New bipolar host materials based on methyl substituted pyridazine for high-performance green and red phosphorescent OLEDs

Accepted Manuscript New bipolar host materials based on methyl substituted pyridazine for highperformance green and red phosphorescent OLEDs Bin Jia, Hong Lian, Tijian Sun, Jiancong Wei, Jianhai Yang, Haitao Zhou, Jinhai Huang, Qingchen Dong PII:

S0143-7208(19)30385-7

DOI:

https://doi.org/10.1016/j.dyepig.2019.04.058

Reference:

DYPI 7513

To appear in:

Dyes and Pigments

Received Date: 18 February 2019 Revised Date:

12 April 2019

Accepted Date: 23 April 2019

Please cite this article as: Jia B, Lian H, Sun T, Wei J, Yang J, Zhou H, Huang J, Dong Q, New bipolar host materials based on methyl substituted pyridazine for high-performance green and red phosphorescent OLEDs, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.04.058. 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

New Bipolar Host Materials Based on Methyl Substituted Pyridazine

for

High-Performance

Green

and

Red

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Phosphorescent OLEDs

Bin Jiaa,*, Hong Lianb, Tijian Suna Jiancong Weia, Jianhai Yangc, Haitao Zhoud, Jinhai

a

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Huangd,*, Qingchen Dongb,*

School of Basic Medical Science, Shanxi Medical University, Taiyuan 030001,

China. E-mail: [email protected]

MOE Key Laboratory of Interface Science and Engineering in Advanced

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b

Materials and Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, 79 Yingze West Street, Taiyuan 030024, P. R. China.

Fax:

+86-351-6010311;

c

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[email protected].

Tel.:

+86-351-6010311;

E-mail:

Xi’an Research Institute of Hi-Tech, Xi’an, 710025, China.

d

Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, P. R. China. E-mail:

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[email protected]

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Graphical abstract

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Abstract In this work, two host materials, namely DAMP and DCMP were designed and synthesized based on 4,5-dimethylpyridazine and triphenylamine or carbazole. These two compounds exhibit good thermal stability, suitable highest occupied molecular

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orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and balanced carrier transport properties. Green phosphorescent organic light-emitting devices (PhOLEDs) based on DAMP and DCMP exhibit excellent performance with the maximum external quantum efficiencies (EQEs) of 19.5% and 22.2%, respectively.

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At the luminance of 1000 cd/m2, the EQE of the device based on DCMP can still reach to 21.4%. Besides, red PhOLEDs hosted by DAMP and DCMP also showed

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satisfied performance, with the maximum EQE of 15.3% and 20.0%, respectively. These results suggest that DAMP and DCMP are promising host materials for green and red PhOLEDs.

Keywords

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bipolar host materials

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Phosphorescent organic light-emitting diodes, pyridazine derivatives, high efficiency,

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1. Introduction Organic light-emitting diodes (OLEDs) have been intensively studied for application as new-generation flat-panel displays and energy-saving lighting sources.[1-5] Some OLED devices, such as smart phones, are beginning to be commercialized.

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Phosphorescent OLEDs (PhOLEDs) have attracted more and more attention because they can achieve 100% internal quantum efficiency (IQE) by heavy atom effect.[6-8] To avoid competitive factors such as triplet-triplet annihilation (TTA), PhOLEDs usually adopt host-dopant structure. Host materials are as important as emitters and

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impose significant effects on the electroluminescence (EL) performance of OLEDs.[9-10] An ideal host material must satisfy at least the following requirements:

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(1) high triplet energy level (ET) to ensure energy transfer to the guest; (2) appropriate HOMO/LUMO energy levels for an effective charge injection; (3) high glass transition temperature (Tg) and thermal decomposition temperatures (Td) to ensure device stability; (4) balanced hole and electron transport ability to broaden the exciton recombination zone in the emitting layer (EML).[11-15]

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Bipolar host materials containing both hole transporting and electron transporting units can satisfy the above points. However, the strong intramolecular charge transfer interactions between the donor and the acceptor groups in the bipolar

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hosts may result in low triplet energies and the subsequent back-transfer of energy from the guest to the host, thus reducing the device efficiency. The introduction of

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highly twisted π-conjugation can effectively address this issue.[16-19] In this work, by introducing methyl groups to the classic electron transporting moiety pyridazine, we reported two host materials with high triplet energy levels based on 4,5-dimethylpyridazine, 4,4'-(4,5-dimethylpyridazine-3,6-diyl)bis(N,N-diphenylaniline) 3,3'-(4,5-dimethylpyridazine-3,6-diyl)bis(9-phenyl-9H-carbazole)

namely (DAMP)

and

(DCMP).

Carbazole and triphenylamine were selected as hole transporting units. Methyl substituted pyridazine increased the steric hindrance of molecule and decreased the intramolecular charge transfer. As expected, high ET values of 2.62 and 2.56 eV were

ACCEPTED MANUSCRIPT obtained for DCMP and DAMP, respectively. In addition, these two compounds exhibit good thermal stability, appropriate HOMO/LUMO energy levels, and balanced carrier transport properties. Green devices based DAMP and DCMP exhibit maximum external quantum efficiencies (EQEs) of 19.5% and 22.2%, respectively.

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And the efficiency roll-off of these devices are satisfactory, for example, at the luminance of 1000 cd/m2, the EQE of the device based on DCMP can still reach to 21.4%. Red devices hosted by DAMP and DCMP also exhibit excellent efficiencies with the EQE of 15.3% and 20.0%, respectively.

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2. Experimental 2.1. Materials and measurement

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Chemicals and solvents used in the process were reagent grades and purchased from J&K Chemical Co. and Shanghai Taoe Chemical Co. without further purification. All reactions and manipulations were carried out under N2 atmosphere. Silica gel (300-400 mesh) column chromatography was used as the stationary phase in the column. Both DCMP and DAMP were synthesized via Suzuki coupling. 13

C NMR spectra were measured using a Bruker AM 400

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The 1H and

spectrometer. Infrared spectra were recorded on the Bruker Tensor27 FTIR spectrometer using KBr pellets for solid state spectroscopy. Mass spectra were

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obtained on a Waters LCT Premier XE spectrometer. The ultraviolet-visible (UV-Vis) absorption spectra of the samples were characterized using a Varian Cary 500

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spectrophotometer. Photoluminescence (PL) measurements were conducted by a Varian-Cary fluorescence spectrophotometer at room temperature. The cyclic voltammetry experiments were performed by a Versastat II electrochemical workstation (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, 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dichloromethane solution as the supporting electrolyte with a scan rate of 100 mV/s. The E1/2 values were determined by (Epa + Epc)/2 using ferrocene as an external standard, where Epa and Epc

ACCEPTED MANUSCRIPT were the anodic and catholic peak potentials, respectively. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere using a NETZSCH STA 409 PC/PG instrument with a heating scan rate of 10 oC/min. Thermogravimetric analysis (TGA) was carried out using a TGA instrument under a

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nitrogen atmosphere with a heating scan rate of 10 oC/min.

2.2. Synthesis

2.2.1. Synthesis of 4,4'-(4,5-dimethylpyridazine-3,6-diyl)bis(N,N-diphenylaniline)

A

mixture

of

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(DAMP) 3,6-dibromo-4,5-dimethylpyridazine

(0.2

g,

0.75

mmol),

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(4-(diphenylamino)phenyl)boronic acid (0.46 g, 1.6 mmol) and K2CO3 (0.31 g, 2.26 mmol) in tetrahydrofuran (THF) (5 mL) and deionized water (5 mL) was added to a round bottle flask and bubbled with N2 for 15 min. Then Pd(PPh3)4 (9 mg, 0.008 mmol) was added, and the resulting mixture was refluxed for 5 h under N2 protection. The reaction mixture was cooled down to room temperature, poured into H2O and

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extracted with dichloromethane (DCM) for three times. Then the combined organic layer was dried over anhydrous sodium sulfate and purified by column chromatography, affording the final product as a white solid (0.31 g, 69%). 1H NMR (400 MHz, CDCl3, δ): 7.47 (d, J = 8.8 Hz, 4H), 7.28 (dd, J = 8.0, 4.0 Hz, 8H), 7.17 (d,

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J = 8.4 Hz, 12H), 7.06 (t, J = 7.9 Hz, 4 H), 2.34 (s, 6H). 13C NMR (100 MHz, CDCl3, δ): 159.69, 148.26, 147.50, 134.81, 131.31, 130.52, 129.37, 124.88, 123.31, 122.58,

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16.63. IR (KBr): 3033 (υC-H) cm-1 for phenyl ring, 2960(υC-H) cm-1 for methyl group, 1591, 1488(υC=C) cm-1, 1278(υC-N) cm-1, 745, 695(δC-H) cm-1 for phenyl ring. HRMS (ESI, m/z): [M+H]+ calcd for C42H35N4, 595.2856, found, 595.2266.

2.2.2. Synthesis of 3,3'-(4,5-dimethylpyridazine-3,6-diyl)bis(9-phenyl-9H-carbazole) (DCMP) A

mixture

of

3,6-dibromo-4,5-dimethylpyridazine

(0.2

g,

0.75

mmol),

(9-phenyl-9H-carbazol-3-yl)boronic acid (0.45 g, 1.6 mmol) and K2CO3 (0.31 g, 2.26 mmol) in THF (5 mL) and deionized water (5 mL) was added to a round bottle flask

ACCEPTED MANUSCRIPT and bubbled with N2 for 15 min. Then Pd(PPh3)4 (9 mg, 0.008 mmol) was added, and the resulting mixture was refluxed for 5 h under N2 protection. The reaction mixture was cooled down to room temperature, poured into H2O and extracted with DCM for three times. Then the combined organic layer was dried over anhydrous sodium

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sulfate and purified by column chromatography, affording the final product as a white solid (0.28 g, 63%). 1H NMR (400 MHz, CDCl3, δ): 8.46 (s, 2H), 8.19 (d, J = 7.6, 2H), 7.70-7.61 (m, 10H), 7.56-7.44 (m, 8H), 7.35-7.30 (m, 2H), 2.47 (s, 6H).

13

C NMR

(100 MHz, CDCl3, δ): 160.63, 141.37, 140.92, 137.54, 135.09, 129.99, 129.79,

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127.67, 127.64, 127.16, 126.27, 123.42, 123.39, 121.83, 120.53, 120.26, 110.00, 109.59, 16.82. IR (KBr): 3054(υC-H) cm-1 for phenyl ring, 2854, 2924(υC-H) cm-1 for

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methyl group, 1594, 1495, 1454(υC=C) cm-1, 1373(υC-N) cm-1, 752, 707(δC-H) cm-1 for phenyl ring. HRMS (ESI, m/z): [M+H]+ calcd for C42H31N4, 591.2543, found,

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591.2549.

Scheme 1. Synthetic routes for DAMP and DCMP.

2.3 OLED fabrication and performance measurements Indium tin oxide (ITO)-coated glass substrates were cleaned with detergent, acetone, isopropanol and deionized water. The substrates should be dry under nitrogen and were then subjected to UV−ozone treatment for 15 min. PEDOT:PSS was spin-coated onto the ITO substrate at a speed of 3000 rpm for 1 min and annealed at 120 ºC for 10 min before they were loaded into a vacuum evaporation system. The organic

ACCEPTED MANUSCRIPT compounds were deposition in a high vacuum (~10-4 Pa) at the rate of 1−2 Å s−1. Then a cathode composed of LiF and Al metal was deposited sequentially onto the substrate. The performance parameters, including EL spectra, CIE coordinates, and J−V−B curves of the devices, were measured using a program-controlled Konica Minolta

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CS-2000 photometer and a source-measure-unit Keithley 2400 under ambient conditions at room temperature.

3. Results and discussion 3.1. Synthesis and characterization

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The structures and synthetic routes of target molecules (DAMP and DCMP) were depicted in Scheme 1. Both DAMP and DCMP were synthesized with satisfied yields

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via Suzuki coupling between 3,6-dibromo-4,5-dimethylpyridazine and corresponding arylboronic acid (1 and 2). The structures of DAMP and DCMP were identified and characterized by 1H and

13

C NMR, infrared (IR) spectrometer as well as the

high-resolution mass spectrometry (HRMS). 3.2. Thermal properties

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To guarantee long life time of OLEDs, it is essential for materials to have good thermal stability. Thus, the thermal properties of DAMP and DCMP were investigated by gravimetric analyses (TGA) and differential scanning calorimetry

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(DSC) measurements under nitrogen atmosphere (Table 1 and Fig. 1). The onset thermal decomposition temperatures (Td), corresponding to 5% weight loss, were 384

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and 425 °C for DAMP and DCMP, respectively, which demonstrates good thermal stabilities of both materials. Besides, the glass transition temperature (Tg) of DAMP and DCMP were 123 and 102 oC, respectively. The relatively higher Tg of DAMP could be ascribed to its more steric molecular structure.[20]

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100

DAMP DCMP

40

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60

DAMP DCMP

Exothermic

Weight (%)

80

o

102 C o 123 C

50

100

150

200

o

Temperature ( C) 0 100

200

300

400

500

600

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o

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20

Temperature ( C)

Fig. 1. Thermal properties of DAMP and DCMP.

Table 1 Properties of DAMP and DCMP. λmax

λmax

abs

em

a

(nm)

DCMP

343

a

325

ET

Tg

Td

(eV)b

(eV)c

(eV)d

(eV)e

(oC)f

(oC)f

3.24

5.23

1.99

2.56

123

384

3.47

5.58

2.11

2.62

102

425

(nm) 452

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DAMP

HOMO LUMO

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Eg

Compound

402

Measured in DCM.

b

Estimated from onset of the absorption spectra (Egopt = 1241/λonset).

c

Calculated from cyclic voltammetry.

d

Calculated by the equation EHOMO = ELUMO − Eg.

e

Calculated by the first peak of phosphorescence spectra measured at 77 K.

f

Measured by DSC and TGA.

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a

3.3. Photophysical properties The UV-vis absorption and photoluminescence spectra (at room temperature in DCM)

ACCEPTED MANUSCRIPT were shown in Fig. 2. The low energy band around 340 nm of DAMP can be assigned to the intramolecular charge transfer (ICT) from the electron-donating triphenylamine to the electron-accepting pyridazine moiety, while the π-π* transition of compound DCMP is around 330 nm.[21-22] It can be seen from PL spectra that the maximum

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emission peak of DAMP is 452 nm, but the fluorescence of DCMP is very weak. We speculate that the reason may be the limited conjugate system. Compared with the triphenylamine group in DAMP, the carbazole in DCMP is more rigid. The steric hindrance between methyl substituted pyridazine and rigid carbazole causes DCMP to exhibit a highly twisty structure, thus

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reducing the conjugate system.[23-24]

Additionally, the triplet energy level evaluated from the highest energy vibronic

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sub-band of the low temperature phosphorescent spectrum was 2.62 eV for DAMP and 2.56 eV for DCMP, respectively, which qualify them to be potential hosts for green and red PhOLEDs.

(a)

DAMP DCMP

0.6

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0.8

EP

0.4 0.2

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Normalized Absorption

1.0

0.0

250

300

350

Wavelength (nm)

400

450

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(b) DAMP DCMP

0.8

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0.6 0.4 0.2 0.0 350

400

450

500

0.2

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0.8

0.4

600

DCMP DAMP

EP

Normalized intensity

(c) 1.0

0.6

550

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Wavelength (nm)

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Normalized Intensity

1.0

0.0

450

500

550

600

Wavelength (nm)

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400

Fig. 2. (a) UV-Vis absorption spectra of DAMP and DCMP in DCM at the concentration of 1 × 10-5 mol/L. (b) Room temperature emission spectra of DAMP and DCMP in DCM at the concentration of 1 × 10-5 mol/L. (c) The corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at 77 K.

3.4. Electrochemical properties and density functional theory calculations To investigate the electrochemical properties of DAMP and DCMP, cyclic voltammetry (CV) was performed and the results were outlined in Fig. 3 and Table 1.

ACCEPTED MANUSCRIPT The HOMO levels of DAMP and DCMP were calculated from the onset oxidation potentials by the equation: EHOMO = −Εox − 4.4eV, while the LUMO levels can be estimated from the EHOMO and optical band gap.[11, [25]] Hence, the HOMO/LUMO energy levels of DAMP and DCMP were determined to be −5.23 eV/−1.99 eV and

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−5.58 eV/−2.11 eV, respectively. To further understand the molecular orbitals of these two compounds, density functional theory (DFT) calculation was carried out. As shown in Fig.4, LUMO orbitals are mainly located at the methyl substituted pyridazine and the HOMO levels are distributed on the electron donors tiphenylamine

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and carbazoles. The spatially separated HOMO and LUMO levels confirm the bipolar

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nature of these two materials.

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Current

DAMP DCMP

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0.0

0.5

1.0

1.5

2.0

Potential (V)

Fig. 3. Cyclic voltammograms of compounds DAMP and DCMP in DCM solution containing 0.1 M TBAPF6 electrolytes, scanning rate: 100 mV/s.

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Fig.4. Spatial distributions of the HOMO and LUMO levels for DAMP and DCMP.

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3.5 EL performance

Fig.5. The energy diagram and the molecular structures used in PhOLEDs devices. To further investigate the properties of these two compounds, green PhOLEDs

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utilizing DAMP (device G1) and DCMP (device G2) as host materials were fabricated. The device structures are ITO/PEDOT:PSS (40 nm)/TAPC (40 nm)/ TCTA

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(5 nm)/ Ir(ppy)3-host (5% in volume, 20 nm)/TmPyPB (50 nm)/LiF (0.6 nm)/Al (80 nm). Herein, Ir(ppy)3 was chosen as the green emitter; PEDOT:PSS and LiF were used as hole-injection layer (HIL) and the electron injection layer (EIL); TAPC and TmPyPB served as the hole-transporting layer (HTL) and electron-transporting layer (ETL); TCTA was used as the electron-blocking layer (EBL). The doping concentration of the EML was 5% in volume without further study. The energy diagram and the molecular structures of each layer were shown in Fig. 4.[10, 26-29] As shown in Fig. 6(a) and Table 2, both devices G1 and G2 exhibit quite low turn-on voltages of 3.2 V and 3.4V, respectively. These quite low voltages could be

ACCEPTED MANUSCRIPT explained by the fact that both DAMP and DCMP have appropriate HOMO energy levels, which are very similar to that of TCTA (−5.70 eV), thus improving hole injecting into the emitting layer and reduce the driving voltage. [30-31] The Devices G1 and G2 all exhibit pure green emission with maximum emission peak at 511 nm,

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which indicates a full energy transfer from the hosts to the dopant. Satisfactorily, devices G1 and G2 demonstrate excellent EL performances with the maximum current efficiencies (ηc) of 63.9 cd A-1 (EQE of 19.5%) and 72.5 cd A-1 (EQE of 22.2%), respectively. The outstanding device performances can be explained by the

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following two points. The first one is appropriate HOMO/LUMO energy levels. The HOMO/LUMO energy levels of DAMP and DCMP are −5.23 eV/−1.99 eV and

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−5.58 eV/−2.11 eV, respectively. The small energy barriers between the adjacent carrier transport layer and EML can facilitate carrier injection from the TCTA (−5.70/−2.40 eV) and TmPyPB (−6.68/−2.73 eV) to the EML. The second item is the excellent balanced charge-transport property of DAMP and DCMP, thus improving the EL performance of the devices. Besides, consistently outstanding chromaticity

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was also achieved for these two devices. As shown in Fig. 6(c), the EL spectra of devices G1 and G2 changed very little as the voltage increases from 3 to 10 V. Finally, it is worth mentioning that the both devices G1 and G2 exhibit quite low efficiency

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roll-off. For example, at the luminance of 1000 cd/m2, the EQE of devices G1 and G2 still remain at 19.0% and 21.5%, which are almost the same as the maximum. At the luminance of 5000 cd/m2, the EQE of devices G1 and G2 still can achieve 18.5% and

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17.7%, respectively.

Table 2. Electroluminescence Characteristics of PhOLEDs

devices

hosts

Von(V)a

Lmax(cd m-2)b

ηc (cd A-1)c

ηp (lm W-1)d

EQE (%)c

CIE(x, y)e

G1

DAMP

3.2

49832

63.9 62.1 60.2

47.2

19.5 19.0 18.5

0.32, 0.60

G2

DCMP

3.4

28602

72.5 70.7 57.6

52.5

22.2 21.5 17.7

0.32, 0.60

R1

DAMP

3.4

17544

28.4 19.2 14.6

24.1

15.3 10.2 7.9

0.62, 0.38

R2

DCMP

3.4

34280

37.0 33.2 28.7

28.2

20.0 18.0 15.4

0.62, 0.38

a

Von, turn-on voltage, at 1 cd m-2;

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Lmax, maximum luminance;

c

Order of measured values: maximum, at 1 000 cd m-2, at 5 000 cd m-2;

d

Maximum value,

e

Measured at 6 V.

10

DAMP DCMP

)

3

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10

100

2

10

0 4

6

8

1

Luminance (cd m

-2

10

)

4

200

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-2

Current Density (mA cm

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5

(a)

10

0

10

10

DAMP DCMP

100

AC C

10

10

1

10

100

1000 -2

Luminance (cd m

)

1 10000

EQE (%)

100

EP

Current Efficiency (cd A

-1

)

(b)

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Voltage (V)

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DAMP DCMP

1.0 0.8

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0.6 0.4 0.2 0.0 400

500

600

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Normalized Intensity

(c)

700

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Wavelength (nm)

Fig. 6. (a) Current density-voltage-luminance (J-V-L) characteristics; (b) ηc and EQE versus current density of green PhOLEDs; (c) EL spectra at 6 V.

0

4

6

10

4

10

3

10

2

10

1

10

0

)

5

-2

TE D

EP

100

AC C

Current Density (mA cm

-2

)

DAMP DCMP

10

8

Voltage (V)

10

12

Luminance (cd m

(a) 200

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DAMP DCMP

100

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EQE (%)

100

10

10

1 10

100

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Current Efficiency (cd A

-1

)

(b)

1000

1

10000

-2

1.0

DAMP DCMP

0.6 0.4 0.2

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0.8

EP

Normalized Intensity

(c)

)

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Luminance (cd m

0.0

AC C

400

500

600

700

Wavelength (nm)

Fig. 7. (a) Current density-voltage-luminance (J-V-L) characteristics; (b) ηc and EQE versus current density of red PhOLEDs; (c) EL spectra at 6 V.

To investigate the performance of DAMP and DCMP as host materials for lower triplet energy level iridium dopant, red PhOLEDs utilizing Ir(pq)2acac as emitter were fabricated with the same device structure as the green device. The as-prepared red OLEDs based on DAMP and DCMP were named as R1 and R2,

ACCEPTED MANUSCRIPT respectively. The current-voltage-luminance (J–V–L) characteristics and EL efficiency as well as the EL spectra are shown in Fig. 7 and summarized in Table 2. Benefit from appropriate HOMO/LUMO energy levels, devices R1 and R2 also exhibit low turn-on voltages, which is similar to the case of green PhOLEDs. Device R1 shows the

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maximum ηc of 28.4 cd A-1 and maximum EQE of 15.3%. Device R2 achieves higher performance with ηc of 37.0 cd A-1 and EQE of 20.0%. The efficiencies of red devices are slightly lower than that of green devices. The reason could be that red phosphors have the relatively lower photoluminescence quantum efficiency (PLQY).

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Low efficiency roll-off is also achieved for devices R1 and R2. For example, device R2 shows ηc of 33.2 cd A-1 and EQE of 18.3% at the brightness of 1000

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cd m-2.

4. Conclusion

In this work, two new host materials (DAMP and DCMP) based on methyl substituted pyridazine and triphenylamine/carbazole were designed and synthesized. Both two host materials exhibited good thermal stability, suitable HOMO/LUMO

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energy levels and balanced carrier transport properties. Green and red emitting devices based DAMP and DCMP were fabricated and excellent performances were achieved. Green devices based on DAMP and DCMP exhibit maximum quantum

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efficiencies of 19.5% and 22.2%, and red devices hosted by these two materials also exhibit excellent efficiencies with the EQE of 15.3% and 20.0% for DAMP and

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DCMP, respectively. Benefiting from the bipolar property, the efficiency roll-off of all devices is satisfactory. At the luminance of 1000 cd/m2, the EQE of the green device based on DCMP can still reach to 21.4%. These results suggest that DAMP and DCMP can be ideal host materials for high-performance PhOLEDs.

Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Grant No.: 61774109). This work was also supported by

ACCEPTED MANUSCRIPT the Youth “Sanjin” Scholar Program, Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, the Key R&D Project

of

Shanxi

Province

(International

cooperation

program,

No.

201603D421032), the Natural Science Foundation of Shanxi Province

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(2015011024), the 331 technology cultivation fund plan of school of basic medical science (201422), and the Youth foundation of Shanxi Medical University (No. 057614).

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References

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[1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913−5.

[2] Yang XL, Zhou GJ, Wong WY. Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic

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light-emitting devices. Chem Soc Rev 2015;44:8484−575. [3] Huang JH, Su JH, Tian H. The development of anthracene derivatives for organic light-emitting diodes. J Mater Chem 2012;22:10977−89.

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[4] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012;492: 234−8.

AC C

[5] Farinola GM, Ragni R. Electroluminescent materials for white organic light emitting diodes. Chem Soc Rev 2011;40:3467−82.

[6] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998;395:151−4. [7] Chi Y, Chou PT. Transition-metal phosphors with cyclometalating ligands: fundamentals and applications. Chem Soc Rev 2010;39:638−55. [8] Lee CW, Lee JY. Above 30% External Quantum Efficiency in Blue

ACCEPTED MANUSCRIPT Phosphorescent Organic Light-Emitting Diodes Using Pyrido[2,3-b]indole Derivatives as Host Materials. Adv Mater 2013;25:5450−4. [9] Chiu TL, Chen HJ, Hsieh YH, Huang JJ, Leung MK. High-Efficiency Blue Phosphorescence

Organic

Light-Emitting

Diode

with

Ambipolar

RI PT

Carbazole–Triazole Host. J Phys Chem C 2015;119:16846−52. [10] Jia B, Lian H, Chen Z, Chen Y, Huang J, Dong Q. Novel carbazole/indole /thiazole-based host materials with high thermal stability for efficient

SC

phosphorescent organic light-emitting diodes. Dyes Pigm 2017;147:552-559.

[11] Song WX, Shi LJ, Gao L, Hu PJ, Mu HC, Xia ZY, Huang JH, Su JH.

M AN U

[1,2,4]Triazolo[1,5-a]pyridine as a building block for universal host materials for high-performance RGB and White PhOLEDs. ACS Appl Mater Interfaces 2018; 10:5714−22.

[12] Li W, Li JY, Wang F, Gao Z, Zhang SF. Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs. ACS Appl

TE D

Mater Interfaces 2015;7: 26206−16.

[13] Jhulki S, Seth S, Ghosh A, Chow TJ, Moorthy JN. Benzophenones as Generic Host Materials for Phosphorescent Organic Light-Emitting Diodes. ACS Appl

[14] Cui

EP

Mater Interfaces 2016;8:1527−35. LS,

Kim

JU,

Nomura

H,

Nakanotani

H,

Adachi

C.

AC C

Benzimidazobenzothiazole-Based Bipolar Hosts to Harvest Nearly All of the

Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light-Emitting Diodes. Angew Chem Int Ed 2016;55:6864−8.

[15] Chen Y, Wei X, Cao J, Huang JH, Gao L, Zhang JH, Su JH, Tian H. Novel Bipolar Indole-Based Solution-Processed Host Material for Efficient Green and Red Phosphorescent OLEDs. ACS Appl Mater Interfaces 2017;9:14112−9. [16] Lai CC, Huang MJ, Chou HH, Liao CY, Rajamalli P, Cheng CH. m-Indolocarbazole Derivative as a Universal Host Material for RGB and White

ACCEPTED MANUSCRIPT Phosphorescent OLEDs. Adv Funct Mater 2015;25: 5548−56. [17] Liu H, Cheng G, Hu DH, Shen FZ, Lv Y, Sun GN Yang B, Lu P, Ma YG. A Highly Efficient, Blue-Phosphorescent Device Based on a Wide-Bandgap Host/FIrpic: Rational Design of the Carbazole and Phosphine Oxide Moieties on

RI PT

Tetraphenylsilane. Adv Funct Mater 2012;22:2830−6. [18] Zhang DD, Duan L, Li YL, Li H, Bin ZY, Zhang DQ, Qiao J, Dong GF, Wang LD,

Qiu

Y.

Towards

High

Efficiency

and

Low

Roll-Off

Orange

SC

Electrophosphorescent Devices by Fine Tuning Singlet and Triplet Energies of Bipolar Hosts Based on Indolocarbazole/1,3,5-Triazine Hybrids. Adv Funct

M AN U

Mater 2014;24:3551−61.

[19] Li W, Li JY, Liu D, Li DL, Zhang D, Dual n-type units including pyridine and diphenylphosphine oxide: effective design strategy of host materials for high-performance organic light-emitting diodes. Chem Sci 2016;7:6706−14. [20] Jin JJ, Zhang WZ, Wang B.; Mu GY, Xu P, Wang L, Huang H, Chen JS, Ma DG.

TE D

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 2014;26:2388−95.

EP

[21] Gudeika D, Norvaisa K, Stanislovaityte E, Bezvikonnyi O, Volyniuk D, Turyk P, Hladka I, Yashchuk VM, Grazulevicius JV. High-triplet-energy derivatives of

AC C

indole and carbazole as hosts for blue phosphorescent organic light-emitting diodes. Dyes Pigm 2017;139:487−97.

[22] Liu ZW, Qayyum MF, Wu C, Whited MT, Djurovich PI, Hodgson KO, Hedman B, Solomon EI, Thompson ME. A Codeposition Route to CuI−Pyridine Coordination Complexes for Organic Light-Emitting Diodes. J Am Chem Soc 2011;133:3700−3. [23] Huang JJ, Hung YH, Ting PL, Tsai YN, Gao HJ, Chiu TL, Lee JH, Chen CL, Chou PT, Leung MK. Orthogonally Substituted Benzimidazole-Carbazole

ACCEPTED MANUSCRIPT Benzene As Universal Hosts for Phosphorescent Organic Light-Emitting Diodes. Org Lett 2016;18:672-5. [24] Etherington MK, Gibson J, Higginbotham HF, Penfold TJ, Monkman AP. Revealing the spin-vibronic coupling mechanism of thermally activated delayed

RI PT

fluorescence. Nat Commum 2016;7:13680-1-7. [25] Li W, Li JY, Liu D, Li DL, Wang F. Cyanopyridine Based Bipolar Host Materials for Green Electrophosphorescence with Extremely Low Turn-On Voltages and

SC

High Power Efficiencies. ACS Appl Mater Interfaces 2016;8:21497−504.

[26] Hung WY, Chiang PY, Lin SW, Tang WC, Chen YT, Liu SH, Chou PT, Hung YT,

M AN U

Wong KT. Balance the Carrier Mobility To Achieve High Performance Exciplex OLED Using a Triazine-Based Acceptor. ACS Appl Mater Interfaces 2016;8:4811−8.

[27] Emil J, Boris M, Sigurd S, Hans A. Time-dependent density functional calculations of phosphorescence parameters for fac-tris(2-phenylpyridine)

[28] Boris

TE D

iridium. Chem Phys 2007;333:157−67. M,

Gleb

B,

Hans

A.

Principles of phosphorescent organic light emitting devices. Phys Chem Chem

EP

Phys 2014;16:1719-58.

[29] Gleb B, Boris M, Hans A. Theory and Calculation of the Phosphorescence

AC C

Phenomenon. Chem Rev 2017;117:6500-37.

[30] Zhang D, Duan L, Zhang D, Qiu Y. Towards ideal electrophosphorescent devices with low dopant concentrations: the key role of triplet up-conversion. J Mater Chem C;2014;2:8983-9. [31] Zhang D, Duan L, Zhang D, Qiao J, Dong G, Wang L, Qiu Y. Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet-triplet exchange energy without p- or n-doping layer. Org Electron; 2013;14:260-6.

ACCEPTED MANUSCRIPT Highlights Two host materials DAMP and DCMP based on 4,5-dimethylpyridazine and triphenylamine or carbazole were designed and synthesized. Green and red phosphorescent organic light-emitting devices (PhOLEDs) based

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EP

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M AN U

SC

efficiencies (EQE) of 22.2% and 20.0%, respectively.

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on DCMP exhibit excellent performance with the maximum external quantum