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Two spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives as host materials for green phosphorescent organic light-emitting diodes
Thin Solid Films 642 (2017) 96–102
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Two spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives as host materials for green phosphorescent organic light-emitting diodes
MARK
Yi Chena, Bo Wangb, Jinhai Huangc, Lei Wangb,⁎, Jianhua Sua,⁎ a b c
Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, PR China Shanghai Taoe Chemical Technology Co., Ltd., Shanghai, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Green phosphorescent organic light-emitting diodes Host materials Spiro Thermal stability Indoloacridine Benzofuran Benzothiophene
Two host materials based on spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives were designed and synthesized by introducing the benzofuran/benzothiophene unit to the spiro [fluorene-9,8′-indolo[3,2,1-de]acridine] unit. The two host materials exhibited high triplet energy level (> 2.65 eV) and the benzofuran-substituted derivative presented suitable glass transition temperature (> 130 °C). Their electrochemical and photophysical properties were also determined. The green phosphorescent organic light-emitting diodes based on the two host materials obtained electroluminescence performances with external quantum efficiencies of 14.3 ± 0.1% and 11.9 ± 0.1%, respectively.
1. Introduction In the past two decades, an enormous number of research findings have been devoted to study the organic light-emitting diodes (OLEDs) for their practical application in full color flat panel displays and lowcost solid-state lighting sources. Under electrical excitation, the ratio of singlet and triplet excitons is approximately 1:3, which makes the efficiency of fluorescent OLEDs under 25% with the emission solely from singlet exciton. In 1998, Forrest and Thompson reported highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with the heavy-metal phosphors as the emission layer, which boosted the theoretical internal quantum efficiency from 25% to 100% by utilizing both singlet and triplet excitons [1]. Typical PHOLEDs often adopts hostguest strategy by dispersing the heavy metal-containing complex homogeneously into a host matrix to avoid efficiency roll-off induced by the concentration quenching and triple-triple annihilation [2–3]. In this regard, the design of host materials with great performance is of equal importance to the phosphors to achieve high efficiency. As an ideal host material, one important design principle is that those compounds should obtain a higher triplet energy level (ET) than the dopant, which would be benefit to prevent the back energy transfer from the dopant to host [4]. On the other hand, suitable frontier molecular level, great thermal/morphological stability and balanced carrier mobility should also be taken into consideration in the molecule design [5–8]. One straightforward strategy to achieve eligible host
⁎
material is to use the building block such as the spiro-based heteroaromatic compounds [9–12]. Among these organic molecules, carbazole-based materials, such as 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP), have attracted exceptional attention as their excellent properties [13–18]. However, those carbazole-based materials ineluctable inherited the drawback of CBP with a low glass transition temperature. To cover this shortage, tremendous efforts have been devoted to. For example, Yang et al. reported a host material BCBP, which exhibited a high glass transition temperature (173 °C) and 13.7% of external quantum efficiency (EQE) for green PHOLEDs [19]. Liao et al. developed two fused N-phenylcarbazole ring host materials, which achieved high ET (> 2.70 eV) and excellent electroluminescence (EL) properties with the maximum current efficiencies (CE) of 33.9 cd/A and 40.8 cd/A [20]. Enlighten from the previous work, the indoloacridine derivatives with good hole-transporting ability and high triplet energy could be used as donor. Meanwhile, benzofuran/benzothiophene moieties with good thermal stability could be a candidate as an electron acceptor. We develop a sp3 C as a bridge to connect the donor group and accept moiety to increase their steric conformation and then the triplet energy level tuned, owning to their unique conformation with two mutually perpendicular chromophores [21]. In this article, two spiro-based heteroaromatic compounds spiro[fluoreno[4,3-b]benzofuran-7,8′-indolo [3,2,1-de] acridine] (CZO) and spiro[benzo[b]fluoreno[3,4-d]thiophene-7,8′-indolo[3,2,1-de]acridine] (CZS) were designed and
Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Su).
http://dx.doi.org/10.1016/j.tsf.2017.09.036 Received 22 June 2016; Received in revised form 13 September 2017; Accepted 15 September 2017 Available online 18 September 2017 0040-6090/ © 2017 Published by Elsevier B.V.
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affording the yellow procedure (1.04 g, 3.9 mmol, 77.20%). 1H NMR (400 MHz, CDCl3) δ 7.99 (dd, J = 7.2, 4.0 Hz, 2H), 7.85 (d, J = 7.6 Hz, 1H), 7.73–7.65 (m, 3H), 7.57 (tdd, J = 8.4, 4.0, 1.2 Hz, 2H), 7.44–7.38 (m, 1H), 7.34 (td, J = 7.6, 0.8 Hz, 1H).
synthesized by installing the benzofuran/benzothiophene unit to the indoloacridine moiety via a spiro structure as bridge [22–23]. To further evaluate the EL performances of those host materials, the PHOLED devices using the Ir(ppy)3 as triple emitter were fabricated. Both two materials exhibited low efficiency roll-off with the external quantum efficiencies of 14.3% at 1000 cd/m2 and 10.7% at 10000 cd/m2 for CZO, 11.9% at 1000 cd/m2 and 10.5% at 10000 cd/m2 for CZS.
2.2.2. Synthesis of 7H-benzo[b]fluoreno[3,4-d]thiophen-7-one A mixture of methyl 2-bromobenzoate (2.16 g, 10 mmol), dibenzo [b,d]thiophen-4-ylboronic acid (2.5 g, 11 mmol) and 2 M K2CO3 (10 mL) in tetrahydrofuran (40 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.108 g, 0.092 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was recrystallization by ethyl alcohol, affording yellow solid methyl 2-(dibenzo[b,d]thiophen-4-yl)benzoate (3.0 g, 9.4 mmol, 94.34%). Then, to a round flask added 2-(dibenzo [b,d]thiophen-4-yl)benzoate (1.59 g, 5 mmol) and 15 mL PPA (polyphosphoric acid), and then the mixture was heated to 140 °C and stirred by an electric mechanical agitator for 4 h. After cooling to ambient temperature, 70 mL water was dropwise slowly into the mixture stirring under 0 °C. After 1 h, the mixture was filtered under vacuum and washed with ethyl alcohol (3 × 20 mL). The solid was dried under vacuum for 20 h, affording the yellow procedure (1.03 g, 3.6 mmol, 72.03%). 1H NMR (400 MHz, CDCl3) δ 8.16 (dd, J = 7.6, 0.8 Hz, 2H), 7.50 (td, J = 8.0, 1.6 Hz, 1H), 7.45–7.37 (m, 3H), 7.32–7.27 (m, 2H), 7.07 (d, J = 8.0 Hz, 1H), 7.03 (d, J = 8.0 Hz, 1H).
2. Experimental 2.1. General information Chemicals and solvents used in the process were reagents grades and purchased from J & K Chemical Co. and Aladdin Chemical Co. without further purification. Tetrahydrofuran (THF) was purified by distillation over sodium under N2 atmosphere prior to use. In addition, the boronic acid pinacol ester (3 and 4) were obtained from Shanghai Taoe chemical technology Co., Ltd. All reactions and manipulations were carried out under N2 atmosphere. All materials were purified further by vacuum sublimation prior to fabrication of OLED devices. The 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AM 400 spectrometer with tetramethylsilane as the internal standard. High-resolution mass spectra (HRMS) were measured on a Waters LCT Premier XE spectrometer. The infrared radiation (IR) spectra were recorded in the range 4000–600 cm− 1 using the potassium bromide disk for solid samples by the Fourier transform infrared spectroscopy instrument. The ultraviolet-visible (UV–vis) absorption spectra were obtained on a Varian Cary 500 spectrophotometer. Photoluminescence (PL) spectra were recorded on room temperature by Varian-Cary fluorescence spectrophotometer. The cyclic voltammetry experiments were performed by a Versastat II electrochemical work station (Princeton applied research) using a conventional three-electrode configuration with a glassy carbon working electrode, a Pt wire counter electrode, and a regular calomel reference electrode in saturated KCl solution. The oxidation and reduction potentials were measured in solution tetrahydrofuran containing of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV/s. The differential scanning calorimetry (DSC) analysis was performed on a DSC Q2000 V24.11 Build 124 instrument with a heating scan rate of 5 °C/min from 0 °C to 250 °C under nitrogen atmosphere. Thermo gravimetric analysis (TGA) was carried out on the TGA instrument by measuring weight loss of samples with a heating scan rate of 10 °C/min from 50 °C to 800 °C under nitrogen.
2.2.3. Synthesis of spiro[fluoreno[4,3-b]benzofuran-7,8′-indolo[3,2,1-de] acridine] (CZO) A mixture of 9-(2-bromophenyl)-9H-carbazole (1.61 g, 5 mmol) in the THF (40 mL) was treated with the n-BuLi (2 mL, 2.5 M, 5 mmol) under argon at − 78 °C for 30 min. Then, 7H-fluoreno[4,3-b]benzofuran-7-one (1.35 g, 5 mmol) dissolved in 40 mL THF was added dropwise. The mixture was stirred for 30 min under argon at − 78 °C, and allowed to warm to the room temperature. After 12 h, the organic layer was washed with water and extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the alcohol product was obtained. Then the alcohol was added without further purification to a mixture of 5 mL concentrated aqueous HCl and acetic acid 50 mL. The mixture was heated to reflux and stirred for 2 h. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by column chromatography on silica gel. The solid was dried under vacuum for 20 h, affording the white procedure (1.03 g, 2.1 mmol, 42.04%). 1H NMR (400 MHz, CDCl3) δ 8.37 (d, J = 7.6 Hz, 1H), 8.31 (d, J = 8.4 Hz, 1H), 8.26 (dd, J = 8.4, 0.8 Hz, 1H), 8.20 (dd, J = 7.6, 0.8 Hz, 1H), 7.93 (dd, J = 7.6, 0.8 Hz, 1H), 7.88 (dd, J = 7.6, 0.8 Hz, 1H), 7.74 (dd, J = 8.0, 4.4 Hz, 2H), 7.64 (ddd, J = 8.4, 7.6, 1.2 Hz, 1H), 7.54–7.46 (m, 2H), 7.44–7.39 (m, 1H), 7.39–7.31 (m, 2H), 7.24–7.18 (m, 2H), 7.16–7.11 (m, 1H), 7.06 (dd, J = 9.6, 5.6 Hz, 1H), 6.87–6.79 (m, 1H), 6.66 (dd, J = 8.0, 1.6 Hz, 1H), 6.55 (dd, J = 7.6, 1.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 156.92, 155.71, 155.22, 150.51, 138.56, 137.06, 136.73, 129.44, 129.15, 128.46, 127.13, 126.69, 126.40, 125.49, 124.44, 123.92, 123.54, 123.04, 122.69, 121.19, 120.66, 120.29, 118.20, 114.48, 113.73, 111.95, 57.71. IR (KBr, disk) ν 3060.15, 2925.56, 1676.69, 1631.58, 1594.74, 1394.74, 1226.31, 785.71 cm− 1. HRMS (ESI, m/z): [M + H]+ calcd for: C37H21NO 496.1701, found, 496.1704.
2.2. Preparation of materials 2.2.1. Synthesis of 7H-fluoreno[4,3-b]benzofuran-7-one A mixture of methyl 2-bromobenzoate (2.16 g, 10 mmol), dibenzo [b,d]furan-4-ylboronic acid (2.34 g,11 mmol) and 2 M K2CO3 (10 mL) in tetrahydrofuran (40 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.108 g, 0.092 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was recrystallization by ethyl alcohol, affording yellow solid methyl 2-(dibenzo[b,d]furan-4-yl)benzoate (2.84 g, 9.4 mmol, 94.04%). Then, to a round flask added methyl 2-(dibenzo [b,d]furan-4-yl)benzoate (1.51 g, 5 mmol) and 15 mL PPA (polyphosphoric acid), and then the mixture was heated to 140 °C and stirred by electric mechanical agitator for 4 h. After cooling to ambient temperature, 70 mL water was dropwise slowly into the mixture under 0 °C. After 1 h, the mixture was filtered under vacuum and washed with ethyl alcohol (3 × 20 mL). The solid was dried under vacuum for 20 h, 97
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2.2.4. Synthesis of spiro[benzo[b]fluoreno[3,4-d]thiophene-7,8′-indolo [3,2,1-de]acridine] (CZS) A mixture of 9-(2-bromophenyl)-9H-carbazole (1.61 g, 5 mmol) in the THF (40 mL) was treated with the n-BuLi (2 mL, 2.5 M, 5 mmol) under argon at − 78 °C for 30 min. Then, 7H-benzo[b]fluoreno[3,4-d] thiophen −7-one (1.43 g, 5 mmol) dissolved in 40 mL THF was added dropwise. The mixture was stirred for 30 min under argon at − 78 °C, and allowed to warm to the room temperature. After 12 h, the organic layer was washed with water and extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the alcohol product was obtained. Then the alcohol was added without further purification to a mixture of 5 mL concentrated aqueous HCl and acetic acid 50 mL. The mixture was heated to reflux and stirred for 2 h. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by column chromatography on silica gel. The solid was dried under vacuum for 20 h, affording the white procedure (1.13 g, 2.2 mmol, 44.14%). 1H NMR (400 MHz, CDCl3) δ 8.31 (d, J = 8.4 Hz, 1H), 8.26 (d, J = 8.4 Hz, 1H), 8.20 (d, J = 7.6 Hz, 1H), 8.15 (dd, J = 6.8, 2.0 Hz, 1H), 8.09 (d, J = 7.6 Hz, 1H), 8.04–7.97 (m, 2H), 7.89 (dd, J = 7.6, 0.8 Hz, 1H), 7.69–7.62 (m, 1H), 7.56–7.45 (m, 3H), 7.42 (t, J = 7.6 Hz, 1H), 7.39–7.32 (m, 1H), 7.28 (d, J = 8.0 Hz, 1H), 7.23 (t, J = 3.6 Hz, 2H), 7.05 (t, J = 7.6 Hz, 1H), 6.86–6.79 (m, 1H), 6.64 (dd, J = 8.0, 1.6 Hz, 1H), 6.54 (dd, J = 7.6, 0.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 156.21, 154.37, 139.67, 138.62, 137.12, 136.80, 136.13, 135.05, 133.76, 132.08, 129.42, 128.96, 128.34, 128.09, 126.76, 126.39, 125.62, 124.79, 124.09, 123.36, 123.05, 122.70, 122.09, 121.65, 121.20, 118.23, 114.49, 113.74, 57.44. IR (KBr, disk) ν 3060.16, 2921.05, 1676.69, 1641.35, 1596.24, 1390.44, 1226.31, 784.21 cm− 1. HRMS (ESI, m/z): [M + H]+ calcd for: C37H21NS 512.1473, found, 512.1531.
Scheme 1. The structure and synthetic routes of the host materials. (i) Pd(PPh3)4, K2CO3 (2 M), THF, reflux. (ii) PPA, 120 °C. (iii) n-BuLi (2.5 M), THF, − 78 °C. (iv) HCl (11 mol/ L), AcOH, reflux.
two target compounds were identified and characterized by 1H and 13C NMR, as well as the HRMS. 3.2. Thermal properties The thermal stability and morphological property were investigated by the thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements, which were carried out under nitrogen atmosphere. As the Fig. 1 shown, the thermal decomposition temperatures (Td, corresponding to 5% weight loss) of CZO and CZS were 396 and 410 °C, respectively, and the glass transition temperature (Tg) of CZO was 138 °C. However, the glass transition temperature of CZS was not observed even in the second DSC scan. Both two host materials obtained high Td and Tg, which can be mainly attributed to their steric molecular structure [26]. Simultaneously, the Td of CZS was litter higher than that of CZO, which was closely correlated to the molecule weight. All above, both host materials exhibited great thermal stability, which could be beneficial for keeping morphological stability during the device operating process.
2.3. OLED fabrication measurement In a general operation procedure, OLED devices were fabricated under high vacuum (~ 10− 4 Pa) chamber by thermal evaporation of organic layers onto a clean glass substrate precoated with a 150 nm thick indium tin oxide (ITO) layer. Prior to use, the substrate was degreased in an ultrasonic bath by the following sequence: in detergent, de-ionized water, acetone, and isopropanol, and then cleaned in a UVozone chamber for 15 min. The typical deposition rates, monitored by oscillating quartz, were 1.0, 0.5, and 5.0 Å/s for organic materials, lithium fluoride (LiF), and aluminum (Al), respectively. The device active area defined by the overlap between the electrodes was 3 ∗ 3 mm2 in all case. The EL spectra, CIE coordinates and current-voltage characteristics of the devices were measured with a PHOTO RESEARCH SpectraScan PR 655 photometer and a KEITHLEY 2400 SourceMeter constant current source. All measurements were carried out immediately under ambient atmosphere without device encapsulation after the devices have been fabricated.
3.3. Optical properties The photophysical properties of CZO and CZS were investigated by ultraviolet-visible (UV–vis) and photoluminescence (PL) spectrometers in the tetrahydrofuran. As Fig. 2 shown, both two compounds exhibited similar UV–vis absorption bands between 250 nm and 360 nm. The maximum absorption peak around 350 nm could be assigned to molecular n–π* transitions of carbazole group [27]. At short wavelength, three characteristic peaks around 250 nm, 275 nm and 295 nm were related to the π–π* transitions of the accept units in CZO and CZS [28]. The corresponding optical band gaps of CZO and CZS estimated from the absorption edge of UV–vis spectra were 3.39 eV and 3.38 eV, respectively. As seen from the PL spectrum at the room temperature, the maximum emission peaks of CZO and CZS were 369 nm and 370 nm, respectively, completely overlapped with the maximum absorption peak of Ir(ppy)3 [29], implying that the Förster energy could be efficiently transferred from host to dopant. The fluorescence quantum yields (PLQYs) of CZO and CZS were also measured as 23.92% and 10.66%, respectively. The PLQYs of both two compounds are lower the reported compound SIA-F [10], which could own to the heavy atom effect. Furthermore, the lifetime of two host materials are also tested as 6.26 ns for CZO and 5.48 ns for CZS, respectively. Additionally, the phosphorescence spectra measured at 77 K were also shown in Fig. 2, and the triplet energy levels of CZO and CZS evaluated from the highest-energy vibronic sub-bands [30] of the phosphorescence spectra at 77 K were 2.73 eV and 2.67 eV, which were higher than the green
3. Results and discussion 3.1. Synthesis and characterization The structures and synthetic routes of the two target molecules were outlined in Scheme 1. Two key intermediate compounds 7H-fluoreno [4,3-b]benzofuran-7-one and 7H-benzo[b]fluoreno[3,4-d]thiophen-7one were synthesized through multiple steps according to the previous literatures [24–25]. Subsequently, the desired products could be synthesized by the ring closing reaction between the ketone compounds and 9-(2-bromophenyl)-9H-carbazole in 42.04% and 44.14% yield. The 98
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a) 125
b) CZO CZS
CZO
Endothermic
Weight (%)
100
75
50
25
0 100
200
300
400 500 600 Temperature (OC)
700
800
80
100
120 140 160 Temperature (OC)
180
200
Fig. 1. Differential scanning calorimetry curve of CZO and CZS compounds and thermogravimetry data of CZO compound.
0.0003
1.0 CZO CZS
0.0002 Current (A)
Normalized Intensity
0.8 0.6 0.4
CZO CZS
0.0001 0.0000 -0.0001
0.2
-0.0002 -0.0003 -2
0.0 250
300
350
400 450 500 Wavelength (nm)
550
600
-1
0
1 2 E ( V vs. SCE )
3
4
Fig. 3. The cyclic voltammograms of CZO and CZS compounds in tetrahydrofuran containing of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6), scan rate: 100 mV/s.
Fig. 2. UV–vis absorption spectra, fluorescence spectra and phosphorescence spectra of CZO and CZS compounds.
phosphor Ir(ppy)3 (ET = 2.41 eV) and the commonly used host material CBP (ET = 2.56 eV). All the results indicated that the two compounds sufficiently fulfilled the requirements of good host materials used in the green PHOLEDs.
[(3-pyridyl)-phen-3-yl]benzene, 40 nm)/LiF (1 nm)/Al(80 nm) were fabricated. Fig. 4 showed the energy levels of those materials used in the green PHOLEDs in this paper. In these multilayer device configurations, to reduce the charge injection barriers, TAPC and TmPyPB were utilized as hole transporting layer and electron transporting layer, respectively. To reduce the charge injection barriers, MoO3 and LiF were employed as hole-injection layer (HIL) and electron-injection layer (EIL), respectively. For comparison, the CBP-based device with the same multilayer configuration was also fabricated and tested. The current density-voltage-brightness (J-V-L) characteristics of CZS-based device, CZO-based device and CBP-based device were presented in the Fig. 5 with the corresponding date summarized in the Table 2. The turn-on voltages of CZS-based device and CZO-based device at 1 cd/m2 are 3.2 V and 3.1 V, respectively. The maximum brightness of device CZS-based device is 32,913 cd/m2 at the voltage of 8.6 V, and 46,794 cd/m2 for CZO-based device at the voltage of 8.7 V. Compared to the CBP-based device, both CZS-based device and CZObased device exhibited equivalent turn-on voltage and maximum luminance, suggesting that CZS and CZO could be host materials for application in green PHOLEDs. The dependence of current and power efficiency on luminance measured for CZS-based device and CZO-based device are shown in Figs. 6 and 7, respectively. The maximum external quantum, maximum current and power efficiency of CZO-based device were 14.5%, 50.8 cd/ A, and 42.8 lm/W, respectively, which were quite higher than that of CZS-based device (12.1%, 42.4 cd/A and 34.3 lm/W). In comparison,
3.4. Electrochemical properties and energy level The cyclic voltammetry measurements of CZO and CZS were employed to evaluate their electrochemical behaviors such as energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the results were classified in Fig. 3 and Table 1. It can be seen from the Fig. 3 that both the two compounds exhibited two reversible oxidation peaks, which could be attributed to the oxidation of indoloacridine and the hereroaromatic substituted fluorene ring [10]. The HOMO energy level, calculated from the onset oxidation potentials by the equation: EHOMO = − Eox − 4.4 eV [31], were − 5.76 eV and − 5.74 eV for CZO and CZS, respectively. Meanwhile, the lowest unoccupied molecular orbital (LUMO) energy levels, estimated from the EHOMO and optical band gaps, were − 2.37 eV and − 2.36 eV for CZO and CZS, respectively. 3.5. Electroluminescence To evaluate the practical electroluminescence properties of CZS and CZO as host materials, the multilayer devices configurations of ITO/ MoO3 (10 nm)/TAPC (4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline], 50 nm)/Host: dopant (8%, 15 nm)/TmPyPB (1,3,5-tri 99
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Table 1 Physical properties of CZS and CZO host materials.
CZO CZS
Abs λmax (nm)a
PL λmax (nm)a
Eg (eV)b
HOMO (eV)c
LUMO (eV)d
ET (eV)e
Td (°C)f
Tg (°C)f
Tm (°C)
PLQY (%)g
τ (ns)h
354 355
369 370
3.39 3.38
− 5.76 − 5.74
− 2.37 − 2.36
2.73 2.67
396 410
138 NA
281 321
23.92 10.66
6.26 5.48
a
Measured in tetrahydrofuran at room temperature. Estimated from onset of the absorption spectra (EgOpt = 1241/λonset). Calculated from cyclic voltammetry. d Calculated by the equation EHOMO = ELUMO − Eg. e Calculated by the first peak of phosphorescence spectra measured in the 2-methyltetrahydrofuran at 77 K. f Measured by TGA and DSC. g Measured by fluorescence spectrometry in tetrahydrofuran at room temperature. h Measured by time-correlated single photon counting in tetrahydrofuran at room temperature. b c
Fig. 4. The energy level diagram of device and the chemical structure of materials used in the device.
efficiency of 42.7 lm/W. Both CZS-based device and CZO-based device exhibited competitive EL performances compared to CBP-based device. And the device efficiencies of CZO were slight higher than that of CBP, which could be explained by the fact that the CZO possess higher triplet energy than the CBP. Furthermore, both CZS-based device and CZObased device presented quite lower roll-off efficiency with EQE of 14.3% at 1000 cd/m2 and 10.7% at 10000 cd/m2 for CZO, 11.9% at 1000 cd/m2 and 10.4% at 10000 cd/m2 for CZS, respectively. Apparently, both devices exhibited significantly high performances, which could be attributed to the reasonable HOMO and LUMO energy levels of the two compounds and suitable device structure. In addition, all those devices exhibited pure green emission at 516 nm with the CIE(x,y) (0.33, 0.61) and no residual emission from the host or other layer materials, manifesting that the electroluminescence was originated from the complete energy transfer from host to dopant.
CZS CZO CBP
200
2
10000 1000 2
150
Luminance (cd/m )
Current density (mA/cm )
250
100 100 10 50 1 0 0
2
4 6 voltage (V)
8
0.1 10
Fig. 5. Current density-voltage-luminance curves of PHOLED devices based on CZS, CZO and CBP.
4. Conclusion
the CBP-based device achieved a maximum external quantum of 13.8%, maximum current efficiency of 48.9 cd/A, and maximum power
In summary, Two spiro [fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives were synthesized via Suzuki cross-coupling reaction and ring closing reaction for application in green PHOLEDs. Both two 100
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Table 2 Performance parameters of PHOLEDs with CZS and CZO host materials. Host
Turn-on voltage (V)a
Current efficiency (cd/A)b
Power efficiency (lm/W)b
EQE (%)b
CIE(x,y)c
CZS CZO CBP
3.2 3.1 3.0
40.7/42.1/36.8 49.1/50.9/37.6 48.9/48.7/39.2
33.8/30.3/18.6 42.9/38.8/18.7 42.7/33.7/18.1
11.5/11.9/10.4 13.8/14.3/10.7 13.8/13.7/11.1
(0.33, 0.61) (0.33, 0.61) (0.32, 0.61)
c
At 1 cd/m2. Estimated at the luminance of 100 cd/m2, 1000 cd/m2 and 10,000 cd/m2. Measured from the EL spectra by inverting into chromaticity coordinates on the CIE 1931 diagram.
a) 60 50 Current Efficiency (cd/A)
b) 15
100 CZS CZO CBP
80
40 60 30 40 20 20
10 0 1
10
100 1000 2 Luminance (cd/m )
0 10000
10 EQE (%)
b
Power Efficiency (lm/W)
a
CZS CZO CBP
5
0 1
10
100 1000 2 Luminance (cd/m )
10000
Fig. 6. (a) The current efficiency and power efficiency; (b) external quantum efficiency of the devices based on CZS, CZO and CBP. diodes, Adv. Mater. 25 (2013) 6801–6827. [4] Y. Tao, C. Yang, J. Qin, Organic host materials for phosphorescent organic lightemitting diodes, Chem. Soc. Rev. 40 (2011) 2943–2970. [5] N.J. Lundin, A.G. Blackman, K.C. Gordon, D.L. Officer, Synthesis and characterization of a multicomponent rhenium (I) complex for application as an OLED dopant, Angew. Chem. Int. Ed. 45 (2006) 2582–2584. [6] C.T. Chen, Y. Wei, J.S. Lin, M.V.R.K. Moturu, W.S. Chao, Y.T. Tao, C.H. Chien, Doubly ortho-linked quinoxaline/diphenylfluorene hybrids as bipolar, fluorescent chameleons for optoelectronic applications, J. Am. Chem. Soc. 128 (2006) 10992–10993. [7] H. Inomata, K. Goushi, T. Masuko, T. Konno, T. Imai, H. Sasabe, J.J. Brown, C. Adachi, High-efficiency organic electrophosphorescent diodes using 1,3,5-triazine electron transport materials, Chem. Mater. 16 (2004) 1285–1291. [8] H.H. Chou, C.H. Cheng, A highly efficient universal bipolar host for blue, green, and red phosphorescent OLEDs, Adv. Mater. 22 (2010) 2468–2471. [9] Y.K. Wang, Y.L. Deng, X.Y. Liu, X.D. Yuan, Z.Q. Jiang, L.S. Liao, A facile way to synthesize high-triplet-energy hosts for blue phosphorescent organic light-emitting diodes with high glass transition temperature and low driving voltage, Dyes Pigments 122 (2015) 6–12. [10] S. Thiery, D. Tondelier, B. Geffroy, O. Jeannin, J. Rault-Berthelot, C. Poriel, Modulation of the physicochemical properties of donor-spiro-acceptor derivatives through donor unit planarisation: phenylacridine versus indoloacridine. New hosts for green and blue phosphorescent organic light-emitting diodes (PhOLEDs), Chem. Eur. J. 22 (2016) 1–15. [11] M. Romain, D. Tondelier, B. Geffroy, A. Shirinskaya, O. Jeannin, J. Rault-Berthelot, C. Poriel, Spiro-configured phenyl acridine thioxanthene dioxide as a host for efficient PhOLEDs, Chem. Commun. 51 (2015) 1313–1315. [12] M.M. Xue, C.C. Huang, Y. Yuan, L.S. Cui, Q. Li, Y.X. Li, B. Wang, Z.Q. Jiang, M.K. Fung, L.S. Liao, De novo design of boron-based host materials for highly efficient blue and white phosphorescent OLEDs with low efficiency roll-off, ACS Appl. Mater. Interfaces 8 (2016) 20230–20236. [13] J. Jin, W. Zhang, B. Wang, G. Mu, P. Xu, L. Wang, H. Huang, J. Chen, D. Ma, Construction of high Tg bipolar host materials with balanced electron–hole mobility based on 1,2,4-thiadiazole for phosphorescent organic light-emitting diodes, Chem. Mater. 26 (2014) 2388–2395. [14] G.J. Tian, W.Q. Liang, Y. Chen, N. Xiang, Q.C. Dong, J.H. Huang, J.H. Su, A novel spiro-annulated host based on carbazole with good thermal stability and high triplet energy for efficient blue and green phosphorescent organic light-emitting diodes, Dyes Pigments 126 (2016) 296–302. [15] M. Romain, D. Tondelier, O. Jeannin, B. Geffroy, J. Rault-Berthelot, C. Poriel, Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: new host materials for efficient sky-blue PhOLEDs, J. Mater. Chem. C 3 (2015) 9701–9714. [16] J. Huang, J.H. Su, H. Tian, The development of anthracene derivatives for organic light-emitting diodes, J. Mater. Chem. 22 (2012) 10977–10989. [17] G.J. Tian, X. Wei, N. Xiang, J.H. Huang, J. Cao, Z. Wang, J. Zhang, J.H. Su, Small organic molecules based on oxazole/thiazole with excellent performances in green and red phosphorescent organic light-emitting diodes, RSC Adv. 6 (2016)
Normalized EL Intensity
1.0 CZS CZO CBP
0.8 0.6 0.4 0.2 0.0 400
500 600 Wavelength (nm)
700
800
Fig. 7. The normalized EL spectra of devices based on CZS, CZO and CBP.
compounds exhibited good thermal stability (Td > 390 °C) and high triplet energy (ET > 2.56 eV), which indicated that the introduction of steric spiro structure could maintain both high thermal stability and high triplet energy. Meanwhile, the two hosts based green phosphorescent organic light-emitting devices were also fabricated to evaluate the practical utilities of the two compounds. Compared to CBP-based device, both devices displayed competitive EL performance as the external quantum efficiencies of 14.3% and 11.9% at 1000 cd/m2 for CZO and CZS, respectively. References [1] 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 electroluminescent devices, Nature 395 (1998) 151–154. [2] H. Suzukia, S. Hoshino, Effects of doping dyes on the electroluminescent characteristics of multilayer organic light-emitting diodes, J. Appl. Phys. 79 (1996) 8816–8822. [3] C. Murawski, K. Leo, M.C. Gather, Efficiency roll-off in organic light-emitting
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oxide derivative as a host and exciton blocking material for blue phosphorescent organic light-emitting diodes, Thin Solid Films 518 (2010) 3716–3720. Y. Chen, J. Xie, Z. Wang, J. Cao, H. Chen, J. Huang, J. Zhang, J. Su, Highly efficient bipolar host material based-on indole and triazine moiety for red phosphorescent light-emitting diodes, Dyes Pigments 124 (2016) 188–195. S.C. Dong, Y. Liu, Q. Li, L.S. Cui, H. Chen, Z.Q. Jiang, L.S. Liao, Spiro-annulated triarylamine-based hosts incorporating dibenzothiophene for highly efficient singleemitting layer white phosphorescent organic light-emitting diodes, J. Mater. Chem. C 1 (2013) 6575–6584. S. Thiery, D. Tondelier, C. Declairieux, G. Seo, B. Geffroy, O. Jeannin, J. RaultBerthelot, R. Metivier, C. Poriel, 9,9′-Spirobifluorene and 4-phenyl-9,9′-spirobifluorene: pure hydrocarbon small molecules as hosts for efficient green and blue PhOLEDs, J. Mater. Chem. C 2 (2014) 4156–4166. I. Tanaka, Y. Tabata, S. Tokito, Comparison of phosphorescence properties of greenemitting Ir(ppy)3 and red-emitting Btp2Ir(acac), Jpn. J. Appl. Phys. 43 (2004) 1601–1603. K. Wang, S.P. Wang, J.B. Wei, S.Y. Chen, D. Liu, Y. Liu, Y. Wang, New multifunctional phenanthroimidazole-phosphine oxide hybrids for high-performance red, green and blue electroluminescent devices, J. Mater. Chem. C 2 (2014) 6817–6826. C.J. Tonzola, M.M. Alam, W. Kaminsky, S.A. Jenekhe, New n-type organic semiconductors: synthesis, single crystal structures, cyclic voltammetry, photophysics, electron transport, and electroluminescence of a series of diphenylanthrazolines, J. Am. Chem. Soc. 125 (2003) 13548–13558.
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Two spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives as host materials for green phosphorescent organic light-emitting diodes
MARK
Yi Chena, Bo Wangb, Jinhai Huangc, Lei Wangb,⁎, Jianhua Sua,⁎ a b c
Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, PR China Shanghai Taoe Chemical Technology Co., Ltd., Shanghai, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Green phosphorescent organic light-emitting diodes Host materials Spiro Thermal stability Indoloacridine Benzofuran Benzothiophene
Two host materials based on spiro[fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives were designed and synthesized by introducing the benzofuran/benzothiophene unit to the spiro [fluorene-9,8′-indolo[3,2,1-de]acridine] unit. The two host materials exhibited high triplet energy level (> 2.65 eV) and the benzofuran-substituted derivative presented suitable glass transition temperature (> 130 °C). Their electrochemical and photophysical properties were also determined. The green phosphorescent organic light-emitting diodes based on the two host materials obtained electroluminescence performances with external quantum efficiencies of 14.3 ± 0.1% and 11.9 ± 0.1%, respectively.
1. Introduction In the past two decades, an enormous number of research findings have been devoted to study the organic light-emitting diodes (OLEDs) for their practical application in full color flat panel displays and lowcost solid-state lighting sources. Under electrical excitation, the ratio of singlet and triplet excitons is approximately 1:3, which makes the efficiency of fluorescent OLEDs under 25% with the emission solely from singlet exciton. In 1998, Forrest and Thompson reported highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with the heavy-metal phosphors as the emission layer, which boosted the theoretical internal quantum efficiency from 25% to 100% by utilizing both singlet and triplet excitons [1]. Typical PHOLEDs often adopts hostguest strategy by dispersing the heavy metal-containing complex homogeneously into a host matrix to avoid efficiency roll-off induced by the concentration quenching and triple-triple annihilation [2–3]. In this regard, the design of host materials with great performance is of equal importance to the phosphors to achieve high efficiency. As an ideal host material, one important design principle is that those compounds should obtain a higher triplet energy level (ET) than the dopant, which would be benefit to prevent the back energy transfer from the dopant to host [4]. On the other hand, suitable frontier molecular level, great thermal/morphological stability and balanced carrier mobility should also be taken into consideration in the molecule design [5–8]. One straightforward strategy to achieve eligible host
⁎
material is to use the building block such as the spiro-based heteroaromatic compounds [9–12]. Among these organic molecules, carbazole-based materials, such as 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP), have attracted exceptional attention as their excellent properties [13–18]. However, those carbazole-based materials ineluctable inherited the drawback of CBP with a low glass transition temperature. To cover this shortage, tremendous efforts have been devoted to. For example, Yang et al. reported a host material BCBP, which exhibited a high glass transition temperature (173 °C) and 13.7% of external quantum efficiency (EQE) for green PHOLEDs [19]. Liao et al. developed two fused N-phenylcarbazole ring host materials, which achieved high ET (> 2.70 eV) and excellent electroluminescence (EL) properties with the maximum current efficiencies (CE) of 33.9 cd/A and 40.8 cd/A [20]. Enlighten from the previous work, the indoloacridine derivatives with good hole-transporting ability and high triplet energy could be used as donor. Meanwhile, benzofuran/benzothiophene moieties with good thermal stability could be a candidate as an electron acceptor. We develop a sp3 C as a bridge to connect the donor group and accept moiety to increase their steric conformation and then the triplet energy level tuned, owning to their unique conformation with two mutually perpendicular chromophores [21]. In this article, two spiro-based heteroaromatic compounds spiro[fluoreno[4,3-b]benzofuran-7,8′-indolo [3,2,1-de] acridine] (CZO) and spiro[benzo[b]fluoreno[3,4-d]thiophene-7,8′-indolo[3,2,1-de]acridine] (CZS) were designed and
Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Su).
http://dx.doi.org/10.1016/j.tsf.2017.09.036 Received 22 June 2016; Received in revised form 13 September 2017; Accepted 15 September 2017 Available online 18 September 2017 0040-6090/ © 2017 Published by Elsevier B.V.
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affording the yellow procedure (1.04 g, 3.9 mmol, 77.20%). 1H NMR (400 MHz, CDCl3) δ 7.99 (dd, J = 7.2, 4.0 Hz, 2H), 7.85 (d, J = 7.6 Hz, 1H), 7.73–7.65 (m, 3H), 7.57 (tdd, J = 8.4, 4.0, 1.2 Hz, 2H), 7.44–7.38 (m, 1H), 7.34 (td, J = 7.6, 0.8 Hz, 1H).
synthesized by installing the benzofuran/benzothiophene unit to the indoloacridine moiety via a spiro structure as bridge [22–23]. To further evaluate the EL performances of those host materials, the PHOLED devices using the Ir(ppy)3 as triple emitter were fabricated. Both two materials exhibited low efficiency roll-off with the external quantum efficiencies of 14.3% at 1000 cd/m2 and 10.7% at 10000 cd/m2 for CZO, 11.9% at 1000 cd/m2 and 10.5% at 10000 cd/m2 for CZS.
2.2.2. Synthesis of 7H-benzo[b]fluoreno[3,4-d]thiophen-7-one A mixture of methyl 2-bromobenzoate (2.16 g, 10 mmol), dibenzo [b,d]thiophen-4-ylboronic acid (2.5 g, 11 mmol) and 2 M K2CO3 (10 mL) in tetrahydrofuran (40 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.108 g, 0.092 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was recrystallization by ethyl alcohol, affording yellow solid methyl 2-(dibenzo[b,d]thiophen-4-yl)benzoate (3.0 g, 9.4 mmol, 94.34%). Then, to a round flask added 2-(dibenzo [b,d]thiophen-4-yl)benzoate (1.59 g, 5 mmol) and 15 mL PPA (polyphosphoric acid), and then the mixture was heated to 140 °C and stirred by an electric mechanical agitator for 4 h. After cooling to ambient temperature, 70 mL water was dropwise slowly into the mixture stirring under 0 °C. After 1 h, the mixture was filtered under vacuum and washed with ethyl alcohol (3 × 20 mL). The solid was dried under vacuum for 20 h, affording the yellow procedure (1.03 g, 3.6 mmol, 72.03%). 1H NMR (400 MHz, CDCl3) δ 8.16 (dd, J = 7.6, 0.8 Hz, 2H), 7.50 (td, J = 8.0, 1.6 Hz, 1H), 7.45–7.37 (m, 3H), 7.32–7.27 (m, 2H), 7.07 (d, J = 8.0 Hz, 1H), 7.03 (d, J = 8.0 Hz, 1H).
2. Experimental 2.1. General information Chemicals and solvents used in the process were reagents grades and purchased from J & K Chemical Co. and Aladdin Chemical Co. without further purification. Tetrahydrofuran (THF) was purified by distillation over sodium under N2 atmosphere prior to use. In addition, the boronic acid pinacol ester (3 and 4) were obtained from Shanghai Taoe chemical technology Co., Ltd. All reactions and manipulations were carried out under N2 atmosphere. All materials were purified further by vacuum sublimation prior to fabrication of OLED devices. The 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AM 400 spectrometer with tetramethylsilane as the internal standard. High-resolution mass spectra (HRMS) were measured on a Waters LCT Premier XE spectrometer. The infrared radiation (IR) spectra were recorded in the range 4000–600 cm− 1 using the potassium bromide disk for solid samples by the Fourier transform infrared spectroscopy instrument. The ultraviolet-visible (UV–vis) absorption spectra were obtained on a Varian Cary 500 spectrophotometer. Photoluminescence (PL) spectra were recorded on room temperature by Varian-Cary fluorescence spectrophotometer. The cyclic voltammetry experiments were performed by a Versastat II electrochemical work station (Princeton applied research) using a conventional three-electrode configuration with a glassy carbon working electrode, a Pt wire counter electrode, and a regular calomel reference electrode in saturated KCl solution. The oxidation and reduction potentials were measured in solution tetrahydrofuran containing of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV/s. The differential scanning calorimetry (DSC) analysis was performed on a DSC Q2000 V24.11 Build 124 instrument with a heating scan rate of 5 °C/min from 0 °C to 250 °C under nitrogen atmosphere. Thermo gravimetric analysis (TGA) was carried out on the TGA instrument by measuring weight loss of samples with a heating scan rate of 10 °C/min from 50 °C to 800 °C under nitrogen.
2.2.3. Synthesis of spiro[fluoreno[4,3-b]benzofuran-7,8′-indolo[3,2,1-de] acridine] (CZO) A mixture of 9-(2-bromophenyl)-9H-carbazole (1.61 g, 5 mmol) in the THF (40 mL) was treated with the n-BuLi (2 mL, 2.5 M, 5 mmol) under argon at − 78 °C for 30 min. Then, 7H-fluoreno[4,3-b]benzofuran-7-one (1.35 g, 5 mmol) dissolved in 40 mL THF was added dropwise. The mixture was stirred for 30 min under argon at − 78 °C, and allowed to warm to the room temperature. After 12 h, the organic layer was washed with water and extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the alcohol product was obtained. Then the alcohol was added without further purification to a mixture of 5 mL concentrated aqueous HCl and acetic acid 50 mL. The mixture was heated to reflux and stirred for 2 h. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by column chromatography on silica gel. The solid was dried under vacuum for 20 h, affording the white procedure (1.03 g, 2.1 mmol, 42.04%). 1H NMR (400 MHz, CDCl3) δ 8.37 (d, J = 7.6 Hz, 1H), 8.31 (d, J = 8.4 Hz, 1H), 8.26 (dd, J = 8.4, 0.8 Hz, 1H), 8.20 (dd, J = 7.6, 0.8 Hz, 1H), 7.93 (dd, J = 7.6, 0.8 Hz, 1H), 7.88 (dd, J = 7.6, 0.8 Hz, 1H), 7.74 (dd, J = 8.0, 4.4 Hz, 2H), 7.64 (ddd, J = 8.4, 7.6, 1.2 Hz, 1H), 7.54–7.46 (m, 2H), 7.44–7.39 (m, 1H), 7.39–7.31 (m, 2H), 7.24–7.18 (m, 2H), 7.16–7.11 (m, 1H), 7.06 (dd, J = 9.6, 5.6 Hz, 1H), 6.87–6.79 (m, 1H), 6.66 (dd, J = 8.0, 1.6 Hz, 1H), 6.55 (dd, J = 7.6, 1.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 156.92, 155.71, 155.22, 150.51, 138.56, 137.06, 136.73, 129.44, 129.15, 128.46, 127.13, 126.69, 126.40, 125.49, 124.44, 123.92, 123.54, 123.04, 122.69, 121.19, 120.66, 120.29, 118.20, 114.48, 113.73, 111.95, 57.71. IR (KBr, disk) ν 3060.15, 2925.56, 1676.69, 1631.58, 1594.74, 1394.74, 1226.31, 785.71 cm− 1. HRMS (ESI, m/z): [M + H]+ calcd for: C37H21NO 496.1701, found, 496.1704.
2.2. Preparation of materials 2.2.1. Synthesis of 7H-fluoreno[4,3-b]benzofuran-7-one A mixture of methyl 2-bromobenzoate (2.16 g, 10 mmol), dibenzo [b,d]furan-4-ylboronic acid (2.34 g,11 mmol) and 2 M K2CO3 (10 mL) in tetrahydrofuran (40 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.108 g, 0.092 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was recrystallization by ethyl alcohol, affording yellow solid methyl 2-(dibenzo[b,d]furan-4-yl)benzoate (2.84 g, 9.4 mmol, 94.04%). Then, to a round flask added methyl 2-(dibenzo [b,d]furan-4-yl)benzoate (1.51 g, 5 mmol) and 15 mL PPA (polyphosphoric acid), and then the mixture was heated to 140 °C and stirred by electric mechanical agitator for 4 h. After cooling to ambient temperature, 70 mL water was dropwise slowly into the mixture under 0 °C. After 1 h, the mixture was filtered under vacuum and washed with ethyl alcohol (3 × 20 mL). The solid was dried under vacuum for 20 h, 97
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2.2.4. Synthesis of spiro[benzo[b]fluoreno[3,4-d]thiophene-7,8′-indolo [3,2,1-de]acridine] (CZS) A mixture of 9-(2-bromophenyl)-9H-carbazole (1.61 g, 5 mmol) in the THF (40 mL) was treated with the n-BuLi (2 mL, 2.5 M, 5 mmol) under argon at − 78 °C for 30 min. Then, 7H-benzo[b]fluoreno[3,4-d] thiophen −7-one (1.43 g, 5 mmol) dissolved in 40 mL THF was added dropwise. The mixture was stirred for 30 min under argon at − 78 °C, and allowed to warm to the room temperature. After 12 h, the organic layer was washed with water and extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the alcohol product was obtained. Then the alcohol was added without further purification to a mixture of 5 mL concentrated aqueous HCl and acetic acid 50 mL. The mixture was heated to reflux and stirred for 2 h. The reaction mixture was cooled down to room temperature, poured into H2O and then extracted with dichloromethane for three times. The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by column chromatography on silica gel. The solid was dried under vacuum for 20 h, affording the white procedure (1.13 g, 2.2 mmol, 44.14%). 1H NMR (400 MHz, CDCl3) δ 8.31 (d, J = 8.4 Hz, 1H), 8.26 (d, J = 8.4 Hz, 1H), 8.20 (d, J = 7.6 Hz, 1H), 8.15 (dd, J = 6.8, 2.0 Hz, 1H), 8.09 (d, J = 7.6 Hz, 1H), 8.04–7.97 (m, 2H), 7.89 (dd, J = 7.6, 0.8 Hz, 1H), 7.69–7.62 (m, 1H), 7.56–7.45 (m, 3H), 7.42 (t, J = 7.6 Hz, 1H), 7.39–7.32 (m, 1H), 7.28 (d, J = 8.0 Hz, 1H), 7.23 (t, J = 3.6 Hz, 2H), 7.05 (t, J = 7.6 Hz, 1H), 6.86–6.79 (m, 1H), 6.64 (dd, J = 8.0, 1.6 Hz, 1H), 6.54 (dd, J = 7.6, 0.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 156.21, 154.37, 139.67, 138.62, 137.12, 136.80, 136.13, 135.05, 133.76, 132.08, 129.42, 128.96, 128.34, 128.09, 126.76, 126.39, 125.62, 124.79, 124.09, 123.36, 123.05, 122.70, 122.09, 121.65, 121.20, 118.23, 114.49, 113.74, 57.44. IR (KBr, disk) ν 3060.16, 2921.05, 1676.69, 1641.35, 1596.24, 1390.44, 1226.31, 784.21 cm− 1. HRMS (ESI, m/z): [M + H]+ calcd for: C37H21NS 512.1473, found, 512.1531.
Scheme 1. The structure and synthetic routes of the host materials. (i) Pd(PPh3)4, K2CO3 (2 M), THF, reflux. (ii) PPA, 120 °C. (iii) n-BuLi (2.5 M), THF, − 78 °C. (iv) HCl (11 mol/ L), AcOH, reflux.
two target compounds were identified and characterized by 1H and 13C NMR, as well as the HRMS. 3.2. Thermal properties The thermal stability and morphological property were investigated by the thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements, which were carried out under nitrogen atmosphere. As the Fig. 1 shown, the thermal decomposition temperatures (Td, corresponding to 5% weight loss) of CZO and CZS were 396 and 410 °C, respectively, and the glass transition temperature (Tg) of CZO was 138 °C. However, the glass transition temperature of CZS was not observed even in the second DSC scan. Both two host materials obtained high Td and Tg, which can be mainly attributed to their steric molecular structure [26]. Simultaneously, the Td of CZS was litter higher than that of CZO, which was closely correlated to the molecule weight. All above, both host materials exhibited great thermal stability, which could be beneficial for keeping morphological stability during the device operating process.
2.3. OLED fabrication measurement In a general operation procedure, OLED devices were fabricated under high vacuum (~ 10− 4 Pa) chamber by thermal evaporation of organic layers onto a clean glass substrate precoated with a 150 nm thick indium tin oxide (ITO) layer. Prior to use, the substrate was degreased in an ultrasonic bath by the following sequence: in detergent, de-ionized water, acetone, and isopropanol, and then cleaned in a UVozone chamber for 15 min. The typical deposition rates, monitored by oscillating quartz, were 1.0, 0.5, and 5.0 Å/s for organic materials, lithium fluoride (LiF), and aluminum (Al), respectively. The device active area defined by the overlap between the electrodes was 3 ∗ 3 mm2 in all case. The EL spectra, CIE coordinates and current-voltage characteristics of the devices were measured with a PHOTO RESEARCH SpectraScan PR 655 photometer and a KEITHLEY 2400 SourceMeter constant current source. All measurements were carried out immediately under ambient atmosphere without device encapsulation after the devices have been fabricated.
3.3. Optical properties The photophysical properties of CZO and CZS were investigated by ultraviolet-visible (UV–vis) and photoluminescence (PL) spectrometers in the tetrahydrofuran. As Fig. 2 shown, both two compounds exhibited similar UV–vis absorption bands between 250 nm and 360 nm. The maximum absorption peak around 350 nm could be assigned to molecular n–π* transitions of carbazole group [27]. At short wavelength, three characteristic peaks around 250 nm, 275 nm and 295 nm were related to the π–π* transitions of the accept units in CZO and CZS [28]. The corresponding optical band gaps of CZO and CZS estimated from the absorption edge of UV–vis spectra were 3.39 eV and 3.38 eV, respectively. As seen from the PL spectrum at the room temperature, the maximum emission peaks of CZO and CZS were 369 nm and 370 nm, respectively, completely overlapped with the maximum absorption peak of Ir(ppy)3 [29], implying that the Förster energy could be efficiently transferred from host to dopant. The fluorescence quantum yields (PLQYs) of CZO and CZS were also measured as 23.92% and 10.66%, respectively. The PLQYs of both two compounds are lower the reported compound SIA-F [10], which could own to the heavy atom effect. Furthermore, the lifetime of two host materials are also tested as 6.26 ns for CZO and 5.48 ns for CZS, respectively. Additionally, the phosphorescence spectra measured at 77 K were also shown in Fig. 2, and the triplet energy levels of CZO and CZS evaluated from the highest-energy vibronic sub-bands [30] of the phosphorescence spectra at 77 K were 2.73 eV and 2.67 eV, which were higher than the green
3. Results and discussion 3.1. Synthesis and characterization The structures and synthetic routes of the two target molecules were outlined in Scheme 1. Two key intermediate compounds 7H-fluoreno [4,3-b]benzofuran-7-one and 7H-benzo[b]fluoreno[3,4-d]thiophen-7one were synthesized through multiple steps according to the previous literatures [24–25]. Subsequently, the desired products could be synthesized by the ring closing reaction between the ketone compounds and 9-(2-bromophenyl)-9H-carbazole in 42.04% and 44.14% yield. The 98
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a) 125
b) CZO CZS
CZO
Endothermic
Weight (%)
100
75
50
25
0 100
200
300
400 500 600 Temperature (OC)
700
800
80
100
120 140 160 Temperature (OC)
180
200
Fig. 1. Differential scanning calorimetry curve of CZO and CZS compounds and thermogravimetry data of CZO compound.
0.0003
1.0 CZO CZS
0.0002 Current (A)
Normalized Intensity
0.8 0.6 0.4
CZO CZS
0.0001 0.0000 -0.0001
0.2
-0.0002 -0.0003 -2
0.0 250
300
350
400 450 500 Wavelength (nm)
550
600
-1
0
1 2 E ( V vs. SCE )
3
4
Fig. 3. The cyclic voltammograms of CZO and CZS compounds in tetrahydrofuran containing of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6), scan rate: 100 mV/s.
Fig. 2. UV–vis absorption spectra, fluorescence spectra and phosphorescence spectra of CZO and CZS compounds.
phosphor Ir(ppy)3 (ET = 2.41 eV) and the commonly used host material CBP (ET = 2.56 eV). All the results indicated that the two compounds sufficiently fulfilled the requirements of good host materials used in the green PHOLEDs.
[(3-pyridyl)-phen-3-yl]benzene, 40 nm)/LiF (1 nm)/Al(80 nm) were fabricated. Fig. 4 showed the energy levels of those materials used in the green PHOLEDs in this paper. In these multilayer device configurations, to reduce the charge injection barriers, TAPC and TmPyPB were utilized as hole transporting layer and electron transporting layer, respectively. To reduce the charge injection barriers, MoO3 and LiF were employed as hole-injection layer (HIL) and electron-injection layer (EIL), respectively. For comparison, the CBP-based device with the same multilayer configuration was also fabricated and tested. The current density-voltage-brightness (J-V-L) characteristics of CZS-based device, CZO-based device and CBP-based device were presented in the Fig. 5 with the corresponding date summarized in the Table 2. The turn-on voltages of CZS-based device and CZO-based device at 1 cd/m2 are 3.2 V and 3.1 V, respectively. The maximum brightness of device CZS-based device is 32,913 cd/m2 at the voltage of 8.6 V, and 46,794 cd/m2 for CZO-based device at the voltage of 8.7 V. Compared to the CBP-based device, both CZS-based device and CZObased device exhibited equivalent turn-on voltage and maximum luminance, suggesting that CZS and CZO could be host materials for application in green PHOLEDs. The dependence of current and power efficiency on luminance measured for CZS-based device and CZO-based device are shown in Figs. 6 and 7, respectively. The maximum external quantum, maximum current and power efficiency of CZO-based device were 14.5%, 50.8 cd/ A, and 42.8 lm/W, respectively, which were quite higher than that of CZS-based device (12.1%, 42.4 cd/A and 34.3 lm/W). In comparison,
3.4. Electrochemical properties and energy level The cyclic voltammetry measurements of CZO and CZS were employed to evaluate their electrochemical behaviors such as energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the results were classified in Fig. 3 and Table 1. It can be seen from the Fig. 3 that both the two compounds exhibited two reversible oxidation peaks, which could be attributed to the oxidation of indoloacridine and the hereroaromatic substituted fluorene ring [10]. The HOMO energy level, calculated from the onset oxidation potentials by the equation: EHOMO = − Eox − 4.4 eV [31], were − 5.76 eV and − 5.74 eV for CZO and CZS, respectively. Meanwhile, the lowest unoccupied molecular orbital (LUMO) energy levels, estimated from the EHOMO and optical band gaps, were − 2.37 eV and − 2.36 eV for CZO and CZS, respectively. 3.5. Electroluminescence To evaluate the practical electroluminescence properties of CZS and CZO as host materials, the multilayer devices configurations of ITO/ MoO3 (10 nm)/TAPC (4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline], 50 nm)/Host: dopant (8%, 15 nm)/TmPyPB (1,3,5-tri 99
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Table 1 Physical properties of CZS and CZO host materials.
CZO CZS
Abs λmax (nm)a
PL λmax (nm)a
Eg (eV)b
HOMO (eV)c
LUMO (eV)d
ET (eV)e
Td (°C)f
Tg (°C)f
Tm (°C)
PLQY (%)g
τ (ns)h
354 355
369 370
3.39 3.38
− 5.76 − 5.74
− 2.37 − 2.36
2.73 2.67
396 410
138 NA
281 321
23.92 10.66
6.26 5.48
a
Measured in tetrahydrofuran at room temperature. Estimated from onset of the absorption spectra (EgOpt = 1241/λonset). Calculated from cyclic voltammetry. d Calculated by the equation EHOMO = ELUMO − Eg. e Calculated by the first peak of phosphorescence spectra measured in the 2-methyltetrahydrofuran at 77 K. f Measured by TGA and DSC. g Measured by fluorescence spectrometry in tetrahydrofuran at room temperature. h Measured by time-correlated single photon counting in tetrahydrofuran at room temperature. b c
Fig. 4. The energy level diagram of device and the chemical structure of materials used in the device.
efficiency of 42.7 lm/W. Both CZS-based device and CZO-based device exhibited competitive EL performances compared to CBP-based device. And the device efficiencies of CZO were slight higher than that of CBP, which could be explained by the fact that the CZO possess higher triplet energy than the CBP. Furthermore, both CZS-based device and CZObased device presented quite lower roll-off efficiency with EQE of 14.3% at 1000 cd/m2 and 10.7% at 10000 cd/m2 for CZO, 11.9% at 1000 cd/m2 and 10.4% at 10000 cd/m2 for CZS, respectively. Apparently, both devices exhibited significantly high performances, which could be attributed to the reasonable HOMO and LUMO energy levels of the two compounds and suitable device structure. In addition, all those devices exhibited pure green emission at 516 nm with the CIE(x,y) (0.33, 0.61) and no residual emission from the host or other layer materials, manifesting that the electroluminescence was originated from the complete energy transfer from host to dopant.
CZS CZO CBP
200
2
10000 1000 2
150
Luminance (cd/m )
Current density (mA/cm )
250
100 100 10 50 1 0 0
2
4 6 voltage (V)
8
0.1 10
Fig. 5. Current density-voltage-luminance curves of PHOLED devices based on CZS, CZO and CBP.
4. Conclusion
the CBP-based device achieved a maximum external quantum of 13.8%, maximum current efficiency of 48.9 cd/A, and maximum power
In summary, Two spiro [fluorene-9,8′-indolo[3,2,1-de]acridine] derivatives were synthesized via Suzuki cross-coupling reaction and ring closing reaction for application in green PHOLEDs. Both two 100
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Table 2 Performance parameters of PHOLEDs with CZS and CZO host materials. Host
Turn-on voltage (V)a
Current efficiency (cd/A)b
Power efficiency (lm/W)b
EQE (%)b
CIE(x,y)c
CZS CZO CBP
3.2 3.1 3.0
40.7/42.1/36.8 49.1/50.9/37.6 48.9/48.7/39.2
33.8/30.3/18.6 42.9/38.8/18.7 42.7/33.7/18.1
11.5/11.9/10.4 13.8/14.3/10.7 13.8/13.7/11.1
(0.33, 0.61) (0.33, 0.61) (0.32, 0.61)
c
At 1 cd/m2. Estimated at the luminance of 100 cd/m2, 1000 cd/m2 and 10,000 cd/m2. Measured from the EL spectra by inverting into chromaticity coordinates on the CIE 1931 diagram.
a) 60 50 Current Efficiency (cd/A)
b) 15
100 CZS CZO CBP
80
40 60 30 40 20 20
10 0 1
10
100 1000 2 Luminance (cd/m )
0 10000
10 EQE (%)
b
Power Efficiency (lm/W)
a
CZS CZO CBP
5
0 1
10
100 1000 2 Luminance (cd/m )
10000
Fig. 6. (a) The current efficiency and power efficiency; (b) external quantum efficiency of the devices based on CZS, CZO and CBP. diodes, Adv. Mater. 25 (2013) 6801–6827. [4] Y. Tao, C. Yang, J. Qin, Organic host materials for phosphorescent organic lightemitting diodes, Chem. Soc. Rev. 40 (2011) 2943–2970. [5] N.J. Lundin, A.G. Blackman, K.C. Gordon, D.L. Officer, Synthesis and characterization of a multicomponent rhenium (I) complex for application as an OLED dopant, Angew. Chem. Int. Ed. 45 (2006) 2582–2584. [6] C.T. Chen, Y. Wei, J.S. Lin, M.V.R.K. Moturu, W.S. Chao, Y.T. Tao, C.H. Chien, Doubly ortho-linked quinoxaline/diphenylfluorene hybrids as bipolar, fluorescent chameleons for optoelectronic applications, J. Am. Chem. Soc. 128 (2006) 10992–10993. [7] H. Inomata, K. Goushi, T. Masuko, T. Konno, T. Imai, H. Sasabe, J.J. Brown, C. Adachi, High-efficiency organic electrophosphorescent diodes using 1,3,5-triazine electron transport materials, Chem. Mater. 16 (2004) 1285–1291. [8] H.H. Chou, C.H. Cheng, A highly efficient universal bipolar host for blue, green, and red phosphorescent OLEDs, Adv. Mater. 22 (2010) 2468–2471. [9] Y.K. Wang, Y.L. Deng, X.Y. Liu, X.D. Yuan, Z.Q. Jiang, L.S. Liao, A facile way to synthesize high-triplet-energy hosts for blue phosphorescent organic light-emitting diodes with high glass transition temperature and low driving voltage, Dyes Pigments 122 (2015) 6–12. [10] S. Thiery, D. Tondelier, B. Geffroy, O. Jeannin, J. Rault-Berthelot, C. Poriel, Modulation of the physicochemical properties of donor-spiro-acceptor derivatives through donor unit planarisation: phenylacridine versus indoloacridine. New hosts for green and blue phosphorescent organic light-emitting diodes (PhOLEDs), Chem. Eur. J. 22 (2016) 1–15. [11] M. Romain, D. Tondelier, B. Geffroy, A. Shirinskaya, O. Jeannin, J. Rault-Berthelot, C. Poriel, Spiro-configured phenyl acridine thioxanthene dioxide as a host for efficient PhOLEDs, Chem. Commun. 51 (2015) 1313–1315. [12] M.M. Xue, C.C. Huang, Y. Yuan, L.S. Cui, Q. Li, Y.X. Li, B. Wang, Z.Q. Jiang, M.K. Fung, L.S. Liao, De novo design of boron-based host materials for highly efficient blue and white phosphorescent OLEDs with low efficiency roll-off, ACS Appl. Mater. Interfaces 8 (2016) 20230–20236. [13] J. Jin, W. Zhang, B. Wang, G. Mu, P. Xu, L. Wang, H. Huang, J. Chen, D. Ma, Construction of high Tg bipolar host materials with balanced electron–hole mobility based on 1,2,4-thiadiazole for phosphorescent organic light-emitting diodes, Chem. Mater. 26 (2014) 2388–2395. [14] G.J. Tian, W.Q. Liang, Y. Chen, N. Xiang, Q.C. Dong, J.H. Huang, J.H. Su, A novel spiro-annulated host based on carbazole with good thermal stability and high triplet energy for efficient blue and green phosphorescent organic light-emitting diodes, Dyes Pigments 126 (2016) 296–302. [15] M. Romain, D. Tondelier, O. Jeannin, B. Geffroy, J. Rault-Berthelot, C. Poriel, Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: new host materials for efficient sky-blue PhOLEDs, J. Mater. Chem. C 3 (2015) 9701–9714. [16] J. Huang, J.H. Su, H. Tian, The development of anthracene derivatives for organic light-emitting diodes, J. Mater. Chem. 22 (2012) 10977–10989. [17] G.J. Tian, X. Wei, N. Xiang, J.H. Huang, J. Cao, Z. Wang, J. Zhang, J.H. Su, Small organic molecules based on oxazole/thiazole with excellent performances in green and red phosphorescent organic light-emitting diodes, RSC Adv. 6 (2016)
Normalized EL Intensity
1.0 CZS CZO CBP
0.8 0.6 0.4 0.2 0.0 400
500 600 Wavelength (nm)
700
800
Fig. 7. The normalized EL spectra of devices based on CZS, CZO and CBP.
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