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Highly efficient bipolar host material based-on indole and triazine moiety for red phosphorescent light-emitting diodes
Accepted Manuscript Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-Emitting Diodes Yi Chen, Jingwei Xie, Zixing Wang, Jin Cao, Hongwei Chen, Jinhai Huang, Jianhua Zhang, Jianhua Su PII:
S0143-7208(15)00358-7
DOI:
10.1016/j.dyepig.2015.09.011
Reference:
DYPI 4922
To appear in:
Dyes and Pigments
Received Date: 20 August 2015 Revised Date:
9 September 2015
Accepted Date: 10 September 2015
Please cite this article as: Chen Y, Xie J, Wang Z, Cao J, Chen H, Huang J, Zhang J, Su J, Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent LightEmitting Diodes, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Highly Efficient Bipolar Host Material Based-on Indole and Triazine
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Moiety For Red Phosphorescent Light-Emitting Diodes
Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai Huang,a* Jianhua Zhangb* and Jianhua Sua
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Graphical abstract
ACCEPTED MANUSCRIPT
Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-Emitting Diodes
a
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Huang,a* Jianhua Zhangb* and Jianhua Sua
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Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East
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China University of Science & Technology, Shanghai 200237, P. R. China. Fax: (86) 21-64252288; Tel: (86) 21-64252288; E-mail: [email protected]. b
Key Laboratory of Advanced Display and System Applications, Ministry of
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Education, Shanghai University, 149 Yanchang Rd, Shanghai, 200072, China.
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E-mail: [email protected]
1
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Abstract Two novel indole-based, bipolar, host materials were designed and synthesized by introducing the triazine unit to the 5-position of indole moiety with a meta-linking
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strategy. The two host materials exhibited excellence bipolar transport abilities and the meta-linking analog presented a high Tg value (>120 oC). Furthermore, their electrochemical and photo-physical properties were also fully characterized and all
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the results exhibited that the meta-linking strategy made a significant effect on the
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physical properties of the two hosts. Moreover, the PHOLED devices using 9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) phenyl)-9H-carbazole
and
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
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le as hosts were also fabricated to evaluate the practical utilities of host materials. The devices achieved the maximum external quantum efficiencies of 17.53 % for para-linking compound and 14.53 % for meta-linking analog, respectively. In
efficiency
was
twice
that
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quantum
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particular, para-linking compound based device revealed the maximum external
4,4′-bis(N-carbazolyl)-1,10-biphenyl as the host.
2
of
the
device
using
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Keywords
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Red phosphorescent OLEDs; Host materials; Bipolar; indole- triazine derivatives;
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High efficiency; Thermal stability.
3
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1. Introduction
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Phosphorescent organic light-emitting devices (PHOLEDs) have been deemed to be potential candidates for the next generation technology for flat-panel display and solid-state lighting sources, as the heavy-metal phosphors can theoretically achieve 100
%
internal
quantum
efficiency
by
harvesting
both
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approximately
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electro-generated singlet and triplet excitons. [1] To avoid efficiency roll-off induced by the concentration quenching and triple-triple annihilation, those heavy-metal phosphors are normally homogeneously dispersed into a host matrix. [2] Thus, the design of host material with great performance is of equal importance to triple guest
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to achieve highly device efficiency.
Generally, an appropriate host material is required to satisfy several conditions. Firstly, the triplet energy level (ET) of host material should be higher than that of
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dopant to facilitate the total energy transfer from host to phosphorescent emitter and
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to better confine the triple exciton within the emitting layer. [3-5] To this goal, several strategies have been put forward to develop the host materials with limited π-conjugation. For example, either silicon or sp3 carbon was introduced into host material as the building blocks to decrease the π-conjugation, and the method of controlling the interconnection position between aromatics is normally implied to increase the distortion of the backbone structure and further enhance the triplet energy level (ET) of host materials. [6-8] Besides the appropriate triplet energy level, the 4
ACCEPTED MANUSCRIPT carrier transport property is another criteria to be met, as the occurrence of charge hopping in the emission layer would decrease the carrier recombination zone and directly lower the device efficiency. [9] To meet this requirement, both electron
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accepter moieties such as benzimidazole, [10] phosphine oxide, [11] triazine, [12] and oxadiazole [13] and electron donors groups like carbazole, [14] amine, [15] fluorine [16] and dibenzothiophene [17] should be introduced to modify the bipolar carrier
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abilities of hosts. In addition, a desired host material should still possess a high glass
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transition temperature (Tg) that could enhance the thermal and morphological stability and consequently extend the operational lifetime of the device. As known to all, 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) is widely used as a host material for red and green PHOLEDs due to its high triplet energy level and
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excellent hole transporting property. [18] Unfortunately, this carbazole-based derivative suffered from a low glass transition temperature, which would adversely affect the device lifetime and restrict their commercial application. [19] Over the
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years, tremendous efforts have been devoted to developing improved host materials to
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replace CBP. [20-23] It was reported that, indole type materials, which also possess high triple energy level and exceptional hole transporting properties, would be the potential host material for PHOLEDs. [24] Grigalevicius et al. reported a series of indole-based polymer host materials with high thermal stability. The device based on a polyether containing 2-phenylindol-1-yl moieties achieved the best performance with maximum power efficiency of 8.46 lm/W and current efficiency of 718 cd/m2. [25] However, the indole derivatives based small molecule host materials are still rare. 5
ACCEPTED MANUSCRIPT Taking the advantage of the indole-structure in the hosts, we designed and synthesized two novel host materials incorporating the indole-carbazole moiety and triazine unit with
a
meta-linking
strategy
[26]
in
this
article.
The
meta-linking
phenyl)-9H-carbazole
(MTZ)
and
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9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) para-linking
analog
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
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le (PTZ) were obtained through connecting the triazine unit to the 5-position of an
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indole unit. By installing the triazine unit to the indole, the two indole-triazine derivatives not only inherit good hole transport property, but also exhibit enhanced electron transport ability and more balanced bipolar charge transport ability. In addition, the MTZ presented a high Tg values above 120 oC. To further evaluate the
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EL performances of those host materials, the PHOLED devices using the Ir(mphmq)2acac as triple emitter were fabricated. The maximum external quantum efficiencies of the red devices based on PTZ and MTZ as host were 17.53 % and
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14.53 %, respectively, which show comparatively better device performances
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compared to the 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) based device. In particular, the para-maximum EQE of PTZ based device is twice as high as that of the device using CBP as host. 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. In 6
ACCEPTED MANUSCRIPT addition, the boronic acid pinacol esters (3 and 4) were obtained from Shanghai Taoe chemical technology Co., Ltd. Tetrahydrofuran (THF) was purified by distillation over sodium under a N2 atmosphere prior to use. All reactions and manipulations were
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carried out under a N2 atmosphere. All column chromatography was performed on silica gel (300-400 mesh) as the stationary phase in the column. All materials were purified further by vacuum sublimation prior to fabrication of OLED devices. 13
C NMR spectra were recorded on a Bruker AM 400 spectrometer
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The 1H and
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with tetramethylsilane as the internal standard. High-resolution mass spectra were measured on a Waters LCT Premier XE spectrometer. The IR spectra were recorded in the range 4000-600 cm-1 using the potassium bromide disk for solid samples by the FTIR instrument. The ultraviolet–visible (UV-Vis) absorption spectra were obtained
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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
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work station (Princeton applied research) using a conventional three-electrode
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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 dichloromethane/acetonitrile (7:3, v/v) solution 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 10 °C/min from 0 °C to 250 °C under nitrogen 7
ACCEPTED MANUSCRIPT 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.
2.2.1.
The
synthesis
of
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2.2. Preparation of materials 9-(4-iodophenyl)-9H-carbazole
9-(4-(5-bromo-1H-indol-1-yl) phenyl)-9H-carbazole (2).
(1)
and
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A mixture of 9-(4-bromophenyl)-9H-carbazole (7.66 g, 23.86 mmol) and dried
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tetrahydrofuran (100 mL) were added into a three-necked flask and bubbled with argon stirring for 15 min under -78 °C, then 2.5 M n-butyllithium (9.5 mL 23.86 mmol, n-hexane) were added to the mixture by a disposable syringe and continue to stirring for 30 min. Subsequently, iodine (7.27 g, 28.63 mmol) dissolved in
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tetrahydrofuran (50 mL) were added dropwise slowly to the reaction stirring for 1 h and resulting mixture was reacted overnight at room temperature. After cooling to ambient temperature, the mixture was extracted with water and dichloromethane. The
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organic layer was evaporated in vacuum affording yellow solid 1, which was used in
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the next reaction without future purification. To a two-necked flask added the obtained 1, 5-bromo-1H-indole (4.92 g, 25 mmol) in mesitylene (30 mL), potassium hydroxide (2.35 g, 19 mmol). Under argon atmosphere, cuprous iodide (0.14 g, 0.74 mmol) and phenanthrolene (0.13 g, 0.72 mmol) were added to the mixture, and the resulting mixture was refluxed for 8 h. After cooling to room temperature, the mixture was extracted with water and dichloromethane, and then the organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude 8
ACCEPTED MANUSCRIPT product was purified by column chromatography, affording a white solid 2 (2.56 g, 5.87 mmol, 28 %). mp: 152-154 °C. IR (KBr, disk) ν 3049.22, 1515.62, 1444.97, 1230.09, 744.40, 717.91 cm-1. 1H NMR (CDCl3, 400 MHz) δ 8.17 (d, J = 7.6 Hz, 3H),
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7.71 (dd, J = 8.8, 4.4 Hz, 5H), 7.49 – 7.45 (m, 5H), 7.35 – G7.30 (m, 3H), 6.69 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 140.77, 138.28, 136.24, 134.58, 132.01,
120.28, 113.84, 112.48, 111.96, 109.65, 103.62. The
synthesis
of
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2.2.2.
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131.15, 130.98, 128.99, 128.38, 126.13, 125.55, 125.50, 123.81, 123.53, 120.47,
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) phenyl)-9H-carbazole (PTZ).
A mixture of 2 (1.31 g, 3 mmol), boronic acidester 3 (1.57 g 3.6 mmol) and 2 M
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aq. K2CO3 (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.069 g, 0.006 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere.
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The reaction mixture was cooled down to room temperature, poured into H2O and
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then extracted with dichloromethane (3*20 mL). The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by SiO2 column chromatography, affording yellow solid PTZ (1.05 g, 1.78 mmol, 52.7 %). mp: 240-241 °C. IR (KBr, disk) ν 3052.17, 1603.92, 1586.32, 1515.52, 1446.92, 1230.09, 832.71 cm-1. 1H NMR (CDCl3, 400 MHz) δ 8.88 (d, J = 8.4 Hz, 2H), 8.82 (dd, J = 8.0, 1.6 Hz, 4H), 8.19 (d, J = 7.6 Hz, 2H), 8.08 (d, J = 1.2 Hz, 1H), 7.91 (d, J = 8.4 Hz, 2H), 7.85 – 7.74 (m, 5H), 7.70 – 9
ACCEPTED MANUSCRIPT 7.57 (m, 7H), 7.55 – 7.44 (m, 5H), 7.37 – 7.31 (m, 2H), 6.87 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 171.61, 146.26, 140.83, 138.63, 136.38, 136.00, 135.72, 134.44, 133.33, 132.49, 130.12, 129.51, 129.00, 128.67, 128.39, 127.48, 126.14,
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125.46, 123.53, 122.44, 120.47, 120.25, 120.12, 110.98, 109.70, 104.77, 100.12. HRMS (ESI, m/z): [M+H]+ calcd for: C47H32N5, 666.2658, found, 666.2657. 2.2.3.
The
synthesis
of
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9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
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phenyl)-9H-carbazole (MTZ).
A mixture of 2 (1.05 g, 2.4 mmol), boronic acidester 4 (1.25 g 2.9 mmol) and 2 M aq. K2CO3 (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.055 g, 0.0048 mmol)
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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 (3*20 mL). The combined organic
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layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum
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and the crude product was purified by SiO2 column chromatography, affording yellow solid MTZ (0.84 g, 1.43 mmol, 52.8 %). mp: 243-244 °C. IR (KBr, disk) ν 3063.94, 1586.26, 1515.62, 1447.91, 1333.12, 1230.09, 800.33, 700.09 cm-1. 1H NMR (CDCl3, 400 MHz) δ 9.08 (s, 1H), 8.80 (dd, J = 8.0, 1.6 Hz, 4H), 8.75 (d, J = 7.6 Hz, 1H), 8.18 (d, J = 7.6 Hz, 2H), 8.08 (d, J = 1.2 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 7.86 – 7.72 (m, 5H), 7.71 – 7.55 (m, 8H), 7.54 – 7.42 (m, 5H), 7.37 – 7.29 (m, 2H), 6.87 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 171.82, 171.69, 142.73, 140.85, 138.72, 10
ACCEPTED MANUSCRIPT 136.75, 136.30, 135.94, 135.52, 133.92, 132.55, 131.59, 130.11, 129.04, 128.68, 128.39, 127.93, 127.25, 126.14, 125.47, 123.53, 122.64, 120.48, 120.25, 120.10, 110.93, 109.72, 104.70. HRMS (ESI, m/z): [M+H]+ calcd for: C47H32N5, 666.2658,
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found, 666.2659. 2.3. OLED fabrication measurement
In a general operation procedure, OLED devices were fabricated under high
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vacuum (~10-4 pa) in a chamber by thermal evaporation of organic layers onto a clean
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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 UV-ozone chamber for 15 min. The typical deposition rates, monitored by oscillating quartz,
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were 0.5, 0.1, 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 cases.
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The current-voltage characteristics of devices were measured by a Keithley 2400
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electrometer in the dark at room temperature and only the luminance from the front face of the devices was collected with MinoltaL110 luminance meter. The electroluminescence (EL) spectra were measured with the PR650 spectrometer. All measurements were carried out under an ambient atmosphere without device encapsulation, immediately after the devices have been fabricated. 3. Result and discussion 3.1. Synthesis and characterization 11
ACCEPTED MANUSCRIPT Scheme 1 depicts the synthetic route to the target molecules MTZ and PTZ. Firstly, the 9-(4-bromophenyl)-9H-carbazole was treated with n-BuLi at -78 °C, followed by reaction with iodine affording the iodo compound 1. The iodo
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intermediate 1 was used without further purification to afford the key intermediate 2 by the copper-catalyzed Ullmann reaction. The desired products (MTZ and PTZ) could be achieved by the palladium-catalyst Suzuki–Miyaura reaction of the boronic
13
C NMR spectral as well as the high-resolution mass
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fully characterized by 1H and
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acid pinacol esters (3 and 4) and intermediate 2 in 53 % yield. All final products were
spectrometry (HRMS). Further purification was accomplished by train sublimation under vacuum. 3.2. Thermal properties
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The thermal properties of the compounds MTZ and PTZ were measured by the thermalgravimetric analyses (TGA) and differential scanning calorimetry (DSC). As
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shown in the Fig. 1 and Table 1, both materials exhibited the decomposition temperatures (Td, corresponding to 5 % weight loss) at 460 °C for MTZ and at 452 °C
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for PTZ, and the glass transition temperature (Tg) of MTZ was 124 ºC. However, a glass transition temperature of PTZ was not observed even in the second DSC scan. It was worth noting that MTZ showed both high Tg and Td values, mainly owning to its steric molecular structure. [27] The highly thermal stability of the new host materials would be desirable for enhancing the film morphology and device lifetime by reducing the crystallization and phase separation during device operation. 3.3. Photophysical properties 12
ACCEPTED MANUSCRIPT To investigate the photophysical properties of MTZ and PTZ, their UV-Vis absorption spectra and photoluminescence spectra as films and in the toluene and their phosphorescent spectra in 2-methyltetrahydrofuran at 77 K were presented in the Fig.
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2 and Table 1. As shown in the UV-Vis absorption spectra, a characteristic absorption peak around 340 nm was presented, which could be attributed to the n–π* transitions of N-phenylcarbazole. However, a higher frequency energy band of PTZ could be
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observed around 300 nm, which was assigned to the π–π* transitions of
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N-phenylindole. [28] The corresponding optical band gap of MTZ was 3.5 eV, estimated from the absorption edge of UV-vis spectra, which was higher than that of PTZ by 0.3 eV.
It can be seen from the PL spectra that, the maximum emission peaks of MTZ
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and PTZ were 407 nm and 409 nm in solution, whereas the maximum emission peaks of MTZ and PTZ in the solid state were red shifted to 425 nm and 428 nm, respectively, owing to the intermolecular π-stacking in the solid state. Meanwhile, the
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PTZ exhibited small red-shift compared to MTZ both in toluene and in the solid state,
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indicating that the para- substituent on the triazine unity would be beneficial for the delocalization of the π electron cloud. The relative fluorescence quantum yield were also tested with the method of using ratio value of integrated area under corrected fluorescence spectra and the anthracene in ethanol as the reference substance. As shown in Table 1, the fluorescence quantum yield was 0.19 for MTZ and 0.24 for PTZ. Additionally, the triplet energy level evaluated from the highest-energy vibronic 13
ACCEPTED MANUSCRIPT sub-bands of the low temperature phosphorescent spectra was 2.66 eV for MTZ and 2.47 eV for PTZ, which qualify them to be potential hosts for red PHOLEDs. Compared to the PTZ, the meta-disposition linkage MTZ exhibited a dramatically
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higher ET and a larger band gap, indicating that the meta-linking design strategy could partially reduce the π conjugation between the electron-withdrawing core and electron-deficient group. [29]
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3.4. Electrochemical properties and theoretical calculation
voltammetry
(CV)
using
tetra-n-butylammonium
a
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The electrochemical behaviors of MTZ and PTZ were investigated by cyclic conventional
hexauorophosphate
tri-electrode
system
(TBAPF6)
with in
dichloromethane/acetonitrile (7:3, v/v) as supporting electrolyte and the related date
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are presented in the Fig. 3 and Table 1. The MTZ and PTZ exhibit the onset oxidation potentials at 1.10 V and 1.16 V, respectively. Hence, the highest occupied molecular orbital (HOMO) energy levels, estimated from the onset oxidation potential
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by comparison to ferrocene (4.4 eV versus vacuum), [30] were calculated as -5.50 eV
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for MTZ and -5.56 eV for PTZ. Meanwhile, the lowest unoccupied molecular orbital (LUMO) energy levels, determined by the EHOMO and optical band gap, were -2.00 eV for MTZ and -2.36 eV for PTZ. To better understand the electronic structure of the two materials, density
functional theory (DFT) calculations were also carried out using B3LYP hybrid functional to simulate the HOMO-LUMO spatial distributions. It can be seen from the Fig. 4 that, the HOMO orbital distribution of the MTZ is mainly located on the indole 14
ACCEPTED MANUSCRIPT and carbazole moieties and the LUMO orbital distribution of MTZ is distributed on the triazine unities. Compared to the electron density distribution of MTZ, the PTZ exhibits rather dispersed over the molecule. The LUMO orbital distribution of PTZ is
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mostly located on the triazine unity and slightly extended to the indole group, while the HOMO orbitals distribution of PTZ is similar to that of MTZ. The different LUMO orbitals distribution between MTZ and PTZ could be attributed to the
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different linkage position, as the meta-linking strategy was beneficial to separate the
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HOMO and LUMO orbitals of those donor-acceptor type host materials and further improve their bipolar charge properties.
3.5. Bipolar transporting characteristics
To assess the bipolar transporting characteristics of the two host materials,
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single-carrier devices with the structure of [ITO / compounds (100 nm) / MoO3 (5 nm) / Al (80 nm)] for hole-only device and [ITO / LiF (1 nm) / compounds (100 nm) / LiF (1 nm) / Al (80 nm)] for electron-only device were fabricated. To better understand
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the bipolar carrier-transport ability of the two novel host materials, single-carrier
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devices based on the widely used host material CBP with the same architecture were also fabricated for comparison. It can be seen from Fig. 5, the J-V characteristics of these devices displayed that the current density of holes and electrons was smoothly rising with the increase of voltage, suggesting that these host materials exhibited good bipolar carrier-transport ability. Compared to the CBP, both PTZ and MTZ revealed relatively balanced hole and electron transport abilities with more improved electron-transport ability than hole-transport ability. We attributed this to the 15
ACCEPTED MANUSCRIPT introduction of the electron-transport moiety, which induced the separation of HOMO and LUMO orbitals and provided the transport channels for both holes and electrons, thus improving the balance of bipolar carrier transportation. To further investigate
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their bipolar character, the carriers mobility was also obtained by fitting the current-voltage characteristics of the only devices to the space-charge limited current (SCLC) model. The electron mobilities of PTZ and MTZ are around 9.88×10-7 and
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6.23×10-8 cm2 V-1 s-1, while the hole mobilities are around 8.61×10-9 and 5.70×10-8
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cm2 V-1 s-1, respectively, which were consistent with the single-carrier device.[31] It was worth noting that the MTZ based devices showed more balanced bipolar carrier transportation than that of PTZ. With the similar HOMO energy levels, this difference could be ascribed to that the LUMO energy level of para-substituted compound PTZ
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was lower than that of the meta-substituted compound MTZ. This leads to a further separation between HOMO and LUMO energy level, which favors the balanced transportation of holes and electrons. [32]
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3.6. Phosphorescent OLEDs
AC C
To evaluate the practical electroluminescence properties of MTZ and PTZ as host
materials,
the
multilayer
device
configurations
of
[ITO
/
N1-(naphthalen-2-yl)-N4,N4-bis(4-(naphthalen-2-yl(phenyl)amino)phenyl)-N1-phenylb enzene-1,4-diamine (2-TNATA, 60 nm) / tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA,
10
nm)
/
host:
dopant
1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene
(8
%, (TPBi,
25 10
nm)
/
nm)
/
4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm) / LiF (1 nm) / Al (80 nm)] were 16
ACCEPTED MANUSCRIPT fabricated. The device structure and the energy levels of employed materials is presented in the Fig. 6. For this multilayer device configurations, the red emissive Ir(mphmq)2acac [33] was doped in PTZ and MTZ with a doping concentration of
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8wt % to form emitting layers (EML) of device R1 and R2, respectively. To confine the excitions within the EML, TCTA and TPBi were utilized as electron-blocking layer (EBL) and hole-blocking layer (HBL), respectively. Moreover, to reduce the
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charge injection barriers, the 2-TNATA was used as hole-injection layer / transporting
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layer (HIL / HTL), while BPhen and LiF were employed as electron-transporting layer (ETL) and electron-injection layer (EIL), respectively. For comparison, the device R3 with the same multilayer configuration applying the commonly used red phosphorescent host CBP was also fabricated and tested.
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The current density-voltage-brightness (J-V-L) characteristics, curves of current and power efficiencies versus luminance and the electroluminescence (EL) spectra of devices were presented in the Fig.7 and the related data were summarized in the
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Table 2. As shown in Fig. 7, devices R1 and R2 exhibited the turn-on voltage of 4.6
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eV and 4.1 eV, respectively, which were higher than that of R3 (3.7 eV). We assigned this phenomenon to higher hole transporting ability of CBP than that of the synthesized host materials PTZ and MTZ. The maximum external quantum, maximum current and power efficiency of
device R1 hosted by PTZ were 17.53 %, 17.33 cd/A, and 11.17 lm/W, respectively, which were quite higher than those of MTZ based device R2 (14.53 %, 14.15 cd/A and 8.63 lm/W). In comparison, the device R3 using CBP as host material achieved a 17
ACCEPTED MANUSCRIPT maximum external quantum of 9.2 %, maximum current efficiency of 8.88 cd/A, and maximum power efficiency of 6.09 lm/W. Both devices, R1 and R2, exhibited significantly higher performances than device R3. Especially the maximum external
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quantum of device R1 was about twice higher than that of R3, which could be explained by the fact that the PTZ and MTZ possess a more balanced transportation of hole and electron carriers than the CBP as mentioned in the single-carrier devices.
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Therefore, a better carrier recombination within the emitting layer was achieved. [34]
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Furthermore, the device R1 presented quite better performance, even though the MTZ based single-carrier device shown a more balanced bipolar carrier-transporting ability than that of PTZ. A possible reason was that, generally, the migration rate of hole transport layer (HTL) was higher than that of electron transport layer (ETL) in
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the devices. As thus, the carrier-transport ability of the host made a greater effect on the device performance. The single-carrier device of PTZ revealed slightly higher electron-transportability than that of MTZ, and lower hole-transport ability than that
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of MTZ, causing the PTZ based device R1 to achieve a better carrier balance and
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then higher efficiency. In addition, all of those devices exhibited pure red emission at 618 nm with the CIE(x,y) (0.65, 0.34) and no residual emission from the host or other layer materials, manifesting that the electroluminescence was solely originated from the complete energy transfer from host to dopant. 4. Conclusion In summary, two novel indole-based bipolar host materials have been developed successfully via the palladium-catalyst Suzuki–Miyaura reaction for application in red 18
ACCEPTED MANUSCRIPT PHOLEDs. By installing the triazine unit to the 5-position of indole fragment, both host materials exhibit enhanced electron transport ability and then balanced transmission rate of holes and electrons. Meanwhile, the introduction of a
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meta-linking strategy extensively limits the π-conjugation, resulting in a steric bulk and high Tg value (>120 oC) for MTZ. Furthermore, the two hosts based red phosphorescent organic light-emitting devices were also fabricated to evaluate the
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practical utilities of the host materials. Both devices displayed excellent performance
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as the maximum EQEs of 17.53 % and 14.53 % were achieved for PTZ and MTZ, respectively. For comparison, the maximum EQE of PTZ based device is twice as high as that of the device using CBP as host. Acknowledgements
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Z. Wang thanks the financial support from National Natural Science Foundation of China (No.21302122), the National Basic Research Program of China (973 program 2015CB655005), Science and Technology Commission of Shanghai
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Municipality (13ZR1416600).
§
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Notes and References
These authors contributed equally to this work.
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ACCEPTED MANUSCRIPT Table 1. Physical properties of the new host materials MTZ and PTZ.
λabs max
λ
em max
Eg
HOMO
LUMO
ET
Td
Tg
[eV]d
[eV]e
[eV]f
[eV]g
[°C]h
[°C]h
Hole
Electron
mobility
mobility
c
φ
[nm]
[nm]b
a
[cm2 V-1
MTZ
340
407,425
0.19
3.5
-5.50
-2.00
2.66
PTZ
341
409,428
0.24
3.2
-5.56
-2.36
2.47
460
124
5.70 *10-8
452
NA
8.61*10-9
Measured in toluene solution at room temperature. b Measured in toluene and as film. c Relative fluorescence quantum yield were calculated using the method of ratio value of integrated area under corrected fluorescence spectra and the anthracene in ethanol as the reference substance. d Estimated from onset of the absorption spectra (EgOpt = f Calculated by the 1241 / λonset). e Calculated by the equation EHOMO = - 4.4 - Eox onset. g equation EHOMO = ELUMO - Eg. Calculated by the first peak of phosphorescence spectra measured at 77 K. h Measured by DSC and TGA. i the bipolar carrier mobility were calculated by fitting the current-voltage characteristics of the only devices to the space-charge limited current (SCLC) model.
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a
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s-1]i
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ηceb
ηpeb
ηextc
CIEd
[cd A-1]
[lm W-1]
[%]
[x,y]
Host
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Device
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Table 2. Electroluminescence characteristics of device.
R1
PTZ
17.33
11.17
17.53
0.65, 0.34
R2
MTZ
14.15
8.637
14.53
0.65, 0.34
R3
CBP
8.88
6.09
9.2
0.65,0.34
Efficiencies in the maxima. from the EL spectra at 8 V.
b
Maximum external quantum efficiency.
26
c
Measured
[cm2 V-1 s-1]i 6.23*109.88*107
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Fig. 1. The TGA of MTZ and PTZ. Fig. 2. (a) UV-Vis absorption spectra of MTZ and PTZ in toluene solution. (b)
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Room-temperature emission spectra of MTZ and PTZ as films and in the toluene (c) the corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at
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77 K.
Fig. 3. Cyclic voltammograms of compounds PTZ and MTZ.
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Fig. 4. Spatial distributions of the calculated HOMO and LUMO energy levels of PTZ and MTZ.
Fig. 5. J-V curves of the hole-only devices and electron-only devices for PTZ, MTZ
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and CBP.
Fig. 6. Schematic energy-level diagram of the red phosphorescent OLEDs and the molecular structures of the materials used.
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Fig. 7. (a) J-V-L characteristics, (b) current efficiencies and power efficiencies versus
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luminance curves and (c) electroluminescence (EL) spectra and the corresponding CIE1931 coordinates for devices based on PTZ and MTZ at 8 V.
27
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100
MTZ PTZ
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Weight(%)
80 60
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40
0 200
300
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20
400
500
600
700
800
o
Temperture ( C )
MTZ PTZ
Abs. (a.u.)
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a) 0.10
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Fig. 1. The TGA of MTZ and PTZ.
0.05
0.00 300
350 Wavelength (nm) 28
400
450
ACCEPTED MANUSCRIPT b)
1.0 MTZ solution PTZ solution MTZ film PTZ film
0.6
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Normalized (a.u.)
0.8
0.4
0.0 350
400
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0.2
450
500
550
600
Wavelength (nm)
c)
ET
TE D
MTZ PTZ
EP
0.5
ET
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Normalized (a.u.)
1.0
0.0 400
450
500
550
600
650
700
Wavelength (nm) Fig. 2. (a) UV-Vis absorption spectra of MTZ and PTZ in toluene solution. (b) Room-temperature emission spectra of MTZ and PTZ as films and in the toluene (c) the corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at 77 K. 29
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-1.0
-0.5
0.0
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MTZ PTZ
0.5
1.0
1.5
2.0
2.5
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Voltage (V)
HOMO
LUMO
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Fig. 3. Cyclic voltammograms of compounds PTZ and MTZ.
PTZ
MTZ
Fig. 4. Spatial distributions of the calculated HOMO and LUMO energy levels of PTZ and MTZ. 30
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4
10
3
10
Electron Only PTZ MTZ CBP Hole Only PTZ MTZ CBP
2
10
1
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10
0
-1
10
-2
10
-3
10
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2
J (mA/cm )
10
-4
10
-5 -6
10
-7
10
0
3
6
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10
9 12 Voltage (V)
15
18
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Fig. 5. J-V curves of the hole-only devices and electron-only devices for PTZ, MTZ and CBP.
Fig. 6. Schematic energy-level diagram of the red phosphorescent OLEDs and the 31
ACCEPTED MANUSCRIPT molecular structures of the materials used.
10
2
10
1
10
0
10
600
2
500
-1
3
4
5
Current Density (mA/cm )
10
3
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4
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10
700
PTZ MTZ CBP PTZ MTZ CBP
6
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2
Luminance (cd/m )
(a) 105
7
8
9
10
400 300 200 100 0
11
12
20
15
18 16 14 12
10
10 8 6
5
4 2
0 -1 10
10
0
1
2
10 10 10 2 Luminance (cd/m )
32
3
4
10
0
5
10
Power efficiency (lm/W)
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PTZ MTZ CBP PTZ MTZ CBP
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Current Efficiency (cd/A)
(b) 20
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Voltage (V)
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PTZ-8V MTZ-8V CBP-8V
0.5
0.0 400
450
500
550
600
650
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350
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EL intensity (a.u.)
(c) 1.0
700
750
Wavelength (nm)
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Fig. 7. (a) J-V-L characteristics, (b) current efficiencies and power efficiencies versus luminance curves and (c) electroluminescence (EL) spectra and the corresponding CIE1931 coordinates for devices based on PTZ, MTZ and CBP at 8 V.
33
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Scheme 1. Synthetic routes of new host materials MTZ and PTZ.
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Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-Emitting Diodes
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Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai Huang,a* Jianhua Zhangb* and Jianhua Sua
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Highlights
Two novel indole- triazine based bipolar host materials were developed.
•
The meta-linking analog presented relatively high glass transition temperature of
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•
124 ºC.
Phosphorescent organic light-emitting diodes with the two host materials possess
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a high maximum external quantum efficiencies >14%.
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•
S0143-7208(15)00358-7
DOI:
10.1016/j.dyepig.2015.09.011
Reference:
DYPI 4922
To appear in:
Dyes and Pigments
Received Date: 20 August 2015 Revised Date:
9 September 2015
Accepted Date: 10 September 2015
Please cite this article as: Chen Y, Xie J, Wang Z, Cao J, Chen H, Huang J, Zhang J, Su J, Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent LightEmitting Diodes, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highly Efficient Bipolar Host Material Based-on Indole and Triazine
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Moiety For Red Phosphorescent Light-Emitting Diodes
Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai Huang,a* Jianhua Zhangb* and Jianhua Sua
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Graphical abstract
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Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-Emitting Diodes
a
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Huang,a* Jianhua Zhangb* and Jianhua Sua
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Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East
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China University of Science & Technology, Shanghai 200237, P. R. China. Fax: (86) 21-64252288; Tel: (86) 21-64252288; E-mail: [email protected]. b
Key Laboratory of Advanced Display and System Applications, Ministry of
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Education, Shanghai University, 149 Yanchang Rd, Shanghai, 200072, China.
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E-mail: [email protected]
1
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Abstract Two novel indole-based, bipolar, host materials were designed and synthesized by introducing the triazine unit to the 5-position of indole moiety with a meta-linking
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strategy. The two host materials exhibited excellence bipolar transport abilities and the meta-linking analog presented a high Tg value (>120 oC). Furthermore, their electrochemical and photo-physical properties were also fully characterized and all
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the results exhibited that the meta-linking strategy made a significant effect on the
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physical properties of the two hosts. Moreover, the PHOLED devices using 9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) phenyl)-9H-carbazole
and
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
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le as hosts were also fabricated to evaluate the practical utilities of host materials. The devices achieved the maximum external quantum efficiencies of 17.53 % for para-linking compound and 14.53 % for meta-linking analog, respectively. In
efficiency
was
twice
that
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quantum
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particular, para-linking compound based device revealed the maximum external
4,4′-bis(N-carbazolyl)-1,10-biphenyl as the host.
2
of
the
device
using
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Keywords
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Red phosphorescent OLEDs; Host materials; Bipolar; indole- triazine derivatives;
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High efficiency; Thermal stability.
3
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1. Introduction
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Phosphorescent organic light-emitting devices (PHOLEDs) have been deemed to be potential candidates for the next generation technology for flat-panel display and solid-state lighting sources, as the heavy-metal phosphors can theoretically achieve 100
%
internal
quantum
efficiency
by
harvesting
both
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approximately
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electro-generated singlet and triplet excitons. [1] To avoid efficiency roll-off induced by the concentration quenching and triple-triple annihilation, those heavy-metal phosphors are normally homogeneously dispersed into a host matrix. [2] Thus, the design of host material with great performance is of equal importance to triple guest
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to achieve highly device efficiency.
Generally, an appropriate host material is required to satisfy several conditions. Firstly, the triplet energy level (ET) of host material should be higher than that of
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dopant to facilitate the total energy transfer from host to phosphorescent emitter and
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to better confine the triple exciton within the emitting layer. [3-5] To this goal, several strategies have been put forward to develop the host materials with limited π-conjugation. For example, either silicon or sp3 carbon was introduced into host material as the building blocks to decrease the π-conjugation, and the method of controlling the interconnection position between aromatics is normally implied to increase the distortion of the backbone structure and further enhance the triplet energy level (ET) of host materials. [6-8] Besides the appropriate triplet energy level, the 4
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accepter moieties such as benzimidazole, [10] phosphine oxide, [11] triazine, [12] and oxadiazole [13] and electron donors groups like carbazole, [14] amine, [15] fluorine [16] and dibenzothiophene [17] should be introduced to modify the bipolar carrier
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abilities of hosts. In addition, a desired host material should still possess a high glass
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transition temperature (Tg) that could enhance the thermal and morphological stability and consequently extend the operational lifetime of the device. As known to all, 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) is widely used as a host material for red and green PHOLEDs due to its high triplet energy level and
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excellent hole transporting property. [18] Unfortunately, this carbazole-based derivative suffered from a low glass transition temperature, which would adversely affect the device lifetime and restrict their commercial application. [19] Over the
EP
years, tremendous efforts have been devoted to developing improved host materials to
AC C
replace CBP. [20-23] It was reported that, indole type materials, which also possess high triple energy level and exceptional hole transporting properties, would be the potential host material for PHOLEDs. [24] Grigalevicius et al. reported a series of indole-based polymer host materials with high thermal stability. The device based on a polyether containing 2-phenylindol-1-yl moieties achieved the best performance with maximum power efficiency of 8.46 lm/W and current efficiency of 718 cd/m2. [25] However, the indole derivatives based small molecule host materials are still rare. 5
ACCEPTED MANUSCRIPT Taking the advantage of the indole-structure in the hosts, we designed and synthesized two novel host materials incorporating the indole-carbazole moiety and triazine unit with
a
meta-linking
strategy
[26]
in
this
article.
The
meta-linking
phenyl)-9H-carbazole
(MTZ)
and
RI PT
9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) para-linking
analog
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
SC
le (PTZ) were obtained through connecting the triazine unit to the 5-position of an
M AN U
indole unit. By installing the triazine unit to the indole, the two indole-triazine derivatives not only inherit good hole transport property, but also exhibit enhanced electron transport ability and more balanced bipolar charge transport ability. In addition, the MTZ presented a high Tg values above 120 oC. To further evaluate the
TE D
EL performances of those host materials, the PHOLED devices using the Ir(mphmq)2acac as triple emitter were fabricated. The maximum external quantum efficiencies of the red devices based on PTZ and MTZ as host were 17.53 % and
EP
14.53 %, respectively, which show comparatively better device performances
AC C
compared to the 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) based device. In particular, the para-maximum EQE of PTZ based device is twice as high as that of the device using CBP as host. 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. In 6
ACCEPTED MANUSCRIPT addition, the boronic acid pinacol esters (3 and 4) were obtained from Shanghai Taoe chemical technology Co., Ltd. Tetrahydrofuran (THF) was purified by distillation over sodium under a N2 atmosphere prior to use. All reactions and manipulations were
RI PT
carried out under a N2 atmosphere. All column chromatography was performed on silica gel (300-400 mesh) as the stationary phase in the column. All materials were purified further by vacuum sublimation prior to fabrication of OLED devices. 13
C NMR spectra were recorded on a Bruker AM 400 spectrometer
SC
The 1H and
M AN U
with tetramethylsilane as the internal standard. High-resolution mass spectra were measured on a Waters LCT Premier XE spectrometer. The IR spectra were recorded in the range 4000-600 cm-1 using the potassium bromide disk for solid samples by the FTIR instrument. The ultraviolet–visible (UV-Vis) absorption spectra were obtained
TE D
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
EP
work station (Princeton applied research) using a conventional three-electrode
AC C
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 dichloromethane/acetonitrile (7:3, v/v) solution 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 10 °C/min from 0 °C to 250 °C under nitrogen 7
ACCEPTED MANUSCRIPT 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.
2.2.1.
The
synthesis
of
RI PT
2.2. Preparation of materials 9-(4-iodophenyl)-9H-carbazole
9-(4-(5-bromo-1H-indol-1-yl) phenyl)-9H-carbazole (2).
(1)
and
SC
A mixture of 9-(4-bromophenyl)-9H-carbazole (7.66 g, 23.86 mmol) and dried
M AN U
tetrahydrofuran (100 mL) were added into a three-necked flask and bubbled with argon stirring for 15 min under -78 °C, then 2.5 M n-butyllithium (9.5 mL 23.86 mmol, n-hexane) were added to the mixture by a disposable syringe and continue to stirring for 30 min. Subsequently, iodine (7.27 g, 28.63 mmol) dissolved in
TE D
tetrahydrofuran (50 mL) were added dropwise slowly to the reaction stirring for 1 h and resulting mixture was reacted overnight at room temperature. After cooling to ambient temperature, the mixture was extracted with water and dichloromethane. The
EP
organic layer was evaporated in vacuum affording yellow solid 1, which was used in
AC C
the next reaction without future purification. To a two-necked flask added the obtained 1, 5-bromo-1H-indole (4.92 g, 25 mmol) in mesitylene (30 mL), potassium hydroxide (2.35 g, 19 mmol). Under argon atmosphere, cuprous iodide (0.14 g, 0.74 mmol) and phenanthrolene (0.13 g, 0.72 mmol) were added to the mixture, and the resulting mixture was refluxed for 8 h. After cooling to room temperature, the mixture was extracted with water and dichloromethane, and then the organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude 8
ACCEPTED MANUSCRIPT product was purified by column chromatography, affording a white solid 2 (2.56 g, 5.87 mmol, 28 %). mp: 152-154 °C. IR (KBr, disk) ν 3049.22, 1515.62, 1444.97, 1230.09, 744.40, 717.91 cm-1. 1H NMR (CDCl3, 400 MHz) δ 8.17 (d, J = 7.6 Hz, 3H),
RI PT
7.71 (dd, J = 8.8, 4.4 Hz, 5H), 7.49 – 7.45 (m, 5H), 7.35 – G7.30 (m, 3H), 6.69 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 140.77, 138.28, 136.24, 134.58, 132.01,
120.28, 113.84, 112.48, 111.96, 109.65, 103.62. The
synthesis
of
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2.2.2.
SC
131.15, 130.98, 128.99, 128.38, 126.13, 125.55, 125.50, 123.81, 123.53, 120.47,
9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) phenyl)-9H-carbazole (PTZ).
A mixture of 2 (1.31 g, 3 mmol), boronic acidester 3 (1.57 g 3.6 mmol) and 2 M
TE D
aq. K2CO3 (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.069 g, 0.006 mmol) was added to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere.
EP
The reaction mixture was cooled down to room temperature, poured into H2O and
AC C
then extracted with dichloromethane (3*20 mL). The combined organic layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude product was purified by SiO2 column chromatography, affording yellow solid PTZ (1.05 g, 1.78 mmol, 52.7 %). mp: 240-241 °C. IR (KBr, disk) ν 3052.17, 1603.92, 1586.32, 1515.52, 1446.92, 1230.09, 832.71 cm-1. 1H NMR (CDCl3, 400 MHz) δ 8.88 (d, J = 8.4 Hz, 2H), 8.82 (dd, J = 8.0, 1.6 Hz, 4H), 8.19 (d, J = 7.6 Hz, 2H), 8.08 (d, J = 1.2 Hz, 1H), 7.91 (d, J = 8.4 Hz, 2H), 7.85 – 7.74 (m, 5H), 7.70 – 9
ACCEPTED MANUSCRIPT 7.57 (m, 7H), 7.55 – 7.44 (m, 5H), 7.37 – 7.31 (m, 2H), 6.87 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 171.61, 146.26, 140.83, 138.63, 136.38, 136.00, 135.72, 134.44, 133.33, 132.49, 130.12, 129.51, 129.00, 128.67, 128.39, 127.48, 126.14,
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125.46, 123.53, 122.44, 120.47, 120.25, 120.12, 110.98, 109.70, 104.77, 100.12. HRMS (ESI, m/z): [M+H]+ calcd for: C47H32N5, 666.2658, found, 666.2657. 2.2.3.
The
synthesis
of
SC
9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
M AN U
phenyl)-9H-carbazole (MTZ).
A mixture of 2 (1.05 g, 2.4 mmol), boronic acidester 4 (1.25 g 2.9 mmol) and 2 M aq. K2CO3 (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.055 g, 0.0048 mmol)
TE D
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 (3*20 mL). The combined organic
EP
layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum
AC C
and the crude product was purified by SiO2 column chromatography, affording yellow solid MTZ (0.84 g, 1.43 mmol, 52.8 %). mp: 243-244 °C. IR (KBr, disk) ν 3063.94, 1586.26, 1515.62, 1447.91, 1333.12, 1230.09, 800.33, 700.09 cm-1. 1H NMR (CDCl3, 400 MHz) δ 9.08 (s, 1H), 8.80 (dd, J = 8.0, 1.6 Hz, 4H), 8.75 (d, J = 7.6 Hz, 1H), 8.18 (d, J = 7.6 Hz, 2H), 8.08 (d, J = 1.2 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 7.86 – 7.72 (m, 5H), 7.71 – 7.55 (m, 8H), 7.54 – 7.42 (m, 5H), 7.37 – 7.29 (m, 2H), 6.87 (d, J = 3.2 Hz, 1H). 13C NMR (CDCl3, 101 MHz) δ 171.82, 171.69, 142.73, 140.85, 138.72, 10
ACCEPTED MANUSCRIPT 136.75, 136.30, 135.94, 135.52, 133.92, 132.55, 131.59, 130.11, 129.04, 128.68, 128.39, 127.93, 127.25, 126.14, 125.47, 123.53, 122.64, 120.48, 120.25, 120.10, 110.93, 109.72, 104.70. HRMS (ESI, m/z): [M+H]+ calcd for: C47H32N5, 666.2658,
RI PT
found, 666.2659. 2.3. OLED fabrication measurement
In a general operation procedure, OLED devices were fabricated under high
SC
vacuum (~10-4 pa) in a chamber by thermal evaporation of organic layers onto a clean
M AN U
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 UV-ozone chamber for 15 min. The typical deposition rates, monitored by oscillating quartz,
TE D
were 0.5, 0.1, 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 cases.
EP
The current-voltage characteristics of devices were measured by a Keithley 2400
AC C
electrometer in the dark at room temperature and only the luminance from the front face of the devices was collected with MinoltaL110 luminance meter. The electroluminescence (EL) spectra were measured with the PR650 spectrometer. All measurements were carried out under an ambient atmosphere without device encapsulation, immediately after the devices have been fabricated. 3. Result and discussion 3.1. Synthesis and characterization 11
ACCEPTED MANUSCRIPT Scheme 1 depicts the synthetic route to the target molecules MTZ and PTZ. Firstly, the 9-(4-bromophenyl)-9H-carbazole was treated with n-BuLi at -78 °C, followed by reaction with iodine affording the iodo compound 1. The iodo
RI PT
intermediate 1 was used without further purification to afford the key intermediate 2 by the copper-catalyzed Ullmann reaction. The desired products (MTZ and PTZ) could be achieved by the palladium-catalyst Suzuki–Miyaura reaction of the boronic
13
C NMR spectral as well as the high-resolution mass
M AN U
fully characterized by 1H and
SC
acid pinacol esters (3 and 4) and intermediate 2 in 53 % yield. All final products were
spectrometry (HRMS). Further purification was accomplished by train sublimation under vacuum. 3.2. Thermal properties
TE D
The thermal properties of the compounds MTZ and PTZ were measured by the thermalgravimetric analyses (TGA) and differential scanning calorimetry (DSC). As
EP
shown in the Fig. 1 and Table 1, both materials exhibited the decomposition temperatures (Td, corresponding to 5 % weight loss) at 460 °C for MTZ and at 452 °C
AC C
for PTZ, and the glass transition temperature (Tg) of MTZ was 124 ºC. However, a glass transition temperature of PTZ was not observed even in the second DSC scan. It was worth noting that MTZ showed both high Tg and Td values, mainly owning to its steric molecular structure. [27] The highly thermal stability of the new host materials would be desirable for enhancing the film morphology and device lifetime by reducing the crystallization and phase separation during device operation. 3.3. Photophysical properties 12
ACCEPTED MANUSCRIPT To investigate the photophysical properties of MTZ and PTZ, their UV-Vis absorption spectra and photoluminescence spectra as films and in the toluene and their phosphorescent spectra in 2-methyltetrahydrofuran at 77 K were presented in the Fig.
RI PT
2 and Table 1. As shown in the UV-Vis absorption spectra, a characteristic absorption peak around 340 nm was presented, which could be attributed to the n–π* transitions of N-phenylcarbazole. However, a higher frequency energy band of PTZ could be
SC
observed around 300 nm, which was assigned to the π–π* transitions of
M AN U
N-phenylindole. [28] The corresponding optical band gap of MTZ was 3.5 eV, estimated from the absorption edge of UV-vis spectra, which was higher than that of PTZ by 0.3 eV.
It can be seen from the PL spectra that, the maximum emission peaks of MTZ
TE D
and PTZ were 407 nm and 409 nm in solution, whereas the maximum emission peaks of MTZ and PTZ in the solid state were red shifted to 425 nm and 428 nm, respectively, owing to the intermolecular π-stacking in the solid state. Meanwhile, the
EP
PTZ exhibited small red-shift compared to MTZ both in toluene and in the solid state,
AC C
indicating that the para- substituent on the triazine unity would be beneficial for the delocalization of the π electron cloud. The relative fluorescence quantum yield were also tested with the method of using ratio value of integrated area under corrected fluorescence spectra and the anthracene in ethanol as the reference substance. As shown in Table 1, the fluorescence quantum yield was 0.19 for MTZ and 0.24 for PTZ. Additionally, the triplet energy level evaluated from the highest-energy vibronic 13
ACCEPTED MANUSCRIPT sub-bands of the low temperature phosphorescent spectra was 2.66 eV for MTZ and 2.47 eV for PTZ, which qualify them to be potential hosts for red PHOLEDs. Compared to the PTZ, the meta-disposition linkage MTZ exhibited a dramatically
RI PT
higher ET and a larger band gap, indicating that the meta-linking design strategy could partially reduce the π conjugation between the electron-withdrawing core and electron-deficient group. [29]
SC
3.4. Electrochemical properties and theoretical calculation
voltammetry
(CV)
using
tetra-n-butylammonium
a
M AN U
The electrochemical behaviors of MTZ and PTZ were investigated by cyclic conventional
hexauorophosphate
tri-electrode
system
(TBAPF6)
with in
dichloromethane/acetonitrile (7:3, v/v) as supporting electrolyte and the related date
TE D
are presented in the Fig. 3 and Table 1. The MTZ and PTZ exhibit the onset oxidation potentials at 1.10 V and 1.16 V, respectively. Hence, the highest occupied molecular orbital (HOMO) energy levels, estimated from the onset oxidation potential
EP
by comparison to ferrocene (4.4 eV versus vacuum), [30] were calculated as -5.50 eV
AC C
for MTZ and -5.56 eV for PTZ. Meanwhile, the lowest unoccupied molecular orbital (LUMO) energy levels, determined by the EHOMO and optical band gap, were -2.00 eV for MTZ and -2.36 eV for PTZ. To better understand the electronic structure of the two materials, density
functional theory (DFT) calculations were also carried out using B3LYP hybrid functional to simulate the HOMO-LUMO spatial distributions. It can be seen from the Fig. 4 that, the HOMO orbital distribution of the MTZ is mainly located on the indole 14
ACCEPTED MANUSCRIPT and carbazole moieties and the LUMO orbital distribution of MTZ is distributed on the triazine unities. Compared to the electron density distribution of MTZ, the PTZ exhibits rather dispersed over the molecule. The LUMO orbital distribution of PTZ is
RI PT
mostly located on the triazine unity and slightly extended to the indole group, while the HOMO orbitals distribution of PTZ is similar to that of MTZ. The different LUMO orbitals distribution between MTZ and PTZ could be attributed to the
SC
different linkage position, as the meta-linking strategy was beneficial to separate the
M AN U
HOMO and LUMO orbitals of those donor-acceptor type host materials and further improve their bipolar charge properties.
3.5. Bipolar transporting characteristics
To assess the bipolar transporting characteristics of the two host materials,
TE D
single-carrier devices with the structure of [ITO / compounds (100 nm) / MoO3 (5 nm) / Al (80 nm)] for hole-only device and [ITO / LiF (1 nm) / compounds (100 nm) / LiF (1 nm) / Al (80 nm)] for electron-only device were fabricated. To better understand
EP
the bipolar carrier-transport ability of the two novel host materials, single-carrier
AC C
devices based on the widely used host material CBP with the same architecture were also fabricated for comparison. It can be seen from Fig. 5, the J-V characteristics of these devices displayed that the current density of holes and electrons was smoothly rising with the increase of voltage, suggesting that these host materials exhibited good bipolar carrier-transport ability. Compared to the CBP, both PTZ and MTZ revealed relatively balanced hole and electron transport abilities with more improved electron-transport ability than hole-transport ability. We attributed this to the 15
ACCEPTED MANUSCRIPT introduction of the electron-transport moiety, which induced the separation of HOMO and LUMO orbitals and provided the transport channels for both holes and electrons, thus improving the balance of bipolar carrier transportation. To further investigate
RI PT
their bipolar character, the carriers mobility was also obtained by fitting the current-voltage characteristics of the only devices to the space-charge limited current (SCLC) model. The electron mobilities of PTZ and MTZ are around 9.88×10-7 and
SC
6.23×10-8 cm2 V-1 s-1, while the hole mobilities are around 8.61×10-9 and 5.70×10-8
M AN U
cm2 V-1 s-1, respectively, which were consistent with the single-carrier device.[31] It was worth noting that the MTZ based devices showed more balanced bipolar carrier transportation than that of PTZ. With the similar HOMO energy levels, this difference could be ascribed to that the LUMO energy level of para-substituted compound PTZ
TE D
was lower than that of the meta-substituted compound MTZ. This leads to a further separation between HOMO and LUMO energy level, which favors the balanced transportation of holes and electrons. [32]
EP
3.6. Phosphorescent OLEDs
AC C
To evaluate the practical electroluminescence properties of MTZ and PTZ as host
materials,
the
multilayer
device
configurations
of
[ITO
/
N1-(naphthalen-2-yl)-N4,N4-bis(4-(naphthalen-2-yl(phenyl)amino)phenyl)-N1-phenylb enzene-1,4-diamine (2-TNATA, 60 nm) / tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA,
10
nm)
/
host:
dopant
1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene
(8
%, (TPBi,
25 10
nm)
/
nm)
/
4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm) / LiF (1 nm) / Al (80 nm)] were 16
ACCEPTED MANUSCRIPT fabricated. The device structure and the energy levels of employed materials is presented in the Fig. 6. For this multilayer device configurations, the red emissive Ir(mphmq)2acac [33] was doped in PTZ and MTZ with a doping concentration of
RI PT
8wt % to form emitting layers (EML) of device R1 and R2, respectively. To confine the excitions within the EML, TCTA and TPBi were utilized as electron-blocking layer (EBL) and hole-blocking layer (HBL), respectively. Moreover, to reduce the
SC
charge injection barriers, the 2-TNATA was used as hole-injection layer / transporting
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layer (HIL / HTL), while BPhen and LiF were employed as electron-transporting layer (ETL) and electron-injection layer (EIL), respectively. For comparison, the device R3 with the same multilayer configuration applying the commonly used red phosphorescent host CBP was also fabricated and tested.
TE D
The current density-voltage-brightness (J-V-L) characteristics, curves of current and power efficiencies versus luminance and the electroluminescence (EL) spectra of devices were presented in the Fig.7 and the related data were summarized in the
EP
Table 2. As shown in Fig. 7, devices R1 and R2 exhibited the turn-on voltage of 4.6
AC C
eV and 4.1 eV, respectively, which were higher than that of R3 (3.7 eV). We assigned this phenomenon to higher hole transporting ability of CBP than that of the synthesized host materials PTZ and MTZ. The maximum external quantum, maximum current and power efficiency of
device R1 hosted by PTZ were 17.53 %, 17.33 cd/A, and 11.17 lm/W, respectively, which were quite higher than those of MTZ based device R2 (14.53 %, 14.15 cd/A and 8.63 lm/W). In comparison, the device R3 using CBP as host material achieved a 17
ACCEPTED MANUSCRIPT maximum external quantum of 9.2 %, maximum current efficiency of 8.88 cd/A, and maximum power efficiency of 6.09 lm/W. Both devices, R1 and R2, exhibited significantly higher performances than device R3. Especially the maximum external
RI PT
quantum of device R1 was about twice higher than that of R3, which could be explained by the fact that the PTZ and MTZ possess a more balanced transportation of hole and electron carriers than the CBP as mentioned in the single-carrier devices.
SC
Therefore, a better carrier recombination within the emitting layer was achieved. [34]
M AN U
Furthermore, the device R1 presented quite better performance, even though the MTZ based single-carrier device shown a more balanced bipolar carrier-transporting ability than that of PTZ. A possible reason was that, generally, the migration rate of hole transport layer (HTL) was higher than that of electron transport layer (ETL) in
TE D
the devices. As thus, the carrier-transport ability of the host made a greater effect on the device performance. The single-carrier device of PTZ revealed slightly higher electron-transportability than that of MTZ, and lower hole-transport ability than that
EP
of MTZ, causing the PTZ based device R1 to achieve a better carrier balance and
AC C
then higher efficiency. In addition, all of those devices exhibited pure red emission at 618 nm with the CIE(x,y) (0.65, 0.34) and no residual emission from the host or other layer materials, manifesting that the electroluminescence was solely originated from the complete energy transfer from host to dopant. 4. Conclusion In summary, two novel indole-based bipolar host materials have been developed successfully via the palladium-catalyst Suzuki–Miyaura reaction for application in red 18
ACCEPTED MANUSCRIPT PHOLEDs. By installing the triazine unit to the 5-position of indole fragment, both host materials exhibit enhanced electron transport ability and then balanced transmission rate of holes and electrons. Meanwhile, the introduction of a
RI PT
meta-linking strategy extensively limits the π-conjugation, resulting in a steric bulk and high Tg value (>120 oC) for MTZ. Furthermore, the two hosts based red phosphorescent organic light-emitting devices were also fabricated to evaluate the
SC
practical utilities of the host materials. Both devices displayed excellent performance
M AN U
as the maximum EQEs of 17.53 % and 14.53 % were achieved for PTZ and MTZ, respectively. For comparison, the maximum EQE of PTZ based device is twice as high as that of the device using CBP as host. Acknowledgements
TE D
Z. Wang thanks the financial support from National Natural Science Foundation of China (No.21302122), the National Basic Research Program of China (973 program 2015CB655005), Science and Technology Commission of Shanghai
EP
Municipality (13ZR1416600).
§
AC C
Notes and References
These authors contributed equally to this work.
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RI PT
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TE D
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AC C
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ACCEPTED MANUSCRIPT Table 1. Physical properties of the new host materials MTZ and PTZ.
λabs max
λ
em max
Eg
HOMO
LUMO
ET
Td
Tg
[eV]d
[eV]e
[eV]f
[eV]g
[°C]h
[°C]h
Hole
Electron
mobility
mobility
c
φ
[nm]
[nm]b
a
[cm2 V-1
MTZ
340
407,425
0.19
3.5
-5.50
-2.00
2.66
PTZ
341
409,428
0.24
3.2
-5.56
-2.36
2.47
460
124
5.70 *10-8
452
NA
8.61*10-9
Measured in toluene solution at room temperature. b Measured in toluene and as film. c Relative fluorescence quantum yield were calculated using the method of ratio value of integrated area under corrected fluorescence spectra and the anthracene in ethanol as the reference substance. d Estimated from onset of the absorption spectra (EgOpt = f Calculated by the 1241 / λonset). e Calculated by the equation EHOMO = - 4.4 - Eox onset. g equation EHOMO = ELUMO - Eg. Calculated by the first peak of phosphorescence spectra measured at 77 K. h Measured by DSC and TGA. i the bipolar carrier mobility were calculated by fitting the current-voltage characteristics of the only devices to the space-charge limited current (SCLC) model.
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a
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s-1]i
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ηceb
ηpeb
ηextc
CIEd
[cd A-1]
[lm W-1]
[%]
[x,y]
Host
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Device
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Table 2. Electroluminescence characteristics of device.
R1
PTZ
17.33
11.17
17.53
0.65, 0.34
R2
MTZ
14.15
8.637
14.53
0.65, 0.34
R3
CBP
8.88
6.09
9.2
0.65,0.34
Efficiencies in the maxima. from the EL spectra at 8 V.
b
Maximum external quantum efficiency.
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c
Measured
[cm2 V-1 s-1]i 6.23*109.88*107
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Fig. 1. The TGA of MTZ and PTZ. Fig. 2. (a) UV-Vis absorption spectra of MTZ and PTZ in toluene solution. (b)
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Room-temperature emission spectra of MTZ and PTZ as films and in the toluene (c) the corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at
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77 K.
Fig. 3. Cyclic voltammograms of compounds PTZ and MTZ.
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Fig. 4. Spatial distributions of the calculated HOMO and LUMO energy levels of PTZ and MTZ.
Fig. 5. J-V curves of the hole-only devices and electron-only devices for PTZ, MTZ
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and CBP.
Fig. 6. Schematic energy-level diagram of the red phosphorescent OLEDs and the molecular structures of the materials used.
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Fig. 7. (a) J-V-L characteristics, (b) current efficiencies and power efficiencies versus
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luminance curves and (c) electroluminescence (EL) spectra and the corresponding CIE1931 coordinates for devices based on PTZ and MTZ at 8 V.
27
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100
MTZ PTZ
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Weight(%)
80 60
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40
0 200
300
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20
400
500
600
700
800
o
Temperture ( C )
MTZ PTZ
Abs. (a.u.)
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a) 0.10
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Fig. 1. The TGA of MTZ and PTZ.
0.05
0.00 300
350 Wavelength (nm) 28
400
450
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1.0 MTZ solution PTZ solution MTZ film PTZ film
0.6
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Normalized (a.u.)
0.8
0.4
0.0 350
400
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0.2
450
500
550
600
Wavelength (nm)
c)
ET
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MTZ PTZ
EP
0.5
ET
AC C
Normalized (a.u.)
1.0
0.0 400
450
500
550
600
650
700
Wavelength (nm) Fig. 2. (a) UV-Vis absorption spectra of MTZ and PTZ in toluene solution. (b) Room-temperature emission spectra of MTZ and PTZ as films and in the toluene (c) the corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at 77 K. 29
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-1.0
-0.5
0.0
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MTZ PTZ
0.5
1.0
1.5
2.0
2.5
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Voltage (V)
HOMO
LUMO
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Fig. 3. Cyclic voltammograms of compounds PTZ and MTZ.
PTZ
MTZ
Fig. 4. Spatial distributions of the calculated HOMO and LUMO energy levels of PTZ and MTZ. 30
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4
10
3
10
Electron Only PTZ MTZ CBP Hole Only PTZ MTZ CBP
2
10
1
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10
0
-1
10
-2
10
-3
10
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2
J (mA/cm )
10
-4
10
-5 -6
10
-7
10
0
3
6
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9 12 Voltage (V)
15
18
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Fig. 5. J-V curves of the hole-only devices and electron-only devices for PTZ, MTZ and CBP.
Fig. 6. Schematic energy-level diagram of the red phosphorescent OLEDs and the 31
ACCEPTED MANUSCRIPT molecular structures of the materials used.
10
2
10
1
10
0
10
600
2
500
-1
3
4
5
Current Density (mA/cm )
10
3
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4
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10
700
PTZ MTZ CBP PTZ MTZ CBP
6
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Luminance (cd/m )
(a) 105
7
8
9
10
400 300 200 100 0
11
12
20
15
18 16 14 12
10
10 8 6
5
4 2
0 -1 10
10
0
1
2
10 10 10 2 Luminance (cd/m )
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3
4
10
0
5
10
Power efficiency (lm/W)
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PTZ MTZ CBP PTZ MTZ CBP
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Current Efficiency (cd/A)
(b) 20
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Voltage (V)
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PTZ-8V MTZ-8V CBP-8V
0.5
0.0 400
450
500
550
600
650
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350
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EL intensity (a.u.)
(c) 1.0
700
750
Wavelength (nm)
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Fig. 7. (a) J-V-L characteristics, (b) current efficiencies and power efficiencies versus luminance curves and (c) electroluminescence (EL) spectra and the corresponding CIE1931 coordinates for devices based on PTZ, MTZ and CBP at 8 V.
33
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Scheme 1. Synthetic routes of new host materials MTZ and PTZ.
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Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-Emitting Diodes
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Yi Chen,a§ Jingwei Xie,b§ Zixing Wang,b Jin Cao,b Hongwei Chen,a Jinhai Huang,a* Jianhua Zhangb* and Jianhua Sua
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Highlights
Two novel indole- triazine based bipolar host materials were developed.
•
The meta-linking analog presented relatively high glass transition temperature of
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124 ºC.
Phosphorescent organic light-emitting diodes with the two host materials possess
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a high maximum external quantum efficiencies >14%.
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•