) 1215–1219.  Y.H. Chung, L. Sheng, X. Xing, L.L. Zheng, M.Y. Bian, Z.J. Chen, L.X. Xiao, Q.H. Gong, A pure blue emitter (CIEy approximate to 0.08) of chrysene derivative with high thermal stability for OLED, J. Mater. Chem. C 3 (2015) 1794–1798.  W.C. Chen, C.S. Lee, Q.X. Tong, Blue-emitting organic electroﬂuorescence materials: progress and prospective, J. Mater. Chem. C 3 (2015) 10957–10963.  Q. Zhang, S.P. Xiang, Z. Huang, S.Q. Sun, S.F. Ye, X.L. Lv, W. Liu, R.D. Guo, L. Wang, Molecular engineering of pyrimidine-containing thermally activated delayed ﬂuorescence emitters for highly eﬃcient deep-blue (CIEy < 0.06) organic light-emitting diodes, Dyes Pigments 155 (2018) 51–58.  X.Y. Tang, Q. Bai, Q.M. Peng, Y. Gao, J.Y. Li, Y.L. Liu, L. Yao, P. Lu, B. Yang, Y.G. Ma, Eﬃcient deep blue electroluminescence with an external quantum eﬃciency of 6.8% and CIEy < 0.08 based on a phenanthroimidazole-sulfone hybrid donor-acceptor molecule, Chem. Mater. 27 (2015) 7050–7057.  Y. Tan, Z. Zhao, L. Shang, Y. Liu, C. Wei, J. Li, H. Wei, Z. Liu, Z. Bian, C. Huang, Novel bipolar D- π -A type phenanthroimidazole/carbazole hybrid material for high eﬃciency nondoped deep-blue organic light-emitting diodes with NTSC CIEy and low eﬃciency roll-oﬀ, J. Mater. Chem. C 5 (2017) 11901–11909.  X.J. Zhan, N. Sun, Z.B. Wu, J. Tu, L. Yuan, X. Tang, Y.J. Xie, Q. Peng, Y.Q. Dong, Q.Q. Li, D.G. Ma, Z. Li, Polyphenylbenzene as a platform for deep-blue OLEDs: aggregation enhanced emission and high external quantum eﬃciency of 3.98%, Chem. Mater. 27 (2015) 1847–1854.  Z.L. Zhu, M. Chen, W.C. Chen, S.F. Ni, Y.Y. Peng, C. Zhang, Q.X. Tong, F. Lu, C.S. Lee, Removing shortcomings of linear molecules to develop high eﬃciencies deep-blue organic electroluminescent materials, Org. Electron. 38 (2016) 323–329.  M.Y. Wong, S. Krotkus, G. Copley, W. Li, C. Murawski, D. Hall, G.J. Hedley, M. Jaricot, D.B. Cordes, A.M.Z. Slawin, Y. Olivier, D. Beljonne, L. Muccioli, M. Moral, J.-C. Sancho-Garcia, M.C. Gather, I.D.W. Samuel, E. Zysman-Colman, Deep-blue oxadiazole-containing thermally activated delayed ﬂuorescence emitters for organic light-emitting diodes, ACS Appl. Mater. Interfaces 10 (2018) 33360–33372.  L.S. Cui, H. Nomura, Y. Geng, J.U. Kim, H. Nakanotani, C. Adachi, Controlling singlet-triplet energy splitting for deep-blue thermally activated delayed ﬂuorescence emitters, Angew. Chem. Int. Ed. 56 (2017) 1571–1575.  C.Y. Chan, L.S. Cui, J.U. Kim, H. Nakanotani, C. Adachi, Rational molecular design for deep‐blue thermally activated delayed ﬂuorescence emitters, Adv. Funct. Mater. (2018) 1706023.  Z. Huang, B. Wang, Q. Zhang, S.P. Xiang, X.L. Lv, L.X. Ma, B. Yang, Y. Gao, L. Wang, Highly twisted bipolar emitter for eﬃcient nondoped deep-blue electroluminescence, Dyes Pigments 140 (2017) 328–336.  B. Liu, Z.W. Yu, D. He, Z.L. Zhu, J. Zheng, Y.D. Yu, W.F. Xie, Q.X. Tong, C.S. Lee, Ambipolar D-A type bifunctional materials with hybridized local and chargetransfer excited state for high performance electroluminescence with EQE of 7.20% and CIEy similar to 0.06, J. Mater. Chem. C 5 (2017) 5402–5410.  M.Y. Bian, Z.F. Zhao, Y. Li, Q. Li, Z.J. Chen, D.D. Zhang, S.F. Wang, Z.Q. Bian, Z.W. Liu, L. Duan, L.X. Xiao, A combinational molecular design to achieve highly eﬃcient deep-blue electroﬂuorescence, J. Mater. Chem. C 6 (2018) 745–753.  P.Y. Chou, H.H. Chou, Y.H. Chen, T.H. Su, C.Y. Liao, H.W. Lin, W.C. Lin, H.Y. Yen, I.C. Chen, C.H. Cheng, Eﬃcient delayed ﬂuorescence via triplet-triplet annihilation for deep-blue electroluminescence, Chem. Commun. 50 (2014) 6869–6871.  J.Y. Hu, Y.J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Bisanthracenebased donor- acceptor- type light- emitting dopants: highly eﬃcient deep- blue emission in organic light- emitting devices, Adv. Funct. Mater. 24 (2014) 2064–2071.  C. Tang, F. Liu, Y.J. Xia, J. Lin, L.H. Xie, G.Y. Zhong, Q.L. Fan, W. Huang, Fluorenesubstituted pyrenes - novel pyrene derivatives as emitters in nondoped blue OLEDs, Org. Electron. 7 (2006) 155–162.  W. Qin, Z.Y. Yang, Y.B. Jiang, J.W.Y. Larn, G.D. Liang, H.S. Kwok, B.Z. Tang, Construction of eﬃcient deep blue aggregation-induced emission luminogen from triphenylethene for nondoped organic light-emitting diodes, Chem. Mater. 27
Organic Electronics 69 (2019) 77–84
Y. Tan, et al.
 Z. Gao, Y.L. Liu, Z.M. Wang, F.Z. Shen, H. Liu, G.N. Sun, L. Yao, Y. Lv, P. Lu, Y.G. Ma, High-eﬃciency violet-light-emitting materials based on phenanthro 9,10-d imidazole, Chem. Eur J. 19 (2013) 2602–2605.  Z. Li, Z. Wu, W. Fu, P. Liu, B. Jiao, D. Wang, G. Zhou, X. Hou, Versatile ﬂuorinated derivatives of triphenylamine as hole-transporters and blue-violet emitters in organic light-emitting devices, J. Phys. Chem. C 116 (2012) 20504–20512.
 S.G. Fan, J. You, Y.Q. Miao, H. Wang, Q.Y. Bai, X.C. Liu, X.G. Li, S.R. Wang, A bipolar emitting material for high eﬃcient non-doped ﬂuorescent organic lightemitting diode approaching standard deep blue, Dyes Pigments 129 (2016) 34–42.  Y. Zhang, S.L. Lai, Q.X. Tong, M.F. Lo, T.W. Ng, M.Y. Chan, Z.C. Wen, J. He, K.S. Jeﬀ, X.L. Tang, W.M. Liu, C.C. Ko, P.F. Wang, C.S. Lee, High eﬃciency nondoped deep-blue organic light emitting devices based on imidazole-pi-triphenylamine derivatives, Chem. Mater. 24 (2012) 61–70.